INFRARED-SPECTROSCOPY OF ADSORBED CO AND INTERMEDIATE SPECIES IN ELECTROCHEMICAL REDUCTION OF CO2 TO HYDROCARBONS ON A CU ELECTRODE

Voltammetric measurements showed in the previous paper that charge transfer is accompanied with CO adsorption on Cu electrode. Adsorbed CO is present at Cu electrode surface below the potential of the cathodic charge transfer, and not present above the potential, as confirmed by infrared spectroscopy. This article reports significant features of in situ infrared spectra of adsorbed CO in detail, obtained with a Cu polycrystal electrode in aqueous electrolytes at 0 to 2 degrees C with regard to electrochemical reduction of CO2 to hydrocarbons. Two infrared absorption peaks were assigned to adsorbed CO. A sharp absorption band appeared at ca. 2080 cm(-1) between -0.8 and -1.0 V vs. nhe in 0.2 M KHCO3. Another absorption peak was broad at 2040 cm(-1) between -0.7 and -0.8 V in the same solution. The infrared absorption intensity is closely related with the amount of charge accompanied with CO adsorption. The infrared absorption bands were studied using D2O electrolyte solutions; the results agreed well with those obtained in H2O electrolyte system. Thus the spectra will not be affected by hydrogen. Infrared absorption bands of reduced CO2 were measured as well, showing identical spectroscopic features with adsorbed CO. The reduced CO2 is present at -1.0 V at Cu electrode and not present at -0.4 V. The reduced CO2 at Cu electrode is thus identified as adsorbed CO.