Effect of nature of ceria supports on the growth and sintering behavior of Au nanoparticles

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Effect of nature of ceria supports on the growth and sintering behavior of Au nanoparticles Yinghui Zhou a,b , Erik Wayne Peterson a , Jing Zhou a,∗ a b

Department of Chemistry, University of Wyoming, Laramie, WY 82071, United States Department of Physics, Xiamen University, Xiamen 361005, PR China

a r t i c l e

i n f o

Article history: Received 18 February 2014 Received in revised form 20 April 2014 Accepted 21 April 2014 Available online xxx Keywords: Ceria Gold Nanoparticle Scanning tunneling microscopy X-ray photoelectron spectroscopy Catalysis

a b s t r a c t The growth and sintering behavior of gold have been investigated on reducible CeOx (1 1 1 ) (1.5 < x < 2) thin films with controlled oxidation states under ultrahigh vacuum conditions. Scanning tunneling microscopy studies reveal that Au experiences the transition of the three-dimensional particle growth on the fully oxidized ceria surface to the two-dimensional growth on the reduced ceria at room temperature. X-ray photoelectron spectroscopy data show a positive shift of up to 0.7 eV for the Au 4f core level with the increase of the degree of Ce reduction in the film, which is attributed to the Au particle size effect. Upon heating to higher temperatures, Au agglomerates to form large particles with a well-defined hexagonal shape. The temperature of transformation depends on the cerium oxidation states. These Au particles exhibit the 4f binding energies that are characteristics for the bulk gold. Instead of nanoparticles, layered films can be formed on the reduced ceria surface when Au is deposited at 500 K. Our study demonstrates that the structure of Au and correspondingly the interfaces between Au and ceria can be controlled by varying the nature of ceria surfaces as well as annealing/deposition temperatures. Such study can play a role in the understanding of the structure–reactivity relationship of ceria-supported gold catalysts. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Ceria-supported Au nanoparticles have attracted a lot of attention in recent years. Current research has demonstrated that they can exhibit promising reactivity in many applications including water–gas shift reaction, NO reduction, as well as CO oxidation [1–10]. Although the nature of the unique reactivity of ceriasupported Au is still a subject of debate in the literature, strong evidence in the literature is provided that ceria supports play a major role in the chemistry of deposited Au nanoparticles. The unique redox properties and oxygen storage capacity of ceria supports, which can reversibly transform between the Ce4+ and the Ce3+ oxidation states and create oxygen vacancies on the surface, can enhance the Au activity [2,6,11,12]. To elucidate the chemistry of ceria-supported Au catalysts, it is of significance and practical importance to gain a fundamental understanding of its growth and nucleation process on ceria and have a thorough examination of the effect of the redox properties on ceria on the electronic and geometric structures of Au

∗ Corresponding author. Tel.: +1 307 766 4335. E-mail address: [email protected] (J. Zhou).

particles. It is known that nanostructures and redox properties of oxide supports can affect the dispersion, morphology, and electronic properties of supported Au metal particles, which controls the reactivity [4,8,13–17,18]. In the study of the Au growth, it is also necessary to investigate the sintering behavior. Understanding the sintering of oxide-supported metal particles is a central theme for catalysis research. Nanosized metal particles supported on oxide surfaces can lose the activity as their sizes increase upon heating to higher temperatures. Up to date; much progress has been made on the correlation of the Au metal chemistry with the nonstoichiometry of the ceria surface [11,19]. There are several reports on the growth of Au with stoichiometric and slightly reduced CeO2 (1 1 1) [4,13,14,16]. These studies mostly focused on the structure and interaction of Au with CeO2 at room temperature and low temperature of 10 K. It has been shown that Au forms three-dimensional particles on ceria at 300 K. Step edges on CeO2 (1 1 1) are the preferred nucleation sites for Au and thus a uniform distribution of Au was observed, while surface oxygen vacancies are more desirable sites to bind Au on weakly reduced CeO2 (1 1 1). At 10 K, Au atoms tend to bind to top or bridge oxygen sites. However, a systematic examination of the structure of Au nanoparticles, in particular with respect to the sintering behavior, as a function of redox properties of ceria is still lacking in the current literature; and this motivates our study.

