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.

中文翻译：

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探索在具有 OCM 活性的 La2O3 催化剂上形成碳酸盐

La ₂ O ₃催化的甲烷氧化偶联（OCM）是将甲烷直接转化为更有价值的C ₂产物的有前途的催化部分氧化过程之一。先前的优化研究发现纳米棒形状的 La ₂ O ₃样品（n-La ₂ O ₃）表现出最佳的低温 OCM 活性。我们之前的结果将大量 La ₂ O ₂ CO ₃的形成与 OCM 中的中毒效应相关联。在这项研究中，耦合在线 MS 和原位XRD 被应用于进一步阐明这种中毒效应。在相同温度范围内，n-La ₂将 O ₃样品与商业各向同性 La ₂ O ₃ (M-La ₂ O ₃ ) 催化剂的 OCM 性能和在各种 CO ₂浓度下形成 La ₂ O ₂ CO _{3 的}倾向进行比较。发现n-La ₂ O ₃样品比M-La ₂ O ₃样品更能抵抗形成La ₂ O ₂ CO ₃。原位XRD 结果显示，在同样暴露于 10%、30% 和 50% CO _{2 后}在大约 550 °C 时，M-La ₂ O ₃向 La ₂ O ₂ CO ₃的相变完成，而 n-La ₂ O ₃仅部分转化。此外，耦合在线 MS 和原位XRD 结果表明，在相同的 CO ₂吸附量后，n-La ₂ O ₃样品能够保持比 M-La ₂ O ₃样品更大的 La ₂ O ₃晶粒尺寸。. 阿伦尼乌斯图证实，在相同的温度范围内，OCM 的表观活化能约为 60 kJ mol ^-1n-La ₂ O ₃比M-La ₂ O _{3 低}。这些结果有力地支持碳酸盐的形成抑制了 OCM 性能，这可以作为开发更有效的 La ₂ O ₃基催化剂的指标。