Empirical Insights into the CO2 Reduction Reaction Mechanism: A Study of the Reduction of CO2, CO and Formaldehyde on Cu Electrodes By Differential Electrochemical Mass Spectrometry

This work describes results from a first-ever attempt to employ differential electrochemical mass spectrometry (DEMS) of selectively pre-adsorbed reactants and (postulated) intermediates as a supplementary experimental approach in the study of electrocatalytic reaction mechanisms. For the specific case of the Cu-catalyzed electrochemical reduction of CO₂, dilute solutions of CO₂, CO and HCHO in aqueous KHCO₃ were placed separately in a DEMS stationary thin-layer electrochemical cell in contact with a Cu electrode. Simultaneous faradaic-current (CV) and mass-spectrometric-current (MSCV) voltammograms were then obtained for each solution. The only reduction products detected by mass spectrometry were H₂, CH₄, H₂C=CH₂ and CH₃CH₂OH; CO was also assayed but only from the reduction of CO₂.

The results prompt the following empirical inferences: (i) The slight but not imperceptible drop in the hydrogen-evolution current may not be caused by CO poisoning. It also exists for HCHO, a more feeble adsorbate, in the absence of the monoxide. In addition, the adsorption strength of H₂ is four times greater than that of CO. (ii) CO is the first product of CO₂ reduction, as well as the first intermediate in more advanced reactions that include formation of pure and oxygenated hydrocarbons; this is in conformity with the (almost) unanimously held view. (iii) HCHO is not a precursor for C=C double-bond formation. (iv) HCHO is a secondary intermediate for the production of methane and ethanol. (v) The generation of CH₄ and CH₃CH₂OH from adsorbed CO occurs via two pathways: one requires a theoretically postulated surface species, CO protonated on the C atom, and the other involves adsorbed HCHO, constituted after the rate-limiting protonation step. (vi) The generation of CH₄ and CH₃CH₂OH from CO has a much higher activation barrier than conversions from HCHO; not unexpected since the reactions transpire after the slow Cu–OCH⁺ formation and, consequently, are not highly activated.