Exploring the formation of carbonates on La2O3 catalysts with OCM activity

Abstract

La₂O₃ catalyzed oxidative coupling of methane (OCM) is one of the promising catalytic partial oxidation processes that converts methane directly into more valuable C₂ products. Previous optimization studies found a nanorod shape La₂O₃ sample (n-La₂O₃) to exhibit the best low temperature OCM activity. Our previous results correlated the formation of bulk La₂O₂CO₃ with a poisoning effect in OCM. In this study, coupled online MS and in situ XRD are applied to further elucidate this poisoning effect. In the same temperature range, the n-La₂O₃ sample is compared with a commercial isotropic La₂O₃ (M-La₂O₃) catalyst for their OCM performance and propensity to form La₂O₂CO₃ under various CO₂ concentrations. The n-La₂O₃ sample is found to be far more resistant against forming La₂O₂CO₃ than the M-La₂O₃ sample. In situ XRD results show that after identical exposures to 10%, 30%, and 50% CO₂ at around 550 °C, the phase transition to La₂O₂CO₃ is complete for M-La₂O₃, while n-La₂O₃ is only partially converted. In addition, coupled online MS and in situ XRD results indicate that the n-La₂O₃ sample is able to maintain larger grain sizes of La₂O₃ than the M-La₂O₃ sample after the same adsorption amount of CO₂. Arrhenius plots confirm that in the same temperature range the apparent activation energy for OCM is around 60 kJ mol⁻¹ lower for n-La₂O₃ than for M-La₂O₃. These results strongly support that carbonate formation suppresses the OCM performance, which may serve as an indicator in developing more efficient La₂O₃ based catalysts.