TiO2: Thermally and photolytically activated reactions

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surface science reports Surface Science Reports 71 (2016) 77–271 www.elsevier.com/locate/surfrep

Surface chemistry of Au/TiO2: Thermally and photolytically activated reactions Dimitar A. Panayotovb, John R. Morrisa,n b

a Department of Chemistry, Virginia Tech, Blacksburg, VA 24061-0212, USA Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria

Received 28 July 2015; received in revised form 13 November 2015; accepted 16 November 2015 Available online 5 May 2016

Abstract The fascinating particle size dependence to the physical, photophysical, and chemical properties of gold has motivated thousands of studies focused on exploring the ability of supported gold nanoparticles to catalyze chemical transformations. In particular, titanium dioxide-supported gold (Au/TiO2) nanoparticles may provide the right combination of electronic structure, structural dynamics, and stability to affect catalysis in important practical applications from environmental remediation to selective hydrogenation to carbon monoxide oxidation. Harnessing the full potential of Au/TiO2 will require a detailed atomic-scale understanding of the thermal and photolytic processes that accompany chemical conversion. This review describes some of the unique properties exhibited by particulate gold before delving into how those properties affect chemistry on titania supports. Particular attention is given first to thermally driven reactions on single crystal system. This review then addresses nanoparticulate samples in an effort begin to bridge the so-called materials gap. Building on the foundation provided by the large body of work in the field of thermal catalysis, the review describes new research into light-driven catalysis on Au/TiO2. Importantly, the reader should bear in mind throughout this review that thermal chemistry and thermal effects typically accompany photochemistry. Distinguishing between thermallydriven stages of a reaction and photo-induced steps remains a significant challenge, but one that experimentalists and theorists are beginning to decipher with new approaches. Finally, a summary of several state-of-the-art studies describes how they are illuminating new frontiers in the quest to exploit Au/TiO2 as an efficient catalyst and low-energy photocatalyst. & 2016 Elsevier B.V. All rights reserved.

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Introduction: Au nanostructures – overview of a fascinating material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 1.1. Historical perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 1.2. Au NPs in chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 1.3. Au NPs in photochemistry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 1.4. Review motivation and organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Gold nobleness, relativistic effects and properties of gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.1. Relativistic effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 2.2. Physical properties of gold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 2.3. Chemical properties of gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Isolated gold clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.1. Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3.2. Adsorption of molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

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Corresponding author. Tel.: +540 231 2472. E-mail address: [email protected] (J.R. Morris).

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3.2.1. Carbon monoxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.2.2. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 3.2.3. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 3.2.4. Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 3.3. Reactivity of gold clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.3.1. CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.3.2. Hydrogen peroxide formation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.3.3. Methanol oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.3.4. Activation and oxidation of alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 3.3.5. Activation and transformation of methane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Surface structure and reactivity of gold single crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 4.1. Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 4.2. Adsorption of molecules and atoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 4.2.1. Carbon monoxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 4.2.2. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 4.2.3. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 4.2.4. Water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 4.2.5. Nitrogen oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.2.6. Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.2.7. Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 4.3. Key oxidative reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 4.3.1. CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 4.3.2. Methanol oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 4.3.3. Propene oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Gold structures on ordered titania supports: planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 5.1. The interaction of gold with the oxide support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 5.2. Nucleation and growth of gold on oxide surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 5.2.1. Au/TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 5.2.2. Au/CeO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 5.2.3. Au/MgO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 5.2.4. Au/Al2O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 5.2.5. Au/SiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 5.2.6. Au/FeOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 5.3. Sintering of gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 5.4. Design and synthesis of sinter-resistant supported gold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 5.5. Titania: the most popular support for Au-based catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 High surface area powder gold–titania catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6.1. Design strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 6.2. Methods for deposition of gold on TiO2 powder supports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Adsorption of molecules on gold–titania surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 7.1. Carbon monoxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 7.1.1. Au/TiO2 planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 7.1.2. Au/TiO2 high surface area catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 7.2. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 7.2.1. Au/TiO2 planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 7.2.2. Au/TiO2 high surface area catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 7.3. Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7.3.1. Au/TiO2 planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 7.3.2. Au/TiO2 high surface area catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 7.4. Water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 7.4.1. Au/TiO2 planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 7.4.2. Au/TiO2 high surface area catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 7.5. Nitrogen oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 7.5.1. Au/TiO2 planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 7.5.2. Au/TiO2 high surface area catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 7.6. Alcohols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 7.6.1. Au/TiO2 planar model catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 7.6.2. Au/TiO2 high surface area catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 7.7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 Thermal activation of catalytic gold–titania systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 8.1. Factors for catalytic activity enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

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8.1.1. Quantum size effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2. Electronic effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.3. Low coordination sites in the clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.4. Fraction of perimeter sites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. Key catalytic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1. CO oxidation (planar model and high surface area catalysts). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2. H2 oxidation (planar model and high surface area catalysts) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3. Preferential CO oxidation in presence of H2 (PROX reaction) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4. Water–gas shift reaction (WGSR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5. Selective oxidation of ethene and propene (planar model and high surface area catalysts) . . . . . . . . . . . . . . . . . . 8.2.6. Selective oxidative dehydrogenation of alcohols and carboxylic acids. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.7. Total oxidation of volatile organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9. Nanoparticulate gold–titania photocatalytic systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. Strategies for light induced photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Major processes in light induced energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1. UV excitation of TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.2. UV excitation of Au/TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.3. VIS excitation of Au/TiO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. Surface plasmon resonance in gold nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1. Light–metal nanoparticle interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2. Models and calculations of SPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1. The dipolar approximation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2. The effective dielectric function theories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3. Effects of particle size and shape, surrounding medium, and metal NPs’ interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1. Size effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2. Shape effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.3. Surrounding medium effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.4. Interactions of metal NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Plasmonic gold–titania photocatalytic systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Characteristics for plasmonic photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Design strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1. Sole-metal form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2. Embedded form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3. Encapsulated form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.4. Isolated form. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3. Preparation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1. Incorporation of metal NPs onto semiconductors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2. Shape and configuration control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12. Photophysical mechanisms for plasmon-mediated photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1. Direct photocatalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2. Indirect photocatalysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1. SPR-mediated metal-semiconductor charge injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.2. Near-field effects and scattering mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.3. Resonant energy transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.4. The role of interband transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.5. Light-to-heat conversion: Role of the irradiation regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13. Key photocatalytic reactions under vis-light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1. Dissociation of diatomic molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.1. Hydrogen dissociation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.2. Oxygen dissociation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2. Hydrogen generation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1. Water splitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.2. Hydrogen evolution from water/sacrificial agent mixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.3. LSPR enhanced reverse water gas shift reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3. Photodegradation of organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.1. Photodegradation of organics in liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.3.2. Photodegradation of volatile organic compounds (VOCS) in the gas phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14. Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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161 164 165 168 170 170 179 181 183 186 188 194 196 196 197 197 199 201 203 203 204 206 207 208 208 209 211 211 212 212 212 212 213 213 213 213 214 215 215 218 219 220 223 225 227 228 230 230 230 231 232 232 236 239 239 239 248 251 256 256

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1. Introduction: Au nanostructures – overview of a fascinating material 1.1. Historical perspective Gold, because of its natural beauty, immutability, and rarity, has arguably had a greater impact on civilization than perhaps any other material on earth. This fascinating metal has driven economies for millennia, been used to fashion the most valued works of art ever created, and sparked the rise and fall of entire empires. Many civilizations throughout history have adopted gold as a symbol of divinity, wealth, and power [1–3]. For example, archaeological discoveries showed that gold was used in divination or funeral rituals in the Varna cemetery (Bulgaria) (dated circa 5th millennium B.C.), in the tombs of Egyptian pharaohs (around the 5th or 4th century B.C.), and in ancient China. The use of gold as currency originated in ancient Greece; the oldest gold coins found in Lydia are dated to the 6th century B.C. “Soluble” gold was also used in funeral rituals in Egypt and China [3–5]. In Roman Imperial times, colloidal gold was the critical component in ruby glass and for coloring ceramics [6]. Until the Middle Ages, soluble gold was used in medicine to treat various diseases. One of the most famous and often cited examples, the Roman Lycurgus Cup (5th to 4th century B.C.), is red in transmitted light and dark green upon reflection due to the presence of gold colloids [3,7]. Fig. 1.1 shows photographs of the cup under a variety of lighting conditions. The photon scattering and plasmon absorption phenomena responsible for these effects have been the subject of intense scientific interest over the centuries [7].

In the 17th century, metallic nanoparticles that produce extraordinary colors in colloidal suspension were used to make the brilliantly colored stained glass used in cathedrals at that time [8]. Several books published during the 17th and 18th centuries described the formation of colloidal gold and its uses in medicine [3]. Finally, in 1857, the art of preparing gold in the form of nanoparticles was systematically studied and described by Michael Faraday in his pioneering work [9]. In that publication, Faraday describes gold as being present in solution in a “finely divided metallic state” [9]. For many years however, metallic gold had been considered to be inert and relatively uninteresting from a synthetic chemist's point of view [3]. Despite the relatively low interest in gold from a synthetic chemist's perspective, the chemical physics community has long studied the material in pursuit of a fundamental understanding of its interesting physical properties. This interest was significantly buoyed by the development of nanotechnologies in the final two to three decades of the 20th century [3]. New methods for creating particles of gold enabled scientists to revisit many previously observed phenomena on much better characterized systems. In fact, the unique stability of gold makes it particularly suitable for the synthesis of nanoscale structures in which the majority of atoms are present at the surface. Most other metals are so unstable under ambient conditions that passivating oxide films rapidly covers them. Thus, gold holds great appeal for the exploration of fundamental properties and practical applications in the field of nanoscience [10,11]. The development of new synthetic methods, aided by our progressively improved understanding of nanoscale materials, has helped material scientists and chemists design stabilized

Fig. 1.1. The Lycurgus Cup. Origin of the extraordinary optical response: artifact illuminated from the outside, reflected light (a), and (b) from the inside, transmitted light. Reprinted with permission from Ref. [7]. Copyright 2007 World Gold Council.

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gold clusters with precise control over the size and size distribution. For example, Schmid and co-workers [12] reported in 1981 the delicate synthesis of a Au55(PPh3)12Cl6 cluster with unique narrow dispersity (1.4 7 0.4 nm) for use in studies of quantum-dot nanomaterials. Further progress in fabrication showed that thermally stable and air stable gold nanoparticles could routinely be created with detailed control over size and dispersity [3,13–15]. Parallel to efforts in particle synthesis, the development of advanced characterization techniques was absolutely crucial to designing gold nanostructures. High resolution imaging and spectroscopic methods greatly improved the understanding of gold size-structure dependence and how to synthesize and stabilize particles at the nanoscale [3]. Fig. 1.2 shows a series of high-resolution STEM images recorded for Au-decorated TiO2 surfaces that exemplify how exquisitely one can control and characterize the physical dimensions of particles. Such advances have enabled scientists to create colloidal suspensions that push the color spectrum far beyond that employed in ancient Roman artwork. Fig. 1.3 shows colloidal solutions for particles with sizes between 3 and 100 nm. In fact, scientists can now use particle size and geometry to span a tremendous spectral range of absorptivities. Fig. 1.3c shows UV– vis spectra for a series of particles of variously sized spherical shapes; the peak in the plasmon resonance for particles occurs between
512 and
572 nm as the size of AuNPs increases.

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1.2. Au NPs in chemistry Although the first reported example of using gold in catalysis appeared in 1925 [18], the chemistry of gold did not received significant attention until the 1970s [19–24]. In 1985, Hutchings [25] helped discover the application of supported gold as a catalyst in the hydrochlorination of acetylene. Although Hutchings' work received a considerable amount of attention, it was not until the publication of two ground-breaking studies by Haruta and co-workers in 1987 and 1989 [26,27] that interest in the chemical properties of gold particles began to develop. The Haruta work showed, for the first time, that finely dispersed small supported gold nanoparticles (Au NPs) could catalytically convert CO to CO2, even well below room temperature. That work demonstrated to the world the remarkable catalytic potential of supported Au NPs. At the time this review was written, the 1987 publication by Haruta had received 1500 citations – a number that continues to grow significantly each month. The importance of these publications stems from the remarkable data provided in Fig. 1.4. Fig. 1.4a shows that the most active catalyst sample identified in the Haruta work contained AuNPs with mean diameters of 4.57 1.6 nm and exhibited unprecedented (at the time) high efficiency towards CO oxidation: 100% at temperatures as low as 203 K, far below results obtained for other analogous systems [27].

Fig. 1.2. High-resolution STEM image of epitaxial AuNP with the epitaxial relationship Au(111)[1 10]||TiO2(110)[001]: (a) 4.3 nm width; (b) 10.9 nm width; and (c) the best-fit Wulff shape has been overlaid on one half of the 10.9 nm AuNP. Reprinted with permission from Ref. [16]. Copyright 2010 American Physical Society.

Fig. 1.3. Colloidal aqueous solution of gold nanoparticles with sizes from 3 to 100 nm showing orange, red or purple coloring (a). Concentration: 0.01% (w/v) based on gold. (b) TEM image of AuNPs; (c) UV–vis spectra of AuNPs with increasing size (from left to right). Reprinted with permission from Ref. [17]. Copyright 2016 Nanocos Inc. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 1.4. (a) Oxidation efficiencies of CO over various catalysts as a function of temperature. (1) Au/α-Fe2O3 (Au/Fe¼ l/19, coprecipitation, 400 1C); (2) 0.5 wt% Pd/γ-Al2O3 (impregnation, 300 1C); (3) Au fine powder; (4) Co304 (carbonate, 400 1C); (5) NiO (hydrate, 200 1C); (6) α-Fe2O3 (hydrate, 400 1C); (7) 5 wt% Au/αFe2O3 (impregnation, 200 1C); (8) 5 wt% Au/γ-Al2O3 (impregnation, 200 1C). Reprinted with permission from Ref. [27]. Copyright 1989 Elsevier B.V. (b) TEM image of Au/α-Fe2O3 (Au/Fe¼l/19, coprecipitate calcined at 400 1C), mean diameters of AuNPs 4.5 nm with a standard deviation of 1.6 nm, 3146 particles scanned. Reprinted with permission from Ref. [28]. Copyright 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Six to eight years after the initial Haruta publications, Hammer and Norskov introduced a simple concept to explain the nobleness of bulk gold metal, as well as trends in catalytic activity for transition metals [29]. The so-called d-band model poses that molecular dissociative adsorption (hence, catalytic activity) is enhanced for transition metals with d-band energies above the Fermi level. However, the d-band of Au, located below the Fermi level, is filled; therefore, both the bonding and antibonding orbitals, which are formed upon hybridization of the adsorbate level with the metal, are typically filled. A filled antibonding state leads to high barriers for adsorption and reaction. This model has proven extremely effective in predicting the catalytic activity of many systems [30] and has also been invoked to explain the activity of small Au NPs. That is, this model holds if the d-band of gold shifts partially above the Fermi level with decreasing particle size [31]. However, Wang and Hammer [32] have suggested that the activity of small clusters on reducible oxide supports is more intimately tied to charge transfer from peripheral Au atoms to oxygen adatoms, which leads to an interfacial dipole that plays a major role in activating chemistry, especially in the presence of polar reactants. Such on-going discussions in the literature further the intrigue associated with this fascinating metal catalyst made from the most noble metal. The growth in research interest in the field of gold catalysis over the latter part of the 20th century and into the new millennium is summarized in many excellent review articles, including those by Bond and Thompson [33–35], Haruta [36–38], and Hashmi and Hutchings [39]. Several important discoveries highlight gold-based systems as the catalyst of choice for a number of reactions. In fact, Hutchings suggests that “gold is not just a good catalyst, it is the best” [11]. In addition to high activity under mild temperature and pressure conditions, gold catalysts have demonstrated remarkable selectivity towards specific reactions, e.g. selective oxidation of hydrocarbons and alcohols [40–42]. Further, small gold clusters (3–10 atoms) generated in situ from gold salts or other complexes were found to be extremely active species in homogeneous gold-

catalyzed organic reactions [43]. Such homogeneous chemistry is closely related to heterogeneous catalysis in this case. In fact, Corma and Garcia emphasized the convergence of homogeneous and heterogeneous catalysis by gold nanoparticles [44,45]. Parallel to the more “realistic” high area catalysts, model catalytic systems utilizing gold nanostructures dispersed on flat single crystal support surfaces have also been explored. Such elegant studies, particularly from the fields of surface science and spectroscopy, are providing deep insight into why gold can function as a special catalyst for a broad range of transformations [11,46–52]. The reader is referred to several important monographs and reviews for thorough coverage of this field [53–56]. Special issues of journals, such as Chemical Society Reviews (37 from 2008), Faraday Discussions (152 from 2011) and Accounts of Chemical Research (47 from 2014) have presented collections of reviews that describe the current state-of-the-art research in gold chemistry and materials science, including catalysis. 1.3. Au NPs in photochemistry Beyond the remarkable activity of Au NPs for thermal catalysis, these particles have been shown to be highly effective at sensitizing photocatalytic chemistry. Au NPs have been shown to enhance UV photochemistry by serving as an electron sink for conduction band electrons generated in TiO2, thereby enhancing electron–hole pair separation and lifetimes. Such charge separation is enhanced by the formation of a Schottky junction, a natural result of the contact between the metal nanoparticle and semiconductor. The junction creates a space-charge region that can force the electrons and holes to move in different directions, thus suppressing the electron– hole recombination rate [57,58]. However, one should recognize that the Schottky barrier height (SBH) depends entirely upon the energy of the work function of the semiconductor and the Fermi energy of the metal, as well as the extent of band bending, which does not occur for very small particles [58].

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Furthermore, the actual distribution of charge at the metal– semiconductor interface does not follow a simple superposition principle; new “interface states” (analogous to surface states) depend on the precise atomic arrangement at the interface separating the two materials. These effects highlight the reality that the nature of the Schottky junction may change significantly from sample to sample as the electronic structure of particles is affected by the properties of the particles and the character of the interface [59]. Unfortunately, existing models of the SBH do not accurately describe this complex electronic structure [59]. Such major research challenges should be borne in mind while exploring the work cited within this review. Remarkably, Au NPs have also been shown to enable photocatalytic processes in the visible range, even for large bandgap semiconductors. The visible light-driven activity is related to the generation of surface plasmons. Surface plasmons occur when “the photon frequency matches the natural frequency of metal surface electrons oscillating against the restoring force of their positive nuclei” [60]. The plasmon resonance of dilute suspensions of spherical gold particles, which have a red–purple color (see Fig. 1.3), is observed as a broad absorption peak centered at ca. 520 nm [4]. After the remarkable studies of Faraday, much of which focused on the intense colors exhibited by colloidal gold, the theoretical work of Gustav Mie from 1908 explained the phenomena by solving Maxwell’s equations for the absorption and scattering of electromagnetic radiation by spherical metallic particles. Later in 1912, Richard Gans generalized Mie theory to spheroidal particles of any aspect ratio in the small particle approximation. More modern, yet still pioneering, studies in the groups of El-Sayed [8,61,62] and Schatz [63,64] showed that the wavelength and intensity of surface plasmon resonance (SPR) depend not only on the nature of the metal, but also on the size and shape of metal nanoparticles and the surrounding media. Further experimental and theoretical work found that an SPR leads to a build-up of an intense, spatially non-homogeneous, oscillating electric field in the neighborhood of the nanoparticle. Thus, surface plasmons essentially act as a concentrator of the energy of incoming photons in a small volume surrounding the nanostructure, which makes the plasmonic nanostructures very suitable for various applications involving photocatalysts, sensors, optoelectronic devices, etc. [8]. In 1981, Nitzan and Brus [65] predicted that the

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electromagnetic field concentration via surface plasmons can lead to a significant enhancement in the rate of photochemical transformations. Soon thereafter, photocatalysis based on plasmonic nanostructures emerged as a new field that combined the fields of plasmonics and catalysis. The excitation of localized surface plasmon resonance (LSPR) in metal nanoparticles can drive the catalytic reaction directly on the surface of metal nanoparticles or remotely on the surface of nearby support nanoparticles. In direct photocatalysis, the metal nanoparticles act as the light absorbers and the active sites (see Fig. 1.5a), whereas in indirect photocatalysis, the energy of an SPR is transferred from the metal nanoparticles to the nearby support where catalysis occurs (see Fig. 1.5b). The support material can be inert (e.g. textile fibers), conductive (e.g. graphene and graphene oxide), or semiconducting (e.g. titanium oxide). A clear indication that excitation of gold/support nanocomposites can drive photocatalysis was provided by numerous studies, which demonstrated that wavelength-dependent reaction rates peak at the frequency where the plasmon intensity is the highest. Since gold nanoparticles have large SPR absorption cross-sections that fall in the visible region of the solar electromagnetic spectrum, the development of gold-based plasmonic photocatalysts appears very promising for real applications such as environmental remediation, photocatalytic hydrogen generation, etc.

1.4. Review motivation and organization Although many excellent review articles have been published on thermal catalysis, and many others on the photocatalysis of Au NPs supported on various metal oxides, these two critical aspects of the overall chemistry of Au NPs on TiO2 supports have not been included in a single review article. As will be highlighted throughout the current work, thermal chemistry and thermal effects always accompany and cannot be separated from the photochemistry. This review strives to bring the two fields together in a way that provides a comprehensive overview of Au/TiO2 catalysis in its many forms. There are several overarching themes that emerge from such a review, which are instructive to highlight here as background for the remainder of this document:

Fig. 1.5. (a) Schematics of plasmon-induced H2 dissociation on the AuNP surface. Reprinted with permission from Ref. [66]. Copyright 2013 American Chemical Society. (b) Major processes in light-induced photocatalysis on Au/TiO2. Reprinted with permission from Ref. [67]. Copyright 2013 IOP PUBLISHING.

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(1) Both components of the catalyst, the Au NPs and the TiO2 support, play critical roles in the majority of the key mechanistic steps of thermally activated and photolytic reactions [68,69]. (2) Strong electronic effects caused by the coexistence of these two materials facilitate a great deal of unique chemistry [70]. (3) The results from many seemingly similar studies can differ significantly from one another because the sample particle size, geometry, preparation conditions, and history all affect the nature of the catalyst in ways that are not always accounted for experimentally. (4) Although seldom studied directly, the geometry and electronic structure of small Au NPs are likely to be dynamic, as this high-energy material with many low-coordinated atoms tends toward lower energy configurations during surface adsorption and release of molecules. (5) Synergistic effects, such as the adsorption of one reactant affecting the energetics of the other reactant, likely play a major role in the overall chemistry. (6) Finally, despite the tremendous research effort currently underway to understand Au/TiO2 catalysis and photocatalysis, there is tremendous need for new studies that can provide a clearer understanding as to precisely which thermal reaction mechanisms, which plasmon decay processes, and which charge transfer phenomena are active under different conditions. Fortunately, any future research efforts in this area will build on the large body of work associated with the support, TiO2, which represents one of the best studied oxide systems, both as a single crystal and a high surface area material. The reader is encouraged to first study the classic Surface Science Review article by Diebold [71] (more than 4500 citations) and the recent review by Thornton et al. [72] prior to pursuing the material contained here. In addition, the photocatalytic properties of TiO2 have been well documented in the literature. The reader is encouraged to seek important reviews by, for example, Fujishima et al. [73] and Henderson [74]. The following review is constructed by building up from simple, isolated gold atoms, to clusters, then to bulk single crystal model systems. The review of such isolated systems provides a foundation for discussions of seminal studies involving more complex model catalysts composed of sizeselected or characterized gold nanoparticles deposited onto single crystal supports. Finally, studies of more realistic (actual) catalysts composed of gold, often having inexact sizes and structures, deposited on high surface area titania supports are discussed. In this review, the term planar model catalysts is used to describe gold catalysts deposited on single crystal or other planar supports, while the term high surface area catalysts describes gold-based catalysts deposited on high surface area powdered, nanoparticulate, and 3-dimensional supports. These terms were originally used by Goodman et al. [75,76] to describe these classes of systems. The first part of this review (Sections 2–8) is focused on the physicochemical properties of gold metal, with an emphasis on

the preparation methods and physical characterization of nanocluster and nanoparticulate gold systems. It examines the chemical reactivity of these systems towards probe– adsorbates, explores the factors that control the thermal activation of adsorbate–catalyst systems, and finally, probes some key catalytic reactions of fundamental and practical interest. The second part of this review (Sections 9–13) first describes the nature of the SPR in gold nanostructures. In these sections, an emphasis is placed on factors that control the optical properties of supported gold nanoparticles, as well as the photophysical mechanisms that control their activity under UV and visible light irradiation. Then, some key photocatalytic reactions activated by visible light irradiation of supported gold catalysts are examined. The concluding section, Section 14, provides some perspectives for application of the unique catalytic and photocatalytic properties offered by supported gold catalysts in both fundamental and practical contexts. Further, this section contains suggestions for future research that will help elucidate the atomic-scale mechanisms of catalysis in a way that enables researchers to design new catalysts for targeted chemistries. 2. Gold nobleness, relativistic effects and properties of gold The ability of gold to catalyze a chemical reaction depends on the nature of the reactant binding site [33,53,77]. The interaction of a molecule with a catalytically active gold surface must alter the electronic structure of the molecule, weaken (activate) existing bonds, produce high energy intermediate species that are poised to transform into more stable products, and finally release products efficiently in a way that returns the catalyst to its original state. Owing to the inherent inert character of gold, most closed-shell reactant molecules do not interact strongly with its surface. In general, chemisorption requires a number of low-coordination Au atoms to be present on the surface, which can be accomplished by reducing the diameter of particles, thereby increasing the curvature of the surface and presenting more edge, corner, and steps to the gas or liquid phase (catalysis feedstock). Beyond simply altering the number of low-coordination high-energy surface sites, the electronic structure of small particles (and, in fact, thin films) differs from that of bulk gold, which also affects its reactivity. As alluded to quite frequently in the literature (the two seminal publications [27,78] combine for over 3000 citations as of December 2015), Haruta and co-workers were among the first recognize and take advantage of the unique properties of nanoscale Au particles when they demonstrated that gold could be made to be catalytically active for the oxidation of CO and H2 [27,78]. For centuries, prior to Haruta's initial studies, gold was considered to be the most noble metal because, on the macro scale, this metal exhibits remarkably low reactivity in the absence of a native oxide. Early work by Sault, Madix and Campbell showed that clean Au(110)-(1  2) does not dissociatively adsorb oxygen, even at pressures up to 1400 Torr, and temperatures between 300 and 500 K [77]. In addition, H2

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does not adsorb on the surface of bulk gold, so the material is completely inactive toward the oxygen reduction reaction. To explain the inertness of gold metal, Hammer and Norskov [29] introduced a simple concept that relates the nobleness of metals to two characteristics: (1) the degree of filling of the antibonding adsorbate–metal d states, and (2) the degree of overlap between the interacting atoms or molecules and the gold–d states [29]. These two factors determine the strength of the adsorbate–metal bonding and the activation energy for adsorbate dissociation. The trends associated with these two factors for the metals surrounding gold on the periodic table is shown in Fig. 2.1. The net result of this trend for Au is that it typically exhibits both a filled antibonding adsorbate–metal d state and the largest coupling matrix element. The co-existence of the two properties renders gold the most noble metal. 2.1. Relativistic effects Along with Cu and Ag, gold belongs to the periodic table's group 11 elements. Gold is a late transition metal with a doublet 2S([Xe] 6s14f145d10) atomic ground state. The large atomic mass of Au, 196.97 amu, predisposes the metal to relativistic effects on the electronic energy levels. That is, with the heavy elements, the core electrons experience the unshielded potential of the nucleus, and they acquire those of light to balance the strong electrostatic field [79]. According to Einstein's Theory of Special Relativity, the mass (m) of electrons with large speed increase according to the equation m ¼ m0 =ðl–ðυ=cÞÞ1=2

ð2:1Þ

where m 0 is their rest mass, υ their speed, and c is the speed of light. This description is the basic foundation of the relativistic effect, i.e. the introduction of a velocity

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dependent mass [79]. The relativistic energy contribution exists in all atoms, but becomes particularly large for atomic numbers greater than about 50 (Sn). The 1s electrons in gold approach 60% the speed of light, where they are 1.51 times more massive than while at rest. In addition, as a consequence of relativity, the electron’s spin and orbital angular momentum coupling exceeds that which would be found for lighter elements [51]. Because of the velocity-dependent mass of electrons, the core of heavy elements is contracted, and because the higher s levels must remain orthogonal to the core states, they experience a similar contraction. Fig. 2.2 presents the contraction of the 6s shell for elements Cs (Z ¼ 55) to Fm (Z ¼ 100), as taken from a review by Pyykkö [79]. For Au, the contraction reaches a remarkable 20%, which is the maximum for the stable elements. The contraction stabilizes the 6s level of the valence states of Au while it destabilizes the 5d levels, which brings them closer together in energy. Thus, the presence of relativistic effects leads to an enhancement in the sd hybridization. A theoretical treatment of the relativistic effects requires solution of the relativistic wave equation. In 1928, British physicist Paul Dirac formulated a four-component equation that provides a logical explanation for the existence of spin in a way that nonrelativistic descriptions cannot. However, the solution to Dirac’s equation is challenging, even with modern methods. For an overview of the theoretical methods available for the treatment of electron–electron interactions and relativistic effects, refer to the insightful review by Willock and coworkers [51]. The general physical and chemical consequences of electron–electron interactions and relativistic effects will be briefly discussed in the following sections, which summarize very helpful reviews on this topic by Bond et al. [33,53,54]. 2.2. Physical properties of gold Gold is such a unique metal because of its combination of physical and chemical properties on both macroscopic and microscopic levels [33,53,54]. From a macroscopic perspective, gold is characterized by a distinct yellow color, high chemical

Fig. 2.1. The s–d coupling matrix element (Vsd), the filling of the metal d bands, and the cohesive energy for metals in the vicinity of gold in the periodic table. The filling of the metal d bands is taken as an indication of the propensity for filling of the adsorbate–metal d antibonding state. The largest adsorbate–metal d repulsion, and hence the largest nobleness in terms of the surface reactivity, is obtained by maximizing Vsd and having an antibonding state filling of one. This is obtained for gold. The nobleness in terms of ability to resist corrosion and dissolution further involves the cohesive energy of the metals. This energy is largest for the 5d metals like Ir and Pt and adds particularly to the nobleness of these metals. Reprinted with permission from Ref. [29]. Copyright 1995 Nature Publishing Group.

Fig. 2.2. The relativistic contraction of the 6s shell in the elements Cs (Z¼55) to Fm (Z¼ 100) shown as the ratio of the average electron-nuclear distances obtained from a relativistic, 〈r〉R, and non-relativistic 〈r〉NR calculation. Reprinted with permission from Ref. [79]. Copyright 1988 American Chemical Society.

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stability, and high redox potential, which are the consequences of the material's electronic structure and relativistic effects. On the nanoscale, the combination of gold's unusual electron configuration with structural effects leads to (i) a significantly larger number of surface and near-surface atoms than bulk atoms, so that the overall properties are governed by the surface; (ii) a confinement of the electromagnetic field when an optical wave interacts with a gold nanoparticle (Au NP), which enhances light absorption in the visible range due to a localized plasmon resonance (giving particles a specific size-dependent color); and (iii) quantum effects that can change particles from metallic to more semiconducting in nature, depending on overall dimensionality [54]. Table 2.1 summarizes key physical characteristics of gold, as well as those of copper and silver [54]. Gold crystallizes in the face-centered cubic (fcc) structure, with a metallic radius less than that of silver. Gold is highly malleable: 1 g of gold can be extruded into а foil of
1 m2 in area, which is remarkably thin at less than 250 atomic diameters. The same amount of gold can be formed into a wire 165 m long and 20 μm in diameter. Though largely inert, gold can alloy with several other elements. When alloyed, the color of gold changes: e.g. the addition of copper produces “rose gold”, aluminum alloys lead to purple coloration, the addition of indium produces blue, and the color of cobalt alloys with gold is generally black. When compared to copper and silver, gold exhibits some properties (e.g. density) that follow from the atomic mass trend. In many cases, however, this trend is reversed: e.g., the melting point and sublimation enthalpy for gold are very similar to those of copper and silver; gold exhibits a relatively high Au–Au bond energy and shorter than expected bond length. The optoelectronic properties of gold are also unpredictable based on trends in Group 11, e.g. its resistivity is surprisingly greater than that of silver. Gold absorbs electromagnetic radiation in the visible region of the spectrum because the gap between the center of the 5d band and the Fermi level is uniquely low due to relativistic effects. Table 2.1 Physical properties of gold compared to those of copper and silver (column 11 elements). Property

Cu

Ag

Au

Atomic number Atomic mass (amu) Atomic configuration Structure Metallic radius Density (g cm  3) Melting temp (K) Sublimation enthalpy (kJ mol  1) 1st ionization energy (kJ mol  1) Electrical resistivity at 293 K/ micto ohm/cm Interband transition threshold /eV /nm

29 63.55 [Ar]3d104s1 fcc 0.128 8.95 1356 33776 745 1.67

47 107.868 [Kr]4d105s1 fcc 0.14447 10.49 1234 2857 4 731 1.59

79 196.9665 [Xe]4f145d106s1 fcc 0.14420 19.32 1337 3437 11 890 2.35

3d-4s 2.1 590

4d-5s 3.9 318

5d-6s 1.84 674

* Reprinted with permission from Ref. [54]. Copyright 2012 Imperial College Press.

As reported in Table 2.1, the interband transition for silver occurs in the UV range, while it is in the red range for gold. For further details regarding light absorption properties of gold nanoparticles, see Section 10. 2.3. Chemical properties of gold The chemical behavior of gold (5d106s1) is determined largely by the properties of the 5d electrons and the tendency of gold to acquire an electron for completion of the 6s2 level. This latter effect is responsible for the higher electron affinity and higher first ionization potential as compared to silver and copper (see Table 2.1). As a result, Au readily forms the Au  I state. The former effect explains the predominance of the AuIII state, which possesses a stable 5d8 configuration [80]. The electronegativity of gold (2.4 in Pauling units) equals that of selenium and is close to that of sulfur and iodine (2.5); thus, under certain conditions, gold can exhibit properties similar to those of a halogen. The electronic structure of gold defines its nobility. When in the bulk macroscopic form, gold does not react vigorously with oxygen and sulfur compounds, i.e. it does not appear to tarnish like copper and silver. This is because of the low stability of gold oxide (Au2O3), which decomposes at about 443 K and has a positive heat of formation. Gold sulfides (Au2S and Au2S3) also exhibit low stability. The dissolution of gold requires the presence of both an oxidant and a ligand to stabilize the resulting cation. Thus, gold dissolves in aqua regia (conc. HCl:conc. HNO3 ¼ 3:1) to form AuCl3. It can dissolve in aqueous CN– solutions in the presence of oxygen to form the [Au(CN)2]  anion. Despite its inertness, gold can be encouraged to form such compounds as hydrides AuН3 (i.e. НAu(Н2)) and AuН5 (i.e. H3Au(H2)), and, AuXe þ and AuXe2þ , which have been detected in low-temperature matrices [81]. Gold complexes containing two or more AuI ions, where the distance between such ion pairs is unusually short, are also known [82]. This indicates the existence of some degree of bonding between gold ions and such effects are termed aurophilic attractions or aurophilicity [81–84]. Theoretical studies have shown that the aurophilicity is due to dispersion forces of the type that hold molecules together in liquids and solids, but are stronger than typical van der Waals forces [84,85]. Depending on the separation between the gold atoms, the strength of this effect can range between 10 and 100 kJ mol  1, i.e. a kind of bonding like that of hydrogen bonding in water and alcohols. 3. Isolated gold clusters Since the work of Haruta, Goodman, and others, which established a correlation between the catalytic activity of oxide-supported gold catalysts and the degree of Au particle dispersion [27,78,86–90], experiments with finite, massselected gas-phase Au clusters have been pursued [91–96] to determine the size-dependence to gold reactivity. Isolated gas phase metal clusters provide model systems for detailed mechanistic studies into the energetics and kinetics of metalmediated catalytic reactions [96]. These studies, by providing

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baseline information and direct insight into reaction pathways, can yield [97] promising insight into the development of new catalysts [94]. Furthermore, by increasing the cluster size atom-by-atom, a fundamental understanding of the chemistry can be obtained from single atoms to nanoparticles [98,99]. Therefore, a few select key discoveries within the fascinating field of gold cluster chemistry are reviewed herein. 3.1. Structures To understand the unique activity of small clusters and nanoparticles of gold, one should initially focus on the obvious question of how the electronic structure of clusters is related to the 6s free electron structure of bulk gold [49]. Hence, as described in Section 2, relativistic effects play a significant role and must be considered [79,85]. In two recent review articles, Häkkinen [49] and Willock and co-workers [51] discussed the application of computational methods, calculations based on selected density-functional-theory, and other approaches for describing the atomic and electronic structure of gold nanoclusters and their catalytic behavior. In addition, Pyykko [80] has summarized the findings of a large number of theoretical and experimental studies that highlight several interesting characteristics of Au clusters. Some of the most important results described within these reviews are the observations that clusters of particular sizes can exhibit both enhanced stability and enhanced catalytic activity [80]. Fig. 3.1 presents data from one of the first systematic studies of cationic gold Aunþ clusters containing various numbers of atoms (N) in the range of 4 o N o 13. The figure provides the experimentally determined collision cross sections for Au cations as a function of N compared to theoretical (geometrical) cross sections determined for a large number of relaxed gold cluster isomers. Fig. 3.1 shows that for N ranging from 1 to 7, the measured values essentially coincide with the calculated data for the ground-state structures. All structures for N ¼ 3–7 are planar: a triangle, a rhombus, an hour-glass shape, a triangle, and a centered hexagon, respectively. These small clusters simply represent fragments of a closely-packed hexagonal plane. For 8 r N r13, an equally good match is observed for threedimensional structures, which in many cases can be described as slightly relaxed fragments of an fcc bulk structure. Wu et al. used DFT calculations (in GGA) to identify stable isomers for cationic, neutral, and anionic clusters in the size rang of Aun (n ¼ 1–6) [100]. Their calculated low-energy structures of both the neutral and the charged clusters are in overall good agreement with other reports [101–104]. As shown in Fig. 3.2a, the structures of the gold clusters depend significantly on their charge states. For example, the anionic Au–3 cluster prefers an equilateral triangle structure, while the cationic Au3þ prefers a linear structure. For the tetramers, both the cation and the neutral have a rhombus structure. The Au4 prefers a zigzag geometry, but there is little energetic difference between this structure and the Y-shaped motif. For the pentamer, the X-shaped structure is preferred for the cation, which exhibits a total

Fig. 3.1. Experimentally determined and theoretically (DFT calculations) predicted ion mobility cross sections for gold cluster cations. Reprinted with permission from Ref. [93]. Copyright 2002 American Institute of Physics.

energy that is 0.15 eV lower than the planar trapezoidal structure that is the most stable for the neutral and the anionic pentamer. The planar triangle structure is the most stable one identified for the hexamer, irrespective of the charge state. Finally, the binding energies, EB, of the clusters are shown in Fig. 3.2b. As expected, the EB values are found to increase with cluster size. In complementary studies, Olson et al. applied several levels of theory to help identify the lowest energy isomers of Au6 and Au8 clusters [105]. For the Au6 cluster, the results of this study were supportive for the previously achieved consensus that the global minimum structure is planar. For the Au8 cluster, both Gaussian-based and plane wave DFT methods predicted planar structures for the two lowest energy isomers. Xiao et al. have found that the geometric diversity of Au clusters increases with cluster size, generally leading to linear, zigzag, planar (2D), and 3D structures [106]. For cluster sizes up to Au13, the most stable clusters are realized for 2D geometries, while 3D geometries appear to be more probable for clusters with Au14 and above, as shown in Fig. 3.3 [106]. The greater stability of the 2D versus the 3D structures was attributed to the relativistic effects that affect the energy of the 6s and 5d orbitals. Due to relativity, these orbitals are closer together in energy than would otherwise be expected and thus, the sd hybridization that leads to bonding in the plane is favored. Linear and zigzag Aun clusters are competitive for n up to 5, after which differences in EB increase to 0.3 eV and above. For cluster sizes of Au7 and above, the difference in EB between the 2D and 3D forms is

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Fig. 3.2. (a) Lowest-energy structures of cationic, neutral, and anionic Aun (n¼ 1–6) clusters. Bond lengths are listed in Å. (b) Binding energy per atom as a function of cluster size for cationic (solid line), neutral (dashed line), and anionic (dotted line) gold clusters. Reprinted with permission from Ref. [100]. Copyright 2002 American Institute of Physics.

Fig. 3.3. The binding energies of alternative structure classes for Aun clusters as a function of cluster size, n. Reprinted with permission from Ref. [106]. Copyright 2006 American Institute of Physics.

only 0.1 eV, or less. As previously shown by Olson et al. [105] high level theoretical methods, including second-order perturbation theory (MP2) and coupled cluster methods (CCSD(T)), predicted that the global minimum structure for the Au8 cluster is nonplanar. In agreement with these results, Ferrighi et al. [107] employed meta-generalized gradient approximation (MGGA) functionals to suggest that the 2D to 3D transition for Aun (n o 13) depends on the charge state of the Au cluster. The change from 2D to 3D

structures was suggested to occur between n ¼ 8 and 12 for cationic and anionic clusters, respectively. The 2D–3D structural crossover at Au12 for anionic clusters was further confirmed in experimental investigations by photoelectron spectroscopy [108,109] and electron diffraction [110]. Furthermore, these investigations showed a gradual transformation of the optimal structure from a near-planar, flat ‘‘cage’’ (N¼ 13, 14) to a tetrahedral cage, and to finally evolving as tubular structures for N¼ 20. For neutral gold clusters, transformation from flat-cage to a hollow-cage structure was found at Au17 [111], in contrast to the anion counterparts for which the structural transition was at Au16 [112]. For neutral gold clusters, the shell-like flat-cage structures dominated for Au15 and Au16, while for Au17 and Au18 clusters, spherical-like hollow-cage structures were favorable. Interestingly, the hollow-cage structure of Au17 (with C2υ point-group symmetry) showed high stability, either in neutral or in anionic form [111]. For small neutral gold clusters, however, other research showed a tendency for them to form 2D structures up to Au 13 [109]. Importantly, Assadollahzadeh and Schwerdtfeger carried out a systematic search for the minimum energy structure for isomers of small neutral gold clusters Au n (n ¼ 2–20) using density functional theory together with a relativistic pseudo potential [113]. A rather slow convergence was found for all of the properties toward the bulk limit. Based on these results, it was concluded that the onset of metallic character with increasing cluster size could not be predicted for sizes as high as Au 20. Thus, they surmised that much larger clusters should be examined before conclusions about the bulk limit can be drawn. Interestingly, a large odd–even cluster size oscillation was established for ionization potentials (IP) [114] and electron affinities (EA) [109,115] that correlated well with reported experimental data for IP and EA. Based on the calculated polarizabilities, a clear transition to compact threedimensional structures was obtained at Au 14. They

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proposed that cluster polarizability measurements can be employed for identifying the 2D-3D transition for gold at low temperatures [113]. Landman and co-workers used photoelectron spectroscopic measurements and first principle DFT calculations to investigate size dependent structural effects and chemical reactivity of gold cluster anions, Aun (n ¼ 15–24) [108]. Their work suggested that the ground state electronic densities of states provide the best fit (in many, but not all cases) to the measured PES data. The calculated ground states revealed an interesting evolution of gold cluster structure. Specifically, the structure of Aun was described as changing from 2D “flaky” structures for n ¼ 15 and 16 to 3D “cages” for n ¼ 17 and 18, tetrahedra for n ¼ 19–21, doublelayer (“pita-pocket-like”) structures for n ¼ 22 and 23, and a “capsule (capped nanotube-like)” for n ¼ 24 [108]. Rösch and co-workers [116] have theoretically investigated a series of gold clusters from Au 6 to Au147 with diameters from 0.7 to 1.7 nm and forming icosahedral, octahedral, and cuboctahedral structures. A convergence of the properties of the clusters to the bulk limit was observed by scalar relativistic density functional calculations that employed linear combinations of Gaussian-type orbitals. Further, Barnard and Curtiss [117] used first-principle calculations to investigate the preferred shape of gold clusters from Au19 to Au146 with diameters less than 3 nm. Their work showed that the equilibrium shape of fcc gold nanoparticles less than 1 nm in size is cuboctahedron. This shape becomes energetically unfavorable with size and rapidly changes to the truncated octahedron, octahedron, and truncated cube shapes. Roldán et al. also explored the scalability of the coinage metal (Cu, Ag and Au) properties from nanoparticles to bulk metals [118]. During these studies, they found that unusual features in the overall density of states (DOS) may lead to major differences in particle reactivity, which could have very important implications for catalysis. Recently, Albuquerque and co-workers [119] demonstrated that decreasing the Au NP size from 1289 to 85 atoms (both magic numbers corresponding to closed-shell structures, truncated octahedron and octahedron, respectively) results in a substantial increase in the local atomic mobility of corner and edge regions, which might be reflected in the enhanced local adsorption ability of these regions. Exploring a combination of quantum and classical properties, importantly, the d-band center with respect to the Fermi level and the short time scale mean-square displacement (MSD), the authors provided some insight into the understanding of how the catalytic activity may change locally over the NP surface [119]. 3.2. Adsorption of molecules Quantitative information regarding the binding energies of probe gas molecules to gold clusters can provide important molecular level information that is relevant to elementary steps involved in reactions catalyzed by gold nanoparticles [99].

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This section examines the energetics of activation and bondmaking processes that occur upon adsorption of simple gas molecules at isolated gold clusters. Observed trends for the binding of molecules to clusters as a function of particle size are specifically highlighted. These types of studies provide a sound foundation for understanding how adsorption, activation, and ultimately catalysis are affected by particle–support interactions, a critical aspect of the overall chemistry. While this section presents a brief overview of adsorption at molecular clusters, important details concerning the electronic structure effects in molecule–surface bonding at bulk metals are discussed in Section 4.2. 3.2.1. Carbon monoxide Carbon monoxide has been employed extensively as a molecular probe of the nature and the strength of active adsorption sites on metal and metal oxide surfaces. The utility of CO stems from the high sensitivity of its vibrational frequency to the electronic structure of its bonding site, environment, geometry, and strength of adsorption [120– 122]. This, combined with the significant transition strength of the carbonyl in the mid-infrared range, make standard FTIR spectroscopic measurements of CO on surfaces an excellent means by which to characterize the nature of metallic surfaces. In their recent review, Willock and co-workers critically analyzed data from an extensive set of publications (155 references) describing theoretical calculations of the adsorption and reaction of CO and O2 on isolated and supported Au clusters [51]. According to these data, CO adsorption on Au clusters occurs through the C atom. The geometry of the adsorbed states can be described by common metal coordination complex nomenclature. Accordingly, a ηm coordination describes CO adsorption at a single metal site with m atoms of the adsorbate in contact with the metal atom. A μn coordination designates the geometry where an adsorbate molecule bridges n number of metal atoms [51]. As has been shown with structural studies and calculations for Au clusters, an accurate description of bonding to gold clusters requires a relativistic treatment. Thus, the theoretical results for CO adsorption on Au clusters may be substantially influenced by the basis sets used in the calculations [51]. Au1 species have been shown to adsorb CO molecules very weakly. Okumura et al. [123] employed B3LYP calculations to suggest that anionic Au1 exhibits negligible binding energy for CO adsorption, while neutral Au1 binds CO with energies in the range of 20–30 kJ mol  1, roughly equal to that of a common hydrogen bond. These data have been shown to be in good agreement with results obtained on matrix isolated complexes from laser ablated Au atoms [124]. The adsorption of CO on coinage metal Cu, Ag, and Au atoms and cations to form neutral M(CO)n (n¼ 1–3) and cationic M(CO)n þ (n¼ 1–3) carbonyls at 4 K was found to follow the classical s donation and π backdonation bonding mechanism for metal carbonyls [124]. The CO binding energy is also found to depend significantly on the charge state of the metal. The binding of CO to anionic and neutral Au2 has been shown to be significantly stronger than binding to Au1, while Au2þ binds CO slightly weaker than

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Fig. 3.4. (a) Ground-state (GS) structure (S¼ 1/2) of Au2CO–; (b) contour plot of density difference, Δρ ¼ρ(Au2CO  )ρ(Au2 ) ρ(CO); (c) “side-bonded” isomer (S¼ 1/2). BE is the binding (adsorption) energy of CO, vDE is the vertical electron detachment energy. The contours of Δρ are shown in the range from  0.015 to 0.015 au with 0.006 au intervals. Dashed (solid) line denotes depletion (accumulation) of the density. The indicated bond lengths are given in Å. Reprinted with permission from Ref. [125]. Copyright 2001 American Chemical Society.

Fig. 3.5. Calculated binding energies, Ead, for CO adsorbed to Aun þ , Aun0 and Aun  as a function of cluster size. Results from PW91 (solid symbols) and B3LYP (open symbols) exchange–correlation functionals are compared. Reprinted with permission from Ref. [100]. Copyright 2002 American Institute of Physics.

does Au þ (according to PW91 results). For Au2 , Häkkinen and Landman [125] identified two configurations of CO, as shown in Fig. 3.4. The ground-state (GS) bent η1 geometry (Fig. 3.4a) significantly elongates both the Au–Au and C–O bonds and exhibits a CO adsorption energy of
93 kJ mol  1. The alternative μ2 geometry (Fig. 3.4c) is less favorable than the GS by
40 kJ mol  1. In this configuration, CO adsorption induces rupture of the Au–Au bond. Such varying adsorption site behavior is characteristic for bonding of CO to most transition metal surfaces (e.g., on-top, bridge, etc.). Fig. 3.4b shows the results of calculations for the charge density difference between the η1 complex and a reference geometry where Au2 and CO are in the same positions but are

non-interacting. Following the dashed (solid) lines, one can see how the charge is depleted (accumulated) upon the interaction of the two species. The η1 bent geometry opens the door for donation into the CO(2π*) orbital from Au2 orbitals. Analysis of charge spatial distributions revealed that 0.24e is donated from CO(5s) to gold, while 0.4e is back donated to the CO (2π*) orbital; hence, a transfer of 0.16e to CO may occur upon binding. This transfer causes elongation of the C–O bond length reflected in
300 cm  1 red shift in the calculated vibration frequency (1841 cm  1) as compared to that calculated for gas-phase CO (2145 cm  1). A similar bonding pathway for the “side-bonded” configuration (Fig. 3.4C) resulted in much less charge transfer. For adsorption of CO on Au3 clusters, the choice between linear and triangular 2D structures is found to depend on the cluster charge [126]. Anionic and positively charged 2D clusters showed similar strength for binding of CO, while binding to linear structures is less favorable. For CO adsorption on Au4 anions, similar preference of CO binding to 2D over linear structures was observed, although the calculated adsorption energy difference was only 10 kJ mol  1 for these different motifs. The adsorption energy of CO on Aun  (n ¼ 2–7) clusters decreased with the number of CO molecules, i.e. the adsorption of CO exhibits saturation behavior on Aun  , which was determined by combined geometric- and electronictype calculations of gold clusters. Interestingly, the CO adsorbate was found to transform the geometric structure for Au–3. Such adsorbate-induced surface reconstructions have been shown to be an important component for many catalysts. In fact, the 2007 Nobel Prize was awarded to Gerhard Ertl for discovering (among other things) the central role of surface reconstruction during CO oxidation over catalysts. Similar processes likely play a major role in Au-based catalysis; accordingly, evidence for structural reconstructions during

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catalysis on Au-based systems will be described in subsequent sections of this review. For adsorption of CO on cationic Aun (n¼ 1–6), Wu et al. have established a general trend of decreasing adsorption energies with increasing cluster size [100]. This trend is clearly seen from the plot of data (DFT calculations in GGA) shown in Fig. 3.5. The opposite trend is observed for anionic Au and neutral Au exhibiting a maximum in the binding energy for CO at n¼ 3–4. For each system, the cluster geometries were optimized for the lowest energy configuration. Irrespective of the Aun cluster charge state, the bonding of CO in one-fold coordinated on-top geometry was the most favorable. Planar Au structures were preferred even with CO adsorbed on the clusters. Interestingly, the calculated C–O bond length in cationic complexes was always greater and the calculated frequency was higher than that calculated for a reference free CO molecule. This observation may be explained by an electrostatic effect [127,128] related to the change in polarization of the bonding orbitals, caused by the positive charge of the gold cluster [100]. With the anionic complexes, the C–O bond length was also found to increase slightly, but the ν(CO) frequency decreased upon CO bonding to Aun–. This complex effect is likely due to the combined influences of back donation and electrostatic interactions. Bagus and Pacchioni [129] used a Au5COq cluster model to learn about the bond strength and CO vibrational frequency at neutral or charged gold (q ¼ þ 1, 0,  1). They employed a constrained space orbital variation (CSOV) method to decompose the various contributions to binding. While the adsorption energy was found to depend little on the value of q for this size of cluster (consistent with Fig. 3.5), the CO stretching frequency was substantially affected by q (a demonstration of the utility of CO as an effective probe molecule for IR studies of Au oxidation state). Two major factors were found to affect the CO stretching frequency for different charge states, q: (1) the electric field–charged nanoparticle interaction (Stark effect), and (2) the back electronic charge donation (Au-CO π*). Importantly, the CO-Au s donation was not nearly as important as the π* back-donation. The presence of a net charge (the Stark term) confined in the small volume of the metal cluster results in a larger CO frequency for q¼ þ 1 and a lower frequency for q¼  1. However, this effect likely decreases significantly for larger particles and bulk surfaces, where the charge is delocalized on a macroscopic scale and further from or relatively lower at the CO adsorption site. For Aun clusters (n ¼ 5–10), Fernández et al. [130] have shown theoretically that CO adsorption occurs at the least coordinated Au atom. In the cases of Au5 and Au7, however, a bridge position was preferred. For the bridge sites, the charge and CO bond length were found to be larger than the values estimated for CO adsorbed on top sites. Based on an analysis of the partial density of states, these trends were explained by the enhancement of π back-donation for bridge isomers. Phala et al. [131], in studies of adsorption on Aun (n¼ 1–13) clusters, found that the CO binding energies correlated with the variation of frontier orbital energies of the bare gold clusters,

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as previously established [100,101]. Binding of CO onto the on-top site of the clusters was the most favorable configuration for all clusters except for Au13. The preference for the on-top site was suggested as an indication of a larger contribution of the 5s–sd interaction as compared to that of the 2π*–sd interaction. For the interesting case of 3D Au13 clusters, Okumura et al. [123] showed that the adsorption energy for CO to neutral and positively charged clusters was about the same,
88 kJ mol  1. The calculated Mulliken charges indicated that the Au atom in direct contact with the CO molecule tends to become positively charged, while charges of the other Au atoms were negative. Huang et al. have studied both experimentally and theoretically CO chemisorption on golden cages, Aun  (n ¼ 16–18)  [132]. The gold cages Au–16 and Au18 were found to undergo a cage-to-cage structural transformation upon CO adsorption.   The Au16 cage transformed to a structure similar to Au17 upon the adsorption of CO. Two nearly degenerate initial structures  for the free Au18 clusters, i.e., a cage and a pyramidal isomer, were found to form only the cage isomer upon CO adsorption.  The Au17 cage showed high stability and retained its initial structure upon CO adsorption. Similar robustness has also been observed for the free neutral cage Au17 [111]. Several groups have reported on pressure- [92,133–135] and temperature-dependent [136] adsorption of CO on both anionic and neutral gold clusters. Wallace et al. [133] have established that subsequent CO adsorption steps are even more sensitive to cluster size than to the initial adsorption process. Upon saturation, a pronounced odd–even pattern was observed for clusters Au–n with n ¼ 6–13, where even-n sized clusters remain partially unsaturated, while odd-n clusters approach saturation values. Veldeman et al. reported a significant temperature and size dependence for CO adsorption on neutral Aun clusters, n¼ 9–68 [136]. Fielicke et al. [137] reported a coverage, cluster size, and charge dependency to the CO stretching frequency. Adsorption at high partial pressures of CO resulted in saturated Aun(CO)–m complexes that were in close agreement with studies reported by Wallace and Whetten [133]. In general, the ability of gold clusters to bind to more than one CO molecule is related to the flexibility of their geometry [138–141]. Recently, Pal et al. [141] reported on the chemisorption-induced 2D–3D–2D structural transitions in gold anionic heptamers, (CO)nAu7 (n ¼ 1–4). Using both photoelectron spectroscopic (PES) experiments and ab initio calculations, these authors were able to follow the structural changes in the anionic Au–7 cluster during attachment of CO molecules, as shown in Fig. 3.6. The gold cluster motif in the most stable COAu7 cluster was found to exist as an intermediate between the two stable 2D isomers of Au7 . The same planar Au7 motif appears to be stable for the (CO)2Au7 . Two isomers were found for (CO)3Au–7: one included a planar hexagonal Au7 motif and a second one had the global-minimum structure of the Au–7 cluster. The structure of (CO)4Au–7 was found to be markedly different; it consists of the global-minimum structure of Au–7, three terminal CO molecules, and one bridging CO. The

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Fig. 3.6. Optimized structures of low-lying isomers of (CO)nAu–7 (n¼ 1–4). Reprinted with permission from Ref. [141]. Copyright 2011 American Chemical Society.

extreme flexibility of the Au–7 cluster to undergo 2D–3D–2D structural transitions upon CO chemisorption suggests that gold structure can be strongly dependent on the CO coverage. As alluded to above, this structural flexibility plays an important role in the catalytic performance of gold. 3.2.2. Oxygen Unlike usual transition-metal oxidation catalysts, which typically perform oxidation reactions via formation of a O2– key intermediate in a Mars-van-Krevelen mechanism, such a mechanistic step is thermodynamically prohibited for gold and the way gold nanoparticles activate O2 remains unresolved [142]. In the electronic structure of O2, the triplet ground state has two unpaired electrons that occupy the degenerate 2πu* orbitals. The electrophilic O2 molecule (the electron affinity of O2 is 0.44 eV [143]) can accept charge leading to formation of the superoxo anion, O2 (doublet), and the peroxy anion O22  (singlet) [51]. The excess charge populates the anti-bonding orbitals and thus the O–O bond length is greater than that of the neutral molecule. Therefore, charge transfer from a surface to O2 can effectively activate the molecule. A critical analysis of a large number of data, mostly from theoretical studies, on the adsorption of O2 on isolated Au clusters can be found in the recent review by Willock and coworkers [51]. As with CO adsorption, the Au cluster size and charge state have a significant influence on O2 adsorption. The neutral Au1 cluster adsorbs O2 molecules more weakly than either of charged species, Au–1 or Au1þ . The calculated (B3LYP) adsorption energy for the neutral, anionic and cationic Au was o 10,
20 and
47 kJ mol  1, respectively. In each of these cases, the O–O bond length remained essentially the same, close to the equilibrium length for free molecular O2 [144]. Similarly, the neutral Au2 cluster adsorbs molecular O2 much more weakly than the charged Au species. The calculated (B3LYP) adsorption energy for neutral Au2 was only
6 kJ mol  1. The anionic Au–2 exhibited an adsorption energy of 92 kJ mol  1, which was found to be much higher than the 27 kJ mol  1 of cationic Au2þ . The strong adsorption of O2 on the anionic cluster leads to an elongation of the O–O bond from 1.20 to 1.31 Å [144].

As noted above, the Au3 cluster has linear and triangular isomers, the latter showing a Jahn–Teller distortion. For the anionic Au–3 cluster, most DFT calculations find the ground state cluster to be linear, where terminal binding shows weak O2 adsorption with an energy of only 4 kJ mol  1 (using hybrid functional B3LYP). The changes in the O–O bond length in this configuration are negligible [144]. For the 2D geometry, the calculated adsorption energy of O2 was found to be strongly method dependent. Calculations based on the PW91 functional [126], reported an energy of 121 kJ mol  1, while CCSD(T) calculations [145] actually showed a negative binding energy. Adsorption of O2 on neutral Au3 clusters can result in either the η1, end-on geometry for Au–3 and all smaller clusters, or in a μ1,1, side-on configuration. The μ1,1 configuration has a higher adsorption energy, as found by both BPW91 [146] and B3LYP [144] methods, if results using the same functional are compared. The reaction of O2 with gas-phase Au–n anion clusters exhibits an even–odd pattern of reactivity for n o 20 [91,108,130,135,147–151]. The even-numbered Au–n clusters bind one molecule of O2 per cluster, while the odd-numbered clusters show limited reactivity. Salisbury et al. [91] postulated that O2 can chemisorb as an O2 species if formed via charge transfer from a Aun  cluster with an unpaired electron. That is, a Au atom has 11 “valence” electrons, which requires that a Aun– cluster possesses an unpaired electron when n is even. Thus, within this model, O2 will chemisorb only on anionic clusters with even n. For charged and neutral gold clusters, Au–n and Aun (n ¼ 1– 8), Häkkinen and co-workers have also established a pronounced sensitivity of O2 adsorption to the cluster size and charge state [149]. In their study, density functional theory with generalized gradient corrections and scalar relativistic pseudopotentials were employed. As in the studies described above, the binding energy of oxygen was found to be larger for the anionic gold clusters. In addition, the energy exhibited an odd–even alteration as a function of the number of gold atoms, with maxima appearing for the even-n AunO–2 complexes. These results support the idea that adsorption of O2 occurs via charge transfer from the Au cluster to O2, which leads to a superoxo state and activation of the O–O bond. As depicted in Fig. 3.7, the adsorption mode of oxygen, molecular or dissociative, was clearly shown to depend on the number of gold atoms in the cluster. Molecular adsorption of O2 is preferred for Aun clusters with n r 3, while dissociative adsorption is favored on large clusters. However, the large energetic barriers obtained for O2 dissociation,
1 eV, preclude bond rupture under most experimentally relevant conditions. Notwithstanding, atomic oxygen or ozone sources have been used to produce surface-bound oxygen atoms [149]. It is predicted that the dissociative adsorption of oxygen will induce large relaxations in the gold part of the [AunO–2] complex. For neutral and cationic gold clusters, the interaction with O2 is much weaker. Although binding oxygen, they appear to be incapable of driving O–O bond activation [149]. Okumura et al. have used DFT B3LYP/LANL2DZ calculations to study the interaction of O2 with an icosahedral Au13

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Fig. 3.7. Binding energy of an oxygen molecule to gold cluster anions. N is the number of gold atoms in the cluster. The dissociative adsorption (DA) mode and the molecular adsorption (MA) one are denoted by a filled square and an empty circle, respectively. Reprinted with permission from Ref. [149]. Copyright 2003 American Chemical Society.

cluster [123,152]. This theoretical study provided insight into the charge transfer (CT) interactions at active Au sites. They found a significant accumulation of electron density near the surface. In fact, the surface atoms exhibited a partial negative charge of
0.7. Such charge build-up is often advantageous for adsorption of molecules, like oxygen, with a high electron affinity. Based on the Mulliken electron densities for the Au– O–O configuration, they demonstrated that transfer of charge to surface O2 molecules occurs. The estimated binding energy between the Au13 cluster and O2 was reportedly close to that calculated for the (Au–O2)  complex,
50 kJ mol  1. Other researchers, including Gong [153] and co-workers and Roldán et al. [154,155], have studied how Au particle size and structure affect the nature of O2 adsorption. Results revealed that O2 can adsorb dissociatively, via charge transfer, at ground state structures of both cage-like Au32(Ih)O2 (of icosahedral symmetry) and amorphous Au32(C1)O2 [153]. In addition, the surface of other small Au nanoparticles (Au25 and Au38) were shown to adsorb O2 dissociatively [154,155]; however, larger particles (Au55 and Au79) were suggested to adsorb O2 molecularly [154,155]. Experimental studies of mass spectrometric size-selected gas phase clusters first discovered that the reactivity of cluster anions with molecular oxygen depended on whether there were even or odd numbers of atoms in the cluster [147]. In accord with theoretical predictions, the proposed activation mechanism for O2 involved the transfer of an electron from gold into the highest occupied molecular orbital of oxygen. Subsequent studies that employed spectroscopic methods such as anion photoelectron spectroscopy (PES) and infrared multiple photon dissociation (IRMPD), supported this description of oxygen–cluster interactions [94,142,156]. Huang et al. performed a systematic PES study of AunO–2 (n¼ 1–7) cluster complexes [157]. Their spectral data showed evidence for molecular chemisorption of O2 for evensized gold clusters, i.e. AunO–2 (n¼ 2, 4, 6), and at the same time they suggested an inertness of odd-sized clusters, AunO–2 (n¼ 1, 3, 5, 7). However, by measuring both the electron-binding energies and O–O vibrational frequencies, the authors reported an inconsistency between the observed frequency and the

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proposed inertness of even-sized neutral Aun clusters with respect to O2. Specifically, the observed frequency of the ν(O–O) vibration (1360 cm  1) for the ground state of the neutral AunO2 complex was found to be between those of free O2 (1580 cm  1) and O2 (1090 cm  1). This implied chemisorption of O2 and partial charge transfer from the Aun cluster. To explain these and other observations, the authors proposed the formation of a double-well potential, consisting of a relatively shallow van der Walls potential energy well for the ground state at extended AunO2 distances and a more attractive potential at small Aun-O2 distances, derived from singlet O2 (1Δg) [157]. Using infrared multiple photon dissociation methods, Fielicke and co-workers studied the interaction of molecular O2 with anionic AunO–2 (n r 7) [142] and neutral AunO2 (4r n r 21) [158] complexes by probing the highly sensitive (to electronic structure) O–O vibrational stretch frequency. With the anionic gold clusters, the observed IR-MPD spectra provided direct experimental evidence for the formation of a superoxo moiety upon O2 adsorption. However, they observed an anticorrelation between the frequency of the ν(O–O) vibration (i.e. the extent of activation) and the electron affinity of the gold cluster [142], which contrasted with the direct correlation previously established [91] for the reactivity of gold anions with O2. The anticorrelation means that those clusters with a low EA exhibit a stronger O–O bond (reflected by a high ν(O–O)), which is counter to the expectation that gold clusters with a lower EA are more likely to donate electron density into the O2 π* orbital, and thus, weaken the O–O bond (lower the ν(O–O)). This observation was explained in the following way: when the EA of the cluster is low, the HOMO is high-lying and significantly off resonance with the O2 π* orbital. Thus, the energetic overlap between the anionic gold cluster HOMOs and the O2 π* orbital is reduced, relative to clusters with larger electron affinities, resulting in lower charge transfer into the O2 π* orbital, a stronger O–O bond, and a higher ν(O–O). As revealed by the IR-MPD spectra, neutral gold clusters behaved similarly to anionic clusters in binding oxygen [158]. The observed superoxo (O2 ) species appears to arise from an electron transfer from odd-sized AunO2 (n ¼ 7, 9, 11, 21) gold clusters. Thus, superoxo (O2 ) species are bound to formally cationic gold clusters where the energy required for ionization is offset by the new ion–ion interaction in the complex. Conversely, the even-sized clusters were found to be largely unreactive to O2, with the exceptions of Au4, Au10, and Au12. Importantly, DFT calculations showed that charge transfer leads to rearrangement of the cluster atoms until the final structure is similar to that of the cationic cluster. The energy minimized structures are illustrated in Fig. 3.8. As with many other systems [159,160], this type of dynamic structural reconstruction upon adsorbate activation likely plays a role in supported Au catalysis as well. 3.2.3. Hydrogen As with CO and O2 adsorption, the adsorption and dissociation of H2 on small unsupported (gas phase) gold clusters depend not only on the cluster size but also on the charge state

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Fig. 3.8. Gold cluster structural rearrangement upon adsorption of oxygen. Reprinted with permission from Ref. [158]. Copyright 2013 American Chemical Society.

of the cluster [131,147,161]. Dissociative chemisorption of H2 on gold, as with most transition metals, is a non-activated process that occurs at near gas kinetic collision rates with small metal clusters (30–40 atoms in size) [147]. Two mechanisms for H–H bond activation on metal clusters are possible: (i) the ionization potential (IP) or work function (φ) of the metal clusters may be sufficiently low, such that the metal acts as a good charge donor with respect to the adsorbate molecule (large clusters), or (ii) the IP of the metal cluster may be high enough to serve as a good charge acceptor (smaller clusters) [147]. However, in an early work, Cox et al. found experimentally that only small unsupported gold cluster cations are reactive towards D2, while clusters containing more than
15 atoms were inert [147]. The obtained reactivity order towards H2, as a function of the cluster charge state, was shown to follow cations4neutrals4 4anions, which is opposite of that established for O2. Further, no even-odd pattern of reactivity variation was observed for H2 (as has been demonstrated for O2). These authors concluded that the electronic character of both the cluster and adsorbate contribute to H2 activation and dissociation in ways that make the reactivity less sensitive to the number of atoms in the cluster [147]. By employing DFT, second order perturbation theory (MP2), and coupled cluster (CCSD(T)) methods, Varganov et al. confirmed the experimental results described above. While binding of molecular hydrogen to neutral Au2 and Au3 clusters was shown to occur easily, with energies of 55 kJ mol  1 and 71 kJ mol  1, respectively, it appears unfavorable on negatively charged Aun– clusters [161]. The energy barriers for H2 dissociation on neutral Au2 and Au3 clusters with respect to AunH2 complexes are 110 kJ mol  1 and 59 kJ mol  1, respectively. These barriers are higher for charged Aun– clusters: 93 kJ mol  1 for Au–2 and 139 kJ mol  1 for Au3 . Overall, the binding of H2 to gold clusters was shown to be quite different from that of O2. Both hydrogen atoms are coordinated to gold clusters [161], whereas, only one oxygen atom of O2 is thought to be involved in coordination for clusters of this size [145]. In their hybrid DFT calculations, Okumura et al. have also studied the adsorption of H2 on gold clusters [123]. The obtained absolute value for the H2 dissociation energy on Au13 clusters was about one-third of the transition energy calculated for the H2 dissociation on the surface of bulk Au. We may recall here that Nørskov et al. [29] calculated an H2 dissociation energy of about 1 eV for single crystal Au (111). Thus, Okumura et al. [123]

concluded that Au nanoparticles should have a stable chemisorption state for atomic hydrogen and an ability to dissociate H2 at low temperatures. In agreement with this prediction, Kang et al. [162] found theoretically that H2 dissociation is facile at low temperatures on both neutral and cationic Au4 and Au5. Neutral Au4 and Au5 clusters, however, are the most active for H2 dissociation from both thermodynamic and kinetic points of view. No correlation was observed between the coordination number of the Au atom and the rate of H2 dissociation. In addition, the adsorption of H2 on a Au32 cluster has been shown to follow both molecular or dissociative pathways, depending on the Au cluster structure, as established by Wang and Gong [153]. In cases where uptake is governed by dissociation, each of the two H atoms tends to be stabilized through interactions with two or three Au atoms. Phala et al. [131] found, by DFT calculations, that the adsorption of atomic hydrogen is more favorable onto oddnumbered neutral Au1–Au13 clusters due to their smaller HOMO–LUMO gap. Beyond very small clusters, Okamoto used their DFT calculations to study the adsorption of atomic hydrogen on the surface of a Au55 cluster [163]. The adsorption of H at the top of the Au55 cluster was found to be endothermic, but slightly energetically favorable for fcc and hcp hollow sites. One notes that because the adsorption energy for hydrogen is a good predictor of H2 oxidation [130], these results suggest that Au55 may be an active catalyst for a H2 oxidation. For reference, Au55 corresponds to a 1.4 nm nanoparticle [164]. Other researchers have explored the adsorption of molecular [162,165] and atomic [166] hydrogen on planar Aun, which appears to occur preferentially in the plane of the clusters. Molina and Alonso theoretically probed (DFT) the binding of H atoms to planar Aun clusters with n up to 10 [166]. In their work, a strong directionality of the bonds for atomic hydrogen binding was observed with bond direction contained in the plane of the Au cluster being preferred; bond direction perpendicular to the cluster plane was not favorable. In addition, a strong odd–even variation of the reactivity with cluster size was observed. These observations were rationalized on the basis of the spatial distribution of the frontier orbitals (HOMO and LUMO) and the Fukui functions. For a system with N electrons, the two Fukui functions are f þ (r) ¼ nN þ 1(r) nN(r) and f–(r) ¼ nN(r)  nN  1(r) where n represents the ground state densities of systems with N, Nþ 1, and N  1 electrons. Fukui functions give a measure for the chemical potential change as the number of electrons is varied. Regions with high values for f þ (r) generally accept charge from electron donors (nucleophiles), whereas regions with high values of f  (r) easily donate charge and thus stabilize electron acceptors (electrophiles). The Fukui functions can be approximated by the electron densities of the LUMO and HOMO orbitals. It was proposed that the most reactive cluster sites for H adsorption are those in which the frontier orbitals or Fukui functions are localized. Thus, the expected active role of the frontier orbitals in determining the chemical properties of small gold clusters was confirmed (as in the case of O2 binding, above).

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Roncero and co-workers have also studied the interactions of H2 with planar Auqn (n¼ 4–10 and q¼ 0, 7 1) clusters and how those interactions depend on cluster geometry [167]. Their calculations were performed by DFT (GGA) with PW91 functional and showed good agreement with CCSD (T) calculations. As found previously, H2 dissociation was found to be possible when it approaches the perimeter of the gold clusters in the plane [65,66]. Fig. 3.9 presents the prototypical cases of H2 þ Au7 and H2 þ Au6þ , where the minimum energy path (MEP) is calculated as a function of r (the H–H distance) and R (the distance from the closest gold atom and the H2 center of mass). Three key coordinates are specifically highlighted in Fig. 3.9: (A) a point in the entrance channel where the H–H distance is little changed from equilibrium, (B) the top of the barrier, or the transition state, where H atoms are already separated, and (C) the Au–H–Au–H–Au double-bridge potential energy well for hydrogen chemisorption. A careful examination of MEPs showed that such low barriers for dissociation appear only when the entrance channel is characterized by a significant well for adsorption of E 0.4–0.6 eV. For acute angles, the energy barrier for H2 dissociation was always relatively high (except for Au–9); however, when neighboring gold atoms are available to participate, such as at broad angles, the reaction barrier is much lower. When Au–n anions have potential energy wells along the entrance channel, their barrier to reaction tends to be below that of the analogous cation or neutral clusters. Despite some of these observations, there was no clear trend in reactivity with the number of

Fig. 3.9. Potential energy surface (eV) for the H2 þAu7 reaction (bottom panel) and for the H2 þAu6þ (side) (top panel), as a function of the H–H distance, r, and the distance from the closest gold atom and the H2 center of mass, R. The geometry of the stationary points along the MEP is shown as insets. Distances are in angstroms. Reprinted with permission from Ref. [167]. Copyright 2010 American Chemical Society.

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electrons or gold atoms. Taken together, the observations were used to conclude that the necessary condition for binding an H2 molecule at a cluster site is the presence of a deficit in the electronic density (or partial positive charge) at that site. The importance of this interpretation will be highlighted throughout the subsequent sections, which describe the details of reactions on Au clusters and nanoparticles. 3.2.4. Methanol In addition to CO, methanol is an often-used probe molecule applied to the study of size-selected gold clusters. Methanol behaves as a sensitive sensor molecule that can be used to define the real-space structures of small metal clusters. Rousseau et al. [103,168] have studied the structural, dynamic, and electronic properties of adducts formed by adsorbing methanol onto sizeselected gold clusters, Aun þ CH3OH (n¼ 1, 3, 5, 7, 9 and 15). Infrared multiple-photon dissociation spectroscopy of trapped ions and theoretical Car–Parrinello calculations were combined in their work. Indeed, they helped to establish that the C–O vibrational frequency (within adsorbed methanol) is sensitive to gold cluster size. Notably, it was sensitive to a change in the dimensionality from 2D to 3D structures. The formation of gold–methanol adducts decreases the C–O frequency as compared to free methanol (ν¼ 1014 cm  1, calculated), signaling a weakening of the C–O bond. As reported in Fig. 3.10, the observed red shift Δν was largest, 4 100 cm  1, for the limit of n ¼ 1. This red shift decreased significantly with the increase of the cluster size to n¼ 3, but remained around 75 cm  1 for clusters with n up to 7. Further increases of n from 8 to 15 caused a second less pronounced step-like change of the frequency shift, leveling off at about 60 cm  1. Note that the red shift exhibited by the neutral Au4CH3OH adduct was only about 30 cm–1. The calculations showed that the cluster–methanol interaction is mediated via a direct Au*–O bond between one of the lowestcoordinated gold atoms, Au* of the gold cluster, and the methanol molecule. There are two such atoms for clusters with no 7 and three or four for the 3D n Z 9 clusters. This indicates the dimensionality dependence of methanol bonding to gold clusters. Using the same experimental and theoretical approaches, Dietrich et al. [169] have studied the structural and energetic properties of methanol adducts on size-selected gold clusters Aunþ (CH3OH)m for n¼ 1–10 and 15 and m ¼ 1–3. Comparison of the experimental ν(C–O) with those obtained by Car–Parrinello calculations revealed that a discontinuous red shift in ν(C–O) was related to the dimensionality of the gold clusters, in agreement with previous findings [168]. The red shift also decreased when the number of adsorbed methanol molecules increased. Interestingly, in the majority of these early studies the dissociation of methanol to methoxy species was not discussed. Hence, one might surmise that, in the absence of a promoting co-adsorbate like O2, methanol is adsorbed and desorbs as a molecule without O–H scission and formation of cluster– methoxy complexes. The interaction between cluster-methoxy complexes and co-adsorbed O2 has been treated theoretically and those studies will be discussed in Section 3.3.2.

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Fig. 3.10. Red shifts of the C–O stretching frequency Δν¼ ν(CH3OH)–ν (Aunþ CH3OH) of Aun þ CH3OH, calculated as a function of cluster size, n. Filled squares, experiment; open circles, calculations for lowest-energy adducts; open rhombus, calculations for higher-energy isomers. Reprinted with permission from Ref. [168]. Copyright 1998 Elsevier B.V.

3.3. Reactivity of gold clusters As discussed above, a large number of experimental and theoretical studies have been devoted to building an understanding of the structural, electronic, and catalytic properties and reactivity of gold clusters, Aun, in the subnanocluster size range where no 100. Recent review articles by Häkkinen [49], Coquet et al. [51] and Pyykko [80] have shown how the very high relativistic effects in gold control its physical and chemical properties in ways that have significant effects on the overall reactivity toward adsorbate molecules like CO, O2, H2, CH4, CH3OH, alkenes etc. One of the most important discoveries in this field was that addition or removal of a single Au atom alters the cluster properties dramatically. Given the sensitivity of reactivity to particle size and structure, it is not surprising that reported results in the literature can often vary significantly across different laboratories. Small changes in particle preparation methods, history, and treatment can lead to large differences in experimental observations [51]. Martínez [170] recently applied a simple model based on density functional theory to analyze the electron donor– acceptor properties (characterize the reactivity) of planar and three dimensional neutral gold clusters, Aun (n¼ 2–20). In most reactions that occur on small neutral gold clusters, partial charge transfer between the gold cluster and the adsorbed molecule was found to control the reactivity of the cluster. The direction of charge transfer and the capacity of the system to donate or accept charge are determined by the chemical potential, μ, at a constant external potential. This work also explored how the chemical potential changes as a function of the number of atoms within a gold cluster [170]. To determine the size-site-reactivity relationship in adsorbate–cluster complexes, theoretical studies have also explored the Fukui function. The Fukui function [171], or frontier function, is presented as the sensitivity of the chemical potential to an external perturbation at a particular point and can be used to describe the site reactivity or site selectivity to nucleophilic, electrophilic, or radical attack [171]. The HOMO

and LUMO frontier orbitals are, in this way, considered to be the principal factors that govern the chemical reactivity and the stereo-selective reaction path [172]. Using Fukui condensed functions [172], Pal and co-workers [173,174] studied the influence of relativistic effects on the structure and reactivity of gold clusters. Their efforts were aimed at obtaining a general rule of how the size and shape of gold clusters control the number and location of Au sites that are active for electrophilic and/or nucleophilic attack. As they highlight, the chemical, and particularly the catalytic, behavior of a given gold cluster is a complex function of the basic electrophilicity and nucleophilicity of various sites in the cluster. As one example of this work, they established that the structural properties of tertrahedral gold clusters (Au19 and Au20) are affected in significant ways by relativistic effects. However, based on Fukui function calculations, the reactivity of various atoms within these clusters indicated that the trends were unaffected by relativistic effects [173]. As shown in Fig. 3.11, the atoms in the clusters studied were divided into various classes depending upon their symmetry types or environment (coordination number). They established that each class of atoms likely exhibits a different reactivity. In the Au20 cluster, the vertex atoms were found to be the most reactive toward a nucleophilic attack. In the Au19 cluster, atoms connecting the missing vertex edge with the pyramid base along with the vertex atom were the most reactive for a nucleophilic attack. In both the clusters, atoms lying at the center of each face were shown to be favorable for an electrophilic attack. The reactivity of Aun (n¼ 6–13) clusters was also studied and was found to depend on the position and number of electrophilic and nucleophilic sites in the cluster [174]. While 2D conformations were found to be the best catalysts for gas phase catalytic redox reactions, 3D conformations were better for catalytic reactions, where adsorption of reactants and subsequent oxidation and reduction occur [174]. Among various reactions catalyzed by gold clusters and nanoparticles, aerobic oxidation, i.e. oxidation in the presence of molecular oxygen, has received special attention because such reactions have been shown to provide high activity and selectivity to the desired products under ambient conditions and at low temperatures. One of the crucial factors for the reaction rate on gold nanoclusters was found to be related to the coadsorption energy of oxidized reductant molecules (CO, alcohol, alkene etc.) and the oxidant (O2) on these clusters. The following discussion focuses on selected catalytic reactions that have been studied on gold clusters and nanoparticles using a range of theoretical and experimental techniques.

3.3.1. CO oxidation The oxidation of CO has been studied extensively and it has become a benchmark reaction for examining the catalytic behavior of nanosized gold. In general, there are two basic reaction paths that have been considered for the oxidation of CO over Au particles: an indirect path that includes O2 dissociation and a direct path where adsorbed O2 reacts with adsorbed CO [51].

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Fig. 3.11. Projection views of the clusters Au19 and Au20 in their ground state configurations. Classes of atoms: Site “A” – vertex atoms of the pyramid base, coordinated to three atoms; Site “B” – atoms lying at the center of each face, coordinated to nine atoms; Site “C” – edge atoms of the pyramid-base, coordinated to six atoms; Site “D” – atoms with missing vertex on the top, coordinated to five atoms; Site “E” – atoms connecting the base of the pyramid and the missing vertex, coordinated to six atoms. Reprinted with permission from Ref. [173]. Copyright 2009 American Chemical Society.

As described above, early gas-phase studies on the reactivity of charged gold clusters indicated that the adsorption of simple molecules is both strongly size- and charge-state dependent [135,147]. Häkkinen and Landman [125] predicted theoretically that even charged gold dimer anions, Au–2, may catalyze carbon monoxide oxidation to carbon dioxide. Spurred by these and other findings, Wallace and Whetten [92] have studied the coadsorption of CO and O2 on selected anionic gold clusters, Aun , in the gas phase. In their work, highly diluted gold clusters were exposed to reactants (CO and O2) and the products were detected by time-of-flight mass spectrometry. Oxygen was found to adsorb as a one-electron acceptor on the Au–n clusters, with even-n clusters exhibiting varying reactivity toward O2, while odd-n clusters showed no evidence of reactivity [91]. A highly size-dependent reactivity toward CO was observed for Au–n with n ¼ 4–19, but no adsorption of CO was detected for gold dimer or trimmer [133]. However, the exposure of gold clusters to both reactants, either simultaneously or sequentially, showed a striking result: the existence of a cooperative coadsorption. In other words, the preadsorption of one reactant on a cluster led to the increased reactivity of the cluster toward other reactant, but the reactants were found to not compete for adsorption sites. This synergistic coadsorption behavior was explained by key changes in the electronic structure of the clusters following initial adsorption. Adsorption of O2 may remove the charge of anionic Au clusters, for example, such that the metal then appears neutral to an approaching CO molecule. It is known that CO binds much more tightly to neutral Aun than to the anion [125]. In the experiments presented herein [92], Au showed an increased cooperative coadsorption with increasing numbers of

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preadsorbates. The coadsorption of CO and O2 on Au–6 led to new peaks in the mass spectrum, which were attributed to the loss of CO2. Based on these results [92], the authors proposed a mechanism for the room-temperature reaction of O2 þ CO at low coverage. A schematic of the reaction pathway is provided in Fig. 3.12a. The mechanism of Fig. 3.12a describes a coadsorption and catalytic combustion cycle where (I) the bare equilibrium Au6 adsorbs molecular oxygen to form a superoxide adsorbate (II). Subsequently, (III) promoted coadsorption of CO occurs to initially yield a Au6CO3 species, which rearranges to produce the very stable CO3 adsorbate (IV). Decomposition yields CO2 and leaves the Au6O  (V). A second CO is adsorbed to form the Au6CO–2 species (VI), which may desorb a second CO2 and return the Au–6 catalyst. The measured integral reaction rate for CO oxidation on gas-phase Au6 anions at room temperature was 100 times larger than that reported for commercial or model gold catalysts. Socaciu et al. [176], who carried out an analogous study to that described in the preceding paragraph, found that at cryogenic temperatures, T ¼ 100 K, coadsorption of carbon monoxide and molecular oxygen can occur even on smaller gold-cluster anions, i.e. on the dimer and trimmer. Their massspectrometric measurements showed catalytic CO2 formation inside an ion trap; importantly, they reported the observation of a metastable intermediate with a mass corresponding to that of the ion, Au2CO3 . On the basis of a detailed kinetic analysis in the temperature range of 100–300 K and first-principle calculations of the energetics and structures of the intermediates, the mechanism of the catalytic reaction was deduced, which is shown in Fig. 3.12b [175]. The hypothesized intermediate was speculated to have one of two structures, either that of a digold carbonate, Au2CO–3, or a peroxyformate-like, Au2(CO)O–2. The peroxyformate-like species was suggested to form via an Eley–Rideal (ER) mechanism at no energy expense, while an alternative Langmuir–Hinshelwood (LH) mechanism was shown to occur at a significant energy cost of about 1.1 eV. Both the carbonate and peroxyformate species exhibited low activation barriers in the model calculations, 0.3–0.5 eV, for conversion to CO2, thus possibly facilitating the observed lowtemperature catalytic oxidation of CO on gold dimers. Lopez and Nørskov [177] used DFT calculations with GGA to track the minimum reaction paths for the elementary steps during oxidation of CO on a Au10 cluster. Two different reaction paths were considered: first, an indirect path that includes O2 dissociation, and then a more direct path where adsorbed O2 simply reacts with adsorbed CO. In both reaction paths, adsorption of O2 in an O–2 superoxide-like state was identified as the first reaction step. The obtained reaction barriers for both reaction paths were less than 0.4 eV, indicating that CO oxidation on the Au10 cluster should be facile at temperatures well below room temperature. However, Stolcic et al. concluded from their experimental study with gas-phase anionic gold clusters [178] that, at room temperature, adsorption of oxygen on Au–n occurred through a nondissociative pathway. Yuan and Zeng [126] theoretically explored the cooperative adsorption between CO and O2 to arrive at a

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Fig. 3.12. Schematic illustration of the gas-phase catalytic cycles for oxidation of carbon monoxide by gold anions: (a) gold hexamer, Reprinted with permission from Ref. [92]. Copyright 2002 American Chemical Society. (b) gold dimer, Reprinted with permission from Ref. [92]. Copyright 2003 American Chemical Society.

similar conclusion: the dissociation of O2 likely does not occur for gas-phase gold cluster anions, even in the presence of preadsorbed CO. In fact, their work suggests that adsorbed CO stabilizes oxygen in its superoxo form, O–2. In this way, the preadsorption of CO may increase the rate of CO oxidation. These issues have motivated a tremendous amount of research in this area over the past decade. Molina et al. [179] focused their theoretical ab initio DFT simulations on the goal of uncovering the influence of multiply-adsorbed CO molecules on the reaction mechanisms of CO oxidation with O2. Both small nano-clusters (Au4, Au5) and a model of gold nanoparticle edge sites were explored. In the simulations with edge sites of a large gold nanoparticle, a simplified model was considered, including a slab of the missing-row Au(110) surface. It was found that the interaction of CO with an O2–adsorbed Au5 cluster partially displaces the O2 molecule, although oxygen remains close and eventually reacts with CO to form OCOO  radicals. Sequential interaction with neighboring co-adsorbed CO leads to direct formation of two CO2 molecules. In the case of the model for the nanoparticle edge sites, similar behavior was observed, although the much more weakly bound O2 limits the overall activity relative to that of small clusters. In another theoretical study of CO oxidation, Wang et al. [180] explored cationic, neutral, and anionic Au trimers. Three possible reaction pathways were examined: (1) the reaction of O2–adsorbed Au3 complexes with CO, (2) CO–adsorbed Au3 reacting with O2, and (3) a self-promoting mechanism in which a bound CO molecule is activated by preadsorbed CO. They suggested that all three pathways are actually viable, based on low calculated barriers and high exothermicity. In addition, every possible Au species, i.e. cationic, neutral, and anionic, were found to exhibit activity toward CO oxidation. Although Au-carbonates were found to be a stable species produced along the reaction pathway, they were not necessarily

identified as reaction intermediates. Additional molecular dynamics simulations indicated that the formation of the stable Au-carbonate moiety was due to a collisional attack of CO2 at a surface O atom in the Au oxides. Bürgel et al. [181] reported self-promoted CO oxidation on cationic gold oxide clusters as a consequence of the large energy of CO adsorption on these clusters. The high adsorption energy of two CO molecules on cationic oxide clusters was found to help activate complete CO oxidation via either an Eley–Rideal-like or a Langmuir–Hinshelwood-like mechanism involving multiple collisions. On anionic clusters, the reaction was suggested to proceed via an Eley–Rideal-like mechanism, where CO preferentially attacks adsorbed oxygen sites rather than bare gold. Further computational studies were performed by Gao et al. [182] who employed ab initio calculations to estimate the energies of both CO and O2 molecules when they co-adsorb on various surface sites of the anionic and neutral counterparts of 12 different sized clusters. In general, the anionic clusters were found to adsorb CO and O2 more strongly than neutral clusters, as illustrated in Fig. 3.13 for clusters Au16–Au20 (see the figure caption for an explanation of how energy corresponds to the colors employed in the figures). As depicted in Fig. 3.13a, CO adsorption energy decreases with size for both anionic and neutral clusters, while O2 adsorption becomes more favorable for the anionic species. In addition, the adsorption of O2 on the corner sites of pyramidal Au–20 clusters was found to be the strongest among cluster sizes ranging from Au–16 to Au–20. As shown in Fig. 3.13b, the cone angle θ, defined as the angle between one bond and the opposing bond midpoint, was chosen as another geometric indicator of surface site reactivity. The details of their analysis provide insight into the structure– function relationship for these Au clusters. Overall, the authors found that, despite the strong adsorption of CO on neutral clusters, the coadsorption of CO and O2 on anionic clusters may lead to higher catalytic activity. Fig. 3.14 shows the

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Fig. 3.14. Calculated reaction pathways corresponding to five surface sites  (A–E) on Au34 . The five sites are highlighted on the cluster (inset). Reprinted with permission from Ref. [182]. Copyright 2011 American Chemical Society.

Fig. 3.13. (a) Schematic illustration of adsorption energies of CO and O2 on anionic and neutral clusters Au16–Au18, and Au20. Adsorption energy color code: dark green, o 0.9 eV; green,  0.5 to 0.9 eV; orange, 0.2 to  0.5 eV; gold yellow, 0.0 to  0.2 eV; blue, no binding. Negative sign represents exothermic process in adsorption. (B) Definition of the cone angle θ for a surface site (red), which is constructed based on the central surface site (red) and one of the nearest-neighbor sites, and a midpoint on the opposing bond (connected by green dashed line). Although this angle is dependent on the chosen nearest-neighbor site, its variance is typically less than 51 from the average value. Reprinted with permission from Ref. [182]. Copyright 2011 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

adsorption [182], which is in agreement with other theoretical studies [183]. Finally, this review highlights an interesting reaction pathway identified recently by Liu et al. [184] for the selfpromoting oxidation of CO at a unique triangular Au3 active site on nanosized (0.3–0.8 nm) gold clusters. DFT calculations helped to show that a coadsorbed CO molecule at a Au3 active site may be able to promote the scission of an O–O bond. The O–O bond scission was shown to result in the spontaneous production of two CO2 molecules, as illustrated in Fig. 3.15. The key step, bond breaking in the OCOO* intermediate, was found to be accelerated by the electrophilic attack of a neighboring CO molecule. Importantly, Liu et al. established that CO self-promoting oxidation may also be active in supported gold clusters such as Aun/MgO and bilayer Au/TiO2 systems [184]. These ideas will be further explored in subsequent sections of this review.

reaction pathways calculated at five surface sites (A–E) on the “magic number” cluster Au–34. Site A and site E are both relatively strong coadsorption sites for CO and O2 (site A,  1.22 eV; site E,  1.04 eV). The estimated reaction barrier at the first transition state (TS1) was found to be only 0.21 and 0.27 eV, respectively. Site C and site D are relatively weaker sites for coadsorption of CO and O2 (site C,  0.93 eV; site D,  0.98 eV), but the barrier at the TS1 state is even lower (0.16 and 0.01 eV, respectively). Within their model [182], the first step in oxidation is most likely to occur at site A, which exhibits the strongest coadsorption energy for CO and O2, because the energy level of TS1 is the lowest among the five sites considered. In the second reaction step, the intermediate O–C–O–O* decomposes to form a free CO2 molecule and an atomic O. In this final step, site A gives rise to the lowest reaction barrier (0.24 eV) and site C gives the second lowest barrier (0.34 eV). The reaction barrier for the other three sites considered was relatively higher (
0.55 eV). These results indicate that the strength of reactant adsorption is not directly related to the second reaction barrier. These researchers concluded that the rate limiting step of the reaction likely involves initial CO

3.3.2. Hydrogen peroxide formation The direct synthesis of H2O2 from its constituents, molecular hydrogen and molecular oxygen, is one of the key catalytic reactions currently considered for “green chemistry”, e.g. for selective redox processes [185–188]. There are indications in the literature that highly dispersed Au nanoparticles hold potential for use as hydrogen peroxide formation catalysts [185,189–192]. For example, Olivera et al. [192] predicted that gold-based catalysts would perform better in the direct synthesis of H2O2 than Pd, Pt, or Ag based catalysts. In addition, Thomson and co-workers [146,193] employed DFT calculations to predict that hydrogen peroxide can be formed directly from molecular hydrogen and oxygen over both neutral and charged gold clusters Aun with n r 5. For each adsorption and reaction step, multiple starting geometries were explored and optimized [193]. They suggested that the mechanism of hydrogen peroxide formation involves breaking of the H–H bond, but the O–O bond remains intact. In the entrance channel, one H atom is captured by the adsorbed O2 (a Au3O2 structure) to create the hydroperoxy intermediate (OOH), while the other H atom binds to a second Au atom.

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Fig. 3.15. Trimolecular LH mechanism involving CO self-promoting oxidation via CO induced O  O bond scission at unique triangular Au3 active site. Reprinted with permission from Ref. [184]. Copyright 2013 American Chemical Society.

