The increasing availability of natural gas, driven by advancements in exploration and extraction, has spurred interest in directly converting methane into higher-value products through catalytic reactions like oxidative coupling of methane (OCM). This shift towards economically viable applications for methane holds significant environmental relevance, offering potential solutions to mitigate methane, CO, and CO₂ emissions. The successful utilization of oxides as catalysts for OCM is intricately linked to the mechanisms of defect formation and properties such as ionic conductivity. These characteristics are closely tied to the disorder in the distribution of cations and vacancies within the crystal lattice.

Our previous studies suggest that for oxides with a fluorite structure, local vacancy ordering enhances the selectivity of the OCM reaction, facilitating the direct conversion of methane into C₂ products. This current research focuses on controlling the structural order-disorder dynamics within mixed oxides of cerium and lanthanum, both known for their high catalytic activity. Utilizing X-ray diffraction and Raman spectroscopy, we have observed that the inclusion of zirconium induces a transition from a disordered phase to pyrochlore, an ordered superstructure of fluorite. OCM reaction tests on La₂Ce_xZr_2-xO₇ samples revealed a notable improvement in selectivity towards C₂ product formation with the incorporation of zirconium into the crystalline structure. This enhancement underscores the significance of structural modifications in influencing catalytic performance.

We speculate that the presence of metal-vacancy-metal clusters or the emergence of biphasic fluorite/C-type or fluorite/pyrochlore structures at elevated temperatures could explain observed improvements in catalyst performance. These arrangements may locally lower oxygen ion diffusion, reducing the availability of molecular oxygen species at the surface and preventing the complete oxidation of methane in the OCM reaction.

In summary, our investigation into defect and disorder engineering on tailored rare-earth oxide nanocatalysts underscores the importance of structural modifications in influencing catalytic performance. By elucidating the role of local vacancy ordering and incorporating elements like zirconium, we have observed notable enhancements in selectivity towards C₂ product formation in methane conversion reactions. These findings hold promise for the development of efficient and environmentally sustainable catalytic processes.