Thermal and kinetic analysis of uranium salts

In this study the thermal decomposition kinetics of uranyl acetate dehydrate [UO₂(CH₃COO)₂·2H₂O] were studied by thermogravimetry method in flowing nitrogen, air, and oxygen atmospheres. Decomposition process involved two stages for completion in all atmosphere conditions. The first stage corresponded to the removal of two?moles of crystal water. The decomposition reaction mechanism of the second stage in nitrogen atmosphere was different from that in air and oxygen atmospheres. Final decomposition products were determined with X-ray powder diffraction method. According to these data, UO₂ is the final product in nitrogen atmosphere, whereas U₃O₈ is the final product in air and oxygen atmospheres. The calculations of activation energies of all reactions were realized by means of model-free and modeling methods. Kissinger?CAkahira?CSunose (KAS) and Flynn?CWall?COzawa (FWO) methods were selected for model-free calculations. For investigation of reaction models, 13 kinetic model equations were tested. The model, which gave the highest linear regression, the lowest standard deviation, and an activation energy value which was close to those obtained from KAS and FWO equations, was selected as the appropriate model. The optimized value of activation energy and Arrhenius factor were calculated using the selected model equation. Using these values, thermodynamic functions (??H*, ??S*, and ??G*) were calculated.     

Thermal and kinetic analysis of uranium salts

The thermal decomposition kinetics of UO₂C₂O₄·3H₂O were studied by TG method in a flowing nitrogen, air, and oxygen atmospheres. It is found that UO₂C₂O₄·3H₂O decomposes to uranium oxides in four stages in all atmosphere. The first two stages are the same in the whole atmosphere that correspond to dehydration reactions. The last two stages correspond to decomposition reactions. Final decomposition products are determined with X-Ray powder diffraction method. Decomposition mechanisms are different in nitrogen atmosphere from air and oxygen atmosphere. The activation energies of all reactions were calculated by model-free (KAS and FWO) methods. For investigation of reaction models, 13 kinetic model equations were tested and correct models, giving the highest linear regression, lowest standard deviation, and agreement of activation energy value to those obtained from KAS and FWO equations were found. The optimized value of activation energy and Arrhenius factor were calculated with the best model equation. Using these values, thermodynamic functions (??H ^*, ??S ^*, and ??G ^*) were calculated.     

The non-isothermal decomposition of cobalt acetate tetrahydrate

The non-isothermal decomposition of cobalt acetate tetrahydrate was studied up to 500°C by means of TG, DTG, DTA and DSC techniques in different atmospheres of N₂, H₂ and in air. The complete course of the decomposition is described on the basis of six thermal events. Two intermediate compounds (i.e. acetyl cobalt acetate and cobalt acetate hydroxide) were found to participate in the decomposition reaction. IR spectroscopy, mass spectrometry and X-ray diffraction analysis were used to identify the solid products of calcination at different temperatures and in different atmospheres. CoO was identified as the final solid product in N₂, and Co₃O₄ was produced in air. A hydrogen atmosphere, on the other hand, produces cobalt metal. Scanning electron microscopy was used to investigate the solid decomposition products at different stages of the reaction. Identification of the volatile gaseous products (in nitrogen and in oxygen) was performed using gas chromatography. The main products were: acetone, acetic acid, CO₂ and acetaldehyde. The proportions of these products varied with the decomposition temperature and the prevailing atmosphere. Kinetic parameters (e.g.E and lnA) together with thermodynamic functions (e.g. °H, C _p and °S) were calculated for the different decomposition steps. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Thermogravimetric study of uranium phosphates

The thermal decomposition of (UO₂)₃(PO₄)₂ and U(HPO₄)₂ ·xH₂O in the temperature range 25–1600?, was investigated. (UO₂)₃(PO₄)₂ decomposed first to 1/3[U₃O₈ + 3U₂O₃P₂O₇] and then to U₃O₅P₂O₇ before a loss of phosphorus was observed above 1350?. Decomposition in air and in inert atmospheres was nearly identical. Reduction with H₂ or with carbon black in argon gave U₃O₅P₂O₇ and [UO₂ + + (UO)₂P₂O₇] before pure UO₂ was formed. U(HPO₄)₂ ·xH₂O decomposed to UP₂O₇ in argon. It oxidized partly in air before the same product was obtained. The high temperature stability of UP₂O₇ and U₃(PO₄)₄ was also investigated.     

