Electrochemical study of aluminium ion reduction in acidic AlCl3-n-butyl-pyridinium chloride melts

Electrochemical reduction of AlCl₃ dissolved in acidic AlCl₃-n-butyl-pyridinium chloride melt was studied by linear sweep voltammetry and chronopotentiometry at tungsten and platinum electrodes, in the Al₂Cl ₇ ^? concentration range 0.3 to 0.5 M between 30 and 60°C. Al₂Cl ₇ ^? bulk reduction was preceded by a nucleation (tungsten) or alloy formation phenomenon (platinum). The overall results agree rather well with the mechanism: $$\begin{gathered} 2AlCl_4^ - \rightleftarrows Al_2 Cl_7^ - + Cl^ - \hfill \\ 4Al_2 Cl_7^ - + 3e \rightleftarrows Al + 7AlCl_4^ - \hfill \\ \end{gathered} $$ The electrochemical reaction appeared quasi-reversible. Calculated values of the product of the transfer coefficient by the number of the electron exchanged in the rate determining step were in the range 0.45 to 0.7. Diffusion coefficients for Al₂Cl ₇ ^? were calculated.     

Aluminium dissolution in the NaF-AlF3-Al2O3 system: Effects of alumina,temperature, gas bubbling and dissolved metal

Current reversal chronopotentiometry, with and without a delay time between the forward and reverse current pulses, was employed to evaluate the effects of temperature, alumina content, gas bubbling (argon and carbon dioxide) and dissolved metal on the rate of aluminium dissolution in NaF–AlF₃–Al₂O₃ molten bath. The working electrode was a tungsten wire electrode and the temperature range studied was 824–1040°C. The effect of the alumina content was determined in melts with CR=1.45 and CR=4.3 at 1029±3°C (CR = mol NaF/mol AlF₃). The experiments involving gas bubbling and dissolved metal were carried out in melts similar to industrial compositions, i.e. CR=2.4, 4.8 wt. % Al₂O₃ at 980°C. In general, the dissolution rate of aluninium increased with increasing temperature, decreased slightly with increasing alumina content in acidic melts (CR<3) but changed little in basic melts (CR>3), increased with bubbling and decreased in the presence of dissolved metal. The rate of Al dissolution is thus mass transport controlled.     

Chemical and electrochemical behaviour of nickel ions in the ZnCl2-2NaCl melt at 450°C

The stability of nickel chloride and oxide as well as the electrochemical behaviour of the Ni²⁺ ion have been studied in ZnCl₂-2NaCl melts at 450°C by means of X-ray diffractions (XRD), potentiometry, cyclic voltammetry, chronoamperometry and chronopotentiometry. The standard potential of the redox couple Ni(II)/Ni(O) and solubility product of nickel oxide have been determined (E°Ni(s)/Ni(II) = ?1.006 ± 0.001 V (against Cl₂(1 atm)/Cl^?), pKs = 4.8 ± 0.1 in molality scale). These results have allowed the construction of E-pO^2? equilibrium diagrams. Nickel (II) reduction is close to the reversibility according to the scheme: $${\text{NiCl}}_{\text{2}} ({\text{solvated}}){\text{ + 2e}}^ - \Rightarrow {\text{Ni(s) + 2Cl}}^ - $$ with a diffusion coefficient, D _Ni, close to 3 × 10 ^?6cm²s^?1.     

The thermoelectric power in Bi2O3

The thermo electric power, ΔE/ΔT, of the cell $$\begin{gathered} O_2 + N_{2, } Pt/Bi_2 O_3 (\delta phase)/Pt, O_2 + N_2 \hfill \\ (T + \Delta T) (T) \hfill \\ \end{gathered}$$ has been measured as a function of oxygen pressure (10^?4 atm ? p(O₂) ? 1 atm) in the temperature range 650–800° C. The experimental result can be described by: $$[ \in ({\rm O}_2 /{\rm O}^{2 - } ) - \in (e, Pt)] = [45.6 \pm 5.6 log p(O_2 ) - 261](\mu VK^{ - 1} )$$ within experimental error, where ε(O₂/O²), the Seebeck coefficient ofδ-Bi₂O₃, stands for \(\mathop {\lim }\limits_{\Delta T \to 0} \Delta E/\Delta T\) The change of ΔE/ΔT with oxygen pressure corresponds to the change of the partial molar entropy of O₂. The heat of transport of O^2? ions is calculated to be 0.13 eV ± 0.01 whereas the activation enthalpy for ionic conduction is 0.30 eV. From this discrepancy it is concluded that the free ion model of Rice and Roth cannot be applied, while the extended lattice gas model of Girvin might explain the results when strong polaron coupling is assumed.     

