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

Abstract

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.