NEW APPROACHES TO THE STUDY OF ELECTROCHEMICAL DECARBOXYLATION AND THE KOLBE REACTION: PART III. QUANTITATIVE ANALYSIS OF DECAY AND DISCHARGE TRANSIENTS AND THE ROLE OF ADSORBED INTERMEDIATES

Abstract

Galvanostatic cathodic discharge and open-circuit decay transients have been obtained for the decarboxylation of formate in formic acid and interpreted quantitatively in terms of the adsorption of intermediates in the reaction. These intermediates are identified with HCOO• radicals. Extended anodic polarization at platinum and particularly at palladium leads to the formation of films of an anodic product which are considerably thicker than a monolayer. After relatively long times (> 100 seconds) of anodic polarization, the film growth obeys the inverse logarithmic rate law deduced by Mott and Cabrera. The thick films which are 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 pseudocapacitance and charge associated with the ad-layer from open-circuit decay curves and Tafel parameters is used to obtain the pseudo-capacitance and charge associated with the transition region in the current–potential curves for the formate decarboxylation. It is shown that this region corresponds to filling of the surface with adsorbed intermediates formed in the reaction. These observations are shown to support the reaction mechanism proposed in Part I.