Electrochemical study on the cationic promotion of the catalytic SO2 oxidation in pyrosulfate melts

The electrochemical behavior of the molten V2O5-M2S2O7 (M = K, Cs, or Na) system was studied using a gold working electrode at 440 degrees C in argon and air atmosphere. The aim of the present investigation was to find a possible correlation between the promoting effect of Cs+ and Na+ ions on the catalytic oxidation of SO2 in the V2O5-M2S2O7 system and the effect of these alkali cations on the electrochemical behavior of V2O5 in the alkali pyrosulfate melts It has been shown that Na+ ions had a promoting effect on the V(V) reversible arrow V(IV) electrochemical reaction. Sodium ions accelerate both the V(V) reduction and the V(IV) oxidation, the effect being more pronounced in the case of the V(IV) oxidation. Sodium ions also had a significant (almost 0.2 V) depolarization effect on the V(IV) --> V(V) oxidation. The peak current of the V(IV) --> V(V) oxidation waves vs. Na2S2O7 concentration plots had maximums at ca. 8.5 mol % of Na2S2O7 in air atmosphere for all the studied potential scan rates. In the Cs(2)S(2)O7-K2S2O7 (1:1) melt the V(IV) --> V(V) oxidation was affected by Cs+ ions with a depolarization effect of 0.2 V and an even more significant acceleration than the in molten V2O5-Na2S2O7-K2S2O7 system. The V(IV) --> V(V) oxidation peak currents were approximately 1.5 times higher than in the V2O5-K2S2O7 system at all studied potential scan rates. No correlation has been found between the described effects and the electric conductivity of the systems. The rate-determining stage in the catalytic SO, oxidation most likely is the oxidation of V(IV) to V(V) and the Na+ and Cs+ promoting effect is based on the acceleration of this stage. It has also been proposed that voltammetric measurements can be used for fast optimization of the composition of the vanadium catalyst (which is approximately V2O5-M2S2O7) for sulfuric acid production.