New approaches to the study of electrochemical decarboxylation and the Kolbe reaction.

Abstract

A critical review of previous work on the Kolbe reaction is given where it is shown that very few applications of modern electrochemical techniques and no applications of recent electrochemical kinetic theory have been made previously to this reaction. No work on the electrochemical kinetics of formate decarboxylation has been carried out previously. The kinetics of electrochemical decarboxylation formate ions in pure anhydrous formic acid have been studied in detail for the first time as a model reaction for examination of the decarboxylation and radical coupling reactions that occur with higher aliphatic acids. Current-potential relations (Tafel slopes) have been obtained which show a sharp transition, not diffusion controlled, which is characteristic of passivation phenomena. The behavior is observed at platinum, palladium, gold and palladium-gold alloy electrodes. Tafel slopes are derived for "Langmuir" and "Temkin" conditions of surface coverage. Reversible potentials and exchange currents have been evaluated for the reaction. Galvanostatic charging, cathodic discharge and open circuit decay transients have been obtained for the decarboxylation of formate in formic acid which indicate the formation of films and are interpreted quantitatively in terms of the adsorption of reaction intermediates on the electrode surface. These intermediates are surmised to be HCOO· radicals, and the transition regions in the Tafel plots are identified with the formation of this adsorbed layer. Extended anodic polarisation at platinum and particularly at palladium leads to the formation of films of an anodic product which are considerably thicker than a monolayer, but at gold the film thickness does not exceed that of a monolayer. Following relatively long times (> 100 sec.) of anodic polarisation, film growth obeys the inverse logarithmic rate law deduced by Mott and Cabrera. During the course of the work, an interesting and evidently unique phenomenon of delayed gas evolution was discovered during studies of the formate decarboxylation reaction at palladium electrodes. The electrochemistry of this phenomenon has been studied in some detail and the delayed gas evolution is shown to be associated with attainment of a critical potential in the self-discharge process on open-circuit e.m.f. decay. The thick films formed at palladium are believed to be responsible, upon autocatalytic decomposition, for the delayed gas evolution phenomenon observed at this metal. A new method for deduction of adsorption pseudo-capacitance and charge associated with the adsorbed layer from open-circuit decay transients and Tafel parameters is used to obtain the pseudo-capacitance and charge associated with the transition region in the current-potential relationships for the formate decarboxylation. It is shown that this region corresponds to filling of the surface with adsorbed intermediates formed in the reaction. Possible reaction mechanisms are proposed and are examined in relation to the observed experimental results. The galvanostatic results enable distinctions to be made between some of the possible reaction mechanisms proposed. In the aqueous formate systems, current-potential relationships have been obtained which also exhibit transition regions; multilayer formation is indicated, including deposition of layers of different species corresponding to different potential arrest regions in the galvanostatic discharge transients. Reversible potentials and exchange currents have been evaluated for the reaction. Experimental results in the non-aqueous and aqueous formate systems are compared; It is suggested that the layers formed in the aqueous system may consist of oxide and discharged carboxylate and that the transition behavior and critical currents in the Kolbe reaction correspond to conditions for passivation of the metal by oxide and discharged carboxylate species. Electrochemical kinetic studies on the Kolbe reaction have been carried out with potassium trifluoroacetate in trifluoroacetic acid (100%). High yields of the Kolbe product C2F6 and CO2 are obtained and current-potential plots exhibiting transition behavior and linear Tafel regions are found. Tafel slopes are deduced for "Langmuir" and "Temkin" surface coverage conditions by surface intermediates and compared with experimental results. Plausible reaction mechanisms are proposed, which are supported by galvanostatic discharge transients; the latter indicate that the coverage by adsorbed intermediates in the trifluoroacetate reaction approaches a monolayer.