Mechanistic Insights into the Reduction of CO₂ on Tin Electrodes using in Situ ATR-IR Spectroscopy

The reduction of CO₂ on tin cathodes was studied using in situ attenuated total reflectance infrared spectroscopy (ATR-IR). Thin films of a mixed Sn/SnO_x species were deposited onto a single-crystal ZnSe ATR crystal. Peaks centered at about 1500, 1385, and 1100 cm^-1, attributed to a surface-bound monodentate tin carbonate species, were consistently present under conditions at which CO₂ reduction takes place. It was shown that these peaks are only present at potentials where CO₂ reduction is observed. Moreover, these peaks disappear if the pH of the reaction is too low or if the tin surface is chemically etched to remove surface oxide. Sn₆O₄(OH)₄ and SnO₂ nanoparticles were shown to be catalytically active for CO₂ reduction, and insights into the oxidation state of the catalytically active species are gained from a comparison of the catalytic behavior of the two nanoparticle species. From these experiments, a mechanism governing the reduction of CO₂ on tin electrodes is proposed. (Chemical Equation Presented).