After tightly sealing the vessel with a polytetrafluoroethylene (PTFE) O-ring compressed between the vessel and cap, the headspace (29.7 mL) was evacuated using a rotary pump (GCD-136X, ULVAC). This was followed by the addition of 0.3 MPa of H2, and the subsequent introduction of CO2 from a gas cylinder (47 L) filled with 6 MPa of CO2. The mass of introduced CO2 was 5.1±0.5 g. Thereafter, the vessel was heated at 200 °C for 7 days in a laboratory oven (DSN-111, ISUZU).