First-principles investigation of transition metal atom M (M = Cu, Ag, Au) adsorption on CeO2(110)

Transition metal atom M (M = Cu, Ag, Au) adsorption on CeO2(110), a technologically important catalytic support surface, is investigated with density-functional theory within the DFT + U formalism. A set of model configurations was generated by placing M at three surface sites, viz., on top of an O, an O bridge site, and a Ce bridge site. Prior to DFT optimization, small distortions in selected Ce-O distances were imposed to explore the energetics associated with reduction of Ce4+ to Ce3+ due to charge transfer to Ce during M adsorption. Charge redistribution is confirmed with spin density isosurfaces and site projected density of states. We demonstrate that Cu and Au atoms can be oxidized to Cu2+ and Au2+, although the adsorption energy, E-ads, of Au2+ is less favorable and, unlike Cu2+, it has not been experimentally observed. Oxidation of Ag always results in Ag+. For M adsorption at an O bridge site, E-ads(2NN) > E-ads(3NN) > E-ads(1NN) where NN denotes the nearest neighbor Ce3+ site relative to M. Alternatively, for M adsorption at a Ce bridge site, E-ads(3NN) > E-ads(2NN) > E-ads(1NN). The adsorption behavior of M on CeO2 (110) is compared with M adsorption on CeO2(111).