Studies on nonisothermal solid state galvanic cells — effect of gradients on EMF

An expression for the EMF of a nonisothermal galvanic cell, with gradients in both temperature and chemical potential across a solid electrolyte, is derived based on the phenomenological equations of irreversible thermodynamics. The EMF of the nonisothermal cell can be written as a sum of the contributions from the chemical potential gradient and the EMF of a thermocell operating in the same temperature gradient but at unit activity of the neutral form of the migrating species. The validity of the derived equation is confirmed experimentally by imposing nonlinear gradients of temperature and chemical potential across galvanic cells constructed using fully stabilized zirconia as the electrolyte. The nature of the gradient has no effect on the EMF.Nomenclature J _i flux of speciesi - X _i generalized forces - L _ij Onsagar coefficient - ₁ electrochemical potential of ions - ₂ electrochemical potential of electrons - T absolute temperature - U ₁ ^* total energy of transfer of the ion - partial molar enthalpy of the ion - Q ₁ ^* heat of transport of the ion - Z ₁ charge on the ion - F Faraday constant - electrostatic potential - ₂ chemical potential of the electron - ₁ chemical potential of the ion - partial entropy of the ion - E _SE EMF developed across the solid electrolyte - E _Pt EMF developed across the platinum lead - ( ₂)_Pt chemical potential of electrons in platinum - partial entropy of electrons in platinum - (Q ₂ ^* )_Pt heat of transport of electrons in platinum - E _cell EMF developed across the whole cell - chemical potential of oxygen - chemical potential of oxygen in its standard state - R universal gas constant - partial pressure of oxygen - relative chemical potential of oxygen - _M relative chemical potential of metal M - a _M activity of metal M     

Calculation of mean current densities in a two rotating disc electrodes system

A theoretical relationship for mass transfer in the laminar flow region of streaming in a rotating electrolyser was derived by the method of similarity of the diffusion layer for electrodes placed sufficiently far from the rotation axis. The obtained relationship was compared with the known equations valid for systems with axial symmetry. The mean current densities were found from the numerical solution of the convective diffusion equation by the finite-element method and were compared with experimental results.Nomenclature a constant, exponent - c concentration - c ₀ concentration in the bulk phase - C _ij matrix coefficient - D diffusion coefficient - F Faraday constant, 96487 C mol^–1 - h interelectrode distance - j current density - mean current density - J mass flux density - L _j base function - n number of transferred electrons in electrode reaction - n _r outer normal to the boundary - mass flux - N number of nodal points in an element - Q volume rate of flow - mean volume rate of flow - r radial coordinate - r ₀ inner electrode radius - r _l outer electrode radius - r _v radius of inlet orifice - r _d outer disc radius - v _r radial velocity component - v _z normal velocity component - z normal coordinate - thickness of the layer in which the equation of convective diffusion is solved - boundary of the integration domain - thickness of the diffusion layer - _N thickness of the Nernst diffusion layer - v kinematic viscosity - angular velocity - surface Criteria Re _chan channel Reynolds numberQ/hv - Re _loc local Reynolds number,Q/(r + r ₀) - local Reynolds number at mean electrode radius,Q/v(r ₁ +r ₀) - Re _rot rotation Reynolds number, r _d ² /v - modified rotation Reynolds number at mean electrode radius, (r ₁+r ₀)²/4v - Ré _rot modified rotation Reynolds number, (r+r ₀)²/4v - Sc Schmidt number,v/D - Sh _r local Sherwood number,j(r-r ₀)/nFDc _o - mean Sherwood number, - Ta Taylor number,h(/v)^1/2     