Thus, the formed hydroperoxy intermediate further acts as the precursor species for the closed-loop catalytic cycle. The addition of a second H2 to the OOH intermediate results in the formation of H2O2, which was identified as the ratedetermining step of the catalytic cycle. This is a common feature of the reaction pathways for H2O2 formation on all the clusters explored. After the H2O2 desorption, the remaining AunH2 reacts with O2 to form the hydroperoxy intermediate and thus the reaction cycle is closed. The critical requirement for this chemistry is that a balance be struck between relatively weak Aun–O2 interactions, favorable H2 dissociation, and strong interactions between these species, especially when O2 coverage is high. The authors concluded that the formation of the hydroperoxy intermediate is both thermodynamically and kinetically accessible only for a narrow range of O2 binding energies from 40 to about 50 kJ mol  1 [193]. Ding and co-workers [194] have theoretically investigated the direct formation of H2O2 from H2 and O2 over anionic gold clusters Aun (n¼ 1–4). The H2O2 formation was found to proceed via two elementary steps. In the first step, an OOH-containing intermediate may be formed via the dissociation of H2 and in the second, this intermediate is transformed via isomerization into a product-like intermediate. Their calculated energy barriers for the formation of H2O2 were very close (only 2.5 kJ mol  1 difference) to those reported by Thomson et al. [193]. Others such as Häkkinen and co-workers [195] have used firstprinciple-based simulations to study the energetics and pathways for hydrogen peroxide formation from H2 and O2 bound to neutral gold dimers and tetramers. In their work, two competing channels, with production of H2O2 and H2O as end products, were considered. In addition to the T¼ 0 K geometry optimizations, this work also employed constrained Car–Parrinello molecular dynamics simulations at T¼ 300 K and metadynamics at T¼ 300 K. Their calculations showed a gold cluster-size dependence to hydrogen peroxide formation. Binding of a Au4 cluster with H2 and O2 was found to transform the ground state rhombus geometry into a linear gold tetramer. As shown in Fig. 3.16 for the case of a Au4 cluster, O2 is tilted and points to the proton (O–H distance 1.98 Å) that is connected to two Au atoms; the activation barrier to achieve this TS is 0.86 eV. Next, the reaction path was found to proceed downhill until the Au cluster recovers its initial rhombus geometry. Within this picture, the product hydrogen peroxide is bound to a “Y-shaped”

Au4 isomer with a Au–O distance of 2.24 Å and Au4/H2O2 binding energy 0.57 eV; water is bound to a rhombus-like Au4O isomer with a Au–O distance of 2.30 Å and Au4O/H2O binding energy of 0.62 eV. Overall, this work [195] demonstrated the importance of geometric fluxionality (also demonstrated for CO oxidation) for the catalytic performance of Au clusters. For further details and key references related to the role of Au fluxionality in catalytic reactions, see Sections 4.2 and 8.2. 3.3.3. Methanol oxidation Methanol oxidation by molecular oxygen is another benchmark catalytic reaction that has been explored with the goal of developing an understanding of the structural, chemical, and catalytic properties of nanosized gold clusters. Further, the selective oxidation of methanol is an important industrial process that is currently explored for the production of formaldehyde [196], as well as molecular hydrogen [197]. Currently, the two main catalysts that are used in formaldehyde production are based on Ag and Fe–Mo oxide that operate at 923 and 623 K, respectively [196]. Finding a new catalyst that has high activity and high selectivity for the oxidation of methanol to formaldehyde at lower temperatures is of practical importance. The ability of supported gold catalysts to activate molecular oxygen for low temperature (o 373 K) selective oxidation of alcohol to carbonyl compounds gives some promise in this direction [52]. As discussed above, experimental and theoretical studies have revealed that anionic gold clusters (Au–n) with small even-numbered (nr 14) and “magic” (n ¼ 18, 20) sizes can generate superoxo-like species via electron transfer from the HOMO of the gold cluster to the LUMO (π*) of the O2 molecule [94,148,149,152]. As for CO oxidation, the superoxo species can initiate the oxidation of alcohols, alkenes, etc. Tsukuda and co-workers [198,199] have demonstrated that the charge-transfer activation of O2 by Au is the primary step for aerobic oxidation of alcohols. In complementary work, Ehara and co-workers [200,201] employed DFT calculations with the M06 functional to study the aerobic oxidation of methanol to formic acid on two model gold catalysts: the charged Au–20 and Au–8 clusters. For reference, the 20-atom gold cluster has a diameter of 1.1 nm. Both clusters showed low electron affinity, which favors electron transfer from the cluster to the 2π* orbital of the oxygen molecule, resulting in the non-dissociative

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Fig. 3.16. The optimized 0 K configurations for Au4 catalyzed reactions. Reprinted with permission from Ref. [195]. Copyright 2009 The Royal Society of Chemistry.

chemisorption of O2 on Au–n. The calculated adsorption energy of O2 on the Au–8 cluster [201] was 52 kJ mol  1, while on the Au–20 cluster [200] it varied from 25 kJ/mol to 70 kJ mol  1. These results are consistent with other reports [151,154]. From a structural perspective, O2 was found to adsorb preferably at apex and edge sites [200,201]. As the discussion associated with Fig. 3.12 described, high electron densities normally appear at low-coordination sites [173] (in agreement with the experimental observations that rough gold surfaces are more active for the adsorption of O2 than flat surfaces). Upon adsorption, the O–O bond was found to elongate from 1.277 to 1.305 Å with the Au–8 cluster [201] and from 1.289 to 1.351 Å for the Au–20 cluster [200]. In both cases, however, the O–O bond does not rupture; rather, the superoxo-like anion abstracts hydrogen from a C–H group of a co-adsorbed methoxy species. The transfer of hydrogen to the superoxo-like anion results in the formation of formaldehyde adsorbed on a hydroperoxyl-like complex, AunOOH. This step is considered to be the ratedetermining step for the overall reaction with an activation barrier of about
94 kJ mol  1 [200]. Structural distortion of the gold cluster, together with charge re-distribution, serves to stabilize the transition state structures. Subsequently, formaldehyde is converted to a hemiacetal intermediate via intramolecular insertion by the hydroxyl group of the hydroperoxyl-like complex. Finally, the hemiacetal intermediate was found to react with atomic oxygen attached on the gold cluster to produce formic acid. Ehara and co-workers [202] revisited the above-discussed energetics for methanol oxidation to formic acid on Au8 clusters by exploiting renormalized coupled-cluster and DFT calculations. A reasonable agreement was found between results of the previous DFT calculations (with M06 hybrid functional) and those obtained by the CR-CC(2,3) method. The researchers demonstrated that CR-CC(2,3) calculations confirm the reaction mechanism discussed above, which places the rate-determining transition state (hydrogen transfer from methoxy to the molecular oxygen) at about 96 kJ mol  1 above the reactants. This work provides critical information for future theoretical and experimental studies of reactivity on metal nanoparticles.

3.3.4. Activation and oxidation of alkenes Along with catalytic oxidation of carbon monoxide and the more complex oxidation of alcohols, gold nanoclusters have

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been shown to be active catalysts for epoxidation of alkenes at mild temperatures – a reaction with industrial importance in which an oxygen atom modifies the carbon–carbon double bond. An early study by Nakatsuji and co-workers [203] addressed the question of why “only silver is an effective catalyst for the partial oxidation of ethylene” by studying the reactivity and the stability of oxygen on Cu, Ag, and Au surfaces. They showed that oxygen forms superoxide on Ag, which subsequently reacts with ethylene to produce ethylene oxide. On the surface of Au, their calculations (dipped adcluster model [204]) showed that electron flow from Au metal into O2 does not occur. Therefore, they found that adsorbed molecular oxygen does not survive for sufficient durations on a clean gold surface to form Au2O2 or Au4O2 adcluster, and thus to catalyze the epoxidation. The authors predicted that Au may show catalytic activity toward epoxidation if its electron donating ability to O2 is increased. Indeed, soon after this publication [203], Haruta and co-workers discovered that gold nanoparticles supported on TiO2 as hemispherical particles can catalyze the selective oxidation of propylene in an oxygen and hydrogen rich atmosphere [205]. Further research and developments have shown that supported gold-based catalysts are promising systems for direct epoxidation of higher olefins by molecular oxygen (see Section 8.2.5) [185,206,207]. Despite the potential practical importance of this chemistry, there are relatively few examples of theoretical studies of catalytic oxidation of alkenes on gold clusters. Notwithstanding, Lee et al. reported the epoxidation of propene on immobilized Au6–Au10 clusters [208]. In addition, ethylene epoxidation by atomic oxygen on Au29 clusters [209] and on Au(111) has been explored [209,210]. By using DFT calculations with the B3PW91 functional, Lyalin and Taketsugu [211,212] have shown that cooperative adsorption of O2 and C2H4 on small gold clusters can stimulate the dissociation of O2. The cooperative effect, which stabilizes the O2–Aun–C2H4 system, may play a major role in the epoxidation of alkenes. In agreement with other studies [51,130,144,149], individual O2 adsorption on small gold clusters showed odd–even oscillations in Eads(O2), r(O–O), and Δq(O2), as a result of the electronic shell effects. However, this study seems to have been the first to report the adsorption of ethylene on small Aun clusters. The mechanism of ethylene binding was found to be quite different from that of the O2– Aun interaction. Specifically, ethylene was shown to bind to gold clusters via electron transfer from the filled π orbital to the metal, along with a back-donation from the d orbital of gold to the empty π* C2H4 orbital, in accordance with the Dewar– Chatt–Duncanson description [213]. Both the donation and back donation of electrons cause a weakening of the C–C bond in C2H4. Thus, adsorption of C2H4 on gold clusters promotes its activation and a probable oxidation. Compared with sequential adsorption, coadsorption of O2 and C2H4 on oddn gold clusters was energetically favorable, with C2H4 weakening the O–Au bond. This effect was rationalized by reference to strong charge transfer effects in the system. That is, adsorption of O2 on Aun with an odd number of atoms

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likely leads to charge transfer from the cluster to the 2π* orbital of O2. Thus, C2H4 favorably adsorbs on the positively charged gold cluster. The excess positive charge on the cluster stabilizes the π mode of adsorption, thereby strengthening the Au–C bond. An analogous process was also predicted for CO [100] and propene [214] adsorption on O2–Aun clusters. Beyond the synergistic effects of O2 adsorption on ethylene binding, the above-referenced study [212] suggests that coadsorption of C2H4 can promote oxygen dissociation. The general conclusion of that work was that the nature of O2 dissociation on gold clusters depends on the presence of other co-adsorbates (including O2 itself). As was discussed for CO and CH3OH oxidation, this effect is of particular importance for describing the catalytic oxidation of ethylene and other hydrocarbons on small gold clusters. Further studies of alkene oxidation have been pursued by Metiu and co-workers [214], who employed DFT calculations to investigate the bonding of propene to small gas-phase gold clusters and to Au (111). The calculated desorption energies and geometries of the isomers of the complexes [Aun(C3H6)]q (for n¼ 1–5, 8 and q¼  1, 0, þ 1) were used to predict trends on propene–cluster bond strength and the bonding site of propene on the cluster. The authors proposed a set of qualitative rules for predicting the binding site and the structure of the most stable isomers. These rules, based on the calculated LUMOs of the Au cluster and HOMO of propene, can be summarized in the following way: (1) Upon binding of propene to gold, electron density is transferred from the HOMO of the C3H6 to one of the LUMO orbitals of gold. The easier this transfer is, the stronger the bond, i.e. the strength of C3H6 bonding to clusters decreases in the following order Aunþ 4Aun 4Aun . (2) The most stable isomer of [Aun(C3H6)]q (q¼  1, 0, þ 1) is the one in which propene adsorbs at a site with LUMO1 having the most prominent lobe. LUMO1 denotes the empty orbital with the lowest energy. This rule accurately predicts that flat faces of a cluster or flat surfaces of the bulk do not bind propene. (3) The desorption energy of propene from the most stable isomer of [Aun(C3H6)]q (q¼  1, 0, þ 1) correlates with the energy of LUMO1. The same correlation exists for CO[100] and propene oxide adsorbed on gold clusters. (4) If there are two prominent lobes for LUMO1, the lowest energy isomer is obtained when C3H6 binds to the Au site with the lowest coordination. 3.3.5. Activation and transformation of methane The activation and catalytic transformation of methane into “more valuable” products such as methanol, formaldehyde, or light olefins is of significant economic importance but presents a challenging task for chemists [215]. As it was once dubbed by Barton, it is a “search for the chemist's Holy Grail” [215]. The reason for such difficulty is the high energy required for activation of the stable C–H bond in methane; the bond dissociation energy is 440 kJ mol  1 [216]. Activation and dehydrogenation of methane (by forming MCH2þ ) has been found viable on a variety of third-row transition metal cations (M þ ) [217]. However, Au þ is one of a few 5d metals that is completely unreactive with respect to methane activation [217], and gold clusters Aux þ were shown to be unreactive

toward CH4 under single-collision conditions [218]. Interestingly, conditions under which multiple collisions occur have been shown to promote CH4 uptake [219]. Lang et al. [220] have studied the interaction of CH4 with free Au2þ ions by combining radio-frequency (RF) ion-trap mass-spectrometry measurements and first-principles theoretical calculations. Measurements of temperature-dependent product-ion mass distributions and kinetics revealed the lowtemperature (
250 K) activation and dehydrogenation of methane leading to selective formation of ethylene. Noteworthy, for both the C–H bond activation and subsequent ethylene release, cooperative effects involving multiple adsorbed molecules were required. Analysis of kinetic data and DFT calculations led to the development of mechanistic steps for the catalytic cycle, shown in Fig. 3.17. In a follow up study, Lang et al. [99] further explored the interaction of CH4 with a series of small gold cluster cations Aun þ (n ¼ 2–6) by combined gas-phase kinetic measurements and first-principles DFT calculations. The experimental data were analyzed by the Lindemann energy transfer model for association reactions [221] to determine the cluster-sizedependent binding energetics of methane to cluster cations. For this purpose, “tight” and “loose” transition-state (TS) models were employed. The “tight” TS model, typically employed to describe rearrangement type reactions, yields the lower limit for the binding energy. In contrast, the “loose” TS describes bond cleavage reactions. For both TS models, the binding energies were found to decrease continuously with increasing cluster size. The largest binding energy of 62.77 3 kJ mol  1 and 87.8 7 4 kJ mol  1 for a “tight” and “loose” TS model, respectively, was determined for the Au2þ cluster. The binding energies obtained by employing a “loose” TS model were in good agreement with the theoretical values at the optimal adsorption geometries. In addition, these binding energies considerably exceed the value (14.5 7 0.2 kJ mol  1) reported for the binding of CH4 to an extended Au(111) surface [222] (see Section 4.2.6). Detailed computational analysis of the Kohn–Sham molecular orbitals for the Au2– CH4þ and Au6–CH4þ structures showed the existence of two bonding orbital types: type (I) where the bonding orbital is localized mainly on the binding (proximal) Au atom and the adsorbed molecule; and type (II), consisting of orbitals that are delocalized over the entire system [99]. Recently, Mowbray et al. [223] employed DFT calculations to systematically study the adsorption of CH4 and O2 on gold clusters and nanoparticles. This study identified a linear correlation between methane to gold charge transfer and CH4 adsorption energy. The obtained correlation was independent of the size of the gold clusters (Aun for n ¼ 2, 6, 7, 55, 201) and the coordination number of the adsorption site (Nc ¼ 1, 2, 3, 5, 6). Importantly, this trend appears to hold for gold particles in vacuum, as well as on a pristine or defective rutile TiO2(110) support. Because the gold 5d levels are filled, the Aun–CH4 interaction is suggested to occur primarily via the 6s levels. Thus, the 5d levels of gold contribute only though a weak repulsive interaction. In contrast to CH4, the adsorption energy of oxygen depends only weakly on charge transfer for

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from key studies are provided, which will help to explain some of the chemistry on clusters described above and prepare the reader for the subsequent sections that focus on gold nanoparticles and supported nanoparticles. 4.1. Structures

Fig. 3.17. Reaction mechanism that best fits the kinetic data. Reprinted with permission from Ref. [220]. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

positively charged gold nanoparticles and it is rather strongly dependent on the gold particle size, as well as the coordination number at the adsorption site [224]. This suggests that one may independently affect methane and oxygen adsorption at positively charged gold nanoparticles (through surface or electrostatic doping), and hence control the catalytic performance of gold nanoparticles in delicate oxidations [223]. 3.4. Summary Many of the concepts described here help to predict the activity of larger Au nanoparticles dispersed on metal-oxide supports. Importantly, they highlight the structural modifications during adsorption, the synergistic effects of co-adsorbate binding, and the critical role of charge transfer during activation of reactant molecules. These effects are mitigated to a large extent in extended bulk single-crystal systems; however, a complete understanding of chemistry on supported Au-nanoparticles requires a brief review of gas-surface interactions on bulk Au, the topic of the next section. 4. Surface structure and reactivity of gold single crystals Well-defined extended crystalline gold surfaces can serve as important modes for uncovering how gold clusters and nanoparticles catalyze chemical transformations. Single-crystal surfaces can be employed to systematically explore how molecule– surface interactions depend on crystal face orientation, as well as the presence of steps, kinks, and other specifically targeted defects. In addition, studies on crystalline surfaces provide critical benchmarks for computational chemistry and references for experimental techniques. In these ways, experimental and theoretical studies on single crystals must be considered in building a comprehensive understanding of Au/TiO2 catalysts. Several excellent reviews have been published recently on this and closely related topics [225–227]. Below, a few highlights

Crystalline gold resides as a face-centered cubic (fcc) structure and the three low Miller index surfaces (100), (110), and (111) are, by far, the most widely explored (see Fig. 4.1) [226]. The coordination number of Au surface atoms is 7, 11 and 9 for Au(100), Au(110), and Au(111) facets, respectively. These differing degrees of coordination naturally present different surface energies of gold 0.08, 0.10, and 0.05 eV/Å2 for the (100) [228], (110) [229] and (111) [230] surfaces, respectively. However, the structure of each low-index surface reconstructs as it strives to minimize free energy [228–232]. Thus, different patterns of reconstruction have been observed for different surfaces, e.g. a c(26  68) for the Au(100), a (1  2) pattern for Au(110), and a reconstruction with a [(22 7 1)  √3] unit cell for the (111) surface. Gold is the only face-centered-cubic (fcc) metal for which the closepacked Au(111) surface has a reconstructed ground state. The beautiful (22 7 1)  √3 “herringbone” reconstruction, shown in Fig. 4.2a, is the consequence of two competing effects [226]: a contraction in response to reduced coordination versus a substrate potential that energetically favors a commensurate surface layer [233]. The substrate layer and the surface layer misfit results in a periodic array of pairs of partial dislocations. This causes spontaneous formation of alternating domains composed of surface atoms that occupy a fcc sites and hexagonal closepacked (hcp) sites, as shown in Fig. 4.2b [226,233,234]. Thus, the surface gold atoms located at the dislocation lines occupy bridge, as opposed to hollow, sites. Like its color, the reconstruction of gold is a consequence of relativistic effects, i.e. the interaction of sp and d states that results in unique surface states [235]. The reconstructed surface is stable up to 865 K, but the long range order is lost at higher temperatures [236]. The (1  2) pattern of Au(110) is formed by a “missing-row” reconstruction along the 〈110〉 direction. The reconstructed surface has gold atoms that reside in top, side of row, and trench sites, each with different coordination numbers. Importantly, this structural reconstruction has a significant effect on molecular adsorption at the surface [226,237]. Scientists have suggested that delocalized 5d electrons interact to stabilize the reconstruction [238]. The reconstructed surface is stable up to
650 K; however, roughening along and adjacent to step edges has been reported at higher temperatures [239]. Comparing the three low indexed surfaces, the actual reconstructed structure of Au(100) is somewhat controversial and various models have been suggested, including a (5  20) with rotation [240], a c(26  68) [231], a (24  48) with rotation [241] and a hexagonal (5  28)R0.61 [242]. Researchers have suggested that the reconstructions might exhibit various combinations of these structures at different temperatures and for different

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Fig. 4.1. (100), (110), and (111) fcc crystal surfaces. Reproduced with permission from the NIST Surface Structure Database.36. Reprinted with permission from Ref. [226]. Copyright 2012 American Chemical Society.

Fig. 4.2. (a) STM image of the reconstructed Au(111) surface. Inset shows atomic resolution of the edge dislocations (depressions) which are present at the elbows of the herringbone reconstruction. (b) In-plane structure of the Au(111) reconstructed surface. The circle and crosses correspond to atoms in the first and second surface layers, respectively. The original figure from Ref. [233]. Reprinted with permission from Ref. [226]. Copyright 2012 American Chemical Society.

step densities [243]. This reconstruction is governed in large part by relativistic effects due to the high degree of d orbitals participation in bonding [228,244]. Upon heating to 1170 K, the reconstruction is removed and only the (1  1) phase is observed, revealing an order–disorder transition [245,246]. Bulk gold crystals cleaved to expose high Miller index (stepped) surfaces have also been studied for a variety of purposes, including their relevance to catalysis, as the highly curved nature of particulate surfaces present many high stepped motifs to the reactant feed. The structure of such systems is shown schematically in Fig. 4.3 [227]. The Au(211) stepped surface is characterized by three-atomwide terraces of (111) orientation and a monatomic step with a (100) orientation, or 3(111)  (100) in microfacet notation. The (310) surface has terraces with a (100)-like structure, whereas the terraces of the (321) surface have a (111)-like structure. The atoms at the steps of both surfaces are 6-coordinated and atoms at terraces are both 8 and 9-coordinated. Au(321) exhibits five unique classes of surface atoms, while the Au(310) surface is composed of three different types of atoms. The Au(532) surface is characterized by a large number of low coordination (6) kink sites [249]. Low-Miller-index (111) and (100), stepped (211), and kinked (532) gold surfaces exhibit a strong reactivity dependence on the local coordination number of the surface atoms. Low Au atom coordination is typically associated with higher binding energies [224] of adsorbates and thermally induced chemical

transformations. These chemical interactions are of fundamental importance to the overall understanding of Au-based catalysts; however, these extended bulk systems are slightly outside the scope of this review. Therefore, only a summary of a few key discoveries related to adsorption and catalytic reactions on single crystal gold surfaces will be provided in the following sections, which begin with a brief overview of simple bond formation during physisorption and chemisorption at gold surfaces. 4.2. Adsorption of molecules and atoms As a molecule approaches a metal, it may simply skip off the surface following an impulsive collision, bounce along the surface prior to recoiling back into the ambient, or become trapped by an attractive potential energy well. A fully trapped molecule that, even momentarily, becomes an adsorbate is considered to have reached full mass accommodation. For many (but not all) reactions, mass accommodation is a necessary first step to catalysis. The initial energy transfer events and subsequent accommodation efficiencies are determined by the molecule–surface approach geometry, energetics, and the overall potential energy surface [250]. A weakly trapped molecule may bond to the surface through dispersion forces, such as electrostatic polarization and dynamic dipole correlations [251]. An example of such bonding, characterized as physisorption, is that which occurs

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Fig. 4.3. (a) Surface structures of the Au(211), Au(310), Au(321) and Au(532) surfaces (generated with Surface Explorer Ref. [247]). Reprinted with permission from Ref. [227]. Copyright 2009 World Gold Council. (b) Top and side view of the Au(310) and Au(321) surfaces. The coordination number for the different types of surface atoms is indicated in the (b). Reprinted with permission from Ref. [248]. Copyright 2009 Elsevier B.V.

between linear hydrocarbons (alkanes) and Au(111) where the bond strength is
6 kJ mol  1 (ca. 0.06 eV) per methylene subunit [222]. In this case, the separation between the adsorbate and the surface atoms is much longer than a typical covalent bond. Although available for reactions with other molecules or at special surface sites, physisorbed molecules are not typically considered to be activated for catalysis. Stronger interactions that lead to rearrangements of the surface and adsorbate electronic structure (chemistry) require molecule–surface separations that provide overlap of electronic wavefunctions [251]. An example for such strong bonding is the chemisorption of CO on late transition metals, where adsorption energies in the range of
50–190 kJ mol  1 (0.5–2 eV) have been reported [251,252]. Interestingly, CO (and other molecules) do not interact nearly this strongly with the coinage metals Cu, Ag and Au. Building a fundamental understanding for the physics that governs the strength of adsorbate–surface bonding is critical to predicting the fate of molecules on surfaces, interpreting experimental observations, and designing new catalysts [29,251,253]. Within this context, many groups have explored the surface adsorption characteristics of the excellent probe molecule CO. Studies of surface bonding for CO and the isoelectronic molecule, N2, on late transition metals provide critical insight into the electronics of molecular adsorption [254]. Over the years, researchers have often struggled to develop a complete understanding of adsorption, even for these small molecules at surfaces. Because the adsorption energies are typically very low, relative to the molecular dissociation energy (5–10%), scientists have relied on the assumption that adsorption involves only small modifications to the molecular orbital structure of the adsorbate. Thus, many models treated the molecule as a unit, where only the frontier-orbitals (the HOMO and the LUMO) were considered, as described in Section 3 [254].

Fig. 4.4 illustrates the most common description of the bonding of CO to metals through a frontier-orbital picture [253,254]. In CO, the 5s orbital is the highest occupied molecular orbital (HOMO) and the 2π* orbital is the lowest unoccupied molecular orbital (LUMO). “A dative bond between the CO 5s and metal states of s symmetry is formed, leading to charge donation into the metal which is compensated by a back donation into the CO 2π*” [253,254]. The charge transfer of an electron from the 5s orbital of CO to a metal center (M’CO) would decrease the population of the slightly anti-bonding 5s orbital [255] (see Fig. 4.4a), thereby stabilizing the molecule (strengthening the CO bond). This effect would produce a blueshift of the observed CO stretching frequency relative to gas phase CO. However, the main contribution to CO bonding has been described as arising from the back donation of an electron from the metal d orbitals to the empty anti-bonding 2π* molecular orbital, i.e. M-CO, as shown in Fig. 4.4b. As with metal hexacarbonyl complexes [256], back donation leads to elongation of the C–O distance and weakening of the CO bond. In the above frontier-orbitals picture, an intuitive synergy between the s and π systems is easy to understand, where the internal C–O bond is weakened due to the raised population in the antibonding CO 2π* state, resulting from the back donation [253,254]. This description has been used extensively to explain spectroscopic observations of CO vibrational frequency changes upon adsorption of the molecule at different metal surfaces and surface sites [257]. However, this frontierorbitals description includes only the first step in the reaction path that leads to the final adsorbate electronic structure. A complete explanation for surface bonding must involve descriptions of the complete electronic structure [30]. Periodically, other concepts have challenged the frontier-orbitals model by describing the repulsive s interaction [258] and substantial mixing between the s orbitals [259], for example.

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Fig. 4.5. Schematic illustration of the formation of a chemical bond between an adsorbate valence level (dark blue) and the s (light blue) and d (red) states of a transition-metal surface. The bond is characterized by the degree to which the antibonding state between the adsorbate state and the metal d states is occupied. Ref. [1232]. Copyright 2005 Springer Science+Business Media, LLC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 4.4. Schematic picture of the frontier orbital description of CO metal bonding via 5s donation (a) and 2p back-donation (b) interactions with metal electron states. Reprinted with permission from Ref. [254]. Copyright 2004 Elsevier B.V.

As highlighted in previous sections, Nørskov and Hammer [29], described a basic concept, borne out of the Newns–Anderson model [260,261], to explain trends in adsorption strength along the series of transition metals. This so-called d-band model has become one of the most used concepts in catalysis due to its success in predicting trends in reactivity for different metals. According to this description, the trends in molecule–surface bonding stem from the formation of bonding and antibonding “orbitals” between renormalized valence states and metal d-states. The d-band model, illustrated in Fig. 4.5, is an approximate description of the interactions between the adsorbate valence levels and the s and d energy levels of a transition-metal surface. The schematic illustrates how the adsorbate state may split into bonding and anti-bonding resonances upon coupling to the d states of the metal. Because all transition metals have similar broad halffilled s-bands, the relative strength of the molecule–surface bond depends almost entirely on the interaction of the single renormalized resonance with the metal d states. For the weak coupling cases, the bonding energy depends (to a first approximation) only on the position of the d-states relative to the Fermi level. When the d-band is higher than the Fermi level (unfilled), the molecule– surface antibonding orbital does not become occupied, which produces a strong adsorbate–surface bond. In contrast, when the dband is fully occupied, the antibonding orbital is filled and the overall interaction is weak or repulsive, as is the case for CO adsorption on gold [262]. Actual molecule–surface interactions usually lie somewhere between these extremes. Nilsson et al. [253,254] combined X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) measurements, together with density functional theory (DFT) calculations, to explore the electronic structure of CO and N2, the surface, and the molecule–surface bond upon adsorption. For bonding to a Ni atom, their results demonstrated the critical role of orbital mixing within the π system of these molecules. The mixing of the π orbitals results in the formation of a bonding, non-bonding, and an antibonding orbital resembling an allylic molecular orbital

configuration between the metal atom and the molecule. In addition, the CO s orbital changes as it is depopulated and polarized away from the metal toward the outer oxygen atom of CO. The changes in the population of the molecular orbitals are shown Fig. 4.6. Energetically, the π- and s-systems were found to behave in opposite ways: population of the new π bonding orbital stabilizes the adsorbate–substrate complex, whereas the s-interaction leads to repulsion. Within this model, the adsorption energy is then dictated by the degree of population of the new bonding and antibonding orbitals formed between the adsorbate and the metal, which (as described in the Norskov and Hammer d-band model) depends on the energy (and width) of the d-band. In addition, the s-repulsion should also depend on the d-band characteristic because a less filled d-band results in lower Pauli repulsion. Overall, the important conclusion from these studies is that the adsorption energy for CO is predicted to increase as the d-band center shifts toward the Fermi level in the metal. Hence, adsorption energy, activation energies, and transition state energies should all depend on the identity of the metal, crystal structure, and surface site. For real catalysts, discussed in subsequent sections, these energetics (mediated by the d-band structure) should also depend on metal support, particle size, particle shape, and interfacial structure. Recently, Pettersson and Nilsson [30] analyzed experimental and theoretical work on chemical bond-formation at surfaces [253,254,263] within the framework of the d-band model [29]. Particular focus in their work was placed on the states (empty and occupied) available on the adsorbate. Within this analysis, the authors described several different bonding configurations, which are reproduced here in Fig. 4.7. In this description, there is a radical state with an unpaired electron, which can directly interact with the metal d-band, i.e. in (a) an atomic radical – the adsorbate has inherent radical character; in (b) a diatomic molecule (CO, N2) – rehybridization creates a virtual radical state; in (c and d) unsaturated hydrocarbons and O2 – rehybridization creates a real radical state; in (e) saturated hydrocarbons – polarization enables a weak bond formation. As described above, the radical state

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whether the adsorptive and reactive properties of stepped, kinked, and rough single-crystal gold surfaces approximate those of supported Au particles, or if the underlying support dramatically affects the electronic and physical structure of the surface atoms. For example, it has been reported that oxygen molecules dissociate more easily on step and kink sites and CO binds more strongly to such sites (vide infra). Thus, by exploring the uptake characteristics of various adsorbates, researchers have found that the particular electronic (ligand) and geometric (ensemble) characteristics in these systems can reproduce chemistries also observed in particulate catalysts [267–270]. A recent review by Gong nicely presents historical, experimental, and theoretical work that examined adsorption at gold single crystal surfaces [226], while updating key information from previous reviews. For a more in-depth analysis of this chemistry, the reader is encouraged to seek out that work [186,225,227]. In what follows, an overview of experimental and computational observations regarding molecular adsorption is provided. Although not explicitly discussed in much of the work cited below, the bonding descriptions highlighted above account for much of the observed chemistry.

Fig. 4.6. Charge density difference plots of CO adsorbed on a Ni13 cluster. Regions of electron loss are indicated with dashed outer line (red) and increase (blue) with full line. A plane containing the interacting metal atom with one CO molecule in the same plane is shown. Reprinted with permission from Ref. [254]. Copyright 2004 Elsevier B.V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

produces bonding and antibonding orbitals. The population of these orbitals depends on the location of the d-band center relative to the Fermi level [30]. Insightful work by Nørskov and coworkers [262,264] explored how the d-band model could be used to explain differences in the energy of adsorption for CO and O-atoms on different surface structures of the same metal. The key conclusion from that work is that the energetics of adsorption depends on surface structure precisely because the center of the d states depends on the width of the band, which changes with coordination number. One important result from these studies [224] is provided by the data in Fig. 4.8. According to these results, step-metal atoms form stronger bonds to adsorbates than atoms at terrace sites because the d-band center is closer to the Fermi level for low-coordinated atoms [224,251]. This result also helps to explain trends in reactivity with cluster/ particle size (see Section 8.1.3) [265,266]. The above discussion clearly shows that the structure has a strong influence on the energetics of molecular bonding [225,227,262]. In this view, an interesting question arises as to

4.2.1. Carbon monoxide Very early studies found that unlike transition metals such as Ni, Pd, Rh, Pt that chemisorb CO strongly at room temperature, Au and Cu films reversibly chemisorb CO, exhibiting enthalpies of adsorption in the range of 36–39 kJ mol  1 [271]. However, on smooth gold films, no adsorbed CO was observable in one study, via IR spectroscopic measurements, even at 25 K [272]. In contrast, adsorption of CO appeared to occur on rough gold films at surface temperatures up to 170 K, as evidenced by an IR band at 2125 cm  1 [272]. Further insight into CO adsorption was obtained by Piccolo et al., who studied the adsorption of CO on closely packed Au (111) at pressures in the range of 10–3–103 Torr at room temperature [273]. Clear evidence was found for chemical interactions between CO and “herringbone” 22  √3 reconstructed clean Au(111) via on-top adsorption (a RAIRS study), which drove a surface deconstruction. Their STM study showed that the number of step and kink sites increases upon CO exposure at PCO E 0.13 kPa. A gradual 22  √3-1  1 deconstruction transition occurred at terraces and appeared to be complete at
33 kPa. This CO-induced deconstructing of the herringbone superstructure leads to formation of island-like heterogeneous structures. A more recent study demonstrated that extensive CO exposure at pressures up to 0.1 kPa can facilitate atomic rearrangement on the roughened surface, even near room temperature [274]. The rearrangement tends to drive the surface toward the Au(111) structure, which occurs simultaneously with a reduction in the ability to bind CO. The adsorption of CO on the more open Au(110)-(1  2) surface exhibited an isosteric heat of adsorption of 59 kJ mol  1 for θCO-0, which decreased strongly with coverage [275]. This indicated weak chemisorption and substantial CO–CO repulsion on this surface. Under vacuum, the (1  2) reconstructed Au(110) surface showed large-scale alignment of terraces in the [1 10] direction [276]. Exposure to CO at
1 Pa induced a slow transition deconstruction of the (1  2) surface to a (1  4)

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Fig. 4.7. Schematic illustrations of the different types of chemical bond formation on metal surfaces. Reprinted with permission from Ref. [30]. Copyright 2014 Springer Science+Business Media, LLC.

Fig. 4.8. Illustration of the extent of the d-band model. Calculated CO and O adsorption energies for a range of different Au (a) and Pt (b) surfaces including 12 atom clusters are seen to correlate with the calculated d-band center (εd). Reprinted with permission from Ref. [224]. Copyright 2009 American Chemical Society.

structure at PCO E13 Pa. Exposure to
4 kPa CO caused a dramatic restructuring to yield atomic-high islands. Further increase in PCO to 13.3 kPa led to a (1  1) unreconstructed surface, while an increase to 67 kPa led to the appearance of
0.05 nm deep holes arranged in a c(4  4) array [276]. Studies of CO at other crystal faces show similar interesting dynamics and provide some insight into what may happen on highly curved particulates. The Au(211) surface has been shown to exhibit higher CO binding energies and enhanced reactivity relative to the flat Au(111) surface [277]. On the Au (211) surface, CO has been shown to bind more strongly at step sites (50 kJ mol  1) than at terrace sites (27–38 kJ mol  1). The sticking coefficient for CO occupying step sites was also

higher (
5  ) as compared to populating terrace sites at higher coverages. The IRAS-measured frequency of adsorbed CO suggested bonding in atop configurations at all coverages and conditions. As found for other systems, a small red shift in ν(CO) from 2126 to 2112 cm  1 was measured with increasing CO coverage [272,278,279]. On Au(332), CO adsorption occurs with an isosteric enthalpy of 55 kJ mol–1 at low coverage, which decreases to 20 kJ mol  1 at saturation [278,279]. At this surface, an IR band at 2120 cm–1 was detected at low coverage, which red shifted with increasing coverage. By using a Cl6O–C18O isotopic mixture, the researchers succeeded in resolving four separate components in a set of complex IR spectra. The complexity of the CO spectrum was explained in terms of CO-induced surface reconstruction with three of the components associated with kinks, terraces, and steps. The fourth low-intensity component, which appeared at 22 cm–1 below the principal component and was registered only at higher coverages, was not conclusively assigned, but was most likely attributed to the effects of surface reconstruction [278]. Further experimental work by researchers such as Weststrate et al. [248] explored CO adsorption on vicinal Au(310) and Au (321) surfaces. These surfaces both exhibit 6-fold coordinated atoms at the step edges, but they have different terrace structures. The characteristics of CO adsorption was found to be similar on both surfaces, indicating that CO is adsorbed on the 6-fold coordinated step atoms rather than on 8-fold coordinated terrace atoms. TDS peaks at 120 and 180 K were attributed to CO at step atoms, each corresponding
25% coverage. This study showed that long-range substrate-mediated repulsive interactions govern CO adsorption on Au surfaces, which demonstrates that the coordination number at the adsorption site is not the only factor in determining CO adsorption energy.

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Fig. 4.9. (a) Top view of the CO adsorption on Au(110)-(1  2) surface. (b) Stretching normal modes of the (2  1) adsorption structure where the CO molecules adsorb on the ridge of the missing row and on the Au adatom along the missing row. Adapted with permission from Ref. [280]. Copyright 2005 American Chemical Society.

Theoretical calculations have also been performed to help elucidate the adsorption configuration and energetics of CO at gold surfaces. For example, Loffreda and Sautet [280] performed DFT based analysis of the CO stretching frequency at a variety of Au(110) surfaces. The atop bonding was found to be the most favorable adsorption configuration on both unreconstructed and reconstructed surfaces. An equilibrium-thermodynamic model based on GGA total energy calculations, together with analysis of the CO vibration, indicated the existence of two competitive types of CO adsorption structures under a variety of conditions. In the case of a reconstructed Au(110)-(1  2) surface, the first structural type includes CO adsorption along the ridges of the missing-row (Fig. 4.9a, occupation of sites A and C). The second type is a coadsorption motif where CO is adsorbed both along the ridges and at gold adatoms found in the otherwise vacant troughs (Fig. 4.9a, occupation of sites Aþ Bþ C). Dipolar coupling between the CO adsorbates along the ridges and on the adatoms produces fascinating, albeit complex, normal modes (See Fig. 4.9b and the associated reference). Henry et al. used ab initio calculations to study the energetics of CO adsorption on flat [(Au(111) and Au(100)], stepped [Au (211)], and kinked [Au(532)] gold surfaces [249]. In their work, the flat Au(111) surface exhibited the lowest adsorption energy (27 kJ mol  1), where the energy difference for CO adsorption on various sites (fcc and hcp hollow, top, and bridge) was less than 1 kJ mol  1. The calculated adsorption energy for the (110) surface was 37 kJ mol  1; the bridge sites were slightly energetically preferable over atop sites with a slight energy difference of 4 kJ mol  1. CO adsorbed at atop and bridge sites on the step edge of the Au(211) surface exhibited an adsorption energy of 52 kJ mol  1, which increased to 66 kJ mol  1 for CO binding at the kink sites on the Au(532) surface. In all cases, the CO adsorption energy was found to depend inversely on the coverage. It is worth noting that the vibrational frequency of COad exhibited only a small drop compared to that of free CO, despite the significant binding energy. Further DFT calculations by Hussain et al. showed that the presence of defects, steps, and kinks on the gold surface affects the activity [281]. The adsorption energy of CO was found to decrease in the following order: Au/Au(100) 4Au(310) 4 Au (110) E Au(100) 4 Au(111). Not surprisingly, the bridge sites were preferable for CO adsorption on all the surfaces except for Au(310), where atop coordination was favored. In all

configurations, the axis of the C–O adsorbate was oriented perpendicular to the plane of the surface. A detailed description of molecular orientation, rehybridization costs, and final binding energetics for CO (as well as other species) can be found in Pettersson et al. [30], which brings together benchmark experimental measurements with theoretical insight. In line with many of the studies highlighted above [273,282], Hrbek et al. demonstrated that CO adsorption on defect-modified Au(111) produces morphological surface changes even at low temperatures [283]. These findings are consistent with a number of experimental and theoretical investigations that show the importance of dynamic active sites in heterogeneous catalysis, which replaces the previouslyheld notions of a static atomic configurations [284–287]. For example, IRAS studies have shown that CO adsorption is very limited on annealed “smooth” Au(111) surfaces [272,278]; however, surfaces patterned by vacancies exhibit a drastic increase in the concentration of adsorbed CO [283]. After adsorption–desorption cycling, readsorption of CO revealed irreversible surface changes, as indicated by a significant (72%) reduction in IR absorbance. STM images, recorded at various stages of adsorption–desorption cycles (see Fig. 4.10), showed that the well-defined hexagonal shape of vacancy islands was transformed into a round shape. More specifically, the straight steps were rounded out following the adsorption– desorption cycling and the islands were found to be decorated by nanosized particles along the upper step edge. When this surface was annealed to 400 K, the nanoparticles were removed, which restored the hexagonal shape of the vacancy islands. Based on these observations, it was concluded that even weakly adsorbed CO can drive morphological changes on the close-packed gold surface at temperatures as low as 273 K and below. As discussed in subsequent sections, this dynamic behavior caused by adsorbate-induced mobility of Au atoms, can play an important role in catalysis on supported gold particles. In this way, the extensive studies performed on well-defined Au surfaces have led the way to a better understanding of the chemistry of nanoparticle systems. 4.2.2. Oxygen Single-crystal gold surfaces cannot activate oxygen and thus do not chemisorb measureable concentrations of oxygen under UHV conditions or at elevated temperatures and pressures

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[186,226,288]. For example, an early report by Sault et al. indicated that O2 does not adsorb dissociatively on Au(110)1– (1  2) at O2 pressures up to 133 kPa and temperatures in the range of 300–500 K [77]. However, in the presence of a hot filament, oxygen adatoms were detected on their gold surface. In that work, recombinative desorption of oxygen atoms to produce O2 showed first order kinetics with a high activation barrier of 132 kJ mol–1 and a pre-exponential factor of 1.3  1012 s  1. Likewise, Kim et al. concluded that the relaxed Au(211) surface does not exhibit reactivity for dioxygen dissociation, even at elevated pressures [277]. Nevertheless, oxygen adatoms were observed to bond more strongly on surface steps than on terraces. In line with the experimental work, DFT has been employed to show that dissociation of O2 on Au is energetically highly unfavorable [227]. Despite the stability of molecular oxygen on Au, researchers have developed methods that produce O-covered single-crystal Au surfaces under UHV conditions. Thermal dissociation of gaseous O2 over hot filaments, exposure to ozone (O3), application of a radiofrequency-generated plasma source, and

active ion sputtering with O2þ have proven successful in generating surfaces covered by oxygen adatoms [186,226,288]. Note, however, that the maximum oxygen coverage obtained is highly dependent on the method of atomic oxygen generation. In addition, the preparation method impacts the surface morphology while defining the distribution of surface oxygen [186]. Similar to the dynamic response of Au surface atoms upon CO adsorption, oxygen adatom binding appears to lead to significant structural changes. For Au(111), Friend and coworkers demonstrated that the deposition of O (or S) lifts the reconstruction [186,234,289]. Specifically, the adsorption results in a removal of gold atoms from the herringbone structure. At high adsorbate coverage, clusters of gold oxide (or sulfide) are observed, which may occur through abstraction of atoms from terrace sites. Concomitant with the reconstruction, the freeing of atoms from the herringbone structure creates a higher density of low-coordinated Au atoms by forming zigzag step edges or islands. These under-coordinated Au atoms most likely play an essential role in the reactivity and catalytic behavior of Au particulates [289]. At 1.0 ML

Fig. 4.10. Room temperature STM images (200 nm  200 nm; 27 nm  27 nm inset) of the vacancy islands modified Au(111) surface before CO adsorption (a) and after CO adsorption–desorption cycle (b). Ball model of the modified Au(111) surface with five adsorption sites labeled (c); plot comparing the reaction energies versus reaction barriers for a climb of an Au atom from five different sites to the upper terrace before and after CO adsorption (d). Adapted with permission from Ref. [283]. Copyright 2008 American Chemical Society.

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oxygen coverage on Au(111), the work function was found to increase by þ 0.80 eV, which was explained by an electron transfer into the oxygen adlayer. Based on the interpretation of results from X-ray photoelectron and vibrational spectroscopic studies, Friend and co-workers identified three distinct types of oxygen: (1) chemisorbed oxygen, (2) surface oxide, and (3) bulk oxide. Importantly, these oxygen motifs play a major role in oxidation catalytic chemistry [186,290]. Further studies of atomic oxygen on Au have been performed by Gibson and Sibener, who described the Oatom-induced deconstruction of 23  √3 Au(111) [291]. Charge depletion from the surface via the electronegative oxygen was invoked as the probable cause for the observed reversion of the surface to the (111) structure. Their work showed the important role that diffusion of O atoms away from the terraces and trapping at steps or defects plays in the overall dynamics of surface restructuring. In related work, Koel and co-workers used ozone decomposition on the Au(111) surface to study the reactivity of oxygen adatoms toward small molecules such as CO, CO2, NO2, H2O, CH3OH, and C2H4 [292]. Ozone adsorption on Au(111) was found to produce electronegative surface oxygen that, as described for O-atom adsorption, increases the work function. Specifically, an energy shift in the O 1s XPS signal was attributed to electron transfer from the substrate to an oxygen adlayer [293]. The same group also studied the adsorption of oxygen on Au(211) by exploring exposure to O2 at high pressure and O3 under UHV conditions [294]. As described above, their work showed that molecular oxygen (O2) can not be activated to dissociate on stepped Au(211), even at high-pressures (700 Torr) and at elevated sample temperatures (300–450 K). Exposure to O3, however, was found to produce oxygen adatoms on the surface at coverages up to θO ¼ 0.90 ML. As probed by TPD of O2, oxygen adatoms were suggested to bind more tightly at step sites than at terrace sites. The estimated activation energy for oxygen desorption from step sites was reported to be 142 kJ mol  1, whereas that from terraces was 138 kJ mol  1 [294]. These values were similar to others reported in the literature, e.g. a coverage-dependent activation energy for desorption between 126 and 134 kJ mol  1 was reported for oxygen from Au(111) [288]. Thus, Au(111) and Au(211) surfaces may not behave differently towards oxygen desorption, despite some contrary conclusions drawn by computational work [266]. Fujitani and co-workers also studied the dissociation of O3 on different single crystal gold surfaces, namely Au(110), Au (111) and Au(311) [295]. For the Au(111) surface, a saturation coverage of 1.1 ML of atomic oxygen was estimated, in accord with other reports [293]. Interestingly, no adsorbed atomic oxygen was registered for O3-exposed Au(100), even at 1500 L dose, contrary to previously reported observations [296]. One key result from this work is the observation that the saturation coverage of atomic oxygen on Au(311) was about half of that observed for the Au(111) surface [295]. Because the fraction of the (111) face exposed on a Au(311) surface is 50% of that exposed on Au(111), these results suggest that O3 dissociates selectively at the (111) face of gold. As alluded to above, computational chemistry has also been applied in the study of adsorption of atomic oxygen on gold.

111

German and Efremenko calculated an activation barrier of 178 kJ mol  1 for O2 dissociative adsorption on Au(111) [297], which is close to experimentally estimated values (134– 142 kJ mol  1) [293]. Shi and Stampfl also studied the adsorption of oxygen on Au(111) [298]. Their DFT work suggested that atomic oxygen is only weakly adsorbed. The most favorable structure they identified involved a thin surface-oxide-like configuration. In this structure, oxygen atoms were quasi three-fold-coordinated to gold atoms, while the gold atoms in the surface layer were two-fold, linearly coordinated to oxygen atoms. Ab initio atomistic thermodynamic calculations identified this configuration as the most stable one that would exist under actual catalytic conditions, e.g. temperatures up to 420 K and atmospheric pressure [298]. In a similar vein, Baker et al. used DFT to study the role of defects, such as vacancies and steps, in the adsorption of oxygen at the Au(111) surface [299]. Their work suggests that an attractive force exists between oxygen atoms and surface vacancies, which, in turn, decreases the barrier for creating additional surface vacancies by 19 kJ mol–1. However, the interaction of oxygen with gold adatoms was found to be repulsive. Oxygen binding was also studied theoretically on other single-crystal gold surfaces, including Au(110) [300], Au(100) [301], Au(211) [302] and Au(321) [303,304]. As with Au (111), the interaction of O2 with Au(110) and Au(100) surfaces was very weak, and no chemisorption was observed. However, DFT calculations by Xu and Mavrikakis concluded that the stepped Au(211) surface can dissociate dioxygen [302]. The dissociation barrier for O2 was found to be lower by 0.6 eV on a stepped surface. As a result of this work, stretched step edges were suggested to serve as active sites for O2 adsorption and activation on Au nanoparticles. Motivated by this work, Gomes and co-workers studied the adsorption of oxygen on stepped Au(321) [303]. Oxygen atoms were found to prefer interaction with surface cavities, while O2 molecules appeared to prefer final configurations with their axis parallel to the terraces at bridge or close to bridge sites. In a follow-up study [304] they found that a single oxygen atom prefers to adsorb at a fcc hollow site on (111) terraces adjacent to a step, while two oxygen atoms co-adsorb on the Au(321) surface, preferring hollow sites nearby the step. Overall, research has shown that, while molecular oxygen does not adsorb at gold surfaces under typical conditions, atomic oxygen does stick to Au(111) and other surfaces at fcc hollow sites. The activation energy for recombinative desorption of oxygen from gold appears to be on the order of
130 kJ mol  1. Finally, as will be discussed further, adsorbed atomic oxygen on gold appears to be highly reactive toward the oxidation of CO and other species, which has strong implications regarding the mechanisms of action for supported gold nanoparticles. 4.2.3. Hydrogen Research has established that molecular hydrogen, under typical catalytic conditions, does not dissociate on single crystal gold. In fact, Hammer and Norskov [29] employed hydrogen as a test molecule in their landmark paper, which

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explained why this molecule does not react on the surface of bulk gold – although it readily dissociates on many other transition metals. Nieuwenhuys and co-workers were unable to detect adsorbed hydrogen on Au(310) under temperatures in the range of 80–500 K and H2 pressures up to 5  10  7 kPa [305]. Sault et al. successfully produced hydrogen adatoms on Au(110)–(1  2) with the aid of a hot filament to thermally decompose H2. Recombinative desorption of H2 was observed in a single TPD peak at 216 K [77]. Stobinski et al. reported the chemisorption of only small amount of deuterium (or hydrogen) on unsintered thin gold films (deposited on a Pyrex glass cell) at 78 K [306,307]. They suggested that chemisorption of hydrogen may occur at low coordinated Au atoms to form AuH2 complexes, similar to the compounds arising from the interaction of H2 with isolated Au atoms [307]. Using DFT calculations, Okamoto showed that the adsorption energy of atomic hydrogen on Au(111) is much weaker than that on Pt(111) [163]. Their work reported a negative adsorption energy, which suggests that H atoms on the Au (111) surface are less stable than a free H2 molecule. Recall from above that the adsorption of atomic H at the top of a Au55 cluster was also endothermic in general, but mildly exothermic for hollow fcc and hcp sites. Barrio et al. also used the DFT approach to study the coadsorption of O2 and H2 or H on Au(111) and Au(100) single-crystal surfaces [301]. As reported above, no significant interaction was found between the flat clean metal surface and O2. However, when pre-dissociated hydrogen was present on the gold surface, the adsorption energy of oxygen was much higher and co-adsorption was found to occur. The origin of this synergistic effect for the coadsorption of hydrogen and O2 was attributed to the formation of an O–H bond that subsequently enhanced the O2Au interaction [301]. Because molecular hydrogen has such a high energetic barrier for dissociation on gold, researchers have turned to other means of activating hydrogen at Au. Mullins and coworkers, for example, have employed a gas-phase source of H atoms to populate a gold surface and then explored chemical reactions of environmental and technical relevance [308–310]. Some of their work is highlighted in Sections 4.2.5 and 4.3. 4.2.4. Water Water has been found to adsorb reversibly on clean Au (111), exhibiting a single desorption peak at
160 K and following zeroth-order desorption kinetics [311,312]. For an extensive range of water coverages, infrared spectroscopic studies have suggested the exclusive presence of molecular water, indicating that water does not adsorb dissociatively to form stable surface hydroxyls. However, substantial (up to 70%) isotopic exchange between surface 16O and water H18 2 O has been observed during TPD, indicating the formation of transient hydroxyl species. The oxygen adatoms were likely critical for the formation of these hydroxyls and water formation, whereas no contribution from a gold oxide could be identified in this work [312]. Using a low-temperature STM, Ikemiya and Gewirth observed the initial stages of H2O adsorption on a clean Au

(111) surface [313]. Their STM images revealed the presence of regions of bare reconstructed Au(111), regions of a planar water film, and regions where water clusters of a few nanometers in diameter were growing in three dimensions. The Au(111) surface remained reconstructed throughout their studies, which implies only weak binding. Not surprisingly, the growth dynamics of amorphous solid water films on Au (111) were found to depend on deposition temperature, annealing temperature, and annealing time [313,314]. Recently, Corem et al. employed helium atom scattering (HAS) to reveal the presence of highly commensurate wellordered islands of H2O on Au(111) in the temperature range of 110–130 K [315]. These islands of H2O were characterized by a well-defined height of
5 Å. On clean Au(110), water has also been found to adsorb reversibly, where two desorption peaks, one at 185 K and another at 190 K, were assigned to multilayers and a monolayer, respectively [316]. The estimated activation energies for desorption were in the range of 46–50 kJ mol  1, assuming a preexponential factor of 1  1013 s  1. Following water desorption, no residual oxygen was detected on the Au(110) surface, indicating that water was adsorbed molecularly without decomposition. When water was adsorbed on highly oxidized Au (110), a new peak in the TPD distributions appeared at 215 K, which was likely associated either with oxygen-stabilized water or the disproportionation of surface hydroxyls [312]. In similar work as that discussed above, desorption of water from Au(310) was characterized by two separate desorption states [317]. The peak in the TPD distributions obtained at the lowest H2O doses was found to shift from 158 K to 172 K, where the overlapping leading edges indicated zeroth-order desorption kinetics. Prior to saturation, a second desorption feature appeared as a shoulder at
165 K, which was attributed to an adsorption state associated with 6-fold coordinated Au atoms. Monte Carlo simulations for the adsorption of water on Au (210) showed that most of the adsorbed water molecules are oriented with their molecular plane parallel to the surface, which maximizes dispersive interactions [318]. Phatak et al. studied the adsorption and dissociation of water on (111) surfaces of several metals, including Au [319]. Their DFT-calculations of activation energies for H OH and O H dissociation showed that both steps are highly endothermic on Au(111) and slightly endothermic on Pt(111) and Pd(111). Gomes et al. also used a DFT approach to investigate water adsorption and dissociation on a zigzag stepped Au(321) surface. In their work, molecules were found to interact directly with the lowest 6-fold coordinated gold atoms, which have one of their hydrogen atoms pointing towards the (111) terraces. The dissociation of H2O occurs via a TS state, as shown in Fig. 4.11. The O–H bond that is being cleaved has a length of 1.86 Å (0.98 Å in the initial state) and the H atom is approaching (and interacting with) the terrace bridge site. The dissociation of water was found to be 1.19 eVendothermic with a barrier of 1.33 eV. Overall, these results suggest that the high activity of some Au catalysts in the water gas shift reaction (WGSR) is not related to the presence of low-coordinated atoms [320].

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4.2.5. Nitrogen oxides The adsorption and interaction of nitrogen oxides with various Au single-crystal surfaces has been investigated experimentally and theoretically. Similar to the results for oxygen adsorption, experiments have shown that NO is unable to absorb on clean Au(111), even at temperatures as low as 95 K [321,322]. From a computational perspective, DFT calculations estimated an adsorption energy of 20 kJ mol  1 for NO adsorption on Au(111) surface [323,324]. Beyond the (111) face of Au, Rienks et al. found that NO can adsorb on Au(100) with an adsorption energy of
57 kJ mol  1 [325]; however, this energy is also insufficient to drive the dissociation of the molecule. Notwithstanding, molecular chemisorption of NO on Au(100) was observed to be sufficiently strong to drive a phase transition from a denser hex reconstructed Au(100) surface to a more open bulk terminated (1  1) surface. Such adsorbate-induced phase transitions were also observed for Pt(100). This transition appears to occur at low NO exposures and in a wide range of temperatures, extending from room temperature to below 100 K [326,327]. On Au(100), however, this process was restricted to a temperature window around 170 K; transition to an ideally terminated phase was not observed after exposure at 150 or 215 K. The reduced temperature ensures a sufficiently high NO coverage, while the upper temperature limit reflects the relatively weak adsorption of NO. These observations suggest that the process of phase transition on Au(100) is more strongly activated than that on Pt(100). On Pt(100), NO can dissociate easily even at room temperature [328]. Other vicinal gold surfaces, however, react differently to adsorption of nitric oxide. Vinod et al. observed dissociative adsorption of NO on Au(310), which is composed of (100) terraces and (110) steps, to form N2Oad and Oad on this surface [305]. The decomposition of NO to N2O was observed at temperatures as low as 80 K. Interestingly, on the same Au (310) surface, no adsorption of O2, H2, CO and N2 was observed over a wide range of temperatures 80–500 K and pressures up to 5  10  5 kPa. The maximum in the TPD distribution for NO was reported to be 120 K, which corresponds to an adsorption energy of
30 kJ mol  1 (Redhead analysis). One of the proposed mechanisms for the observed N2O production involves the formation of an intermediate Nad species. While XPS did not indicate the presence of Nad (binding energy 397 eV), the O 1s spectrum, obtained at 160 K, provided a signature of NO decomposition. The XPS and TPD studies showed that the product N2Oad was stable up to
220 K; however, the same N2Oad species created via pure N2O dosing was found to desorb at much lower temperatures,
120 K. Based on these observations, the authors proposed that NOad decomposes to Nad, which immediately reacts with NO molecules to leave N2Oad and Oad on the surface [305]. The significant structural sensitivity of NO chemistry on Au surfaces was further explored by the recent work of Huang and co-workers [329]. They explored the adsorption and decomposition of NO on Au(997) and Au(110)-(1  2) surfaces, which both expose 7-fold coordinated Au atoms as low-coordinated adsorption sites. On Au(997), α-NO species dominate the adsorption

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Fig. 4.11. Transition state (TS) structure for water dissociation on the Au(321) surface. Reprinted with permission from Ref. [320]. Copyright 2010 Elsevier B.V.

layer, while the Au(110)-(1  2) also exhibits another less stable and more abundant β-NO species. A fraction of the α-NO adsorbate was found to decompose into O adatoms and N2O upon heating and the less stable β-NO species exhibited much higher reactivity. This is in contrast to the results of Vinod et al. [305], who found that (NO)2 dimer species, instead of NOad, is the active surface species for NO decomposition into the O adatom and N2O [329]. Depending on the Au surface structure, various types of (NO)2 dimers were observed. Complementary DFT calculations suggest that two NOad molecules adsorbed at neighboring ridge Au sites (for both Au(997) and Au(110)(1  2)) are capable of (NO)2 dimer formation. The adsorption energy of these dimers was calculated to be 72 and 68 kJ mol  1, respectively [329]. The adsorption of other nitrogen-containing oxides, such as NO2, N2O3 and N2O4, on Au(111) has been studied by Koel and co-workers [321,330] and by other groups [331,332]. The general consensus of these studies is that NO2 adsorbs reversibly on Au(111), exhibiting a multilayer desorption at 150 K and a monolayer desorption peak at 220 K (with an activation energy of
60 kJ mol  1) [321,330]. Vibrational spectroscopies [330,331] and DFT calculations [333] showed that the molecule is bound in an O,O0 -chelating geometry with C2ν symmetry through its two oxygen atoms. In addition, dimerization of NO2 to N2O4 was reported to occur within multilayer films [332]. As already discussed, the presence of chemisorbed atomic oxygen has a dramatic effect on the reactive properties of Au (111) toward many gas phase molecules, including nitrogen oxides. Mullins and coworkers found that NO uptake on Au (111) is facilitated by the presence of oxygen adatoms [322]. They established that at temperatures above 200 K, NO2 production is limited by the residence time of surface-bound NO, whereas at temperatures below 200 K, NO reacts with surface oxygen to form a chemisorbed NO2. Torres et al. used DFT to study the structure and energetics of low-coverage complexes formed upon adsorption of NO on atomic oxygen pre-covered Au(111) [323]. Their calculated

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adsorption energies were very small, i.e. 19 and 39 kJ mol  1 for fcc sites and defect sites, respectively. In another DFTbased study, Yang and co-workers found that clean Au(111) only weakly binds NO, whereas atomic oxygen pre-covered Au(111) is very active to NO [324]. Specifically, on a 0.33 ML oxygen precovered surface, NO was found to react with the chemisorbed oxygen to form NO2. This reaction readily proceeded over a small energy barrier of about 17 kJ mol  1. The calculated desorption energy for NO2 was 62 kJ mol  1. For a surface with 1.0 ML oxygen coverage, NO interacted with surface oxygen without a barrier to form NO2, which readily desorbed. This reaction was proposed to proceed via an Eley–Rideal (ER) mechanism [324]. As discussed in Section 4.2.1, Hussain et al. used DFT calculations to study CO and NO adsorption on Au(111), (100), (110) and (310) surfaces, as well as on Au adatoms on Au(100). Their observed trend for the energy of adsorption of CO also held for NO, i.e. Au/Au(100)4Au(310)4Au(110)EAu(100) 4Au(111). The results clearly showed that the reactivity of Au atoms increases significantly with the degree of coordinative unsaturation. Thus, the strongest interaction for both CO and NO with Au was obtained for step edges or Au adatoms on terraces. These results are in support of the understanding that defects, steps, and kinks also govern the activity of gold catalysts [227]. Both NO and CO molecules prefer to adsorb at bridge sites on low index surfaces, while atop configuration is preferred on the stepped (310) surface [281]. In all configurations, the molecular axes are perpendicular to the surfaces, except for NO on the stepped Au(310) surface [281]. Gomes and co-workers theoretically predicted that co-binding of hydrogen is required for NO dissociation on a zigzag stepped Au(321) surface [334]. DFT calculations with a periodic supercell approach showed that NO likely binds at sites near the steps on this surface. H2 dissociation was found to occur near the steps with a barrier of 54 kJ mol  1, which is significantly smaller than that obtained for the Au(111) surface, 109 kJ mol  1. In contrast, the barrier calculated for NO dissociation on the clean surface was very high (336 kJ mol  1). However, the hydrogenations of NO to NOH and of NOH to NHOH were thermodynamically more favorable with energy barriers of only 52 and 27 kJ mol  1, respectively. These authors concluded that NO dissociation can occur via competing pathways that involve the interaction of an additional hydrogen atom with either the N or O sites of the NOH intermediate [334]. In support of the above theoretical predictions, Mullins and co-workers recently showed experimentally that pre-covering a Au(111) surface with atomic hydrogen dramatically enhances the reactivity of gold towards nitrogen oxides [310]. The experimental results showed that NO2 can be converted to NO on atomic hydrogen pre-covered Au(111), even at cryogenic temperatures (77 K) [310]. The conversion efficiency of NO2 was 100% with selectivity towards NO of 100% at temperatures as low as 120 K. RAIRS measurements and DFT calculations showed that HNO2 and N2O3 were intermediates in this process. It was further shown experimentally that weakly chemisorbed H-adatoms are the key players in hydrogenation reactions at gold surfaces [335].

4.2.6. Hydrocarbons Current interest in the adsorption of hydrocarbons on Au surfaces is inspired by the high activity and selectivity of goldbased catalysts in a wide range of heterogeneous reactions, such as oxidation and dehydrogenation of hydrocarbons and oxygenated hydrocarbons, hydrogenation of unsaturated substrates, etc. As reviewed by Gong, there is a general consensus that hydrocarbons and oxygenates adsorb weakly and molecularly on flat gold surfaces [226] For example, early work by Chesters and Somorjai [336] found that ethylene, cyclohexene, n-heptane, and benzene cannot chemisorb under low pressure conditions on either the Au(111) or on the stepped Au(766) surface. However, naphthalene exhibited dissociative chemisorption on both surfaces. In fact, hydrocarbon fragments were observed as strongly bound species on the gold surfaces, but the activation energy for dissociative chemisorption of lighter hydrocarbon molecules appeared to be high [336]. To overcome the energy barrier to chemisorption, the incident molecules required either increased translational energy or higher surface temperatures [222]. Many of the electronic interactions that control the initial stages of hydrocarbon adsorption have been explored theoretically [30], and researchers are beginning to build a comprehensive understanding for adsorption energetics as well as transition state energetics (two properties that are intimately linked) [251]. An alternative pathway to chemisorption was found to be through a physisorbed precursor-mediated process. Wetterer et al. [222] studied the energetics and kinetics of physisorption for hydrocarbons on Au(111) by helium atom reflectivity measurements. Specifically, the adsorption of n-alkanes, 1-alkenes, and cyclic hydrocarbons on Au(111) surfaces was investigated. For a series of long-chain n-alkanes (from C6H14 to C12H26), the physisorption energy was found to increase linearly with chain length by 6.270.2 kJ mol  1 per methylene unit [222]. This type of adsorption energy increase with methylene units is a general result owing simply to enhanced van der Waals (dispersion) interactions. Generally, hydrocarbons have been observed to physisorb molecularly even on reactive transition metals (Cu(100)) and noble metals (Pt(111), Pt(110), etc.), where similar linear chain length dependencies on adsorption energy were established [222,337,338]. In an effort to reveal the physics behind these dependencies, the physisorption of hydrocarbons on Au(111) was modeled theoretically [222,339]. Wetterer et al. [222] used a bond-additive model while Baxter et al. [339] used a combination of three different approaches to predict the adsorption energy of various hydrocarbons. In both studies, a good agreement with the experimental results was achieved. Outka and Madix [316] have further explored ethylene (C2H4) and acetylene (C2H2) adsorption at Au(110). Their TPD data suggested that acetylene was molecularly adsorbed on the clean surface. These researchers estimated the activation energy for desorption of acetylene to be 42 kJ mol  1. Initially thought to be molecularly adsorbed on the clean surface, acetylene was found to decompose on the Au(110) surface in the presence of oxygen adatoms. The oxidation products were identified as H2O and CO, which desorbed from the surface at 205 K and 525 K, respectively.

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In accord with other measurements, Davis and Goodman [296] demonstrated that the adsorption of propylene on both Au (111) and Au(100) surfaces followed non-reactive pathways. For their propylene-covered surfaces, they observed two desorption peaks – one originating from a multilayer at 120 K and the other from a monolayer, which desorbed at 140 K [for (111)] and 145 K [for (100)]. From this data, the authors estimated a desorption activation energy for propylene from both surfaces of about 39 kJ mol  1. Very small shifts in the vibrational frequencies of the adsorbates relative to their gasphase values suggested only weak interactions between propylene and the gold surfaces. From an oxygen-covered surface, propene was found to desorb at 150 K with a shoulder at 200 K. The species leaving the oxygen-covered surface consisted of propene and its total oxidation products (see Section 4.3.3). It should be mentioned that DFT calculations by Chretien et al. [214] showed only a very weak bonding energy (
7 kJ mol  1) for propene on clean Au(111). However, the difference between the experimentally measured (desorption energy) and theoretically calculated (binding energy) likely originates from the significant role of dispersion forces, which the functionals fail to describe. Beyond simple alkanes, several groups have explored the uptake and desorption energetics of C6-based molecules on Au (111) [340–342]. In agreement with Somorjai et al. [336], Koel et al. found that benzene [342], cyclohexane (c-C6H12) and cyclohexene [343] are reversibly adsorbed on Au(111). The observed peak intensities in their TPD studies occurred at 239, 198 and 213 K, respectively, which were attributed to desorption from monolayer films [342,343]. 4.2.7. Methanol Methanol is a simple, yet extremely practically relevant, organic molecule that has been extensively explored in UHVbased surface science studies and theoretical calculations [344,345]. The presence of three heteroatom bonds in methanol, which can be activated through many different pathways, makes this molecule an excellent model system with which to study the reactivity of transition metal single-crystal surfaces. Methanol appears to adsorb molecularly on clean Au(111) [292,346] and Au(110) [316] at cryogenic temperatures. Koel and co-workers [292] found that, at low coverages on Au(111), CH3OH desorbs during TPD with a maximum desorption rate at 184 Κ. When the exposures were 1 L or larger, desorption from multilayers appeared near 140 K. No products of decomposition were observed in that work and no residual carbon or oxygen was found on the surface following TPD [226,346]. Other TPD studies reported features at
200 K for Au(100) [316] and at
155 K for Au(111) [346]. The activation energy for desorption from Au(100) was estimated to be
50 kJ mol  1, assuming a preexponential factor of 1x1013 s–1 [316]. In a complementary study, the TPD peak at 155 K (β phase) shifted to a lower temperature with increasing methanol coverage on Au(111). This shift was attributed to repulsive interactions between the adsorbates. Based on detailed numerical analysis, desorption energies of 40.6 7 1.6 kJ/mol and 34.8 7 1.4 kJ/mol were estimated for

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methanol coverages of 0.2 and 1.0 ML, respectively [346]. At methanol coverages exceeding 1.0 ML, a low temperature feature at 175 K with zeroth-order behavior was observed for the Au(110) surface, indicative of multilayer adsorption [316]. Two desorption features were also observed for Au(111). At
2.14 ML coverage, a desorption signal at 145 K was attributed to amorphous multilayers (α2 phase), while a TPD feature at 134 K was assigned to crystallized multilayers (α1 phase) adsorbed on top of the α2 phase [346]. As with many other co-adsorbates, methanol was found to react on Au in the presence of oxygen adatoms [226]. Preadsorbed oxygen adatoms on both Au(110) [316] and Au(111) [346] surfaces have been shown to react like a Brønsted base with the methanolic hydroxyl group [316]. On Au(111) [292], surface oxygen increased the adsorption energy of CH3OH significantly. Co-adsorbed oxygen adatoms were found to activate CH3OH oxidative degradation to produce CO2 and H2O. This chemistry on Au(111) is very similar to what others found for Au(110). On oxygen adatom covered Au(110) [316], the first reaction of methanol occurred at 200 K to form H2O and a second reaction started at 250 K, where methanol, methyl formate, water, and traces of hydrogen were observed. Experiments with isotopically labeled CD3OH showed that oxygen adatoms could react with methanol by abstracting the acidic hydroxyl hydrogen [316]. Subsequent reactions of the resulting surface methoxy group resulted in the desorption of completely deuterated products, CD3OD and DCOOCD3 [316]. In agreement with these early studies, Mullins and coworkers found that on clean Au(111), methanol only weakly adsorbs and desorbs molecularly [346]. By using molecular beam reactive scattering, TPD experiments, and theoretical calculations, these researchers mapped the coveragedependent desorption energetics. For methanol coverages of 0.2 and 1.0 ML, desorption energies of
41 kJ mol  1 and 35 kJ mol  1 were estimated. For oxygen atom pre-covered Au(111), TPD of adsorbed methanol showed desorption of oxygenated products including H2O, CO, and CO2; however, no other partial oxidation products were observed. Based on these studies, researchers concluded that abstraction of hydroxyl hydrogen is the first step in the surface decomposition of methanol and that the surface-bound methoxy group (CH3Oad) is the primary intermediate in methanol oxidation chemistry [346]. This study demonstrated that gold surfaces with adsorbed oxygen adatoms can be catalytically active. Indeed, Friend and co-workers showed that by using O3 decomposition, oxygen adatoms render gold active for methanol decomposition/oxidation chemistry [347]. The oxidation of Au(111) by ozone at 200 K resulted in the formation of gold nanoclusters (80% of which had diameters of approximately 2 nm), where oxygen atoms were bound. This highly active surface was found to promote the conversion of methanol to methyl formate, formaldehyde, and formic acid. The esterification reaction occurred at oxygen coverages up to 0.5 ML, but at higher oxygen coverages the overall reactivity of the surface decreased significantly. A study by Vinod et al. provides further insight into how surface structure affects methanol adsorption/decomposition

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chemistry by utilizing the stepped Au(310) surface [348]. In their work, the clean stepped surface was found to be much more reactive than flat surfaces: O–H bond rupture occurred above 150 K to produce adsorbed methoxy groups that were found to be remarkably stable up to 500 K. Most importantly, their results demonstrated that the Au(310) surface is active for methanol decomposition to a methoxy without the aid of adsorbed oxygen. The high activity of Au(310) towards methanol was rationalized by the existence of high energy sites near (110) steps and (100) terraces. The adsorption and dissociation of methanol on gold has also been studied theoretically. Chen and co-workers have used DFT calculations to estimate the adsorption energies of CH3OH, CH3O, and HCHO, as well as identify possible reaction pathways for methanol on Au(111) [349]. The most stable adsorption configuration of methanol was found to occur at an on-top site with an adsorption energy of 55.3 kJ mol  1. This energy decreased to 38.4 kJ mol  1 for the bridge configuration and remained almost the same for bonding at hcp and fcc sites. The methoxy group was found to adsorb preferentially on the bridge and hcp sites in an up-right configuration with adsorption energies of 96.4 and 95.1 kJ mol  1, respectively. From Mulliken charge population analysis, these authors concluded that some charge was transferred from the substrate Au to the methoxy group. Formaldehyde was determined to adsorbed only weakly on Au(111) with an adsorption energy of 23.6 kJ mol  1 [350]. Recently, the selective dehydrogenation/oxidation reactions of methanol with atomic oxygen-covered Au (111) surfaces were systematically studied by DFT approaches [351,352]. The computed barriers for methanol degradation to methoxy (CH3O) and hydroxyl groups (OH) was 40 kJ mol  1, compared with 152 kJ mol  1 calculated for the transfer of H to the clean Au surface [351]. At low oxygen coverage on Au (111), the production of CH2O and CO was found to start from α-H elimination and β-H elimination, respectively [352]. This oxidation pathway was proposed to be controlled by the thermodynamics of the first step rather than by the kinetics, where the overall energy barrier to produce CO was 38 kJ mol  1 relative to gas-phase methanol. At high oxygen coverage, the elimination of α-H and one β-H was suggested to occur simultaneously to form CH2O via a cooperative interaction with two nearby oxygen adatoms, as illustrated in Fig. 4.12. Fast interaction of CH2O with an oxygen atom and hydroxyl to produce a CH2OO(H) intermediate, which could be further oxidized to formate (CHO2ad), was evoked to explain the absence of CH2O(g) production [352]. 4.3. Key oxidative reactions As highlighted in publications throughout this field, identification of the active site of supported Au nanoparticles and the chemical nature of the active oxygen species remain elusive, despite extensive research. Systematic studies on model surfaces, performed under well-controlled conditions, are typically motivated by the importance of finding solutions to this problem. For most supported gold nanoparticles, a small

number of low-index facets, such as (111), predominate on the particle surface. Therefore, a molecular-level understanding of oxidative chemistry on highly characterized single crystals could provide mechanistic insight into the nature of Au-based catalysts, which may eventually lead to improvements. Recently, several excellent reviews have been published that focus on this topic [186,225,227,288]. Following the above discussion on the adsorption configuration and energetics of oxygen and other small molecules on a variety of gold single crystal surfaces, the current review expands into providing some information on the mechanistic details for key oxidative catalytic reactions. 4.3.1. CO oxidation Outka and Madix were the first to publish a fairly comprehensive study of the oxidation of CO on single crystal gold [353]. Atomic oxygen pre-covered Au(110) (at 0.25 ML initial coverage) was found to oxidize CO to CO2 at temperatures as low as 273 K. An apparent activation energy of 8 7 4 kJ mol  1 was estimated for the range 273–440 K. While studying CO oxidation on Au(110)-(1  2), Gottfried and Christmann found that at low O-coverage, the reaction was first-order in θO with an apparent activation energy of  1.8 kJ mol–1 [354]. Assuming a Langmuir–Hinshelwood type of reaction mechanism, a true activation energy of 57 kJ mol  1 was reported. An increase in the initial reaction rate was observed between 60 and 180 K, followed by a decrease at up to 400 K; thus, a maximum initial reaction rate was identified at 180 K. Koel and co-workers found that the CO oxidation reaction occurs rapidly on Au(111) at room temperature and below [292]. At 250 Κ with θO E1 ML, a turn-over frequency (TOF; molecules CO2 formed per Au atom) of
2.5  10  3 was obtained immediately after reaction initiation. After 800 s of reaction time, this value decreased by an order of magnitude under constant CO pressure. The CO oxidation rate was approximately first order in CO pressure and in oxygen coverage. Interestingly, Friend and co-workers obtained a similar dependence of CO oxidation rate on temperature for Au(111) as that reported for Au(110) [290]. In that work, the highest initial rate of reaction was obtained at
200 K for Au(111) oxidized at 200 K by ozone. As shown in Fig. 4.13, the initial rate of CO2 production at 200 K was found to strongly depend on the thermal conditions under which the oxygen ad-layer was prepared. Furthermore, the rate of CO2 production was found to depend on oxygen coverage, but to different extents, for the surfaces prepared under the two temperature regimes. While Fig. 4.13 shows that the initial rate of CO2 production on an O-covered surface prepared at 200 K passes through a maximum at 0.5 ML of oxygen, the rate was nearly independent of oxygen coverage – providing that the oxygen layer was prepared at 400 K. Importantly, the rate of CO oxidation obtained in the first case was almost three times higher than that achieved in the second, even though the temperature of rate measurements was the same, 200 K. These differences in the catalytic behavior of the two Au surfaces were attributed to the different structures of the oxygen overlayers. The chemisorbed

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Fig. 4.12. Transformation of CH3OH(a) to CH2O(a) on high oxygen coverage surface: CH3OH(a)þ 3O(a)-CH2O(a)þ 2OH(a)þO(a). The key bond lengths (angstroms) are indicated by black arrows and labeled with black numbers. The red numbers with and without parentheses show the energy barriers (electron volts) of the elementary steps with and without zero-point energy corrections. Reprinted with permission from Ref. [352]. Copyright 2014 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

oxygen obtained by deposition at 200 K appears much more active than the large islands of surface oxide (with a wellordered 2D structure) formed when oxygen is deposited at 400 K. Therefore, it was concluded that chemisorbed metastable oxygen plays a key role in the CO oxidation chemistry. The idea that metastable Au–O phases enhance the reactivity of gold surfaces and control the rates of catalytic oxidative reactions likely applies to many systems, including supported nanoparticulate Au [290]. By definition, the free energy of metastable phases is higher than that of stable phases; thus, reactions involving such phases are energetically more favorable than their lower energy counterparts [290]. One should also bear in mind that the coverage of oxygen or other adsorbates may affect the electronic structure (width and position of the d band) on the metal, thereby affecting the binding energy and subsequent reaction barriers for catalysis [265]. Mullins and co-workers explored the role of oxygen overlayer structure in the catalytic oxidation of CO on Au(111) [288,355,356]. By employing molecular beam scattering techniques, the oxidation of CO was studied under ultrahigh vacuum (UHV) conditions. Interestingly, the initial adsorption probability for an impinging CO molecule at 77 K was found to not vary significantly with oxygen coverage (above the limit of zero coverage). For all oxygen coverages, prompt CO2 generation was detected, followed by a steep decrease in the rate of CO2 production due to CO accumulation on the surface at 77 K [355]. On the topic of low-temperature CO oxidation, carbonate formation and decomposition (CO32CO2 þ Oad) is an expected reaction that is related to the intrinsic activity of metal surfaces. For example, surface-bound carbonate is formed efficiently when oxygen pre-covered Ag(110) is exposed to CO2 at 300 K [357–359]. 16O/Au(111) surfaces and gaseous C18O2 have been used to show the importance of carbonate formation and decomposition for two oxygen coverages (0.5 and 1.0 ML) [356]. Under both O-coverages, an Arrhenius plot of the reaction probability showed similar apparent activation energies of Ea ¼  147 8 kJ mol  1, which suggests the existence of competing pathways: carbonate formation versus CO2 desorption. These authors concluded that the barrier for CO3 formation on Au(111) is significant and CO3 is bound much more strongly on Au than on Ag surfaces. When the order of deposition of CO2 and O was changed in that work, however, different reaction characteristics were identified. Exposure of a C18O2 pre-adsorbed Au

Fig. 4.13. Initial rate of CO2 production as a function of oxygen coverage deposited at 200 (filled circles) and 400 K (open circles). Reprinted with permission from Ref. [290]. Copyright 2006 American Chemical Society.

(111) surface to a beam of 16O atoms led to a significant increase in the amount of carbonate formed (by a factor of
4), as compared with exposing C18O2 to the 16Oad precovered Au(111) surface. This result clearly indicates that the state of the atomic oxygen adsorbate (i.e., excited or ground state) is of significant importance in carbonate formation reactions [288,356]. These results provide important information that oxidative reactions on gold surfaces could follow a hot-precursor-mediated mechanism. In addition, this group showed that the presence of water can significantly (by 70%) increase the oxidation of impinging CO molecules on oxygen atom-pre-covered Au(111) [311,360]. These results were complemented by DFT calculations, which suggested that adsorbed water decreases the energetic barrier for CO oxidation [288]. Beyond the simple (111) surface, researchers have probed CO oxidation at stepped surfaces that may be more representative of the sites present on nanoparticulate catalysts. However, a study of oxidation on Au(211) suggested that this surface provides little enhancement of the CO oxidation probability [277]. This study [277] concluded that the types of facets on Au(211) stepped surfaces (which are likely present on Au nanoparticles) do not account for the remarkable catalytic activity of supported gold catalysts. Theoretical studies of the oxidation of CO on gold singlecrystal surfaces have suggested that the rate-limiting step is the dissociation of O2. However, the activation energy for dissociative uptake of O2 on Au exceeds the small enthalpy of

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adsorption. Notwithstanding, DFT calculations on stepped surfaces by Liu et al. [Au(221)] [361] and by Fajin et al. [Au(321)] [362] suggest that molecularly adsorbed oxygen may actually be active, albeit with a higher barrier, for CO oxidation. Probable intermediates could be atomic or molecular oxygen, but other oxygen-containing species, such as peroxides or carbonates, were proposed as well [302,363]. Friend and co-workers used dynamical ab initio molecular dynamics simulations to study the role of Au atom release (see Section 4.2.2) in oxidation of CO on oxygen covered Au(111) [364]. Adsorption in the absence of metal atom release was found to be energetically preferable at low oxygen coverages, while gold incorporation was favored at higher oxygen coverages. This work suggests that chemisorbed oxygen in three-fold sites is much more reactive than gold-containing structures (surface oxides and sub-surface oxygen). For example, at fractional oxygen coverages of 0.22 ML, 80% of the surface was initially composed of chemisorbed oxygen, while the remainder of the surface was occupied by the oxide. However, the vast majority (
86%) of oxygen responsible for CO oxidation was identified as the chemisorbed oxygen. At 0.33 ML of oxygen, again, 83% of the reactive atoms were chemisorbed oxygen, though the surface contained only 60% chemisorbed oxygen. This finding indicates that the chemisorbed oxygen was the most reactive type of active oxygen species on the surface. These results suggest that a high activity of gold may be achieved at low temperatures and low coverages, consistent with experimental findings [364]. In another theoretical study, Hussain et al. [365] searched for particular structures of extended surfaces that have high reactivity towards either O2 dissociation, after which CO þ O is energetically favorable, or to CO2 formation via CO þ O2 reactions. Hence, they first assessed the adsorption energies of the reactants, as shown in Fig. 4.14. Fig. 4.14 clearly shows the not-unexpected trend that stronger binding occurs for CO when the coordination number of the adsorbing gold site decreases. Thus, the strongest binding ( 0.88 eV) is realized at four-fold coordinated gold adatoms on the Au(100) surface. On the stepped Au(310) surface, CO(a) also exhibited substantial bond strength, between  0.73 and  0.79 eV. As expected, the characteristics of oxygen adsorption are more critical than those of CO in predicting subsequent reactivity. Irrespective of whether CO oxidation occurs with dissociated or molecular oxygen, the O2 molecule must bind sufficiently strongly to the surface, such that the activation energy for the subsequent step in the reaction does not exceed the adsorption energy; otherwise O2 will desorb prior to reaction. As suggested by the results provided in Fig. 4.14, only the diatomic row and the Au38 cluster form sufficiently strong bonds with O2. The strong bonding is related to the surface structures, which expose groups of four Au atoms of relatively low coordination in a square array. On the diatomic row, each Au atom of the square ensemble has a coordination number of seven (N ¼ 7). O2 binds at such structural motifs in a “hollow-four center bridge” configuration with its molecular axis parallel to the surface; subsequently, the site is specifically configured to host the two O-atoms upon O2 dissociation.

The results of the work highlighted in Fig. 4.14 demonstrated that sites consisting of low coordinated gold atoms in a square-like geometry could initiate the dissociation of oxygen [365]. Specifically, oxidation of CO via O2 dissociation seems to be possible on the Au38 cluster, according to calculations, but this reaction appears less feasible on the extended diatomic rows of Au on Au (100). The reaction mechanism via direct interaction of CO with molecular O2 was also considered in this work. Although CO oxidation may be energetically feasible in this case, the low barrier for O2 surface diffusion may encourage the molecule to move away, instead of overcoming the activation barrier to form an OCOO complex and to produce CO2 [365]. Much of the preceding description is consistent with the models described by Hammer and others that provide further insight into electronic structure effects on reactivity for this system [251]. 4.3.2. Methanol oxidation Another key oxidative catalytic reaction that has attracted significant attention is the selective oxidation of alcohols on single crystal gold surfaces. The selective conversion of alcohols into aldehydes or ketones is of a significant practical interest, e.g. in the synthesis of fine chemicals [366] and the development of fuel cells [367]. As noted in Section 4.2.6, Madix and co-workers demonstrated that the Brønsted-base behavior of oxygen adatoms on gold can induce acidic hydrogen removal from different adsorbates [316,368]. Therefore, atomic oxygen pre-covered Au single crystals have been used as models for uncovering the details of alcohol surface chemistry [288]. Alcohol oxidation on gold generally exhibits similar characteristics as on other coinage metals, but the unique reactivity of alkoxy-containing intermediates leads to different final products [288,369]. On oxygen pre-covered Au(110), Outka and Madix found that CH3OH reacts via abstraction of the hydroxyl hydrogen to form surface-bound CH3Oad and OHad [316]. Methoxy decomposes to form surface formaldehyde. The researchers showed that surface-bound formaldehyde appears to react with methoxy or oxygen adatoms/hydroxyls, leading to the formation of methyl formate or formate groups. The surface-bound methyl formate desorbs at a surface temperature of
270 K; however, the more stable formate remains intact until
340 K at which point it decomposes to CO2. Methanol exhibits similar behavior on O-Au(111) as far as the surface methoxy intermediate is concerned [346,347]. However, with the O-Au(111) surface, no formation of methyl formate from adsorbed formaldehyde and methoxy was observed. Therefore, the surface chemistry of methanol strongly suggests that gold surfaces exhibit a significant structural sensitivity towards oxidative reactions. As a further example, Vinod et al. [348] found that O–H bond scission in CH3OH can occur on clean Au(310) above 150 K without the necessity of reactive oxygen, as with Au(110) [316] and Au(111) [346,347]. This difference in reactivity was attributed to the presence of unique sites at (110) steps and (100) terraces on the Au(310) surface [348]. Interestingly, Friend and co-workers [347] reported that an O-Au(111) surface, prepared by exposure to ozone at 200 K, can

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Fig. 4.14. Overview of adsorptions and reactions relevant for CO oxidation on different gold surfaces. Reprinted with permission from Ref. [365]. Copyright 2011 Springer Science+Business Media, LLC.

promote the conversion of methanol to methyl formate, formaldehyde, and formic acid. The esterification reaction to methyl formate was found to occur at 220–280 K. At low oxygen coverages (below 0.5 ML), methyl formate and formaldehyde products were observed to desorb at 220 and 250 K, respectively. Formic acid, CO2, and H2O evolved concomitantly at 280 K, indicating that these products are formed from the same intermediate via the same rate-limiting step. Formate was identified as the reactive intermediate leading to the formation of both formic acid and CO2. The results of this work showed that the oxygen coverage, the bonding environment of oxygen, and water desorption play essential roles in gold-mediated oxidative reactions of methanol. To further evaluate the importance of these factors in methanol thermal chemistry, the selective coupling of methanol on Au(111) was studied theoretically by the same group [351]. DFT calculations showed that the barrier for formation of methoxy (CH3Oad) and OHad is only
40 kJ mol  1 when oxygen is present on the Au(111) surface. Of course, the barrier for H transfer to a clean Au surface was found to be much higher, 152 kJ mol  1. Several mechanistic pathways were considered for the β-H elimination from CH3Oad. Specifically, attack by adsorbed O, OH, or a second CH3O was explored with the latter exhibiting the lowest energetic barrier (47 kJ mol  1). That reaction leads to formation of surface formaldehyde (H2C¼ Oad) and OHad. Subsequent coupling of methoxy and formaldehyde was found to be without barriers. This result was consistent with the experimental conclusion [347] that β-H elimination is the rate-limiting step for the overall reaction. With the exception of oxygen, the diffusion barrier for most of the surface species was found to be relatively low, which generally favors coupling reactions [351]. Recent theoretical work by Wang et al. [352] complemented the Friend work described above. These studies provided further energetic details about the dehydrogenation and oxidation of methanol on atomic oxygen-covered or OH-covered Au

(111) surfaces. On O-Au(111) with low oxygen coverage, CH2O and CO production starts from α-H elimination and β-H elimination, respectively, as reported by others [351]. At high oxygen coverage, the dehydrogenation of methanol to CH2Oad on O-Au(111) has been shown to proceed in one-step via concomitant α-H and one β-H eliminations. As shown in Fig. 4.15, the energy barrier calculated for this reaction is 0.47 eV. Within this model, the CH2Oad is hydrogen bonded to the new surface hydroxyls with an adsorption energy of 0.40 eV. The CH2Oad and the hydroxyl group then react to form H2COOH with a 0.06 eV barrier and a  0.54 eV energy release. Further dehydrogenation of CH2OOHad can proceed via α-H elimination by surface atomic oxygen or hydroxyl moieties with an energy barrier of 0.21 or 0.19 eV, respectively. The relatively low energy barriers are due to formation of a CH2O2ad intermediate. Once CH2O2ad is formed on the surface, it is readily dehydrogenated to CHO2ad with or without involvement of atomic oxygen or hydroxyls. The last step in the formation of CO2 is the dehydrogenation of CHO2ad. The energy barriers for this step are 0.95, 1.16, and 1.01 eV when assisted by clean Au(111), surface atomic oxygen, or hydroxyl groups, respectively. These results suggest that the dehydrogenation reaction may proceed on clean Au (111) surfaces once the initial barrier is surmounted. 4.3.3. Propene oxidation Propene oxidation on gold single-crystal surfaces is another key reaction that has been studied by surface science methods and theoretical approaches with the goal of developing an understanding for how gold may catalyze the selective oxidation of olefins. Gold-based supported catalysts have attracted significant interest due to their potential utility for epoxidation of terminal alkenes, e.g. ethene, propene and styrene [205,296,341]. In their work discussed above (see Section 4.2.6), Davis and Goodman [296] found that the propene–oxygen interaction on

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oxygen precovered Au(111) and Au(100) surfaces is likely very weak. At high oxygen coverages, however, propene appears to react with oxygen to produce CO2, CO, and H2O. Appearance of masses 56 and 58 during the desorption experiments indicated that partial oxidation of propene over gold can occur at bulk gold surfaces, albeit in very small quantities. Therefore, it was concluded that the facile propene epoxidation observed on Au nanoparticles is a manifestation of the unique properties of nanosized Au catalysts. Interestingly, in a similar study, Friend and co-workers [370] found that the reactivity of propene on oxygen pre-covered gold surfaces strongly depends on the method of oxygen deposition (as observed for methanol oxidation). Partial oxidation of propene was promoted when atomic oxygen (0.3 ML) was deposed on Au(111) via O3 decomposition at 200 K. That surface was also shown to be highly reactive towards CO and methanol oxidation (see above discussion). The ozone-prepared O-Au(111) surface was able to convert propene to several partial oxidation products, including acrolein, acrylic acid, and carbon suboxide (O¼ C¼ C¼ C¼ O), in competition with combustion to CO2 and H2O. Acrolein was the primary product of partial oxidation, proposed to derive from an allyloxy intermediate that can form from oxygen insertion into the allylic C–H bond. The labile allylic hydrogen in propene was suggested to be responsible for the absence of propene epoxide. Interestingly, the epoxide could be formed if the allylic hydrogen activation process was suppressed, as in experiments that explored the deuterium kinetic isotope effect on this chemistry. These results revealed that small changes in the energetics of C–H activation can dramatically affect selectivity. Therefore, modifying the properties of oxygen on Au may be an effective strategy for obtaining desirable epoxidation products. That is, oxygen on small gold particles supported on an active oxide material may exhibit advantageous characteristics for selective oxidation. Metiu and co-workers [214] have explored the key factors that may produce more active gold-based catalysts. Some results from their work are discussed in Section 3.3.4, which describes DFT studies of the interactions of propene with small gas-phase Aun (n¼ 1–5, 8) clusters and with Au(111) and Au/ Au(111) model surfaces. Their work established that adsorption of O2 on Aun is governed by the transfer of electrons from the cluster to the 2π* MO of the O2 molecule. In this way,

Fig. 4.15. Schematic diagram for methanol oxidation to CO2 on O–Au (111) surface at high oxygen coverage. Reprinted with permission from Ref. [352]. Copyright 2014 American Chemical Society.

binding of O2 to the cluster renders the cluster electron deficient, which increases the cluster's interaction energy and oxidation potential toward propene. This result also leads to the prediction that the binding of propene to AunO2 is stronger than propene binding to bare Aun. As highlighted throughout this review, this cooperative effect plays a major role in the oxidative chemistry of Au-based catalysts, as has been observed experimentally for the case of CO and O2 simultaneous adsorption on anionic Au clusters [92,176]. This effect will be revisited below, where the discussion will turn to real nanoparticulate catalysts. 5. Gold structures on ordered titania supports: planar model catalysts The discussions in Section 3 highlighted several studies that clearly demonstrated the potentially high chemical activity of small gold clusters. Extending these observations to the creation of a practical catalyst requires deposition of the particles onto a high surface area support, usually a metal oxide, where the clusters may be stabilized against sintering and expose a high fraction of reaction sites to the reactant feedstock. In addition, many supports play an important and active role as a co-catalyst by providing binding sites for preconcentrating reactants, altering the electronic structure of the metal in ways that are advantageous for catalysis, and providing chemically unique interfacial sites that may activate oxygen and other reactants. Not surprisingly, researchers recognized very early that the size and structure of the metal particles are critically important parameters in determining activity (as is the case for free unsupported gas-phase clusters) and that the electronic, structural, morphological, and chemical nature of the support may be equally important [371]. In the case of high surface area supported gold catalysts, Haruta and co-workers discovered that the method of preparation can be extremely important [371]. However, the complexity of such “real word” high surface area oxide supported gold catalysts makes the study of their atomic-scale properties challenging. In many studies, difficulties arise from uncertainties in the gold particle size and morphology, the poorly defined and controlled structure of high surface area supports, and the possible accumulation of contaminants during experiments. These difficulties obscure the development of a fundamental description of the nature of the catalyst and the catalytic process [76]. Therefore, as the case with many practically important chemical processes, scientists have turned to model systems that lend themselves to systematic fundamental studies of how structure and functionality affect performance. Fig. 5.1 presents a schematic model of the type of planar oxide-supported gold catalysts that can be used to investigate critical properties, including the electronic structure, physical structure, and metal–support interactions of highly dispersed gold supported on oxide surfaces. Such model catalysts can be reproducibly prepared in a controllable manner by using UHV deposition of metals onto various oxide thin films supported on refractory metal single crystals [48,76,226,371,271]. These model catalysts can serve

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as a complementary intermediate to bridge the “material gap” between single crystals and “real world” catalysts. The model planar systems are relatively simple and are suitable for study by UHV-based traditional surface science characterization techniques such as Auger electron spectroscopy (AES), temperature programmed desorption (TPD), low-energy electron diffraction (LEED), ion scattering spectroscopy (ISS), Xray and ultraviolet photoelectron spectroscopy (XPS and UPS), high resolution electron energy loss spectroscopy (HREELS), in situ infrared reflection absorption spectroscopy (IRAS), transmission electron microscopy (TEM), UHV-STM and AFM [75], and other newly developed methods [226]. Thus, the application of “model” planar catalyst systems helps to avoid many of the intrinsic complexities of “real” high surface area supported metal catalysts. Additionally, the so-called “pressure gap” problem can also be addressed (to varying degrees) through the employment of combined UHV-high pressure systems. In such experimental setups, the sample is first prepared and characterized in UHV and then transferred to a high pressure reactor cell. Following catalysis, the sample is returned back to the UHV chamber for post reaction analysis. This experimental approach allows special issues, including structural sensitivity and the role of intermediates, promoters, and inhibitors, to be addressed through a direct comparison between the kinetics measured on single crystal model catalysts with those obtained for high surface area supported catalysts [76]. 5.1. The interaction of gold with the oxide support Due to its significant electronegativity (2.31 eV) and ionization potential (9.22 eV, experimental values [1230]), gold is a poor electron donor and thus it interacts only weakly with most oxides. However, those interactions can be critically important for catalysis. Two important roles of the support include (i) serving as a scaffold for the adsorption, nucleation, and growth of gold nanoparticles, and (ii) providing sites for the adsorption and activation of reagents as well as a holding place for reaction products. The interaction of gold particles with an oxide support depends on the nature of the support, preparation method, pretreatment, and reaction environment. The tremendous variability in particle–support properties can be reduced significantly through UHV methods, which enable a systematic approach to building structure–function relationships in catalysis. Importantly, oxide films can be prepared on metal substrates in nearly pristine form via careful annealing methods. Further, the stoichiometry of the oxide surface, and thus the availability of surface defects (that can serve as gold nucleation centers), can be well controlled by deliberate oxidation (molecular O2) and reduction (thermally or by electron and ion bombardment) treatments. An outstanding fundamental issue for Au/TiO2 catalysts is characterizing the oxidation state of the active site. While theoretical calculations and ultrahigh vacuum (UHV) studies suggest that the active charge state of gold is anionic Auδ  (created by charge transfer from the support to the Au) [46,373–375], steady-state experiments have previously suggested that oxidized or cationic Auδ þ is responsible for

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Fig. 5.1. Schematic representation of a model planar oxide-supported gold catalyst. Reprinted with permission from Ref. [48]. Copyright 2008 American Chemical Society.

unexpected instances of gold activity [47,376,377]. Related to this issue is how the oxide support affects other properties of the Au. Some studies suggest that reducible oxides, like TiO2 and Fe2O3, promote higher activity of supported Au than irreducible oxides, such as SiO2 and Al2O3, under similar conditions and for identical size of Au particles [378–380]. Reducible oxides have lower energy metal–oxygen bonds and smaller band gap energies than irreducible oxides, which may affect the electron density of the Au particles on the surface, which may in turn influence the chemistry. Of course, the support may affect the position of the Au d-band relative to the Fermi level, which can significantly affect the energy of adsorption, the activation barriers for dissociation, and the bond energy of final products – all of which are related through Brønsted–Evans–Polanyi descriptions [381]. Comparing low temperature (273 K) CO oxidation activity of oxide-supported Au, Haruta and co-workers [86] found that the turnover frequencies (TOFs) were almost independent of the chemical identity of the metal oxide support. This result was particularly surprising because the naked supports in isolation all exhibited different levels of activity: TiO2 – poor, Fe2O3 – moderate, Co3O4 – high. In contrast to the insensitivity to the support, the TOFs were strongly dependent on the diameter of gold nanoparticles and showed a sharp increase for gold diameters below 4 nm. Analogous results were found for Al2O3, SiO2, and TiO2 supports. These results indicated that CO oxidation on gold nanoparticles is much more strongly dependent on particle size than other noble metals. Notwithstanding, Schubert et al. [380] have demonstrated that the TOF can be independent of the Au particle size in cases were gold is supported on reducible transition metal oxides such as Fe2O3; in this case, oxygen dissociation is not rate limiting. However, this independence applies only to low metal loadings, where the metal particles are well separated and the supply of oxygen does not limit the rate [380]. Additional studies that widen the range of oxide supports revealed another interesting trend [137]: Strongly acidic materials, such as Al2O3–SiO2 and WO3, exhibited poor activity (4 470 K), less acidic SiO2, Al2O3, and TiO2 or slightly basic Fe2O3, Co3O4, and NiO were highly active (
300 K), and strongly alkaline Be(OH)2 and Mg(OH)2 showed the highest activity (
196 K). In their theoretical work, Wang and Hammer [32] considered the importance of this trend, as far as it predicts that Au particles will be more oxidized on supports that are more basic. Thus, for rutile TiO2(110)-supported Au7 nanoclusters, large adhesion

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energies of gold were found only when the cluster becomes cationic with Au þ ions in Au–O or Au–OH bonds. Other studies have emphasized the importance of particle size over the nature of the support. For example, Freund and coworkers [382] found that adsorption of CO on gold nanoparticles deposited on well-ordered alumina and iron oxide films showed a size effect in that small particles adsorb CO more strongly. However, the maximum CO desorption temperature (at
170 K) for a given size (
3 nm) of gold nanoparticles was essentially independent of the support. This study concluded that the support effects observed for real catalytic systems toward CO oxidation likely originate from the characteristics of oxygen within these systems, rather than CO. The structural sensitivity suggested by Haruta et al. [86] has also been observed for model Au/TiO2(110) catalysts in a series of studies by Goodman’s group [88,383,384]. For the model Au/TiO2(110) catalysts, the size of the Au clusters was found to have a significant effect on CO oxidation rate, with a peak in catalytic activity for
3 nm Au clusters. In addition, it was found that the deposition of gold on O2-exposed, rough TiO2 prevents sintering of the Au clusters under CO oxidation conditions [88]. These studies highlighted the importance of characterizing the nature of the metal/oxide bond to understand not only the formation mechanism of the metal particles, but also to reveal the specific property that mediates the catalytic activity. For another model Au catalyst supported on MgO (100), it was shown both experimentally and theoretically that Au8 clusters are active for CO oxidation only if the Au clusters nucleate at oxygen vacancies [90]. Wahlström et al. [385], combining STM study and DFT calculations, showed that bridging oxygen vacancies are the active nucleation sites for Au clusters on the surface of rutile TiO2(110). Research showed that a single oxygen vacancy binds an average of 3 Au atoms. Chrétien and Metiu studied the interaction between small Au7 clusters and stoichiometric [386], partially reduced [387], or partially hydroxylated rutile TiO2(110) surface via DFT. For stoichiometric TiO2(110), analysis of the electronic structure revealed that the main contribution to the binding energy comes from the overlap between the HOMO of Au clusters and the Kohn–Sham orbitals localized on the bridging and the in-plane oxygen of the rutile TiO2(110) surface. The binding energy of all Au clusters to the partially reduced surface was found to be larger by
0.25 eV than the binding energy to a stoichiometric surface. Further, oxygen vacancies on reduced TiO2 influences the binding energy of Aun clusters only when they are in direct contact with the defect. Finally, their models showed that the total charge on clusters that are close to, but do not overlap with, a vacancy site differ little from the charge they have when the cluster is adsorbed on a stoichiometric surface. Bansmann and co-workers [388], studying planar Au/TiO2 model catalysts, showed that the catalytic activity depends a great deal on the conditions under which the underlying support was prepared. Nanoparticles of gold deposited under UHV conditions on a stoichiometric support showed very low catalytic activity, compared to a reduced TiO2(110) support. However, they suggested that surface oxygen vacancies alone

do not completely account for variances in the catalytic activity. The migration of Ti from bulk TiO2 to subsurface regions in the presence of reactive gases was proposed to play an important role for lowering the energetic barriers for the CO oxidation. STM studies by Goodman and co-workers [389] explored the effects of the support on the nucleation, growth, and morphology of gold nano-clusters. Specifically, they employed TiOx/SiO2 thin films grown on Mo(112) to directly compare the resulting morphology of Au clusters on three classes of support: irreducible, less reduced TiOx (x
2), and highly reduced TiOx (x
1). Fig. 5.2 provides their key results, which suggest that nucleation and growth of Au clusters on TiOx is somewhat more prevalent. In addition, Au clusters formed at high densities on the most reduced TiOx. These results indicate that reduced Ti sites are likely important active sites for the nucleation and growth of Au clusters. Chen and Goodman [390] employed STM and ultra-violet photoemission spectroscopy (UPS) to study the role of reduced Ti in the interaction of deposited Au with titania surfaces for two model Au catalysts, rutile TiO2(110) and an ordered TiOx/ Mo(112) thin film. When deposited on defective TiO2(110), Au particles appeared to bind initially at the oxygen vacancy sites. Gold was found to completely wet the oxide on ordered and reduced Ti3 þ Ox/Mo(112) films. Two ordered structures, a (1  1)–monolayer and a (1  3)–Au/TiOx/Mo(112) bi–layer, were atomically resolved by STM in these studies. Laursen and Linic [391] combined density functional calculations and ab initio thermodynamic studies in their work into how the oxidation state of Au nanostructures depends on the nature of the support. The authors found a fundamental difference in the electronic structures of Au on rutile TiO2(110) and SiO2(110). The key difference in these two supports is that the extent of redistribution of electron density at oxygen vacancies depends significantly on the type of oxide. As shown in Fig. 5.3a, the electron density was found to shift to low laying 3d bands of bridge Ti atoms upon reduction of TiO2 (R-TiO2). In the stoichiometric TiO2 (S-TiO2), these orbitals are unoccupied. The charge redistribution provides the bridge Ti atoms with a net gain in charge of  0.4e  and extends them by 1 Å, compared to the Ti–Ti distance in STiO2. For stoichiometric SiO2 (S-SiO2), the electron density created at an oxygen vacancy forms an Si–Si bonding level at  0.85 eV below the Fermi level (Fig. 5.3b). These differences in charge redistribution affect the properties of Au clusters grown on these supports. The accumulated electron density at Ti 3d states in the reduced TiO2 system is found to transfer to Au atoms as Au–Ti bonds are formed. The implied formation of anionic Auδ  upon deposition is consistent with other computational studies [374] and with UHV experiments [373]. An important result from the work highlighted in Fig. 5.3 is that the interaction between Au and reduced SiO2 appears to be weaker than between Au and reduced TiO2; however, highly anionic Au, formed on Au/R–TiO2, binds oxygen relatively strongly. This finding might explain the above discussed apparent paradox, where UHV experiments and DFT calculations indicate formation of negatively charged (anionic) Au,

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while catalytic studies (steady-state conditions) suggest that the Au/oxide activity depends on the presence of cationic Au atoms. Fig. 5.3c shows how the Au electronic state depends on external conditions and support. For conditions in which the chemical potential of oxygen is low, i.e. UHV environment and elevated temperature, anionic Auδ– is formed on both oxides. When the chemical potential of oxygen increases, the interaction between oxygen and Au/oxide is enhanced, which drives oxidation and (cationic) Auδ þ formation [391]. Further theoretical study by Laursen and Linic [392] demonstrated a strong interaction between Au and defects on TiO2(110). Three different Au-oxide model interfaces were considered: (i) Au on stoichiometric TiO2 (Au/S-TiO2), (ii) Au on reduced TiO2 (Au/R-TiO2), characterized by the absence of one bridging oxygen per unit cell, and (iii) Au on oxidized TiO2 (Au/O-TiO2), in the presence of an additional oxygen atom was present. Thus, the three interfaces differed only by the amount of oxygen at the metal–oxide interface. DFT and ab initio thermodynamic calculations showed that these stoichiometric faults are quite thermodynamically stable. Further, they found that Au nanostructures exhibit anionic or cationic electronic character, depending on the nature of the surface defect closest to the particle: an oxygen vacancy or an extra oxygen atom, respectively, see Fig. 5.4. Importantly, these differences in the electronic structure may be correlated to the catalytic activity of supported Au nanoparticles for a broad range of oxygen chemical potentials, as mediated by specific operating conditions. Important work performed by Besenbacher and coworkers [393] focused on unraveling the discrepancies between surface science studies of model systems and studies of high area (nanoparticulate) supported materials. Studies of nanoparticulate supported materials tend to lead toward hypotheses that suggest oxygen vacancies are responsible for the stabilization and activity of Au clusters [385,389,394–396], whereas investigations of model systems report that slightly oxidized gold (Au δ þ ) or metallic gold (Au0) governs the activity of dispersed Au catalysts

Fig. 5.2. A plot of the number density of Au clusters with respect to Au cluster size deposited on SiO2 and TiOx/SiO2 thin films grown on a Mo(112) single crystal. Reprinted with permission from Ref. [389]. Copyright 2006 Elsevier B.V.