Thermal analysis of the decomposition of ammonium uranyl carbonate (AUC) in different atmospheres

The intermediate products formed during thermal decomposition of ammonium uranyl carbonate (AUC) in different atmospheres, (air, helium and hydrogen) have been determined by thermal analysis, (TG, and DTA) and X-ray analysis. The endproducts observed are U₃O₈ and UO₂ in air/He and hydrogen, respectively. The following intermediate products were observed in all atmospheres:

X-ray diffraction analysis showed that these phases were amorphous.     

均匀棒状过氧化氢合碳酸钠制备及非等温热分解动力学

本文制备了均匀棒状过氧化氢合碳酸钠，化学分析确定其组成为Na₂CO₃·1.5H₂O₂，用DTA-TG-DTG技术并辅以X-ray分析研究了它在静态空气、流动氧气和流动氮气气氛中的非等温热分解过程及动力学。     

The thermal stability of calcium uranates in a hydrogen atmosphere

The stability and decomposition of CaUO₄, Ca₂UO₅, and Ca₃UO₆ on heating in hydrogen were investigated by X-ray powder diffraction and thermogravimetry. Ca₂UO₅ decomposes at 450°C into Ca₂UO_4.5 with a triclinic unit cell. At 850°C, it changes to monoclinic Ca₂(Ca_0.67U_0.33)UO₆ which loses some oxygen up to the composition Ca₂(Ca_0.67U_0.33)UO_5.83. At 1100°C, it decomposs to UO₂ solid solution and CaO. CaUO₄ decomposes at 900°C to Ca₂(Ca_0.67U_0.33)UO_5.83 and CaU₂O₆. The decomposition products of Ca₃UO₆ at 850°C are Ca₂(Ca_0.67U_0.33)UO_5.83 and CaO.     

Simultaneous TG/DTG–DSC–FTIR characterization of collagen in inert and oxidative atmospheres

In this paper, a TG/DTG–DSC–FTIR study of type I collagen extracted from bovine Achilles tendon both in inert (nitrogen) and oxidative atmosphere (synthetic air and oxygen) from room temperature to 700 °C was performed. The thermal analysis results have shown that after initial dehydration, collagen exhibits a single decomposition step in nitrogen (due to pyrolysis), while in air and oxygen two steps are observed due to thermo-oxidative decomposition, the latter being highly exothermic. The CO₂ bands dominate the FTIR spectra of evolved gases in all atmospheres (especially in air and oxygen), along with the characteristic bands of ammonia, water, HNCO, methane. In nitrogen, the bands of pyrrole, HCN, and ethane were also identified, while in oxidative atmospheres, nitrogen oxides and CO are released. A study was also performed by comparing the DTG and gas evolution curves observed for the three atmospheres.     

Thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O: II. Identification of the gaseous products

The main goal of the presented work was to verify the previously assumed decomposition stages of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O (HACOT) [Thermochim. Acta 354 (2000) 45] under different atmospheres (inert, oxidising and reducing). The gaseous products of the decomposition were qualitatively and quantitatively analysed by mass spectrometry (MS) and Fourier-transformed infrared spectroscopy (FT-IR). It was confirmed that the gaseous products of HACOT decomposition under studied atmospheres there were H₂O (stage I) and NH₃, CO₂ (stage II). The main gaseous products in the third stage in argon and hydrogen (20 vol.% H₂/Ar) were CO and CO₂, whereas in air (20 vol.% O₂/Ar) only CO₂ was identified. Under the oxidising as well as reducing atmospheres the influence of secondary reactions on the composition of both, solid and gaseous products was found particularly strong during the third stage of the process. The studies of the multistage decomposition of HACOT, additionally complicated by many secondary reactions, required application of the hyphenated TA-MS or TA-FT-IR techniques combined with the pulse thermal analysis PTA^® allowing quantification of the spectroscopic signals and investigation of gas-solid and gas-gas reactions in situ.     

