Sulfuric Acid Formation via H2SO3 Oxidation by H2O2 in the Atmosphere.

Abstract

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.