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[32,34,225,376,397,398]. Using high-resolution STM, the deposition and nucleation of Au nanoparticles was studied on TiO 2(110) surfaces in varying states of oxidation: (i) reduced (bridge oxygen vacancies), (ii) hydrated (bridge hydroxyl groups), and (iii) oxidized (oxygen adatoms). The STM images of these three TiO 2 surfaces before exposure to gold are shown in Fig. 5.5A–C. The bright rows in STM images of the stoichiometric TiO 2(110) surface correspond to Ti troughs and the spatially higher O br atoms appear dark [71]. The light features that span the bright Ti troughs are assigned to O br vacancies (Fig. 5.5A) on partially reduced TiO2 (110) [r-TiO 2(110)] [399]. By allowing water to dissociate at O br vacancies (Fig. 5.5B), a hydrated TiO 2(110) [h-TiO 2(110)] surface was formed. Capping H adatoms on some of the O br atoms form bridging hydroxyls (OHbr ) that image as brighter spots between the Ti rows [399,400]. The oxidized TiO 2(110) [o-TiO2(110)] surface was prepared by letting O 2 dissociate in Obr vacancies (Fig. 5.5C). The oxidized surface was characterized by defect-free O br rows and many O adatoms (Oot ) along the Ti troughs. Exposing these three different TiO 2(110) surfaces to gold at room temperature, so that
3% of a monolayer (ML) coverage was formed, led to quite different Au cluster morphologies, Fig. 5.5D–I. On the r-TiO2 (110) surface, the numerous Au clusters appear rather small and are distributed homogeneously on the terraces (Fig. 5.5A, D and G). In contrast, on the hTiO 2(110) surface, relatively large Au clusters are deposited along the step edges of the substrate, but no small clusters were detected on the terraces (Fig. 5.5A, E and H). On the o-TiO2 (110) surface, the deposited Au clusters were homogeneously nucleated on the terraces (Fig. 5.5A, F and I). The absence of homogeneously dispersed Au nanoclusters on the h-TiO2(110) surface suggest that Au atoms and smaller nano-scale clusters diffuse rapidly at near room temperature on the hydrated surface. Further, STM images for Au/r-TiO2(110) showed that sintering occurs near 340 K, which is in contrast to the Au/o-TiO2(110) system. With support from DFT calculations, two gold-TiO2(110) binding mechanisms were identified for the reduced and oxidized supports. A pathway involving cationic Au was proposed for the oxidized support [o-TiO2(110)], which produces much stronger Aun–support forces than on the r-TiO2(110) surface, in agreement with previous work [32]. The authors then made the general prediction that cationic gold at the Au–support interface is most likely present within the most active Au/TiO2 catalysts. The results of Besenbacher's work, discussed above [393], for the role of TiO2 hydration on Au cluster deposition were further supported by a recent study by Tong et al. [401]. Using variable-temperature STM and DFT calculations, these authors showed that a Au atom and a water molecule may compete for oxygen vacancies on the reduced TiO2 surface. Water displaces the Au atom to form a stable OH–Au–TiO2 complex on the surface. Deposition of Au1þ onto hydroxylated TiO2(110)(1  1) under so-called “soft–landing conditions” at 600 K leads to decoration of the surface with isolated gold atoms

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Fig. 5.3. (a) Bridge Ti–projected LDOS for S–TiO2 and R–TiO2. The inset shows the Wannier orbital corresponding to Ti 3d state that gains electron density. (b) Bridge Si-projected LDOS for S–SiO2 and R–SiO2. The inset shows the Wannier orbital corresponding to the Si–Si binding state of R–SiO2. (c) The lines depict external pressure and temperature at which the average electronic fingerprint of Au changes from anionic to cationic (oxidized). The large dot represents typical CO-oxidation conditions. Adapted with permission from Ref. [391]. Copyright 2006 American Physical Society.

Fig. 5.4. Difference in the calculated self-consistent electron density of Au/RTiO2, Au/S-TiO2, and Au/O-TiO2, and the independently calculated electron density of the Au nanostructure and the support (R-TiO2, S-TiO2, and O-TiO2). The Bader charge of the Au atoms interacting with the defect is also shown. Au on stoichiometric TiO2 was calculated to be charge neutral. A negative charge means there is a shift of electronic density to the Au atoms, while a positive charge implies a shift in the opposite direction. The white line shows the interface between Au and TiO2 (cutting through the plane of bridging oxygen). Reprinted with permission from Ref. [392]. Copyright 2009 American Chemical Society.

interstitials on TiO2 (110)) that accept excess charges from the reduced Ti3 þ sites act as charge (electron) donor sites for reactants. Thus, the bonding of Au atoms appears to be greatly enhanced at the sub-stoichiometric locations. In contrast, fully stoichiometric TiO2 surfaces provide enhanced Au bonding owing to the singly coordinated oxygen atoms, which serve as strong electron acceptors. As discussed above, the nature of the surface sites depends entirely on the sample preparation conditions, history, and treatment (activation) prior to use. An important conclusion that can be drawn from the studies highlighted in this section is that the amount of background water present during, and even after, preparation of a particular Au/TiO2 catalysis may affect the properties of the material. Along with the sensitivity of final particle size, heterogeneity, and electronic environment to the thermal history of the sample, [226, 403, 404] these observations help account for some of the variability reported in experimental studies of thermal and photocatalysis across research groups (see Section 8-13).

5.2. Nucleation and growth of gold on oxide surfaces bound to oxygen vacancies. In contrast, deposition of Au at 300 K produces large, sintered islands of Au, in agreement with the results reported in Fig. 5.5E, H [393]. The isolated Au atoms change locations from immediately above the bridging oxygen rows to on top of 5c-Ti atoms when the substrate temperature decreases from 600 to 300 K. Upon heating above 600 K, the Au atoms return to directly over the bridging oxygen rows, which further demonstrates the significant mobility of the surface species and the reversibility of this process. Recently, Park et al. [402] demonstrated experimentally (STM) and theoretically (DFT calculations) that partially reduced TiO2 forms at defect sites. Different types of surface defects were observed and related to Ti interstitials on TiO2(110). The nature of gold deposition onto these supports was found to depend significantly on the defect physical and electronic structure. Sub-stoichiometric strands (formed by Ti

Experimental and theoretical studies of model catalysts have improved our understanding of fundamental issues related to the nucleation and growth, structure, sintering, electronic, and catalytic properties of various metallic overlayers on many different oxide surfaces. Numerous excellent review articles have given a general description of the behavior of metals on oxides [372,405,406], including gold supported on TiO2 [407]. More recent reviews by Chen and Goodman [46,408], Freund and co-workers [48,225] Gong [226] and Cosandey [403] have discussed the current progress in the study of nucleation and growth of gold nanoparticles on different planar oxide supports. Along with describing the structure of single crystal surfaces of support oxides, the adsorption, nucleation, and growth of gold nanoparticles on oxide surfaces with different stoichiometry and defects structure have also been discussed. Therefore, the following discussion is limited to only

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Fig. 5.5. STM images of r–(A), h–(B), and o–(C) TiO2(110) surfaces before Au exposure (130  130 Å2). Symbols indicate Obr vacancies (square), H adatoms on Obr sites (hexagon), and Oot atoms in the Ti troughs (circle). Insets (30  30 Å2) show the point defects of interest enlarged. (D to I) STM images after 3% ML Au exposure at RT [1ML is defined as 1.387  1015 atoms per cm2 corresponding to Au(111)]. Image sizes are 130  130 Å2 ((D) to (F)) and 350  350 Å2 ((G) to (I)), respectively. In (D) to (F), the heights of the Au clusters are given by contour lines at 1.2, 3.2, and 5.2 Å above the terrace. Adapted with permission from Ref. [393]. Copyright 2007 AAAS.

highlighting some of the most important trends and conclusions from the field. 5.2.1. Au/TiO2 By a wide margin, the most studied crystal face of titania, especially for Au nucleation and growth investigations, is TiO2(110). The classic review by Diebold [71] describes, very clearly, the surface structure and the chemistry of TiO2. The stoichiometric surface of rutile TiO2(110) (1  1) possesses rows of alternating titanium and oxygen with 50% of the titanium atoms covered by bridging oxygen, as shown in Fig. 5.6. The geometry of reduced 1  1 surfaces, added-row 1  2 and TiO 1  2 reconstructions are also depicted in Fig. 5.6 [404]. Recently, Yu and Trinkle [404] used DFT energy density calculations to determine the equilibrium structure that matches experimental measurements [16] for different Au/

TiO2 interfaces, based on the TiO2(110) surface reconstruction shown in Fig. 5.6. Table 5.1 summarizes data for the work of adhesion Eadh and the Au–Ti separation distance dAu–Ti for the Au(111)//TiO2(110) interfaces considered and compares them to experimentally defined values. The data from Table 5.1 indicate that the TiO reconstruction provides the most stable configuration with interfacial distances of 2.45 Å, and work of adhesion of 29 meV/Å2. This new surface reconstruction is stabilized solely by the presence of the Au overlayer. The variations in calculated energy for each Au atom during the formation of the Au//TiO2 interface demonstrated that the attractive forces present in the top Au layer stabilize the overall structure [404]. Anderson and co-workers [409] studied the agglomeration characteristics of 0.05 ML of Au deposited as Au þ on rutile TiO2(110). Their work combined ion scattering, XPS, CO adsorption, and CO–TPD to develop a fairly comprehensive

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set of experimental data. The thermal stability of both approximately stoichiometric (UHV-annealed) TiO2 and high defect-density TiO2 (oxygen vacancies created by He þ bombardment) were studied over temperatures ranging from 115 K to 800 K. At low temperatures, Au was found to be atomically dispersed, but this changes into complex structures near room temperature and at oxygen vacancies, and finally agglomeration into small clusters occurs at Tanneal Z 450 K. Parker and Campbell [410] employed vapor-deposition of gold on TiO2(110) to demonstrate that only after achieving a critical coverage of gold does the growth produce three dimensional structures. This is a somewhat unexpected result because gold–gold bonds are much more energetically favorable than gold-oxide bonds; therefore, one might expect the growth mode to be a Volmer–Weber type, [411] where 3Dislands are formed from the onset of growth. Instead, the initial growth of Au particles was shown to be kinetically limited to 2D-islands at room temperatures and below. As a complement to these previous studies, Gong et al. [412] systematically investigated the adsorbed Au and Pt monomers, dimers, and trimers at anatase TiO2(101) terraces and two major step edges, as well as near O-vacancies. Theoretical predictions were tested by vapor-deposition of Au and Pt on TiO2(101) at small coverages followed by STM imaging. On clean anatase (101), Au preferably nucleated on step edges to form large clusters, whereas Pt was shown to form monomers on terrace sites with a large binding energy of 2.2 eV. Oxygen vacancies, created by electron irradiation, significantly affected the growth of Au clusters, while the nucleation characteristics of Pt was much less influenced. An analysis of the results from numerous experimental and theoretical studies enabled Gong [226] to summarize trends for the growth of gold on reduced rutile TiO2(110), which show

Fig. 5.6. Geometry for four different TiO2(110) surface structures. Upper two are stoichiometric 1  1 and reduced 1  1; bottom two are added-row 1  2 reconstruction and TiO 1  2 reconstruction. The stoichiometric structure has bridging oxygen (2c)b atoms above the flat titanium (5c)/(6c) and oxygen (3c) plane. Removal of the bridging oxygens produces a reduced surface, with 4- and 5-fold coordinated titanium. The added-row reconstruction removes every other row of Ti (4c) atoms with subsurface bridging oxygens (3c)sub for a 1  2 reconstruction, with 2-fold coordinated oxygen. Finally, additional reduction of the added-row reconstruction, by removing the oxygen (2c) atoms neighboring the removed row, produces the TiO reconstruction with 3-fold coordinated titanium. Reprinted with permission from Ref. [404]. Copyright 2011 American Chemical Society.

the following: (1) at low coverage ( o 0.1 ML), growth tracks a two-dimensional mode. Upon increasing coverage, a threedimensional growth mode takes over; (2) during early growth, Au particles elongate along bridge-bonded oxygen rows; (3) the nucleation of Au particles occurs preferably at the oxygen vacancy sites; and (4) with an increasing number of oxygen vacancies, the Au particle number density increases, while the mean particle size decreases. Chen and Goodman have also studied, experimentally and theoretically, the formation of 1-D, 2-D, and 3-D Au structures with one, two, and three atomic-layers (see Fig. 5.7) [46]. As suggested by the STM images and illustrated by the model structures, Au nanoparticles nucleate at surface defects, developing from 1-D to 2-D, before evolving into larger 3-D structures. The transition from a 2D to 3D growth mode occurs at Au coverages that depend on the defect density on the TiO2 surface. On a pristine surface, ~0.09 ML of Au coverage was required to initiate 3D growth – in comparison to ~0.22 ML of Au for a sputtered surface [413]. The critical Au coverage was shown to decrease with temperature. The tipping-point coverage for 3D growth was shown to be 0.19 ML at 160 K and only 0.09 ML at 300 K. Santra et al. [414] further observed that the density of nucleated particles at the step edges saturates at a higher rate than on the terraces. In addition, the height of particles nucleated at step edges develops at a higher rate than particles located at terraces. During growth on rutile, the Au lattice does not appear to deform beyond the minimal strain required to match the TiO2(110) crystal register [415]. This suggests that the forces between gold and rutile are rather minimal (as predicted theoretically). The highest adhesion energy for the Au(111)// TiO2(110) interface with the strongest metal–support interaction has been observed for the most reduced (1  2) reconstructed TiO2 surface [403,404]. On the surface of the anatase phase, the situation is qualitatively different; a strong epitaxial relationship was observed between Au and anatase due to the small misfit of only 0.9% (compare to 2.9% for the most stable reduced (1  2) rutile in Table 3) between the Au(111) face and the TiO2(112) plane [415]. Recently, Cosandey [403] reviewed results from experimental and theoretical studies on epitaxial orientation relationships, interfacial energy, and atomistic studies of the Au/TiO2 interface for model Au//(110)TiO2 systems. One of the key studies discussed in that work was by Sivaramakrishnan et al. [16] where experimental results for the epitaxial orientation Table 5.1 Comparison of different Au(111)//TiO2(110) interfacesa.

Stoichiometric Reduced Added-row TiO Experiment [16]

11 11 12 12 12

Eadh [meV/Å2]

dAu–Ti [Å]

Misfit (%)

7|4 71 54|53 71 –9|– 971 22|29 71 2877

3.90 2.79 3.00 2.45 2.357 0.16

3.6 3.6 2.9 2.9

a Reprinted with permission from Ref. [404]. Copyright 2011 American Chemical Society.

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relationship Au(111)–10] [1 ||TiO2(110)[001] were presented. The triple line energy (TLE) between gold, rutile titanium dioxide, and air was also measured for Au clusters grown on reduced rutile TiO2(110) (followed by annealing in air at 975 K). The strong size–shape–activity relationship for Au/TiO2 catalysts, highlighted by many researchers over the years (see subsequent sections), has been attributed to the unique properties of Au nanoparticles that are acquired during growth on oxide supports [384]. In general, three key phenomenon affect reactivity: (1) the existence of a unique Au–TiO2 interfacial structure where the Au clusters are strongly bonded to the TiO2 surface, which affects the Au–Au spacing and electronic structure, (2) the presence of special sites at the perimeter of the Au/TiO2 interface that may activate O2 and other molecules, and (3) the existence of a high density of steps, edges, and corners that contain lowcoordinated Au atoms that may activate reagents [403]. These unique properties of supported Au nanoparticles are discussed further in Sections 7 and 8. Here, the dynamics of 3-D particle growth on model TiO2 are briefly described. For a metal cluster with isotropic surface energy, the cluster assumes aspherical shape [403]. The contact angle given by the Young–Dupre relationship is: cos (θ) ¼ (γo  γmo)/γm, where θ, γo, γm, and γmo are the contact angle, oxide surface energy, metal surface energy, and metal–oxide interfacial energy, respectively, as shown in Fig. 5.8a. For a metal cluster with anisotropic surface energy, the clusters tend to form facets with equilibrium facet lengths given by the Wulff construction

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[416,1231]. In this case, the contact angle is representative of the relative facet angle with respect to the substrate, as shown in Fig. 5.8b, [1231]. The cross-section of the cluster appears as if the cluster “sinks” into the substrate by an amount Δh, which depends on the metal cluster surface energy γm and adhesion energy Eadh defined as Δh/h ¼ Eadh/γm with Eadh ¼ γo þ γm–γmo. These relationships, however, do not take into account the energy contribution of the triple junction line energy. This assumption becomes relevant for very small clusters that are in the range of a few nanometers. By taking into account the interfacial triple line energy (τInt) for a supported cluster, Yu and Trinkle [404] modified the Wulff–Kaishew principle to


 ðΔh hÞ=h ¼ γ TiO2 –γ Int =γ Au – τInt =γ Au  dlInt =dAInt where AInt and lInt are the interfacial area and interfacial perimeter, respectively. The “geometric factor” of dlInt/dAInt depends on the shape of the interface as illustrated in Fig. 5.9. Neglecting the effects of change in strain relaxation and surface energy with nanocrystal size, the lower limit of triple line energy (Fig. 5.9f) for Au nanocrystals supported on TiO2(110) was estimated to be 0.287 0.08 eV/Å (4.53 7 1.27  10  10 N) by using γAu(111) ¼ 1.283 J m  2 and γTiO2(110) ¼ 0.33 J m  2. If strain contributes significantly to shape change by H/w induced change, then the actual value of TLE must be larger than the measured one to overcompensate for the hypothetical strain-induced decrease in Δh/h for larger nanocrystals. Similarly, an increase in surface energy for

Fig. 5.7. Schematic structural models for 1-D, 2-D and 3-D structures with two-atomic and three-atomic-layers thick Au particles on the TiO2(110). STM images of Au/TiO2(110)-(1  1) with a Au coverage of o0.05 ML and 0.25 ML (1 ML: one monolayer corresponds to one Au atom per surface five-coordinate-Ti4 þ ) deposited on a TiO2(110) single crystalline surface at 300 K followed by an anneal at 850 K for 2 min. Reprinted with permission from Ref. [46]. Copyright 2008 The Royal Society of Chemistry.

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smaller nanocrystals should result in an increase in Δh/h for smaller nanocrystals [16]. 5.2.2. Au/CeO2 Under ambient conditions, bulk CeO2 crystallizes in the cubic fluorite structure, with the Fm3m space group, consisting of a cubic close-packed array of metal atoms with all tetrahedral holes filled by oxygen [417]. CeO2 belongs to socalled reducible oxides where metal cations can change the oxidation state depending on ambient conditions. Reduction of CeO2 at elevated temperatures leads to formation of a continuum of oxygen-deficient, nonstoichiometric CeO2-x oxides, whereas reduction at lower temperatures (T o 723 K) results in formation of a series of discrete compositions. CeO2 remains in its fluorite crystal structure even after loss of considerable amounts of oxygen from its lattice and formation of a large number of oxygen vacancies. The suboxides can be readily reoxidized to CeO2 by exposure to an oxidizing environment [417]. Thin films of ceria on metal substrates exhibit excellent electrical and thermal conductivity and thus are suitable for employment by surface-sensitive analytical tools [418–421], particularly to form model planar CeO2supported Au catalysts [422]. Clean CeO2(111) films basically expose flat terraces, mostly circular in shape, that may consist of several domains. The steps at terraces present a variety of low-coordinated atoms located on edges, which may provide centers for nucleation of gold particles. Freund and co-workers reported that gold interacts more strongly with the step edges

Fig. 5.8. Schematic representation of equilibrium cluster morphology for (a) a cluster with isotropic surface energy; (b) a cluster with anisotropic surface energies and equilibrium faceted shape given by the Wulf construction. The cluster is truncated at the interface by an amount, Δh, which is dependent on the interfacial energy. Adapted with permission from Ref. [1231]. Copyright 2010 The American Physical Society (APS).

than with flat terrace regions [418]. At low Au coverage, only a small number of Au particles were detected on the terraces, which were likely associated with defects sites [422]. At high gold coverage, particles nucleate at the step edges and only after saturation of these sites do the particles grow randomly over the entire surface [422]. A similar observation has been reported by Weststrate et al. [423] for both oxidized and reduced CeO2(111) films. In agreement with experimental work, Castellani et al. [424] predicted theoretically that the majority of Au particles nucleate on the step edges [421] and that eventual sintering of Au particles also proceeds mainly along the step edged of the CeO2(111) support [422]. XPS has been employed by Rodriguez et al. [425,426] to show that, at 300 K, gold nucleates on oxygen vacancies to form 3D islands at CeO2(111). Akita et al. [427–429] employed high-resolution TEM to show Ostwald ripening through surface diffusion of Au atoms to form 3D islands of Au particles on polycrystalline CeO2, see Fig. 5.10. The small Au particle indicated by an arrow in Fig. 5.10a disappears at 870 K, as shown in Fig. 5.10b. Small particles under 5 nm in diameter almost disappear at 910 K (Fig. 5.10c), and only large Au particles remain. The large hemisphere-like Au nanoparticles have a top facet with a hexagonal shape, suggesting a (111) surface, and side facet of (100) character. Successive HRTEM images created during heating at 1000 K showed that on the CeO2(111), a small Au particle remained intact with a clear epitaxial relationship of CeO2(111)//Au(111) [427]. In this work, high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) images of the Au/CeO2 interfaces with the orientation relationship of (111)[1 10]Au//(111)[1 10]CeO2, (111)[ 110]Au//(111)[1 10]CeO2 and atomic columns of Au and Ce were successfully resolved for the first time [428]. 5.2.3. Au/MgO MgO, which exhibits cubic crystals with a rock salt structure [430], has been explored as the prototypical ionic oxide support [431]. Small gold clusters deposited on electronbeam-cleaved MgO surfaces showed high mobility – diffusing even at room temperature to form favorable epitaxial orientations such as Au(111)//MgO(001) or Au(110)//MgO(110). This indicates that the binding energy of the gold clusters on this substrate is low even at
300 K [431]. Højrup-Hansen et al. [432] have studied the nucleation and growth of Au nanoparticles on MgO(100) using atomic force microscopy (AFM). They established that the nucleation kinetics follow a general scheme consisting of three stages: nucleation, growth and coalescence. As a function of the deposition time, the density of clusters first increases (nucleation stage), then remains constant (growth stage), and finally decreases (coalescence stage). The saturation density of clusters was found to decrease with the substrate temperature, following an Arrhenius law with activation energy of 0.12 eV [432]. In several reports, Freund and coworkers [433–436] documented the interaction of gold clusters with thin MgO(001) films prepared on Mo(001) or Ag(001) substrates. The researchers confirmed that pristine MgO films did not exhibit a measurable amount of point defects (color centers). When color centers were created

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Fig. 5.9. Gold nanocrystals on reduced TiO2: (a) 10.9 nm Au cluster, (b) 4.3 nm Au cluster, (c) model of Au nanocrystal, and (d) schematic of Au nanocrystal shape showing measurements of nanocrystal dimensions. (e) Plot of Δh/h versus nanocrystal width for Au nanocrystals with size in the range of 4–11 nm; (f) plot of (Δh– h)/h versus 1/αInt, where αInt is defined as αInt ¼ wþ 2Δh/tan(θ). The TLE and interfacial energy have been thus measured from the slope and intercept, respectively. Adapted with permission from Ref. [16]. Copyright 2010 American Physical Society.

by electron bombardment, Au nucleated at these centers as well as on regular terrace sites [434,435]. Based on STM measurements, nucleation sites were identified along the step edges of the MgO surface [434]. The experimental results showed that the crossover in dimensionality from planar to three-dimensional Au clusters occurs for MgO film thicknesses between 3 ML and 8 ML [433]. In particular, on an 8 ML thin MgO film, Au atoms were adsorbed preferentially at oxygen sites and, after annealing to room temperature, formed 3Dclusters. In contrast, on a 3-layer MgO film Au was adsorbed at both magnesium and oxygen sites and remained two dimensional even after annealing [433]. Pacchioni and coworkers [437] used first-principles calculations to comparatively investigate the energetics, atomic structure, dimensionality, and chemical properties of Aun nanoclusters adsorbed on the MgO(100) crystalline surface and on MgO thin films grown on a Mo(100) substrate. The optimal geometries of Aun clusters adsorbed on MgO(100) were three-dimensional (3D), whereas planar configurations were preferable for Au clusters on MgO thin films, MgO//Mo(100) [437]. An important conclusion from these studies is that the adhesive energy for Aun adsorption on MgO films originates from charge redistribution caused by the underlying metallic substrate that supports the MgO. 5.2.4. Au/Al2O3 Unlike on TiO2(110), the growth of gold films and particles on Al2O3 has been reported to develop in three dimensions, even under low exposures and low-coverages [382,438]. Model films of alumina have been grown in a highly controlled way over NiAl(110) supports [48,439]. The thin alumina films

exhibit large terraces, but these flat regions contain defects that likely serve as nucleation sites for metal growth. Shaikhutdinov et al. [382] used STM imaging to show that Au nucleates on Al2O3/NiAl(110) films on the terraces as well as at defects, such as steps and line faults. The absence of preferential nucleation suggests that the mobility of Au on the alumina surface is very low, even at 300 K. This result was in agreement with the observed low mobility of gold on alumina demonstrated in work by Winkler et al. [438]. Even at 0.05 ML of gold coverage, Au particles exhibited a hemispherical shape, indicating 3-D growth from the onset [382,439]. These Au particles also showed very narrow size distributions. In addition, the system morphology was quite thermally stable up to 600 K, as evidenced through STM and LEED measurements. For gold nanoparticles on amorphous alumina films, Carrey et al. [440] reported the somewhat surprising result that large gold particles, under certain conditions, tend to have a higher mobility than single atoms or small clusters. Hence, the deposition resulted in a bimodal distribution of particle sized because the large particles readily diffuse and coalesce during deposition, while trapping at defects confines the smaller particles. 5.2.5. Au/SiO2 Wendt et al. [441] have explored the properties of ultrathin SiO2 films that were grown on an Mo(112) substrate to learn about the molecular and electronic structure of this model material [442]. The physical properties of the thinnest SiO2 films were affected by the Mo substrate due to strong Si–O– Mo linkages. Silica films thicker than two monolayers exhibited characteristics more similar to bulk-like silica.

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Fig. 5.10. HRTEM images of Au particles on CeO2 during heating at 820 K (a), 870 K (b), and 910 K (c). Reprinted with permission from Ref. [427]. Copyright 2007 Elsevier B.V.

Growth of gold on the silica thin films exhibited a growth very similar to that observed for Au on TiO2(110) [442]. At 300 K, the growth mode of Au at fractional monolayer coverages (0.1 ML) was found to be quasi-two-dimensional (2D). At higher Au coverages, three-dimensional (3D) growth was observed, while no evidence was found for significant chemical interactions between gold and silica. Au sintering was observed upon heating the sample to 600 K. Above a surface temperature of 600 K, sintering diminished before gold was found to desorb at higher temperatures. LEIS measurements for Au/SiO2/Mo(110) and the Au/TiO2/ Mo(100) model catalysts indicated that the rate of gold diffusion is very similar for both surfaces [442,443]. However, Au clusters preferentially nucleate and grow on TiOx (x
2) islands compared to the bare SiO2 surface. In addition, higher number densities of Au clusters were detected on highly reduced TiOx (x
1), which suggests that reduced Ti sites play a role as active nucleation sites (see Fig. 5.2). Importantly, for catalytic chemistry, the majority of the Au clusters were found to reside at the TiOx boundaries on the SiO2 surface [389]. Pacchioni and co-workers [444] have computationally investigated the role of point defects on SiO2/Mo(112) thin films in directing particle growth. Point defects at the surface of SiO2/Mo(112) were found to act as strong adsorption sites for the Au atoms. Interestingly, the same defects generated on an unsupported silica film with the same structure exhibited quite different behavior.

5.2.6. Au/FeOx Thin films of well-ordered iron oxide have attracted scientific attention as an potentially active support for Au-based catalysts [225,226]. Model supports of FeO(111), Fe3O4(111), and Fe2O3(0001) can be readily synthesized by iron deposition and oxidation cycles on Pt(111) [48,445]. The thin FeO(111) film grown on Pt(111) exhibits an O–Fe–Pt–Pt stacking and a strongly relaxed surface structure with a
10% mismatch between FeO(111) and Pt(111) lattices [446]. As observed with other oxide supports, adsorption of gold atoms on the FeO(111) film first occupy step site “holes” in the oxide film, which are composed of o3% of the surface. Under continued exposure, small Au clusters develop at the terraces as well [382,447]. At high coverages, Au nanoparticles were found to be generally homogeneously dispersed on the FeO(111) surface. The density of nucleated gold on the FeO film was approximately half of that observed for gold on Al2O3/NiAl(110). This implied that the two supports have quite different diffusivity of gold, which is probably related to lower defect density on the FeO film as compared to alumina [225]. Good epitaxy with the FeO(111) support was observed for monolayer islands of Au, which implied that larger Au particles evolve to expose (111) top facets [448]. At increased gold coverage, nanoparticles with a diameter as large as 7 nm were observed [448]. Studies on Fe3O4(111) films formed on Pt(111) single crystals [225] have shown that the regular surface is terminated by 1/4 of a monolayer (relative to the oxygen sub-layer) of Fe3 þ cations [445]. The nucleation and growth of Au particles on the cationic surface is controlled by terrace steps and the structural defects, which include iron vacancies and, in some cases, small domains that exhibit “biphase ordering” [449,450]. Because of vacancy defects, the diffusion of gold atoms on Fe3O4(111) films is rather limited as compared to FeO and prevents the Au particle sintering at 300 K [225]. At this temperature, the Au particle size distribution appears to be relatively broad for Au/Fe3O4. Heating at temperatures of up to 500 K caused an aggregation of Au particles to an average size of
4 nm, as observed by STM. In this work, a histogram analysis showed evidence for narrowing of the size distribution and indicated that the number of layers in the cluster increase as particles grow [382]. The annealed particles were well ordered and exhibit mostly hexagonal and trigonal structures. Finally, a small lattice mismatch (
3%) was observed between Au(111) and Fe3O4(111) and the (111) face of the Au particles was the most exposed. The STM results for gold deposition on the α-Fe2O3(0001) films were very similar to those observed for the Fe3O4 films: 3D-Au particles were uniformly distributed on the oxide surface and exhibited a hemi-spherical shape [225]. This result was in agreement with the findings of Haruta et al. that gold exists as hemispherical, epitaxial oriented particles (111)Au// (110)Fe2O3 on the oxide, as observed by TEM analysis [86]. 5.3. Sintering of gold nanoparticles As discussed above, the catalytic activity of supported gold particles is strongly size-dependent. The maximum activity

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(TOF) appears to occur for most studies in the 3–6 nm range. Therefore, sintering of as-prepared particles during catalysis or thermal activation of the system may be detrimental to performance [410]. Goodman and co-workers observed that gold particles deposited on a TiO2(001) thin film at 120 K underwent significant sintering between 120 and 400 K, but then plateaued and remained stable up to 800 K. Other groups [451–453] have established that annealing of Au/TiO2(110) to temperatures 700–775 K results in less substrate covered by gold, due to gold island growth. However, no evidence was found for encapsulation of Au particles by titanium suboxides, as was reported for other metals. Egdell and co-workers [453] utilized STM to observe Ostwald ripening at high temperature in their stability studies of small gold particles on TiO2(110). One of the main mechanisms responsible for Au-particle sintering is the Ostwald ripening mechanism [384,453,454], although diffusion/coalescence of particles can also contribute and even be a dominant sintering mechanism under some conditions [453,455–457]. Ostwald ripening (see Fig. 5.10) is a thermodynamically-driven phenomenon that results from the relative stability of larger particles over smaller particles, i.e. the larger particles grow larger while the smaller particles diminish, due to the movement of individual atoms from one island to another [410]. Wynblatt and Gjostein [458] built an atomistic model (through the Gibbs–Thomson relation) of sintering kinetics at the “interface-controlled” limit [410]. Parker and Campbell [410,459] used a modified pairwise bond-additivity (MBA) approach to develop a new kinetic model for sintering of Au/TiO2(110) catalysts based on Ostwald ripening. This new kinetic model allows accurate predictions to be made for the long-term sintering behavior of Au/TiO2 catalysts by extrapolating data from short-term sintering kinetic measurements. In these studies, XPS measurements were used to quantitatively evaluate the amount of gold deposited on the surface, while low-energy ion scattering (LEIS) was used to quantitatively determine how much of the surface was covered by at least one atomic layer thick gold (or a so-called “Au area fraction”). The area fraction of the surface covered by gold was monitored as a function of temperature during a linear ramp using temperature programmed low energy ion scattering (TP-LEIS). The TP-LEIS is unique in that it provides the apparent activation energy involved in long-term sintering through a single short-term experiment [410,459]. Although the initial nucleation and growth behavior of gold is critically important to catalyst development, the character of supported gold under conditions relevant to catalysis (in operando) is essential for predicting its ultimate activity. Most important to note is that gold particles on titania have been shown to undergo a form of Ostwald ripening when exposed to a feedstock of CO þ O2 (10 Torr, 2:1 CO/O2) at room temperature [384]. Specifically, 2.6 nm clusters with a narrow size distribution were observed to grow to 3.6 nm in diameter. Exposing larger particles (d ¼ 4.2 nm) to 10 Torr of oxygen at room temperature caused virtually no changes, i.e. no ripening was observed. However, Sykes et al. [456] observed tremendous sintering of gold upon exposure to air or oxygen at room

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temperature for extended durations (20 h). The role of oxygen was hypothesized to be that it removes vacancies, which otherwise anchor the gold particles/atoms to the surface. SaintLager and co-workers [460,461] have also found experimental evidence that CO oxidation to CO2 induces sintering of Au particles. These authors used in situ grazing incidence smallangle X-ray scattering (GISAXS) to determine the morphology (size and shape), as well as the spatial distribution of gold nanoparticle assemblies on the substrate. By using in operando measurements, new insights were gained for the relationship between the size and the catalytic activity of supported gold nanoparticles on TiO2(110). Sintering was found to be more prominent when Au particles are initially small, such particles being naturally higher in energy. They showed the sintering rate to depend on the TiO2(110) surface direction, with greater gold atom mobility along the [001] direction. An important final aspect of this chemistry to note is that these researchers suggested that the sintering results from a more complex process than simple reaction-induced local heating. 5.4. Design and synthesis of sinter-resistant supported gold Because particle size, the dynamic nature of which is well documented, has such a critical effect on catalytic efficiency, new research efforts have emerged that aim to stabilize small supported Au particles. One of the strategies favoring sinterresistant supported Au catalysts is design supports that restrict movement through a reduction in the diffusivity of individual Au atoms and trapping of the particles themselves. Surface science studies of planar model catalysts have proven to be very helpful in guiding strategy development for sinterresistant supports. Chen and Goodman [46], for example, have presented some strategies for the development of functional oxide supports that may enhance the gold-support interactions in a way that leads to inhibition of gold particle sintering. One example of such an oxide support is a Ti-doped SiO2 film prepared on a Mo singlecrystal surface [462,463]. First, a monolayer SiO2 film was deposited on a single-crystal Mo surface. The observed well– defined c(2  2) LEED pattern indicated that their SiO2 film was well–ordered. Following formation of the silica overlayer, Ti was deposited at room temperature; this structure was oxidized at
850 K and finally annealed to
1050 K. Ti was found to incorporate into the surface where it formed Ti–O–Si linkages at low coverage (
8 %) (Fig. 5.11a) and TiOx 3-D islands at a relatively higher (
17%) coverage (Fig. 5.11b). On the 8% Ti-doped SiO2 surface, gold nucleated primarily at the Ti defects (Fig. 5.11a) whereas on the 17% Tidoped SiO2 surface, gold primarily decorated the extremities of the TiOx islands (Fig. 5.11b). The important observation from this work is that the thermally induced Au sintering on the Ti-doped surfaces was greatly inhibited relative to Au on SiO2, an effect that the authors attributed to well-separated anchor sites on the surface [46,464]. Rashkeev et al. [465] performed computational studies .of the experimentally observed stabilization of Au

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nanoparticles when a partial monolayer of SiO2 is deposited on a Au/TiO2 catalyst. They found that deposition of SiO2 structures on a pure TiO2 substrate leads to lattice-mismatch instabilities that result in the formation of additional strong binding sites for Au atoms/nanoparticles. They further described the effect as that of the partial monolayer of SiO2 introducing an atomic-scale roughness, which inhibits the diffusion of gold atoms at the surface and restricts nanoparticle growth and sintering. In closely related work, Goodman and co-workers [466] succeeded in preparing ordered Au/TiOx/Mo(112) films where the strong interaction between Au and the reduced titania surface was the driving force for Au wetting of this oxide support. The titania films were synthesized by depositing
1 ML Ti onto SiO2(1 ML)/Mo(112), followed by oxidation at 850 K, annealing at 1200 K, and decomposition at 1400 K. After the 1400 K annealing, Si was completely removed from the surface, resulting in a well-ordered TiOx/Mo(112) surface. STM and LEED data revealed the formation of an (8  2)-TiOx reduced titania surface where there are seven Ti atoms for every eight Mo atoms traversing the Mo(112) trough, which binds to the surface via Ti–O–Mo linkers and to each other via Ti–O–Ti bonds. This well-ordered titania film exhibited a single phonon feature at 84 meV in HREEL, which, as supported by XPS results, is consistent with a reduced titania film. Thus, for the (8  2)-TiOx thin film, a full monolayer of reduced Ti3 þ sites was suggested to exist, which the authors hypothesized would facilitate strong binding between deposited Au and the TiOx surface. Indeed, annealing a Au-exposed (8  2)-TiOx surface to 900 K led to complete wetting of the substrate. The Au/Mo AES ratio and the corresponding νCO intensity recorded via FTIR supported these conclusions. Finally, TPD of Au from TiOx confirmed that the Au–TiOx binding energy exceeded the strength of Au–Au bonds in the bulk material, which agrees with the high apparent stability of ordered Au nanofilms. Additional studies by Behm and co-workers [467] demonstrated that Au/TiO2/Ru(0001) model catalysts may inhibit sintering during catalysis. In their work, epitaxial thin TiO2 films were grown on Ru(0001) by deposition of Ti in an O2 atmosphere, followed by annealing and cooling in an O2

atmosphere. By optimizing the pre-treatment conditions for deposition and post-deposition processing, flat films of fully oxidized TiO2 or as partly reduced TiOx were readily synthesized. The reduced films consisted of a mixture of TiO2 and Ti2O3. Au particles deposited on the oxidized TiO2 were slightly larger (higher) than those on reduced TiO2. The Au and TiO2 were found to interact only weakly and annealing at 770 K induced Au particle growth, resulting in more hemispheric particles and a larger number of higher, 3D islands. Compared to Au on fully oxidized TiO2, Au islands grown on reduced TiOx were more stable, which was attributed to O-vacancies in the underlying TiOx substrate. These observations have important implications for the choice of an appropriate support for potential applications in catalysis. 5.5. Titania: the most popular support for Au-based catalysis As highlighted herein, gold deposited on planar TiO2 supports provides a model system that has been extensively examined with a wide range of surface science analytical techniques to obtain a fundamental understanding of the electronic, structural, morphological, chemical, and catalytic properties of supported gold nanoparticles. Over the years, Au/ TiO2(110) has become the most studied model gold catalyst, both experimentally and theoretically [403,407]. There are two main reasons for the tremendous interest in Au/TiO2 model systems. First, TiO2 itself is an inherently promising material with applications that span from solar energy conversion to photocatalysis [74]. Second, one can precisely control the size and shape of deposited gold nanoparticles and thus vary their physical properties and catalytic performance. The properties of model Au/TiO2 can be predictably manipulated by the proper choice of TiO2 pre-treatment (i.e. by controlling the surface stoichiometry and structure of the TiO2 support) and by varying the parameters of gold deposition (i.e. by controlling the gold coverage and deposition temperature). Most importantly, "real" catalysis developed from Au deposition on P25-TiO2 has exhibited excellent activity for many catalytic reactions [468]. These include CO oxidation [27], aniline oxidation [469], benzylamine oxidation [470] and selective nitroaromatics hydrogenation [471]. In fact, this system has

Fig. 5.11. 3D STM images of (a) Au(0.04 ML)/TiOx(8%)–SiO2 and (b) Au(0.08 ML)/TiOx(17%)–SiO2 showing that both Ti defects and TiOx islands play a role as nucleation sites for Au nanoclusters. Reprinted with permission from Ref. [464]. Copyright 2006 American Chemical Society.

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shown so much potential that the World Gold Council (WGC) developed and, at one point in recent history, marketed a TiO2supported Au powder as a reference (catalyst type A).; thus, well-characterized “standard” samples of Au/TiO2 were at one time commercially available [471]. 6. High surface area powder gold–titania catalysts The natural abundance, ease of preparation, reducibility, and photoactivity of TiO2 all contribute to the tremendous popularity of the semiconductor as a heterogeneous catalyst support. Most importantly, TiO2 appears to directly participate in the chemistry as an “active support material” [380,393,464]. However, the method of preparation, which affects sintering rates (see above) and the Au–TiO2 electronic structure, is crucial for full realization of this activity. 6.1. Design strategies Shortly after discovering the unique catalytic properties of highly dispersed hemispherical gold nanoparticles on TiO2, Haruta and co-workers developed several preparation techniques for depositing Au nanoparticles on metal oxides [26,27,472–475]. Early reviews by Bond and Thompson [33] and Haruta [37] classified these methods into three general categories. The first category makes use of well-mixed precursors. Such precursors can be hydroxide, oxide, or metal mixtures of Au prepared through co-precipitation [27,78], cosputtering [476], or amorphous alloying [477]. Calcination in air at temperatures above 550 K transforms the mixtures into metallic Au nanoparticles strongly attached to the crystalline metal oxides. The second class of preparation involves the deposition of Au-containing compounds; for example, Au hydroxide (so-called deposition–precipitation (DP)) [86,472], direct anionic exchange (DAE) [478], or organo-gold complex by gas phase grafting (GG) [479,480] and liquid phase grafting (LG) [481,482]. The last category is based on utilization of monodispersed Au colloids that are stabilized by organic ligands or polymeric compounds [198,483]. Others have similarly categorized routes to Au/TiO2 catalyst preparation. The general categories are briefly described below. 6.2. Methods for deposition of gold on TiO2 powder supports TiO2 is an amphoteric oxide with an isoelectric point (IEP) of 6 (IEPP25 ¼ 6.2) [484,485], which makes it an excellent candidate for deposition–precipitation in NaOH (DP NaOH) for generation of Au/TiO2 powdered catalysts [472,473]. The deposition–precipitation protocol developed by Haruta and coworkers [86] was employed for some time by the WGC for the preparation of a number of Gold Reference Catalysts [486], with the main objective of providing researchers with a benchmark for comparing their own catalyst formulations [487]. Unfortunately, such standardized samples were no longer available at the time this review was published. Soon after its original development [86], the DP method became the most recommended approach for synthesizing

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small supported gold catalysts [1]. With a nominal amount of
13 wt% of Au in solution and a pH between 7 and 10, the DP method allows the deposition of up to 3 wt% of Au and an average size of Au nanoparticles of about 3 nm. One should note that the amount of deposited gold on TiO2 by DP NaOH is always lower than the amount of gold contained in solution (the yield of DP is lower than 100%). Louis and co-workers [488] developed an alternative method for the preparation of gold nanoparticles supported on TiO2 by deposition–precipitation. Their DP method was originally developed by Geus et al. [489,490] and uses CO (NH2)2 (DP urea) as the precipitating base. The DP-urea method permits the gradual and homogeneous addition of hydroxide ions throughout solution, CO(NH2)2 þ 3H2O2NH4þ þ CO2 þ 2OH  , and avoids local increase in pH. According to the standard preparation procedure, 1 g of TiO2 was added to 100 mL of an aqueous solution of HAuCl4 (4.2  10  3 M) and of urea (0.42 M). The initial pH was
2. The suspension, thermostated at 353 K, was vigorously stirred for 4 h (pH increases) and then centrifuged, washed, and dried. Samples prepared in this way must be kept in the dark and preferably under vacuum until they are ready to use. Louis et al. [491] employed different analytical techniques, including UV–visible absorption, XANES, EXAFS, and TEM, to characterize Au/TiO2 catalysts prepared by deposition–precipitation with NaOH and urea. The achieved Au loading on the same TiO2 P25 support (surface area of 45 m2 g  1) was
3 wt% Au for the DP NaOH prepared sample while it was much higher
8 wt% for the DP-urea sample [488,491]. After preparation, the deposited gold species were in the oxidic state III for both samples. However, they transformed into metallic gold in air at
373 K for DP NaOH and at
423 K for DP urea. At 473 K, the gold was found to be metallic for both preparations. Fig. 6.1 presents HRTEM images of gold nanoparticles deposited on TiO2 P25 by the DP-urea method and reduced in H2 at two different temperatures. After reduction at 473 K, the Au nanoparticles were single crystals of
2 nm diameter (Fig. 6.1a) and presented some tiny facets and rounded parts. They were stable under the electron beam. Fig. 6.1b presents Au particles from the same initial preparation after pretreatment at 753 K under H2. The particles were perfectly faceted and presented the unique shape of a truncated octahedron exposing (111) and (100) facets. They were stable under electron beam irradiation, and their interface was flat. Noteworthy, Fig. 6.1b demonstrates the gold particle epitaxy on the TiO2 substrate. The (111) plane of gold is parallel to the (110) plane of the rutile (the titania support is composed of 30% of rutile). The calcination temperature resulted in an increase in the Au particle size from
1.5 to 3.5 nm. The CO oxidation activity increased with the percentage of metallic gold (which was higher for DP-urea), and maximized after calcination at 473 K for both types of preparation. However, the estimated activities (per mole of Au) and TOFs were the same for the two types of samples. An increase in the calcination temperature from 473 to 673 K led to a reduction in

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activity attributed to a change of particle shape, rather than to an increase in the overall size of the particles. Louis and co-workers [492] also studied the mechanism of gold precursor deposition onto TiO2 with either NaOH or urea. For the DP NaOH prepared sample, the main species present at pH ¼ 8 are [AuCl(OH)3]– ions that react with hydroxyl groups (a limited number) at the TiO2 surface to form a grafted hydroxy-gold compound, likely Ti[OAu(OH)3]–. For this method, the limiting reagent (TiO2 hydroxyl groups) mediates the amount of deposited gold. In contrast, the DP urea method deposits nearly 100% of the gold present in solution. The key difference is that gold(III) is deposited as a precipitate with an atomic composition of AuN2.2O1.2C0.9H4.2Cl0.1 [492]. This compound, which is clearly not gold(III) hydroxide, is suggested to arise from reactions between the gold precursor and decomposition products of urea. The small Au particle size observed, via TEM, for the Au/TiO2 sample prepared by the DP-urea method suggests that a strong interaction occurs between the TiO2 support and the gold precipitate during the deposition, which leads to highly dispersed and strongly anchored metallic gold [492]. In a series of studies complementary of the work by Louis et al. [492], Moreau et al. [493–495] have systematically studied the role of pH changes during DP synthesis on the resulting activity of catalysts. In their work, Au/TiO2 catalysts were prepared with varying initial pH values (from 4 to 11, as controlled by the concentration of NaOH) of the HAuCl4 solution. Using the equilibrium constants measured by Nechayev and Nikolenko [496], Moreau et al. [493] calculated the relative concentration of gold complexes as a function of the solution pH. The results of these calculations are presented in Fig. 6.2. These results indicate that the neutral AuCl3.H2O is the primary species at pH of about 3–4; at pH 7 the AuCl(OH)3 anion is prevalent; above pH ¼ 10, the dominant anion species is Au(OH)4 . TEM images of the Au/TiO2 samples synthesized by Moreau et al. [493–495] directly showed the dependence of

gold particle size on the final solution pH. At a final pH of 6, the prepared samples exhibited a fairly broad distribution of gold particle diameters in the range of 4–20 nm and an average diameter of 10 nm. In contrast, at a final pH of 8.5, the diameter of prepared particles ranged from 1.5 to 4 nm with a much smaller mean diameter of only 2 nm. The catalytic activity of uncalcined samples as measured by T50 (i.e. the temperature for 50% conversion) was highest for catalysts prepared at final pH 8.5 (T50 ¼ 243 K), which coincides with the lowest mean particle diameters for all of the preparation conditions. However, the calculated specific activity (per unit mass of gold) was the same for all of the uncalcined catalysts with gold loading ranging from 0.05 to 1.9 wt%, giving a value of 3.9 (7 0.4)  10  4 molCO s  1 g  1 Au [474,497]. The authors suggested that the constant specific activity is explained by the fact that their preparation method creates consistent particles sizes, independent of loading, and hence the population of active sites simply increases linearly with gold content. For catalysts prepared at pH 9, the catalytic activity was unaffected by calcination up to 573 K, while for particles prepared at pH 6.4, calcination at temperatures as low as 473 K appeared to significantly reduce activity. Two possible factors were suggested to contribute to the activity drop: (i) at pH 6.4 the gold ions in solution remain rich in chlorine, which may assist the mobility and thus the aggregation of gold particles; (ii) at pH 6.4 a number of large particles develop, whereas at pH 9 the particle sizes are restricted. In the former case, the differences in particles sizes for a single system provides a driving force for Ostwald ripening, which is absent for samples with uniformly small particles. The most important results from this work are that the adsorbed precursor (which determines the ultimate fate of the particle growth) is in rapid equilibrium with solution-phase gold-containing species, which provides a handle to control the gold particle size during the synthesis [495]. Furthermore, control over the adsorption equilibrium by pH alteration enables sintered gold particles or poorly dispersed precursors to be re-dissolved in

Fig. 6.1. HRTEM images of gold particles in the profile view in the DP-urea sample reduced under Hþ (a) at 473 K, (b) at 773 K (the view axis, perpendicular to the image, is in the (110) direction). Reprinted with permission from Ref. [491]. Copyright 2004 Elsevier B.V.

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aqua regia and the gold precursor again deposited at the most appropriate pH, thus potentially restoring the activity of a defunct Au/TiO2 catalysts [495]. Behm and co-workers [498,499] used a modified DP procedure and a unique reductive conditioning approach to prepare Au/TiO2 catalysts with small metallic Au particles (o 2 nm) that exhibited excellent activity for low-temperature CO oxidation. According to this modified DP procedure, an aqueous solution of HAuCl4 was added to a suspension of annealed TiO2 powder (Anatase, Sachtleben VP9413/3, annealed in air at about 973 K for 30 min, TiO2 particle sizes 21–25 nm) while stirring at 333 K. During this process, the suspension was kept at a pH of about 5.5 by adding 0.16 M Na2CO3. Following 30 min of continued stirring, the precipitate was cooled, filtered and washed. The filtrate was then dried overnight at room temperature under vacuum. Prior to catalytic experiments, the Au/TiO2 samples were conditioned either by calcining in 10% O2/N2 at 673 K for 30 min or by a reductive conditioning procedure that involved 45 min annealing in 10% H2 (in a N2 flow) at 473 K. Both conditioning procedures resulted in fully reduced metallic Au particles. The particle sizes centered around 2.5–3 nm after simple calcination in oxygen and about 1.8 nm after the reductive treatment. Due to the lower sizes of Au nanoparticles, the catalyst obtained after reductive conditioning showed a significantly higher rate and a higher TOF for CO oxidation than the conventionally treated sample. Another important note is that these researchers found that the natural ageing of the Au/TiO2 catalysts can be avoided by storing the samples in the dark. An alternative approach to synthesis by Iwasawa and coworkers [89,481,500] that employed Au phosphine complexes as particle precursors and as-precipitated metal hydroxides as oxide precursors provided remarkably active catalysts for low-temperature CO oxidation. The authors strongly emphasized that the “as-precipitated” Ti(OH)4 was essential for creating well-dispersed active catalysts. Choudhary et al. [501] also reported highly dispersed Au catalysts with sub 5 nm particles that were created through a wet-impregnation method to deposit [Au6(PPh3)6](BF4)2 onto a TiO2 support.

Fig. 6.2. Relative equilibrium concentration of gold complexes ([Cl] ¼ 2.5  10  3 M) as a function of the pH of the solution, calculated with equilibrium constants reported by Nechayev et al. [496]. Reprinted with permission from Ref. [493]. Copyright 2005 Elsevier B.V.

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High-temperature reduction–oxidation cycling of the [Au6(PPh3)6](BF4)2/TiO2 resulted in well-dispersed gold catalysts that possessed remarkably high reported activities [481]. Gates and co-workers [502,503] further investigated preparation methods and demonstrated the synthesis of TiO2supported gold nanoclusters from adsorbed AuIII(CH3)2 (C5H7O2) (dimethyl acetylacetonate gold(III)). This compound, first used to prepare supported gold by Okumura et al. [479], provides a route to avoiding contamination of the catalyst by chlorine or phosphorous. Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies were used to study the cluster size and oxidation state of the deposited gold. In these studies, a Au/TiO2 sample with 1 wt% Au loading was prepared by stirring Au(CH3)2(acac) in n-pentane with TiO2 (P25) powder (for 1 day). The TiO2 support was partially dehydroxylated under vacuum at 673 K. The solvent was removed by evacuation (o 10  3 Torr) for 1 day. XANES and EXAFS data demonstrated the formation of mononuclear Au(III) complexes as the predominant surface gold species in the as-prepared sample. The gold mononuclear complex was bonded to two titania lattice oxygen atoms with an Au–O distance of 2.16 Å. Treatment of the as-deposited gold species in He and H2 at 323–573 K led to the formation of metallic Au clusters on the TiO2 with average cluster size of 1.5 nm. In a relatively novel approach, Rolison and co-workers [504–508] employed the general modification of guest  host composite aerogels for the generation of active Au/TiO2 catalysts. In their method, the nanoscale guest, when added during host sol gelation, becomes suspended in a 3-D network of interconnected TiO2 particles. The suspended Au particles remain accessible to the reactant feedstock as it diffuses through the pore network [506]. The covalently linked TiO2host particles range in size from 10 to 15 nm, while the Au– guest particles typically exhibit comparable sizes in the range of 5–6 nm. Composite materials created in this way facilitates multiple interfacial contacts between the Au particles and the surrounding titania support. The authors attributed the observed catalytic [506] and photocatalytic [508] performance in these materials to the extended 3-dimensional metal-oxide interfacial structure. Such structure may provide multiple interfacial sites not offered by catalysts created by the DP method, which may help explain why the aerogel catalysts are active despite the large Au particle sizes [506]. In another approach, Stucky and co-workers [509] developed an efficient one-step route to the synthesis of metallic nanoparticles. Their method involved the use of amine-borane complexes as reducing agents. With careful selection of solvents and reaction temperatures, they synthesized gold nanoparticles with sizes of 3.5 7 0.5, 6.37 0.5, and 8.27 0.9 nm [510]. The stable gold nanoparticles were functionalized by long chain alkyl thiols, which rendered them highly soluble in organic solvents. Taking advantage of the relatively weak interaction between the hydrophobic gold nanoparticles and hydrophilic oxide substrates (TiO2, SiO2, ZnO, Al2O3, etc.), an aprotic solvent could be used to create a homogeneously loaded sample of the gold nanoparticles. The

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dispersion of gold nanoparticles was then locked in place by calcination. The capping ligands were eliminated by annealing the composites in air at 573 K. Interestingly, the 6.3 nm gold nanoparticles supported on SiO2 showed the highest TOF for selective oxidation of ethanol to acetaldehyde [510]. Cuenya and co-workers [511,512] applied the method of micellular encapsulation [513] in PS-P2VP diblock copolymers [polystyrene-block-poly(2-vinylpyridine) diblock copolymers] to the synthesis of Au/TiO2 and Au/SiO2 catalysts. By dissolving PS-P2VP polymers in toluene, inverse micelles were formed with the nonpolar polystyrene (PS) tails extending outward. Chloroauric acid (HAuCl4.3H2O) was used to attach AuCl4 to the pyridine groups in the P2VP core. The micelles containing Au nanoparticles were deposited onto different supports by dip-coating at a rate of 1 μm/min. The length of the polymer tail (PS) was found to determine the final inter-particle separation distance [511]. The polymer from the Au nanoparticle surface was then removed by exposure to an O2 plasma at low temperature (150 K). Daniele and co-workers [487] reported a “supported-Turkevitch”-type synthesis of Au/TiO2 catalysts. According to this approach, a high surface area titania support was first functionalized with heteroleptic titanium alkoxides containing citrate moieties. Subsequently, during the sol–gel process, this tunable citrate functionalization allows the reduction of gold precursors to proceed in situ, directly at the solid surface. Thus, the anchored citrate functionality plays the role of reducing/stabilizing agent. The catalytic activity of such a Au catalyst sample, containing 0.2 wt. % Au deposited on (Cit)1(TiO2)20 hybrid support (480 m2 g  1), was only slightly lower than that observed over a reference sample of Au/TiO2 Type A Gold Reference Catalysts [486] obtained from World Gold Council. Recently, Flytzani-Stephanopoulos and co-workers [514] demonstrated that appreciable loadings (
1 wt %) of isolated gold atoms can be stabilized on TiO2 supports by the use of a new method that combines traditional gold deposition/precipitation with UV irradiation. Au catalysts prepared in this way have been shown to be active for the low-temperature water–gas shift reaction, where dissociation of H2O is facile on the Au O TiOx sites. These authors demonstrated that nanoparticles formed at increasing gold loadings actually do not contribute to the catalytic activity. That is, when the “excess” gold was removed by sodium cyanide leaching, the atomically dispersed gold that remained on the titania showed remarkable catalytic activity. These new materials may catalyze a number of other reactions that require oxidized metal centers and represent an exciting area of research for future development [514]. 7. Adsorption of molecules on gold–titania surfaces The chemical synergy between the support and metal in the performance of Au/TiO2 catalysts has been known since the early work of Haruta et al. [78,86]; however, scientists are only beginning to understand the origin of this phenomenon. Of critical importance are the factors that affect the first stages of molecular adsorption and activation on the surfaces of such

materials. This initial step in any catalytic reaction is crucial because reactants must interact sufficiently strongly with the material to become activated, yet intermediates must interact weakly enough to readily diffuse to and react at sites where an activated co-reactant resides [224]. The strength of bonding, and thus the intrinsic catalytic activity towards chemical transformation of an adsorbate, is determined by the structure dependence of the adsorbate–surface interaction [224]. As discussed in Section 4, the adsorption energy of CO and O on transition and noble metal surfaces varies with the d-band center of the surface atoms (see Fig. 4.8). According to the dband model [515], the bonding strength of an adosrbate increases as the d states are shifted up in energy, i.e. as the d-band center approaches the Fermi energy. Thus, for any discussion of reactivity involving small particles and defective surfaces, one must bear the following relationship in mind: "The lower the metal coordination number is, the higher the d states are in energy, and the stronger they interact with adsorbates” [224]. As such, steps, kinks, and small particles involving low-coordinated Au atoms constitute the most reactive surface structures. Further, the support can play a major role in catalysis by affecting the electronic structure, particle size, and defect density of the metal. Just as importantly, the support lends "dual" active sites where co-reactants may be activated through binding simultaneously to the metal and metal-oxide [224]. In an effort to understand the initial interaction between an adsorbate and metal surface, researchers have extensively explored the uptake of reactants on the surface of both "model" Au/TiO2 catalysts consisting of well-defined single crystalline TiO2 surfaces and “real” high surface area systems comprised of TiO2 nanoparticles. Interestingly, under certain conditions, model systems may behave very similarly to the more realistic nanoparticulate catalysts. For example, the turnover frequencies (TOFs) for the CO þ O2 reaction, i.e. the number of CO2 product molecules/site/s as a function of Au cluster size, has been shown (see Fig. 7.1) to reach a maximum for specific cluster sizes for both real (A) and model (B) catalysts [37,384,516]. Many groups have therefore studied CO, O2, and other small molecule adsorption on such systems to construct an understanding for the first steps in catalysis. 7.1. Carbon monoxide 7.1.1. Au/TiO2 planar model catalysts Adsorption of CO at the Au/TiO2 surface is a key mechanistic step in the oxidation of CO, as well as in other transformations (e.g., methanation or NO reduction). As discussed in Section 4.2, the uptake of CO on both single crystal [275,517] and on a sputtered [275] Au surfaces is sensitive to surface structure and coordination number of the surface atoms. Therefore, it is not surprising that the adsorption of CO on TiO2-suppored Au particles depends heavily on the structural and electronic properties of the particles. Lee et al. have studied CO adsorption on a model Au/TiO2(110) system created by ion beam deposition at low temperature (115 K) [409]. Both near-stoichiometric and highly defective TiO2 supports were used to deposit 0.05

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ML of Au as Au þ . This study combined ion scattering, X-ray photoelectron spectroscopy (XPS) and CO temperatureprogrammed desorption (TPD) to track the thermal transformations of Au deposited over a wide temperature range (115– 800 K). The Au4f XP spectra revealed two key developments in the thermal modification of deposited Au: (1) automatically dispersed gold deposited at 115 K began to associate with oxygen vacancies as the temperature was raised to 300 K; and (2) at temperatures above 450 K, the Au atoms formed small clusters. Interestingly, in the presence of adsorbed CO, the XP signal for carbon exhibited a large shift to higher binding energy, consistent with strong CO bonding to isolated Au on stoichiometric sites. In contrast, the annealed Au/TiO2 samples (to 300 K) and those prepared with highly defective TiO2 exhibited only small changes in binding energy, indicating weak CO interactions with Au–vacancy complexes. XPS data showed that only limited amount of gold was agglomerated in clusters at 300 K. Using CO vibrational spectroscopy and DFT calculations, Wörz et al. studied the interactions of CO at Au atoms that had been deposited on TiO2 thin films grown on Mo(110) [396]. They used the vibrational frequency of adsorbed CO to help distinguish between Au atoms bound at regular sites on the titania support as Auδ þ and Au atoms on oxygen vacancies, which likely exists as Auδ . As previously predicted [373,518,519] – and reviewed in prior sections – DFT calculations showed that Au atoms can diffuse and become trapped at oxygen vacancies where they bind strongly to the TiO2 substrate (computed binding energy of 1.7 eV). On such trapped Au sites, TiO2 charge transfer to Au creates formally Au  . The excess negative charge on the AuCO complexes formed at oxygen vacancies is responsible for a relatively weak Au–CO bond (0.4 eV) and a lower CO vibrational frequency. CO adsorption on Au atoms at regular bridging oxygen sites results in the opposite effects due to the formation of Auδ þ at these sites. Importantly for catalysis, reduced surfaces produce weaker AuCO complexes that are stable only up to 150 K, while

Fig. 7.1. TOFs for CO2 production as a function of the size of Au cluster supported on TiO2: (A) “Real” high surface area Au/TiO2 catalysts prepared by a deposition–precipitation method, data taken from Ref. [474]. (B) “Model” Au/TiO2 catalysts prepared by vapor-deposited Au atoms on planar TiO2 films on Mo(100). Reprinted with permission from Ref. [384]. Copyright 2000 Elsevier B.V

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on oxidized TiO2 surfaces, AuCO appears to be thermally stable above 350 K. The charge transfer from TiO2 to Au clusters, which plays such a large role in the energetics of CO adsorption, has been studied by a number of groups. For example, Jiang et al. [520] employed XPS to explore charge transfer in model systems. On the stoichiometric surface, they found that the Au 4f binding energy monotonically shifted toward lower binding energy with the thickness of the Au. On the reduced surface, the Au 4f binding energy monotonically shifted downward with Au thickness up to 1.5 Å. These Au 4f binding energy shifts on the reduced surface were attributed to a collective contribution from the particle size effect and the charge transfer from the reduced surface to Au clusters. Goodman et al. have explored the influence of metal cluster size on chemisorption and binding energies of CO on Au clusters supported on TiO2 by infrared reflection absorption spectroscopy (IRAS) [521]. In their studies, Au was deposited at controlled coverages on the surface of TiO2 thin films (on a Mo (110) crystal) via vapor deposition. STM analysis provided average cluster sizes for the deposited gold [383]. Their IRAS spectra of CO adsorbed on 4 MLE (monolayer equivalent calibrated by Auger break point) Au/TiO2 exhibited only a single feature at 2122 cm  1 and no observable secondary features that could be related to a chemically distinct interfacial site or to CO bound at the TiO2 surface were observed. This simple result suggests that the size of the Au clusters for the 4 MLE Au/TiO2 were sufficiently large to exhibit CO adsorption characteristics similar to those of bulk Au (see Section 4.2). For this system, the Langmurian isotherms for CO adsorption suggested relatively weak dipole–dipole interactions between co-adsorbed CO molecules. From the slopes of isosteric plots created in this work, the heat of CO adsorption as a function of coverage was derived. As Au coverage (and therefore cluster size) decreased, they found that the heat of adsorption, –ΔHads, passed through a maximum (
76 kJ mol–1) at approximately the same cluster size for which maximum in catalytic activity was identified (see Fig. 7.1) [88]. In a related study by Campbell's group [522], Au/TiO2 systems, with Au clusters in the critical size regime for catalytic activity, demonstrated the strongest bonding for atomic oxygen. Based on these and related studies [523–525], an understanding has emerged that enhanced binding of CO and oxygen at nanosized Au clusters on TiO2 is important for activating the COþ O2 reaction. As the size of Au particles decreases below 10 nm, the density of coordinatively unsaturated Au atoms at the corner, edge, and terraces increases relative to the total number of Au atoms in the cluster [177,266,526], and CO and O2 are found to bind stronger at defects (corners, edges, and steps) smooth terrace regions [177,302,526]. However, given these observations, other groups have reported a lower catalytic activity for monolayer Au (coordination number of 3–4) as compared to that of bilayer Au (coordination number of
6) [394]. Furthermore, the catalytic rate actually decreases as the size of supported Au particle decreases for particles below 3 nm [384] (Fig. 7.1). We will return to the discussion of the nature of catalytically active centers on Au/TiO2 systems in Section 8.

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For the more general discussion of the nature of adsorbed CO, further research has demonstrated the sensitive interplay between the electronic character of the Au particles, the strength of the CO–Au bond, and the resulting vibrational frequency of the CO stretching mode. In fact, infrared spectroscopy of adsorbed CO has emerged as one of the most useful tools in characterizing the electronic nature of gold particles on titania [129,464,524,527]. Infrared spectroscopic data from a series of studies have been summarized in a relatively recent publication [464], which serves as an excellent reference source for anyone performing studies in this field. The main results from that work are summarized in Fig. 7.2, which provides peak wavenumbers for the absorptivity of CO on different Aucontaining samples (at low coverage). The CO spectral features reported here correspond to CO bound at atop Au sites. The ν (CO) mode shifts to lower frequency as the electron density at the Au clusters increases and to higher frequency on electrondeficient clusters. The extent of this shift in the vibrational energy for CO correlates with the electronic charge on Au site to which the CO is bound. As displayed in Fig. 7.2, the observed ν(CO) frequencies follow the sequence
2124, 2112, 2107, and 2095 cm  1 for CO adsorbed at: multilayer Au on Mo, a Au bilayer on titania, a Au monolayer on titania, and a Au monolayer on Mo, respectively. This sequence directly follows the trend in the heats of adsorption for CO on these surfaces [528], which is correlated with the charge state of Au [528]. Charge transfer between the TiO2 support and Au, which plays a major role in activating compounds on the surface, will be examined further in subsequent sections. The most important conclusions from these trends, especially as they relate to the Au/TiO2 model systems, are as follows [464]: (1) the thickness of Au particles may be more important than the particle diameter for activation and catalytic activity, and (2) bilayer films seem to increase the active site density by 1–3 orders of magnitude over conventional supported Au catalysts. In addition to the model systems discussed above, others have focused on CO interactions and activation on planar Au/ TiO2/Ru(0001) (see Section 5.1.1) [467,529,530]. Based on XPS data, the interaction between Au particles and the TiO2 substrate was found to be rather weak in this system [467]. The IR spectrum of CO adsorbed on 0.2 ML Au/TiO2/Ru(0001) (at 1  10  4 mbar and Ts ¼ 100 K) exhibited bands at 2185 cm  1 and 2105 cm  1 that were attributed to CO bound molecularly on the TiO2 substrate and Au nanoparticles, respectively. In addition, thermal desorption measurements showed a clear size and shape effect for CO adsorption energetics on the Au nanoparticles within this system [467,529]. Specifically, the activation energy for CO desorption decreased from 65 kJ mol  1 for
2 nm Au particles (0.1 ML Au on oxidized TiO2) to 51 kJ mol  1 for
4 nm Au particles (1.5 ML Au) [467]. These values are in agreement with the adsorption energies of 70–75 kJ mol  1 determined for CO on Au/TiO2/Mo(110) with similar Au coverage (and likely particle sizes) [521]. The agreement in the results of both groups extends also to the finding that Au particle thickness is a key parameter: the

bonding of CO to monolayer and bilayer Au islands on TiO2 is much stronger than on multilayer Au particles that exhibit more bulk-like properties. For single crystal Au(110) [517], Au(110) [275], Au(100) [531] and Au(332) [278] surfaces, adsorption energies of 46, 59, 58, and 55 kJ mol  1, respectively, were reported for CO adsorption at low coverage (θCO-0). Further information on this topic can be found in a helpful review by Freund and co-workers [225], which summarizes data from a large number of IR spectroscopic studies for CO adsorption on various gold surfaces, supports, and model systems. 7.1.2. Au/TiO2 high surface area catalysts Since the appearance of the first papers by Haruta and coworkers [27,78], several other studies [86,532] have embarked on developing an understanding for catalysis on nanoparticulate TiO2-supported Au catalysts. The so-called “real” catalysts prepared via deposition–precipitation (DP) methods have developed into benchmark systems in the study of catalytic properties of highly dispersed gold nanoparticles on reducible oxide supports [86]. The DP method produces small hemispherical gold particles stabilized on the support by epitaxial contact, dislocations, or contact with the amorphous oxide layer [86]. Studies of CO adsorption on such systems have shown that CO binds to
3.5 nm Au/TiO2 at two types of sites: normal terrace sites and sites along the Au particle perimeter. Both CO and O2 have been shown to adsorb at the same time on 1 wt% Au/TiO2 to produce carbon dioxide and carbonate-like species [533]. Based on these studies, researchers hypothesized that the CO þ O2 reaction most likely occurs at the interfacial perimeter sites between gold and the support [86,533]. Iizuka et al. [534] have performed detailed studies of the adsorption of CO on 3.3 wt% Au/TiO2 with 3.5 nm mean particle diameter prepared by DP-method. In their work, both

Fig. 7.2. Comparison of the stretching frequencies for CO adsorption on various supported Au catalyst. Reprinted with permission from Ref. [464]. Copyright 2006 American Chemical Society.

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static and recirculating methods were applied under constant CO pressure within the temperature range 253–303 K. Under these conditions, the majority of CO (90%) followed reversible uptake with a Langmurian-type dependence on the CO pressure. A large portion of this reversible adsorption was on the TiO2 support. The irreversible component of CO adsorption was found to produce CO2 which was found to accumulate on the TiO2 surface as carbonate-like species. The role of Au particle size for CO adsorption (and CO oxidation) has further been studied for a 1 wt% Au/TiO2 catalyst having Au particles with mean diameters in the range of 2–10 nm [524]. The most active sample (2.5 nm Au calcined at 573 K) exhibited an infrared absorption peak for CO at 2098 cm  1, which was assigned to CO linearly bound to Au sites, see Fig. 7.3. Interestingly, when the sample was calcined at 873 K, the Au particle size increased to 10 nm in diameter and the ΙΡ band for CO vanished. These authors concluded that CO adsorption take place only on steps, edges and corners of Au particles as opposed to the terrace regions that are dominant feature of larger particles [36,524]. These conclusions are consistent with the studies by Goodman et al., discussed above, that revealed that the thickness of gold islands controls the adsorption (and activity) of CO [88,383]. The nature of the active Au sites on Au/TiO2 particulates was further examined in a series of studies that combined chemisorption and spectroscopic measurements [535,536]. Menegazzo et al. [536] have combined quantitative measurements of CO chemisorption and FTIR spectroscopy to explore the active gold sites on a 1.51 wt% Au/TiO2 reference catalyst, provided by the World Gold Council. According to TEM determination, these gold particles exhibit a Wulff-like morphology with mean diameter of 3.87 1.5 nm and height 1 nm. The Au/TiO2 sample was mildly reduced and hydrated before the CO pulse chemisorption and the spectroscopic measurements to avoid CO chemisorption on the TiO2 support. The FTIR spectrum of CO adsorbed on the Au/TiO2 sample exhibited a band at 2098 cm  1 assigned to CO adsorbed at Au0 sites. The molar absorption coefficient for CO was calculated using the ratio between the integrated area of the FTIR band of adsorbed CO and the mol of chemisorbed CO, both per mol of Au. Based on reports [533] that only poorly coordinated Au atoms at step and edge sites are active for CO adsorption, a CO:Au molar ratio of 0.28:0.31 per step-edge Au atom was estimated for the temperature range 140–180 K. This result is in good agreement with the estimated coverage of 30% obtained at 170 K for CO adsorption on the Au(110)(1  2) single crystal surface [517]. In a similar study, Derrouiche et al. [535] have reported heats of adsorption of CO as a function of coverage on a 1 wt% Au/TiO2 sample (3–5 nm Au particles) prepared by DP method. The evolution of the IR bands for the adsorbate with the surface temperature, Ts (Ts ¼ 300–400 K), were followed at a constant CO partial pressure (PCO ¼ 1–10 kPa). In this way, the isosteric heats of adsorption of CO have been determined using the Clausius–Clapeyron equation. This method was previously explored for CO adsorption on Au single crystal [275,517] and model Au/TiO2 systems [521]. At 300 K, two

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IR bands were observed at 2184 and 2115 cm  1, which were assigned to CO species adsorbed on Ti4 þ cationic and Au0 sites, respectively. When the CO partial pressure was systematically increased from 1 to 15 kPa, the band for CO adsorbed at Au0 sites increased in intensity and shifted to 2110 cm  1. The band for CO adsorbed at Ti4 þ sites also increased in intensity and shifted to 2182 cm  1. At 15 kPa CO pressure, the Au sites were saturated with CO while saturation of the Ti4 þ sites was attained at much higher pressures (4 20 kPa). Not surprisingly, the behavior of the CO band for adsorption on Au0 sites, is similar to that of a band at
2110 cm  1 observed at high CO coverage at single crystal surfaces (see Section 4.2) Au(110) [276,517] and Au(332) [278]. Adsorption of CO is thought to cause a reconstruction of Au single crystalline surface as well as carbon deposition at long durations, even at 300 K [276]; however, such changes were not reported with the 1 wt% Au/TiO2 sample [535]. There was also agreement with the results obtained with a model Au/ TiO2/Mo(110) system, where the position of the νCO band was found to depend on the Au particle size: the band was found to blue-shift by 4 cm  1 with increasing particle size from 1.8 to 3.1 nm [521]. Interestingly, when the temperature for the CO adsorption experiments described above on the 1 wt% Au/TiO2 sample [535] was increased from 300 K to 407 K at constant CO partial pressure (10 vol% CO/He), the intensities of the IR bands for Ti4 þ CO and Au0CO progressively decreased. At a high temperature of 407 K, only the Au0CO band was present and was shifted to 2115 cm  1 [535]. In addition to the frequencies for these modes, the heats of adsorption for the two species, Ti4 þ CO and Au0CO, also vary with coverage: from 50 to 40 kJ mol  1 at coverages 0 and 1, respectively, for the Ti4 þ CO species, and, from 74 to 46 kJ mol  1 at coverages 0 and 1 for the Au0CO species. The values for the heat of CO adsorption at Au sites are slightly higher than, but consistent with, those measured for CO adsorbed on single crystal Au (110) [517], Au(110) [275], Au(100) [531] and Au(332) [278] surfaces: 46, 59, 58 and 55 kJ mol  1, respectively (for low CO coverages). These values are also in agreement with the CO adsorption energies measured for Au/TiO2 model systems with

Fig. 7.3. IR absorption spectra produced by adsorption of 4 mbar of CO at 90 K on 1 wt% Au/TiO2 calcined at 473, 573 and 873 K [524]. Reprinted with permission from Ref. [36]. Copyright 2002 Springer Science and Business Media.

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similar Au particle sizes (2–4 nm): 70–75 kJ mol  1 (Au/TiO2/ Mo(110)) [521] and 65–51 kJ mol  1 (Au/TiO2/Ru(0001) [467]. The heats of adsorption determined for the Ti4 þ CO species are also in good agreement with experimentally [537– 539] and theoretically [540–542] defined CO adsorption energies on different TiO2 surfaces. Together, these studies provide a converging understanding for CO adsorption. Not surprisingly, the wavenumber of the C–O stretching vibration is closely correlated to the bonding geometry of adsorbed carbon monoxide on metals [121]. As with Au single crystal systems and model Au/TiO2 systems, adsorption of CO on Au0 sites of real Au/TiO2 systems produces a single band in the IR spectrum at a wavenumber somewhat greater than 2110 cm-1 that is attributed to linearly bonded (singly-coordinated) CO [278]. For these systems, the band shows anomalous behavior: the wavenumber decreases slightly as the CO surface coverage increases, which contrasts the typical response of the CO vibrational frequency with coverage reported for other systems due to dipole coupling effects [278,543]. Behm and co-workers [499] have found that the vibrational frequency of adsorbed CO on Au/TiO2 is largely insensitive to the CO partial pressure, with only a small red shift from 2112 cm  1 at 0.05 kPa CO to 2108 cm  1 at 1.5 kPa CO partial pressure. Similar behavior was observed for CO adsorption on Au/Fe2O3 [544] and on massive Au, from 2129 cm  1 at 1.33  10  4 kPa of CO to 2106 cm  1 at atmospheric pressure [545]. This anomalous chemical shift has been interpreted as evidence for a paucity of back charge donation from Au d-electrons into the 2π∗ level of CO; as a result, the Au–CO bond is mainly due to charge transfer from the 5s orbital of CO, which is slightly antibonding for the C–O bond, to the metal [545]. The anomalous behavior of ν(CO) observed with CO coverage increase on Au surfaces as a relatively small negative shift is a consequence of two much larger but opposing effects: a coupling shift leading to a wavenumber increase, as expected, and a chemical shift leading to a frequency reduction. The relative importance of these interactions has been explored through the use of isotopically-labeled mixtures of CO (12CO–13CO or C16O–C18O) [527,546]. Boccuzzi and coworkers employed FTIR spectroscopy with the probe molecule, CO and 12CO–13CO isotopic mixtures to study the properties of the different gold catalysts [527,546]. As discussed above, the authors have estimated that only 30% of the exposed Au sites within a 1.5 wt% Au/TiO2 sample (3.8 nm Au, World Gold Council) chemisorb CO [536,546]. Fig. 7.4 summarizes some of their results for the adsorption of CO and 12CO–13CO. The IR spectrum exhibits a band at 2110 cm  1 attributed to CO linearly bound to metallic gold sites (Fig. 7.4a), which is asymmetrically broadened to the low frequency side. The position of this band systematically blue shifts to 2117 cm  1 as the 12CO pressure, and thereby 12CO coverage, decreases. Note that the linewidth and the asymmetry of band persist throughout this pressure regime. Adsorption of the 12CO–13CO mixture (1:1) produces a significantly redshifted 12CO band at 2096 cm  1 and a new band for the isotope at 2049 cm  1. The band for 13CO is less intense,

broader and more symmetric than the band of pure 12CO shown in Fig. 7.4a. As shown by the insets, each band can be modeled by two Lorentzian functions, which implies that there are two distinct adsorption sites for CO. At high 12CO coverage, dipolar coupling tends to transfer intensity from the low wavenumber band to its higher wavenumber counterpart. The coupling is reduced at low CO coverages, as well as for the isotopic mixture. Such dipolar coupling behavior is characteristic of a high degree of homogeneity in the types of exposed gold sites. On the basis of these data, along with an analysis of frequency shifts for the isotopic mixtures, scientists assigned the two IR modes to CO at two different, but interacting, Au sites: 6-coordinated corner sites and 7coordinated edge sites. The 8-coordinated terrace sites do not appear to adsorb CO even at 90 K [527,546]. Hadjiivanov and co-workers [547] have used IR spectroscopy of adsorbed CO probe molecule to study the role of activation temperature on the state of gold supported on titania. The Au/TiO2 (P25) sample was prepared by deposition– precipitation with urea and contained
0.7 wt% Au with average particles diameter
3 nm. On the as-prepared Au/ TiO2 sample, gold existed in Au3 þ form, which was not affected by evacuation at ambient temperature. Upon adsorption of CO at ambient temperature, the Au3 þ sites were readily reduced, producing the immediate rise of a band at 2168 cm  1 attributed to Au þ CO species, which were further slowly converted into Au0CO species exhibiting a band at 2106 cm  1. This finding is in agreement with the results reported previously by Maciejewski et al. [548]. Elevation of pretreatment temperature to 473 K causes formation of metallic gold, producing carbonyl bands at 2130 and 2105 cm  1. The band at 2105 cm  1 was attributed to CO adsorbed on small three-dimensional metal gold particles in accordance with previous assignments [279,524,549,550]. The adsorption centers were probably low-coordinated step and/or edges and/or corner Au sites because gold terrace sites do not adsorb CO even at low temperature [276,448,524,549]. The band at 2130 cm  1 was tentatively assigned to carbonyls formed with the involvement from positively polarized (Auδ þ ) sites. This hypothesis was supported by the finding that the 2130 cm  1 band was the principal and most stable gold carbonyl band detected on the sample treated under oxygen at 573 K. After such treatment, one could expect the existence of partially oxidized Auδ þ sites. Metallic gold on Au/TiO2 was stable towards heating in an oxygen atmosphere, but was easily oxidized by a NO þ O2 mixture at 573 K. In an important contribution, Wang and Hammer [551] employed DFT calculations to study the effect of oxidation state of oxide supported Au clusters (Au8/MgO, Au7/TiO2 and Au10/TiO2) on the adsorption characteristic (CO and O2). They found that CO prefers adsorption sites that are far from the oxide support, while O2 adsorbs on sites where bonding to both Au atoms and the oxide support (cations) can be simultaneously achieved (see Fig. 7.5a–j). For a Au7 cluster on the reduced TiO2 surface with Obr vacancies, the preferred CO binding sites were identified to be the top two layers of the cluster (Fig. 7.5a, b).

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Fig. 7.4. IR absorption spectra produced by room temperature adsorption of 10 mbar of 12CO (section a) and of 1:1 12CO–13CO mixture (section b) at decreasing pressure on reduced Au/TiO2. Inset: Curvefit of spectra taken in 10 mbar of 12CO (section a, black line) and 10 mbar of 1:1 12CO-13CO mixture (section b, black line) on reduced Au/TiO2. Reprinted with permission from Ref. [527]. Copyright 2009 World Gold Council.

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calculated vibrational frequencies (2149 and 2187 cm  1) for a highly reduced state of the support (Fig. 7.5a, b) should be considered exceptions to the above general picture. Indeed, charge density analysis of the Au7 cluster indicated that the top Au atoms are likely slightly positively charged, which helps to account for trends in the calculated CO vibrational frequencies. As shown in Table 7.1, there is a good agreement between the CO vibrational frequencies calculated in the work by Hammer [551] and those obtained by IR spectroscopic studies; the later data are presented according to the assignments for CO adsorbed on Au  , Au0, or Au þ used in the literature. According to this data, the Au1 and Au3 clusters on reduced TiO2 tend to accumulate net negative charge, the extent of which correlates with CO vibrational frequencies. However, one should note that contrary to the experiments, these calculations indicate that CO is only weakly adsorbed to the small Au particles. Beyond CO adsorption, Fig. 7.5 presents two O2 adsorption configurations on a Au7 cluster that resides on reduced TiO2 and on hydroxylated TiO2. Although the oxidation states of these two model catalysts are much different, the calculations clearly predict that O2 adsorbs exothermically only for molecules that bond to dual sites; that is, O2 binds simultaneously to a cationic site in the support and to the Au cluster. Molina et al. [525,554] reported a similar observations for Au rods on MgO and TiO2. These authors further hypothesized that the Au/oxide interface is the only active site for O2 adsorption, activation, and subsequent CO oxidation. The role of these dual binding sites in the activation of adsorbing molecules [68,525,554,555] is examined further in subsequent sections of this review. 7.2. Oxygen

On stoichiometric TiO2, two conformers of the Au7 cluster were explored (Fig. 7.5d, e); again, CO was found to bind strongly on the top of the Au cluster. On hydroxylated TiO2, CO binding was strong for Au atoms far from the support, Fig. 7.5f, while weaker CO bonding was realized for Au atoms coordinated directly to the support, Fig. 7.5g, h. When the TiO2 support was further oxidized, Fig. 7.5i, j, the CO adsorption energy remained significant on a selected Au site that neighbored the oxidized Au at the interface. Fig. 7.5k, l shows the CO bonding configurations for Au1 and Au3 clusters on reduced TiO2 with Obr vacancies [551]. In the work highlighted in Fig. 7.5, calculated CO vibrational stretch frequencies revealed details about the electronic structure of the Au clusters. For Au7 clusters on the most oxidized TiO2 support, the calculated CO vibrational energy was relatively high, 2167–2174 cm  1. Such high frequency modes are usually attributed to the electron deficiency of the adsorption site caused by its surroundings. The electron deficiency prevents the electron donation to the CO 2π* antibonding orbital that would weaken the C–O bond. For a Au7 cluster on less oxidized supports (Fig. 7.5c–f), the CO vibrational mode appears at lower frequencies, 2089– 2112 cm  1. In these cases, electron donation to CO becomes possible because of the more electron-rich environment. The

As discussed in Section 4.2.2, oxygen does not readily chemisorb on gold single crystals [353,556,557], either dissociatively or molecularly. However, production of atomic oxygen at metallic Au sites has been implicated as a key mechanistic step for the oxidation of CO, olefins, and alcohols and amines [558]. For gold nanoparticles supported on reducible oxides like TiO2, it has been suggested that the activation step proceeds on the support, which then provides active oxygen to the gold cluster [36]. However, the exact mechanism for O2 activation remains unresolved. 7.2.1. Au/TiO2 planar model catalysts The adsorption of oxygen on 2D and 3D gold islands on TiO2(110) has served as a standard model system for exploring the interactions of oxygen with Au particles [413,522,559]. Vapor deposition of gold onto the titania surface produces single-atom thick 2D islands at low coverage, while thicker 3D islands develop at higher Au loadings [413]. Following sample preparation and characterization, the Au/TiO2(110) model systems were then exposed to atomic oxygen produced on a hot filament. TPD showed evidence for recombinative desorption of O2, which exhibited a maximum at 741 K for oxygen desorption from 2-D particles on TiO2(110), while the TPD

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Fig. 7.5. Calculated adsorption potential energies and vibrational frequencies (in cm  1) for CO on Au7 supported by rutile TiO2(110) in various different oxidation states, (a–j); CO properties on Au1 and Au3 on reduced TiO2, (k, l); O2 adsorption potential energies on Au7, (m, p). Negative potential energies reflect exothermic adsorption reactions. Reprinted with permission from Ref. [551]. Copyright 2007 Springer Science+Business Media, LLC. Table 7.1 Experimental and theoretical stretch frequency of CO on various Au/TiO2 systems. Adapted with permission from Ref. [551]. Copyright 2007 Springer Science+Business Media, LLC. Species

Experimental (cm  1)

Theoretical (cm  1)

Au–/TiO2–CO

2096, 2104, 2108 [528]

Au0/TiO2–CO Au þ /TiO2–CO

2113–2105 [68,552] 2119 [69,89] 2112 [70,499] 2168, 2176 [64,547]

Au1: Au3: Au7: Au7:

Ti4 þ –CO Ti3 þ –CO

2209, 2192 [71,553] 2131 [71,553]

2097 2098 2091 2089–2112

Au7: 2167–2174 Au7: 2149, 2187 2265 –

maximum for thicker particles was shifted to lower temperature, i.e. 545 K. This result suggests that thin islands of Au bind atomic oxygen much more strongly than do large Au islands. These studies concluded that ultrathin gold particles should provide a lower activation barrier for dissociative adsorption of O2 than thick gold particles, as explained by the potential energy diagram shown in Fig. 7.6, and this difference is evidenced in the reverse process of recombinative desorption. The potential energy diagram in Fig. 7.6 depicts the activation energy for desorption of 2Oa from 2D Au islands 1 (EThin larger than that for desorption from des ) as
34 kJ mol Thick thick Au islands (Edes ). This suggests that 2Oa on 2D Au islands must be more than 34 kJ mol  1 more stable than 2Oa on thick islands. Hence, ultrathin gold particles are expected to dissociatively adsorb O2 more readily than large gold particles. Thus, the higher catalytic activity of thin (small) Au particles on TiO2 for the CO oxidation reaction is likely related to the influence of strong atomic oxygen binding [522]. Unfortunately, efforts to study the reactivity of molecular O2 under high O2 pressure (250 mbar) conditions were unsuccessful, probably because low-level background impurities rapidly removed the adsorbed oxygen species [522].

