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

Abstract

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 onsurface 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 CH 3 * to CH 2 * 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 CH 2 * 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 CH 2 * 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 C 2 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 steplike 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.

Coupling of Methane and Carbon Dioxide Mediated by Diatomic Copper Boride Cations

“…The formation of CH x ( x =2, 3) species has been reported as the first step in the direct coupling of CH 4 and CO 2 to generate the C 2 oxygenated chemicals . Recently, Trinh and co‐workers predicted that boron‐doped copper is a potential catalyst to activate methane to form the absorbed CH x ( x =2, 3) on the catalytic surface and selectively promote the C−C coupling reaction to produce ethylene . Inspired by this prediction, we prepared Cu x B y + ions by laser ablation of a 63 Cu/ 11 B mixed sample disk.…”

Coupling of Methane and Carbon Dioxide Mediated by Diatomic Copper Boride Cations

“…[9] Themonoatomic Ta + is the only gas-phase species reported to facilitate the coupling of CH 4 and CO 2 to generate ketene, [10] H 2 C=C=O, which is an important intermediate for syngas-toethylene conversion [11] and acetylation of chemical compounds. [14] Inspired by this prediction, we prepared Cu x B y + ions by laser ablation of a 63 Cu/ 11 Bm ixed sample disk. Theformation of CH x (x = 2, 3) species has been reported as the first step in the direct coupling of CH 4 and CO 2 to generate the C 2 oxygenated chemicals.…”

“…[3,13] Recently,T rinh and co-workers predicted that boron-doped copper is apotential catalyst to activate methane to form the absorbed CH x (x = 2, 3) on the catalytic surface and selectively promote the C À Cc oupling reaction to produce ethylene. [14] Inspired by this prediction, we prepared Cu x B y + ions by laser ablation of a 63 Cu/ 11 Bm ixed sample disk. TheC uB + species with the highest ion intensity was mass-selected, confined and cooled, and then successively reacted with CH 4 /CD 4 and CO 2 in al inear ion trap reactor [15] (see the Supporting Information for more details).…”

Investigating metal oxides for C1 chemistry : a computational and experimental study

“…Transition metal oxides (TMOs) are widely used in the form of pure, supported, and zeolite incorporated structures as catalysts for various industrially important reactions like chemical looping combustion, 27,110 selective oxidation, [111][112][113][114][115][116][117] and dehydrogenation of hydrocarbons to produce multiple highvalue chemicals. [117][118][119] Catalytic performance of TMOs for a particular reaction or a product is an interplay between (i) surface characteristics, such as the degree of unsaturated sites, acid-base characteristics, facet distribution (presence of one or more dominant facets), 51,119 lattice oxygen binding energies, ease of vacancy formation, presence of cationic or anionic vacancies, and the type and degree of doping…”

“…111,116,120 and, (ii) adsorbate-surface interactions. Presence of different kinds of metal and oxygen sites51,113,119,121 adds to the limitations of experimental surface science techniques59,122 in the identification and quantification of the number of active sites, 59, 122-124 subsequently making it challenging to determine the turn-over-frequency for metal oxides.…”