NATURE OF METAL-METAL BONDING IN MIXED-METAL CATALYSTS

The nature of the electronic and physical properties Of ultrathin Cu and Pd films on Ta(110) have been studied using X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). The results indicate that monolayers of Cu and Pd on Ta(110) have lower electron densities than that measured for the surface atoms of Cu(100) and Pd(100), respectively. By comparison of the present results with those for other Cu, Ni and Pd films, correlations are found among the electronic perturbations of the monolayer films, the metal-substrate cohesive bond strength and the chemisorption properties of the monolayer films toward CO. A simple model is described to explain the direction and magnitude of the electronic perturbations. Surface electronegativity is defined and is found to be quite different from electronegativities in bulk alloys. The electronic state of the metal overlayer is determined to be an excellent indicator of the ability to chemisorb CO.