Monolayer Systems

7.06

Monolayer Systems

IE Wachs and CJ Keturakis, Lehigh University, Bethlehem, PA, USA ã 2013 Elsevier Ltd. All rights reserved.

7.06.1 Introduction 7.06.2 Characterization Methods 7.06.2.1 Raman Spectroscopy 7.06.2.2 IR Spectroscopy 7.06.2.3 Ultraviolet–Visible–Near–Infrared Spectroscopy 7.06.2.4 x-Ray Absorption Spectroscopy 7.06.2.5 Nuclear Magnetic Resonance Spectroscopy 7.06.2.6 Electron Paramagnetic (or Spin) Resonance Spectroscopy 7.06.2.7 Mo¨ssbauer Spectroscopy 7.06.2.8 Scanning Transmission Electron Microscopy 7.06.2.9 x-Ray Photoelectron Spectroscopy 7.06.3 Synthesis of Supported Metal Oxide Phases 7.06.3.1 Wet Impregnation Methods 7.06.3.1.1 Incipient wetness impregnation 7.06.3.1.2 Ion exchange 7.06.3.1.3 Sol–gel method 7.06.3.1.4 Anchoring and grafting 7.06.3.2 Vapor-Phase Methods 7.06.3.2.1 Anchoring and grafting 7.06.3.2.2 Chemical vapor deposition 7.06.4 Molecular and Electronic Structures 7.06.4.1 Precursor Solutions and Calcination 7.06.4.2 Ambient Conditions 7.06.4.3 Dehydrated Conditions 7.06.4.4 Reaction Conditions 7.06.4.4.1 Time-resolved operando molecular spectroscopy 7.06.4.5 Influence of the Oxide Support 7.06.4.5.1 Influence of different oxide supports 7.06.4.5.2 Influence of oxide support phase 7.06.4.5.3 Influence of oxide support dimension 7.06.4.6 Mixed Oxide Supports 7.06.5 Structure–Activity Relationships 7.06.5.1 Redox Catalysis 7.06.5.1.1 SO2 oxidation to SO3 7.06.5.1.2 Propane oxidative dehydrogenation to propylene 7.06.5.2 Acid Catalysis 7.06.5.2.1 Methanol dehydration to dimethyl ether 7.06.5.2.2 n-Pentane isomerization 7.06.5.3 Bifunctional Redox–Acid Catalysts 7.06.5.3.1 n-Butane oxidation to maleic anhydride 7.06.5.3.2 SCR of NO with NH3 to N2 7.06.6 Presence of Surface Metal Oxides in Other Mixed Oxide Systems 7.06.7 Conclusion Acknowledgments References

7.06.1

Introduction

When a metal oxide is deposited on a high surface area oxide support a two-dimensional (2D) surface metal oxide phase can form for many supported metal oxide systems (also commonly referred to as monolayer oxide-on-oxide systems).1–3 The

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supported metal oxide surface coverage, ranging from sub-monolayer to above monolayer (monolayer being the maximum dispersion limit of the 2D surface metal oxide phase), is a large controlling factor of the metal oxide molecular structures that form on the oxide support. Acidic metal oxides (e.g., WOx, CrOx, VOx, and MoOx) usually anchor to the support by titrating

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the more basic surface hydroxides while basic metal oxides (e.g., NiOx, FeOx, and CoOx) tend to titrate surface Lewis acid sites, such as surface oxygen vacancies.4,5 Below monolayer surface coverage, isolated and polymeric surface metal oxide species are typically present in the surface metal oxide phase, while exceeding monolayer surface coverage ushers the formation of crystalline nanoparticles (NPs) on top of the surface metal oxide monolayer.6–8 Thus, monolayer coverage is a critical parameter for supported metal oxide systems since it usually represents a transition from 2D to 3D metal oxides that possess different chemical characteristics (molecular structure, electronic structure, and chemical reactivity). Supported metal oxide catalysts have been extensively studied in the last four decades and are employed in many industrial catalytic processes9: automotive exhaust combustion over supported PtOx/CeO2,10 NOx storage over supported PtOx/ BaOx/Al2O3,11 selective catalytic reduction (SCR) of NOx to N2 over supported V2O5–WO3/TiO2 and V2O5–MoO3/ TiO2,12–18 hydrogen production via high-temperature water– gas shift (WGS) over supported CrO3/Fe2O3,19 olefin metathesis over supported Re2O7/Al2O3 and WO3/SiO2,20,21 o-xylene oxidation to phthalic anhydride over supported V2O5/ TiO2,22,23 H2S oxidation to elemental sulfur over supported Fe2O3/SiO2 and MnOx/SiO2,24 ethylene polymerization over supported CrO3/SiO2,25,26 and hydrodesulfurization (HDS) of refined petroleum products over supported CoO/Al2O3, MoO3/Al2O3, or CoO-MoO3/Al2O3 activated by sulfidation treatment.27,28 Due to the ability to control surface coverage, and consequently control molecular surface structures, supported metal oxide catalysts serve as well-defined model systems for fundamental catalytic reaction studies. The 2D nature of surface metal oxide phases readily lends itself to surface characterization by both surface and bulk spectroscopic techniques since the supported metal oxide is only present as a surface oxide overlayer in the sub-monolayer region. Furthermore, the marriage of catalytic performance and spectroscopic characterization allows for the establishment of fundamental molecular/electronic structure–activity/selectivity relationships that facilitate the rational design of advanced metal oxide catalytic materials. The focus of this chapter is on supported metal oxide catalysts with a strong emphasis on work published in the open literature from 2000 until present involving in situ characterization.

energy exchange occurs in the vibrational region and provides information about the molecular structure and properties of molecules from their vibrational transitions. Specifically, Raman spectroscopy detects vibrations that involve a change in polarizability, or deformation of the electron cloud. Typically, this results in Raman spectroscopy being sensitive to symmetric vibrations, whereas infrared (IR) spectroscopy is complementary and sensitive to asymmetric vibrations. Since Raman spectroscopy is not sensitive to changes in dipole moments, spectral interference from water and hydroxides is not a problem and diatomic molecules can readily be analyzed. An important consequence of the Raman scattering selection rules is the ability to characterize and monitor samples in situ in the aqueous, gas, or solid phases under all environmental conditions.29 In practice, Raman spectroscopy detects critical metal oxide vibrations corresponding to inorganic catalytic active sites such as M¼O, M–O–M, M–O–M0 , and M–O vibrations. IR spectroscopy may also be able to detect surface M¼O vibrations which can be compared to Raman data for very complete structural assignments. With Raman spectroscopy’s ability to operate under all environmental conditions, supported metal oxide molecular structures can be monitored starting with sample preparation utilizing metal oxide precursor solutions to reaction conditions involving gas-phase reactions over solid catalysts. The reader is referred elsewhere for in-depth detail of all aspects of Raman spectroscopy of supported metal oxides.29–32

7.06.2.2

IR Spectroscopy

While a myriad amount of physical and chemical characterization techniques exist, very few can be performed in situ, or under reaction conditions. This section focuses only on characterization techniques that can be performed in situ, since only data from these physical techniques can allow for the establishment of molecular structure–activity relationships under realistic and industrially relevant reaction conditions.

Complementary to Raman spectroscopy, IR spectroscopy takes advantage of direct resonance between the frequency of IR radiation and the vibrational frequency of a specific molecular vibration mode. Molecular vibrations that involve a change in dipole moment, or a net change in the separation distance of a molecule’s negative and positive ends (dipoles, literally ‘two poles’), are IR active and readily absorb IR radiation corresponding to the frequency of vibration. Typically, this results in IR spectroscopy being sensitive to asymmetric vibrations, as previously mentioned. As a consequence of IR spectroscopy’s sensitivity to dipole moment changes, water and hydroxide analyses result in large signals that usually dominate important regions of the IR spectrum, and diatomic molecules cannot be detected. However, IR spectroscopy gives rise to strong signals from surface chemical probes or surface reaction intermediates (CO, CO2, CH3OH, pyridine, HCOOH, etc.), support hydroxides (M–OH), and occasionally surface M¼O vibrations. Attenuated total reflectance-IR (ATR-IR) spectroscopy has been developed to overcome analysis limitations of aqueousphase reactions/samples. Both traditional IR (transmission or diffuse reflectance modes) and ATR-IR can be operated in situ. Additionally, the kinetics of reaction products/intermediates can be followed quantitatively and nondestructively by IR peak integration using both traditional IR modes33,34 or ATRIR.35 The reader is referred elsewhere for additional details on traditional IR methods32,34,36–38 and ATR-IR methods.35,39

7.06.2.1

7.06.2.3

7.06.2

Characterization Methods

Raman Spectroscopy

The Raman effect is a product of inelastic scattering of electromagnetic radiation upon interaction with matter. The inelastic

Ultraviolet–Visible–Near–Infrared Spectroscopy

Ultraviolet–visible–near–infrared (UV–vis–NIR) spectroscopy analyzes a large wavelength range of  50 0004000 cm1

Monolayer Systems

(200–2500 nm), which is divided into the appropriate UV, vis, and NIR regions of the electromagnetic spectrum. Usually, the UV–vis regions probe electronic transitions of molecules while the NIR region probes the overtone and combination vibrations of specific vibration modes seen in regions analyzed by Raman and IR spectroscopy (100–4000 cm1). While the NIR region is less commonly examined in metal oxides, molecules that contain p electrons or nonbonding electrons (n-electrons) give rise to electronic transitions in the UV–vis region and provide a wealth of information. Absorption of energy in the form of UV or visible light radiation excites electrons from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecule orbital (LUMO), resulting in excited states or antibonding states. By probing these electronic transitions information about oxidation states, coordination of metals, charge transfers, dispersion, composition, and band gaps can be obtained. UV–vis–NIR spectroscopy can be performed in both transmission and diffuse reflectance modes, with transmission preferred for aqueous-phase reactions/samples and diffuse reflectance for solid samples (with the exception of transparent crystals). Transmission mode relies only on the samples absorption coefficient and utilizes the Beer–Lambert law for the calculation of UV–vis–NIR absorbance whereas diffuse reflectance mode relies on both scattering and absorption coefficients, using the Kubelka–Munk function to represent UV–vis–NIR absorption. This makes quantification more difficult with UV–vis–NIR diffuse reflectance spectroscopy. Despite this limitation, UV–vis–NIR spectroscopy can be performed over a wide range of temperatures with sub-second time resolution. The reader is referred elsewhere for detail on UV–vis–NIR theory and applications.40–42

7.06.2.4

x-Ray Absorption Spectroscopy

x-Ray absorption spectroscopy (XAS) is differentiated from the optical Raman, IR, and UV–vis–NIR spectroscopies on the basis of the use of high-energy x-ray photons. Instead of probing valence electrons directly, x-ray photons are of such high energy that they physically eject core electrons from the s (K-edge absorption) or p (L-edge absorption) orbitals. These core photoelectrons (ejected electrons) have kinetic energy and can interact with surrounding atoms. The interaction of low kinetic energy photoelectrons with valence electrons, which is the transition of core electrons into unoccupied electronic states (e.g., s ! p or p ! d), is called x-ray absorption nearedge structure (XANES) and provides information on oxidation state/electronic structure and local geometry. High kinetic energy photoelectrons do not usually interact with valence electrons, but instead act as outgoing spherical waves which are backscattered from nearest neighbor atoms. The backscattering waves create an interference pattern with the outgoing wave which results in what is called extended x-ray absorption fine structure (EXAFS). The data contained in the EXAFS interference pattern can provide information on local structure and coordination numbers, bond lengths, structural disorder, and even size, shape, and surface morphology of 1–2 nm clusters.43,44 This information is extracted by considering all possible scattering paths of the photoelectron waves in the material and fitting the data with such a model. Although

