Reducibility of Ag- and Cu-Modified Ultrathin Epitaxial Cerium Oxide Films

The functionality of cerium oxide, and in particular its reactivity, can be significantly altered by the addition of diluted cationic species with different electronic properties as compared to cerium. We investigate the modifications induced by Ag and Cu as modifier cations in cerium oxide ultrathin epitaxial films. The reducibility is assessed by following the modifications of the oxidation state of surface Ce ions by X-ray photoemission spectroscopy, during thermal treatments in ultrahigh vacuum and oxygen partial pressure. A significantly higher reducibility is observed in Ag- and Cu-modified films as compared to pure CeO2 films of the same thickness. The thermal stability of the cation modifier concentration and the changes of the surface structure with the reducing treatments are also discussed. The modifications induced in the material are explained by comparison with density functional theory calculations, which indicate that the oxygen vacancy formation energy is significantly modified by the addition of Ag or Cu in the cerium oxide matrix. The obtained results are of help in view of a rational design of catalysts with optimized performance.