Copper recovery from dilute solutions in the Falling-Film cell

The removal of copper from dilute solutions is examined in electrochemical reactors where the electrolyte flows as a thin film in an inclined channel between a plane plate and a sheet of expanded metal (Falling-Film cell). Copper is recovered as a thin sheet from the plane plate. The results are compared with a known simplified model and the variations of the faradaic yield with the operating conditions are discussed.Nomenclature A _e electrode surface area - b width of inclined channel - C(t) copper concentration at timet - C ₀ initial copper concentration - d interelectrode distance - overall current density - overall limiting current density - overall mass transfer coefficient - L length of the channel - Q _v volumetric flow rate - Q _vl volumetric flow rate per unit of channel width (=Q _v/b) - t time - t _s residence time in the reactor, defined by Equation 1 - mean flow velocity of the liquid film, defined by Equation 2 - V volume of electrolyte in the circuit - V _R reactor volume - v _sn normalized space velocity, defined by Equation 9 - inclination angle with respect to the horizontal - instantaneous faradaic yield - time-averaged faradaic yield - v _e number of electrons exchanged in the electrochemical reaction     

Optimization of an electrochemical hydrogen-chlorine energy storage system

A mathematical model is presented for the optimization of the hydrogen-chlorine energy storage system. Numerical calculations have been made for a 20 MW plant being operated with a cycle of 10 h charge and 10h discharge. Optimal operating parameters, such as electrolyte concentration, cell temperature and current densities, are determined to minimize the investment of capital equipment.Nomenclature A _ex design heat transfer area of heat exchanger (m²) - a _F electrode area (m²) - heat capacity of liquid chlorine (J kg^–1K^–1) - heat capacity of hydrogen gas at constant volume (J kg^–1 K^–1) - c _p,hcl heat capacity of aqueous HCl (J kg^–1 K^–1) - C _$acid cost coefficient of HCl/Cl₂ storage ($ m^–1.4) - C _$ex cost coefficient of heat exchanger ($ m^–1.9) - C _$F cost coefficient of cell stack ($ m^–2) - cost coefficient of H₂ storage ($ m^–1.6) - C _$j cost coefficient of equipmentj ($/unit capacity) - C _$pipe cost coefficient of pipe ($ m^–1) - C _$pump cost coefficient of pump ($ J^–0.98 s^–0.98) - E cell voltage (V) - F Faraday constant (9.65 × 10⁷ C kg-equiv^–1) - F _j design capacity of equipmentj (unit capacity) - G _D design electrolyte flow rate (m³ h^–1) - heat of formation of liquid chlorine (J kg-mol^–1 C1₂) - H _f ⁰ ,_HCl heat of formation of aqueous HCl (J kg-mol^–1HCl) - H _m total mechanical energy losses (J) - I total current flow through cell (A) - i operating current density of cell stack (A m^–2) - L length of pipeline (m) - N number of parallel pipelines - n_HCl change in the amount of HCl (kg-mole) - P pressure of HCl/Cl₂ storage (kPa) - p ₁ H₂ storage pressure at the beginning of charge (kPa) - p ₂ H₂ storage pressure at the end of charge (kPa) - –Q _ex heat removed through the heat exchanger (J) - R universal gas constant (8314 J kg-mol^–1 K^–1) - the solubility of chlorine in aqueous HCl (kg-mole Cl₂ m^–3 solution) - T electrolyte temperature (K) - T ₂ electrolyte temperature at the end of charge (K) - T _max maximum electrolyte temperature (K) - T _min minimum electrolyte temperature (K) - t final time (h) - t _ex the length of time for the heat exchanger operation (h) - Uit _ex overall heat transfer coefficient (J h^–1 m^–2 K^–1) - V _acid volume of HCl/Cl₂ storage (m³) - } volume of H₂ storage (m³) - v design linear velocity of electrolyte (m s^–1) - amount of liquid chloride at timet (kg) - amount of liquid chlorine at timet ₀ (kg) - w _hcl amount of aqueous HCl solution at timet (kg) - W _p design brake power of pump (J s^–1) - X electrolyte concentration of HCl at timet (wt fraction) - X _f electrolyte concentration of HCl at the end of charge (wt fraction) - X _i electrolyte concentration of HCl at the beginning of charge (wt fraction) - X ₀ electrolyte concentration of HCl at timet ₀ (wt fraction) - Y objective function to be minimized ($ kW^–1 h^–1) - _j the scale-up exponent of equipmentj - overall electric-to-electric efficiency (%) - _acid safety factor of HCl/Cl₂ storage - fractional excess of liquid chlorine - _p pump efficiency - average density of HCl solution over the discharge period (kg m^–3)     

