Water vapor effect on the physico-geometrical reaction pathway and kinetics of the multistep thermal dehydration of calcium chloride dihydrate.

This study investigated how water vapor influences the reaction pathway and kinetics of the multistep thermal dehydration of inorganic hydrates, focusing on CaCl2·2H2O (CC-DH) transforming into its anhydride (CC-AH) via an intermediate of its monohydrate (CC-MH). In the presence of atmospheric water vapor, the thermal dehydration of CC-DH stoichiometrically proceeded through two distinct steps, resulting in the formation of CC-AH via CC-MH under isothermal conditions and linear nonisothermal conditions at a lower heating rate (ß). Irrespective of atmospheric water vapor pressure (p(H2O)), these reaction steps were kinetically characterized by a physico-geometrical consecutive process involving the surface reaction and phase boundary-controlled reaction, which was accompanied by three-dimensional shrinkage of the reaction interface. In addition, a significant induction period was observed for the second reaction step, that is, the thermal dehydration of CC-MH intermediate to form CC-AH. With increasing p(H2O), a systematic increase in the apparent Arrhenius parameters was observed for the first reaction step, that is, the thermal dehydration of CC-DH to form CC-MH, whereas the second reaction step exhibited unsystematic variations of the Arrhenius parameters. At a larger ß in the presence of atmospheric water vapor, the first and second reaction steps partially overlapped; moreover, an alternative reaction step of the thermal dehydration of CC-MH to form CaCl2·0.3H2O was observed between these reaction steps. The physico-geometrical phenomena influencing the reaction pathway and kinetics of the multistep thermal dehydration were elucidated by considering the effects of atmospheric and self-generated water vapor in a geometrically constrained reaction scheme.

The physico-geometrical reaction pathway and kinetics of multistep thermal dehydration of calcium chloride dihydrate in a dry nitrogen stream.

Several inorganic hydrates exhibit reversible reactions of thermal dehydration and rehydration, which is potentially applicable to thermochemical energy storage. Detailed kinetic information on both forward and reverse reactions is essential for refining energy storage systems. In this study, factors determining the reaction pathway and kinetics of the multistep thermal dehydration of inorganic hydrates to form anhydride via intermediate hydrates were investigated as exemplified by the thermal dehydration of CaCl2·2H2O (CC-DH) in a stream of dry N2. The formation of CaCl2·H2O (CC-MH) as the intermediate hydrate is known during the thermal dehydration of CC-DH to form its anhydride (CC-AH). However, the two-step kinetic modeling based on the chemical reaction pathway considering the formation of the CC-MH intermediate failed in terms of the reaction stoichiometry and kinetic behavior of the component reaction steps. The kinetic modeling was refined by considering the physico-geometrical reaction mechanism and the self-generated reaction conditions to be a three-step reaction. The multistep reaction was explained as comprising the surface reaction of the thermal dehydration of CC-DH to CC-AH and subsequent contracting geometry-type reactions from CC-DH to CC-MH and from CC-MH to CC-AH occurring consecutively in the core of the reacting particles surrounded by the surface product layer of CC-AH. The acceleration of the linear advancement rate of the reaction interface during both contracting geometry-type reactions was revealed through multistep kinetic analysis and was described by a decrease in the water vapor pressure at the reaction interface as the previous reaction step proceeded and terminated.

Efflorescence kinetics of sodium carbonate decahydrate: a universal description as a function of temperature, degree of reaction, and water vapor pressure.

The efflorescence of sodium carbonate decahydrate (SC-DH) required to form its monohydrate (SC-MH) was systematically studied under isothermal and linear nonisothermal conditions at different atmospheric water vapor pressures (p(H2O)) using a humidity-controlled thermogravimetry instrument equipped with a cooling circulator. The universal kinetic description at various temperatures (T) and p(H2O) values was evaluated using the extended kinetic equation with an accommodation function (AF) comprising p(H2O) and the equilibrium pressure of the reaction (Peq(T)). By optimizing two exponents in the AF, all kinetic data were universally described in terms of the isoconversional kinetic relationship examined at individual degrees of reaction (α). This enabled the examination of the isothermal kinetic relationship and the parameterization of the contribution of the self-generated water vapor, allowing the incorporation of kinetic data recorded in a stream of dry N2 into the universal kinetic description as a function of T, α, and p(H2O). The results indicated that the reaction is physico-geometrically controlled by the surface reaction at the hemispherical top surface of SC-DH particles and subsequent advancement of the reaction interface toward the center and bottom of these particles, where the interfacial process is regulated by an elementary step of the consumption of H2O vacancies to form the SC-MH building unit. The apparent activation energy (Ea) of ∼178 kJ mol-1 was determined using the extended kinetic approach considering the effect of p(H2O) correlated with the intrinsic Ea of the Arrhenius-type temperature dependence (∼63 kJ mol-1) by subtracting the contribution of the temperature dependence of Peq(T) in the AF.

