Role of a Hydroxide Layer on Cu Electrodes in Electrochemical CO₂ Reduction

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.