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powerful, new chemistry may not be realized by EXAFS because a model can only be created from known structures and scattering paths. An x-ray absorption spectrum can be measured by either transmission or fluorescence methods, with transmission being the most common. Fluorescence detection is typically used with highly diluted/low concentration samples. XAS can be performed at nearly all temperatures and pressures and includes aqueous-phase reactions. There has also been a significant push in the recent decade to combine XAS with other vibrational spectroscopy/diffraction techniques.45–47 The reader is referred elsewhere for further details of theory, equipment, and applications of XAS.43,44,48

7.06.2.5

Nuclear Magnetic Resonance Spectroscopy

Nuclear magnetic resonance (NMR) spectroscopy probes the nuclei of atoms rather than electrons or vibrations. It is typically performed as solid-state magical angle spinning (MAS) NMR, in order to obtain adequate resolution, for heterogeneous metal oxide catalysts. Quantum electrodynamics (QED) permits that a nucleus can possess spin, giving it angular momentum in the quantum sense and, thus, a magnetic moment. In the presence of an external magnetic field, the nuclei magnetic moments align with the field causing the spins to split and possess different energies, known as the nuclear Zeeman effect. While in a magnetic field, the sample is irradiated with photons (usually radio radiation) of a specific resonant frequency, causing the spin to jump to another other energy level. Thus, NMR probes the amount of energy required to change the spin of a nucleus in the presence of a magnetic field. Only nuclei with a spin >0 are NMR active, with the most common active nuclei being 1H and 13C, though many more exist. NMR can provide a wealth of information such as distinct Brnsted acid site identification from 1H NMR, local coordination or characterization of NMR active metals, such as Al, by 27 Al NMR, or hydrocarbon reaction intermediates by 13C NMR. The reader is referred elsewhere for information on NMR theory49 and in situ applications of the technique.50–52

7.06.2.6 Electron Paramagnetic (or Spin) Resonance Spectroscopy Paramagnetism is the result of an unpaired electron having a magnetic dipole moment. QED dictates that the magnetic moment of an electron arises from its properties of spin and orbital motion, giving it angular momentum in the quantum sense. In the presence of an external magnetic field, the electrons magnetic moments align with the field and each other. The magnetic field causes the electron spins to split and possess different energies, known as the Zeeman effect. While in a magnetic field, the sample is irradiated with photons (usually microwave radiation) of a specific resonant frequency, causing a spin to flip to the other energy level. Thus, electron paramagnetic (or spin) resonance (EPR (or ESR)) probes the amount of energy required to reverse the spin of an electron in the presence of a magnetic field. While this explanation of EPR spectroscopy is grossly oversimplified and neglects discussion on the nuclear Zeeman effect interactions, (hyper) fine structure

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Monolayer Systems

interactions, and the complex quantum mechanics behind them, several reviews and textbooks exist that delve into such detail.53–55 In practice, EPR utilizes a monochromatic radiation source while the magnetic field is varied. It is capable of analyzing aqueous and solid samples under reaction conditions, limited by the reaction cell. EPR has also been combined with other vibrational spectroscopy techniques such as Raman and UV–vis spectroscopies.56 Not only can EPR monitor paramagnetic species in a catalyst, but radicals formed during reaction, ferromagnetic particles, and conduction electrons are also able to be detected.57

7.06.2.7

Mo¨ssbauer Spectroscopy

Mo¨ssbauer spectroscopy utilizes the radioactive decay and g-ray emission of an unstable atomic nucleus to cause resonant absorption in an examined sample (the absorber). This technique is not commonly seen in the catalysis literature as a Mo¨ssbauer spectrum cannot be obtained for all elements, a serious limitation. A Mo¨ssbauer spectroscopy experiment can be described as (1) A g-ray is emitted by an element specific Mo¨ssbauer isotope source by transition from the sources’ nuclear excited state to a nuclear ground state. The g-ray energy is modulated by inducing a small relative velocity between the source and absorber (the Doppler effect). (2) Resonant absorption of the g-ray will take place in the absorber (sample) only if the absorber is the same element as the isotope source (e.g., Fe atoms will absorb g-rays from the decay of a 57Fe isotope source). (3) Transmitted g-rays are detected and plotted versus the modulation energy. Shifts and splitting of the Mo¨ssbauer resonance are observed and provide information about the chemical nature of the absorbing atom. Some isotopes of interest to the metal oxide catalysis community include 57Fe, 101,99 Ru, 119Sn, 131Sb, 181Ta, 182–184W, 195Pt, 67Zn, 187Re, 61Ni, 107 Ag, and 197Au. Although there exist 79 isotopes for which the Mo¨ssbauer effect has been observed, not all isotopes will provide chemical information and some have a half-life of <1 day. The reader is referred elsewhere for information about the physics behind shifts and splitting of the Mo¨ssbauer resonance58 and in situ applications of the technique.59

7.06.2.8

Scanning Transmission Electron Microscopy

Scanning transmission electron microscopy (STEM) is a wellknown microscopy technique whereby a beam of electrons is transmitted through a thin sample forming an atomic resolution image. A variety of signals are generated when electrons interact with a sample, and thus STEM can provide more information than just an image. For example, inelastically scattered electrons from the sample can be analyzed to provide electronic structure, oxidation states, and chemical composition, a technique called electron energy-loss spectroscopy (EELS). High-angle scattered electrons can be collected by an annular detector which provides elemental contrast to the STEM image, a technique known as high-angle annular darkfield (HAADF) imaging. Though STEM is typically performed under ultra-high vacuum (UHV) conditions, research in the recent decade has seen the development of in situ cells for (S) TEM analysis. Various in situ cell designs are capable of

reaching temperatures of 1000  C,60 gas pressures of one atmosphere,61 and even using liquid reactants.62 In the near future, a combination of the latest aberration-corrected STEM and in situ cell technology could provide unprecedented detail on the dynamic structural evolution of active surface species under reaction conditions.60,63,64

7.06.2.9

x-Ray Photoelectron Spectroscopy

x-Ray photoelectron spectroscopy (XPS) follows the same principles as XAS but differs in that it detects the number of photoelectrons that completely escape from the material. Only photoelectrons from the top 1–10 nm of a material can escape, thus making XPS a surface sensitive technique. Typically, XPS is performed in UHV conditions since escaped electrons can interact with a gaseous atmosphere; however, recent advances in technology have produced systems capable of in situ characterization at pressures of 1 torr.51,65–67 XPS can detect oxidation states, local coordination, and adsorbed species. Thus, after further development, in situ XPS may prove to be a useful tool for analyzing reactions at a surface. More information on XPS is available elsewhere.68

7.06.3

Synthesis of Supported Metal Oxide Phases

It is now accepted that the final surface metal oxide species present on supports is thermodynamically driven and independent of preparation method.2 Careful studies have shown that a series of supported MoO3/TiO2 and V2O5/TiO2 catalysts prepared by a multitude of synthesis methods all resulted in the same final surface metal oxide species, according to Raman spectroscopy analysis.69,70 Other similar studies with MoOx supported on SiO2 reached the same conclusions.71,72 However, the concentrations of surface MOx species and their corresponding crystalline MOx NPs are significantly affected by different synthesis methods.73–77 Detailing every synthesis method that has appeared in the literature is beyond the scope of this chapter. As such, the most common wet impregnation and vapor-phase synthesis methods are discussed. The reader is referred to two comprehensive synthesis textbooks for detailed information on a multitude of preparation methods.78,79

7.06.3.1

Wet Impregnation Methods

7.06.3.1.1 Incipient wetness impregnation An impregnation solution must first be created by dissolving an active precursor in an aqueous medium (e.g., Cr(NO3)3 in H2O for Cr deposition). An amount of impregnation solution equal to the pore volume of the support is then mixed with the dry support and the solution is drawn into the support pores via capillary action. This volume, or incipient wetness point, is critical as an excess volume triggers a change in the solution transport from capillary action to diffusion, a much slower process. The loading amount that can be achieved via incipient wetness impregnation (IWI) is governed by the solubility of the active precursor in the aqueous medium chosen. After impregnation, the mixture is typically calcined to remove any volatile components of the impregnation solution that are left over

Monolayer Systems

(surface nitrates in the Cr(NO3)3 case) and stabilize the active surface species in their final state. This preparation method is rapid and can produce catalysts ready for calcination in less than a day.80

7.06.3.1.2 Ion exchange A solution containing the desired species for exchange is created by dissolving an active precursor in solution. Differing from the IWI method, ion exchange requires solution in excess of the supports total pore volume. As the name implies, ions in the support are exchanged with specific ions in the precursor solution after complete immersion. The exchange process is allowed to slowly reach equilibrium before calcination. Some supports naturally possess charged ions within their framework, such as zeolites, while others are amphoteric. Zeolites have a finite amount of exchange sites which are independent of the precursor solution pH. Amphoteric supports, such as g-alumina, require the precursor solution pH to be tailored to the specific support. Adjusting the precursor solution pH changes the support surface hydroxides creating a net surface charge, allowing for ion exchange. There exists a pH value for each support at which it will possess zero charge, called the point of zero charge (PZC). Generally, when an amphoteric support is immersed in a precursor solution with a pH > PZC, it will possess a negative surface charge allowing for cation exchange, while a pH < PZC favors anion exchange. Subsequent filtration and calcination remove any remaining volatile components of the precursor solution.81 A similar variation of the ion exchange technique, called strong electrostatic adsorption (SEA), utilizes an optimum solution pH to facilitate a strong precursor interaction with the support. However, SEA does not necessarily involve ion exchanges.82

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(called xerogel), supercritical drying (called aerogel), or freeze-drying (called cryogels). Even when dried, the gels still contain residues from preparation. Subsequent calcination finalizes the supported metal oxide catalyst and removes any remaining residues. The reader is referred elsewhere for information on all facets of sol–gel preparation.83

7.06.3.1.4 Anchoring and grafting Anchoring and grafting methods both involve the formation of covalent bonds between a transition metal complex and an inorganic support, with very subtle differences between the two, not to be discussed here. A wide range of precursors can be used including metal halides and oxyhalides, metal alkoxides, and organometallics. The support typically undergoes a thermal treatment to remove physisorbed water and other surface contaminants that might prevent anchoring and grafting. The metal oxide precursor chosen reacts with support hydroxides to form an anchored complex. Any remaining physisorbed complexes are eliminated by a solvent wash. The final surface phase is obtained by either hydrolysis of the anchored complex with water or decomposition by a mild thermal treatment, depending on the precursor used. In theory, this method cannot exceed monolayer coverage; however, several cycles of the preparation method can lead to multilayer coverages. Details on the subtleties of anchoring and grafting as well as specific treatments based on the chosen precursor are given elsewhere.84

7.06.3.2

Vapor-Phase Methods

7.06.3.2.1 Anchoring and grafting This method is described in Section 7.06.3.1.4 and can, alternatively, be performed in the vapor phase.

