Constituent selection and performance characterization of catalysts for oxidative coupling of methane and oxidative dehydrogenation of ethane

摘要

The catalytic property of BaF_2-doped tetragonal LaOF catalysts for oxidative coupling of methane and oxidative dehydrogenation of ethane was evaluated in detail by independently varying the concentration of BaF_2 in the catalysts. The results indicate that addition of BaF_2 into the tetragonal LaOF significantly improves the catalytic performance of LaOF for both reactions, and the best results are observed over the BaF_2/LaOF catalysts with BaF_2 content of ca. 10 mol%. The XRD analysis of the fresh catalysts suggests that, for the catalysts with BaF_2 content less than 15 mol%, most of the Ba~(2+) ions together with the accompanying F~- and O~(2-) may be dispersed in the LaOF, forming a Ba~(2+)-doped LaOF phase in which some of the La~(3+) lattice points might be replaced by the less positively charged Ba~(2+), leading to the formation of an F-center, anion vacancies or O~- species in the LaOF lattice which will be favorable to the adsorption and activation of O_2. The results of in situ Raman characterization of the surface dioxygen species over the LaOF and 10% BaF_2/LaOF catalysts suggest that the improvement of alkane conversions on a BaF_2-doped LaOF may be explained by the relative abundance of the dioxygen adspecies over the catalyst. While one of the possible reasons for the significant improvement of the selectivities may have resulted from the favorable effect of F~- ions for the isolation of the surface oxygen adspecies.