Search Results
Abstract
The thermal behaviour of Ba[Cu(C2O4)2(H2O)]·5H2O in N2 and in O2 has been examined using thermogravimetry (TG) and differential scanning calorimetry (DSC). The dehydration starts at relatively low temperatures (about 80°C), but continues until the onset of the decomposition (about 280°C). The decomposition takes place in two major stages (onsets 280 and 390°C). The mass of the intermediate after the first stage corresponded to the formation of barium oxalate and copper metal and, after the second stage, to the formation of barium carbonate and copper metal. The enthalpy for the dehydration was found to be 311±30 kJ mol−1 (or 52±5 kJ (mol of H2O)−1). The overall enthalpy change for the decomposition of Ba[Cu(C2O4)2] in N2 was estimated from the combined area of the peaks of the DSC curve as −347 kJ mol−1. The kinetics of the thermal dehydration and decomposition were studied using isothermal TG. The dehydration was strongly deceleratory and the α-time curves could be described by the three dimensional diffusion (D3) model. The values of the activation energy and the pre-exponential factor for the dehydration were 125±4 kJ mol−1 and (1.38±0.08)×1015 min−1, respectively. The decomposition was complex, consisting of at least two concurrent processes. The decomposition was analysed in terms of two overlapping deceleratory processes. One process was fast and could be described by the contracting-geometry model withn=5. The other process was slow and could also be described by the contracting-geometry model, but withn=2. The values ofE a andA were 206±23 kJ mol−1 and (2.2±0.5)×1019 min−1, respectively, for the fast process, and 259±37 kJ mol−1 and (6.3±1.8)×1023 min−1, respectively, for the slow process.
Abstract
Oxygen depletion and reoxidation of bismuth molybdates (2∶1; 2∶3) have been studied by means of isothermal thermogravimetry and DTA measurements. From the isothermal curves, Arrhenius energies were obtained between 411 and 683 K. The activation energies for oxygen depletion from Bi2O3·MoO3 were lower than those for Bi2O3·3MoO3. Two kinetically different types of oxygen release were identified for both molybdates. Arrhenius plots were also obtained from reoxidation experiments: Bi2O3·MoO3 was more easily reoxidable than Bi2O3·3MoO3. The substantial closeness of the respective activation energies suggests that depletion and reoxidation follow the same mechanistic steps. Some DTA measurements confirm the existence of at least two types of reoxidation sites for both oxysalts.
by thermogravimetric analysis (TG) using isothermal and non-isothermal methods. There are certain limitations of both methods. Isothermal TG is considered more appropriate than non-isothermal TG for the thermal decomposition of general chemical
performed by a calibrated hygrometer. The weight change of each sample was monitored over a period of 6 months and the solid recovered at the end of the study was characterized by PXRD and thermal analysis. Isothermal TG
the mass loss of a sample is measured against temperature under controlled heating rate and gas atmosphere, and then recorded in the form of TG curves [ 8 ]. TG can be run in either isothermal or non-isothermal (dynamic) mode. Non-isothermal TG is
correlate with the percentage of the char residue determined from non-isothermal TG runs; those at 130 °C with an increasing percentage of the char residue increase ( Fig. 5 ). Although the extrapolation of rate constant to 130 °C from non-isothermal TG is
isothermal tests were performed in air with the flow rate of 100 mL min −1 . The analyses of isothermal TG data were carried out using the same software as those used for the nonisothermal TG data. Morphological characterization
were supported on a copper mesh before observation under the microscope. Non-isothermal TG measurements TG measurements were performed with 10–12 mg of samples using a TGA Q50 V6.1, TA Instruments, USA, under a
curves were obtained in the same equipment under the same conditions of air and mass from dynamic thermogravimetric curves. The temperatures used for isothermal studies were: 165, 175, 185, 195, and 205 °C. The isothermal TG data were analyzed using the
3 by both isothermal and non-isothermal TG. Reaction kinetics by TG The most common experimental technique employed to study kinetics of thermally activated reactions is TG, under the conditions of isothermal and
