The thermal decomposition temperatures of ionic metal oxalates

Previous investigations linking the thermal decomposition properties of metal oxalates to the nature of the bonding have been successful only in establishing qualitative relationships. A quantitative relation is now revealed permitting prediction of the thermal decomposition temperaturesT _d (°C) of metal oxalates:T _d =516–1.4006r _c /r _i I wherer _c /r _i is the ratio of the Pauling covalent radius and the ionic radius of the metal atom in hexacoordination, andI _i is the sum of the ionization potentials of the metal atom in kJ mol^–1.

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Zusammenfassung Frühere Untersuchungen über Beziehungen der Eigenschaften thermischer Zersetzung von Metalloxalaten in Bezug auf Bindungseigenschaften, waren nur hinsichtlich der Feststellung qualitativer Zusammenhänge erfolgreich. Es wurde ein quantitativer Zuhammenhang gefunden, welcher die Voraussage der thermischen ZersetzungstemperaturenT _d (°C) der Metalloxalate gestattet:T _d =516–1.4006r _c /r _i , I _i wobeir _c /r _i das Verhältnis der Pauling'schen kovalenten Radiusen und des Ionenradius des Metallatoms in sechsfacher Koordination ist undI _i die Summe des Ionisierungspotentials des Metallatoms in kJ·mol^–1.

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Résumé Les études antérieures qui reliaient les caractéristiques de la décomposition thermique des oxalates métalliques à la nature de la liaison sont restées limitées à des relations qualitatives. Une relation quantitative qui permet de prédire les températures de décomposition thermiqueT _d (°C) des oxalates de métaux est présentée ici:T _d =516–1.4006r _c /r _i , I _i oùr _c /r _i est le rapport du rayon covalent de Pauling au rayon ionique de l'atome métallique hexacoordonné etI _i la somme des potentiels d'ionisation de l'atome de métal en kJ·mol^–1.

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, , . , T _d (°) :T _d =516–1.4006r _c /r _i ,I _i r _c /r _i — , aI _i — , · ^–1.

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Excerpt from a Dissertation by I. A. Kahwa, in fulfilment of the requirements for the degree of M. Sc. at the University of Dar es Salaam.

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The authors wish to thank the University Research and Publications Committee for grants which enabled carrying out of the preliminary studies, and the Staff Development Committee of the University of Dar es Salaam for financial support to I. A. Kahwa.     

Synthesis and thermal decomposition of mixed Gd-Nd oxalates

Several (Gd_1−xNd_x)₂[C₂O₄]₃·nH₂O samples (0≤x≤1) were prepared by a coprecipitation method: the precipitation is quantitative and all the samples are homogeneous in stoichiometry. XRD analyses have shown that a complete solid solution is formed over the whole range of compositions. The dried Gd rich oxalates have initially a low water content which gradually increases with the Nd content. All the oxalates decompose in O₂ around 700°C either into a single mixed oxide or in a mixture of oxides through several steps, which can be ascribed to the loss of water and CO₂.     

The importance of the CO 2 2− anion in the mechanism of thermal decomposition of oxalates

The effects of active (carbon dioxide and carbon monoxide) and neutral (helium, argon and nitrogen) atmospheres on the course of thermal decomposition of oxalates have been studied and compared. A mechanism of thermal decomposition has been proposed on the basis of the results obtained, the first stage of which consists in a heterolytic dissociation of the C-C bond, with the formation of carbon dioxide and the CO ₂ ^2– anion.

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Zusammenfassung Die Effekte von aktiven (Kohlendioxid und Kohlenmonoxid) und neutralen (Helium, Argon, Stickstoff) Atmosphären auf den Verlauf der thermischen Zersetzung von oxalaten wurden untersucht und miteinander verglichen. Aus den erhaltenen Ergebnissen wird ein Mechanismus der thermischen Zersetzung abgeleitet, dessen erster Schritt die heterolytische Dissoziation der C-C-Bindung unter Bildung von CO₂ und des CO ₂ ^2– -Anions ist.

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(- ) (, ) . , C-C CO ₂ ^2– .

