(658g) Theoretical Insights into Selectivity Differences to Form Ethylene and Ethanol in the Electrocatalytic Reduction of CO2 over Cu Electrodes

The selective conversion of CO₂ into chemicals and fuels can play a critical role in reducing CO₂ emissions. The electrocatalytic reduction of CO₂ (CO2RR) over Cu and Cu-based alloys has gained considerable interest due to the higher production of multi-carbon products such as ethylene and ethanol. The mechanisms and paths responsible for these transformations are complicated due to the number of electron-proton transfer steps involved and the influence of the complex metal/solution interfacial environment that drives these reactions.

In this work, we examine the elementary steps involved in controlling the selectivity differences between forming ethylene vs. ethanol. In an effort to elucidate the influence of complex electrochemical metal/solution interface, we carry out potential-dependent ab-initio molecular dynamics and density functional theory (DFT) calculations over the Cu(111) surface in the presence of explicit solvent and electrolyte molecules. Previous simulations indicate that the CO-COH* may be a likely C-C coupling intermediate. As such, we examine in detail the elementary paths involved in the reduction of CO-COH* to form ethylene and ethanol.

We first examine and compare the proton-coupled electron transfer (PCET) and direct hydrogen atom transfer paths involved in the initial reduction of CO-COH*. Our potential-dependent simulations show that each mechanism dominates at different potential ranges. Generally, the PCET paths preferentially add hydrogen to oxygen atoms over carbon atoms in the surface species, thus driving favorable dehydration steps. Through a series of PCET steps, we examine different reduction paths from CO-COH* as well as branching points that determine the bifurcation to C₂H₄ and C₂H₅OH as a function of potentials. In addition, the effects of KOH electrolytes on the reaction mechanics and product selectivity are also discussed. Our findings provide insights to improve the selectivity towards desired multi-carbon products.