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To contribute to the understanding of the effect of the nanostructures and redox properties of ceria on the Au growth and sintering at the fundamental level, reducible CeOx (1 1 1) (1.5 < x < 2) thin films grown on Ru(0 0 0 1) was used as model supports in this study. The structure of the Au nanoparticles deposited on ceria surfaces with different degrees of cerium reduction was examined using scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. Their interactions with ceria supports were probed by X-ray photoelectron spectroscopy (XPS). The effects of the deposition temperature on the growth mode of Au as well as its sintering behavior upon heating to high temperatures were also investigated.

can be overestimated due to the tip convolution effects, but they are self-consistent. STM experiments were conducted with an etched W tip. All STM images were collected with a constant current mode (sample bias: 2.0–3.0 V; tunneling current: 0.05–0.1 nA). All XPS spectra were taken using a Mg K␣ radiation (15 kV, 20 mA) with a fixed electron passing energy of 50 eV and an entrance slit size of 6 × 12 mm2 . A typical XPS spectrum was collected with a 0.020 eV step and 0.200 s dwell time and averaged over two scans. Our XPS spectrometer was calibrated by setting the 4f7/2 binding energy (BE) of a bulk gold foil (Alfa Aesar, 0.1 mm thick, 99.9975+%) at 84.0 eV. LEED patterns of the Ru(0 0 0 1) and CeOx (1 1 1) surfaces were collected at the beam energy of 60 eV.

2. Materials and methods 3. Results and discussion The experiments were carried out in a multi-technique surface analysis ultrahigh vacuum chamber with a base pressure less than 5 × 10−11 Torr. This chamber consists equipment manufactured by Omicron Nanotechnology including a variable-temperature scanning tunneling microscope (VT STM XA650), an EA 125 U1 hemispherical electron spectrometer and a DAR 400 twin-anode X-ray source for XPS studies, 4-grid SPECTALEED optics for low energy electron diffraction (LEED) studies, an ISE 5 cold cathode sputtering ion source, a sample manipulator and a fast-entry load lock. Additionally, a quadrupole mass spectrometer (Hiden HAL/3F PIC), home-made metal evaporation sources, as well as gas handling lines were installed onto the system. A Ru(0 0 0 1) single crystal (Princeton Scientific Corp., one side polishing, roughness < 0.03 micron, orientation accuracy < 0.1 degree) was used as a substrate for the growth of ceria thin films as described in our previous studies [20,21]. The crystal was cleaned by Ar ion bombardment with a sample current of ∼3 ␮A followed by annealing at 1300 K for 45 s. STM, XPS and LEED were used to confirm the surface order and cleanliness of Ru(0 0 0 1) prior to the deposition of ceria thin films. Fully oxidized CeO2 (1 1 1) thin films were grown on Ru(0 0 0 1) by deposition of Ce with a flux of ∼0.2 ML/min in 2 × 10−7 Torr O2 at 700 K followed by subsequent heating the surface to 1150 K [20,22,23]. Reduced CeOx (1 1 1) thin films can be grown by decreasing the oxygen pressure during Ce deposition. After the film growth, the degree of cerium reduction was monitored by collecting Ce 3d XPS spectrum. The stoichiometric value x in CeOx can be determined by fitting the spectrum with respect to reference Ce 3d XPS data collected from CeO2 and CeO1.5 as Ce4+ and Ce3+ cations exhibit characteristic peaks due to different 4f configurations. LEED was used to examine the long-range order of the films. All the ceria thin films exhibits a sharp p(1.4 × 1.4) LEED pattern confirming the formation of well-ordered ceria surfaces with (1 1 1) orientation on Ru. Their atomic structures were resolved by STM [20]. STM line profile measurement suggests that the films have 5–6 Ce–O–Ce sandwich layers. Au was vapor-deposited onto the ceria films at temperatures of 300 and 500 K using a home-made evaporation source at a rate of ∼0.3 ML/min. The Au metal source was constructed by wrapping the Au wire (Alfa Aesar, 99.999%, 0.25 mm in diameter) around a tungsten wire (Alfa Aesar, 99.95%, 0.25 mm in diameter). The Au evaporation was obtained by applying current through the tungsten filament and heating the Au wire. Coverage was estimated from the average sizes and densities of the Au particles from the STM measurements as described in detail elsewhere [18]. One monolayer (ML) of Au is calculated to be 1.40 × 1015 atoms/cm2 with respect to the packing density of the (1 1 1) surface. It is known that the STM tip can influence the size and structure of the metal particles and thus affect the coverage measurement. Therefore, during the STM experiments, special attention was paid to ensure the good quality of the tip. The reported Au coverage in the paper

