SURFACE SCIENCE STUDIES OF THE ELECTRONIC AND CHEMICAL-PROPERTIES OF BIMETALLIC SYSTEMS

Recent studies dealing with the electronic, chemical, and catalytic properties of well-defined bimetallic surfaces (prepared by vapor-depositing one metal onto a crystal face of a second metal) are discussed. The results show that a metal atom supported on a dissimilar metal can be electronically perturbed and that this perturbation can dramatically alter the chemical and catalytic properties of both constituents of the bimetallic system. In many cases, the metal adatoms exhibit properties toward the chemisorption of H2, O2, CO, and CO2 and reactivities toward small hydrocarbons that are significantly different from those seen for the pure metal. For supported monolayers of Ni, Cu, and Pd a correlation is observed between shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. The shifts in core-level binding energies and CO desorption temperatures are a consequence of (1) electronic interactions between the metal overlayer and metal substrate and (2) variations that occur in the admetal-admetal interactions when the admetal adopts the lattice parameters of the substrate. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behavior of high surface area supported bimetallic catalysts. The coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with an ultrahigh-vacuum system for surface analysis allows detailed studies of structure sensitivity, the effects of surface composition on catalytic activity, and, in certain cases, identification of reaction intermediates by postreaction analysis. The roles of "ensemble" and "ligand" effects in mixed-metal catalysts are discussed in the light of data obtained by using well-defined bimetallic surfaces.