Thermal decomposition of aluminium hydroxides

The thermal decompositions of the crystalline aluminium hydroxides hydrargillite, bayerite and nordstrandite were investigated by thermogravimetry, differential thermal analysis, X-ray diffraction and infrared spectrophotometry. It was found that these aluminium hydroxides undergo thermal decomposition in the following sequences: hydrargillite-1

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Zusammenfassung Die thermische Zersetzung von kristallinen Aluminiumhydroxiden, wie Hydrargillit, Bayerit und Nordstrandit, wurde thermogravimetrisch, differentialthermoanalytisch, röntgendiffraktometrisch und IR-spektrophotometrisch untersucht. Es wurde festgestellt, da\ diese Aluminiumhydroxide in der folgenden Reihenfolge zersetzt werden: Hydrargillit-I

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Thermal decomposition of silylated layered double hydroxides

Anionic surfactant and silane modified layered double hydroxides (LDHs) were synthesized through an in situ coprecipitation method. The structure and morphology were characterized by XRD and TEM techniques, and their thermal decomposition processes were investigated using infrared emission spectroscopy (IES) combined with thermogravimetry (TG). The surfactant modified LDHs (H-DS) shows three diffractions located at 1–7° (2θ), while there is only one broad reflection for silane grafted LDHs (H–Si) in this region. The morphologies of the H-DS and H–Si show fibrous exfoliated layers and curved sheets, respectively. The IES spectra and TG curves indicate that alkyl chain combustion and dehydroxylation are overlapped with each other during heating from 373 to 723 K in H-DS and to 873 K in H–Si. Sulfate anion transformation process occurs at 473 K in H-DS and 523 K in H–Si. The derivant of sulfate can exist even above 1073 K. After further decomposition, the metal oxides and the new type of Si–O compounds are formed beginning at around 923 K in silane modified sample.     

Thermal decomposition of 8-hydroxyquinoline complexes with aluminium,cobalt, manganese and nickel

The conditions of thermal decomposition of aluminium(III), cobalt(II), manganese(II) and nickel(II) 8-hydroxyquinoline complexes have been studied by TG-DSC analysis. The thermal decomposition of these complexes has two stages: dehydration and loss of 8-hydroxyquinoline. The final solid product is an oxide.     

Thermal decomposition of nickel azide

The results on the thermal decomposition of anhydrous nickel azide prepared by reacting nickel azide solution with AnalaR Me₂CO are reported in the temperature range 490–525 K. The sample starts to decompose immediately after it is raised to the decomposition temperature and the rate of decomposition continuously decreases. The decomposition kinetics have been explained in terms of exponential decay law for α?0.19 and contracting volume law for α > 0.19. The domain of exponential decay law corresponds to the initial decomposition pertaining to surface nucleation and two dimensional surface growth of the product phase, while contracting volume law explains the growth of the product phase into the bulk. The role of defects has been explained by electrical conduction studies on the sample.     

Thermal decomposition of mineral zircon by alkali metal hydroxides

Thermal decomposition of zircon mineral was studied in the presence of sodium and potassium hydroxides and in the presence of a waste mixture of hydroxides containing NaOH and KOH in a weight ratio of 23. The process was controlled in such a way that the decomposition products might contain the minimum possible amount of alkali and could thus be applied for the syntheses of zircon pigments without any further exacting treatment.

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Zusammenfassung Es wurde die thermische Zersetzung eines Zirkonminerals in Gegenwart von Natrium- und Kaliumhydroxid-sowie in Gegenwart einer Altlauge mit einem Gehalt an NaOH und KOH im Verhältnis zwei zu drei untersucht. Der Prozeß wird derart gesteuert, daß die Zersetzungsprodukte einen möglichst geringen Anteil an Lauge besitzen und so ohne weitere arbeitsaufwändige Behandlungen zur Synthese von Zirkonpigmenten verwendet werden können.

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Thermal decomposition of mixed oxalates

The thermogravimetric analysis of several coprecipitated zinc-copper oxalates in nitrogen is reported. The thermal decompositions of these mixed oxalates show a separate single step for dehydration and decomposition in the mass loss versus temperature curve. It is found that the onset temperature decreases with composition. For example, it decreases from 380 for zinc oxalate to 260 for copper oxalate. The end-product is mixed oxides and copper metal. These studies indicate the formation of an interpenetrating structure or mixed crystals during coprecipitation. Rate parameters have been calculated for dehydration and decomposition.One of us (B. D. D.) is grateful to the University Grants Commission, New Delhi, India, for the award of a research fellowship.     

