Indirect high current density reductions using cathodically generated solvated electrons

Abstract

It is usually economically desirable in view of the high capital cost of plant and high interest rates, to perform electrosyntheses at high current densities. However, the rate of mass transfer often prevents the optimum value being achieved, particularly when the concentration of the substrate has to be kept low due to solubility or mechanistic reasons. Many attempts have been made to develop compact electrodes with large surface area to overcome this difficulty e.g. porous electrodes, slurry electrodes, particulate electrodes. In situations where indirect reaction is feasible, e.g. oxidation via a higher valency state of a transition metal [1], then a high current density can be achieved by using a high concentration of the intermediate, namely, the corresponding metal ion. The limiting factor then becomes the diffusion limited rate of mass transfer of the metal ion through the boundary layer. In the case of indirect reductions using electrochemically generated solvated electrons [2] the situation is even more favourable. Owing to the very high mobility of solvated electrons which, at high concentrations, is several orders of magnitude higher than for normal species [3], very high rates of electron transfer can be achieved through the stagnant boundary layer and gentle stirring suffices to balance this rate with the subsequent homogeneous reaction. Reductions using solvated electrons produced at very high controlled current densities in the presence of low concentrations of an organic substrate were performed in the hexamethylphosphoramide (HMPA)/lithium chloride