Kinetics Of Dehydroxylation Research Articles

Comparative study on non-isothermal dehydroxylation kinetics of talc based on multi-scan thermogravimetry and thermodilatometry methods

Comparative study on non-isothermal dehydroxylation kinetics of talc based on multi-scan thermogravimetry and thermodilatometry methods

Investigating the thermal behavior and dehydroxylation kinetics of bentonite: isothermal and non-isothermal study

Investigating the thermal behavior and dehydroxylation kinetics of bentonite: isothermal and non-isothermal study

Kinetics of antigorite dehydroxylation for CO2 sequestration

Heat-treatment of serpentine minerals generates structural amorphicity and increases reactivity during subsequent mineral carbonation, a strategy for large-scale sequestration of CO2. This study employs thermal analyses (TGA-DSC) in conjunction with in-situ synchrotron powder X-ray diffraction (PXRD) to record concurrent mass loss, heat flow, and mineralogical changes during thermal treatment of antigorite. Isoconversional kinetic modelling demonstrates that thermal decomposition of antigorite is a complex multi-step reaction, with activation energies (Eα) varying between 290 and 515 kJ mol−1. We identify three intermediate phases forming during antigorite dehydroxylation, a semi-crystalline chlorite-like phase (γ-metaserpentine) showing an additional reaction pathway for the decomposition of Al2O3-rich antigorite into pyrope, and two distinct amorphous components (α and β-metaserpentine) which convert into forsterite and enstatite at higher temperature, respectively. The combination of isoconversional kinetics with in-situ synchrotron PXRD illustrates, for the first time, that local crystal structure changes, related to intermediate phase and forsterite formation, are responsible for the steep increase in activation energy above 650 °C and only 49% dehydroxylation can be achieved prior to this increase. This suggests that the high thermal stability of Al2O3-rich antigorite would severely limit Mg extraction during application of mineral carbonation under flue gas conditions.

Kinetics of Dehydroxylation and Decarburization of Coal Series Kaolinite during Calcination: A Novel Kinetic Method Based on Gaseous Products.

The analysis of gaseous products reveals the characteristics, mechanisms, and kinetic equations describing the dehydroxylation and decarburization in coal series kaolinite. The results show that the dehydroxylation of coal series kaolinite arises from the calcination of kaolinite and boehmite within the temperature range of 350–850 °C. The activation energy for dehydroxylation is 182.71 kJ·mol−1, and the mechanism conforms to the A2/3 model. Decarburization is a two-step reaction, occurring as a result of the combustion of carbon and the decomposition of a small amount of calcite. The temperature range in the first step is 350–550 °C, and in the second is 580–830 °C. The first step decarburization reaction conforms to the A2/3 mechanism function, and the activation energy is 160.94 kJ·mol−1. The second step decarburization reaction follows the B3 mechanism function, wherein the activation energy is 215.47 kJ·mol−1. A comparison with the traditional methods proves that the kinetics method utilizing TG-FTIR-MS is feasible.

Mechanism and Kinetics of Dehydroxylation and Decarbonation of Mg-Fe Hydrotalcite.

Thermal behavior of hydrotalcites, which is a calcination process, is critical to prepare the metal oxide catalysts with high performances in the practical applications. In this paper, the MgFe-LDH with Mg/Fe molar ratio of 3.0 was prepared by urea method and the calcined products are obtained by calcining at different temperatures (473 K, 573 K, 673 K, 773 K, 873 K and 973 K) under a N2 atmosphere for 4 h. The structure, morphology, texture, pyrolysis kinetics and mechanism of the MgFe-LDH were studied in detail. On one hand, based on the TG/DSC curves, Starink, Kissinger and Flynn-Wall-Ozawa (FWO) methods were used to calculate the activation energy, on the other hand, the Šatava-Šesták, Achar and Málek methods were used to define the most probable reaction mechanisms of pyrolysis behavior. The results suggested that the thermal decomposition of the LDH experienced two steps, i.e., removal of the interlayer water, followed by dehydroxylation and decarbonation. Moreover the Mákel method was used to define the most probable reaction mechanisms of the pyrolysis behavior.

