Kinetics and mechanism of the Al(iii)/Al electrode reaction in cryolite-alumina melts

Using the relaxation method (RM) with galvanostatic perturbation and electrochemical impedance spectroscopy (EIS), exchange current densities and activation parameters were determined for the electrode reaction on the aluminium electrode in pure cryolite melt and in cryolite-alumina melts with the addition of 2–12 wt % Al₂O₃. In all these melts a three step electrode process was observed, comprising a preceding chemical reaction followed by two charge transfer steps. The exchange current densities for two charge transfer steps were determined as a function of temperature, together with the equilibrium constant of the preceding chemical reaction and its kinetic and diffusion impedance. The third step was found to be independent of diffusion and of the concentration of alumina, whereas the second step showed mixed characteristics. The exchange current densities were of the order of 5–15 A cm^–2.

Abbreviations

List of symbols A electrode area (cm²) - A, B, C coefficients of Equation 12 (V) - C _dl double layer capacitance (F) - c concentration (mol cm^–3) - D _O; D _R diffusion coefficients of the oxidized and reduced ionic species (cm² S^–1) - E _A activation energy (kJ mol^–1) - F Faraday constant (C mol^–1) - j exchange current density (A cm^–2. - K equilibrium constant (dimensionless) - k sum of the forward (k ₁) and backward (k ₂) rate constants (s^–1) - k ₀ standard rate constant (cm s^–1) - L ₁, L ₂ high frequency and low frequency inductances (H) - L _out outer inductance (H) - R _el electrolyte resistance () - n number of electrons (dimensionless) - R molar gas constant (J mol^–1 K^–1) - R _ct charge transfer resistance () - s ₁, s ₂ angular frequencies (s^–1) - T temperature (K) - t time (s) - Z complex impedance () - Z, Z real and imaginary parts of the complex impedance () - Z _w Warburg diffusion impedance ( s^–1/2) Greek symbols _c overall cathodic transfer coefficient (dimensionless) - , coefficients in Equation 12 (s^–1/2) - coefficient in Equation 12 (s^–1) - overpotential (V) - angular frequency (s^–1) - _O, _R Warburg coefficients of the oxidized and reduced ionic species ( s^–1/2)     

Stability of a boron-doped diamond electrode in molten chloride systems

Stability of a boron-doped diamond as an oxygen evolution electrode material was evaluated at 773 K in molten LiCl–KCl (58.5:41.5 mol%), LiCl–KCl (75:25 mol%), LiCl–CaCl₂ (64:36 mol%), LiCl–NaCl–CaCl₂ (52.3:13.5:34.2 mol%). In molten LiCl–KCl systems, the BDD is stable at 773 K regardless of the concentration of oxide ion and the composition of the melt. In molten LiCl–CaCl₂ and LiCl–NaCl–CaCl₂, the BDD electrode is less stable than in molten LiCl–KCl systems.     

The use of molten magnesium chloride in the preparation of crystalline ceramic powders

The interaction of molten MgCl₂ with various oxides and oxide minerals is described. Whithin several hours, at temperatures of 700–1100°C, components such as oxides of Fe, K, Ca and — under proper conditions — Ti are largely converted into the corresponding chlorides, the more volatile of which evaporate, the others being extracted into the liquid phase. By proper choice of stoichiometry, crystalline cordlerite, forsterite, enstatite and spinal could be prepared; they contained a total < 0.2% of oxides of Na, K, Ca and Fe, and their average-diameter range was usually 1–10 μm (or larger for forsterite). The preparation of other microcrystalline powders in molten MgCl₂ is briefly discussed.     

Kinetics and mechanism of urea-formaldehyde reaction

The various initial reactions in the urea-formaldehyde reaction have been isolated and the course of the reaction investigated using a quantitative thin-layer chromatographic technique developed for the purpose. The rate constants for the formation of monomethylol urea pass through a minimum in the pH range 4.5–8, thereby proving catalysis by H⁺ and OH^?. Whereas the reaction under alkaline conditions leads to methylol formation, acidic conditions favour formation of methylene bridges. It was found that the higher homologues were formed through the methylolation of methylene urea followed by its condensation with free urea and not by the reaction of methylene urea with methylol urea.     

Kinetics and reaction scheme of coal liquefaction over molten tin catalyst

Rates of formation of gases, oils, asphaltenes and preasphaltenes during non-solvent liquefaction of coal over molten tin catalyst have been measured. A probable reaction scheme and the rate constants for the pathways comprising the scheme have been presented. The results show that the catalyst greatly accelerates the conversion of preasphaltenes to asphaltenes. It also accelerates two other reactions, i.e., coal to preasphaltenes and coal to asphaltenes. By contrast, the catalyst does little to promote gasification and formation of oils.     

