Molecular orbital studies of dissociative chemisorption of first period diatomic molecules and ethylene on (100) W and Ni surfaces

The extended Hückel molecular orbital method is used to examine interactions of Li2, B2, C2, N2, CO, NO, O2, and F2 with nine atom clusters representing W(100) and Ni(100) crystal surfaces. The following predictions are made and are corroborated by experimental facts when available: (1) A strong tendency for charge transfer between the substrate and adsorbate exists as would be expected from electronegativity differences. (2) The adsorbed molecules display a tendency to dissociate because of Coulombic repulsions and frequently because of the filling of antibonding levels as well. At the same time Li2 and F2 form ionic bonds with the surface and covalent character is also evident for the others. (3) Strong bonding interactions form between the adsorbates and surfaces for any adsorbate orientation or position. Thus a connection is made with physical theories which exclude atomic detail. Such surface homogeneity can be resolved into detail with molecular orbital methods, but the emphasis in this paper is on examining orbital interactions between adsorbates and substrates with small regard to energy changes. (4) Active sites, which are atoms on a surface which have stronger chemisorptive capability, are found on steps and corners of metal clusters. These atoms, which stick out from the bulk, gain extra electronic charge as a result of orbital orthogonality and they form stronger bonds with electrophilic adsorbates, sometimes also strengthening the diatomic bond a little. This suggests rough surface areas might initially attract electrophilic adsorbates, but that dissociation could happen in a nearby region. (5) Photoemission spectra for CO on W(100) and Ni(100) and ethylene on Ni(111) are considered in some detail. The calculations corroborate the spectra. Comments are made on spectra of N2, N2O, NO, and O2 on W(100), CO on W(110), and ethylene on W(100) and Ni(100). Bandwidths and positions are discussed. In general the extended Hückel molecular orbital method appears to be a valuable tool in surface chemisorption and catalysis studies.