Interestingly, fairly recent work has revealed that exposure of 2 ML Au/TiO2(110) model catalyst to a plasma of oxygen at 77 K results in a surface that contains both atomic Oa and molecular O2,a chemisorbed species [560]. Using isotopically labeled O2 and CO, this work focused on the reactivity of atomically and molecularly adsorbed oxygen with CO on the Au/TiO2(110) model system [561]. To produce surfaces populated with Oa species only, the O2,a species were cleared from the sample by employing collision-induced desorption with high kinetic-energy beams of Kr or Ar. It was observed that the population of a sample with both Oa and O2,a chemisorbed species results in consistently larger production of CO2 than in the case when the sample contained only Oa species. This work was extended in a study in which the reactivity of molecularly adsorbed oxygen on five different Au/TiO2(110) model systems having 0.5, 0.75, 1, 1.25, and 2 ML Au coverage was examined [562]. In this work, the samples were produced by vapor deposition of gold and were equivalently exposed to oxygen generated by the plasma-jet. The results from this work demonstrated that the greatest yield for CO2 production occurred for the sample that contained 1 ML-equivalent of Au. However, uncertainties in the concentration of molecularly chemisorbed oxygen precluded any unambiguous conclusions to be made regarding the role of gold particle size on the CO oxidation chemistry by adsorbed molecular oxygen [562]. Spurred by the efficacy of supported Au NPs as an oxidation catalyst, scientists from several labs over the globe combined their efforts to study the reactivity of O2 with Au NPs [563]. In a key study, in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was utilized to study the reactivity of model Au/TiO2(110) towards molecular O2 at pressures up to 1 Torr [564]. As established by others, [522] molecular oxygen was shown to interact only weakly with Au surfaces at room temperature. However, these authors found that molecular oxygen could be activated on both bulk Au and TiO2(110)– supported Au NPs by X-ray irradiation to generate oxidic Au at room temperature. Production of chemisorbed oxygen species induced by X-ray irradiation has been reported by

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Fig. 7.6. Potential energy diagram for the interaction of oxygen with thin (2D, - - -) oEThick . Reprinted with permission and thick (3D, —) Au islands on TiO2:EThin a a from Ref. [522]. Copyright 1999 Springer Science+Business Media, LLC.

other groups, however at much lower temperature of O2 exposure (below 30 K) [557,565]. The oxidic Au formed on the Au/TiO2(110) surface was found to be unstable due to the likely facile reducibility of the TiO2 support exposed to X-ray irradiation [566] and the possible spillover of oxygen from Au to TiO2 [511]. The authors concluded that the widely proposed gold-only activation mechanism of oxygen is unlikely under the reaction conditions explored in this work. The XPS results discussed above, which are based on observations of a Au 4f core-level shift due to the formation of oxidic Au under XPS irradiation of a model Au/TiO2(110) surface [563], conflict to some degree with prior data obtained with Au single crystal [557] and real nanoparticulate Au/TiO2 systems [511,567]. Recently, researchers have studied a model Au/TiO2(110) system in an attempt to reconcile these previous conflicting XPS data. In this work, the vapor deposited Au nanoparticles were in two size ranges:
2–2.5 nm (0.14–0.2 ML Au) and
3.3 nm (0.4 ML Au). The influence of oxygen (O2) and carbon monoxide (CO) on the Au 4f photoelectron spectrum of particles was studied by in situ (high pressure) XPS at 300 K, for O2 and/or CO pressures from 0.1 to 1 mbar. The key results from this study are shown in Fig. 7.7. As shown in Fig. 7.7, a new Au 4f component appeared at 2.4–2.6 eV higher binding energy (relative to the Au(0) bulk signal) was observed when O2 pressure was increased over the Au/ TiO2(110) sample. This photoelectron signal was observed with all particle sizes and was attributed to oxidized gold

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species induced by X-ray radiation in the presence of oxygen. The pressure-dependent activation of oxygen was most efficient for the
2–2.5 nm Au particles, which helps to reconcile the previously conflicting XPS data [563]. In addition, this work showed evidence that the oxygen species produced on the Au/TiO2(110) sample with
2 nm Au particles are the most active for oxidation of CO. Recently, ambient-pressure X-ray photoelectron spectroscopy (APXPS) was applied to study changes in charge state of Au nanoparticles supported on a rutile TiO2(110) singlecrystal surface at elevated-pressure [569]. This work demonstrated that the net charge on Au nanoparticles does not change significantly upon exposure to CO or O2. However, a reversible band-bending effect was observed in the Au/TiO2 system, attributed to reversible adsorption and charge transfer between O2 molecules and TiO2(110). Charge transfer at a surface can be revealed in XP spectra because the electron binding energy (BE) is affected by the electronic configuration of the surface species. Donation of electrons results in a shift to a higher apparent BE, whereas acceptance of electrons causes a shift to a lower BE relative to the neutral species. The correct assignment of the origin of such a peak shift requires the use of a proper reference that is unaltered during the experiment. In this work, the authors used a specially designed sample (Fig. 7.8a) to establish a proper reference system for the binding energy measurements. In their arrangement, the TiO2(110) substrate had two types of Au coverage: the first section (1) was created with a 4 mm Au particles and was tied to ground potential; the second area (2) contained an array of 200 μm-diameter electrically floating Au islands. The Au 4f spectra shown in Fig. 7.8b were recorded for both areas in different gas atmospheres. These spectra do not show a shift in the Au 4f energy for the electronically floating Au (Area 2) relative to grounded Au (Area 1). The coincidence of these spectra demonstrated to the authors that the Au islands in Area 2 do not accumulate charge during XPS measurements. Apparently, the conductivity of the TiO2 support is sufficient to neutralize any charge generated from the photoelectron emission. This approach was then used to calibrate the binding-energy scale based on the Au 4f7/2 peak position of grounded Au. As was discussed above, Au can be oxidized by extended exposure to X-rays at high oxygen pressures; thus, special care was taken to assure the chemical stability of the reference material. Finally, the authors measured the chemical shifts of the core levels of the Ti and O atoms, the adsorbates, the gas molecules, and the nanoscale Au in a Au/TiO2 sample grown by thermal evaporation. The data shown in Fig. 7.9 reveal that the peaks stemming from TiO2: Ti 2p, Ti 3p, and O 1s, change in concert and by the same amount upon O2 exposure, while the peaks associated with the supported Au nanoparticles remain unchanged. Analysis of these data suggests that the reversible changes in peak position for titania are due to band bending caused by O2 adsorption and charge transfer to the TiO2 substrate. Importantly, the results of this work help to clarify the origin of peak shifts for Au nanoparticles and contribute to the fundamental understanding

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Fig. 7.7. In situ XP spectra of the Au 4f region of 0.2 ML Au (
2.5 nm Au clusters) on TiO2(110). (a) As-prepared Au particles on TiO2(110) at 300 K in UHV; (b) XPS in 0.1 mbar O2, 1 h total exposure to X-rays; (c) XPS in 1 mbar O2, 10 min exposure to X-rays; (d) XPS in 1 mbar O2, 1 h exposure to X-rays; (e) after (d), but without X-rays for 30 min at 1 mbar O2; (f) UHV XPS measurement after exposure to X-rays for 30 min in UHV; (g) UHV XPS measurement after 1 h in UHV. Reprinted with permission from Ref. [568]. Copyright 2012 Elsevier B.V.

of the nature of adsorbed O2 and O–2 species in supplying reactive oxygen for surface-catalyzed reactions. 7.2.2. Au/TiO2 high surface area catalysts The formation of chemisorbed oxygen, surface oxides, or subsurface oxygen with nanoparticulate Au/TiO2 catalysts under realistic reaction conditions has also been a subject of intense research. Although the mechanisms of oxidation catalysis will be discussed in subsequent sections, the nature of surface oxygen within nanoparticulate systems (the topic of the current section) is closely tied to how it reacts with other species, like CO. For example, a series of important studies have employed isotopic labeling and CO oxidation rates to test

the stability of surface oxygen on TiO2 [89]. In that work, Fourier-transform infrared spectroscopy (FTIR), electron spin resonance (ESR), and temperature-programmed desorption (TPD) were employed to study a Au/Ti(OH)4 catalyst that was prepared by supporting a Au–phosphine complex on asprecipitated wet titanium hydroxide followed by calcination at 673 K. The gold loading was
3 wt% [89]. At room temperature, the Au/Ti(OH)4* was found to be inactive for oxygen isotopic exchange either between gas phase O2 molecules or between gaseous oxygen and lattice oxygen atoms. Interestingly, lattice oxygen atoms of the Au/Ti(OH)4* were active in oxygen exchange with the product CO2 but not with gas phase CO. In this work, the oxygen was thought to adsorb at vacancies on the TiO2 surface in the form of superoxide O2 . The O2 species that formed close to Au particles were hypothesized to react with CO adsorbed at the Au particle surface. This finding somewhat contradicted previous work, which suggested that oxygen is irreversibly adsorbed at the Au-TiO2 interface by dissociation and forming a OC–Au–O intermediate [475,570]. The role of oxygen vacancies for stabilization of O2 radicals was explored in an EPR study, where gold nanoparticles were supported on the reducible oxide, TiO2, and on a non-reducible oxide, Al2O3 [571]. Surprisingly, it was found that the generation of superoxide O–2 did not require a reducible support. At room temperature, both catalysts stabilized O–2 radicals at cation sites. Because the Au/Al2O3 system was found to catalyze CO oxidation, one can conclude that electron transfer to O2 occurs from the Au particle itself. In contrast, charge transfer in the Au/TiO2 system may also occur (with lower activation energy) through Ti ions due to an electron transfer from Au to TiO2, which is suggested to be stimulated by coadsorption of CO [571]. Key insight into this system has also been provided by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) studies, which probed the relationship between Au–Au bond length and reactivity toward oxygen on various oxide supports [572]. In that work, several methods were applied for the preparation of Au catalysts supported on silica, alumina, titania, zirconia, ceria, and niobia at different loadings (dispersion) and all catalysts were reduced (at 423–523 K) before the EXAFS measurements. This work demonstrated that Au–Au bond length within the supported particles decreased as the size of Au particles decreased. Such contraction of the Au–Au bond was similar to that previously reported for unsupported Au particles and did not appear to depend on the type of oxide support [573–575]. Further, this work showed that while large particles were inert to oxygen, small particles (o3 nm) were reactive to air and about 10% of the Au atoms could be oxidized [572]. The observation that O2 could be activated on Au-based catalysts supported on reducible, e.g. TiO2 [572] and on nonreducible, e.g. Al2O3 [576], oxides, suggests that both the gas-phase oxygen as well as the support are critical in providing active oxygen for the gold cluster [380,475,577]. Moreover, DFT calculations predicted that both the Au particles and the support oxide might activate oxygen

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molecules [50,578]. Strong experimental evidence suggests that gold nanoparticles supported on a reducible oxide (TiO2) can form activated gold-oxygen complexes [579]. In addition,

Fig. 7.8. (a) Sketch of the fabricated sample of gold evaporated on top of a rutile TiO2(110) single crystal (sample 1) (see text for details). (b) XPS spectra of Au in the two sample areas for different gas exposures at 1 Torr total pressure. Reprinted with permission from Ref. [569]. Copyright 2011 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

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studies have suggested that fully reduced gold particles may be responsible for the activation of molecular oxygen. In that work, Au/TiO2 catalysts were prepared via depositionprecipitation with urea at two different nominal gold loadings, i.e. 3.2 wt% and 1.0 wt%. The authors employed X-ray absorption near-edge structure (XANES) spectroscopy to track signal from the Au L3-edge, which is due dipole-allowed, intra-atomic electronic transitions from a 2p3/2 core level to unoccupied d-states in the Au valence region [579]. The Au L3 XANES spectra for compounds containing cationic gold, e.g. AuCl3 as shown in Fig. 7.10, exhibit an intense near-edge resonance (“white line”) that arises from unoccupied d-states. Spectrum b in Fig. 7.10 shows that the L3-near-edge (in He) of a reduced 4 wt % Au/TiO2 catalyst resembles the spectrum of Au foil (spectrum a), indicating that the gold resides in metallic clusters. No presence of cationic gold could be identified in the reduced Au/TiO2 sample. Subsequent exposure of this reduced sample to O2 (spectrum c) resulted in an increased intensity in the d-band region (see the difference spectrum, c–a). This change is an indication that the interaction of Au particles with adsorbed oxygen decreases the d-band occupancy of Au. This decrease was even more pronounced for the 1 wt % catalyst, implying (as one would expect) that the fraction of Au atoms interacting with oxygen was larger for the sample with the higher dispersion [579]. These observations suggested to the authors that gold supported on the reducible oxide, TiO2, can form an activated Au–O complex, similar to that observed for Au on a nonreducible support [576]. The role of the support, i.e. reducible versus nonreducible, and the size (height) of Au particles for formation of oxidized gold species were also explored by other researchers [511,580– 582]. In these studies, the authors employed diblock copolymer micelles to synthesize supported Au systems (Au/TiO2, Au/SiO2 and Au/Al2O3) with well-controlled size and dispersion of Au nanoparticles (see Section 6.2). As expected from the generally accepted understanding of the role Au particle size [266,384,464], the support and the particle size were

Fig. 7.9. (a) Series of O 1s and Ti 2p spectra measured on TiO2(110) (sample 2), and, (b) O 1s and Au 4f XPS spectra measured on Au/TiO2(110) (sample 3) for different gas and pressure conditions. The spectra acquired in UHV (black lines) are the first in the series. Reprinted with permission from Ref. [569]. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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found to mediate the reactivity of oxygen [580]. For Au particles in the size range of 1–6 nm, the activation of oxygen was strongly related to electrocatalytic oxidation of CO. The most active Au particle sizes for this reaction were reported to have a diameter of 1.5 nm, which appeared to present the largest fraction of oxygen at the particle surface, as measured by the Au3 þ /Au0 X-ray photoemission intensities. Importantly, the stabilization of Au3 þ oxide on the semiconducting TiO2 oxide was found to be greater than on insulating oxides like SiO2 [580]. These authors further studied the size and support effects on the formation and thermal stability of oxidized gold. They investigated the oxygen reactivity toward gold nanoparticles that had the same two size distributions (average sizes of
1.5 and
5 nm), but were deposited on different supports: reducible TiO2 and nonreducible SiO2 [511]. Exposure of the AuNPs to atomic oxygen at lowtemperatures (150 K) produced surface as well as subsurface gold oxide, Au2O3. Based on XPS measurements, they suggested that the thickness of the Au2O3 shell formed on AuNPs depends on the size of the NPs. On large 5 nm particles, the thickness of the Au oxide shell was 0.79 7 0.02 nm for Au/SiO2 and 0.83 7 0.02 nm for TiO2, while for small 1.5 nm AuNPs it was 0.38 7 0.02 nm and 0.31 7 0.01 nm, respectively. The size effect on the thermal decomposition of the Au2O3 shell was discussed in the context of both geometrical and electronic effect models, previously proposed for understanding reduction rates of metal nanocatalysts [583,584]. Specifically, geometrical effects that depend on particle size influence the types of sites that are available for adsorption and size-dependent changes in the electronic structure of small particles lead to electronic effects. Thus, geometrical effects are thought to dominate for large AuNPs (
5 nm), whereas electronic effects may control the reduction of small clusters (
1.5 nm); these influences were used to explain the size-dependent trend observed for the Au/TiO2 system. At temperatures up to 300 K, the extent of decomposition of the Au2O3 shell on large particles was
50 % at
310 K; in contrast, the same level of decomposition was

Fig. 7.10. Au L3 XANES of 4 wt% Au/TiO2 catalyst exposed to O2 and He; also shown are Au3 þ (AuCl3) and Au metal references. Reprinted with permission from Ref. [579]. Copyright 2007 American Chemical Society.

achieved at just
265 K for the smaller particles. For the Au/SiO2 system, the Au2O3 shell on smaller AuNPs displayed a slightly higher thermal stability at To 450 K, i.e. 50% decay was measured at 430 K as compared to a similar decay at 410 K for the large particles. At T¼ 550 K, the Au2O3 shell was nearly completely decomposed on the small particles, while it was still present (
25%) on the large particles. For bulk Au systems (see Section 4.2.2), the reported temperatures for decomposition of Au2O3 were in the range 360–450 K [585,586]. It should be noted, however, that a more stable form of gold oxide was reported to exist on Au(111) even at 1073 K [336]. As these results suggest, the size-dependent stability of Au2O3 is different for similarly sized Au clusters deposited on reducible TiO2 and nonreducible SiO2 supports. To further clarify these effects, the role of oxygen vacancies on titania in the overall chemistry was studied [511]. Although the specific role of vacancies in the thermal decomposition of Au2O3 was not elucidated in this study, the results suggested that the higher stability of Au2O3 on large AuNPs is due to a larger energy barrier that atomic oxygen experiences when passing through the Au|| TiO2 interface. That is, it was suggested that oxygen diffusion is the rate-limiting step for Au2O3 decomposition. Fig. 7.11 illustrates the model proposed by the authors for the thermal decomposition of Au2O3 on size-selected Au NPs supported on SiO2 and TiO2. Three main processes are suggested to occur on both systems during Au2O3 shell decomposition: (1) direct desorption of atomic oxygen; (2) recombinative production and desorption of molecular oxygen; and (3) diffusion of subsurface oxygen to the surface within the Au2O3 shell. In the case of Au/TiO2, one additional decomposition pathway was considered, i.e. (4) oxygen atom spillover from the Au2O3 shell to the TiO2 substrate, to replenish O vacancies on the slightly reduced oxide. These scientists [511] attempted to determine the preexponential factor and the activation energy for the decomposition of the Au2O3 shell based on Arrhenius-type experiments using TPD; however, these results were criticized in the literature [587]. The authors subsequently clarified that their published results were obtained for a desorption order of n ¼ 1, which lead to a desorption energy of Edes ¼ 1.0 7 0.1 eV for only the small (
1.5 nm) NPs [582]. Notwithstanding, the microscopic complexity of the desorption process, as illustrated in Fig. 7.11, likely necessitates reexamination of the TPD procedure and analysis for a more reliable determination of activation parameters for O2 desorption [582,587]. Despite the challenges associated with these types of studies, accurate measurements of the overall energetics and mechanisms of oxygen desorption remain of critical importance. The presence of surface and subsurface oxides on Au has been repeatedly confirmed by studies such as those that employ CO(a) as a probe of surface oxidation state [511]. Moreover, the role of the reducible oxide support was found to be critical for the catalytic oxidation of CO and other molecules. For example, the catalytic activity of Au nanoparticles supported on TiO2 was significantly higher than that of NPs supported on SiO2 [588]. As described above, the activation of oxygen at supported Au nanoparticles includes formation of active surface and

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subsurface oxygen species. Motivated by this fact, the role of the oxide has been explored in depth for the Au/TiO2 system [589,590]. Using temporal analysis of products (TAP) measurements, the authors first focused their study on two questions: (i) whether reactive oxygen species can be stored in Au/TiO2 at a temperatures typically used for CO oxidation (353 K), and (ii) whether the reversibly stored (removed) oxygen species are also the active species in the oxidation reaction [589]. In addition, these scientists focused on the location of the active oxygen at the catalyst surface [590]. Fig. 7.12 illustrates some of the important results of their study. The upper two panels of Fig. 7.12a show that, in concert with the loss of CO, the formation of CO2 proceeds during a typical pulse of CO gas, despite the fact that no O2 was present in the gas phase. This indicated that CO reacts with oxygen species that are already present on the surface of the Au/TiO2. The reactive oxygen species found to be restored during a subsequent re-exposure to O2(g). Importantly, they found that the amount of reactive oxygen decreased with the gold particles size, i.e. the perimeter length, as shown in Fig. 7.12b. These experiments represent one of many demonstrations that reactive oxygen can be repeatedly re-produced on the surface of Au/TiO2 and that this oxygen is critical for CO oxidation [589]. Worth mentioning is the fact that thermal activation has been shown to increase the amount of active oxygen depleted during reduction, but not the absolute level of active oxygen present on the surface after reoxidation by O2

Fig. 7.11. Schematic model illustrating different mechanisms for Au2O3 decomposition on large and small NPs supported on SiO2 and TiO2. Four processes are depicted: (1) direct desorption of atomic oxygen; (2) recombination of atomic oxygen and desorption as molecular oxygen; (3) segregation of subsurface oxygen to the NPs surface; and (4) atomic oxygen from the NP shell spills over to the TiO2 substrate and replenishes oxygen vacancies created on TiO2 upon sample annealing. Reprinted with permission from Ref. [511]. Copyright 2008 American Chemical Society.

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pulses. These data were interpreted in terms of creating oxygen vacancies, i.e., removing active oxygen from surface lattice sites along the Au||TiO2 interface perimeter; upon exposure to O2 pulses these oxygen vacancies can subsequently be replenished [590]. These data definitely contradict the above-discussed data from Iwasawa group that lattice oxygen does not contribute to the CO oxidation reaction on hydroxylated Au/Ti(OH) 4* catalyst [89]. We will discuss this discrepancy concerning more details of the mechanism of CO oxidation in Section 8.2.1. 7.3. Hydrogen Although the adsorption and activation of H2 on gold nanoparticles have attracted much less attention than CO and O2 activation, a deep understanding for the fundamental chemistry of hydrogen on Au/TiO2 could lead to important practical applications such as hydrogenation of organics and other transformations [591–593]. As with the other molecules, the unanswered scientific questions about H2 activation include: Does the size of Au nanoparticles control the reactivity of surface hydrogen? What is the role of the oxide–support? What the role of the particle-support interface in driving the chemistry? Furthermore, other important issues such as whether H2 dissociation is hemolytic or heterolytic remain unresolved. Notwithstanding, the original d-band model description of H2 binding to Au presented by Hammer and Norskov accounts for many of the experimental observations. As discussed in Section 3.2, theoretical calculations showed that the reactivity of small unsupported Au clusters towards H2 depends not only on the cluster size but also on the charge state of the cluster [131,161]. Adsorption of H onto oddnumbered clusters Aun (n o 13) was more favorable than that onto even-numbered clusters. The transfer of an electron into the LUMO of Au is important, and thus an energy match of the Aun-LUMO and H-1s levels is a requirement for optimal adsorption [131]. Molecular hydrogen easily binds to neutral Au2 and Au3 clusters with binding energies of 0.55 eV and 0.71 eV, respectively. Although negatively charged Au2 clusters do not bind molecular hydrogen, H2 dissociation can occur with energy barriers of 0.93 eV for Au2 and 1.39 eV for Au3 [161]. On model planar systems, hydrogen is proposed to dissociate primarily at Au sites around the perimeter of the particles [594]. However, others suggest that H2 dissociation occurs at low coordinated (LC) metallic Au sites remote from the Au||TiO2 interface [595–597]. Like the experimental studies, theoretical predictions show evidence for both H2 dissociation at boundary sites [598,599] and at LC Au sites away from the interface [600]. 7.3.1. Au/TiO2 planar model catalysts The H2–D2 exchange reaction has been employed to study the role of Au particle size in H2 dissociation on a model Au/ TiO2(110) system [594]. In this work, the authors employed a cathodic ark plasma (CAP) deposition method (see

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Fig. 7.12. (a) Upper panels: Signals for CO and CO2 obtained during the first sequence of CO/Ar pulses [CO(1)] over the Au/TiO2 catalyst at 353 K directly after pretreatment (at 673 K). Lower panel: O2 signal obtained during the first sequences of O2/Ar pulses [O2(1)] at 353 K over the Au/TiO2 catalyst after reduction by CO/Ar pulses. Data from the second sequence of CO/Ar and O2/Ar pulses that were present in the original Fig. 1 of Ref. [590] are not shown here. (b) Oxygen storage capacity of the Au/TiO2 catalysts in multi-pulse experiments at 353 K against the estimated length of the Au–TiO2 interface perimeter. The catalysts are denoted as C400–C700, where the number corresponds to the temperature of calcination (in Celsius) in air for 2 h (before the pulse experiments) to obtain AuNPs with different mean particle diameter Adapted with permission rom Ref. [589]. Copyright 2009 Elsevier B.V.

Section 6.2.1) to obtain Au/TiO2(110) with tunable Au particle size – but having Au-loading limited to one monolayer equivalent (MLE). The mean Au particle diameter for these samples was controllably varied between 1 and 10 nm, and their activity in H2–D2 exchange was followed by the rate of HD formation in the temperature range 300–500 K. Separate experiments conducted with Au(111) and Au(311), as well as with TiO2(110) single-crystal surfaces, showed that neither of these surfaces dissociate hydrogen. However, the Au/ TiO2(110) surface does dissociate H2 and the rate of HD production (via a dissociative reaction path) exhibits strong dependence on the size of Au particles, as shown in Fig. 7.13a The calculated initial rate of HD formation at 425 K markedly increased for gold particles smaller than 2 nm, although all the samples have the same loading (Fig. 7.13b, left side scale). Gold particles of diameter 1.3 nm produced 30 times more HD molecules as compared to gold particles with d 4 5.5 nm (Fig. 7.13b). However, the normalization of HD production rate to the total number of Au atoms at the perimeter of gold particles, i.e. the estimated turnover frequencies (TOFs) for each sample, exhibited a surprising result: the TOFs are almost independent of the mean diameter of the Au particles (Fig. 7.13b, right side

scale). Arrhenius plots from the kinetic data showed an apparent activation energy of
36.5 kJ mol  1 for the H2–D2 exchange reaction on the samples, independent of gold particle size. These results strongly suggest that the active sites for H2 dissociation are Au atoms at the gold particle–titania interface. As the fraction of “free” edge and corner Au sites (void of direct support contact) is strongly dependent on the mean diameter of Au particles [37,266], the independence of TOFs on Au particle diameter suggests that the low coordinated gold atoms (away from the perimeter) are not involved in the dissociation of H2 [594]. The identification of perimeter Au sites as the active sites for the H2–D2 exchange reaction was confirmed by further studies of the inversed system TiO2/Au (111) with (ΘTi ¼ 0.6) [594]. The estimated activation energy of the reaction was practically the same (36.2 kJ mol  1) as that measured for the Au/TiO2(110) system. In combination with XPS results and an argument based on DF calculations [594], the authors hypothesized that the H2 dissociation is not activated by the gold atoms alone, but instead by dual sites where interfacial gold atoms and oxygen atoms from the TiO2 play a synergistic role in the chemistry. The perimeter sites hypothesis is further supported by recent DFT theoretical contributions. Such studies [598,601] have

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found that adsorption of H2 on Aun (n¼ 1, 2, 8, 20) strongly depends on cluster size, geometry, flexibility, and support interactions. The active sites for H2 dissociation were identified to be located at gold nanoparticle corners and edges located near the support. Further, periodic DFT calculations have been employed to study the dissociation of molecular hydrogen at perimeter sites and the spillover of hydrogen atoms from the gold to the support [599]. In their model, the authors used a gold strip cleaved from an Au(111) surface and a Rutile (110) support modeled by three-layer slabs and 4  2 surface super cells. Their results suggested that molecular hydrogen can dissociate at the perimeter sites and that heterolytic dissociation is favored. In accordance with previous experimental data [597], it was found that the reactive oxygen from TiO2(110) at the Au||TiO2 interface can be readily passivated by newlyformed OH groups, suggesting that further dissociation of molecular hydrogen may occur only at pure gold sites. In contrast to the studies discussed above, a series of related investigations [596,600,602] suggest that gold sites in direct contact with the TiO2 support are not active for H2 dissociation. In these studies, the sites for H2 adsorption and activation were explored for a gold nanoparticle containing 13 atoms. Three different isomers of Au13 containing one (1L), two (2L), and three (3L) atomic layers were considered. The anatase polymorph (001) surface was chosen as the TiO2 support and represented by a (4  4) supercell slab model containing three TiO2 layers. The reactivity towards H2 was studied for Au particles of different shapes while supported on stoichiometric and reduced TiO2 surfaces. On stoichiometric TiO2, Au particles were found to obtain a positive charge; in contrast, they are negatively charged on reduced titania. However, the charge of Au was found to be irrelevant to the activation of H2. The most important findings suggest that active Au sites most likely have a net charge near zero, are located at low coordinated positions, and are bonded directly to the support.

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These interesting hypotheses have been tested in many subsequent studies, some of which are described below for nanoparticulates. 7.3.2. Au/TiO2 high surface area catalysts Despite the importance of H2 activation chemistry in hydrogenation reactions over real catalysts [591,592,600,603], there have been only a limited number of studies into this system [549,595,600,603]. Studies have shown completely reversible hydrogen adsorption on low temperature (473 K) reduced 2.3 wt % Au/TiO2, while no detectable adsorption was observed in these studies on nonreduced Au/TiO2, as studied by H2 adsorption isotherms [603]. The uptake of H2 on a reduced sample was greater at 473 К than at 300 К, indicating that this process is activated. Infrared spectroscopy has also been employed in the study of H2 activation on both oxidized and reduced 3 wt % Au/TiO2 at room temperature. [549] Spectroscopic evidence was reported that H2 dissociatively adsorbs at room temperature at Au/TiO2. The spectral signature for this hydrogen production arose from the formation of OH groups following dissociation and reaction with adsorbed oxygen atoms to produce surface OH groups, or from spillover where they can reduce the support. Reduction creates Ti3 þ shallow donor levels in TiO2, which cause a continual increase in the background absorbance of the sample. Details for the hydrogen spillover phenomena can be found in a recent review [604]. Other groups have used IR spectroscopic studies of trapped and conduction band electrons to study adsorption of H2 molecules on nanoparticulate Au/TiO2. In these experiments, gold particles with mean diameter
2.7 nm were deposited on Degussa P25 TiO2 by the DP method [595,605]. Hydrogen interacting with Au/TiO2 was found to produce a broad largely monotonic IR absorbance from 4000 to 1000 cm  1, while no such spectral changes were observed with a pure TiO2 sample. These observations showed that Au nanoparticles were responsible for the background changes within the Au/TiO2 sample.

Fig. 7.13. H2–D2 exchange reaction on 1 ML Au/TiO2(110) surfaces having different mean diameters of gold particles. The reaction is performed in a batch mode by using a mixture of 6 Torr H2 and 6 Torr D2 at 425 K. (a) Time course of HD production Ref. [594] SI. (b) The initial rate of HD formation for each catalyst sample and the turnover frequencies based on the length of the perimeter interface as a function of the mean diameter of gold particles. Adapted with permission from Ref. [594]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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As shown in Fig. 7.14A, the rise in the IR background, as measured by the IR absorbance at 1740 cm  1, started immediately after the introduction of H2, terminated after evacuation of H2, and then slowly declined under dynamic vacuum. The predominant rise in the background absorbance was ascribed to the interaction of IR photons with free (delocalized) CB electrons and trapped (localized) electrons at shallow donor levels in the semiconductor [595,605]. Thus, the observed IR electronic signal was directly proportional to the number of electrons donated to TiO2 via atomic H addition and TiO2 reduction. Measurements of the initial rate of IR signal increase due to electronic excitations (over the range of hydrogen pressures from 0.2 to 20 Torr at 298 K) revealed a ½ hydrogen pressure dependence. This result showed that the H atom flux is a result of hydrogen dissociation and spillover from the Au to the TiO2. The estimated activation energy for H2 dissociation on Au particles was
50 kJ mol  1 [595]. After the spillover to the TiO2 support, a hydrogen atom forms localized Ti–O(H)–Ti species; this species donates an electron to a shallow level in the band gap of TiO2, which results in ndoping of the TiO2 support. As shown in Fig. 7.14B, the admission of H2 to an oxidized Au/TiO2 sample caused, in addition to the background rise (Fig. 7.14A), formation of surface hydroxyls for some short initial period. This formation of surface OH was irreversible (remained during evacuation), but the population of OH groups appeared to saturate upon repeated exposure to H2. In contrast, the rise in the broad IR absorbance related to delocalized and localized electrons continued after readmission of hydrogen. This observation suggested that the interaction of hydrogen with active oxygen from TiO2 to produce OH groups is not directly related to the H atom n-doping of TiO2. These observation suggest that hydrogen dissociation occurs, not at dual perimeter sites (i.e., sites requiring simultaneous interactions with a Au atom and an O atom from TiO2 at the Au||TiO2 interface) [594], but at sites on the surface of the Au particles. In an effort to help identify the Au sites where H2 activation and dissociation occurs, CO has been used to as a spectroscopic marker to characterize the surface. As described throughout this review, the vibrational frequency of CO is sensitive to the charge and coordination number of the site to which it is bound [600]. Therefore, in these studies a number of Au/TiO2 samples with similar gold particles sizes were prepared by DP method and subsequently characterized by CO (a) IR spectroscopy. The nature of different gold sites was elucidated with the help of DFT calculations and the IR bands for CO(a) were assigned to particular gold sites. The IR bands between 2070 and 2110 cm  1 were attributed to CO adsorbed at low coordinated neutral Au0 sites, bands at
2120 cm  1 were assigned to CO interactions with moderately positively charged Auδ þ atoms within Au–O–Ti linkages, and the IR band that appeared at 2135 cm  1 was assigned to CO weakly bound at positively charged Auδ þ gold atoms interacting with O adatoms. The concentration of these sites was found to depend on the overall morphology of Au particles. Following sample characterization, the rate of H2/D2 exchange was measured for several different samples and the rate of exchange was correlated with

Fig. 7.14. Comparison of the kinetics of production of CBE and Ti-OH groups on Au/TiO2 at 295 K. Data from Reprinted with permission from Ref. [595]. Copyright 2007 American Chemical Society.

concentration of particular Au surface sites. The data from this study are re-presented in Fig. 7.15. Fig. 7.15a shows that the rate of H/D exchange and the number of Au0 gold sites (CO bands at 2077–2110 cm  1) are strongly correlated. However, as seen in Fig. 7.15b, there is no or only weak correlation with the concentration Auδ þ gold sites that exist at the Au||TiO2 interface (as identified by bands at 2125 and 2135 cm  1). Further, the H/D exchange rate was found to be the highest for the Au/TiO2 sample with the largest concentration of LC neutral Au0 sites (Figure 15a) and a negligible amount of Auδ þ gold sites (Fig. 7.15b). These observations suggest that gold atoms at Au–O–Ti linkages are not responsible for the dissociation of H2. Another important conclusion from these studies was that the morphology of Au NPs and the position of the gold atoms strongly affected the activity of the supported gold catalysts towards H2 dissociation, independent of Au particle size. In agreement with the results discussed in the preceding paragraph [600], others [597] have employed spectroscopic probes to provide evidence that LC gold sites are involved in the dissociative adsorption of H2 molecules on Au/TiO2 nanoparticles. In these experiments, the authors first adsorbed CO as a spectroscopic probe of the nature of the Au surface sites, then H2 gas was admitted and both the extent of TiO2 reduction by hydrogen spillover and the character of surface Au were tracked with infrared spectroscopy. The data from this experiment are presented in Fig. 7.16.

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The IR spectrum of CO adsorbed on
2.7 nm Au particles of an oxidized Au/TiO2 sample, which exhibited two primary types of active Au sites, is shown in Fig. 7.16 panel B and is highlighted by the inset shown in panel C. Partially oxidized gold sites, which display the character of a Auδ þ –CO species (giving rise to the ν(CO) mode at 2141–2129 cm  1), likely reside around the perimeter of the Au nanoparticles [547,548,597,606]. The sites for CO adsorption appeared to be dominated by neutral Au0 sites, detected as a Au0–CO species with a broad spectral feature from 2126 to 2112 cm  1, located at low coordinated positions on the gold particles as steps, edges, corners etc.. Immediately upon the introduction of H2 to the CO-covered particles, a broad band at 3290 cm  1 due to the formation of OH groups developed (Fig. 7.16A) indicating that atomic hydrogen migrated to react with a small concentration of oxygen located around the perimeter of the Au particles. As this active oxygen (from TiO2) at the periphery of Au particles was depleted during reactions with hydrogen, a featureless infrared signal emerged across the entire spectrum (Fig. 7.16A, B). As the preceding discussion of Ref. [595], this spectral response of the TiO2 support is due to the sequential processes of H2 dissociation on the Au, spillover of H atoms to the TiO2, and hydrogenic ndoping of TiO2. Concomitant with the removal of interfacial oxygen (due to OH formation), the frequency of both ν(CO) modes red-shifted, Fig. 7.16C. The intensity and wavenumber change of both CO modes as a function of the increase in the signal due to population of shallow trapped states are shown in Fig. 7.16, panels D and E. As charge is accumulated in Au/ TiO2, the frequency of ν(CO(a)) at both surface sites is found to further shift to the red. The red shift of the highest frequency band, initially at 2129 cm  1, to 2119 cm  1 is ascribed to the conversion of Auδ þ –CO species into Au0–CO. As theoretically predicted by Yang et al. [599], the perimeter oxygen is readily passivated by the formation of OH groups, thus further dissociation of H2 (as observed in this work) most likely occurred at pure gold sites. Indeed, the data from panels D and E in Fig. 7.16 suggest that neutral Au0 atoms at low

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coordinated sites are present throughout the creation of shallow trapped states by hydrogen spillover. These results are consistent with the findings of others who have studied the H2 þ O2 reaction over nanoparticulate Au/ TiO2 [69]. In a combined experimental and theoretical study, they established that the activation barrier for H2 dissociation on Au is substantially lower if O2 is present in the gas phase [69]. In the absence of oxygen, H2 molecules dissociate on Au particles and the produced H atoms migrate to the TiO2 surface where they dissolve into the bulk [595,597]. Further, hydrogen atoms may reduce TiO2, which results in injection of electrons into the CB, as suggested by IR spectroscopic studies. For the anoxic conditions under which these experiments were conducted, DFT calculations predicted dissociation of H2 at low coordinated (CN ¼ 7) Au atoms that exist on the surface of the Au cluster (as opposed to at the Au||TiO2 interface). The calculated activation barrier for H2 dissociation on these low coordinated Au sites was 48 kJ mol  1, which is close to the experimentally obtained 52 kJ mol  1 [595]. This barrier rises to 92 kJ mol  1 for high coordination Au sites (CN ¼ 9). These results are in accord with several predictions and experimental results [595,597,600]. In the presence of preadsorbed O2, however, H2 appears to adsorb at Au perimeter interfacial sites where the H–H dissociation barrier is only 16 kJ mol  1. The effect of interfacial hydrogen stabilization through the formation of a Ti-OOH adsorbate and the fascinating role of this species in the H2 þ O2 reaction on Au/TiO2 will be discussed in Section 8.2.2. Based on the above discussion, the following physical picture emerges for H2 adsorption on the Au/TiO2 surface: hydrogen molecules are activated and dissociate at neutral sites in low coordinated positions on Au nanoparticles [69,595– 597,600]; the produced H atoms migrate and spillover onto the TiO2 support where they protonate and n-dope the material [69,595,597]. As recently reported [607,608], hydrogen dissociation at low coordinated gold atoms can be a ratedetermining step in hydrogenation catalytic reactions on Au/ TiO2 catalysts.

Fig. 7.15. Relationship between the H/D exchange determined on different Au/TiO2 samples and the area of the IR bands at 2110–2077 cm  1 (a) and 2136– 2124 cm  1 (b). Reprinted with permission from Ref. [600]. Copyright 2009 American Chemical Society.

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Fig. 7.16. CO probe of active Au sites for H2 dissociation at 295 K. (A and B) Following equilibration with 10 Torr CO (the violet trace), the Au/TiO2 sample was exposed to 10 Torr of H2, and a series of IR spectra (the gray traces) were recorded to track the changes in the surface during H2 dissociation and n-doping of Au/TiO2. The expanded region of panel B focuses on changes of the CO mode during H2 exposure. (C and D) The changes in the intensity (C) and in the wavenumber (D), respectively, of the ν(CO) modes for CO molecules bonded to perimeter and low coordinated Au sites, as a function of the increase in the electronic absorbance signal (as measured at 1740 cm  1). For clarity, the color-labeled points correspond to the spectra shown in panels A–C. Reprinted with permission from Ref. [597]. Copyright 2011 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

7.4. Water The interactions of H2O(g) with gold nanoparticles supported on reducible oxides, such as TiO2, play an important and perhaps critical (although often overlooked) role in the overall chemistry of this system. Even traces of water (with

partial pressures in the UHV range) can significantly alter the Au/TiO2 chemistry and physics. There are two main aspects associated with the role of surface adsorbed water: first, water can change the stability of deposited gold clusters (their mobility and agglomeration), and second, water can influence (promote or inhibit) specific reaction steps involved in

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catalysis at the gold particle||support interface. The role of water in the formation of gold clusters on the surface of TiO2 was briefly discussed in Section 5.2. In the current section, the surface chemistry of H2O (g) on Au/TiO2 materials are discussed. 7.4.1. Au/TiO2 planar model catalysts Water is known to play a role in the bonding of Au clusters to TiO2 [393] through the hydroxylation of O-vacancy sites [393]. On freshly annealed TiO2(110), O-vacancies are very efficient in dissociating adventitious water, even under UHV conditions. Dissociated water appears to fill the initial O vacancy upon formation of a pair of surface hydroxyl groups. The hydroxylated sites diffuse, by H-atom migration, to adjacent bridging O atoms [385,399,609]. Similar results have been reported by a variety of groups [400,610]. The role of water in agglomeration of gold has been studied for model catalysts prepared by deposition of size-selected Aun on rutile TiO2(110) [611,612]. The highest dispersion for Au was obtained when gold was deposited as Aunþ [611]. However, a STM work has found that gold agglomerates in large particles when deposited under similar conditions (RT deposition as Aunþ ) [613]. The discrepancy between these results appeared to be a consequence of the different time scale of both measuring techniques: the rapid ion-scattering spectroscopy (ISS) measurements as compared to the much slower STM, which allows for larger background water exposures. A combined ISS, TPD and XPS was applied to study the role of Au particles for O-vacancy assisted adsorption and dissociation of water on slightly reduced Au/TiO2(110) and TiO2(110) surfaces [612]. Using H18 2 O, the water TPD study of TiO2(110) showed three main desorption forms of water: desorption from multilayer (i.e., second layer) sites (a TPD peak at
145 K), from two–coordinate O2  (2cO ¼ bridging oxygen) sites (at
160 K) and from five–coordinate Ti (5cTi) sites (at
250 K), in accordance with previous reports [614,615]. The same TPD features with practically identical integrated intensities for the 2cO and 5cTi desorption states were observed with 0.05 ML Aun/TiO2(110). On both samples, no measurable water–surface oxygen exchange was established (up to 300 K) for water bound at 2cO and 5cTi sites, consistent with previous TPD and HREELS results [616]. This implies that water adsorbs molecularly at these sites. An analysis of the much weaker desorption feature appeared between 300 and 600 K showed that at least 90% of water was desorbed as 18 H16 O – 16O exchange during the recombina2 O, suggesting tive desorption of water as previously established for single crystal TiO2.[614,615,617] However, the measured recombinative desorption signal (ISS) for Au/TiO2 was
40% smaller than that for TiO2. As the initial density of O-vacancy sites was identical for both samples, the decreased recombinative desorption of water was attributed to the blocking effect of Au on the initial vacancy sites, presumably by binding there. This blocking effect of Au for O-vacancies was further studied by measuring the ISS Au/total O ratios after a series of annealing steps for 5% ML Au/TiO2(110) and for 5% ML Au/TiO2(110) exposed to water after and before Au deposition, as shown in

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Fig. 7.17a. Note that the Au intensity at each point was normalized to the total O intensity. The TPD experiments[612] and XPS results[611] suggested that only a minimal amount of gold deposited at
120 K on TiO2 was bonded to vacancy sites while the rest was bound to "nondefective" sites. Upon heating to 300 K, mobile Au atoms tended to move to vacancy sites (either empty or preoccupied with Au). Heating at 450 K and above caused a strong decrease in the Au ISS intensity (in all the three cases, see Fig. 7.17a), which was attributed to agglomeration, in agreement with XPS measurements. As seen in Fig. 7.17, the Au: total O ratio in the experiments with water exposure (before or after Au deposition) was significantly lower throughout the annealing/agglomeration process than the result under dry conditions. The explanation was that water exposure promotes the surface diffusion and agglomeration of gold at room temperature, presumably by occupying vacancy sites (through hydroxylation), which would otherwise tend to stabilize Au atoms or small clusters. A variable-temperature STM study helped to directly show a reversible water-induced relocation of gold atoms from an oxygen vacancy on the TiO2(110) surface [401]. In these studies, gold deposition as Au1þ under soft-landing conditions at high temperature, 600 K, resulted in a gold atom-decorated surface, in significant contrast to the large sintered islands of particles formed at 300 K. At 600 K, the isolated Au atoms were found to be deposited directly above the bridging oxygen rows, i.e. bound at 2cO sites. Cooling the substrate from 600 to 300 K resulted in the relocation of Au atoms, and they were bounded directly above 5cTi atoms. When the temperature was returned to 600 K, the Au atoms returned to bind directly over the bridging oxygen rows, indicating that this process is reversible. An STM snapshot of Au atom positions at these two temperatures is shown in Fig. 7.17b and is consistent with the findings in the discussion associated with Fig. 7.17a. The change in the Au binding site between 2cO and 5cTi was attributed to the ability of an adsorbing water molecule to compete with a deposited Au atom for the binding site. DFT calculations predicted that water adsorbs dissociatively at an oxygen vacancy site occupied by an Au atom, displacing the Au atom, and forming a stable OH–Au–TiO2 complex [401] on the surface. Beyond the influence of adsorbed water on Au cluster formation, water has also been shown to affect the adsorption of oxygen (and hence overall catalytic activity) on the surface of Au/TiO2, a topic of a recent review [618]. In the case of CO oxidation on Au/TiO2, moisture in the feed gas has been shown to have two effects: it activates O2 molecules and it decomposes a carbonate species [619]. Motivated by results such as these, TPD under UHV conditions has been employed to study the effect of moisture on CO oxidation at Au/ TiO2(110) model catalysts [620]. The interaction of H18 2 O with 16Oa precovered Au/TiO2(110) surface (0.5 ML Au, dAu E2–4 nm) showed formation of C16O18O and C16O16O. The amount of produced C16O18O increased with increasing water coverages; however, the total amount of produced CO2 decreased. The formation of C16O18O indicated that the

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Fig. 7.17. (a) ISS Au/total O ratios after a series of annealing steps for 5% ML Au/TiO2(110) and 5% ML Au/TiO2(110) with water exposure after and before Au deposition. Reprinted with permission from Ref. [612]. Copyright 2008 American Chemical Society. (b) Variable-temperature scanning tunneling microscopy (STM) snapshot of gold deposited as Au1þ on the TiO2(110)-(1  1) surface showing the reversible relocation of isolated Au atoms while temperature was changed between 600 and 300 K. Reprinted with permission from Ref. [401]. Copyright 2010 American Chemical Society.

oxygen in H18 2 O is activating the CO oxidation. The absence of C18O18O or C18O products implied that the C–O bond in carbon monoxide was not broken in the oxidation process. The results of this study clearly showed that water adsorption might affect the adsorption of both O2 and CO and as well the oxidation of CO in multiple complex ways. The complexity of water chemistry on the model Au/TiO2(110) catalysts has been recently studied and reported for CO oxidation. At reaction temperatures of 300 K, the amount of produced CO2 on Au/TiO2(110) was found to increase with increasing the H2O pressure up to 0.1 Torr, but then decrease at H2O pressure of 0.5 Torr [621]. In contrast, at 400 K, CO oxidation proceeded without addition of H2O, and the rate of CO2 formation was invariant on the water partial pressure. These results implied that water has a positive effect on the activation of oxygen for CO oxidation only at low temperatures, while at higher temperatures oxygen can be activated directly on the catalyst surface without the presence of H2O in the gas phase. Further details on the binding of water at Au||TiO2 interface sites under reaction conditions during CO oxidation will be discussed in Section 8.2.1. 7.4.2. Au/TiO2 high surface area catalysts The effect of water, positive or negative, on the efficiency of supported gold nanoparticulate catalysts has been extensively studied [33,552,622]. On Au/Ti(OH)4 [89] and Au/TiO2 [552], water was found to poison the surface, perhaps by occupying anion vacancies (i.e. hydrating the surface), and preventing oxygen adsorption [622]. How water adsorption affects catalytic activity in these systems has been progressively better understood by studies of the well defined model systems where the role of water for Au particle/support interactions, O2 activation, and CO uptake have been examined (see the previous section). Most of the early studies on real systems were qualitative, often exploring preactivated catalyst surfaces with unspecified water coverages [33]. In some of these studies, humidity was found to enhance the reaction rate by more than 10 times at concentrations up to 200 ppm of water vapor for CO oxidation on
1 wt % Au/TiO2 and Au particle mean diameter
3 nm [623]. Surprisingly, the apparent activation energy was independent of the

amount of moisture in the feedstock [619]. These studies helped to establish that the rate of low-temperature CO oxidation is most significantly affected by the amount of adsorbed moisture on the catalyst surface rather than by the water content in the gas phase. This implied that the oxidation of CO over Au/TiO2 was influenced by some water-derived species on the catalyst surface [619], as previously proposed [33]. At water concentrations of 0.1 ppm, such water-derived species could be surface OH groups that may activate O2 molecules or modify the electronic state of the gold atoms exposed at the surface [619,623]. Analyzing the results of a detailed study on water effects in CO oxidation at Au/TiO2, Au/Al2O3 and Au/SiO2 catalysts, researchers arrived at the conclusion that water plays two major roles in the reaction: (1) the activation of oxygen, and (2) the facilitated decomposition of carbonate. A mechanistic model for water action was also proposed (see Section 8.2.1) [619]. As previously described, for Au single crystal surfaces (Section 4.2.4) and for model Au/TiO2(110) systems (Section 7.4.1), there is an emerging understanding that water reacts with coadsorbed oxygen to form a pair of surface-bound hydroxyl groups. Surface hydroxyl groups that are formed at the perimeter of Au nanoparticles may play a major role in the mechanism of catalytic processes such as CO oxidation and other reactions on supported Au catalysts. The details of these mechanisms will be further discussed in Section 8.2. 7.5. Nitrogen oxides Nitrogen oxides exhibit rich chemistry on both single crystal and nanoparticulate gold surfaces. The adsorption and interaction of nitrogen oxides on single crystal gold surfaces have been studied both experimentally and theoretically and discussion on this topic can be found in Section 4.2.5. Surprisingly, there are few known reports on the adsorption of nitrogen oxides on model Au/TiO2(single crystal) surfaces. Therefore, to prepare the reader for the discussion on the adsorption of nitrogen oxides on real systems, some of the key findings uncovered in studies of single crystal gold surface will be revisited. On both Au(100) [325] and A(111) [53] surf-

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aces, the adsorption of NO is weak and NO desorbs at 170–230 K from the former surface and at 95 K from the latter. The co-adsorption of NO with atomic or molecular oxygen has also been studied, because NO oxidation during the NOx storage process occurs under oxygen rich conditions. The adsorption of NO on Au(111) precovered with atomic oxygen leads to the formation of surface nitrogen dioxide, NO2, at Ts ¼ 85–200 K with a very low activation energy
20 kJ mol  1 [322]. At temperatures higher than 200 K, the short lifetime of adsorbed NO limits the production of NO2, Theoretical calculations [323] supported these results by showing that the reaction of NO with oxygen adatoms proceed with a low activation barrier (
20 kJ mol  1) on Au(111) and suggest that the reaction occurs at defect sites. However, subsequent calculations [624] found the adsorption of NO on defect-free Au(111) favorable via the N atom of NO at on-top sites. In those studies, the oxidation of NO by oxygen adatoms was reported to occur without an activation barrier, but the overall reaction was limited by the energy required for NO2 desorption (75 kJ mol  1. The interaction with molecular oxygen proceeded via single– (ONOO*) or double–bonded (ON*OO*) intermediates with slight energetic differences. On a stepped Au(321) surface, DFT calculations found that O adatoms bounded at hollow sites nearby the steps interact with NO adsorbed at on-top sites [625]. When co-adsorbed with O2, NO forms an ONOO* stable intermediate at sites near the steps that is converted into strongly adsorbed (Eads ¼  1.10 eV) NO2 product. On stepped Au(310), NO was found to adsorb dissociatively to form N2O at temperatures as low as 80 K. A TPD study showed that NO desorbs from this surface at about 120 K while the product N2O desorbs at slightly higher temperature, 150 K [305]. Calculations indicated that NO adsorption at undercoordinated Au sites on zigzag stepped Au(321) could also be dissociative, but it requires the presence of atomic hydrogen [334]. Further discussion of NO interactions with co-adsorbed atoms and molecules can be found in Section 4.2.5. 7.5.1. Au/TiO2 planar model catalysts As mentioned above, there are few-to-no known reports of NO adsorption and reaction on model Au/TiO2 (single crystal) systems. Therefore, there are some opportunities for new research in this area. Interestingly, studies of pure Au single crystals have revealed reactivity toward NO. Therefore, it is not surprising that several experiments have reported that nanoparticulate catalysts composed of Au/TiO2 exhibit significant activity toward NO. Several of those studies are reviewed below. 7.5.2. Au/TiO2 high surface area catalysts Studies of NO adsorption on real nanoparticulate Au/TiO2 are complex because NO not only reacts on the Au sites, but researchers have established that NO readily adsorbs on the TiO2 support, giving rise to a variety of NO-derived surface species, as observed by IR spectroscopy [626,627]. Specifically, scientists have identified chemisorbed NO (nitrosyl groups) and N2O (on reduced Ti3 þ surface sites) on TiO2 surfaces [626]. Nitrites and nitrates were also observed after

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long exposures to NO. Others [627] established that adsorbed NO on TiO2 (Degussa P-25) disproportionates to give NO  (1170 cm  1) and N2O22  (1335 cm  1) surface species, as well as nitrates (1650–1550 and 1240–1220 cm  1). In those studies, co-adsorption of NO and O2 led to significant production of nitrate related species and NO þ (2206 cm  1). In addition, nitro-complexes (1520 and 1284 cm  1) and nitrosyls (1950–1800 cm  1) were observed, along with a multiple of other types of adsorbates [628–630]. Beyond these studies of adsorption and decomposition, researchers have applied ambient pressure photoelectron spectroscopy (APPES) to the study of nitric oxide and molecular oxygen interactions on gold nanoparticles deposited on TiO2 and SiO2 [631]. In those studies, both gold-based model catalysts consisted of mono-dispersed gold nanoparticles with different diameters (2–5 nm) and polycrystalline silica and titania nanoporous thin films as oxide supports. APPES is an in situ technique [563,588] that provides information (XPS spectra) about chemical transformations that occur on a catalyst surface under reaction conditions. Fig. 7.18 presents the XPS spectra of mono-dispersed (4 nm) Au nanoparticles supported on a nanoporous TiO2 thin film under exposure to 0 (UHV) – 470 mTorr NO at room temperature [631]. The XPS spectra under UHV conditions were obtained following sample annealing to 383 K to remove weakly bound species from the surface. Adsorption of NO on this clean surface caused a broadening in the peaks: from 0.99 eV FWHM in UHV to 1.24 eV under 470 mTorr of NO (Fig. 7.18a). The two spectral features that appear in the O1s region under UHV (Fig. 7.18b) were assigned to the lattice oxygen in TiO2 (peak at 531.0 eV), and surface hydroxyl groups (peak at 532.6). The source of hydroxyl groups was attributed to background vapors of H2O (inevitably present even under UHV conditions) that dissociate at the TiO2 surface [632]. Both O1s peaks were found to shift during NO exposure. The binding energy shift in the O1s region was attributed to band bending in the TiO2, as previously found for oxygen adsorption on Au/TiO2 [569] (see Section 7.2.1). Researchers propose that the work function of TiO2 increases due to formation of surface O2 , as shown by Eqs. (7.1) and (7.2) for the cases of O2 and NO: O2 þ Ti3 þ -Ti4 þ þ O2

ð7:1Þ

2NO þ 2Ti3 þ -2Ti4 þ þ O2 þ N2 O

ð7:2Þ

In the case of NO adsorption, the rise in the work function of TiO2 was 1.6 eV, which was much higher than that (0.7 eV) observed for O2 adsorption under the same conditions. This difference is attributed to the stronger oxidizing character of NO (stronger electron affinity). On oxidized TiO2, NO can form charged NO and nitrate species that also cause a rise in the work function of the substrate. The two supports, TiO2 and SiO2, were found to behave differently towards the adsorption of NO as revealed by comparison of the N 1s core level XPS spectra shown in Fig. 17. Upon exposure to 0.5 Torr of NO, adsorbed NO and several N-containing species were observed on the Au/TiO2 catalysts, while no changes were detected on

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the Au/SiO2 samples under the same conditions. On the Au/ TiO2 sample, surface species including atomic N (399.3 eV) and NO3 (405.1 eV) were identified (Fig. 7.18c). In addition, NO dimers or dinitrosyl species (401.9 eV) were observed – probably intermediates from NO transformation into N2O. As discussed above, these species were described by Debeila and co-workers for nanoparticulate Au/TiO2 samples exposed to NO at room temperature [630]. Exposure of bare TiO2 to nitric oxide under the same conditions resulted in an identical N 1s XPS spectrum as that observed for Au/TiO2. This result helped to confirm that NO chemisorbs only on the TiO2 support. It was also established that the band bending effect of adsorbed NO was reversible and is related mostly to the adsorbed species containing oxygen and nitrogen, while atomic nitrogen has a minor effect. This section examined the adsorption and interaction of nitrogen oxides on nanoparticulate Au/TiO2 that produce a wide number of surface species primarily adsorbed on the TiO2 support. The formation of NO-derived surface species may play important role in the catalytic reduction of NOx by reducing agents such as CO, H2, hydrocarbons, etc. We will discuss this topic in Section 8.2.6. 7.6. Alcohols Studies of the adsorption and chemical transformations of simple alcohols, like methanol and ethanol, at titania-supported nanoparticulate metals (including Au) have fundamental importance for developing a full understanding of the mechanisms of catalysis and photocatalysis. From a practical point of view, methanol and ethanol may serve as renewable energy carriers for generating hydrogen, as well as renewable sources for specialty chemicals. A wide variety of Au/TiO2-based systems have been examined as potential catalysts for chemical and photochemical transformations of alcohols. This section focuses

on a few representative studies targeting the adsorption and thermal decomposition of methanol and ethanol. Titania, either as a bulk single-crystal, crystalline film, or powder, has been shown to adsorb alcohols molecularly and dissociatively – the latter depending on the nature of the TiO2 polymorph, its stoichiometry and hydroxylation, particle size, etc. On flat Au(110) [316] and Au(111) [346] single crystal surfaces, methanol has been found to adsorb molecularly, and may decompose via O–H bond scission on stepped Au(310) surfaces [348]. Reactions of alcohols on supported Au clusters are likely driven by the high activity of undercoodinated Au atoms for O–H and C–H bond scission, as observed for methanol on Au single-crystal surfaces when precovered with oxygen (see Section 4.2) [316,346–348,633]. As discussed above for other particulate-adsorbate types of interactions, there is also compelling evidence that the Au–titania interfacial regions around the periphery of particles play a critically important role in the overall chemistry. 7.6.1. Au/TiO2 planar model catalysts Experimental and theoretical evidence has shown that both Au and TiO2 components within model catalysts likely participate in the adsorption and subsequent decomposition of alcohols. As described in the following sections, the chemistry of the independent components of the catalysts is discussed, which provides a foundation for understanding the composite materials. 7.6.1.1. TiO2 single crystal surfaces. Methanol adsorption on single crystal TiO2 surfaces has been the subject of fairly extensive experimental and theoretical work [344]. Results from TPD work involving methanol adsorption on anatase (101) [634] and rutile (110) [635,636] were found to be qualitatively very similar. Methanol appears to adsorb on TiO2 surfaces both molecularly and dissociatively, dependent on the properties of the surface. It was suggested that CH3OH

Fig. 7.18. (a) Au 4f XPS spectra of 4-nm mono-dispersed Au nanoparticles supported on nanoporous TiO2 in UHV and under various pressures of NO. (b) Corresponding O1s XPS spectra. The binding energy scale was calibrated using the Au 4f7/2 peak (84.0 eV). (c) N1s XPS spectra of the bare TiO2 thin film and that with deposited 4-nm Au nanoparticles in the presence of 240 mTorr NO. All the spectra were acquired at room temperature. Reprinted with permission from Ref. [631]. Copyright 2011 Elsevier B.V.

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dissociates at oxygen-atom vacancies on reduced TiO2 surfaces [635–638]. Some indirect evidence for methanol dissociation at Ti5c sites was found in 2PP spectroscopic studies [639]. As methanol approaches the surface of rutile TiO2(110), the molecules may simply reflect from the substrate, bind momentarily through dispersion interactions, or find high energy surface sites such as oxygen vacancies. Direct evidence for dissociation at such vacancies has been provided by STM work conducted at low methanol exposures [637]. In those experiments, bright spots in atomically resolved images, assigned to Obr-vacancies, were found to convert to features assigned methoxy and OHbr pairs upon methanol exposure [609,637]. As with any other surface, this study pointed to the importance of defects in driving adsorption, a concept that motivates other work into adsorption on more intentionallydefective materials. For example, on a defective (electron beam irradiated) surface, a significant fraction of methoxy groups (28–36%) were found to react to form methane. It was suggested that methane formation from methanol is favored at defects, because the thermodynamic driving force for healing such high energy sites effectively pulls oxygen atoms from trapped methanol [638]. Interestingly, when O2 was deliberately pre-dosed onto the UHV annealed rutile TiO2(110) surface prior to methanol exposure, a small fraction (
15 %) of formaldehyde evolved at elevated surface temperatures together with molecular methanol [638]. Beyond experimental studies, the complexity of surface dynamics under methanol adsorption on single crystal anatase (101) [640] and rutile (110) [639,641] has been demonstrated in theoretical work. Molecular dynamics simulations showed that on stoichiometric anatase (101), molecular adsorption of methanol is always favored, even though dissociation becomes increasingly more favorable at high coverages [640]. Of course, model systems demonstrated that a surface characterized by oxygen vacancies shifted the energy balance in favor of the dissociated state, whereas a hydroxylated surface seemed to lead to an even competition between molecular and dissociative adsorption. Interestingly, methanol reactions and interactions may induce states close to the top of the O-2p valence band, which are suggested to play a role in the ability of methanol to serve as a hole scavenger in photocatalysis [640]. On the rutile (110) surface, static DFT calculations combined with molecular dynamics simulations demonstrated that both molecular adsorption and dissociation at Ti5c sites are possible [641,642]. Bridging oxygen atoms that can be easily removed to produce vacancies were found to act as preferential adsorption sites [641]. Most importantly, in-plane oxygen atoms (considered previously have a minor role) were found to be required for the dissociation of methanol, since they appear to be sites that play a role in stabilizing intermediates [642]. In this mechanism, a methanol molecule first dissociates, transferring a hydrogen atom to an in-plane oxygen to form a hydroxyl group (OHin), and next, the hydrogen is transferred to the bridging oxygen. However, it must be noted that dissociation to form a hydroxyl group may be nearly isoenergetic with simple molecular adsorption [642].

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Another DFT-based study found that the relative stabilities of different structures of adsorbed methanol on TiO2(110) depend on several factors such as the extent of chemisorptioninduced charge transfer, the relative strength of different types of hydrogen bonds, and the steric hindrance between a methyl group and the surface. Perhaps most important to the chemisorption of CH3OH on TiO2 is the adsorbate-substrate charge transfer. Upon formation of a Ti-O bond, some degree of charge transfer occurs from a Ti5c atom to the O atom of methanol or methoxy. Thus, the net positive charge on Ti5c and negative charge on O of the adsorbate give the Ti-O bond partially ionic character [639]. 7.6.1.2. Au single crystal surfaces. Methanol has been found to adsorb molecularly on clean single crystal Au(100) [316] and Au(111) [346] surfaces, whereas on stepped surfaces as Au(310) methanol decomposes via O–H bond scission, leading to formation of surface methoxy and hydroxyl adsorbates [348]. The interesting aspects of methanol uptake on Au are discussed Section 4.2.7 and will not be repeated here. However, that section provides a review of key background information that can be used, along with the following discussion, to learn about the differences between chemistry on particulate composites and on planar gold surfaces. 7.6.1.3. Au/TiO2 planar model systems. Recently, methanol reactions at model Au/TiO2(110) surfaces have been explored in detail by a number of groups [345,643]. These investigations have provided valuable insight into understanding the chemistry of alcohols at Au–titania surfaces and have demonstrated the synergy between the Au and TiO2 components of the catalysts for the oxidation of methanol to formaldehyde and other products. In one such study, gold was vapor-deposited at room temperature on a slightly reduced (briefly heated above 950 K) TiO2(110)-(1  1) surface at varying Au coverage between 0.25 and 5 ML [345]. An STM study confirmed that the number of accessible and active interfacial metal–metal oxide sites is inversely proportional to Au coverage. For example, the Au clusters for the 0.25 ML sample exhibited the smallest diameters and the largest population of Au–titania interfacial sites. At 2 ML Au loading, larger clusters were formed due to Au particle coalescence, and consequently this led to decreased number of perimeter sites. For 5 ML loading, the cluster coalescence was even more pronounced. In fact, the surface resembled a Au film rather than dispersed clusters. In addition, gold-free TiO2(110) was also studied under the same conditions. Using isotopic labeling experiments (i.e., CD3OH) it was found that the reaction of methanol with Au/TiO2 occurs via O–H bond rupture to form a methoxy intermediate bonded to TiO2 regions of the sample. Indeed, XPS measurements indicated that the titania surface was slightly oxidized following methanol exposure in the presence of Au clusters. Following exposure, TPD studies of methanol on Au/TiO2 were conducted under UHV conditions and showed that both Au and titania sites are necessary for methanol oxidation to formaldehyde; no formaldehyde production was found during TPD of methanol

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adsorbed on the pure TiO2 surface. Detailed TPD experiments showed low temperature desorption of H2 (245 K) and water (275 K) from the Au/TiO2 surface. Therefore, Au sites were found to facilitate H-removal and prevent hydrogen–methoxy group recombination to form methanol. This allows methoxy adsorbates to undergo C–H bond scission to produce formaldehyde and methyl in the temperature range 450–600 K. Thus, a general scheme of the reaction mechanism was proposed by which the role of TiO2 is to dissociate the O–H bond and to form a reactive methoxy intermediate, while Au sites participate by facilitating surface hydrogen elimination through the formation of H2, which prevents the recombination with methanol fragments. As shown in Fig. 7.19, the yields for the main products, formaldehyde and methyl, were found to depend on Au coverage. The production of both formaldehyde and methyl remained near their maxima for Au loadings between 0.25 and 1 ML, suggesting that the Au||TiO2 interfacial region likely constitute the active sites for methanol transformations. Upon heating the 0.25 ML Au/TiO2 sample, Au clusters grow (observed by STM), which correlated with a decrease in the yield of formaldehyde (the blue squares). This is explained by the loss of sites around the Au particle perimeter due to sintering. In contrast, the yield for thermally treated 5 ML Au clusters increases with temperature, presumably because annealing of this film opens the Au||TiO2 interface. Beyond studies that focus on adsorption and desorption, the thermal- and photo-catalysis of alcohols on Au/TiO2(110) has been studied by a large number of researchers. The thermal activation of methanol and the role of surface oxygen in the reaction mechanism are discussed in Section 8.2.5. 7.6.2. Au/TiO2 high surface area catalysts 7.6.2.1. TiO2 nanoparticles 7.6.2.1.1. Methanol. As the simplest alcohol, methanol adsorption on the surface of TiO2 nanoparticles provides a logical starting point for developing an understanding for reactions involving more complex OH-containing molecules. In an early study, researchers showed that the pathways for methanol uptake depend on the crystal modifications of TiO2 [644]. Acidic sites on the anatase surface were suggested to govern molecular adsorption of methanol. In contrast, methanol uptake appears to follow a dissociative pathway to form methoxy groups on rutile [644]. Other studies examined adsorption of methanol on several different types of TiO2 [626,645–649] and established that both molecular and dissociative forms of methanol may co-exist on the surface. The two forms of adsorbed methanol are typically distinguished by their IR absorption frequencies: bands at
2850 and
2950 cm  1 are assigned to the symmetric and asymmetric C–H stretching frequencies of the molecularly adsorbed state, CH3OHa, while bands at
2830 and
2930 cm  1 are assigned to the dissociated state, CH3Oa [646]. As the amount of CH3OHa increases, the bands associated with residual isolated Ti–OH groups (
3730–3670 cm  1) disappear and a new broad band at 3500–2700 cm  1, due to the vibrations of hydrogen–bonded O–H groups, emerges. Molecular adsorption of methanol is also evidenced by a ν(OC) mode at 1050 cm  1,

while pairs of bands at 1440 cm  1 [δ(CH3)] and 1109 cm  1 [ν (OC)]; at 1460 cm  1 [δ(CH3)] and
1060 cm  1 [ν(OC)]; and at 1460 cm  1 [δ(CH3)] and 1020 cm  1 [ν(OC)] signal the existence of mono, double, and triply bonded methoxy species, respectively, that are products of methanol dissociation [649]. For example, the methanol adsorption on clean 4 nm rutile particles has been shown to be affected by both molecular and dissociative pathways that occur primarily at surface OH groups and at Lewis acid sites, respectively [649]. In this study, the Lewis acidity of the coordinatively unsaturated sites were found to decrease (as gauged by the infrared stretching frequency of the probe molecule, CO) with methanol coverage [649]. At low methanol coverage, protons from dissociated CH3OH(a) were suggested to react with residual Ti–OH groups to produce adsorbed H2O; the remaining methoxy

Fig. 7.19. A plot of normalized product yields from methanol reaction as a function of Au coverage for (a) formaldehyde (30 amu) and (b) methyl (15 amu). The red circles represent experiments on unannealed Au clusters, and the blue squares are from reaction on Au clusters annealed to 800 K for 1 min. All values were normalized to the integrated signal for reaction on the unannealed 0.25 ML Au clusters. Yields were determined from the integrated peak intensities at 30 amu in the TPD experiments, and the 15 and 30 amu signals were corrected for cracking contributions from methanol. The error bars shown for the 0.25 ML coverage are the standard deviations from 4 experiments for the unannealed clusters and 2 experiments for the annealed clusters. Reprinted with permission from Ref. [345]. Copyright 2012 Elsevier B.V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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groups CH3O(a) were found to be stable up to 673 K [646,649]. Others studying methanol adsorption on Degussa P25 have found, by IR and TPD measurements, that a variety of decomposition products are likely formed during sample annealing [646]. At temperatures below 673 K, recombined CH3OH, CH3OCH3, and CH2O were found to leave the surface, while CO, CO2 and CH4 were detected at higher temperatures. Similar results have been obtained [650] for methanol adsorbed on TiO2 (P25) in a combined DRIFTS and TPD study. At a full monolayer coverage, methanol was found to mostly adsorb in a dissociative manner, forming methoxy groups associated with the cationic sites, and hydroxyl groups at the anionic sites. The methoxy species were relatively stable up to 523 K, at which point they began to decompose to produce dimethyl ether through a bimolecular surface reaction. As the surface was exhausted of methoxy groups, a deoxygenation pathway dominated the dynamics and production of methane and water (at 603 K in TPD) became prominent pathways, along with the production of CO and hydrogen. As posited in the literature [626], most of the results for methanol adsorption on TiO2 suggest that the difference between anatase and rutile is not connected solely with the crystal structure, but more directly with the resulting morphology of relatively high-surface-area preparations of TiO2 nanoparticles. Recently, scientists observed almost identical spectra for methanol adsorbed on mixed phase Degussa P25 TiO2 (80% anatase (average size 25 nm) and 20% rutile (33nm average diameter), SBET ¼ 50 m2 g  1) and mesoporous 3D networked TiO2 (anatase, SBET ¼ 150 m2 g  1). On both TiO2 samples, methoxy groups were the dominant surface species after saturation with methanol. In addition, the observed positions and intensity ratios for the asymmetric ν(CH3) (a) and the symmetric ν(CH3)(s) modes of corresponding methanol and methoxy surface species were similar for these two samples. Most importantly, spectral similarities were observed for the ν(OC) modes of methoxide groups, in single-, double-, and triple-bond arrangements at Ti sites. These spectral observations support the suggestion [626] that the crystal structure of TiO2 alone does not govern the initial methanol adsorption pathways. 7.6.2.1.2. Ethanol. Like methanol, the adsorption of ethanol on the surface of nanoparticulate TiO2 is significantly dissociative to yield alkoxides and surface hydroxyl groups. A recent study [651] focused effort on employing TPD and IR measurements to compare the reactivity of ethanol on TiO2 and Au/TiO2 in an effort to understand the effect of the goldtitania interface on chemistry. In that work, a nanoparticulate TiO2 anatase sample was prepared by the sol–gel hydrolysis of Ti(IV) isopropoxide. Ethanol TPD on H2-reduced TiO2 showed ethanol desorption in the temperature range 380–700 K, which accounted for only
4 % of the total products. The main reaction product (665 K) was ethylene (with a carbon selectivity of about 70%). The dehydration reaction of ethanol to form ethylene was linked to a combination of surface oxygen defects created prior to adsorption and those formed during TPD. Other minor products of ethoxide interactions were acetaldehyde, butene, and crotonaldehyde, in decreasing

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order of yield. The dehydrogenation reaction channel to acetaldehyde was found to be far less prominent than the dehydration reaction. 7.6.2.2. Au/TiO2 high surface area catalysts. Although the interactions of alcohols on nanoparticulate Au/TiO2 have been studied extensively under various pressures and thermal conditions, fundamental studies of binding sites, surface adsorbate, and evolved gas phase species have been reported by only a few research groups. Some of that work is reviewed below. 7.6.2.2.1. Methanol. IR spectroscopy, combined with quadrupole mass spectrometry, has been used [652] to study the adsorption of methanol on a 3 wt. % Au/TiO2 sample with
4 nm mean diameter Au particles, prepared by the deposition–precipitation (DP) method. In that work, adsorption of methanol on the reduced Au/TiO2 sample at room temperature produced an IR spectrum characteristic of both molecular and dissociated methanol derived species, similar to that previously reported in the literature for pure TiO2 (see the discussion above) [626,653]. Hence, the bands in the region of ν(OC) at 1157, 1128 and 1048 cm  1 were assigned to coordinated methoxy species adsorbed on the support. The band at 1157 cm  1 was tentatively assigned to on-top methoxy species bounded at Ti3 þ sites near an oxygen vacancy on TiO2 and metallic gold. The bands at 1128 cm– 1 and at 1048 cm  1 were attributed to on-top and two fold bridge methoxy species, respectively, bound to surface Ti4 þ sites. By heating sequentially to 403 and 473 K, the bands related to methoxy groups decreased in intensity and weak bands appeared at 2864, 1567, 1380, 1359 cm–1 that were assigned to two-fold bridge formates at Ti4 þ sites of titania. When oxygen and water were added to the gas phase, intense bands assigned to Au-bound formate (2950, 1632 and 1312 cm–1) and formaldehyde (2844, 2731, 1596 cm  1) species were also identified (at 403 K). After heating to 523 K, only trace amounts of adsorbed species were observable. QMS analysis of the gas phase after
8 hours post methanol exposure to the Au/TiO2 surface at 523 K showed the emergence of hydrogen and CO, together with significant amounts of methane, CO2, and traces of formaldehyde; however, no traces of methanol were reported. The results of that work suggest that the methoxy species adsorbed near oxygen vacancies and gold particles are reactive under very mild conditions; these methoxy species interact with oxygen activated at gold sites near the vacancies to produce formate and formaldehyde. Similar results in studies of methanol on nanoparticulate Au/ TiO2 have been reported by others [650]. By exploring the thermal chemistry of methanol on both TiO2 and 1 wt. % Au/ TiO2, the authors obtained comparative information about the decomposition pathways of methanol and the temperature intervals for activation. A decomposition pathway to form methane appeared to be active on both materials; however, the evolution of CO2 and H2 occurred at much lower temperatures (475 K, instead at 603 K) and the pathway leading to ether production seemed to be absent in the presence of Au. The authors related this difference in the behavior of TiO2 and Au/

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TiO2 to the production of a formate species on the surface of the Au component of Au/TiO2. In work complementary to that described in the preceding paragraph, researchers [508] have observed that although the ν (OC) modes for methanol-derived adsorbates appear at nearly identical wavenumbers for methoxy formed on Au  TiO2 and pure TiO2 aerogels, they exhibit significantly different intensities on the two samples. With the assumption of identical absorptivities for the ν(OC) modes, it was hypothesized that the monodentate alkoxy groups were the predominant surface adsorbates on both materials. An intense ν(OC) band at 1158 cm  1 was observed exclusively in the presence of Au particles, which suggested that this mode, in agreement with previous assignments [652], was due to on–top methoxy adsorbates adjacent to the Au||TiO2 interface. Further investigations into this system that employed CO as a probe molecule showed that on both clean Au  TiO2 and TiO2 aerogel surfaces, the adsorption of methanol eliminates Ti(CUS) sites. On the Au  TiO2 sample, CO helped to identify two types of Au sites. The first type of site produced a high frequency band at 2143–2131 cm  1, which was assigned to a Auδ þ CO species. Such Au sites involved in Au  O  Ti linkages have been assumed to form at the metal  support interface around the perimeter of the gold nanoparticles. The second type of Au site was responsible for a low frequency band at 2126– 2119 cm  1, which was assigned to Au0CO. The neutral Au0 sites are assumed to exist at low coordinated positions at steps and edges on the Au nanoparticles. Interestingly, upon methanol/methoxy saturation of the Au/TiO2 sample, the band for Auδ þ CO species vanished, while the band for Au0CO species simply red-shifted. Simultaneously, bands due to carboxylate-carbonate structures developed in the spectra. These spectral changes suggested that perimeter Au sites or the Au  O  Ti linkages were involved in the oxidation of methanol-derived species. As already discussed for the adsorption of a variety of other molecules on Au/TiO2, the perimeter Au sites at the Au||TiO2 interface are largely responsible for the activation and chemical transformations of surface-bound species (See Section 8.1). 7.6.2.2.2. Ethanol. Although the initial stages of ethanol uptake appear to be similar to methanol, the two molecules may ultimately react very differently on the surface of Au/TiO2 nanoparticulate materials. A study by the Idriss group, for example, reports on ethanol adsorption on 8 wt. % Au/TiO2 catalyst, obtained by the deposition–precipitation with urea (DPU) method [651]. Their infrared spectroscopic studies of H2–reduced Au/TiO2  x after exposure to ethanol at 300 K showed clear evidence for both molecular and dissociative adsorption forms of ethanol. The molecular adsorption of ethanol at room temperature was characterized by bands at 1262 cm  1, 1310 cm  1, and 1398 cm  1 due to O–H bending, CH2 wagging, and CH3 symmetric deformation of C2H5OHa species, respectively, formed at TiO2 regions of the sample. The dissociative adsorption of ethanol produced surface ethoxide groups, C2H5O(a), characterized by bands at 1047 cm  1, 1073 cm  1, 1093 cm  1 and 1122 cm  1, as previously observed for pure TiO2 [647,648]. The bands at

1047 cm  1, 1122 cm  1, and 1146 cm  1 were assigned to the C–O stretching vibrations and the bands at 1071 cm  1 and 1093 cm  1 to the C–C stretching of ethoxides. Among C–O stretching bands, the band at 1047 cm  1 was assigned to bidentate type binding, while a band at 1122 cm  1 was assigned to a monodentate ethoxide species. In this particular study [651], the authors indicated that they found no spectroscopic evidence that the presence of Au had any, even very small, effect on ethanol adsorption pathways at room temperature. However, their TPD distributions for the two samples were considerably different. First, reaction product desorption occurred at much lower temperatures for their Au/TiO2 sample compared to that for bare titania. Second, the main desorption product from the ethanol-exposed Au/TiO2 sample was benzene (ca. 60.7% carbon yield), not ethylene as observed for TiO2. Finally, IR bands attributed to adsorbed crotonaldehyde were seen upon flashing the surface temperature to 570 K. As illustrated in Fig. 7.20, the high selectivity of the Au/TiO2 catalyst for benzene formation was rationalized in terms of a mechanism whereby β-aldolisation reactions of acetaldehyde create a surface 2,4-hexadienal intermediate. This intermediate subsequently undergoes C–H bond scission of the methyl group, induced by the Au particles, followed by intramolecular cyclisation and H2O elimination to give benzene. The results from this work suggest that Au/TiO2 nanoparticulate catalysts may be useful in the synthesis of aromatic molecules from aliphatic alcohols. 7.7. Conclusion Although there are several excellent examples of chemistry on Au for many other adsorbates not discussed here, much of the photocatalytic chemistry reviewed in subsequent sections has been explored only for small test molecules like CO and CH3OH. In addition, an adequate understanding of photocatalytic reaction mechanisms requires a sound understanding of thermal catalysis and the results described above provide an adequate context for presentation of the catalytic chemistry of small molecules on Au/TiO2, the main topic of Section 8. 8. Thermal activation of catalytic gold–titania systems As described above, the most noble metal can under certain conditions become catalytically active [27,78,86]. In particular, titania-supported gold nanoparticles exhibit high activity for CO oxidation even at temperatures well below room temperature [86,408]. In fact, supported Au particles have been found to activate a wide range catalytic reactions [55], such as propylene epoxidation [205,214,654–656], propane dehydrogenation [657], water gas shift reaction [549,658–660], NO reduction/ dissociation [630,661], hydrogenation [22,592,603], SO2 dissociation [662], partial oxidation of alcohols [345,663,664], and oxidation of volatile organic compounds [55,660,665,666]. Although a large number of experimental/theoretical studies and review reports have been published on this topic, the nature of the active Au species, its structure, and particular sites remain unclear [55,408]. There is a general agreement that the catalytic

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Fig. 7.20. Schematic of proposed mechanism for benzene formation via a surface 2,4-hexadienal intermediate over Au/TiO2. Reprinted with permission from Ref. [651]. Copyright 2012 Elsevier B.V.

activity of gold is, to a large extent, mediated by the size of Au particles; however, other characteristics such as the particle shape, the character of the support material, the Au–support interface, the method of Au particle deposition, and the pretreatment procedure of the catalyst are also of fundamental importance for catalytic performance of Au/TiO2 systems. Many of these issues have been discussed in a number of recent review and summary articles [55,226,335,660,666]. The emerging understanding from these studies is that catalytic activity is ultimately controlled by the density of states of the Au particles, charge exchange between the particles and the support, and the unique nature of dual catalytic sites where the metal atoms and the oxide atoms participate in synergistic ways. These critical aspects likely depend on one another and are all affected by sample history and preparation conditions. The following section describes the role that some of these effects play in defining the catalytic activity of Au/TiO2 systems. 8.1. Factors for catalytic activity enhancement In wide range of catalytic applications, the activity of finely dispersed metal particles, including that of gold nanoparticles, is controlled by a variety of nanosize-related effects. Early attempts to understand the reaction rate dependence on particle size were made by Taylor [667] (in the 1920s) and Kobosev [668] (in the 1930s). Boreskov et al. (in the 1950s) were the first to complete a systematic investigation of the relationship between particle size and catalytic activity [669]. Since the 1960s, the development of powerful surface science techniques that use single crystal model catalysis allowed scientists to study the nature of processes such as adsorption, desorption and dissociation that control the catalytic transformations at supported metal surfaces. Poppa and co-workers have used model catalysts consisting of metal particles (containing hundreds to thousands of atoms) deposited on substrates like mica to study the role of size and morphology on their catalytic behavior [371,670]. In catalysis, the term “structure insensitivity” was introduced by Boudart for catalytic reactions very insensitive to the degree of dispersion of the supported metal [671]. Che and Bennett, in their review work on the relationship between particle size and catalytic properties of supported metals, have discussed the variation of turnover frequency (TOFs) with particle size and fraction of the atoms exposed [672]. Fig. 8.1 shows these dependencies for a structureinsensitive reaction (curve A) and structure-sensitive reactions (B–D) (adapted from Ref. [673]). At the beginning of our discussion in Section 7 we showed that the reaction of catalytic oxidation of CO on titaniasupported Au nanoparticles exhibit a strong structure

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sensitivity. As shown in Fig. 7.1, the turnover frequencies for the CO catalytic oxidation on Au/TiO2, i.e. the number of CO2 molecules produced by a single Au site for a second reaction time, depends non-monotonically on the size of Au cluster and maximizes for specific cluster sizes in the 2–4 nm range for both the “real” (A) [474] and the “model” (B) [384] systems. The “structure sensitivity” criterion clearly shows that size dependence in catalysis by gold is complex, as in the case of other nanoparticulate supported metal catalysts. However, there is no single theory that can explain all the nanosizerelated phenomena observed. Recently, Strizhak summarized the following 19 principal nanosize effects that have been discussed in the literature for different heterogeneous catalytic reactions occurring on nanometer supported metal catalysts (reproduced from Ref. [674]): (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)

Effect of the total surface area. Effect of increase in the number of surface atoms. Effect of the number of defects. Effect of the number of low-coordination atoms. Catalysis at the boundary (along the perimeter) of the nanoparticle. Effect of the fractality (non-uniformity) of the surface. Effect of the distribution function. Effect of the surface structure. Effect of the form of the nanocrystal. Nanoparticle–liquid phase transition. Effect of the reaction medium. Nanoparticle–support interaction. Quantum-size effect. Surface composition. Effect of impurity atoms. Effect of the nanoparticle charge. Appearance of chemically induced current. Features of the mechanism/kinetics. Dependence of the adsorption/activation energy on the size.

Each of these effects, either alone or in combination, can contribute to explaining the non-monotonic relationship between catalytic activity and particle size. The effects listed as 1–9 are primarily related to geometry and structure, but accurate determination of active centers to account for these effects is challenging from both an experimental and theoretical perspective. The effects of conditions or sample characteristics like the reaction medium, the identity of the support, the surface composition, and the presence or absence of impurity atoms are critical to the formation of the active center, but do not necessarily provide an explanation for catalytic activity [674]. The following discussion focuses on the three nanosize-related effects that have been considered to be the most important for the catalytic activity of Au nanoparticles supported on titania. 8.1.1. Quantum size effects Perhaps not surprisingly, given the discussion in Section 3 and the d-band model discussed in Section 4, quantum size effects [675–677] have been found to dictate many physical (electronic,

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Fig. 8.1. Variation of turn over frequency with particle size and fraction of the atoms exposed for structure insensitive reaction (curve A) and structure sensitive reactions (B–D) as discussed in Ref. [672]. Adapted with permission from Ref. [673]. Copyright 2010 Elsevier B.V.

magnetic, optical etc.) and chemical (adsorption, dissociation, catalytic transformation) properties of nano-sized metal and metal oxide materials. Quantum size effects have also been invoked to explain the unique properties of nano-scale gold catalysts [408,464,678]. As shown in Fig. 7.1, model catalysts that utilize ultra-small Au nanoparticles deposited on a single crystal TiO2 surface exhibit similar behavior for the dependence of CO oxidation on Au particle size as that observed for realistic high surface area Au/TiO2 systems. Hence, the well-defined catalytic surface of model catalysts, when combined with surface sensitive analytical techniques, can provide a direct probe for structure/ function relationships to obtain atomic–level understanding for catalytic activity of gold. In their important paper published in Science in 1998, Valden and Goodman [88] investigated the origin of the high activity for Au particles with diameters of about 3 nm by measuring the STM bias-dependent tunneling current for several Au particle sizes deposited on TiO2(110)– (1  1). The catalytic activity was measured for a CO þ O2 reaction mixture at realistic gas pressures and at a temperature of 350 K. The activity data obtained for various Au coverages in the range of 0.2 to 4.0 ML are presented in Fig. 8.2 (upper right). The band gap determinations of the Au particles versus their size for 2D and 3D clusters are shown in the middle right. Their STM image for 0.25 monolayers (ML) of Au on TiO2(110)–(1  1) is reproduced in Fig. 8.2 (lower left). The signal from Au particles is displayed as bright protrusions that, for this image, exhibit an average diameter of
2.6 nm and height above the substrate of
0.7 nm, which corresponds to two or three atomic layers, as shown schematically in the upper left part in Fig. 8.2 [88,408]. A metal-to-insulator transition as particle size decreases, i.e. a quantum size effect, was observed for particles with a diameter below 3.5 nm and a height of 1.0 nm (
300 atoms per particle). One atom thick particles exhibited relatively large band gaps, whereas three or more atomic layer particles possessed metallic properties. Fig. 8.2 (lower right) also demonstrates that the relative population of particles in the 0.2–0.6 V band gap range is composed mainly of Au particles with thicknesses of two atomic layers.

As discussed in Section 7.2.1, Campbell [413] and Madey [407] established that the vapor deposition of Au on TiO2(110) initially forms two-dimensional (2-D) clusters until the exposure reaches critical Au coverage. This critical coverage is strongly related to temperature and defect density on the TiO2 surface. At increasing Au coverages, Au nucleates on top of 2-D islands to form 3-D islands that are thermodynamically favored. Quantum size effects in the behavior of Au/TiO2 catalysts, as inferred from the metal–to–insulator transition, were observed by Haruta and co–workers [679,680] for a model system by STM measurement of the local barrier height (LBH) of Au nanoparticles on a TiO2(110)–(1  2). Measurement of LBH is a powerful tool to study the initial stages of thin metallic film growth [681]. Using STM, spatially resolved maps of the BH, i.e. dI(x,y)/dz images, are obtained by means of a tip–sample distance z modulation technique. LBH is estimated from 0.95(d ln I/dz)2, where I is the tunneling current and z is tip–sample distance [679,681]. Measurements on the clean TiO2(110) surface showed that the LBH values at (1  1) and (1  2) rows were very similar, while defect sites exhibited much smaller values. It was inferred from these results that the topmost oxygen atoms define the LBH on TiO2. When Au particles were present on the TiO2 surface, the LBH was dependent on the height of Au particles. As shown in Fig. 8.3a, the relative LBH (ΔLBH) values for the large Au particles was about þ 0.3 eV. As the height of the particles decreased below 0.4 nm, the ΔLBH decreased to zero with height and the ΔLBH of the small particles was distributed from þ 0.2 to  0.4 eV. Fig. 8.3b shows the energy gap dependence on size, as obtained by current image tunneling spectroscopy (CITS). These measurements indicated that at the critical height of 0.4 nm, a metal–to–nonmetal transition occurred. This height was very similar to that at which the ΔLBH started to decrease. These results imply that the LBH of the Au particles correlates with the metal–to–nonmetal transition, i.e. the quantum size effect. Upon analysis of these results, researchers suggested that a negatively charged region was generated in metallic Au particles (4 0.4 nm in height), and this region diminished when the particles became nonmetallic (o 0.4 nm in height) [679,680]. This conclusion was found to be consistent with the fact that the catalytic activity of Au particles for CO oxidation diminishes as the particles transition from fully metallic to partially non-metallic, as observed in the work by Goodman et al. [88]. Importantly, they demonstrated that negatively charged Au was the likely species to react with O2 and CO. Studies by Goodman et al. into the adsorption of CO on Au have provided some insight into quantum size effects [46,517,521]. This reaction, the simple adsorption (and activation) of CO on Au particles, may ultimately determine the rate of CO catalytic oxidation on Au/TiO2 [36]. In one of these studies, Goodman and coworkers [521] assessed the energetics of CO adsorption on Au clusters (diameter 1.8–3.1 nm) supported on TiO2(110) using infrared reflection absorption spectroscopy (IRAS). With decreasing Au particle size, the vibrational frequency of CO adsorbed on Au nanoparticles was

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Fig. 8.2. A model structure of bilayer Au island on TiO2(110) surface, and a STM image of TiO2(110)-(1  1) with Au coverage of 0.25 ML (left). The activity for CO oxidation at 350 K as a function of the Au particle size supported on TiO2(110) assuming total dispersion of the Au. The CO:O2 mixture was 1:5 at a total pressure of 40 Torr. Activity is expressed as (product molecules)  (total Au atoms)  1 s  1; Particle band gap measured by STS as a function of the Au particle size: () two-dimensional (2D) clusters; (◊) 3D clusters, two atom layers in height; (▲) 3D clusters with three atom layers or greater in height; and, relative population of the Au particles (two atom layers in height) that exhibited a band gap of 0.2–0.6 V as measured by STS from Au/TiO2(110) [88,408]. Reprinted with permission from Ref. [408]. Copyright 2006 Elsevier B.V.

found to slightly blue-shift (approximately 4 cm–1) relative to CO on bulk Au, but the heats of adsorption increased dramatically, from 12.5 to 18.3 kcal mol  1 (52.3 to 76.6 kJ mol  1) [517,521]. Importantly, the maximum heat of adsorption appeared to coincide with the particle size for which the catalytic activity reached a maximum [88] and where a metal– to–insulator transition has been observed [88,679]. Therefore, particles that exhibited a distinct band gap (2D clusters) were suggested to be particularly well-suited for catalytic oxidation of CO on Au/TiO2. Interestingly, this result can be compared to work by Campbell and researchers who found that O2 bonds more strongly to 2D Au islands than to 3D islands of Au deposited on TiO2(110) [522]. The higher catalytic activity of thin (2D) Au particles on TiO2 for CO oxidation was related, in that study, to the stronger bonding of atomic oxygen by 2D Au. In both cases, the quantum size effects were suggested to be of critical importance. However, one should note that many of the conclusions highlighted above are not consistent with some recent studies into the adsorption properties of CO on Au islands. Using CO as a probe molecule, Freund and co–workers [682] found that monolayer islands of Au on FeO(111) do not, in fact, seem to behave different than bulk gold toward the adsorption characteristics of CO. They concluded that the exceptional activity of gold nanoparticles in CO oxidation does not arise from quantum size

effects; rather, the Au particle activity was related to highly under–coordinated, thus highly energetic, Au sites. Motivated by somewhat conflicting explanations for the high activity of Au/TiO2, researchers have re-examined the relationship between catalytic activity and the size or shape of Au nanoparticles during CO oxidation [460]. In this study, the Au nanoparticles were vapor deposited at 300 K under UHV and the average thickness of the gold deposit varied between 0.05 and 3 monolayers. The geometry of the Au nanoparticles was monitored in the presence of O2, Ar, or a mixture of O2 þ CO and of Ar þ CO by grazing incidence small-angle X-ray scattering (GISAXS) while measuring the catalytic activity under operational conditions. Gold nanoparticles with diameter in the range of 1-5 nm showed catalytic activity and a well-defined maximum appeared in a narrow range, between 2 and 3 nm, as shown in Fig. 8.4. Thus, the identification of an optimum for the size of Au particles, as first evidenced by Valden et al. [88], was reproduced by others. Interestingly, although the optimum particle sizes were consistent (2.1 7 0.3 nm versus 3–3.5 nm in these two studies), the particulate structures that produced the maximum activity differed. That is, the rate-to-height correlation shown in Fig. 8.4 indicates that maximum activity may occur for small particles that are nearly 6 atomic layers high, as opposed to the originally proposed 2 adlayer thick particles

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Fig. 8.3. Size dependence of (a) the relative LBH and (b) the energy gap of Au particles on the TiO2(110)-(1  2) surface. The ΔLBH was defined as LBH difference between the particles (LBHAu) and the substrate (LBHTiO2): ΔLBH¼LBHAu–LBHTiO2. Reprinted with permission from Ref. [679]. Copyright 2004 Elsevier B.V.

[88]. This finding is in general agreement with the report by Freund et al. [682] and results obtained with a dispersed Au/ TiO2 catalyst [491]. Notwithstanding interpretations to the contrary, the repeated observations [88] of high catalytic activity at particles sizes where a metal-to-nonmetal transition occurs strongly suggest the existence of quantum size effects in the catalytic performance of gold nanoparticles. However, the fundamental nature of this relation has not been completely elucidated. That is, the question remains: how does the quantum size effect render the Au particles catalytically active? The answer to this issue lies in studies that probe the electronic effects on particle size.

8.1.2. Electronic effects In a macroscopic bulk metal sample, the electron-energy spectrum is usually considered to be a continuum; however, descriptions of the electronic properties of particles comprising 10 to 100 atoms exhibit non-metallic or behavior, which presents additional challenges to electronic structure studies

[676,678]. Although tremendous progress has been made toward developing an understanding of the relationship between the geometric, electronic, and catalytic characteristics of very small metal particles, this problem remains a challenge [262,678,683–686]. Generally, the high catalytic activity of small particles has been attributed to the presence of electrondeficient metal surface atoms, i.e. sites with lower electron density compared to that of the bulk metal [687]. Such locally energetic sites naturally present higher reactivity toward incoming molecules. However, the particle size effects are also likely related to changes in the overall electronic structure of the metal. As the d-band model of Hammer and Nørskov proposes (see Section 2 and 4.2), the closer the d orbital energy is to the Fermi level, the stronger the adsorbate-surface bonds become. However, as the size of metal particles becomes smaller than about 3 nm (as in highly reactive particles), the metal–metal bond length tends to decrease, the d-band splitting is affected, and the local density of states at or near the Fermi level is altered. For example, 3-nm two-dimensional Au particles that contain
200 atoms supported on TiO2(110) showed a band gap of 0.2–0.6 eV, and thus were less metallic than bulk gold; these particles also exhibited the greatest turnover rate [88]. Low coordination sites, the position of the d-band center, and the formation of a band gap all may play a role in catalytic activity. Using X-ray absorption near-edge spectroscopy (XANES) and theoretical calculations, van Bokhoven et al. [572,683] studied the effect of particle size on the intrinsic reactivity of small gold particles, as well as the role of the support. X-ray absorption spectroscopy probes the excitation of 2p core electrons in an atom to unoccupied d-like states near the Fermi level. The first feature in LIII-edge spectra is called the whiteline and its intensity is used to measure changes in dstate occupancy. Further, the shape of the whiteline is sensitive to Au–Au bond length and particle shapes. In bulk gold, the hybridization of the s, p, and d orbitals creates holes in the d band, which leads to a transfer of a small amount of 5d electron (and 6s electron) density to the 6p orbitals. This effectively diminishes the d-band electron population, which is reflected in large whiteline for bulk gold. As Au particle size decreases, the intensity of the whiteline decreases due to an increase in the number of d electrons. In agreement with these experimental results, calculations showed that the decrease in Au nanoparticle size changes not only the occupancy of the d band, but also leads to a d band narrowing and shift in energy closer to the Fermi level [688]. These changes render the nanoparticles intrinsically more reactive and catalytically more active than larger particles. The results of this work [683] are consistent with the previously observed high coverages of hydrogen [689] and oxygen [572,576] on supported Au nanoparticles, while single crystals do not chemisorb these molecules [557,689,690]. Likewise, CO binds more strongly on small Au nanoparticles, which correlates with the higher activity of supported Au catalysts in CO oxidation [382]. Interestingly, XANES experiments detected no change in the whiteline intensity, i.e. the number of holes in the d band, for different supports (Al2O3, TiO2, SiO2, Nb2O5, CeO2, and ZrO2).

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strain effect [302]. These studies [138,691,692] help to explain the trends in catalytic activity with particle size that have been observed in a great deal of work cited throughout this review.

