Influence of gas media on the thermal decomposition of second valence iron sulphates

FeSO₄·H₂O and FeSO₄ represent the second valence of iron sulphates. Number of studies has been done to understand formation of intermediate sulphates like FeOHSO₄ and Fe₂O(SO₄)₂, representing the oxidation of Fe²⁺ to Fe³⁺. At selected temperatures, both the thermo-dynamical equilibrium in the Fe–S–O system and the formation of the crystal structures in the solid phase are controlled by the partial pressure of water vapour and oxygen in the gas phase. The effects of the temperature and the partial pressure of gas components on the solid-phase content are demonstrated by phase diagrams. The study puts the accent on the influence of oxygen content in gas environment on processes of thermal decomposition of FeSO₄·H₂O and FeSO₄. At three quantities of oxygen content—0% (100% Ar), 21% (dry air) and 100% (pure O₂) the processes of oxidation and formatting metastable iron sulphates were examined by several experimental techniques. The thermal decomposition of samples was investigated by TG–DTG–DTA method in the temperature range 293–1400 K. Partial pressure of water vapour was determined by the quantity of water released from dehydration process of FeSO₄·H₂O. Infrared spectroscopy, Mössbauer spectroscopy and X-Ray powder diffraction method were used for identification of the new formed solid structures and for characterization of the content of the iron sulphates with different valencies of iron. The experimental data and their analyses give the possibility to determine the different stages of decomposition, related to the formation of intermediates. Depending on gas environment, the basic relationships for reaction kinetics is drawn. It is demonstrated on that correlation exists between the kinetic’s parameters and the content of oxygen in the gas phase.     

Kinetic studies on the thermal decomposition of phosphate-doped sodium oxalate

The thermal decomposition kinetics of sodium oxalate (Na₂C₂O₄) has been studied as a function of concentration of dopant, phosphate, at five different temperatures in the range 783–803 K under isothermal conditions by thermogravimetry (TG). The TG data were subjected to both model-fitting and model-free kinetic methods of analysis. The model-fitting analysis of the TG data of all the samples shows that no single kinetic model describes the whole α versus t curve with a single rate constant throughout the decomposition reaction. Separate kinetic analysis shows that Prout–Tompkins model best describes the acceleratory stage of the decomposition, while the decay region is best fitted with the contracting cylinder model. Activation energy values were evaluated by both model-fitting and model-free kinetic methods. The observed results favour a diffusion-controlled mechanism for the thermal decomposition of sodium oxalate.     

The influence of oxygen pressure on the kinetics of the thermal decomposition of Co3O4

The kinetics of the thermal decomposition of Co₃O₄ has been examined in the 1123–1200 K temperature and 2.66–20.73 kPa oxygen pressure range. The kinetics of this process has been deseribed in terms of a mixed-control model of reaction. The values of activation energies of diffusion and chemical reaction as well as the observed activation energy have been given. The strong dependence of the decomposition rate on temperature and oxygen pressure has been explained.     

Simulation approach to decomposition kinetics and thermal hazards of hexamethylenetetramine

The thermal stability of HMT under dynamic, isothermal and adiabatic conditions was investigated using differential scanning calorimeter (DSC) and accelerating rate calorimeter (ARC), respectively. It is found from the dynamic DSC results that the exothermic decomposition reaction appears immediately after endothermic peak, a coupling phenomenon of heat absorption and generation, and the endothermic peak and exothermic peak were indentified at about 277–289 and 279–296 °C (T_peak) with the heating rates 1, 2, 4 and 8 °C min⁻¹. The ARC results reveal that the initial decomposition temperature of HMT is about 236.55 °C, and the total gas production in decomposition process is 6.9 mol kg⁻¹. Based on the isothermal DSC and ARC data, some kinetic parameters have been determined using thermal safety software. The simulation results show that the exothermic decomposition process of HMT can be expressed by an autocatalytic reaction mechanism. There is also a good agreement between the kinetic model and kinetic parameters simulated based on the isothermal DSC and ARC data. Thermal hazards of HMT can be evaluated by carrying out thermal explosion simulations, which were based on kinetic models (Isothermal DSC and ARC) to predict several thermal hazard indicators, such as T_D24, T_D8, TCL, SADT, ET and CT so that we can optimize the conditions of transportation and storage for chemical, also minimizing industrial disasters.

