Temperature- and pH-dependent mechanism of hydrogen production from hydrothermal reactions of sulfide

The effect of pH and temperature on hydrogen production from an aqueous alkaline solution with sulfide (HS− and S2−) as a reducing agent of water has been studied at pH 9–13 and at temperatures between 230 and 320 °C, under corresponding saturated vapor pressure, in a Hastelloy C-22 reactor. A reaction time of 60 min at all pH values produced a significant amount of hydrogen at ≥280 °C and corresponding saturated vapor pressures. Hydrogen production increased with both pH and temperature, but was more significant with temperature. Sulfide consumption also increased with temperature, but its pH dependence was generally insignificant. The ratio of hydrogen produced to sulfide consumed (mol/mol) was 0.6–3.4, and the amount and/or oxidation state of sulfur product increased with pH and temperature. Results of this study confirm that hydrogen is produced from water reduction by sulfide under the experimental conditions, where the predominant reaction was pH- and temperature-dependent. Results of this study also suggest that optimum hydrogen production conditions via a sulfur redox cycle, based on the sulfide regeneration process, is optimized at 300 °C and pH 13, and at 320 °C and pH 11.

► Mechanism of the H2 production is temperature and pH-dependent.
► High temperatures and high pH values led to higher H2 production.
► Predominant reaction shifts from formation of Sn2− to SxOy2− with pH/temperature.
► Optimum H2 production conditions are at 300 °C and pH 13, and at 320 °C and pH 11.