The thermal decomposition of ammonium metavanadate

On heating, ammonium metavanadate (AMV) decomposes in several atmospheredependent stages. An important decomposition intermediate, ammonium hexavanadate (AHV), may also be prepared by wet-chemical methods and the kinetic parameters for the thermal decomposition of AMV and of the AHV preparation have been obtained. The kinetic study has been supplemented by surface-area measurements and by electron microscopic examination of the surfaces of reactant, intermediate and product crystallites. On the basis of the type of decomposition curve, the measured activation energies, and the effects of oxygen and water vapour on the decomposition rate, it has been concluded that in vacuum and in inert atmospheres the evolution of ammonia is the rate-determining step, while in oxidizing atmospheres evolution of water is rate determining. Comparison of the kinetic parameters with thermodynamic data for the decomposi. tion has led to suggestions as to the nature of the activated complexes involved.     

The thermal decomposition of ammonium metavanadate,III

The kinetics and thermodynamics of the thermal decomposition of ammonium metavanadate (AMV) are combined with the structural information available for AMV, for the important decomposition intermediate, ammonium hexavanadate (AHV), and for vanadium pentoxide, the product of the decomposition in non-reducing atmospheres, to enable the atomic movements involved in the course of decomposition to be discussed in detail.The decomposition of AMV involves scission of the V-G chains in the AMV structure (accompanied by simultaneous evolution of gaseous ammonia and water) with subsequent rearrangement and cross-linking of discrete units, based on V₃O ₈ ^– , to form AHV. Further decomposition to V₂O₅ is a similar but less ordered process.

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Zusammenfassung Die Kinetik und thermische Zersetzung von Ammoniummetavanadat (AMV) wurden mit der zu erhaltenden Information über die Strukturen des AMV, des wichtigen Zwischenproduktes der Zersetzung, des Ammoniumhexavanadats (AHV) sowie des Vanadiumpentoxids verbunden. Das in nichtreduzierenden Atmosphären erhaltene Zersetzungsprodukt wurde eingehend erörtert um die in der Zersetzung mit inbegriffenen Atombewegungen zu studieren.Bei der Zersetzung von AMV spielt eine Spaltung der V- O-Ketten in der Struktur des AMV mit (welche durch eine gleichzeitige Entwicklung von gasförmigen Ammoniak und Wasser begleitet wird), dieser folgt eine Neuordnung und Raumvernetzung diskreter Einheiten auf V₃O ₈ ^– -Basis um zur Bildung von AHV zu führen. Die weitere Zersetzung zu V₂O₆ ist ein ähnlicher, obwohl weniger geordneter Vorgang.

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Résumé On a rapproché les données structurales du métavanadate d'ammonium (MVA) avec celles relatives à la cinétique et à la thermodynamique de la décomposition thermique du MVA, pour étudier le produit intermédiaire important de la décomposition, l'hexavanadate d'ammonium (HVA) et le pentoxyde de vanadium. On discute en détail le produit de la décomposition obtenu en atmosphère non réductrice afin d'obtenir des données sur les mouvements atomiques mis en jeu pendant la décomposition.La décomposition du MVA s'effectue avec scission des chaines V—O dans la structure du MVA, accompagnée d'un dégagement de gaz ammoniac et d'eau et suivie d'un réarrangement et d'une réticulation d'unités discrètes V₃O ₈ ^– , pour former HVA. La décomposition ultérieure en V₂O₅ suit un processus similaire mais moins bien ordonné.

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() , , ( ) . , . - ( ) , V₃O ₈ ^– . V₂O₅ , .

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Financial support from the National Institute for Metallurgy, Johannesburg, and the South African Council for Scientific and Industrial Research is gratefully acknowledged.     

The thermal decomposition of ammonium metavanadate,I

The stoichiometry of the various stages involved in the thermal decomposition of ammonium metavanadate has been shown to correspond to a stepwise decrease in the ratio of ammonia and water to V₂O₅, with V₂O₅ being the final product in vacuum, in air and in argon. In ammonia, VO₂ is formed. The actual stages and intermediates are dependent upon the prevailing atmosphere. Chemical analyses, together with infrared absorption spectra and X-ray powder data, have enabled the intermediates and products to be characterized and the structural changes involved in the decomposition to be discussed.

