Pathways and Kinetics for Autocatalytic Reduction of CO2 into Formic Acid with Fe under Hydrothermal Conditions

The utilization of CO2, as a cheap and abundant carbon source to produce useful chemicals or fuels, has been regarded as one of the promising ways to reduce CO2 emissions and minimize the green-house effect. Previous studies have demonstrated that CO2 (or HCO3–) can be efficiently reduced to formic acid with metal Fe under hydrothermal conditions without additional hydrogen and any catalyst. However, the pathways and kinetics of the autocatalytic CO2 reduction remain unknown. In the present work, the reaction kinetics were carefully investigated according to the proposed reaction pathways, and a phenomenological kinetic model was developed for the first time. The results showed that the hydrothermal conversion of HCO3– into formic acid with Fe can be expressed as the first-order reaction, and the activation energy of HCO3– is 28 kJ/mol under hydrothermal conditions.