SURFACE COLOR-CENTERS ON CALCIUM-OXIDE - AN ELECTRON-PARAMAGNETIC-RESONANCE INVESTIGATION

Various types of surface color centers at the surface of high surface area calcium oxide have been investigated by electron paramagnetic res onance. The defect centers, basically consisting in surface-trapped el ectrons, were generated either by UV irradiation of the solid in hydro gen (or deuterium) atmosphere or by addition to the oxide of low ioniz ation energy metal (Mg,K) vapors. in the former case F-s(H)(+) defects have been obtained and characterized by magnetic interaction between the trapped electron and the proton of a nearby surface hydroxyl. In t he second case F-s(+) centers and, in the case of the addition of pota ssium, a different center labeled F-s(K)(+) and characterized by inter action ofthe unpaired electron and a single potassium nucleus have bee n observed. The reactivity of all centers with oxygen to form adsorbed superoxide radical anions has been also investigated. The structural features ofthe adsorbed superoxide ion have been determined in the cas e ofthe reaction between F-s(H)(+) centers and O-2, again on the basis of the observed magnetic interaction between the adsorbed moiety and a nearby hydoxyl group. The reported data are strictly analogous to ea rlier data on MgO so that the whole set of experimental results can be generalized and interpreted in terms ofthe global surface defective p roperties ofthe alkaline-earth metal oxides.