Thermal decomposition of scandium(III) benzenetricarboxylates in air and nitrogen atmospheres

The conditions of thermal decomposition of scandium(III) hemimellitate, trimellitate and trimezinate in air and nitrogen atmospheres have been studied. On heating, the benzene-tricarboxylates of Sc(III) decompose in two stages. First, the hydrated complexes lose crystallization water; heating in air finally yields Sc₂O₃, and heating in a nitrogen atmosphere Sc₂O₃ and C. The dehydration of the complexes is associated with strong endothermic effects. The decomposition of benzenetricarboxylates in air is accompanied by an exothermic effect and in nitrogen by an endothermic effect. The activation energies of the dehydration and decomposition reactions have been calculated for the Sc(III) benzenetricarboxylates.     

Thermal decomposition of cesium hexanitratouranium(IV)

As cesium hexanitratouranium(IV), Cs₂U(NO₃)₆, has the same Cs:U stoichiometry as that of Cs₂UO₄, thermal decomposition of this nitrato complex in air and nitrogen was studied in detail as a possible alternate method of preparing pure Cs₂UO₄. The volatility of cesium nitrate, which is one of the intermediate products, changed this Cs:U ratio during thermal decomposition. Hence, only Cs₂U₂O₇ was obtained on heating the sample to 775 K or higher. A scheme for the thermal decomposition of Cs₂U(NO₃)₆ is given by combining the observed TG, XRD and IR data.     

Investigation of UO4·2NH3·2HF by simultaneous thermogravimetry-differential thermal analysis-mass spectrometry technique

Thermal decomposition of UO₄·2NH₃·2HF was studied under high vacuum and in different gas atmospheres (N₂, O₂, synth. air). Gaseous decomposition products were analyzed and recorded using a quadrupole mass spectrometer.By discussing TG and MS data as well as X-ray analysis of intermediate products an attempt is made, to explain decomposition mechanisms under varied experimental conditions.Thermal decomposition strongly supports the results of X-ray analysis leading to the formula UO₄·2NH₃·2HF.     

Thermal and X-ray diffraction analysis studies during the decomposition of ammonium uranyl nitrate

Two types of ammonium uranyl nitrate (NH₄)₂UO₂(NO₃)₄·2H₂O and NH₄UO₂(NO₃)₃, were thermally decomposed and reduced in a TG-DTA unit in nitrogen, air, and hydrogen atmospheres. Various intermediate phases produced by the thermal decomposition and reduction process were investigated by an X-ray diffraction analysis and a TG/DTA analysis. Both (NH₄)₂UO₂(NO₃)₄·2H₂O and NH₄UO₂(NO₃)₃ decomposed to amorphous UO₃ regardless of the atmosphere used. The amorphous UO₃ from (NH₄)₂UO₂(NO₃)₄·2H₂O was crystallized to γ-UO₃ regardless of the atmosphere used without a change in weight. The amorphous UO₃ obtained from decomposition of NH₄UO₂(NO₃)₃ was crystallized to α-UO₃ under a nitrogen and air atmosphere, and to β-UO₃ under a hydrogen atmosphere without a change in weight. Under each atmosphere, the reaction paths of (NH₄)₂UO₂(NO₃)₄·2H₂O and NH₄UO₂(NO₃)₃ were as follows: under a nitrogen atmosphere: (NH₄)₂UO₂(NO₃)₄·2H₂O → (NH₄)₂UO₂(NO₃)₄·H₂O → (NH₄)₂UO₂(NO₃)₄ → NH₄UO₂(NO₃)₃ → A-UO₃ → γ-UO₃ → U₃O₈, NH₄UO₂(NO₃)₃ → A-UO₃ → α-UO₃ → U₃O₈; under an air atmosphere: (NH₄)₂UO₂(NO₃)₄·2H₂O → (NH₄)₂UO₂(NO₃)₄·H₂O → (NH₄)₂UO₂(NO₃)₄ → NH₄UO₂(NO₃)₃ → A-UO₃ → γ-UO₃ → U₃O₈, NH₄UO₂(NO₃)₃ → A-UO₃ → α-UO₃ → U₃O₈; and under a hydrogen atmosphere: (NH₄)₂UO₂(NO₃)₄·2H₂O → (NH₄)₂UO₂(NO₃)₄·H₂O → (NH₄)₂UO₂(NO₃)₄ → NH₄UO₂(NO₃)₃ → A-UO₃ → γ-UO₃ → α-U₃O₈ → UO₂, NH₄ UO₂(NO₃)₃ → A-UO₃ → β-UO₃ → α-U₃O₈ → UO₂.     

