Theoretical\nInvestigation of Small Transition-Metal Clusters Supported on the\nCeO₂(111) Surface

Abstract

We\nhave performed a systematic investigation of 4-atom transition-metal\n(TM) clusters (TM = Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) supported\non the unreduced CeO₂(111) surface using density functional\ntheory calculations within the Perdew–Burke–Ernzerhof\nfunctional and on-site Coulomb interactions for the Ce <i>f</i>-states. We found two structure TM₄ patterns on CeO₂(111), namely, two-dimensional (2D) arrays with zigzag orientation\nfor Ru, Rh, Os, and Ir and tetrahedral three-dimensional (3D) configurations\nfor Cu, Pd, Ag, Pt, and Au. Our analyses indicate that the occupation\nof the antibonding <i>d</i>-states and the hybridization\nof the TM <i>d</i>-states with O <i>p</i>-states\nplay a crucial role in the magnitude of the TM–TM and TM–O\ninteractions and determine the formation of the 2D and 3D configurations\non CeO₂(111). The interaction of TM₄ with the\nCeO₂(111) surface changes the nature of the occupied Ce <i>f</i>-states from itinerant (Ce^IV in the clean surface)\nto localized (Ce^III) states; hence, it increases the atomic\nsize of Ce^III compared with Ce^IV by 4.4%, which\nplays a crucial role in building in a lateral tensile strain in the\ntopmost Ce layer in the surface. Furthermore, we found an enhancement\nof the electron localization of the TM <i>d</i>-states upon\nthe adsorption of TM₄ on CeO₂(111). We found\nthat the number of Ce atoms in the Ce^III oxidation state\ndepends on the TM element and structure. For Ru, Rh, Os, and Ir on\nCeO₂(111), all the Ce atoms in the topmost Ce layer change\nthe oxidation state from IV to III (i.e., 100%), while for (Pd, Pt)\nand (Cu, Ag, Au) on CeO₂(111), only 25% and 50% of Ce atoms,\nrespectively, convert the oxidation state from IV to III.