EPR STUDY OF THE STABILITY AND THE ROLE OF THE O-2-SPECIES ON LA2O3 IN THE OXIDATIVE COUPLING OF METHANE

The paramagnetic superoxide ion detectable by EPR spectroscopy, can be generated on fully decarbonated La2O3 by oxygen adsorption. After pretreatment in vacuo at 650-degrees-C, oxygen may be admitted at two different temperatures: i) at room temperature: all the O2- ions are located on the La2O3 surface; ii) at 650-degrees-C: 85% of the O2- species are on the surface and 15% are between the (LaO)2(2+) layers of La2O3. The O2- radicals are no longer observed by EPR after the oxidative coupling of methane reaction. The study of their stability in the presence of the reactant gases and oxidation products of the reaction, shows that the main reason for the disappearance of the O2- species is that they are unstable under the low oxygen partial pressure of the gas feed (15 Torr). It appears unlikely that the O2- species are the active sites for the activation of methane.