Thermal dehydration of D-glucose monohydrate in solid and liquid states.

The physico-geometrical reaction pathway and kinetics of the thermal dehydration of D-glucose monohydrate (DG-MH) dramatically alter by the melting of the reactant midway through the reaction. By controlling the reaction conditions, the thermal dehydration of DG-MH was systematically traced by thermoanalytical techniques in three different reaction modes: (1) solid-state reaction, (2) switching from a solid- to liquid-state reaction, and (3) liquid-state reaction. Solid-state thermal dehydration occurred under isothermal conditions and linear nonisothermal conditions at a small heating rate (ß ≤ 1 K min-1) in a stream of dry N2. The kinetic behavior comprised the presence of an induction period and a sigmoidal mass loss process characterized by a derivative mass loss curve with a symmetrical shape under isothermal conditions, resembling the autocatalytic reaction in homogeneous kinetic processes. When DG-MH was heated at a larger ß (≥2 K min-1), the melting of DG-MH occurred midway through the thermal dehydration process, by which a core-shell structure of molten DG-MH and surface product layer of crystalline anhydride was produced. Subsequently, thermal dehydration proceeded as a complex multistep process. Furthermore, the thermal dehydration initiated at approximately the melting point of DG-MH upon the application of a certain water vapor pressure to the reaction atmosphere, and proceeded in the liquid-state, exhibiting a smooth mass loss process to form crystalline anhydride. The reaction pathway and kinetics of the thermal dehydration of DG-MH and the corresponding changes with the sample and reaction conditions are discussed on the basis of the detailed kinetic analysis.

Physico-geometrical kinetic insight into multistep thermal dehydration of calcium hydrogen phosphate dihydrate.

The origin of the multistep thermal dehydration of calcium hydrogen phosphate dihydrate (dibasic calcium phosphate dihydrate (DCPD)) to form γ-calcium diphosphate (γ-calcium pyrophosphate (γ-CPP)) via calcium hydrogen phosphate anhydride (dibasic calcium phosphate anhydride (DCPA)) was investigated from a specific viewpoint of physico-geometrical constraints generated during the reaction. The overall thermal dehydration was separated into five partially overlapping steps through systematic kinetic analysis. The first three steps and the residual two steps were attributed to the thermal dehydration of DCPD to form DCPA and of DCPA to form γ-CPP, respectively. The first to third steps were kinetically characterized by the surface reaction of plate-like particles controlled by nucleation and growth, the movement of the reaction interface inward to the plate by releasing water vapor through voids formed in the surface product layer, and the rapid escape of water vapor accompanied by the cleavage of plate-like particles into slices, respectively. The contributions of each component step varied with the heating conditions and atmospheric water vapor pressure. The subsequent dehydration of DCPA proceeded in two steps by the release of trapped water molecules in amorphous DCPA induced by its gradual crystallization and the dehydration of DCPA to form poorly crystalline γ-CPP, which continued to grow during the fifth mass loss step and exhibited a detectable exothermic phenomenon after the mass loss was completed. The possible causes of the variation in the multistep reaction features with reaction conditions were discussed by correlating the kinetic analysis results with the crystallographic and morphological findings.

Effect of atmospheric water vapor on independent-parallel thermal dehydration of a compacted composite of an inorganic hydrate: sodium carbonate monohydrate grains comprising crystalline particles and a matrix.

