Cu is known as one of the most promising metallic catalysts for conversion of CO2 to hydrocarbons such as methane, ethylene, and ethanol by electrochemical reduction. The oxide-derived Cu (OD-Cu) moiety has been investigated as a candidate for enhancing the activity for CO2 electrochemical reduction to C2+ products. The reduction process is affected by catalytic grain boundary, local pH, residual oxygen atoms, and other features of the catalysts. In order to understand the detailed mechanism, we performed in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (in situ ATR-SEIRAS) measurements for CO2 reduction using several different Cu electrodes whose oxidation states are controlled. The spectroscopic investigations demonstrate that a copper oxide electrode (Cu2O) has low activity against CO2 reduction on the basis of low-level detection of CO as an intermediate of CO2 reduction. On the other hand, other Cu electrodes possessing an OH layer on the Cu surface (Cu­(OH)2/Cu) and metallic Cu exhibit higher CO2 reduction activity with significantly greater detection of produced CO. When the metallic Cu electrode is used, only one peak (2060 cm–1) assignable to CO bound to the atop site of Cu is observed. However, additional peaks are detected in the range of 1900–2100 cm–1 when the Cu­(OH)2/Cu electrode is used. To understand these findings, the adsorption energy of CO on a Cu­(OH)2/Cu electrode and the CO stretching frequency were evaluated by performing DFT calculations. The adsorption energy is enhanced and the CO stretching frequencies are shifted to lower energy in comparison with those using a metallic Cu electrode. These results indicate that it is predominantly favorable to adsorb some CO molecules near the OH moiety of the Cu­(OH)2/Cu electrode and to induce interactions of CO molecules with each other. This observation is consistent with the results of controlled potential electrolysis (CPE), which generates C2+ products as previously reported. When CPE is carried out in D2O solution, residual and/or adsorbed OD– groups on Cu are detected by ATR-SEIRAS and the surface of the Cu­(OH)2/Cu electrode is confirmed to be metallic Cu, as measured by in situ Raman and XPS. From the ATR-SEIRAS experiments when switching from under CO2 to Ar during the electrochemical reduction, the OH layer is suggested to prevent deactivation of the Cu electrode via formation of the CO layer, which is detected as a bridge-bounded form on the metallic Cu electrode. The above findings indicate that the OH layer provides the advantage of attracting CO molecules closer to each other while reducing them to C2+ products without any degradation during electrolysis.

Cu electrode-based electrochemical CO 2 reduction using renewable energy is a promising method for conversion of CO 2 to useful compounds such as methane, ethylene, and ethanol. Heteroatom-doped and/or -derived Cu as oxide-derived Cu has been investigated in context of development of a stable catalyst with high selectivity, whereas the role of heteroatoms is not yet well understood. It is not known whether heteroatoms act as a moiety of the catalyst or simply induce reconstruction of the catalyst. This work is an investigation of the role of the heteroatom in electrocatalytic CO 2 reduction with a Cu electrode modified with methanethiol monolayers (MT−Cu), which is able to distinguish the presence of heteroatom contamination originating from electrolyte or air. Controlled potential electrolysis of CO 2 using an MT−Cu electrode at −1.8 V at Ag/AgCl exhibits greater selectivity for C 2 products than an unmodified polycrystalline Cu electrode (bare Cu). On the other hand, a sulfur-modified Cu (S−Cu) electrode predominantly generates formate as a CO 2 reduction product. In an investigation of the mechanism, an in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy instrument is used as a powerful surface analyzer. Scanning electron microscopy, grazing-incidence wideangle X-ray scattering (GIWAXS), and X-ray spectroscopy (XPS) are also employed in the investigation. The spectroscopic data show that reconstruction and formation of Cu + on the Cu surface occur at negative potential greater than −1.4 V vs Ag/AgCl by electrochemical reduction of methanethiol monolayers. DFT calculations are also performed under conditions close to the experimental conditions of electrical bias and aqueous electrolyte. The results indicate that a roughened surface is favorable for generating C 2 products. In addition, the Cu + moiety promotes generation of C 2 products, demonstrating that the doped heteroatom plays a crucial role in electrochemical CO 2 reduction.

The catalytic conversion of CO 2 to useful compounds is of great importance from the viewpoint of global warming and development of alternatives to fossil fuels. Electrochemical reduction of CO 2 using aromatic Nheterocylic molecules is a promising research area. We describe a high performance electrochemical system for reducing CO 2 to formate, methanol, and CO using imidazole incorporated into a phosphonium-type ionic liquid-modified Au electrode, imidazole@IL/Au, at a low onset-potential of −0.32 V versus Ag/AgCl. This represents a significant improvement relative to the onset-potential obtained using a conventional Au electrode (−0.56 V). In the reduction carried out at −0.4 V, formate is mainly generated and methanol and CO are also generated with high efficiency at −0.6 ∼ −0.8 V. The generation of methanol is confirmed by experiments using 13 CO 2 to generate 13 CH 3 OH. To understand the reaction behavior of CO 2 reduction, we characterized the reactions by conducting potential-and time-dependent in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (SEIRAS) measurements in D 2 O. During electrochemical CO 2 reduction at −0.8 V, the C−O stretching band for CDOD (or COD) increases and the CO stretching band for COOD increases at −0.4 V. These findings indicate that CO 2 reduction intermediates, CDOD (or COD) and COOD, are formed, depending on the reduction potential, to convert CO 2 to methanol and formate, respectively.