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:

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.

     

Kinetics and mechanism of the low-temperature oxidation of carbon monoxide with oxygen on a PdCl<Subscript>2</Subscript>–CuCl<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst

A systematic study of the kinetics of the low-temperature oxidation of carbon monoxide with oxygen on a PdCl₂–CuCl₂/γ-Al₂O₃ supported catalyst was carried out over a wide range of the partial pressures of oxygen, water, and CO in order to test hypotheses on the reaction mechanism. It was shown that, as the temperature was increased from 20 to 38°C, rate of formation of CO₂ decreased and the apparent activation energy was about–40 kJ/mol. The hypotheses of different degrees of complexity concerning the reaction mechanism were formulated based on physicochemical data and a Langmuir–Hinshelwood model. Mechanisms in which carbon dioxide is formed on the interaction of the surface Pd(I) and Pd(II) complexes that include carbon monoxide and water with the surface complex of Cu(I) that coordinates oxygen were recognized as the most probable.     

Synthesis of SnO<Subscript>2</Subscript>–MoO<Subscript>3</Subscript> solid solution and its properties in the reaction of methanol dehydrogenation

X-ray diffraction analysis, synchronous-thermal analysis, and IR spectroscopy were used to confirm the possibility of obtaining a tin–molybdenum solid solution by the closed-cycle technology via joint mechanochemical activation of tin oxide (SnO₂) and molybdenum oxide (h-MoO₃), followed by a thermal treatment at 650°C. An analysis of the catalytic properties demonstrated that various target reaction products (methyl formate and formaldehyde) can be obtained by varying the temperature of the methanol dehydrogenation process. It was found that the selectivity toward methyl formate reaches a maximum of 99.88% at a temperature of 180°C. Raising the temperature further results in that the selectivity toward formaldehyde increases and reaches its maximum value at 260°C.     

The sensor properties of SnO<Subscript>2</Subscript> · In<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposite oxides in the detection of hydrogen in air

The sensor properties of In₂O₃ · SnO₂ polycrystalline films having different compositions were studied in the detection of 2% hydrogen in air over the temperature range 330–530°C. Films containing 19% In₂O₃ were most sensitive to hydrogen. The temperature dependence of the sensitivity of sensors passed a maximum, the position of which depended on the composition of the film. The temperature at which sensor sensitivity was maximum decreased as the content of indium oxide increased. This temperature was 485°C for the SnO₂ film and 425°C for the In₂O₃ film. The response and relaxation times of sensors also decreased as the amount of In₂O₃ in the composite metal oxide film increased. Possible mechanisms of the sensor sensitivity of the films are discussed.     

Synthesis of hydroxy derivatives from adamantanecarboxylic acids in the system MnO<Subscript>2</Subscript>–H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A convenient procedure has been developed for the synthesis of mono- and dihydric cage alcohols from adamantanecarboxylic acids and their esters using the MnO₂–H₂SO₄ system. The reaction at elevated temperature involved both decarboxylation and decarbonylation of the initial acid or ester.     

On the Control of Plasma Parameters and Active Species Kinetics in CF<Subscript>4</Subscript> + O<Subscript>2</Subscript> + Ar Gas Mixture by CF<Subscript>4</Subscript>/O<Subscript>2</Subscript> and O<Subscript>2</Subscript>/Ar Mixing Ratios

The effects of both CF₄/O₂ and Ar/O₂ mixing ratios in three-component CF₄ + O₂ + Ar mixture on plasma parameters, densities and fluxes of active species determining the dry etching kinetics were analyzed. The investigation combined plasma diagnostics by Langmuir probes and zero-dimensional plasma modeling. It was found that the substitution of CF₄ for O₂ at constant fraction of Ar in a feed gas produces the non-monotonic change in F atom density, as it was repeatedly reported for the binary CF₄/O₂ gas mixtures. At the same time, the substitution of Ar for O₂ at constant fraction of CF₄ results in the monotonic increase in F atom density toward more oxygenated plasmas. The natures of these phenomena as well as theirs possible impacts on the etching/polymerization kinetics were discussed in details.     

