Showing papers on "Thermal decomposition published in 2001"

Nanoscale Magnesium Hydroxide and Magnesium Oxide Powders: Control over Size, Shape, and Structure via Hydrothermal Synthesis

Abstract: Mg(OH)2 nanocrystallines with rod-, tube-, needle-, or lamella-like morphologies have been synthesized by a hydrothermal reaction using different magnesium precursors and solvents as the reactants. The products appeared to have narrow size distributions with a monodisperse nature. Subsequent thermal decomposition at 450 °C gave nanosized MgO, which preserved well the morphological features of the Mg(OH)2 samples. The specific surface areas of the MgO samples were determined by the BET technique, which gave a feature of high surface area generally larger than 100 m2/g. The channels formed in the thermal dehydroxylation process may account for this feature of the MgO nanocrystallines.

The thermal decomposition of silver (I, III) oxide: A combined XRD, FT-IR and Raman spectroscopic study

Abstract: FT-IR and Raman spectra for polycrystalline powders of silver (I, III) oxide, AgO, and silver (I) oxide, Ag2O, are reported. The vibrational spectra for each oxide are discussed in relation to its crystal structure, and were found to be consistent with factor group analysis predictions. Infrared and Raman spectroscopy, in conjunction with powder XRD, were also used to follow the thermal decomposition of AgO powder in air. Supplementary studies employing differential scanning calorimetry (DSC) and temperature programmed reaction (TPR), provided additional information relevant to the decomposition process. In agreement with mechanisms previously reported, AgO was thermally reduced to metallic silver ia two non-reversible steps, with the intermediate formation of Ag2O. The transformation of AgO to Ag2O occurred with heating in the 373–473 K region, while the product of this reaction remained stable to temperatures in excess of 623 K. Complete thermal decomposition of the Ag2O intermediate to Ag and O2 occurred at 673 K.

Preparation of Monodispersed Fe−Mo Nanoparticles as the Catalyst for CVD Synthesis of Carbon Nanotubes

Abstract: Uniform iron−molybdenum nanoparticles were prepared by thermal decomposition of metal carbonyl complexes using a mixture of long-chain carboxylic acid and long-chain amine as protective agents. The sizes of the nanoparticles can be systematically varied from 3 to 14 nm by changing the experimental conditions. High-resolution TEM images and EDX data show that the prepared nanoparticles are highly crystalline iron nanoparticles containing ≈4% molybdenum. The effects of the concentration, reaction time, the ratio of metal carbonyl complexes versus protective agents, and the ratio of acid/amine of the protective agents on the sizes of the produced nanoparticles were systematically studied. The prepared nanoparticles were used as catalysts for single-walled carbon nanotube growth and the results indicate that there is an upper limit for the size of the catalyst particles to nucleate single-walled carbon nanotubes.

Thermal behaviour of poly(propylene) layered silicate nanocomposites

Abstract: The thermal degradation behaviour of nanocomposites based upon poly(propylene)/organoclay, modified with protonated octadecyl amine (C18) in comparison to that of non-exfoliated microcomposites based upon organoclay, modified with protonated butyl amine (C4), was studied by thermogravimetry. In the case of the nanocomposite, the temperature at which volatilisation occurs increases as compared of the microcomposite. Moreover, the thermal oxidation process of the polymer is strongly slowed down in the nanocomposite with high char yield both by a physical barrier effect, enhanced by ablative reassembling of the silicate, and by a chemical catalytic action due to the silicate and to the strongly acid sites created by thermal decomposition of the protonated amine silicate modifier.