7.06.3.2.2 Chemical vapor deposition 7.06.3.1.3 Sol–gel method Though typically used for the synthesis of bulk mixed metal oxides, the sol–gel method can also result in highly dispersed supported metal oxide systems. Preparation of the support usually involves a metal alkoxide precursor of the formula M (OR)x where M is a metal, R is an alkyl group, and x is the valence state of the metal. The simplified route to final product first involves the hydrolysis of the metal alkoxide [M(OR)x] in solution into a metal hydroxide [M(OH)x], whereby condensation (of either water or alcohol) ushers the formation of the metal oxide network [MOx]. Supported metal oxide species can be synthesized with either a one-step process (adding the surface metal precursor with support precursor) or a two-step process (adding the surface metal precursor after the support is formed). The one-step process can prove difficult at times since the surface metal precursor will often reduce and precipitate out of the sol–gel before it is anchored on the support, or the active metal will end up in the bulk of the support. A complexing agent is sometimes used to circumvent the one-step process difficulties. While the one-step process shortens preparation time, the two-step process negates the difficulties mentioned and often produces more highly dispersed surface metal oxide species. Carefully drying the resulting sol–gel is a critical step in order to prevent the collapse of the pore networks from intense capillary pressure. Gels can be dried by evaporative drying

Chemical vapor deposition (CVD) creates a surface phase by either adsorption or reaction of metal precursors in the vapor phase with the support. Besides adsorption, CVD can utilize many reactions to deposit metals on the surface including pyrolysis, hydrolysis, reduction, oxidation, carburization, and nitridization. The reactions can also be used in combination to create complex surfaces. Choice of precursor is critical and must meet certain chemical standards such as stability at room temperature, sufficient volatility at low temperature, and ability to react with the support without producing side reactions. Reaction of gas-phase precursors typically occurs with metal oxide support hydroxides, though not exclusively. Alternatively, precursors can be adsorbed to the support and subsequently decomposed to create surface phases. An advantage of CVD is the ability to control coverage and create uniformly dispersed particles. Many derivatives of this technique exist for preparing just about any surface on a material such as atomic layer deposition (ALD)85 and specialized CVD techniques.86

7.06.4 7.06.4.1

Molecular and Electronic Structures Precursor Solutions and Calcination

The choices of precursor and synthesis conditions are important parameters in the overall composition of the final catalyst.

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Monolayer Systems

While it is known that the final surface metal oxide species present on supports is thermodynamically driven and independent of preparation method,2 synthesis choices can allow one to tailor the surface of a catalyst to one or more of the thermodynamically stable species. Aqueous molybdenum chemistry has received much attention in the last decade due to its use in preparing HDS catalysts. In aqueous solutions, only three Mo oxide anions are present (MoO42, Mo7O246, and Mo8O264), and their relative distribution is dependent on the solution pH and the Mo oxide concentration (see Figure 1 for their structures).6,87 Bergwerff et al. performed a series of impregnations on alumina extrudates using dissolved (NH4)6Mo7O24 solutions of varying pH and concentrations.88 They found, using Raman spectroscopy, that an acidic solution (pH ¼ 6) established MoOx on alumina as Mo7O246, MoO42, and Al(OH)6Mo6O183 anions 1 h after impregnation, while a basic solution (pH ¼ 9) resulted in mainly MoO42 anions. After drying, it was found that a layer of (NH4)3[Al (OH)6Mo6O18] formed on the outer catalyst surface (by Raman and SEM) using acidic solutions due to ligand-promoted dissolution of the support. This led to the formation of bulk MoO3 after calcination. Drying of basic solutions led to a redistribution of Mo complexes which, in turn, can lead to areas of high Mo loadings. Subsequent calcination caused the formation of MoO3 and Al2(MoO4)3 crystals from these areas of high Mo concentration. In addition, Bergwerff et al. found that the addition of citric acid to the precursor solution helped maintain good Mo oxide distribution throughout impregnation, drying, and calcination, and prevented the formation of MoO3 and Al2(MoO4)3 phases. This example demonstrates the importance of understanding the often-overlooked chemistry behind the preparation methods that exist in the scientific literature.

7.06.4.2

the atmosphere during transfer to storage vials and in situ reactors. Raman and UV–vis spectra were collected before any pretreatment was performed on the supported catalysts. As mentioned above, only the MoO42, Mo7O246, and Mo8O264 anions are present in aqueous solutions and are shown in Figure 1. On a SiO2 support, Raman and UV–vis spectroscopy revealed that Mo oxide is present as hydrated Mo7O246 clusters on the surface of the support. As illustrated in Figure 2, low Mo oxide coverage on Al2O3 (1% MoO3/ Al2O3) primarily gave rise to hydrated MoO42 isolated species (bands at 912, 846, and 320 cm1), while high Mo coverage (20% MoO3/Al2O3) favored hydrated Mo7O246 clusters (bands at 950, 846, 367, and 210 cm1). The same trend was followed on a ZrO2 support as for Al2O3 with also a trace amount of hydrated Mo8O264 present at high Mo oxide loadings being detected by UV–vis spectroscopy. Although XANES analysis can readily distinguish between MoO4 and MoO6 coordination, it is not able to readily distinguish between the two MoO6-coordinated Mo7O246 and Mo8O264 hydrated clusters. The presence of the different oxide anions was found to track the surface pH of the hydrated supported metal oxide catalysts, also known as PZC or isoelectric point (IEP), since the surface pH values of the oxide supports varied dramatically (SiO2 (PZC  2–4), ZrO2 (PZC  6), and Al2O3 (PZC 9)).89 A series of supported WOx catalysts were prepared similar to the supported MoOx catalysts.8 The spectra were collected after sample calcination and under ambient conditions. Aqueous tungsten oxide solutions contain WO42, HW6O215, and W12O396anions as well as several other species with their relative amounts dependent on the solution pH and W oxide concentration.87 On a SiO2 support, Raman and UV–vis spectroscopy revealed that W oxide is present as a combination of hydrated monotungstate WO42 isolated anions and hydrated polytungstate W12O396 clusters at all loadings. For the Al2O3 support, isolated WO42 hydrated anions are present at low loadings and primarily W12O396 hydrated clusters at high loadings. The ZrO2 support exhibited isolated WO42 hydrated anions at low W oxide loadings and HW6O215 hydrated clusters at high W oxide loadings. The hydrated W oxide anions were shown to follow the aqueous solution chemistry of W oxides and depend on their concentration and net pH at PZC of the catalyst system as shown in Figure 3 for the supported WO3/Al2O3 catalyst system.89,90 A series of supported V oxide catalysts were prepared in the same manner as the above examples and analyzed by UV–vis, Raman, and solid-state 51V NMR spectroscopy to determine the local VOx structures under ambient conditions.89,91,92 The

Ambient Conditions

Supported metal oxides have been found to contain about  20 layers of adsorbed moisture when exposed to the atmosphere after synthesis and calcination. The adsorbed moisture converts the molecular structure of the supported metal oxide phases to the same ionic species found in the aqueous precursor solutions.29,89 Recent work has examined several supported metal oxide systems under both ambient and dehydrated conditions using UV–vis, Raman, and XANES spectroscopy to determine the local molecular structures. A series of supported MoOx catalysts were prepared by IWI on various supports and calcined in air.6 After calcination, the catalysts were exposed to Monomolybdate

Heptamolybdate 2-

O

O O

Mo

O O O

Mo O O

O

Mo O O

MoO42-

Mo

O

O

O

O

O O

Mo

O

O

Mo

O O

Octamolybdate 6-

O O

O

O Mo

O Mo O

O

O

O O

O O O

Mo

O

O

O

Mo

Mo

O O

O

O

4-

O

Mo O O Mo

O O

Mo O

O

Mo

Mo7O246-

Figure 1 Structures of molybdenum oxide anions found in solution.

O

O

Mo O O

Mo8O264-

O O

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Monolayer Systems

1006 20% MoO3/Al2O3,

940 870

Dehydrated

210 377 590

Intensity (a.u.)

990

1% MoO3/Al2O3,

838 452

345

Dehydrated

950

20% MoO3/Al2O3,

846

Ambient

210 912

367 561

846

1% MoO3/Al2O3,

320

Ambient

Al2O3

1200

1000

800

600

400

200

Raman shift (cm-1) Figure 2 Raman spectra of supported MoO3/Al2O3 catalysts under ambient and dehydrated conditions as a function of surface molybdena coverage. Reproduced from Tian, H.; Roberts, C. A.; Wachs, I. E. J. Phys. Chem. C 2010, 114, 14110–14120.

7.06.4.3

Dehydrated Conditions

Upon heating to  300  C, the moisture layers present on supported metal oxide systems from ambient moisture become negligible and the hydrated metal oxide anions dehydrate and

0 W12O4110-1

-2

Log m W (VI)

vanadate species present in aqueous solutions are depicted in Figure 4. For a ZrO2 support, sub-monolayer coverage revealed the presence of hydrated VO2(OH)2 anions and monolayer coverage revealed primarily hydrated V10O286 anions. Submonolayer coverages on Al2O3 possessed V oxide as isolated VO3(OH)2, polymerized (VO3)nn, or V10O286 hydrated species, while monolayer coverage did not possess any isolated anions. Sub-monolayer and monolayer coverages on SiO2 possessed the same UV–vis edge energies corresponding to hydrated decavandate-like species while Raman could only detect one broad peak at sub-monolayer coverage related to decavandate-like anions. Solid-state 51V NMR revealed a chemical shift of 300 ppm for sub-monolayer coverage, which further confirms the decavandate-like species. These careful studies illustrate that aqueous chemistry plays an important role in supported metal oxide systems after calcination when samples are exposed to ambient conditions that contain moisture. A unique consequence of the aqueous layers present on metal oxide supports is that the oxide support can also dissolve into the aqueous surface layers, under specific pH conditions, to form new complexes. Under flowing H2O/O2 at room temperature, supported MoO3/SiO2 systems convert surface Mo7O246 anions to silicomolydic acid SiMo12O404 Keggin clusters. These Keggin clusters only form at room temperature conditions and thermally decompose as surface moisture is evaporated upon heating to  300  C. Thus, these unique metal oxide structures can only serve as catalytic active sites for catalytic reactions occurring below  300  C since they are not thermally stable at higher temperatures.93–95

W6O20(OH)5-

W12O396-3

10% 25% 5%

15% -4

WO42-

1%

-5 H2WO4 -6

1

2

3

4

5

6

7

8

pH Figure 3 Predominance diagram for aqueous W(VI) species (ionic strength ¼ 3 M; T ¼ 50  C). Lines correspond to conditions that produce equimolar amounts of the adjacent species. The ordinate is the common logarithm of the molality of the W(VI) species. Dashed trajectory added by the current authors based on the results of a previous investigation,90 where the key is as follows: 1% WO3/Al2O3 (○); 5% WO3/Al2O3 (□); 10% WO3/Al2O3 (D); 15% WO3/Al2O3 (*); and 25% WO3/Al2O3 (r). This figure is reprinted with permission of John Wiley & Sons, Inc. from Baes, C. F.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons, New York, 1976.

disperse on the oxide supports, typically anchoring to the support by titrating basic surface hydroxides and Lewis acid sites as schematically depicted in Figure 5 for supported MoOx catalysts.2 The dehydrated molecular structures of surface metal oxide species are very different from their hydrated counterparts. In order to accurately probe the dehydrated surface metal oxide phases, the use of in situ reactors and spectroscopy techniques is necessary.

