(230b) Supercritical Hydrothermal Synthesis - Solvent Effect on Kinetics, Reaction Equilibrium and Solubility in Supercritical Water

30 years has been passed since Supercritical Hydrothermal Synthesis was proposed. Many researches have been reported on nano particles synthesis of variety of materials, kinetic study, in-situ monitoring of the reactions (mechanistic study), and process development on mixing regime, etc. One of the key features of the reaction in supercritical water is the drastic variation of solvent effect on reactions. For the organic reactions or ionic systems, some studies have been reported, and the Kirkwood Equation with considering the local dielectric constant was used to explain the solvent effect. However, few research has been reported from this point of view, namely the solvent effect on the hydrothermal reaction.

Solubility of metal oxides is a critical information for understanding nucleation and particle growth in the hydrothermal synthesis. The solubility of metal oxides formed through the hydrothermal synthesis can be estimated through the combination of mass balance, charge balance and the reaction equilibrium that changes greatly around the critical point. HKF model is a widely accepted approach for the quantitative prediction of the equilibrium constant in supercritical conditions. Parameters needed can be obtained from the data base, but sometimes some are not available, and thus modified HKF model was proposal. The solubility of some metal oxides estimated by the above method was compared with the experimentally obtained solubility to confirm the availability of this methods.

Also, the solvent effect on kinetics around the critical point was evaluated. In this study, NiO synthesis from Ni nitrate was employed as a model reaction. For this study, precise evaluation of reaction rate is essential, and thus a flow type tubular reactor with rapid heating and cooling system was employed, with which residence time less than 0.1 second can be precisely controlled. From the conversion of precursor at different reaction time, the first order reaction rate was evaluated. Thus evaluated apparent reaction rate was compared with the mixing rate, namely the Re number. From this relation, intrinsic rate constant was evaluated at different temperatures and pressures over subcritical to supercritical conditions. In the supercritical conditions, with increasing pressure/density of water, reaction rate was decreased The pressure dependence can be explained well with Kirkwood equation, namely as a function of dielectric constant. The sensitivity on the dielectric constant is determined by the polarity of precursor and the intermediate species. It was estimated by using molecular orbital simulation, with MOPAC, and compared with the experimentally obtained parameter. The effect of the solvation around the molecules was also taken into account. Using the parameter evaluated the effect of temperature on the kinetics was estimated and confirmed that the experimentally obtained rate constant can be fairly well explained。References