The effect of atmospheric water vapor on the thermal dehydration of sodium carbonate monohydrate (SC-MH), which was characterized as cubic grains of a compacted composite comprising columnar SC-MH crystals and a matrix, was systematically assessed using a humidity-controlled thermogravimetry system at various atmospheric water vapor pressures (p(H2O)). The thermal dehydration of the SC-MH compacted composite occurred via an induction period (IP) and partially overlapping two-step mass loss steps due to the thermal dehydration of the SC-MH matrix and columnar crystals. All component reaction steps were retarded with an increase in the p(H2O) value. The kinetics of individual reaction steps were universally described over different temperatures and p(H2O) values based on a kinetic equation that considered p(H2O) and the equilibrium pressure of the thermal dehydration. Additionally, the physico-geometrical consecutive surface reaction (SR) and subsequent phase boundary-controlled reaction (PBR) model was employed to describe the first mass loss step. The difference between the effects of atmospheric p(H2O) on SR and PBR processes was parameterized via an advanced kinetic analysis. The kinetic behavior of the second mass loss step was discussed based on a three-dimensional contracting geometry model with accelerating reaction interface advancement, where the changes in the rate behavior with atmospheric p(H2O) were explained by the total effect of atmospheric and self-generated p(H2O) on the kinetics. The present results provide additional insights into the independent-parallel thermal decomposition kinetics of composite materials by considering the effects of atmospheric and self-generated gases.

Physico-geometrical kinetics of the thermal dehydration of sodium carbonate monohydrate as a compacted composite of inorganic hydrate comprising crystalline particles and matrix.

The kinetics of the thermal dehydration of compacted composite grains of Na2CO3·H2O (SC-MH) comprising columnar SC-MH crystalline particles and an SC-MH matrix were investigated as a model system for composites of the same compound with a porphyritic texture. The presence of an induction period was confirmed as a novel finding for the thermal dehydration of SC-MH. The subsequent mass loss process was characterized as a partially overlapping two-step process attributed to the consecutive reactions of SC-MH matrix and columnar SC-MH crystalline particles. The overlapping nature of two reaction steps was revealed by determining the contributions and kinetic parameters of the individual reaction steps via a kinetic deconvolution analysis. Furthermore, the initial mass loss process caused by the thermal dehydration of the SC-MH matrix was characterized as a physico-geometrical consecutive process comprising a surface reaction and a subsequent three-dimensional (3D)-phase boundary-controlled reaction. The subsequent thermal dehydration of the columnar SC-MH crystalline particles compacted in the grains was characterized as being geometrically constrained by 3D-interface shrinkage, forming two reaction interfaces during the overlapping stage of the two reaction steps. It was expected from the kinetic results that the linear advancement rate of the second reaction interface was influenced by the water vapor produced at the reaction interface of the first reaction step. This caused the linear advancement rate of the second reaction interface to accelerate as the reaction proceeded due to contraction of the first reaction interface and completion of the first reaction step.

Individual effects of atmospheric water vapor and carbon dioxide on the kinetics of the thermal decomposition of granular malachite.

This study examined the effects of atmospheric water vapor and CO2 on the thermal decomposition of granular malachite as a model process for the thermal decomposition of large and compact agglomerate solids. In previous studies in a dry N2 gas stream, the thermal decomposition of the granular malachite exhibited physico-geometrically constrained two-step mass loss behaviors accompanied by the swelling of granular particles and crack formation in the surface product layer of each granule. In the presence of atmospheric water vapor, the reaction was shifted systematically to lower temperatures with increasing atmospheric water vapor pressure (p(H2O)) by maintaining the two-step mass loss behavior. Kinetic analyses of the two-step heterogeneous process and subsequent universal kinetic description for each reaction step over different p(H2O) values demonstrated that the catalytic effect of atmospheric water vapor is more significant in the first reaction step because of the surface reaction. Conversely, in the presence of atmospheric CO2, the reaction shifted systematically to higher temperatures with increasing partial pressure of CO2 where the surface and internal reaction steps were more clearly separated, and additional mass-loss steps appeared to complete the reaction. The enhanced retardation effects of atmospheric CO2 as the mass-loss process advanced were confirmed by kinetic analyses of the empirically deconvoluted five-step reaction process. This phenomenon was explained by the effects of atmospheric CO2 on the construction of the surface product layer that helps block the diffusional removal of the gaseous product and increases the internal gaseous pressure.

An advanced kinetic approach to the multistep thermal dehydration of calcium sulfate dihydrate under different heating and water vapor conditions: kinetic deconvolution and universal isoconversional analyses.

