A Comparative Study of the p(2 × 2)-CO/M(111), M=Pt,Cu,Al Chemisorption Systems

Abstract

We propose a model for CO chemisorption on late transition metal, noble metal, and main-group surfaces based on the results of energy partitioning studies of surface -CO bonding for the CO/M(111), M)Pt,Cu,Al chemisorption systems. Plane-wave density functional theory was used to calculate the chemisorption geometry for CO on the top, bridge, and hollow sites of the M(111) (M)Pt,Cu,Al) surfaces and to verify the experimentally determined preference for top site CO chemisorption on all three surfaces. To construct a chemically intuitive, molecular orbital based model of surface-CO bonding, an energy partitioning analysis of surface-CO bonding was carried out within a tight binding scheme based on the extended Huckel method. Although within this one-electron formalism we are no longer able to make quantitative assesments of bonding, we are able to readily extract surface-CO bonding trends. By expanding the orbital basis on CO to include energetically low-lying nonfrontier orbitals and explicitly evaluating the role of the surface s and p bands in surface-CO bonding, we note several discrepancies between our model and traditional, frontier orbital based models of surface-CO interaction. Especially important is the role of the CO(4U) orbital. We note that for CO chemisorption on all three surfaces, the energetic preference for top site chemisorption is the result of a balance between the stabilization associated with the formation of the surface -CO bond and chemisorption- site-dependent changes in both C-O bonding and M- M( M)Pt,Cu,Al) bonding within the surface layer on chemisorbing CO. Further, by choosing to consider CO chemisorption on the Cu(111) surface as part of a continuous transition from CO chemisorption on late transition metal surfaces to CO chemisorption on sp- metal surfaces, we are able to assess the degree to which we may refer to copper as an sp-metal.