Activation of O-2 over SrF2- and BaF2-promoted SmOF catalysts for the oxidative dehydrogenation of ethane

Promotion of rhombohedral SmOF by SrF2 or BaF2 led to significant gain in ethane conversion but little change in ethene selectivity in the oxidative dehydrogenation of ethane. At 893 K, C2H6 conversion and C2H4 selectivity over SmOF were, respectively, 38.8 and 71.2%. With 10% SrF2/SmOF and 20% BaF2/SmOF catalysts, C2H6 conversion became 69.7 and 68.4%, respectively, while C2H4 selectivity was 67.7%. X-Ray diffraction (XRD) studies revealed a small amount of cubic SrF2 but no crystal phase of BaF2 in the promoted catalysts. The SrF2 lattice contracted slightly and the SmOF lattices expanded slightly. We believe that such deformities resulted from ionic exchange/substitution occurring between/within the SrF2 and SmOF phases with creation of active centres. The Raman and O-2 temperature programmed desorption (TPD) results, indicated that, above 973 K, dioxygen species on SmOF were converted completely to mono-oxygen species. With the promoted catalysts, such conversion was complete at around 573 K. Because Sm3+ is paramagnetic and owing to the effect of spin-spin interaction, we could not obtain any EPR signals of O-2(-) or O- over SmOF. Over the promoted catalysts, however, EPR signals corresponding to O-2(-) and O- located in the proximity of non-paramagnetic Sr2+ and Ba2+ ions were observed. The conversion of O-2(-) to O- was confirmed and monitored by EPR spectroscopy. We conclude that the deformities generated in the promoted catalysts have facilitated the conversion of dioxygen species to O- species at lower temperature, resulting in better catalytic performance.