A computational DFT study of CO oxidation on a Au nanorod supported on CeO2(110): on the role of the support termination

Abstract

Possible reaction paths for CO oxidation on ceria-supported Au nanoparticle catalysts were modeled by placing a Au nanorod on a CeO2(110) surface. The results are discussed against experimental and computational data in the literature for Au/CeO2 with emphasis on the role of the ceria surface termination and involvement of ceria lattice oxygen atoms. Three CO oxidation mechanisms were modeled using density functional theory calculations: (i) reaction of adsorbed CO with ceria lattice O atoms (Mars–van Krevelen mechanism), (2) reaction of adsorbed CO with co-adsorbed O2 (co-adsorption mechanism) and (3) dissociation of adsorbed O2 followed by CO oxidation (stepwise mechanism). All three candidate mechanisms are relevant to CO oxidation catalysis as they exhibit nearly similar overall reaction barriers. The Mars–van Krevelen mechanism is consistent with experimental findings on the involvement of lattice O atoms in CO oxidation. This mechanism is prohibitive for CeO2(111) because of too high oxygen vacancy formation energy. Besides, the specific surface termination of CeO2(111) prevents O2 adsorption at its interface with Au due to repulsive interactions with the lattice O atoms. Molecular O2 adsorption is possible on CeO2(110) because of the presence of Ce4+ ions in the top layer of the surface. O2 adsorption can occur on a defective Au/CeO2(111) surface (J. Am. Chem. Soc., 2012, 134, 1560), because exposed Ce3+ ions are available. However, it is established here that O2 dissociation will heal the vacancies and deactivate Au supported on the CeO2(111) surface. The importance of Mars–van Krevelen and stepwise mechanisms in CO oxidation by Au/CeO2 strongly depends on the surface plane of the ceria support.