The growth of Au was first investigated as a function of cerium oxidation states at 300 K. Shown in Fig. 1a–c are STM results of 0.4–0.5 ML Au deposited on fully oxidized CeO2 (1 1 1) and partially reduced ceria (CeO1.88 and CeO1.75 ). Au forms three-dimensional (3D) particles on CeO2 with measured average diameter and height ˚ respectively. The aspect ratio (height to diamof 24.9 and 7.1 A, eter) is ∼0.3 which suggests that the particles are hemispherical [13]. They preferentially nucleate at the step edges on CeO2 (1 1 1). Only a small amount of Au particles were observed on the terraces; and they exhibit a larger particle size than those at the step edges. On the reduced ceria surfaces, uniformly distributed Au particles were observed. Moreover, the particle height decreases with the increase of cerium reduction in the film. Au exhibits an average particle height of 6.1 A˚ on CeO1.88 . Extensive decrease in the Au particle height was observed on CeO1.75 (1 1 1). The particles are about 3.4 A˚ high. Nevertheless, Au particle diameter on the reduced ceria is comparable to that on the oxidized one. Particle diameter, height and density on these three ceria surfaces are tabulated in Table 1. Au grows three-dimensional particles on CeO2 (1 1 1). This is consistent with the thermodynamic prediction based on the surface free energies. The surface free energy of Au is 1.13 J/m2 which is larger than that of CeO2 (∼0.7 J/m2 ) [24–26,27]. The interaction between the Au and stoichiometric ceria is weak [14,28]. Thus a 3D particle growth would be expected. Nanostructures of ceria thin films play a significant role in the nucleation and growth of gold. The fully oxidized CeO2 (1 1 1) surface presents a very low percent of point defects on the terraces. As a result, most of the Au preferentially nucleates at the step edges of the ceria surfaces and forms particles. In addition to step edges, the reduced ceria films exhibit various types of terrace point defects, which can also act as the nucleation sites for the Au. Therefore, Au particles were observed both on the terraces and step edges on the reduced ceria surfaces resulting a uniform distribution. Furthermore, it is shown in the literature that Au interacts stronger with the defective ceria surface than the stoichiometric one [29,30]. It has a shorter diffusion length on the defective surface which results in smaller particles with a larger particle density. Our findings are consistent with previous studies of Au on ceria as well as other metals on ceria [3,4,18,31,32,33]. To investigate the sintering behavior of Au, all three Au/ceria surfaces were subsequently annealed to the indicated higher temperatures for 4 min (Fig. 1d–i). There is a slight increase in the particle size of Au on CeO2 upon heating to 500 K. At 700 K, Au experiences extensive particle aggregation. The average particle ˚ respectively. The particles diameter and height are 37.8 and 9.9 A, are exclusively found at the step edges. Further annealing at 800 K causes the formation of hexagonal crystalline Au particles with the diameter and height values of 51.6 and 20.4 A˚ (data not shown). On CeO1.88 , Au particles coalesce and develop into shaped particles,

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Fig. 1. (a–c) STM images of 0.4–0.5 ML Au deposited on CeO2 (1 1 1), CeO1.88 and CeO1.75 at 300 K as well as corresponding surfaces after heating to 500 K (d–f) and 700 K (g–i). All images are 100 nm × 100 nm. Table 1 Mean particle diameter, height and density of 0.4–0.5 ML Au on CeOx upon deposition at 300 K followed by heating to 500 and 700 K as well as by direct deposition at 500 K. CeOx

Temperature (K)

Mean particle diameter (Å)

Mean particle height (Å)

Particle density (×1012 cm−2 )

Au 4f7/2 BE (eV)

CeO2

300 500 700

24.9 ± 6.0 25.1 ± 4.8 37.8 ± 8.2

7.1 ± 3.1 7.8 ± 2.2 9.9 ± 3.0

4.1 2.3 0.9

84.2 84.1 84.0

CeO1.88

300 500 700

20.2 ± 4.3 31.5 ± 7.4 77.3 ± 13.6

6.1 ± 1.6 3.5 ± 0.7 12.7 ± 2.8

6.5 2.5 0.1

84.3 84.3 84.0

CeO1.75

300 500 700

23.8 ± 4.7 37.1 ± 5.6 61.9–320.2

3.4 ± 0.6 5.1 ± 0.4 3.4–9.2

6.1 3.2 0.1

84.7 84.3 84.1

CeO2 CeO1.75

500 500

45.9 ± 7.3 70.0–190.0

14.4 ± 2.8 2.5–10.9

0.7 0.1

84.0 84.3

mostly with a hexagonal structure at 500 K. Heating the surface to 700 K causes the extensive agglomeration of the Au particles to form large particles. They exhibit a well-defined hexagonal shape which likely exposes the Au(1 1 1) surface. On average,

these particles are 77.3 A˚ wide and 12.7 A˚ high. On a more reduced CeO1.75 surface, hexagonal Au particles were also observed at 500 K ˚ The with an average particle diameter and height of 37.1 and 5.1 A. formed Au particles are about two-atomic layer high assuming the