Hydrogen Production by Catalytic Decomposition of Methane

Abstract: Traditionally, hydrogen is produced by reforming or partial oxidation of methane to produce synthesis gas, followed by the water-gas shift reaction to convert CO to CO2 and produce more hydrogen, followed in turn by a purification or separation procedure. This paper presents results for the catalytic decomposition of undiluted methane into hydrogen and carbon using nanoscale, binary, Fe−M (M = Pd, Mo, or Ni) catalysts supported on alumina. All of the supported Fe−M binary catalysts reduced methane decomposition temperature by 400−500 °C relative to noncatalytic thermal decomposition and exhibited significantly higher activity than Fe or any of the secondary metals (Pd, Mo, and Ni) supported on alumina alone. At reaction temperatures of approximately 700−800 °C and space velocities of 600 mL g-1 h-1, the product stream was comprised of over 80 volume % of hydrogen, with the balance being unconverted methane. No CO, CO2, or C2 and higher hydrocarbons were observed in the product gas. High-resolution SEM and...

Implication of the microstructure of binary Cu/ZnO catalysts for their catalytic activity in methanol synthesis

Abstract: Binary Cu/ZnO catalysts with varying molar ratios (90/10 through 10/90) were studied under methanol synthesis conditions at 493 K and at atmospheric pressure. The methanol synthesis activity of the catalysts was correlated to their specific Cu surface area (N2O reactive frontal chromatography, N2O RFC) after reduction in 2 vol% H2 at 513 K. Activity data were supplemented with a detailed analysis of the microstructure, i.e., crystallite size and strain of the reduced Cu and the ZnO phases after reduction using X-ray diffraction line profile analysis. The estimated copper surface area based on a spherical shape of the copper crystallites is in good agreement with data determined by N2O RFC. A positive correlation of the turnover frequency for methanol production with the observed microstrain of copper in the Cu/ZnO system was found. The results indicate a mutual structural interaction of both components (copper and zinc oxide) in the sense that strained copper particles are stabilized by the unstrained state of the zinc oxide microcrystallites. The observed structural deformation of ZnO in samples with higher Cu loading can originate, for instance, from epitaxial bonding of the oxide lattice to the copper metal, insufficient reduction or residual carbonate due to incomplete thermal decomposition during reduction. Additional EXAFS measurements at the Cu K and the Zn K edge show that about 5% ZnO are dissolved in the CuO matrix of the calcined precursors. Furthermore, it is shown that the microstructural changes (e.g., size and strain) of copper can be traced back to the phase composition of the corresponding hydroxycarbonate precursors.

The Reaction of Charged Cathodes with Nonaqueous Solvents and Electrolytes: I. Li0.5CoO2

Abstract: The thermal decomposition of Li 0 5CoO2 was studied by accelerating rate calorimetry and X-ray diffraction. Oxygen loss from the material according to the reaction Li 0.5 CoO 2 → 0.5 LiCoO 2 + 1/6Co 3 O 4 + 1/6O 2 occurs at temperatures above 200°C. By contrast, the reaction of Li 0.5 CoO 2 with ethylene carbonate:diethyl carbonate (EC:DEC) solvent initiates at temperatures as low as 130°C, which is much lower than the decomposition temperature of Li 0.5 CoO 2 itself, and reduction to CoO occurs. We believe that this is caused by the reducing power of the solvent. The heat generated by this reaction is consistent with that expected from the combustion of the solvent hy the oxygen liberated during the decomposition of the solid. The reaction of Li 0.5 CoO 2 with xM LiPF 6 /EC:DEC (0 < x < 1.5) electrolyte was also studied. As the salt concentration is increased, the solvent combustion reaction is suppressed, suggesting that increased LiPF 6 concentration can slow the cathode/electrolyte reaction in practical Li-ion cells under conditions of electrical or mechanical abuse.

