The Role of Charge Transfer in the Oxidation State Change of Ce Atoms in the TM

摘要: Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO2), fundamental issue as the role of the TM atoms in the change of the oxidation state of Ce atoms is 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 CeIV (itinerant Ce f-states) to CeIII (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 decreasing in the ionic charge of the O ions located below the cluster and an increasing 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 CeIV atoms to the O atoms with the same magnitude as in the clean CeO2 (111) surface. Consequently, the …