CHEMICAL AND ELECTRONIC-PROPERTIES OF PT IN BIMETALLIC SURFACES - PHOTOEMISSION AND CO-CHEMISORPTION STUDIES FOR ZN/PT(111)

On platinum zinc surfaces at 150-300 K, CO adsorbs only on Pt sites. CO molecules bonded to these sites exhibit a significantly lower adsorption energy (4-8 kcal/mol) and a larger O 1s binding energy (0.2-0.4 eV) than molecules bonded to clean Pt(111). To explain these changes in the Pt↔CO interaction, we have examined the electronic properties of Zn/Pt(111) surfaces using core and valence level photoemission, and ab initio self-consistent-field calculations. At 80 K, Zn atoms deposited on Pt(111) remain on top of the surface. Above 400 K, alloy formation occurs. In general, the formation of Pt-Zn bonds produces a large depletion in the density of Pt 5d states around the Fermi level, with a shift in the centroid of the Pt 5 d band and 4f core levels toward higher binding energy. This is accompanied by an important redistribution of charge, in which Pt loses 5d electrons and gains (6s,6p) electrons. The electronic perturbations induced by Zn on Pt reduce its CO-chemisorption ability by weakening the strength of the Pt(5d)-CO(2π*) bonding interactions. The changes in the Pt properties observed after bonding this metal to Zn are as large as those found for Pt bonded to early transition metals, and much larger than those found when Pt is bonded to late-transition metals. In the bimetallic surfaces, the perturbations in the Pt properties increase when the fraction of empty states in the valence band of the metal bonded to Pt rises.