An Investigation into Silane Evolution from Porous Silicon by Temperature Programmed Desorption Method

The gaseous species desorbed from porous silicon (PS) were investigated using the method of temperature programmed desorption (TPD) and fourier transform infrared spectroscopy (FTIR). Silicon wafers (25–50 cm, p^–, FZ) were anodised in 40% HF and HF/C₂H₅OH electrolytes. The PS samples were linearly heated at 1.5 K s^–1 using a custom built heating unit in a oil-free pump backed vacuum chamber at a base pressure of <10^–8 torr. A quadrupole mass spectrometer, which was used as the detector, was fitted in line of sight of the sample at a distance of about 6 mm. It was observed that silane was liberated during the heating of porous silicon samples produced from both electrolytes. The peak temperature at which this occurred was at 570 ± 10 K. This temperature coincides with the temperature of silicon-silicon bond breakage in Si–SiH₃ groups on the pore walls, as shown by the FTIR results. It is proposed that silane formation involves the reaction of the Si-silyl group with moisture: Si–SiH₃ + H₂O Si–OH + SiH₄.     

Electrical Properties and the Structure of Glasses in the xNa2O–(1 – x)2PbO · B2O3 System

The temperature–concentration dependences of the electrical conductivity and the activation energy for electrical conduction of glasses in the Na₂O–B₂O₃ and Na₂O–2PbO · B₂O₃ systems are studied. The investigation into the nature of the electrical conduction in these glasses reveals that the contribution from the electronic component (10^–3%) of the conductivity is within the sensitivity of the Liang–Wagner technique. A considerable alkali conductivity is observed upon introduction of more than 12 mol % Na₂O. The true transport number of sodium _Na is as large as unity at [Na₂O] 15 mol %. It is shown that the observed temperature–concentration dependences of the electrical and transport properties are governed by the ratio between the concentrations of polar and nonpolar structural–chemical units of the Na⁺[BO_4/2]^–, Na⁺[O^–BO_2/2] Na⁺[O^–BO_2/2], Pb²⁺ _1/2[BO_4/2]^–, Pb²⁺ _1/2[O^–BO_2/2], and [BO_3/2] types.     

The oxidative coupling of methane and the activation of molecular O2 on CeO2/BaF2

CeO₂/BaF₂ was used as the catalyst for the oxidative coupling of methane (OCM). At 800°C and CH₄O₂=2.71,CH₄ conversion of 34% with C₂ hydrocarbon selectivity of 54.3% was obtained. XRD measurement showed that partial anion (O^2–,F^–) and/or cation (Ce⁴⁺,Ba²⁺) exchange between CeO₂ and BaF₂ lattices occurred. ESR study showed that O^– species existed on degassed catalyst. XPS study revealed that, when BaF₂ was added to CeO₂, the binding energy of Be 3d_5/2 was 2.2 eV lower than that in CeO₂, and the electron-enriched lattice oxygen species was detected. XPS, ESR and Raman study showed that, under O₂ adsorbing conditions, O ₂ ^2– and O _– ² species were detected on CeO₂/BaF₂.This work was supported by the State Key Laboratory for Physical Chemistry of the solid surface and the National Science Foundation of China.     

Local Symmetry of Cu2+ Ions in Sodium Silicate Glasses from Data of EPR Spectroscopy

Sodium silicate glasses with a constant ratio of oxide concentrations (mol %) SiO₂/Na₂O = 2.4 and with copper ions introduced in the form of CuO (from 1 to 10 mol %) are studied by the EPR method. The shape and width of the EPR line of copper ions are analyzed, and the spin-Hamiltonian parameters g _||, g , A _||, and A are determined by simulating the EPR spectrum and comparing the simulated and experimental spectra. The EPR spectrum of copper ions (1 mol %) is characterized by the parameters g _|| = 2.35, g = 2.065, A _|| = 135 × 10^–4 cm^–1, A = 7 × 10^–4 cm^–1, and H = 25 G. An analysis of this spectrum shows that the nearest environment of the Cu²⁺ ion has the shape of an elongated octahedron. The EPR spectrum of the sodium silicate glass containing 10 mol % Cu is a superposition of the spectrum of an octahedral complex (g _|| = 2.35, g = 2.075, A _|| = 135 × 10^–4 cm^–1, H = 40 G) and the spectrum of a cluster (g _|| = 2.35, g = 2.15, A _|| = 135 × 10^–4 cm^–1, H = 50 G).     

