Methanethiol SAMs Induce Reconstruction and Formation of Cu+ on a Cu Catalyst under Electrochemical CO2 Reduction

Cu electrode-based electrochemical CO2 reduction using renewable energy is a promising method for conversion of CO2 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 CO2 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 CO2 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 CO2 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 CO2 reduction.