Formation of a Rhodium Surface Oxide Film in Rhn/CeO2(111) Relevant for Catalytic CO Oxidation: A Computational Study

The structure of small Rh (n = 1-10) clusters and corresponding Rh-oxide (RhnOm) clusters (n = 1-4; m = 1-9) supported on a stoichiometric CeO2(111) surface has been investigated using density functional theory corrected for on-site Coulombic interactions (DFT+U) with the goal to identify a realistic model for Rh/CeO2-based CO oxidation catalysts. Rh-n clusters on ceria prefer to adopt a three-dimensional morphology. The adsorption of oxygen leads to the reconstruction of such clusters into a two-dimensional Rh-oxide film. The stability of RhnOm, species is determined by evaluating the reaction energy for the stepwise oxidation of Rh-n which is to be compared with data for the experimental fresh catalysts. It is found that with increasing cluster size the surface oxide phase becomes increasingly stable against the isolated RhO3 form under oxidative conditions. The Helmholtz free energy change for RhnOm clusters with varying m was determined for the reduction by CO and oxidation by O-2. In this way, it was found that Rh-oxide species are more stable than the corresponding pure Rh clusters when supported on CeO2(111). This suggests that the active site for CO oxidation is a Rh surface-oxide.