This study aims to identify the kinetic features of individual reaction steps of the multistep thermal dehydration of calcium sulfate dihydrate (CS-DH) to anhydride via a hemihydrate (CS-HH) intermediate by achieving the universal kinetic description of each reaction step under different heating and water vapor pressure (p(H2O)) conditions. The mass loss processes of the thermal dehydration of CS-DH were systematically traced via humidity-controlled thermogravimetry under isothermal and linear nonisothermal conditions at various atmospheric p(H2O) values. After reconfirming the variation in the thermal dehydration pathway from a single-step dehydration to anhydride to a multistep process via the CS-HH intermediate with an increase in the p(H2O) value, the kinetic curves for each component reaction step were obtained by separating each component process from the partially overlapping mass-loss curves by kinetic deconvolution analysis as required. The induction period (IP) and the mass-loss processes of the thermal dehydrations of CS-DH to anhydride and CS-HH intermediate were compared, wherein more significant retardation effects of water vapor were observed for the IP process followed by direct dehydration to anhydride and for the mass-loss process from CS-DH to the CS-HH intermediate. The universal kinetic behavior of the thermal dehydration of the CS-HH intermediate to anhydride was compared with that of the CS-HH reagent; thus, comparable universal kinetic behaviors were observed except the reaction geometry. Based on the universal kinetic results, the key kinetic phenomenon for regulating the variation of the thermal dehydration pathway of CS-DH was discussed.

Thermally induced dehydration reactions of monosodium L-glutamate monohydrate: dehydration of solids accompanied by liquefaction.

In this study, we investigated the mechanistic features and kinetics of the thermal decomposition of solids accompanied by liquefaction as exemplified by the thermal dehydration reactions of monosodium L-glutamate monohydrate (MSG-MH). The thermal dehydration of MSG-MH occurs via two mass-loss processes comprising the elimination of crystalline water and intramolecular dehydration. Multistep kinetic behaviors and the liquefaction during both thermal dehydration processes were evidenced by systematic thermoanalytical measurements and in situ microscopic observations. During the thermal dehydration of crystalline water, the liquefaction of the surface product layer occurred midway through the reaction, and the subsequent reaction proceeded with a geometrical constraint, where the solid reactant was covered by a liquid surface layer, affording a solid anhydride. The intramolecular dehydration of the solid anhydride yielded a liquid product on the surface of the reacting particles, and the internal solid reactant dissolved in the liquid product. Subsequently, the intramolecular dehydration proceeded in the liquid phase to afford liquid sodium pyroglutamate. The net kinetic behavior of the physico-geometrical reaction steps in each thermal dehydration process was revealed using kinetic approaches based on cumulative and conjunct kinetic equations. The advanced kinetic approaches employed to reveal the specific kinetic features of the heterogeneous reaction processes in solid-liquid-gas systems are described in this article.

Geometrical constraints of thermal dehydration of ß-calcium sulfate hemihydrate induced by self-generated water vapor.

The thermal dehydration of calcium sulfate dihydrate exhibits a complex reaction behavior, in which the reaction pathway and kinetics vary depending on water vapor pressure (p(H2O)) applied as the atmospheric condition and generated in the course of the reaction. Under high p(H2O) conditions, a crystalline hemihydrate is produced as an intermediate, which subsequently dehydrates to form anhydride. In this study, the thermal dehydration of calcium sulfate hemihydrate under different self-generated p(H2O) conditions was investigated to gain further insight into the reactions in the calcium sulfate-water vapor system. The thermal dehydration of the hemihydrate under two sets of sampling conditions, namely, in open and lidded (semi-closed) pans, was systematically investigated via thermogravimetry (TG) in different heating program modes. The experimentally resolved TG curves were analyzed using the formal kinetic calculation methods based on isoconversional and isothermal kinetic relationships. Under both the sampling conditions, the thermal dehydration reaction was significantly influenced by self-generated p(H2O), which regulated the reaction proceeding from the top surface of the sample bed to the bottom. Under higher self-generated p(H2O) conditions in a lidded pan, the thermal dehydration under different heating program modes exhibited an invariant kinetic behavior characterized by a single set of kinetic parameters, whereas in an open pan the kinetic behavior varied between the reactions under isothermal and other heating modes. Based on the results of the formal kinetic analysis, an advanced kinetic modeling based on a physico-geometrical consecutive reaction model was examined to describe in detail the specific kinetic features of the reaction under self-generated p(H2O) conditions.

