First-principles study of CO bonding to Pt(111): validity of the Blyholder model

The chemisorption of CO molecules on Pt(111) is studied by first-principles calculations based on the local density functional formalism with a slab model to represent the extended metal surface. We have developed a population analysis scheme which is applicable to calculations with plane wave basis sets. By applying the scheme to CO/Pt (111), the 4 sigma and 1 pi orbitals of CO are found to be completely filled, showing that they do not play a role in the bonding of CO to the surface. On the other hand, the calculated populations of the 5 sigma and 2 pi orbitals indicate that there is substantial 5 sigma donation and 2 pi backdonation, supporting the Blyholder model of CO chemisorption, the validity of which has recently been questioned. These results also suggest that a molecular-orbital based picture such as the Blyholder model is appropriate for describing simply the chemisorption bond, in spite of the fact that the molecular orbitals of CO rehybridize with each other upon adsorption, as has been shown recently by X-ray emission spectroscopy. (C) 1998 Elsevier Science B.V.