CO2 Chemisorption and Its Effect on Methane Activation in La2O3-Catalyzed Oxidative Coupling of Methane

Density functional theory and coupled cluster theory calculations were carried out to study the formation of the carbonate species on La2O3 catalyst using the cluster model and its effect on subsequent CH4 activation. Physisorption and chemisorption energies as well as energy barriers for the reaction of CO, and La2O3 clusters, and the reaction of CH4 with the CO32- site on the resulting clusters, were predicted. Our calculations show that CO, chemisorption at the La3+-C-3(2-) pair sites is thermodynamically and kinetically very favorable due to the strong basicity of the O2- site on La2O3, which leads to the formation of the La3+CO32- pair sites. In addition, CH4 activation at the La3+-C-3(2-) pair sites is similar to that at the La3+-C-3(2-) pair sites, which results in the formation of the bicarbonate species and the La-CH3 bond, although the La3+-C-3(2-) pair sites are much less reactive with CH4 in terms of both thermodynamics and kinetics. Further thermodynamical calculations show that the CO32- species in these clusters dissociate between 500 to 1250 K, with half of them completely dissociated at 873 K, consistent with the experimental observation. Our studies suggest that the CO32- site is unlikely to be the active site in La2O3-catalyzed oxidative coupling of methane, and CO, as a major byproduct is likely to act as a poison to the (LaO3)-O-2-based catalysts especially at modest reaction temperature.