Synthesis of iron nanoparticle: Challenge to determine the limit of hydrogen reduction in supercritical water

Supercritical hydrothermal syntheses of metal nanoparticles were investigated. Organic metal salt and hydrogen gas produced by water catalyzed decomposition of formic acid was employed as metal sources and reduction agent, respectively. The formation of iron was verified by measuring the magnetic property of the products by superconducting quantum interference device (SQUID) magnetometer as well as crystallographic analysis by X-ray diffraction (XRD). As predicted by the free energy calculation of reduction of metal oxides by hydrogen molecule, silver, palladium, copper, nickel and cobalt nanoparticles were synthesized without using surface modifier, whereas, iron could be synthesized at small yield. The main product was iron oxides (mainly magnetite). In order to increase the yield of iron, hexanoic acid was employed as an in situ surface modifier of the synthesis. The surface modification lessened the size of the synthesized nanoparticles and increased the yield of iron. The optimum condition for iron synthesis was also investigated, as a result, 7.6% yield of iron was achieved.

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► The limit of hydrogen reduction in supercritical water (SCW) was examined by means of supercritical hydrothermal synthesis of metal nanoparticles.
► Hydrogen obtained as a decomposition product of formic acid in SCW is employed as a reducing agent.
► As a results, silver, palladium, copper, nickel and cobalt were successfully synthesized without using surface modifier, whereas, iron was synthesized at very small yield (3% without and 7% with hexanoic acid surface modifier).