Mechanism and Kinetic Parameters of the Thermal Decomposition of Gibbsite Al(OH)3 by Thermogravimetric Analysis |
D. Redaoui, F. Sahnoune, M. Heraiz and A. Raghdi
Physics and Chemistry of Materials Lab., Department of Physics, University Mohamed Boudiaf of M'sila, 28000, M'sila, Algeria |
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In this study, the mechanism and the kinetic parameters of the thermal decomposition of gibbsite Al(OH)3 were studied by differential thermogravimetry technique under non-isothermal conditions, between room temperature and 1200 K at heating rates of 5, 10, 15 and 20°C min-1. The obtained differential thermogravimetry curves show clearly three distinct peaks. The first peak is due to the partial dehydroxylation of gibbsite. Among the 32 types of differential equations of non-isothermal kinetics, we have found that the most suitable mechanism is (A3/2: g(x)=[-ln(1-x)]2/3) also called Avrami-Erofeev equation of order 2/3. The values of the activation energy EA and of the pre-exponential factor K are 157 kJ mol-1 and 7.58×1015 s-1, respectively. The second peak corresponds to the decomposition of gibbsite to boehmite. Decomposition is controlled by the rate of second-order reaction (F2: g(x)=(1-x)-1-1), under the applied conditions. The activation energy EA and pre-exponential factor K correspond to 243 kJ mol-1 and 3.73×1022 s-1, respectively. The third peak is due to transformation of boehmite to alumina. However the mechanism for such transformation is better described by the 3/2 rate order reaction (F3/2: g(x)=(1-x)-1/2-1). In addition, the values of EA and K were determined to be around 296 kJ mol-1 and 1.82×1019 s-1, respectively. The results of differential thermogravimetry were supplemented by the differential thermal analysis. X-ray powder diffraction analysis was carried out for samples of gibbsite treated at different temperatures between 200 and 1200°C in 200°C steps. |
DOI: 10.12693/APhysPolA.131.562 PACS numbers: 81.70.Pg, 81.05.Je, 81.05.Mh, 81.70.Pg, 81.30.Mh, 07.85.Nc |