C−C Coupling Is Unlikely to Be the Rate‐Determining Step in the Formation of C2+ Products in the Copper‐Catalyzed Electrochemical Reduction of CO

The identity of the rate-determining step (RDS) in the electrochemical CO reduction reaction (CORR) on Cu catalysts remains unresolved because: 1) the presence of mass transport limitation of CO; and 2) the absence of quantitative correlation between CO partial pressure (pCO ) and surface CO coverage. In this work, we determined CO adsorption isotherms on Cu in a broad pH range of 7.2-12.9. Together with electrokinetic data, we demonstrate that the reaction orders of adsorbed CO at pCO <0.4 and >0.6 atm are 1st and 0th , respectively, for multi-carbon (C2+ ) products on three Cu catalysts. These results indicate that the C-C coupling is unlikely to be the RDS in the formation of C2+ products in the CORR. We propose that the hydrogenation of CO with adsorbed water is the RDS, and the site competition between CO and water leads to the observed transition of the CO reaction order.