Intercepting Elusive Intermediates in Cu-Mediated CO Electrochemical Reduction with Alkyl Species

Abstract

Understanding of the reaction network of Cucatalyzed CO 2 /CO electroreduction reaction [CO (2) RR] remains incomplete despite intense research efforts. This is in part because the rate-determining step occurs early in the reaction network, leading to short lifetimes of subsequent surface-bound intermediates, the knowledge of which is key to selectivity control. In this work, we demonstrate that alkyl groups can effectively couple with surface intermediates in the Cu-catalyzed CORR and, for the first time, intercept elusive C 1 and C 2 intermediates. Combined reactivity data and in situ spectroscopic results demonstrated that surface-bound alkyl groups derived from the corresponding alkyl iodides are able to couple with adsorbed CO to form carboxylates and ketones via one and two successive nucleophilic attacks, respectively. Leveraging this new chemistry, CH x (x ≤ 3) and C 2 H x (x ≤ 4) are intercepted and identified as precursors for methane and n-propanol in the CORR, respectively. Importantly, reaction pathways leading to methane and C 2+ products are not intrinsically orthogonal, but their connection is mainly impeded by low coverages of energetic intermediates. This study shows that perturbing the reaction of interest by introducing a slightly interacting probe reaction network could be an effective and general strategy in mechanistic studies of catalytic reactions.

Mechanistic Insights into the Formation of Hydroxyacetone, Acetone, and 1,2-Propanediol from Electrochemical CO2 Reduction on Copper

“…The blue mechanism in (b) represent the likely pathway for ethylene glycol, glycolaldehyde, and glyoxal based on the results in this work, as also proposed by Garza et.al . The pathway in green is based on the results shown by Li et al and the trends observed in this work and the work of Delmo et al…”

“…This approach is a useful strategy to experimentally identify potential intermediates, although it does not rule out the possibility of alternative C 2 intermediates participating in the reaction pathway. The reaction pathway for 1-propanol will not be discussed here as it has been discussed in detail elsewhere. − Additionally, a small amount of hydroxyacetone (FE < 0.05%) was detected, which was also observed by Kuhl et al However, unlike 1-propanol, the reaction mechanism for hydroxyacetone formation has not yet been investigated. Interestingly, based on the reduction of hydroxycarbonyls, as shown in Figure S7, as hydroxyacetone was detected, it is also expected to form 1,2-propanediol and acetone since its reduction leads to these molecules at −1.0 V. Nonetheless, it is possible that these products were formed, but their productivity was below the chromatograph’s detection limit.…”

“…The main C 3 product detected was 1-propanol, which is primarily formed through methylcarbonyl−CO coupling, as previously shown by Xu's group. 15,16 The formation of acetone was also investigated and was attributed to methylcarbonyl−CH 3 * coupling, as also suggested by Li et al 15 Hydroxyacetone and acetone exhibit very low faradaic efficiencies during CO 2 RR. 23,24 Interestingly, acetone can be easily formed on Pd 25 and Pt 26,27 electrodes through the reduction of hydroxyacetone.…”

“…Only 1-propanol was observed as C 3 compound, as also observed before. 15,23,35 The formation of hydroxyacetone and 1,2-propanediol was found to be higher in the electrolyte that contained glycolaldehyde compared to the one containing glyoxal. This observation suggests that glycolaldehyde is the most likely intermediate for hydrox-yacetone formation.…”

“…As a result, their formation pathways are not fully understood, especially in the case of the minor C 3 compounds. Recently, Li et al 15 used alkyl iodides to intercept elusive C 1 and C 2 intermediates during CORR, providing insights into the reaction pathways of major products such as ethanol, ethylene, and 1-propanol. They have also found insights about acetone formation, one of the minors C 3 -oxygenates, where the coupling between methylcarbonyl and CH 3 * was the suggested likely pathway for its formation.…”