Reaction Pathway\nin the Hydrothermal Synthesis of\nMetal Nanoparticles Using Formic Acid

We revealed the reaction pathway in the hydrothermal\nsynthesis\nof Cu nanoparticles from an aqueous solution of copper formate at\nrelatively low temperatures up to 573 K by <i>in situ</i> synchrotron X-ray diffraction. The two possible pathways were reduction\nreaction caused by (i) formic acid adsorption on an intermediate oxide\nand (ii) hydrogen gas generated through the water-catalyzed decomposition\nof formic acid under hydrothermal conditions. We observed the formation\nof intermediate Cu<sub>2</sub>O under all conditions within the temperature\nrange of 473–673 K and pressure range of 5–30 MPa, despite\nrapid heating within a few seconds. The induction time of Cu formation\ndecreased exponentially with increasing temperature. The pressure-independent\nactivation energy of 0.87(7) eV was estimated for Cu formation between\n498 and 573 K from the Arrhenius plot of induction time. Compared\nwith the reported values, the pressure-independent activation energy\nwas attributed to reduction reaction (i). In addition, we suggest\nthat reaction (i) may also dominate in the supercritical and subcritical\nhydrothermal synthesis, which was originally designed with reaction\n(ii). This is due to the observation of intermediate Cu<sub>2</sub>O and the fact that particle growth and induction time can be explained\nby reaction (i) continuously from lower temperatures. While further\ninvestigation is necessary to confirm the reaction pathway under supercritical\nand subcritical conditions, the present findings can change a synthesis\nstrategy from suppressing to utilizing the intermediate.