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Zusammenfassung Es wurde gezeigt, daß die verschiedenen Stufen in der thermischen Zersetzung von Ammoniummetavanadat dem stufenweisen stöchiometrischen Verlust von Ammonia und Wasser entsprechen. In Vakuum, Sauerstoff und Argon ist V₂O₅, in Ammoniak VO₂ das Endprodukt. Die Zwischenprodukte der einzelnen Stufen sind von der umgebenden Gasatmosphäre abhängig. Durch chemische, infrarot- und röntgenspektroskopische Analyse gelang es, diese zu charakterisieren und so die durch die Zersetzung hervorgerufenen strukturellen Umlagerungen zu deuten.

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Résumé On montre que les différentes étapes de la décomposition thermique du métavanadate d'ammonium correspondent à la diminution progressive de l'eau et de l'ammoniac par rapport à V₂O₅; cet oxyde constitue le produit final dans le vide, dans l'air et dans l'argon. Dans l'ammoniac, c'est VO₂ qui se forme. Les étapes respectives et les intermédiaires dépendent de l'atmosphère qui prévaut. A l'aide de l'analyse chimique, des spectres d'absorption infrarouge et des données de rayons X sur poudre, on a pu caractériser les intermédiaires et les produits formés, ainsi que les changements structuraux provoqués par la décomposition.

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, V₂O₅. , V₂O₅. . , , .

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We wish to acknowledge helpful comments from Prof. L. Glasser and Dr. N. H. Agnew and financial support from the National Institute for Metallurgy, Johannesburg, South Africa.     

Thermal decomposition of ammonium metavanadate

Ammonium metavanadate is subjected to thermal treatment in the range 180–550°C and structural and surface area changes are studied by X-ray diffraction and N₂ adsorption. DTA and TGA show the existence of a large endotherm (260°C) possibly composed of several stages, and a much smaller one (340°C) which is responsible for evolution of the molecule of water associated with the V₂O₅ initially formed. Three exotherms also appear and explanations for their presence are given. The phases formed, as well as the specific surface areas, are determined for the products obtained in a vacuum and in the presence of water vapour, and changes in surface areas are related to the phase transformations and dehydration of the products formed during thermal treatment.     

Activation energy of the thermal decomposition of ammonium metavanadate. A thermogravimetric study

The thermogravimetric curves of the title compound (AMV), recorded at two rates of heating (10 and 1.25 deg min^–1), exhibited two separate steps of weight loss (at 423–513 and 513–613 K). The TG curves were analysed by testing 18 solid-state reaction equations associated with the methods of Coats and Redfern (1964), atava and kvára (1969) and Blazejowski et al. (1983). On the basis of linearization procedures, high-order reactions with n=1.5 and 2.0 best described the two TG steps. The activation energies of the two conversion stages are: first stage: 163.6 and 152.6 kJ mol^–1, and second stage: 238.1 and 167.7 kJ mol^–1 (low and high heating rates, respectively).

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Zusammenfassung Die thermogravimetrische Untersuchung der Titelverbindung (AMV) ergab bei zwei Aufheizgeschwindigkeiten (10 und 1,25 deg·min^–1) zwei verschiedene Gewichtsverlustintervalle (423–513 und 513–613 K). Die Analyse der TG-Kurven wurden anhand der Gleichungen von 18 Festkörperreaktionen mittels der Coats-Redfern Methode (1964), der Methode von atava und kvára (1969) sowie der Methode von Blazejowski und Mitarb. (1983) durchgeführt. Auf Grund des Linearisierungsverfahrens erwiesen sich höhere Reaktionsordnungen mit n=1,5 und 2 am geeignetsten zur Beschreibung beider TG-Kurven. Die Aktivierungsenergie der beiden Konversionsschritte (für die kleinere und größere Aufheizgeschwindigkeit) beträgt: 1. Schritt: 163,6, 152,6 und 2. Schritt: 238,1 und 167,7 kJ·mol^–1.

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, (10 1,25 ·^–1), 423–513 513–613 . 18 , - , -, . , =1,5 2,0 . , , , 163,6 152,6 ·^–1, – 238,1 167,7 ·^–1.

     

The influence of dissociation reaction on ammonium nitrate thermal decomposition reaction

In order to investigate the influence of dissociation reaction on thermal decomposition of ammonium nitrate (AN), biochar was selected as an adsorbent to interfere with the dissociation of AN. The TG-DSC results showed that the notable exothermic reaction of AN with the presence of 2% or 7% biochar took place. The decomposition temperature of AN decreased with increasing amount of biochar. The notable knee point was found in the TG curves. The activation energy of AN with biochar in the initial stage was higher than that of AN itself. Remote sensing Fourier transform infrared experiments found biochar induced AN decomposition at about 190 °C, which was also confirmed by the TG-MS results. After dissociation reaction, HNO₃ (g) and NH₃ (g) were adsorbed and crystalline of AN was formed on the surface of biochar. With the increasing temperature, NH₃ escaped from the surface of biochar, while HNO₃ (g) was stayed in biochar. HNO₃ (g) catalyzed the thermal decomposition of AN and also reacted with biochar. The results indicated that dissociation reaction of AN played an important role during AN thermal decomposition process. When dissociation reaction was changed, the thermal decomposition reaction of AN would also change, catalysis or inhibition AN thermal decomposition. It is a useful reference to guide the AN additives selection and to understand the mechanism for the AN decomposition accident.

