The 10 mol% BaBr2/La2O3 catalyst is superior to the 10 mol% BaF2/La2O3 catalyst in CH4 conversion but slightly inferior in C2 selectivity. At 750°C, the former has a CH4 conversion of 44.7% and a C2 selectivity of 40.5% (C2 yield = 18.1%); whereas the latter has a CH4 conversion of 27.0% and a C2 selectivity of 43.7% (C2 yield = 11.8%). The halide ions behave very differently in the two catalysts. The Br- ions disperse on La2O3 while the F- ions diffuse into the La2O3 lattice, leading to the formation of hexagonal LaF3 and rhombohedral LaOF. In situ Raman studies of the two catalysts confirmed the generation of BaCO3 and La2O2CO3 during OCM reactions. The BaBr2/La2O3 catalyst has higher CO2 adsorption capacity and is superior to BaF2/La2O3 in forming the crystal phase of hexagonal La2O2CO3 as well as forming O2 n- (1 < n < 2)ions in oxygen. We conclude that the dispersion of Br- ions on the surface has enhanced the basicity of the catalyst. Consequently, both H-abstraction ability and CO2 adsorption capability of the catalyst increase. The novel role of Br- ions is to destabilize the La2O2CO3 being formed. Influenced by the presence of Br- ions, the La2O2CO3 formed in OCM conditions decomposes at 690°C instead of 790°C. We speculate that the weakening of C-O bonds in CO3 2- is due to the delocalization of Br- electron into the π* orbital of CO3 2-. During OCM reactions above 690°C, decomposition and formation of CO3 2- occur simultaneously and randomly in the crystal frame of hexagonal La2O2CO3, creating active sites which are beneficial for methane activation.
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- Physical and Theoretical Chemistry