Synthesis, Characterization and Optimum Reaction Conditions of Oligo-N, N′-bis (2-hydroxy-1-naphthalidene) Thiosemicarbazone

The oxidative polycondenzation reaction conditions of N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone (HNTSC) using air oxygen, H₂O₂ and NaOCl were studied in an aqueous alkaline medium between 50–90°C. Oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone was characterized by ¹H-NMR, FT-IR, UV-Vis, size exclusion chromatography (SEC) and elemental analysis techniques. Solubility testing of oligomer was investigated using organic solvents such as DMF, THF, DMSO, methanol, ethanol, CHCl₃, CCl₄, toluene acetonitrile, ethyl acetate, concentrated H₂SO₄ and an aqueous alkaline solution. Using NaOCl, H₂O₂ and air O₂ oxidants, conversion to oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone (OHNTSC) of N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone was found to be 85, 80 and 76%, respectively, in an aqueous alkaline medium. According to the SEC analyses, the number-average molecular weight, weight-average molecular weight and polydispersity index values of OHNTSC synthesized were found to be 1050 gmol^–1 1715 gmol^–1 and 1.63, using NaOCl, and 2137, 2957 gmol^–1 and 1.38, using air O₂ and 2155 gmol^–1 4164 gmol^–1 and 1.93, using air H₂O₂, respectively. Also, TG analysis was shown to be unstable of oligo-N, N-bis (2-hydroxy-1-naphthalidene) thiosemicarbazone against thermo-oxidative decomposition. The weight loss of OHNTSC was found to be 97.29% at 900°C.     

Isotopic exchange between H2 and D2 by the Rideal-Eley mechanism

The rate of the reaction, H₂+D₂ 2HD, is very rapid at –160 °C on the catalyst Cp₂Th-(CH₃)₂/dehydroxylated Al₂O₃, activated in H₂+D₂ and believed to contain supported Cp₂ThH⁺(Cp = ⁵-pentamethylcyclopentadienyl). Even at –195 °C, the turnover frequency is greater than 0.1 sec^–1 at atm. Strong evidence is adduced that the exchange proceeds by a Rideal-Eley process, i.e. by a four-center transition state involving Th-H+D-D.     

Applications of magnetoelectrolysis

A broad overview of research on the effects of imposed magnetic fields on electrolytic processes is given. As well as modelling of mass transfer in magnetoelectrolytic cells, the effect of magnetic fields on reaction kinetics is discussed. Interactions of an imposed magnetic field with cathodic crystallization and anodic dissolution behaviour of metals are also treated. These topics are described from a practical point of view.Nomenclature 1, 2 regression parameters (-) - B magnetic field flux density vector (T) - c concentration (mol m^–3) - c bulk concentration (mol m^–3) - D diffusion coefficient (m² s^–1) - d _e diameter of rotating disc electrode (m) - E electric field strength vector (V m^–1) - E _i induced electric field strength vector (V m^–1) - E _g electrostatic field strength vector (V m^–1) - F force vector (N) - F Faraday constant (C mol^–1) - H magnetic field strength vector (A m^–1) - i current density (A m^–2) - i _L limiting current density (A m^–2) - i _L ⁰ limiting current density without applied magnetic field (A m^–2) - I current (A) - I _L limiting current (A) - j current density vector (A m^–2) - K reaction equilibrium constant - k reaction velocity constant - k _b Boltzmann constant (J K^–1) - _{m 1}, m ₂ regression parameters (-) - n charge transfer number (-) - q charge on a particle (C) - R gas constant (J mol^–1 K^–1) - T temperature (K) - t time (s) - V electrostatic potential (V) - v particle velocity vector (m s^–1) Greek symbols transfer coefficient (–) - velocity gradient (s^–1) - _MS potential difference between metal phase and point just inside electrolyte phase (OHP) - diffusion layer thickness (m) - ₀ hydrodynamic boundary layer thickness without applied magnetic field (m) - density (kg m^–3) - electrolyte conductivity (^–1 m^–1) - magnetic permeability (V s A^–1 m^–1) - kinematic viscosity (m² s^–1) - vorticity     

