The enhancement effects of BaX2 (X = F, Cl, Br) on SnO2-based catalysts for the oxidative coupling of methane (OCM)

Abstract

In this study, the promotional effects of different barium halides (BaF2, BaCl2 and BaBr2) on SnO2 for OCM reaction have been investigated. It is observed that the addition of all the barium halides can improve the reaction performance of SnO2, following the sequence of BaBr2 > BaCl2 > BaF2. Raman and XRD results have substantiated that adding barium halides to SnO2 can increase the amount of surface vacancies/defects, hence creating more abundant surface OCM reactive oxygen sites by anion substitution. At the same time, the amount of the moderate basic sites contributing to OCM reaction is also improved. During OCM reaction, the amount of the OCM reactive oxygen species as well as the moderate basic sites can be further increased. As proved by In situ Raman and XPS results, on BaF2:SnO2 = 1:1 and BaCl2:SnO2 = 1:1, O2− is the OCM reactive oxygen sites. In contrast, on BaBr2:SnO2 = 1:1, O22- is the OCM reactive oxygen sites, which is induced by the formation of BaSnO3 perovskite phase. Moreover, O22- anions are reported to be favourable for the direct formation of C2H4 through carbene mechanism, which explains the high ethylene selectivity on BaBr2-modified catalyst. The amount and properties of the surface active oxygen species and basic sites are of great importance for OCM reaction, and their concerted interaction determines the reaction performance of BaX2/SnO2 catalysts. BaBr2:SnO2 = 1:1 contains the apporiate amount of both types of active sites, hence displaying the best OCM performance among all the catalysts.