Electrostatic adsorbate-adsorbate interactions: The poisoning and promotion of the molecular adsorption reaction

The electrostatic interaction between two adsorbates, and, in particular, between an adsorbed atom and an adsorbed or adsorbing molecule is studied. Based on self-consistent calculations of the electrostatic potential around a series of atoms outside a jellium surface, it is shown that a simple electrostatic interaction can explain a large number of experimental observations concerning the influence of pre-adsorbed atoms on the adsorption rate, stability and adsorption configuration of simple molecules on metal surfaces. The role of pre-adsorbed alkalis as promoters and of electronegative atoms like P, S, Cl and O as poisons for the adsorption of electron acceptor molecules like H2, O2, N2 and CO is discussed, as well as the relative magnitude of the influence of the alkalis and the electronegative atoms. The peculiar effects that pre-adsorbed atoms have on molecules like H2O and NH3 are ascribed to the large intra-molecular electron transfer in these molecules.