Synthesis and Redox Properties of a π-Conjugated Cyclobutadienecobalt Polymer Containing Ferrocenyl Groups

The reaction of CpCo(PPh₃)₂, in which Cp= ⁵-cyclopentadienyl, with a -conjugated diacetylene, FcCC–o-C₆H₄–CCFc, in which Fc=ferrocenyl, was found to give a cyclobutadienecobalt mononuclear complex, { ⁴-C₄Fc₂(o-FcC₆H₄)₂}CoCp (1), the crystal structure of which was determined by X-ray crystallography. In contrast, the reaction of CpCo(PPh₃)₂ with FcCC–p-C₆H₄–CCFc affords a cyclobutadienecobalt polymer, [p-C₆H₄( ⁴-C₄Fc₂)CoCp] _n (2). The monocobalt complex 1 shows reversible 1e^– and 3e^– redox waves at E ⁰=0.116 and 0.350 V vs Ag/Ag⁺, and the polymer complex 2 shows two chemically reversible redox waves at E ⁰=0.143 and 0.219 V for the oxidation of the ferrocenyl moieties in the cyclic voltammogram. Crystal data are as follows: (1, C₆₅H₄₉CoFe₄), triclinic, space group P\={1} (No. 2), a=13.547(4), b=16.197(4), c=11.763(4) Å, =106.79(2), =97.93(3), =97.12(3), V=2410(1) ų, Z=2.     

Mass diffusion-controlled bubble behaviour in boiling and electrolysis and effect of bubbles on ohmic resistance

A survey is given of theoretical asymptotic bubble behaviour which is governed by heat or/and mass diffusion towards the bubble boundary. A model has been developed to describe the effect of turbulent forced flow on both bubble behaviour and ohmic resistance. A comparison with experimental results is also made.Nomenclature ga liquid thermal diffusivity (m² s^–1) - B width of electrode (m) - c liquid specific heat at constant pressure (J kg^–1 K^–1) - C ₀ initial supersaturation of dissolved gas at the bubble wall (kg m^–3) - d bubble density at electrode surface (m^–2) - D diffusion coefficient of dissolved gas (m² s^–1) - D _h –4S/Z, hydraulic diameter, withS being the cross-sectional area of the flow andZ being the wetted perimeter (m) - e base of natural logarithms, 2.718... - f local gas fraction - F Faraday constant (C kmol^–1) - G evaporated mass diffusion fraction - h height from bottom of the electrode (m) - h _w total heat transfer coefficient for electrode surface (J s^–1 m^–2 K^–1) - h _w,conv convective heat transfer coefficient for electrode surface (J s^–1 m^–2K^–1) - H total height of electrode (m) - i electric current density (A m^–2) - j, j ^* number - J modified Jakob number,C ₀/ ₂ - enthalpy of evaportion (J kg^–1) - m density of activated nuclei generating bubbles at electrode surface (m^–2) - n product of valency and number of equal ions forming one molecule; for hydrogenn=2, for oxygenn=4 - p pressure (N m^–2) - p excess pressure (N m^–2) - R gas constant (J kmol^–1 K^–1) - R ₁ bubble departure radius (m) - R ₀ equilibrium bubble radius (m) - R/R relative increase of ohmic resistance due to bubbles, R, in comparison to corresponding value,R, for pure electrolyte - Re Reynolds number,D _h/ - Sc Schmidt number,/D - Sh Sherwood number - t time (s) - T absolute temperature (K) - T increase in temperature of liquid at bubble boundary with respect to original liquid in binary mixture (K) - gu solution flow velocity (m s^–1) - x mass fraction of more volatile component in liquid at bubble boundary in binary mixture - x ₀ mass fraction of more volatile component in original liquid in binary mixture - y mass fraction of more volatile component in vapour of binary mixture - contact angle - local thickness of one phase velocity boundary layer (m) - _m local thickness of corresponding mass diffusion layer (m) - ^* local thickness of two-phase velocity boundary layer (m) - _o initial liquid superheating (K) - constant in Henry's law (m² s^–2) - liquid kinematic viscosity (m² s^–1) - ^* bubble frequency at nucleus (s^–1) - ₁ liquid mass density (kg m^–3) - ₂ gas/vapour mass density (kg m^–3) - surface tension (N m^–1) Paper presented at the International Meeting on Electrolytic Bubbles organized by the Electrochemical Technology Group of the Society of Chemical Industry, and held at Imperial College, London, 13–14 September 1984.     

