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(SnOx)-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.