     

Kinetics of the thermal decomposition of anhydrous cobalt nitrate by SCRT method

It has been shown the ability of the Sample Controlled Reaction Temperature (SCRT) method for both discriminate the kinetic law and calculate the activation energy of the reaction. This thermal decomposition is best described by a Johnson–Mehl–Avrami kinetic model (with n = 2) with an activation energy of nuclei growth which fall in the range 52–59 kJ mol^?1. The process is not a single-step because the initial rate of decomposition is likely to be limited by nucleation. The results reported here constitute the first attempt to use the new SCRT method to study the kinetic of the thermal decomposition of cobalt nitrate.     

Evolution of yttrium trifluoroacetate during thermal decomposition

A detailed analysis of the thermal decomposition of yttrium trifluoroacetate under different atmospheres is presented. Thermogravimetry, differential thermal analysis, and evolved gas analysis have been used for this in situ analysis. Solid residues at different stages have been characterized by means of X-ray diffraction, elemental analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. The first decomposition stage (310 °C) is exothermic and involves the complete removal of carbon (organic part) and the formation of yttrium fluoride. This process is characterized by a fast mass loss rate. Afterwards, yttria (Y₂O₃) is formed at 1200 °C through a slow process controlled by the out diffusion of fluorine that involves the formation of yttrium oxyfluoride as an intermediate. The evolution of the mass during the decomposition and the structure of the yttria particles is not affected by the presence of oxygen or water. However, when the oxygen (water) partial pressure is as low as 0.02% (<0.002%), the kinetics and final particle structure are strongly affected.     

Synthesis and thermal behavior of gem-dinitro valerylated polystyrene

Commercial polystyrene has been chemically modified with 4,4-dinitro valeryl chloride by use of Friedel–Crafts acylation reaction in the presence of anhydrous aluminum chloride in a mixture of 1,2-dichloroethane and nitrobenzene. The modified polystyrene containing –COCH₂CH₂C(NO₂)₂CH₃ fragments in side phenyl rings, named gem-dinitro valerylated polystyrene (GDN-PS), was characterized by an Ubbelohde’s viscometer, FTIR, and ¹H NMR spectroscopy. Simultaneous thermogravimetry–differential thermal analysis and differential scanning calorimetry (DSC) have been used to study thermal behavior of the polymer. The results of TG analysis revealed that the main thermal degradation for the GDN-PS occurs during two temperature ranges of 200–300 and 300–430 °C. The DTA curve of GDN-PS is showing a visible exothermic peak at 253.8 °C corresponding to the decomposition of gem-dinitro valeryl groups. The decomposition kinetic of the gem-dinitro groups for GDN-PS with degree of substitution (DS) 11 % was studied by non-isothermal DSC under various heating rates. Kinetic parameters such as activation energy and frequency factor for thermal decomposition of GDN-PS with DS 11 % were evaluated via the ASTM E698 and two isoconversional methods.     

Influences of product gases on the kinetics of thermal decomposition of synthetic malachite evaluated by controlled rate evolved gas analysis coupled with thermogravimetry

An instrument of controlled rate evolved gas analysis (CREGA) coupled with TG‐DTA was constructed for analyzing the influences of product gases on the kinetics and mechanism of the thermal decomposition of solids that produce more than one gaseous products at the same stage of reaction. The thermal decomposition of synthetic malachite, Cu₂(OH)₂CO₃, was subjected to the measurements of CREGA‐TG under controlled concentrations of H₂O and CO₂ in the reaction atmosphere with taking account of self‐generated H₂O and CO₂ during the course of reaction. By a series of CREGA‐TG measurements carried out under various atmospheric conditions, it was reconfirmed that the reaction is accelerated and decelerated by the effects of atmospheric H₂O and CO₂, respectively. From the kinetic analysis of the CREGA‐TG curves and results of high temperature X‐ray diffraction measurements under various reaction atmospheres, it was revealed that the anomalous effects of atmospheric H₂O on the reactivity and on the reaction rate of the thermal decomposition of synthetic malachite appear at the early stage of the reaction. Usefulness of the CREGA‐TG technique for measuring the kinetic rate data for the thermal decomposition of solids was demonstrated in the present study, by emphasizing the importance of quantitative control of self‐generated reaction atmosphere. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 346–354, 2005     

Effect of atmosphericwater vapor on the kinetics of thermal decomposition of copper(II) carbonate hydroxide

