ELECTROCHEMICAL-BEHAVIOR OF AQUEOUS SULFUR-DIOXIDE AT POLYCRYSTALLINEPT ELECTRODES IN ACIDIC MEDIUM - A VOLTAMMETRIC AND IN-SITU FT-IR STUDY .2. PROMOTED OXIDATION OF SULFUR-DIOXIDE - REDUCTION OF SULFUR-DIOXIDE

The promoted electro-oxidation of aqueous sulphur dioxide at platinum electrodes has been studied in acidic medium with the aid of cyclic vo ltammetry and in-situ FT-IR spectroscopy. Promotion of SO2 oxidation i s achieved when adsorbed SO2 is reduced previously to adsorbed sulphur . On a platinum surface covered by sulphur in this way, an enhancement of SO2 oxidation is attained. Spectroscopic evidence demonstrates tha t, like oxidation of SO2 in the oxygen adsorption region, soluble S(VI ) is the ultimate reaction product of the catalysed SO2 oxidation. The electroreduction of SO2 has been dealt with by using the same techniq ues. Besides a surface process converting adsorbed SO2 into adsorbed s ulphur, bulk SO2 reduces irreversibly giving rise to a diffusion-limit ed voltammetric peak. The absence of significant IR bands in the poten tial region at which reduction of built SO2 takes place, allows discar ding the generation of S-H containing species. A sulphur + polysulphid e mixture is suggested as the ultimate product, yet whether this mixtu re is formed directly in the electron-transfer step or stems from chem ical decomposition of sulphur-oxygen short lifetime intermediates is n ot clear. Reduction of SO2 leads to a progressive accumulation of sulp hur on the platinum surface. An excess of adsorbed sulphur negatively affects the kinetic stages of both oxidation and reduction of bulk SO2 .