Mechanism for acetate formation in CO(2) reduction on Cu: Selectivity trends with pH and nanostructuring derive from mass transport

Abstract

Nanostructured Cu catalysts have increased the selectivities and geometric activities for high value C-C coupled (C2) products in the electrochemical CO(2) reduction reaction (CO(2)RR). The selectivity among the high-value C2 products is also altered, where for instance the yield of acetate increases with alkalinity and is dependent on the catalyst morphology. The reaction mechanisms behind the selectivity towards acetate vs. other C2 products remain controversial. In this work, we elucidate the reaction mechanism behind selectivity towards acetate by using ab-initio simulations, a coupled kinetic-transport model, and loading experiments. We find that trends in acetate selectivity can be rationalized from variations in electrolyte pH and the local mass transport properties of the catalyst and not from changes of Cu’s intrinsic activity. The selectivity mechanism originates in the transport of ketene, a stable (closed shell) intermediate, away from the catalyst surface into solution where it reacts to acetate. While such a mechanism has not yet been discussed in CO(2)RR, variants of it may explain similar selectivity fluctuations observed for other stable intermediates like CO and acetaldehyde. Our proposed mechanism suggests acetate selectivity to increase with increasing pH, decreasing catalyst roughness and to significantly vary with applied potential.

Modeling Operando Electrochemical CO2 Reduction

“…Heenen et al employed an ab initio kinetic-transport model to describe the mechanism for acetate formation on copper, proposing H 2 CCO coupling to OH – in solution as the main precursor toward CH 3 CO 2 – with ketene as common C 2+ intermediate (Figure b, further details on the mechanism in section ). Such analogy between acetate and ethanol reaction route is further supported by 13 CO 2 - 12 CO electroreduction experiments on OD-Cu copper. Different 13 C/ 12 C ratios were detected in the reaction products, namely ethylene, ethanol, acetate, and n -propanol .…”

“…More recently, Chan and co-workers used a coupled microkinetic and transport model to study the mechanism for acetate formation on Cu surfaces . The authors proposed a reaction equation where the surface intermediate ketene (*CH 2 CO) desorbs and reacts with OH – in solution.…”

“…The excluded alternative reduction steps are reported in the bottom left. Adapted with permission from ref . Granted by the authors under Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License…”