A tight-binding study of the electronic structure of and chemisorbed CO

Abstract

Experiments on CO chemisorption at the transition-metal alloy find the molecule bound more weakly than at Pt(111). There was, however, disagreement in experimental interpretation supporting either the bulk termination or pure platinum surface. A recent LEED study strongly supports the latter possibility. We have performed extended self-consistent tight-binding calculations for bulk and surface electronic structure of the alloy and for CO chemisorption at selected adsorption sites. For the bulk we get in the initial-state approximation a good agreement with the measured core-level shifts. For CO we find for both the surface models sites at which the adsorption energy is reduced in agreement with measurements, but also sites at which the adsorption energy practically coincides with that at Pt(111). The calculated core-level shifts at Pt sites in both models are close to experimental values. We predict, however, a distinct CLS for Pt atoms at Ti sites in the Pt overlayer which is not reported experimentally. The possible enrichment of the subsurface layer by titanium might reconcile the experimental and theoretical findings. Correlations between surface core-level shifts and reactivity are also discussed.