Improving the visible-light photocatalytic activity of SnO<Subscript>x</Subscript>·SiO<Subscript>2</Subscript> glass systems by introducing SnO<Subscript>x</Subscript> nanoparticles

Tin silicate glass without SnO_x nanoparticles (SiO₂·SnO_x), a silica glass containing only SnO_x nanoparticles (SiO₂·SnO_xNP) and the improved product, which combines the tin silicate glass with SnO_x nanoparticles (SiO₂·SnO_x·SnO_xNP) was prepared. For the structural analysis ¹¹⁹Sn Mössbauer spectroscopy and X-ray diffraction were applied. The ¹¹⁹Sn Mössbauer spectra showed that the SiO₂·SnO_x·SnO_xNP sample had the largest Sn^II content (12.0%). It also had an outstanding methylene blue degradation with the first-order rate value with (18?±?2) × 10^?3 min^?1 with visible light irradiation.     

Kinetics and thermodynamics of thermal decomposition of NH<Subscript>4</Subscript>NiPO<Subscript>4</Subscript>·6H<Subscript>2</Subscript>O

The single phase NH₄NiPO₄·6H₂O was synthesized by solid-state reaction at room temperature using NiSO₄·6H₂O and (NH₄)₃PO₄·3H₂O as raw materials. XRD analysis showed that NH₄NiPO₄·6H₂O was a compound with orthorhombic structure. The thermal process of NH₄NiPO₄·6H₂O experienced three steps, which involves the dehydration of the five crystal water molecules at first, and then deamination, dehydration of the one crystal water, intramolecular dehydration of the protonated phosphate groups together, at last crystallization of Ni₂P₂O₇. In the DTA curve, the two endothermic peaks and an exothermic peak, respectively, corresponding to the first two steps’ mass loss of NH₄NiPO₄·6H₂O and crystallization of Ni₂P₂O₇. Based on Flynn–Wall–Ozawa equation, and Kissinger equation, the average values of the activation energies associated with the thermal decomposition of NH₄NiPO₄·6H₂O, and crystallization of Ni₂P₂O₇ were determined to be 47.81, 90.18, and 640.09 kJ mol⁻¹, respectively. Dehydration of the five crystal water molecules of NH₄NiPO₄·6H₂O, and deamination, dehydration of the crystal water of NH₄NiPO₄·H₂O, intramolecular dehydration of the protonated phosphate group from NiHPO₄ together could be multi-step reaction mechanisms. Besides, the thermodynamic parameters (ΔH ^≠, ΔG ^≠, and ΔS ^≠) of the decomposition reaction of NH₄NiPO₄·6H₂O were determined.     

State of active components on the surface of the PdCl<Subscript>2</Subscript>-CuCl<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the low-temperature oxidation of carbon monoxide

The state of the active constituents of the freshly prepared PdCl₂-CuCl₂/γ-Al₂O₃ catalyst for the low-temperature oxidation of the carbon monoxide by molecular oxygen was studied by X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). It was shown that copper in the form of a crystalline phase of Cu₂Cl(OH)₃ with the structure of the mineral paratacamite and palladium chloride in an amorphous state occurred on the surface of γ-Al₂O₃. According to XAS data, the local environment of palladium consisted of four chlorine atoms, which formed a flat square with an increased distance between palladium and one of the chlorine atoms. The evolution of the local environments of copper and palladium upon a transition from the initial salts to the impregnating solutions and chlorides on the surface of γ-Al₂O₃ was considered. The role of γ-Al₂O₃ in the formation of the Cu₂Cl(OH)₃ phase was discussed. It was found by the DRIFTS method that linear (2114 cm⁻¹) and bridging (1990 and 1928 cm⁻¹) forms of coordinated carbon monoxide were formed upon the adsorption of CO on the catalyst surface. The formation of CO₂ upon the interaction of coordinated CO with atmospheric oxygen was detected. Active sites including copper and palladium were absent from the surface of the freshly prepared catalyst.     