A study on the thermal decomposition of iron-cobalt mixed hydroxides

Thermal decomposition of pure Fe(OH)₃ and mixed with Co(OH)₂ were studied using TG, DTA, kinetics of isothermal decomposition and electrical conductivity measurements. The thermal products were characterized by X-ray diffraction and IR spectroscopy. The TG and DTA analysis revealed the presence of Co²⁺ retards the decomposition of ferric hydroxide and the formation of -Fe₂O₃. The kinetics of decomposition showed that the mixed samples need higher energy to achieve thermolysis. The investigation of thermal products of mixed samples indicated the formation of cobalt ferrite on addition ofx=1 or 1.5 cobalt hydroxide. The electrical conductivity accompanying the thermal decomposition decreases in presence of low ratio of Co²⁺ (x=0.2) via the consumption of holes created during thermal analysis. The continuous increase in values on increasing of Co²⁺ concentration corresponded to the electron hopping between Fe²⁺ and Co³⁺.

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Zusammenfassung Mittels TG, DTA und der Kinetik von Messungen der isothermen Zersetzung und der elektrischen Leitfähigkeit wurde die Zersetzung von Fe(OH)₃ in reinem Zustand und vermengt mit Co(OH)₂ untersucht. Die thermischen Produkte wurden mittels Röntgendiffraktion und IR-Spektroskopie charakterisiert. TG und DTA zeigen, daß die Zersetzung von Eisen(III)-hydroxid und die Bildung von -Fe₂O₃ durch Gegenwart von Co²⁺ verzögert wird. Die Zersetzungskinetik zeigt, daß die Mischproben mehr Energie für die Thermolyse benötigen. Die Untersuchung der thermischen Produkte zeigt die Bildung von Cobaltferrit bei Zusatz vonx=1 oder 1,5 Cobalthydroxid. Die elektrische Leitfähigkeit nimmt bei der thermischen Zersetzung in Gegenwart von niedrigen Co²⁺-Konzentrationen (x=0.2) durch Verbrauch der bei der Thermoanalyse geschaffenen Löcher ab. Das monotone Ansteigen der -Werte bei steigender Co²⁺-Konzentration stimmt mit dem Überspringen von Elektronen zwischen Fe²⁺ und Co³⁺ überein.

     

Thermal decomposition of sodium aluminium sulphate crystallohydrate

Dynamic TG investigations were carried out to elucidate the mechanism of thermal decomposition of aluminium sodium sulphate crystallohydrate. Shimadzu 31H and MOM Derivatograph X-ray diffraction and other techniques were used to determine data on the decomposition, activation energy, structure and phases in the solid products. Isothermal study in the temperature ranges 883–958 K and 983–1113 K in air or a reducing gas atmosphere revealed different reaction mechanisms. Depending on the experimental conditions, mainly Al₂O₃ can be obtained.     

Thermal decomposition kinetics of nickel fluorenone semicarbazone

Thermal decomposition of nickel fluorenone-semicarbazone has been studied by TG. Thermoanalytical data (TG and DTG) of the chelate are presented. Mass loss considerations at the main decomposition stages indicate conversion of the complex to oxide. Mathematical analysis of TG data shows that first order kinetics is applicable for the first stage and second order to the second stage. Kinetic parameters (energy and entropy of activation and preexponential factor) are reported.

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A product of the thermal decomposition of mixed hydroxides of calcium and titanium

Re-examination of the X-ray powder diffraction data obtained from a compound in the CaOTiO₂ system, reported to be Ca₂Ti₅O₁₂, suggests that the compound is a hitherto unknown defect pyrochlore of composition Ca₂Ti₂O₆. The lattice parameter is 9.953 ± 0.002 Å.     

Thermal decomposition of nickel and zinc peroxotitanates to metatitanates

Methods of DTA, TG, X-ray phase analysis and IR spectroscopy were used to study the thermal dehydration and decomposition of Ni²⁺ and Zn²⁺ peroxotitanates to the corresponding metatitanates. The course of the process was established and the intermediate phases were identified. The information obtained was utilized to determine the optimum temperatures of heating the initial peroxotitanates for conversion to metatitanates with a fairly high degree of crystallinity (for ZnTiO₃ the optimum temperature is 600°C, while for NiTiO₃ it is 550°C).     

