Rates and Mechanism of Carbonyl Sulfide Oxidation by Peroxides in Concentrated Sulfuric Acid

We measured the rates of carbonyl sulfide (OCS) oxidation by hydrogen peroxide H_2O_2 (HP) and peroxymonosulfuric acid HOOSO_2OH (PSA) in 13.5−18.0 M (76−96 wt %) sulfuric acid (SA) between 290 and 306 K. The reaction is first order in both [OCS] and [peroxide], having nearly identical second-order rate constants k_1 for HP or PSA as oxidants. k_1 increases exponentially with Hammett acidity Ho in the range investigated:  log(k_1/M^(-1) s^(-1)) = − (9.57 ± 0.41) − (0.80 ± 0.05) H_o, (H_o < 0), at 306 K. OCS is, however, inert toward HP in concentrated perchloric acid at equivalent H_o values. We infer that OCS oxidation by HP in SA proceeds by an acid-catalyzed process involving the intermediacy of PSA rather than H_3O^(2+). k_1 depends on temperature according to log(k_1/M^(-1) s^(-1)) = (6.64 ± 1.58) − (2606 ± 472)/T, in 18 M SA. The observed kinetic behavior and parameters are typical of sulfide oxidations by PSA in very acidic media. Present data, in conjunction with data on ambient HP and sulfate aerosol levels in the lower stratosphere, lead to reaction lifetimes that are many orders of magnitude longer than the currently estimated OCS atmospheric residence time of about 4 years.