Interaction of Zinc with Transition-Metal Surfaces: Electronic and Chemical Perturbations Induced by Bimetallic Bonding

The addition of zinc can induce significant changes in the chemical and catalytic properties of a transition-metal surface. The properties of a series of bimetallic surfaces that combine Zn with Rh or group 10 elements (TM = Ni, Pd, or Pt) have been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, CO chemisorption, and ab initio self-consistent-field calculations. The deposition of Zn on Rh(111) or polycrystalline surfaces of group 10 metals leads to the formation of alloys. These surface alloys decompose at high temperatures: 600−800 K, ZnNi; 650−850 K, ZnRh; 750−950 K, ZnPt; 850−1000 K, ZnPd. In the alloys, the core levels and valence d band of the transition metals exhibit positive binding energy shifts, while negative shifts are observed for the Zn 2p levels and 3d band. For CO/ZnRh and CO/ZnTM surfaces, the CO adsorption bond is weaker than for CO/Rh and CO/TM surfaces: 1−2 kcal/mol on ZnNi; 4−5 kcal/mol on ZnRh; 4−8 kcal/mol on ZnPt; and 12−16 kcal/mol on ZnPd. For the ZnRh and ZnTM systems, a very good correlation exists between the strength of the bimetallic bond and changes in the electronic and chemical properties of the metals. In these systems, there is an important redistribution of charge that shifts d electrons from the transition metal toward Zn, producing an accumulation of electrons around the bimetallic bonds. The larger this shift of d electrons, the stronger the bimetallic bond, and the bigger the changes in the band structure and chemical properties of the transition metal.