Embedded cluster model studies of impurities at metal surfaces

Abstract

A knowledge of the electronic properties of impurities at metal surfaces is of great value in the understanding of such important phenomena as chemisorption and surface segregation in alloys. We have adopted here a unified approach based on an Embedded Cluster model to study the properties of surface impurities. We have mainly concentrated on hydrogen impurities either adsorbed above the surface or incorporated into the bulk of metals. We have also considered the case of substitutional metal impurities at the surface of host metals. For hydrogen chemisorption we have considered such substrates as free-electron, transition and noble metals as well as bimetallic substrates composed of a single metal impurity in a host matrix or a metallic overlayer on a metal support. The electronic structure of the chemisorbed system is compared to photoemission data when available, from which interpretation of the details of the experimental spectra may be made. It is found that hydrogen adsorption on transition and noble metals results in the formation of a pair of bonding/antibonding resonances on either side of the metal d-band, while for hydrogen on free-electron metals a single hydrogen induced resonance is observed. One-electron energy differences between the H on jellium and H on metal systems are estimated and trends in such energies across the 3d and 4d transition series are compared to the trends in experimental chemisorption energies for H on these metals. The change in hydrogen chemisorption capacity of an inert substrate due to the introduction of chemically active impurities is investigated. The different properties of Pd overlayers with respect to Pd surfaces are also investigated. Interaction energies between adatoms on surfaces are estimated in order to predict the geometry of ordered structures on surfaces. One-electron heats of segregation for binary alloys are calculated. These show a strong solute surface segregation for noble metal impurities in group VIII metals, which is due to the higher d-band occupancy of the noble metal.