摘要
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 (O-2(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 O-2(2-) site were predicted, with the less stable structure (type II) more reactive with CH4 than the more stable structure (type I). CH4 activation at the O-2(2-) site can always occur via the above pathway, and for the smaller La2O4 cluster and the type I 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 I 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 II structure of La4O7. The O-2(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 O-2(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 O-2(2-) sites may still play important roles in the whole catalytic cycle.
