Investigation of the structural and electronic differences between silver and copper doped ceria using the density functional theory

Using the density functional theory (DFT) we have investigated how Ag and Cu atoms, substitutional to Ce, arrange themselves within Ceria (CeO2) and their effect on the ceria lattice, the oxidation states of the metal atoms, and the oxygen vacancy formation energies. Noble metal doped ceria has been proposed in substitution of platinum for a number of catalytic reactions. We have considered single noble metal atoms substituting Ce atoms in the (111) CeO2 surface unit cell, and investigated the thermodynamic stability of few configurations of one, two, and four Ag or Cu atoms in the unit cell. We have found that the noble metal atoms prefer to be located in the cation layer closer to the surface. An interesting result is that Cu causes a strong lattice distortion contrary to Ag. Ag, instead, causes a stronger reduction (oxygen loss) than Cu, since the electrons released in the oxygen vacancy formation are transferred mainly to Ag atoms, whose oxidation state tends towards +1, their preferred one, as found experimentally. In the Cu doped ceria, instead, part of the electrons reduces cerium atoms from +4 to +3 since Cu tends to stay in the +2 oxidation state, its preferred one. When we increase the concentration of Ag and Cu, substituting four Ce atoms, the noble metal atoms prefer to sit closer to each other, forming structural motifs resembling those typical of the Ag2O and CuO oxides.