Photoelectrochemical Oxidation of Sulfur Dioxide in Strong Acid Solution: Iodide-Mediated Oxidation at Illuminated Metal Dichalcogenide Electrodes

Article abstract of DOI:10.1021/ja00411a001

In strong acid solution, n-type MoS2, MoSe2, or WS2 can be used as a photoanode to effect the oxidation of SO2 to SO4(2-).The oxidation can be driven with visible light, hν > 1.1 eV, with an output photovoltage of ca. 0.6 V relative to E_r(SO4(2-)/SO2) in 6 M H2SO4.The oxidation of SO2, however, requires the presence of I(-) as a mediator serving to (1) alter the MY2/liquid energetics to shift the band edges to a more negative potential to yield a photovoltage relative to E_r(SO4(2-)/SO2) and (2) improve the kinetics for SO2 oxidation presumably through the intermediate formation of I3(-).It is noteworthy that MY2 is stable in the presence of strong acid, even concentrated H2SO4; the photooxidation of SO2 can be sustained without photoanodic corrosion of the MY2 at current densities of 20 mA/cm² and at potentials ca. 0.6 V more negative than E_r(SO4(2-)/SO2).The cathode reaction is H2 evolution, and the overall reaction is 2H2O + SO2 -> H2SO4 + H2 that comprises one part of a hybrid cycle for splitting H2O to H2 + 1/2 O2.The cycle is completed, in principle, by thermolyzing H2SO4 to H2O + SO2 + 1/2 O2.Thus, light and heat can be used to effect water splitting.Fundamentally, the noteworthy finding is that I(-) allows a good rate (current) for SO2 oxidation and also favorably affects the energetics to improve the photovoltage.This new concept may be exploited to illustrate processes that can be uniquely done at semiconductor photoelectrodes.The efficiency for conversion of 632.8-nm light (ca. 50mW/cm²) in 6 M H2SO4/ca.1 M SO2/5 mM I(-) is ca. 8percent using an n-type WS2 photoanode.