Thermal decomposition of spherically granulated malachite: physico-geometrical constraints and overall kinetics.

The thermal decomposition of spherically granulated malachite particles was investigated to unveil the specific kinetic features of the reaction in samples in granular form toward the improvement of the thermal processing of malachite as a precursor of functional CuO. Granular malachite underwent thermal decomposition via a partially overlapping two-step mass loss process upon heating the sample in a stream of dry N2 gas. Morphologically, the process was characterized by swelling of the granular particles and cleavage divisions of the surface layer. The kinetics of the thermal decomposition was investigated through step-by-step kinetic analyses of the systematically recorded thermoanalytical curves. Finally, the kinetics of the component reaction steps was separately characterized by performing a kinetic deconvolution analysis. The first reaction step, which contributed approximately 25% to the overall reaction and followed pseudo-first-order kinetics, was attributed to the thermal decomposition of the granular particle surface. The as-produced surface product layer impeded the diffusional removal of the gaseous products, i.e., CO2 and water vapor, from the interior of the granular particles, which caused swelling of the granular particles owing to an increase in the internal gaseous pressure and the cleavage division of the surface product layer by crack formation. The second mass loss step occurred inside the granular particles under significant variations in the self-generated reaction conditions and geometrical constraints and reached its maximum rate midway through the reaction. Possible causes of the observed specific rate behavior are discussed from the viewpoint of physico-geometrical kinetics in the solid-gas system.

Thermally Induced Aragonite-Calcite Transformation in Freshwater Pearl: A Mutual Relation with the Thermal Dehydration of Included Water.

This study focuses on the relationship between the aragonite-calcite (A-C) transformation and the thermal dehydration of included water in the biomineralized aragonite construction using freshwater pearl. These thermally induced processes occur in the same temperature region. The thermal dehydration of included water was characterized through thermoanalytical investigations as an overlapping of three dehydration steps. Each dehydration step was separated through kinetic deconvolution analysis, and the kinetic parameters were determined. A single-step behavior of the A-C transformation was evidenced using high-temperature X-ray diffractometry and Fourier transform infrared spectrometry for the heat-treated samples. The kinetics of the A-C transformation was analyzed using the conversion curves under isothermal and linear nonisothermal conditions. The A-C transformation occurred in the corresponding temperature region of the thermal dehydration, ranging from the second half of the second dehydration step to the first half of the third dehydration step. Because the thermal dehydration process is constrained by the contracting geometry kinetics, the movement of the thermal dehydration reaction interface can be a trigger for the A-C transformation. In this scheme, the overall kinetics of the A-C transformation in the biomineralized aragonite construction is regulated by a contracting geometry.

Apparent autocatalysis due to liquefaction: thermal decomposition of ammonium 3,4,5-trinitropyrazolate.

Thermal decomposition of solids is often accompanied by autocatalysis, one of the possible causes of which is the formation of a liquid phase. The kinetic model considering the liquefaction of solid reactants under isothermal conditions was proposed by Bawn in the 1950s. The present study reports the application of the Bawn model to the thermolysis of 3,4,5-trinitropyrazole ammonium salt (ATNP) under nonisothermal conditions. The thermal decomposition of ATNP is comprised of low-temperature and high-temperature stages. The low-temperature stage exhibits two distinct exothermic peaks in differential scanning calorimetry (DSC), fitted by two consecutive autocatalytic reactions with a model-fitting kinetic analysis. The liquefaction of the solid reactant during the first reaction is directly observed, giving mechanistic evidence for the Bawn model. We have expressed the Bawn model by a combination of two extended Prout-Tompkins (ePT) equations with the activation energy for the leading liquid-state reaction of Ea = 140.6 ± 0.3 kJ mol-1. The release of ammonia is detected from the beginning, suggesting that the thermal dissociation of ATNP to 3,4,5-trinitropyrazole is an initiation reaction of the thermal decomposition. We proposed ATNP liquefication, leading to the apparent autocatalytic behavior of the first global decomposition reaction, is caused by the eutectic formation between ATNP and 3,4,5-trinitropyrazole, as it was confirmed by DSC analysis of the artificial mixture. The presented approach of the combination of ePT formalism with a Bawn model is generally applicable to a broader range of thermal processes accompanied by liquid phase formation and apparent acceleration.