The effect of atmospheric water vapor on the kinetic rate behavior of the thermal decomposition of copper(II) carbonate hydroxide, Cu₂CO₃(OH)₂, was investigated by means of TG-DTA coupled with a programmable humidity controller. With increasing water vapor pressure p(H₂O) from 0.8 to 10.6 kPa, a systematic reduction of the reaction temperature of the thermal decomposition was observed as the continuous trend from the previous works at the lower p(H₂O). Through a comparative kinetic analysis of the reaction at different p(H₂O), a catalytic action of the atmospheric water vapor on the nucleation process at the first half of the reaction was identified as the possible origin of the reduction of the reaction temperature.     

The study of thermal decomposition kinetics of zinc oxide formation from zinc oxalate dihydrate

This study is devoted to the thermal decomposition of ZnC₂O₄·2H₂O, which was synthesized by solid-state reaction using C₂H₂O₄·2H₂O and Zn(CH₃COO)₂·2H₂O as raw materials. The initial samples and the final solid thermal decomposition products were characterized by Fourier transform infrared and X-ray diffraction. The particle size of the products was observed by transmission electron microscopy. The thermal decomposition behavior was investigated by thermogravimetry, derivative thermogravimetric and differential thermal analysis. Experimental results show that the thermal decomposition reaction includes two stages: dehydration and decomposition, with nanostructured ZnO as the final solid product. The Ozawa integral method along with Coats–Redfern integral method was used to determine the kinetic model and kinetic parameters of the second thermal decomposition stage of ZnC₂O₄·2H₂O. After calculation and comparison, the decomposition conforms to the nucleation and growth model and the physical interpretation is summarized. The activation energy and the kinetic mechanism function are determined to be 119.7 kJ mol^?1 and G(α) = ?ln(1 – α)^1/2, respectively.     

Studies on the thermal decomposition kinetics and mechanism of ammonium niobium oxalate

Ammonium niobium oxalate was prepared and characterized by elemental analysis, XRD and FTIR spectroscopy analysis, which confirmed that the molecular formula of the complex is NH₄(NbO(C₂O₄)₂(H₂O)₂)(H₂O)₃. Dynamic TG analysis under air was used to investigate the thermal decomposition process of synthetic ammonium niobium oxalate. It shows that the thermal decomposition occurs in three stages and the corresponding apparent activation energies were calculated with the Ozawa–Flynn–Wall and the Friedman methods. The most probable kinetic models of the first two steps decomposition of the complex have been estimated by Coats–Redfern integral and the Achar–Bridly–Sharp differential methods.     

Kinetics and mechanism of the thermal chlorination of chloroform in the gas phase

The gas‐phase thermal chlorination of CHCl₃ has been studied up to high conversions by photometry and gas chromatography in a conditioned static quartz reaction vessel between 573 and 635 K. The initial pressures of both CHCl₃ and Cl₂ ranged from about 10–100 Torr, and the initial total pressure was varied between about 30–190 Torr. The reaction is rather complex because the produced CCl₄ is not stable. The rate of consumption of Cl₂ therefore increases in the course of time. This acceleration is explained quantitatively in terms of a radical mechanism and its kinetic and thermodynamic parameters. This reaction model is based on a known model for the pyrolysis of CCl₄ to which only one reaction couple involving CHCl₃ has been added. Analyses of the rates of the homogeneous elementary steps show that the primary source of Cl atoms is the second‐order dissociation of Cl₂, which is rapidly superseded by a secondary source, the first‐order dissociation of the CCl₄ primary product. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 466–472, 2000     

Kinetic mechanism of the thermal decomposition of tobacco

Equations have been derived to describe the chemical kinetic factors that affect the rate of formation of products when a mixture of solid components (tobacco) decomposes on heating. Using these equations, a computer model of tobacco pyrolysis has been constructed which can calculate the gas formation rate/temperature profile from a given set of reaction parameters. By comparing the predictions of the model with experimental results at heating rates between 0.8 and 25 deg C s^?1, a generalised kinetic mechanism for the thermal decomposition of tobacco has been developed. For carbon monoxide and other low molecular weight gases, the mechanism is an independent formation of each gas from one solid tobacco component in each temperature region. Pyrolysis of some individual tobacco components in other studies suggests that each gas is actually produced from many components in each temperature region. This more complex mechanism is kinetically equivalent to the deduced mechanism of independent formation from one component.The region in which a given decomposition reaction takes place moves to higher temperatures as the heating rate increases. The amounts of gases formed over any temperature region from 200 to 900°C can be calculated for a given heating rate using the mechanism and the kinetic constants. The present results imply that 75–90% of the carbon monoxide produced by tobacco decomposition at temperatures up to 900°C during a puff on a cigarette corresponds to that formed in the “low temperature region” (200–450°C) defined for pyrolysis experiments at the lower heating rates of 1–10 deg C s^?1.     

