Crystallization study of Se<Subscript>86</Subscript>Sb<Subscript>14</Subscript> glass

The present study concerns with high-accuracy determination of crystallization activation energy (\(E_{\text{c}}\)), the frequency factor (\(k_{0}\)), the kinetic exponent (n) for Se₈₆Sb₁₄ glass. Different three methods have been used to investigate the \(E_{\text{c}} \,{\text{and}}\,k_{0 }\) values. It was found that the deduced value of k ₀ based on Kissinger’s method is too small compared with the others. Therefore, it can’t be used to investigate k ₀ value. Where \(E_{\text{c}} \,{\text{and}}\,k_{0}\) values are already known, the overall reaction rate \(k = k_{0 } { \exp }\left( { - E_{\text{c}} /\left( {R \cdot T} \right)} \right)\) at any temperature can be calculated. Now, Avrami’s equation (\(\chi = 1 - { \exp }\left( { - \left( {kt} \right)^{\text{n}} } \right)\)) contains only one unknown which is the kinetic exponent (n). This method enables us to determine n value without any approximations. The values’ crystallization fraction \((\chi_{\text{th}} )\) that theoretically calculated is the same as that experimentally investigated \((\chi_{{{ \exp } .}} )\).     

Thermodynamic Protonation Parameters of some Sulfur-Containing Anions in NaCl<Subscript>aq</Subscript> and (CH<Subscript>3</Subscript>)<Subscript>4</Subscript>NCl<Subscript>aq</Subscript> at <Emphasis Type="Italic">t</Emphasis>=25 °C

Protonation constants of one thiocarboxylate (thioacetate) and four sulfur-containing carboxylates (2-methylthioacetate, thiolactate, thiomalate, 3-mercaptopropionate) were determined by potentiometric measurements in a wide ionic strength range [0≤I≤5 mol⋅L⁻¹ in NaCl and 0 ≤I≤3 mol⋅L⁻¹ in (CH₃)₄NCl] at t=25 °C. For two of these ligands (2-methylthioacetate and thiolactate), the protonation enthalpies were also determined by calorimetric measurements in NaCl ionic medium [0 ≤I≤5 mol⋅L⁻¹] at t=25 °C. Individual UV spectra of the protonated and unprotonated 3-mercaptopropionate species, together with values of the protonation constants, were obtained by spectrophotometric titrations. Results were analyzed in terms of their dependence on the ionic medium by using different thermodynamic models [Debye-Hückel type, SIT (Specific ion Interaction Theory) and Pitzer’s equations]. Differences among protonation constants obtained in different media were also interpreted in terms of weak complex formation.     

Isopiestic Determination of the Osmotic Coefficients and Pitzer Model Representation for the Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>+LiCl + H<Subscript>2</Subscript>O System at <Emphasis Type="Italic">T</Emphasis>=298.15 K

Isopiestic molalities and water activities have been measured for the Li₂B₄O₇+LiCl + H₂O system at T=298.15 K using an improved isopiestic apparatus. Two types of osmotic coefficients, φ _S and φ _E, were determined, where φ _S is based on the stoichiometric molalities of the solute Li₂B₄O₇(aq) and φ _E is based on equilibrium molalities calculated by consideration of the equilibrium speciation of Li₂B₄O₇ to partially form H₃BO₃, B(OH)₄⁻ and B₃O₃(OH)₄⁻. The stoichiometric equilibrium constant K _m for the aqueous speciation reaction was estimated. Two representations of the osmotic coefficients of Li₂B₄O₇ + LiCl + H₂O were made with Pitzer’s ion-interaction model. Model (1) involved representing the φ _S values with six parameters based on considering the ionic interactions between Li⁺, Cl⁻, and B₄O₇²⁻; and model (2) involved representing the φ _E values based on the calculated equilibrium speciation. Reasonable agreements were obtained between the experimental osmotic coefficient data and those calculated using the above models, with standard deviations of 0.075 and 0.0229, respectively, for these two models. The thermodynamic osmotic coefficients for the complex system containing polymeric boron anions and lithium cation was modelled and explained by use of Pitzer’s ion-interaction model, with minor modifications in combination with speciation reaction equilibria.     