138

Monolayer Systems

Orthovanadate

Metavanadate chains

Decavanadate

3-

O

O O

O O

V O

V O O

O

O

V

O

V

O

VO43-

O O

O

O

O

n-

V

n

O

O

(VO3)nn-

O O

6-

O

V

V

O

O

O O

O O O O O V V O O O V V V O O O O O O V V O O O V

V10O286-

Figure 4 Structures of vanadium oxide anions found in solution.

OH

H 2O OH

H2O

[Mo7O24]6-

H 2O

MoOx

OH

MoOx

MoOx

MoOx

MoOx

Dehydration

Oxide support

Oxide support

D

Figure 5 Schematic depicting dehydration and dispersion of hydrated molybdenum oxide anions on an oxide support.

In general, at low surface coverage Mo oxide systems possess isolated surface dioxido (O ¼)2MoO2 species on SiO2, Al2O3, and ZrO2 supports under dehydrated conditions (see Figure 6). Monolayer molybdena surface coverage on Al2O3 and ZrO2 favors the formation of both dehydrated monomolybdate species as well as dehydrated mono-oxo O¼MoO4 polymolybdate species (as depicted in Figure 6), according to in situ UV–vis and Raman spectroscopy.6 The inability to form polymolydate species on SiO2 also results in the formation of isolated mono-oxo O¼MoO4 as shown in Figure 6. The Raman spectra of the dehydrated supported 1% MoO3/Al2O3 (0.05% monolayer coverage) and 20% MoO3/Al2O3 (monolayer coverage) catalysts are given in Figure 2. For supported 1% MoO3/Al2O3, the band at 990 cm1 corresponds to the symmetric stretch of the isolated dioxo surface (O¼)2MoO2 species with the corresponding MoO4 bending mode occurring at 345 cm1 and the band at 838 cm1 associated with the bridging Mo–O–Al vibrations. For monolayer coverage-supported 20% MoO3/Al2O3, polymolybdate species 1 are confirmed by the bands at 590 and 210 cm that are associated with the symmetric stretch and bending modes of bridging Mo–O–Mo bonds, respectively. Also present are bands at 990, 940, and 377 cm1 related to the symmetric stretch, asymmetric stretch, and bending modes of the surface polymolybdate species, respectively. The Raman spectra of the dehydrated supported MoO3/Al2O3 catalysts in Figure 2 are quite different from their corresponding spectra under ambient conditions. Above monolayer surface coverage, crystalline MoO3 NPs also form, often blocking active sites.96 Sub-monolayer W oxide systems possessed isolated monooxo monotungstate O¼WO4 surface species on Al2O3 and ZrO2 supports and isolated dioxo monotungstate (O¼)2WO2 on SiO2 supports, as shown in Figure 7, according to in situ Raman and UV–vis spectroscopy. Monolayer coverage saw the transformation to a combination of mono- and polytungstate species, with the polytungstate species terminating in the monooxo fashion on all three supports.8 It has been determined in previous studies, by Raman spectroscopy, that exceeding monolayer coverage causes formation of crystalline WO3 NPs.97

Similarly, sub-monolayer coverages for VOx systems revealed the presence of isolated VO4 species for all three supports, while Al2O3 also possessed polymeric VO4 species as depicted in Figure 8. Monolayer coverage on ZrO2 formed isolated and polymeric VO4 species, Al2O3 formed polymeric VO4 and polymeric VO5/VO6 species, and SiO2 formed isolated VO4 species and V2O5 NPs.91 VOx in excess of monolayer coverage forms V2O5 NPs.98,74,99 For the supported MoOx, WOx, and VOx systems discussed, correlations between the UV–vis edge energy (Eg) and the number of metal–O–metal bonds were established, as demonstrated by Figure 9, allowing the determination of dehydrated local molecular structures from a simple Eg value. It was also shown for MoOx and WOx systems that a direct correlation between the Eg value and number of metal atoms in a metal cluster does not exist as has been claimed in the catalysis literature.6,8 A two part series by Lee and Wachs was recently published that details the different dehydrated molecular structures via Raman spectroscopy and 18O–16O isotopic exchange of groups 5–7 transition metal oxides (V2O5, Nb2O5, Ta2O5, CrO3, MoO3, WO3, and Re2O7) impregnated on SiO2.100,101 This study reveals that at sub-monolayer coverage V, Nb, and Ta prefer a mono-oxo O¼MO3 coordination, Cr, Mo, and W can be present as either dioxo (O ¼)2MO2 or mono-oxo O¼MoO4 coordination with their relative concentrations dependent on the oxide support surface properties, and Re is exclusively present as a tri-oxo coordinated (O ¼)3MO surface species. These papers illustrate that the thermodynamically stable phase of surface metal oxide species under dehydrated conditions not only depends on the oxide support, but on the nature of the impregnated metal oxide as well.

7.06.4.4

Reaction Conditions

Reaction environments at elevated temperatures can alter the surface metal oxide catalytic active site and, thus, it is necessary to perform characterization under relevant reaction conditions. A study on the CH4 oxidation to HCHO over supported MoOx/SiO2 revealed that the isolated surface (O ¼)2MoO2

Monolayer Systems

Dioxo O

O Mo

O

Mono-oxo polymolybdate O

MoO3 crystals

O

O

O Mo

O

Mono-oxo

Mo

Mo

O O O O O O

O OO O

Oxide support

139

corresponding to deposited coke appeared under reaction conditions, suggesting that the catalyst is deactivated through a coking mechanism. Switching from reaction conditions to oxidizing conditions was found to reoxidize the surface RhOx and burn off the deposited coke. It has been adequately demonstrated that, under reaction conditions, the surface of supported metal oxide catalysts is dynamic and the surface species present are dependent on the specific reaction environment.

7.06.4.4.1 Time-resolved operando molecular spectroscopy Figure 6 Structures of surface MoOx species found on metal oxide supports under dehydrated conditions.

Dioxo O

O W

O

Mono-oxo polytungstate O

W O

Mono-oxo

WO3 crystals

O

O

O

W

W

O O O O O O

O OO O

Oxide support

Figure 7 Structures of surface WOx species found on metal oxide supports under dehydrated conditions.

Mono-oxo poly VO4

Mono-oxo O V OOO

)

O

O

O V O

O

V O

V2O5 crystals

)

O

Oxide support

Figure 8 Structures of surface VOx species found on metal oxide supports under dehydrated conditions.

species are stable during reaction at 550  C, from in situ Raman spectroscopy experiments.102 On the other hand, the surface (O ¼)2MoO2 species on SiO2 were unstable during methanol oxidation at 230  C and converted to crystalline b-MoO3 NPs under the milder reaction conditions in the presence of the corrosive methanol environment.95 It is, thus, critical to determine the actual molecular structures present under reaction conditions in order to establish fundamental structure–activity relationships that can be used to molecularly design improved supported metal oxide catalysts. In situ spectroscopy under reaction environments can also provide information about the catalytic deactivation mechanism that may take place. A study on the autothermal reforming of propane at 1000  C, over supported RhOx/Al2O3 catalysts, revealed that the surface RhOx species are reduced under reaction conditions, in accordance with the observed in situ Raman spectra.29 Additionally, Raman peaks

Operando spectroscopy is a coined term indicating that catalyst characterization is performed under reaction conditions and is combined with simultaneous measurement of catalytic activity/selectivity.103 This cutting-edge technique provides timeresolved in situ spectral data of the dynamics of surface metal oxide phases present during realistic reaction conditions and provides time-resolved activity/selectivity data all in a single experiment. The unprecedented level of molecular insight gained from this type of experiment definitively establishes fundamental structure–activity relationships for working catalysts, advancing the field of catalysis science and (in)organic chemistry. For selective oxidation reactions, it is critical to determine if the source of oxygen involved in the rate-determining step (rds) is from dissociated surface Oads that is in equilibrium with gaseous O2 (Langmuir–Hinshelwood mechanism) or from the catalyst metal oxide lattice (Mars–van Krevelen mechanism).104 An operando spectroscopy experiment can distinguish between these two mechanistic scenarios through steady-state isotopic transient kinetic analysis (SSITKA). Presented in Figure 10 is an operando spectroscopy SSITKA study that was performed for propylene oxidation to acrolein over a supported V2O5/Nb2O5 catalyst, containing a monolayer of surface O¼VO3 species, at 300  C.105 It was found that switching the gas phase from 16O2 to 18O2 did not perturb the production of H2C¼CHCH16O, which continued in the presence of gaseous molecular 18O2. The absence of an isotopically labeled product, H2C¼CHCH18O, upon the initial isotopic switch in the mass spectrometer signal (Figure 10(a)) indicated that oxygen involved in the rds of acrolein formation came from the catalyst lattice (Mars–van Krevelen mechanism) which still contained 16O. Analyses of the operando Raman spectra (Figure 10(b)) that were collected simultaneously during the isotope exchange revealed that the surface O¼VO3 species retain their original 16O, which was determined by the absence of a shift in the Raman spectra from 16O¼VO3 (1036 cm1) to 18O¼VO3 ( 995 cm1). Additional insight was gained by complementing the operando Raman experiments with operando IR experiments performed in the temperature-programmed (TP) mode, both with and without the presence of gas-phase O2. The mass spectrometry data revealed that no acrolein was formed without the presence of gaseous O2. However, the corresponding operando IR spectra revealed that the same adsorbed surface allyl intermediate (CH2¼CHCH2*) was present both with and without gaseous O2. This suggested that the presence of O2 in the gas phase is necessary for reaction completion (Langmuir–Hinshelwood mechanism), but not for reaction intermediate formation. Additional experiments revealed that the gas-phase O2 is required to oxidize the surface H* (leftover

140

Monolayer Systems

Average no. of Mo–O–Mo bonds

5

4 XMo12O40

Clusters XMo8O26 cluster

Keggin cluster 3

XMo7O24 cluster

2

Polymeric chain

Alternating MoO4/MoO6 chain 1

Dimeric

Mo2O7 dimer

0

Monomeric

Isolated MoO4/MoO6

2.5

3.0

3.5

4.0

4.5

5.0

Edge energy, Eg (eV) Figure 9 Correlation between the UV–vis DRS edge energy (Eg) and number of bridging Mo–O–Mo bonds in bulk mixed-metal molybdates, ammonium salts, and aqueous solution molybdena anions present in the different Mo(VI) structures. Reproduced from Tian, H.; Roberts, C. A.; Wachs, I. E. J. Phys. Chem. C 2010, 114, 14110–14120.

from the allyl intermediate formation) to H2O, otherwise, the surface H* would hydrogenate the surface allyl intermediate back into propylene. This operando molecular spectroscopy study culminated in the first experimental observation of a combined Langmuir–Hinshelwood and Mars–van Krevelen reaction mechanism taking place. As can be appreciated from this study, operando molecular spectroscopy is extremely informative and can provide information about molecular structures, oxidation states, and reaction intermediates under realistic reaction conditions, limited only by the in situ capability of the characterization technique employed.

