Role of Peroxides on La₂O₃ Catalysts in Oxidative Coupling of Methane

Density functional theory and coupled cluster theory [CCSD­(T)] calculations reveal an important pathway for the one-step CH₃OH formation upon CH₄ activation at the peroxide (O₂^2–) site of La₂O₃-based catalysts for the oxidative coupling of methane (OCM) reaction. Using modest-sized La₄O₇ and La₆O₁₀ clusters as catalyst models, two types of structures for the O₂^2– site were predicted, with the less stable structure (type <b>II</b>) more reactive with CH₄ than the more stable structure (type <b>I</b>). CH₄ activation at the O₂^2– site can always occur via the above pathway, and for the smaller La₂O₄ cluster and the type <b>I</b> structure of La₄O₇, an alternative pathway leading to La–CH₃ bond formation was also predicted, similar to that at the oxide (O^2–) site from our previous study. For the type <b>I</b> structure of La₄O₇, the energy barrier for La–CH₃ bond formation is lower than that for CH₃OH formation, but both are higher than that for CH₃OH formation for the type <b>II</b> structure of La₄O₇. The O₂^2– site was predicted to be much less reactive with CH₄ than the oxide (O^2–) site, and can lead to CH₃OH formation, which is considered as a side reaction. Thus, our calculations do not appear to support the central role previously proposed for the O₂^2– site for La₂O₃-based catalysts for the OCM reaction. However, considering the catalytic and redox nature of this reaction, both the O^2– and O₂^2– sites may still play important roles in the whole catalytic cycle