Adsorption of chlorobenzene vapor on V<Subscript>2</Subscript>O<Subscript>5</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst under dynamic conditions

Adsorption dynamics of chlorobenzene vapors on a 5% V₂O₅/Al₂O₃ catalyst has been investigated using the frontal chromatography technique. The uptakes of chlorobenzene have been measured as a function of vapor concentration and adsorption equilibrium has been found to follow formally the Langmuir isotherm. The breakthrough time proved to be a linear function of the column length as expected. Breakthrough profiles have been reported for different experimental conditions and quantitatively fitted by a reduced lumped diffusion model. This model provides an analytical solution that facilitates engineering calculations. Model parameters show complex behavior as functions of stream characteristics and depend on column length. When empirical expressions relating model mass transfer coefficients with influencing variables are found the model demonstrates good accuracy in predicting column performance.     

On the charge storage mechanism at RuO<Subscript>2</Subscript>/0.5 M H<Subscript>2</Subscript>SO<Subscript>4</Subscript> interface

Comparative study of capacitative properties of RuO₂/0.5 M H₂SO₄ and Ru/0.5 M H₂SO₄ interfaces has been performed with a view to find out the nature of electrochemical processes involved in the charge storage mechanism of ruthenium (IV) oxide. The methods of cyclic voltammetry and scanning electron microscopy (SEM) were employed for the investigation of electrochemical behavior and surface morphology of RuO₂ electrodes. It has been suggested that supercapacitor behavior of RuO₂ phase in the potential E range between 0.4 and 1.4 V vs reference hydrogen electrode (RHE) should be attributed to double-layer-type capacitance, related to non-faradaic highly reversible process of ionic pair formation and annihilation at RuO₂/electrolyte interface as described by following summary equation:

Rapid synthesis,kinetics and thermodynamics of binary Mn<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>2</Subscript> · H<Subscript>2</Subscript>O

The non-isothermal kinetics of dehydration of AlPO₄·2H₂O was studied in dynamic air atmosphere by TG–DTG–DTA at different heating rates. The result implies an important theoretical support for preparing AlPO₄. The AlPO₄·2H₂O decomposes in two step reactions occurring in the range of 80–150 °C. The activation energy of the second dehydration reaction of AlPO₄·2H₂O as calculated by Kissinger method was found to be 69.68 kJ mol⁻¹, while the Avrami exponent value was 1.49. The results confirmed the elimination of water of crystallization, which related with the crystal growth mechanism. The thermodynamic functions (ΔH*, ΔG* and ΔS*) of the dehydration reaction are calculated by the activated complex theory. These values in the dehydration step showed that it is directly related to the introduction of heat and is non-spontaneous process.     

Mass attenuation coefficients of B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–CaF<Subscript>2</Subscript> glass system at 0.662 and 1.25 MeV gamma energies

The total mass attenuation coefficients, partial interaction and the effective atomic numbers (Z_eff) of glass system (80−x)B₂O₃–10Al₂O₃–10SiO₂–xCaF₂ (where x = 5, 10, 20, 25, 30, 35 and 40 mol %) have been calculated at photon energies 0.662 and 1.25 MeV using the WinXCom software on the basis of mixture rule. Results indicated that the total mass attenuation coefficients showed a decrease with increasing the CaF₂ concentration, due to a decrease in Compton scattering probability, which gave a dominant contribution to the total mass attenuation coefficients for the studied glass samples at both energies. However, the photoelectric absorption and coherent scattering showed an increase with increasing the CaF₂, concentrations at same energies. For a comparison, the total mass attenuation coefficients of the glass system had lower values at the energy 1.25 MeV than that at 0.662 MeV. Z_eff was found to increase linearly with the increase of CaF₂ concentrations. It was concluded that low CaF₂ concentrations in glass system, under study, have Z_eff close to that of human tissue and have higher total absorption coefficients at energy of 0.662 MeV than that at 1.25 MeV. These results are very useful in designing gamma radiation detectors using thermoluminescence technique. Therefore, it is recommended to use low CaF₂ concentration of our glass system as good gamma detectors at energy of 1.25 MeV.     

