Spectroscopic evidences of the presence of hydrogenated species on the surface of copper during CO_2 electroreduction at low cathodic potentials

摘要

Carbon dioxide electroreduction on copper electrode was studied by surface enhanced Raman scattering (SERS) in K_2SO_4 aqueous solutions with different pH values. CO_2 was bubbled into the solution at 0 V vs. Ag/AgCl, i.e., on an oxidized copper surface. In acidic solutions (pH around 2.5), at -0.2 V, bands indicative of the presence of ethylene on the electrode surface were detected. Although ethylene is knowledgably a product of CO_2 electroreduction on copper, it was not experimentally identified on the electrode's surface at such a low cathodic potential in prior works. In solutions with pH around 2.5, CO bands were not observed, suggesting that hydrocarbons could be formed by a pathway that does not occur via adsorbed CO. In solutions with higher pHs, a complex spectral pattern, between 800 and 1700 cm~(-1), was observed at approximately -0.4 V. The observed spectrum closely resembles those reported in the literature for adsorption of monocarboxylic acids with small chains. The spectral features indicate the presence of a structure containing a double C=C bond, a carboxyl group, and C-H bonds on the electrode's surface. SERS spectra obtained in CO-saturated solution are also presented. However, in this case, no SERS bands were observed in the region between 800 and 1700 cm~(-1) at low cathodic potentials.