Bonding of CO to metal surfaces: A new interpretation

Abstract

The interaction of CO with Na, Mg, and Al surfaces has been studied using the molecular-orbital cluster model. These metals are chosen since we wish to study the CO interaction with the metal valence sp electrons in the absence of d electrons. Our conclusion is that there is substantial σ repulsion and that the bonding arises primarily from the metal π to CO 2π* interaction and donation. This is based on the analysis of self-consistent-field calculations for linear XCO and two-layer X4(1,3)CO clusters, where X represents the metal atom. They are chosen to simulate normal adsorption at the on-top site with the C atom adjacent to the surface, a geometry known to be appropriate for certain transition-metal atoms. The potential curve for the interaction as a function of metal-C distance has an attractive or repulsive character directly related to the amount of adsorption-site σ and π character in the bare (metal atoms only) cluster. A corresponding orbital analysis unambiguously shows that the free CO orbitals are essentially unchanged upon interaction with the metal; there is very little CO 5σ to metal donation. It also shows that the binding of CO to the metal is associated with a considerable change and donation of the metal π electrons to CO 2π*, The σ orbitals of primarily metal character hybridize and polarize away from CO in order to reduce the metal CO 5σ repulsion. This interpretation, which is different from the usual picture of the metal-CO bond, is supported by an extensive set of results. We also consider the effects of the interaction for the valence photoelectron peaks of chemisorbed CO and obtain a new interpretation for the significance of the shift of the 5σ ionization toward the 1π. Predicted features of the CO ionization are compared to experimental photoemission spectra for CO/A1(111). Their agreement provides support for an adsorption geometry with CO normal to the surface. © 1983 The American Physical Society.