Supercritical hydrothermal synthesis of metallic cobalt nanoparticles and its thermodynamic analysis

Metallic cobalt nanoparticles could be synthesized via supercritical hydrothermal reduction process using decomposition of formic acid (340 °C to 420 °C, 22.1 MPa and 10 min). To obtain metallic cobalt nanoparticles, several series of experiments were conducted with changing amount of formic acid. The produced cobalt nanoparticles were characterized with XRD, TEM and SEM. To estimate the required amount of H2, EOSs were employed (ideal gas law, SRK EOS and PSRK EOS). The results of PSRK EOS were well matched with experimental data and gave good explanation for formation of metallic cobalt nanoparticles. Important thing is that required amount of H2 was much smaller than estimated by using ideal gas law. This result suggests that around the critical point of water, fugacity of H2 increased drastically and this leads to reduce the required amount of H2 for the synthesis of cobalt nanoparticles.

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► We synthesized cobalt nanoparticles via a supercritical hydrothermal reduction process.
► We suggested a possible mechanism for the formation of cobalt nanoparticles in the reaction.
► We calculated Keq values by using ideal gas law, SRK EOS and PSRK EOS.
► The values calculated from the PSRK EOS yielded reasonable prediction values rather than those from the ideal gas law and SRK EOS.
► Around the critical point of water, fugacity of H2 increased drastically and this leads to reduce the required amount of H2 for the synthesis of cobalt nanoparticles.