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A series of silica-supported nickel catalyst precursors was synthesized with different SiO2/Ni molar ratios. Reduction of Ni catalyst precursors with different SiO2/Ni molar ratios under a hydrogen atmosphere was investigated at different heating rates. Kinetic parameters were determined using Kissinger–Akahira–Sunose isoconversional and invariant kinetic parameter methods. It was found that for all molar ratios, the apparent activation energy (Ea) is practically constant in the conversion range of 0.20 ≤ α ≤ 0.80. In the considered conversion range, following values ofEawere found: 134.5 kJ mol−1(SiO2/Ni = 0.20), 139.6 kJ mol−1(SiO2/Ni = 0.80), and 128.3 kJ mol−1(SiO2/Ni = 1.15). It was established that the reduction of Ni catalyst precursors with different SiO2/Ni molar ratios is a complex process and can be described by the Šesták–Berggren autocatalytic model. It was found that the reaction is more Langmuir–Hinshelwood type, as hydrogen dissociates rapidly on surface nuclei and the dissociated hydrogen reacts with the Ni–O active system. It was concluded that the reduction process proceeds through bulk nucleation, which is a dominant mechanism, where three-dimensional growth of crystals with polyhedron-like morphology exists. It was found that the Ni/Si ratio decreases after the reduction process. This has been explained by low Ni and higher Si surface concentrations. It has been disclosed that Ni dispersion decreases. [ABSTRACT FROM PUBLISHER]

Thermal decomposition of zinc leach residue has been studied in a tubular furnace under a constant nitrogen gas flowing, at four different operating temperatures (600°C, 750°C, 950°C, and 1150°C). Using a detailed kinetic analysis, it was shown that the investigated process can be described by a two-parameter autocatalytic Šesták–Berggren reaction model. It was noted that the apparent activation energy values Eaincrease progressively with a degree of conversion, accompanied by the appearance of a convex Arrhenius dependence. This behavior is a characteristic of a system of parallel competing reactions. It was concluded that the investigated isothermal decomposition process is characterized by unusually very low preexponential factor and low values of the apparent activation energy. Based on the derived density distribution function of Eavalues, it was concluded that the isothermal decomposition process probably occurs through four reaction steps, where each step is characterized by one parallel reaction. [ABSTRACT FROM PUBLISHER]

Isothermal degradation process of commercial Creosote was analyzed by the thermogravimetric (TG) technique in a nitrogen atmosphere, at four different operating temperatures (230, 250, 270 and 290°C). The kinetic triplet [Ea,Aandf(α)] and the thermodynamic parameters (ΔH≠, ΔS≠and ΔG≠) for investigated Creosote samples were calculated. It was found that two-parameter autocatalytic Šesták-Berggren (SB) kinetic model best describes the process, but in the form of accommodation function with phenomenological character. Applying the multiplicative factor, the true value of activation energy (Etruea) was calculated. The experimental density distribution function of the apparent activation energy values was evaluated from isoconversional kinetic analysis. Based of the characteristic shape of distribution curve, it was concluded that the isothermal degradation of Creosote represents a complex physico-chemical process, given the chemical structure of the studied system. It is assumed that the considered process probably includes primary and secondary (autocatalytic) pyrolysis reactions, together with various decomposition reactions and radicals recombination pathways. The objective of the presented work is the proof of principle of the pyrolysis-based thermo-chemical conversion technologies for the production of value-added chemicals from the complex organic compounds, which even include chemical contaminants (such as PAHs). Also, the present work allows us that by using a unified kinetic approach we can obtain a significant physico-chemical characteristics of the tested system, which can then be used in the procedure for the separation of organics from creosote-treated woods and creosote-contaminated soils. The significance of this research is to identify the global kinetic behavior of some target contaminant compounds for pyrolysis, which are primarily PAHs. [ABSTRACT FROM AUTHOR]

Tritium activity concentration were monitored in monthly precipitation at five locations in Serbia (Meteorological Station of Belgrade at Zeleno Brdo, Vinča Institute of Nuclear Sciences, Smederevska Palanka, Kraljevo and Niš) over 2005, using electrolytic enrichment and liquid scintillation counting. The obtained concentrations ranged from 3.36 to 127.02 TU. The activity values obtained in samples collected at Zeleno Brdo were lower or close to the minimum detectable activity (MDA), which has a value of 3.36 TU. Significantly higher tritium levels were obtained in samples collected in Vinča Institute of Nuclear Sciences compared with samples from the other investigated locations. Amount of precipitation were also recorded. A good linear correlation (r = 0.75) for Zeleno Brdo and VINS between their tritium activity was obtained. It was found that the value of the symmetrical index n (which indicates the magnitude of tritium content changes with time (months) through its second derivative) is the highest for Vinča Institute of Nuclear Sciences compared to other locations, which is in accordance with the fact that the highest concentrations of tritium were obtained in the samples from the cited place. [ABSTRACT FROM AUTHOR]

Aim: The purpose of this article is to investigate the thermal stability and nonisothermal kinetics of Folnak® drug degradation process using different thermoanalytical techniques. Methods: The nonisothermal degradation of Folnak® powder samples was investigated by simultaneous thermogravimetry–differential thermal analysis, in the temperature range from ambient to 810°C. Results: It was found that the degradation proceeds through five reaction stages, which include the dehydration, the melting process of excipients, the decomposition of folic acid, corn starch, and saccharose. The presence of compounds such as excipients increases the thermal stability of the drug and some kind of solid–solid and/or solid–gas interaction occurs. Conclusion: It was concluded that the main degradation stage of Folnak® sample represents the decomposition of folic acid. It was established that the folic acid decomposition cannot be explained by simple reaction order model ( n = 1) but with the complex reaction mechanism that includes higher reaction orders ( n > 1). The isothermal predictions of the folic acid decomposition at four different temperatures ( Tiso = 180°C, 200°C, 220°C, and 260°C) were established. It was concluded that the shapes of conversion curves at lower temperatures (180–200°C) were similar, whereas they became more complex with further temperature increase because of the complexity of the decomposition reaction. [ABSTRACT FROM AUTHOR]