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+ an d Na+ ions on the catalytic oxidation of SO2 in the V2O5-M2S2O7 system and the effect of these alkali cations on the electrochemical behavio r of V2O5 in the alkali pyrosulfate melts It has been shown that Na+ i ons had a promoting effect on the V(V) reversible arrow V(IV) electroc hemical reaction. Sodium ions accelerate both the V(V) reduction and t he V(IV) oxidation, the effect being more pronounced in the case of th e V(IV) oxidation. Sodium ions also had a significant (almost 0.2 V) d epolarization 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 th e studied potential scan rates. In the Cs(2)S(2)O7-K2S2O7 (1:1) melt t he V(IV) --> V(V) oxidation was affected by Cs+ ions with a depolariza tion effect of 0.2 V and an even more significant acceleration than th e in molten V2O5-Na2S2O7-K2S2O7 system. The V(IV) --> V(V) oxidation p eak currents were approximately 1.5 times higher than in the V2O5-K2S2 O7 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+ pro moting effect is based on the acceleration of this stage. It has also been proposed that voltammetric measurements can be used for fast opti mization of the composition of the vanadium catalyst (which is approxi mately V2O5-M2S2O7) for sulfuric acid production.