Theoretical Investigation of Small Transition-Metal Clusters Supported on the CeO2(111) Surface

Abstract

We have performed a systematic investigation of 4-atom transition-metal (TM) clusters (TM = Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) supported on the unreduced CeO2(111) surface using density functional theory calculations within the Perdew–Burke–Ernzerhof functional and on-site Coulomb interactions for the Ce f-states. We found two structure TM4 patterns on CeO2(111), namely, two-dimensional (2D) arrays with zigzag orientation for Ru, Rh, Os, and Ir and tetrahedral three-dimensional (3D) configurations for Cu, Pd, Ag, Pt, and Au. Our analyses indicate that the occupation of the antibonding d-states and the hybridization of the TM d-states with O p-states play a crucial role in the magnitude of the TM–TM and TM–O interactions and determine the formation of the 2D and 3D configurations on CeO2(111). The interaction of TM4 with the CeO2(111) surface changes the nature of the occupied Ce f-states from itinerant (CeIV in the clean surface) to localized (CeIII) states; hence, it increases the atomic size of CeIII compared with CeIV by 4.4%, which plays a crucial role in building in a lateral tensile strain in the topmost Ce layer in the surface. Furthermore, we found an enhancement of the electron localization of the TM d-states upon the adsorption of TM4 on CeO2(111). We found that the number of Ce atoms in the CeIII oxidation state depends on the TM element and structure. For Ru, Rh, Os, and Ir on CeO2(111), all the Ce atoms in the topmost Ce layer change the oxidation state from IV to III (i.e., 100%), while for (Pd, Pt) and (Cu, Ag, Au) on CeO2(111), only 25% and 50% of Ce atoms, respectively, convert the oxidation state from IV to III.