Preparation and Photocatalytic Activity of SnO<Subscript>2</Subscript>@TiO<Subscript>2</Subscript> Core–Shell Composites Modified by Ag

Photocatalytic degradation is an important method to mediate organic pollution in the environment. This article reports Ag-modified SnO₂@TiO₂ core–shell composite photocatalysts prepared via a hydrothermal method. The Ag modification and core structure in the composite enhanced the photocatalytic activity and stability of TiO₂ in Rhodamine B degradation under visible light irradiation. The composite modified in 0.15 M AgNO₃ showed an optimal level of photocatalytic activity, as it degraded 99.14 % Rhodamine B in 60 min while pure TiO₂ only degraded 45.7 % during the same time.     

Electrochemical and kinetic behavior of the MnO<Subscript><Emphasis Type="Italic">X</Emphasis></Subscript>/C–β-PbO<Subscript>2</Subscript> electrode

The MnO_X/C–-PbO₂ electrode was obtained by admixture of -PbO₂ to MnO_X/C. Slow scan voltammetry shows a double reduction peak which is discussed. Galvanostatic intermittent titration technique curves are also used and the value of the proton diffusion coefficient, D(H⁺), is determined. All studies are made in 2 M KOH.Presented at the 3rd International Meeting on Advanced Batteries and Accumulators, 16–20 June 2002, Brno, Czech Republic     

Dissociation enthalpies and phase equilibrium for TBAB semi-clathrate hydrates of N<Subscript>2</Subscript>, CO<Subscript>2</Subscript>, N<Subscript>2</Subscript> + CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> + CO<Subscript>2</Subscript>

Tetra-n-butyl ammonium bromide (TBAB) semi-clathrate (sc) hydrates of gas are of prime importance in the secondary refrigeration domain and in the separation of gas molecules by molecular size. However, there is a scarcity of dissociation enthalpies under pressure of pure gases and gases mixtures for such systems. In addition, the phase equilibrium of TBAB sc hydrates of several pure gases is not well defined yet as a function of the TBAB concentration and as a function of the pressure. In this paper, dissociation enthalpies and the phase equilibrium of TBAB sc hydrates of gas have been investigated by differential scanning calorimetry (DSC) under pressure. Pure gases such as N₂ and CO₂ and gases mixtures such as N₂ +  CO₂ and CH₄ +  CO₂ were studied. To our knowledge, we present the first phase diagram of TBAB sc hydrates of N₂ for different pressures of gas in the TBAB concentration range from 0.170 to 0.350 wt. Enthalpies of dissociation of TBAB sc hydrates of pure gases and gases mixtures were determined as a function of the presssure for a compound with a congruent melting point whose hydration number corresponds to 26.     

Comparison of the Active Species in the RF and Microwave Flowing Discharges of N<Subscript>2</Subscript> and Ar–20 %N<Subscript>2</Subscript>

We report a detailed comparison between RF and microwave (HF) plasmas of N₂ and Ar–20 %N₂ as well as in the corresponding afterglows by comparing densities of active species at nearly the same discharge conditions of tube diameter (5–6 mm), gas pressure (6–8 Torr), flow rate (0.6–1.0 slm) and applied power (50–150 W). The analysis reveals an interesting difference between the two cases; the length of the RF plasma (~25 cm) is measured to be much longer than that of HF (6 cm). This ensures a much longer residence time (10^?2 s) of the active species in the N₂ RF plasma [compared to that (10^?3 s) of HF], providing a condition for an efficient vibrational excitation of N₂(X, v) by (V–V) climbing-up processes, making the RF plasma more vibrationally excited than the HF one. As a result of high V–V plasma excitation in RF, the densities of the vibrationally excited N₂(X, v > 13) molecules are higher in the RF afterglow than in the HF afterglow. Destruction of N₂(X, v) due to the tube wall is estimated to be very similar between the two system as can be inferred from the γ_v destruction probability of N₂(X, v > 3–13) on the tube wall (2–3 × 10^?3 for both cases) obtained from a comparison between the density of N₂(X, v > 3–9) in the plasmas to that of the N₂(X, v > 13) in the long afterglows. Interestingly enough, densities of N-atoms and N₂(A) metastable molecules in the afterglow regions, however, are measured to be very similar with each other. The measured lower density of N₂ ⁺ ions than expected in the HF afterglow is rationalized from a high oxygen impurity in our HF setup since N₂ ⁺ ions are very sensitive to oxygen impurity .     

