Coordination of carbon monoxide to transition-metal surfaces

Abstract

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 determine the stability of adsorption complexes. This will be demonstrated using a simple quantum-chemical approach, which is the surface analogue of the Hückel molecular-orbital method. Analysis of chemisorption on the surface of an s-band lattice as a function of band occupation shows the following: multi-atom coordination is favoured 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 change in surface geometry on chemisorption is a function of band occupation.

Chemisorption of CO on platinum is discussed in detail. It is shown that CO prefers coordination to a single atom because of the relative large interaction of the CO 5σ orbital with the highly occupied d-valence electron band.

The increase in 2π^*-occupation, deduced from i.r. studies, for CO adsorbed on a single-atom position with increased Pt surface-atom unsaturation is found to depend critically on the occupation of the Pt surface d-valence electron band.

Coadsorption effects of potassium and sulphur are discussed. Sulphur coadsorption induces changes in the electronic structure that can be understood on the basis of changes in covalency. Low-coverage alkali-metal coadsorption has two effects on CO chemisorption. Direct interaction with adsorbed alkali metal occurs, resulting in very large decrease 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 calculated to be due to the changed electrostatic potential at the metal surface.