Redox-thermal behavior of archaeological ceramics from the North Caucasus (Russia, Bronze/Iron Age)

The firing behavior of illite-based archaeological ceramics and corresponding firing conditions were investigated, in order to identify the pyrotechnology of ceramic production at archaeological sites in the North Caucasus (Russia, Bronze and Iron Age). Direct observations of the pyrometamorphic degree in the objects by scanning electron microscope (SEM), X-ray powder diffraction (XRD), Raman and Fourier-transform infrared spectroscopy (FT-IR) revealed the thermally induced localization of the redox state within a single object and its influence on the structural distortion, dehydroxylation and total collapse of illite in the ceramics. Fundamental approaches to illite dehydroxylation kinetics and numerical simulations of oxygen diffusion and heat transfer revealed that the firing temperature and time and thickness of the sample, reactivity between oxygen and ceramic pastes and porosity evolution played a decisive role in the firing behavior of the ceramics during the firing at the sites.

Application of logistic function to describe kinetics of non-isothermal dehydroxylation of fullerol

A sample of fullerol C60(OH)24 was synthesized. The basic physicochemical characterization of the synthesized fullerol was done. The non-isothermal kinetics of C60(OH)24 dehydroxylation has been investigated. The thermogravimetric curves have been recorded at different heating rates ranging from 5 to 25 K min−1. By application of the Kissinger–Akahira–Sunose isoconversion method, it was found that the activation energy complexly changes with the dehydroxylation degree. The possibility of mathematical description of the kinetics of fullerol dehydroxylation by logistic function was investigated. Fullerol dehydroxylation conversion curves were completely mathematically described by the linear combination of two logistic functions at all of the investigated heating rates. The changes in the values of parameters of logistic functions with heating rate were established. It was shown that complex kinetics of C60(OH)24 dehydroxylation consists of two consecutive dehydroxylation reactions (low-temperature and high-temperature components). The values of the kinetic parameters for two components of dehydroxylation process were calculated. A model for fullerol dehydroxylation has been discussed.

Dehydroxylation Kinetics of Clay Minerals and Its Application to Friction Heating Along an Imbricate Thrust in an Accretionary Prism

AbstractDehydroxylation of clay minerals within fault gouges is significant for assessing transient thermogenesis due to high‐velocity, frictional slip along fault zones. The clay minerals kaolinite and chlorite are common in fault zones hosted in sedimentary rocks at subduction margins. To better understand the dehydroxylation processes of these clay minerals, high‐temperature X‐ray diffraction analyses were carried out by using a 1:1 mixture of kaolinite and chlorite standard samples. We evaluated the kinetic parameters of each dehydroxylation reaction by thermogravimetric analysis using the Friedman method. For kaolinite, the thermogravimetric data are fitted with a one and a half order equation (F3/2) with an activation energy of 171 kJ/mol and a frequency factor of 5.6 × 108 s−1. The data for chlorite are analyzed by the geometrical contracting model equation (R2) with an activation energy of 197 kJ/mol and a frequency factor of 4.5 × 109 s−1. Thermal models of frictional heating employing this calibration show that the frictional heating can explain the reported clay mineralogy in a fossil imbricate thrust from a shallow part in an ancient accretionary prism (Shirako Fault, Japan). This result supports the previous assertion, and the observed temperature anomaly appears to demonstrate the frictional heating caused by coseismic slip on this fault.

Thermal Dehydroxylation Kinetics of Algerian Halloysite by Differential Thermal Analysis

Thermal Dehydroxylation Kinetics of Algerian Halloysite by Differential Thermal Analysis

Application of the Suzuki–Fraser function in modelling the non-isothermal dehydroxylation kinetics of fullerol

The possibility of fitting the non-isothermal kinetics of C60(OH)27 dehydroxylation by the Frazer–Suzuki equation has been investigated. Thermogravimetric curves of fullerol dehydroxylation have been recorded at different heating rates ranging from 5 to 25 K min−1. The curves of fullerol dehydroxylation reaction rate were completely deconvoluted with the two Frazer–Suzuki functions at all heating rates and hence, it was concluded that mechanism of C60(OH)27 dehydroxylation consisted from two dehydroxylation reactions. Reaction components can be connected with the existence of two clusters on fullerene surface that are formed from OH groups. Using Vyazovkin’s isoconversional method, it was found that, for both components, apparent activation energy changes with dehydroxylation degree, meaning that deconvolution using the Frazer–Suzuki function do not decomposes complex reaction of fullerol dehydroxylation on elementary steps. The dependence of activation energies on their dehydroxylation degree for each of the reaction components is the consequence of the existence of activation energy distribution in the form of a narrow peak, which can be further related with unique reaction model function for each dehydroxylation component.

