The electrochemical reduction of CO2 on a copper electrode in 1-n -butyl-3-methyl imidazolium tetrafluoroborate (BMI.BF4 ) monitored by surface-enhanced Raman scattering (SERS)

Abstract

The electrochemical conversion of CO2 into value-added products using room temperature ionic liquids as solvent/electrolyte has been proposed as an alternative to minimize the environmental effects of CO2 emissions. A key issue in the design of electrochemical systems for the reduction of CO2 is the in situ identification of intermediate surface species as well as reaction products. Copper electrodes, besides being used as cathodes in the electrochemical reduction of CO2, present surface-enhanced Raman scattering (SERS) when properly activated. In this sense, the electrochemical reduction of CO2 over a copper electrode in the room temperature ionic liquids 1-n-butyl-3-methyl imidazolium tetrafluoroborate (BMI.BF4) was investigated by cyclic voltammetry and by in situ SERS. The cyclic voltammetries have shown that the presence of CO2 on the BMI.BF4 anticipates the reduction of BMI+ to the corresponding carbene. Fourier-transform-SERS spectra excited at 1064 nm and SERS spectra excited at 632.8 nm have shown vibrational signals from adsorbed CO. These SERS results indicated that CO adsorbs on the copper surface at two different surface sites. The observation of a 2275 cm−1 vibration in the SERS spectra also confirmed the presence of chemically adsorbed CO2. Other products of CO2 reduction in BMI.BF4, besides CO, were identified, including BMI carbene and the BMI-CO2 adduct. The SERS results also suggest that the presence of a thin film of Cu2O on the copper surface anticipates the reduction of CO2 to CO, an important component of syngas. Copyright © 2016 John Wiley & Sons, Ltd.