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  • Author or Editor: S. Halawy x
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Abstract  

Non-isothermal dehydration of copper chloride dihydrate and nickel chloride hexahydrate were studied by using TG, DTG, DTA and DSC measurements. The copper chloride salt loses its two water molecules in one step while nickel chloride salt dehydrates in three consecutive steps. The first two steps involve the loss of 4 water molecules in two overlapped steps while the third step involves the dehydration of the dihydrate salt to give the anhydrous NiCl2. Activation energies (ΔE) and the frequency factor (A) were calculated from DTG and DTA results. We have also calculated the different thermodynamic parameters, e.g. enthalpy change (ΔH), heat capacity (C p) and the entropy change (ΔS) from DSC measurements for both reactants. The isothermal rehydration of the completely dehydrated salts was studied in air and under saturated vapour pressure of water. Anhydrous nickel chloride was found to rehydrate in three consecutive steps while the copper salt rehydrated in one step.

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Abstract  

The thermal decomposition of pure ammonium heptamolybdate tetrahydrate (AHMT), and doped with Li+, Na+ and K+ ions was investigated using thermogravimetry, differential thermal analysis, infrared and X-ray diffraction techniques. Results obtained revealed that the decomposition of AHMT proceeded in three decomposition stages in which both NH3 and H2O were released in all stages. The presence of 0.5 mol % alkali metal ions enhances the formation of the intermediateb (NH4)2MO7O22·2H2O while the decomposition of this intermediate into MoO3 is slightly affected in the presence of all dopant concentrations used. The infrared absorption spectra of the thermal products of AHMT treated with 10 mol% alkali metal ions (AMI) at 350°C indicated a reduction of some Mo6+ ions. By heating of AHMT above 500°C in presence of 5 or 10 mol % of AMI, a solid-solid interaction between alkali metal oxides and MoO3 giving rise to well crystallized alkali metal molybdates. finally the activation energies accompanied various decomposition stages were calculated.

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Abstract  

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.

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Abstract  

Non-isothermal decomposition of chromium chromate hexahydrate, Cr2(CrO4)3–6H2O, was studied on heating up to 600°C in different dynamic atmospheres of N2, O2 and H2, using thermogravimetry (TG), derivative thermogravimetry (DTG) and differential scanning calorimetry (DSC). The results obtained at various heating rates (2–20°C min–1) were used to derive kinetic (E a and lnA) and thermodynamic ( H, C pand S parameters.It has been found that the activation energies of the dehydration and decomposition steps in N2 are generally larger than in H2 atmosphere, and the reverse is true for the enthalpy change of the decomposition. Thus, it has been concluded that the reductive decomposition (in H2) is easier than the thermal decomposition (in N2 or O2) of the chromate. Irrespective of the gas atmosphere applied, the eventual decomposition product was a mixture of -Cr2O3 and non-crystalline chromate species, -Cr2O3+x. Above 400°C in H2 atmosphere, more deoxygenation of the non-crystalline chromate takes place at high rates of heating to give -Cr2O3.

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