7.06.4.5

Influence of the Oxide Support

A significant amount of supported metal oxide catalysis literature is focused on screening various chemical components to find the most active combination of support and surface dopants, sometimes invoking the claim that particular oxide support phases enhance catalytic performance. The lack of systematic characterization in catalyst screening leads to many contradictions among catalysis scientists studying the ‘same’ materials. Thus, support effects will be discussed in the form of three critical parameters.

7.06.4.5.1 Influence of different oxide supports Enhanced catalytic activity with different oxide supports, for example, TiO2 versus SiO2, is a real effect, with the perfect marriage of oxide support properties and metal oxide surface phases often the goal of catalyst development. Oxide supports typically exhibit acidic, basic, or redox properties, and sometimes even their combinations (redox–acid, redox–base, or acid–base). The nature of many common metal oxide supports has previously been determined qualitatively and quantitatively (for methanol oxidation) through the use of methanol as a probe molecule.106 An intricate study by Deo and Wachs

determined the turnover frequency (TOF, the number of reactant molecules converted per metal atom per second) of the partial oxidation of methanol by supported VOx redox sites on various oxide supports.107 They found that the TOF varied by three orders of magnitude from CeO2 (100 s1) to SiO2 (103 s1). A similar TOF behavior, but inverse relationship, was found for methanol dehydration by supported WOx acid sites.97 The inverse relationship for surface VOx redox and WOx acid sites with the support is related to the different electronic requirements of redox (involving electron transfer) and acid (not involving electron transfer) catalyzed reactions. Additionally, for the supported V oxide catalysts the TOF was found to be independent of VOx loading in the sub-monolayer coverage region indicating that isolated and polymeric surface VOx species possess the same reactivity for redox reactions and the apparent activation energies were similar across all the supports (19.6  2.3 kcal mol1). Similar activation energies and a support-dependent TOF indicate that the nature of the specific oxide support controls the catalyst activity.

7.06.4.5.2 Influence of oxide support phase Another careful study by Deo et. al. examined the influence of different phases of TiO2 (anatase, rutile, brookite, and B-phase) for partial oxidation of methanol over supported VOx catalysts using XPS, in situ Raman and solid-state 51V NMR spectroscopy.108 The supports were synthesized and examined for impurities by XPS before VOx impregnation. Raman and 51 V NMR results indicated that the same dehydrated surface VO4 species were present on all supports irrespective of the bulk titania crystal structure. The TOF for the partial oxidation of methanol was found to be between 1.4 and 2.8 s1 and the selectivity to formaldehyde and methyl formate ranging from 92% to 96% for all supported VOx catalysts. Thus, since the catalytic properties were found to be virtually invariant of support phase, it was concluded that the oxide support phase

Monolayer Systems

C3H6 + 16O2

C3H6 + 18O2

C3H6 + 16O2

6

MS intensity (a.u.)

141

6.0 ´ 10 3.0 ´ 106 0.0 6.0 ´ 106 3.0 ´ 106 0.0 3.0 ´ 104

16

O2

18

O2

C3H416O

0.0 3.0 ´ 104

C3H418O

0.0 3.00 ´ 105 1.50 ´ 105 0.00 3.00 ´ 105 1.50 ´ 105

C16O2 C18O2

0.00 20

25

30

35

40

45

50

Time (min)

(a)

Operando raman spectra Transient isotopic 16O2–18O2 switching 300 ⬚C

Intensity (a.u.)

C. Switch back to 16O2

B. Switch to 18O2

1036

A. Flow 16O2

1200

1150

1100

(b)

1050

1000

950

900

850

800

Raman shift (cm-1)

Figure 10 Operando Raman-MS SSITKA experiment. (a) Operando MS signals for O2, H2C¼CHCHO, and CO2, with both 16O2 and 18O2, during the transient isotopic 18O2–16O2 switching experiment for steady-state propylene oxidation at 300  C. (b) Operando Raman spectra during the transient isotopic 18O2–16O2 switching experiment for steady-state propylene oxidation at 300  C. Reproduced from Zhao, C.; Wachs, I. E. J. Phys. Chem. C 2008, 112, 11363–11372.

does not influence TOF and that only impurities from support synthesis are responsible for minor variations in activity seen across similar oxide support phases.

7.06.4.5.3 Influence of oxide support dimension A recent study by Ross-Medgaarden et al. sought to determine the effect of TiO2 particle size on the catalytic properties and molecular structures of a series of 5% WO3/x% TiO2/SiO2 and 5% V2O5/x% TiO2/SiO2 catalysts.109 This systematic study utilized high-resolution-transmission electron microscopy (HR-TEM), in situ Raman and in situ UV–vis spectroscopies, in situ CH3OH-IR spectroscopy, and CH3OH-temperatureprogrammed surface reaction (TPSR) spectroscopy to determine the structure–activity relationship for TiO2 domains. TiO2 domains in the sub-10 nm range were synthesized on

an inert SiO2 support and further impregnated by IWI to form surface VOx or WOx species. The catalytic active mono-oxo surface redox O¼VO3 and acidic O¼WO4 sites were found to preferentially coordinate to the TiO2 domains on the SiO2 support. For the surface O¼VO3 redox sites, increasing the TiO2 domain size significantly enhanced catalytic activity by delocalization of the TiO2 electrons. For surface O¼WO4 acidic sites, decreasing the TiO2 domain size significantly enhanced catalytic activity by localization of the TiO2 electrons. For the first time, experimental evidence for the influence of metal oxide support dimensions on catalytic activity was obtained through careful systemic characterization. Thus, a structure–activity relationship between local electron density (varied through the use of TiO2 domain size) and catalytic activity was established.

142

Monolayer Systems

7.06.4.6

Mixed Oxide Supports

As previously discussed in Section 7.06.4.5.3, a series of 1– 60% TiO2/SiO2 mixed oxide supports were impregnated with 5% V2O5 or 5% WO3 to determine the influence of the TiO2 particle dimensions on catalytic properties and activity.109 This study revealed that surface VOx and WOx species prefer to anchor on the TiO2 particles rather than the SiO2 because of higher surface-free energy of the exposed TiOx sites. Similar studies on V2O5/Al2O3/SiO2 and V2O5/ZrO2/SiO2 concluded that the same surface redox VO4 species were present as in traditional supported catalysts and preferentially anchor to Al2O3 or ZrO2 instead of SiO2.76,77 These studies aptly demonstrate that thermodynamics governs the molecular surface structures present for mixed metal oxide supports.

7.06.5

Structure–Activity Relationships

The purpose of this section is not to provide an exhaustive review of each topic, but rather to highlight important scientific progress made during the past decade in developing catalytic structure–activity relationships through the use of in situ techniques and selected ex situ experiments.

7.06.5.1

Redox Catalysis

7.06.5.1.1 SO2 oxidation to SO3 Depending on the manufacturing process, the oxidation of SO2 to SO3 is either an unwanted side reaction or a necessary intermediate reaction on the route to final products. Sulfuric acid is one of the largest volume chemicals currently produced in the United States, with  71.5 billion pounds (32.5 million metric tons) produced in 2010.110 High-temperature (450–610  C) sulfur dioxide oxidation to sulfur trioxide, over a supported liquid-phase vanadium catalyst, is a critical intermediate step in the production of sulfuric acid. By contrast, SO2 oxidation to SO3 is an undesirable side reaction during the SCR of environmentally harmful nitrogen oxides (NOx) in power plant flue gas. First, SO3 is an acid rain component. Second, SO3 can combine with ammonia not consumed in the reactor at temperatures below 250  C to form ammonium sulfates that block the SCR catalyst’s pores and foul downstream heat exchangers.12 Thus, fundamental research on SO2 oxidation is widely applicable and desirable in order to either produce more value-added sulfur-containing chemicals or reduce unwanted side reactions and the emission of toxic sulfur oxides (SOx). A series of supported V2O5 binary catalysts (V2O5/MxOy) and ternary catalysts (V2O5–MxOy/TiO2) were investigated by Dunn et al. for the oxidation of SO2 to SO3 using a combination of in situ spectroscopy and kinetic analysis.111–114 For binary systems of the type V2O5/MxOy (M ¼ Si, Al, Ti, Zr, and Ce), the surface vanadia species exist as isolated mono-oxo (M–O)3Vþ5¼O surface species at low vanadia loading and tend to polymerize at high vanadia loadings. Polymerization proceeds by breaking the V–O–M support bonds to form bridging V–O–V bonds. The TOF values for supported V2O5/ TiO2 catalysts were found to be independent of the surface vanadia loading, suggesting that only one surface vanadia

catalytic active site is required for the oxidation of SO2 to SO3 (a two-electron reaction). By varying the oxide support, the TOF for SO2 oxidation of the supported vanadia catalysts can be changed by an order of magnitude (Ce > Zr  Ti > Al > Si), suggesting that the bridging V–O-support bond is important for oxygen insertion into SO2 during the rate-determining-step of SO2 oxidation to SO3. Supported vanadia ternary systems, V2O5–MxOy/TiO2 (M ¼ Re, Cr, Nb, Mo, W, and Fe), were also investigated in the same manner as the binary systems to determine the origin of synergistic effects, if any, between the additional surface metal oxide and VO4 species on the TiO2 support. In situ Raman spectroscopy revealed that only minor structural interactions occurred between the surface VO4 species and the other surface metal oxides. The corresponding catalytic activity confirmed the absence of synergistic interactions between the surface metal oxide species since the overall activity was just the sum of the contributions of the individual surface metal oxides. The absence of synergistic effects between the surface metal oxide sites is a consequence of the SO2 oxidation reaction proceeding over only a single catalytic active site and is a general phenomenon for two-electron reactions. A structure–activity relationship was established for SO2 oxidation to SO3 over supported vanadia catalysts by combining systematic in situ characterization studies with kinetic analysis. SO2 oxidation over supported vanadium oxide catalysts was found to occur over a single catalytic active site, represented by the bridging V–O–M bond, and the TOF can be tuned by varying the oxide support ligand (M). Thus, the SO2 reaction redox mechanism involves the adsorption of acidic SO2(g) onto the somewhat basic bridging V–O–M oxygen of the surface (M–O)3Vþ5¼O species, followed by cleavage of the Vþ5–O–SO2 to form SO3(g). The vanadium oxide becomes reduced to Vþ3 and is rapidly reoxidized to surface Vþ5 species by atomic oxygen formed by dissociative adsorption of molecular O2.

