Sub-Surface Boron-Doped Copper for Methane Activation and Coupling: First-Principles Investigation of the Structure, Activity, and Selectivity of the Catalyst

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• 作者：Quang Thang TrinhArghya Banerjee ; Yanhui Yang ; Samir H. Mushrif
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：January 19, 2017
• 年：2017
• 卷：121
• 期：2
• 页码：1099-1112
• 全文大小：922K
• ISSN：1932-7455

文摘

Copper (Cu) is a commercial catalyst for the synthesis of methanol from syngas, low-temperature water gas shift reaction, oleo-chemical processing, and for the fabrication of graphene by chemical vapor deposition. However, high barriers for C–H bond activation and the ease of formation of carbon/graphene on its surface limits its application in the utilization and conversion of methane to bulk chemicals. In the present paper, using first-principles calculations, we predict that Cu catalyst doped with a monolayer of sub-surface boron (B–Cu) can efficiently activate the C–H bond of methane and can selectively facilitate the C–C coupling reaction. Boron binds strongest at the sub-surface octahedral site of Cu and the thermodynamic driving force for the diffusion of B from an on-surface to the sub-surface position in Cu is stronger than that for the experimentally synthesizable B–Ni (sub-surface boron in nickel) catalyst, providing a proof of concept for the experimental synthesis of this novel catalyst. Additionally, the first-principles computed free energy of the reaction to form B–Cu from boron precursor and Cu is also favorable. The presence of the monolayer sub-surface B in Cu creates a corrugated step-like structure on the Cu surface and significantly brings down the methane C–H activation barrier from 174 kJ/mol on Cu(111) to only 75 kJ/mol on B–Cu. The subsequent dehydrogenation of the adsorbed CH₃* to CH₂* is also kinetically and thermodynamically feasible. Our calculations also suggest that, unlike most of the transition metals, complete decomposition of methane to carbon would not be favored on B–Cu. The dissociation of the surface CH₂* moiety on B–Cu is limited due to the high activation barrier of 161 kJ/mol and lower relative stability of the resultant CH* species, under reaction conditions. The coupling of CH₂* fragments however is kinetically and thermodynamically favorable, with an activation barrier of only 92 kJ/mol; suggesting that B–Cu catalyst would have higher selectivity toward C₂ hydrocarbons. Furthermore, the formation of carbon from the adsorbed CH* moiety has a very high activation barrier of 197 kJ/mol and the completely dehydrogenated C* is relatively much less stable than CH*, under reaction conditions; predicting that coking might not be an issue on the B–Cu catalyst. Evaluation of C–H activation on Cu(110) surface, which has a similar step-like surface structure as B–Cu, and Bader charge and density of states analyses of B–Cu reveal that the geometrical/corrugation effect and the charge transfer from B to Cu synergistically promote the C–H activation on B–Cu, making it as active as other expensive transition metals like Rh, Ru, Ir, and Pt.