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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.