SCHEDULED MAINTENANCE
Exploring the formation of carbonates on La2O3 catalysts with OCM activity†
a
and
Yong
Yang
*a
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.
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