Aspects of coordination polymerization in heterogeneous and homogeneous catalysis. A computational survey

Theoretical studies on ethylene polymerization by single site homogeneous catalysts, as well as by Ziegler–Natta heterogeneous catalysts, have been reviewed. Studies on cation–anion interactions in ion-pair systems of the type [LLMR]⁺[A]^– [L, L=Cp, NCR₂, NPR₃; M=Ti, Zr; A=MeB(C₆F₅)₃^–, MAOMe^–, B(C₆F₅)₄^–] provided indications as to which ancillary ligands on the cation acted as good electron donors, and also revealed the importance of having a weakly coordinating anion to enable the ion-pair to separate more easily in solution. Subsequent calculations of barriers to ethylene insertion into the metal-methyl bond in ion-pair systems of the type LLMeM--B(C₆F₅)₃[M=Ti, Zr] revealed that the ease of separation of the anion from the cation in solution was an important criterion in determining the activity of the catalysts. The barrier to insertion was found to be the rate determining step during the first insertion of ethylene into the metal–methyl bond, but second insertion studies conducted for two different ion-pair systems showed that the barrier to uptake of the monomer became the rate determining step during the second insertion of the ethylene monomer. Theoretical studies conducted on TiCl₄/MgCl₂ heterogeneous Ziegler–Natta catalysts provided insights into the nature of the binding of the titanium catalysts (with the Ti in different oxidation states) on to the MgCl₂ support. The results indicated that the support worked best when a few Ti atoms replaced Mg atoms in the crystal lattice of the support. Ethylene polymerization studies, along with investigations on chain termination, led to the conclusion that the TiCl₃-based edge site on MgCl₂ would be the most promising model for the actual catalytic species. Studies on addition of tetrahydrofuran (THF) to the active sites showed that the presence of the base would lead to higher molecular weights of the polymers, as it acted to increase the barrier to termination (by chain transfer to monomer) more than the barrier to insertion.     