The effect of Me<Subscript>4</Subscript>NBr,Et<Subscript>4</Subscript>NBr,Bu<Subscript>4</Subscript>NBr,and (EtOH)<Subscript>3</Subscript>EtNBr on the Temperature of Maximum Density of Water

The densities of aqueous solutions of Me₄NBr, Et₄NBr, Bu₄NBr, and Et(OH)₃EtNBr were measured in the concentration range 0.002 to 0.05 mol⋅kg⁻¹. The temperature of the determinations ranged from 275.15 to 279.15 K in 0.5 K steps, and the uncertainty of the densities was around ±1×10⁻⁶ g⋅cm⁻³. Eleven concentrations were used for each of the salts. It was found that all the solutes follow Despretz’ law. The absolute value of the Despretz’s constants increases with increasing number of carbon atoms in the cation, except for Et(OH)₃EtNBr which has the highest value. The ionic contributions to the Despretz’s constants were calculated. The volumetric data obtained allows the calculation proposed by Kalgud and Pokale. The effective ionic radii were calculated using a semi-empirical equation, as proposed previously by several workers. The nonlinearity of the plot of the ionic Despretz constants versus effective ionic radius is confirmed.     

Thermodynamic Model for ThO<Subscript>2</Subscript>(am) Solubility in Alkaline Silica Solutions

No thermodynamic data for Th complexes with aqueous Si are available. To obtain such data, extensive studies on ThO₂(am) solubility were carried out as functions of: (1) a wide range of aqueous silica concentrations (0.0004 to 0.14 mol⋅L⁻¹) at fixed pH values of about 10, 11, 12, and 13; and (2) and variable pH (ranging from 10 to 13.3) at fixed aqueous Si concentrations of about 0.006 mol⋅L⁻¹ or 0.018 mol⋅L⁻¹. The samples were equilibrated over long periods (ranging up to 487 days), and the data showed that steady-state concentrations were reached in < 29 days. X-ray diffraction, FTIR, and Raman analyses of the equilibrated solid phases showed that the Th solids were amorphous ThO₂(am) containing some adsorbed Si. The solubility of ThO₂(am) at pH values ranging from 10 to 13.3 at fixed 0.018 mol⋅L⁻¹ aqueous Si concentrations decreases rapidly with an increase in pH, and increases dramatically with an increase in Si concentrations beyond about 0.003 mol⋅L⁻¹ at fixed pH values > 10. The data were interpreted using both the Pitzer and SIT models, and required only the inclusion of one mixed-hydroxy-silica complex of Th [Th(OH)₃(H₃SiO₄)₃²⁻]. Both models provided similar complexation constant values for the formation of this species. Density functional theory calculations predict complexes of this stoichiometry, having six-fold coordination of the Th cation, to be structurally stable. Predictions based on the fitted value of log ₁₀ K ⁰=−18.5±0.7 for the ThO₂(am) solubility reaction involving Th(OH)₃(H₃SiO₄)₃²⁻[ThO₂(am)+3H₄SiO₄+H₂O↔Th(OH)₃(H₃SiO₄)₃²⁻+2H⁺], along with the thermodynamic data for aqueous Si species reported in the literature, agreed closely with the extensive experimental data and showed that under alkaline conditions aqueous Si makes very strong complexes with Th.     

Solubility and Ionic Interaction of the CaF<Subscript>2</Subscript>–CaSO<Subscript>4</Subscript>–H<Subscript>2</Subscript>O System at 298.1 K