Structural entities in NaF---AlF3 melts containing alumina

An ionic structure model is developed for NaFAlF₃ melts containing alumina. The model can be described by the following dissociation equilibria,

Na⁺ is proposed to be the only cation present in the system. The model is derived from activity data for NaF and AlF₃ in melts saturated with alumina at 1285 K and the assumption of an ideal ionic mixture, with randomly distributed anions on the anion positions in the melt. The available data for the system seem to fit the model reasonably well. The entity Al₂F^?₇ will only be present in AlF₃ rich melts. Oxygen atoms in the complexes are most probably involved in bridging bonds of the type AlOAl and

. It appears that aluminium atoms are involved in 4 bonds in AlF₃-rich melts. The number of bonds changes partly towards 5 and/or 6 with increasing NaF content of the melt.     

Interfacial tension between aluminum and cryolite melts during electrolysis of the systems Na3AlF6–AlF3 (NaF)–Al2O3

The interfacial tension between aluminum and cryolite melts containing different salt additions has been measured by the capillary depression method. The technique is based on the measurement of the capillary depression occurring when the capillary, which is moved vertically down through the molten salt layer, passes through the salt/metal interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary. The interfacial tension was found to be strongly dependent on the n(NaF)/n(AlF₃) ratio (cryolite ratio, CR). At the cryolite ratio 2.28 (80 wt.% Na₃AlF₆ + 10 wt.% AlF₃ + 10 wt.% Al₂O₃ // Al, t = 1000 °C) the interfacial tension was 546 mN m⁻¹, while it was 450 mN m⁻¹ at the cryolite ratio 4.43 (80 wt.% Na₃AlF₆ + 10 wt.% NaF + 10 wt.% Al₂O₃ // Al, t = 1000 °C). Experiments under current flow conditions were also performed. During the electrolysis the interfacial tension at n(NaF)/n(AlF₃) ratio 2.28 decreased from 546 mN m⁻¹ at zero current to 518 mN m⁻¹ at 0.112 A cm⁻². The same trend was observed in the system with a cryolite ratio 4.43. The interfacial tension decreased from 450 mN m⁻¹ at zero current to 400 mN m⁻¹ at 0.112 A cm⁻². The consequent increase in interfacial tension of these systems caused by interruption of electrolysis was observed. Electrolysis of the system 25 wt.% NaF + 75 wt.% NaCl (eutectic mixture)/Al indicated no influence of applied current on the interfacial tension at 850 °C.     

Comportement electrochimique de NbCl5 en milieu NaCl-KCl pur et additionne d'ions fluorure

Dans le domaine de température 700–800°C, les solutions d'ions niobium obtenues par addition de NbCl₅ dans le melange équimolaire NaCl-KCl, sont réduites jusqu'au métal en une seule étape: $${\text{Nb(IV) }} + {\text{ 4e}}^ - \Leftrightarrow {\text{Nb(o)}}$$ Cet échange est réversible, il lui correspond le potentiel standard apparent: $$E_{Nb(IV)/Nb}^{'0} = - 0.64V(Ag - AgCl) \pm 0.01V$$ Les espéces Nb(iv) sont oxydées selon le processus réversible: $${\text{Nb(IV)}} \Leftrightarrow {\text{Nb(v)}} + {\text{e}}^ -$$ Le potentiel standard apparent associé est: $$E_{Nb(IV)/Nb}^{'0} = - 0.74V(Ag - AgCl) \pm 0.05V$$ L'ajout d'ions fluorure déstabilise le complexé NbCl₆ ^2? au profit du complexe NbF₆ ^2? . Ceci se traduit par un déplacement du pie cathodique vers des potentiels plus cathodiques mais le mécanisme de réduction comporte toujours une seule étape mettant en jeu quatre électrons. Dans ces milieux des dépôts de niobium métallique ont eté obtenus caractérisés par rayon X. In the 700–800°C temperature range, NbCl₅ solutions in equimolar NaCl-KCl mixtures are reduced to the metal through a single step: $${\text{Nb(IV)}} + 4{\text{e}}^ - \Leftrightarrow {\text{Nb(o)}}$$ This exchange is reversible and the corresponding apparent standard potential is: $$E_{Nb(IV)/Nb}^{'0} = - 0.64V(Ag - AgCl) \pm 0.01V$$ The Nb(iv) species are oxidized according to the following reversible process: $${\text{Nb(IV)}} \Leftrightarrow {\text{Nb(v)}} + {\text{e}}^ -$$ The associated apparent standard potential is: $$E_{Nb(IV)/Nb}^{'0} = - 0.74V(Ag - AgCl) \pm 0.05V$$ The addition of fluoride ions destabilizes the NbCl₆ ^2? complex and yields the NbF₆ ^2? complex. The cathodic peak potential moves toward more cathodic potentials, but the reduction mechanism still involves a single step with four electrons exchanged. In these media, metallic niobium deposits have been obtained, and characterized through X-ray analysis.     

