Reduction kinetics of Ni(OH)2 to nickel powder preparation under hydrothermal conditions

The reduction kinetics from Ni(OH)₂ to fine Ni metal powder under hydrothermal and H₂ pressure conditions using anthraquinon as an activator were investigated. The reduction ratio increased with temperature up to 225 °C. The H₂ molecule was activated by anthraquinone. This activated hydrogen acts as a reduction reagent from Ni²⁺ to Ni⁰. The process may proceed by heterogeneous reaction of Ni²⁺ ion and solid anthraquinon as follows: (1) anthraquinon is hydrated, (2) Ni²⁺ ion is absorbed on anthraquinon hydride, and (3) this complex decomposes to Ni⁰ powder, anthraquinon, and H₂O. The particle size of nickel powder obtained increased with increasing temperature and with decreasing pH. The average particle size was about 350 nm. The reduction kinetics were in good agreement with a core model equation with surface reaction, that is, 1 − (1 −x)^1/3 =kt, wherex is a reaction ratio andt is reaction time. Arrhenius plots showed two slopes with activation energies 65.9 kJ/mol at higher temperature and 377.2 kJ/mol at lower one. This result shows that the hydration of anthraquinon and adsorption of Ni²⁺ ion onto anthraquinon hydride and decomposition to Ni metal and anthraquinone proceed by consecutive reactions and rate determining steps change in each temperature range.