Effect of gold and magnetite on the decomposition kinetics of formic acid at 200 °C under hydrothermal conditions

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Formic acid and formate ions are intermediate compounds in the water gas shift reaction as well as in the abiotic formation of organic molecules from dissolved CO2 and H2 under hydrothermal conditions. The decomposition kinetics of a 0.1 M formic acid solution has been investigated using PTFE-lined reactors at 200 °C on the liquid-vapour equilibrium of water for calculated in-situ pH comprised between 2.8 and 3.6. Under these conditions, formic acid is the dominant aqueous species. Formic acid aqueous decomposition follows a first-order kinetics with a constant k (s−1) such that ln(k) = −13.6 ± 0.6 at 200 °C. The addition of gold chips in the reactor is found to promote formic acid decomposition and the kinetics constant of the reaction is a linear function of the exposed gold surface by unit volume of solution (S/V). Consequently, experiments run in gold tubes as those typically used in cold-seal vessels can be by more than two orders of magnitude faster than those performed in PTFE reactors under the same conditions. The addition in the PTFE-lined reactor, of particles of magnetite (Fe3O4) with sizes centred on 300 nm or the presence of aqueous Fe at the millimolal level does not catalyse the hydrothermal decomposition of formic acid at 200 °C.