FORMATION OF SUPEROXIDE IONS UPON OXYGEN-ADSORPTION ON MAGNESIUM-DOPED MAGNESIUM-OXIDE - AN EPR INVESTIGATION

Oxygen adsorption on the surface of MgO, previously submitted to magnesium metal vapour, give rise to the formation of at least five distinct species of adsorbed superoxide (O2-) characterized by different g(zz) values. Besides species previously observed on MgO, where O2- is generated by other methods, two new species are observed, having g(zz) = 2.091 and g(zz) = 2.064 respectively. The latter species is tightly bound to the surface and is assigned to O2- adsorbed on top of an array of five Mg2+ ions exposed because of the presence of an anion vacancy. The former is assigned to O2- weakly adsorbed on top of a five-coordinated Mg2+ ion at the (100) planar face of the oxide. This species differs from the other species present at the surface in that it is mobile and, as the temperature is raised from 77 to 298 K, exhibits rotation around the axis perpendicular to the surface. Spectra obtained by employing O-17-enriched O2 indicate that both superoxide species are symmetrically adsorbed on the surface.