(131g) Decoding the Mechanistic Pathways in Electrochemical Acetate Oxidation: The Impact of Electrolysis Conditions on Product Selectivity

Electrochemical partial oxidation of biomass-derived carboxylic aids such as acetate presents a sustainable route for producing renewable fuels and value-added chemicals under ambient conditions. Unlike the thermocatalytic process, electrosynthesis enables selective transformations by leveraging electricity from renewable sources [1]. Among carboxylic acids, acetate is a key platform intermediate generated during biomass upgrading and anaerobic digestion. Its oxidation, however, is governed by complex multielectron transfer mechanisms involving radical intermediates and is highly sensitive to catalyst surface states, electrolyte environment, and reactor configuration [2]. Given the similarities in intermediates with methane electrooxidation, unraveling the intricacies of acetate oxidation could also inform broader electrocatalytic strategies for C-H bond activation.

The current study provides a comprehensive mechanistic exploration of the electrochemical oxidation of acetate (AcOR) over noble metal electrodes, such as Pt, Au, Pd, in aqueous CH₃COOK electrolytes. A key focus is placed on determining the effect of surface oxidation states, membrane types, electrolyte concentrations, and electrolysis protocols in governing the product selectivity. Isotope labeling experiments using ¹³C-labeled acetate elucidate the fate of individual carbon atoms and confirm the generation of surface-bound methyl radicals as primary intermediates. When ¹³C is introduced in the carboxyl group, labeled CO₂ is detected, while labeling the methyl group leads to incorporation into hydrocarbon and oxygenate products, confirming decarboxylation as the initiating step. These radicals subsequently participate in Kolbe coupling or oxygenate formation via the Hofer-Moest mechanism, depending on electrochemical conditions.

Catalyst-dependent pathways are investigated by comparing product distributions and surface reconstruction across Pt, Au, and Pd. Pt surfaces favour stable Kolbe and other partial oxidation products due to persistent PtO₂ formation across a wide potential window [3]. In contrast, Au exhibits a narrower stability range for active surface oxides such as Au(OH)₃, AuOOH, Au₂O_3, which supports oxygenate production but quickly transitions to overoxidation at elevated potentials [4]. Pd, despite its position in the platinum group, primarily facilitates CO₂ and O₂ formation due to facile transitions to high valency oxides and susceptibility to dissolution [5].

Electrolyte composition emerges as a critical factor for tuning product selectivity. Increasing acetate concentration from 0.5M to 5.0 M not only buffers pH fluctuations but also stabilizes the availability of reactants at the interface, improving yields of desired products like methanol and formaldehyde. Similarly, the use of anion exchange membranes (AEMs) instead of proton exchange membranes (PEMs) significantly mitigates pH drift during electrolysis and enhances selectivity for oxygenates by maintaining favourable surface conditions and increasing hydroxide ion transport.

Finally, the scalability of AcOR is assessed using a membrane electrode assembly (MEA) based flow cell system, achieving higher current densities and product formation rates compared to batch cells. Pt/C catalysts maintain similar product distribution while overall faradaic efficiency increases under high mass transport conditions. Although CO₂ remains a dominant product under these conditions. Although CO₂ remains a dominant product under these conditions, selectivity towards formaldehyde is preserved, and a single pass conversion of 14% is achieved, indicating potential for industrial application.

Together, this study delivers a mechanistic framework for controlling acetate electro-oxidation through engineering catalyst surface oxidation states, electrolyte compositions, and electrochemical protocols. The results not only advance the understanding of AcOR but also provide insights into applicable broader challenges in selective C-C and C-H bond activation within electrochemical systems.

References:

1. Palkovits, R. (2018). Sustainable carbon sources and renewable energy: challenges and opportunities at the interface of catalysis and reaction engineering. Chem. Ing. Tech, 90, 1699-1708.

2. Nordkamp, M. O., Mei, B., Venderbosch, R., and Mul, G. (2022). Study on the Effect of Electrolyte pH during Kolbe Electrolysis of Acetic Acid on Pt Anodes. ChemCatChem, 14, e202200438.

3. Conway, B. E. (1995). Electrochemical oxide film formation at noble metals as a surface-chemical process. Prog. Surf. Sci., 49, 331-452.

4. Diaz-Morales, O., Calle-Vallejo, F., de Munck, C., and Koper, M. T. (2013). Electrochemical water splitting by gold: evidence for an oxide decomposition mechanism. Chem. Sci., 4, 2334-2343.

5. Grdeń, M., Łukaszewski, M., Jerkiewicz, G., and Czerwiński, A. (2008). Electrochemical behaviour of palladium electrode: Oxidation, electrodissolution and ionic adsorption. Electrochim. Acta, 53, 7583-7598.