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

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• 作者：Changqing Chu ; Yonghui Zhao ; Shenggang Li ; Yuhan Sun
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：February 11, 2016
• 年：2016
• 卷：120
• 期：5
• 页码：2737-2746
• 全文大小：556K
• ISSN：1932-7455

文摘

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 CO₃^2– site on the resulting clusters, were predicted. Our calculations show that CO₂ chemisorption at the La³⁺–O^2– pair sites is thermodynamically and kinetically very favorable due to the strong basicity of the O^2– site on La₂O₃, which leads to the formation of the La³⁺–CO₃^2– pair sites. In addition, CH₄ activation at the La³⁺–CO₃^2– pair sites is similar to that at the La³⁺–O^2– pair sites, which results in the formation of the bicarbonate species and the La–CH₃ bond, although the La³⁺–CO₃^2– pair sites are much less reactive with CH₄ in terms of both thermodynamics and kinetics. Further thermodynamical calculations show that the CO₃^2– 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 CO₃^2– 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.