A study of electrochemical kinetics of copper deposition under pulsed current conditions

The investigation is concerned with the drop of current efficiency (CE) of copper deposition under pulsed current conditions. A mathematical model which is based on different charge transfer rates between the following two reactions, (1) $$Cu^{2 + } + e \to Cu^ + $$ and (2) $$Cu^ + + e \to Cu$$ has been formulated to describe the behaviour of a Cu/CuSO₄ system under pulsed current conditions on a rotating disc electrode. The results indicate that the CE drops continuously as the difference between the exchange current densities of the two reactions increases. The exchange current densities of Reactions 1 and 2 were estimated to be 0.034 mA cm^?2 and 0.024 mA cm^?2, respectively. Prediction of CE using the mathematical model agreed to within 3.5% with experimental data over a range 80.4–93.7%.     

Structure and reactivity of MoO3-MgO catalysts

Surface of OH groups on reduced MoO₂-MgO catalysts such as $$ - - Mg - - O - - \begin{array}{*{20}c} {||} \\ {Mo} \\ | \\ \end{array} - - OH$$ may act as an active site for hydrogenation of propene. The surface hexa-coordinated Mo⁵⁺ ion (MO _6c ⁵⁺ ) was reduced to a lower number of cation such as Mo⁴⁺ or Mo³⁺ which act as an active site for metathesis of propene.     

Electrochemical study of mass transfer in Li-Mg and Li-Mg-Al alloys

The interdiffusion coefficients in Li-Mg alloys and Li-Mg-Al alloys were evaluated using transient techniques such as chronopotentiometry and chronoamperometry. Anodic or cathodic pulses were imposed on the alloy electrodes under galvanostatic and potentiostatic conditions. Taking into account charging of the double layer, ohmic drop, adsorption of diffusing species and electrolyte-electrode boundary shift, the diffusion coefficients of lithium in Li-Mg alloys (α-phase and β-phase) and in Li-Mg-Al alloys were estimated at around 420°C. In the case of Li-Mg α-phase alloys, the values of the diffusion coefficients,D _Li, can be represented in a polynomial expansion of the composition of the alloy,X _Li (mol%) as follows: $$ln D_{Li} = - 19.850 - 0.4294X_{Li} + 0.0249X_{Li}^2$$ The diffusion coefficients of lithium in Li-Mg (β-phase) alloys show extremely large values (?10^?6 cm²s^?1) as also in the Li-Al β-phase alloys.     

Determination of equilibrium silver sulphate concentration in the system Cu-H2SO4-CuSO4-H2O-Ag2SO4-Ag as a function of composition