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(a) Au 4f

(b) Ce 3d

CeO1.75

Intensity (a.u.)

Intensity (a.u.)

CeO1.75

CeO1.88

CeO2

90

85

80

Binding Energy (eV)

CeO1.88

CeO2

920

910

900

890

880

870

Binding Energy (eV)

Fig. 2. (a) Au 4f XPS spectra collected from CeO2 , CeO1.88 and CeO1.75 upon deposition of 0.4–0.5 ML Au at 300 K and (b) Ce 3d XPS spectra collected from as-grown CeO2 , CeO1.88 and CeO1.75 films (red solid line) as well as after Au deposition (black dash line). The spectra of Au 4f XPS from CeO1.88 and CeO1.75 were normalized to that from CeO2 . (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

thickness of one-atomic Au layer is 2.6 A˚ [14]. Upon heating to 700 K, Au develops larger hexagonal domains with the height ˚ These domains have flat value varying between 3.4 and 9.2 A. tops. Furthermore, the edges of domains parallel to each other indicating preferred orientations. Au particles formed on ceria surfaces at 300 K exhibit binding energies higher than that is characteristic of a bulk Au sample (Fig. 2a). Our XPS data shows that Au on CeO2 (1 1 1) has a 4f7/2 binding energy of 84.2 eV. The Au 4f7/2 binding energy value is similar on CeO1.88 , but significantly shifts to 84.7 eV on the more reduced ceria surface (CeO1.75 ). The shift of the Au4f7/2 binding energy can be well related with the particle size effect (Fig. 2b). Compared to bulk Au, the Au particles have a relative smaller size with an average height of 7.1 A˚ on CeO2 as shown by our STM data. The Au particles further decrease the height to 6.1 A˚ on CeO1.88 (1 1 1) and 3.4 A˚ on CeO1.75 (1 1 1). The shift of the Au core-level binding energy to higher energy values with the decreasing of Au particle size on oxide supports has been observed in the literature [4,34,35]. Studies have shown that Au mainly binds to CeO2 in the neutral charge state [14,28]. However charger transfer has also been reported to occur between Au and ceria [6,8,15,17,19,29]. Au can be positively charged as a result of Ce4+ reduction which can shift the BE energy to a higher value [17]. As reflected in the Ce3d XPS data (Fig. 2b), deposition of Au on all three ceria surfaces at 300 K does not cause detectable reduction of Ce4+ states. Our results suggest the BE shift of Au is consistent with the formation of small Au particles on ceria. Upon heating to 700 K, Au particles grow larger as a result of sintering. The 4f7/2 binding energy shifts to the value (∼84.0 eV) for bulk Au. The effect of the deposition temperature on the Au growth was also studied. Shown in Fig. 3 are results of ∼0.5 ML Au deposited on CeO2 and CeO1.75 at 500 K. Deposition of Au on CeO2 produces particles exclusively at the step edges with an average diameter ˚ They exhibit about 80–90% increase and height of 45.9 and 14.4 A. in diameter and height compared to the particles deposited at 300 K and subsequently annealed to 500 K. This is due to the enhanced diffusion of Au at elevated temperatures. These Au particles exhibit bulk characteristics with the 4f7/2 component at 84.0 eV (Fig. 3c). Instead of nanoparticles, flat films with discrete step heights are produced upon deposition of Au on CeO1.75 . One to four layer thick Au films can be formed although most of the Au films are one and

