Electrostatic interactions and their role in coadsorption phenomena

A theoretical framework for comparing binding energies and activation energies for adsorption on surfaces is presented. It is applied to microscopic description of the mechanisms underlying the promotion of the molecular adsorption process on metal surfaces by coadsorbed species. Estimates of the range of this effect, its magnitude and the important factors which differentiate promotional efficiency within a particular chemical group are discussed. It is shown that while correlating surface activity with the macroscopic work function may give useful trends, it is in fact the local derivative of the electrostatic potential with respect to the surface normal which governs the free energy difference when a molecule is adsorbed in the presence or absence of a promoter.