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

Wednesday, October 14, 2015
West Hall 1 (Phoenix Convention Center)
A. Javier, B. Chmielowiec, J. Sanabria-Chinchilla, Y. G. Kim, J. H. Baricuatro (California Institute of Technology), and M. P. Soriaga (California Institute of Technology)
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.