All chemicals were purchased from FUJIFILM Wako Pure Chemical Corporation as reagent grade except for 13 C-labeled sodium formate (H 13 COONa) and a standard gas of OCS (10 vol% OCS in Helium), which were sourced from the Taiyo Nippon Sanso Corporation. Deaerated Milli-Q water (18.2 MΩ cm) was used as the solvent. A gas cylinder of CO2 (47 L, 6 MPa, >99.995% purity) was obtained from Taiyo Nippon Sanso Corporation. H2 gas (>99.99996%) was generated with NM Plus 100 Hydrogen Generator (LNI Schmidlin SA). To prevent oxidation by atmospheric O2, all aqueous solutions were prepared in an anaerobic chamber filled with nitrogen (N2) and H2 gases (the volumetric ratio = 96:4).