Reduction of CO2 with water splitting hydrogen under subcritical and supercritical hydrothermal conditions

• Reduction of CO2 was carried out under subcritical and supercritical hydrothermal conditions.
• Temperature influenced the generation of H2, and H2 produced largely under supercritical condition especially at 425 °C.
• Formic acid yield was produced from CO2 reduction, and the temperature exerted a great effect on the formic acid yield.
• Decomposition of formic acid became quick from 350 °C, which was decomposed into methane.

Hydrothermal conversion of CO2 into organic chemicals in the presence of Fe and Ni powder under subcritical and supercritical hydrothermal conditions has been carried out, and the results showed that H2 could be obtained from water splitting under hydrothermal condition. According to Fe3O4 formation, the reaction temperature seemed to influence the generation of H2, and H2 could be formed largely at supercritical condition especially at 425 °C. The formic acid could be produced from CO2 reduction, and the reaction temperature had a great effect on the formic acid yield. The increase in temperature could improved the formic acid yield, however higher temperature (≥275 °C) seemed to exert a minus influence on the formic acid yield due to decomposition of formic acid. The decomposition of formic acid became quick from 350 °C, and the results showed that the formic acid was decomposed into methane at higher temperature (350 °C).