Joule heating induced local electrodeposition for microelectronic circuits

A fundamental study is performed for local electrodeposition of copper utilizing thermal potential induced by Joule heating. The feasibility of the process for microelectronic applications is assessed by both experiment and mathematical modeling. The results of the investigation show that (i) a copper wire is coated under conditions of a.c. 50 Hz Joule heating in electrolyte containing 1.0 M CuSO₄ and 0.5m H₂SO₄ with relatively high deposition rate of about 0.4 µm min^–1, (ii) the Joule heating current should be kept below the boiling point of the solution to realize uniform deposition, and (iii) results of calculations by the present model based on one-dimensional heat conduction agree well with experimental results.Nomenclature D diameter of wire (m) - D ₀ initial diameter of wire (m) - F Faraday constant (96 487 C mol¹ ) - g acceleration due to gravity (9.807 m s²) - Gr Grashof number - H thickness of electrodeposit (m) - I current (A) - i ₀ exchange current density (Am^–2) - i _n current density normal to electode (Am^–2) - J current density (I/S) (Am^–2) - L length of wire (m) - M molar concentration of electrolyte (mol dm^–3 or M) - m atomic weight (kg mol^–1) - n number of electrons participating - n unit normal vector to boundary - Nu Nusselt number - Pr Prandtl number - q heat per unit volume (W m^–3) - R universal gas constant (8.314 3 J mol^–1 K^–1) - (r, z) cylindrical coordinate (m) - S cross section of wire (m²) - T temperature (K) - T ₀ fixed temperature at both ends of wire (K) - T _y temperature of electrolyte (K) - t time (s) - x longitudinal coordinate over wire (m) Greek symbols heat transfer coefficient (W m^–2 K^–1 - _a,c anodic (a) and cathodic (c) transfer coefficient - thermal expansion coefficient of solution (K^–1) - specific heat (J kg^–1K^–1) - potential (V) - _e electrode potential (V) - thermal conductivity (W m^–1 K^–1 ) - _y ionic conductivity of electrolyte (^–1m^–1) - _e electronic conductivity of electrode (^–1 m^–1) - kinematic viscosity (m²s^–1) - surface overpotential ( _e – ) (V) - time constant (s) - density (kg m^–3) This work was presented at The 7th International Microelectronics Conference, Yokohama, Japan (1992).     

Interfacial electrical properties of electrogenerated bubbles

Electrophoresis measurements on bubbles of electrogenerated hydrogen, oxygen and chlorine rising in a lateral electric field, are reported. In surfactant-free solutions, all bubbles displayed a point of zero charge of pH 2–3, i.e. they were negatively charged at pH > 3 and positively charged at pH < 2. The bubble diameter and electric field strength dependence of the electrophoretic mobilities, coupled with bubble rise rate measurements, indicated that the gas—aqueous solution interface was mobile, such that classical electrophoresis theory for solid particles could not be applied. Adsorption of anionic or cationic surfactants, in addition to modifying the apparent bubble charge, also tended to rigidify the bubble surface, so that at monolayer coverage the bubbles behaved as solid particles.Nomenclature c electrolyte concentration (mol m^–3) - d bubble diameter (m) - E electric field (V m^–1) - g gravitational constant (9.807 m s^–2) - n ₀ ionic number density (m^–3) - q charge density [(, m) Cm^–2] - Q charge [(, m) C] - r bubble radius (m) - R universal gas constant (8.314 J mol^–1 K^–1) - T absolute temperature (K) - u electrophoretic mobility (m² s^–1 V^–1) - electrophoretic velocity (m s^–1) - electrolyte permittivity (F m^–1) - electrolyte viscosity (N m^–2 s) - surface concentration (mol m^–2) - k Debye-Huckel parameter (m^–1) - electrolyte density (kg m^–3) - gas density (kg m^–3) - zeta potential (V) Paper presented at the International Meeting on Electrolytic Bubbles organized by the Electrochemical Technology Group of the Society of Chemical Industry, and held at Imperial College, London, 13–14 September 1984.     