     

Influence of metal oxide additives on the kinetics of isothermal decomposition of ammonium metavanadate

The kinetics of isothermal decomposition of ammonium metavanadate (AMV) and its mixtures with MgO, CaO, CuO and NiO — in the molar ratio of 21 — were investigated. The computer — oriented kinetic analysis of the-t data reveals the validity of Ginstling-Brounshtein equation to describe these reactions. The accelerating effect accompanying the MgO addition can be attributed to its affinity to abstract protons. In case of CaO, the formation of the relatively stable Ca(OH)₂ during the proceeding of the decomposition process results in a retarding effect. On the other hand, the higher calculatedE _a, value for the AMV + CuO mixture was explained in terms of a solid-solid interaction between CuO and the solid decomposition product, leading to the formation of Cu₃(VO₄)₂. According to electronic factors, the effect of NiO addition on the decomposition process was discussed. It was found that the decomposition process is less favourable in presence ofp-type semiconducting additive.

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Zusammenfassung Es wurde die Kinetik der isothermen Zersetzung von Ammoniummetavanadat (AMV) und von Gemischen aus AMV mit MgO, CaO, CuO und NiO im Molverhältnis 21 untersucht. Eine computergestützte kinetische Analyse dert Daten erwies die Gültigkeit der Ginstling-Brounshtein-Gleichung zur Beschreibung dieser Reaktionen. Der beschleunigende Effekt bei Zugabe von MgO kann dessen Neigung zum Entzug von Protonen zugeschrieben werden. Im Falle von CaO führt die Bildung von relativ stabilem Ca(OH)₂ während des Zersetzungsvorganges eher zu einer Verlangsamung. Die hohen berechneten E_a2-Werte für das AMV + CuO-Gemisch können mit einer Feststoff-Feststoff-Wechselwirkung zwischen CuO und dem festen Zersetzungsprodukt erklärt werden, wobei Cu₃(VO₄)₂ gebildet wird. Der Einfluß von NiO-Zugabe auf den Zersetzungsprozeß wurde hinsichtlich elektronischer Faktoren diskutiert. Man fand, daß die Gegenwart vonp-Halbleiterzusätzen für den Zersetzungsvorgang weniger günstig ist.

     

Thermal decomposition of ammonium metavanadate doped with Fe,Co or Ni hydroxides

The thermal decomposition of pure ammonium metavanadate (AMV) and of AMV doped with Fe³⁺,Co²⁺ or Ni²⁺ ions was investigated by TG, DTA, IR and X-ray diffraction. The results obtained revealed that the presence of these dopants enhances the formation of the intermediate compounds V₂O₅ solid. Some of the V⁵⁺ ions of the V₂O₅ lattice seemed to be reduced to V⁴⁺ ions. The activation energies of the different decomposition stages for all samples were calculated. The doped solids calcined at 450°C were characterized by the determination ofSbet and by electrical conductivity measurements. The mechanisms by which these ions affect the properties of the solids produced are discussed in relation to the defect structure created by the doping process.

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Zusammenfassung Mittels TG, DTA, IR und Röntgendiffraktion wurde die thermische Zersetzung von reinem Ammoniummetavanadat (AMV) und von AMV, versetzt mit Fe³⁺, Co²⁺ oder Ni²⁺, untersucht. Die Ergebnisse zeigen, daß die Bildung von Zwischenprodukten und festem V₂O₅ durch die Gegenwart der Zusätze begünstigt wird. Einige der V⁵⁺-Ionen des V₂O₅ Gitters scheinen zu V⁴⁺-Ionen reduziert zu sein. Für alle Proben wurde die Aktivierungsenergie der verschiedenen Zersetzungsschritte berechnet. Die versetzten Feststoffe, die bei 450°C kalziniert wurden, konnten durch die Bestimmung vonSbet und durch elektrische Leitfähigkeitsmessungen charakterisiert werden. Der Mechanismus, über welchen diese Ionen die Eigenschaften der Feststoffe beeinflussen, wurde hinsichtlich der hervorgerufenen Defektstruktur diskutiert.

