Strategies To Improve the Activity While Maintaining the Selectivity of Oxidative Coupling of Methane at La2O3: A Density Functional Theory Study

Oxidative coupling of methane (OCM) holds the promise to achieve high-value-added products directly from methane, and the strategies to improve the catalytic performance of this process are highly desired. In this work, we performed extensive density functional theory (DFT) calculations to systematically study the activity and selectivity of the OCM reactions on several types of the La2O3 and CeO2 catalysts. We theoretically evidenced that the La2O3 catalyst has high hydrocarbon selectivity but low activity, while the CeO2 shows an opposite performance. These results can be largely rationalized by the calculated reaction energetics in generating the key CH3 center dot intermediates and further protecting them from excessive oxidation on the surface. We then proposed two strategies to improve the OCM activity while maintaining the selectivity of the La2O3 catalyst. Geometrically, by constructing the stepped La2O3(210) surface exposing lattice oxygens with low coordination numbers, both the heterolytic cleavage of the C-H bond in methane and the occurrence of the key CH3 center dot intermediates could be promoted. Electronically, codoping of Sr and Ce into La2O3 could favor the direct formation of CH3 center dot and further avoid its deactivation on the surface, leading to the significantly improved OCM performance. By conducting further analyses on the thermostability of the catalysts and calculating catalytic energetics of the complete reaction cycle, we excluded the practical application of the high-Miner-index La2O3 surfaces, but we theoretically predicted the Sr/Ce-La2O3 catalyst with a proper doping concentration to be highly efficient for catalyzing the OCM reactions.