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 of crystalline powders were prepared through an oxidative hydrolysis of iron(II) chloride solution using uea decomposition. Systematic thermogravimetric measurements under isothermal and non-isothermal conditions were performed in order to characterize kinetically the thermal dehydration and dehydroxylation processes. The dehydration of the bound water on the surface of gamma-FeO(OH) observed at around 150 degrees C obeyed 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 crystalline powders. As for the hydrated gamma-FeO(OH), the thermal dehydroxylation process at around 280 degrees C, subsequent to thermal dehydration of the bound water, was well characterized by the two-dimensional phase boundary controlled reaction law R(2) with apparent activation energy of about 150 kJ mol(-l), implying shrinkage of the reaction interface from the edge of the rectangular plate. The Avrami-Erofeyev A(m) law was estimated for the dehydroxylation process of the anhydrous samples, in which the kinetic exponents m in the A(m) law increased with increasing specific surface area and the aspect ration of the rectangular plate of the sample powders.