A dehydroxylation kinetics study of brucite Mg(OH)2 at elevated pressure and temperature

We performed an in situ dehydroxylation kinetics study of brucite under water-saturated conditions in the pressure and temperature ranges of 593–633 K and 668–1655 MPa using a hydrothermal diamond anvil cell. The kinetic analysis of the isothermal–isobaric data using an Avrami-type model involving nucleation and growth processes yields values for the dehydroxylation rate and reaction order compatible with a reaction mechanism limited by the monodimensional diffusion of water molecules from structural OH groups. Our results show a negative pressure dependence on the reaction rate k and a positive temperature dependence on the k. The dehydroxylation of brucite yields an activation volume ∆V value of 5.03 cm3/mol. Following the Arrhenius relationship, the apparent activation energy E a of the process is calculated to be 146 kJ/mol within the experimental P–T ranges. It is determined that the fluid production rates varying from 4.4 × 10−7 to 10.7 × 10−7 $${\text{m}}_{\text{fluid}}^{3} \;{\text{m}}_{\text{rock}}^{ - 3} \;{\text{s}}^{ - 1}$$ are a few orders of magnitude greater than the strain rate of the mantle serpentinites, which may be fast enough to result in the brittle fracture of rocks. Moreover, the rate of fluid production will be enhanced when brucite occurs in the non-/low H2O environments of the subduction zone.

Effects of Er(NO3)3, Nd(NO3)3 and Y(NO3)3 on kinetics of dehydroxylation of kaolinite

Comprehensive thermal analysis technologies of thermogravimetry (TG) and differential thermal analysis (DTA) were applied in this paper to explore effects of Er(NO3)3, Nd(NO3)3 and Y(NO3)3 on kinetics of dehydroxylation of kaolinite, and pertinent kinetic parameters in dehydroxylation of kaolinite were calculated through the Coats–Redfern integral method and Achar differential method, leading to the control mechanism and pertinent kinetic parameters, i.e. apparent activation energy (EA) and pre-exponential (frequency) factor(A), etc. in dehydroxylation of kaolinite, and the influences of respective incorporation of three kinds of rare earth nitrates on kinetic parameters in dehydroxylation processes of kaolinite were analyzed. The activation energy was validated by Ozawa method. The obtained results showed that the course of dehydroxylation of kaolinite was controlled by the rate of the third-order chemical reaction (F3) under applied condition. The median values of EA and lnA corresponded to 307.94KJ·mol−1 and 47.8980, respectively. The control mechanism in dehydroxylation of kaolinite was transformed to the rate of the second-order chemical reaction (F2) after incorporation of the three kinds of rare earth nitrates, respectively. The values of EA and lnA for the dehydroxylation of kaolinite mixed with Er(NO3)3, Nd(NO3)3, and Y(NO3)3, respectively were: 192.24KJ·mol−1 and 29.2753, 198.50KJ·mol−1 and 29.8658, and 195.03KJ·mol−1 and 29.4592, which means incorporations of three kinds of rare earth nitrates significantly decreased both EA and A for the dehydroxylation of kaolinite. The activation energy obtained by Ozawa method corresponded to the average value obtained by the Coats–Redfern integral method and Achar differential method. Participation of the –OH connects to –Al in the coordination of rare earth ion leads to decrease of activation energy.