Fig. 8.4. Reaction rate as a function of the mean particle diameter (red circles). The labels correspond to the different samples. Blue triangles: variation of the measured mean height (in number of atomic layers) as a function of the diameter. The dashed lines are guide for eyes, and the error bars represent the width of the size distribution. Reprinted with permission from Ref. [460]. Copyright 2011 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Therefore, no charge transfer between the support and the d band of the gold nanoparticles appears to occur. However, these results do not exclude the possibility that dual-sites at the perimeter of particles play a critical role in catalysis. Using size-selected Au clusters “soft landed” on oxide films (MgO), Landman and co–workers examined quantum size effects on Au cluster properties [138,691]. The size of Au clusters was, of course, found to be important - but not the only factor that contributes to the reactivity of model catalysts. Importantly, their measured activity correlated with the electronic structure of the catalyst, specifically, the energetic width and location of the d-band of the model gold catalyst. Their work showed that at least 8 atoms are necessary for the catalytic oxidation of CO, because these particles can form structures stable enough to sustain the catalytic reaction. Visikovskiy et al. [692] carried out a systematic experimental study of the d-band properties of Au nanoclusters grown on α-C amorphous carbon. The glassy carbon has a simple valence band structure and provides only weak bonding of Au; thus, it is an advantageous support to employ in studies of particulate electronic structure. The d-band width (Wd ), d-band center position (Ed ), and apparent 5 d3/2–d5/2 spin-orbit splitting were investigated as a function of the number of Au atoms per cluster (nA) and the average coordination number over a wide range of nA(11onA o1600). These results are presented in Fig. 8.5. These experiments revealed that the width of the d-band and the spin-orbit splitting decreased steeply for nA r150 atoms/ cluster, which corresponds to a "critical diameter" of
2.6 nm. As expected, the d-band position was found closer to the EF than the bulk value; however, the band tended to move away from EF as particle size decreased further. This behavior was attributed to the contraction of the Au–Au bond length with the decrease in clusters size, suggesting the importance of a support-mediated

8.1.3. Low coordination sites in the clusters As alluded to above, many of the important properties of particles that affect catalysis are ultimately due to electronic effects. Clearly, the dependence in the adsorption energy of gas-phase molecules on the active metal site coordination number is related to changes in the electronic structure of surface site to which binding occurs [682]. This is because the high lying metal d-states in low coordinated Au atoms are better positioned for interactions with the valence state of the adsorbate than the low-lying states of highly coordinated atoms on close packed surfaces. In fact, it is generally recognized that atoms in low coordinated positions at surfaces are more reactive than coordinated metal atoms precisely because of their higher lying d-states [29,224,265,693]. Fig. 8.6 shows structures for which the lowest coordination number NC decreases from 9 to 6, starting from a close-packed surface (111), a stepped surface (211), and a surface with both steps and kinks (532). The coordination number is 5 for a “magic size” 55 atom cuboctahedral cluster and 4 for a small, 12 atom, cluster [224]. The primary differences in the reactivity between these geometries was found to be related to the “openness” of the surface, which is an electronic effect: the lowest coordinated metal atoms, which most strongly bind adsorbates, accommodate the highest energy d states. Based on these observations, scientists offered a simple rule: “the lower the metal coordination number, the higher the d-states are in energy, and the stronger they interact with adsorbates” [224]. Further insight into this aspect of adsorption and catalysis is provided by a key study [266] that employed a Wulff-like construction and measured aspect ratios of Au particles on TiO2 to calculate the percent of step sites as a function of Au cluster size. The model showed a clear correlation between decreasing Au coordination number and enhanced adsorbate binding strength, and thus helped to explain the size-dependence of catalytic activity for supported gold particles. Others [523,693,694] continued the examination of size-dependent activity of Au nanoparticles deposited on different support materials. As shown in Fig. 8.7, data summarized from different studies clearly show that the activity depends strongly on Au nanoparticle size for all support materials. Note, the data for Au/TiO2 catalysts show that the activity of clusters ranging in size from 2 nm to 20 nm differs by over a factor of 100; whereas, differences between activities of particles of the same size but different supports are much smaller (no more than a factor of 2–3). The reaction rate data expressed as atomic rates (mol CO/(total mol Au s)) was found to follow a d–3 dependence on the size (d) of Au particles [523] and it was suggested that such dependence applies if only corner-type sites on Au particles play a direct role in the catalysis. The obtained power law, a d–3 dependence of CO oxidation rate on the Au particle size [523], suggests that the dimensionality

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Fig. 8.5. (a) The d-band center derived from the deconvoluted spectra of Au nanoclusters. (b) The comparison between the d-band width, Wd, measured before (open symbols) and after (filled circles) deconvolution of the spectra. (c) The comparison between the apparent d-band spin–orbit splitting ESO measured before (open symbols) and after (filled circles) deconvolution of the spectra. The nA values are given in logarithmic scale for the latter two plots to improve data readability. Reprinted with permission from Ref. [692]. Copyright 2011 American Physical Society.

of the most important active site is critical for controlling the activity of gold nanoparticles. Following this approach, key studies [696] have explored the activity of two separate Au/TiO2 (based on P25) catalysts with 4.5 and 7.2 wt% gold loading. In each series, the gold loading was the same but Au particle size was varied (2–10 nm) by applying varying annealing procedures. In this way, the size-dependent activity was obtained for a single catalyst, while avoiding the influence of factors such

as Au loading, nature of support, synthesis approach, or incidental impurities. Using an EXAFS-defined particle size (derived from Au–Au coordination number) and CO oxidation activity measurements, the authors were able to evaluate the role of dimensionality of Au particles on the reaction rate. These data are presented as log–log plots in Fig. 8.8. From the slope in these plots, it was estimated that the CO oxidation rate varies from d  2.1 to d  2.4 for the catalyst with a 4.5 wt% Au loading, and from d  2.9 to d  3.0 for the catalyst with a 7.2 wt% loading. These data indicate that for Au particles in the range 2 – 10 nm, the size dependence of activity was steeper than expected for step sites (d  1.6 for step sites), as proposed in earlier work [266]. However, the values obtained with the 4.5 wt% Au loading are close to the d–3.0 dependence defined by others [523], which accounts for only the contribution of corner-type sites to the activity (see the right axis of Fig. 8.7). Although the power law relationship is easy to apply, it does not account for particle size distributions and differences in particle shape. For free gold cluster (see Section 3.1) less than 3 nm in size (ranging from 13 to 171 atoms), researchers predicted that, among the geometrically “ideal” shapes, the cuboctahedral shape is energetically preferred for small sizes (less than approximately 1.5 nm), and the truncated octahedron dominates at larger sizes (over
1.5 nm) [117]. Furthermore, it was shown that an octahedral morphology and a truncated cubic morphology are expected to be lower in energy than a cuboctahedron [117]. The challenge of predicting how particle size and shape affect activity was addressed in a slightly more advanced approach [695]. The method is based on measurements of particle size and volume by Scanning Transmission Electron Microscopy (STEM) and M–M coordination number as estimation from EXAFS measurement. Based on these results, the geometry of supported Au particles was described by the top slice of a truncated octahedron, a model that is consistent with an fcc crystal structure [697]. By independently varying edge length of the truncated octahedron (m) and the particle thickness (l), a geometric model of the supported Au particles was constructed. The model is composed of a distribution function where m and l describe geometrical parameters, as presented in Fig. 8.9. Having established these geometrical parameters, the authors [698] then combined data from Au particle geometry analysis and CO oxidation rate measurements to calculate the turn-over frequency at specific sites, e.g., corners. Using this approach, the behavior of three Au catalysts with different supports TiO2, MgAl2O4 and Al2O3 and the same
4 wt% Au loading was analyzed. The factor of two higher activity of the Au/TiO2 catalyst as compared to the Au/MgAl2O4 catalyst was explained by the larger amount of corner-Au atoms in the former catalyst. Gold particles on Al2O3 were found to be much flatter, revealing the effect of the support on the particle shape, i.e. the metal-support interfacial energy is different, which in turn affects shape, corner sites, and the turnover. In closely related work [693,694], DFT was employed to estimate the atomic fractions for corner, edge, or surface regions of a

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Fig. 8.6. Structural schematics and lowest coordination number NC for the Au (111) close-packed surface (a), Au (211) stepped surface (b), Au (532) kinked surface (c), Au 55 atom cuboctahedral cluster and Au 12 atom cluster corner model (d). Reprinted with permission from Ref. [224]. Copyright 2009 American Chemical Society.

truncated octahedron as a function of diameter; a summary of these data is shown in Fig. 8.10. These calculations showed that the total number of surface atoms on any given Au particle is largely independent of particle size from 10 nm to 2 nm. However, for particle sizes below 4 nm, the fraction of corners increases significantly and scales as
d  3 with diameter. The behavior of the fraction of corner atoms with particle size coincides with the widely reported increase in CO oxidation activity with decreasing Au particle size, as shown in Fig. 8.7. As highlighted above, low coordinated metal sites exhibit higher energy d-states, which leads to stronger interactions with adsorbates [224]. The models discussed thus far have employed relatively simple descriptions of the particle shape based on the Wulff construction. This description provides the most thermodynamically stable crystal shape, i.e. the shape with minimal surface energy at that volume, while accounting for surface energy anisotropy. It must be stressed that this approach is correct only for large particles where the material is continuous and where the edges and corners contribute little to the total surface energy. That is, this approach is actually not suitable for describing the shape of particles that are only a few nanometers in diameter [699]. In this study [699], modeling of Au nanoparticles began with first characterizing the overall particle shapes. Then, the reactivity of lowcoordinated active sites was estimated from models based on DFT calculations. This enabled the authors to determine the

chemical activity of particles in the same ranges accessible experimentally. Fig. 8.11a shows the configuration with the lowest energy found in the simulations. Fig. 8.11b displays the fraction of atoms in the particle with a coordination number (CN) in the range of four to nine, versus the cluster diameter. As noted in Section 3.1 for coinage metals, the DOS parameters start to be scalable to those of the bulk only for clusters containing 80 or more metal atoms [118]. One can then apply these descriptions to distinguish between metallic clusters and particles. As illustrated in Fig. 8.11a, it is assumed that the overall particle structures are fcc-based, as opposed to icosahedral or decahedral. Although this assumption likely fails for the smallest clusters, the authors anticipate that these general trends hold even for the smallest systems. Importantly, the calculations showed that the catalytic activity is greatly favored for atoms with coordination numbers below 7, i.e., atoms in a “corner-like” coordination. These results predict that activity should scale with diameter according to dx with x near  3, as expected for corner–dominated activity [693]. Thus, the main conclusion of this work [699] is that the catalytic activity for small gold clusters may be attributed to lowcoordinated gold atoms that are in corner-like locations, which is consistent with the idea that the reactivity of metal sites depends on the d-state energy [224]. The assumption of this work that, even for particle sizes below 2–3 nm in diameter, the gold nanoparticles maintain a

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Fig. 8.7. Reported catalytic activities (in mmol/g Au s, left axis) for CO oxidation at 273 K over different Au-based catalysts as a function of the Au particle size (d, in nm), see Ref. [693] for the literature sources. The different supports are indicated by the symbol shape, open symbols correspond to reducible supports, closed symbols to irreducible supports. The solid curve shows the calculated fraction of atoms (right axis) located at the corners of the nanoparticles as a function of particle diameter for uniform particles shaped as the top half of a regular cuboctahedron (see also Fig. 8.11). The arrows point to the measured activities in Ref. [695]. Reprinted with permission from Ref. [693]. Copyright 2007 Springer Science+Business Media, LLC and is an update from Ref. [523].

face-centered cubic structure is supported by recent experimental studies [700]. As discussed above (see discussion associated with Fig. 8.4), it has been established that the catalytic activity maximizes for gold nanoparticles with a diameter of 2–3 nm and a height of 6–7 atomic planes [460]. In a series of studies [460,461,700], the dependence of catalytic activity on the morphology of Au nanoparticles in model Au/TiO2(110) catalysts was explored in operando. Specifically, the morphology of particles, identified by Grazing Incidence Small Angle X-ray Scattering (GISAXS), and the atomic structure, obtained by Grazing Incidence X-ray Diffraction, were studied as a function of the catalytic activity towards CO oxidation. In that work, researchers noted that images obtained by atomically-resolved TEM typically evidence a shape close to cubo octahedrons for particle sizes in the range where catalytic activity appears [700]. Moreover, it has been noted [700] that [111] faces are usually found to directly contact the substrate surface [701–703]. The diffraction data obtained in this work for model Au/TiO2(110) samples, recorded under reaction conditions, revealed the structure to be fcc with the o 111 4 direction perpendicular to the surface. Therefore, a truncated sphere was used to approximate nanoparticulate structure for analysis of the GISAXS data. In this way, two parameters were used to define the corresponding truncated cubo octahedrons, m and l, the number of atoms at the edge and atomic planes parallel to the surface, respectively. Based on relationships of these parameters, the number of atoms along the diameter D, i.e. ND, was also estimated. The most important results of this work, which show how the reaction rate of CO depends on these parameters, are summarized in Fig. 8.12, where the rate is illustrated by the size of each symbol for a number of particle types (various m and l values).

Fig. 8.8. Summary of the activity at 298 K (bottom) and at 273 K (top) for the two samples following all calcination steps leading to increasing particle size. Rate is expressed per total amount of Au (atomic rate). Best power law fits to the data (excluding the two encircled points) are shown for each catalyst. Reprinted with permission from Ref. [696]. Copyright 2006 Elsevier B.V.

Fig. 8.12 indicates that the highest rate for CO oxidation occurs for m close to 3 and l close to 7. The main advantage of this particular structure is that it presents edges parallel and in close proximity to the surface of the substrate. The edges join a (111) hexagonal face to a (100) square face in a way that leads to the creation of a sharp angle of 551 with the TiO2 surface. The combined effects of a sharp angle with low-coordinated atoms near the Au–substrate interface, generate a particular geometric motif that likely is of critical importance for CO oxidation [700]. 8.1.4. Fraction of perimeter sites In addition to the established importance of corner sites in small Au particles for the binding and activation of some adsorbates, like CO, research has clearly identified that the AuTiO2 interfacial perimeter also plays an extremely important role in the overall reaction mechanism and rate. During the earliest studies of the catalytic behavior of supported metallic nanoparticles, including Au/TiO2 systems, a hypothesis was proposed that the high activity might be attributed to sites at

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Fig. 8.9. Example of a two-dimensional particle geometry distribution for a Au/TiO2 catalyst containing 4.4 wt% Au after heating to 673 K for 1 h and approximately 15 h on stream at 273 K in an atmosphere of 1% CO/21% O2 in Ar. The particle geometry is determined from a High Angle Annular Dark Field (HAADF) and STEM characterization of the catalyst after reaction. The area of the circles indicates the fraction of the Au particles with the indicated edge length (horizontal axis) and number of layers (vertical axis). The two most abundant geometries are drawn as examples of the general Au particle geometry in the geometric model. Reprinted with permission from Ref. [693]. Copyright 2007 Springer Science+Business-Media, LLC, data from Ref. [698].

Fig. 8.10. Calculated fractions of Au atoms at corners (red), edges (blue), and crystal faces (green) in uniform nanoparticles consisting of the top half of a truncated octahedron as a function of Au particle diameter. The insert shows a truncated octahedron and the position of representative corner, edge, and surface atoms. Reprinted with permission from Ref. [694]. Copyright 2007 Elsevier B.V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the metal–support interface [36,86,475,533,552,622]. In fact, many have concluded that the nature of the metal-metal oxide interface is the most important factor for the catalytic

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performance of Au nanoparticles [36,86,622]. Interestingly, research into the influence of perimeter sites on activity has found that the method of catalyst preparation [36,37,86,622] and the pretreatment [552,622,704] before use are of particular importance for reaction rates, especially for CO oxidation. The first proposed mechanisms [86,475,533] for the catalytic oxidation of CO, in the context of the perimeter model, suggested that CO is reversibly adsorbed on the surface and around the perimeter of Au particles and irreversibly adsorbed at the Au||TiO2 interface to form an OC–Au–O intermediate. In this case, oxygen was proposed to adsorb dissociatively to ultimately form CO2 that readily desorbs from the interfacial sites. A study on highly active Au/Ti(OH)*4 proposed that CO molecules are reversibly adsorbed on both Au particles and Ti4 þ sites but only the CO molecules adsorbed on Au contributed to the catalytic oxidation. Oxygen adsorbed as superoxide O–2 at oxygen vacancies of TiO2 was the oxidant species, and thus the reaction proceeds at the metal-support interface [89]. In agreement with numerous experimental results [36,89,533,622], several theoretical studies have found that O2 adsorption at the interface between the Au particle and the TiO2 support is the key step in CO oxidation. The energy barrier for the reaction is very low (0.1–0.4 eV) at the interface [50,525,578,705–707]. Okumura et al. [123] proposed a dynamic charge polarization description in which a heterojunction between surface Au particles and the underlying support is required for activating adsorbates. Negatively biased atoms along the perimeter of Au clusters were attributed to localized columbic effects, while positively charged Au atoms were attributed to polarization and back donation by adsorbed CO. Thus, the picture that emerges is that O2 activation occurs at anionic Au atoms around the perimeter region while CO is strongly bounded to cationic Au atoms. As discussed above, low-temperature (as low as 150 K) adsorption of atomic oxygen at Au clusters may form gold oxide on both TiO2 or SiO2 supports [511]. The reducible substrate, TiO2, appears to lower the gold oxide stability relative to, SiO2, possibly due to the effects of oxygen vacancies within titania. It was also found that the reversibly stored oxygen in high surface area Au/TiO2 catalysts can react with CO to produce CO2 [589]. The amount of surface stored oxygen is characteristic for a given catalyst and increases approximately linearly with increasing number of Au perimeter sites, as highlighted by the data presented in Fig. 8.13. This relationship strongly supports the identification of perimeter sites as the active sites for the adsorption of stable, but reactive oxygen. Under real reaction conditions, typically under high oxygen pressures, gold particles are thought to be stabilized via Au– O–Ti bonds. Such stabilization occurs to a greater extent on oxidized supports than on oxygen vacancy-rich (reduced) supports [393]. The adhesion of gold clusters is strongest on the oxidized support as compared to reduced and hydrated TiO2(110) surface. In addition to more stable structures, Au catalysts supported on reducible oxides like TiO2 are considered to be better catalysts than those supported on non– reducible ones, because reducible oxides are more capable of

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Fig. 8.11. (a) Pictures of the configuration with lowest energy found in the simulations, for different particle sizes. It is seen that the nine-coordinated (111) surfaces dominate the non-bulk of the particles. When the particle become sufficiently small, the fractions of six- and seven-coordinated corner and edge atoms become comparable to the fraction of surface atoms. Note how the particle often can accommodate the arbitrary number of atoms by departing from Wulff-like shapes either by being elongated (the 150 atom cluster) or by breaking the symmetry between otherwise equivalent surfaces (most obvious on the 617 atom particle, but seen on almost all particles). (b) Fraction of atoms with coordination numbers from 4 to 9. Adapted with permission from Ref. [699]. Copyright 2011 Elsevier B.V.

forming O-rich Au–support interfaces, which are critical for the activation of oxygen [393]. Recently Wang et al. [708] have shown that a Au20 cluster supported on TiO2(110) may act as a reservoir of charge that can activate the O2 molecules at the Au||TiO2 interfacial perimeter, as shown in Fig. 8.14. As illustrated in Fig. 8.14B, the suggested mechanism is one in which the O2 molecules gain 0.1 0.3 eV in energy as they transform from a pre–adsorbed state to an interfacial configuration. Within this model, the fundamental importance of the Au20 cluster on TiO2 is to enhance the available surface charge for O2 reduction, which increases the concentration of surface O2 (see Fig. 8.14D). Others have recently proposed a dual-catalytic perimeter site model to explain the low temperature catalytic activity of high surface area Au/TiO2 [68,69,709]. In those studies, the catalytic activity was thought to be localized at the Au particle perimeter where Au atoms are adjacent to the TiO2 support. Dual-catalytic sites involving Au atoms and Ti4 þ ions were suggested to work together to activate molecular oxygen for CO oxidation [68,709]. Further details on this and many other suggested mechanisms are provided below.

8.2. Key catalytic reactions The discussion in the previous section provided an introduction to the current state of understanding of how and why certain properties affect the catalytic activity of supported Au nanostructures. This section focuses on the fundamental question of how the tandem Au nanostructure – TiO2 support facilitates catalysis. The current state of understanding for these systems has been achieved by the successful and often impressive collaborations (both direct and indirect through the literature) between theorists and experimentalists.

Fig. 8.12. Reaction rate of the Au NPs as a function of the size of the corresponding truncated cubo octahedrons with, on the vertical axis the number of atomic layers l, and on the bottom horizontal axis, m, the number of atoms on the edges and on the upper one, ND, the number of atoms along the diameter; the scales for l, ND and m are proportional to the value of d⟂, d// and 3d//, respectively. The red spots are proportional to the reaction rate for CO oxidation, as deduced from the experimental data. They are located at the m and l values deduced from the mean D and H. Schematic representations of cubo octahedrons are given for several couples of (m, l) pointed by the blue arrows. The couples (m, l) corresponding to complete cubo octahedrons are marked by dark blue diamonds. On the lower grey dashed line, the aspect ratio H/D is 0.5 and it is 0.8 on the upper one that crosses the dark blue diamonds. The part of the diagram above is shaded since the corresponding (m, l) couples cannot be assigned to the cubo octahedral geometry. In the insert, the experimental data are also plotted as a function of l (vertical axis) and ND (upper horizontal axis) with their error bars calculated from the width of the Gaussian distribution of the Au NPs size given by the GISAXS analysis. Reprinted with permission from Ref. [700]. Copyright 2013 The Royal Society of Chemistry. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

8.2.1. CO oxidation (planar model and high surface area catalysts) It is now widely accepted that high catalytic activity of Au catalysts towards ambient (and even much lower) temperature

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CO oxidation necessitates the presence of Au particles with an average diameter of less than 5 nm deposited on an oxide support. This general requirement is well demonstrated in Fig. 8.15, taken from the recent work by Laoufi et al. [460], which resembles the catalytic activity versus size dependence already discussed throughout previous Sections. Fig. 8.15 nicely summarizes data from published work for TiO2 supported gold prepared by various methods from model Au/TiO2(110) catalysts prepared in UHV [88,460] to real catalysts Au/TiO2 catalysts obtained by deposition-precipitation, calcination [474,491], or deposition of gold particles on titania-coated silica aerogel [710]. As all these experiments were performed in very different conditions, a rescaling was performed to account for the temperature, and oxygen (PO2) and CO (PCO) partial pressures [460]. This comparison shows a remarkable similarity of the catalytic activity versus size for a wide variety of studies, despite differences in the experimental conditions. Moreover, this group of studies has further substantiated that Au, deposited on reducible oxides (TiO2, Fe2O3), typically displays greater activity than Au deposited on irreducible supports (SiO2, MgO) under similar conditions. Two general reaction schemes for CO oxidation have been suggested in the

Fig. 8.13. Relative conversion of CO at 353 K during simultaneous CO/Ar and O2/Ar pulses over the Au/TiO2 catalyst after calcination in air for 2 h at various temperatures plotted against the length of the Au–TiO2 interface perimeter. Reprinted with permission from Ref. [589]. Copyright 2009 Elsevier B.V.

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literature: a Langmuir–Hinshelwood mechanism on active supporting materials (TiO2) [705], and an Eley–Rideal mechanism on non–active supporting materials (MgO) [554]. As generalized in Fig. 8.16, one of the major scientific problems associated with the reaction has been whether the CO oxidation involves initial dissociation of O2 or a direct reaction with adsorbed molecular O2 [711]. Most of the theoretical studies performed thus far appear to be in agreement that large energy barriers exist for the dissociative adsorption of O2, while reaction between CO and adsorbed molecular O2 appears to be efficient with significantly lower energetic requirements [706]. Molina and Hammer [706] related this to the “nobleness” of gold, which binds adsorbates only when Au is present at low-coordinated sites; otherwise, the rupture of the O¼ O bond is energetically very expensive. However, it is precisely the noble character of Au that enables it to behave as a "gentile catalyst" under the right conditions [706]. In such a scenario, the O¼ O double bond may first weaken to a peroxo bond with the eventual help of a reducing adsorbate. Scission of the weakened bond then can occur. This pathway is illustrated in the upper part of Fig. 8.17. The weak binding of adsorbates involved in the reaction to the gold particles actually facilitates desorption, thereby reducing the deleterious effects of surface poisoning. Within this understanding, the limiting aspect of gold catalysis is the binding of reaction adsorbates in sizable quantities (in order to obtain “sizable reaction rates” [706]) and bringing them together. In addition to the above discussed factors related to the smallness of Au particles and low coordination of Au sites (see Section 8.1), other aspects of the reaction must be considered. Such as the charge state of Au sites, special Au–oxide interfacial sites, and thermal stability of the Au clusters [706]. Based on the literature for CO oxidation at oxide supported Au catalysts, Long et al. made two reasonable assumptions: “(1) CO binding is fast and strong relative to oxygen binding, thus CO is readily available at or near the active site, and (2) O2 binding and activation is the key kinetic step in the catalysis.”[712] As discussed in Section 8.1.4, a number of studies have shown the importance of Au||support interfacial

Fig. 8.14. O2 adsorption on Au20/TiO2. (a) The most stable adsorption configuration with three O2 molecules adsorbed at the Au/TiO2 interface. The other possible binding sites are also denoted as “1” (close to Au) and “2” (away from Au). (b) The adsorption energies for O2 adsorbed at different binding sites. (c, d) The electron density difference δρ. (c) Au20/TiO2  x(110) without O2 adsorption, δρ¼ ρtot ρAu20  ρTiO2  x. (d) Au20/TiO2  x(110) with three O2 adsorbed at the interface, δρ¼ ρtot ρAu20 ρTiO2  x  ρO2. The purple surface indicates the increase of electron density, and the green surface indicates the decrease of electron density. Adapted with permission from Ref. [708]. Copyright 2013 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8.17. Schematic representation of the reactions COþ O2-CO2 þ O (a) (upper panel) and COþO-CO2 (lower panel). Reprinted with permission from Ref. [706]. Copyright 2005 Elsevier B.V.

Fig. 8.15. Comparison of the catalytic activity versus Au particle size dependence for Au/TiO2 catalysts prepared by various chemical ways. Red points and dashed line (Laoufi et al. [460]): PO2 ¼2  103 Pa, PCO ¼ 20 Pa, and T¼473 K. Green crosses (Valden et al. [88]): PO2 ¼ 4.4  103 Pa, PCO ¼0.9 Pa, T¼ 350 K. Blue triangles (Bamwenda et al. [474]): PO2 ¼2.1  103 Pa, PCO ¼103 Pa, T¼273 K; Black diamonds (Zanella et al. [491]): PO2 ¼ 4  103 Pa, PCO ¼ 103 Pa, T¼ 278 K. Orange squares (Tai et al. [710]): PO2 ¼ 2.1  104 Pa, PCO ¼ 103 Pa, T¼300 K. A rescaling of data is performed to account for the different temperature, and, oxygen (PO2) and CO (PCO) partial pressures, see the original paper for more details [460]. Reprinted with permission from Ref. [460]. Copyright 2011 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8.16. General mechanism for CO oxidation on Au/oxide. Reprinted with permission from Ref. [711]. Copyright 2007 Springer Science+Business Media, LLC.

perimeter sites, where a cationic site of the support and one Au atom (or more) localized at the Au particle perimeter may act as a dual adsorption center to bind molecular oxygen in a peroxo-type configuration. Thus, small Au particles with a high number of interfacial perimeter sites per Au atom will exhibit the highest catalytic activity in oxidation reactions. Fig. 8.18a and b present two similar mechanisms, proposed by different researchers, for CO oxidation at the Au particle– support interfacial region [36,622]. Both models indicate, in accord with the numerous studies described above, that CO is chemisorbed at low-coordinated gold atoms on the Au particles, while O2 is activated at sites close to the Au||support interface. According to the model shown in Fig. 8.18a, a hydroxyl group (originally at the support) migrates to a AuIII ion at the perimeter, thereby creating an anion vacancy. An oxygen molecule from gas phase is then posited to occupy the anion vacancy as O–2. The slightly alternative model, depicted

in Fig. 8.18b, proposes that oxygen dissociation occurs directly at the Au||TiO2 interface. One of the most important common themes of these models is that they both suggest that the periphery acts as a natural meeting point for both reactants: CO adsorbed at the Au particle and O2 preferentially bonded at either TiO2 or the Au||TiO2 interface. One should note that, recently, the role of surface Ti–OH groups in promoting O2 adsorption and activation in CO oxidation reactions on Au/TiO2 surfaces was investigated [713]. DFT investigations into the energetics of CO oxidation on model catalysts composed of an infinite array of Au rods (Molina et al. [525]) and on a two-layer Au strip (Liu et al. [705]), both supported on rutile TiO2(110), have shown similar results: in the minimum energy oxidation path, CO and O2 are adsorbed near the Au/TiO2 interface (see the discussion with Fig. 8.19). Beyond this work, researchers studied CO oxidation at the Au||support interface of finite Au7 [32,551] and Au10 [551,578] clusters deposited on rutile (TiO2). In addition to interfacial sites, Remediakis et al. [578] have also examined the activity of corner Au atoms that are remote from the Au||support interface. The Au10 cluster located on TiO2(110) adjacent to three oxygen vacancies (shown in Fig. 8.19A), which makes the cluster highly anionic and thus strongly binding toward O2. Two plausible mechanisms for CO oxidation at the Au10 cluster were identified. One pathway resembles those reported for infinite Au structures [525,705], and operative at the edge of the Au/TiO2 interface, as shown in Fig. 8.19B, a. The second pathway proceeds solely on the gold particle (a “Au-only” reaction pathway), as depicted in Fig. 8.19B, b. In this latter case, the effect of the support on the energetics of reaction is negligible. Both reaction pathways involve low-coordinated Au atoms that stabilize both reagents, CO and O2. Further, these two pathways proceed through a similar activation barrier for CO oxidation: in the range 0.36–0.40 eV, which is very close to previously reported values: 0.36 eV (Haruta et al. [86]); 0.15–0.25 eV (Valden et al.[383]); and, 0.16–0.60 eV as reported by Bamwenda et al. [474] and Choudary et al. [501]. In the majority of the experimental investigations involving model Au/TiO2 (single crystal) catalysts, the Au nanostructures were deposited onto the support under UHV conditions. Likewise, most of the theoretical studies necessarily neglect alterations of the Au oxidation state that may be induced by impinging

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Fig. 8.18. Reaction mechanisms proposed for CO oxidation over supported Au nanoparticles according to (a) Reprinted with permission from Ref. [622]. Copyright 2000 World Gold Council and (b) Reprinted with permission from Ref. [36]. Copyright 2002 Springer Science and Business Media.

gases. However, in real catalysis, Au particles are most often produced by wet chemistry and studied by techniques under which the catalysts are much less well defined. One of the most striking differences between studies of the model and real systems is that the approaches likely employ Au particles with different oxidation states: anionic Au is often characteristic of the UHV-prepared model systems while cationic Au is a common state in much of the real catalytic systems. Wang and Hammer [551] have studied the thermodynamically favored oxidation state of supported Au nanoclusters in the presence of O2 (g). As shown in Fig. 8.20, small Au7 and Au10 clusters and their TiO2 support are oxidized by O2 or H2O in ways that ultimately increase the adhesion energy between the clusters and the support. For the Au7/TiO2(110) system, Fig. 8.20A,a, the thermodynamically preferred state involves atomic hydrogen adsorption to a bridging oxygen and the addition of dual oxygen atoms at perimeter of the Au cluster. In the case of Au10/TiO2(110), Fig. 8.20A,b, the favored configuration presents a singe atomic oxygen at the Au||support interface. These structures differ from those identified in UHV-based work [578], i.e. where oxygen vacancies readily become trapped under the Au particles. Fig. 8.20B schematically shows how the most stable oxidation states and structures of Au may change with cluster size. The smallest particles show evidence (in this work) for the existence of highly oxidized gold atoms at the Au/ TiO2 perimeter. This one atom-thick oxide appears to be largely insensitive to particle size and remains intact as for larger particles. As illustrated, the hypothesized reaction pathway involves: (1) CO approaches the O atom located at the Auoxide sites; (2) abstraction of the oxygen forms CO2, together with a Au-oxide oxygen vacancy; (3) molecular O2 binds at the vacancy; (4) additional CO attacks the vacancy–O2 species; and (5) abstraction of one of the oxygen atoms at the vacancy forms CO2 and leaves behind re–oxidized Au particles [551]. This proposed model offers an explanation for why the reaction rates measured for CO oxidation vary relatively moderately from one to another oxide support, as highlighted by the text associated with Fig. 8.7 [523,693]. Wang and Hammer [551] concluded from this stage of their work that oxidation of the interface or interfacial perimeter

region produces active sites that may be similar across different supports. The assertion that Au nanoparticles remain largely in the metallic state while the oxidized Au resides primarily at the interface is reminiscent of the model proposed in earlier studies by Bond and Thompson (see Fig. 8.18a) [622]. However, according to that model, active oxygen originates from the lattice of the support, [622] while the model illustrated in Fig. 8.20 suggests that the active oxygen evolves from the Au oxide. In their report from 2007, H. Kung and co-workers [714], analyzing the results from previous experimental and theoretical work, found that the understanding for low-temperature Au-catalyzed CO oxidation had four unresolved issues: “(1) the importance of the nature of the support on catalyst activity; (2) the Au oxidation state necessary for high activity; (3) the sensitivity of the activity to the moisture level in the reaction feed; and (4) reasons for the high activity in small Au particle size and for the strong dependence on particle size and specific morphology.” From their discussion, it was deduced that the first “support issue” plays an indirect (secondary) role in influencing the reactivity of Au catalysts. The direct participation of the support by supplying active oxygen was determined to be unlikely. Although metallic Au was recognized as necessary for activity, the role of cationic Au was highlighted as an issue where deeper understanding was needed. Not surprisingly, the mode of oxygen activation was, and in many ways remains, the least understood. A deep understanding of the oxygen activation mechanism is critical, not only for revealing the details of CO oxidation, but also for exploiting Au catalysis for other oxidation reactions, such as selective epoxidation and oxidation of hydrocarbons. As discussed above, the importance of small metallic Au particles is likely closely related to the fact that they have a higher density of active sites than large particles. For the issue of moisture, two possible effects were identified: formation and then regeneration of the active Au site. A model of the active site was proposed that was similar to that proposed by Bond and Thompson [622], shown in Fig. 8.18a. Interestingly, recent FTIR study by Hadjiivanov et al. [715] established that co-adsorption of CO and O2 on a Au/SiO2

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Fig. 8.19. (A) The relaxed geometry for [Au10] cluster supported at O2c defect sites on TiO2(110). (B) (a, b) Relaxed geometries of the initial, transition, and final states, and (c) energy profiles for CO oxidation on a [Au10] cluster. Blue line: [Au10] supported on TiO2(110), CO oxidation takes place at the Au||TiO2 interface (path a); black line: [Au10] supported on TiO2(110), CO oxidation takes place solely on the Au particle (path b); red line: unsupported cluster with the bottom three atoms kept fixed at the positions as they would be if the oxide were present. Adapted with permission from Ref. [578]. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

catalyst (note that SiO2 is a non-reducible oxide) leads to conversion of Au0 into Auδ þ sites and that CO could be oxidized under their experimental conditions, even at ambient temperature. Their Au/SiO2 sample was prepared by ammoniaassisted grafting with HAuCl4, which produced a catalyst that contained 1.84 wt % Au with an average particle diameter of 5 nm, as measured after post-activation at 673 K. According to several authors [375,435,555], surface defects, e.g., oxygen vacancies (F-centers), can impart negative charge to gold nanoparticles adsorbed on reduced supports. These studies suggested that experimental evidence for Auδ– can be attributed to negative Au ions located on metallic gold particles. Upon O2 exposure, the Auδ– species were found to easily oxidize to Au0 and beyond to Auδ þ . Interestingly, these studies found evidence for the coexistence of Auδ þ , Au0, and Auδ–on the sample [375,435,555,715]. The previous studies described thus far have clearly demonstrated that the catalytic chemistry of supported Au is complex. Complicating direct comparisons of studies between different research groups is the fact that the performance of Au catalysts is significantly affected by the external operating conditions, precursors and conditions used in the preparation of catalysts, and the pretreatment of the catalysts. These complications highlight the need for high-level theoretical

studies, such as the quantum chemical and ab initio thermodynamic calculations by Laursen and Linic [716], which focused on detailing the geometric and electronic characteristics of active sites for CO oxidation over a Au/TiO2 catalyst. In their work, the Au/TiO2 catalyst was modeled by 2-layerthick Au nano-rods supported on the TiO2(110) surface. The orientation of Au nano-rods on the TiO2(110) surface was optimized for minimum strain within the Au nano-structure. These studies provided evidence that the strongest oxygen binding sites were located at the Au||TiO2 interface and these sites were populated at low oxygen chemical potentials. The oxygen adsorption site is characterized by an oxygen atom interacting simultaneously with a Au atom and a coordinately unsaturated (CUS) Ti þ 4 atom on TiO2. Oxygen adsorption at the Au||oxide interface site is more favorable than adsorption away from the interface by approximately 0.75 eV. CO adsorbates were found to occupy sites on the Au nanostructure, remote from the interface. Further, these authors showed that the electronic structure of the interfacial Au atoms that bind O atoms is characterized by unpopulated d-states above the Fermi level [716]. Experimentally, this prediction was supported by the appearance of high-intensity white lines in X-ray absorption near edge spectroscopy (XANES) measurements [579]. CO adsorbates are then thought to diffuse on

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Fig. 8.20. (A) Free energy diagrams as a function of the oxygen chemical potential for (a) Au7/TiO2(110), and (b) Au10/TiO2(110). Blue lines are for systems with O vacancies, black lines are for stoichiometric systems, red lines are for oxygen rich systems, and green lines are for systems with water fragments (plus oxygen in some cases). The dashed lines are for systems that contain molecular O2. The chemical potential of H2O is chosen to 2.96 eV, corresponding to p(H2O)¼ 10  12 kPa at 298.15 K. (B) Schematic of the expected evolution of the thermodynamically most stable oxidation state of Au clusters with increasing size Adapted with permission from Ref. [551]. Copyright 2007 Springer Science+Business Media, LLC. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

the surface of the Au nano-structure to meet O atoms at the Au||TiO2 interface, where the interaction between CO and O results in highly exothermic formation of CO2 [716]. The postulated mechanism is consistent with that proposed in earlier work by Hammer and coworkers [32,551]. Using DFT calculations and micro-kinetics analysis, Li et al. [717] systematically investigated the size and shape dependence to catalytic activity of sub-nanometer Aun clusters supported on TiO2(110) surface (n¼ 1 4, 7, 16 20). The activity of the Aun/ TiO2 systems was found to increase with the size up to Au18. The Au18 isomer with a hollow-cage structure exhibited the highest activity with CO oxidation rates
30 times higher than that found for the Au7/TiO2 system. Moreover, the hollow-cage Au18 cluster exhibited good stability during the soft-landing on a surface and structure stability upon CO and O2 co–adsorption. In agreement with previous studies, the boundary sites at the Au– cluster||TiO2 interface were found to promote CO oxidation. In the proposed mechanism, CO adsorbates at perimeter Au sites readily interact with dangling O2 adsorbates residing on neighboring Ti5f sites. The calculated reaction barrier of such a dualperimeter-site pathway was calculated to be as low as 0.3 eV. Importantly, the second-layer Au atoms (above the TiO2 support) was shown to play a critical role in that it can enhance the adsorption of O2 via the d π orbital interaction on perimeter Ti sites. This enhancement increases the ratio of O2-to-CO adsorption energies and thus the reaction rates. Fig. 8.21a shows a schematic and a contour plot of reaction rates developed in this

comprehensive work. In this plot, a critical line corresponding to a reaction rate of Rc ¼ 106 s  1 is designated as a benchmark to predict the optimal adsorption conditions for CO and O2 on Au/ TiO2 to achieve high reaction rates. Clearly, these data suggests that, for a given value of CO adsorption energy, the O2 adsorption must be sufficiently strong to achieve a reaction rate larger than the critical value. For isolated, support-free Au clusters, the O2 adsorption energy is much lower than the CO adsorption energy; so, free particles clearly do not satisfy the critical conditions for catalysis mapped in Fig. 8.21a. The presence of a TiO2 support is critical for altering the energetics for CO and O2 interactions and ultimately activation. The critical role of the oxide support was further highlighted in studies of CO oxidation on Au/TiO2(110) model catalysts by Kido and co-workers [718–720]. Their work revealed that the O ad–atoms (Oad) adsorbed on the 5-fold Ti rows of O-rich TiO2(110) can be abstracted by CO to form CO2 even at moderate temperatures [718]. Further, the CO oxidation reaction appeared to be enhanced by deposition of Au clusters on the oxygen-rich titania [719,720]. Optimum structures for catalysis were identified as 2D gold clusters with a diameter of
1.5 nm and two-atomic layers in height. Particles of this size contain approximately 50 Au atoms [719]. Importantly, charge transfer from the Au clusters to the TiO2(110) support may play a key role in the chemistry. These researchers suggest that an interfacial dipole, formed as Au transfers an electron, polarizes surface sites in ways that serve to attract both oxygen and CO reactants. This

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mechanistic description resembles the reaction path predicted in the theoretical work by Wang and Hammer [32]; however, Kido et al. suggest that the role of Au nanoclusters is to lower the activation barrier for the reaction between Oad and CO on the surface of Au/O-TiO2(110). In addition, they imply that catalysis is further facilitated by the fact that the Au/O-TiO2 system increases the number of Oad ad–atoms [720]. Work by Yates et al. [68] focused on how the support-particle interface provides dual catalytic sites at the gold/support interface perimeter of high-area Au/TiO2 catalyst. In that work, gold clusters with a mean diameter of 3 nm were supported on highsurface-area powdered TiO2 via the deposition-precipitation method [488]. The low temperature catalytic CO oxidation was studied by transmission IR spectroscopy [68]. Fig. 8.22A presents time resolved spectra obtained when a CO saturated (8  10  3 kPa) Au/TiO2 sample (spectrum a) was exposed to 0.133 kPa of molecular O2 at 120 K (spectra b-z). As these spectra show, exposure of the CO-saturated surface to O2(g) immediately initiated the CO oxidation reaction, as indicated by the decrease in CO absorbance on TiO2 sites (see band at
2179 cm  1) and the evolution of the CO2 feature on TiO2 sites (see band at 2341 cm  1). As shown by the inset of Fig. 8.22A, the main participant (as identified by CO stretching frequency) in the reaction is the CO bound directly to TiO2, which depleted completely after 120 min of reaction. At the same time, only about 12% loss in IR signal attributed to CO bound to Au occurred. The oxidation of the CO/TiO2 species showed first order kinetics in CO/TiO2 coverage for the temperature interval 110 to 130 K and an Arrhenius plot of the data yielded an apparent activation energy of
15 kJ mol  1

for the overall reaction, in accord with previous work [619,712]. Gradient-corrected DFT calculations, highlighted in Fig. 8.23, helped to identify the active sites for the low temperature CO oxidation at the Au||TiO2 interface. The calculations helped to show that the adsorption/activation of O2 is a critical first step in CO oxidation. As discussed above, it is postulated to occur at the perimeter of Au particle [32,525,551,578,706] either through direct O2 dissociation (see Fig. 8.20B) or through the assistance of a nearby CO in a bimolecular CO-O2 reaction, vide infra. In light of the experimental results (part of them presented in Fig. 8.22), the theoretical study was focused on the mechanism of CO oxidation that occurs at sites in the perimeter region between Au particle and TiO2, assigned as “Reaction zone” in Fig. 8.23a [68]. Upon consideration of the possible adsorption sites at the Au/TiO2 interface, the Ti5c site adjacent to a Au atom at the perimeter was identified as the most likely site to bond O2 [68]. This location appears to be the only site for which O2 can binds to both the Ti5c site of the support and to Au site at the interface in a di-s configuration. This configuration stabilizes adsorbed O2 by a charge transfer from the Au to the Ti and results in the lowest calculated barrier of 0.16 eV for the activation of O2. Dissociation of O2 on Ti5c sites may further be assisted by CO adsorption at neighboring Ti5c sites. The resulting O/Ti5c species are thought to readily react with CO that diffuses in from neighboring TiO2 regions of the material (Fig. 8.23b, steps D – E). The resulting CO2 can either migrate from the active site or desorb into the vapor phase through a barrier of only 0.20 eV (19 kJ mol  1, step F). The activation energies presented in Fig. 8.23b suggest that the rate-limiting

O2⧧ Fig. 8.21. (a) Contour plot of reaction rates versus CO (ECO⧧ ad ) and O2 (Ead ) adsorption energy. Open squares and black triangles and stars represent reaction rates on the gas-phase Aun, pyramid-Aun/TiO2 and hollow-cage-Aun/TiO2, respectively. The dashed–dotted pink line refers to the designated critical line for which the reaction rate corresponds to 106 s  1. The dark-blue region refers to an unfavorable region where the weak adsorption of CO and O2 would lead to high reaction O2 barriers. (b) The difference between the actual O2 adsorption energy (EO2⧧ ad ) and the corresponding value taken from the critical line (Ec ) for the gas-phase Aun and Aun/TiO2 (n¼ 18-pyrd, 19, and 20) systems. Here, the value of EO2 is taken from the cross-point between the critical line (pink line in (a)) and the vertical line c drawing from the interested point (e.g., an open square for gas-phase Aun). Reprinted with permission from Ref. [717]. Copyright 2013 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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step for low temperature oxidation of CO on this particular Au/ TiO2 sample is the diffusion of CO from TiO2 support to the Ti5c sites along the perimeter of the Au particles [68]. The dual-perimeter concept [68] predicts that the role of the support is to supply active species for oxidation at the Au/TiO2 interface. Liu et al. [721] have also explored such processes by studying how the TiO2 crystal-plane affects the catalytic property of a Au/TiO2 system. In this work, Au nanoparticles with similar particle sizes (
3.6 nm) were loaded on anatase TiO2 nano–crystals (50–70 nm) with different crystal planes exposed, i.e. (100), (101), and (001) planes. Fig. 8.24a shows Arrhenius plots obtained in the temperature range 260–410 K for the reaction of CO oxidation carried out on these model Au/TiO2 catalysts. The TOFs were calculated by counting only the perimeter Au atoms. The highest TOF is obtained with the Au/Ti-100 sample, while the Au/Ti-001 sample showed the lowest TOF. Above 320 K, the estimated values of Ea (apparent activation energy) were found to be very low (2–7 kJ mol  1) for the three samples. Below 320 K, however, the values of Ea are much larger and the differences between the three samples become obvious. These findings are consistent with several precious studies [33,37,408,621,722]. The existence of two temperature ranges with different Ea for CO oxidation on Au/TiO2 was reported by Haruta et al. [37] and attributed to a change in the reaction mechanism with temperature. Fujitani and Nakamura [621] proposed that at low temperature, water promotes the CO oxidation by forming hydroperoxides at the Au||TiO2 interface, while at higher temperatures O2 activation occurs more directly at lowcoordinated Au sites and reacts with CO on the Au NPs. In the work of Liu et al. [721], which is somewhat representative of these other studies, the dynamic changes that occur within a Au/TiO2 sample after exposure to different atmospheres are illustrated in Fig. 8.24b. Combining the results from the ESR study and CO oxidation activity, Liu et al. [721] concluded that the Au/Ti-100

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sample is capable of activating molecular O2 at low temperature, whereas Au/Ti-001 is inert. In addition, their XPS measurements indicated a correlation between the surface hydroxyl coverage and the activity, supporting a mechanism where surface OH groups are involved in the activation of oxygen at low temperatures (o 320 K). At high temperatures (4 320 K), the activation of lattice oxygen species at the perimeter of the Au/TiO2 interface was found to be important. The migration of oxygen on (100) and (101) planes is likely more facile than on (001) planes, which results in their high activity at temperatures above 350 K. In-situ DRIFTS experiments indicated that the formation of carbonates is strongly affected by the crystal plane of the TiO2 support. With the highly active Au/Ti(100), carbonates were transferred to the TiO2 support after their formation at the Au||TiO2 interface; whereas with the inactive Au/Ti(101), carbonates were strongly bonded to Au NPs and inhibit the catalytic reaction. The authors concluded that the activation of O2 and formation, distribution, and desorption of carbonates are highly dependent on the TiO2 crystal plane [721]. However, the fundamental origins of the TiO2 crystal-plane effects are not completely understood at the atomic and molecular levels and further research in this area is certainly warranted. Recent theoretical studies into the details of how dualperimeter sites catalyze chemistry for different crystal planes were performed by Vilhelmsen and Hammer [685,723]. Using DFT calculations, these authors studied the CO oxidation reaction on a model Au/TiO2(110) catalysts, containing a Au cluster bound at a rutile TiO2(110) slab. Applying a genetic algorithm search, the global minimum of the Au clusters was established. In this work, the interface between the Au cluster and the support surface was oxidized with O atoms at all 5f-Ti atoms residing underneath the Au cluster [723]. As in the studies describe above [685], the activity and reactivity of sites at the perimeter of the Au||TiO2 interface were found to depend strongly on the surface direction. Specifically, Fig. 8.25

Fig. 8.22. (A) IR spectral development during the CO oxidation reaction on Au/TiO2 under 1 Torr of O2 pressure at 120 K: a, before O2 introduction; z, after 120 min of reaction. The CO/TiO2 oxidation was found to continue when the CO coverage was replenished from spectrum z, indicating that the CO2(a) accumulation did not block the active sites. (Inset) The plot of normalized integrated absorbance of CO/Au (red) and CO/TiO2 (black) against time during the experiment. (B) IR spectral development during temperature-programmed desorption of CO. (Inset) The normalized integrated absorbance of CO/Au (red) and CO/TiO2 (black) against temperature. Adapted with permission from Ref. [68]. Copyright 2011 AAAS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 8.23. (a) Schematic of the mechanism of low-temperature CO oxidation over a Au/TiO2 catalyst at a perimeter zone of reactivity. Experiments directly observing CO/TiO2 and CO/Au surface species show that processes 2 and 3 are fast compared with process 4. (b) CO oxidation cycle proposed from DFT calculation results. The Au atoms and Ti atoms are shown in yellow and gray, respectively, whereas the O in TiO2 lattice and adsorbed O and C atoms are shown in pink, red, and black, respectively. ΔEads, Ea, Ediff, ΔEdes, and ΔH refer to the binding energy, activation barrier, diffusion barrier, desorption energy, and reaction enthalpy, respectively. The elementary steps depicted include the adsorption of O2 (A to B), interaction between O2 and CO (B to C), reaction of O2 and CO (C to D), reaction of adsorbed atomic oxygen and CO (D to E), diffusion of CO (E to F and G to H), and desorption of CO2 (F to G and H to A). (a) indicates adsorbed species. Adapted with permission from Ref. [68]. Copyright 2011 AAAS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

illustrates results that show that the oxidation of CO occurs preferentially along the [11̅0] direction of the support [685]. Along the [001] direction, the binding energy of CO is too weak for sufficient oxidation to occur. Both the [11̅0] direction and the corner of Au24 clusters were hypothesized to catalyze the CO oxidation and the calculations showed that the activation energy may be 0.7 eV (68 kJ mol  1). As shown by graph in Fig. 8.25b, the reaction along the [11̅0] direction includes a pair of barriers: the first one (dotted line) being related to the breakage of the O– O bond in O2, whereas the second one originates from the reaction of CO with O. At the corner site, the CO directly react with the O2. In this latter case, the barrier is due to the energetic expense of the CO molecule migrating away from a highly stable site. An increase of the CO coverage was found to reduce the barrier for corner sites by at least 0.05 eV; however, the activation energy, Ea E0.7 eV, reported in this work is higher than the values previously reported in the literature, i.e. 0.16 eV to 0.59 eV (15–57 kJ mol  1) [68,474,709,724,725]. Several reasons for this discrepancy were considered: first, the “global minimum structure” employed in this work is inherently less reactive than higher energy structures; second, the CO coverage effects were not included in the calculation of activation energy; lastly, structural fluxionality of the Au nanoparticle was not explicitly considered, although it was shown to reduce the reaction barriers [138,691,726]. Studies by Häkkinen et al. [138] and Landman et al. [691] have found that at a finite temperature, supported Au clusters may exist as an equilibrium ensemble of coexisting structural configurations. This property of small clusters was called

“dynamic structural fluxionality” [138] and is expressed in transformation of ensembles between various energetically accessible structural isomers in the course of chemical reactions, thus enhancing the rates for overcoming reaction barriers. This structural fluxionality may be essential for catalysis because structurally constrained metal clusters may prevent the adsorption and activation of reagents, e.g. adsorption of O2 on model Aun/MgO(100) [138]. In the case of model Aun/TiO2(110) catalysts, Friend and co-workers [727] have found that the adsorbed O2 induces redistribution of charge from the Au cluster to the Au–titania interface, which responds through structural distortion that lowers the total energy of the system. As demonstrated by Wang and Hammer [551] (see Fig. 8.20), the charge state of a Au nanoparticle depends strongly on conditions that affect the oxidation state of the local environment: under reducing conditions, Au is negatively charged, while it is positively charged under oxidizing environments. Thus, tracking the charge state of the Au (under operando conditions) is critically important to deciphering the overall mechanisms of oxidation-reduction processes and catalysis. The importance of particle oxidation state, as affected by adsorbates, on the mechanism of CO catalytic oxidation over TiO2 supported Au clusters has been recently studied by Wang et al. [708]. In that work, ab initio molecular dynamics (AIMD) simulations were conducted on a prototypical Au20 particle that resided on reducible TiO2(110) to help uncover how charge transfer and thermal dynamics affect catalytic performance of TiO2-supported Au particles. A detailed description of this system has already been discussed in

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Fig. 8.24. (a) Kinetic studies of CO oxidation reaction on Au/TiO2 catalysts with different TiO2 crystal planes. (b) Schematic illustration of dynamic changes of Au NPs in Au/Ti-100 (top), Au/Ti(101) (middle), and Au/Ti(001) (lower) in different atmospheres at 303 K. Reprinted with permission from Ref. [721]. Copyright 2013 American Chemical Society.

Section 8.1. Figure 8.14 illustrates the influence of finite temperature dynamics on an isolated Au20 cluster, by both a stoichiometric and a reduced TiO2 substrate. AIMD simulations confirmed that the isolated Au20 cluster has high stability and maintains a tetrahedral geometry even at temperatures of 700 K. Supported Au20 on stoichiometric TiO2 exhibits stability in line with that observed for the isolated gas phase cluster. On partially reduced TiO2, however, an oxygen vacancy causes dramatic change to the supported Au20 cluster. One of the 4 apex Au atoms of the tetrahedral Au20 cluster is strongly bonded to the O-vacancy and extended regions of the cluster play a role in bonding with Ti ions along the Ti5c row. As discussed above in the text associated with Fig. 8.14, O2 chemisorption on the Au20/TiO2 surface preferentially occurs with one O atom binding at a perimeter Au site and the other O atom binding at a Ti5C site [68,551,705]. In agreement with previous experimental and theoretical reports of CO adsorbate induced changes in morphology (fluxionality) of small Au clusters [139–141,283,728,729], the AIMD modeling showed evidence that Au20 clusters are labile and tend to expose lowcoordinated sites (such as 4-fold or lower) where CO molecules bind [708]. Fig. 8.26a illustrates an example of eight CO molecules co-adsorbed on a Au20 cluster with a pair of oxygen molecules. Binding of CO to the cluster extrudes lowcoordinated Au sites. Fig. 8.26b shows the increase in the statistical number of low-coordinated Au atoms with CO adsorption, evidencing a significant adsorption-induced surface reconstruction [139–141]. The average binding energy per adsorbed CO molecule was 0.94 eV. In addition to structural changes, the charge of the Au20 cluster, estimated for the case when 2 O2 and 10 CO molecules are adsorbed on the surface, was þ 0.80 e  , which is only 0.10 e  lower than that with 2 O2 molecules adsorbed without any CO. This result implies that adsorbed CO does not affect the charge of the Au20 cluster. Beyond information about the oxidation state of the Au, the AIMD simulations provide possible insight into the

mechanism of CO transport toward the Au||TiO2 interface where activation occurs. Because of its strong bonding to Au, CO diffusion is slow from one Au site to another [68] and thus the CO transport process may not be accomplished by diffusion of CO itself; rather, the whole Au  CO unit may migrate. The liquid-like properties of Au20 can even appear in operando conditions at temperatures as low as 120 K. The emerging picture is that the charge transfer that induces morphological restructuring of the Au cluster also affects the transport of species over the Au particle. Further, CO molecules quickly attach to the O2 species at the Au||TiO2 interfacial perimeter to form a CO  O2 intermediate that reacts within 4 ps to produce CO2. This mechanism is consistent with previous findings that the oxidation of CO occurs without rupturing the O  O bond a priori [32,68,525,561]. This understanding, along with a detailed description of the energetics and charge dynamics, is presented in Fig. 8.27, which shows the lowest energy pathway for CO oxidation on a Au20 cluster supported by TiO2 as determined by the AIMD simulations [708]. 8.2.2. H2 oxidation (planar model and high surface area catalysts) In a relatively early study into the extraordinary catalytic behavior of supported gold, Haruta et al. reported that the oxidation of hydrogen is dependent on the Au particle diameter, i.e. on the exposed surface area of active gold [27]. Later, they found that the oxidation of propene to propene oxide is enhanced in the presence of an O2/H2 mixture [205]. However, this raised the question of how hydrogen, a reducing agent, could possibly promote the oxidation. The formation of surface hydroperoxy species as intermediates was the logical hypothesis to explain the activation mechanism [36,205]. Since then, a great deal of attention has focused on the simple H2 þ O2 interaction and reaction on gold – not only due to its fundamental importance, but also due to its role in a number of catalytic applications. These include the direct H2O2

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Fig. 8.25. (a) A schematic of the Au24/TiO2(110) surface, illustrating that the CO oxidation reaction occurs preferentially along the [11̅0] direction of the support and not along the [001] direction. (b–d) Summary of the reaction steps with the highest activation barriers for each catalytic cycle investigated. The graph in (b) shows the energetics along the path and the drawings in (c) and (d) schematically show the transition states marked with a filled disk in (b). The dashed part of the blue line indicates O2 dissociation, whereas the other transition states stems from CO reacting with the O atom ([001]) or with the O2 molecule (corner). Adapted with permission from Ref. [685]. Copyright 2014 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 8.26. (a) The morphology of Au cluster changes upon CO adsorption; (b) The statistical distribution curve of low coordinated Au atoms for Au20/TiO2, Au20/ TiO2  x, and CO@Au20/TiO2  x. Reprinted with permission from Ref. [708]. Copyright 2013 American Chemical Society.

synthesis [189,190,730], the direct epoxidation of propylene, in the presence of H2 [36,205,731], and, the preferential CO oxidation (PROX) reaction for removal of CO from hydrogenrich streams in polymer electrolyte fuel cells [499,732–734]. Goodman and co-workers experimentally established the formation of one or more hydrogen peroxide/surface hydroperoxy species on nano–sized Au clusters during the interaction of molecular H2 and O2 over Au/TiO2 at 523 K [190]. The hydroperoxy species identified in the inelastic neutron scattering (INS) spectrum were likely either bound to Au particles or complexed with water that was also formed in this reaction. As previously proposed, the formation of H2O2, in contrast to H2O formation, would be favored on Au surfaces because of the relative stability of OH and OOH surface species [192]. Barton and Podkolzin studied, experimentally and via DFT calculations, the kinetics and mechanism of direct water synthesis over Au nanoparticles deposited on titanium silicate

Ti-MFI (TS-1 with 1 wt % Ti) [730]. The proposed mechanism for H2 þ O2 interaction includes the formation of an OOH intermediate. This hydrogen peroxy intermediate can react with hydrogen to form H2O2. Hydrogen peroxide decomposes to OH groups that react further with additional hydrogen to form water. Based on this mechanism, a rate expression was derived that accurately described the obtained experimental kinetic data. The effects of hydrogen on the reactivity of oxygen toward gold nanoparticles and surfaces have been further studied theoretically by DFT calculation in a work by Barrio et al. [301]. The coadsorption of O2 and H2 or H was examined on Au(111) and Au (100) and on free pyramidal clusters (Au25 and Au29). As discussed in Section 3.2, the reactivity of gold clusters (Au14, Au25, and Au29) toward O2 was strongly depended on the type of exposed low-coordinated Au sites, ensemble effects, and the fluxionality of the gold nanoparticles. Oxygen adsorption on the Au14 and Au29 clusters produced superoxo species and peroxo moieties,

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Fig. 8.27. Catalytic cycle and corresponding charge cycle for CO oxidation at the interface of Au20 supported by reduced TiO2 support. The energies are shown in the outer circle and the charges of the Au20 cluster are shown in the two inner circles. The red values correspond to Au/TiO2 x system with two O2 adsorbed (positively charged Au20), while those in blue corresponds to that with only one O2 adsorbed (negatively charged Au20). Reprinted with permission from Ref. [708]. Copyright 2013 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

respectively. When atomic hydrogen was present on the Au surface (obtained from H2 dissociation), the adsorption of oxygen was accompanied by the formation of a hydroperoxo species. Thus, the origin of the synergistic effect observed for the co-adsorption of H2 and O2 was explained by the formation of O–H bonds that enhance the O–Au interactions, reducing (by 0.15–0.30 Å) the O–Au bond lengths. In this work, the reaction H(a)þ OOH(a) - H2O2(a) was reported to be highly exothermic on the Au clusters. Yates and co-workers also explored the low-temperature catalytic oxidation of H2 over a Au/TiO2 catalyst [69]. The primary analytical probe of this chemistry was infrared spectroscopy, which was employed initially to track the interaction of H2 with Au/TiO2. During H2 exposure a very broad absorbance that spanned the entire mid-IR region of the spectrum was observed. This behavior is characteristic of the accumulation of electrons into the CB of TiO2. Population of the CB during H2 exposure is thought to occur through dissociation on the Au particles and atomic hydrogen spillover to TiO2 [595]. An atomic hydrogen binds to surface oxygen losing an electron that is injected into the CB (see Section 7.3.2). When a small quantity of O2 was introduced to the Hrich surface, the oxygen quenched the CB electrons and the IR background returned to baseline [69]. Simultaneously, the oxidation of H2 on the Au/TiO2 surface proceeded with the formation of H2O(a) as detected by the IR band at 1620 cm  1 (assigned to δH2O mode). A detailed kinetic study allowed the estimation of apparent activation energy of
22 kJ mol  1 for the process induced by O2 admission in the temperature region 200–220 K. The Au structure on the rutile TiO2(110) surface was simulated in DFT calculation with a Au nano–rod

covalently bound to the surface. In agreement with previous experimental result [595], the DFT calculations showed that the lowest H–H dissociation barrier on Au in the absence of O2 is approximately 50 kJ mol  1. In the presence of adsorbed O2, this energy was lowered to 16 kJ mol  1. This cycle is highlighted schematically in Fig. 8.28a, b, along with calculated energies of particular structures. Overall, this work suggests that adsorption of O2 at the Ti5c perimeter promotes the dissociative adsorption of H2(g) at neighboring Au perimeter site. Interestingly, these IR studies provided no direct evidence for the formation of hydroperoxo moieties, which have been suggested in other work as an essential catalytic intermediate [190,735]. 8.2.3. Preferential CO oxidation in presence of H2 (PROX reaction) The interest in the effect of H2 on CO þ O2 reactions over supported metal catalysts, including Au, stems from the potential employment of these or similar catalysts for the removal of CO from H2-rich feed gases prior to their introduction into fuel cells [544,736,737]. These gases are generally produced by steam reforming or oxidation of hydrocarbons and other small molecules [732,733]. Imai et al. [738] pointed out that supported Au nanoparticle catalysts have demonstrated intriguing predominance of CO oxidation reaction over H2 oxidation [736,739], and this behavior contradicts that of noble-metal catalysts, such as Pt or Pd. Goodman and co-workers concluded that an effective PROX catalyst is required that provides high CO oxidation activity with low H2 oxidation activity [501].

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Behm and co-workers have used kinetic measurements and in situ IR spectroscopy to study the influence of hydrogen on CO oxidation over Au/TiO2 catalysts prepared by the DP method (with control over Au loading) [499]. CO interactions with Au/ TiO2 at 330 K in an inert N2 gas flow (in the absence of H2) resulted in formation of on-top CO moieties at Au sites, with an IR band at 2112 cm  1, which red-shifted with increasing coverage to 2108 cm  1. With time, formate, carboxylate, and carbonate species were produced, as identified by their characteristic vibrational bands in the 1300–1700 cm  1 region and, for formates, by C–H vibrations between 2800 and 3000 cm  1. The critically important aspect of this work is that it demonstrated that admission of H2 to the N2 gas flow had a significant effect on the characteristics of CO interactions with the Au/TiO2 catalyst. Initially, the ν(C–O) was slightly red-shifted by 3 cm  1, which was hypothesized to be due to either charge transfer or to interactions between the co-adsorbed H and CO species at the Au nanoparticles. With extended reaction time, the TiO2 support was hydroxylated and the band assigned to linear CO species at 2107 cm  1 was converted into a new, asymmetric band with maximum at 2032 cm  1. Based on the wavenumber and shape, this new feature was assigned to an H/ CO complex. Kinetic measurements then were used to suggest that the presence of H2 in the gas phase affects CO oxidation by competitive hydrogen adsorption on the Au nanoparticles and by competitive reaction with oxygen, which seemed to result in an increased CO reaction order. Furthermore, water, formed upon H2 oxidation or admitted directly into the gas flow, was found to enhance the CO oxidation likely by lowering the concentration of surface formate/carbonate species that otherwise inhibit the reaction. Recently, a study by Behm's group [740] found that the active oxygen species (Oact) for the H2 oxidation and the CO oxidation are identical and are formed at

the perimeter of the Au–TiO2 interface. It is proposed that TiO2 surface lattice oxygen located at the Au–TiO2 interface perimeter supply the Oact species that are involved in H2 oxidation via a Mars-van-Krevelen mechanism. However, this process is less effective than the competing oxidation of CO that preferentially occurs. In a complementary work, Rousset and co-workers found that the ability of Au catalysts to oxidize CO preferentially over H2 is not a prerequisite for efficient PROX activity [734]. Although three Au catalysts deposited on three very different supports (TiO2, ZrO2 and Al2O3) showed different activity in the pure oxidation of CO, their PROX activity was found to be quite similar; however, all three catalysts exhibited a similar activity toward the oxidation of H2. In a similar study, Imai et al. explored the activity and selectivity of Au catalysts supported on TiO2, Fe2O3, and ZnO towards the preferential oxidation of CO in a H2-rich atmosphere (gas flow 1 vol.% CO þ 0.5 vol.% O2 þ 98.5 vol.% H2 at 20 000 h  1 mL/g-catalyst) [738]. At low temperatures, they found that Au loading was the main factor that affected the activity towards the CO PROX reaction. In contrast, at high temperatures the selective oxidation of CO to CO2 was suppressed by the competing reaction of H2 with O2 to form H2O, which was effectively independent of Au loading or even the identity of the support. Two key phenomena have been suggested to occur simultaneously during PROX reaction on Au catalysts: a competition between H2 and CO for adsorption on Au particles (which has a negative effect on CO oxidation) [499,734,738], and the appearance of additional reactive intermediates such as hydrogen peroxy-like species [190,191,730,734] produced in the presence of H2 (which have a positive effect on CO oxidation). The competition between these two phenomena is suggested to

Fig. 8.28. Catalytic reaction cycle and the corresponding activation barriers for the individual steps of the mechanism for the oxidation of H2 to form H2O over model Au/ TiO2 structures. The Au atoms and Ti atoms are shown in yellow and grey, respectively, whereas the O in the TiO2 lattice, adsorbed O, and H atoms are shown in pink, red, and cyan, respectively. Ea and ΔH represent activation barriers and reaction energies separately. Reprinted with permission from Ref. [69]. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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depend on the reaction conditions, i.e. the partial pressure of CO, O2, and H2, and the temperature. The extreme sensitivity to reaction conditions for this process has been demonstrated by the Rousset group in the case of a powder gold catalyst [741]. In addition to the Rousset work cited above, the existence of a favored mechanism for the oxidation of CO involving HxOy species for a CO þ O2 þ H2 gas mixture was proposed by Piccolo et al. [735] in their operando infrared spectroscopic study. As shown in Fig. 8.29a, the stepwise addition of H2 to the CO þ O2 feed caused a rise in the rate of CO oxidation by a factor of up to
2. The total loss of selectivity towards CO2 was not substantial (from 100 to 80 %). The DRIFTS data in Fig. 8.29b and c show formation of Auδ þ –CO, Ti4 þ –CO, carbon dioxide and carbonate species during the CO þ O2 reaction, while Au0–CO, water and hydroxyl groups were the main species observed upon the CO PROX reaction associated with the CO þ O2 þ H2 feed. Comparing the activity of several Au catalysts deposited on TiO2 and Al2O3 supports towards CO oxidation, CO PROX, H2 oxidation and H2 PROX reactions, Rousset and co-workers [742] established that Au/TiO2 is more active (for every reaction) than Au/Al2O3 catalysts. Support-dependent particle morphologies and additional support-mediated reaction pathways were the probable causes for this difference. However, it was clearly shown that H2 greatly promotes CO oxidation over Au/Al2O3, whereas the H2 effect on CO oxidation over Au/TiO2 was relatively small. Therefore, the presence of H2 reduces the role of the support for PROX reactions and thus a poor CO-oxidation catalyst such as Au/Al2O3 may be an efficient PROX-catalyst. The promotional effect of H2 on CO oxidation was suggested to originate from reactive intermediates or co-reactants formed in the presence of H2 [742]. The mechanism for the PROX reaction on Au/TiO2, described in many studies, is illustrated well by Fig. 8.30, which details the conceptual understanding of Rousset and coworkers [742]. This picture appears to be consistent with a number of other experimental and theoretical studies [190,301,730,734,735]. An important aspect of this mechanism is that H2 dissociative adsorption and O2 molecular adsorption on gold lead to formation of an OOH* moiety. All the proposed intermediate species have been previously identified spectroscopically, with the exception that Au–OH could not be distinguished from Au–OOH [735]. Note, that this mechanism includes the formation of a hydroxycarbonyl intermediate proposed by Bond and Thompson [622] and Kung et al. [714]. 8.2.4. Water–gas shift reaction (WGSR) In the industrial catalytic production of liquid fuels via Fischer–Tropsch synthesis and in methanol synthesis, gas mixtures COþ H2 with suitable CO/H2 compositions are required. The CO/H2 ratio in these gas feeds can be tuned via the water–gas shift reaction, WGSR: COþ H2O¼ CO2 þ H2, or reverse water–gas shift, CO2 þ H2 ¼ COþ H2O [743]. Perhaps more importantly, a combination of the WGS and PROX processes can be an efficient means of upgrading the purity of H2 in fuel cell applications, ammonia synthesis, and selective

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hydrogenation processes. Not surprisingly, based on the above discussion, supported Au catalysts have been explored for utilization in the WGSR [660]. Motivated by the early reports of Haruta and co-workers [78,86] along with the well known activity of α-Fe2O3 in the water–gas shift reaction, Andreeva and co-workers embarked on a study of the activity of Au/α-Fe2O3 in the WGSR, i.e. CO þ H2O [744,745]. They found that the Au/αFe2O3 catalyst exhibited activity for the WGSR at lower temperatures (T 4 400 K) than did (T 4 470 K) a sample of one of the most efficient commercial WGSR systems, i.e. a copper–zinc–aluminum (CuO/ZnO/Al2O3) catalyst [744]. This work spawned further studies into the activity of Au/αFe2O3 and Au/TiO2 for the WGSR, showed a strong dependence on the preparation method and the size of gold particles [746–748], a general trend for the activity of nano–sized gold already discussed for CO oxidation (see Sections 7.1, 8.1 and 8.2.1). Based on a combined IR spectroscopic and TPD study, it was concluded that the water-CO interaction at temperatures from 300 K to 523 K proceeds on the surface of small metallic gold particles [549]. Fig. 8.31 presents IR spectra obtained under the conditions of the WGSR for the Au/TiO2 catalyst. The exposure of the Au/TiO2 sample to water, with pre– adsorbed CO at 300 K, produced CO2, bicarbonates and systematic changes in the carbonyl region of the spectra (Fig. 8.31a, curves 2–4). The decrease in the intensity of bands associated with CO adsorbed on Ti4 þ surface cations (
2180 cm  1) was attributed to the displacement of CO adsorbates by molecularly adsorbed water on the same sites. The systematic changes in the carbonyl bands at 2110, 2030 and
1990 cm–1 indicated gradual transformation of on top CO species into bridged species bounded to negatively charged Au clusters. Simultaneously, bands at 1582, 1413, and 1220 cm–1, attributed to a bicarbonate species, increased significantly in intensity. The 2110–1 band assigned to a CO moiety bound at Au0 sites appeared directly related to the CO2 formation. The bridge-bonded CO species at Auδ– sites showed high stability and thus were suggested as spectator species. These results suggested that the role of water was to donate oxygen to gold nanoparticles, causing concomitant changes in the coordination of adsorbed CO. Upon temperature increase to 523 K, all the carbonyl bands and the bicarbonate bands were depleted, while bands at 2930 and 2851 cm–1 in the region of formate C–H stretch vibrations intensified. The authors reasoned that formates were produced via the decomposition of bicarbonate species. Parallel quadrupole mass analysis of the gas phase products evidenced the production of significant amounts of H2 and CO2, even at 373 K, which are clear products of the WGSR. Using the same experimental setup, the authors have also studied the reversible WGS reaction over the same Au/TiO2 catalyst [549]. Immediately after the introduction of a CO2 þ H2 gas mixture, production of a CO moiety, adsorbed on-top at Au0 sites (band at 2110 cm  1), together with CO2, weakly adsorbed on the support, and carbonate and bicarbonate species were observed. Importantly, they found

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Fig. 8.29. CO oxidation over Au/TiO2 reference catalyst at 323 K during exposure to a feed of 2 mol% COþ2 mol% O2 in He, and, PROX reaction upon addition of 2, 5, 10 and 16 mol% H2. (a) Mass spectrometry signals for CO2 (m/z¼44, red, top curve) and H2O (m/z¼ 18, blue, bottom curve). (b) Position (red squares/left scale) and intensity (blue disks/right scale) of the main Au-carbonyl peak. (c) Overview of the features detected by DRIFTS: top spectrum, recorded at t¼87 min during COþO2 reaction, and, bottom spectrum, recorded at t¼322 min during PROX reaction. Corresponding vibration modes and surface species are indicated. Adapted with permission from Ref. [735]. Copyright 2009 Elsevier B.V. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

that no IR bands that could be assigned to CO(a) appeared in the spectrum if CO2 was adsorbed in the absence of hydrogen. Based on these results and previous reports, two reaction mechanisms have been presented for the WGS reaction: a regenerative, redox mechanism and an associative mechanism [549,658]. The former mechanism comprises successive oxidation and reduction stages of the surface, while the latter mechanism involves production of surface formate intermediates. The redox mechanism involves complete dissociation of H2O to form surface Oa and Ha where the Oa species reacts with CO. The mechanism involving formate requires the dissociation of H2O to form OHa, which subsequently produces HCOOa upon reaction with CO. Formates are then suggested to decompose to H2 and CO2 products. The participation of carbonate and bicarbonate intermediates, instead of formates, may also be a possibility in this process. The redox mechanism is believed to occur mainly at high-temperatures in the presence of reducible supports. It is well known that the partial reduction of the support may form oxygen vacancies that further promote the WGS catalysis [658]. Although variations in the composition, preparation method, structure, kinetics, and reaction mechanism have been widely investigated and shown to be extremely important, the activity of gold catalysts in WGSR is not well understood and rationalized [658,743,749–755]. Based on published work,

Burch [749] analyzed the relationship between activity, structure, and reaction mechanism for a large number of gold catalysts studied for the WGSR [749]. From this comprehensive analysis, a universal mechanism was proposed, which accounts for many factors that play an important role in the catalytic efficiency: the choice of catalyst, the temperature, and the composition of the reaction gas mixture. All these factors influence the formation of reaction intermediates and thus the mechanism of WGSR. The proposed universal mechanism (Fig. 8.32) for the WGSR is consistent with most of the available experimental and theoretical results published to date. A key for better understanding this mechanism is the realization that all previously proposed mechanisms are “subsets of a single process.” For example, in the “formate” model the decomposition of a “carbonate” is required to complete the catalytic cycle. When the first and last steps of the reaction mechanisms are separated in time, then this mechanism is described by the “redox” model. While Fig. 8.32a shows the mechanism for the reverse WGSR, Fig. 8.32b presents the corresponding, identical, mechanism for the forward WGSR. The principle of microscopic reversibility is reflected in the “universal” mechanism presented in Fig. 8.32c, i.e. the precise details of the reaction mechanism can be defined by the experimental parameters. In its simplest terms, the role of water in the WGSR is to leave oxygen on the surface and to release

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Fig. 8.30. Mechanistic scheme for H2-promoted oxidation of CO (CO PROX). Reprinted with permission from Ref. [742]. Copyright 2009 Elsevier B.V.

hydrogen into the gas phase, while the role of carbon monoxide is to remove oxygen from the surface and to release carbon dioxide. The following discussion clearly shows that a key point to bear in mind for water–gas shift catalysts is that the reaction conditions define the nature of the active catalyst surface. Rodriguez and co-workers [756] were among the pioneers in applying surface science approaches to the study of the lowtemperature WGSR on catalysts containing Au nanoparticles supported on well-characterized flat CeO2(111) or rough CeO2 films as model catalysts. These authors also applied an interesting approach in studying the WGSR over inverse catalytic systems having TiO2 and CeO2 nanoparticles grown on Au(111) [757]. Although clean Au(111) is inactive in the WGS reaction, covering 20 to 30% of this surfaces with ceria or titania nanoparticles produced systems with activities similar to those of high performing WGS catalysts such as Cu(111) or Cu(100). In these inverse TiO2-x/Au(111) and CeO2-x/Au(111) systems, O vacancies of the oxide nanoparticles serve to dissociate water, while Au sites located nearby sequester CO, after which the metal-oxide interface provides sites for the subsequent WGS reaction steps. Thus, the mild chemical activity of bulk gold, coupled to the more highly reactive oxide, is found particularly advantageous for these reactions. Based on these results, new mixed-metal oxide systems were designed such as Au/CeOx/TiO2 [758] and Au/ MxOy/TiO2 (MxOy ¼ Al2O3, CaO, Y2O3, and ZrO2) [759] in attempts to improve the activity and thermal stability of Au catalysts in the WGSR. Recently, Ribeiro and co-workers reported a series of studies [753–755] whereby the detailed nature of active sites were investigated. Specifically, the performance of Au/TiO2 and Au/Al2O3 catalysts in the WGSR was investigated at atmospheric pressure conditions in the temperature range 350– 420 K. Time-resolved IR spectroscopic data obtained upon transient isotopic experiments showed that CO adsorbed on Au0 sites is involved in the reaction pathway, while the surface

Fig. 8.31. (a) FTIR spectra scanned before and during the WGS reaction performed on reduced Au/TiO2: curve 1, 25 mbar CO; curve 2, immediately after admission of the first dose of H2O (3.6  10  3 kPa) on the IR sample cell, already containing a much higher CO pressure; curve 3, immediately after admission of the second dose of H2O (2.5  10  2 kPa); curve 4, after 1 h of reaction at RT; curve 5, after 10 min at 523 K. The carbonyl region is magnified in the inset. (b) Effect of CO admission on the FTIR spectra of Au/TiO2 pre-covered with H2O, in the presence of the water–vapor phase (thin curve) or in its absence (bold curve). Reprinted with permission from Ref. [549]. Copyright 1999 Elsevier B.V.

formate and carbonate species are spectators under their reaction conditions [760]. The dominant active sites were found to be the low coordinated corner Au sites, which were suggested to be much more reactive than perimeter Au sites [754]. Perhaps not entirely surprisingly, given the complex nature of this chemistry, the studies described above often yield contradictory information about the performance of supported gold catalysts in the WGSR depending on the reaction conditions: low pressure and low temperature, or atmospheric pressure and moderate temperature. Some authors propose the direct participation of the oxide-metal interface [757] (a common theme and constant re-discovery presented throughout the literature), while others predict the active role for the low coordinated corner Au0 sites that are remote from the interface [753]. Further, the nature

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Fig. 8.32. (a) Proposed “carbonate/carboxylate” mechanism for the reverse WGS reaction. (b) Proposed “carbonate/carboxylate” mechanism for the forward WGS reaction. (c) Proposed “universal” mechanism for the WGS reaction. Note H species attached to Au particles are not intended to imply a stable species, but simply a very transitory species that provides a route to spillover hydrogen and may be formed at surface defects or via assisted adsorption. Reprinted with permission from Ref. [749]. Copyright 2006 The Royal Society of Chemistry.

of the surface-active species involved in the reaction pathway remains unresolved in terms of which species are spectators in the reaction: CO or formate-carbonates [761]. Overall, there remains a significant opportunity for further research in this area.

8.2.5. Selective oxidation of ethene and propene (planar model and high surface area catalysts) Selective oxidation reactions of terminal alkenes, such as the epoxidation of ethene, propene and styrene on supported Au and Ag catalysts, have attracted significant attention over the last decade because of the importance of alkene oxides as starting chemicals in different fine chemical synthesis processes [39,188]. A process for the direct epoxidation of alkenes is highly desirable, as currently available processes need optimization, e.g. operation under mild conditions and without solvent. Although the epoxidation of ethene with O2 is realized as an industrial scale process, i.e. based on supported Ag catalyst and giving high selectivity ( 4 90%) [39], the epoxidation of propene is more problematic and gives low selectivity ( o 10%) with different catalysts. As such, gold– titania catalysts have been investigated as possible candidates for production of alkene oxides. As with the other reactions discussed above, these alkene-oxidation catalysts are suggested to act as bifunctional, i.e. both Au NPs and TiO2 are involved in the hydroepoxidation reaction [762]. The primary role proposed for Au NPs is to produce peroxide species, which thereafter epoxidize propene over a titanium site.

8.2.5.1. Ethene. Since the discovery of ethene epoxidation on silver surfaces [763], the ethene oxidation mechanism was found to follow two parallel paths [764,765]. As illustrated in Fig. 8.33a, selective (k1) and non-selective (k2) parallel reactions take place on the metal particles, whereas consecutive isomerization of ethene oxide (EO) to acetaldehyde (AA) (k3) is found to be sensitive to the acidity of the support material. Once formed, AA rapidly combusts (k4) to CO2 and H2O. Although the combustion of ethene is the thermodynamically favored reaction path, selective production of EO is a kinetically controlled reaction [766]. Early studies considered both the molecularly adsorbed oxygen [203,767] and adsorbed atomic oxygen [768] on the metal surface as the active species for the selective channel. A theoretical study of the reactivity and the stability of oxygen on Cu, Ag, and Au surfaces showed that both Cu and Au surfaces display reactivity similar to that of Ag, leading to EO formation [203]. On Ag, superoxide was found to have a sufficient lifetime to react with ethene and lead to EO production. This unique aspect of the Ag chemistry is related to the electron transfer efficiency and to the geometry of the Ag surface. On Au, the electron transfer from the metal to the adsorbate is less effective, thus the molecular and dissociative oxygen species are not sufficiently stabilized to perform epoxidation [203]. 8.2.5.2. Propene. Since the discovery of selective epoxidation of propene by gold catalysts [205], the partial oxidation of propene on supported Au catalysts has attracted significant research interest [186,188,769]. Although propene oxidation by gold demonstrates some of the special catalytic issues

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characteristic of CO oxidation, the complexity of the propene molecule requires the consideration of more reaction pathways, as highlighted in Table 8.1 [186]. Early studies showed that propene adsorbs weakly on clean single-crystal Au surfaces in a slightly tilted configuration with a desorption activation energy of only 9.4 kcal/mol [296]. On oxygen-precovered surfaces, the tilting of propene molecule increases, indicating a weak interaction with the oxygen. At high oxygen coverages, the propene oxidation reaction is enhanced and results in CO2, CO and H2O production. Later studies identified the formation of products of partial oxidation [acrolein, acrylic acid, and carbon suboxide (O ¼ C ¼ C ¼ C ¼ O)] and combustion (CO2 and H2O) [370]. It was suggested that modifying the properties of the O–Au bond, e.g. via addition of an oxide support, might engender enhanced selectivity toward the epoxidation process. Thermal desorption studies of propene on a model Au/ TiO2(110) catalyst revealed the presence of a high energy desorption feature, different from that observed with either TiO2(110) or low index single crystal Au surfaces [774]. This feature was attributed to propene desorbing from Au sites at the perimeter of gold islands on the TiO2(110), where the molecule interacts simultaneously with both the perimeter gold sites and the TiO2(110) surface. The facile diffusion of propene on the surface to bind at the edges of Au islands was suggested to be important for its interaction with eventual peroxide species formed on Au islands. For Au nanoparticles deposited on high surface area supports, several major issues were found to be important for their activity and selectivity towards propene partial oxidation, such as the loading and size of gold nanoparticles and method of preparation [769]. Furthermore, the presence of promoters such as alkaline and alkaline earth salts can also be beneficial [769]. In a series of experiments, Nijhuis et al. studied the epoxidation of propene on Au/TiO2 catalyst in the presence of a H2 þ O2 mixture [731,762,775–777]. They found that the titania support is involved in the formation of a bidentate propoxy species [731,775]. The Au nanoparticles promote the further oxidation of the bidentate propoxy species to carbonate/ carboxylate moieties, which, however, appear to possibly deactivate the catalyst [776]. In the presence of hydrogen

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and oxygen, hydroperoxide species (*OOH or H2O2) have been shown to form on gold clusters [146] and nanoparticles [190,730]. The peroxide species on gold can then promote the desorption of bidentate propoxy compounds from titania, producing propene oxide (PO) and water to restore the titania to its original state [731,776,777]. Lee et al. [208] have studied experimentally and theoretically the epoxidation of propene on model gold catalysts (Au6– Au10 clusters deposited on Al2O3 and TiO2). Their experiments were conducted with various gas mixtures C3H6/O2, C3H6/O2/H2, and C3H6/O2/H2O (in helium). Surface hydroxyl groups at the perimeter of the Au clusters were found to be crucial for the epoxidation reaction. As depicted in Fig. 8.34, DFT calculations suggest three different possibilities for adsorption of propene on the catalyst surface. The first structure highlighted in Fig. 8.34 involves the interaction of the double C ¼ C bond of propene and a lowcoordinate gold atom (Fig. 8.34a and d). Such bonding is rather strong (1.09 and 0.81 eV for Au7/Al2O3 and Au/TiO2, respectively) and is consistent with previously reported π-binding of propene to Au/TiO2 [775] and Au/SiO2 [778]. A probable second step of the reaction includes diffusion of the molecule to the gold/oxide interface, which converts the double C ¼ C bond into a single C–C bond to form the two new covalent C–surface bonds (Fig. 8.34b and e). This conversion is almost barrierless and leads to a highly stable final state. The conformer that bridges a surface oxygen anion and a gold atom resembles the metallacycle (C3H6O) intermediate already shown for ethene epoxidation (Fig. 8.33b) [210]. Interestingly, the mechanisms of metallacycle intermediate formation was different for the two systems: peroxo •OOH radicals were needed for formation of C3H6O on Au/TiO2, while on Au7/ Al2O3 the C3H6O species might form directly after C3H6 adsorption [208]. On both surfaces, a neighboring hydroxyl group was found to promote the detachment of propene oxide metallacycles from the substrate and thus to favor the epoxidation [208]. Indeed, recent studies demonstrated that *OOH species might form from H2O/O2 mixtures and act as a peroxide precursor in epoxidation of propene to propene oxide on Au/TiO2 catalysts [777,779].

Fig. 8.33. (a) Molecular mechanism for ethene epoxidation. Reprinted with permission from Ref. [764]. Copyright 2012 Springer Science+Business Media, LLC. (b) Schematic representation of the three transition-state structures involved in the epoxidation mechanism on Au(111) surface. Reprinted with permission from Ref. [210]. Copyright 2006 American Chemical Society.

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Table 8.1 Partial oxidation of propene on gold catalystsa. Catalyst

Oxidant/Reductant

Product

Selectivity (%)

Ref.

Au(111) Au/SiO2

O O2

Acrolein Acrolein Acetone Ethanal Propanal Acrolein Propane Propene oxide Propene oxide Ethanal Propanal Propane Ethanal Propanal Propene oxide Propane


53 33 3.5
75
15
10

[370] [770]

O2 þ H2

Au/TiO2

H2 or D2 O2 þ H2

Au/MgO

O 2 þ H2

H2 or D2

490 67
15
30 99 (plus any CO2)
65
20
20

[771]

[772] [769] [771]

[773] [771]

[772]

a Adapted with permission from Ref. [186]. Copyright 2007 American Chemical Society.

8.2.6. Selective oxidative dehydrogenation of alcohols and carboxylic acids The conversion of biomass-derived feedstock into fuels and chemical intermediates via catalytic oxidative dehydrogenation is currently an issue of paramount importance for developing technologies to reduce the world's dependence on fossil fuels [780]. A key aspect of this chemistry will require effective oxidative dehydrogenation catalysts. Metal-oxide supported gold systems have been found to serve as active and selective catalysts in oxidative dehydrogenation of alcohols and carboxylic acids at relatively low temperatures [188]. Dehydrogenation [781–784], selective oxidation [785–788] and carbonylation [789] of alcohols, and selective oxidation of carboxylic acids [790–792] have been studied on model (see Section 4.3) and real supported Au catalysts; nonetheless, the role of the Au particles themselves remains a subject of fundamental interest. For oxidation and dehydrogenation reactions on supported Au-catalysts, some authors have proposed that the activation of alcohols proceeds on the gold surface [793], while others have suggested this to occur on the support [794,795]. It was also proposed that supported gold catalysts are bifunctional, i.e. the support surface provides sites for activation of alcohols whereas gold particles present sites for hydrogen abstraction and recombination [782,795]. Finally, unique bifunctional characteristics of Au Ti4 þ perimeter sites have been highlighted as the sites for oxidative-dehydrogenation of carboxylic acids on TiO2 supported Au nanoparticles [791]. 8.2.6.1. Methanol (planar model and high surface area catalysts). In addition to their potential practical importance as sources of H2, methanol and ethanol oxidation are relatively simple probe reactions with fundamental importance [796,797] for understanding the chemistry of thermally and photoactivated oxidation reactions. The oxidation chemistry of methanol has been well studied on Au single crystal surfaces as well as on a variety of titania surfaces: single crystals,

polycrystalline films and powders. The adsorption and thermal decomposition of methanol on single crystal Au and on Au nanoparticles supported on planar and high area TiO2 supports were discussed in Sections 4.2.6 and 7.7.1, respectively. Surprisingly, the oxidation of methanol on Au nanoparticles supported on TiO2 has been relatively infrequently studied since the first reports on full [798] and selective [663] oxidation of methanol were published. Importantly, Haruta et al. found that the oxidation of methanol and its decomposition derivatives on supported gold catalysts, including Au/TiO2, is markedly dependent on both the metal oxide support and the particle size of Au [798]. Others have corroborated this finding [663]. Chang and co-workers have studied the details of the chemistry and factors defining the catalytic activity of Au/ TiO2 catalysts for the partial oxidation of methanol (POM) to produce hydrogen [663]. In their study, the Au/TiO2 catalysts were prepared by the DP method and the size of gold particles was finely tuned between 3.5 and 1.9 nm by controlling the pH from 6 to 9. They found that high calcination temperatures, up to 673 K, resulted in an increase of Au particle diameter from 2.9 to 4.3 nm. According to XPS analysis, gold in uncalcined catalysts existed in three different states defined as metallic gold (Au0), non-metallic gold (Auδ þ ) and Au2O3. After the calcination at 573 K, all the deposited gold converted to its metallic state. In general agreement with the Haruta's work, [798] the catalytic activity of the Au/TiO2 catalysts was found to be strongly dependent on the size of Au particles. The highest selectivity for hydrogen production showed the catalyst precipitated at pH 8, containing nominally 1.35 wt. % Au with average particle size of 2.9 nm. The POM reaction caused aggregation of smaller Au particles resulting in large size Au crystallites, i.e. the average diameter increases from 2.9 to 7.4 nm. Further, they demonstrated that the O2/CH3OH molar ratio was a key factor in determining the final product distribution. At molar ratio 0.3, the selectivity towards hydrogen production was almost complete over the temperature range (483–583 K) studied. When increasing the O2/CH3OH molar ratio to 0.5, a trace amount of methane was observed but no traces of dimethyl ether, formaldehyde and/or methyl formate were detected. The suggested reaction pathway for selective oxidation of methanol to hydrogen includes consecutive stages of methanol combustion, partial oxidation, and steam reforming. Chang and co-workers also examined possible strategies for improving the performance of Au/TiO2 catalyst towards the POM reaction via addition of a second support TiO2 – MOx (M ¼ Fe, Co, Zn) [799], or by addition of a second metal (Cu) [800]. Gold particles deposited on a composite support TiO2 – MOx were found to be more stable towards aggregation and showed increased activity. A combination of factors, such as enhanced mobility of lattice oxygen that maintains the oxidation state of active gold sites and preserves the size of Au particles, was suggested to be responsible for the better performance of composite systems [799]. In addition, the interaction between Au and Cu helped to create smaller metal particles in bimetallic systems, which stabilized the active component for hydrogen production [800].