Simultaneous use of isothermal,nonisothermal, and constant rate thermal analysis (CRTA) for discerning the kinetics of the thermal dissociation of smithsonite

The mechanism of the thermal decomposition of smithsonite has been determined from a comparison of the results obtained from isothermal, linear heating rate (TG), and Constant Rate Thermal Analysis (CRTA) experiments. Two important precautions have been taken in this work. Firstly, the chemical composition of the sample has been checked in order to be sure that pure anhydrous zinc carbonate has been used. Secondly, the experimental conditions have been selected in such a way to avoid the influence of heat and mass transfer phenomena on the forward reaction. It has been shown that the mechanism for the thermal decomposition of smithsonite depends upon temperature. Thus, at temperatures lower than 650 K, approximately, an A_0.5 kinetic model describes the reaction, whereas, at temperatures roughly higher than 690 K the above reaction obeys a F₁ kinetic law. An interpretation of this behavior is given. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 647–655, 1998     

A kinetic and mechanistic study of thermal decomposition of strontium titanyl oxalate

Thermal decomposition of anhydrous strontium titanyl oxalate proceeds through a series of complex reactions to form strontium metatitanate at high temperature. Among them the decomposition of oxalate is the first major thermal event. A kinetic study of oxalate decomposition in the temperature range 553-593 K has been carried out by cooled gas pressure measurement in vacuum. Results fitted the Zhuravlev equation for almost the entire α-range (0.05-0.92) indicating the occurrence of a diffusion-controlled, three-dimensional rate process. The activation energy has been calculated to be 164 ± 10 kJ mol⁻¹. Results from elemental analysis, TGA, IR and SEM studies of undecomposed and partially decomposed samples have been used to supplement kinetic observations in formulating the mechanism for oxalate decomposition.     

Correlation of thermal degradation mechanisms: Polyacetylene and vinyl and vinylidene polymers

The thermal degradation of poly(vinyl bromide) (PVB), poly(vinyl chloride) (PVC), poly(vinyl alcohol) (PVA), poly(vinyl acetate) (PVAc), poly(vinyl fluoride) (PVF), poly(vinylidene chloride) (PVC₂), and poly(vinylidene fluoride) (PVF₂) has been studied by direct pyrolysis–mass spectrometry (DP-MS) and flash pyrolysis–gas chromatography–mass spectrometry techniques. Vinyl and vinylidene polymers exhibit two competitive thermal degradation processes: (1) HX elimination with formation of polyene sequences which undergo further moleculaar rearrangements, and (2) main-chain cleavage with formation of halogenated or oxigenated compounds. The overall thermal degradation process depends on the prevailing decomposition reaction in each polymer; therefore, different behaviors are observed. The thermal degradation of polyacetylene (PA) has also been studied and found important for the elucidation of the thermal decomposition mechanism of the title polymers.     

Thermal behavior and kinetic modeling of (NH4)4UO2(CO3)3 decomposition under non-isothermal conditions

The thermal behavior and kinetic analysis of ammonium uranyl carbonate decomposition has been studied in inert gas, O₂, and 90%Ar–10%H₂ atmospheres under non-isothermal conditions. The results showed a dependence on specific surface area with the decomposition temperature of ammonium uranyl tri-carbonate (AUC). Specific surface area increases and reaches a maximum between 300 and 400 °C and decreases at T > 400 °C. The reaction paths of AUC decomposition under the three atmospheres were proposed. The integral methods Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) were used for the kinetic analysis. The activation energy averages are 58.01 and 56.19 kJ/mol by KAS and FWO methods, respectively.

