The characterization of BaCO₃-modified LaOF catalysts for the OCM reaction

In this paper, BaCO₃ is reported to be a good promoter for rhombohedral LaOF. With the addition of 5-20 mol% BaCO₃, methane conversion at 800°C was increased by ca. 40% with only slight loss in C₂ selectivity. At CH₄:O₂:N₂ = 2.6:1:12 and a contact time of 0.6 g s ml^-1, the methane conversion was 36.1%, while the C₂ selectivity was 45.3%, giving a C₂ yield of 16.4% over the 10 mol% BaCO₃/LaOF catalyst at 800°C. Comparing to LaOF, there was a 30% increase in C₂ yield. Moreover, similar to the case of BaCO₃/LaOBr, the C₂H₄/C₂H6 ratio was enhanced by the presence of 5-20 mol% BaCO₃. The loaded barium was found to remain on the surface of the catalyst. XRD, O₂-TPD, and CO₂-TPD studies revealed that the BaCO₃/LaOF catalysts changed greatly in composition, surface site basicity, and active site concentration during OCM reaction. In situ Raman studies disclosed that at 700°C under CH₄/O₂ (2.6/1), O^2-₂ species were present on the 10 mol% BaCO₃/LaOF catalyst, while none was observed on LaOF. We conclude that the enhancement in methane conversion over the 5-20 mol% BaCO₃/LaOF catalysts was due to the generation of surface BaCO₃ clusters on LaOF, which could decompose at the temperature range (700-850°C) adopted for OCM reaction to give BaO entities capable of activating O₂ to adsorbed O^2-₂.