The mechanism of the nitrate (platinum) electrode in molten alkali nitrates

The mechanism for the nitrate electrode, (Pt)NO₂, O₂/NO^?₃, is shown to be: NO^?₂ = NO₂ + e; NO₂ = NO + $\text{1}\text{2}$O₂; NO^?₃ + NO = NO^?₂ + NO₂; with the overall reaction: NO^?₃ = NO₂ + $\text{1}\text{2}$O₂ + e.     

Electrochemical studies on the redox mechanism of uranium chloride in molten LiCl-KCl eutectic

The reduction and oxidation processes on platinum and glassy carbon electrodes in molten LiCl-KCl eutectic containing UCl₃ were investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in the temperature range 660-780 K. Two redox peaks have been observed in the cyclic voltammograms corresponding to the two redox reactions U(IV)/U(III) and U(III)/U which are found to be reversible and quasi-reversible, respectively. The reduction potentials of U(IV)/U(III) and U(III)/U are −0.325 and −1.490 V versus reference electrode (0.1 mol% AgCl in the LiCl-KCl), respectively at 700 K. Chronopotentiometric measurements confirm the three-electron transfer during the reduction of U(III) to U metal. Electrochemical impedance spectroscopy data at various potentials were interpreted either as diffusion, adsorption or reduction process by nonlinear fit of the impedance data to the simulated equivalent circuits.     

煤熔渣对镁铝尖晶石质耐火材料的侵蚀机制

为了实现水煤浆气化炉炉衬材料的无铬化,研究开发合适的耐火材料代替水煤浆气化炉用高铬砖,以尖晶石骨料及细粉、α-Al_2O_3微粉和轻烧Mg O微粉为原料,于1 600℃保温5 h烧成,制备了φ50 mm×40 mm、内孔为φ25 mm×25 mm的镁铝尖晶石质坩埚试样。采用静态坩埚法,借助XRD、SEM+EDS研究了高温煤熔渣对试样的侵蚀行为。结果表明:1)侵蚀后的镁铝尖晶石材料结构疏松,出现较明显的裂纹,煤熔渣完全渗入试样内部。2)经煤熔渣侵蚀后的镁铝尖晶石材料,物相组成发生变化,除原有的镁铝尖晶石外,还有新物相镁铁铝复合尖晶石相存在。3)煤熔渣对镁铝尖晶石材料的侵蚀机制是物理渗透为主,化学熔蚀为辅。     

Kinetics and mechanism of the hydrogen-evolution reaction on mercury in HCl-dimethylsulphoxide solutions

Rapid galvanostatic and potentiostatic stationary E/I curves, E/time characteristics at current interruption and electrode capacitance measurements were made to investigate hydrogen-ion discharge on mercury in HCl-DMSO solutions, with and without the addition of LiCl as supporting electrolyte. Kinetic parameters and their temperature and concentration dependences are discussed in terms of the initial electron-trnsfer reaction as rate-determining.     

氯化铁的催化活性及其机理的探讨

报道了氯化铁在Ｆｒｉｅｄｅｌ－Ｃｒａｆｔｓ反应，酯化、酯交换、合成缩醛（或缩酮）、片呐重排和氧化反应中的应用并探讨了其反应机理。     

Kinetics of hydrogen evolution reaction on a stainless steel electrode

The kinetic parameters for the hydrogen evolution reaction on a stainless steel substrate have been obtained from a study of the steady-state polarization curves as well as the galvanostatic transients. The high Tafel slope obtained in the steady-state polarization measurements was ascribed to the presence of an oxide film present on the surface of the stainless steel electrode.     

Kinetics and mechanism of the alkali-catalyzed p-cresol–formaldehyde reaction

The kinetics of the reaction of p-cresol with formaldehyde in relation to the functionality of p-cresol using NaOH as catalyst has been studied at temperatures of (65 ± 0.05)°C, (70 ± 0.05)°C, (75 ± 0.05)°C, and (80 ± 0.05)°C. The pH maintained was 7.0, 8.0, 9.0, 9.4, and 10.0. The reaction follows a second-order rate law. The rate was found to increase with increase in pH. The stepwise rate constants (k₁ and k₂) for the formation of monomethylol-p-cresol and dimethylol-p-cresol, respectively, were calculated from the overall rate constant k. The values of Arrhenius parameters and the entropy of activation for the overall as well as the stepwise reactions were calculated. The experimental and calculated values of k at pH 10.0 and temperatures 65, 70, 75, and 80°C were found to agree well within experimental errors. A mechanism conforming to the energies and entropies of activation of the reaction is suggested.     

Kinetics of the reaction of hafnium tetrachloride vapour with solid sodium chloride

The kinetics of the reaction of hafnium tetrachloride vapour with solid sodium chloride spheres to form sodium hexachlorohafnate have been studied as a function of pressure, temperature and sphere size. The course of the reaction was followed by weight gain measurements in an all-glass closed system using a quartz spring. Various kinetic models for gas-solid reactions were tested against the experimental data. Only the Carter-Valensi kinetic model gave a consistently good fit. Possible reaction mechanisms are discussed in the light of the pressure and temperature dependence of the specific rate constant in the Carter-Valensi model and in the inert marker experiments.     