Thermal dehydration of calcium sulfate dihydrate: physico-geometrical kinetic modeling and the influence of self-generated water vapor.

Complex kinetic behaviors in the thermal dehydration of CaSO4·2H2O under varying water vapor pressure (p(H2O)) conditions impel researchers in the field of solid-state kinetics to gain a more comprehensive understanding. Both self-generated and atmospheric p(H2O) are responsible for determining the reaction pathways and the overall kinetic behaviors. This study focuses on the influence of the self-generated water vapor to obtain further insights into the complexity of the kinetic behaviors. The single-step mass-loss process under conditions generating a low p(H2O) was characterized kinetically by a physico-geometrical consecutive induction period, surface reaction, and phase boundary-controlled reaction, along with the evaluation of the kinetic parameters for the individual physico-geometrical reaction steps. Under the conditions in which more p(H2O) was generated, the overall reaction to form the anhydride was interpreted as a three-step process, comprising the initial reaction (direct dehydration to the anhydride) and a subsequent two-step reaction via the intermediate hemihydrate, which was caused by the variations in the self-generated p(H2O) conditions as the reaction advanced. The variations in the reaction pathways and kinetics behaviors under the self-generated p(H2O) conditions are discussed through a systematic kinetic analysis conducted using advanced kinetic approaches for the multistep process.

Kinetics of contracting geometry-type reactions in the solid state: implications from the thermally induced transformation processes of α-oxalic acid dihydrate.

This study focuses on the physico-geometrical constraints of the kinetics of the thermal decomposition of solids as exemplified by the thermal dehydration of α-oxalic acid dihydrate and the subsequent thermally induced sublimation/decomposition of the as-produced anhydride using the samples of crystalline particles (CPs) and a single crystal (SC) form. The CP and SC samples possess approximately similar geometrical figures with different sizes. The shapes of the original dihydrate and the as-produced anhydride from thermal dehydration are practically congruent. Therefore, proper evaluations of the current kinetic understanding of contracting geometry-type reactions were expected by the comparisons of the kinetic behaviors among different sample forms and thermally induced processes. The kinetic analysis of the thermal dehydration process revealed that the consecutive physico-geometrical processes comprised of an induction period, a surface reaction, and a phase boundary-controlled reaction, where distinguishable differences in the rate behavior were observed between the CP and SC samples for the surface reaction. On the other hand, the thermally induced sublimation/decomposition of the anhydride was described as an ideal single-step geometry contraction process, for which the CP and SC samples exhibited the same rate variation behavior under isothermal conditions. However, the sublimation/decomposition processes of the CP and SC samples were characterized by the different Arrhenius parameters, in which the compensative changes in the apparent activation energy and preexponential factor were apparent. Implications for the kinetic modeling of the solid-state reactions and the interpretation of kinetic results were obtained from the results of the comparative kinetic study for different sample forms and thermally induced processes.

Revealing the effect of water vapor pressure on the kinetics of thermal decomposition of magnesium hydroxide.

This study aims to establish an advanced kinetic theory for reactions in solid state and solid-gas systems, achieving a universal kinetic description over a range of temperature and partial pressure of reactant or product gases. The thermal decomposition of Mg(OH)2 to MgO was selected as a model reaction system, and the effect of water vapor pressure p(H2O) on the kinetics was investigated via humidity controlled thermogravimetry. The reaction rate of the thermal decomposition process at a constant temperature was systematically decreased by increasing the p(H2O) value, accompanied by an increase in the sigmoidal feature of mass-loss curves. Under nonisothermal conditions at a given heating rate, mass-loss curves shifted systematically to higher temperatures depending on the p(H2O) value. The kinetic behavior under different temperature and p(H2O) conditions was universally analyzed by introducing an accommodation function (AF) of the form (P°/p(H2O))a[1 - (p(H2O)/Peq(T))b], where P° and Peq(T) are the standard and equilibrium pressures, respectively, into the fundamental kinetic equation. Two kinetic approaches were examined based on the isoconversional kinetic relationship and a physico-geometrical consecutive reaction model. In both the kinetic approaches, universal kinetic descriptions are achieved using the modified kinetic equation with the AF. The kinetic features of thermal decomposition are revealed by correlating the results from the two universal kinetic approaches. Furthermore, advanced features for the kinetic understanding of thermal decomposition of solids revealed by the universal kinetic descriptions are discussed by comparing the present kinetic results with those reported previously for the thermal decomposition of Ca(OH)2 and Cu(OH)2.