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Fig. 8.34. Most relevant relaxed structures and binding energies (with respect to gas-phase propene) for propene adsorption at (a–c) for Au7/Al2O3 and (d–f) for Au/ TiO2. Al green, Au yellow, H white, O red, C gray, Ti light gray. Adapted with permission from Ref. [208]. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

UHV surface science studies have been employed by Chen and co-workers [345] in the study of the partial oxidation of methanol over Au clusters vapor-deposited on TiO2 (110), where the Au loading was between 0.25 and 5 ML. The adsorption and thermal decomposition of methanol over these systems is fully described in Section 7.7.1. Chen's results showed that both TiO2 and Au nanoparticles were necessary for thermal dehydrogenation of methanol to formaldehyde. The TiO2 support was suggested to be responsible for the rupture of the O–H bond in methanol and produce a reactive methoxy intermediate. The role of Au particles was to provide sites for recombination of H atoms into H2. Otherwise, on clean TiO2 hydrogen atoms recombine with surface methoxy groups to reform methanol. Formaldehyde and methyl were the main products of the thermal dehydrogenation of methanol and both appeared in the temperature region of 450–600 K during TPD experiments with near stoichiometric Au/TiO2 (110). The oxidation of the Au/TiO2 (110) surface was found to promote formaldehyde production regardless if oxidation was applied before or after the Au deposition. As shown in Fig. 8.35, the reaction of CD3OH with Au/TiO2 produced only H-containing products at temperatures below 350 K, while D-containing products dominated at higher temperatures. This finding is consistent with the conclusion that the initial reaction of methanol occurs via O–H bond scission to form methoxy groups at low temperature. The fact that only CD2O formaldehyde species evolved at high temperatures implies that the selective C–D bond scission occurred in the methyl group. The deuterium atoms liberated from C–D scission in methoxy mostly combine to form D2 rather than to diffuse to another neighboring methoxy to recombine and produce methanol. The absence of methyl formate production indicated the low mobility of methoxy and formaldehyde on Au clusters, which is the opposite from what was observed for bulk Au surfaces [801]. The observed relationship between formaldehyde yields

and Au particle size is consistent with formaldehyde production at the interfacial sites, in that the formaldehyde yield increases with the number of sites available at the perimeter of the Au clusters. Recently, Camellone et al. reported that the high catalytic activity and selectivity of Aun/TiO2 (110) model catalysts in oxidative dehydrogenation of methanol to formaldehyde originates from interfacial sites [664]. By means of GGA þ U calculations, Aun/TiO2 (110) was modeled by growing clusters, Aun, at an F-center on TiO2 (110). This F-center was created by a surface O vacancy that acted as an anchoring site for initial nucleation of Au clusters. This research suggests that the activation of molecular oxygen occurs at a dual perimeter sites [68], between the Au11 clusters and the TiO2 (110) surface. In the reaction, charge is transferred from Au11/TiO2 (110) to oxygen by forming Oδ2  with a strong binding energy of
2.15 eV. Calculations showed that the charge transferred to O2 at the interfacial sites was almost independent of the size of the Aun (n ¼ 11–16) cluster and its shape. Once formed, Oδ– 2 is suggested to react with incoming methanol to form a CH3OH–O2 intermediate at the dual perimeter sites. Upon its interaction with CH3OH, the O–O bond is weakened and dissociates with an activation barrier of 0.05 eV to form Ti5cOH and Au-CH3O. The Au-CH3O intermediate loses an H atom to a Au site at the metal cluster, thus transforming itself into Au-CH2O. The calculated activation energy for this step was 0.69 eV. In this way, the gold cluster behaves as a hydrogen storage device. The energy barrier for CH2O desorption was determined to be 0.29 eV. The H atom needs to overcome a barrier of 1.5 eV to migrate over the gold cluster and reach an activated O atom at a perimeter Ti site. The reaction of hydrogen and the oxygen atom forms an OH intermediate. The hydroxyl species subsequently may react with a neighboring OH group to produce an H2O molecule. Overcoming an energy barrier of 0.9 eV, water can desorb and

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thereby complete the cycle of methanol transformation. The O atom species left behind at the gold-titania interface is then available for the beginning of a new reaction cycle. The corresponding experimental examination of the aboveproposed mechanism was performed with model Au/TiO2 (110) planar systems using HREELS and TDS studies [664]. In that work, gold nanoparticles were deposited at 300 K onto vacuum-annealed TiO2 (110) containing
10% of surface oxygen vacancies. Based on the STM, AFM and EELS data, the structural transition of Au particles from the 2D to 3D configuration was found to occur at around 0.2 ML of overall gold coverage [802]. This transformation was indicated by a feature peak at
2.5 eV in the EELS spectrum, assigned to the surface plasmon excitation of Au clusters [802]. Another loss at 0.8 eV was attributed to Ti3 þ sites at oxygen vacancies [803] and indicated a charge transfer from Au to the TiO2 substrate, as was predicted by the DFT calculations [664]. This peak diminished at larger Au-coverages due to the increased

Fig. 8.35. Temperature programmed desorption data for a saturation exposure of CD3OH adsorbed at 100 K on 0.25 ML of Au on TiO2. Reprinted with permission from Ref. [345]. Copyright 2012 Elsevier B.V.

screening in 3D Au clusters. HREELS spectra obtained during low temperature (95 K) adsorption of CO on as-prepared Au/TiO2(110) revealed the existence of coordinatively unsaturated Ti4 þ sites on the “free” TiO2(110) surface (band at 2189 cm–1) [524,600,649,804] and undercoordinated neutral Au sites at the Au/TiO2 interface (band at 2104 cm–1) [68]. When CO was adsorbed on Au/TiO2(110) pre-exposed to O2 at 95 K, a new band at 2136 cm  1 appeared, indicating CO adsorption at positively charged Au sites [68,524,597,600]. It is noted that this band was previously attributed to CO bound at low coordinated Au sites in the presence of O2 to form peroxo/superoxo-type Oδ2  –Auδ þ –CO species at the interface [524]. The 32 cm–1 blue-shift of this band reflects the decreased electron back-donation from Au to the CO 2π* antibonding orbital in the presence of Oδ2  , in excellent agreement with the prediction of DFT calculations. The TDS revealed desorption of CH3OH, CH2O, and H2O after low temperature (94 K) adsorption of methanol on Au/ TiO2(110) for all Au-coverages (0 – 0.8 ML). The data from the TDS study were in general agreement with the results previously reported for the activity of Au/TiO2(110) [345] and TiO2(110) [636,638] surfaces. The CH2O evolution was found to maximize for the 0.4 ML sample, indicating that this is the optimal Au coverage for achieving maximum concentration of active interfacial sites. This coverage corresponds to an average Au particle size of 3–3.5 nm [68,88,373,384,388,394,564]. Moreover, this reaction was proposed to occur at dual perimeter sites between Au and TiO2 [68]. The above-discussed experimental and theoretical results of this work consistently show that the interfacial sites between Au and TiO2 govern the activity of Au/TiO2 in a low-temperature reaction channel for oxidative dehydrogenation of methanol to formaldehyde [664]. Another important research contribution in this field was aimed at addressing the fundamental question of whether characteristics, such as Au particle size and the nature of interfacial sites at the Au perimeter, affect chemistry in a general way that could be applied to other technologically important reactions. Specifically, Hong and Rahman [805] employed DFT calculations to provide a rationale for the high reactivity of interfacial sites in the decomposition of methanol on a Au13/TiO2(110) model catalyst. They established, that the high activity of interfacial sites in this reaction originates from coulombic interactions, induced by charge transfer, among the gold, reactant, and reducible TiO2 support. Specifically, the formation of an ionic O  Au bond between a perimeter gold site and methoxy was identified as a prerequisite for the high reactivity of the gold–titania interface. In the work described above [805], the Au13/TiO2(110) was modeled by placing a Au13 cluster on a 30% reduced TiO2(110) surface in a hemispherical geometry, i.e. interfacing with the support at 4 Ticus sites, as shown in Fig. 8.36a. The hemispherical geometry on reduced TiO2 was the most energetically favored (among those investigated) and exhibited a high density of interfacial sites available for catalytic reaction [805]. In this system, methanol was found to adsorb dissociatively on TiO2(110) with no energetic barrier for O  H rupture on the fully reduced surface, or when it reacts with oxygen ad-

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atoms at terminal Ticus on the stoichiometric surface. The resulting methoxy was identified as the active intermediate species of methanol on TiO2(110). On partially reduced Au13/ TiO2(110), O-vacancy (Ovac) is the most probable site for methoxy adsorption. A comparison of Fig. 8.36c and d shows the changes in the adsorption geometries of the methoxy group when the bonding site changes from noninterfacial to interfacial: (i) an Au–O bond is formed; (ii) tilting of the methoxy axis increases and the O–Ti and O–C bonds are elongated. The calculations yielded a much lower energetic barrier for the oxidation of methoxy to formaldehyde via C  H bond scission at the interfacial sites (0.78 eV) as compared noninterfacial sites (1.29 eV). In addition to the above work, the authors [805] endeavored to answer another fundamental question: “What particular electronic property of the interfacial site causes the marked differences described above?” Calculations revealed a strong mixing of the charge densities. Wave functions of the reactants, the gold NP, and the support appear to be mixed in a way that leads to charge accumulation within the methoxy group, i.e. at the O atom and in the C moiety of the methoxy, while charge is depleted from the Au atom that is bonded to the O atom. Thus, the Au atom, which donates an electron to the O atom of the methoxy, becomes cationic, as demonstrated by results of Bader analysis, shown in Fig. 8.37. Fig. 8.37a illustrates the results that many of the Au atoms within the particle remain neutral in the absence of methanol, with the exception of the atoms on the upper surface and left edge of the particle (which exhibit cationic character) and those on the lower surfaces (which appear to be anionic) [386,806,807]. Note that the three central Au atoms, strongly bonded to O vacancies, are all anionic. Fig. 8.37b shows that adsorption of methoxy at the interfacial site alters only the interfacial Au atom that is directly bonded to the methoxy. This low-coordinated interfacial Au atom is found to donate a

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partial charge (
0.11e) to the oxygen on methoxy. On the whole, both the Au particle and TiO2 donate charge to methoxy, i.e. 0.15e and 0.17e, respectively. The overall charge distribution between the Au atom, the methoxy, and the support is presented in Fig. 8.37c. It becomes clear from the Bader analysis that the new O  Au bond is ionic and that the methoxy effectively oxidizes the Au and TiO2 support, as proposed by Chen et al. [345]. The positively charged Au atom and C moiety of the methoxy thus experience a net repulsive interaction. A direct consequence of this repulsion is the large tilting of the C  O axis of the methoxy. The Au 2 C repulsion causes the energy of the positive Au atom to shift up while that of the negatively charged O atom shifts down. Thus, the O atom becomes stabilized (less reactive) but the positively charged C destabilizes (becomes more reactive), which tends to activate the C  H moiety for bond scission. To summarize, “charge-transfer induced activation of methoxy (by the repulsive potential of the cationic gold) together with the enhanced H  Obr interaction by the large tilting of methoxy axis contribute to reduction of the energy barrier for C  H scission reaction in the interfacial site” [805]. The oxidation of gold, as a critical aspect of catalytic reactions, appears to be a general phenomenon that is not restricted to methanol reactions or to TiO2 as the support. As already discussed in previous sections, cationic Au has been proposed to induce strong binding of the particles to a support [578] and to create active Au–oxide at the Au–support interface [551,808]. In the cases of CO and H2 oxidation on Au/ TiO2, there is a consensus that O2 adsorption and activation occurs on TiO2 and that the catalytic reaction with O2 mainly occurs at the interfacial sites [68,69,578,705,706]. The concept for activation of O2 at interfacial sites, which naturally involves charge transfer, will be further examined, vide infra, for other thermally and photolytically activated catalytic reactions on TiO2 supported gold nanoparticles.

Fig. 8.36. (a, b) Adsorption of Au13 cluster on the partially reduced TiO2(110) surface with triple vacancies. Schematics of methoxy (c, d) and formaldehyde and hydroxyl (e, f) adsorption on Au13/30%-reduced TiO2(110) surface at (c, e) a noninterfacial and (d, e) an interfacial site. Light-blue spheres are used for Ti, yellow for Au, red for support, dark blue for molecular O, green for C, and gray for H. Adapted with permission from Ref. [805]. Copyright 2013 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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8.2.6.2. Ethanol (planar model and high surface area catalysts). A study by Biella and Rossi has shown that relatively large Au nanoparticles ( 4 15 nm) supported on silica are efficient in the catalytic oxidation of alcohols to aldehydes and ketones exhibiting high selectivity [786]. In part, these results have inspired some surface science studies on the oxidative chemistry of alcohols on single crystal Au surfaces [809,810]. Mullins et al., studying the oxidative conversion of ethanol into acetaldehyde on atomic oxygen precovered Au(111), discovered that the reaction begins with initial O-H bond rupture to yield ethoxide followed by β-C–H bond activation to form acetaldehyde and water [809]. At low O-coverages (o 1 ML), the β-C–H bond scission with selective oxidation to acetaldehyde dominated the reaction. At greater O-coverages, cleavage of the γ-C–H bond and the C–C bond also proceeded via nonselective oxidation to CO2 and H2O, in addition to acetaldehyde. Further mechanistic details in ethanol oxidation on O-covered Au(111) were reported in a study by Friend and co-workers [810]. That study showed that the concentration of adsorbed oxygen plays a pivotal role in governing the selectivity of ethanol oxidation. The production of the ester was found to maximize at initial O coverage of
0.2 ML. At these low O coverages, Au nanoparticles were formed from O-induced release of Au from the surface. These particles exhibited local 3-fold coordination of O and their diameters were
2 nm [290]. With increasing oxygen coverage, products of secondary oxidation formed at the expense of the ester, i.e. acetic acid, ketene, and CO2. At higher O coverages the diameter of 2-D “oxide” islands increased to 4 20 nm [290,558]. This study highlighted the existence of competing pathways involving two key intermediates: ethoxy and acetate.

In general agreement with these experimental findings are the results of the recent theoretical study by Meng et al. [811]. Comprehensive periodic density functional theory calculations were applied to examine the reaction pathways for selective oxidation of ethanol on the Au(111). Ethanol dissociation via the O–H bond scission and transfer of the H atom to oxygen or surface hydroxyl groups (rather than to gold) was found to have a low activation barrier, i.e. 0.20 or 0.17 eV, respectively. Extraction of hydrogen via selective β-C–H bond scission followed by transfer of an H atom to surface O atoms or another ethoxyl was also predicted to experience low energy barriers of 0.29 and 0.27 eV, respectively. The preferred adsorption site, configuration and respective activation barriers for radical intermediates as well as final products, were examined in this work. Acetaldehyde, acetic acid, and ethyl acetate were the primary products of ethanol oxidation, formed through mechanistic steps close to those reported in the previous experimental work [809,810]. The low diffusion barriers for all the intermediates, except for surface oxygen and bidentate acetate, allowed facile rearrangement and migration of these moieties from the initially preferred adsorption site to the ideal configuration for reactions to occur. The activation barrier for the β–H elimination of ethoxy was found to exceed that for ethanol activation and formation of ethyl acetate. Production of acetic acid via interaction of acetaldehyde with adsorbed O atoms was not favored energetically [811]. The important conclusion from the above experimental and theoretical studies with extended Au surfaces is that when oxidized, these Au surfaces are capable of selective oxidation of alcohols. The selectivity of alcohol oxidation is mediated by the concentration of adsorbed oxygen with probable formation

Fig. 8.37. (a, b) Oxidation state of gold atoms (and methoxy molecule) before (a) and after (b) methoxy adsorption at the interfacial site on the partially reduced TiO2(110) surface. Red, black, and blue represent positive, neutral, negative charge, respectively, while relative brightness indicates magnitude of the charge. The brightest blue and red, respectively, represent 0.27e and þ 0.1e in case of gold and 0.9e and þ 0.33e in case of methoxy, with respect to their neutral atomic charge. (c) Bader charge of gold, methoxy, and support at the interfacial site. Adapted with permission from Ref. [805]. Copyright 2013 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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of 2-D “oxide” islands ( 420 nm) at high O coverage. As already discussed, catalytic behavior of supported gold is strongly size-dependent and usually particles with dimension below 10 nm show high activity toward a variety of reactions, whereas bulk gold is (of course) generally quite inactive toward the activation of even larger organic adsorbates. As mentioned above, the Biella and Rossi work showed that relatively large
15 nm gold particles supported on non reducible SiO2 (1% Au/SiO2) exhibit remarkable selectivity in catalytic oxidation of aliphatic primary alcohols to aldehydes and secondary alcohols to ketones at 573 K [786]. After testing, the mean diameter of Au particles increased to 25 nm, but then remained stable after 60 h. To probe the sizedependency of catalytic oxidation of alcohols, Zheng and Stucky studied three SiO2-supported catalysts with 3.5, 6.3, and 8.2 nm gold nanoparticles [510]. At 2 wt. % Auloading, the catalyst with 6.3 nm gold nanoparticles showed the highest activity under mild conditions (373 K) towards the selective oxidation of ethanol to ethyl acetate (86 %) and acetaldehyde (14%). Guan and Hensen [781] also examined the effect of the gold particle size as well as the presence of oxygen on the dehydrogenation of ethanol by silica-supported gold catalysts. In their work, the Au particle size was varied between 1.7 and 15 nm by varying the silica support and the gold loading procedure. The non-oxidative dehydrogenation of ethanol was found to depend strongly on the Au particle size and the reaction rate appeared to be the highest for gold nanoparticles of about 6 nm. This size-dependent activity was explained by the structural sensitivity of the reaction of β-H atoms removal from adsorbed ethoxide, i.e. it was attributed to the existence of surface steps with a suitable geometry. The density of such stepped sites is expected to maximize for particles with intermediate sizes. Sobolev et al. [788] have shown that gold nanoparticles supported on TiO2 are active towards selective oxidation of ethanol in air at temperatures as low as 400 K, while gold supported on SiO2 and Al2O3 exhibited activity at higher temperatures,
480 K. The Au/TiO2 catalysts with Au loadings varying from 2 wt. % to 7 wt. % displayed two distinct thermal regimes for the catalytic oxidation of ethanol to acetaldehyde. However, Au/TiO2 catalysts with much lower loadings, i.e. 0.5 wt. % and 1 wt. % Au, showed the usual activity profile with a steady increase of ethanol conversion above 480 K, a behavior that was close to that of 2 wt. % Au/Al2O3 and 1 wt. % Au/SiO2 catalysts. Note that the main product of the reaction was also acetaldehyde. For these reactions, the clean TiO2 support also exhibited activity, but TiO2 induced mainly acid-catalyzed reactions, giving diethyl ether and ethylene rather than acetaldehyde. At 400 K, the 5 wt. % Au/TiO2 catalyst was the most efficient one towards the selective oxidation of ethanol to acetaldehyde. However, the accumulation of organics on the surface via coking was found to cause catalyst deactivation, which was reflected in an observed double-peak conversion profile with temperature. Another possible reason for the conversion was also suggested: divergent oxidative and non-oxidative, reaction pathways for ethanol dehydrogenation depend on temperature.

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This implies that molecular oxygen plays the role of an intrinsic promoter for ethanol dehydrogenation, as was posed for aerobic and anaerobic reactions induced by gold particles supported on silica [781]. Two recent contributions to the understanding of anaerobic dehydrogenation of ethanol on Au/TiO2 catalysts reported that the deposition of Au nanoparticles on TiO2 dramatically changes the surface chemistry, surface energetics, and relative distribution of dehydrogenation products [651,812]. In one such study, Idriss and co-workers [651] combined TPD and IR spectroscopic studies to follow the surface reactions of ethanol over TiO2 and Au/TiO2 catalysts. The Au/TiO2 (anatase) catalysts contained 1–8 wt.% Au loading and showed loadingaffected product distributions. This effect was most pronounced for the 8 wt.% Au. With the H2-reduced TiO2, the reaction products of surface ethoxides were evolved in one single domain at
660 K. The main decomposition product was ethylene with a carbon selectivity of
70%; the other minor products that evolved (in decreasing yield) were acetaldehyde, butene, and crotonaldehyde. With the H2reduced 8 wt.% Au/TiO2, a large fraction of the reaction products evolved at
600 K where benzene was the main desorption product, showing a selectivity of
61 %; the other products were ethylene and acetaldehyde, along with very small amounts of butene, crotonaldehyde and furan. The main reactions with surface ethoxides include dehydration (producing ethylene), dehydrogenation (acetaldehyde), and condensation and coupling (butene, crotonaldehyde and furan). The high selectivity of the Au/TiO2 catalyst for benzene formation was suggested to occur via a sequence of steps starting with βaldolization reaction of acetaldehyde [651]. In a similar complementary study to that described above, Gonzalez-Yãnez et al. [813] investigated by IR spectroscopy and mass spectrometry the interaction of ethanol with the surfaces of TiO2 and TiO2-supported gold. Gold particles with an average diameter of 7.5 nm were deposited on TiO2 (P25) via the DP method at a nominal gold loading of 5 wt. %. XANES spectra showed no characteristic features for cationic gold and it was concluded that gold was present predominantly as Au0 on the calcined Au/TiO2 sample. According to the IR spectral data, ethanol was found to adsorb in both molecular and dissociative forms on the surfaces of both, the Au/TiO2 sample and a bare TiO2 support. However, no spectral features specific for ethanol or ethoxy species bonded to Au sites were observed with Au/TiO2 and it was concluded that all ethanolderived adsorbates reside on the support. The primary types of ethoxy species identified through IR spectroscopy were attributed to linearly and doubly bound moieties to Ti4 þ sites. When the bare TiO2 support was treated in flowing ethanol at increasing temperature, production of acetaldehyde at
500 K, and formation of butene and water at
573 K, were observed. No production of hydrogen was registered with the bare TiO2 sample. The main products with the Au/TiO2 sample were acetaldehyde and H2, although some butene and water were also formed at high temperatures. The suggested role of gold particles was to subtract hydrogen atoms from the β-C–H bond of neighboring ethoxy species to give η1(O)-

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acetaldehyde. This conclusion was based on the observation of a νCO vibration mode at approximately 1700 cm  1, which was attributed to η1(O)-acetaldehyde species linearly bounded at Ti4 þ sites on the support. Following their abstraction from the β-C–H moiety of ethoxides, hydrogen atoms bond at gold sites were suggested to migrate and recombine over Au particles to yield H2. The absence of H2 for the bare TiO2 support – but rather the evolution of water – indicated to the authors that H atoms extracted from ethoxides interact with lattice oxygen of the TiO2. In this case, the νCO band of η1(O)-acetaldehyde was missing, while new bands at 1407 and 1122 cm  1 appeared in the spectra. These bands were attributed to νCO and νCC vibration modes, respectively, of η2(O)-acetaldehyde. A schematic representation of the proposed mechanisms for the reactions of ethanol on the Au/TiO2 catalyst and the bare TiO2 support is shown in Fig. 8.38.

8.2.6.3. Carboxylic acids (planar model and high surface area catalysts). In addition to a variety of alcohols, supported Au catalysts also exhibit activity toward C–H and C–O bonds cleavage in carboxylic acids, leading to their selective de-oxygenation to useful products. Yates et al. [790,791] have recently reported on partial oxidation of acetic acid at the dual perimeter sites of Au/TiO2 that results in formation of a ketenylidene (C ¼ C ¼ O) species bound to the Au nanoparticle rather than the TiO2 support. The dual Au–Ti4 þ perimeter sites have already been shown to catalyze the activation of O–O and O addition to small molecules as H2 [69] and CO [68]. The Au/TiO2 catalyst used in these studies was prepared via the DP [488] and contained
3 nm Au nanoparticles at
8 wt % loading. FTIR spectroscopic studies and DFT calculations found the adsorption of acetic acid on the Au/TiO2 surface to be dissociative at 400 K, resulting in carboxylate (CH3COO  ) intermediates on the TiO2 support. Molecular oxygen was previously found to adsorb preferentially at Au–Ti4 þ dual perimeter sites, where the active surface oxygen atoms are formed [68,69]. When the adsorbed acetate on the Au/TiO2 catalyst was exposed to O2 at 400 K, surface ketenylidene species due to CH3COO/ TiO2 oxidation were formed, as signaled by an IR band at 2040 cm  1. No reaction was observed with a TiO2 sample without Au nanoparticles. In addition, a Au/SiO2 sample with a similar average gold particle size and loading showed no formation of ketenylidene species under the same conditions. These findings suggested that the presence of both Au nanoparticles and TiO2 is necessary for partial oxidation of acetic acid to ketenylidene species. Kinetic experiments carried out with CH13 3 COO/TiO2 and CD3COO/TiO2 revealed that the C–O bond scission of the HCCOO/TiO2 intermediate is the rate-limiting step for acetate partial oxidation. Based on the DFT simulations it was proposed that the Au–Ti4 þ dual perimeter site is crucial for four processes: (1) activating O2; (2) assisting the dehydrogenation steps; (3) disruption of a C–O bond in the COO moiety; and (4) stabilizing the intermediates [791]. The calculated overall activation barrier for ketenylidene formation on the Au/TiO2 catalyst was

1.80 eV consistent with the experimentally measured value of 1.7 eV for the apparent activation energy. Beyond acetic acid, selective C¼ C double bond formation via oxidative-dehydrogenation was also observed with higher carboxylic acids [792]. Combining FTIR, TPD and DFT studies, it was found that the same Au/TiO2 catalyst is active in partial oxidation of propionic acid (H3CCH2COOH) and butyric acid (H3CCH2CH2COOH) to unsaturated acrylate (H2C¼ CHCOO) and crotonate (CH3CH¼ CHCOO), respectively [792]. These studies suggested reaction pathways that might be operative in many systems. The activation of oxygen is suggested to occur at Au Ti4 þ site pairs at the Au||TiO2 interface to form atomic oxygen (Au O*) surface species [68]. Atomic oxygen (bound weakly to Au) behaves like a base in the activation of C H bonds. In this work [792], for both the C2 and C3 acids, the activation of C H at the C2 atom of the alkyl backbone is the first stage of oxidative-dehydrogenation, followed by C H activation at the C3 atom to form a C¼ C species. Fig. 8.39a presents the calculated potential energy characteristics for the dehydrogenation of propionate adsorbates to an acrylate intermediate. The O H bond of propionic acid is readily activated and cleaved at surface bridging oxygen sites of the TiO2, leading to the generation of surface-bound propionate intermediates at the Au||TiO2 interface. The activation of the C2  H bond via O*– Au occurs over a barrier of 0.53 eV before forming a CH3CH COO* species bound at the TiO2 surface. The produced OH* species has a higher basicity than even O* and subsequently can readily activate (at a barrier of only 0.28 eV) the C3  H bond of the CH3CHCOO/Ti to form an acrylate (CH2 ¼ CH COO/Ti) species. 8.2.7. Total oxidation of volatile organic compounds Beyond small molecule transformations, there is growing interest in the complete oxidation of volatile organic compounds (VOCs) on supported catalysts. The practical interest in such catalytic systems is mainly related to environmental remediation, as recently highlighted in other review publications [55,660,666]. Depending on source, VOCs can include a large variety of species, such as alkanes, alkenes, alcohols, ketones, aldehydes, aromatics, halogenated hydrocarbons. Among the non-halogenated compounds, the most common and toxic are formaldehyde, benzene, toluene, propene, phenol, acetone, styrene [666]. Two classes of unburned hydrocarbons, emitted

Fig. 8.38. Schematic representation of formation of η1(O)-acetaldehyde on Au/ TiO2 and η2(O)-acetaldehyde on the bare TiO2 support. The gold particles are proposed to act as sites for hydrogen subtraction from the ethoxy species. Reprinted with permission from Ref. [812]. Copyright 2013 Elsevier B.V.

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by industrial and automotive exhausts, include small molecules that are generated upon C–C bond rupture (e.g. CH4, C2H6, C2H4, C2H2, C3H8, C3H6) and aromatic hydrocarbons (such as benzene, toluene, xylenes, etc.) formed by pyrosynthesis in fuelrich high temperature zones within flames [666]. Over the years, a variety of processes for efficient removal of VOCs from exhaust streams have been developed. Among these technologies, catalytic oxidation is the most efficient for removal of small amounts of VOC emissions; in addition, catalytic combustion requires relatively low temperatures and thus suppresses the emission of NOx [55,814]. Catalytic combustion of VOCs usually utilizes two classes of catalysts: supported noble metals (often palladium and platinum) and transition metal oxides [666]. The unique catalytic performance of gold NPs supported on base metal oxides in lowtemperature CO oxidation [78,86] have spurred research into gold-based systems [798,815]. Recent reviews [55,660,666] have critically analyzed the growing literature in the field of catalysis by gold, focusing specifically on the application of supported gold catalysts in the total oxidation of volatile organic compounds. Special attention has been given to the effectiveness of Au-containing catalysts towards removal of different families of VOCs such as saturated and unsaturated aliphatic compounds, aromatic hydrocarbons, oxygenated VOCs, and, Cl-, N- and S-containing VOCs. Not surprisingly, alkanes were found to be the least reactive VOCs: the activation of the C–H bond in saturated hydrocarbons is the crucial step in the combustion of these compounds. Importantly, the above-cited reviews [6,16,27] discuss the influences of several factors on the catalytic behavior of supported Au NPs. As is the case for other chemical transformations, some of the most important factors in the low-temperature catalytic combustion of VOCs is the choice of the support and the related preparation method [55]. In particular, the right methods and support help assure the generation of finely divided gold particles that strongly interact with the support [570]. At particular Au-loadings, Au nanoparticles sizes, shapes, and size-distributions, the gold/support interface

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appears to provide sites where the low temperature activation of oxygen may occur. Researchers have also shown that the support can be critically involved in the initial adsorption and subsequent activation of VOCs where surface hydroxyl groups or surface defect sites play a critical role [660,666]. Moreover, the support lattice oxygen of reducible oxides can participate in the reaction through a redox Mars–van Krevelen (MKV) mechanism. In the case of naphthalene oxidation over a Auceria catalyst, Solsona et al. [816] proposed that active sites involve surface hydroxyl groups within the support, where naphthalene can adsorb and react, and lattice oxygen can react through a MVK mechanism. Unexpectedly, this study found that Au nanoparticles play only an indirect role in the reaction by removing the post-synthesis carbonate species from the ceria surface. Other studies have established a direct relationship between the surface oxygen mobility of the gold/support system and the catalytic activity towards the oxidation of VOCs [666], e.g. oxidation of: light hydrocarbons as methane [577,817,818], ethane [818] and propane [818]; n-hexane and benzene [819], 2-propanol [819,820], methanol and toluene [820]. In these reactions, a mutual promoting action between the support and gold was posited to exist. Specifically, the support appears to enhance the fixation of gold, its dispersion, and its oxidation state, while gold increases the mobility of the support lattice oxygen [666]. However, to derive such a general trend of co-operative synergistic effect is a formidable task. Nieuwenhuys and co-workers [821] applied an interesting approach to obtain information about the role of gold and its support in the catalytic oxidation of VOCs. These authors used different type of promoters MOx-(M: alkali (earth), transition metal and cerium) to evaluate the catalytic performance of Au/ Al2O3 in combustion of saturated hydrocarbons including methane and propane. These studies suggest that transition metal oxides (ceria was the most active) play a dual role in catalysis – structural and chemical – i.e. to stabilize gold particles against sintering and to provide active oxygen species for the reaction, respectively [821]. Ousmane et al. [822] arrived at analogous conclusions for the role of the reducible

Fig. 8.39. (a) DFT-calculated potential energy diagram for the oxidative dehydrogenation of propionate at the Au||TiO2 interface to form an acrylate surface intermediate. (b) Structure of acrylate. Reprinted with permission from Ref. [792]. Copyright 2014 American Chemical Society.

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oxide support when comparing the activity of Au catalysts supported on mesoporous oxides as TiO2, ZnO and Al2O3. The Au/TiO2 catalyst that best stabilized highly dispersed Au0 nanoparticles (2 nm) exhibited the best activity in propene oxidation, while maintaining particle size after reaction. The activity of gold catalysts towards total oxidation of oxygenated VOCs, such as alcohols (methanol, ethanol, 2-propanol, n-propanol), aldehydes (formaldehyde), ketones (acetone) and esters (ethyl acetate) has been studied by a number of groups [53,666,823]. In general, the reactivity order established was alcohols4aldehydes4ketones4esters [53,823]. At low temperatures, formation of some intermediate oxidation products has also been detected [666]. Formaldehyde (HCHO), a typical indoor air contaminant and a probable human carcinogen, has attracted much attention in studies employing supported gold catalysts for total oxidation of VOCs. Gold catalysts were found effective in low-temperature HCHO combustion, as reported by Haruta and co-workers [798]. The activity measured for a Au/α-Fe2O3 catalyst was comparable to that of Pd/γAl2O3. In agreement with these results, Li et al. [824] found that HCHO is completely burned at 353 K on a gold/iron-oxide catalyst. In their proposed mechanism, formaldehyde adsorbs on gold particles while oxygen is activated at the perimeter ferrihydrite sites. Gold nanoparticles promote the mobility of support lattice oxygen, which is probably involved in the HCHO oxidation through a MVK reaction mechanism [824]. Hence, the total oxidation of formaldehyde was found to be crucially dependent of the reducibility of the support and the stabilization of gold particles on the support [666]. Thus, Zhang et al. [825] found that gold supported on three dimensionally ordered macroporous CeO2 exhibits an improved catalytic activity, with a full oxidation of HCHO at 348 K [825]. The uniform macro-porous structure was suggested to provide a good distribution of Au nanoparticles with little to no aggregation. The oxidation of aromatic hydrocarbons was found to require higher temperatures than aliphatic alkenes. As reviewed by Scirè et al. [666], a wide range of oxide supports, preparation methods and promoters has been explored for these reactions. Full oxidation of benzene can be achieved between 473 and 573 K and between 573 and 723 K for toluene, depending on the nature of the support. In these oxidation reactions, the positive influence of high surface area mesoporous supports, like ceria, titania or zirconia, on the reaction rates has been reported [826–831]. Furthermore, the addition of MOx promoters such as vanadia was found beneficial [826–829]. In addition, gold was found to have a promotional effect on Pd- [831] and Pt-based [832] bi-metallic catalysts for full oxidation of toluene. The total oxidation of chlorinated hydrocarbons was found to occur at higher temperatures as compared to the nonchlorinated compounds [666]. Della Pina et al. [833] observed that chlorobenzene conversion curves shift to higher temperatures relative to those of benzene on gold catalysts supported on different oxides. However, a strong deactivation of gold catalysts was observed during chlorobenzene conversion due

to irreversible Au-Cl bond formation and likely contamination of high energy interfacial sites. Finally, it should be noted that mineralization of N- and S-containing VOCs has been studied on Au catalysts supported on different oxides and binary component catalysts [666]. Okumura et al. [834] reported oxidation of trimethylamine, o-chlorophenol and of dioxin derivatives over Aucontaining multi-component noble metal catalysts. Oxidation of dimethyldisulfide (DMDS) was studied on gold supported on TiO2 and zeolite (MCM-41, HZSM-5 and H-beta) supports [835]. For the Au/TiO2 catalyst, the removal of DMDS occurred at 430 K where formation of SO2 (around 35%) and elemental S (around 65%) was measured. Panayotov and Morris [665] studied by IR spectroscopy the oxidation of the chemical warfare agent simulant dimethyl methylphosphonate (DMMP) on nanoparticulate Au/TiO2 and pure TiO2. In their work, the Au/TiO2 catalyst was prepared by the DP-urea method [488] and contained 2.7 nm gold nanoparticles at 8% wt gold loading [597]. The proposed oxidation mechanism involves charge exchange between the Au particles and adsorbed oxygen (from the gas phase), which subsequently decomposes DMMP that is adsorbed on the TiO2 regions of the material. In agreement with theoretical predictions [391], this study suggested that the chemical potential of oxygen and surface bound reductants govern the charge state of Au nanoparticles and thus the reactivity of the supported gold catalysts [665].

9. Nanoparticulate gold–titania photocatalytic systems 9.1. Strategies for light induced photocatalysis The discussion in Sections 7 and 8 provide several examples of the critical role of the synergy between gold and titania in the thermal activation of heterogeneous catalytic reactions over Au/ TiO2. As those sections illustrate, developing an understanding of the initial uptake, adsorption, and thermal chemistry is the first step toward elucidating the overall catalytic cycle. Such insight is also critical to understanding the mechanisms of photocatalysis, one of the most intriguing and fastest growing fields associated with Au/TiO2 catalysis. Titania is a wide band-gap (Egap E3 eV) semiconductor that requires high-energy (43 eV) photons for the generation of electron-hole pairs that drive catalytic oxidation-reduction chemistry. Because high-energy photon sources have many practical limitations and only 3 5% of the sunlight that reaches the earth falls into this region of the electromagnetic spectrum, [836] great effort has been expended in recent years toward the development of new TiO2-based materials that absorb visible light. One strategy is to dope TiO2 with ions to narrow the band gap or to create states within the band gap that lead to facile electron–hole separation under UV and longer-wavelength light irradiation. Several excellent reviews have been developed on the topic of titania doping [71,74,837–840]. The primary focus of the following sections in this review is on the use of metal

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nanoparticles to enhance the UV and visible light-induced charge separation within titania-based photocatalysts. In the UV region of the electromagnetic spectrum, metallic particles on the surface of titania can enhance the overall absorptivity, serve as charge traps, and facilitate charge injection into the semiconductor to enhance photocatalytic efficiency. Remarkably, these catalysts also exhibit photocatalytic activity in the visible range due primarily to the plasmonic properties of TiO2-supported metallic nanostructures. Efficient absorption of light in such metalsemiconductor composites is due to resonance of the incident photons with collective oscillations of confined metallic valence electrons, known as plasmons. The exact energy profile of the surface plasmon band of metallic nanoparticles depends on factors such as the dielectric constant of the substrate, the size and shape of the particles, and their charge – all of which subsequently affect the photophysical mechanism by which SPR mediates photocatalytic reactions. Recently, strategies for enhancing charge carrier separation for improved photocatalytic activity have been reviewed by Devi et al. [841] and Marschall [842]. In the following sections, several key studies that have helped to elucidate the fascinating photochemistry of Au/TiO2 catalysis are reviewed. 9.2. Major processes in light induced energy transfer When a TiO2-based catalyst is exposed to radiation, a wide variety of processes may occur that depend on properties such as photon energy and intensity, adsorbate electronic structure and coverage, particle geometry and phase, and metal nanoparticulate identity and structure. The primary processes that lead to charge separation or migration, which ultimately drives photochemistry, will be briefly described in this section, with special attention paid to how Au particles affect the lightmatter interaction. 9.2.1. UV excitation of TiO2 Efficient photoinduced electron hole (e   h þ ) pair generation is responsible for the emergence of numerous photochemical applications of TiO2 [539,843,844], including solar energy conversion [845] and heterogeneous reactions of environmental pollutants and other organic precursors [73,74,846–848]. Motivated by these practical applications, researchers have focused extensively on both powder and single crystal TiO2 surfaces. In particular, TiO2 has become an excellent model system for probing photochemical and photocatalytic reactions and numerous recent reviews [58,73,74,837,849–852] have provided an overview of different aspects of the photophysics and photochemistry of TiO2 surfaces. Fig. 9.1 illustrates the main processes that occur in conventional TiO2 heterogeneous photocatalyst (i.e. void of Au nanoparticles) when irradiated with UV photons. A UV photon of the appropriate energy and momentum may produce an exiton within the bulk of a TiO2 nanoparticle. Exitons are weakly associated and readily separate into electron-hole (e–h) pairs. The separated electron and hole may simply recombine to produce a photon or recombine nonradiatively to heat the lattice (Fig. 9.1a). The

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majority of photogenerated charge carriers that occupy the semiconductor conduction band (CB) and valence band (VB) undergo facile nonradiative electron hole pair recombination:  þ þ hVB . Charge carriers that do not recombine may become eCB þ trapped within band gap states, characterized as hole traps (hVB þ   - hT ), shallow electron traps (eCB - eST), and deep electron   traps (eCB - eDT ). Finally, when a TiO2 nanoparticle is immersed in a medium (gas, liquid, or even solid) of reactant molecules (electron acceptors and donors), the photogenerated electron may be captured by an acceptor molecule, A, in the media and reduce (Aþ e  - A  ); similarly, the photogenerated hole may extract an electron from a donor molecule through the reaction Dþ h þ - D þ . Although electron injection into the CB and hole generation in the VB occur over femtoseconds [73,853,854], electron and hole trapping into the gap states [855–858] is a diffusionlimited process that occurs over a range of time scales from femtoseconds [856] to minutes [73,605,859,860]. Specifically, the trapping of CB electrons has been shown to occur at particulate surfaces within 30 ps, while holes have been shown to require up to 250 ns before they populate trap states after excitation [861]. Electrons initially trapped at surface states can decay further into deeper states within 500 ps [856]. In the absence of charge scavengers, trapped electrons and holes can eventually recombine. According to one study [862], electronhole recombination occurs primarily on the ns time scale in nanocrystalline TiO2 with 80% of charge carriers decaying in 200 ns [862], or 90% in 10 ns as observed in another study [863]. A single e   h þ pair created within a TiO2 particle may have a lifetime as long as 30 ns [861]. At low e   h þ pair concentrations, intra-particle recombination generally follows first-order kinetics. Unfortunately, especially for photochemical applications, only a minute fraction of photogenerated charge carriers reach the surface of TiO2 where they become available for redox chemistry [73,74,539,844,850,864]. The quantum yield, Φ, for such a photochemical processes is presented as [850] Φ ¼ kCT =ðk CT þ kR Þ

ð9:1Þ

where kCT is the rate constant for charge transfer and kR is the rate constant for recombination. The quantum yield would approach unity if recombination processes were eliminated, but this is never the case. The holes generally localize at deep trap states that exist both at the surface and in subsurface regions of the nanoparticles. In addition to hole-traps, electrons can be readily localized in the TiO2 lattice. While a number of studies have suggested that Ovac sites are responsible for the band gap states (BGS) in TiO2 [71,865–868], others suggest that BGS are due to titanium interstitial defects (Tiint) [869–873]. As with other semiconductors, the lattice structure of the TiO2 material affects its band gap characteristics, which leads to different bulk band gaps for anatase and rutile polymorphs of TiO2 [71,848]. Further, the absorption edge in nanoscale TiO2 has been shown to be
0.2 eV higher in energy than the bulk for particles below
10 nm diameter [848,874]. Nanoparticulate anatase, with diameters typically less than 50 nm,

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Fig. 9.1. (a) Schematic representation of the major processes occurring when a TiO2 particle immersed in the media of reactant molecules (acceptor and donor) is irradiated with UV light. Adapted with permission from Ref. [850]. Copyright 2009 Elsevier B.V. (b) Surface and bulk electron carrier trapping. Reprinted with permission from Ref. [850]. Copyright 2009 Elsevier B.V.

exhibits a band gap of
3.2 eV and UV light absorption at wavelengths below 385 nm [848,875]. The thermodynamically more stable and typically larger rutile nanoparticles (d 4200 nm) have a slightly smaller band gap than that of anatase at
3.02 eV and can thus be excited with longer wavelength radiation,
410 nm [848,875]. Recently, Deák et al. [876], Scanlon et al. [877], as well as Pfeifer et al. [878] have found theoretical and experimental indications that when anatase and rutile are brought into contact a staggered energy band alignment occurs. Thus, the valence band maximum of rutile is found at 0.77 0.1 eV (experimental) [878] or 0.63 eV (theoretical) [876] above that of anatase. Analysis of the electronic structure has shown evidence for substantial splitting of the resulting bands owing to the more significant overlap of the O 2pz orbitals in rutile relative to the orbitals in anatase, which is the reason for the higher maximum of the valence band of rutile. The staggered band alignment can explain the observed enhancement of photocatalytic activity for mixed phase TiO2 particles [875,879,880], because the energy structure provides a gradient for separation of photoexcited e   h þ pairs [878]. Transient absorption spectroscopy has been employed to investigate the dynamics of photoinduced charge carriers [857,881–887]. In addition, charge-scavenging probes under pulse [857,882–884,886] or steady state [887,888] UV-irradiation have provided insight into the rates and mechanisms of charge recombination and trapping in TiO 2. Three of the most important reactive species have been identified under UV irradiation of nanocrystalline TiO2 : trapped holes, trapped electrons, and free electrons [886,887]. Using transient absorption spectroscopy, Yoshihara et al. [887] showed that trapped holes exhibit an absorption band near 520 nm, trapped electrons display a band at 770 nm, and free conduction band electrons produce broad absorption band that spans the entire region from the visible to the infrared. Importantly for chemical reactions, the trapped charge carriers appeared to be localized at the surface of the particles, but the free CB electrons most often were identified to be in the bulk [887]. Berger et al. [859] combined electron paramagnetic

resonance (EPR) and infrared spectroscopic measurements to observe photogenerated electrons and holes produced by band gap UV irradiation of powdered anatase TiO 2 under vacuum. Trapped holes were detected by EPR as O – , generated from lattice O2– in the valence band. The electrons were detected either as Ti 3 þ (EPR detection) or as electrons populating the conduction band (IR spectroscopic detection). At 90 K, both electrons and holes were trapped for very long durations, on the order of hours. At 140 K, Ti 3 þ trapped electrons can recombine with holes, whereas conduction band electrons were stable, exhibiting broad IR absorption (4000–1000 cm  1). At 298 K, stable trapped hole and trapped electron states were lost from TiO 2. The presence of O 2 during UV excitation at 140 K lead to the formation of two long-lived superoxy O –2 states (EPR) associated with different titanium cation sites on the surface. Oxidative reactions on TiO2 nanoparticles have been suggested to occur via photogenerated holes [885–887], as opposed to OH radicals [889]. A recent review by Henderson [74] highlights unresolved issues in the current understanding of different photocatalytic reactions and points to several studies that present contradictory interpretations reaction pathways and dynamics. For example, one body of work suggests that the photooxidation of methanol is a direct hole-mediated process [890–895], while others propose that indirect O2  mediated oxidation is involved [894,896,897]. In one set of studies, formate and formaldehyde are reported to be the primary intermediates of direct and indirect oxidation reactions, respectively [893,898,899], whereas other studies draw contradictory conclusions [892,894]. The role of oxygen as an electron scavenger that assists the charge separation and thus facilitates photooxidation by reducing the rate of electron  hole recombination appears to be well established [74]. Oxygen can also be indirectly involved in the photogeneration of other oxidants (such as OH radicals and H2O2) [74,864]. In their recent review, Henderson and Lyubinetsky [851] analyzed the results of numerous studies regarding the photochemistry of O2 on TiO2. Several of the highlights of this review, which are critical to

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the overall understanding of photochemistry on Au/TO2, are re-presented here. Some of the most extensive research on O2 photochemistry has been carried out by Yates and co-workers [864,888,900,901] in an effort to obtain better understanding of the electron hole recombination processes. Using a surface science approach, this group developed a hole-mediated model for O2 desorption where photogenerated holes migrate to the surface where they attack chemisorbed O2 anions, to produce neutral O2 that readily þ  desorbs from the surface, i.e. the reaction O(a) 2 þ h -O2(g) can occur upon electron-hole pair creation in O2-covered TiO2. They established that the kinetics of O2 photo-stimulated desorption (PSD) is first order with respect to the concentration of charge carriers when surface traps play a strong role in recombination [901]. In contrast, for bulk charge carriers, O2 PSD tracks secondorder kinetics and is governed largely by direct recombination because the bulk is less defective than any surface [864]. Studying the O2 PSD versus O2 coverage, Petrik and Kimmel [902,903] found that the yield of O2 via PSD depends on O2 coverage in complex ways and increases linearly, but with varying rates over two ranges of coverage. The authors proposed that oxygen coverage influences the photodesorption process because the charge of the chemisorbed O2 is concentration dependent. However, they found that less than 40% of the chemisorbed O2 could be effectively desorbed during illumination, which seemed to indicate that a significant amount of adsorbed oxygen is inactive for PSD. Indeed, recent STM studies by Lyubinetsky and co-workers [904] demonstrated that oxygen species bound at non-VO sites (Ti5c) must be taken into account in order to properly describe the photochemistry of O2 on TiO2(110). Using TPD of O2 (after chemisorption of O2 on a UV irradiated surface), Petrik and Kimmel [902,903] found that photolysis results in dissociation of chemisorbed O2 at the VO sites. It was proposed that O2 photolysis is an electron-mediated process that results in healing of the original VO and formation of Oa at an adjacent Ti5c site, i.e. the process is thought to be 2   O2 2 /Vþ e -Ob þ Oa /Ti5c. Henderson and Lyubinetsky [851] noted that both photostimulated processes with O2, i.e. the e--mediated dissociation and h þ -mediated desorption, should be considered as complementary reductive and oxidative surface reactions, respectively. Thus, both reactions should occur at substantial and balanced rates for effective photocatalysis on TiO2. However, differences in the electron and hole transfer dynamics to chemisorbed O2/VO and O2/Ti5c may result in significant reaction rate disparities [74,837]. As well documented and specifically noted in their review, the complex site-specific and coverage-dependent photochemistry of O2 on TiO2(110) is clearly far from well-understood and further study is needed. 9.2.2. UV excitation of Au/TiO2 The competition between charge recombination and interfacial charge transfer between photoexcited TiO2 and adsorbates appears to limit the overall photocatalytic reaction rate on pure titania surfaces [74,905]. For example, recombination can occur on the femtosecond timescale, while the transfer of trapped electrons to O2, has been reported to occur over time periods from ms (0.33 ms) [906] to hours [859]. Further, numerous studies

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have shown that interfacial charge transfer is strongly sensitive to the surface properties of the semiconductor [74]. Modification of the surface can affect the charge transfer process in various ways, such as by creating additional surface traps or charge reservoir sites, by altering of the intrinsic charge transfer rate, by changing reactant adsorption/desorption rates, and by introducing visible light sensitizers [74,840,907]. One of the most often-employed modification methods intended to enhance the photochemistry is to decorate the TiO2 surface with nanosized metallic particles via photodeposition, impregnation, etc. Various metal nanoparticles such as Pt, Au, Pd, Ru, Rh, and Ag have been employed for modification of TiO2 photochemical properties. Deposition of noble metal nanoparticles on the surface of TiO2 can lead to interfacial charge transfer and separation, thus decreasing charge recombination rates [58,74,840,907–910]. Hence, the photocatalytic activity of a metal/semiconductor (MS) system can be enhanced in this way by extending the electron–hole pair separation lifetime. However, we point out that, near the time of completion of this review, researchers have shown compelling evidence that the role of the metal sites in the photochemical production of hydrogen from water under UV irradiation is to serve as recombination sites for hydrogen atoms produced via proton reduction on the TiO2 surface [1224]. As the field progresses, these new results must be viewed in the context of the existing body of work for UV-photochemical reactions at Au/ TiO2 catalysts, much of which is reviewed below. One of the most critical characteristics of an MS junction is the development of a potential at the interface that results in an energetic barrier for charge transport. The magnitude of the potential, called the Schottky-barrier height (SBH), is caused by the energy level mismatch across the MS interface [59,911]. In MS systems (such as the Au/TiO2 interface) the metal work function (ϕM) may be greater than that of the semiconductor (ϕSC). When two materials with mismatched work functions are brought together to form a junction, electrons from the semiconductor flow into the metal, until the Fermi levels of metal (EF,M) and semiconductor (EF,SC) are equilibrated [58,912]. Upon charge exchange and alignment of the Fermi levels, a Helmholtz double layer or surface space charge region forms. An n-type depletion region (as expected at the Au/TiO2 interface) is shown schematically in Fig. 9.2b. This region is a layer with high resistance owing to the repulsion of additional electrons or holes from the interface after equilibration [912]. Schottky-Mott theory proposes that the SBH between a metal and a semiconductor with an electron affinity of χSC should equal ΦnSB ¼ ϕM – χSC, where ΦnSB denotes the SBH measured for the n-type semiconductor. Far from the interface, the energy band edges in the semiconductor approach their values at infinite separation, causing a continuous band bending at the interface [912]. However, we note that the double layer may be 10  4–10  6 cm wide [912], which is larger than the individual components of many supported particulate catalysts. As a result, the degree of band bending is limited for nanoscale systems [58]. For more details on the physics and chemistry of the Schottky barrier height, the reader is directed to the excellent review papers by Tung [59,911]. The concepts of band

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bending in semiconductor and metal/semiconductor systems, along with the chemical, physical and photochemical consequences of band bending were also discussed in a recent review by Zhang and Yates [58]. Several different methods have been employed for measuring the SBH in Au/semiconductor systems [913–915]. For example, Kaiser and Bell employed ballistic electron emission microscopy (BEEM) to detect a SBH at the Au/Si(100) of 0.92 eV [913,914]. Tang et al. combined current–voltage (I–V) measurements and thermionic emission theory modeling to estimate the SBH in Au/TiO2 [915]. For an electrodeposited gold film, the estimated SBH was larger (1.1 eV) than that for an evaporated film (0.9 eV). Maeda et al. found that the relative local barrier height values (ΔLBH) of Au particles to the TiO2(110)-(1  2) substrate is a function of Au particle height, see Section 8.1.1 [679]. As shown in Fig. 8.3 for small particles (particle height o 0.4 nm), the values of ΔLBH are primarily between  0.4 and 0 eV, which suggests that small Au particles may actually donate electrons to TiO2 to yield a positively charged Au particle. For particle heights exceeding 0.4 nm, the ΔLBH values range from 0 to 0.8 eV, which suggests that large particles generally accept charge from the TiO2 to result in a negatively charged Au particle [679]. As highlighted in Tung’s review, the formation of the Schottky barrier height is a complex problem due to the strong dependence of the SBH on the atomic structure of the MS interface [59]. In addition, factors such as the defect density, preparation method, and sample history are also likely important for the SBH and band bending phenomena [59]. Therefore, caution should be exercised when one applies previously measured values to new catalysts. The photophysical mechanisms responsible for enhanced charge separation in metal-decorated TiO2 have been explored by a number of groups. Murray and co-workers [916], for example, have found that gold nanoparticles undergo quantized charging that makes them a unique candidate to achieve Fermilevel equilibration. As they describe the process, double-layer charging around the metal nanoparticle promotes the storage of the electrons within the gold nanoparticle. With the metal nanoparticle and the semiconductor in electrical contact, photogenerated electrons can be re-distributed between the TiO2 and Au nanoparticles. This occurs because the EF level of the TiO2,

which is in initial equilibration with the EF of the Au, shifts to higher energies upon UV illumination and population of the CB [910,917]. Hence, during the initial stage of illumination, there is a thermodynamic driving force for the transfer of electrons into the Au particles. The electron transfer from photo-excited TiO2 into Au, in turn, shifts the Fermi level of Au higher, toward the semiconductor CB; subsequently, the two systems re-equilibrate, as schematically shown in Fig. 9.3. In their experiments, Kamat and co-workers [919,920] made use of the equilibration between the semiconductor (TiO2 or TiO2/Au) and a C60/C•60 redox couple (in an ethanol-toluene solvent) to determine the flat-band potential of the semiconductor. The maximum yield of C•60 observed for the TiO2/Au system was significantly higher than that observed with pure TiO2 suspensions. The enhanced reduction efficiency provide evidence that the gold nanoparticles on TiO2 can serve as a reservoir for photogenerated electrons. Thus, the improved separation of charge carriers in the composite system reflects an enhanced photocatalytic activity. In those studies, it was also established that the apparent shift of Fermi level, EF*, is affected by the size of Au particles for sizes in the range 3–8 nm [920]. The smaller Au particles induced a greater shift in EF* than the larger particles. For example, the shift in flat band potential achieved with 3-nm-diameter gold nanoparticles was  60 mV while it was  20 mV when 8-nm Au nanoparticles were utilized. This indicated that the apparent Fermi level of the composite system could be tuned by controlling the size of the Au nanoparticles. In a similar vein to the above-cited work, Kiyonaga et al. [921] employed Au/TiO2, prepared by the DP method, to study the role of Au particle size on the redox activity of the composite material. The Fermi energy of Au nanoparticles at the photostationary state 0 (EF) was determined in water by using the S/S2 couple as a 0 redox probe. The EF in water was found to increase as the mean size of Au nanoparticles (d) increased at 3.0 r d r 13 nm. The results of this work showed that in water solutions, despite the increased efficiency of the photoinduced electron transfer from TiO2 to Au with increasing d, the stabilization of charged Au NPs decreased due to enhanced water solvation. Tada and co-workers [922] formulated a concept for achieving a so-called “reasonable delivery photocatalytic reaction systems (RDPRSs)” for application of photocatalysts

Fig. 9.2. Basic energy diagram at a metal–semiconductor interface before (a) and after (b) contact for high work function metal and n-type semiconductor. Adapted with permission from Ref. [912]. Copyright 1982 Elsevier B.V.

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based on noble metal nanoparticle-loaded TiO2, i.e. M/TiO2. The RDPRS concept explored conditions for the enhancement of charge separation due to interfacial electron transfer from semiconductors to metals [910,923]. In that work, three key conditions were identified and highlighted [922]. The first condition requires separation of sites where the oxidation (TiO2) and reduction (M) proceed. The second condition is that the supply of reductants and oxidants needs to be abundant and selective to the oxidation and reduction sites, respectively. The third condition requires the inhibition of product readsorption on the photocatalyst surface. This RDPRS was exemplified by 2e  reduction of 2,20-dipyridyl disulfide (PySSPy) to 2-mercaptopyridine (PySH) on Au/TiO2 photocatalysts [922]. Noticeably, the reduction was drastically enhanced by the incorporation of even a small amount of Ag, and the accelerating effect obtained with Au nanoparticles was greater than that of Ag at the same loading level. Further, the photocatalytic activity significantly increased with the Au– loading. However, when too many metal particles were loaded on a single TiO2 particle, the charge separation effect was impaired due to the overlap of the interfacial electric fields. Ismail et al. [924] reported an enhanced photonic efficiency for gold nanoparticles deposited on mesoporous networks of TiO2 under UV illumination. In agreement with the results of Kamat et al. [920], these authors established that the Au/TiO2 nanocomposites with smaller gold nanoparticles (0.5 wt % Au/ TiO2) exhibit, in general, a higher photochemical activity towards methanol photooxidation than those with larger particles ( Z 1 wt.% Au/TiO2) [924]. It was suggested that the smaller gold particles in contact with charged TiO2 induce more negative apparent Fermi level shift than bigger particles, leading to a better charge separation and thus a higher reductive power for the photocatalyst. The three-dimensional mesoporous TiO2 network was proposed to act as an antenna that enhances transfer of the photogenerated electrons in TiO2 to the Au particle. Electrons are transferred from the location of light absorption through the interparticle TiO2 network to a suitable interface with the Au particle and subsequently to the Au nanoparticle where the actual electron-transfer reaction to

Fig. 9.3. Schematic representation of shift in Fermi level of the composite as a result of electron storage. Reprinted with permission from Ref. [918]. Copyright 2007 American Chemical Society.

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O2 can occur to form O•– 2 radicals. The valence band holes, photogenerated concomitantly with electrons in TiO2, are suggested to migrate to the TiO2 surface where they extract electrons from adsorbed hydroxide ions or water molecules, to generate highly reactive adsorbed •OH radicals [924]. In a similar vein, Caruso and co-workers [925] used a solvothermal method for in situ reduction of a gold precursor to gold nanoparticles embedded in porous titania networks to obtain a Au/TiO2 composite with enhanced photocatalytic activity towards degradation of methylene blue in aqueous solution. The photocatalytic activity was found to be affected by four key factors: the relative peak intensity of the anatase phase in their XRD data, the gold content, the surface hydroxyl groups (correlated to surface defects) in the final composite, and the charge separation capability of the deposited gold nanoparticles. The optimum photocatalytic efficiency, which was a factor of two higher than that observed for Au-free TiO2, was obtained for Au loading of 2–4 wt. %. In studies that also explored the UV-driven photochemistry of networked TiO2, the UV oxidation of methanol at 3D mesoporous TiO2 and Au TiO2 was explored by Panayotov et al. [508]. Their work showed evidence for more efficient separation of electron hole pairs for the 3D networked TiO2 aerogel than for a commercial nanoparticulate TiO2. Interestingly, both TiO2 and Auimpregnated TiO2 aerogel networked systems showed comparable activity and kinetics for methanol to formate photooxidation under broadband-UV illumination; however, exposure of Au TiO2 to a visible light source, centered at 550 nm, also converted methoxy groups to formates. The visible light chemistry was found to occur at rates comparable to the same chemistry on the TiO2 aerogels (without Au particles) under UV illumination (note: no photooxidation activity was observed in their studies for non-goldcontaining TiO2 aerogels under irradiation at wavelengths longer than 400 nm). The results of this work indicate that the combination of Au nanoparticles with 3D-networked TiO2 (aerogels) is a promising strategy for driving photooxidation in the visible range of the electromagnetic spectrum. This conclusion, supported by a number of previous and subsequent studies (see below), is important for the practical application of photocatalysts where energy consumption and environmental applications may be critical considerations. 9.2.3. VIS excitation of Au/TiO2 High-energy (UV) photon sources, though required for many photocatalytic reactions, suffer from several practical limitations and only a small percentage (
3–5 %) of sunlight at the earth's surface falls into this region of the electromagnetic spectrum; therefore, new TiO2-based materials that absorb visible light are the subject of numerous investigations [840,926]. Such strategies include band gap engineering to match the solar spectrum by including nonmetal and metal ions in TiO2 [839,927–930], by construction of hybrid composites of TiO2 and other semiconductors [926,931–934], and by heterojunction photocatalysts [926,935–937]; localization of energetic charge carriers at specific surface sites by dyesensitized photocatalysts [938–940], and other composite semiconductor photocatalysts [940,941]. In all these cases,

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visible-light absorption and the separation of photogenerated charge carriers represent critical challenges for achieving effective photocatalytic performance. TiO2-supported metallic nanostructures have been shown to absorb visible radiation and effectively convert visible photonic energy to electronic, thermal, or chemical energy for catalysis. Efficient absorption by oxide-supported metal nanoparticles is due to resonance of the incident photons with collective oscillations of confined metallic conduction electrons, known as plasmons [64], as illustrated schematically in Fig. 9.4. Within this conceptual framework, displacement of the electron cloud relative to the nuclei of the metal causes a restoring force to arise from Coulombic attraction between electrons and nuclei. This results in an effective oscillation of the electron cloud relative to the central nucleus [64]. Plasmonic metal nanostructures exhibit significant absorption cross-sections that are, under resonant conditions, approximately five orders of magnitude greater than the absorption cross-section of conventional dye sensitizers [942]. The oscillation frequency of a particular plasmon depends on four properties: the density of electrons, the effective electron mass, and the shape and size of the charge distribution. Importantly for photochemical applications, the precise energy profile and the intensity and wavelength of the SPR band can be altered by tuning the size, shape, and dielectric surrounding of the plasmonic NPs. A number of detailed experimental studies has shown that both metal and TiO2 are needed for obtaining visible-light photocatalytic activity; moreover, numerous configurations have been reported for Au–TiO2 nanocomposites. These include Au NPs deposited onto TiO2 spheres, NPs, nanotubes, nanosheets, films, mesoporous TiO2, coatings over SiO2 spheres, etc. [907]. Not surprisingly, but certainly worth noting, various methods have been explored for incorporating Au NPs into TiO2, including chemical, thermal or hydrothermal reduction, photoreduction, deposition–precipitation method, sputtering of Au NPs, and incorporation of pre-synthesized Au NPs on TiO2, as recently reviewed by Wang et al. [943], Kim and coworkers [907], and Louis [944]. The preparation methods employed for Au-titania nanocomposites for photochemical applications are reviewed further in Section 12. A clear indication that excitation of surface plasmons can drive photocatalysis in metal-semiconductor nanocomposites has been confirmed by numerous studies, which demonstrate that wavelength-dependent reaction rates peak at the frequency where the plasmon intensity is the highest [945–952]. The wavelength dependence of the quantum efficiency for a particular reaction, the so-called action spectrum, typically maximizes precisely at the excitation energy of the local surface plasmon resonance (LSPR) of the material [836,946,947,951–953]. Kowalska et al. [946], studying the visible-light photoinduced oxidation of 2-propanol to acetone over Au–TiO2, found that the calculated apparent quantum yield (Φ) resembles the respective diffuse reflectance spectrum of the sample, as shown in Fig. 9.5. The apparent quantum yield was calculated as the ratio of the number of consumed electrons (assuming two–electron oxidation to acetone) to the number of impinging photons. These results indicate that the LSPR excitations lead to electron-induced “chemical” transformations. Other authors

arrived at analogous conclusions in studies where the SPR intensity and wavelength were altered through changes to the composition, shape, or size of plasmonic nanostructures [948,950,954]. Thus, plasmonic nanostructures can yield chemically useful energetic electrons when irradiated with visible light. These electrons can be used in tandem with thermal energy (i.e. via vibrationally excited states) to drive catalytic chemical transformations at temperatures significantly lower than those of pure thermal processes [949]. Fig. 9.6 shows a schematic of the major processes that occur in a plasmonic Au/TiO2 photocatalyst, as represented by Tsai and co-workers in their review on plasmonic photocatalysis [67]. Here, the gold nanoparticle, capable of absorbing visible light, is supported on an n-type semiconductor TiO2 nanoparticle, which independently absorbs UV light itself. The incident visible light excites LSPR in the Au nanoparticle via a coherent oscillation of electrons, which is suggested to excite electrons and holes within TiO2 (depicted as (A)) by energy transfer and/or charge carrier transfer to TiO2 (although the precise mechanism remains a topic of discussion; further details will be provided in Section 11). The general consensus in the literature is that these processes are essential for plasmonic photocatalysis and enable the creation of active electrons and holes in TiO2 nanoparticles, even in the absence of direct light absorption by TiO2 [67]. Although the enhancement in photocatalytic activity at the plasmon resonance frequency of TiO2-incorporated gold nanoparticles is well established, the mechanism by which energy is transferred from gold to titania is still under discussion [955,956]. Two main mechanisms have been highly cited in the literature regarding plasmonic enhancement of photo catalytic reactions: direct charge transfer and local electric field enhancement. Tatsuma and co-workers [945,952] proposed a charge transfer mechanism in which an excited plasmon in Au or Ag generates conduction band electrons with energy sufficient to traverse the Schottky barrier and enter the conduction band of the adjacent TiO2. The alternative mechanism [950,955–957] involves the well-known local electric field enhancement. Elevated electric fields (enhanced by plasmonic absorption and concentration) near the interface have been shown to excite electron  hole pairs in the semiconductor, which leads to enhanced photocatalytic activity. Further, Cushing et al. [958] proposed an alternative, nonradiative mechanism, where plasmonic energy transfer occurs from a resonant energy transfer (RET) process. The

Fig. 9.4. Schematic of plasmon oscillation for a sphere, showing the displacement of the conduction electron charge cloud relative to the nuclei. Reprinted with permission from Ref. [64]. Copyright 2003 American Chemical Society.

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Fig. 9.6. Schematic representation of the major processes in plasmonic photocatalysis. Reprinted with permission from Ref. [67]. Copyright 2013 IOP Publishing Ltd. Fig. 9.5. Action spectrum of 2-propanol oxidation on Au/TiO2: (■) Au-modified, (□) bare and the diffuse reflectance spectrum of Au/TiO2. Reprinted with permission from Ref. [946]. Copyright 2009 The Royal Society of Chemistry.

RET process describes direct excitation of electron  hole pairs in the semiconductor through the relaxation of the localized surface plasmonic dipole. Finally, Boyd and co-workers [959] demonstrated that plasmonic excitation may be employed to directly heat the catalyst particles using a low-power laser. Several published articles that have evoked these (and other) photophysical mechanisms to describe surface plasmon resonance mediated enhancement of photocatalytic activity will be described in subsequent sections. However, the stage for this discussion is first set by providing an overview of SPR generation in gold nanoparticles, the topic of Section 10. 10. Surface plasmon resonance in gold nanoparticles 10.1. Light–metal nanoparticle interaction Many conceptual models of metallic NPs describe them as a lattice of ionic cores surrounded by confined but freely-moving electrons (the Fermi sea) [960]. The force exerted by an external oscillating electromagnetic field on the electrons of a metal can result in coherent oscillations of electrons in space and energy [64]. These oscillations of the free electron gas are referred to as plasmons [62,64,961]. In the bulk, plasmons occur at energies specific to each metal, approximated by the equation: E p ¼ ðne2 =mε0 Þ 12 ;

ð10:1Þ

where ε0 is the permittivity of free space, n is the electron density, e is the electron charge, and m is the electron mass; "bulk" refers to a material that extends, in three dimensions, well beyond the wavelength of light [961]. At a metal-dielectric interface, plasmons take the form of surface plasmon polaritons (SPPs) as illustrated in Fig. 10.1a; for simplicity, they are also referred to as surface plasmons (SPs) [64]. At the interface, optically excited surface plasmons and incident light are coupled into surface plasmon modes that may be standing or propagating waves. Light incident at high angles relative to the local surface normal (wave vector nearly parallel to the surface) is known to couple most efficiently.

When the incident light impinges on a particle of a size comparable to the wavelength of light, the excited plasmon is confined to the particle itself and it is termed a localized surface plasmon (LSP) [64]. The conceptual framework for this phenomenon is that the electromagnetic field imposes a potential energy gradient on the free conduction electrons, which moves them towards the NP surface. Thus, negative charge accumulates on one side and positive charge accumulates on the opposite side of the NP, thereby creating an electric dipole, as shown pictorially in Fig. 10.1b. This dipole, in turn, generates an electric field across the NP (opposite to that of the electric field vector of the impinging light), which forces the electrons to return to their equilibrium position. The magnitude of the restoring force is naturally proportional to the extent of the electron displacement [960]. In his recent review, Garcia [960] describes a simplified classical picture that can be useful in the development of a conceptual understanding of SPs. If the external electric field is removed when the electrons are displaced from their equilibrium position, they will continue to oscillate at a frequency referred to as the plasmonic frequency. In the general case of a linear oscillator, the system will oscillate at the frequency of the external force, but with amplitude and phase that depend on both the force and the physical characteristics of the oscillator. As shown in Fig. 10.2a, the oscillating amplitude maximizes at the resonant frequency, but far away from this frequency, the movement of electrons is restricted. In metallic NPs, the resonant frequency for electronic oscillations corresponds typically to light in the UV–Vis region of the electromagnetic spectrum, thus the SPs will exhibit an absorption band in this region, as Fig. 10.2b illustrates. The absorption efficiency of a particle is indicated by the particle's absorption cross section. Classically, an absorption cross section can be presented as the geometrical section of an ideal opaque particle that would absorb equal number of photons as the real particle. This concept is illustrated in Fig. 10.2c. In addition to absorption, light can also experience scattering that alters its propagation direction and energy, as Fig. 10.2d illustrates. The scattering cross-section can also be presented as the geometrical section of an ideal particle; the overall scattering efficiency of this ideal particle is, of course, equal to that of the real one. The sum of the absorption and the

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scattering cross sections defines the extinction cross section of the particle, which represents the efficiency of photon removal from the incident beam of light. For NPs composed of noble metals, the extinction cross section can be an order of magnitude greater than their geometrical section [960]. In his review on optical studies of dynamics in noble metal nanostructures, Hartland [962] discussed the sequence of events that occur following photon absorption: LSPR dephasing, internal relaxation of the electrons via electron-electron scattering and electron-phonon coupling, and energy dissipation to the environment. Schematic representations of the sequence of events, along with corresponding timescales, following optical excitation is presented in Fig. 10.3. Partitioning of the dynamics into consecutive dephasing (10 fs time scale), electron-electron scattering (100 fs), electron-phonon coupling (1–5 ps), and heat dissipation

processes (10–100 ps) is commonly employed in the categorization and discussion of experimental data [962]. Understanding these photophysical processes have helped scientists to develop many applications of metal nanoparticles, including visible light stimulated photocatalysis. Below, we will briefly present the basic theoretical concepts describing photon absorption and energy relaxation processes. 10.2. Models and calculations of SPs The above-described simplified photophysical picture of separable electronic and vibrational motions breaks down for visible to near-UV excitation of metals due to the high density of empty states near the Fermi level. For metals, optical properties of a spherical particle can be described by Mie theory, which requires knowledge of the dielectric

Fig. 10.1. Illustrations of (a) surface plasmons and (b) a localized surface plasmon. Adapted with permission from Ref. [961]. Copyright 2011 American Chemical Society.

Fig. 10.2. (a) Oscillation amplitude for a linear oscillator as a function of the external force frequency. (b) Optical absorption spectrum corresponding to 10 nm silver NPs embedded in a silica glass. (c) Illustration of absorption cross section concept. (d) Picture describing transmission, absorption and scattering processes. Adapted with permission from Ref. [960]. Copyright 2011 IOP Publishing Ltd.

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constants of the particle and the surrounding media [62,962]. The dielectric constants of metals are strongly dependent on frequency and they contain both real and imaginary components, where the real part provides the position of the LSPR and the imaginary part reflects the dephasing, as detailed below [962]. The simplest model for describing the dielectric constant of a metal is the Drude or free electron model, which describes the motion of electrons exposed to an electric field and gives the following expression: [62,962]

The Drude model describes the dielectric constant of metals, but it fails in the visible to near-UV region for systems such as Au and Ag due to interband transitions. The onset of the interband transitions in these metals is at approximately 2.4 eV for Au, and 3.9 eV for Ag. These excitations give rise to a frequency-dependent damping that can be accounted for in the dielectric constant by adding an extra term: [962]

ω2p εðωÞ ¼ 1  ; ωðω þ iγ b Þ

where εib(ω) is the contribution of interband transitions. Fig. 10.4 shows a plot of the real (ε1) and imaginary (ε2) components of the dielectric constant for bulk Au. Note that the possibility of interband transitions leads to an increase in damping at energies exceeding 2.4 eV. The inset of Fig. 10.4 illustrates the band structure of gold. The increased damping is related to scattering of electrons into empty levels of the conduction band (so-called Landau damping) [962]. Fig. 10.5 presents the optical absorption spectrum of Au and Ag NPs [960]. In the spectrum of Ag NPs, both absorptions, i.e. the surface plasmon band and that of interband transitions, are well resolved. In the spectrum of Au NPs, both absorptions overlap. We note that this band overlap should always be considered when analyzing SP bandwidths for various applications. Although the classical Drude model can be helpful for providing a general physical description of surface plasmons, it is too simple for highly accurate approximations. The spectrum of metal particles can be more precisely calculated by solving Maxwell's equations within the NP region using the proper boundary conditions. Mie theory provides an exact analytic solution to Maxwell’s equations for small (d{ λ), noninteracting spheres (i.e., the electric field created by one NP does not affect the neighboring particles) [62,962]. Mie theory gives the following expressions for total scattering, extinction, and absorption cross-sections: [962]

ð10:2Þ

where ωp is the plasma frequency and γb is the bulk damping constant describing the damping due to the scattering of the oscillating electrons with the ionic cores, which is related to the mean free path l of the electrons by γb ¼ υF/l where υF is the velocity of the conduction electrons (Fermi velocity). The plasmonic frequency can be found using Eq. (10.1). For a Au nanoparticle, the mean free path of electrons at room temperature is approximately 20 nm [962]. For a particle with dimensions smaller than the mean free path of the bulk material, surface collisions must be included as modifications to the damping constant in Eq. (10.2) [62,962] γðleff Þ ¼ γ b þ

AvF ; leff

ð10:3Þ

where leff is the effective path length of the electrons (i.e. the mean path traveled before scattering off a surface), and A is a material-dependent constant accounting for the details of the electron-surface scattering. The constant A is considered to be a phenomenological parameter although its magnitude depends on properties such as crystal quality, facets, capping species, strain, etc. [62,960,963].

εðωÞ ¼ εib ðωÞþ 1 

ω2p ω½ω þ iγðleff Þ

;

ð10:4Þ

ssca ¼

1 2πR2 X ð2n þ 1Þfjan j2 þ jbn j2 g; x2 n ¼ 1

ð10:5aÞ

sext ¼

1 2πR2 X ð2nþ 1ÞRe½an þ bn ; x2 n ¼ 1

ð10:5bÞ

sabs ¼ sext  ssca ;

ð10:5cÞ

where x¼ 2πRnm/λ, nm is the refractive index of the medium, and R is the radius of the particle. The different terms n in Eqs. (10.5a) and (10.5b) correspond to the dipole (n ¼ 1), quadrupole (n¼ 2), hexapole (n ¼ 3), etc., contributions. The an and bn factors are composed of the Riccati–Bessel functions ψn and ξn and are given by [962] 0

0

ψ ðmxÞψ n ðxÞ mψ n ðmxÞψ n ðxÞ an ¼ n0 ; 0 ψ n ðmxÞζ n ðxÞ mψ n ðmxÞζ n ðxÞ 0

Fig. 10.3. Sequence of events and approximate time scales following absorption of photons by a metal nanoparticle. Reprinted with permission from Ref. [962]. Copyright 2011 American Chemical Society.

bn ¼

ð10:6aÞ

0

mψ n ðmxÞψ n ðxÞ ψ n ðmxÞψ n ðxÞ ; 0 mψ 0n ðmxÞζ n ðxÞ ψ n ðmxÞζ n ðxÞ

ð10:6bÞ

where m ¼ np/nm, np is the refractive index of the par-

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ticle. Using available computational approaches, one can use Eqs. (10.5a) and (10.5b) to determine the extinction and scattering spectra of metallic nanoparticles [960]. Fig. 10.6 presents the calculated extinction and scattering spectra for gold and silver nanoparticles with radii of 25 and 50 nm [962]. One notes that for this size range, only the dipole and quadrupole terms are significant. From Fig. 10.6, one may note that increasing the metal particle radius causes two effects: a red shift and broadening of the plasmon resonance band [962]. The red shift is a retardation effect caused by non-uniformity in the electric field across the particle. The broadening effect is caused by the radiation

damping. A particle size increase leads to a strong rise in the electric field to electron coupling at the LSPR. Thus for large particles, the light scattering is significant and gives rise to an extra damping effect that is not present in the bulk metal. Fig. 10.7 illustrates the role of particle radius on the LSPR linewidths for different sized spherical Ag particles; calculations are performed using Eq 10.5a [962,964]. The broadening of the line width is caused by the increased radiation damping and is proportional to the volume of the particle. Note, the effect is stronger in higher dielectric constant environments [964]. These calculations demonstrate that for Ag nanoparticles, radiation damping strongly influences the line width for particles larger than 20 nm radius. 10.2.1. The dipolar approximation The dipolar approximation allows the calculation of light absorption by metallic NPs, provided that the particulate size is smaller than the wavelength of light [960]. For the plasmonic spectral region (
360–720 nm, i.e. visible), the condition of “small NPs” is usually achieved for diameters below
50 nm. In these cases, the impinging electromagnetic field is approximately constant, and the interaction with metallic NPs is governed by electrostatics rather than electrodynamics [64]. As shown in Fig. 10.8, the electric field inside such small NPs can be considered uniform and the particle can be represented by an electric dipole. This approximation corresponds to the case when the first term in Eqs. (10.5a) and (10.5a) is n ¼ 1. (Note that the term n ¼ 0 corresponds to zero total electric charge of the NPs). Thus, for small particles (o 50 nm), the dipole contributions in Eqs. (10.5a) and (10.5b) dominate and the extinction is governed by absorption. Within this approximation, the extinction and scattering cross-sections are given by [960,961] 3=2

Fig. 10.4. Real and imaginary dielectric constant data for gold (taken from Ref. [53] of Hartland [962] review). The dashed lines show a fit to the data using the Drude model (Eq. (10.2)). The inset shows a cartoon of the band structure of gold, where εF is the energy of the Fermi level, and ωib is the frequency of the gold interband transitions. Reprinted with permission from Ref. [962]. Copyright 2011 American Chemical Society.

sext ¼

18πεm V ε2 ðλÞ ; λ ½ε1 ðλÞþ 2εm 2 þ ε2 ðλÞ2

ð10:7aÞ

ssca ¼

32π 4 ε2m V 2 ðε1  εm Þ2 þ ðε2 Þ2 ; λ4 ðε1 þ 2εm Þ2 þ ðε2 Þ2

ð10:7bÞ

where λ is the light wavelength, εm is the real dielectric function

Fig. 10.5. Optical absorption spectra for (a) Au and (b) Ag NPs with 40 nm size (embedded in a silica matrix with ε¼ 2.25). For Au NPs, the contributions to the optical absorption of interband transitions and SPs are resolved. Reprinted with permission from Ref. [960]. Copyright 2011 IOP Publishing Ltd.

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Fig. 10.6. Calculated extinction (solid lines) and scattering (dashed lines) spectra for different sized Au and Ag nanoparticles in water (radii are 25 or 50 nm and n¼1.33). Only the dipole and quadrupole terms in Eqs. (10.5a) and (10.5b) were included. Reprinted with permission from Ref. [962]. Copyright 2011 American Chemical Society.

particles relatively well, single-particle approaches fail in describing the light-matter interactions for arrays of particles. Another way to calculate the optical properties of metallic NPs is based on the effective medium approach, i.e. the heterogeneous system consisting of the NPs and the surrounding media is replaced by a homogeneous medium with an effective dielectric function eeff, exhibiting the same dielectric polarization upon light illumination. These descriptions are known as "effective medium theories." One of the most enduring models describing the permittivity of random composites is the Maxwell-Garnett approximation [960,966]. This model derives from the Clausius–Mossotti formula, which links the polarizability α of the small particles in an array to the effective dielectric function of such material eeff according to: [966] Fig. 10.7. Linewidth of the LSPR versus radius for Ag nanoparticles in different media. The linewidths were obtained from the scattering cross sections versus energy using the full Mie theory calculations. Reprinted with permission from Ref. [964]. Copyright 2008 The Royal Society of Chemistry.

of the medium, and ε1 þ iε2 is the complex dielectric function of the metal. While the small particle approximations apply only for very small particles (those below 10 nm in diameter), their predictions appear to be nearly accurate for larger particles (
60 nm). However, the uniformity of the electric field within the NP becomes more anisotropic for larger particles (Z 100 nm) [965]. In that case, more multipolar terms (i.e. more terms in equation 10.6) must be included in the model.

10.2.2. The effective dielectric function theories Although Mie theory describes the scattering and absorption of light in dilute dispersions of weakly-interacting, spherical

εeff  εm α ¼ ; 3υεm εeff þ 2εm

ð10:8Þ

where υ is the average volume per particle. Although this formula does not take into consideration non-dipolar interactions and the effect of local order is neglected, it works quite well for dilute samples where only dipolar interactions are important. Maxwell–Garnett theory gives the following for the effective dielectric function [966] εeff ¼ εm

εð1 þ 2f Þþ 2εm ð1  f Þ ; εð1  f Þþ εm ð2 þ f Þ

ð10:9Þ

where f is the filling fraction of metal NPs (described by ε) inside the matrix (described by εm). The Maxwell–Garnett model produces the same result as the dipolar approximation model in the case of spherical, small and isolated NPs. An advantageous aspect of this model is that it can be modified to account for particle–particle interactions or the

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Fig. 10.8. Electric field and charge distribution at the surface of NPs for a size much smaller (a) and comparable (b) to the light wavelength. Reprinted with permission from Ref. [960]. Copyright 2011 IOP Publishing Ltd.

presence of non-spherical NPs [960]. For example, Maxwell– Garnett theory was found to explain very well the optical properties of composites consisting of silica-coated gold particles with volume fractions of at least f¼ 0.3, where the volume fraction was controlled by varying the silica shell thickness [967]. When metal NPs are close together, higher-order multipoles beyond the dipole can be relevant. In these cases, the effective dielectric functions can still be determined, but their calculation becomes more complicated, although quasianalytical solutions to this problem have been postulated [968]. 10.3. Effects of particle size and shape, surrounding medium, and metal NPs’ interaction 10.3.1. Size effects For metal NPs with a given shape and material composition, the size of NPs has a dramatic effect on the SP resonance and the optical characteristics of the NPs [62]. According to Mie theory, for particles of radius well below the excitation wavelength, the magnitude of the absorption cross-section is proportional to R3, while that of scattering is proportional to R6 (see Eqs. (10.7a) and (10.7b)). For the smallest particles, LSPR extinction is due largely to absorption, and as size increases, scattering becomes more important [942,961]. As underscored by Garcia [960], there is a significant size dependence (among other characteristics) to dependence to LSPR properties. For gold nanospheres, an important phenomenological transition occurs around 40–50 nm in particle radius [960,965]. 10.3.1.1. Small NPs (radius up to
50 nm). For this size range, one may assume that the optical properties of spherical NPs can be described by a dielectric dipole (Fig. 10.8a) [960]. While the width and the intensity of the resonance band are strongly affected by the radius, the resonance wavelength remains virtually unperturbed [963]. In this range, intrinsic and extrinsic effects can be distinguished. The intrinsic electron oscillation damping is due to scattering with the ionic cores and with the surface. The damping constant for the electron

oscillations is given by Eq. (10.3). The first term (γb) in Eq. (10.3) accounts for damping caused by ionic core scattering effects. This term depends on the nature of the metal and its crystal structure [960]. The second term in Eq. (10.3) is related to scattering from the particle surface. The fraction of electrons close to the surface is lower for larger particles than for small particles, thus the total damping is reduced relative to that of small particles. In contrast, the greater the velocity of the oscillating electrons (which is near the Fermi velocity), the greater the scattering cross section. Hence, the surface damping is inversely proportional to the particle radius, but directly proportional to the Fermi velocity [960]. This concept is illustrated by the data provided in Fig. 10.9, which shows that the intensity of the LSPR for Ag NPs embedded in silica increases with particle radius, while the resonance peak wavelength is only slightly affected. The FWHM of the resonance band was found to be inversely proportional to the particle size, for NP radius below 25 nm. In addition to the intrinsic effect of the NP size on the LSPR, reports have shown that the dielectric function of small NPs depends on the radius [969]. As discussed above, interband transitions alter the simple form of ε(ω) for free electrons and in most metals they are important in the spectral region of LSPR resonances [969,970]. Within the nanoscale, the band structure depends on size, and consequently the contribution from the interband transitions to the dielectric function is also size dependent. In real metals, the contribution from the interband transitions can lead to increased linewidth of LSPR for very small metal particles with radius r 5 nm [969,971]. The surface-induced electron-electron interaction in these particles leads to quasiparticle scattering that is mediated by the collective excitation [972]. The sizedependent mechanism results in a surface-plasmon-assisted resonant scattering of d-holes into the conduction band. The main role of the SP-assisted d-hole scattering is to change the absorption line shape near the SP resonance, an important effect that has been observed with ultrafast pump-probe spectroscopy [972]. Pinchuk et al. [973] studied theoretically the influence of the interband electronic transitions on the optical properties of small metallic nanoparticles with sizeso50 nm, i.e. the influence on frequency, amplitude, and bandwidth of the surface plasmon resonance. The applied simple approach considered the dielectric permittivity of the constituent metal. The results of this work showed that the size and interface decay channels of the SPR must include the influence of the interband electronic transitions for noble metals, especially in the case of gold nanoparticles; otherwise, the decay rate may be strongly underestimated. Recently, Ochoo et al. [974] reconsidered the damping effect of the inner band (IB) electrons on the optical absorption and bandwidth of metal nanoparticles. The developed model revealed that strong coupling of IB and CB electrons drastically alters the absorption spectra, splitting it into distinctive dipole and quadrupole modes. The model finds that the IB damping effect is mainly frequency dependent and only partly size dependent. Moreover, the coupling of IB and CB electrons may even introduce a behavioral switch, especially in the cases

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Fig. 10.9. Optical absorption spectrum for Ag NPs embedded in silica (n¼ 1.5) with different sizes calculated according to the Mie theory. Reprinted with permission from Ref. [960]. Copyright 2011 IOP Publishing Ltd.

in Au and Cu, but not for Ag. The proposed model can be used as a tool for establishing NP size from the correlation of the spectroscopic parameters, mainly the wavelength at maximum absorption and the bandwidth. 10.3.1.2. Large NPs (radius larger than
50 nm). Metal NPs with radii exceeding
50 nm are sufficiently close in size to the wavelength of the excitation radiation such that dipolar descriptions of the LSPR may not adequately describe the electronic response. Thus, further multipolar terms are required in the overall description of the LSPR. For larger particles, the resonance band splits into several contributions: two parts for quadrupole, three parts for an octopole, etc. Fig. 10.10 displays how the different terms contribute in Eqs. 10.5a and 10.5b to the absorption cross section of Au NPs [960]. For Au NPs with a radius of 40 nm and smaller, the quadrupolar term (L ¼ 2) is insignificant and the dipolar model can be employed. However, for particles with radius 60 nm, multipolar effects are important. For even larger particles, the SPs are described as propagating waves with well-defined modes [960]. Beyond multipolar effects, another reason for experimentally observed broadening of the absorption band of materials containing metallic NPs is the size dispersion. It is well established that size dispersion broadens of the absorption band linewidth. For example, a sample of Ag NPs with an average size of 10 nm and a dispersion 74.2 nm produced a SP absorption band that was three times more narrow (according to the FWHM) than that measured for the same size particles with a dispersion of78.5 nm [960]. One notes here that the size of the NPs cannot be determined exclusively from the FWHM of the absorption band because it is dependent on two parameters: average size and size distribution. 10.3.2. Shape effects The same size-dependent phenomena that affect the nature of SPs are strongly affected by particle shape. For example, the change in the shape of NPs from spherical to elongated rods leads to significant changes in the optical characteristics of metallic NPs [975,976]. The shape of metallic NPs influences the SPs because geometry influences the charge accumulated at

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the particle surface and thus the restoring force, i.e. it is related to the particle polarizability. Metallic nanorods provide a classic example of these shape effects [975,976]. As illustrated in Fig. 10.11a [960], the accumulation of charges at the NP surface during electron oscillations is different along the rod axis than along a perpendicular direction. The charge accumulation maximizes for the transverse plasmons and is minimized for electron displacement along the rod axis or for longitudinal plasmons. The restoring forces are proportional to the charge accumulation. Therefore, for axial oscillations of electrons, smaller restoring forces are expected, which results in lower resonance frequencies. For nanorods, the transversal resonance band appears at slightly smaller wavelengths as that for spherical NPs. The longitudinal plasmonic resonance shifts toward the red with the nanorod aspect ratio, as demonstrated by the data presented in Fig. 10.11b. This behavior provides a strategy for tuning the SP band through control of the aspect ratio, which is especially useful for a variety of applications. If independent wellseparated rods are not oriented on an overall macroscopic scale (as is the case for supported metal NPs), the absorption spectrum will be a weighted average of the possible orientations with both longitudinal and transversal resonance bands. Other particulate geometries, including triangular prisms, nanocubes, and nanocages, give rise to even more complex effects on the SP. In general, deviations from spherical shapes leads to a red shift in the resonance band [976]. Unfortunately, Maxwell's equations can be solved analytically only for spherical nanoshells or ellipsoids. The SP spectra for non-spherical particles must be modeled by other means, such as Gans theory [61,961,975]. Within this model, the absorption cross section for a prolate spheroid, analogous to that in Eq. (10.7a) for a sphere, is given by: [961] sabs ¼

ð1=P2j Þε2 ω 3=2 X εm V ; 2 2 3c j fε1 þ ½ð1 Pj Þ=Pj εm g þ ε2

ð10:10Þ

where V is the volume of the rod. The sum over j considers the three dimensions of the particle. Pj includes PA, PB, and PC, which are the depolarization factors for each axis of the particle, where A 4B ¼ C (A ¼ length, B ¼ C ¼ width) for a prolate spheroid. The depolarization factors anisotropically alter the values of ε1 and ε2 and the resulting LSPR peak frequencies. They are given by:     1 e2 1 1þe ln PA ¼ 1 ; ð10:11aÞ 1e e2 2e PB ¼ PC ¼

1  PA ; 2

ð10:11bÞ

where e is a factor that includes the particle aspect ratio R: "

 2 #1=2   B 1 1=2 e ¼ 1 ¼ 1 2 ; A R

ð10:12Þ

Fig. 10.12 presents absorption spectra of elongated gold ellipsoids calculated by Eq. (10.10), using known values for

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Fig. 10.10. Optical absorption spectrum for Au NP with different sizes calculated according to the Mie theory for nm ¼1.5 and the contribution of the different multipolar terms. Reprinted with permission from Ref. [960]. Copyright 2011 IOP Publishing Ltd.

Fig. 10.11. (a) Illustration of charge accumulation for longitudinal and transversal SPs. (b) Calculated absorption spectra for Au nanorods with different aspect ratios indicated in the figure. (c) Resonant position and intensity of the longitudinal SP as a function of the nanorods aspect ratio. Reprinted with permission from Ref. [960]. Copyright 2011 IOP Publishing Ltd.

the dielectric constant of gold and a fixed dielectric constant for the medium [61]. The two maxima in the simulated absorption spectra correspond to transverse and longitudinal components. The maximum absorbance for the transverse mode shifts slightly to the blue with increasing aspect ratio, while that of the longitudinal mode is significantly red-shifted by over 100 nm. The simulations also predict that the ratio of the longitudinal to the transverse mode intensities increases with aspect ratio [61]. Numerical methods for simulating the SP include boundary element method (BEM) [977], finite element method (FEM) [978], discrete dipole approximation (DDA) [978,979], and

finite difference time domain (FDTD) [966,978,980]. The interested reader is encouraged to explore these methods further in several key literature references. Recently, Schatz and co-workers [978] published an introduction to the application of FEM, DDA and FDTD methods for describing the electromagnetic properties of silver and gold nanoparticles and their dependence on the nanoparticle shape, size, and dielectric environment. In addition, Garcia de Abajo and co-workers [966,977] reported a critical comparison of the performance of BEM, DDA, and FDTD methods for predicting and understanding the optical response of gold nanoparticles. Texeira [980] analyzed the development and application of finite

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difference time domain (FDTD) and finite-element time domain (FETD) methods for solving different problems in photonic nanostructures. 10.3.3. Surrounding medium effects In addition to size and shape, the optical response of gold nanoparticles depends significantly on the surrounding medium. Two clear effects of the surrounding medium can be distinguished for the SP excitation process [960]. The first effect is related to the dielectric of the surrounding medium, which affects the wavelength of light near the NP. This effect alters the spatial characteristics of the electric field at the surface of the NPs, and thus the SPR spectrum. The second and most significant effect is caused by polarization of the medium. During SP excitation, anisotropic charge accumulation in the NPs creates an additional electric field (to that of the incident light) in the surroundings of the NPs. This field induces a polarization of the dielectric medium, resulting in charge accumulation at the interface between the dielectric and the metallic NPs. Consequently, the charge accumulation in the NP (due to the movement of conduction electrons) is partially compensated. This charge reduction is determined by the dielectric function of the medium (εm ); the larger the εm , the larger the polarization charge, and hence, the larger the effect on the SP [960]. A decrease in the net charge at the NP surface will reduce the restoring force, and thus the SPR spectrum. Because lower restoring forces naturally lead to lower resonant frequencies, an increase in the ε m of the surrounding medium is reflected in a shift of the SP resonant band towards longer wavelengths (lower frequencies) [960]. Fig. 10.13 presents the calculated absorption spectra of elongated gold ellipsoids for different medium dielectric constants using Eq. (10.10) [61]. The calculations predict a strong monotonic SP shift to longer wavelengths, along with

Fig. 10.12. Calculated absorption spectra of elongated ellipsoids with varying aspect ratios R using Eq. (10.10). The medium dielectric constant was fixed at a value of 4. The inset shows a plot of the maximum of the longitudinal plasmon band determined from the calculated spectra as a function of the aspect ratio. Reprinted with permission from Ref. [61]. Copyright 1999 American Chemical Society.

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an absorbance increase (for both resonances), with increasing dielectric constant of the medium. The increased resonance intensity with the dielectric of the medium is expected behavior for a forced oscillator because a decrease in the resonance frequency causes a rise in the amplitude. Mathematically, this effect is reflected in Eq. (10.7a) where the absorption cross-section is proportional to ε3/2 m .

10.3.4. Interactions of metal NPs The previous section focused on the optical properties of isolated metallic nanoparticles surrounded by a dielectric. In photocatalytic materials developed for practical applications, the metallic nanoparticles may more likely be located closely enough, as a "nanoparticle ensemble" to result in plasmonic coupling [67,960]. Plasmonic coupling within ensembles could occur through either near-field dipolar coupling or far-field diffraction coupling [981]. The NP dipole contribution to the electric field outside of the NP falls off very quickly; it decreases with distances as 1/r3, where r is the distance to the particle center. Thus, one can predict that the dipolar coupling scales with interparticle distance as d  3 and becomes negligible for d4 λ. The farfield coupling can play a significant role in the photodynamics of 1D and 2D arrays of metal nanoparticles [982] . Fig. 10.14A illustrates the mechanism of the near-field coupling for a chain of nanoparticles [67,981]. Under polarization perpendicular to the chain axis (Fig. 10.14A, a), the transverse mode is excited and electrons of all the nanoparticles oscillate in phase. The restoring force of the dipole in one particular particle increases as the charge distribution in a neighboring particle repels the dipole. This results in a blue shift of the resonant wavelength relative to that of the isolated nanoparticle. Under longitudinal mode excitation (Fig. 10.14A, b), the dipole of a particle is attracted to charges on neighboring particles, which results in a lower restoring force. As a consequence of the change in the restoring force, the longitudinal resonance is red-shifted with respect to isolated structures. These effects for coupled nanoparticles produce split absorption bands that span a wide energy range, which can be advantageous for photocatalytic applications [67]. Fig. 10.14B provides the electric field distribution for an isolated Au nanoparticle (diameter 100 nm) compared to that of a dimer (with a spacing of
30 nm) [983]. Clearly seen in this figure is a field-enhanced region that spills over to within just 10 nm of the spherical surface for the single nanoparticle; however, the field spans the entire 30 nm gap for the coupled particles. Fig. 10.14C further explores the characteristics of local fields generated within dimer gaps by providing a plot of the extinction spectra of the transverse and longitudinal modes for different dimer gaps. For the transverse mode (Fig. 10.14C, a) the reduction of the dimer gap induces stronger overall plasmonic coupling, which results in a blue-shift of the resonant wavelength. This blue shift is due to a repulsion of the dipoles in the two nanoparticles. In contrast, the longitudinal mode exhibits a resonant wavelength redshift upon gap reduction (see Fig. 10.14C, b).

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Fig. 10.13. Calculated absorption spectra of elongated ellipsoids with varying medium dielectric constant εm using Eq. (10.10). The aspect ratio was fixed at a value of 3.3. The inset shows a plot of the maximum of the longitudinal plasmon band determined from the calculated spectra as a function of the medium dielectric constant. Reprinted with permission from Ref. [61]. Copyright 1999 American Chemical Society.

Together, the results (both theoretical and experimental) described above suggest strategies for enhancing energy transfer, charge separation, and photochemistry for plasmonic photocatalysis. These strategies are currently under investigation by many groups. A subset of many relevant studies (both fundamental and practical) is reviewed in the following sections.

11. Plasmonic gold–titania photocatalytic systems 11.1. Characteristics for plasmonic photocatalysts The above discussion indicates that the morphology of plasmonic metal nanoparticles, their size, shape, and density in the composite systems, are parameters that strongly influence their optical properties, i.e. the bandwidth, position, and intensity of the LSPR. In addition, a uniform distribution of metallic NPs in the plasmonic composite system is required for the highest photocatalytic activity because aggregation of plasmonic NPs may result in rapid charge recombination and reduced quantum efficiency. Further, the magnitude of the SPRinduced rate enhancement can be affected through alterations of the metal and semiconductor optical properties, as well as through modifications to the geometric arrangement of the composite system [955]. In these systems, research has shown that the spectral overlap among the illumination source, the metal nanoparticle SPR band, and the semiconductor absorbance can be used to predict the SPR-induced rate enhancement for composite photocatalysts of a given type [957]. Therefore, preparation methods that provide detailed and precise control over the design parameters are required if the full potential of Au/TiO2 photocatalysts is to be realized [907]. One of the key design parameters, the relative geometric arrangement of the metallic and semiconductor components to the photocatalysts, is critically important to the photocatalytic behavior because the LSPR is highly sensitive to the local environment. Interestingly, noble metal nanoparticles can exhibit

a variety of interfacial contact characteristics on the same support. For example, the metal and semiconductor could contact only through the surfaces, or the metal nanoparticles could actually be embedded (fully or partially) in the semiconductor. When the building blocks are in direct contact, a number of photophysical mechanisms may play a role separately or in concert [955,956]. As discussed further in Section 12, these photophysical mechanisms include metal-to-semiconductor charge transfer, near-field enhancements, local heating and scattering enhancements. For plasmonic photocatalyst, where the metal and semiconductor are spaced apart by non-conductive organic molecules [948,954] or porous inorganic films [985,986], the near-field and scattering mechanisms can dominate the photophysical reaction mechanisms. A design of composite systems, in which plasmonic nanostructures are positioned in such a way in the semiconductor matrix that resonant photons are allowed to make multiple passes through the semiconductor, can lead to very strong scattering effects. On the other hand, a design of composite systems that have a large interfacial area between the plasmonic-metal and the semiconductor could result in strong near-field effects [955,987]. A brief overview of current design strategies is provided below for general reference.

11.2. Design strategies The use of LSPR for enhancement of chemical reactions can be primarily divided into two categories [67]: (a) indirect photocatalysis, where the energy of incident light induces a LSPR in metal nanoparticles and the absorbed energy (electronic or thermal) is then transferred to a nearby semiconductor [946,948,954,988], molecular photocatalyst, or other metal [989] to drive chemistry remotely, and (b) direct photocatalysis, where plasmonic metal nanoparticles simultaneously serve as the chromophore and the site for catalysis [990]. In the latter case, the direct photocatalysis can be thought of as a chemical transformation that occurs on the surface of the plasmonic nanostructure in response to photoexcitation of LSPR. Tsai and co-workers classified the various plasmonic photocatalytic systems into the following four forms depending on the contact configuration of plasmonic metal nanoparticles and the semiconductor [67]. This classification covers most of the systems that have demonstrated plasmonic rate photoenhancement [67].

11.2.1. Sole-metal form This is the simplest configuration, which is composed of plasmonic metal nanoparticles and an inert support (i.e. textile fibers) or a conductive support (i.e. graphene and graphene oxide), which does not possess any photoactivity. For these photocatalyst configurations, the photocatalysis relies solely on the LSPR of the metal nanoparticles. In one mechanism, the energy of photons is utilized through localized heating to drive thermolysis or temperature-dependent enhancement of chemical reactions [959]. Another mechanism is the direct plasmonic photocatalysis, where the energy of photons is directly transferred to adsorbates via the LSPR [987].

