Theoretical investigation of small transition-metal clusters supported on the CeO₂(111) surface

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 (Ce^IV in the clean surface) to localized (Ce^III) states; hence, it increases the atomic size of Ce^III compared with Ce^IV 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 Ce^III 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.