The role of charge transfer in the oxidation state change of Ce atoms in the TM sub(13)-CeO sub(2)(111) systems (TM = Pd, Ag, Pt, Au): a DFT + Uinvestigation

Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO sub(2)), fundamental issues such as the role of the TM atoms in the change in the oxidation state of Ce atoms are still not well understood. In this work, we report a theoretical investigation based on static and ab initiomolecular dynamics density functional theory calculations of the interaction of 13-atom TM clusters (TM = Pd, Ag, Pt, Au) with the unreduced CeO sub(2)(111) surface represented by a large surface unit cell and employing Hubbard corrections for the strong on-site Coulomb correlation in the Ce f-electrons. We found that the TM sub(13) clusters form pyramidal-like structures on CeO sub(2)(111) in the lowest energy configurations with the following stacking sequence, TM/TM sub(4)/TM sub(8)/CeO sub(2)(111), while TM sub(13) adopts two-dimensional structures at high energy structures. TM sub(13) induces a change in the oxidation state of few Ce atoms (3 of 16) located in the topmost Ce layer from Ce super(IV) (itinerant Ce f-states) to Ce super(III) (localized Ce f-states). There is a charge flow from the TM atoms to the CeO sub(2)(111) surface, which can be explained by the electronegativity difference between the TM (Pd, Ag, Pt, Au) and O atoms, however, the charge is not uniformly distributed on the topmost O layer due to the pressure induced by the TM sub(13) clusters on the underlying O ions, which yields a decrease in the ionic charge of the O ions located below the cluster and an increase in the remaining O ions. Due to the charge flow mainly from the TM sub(8)-layer to the topmost O-layer, the charge cannot flow from the Ce super(IV) atoms to the O atoms with the same magnitude as in the clean CeO sub(2)(111) surface. Consequently, the effective cationic charge decreases mainly for the Ce atoms that have a bond with the O atoms not located below the cluster, and hence, those Ce atoms change their oxidation state from IV to III. This increases the size of the Ce super(III) compared with the Ce super(IV) cations, which builds-in a strain within the topmost Ce layer, and hence, also affecting the location of the Ce super(III) cations and the structure of the TM sub(13) clusters.