Thermal decomposition of cerium oxalate and mixed cerium-gadolinium oxalates

Cerium oxalate and mixed cerium-gadolinium oxalates containing 20 and 50 mol% gadolinium were subjected to thermal decomposition. Thermal analysis showed that cerous oxalate is transformed to cerium oxide in two steps. The first step involves the endothermic removal of 10 mol of water, with a calculated activation energy of 78.2 kJ/mol. The second step involves the exothermic decomposition of the anhydrous oxalate, with an activation energy of 112.6 kJ/mol. The water content in the mixed cerium-gadolinium oxalates decreases with increasing gadolinium content, while the temperature of exothermic decomposition of the anhydrous oxalate increases with it.

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Zusammenfassung Zeroxalat und Zer-Gadolinium-Mischoxalat mit 20 bzw. 50 mol% Gadolinium wurden einer thermischen Zersetzung unterzogen. Die Thermoanalyse zeigte, da\ Zeroxalat in zwei Schritten in Zeroxid überführt wird. Der erste Schritt mit der Aktivierungsenergie von 78.2 kJ/mol besteht in der endothermen Abgabe von 10 mol Wasser. Der zweite Schritt mit der Aktivierungsenergie 112.6 kJ/mol umfa\t die exotherme Zersetzung des wasserfreien Oxalates. Der Wassergehalt der Zer-Gadolinium-Mischoxalate nimmt mit steigendem Gadoliniumgehalt ab und die Temperatur für den exothermen Zersetzungsvorgang der wasserfreien Oxalate nimmt mit steigendem Gadoliniumgehalt zu.

     

Thermal, solution and reductive decomposition of Cu-Al layered double hydroxides into oxide products

Cu-Al layered double hydroxides (LDHs) with [Cu]/[Al] ratio 2 adopt a structure with monoclinic symmetry while that with the ratio 0.25 adopt a structure with orthorhombic symmetry. The poor thermodynamic stability of the Cu-Al LDHs is due in part to the low enthalpies of formation of Cu(OH)₂ and CuCO₃ and in part to the higher solubility of the LDH. Consequently, the Cu-Al LDH can be decomposed thermally (150 °C), hydrothermally (150 °C) and reductively (ascorbic acid, ambient temperature) to yield a variety of oxide products. Thermal decomposition at low (400 °C) temperature yields an X-ray amorphous residue, which reconstructs back to the LDH on soaking in water or standing in the ambient. Solution decomposition under hydrothermal conditions yields tenorite at 150 °C itself. Reductive decomposition yields a composite of Cu₂O and Al(OH)₃, which on alkali-leaching of the latter, leads to the formation of fine particles of Cu₂O (<1 μm).     

Thermal decomposition of basic aluminium potassium sulphate. Part I. Stages of decomposition

The stages of thermal decomposition of basic aluminium potassium sulphate were studied with the use of differential thermal analysis (DTA), thermogravimetry (TG), mass spectrometry (MS), IR spectroscopy (IR), and X-ray diffraction phase analysis (XRD). The individual stages of the decomposition were determined and both solid and gaseous products of the dissociation were identified. On the basis of spectrometric data and thermodynamic calculations the possibility of the occurrence of the SO in the gaseous reaction products is discussed.     

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 dehydroxylation of aluminium hydroxides and the kinetics ofα-Al2O3 formation

Studies were made of the dehydroxylation of several aluminium hydroxide modifications and the kinetics of-Al₂O₃ formation. The investigated samples differed in both mineral composition and the level of alkali admixtures. It was found that the rate of formation and the quantity of-Al₂O₃ depend mainly on the purity of the aluminium hydroxides, while the transition forms of alumina depend on the initial type of the aluminium hydroxide.

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Zusammenfassung Es wurde die Dehydroxylierung verschiedener Aluminiumhydroxidmodifizierungen sowie die kinetische Untersuchung der Bildung von-Al₂O₃ durchgeführt. Die untersuchten Proben unterschieden sich sowohl in ihrer Mineralienzusammensetzung als auch im Grad der Verschmutzung mit Alkalien. Man stellte fest, daß sowohl Geschwindigkeit als auch Menge von-Al₂O₃ hauptsächlich von der Reinheit des Aluminiumhydroxides abhängen und daß die Übergangsformen von Aluminiumoxid von ihrem ursprünglichen Typ von Aluminiumhydroxid abhängen.