Solubility isotherms of the sparingly soluble salts CaF_2(s) and CaSO₄·2H₂O_(s) in their mixed aqueous solutions have been measured at 298.1 K. It was found that the CaF_2(s) solubility decreases with increasing CaSO₄ concentration in the solution and reaches about 1/3 of the CaF_2(s) solubility in pure water in the CaSO₄·2H₂O_(S) saturated solution. A thermodynamic model was developed to predict the CaF_2(s) solubility isotherm in this system, in which the short range interactions of the species in the aqueous solution are represented by ion-association constants reported in literature, and the long range interaction, i.e., the electrostatic term, is represented by the well known Davies equation. The predicted solubility isotherm reasonably agrees with the experimental results. The contributions of the long-range term and the short-range term to the calculated solubility isotherm were investigated. It was concluded that the ionic association combining with the Davies equation is sufficient to represent the excess interaction of the CaF₂ + CaSO₄ aqueous solution at 298.15 K. This model approach could be applicable for other dilute mixed electrolyte systems in which component activity coefficients are lacking and model parameters are difficult to determine.     

A kinetic and statistical model for carbon deposition on Ni/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in the steam methane reforming

An experimental and statistical study was performed for the carbon deposition on Ni/Al₂O₃ catalyst in the methane steam reforming process. Carbon deposition plays a significant role in the catalyst deactivation. Thus, applying a statistical model and a kinetic rate for carbon deposition is so valuable. The central composite design (CCD) was used for the modeling of the carbon deposition process. The statistical analysis of model, as obtained from the CCD method, revealed that a polynomial equation with the F-value = 456.94, the p value < 0.0001, and the R ² = 0.9919, could appropriately predict the experimental data. Based on the established models, an increase in steam to methane ratio (S/C) caused carbon deposition sharply decreased. As pressure increased from 1.81 to 4.19 bar, carbon deposition slightly increased. When temperature varied from 540 to 600 °C, whisker carbon was produced and its activity increased with temperature. As temperature exceeded 600 °C, carbon deposition slightly increased that can be attributed to formation of pyrolytic carbon. The minimum of carbon deposition was occurred in low pressure, high S/C and at 600 °C. So, the kinetic rate of carbon deposition was suggested in these conditions using generalized reduced gradient nonlinear method. The proposed kinetic rate of methane decomposition reaction can accurately predict the experimental rate data.     

Non-isothermal decomposition kinetics of synthetic serrabrancaite (MnPO<Subscript>4</Subscript> · H<Subscript>2</Subscript>O) precursor in N<Subscript>2</Subscript> atmosphere

The thermal decomposition of synthetic serrabrancaite (MnPO₄ · H₂O) was studied in N₂ atmosphere using TG-DTG-DTA. Thermal analysis results indicate that the decomposition occurs in two stages, which are assigned to the dehydration and the reduction processes and the final product is Mn₂P₂O₇. X-ray powder diffraction, FT-IR and FT-Raman techniques were used for identification of the solid decomposition product. The decomposition kinetics analysis of MnPO₄ · H₂O was performed under non-isothermal condition through isoconversional methods of Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS). The dependences of activation energies on the extent of conversions are observed in the dehydration and the reduction reactions, which could be concluded the “multi-step” processes.     

Sol–gel synthesis and characterization of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> · CeO<Subscript>2</Subscript> doped with Pr ceramic pigments

Undoped x · α-Fe₂O₃ y · CeO₂ and doped with praseodymium ceramic pigments were obtained by the sol–gel method after heat treatment at 800 °C for 2 h. These pigments were characterized by XRD, nitrogen adsorption, scanning electron microscopy, ultraviolet-visible absorption spectroscopy and colorimetrical measurements. Red and brown colors with several tonalities were observed after changes with Ce and Pr concentration.     

Thermoanalytical and NMR investigation of NaBH<Subscript>4</Subscript> × 2H<Subscript>2</Subscript>O thermolysis process

This paper describes the thermal investigations and kinetic analysis regarding the solid-state degradation of three compounds used as mental disorder therapeutic agents (antidepressants), namely amitriptyline, desipramine and imipramine. The study was carried according to ICTAC 2000 recommendations, by using three isoconversional methods, namely Flynn–Wall–Ozawa, Kissinger–Akahira–Sunose and Friedman. The differential method of Friedman indicated multistep degradation, which was later confirmed by the nonparametric kinetic method (NPK). NPK method showed that all three tricyclic antidepressants are degraded by two processes. In terms of apparent activation energies for decomposition, the NPK method indicated 123.4 kJ mol^?1 for imipramine, 112.3 kJ mol^?1 for desipramine and 82.9 kJ mol^?1 for amitriptyline, and the results are in good agreement with the ones suggested by isoconversional methods.     