In situ investigation of thethermal decomposition of Co–Al hydrotalcite in different atmospheres

Abstract: High temperature X-ray diffraction (HT-XRD), thermal analysis (TGA-DTA), mass spectrometry (MS), in situ Fourier transform infrared (FT-IR) spectroscopy, and in situ Raman spectroscopy have been used to characterize the thermal decomposition of Co–Al hydrotalcite, [Co6Al2(OH)16](CO3)·4H2O, in air and inert atmospheres. In the first decomposition step, water is removed from the structure, a process which is complete at 150–200 °C. This transition is followed by dehydroxylation and decarbonation, as well as carbonate reorganization in the interlayer space. These processes require higher temperatures under inert atmospheres than in air. The transition temperatures also depend on the nature of the technique applied (static vs. dynamic operation). An intermediate metastable mixture of phases is identified, which contains the dehydrated layered structure and an emerging spinel-like mixed oxide phase. This phase is formed in the region of 150–175 °C in air and was not observed under inert atmospheres. Dehydroxylation leads to the collapse of the hydrotalcite phase and is complete at 250–300 (air) and 350–400 °C (inert gas). Carbonate removal is coupled with the dehydroxylation process, although removal of carbonate groups is only complete at 450 (air) and 600 °C (inert gas). Thermal treatment in air finally leads to a solid solution of cobalt spinels [Co(Co,Al)2O4]. Mixtures of CoO and CoAl2O4 are formed upon treatment under inert atmospheres. Based on the analytical results, a simplified structural model for the decomposition process is presented. The presence of oxidizable Co2+ cations in the octahedral sheets and the diffusion of Co3+ to the interlayer space in the dehydrated layered structure, and the stability of the solid solution of Co-spinels formed are identified as key factors in the low thermal stability of the hydrotalcite precursor in air.

Thermal and carbothermic decomposition of Na 2 CO 3 and Li 2 CO 3

Abstract: In order to elucidate the decomposition mechanism of Na2CO3 and Li2CO3 in mold-powder systems employed in the continuous casting of steel, decompositions of Na2CO3 and Li2CO3 were investigated using thermogravimetric (TG) and differential scanning calorimetric (DSC) methods at temperatures up to 1200 °C, under a flow of argon gas. For the case of pure Na2CO3, the thermal decomposition started from its melting point and continued as the temperature was increased, but at a very slow rate. For Li2CO3, however, the decomposition occurred at much faster rates than that for Na2CO3. When carbon black was added to the carbonate particles, the decomposition rates of both Na2CO3 and Li2CO3 were significantly enhanced. From mass-balanced calulations and X-ray diffraction (XRD) analyses of the reaction products, it is concluded that decompositions of Na2CO3 and Li2CO3 with carbon black take place according to the respective reactions of Na2CO3 (1) + 2C (s) = 2Na (g) + 3CO (g) and Li2CO3 (l) + C (s) = Li2O (s) + 2CO (g). It was found that liquid droplets of Na2CO3 were initially isolated due to carbon particles surrounding them, but, as the carbon particles were consumed, the liquid droplets were gradually agglomerated. This effected a reduction of the total surface area of the carbonate, resulting in a dependence of the decomposition rate on the amount of carbon black. For the case of Li2CO3, on the other hand, hardly any agglomeration occurred up to the completion of decomposition, and, hence, the rate was almost independent of the amount of carbon black mixed. The apparent activation energies for the decomposition of Na2CO3 and Li2CO3 with carbon black were found to be similar and were estimated to be 180 to 223 kJ mole−1.

Decomposition pathways of hydrotalcite-like compounds Mg1-xAlx(OH)2(NO3)x.nH2O as a continuous function of nitrate anions

Abstract: Thermal decomposition pathways of our recently prepared hydrotalcite-like compounds Mg1-xAlx(OH)2(NO3)x·nH2O in x = 0.20−0.34 (J. Phys. Chem. B 2001, 105, 1743−1749) have been investigated with XRD, DTA, TGA, FTIR, and combined TGA/FTIR techniques. It has been found that, unlike those in carbonated hydrotalcites, the dehydroxylation and decomposition of anions in low nitrate-content (x) hydrotalcites are separated, while the two processes in the high x samples are overlapped. In line with our recent structural models, the dehydroxylation process in the samples with high x value can be further differentiated into steps, depending on chemical nature of hydroxyl group and nitrate content. The layered structure of these hydrotalcite compounds becomes thermally more stable when more nitrate ions are intercalated. The depletion of nitrate anions (decomposed into NO2 and O2) in the low x compounds is a continuous process, whereas that in the high x compounds is a discrete one. At 400 °C, most nitrate anions are ...

