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 KO). A mechanism involving a reversible inner sphere one-electron reduction of CO2 followed by a rate-determining CO2 center dot- 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.