two-layer thick as indicated in Fig. 3b. The thickness for each layer is ∼2.5 A˚ as measured by STM. Similar to Au particles with less than three atomic-layer high formed on ceria at 300 K, Au on CeO1.75 shows the 4f7/2 Peak centered at 84.3 eV. No change of the ceria stoichiometry was observed upon Au deposition (data not shown). Our data have shown that Au grows 3D clusters on CeO2 (1 1 1). However, a new growth and sintering phenomena was observed for Au on more reduced ceria such as CeO1.75 . Hexagonal shapes of Au extended islands or films can be formed upon heating the room-temperature clusters to higher temperatures (Fig. 1i) or by depositing Au at elevated temperatures (Fig. 3b). We believe the transition of 3D particle to 2D film growth is associated with the intrinsic defects present on the reduced ceria as well as the defect density. Surface defect sites on oxides can bind Au strongly and thus influence the shape and size of Au clusters. For instance, the growth of Au on TiO2 (1 1 0) greatly depends on the number of oxygen vacancies on terraces [36–38]. At low coverage, Au forms 2D particles at O vacancies on TiO2 due to a stronger Au–titania interaction than that of Au–Au. It transfers to 3D particle growth with the increase of coverage. The coverage at which the transformation of Au from 2D to 3D occurs increases with the defect density. Wetting behavior of Au was observed on a highly ordered and reduced TiOx thin film grown on Mo(1 1 2) after a high-temperature annealing [36,38]. Reduced ceria such as CeO1.75 has a larger number of surface defects (e.g., O vacancies) present on terraces. Our STM studies did not resolve detailed structures of the reduced ceria surface. However, ordering of vacancies on reducible CeO2 (1 1 1) have been reported in the literature [39,40]. The strong interaction between Au and surface defects can cause the formation of Au extended islands or films on the reduced ceria. Our study suggests that nanostructures and redox properties of ceria not only can influence the size and distribution of deposited Au particles, but also the structure of Au. Chemical reactions on gold surfaces have received much attention owing to their unusual catalytic properties in many important catalytic reactions including CO oxidation and water–gas shift reactions. In general, three main factors can attribute to the activity of gold catalysts: (1) the size and morphology of gold particles, (2) the oxidation state of gold, and (3) the strong interaction between the gold and support. In spite of a number of studies in the field, the nature of the Au chemistry on ceria is still a subject of research and debate. On one

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Fig. 3. STM images of ∼0.5 ML Au deposited on (a) CeO2 (1 1 1) and (b) CeO1.75 at 500 K. The numbers in (b) indicate the layer thickness of the Au films. All images are 100 nm × 100 nm. (c) Au 4f XPS spectra collected from surfaces (a) and (b).

hand, the unique reactivity of the system has been attributed to the presence of cationic Au [6,41]. However, other studies indicate that the cationic Au species are not stable under the reaction conditions and can transfer to the metallic Au aggregates. Furthermore, Rodriguez and co-workers have demonstrated that the structure of the Au/oxide interface rather than the dimensions of Au particles that governs the water–gas shift reaction [2,3]. As shown in our study, 2D/3D/hexagonal Au particles as well as Au films can be obtained. These Au-ceria systems present different Au-ceria interfacial sites which can have a significant effect on the adsorption of molecules such as CO for the CO oxidation and water–gas shift reactions and thus the Au reactivity. Our model Au/ceria systems are well-suited for the investigation of the catalytic activity of gold particles with controlled sizes and structures. Our results can also shed light on the structural aspects of catalysis over ceria-supported gold that is lacking in the current literature. 4. Conclusions The detailed role of the redox properties associated with oxygen vacancies in ceria in the growth and sintering behavior of Au was investigated using STM and XPS under the ultrahigh vacuum conditions. Our results suggest that nanostructures and redox properties of ceria play a key role in the size, structure, and distribution of deposited Au particles. Both Au nanoparticles and well-ordered films can be prepared on ceria dependent on the ceria oxidation states as well as the growth conditions. Our study can provide structural insight for the understanding the Au/ceria chemistry. Acknowledgements The research is sponsored by University of Wyoming start-up funds, Wyoming NASA EPSCoR grant (NNX07AM19A) as well as National Science Foundation (Grant No. CHE1151846). References [1] A. Trovarelli, Catalysis by Ceria and Related Materials, Imperial College Press, London, 2002. [2] J.A. Rodriguez, Catal. Today 160 (2011) 3–10. [3] J.A. Rodriguez, P. Liu, J. Hrbek, J. Evans, M. Perez, Angew. Chem. Int. Ed. 46 (2007) 1329–1332. [4] M. Baron, O. Bondarchuk, D. Stacchiola, S. Shaikhutdinov, H.J. Freund, J. Phys. Chem. C 113 (2009) 6042–6049.

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