     

The thermal decomposition of oxalates Part 14. Dehydration of magnesium oxalate dihydrate

The isothermal dehydration of magnesium oxalate dihydrate has been studied under various pressures of water vapour by use of a thermogravimetric balance. The rate of dehydration was found to be dependent upon the water vapour pressure.The reaction rate at temperatures below 124°C decreases sharply with an increase in water vapour pressure up to 0.5 mm Hg. With further increase in pressure an increase in rates is observed; this rises to a maximum and then falls again as the pressure is increased. The limitation of this phenomena to a limited temperature range is shown in the case of magnesium oxalate dihydrate. Above 124°C the initial fall in rate is not observed, the rate rises with increasing pressure from vacuum to a maximum and the falls. X-ray diffraction studies indicated that the anhydrous product prepared in the second region where a decrease of rate of dehydration occurred was crystalline but the sample dehydrated under vacuum or in the first region produced an amorphous anhydrous salt. A compensation effect is demonstrated with plots of the activation energy against the logarithm of the pre-exponential term in the Arrhenius equation.     

Theoretical studies of thermal decomposition of anhydrous cadmium and silver oxalates

Detailed analysis of the results of full potential linearized augmented plane wave (FP LAPW) ab initio calculations for anhydrous silver and cadmium oxalates, reported in first part of this paper [1] has been presented. Additional calculations of Bader’s AIM (Atoms in Molecules) topological properties of the electron density, bond orders (Pauling, Bader, Cioslowski and Mixon) and bond valences according to bond valence model have been done. The obtained results show the similarities in electronic structure of both compounds and support the conclusion, that during the thermal decomposition process, these compounds should most probably decompose to metal and carbon dioxide, in agreement with the experiment.     

Theoretical studies of thermal decomposition of anhydrous cadmium and silver oxalates

The results of first principles calculations of band structure, density of states and electron density topology of CdC₂O₄ and Ag₂C₂O₄ crystals are presented. The calculations have been performed with WIEN2k ab initio program, using highly precise full potential linearized augmented plane wave (FP LAPW) method within Density Functional Theory formalism. The obtained SCF electron density has been used in calculations of Bader’s AIM (atoms in molecules) topological properties of the electron density in crystal. The obtained results show important similarities in electronic structure and electron density topology of both compounds and allow supposing, that during the thermal decomposition process these compounds should behave similarly, which is in agreement with the experiment.     

Preparation and thermal decomposition of some oxomolybdenum(VI) oxalates

Anionic oxomolybdenum(VI) oxalates having the general formula A₂[Mo₂O₆(C₂O₄)], where A = K⁺ and NH⁺₄, are prepared and characterized by chemical analysis and IR spectra, and their thermal decomposition studied using TG and DTA techniques. Both the compounds are anhydrous and the decomposition of oxalate takes place in a single step. The ammonium compound decomposes between 255 and 320°C to give MoO₃ as the end product, while the potassium compound decomposes between 300 and 380°C to give K₂Mo₂O₇ as the end product. Both the products were characterized by chemical analysis, IR and X-ray studies. The X-ray diffraction patterns of the two oxalato complexes confirm that they are crystalline compounds.     

Thermal decomposition of zirconyl oxalates

Conditions for the preparation of stoichiometric barium zirconyl oxalate heptahydrate (BZO) have been standardized. The thermal decomposition of BZO has been investigated employing TG, DTG and DTA techniques and chemical and gas analysis. The decomposition proceeds through four steps and is not affected much by the surrounding gas atmosphere. Both dehydration and oxalate decomposition take place in two steps. The formation of a transient intermediate containing both oxalate and carbonate groups is inferred. The decomposition of oxalate groups results in a carbonate of composition Ba₂Zr₂O₅CO₃, which decomposes between 600 and 800° and yields barium zirconate. Chemical analysis, IR spectra and X-ray powder diffraction data support the identity of the intermediate as a separate entity.

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Zusammenfassung Die Bedingungen für die Herstellung von stöchiometrischem Barium-zirconyl-oxalat Heptahydrat (BZO) wurden standardisiert. Die thermische Zersetzung von BZO wurde unter Einsatz der TG-, DTG- und DTA, sowie der chemischen und Gasanalyse untersucht. Die Zersetzung verläuft über vier Stufen und wird von der umgebenden Gasathmosphäre nicht besonders beeinflusst. Sowohl die Dehydratisierung als auch die Oxalatzersetzung erfolgt in zwei Stufen. Die Bildung einer intermediären Übergangsverbindung mit sowohl Oxalat- als auch Carbonatgruppen wirken hierbei mit. Die Zersetzung der Oxalatgruppen ergibt ein Carbonat der Zusammensetzung Ba₂Zr₂O₅CO₃, das zwischen 600 und 800° zersetzt wird und Bariumzirconat ergibt. Die Angaben der chemischen Analyse, der IR-Spekren und der Röntgen-Pulver-Diffraktion unterstützen die Identität der Intermediärverbindung als eine separate Einheit.

