ADSORPTION OF SMALL MOLECULES ON METAL-OXIDES

Several different examples of molecular or atomic adsorption on rock-salt structured oxide surfaces are discussed with particular emphasis on the sensitivity to the crystal potential. The CO and NO adsorption energies and geometries for adsorption on the (100) faces of the sequence MnO, FeO, CoO and NiO are compared. CO is always found more strongly adsorbed than NO, in contradiction with experiment. Decreasing the crystal potential, i.e. assuming a lower ionicity, always leads to stronger binding. Embedding of an anion-terminated MgO5 cluster in a surrounding of total-ion model potentials leads to an essentially unbound CO after corrections for superposition errors. This is verified in large, all-electron calculations on an Mg14O5 cluster model. Extensive dynamic correlation leads to a weakly bound (0.07 eV) CO on MgO5. We also report a simple model reaction for the oxidation of CO.