A New Perspective on the Green Strategy of Close Cycle Dissociation of H₂S

Abstract

Extremely toxic H₂S gas is produced in huge quantities across the world as a byproduct of the desulfurization of hydrocarbon fuels. This H₂S gas is further converted into sulfur in sulfur recovery units. However, the valuable hydrogen content is lost as water vapour. It emphasizes the importance of proper exploitation of this resource. A continuous two-steps cyclic process for producing H₂ and sulfur simultaneously has been derived in this study. The first well-studied photocatalytic step generates hydrogen by water splitting in the presence of sulfide (S^2–) and sulfite (${\mathrm{SO}}_{3}^{2-})$, the sacrificial agents. Here, these sacrificial agents are converted into thiosulfate (${\mathrm{S}}_{2}{\mathrm{O}}_{3}^{2-}$). In the second step, this ${\mathrm{S}}_{2}{\mathrm{O}}_{3}^{2-}$ reacts with hydrogen sulfide (HS^–) to form elemental sulfur, along with generation of S^2– and ${\mathrm{SO}}_{3}^{2-}$, which are again reused in the first step. The kinetics of the reaction involved in the second step (${\mathrm{HS}}^{-}+ {\mathrm{S}}_{2}{\mathrm{O}}_{3}^{2-}+\mathrm{ O}{\mathrm{H}}^{-} \to \mathrm{S}+{\mathrm{S}}^{2-}+\mathrm{ S}{\mathrm{O}}_{3}^{2-}+ {\mathrm{H}}_{2}\mathrm{O}$) were studied in the present work. The reaction was found to be non-elementary with a rate law of $k{[{\mathrm{HS}}^{-}]}^{1/2}{[{\mathrm{S}}_{2}{\mathrm{O}}_{3}^{2-}]}^{3/2}$. The observed rate law is of the order of 1.5 with respect to thiosulfate and 0.5 with respect to HS^– ions. A reaction mechanism for the second step consistent with the kinetics was also proposed. The rate constant was observed to be $1.075\times {10}^{10}{\mathrm{e}}^{- 62280/RT}/\mathrm{ M}/\mathrm{s}$. This new proposed close cycle may simultaneously achieve green and renewable H₂ production along with H₂S removal.