Plasmonic-Promoted Formation of Surface Adsorbed Stochastic CO during Electrochemical CO₂ and CO Reduction on Cu at Extreme Low Overpotentials

Reducing the formation overpotential of key reaction intermediates represents a major challenge in developing broad electrocatalytic reactions. Recent vibrational spectroscopic studies of electrochemical CO₂ or CO reduction reaction (CO₍₂₎RR) characterized an interesting formation of stochastic CO (CO_stochastic) intermediate with negligible energy losses under certain circumstances. Yet, the precise formation conditions and mechanisms remain unclear, hindering the correct understanding of related spectroscopic results and utilization of these effects to develop the CO₍₂₎RR and other electrocatalytic reactions. Herein, we combine in situ attenuated total reflection infrared absorption (ATR-IR) and Raman spectroscopies to systematically study the origins and generation mechanisms of these CO_stochastic. We reveal for the first time that the CO_stochastic originates from plasmonic excitation of Cu by laser excitation and exists only in in situ Raman but not in ATR-IR measurements utilizing the exact same catalysts and electrochemical cell. Both the illumination wavelength and the fluence are important for the formation of CO_stochastic. Furthermore, the surface speciation of the catalyst, i.e., the copresence of mixed metallic Cu⁰ and Cu⁺ states, is discovered to be crucial for the formation of CO_stochastic. These results demonstrate that mechanistic studies of the CO₍₂₎RR and other electrocatalytic reactions utilizing in situ Raman spectroscopy must carefully control the excitation fluence to avoid signals due to plasmonic excitations of the catalysts other than the electrochemical processes. Meanwhile, plasmon-enhanced electrochemistry, although still impractical for promoting CO₂RR under ordinary illumination, may inspire new strategies for designing light-driven electrochemical reactions at extremely low overpotentials.