Hydrogen Sulfide Splitting via the Iodine Thermochemical Cycle: A Theoretical and Experimental Study of a Potential Route to Hydrogen Production

Hydrogen sulfide (H2S) is a hazardous and toxic gas that is often found as a byproduct in the oil and gas industry processes. Traditional methods for converting H2S into less harmful chemicals, such as the Claus process, are energetically demanding and produce sulfur oxides. In this study, we present an aqueous phase thermochemical cycle that utilizes H2S and iodine (I2) to produce hydrogen iodide (HI)–an intermediate for hydrogen (H2) production. This cycle was characterized by combined theoretical and experimental work, predicting two possible stepwise reaction mechanisms for the degradation of H2S by I2 and the production of HI. One mechanism is more prone to take place at high H2S concentrations, with an overall reaction of 8H2S + 7I2 → S8 + H2 + 14HI and thus resulting in an HI:H2S = 1.75:1 ratio. The other takes place at lower H2S concentrations involving water (H2O) as a reactant as per the overall reaction H2S + 3I2 + 2H2O → SO2 + 6HI, resulting in a higher yield of HI according to a HI:H2S = 6.00:1 ratio. Our quantum chemistry and kinetics calculations indicate that both mechanisms are kinetically and thermodynamically hindered, especially the latter with a higher standard Gibbs free energy barrier height. Our pH measurements in a reactor at different H2S concentrations, H2S:I2 ratios, and temperatures were used to monitor the H+ and H+/H2S mole profiles as a function of time. The resulting profiles indicate a transition between two mechanisms whose relative prominence is controlled by an interplay between the investigated initial conditions and that may correspond to those characterized by our calculations. Our newly proposed mechanisms in the aqueous phase differ from those previously proposed by other authors, and the gathered fundamental mechanistic details may facilitate the design of catalyzed processes that would make this H2S-I2 thermochemical cycle more competitive for relevant industrial processes such as H2 production and H2S removal.