Exploring the formation of carbonates on La2O3 catalysts with OCM activity

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.