The influence of thermal treatment on properties of kaolin

The kinetics of dehydroxylation of South African kaolin revealed that both the inner and the surface hydroxyl groups disappear according to first order kinetics, however, the surface group disappeared faster than the inner groups, showing that diffusion control kinetics is also important. The temperature dependent transformations of the kaolin was measure by means of fractional conversion of the ratios between AlO6:AlO4 and Si-O-Al:Si-O-Si, which showed kobs values 0.0168 s-1 and 0.0089 s-1 for the transformation to the spinel phase and values of 0.0165 s-1 and 0.0156 s-1 for the transformation to mullite respectively. The pozzolanic activities of the kaolin calcined at different temperatures showed a maximum pozzolanic activity when the kaolin is calcined at 650?C and the pozzolanic activity for mullite is even less than for the uncalcined kaolin. XPS revealed that the atomic ratio between Si and Al did not change from kaolin to metakaolin (Si:Al = ca. 1.2) however the mullite showed a atomic ratio of Si:Al = 1.52, implying that some deallumination occurred during calcination at high temperatures.

Thermal Analysis on Dehydroxylation of Sol-Gel Derived Zinc Doped Calcium Phosphate Powders

The dehydroxylation of Zn free and 4 mol% Zn CaP powder was investigated using thermogravimetric analysis over the range of room temperature to 1200 °C. The kinetic result of dehydroxylation of Zn free and 4 mol% Zn CaP was calculated by means of the Ozawa–Flynn–Wall method. The XRD result indicated that the amount of Zn incorporated in HA lattice influences the phase stability of HA as it decreases with an increase in Zn concentration. According to calculated activation energy and conversion degree, the kinetics of HA dehydroxylation was identified, which included four successive conversion stages kinetically controlled by different rate-controlling processes. The dehydroxylation analysis of TG/DTG data show that Zn incorporation in HA lattice influences the phase stability of HA.

Thermal behavior study of pristine and modified halloysite nanotubes

Pristine halloysite nanotubes (HNTs) were studied by thermogravimetry (TG) up to 800 °C. Etching of alumina from inside the tube (causing a significant increase in tube lumen) was realized by treating the material with an acidic H2SO4 solution at 50 °C. Both materials were characterized by TG-FTIR techniques and their thermal behaviors were compared with that of kaolinite. The coupling of TG with FTIR enables to detect the gases evolved during the TG experiments, thus confirming that only pristine HNTs undergo dehydration with the loss of interlayer water molecules at around 245 °C, while dehydroxylation occurs in all these materials in close temperature ranges around 500 °C. TG runs at five different heating rates (2, 5, 10, 15 and 20 °C min−1), was carried out in the same experimental conditions used for the thermal analysis study with the aim to investigate dehydration and dehydroxylation kinetics using some isoconversional methods recommended by the ICTAC kinetic committee, and thermogravimetric data under a modulated rising temperature program. Finally, the results of the kinetic analysis were discussed and explained in terms of the strengths of the hydrogen bonds broken during these processes.

Surface Thermodynamics and Kinetics of MgO(100) Terrace Site Hydroxylation

Surface thermodynamic and kinetic models are applied to MgO hydroxylation data obtained under adsorption–desorption water vapor conditions using ambient pressure X-ray photoelectron spectroscopy. Experimental conditions range from 0.67–67 Pa and 270–600 K (10–5 to 20% relative humidity). The resulting standard state enthalpy (ΔH°) and entropy (ΔS°) for terrace site hydroxylation are −40 ± 1 kJ mol–1 and −50 ± 3 J mol–1 K–1, respectively, and independent of OH coverage. At 298 K, this gives a Gibbs free energy of ΔG° = −25 kJ mol–1. The bulk reaction of water vapor with MgO to form Mg(OH)2 (brucite) is ΔG° = −36 kJ mol–1, suggesting that the hydroxylation of the top layer of MgO is a metastable state. Hydroxylation (adsorption) follows zero order kinetics with respect to surface site concentrations, suggesting water is a precursor prior to dissociative adsorption. Dehydroxylation (desorption) kinetics are first order, with a frequency factor of ν = 1.4 (±1.2) × 1011 s–1, suggesting that recombinative desor...

