CO₂ Reduction Promoted by Imidazole Supported on a Phosphonium-Type Ionic-Liquid-Modified Au Electrode at a Low Overpotential

The catalytic conversion of CO₂ to useful compounds is of great importance from the viewpoint of global warming and development of alternatives to fossil fuels. Electrochemical reduction of CO₂ using aromatic N-heterocylic molecules is a promising research area. We describe a high performance electrochemical system for reducing CO₂ 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 ¹³CO₂ to generate ¹³CH₃OH. To understand the reaction behavior of CO₂ reduction, we characterized the reactions by conducting potential- and time-dependent in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (SEIRAS) measurements in D₂O. During electrochemical CO₂ 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 CO₂ reduction intermediates, CDOD (or COD) and COOD, are formed, depending on the reduction potential, to convert CO₂ to methanol and formate, respectively.