Electric field effects in heterogeneous catalysis

Abstract

Coadsorbed ions on metal surfaces generate electrostatic fields of the order of V A ̊ which induce substantial changes in the rate of catalytic reactions. The mechanism of the interaction of the electric field with adsorbed reactants or products on the catalyst surface is analyzed by means of ab initio cluster model wavefunctions. Two examples of catalytic processes modified by coadsorbed ions are considered: (a) The trimerization of acetylene to benzene on the Cu(110) surface and (b) the oxidation of ethylene and other organic molecules on supported Pt catalysts under the effect of an external applied potential. In both cases the trend observed experimentally is reproduced by the calculations and attributed to a field-induced change in the strength of the bond of the reactants or products. The way the field affects the bond, however, is different in the two cases. This can be understood in terms of ionic-covalent character of the adsorbates. For covalent adsorbates, like benzene on Cu(110), the field changes the metal work function and consequently the extent of the adsorbate-surface dative bonding. The effect is entirely chemical. For ionic or partially ionic adsorbates, like the oxygen atoms involved in the oxidation reactions, the effect is largely electrostatic and arises primarily from the interaction of the non-uniform electric field with the polar metal-oxygen bond.