Direct Detection of Electron Transfer Reactions Underpinning the Tin-Catalyzed Electrochemical Reduction of CO₂ using Fourier-Transformed ac Voltammetry

Two underlying electron transfer processes that directly underpin the catalytic reduction of carbon dioxide (CO₂) to HCOO^– and CO at Sn electrodes have been detected using the higher order harmonic components available in Fourier-transformed large-amplitude ac voltammetry. Both closely spaced electron transfer processes are undetectable by dc voltammetry and are associated with the direct reduction of CO₂ species and have reversible potentials of approximately −1.27 and −1.40 V vs Ag/AgCl (1 M KCl). A mechanism involving a reversible inner-sphere one-electron reduction of CO₂ followed by a rate-determining CO₂^•– protonation step is proposed. Molecular CO₂ has been identified as the dominant electroactive species that undergoes a series of coupling electron transfer and chemical reactions to form the final products. The substantial difference in the catalytic responses of Sn­(SnO_<i>x</i>)-modified glassy carbon and Sn foil electrodes are attributed to their strongly preferred Sn (200) orientation and polycrystalline states, respectively. The Fourier-transformed ac technique should be generally applicable for predicting the performance of Sn catalysts