Alternative source of H2 is the reaction between H2S and iron sulfide minerals. In our scCO2 experiment using mackinawite in place of MoS2, a complete conversion of mackinawite to pyrrhotite was observed after the 7-day reaction at 200 °C (SI Appendix, Fig. ; H2 was excluded from the starting material in this run). The generated H2 was 114 μmol, which corresponded to 2.0 mol% of the mackinawite used (500 mg). It was expected that mackinawite initially reacted with H2S to form pyrite and H2 (53, 54):