Electrochemical reduction of CO₂ on silver as probed by surface‐enhanced Raman scattering

Surface‐enhanced Raman scattering (SERS) spectroscopy was used to explore the electrochemical reduction of CO₂ at a polycrystalline Ag electrode in KHCO₃ aqueous solution saturated with CO₂ at room temperature. Raman scattering from the oxidation‐reduction cycled Ag electrode suggests the formation of CO and HCOOH by CO₂ reduction. At an electrode potential of 0.08 V (vs. Ag/AgCl), adsorbed CO gives rise to a C‐O stretch band at 2128 cm⁻¹. This band decreases in intensity and shifts to higher wavenumber with an increase of negative potential in the range from 0.0 to − 0.24 V. A different potential dependence is observed for the C‐H stretch mode of adsorbed HCOO⁻ (2870 cm⁻¹ at − 0.8 V). The O‐C‐O antisymmetric and symmetric stretch modes of HCOO⁻ observed at near 1550 and 1350 cm⁻¹ decrease in intensity with increasing negative potential, presenting a striking contrast to the C‐H stretch mode. The SERS experiment with a CO₂‐saturated KHCO₃D₂O solution is also described and discussed.