6900
J. Phys. Chem. C 2008, 112, 6900-6906
CO Adsorption on a Au/CeO2 (111) Model Catalyst C. J. Weststrate,* A. Resta, R. Westerstro1 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 ReceiVed: December 14, 2007; In Final Form: February 18, 2008
We prepared a Au/CeO2 (111) model catalyst by depositing a thin cerium oxide film on a Ru(0001) surface and subsequently depositing gold. This model system was investigated using high-resolution photoemission spectroscopy. Gold forms metallic nanoparticles on CeO2 with an average particle size that depends on the Au dose. At 80 K adsorption of CO was observed on the supported Au particles, which induces a chemical shift of +0.9 eV in the Au 4f level of the Au atoms directly involved in the Au-CO bond. CO adsorption also induces an additional, particle-size-dependent shift, which affects all Au atoms in the particle; i.e., the whole Au particle is affected by CO adsorption. The fraction of surface atoms involved in CO bonding decreases with increasing gold particle size, from ∼60-70% for small particles to 15-20% for large particles. It is concluded that CO only adsorbs on defects (low-coordinated Au atoms). The CO desorption temperature decreases with increasing particle size. This is explained as follows: on small particles the most abundant defects are corner atoms and kinks (6-coordinated), which interact strongly with CO. On large particles the most abundant defects are edges between two planes (7-coordinated), which interact less strongly with CO.
I. Introduction Recent literature reports showed that Au/CeOx catalysts have a remarkable activity both for CO oxidation and for the watergas-shift (WGS) reaction.1-5 Low-temperature CO oxidation by supported Au nanoparticles, first reported by Haruta et al.,6 is interesting from a fundamental point of view, as the exact origin of the catalytic activity is much debated.7 For example, several authors assigned the activity of the Au/CeOx catalyst in the WGS reaction to the presence of positively charged gold (Auxδ+).3,8 The role of ceria in this case is to stabilize the cationic Au species.8 Others found that metallic Au nanoparticles are responsible for the WGS activity at higher temperature.9 Cerium oxide is known to have a positive effect on a number of catalytic oxidation reactions, and it is therefore used as an additive in heterogeneous catalysts, for example in the threeway catalyst (TWC). The promoting effect of cerium oxide is most commonly explained by its redox behavior; i.e., Ce4+ is easily reduced to Ce3+, thus providing active oxygen in oxidation reactions. In this paper we report on our results obtained using a welldefined model system, consisting of a Ru(0001) surface covered with a thin, well-ordered CeO2 film on which Au is deposited. Evaporation of metallic Ce in an oxygen atmosphere on a wellordered metallic substrate results in epitaxial growth of a thin CeO2 film.10,11 Ru(0001) was found to be the best substrate, yielding a stable CeO2 film with a CeO2 (111) surface.10 An STM study performed by Lu et al. showed that the film roughness (step density) is determined by both the substrate temperature during CeOx deposition and the temperature treatment after the CeOx deposition.12 These authors also studied Au deposition on such thin CeOx films.12,13 This study showed that Au on oxidized films adsorbs at step edges. The average Au particle size is a function of the amount of deposited gold. Oxygen vacancies can be created in two ways: either by * Corresponding author. E-mail:
[email protected].
lowering the oxygen pressure during CeOx deposition10,14 or by heating to ∼1200 K in vacuum. The latter procedure is also used to create vacancies in CeO2 single-crystal surfaces.15-17 Thus, our system is a versatile model catalyst in which several key properties (number of oxygen vacancies, CeO2 step density, Au particle size) can be varied. In this study we report our results obtained on an oxidized CeO2 surface. We studied a rough CeO2(111) surface (small terraces, high step density). Since gold nucleates at the step edges on an oxidized surface,13 the Au dispersion is enhanced by using such a rough cerium oxide surface. We have used synchrotron-based high-resolution core level photoemission spectroscopy (HRCLS) to investigate the influence of CeO2 on the oxidation state of the gold. We furthermore investigated the adsorption of CO on the supported Au nanoparticles and the effect of the Au particle size on CO adsorption. II. Experimental Section The HRCLS measurements were performed using beamline I311 at MAX-lab, Lund, Sweden. The experimental system is described in detail elsewhere.18 In brief, the preparation chamber (∼1 × 10-10 mbar) contains a sputter gun for sample cleaning and LEED optics. The core level spectra were measured in the analysis chamber (
