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.