Study of Mechanism for Hexane Decomposition with Gliding Arc Gas Discharge

The mechanism of hexane decomposition under gliding arc gas discharge conditions is studied from both qualitative and quantitative analyses of its products for various hexane initial concentrations and different background atmospheres : nitrogen, argon, air (O₂ 21% N₂ 79% vol.) and N₂–O₂ mixtures. The decomposition rate, which decreases with increasing hexane initial concentration, can reach 94% when the carrier gas is air. Due to the electron energy consumed by the dissociation of nitrogen, the decomposition rate of hexane in nitrogen is lower than in argon. The radical channel plays a predominant role in the hexane decomposition process. With increasing oxygen concentration in the carrier gas, the hexane decomposition rate increases and promotes the conversion of CO– CO₂, but it also leads to the formation of NO₂.     

Chemistry of the CARBEX process: Identification of absorption bands of the ligands in the electronic spectra of aqueous solutions of Na<Subscript>4</Subscript>[UO<Subscript>2</Subscript>(O<Subscript>2</Subscript>)CO<Subscript>3</Subscript>)<Subscript>2</Subscript>]

On the basis of consideration of dissociation, hydration, association, and ligand exchange, the assignment of absorption bands in the electronic spectra of aqueous solutions of the Na₄[UO₂(O₂)CO₃)₂] complex has been performed. It has been demonstrated that the absorption in the range 190–400 nm is caused by the oxygen atoms of the O₂^2- and CO₃^2- groups and hydration water molecules of dissociated and neutral complex species Na₃[UO₂(O₂)(CO₃)₂]^–, Na₂[UO₂(O₂)(CO₃)₂]^2–, and Na₄[UO₂(O₂)(CO₃)₂].     

Thermal and thermo-oxidative degradation of poly(hydroxy ether of bisphenol-A) studied by TGA/FTIR and TGA/MS

The nature and the extent of degradation of poly(hydroxy ether of bisphenol-A) phenoxy resin were analysed by thermogravimetry (TGA/DTGA) under nitrogen and air atmosphere. Decomposition kinetics were elucidated according to Flynn-Wall-Ozawa, Friedman and Kissinger methods. The evolved gases during degradation were inspected by a thermogravimetry analyser coupled with Fourier Transform Infrared Spectrometer (TGA/FTIR) and also with a TGA coupled to a Mass Spectrometer (TGA/MS). Mass spectra showed that chemical species evolved in phenoxy decomposition in air were very similar to those assigned from degradation in nitrogen (water, methane, CO, CO₂, phenol, acetone, etc.). However, these species appear in different amount and at different temperatures in both atmospheres. FTIR analysis of the evolved products showed that water and methane were the beginning decomposition products, indicating that decomposition is initiated by dehydration and cleavage of C-CH₃ bond in the bisphenol-A unit of phenoxy resin. After this initial stage, random chain scission is the main degradation pathway. Nevertheless, in air atmosphere, previously the complete decomposition of the phenoxy obtaining fundamentally CO₂, and water, the formation of an insulated surface layer of crosslinked structures has been proposed.     

Thermoanalytical study on the oxidation of uranium dioxides derived from uranyl nitrate,uranyl acetate and ammonium diuranate

The oxidation of UO₂ was investigated by TG, DSC and X-ray diffraction . UO₂ samples were prepared by the reduction of UO₃ at P_H2 + P_N2 = 100 + 50 mm Hg and 5°C min^?1 up to 800°C. In order to obtain six UO₂ samples with different preparative histories, UNH, UAH and ADU were used as starting materials and their thermal decomposition was carried out at 450–625°C for 0–9 h at an air flow rate of 100 ml min^?1. α-UO₃, γ-UO₃, UO₃ - 2 H₂O, and their mixtures were obtained. The reduction of UO₃ gave β-UO_2+x with different x values from 0.030 to 0.055. The oxidation carried out at P_O2 = 150 mm Hg was found to consist of oxygen uptake at room temperature. UO₂ - U₃O₇ (Step I) and U₃O₇ → U₃O₈ (Step II). TG and DSC curves of the oxidation showed two plateaus and two exothermic peaks corresponding to Steps I and II. In the case of two of the samples, the DSC peak of Step II split into two substeps, which were assumed to be due to the different reactivities of U₃O- formed from α-CO₃ and that from other types of UO₃. The increase in O/U ratio due to the oxygen uptake at room temperature changed from 0.010 to 0.042 except for a sample prepared from ADU which showed an extraordinarily large value of 0.445. TG curves showed an increase in O/U from room temperature to near 250°C for Step I and the plateau at 250–350°C where O/U was about 2.42, and showed a sharp increase in O/U above 350°C for Step II and the plateau above 100°C where O/U was 2.72–2.75. It is thought that the prepared UO₂ had a defective structure with a large interstitial volume to accommodate the excess oxygen.     

