INFLUENCE OF THE SURFACE ATOM METALLIC COORDINATION IN THE ADSORPTIONOF ETHYLENE ON A PLATINUM SURFACE - A THEORETICAL-STUDY

A theoretical analysis of the chemisorption of ethylene on a platinum surface is presented as a function of the metallic coordination of the platinum surface atom. Adsorption sites with coordinations ranging fr om 9 to 3 are considered and modeled by clusters of size between 71 an d 114 atoms, with a special emphasis on step and kink sites. From exte nded-Huckel calculations, the ethylene molecule is shown to adsorb in a di-sigma mode for sites with high metallic coordinations while the p i chemisorption becomes more stable for coordinations 7 and below. The binding energy of the molecule is increased when the surface atom coo rdination is reduced. These changes are explained by a strong reductio n of the four-electron repulsion between occupied molecular orbitals o f ethylene and filled states of the cluster for sites with low metal c oordination. This sensitivity of the molecule adsorption on the struct ure of the surface could have important implications on the ethylene r eaction pathway.