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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.

The role of metastable surface oxides in the reduction of CO 2 on lead, bismuth, tin, and indium electrodes was probed using in situ attenuated total reflectance infrared (ATR-IR) spectroelectrochemistry. The effect of the surface oxide on the Faradaic efficiency of CO 2 reduction to formic acid was studied by etching and anodizing the electrodes, and the results were correlated with respect to the observed spectroscopic behavior of the catalysts. A metastable oxide is observed on lead, tin, and indium cathodes under the electrochemical conditions necessary for CO 2 reduction. Spectroscopic evidence suggests that bismuth electrodes are fully reduced to the metal under the same conditions. The dynamics of the electroreduction of CO 2 at lead and bismuth electrodes appears to be different from that on on tin and indium electrodes, which suggests that these catalysts act through different mechanistic pathways. The post-transition-metal block can be divided into three classes of materials: oxide-active materials, oxide-buffered materials, and oxide-independent materials, and the mechanistic differences are discussed.

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