Continuous Production of Nickel Fine Particles by Hydrogen Reduction in Near-Critical Water

Nickel fine particles with diameters under 600 nm were synthesized rapidly and continuously in water at temperatures ranging from 300 to 380 °C and at 30 MPa. Nickel particles formed through the following two steps: formation of Fe 3 O 4 nuclei by hydrothermal synthesis from FeSO 4 aqueous solution and precipitation of nickel on the surfaces of the nuclei (i.e., Ni coating) by hydrogen reduction from Ni(CH 3 COO) 2 aqueous solution. The substance 1,10-phenanthroline was used as a complex for preventing hydrolysis of Ni 2+ because hydrolysis promotes formation of nickel oxide particles at higher temperatures. Thickness of Ni shell on the Fe 3 O 4 nuclei increased with increasing reaction temperature and Ni/Fe molar ratio in the feed solution. The 1,10-phenanthroline was thermally stable up to 380 °C for residence times of ca. 19 s, which means that recycle after product formation is probably possible.