Analysis of the bonding mechanism of benzene on Cu(110), Cu(111), Pd(111) and the effect of coadsorbed Cl atoms

We have studied, by means of ab initio cluster model wavefunctions, the bonding mechanism of benzene adsorbed on Cu and Pd surfaces together with the effect of coadsorbed Cl atoms. We have found that the experimentally observed enhancement of the chemisorption bond strength of benzene on Cu(110) in the presence of coadsorbed Cl can be explained by a simple electrostatic model which assumes that the dominant effect of the Cl atoms is to create a surface dipole-layer which lowers the metal Fermi-level. This induces an increase in charge donation from benzene to the Cu substrate which overcompensates the reduction in charge back-donation and results in a reinforcement of the chemisorption bond strength. We suggest that on Pd the effect of Cl is not observed because the metal back-donation contribution to the bonding of benzene to the Pd surface is more important than on Cu; i.e. the increase in benzene-Cu charge donation in the presence of Cl on Pd is compensated by the reduction in back-donation, resulting in virtually no change in the chemisorption bond strength. Additionally, the effect of the surface dipole-layer on the Pd Fermi-level is very much less pronounced than that found with Cu, and hence the effect on the charge-transfer mechanisms in the benzene-substrate bonding is greatly reduced in the case of Pd.