On the determination of oxygen potentials and O/M ratios in mixed U-Pu oxides by means of solid state galvanic cells

Measurements of oxygen potential on uranium-plutonium mixed oxides by means of solid state galvanic cells are discussed and compared with precedent data. Discrepancies in the values at low temperature (1000 K) as well as in the versusT slopes are found, which disappear when the samples are thermally cycled. An analysis is performed of this effect, showing that the absence of a good homogeneization of the sample in oxygen and/or in plutonium content can give rise to mixed potentials and hence to erroneous results.     

Nitrogen transformations in a flooded soil in the presence and absence of rice plants: 2. Denitrification

Denitrification rates (d) in a flooded alkaline clay were measured following addition of either to the floodwater, by collecting evolved N₂ + N₂O in an enclosure in the absence or presence of rice plants. Similar estimates of d were obtained in the treatment when the isotopic composition of the enclosed atmosphere was determined using arc redistribution or direct mass spectrometric analysis. Approximately 90% of the gaseous products of denitrification were physically trapped in the soil five days after addition. Mechanical shaking of the soil-water system was an effective method for releasing entrapped gas. Denitrification showed a marked diurnal variation in both and treatments planted to rice, with higher rates during the day than at night. Measured rates of denitrification were higher in planted than in unplanted pots for both and treatments for normal gas sampling. However, evidence was obtained that this was not a real effect, but was due to release of entrapped gas. Denitrification losses corrected for gas entrapment were estimated at <5% of applied . The ¹⁵N mass balance indicated that a much larger amount of applied ammonium (15–25%) was lost by NH₃ volatilisation. The rate of denitrification corrected for gas entrapment was similar to the rate of nitrification estimated by inhibition of ammonium oxidation. Although the inhibitors 2-ethynylpyridine and acetylene prevented denitrification by effectively inhibiting nitrification of , the total recovery of ¹⁵N in the soil-plant system did not increase. The total recovery of was 7–9% higher in the presence than in the absence of rice.     

The performance of porous and gauze electrodes in electrolysis with parabolic velocity distribution

An approximate numerical method for the estimation of the velocity exponent in (small-scale) flow-through porous and gauze electrodes is presented. The method can also be employed to determine if a plug-flow or a parabolic-flow model offers a more reliable representation of the experimental behaviour of the electrode.Nomenclature a cross sectional area of the electrode - B integration parameter (Equations 7 and 8) - c exit active ion concentration, its mean measured value in the case of parabolic flow,c _o its inlet value;c _m its mean value; its mean calculated value in the case of parabolic flow;c ^* dimensionless concentration, equal toc/c _o; mean dimensionless concentration, equal to /c _o - F Faraday's constant - i _L mean limiting current density (geometric-area base) - j proportionality factor (Equation 1) - k _m mass transport coefficient, its mean value - L length of the electrode - n number of electrons involved in the electrode reaction - N ionic flux - r radial coordinate - R _E geometric radius - R limiting degree of conversion - s specific surface area of the electrode (surface per volume) - u linear solution velocity; u_o its maximum (centreline) value; its mean value (=u_o/2) - v volumetric flow rate; its mean value - x transform variable forz - z dimensionless radial distance - velocity exponent for mass transport (Equation 1)     

Flow-through and flow-by porous electrodes of nickel foam. I. Material characterization

