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, γ-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^–₂)_s and (O^–₃)_s radicals, at concentrations of about 5.5 × 10¹⁵ and 1.5 × 10¹⁵ spins m^–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 (O^–₂)_s. Pre-exposure to H₂ also increases the (O^–₂)_s concentration on CaO (from 5.5 to 14.0 × 10¹⁵ spins m^–2), whereas it destroys the activity of this oxide for (O^–₃)_s formation. Neither treatment (thermal activation in vacuo with or without exposure to H₂) is efficient in the case of the γ-Al₂O₃ surface, which, however, produces (O^–₂)_s if irradiated by ultraviolet.

The electron donor power of oxides, measured by the (O^–₂)_s concentration, is found to decrease in the order CaO > MgO > γ-Al₂O₃. Surface centres for (O^–₂)_s formation are believed to be O^2– ions in surface sites of low coordination. Centres for (O^–₃)_s formation are O^–-ions, possibly in clusters, present on the CaO surface.

The results on ZnO, which also adsorbs oxygen as (O^–₂)_s, indicate that the electron donor centres in this oxide are different in nature from those active in MgO, CaO and γ-Al₂O₃.