Hydrogen Production via Photolytic Oxidation of Aqueous Sodium Sulfite Solutions

Sulfur dioxide (SO2) emission from coal-burning power plants and refinery operations has been implicated as a cause of acid rain and other air pollution related problems. The conventional treatment of SO2−contaminated air consists of two steps: SO2 absorption using an aqueous sodium hydroxide solution, forming aqueous sodium sulfite (Na2SO3), and Na2SO3 oxidation via air purging to produce sodium sulfate (Na2SO4). In this process, the potential energy of SO2 is lost. This paper presents a novel ultraviolet (UV) photolytic process for production of hydrogen from aqueous Na2SO3 solutions. The results show that the quantum efficiency of hydrogen production can reach 14.4% under illumination from a low pressure mercury lamp. The mechanism occurs via two competing reaction pathways that involve oxidation of SO3 2− to SO4 2− directly and through the dithionate (S2O6 2−) ion intermediate. The first route becomes dominant once a photostationary state for S2O6 2− is established. The initial pH of Na2SO3 solution plays an important role in determining both the hydrogen production rate and the final products of the photolytic oxidation. At initial solution pH of 9.80 Na2SO3 photo-oxidation generates Na2SO4 as the final reaction product, while Na2S2O6 is merely a reaction intermediate. The highest hydrogen production rate occurs when the initial solution pH is 7.55. Reduction in the initial solution pH to 5.93 results in disproportionation of HSO3 − to elemental sulfur and SO4 2− but no hydrogen production.