This paper deals with the characterization of three nickel foams for use as materials for flow-through or flow-by porous electrodes. Optical and scanning electron microscope observations were used to examine the pore size distribution. The overall, apparent electrical resistivity of the reticulated skeleton was measured. The BET method and the liquid permeametry method were used to determine the specific surface area, the values of which are compared with those known for other materials.Nomenclature a _e specific surface area (per unit of total volume) (m^–1) - a _s specific surface area (per unit of solid volume) (m^–1) - (a _e)_BET specific surface area determined by the BET method (m^–1) - (a _e)_Ergun specific surface area determined by pressure drop measurements (m^–1) - mean pore diameter (m) - mean pore diameter determined by optical microscopy (m) - mean pore diameter using Ergun equation (m) - e thickness of the skeleton element of the foam (m) - G grade of the foam (number of pores per inch) - P/H pressure drop per unit height of the foam (Pa m^–1) - r electrical resistivity ( m) - R _h hydraulic pore radius (m) - T tortuosity - mean liquid velocity (m s^–1) Greek symbols mean porosity - circularity factor - dynamic viscosity (kg m^–1 s^–1) - liquid density (kg m^–3) - pore diameter size dispersion     

Influence of the partial pressure of oxygen on the redox chemistry of multivalent elements in molten sodium tetraborate

Voltammetric determinations based on current measurements have been developed in order to allow anin situ evaluation of the redox ratios of multivalent elements in molten sodium tetraborate. The same technique has been used to study the influence of the partial pressure of oxygen in the gaseous atmosphere on the redox properties of sodium tetraborate melts. Experiments were carried out at 1273 K in solutions of chromium with oxidation states VI and III and of antimony with oxidation states V and III; the values ranged from 5×10^–5 atm to 1 atm. Log [Ox]/[Red] plotted against log shows linear relationships with slopes of 0.72 and 0.54 in the case of chromium and antimony solutions, respectively. These values lie close to the theoretical values of 0.75 and 0.50, respectively, which are deduced assuming conditions of equilibrium between oxygen and the solute. In agreement with these results, voltammetric and potentiometric determinations confirm Nernstian behaviour of the oxygen-oxide redox system.     

Measurement and regulation of oxygen content in selected gases using solid electrolyte cells. IV. Accurate preparation of CO2-CO and H2O-H2 mixtures

Mixtures of CO₂-CO, H₂O-H₂ and Ar-H₂O-H₂ of precise composition were prepared using a zirconia pump and analysed with a zirconia gauge. The ratio was varied from 5×10^–2 to 10⁴ and the ratio from 3×10^–4 to 10^–2. A Faraday's Law test proved to be a simple and reliable procedure for checking the conditions of utilization of these gaseous mixtures and for verifying that no significant disproportionation of CO or leakage along the gas circuit altered the prepared composition. From a practical point of view the best methods of preparing mixtures with low oxygen activity are reduction of carbon dioxide in the range 5×10^–11 to 10^–17 atm and oxidation of inert gas-H₂ in the range 10^–19 to 10^–27 atm at 800°C.     

Etude de la réaction de réduction électrochimique de l'aluminium dans les mélanges de chlorures alcalins et de chlorures-fluorures alcalins fondus à 450°C

Résumé La réduction électrochimique de AlCl₃ dans les mélanges NaCl-KCl-LiCl et NaCl-KCl-LiCl-LiF fondus à 450°C, a été étudiée par chronopotentiométrie et chronoamperométrie sur électrode d'argent. Les chronopotentiogrammes et les chronoamperogrammes obtenus, sont interpretés comme un processus simple, regi par la diffusion, entre espèces solubles qui met en jeu l'échange d'un nombre apparentn d'électrons dont la valeur décroit, tout en restant cependant voisine de 2, au fur et a mesure que la densité du courant imposé diminue.Nous avons également étudié le profil de diffusion du dépôt formé à l'intérieur de l'électrode d'argent à l'aide de la microsonde électronique et calculé en utilisant les courbes chronoamperométriquesI = f(t), une valeur approximative du coefficient de diffusion de l'aluminium, , réduit sur cette même électrode, qui est du même ordre de grandeur dans les deux électrolytes étudiés. 10 × 10^–12 cm²s^–1 pourn=3.