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Fig. 10.14. (A) Schematic of near-field dipolar coupling of nanoparticles. Reproduced from Fig. 5.10 of [981] with permission of Professor Stefan Maier, and with kind permission of Springer Science and Business Media. (B) Comparison of the electric field distributions of (a) a single Au nanoparticle and (b) nanoparticle dimer. Reproduced from Fig. 1 of [983] with permission of Professor Byoungho Lee, and with kind permission of MDPI Publishing. (C) The extinction log(1/transmission) spectra of the transverse resonance (a) and the longitudinal resonance (b) of an array of Au dimers. The dimer has a center-to-center gap of 300 nm and the adjacent dimer columns are spaced by 900 nm center-to-center. The particle diameter is 150 nm and the particle height is 17 nm. Reproduced from Fig. 1 of [984]. Copyright 2003 Elsevier B.V.

11.2.2. Embedded form In this configuration, the plasmonic metal nanoparticles are partially embedded in the semiconductor and are also partially exposed to the surrounding medium, gas atmosphere, or solution. The metal nanoparticle and the semiconductor are in direct electrical contact. This arrangement describes many previously studied systems that report the visible light photocatalytic activity of TiO2 nanoparticles (or thin films) decorated with plasmonic noble metal nanoparticles [468,508,945,947,950,952,991–996]. These are non-centrosymmetric particles with two distinct sides, e.g. one side is made of gold and the other side is a semiconductor (TiO2) [997,998]. 11.2.3. Encapsulated form This configuration consists of plasmonic metal nanoparticles that are fully buried in the semiconductor and are not directly exposed to the surrounding medium, gas atmosphere, or solution. There is, however, direct physical, thermal, and electrical interactions between the metal and the semiconductor. This arrangement has been widely applied in a variety of fundamental and practical studies [952,999,1000]. 11.2.4. Isolated form In this configuration, the metallic nanoparticle and the semiconductor support are isolated from each other by an insulating layer. As a result, the metal nanoparticle is not exposed directly to the surrounding medium [1001–1004]. The isolation eliminates direct electron transfer, while maintaining transfer pathways that follow a more indirect mechanism. One particularly

intriguing aspect of this configuration is that the field effects that influence indirect transfer can be studied by systematically altering the thickness of the insulating layer [985,1005].

11.3. Preparation methods A diversity of synthetic methods has been explored for preparing plasmonic photocatalytic materials. Some of these methods include deposition–precipitation [921,951,953,1006–1009], photoreduction [1010–1012], ionexchange [1013], impregnation [988,1014], chemical reduction [948,949,957,1015], physical vapor deposition [950,994,1016], hydrothermal [1017,1018], solvothermal [925], and encapsulation [985,1005,1019–1022]. Importantly, the recognition that anisotropic shapes and interactions, via chemical "patchiness" (i.e. ordered arrays formed via specific building block interactions), can serve as excellent tools for engineering the assembly of targeted nanostructures has brought new perspectives to the field [1023]. Reviews by the research groups of Glotzer and Solomon [1023], Mulvaney [1024], Kim [907], and Yu [1025] have summarized recent research on methods for preparing nanostructured plasmonic photoactive materials. Researchers in this field are encouraged to search these reviews for new and highly effective methods for controlling numerous fundamental properties and characteristics of semiconductor-support metal catalysts and photocatalysts.

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11.3.1. Incorporation of metal NPs onto semiconductors 11.3.1.1. Deposition–precipitation. One of the most frequently adopted synthetic methods for preparing plasmonic photocatalysts with incorporated Au and Ag is deposition– precipitation, with subsequent rinsing and further reduction. The DP method renders very small metal NPs (e.g., less than 5 nm) and narrow size distributions onto support materials that are catalytically very active [38], as discussed in Section 6.2. The gold precursor comes from aqueous solutions containing Au3 þ ions, usually HAuCl4. The size and the loading of a noble metal can be controlled through the solution pH and the final calcination temperature. With pH above 4, the hydrolysis of AuCl–4 to [Au(OH)xCl4–x]– gold complex is accelerated. The pH is adjusted with respect to the isoelectric point of the support material by adding a solution such as NaOH, NH3.H2O, Na2CO3, urea etc. [38,488]. Upon reaching the support's isoelectric point, metal NPs and hydroxyl groups deposit onto the surface of the support. One should note that, for this reason, materials with a low isoelectric point, such as SiO2 and carbonbased materials, cannot be efficiently loaded with noble metal NPs through the DP method. For those systems where deposition occurs, the as-prepared gold/support composite is typically further reduced by calcination in air [953], thermal treatment under hydrogen gas [1008], or other reducing atmosphere. The solid product can also be reduced by suspension in alcohols or other reducing media. 11.3.1.2. Photodeposition. Photoreduction-assisted deposition of noble metal NPs on semiconductor supports like TiO2 and ZnO has been often employed in the synthesis of plasmonic photocatalysts. In this approach, the semiconductor NPs are first dispersed in an ionic precursor containing the metal, which leads to adsorption of metal ions. For this first step, other methods such as deposition–precipitation, impregnation, and ion exchange could actually be employed as well [1025]. The metal precursor is then photoreduced via exposure to UV light. Upon irradiation, electrons and holes are photogenerated in the support NPs (TiO2 or ZnO). These electrons are utilized for the reduction of the metal precursor to metal, which consequently is deposited on the semiconductor surface [1025]. The inclusion of a hole scavenger (e.g. alcohol) is required to avoid the accumulation of charge during the photodeposition. The key control parameter in this method is the light intensity. Increasing the intensity of light causes a rise in the average particle size of deposited plasmonic nanoparticles [946,1026]. Photoreduction can lead to relatively high deposition ratios of noble metals compared to other methods. For instance, Kowalska et al. [946,947] and Kominami et al. [1027] prepared supported Au-photocatalysts under light irradiation with nearly complete gold deposition. Moreover, further calcination is not needed, and thus, thermally-induced aggregation of particles can be avoided. Using this method, a variety of metals can be deposited on a single support through simultaneous or successive reduction. One should mention that the major disadvantage of the photoreduction method is that it tends to offer low reproducibility [1025].

11.3.1.3. Ion exchange. The ion exchange [1025] (or chemical [907]) method is often utilized for synthesis of plasmonic photocatalysts, when the solubility of the exchange reaction products is lower than the reactants [1025]. This is perhaps the simplest approach, which exploits in situ chemical reduction following adsorption of metallic precursors onto a metal oxide semiconductor. For fabrication of photocatalysts containing Au NPs, there are different methods of preparation of colloidal suspensions in suitable solvents. Among these, the citrate reduction method (Turkevich method) [1028], the Brust method [14], and reduction by NaBH4 are the most convenient approaches for Au NP deposition on the surface of semiconductors. Adsorption of pre-synthesized or "stock" Au NPs on the support surface can be achieved by simply creating a dispersion of a sample of Au colloids and the semiconductor of interest [1020,1029–1031]. Aggregation of NPs during the adsorption stage is a commonly observed disadvantage of this method, which can be overcome by the application of suitable bifunctional linker molecules. Such molecules are typically employed to anchor Au NPs onto the chemically modified surface of the semiconductor [1032–1034]. 11.3.1.4. Hydrothermal/solvothermal technique. Hydrothermal/solvothermal techniques have been used to prepare inorganic crystalline materials, where an amorphous inorganic species is allowed to crystallize at a relatively low temperature under high pressure, generated by heating an aqueous solution (hydrothermal) or organic solvent (solvothermal) in a closed vessel [925]. This technique allows chemical interactions to proceed that otherwise are not possible at atmospheric pressures and low temperatures. In the hydrothermal process, the metal oxide (or metal oxide precursor) is mixed with a metal precursor and a base in water and an autoclave is employed in the thermal processing [1035–1037]. During the process, the metal precursor adsorbed on the surface of the semiconductors can be reduced chemically [1036]. An analogous procedure is utilized in the solvothermal process, except that water is replaced with an appropriate solvent [925]. An additional advantage of these processes is that by simply adjusting the conditions, such as solution pH; addition of extra reactants; and varying temperature and time of the process, diverse crystalline products with different crystal composition, structure, and morphology can be achieved. Although these approaches are cost effective and quite simple, they can lead to aggregation of metallic NPs [1036]. 11.3.1.5. Sputtering process. In this approach, metal atoms from a target are ion-sputtered and directed toward a substrate [1038–1040]. The ejected atoms react with created gas-phase particles before depositing on the substrate in oxidized form. This vacuum-based method is usually conducted via an argonbased plasma. This approach produces thin and uniform films that are often created for studies of metal-semiconductor hybrid systems on bulk supports. The advantages of this technique include high reproducibility and the ability to achieve well-controlled Au-loading on the substrate.

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11.3.1.6. Encapsulation. Enclosing noble metal NPs inside a semiconductor matrix leads to fascinating hybrid photocatalysts that exhibit high stability and often enhanced photocatalytic activity. In these systems, the semiconductor matrix inhibits both the aggregation and the dissolution or contamination of metal NPs in harsh environments [907,1025]. For fabrication of these systems, a variety of techniques, including physical vapor deposition, spincoating, etc., can be utilized. Bian et al. [1000] reported the preparation of Au–TiO2 core– shell structures by in situ encapsulation of Au particles into core–shell TiO2 spheres, through consecutive solvothermal and hydrothermal processes. In this method, TiO2 and Au precursors are added to a solution of glycerol and ethyl ether in an autoclave and subjected to a solvothermal process. Zhang et al. [1020] applied another preparation approach for the synthesis of a sandwich-like TiO2/Au@SiO2 photocatalyst. These scientists combined simple sol–gel and calcination processes to fabricate a nanostructure composed of a SiO2 core, a layer of Au NPs, and a doped TiO2 shell. Compared to a traditional TiO2–Au structure, in which Au NPs can aggregate at elevated temperatures, the sandwich structure appears to greatly increase the stability of the deposited Au NPs. In their work, the authors suggested that the contact between the Au NPs and the TiO2 matrix facilitates the transportation of photogenerated electrons and charge separation [1020]. Similarly, sandwichstructured TiO2/Ag@SiO2 was prepared by Awazu et al. [985]. Using the sputtering approach, Ag NPs were embedded in SiO2 to form Ag@SiO2. In the next step of their process, a TiO2 film was spin-coated onto the Ag@SiO2 to form the final TiO2/Ag@SiO2 composite. The SiO2 shell prevents the oxidation of Ag. Composites prepared in this way have been shown to exhibit a tremendous increase in the amplitude of the near-field electromagnetic field upon generation of localized plasmons, which enhances the photocatalytic activity [985]. Kumar et al. [986] reported the preparation of TiO2/SiO2/Ag nanoparticle architectures on Si substrates, where the thickness of the SiO2 interlayer was systematically varied. Using atomic layer deposition (ALD), the authors achieved controllable coating of both the SiO2 and the TiO2 thin films that cannot be achieved through other coating approaches such as sputtering, thermal evaporation, or sol-gel processes. Through systematic control of nanoparticle density and Ag–TiO2 spacing, the mechanisms responsible for plasmonic enhancement of photocatalysis have been investigated [986]. Finally, Pietron et al. [506,508] used a guest host composite aerogels strategy to incorporate gold nanoparticles into the nanoparticle network of TiO2 aerogel. The obtained 3D Au TiO2 network contains Au-guest particles (
5 6-nm average diameter) and TiO2-host particles (
10 15 nm covalently bonded nanoparticles that comprise the networked support). This method provides isolated Au particles with multiple anchor points at the surrounding titania support, which has been shown to promote oxidative thermal [506] and photo-assisted [508] catalysis. 11.3.2. Shape and configuration control Because of the significant advances in the field of SPR-based photocatalysis, novel synthesis and assembly methods for

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creating and controlling plasmonic nanoarchitectures are key objectives in this field. Particularly important is the goal of preparing monodispersed noble metal particles with controllable interparticle distances on any desired substrate. The past decade has brought advances in "bottom up" techniques including templating, chemical, electrochemical, sonochemical, thermal, and photochemical reduction methods for shape control of lowdimensional nanomaterials [976]. Bottom up synthesis usually employs an agent to stop growth of the particle at the nanoscale [917]. Capping by surfactants or polymers prevents aggregation and precipitation of the metal nanoparticles. The size and shape of nanoparticles can be controlled by the proper choice of reductant, time, and capping material [976]. Hu et al. [975], Lu et al. [1041] and Grzelczak et al. [1024] have discussed different chemical approaches that have been employed to obtain a variety of metal nanoparticle shapes with tunable optical properties for different applications. Within the last decade, a large variety of geometries have been produced, including spheres, rods, cubes, disks, wires, tubes, branched, triangular prisms, and tetrahedral nanoparticles of gold, silver, and platinum [975,1024]. Although some of the chemical mechanisms that have been proposed to explain the anisotropic growth and the formation of gold NPs with different morphologies were the focus in previous reviews [1024,1041], the fundamental parameters involved are still far from clear. In addition, characteristics such as the semiconductor crystalline form, NP size and shape, and the contact configuration of the plasmonic-metal-nanoparticle/semiconductor all influence charge and energy transfer and, thus, the overall photocatalytic efficiency. For example, Kowalska et al. [1042] have shown that, in contrast with the nano-scale Au particles required for thermal catalysis, photocatalytic activity under visible-light can be achieved with larger polydisperse gold NPs. Further, they found that rutile-based Au/TiO2 photocatalyst containing large gold and titania NPs had higher photoactivity than small gold-on-anatasebased titania NPs [1042]. These and similar studies alluded to above highlight the need for exquisite control over particle size, loading and spacing in the design and synthesis of Au/TiO2 photocatalysts for systematic studies of reaction mechanisms. Furthermore, comparisons of results from different studies across different research laboratories requires standard materials that are synthesized under identical conditions to produce nearly identical samples. Although standards and exact control over key characteristics remains a challenge, the following sections provide an overview of the current state of the field, which has been developed using materials synthesized via the variety of methods currently available. 12. Photophysical mechanisms for plasmon-mediated photocatalysis As discussed above, strong photon absorption, scattering, and local-field enhancements occur at the plasmonic frequency of metal particles because of the large optical polarization that accompanies the collective oscillations of electrons [6]. These phenomena are of interest for a wide range of practical and scientific applications, including plasmon-driven photocatalysis.

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The conversion of photonic energy into electronic or thermal energy through the excitation of surface plasmons occurs through plasmon damping, i.e. the optical polarization dephasing associated with collective electron oscillations [1043]. Briefly, the dephasing rate (with a time constant, T2) is governed by the inelastic decay of the plasmon population, which is characterized by the time constant, T1, and related to T2 through T2 1 ¼ T1 1/2 þ T*–1, where T* accounts for elastic phase-loss of the plasmon [1043]. The properties of the SP are strongly influenced by its dephasing time, T2 ¼ 2ћ/Γhom, Γhom, being the homogeneous line width of the SP resonance [1044]. For instance, the relationship between the dephasing time and the local field enhancement factor, |f|, is given by |f|p T2. As illustrated in Fig. 12.1a, the decay of surface plasmons may occur though two pathways: (1) radiative damping, which describes the transformation of plasmons into photons, and (2) nonradiative decay, which works to produce electron–hole excitations. The nonradiative decay channel can occur via two mechanisms: intraband conduction band excitations and transitions between specific bands (e.g., the d-band) and the conduction band or interband excitations. The decay into electron–hole excitations and how that decay is related to the scattering of electrons are still a matter of discussion in the literature. The main mechanisms include scattering of the electrons at: phonons (electron  phonon scattering), the particle surface (electron-surface scattering), and with electrons (electron  electron scattering) [1045–1048]. Further, electron scattering off the particle surface and interactions with adsorbates may also contribute to the decay, a process referred to as interface damping [1043]. In the following section the influences of each channel on the overall decay rate are discussed. Electron  Phonon Scattering. The oscillating conduction band electrons can couple to the occupied phonon modes of the particle and surrounding support, which results in plasmon dephasing [1045,1046]. The electron  phonon coupling is temperature dependent and can be described by two models: one that accounts for temperatures below the Debye temperature of the material and one that covers temperatures above the Debye temperature [1046]. One such description is a solidstate Debye model. It considers the partial occupation of

phonon modes and applies for temperatures lower than the ΘDebye (170 K). The application of this model requires the plasmonic energies to be above the thermal energy, but below the Fermi energy of the material. Under other conditions, interband transitions may occur, which changes the population of phonons and electrons. This model describes the electron phonon coupling approaching a constant minimum of 8.8 meV for Tr 50 K for gold [1046]. For T 4 ΘDebye, the electron–phonon coupling linearly increases with T as all phonon modes become occupied. Electron  Surface Scattering. This damping mechanism describes the inelastic scattering of the coherently oscillating electrons at the metal particle surface [1047]. This process becomes important for nanoparticles with a smaller radius than the free path of the electrons at the Fermi edge. The extent of damping is determined by the Fermi velocity, υF, divided by the particle radius, i.e. the inverse dephasing time is proportional to the inverse radius: 1/T2 p υF/R. At the interface between the particle and the surrounding medium, the scattering of plasmonic electrons is a particle size and shape dependent dephasing mechanism [1046,1049]. This means that the scattering at the particle surface becomes more probable when surface area increases. Furthermore, the resonant frequency of plasmonic metals is dependent on the particle size and shape. Electron  Electron Scattering. The scattering of electrons via electron  electron collisions is known to dampen phonons [1045]. The probability for this mechanism to proceed mainly depends on the difference in energy of the electron energy and the Fermi level; thus, it depends on the distribution of electrons near the Fermi level [1045,1050]. As the photon energy overwhelms the thermal energy (kBT), the e  e scattering is regarded as temperature independent. In previous work, increased electron–electron interactions were reported with particle size reduction and were ascribed to local reduction of the Coulomb potential screening, due to the spillout of the conduction electrons and to the core electrons exclusion from the surface vicinity [1051]. Radiative Damping. Radiative damping, which contributes to the total dephasing process, depends on size and shape of the particle. The radiative damping becomes stronger with increased the sizes of Au particles, while the enhancement

Fig. 12.1. (a) Schematic representation of radiative (left) and nonradiative (right) decay of particle plasmons in noble-metal nanoparticles. The nonradiative decay occurs via excitation of electron–hole pairs either within the conduction band (intraband excitation) or between the d band and the conduction band (interband excitation). Reprinted with permission from Ref. [1043]. Copyright 2002 American Physical Society. (b) Sketch of the energy relaxation processes after selective excitation of nonequilibrium electrons by a femtosecond pulse (hν) in a metal nanoparticle. τth, τe-ph, and τp-m are the characteristic times for internal thermalization of the electron gas, electron-lattice thermalization, and particle–matrix energy exchanges, respectively. Reprinted with permission from Ref. [1045]. Copyright 2001 American Chemical Society.

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produced by small particles is limited by surface scattering [1052]. The quantum efficiency for resonant light scattering, in the case of a Au nanorod, was found to be considerably higher than that obtained with a sphere of the same volume. Researchers proposed that the higher quantum efficiency was a result of a reduction in the nonradiative decay rate because interband dampening was suppressed by the nature of plasmons within the rod [1043]. This mechanism, illustrated above in Fig. 12.1a, has been studied by several groups [1043,1052]. Scientists have applied different experimental techniques and theoretical modeling to study the electron dynamics in SP excited Au nanoparticles in order to obtain a fundamental understanding of the plasmon decay and damping processes. Shahbazyan et al. [972] found that the e-e interactions inside small particles are strongly modified by surface-induced dynamical screening. This originates from the size-dependent contribution to the interband dielectric function arising via an interband d-hole scattering mechanism, absent in bulk metals. Thus, in noble-metal nanoparticles this size-dependent mechanism results in a surface-plasmon-assisted resonant scattering of d-holes into the conduction band. Varnavski et al. [1053] studied the femtosecond visible photoluminescence arising from the interband recombination of d-holes with electrons in the conduction band near a Fermi surface. The decay of the photoluminescence was found to be very fast, less than 50 fs. The spectrum and efficiency of this emission was found to be the same for the two excitations regimes, UV light (4.65 eV) or visible light (3.02 eV), for each of the investigated geometries (nanospheres and nanorods of two different aspect ratios). The thermalization time scale was found to remain much slower (hundreds of femtoseconds) than that for the photoluminescence process. Konrad et al. [1046] explored single photon luminescence for plasmonic particles to determine the underlying mechanisms for plasmonic decay and damping. Their approach involved exciting plasmons along the long axis of single gold nanorods and observing the system response via confocal microscopy over the temperature range of 1.6 to 295 K. The gold nanorods employed in their work were of 10 nm width and 38 nm length (aspect ratio of 3.8). These Au NRs exhibited mean band maxima for excitation of the SP at 1.77 7 0.02 eV (1.6 K) and at 1.76 7 0.05 eV (295 K); the average values of the FWHM for these features were measured to be 46.57 2.5 meV (1.6 K) and 76.5 7 2.8 meV (295 K). The widths of the bands were analyzed to provide the dephasing times, which were found to vary between 22 and 35 fs at 1.6 K and between 15 and 21 fs at 295 K. Based on these measurements and subsequent modeling, the following contributions were identified for the dephasing processes: electron  surface scattering at 20.7 meV, electron  electron scattering at 15.8 meV, and radiative damping at 1.6 meV. Finally, electron  phonon scattering was found to range from 8.8 meV (1.6 K) to 38.8 meV (295 K) [1046]. The decay of a plasmon resonance, occurring on the femtosecond time scale, can be caused by several important processes that have been widely discussed in the literature [1047,1048]. These damping processes are radiating damping,

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surface scattering, Landau damping, chemical interface damping (CID), and direct emission of electrons. For these dephasing processes, the inverse dephasing time is proportional to the inverse radius of a particle (R). This 1/R dependence of the dephasing time is defined by the Matthiessen law: [1048] P Ai 1 1 1 A ð12:1Þ ¼ 1þ i ¼ 1þ ; T2 T2 T2 R R with A¼ Σi Ai. T1 2 being the dephasing time included in the dielectric function of the bulk, A is the damping parameter, and Ai is the corresponding damping parameter, quantifying the size dependence of the respective contributions [6]. Recently, Hubenthal and co-workers [1047] measured T2 of gold nanoparticles as a function of nanoparticle size (R ¼ 1– 20 nm) at the onset of the interband transition, i.e., at hν ¼ 1.85 eV. This work raised the issue of whether the damping was due to Landau damping, i.e., intraband transitions, or due to band structure changes that influence the interband transitions. In a follow up study [1048], these researchers experimentally quantified the parameter, A, for gold nanoparticles and thus attempted to clarify the influence of interband transition on the damping of the plasmon resonance [1048]. First, dephasing times ranging from T2 ¼ 5 fs to T2 ¼ 17 fs were measured under ultrahigh vacuum conditions as a function of photon energy (1.25–2.75 eV). Subsequently, the damping parameter, A, was extracted from the data and a strong resonance-like damping of the plasmon, in the vicinity of the onset of the interband transition, was established. For photon energies below hν ¼ 1.70 eV, the value for the damping parameter was distributed statistically around a value of A ¼ 0.19 nm/fs, while it increased rapidly to 0.32 nm/fs for hν ¼ 1.85 eV. For higher photon energies, A decreased steadily to A ¼ 0.24 nm/fs at hν ¼ 2.15 eV. A comparison to former measurements, as well as to theoretical predictions, revealed that the surface scattering and a discretizing and broadening of the band structure (which influence the interband transition) are the most dominant size-dependent aspects of the system that play a key role in damping. Scientists concluded from this work that the energy of a plasmon can be transferred to a single electron, which excites a transition from the d-band to the sp-conduction band, and, consequently, a plasmon should to be treated as a two-level system [1048]. Importantly for catalysis, Bauer and co-workers [1054–1057] introduced a phenomenological model in which the interface between a Au NP and an adsorbate adlayer contribute to an additional decay mechanism. They suggest that the resident time of electrons in the adsorbates is long enough to break the plasmon phase coherence, which reduces the surface plasmon lifetime. Thus, the damping parameter, A, is divided into two terms, A¼ Asize þ Aadsorbates. The Asize gives the contribution from pure size (bulk) properties whereas the Aadsorbates is related to the chemical environment directly in contact with the metal nanoparticle surface, i.e., the adsorbates. In the presence of adsorbate molecules, the rate at which plasmonic electrons equilibrate changes, providing pathways for redistribution of the

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electronic kinetic energy. The authors used the phrase “ultrafast chemical interface scattering (UCIS)” for this mechanism, as it is consistent with chemistry-induced localized surface plasmon damping. Fig. 12.2 illustrates this UCIS process in the metal/adsorbate system. Within this picture, femtosecond pulse absorption creates a nonequilibrium distribution electrons (type-I) within 5–10 fs (presumably due to fast plasmon decay) with energies, Eini, in the range of intraband transitions EF rEini o(EF þ hν). The electron is suggested to be transferred to occupy an empty electronic level with an energy, Eini. Consequently, an intermediate negative ion state is created. The electron may then scatter back with energy, Efin, into the conduction band of the gold (see path 1). During the electron-adsorbate interaction, the nascent nonthermal electrons deposit an amount of energy equal to ΔE¼ Eini–Efin into the electronic levels of the adsorbate, which is converted to molecular vibrational energy. Type-I hot electrons thermalize in
800 fs to form Type-II hot electrons. Type II electrons represent those that are in thermal equilibrium with other electrons, but they have yet to equilibrate with the lattice. These studies demonstrate the coupling between photonic energy at plasmonic resonances and molecular excitation of adsorbates, which suggests that such photochemistry can be used in a directed way for catalysis. In the early 1980s, Nitzan and Brus [65] predicted that the electromagnetic field concentration via surface plasmons can lead to a significant enhancement of the rate of photochemical transformations. Soon thereafter, plasmonic photocatalysis emerged as a new field that couples the fields of plasmonics and heterogeneous catalysis. Through the ensuing years, plasmon-mediated photocatalysis has been developed as a technology for conversion of low-intensity solar light into chemical energy. The excitation of LSPR in metal nanoparticles can drive catalytic reactions directly on the surface of metal nanoparticles or remotely on the surface of nearby semiconductor nanoparticles. Thus, the utilization of LSPR excitation to sustain photocatalysis can be divided into two categories: direct and indirect photocatalysis. In direct photocatalysis, the metal nanoparticles act as the photon absorber and the catalytically active site, i.e. the energy is transferred directly between the excited plasmon and the molecule. In indirect photocatalysis, the energy of the LSPR is transferred

from metal nanoparticles to a nearby semiconductor were the catalysis occurs [987]. 12.1. Direct photocatalysis Christopher and co-workers [987] advanced several fundamental concepts in the development of direct plasmon–driven photocatalysis “as a route to concentrate and channel the energy of low intensity visible light into adsorbed molecules, enhancing the rates of chemical transformations, and offering pathways to control reaction selectivity.” However, early theoretical work by Nitzan and Brus [65] regarded the phenomenon of direct plasmonic photocatalysis as improbable because charge carriers created from the excitation of a plasmon are very short-lived and electronically excited adsorbates on metal surfaces are rapidly quenched. Further, Hou and Cronin [956] have argued that direct excitation cannot occur because the energy of plasmonic electrons is insufficient to drive chemical reaction. Despite these reports, may researchers continue to provide evidence that charge carriers and excited states can be efficiently generated and used for direct photocatalytic reactions. The consensus from the experimental results presented in the literature is overwhelmingly in agreement that LSPR excitation is capable, under certain conditions, of driving photochemical transformations through the generation of strong local electric fields, local heating, and energetic electrons at nanostructured metal surfaces [62,1058]. The coherent electronic oscillation lifetime due to a SP excitation in a plasmonic metal has been established to be
5 100 fs, which appears to be of sufficient duration to drive surface chemistry as the plasmon decays [971]. As mentioned above and re-illustrated in Fig. 12.3, SPRs can dephase and decay through three mechanisms [987]: (1) elastic radiative decay to generate photons, (2) nonradiative Landau–type damping that produces charge separation within the metal particle, and (3) interactions and charge transfer between the surface plasmons and vacant adsorbate states or so-called "chemical interface damping" (CID). Although the three plasmon decay processes have different mechanisms, they all appear to have the potential of depositing energy into adsorbates [987]. In the radiative plasmon decay process (1), the energy of reradiated photons from the plasmonic nanostructure can be injected into the adsorbed

Fig. 12.2. Schematic drawing of the ultrafast chemical interface scattering (UCIS) process in Au NPs with a diameter of 4.2 nm wrapped in a shell of sulfate. Reprinted with permission from Ref. [1057]. Copyright 2006 American Chemical Society.

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molecules. In order for this to occur, the adsorbates must have an intramolecular, allowable electronic transition, with an energy very close to that of the re-radiated photons. One important aspect of note related to the electronic structure of adsorbates is that near a surface, the energy of the occupied and unoccupied levels shift because certain electron states are stabilized by image charges and other interactions. Further, the lifetime of particular states can be very short lived at a surface, which significantly broadens states that have narrow distributions in the gas phase. The excitation/de-excitation processes of an adsorbed molecule are described by photon absorptioninduced vibronic energy exchange and are governed by the Franck  Condon Principle [1059]. Plasmon decay process (2) leads to the transfer of energy to adsorbates via rapid, but transient, transfer of plasmon–generated electrons (holes) between the metallic particle and a vacant (occupied) orbital on the surface adsorbates [987]. Plasmons that decay through Landau damping (2) convert the energy of the impinging photon to single electron/hole pair excitations that occur in the range of
10 fs after the initial plasmonic excitation [1060]. Landau damping is the result of intraband transitions between levels of sp character that reside above and below the Fermi level energy, Ef, in coinage metals [1061]. Thus, the excitations of single electrons may occur from below the Ef to above the Ef, where the probability distribution of excited electrons will be relatively constant within the energies Ef and Ef þ hν. The excited electrons experience the effects of other electrons through Columbic inelastic scattering. These scattering interactions lead to an energy cascade that effectively distributes the energy of the primary electron throughout the sea of electrons that compose the particle. As the initial electronic energy is distributed throughout the metal, electron-phonon coupling can begin to drive local heating of the lattice. This heating can occur on a characteristic time scale of about 1 ps in bulk noble metals [1045]. Heat dissipation to the surrounding environment proceeds within 10  100 ps and may thermally excite surface adsorbates at or adjacent to the metal. Accommodation of thermal energy by the adsorbate molecules may, in turn, activate chemical transformations, where an Arrhenius dependence of reaction rate on surface temperature is observed [987]. Importantly, investigations of plasmon–mediated local heating have shown that, under illumination by solar radiation (100 mW/cm2), maximum transient temperatures reach only about 10  2 K in many systems [949,987,1062]. Thus, Christopher et al. point out that a solar intensity of 106 mW/cm2 would be required to produce just a factor of two enhancement, relative to room temperature and in the dark, in a particular reaction rate (for a hypothetical activation barrier of
100 kJ/mol) due to plasmonic heating [987]. This simple rate analysis and enhancement estimation suggest that typical solar illumination may not be effective in driving catalysis through surface plasmon generation and local heating [987]. However, the estimation does not consider non-Arrhenius type behavior that may accompany local heating effects. The two plasmon decay processes described above occur largely independently of environmental conditions. However,

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Fig. 12.3. Schematic showing the three dephasing processes of oscillating surface plasmons, which can deposit energy into adsorbates. Reprinted with permission from Ref. [987]. Copyright 2014 American Chemical Society.

researchers have suggested that the presence of adsorbates on the surface of plasmonic nanostructures can induce an alternative, ultrafast dephasing pathway (3), CID, which has a characteristic time of just
5 fs [987]. CID is proposed to occur via the direct transfer of charge carriers to unoccupied adsorbate states at the instant of plasmon dephasing. Although such transfer may be a component of Landau damping, CID is fundamentally different in several ways, including the overall time scale, a coherence in energy exchange, and a possible efficiency dependence on incident wavelength [987]. Due to temporal differences, photocatalytic reactions that proceed through the various mechanisms are expected to exhibit quite different characteristics that affect product yields, rates, and branching ratios [987]. Overall, the relationship between adsorbate electronic structure and the mechanistic characteristics of plasmonic photocatalysis is of significant importance for developing systems with high reaction yields and selectivities [987]. A more detailed discussion of plasmon-driven direct photocatalysis and its mechanistic characteristics can be found in the review paper by Christopher and co-workers, which has served as the basis for much of the preceding discussion [987]. 12.2. Indirect photocatalysis The transfer of energy from a plasmon to a nearby material, such as a semiconductor, called plasmon resonance energy transfer (PRET), has stimulated significant interest in several areas beyond solar energy conversion, including photocatalysis [1063,1064]. This type of indirect photocatalysis utilizes the excitation of a LSPR in plasmonic nanoparticles to transfer photon energy to a nearby semiconductor, where the chemical transformation of adsorbates may occur. The spatial distribution of plasmonically excited electrons and holes by PRET depends on the electric field intensity and can be described by reliable theoretical methods, such as discrete dipole approximation (DDA) or finite difference time domain (FDTD) methods [978,1064]. The plasmon dephasing rate is the sum of elastic dephasing, radiative decay, and nonradiative decay processes. In general, the probability for PRET will be enhanced for plasmonic nanoparticles that have low

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rates of dephasing. Therefore, it is important that the rate of PRET is high compared to such competing processes. As shown in Fig. 12.4, the dephasing time constant is strongly dependent on the size of plasmonic metal nanostructures, with smaller particles having longer dephasing times [1065]. The competing channels include emission of a photon (i.e., scattering) and conversion of the plasmon to thermal energy, which can be minimized by tuning particle size, shape, and composition [1064]. When the lifetime is analyzed as a function of the size of spherical nanoparticles, it is typically found that particles with a 20 nm diameter exhibit the largest lifetimes. Other shapes, such as nanorods, have been found to have even longer dephasing times [1043]. For plasmonic-metal/semiconductor nanostructures, strong evidence that PRET plays an important role in enhancing the rate of a given photocatalytic reaction can be obtained by measurements of the reaction rate enhancement as a function of the excitation wavelength, the so-called action spectrum. For a wide range of reactions, the highest rate enhancements have been reported at wavelengths corresponding to the metal SPR. Moreover, modulation of the SPR intensity and wavelength by altering the nature, shape, or size of plasmonic nanostructures has led to identical conclusions [948,954]. Based on the established positive relationship between SPR intensity and reaction rates, a hypothesis was derived that the excitation of the SPR accelerates the rates of photocatalytic reactions that occur on the nearby semiconductor. It is now generally accepted that metal nanoparticle–to–semiconductor PRET plays a crucial role in photocatalysis [955,1066]. This energy transfer leads to an increased steady-state concentration of “chemically useful” energetic charge carriers in the semiconductor [955]. Linic and co-workers [955] used the term “chemically useful” to describe energetic charge carriers at the surface of a semiconductor that are thermodynamically available for photochemical transformations. Several non-mutually exclusive PRET mechanisms, described below, have been evoked to explain SPR-induced generation of charge carriers in a nearby semiconductor for enhancing photocatalytic reaction rates. A fundamental issue that remains unresolved, but is of paramount importance for understanding plasmonic photocatalysis, is related to the detailed mechanism for how charge and energy transfer from the metal nanoparticle effectively compete with intrinsic ultrafast (hundreds fs) energy relaxation in the metal [67,1067].

12.2.1. SPR-mediated metal-semiconductor charge injection Kozuka et al. [1068] have observed that visible light irradiation of gold and silver nanoparticles that decorate TiO2 films produces anodic photocurrents. To obtain action spectra in their work, they applied a large bias voltage. However, similar results were obtained by applying appropriate electron donors (in the presence of I–/I–3 or Fe2 þ /3 þ ). In similar work, Tian and Tatsuma [945,952] demonstrated a strong correlation (without a bias voltage) between plasmon absorbance and photopotential and photocurrent action spectra for silver and gold nanoparticles incorporated within nanoporous TiO2 films.

Fig. 12.4. Plasmon dephasing time as a function of resonance energy and nanoparticle size. Reprinted with permission from Ref. [1065]. Copyright 2002 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

These authors [945] hypothesized a mechanism involving charge transfer to rationalize the experimentally observed photon-to-current conversion efficiency (IPCE) of their plasmonic systems under visible light irradiation. The mechanism is schematically illustrated in Fig. 12.5a. According to this charge transfer mechanism, the plasmon resonance excites hot electrons in Au or Ag (see the above discussion about plasmonic relaxation pathways), some of which have energies in excess of the Shottky barrier at the interface that then transfer to the conduction band of the adjacent TiO2 semiconductor. Simultaneous transfer of compensatory electrons occurs from the solution-phase electron mediator (Fe2 þ /3 þ ) to the Au NPs, which results in the achievement of spatial charge separation in the Au/TiO2 system [945]. The potential of the Fe2 þ /3 þ mediator is more positive (Fe3 þ þ e– - Fe2 þ , þ 0.77 V versus NHE) than that of the TiO2 CB ( 0.3 V versus NHE) and that of the O2 reduction potentials (single-electron reduction process: O2 þ e– - O2 ;  0.56 V versus NHE; or multielectron reduction process: O2 þ 2H2Oþ 4e– - 4OH–; þ 0.40 V versus NHE), so that Fe2 þ /3 þ pairs (rather than the CB of TiO2) give electrons to the Au NPs [1069]. Fig. 12.5b shows a clear coincidence of the photocurrent action spectrum and the absorption spectrum of gold nanoparticles in the TiO2 film. Note that the maximum of IPCE (
12%) is observed at around the SPR maximum at 560 nm. These data indicate that SPR generation of plasmonic Au nanostructures is the cause for the observed electroninduced “chemical” transformations in the system. The charge transfer mechanism to the CB of TiO2, highlighted in Fig. 12.5a, is analogous to that proposed decades ago by O'Regan and Gratzel for the dye-sensitized TiO2 solar cell [1070]. In their work and many subsequent studies, they showed that a semiconductor–bound dye molecule may absorb a photon to generate an energetic charge carrier that migrates to the semiconductor [940]. In an analogous fashion, metallic nanoparticles (NPs) appear to be very promising sensitizer candidates because of their localized surface plasmons (LSPs), which have optical cross-sections up to 105 greater than those of molecular dyesensitizers [942]. However, there are some key differences between the excitation of and charge injection from a discrete molecular orbital in a dye and the excitation, decay, and charge injection from a plasmonic surface structure (see below) [956].

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Notwithstanding, Tian and Tatsuma [945] demonstrated that photocatalyst systems based on Au- and Ag-TiO2 could oxidize ethanol and methanol under visible light. The charge transfer mechanism has been proposed to explain other SPR-induced photocatalytic reactions, such as water splitting [953], methyl orange decomposition [1071], photo-oxidation of 2-propanol [946,947] and formaldehyde [1072] observed under visible light illumination. In these studies, the charge injection mechanism was used to explain photochemistry in composite systems where direct electrical contact defines the plasmonic particle–semiconductor interface, which affords efficient transfer of charge carriers. In their studies, Tachiya and co-workers [1073,1074] provided spectroscopic evidence of plasmon-induced electron injection from gold NPs to TiO2 film. Using femtosecond time-resolved infrared (IR) probe transient spectroscopy, these researchers observed electron transfer from 10-nm gold NPs to TiO2 P25 NPs to occur within 240 fs, with an injection yield of
40%. Subsequent detailed studies [1067,1075] revealed that electron transfer was complete within 50 fs, where the electron transfer yield ranged from 20–50% under 550 nm radiation [1075]. As mentioned above, the electronic relaxation through electron–electron scattering occurs on sub–100 fs timescales. Therefore, in these experiments the excited electrons in the gold NPs were transferred to the conduction band of TiO2 before or during the electron–electron scattering process [1067]. The electron injection mechanism suggested by Furube et al. [1073] relies on the energetic overlap of the plasmon band with interband transitions in gold, which can lead to the excitation of electrons in the filled d-band to levels above the Fermi level. In their work, it was proposed that the coherent electronic excitation could provide sufficient energy to some of the electrons, such that they could overcome the Au–TiO2 Schottky barrier, which is about
1.0 eV [915,1076].

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Subsequently, another mechanism for charge injection has been discussed. A plasmon may decay into an electron-hole pair, although factors affecting yield of such path have not been fully elucidated [1067]. Efficient electron injection could also be expected if the absorbed light is concentrated in nanospace at a specific localized position, i.e. if electric field enhancement is realized (see the following section) [1067,1075]. Moskovits et al. [999] further explained the charge transfer mechanism, in which the surface plasmonic decay generates electron-hole pairs in the gold. Their experimental results showed that a large fraction of the resulting hot electrons may tunnel into the conduction band of the adjacent semiconductor, leading to electrical current in the TiO2 when the device is irradiated with sub–band gap energy photons. Sá et al. [1077] used high-resolution X-ray absorption spectroscopy (HR-XAS) at the Au LIII-edge to probe the creation of hot electrons due to LSP excitation in 40 nm wellseparated Au NPs supported on a passivized-Si substrate. HRXAS is a sensitive probe of the unoccupied density-of-states in the metal. The Au NP spectrum was characterized by a resonance threshold at 11923 eV (whiteline) associated with the 2p3/2 - 5d dipole transition, reflecting the unoccupied d density of states above the Fermi level. The unoccupied states above the Fermi level in Au (5d106s1) arise from the s–d hybridization. The observed increase in the whiteline intensity (11923 eV) was ascribed to an increase in the population of holes in the Au d-band (valence states). Along with this effect, a transient broadband mid-IR (infrared) absorption provided evidence for increased concentration of free and trapped electrons in the conduction band of the TiO2 semiconductor. Therefore, the data of this study are consistent with the formation of electron–hole pairs in Au nanoparticles under

Fig. 12.5. (a) Proposed mechanism for the photoelectrochemistry. Charges are separated at a visible-light-irradiated gold nanoparticle-TiO2 system. (b) The IPCE of the gold–TiO2 film in a N2-saturated acetonitrile and ethylene glycol (in response to visible light irradiation (1.36  1014 photons cm  2 at each wavelength). Adapted with permission from Ref. [945]. Copyright 2005 American Chemical Society.

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plasmonic excitation, where some of electrons have sufficient energy to overcome the Schottky barrier and be injected into the TiO2 conduction band. Thus, it was concluded that metal plasmonic nanostructures supported on semiconductor oxides can act as light sensitizers and photocatalysts. Tada and co-workers [1006,1078], studying the visible activity of Au/TiO2 for the oxidation of alcohols and thiols, found that generation of the Au NP plasmon was the driving force for the oxidation. Based on photocurrent action spectral analysis, the authors conclude that the LSPR excitation of Au NPs triggers the injection of electrons into TiO2. A Fano analysis of the absorption spectrum revealed a strong coupling between the LSPR and interband transitions. This coupling induces a rapid damping of the LSPR to decrease the efficiency of the LSPR-driven electron transfer from Au NPs to TiO2 and the overall reactions. Subsequent studies by Tada's group [951] suggest that rutile provides much higher visible-light activity than does anatase as the support for Au particles in the oxidation of alcohols to carbonyl compounds. Based on their photoelectrochemical measurements, they concluded that an extended LSPR lifetime on the rutile sample, relative to the anatase-based catalyst, was the primary reason that they observed an enhancement in alcohol oxidation for the Au/rutile system. Halas and co-workers [1079] recently reported an active optical antenna device using Au nanorods for light detection. In that work, visible light photons were coupled into Au nanoantennas through resonant plasmonic excitations, which decayed into energetic “hot” electrons that were injected into the semiconductor for photocurrent generation. In their design, the spacing between Au nanorods was large enough to eliminate the effects of near-field interantenna coupling. As alluded to in subsequent sections, Cronin et al. [950,956] analyzing the mechanisms of visible plasmonic photocatalysis, noted that surface plasmons consist of a "charge density wave on the surface of the metal. Therefore, SPs cannot be described by simple HOMO-LUMO analogies that promote charge transfer within dye-sensitized semiconductors. According to these authors, plasmonic electrons (holes) reside at the Au Fermi energy; as such, they should not have sufficient energy to drive most reduction (oxidation) half-reactions [956]. However, such arguments are not consistent with the chemical interface damping (CID) mechanism (and other observations, vide infra) [987,1080]. A large damping has been observed for surface plasmon excitations in “reactive” matrix environments, such as under vaporous CO or C2H4 [1080]. Using single-particle dark-field scattering and photoluminescence spectroscopy, Link and coworkers [1081] compared the homogeneous surface plasmon resonance linewidth of gold nanorods on a graphene support to that of nanorods on a quartz support. The damping of the plasmon resonance, as evidenced by an observed increased linewidth for the graphene-supported sample, was attributed to the transfer of plasmon-generated hot electrons from the Au nanorods to the graphene support. Although the energy of the electrons was not explicitly probed, one speculates that they may be available for reduction reactions. Similar observations of charge transfer have been reported in work by Nadtochenko

and co-workers [1082] who used femtosecond transient absorption spectroscopy to study the electron dynamics in Au NPs (possessing an average diameter of
16 nm) embedded in mesoporous TiO2. Their transient spectra were explained by a charge transfer event from the Au NP to TiO2. Recently, Brückner and co-workers [1083] reported the first direct evidence of SPR-excited “hot electrons” that migrate from the Au conduction band to a TiO2 support. The electrons were detected via EPR as they were trapped at oxygen vacancies in near the Au-TiO2 interface. Photocatalytic water reduction on a Au-TiO2 (P25, Degussa) catalysts was monitored under irradiation at distinct UV and visible wavelengths, using in situ EPR spectroscopy to track the population of unpaired electrons. Fig. 12.6a presents the observed EPR spectra of light-induced generation of paramagnetic species, collected both in the dark and under illumination by visible and UV/Vis light. The signal A is assigned to an electron trapped at an oxygen vacancy in TiO2 that is probably in close vicinity to a Au nanoparticle. This signal decreases under irradiation in the presence of O2, suggesting electron transfer from species A to O2. Signals B and C are attributed to Ti3 þ in rutile and anatase phases, respectively, of the P25 TiO2 support, which become much more apparent under irradiation. Signal A strongly increases for the Au-loaded sample when excited with visible-light in the absence of reactants. The authors propose that Au nanoparticles absorb light through their SPR-induced intraband and d–sp interband transitions (see Fig. 10.4) [1067]. Signal D is assigned to O•– radicals, formed in TiO2 via trapping of the positive holes by lattice O2  species, under UV/Vis irradiation at low (90 K) temperature. Signal D is much stronger for the pure TiO2 sample than for the Au-TiO2, most likely due to the loading of Au nanoparticles increasing the number of oxygen vacancies, leading to a deficiency of O2  species. Note that visible light (λ 4 420 nm) irradiation of the Au-TiO2 sample does not produce species D, i.e. trapped holes, whereas the signal of species A is as strong as under UV/Vis irradiation. It is inferred that the visible photoexcitation of Au-TiO2 injects electrons into the TiO2 conduction band, where these electrons become trapped at surface oxygen defect vacancies near the Au particle||TiO2 junction. As discussed in Section 8.1.4, such surface defect sites likely play a crucial role in various thermally catalyzed heterogeneous reactions because they provide possible sites for binding reactants. Fig. 12.6b shows that the emergence of signal, A, in the AuTiO2 sample under irradiation with varying energy photons, is closely related to the visible-light absorption spectrum, which demonstrates a strong correlation between the lightgenerated electron transfer and the Au surface plasmon resonance. Importantly, the additional EPR signal intensity in the region 400–500 nm can be explained by interband Au electronic transitions [1067]. Thus, it was suggested that the electron transfer from Au to TiO2 involves two excitation pathways: (a) d–sp interband transitions at short wavelengths, and (b) SPR-driven intraband transitions at the longer visible wavelengths.

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A theoretical treatment of the photophysical mechanism of hot plasmonic carrier generation and transfer to surface molecules and semiconductors has been reported recently by Govorov’s group [1084,1085]. In a standard photoexcitation description of a metal-semiconductor-Schottky-barrier device, hot electrons generated in the metal have an isotropic distribution in momentum space. Only the electrons within a specific region momentum space can propagate into the semiconductor [1084]. Remarkably, this description was treated theoretically as early as 1931 by Fowler [1086]. Fowler's theory was further developed to treat electronic waves in small metal particles, including different plasmonic nanostructured devices [1087–1089]. Employing a quantum theory based on the density matrix equation of motion, Govorov et al. [1084,1085] investigated the creating and injection of charge carriers from an SPR-excited metal nanostructure to a semiconductor or to molecular adsorbates. Most importantly, the energy distributions of plasmonically excited hot carriers were found to be dramatically different for metal nanostructures with large and small sizes [1084]. As could be expected for large nanocrystals, the excited-carrier distributions resembled that for a plasmon wave in the bulk; the majority of hot carriers had very small excitation energies. When the size of nanocrystal was reduced to 20 nm or smaller, the carrier distribution extended to larger energies and occupied the whole region from the Fermi level to the energy of the photon (EF oεoEF þ hν). The physical reason for these observations was attributed to the more efficient nonconservation of electron momentum in small nanocrystals [1084]. Charge confinement and reflection within small particles generates a large number of energetic (hot) carriers. The

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plasmonic field enhancement factor and the electromagnetic field inhomogeneity within metal nanostructures are the other important photogeneration effects. These effects were found to depend strongly on the nanocrystal shape and geometry [1085]: a nanocube is likely more efficient for plasmonic photoelectron generation than a nanosphere, which creates higher energy carriers than a plasmonic slab [1085]. In contrast to the many reports of hot electron generation and transfer to the semiconductor, there are only a few reports of energetic holes that are retained in plasmonic-metal particles. The reason for this paucity of studies that focus on holes is that the available experimental tools are currently insufficient for elucidating hole dynamics. Notwithstanding, studies have shown that very small particles of Au,
2 nm diameter, may retain holes that have sufficient energy to drive the oxygen-evolving half-reaction (in water splitting) on the surface of the metal [468,953]. 12.2.2. Near-field effects and scattering mechanisms As discussed in Section 10, irradiating metal nanoparticles at frequencies close to their plasmonic resonance can generate intense local electric fields near the surface of the nanoparticles [955]. Electrodynamic calculations based on the finite-difference time-domain (FDTD) method have shown that the intensity of induced local electric fields, i.e. the intensity of plasmonic “hot spots,” can be much greater (up to three orders of magnitude) than that of the electric field source [956,1090,1091]. Researchers have suggested that electron-hole pairs can be locally generated in the semiconductor TiO2 due to the local field

Fig. 12.6. In situ EPR spectra of Au(1.0 wt%)–TiO2 (dAu ¼10–15 nm) (a) at 90 K compared to pure TiO2 (gray) in the dark and under pure visible and UV/Vis light; (b) double integrals of the EPR signal A (symbols) as a function of irradiation wavelength in comparison to the SPR absorption band of Au–TiO2. Signals: (A) e–cb trapped at O vacancies (gI ¼ 2.005); (B) rutile Ti3 þ (gI ¼1.975); (C) anatase Ti3 þ (gI ¼1.975); (D) Ti4 þ –O––Ti4 þ –OH  (gI ¼2.018); (E) Ti4 þ –O2  (gI ¼ 2.026). Adapted with permission from Ref. [1083]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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enhancement generated by plasmonic nanoparticles at such "hot spots" [956,985]. These induced local electric fields are highly non-homogenous, with a high intensity at the surface of the plasmonic nanostructure that exponentially decreases with surface distance for the first
20–30 nm, but with a linear distance dependence at larger distances [955]. The magnitude of the induced local electric field can be particularly large when the plasmonic metal nanomaterials have sharp points, sides, and concave curvatures, like those found at the surface of nanowires, cubes, triangular plates, and NP junctions [60]. The phenomenon of local field enhancement is illustrated in Fig. 12.7. When a photo-excited plasmonic nanostructure is in direct contact with titania or other semiconductors, electrons and holes within the material may be affected by the intense fields. Because the rate of semiconductor electronic excitations, e  –h þ pair formation, is proportional to the intensity of the electromagnetic field at the site of excitation (actually, |E|2), [1092] e  –h þ pair formation rates within local regions of the semiconductor may increase by a few orders of magnitude due the presence of a photo-excited plasmonic nanostructure [948]. However, one must bear in mind that the typical energy for driving plasmons within Au or other metallic nanoparaticles is below that required for direct e  –h þ pair generation within large band-gap semiconductors like titania. Therefore, plasmonically enhanced local fields will only generate e  –h þ pairs within a titania support through multi-step processes likely requiring initial excitations into defect trap states followed by further excitation into the CB. Notwithstanding, this local field effect on e  –h þ pair formation rate may provide some critical advantages compared to bulk semiconductor photoexcitation: (1) the photogenerated charges may be more efficiently separated from each other due to the effects of the surface potential; and (2) the charge carriers are generated close to the surfaces of the materials, where adsorbates reside; hence, they are more readily available to perform photocatalytic transformations [948,950,956]. In these ways, the probability of photoreactions within such systems is enhanced relative to the probability for chargecarrier recombination. For the experiment shown in Fig. 12.7, an enhancement factor of 9 was experimentally observed for the photocatalytic degradation of methyl orange [1016]. Mizeikis et al. [1093] performed FDTD theoretical calculations of optical field enhancements in a system consisting of spherical and hemispherical noble metal nanoparticles on a smooth titania surface. For hemispheres, the field is strongly localized at the metal||substrate interface, where an intensity enhancement of up to 104 times is reported. Moreover, the field is predominantly polarized along the normal to the substrate. These findings suggest a potential application of hemispheresubstrate systems in photocatalysis and light harvesting. In related work, Chen et al. [1094] employed one-dimensional ZnO nanorods that were decorated with gold nanospheres in studies of plasmonically assisted water splitting. The Au/ZnO exhibited optical absorbance in the visible range of the electromagnetic spectrum due to the gold nanospheres, while the ZnO nanorod support absorbed in the ultraviolet range. The X-ray absorption near edge structure (XANES) of the Zn K-edge was

used in situ as a probe of the ZnO conduction band structure. Under visible illumination at 530 nm, the strong electromagnetic field generated near the gold nanoparticles was found to significantly influence the electronic structure of the ZnO, which resulted in rapid transport of photogenerated electron-hole pairs. Importantly, the results from photocurrent measurements suggest that the injection of hot electrons from Au into the semiconductor support increase: the probability of photochemical water-splitting. The authors proposed that the hot electrons are injected into the CB of the semiconductor, while the SPR-induced high local electromagnetic fields generate conduction band vacancies at the surface of ZnO, which promote the separation of photogenerated electrons and holes [1094]. SPR-induced enhancement in the photocatalysis of a semiconductor has also been observed for systems where thin insulating spacers between the semiconductor and plasmonic metal prevent direct metal-semiconductor charge transfer. Specifically, Awazu and co-workers [985] studied a plasmonic photocatalyst that consisted of a Ag NP core surrounded by a silica (SiO 2) shell. The core-shell particles were supported on a titania substrate. The thickness of the SiO 2 shell was optimized to give rise to LSP effects. The enhancement of the photocatalytic activity was found to increase by decreasing the thickness of the SiO 2 shell. For a shell thickness of 5 nm, a 7-fold enhancement of the photocatalytic activity was observed (as compared to the activity of TiO2 alone) towards the decomposition of methylene blue, under near UV illumination. Because the plasmonic Ag nanoparticles and the titania were electronically isolated and the energy of the near UV radiation was insufficient for direct excitation of the titania, the authors concluded that the observed photocatalytic activity was obtained exclusively from the plasmonically induced near-field that generated e  –h þ pairs in the TiO2 [985]. Further demonstrations of similar effects have come from work by Thomann et al. [1021], who reported enhancement factors of 20 in photocurrent, while using plasmonic structures for the water splitting reaction. These results were obtained with Au@SiO2 nanoparticles placed on top of a thin layer of Fe2O3, which in turn coated a glass slide via a transparent conductive oxide (TCO) layer. The Au nanospheres were 50 nm in diameter, the SiO2 layer was 10 nm thick and the Fe2O3 layer was 90 nm thick. The photocurrent enhancement spectra closely resembled the absorption enhancement spectra in the peak positions. This enhancement of the photocurrent, and thus the generated electrons and holes, was attributed to the LSPR-induced enhancement of the total electric field (scattered- and near-field) around Au nanoparticles. The phenomenon of near-field enhancement in photochemistry was also evidenced in studies of how the rate of the oxygenevolution half-reaction depended on light intensity for a composite catalyst composed of plasmonic Ag nanostructures supported on N-doped TiO2 (N-TiO2) semiconductor electrodes [948]. The composite photocatalysts were comprised of a physical mixture of plasmonic Ag nanocubes (
120 nm edge length) and N-TiO2 (
25 nm particles), separated by thin, non-conductive spacers that were designed to restrict direct charge exchange between the two materials. Fig. 12.8 shows that the pure N-TiO2 exhibited

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approximately a half-order dependence of the photocurrent on the light intensity, while the composite Ag/N-TiO2 exhibited approximately first-order dependence on intensity. Other researchers [901] established that the surface concentration of holes for charge carriers photogenerated in the bulk of TiO2 depends on the square root of the light intensity, whereas holes that are generated at the surface linearly depend on light intensity. Based on these reports, scientists [948] concluded that data presented in Fig. 12.8 was an indication that charge carriers were photogenerated in the semiconductor surface layer of the composite Ag/N-TiO2 system. Ishihara and co-workers [1095,1096] have shown theoretically that near-field radiation can activate even a forbidden optical transition due to the breakdown of the long wavelength approximation. They found that under particular resonance conditions, metal nanostructures serve as a light-harvesting antenna to concentrate photon energies into dimer molecules without absorption by the metal structures. The localized light field is particularly enhanced by the plasmon resonance at nanogaps between particles. Following these findings, Tachiya et al. [1067] proposed that the near-field at or in the vicinity of the Au/TiO2 interface may be able to excite electrons in gold, even if such optical transitions are forbidden for far-field light. However, further experimental and theoretical investigations are required before a complete understanding of these processes can be provided. One should note that an additional important effect on absorption cross-sections for small particles may contribute to rate enhancements for such photochemical processes. For plasmonic metal nanostructures with sizes larger than
50 nm, the metal SPR is accompanied by resonant photon scattering [955,1092,1097–1099]. Photon scattering by plasmonic nanostructures increases the average path length of the photons in the composites and, thus, also likely contributes to enhancing electron-hole pair generation in the semiconductor. In this conceptual understanding, the plasmonic nanoparticles are described as nanomirrors. As illustrated in Fig. 12.9, some resonant, but non-initially absorbed, photons may be scattered by metal nanoparticles; thus, those photons may effectively undergo multiple passes through the system. This scattering mechanism may contribute to an increase of
25% in absorbance in the case of Ag/N-TiO2 composites (Ag nanocubes) as compared to the N-TiO2 semiconductor [948]. However, this relatively small increase in the effective number of photons does not fully explain the observed enhancement in the reaction rate, as described by the data presented in Fig. 12.8. Thus, the main contribution to the enhancement of the photocatalytic reaction studied was attributed to the SPR-induced near-field in Ag nanostructure.

Fig. 12.7. (a) SEM image of a 5 nm thick Au island film deposited on anodic TiO2. (b–d) Electric field intensity at the interface of Au–TiO2 calculated using FDTD. Reprinted with permission from Ref. [950]. Copyright 2011 American Chemical Society.

12.2.3. Resonant energy transfer Cushing and co-workers [958] have reported on the resonant energy transfer (RET) mechanism for generation of electron  hole pairs in the semiconductor. In their description,

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dipole  dipole interactions between the metal (donor) and semiconductor (acceptor) enhance plasmon-driven photocatalysis. Whereas the above–discussed mechanism involves the generation of high local electromagnetic fields that may drive interband transitions in the semiconductor, the RET mechanism describes the direct excitation of e  –h þ pairs in the semiconductor through coupling to the localized surface plasmonic dipole. Interestingly, the nonradiative dipole  dipole energy transfer RET pathway for e  –h þ pair generation is not limited by the matching of electronic band structure and equilibration of charge, as in the case of dye-sensitized semiconductors. Cushing et al. [958] suggested that the RET process can generate semiconductor e––h þ pairs at photonic excitation energies both greater than and lower than the band gap energy due to the dipole coupling with states at the band edge. Further, weak or even optically forbidden transitions would be accessible through this dipole-driven process. To study the likelihood of plasmonic energy transfer by RET, researchers [958] synthesized core-shell Au@Cu2O and sandwiched Au@SiO2@Cu2O nanoparticles. Gold and Cu2O were chosen because of the match between the LSPR band of the Au core and the absorption band gap of Cu2O, which is energetically well situated for RET. The SiO2 interlayer prevents direct charge transfer between the particles, but allows optical field propagation throughout the system. Fig. 12.10a shows results from the transient-absorption measurements used to monitor the charge dynamics and relaxation times of the plasmon mediated energy-transfer mechanism in the Au@Cu2O core-shell and the Au@SiO2@Cu2O nanoparticles. In this work, the relative transmission change |ΔT/T| is assumed to be proportional to the relative carrier population created in the Cu2O by plasmonic energy transfer. For the Au@Cu2O sample, two components of the transient signal were well resolved, a fast component (o 100 fs) and a slow component (
2 ps). The fast

component was attributed to direct interactions in the metal. For the insulated Au@SiO2@Cu2O sample, only the slower component appeared because these interactions were not present. For both samples, the carrier decay rates in the Cu2O showed a time constant of several hundred picoseconds, which is a timescale that is consistent with interband recombination. Fig. 12.10b illustrates the author-suggested [958] plasmon-induced charge separation mechanisms for DET and RET processes. They suggest that a DET mechanism should create a wavelength-dependent excited carrier density that mirrors the plasmon absorption line shape, which was not observed. Instead, they found that the number of generated carriers tracked the overlap integral between the plasmon and the Cu2O density of states, which could be explained by the RET mechanism for electron–hole pair generation. This study [958] suggests that a metallic plasmonic nanostructure can act very efficiently as a photosensitizer and that the dynamics may be explained by the RET process. The RET dipole coupling pathway leads to an effective transition probability increase, which allows optically inefficient and dark states to be involved in photocatalysis [1100,1101]. Importantly, dipole coupling involving sub CB edge states for the RET mechanism may essentially extend the band gap to lower energies [1102–1105].

Fig. 12.8. Photocurrent as a function of broadband visible-light intensity for N-TiO2 and composite Ag/N-TiO2 samples. Ag/N-TiO2 exhibits approximately a linear (first-order) dependence on the light intensity, while N-TiO2 exhibits approximately half-order dependence. Reprinted with permission from Ref. [948]. Copyright 2011 American Chemical Society.

Fig. 12.9. A schematic of the proposed active complex of plasmonic Ag particles that can support a super-linear rate is shown. Photons interacting with the active complex are (1) from the source, (2) elastically scattered from other nanoparticles in the reactor or (3) inelastically scattered from the adsorbates. Reprinted with permission from Ref. [1099]. Copyright 2012 Macmillan Publishers Ltd.

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12.2.4. The role of interband transitions In addition to the intraband absorption (
520 nm) due to the LSPR effects, gold NPs also exhibit a broad interband absorption starting from
750 nm to the ultraviolet region, as illustrated in Fig. 12.11 [1106]. The optical absorption coefficient of small nanoparticles (dNP o λincident) is given by Mie theory, where the dielectric function is written as a combination of an interband term and an intraband term (a Drude term) [962]. The Drude term relates to the optical response of the conduction electrons (excitation of the high sp band), which give rise to the surface plasmon band in the visible region. The absorption of UV light excites transitions of 5d electrons to the 6sp band (interband transition) as discussed in Section 10.2 [6,962]. This suggests that the full solar spectrum may be employed for driving chemical reactions by Au-NP-based photocatalysts. The broad spectrum photoactivity of Au-NP-based systems has been demonstrated by many workers, including Zhu and collaborators [1107]. Their photocatalyst contained about 8% gold (metallic state) loaded onto zeolite (Y, SiO2 and ZrO2) supports by the impregnation method. These supports have a band gap of
5 eV (ZrO2) and above (zeolite Y and SiO2), so they exhibited very little light absorption and no photoactivity when used alone (without deposited Au-NPs), even under UV irradiation. Based on the UV–visible spectra, surface photocurrent spectra, and transient photovoltage spectra, the scientists concluded that light absorption by Au-NPs can induce electron transfer from the Au particle to a molecule, such as O2. Surface photocurrent measurements indicated that the interband UV absorption resulted in a much larger fraction of electron transfer from the Au-NPs to oxygen molecules than the intraband SPR absorption in the visible, as illustrated in Fig. 12.12. Consequently, more positive charges were thought to be left in lower energy levels (in 5d band) of the Au-NPs for the UV light exposure. The trapped holes in gold's 6sp band (created by visible light absorption due to the SPR) are thought to effectively capture electrons from easily oxidizable molecules, such as dyes, HCHO, and methanol [1014]. Molecules with higher oxidation potentials, such as phenols, require ultraviolet light absorption by the AuNPs for the creation of more energetic holes that reside in the 5d band of gold (schematically illustrated in Fig. 12.12) [1107]. Schemes such as those depicted in Fig. 12.12, which show the energetic overlap between the 6sp band and 5d band, suggest a means by which to selectively switch between specific reactions depending on the wavelength of the irradiating light [1107]. Similar strategies have been applied by Cronin and coworkers [994] in studies of the role of plasmon-enhanced absorption and interband transitions in the photocatalytic conversions of CO2/H2O into hydrocarbon products. In that work, four excitation energies (two in the visible range and two in the UV range) were applied to explore photochemistry associated with the plasmon resonance, as activated in the visible range, and photochemistry driven by electronic transitions, as initiated by UV radiation. Three samples were explored in their work: (1) bare TiO2, (2) Au nanoparticles deposited on top of TiO2, and (3) bare Au nanoparticles. Under

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Fig. 12.10. Ultrafast pump–probe measurements for core–shell and sandwich structures. (a) Transient absorption for Au@Cu2O and Au@SiO2@Cu2O nanostructures acquired with a wavelength of 650 nm and a laser fluence of 7 mJ/cm2. Decays are fit showing nearly identical recombination rates. (b) Schematic representation of the various transfer mechanisms that can occur in the Au@Cu2O structure. Also shown in the diagram are the pump, probe (free-carrier absorption), and recombination paths. Adapted with permission from Ref. [958]. Copyright 2012 American Chemical Society.

UV irradiation (a 254 nm mercury lamp source), methane was the only product detected with the bare TiO2 catalyst, whereas ethane, formaldehyde, and methanol were the products detected with both the Au/TiO2 photocatalyst and bare Au nanoparticles deposited on glass, indicating that the CO2 photoreduction reaction takes place on the Au surface and that the titania did not play a role in the overall chemistry. Thus, it was suggested that the excited electrons within Au transfer to CO2 and co-react with water to produce the reduction products. Simultaneously, energetic holes apparently drive the formation of O2 to complete the reduction/oxidation cycle [994]. In contrast to the minor role of the TiO2 support in the UV-driven photochemistry, irradiation of a 5 nm thin Au island film deposited on TiO2 caused a 24-fold enhancement in visible (532 nm) photocatalytic activity. This large enhancement was attributed to the strong local electric field generated by the excitation of the surface plasmon at the Au NP surface. The enhanced electric field was hypothesized to excite e  –h þ

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Fig. 12.11. Absorption spectra of gold nanoparticles with different average diameters. Reprinted with permission from Ref. [1106]. Copyright 2007 American Chemical Society.

pairs in the surrounding TiO2 much more rapidly than would otherwise be possible with visible light [994]. 12.2.5. Light-to-heat conversion: Role of the irradiation regime Under photo-illumination, Au NPs are well known to efficiently generate heat [1108,1109]. Thermal effects become especially strong at wavelengths close to the plasmon resonance frequency of Au NPs because the oscillating plasmonic electrons may couple to the phonons of the support. The energy transfer occurs on a time scale of 2 5 ps, followed by phonon  phonon energy dissipation throughout the surrounding medium, which heats the support on time scales of 100  380 ps [1110,1111]. The precise heat dissipation rate depends on the laser source, the particle size, and the chemical identity and phase of the medium [1108,1112]. Interestingly, the large optical cross-section of metal nanoparticles, as compared to their geometric cross-section [942], helps to produce very high temperatures around and within a single AuNP [1113,1114]. Temperatures have been reported to be sufficient to vaporize a solution [959] or even to melt the metal nanoparticle [1115]. Therefore, one must always recognize that thermal chemistry is a critical aspect of photocatalysis at plasmonic frequencies. Wang and co-workers [1116] reported on the plasmondriven photo-to-thermal energy conversion for Au NPs. In their work, this group employed a 146 mW continuous solidstate laser (785 nm) and ambient photothermal detection to record a temperature rise of
30 K upon irradiating an aliquot of an aqueous suspension of one-dimensional gold nanoparticles (nanorods) for 5 s. In the low laser intensity range 0–1.5 W, Roper and coworkers [1111] found that the temperature in an aqueous suspension of 20-nm gold particles irradiated at 514 nm increased in proportion to the applied laser power. The

Fig. 12.12. The diagram of band structures of supported Au-NPs and the proposed mechanism for photocatalysis using supported Au-NPs. SRB stands for sulforhodamine-B dye. Reprinted with permission from Ref. [1107]. Copyright 2009 The Royal Society of Chemistry.

temperature rise was found to be also proportional to Au NP concentration. This work further showed that the efficiency of incident resonant light to heat transduction can be raised from 3.4% to 9.9% by temporally modulating the irradiation. Light in resonance with a SPR, even of relatively weak intensity, has been shown to melt ice with embedded Au NPs, whereas even a very intense laser beam does not melt a pure ice sample [1117]. Richardson and co-workers [1117,1118] applied thermooptical spectroscopy to investigate optically excited gold NPs in an ice matrix. Theoretical calculations and experimental data were combined to estimate the heat created by excited Au nanoparticles and agglomerates. As shown by the inset of Fig. 12.13a, the heating rate in the case of water and Au NPs was relatively fast. For example, for Au particles with RNP ¼ 30 nm, the estimated characteristic thermal ramp time was 6 ns. The calculations showed (Fig. 12.13b) that a heating effect of a few K can be achieved with light fluxes of 103–106 W/cm2 by using NPs of relatively large radius (RNP 410 nm). The duration required to melt ice and a polymer were in the sub-ms-range and the ms-range, respectively, for photoexcitations of plasmonic materials [1117]. The same authors studied, experimentally and theoretically, the heat transfer for optically excited Au NPs embedded in water droplets [1119]. In those systems, the light-to-heat conversion efficiency (η) was found to be close to unity (0.97 o η o 1). Recently, Pinchuk and co-workers [1120] investigated the light-to-heat energy transfer for Au nanoparticles ranging in size from 5 to 50 nm. In their work, a gold nanoparticle suspension was exposed to a continuous green laser (532 nm) with power
228 mW. They found that the conversion efficiency rose from 0.65070.012 to 0.80370.008 with decreasing particle sizes from 50.0972.34 to 4.9870.59 nm, respectively, which demonstrated a tenability to the photothermal properties of these systems. In this work, Mie theory was used to account for the variation in the heating efficiency with particle size, which was shown to correlate to the absorption/extinction ratios.

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As mentioned above, the correlation between quantum efficiency and photon flux is a key characteristic of sample heating with noble metal NPs, which has been shown to affect plasmonic photocatalysis [60]. For example, Chen et al. [1014] found that the rate of visible-light-driven aerobic oxidation of HCHO increased exponentially from 9% to 64% as light intensity increased from 0.02 W cm  2 to 0.17 W cm  2. The authors estimated a heating rate of 3–5 K s  1 with a resulting temperature above 373 K during irradiation of Au@ZrO2 nanoparticles. Christopher et al. [1099] investigated the rate of ethylene photocatalytic epoxidation (which is limited by the dissociation of O2) as a function of the light source intensity at several temperatures. At intensities below 300 mW cm  2, a linear dependence on light intensity was observed, while at higher intensities the relationship deviated significantly from linear; these results still suggest a thermally-driven process caused by excitation of surface plasmons. Boyd and co-workers [959] recently demonstrated a new method for performing different types of heterogeneous catalytic reactions that makes use of the plasmon resonance in nanoscale metal nanostructures to provide the necessary heat of reaction when illuminated with a low-power laser. This method allowed the authors to initiate catalytic steam reforming of ethanol inside a microchannel at room temperature. The experiments were performed within a linear microfluidic channel (100  40 μm, 25 mm length), which allowed for precise control of liquid feedstocks and the collection of small amounts of reaction products. Arrays of gold nanoparticles (
20 nm in diameter) were produced by block copolymer lithography and were deposited on glass microscope slides by spin-coating. Within this arrangement, a continuous green laser (532 nm) at a power density of 6.4  108 W/m2 was employed to vaporize the liquid, which produced a bubble (r¼ 5 μm) that led to a pressure increase estimated to be as high as 28 kPa and a temperature rise of 108 K. When a mixture of alcohol and water was used, GC analysis showed steam reforming to occur with selectivities of 19%, 75%, and 6% for H2, CO2, and CO, respectively. The localized heating effect has also been shown to thermally activate catalytic reactions at relatively low overall sample temperatures, which otherwise require much higher temperatures. This effect was demonstrated recently by Christopher et al. in the oxidation of ethylene, CO and NH3 [949]. In their work, a Ag/Al2O3 catalyst containing Ag nanocubes (
60 nm edge length) supported on α-Al2O3 particles was used. For the oxidation of ethylene at 430 K, the reaction rate was found to increase by a factor of 3–8 when the sample was exposed to visible light (250 mW cm  2). To reach the same reaction rate in the dark, the sample required temperatures of 470 K. Based on kinetic isotope experiments and DFT, the authors postulated that the excited plasmons on the silver surface create energetic electrons that transfer to the 2π* state of adsorbed O2. The population of antibonding O2 orbitals accelerates the rate-limiting step of O2-dissociation. In a later study, these authors also detailed how thermal energy exchange can also excite vibrations of surface adsorbates, thereby further accelerating photocatalytic reaction rates [1099].

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Fig. 12.13. Calculated temperature increase at the surface of Au NP in the water as a function of wavelength (a) and illumination power (b), I0 is the light intensity inside the matrix. The graph (b) is given at the plasmon peak wavelength. A matrix is the water with ε0 ¼1.8. Inset: spatial distribution of temperature at different times. Reprinted with permission from Ref. [1117]. Copyright 2006 Springer Science+Business Media, LLC.

In a similar vein, Zhang et al. [1121] reported a strong dependence on the light irradiation regime (i.e. the intensity and wavelength) for the photooxidation of benzyl alcohol on zeolite-supported gold nanoparticles. They suggested that stronger light intensities excite more energetic electrons in a Au-NP, which enhances the electromagnetic field around the particle. This effect strengthens the interaction between AuNPs and benzyl alcohol, which leads to a higher conversion rate. The range of wavelength also influences the photooxidation process. The SPR effect of Au-NPs makes a primary contribution to the photooxidation reaction at wavelengths between 515 to 535 nm. Their [1121] kinetic studies indicated a much lower apparent activation energy (49.8 kJ mol  1) for photooxidation upon visible-light irradiation than that (83.3 kJ mol  1) required under thermal reaction conditions. A tentative mechanism was proposed to explain the SPR-induced rate enhancement. Specifically, benzyl alcohol molecules are thought to initially adsorb on the surface of the zeolite support through the formation of hydrogen bonds. Upon visible-light irradiation, Au-NPs become electronically polarized due to the SPR effect. Then, Au–H species are formed by abstraction of an α–H atom from the cleavage of the C–H bond on the methylene group (–CH2–) of benzyl alcohol. On other hand, scavenging of electrons from polarized Au-NPs by molecular oxygen forms activated O2 that further interacts with the Au–H species to yield water.

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13. Key photocatalytic reactions under vis-light The unique ability of nanoscale gold particles to absorb, scatter, and redistribute radiative energy at wavelengths that span large region of the electromagnetic spectrum renders them excellent candidates for plasmon-driven photocatalysis, the efficiency of which depends on the nanoparticle size and geometry, the support medium, and nanoparticle/support interfacial properties. These properties, along with the relatively low energy requirements for generating visible light, have motivated tremendous research into plasmon-enhanced photocatalytic reactions. The preceding section describes general aspects of the mechanisms that have been evoked to account for plasmonically-driven photocatalysis. The current section focuses on more specific classes of reactions. Several types of reactions have been found to proceed though plasmonenhancement, including diatomic molecular dissociation [66,949,1122–1124], water splitting [507,948,953,1064,1125, 1126], H2 production from alcohol [998,1127], liquid and gas phase oxidation reactions [508,946,949,1128], hydrocarbon conversion [994], etc. Recently, a number of reviews have described different aspects of the photocatalytic activity of plasmonic metal nanostructures under UV and visible light irradiation [60,67,907,1066,1129]. Here, we attempt to bring some of the most important discoveries and insights together into a comprehensive overview on the topic. 13.1. Dissociation of diatomic molecules As described in Section 12, excited LSPs in metal NPs can nonradiatively decay as they produce hot electrons that possess energies between the Fermi and vacuum levels. The excited electrons may then dissipate their energy through electron  phonon energy exchange or, when molecular adsorbates reside on the surface, the electrons may move to an unoccupied state of the adsorbate [1130–1132]. The resultant increase in internal energy of the molecule may provide an alternative pathway for dissociation, which may trigger further chemical transformations. The time required for vibrational energy redistribution on a metal surface is typically picoseconds [1133]. With the oscillation time for the vibration of an adsorbate being
50 fs, vibrationally activated molecules generally have ample time to react before the additional energy is dissipated. Therefore, light-induced excitation of adsorbates through surface plasmon generation may activate an adsorbate for catalysis at overall surface temperatures much lower than analogous thermally-driven catalytic reactions. 13.1.1. Hydrogen dissociation The adsorption and dissociation of H2 on metal surfaces is perhaps the most important reaction for heterogeneous catalysis. The enthalpy for dissociation of H2 is 436 kJ/mol (4.51 eV), which cannot be surmounted through thermal activation alone. As discussed in Section 7.3.2, Panayotov and Yates [595] have shown that molecular H2 (D2) can dissociate homolytically to atomic hydrogen over gold nanoparticles (
2.7 nm) supported on TiO2 with an activation

barrier of
50 kJ mol  1 (0.52 eV). For this Au/TiO2 catalyst, H2 dissociation occurs in dark conditions and at temperatures well below the ambient, i.e. at 250 K. The thermal H2/D2 isotopic exchange reaction has also been reported to occur on Au NPs supported on Al2O3 (
300 K) [689] and SiO2 (450 K) [1123]. These results beg the question of possible photocatalytic activity for H2 to further lower the barrier. Recently, Halas and co-workers [66] reported the first experimental evidence that gold nanoparticles supported on a TiO2 matrix can activate photolytic dissociation of H2 (D2) under visible light irradiation. In their work, the activation energy for H2 dissociation was reduced to 1.7 1.8 eV through a plasmon-mediated process. The dissociation was suggested to be enabled through the involvement of an excited Feshbach resonance [1134]. According to this mechanism, hot electrons result in a transient population of an H2 excited state (occupation of the s* orbital of the molecule). A schematic of the overall process is provided in Fig. 13.1. In the work highlighted in Fig. 13.1, the dissociation rate of H2 was indirectly measured by exposing Au nanoparticles to an H2/D2 source of gas and monitoring the HD formation rate as a function of photon intensities and wavelengths [66]. Under dark conditions, HD formation was minimal at room temperature. Under excitation with photons from a visible laser source, a 6fold increase in the rate of HD production occurred [66]. A steady increase in the rate of molecular hydrogen production, with increasing Au loading, demonstrated a strong influence of Au nanoparticle concentration on the reaction. Finite-difference time-domain (FDTD) calculations for Au NPs sizes, ranging from 6 nm to 21 nm at 298 K and incident laser power up to 2.4 W cm  2, showed a negligible photothermal heating effect. The calculated near-field enhancements clearly increased with Au NPs size; however, the size effect was contrary to the observed photoreaction trends, which strongly suggests that the near-field effects discussed above in Section 12.2.2 were not responsible for the H2 dissociation. That is, for the same 1 wt % Au NP loading, the photocatalytic rate measured for 16 nm Au nanoparticles was 50% of that for much smaller particles. In addition, the photocatalytic rate appeared to depend linearly on laser intensity, which eliminated the possibility that photon excitations of electrons in the TiO2 were responsible for H2photodissociation. Overall, the observed direct correlation between the absorption cross section and the rate of hot electron production (as detected by the HD generation) provided direct evidence for SPR-induced hot electron-driven H2 photodissociation on Au NPs [66]. The proposed mechanism [66] of hot electron-induced dissociation of H2 on Au is illustrated in Fig. 13.1. Visible light irradiation excites the SPR of the Au NPs where subsequent plasmonic decay creates an electron-hole pair (Fig. 13.1a), which leads to a nonequilibrium energy distribution of electrons (Fig. 13.1b). Electrons in the highenergy (above the Ef) portion of this distribution may come into resonance with and transfer into the antibonding (AB) state of the H2 molecule. The transient negative ion, Hδ2  (Fig. 13.1c) would be stabilized by its image potential in the metal [1135].

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The Hδ2  species, necessarily formed far from its equilibrium molecular bond distance, then moves in response to the gradient of the excited state potential energy surface (Fig. 13.1c). As the molecule proceeds along this trajectory, the Hδ may transfer its electron back to the Au NP, which 2 could place the H2 molecule onto the repulsive part of the ground potential energy surface where it readily dissociates as the atoms are stabilized through interactions with the metal [66]. This hot-electron-induced mechanism for H2 dissociation received additional support by the results of a follow-up study where Au NPs were supported on insulator oxides such as SiO2 or Al2O3 [1123]. In those studies, researchers observed that the change of the support from TiO2 to SiO2 led to a dramatic increase in the rate of H2 dissociation. Upon photonic excitation, the rate of HD formation on the Au/SiO2 instantaneously increased by a factor of
150, whereas the enhancement on the Au/TiO2 was only 6 fold, although both photocatalysts contained the same loading of similarly sized (5 30 nm) Au NPs. This was because in the case of TiO2 the hot electrons were transferred directly into the conduction band of the support rather than the antibonding orbital of the adsorbate. This result provided strong evidence that the reaction of H2/D2 exchange proceeds on the surface of SPRexcited Au NPs, mediated by hot-electron transfer to the adsorbate molecules, such that the dielectric support was not actively involved in the chemical transformation. Furthermore, this chemical transformation was also activated on Au NPs supported on Al2O3 under the same irradiation conditions. Finally, detailed experimental and theoretical studies showed that the contributions from photothermal and near-field effects were not substantial for this reaction. These results strongly support the proposed mechanism that SPR-excited Au NPs generate hot electrons that transfer to the H2 molecules, substantially reducing the barrier for H2 dissociation [1123]. 13.1.2. Oxygen dissociation Linic et al. [949] have proposed an SRR-induced mechanism, similar to that described above, for O2 that leads to the activation of the O–O bond in the photolytic epoxidation of ethylene and

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oxidation of CO and NH3. These photothermal oxidation reactions were found to occur at temperatures lower than that for thermally activated reactions. Using kinetic isotope experiments and density functional calculations, these authors postulated a mechanism in which surface plasmon-generated hot electrons populate the antibonding orbitals of oxygen to form a negative-ion state, which initiates O2 dissociation [949]. The plasmonic nanostructure used in their study contained Ag nanocubes of
60 nm edge length supported at
20% wt loading on α-Al2O3 particles. Cubic nanostructures have demonstrated superior selectivity towards ethylene epoxidation as compared to that of nanospheres and nanowires [1136,1137]. In addition, plasmonic intensities observed with cubic nanostructures are greater than those measured with spheres and nanowires [954]. The size of the catalytic particles was optimized based on catalytic performance and optical properties. The rate of ethylene epoxidation was measured at 450 K and atmospheric pressure with visible light
2.1 eV (590 nm) irradiation or under dark conditions. Under 250 mW cm  2 irradiation, a fourfold increase in the steady-state reaction rate was observed compared to that measured for the thermal process (no light). Hence, photolytic activation was found to produce a 40 K decrease in the operating temperature for the overall reaction. In their work, control studies in which the intensity of visible light was varied resulted in a linear relationship between the photoinduced rate of oxidation and light intensity. This linear dependence is a signature of an electron-driven chemical process, primarily because no other mechanism, such as heating, exhibits this behavior [949,1138]. Thus, energetic electrons produced by the excitation of surface plasmons are suggested to be responsible for the observed photoactivity. The hypothesis is that energetic electrons (presumably produced through plasmonic decay and electron–hole pair generation) accelerate the elementary step of molecular O2 dissociation on silver to form adsorbed atomic oxygen. To prove the validity of this physical picture, the authors performed kinetic isotope studies to compare the steady-state rates of ethylene epoxidation using unlabeled (16O2) and labeled (18O2) oxygen. On metal surfaces, electron-driven elementary surface reactions have been shown to exhibit larger kinetic

Fig. 13.1. Mechanism of hot electron-induced dissociation of H2 on Au nanoparticles. (a) Schematics of hot electron excitation in AuNP showing d-band electron– hole pair excited above the Fermi level upon plasmon decay. The narrow bonding and broad antibonding states of adsorbed H2 are denoted as B and AB, respectively. (b) Schematic of Fermi–Dirac type distribution of hot electrons permitting hot electron transfer into the antibonding state of H2. (c) Proposed mechanism of hot-electron induced dissociation of H2 on AuNP surface. Adapted with permission from Ref. [66]. Copyright 2013 American Chemical Society.

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isotope effects (KIEs) (16O rate/18O rate) than thermal phonondriven reactions [1139]. As shown in Fig. 13.2a, the steady-state rate of the photothermal ethylene epoxidation reaction (light on) measured under identical conditions (450 K) was larger for 16O2 than for 18O2. The estimated KIE for the photothermal process was 1.19þ 0.01, as compared to a KIE of 1.06þ 0.02 for the thermal (light off) process. The larger KIE for the photothermal process, compared to that of the thermal process, was suggested to be another signature of the electron-driven process depicted in Fig. 13.2. The transfer of an energetic electron from Ag to the antibonding O–O 2π*-state of molecular O2 adsorbed on silver is postulated to form an O2 ion (an O2 molecule with an extra electron in the 2π*-state), which facilitates O2-dissociation. To obtain more details on this physical picture, the authors employed DFT calculations [949]. These calculations showed that, for O2 adsorbed on the silver surface, the antibonding orbital is only partially occupied in the electronic ground state (at low temperatures), which keeps O2 in the undissociated molecular state, see Fig. 13.2b. Figure 13.2c shows the potential energy surfaces for O2 and O–2 where an electron was removed from the Fermi level of silver and placed in the O2 2π*-state of O2 adsorbed on the Ag(100) surface. The energy required to populate the antibonding orbital of O2 adsorbed on silver is
2.4 eV. This energy matches the energy of silver surface plasmons, and thus, the energy of hot electrons formed in the process of plasmon excitation and decay on the silver nanocubes. This result implies that an efficient resonant-electron transfer to the antibonding state is possible. The scattering of energetic electrons into the antibonding orbital of O2 results in stretching of the O–O bond. That is, the lowest energy configuration for the charged O2 state is characterized by a larger O–O bond distance (as described above for H2). The decay of the electron from O–2 back to the metal results in the collapse of O2 into a vibrationally excited state of the ground-state potential energy surface. Overall, the transfer of an electron from Ag to the antibonding orbital and then a back-transfer to the metal, deposits energy into the O–O vibrational mode. Because the oscillation time for the vibration of adsorbed O2 is nominally in the range of
50 fs [1130,1131], it was suggested that the molecule will not have enough time to react before this vibrational energy is dissipated. Therefore, the net effect of the SPR-induced electron-transfer will be that the O2 adsorbate becomes activated at a temperature that is lower than that for the thermal (light off) process, Fig. 13.2d. Because of the difference in the masses of the 16O2 and 18O2, the 16O–16O bond will be stretched further than the 18O–18O bond. That is, the vibrational energy associated with the 16O2 isotope is larger and therefore it will be easier to activate 16O2 than 18O2. Although the results of this study [949] appear to demonstrate that SPR-induced transfer of energetic electrons from the metal to the adsorbate leads to enhancement of the catalytic reaction, the detailed mechanism is still not completely understood or even established. Another possibility for electron transfer is the direct interaction of surface plasmons with adsorbates. In addition, the effects of spatially inhomogeneous

distributions of plasmons on the silver nanostructures and ‘hot spots’ formed between multiple plasmonic particles can play a critical role for O2-dissociation [949]. 13.2. Hydrogen generation Solar energy is considered to be a renewable resource that holds tremendous potential in assuring energy sustainability (at least for the next several billion years, the expected life span of this star). However, the conversion of solar photons into electricity and fuels is a great scientific and technological challenge [1140]. One of the most attractive solutions is to store solar energy in the form of chemical fuels, such as hydrogen gas and alcohols. As a fuel, hydrogen gas has a highenergy capacity; it is a clean, carbon-free renewable fuel with the potential to reduce many of our energy demands. Researchers, being inspired by natural photosynthesis, have been seeking to develop artificial processes that use photoactive materials. Clean hydrogen generation is one of the major motivations for developing photoelectrochemical (PEC) water splitting and hybrid solar H2 production through alcohol steam reforming. In recent years, several excellent reviews have been devoted to solar photocatalytic water splitting, where the beneficial role of surface plasmon resonance excitations have been explored [1141–1145]. 13.2.1. Water splitting Photocatalytic water splitting has been studied extensively since the discovery of the Honda-Fujishima effect, which demonstrated that a photoelectrochemical (PEC) cell with a titanium dioxide photoanode can decompose water to hydrogen and oxygen by photo-illumination [1146]. Fig. 13.3a illustrates the configuration of a PEC cell with an n-type semiconductor TiO2 photoanode and a Pt counter electrode. Contact between the semiconductor and the electrolyte leads to interfacial charge transfer as electrons equilibrate, which leads to an upward shift of the electronic bands. This band bending creates a potential barrier known as a Helmholtz barrier, which depends on the nature of the aqueous electrolyte and the semiconductor electrode [1147]. Under supra band gap UV illumination of the semiconductor, electrons from the valence band are promoted to the conduction band and, consequently, electron-hole pairs are generated. The interfacial potential barrier can facilitate the separation of electron and hole pairs and thus increase their lifetime. The photoexcited electrons may then transfer to the Pt counter electrode and reduce water to generate H2, while holes diffuse to the surface of the semiconductor where they are available to oxidize water to form O2 [1143,1148]. The reaction on each electrode can be presented by the following equations: [1143] Photoanode : H2 O þ 2h þ -2H þ þ 1=2O2 E0ox ¼  1:23 V Cathode : 2H þ þ 2e  -H2 E0red ¼ 0 V Therefore, to drive this reaction, the photon energy must exceed 1.23 eV (Ei ¼ ΔGO(H2O)/2NA, where ΔGo(H2O)¼

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Fig. 13.2. Molecular mechanisms for electron-assisted O2 dissociation. (a) Steady-state rate for photothermal reaction (light on) measured at 450 K for 16O2 and 18 O2 reactants. (b) Molecular density of states projected on O2 adsorbed in the lowest energy configuration on the Ag(100) surface (the dominant surface facet on silver nanocube catalysts [1136]), calculated using DFT. The Fermi level is depicted by the horizontal line at energy equal to 0 eV. The 1π-bonding and 2π*antibonding orbitals of adsorbed O2 are shown, as is the model system used in the calculations (the red spheres are oxygen atoms and the grey spheres are silver atoms). (c) DFT-calculated potential energy surface for O2 and O2 on Ag(100). Excitation of silver surface plasmons allows for the transfer of an excited electron to O2, which forms O2 . The O2 becomes negatively charged and nuclear motion is induced along the O2 potential energy surface. *¼ equilibrium state; ads ¼adsorbed. τ depicts the progression of the O2 molecule on the negative ion potential energy surface as a function of time. (d) Schematic of the proposed mechanism of photothermal O2 dissociation. Reprinted with permission from Ref. [949]. Copyright 2011 Macmillan Publishers Ltd. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

237.141 kJ mol  1; NA ¼ Avogadro's number¼ 6.022  1023 1 mol ) [1149]. Unfortunately, one of the major obstacles in large-scale PEC utilization is its low activity due to the high recombination rate of photogenerated electron hole pairs, setting a limit of light energy conversion efficiency. Many efforts have been devoted toward improving the quantum efficiency of semiconductorbased photocatalytic water splitting, as recently reviewed by Chen et al. [1142]. Although water-splitting photocatalysts are mainly composed of semiconductor materials, silver and gold plasmonic nanostructures hold promise as strategies for promoting photocatalytic water splitting. As illustrated in Fig. 13.3b,

plasmonic metal nanostructures may replace dye sensitizers, whereby they efficiently absorb photons and transfer energetic electrons to the nearby semiconductor. As described extensively above, plasmonic metal nanostructures appear to be very promising photocatalysts because of their excellent charge mobility and very high absorption cross-sections, up to 105 larger than that of typical dye-sensitizers [942]. Furthermore, the resonance wavelength of such nanostructures can be tuned by changing their size or shape, which provides a strategy for utilizing the entire solar spectrum for photocatalysis [955]. Linic et al. [955] pointed out some critical energetic considerations related to the electronic structure of plasmonic nanostructures that are important for realizing efficient

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photocatalytic water splitting. Most importantly, many practically useful semiconductors exhibit their conduction band edges at energies between  1.0 and 0 V on the normal hydrogen electrode (NHE) scale, while their valence band edges lie around 2.0 and 3.5 V (electron energy between  2.0 and  3.5 eV) on the NHE scale. Plasmonic metal nanostructures typically exhibit SPR energies from 1.0 to 4.0 eV relative to the Fermi level, which is located near 0 V on the NHE scale. Despite energetic alignment (through charge equilibration) of the electronic states at the metal/semiconductor junction [58], only highly energetic electrons excited (by SPR decay) in the metal may be transferred to the semiconductor. If these electrons have sufficient energy, they may be effective at executing the hydrogen-evolution half-reaction on the semiconductor. In terms of the oxygen-evolution half-reaction, there are only a few reports that suggest holes retained on very small plasmonic-metal particles are sufficiently energetic to drive oxidation [953]. Silva et al. [953] performed one of the first studies of visible photocatalytic water splitting that employed plasmonic metal Au nanoparticles supported on TiO2 (P25) semiconductor nanoparticles. The photoactivity was studied under both UV and visible light (532 nm laser or polychromatic light λ 4 400 nm) irradiation regimes. They established that the efficiency and mechanism for water splitting depend on whether excitation is mediated through the titania semiconductor or through the gold plasmon. Irradiation with photons of energy larger than the TiO2 bandgap (hν 4 3.3 eV) generates electrons in the TiO2 semiconductor conduction band and holes in the valence band. Electrons from the CB of the semiconductor are thought to then migrate to the gold nanoparticles where they are used for hydrogen generation. The positive holes were quenched by EDTA in their work, which was used as a sacrificial electron donor. Using

(NH4)2Ce(NO3)6 as a sacrificial electron acceptor, production of O2 was observed with the Au(1.5 wt %)/TiO2 and P25 TiO2 samples. In addition to UV excitation, visible light (λ 4 400 nm) excitation at wavelengths (523 nm) very close to the gold plasmon band (λmax ¼ 532 nm) resulted in hydrogen evolution with the Au/TiO2 sample. The proposed pathway for hydrogen generation in this energy regime is analogous to that depicted in Fig. 13.3b (but notably different from that depicted in 13.1). Upon SPR-excitation of Au nanoparticles, highly energetic electrons from Au are suggested to be injected into the CB of TiO2 (presumably through plasmonic decay, followed by electron transfer), leading to the creation of holes in the Au nanoparticles and energetic electrons in the TiO2 CB. The electrons lead to hydrogen generation, while the holes were quenched by the EDTA [953]. Chen et al. [1031] have proposed a somewhat alternative mechanism to explain their experimental observation of SPRinduced photocatalytic water splitting to produce H2 on a Au (3 wt %)/TiO2 photocatalyst. Using finite element method (FEM) electromagnetic simulation, these authors found a 4fold intensification of the electric field at the interface between the Au particle and the subdomain of TiO2. Thus, it was concluded that the function of the SPR was to provide an extra electromagnetic field, which may play an important role in the enhancement of H2 production in photocatalytic water splitting. The importance of the local electric field has also been highlighted by Linic and co-workers [948], who found that the high local fields at the metallic particles can generate e  –h þ pairs in the surface region of a semiconductor support. When formed near the semiconductor surface, charge carriers can reach the active surface sites more readily than charges formed in the bulk. This prevents the fast charge-carrier recombination and thus enhances the water splitting reaction. Within this mechanism, the overlap between the plasmonic metal SPR

Fig. 13.3. Photoelectrochemical water splitting. (a) Semiconductor-based photocatalytic process. Reprinted with permission from Ref. [1143]. Copyright 2014 Elsevier B.V. (b) Plasmonic nanostructure based photocatalytic process: plasmon-induced charge transfer mechanism. Reprinted with permission from Ref. [955]. Copyright 2011 Macmillan Publishers Ltd.