Kinetics for the Oxygen Evolution Reaction and Application of the Ti/SnO<Subscript>2</Subscript> + RuO<Subscript>2</Subscript> + MnO<Subscript>2</Subscript> Electrode

A Ti/SnO₂ + RuO₂ + MnO₂ electrode was prepared by thermal decomposition of their salts. Results from SEM and XPS analyses, respectively, indicate that the coating layer exhibits a compact structure and the oxidation state of Mn in the coating layer is +IV. The experimental activation energy for the oxygen evolution reaction, which increased linearly with increasing overpotential, is about 8 kJ⋅mol⁻¹ at the equilibrium potential (η=0). The electrocatalytic characteristics of the anode are discussed in terms of ligand substitution reaction mechanisms (Sn1 and Sn2). It was found that the transition state for oxygen evolution at the anode in acidic solution follows a dissociative mechanism (Sn1 reaction). The Ti/SnO₂ + RuO₂ + MnO₂ anode in conjunction with UV illumination was used to degrade phenol solutions, where the concentration of phenol remaining was determined by high-performance liquid chromatography (HPLC). The results indicate that the degradation efficiency of phenol on the anode can reach 96.3% after photoelectrocatalytic oxidation for 3 h.     

A magnetic CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposite fabricated by the sol-gel method for electrocatalytic oxidation and determination of L-cysteine

Magnetic CoFe₂O₄/SiO₂ spinel-type nanocomposites have been fabricated by a sol-gel method in the presence of various acids. Their structural, morphological and magnetic properties were characterized by XRD, SEM, TEM, FTIR, VSM and EDX which revealed that they are formed in the presence of all precursors. TEM analysis indicates homogeneous and porous spherical morphology with nanosize grains 10–20 nm in diameter in the presence of salicylic acid. Electrochemical sensor application of nanocrystalline CoFe₂O₄/SiO₂ synthesized by salicylic acid for determination of L-cysteine was investigated using differential pulse voltammetry (DPV) and cyclic voltammetry (CV). DPV indicates that the sensor shows remarkable sensitivity for the determination of L-Cys. The response of a glassy carbon electrode modified with CoFe₂O₄/SiO₂ is linear in the 0.02–425 μM L-Cys concentration range, with a 0.20 μM detection limit (at an S/N ratio of 3). The electrode produces a negligible current response for tryptophan, glutamic acid and citric acid at the working potential applied (+0.748 V vs Ag/AgCl). The electrode is reliable, simple, rapidly prepared, precise, and the method does not require extensive sample treatment.

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Graphical abstract A CoFe₂O₄/SiO₂ magnetic nanocomposite was synthesized by a sol-gel auto combustion method in the presence of various acids as precursors. A highly sensitive electrochemical sensor was fabricated for determination of trace amount of L-cysteine using a glassy carbon electrode modified with the nanocomposite.

     

Some thermodynamic functions and kinetics of thermal decomposition of NH<Subscript>4</Subscript>MnPO<Subscript>4</Subscript> · H<Subscript>2</Subscript>O in nitrogen atmosphere

The ammonium manganese phosphate monohydrate (NH₄MnPO₄ · H₂O) was found to decompose in three steps in the sequence of: deammination, dehydration and polycondensation. At the end of each step, the consecutive one started before the previous step was finished. The thermal final product was found to be Mn₂P₂O₇ according to the characterization by X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy. Vibrational frequencies of breaking bonds in three stages were estimated from the isokinetic parameters and found to agree with the observed FTIR spectra. The kinetics of thermal decomposition of this compound under non-isothermal conditions was studied by Kissinger method. The calculated activation energies E_a are 110.77, 180.77 and 201.95 kJ mol⁻¹ for the deammination, dehydration and polycondensation steps, respectively. Thermodynamic parameters for this compound were calculated through the kinetic parameters for the first time.