Diffusion on Metal Surfaces:Formalism and Application to CO Diffusion

摘要

An analytical approach searching for the saddle point on a semiempirical adiabatic potential surface(SP-SE-APES)is proposed in order to study diffusion of adsorbed molecules on a metal surface.Three reaction coordinates are used to construct this potential surface:the perpendicular adsorption height z,the distance x between adsorption sites(top,bridge,or hollow),and coordinate theta describing phonon vibrations of the metal atoms.Harmonic approximation is used to describe the potential energy along the theta coordinate,and anharmonic Morse functions are used to describe the potential energies along z and x coordinates.The theory is applied to study CO diffusion on five transition metals(rhodium,ruthenium,palladium,iridium,and platinum),and the activation energies and the prefactors are calculated for various hopping mechanisms.The one-step top-bridge-top path with the bridge state as a transition state,which was generally assumed to be the dominant mechanism of CO diffusion on these metals,is shown not to be the most likely path.Results of calculation of the diffusion coefficients are compared with experimental data.