ON THE COADSORPTION OF CO AND ALKALI ATOMS AT TRANSITION-METAL SURFACES - A LCGTO-LDF CLUSTER STUDY

The coadsorption system consisting of carbon monoxide and alkali atoms on transition metal surfaces has been studied theoretically by first principles electronic structure calculations. Special attention has been paid to the reduction of the CO stretching frequency in the presence of coadsorbed alkali atoms, an effect that indicates a weakening of the CO bond. To investigate CO coadsorbed with alkali atoms, a hierarchy of models has been constructed based on the clusters CO, Ni2CO and Ni(n)(CO)K2 (n = 8, 14). These models permit the separate and joint study of several interaction mechanisms and the evaluation of their relative contributions. In the smaller clusters, the electric field of the surface dipole layer is modeled by point charges. The electronic structure calculations have been carried out using the self-consistent linear combination of Gaussian-type orbitals local density functional (LCGTO-LDF) method. The calculated values for the reduction of the CO stretching frequency and the shifts of core and valence levels of CO and alkali atoms are in good agreement with experimental data. A comprehensive model for the CO/alkali coadsorption on transition metal surfaces emerges which allows the explanation of a variety of experimental findings. This model is corroborated by a detailed analysis of the electronic structure of the coadsorption system. In quantitative agreement with experimental data, about half of the reduction of the CO vibrational frequency has to be attributed to substrate-induced backbonding into the antibonding 2pi* orbital of CO as first suggested by Blyholder. The alkali-induced additional frequency shift is dominated by the electrostatic interaction between CO and the surface dipole layer which is modified by alkali coadsorbates. To a smaller extent, this frequency shift is also affected by enhanced backbonding due to a raised Fermi energy of the substrate. Direct orbital interactions between the coadsorbates were found to be negligible. Ionic models ot rehybridization models are not supported by the present study.