Comparative kinetic study of formic acid decomposition for hydrogen production in supercritical water and gas phase: Effect of water catalysis

Formic acid is attracting increasing attention due to is important role for hydrogen production in different practical scenarios. This work presents a comparative kinetic study of formic acid decomposition in supercritical water (SCW) and gas phase. At first, the decomposition of formic acid in SCW was investigated at a pressure of 25.0 MPa, temperatures of 773-848 K and residence times of 2.1-8.5 s in an isothermal flow reactor. Hydrogen and carbon dioxide were found to be the major gaseous products, indicating that decarboxylation was the predominant reaction pathway. A detailed kinetic model was developed with consideration of water catalysis and was used to interpret the present data as well as data from literature. For gas-phase conditions, the kinetics of formic acid decomposition were well characterized by analyzing different types of experimental data in literature. The Machado model was proved to provide satisfactory description of the kinetic behavior of formic acid decomposition. Based on the developed models, a comparative kinetic analysis was further conducted to explore the differences of characteristics of formic acid decomposition under the two conditions. Water catalysis effectively reduced the onset temperature and facilitated the reaction rate for formic acid decomposition in SCW. Finally, the catalytic mechanism of water was elucidated in detail.