Copper (Cu) is the only elemental metal that electrochemically catalyzes formation of significant amounts 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 (H2), ethylene (C2H4), and methane (CH4)] plus small amounts of other C2’s, C3’s and oxygenates. Due to its unique ability to catalyze hydrocarbon formation, Cu is a prototype to determine 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) calculations with an explicit description of water on the Cu(100) surface (experimentally shown to be stable under CO2RR conditions) to examine the initial reaction pathways to form CO and formate (HCOO–) from CO2 through free energy calculations at 298K and pH 7. We find that CO formation proceeds from physisorbed CO2 to chemisorbed CO2 (*CO2δ-), with a free energy barrier of ΔG‡=0.43eV, the rate determining step (RDS). The subsequent barriers of protonating *CO2δ- to form COOH* and then dissociating COOH* to form *CO are 0.37 eV and 0.30 eV, respectively. HCOO– formation proceeds through a very different pathway in which physisorbed CO2 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 CO2 binding, which might involve controlling pH, alloying or changing the structure at the nanoscale.

Reaction mechanisms for the electrochemical reduction of CO2 on the Cu(100) surface from quantum mechanics free energy calculations with explicit water

253rd National Meeting (2017)

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