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The electrochemical reduction of AlCl₃ in the molten electrolytes NaCl-KCl-LiCl and NaCl-KCl-LiCl-LiF at 450°C was studied by means of chronopotentiometry and chronoamperometry on a silver electrode. We interpreted the obtained chronopotentiometric and chronoamperometric curves by means of a simple, diffusion-controlled reaction between soluble species involving the exchange ofn electrons. The calculated apparent value ofn decreased as the applied current density was reduced, but it remained always approximately equal to 2.We also investigated the diffusion profile of the deposit in the bulk of the silver electrode by means of a microprobe analyser and calculated, using theI=f(t) curves, an approximate value of the diffusion coefficient , of aluminium reduced on this electrode. This value is approximately the same for both electrolytic melts used, i.e. 10×10^–12 cm²s^–1, assuming thatn=3.

     

Enhanced mass transfer in electrochemical cells using turbulence promoters

Many electrochemical processes suffer in varying degrees from mass transfer limitations. These limitations may require operation at considerably less than economic optimum current densities. Mass transfer to a surface may be considerably enhanced by insertion of turbulence promoters in the fluid flow path near the affected surface.An instrument was developed to measure local current densities in the hydrodynamically very difficult region near the turbulence promoter. A general method for the relative evaluation of hydrodynamic conditions has been developed. Generalization of the data permits optimization of hydrodynamic cell design using the promoter shapes investigated.

Notation

Symbols A Coefficient for cell power costs, $ m² (As)^–1 - A _c Cell area, m² - a Constant in Equation 4 - B Coefficient for area-proportional costs, $ A (m² s)^–1 - C Coefficient for pumping power costs, $ A (m² s)^–1 - C _b Bulk concentration, kg mol m^–3 - C _bi Inlet bulk concentration, kg mol m^–3 - C _e Energy cost, $ (Ws)^–1 - C _i Interfacial concentration, kg mol m^–3 - ¯C _s Amortized area cost, $ (m² s)^–1 - D Current—density-insensitive costs, $ s^–1 - D _e Equivalent diameter, m - D Diffusion constant, m² s^–1 - e Current efficiency - F _d Cell feed rate, m³ s^–1 - F 96.5×10⁶ A s kg eq^–1 - g Channel width, m - h Channel height, m - i Current density, A m^–2 - i _opt Economic optimum current density, A m^–2 - K Total costs of running cell, $ s^–1 - (K–D)_ideal Total sensitive costs under hydrodynamically ideal conditions, $ s^–1 - k _c Convective mass transfer coefficient, m s^–1 - L Total length of flow path, m - l Promoter spacing, m - N Mass flow rate to surface due to convection, kg mol m² s^–1 - n _e Number of electrons transferred in electrode reaction - P _c Power required by cell, W - P/L Average pressure gradient in channel, N m^–3 - R _av Effective cell resistance, m² - S Open channel cross-section, m² - S ₀ Minimum channel cross-section at promoter, m² - s _i Stoichiometric coefficient of species i - t _i Transport number of species i in solution - ¯t _i Effective tranport number of species at polarized surface - V Average fluid velocity, m s^–1 - x Distance from inception of concentration disturbance, m - ₁ Electrical power conversion efficiency - ₂ Pumping power conversion efficiency - Solution viscosity, kg (m s)^–1 - Solution density, kg m^–3 Dimemionless groups Fanning friction factor - Reynolds number - R h/g Channel aspect ratio - D _e/l Promoter frequency - S/S ₀ Contraction coefficient - Sherwood number - Degree of reaction - Dimensionless total sensitive - Dimensionless current density - Energy cost ratio     

Fine structural characteristics and physical properties of raw silk

Summary The theories of KRATKY, DEBYE and BUESCHE and POROD have been applied to evaluate macromolecular parameters which speak of the fine structural characteristics of raw silk — a natural polymer in the solid state. The small-angle KRATKY camera has been utilised for the measurements of the scattering intensities. The macromolecular parameters evaluated are the percentage of void (w₁), the specific inner surface (O/V), length of coherence (1_c), range of inhomogeneity (1_r), transversal length –s – and which were found to be equal to 0.13%, 25.15 × 10^–4 Å^–1, 21.84 Å, 2.11 Å, 1.59 × 10³ Å and 2.11 Å respectively The physical properties as evaluated by Scott's IP2 Tester have been reported.     

