Mechanistic In Situ ATR-FTIR Studies on the Adsorption and Desorption of Major Intermediates in CO2 Electrochemical Reduction on CuO Nanoparticles.

Increasing levels of carbon dioxide (CO2) from human activities is affecting the ecosystem and civilization as we know it. CO2 removal from the atmosphere and emission reduction by heavy industries through carbon capture, utilization, and storage (CCUS) technologies to store or convert CO2 to useful products or fuels is a popular approach to meet net zero targets by 2050. One promising process of CO2 removal and conversion is CO2 electrochemical reduction (CO2ER) using metal and metal oxide catalysts, particularly copper-based materials. However, the current limitations of CO2ER stem from the low product selectivity of copper electrocatalysts due to existing knowledge gaps of the reaction mechanisms using surfaces that normally have native oxide layers. Here, we report systematic control studies of the surface interactions of major intermediates in CO2ER, formate, bicarbonate, and acetate, with CuO nanoparticles in situ and in real time using attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). Spectra were collected as a function of concentration, pH, and time in the dark and the in absence of added electrolytes. Isotopic exchange experiments were also performed to elucidate the type of surface complexes from H/D exchange. Our results show that the organics and bicarbonate form mostly outer-sphere complexes mediated by hydrogen bonding with CuO nanoparticles with Gibbs free energy of adsorption of about -25 kJ mol-1. The desorption kinetics of the surface species indicated relatively fast and slow regions reflective of the heterogeneity of sites that affect the strength of hydrogen bonding. These results suggest that hydrogen bonding, whether intermolecular or with surface sites on CuO nanoparticles, might be playing a more important role in the CO2ER reaction mechanism than previously thought, contributing to the lack of product selectivity.