Nonisothermal kinetic analysis of decomposition of CuCO3.Cu(OH)2 and 2ZnCO3.3Zn(OH)2
Abstract
Decomposition of copper (II) carbonate hydroxide (CCH), CuCO3.Cu(OH)2 and zinc hydroxide carbonate (ZHC), 2ZnCO3.3Zn(OH)2 is widely used for the synthesis of copper and zinc oxides. Kinetic modelling, identification of the rate-controlling step and change in thermodynamic properties of these decomposition reactions have not been reported adequately in the literature. In this work, non-isothermal kinetics of thermal decomposition of CuCO3.Cu(OH)2, and 2ZnCO3.3Zn(OH)2 has been investigated using thermogravimetric (TG) analysis, employing both model-free iso-conversional and model-fitting methods. Mean apparent activation energy (Ea) values, estimated by various iso-conversional methods (FWO, KAS, Starink, Kissinger and Vyazovkin), were found to be in close agreement (104.9-111.2 kJ.mol-1 for CuCO3.Cu(OH)2 and 192.8-197.9 kJ.mol-1 for 2ZnCO3.3Zn(OH)2). The Coats-Redfern method showed a different value for 2ZnCO3.3Zn(OH)2 (175.6 kJ.mol-1). Ea decreased significantly with an increase in conversion for CuCO3.Cu(OH)2, while it increased, peaked and then decreased with conversion for 2ZnCO3.3Zn(OH)2. It indicates that the decomposition of both these compounds can not be explained by a single-step reaction over entire conversion range. Avrami-Erofeev model of order 2.5 was found to explain the experimental TG data well for CuCO3.Cu(OH)2 decomposition, while 2ZnCO3.3Zn(OH)2 decomposition followed the chemical reaction model of order 1.75. Positive changes in the thermodynamic properties, enthalpy (DH), and Gibbs free energy (DG) for both samples showed that these endothermic decomposition reactions were not spontaneous. The difference between Ea and DH was found to be low; 3.9-4.5 kJ.mol-1and 4.0-4.6 kJ.mol-1 for CuCO3.Cu(OH)2 and 2ZnCO3.3Zn(OH)2 respectively, indicating the reactions to be favourable. The thermodynamic properties, though not influenced by the iso-conversional method and the rate of heating, varied significantly with the conversion. FTIR analysis of the evolved gases showed that dehydration started before the initiation of decarbonation, followed by simultaneous progress of both these reactions for CuCO3.Cu(OH)2 and 2ZnCO3.3Zn(OH)2.
