Oxidation/reduction Reactions of molybdate ions in sodium sulfate at 1200 K

Cyclic voltammetric studies have been used to elucidate the electrochemical and coupled chemical reactions of solute molybdate ions on sodium sulfate at 1200 K in different melt basicities fixed by SO₂-O₂ atmospheres. The reaction mechanism was determined by use of the Nicholson and Shain diagnostic criteria in combination with the phase stability diagram. Within the regime of sodium sulfate stability, three reduction reactions occur. In a highly acidic melt, the reduction of molybdate ions (Mo in + 6 oxidation state) to Mo as + 4 is coupled to an irreversible chemical reaction whereby molybdenum dioxide precipitates. Molybdenum in oxidation state + 4 is reduced to Mo-(+2)-species which are immediately reoxidized to Mo-(+4)-species by an oxidizing agent in the melt, a so-called catalytic reaction. The reduction of Mo-(+2)-species to molybdenum follows in a very narrow potential range close to the previous reaction and has the same catalytic reaction mechanism. The reduction reactions are reversible. The reverse oxidation of molybdenum to the molybdate ion proceeds in three electron transfer steps.