Reaction mechanisms for the electrochemical reduction of CO_2 on the Cu(100) surface from quantum mechanics free energy calculations with explicit water

Abstract

Copper (Cu) is the only elemental metal that electrochem. catalyzes formation of significant amts. of hydrocarbons, but it requires a high overpotential (0.9 V) for a reasonable current, and it leads to a mix of major products [including hydrogen (H_2), ethylene (C_2H_4), and methane (CH_4)] plus small amts. of other C 's, C 's and oxygenates. Due to its unique ability to catalyze hydrocarbon formation, Cu is a prototype to det. and validate the mechanism of hydrocarbon formation, to serve as the basis for designing new catalysts that increase product selectivity and rates while simultaneously lowering overpotentials. Here, we carry out Quantum Mechanics (QM) calcns. with an explicit description of water on the Cu(100) surface (exptl. shown to be stable under CO2RR conditions) to examine the initial reaction pathways to form CO and formate (HCOO‾) from CO_2 through free energy calcns. at 298K and pH 7. We find that CO formation proceeds from physisorbed CO_2 to chemisorbed CO_2 (*CO_2^(δ-)), with a free energy barrier of ΔG =0.43eV, the rate detg. step (RDS). The subsequent barriers of protonating *CO to form COOH* and then dissocg. COOH* to form *CO are 0.37 eV and 0.30 eV, resp. HCOO‾ formation proceeds through a very different pathway in which physisorbed CO reacts directly with a surface H* (along with electron transfer), leading to ΔG 0.80 eV. Thus, the competition between CO formation and HCOO formation occurs in the first electron transfer step. On Cu(100), the RDS for CO formation is lower, making CO the predominant product. Thus, to alter the product distribution we need to control this first step of CO_2 binding, which might involve controlling pH, alloying or changing the structure at the nanoscale.