     

The thermal decomposition of ammonium heptamolybdate

The thermal decomposition of ammonium heptamolybdate has been investigated by TG. The decomposition is discussed, making use of additional information obtained from isothermal studies, X-ray and IR measurements. The formation of two new compounds, namely (NH₄)₂O · 14 MoO₃ and (NH₄)₂O · 22 MoO₃, prior to the formation of MoO₃ is detected, as well as the main intermediate compounds (NH₄)₂O · 2.5 MoO₃ or (NH₄)₂O · 3 MoO₃ (according to the water content of the starting material) and (NH₄)₂O · 4 MoO₃.     

Kinetics of the thermal decomposition of ammonium thiocarbamate

Results of the kinetic study of the thermal decomposition of ammonium thiocarbamate in vacuum (ca. 10^?2 torr) in the temperature range 303–353 K are presented. Under these conditions ammonium thiocarbamate decomposes into gaseous products (NH₄COSNH₂→2 NH₃ + COS). Fifteen general equations for the kinetics of thermal decomposition of solids were taken into consideration. The following equation describes the experimental results over the whole range of the degree of decomposition: $$\alpha = \frac{{2k_2 }}{{ab}}(t - t_0 )[a + b - 2k_2 (t - t_0 )]$$ whereα=degree of decomposition,t=time,t ₀=time required to attain constant growth rate of nuclei, anda andb are the dimensions of the crystal. The temperature-dependence of the rate constantk ₂ was determined and the activation energy was found to be 10.7 kcal/mole. An experiment planning scheme was adopted in this study: in a series of experiments, each one was followed by statistical analysis in order to plan successive experiments depending on the results of the previous one.     

硫酸铵的热分解

目前 ,关于各种无机酸铵盐的热分解情况都研究得比较清楚 ,但关于硫酸铵热分解情况的报道结果则不一致[1 - 7] 。本文对硫酸铵在各种温度下的常量热分解进行了研究 ,探讨其热分解机理 ,以利于对磷石膏 (主要成分为ＣａＳＯ4)的开发利用提供新的思路和方法[8] 。1　实验坩锅炉中热分解　约 1 0ｇ(ＮＨ4) 2 ＳＯ4置于恒重坩埚中 ,再放入坩埚炉中升温至设定温度 ,恒温0 5ｈ ,取出冷却 ,称重。并将坩埚中残余物用水溶解 ,测定水溶液中的Ｈ+、ＮＨ+4、ＳＯ2 -4 等离子的含量。其中ＮＨ+4含量采用甲醛法测定 ,ＳＯ2 -4 含量采用硫酸钡重量法测定…     

Kinetics of thermal decomposition of ammonium persulfate

Kinetics of thermal decomposition of ammonium persulfate was studied by calorimetry and Raman spectroscopy. The values of activation energies of the overall reaction and its individual stages were estimated.     

The thermal decomposition of doped ammonium perchlorate

The thermal decomposition of ammonium perchlorate doped with Cu²⁺, Mn²⁺, Co²⁺ and Fe³⁺ (with and without H⁺) was studied using DTA and TG. Small concentrations of these ions inhibit the decomposition by eliminating the first exothermic peak. At higher concentrations, a catalyzed decomposition is observed. Either the influence of the doping ions on the mechanism of the thermal decomposition is different from that of added oxides or a modification of the electron transfer mechanism is needed.     

Structure and thermal decomposition of ammonium metatungstate

The structure and morphology of ammonium metatungstate (AMT), (NH₄)₆[H₂W₁₂O₄₀]?4H₂O, and its thermal decomposition in air and nitrogen atmospheres were investigated by SEM, FTIR, XRD, and TG/DTA-MS. The cell parameters of the AMT sample were determined and refined with a full profile fit. The thermal decomposition of AMT involved several steps in inert atmosphere: (i) release of crystal water between 25 and 200 °C resulting in dehydrated AMT, (ii) formation of an amorphous phase between 200 and 380 °C, (iii) from which hexagonal WO₃ formed between 380 and 500 °C, and (iv) which then transformed into the more stable m-WO₃ between 500 and 600 °C. As a difference in air, the as-formed NH₃ ignited with an exothermic heat effect, and nitrous oxides formed as combustion products. The thermal behavior of AMT was similar to ammonium paratungstate (APT), (NH₄)₁₀[H₂W₁₂O₄₂]?4H₂O, the only main difference being the lack of dry NH₃ evolution between 170 and 240 °C in the case of AMT.     