A study of non-isothermal kinetics of limestone decomposition in air (O2/N2) and oxy-fuel (O2/CO2) atmospheres

The non-isothermal experiments of limestone decomposition at multi-heating rates in O₂/N₂ and O₂/CO₂ atmospheres were studied using thermogravimetry. The limestone decomposition kinetic model function, kinetic parameters of apparent activation energy (E), and pre-exponential factor (A) were evaluated by Bagchi and Malek method. The results shown that in 20 % O₂/80 % N₂ atmosphere, the limestone decomposed slowly following the contracting sphere volume model controlled by boundary reaction (spherical symmetry) in two stages, and the E increased by about 50 kJ mol^?1 in the second decomposition stage. But in 20 % O₂/80 % CO₂ atmosphere, the presence of high-concentration CO₂ significantly inhibited the limestone decomposition, and made the decomposition process occur at high temperature with a rapid rate; the decomposition kinetics was divided into three stages, the first stage was an accelerated decomposition process following the Mampel Power law model with the exponential law equation, the second stage followed the nth order chemical reaction model as an α–t deceleration process, and the third stage belonged to the random nucleation and nuclei growth model with the Avrami–Erofeev equation. And with the heating rate increasing, the reaction order n showed a slight rise tendency. The E was about 1,245 kJ mol^?1 in 20 % O₂/80 % CO₂ atmosphere, but was only about 175 kJ mol^?1 in 20 % O₂/80 % N₂ atmosphere. The E and A increased markedly in the O₂/CO₂ atmosphere.     

Thermogravimetric study of uranium phosphates

The thermal decomposition of UO₂NH₄PO₄ · 3H₂O and UO₂HPO₄ · 4H₂O was studied in the temperature range 25–1600?C. Both compounds gave U₂O₃P₂O₇ around 900?C after a two step dehydration and an orthophosphate-pyrophosphate transformation. UO₂NH₄PO₄ · 3H₂O did not form any pure intermediates, but (UO₂)₂P₂O₇ could be prepared from UO₂HPO₄ · 4H₂O. In air, U₂O₃P₂O₇ lost phosphorus above 1250?C. In argon, (UO)₂P₂O₇ was first formed between 1000 and 1290?C and this product only lost phosphorus at still higher temperatures. (UO)₂P₂O₇ was also obtained by reduction of (UO₂)₂P₂O₇ or U₂O₃P₂O₇ at 700?C in H₂ or with carbon black in argon above 1000?C. It oxidised in air above 250?C with the formation of U₂O₃P₂O₇.     

Thermal decomposition of the oxo-diperoxo-molibdenum (VI)-potassium oxalate

The oxo-diperoxo-molibdenum(VI)-potassium oxalate, K₂[MoO(O₂)₂(C₂O₄)] was synthesized using an adapted version of the method suggested by Dengel. The thermal behavior of the synthesized complex was investigated by simultaneous thermal analysis TG/DTG/DTA, in air or nitrogen atmosphere, to identify and characterize the mass-loss decomposition processes. In addition, for the characterization of the observed decomposition steps, the FT-IR spectra for the initial complex, evolved gaseous compounds and isolated complex at 230 and 430/383 °C in air/nitrogen atmosphere, were recorded. On the 35–500 °C temperature range, the K₂[MoO(O₂)₂(C₂O₄)] complex presented three main decomposition steps, accompanied by mass-loss. The first degradation step is due to the elimination of one oxygen molecule, by the breaking of the peroxo groups, with the formation of an intermediary, like [MoO₃L]. The other two degradation steps can be attributed to the decomposition of the organic ligand, with the final formation of a stable metallic oxide.