Electrolytic removal of oxygen from gases by means of solid electrolyte

Nitrogen gas, containing controlled amounts of free oxygen and water or carbon dioxide, was de-oxidized by means of a solid oxide electrolyte tube at temperatures from 800 to 1000°C and at applied voltages of 0.5–3.0 V. The oxygen potential of the resulting gas mixture was measured simultaneously. The efficiency of the removal of oxygen, present in free or combined state, is shown as a function of the initial oxygen content , temperature and applied voltage. The electrolysing current/applied voltage relationships obtained indicate that the resistance of the electrolysing circuit is mainly determined by the initial free oxygen content of the gas mixture. The behaviour of solid oxide electrolytes with externally applied potentials corresponding to pure ionic and mixed conduction is also discussed.     

Flow-through and flow-by porous electrodes of nickel foam. II. Diffusion-convective mass transfer between the electrolyte and the foam

The work described here concerns the diffusion-convective mass transfer to flow-through and flow-by porous electrodes of nickel foam. Empirical correlations giving the product of the mass transfer coefficient and the specific surface areaa _e of the material as a function of the pressure drop per unit electrode height and as a function of the grade characterizing the foam are proposed. The performance of various materials are compared in terms of vs the mean linear electrolyte flow velocity.Nomenclature a _e specific surface area (per unit of total volume of electrode) (m^–1) - A, B Ergun law coefficients determined in flow-by configuration - A, B Ergun law coefficients determined in flow-through configurationA, A (Pa m^–3 s²);B, B (Pa m² s^–1) - C _E entering concentration of ferricyanide ions (mole m^–3) - D molecular diffusion coefficient (m² s^–1) - F Faraday number (C mol^–1) - G grade of the foams - I _L limiting current (A) - mean mass transfer coefficient (m s^–1) - n number of stacked foam sheets in the electrode - P/H pressure drop per unit of height (Pa m^–1) - Q _v volumetric electrolyte flow rate (m³ s^–1) - Re Reynolds number - Sc Schmidt number - Sh Sherwood number - T mean tortuosity of the foam pores - mean electrolyte velocity (m s^–1) - V _R electrode volume (m³) - X conversion - dynamic viscosity (kg m^–1 s^–1) - v number of electrons in the electrochemical reaction - v kinematic viscosity (m² s^–1)     

Residence time distributions and reaction kinetics in a fuel cell anode

A method which treats the fuel cell anode as a chemical reactor is developed to predict fuel cell performance. The method is based on experimentally measured residence time distribution parameters and differential cell kinetic data. The apparatus and experimental technique used to obtain the gas-phase residence time distributions are described. Kinetic data obtained from differential cell tests of the electrodes are used to evaluate an empirical rate expression.Axial dispersion model solutions for flow with volume change are obtained, based on the measured Peclet numbers and empirical rate expressions, and compared with experimental data from operating large high-temperature molten carbonate fuel cells. Agreement between the model and the experimentally determined data is very good, but only for low conversions of the fuel.Notation A cross-sectional area, cm² - C concentration of hydrogen. (g mole/cm³) - c=C/C _o dimensionless concentration of hydrogen - D dispersion coefficient cm²/s - d _e equivalent diameter, cm - F Faraday's constant - I total current, A - J current density, mA/cm² - k reaction rate constant, appropriate units - L length, cm - M number of moles - N =D/UL dispersion number - n order of reaction - n _e number of electrons transferred - –r rate of reaction based on volume of fluid, moles of reactant reacted/ cm³ s - S _e surface of electrode, cm² - T absolute temperature, °K - mean residence time, s - U velocity component in Z direction, cm/s - u = U/U ₀ dimensionless velocity - V _a volume of system, cm³ - V operating voltage, V - v volumetric flow rate, cm²/s - fractional conversion, degree of conversion of hydrogen - y mole fraction of hydrogen - Z space coordinate, cm - z =Z/L fractional length Greek letters coefficient of expansion - _m molar density of fuel, g mole/cm³ - overvoltage, V - dimensional variance, s² - ² dimensionless variance - =V_a/v ₀ space time, s     