7.06.5.1.2 Propane oxidative dehydrogenation to propylene Vanadium oxide catalysts have also been extensively examined for the partial oxidation of light alkanes to their valuable olefin counterparts. Of particular interest is propane oxidative dehydrogenation (ODH) to propylene, the second largest feedstock, after ethylene, for the petrochemical industry. In situ spectroscopy experiments performed in the last decade have provided a wealth of new information about the catalyst molecular structure under reaction conditions. A quantitative method for determination of the fraction of monomeric and polymeric surface VO4 species that are present in vanadate materials was developed based on UV–Vis spectroscopy Eg values since Eg linearly decreases with extent of VO4 polymerization.115 Catalytic TOF values for propane ODH to propylene were found to be independent of the extent of polymerization of surface VO4 species, Eg values, the surface Brnsted acidity, and the reducibility of the surface vanadia species, indicating that the reaction proceeds on a single catalytic active site . The TOF varied by over an order of magnitude (ZrO2 > Al2O3 > SiO2) upon varying the oxide support suggesting that the bridging V–O-support oxygen is the catalytic active oxygen involved in the propane ODH reaction. The TOF dependence on the specific oxide support, for both isolated

Monolayer Systems

100

V2O5/ZrO2

80

TOF (´10-4 s-1)

(low coverage) and polymerized (high coverage) surface VO4 species, and its independence on the extent of polymerization of surface VO4 species are shown in Figure 11.116 The fundamental structure–activity relationships for propane ODH by supported VO4/Al2O3 catalysts were further investigated with a detailed operando Raman–gas chromatography (GC) spectroscopy study by Cortez and Ban˜ares.117 The effect of the O2/C3H8 ratio was examined on a supported VO4/ Al2O3 catalyst with a surface vanadia coverage corresponding to 0.5-monolayer. Varying the O2/C3H8 ratio had only a minor effect on propane conversion and no effect on propylene selectivity. The corresponding Raman spectroscopy findings indicated that polymeric surface VO4 species are more easily reduced than isolated surface VO4 species. The presence of partially reduced surface VOx species under reaction conditions was independently confirmed by in situ EPR/UV–vis spectroscopy.118–120 Furthermore, the disappearance of the terminal V¼O bonds in the Raman spectra did not correspond to changes in catalytic performance suggesting that the terminal V¼O oxygen is not involved in the kinetic rate-determining step during propane ODH.118–120 The influence of synthesis method upon the structure and performance of supported vanadia catalysts on Al2O3, TiO2, and SiO2 oxide supports was recently investigated with three different vanadium precursors: ammonium metavanadate [NH4VO3], vanadium triisopropoxide [VO(OCH(CH3)2)3], and vanadyl acetylacetonate [VO(C5H7O2)2].121 In situ Raman spectroscopic characterization revealed that the vanadium triisopropoxide and vanadyl acetylacetonate precursors resulted in only isolated and polymeric surface VOx species below monolayer coverage, and that crystalline V2O5 NPs only formed above monolayer coverage. By contrast, the ammonium metavanadate [NH4VO3] precursor was found to form crystalline V2O5 NPs in addition to isolated and polymeric surface VO4 species. The propane ODH TOF values for the catalysts prepared with vanadium triisopropoxide or vanadyl acetylacetonate, in which crystalline V2O5 NPs were not present in the sub-monolayer region, were constant with vanadia loadings up to monolayer coverage. The constant TOF value with surface VO4 coverage is a consequence of only a single catalytic active site for the propane ODH reaction that is a two-electron reaction. The catalysts prepared with ammonium metavanadate that contained V2O5 NPs and surface VO4 species in the sub-monolayer region, however, displayed increasing TOF values with vanadia loading below monolayer coverage. In situ Raman spectroscopy revealed that the only difference between the catalysts prepared with ammonium metavanadate and vanadium triisopropoxide or vanadyl acetylacetonate was the co-presence of crystalline V2O5 NPs in the catalysts synthesized with ammonium metavanadate. These careful kinetic studies reveal that the crystalline V2O5 NPs formed in the sub-monolayer region are actually more active than the surface VO4 species for propane ODH. Note that the active V2O5 NPs formed below monolayer coverage should not be confused with large crystalline V2O5 particles that have been shown to be less active for oxidation reactions.107 A structure–activity relationship was established for propane ODH to propylene over supported vanadia catalysts by combining systematic in situ characterization studies with kinetic analysis. Propane ODH over supported vanadium oxide

143

60

40

V2O5/Al2O3

20 V2O5/SiO2 0 0

20

40

60

80

100

Fraction of polymeric surface VO4 Figure 11 Propane ODH TOF as a function of fraction of polymeric surface VO4 species for dehydrated supported V2O5/SiO2, V2O5/Al2O3, and V2O5/ZrO2 catalysts. Reproduced from Tian, H.; Ross, E. I.; Wachs, I. E. J. Phys. Chem. B 2006, 110, 9593–9600.

catalysts was found to occur over a single catalytic active site, represented by the bridging V–O–M bond, and the TOF can be tuned by varying the oxide support ligand (M). Thus, the propane ODH reaction redox mechanism involves the reaction of propane at the somewhat basic bridging V–O–M oxygen of the surface (M–O)3Vþ5¼O species. The surface vanadium oxide becomes reduced by giving up one of its oxygen atoms to the two hydrogen atoms stripped from propane during the ODH reaction. The reduced surface VOx species are rapidly reoxidized to surface Vþ5 species by atomic oxygen formed by dissociative adsorption of molecular O2.

7.06.5.2

Acid Catalysis

7.06.5.2.1 Methanol dehydration to dimethyl ether Production of dimethyl ether (DME) has received significant attention in the recent decade due to its potential as an alternative fuel and means of reducing the dependency on petroleum.122 Though a portion of the recent literature focuses on zeolite or polyoxometalate catalysts, these systems are not discussed since appropriate chapters exist for each of these specific materials (Chapters 7.11 and 7.08). The solid acidity of supported tungsten oxide catalysts has found numerous applications in industrial reactions such as HDS, hydrocarbon cracking, and SCR of NOx with NH3 to form N2.123 The application of supported tungsten oxide catalysts for methanol dehydration has been recently investigated by several research groups.123–126 A series of supported WO3 catalysts were prepared by IWI of ammonium metatungstate as a function of tungsten oxide loading on several precalcined oxide supports (Al2O3, TiO2, Nb2O5, and ZrO2) and investigated for methanol dehydration to DME. The catalysts were physically characterized with in situ Raman spectroscopy, in situ UV–Vis spectroscopy, and HRTEM, and chemically probed with CH3OH-TPSR and steadystate CH3OH dehydration reactions.97,123 Monolayer tungsten

144

Monolayer Systems

oxide coverage was found to be 4.5 W nm2 and the corresponding dehydrated surface WOx species on all supports, as a function of loading, were found to be: surface monotungstate WO4 dioxo species (4.5 W nm2), surface polytungstate WO5/WO6 mono-oxo species (<4.5 W nm2), WO3 NPs (4.5–9 W nm2), and large bulk-like WO3 particles (>9 W nm2). The UV–vis Eg values of all catalysts were found to decrease monotonically with increasing WOx loading, indicating greater electron delocalization for the larger tungsten oxide domains. The specific acidity of all catalysts studied, however, was not found to correlate to either the WOx molecular structures or electronic structures. Instead, the relative catalytic acidity was determined to be a strong function of the specific oxide support. For supported WO3/ Al2O3 catalysts, the surface WOx species were more active acid catalysts than crystalline WO3 NPs. For the remaining supports studied, however, the WO3 NPs were more active than the surface WOx species. This experimental observation reflects the important effect of the oxide support on the relative acidic activity of the various surface WOx species and WO3 NPs. Thus, no general structure–activity relationship exists between the tungsten oxide molecular/electronic structures and catalytic acidic activity because the oxide support ligand dominates the surface acidity properties. It is well documented in the literature that the acidity of supported WO3/ZrO2 catalysts strongly depends on the synthesis method employed, especially the nature of the ZrO2 support.127 Thus, two series of supported WO3/ZrO2 catalysts were synthesized: one with a precalcined crystalline ZrO2 support and the second with an uncalcined amorphous oxyhydroxide ZrOH support.126 Both series of catalysts varied in tungsten loading and calcination temperature and were characterized by x-ray diffraction (XRD), XPS, TEM, in situ Raman, and in situ UV–vis spectroscopy to determine the molecular and electronic tungsten oxide structures present. The systematic side-by-side comparison of the WO3/ZrO2 and WO3/ ZrOH catalysts revealed that the WO3/ZrOH catalyst series also contained Zr-stabilized distorted WO3 NPs (1 nm) that were not present in the precalcined WO3/ZrO2 catalyst series that only possessed crystalline WO3 NPs. Corresponding methanol dehydration kinetic studies demonstrated that the poorly ordered Zr–WO3 NPs are responsible for the enhanced acidic activity of the supported WO3/ZrOH catalyst series. Thus, establishing the structure–activity relationships for supported WO3 on zirconia catalyst systems. A series of supported WO3/ZrO2 catalysts were also prepared by thermal spreading of WO3 powder onto a precalcined ZrO2 support, at different calcination temperatures, and examined for methanol dehydration. The prepared catalysts were physically characterized with XRD and UV–vis spectroscopy, and chemically probed with in situ IR spectroscopy of adsorbed CO2 and pyridine, and steady-state CH3OH dehydration.125 The UV–vis spectroscopy physical characterization studies, however, were performed under ambient conditions that are known to contain hydrated molecular structures different from the final dehydrated active WOx state.8 Nevertheless, it was concluded that multiple surface tungsten oxide species were present on the ZrO2 support as isolated WOx surface species, polytungstate surface species, and WO3 NPs. The corresponding methanol dehydration kinetic studies concluded that the

polymeric surface WOx species and WO3 NPs are more active than the isolated surface WOx species on crystalline ZrO2. From the IR-pyridine adsorption studies, it was also inferred that the catalyst activity is ‘probably’ related to the presence of Bro¨nsted acid sites. It was not clear from this study whether it was the polymeric surface species or the WO3 NPs that were the superacidic catalytic active sites because critical characterization studies of the supported tungsten oxide phase were lacking (e.g., in situ Raman spectroscopy). Although the structure–activity relationship for precalcinedsupported WO3/ZrO2 catalysts for methanol dehydration is not straightforward because of the significant influence of the specific oxide support ligand, it is clear that isolated surface WOx sites are less active than polymeric WOx sites and crystalline WO3 NPs. The use of an uncalcined zirconia oxyhydroxide ZrOH support, however, also forms a new type of tungsten oxide acid site consisting of highly distorted Zr–WO3 NPs that act as superacids.

