EPR STUDY OF THE STABILITY OF THE O2- SPECIES ON LA2O3 AND OF THEIR ROLE IN THE OXIDATIVE COUPLING OF METHANE

The O2- superoxide species can be generated on fully decarbonated La2O3 in two different ways. They can be obtained by oxygen adsorption at room temperature after vacuum treatment at 650-degrees-C, in which case all the O2- species are located on the La2O3 surface. They can also be formed by oxygen treatment at 650-degrees-C and in that case 15% of the O2- species are located between the (LaO)2n(2n+) sheets of La2O3. The adsorption sites are probably vacancies in the coordination sphere of La3+. The O2- radicals are not observed by EPR after the reaction of oxidative coupling of methane. The study of O2- stability in the presence of the different reactant gases and products of the reaction shows that the main reason for the non-observation of the O2- species is that they are unstable under the low oxygen partial pressure of the reactant gas feed (15 Torr). In consequence, the O2- species are probably not the sites directly responsible for the hydrogen abstraction of methane in this reaction. The possible nature of the active sites is discussed.