Electron-donor sites of oxides as investigated on oxygen-17 exchanged CaO surfaces

Abstract

The electron-donor properties of various oxides (BaO, SrO, CaO, MgO, γ-Al₂O₃ and SiO₂–Al₂O₃), activated under vacuum in the temperature range 1000–1273 K, have been investigated by adsorption of 9,10-dimethylanthracene (DMAN), chlorine and oxygen (90%¹⁷O-enriched or non-enriched).

Upon adsorption of DMAN, radical anions are formed on CaO, whereas radical cations are formed on SiO₂–Al₂O₃. On oxides of intermediate base strength (MgO and γ-Al₂O₃) very low radical concentration was obtained. The data allow for a further specification of the correlation between electron acceptor–donor properties of oxides and their acid–base strength.

Upon adsorption of Cl₂, a two g-value e.s.r. signal (g₁= 2.002, g₂= 2.013) was observed on alkaline-earth oxides. A nearly identical signal was also observed on adsorption of O₂ on the same oxides. Upon adsorption of O₂(non-enriched) on CaO samples, previously exchanged with ¹⁷O₂, the above signal showed some relevant modifications, suggesting that the paramagnetic species contains oxygen atoms. The signal is tentatively assigned to a O^3–₃ surface species. This species is thought to be formed on the surface of alkaline-earth oxides in the electron-donation process from low-coordination oxidic sites (O^2–_CUS) toward acceptor molecules. The O^2–_CUS sites are therefore identified as the electron-donor sites.

The work also includes a volumetric study of O₂ adsorption on the various oxides. By comparing volumetric adsorption data with the concentration of radicals (O^–₂ and O^–₃), it is possible to provide a description of the O₂ adsorption mechanism on alkaline-earth oxides.