     

Thermal decomposition of copper(II) and zinc carbonate hydroxides by means of TG-MS

Summary For the quantitative analyses of evolved CO₂and H₂O during the thermal decomposition of solids, calibration curves, i.e. the amounts of evolved gases vs. the corresponding peak areas of mass chromatograms measured by TG-MS, were plotted as referenced by the reaction stoichiometry of the thermal decomposition of sodium hydrogencarbonate NaHCO₃. The accuracy and reliability of the quantitative analyses of the evolved CO₂and H₂O based on the calibration curves were evaluated by applying the calibration curves to the mass chromatograms for the thermal decompositions of copper(II) and zinc carbonate hydroxides. It was indicated from the observed ratio of evolved CO₂and H₂O that the compositions of copper(II) and zinc carbonate hydroxides examined in this study correspond to mineral malachite, Cu₂CO₃(OH)₂, and hydrozincate, Zn₅(CO₃)₂(OH)₆, respectively. Reliability of the present analytical procedure was confirmed by the fairly good agreement of the mass fraction of the evolved gases calculated from the analytical values with the total mass-loss during the thermal decompositions measured by TG.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

An attempt to obtain aluminium hydroxide that could give aluminium oxides of increased thermal stability was made. Aluminium hydroxide was precipitated during a hydrolysis of aluminium chloride in ammonia medium. The influence of preparative conditions, such as a dosing rate of aluminium precursor, pH, duration of the precipitate refluxing and temperature of calcination, on the properties of obtained hydroxides and oxides was investigated. The materials were studied with the following methods: thermal analysis, IR spectroscopy, low-temperature nitrogen adsorption and adsorption–desorption of benzene vapours. Precipitated boehmites had high values of S _BET determined from nitrogen adsorption (220–300 m²g^–1), good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. It has been proved that a high pH value during the precipitation of aluminium hydroxide favoured better crystallisation of boehmite structure, higher temperature of its dehydroxylation into γ-Al₂O₃, and delayed transformation of γ phase into α-Al₂O₃. Aluminium oxides derived from the hydroxides precipitated at a high pH were the most stable at high temperatures, and were characterised with the best surface properties. The online version of the original article can be found at     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The process of hydrolysis of aqueous aluminium sulfate was carried on in ammonia medium at 100°C and for different time intervals (0, 20, 39 or 59 h). The products thus obtained were calcined at 550, 900 or 1200°C for 2 h with the aim to obtain aluminium oxides. The materials were studied with the following methods: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption–desorption of benzene vapours and scanning electron microscopy. Freshly precipitated material was an amorphous basic aluminium sulfate which after prolonged refluxing at elevated temperature in a mother liquor underwent a phase transformation into highly crystalline NH₄Al₁₃(SO₄)₂(OH)₆ containing tridecameric unit Al₁₃. It was accompanied by a decrease of specific surface area and the formation of a porous structure less accessible for benzene molecules. Regardless of the duration of the hydrolysis process, all products were characterised with poorly developed porous structure and hydrophilic character. Their calcination at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ/η- and δ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C had higher values of specific surface area than starting materials due to processes of dehydroxylation and desulfurization. The process of calcination up to 900°C was reflected in developing of not only porous structure but also hydrophobic character of prepared materials. The S _BET values calculated for the oxide samples obtained from aged products of hydrolysis at 1200°C were lower than for the analogous sample prepared without the ageing step. It was concluded that prolonged refluxing at elevated temperature of the products of hydrolysis of aluminium sulfate decreased thermal stability of final aluminium oxides.     

Influence of aluminium precursor on physico-chemical properties of aluminium hydroxides and oxides

The paper concerns aluminium hydroxides precipitated during hydrolysis of aluminium acetate in ammonia medium, as well as aluminium oxides obtained through their calcination at 550, 900 or 1200°C for 2 h. The following techniques were used for analysing of obtained materials: thermal analysis, IR spectroscopy, X-ray diffraction, low-temperature nitrogen adsorption, adsorption-desorption of benzene vapours and scanning electron microscopy. Freshly precipitated boehmite/pseudoboehmite had high value of S _BET, very good sorption capacity for benzene vapours, developed mesoporous structure and hydrophilic character. After prolonged refluxing at elevated temperature its crystallinity increased which was accompanied by a decrease of specific surface determined from nitrogen adsorption, decrease of sorption capacity for benzene vapours and weakening of the hydrophilic character. Calcination of all hydroxides at the temperature up to 1200°C resulted in the formation of α-Al₂O₃ via transition forms of γ-, δ-and θ-Al₂O₃. The samples of aluminium oxides obtained after calcination at 550 and 900°C were characterised with high values of specific surface area and displayed quite high heat resistance, probably due to a specific morphology of starting hydroxides. The process of ageing at elevated temperature developed thermal stability of aluminium oxides.