Electrosynthesis of Н<Subscript>2</Subscript>О<Subscript>2</Subscript> from О<Subscript>2</Subscript> in a Gas-Diffusion Electrode Based on Mesostructured Carbon CMK-3

Mesostructured carbon CMK-3 (Carbon Mesostructured by KAIST) synthesized by the template method is studied as the electrocatalyst for electrosynthesis of Н₂О₂ from О₂ in a gas-diffusion electrode (GDE) in alkaline and acidic solutions. The texture characteristics of the original material and its mixture with hydrophobizer (polytetrafluoroethylene) are studied by the method of low-temperature nitrogen adsorption. The rate constants for hydrogen peroxide decomposition on these materials in alkaline and acidic solutions are calculated. Kinetic parameters of oxygen reduction in alkaline and acidic solutions are determined as well as the capacitance of gas-diffusion electrodes based on mesocarbon. The selectivity of the electrocatalyst is estimated by finding the current fracture γ consumed in oxygen reduction to hydrogen peroxide. Data on the kinetics of hydrogen peroxide accumulation during electrosynthesis of Н₂О₂ from О₂ are obtained. The acidic solution of hydrogen peroxide with the concentration more than 3 M is obtained with the current efficiency higher than 80%.     

Phase equilibria in the Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–CdO system and the thermal stability of Cd<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>7</Subscript> and CdNb<Subscript>2</Subscript>O<Subscript>6</Subscript>

Phase equilibria were studied in the Nb₂O₅–CdO system in the Nb₂O₅-rich region including CdNb₂O₆ and Cd₂Nb₂O₇. It was determined that CdNb₂O₆ and Cd₂Nb₂O₇ in air are stable to 1150 and 1120°C, respectively, and that, above these temperatures, there is solid-phase decomposition of niobates with CdO release in the gas phase. Along with the cadmium oxide evaporation, the Cd₂Nb₂O₇ decomposition is accompanied by the formation of cadmium metaniobate CdNb₂O₆ and the CdNb₂O₆ decomposition results in the formation of niobium oxide Nb₂O₅. No thermal events were observed in the differential thermal analysis curve for a 1: 1 CdNb₂O₆–Cd₂Nb₂O₇ mixture heated to 1100°C in air, which suggests that there are neither phase transformations in cadmium niobates, nor a eutectic within this temperature and concentration ranges. A study of the morphology of compacted samples of niobates determined specific conditions for producing dense composite ceramics, a mixture of niobates, that is suitable for using as a dielectric material.     

Study of kinetics and thermodynamics of the dehydration reaction of AlPO<Subscript>4</Subscript> · H<Subscript>2</Subscript>O

The kinetics and thermodynamics of the thermal dehydration of aluminum phosphate monohydrate, AlPO₄ · H₂O were studied using thermogravimetry (TG-DTG-DTA) at four heating rates in dry air atmosphere. The activation energies of the dehydration step of AlPO₄ · H₂O were calculated through the methods of Friedman (FR) and Flynn–Wall–Ozawa (FWO) and the possible conversion function has been estimated through the Achar and Li–Tang equations. The independent activation energies on extent of conversions and the better kinetic model of the dehydration reaction for AlPO₄ · H₂O indicate single kinetic mechanism and the F _2.05 model as a simple n-order reaction of “chemical process or mechanism no-invoking equation”, respectively. The positive values of ΔH^# and ΔG^# for the dehydration reaction show that it is endothermic and non-spontaneous process and it is connected with the introduction of heat. The kinetic and thermodynamic functions calculated for the dehydration reaction by different techniques and methods were found to be consistent.     

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.     

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.