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J. Phys. Chem. C 2009, 113, 724–728
Influence of Oxygen Vacancies on the Properties of Ceria-Supported Gold C. J. Weststrate,* R. Westerstro¨m, E. Lundgren, A. Mikkelsen, and J. N. Andersen Department of Synchrotron Radiation Research, Institute of Physics, Lund UniVersity, Box 118, S-221 00 Lund, Sweden
A. Resta ESRF, 6, Rue Jules Horowitz, 38000 Grenoble, France ReceiVed: October 06, 2008; ReVised Manuscript ReceiVed: NoVember 18, 2008
The adsorption of CO on nanosized gold particles deposited on a cerium oxide substrate is strongly influenced by the roughness and concentration of oxygen vacancies of the CeOx substrate surface. Literature showed that Au nucleates at step edges on oxidized samples. For oxidized samples we found that the Au dispersion is dependent on the surface roughness. A rough CeO2 surface has a higher step density, which results in a Au higher particle density with a smaller average diameter. Other studies showed that Au nucleates on oxygen vacancies on reduced CeOx substrates. We found that small Au particles adsorbed on oxygen vacancies exhibit a significantly higher Au 4f binding energy than Au particles on oxidized CeO2. Relatively less CO adsorbs on small Au particles on a reduced substrate, with a significantly lower adsorption energy than on Au on an oxidized substrate. For larger particles the influence of oxygen vacancies in the substrate is negligible. I. Introduction Catalysts containing nanosized gold particles exhibit high catalytic activity at low temperatures.1-3 The catalytic activity can be significantly enhanced by using “active” supports or additives, such as TiO2, FeOx, MnOx, and CeOx.4-7 Catalysts based on Au supported on cerium oxide were found to be active for the water-gas-shift reaction.8-12 Rodriguez et al.13 studied a reverse model catalyst, consisting of a Au(111) surface on which CeOx nanoparticles were deposited. The Au acts as CO adsorption site, water dissociates on the cerium oxide defects, and CO oxidation occurs at the Au/CeOx interface. Since Au is primarily responsible for the bonding of CO, the study of CO adsorption on a realistic Au/ CeOx model catalyst is very relevant. The question of how the support influences CO chemisorption on the gold particles is of particular interest. For Rh, for example, it has been shown that the presence of oxygen vacancies in the ceria support has a strong influence on the chemical reactivity of the Rh particles: CO dissociation occurs only on Rh particles on a reduced ceria surface (i.e., with a high oxygen defect concentration).14 Rodriguez et al. reported that decomposition of SO2 only occurred on Au supported on reduced ceria.15 Density functional theory (DFT) calculations by Liu et al.16 predicted strong CO adsorption on a single Au atom adsorbed on an oxidized CeO2 surface, but very weak adsorption of CO on a single Au atom adsorbed on an oxygen vacancy. For Au deposited on substrates such as TiOx and MgO a strong influence of defects in the support was found.17-21 These studies indicate that a strong influence of the support can be expected for Au/ CeOx as well. In a recent publication22 we presented a detailed study of nanometer-sized Au particles supported on a thin, well-ordered * To whom correspondence should be addressed at current address: Laboratory of Inorganic Chemistry and Catalysis, Schuit Institute of Catalysis, Eindhoven University of Technology, P. O. Box 513, 5600 MB, Eindhoven, The Netherlands. E-mail:
[email protected].
CeO2 layer. High-resolution core-level spectroscopy (HRCLS) applied to the Au 4f level was used to study the gold particles directly. The study focused on the oxidized CeO2 surface, and questions regarding the Au particle size and CO adsorption as a function of particle size were discussed. The present paper discusses the influence of oxygen vacancies in the CeOx substrate surface on the CO adsorption on the Au particles. II. Experimental Section The HRCLS measurements were performed using beam line I311 at MAX-Laboratory, Lund, Sweden. The experimental system is described in detail elsewhere.23 The core-level spectra were measured in the analysis chamber (
