Photoassisted Reduction of Sulfur Dioxide in Nonaqueous Solutions at p-Type Semiconductor Electrodes

The electrochemical behavior of S0₂ has been investigated at illuminated (632.8 and 514.5 nm) p-type semiconducting Si, WS₂, and InP in CH₃CN/[n-Bu4N]ClO₄ solutions. SO₂ is photoreducible at each of these materials to form S₂O1_2-, but the electrical power savings efficiencies are variable. The best power savings efficiency, ~ 11% at 514.5 nm (100 mW/cm2), is obtained with a p-type InP photocathode modified by photoelectrodeposition of ~5 × 10-⁸ mol/cm2 of Pt onto the surface. Naked p-InP is considerably less efficient, owing to poor kinetics for the reduction of S0₂. The deposition of Pt onto p-InP forms neither a uniform ohmic contact nor a Schottky barrier; rather, studies of platinized n-InP show directly that the Pt serves as a catalyst for S0₂ reduction. Power savings efficiencies for p-type WS2 and textured Si are low, owing to small output voltages. Preparative, controlled-potential photoelectrochemical reduction of S0₂ at all three photoelectrodes and Pt in CH₃CN/0.1 M SO2/0.5 M [n-Bu4N]C104 has been demonstrated to give >90% current efficiency for formation of S2O2-, which can be precipitated and collected as Na₂S₂0₄.