Kinetics of the reaction of hafnium tetrachloride vapour with solid sodium chloride

The kinetics of the reaction of hafnium tetrachloride vapour with solid sodium chloride spheres to form sodium hexachlorohafnate have been studied as a function of pressure, temperature and sphere size. The course of the reaction was followed by weight gain measurements in an all-glass closed system using a quartz spring. Various kinetic models for gas-solid reactions were tested against the experimental data. Only the Carter-Valensi kinetic model gave a consistently good fit. Possible reaction mechanisms are discussed in the light of the pressure and temperature dependence of the specific rate constant in the Carter-Valensi model and in the inert marker experiments.     

Kinetics and mechanism of the thermal degradation of poly(vinyl chloride)

The kinetics of the thermal degradation of solid powdered poly(vinyl chloride) (PVC) under nitrogen was studied by thermogravimetry, rate of hydrogen chloride evolution, and rate of polyene sequence formation. These results are accommodated by a chain mechanism involving initiation by random dehydrochlorination at normal monomer residues of PVC, and a series of intermediates, each leaking to a stable conjugated polyene sequence. Structural irregularities such as allylic and tertiary chlorine are responsible for a fast initiation process at the very beginning of the degradation. Mean rate constants and activation parameters for random initiation, propagation, and termination reactions of the PVC degradation chain were calculated by simulation. Activation enthalpy/entropy correlations for the experimental data available for dehydrochlorination of chloroalkanes and chloroalkenes in the gas and in the liquid phase or nonpolar solvents and elementary reactions of PVC degradation show that initiation is an HCl elimination through a transition state of four centers requiring a synperiplanar conformation of the >CH–CCl< group, whereas propagation is a dehydrochlorination through a transition state of six centers requiring a cis configuration of the double bond.     

Formation of calcium plumbate: Kinetics and mechanism of reaction

In the presence of excess air the kinetics of reaction between CaO and PbO to form calcium plumbate are governed by the diffusion process described by Jander's kinetic model. The energy of activation of the process, as calculated from the Arrhenius equation, is 50 kcal/mol. In an air and nitrogen atmosphere (ratio 1:16) the reaction rate is 4–5 times slower. It has been shown that formation of calcium plumbate from pure CaO and PbO takes place between 600–800° without formation of intermediate compounds. From the results obtained it is deduced that the mechanism of formation consists of two main stages–diffusion between the CaO and PbO, substantially due to the more diffusible PbO, and chemical reaction in the presence of O₂.     

AlCl3催化合成苯基二氯化膦及其反应机理的探讨

以苯和PCl₃为原料，在三氯化铝催化作用下，通过Friedel-Crafts反应合成了苯基二氯化膦，对该反应机理进行了探讨。研究发现,160 目的NaCl能有效地分离出催化剂三氯化铝，苯基二氯化膦的收率达86.5%。对NaCl与三氯化铝形成配合物的原因进行了探讨。     

The sulphur---sulphide electrode in molten salts—I: Chronopotentiometric behaviour in lithium chloride---potassium chloride eutectic

By means of cathodic chronopotentiometric and potentiometric studies of (nominally) polysulphide solutions in LiCl---KCl eutectic at temperatures between 400 and 440°C, the predominant solute species present at concentrations around 10⁻² M was shown to be polysulphide S²⁻_x. This conclusion is in harmony with earlier spectral and electrochemical studies in LiCl---KCl eutectic.

The chronopotentiometric oxidation of sulphide ion S²⁻ in the same melt is rather complex, but it appears to be controlled by the extensive formation of a polymeric sulphur film, although a mixture of various soluble polysulphides is ultimately formed. Various detailed mechanisms that can account for the observed behaviour are discussed.     

Kinetics and mechanism of the reaction between ozone and hydrogen peroxide in aqueous solutions

A kinetic model has been developed, taking into account both decomposition of ozone molecules and interactions between ozone and hydrogen peroxide for formation of hydroxyl radical and subsequent reactions. Experiments were carried out at 25°C in the pH range of 3 to 13, indicating that the depletion rate of ozone increases with the concentrations of ozone, hydrogen peroxide and hydroxyl ion, as predicted by the kinetic model. Adverse scavenging reactions, however, also play significant roles at sufficient concentration ratios of hydrogen peroxide to ozone and high concentrations of hydroxyl ion in reducing the depletion rate. Results of this research suggest, that it is most desirable to conduct the peroxone oxidation for pollutant destruction by the hydroxyl radical reaction in alkaline solutions of pH below 11, while maintaining about the same concentration of ozone and hydrogen peroxide.