     

Evidence of Kinetically Relevant Consistency in Thermal and Photo-Thermal HCOOH Decomposition over Pd/LaCrO3/C3N4 Composite

Photo-thermal catalysis has been an attractive alternative strategy to promote chemical reactions for years, however, how light cooperates with thermal energy is still unclear. We meet this demand by exploring reaction mechanism via pressure dependency studies as well as H/D exchange experiments with HCOOH decomposition as a probe over a palladium nanoparticle (Pd_n) and isolated Pd (Pd₁) decorated LaCrO₃/C₃N₄ composite catalyst, in which the H₂ formation rate shows a first-order dependence on HCOOH and inverse first-order dependence on CO partial pressures no matter the reaction was driven by thermal or photo-thermal energy. Additionally, negligible kinetic isotopic effects (KIEs: k_H/k_D) were determined under both dark and light conditions at 1.04 and 1.18 when the HCOOH was replaced by HCOOD. Besides, when the reactant HCOOH was further replaced by DCOOD, the KIE values of 1.55 (dark) and 1.92 (light) were obtained, which indicates that the HCOOH decomposition follows kinetically relevant (KR) of C−H bond rupture within HCOOH molecule under both thermal and photo-thermal reaction conditions and the catalytic surface was found to be densely covered by CO based on the pressure dependency studies as well as the in situ Fourier transform infrared spectroscopy (FTIR) analysis. Clearly, the HCOOH decomposition driven by thermal and photo-thermal energy follows the same reaction mechanism. Nevertheless, light induced hot electrons and the derived thermal effect do cause the enhancement of the reaction activity in some circumstances compared with bare thermal catalysis, which clarifies the confusion on cooperation mechanism of photo and thermal energies from the kinetic perspective. Hot electrons induced by photo-illumination was confirmed by in situ FTIR CO chemisorption with ∼10 cm⁻¹ redshift identified of the CO feature once light was introduced. Meanwhile, the photo thermal reaction system suffers from severe electron-hole re-combination at high reaction temperatures and make the thermal effect of photo irradiation dominant with respect to the effect at low reaction temperatures. This research provides insight to the mechanism on how photo-thermal reaction works and draws attention to the photo-thermal reaction process in boosting catalytic activity.     

Atmospheric effect for thermal process of indium hydroxoformate

The influence of thermal process for indium hydroxoformate, In(OH)(HCO₂)₂, used as one of the precursor material of ITO transparent conducting films, has been successfully investigated in some controlled atmospheres by unique thermal analyses equipped with a humidity generator, which are thermogravimetry - differential thermal analysis (TG-DTA), thermogravimetry in conjunction with evolved gas analysis using mass spectrometry (TG-MS) and simultaneous measurement of differential scanning calorimetry and X-ray diffractometry (XRD-DSC). The thermal process in dry gas atmosphere by linear heating experiment was indicated through a single-step reaction between 200 and 300°C, while the thermal process in the atmosphere of controlled humidity proceeded through two-step reactions and the formation of crystalline indium oxide (In₂O₃) was effectively promoted and completed at the lower temperatures with introducing water vapor in the atmosphere. The thermal process changed dramatically by introducing water vapor and was quite different from that in dry gas atmosphere. Pure In₂O₃ was synthesized in inert atmosphere of controlled humidity and could be easily formed at temperatures below 260°C. The XRD-DSC equipped with a humidity generator revealed directly the crystalline change from In(OH)(HCO₂)₂ to In₂O₃ and the formation of the intermediate during the thermal decomposition. A detailed thermal process of In(OH)(HCO₂)₂ and the effect of heating atmosphere are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Studies on kinetics and mechanism of initial thermal decomposition of nitrocellulose

The thermal decomposition of nitrocellulose (NC) 12.1% N, has been studied with regard to kinetics, mechanism, morphology and the gaseous products thereof, using thermogravimetry (TG), differential thermal analysis (DTA), IR spectroscopy, differential scanning calorimetry (DSC) and hot stage microscopy. The kinetics of the initial stage of thermolysis ofNC in condensed state has been investigated by isothermal high temperature infrared spectroscopy (IR). The decomposition ofNC in KBr matrix in the temperature range of 142–151°C shows rapid decrease in O?NO₂ band intensity, suggesting that the decomposition of NC occurs by the rupture of O?NO₂ bond. The energy of activation for this process has been determined with the help of Avrami-Erofe'ev equation (n=1) and is ≈188.35 kJ·mol^?1. Further, the IR spectra of the decomposition products in the initial stage of thermal decomposition ofNC, indicates the presence of mainly NO₂ gas and aldehyde.