Adsorbate–surface and adsorbate–adsorbate interactions and their role in surface reactions

By studying trends in the interaction energies of atoms and molecules with metal surfaces as the metal is changed or the surface is modified by adsorbed impurities, two important surface properties affecting the adsorption or catalytic activity have been uncovered. The first is the degree of filling of the transition metal d band, which determines the trends in the interaction energies along the transition metal series. The ‘‘volcano curve’’ relating catalytic activity with position in the transition metal series for a large number of reactions can be understood on this basis. The second is the electrostatic potential changes induced by steps or adsorbed electropositive or electronegative atoms. The promoting effect of the alkalis and the poisoning effect of the electronegative atoms on the adsorption of molecules like H2, CO, N2, and O2 can be explained by the sign of the electrostatic interaction between these molecules and the preadsorbed atoms. The relative strength of the influence of the alkalis and the electronegative atoms is discussed on the same basis. Finally, the peculiar effect preadsorbed atoms have on the adsorption of NH3 and H2O are ascribed to the large intramolecular charge transfer in these molecules.