Non-Isothermal Kinetic Evaluation of Pyrophyllite Dehydroxylation

Dehydroxylation of clay minerals is an important process with several applications. Despite the industrial use of several well defined clay minerals, only the dehydroxylation kinetics of kaolinite has been reported. Still, the kinetics of these solid state reactions is yet to be understood using unambiguous model. In the present work, we have performed detailed kinetic analysis of dehydroxylation of pyrophyllite clay using non-isothermal kinetics by thermogravimetry (TG) at different heating rates. The TG data has been analysed initially with model-free methods (ASTM E 698, ASTM 1641, Friedman analysis and Ozawa-Flynn-Wall method). Interestingly, both the model-free methods of analysis have unequivocally indicated that activation energy (Eact) changes very marginally with progress of reaction and a single activation energy barrier is present. This surmises that the dehydroxylation process is a single stage process. A multivariate non-linear regression on the dynamic thermal data at different heating rates has been performed to determine the most probable kinetic model based on statistical fit. It has been found that, TG data provides best fit with regression coefficient (r2) value of 0.994, when a single stage Avrami type diffusion reaction model is assumed, and an activation energy of 159 kJ.mol−1.

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

The dehydroxylation reactions of chrysotile Mg3Si2O5(OH)4 and brucite Mg(OH)2 were studied under inert nitrogen atmosphere using isothermal and non-isothermal approaches. The brucite decomposition was additionally studied under CO2 in order to check the influence of a competing dehydroxylation/carbonation/decarbonisation reaction on the reaction kinetics. Isothermal experiments were conducted using in situ high-temperature X-ray powder diffraction, whereas non-isothermal experiments were performed by thermogravimetric analyses. All data were treated by model-free, isoconversional approaches (‘time to a given fraction’ and Friedman method) to avoid the influence of kinetic misinterpretation caused by model-fitting techniques. All examined reactions are characterised by a dynamic, non-constant reaction-progress-resolved (‘α’-resolved) course of the apparent activation energy E a and indicate, therefore, multi-step reaction scenarios in case of the three studied reactions. The dehydroxylation kinetics of chrysotile can be subdivided into three different stages characterised by a steadily increasing E a (α ≤ 15 %, 240–300 kJ/mol), before coming down and forming a plateau (15 % ≤ α ≤ 60 %, 300–260 kJ/mol). The reaction ends with an increasing E a (α ≥ 60 %, 260–290 kJ/mol). The dehydroxylation of brucite under nitrogen shows a less dynamic, but generally decreasing trend in E a versus α (160–110 kJ/mol). In contrast to that, the decomposition of brucite under CO2 delivers a dynamic course with a much higher apparent E a characterised by an initial stage of around 290 kJ/mol. Afterwards, the apparent E a comes down to around 250 kJ/mol at α ~ 65 % before rising up to around 400 kJ/mol. The delivered kinetic data have been investigated by the z(α) master plot and generalised time master plot methods in order to discriminate the reaction mechanism. Resulting data verify the multi-step reaction scenarios (reactions governed by more than one rate-determining step) already visible in E a versus α plots.

Dehydroxylation kinetics of gadolinium hydroxide

The decomposition process of gadolinium hydroxide was studied by means of thermogravimetry in a temperature range from 300 to 900 K. The kinetics of low-temperature dehydroxylation (≈430–600 K) was studied under non-isothermal conditions. A model-free method was used to calculate the activation energy; a nonlinear regression method was applied to calculate the kinetic parameters of multi-stage decomposition reactions. The features of the dehydroxylation kinetics of the multi-stage process can be explained by the formation of GdOOH phase.

The influence of structure order on the kinetics of dehydroxylation of kaolinite

Abstract The structural order of kaolinite is an important factor that shows a substantial effect on the processes which take place during the thermal treatment of kaolin. The influence of structural order on the dehydroxylation process was investigated by simultaneous thermogravimetry and differential thermal analysis (TG-DTA). The thermal analysis was performed on the samples with gradually decreasing structural order prepared by milling procedure. The apparent activation energy of dehydroxylation process decreases with decreasing structural order according to the exponential function. The extrapolation of experimental data leads to the estimation of apparent activation energy of 76.6 kJ mol −1 and of frequency factor of 0.12 × 10 4 s −1 related to completely disordered form of kaolinite, while the ordered form shows the apparent activation energy of 216.17 kJ mol −1 and the frequency factor of 9.26 × 10 4 s −1 . The relationships between features such as the infrared pattern of treated material, the degree of structural order and the apparent activation energy were established.