Analytical approach for estimating kinetics of elementary steps on transition metals: Tunneling, trends and Brönsted-Evans-Polanyi relation

An overall formulation of an adiabatic approach, which combines adsorption characteristics of reactants and products with an analytical method of searching for the saddle point (SP) on semi-empirical adiabatic potential energy surface (APES), is presented for studying surface reactions kinetics. The following cases are analyzed: (i and ii) breaking or formation of a chemical bond, (iii) diffusion hopping of a molecule on a metal surface, and (iv) subsurface penetration of a metal surface. Numerical results for the activation energies and rate constants of elementary steps of CO oxidation, oxygen dissociative adsorption and water dissociation on transition metals (Pt, Pd, etc.) at the zero-coverage limit are presented. Specifically, the reaction energy dependencies of the kinetic characteristics like activation energy are presented as Brönsted-Evans-Polanyi relation. Moreover, the effect of tunneling along reaction coordinate is shown to be significant for several steps. The calculated diffusitivities of CO on (1 1 1) surfaces of several metals are correlated with the corresponding adsorption energies. The mechanism of subsurface oxygen penetration in a cluster resembling a (1 1 1) palladium surface is investigated. Approximate dependence of the activation energy of the oxygen subsurface diffusion on surface coverage is suggested.