Sulfuric Acid Formation via H₂SO₃ Oxidation by H₂O₂ in the Atmosphere

With the help of quantum mechanical methods, the formation of H₂SO₄ by the oxidation of H₂SO₃ with H₂O₂ was studied theoretically. Both stepwise and concerted mechanisms were calculated. It was found that the direct oxidation of H₂SO₃ by H₂O₂ alone requires prohibitive activation energies of >38.6 kcal/mol. However, the addition of one water molecule exhibits a strong catalytic effect that dramatically reduces the overall reaction barrier to 6.2 (2.3 with PCM) kcal/mol. The deprotonated HSO₃^- species also reduces the overall reaction barrier to 5.6 (-5.8 with PCM) kcal/mol. Both of these proceed via concerted pathways. On the other hand, the stepwise mechanisms generally produce intermediates with a hydroperoxy group (-O-O-H), which is a result of a nucleophilic attack by the oxygens of H₂O₂. While studying the catalytic effect of water, a previously unknown hydroperoxy intermediate (HOO)S(OH)₃, where sulfur is coordinated with three OH groups, was found. This work also reveals a rearrangement step of another hydroperoxy intermediate (HOO)SO₂^- to HSO₄^- that was found in earlier experimental studies. For all of the mechanisms calculated, the final H₂SO₄ is formed with a significant exothermicity of >60 kcal/mol. In general, even without sunlight, it was found that the formation of sulfuric acid by hydrogen peroxide can occur in a heterogeneous moisturized environment.