Promoting effect of F- on Sr/La oxide catalysts for the oxidative coupling of methane

The catalytic performance evaluation results showed that the CH4 conversion, C-2 selectivity and ethylene to ethane ratio were significantly higher over LaF3/SrO and SrF2/La2O3 than those over SrO/La2O3 and the maximum C-2 yields of 18-20% were achieved over the LaF3/SrO and SrF2/La2O3 catalysts at 650-750 degrees C. XRD analysis of the fresh catalysts indicated that, with the addition of LaF3 to SrO and treatment in air at high temperature, anionic and/or cationic exchange between oxides and fluorides lattices took place to some extent, leading to the formation of SrF2 and LaOF. The UV spectra of pyridine adsorption and the temperature-programmed desorption spectra of CO2 adsorption on SrF2/La2O3 and SrO/La2O3 showed that surface of the catalyst containing fluoride was more acidic and less basic than that of the corresponding oxide system. The XPS characterizations of O-2-adsorbed catalysts revealed that the catalysts containing F- were more conducive to the formation of the oxygen species with fewer negative charges. The results of in situ IR characterization of the surface oxygen species at 650 degrees C suggested that O-2(-) might be the active oxygen adspecies for the oxidative coupling of methane reaction.