7.06.5.2.2 n-Pentane isomerization Alkane isomerization provides branched high-octane number alkanes for use in gasoline, as well as other fuels. Recently, research has shifted focus to environmentally friendly solid acid catalysts as substitutes for liquid acids that evaporate to cause toxic emissions. Tungstated zirconia (WO3/ZOH) catalysts have received much attention due to their production of desired products, strong acidic nature, and superior structural stability under oxidizing and reducing conditions.128 Several models for the origin of the superacid catalytic active sites for WO3/ZOH catalysts have appeared in the literature: surface WOx monolayer,129 polyoxometalate Keggin (H3ZrW12O40 clusters), conjugate Bro¨nsted–Lewis sites,130,131 and distorted Zr–WO3 NPs.132 The maximum in catalytic isomerization performance per all the W atoms in the catalyst (TOR: turnover rate) was always found to occur slightly above monolayer coverage (>4.5 W nm2) suggesting the critical participation of crystalline WO3 NPs,97 crystalline H3ZrW12O40 Keggins,128,133–136or distorted Zr–WO3 NPs.132 Crystalline WO3 can be ruled out as the catalytic active superacid sites since the presence of crystalline WO3 NPs in precalcined WO3/ ZrO2 catalysts does not contribute to superacidity.97,126 The crystalline H3ZrW12O40 Keggins are not thermally stable above  400  C and, thus, cannot survive the high calcination temperatures of 700–1000  C employed to activate the WO3/ ZrOH catalysts.126,137 In situ Raman spectroscopy and aberration-corrected HAADF-STEM directly observed the presence of the distorted Zr–WO3 NPs (<1 nm) in the superacidic WO3/ZrOH catalysts.126,138 It was further demonstrated that co-impregnation of ammonium metatungstate and ZrOH onto a precalcined catalyst increases the activity for acidcatalyzed reactions.138 Thus, the origin of the superacidity for WO3/ZrOH catalysts is the presence of distorted Zr-WO3 NPs that only form above monolayer coverage and in the presence of uncalcined ZrOH precursors.

7.06.5.3

Bifunctional Redox–Acid Catalysts

7.06.5.3.1 n-Butane oxidation to maleic anhydride Since the 1970s, bulk vanadium–phosphorus oxides (VPOs) have been used industrially for the selective oxidation of

Monolayer Systems

n-butane to maleic anhydride. An extensive amount of research on the nature of the active catalyst has been performed and correlations of catalytic activity and crystalline structure have been previously reviewed.139,140 Despite extensive research, a structure–activity relationship for the catalyst under reaction conditions has yet to be established. The focus here is on the research performed over the last  15 years that utilized in situ spectroscopic techniques to elucidate new structure–activity information about the VPO catalysts under reaction conditions. Early characterization studies of unsupported V–P–O catalysts demonstrated that the stable bulk phase consisted of crystalline vanadyl pyrophosphate (VO)2P2O7, the equilibrated phase, and assumed that the exposed catalytic active surface was just a truncation of vanadyl pyrophosphate (VO)2P2O7 crystal planes.139 Detailed characterization studies, however, revealed that the VOHPO40.5H2O precursor gradually transforms to (VO)2P2O7 and contains an amorphous phase at the surface that encapsulates the crystalline (VO)2P2O7 phase.141–143 Extensive crystallization of this amorphous phase requires about a month under reaction conditions and is responsible for the slow activation of the (VO)2P2O7 catalyst in industrial operation as shown by the data in Figure 12. Furthermore, the P/V ratio of the amorphous phase is much greater than for bulk (VO)2P2O7 as it also contains surface Vþ5 species since (VO)2P2O7 only possesses Vþ4 sites.144 In situ time-resolved XAS and mass spectrometry experiments were performed to determine the kinetic significance of Vþ5.145 By plotting the initial decay of Vþ5 > Vþ4 XANES signal with the initial production of maleic anhydride it was

70

1.2

65

1

Selectivity (mol.%)

0.8

55

0.6

50 45

0.4

40

I(921 cm-1), arbitrary units (200) FWHM, deg. 2Q

60

0.2 35 30

0

5

10

15

20

0 25

Time (days) Selectivity

I(921 cm-1)

(200) FWHM

Figure 12 Evolution of the bulk crystallinity and selectivity of vanadyl pyrophosphate catalyst to maleic anhydride (mol.%) with time under catalytic conditions at 653 K in 1.2% n-butane-air feed at 25 mol.% n-butane conversion. The Raman peak at 921 cm1 is the P–O–P asymmetric stretch of vanadyl pyrophosphate. The ‘(200) FWHM’ plot corresponds to the full width at half maximum (FWHM) of the (200) plane basal faces of vanadyl pyrophosphate in XRD. Reproduced from Guliants, V. V.; Benziger, J. B.; Sundaresan, S.; Yao, N.; Wachs, I. E. Catal. Lett. 1995, 32, 379–386.

145

shown that Vþ5 is intimately involved and responsible for the n-butane oxidation reaction. In situ XAS and in situ XPS were subsequently performed to investigate surface electronic properties under reaction conditions.146,147 The variation of XAS used, V L3 near-edge x-ray absorption fine structure (NEXAFS), is actually surface sensitive due to the soft energy range the technique utilizes, with a probing depth estimated to be 4 nm. Their in situ NEXAFS results, through spectral peak deconvolution, revealed that an unidentifiable phase other than vanadyl pyrophosphate is present on the surface of the catalyst and is a key phase involved in the catalytic active site. Analysis of the equilibrated (VO)2P2O7 catalyst with in situ EPR, XRD, and Raman spectroscopy similarly concluded that the (VO)2P2O7 phase becomes more crystalline over time, but a small amount of structural disorder always remains.148 It was proposed that some amount of structural disorder provides higher catalytic activity and selectivity and gives rise to a more flexible (VO)2P2O7 crystal structure during thermally induced lattice vibrations. This flexibility helps facilitate the transport of oxygen from the bulk to the surface of the catalyst. It was also observed that the surface vanadyl oxidation state can reversibly fluctuate between þ4 and þ5 (redox couple) under reaction conditions. Similarly, in situ environmental high-resolution electron microscopy (EHREM) was used to observe the catalyst defects under different environments.149 Under a butane/air environment, it was found that the VPO crystal is continuously changing to the (201) stable configuration and that reduction of the surface leads to surface restructuring via the formation of (201) glide shear defects. These defects accommodate the strain between anion point defects (from reduction) and the underlying bulk VPO. Thus, the dynamic formation of glide shear defects prevents the collapse of the bulk VPO lattice during reduction. The exact molecular arrangement of the catalytic active surface V–P–O phase, however, is still to be determined. In order to obtain additional fundamental insights into the reaction mechanisms of n-butane oxidation over the amorphous V–P–O phase, a series of well-defined modelsupported VOx catalysts on several oxide supports (SiO2, Al2O3, Nb2O5, ZrO2, CeO2, and TiO2) were synthesized by IWI and characterized by in situ Raman150 and in situ UV–vis spectroscopy.151 In situ Raman spectroscopy revealed that the bridging oxygen, V–O–P or V–O–support, controls the ratedetermining step for this selective oxidation reaction, while the terminal V¼O and polymeric V–O–V bonds did not influence the n-butane oxidation TOF or selectivity. By creating isolated VOx/TiO2/SiO2 sites, it was demonstrated that the oxidation reaction can occur over single surface vanadia sites, but catalysts possessing (two or more) adjacent surface vanadia sites are more efficient. In situ UV–vis spectroscopy revealed that only a small fraction of surface Vþ5 sites (less than 10%) were reduced to Vþ4/Vþ3 under steady-state reaction conditions and the extent of reduction was a function of the specific oxide support: ZrO2 > Al2O3 > SiO2. The maleic anhydride selectivity was also affected by the oxide support ligand: alumina > niobia > titania > zirconia > ceria. The selectivity trend also parallels the Lewis acid strength of the oxide supports: alumina > niobia > titania  zirconia.106,152

146

Monolayer Systems

In fact, by adding surface niobia Lewis acid sites (6% Nb2O5/l % V2O5/TiO2), it was found that the n-butane oxidation TOF increased by a factor of 4 and the maleic anhydride selectivity saw a 1.5  increase. The addition of surface tungsten oxide Brnsted acid sites (6% WO3/l% V2O5/TiO2) resulted in a n-butane oxidation TOF increase by a factor of 3 and no significant improvement in selectivity. These fundamental investigations have revealed the bifunctional requirement of redox–acid sites for n-butane selective oxidation to maleic anhydride and the ability of surface Vþ5 species to participate in this reaction. In summary, the recent catalytic studies have strengthened the understanding of how n-butane oxidation to maleic anhydride proceeds over VPO catalysts. It is now accepted that the vanadyl pyrophosphate (VO)2P2O7 phase serves as an active support to a still not fully understood surface phase. The active surface phase contains Vþ5 species that reduce to Vþ4 during the n-butane oxidation reaction. Cutting-edge operando spectroscopy experiments are poised to provide additional valuable insights into the dynamic nature of the V–P–O catalytic active surface phase in the years to come.

7.06.5.3.2 SCR of NO with NH3 to N2 Introduced in the late 1970s, the SCR of environmentally undesirable NOx by NH3 to environmentally benign N2 and H2O is one of the most important environmental catalytic processes for reducing environmentally undesirable emissions generated during the burning of fuel. The SCR of NOx is still recognized as the best method for NOx removal (DeNOx) from stationary sources.153 The majority of NOx produced from coal-fired power plants is in the form of NO (400–700 ppm) which is typically converted to its benign counterparts over supported V2O5–WO3/TiO2 (1% vanadia and 7% tungsta) catalysts.154 Whereas the vanadia component provides the redox function, tungsten oxide provides the acid function as well as being a promoter that increases the reaction rate and prevents catalyst sintering of the TiO2 support. It is well known that gas-phase NH3 adsorbs on the catalyst surface forming both NH3* on Lewis acid sites and NH4þ* on Brnsted acid sites. Despite over 30 years of research, the basic understanding of the relative contributions of the surface Lewis and Brnsted acid sites (NH3* vs. NH4þ*) for the SCR reaction is still debated in the literature. Several studies employed in situ IR spectroscopy with online activity measurements in order to study ammonia adsorption on Lewis acid sites (NH3*) and Brnsted acid sites (NH4þ*).155,15,156,157 The results suggested that ammonia adsorption on Lewis sites is stronger than on Brnsted sites. Thus, it was concluded that the ammonia adsorbed onto Brnsted acid sites is the active species during SCR of NOx. The influence of surface vanadia coverage, promoters (W, Nb, and sulfates), and various oxide supports (TiO2, Al2O3, and SiO2) on the SCR of NO with NH3 was further investigated with in situ Raman spectroscopy, in situ IR spectroscopy of pyridine adsorption, and steady-state SCR reactivity experiments.16 The SCR TOF was found to be dependent on the vanadia loading or the addition of promoters, suggesting that the reaction proceeds via a dual-site redox–acid mechanism. The TOF was also found to be

dependent on the specific oxide support, varying by a factor of 3 (TiO2 > SiO2  Al2O3). The TOF support dependence along with the observed stability of the terminal V¼O18 bonds during SCR suggests that the V–O-support bond is involved in the rate-determining step. Furthermore, it was concluded that surface NH4þ* species on Brnsted acid sites are more active than surface NH3* species during SCR of NO with NH3 since the surface Brnsted acid sites introduced by the surface WOx promoter were more effective than the surface Lewis acid sites introduced by the surface NbOx promoter. The effect of multiple promoters on supported V2O5/TiO2 catalysts, in the presence of H2O and SO2 (typically present in flue gas streams), upon SCR activity was investigated with the aid of in situ Raman spectroscopy during the SCR reaction and in situ IR spectroscopy for determining the total number of surface Brnsted and Lewis acid sites.17,18 The findings demonstrated that surface Brnsted acid sites are more active than Lewis acid sites for the SCR reaction and that the SCR activity correlates with the total number of Brnsted acid sites (WO3 > MoO3 > Nb2O5 > GeO2 > V2O5 > Fe2O3 > CeO2 > MnO2 > SnO2 > Ga2O3 > La2O3 > ZnO). By contrast, several researchers have proposed that surface NH3* species on Lewis acid sites are the active species for the SCR of NOx. These new findings are in contrast to earlier studies that proposed that the Lewis acid sites were the catalytic active sites.158–161,14,162,163 A major limitation of the earlier in situ IR spectroscopy experiments is that they were performed under vacuum conditions that seemed to not reflect the conditions under realistic SCR reaction conditions. In summary, the SCR of NO by NH3 over supported V2O5– WO3/TiO2 catalysts proceeds via dual redox–acid sites with ammonia adsorbing on both Lewis and Brnsted acid sites as NH3* and NH4þ* species with the NH4þ* species more active than the NH3* species. The adsorbed NH4þ* species preferentially reside on the surface Brnsted acid sites created by the surface WOx species. The function of the surface VO4 sites is to perform the redox reaction between the NH4þ* species and the NO* species to yield N2 and H2O. The oxygen in the bridging V–O–Ti bond is involved in the rate-determining step since the TOF values are strongly dependent on the specific oxide support.

