The role of charge transfer in the oxidation state change of Ce atoms in the TM13–CeO2(111) systems (TM = Pd, Ag, Pt, Au): a DFT + U investigation

Abstract

Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO₂), 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 initio molecular dynamics density functional theory calculations of the interaction of 13-atom TM clusters (TM = Pd, Ag, Pt, Au) with the unreduced CeO₂(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 TM₁₃ clusters form pyramidal-like structures on CeO₂(111) in the lowest energy configurations with the following stacking sequence, TM/TM₄/TM₈/CeO₂(111), while TM₁₃ adopts two-dimensional structures at high energy structures. TM₁₃ induces a change in the oxidation state of few Ce atoms (3 of 16) located in the topmost Ce layer from Ce^IV (itinerant Ce f-states) to Ce^III (localized Ce f-states). There is a charge flow from the TM atoms to the CeO₂(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 TM₁₃ 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 TM₈-layer to the topmost O-layer, the charge cannot flow from the Ce^IV atoms to the O atoms with the same magnitude as in the clean CeO₂(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^III compared with the Ce^IV cations, which builds-in a strain within the topmost Ce layer, and hence, also affecting the location of the Ce^III cations and the structure of the TM₁₃ clusters.