Thermal Decomposition of Maya Blue: Extraction of Indigo Thermal Decomposition Steps from a Multistep Heterogeneous Reaction Using a Kinetic Deconvolution Analysis.

Examining the kinetics of solids' thermal decomposition with multiple overlapping steps is of growing interest in many fields, including materials science and engineering. Despite the difficulty of describing the kinetics for complex reaction processes constrained by physico-geometrical features, the kinetic deconvolution analysis (KDA) based on a cumulative kinetic equation is one practical method of obtaining the fundamental information needed to interpret detailed kinetic features. This article reports the application of KDA to thermal decomposition of clay minerals and indigo-clay mineral hybrid compounds, known as Maya blue, from ancient Mayan civilization. Maya blue samples were prepared by heating solid mixtures of indigo and clay minerals (palygorskite and sepiolite), followed by purification. The multistep thermal decomposition processes of the clay minerals and Maya blue samples were analyzed kinetically in a stepwise manner through preliminary kinetic analyses based on a conventional isoconversional method and mathematical peak deconvolution to finally attain the KDA. By comparing the results of KDA for the thermal decomposition processes of the clay minerals and the Maya blue samples, information about the thermal decomposition steps of the indigo incorporated into the Maya blue samples was extracted. The thermal stability of Maya blue samples was interpreted through the kinetic characterization of the extracted indigo decomposition steps.

Critical Appraisal of Kinetic Calculation Methods Applied to Overlapping Multistep Reactions.

Thermal decomposition of solids often includes simultaneous occurrence of the overlapping processes with unequal contributions in a specific physical quantity variation during the overall reaction (e.g., the opposite effects of decomposition and evaporation on the caloric signal). Kinetic analysis for such reactions is not a straightforward, while the applicability of common kinetic calculation methods to the particular complex processes has to be justified. This study focused on the critical analysis of the available kinetic calculation methods applied to the mathematically simulated thermogravimetry (TG) and differential scanning calorimetry (DSC) data. Comparing the calculated kinetic parameters with true kinetic parameters (used to simulate the thermoanalytical curves), some caveats in the application of the Kissinger, isoconversional Friedman, Vyazovkin and Flynn-Wall-Ozawa methods, mathematical and kinetic deconvolution approaches and formal kinetic description were highlighted. The model-fitting approach using simultaneously TG and DSC data was found to be the most useful for the complex processes assumed in the study.

Impact of atmospheric water vapor on the thermal decomposition of calcium hydroxide: a universal kinetic approach to a physico-geometrical consecutive reaction in solid-gas systems under different partial pressures of product gas.

Thermal decomposition of Ca(OH)2 under atmospheric water vapor exhibits special features, including an induction period (IP) and a subsequent sigmoidal mass-loss behavior under isothermal conditions. Atmospheric water vapor reduces the reaction rate at a specific temperature and causes a systematic shift of the mass-loss curve, which was recorded at a specific heating rate, to higher temperatures as the water vapor pressure, p(H2O), increases. The challenge in this study was to universally describe the kinetics of thermal decomposition under various p(H2O) conditions by introducing an accommodation function in the fundamental kinetic equation. The accommodation function in the multiplied form of two p(H2O) components with a variable exponent in each component was derived on the basis of the classical nucleation and interface reaction theories. The universal kinetic approach was realized by applying the accommodation function to formal kinetic analyses of the Arrhenius plot for the IP and the Friedman plot for the mass-loss process. Furthermore, the overall reaction process under isothermal conditions was analyzed kinetically on the basis of the physico-geometrical consecutive reaction model, which was composed of an IP, a surface reaction (SR), and a phase boundary-controlled reaction (PBR). Subsequently, the kinetic parameters for each physico-geometrical reaction step were determined by the modified Arrhenius plot with the accommodation function. The impact of the atmospheric water vapor on the kinetics of thermal decomposition was characterized in connection with physico-geometrical reaction mechanisms through the interpretation of the kinetic parameters and these variation behavior patterns as the overall reaction advanced.