A KINETIC-STUDY OF THE THERMAL-DECOMPOSITION OF IRON(III) HYDROXIDE-OXIDES .2. PREPARATION AND THERMAL-DECOMPOSITION OF GAMMA-FEO(OH)

Hydrated and anhydrous gamma-FeO(OM) with different sizes and shapes o f crystalline powders were prepared through an oxidative hydrolysis of iron(II) chloride solution using uea decomposition. Systematic thermo gravimetric measurements under isothermal and non-isothermal condition s were performed in order to characterize kinetically the thermal dehy dration and dehydroxylation processes. The dehydration of the bound wa ter on the surface of gamma-FeO(OH) observed at around 150 degrees C o beyed the first-order law with an apparent activation energy of about 100 kJ mol(-1), being recognized as controlled by diffusional removal of evolved water vapor through the stacking assemblage of plate-like c rystalline powders. As for the hydrated gamma-FeO(OH), the thermal deh ydroxylation process at around 280 degrees C, subsequent to thermal de hydration of the bound water, was well characterized by the two-dimens ional phase boundary controlled reaction law R(2) with apparent activa tion energy of about 150 kJ mol(-l), implying shrinkage of the reactio n interface from the edge of the rectangular plate. The Avrami-Erofeye v A(m) law was estimated for the dehydroxylation process of the anhydr ous samples, in which the kinetic exponents m in the A(m) law increase d with increasing specific surface area and the aspect ration of the r ectangular plate of the sample powders.