Redox chemistry of H2S oxidation in the British Gas Stretford Process Part I: Thermodynamics of sulphur-water systems at 298 K

The thermodynamics of aqueous sulphur-water systems are summarized in the form of potential-pH diagrams, calculated from recently reported critically assessed standard Gibbs energies of formation of the species considered. However, there is convincing evidence from the literature that a value of pK _a(HS^–) = 17–19 is appropriate, whereas a value of 13 is widely accepted; hence, the higher value of 19, corresponding to G _f ⁰ (S^2–) = 120.5 kJ mol^–1 , was used in these calculations, rather than G _f ⁰ (S2-) = 86.31 kJ mol^–1 quoted in the main data source.Under ambient conditions, only – 2 (sulphide), 0 (elemental sulphur) and + 6 (sulphate) oxidation states are thermodynamically stable in water, which is predicted to be oxidized by peroxodisulphate (H₂ S₂ O₈/SO ₈ ^2– and peroxomonosulphate (HSO ₅ ^– /SO ₅ ^2– ). However, when sulphate is excluded from the calculations to allow for the large energy of activation/slow kinetics of its formation from sulphide, then other sulphoxy species appear on the diagram for what is then a metastable system. Similarly, if all sulphoxy species (i.e. any species with oxidation states > 0) are excluded, then polysulphide ions (S _n ^2– , 2 n 5) have areas of predominance at high pH, each with a narrow potential window of predominance. Hence, this information is complemented with S _n ^2– /HS^– activity-potential diagrams at pH 9 and 14.Some species have no area of stability even on the metastable diagrams. Hence, potential-pH diagrams are also presented for the sulphite-dithionite system (excluding elemental sulphur), and that involving peroxomonosulphate (HSO ₅ ^– /SO ₅ ^2– ) in place of peroxodisulphate (H₂S₂O₈/SO ₈ ^¨– ).     

Oxidative dehydrogenation of ethane over some titanates based perovskite oxides

A series of perovskite catalysts have been tested for the oxidative dehydrogenation of ethane. The composition of these catalysts covered CaTi_1–x Fe _x O_3–, with 0 x 0.4, SrTi_1–x Fe _x O_3–, with 0 x 1.0, as well as mixtures of these. The latter catalysts containing more basic Sr metal showed higher selectivity to ethene than the former catalysts containing Ca. A few catalysts with Co on B-sites in the lattice were tested, but lost their stability above 923 K, resulting in a substantial change in the product selectivity. The perovskites gained activity when Fe was introduced in the lattice to form hypervalent ions (Fe⁴⁺) which are believed to play a role in the catalytic activity of these materials.     

The role of mass transfer in the electrolytic reduction of hexavalent chromium at gas evolving rotating cylinder electrodes

The rate of electrolytic reduction of hexavalent chromium from acidic solution at a hydrogen-evolving rotating cylinder lead cathode was studied under conditions of different current densities, Cr⁶⁺ concentrations and rotation speeds. The rate of the reaction was found to follow a first order rate equation. The specific reaction rate constant was found to increase with increasing rotation speed until a limiting value was reached with further increase in rotation speed. Mechanistic study of the reaction has shown that at relatively low rotation speeds the reduction of Cr⁶⁺ is partially diffusion controlled, at higher speeds the reaction becomes chemically controlled. The limiting specific reaction rate constant was related to the operating current density by the equationK=0.044i ^1.385. The current efficiency of Cr⁶⁺-reduction was measured as a function of current density, initial Cr⁶⁺ concentration and rotation speed. Possible practical applications are discussed.Nomenclature A electrode area (cm²) - a, b constants in Equations 5 and 13, respectively - C bulk concentration of Cr⁶⁺ at timet(M) - C _o initial concentration of Cr⁶⁺ (M) - C _i interfacial concentration of Cr⁶⁺ (M) - d cylinder diameter (cm) - D diffusivity of Cr⁶⁺ (cm² s^–1) - e _o standard electrode potential (V) - F Faraday's constant (96 487 C) - current consumed in hydrogen discharge (A) - i current density (A cm^–2) - I cell current (A) - K _l mass transfer coefficient (cm s^–1) - K _r mass transfer coefficient due to cylinder rotation (cm s^–1) - K _o natural convection mass transfer coefficient (cm s^–1) - K _g mass transfer coefficient due to hydrogen stirring (cm s^–1) - K ₂ specific reaction rate constant (cm s^–1) - K overall rate constant (cm s^–1) - m theoretical amount of Cr⁶⁺ reduced during electrolysis (g) - P gas pressure (atm) - R gas constant (atm cm³ mol^–1 K^–1) - T temperature (K) - t time (s) - V linear speed of the rotating cylinder (cm s^–1) - hydrogen discharge rate (cm³ cm^–2 s^–1) - V _s solution volume (cm³) - z electrochemical equivalent (g C^–1) - Z number of electrons involved in the reaction - Re Reynolds number (Vd/v) - Sh Sherwood number (K _r d/D) - Sc Schmidt number (v/D) - rotation speed (r.p.m.) - kinematic viscosity (cm² s^–1)     

