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

Chu, Changqing, Zhao, Yonghui, Li, Shenggang, Sun, Yuhan
The Journal of Physical Chemistry C 2016 v.120 no.5 pp. 2737-2746
active sites, bicarbonates, carbon dioxide, carbonates, catalysts, energy, methane, models, temperature, thermodynamics
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 CO₂ and La₂O₃ clusters, and the reaction of CH₄ with the CO₃²– site on the resulting clusters, were predicted. Our calculations show that CO₂ chemisorption at the La³⁺–O²– 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⁺–CO₃²– pair sites is similar to that at the La³⁺–O²– pair sites, which results in the formation of the bicarbonate species and the La–CH₃ bond, although the La³⁺–CO₃²– 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 CO₂ as a major byproduct is likely to act as a poison to the La₂O₃-based catalysts especially at modest reaction temperature.