The value of the ratio \(\gamma _{{\text{Cu}}^{{\text{2 + }}} } /\gamma _{{\text{Ag}}^{\text{ + }} }^2 \) ( \(\gamma _{{\text{Cu}}^{{\text{2 + }}} } ,\gamma _{{\text{Ag}}^{\text{ + }} } \) -are the mean activity coefficients of copper and silver ions, respectively) was calculated from the measured emf of the cell $${\text{Cu(Hg)|H}}_{\text{2}} {\text{SO}}_{\text{4}} {\text{ (}}c_{\text{x}} {\text{)}} - {\text{CuSO}}_{\text{4}} {\text{ (}}c_{\text{y}} {\text{)|Hg}}_{\text{2}} {\text{SO}}_{\text{4}} {\text{, Hg}}$$ and the solubility of Ag₂SO₄ in H₂SO₄ (c _x) and CuSO₄ (c _y) solutions. The concentration of H₂SO₄ in the solution was varied from 0.5 to 2.1 mol dm^?3 that of CuSO₄ from 0.4 mol dm^?3 to saturation. The results were presented as a function: $$\frac{{\gamma _{{\text{Cu}}^{{\text{2 + }}} } }}{{\gamma _{{\text{Ag}}^{\text{ + }} }^2 }} = a_0 + a_1 c_{\text{x}} + a_2 c_{\text{y}} + a_3 c_{\text{x}}^{\text{2}} + a_4 c_{\text{x}} c_{\text{y}} + a_5 c_{\text{y}}^2 .$$ This function allows the estimation of the equilibrium silver ion concentration \(c_{{\text{Ag}}^{\text{ + }} }^{{\text{eq}}} \) in solutions containing both H₂SO₄ and CuSO₄ in the presence of metallic copper. The function is also very useful for the estimation of the \(c_{{\text{Ag}}^{\text{ + }} }^{{\text{eq}}} \) near a working copper electrode.     

Electrochemical behaviour of CuO-TiO2 catalysts

In order to provide further information on the properties of CuO?TiO₂ catalysts, we have investigated their electrochemical behaviour in 1 M LiClO₄-propylene carbonate electrolyte. It appears that TiO₂ is electrochemically reducible at 1.8 V at room temperature, with a faradaic yield of 0.3–0.4 F per mole of TiO₂ with formation of a TiO₂Li_x phase according to the reaction: $$TiO_2 + xe + xLi^ + \leftrightharpoons TiO_2 Li_x $$ The electrochemical study suggests that TiO₂ enhances Cu(II) electroreduction in titania-supported copper catalysts. This electroreduction of Cu(II) occurs either at 2.2 V according to the path: $$Cu(II) + 2e \xrightarrow{{TiO_{2 } support}} Cu(O), TiO_2 $$ or at 1.8 V through an internal electron transfer between TiO₂Li_x and Cu(II) according to the successive reactions: $$\begin{gathered} TiO_2 + xe + xLi^ + \leftrightharpoons TiO_2 Li_x \hfill \\ Cu(II) \xrightarrow{{TiO_{2 } Li_x }} Cu(O), TiO_2 \hfill \\ \end{gathered} $$ This study shows that electrochemistry may be a novel way of determining and controlling the redox states of metal-supported catalysis.     

Electroplating silicon and titanium in molten fluoride media

The electrolytic reduction mechanisms of K₂SiF₆ and K₂TiF₆ solutions in LiF-KF and LiF-NaF-KF eutectic mixtures have been studied at temperatures between 550 and 850°C. The reduction of K₂SiF₆ proceeds by two successive electron transfers, $$Si(IV) + 2e \to Si(II) + 2e \to Si$$ coupled with an antidisproportionation reaction $$Si(IV) + Si \underset{{k_b }}{\overset{{k_b }}{\longleftrightarrow}} 2Si(II)$$ Very pure thin silicon layers, up to 300 μm thick, were obtained on a silver substrate. The cathodic reduction of TiF ₆ ^2? ions occurs in two well separated reversible steps, $$TiF_6^{2--} + e \to TiF_6^{3--} + 3e \to Ti + 6F^--$$ Adherent coatings of pure titanium were found to be linked to the copper substrate by an interdiffusion sublayer comprising Ti₂Cu, TiCu, Ti₂Cu₃ and TiCu₄ which were formed in a narrow potential domain preceding titanium deposition.     

Electrochimie dans la cryolithe fondue—I influence de la teneur en fluorures et en alumine sur le potentiel du systeme A1(III+)/A1(0)

The influence of additions of NaF and AlF₃ on the potential of an aluminium electrode has been studied in molten cryolite. The redox system Al(III+)/Al(0) is reversible, and the value K = 0,03 (ionic fractions) for the dissociation equilibrium (AlF^3?₆ ? AlF^?₄ + 2F^?) has been deduced from potentiometric measurements. Dissolution of Al₂O₃ probably leads to AlOF^2?₃ species.     