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absorption spectrum and the visible absorption spectrum of the semiconductor plays a crucial role. In their work, this overlap was achieved by utilization of nitrogen-doped titania, N-TiO2, as the plasmonic nanoparticle support (see Fig. 12.8) [948]. In related work, Cronin and co-workers [950] observed factors of enhancements of up to 66 in the visible (633 nm) photocatalytic splitting of water on TiO2 with the addition of Au nanoparticles. The anodic TiO2 film employed in their work showed significant absorption in the visible range, due to defect states in the bandgap of TiO2 created by N- and F-impurities during the anodization process. The absorption spectrum of anodic TiO2 covered with a 5 nm island-like film of gold exhibited a slightly higher absorbance in the visible light range than their Au-free sample. FDTD simulations (see Fig. 12.7) highlighted the formation of intense local fields near the TiO2 surface induced by surface plasmons. The authors suggested that their observed enhancement in reactivity was due to the very short minority carrier diffusion length, characteristic for the anodic TiO2, which would otherwise greatly restrict photocatalytic activity [950]. Both the experimental results and the fundamental understanding obtained in this work suggested that solving the problem of photon absorption/electron diffusion length mismatch can open new routes for direct solar–to–fuel production [950]. Using Au-decorated nanostructured-ZnO nanorod arrays as a photoanode, Tsai and co-workers [1094] adopted several strategies to explore mechanistic details of plasmonic chemistry under solar illumination. X-ray absorption near edge structure (XANES) of the Zn K-edge was used to probe the structure of the conduction band of ZnO. This study proposes that the coupling between the SPR-generated hot electrons and the electromagnetic field increases the photochemical water splitting rate. Fig. 13.4 illustrates two of their suggested mechanisms for enhancement by the localized surface plasmon resonance on their Au nanostructures. Absorbing light in the solar range of the spectrum (a xenon lamp with an AM 1.5 filter) upon 100 mW.cm  2 illumination, the gold component of a Au–ZnO photoelectrode is thought to generate hot electrons (likely through plasmonic decay) and an intense local electromagnetic field. The LSPR-excited hot electrons with energies exceeding the Schottky barrier are suggested to be injected into the ZnO CB, where they are driven to the photocathode. Upon reaching the photocathode, electrons react with protons to form hydrogen, while holes remaining within the excited Au nanoparticles accept electrons from the electrolyte (water) to form oxygen. Further, the plasmon-induced electromagnetic field (Fig. 13.4b) may create additional vacancies within the CB of ZnO as compared to pristine ZnO (Fig. 13.4c and d). An important property of the plasmon-induced electromagnetic fields is that the field is spatially nonhomogeneous. That is, the intensity is largest close to the plasmonic metal, thus assuring that the generation of electron–hole pairs is greatest in the part of the semiconductor closest to the gold nanostructures. The advantage of electromagnetic field formation at the photoanode surface is that electron–hole pair separation is improved under the influence of the surface potential, and the shortened distance

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they travel to the surface of the ZnO renders them relatively more accessible to reactants. Moskovits and co-workers [989,1125] have described plasmonic water splitting devices in which they show that 95% of the effective charge carriers were derived from SPR decay to hot electrons. At visible light irradiation, the H2 production efficiencies in their work were up to 20-fold higher than that obtained at UV illumination. Fig. 13.5 illustrates the structure and proposed mechanistic details of such autonomous plasmonic devices for solar water splitting. The device contains an array of aligned gold nanorods that are capped with a crystalline TiO2 layer, which creates a metal–semiconductor Schottky junction. An oxygen evolution catalyst (Co-OEC), which was deposited on the exposed portions of the gold nanorods, served to enhance the O2 evolution [989]. The TiO2 capping layer traps electrons and supports the platinum nanoparticles, which catalyze H2 evolution. Their work provided evidence that essentially all charge carriers involved in water splitting (for this system) arise from the hot electrons produced via SPRdecay in the nanostructured gold. Each Au nanorod of this autonomous device functions without external wiring and is capable of generating 5  1013 H2 molecules per cm2 per s under 1 sun illumination (using AM 1.5 filter and 100 mW cm  2 illumination) while exhibiting very long (60 h) operational stability [989]. Similar strategies of autonomous photocatalysts have been pursued and demonstrated by numerous groups [1148]. Recently, Wang and co workers [1150] have shown that coupling of plasmonic Au nanocrystals with a TiO2-based photonic crystal (PC) substrate can significantly enhance the Au SPR intensity and thus increase the concentration of hot electron that are injected from the Au nanocrystals into the TiO2 CB, leading to a strongly increased water splitting rate under visible light irradiation. In these plasmonic photocatalytic materials, 20 nm Au nanocrystals exhibiting an SPR at 556 nm were assembled onto the photonic crystal, which was connected to a TiO2 nanotube array. Using a two-step anodization process of Ti foil, hierarchical TiO2 nanotube arrays having nanoring/nanotube morphology were obtained. These nanostructures showed typical photonic crystal light absorption spectra exhibiting multiple peaks in the visible range. Photonic crystals are periodic structures of an electromagnetic medium that exhibit photonic band gaps: where light (usually in the visible range) cannot freely propagate but becomes localized and trapped within the structure. These scientists established that matching between the Au SPR wavelength and PC photonic band gap significantly increases the SPR intensity and thus enhances the water-splitting performance. Under visible light irradiation (4 420 nm), the material was found to produce a photocurrent density of
150 μA cm  2, which the authors claimed to be the highest reported in the literature. Employing a similar strategy of materials engineering to achieve effective photocatalytic activity, DeSario et al. [507] developed a catalyst characterized by Au nanoparticles deposited in three-dimensional (3D) networked gold–titania (Au–TiO2) aerogels. Their work showed that the aerogels

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Fig. 13.4. Plasmon-induced effects in photoelectrochemical water splitting on Au nanostructure–ZnO nanorods array photoanode. (a) A model mechanism of the enhancement by LSPR on the Au nanostructure. (b) Electric field intensity at the interface of Au–ZnO calculated using FDTD. (c, d) Relative number of vacancies for ZnO rods (dark condition), ZnO rods (@ UV illumination), Au–ZnO (dark condition), Au–ZnO (@ 530 nm illumination). Adapted with permission from Ref. [1094]. Copyright 2012 American Chemical Society.

enhanced photocatalytic activity towards visible-light-driven splitting of water. The high photocatalytic activity observed in their work was attributed to the more accessible photoelectrochemical reaction interphase, thanks to a three-phase boundary. Further, they suggested that a more efficient conversion of excited surface plasmons into charge carriers in the 3D networked Au–TiO2 aerogels, compared to traditional Au-decorated TiO2 materials, was responsible for their observed activity enhancement. 13.2.2. Hydrogen evolution from water/sacrificial agent mixtures The compilation of results from many studies shows that efficient photocatalytic water-splitting to evolve hydrogen via solar energy harvesting must overcome three main obstacles: (1) inefficient harvesting of visible light, (2) ineffectiveness of charge separation, and (3) efficient recombination of O2 and H2 to form water [1151]. As suggested in the previous section, utilizing plasmonic nanostructures of noble metals may significantly increase solar light harvesting in ways that address these obstacles; however, inefficient charge separation remains a key challenge. A key strategy to address this limitation is to employ sacrificial organic agents, such as alcohols, which may reduce the propensity for e  –h þ pair recombination [1152]. For example, the addition of methanol, which serves as a strong electron donor agent (hole scavenger), to water can eliminate oxygen evolution during the overall reaction [1152]. Fig. 13.6 illustrates the concept that may govern photoelectrochemical water-splitting over plasmonic metal/semiconductor nanostructures in the presence of methanol as a sacrificial organic agent [1083]. Fig. 13.6 (right side) illustrates how, under UV irradiation, the recombination of photogenerated electron-hole pairs in the semiconductor may be inhibited in the presence of metal nanoparticles in electrical contact with the semiconductor. Typically, co-catalysts such as platinum nanoparticles are used in the photocatalytic anaerobic reforming of methanol to improve the ability of the semiconductor to reduce protons to H2 [1152]. Choi and Kang [1149] have found that under UV irradiation the production of H2 from methanol/water

photodecomposition was greater over a Cu/TiO2 photocatalyst than over pure TiO2. Holes are suggested to be scavenged by the sacrificial electron donor, methanol, and thus the recombination of charge carriers is strongly limited. In addition to its role as a hole scavenger, methanol can also contribute to the generation of hydrogen via its decomposition, thus increasing the overall hydrogen production. In addition to platinum and copper co-catalysts, gold nanoparticles, as suggested in Fig. 13.6, may be active for the generation of hydrogen. Selli and co-workers [1153] reported that 1% Au/TiO2 powders, prepared by flame spray pyrolysis and possessing high specific surface area (106 m2g  1) and anatase content (ca. 90%), are active photocatalysts in UV-light driven hydrogen production from methanol/water mixtures. The photocatalytic activity of their synthesized Au/TiO2 catalyst was higher than that of commercial Degussa P25 TiO2 and of 1% Au/TiO2 (P25) obtained via deposition of preformed gold nanoparticles on P25. Further, they showed that changing from a liquid to a vapor phase reactor causes a 30% increase in the activity, which is attributed to the decreased mass transfer rate limitations in liquidphase. Thus, the production of H2 increased up to 10.2 mmol of H2 h  1g  1 cat, with an apparent photon efficiency of 6.3%. The following reactions have been proposed for the methanol decomposition pathway under UV irradiation [1154]: (1) (2) (3) (4) (5) (6) (7) (8)

(TiO2)þ 2hν-TiO2 (2e– þ 2h þ ) 2h þ þ H2O(liq)-1/2O2(g) þ 2H þ 2H þ þ 2e–-H2(g) 2h þ þ CH3OH-2H þ þ HCHO CH3OH(liq) 2 HCHO(g) þ H2(g) HCHO(g) þ H2O(liq)2HCO2H(liq) þ H2(g) HCO2H(liq)2CO2(g) þ H2(g) CH3OH(liq) þ H2O(liq)2CO2(g) þ 3H2(g)

This mechanism includes the following steps: (1) the UV excitation of TiO2 generates an exciton, which dissociates to an electron–hole pair; (2) photogenerated holes induce the split of water molecules to oxygen and protons; (3) the electrons generated on the TiO2 surface are transferred to the metallic particle where they reduce the protons to hydrogen gas; (4) methanol can be

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Fig. 13.5. (a) Schematic of the cross-section of an individual photosynthetic unit showing the inner gold nanorod, the TiO2 cap decorated with platinum nanoparticles, which functions as the hydrogen evolution catalyst, and the Co-OEC material deposited on the lower portion of the gold nanorod. (b) Corresponding transmission electron micrograph (left) and magnified views of the platinum/TiO2 cap (top right) and the Co-OEC (bottom right). (c) Energy level diagram superimposed on a schematic of an individual unit of the plasmonic solar water splitter, showing the proposed processes occurring in its various parts and in energy space. Reprinted with permission from Ref. [989]. Copyright 2013 Macmillan Publishers Ltd.

Fig. 13.6. Illustration of solar photoelectrochemical H2 evolution via UV (right) and visible-light (left) driven water reduction using methanol as sacrificial electron donor. Reprinted with permission from Ref. [1083]. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

involved in a direct oxidation by accepting holes and thus generating protons and the aldehyde. The decomposition of methanol may also proceed to aldehyde and hydrogen (5–7). The overall reaction is presented in (8). Some of these reactions also lead to the evolution of carbon dioxide [1155]. Running the reactions with fully and partially deuterated methanol/water mixtures, Gärtner et al. [1155] concluded that hydrogen gas evolution rates are not significantly influenced by the deuteration grade of the methanol –CH3 groups. Hence, proton reduction (not the oxidation of the methyl group in the sacrificial electron donor) appears to be the rate-limiting step in H2 evolution. The active site for the reaction is likely at the periphery of the metal particles, as the proton reduction rate significantly decreased at increased metal loading, when the number of peripheral sites becomes limited. Thus, the composite metal/semiconductor systems behave as bi-functional catalysts in nature, requiring molecular accessibility to both titania and

metallic sites [1152]. With this design strategy, Idriss and coworkers [1127] demonstrated that the photocatalytic production of H2 on Au–TiO2 nanocomposites under UV irradiation in the presence of ethanol can significantly exceed that reported previously for supported Pd and Pt, indicating that Au catalysts are suitable candidates for fuel cell applications. Oros-Ruiz et al. [1154] also found that Au/TiO2 nanostructures obtained by deposition–precipitation with urea exhibit remarkable photocatalytic activity for hydrogen production under UV–vis irradiation (λmax ¼ 254 nm, 2.2 mWcm  2) in the presence of water/ methanol mixtures. In their work, optimal photocatalytic performance was observed for photocatalyst having 0.5 wt.% Au loading on the TiO2–P25 nanoparticles. This photocatalyst was further augmented by deposition of Cu2O and NiO as cocatalysts [1156]. The improved photocatalytic activity was attributed to the enhancement of electron charge transfer from TiO2 to the Au–MxOy systems and, of course, the influence of the surface plasmon resonance of the gold nanoparticles. In a comprehensive study, Méndez et al. [1157] systematically investigated the role of many material characteristics, including Au-loading and properties of the TiO2-support, on the photocatalytic activity of Au/TiO2 towards production of H2 from water/methanol mixtures. In these studies, commercial and synthesized (by sol-gel methods) TiO2 powders with differing contents of anatase and rutile and a variety of particle diameters were loaded with Au at different concentrations, from 0.2 to 6.0 wt.%. Under illumination at wavelengths 300–400 nm (with a maximum peak at 365 nm), the production of H2 with their Au/TiO2 photocatalysts showed up to two orders of magnitude greater activity than pure TiO2 samples. The highest H2-production activity was measured for commercial titania (Kronos vlp7000, anatase with
7 nm particles) throughout a range of Au–loadings. This high activity was attributed to the anatase composition of the sample, large surface area (260 m2g  1), small size of gold particles, and the homogenous distribution of Au on the photocatalyst surface. Samples with high Au loading and with a particle size lower than 5 nm displayed higher activity at the same photodeposited

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Au percentages. It was established that the main intermediates of methanol degradation were formaldehyde and formic acid [1157]. In a similar vein as those studies described above, Yoshida and co-workers [1158] investigated key factors in hydrogen production for water/ethanol mixtures over Au/TiO2 energized through the LSPR of the Au nanoparticles. The Au/TiO2 catalysts contained Au nanoparticles with diameters of 10– 30 nm photodeposited on TiO2 (anatase and/or rutile) with varying surface areas. Au-loaded samples were prepared by two methods: a conventional photodeposition method and a nanoparticles photodeposition method. The photocatalytic production of hydrogen showed a short induction period under visible light (510–740 nm), which was attributed to reduction of TiO2 that appeared to activate the reaction. Importantly, they found that large spherical Au nanoparticles that exhibit higher efficiencies for excitation of the LSPR were the most effective in the overall reaction. In contrast, aggregated Au nanoparticles were found to exhibit low activity due, in part, to the low efficiency of electron transfer to the conduction band of titanium oxide. Further, an observed higher efficiency of H2-production on short rod-like shaped Au nanoparticles was attributed to the higher efficiency of electron transfer as compared to the spherical Au nanoparticles. Finally, large nanoparticles of anatase TiO2 were found to be more effective in the photocatalytic reaction because of the longer lifetime of CB electrons, i.e. the less charge recombination at the defect sites. As already discussed in Section 12.2.1, Brückner and coworkers [1083] established that the production of H2 through water splitting in the presence of methanol/water mixtures increases significantly when the Au–TiO2 photocatalyst is irradiated at wavelengths larger then 400 nm. They demonstrated that the maximum wavelength in photocatalytic activity coincided with the maximum absorbance of the particles due to SPR excitation. Using EPR spectroscopic measurements to monitor the generation of paramagnetic species, they were able to show strong evidence for visible-light driven electron transfer from the Au conduction band to the TiO2 support. This mechanism for visible light water splitting is illustrated in the left-hand side of Fig. 13.6. Two different electron injection pathways were distinguished: the first was assigned to direct electron–hole pair generation by d–sp interband transition in the shorter wavelength region, while the second was attributed to an SPR-induced electron transfer between Au and TiO2 at higher wavelengths (4 580 nm). As compared to UV-driven photocatalysis, the visible-light driven H2 production showed a 6-fold enhancement under otherwise identical conditions (see Fig. 12.6b) [1083]. Although the visible-light induced interband transitions were proposed to leave oxidative holes in the d-band of gold (the energy required for the interband transition is
2.4 eV, see Section 12.2.4) [1107], Fang et al. [1159] suggested that their observed water reduction reaction was driven by strong plasmon-enhanced localized electric fields. Specifically, these authors reported remarkable visible-light activity in H2 evolution for mesoporous plasmonic Au–TiO2 nanocomposites prepared via a copolymer-assisted sol–gel method. Most

of the deposited Au nanoparticles were embedded in the mesoporous matrix of TiO2. The photocatalytic activity for water reduction was then studied in the presence of ascorbic acid as an electron donor. Comparisons of the visible-light activity of Au–TiO2 and Pt–TiO2 samples led to the hypothesis that defects/impurity states in the TiO2 matrix have only a small effect on reaction rates, while the gold surface plasmons significantly enhance the reaction rate through strong localized electric fields. In addition, they suggest that some "SPR-excited electrons" may have sufficiently high energy to transfer from the Au nanoparticles to the TiO2 support matrix and thus partially contribute to the hydrogen evolution reaction. Other researchers have shown that defect and impurity states in the TiO2 support can significantly affect that photocatalytic activity of metal/semiconductor photocatalysts. For example, doping of boron into TiO2 caused significant improvement of the visible light activity of Au/ B-TiO2 towards hydrogen evolution [1160]. The embedding of Au nanoparticles into the framework of B-TiO2 was found to increase the concentration of interstitial boron species in B-TiO2. This, in turn, gives rise to surface or near-surface states that facilitate further Au nanoparticle development, as demonstrated by the Au 4f XPS spectra. Their work also showed that the hydrogen evolution mainly originates from water reduction rather than methanol reduction, as previously reported by Gärtner et al. [1155]. In both reports, such conclusions were made on the basis of results from isotopic tracer experiments with labeled methanol/water mixtures. Within the general strategy of developing 3-D supported catalyst motifs, Zhan et al. [1161] reported an interesting configuration for plasmonic photoanodes based on Au–embedding in TiO2 nanostructures for efficient visible-light driven H2 production in the presence of methanol as a sacrificial electron donor. Both experimental data and numerical simulations (using the FDTD method) showed that enhancement in the photocatalytic performance of this PEC electrode could be achieved by optimizing the electrode structure. The greater performance of a Au-"in"-TiO2 electrode, as compared to Au"on"-TiO2 electrode, was attributed to the enhanced light absorptivity of the former. It was suggested that the SPRinduced intense local electric fields in the proximate semiconductor surrounding the plasmonic Au nanoparticles greatly enhanced the generation of electron  hole pairs in TiO2, even though the energy of the photons was lower than the TiO2 band gap. As discussed above, Au surface plasmon-mediated e  –h þ pair production in a TiO2 support is necessarily a multi-photon phenomenon. Although the local fields can be intense, the multi-photon transitions are most likely not due to non-linear simultaneous absorptions of coherent radiation. Rather, actual band-gap states (the result of defects and impurities) are thought to be populated first, followed by electronic excitations out of these states and into the conduction band, as suggested by the authors in their phenomenological model of the overall mechanism for photocatalytic water reduction [1161].

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13.2.3. LSPR enhanced reverse water gas shift reaction While the photo-generation of hydrogen from water or hydrocarbons as a potential fuel source holds great promise, hydrogen suffers from a low energy density. In fact, hydrogen is not a viable fuel option for powering heavy machinery and jet engines. In response to such challenges, the reduction of CO2 to useful fuels has received much attention. The initial step of CO2 reduction is the reverse water gas sift (RWGS) reaction, CO2 þ H2 2 CO þ H2O. Although this step is endothermic by over 40 kJ/mol (under standard conditions), it is the most energy-demanding reaction in the conversion of CO2 to useful fuels. Therefore, efficient photocatalysts that harness the energy of visible light for this reaction would be extremely valuable. Motivated by demonstrations that surface plasmons enhance the rate of the water gas shift reaction [1225], Huber and coworkers have shown that the LSPR can enhance the activity of a Au/TiO2 catalyst for the reverse water gas shift (RWGS) reaction [1226]. Through a series of systematic studies that independently probed the photolytic and thermal rates for the RWGS reaction, this research has shown that hot electrons, as opposed to localized heating, are most likely responsible for the measured LSPR enhancement. In fact, they have shown that illumination of a catalyst bed under a feedstock of CO2 and H2 reduces the activation energy for CO2 conversion from 47 kJ/mol (in the dark) to 35 kJ/mol. The dramatic change in the activation energy indicates that illumination affects the overall reaction mechanism rather than enhancing the rate through local heating.

13.3. Photodegradation of organic compounds Another potential practical application of plasmonic nanostructures in the arena of chemical transformations is the degradation of organic compounds. However, solar lightinduced photocatalysis has been reported in only a limited number of studies. For example, researchers have explored exothermic partial oxidation, selective reduction, and organic decomposition reactions on excited plasmonic metal (Au, Ag) semiconductor nanostructures [468,955,1066]. The photodegradation of organic pollutants under solar light irradiation of such nanostructures can utilize both the UV and visible parts of the electromagnetic spectrum. Under UV irradiation, Au NPs have been proposed to function as traps for the electrons photogenerated in TiO2, thereby inhibiting charge recombination, which is the exact opposite role that Au particles may play under visible light irradiation. Visible-light driven photocatalysis in these systems is thought to proceed by Au NPs serving as the photon absorbers via excitation of the SPR and then transferring electrons (generated during plasmonic decay) and/or energy into the TiO2 nanoparticles [1066]. In this section, some of the main discoveries in the field are discussed with special attention given to work that sheds light on possible reaction pathways. Several high fidelity studies have been performed in this area that explored both photonic energy regimes (UV and visible) to help develop a

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fundamental understanding of the photophysical and photochemical mechanisms responsible for catalysis. 13.3.1. Photodegradation of organics in liquid phase Motivated by the need to develop effective purification strategies for addressing a global water demand, scientists have explored Au-NP-based photocatalysts due to their effectiveness in organic transformations [468,1162]. For example, extensive research has focused on the photocatalytic oxidation of various water pollutants, such as toxic organic compounds, by Au-NPs on TiO2 supports under solar irradiation. 13.3.1.1. Degradation of dyes. Photocatalytic degradation of dye molecules has been widely explored as a simple model reaction in the study of solar light-induced chemical transformation of organic compounds (OCs) [468]. Methyl orange (MO), [(CH3)2NC6H4N¼ NC6H4SO3Na], a sodium salt of [4-[[(4dimethylamino)phenyl]-azo]benzenesulfonic acid, is a very stable azo-dye used in the textile industry, but is considered to be a typical water polluting compound. Several studies have shown that Au/TiO2 nanostructures can effectively decompose MO in water under solar light irradiation [958,1016,1163– 1165]. Falaras and co-workers [1163] reported one of the earlier studies on the photocatalytic activity of Au/TiO2 thin films for MO photodegradation under UV light irradiation (λ¼ 350 nm). In their work, gold films were deposited by electron beam evaporation of a Au wire under vacuum onto titania-coated microscope slides. The best activity was obtained for a sample of 0.8 μg cm  2 gold–loaded Au/TiO2 nanocomposite. That sample exhibited a two times faster oxidative degradation (in an oxygen-saturated solution) of MO with respect to the rate obtained with the empty TiO2 material. The enhanced photocatalytic activity was attributed to the action of Au particles as a sink for photogenerated CB electrons in TiO2, resulting in an effective increase in the lifetime of e  –h þ pairs [908]. The long-lived photoinduced electrons were suggested to interact with electron acceptors such as oxygen, creating oxygen radicals (O2•  ), while photogenerated holes migrate to the interface and react with OH  adsorbed onto the TiO2 to create hydroxyl radicals (•OH). These radicals (•OH, O2•  ) possess extremely strong oxidizing properties and were proposed to decompose the MO pollutant through reactions leading to the formation of sulfonated intermediates together with the formation of the  þ inorganic final products (SO2 4 , NO3 , NH4 ), carbon dioxide, and water. Increased Au loading was found to decrease the photoactivity for MO decomposition, which was explained in terms of optimum gold particle size, availability of the semiconductor for light absorption, and open sites for pollutant binding to the semiconductor [1163]. In agreement with the latter finding described above, Cronin and co-workers [1016] observed that heavy Au loading (thin films
5 nm) on TiO2 results in more than a 2-fold decrease in the photodecomposition rate of MO relative to pure TiO2 under UV (λ ¼ 365 nm) irradiation. This decrease in the photoactivity was attributed to the reduction in the active TiO2 surface area caused by the deposited gold. In that study, gold was evaporated as an island-like thin film on the surface

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of an anodic TiO2 substrate (See Fig. 12.7a). The anodic TiO2 was fabricated by electrochemical oxidation of a titanium foil in an ethylene glycol electrolyte containing 0.25 wt.% NH4F at an anodization potential of 30 V. Although Au reduced the UV-driven photoactivity, irradiation of the Au/TiO2 sample with visible light (532 nm laser) caused a more than 9-fold enhancement in the photocatalytic activity for MO decomposition under the same conditions for which pure TiO2 showed negligible activity. As with the many studies highlighted in previous sections, the wavelength of irradiation corresponded to the wavelengths at which a maximum in the optical absorption of Au/TiO2 was observed. Therefore, the enhanced photocatalytic activity was attributed to the excitation of the LSPR in gold nanostructures. Using FDTD electromagnetic simulations, an order-of-magnitude enhancement of the electric field near the Au particles was observed. As illustrated in Fig. 12.7b-d, the formation of “hot spot” regions between randomly distributed gold islands is well expressed [1016]. Thus, the dramatic enhancement in the photocatalytic rate was attributed to defect states in the TiO2, which enable sub-bandgap absorption. The intense local fields then excite electrons from the gap states and into the CB prior to recombination. As previously discussed in Section 12.2.3, Cushing et al. [958] proposed that LSPR-induced resonant energy transfer (RET) can generate electron  hole pairs in the semiconductor by dipole dipole interactions between the plasmonic metal (donor) and semiconductor (acceptor), which can enhance the visible-light photocatalytic degradation of MO. Their designed sandwich Au@SiO2@Cu2O nanostructures contained a thin SiO2 layer that electronically insulated the metal from the semiconductor, thus preventing direct electron transfer between Au and Cu2O. The RET process relies on the overlap between the LSPR-band of the Au core and the band gap absorption of the Cu2O shell. Hence, the plasmonic Au core acted as a photosensitizer via the RET process that induces the photogeneration of charge carriers in TiO2, which in turn react with MO and cause its degradation. Sahu and Parida [1165] reported enhanced photocatalytic degradation of various azo-dyes such as methylene blue, methyl orange, reactive blue-4, and eosin-B under solar irradiation of Au/TiO2 nanostructures. Metallic gold nanoparticles with average diameter of
10 nm were deposited on TiO2 (as prepared by sol-gel method) at loading
1.3 wt % by a borohydrate reduction method. The pH of the solution was found to control the degradation rate of dyes, explained on the basis of point-of-zero charge of Au/TiO2 (pH-pzc ¼ 5.3–5.7). For pH lower than the pH-pzc, Au/TiO2 becomes positively charged, which favors the electrostatic interaction between anionic dyes like MO, RB-4, and EB. In the case of MB, the opposite effect was found, as it is a cationic dye. The higher photocatalytic activity of Au/TiO2 compared to TiO2 was explained in terms of enhanced light absorption, shifting the light absorption from the UV to the visible spectral region and the reduced e––h þ recombination rates.

In addition to its photooxidation activity toward methyl orange decomposition, gold has also been shown to promote the photoreduction of this compound. Recently, Oros-Ruiz and co-workers [1164] reported the photocatalytic reduction of methyl orange (MO) in aqueous solution by gold nanoparticles deposited on TiO2. Gold NPs at various loadings were deposited on TiO2 (Degussa P25) by the deposition–precipitation method, which caused a red shift in the band gap from 3.2 for the pure support to 2.9–2.8 eV for the Au/TiO2 materials. Under irradiation with UV–vis black light lamps, emitting simultaneously at wavelengths λ ¼ 365, 418 and 435 nm, Au/TiO2 samples exhibited higher rate constants for MO reduction by a factor of up to 3.2 as compared to the TiO2 support. The proposed mechanism involves an electron transfer from photoexcited Au/TiO2 to the protonated methyl orange and its subsequent reduction to form a hydrazine derivate. The proposed effect of gold in the photoactivity was related to the electron transfer from the bare semiconductor to the gold nanoparticles (as in the reaction of oxidative degradation of MO reported by Falaras et al. [1163]) and then to the MO molecule. Photocatalytic degradation of methylene blue is another simple model reaction that has been explored in studies of solar light-induced chemical transformations on plasmonic metal nanostructures [925,997,1166,1167]. In an early study, Li and Li [1167] reported that a Au/Au3 þ –TiO2 powder photocatalyst exhibits much higher activity than conventional TiO2 powders in the oxidative photodegradation of methylene blue (MB) in aqueous solutions under visible light irradiation (λ 4 400 nm). A loading of 0.5% molar content of either gold (Au) or gold ion (Au3 þ ) doped in TiO2 showed the highest photodegradation efficiency. Using XRD, UV–vis, XPS and PL spectroscopies, it was established that the presence of Au (0), Au (I), and Au (III) strongly affects the charge trapping, release and migration, recombination, interfacial charge transfer, and thus the photocatalytic behavior. The visible photocatalysis resulted in partial mineralization of MB into several products, including ammonium ion (NH4þ ), nitrite ion (NO2 ), nitrate ion (NO3 ), and sulfate ion (SO24  ). Among those, the ammonium ion was suggested to be an intermediate product, which is finally oxidized to nitrate. Additional important work in this area was conducted by Tsai and co-workers [1004] who explored the photodegradation of MB to study the LSPR-promoted photocatalytic behavior of shell-isolated Au@SiO2/TiO2 plasmonic nanostructures. Their three photocatalysts, Au@SiO2/TiO2, Au/TiO2 and TiO2 showed 95%, 80% and 44% degradation of MB after 5 h simultaneous irradiation with UV(λ¼ 365 nm) and Vis (400 nm oλo700 nm) light. A superior MB photodegradation efficiency of Au@SiO2/TiO2, as compared to that of Au/TiO2 was attributed to the higher LSPR effect of the Au@SiO2/TiO2 than that of the Au/TiO2, even though the 3 nm SiO2 shell insulates the Au (thus reducing its ability to trap electrons). The simulated LSPR spectra and electromagnetic field distribution using the finite element method (FEM) showed a
9-fold increase in the EM field for the SiO2-coated Au as compared to bare Au.

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Pincella et al. [1168] reported the fabrication of a plasmonic device that was designed around the idea of an embedded twodimensional (2D) array of gold nanoparticles (average size
36 nm) on a transparent conductive substrate (10 nm thick indium tin oxide (ITO) layer on quartz), coated with a monolayer of trimethoxyoctylsilane (TMOS), and an on-top deposited layer of titania nanocrystals anchored on the TMOS layer. The LSPR peak of this device was around 700 nm with a full-width at half-maximum of 350 nm. The photocatalytic activity of this device and control samples was measured via oxidative photodegradation of MB in air for both visible (700 nm with 10 nm FWHM) and UV (250–385 nm) irradiation regimes. As expected, the maximum photocatalytic activity was achieved with visible light and the AuNPs 2D array sample. UV excitation actually exhibited 1.7 times lower photoactivity than visible, probably due to the previously proposed [999] charge depletion from the titania to AuNPs (Schottky barrier at the titania/Au interface). Comparison of the photodegradation activity for arrays with and without the titania coating led the authors to conclude that LSPR heating [1109] or energy transfer [1169] effects were not operative; that is, the combination of the AuNP array and the titania layer was essential for achieving visible-light induced photocatalytic activity. Furthermore, the authors concluded that LSPR-induced hot electron transfer from the AuNPs to titania was not responsible for the visible-light activity because the TMOS layer, deposited on the AuNPs, would have inhibited this pathway. Elimination of these mechanistic channels led the authors to suggest that a twophoton absorption process was responsible for the visible light activity of the device. Important support for this hypothesis came from the observation of a quadratic incident light power dependence on the action spectrum (see Fig. 13.7). Based on these results, they proposed that the plasmon-induced locally large electric field (near-field enhancement by AuNPs) was responsible for multi-photon transitions (e––h þ pair generation) within the titania [1168]. Thus, the work highlighted throughout this section demonstrates the promising role of gold nanoparticles as an alternative to organic dyes in photosensitization of wide bandgap semiconductors for efficient photocatalytic degradation of organic contaminants. The orders-of-magnitude larger absorption cross-section of Au NPs (relative to dye molecules) makes them excellent photon-energy conversion mediators for photocurrent production (photovoltaic processes) or chemical oxidation/reduction reactions (photo-catalysis) [1067]. 13.3.1.2. Degradation of organic pollutants, alcohols, acids, esters, etc. Photocatalytic oxidative degradation of organic molecules such as alcohols, acids, etc., has also been explored in many studies into solar light-induced chemical transformation of organic compounds on the surface of photoactive plasmonic materials [468]. In one of their early studies in this field, Tian and Tatsuma [945] found that Au–TiO2 holds potential as a visible-light-sensitive photocatalyst for short chain alcohol oxidation at the cost of O2 reduction. At the time of publication, these observations had little precedence in the photocatalysis by metal–semiconductor nanocomposite–related literature

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[908,910,917]. The data shown in Fig. 13.8 demonstrate that the measured photocurrent action spectrum closely follow the absorption spectrum of their Au/TiO2 film, which provides the SPR response characteristic for gold nanoparticles typical of this size range. Examining the probable products of alcohol oxidative degradation, the authors found that CH3COOH and HCOOH are the final products of the ethanol and methanol photocatalysis, respectively. When O2 was bubbled into the electrolyte containing alcohol, the anodic photocurrent gradually decreased to reach a constant value (10 % of the initial). From this, it was inferred that O2, a well-establish electron acceptor, obtains photo-generated electrons (either from the gold nanoparticles or the TiO2), and thus interferes with the anodic photocurrent. Actually, H2O2 was detected analytically in the solution, indicative of oxygen reduction. As the discussion associated with Fig. 12.5a describes (Section 12.2.1), the proposed mechanism for SPR-induced charge separation and photooxidation of these organics was proposed to involve (1) photoexcitation of the surface plasmon within Au, and (2) transfer of photoexcited electrons into the TiO2 bulk, while the positively charged gold particles oxidize an electron donor in the solution [945]. This mechanism for visible light-induced photocatalytic oxidation of alcohols was also adopted by Kowalska et al. [946] to explain the observed oxidation of 2-propanol by aerated gold-modified titanium(IV) oxide (titania) suspensions irradiated at wavelengths 4450 nm. Gold nanoparticles at 2 wt. % loading were photodeposited onto the surfaces of fifteen commercial titania powders to obtain Au/TiO2 powder samples. Each of these samples exhibited a broad absorption band in the wavelength range of ca. 400–700 nm with a band maximum at ca. 530–610 nm, corresponding to the observed color of the powders (attributed to LSPR of the gold particles on the titania support). These results also indicated that the larger the size of the titania particles, the larger the size of the Au particles, and hence, the peak of LSPR absorption was observed at longer wavelengths. The obtained resemblance between the action spectrum and the absorption spectrum of Au nanoparticles (see Fig. 9.5) proved that the photoactivation of the Au/TiO2 system originates from the visible light-induced LSPR in the gold nanoparticles. Kowalska et al. further established a strong dependence of the photocatalytic activity on gold and titania characteristics, including particle geometry, surface area, and crystal structure [947]. The photocatalytic activity of a variety of Au/TiO2 powder samples was examined both under UV and under visible irradiation (4450 nm) for 2-propanol and acetic acid photooxidation. Gold particles of a wide range of sizes deposited on large rutile particles exhibited the highest photocatalytic activity under vis-light irradiation. This high activity was attributed to the transverse and longitudinal LSPR of rod-like gold particles that decorate the surface of these fairly heterogeneous Au/TiO2 samples, as illustrated in Fig. 13.9. As depicted in Fig. 13.9, electron transfer from Au to TiO2 is proposed to control the photooxidation of 2-propanol to acetone and that of acetic acid to carbon dioxide, in agreement with the work of Tian and Tatsuma [945,952].

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Fig. 13.7. Linear fitting of photocatalytic degradation rate of MB in air versus second power of the incident light. Reprinted with permission from Ref. [1168]. Copyright 2014 NPJ/Nature Partner Journals.

Experiments carried out with gold nanoparticles deposited on silica instead of a titania support, which had similar size distributions of Au NPs, showed much lower reaction rates for OC degradation. It was inferred from these results that the mechanism involving electron transfer through the CB of the semiconductor is required for efficient decomposition of OCs [947]. As further shown by Kowalska et al. [1093] using FDTD calculations, a significant influence associated with localization and intensity enhancement of the optical nearfield can additionally affect the visible light activity of Au/ TiO2 nanocomposites. For hemispheres, the field was strongly localized at the metal-substrate interface, where intensity enhancement of up to 104 times (especially for larger Au nanoparticles, 4 50 nm) was reached. Moreover, the field is predominantly polarized along the normal to the substrate. These findings indicate the great potential of hemisphere-substrate systems for applications such as photochemical reactions and light-to-current conversion. These theoretical findings were further supported experimentally [1042], where twelve different Au/TiO2 preparations obtained via photodeposition of Au NPs on anatase and rutile polymorphs of TiO2 with Au loading 0.05, 0.1, 0.5, 0.75, 1, 1.5, 2, 2.25, 3, 4, 6, and 10 wt. % were studied. The Au loading amount strongly influenced the UV/vis photoactivity for acetic acid oxidation, and three maxima in the photodegradation efficiency were observed for 0.5, 2, and 4 6 wt% of gold. Based on the results obtained with these systems and regardless of Au loading, each rutile Au/TiO2 photocatalyst containing large Au and titania NPs proved to be more active than anatase Au/TiO2 containing small Au and TiO2 nanoparticles. The results suggested that spherical/hemispherical Au NPs are beneficial for enhanced photoactivity under visible irradiation as compared to rod-like Au nanoparticles. The strong dependence of Au/TiO2 photocatalytic activity on the TiO2 crystal form and the particle sizes of Au and TiO2, reported by Kowalska et al. [946,947], was further studied by

Fig. 13.8. Action spectra for incident photon to current conversion efficiency (IPCE) of the Au–TiO2 film in the N2-saturated acetonitrile–ethylene glycol solution containing 0.1 M LiNO3 and 0.5 M ethanol (or methanol) in response to visible light illumination (1.36  1014 photons cm–2 at each wavelength). Absorption spectrum of an unmodified TiO2 film is given in the inset. Reprinted with permission from Ref. [945]. Copyright 2005 American Chemical Society.

Tada and co-workers for both UV and visible light irradiation regimes [951]. Importantly, they found that the Au/anatase system was superior over the Au/rutile system in the UVinduced photocatalytic reduction of nitrobenzene, whereas the Au/rutile system exhibited much higher activity under visible irradiation for the oxidation of alcohols to carbonyl compounds. This conclusion is evident from the results shown in Fig. 13.10a, b, where the TON at irradiation time tp is shown as a function of gold particle diameter, d. The superior nature of a rutile support material for the Au NP photocatalyst (in certain cases) can be rationalized as follows. The much higher permittivity (ε ¼ 114) for rutile, as compared to that of anatase (ε ¼ 48), leads to a redshift in the maximum of the Au NP LSPR band, see Fig. 13.10c, d. This red shift reduces the LSPR–to–interband optical coupling for the rutile support relative to the anatase support, shown in Fig. 13.10e. The reduced coupling slows the damping of the LSPR; the extended LSPR lifetime, in turn, increases the efficiency of the interfacial electron transfer (IET) from Au NPs to TiO2. Therefore, the visible-light photoactivity of Au/ TiO2 may be increased by using rutile as a support for Au NPs. The d-dependence (d¼ NPs diameter) of the photocatalytic activity of Au/TiO2 was further explained based on the following considerations. In the UV light driven nitrobenzene reduction, larger d promotes an upward shift of the gold Fermi energy because of the UV induced IET from TiO2 to Au [922]. Thus, the Au NPs more effectively reduce nitrobenzene to aniline (Fig. 13.10a). In the visible light-driven oxidation, photoactivity increases with increasing d because of the increased LSPR absorption and the increased IET efficiency for d o 5 nm (Fig. 13.10b). However, the photoactivity decreases at d 45 nm, likely due to a major decrease in the surface area of the Au nanoparticles. Therefore, an optimum Au particle size is needed for high visible light photoactivity of the Au/TiO2 nanostructures [951].

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Fig. 13.9. Schematic representation of the mechanism of OCs oxidation by Au/TiO2 under vis irradiation. Reprinted with permission from Ref. [947]. Copyright 2010 The Royal Society of Chemistry.

Shiraishi and co-workers [1170] further contributed to understanding the role of the support structure and gold nanoparticle size in the visible-light driven aerobic oxidation of 1-phenylethanol to acetophenone. This study demonstrates the crucial role of the Au/TiO2 architecture for the activity: small Au particles (dAu o 5 nm) loaded on a mixture of anatase/rutile TiO2 (Degussa, P25) particles provide a good system for activity. In addition, the place where the Au particles are deposited affects the activity. That is, the authors suggest Au particles deposited at the anatase/rutile interface are the most active sites. Based on an ESR spectroscopic study, it was proposed that efficient e  transfer from Au to TiO2 nanoparticles mediates the aerobic oxidation of an organic molecule under solar irradiation. Gold nanoparticles at varying loading were deposited on independent anatase and rutile TiO2 polymorphs and their mixture P25 TiO2 (anatase/rutile E 83/17). Two methods were applied for the AuNPs deposition: deposition precipitation (DP) and photodeposition (photo), known to produce well-controlled deposition of small and large Au nanoparticles, respectively. In addition to the neutral TiO2 support, the basic CeO2 was also explored for Au deposition. While keeping the Au-loading constant (
2 wt. %) the authors were able to manipulate the size of deposited Au nanoparticles and thus to define the effect of dAu and the support crystal form on photoactivity. Fig. 13.11 summarizes some of the most important results of this study. Fig. 13.11a compares the efficacy of the catalysts towards the aerobic oxidation of 1-phenylethanol (1) to acetophenone (2) at room temperature, in the dark (black bars) or under irradiation (Xe lamp, λ4450 nm, white bars). Notably, visible photon irradiation substantially enhances the reaction rate, which is more than four times that obtained from the experiments performed in

243

the dark. These data clearly indicate the superiority of small Au particles loaded on P25 TiO2 in visible-light driven photooxidation of 1-phenylethanol to acetophenone. Measurements of the diffuse reflectance UV vis spectra of the catalysts revealed that the SPR intensities are not consistent with the activity data as previously reported [947,1172]. Thus, the high activity of Au/ TiO2 photocatalysts appears crucially dependent on the combination of small Au particles and the mixed anatase/rutile TiO2 support. The action spectrum analysis confirmed that the enhanced activity of Au/P25 was due to SPR-induced excitation of Au particles. Further, the electron spin resonance (ESR) analysis (77 K) of the catalysts after treatment with O2 at room temperature confirmed an electron transfer mechanism to TiO2, where e  is consumed by the reduction of O2. As shown in Fig. 13.11b, visible-light irradiation (black) produced very weak signals attributable to superoxide-type oxygen anions (O2 ; gxx ¼ 2.003, gyy ¼ 2.009, gzz ¼ 2,025) [1173], formed by e  transfer to O2 adsorbed on the photoexcited TiO2 surface. Visible-light irradiation of Au2(DP673)/P25 created very strong signals (gxx0 ¼ 2.002, gyy0 ¼ 2.004, gzz0 ¼ 2.008) [1171,1174] that were assigned to peroxo type oxygen anions (O O  ). The absence of this signal in the spectra of Au2(DP673)/anatase and Au2(DP673)/rutile catalysts indicates that the O O  species likely do not form on the Au surface, but may form exclusively on the TiO2 regions of the material. Here, the formation of peroxo type O O  species, but not a superoxide anion (O2 ), is rationalized by the reduction of O2 at the Au/TiO2 interface [1173]. The transfer of e  at the Au/TiO2 interface likely forms an oxygen anion, which then interacts with the remaining positive charge on the Au particles, as illustrated in Fig. 13.11c, and thus produces the peroxo-type anion. The absence of the O O  signal in the spectrum of Au2(DP673)/P25 heated at 363 K in the dark indicates that the light-to-heat conversion efficiency in this case is too low to promote the O2 reduction. When the Au loading was increased above
3 wt. %, a deactivation of photocatalysts was observed, inferring that Au loadings of 2 3 wt. % was optimal. Increasing the temperature of sample calcination from 673 K to 873 K resulted in catalyst deactivation, which was explained by the formation of larger Au particles and a decrease in the number of interfacial active sites. The studies described above clearly demonstrate that the characteristics of Au-loaded photocatalysts, such as the nature of the support, the morphology and geometry of the Au particles, and the properties of the interfacial region between gold and the support, all play significant roles in light-induced chemistry. Deciphering the roles of these properties has been a daunting task and there remain many opportunities for important research in this area. Of course, the primary challenge with many such studies lies in the difficulty of systematically changing one property of the system while leaving the others unaffected. For example, a change to Au particle size typically also affects the particle geometry, as well as the chemical and physical nature of the interfacial region. Notwithstanding, researchers have shown that organic molecules, including organic pollutants, can be mineralized completely to water and CO2 over solar light irradiated Au/TiO2 photocatalysts [468], provided the appropriate catalyst conditions are found. This is often accomplished

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Fig. 13.10. (a) Turnover number (TON) for aniline generation at tp ¼ 1 h by the reduction of nitrobenzene as a function of d. (b) TON of the cinnamaldehyde generation at tp ¼6 h by the oxidation of cinnamyl alcohol as a function of d. (c) UV–vis absorption spectra of anatase and Au/anatase with varying Au NP size d and UV–vis absorption spectrum of Au colloid (black broken line). (d) UV–vis absorption spectra of rutile and Au/rutile with varying Au NP size d: F(R1) denotes the Kubelka–Munk function corresponding to the absorption intensity. (e) The optical coupling between the LSPR mode and the interband transition mode. Adapted with permission from Ref. [951]. Copyright 2012 American Chemical Society.

by an Edisonian approach, involving an extensive survey of the catalyst parameter space. Lambert and co-workers [1175–1177] studied the photocatalytic degradation of two water-soluble organic pollutants, tertbutyl ether (MTBE) and 4-chlorophenol, on Au/TiO2 photocatalysts. Under UV irradiation (365 nm, 50 mW/cm  2), they found that Au/TiO2 systems with a small amount of gold nanoparticles increased the reaction rate for the degradation of 4-chlorophenol and MTBE by 50 and 100%, respectively, for the two targets, compared to pure TiO2 [1176]. Their systematic tests showed that the optimum gold loading for MTBE photodegradation was obtained for Au nanoparticles with d¼
3 nm, where an enhancement factor of three was measured, as compared to unmodified titania. Deposition of larger Au nanoparticles (45 nm) led to a MTBE-degradation rate decrease, possibly due to collateral effects, such as decreased surface coverage of hole-trapping Ti–OH groups and/or decreased light absorption [1177]. When Au nanoparticles become very small (o3 nm), they begin to behave as semiconducting rather than as metallic [456]; thus, some have suggested that semiconductor||semiconductor type interfacial electronic structures may facilitate photoexcited electron injection from Au into the TiO2 conduction band [1177]. Centeno et al. [1178] studied the role of Au particle size and loading on the catalytic behavior of a series of Au/TiO2 samples (prepared by deposition–precipitation) in two model reactions: gas-phase oxidation of CO and the degradation of phenol in aqueous media. Varying the gold content from 0.11 to 1.26 wt.

% was found to affect the catalytic performance of Au/TiO2 in both reactions, albeit to differing extents. As expected, for gas phase oxidation of CO, the Au/TiO2 samples exhibited strong dependence of the catalytic activity on the gold particle size and gold content, where the effect of the particle size was most important at low gold loading. Likewise, enhanced UVphotodegradation of phenol was observed for small gold particles (o3 nm) deposited on the TiO2 surface at low loading, up to a maximum content of 0.25 wt. %. In this case, small gold particles were thought to serve as effective traps and "injection centers" for electrons. At higher gold content (40.25 wt. %.), the catalytic activity was found to decrease, possibly due to obstruction or blocking of the active sites at titania. In addition, Au particles may act as electron–hole recombination centers, decreasing the photocatalytic efficiency. In agreement with previous findings [1175,1177], these authors concluded that the photocatalytic performance was affected most by the band gap of the solid (TiO2) and gold loading. However, as described in previous sections of this review, Au loading, particle size, and even catalyst history can have a dramatic effect on not only photocatalysis, but also thermal catalysis. Separating these effects is a scientifically daunting task, but an area ripe for continued research. Recently, Cronin and co-workers [1179] reported on the visible light induced degradation of MO and trichloroethylene (TCE) in aqueous media, catalyzed by Au/SiO2 photocatalysts containing 5 nm Au nanospheres deposited on silica nanoparticles (
100 nm). Increasing the intensity of green laser

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Fig. 13.11. (a) Amounts of acetophenone formed during aerobic oxidation of 1-phenylethanol with respective catalysts in the dark (black) or under visible-light irradiation (λ4450 nm; light intensity at 450–800 nm, 16.8 mW cm  2), (white). The dAu values for the catalysts are given in parentheses. In the assignment of Aux(DPy)/TiO2 samples x is the amount (in wt%) of Au loaded [x wt%¼Au/(Auþ TiO2)  100%] and y is the calcination temperature (in K). (b–e) ESR spectra of the catalysts. The catalysts were treated with 20 Torr O2 in the dark (gray) or under visible-light irradiation at 298 K (black) or in the dark at 363 K (green). After evacuation, the samples were measured at 77 K. The g¼1.997 signal (spectrum b) is assigned to e  at the lattice trapping site of TiO2 [1171]. Adapted with permission from Ref. [1170]. Copyright 2012 American Chemical Society. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(λ ¼ 532 nm) irradiation from 0.2 to 2 W increased the photoconversion efficiency of TCE by
50 %. Reactive species such as hydroxyl radicals (OH•) and superoxide radicals (O•2 ) were suggested to be responsible for their observed photochemistry. In studies also involving reactions in water, Dang et al. [1180] demonstrated that Au@TiO2/graphene (Gr) composites of a specific composition and structure exhibit much higher activity than stand-alone Au@TiO2 in photocatalytic decomposition of 2,4-dichlorophenol (2,4-DCP) under visiblephoton irradiation (4 420 nm). Their systems involved metallic Au nanoparticles (
10 nm in size, round shaped) loaded uniformly on TiO2 particles, followed by grafting onto large (relatively) Gr sheets. The observed enhancement in the photocatalytic performance of Au@TiO2/graphene was attributed to the concentration effect due to the improved adsorption performance introduced by Gr. The mineralization of 2,4-DCP was suggested to occur via IET from Au nanoparticles to TiO2, which mediates the formation of active •OH and O•2 species, the main oxidants. In contrast to many of the studies highlighted thus far, Kowalska et al. [995] recently reported that the best photocatalytic activity for degradation of phenol on Au/TiO2 nanocomposites was obtained for samples with large gold particles (
90 nm) deposited on small (16 nm) rutile nanoparticles. Using a newly developed water-in-oil microemulsion method of preparation, the authors obtained Au/TiO2 samples with well-defined monodispersed Au nanoparticles, which afforded a systematic study of the role of size and loading. The increased visible (4 450 nm) photoactivity of Au/TiO2 samples with large Au particles was explained by an apparent

enhanced photo-absorptivity. In addition, these researchers showed that bimetallic Ag/Au-TiO2 nanocomposites exhibit much greater visible light photoactivity than both Ag-TiO2 and Au-TiO2 photocatalysts, which is an observation that may represent an important step forward in visible light photocatalysis [995]. Beyond degradation of common small organic molecules, scientists have extensively explored the fate of the heteroatoms, usually nitrogen, sulfur, and phosphorous, in the photodecomposition of biologically-relevant compounds. For example, the photooxidation of amino acids has been explored as researchers consider application of this chemistry in biochemical applications. Specifically, Serpone and coworkers [1181] systematically studied the fate of nitrogen during UV light induced decomposition of a series of amino acids in aqueous TiO2 dispersions. Their work highlighted the formation of NH4þ , NO3 and CO2. The nitrogens in the amino acids were photoconverted predominantly into NH3 (analyzed as NH4þ ) and to a lesser extent into NO3 ions. Horváth and co-workers [1182] studied the UV-induced photodegradation of aspartic acid over bare and Ag-loaded TiO2 and found that the amino acid nitrogen was converted primarily into NH3. Under extended times of irradiation, they found that NH4þ was further oxidized through an NO2 intermediate to NO3 . Bürgi and co-workers [1183,1184] described the photocatalytic degradation of organic acids over Au/TiO2 and TiO2 catalysts using attenuated total reflection infrared (ATR-IR) spectroscopy and modulation excitation spectroscopy. In the first of these reports [1183], the UV-induced photodegradation of L-asparagine and L-glutamic amino acids was studied. The

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Au/TiO2 sample was synthesized by adsorption of N-acetyl-Lcysteine-protected gold particles on the surface of TiO2 (P25). The calcined (at 573 K) sample of Au/TiO2 contained 2.7 wt. % Au nanoparticles with an average diameter of 5 nm. On the surface of both Au/TiO2 and TiO2, formation of an oxalate intermediate was registered during the photodegradation of the amino acids. Ammonium was also detected as a product of full degradation (i.e. amino acid mineralization) [1181,1182]. Interestingly, cyanide was found to form on both TiO2 and Au/TiO2 surfaces, where, in the latter case, gold was partially leached into solution as [Au(CN)2]  . Upon UV illumination, the vibrational frequency of gold-adsorbed CN  was found to shift, which was attributed to the transfer of photogenerated electrons from the TiO2 CB to the gold particles. In a second report [1184], the UV and visible-light induced photodegradation of malonic acid was explored in water on core-shell structured Au@TiO2. In that study, adsorption, electron transfer, and photodegradation of malonic acid on the surface of the catalysts were investigated by in situ ATRIR spectroscopy using a dedicated flow-through cell [1183]. Their Au@TiO2 photocatalyst was prepared by mixing a solution of Au-nanorods and titanium(IV) oxide acetylacetonate. The Au-nanorods themselves were synthesized using a seed solution and reduction with ice-cold NaBH4 [1184]. During final calcination at 723 K, the Au-nanorods of the synthesized core–shell structured Au@TiO2 were converted into spherical Au nanoparticles, covered with a 4 nm thick layer of TiO2. XPS measurements showed that the Au nanoparticles were in their metallic state. The IR spectra during exposure to the acids revealed that the adsorbed malonic acid was in deprotonated form, i.e. as malonate, having two equivalent carboxylate groups. This observation indicated that the malonate species was present on the surface in the chelating or bridging adsorption mode. In the dark, the adsorption kinetics of malonic acid was similar on both pure TiO2 and on Au@TiO2; which suggested that the Au nanoparticles in the Au@TiO2 catalysts were encapsulated by a TiO2 layer. Further details regarding the photochemistry of malonatecovered Au@TiO2 showed that UV irradiation led to the formation of an oxalate intermediate. Reactive oxygencontaining species were proposed to play a key role in converting the carbon-centered radical into a carboxylate and thereafter to oxalate. Importantly, the data collected in this work suggested that the lifetime of holes increased in the presence of oxygen. Evidence for this effect came from a broad IR absorption feature that extended from above 2000 to below 1000 cm  1 with increasing intensity at lower wavenumbers during UV irradiation of the malonic acid–covered TiO2 sample. This broad absorbance was attributed to photogenerated electrons trapped in midgap shallow states of TiO2. As described in previous sections, IR observation of free CB and shallow trapped electrons for thermally treated or UV irradiated TiO2 have been well described in the literature by several groups. That is, studies by Hoffmann [1185–1187], Yamakata [883,884,1188], Yates [859,1189–1191], McQuillan [1192–1194], Thachiya [1073–1075,1195], and their co-

workers reported IR observations of CB and trapped electrons in UV excited TiO2. Hu et al. [1184] proposed that electrons were transferred from Au to TiO2 under visible-light SPRexcitation of the Au/TiO2 nanostructures. In the case of malonic acid covered TiO2 (with no Au), malonic acid was suggested to provide trap sites during UV-photogeneration of holes, and thus prevent immediate charge recombination. The reduced propensity for recombination enhanced the transfer of electrons to midgap trap states and to electron acceptors on the surface of TiO2. When Au nanoparticles were present on the catalyst surface, they were proposed to behave as efficient electron acceptors, thereby eliminating electrons from trapping in the midgap shallow states. As a consequence of electron migration into the gold, no electronic IR signal was observed, while the rate of photooxidation increased due to a larger number of long-lived holes residing in the titania support [1184]. For the visible-light irradiation of malonate covered Au@TiO2, Bürgi and co-workers [1148] observed a broad IR absorption, due to trapped and CB electrons, as shown in Fig. 13.12, spectrum a. The addition of Fe3 þ to the solution flow caused an marked decrease of this absorption feature and an increase of the signal attributed to malonate decomposition, as shown in Fig. 13.12, spectrum b. The authors interpreted these results as follows. Upon visible light illumination, SPR-excited electrons (presumably, from plasmonic decay) in Au particles were transferred to the CB of TiO2 and then some of the electrons relaxed to become trapped in the midgap shallow states. Importantly, there was no evident oxalate formation during visible light illumination. Thus, it was concluded that holes left in the Au nanoparticles were not of sufficient energy to oxidize malonate. A required feature of TiO2–based photocatalysis is that oxidation and reduction proceed simultaneously at adjacent active sites on the catalyst surface. Thus, physical separation of photogenerated electrons and holes is critical to activity. When Au particles are present on TiO2, IET can occur from TiO2 to Au nanoparticles under UV light illumination and from Au nanoparticles to TiO2 under visible illumination, which has been suggested to facilitate photocatalysis under both irradiation regimes (see Section 9.2). Tada and co-workers [1196] demonstrated the important potential application of Au/TiO2 photocatalysts for wavelengthswitchable photocatalytic reactions in the oxidation of thiols, as well as disulfide reduction. As they highlight, a reaction of critical importance to biological processes is the reversible formation and cleavage of an S–S covalent bond, i.e., the thiol-disulfide conversion, which needs stoichiometric amounts of oxidant and reductant. Although previous studies revealed that the selectivity of photocatalytic reactions can be tuned by changing the energy of the photons [1197], this group was among the first to report [1196] wavelength-switchable reversible reactions. In this work, a series of Au/TiO2 photocatalysts, having the same Au-loading (0.25 and 0.43 mass %) but varying Au particle size (2–12 nm), were synthesized via the deposition–precipitation method. The visible light-induced (λ 4420 nm, 3.7 mW cm  2) photocatalytic oxidation of 2-mercaptopyridine (PySH) on the surface of Au/TiO2 proceeded under aerobic conditions to give 2,20 -dipyridyl disulfide

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(PySSPy) as follows: Au=TiO2 ; visible ðλ4 420 nmÞ2 PySH þ 1=2 O2 -PySSPy þ H2 O:

The sum of the concentrations ([PySH]þ 2  [PySSPy]) attained was nearly constant for this reaction, indicating that the oxidation of PySH to PySSPy proceeded stoichiometrically. Under UV irradiation (λ 4 300 nm) of Au/TiO2, the catalytic reduction of PySSPy to PySH proceeded: Au=TiO2 ; UV ðλ4 300 nmÞ PySSPy þ H2 O-2 PySH þ 1=2 O2 : While the compounds involved were the same under both UV and visible irradiation, the direction of the reaction was reversed. Therefore, this was a demonstration that the formation and cleavage of the S–S bond can be controlled photocatalytically by changing the wavelength of photoexcitation light. Further, the kinetics of both UV and visible light-induced reactions was studied for the two sets of samples with different Au-loadings for varied Au particle sizes. Based on these studies, the following general scheme for the photoinduced reversible formation and cleavage of the S–S covalent bond was proposed: As illustrated in Fig. 13.13, the selective photocatalytic oxidation of thiol to disulfide proceeds under visible light irradiation of Au/TiO2, whereas the opposite reaction, i.e. the reduction of disulfide to thiol, proceeds under UV light irradiation of Au/TiO2. The key to this process is that the reduction and oxidation potentials for water must be aligned with the energy of the TiO2 CB electrons (for visible reduction) and the TiO2 VB holes (for UV oxidation), while the red-ox potentials for the disulfide must lie very close in energy to the EF of the Au particles. Beyond biologically-relevant hetero-molecules, organophosphorous esters such as pesticides and chemical warfare agents

Fig. 13.12. ATR-IR spectra recorded during visible light illumination of adsorbed malonic acid: (a) illuminated with visible light above 570 nm for 53 min (the background collected after 28 min malonic acid adsorption in the dark) while flowing malonic acid (3  10  4 M) over the Au@TiO2 catalyst, (b) 14 min after addition of 6  10  4 M FeCl3 to the flowing malonic acid solution. Reprinted with permission from Ref. [1184]. Copyright 2012 Elsevier B.V.

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(CWAs) are heteroatom-containing organic compounds that have been studied for their chemical activity within Au/TiO2 systems. The primary challenge in developing chemistries for the thermal and photolytic detoxification of these poisonous compounds is related to the breaking of the P–C or S-C bonds; however, demonstrations have shown that TiO2-based decontamination photocatalysis can significantly degrade and, in some cases, mineralize many of these substances. Hence, this approach holds potential for air or aqueous pollution abatement [665,847,1191,1198–1202]. Recently, Garcia and coworkers [1007] reported the use of titania-supported gold nanoparticles as a photocatalyst for UV and visible lightinduced decontamination of three CWAs. The structures shown in Fig. 13.14 contain a carbon-heteroatom bond, which must be cleaved for successful detoxification. The approach to developing a photocatalyst for the CWA compounds involved the deposition-precipitation method on titania (P25), which was employed to create three Au/TiO2 samples with Au-loadings of 0.4, 0.7, and 1.5 wt%, respectively. For the three Au-loadings, the authors of this work observed no difference in the average size of the deposited gold nanoparticles, as confirmed by TEM analysis. The average diameter of the Au NPs was about 5 nm, which appeared to be maintained throughout the photocatalytic degradation reactions. However, XPS analysis of the Au/ TiO2 samples revealed that the Au state changed from metallic to slightly oxidized during the photocatalytic oxidation of the molecules. Overall, the most important result of the photodegradation study was that the final products appeared to be benign and included only non-toxic phosphonates, sulfides, disulfides, sulfoxides and sulfones. A summary of the results obtained in the UV and visible light photocatalytic degradation of CWAs are listed in Table 13.1. These results show that complete degradation of the three structurally different CWAs can be achieved in short exposure times, i.e.
120 min. Further, the photoactivity of the Au/ TiO2 samples increases with the Au-loading. Note that in some cases 30 min of visible light irradiation was sufficient to mineralize the CWA. In addition, it is clear that the Au/TiO2 samples are less active under UV irradiation, as compared to visible light illumination. This remarkable result suggests that the light absorption by LSPR of gold nanoparticles is responsible for the high photocatalytic efficiency. The authors suggested that the CWAs, adsorbed on the semiconductor surface, may react either with electrons (behave as acceptor molecules) or with holes (electron donor molecules). Further, light irradiation is proposed to generate oxygen radicals (O2•  ) and/or hydroxyl radicals (•OH) that mediate the total degradation of the CWAs, as illustrated in Fig. 13.15. This study, coupled with the others highlighted above, provides strong evidence that Au/TiO2 photocatalysts may serve as an effective aid to driving transformations of organic reactions. Importantly, can such reactions operate in the solar regime of the electromagnetic spectrum? Additional examples of the effectiveness of photochemistry on Au/TiO2 catalysts come from studies into the reactions of volatile organic compounds at the gas-surface interface.

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D.A. Panayotov, J.R. Morris / Surface Science Reports 71 (2016) 77–271 Table 13.1 Photocatalytic performance of the investigated Au/TiO2 catalysts [1007]. Substrate

Irradiation source

Time of exposure/min

Catalysts 0.7% 1.5% 0.4% Au/TiO2 Au/TiO2 Au/TiO2

Soman

UV Visible

Fig. 13.13. Proposed general scheme for reversible formation and cleavage of a S–S covalent bond, i.e., the thiol-disulfide conversion. Reprinted with permission from Ref. [1196]. Copyright 2010 The Royal Society of Chemistry.

VX

UV Visible

Sulfur mustard

UV Visible

Fig. 13.14. From left to right: chemical structures of soman (O-pinacolyl methylphosphonofluoridate), VX (O-ethyl-S2-diisopropylamino ethyl methyl phosphonothionate), and sulfur mustard (bis(2-chloroethyl) sulfide). Reprinted with permission from Ref. [1007]. Copyright 2010 The Royal Society of Chemistry.

13.3.2. Photodegradation of volatile organic compounds (VOCS) in the gas phase As already discussed in Section 8.2, gold nanoparticles supported on metal oxides, particularly TiO2 and other semiconductors, are efficient heterogeneous catalysts for important oxidation processes in the gas phase, including oxidation of CO and H2, selective oxidation of hydrocarbons and oxidation of various volatile organic compounds (VOCs), such as alcohols and aldehydes at ambient or moderately elevated temperatures [38,185,187,665,819]. The combination of the catalytic activity of gold nanoparticles and their high SPR absorptivity makes them promising photocatalyst candidates for decontamination of air pollutants by utilizing solar energy [468]. However, only a limited number of studies have focused on the photocatalytic degradation of gas–phase VOCs on supported gold nanoparticles. One notable study from Gao and co-workers [1014] reported on the photocatalytic oxidation of formaldehyde and methanol to carbon dioxide at Au/ZrO2, Au/CeO2, Au/Fe2O3, and Au/ SiO2 powders. The gold in these systems (2–4 wt. %) was deposited on the oxide supports by the impregnation method. Gold exhibited a broad distribution of particles size on ZrO2 and SiO2 powders with a maximum at 27 and 53 nm, respectively. Most of the Au NPs on Fe2O3 were between 10 and 30 nm, while on CeO2 they were below 10 nm. For the photocatalytic tests, gold containing powders were deposited on glass slides and placed in the catalytic chamber. Formaldehyde (HCHO) or methanol (CH3OH) was injected into the air-filled vessel (100 ppm) and sealed. The content of HCHO or CH3OH and the product CO2 was analyzed chromatographically during the reaction, initiated by blue (λ¼ 400–500 nm, 0.17 Wcm  2) or red (λ¼ 600–700 nm) light irradiation. In this work, the general trend in activity followed: Au/ZrO2 4 Au/CeO2 4 Au/Fe2O3. However, the Au/SiO2 sample exhibited activity only under red

30 120 30 120

15 35 40 85

14 38 42 100

21 42 47 100

30 120 30 120

13 39 45 78

22 40 71 100

17 45 73 100

30 120 30 120

11 54 21 82

28 59 32 100

40 67 37 100

light irradiation. The authors of this work inferred that gold particle size was not a crucial factor for activity; however, the nature of the metal oxide strongly influenced the catalytic activity. Interestingly, the conversion efficiency exhibited an exponential dependence on the light intensity, which the authors used to suggest that light-to-heat conversion, mediated through the SPR-excited Au nanoparticles, was responsible for the photooxidation activity of the catalysts. An estimate showed that gold particles could be heated up to 373 K (even if the energy conversion efficiency is 10% or below) under their experimental conditions, at which point HCHO oxidation can proceed [1111,1116]. The second consequence of SPR excitation within the metal is the interaction between the oscillating local electromagnetic fields and polar molecules also may assist in activating the molecules when oxygen is in close proximity. In this way, adsorbed oxygen on the supports (ZrO2, CeO2 and Fe2O3) was suggested to control the high activity of the catalysts under dry air conditions. In agreement with the above suggestion, Zheng et al. [1203] found that the activity of Au/TiO2 photocatalysts requires adsorption and activation of O2 molecules at the surface. From this work, the process of O2 activation appears to be strongly related to hydroxyl surface coverage and defect concentration at the support. This mechanism was a common theme for the three different catalytic reactions studied: thermally activated CO oxidation in air, UV-light induced photocatalytic decomposition of sulforhodamine B (SRB) dye in water, and the visible light driven decomposition of formaldehyde in air. Four kinds of high surface area (200–300 m2g  1) TiO2 supports (nanotubes or nanorods) with different phases and composition were prepared for this study, and gold nanoparticles were deposited on the supports by NaBH4-reduction to yield four Au/TiO2 (3 wt. %) catalysts. Gold (in its metallic state) was found to be well dispersed on these large surface area supports and showed a narrow distribution of particles sizes,

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Fig. 13.15. Photodegradation of the neurotoxic compounds on the Au/TiO2 catalysts under visible irradiation. Adapted with permission from Ref. [1007]. Copyright 2010 The Royal Society of Chemistry.

dAu ¼ 4–8 nm. Based on FTIR spectroscopic data, it was established that one of the samples containing anatase TiO2 had the highest ability to gain surface OH groups, with effectively adsorbed water. This observation was important because XPS data indicated that CO interacts with surface OH groups to form a CO þ OH complex during the catalytic CO oxidation. Such hydroxycarbonyl species were previously assigned to intermediates formed on Au/TiO2 and were suggested to play a crucial role in low temperature catalytic oxidation of CO [1204]. All of the synthesized Au/TiO2 catalysts exhibited high visible light (λ ¼ 400–500 nm) activity for oxidative photodegradation of HCHO, but the highest conversion was measured for the Au/TiO2 (anatase) sample. An interfacial electron transfer process from SPR-excited Au nanoparticles to the TiO2 support and then to adsorbed oxygen to form O•– 2 was proposed to help explain the photocatalytic activity. Simultaneously, an electron transfer from formaldehyde molecules adsorbed on gold to the metal was suggested to complete the reduction-oxidation cycle. Liu et al. [1205] have also reported that visible light irradiation (λ ¼ 490–780 nm,
200 mWcm  2) of Au/TiO2 catalysts can improve the gas-phase oxidation of CO at room temperature. A Au/TiO2 catalyst containing
0.5 wt. % Au nanoparticles of diameter 3–10 nm was prepared in this group by the deposition–precipitation method. In response to visible light irradiation (λ4 490 nm), the conversion of CO was found to increase from 29.5 % (in dark) to 38.5 % and to decrease back to (only) 32 % when the light was switched off. The CO oxidation rate enhancement was attributed to the SPRmediated light-to-heat conversion upon plasmonic decay. Their FTIR spectroscopic data indicated that the visible irradiation caused a red shift in the vibrational frequency of CO adsorbed on gold nanoparticles from 2104 cm  1 to 2091 cm  1 along with an intensity increase. The reason behind these observations was attributed to an increase in electron density around the low-coordination Au sites where CO was adsorbed. As described above (see Section 3.2.1) electron transfer from the excited 5d orbital of Au to the π* orbital of CO is thought to weaken the C-O bond and thus, to red shift the CO vibrational frequency (νCO) [128,255].

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Spectral changes were also observed in the region of νOH and δH2O modes of surface OH groups and adsorbed water on the TiO2 during visible irradiation in the presence of O2. Based on this and other spectral data, a mechanism was proposed in which CO at Au sites is oxidized by neighboring OH groups siting on the TiO2 surface. However, the source of H2O molecules needed to replenish the surface OH groups during catalysis was not specified in this study. Further research in this area of CO photooxidation has come from the Cronin group [1128] who have shown that integrating strongly plasmonic Au nanoparticles into Fe2O3 leads to a significant enhancement in oxidation rate. The plasmonically driven catalytic reaction rate was at least 2 orders of magnitude higher as compared to that obtained under uniform heating. In this work, the preparation of the Au/Fe2O3 plasmonic composite and the catalytic tests were performed in a specially designed microchannel reactor consisting of a gas delivery system, an optical microscope, and an excitation laser source. The microchannel system was critical for achieving high sensitivity in the mass-spectrometric detection of reaction products (produced from the small catalyst area of the order of the focused laser spot,
1 μm2). Gold nanoparticles were first deposited in the microchannels by electron-beam evaporation. This produced a 5 nm thick island-like film of strongly plasmonic Au nanoparticles separated by only a few nanometers [1206]. A dilute Fe(CO)5/CO gas was then supplied to the microchannel system under irradiation of the Au particles at their SPR-frequency. Thus, a crystalline Fe2O3 (in the hematite phase) was deposited over the Au film to form the catalytically active Au/Fe2O3 plasmonic composite. As seen in Fig. 13.16a, the CO2 and O2 signals remained constant during the first 300 s of visible light irradiation (λ¼ 532 nm, 48 mW), which was in the induction period, during which the Au/Fe2O3 plasmonic composite formed. After this period, the CO2 signal increases and that of O2 decreases as the reaction 2COþ O2 - 2CO2 proceeded. The sudden increase in temperature (Fig. 13.16b) observed after 300 s was caused by the exothermic oxidation of CO (ΔH¼  532 kJ mol  1), which was catalyzed by the newly formed Fe2O3–Au composite. Control studies involving the irradiation of regions with only Au nanoparticles present (no Fe2O3) revealed no change in the CO2 signal, as shown in Fig. 13.16c. Similarly, under irradiation of regions with Fe2O3 alone (no Au nanoparticles) no production of CO2 was registered. Therefore, it was concluded that the CO oxidation was not driven solely by the thermal energy (generated during plasmonic decay) of the irradiated gold nanoparticles. Rather, the presence of both materials in contact is required for this reaction to proceed. Moreover, it was established that merely heating (350 1C) the reaction system did not achieve the same levels of reaction enhancement as the visible light irradiation. Overall, this study showed the light-mediated catalysis was at least 2–3 orders of magnitude higher than that of thermally activated catalysis and that this chemistry relies on the interaction between the plasmonic nanoparticles and the metal oxide support. A number of studies have reported that mesoporous TiO2 is an excellent host for anchoring plasmonic gold to produce nanocomposites with enhanced photocatalytic activity

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[507,508,924,953,1007,1207–1211]. As previously discussed within the context of thermally activated reactions on Au/TiO2 composites (Section 8.2.1), Pietron et al.[506] have shown that this multidimensional metal oxide interface facilitates catalytic oxidation of carbon monoxide at Au particles that others have shown are too large to efficiently catalyze the reaction (see Figs. 7.1 and 8.7). Further, Ismail et al. [924] and Panayotov et al. [508] reported that networked Au particles in oxide nanoarchitectures can promote the photocatalytic oxidation of methanol, which is an important test reaction for learning about photoassisted catalytic processes [899,924,1212,1213]. The scheme in Fig. 13.17 illustrates the mechanism proposed by Ismail et al. [924] to explain the enhanced UV photoactivity of mesoporous Au/TiO2 nanocomposites in methanol oxidation. In this mechanism, the mesoporous TiO2 network behaves like an antenna to improve the intrinsic electron diffusion and enable the electrons to reach the TiO2/Au interface, thereby reducing the extent of charge recombination. Thus, researchers found that even a small number of Au particles that are in electronic contact with the entire TiO2 network can function as good electron relays to enable charge separation and subsequent transfer to the solute. Motivated by the success of the 3-D networked Au TiO2 aerogels to promote CO oxidation, Panayotov et al. [508] explored the UV and visible light-mediated photochemistry of methanol within these materials. Infrared spectroscopic studies within the well-controlled confines of a UHV-based reaction cell afforded studies under well-defined conditions (surface coverage, T, light fluence). Titania aerogels were prepared from titanium(IV) isopropoxide using the sol-gel method as described by Dagan and Tomkiewicz [1214], whereby gold nanoparticles were deposited in TiO2 aerogels using the guest host strategy developed by Rolison and co-workers [504,505] for modification of composite aerogels. The similar dimensions of the TiO2-host particles (
10 15 nm covalently bonded anatase nanoparticles that comprise the networked support) and Au-guest particles (
5 6-nm metallic nanoparticles) enable each Au particle to form many interfacial contacts with the surrounding support. As discussed in Section 7.2.2, methanol uptake on thermally activated samples was found to be predominantly dissociative to produce methoxy groups on both Au-TiO2 [651,653] and TiO2 aerogels [649,650,899,1215]. For a detailed discussion on the FTIR spectra and band assignment for methanol covered samples, see Section 7.2.2.

The light-induced photochemistry of methanol was studied by Panayotov et al. [508] under both anaerobic (in vacuum) and aerobic (in the presence of O2) conditions with methanolsaturated Au–TiO2 and TiO2 aerogels. UV light irradiation of Au–TiO2 and TiO2 aerogels under anaerobic conditions caused a major increase in the IR absorbance across the entire mid-infrared regime (Fig. 13.18a, the red trace). As discussed above, such broad (4000–1000 cm  1) IR absorbance was attributed to both free conduction band and shallow-trapped photoexcited electrons within the TiO2 [859,883,1187,1193]. The adsorbed methoxy groups were found to behave as effective hole traps and extend the lifetimes of conduction band and shallow-trapped electrons [899,1213]. Upon addition of O2, oxygen scavenged the excited electrons in titania, which immediately returned the broad electronic absorbance to baseline (Fig. 13.18a, blue trace). The small negative features for methoxy-derived species together with a positive feature at 2868 cm  1, along with doublets at
1570 cm  1 and at
1370 cm  1, indicated the oxidation of methoxy groups into surface formates (the inset in Fig. 13.18a). Irradiation with low energy visible light (λ¼ 550 nm) produced charge carriers only in the methanol-saturated Au TiO2 (Fig. 13.18b, the red trace), but not in the corresponding TiO2 sample (not shown). Upon quenching of excited electrons by oxygen, the IR spectrum of the Au TiO2 sample revealed the formation of surface formates as products of the photooxidation of methanol (Fig. 13.18b, the blue trace). These data indicate that visible light activity of Au TiO2 is probably mediated by the SPR of gold nanoparticles, which was further checked under aerobic conditions. Fig. 13.19 summarizes the results obtained under aerobic conditions. Irradiation of Au TiO2 and TiO2 aerogels with UV radiation over of 20 Torr of O2 more efficiently generated surface-adsorbed formates. Further, the results demonstrated that a portion of the surface-bound methanol was completely oxidized to CO2 at Au TiO2 (Fig. 13.19a, b). By scavenging electrons, oxygen enables holes to migrate toward the Au||TiO2, apparently causing methoxy group oxidation. This description is consistent with other studies, and in particular that of Bahnemann and co-workers [1212]. Their results implied that methanol may react with surface-trapped holes to form a •CH2OH intermediate and that dissolved O2 scavenges CB electrons to complete the oxidation–reduction cycle.

Fig. 13.16. (a) Quadrupole mass spectrometer data for O2 and CO2 signals. (b) Infrared temperature data taken during laser irradiation in CO. (c) Quadrupole mass spectrometer signals for CO2 during 5 s laser-induced reactions on various substrates. Adapted with permission from Ref. [1128]. Copyright 2010 American Chemical Society.

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Irradiation with visible light (λ ¼ 550 nm) in the presence of O2 caused the oxidation of methoxy to formate to proceed on the Au  TiO2 aerogel sample, while no activity was registered for the pure TiO2 aerogel sample (Fig. 13.19b). In addition, the relative rates for formate generation under UV- and visibleexposure regimes were fundamentally different from one another (Fig. 13.19c). Under UV irradiation, at both aerogel samples, formate production exhibited a biexponential rate dependence. In contrast, visible irradiation of the Au  TiO2 showed a single-exponential dependence to the time evolution of formate production. This difference may be related to differing pathways for the creation and separation of e   h þ -pairs. As seen in Fig. 13.19d, the Au  TiO2 aerogel exhibits an absorption maximum at λ ¼ 550 nm caused by the LSPR in the Au particles. This absorption feature coincides with excitation energy that produces the highest oxidation

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activity; therefore, the photooxidation of methoxy species is most likely related to the LSPR decay-induced production of charge carriers in the Au  TiO2 catalyst upon visible light exposure. However, one should note that local heating effects during LSPR decay and the effect of the local field enhancements have not been explicitly studied in this work. Multiple mechanisms for photoconversion of surface adsorbates in the visible range of the electromagnetic spectrum may be active, to varying degrees, in many of the systems described within the this review. For example, direct electron transfer may occur in some systems along with a local heating effect, both of which facilitate chemical transformations. Future research into the dynamics of charge generation, diffusion, and interfacial crossing offers the opportunity for numerous key fundamental discoveries that will aid in the development of the next generation of low-energy consumption photocatalysts. In addition, the emerging understanding of how the synergy between TiO2 crystal phases (and their relative proximity to Au particles) may enhance catalytic activity will be critical to furthering catalyst development [1227,1228,1229]. 14. Summary and outlook

Fig. 13.17. Schematic illustration of the proposed antenna mechanism to explain the enhanced activity for methanol photooxidation over UV irradiated mesoporous Au/TiO2 nanocomposites as photocatalysts. Reprinted with permission from Ref. [924]. Copyright 2009 American Chemical Society.

Over the past quarter of a century, the combined efforts of scientists from a wide variety of fields, including material science, catalysis, chemical analysis, computational chemistry, etc., have facilitated deep insights into Au-based catalysts. At the same time, available research demonstrates the tremendous challenge that lies in developing a comprehensive understanding of thermal- and photo-catalytic processes, effects, and characteristics for these systems. The discovery by Haruta and coworkers [27,78], which first identified the unique low temperature activity of Au nanoparticles for the oxidation of CO and H2, planted the scientific seeds that have grown into the development of highly active and selective catalytic systems for a broad range of homogeneous [44] and heterogeneous [55] applications. A critical aspect of this development over the years

Fig. 13.18. The IR electronic spectrum of methanol-saturated Au–TiO2 obtained after 60 min of photoexcitation with UV (a) and visible (b) light. Adapted with permission from Ref. [508]. Copyright 2013 American Chemical Society.

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is our enhanced understanding of how characteristics such as the physical and the electronic structure of the material affect function [685]. Unraveling how these properties influence function is a formidable challenge because they often cannot be independently altered in ways that provide an avenue for systematic studies. For example, Au particle size has been shown be one of the most critical parameters affecting activity [464,523]; however, changes in size affect the interfacial structure, the fraction of undercoordinated sites, and the electronic band structure of the catalyst, all of which have a major influence on thermal- and photo-catalytic turnover. For example, the stark difference in the reactivity of bulk Au, which is highly inert, and Au nanostructures, which catalyze reactions, has been related to the higher concentration of undercoordinated sites on the surface of nano-scale gold particles [706]. However, the story is more complex than a simple picture of chemistry occurring at surface defect sites. Theoretical predictions confirm that the reduction of the cluster size shifts the d band closer to EF, which fundamentally changes the material's surface properties by altering the population of bonding and antibonding

orbitals that form during Au-adsorbate hybridization [29,224,523,692]. In addition, very small particles possess an electron structure that resembles that of large molecules rather than metal, which requires a completely different paradigm for understanding catalysis. The same scientific challenges exist for unraveling the role of the support in catalysis [691]. Many of the most important issues associated with building a comprehensive understanding of Au/TiO2 catalysts are listed in Section 8. Notwithstanding these challenges, progress has been made in developing mechanistic insight into Au/TiO2 thermal- and photo-catalysis, as well as demonstrating that the catalyst has applications far beyond facilitating the oxidation of carbon monoxide. Thermal chemistry always accompanies photochemistry. Therefore, this review attempts to first provide an overview of the types of reactions, mechanisms, and kinetics that affect the fate of adsorbates on Au/TiO2 surfaces before presenting details about how those adsorbates may be transformed by photonic energy. Such a comprehensive review requires one to build a conceptual foundation with an initial focus on

Fig. 13.19. UV (a) and visible (b) light-induced photoconversion of methoxy to formate species on Au–TiO2 and TiO2 aerogels under aerobic conditions. (c) Kinetics of formate production during light irradiation of samples. (d) Excitation wavelength dependence of formate production at Au–TiO2 and TiO2 aerogels and plasmon resonance absorbance spectrum of the Au–TiO2 aerogel. The spectra are normalized for sample loading. Reprinted with permission from Ref. [508]. Copyright 2013 American Chemical Society.

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experimental and theoretical studies that have been performed on isolated Au clusters and crystalline materials, followed by a review of model Au catalysts on ordered supports, and finally showing how fundamental studies help to provide insight into chemistry that occurs at real systems composed of small Au particles supported by the high surface area TiO2. As explicitly highlighted in this review, experimental studies with isolated gas-phase Au clusters and complementary computational investigations have provided “a conceptual framework and an efficient means to obtain direct insight into reactivity patterns, the role of differential ligation, the importance of aspects of electronic structure, and the nature of crucial intermediates” [97]. Such studies can contribute significantly to the overall “understanding of the intrinsic operation of practical catalysts at a strictly molecular level” [97] and to open new perspectives for future synthesis routes for the generation of highly sophisticated Au catalytic systems. Therefore, the catalysis community can learn a tremendous amount from the field of gas-phase cluster chemistry. Recognition of the overlap between these fields will help to alleviate the burden of re-discovering aspects of chemistry on small Au particles for catalysis that have been previously revealed through early Au-cluster work. Beyond elegant experiments on Au-clusters, surface science studies of well-defined single crystal Au surfaces and model Au catalysts on ordered supports under ultra-high vacuum conditions have greatly contributed to the current molecular-level understanding of the catalytic behavior of nanoscale gold [46,51,225]. By employing traditional UHV-based surface science techniques and other newly developed characterization methods, scientists have examined electronic (ligand) and geometric (ensemble) variations under adsorption of molecules at the surfaces. Elegant studies of chemistry at Au-particles on single crystal titania, titania on Au, and Au thin films on titania have provided invaluable information into the unique surface chemistry of these systems. Of particular importance are studies that are revealing the dynamic nature of nanoscale particles as they respond to changing surface energies upon molecular adsorption. Though fundamental surface science studies of model catalyst systems are critically important to advancing the field, both materials and pressure gaps must be considered. The pressure gap occurs because model and real catalytic systems are investigated under different pressure regimes, usually UHV and near atmospheric pressure, respectively. The materials gap represents the discrepancy between the design of idealized model systems with well-defined structure and morphology and the structural complexity of real catalyst systems. In recent years, considerable efforts have been devoted to bridge the pressure and materials gap; accordingly, new experimental methods have been developed that are capable of operating under continuously varying conditions [96]. Of particular note is the role of adsorption centers on the oxide support and on sites at the perimeter/contact area of the metal/support interface for activation of reagent molecules [46,47,68,69,186,1216]. The structure of Au particles at the metal–oxide interface, the gold-substrate interaction, and the support's impact on the properties of the adsorbed gold are only beginning to be understood [46,48,389,390,708,1217]. Moreover, Au

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nanoparticles can sinter rapidly under reaction conditions [461]. The recent developments in aberration-corrected scanning transmission electron microscopy (STEM) complemented with first-principles calculations demonstrated the full complexity of these aspects in real heterogeneous catalysts [1218,1219]. The major challenges in this field remain in the identification of particular adsorption sties where molecules are activated. Importantly, identifying strategies for fine-tuning such sites to a specific reaction [40] represents a major opportunity for future breakthroughs in both thermal- and photo-catalysis. At temperatures relevant to technological applications, the nanoscale promotion of reactivity has been shown to be affected greatly by the system's fluxionality, i.e. dynamic changes in shape, symmetry, and coordination of atoms within the gold nanostructure induced by the interaction with adsorbate molecules. This property of gold has been treated theoretically and found to originate from the highly reduced dimensions of the individual metal aggregates [138,301,708,1220]. Fluxionality is closely related to the role of the reducing nature of the support, largely attributed to charge donation to or withdrawal from gold or the possible charge build up at the metal/oxide interface (or both) [708]. Therefore, further surface science studies with model Au catalysts are needed to obtain dynamic information for the size-, shape-, and support-related aspects of the behavior of gold nanoparticles during the catalytic processes. Experiments employing high-energy X-ray absorption for in operando studies offer a promising strategy for building insight into the effects of fluxionality. Further, grazing incidence Xray scattering and grazing incidence X-ray diffraction experiments, coupled with mass spectrometry, were found particularly well suited for such type of studies because they are non-destructive techniques [461,700]. In addition to a Au-cluster's fluxional character, the field has shown that chemistry at nanostructures is sensitive to synergistic effects driven by coadsorption of reactants. For example, the synergistic effect observed for the coadsorption of hydrogen and O2 on Au nanoclusters leads to the formation of O-H bonds that enhance the O2Au bonding interaction [301]. Such chemistry may result in barrierless production of hydrogen peroxide, H2O2, which is a critically-important molecule for practical applications and as an intermediate in driving subsequent chemical transformations in organic synthesis [195]. Great opportunities exist for further theoretical and experimental studies of model catalyst systems that elucidate how such synergistic effects may control the propagation of a reaction. A key experimental requirement for future advancements in this field is the precise control of gold nanoparticle growth during synthesis [1024]. Controlling particle size while preserving monodispersity remains an experimental challenge that will require the combined efforts of expertise in materials science and catalysis. Further, advanced and highly sensitive in situ physical methods like UV  vis spectrophotometry, TEM, X-ray absorption near edge spectroscopy (XANES), smallangle X-ray scattering (SAXS) etc. are critical for tracking nanostructure properties during synthesis. In this respect, developing new, simple, and affordable methodologies that

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afford time-dependent monitoring of parameters during synthesis – such as redox potential, pH, conductivity, and turbidity in aqueous environment – will be of great importance for better understanding and controlling the reduction and nucleation/ growth processes of gold nanoparticles [1221]. The practical application of gold-based plasmonic nanostructures for photocatalysis has made substantial progress over the last 15 years, beginning with pioneering studies [8,61–64] that demonstrated the size-, shape-, and support-dependent optical properties of nanoscale gold. For example, early investigations revealed that the same wavelength dependence is observed for both absorption and scattering for small spherical gold nanoparticles (e.g., 30 nm diameter). However, larger-diameter nanoparticles (e.g., 100 nm) tend to favor scattering relative to absorption in the red regions of the spectra. Following these studies, extensive experimental and theoretical research identified many physical properties that affect the optical response of plasmonic metal nanostructures [907,955,960,966]. These studies of the optical properties of plasmonic gold nanostructures have augmented our overall understanding of how photonic energy may be applied to drive photocatalysis. Importantly, there are now hundreds of demonstrations (highlighted throughout this review) of a wavelength dependence to photocatalytic reaction quantum efficiency that exhibits a maximum precisely at the excitation frequency of the local surface plasmon resonance of the nanocomposite material [945,946]. Such dependence strongly evidences the LSPR-origin of the photocatalytic rate enhancement of chemical transformations by plasmonic nanostructures. Studies into the dependence of a reaction's photocatalytic rate enhancement on the intensity of the incident light (photon flux) have provided insight into the mechanism for driving chemical transformations [60]. Reports indicate that a linear relationship between the measured photocatalytic rate and the calculated plasmonic intensity provides evidence that excitation of surface plasmons is directly responsible for catalysis. In contrast, a relationship with a non-linear correlation between intensity and conversion rate, e.g. a power-law dependence, may be indicative that the reaction is driven by thermal effects through local light-to-heat conversion [60,955]. The emerging advanced understanding of photophysical processes [962] and the transfer of LSPR energy to effectively activate a local absorber at supported metal nanoparticulate structures (i.e. an adsorbate molecule [987] or semiconductor nanoparticle [955,966]) has stimulated significant research in several areas including photocatalysis [60,468,960], solar energy conversion [955], optoelectronics and photonics, medical applications, etc. [960,975]. This research has spurred substantial progress in the design and synthesis of new plasmonic photocatalytic systems [955] with diverse metal particles of various shapes and sizes [1024,1222], along with a variety of metal–support contact configurations [67,989]. Further scientific advances through the use of high resolution spectroscopic techniques that enable nanoscale studies of materials, especially methods that are sensitive to the electromagnetic fields at or near the metal particle surface–e.g., dark field microspectroscopy [965,1223] and surface-enhanced Raman spectroscopy [1058] - are providing time-resolved

insight into the form and function of plasmonic nanostructures [966,1223]. Using such experimental approaches, usually coupled to theoretical methods, scientists are systematically learning how to tune surface plasmon resonances for specific applications [955,960,966,1024,1058,1223]. In addition, research has provided general time scales for light-driven phenomena. In particular, the dynamics of LSPR relaxation has been described to involve a sequence of events that includes initial dephasing of the LSPR (10 fs time scale), internal relaxation of electrons via electron–electron scattering (100 fs), electron–phonon coupling (1–5 ps), and energy dissipation to the environment (10–100 ps) [962]. Despite all of this work, one fundamental issue that remains unresolved for many systems is how charge and energy transfer from the metal nanoparticle can effectively compete with the intrinsic ultrafast (hundreds fs) energy relaxation of LSPR [67,1067]. Although limited in number, studies on direct photocatalysis have shown evidence that metal nanoparticles can act simultaneously as light absorbers and as catalytically active sites, i.e. the energy of an LSPR can be transferred directly from the excited metal nanoparticle to an adsorbate molecule [987,1015]. For example, hot electron-induced dissociation of H2 on gold nanoparticles supported on inert SiO2 appears to be directly photo-activated [1123]. Direct photocatalysis of adsorbates at a metal surface provides a possible avenue for targeted activation of adsorbate bonds that allow rational manipulation of reaction selectivity in a desired direction [987]. Despite the many studies that have provided mechanistic insight into the overall photochemistry, the detailed photophysical pathways involving indirect energy exchange and charge transfer between an LSPR-excited plasmonic metal and an adsorbate molecule, either at the metal, the interface, or on the support, continue to stimulate intense research and discussion [955,966]. Several non-mutually exclusive photophysical mechanisms of energy-transfer have been evoked to explain the SPR-induced enhancement of photocatalytic reactions on supported gold nanostructures: SPR-mediated charge injection from metal to semiconductor, near-field electromagnetic and scattering, resonant energy transfer, and light-to-heat conversion. According to the charge transfer mechanism, electron–hole pairs are formed in Au nanoparticles under LSPR excitation, where some of electrons have sufficient energy to overcome the Schottky barrier, and be injected into the TiO2 conduction band [945]. Femtosecond time-resolved IR transient spectroscopy revealed that electron transfer to TiO2 occurs within 50 fs [1067], fast enough to be completed within the timescale (
100 fs) for relaxation of excited electrons. Importantly, studies with high-resolution X-ray absorption spectroscopy revealed the formation of holes in the Au d-band (valence states) of visible light excited Au/TiO2 [1077]. In addition, results of in situ EPR studies [1083] suggest that electron transfer from Au to TiO2 is induced by the superposition of two different electron excitation pathways within the Au particle: the d–sp interband transitions at short wavelengths and by the SPR intraband transitions at larger wavelengths. However, the plausibility of such a charge transfer mechanism in Au/TiO2 systems has been called into question [956].

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Unfortunately, theoretical treatment of this photophysical mechanism is rather limited; therefore, there exists an opportunity for future work in this area [966]. Notwithstanding, recently reported theoretical work [1084,1085], suggests that the energy distributions of plasmonically excited hot carriers depend significantly on particle size and shape [1084]. Further theoretical studies on charge transfer (CT) will be crucial to designing plasmonic nanostructures for solar and photocatalytic applications in which CT may play a central role in LSPRmediated energy transfer. Along with CT activation pathways, the LSPR-induced local electric field may also affect energy transfer through electronic excitations within the semiconductor support, whereby they drive photochemical transformations. Electrodynamic calculations have shown that the intensity of an LSPR-induced local electric field, i.e. the intensity of plasmonic “hot spots,” can be several orders of magnitude greater than that of the incident electric field [956,1090,1091]. Furthermore, the intensity of the induced local electric field can be even larger when the plasmonic metal nanostructures have sharp tips, edges and concave curvatures, such as nanowires, cubes, triangular plates and NP junctions [60]. Researchers have suggested that electron-hole pairs can be generated locally in regions surrounding semiconductor nanoparticles due to the large fields. A key consequence of such spatially anisotropic and intense fields is that the rate of electron–hole pair generation is largest in the regions of the semiconductor nearest to the plasmonic nanostructure, i.e. near the surface, where it may most effectively contribute to the photochemistry [948]. Resonant energy transfer (RET) is another LSPR-induced photophysical mechanism capable of generating electron hole pairs in the semiconductor, which then can drive photocatalysis. According to this mechanism, dipole dipole interaction between the plasmonic metal (donor) and semiconductor (acceptor) greatly enhances the visible-light photocatalytic activity as compared to the semiconductor alone [958]. Whereas the above-discussed mechanism of radiative local electromagnetic field increases the rate of interband transitions in the semiconductor due to the increased local near field, the RET process excites electron hole pairs directly in the semiconductor nonradiatively through the relaxation of the localized surface plasmon dipole. Thus, via the RET process, the plasmonic metal nanostructure can act very efficiently as a photosensitizer. The LSPR-generated electrons and holes created (via various pathways, described above) inside or near the Schottky junction (at the metal/semiconductor interface) may be well separated due to the build-up of an internal electric field in the space-charge region. Efficient charge separation would suppress the electron–hole recombination rate and thus enhance the photocatalytic efficiency [67]. However, the Schottky barrier height [59] and band bending phenomena [58] in such nanoparticulate systems are sensitive to factors such as the atomic structure of the metal/semiconductor interface, the relative particle sizes, the composition and defect density, preparation method and sample history. Because of these factors, the scientific community is in need of standard samples

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and advanced theoretical models that will better capture the energetics of real systems [58]. Beyond electronic effects, LSPR-excitation in Au NPs is well known to generate heat [1108,1117]. The heating effect, i.e. the photothermal effect, depends on the shape and organization of nanoparticles [1062]. The large optical cross-section of metal nanoparticles, as compared to their geometrical cross-section [942], assures that the temperature around and within a single AuNP could be very high [1113,1114]; sufficient to vaporize a solution [959] or even to melt the metal nanoparticle [1115]. Importantly, the LSPR-induced heating effect has been shown to depend strongly on incident light intensity and wavelength [1062,1121]. In this way, the localized heating effect can be tuned to drive thermally catalytic reactions at a relatively low overall temperatures [949]. Because of such heating effects, thermal chemistry must always be considered in explaining reaction pathways that are initiated by the generation of surface plasmons. Regardless of the specific mechanism, plasmon-enhanced photocatalytic reactions have attracted significant attention due to their high throughput and low energy requirements. This review presents several classes of reactions that have been shown to be initiated through plasmonic excitations including diatomic molecular dissociation [66,1122], water splitting [507,948,953,1064,1125,1126], H2 production from alcohol [998,1127], liquid and gas phase oxidation reactions [508,946], and hydrocarbon conversion [994]. The literature is replete with such examples and new demonstrations emerge each week. Recently, a number of reviews have been published that discuss various aspects associated with the photocatalytic performance of plasmonic metal nanostructures under UV and visible light irradiation [60,67,907,1066,1129]. Such well established precedence, coupled with emerging mechanistic understanding, bodes extremely well for the potential of future practical applications of plasmonic photocatalysis in areas such as environmental remediation, air cleaning, water splitting, molecular synthesis, and CO2 reduction. Even while researchers continue to show how Au/TiO2 nanostructured materials can be used to catalyze thermal- and photo-chemistry, there exist several key areas of inquiry that may lead to important field-advancing discoveries. Perhaps most importantly, a more comprehensive or universal understanding of the photophysical mechanism that controls the transformation of light energy into chemical energy within plasmonic nanostructures is needed. Physical studies on model well-defined photocatalytic systems via in situ methods, especially under real reaction conditions (in operando), are of crucial importance to this endeavor. In many current descriptions found in the literature, the mechanistic descriptions of charge carrier generation, separation, and transfer are vastly oversimplified due to a simple lack of information. Efforts from both experimental and theoretical communities will be required for solving this fundamental challenge. Methods that provide direct insight into structural and electronic changes that particles experience during catalysis, many of which may be transient, will lead to a dramatically-improved understanding of these reactions.

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