Thermal and chemical ionization in flames

Conclusions We have determined the rate constants of the potassium ionization process AA⁺+e^– in the flames of 2H₂+O₂+X (Ar, He) mixtures on the temperature interval 1500–2500° K. The activation energy of this process is close to the ionization potential of potassium (100 kcal).In our experiments the rate of ion formation in the front of a hydrogen flame seeded with potassium exceeded the purely thermal ionization rate by 0.5–2 orders. The presumed cause is recombination ionization of the potassium in the flame front, for example, K+O+OK⁺+O₂+e^–. This is confirmed by the intensification of ionization in the reaction zone in the presence of an excess of oxygen in homogeneous H₂-air and H₂–O₂–(He, Ar) mixtures with alkali impurities.At T=1700° K the recombination coefficient for electrons and potassium ions is close to 1·10^–8 cm³·sec^–1. For a more precise determination it is necessary to know the frequency of electron capture by molecules and atoms under the experimental conditions.Experiments on thermal ionization in turbulent flames confirm the earlier conclusion concerning the important role of mass transfer in the chemi-ionization of hydrocarbon flames.Fizika Goreniya i Vzryva, Vol. 6, No. 1, pp. 37–48, 1970     

Preparation and properties of Raney nickel electrodes on Ni-Zn base for H2 and O2 evolution from alkaline solutions Part II: Leaching (activation) of the Ni-Zn electrodeposits in concentrated KOH solutions and H2 and O2 overvoltage on activated Ni-Zn Raney electrodes

The partial dissolution of zinc from electrodeposited Ni-Zn alloys (withX _Zn ⁰ =22–87.3 mol %) was studied, in cold and nearly boiling 10m KOH. It was found that alloys withX _Zn ⁰ 22 mol % are not dissolved at all. The dissolved zinc fraction,A, increased rapidly with further increase in zinc content and after having passed a maximum withA=82–90% atX _Zn ⁰ =55–58 mol % and a sharp minimum withA=52–65% atX _Zn ⁰ =65–69 mol %, it asymptotically approached toA 100% atX _Zn ⁰ 100 mol %. The discontinuous dependence ofA againstX _Zn ⁰ may be explained by differences in the crystallographic composition of the alloy deposits. Alloys withX _Zn ⁰ <50–60 mol % can be allocated to solid solutions of zinc in the Ni matrix (-phase); the range of 50–60<X _Zn ⁰ <70–80 mol % corresponds to the coexistence of + phases. The pure -phase exists within a narrow range atX _Zn ⁰ =75–80 mol %. No zinc dissolution from Ni-Zn alloys withX _Zn ⁰ 22 mol % was explained by extremely low zinc activities in dilute solid solutions of the -phases shifting the Gibbs energy of the dissolution reaction to very low negative, or even to positive values. The dependence of the hydrogen and oxygen overvoltage atj=0.4 A cm^–2 in 10m, KOH at 100°C on the original zinc contentX _Zn ⁰ showed, in both cases, a clear minimum atX _Zn ⁰ =75–78 mol %. This points to a practically pure -phase in the original Ni-Zn alloy with an approximate composition NiZn₃.     

