The mobility of bonds at metal surfaces (heterogeneous catalysis)

Abstract

Chemical reactions between molecules and atoms may be inhibited by barriers in the potential energy as a function of the coordinate which describes the reaction. Adsorption of the reactants on a metal reduces the barrier by broadening the molecular and atomic levels into resonances whose occupation depends on their energy relative to the Fermi level. This mechanism does not depend on any approximation or simplified model; however in order to explain the effect more clearly and to enable estimates to be made, a variation of the Holstein model (1959) is introduced and applied to H2+H as an example. It is argued that this effect applies to heterogeneous catalysis in that covalent bonds are brought into contact with a metal, and in the opposite extreme to embrittlement when metallic bonds are made more covalent. The general question of mobility of bonds is related to a transition from a metal to a covalent insulator.