MoS2 was prepared by the reduction and sulfidation of molybdate under scCO2 (see Materials and Methods). The obtained MoS2 was a poorly crystalized molybdenite forming submicro-to several micrometer-sized aggregates with occasional plate-like structures (SI Appendix, Fig. ). The MoS2 (50 mg) was deposited in 3.43 mL of an aqueous solution of sodium sulfide (Na2S) and hydrogen chloride (HCl) (400 mmol L -1 for each) in a glass vial. The vial was encased in a titanium reaction vessel (SI Appendix, Fig. ) with the headspace (34.3 mL) filled with CO2 and H2 (6.0 MPa and 0.3 MPa at 25 °C, respectively), and heated at 200 °C for up to 14 days. At the reaction temperature, the dissolved concentration and partial pressure of CO2 were estimated to be 0.9 mol kg -1 and 14 MPa, respectively . The introduced H2 gas set the initial dissolved H2 concentration at 200 °C to be 5 mM , which was within a typical range of H2 concentration in fluids from submarine hydrothermal systems . Considering the pKa of H2S and CO2 at the reaction condition , the neutralization of HS -was expected to lead the solution pH to 6.5, a slightly higher value than the neutral pH at 200 °C . After a certain period of heating, the vessel was cooled under air, depressurized, and opened to collect the aqueous solution. The headspace gas was sampled into a sealed glass vial just before the depressurization.