Study of thermal decomposition of ammonium cerium sulphate

Thermal decomposition of ammonium cerium sulphate has been studied by differential scanning calorimetry and thermogravimetry. The results show that the material decomposes in five steps in the temperature region 364–1116 K in oxygen. Based on the thermal data, elemental analysis and magnetic susceptibility measurements, sequence of decomposition has been established. The final product has been identified as CeO₂ by X-ray diffractometry. Ammonium cerium sulphate and the products of the first and the final transitions contain cerium ion in 4+ oxidation state, while the three intermediate phases have cerium ion in 3+ oxidation state. From the non-isothermal DSC studies, kinetic parameters have been computed. The isothermal data show that the dehydration process follows Ginstling-Brounshtein mechanism, while the next three steps are governed by Mampel's unimolecular law of random nucleation.

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Zusammenfassung Mittels DSC und TG wurde die thermische Zersetzung von Ammoniumzersulfat untersucht. Die Ergebnisse zeigen in Sauerstoff einen fünfstufigen Zersetzungsprozeß im Temperaturbereich 364–1116 K. Ausgehend von den thermoanalytischen Angaben, der Elementaranalyse und den Messungen der magnetischen Suszeptibilität wurde eine Sequenz für die Zersetzung erstellt. Das Endprodukt wurde mittels Röntgendiffraktion als CeO₂ identifiziert. Ammoniumzersulfat und die Produkte des ersten und des letzten Überganges enthalten Zerionen mit der Oxydationsstufe +4, während in den drei Zwischenschritten Zerionen mit der Oxydationsstufe +3 vorkommen. Anhand der nichtisothermen DSC-Untersuchungen wurden die kinetischen Parameter berechnet. Die isothermen Angaben zeigen, daß der Dehydratationsprozeß einem Ginstling-Brounstein Mechanismus folgt, während die folgenden drei Schritte durch das Mampelsche unimolekulare Gesetz der Randomkeimbildung bestimmt werden.

     

CuO nanocrystals in thermal decomposition of ammonium perchlorate

CuO nanocrystals of different surface areas were prepared. All samples were characterized by X-ray diffraction, transition electron microscope, thermogravimetry, Brunauer-Emmett-Teller technique, Fourier transform infrared spectroscopy, and Raman spectroscopy. CuO nanocrystals showed a stable monoclinic structure. With increasing surface areas, the surface hydration became significant, which is followed by shifts in infrared frequencies and Raman phonon modes. CuO nanocrystals were explored as an additive to catalytic decomposition of ammonium perchlorate (AP). AP decomposition underwent a two-stage process. Addition of CuO nanocrystals led to a downshift of high-temperature stage towards lower temperatures.     

Thermal decomposition of ammonium metavanadate supported on Al2O3

The thermal decomposition of ammonium metavanadate supported on aluminium oxide was investigated using DTA, TG and X-ray diffraction techniques.The results obtained revealed that ammonium vanadate decomposed at 225–250°C giving an intermediate compound ((NH₄)₂V₆O₁₆) which decomposed readily at 335–360°C producing V₂O₅. Alumina was found to chance the formation of the intermediate compound and retard its decomposition. Some of the V⁵⁺ ions of V₂O₅ lattice seemed to be reduced into V⁴⁺ and V³⁺ ions by heating in air at 450°C in the presence of Al₂O₃. Such a reaction was attributed to dissolution of some Al³⁺ ions in the V₂O₅ lattice via location in interstitial positions and/or in cationic vacancies. Al₂O₃ was found to interact with V₂O₅ at 650° C giving well-crystalline A1VO₄ which decomposed at about 750°C forming well-crystalline δ-Al₂O₃ and V₂O₅,. Pure Al₂O₃, heated in air at 1000°C, existed in the form of the κ-phase which, on mixing with V₂O₅ (0.5 V₂O₅:1 Al₂O₃) and heating in air at 1000°C, was converted entirely to the well-crystalline α-Al₂O₃ phase.     

Radiation effects on thermal decomposition of ammonium perchlorate

Thermal decomposition of -irradiated (dose: 0–3.6 MGy) ammonium perchlorate was followed. The dynamic heating (range: 100–220 °C) and IR spectral measurements were carried out simultaneously. Temperature and dose brought a lowering in peak intensity of NH ₄ ⁺ and C10 ₄ ^– ions. Radiolytic products C10₃ and NH₃ are considered to initiate the decomposition process.