Thermochemical and electrochemical CO2 reduction on octahedral Cu nanocluster: Role of solvent towards product selectivity

Abstract

There has been a major interest in the development of efficient catalysts for the electrochemical reduction of CO2 into useful chemicals and fuels. In this work, the thermochemical and electrochemical CO2 reduction pathways are systematically studied on the (111) facet of an octahedral copper (Cu85) nanocluster (NC) using density functional theory (DFT) calculations. An investigation is carried out to understand the catalytic activity of the Cu NC towards CO2 reduction to explore its activity and product selectivity (CH3OH vs. CH4; i.e., six-electron vs. eight-electron reduction reaction). Interestingly, CO2 adsorbs strongly the (111) facet of the Cu NC, as opposed to the previous reports on the periodic Cu(111) surfaces. Furthermore, such NC shows excellent catalytic activity towards direct CO bond dissociation, which is again in contrary to what was reported in the literatures. Besides, the CO2 hydrogenation reaction has been investigated using different proton transfer mechanisms (surface-hydrogenation, water-assisted, and water solvated) with/without the help of a solvent water molecule. We find that the water-assisted H-shuttling mechanism lowers the activation barrier significantly and thus favours the CO2 hydrogenation. However, the direct CO bond dissociation is very competing with respect to indirect CO bond dissociation. Based on our reaction free energy calculations, activation barrier calculations, and simulated Pourbaix calculations, we find that the Cu NC shows excellent catalytic activity and selectivity over any catalysts studied to date. Besides, the Cu NC requires a lower overpotential (0.53V) compared to the periodic Cu(111) surface (0.71V) and thus can be a promising catalyst for CO2 reduction reaction.