INTERACTIONS IN BIMETALLIC BONDING - ELECTRONIC AND CHEMICAL-PROPERTIES OF PDZN SURFACES

The properties of PdZn surfaces have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, and CO chemisorption. The formation of Pd-Zn bonds increases the binding energy of the core and valence levels of Pd and reduces the binding energy of the core and valence levels of Zn. An equiatomic PdZn alloy exhibits a large depletion of Pd(4d) states near the Fermi level and a shift of similar to+0.7 eV in the binding energy of the Pd(3d) levels. Ab initio SCF calculations for the PdZn molecule and PdnZnn (n = 2-4) clusters show a decrease in the electron population of the Pd(4d) orbitals as a consequence of Pd(4d) --> Zn(4p) charge transfer and Pd(4d) --> Pd(5s,5p) rehybridization. These electron transfers reduce electron-electron repulsion within a Pd atom, shifting its core and valence levels toward higher binding energy. The electronic perturbations induced by Zn on Pd reduce the CO-chemisorption ability of Pd by weakening the Pd(4d)-CO(2 pi) bonding interactions. PdZn surfaces that are rich in Zn show Pd-CO bonds that are 12-16 kcal/mol weaker than those seen on pure Pd surfaces. The electronic and chemical perturbations observed after bonding Pd to Zn are as large as those found for Pd bonded to early-transition metals and much bigger than those found when Pd is bonded to late-transition metals.