7.06.6 Presence of Surface Metal Oxides in Other Mixed Oxide Systems Mixed oxide compounds are utilized in many disciplines including ceramics, semiconductors, piezoelectronics, electrolysis, and catalysis. Despite their extensive application, detailed surface characterization of mixed oxide materials is scarce and the surfaces of mixed oxides are often assumed to just be truncations of their bulk structure. Since little is known about the outermost surface composition, proposed catalytic active sites and surface reaction mechanisms are discussed in the literature based on bulk mixed oxide structures. This method has been questioned by several recent studies that report TiO2-rich overlayers on reconstructed SrTiO3(0 0 1) model surfaces,164 formation of amorphous oxide overlayers in which there exists surface enrichment under selective

Monolayer Systems oxidation reaction conditions,143,147 and surface enriched VOx and MoOx layers, about one atomic layer thick, on bulk mixed oxides such as ZrV2O7, Ce8Mo12O49, and a-Bi2Mo3O12.165 The lack of detailed knowledge of the surface composition and structure of bulk mixed oxide systems hampers the development of structure–activity relationships and the design of advanced catalytic materials. Characterization techniques that provide true surface composition information are as limited as the number of publications that address such an issue. XPS from laboratory sources has an average sampling depth of  1–3 nm, which results in the outermost surface layer contributing only  30% of the total signal. XPS from synchrotron radiation sources allows for increased surface sensitivity ( 1 nm) by decreasing excitation and photoelectron kinetic energies. True surface composition, however, is only obtained through low-energy ion scattering (LEIS) because ions penetrating below the outermost surface layer become largely neutralized.166 Recent technological advances have increased the LEIS sensitivity by  3000 , termed high-sensitivity low-energy ion scattering (HS-LEIS), which exhibits a detection limit on the order of several ppm and sputter damage from static collection of only 0.2% of a monolayer.167 This quantitative technique is equipped to push the boundaries of our fundamental understanding of the surfaces of bulk mixed oxide materials. Merzlikin et al. recently utilized LEIS, laboratory grade XPS (Leybold surface analysis system), and synchrotron-based XPS (HESGM beamline of BESSYII, Germany) techniques to demonstrate that surface enriched overlayers exist on some bulk mixed oxides.165 Shown in Figure 13, results of LEIS sputter series on bulk mixed ZrV2O7 and supported V2O5/ZrO2 indicated similar surface enrichment on both samples, though only the supported sample was synthesized to have surface

147

VOx enrichment by impregnation of a ZrO2 support with a V-precursor. The asymptotic nature of the Zr/V ratio for bulk ZrV2O7 (Figure 13(b)) indicates that the surface is initially V enriched before reaching the bulk Zr/V ratio of 0.4. Similarly, the low initial Zr/V ratio for supported V2O5/ZrO2 (Figure 13(d)) also indicates that V is surface enriched. The VOx surface layers were also detected by synchrotron-based XPS, but not readily revealed by laboratory grade XPS. By contrast, Merzlikin et al. also reported that some bulk mixed oxides, such as CoMoO4, did not contain surface-enriched phases as determined by all three discussed techniques. Thus, surface-enriched overlayers on bulk mixed oxides is not quite a general phenomenon, but should still be taken into consideration when discussing surface reaction mechanisms. A recent critical review demonstrated the generality of surface vanadium oxide phases in all types of mixed oxide catalysts.7 Over 30 years of research on vanadium-containing molecular sieves, zeolites, clays, hydrotalcites, stoichiometric bulk oxides, and bulk solid solutions revealed that these mixed oxides exclusively contain surface-enriched VOx species. Reactivity studies have shown that surface VOx sites are the catalytic active sites for all V-containing mixed oxide systems investigated thus far. The combination of the low surface free-energy of vanadium oxide and its low Tammann temperature (the temperature at which surface diffusion begins) is responsible for the universal presence of surface VOx sites on all mixed oxide systems. The culmination of decades of extensive characterization has resulted in a paradigm shift in our fundamental understanding of catalysis by mixed oxides and demonstrated that surface metal oxide phases are pervasive in mixed oxide materials, representing the catalytic active sites for chemical reactions. Table 1 highlights well-studied metal oxide systems that have been shown to possess a unique surface

0.6 V2O5 / ZrO2 Izr / Iv

ZrV2O7 200

0.4

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Figure 13 LEIS sputter series and intensity trends measured with Zr/V mixed oxides. (a, b) ZrV2O7, E0 ¼ 1000 eV, (c, d) 4 wt% V2O5/ZrO2 (V content (7.5 atoms nm2) near the theoretical monolayer limit), E0 ¼ 1000 eV. Reproduced from Merzlikin, S. V.; Tolkachev, N. N.; Briand, L. E.; Strunskus, T.; Wo¨ll, C.; Wachs, I. E.; Gru¨nert, W. Angew. Chem. Int. Ed. 2010, 49, 8037–8041.

148

Monolayer Systems

Table 1

";Chronology of metal oxide systems with unique surface oxide layers

Year

Metal oxide system

1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994

MoO3168; supported WO3169,170 V2O5171 Re2O7172,174; supported CrO3173

Supported Supported Supported – – – – Supported PdO175 – – Supported NiO176 Supported CeO2177; supported Nb2O5178; supported Sb2O3179 Supported La2O3180; supported PtO181 – Supported Rh2O3182; supported IrO2182; supported Ga2O3188 Supported SO3183; supported P2O5183; supported Fe2O34; supported TiO2; supported Eu2O3185 – Supported MnO186

Year

Metal oxide system

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Bulk (VO)2P2O7143,147 Supported CoO5 – Supported SnO2187 Supported K2O113; Supported Ta2O5189; Supported ZrO276 – – Bulk MoVWOx195; bulk SrTiO3(001)164 – – Solid solution CoxNi1xO190 –

2007 2008 2009

– – –

2010

Bulk a-Bi2Mo2O12165; bulk a-Bi2Mo3O12165; bulk g(H)-Bi2MoO6165; bulk Ce8Mo12O49165; bulk Fe2(MoO4)3165; bulk ZrV2O7165 Supported CeO2 (more detailed study)194; bulk FeVO4191 Bulk VxFe1xSbO4193

2011 2012

Note: Chronology is mainly based (but not exclusively) on the first analysis of the metal oxide system by Raman spectroscopy, as it was in the original table. Source: Adapted and expanded from Wachs, I. E. In Raman Spectroscopy of Catalysts; Lewis, I. R., Edwards, H. G. M., Eds.; Handbook of Raman Spectroscopy; CRC Press: New York, 2001; pp 799–833.

layer, enriched in a particular metal oxide. Oxide monolayers are present in more systems than given in Table 1, but they have not been as extensively documented.

7.06.7

Conclusion

Supported metal oxide catalysts, the so-called ‘monolayer catalysts,’ are ideal model catalyst systems for investigating catalytic molecular/electronic structure–activity/selectivity relationships for chemical reactions. The number and nature (Brnsted acid, Lewis acid, redox, etc.) of catalytic active sites can be controlled, oxide supports can be varied, and the molecular and electronic structures of surface MOx species can readily be determined by numerous spectroscopy techniques (surface as well as bulk since the active sites are on the surface). With modern advances in technology, it is now possible to monitor surface and bulk catalyst properties under realistically relevant reaction conditions using operando spectroscopic techniques (Raman, IR, UV–vis, NMR, x-ray absorption (EXAFS/ XANES), and XRD). Thus, a unique scientific opportunity exists to exploit the high-quality fundamental catalysis capabilities that are now available and utilize this new knowledge for molecular engineering of advanced catalytic materials with improved performance (activity and selectivity). From Section 7.06.5.1.1 it was determined, through careful in situ spectroscopic and kinetic analyses, that SO2 oxidation to SO3 with supported V2O5/MxOy catalysts occurs over a single catalytic active site via oxygen insertion from the bridging V–O-support bond. These discoveries indicate that, when engineering an advanced SO2 oxidation catalyst, vanadium loadings in excess of monolayer coverage are wasteful and the

choice of catalyst support controls the reactivity. Results from tested ternary systems (V2O5–MxOy/TiO2) showed that the overall activity was just the sum of the contributions of the individual surface metal oxides (i.e., synergistic effects do not exist). Thus, scientists requiring a specific activity can easily calculate and synthesize a ternary system that will provide the desired catalyst performance. For chemical reactions where SO2 oxidation is an undesired side reaction, such as SCR of NOx, scientists may choose to develop their SCR catalyst around a support that provides the worst SO2 oxidation activity. A pinnacle of molecularly engineered, advanced catalysts is the bifunctional redox–acid V2O5–WO3/TiO2 catalyst discussed in Section 7.06.5.3.2 for the SCR of NOx. Through the use of in situ spectroscopic characterization and detailed kinetic studies, the contributions of the dual active sites to the overall SCR activity were unraveled. Ammonia was found to adsorb as both NH3* and NH4þ*, with the NH4þ* species being more active and adsorbing preferentially on surface Brnsted acid sites created by surface WOx species. Surface VO4 sites were found to perform the redox reaction between the NH4þ* species and the NO* species to yield environmentally benign N2 and H2O. Furthermore, the bridging V–O-Ti bond is involved in the rate-determining step since the TOF values are strongly dependent on the specific oxide support. Thus, this bifunctional catalyst can be tailored to a specific activity/selectivity by changing the relative amount of Brnsted acid WOx sites, VO4 sites, or the catalyst support. Advances in characterization techniques in the last three to four decades have provided the scientific community with a wealth of new molecular level information about the structure and function of different catalytic active sites present in

Monolayer Systems

monolayer systems. It is now possible to tailor or ‘molecularly engineer’ supported metal oxide catalysts for maximum, or sometimes minimum, performance. With the continued growth of our molecular level understanding, we will soon be able to tailor supported metal oxide catalysts for all catalytic reaction applications (redox, acid, base, redox–acid, redox– base, and acid–base).

Acknowledgments The financial support of the department of energy – basic energy sciences (grant DE-FG02-93ER14350) and National Science Foundation (grants 0609018 and 0933294) are gratefully acknowledged during the writing of this manuscript.

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