Chemical equilibria and particle morphology of boehmite (AlOOH) in sub andsupercritical water

In a recent study, we proposed a supercritical water crystallization method for production of metal oxide particles. Around the critical point, the mo rphology of boehmite (AlOOH) particles varied greatly with the reaction tem perature, pressure and concentration of aqueous aluminum nitrate solution. In this study, the relationship between the morphologies of particles obtai ned and the chemical species in solution is discussed. For estimation of ch emical species concentrations, evaluation of equilibrium constants of the h ydrothermal reactions around the critical point is required. For this, a mo del based on the Gibbs energy change by temperature, solvent effects and io n-ion interactions is employed. The solvent effect was calculated by the Bo rn equation. The effect of ion-ion interaction was calculated by the extend ed Debye-Huckel equation. Using this model, the distribution of chemical sp ecies for the AlOOH system (Al3+, Al(OH)(2+), Al(OH)(3), Al(OH)(3), Al(OH)( 4)(-), NO3-) in subcritical (350 degrees C, 30 MPa) and supercritical water (400 degrees C, 30 MPa) was estimated. The particle morphology seems to be determined by selective adsorption of positive charged species, Al(OH)(2+) , on the negatively charged faces of AlOOH crystal. (C) 1999 Elsevier Scien ce B.V. All rights reserved.