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.     

Theoretical studies of electronic structure and structural properties of anhydrous alkali metal oxalates

The theoretical analysis of electronic structure and bonding properties of anhydrous alkali metal oxalates, based on the results of DFT FP-LAPW calculations, Bader’s QTAIM topological properties of electron density, Cioslowski and Mixon’s topological bond orders [reported in the first part of this paper by Kole?yński (doi:10.1007/s10973-013-3126-z)] and Brown’s Bond Valence Model calculations, carried out in the light of thermal decomposition pathway characteristic for these compounds are presented. The obtained results shed some additional light on the origins of the complex pathway observed during thermal decomposition process (two stage process, first the formation of respective carbonate and then decomposition to metal oxide and carbon dioxide). For all structures analyzed, strong similarities in electronic structure and bonding properties were found (ionic-covalent bonds in oxalate anion with C–C bond as the weakest one in entire structure and almost purely ionic between oxalate group and alkali metal cations), allowing us to propose the most probable pathway consisting of consecutive steps, leading to carbonate anion formation with simultaneous cationic sublattice relaxations, which results in relative ease of respective metal carbonate formation.     

Theoretical studies of electronic structure and structural properties of anhydrous alkali metal oxalates

Nanocrystalline LiMn₂O₄ was synthesized by calcining LiMn₂(CO₃)_2.5·0.8H₂O above 600 °C in air. The precursor and its calcined products were characterized by thermogravimetry and differential scanning calorimetry, X-ray powder diffraction, and scanning electron microscopy. The result showed that highly crystallization LiMn₂O₄ with cubic structure [space group Fd-3m(227)] was obtained when the precursor was calcined at 600 °C in air for 1.5 h. The thermal process of the precursor in air experienced three steps which involved, at first, the dehydration of 0.8 water molecules, then decomposition of MnCO₃ into Mn₂O₃, at last, reaction of Mn₂O₃ and Li₂CO₃ into cubic LiMn₂O₄. Based on Starink equation, the values of the activation energies associated with the thermal process of LiMn₂(CO₃)_2.5·0.8H₂O were determined. Besides, most probable mechanism functions and thermodynamic functions (ΔS ^≠, ΔH ^≠, and ΔG ^≠) of thermal processes of LiMn₂(CO₃)_2.5·0.8H₂O were also determined.     

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.     

Non-isothermal decomposition of anhydrous silver malonate

The non-isothermal decomposition of anhydrous disilver malonate was studied up to 300°C by means of TG, DTA and DSC techniques in different atmospheres (e.g. N₂, H₂ and air). Acetic acid, CO₂, acetone and CO were identified as the volatile decomposition products using gas chromatography. Silver metal, on the other hand, was identified as the final solid product using X-ray powder diffraction. The mechanism described involves the breakdown of adsorbed radicals, probably including-CH₂COO- and related species, on the surface of the metallic silver product. The activation energy (ΔE) and the frequency factor (InA) were calculated for the decomposition process from the variation of peak temperature (of the DTA curves) with the rate of heating (φ). The enthalpy change (ΔH), the heat capacity (C _p) and entropy change (ΔS) were calculated from the DSC measurements.     

The effect of gamma-irradiation on the thermal decomposition of anhydrous cadmium nitrate

The thermal decomposition of -irradiated anhydrous cadmium nitrate was studied by dynamic thermogravimetry. The reaction order, activation energy, frequency factor and entropy of activation were calculated by the Coats-Redfern method and were compared with those of the unirradiated salt. Irradiation enhances the decomposition and the effect increases with the irradiation dose. The activation energy decreases on irradiation. The mechanism of the decomposition of unirradiated and irradiated anhydrous cadmium nitrate follows the Mampel equation: -ln(1-) for g() and the rate-controlling process is random nucleation with the formation of a nucleus on every particle.     

