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

Abstract

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 La 2 O 3 and CeO 2 catalysts. We theoretically evidenced that the La 2 O 3 catalyst has high hydrocarbon selectivity but low activity, while the CeO 2 shows an opposite performance. These results can be largely rationalized by the calculated reaction energetics in generating the key CH 3 • 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 La 2 O 3 catalyst. Geometrically, by constructing the stepped La 2 O 3 (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 CH 3 • intermediates could be promoted. Electronically, codoping of Sr and Ce into La 2 O 3 could favor the direct formation of CH 3 • 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-Miller-index La 2 O 3 surfaces, but we theoretically predicted the Sr/Ce−La 2 O 3 catalyst with a proper doping concentration to be highly efficient for catalyzing the OCM reactions.

CH3•-Generating Capability as a Reactivity Descriptor for Metal Oxides in Oxidative Coupling of Methane

“…Recently, we have systematically investigated the OCM reaction processes over La 2 O 3 and CeO 2 using DFT calculations. 20 Owing to their different surface geometric and electronic structures, La 2 O 3 and CeO 2 can promote different transformation pathways of the as-generated methyl species, following the first C−H bond breaking. The calculated results suggest that promoting the direct formation of CH 3…”

“…• while the surface reducibility was also used as a descriptor for their catalytic characteristics. 20,23 However, in experimental studies, identifying the intrinsic activity of metal oxides in methane activation is still challenging because very few in situ techniques can be used to quantitatively detect CH 3…”

“…• intermediates and (ii) how they can be protected from excessive oxidation on the surface. 20 In the current work, by studying a series of metal oxides, we successfully integrate the two into a unified parameter, CH 3…”

Extraction of Catalyst Design Heuristics from Random Catalyst Dataset and their Utilization in Catalyst Development for Oxidative Coupling of Methane

“…It was reported that La 2 O 3 surfaces allow oxygen to be present as various anion species, (some of) which are highly active for methane activation [22,23] . Moreover, La 2 O 3 has been frequently coupled with Sr and Ce, [24–27] which is likely consistent with the observed importance of basic oxide‐forming elements. At 900 °C, high performing catalysts often contained both basic oxide‐forming elements and early transition metals as active elements (Figure 1c, d).…”

Recent Developments in Natural Gas Flaring Reduction and Reformation to Energy-Efficient Fuels: A Review

“…Numerous catalysts have been studied for the production of alkenes (C 2+ ) through the OCM process. The summary of various catalysts used in OCM for alkenes is presented in Table . − …”