A particulate vortex bed cell for electrowinning: operational modes and current efficiency

The process of electrowinning of copper ions from dilute solutions has been used as a model system to assess the performance of a vortex bed cell with a three-dimensional cathode of conducting particles. Experiments were carried out under three conditions: with constant cell voltage, with constant cell current throughout the process and with exponential decrease of the operating current with time in order to underfollow the limiting current. Results from a batch recirculating system indicate that exponential decrease of operating current with time effects an improvement in current efficiency over a wide range of concentration.Nomenclature specific surface area of particles (cm^–1) - C, C _i concentration of Cu²⁺ ions at the momentt, and initial concentration, respectively (M) - d _p particle diameter (cm) - F Faraday number (96 487 A s mol^–1) - i current density (Am^–2) (calculated for the surface area of the particles) - i _av average current density obtained in the constant cell voltage process (Am^–2) - I _L(t),I _L o limiting current at timet, and initial limiting current, respectively (A) - k _L mass transfer coefficient (cm s^–1) - n number of electrons transferred in the process - Q volumetric flow rate (dm³ s^–1) - R universal gas constant (J mol^–1 K^–1) - t time (s) - T temperature (K) - U cell voltage (V) - V volume of electrolyte (cm³) - v _o volume of particles (cm³) - overpotential (V) - _e current efficiency - , _o bed porosity and porosity of the fixed bed, respectively - =V/Q residence time (s) - see Fig. 2     

Numerical models for reactions catalysed by homogeneous mediators: the case of Fenton's reagent

A numerical model has been developed to describe the behaviour of a batch reactor in which Fenton's reagent is used for hydroxylating aromatic hydrocarbons under conditions of electrochemical regeneration. The test reaction considered is the conversion of benzene into phenol. Comparison is made with previously published experimental results.Nomenclature A electrode area, m² - a ₁ parameter defined by Equation 21 - C _i concentration of species, i, in the bulk solution, mol m^–3 - c _i local concentration of species, i, in the diffusion layer, mol m^–3 - K _i effective mass-transfer coefficient, m s^–1 - k _j rate constant of reaction j - R _j rate of reaction j, mol m^–3 s^–1 - r _i rate of change of concentration of species i due to chemical reaction, mol m^–3 s^–1 - t time, s - V reactor volume, m³ - x distance from the cathode surface, m - x ^* maximum thickness of the diffusion layer, m - period of diffusion layer renewal, s Subscrpts 1 oxygen - 2 Fe³⁺ - 3 hydrogen peroxide - 4 Fe²⁺ - 5 benzene - 6 phenol - 7 biphenyl This paper was presented at the meeting on Electroorganic Process Engineering held in Perpignan, France, 19–20 September 1985.     

Nickel electrowinning using a Pt catalysed hydrogen-diffusion anode. Part II: Batch tank with a sulphate bath

A batch tank of 1.6L capacity has been designed for Ni electrowinning using a Pt catalysed H₂-diffusion anode and a stainless steel/Ni cathode, both of 100cm² area. The anode has high stability for sulphate baths of pH3.5 with 35gL^–1 H₃BO₃ and Ni²⁺ concentrations from 10gL^–1 up to saturation. Linear correlations between the cell voltage and current intensity are found, as expected for ohmic control of the process. Good electrowinning conditions at room temperature are obtained for Ni²⁺ contents from 50gL^–1, using an interelectrode gap of 2cm and regulating the solution pH by NaOH addition for periods of 30min. Current efficiencies 93% are always found. The energy costs increase linearly with increasing current density from 10 to 50mAcm^–2, with values much lower than those obtained using a conventional lead anode. This indicates that the Pt catalysed H_{_2}-diffusion anode is preferable for Ni electrowinning. The crystals are composed of high-purity Ni and have a face-centred cubic structure. Surface analysis by SEM shows that the grain size gradually increases with increasing current density, with no significant influence of the Ni²⁺ concentration.