Polytetrafluoroethylene decomposition in air and nitrogen

Abstract: The kinetics of the thermal decomposition of polytetrafluoroethylene (PTFE) have been studied using dynamic TG-DTG at heating rates between 1 and 25°C/min at atmospheric pressure. Two different atmospheres were used: on the one hand, an inert atmosphere (N 2 ) in order to study the pyrolysis of the material, and on the other hand an oxidative atmosphere (synthetic air) to study the combustion of the polymer. The same kinetic model has been applied simultaneously to runs performed at different heating rates and different atmospheres allowing a good correlation of the weight loss data. The kinetic model considers that the overall decomposition of the PTFE is done via two different parallel processes. Dynamic measurements were performed by combined thermogravimetry mass spectrometry (TG-MS) in order to determine the decomposition products. The evolution of C 2 F 4 , CF 4 , COF 2 , HF, hydrocarbons, benzene and some other compounds has been also analyzed.

Thermal Decomposition of Cellulose Crystallites in Wood

Abstract: Decomposition of cellulose crystallites in wood during pyrolysis was studied by X-ray diffraction using a tension wood of Populus maximowiczii (cottonwood), which contains highly crystalline cellulose. X-ray diffraction profiles were recorded at varied temperature up to 360°C. By one-hour isothermal treatments, the cellulose crystallites did not decompose at 300°C, but completely decomposed at 340°C. The change in equatorial diffraction profile was studied by temperature scan up to 360°C and by isothermal treatment at the critical temperature of 320°C. Along with the changes by thermal expansion, the changes in diffraction diagram revealed a characteristic discrepancy between the diminishment of crystalline order and the reduction in crystallite size; i.e., the intensity of crystalline reflections diminished steadily while the crystallite size decreased much more slowly. A model of highly heterogeneous decomposition is proposed to explain this behavior.

Novel Ionic Liquid Thermal Storage for Solar Thermal Electric Power Systems

Abstract: Feasibility of ionic liquids as liquid thermal storage media and heat transfer fluids in a solar thermal power plant was investigated. Many ionic liquids such as [C4min][PF6], [C8mim][PF6], [C4min][bistrifluromethane sulflonimide], [C4min][BF4], [C 8mim][BF4], and [C4min][ bistrifluromethane sulflonimide] were synthesized and characterized using thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), nuclear magnetic resonance (NMR), viscometry, and some other methods. Properties such as decomposition temperature, melting point, viscosity, density, heat capacity, and thermal expansion coefficient were measured. The calculated storage density for [C8mim][PF6] is 378 MJ/m 3 when the inlet and outlet field temperatures are 210 o C and 390 o C. For a single ionic liquid, [C4mim][BF4], the liquid temperature range is from –75 o C to 459 o C. It is found that ionic liquids have advantages of high density, wide liquid temperature range, low viscosity, high chemical stability, non-volatility, high heat capacity, and high storage density. Based on our experimental results, it is concluded that ionic liquids could be excellent liquid thermal storage media and heat transfer fluids in solar thermal power plant.

Thermoplastic resin composition

Abstract: PROBLEM TO BE SOLVED: To obtain a polyimide resin exhibiting improved melt stability, having decomposition resistance under an ultimate processing treatment condition SOLUTION: This thermoplastic resin composition having excellent melt flow stability is characterized in that the composition comprises (a) a polyimide resin and (b) a phosphorus-containing stabilizer, (a) the polyimide resin contains a structural unit of formula (I) and (b) the phosphorus-containing stabilizer has 10-100 wt% of the initial weight remaining by a thermogravimetric analysis (nitrogen atmosphere, 20 [ degC/minute] rate of temperature increase, 25-300 degC measurement range) Consequently the composition exhibits extremely improved melt stability and thermal decomposition resistance