Understanding the reaction mechanism of Kolbe electrolysis on Pt anodes

Abstract

Kolbe electrolysis has been proposed as an efficient electro-oxidation process to synthesize (un)symmetrical dimers from biomass-based carboxylic acids, but its mechanism remains controversial. In this work, we develop a microkinetic model based on density functional theory to study the reaction mechanism of Kolbe electrolysis of acetic acid (CH 3 COOH) on both pristine and partially oxidized Pt anodes. We show that the shift in the rate-determining step of the oxygen evolution reaction (OER) on a Pt(111)@α-PtO 2 surface from OH ∗ formation to H 2 O adsorption gives rise to large Tafel slopes, i.e., the inflection zones observed experimentally at high anodic potentials on Pt. Our simulations find that the CH 3 COO ∗ decarboxylation and CH 3 ∗ dimerization steps determine the activity of the Kolbe reaction. This work resolves major controversies in the mechanism of Kolbe electrolysis on Pt anodes: the origin of the inflection zone and the identity of the rate-limiting step. • A microkinetics model for Kolbe electrolysis of acetic acid on Pt anodes is developed • Oxidation of the Pt surface dramatically weakens the adsorption strength of Kolbe species • Suppressed activity of oxygen evolution reaction leads to the inflection zone • Decarboxylation and/or dimerization steps determine activity of Kolbe products Organic electrosynthesis has, in recent years, been revived as a promising pathway for the sustainable production of important chemicals. The advantages of electrosynthesis include mild reaction conditions, the use of green electricity, and the tunability of activity and selectivity with the applied potential. The Kolbe reaction converts biomass-derived carboxylic acids to high-value products through decarboxylation and dimerization. However, the mechanism of this reaction is not fully understood, which limits further optimization of the reaction systems. Building on previous mechanistic studies, this work identifies the key steps involved in Kolbe electrolysis of acetic acid. The mechanistic insights obtained suggest that tuning the relative adsorption strength of carboxylic species versus water in aqueous systems could promote Kolbe products. Electroorganic synthesis from biomass-derived chemicals has been revived in recent years, but reaction mechanisms remain to be established. Liu et al. perform a detailed mechanistic study of Kolbe electrolysis on oxidized platinum by developing a microkinetics model based on density functional theory calculations.