Coordination of carbon monoxide to transition-metal surfaces

The coordination of CO chemisorbed on a transition-metal surface is a sensitive function of the electronic structure of the surface metal atoms. The group orbital concept appears to provide a key to the understanding of the fundamental electronic features that det. the stability of adsorption complexes. This is demonstrated by using a simple quantum-chem. approach, which is the surface analog of the Hueckel mol.-orbital method. Anal. of chemisorption on the surface of an s-band lattice as a function of band occupation shows the following: multi-atom coordination is favored at low electron band occupation and single-atom coordination at high electron band occupation for adsorbate orbitals of s-symmetry; interaction with orbitals of p-symmetry is only possible at bridging positions and increases with band filling; the effect of changes in surface geometry on chemisorption is a function of band occupation. Chemisorption of CO on Pt is discussed in detail. It is shown that CO prefers coordination to a single atom because of the relative large interaction of the CO 5s orbital with the highly occupied d-valence electron band. The increase in 2p*-occupation, deduced from IR studies, for CO adsorbed on a single-atom position with increased Pt surface-atom unsatn. is found to depend critically on the occupation of the Pt surface d-valence electron band. Coadsorption effects of K and S are discussed. Sulfur coadsorption induces changes in the electronic structure that can be understood on the basis of changes in covalency. Low-coverage alkali-metal coadsorption has 2 effects on CO chemisorption. Direct interaction with adsorbed alkali metal occurs, resulting in very large decreases in CO frequency and indirect long-range effects occur, resulting in the case of Pt in a shift of CO from a single-atom to a bridge position. The latter effect is calcd. to be due to the changed electrostatic potential at the metal surface