Thermal dehydration kinetics of a rare earth hydroxide, Gd(OH)3

Abstract

AbstractThis paper reports the synthesis, characterization, and dehydration kinetics of a rare earth hydroxide, Gd(OH)3. Uniform rod‐like Gd(OH)3 powder was prepared by a colloidal hydrothermal method. The powder thus obtained dehydrated into its oxide form in a two‐step process, where crystalline GdOOH was obtained as the intermediate phase. Crystal structure study revealed a monoclinic structure for GdOOH, with space group P2/1m and lattice parameters a = 6.0633, b = 3.7107, c = 4.3266, and β = 108.669. The first‐step dehydration follows the F2 mechanism, while the second step follows the F1 model, indicating that both the steps are controlled by nucleation/growth mechanism. The activation energy Ea and frequency factor A are 231±12 kJ/mol and 2.08 × 1018 s−1 for the first step and 496 ± 32 kJ/mol and 7.88 × 1033 s−1 for the second step, respectively. Such high activation energy calculated from the experimental data can be ascribed to the high bonding energy of GdO bond, and the difference in activation energy for the two steps is due to the change in the bond length of hexagonal Gd(OH)3 and monoclinic GdOOH. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 75–81, 2007