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Résumé On a standardisé les conditions de préparation de l'oxalate heptahydraté de zirconyle et de baryum (BZO) stoechiométrique. On a étudié la décomposition thermique de BZO par TG, TGD et ATD ainsi que par analyses chimiques et analyses des gaz. La décomposition a lieu en quatre étapes et n'est pas trop influencée par l'atmosphère ambiante. La déshydratation et la décomposition de l'oxalate ont lieu en deux étapes. Il se forme un composé intermédiaire de transition contenant à la fois les groupes oxalate et carbonate. La décomposition des groupes oxalate fournit un carbonate de composition Ba₂Zr₂O₅CO₃ qui se décompose entre 600 et 800° pour fournir du zirconate de baryum. L'analyse chimique, les spectres IR et la diffraction des rayons X sur poudre, apportent les preuves de l'existence d'un composé intermédiaire comme entité séparée.

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() . , , . . . , . Ba₂Zr₂O₅CO₃, 600–800 c . , .

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The authors express their sincere thanks to Professor A. R. Vasudevamurthy for his keen interest and constant encouragement. One of us (TG) is grateful to the Council of Scientific and Industrial Research, India, for the award of a research fellowship.     

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.     

Thermal decomposition of oxalates and nitrates of transplutonium elements by differential thermal analysis

Thermal decomposition of oxalate and nitrate hydrates of transplutonium elements has been studied by differential thermal and X-ray analysis, under heating in helium and oxygen. The formation heats of anhydrous crystalline Am(III) and Cm(III) nitrates were estimated from DTA data. Data of the self-decomposition of²⁴⁴Cm(III) salts under the influence of inherent -radiation were obtained.     

Thermal decomposition of titanyl oxalates IV. Strontium and calcium titanyl oxalates

Thermal decomposition of strontium titanyl oxalate tetrahydrate and calcium titanyl oxalate hexahydrate have been studied employing TG, DTA, gas and chemical analysis. The decompositions proceed through three major steps: dehydration, decomposition of the oxalate to a carbonate and the decomposition of the carbonate to yield the final products, the metatitanates. The intermediates of the oxalate decomposition are found to be Sr₂Ti₂O_4+x(CO₃)₂-x(CO₂)_x and Ca₂Ti₂O₄(CO₃)₂, respectively. The entrapment of carbon dioxide in the former and the presence of non-equivalent carbonate groups in the latter are substantiated by their i.r. spectra. The penultimate solid residues are poorly crystalline Sr₂Ti₂O₅CO₃ and amorphous Ca₂Ti₂O₅CO₃. Decompositions of these carbonates are accompanied by growth in particle size of the products, SrTiO₃ and CaTiO₃, respectively.     

Studying of silane thermal decomposition mechanism

The mechanism of silane thermal decomposition is investigated in a flow reactor. The time dependencies of silane consumption and disilane formation were compared with those parameters of solid product (aerosol particles) such as concentration, total hydrogen content in solid product, and fraction of hydrogen contained in solid product as polyhydride groups (SiH₂)_n. Silane loss and gaseous product formation were analyzed using a mass spectrometer. The hydrogen content in solid product was analyzed by the methods of IR-spectroscopy and hydrogen evolution. Based on a simple kinetic scheme we qualitatively explained the experimental dependencies of silane conversion and disilane formation, the effective activation energy of the decomposition process, and the amount of polyhydride groups in the solid product on reaction time and initial silane concentration. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 99–110, 1998.     

