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

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. (Chemical Equation Presented).