Understanding the reaction mechanism of Kolbe electrolysis on Pt anodes

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 (CH₃COOH) 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-PtO₂ surface from OH^∗ formation to H₂O 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 CH₃COO^∗ decarboxylation and CH₃^∗ 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.