Electron spin resonance and volumetric investigations of oxygen adsorption on high surface area CoO–MgO

Abstract

Oxygen adsorption on high surface area (h.s.a. 200 m² g^–1) CoO–MgO samples (cobalt content from 0.05 to 5 atoms per 100 Mg atoms) has been investigated by e.s.r. spectroscopy and volumetric techniques. After activation under vacuum at 873–1173 K and oxygen adsorption the samples show two main e.s.r. signals: the first, for which hyperfine interaction with a cobalt nucleus is observed, arises from a Co³⁺… O^–₂ species; the second arises from an O^–₂ species adsorbed on sites of the MgO matrix (Mg²⁺ or anion vacancies). A relatively weaker three g-value signal, also observed, has been attributed to an O^–₃ radical. Since no hyperfine structure is present the O^–₃ species is believed to be adsorbed on a matrix site.

The relative stability of the surface species is found to decrease in the order Co³⁺… O^–₂ O^–₃-(matrix) O^–₂-(matrix). In particular, the first species is reversibly destroyed by evacuation at 298 K, the second is completely removed at 373K and the third is stable up to 400–500 K.

Comparison of adsorption data (volumetric) with radical concentration values also provides evidence for the presence of diamagnetic forms of oxygen, probably O^2– and O^2–₂; radical concentrations are found to be only a fraction of the total adsorption. Strong O^2– forms appear to be favoured by increasing cobalt content.

The nature of the active sites and the mechanism of oxygen adsorption are discussed. We propose that the first stage of oxygen activation involves electron transfer from Co²⁺ surface ions. The subsequent O—O bond cleavage is accomplished with the participation of matrix sites.