Structure of gold atoms on stoichiometric and defective ceria surfaces

Within the framework of the GGA+U implementation of density functional theory, we investigate atomistic and electronic structures of Au adsorbed on the stoichiometric and the defective CeO(2){111} surfaces, in the latter of which either O or Ce vacancies are presented. We show that on the stoichiometric surface, the most stable adsorption site of Au is not on the top of the outermost O atoms, as previously suggested, but on a bridgelike site in which the Au directly binds to two O atoms. We suggest that on both sites, the original empty Ce 4f states near the Fermi level facilitate the oxidation of the Au; the preference of the Au for being on the bridgelike site is due to the larger O 2p-d(Au) mixing, accompanied by more significant electron redistributions. On the reduced surface with O vacancies, the most stable adsorption site of Au is near the vacancy position. Unlike that on the stoichiometric surface, strong ionic bonding character exists between Au and Ce, as the former becomes Au(delta-) due to the occupation of the 6s(Au) orbitals. Upon substitution for one of the Ce atoms in the lattice, the Au possesses a much stronger positive charge than that in other cases. We find that although Au is strongly bonded when it is at the Ce vacancy site, the overall binding (i.e., with the Ce vacancy formation energy being taken into account) is weaker than that for Au adsorbed at the stoichiometric surface.