Mechanistic Insights into the Electroreduction of Carbon Dioxide to Formate on Palladium

The electrochemical reduction of carbon dioxide (or the CO₂-reduction reaction, CO₂RR) presents a promising strategy to mitigate CO₂ emissions while producing valuable chemical feedstocks. Palladium (Pd) catalysts are particularly interesting for their capacity to selectively produce formate at low overpotentials and carbon monoxide (CO) at higher overpotentials. However, palladium's CO₂-to-formate activity is often hindered by the progressive poisoning of its surface with CO. To shed light on the parameters that control this performance-determining process, in this study we employ Operando grazing incidence X-ray absorption spectroscopy and attenuated total reflectance surface-enhanced infrared absorption spectroscopy to investigate the CO₂RR mechanism on carbon-supported Pd nanoparticles (Pd/C) and a freestanding Pd aerogel with similar electrochemical surface areas but substantial differences in hydride formation, CO poisoning, and catalytic performance. Pd/C demonstrates rapid hydride formation and constant formate activity at -100 and -200 mV vs the reversible hydrogen electrode, revealing an indirect correlation between activity for formate and hydride stoichiometry that strongly indicates the active involvement of the surface hydride in the CO₂RR to formate. In contrast, the Pd aerogel suffers from rapid CO surface poisoning and a concomitantly negligible formate-production activity at the same potentials. These differences in catalytic behavior are linked to an increased presence of grain boundaries in the aerogel's surface that has been tied to a reduction in the activation barrier for CO₂ conversion to the surface-adsorbed *COOH and the subsequent formation of strongly adsorbed CO. As such, our findings highlight how optimizing the structural features of Pd-based surfaces can lead to significant enhancements in their efficiency toward formate production.