New aspects of the electrochemistry of heteropolyacids: Part II. Coupled electron and proton transfers in the reduction of silicoungstic species

Abstract

The reduction process of silicotungstic species has been studied by cyclic voltammetry in strongly acidic aqueous solutions and in dimethylformamide with stepwise addition of perchloric acid. Important morphology changes of the waves are observed in these media. In particular, attention has been focused on the first voltammetric wave which rises from a one-electron, diffusion-controlled process to an overall, apparently direct, two-electron transfer in the range of 3 M to 7 M perchloric acid concentration in water. In large-scale electrolysis on this two-electron wave, hydrogen evolution is observed. Due to solvent effects, much the same phenomena are obtained in dimethylformamide, except that it is not necessary to resort to such extreme perchloric acid concentrations to reach the two-electron process. In this last medium, the behaviours of H 4SiW 12O 40 and H 4SiW 12O 40 have been compared. Hydrogen evolution during electrolysis on the two-electron wave in dimethylformamide has also been observed. A clear difference appears between the mechanisms of formation of the two-electron wave in acidic water and in dimethylformamide. In the former case, the second wave shifts in the positive direction until it merges with the first one, while in the latter medium a new wave more positive than the original waves grows at their expense. These observations are explained on the basis of solvent effects.