Due to the increasing exploration and utilization of natural gas, the efficient conversion of methane into value−added chemicals such as ethylene via the oxidative coupling of methane (OCM) reaction becomes of increasing importance. Among the known OCM catalysts, La2O3−based catalysts exhibited good activity, selectivity and stability. However, the nature and the evolution of the surface oxygen species as well as the detailed catalytic mechanism of the OCM reaction remain elusive. In this work, extensive periodic density function theory calculations were performed to study the evolution of surface oxygen species on the La2O3 catalyst during the OCM reaction and to establish the catalytic mechanism for the selective and nonselective oxidation of CH4. The lattice oxygen site (O2−) was found to activate CH4 via heterolytic CH3−H bond splitting, and competitive adsorption of O2 from the gas phase is necessary for the desorption of the CH3 species. This converts the O2− site to the superoxide site (O2−), which further evolves by activating another CH4 molecule via direct hydrogen abstraction. The resulting O22− site reacts with a third CH4 molecule via an oxygen insertion mechanism leading to the direct formation of CH3OH, which can be further oxidized by the O22− species to yield CO2 in three steps with a similar mechanism. This suggests the O22− site to be primarily responsible for unselective CH4 oxidation in the OCM reaction, in contrast to the previously proposed OCM catalytic mechanism at the peroxide site. Our first principles study gives a comprehensive view on the interaction of CH4 and O2 with the La2O3 catalyst surface, and provides critical insights into the evolution of surface oxygen species as well as the detailed reaction network for CH4 selective and unselective conversions.

Mechanism of selective and nonselective oxidation in La2O3-catalyzed oxidative coupling of methane reaction

256th National Meeting (2018)

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