Kinetics of anodic and chemical reactions in Kolbe's electrosynthesis

Electrode Kinetic Aspects of the Kolbe Reaction

“…Also, it involves an experimentally unlikely assumption of linear dependence of coverage on potential (in the Kolbe region) which the results do not substantiate (164). Recently it has been suggested that anomalously high Tafel slopes arise in the case of the Kolbe reaction owing to specific anion adsorption on the electrode (66,87). However, the relatively high Tafel slopes (~0.2-0.25 v) that are usually observed in case of the Kolbe reaction are probably beyond the values that might be expected on the basis of anion adsorption effects (31).…”

“…Other indirect electrochemical evidence against the formation and homogeneous recombination of R radicals is as follows: (a) Fioshin and Vasil'ev (66) have found that the rate of formation of C2H6 in the acetate Kolbe reaction is independent of rotation speed of a rotating platinum electrode (if radicals were spun off into the solution phase, their recombination would tend to be a less efficient process the more they were dispersed into the solution away from the boundary layer near the electrode); (b) Fleischmann and co-workers (70) have found that with increasing frequency in pulsed electrolysis the Kolbe dimerization becomes less favored than the Hofer-Moest oxidation (see section V.B.2). If the dimerization was an homogeneous process among "free" radicals, it is difficult to see how the observed result could arise since the origin of the effect is regarded as being associated with competitive processes of oxidation and recombination at the surface of the electrode (70).…”

“…Small quantities of ethane were produced in the anode compartment when the solution contained not more than 50% H2SO4. Also, in another investigation (68; also see ref 66) the influence of the nature of the cation on the Kolbe reaction was examined and found to be of little significance in the part of the current-potential curve which represented the occurrence of the Kolbe reaction. However, in the part of the polarization curve that corresponded to oxygen evolution, a cation effect could be observed at low concentrations of the alkali acetate, i.e., below 1 N. It was observed that the larger the cation radius, the lowrer the maximum current density for oxygen evolution and thus the higher the yield of ethane (at a given potential).…”

Understanding the reaction mechanism of Kolbe electrolysis on Pt anodes

“…Kolbe electrolysis of other carboxylic acids on Pt and Ir also show inflection zones at high potentials that accompany the selectivity transition from OER to the Kolbe reaction 31-33 . In the earliest mechanistic studies, Crum-Brown, Walker and co-workers proposed that Kolbe reaction is a free radical reaction (i.e., homogeneous solution phase reaction), which does not directly involve the electrode surface [34][35][36] . This free-radical theory was later refuted by rotating electrode 33 and pulsed electrolysis 37 experiments on Pt anodes, which showed that the electrode surface promotes the reaction rate and that Kolbe species reversibly adsorb on the Pt electrode.…”

“…They postulate that this transition occurs upon sufficient build-up of a "barrier-layer" film of CH3COO* at high enough potentials where CH3COOH is favored. The build-up of such a "barrier layer" would hinder the adsorption of H2O or OHspecies (the reactants of OER) on the surface and thereby suppress OER 33 .…”