First principles studies of thermal decomposition of anhydrous zinc oxalate

The highly reactive and unstable exothermal features of methyl ethyl ketone peroxide (MEKPO) have led to a large number of thermal explosions and runaway reaction accidents in the manufacturing process. To evaluate the self-accelerating decomposition temperature (SADT) of MEKPO in various storage vessels, we used differential scanning calorimetry (DSC) and vent sizing package 2 (VSP2). The thermokinetic parameters were, in turn, used to calculate the SADT from theoretical equations based on the Semenov model. This study aimed at the SADT prediction value of various storage vessels in Taiwan compared with the UN 25 kg package and UN 0.51 L Dewar vessel. An important index, such as SADT, temperature of no return (T _NR) and adiabatic time maximum rate (TMR_ad), was necessary and useful to ensure safe storage or transportation for self-reactive substances in the process industries.     

Non-isothermal decomposition of silver maleate dihydrate and anhydrous silver fumarate

The non-isothermal decompositions of silver maleate dihydrate (C₄H₂O₄Ag₂2H₂O) and anhydrous silver fumarate (C₄H₂O₄Ag₂) were studied up to 500°C, in a dynamic atmosphere of air, by means of TG and DTA measurements. Both compounds showed some sublimation (at 120°C for silver maleate and at 180°C for silver fumarate) prior to the onset of decomposition (at 170°C for silver maleate and at 280°C for silver fumarate). The gaseous decomposition products of both compounds were found, using IR spectroscopy, to be dominated by maleic anhydride and CO₂. Minor proportions of ethylene, ethyl alcohol, acetone, methane and isobutene were also identified. Metallic silver was the final solid product, as identified by X-ray diffractometry. NMR analysis was used to monitor the isomerization of the maleate radical into the more stable fumarate above 230°C. Kinetic parameters (E _a and lnA) were calculated from the effect of heating rate, (2, 5, 10, and 20 deg min^?1) on the DTA measurements. A mechanism is suggested for the decomposition pathways of these compounds, on basis of the results obtained and, also, on similarities with analogous systems.     

Vibrational studies of anhydrous lithium,sodium and potassium oxalates

The IR and Raman spectra of sodium oxalate and the IR spectrum of lithium oxalate are re-examined. The Raman spectrum of lithium oxalate is presented for the first time. A recent structural study is used as the basis for the first detailed vibrational study of anhydrous potassium oxalate, phase II.     

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 mechanism of thermal decomposition of ionic oxalates

A mechanism for the thermal decomposition of ionic oxalates has been proposed on the basis of three quantitative relationships linking the quantitiesr _c/r _i (the ratio of the Pauling covalent radius and the cation radius of the metal atom in hexacoordination) andΣI _i (the sum of the ionization potentials of the metal atom in kJ mol^?1) with the onset oxalate decomposition temperature (T _d) (Eq. 1) the average C-C bond distance (¯d) (Eq. 2), and the activation energy of oxalate decomposition (E _a) (Eq. 3): (1) $$T_d = 516 - 1.4006\frac{{r_c }}{{r_i }}(\sum I_i )^{\frac{1}{2}}$$ (2) $$\bar d = 1.527 + 5.553 \times 10^{ - 6} \left( {122 - \frac{{r_c }}{{r_i }}(\sum I_i )^{\frac{1}{2}} } \right)^2$$ (3) $$E_a = 127 + 1.4853 \times 10^{ - 6} \left( {\left( {\frac{{r_c }}{{r_i }}} \right)^2 \sum I_i - 9800} \right)^2$$ On the basis of these results it is proposed that the thermal decomposition of ionic oxalates follows a mechanism in which the C-O bond ruptures first. From Eq. 3 it is further proposed that strong mutual electronic interactions between the oxalate and the cations restrict the essential electronic reorganization leading to the products, thereby increasingE _a.     

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.     

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.     

Mössbauer and DTA studies of the decomposition of iron oxalates

Results are reported for FeC₂O₄·2H₂O and Fe₂(C₂O₄)₃·5H₂O; various stages in the decomposition are demonstrated. The second compound on decomposition in nitrogen gives rise to finely divided Fe₃O₄ containing a little free metal. The conditions for this are established.     

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 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.