Electron Spin Resonance Studies of the Formation and Thermal Stability of Oxygen Radicals on CaO and some other Oxides

Abstract

The formation of surface radicals by adsorption of oxygen on CaO, MgO, y-Al2O3 and ZnO has been studied by e.s.r. spectroscopy. The oxides, after activation by heating under vacuum, were exposed to oxygen at 298 K. In these conditions (O;)s and (O3~)s radicals, at concentrations of about 5.5 x 1015 and 1.5 x 1015 spins in-2, respectively, were observed on CaO thermally activated at 1173 K. By contrast the MgO sample requires pre-exposure to H2 at 298 K before O2 admission in order to produce (O2)s. Pre- exposurc to H2 also increases the (O2)s concentration on CaO (from 5.5 to 14.0 x 1015 spins m-2), whereas it destroys the activity of this oxide for (O3-)s formation. Neither treatment (thermal activation in vacuo with or without exposure to H2) is efficient in the case of the y-Al2O3 surface, which, however, produces (O2)s if irradiated by ultraviolet. The electron donor power of oxides, measured by the (O2)s concentration, is found to decrease in the order CaO > MgO >y-Al2O3. Surface centres for (O2)s formation arc believed to be O2- ions in surface sites of low coordination. Centres for (O3”)s formation are O_-ions, possibly in clusters, present on the CaO surface. The results on ZnO, which also adsorbs oxygen as (O;)s, indicate that the electron donor centres in this oxide are different in nature from those active in MgO, CaO and y-Al2O3.