Effect of metal oxide additives on the thermal decomposition of perchlorates, oxalates and hydroxides

The effects of -Fe₂O₃ and -Al₂O₃ additives on the thermal decomposition of perchlorates, oxalates and hydroxides were investigated by means of DTA, TG and X-ray techniques. It was found that the oxide additives catalytically promoted the decomposition of perchlorates (NaClO₄, KClO₄ and Mg(ClO₄)₂) and resulted in a lowering of the initial decomposition temperature (T_i). On the other hand, the oxides showed no significant effect on the decomposition of oxalates (FeC₂O₄ and CuC₂O₄) and hydroxides (Mg(OH)₂ and Al(OH)₃).

The thermal decomposition of KClO₄ was chosen to compare the catalytic effect of twelve metal oxides. The results indicated that the transition metal oxides such as Cr₂O₃, -Fe₂O₃ and CuO markedly accelerated the decomposition; these oxides resulted in a solid-phase decomposition before fusion of KClO₄, and the initial decomposition temperature (T_i) of KClO₄ with oxides was about 100–200°C lower than that without catalyst. The oxides such as -Al₂O₃ and MgO resulted in a slight lowering of the temperature of the fusion and promoted the molten-phase decomposition of KClO₄, but their effects were not so remarkable as those of the transition metal oxides. The modified catalytic mechanisms of transition metal oxides were proposed by considering the electron transfer and the oxygen-abstraction models.     

The kinetics and mechanism of thermal decomposition of alkyl isocyanates

The kinetics of the gas-phase decomposition of ethyl, isopropyl, and t-butyl isocyanates have been studied in the temperature range of 380–530°C. t-Butyl isocyanate decomposes almost exclusively by a unimolecular route to isobutene and HNCO, but in EtNCO and i-PrNCO this route competes with a free-radical chain which produces CO, CH₄, and HCN or CH₃CN. In i-PrNCO, however, the chain process is very rapidly inhibited by the propene formed in the parallel unimolecular route. A minor heterogeneous bimolecular decomposition in each case gives rise to carbon dioxide and a carbodiimide. Mechanisms and trends in the alkyl isocyanates from methyl through t-butyl are discussed.     

Thermal decomposition of oxalates. Part 15. Effect of container material on DTA results for thermal decomposition of magnesium oxalate

During the study of the thermal decomposition of magnesium oxalate dihydrate certain anomalies were noted. It was found that even in the presence of oxygen the decarboxylation peak was endothermic except in the presence of a catalyst. In a platinum crucible results were found to be different from those in a porcelain crucible due to the platinum acting effectively as a catalyst.     

Thermal decomposition of zinc and cadmium zirconyl oxalates

The enthalpy of formation at 298.15 K of the polymer Al₁₃O₄(OH)₂₈(H₂O)³⁺₈ and an amorphous aluminium trihydroxide gel was studied using an original differential calorimetric method, already developed for adsorption experiments, and aluminium-27 NMR spectroscopy data. ΔH_f “Al₁₃” (298.15 K) = ? 602 ± 60.2 kJ mole^?1 and ΔH_f Al(OH)₃ (298.15 K) = ? 51 ± 5 kJ mole^?1. Using theoretical values of ΔG_R “Al₁₃” and ΔG_R Al(OH)₃, we calculated ΔG_f “Al₁₃” (298.15 K) = ? 13282 kJ mole^?1; ΔS_f “Al₁₃” (298.15 K) = + 42.2 kJ mole^?1; ΔG_f Al(OH)₃ (298.15 K) = ? 782.5 kJ mole^?1; and ΔS_f Al(OH)₃ (298.15 K) = + 2.4 kJ mole^?1.     

Thermal decomposition features of calcium and rare-earth oxalates

Thermal decomposition of Ln₂(C₂O₄)₃ · 9H₂O concentrate (Ln = La, Ce, Pr, Nd) in the presence of CaC₂O₄ · H₂O was studied by X-ray diffraction, thermogravimetry, and chemical analysis. Annealing at temperatures above 374°C in the absence of calcium oxalate gives rise to the solid solution of CeO₂-based rare-earth oxides. Calcite CaCO₃ is formed in the presence of calcium oxalate at annealing temperatures above 442°C, which impedes the formation of lanthanide oxide solid solution and favors crystallization of oxides as individual La₂O₃, CeO₂, Pr₆O₁₁, and Nd₂O₃ phases. An increase in temperature above 736°C is accompanied by decomposition of calcium carbonate to give rise to an individual CaO phase and an individual phase of CeO₂-based lanthanide oxide solid solution.