Reversible Metal Sulfide Transition in a Two-Step Thermochemical H₂S Splitting

Two-step thermochemical H2S splitting process is a promising method to recover both H2 and S from a toxic waste H2S gas via low/high metal sulfide conversion. Several studies have examined the potential of the process experimentally. Still, there is a lack of comprehensive research into such as to provide a guideline for selecting the appropriate metal for the process. The most common metals in the periodic Table (28 metals) have been examined to find stable low/high metal sulfide couples. A list consisting of 17 metals that can form thermodynamically stable couples has been shortlisted, in which six metals: K, Zr, Hf, Nb, Fe, and Ni, were found to be promising candidates with a high H2S splitting efficiency, high H2 yield and good reversible transition between low and high sulfides. The transition between sulfide crystals has been experimentally demonstrated with the Ni3S2/Ni3S2+x couple. This couple can decompose up to 90 mg H2S/g-Ni3S2 at 500 °C to produce H2 and high sulfides Ni3S2+x that was successfully regenerated at 700 °C for multiple cycles. The thermodynamic calculations revealed that the sulfides of Zr, Hf, and Nb are favorable for the two-step H2S splitting process, but have not been explored experimentally. The sulfide of Zr was examined in this work, but they got oxidized by residual oxygen in the test gas before the crystalline transition was detected and remained as a challenge in the future works.