Étude de la reduction électrochimique de AlCl3 dans le dimethylsulfoxyde à 25°C

The electrochemical reduction of AlCl₃ in dimethylsulphoxide at 25°C was studied by triangular voltammetry on platinum electrodes. The reduction mechanism corresponds to the kinetic scheme: $$Ox(ads) + ne \to Red$$ where Ox is irreversibly adsorbed on the cathode. The reduction peak of the aluminium ions, Al(III), appears at ?1.96±0.05 V versus the reference electrode which is the half cell Ag/AgCl ₂ ^? 10^?3 M, LiCl 0.1 M. It is preceded by another badly defined peak around ?1.74 V, very probably due to the reduction of the protons present in traces in the electrolyte solution. The value of the quantityan _a (wherea is the transfer coefficient andn _a is the number of electrons involved in the rate determining step) was determined as 0.45±0.05 from measurements of all the experimental curves.     

Electrochemical study of catalytic oxidation of naphthalene in molten sulphate pyrosulphate at 425° C with vanadium pentoxide

Naphthalene oxidation in a molten mixture of potassium sulphate pyrosulphate at 425° C in the presence of vanadium pentoxide takes place in three stages. The electrochemical study of each stage has led to the proposal of kinetic equations for the initial rates: (i) naphthalene oxidation by the medium $$V_1^0 = 2 x 10^{ - 2} P_{C_{10} H_8 }$$ (ii) mixture regeneration by V₂O₅ $$V_2^0 = 19.2 x 10^{ - 2} P_{SO_2 }$$ (iii) catalyst regeneration by oxygen $$V_3^0 = 82.8P_{O_2 } [V(IV)]^2$$ This shows that the molten salt acts not only as a solvent but also as a reagent in the catalytic process.     

The mechanism of manganese electrodeposition

The mechanism of manganese electrodeposition from a sulphate bath on to a stainless-steel substrate has been studied by using current efficiency data to resolve the totali-E curves. A simple, two-step electron transfer mechanism: $${\text{Mn}}^{{\text{ + + }}} + {\text{e}}\xrightarrow{{{\text{r}}{\text{.d}}{\text{.s}}}}{\text{Mn}}^{\text{ + }} $$ $${\text{Mn}}^{\text{ + }} + {\text{e}} \to {\text{Mn}}$$ is proposed to explain the following experimentally obtained parameters: cathodic and anodic transfer coefficients, reaction order and stoichiometric number. The mechanism also explains the effect of pH oni _o,Mn and on the corrosion currents.     

Anodization of molybdenum. I. Galvanostatic anodization

Molybdenum was anodized at different current densities (10^?4–10^?2 A cm^?2) in various aqueous solutions. Potential-time curves obtained in strong acid solutions are similar to those usually reported for the valve metals, and the anodization kinetics were found to obey the familiar exponential law $$i = A \exp BH$$ and also to obey the empirical relation, $$(dE/dt)_i = a(i)^b $$ Using both polarization and capacitance measurements, it was found that the field strength,H, the electrolytic parametersA andB and the constantsa andb are comparable with those previously reported for many valve metals. Except in strong acid solutions,E-time curves showed an induction period before oxide formation. The duration time of the induction period,t _i, was found to increase with decrease of solution acidity and current density and with increase of temperature. Although chloride ion may act as a depolarizer,t _i was found to decrease with increase of chloride ion concentration, probably by increasing the anodization field strength.     

Mass transfer study of the electropolishing of vertical cylinders with active ends under natural convection conditions

Rates of electropolishing of vertical copper cylinders with active ends in H₃PO₄ were studied by measuring the limiting current under natural convection. Variables studied were H₃PO₄ concentration, cylinder diameter and aspect ratio. The rate of polishing of the whole cylinder was represented by the mass transfer equation $$Sh = 0.33(Sc Gr)^{0.32}$$ for the range 1.17 × 10¹⁰ < Sc Gr < 5.11 × 10¹¹. Rates of mass transfer were measured also at vertical cylinder with insulated ends, and the upward facing surface (disc). Data for the vertical cylindrical surface were represented for the range 8.75 × 10⁹ < Sc Gr < 1.1 × 10¹² by the equation $$Sh = 1.206(Sc Gr)^{0.255}$$ while data at the upward facing disc were correlated for the range 0.11 × 10¹⁰ < Sc Gr < 46 × 10¹⁰ by the equation $$Sh = 0.17Sc^{0.396} (Sc Gr)^{0.146}$$ A comparison between the measured rate of mass transfer at the whole cylinder and the value calculated by adding the rates of mass transfer at the separate surfaces of the cylinder shows that the measured value deviates from the calculated value, the degree of deviation increases with increasing Sc × Gr. Deviation was attributed to flow interaction at the different cylinder surfaces.