The transient behaviour of a class of semicontinuous tank electrolysers

The transient behaviour and figures of merit of an isothermal electrolyser consisting of a perfectly mixed-flow compartment and a batch compartment separated by an ion-selective membrane are analysed by means of its governing balance equations solved on a microcomputer.Nomenclature c acid concentration - E conversion of the oxidized to the reduced species at the cathode - F Faraday's constant (96487 C mol^–1) - i current density - K cost of operation - k ₁,k ₂ cost coefficients (specific costs) - Q _s charge density; its sample regression - S _O initial slope of the conversion-time curve (Fig. 3) - t time - t _H transport (transference) number of hydronium ions - V magnitude of the imposed voltage - x ₁ ⁰ magnitude of the inlet concentration of the oxidized species - w acid concentration in the anolyte - lumped parameter, defined as 1+^k m ^A/Q _c - sample linear regression parameters (Table 5) - fractional concentration of the oxidized species defined as (concentration — surface concentration)/concentration - _c mean residence time defined asV _c /Q _c Special symbols MU arbitrary monetary unit - * steady-state The majority of symbols is defined in Table 1.     

Dissolution of tin in the presence of Cl− ions at pH 4

The anodic dissolution of tin, investigated in an acidic solution at pH 4 containing 0.1–1 M NaCl at 25°C, displays Tafel behavior as long as the electrode surface is bare (E–0.5 V vs SCE). The main characteristics can be derived from the proposed dissolution mechanism, i.e and . The mechanism involves two consecutive steps, each corresponding to the transfer of one electron, the second step being rate determining. ForE values anodic to –0.5V vs SCE partial coverage of the surface by a corrosion product is observed and the behavior is no longer Tafelian. From 0.4 to 1V vs SCE, a plateau current is observed on logi vsE curves and the anode is completely covdered by a corrosion product. Results obtained with a rotating disc electrode suggest that the rate-determining step of the dissolution process in this region of potential is the diffusion of an ionic species into the solution.     

The influence of total monomer concentration on the “reactivity” of ω-(p-vinylbenzyl ether) macromonomers of poly(2,6-dimethyl-1,4-phenylene oxide) determined from radiacal copolymerization experiments with butyl methacrylate

Summary The influence of the total monomer concentration on the radical reactivity ratio r₁ of butyl methacrylate (BMA) (M₁)--(p-vinylbenzyl ether) macromonomer of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO-VBE) (M₂) monomer pair was investigated. For two different molecular weights of the PPO-VBE macromonomer ( , and ), the determined reactivity ratio r₁ decreases with the increase of the macromonomer concentration. Therefore, the reactivity of the macromonomer, 1/r₁, follows the opposite trend. This dependence is due to micelles formation during copolymerization. This microsegregation process partitionates the comonomer concentrations between the bulk of solvent and around the growing chain and therefore, the experimental r₁ is actually a product of the true reactivity ratio r₁ ⁰ and a partition coefficient k.     

Surface characterization of zirconium titanate (ZrTiO4) powder by measurements of electrical photoconductance and photoassisted oxygen isotope exchange

The photosensitivity of a ZrTiO₄ sample (33 m²/g) prepared by a sol-gel method has been assessed in the presence of O₂ by both photoconductance and oxygen isotope exchange (OIE) measurements at room temperature at wavelengths > 290 nm. For oxygen pressures < ca. 13.3 Pa, the steady-state photoconductance of ZrTiO₄ was unaffected by , which indicated that the direct recombination of the photoproduced charges played the dominant role. At higher pressures, varied as the reciprocal of , which was consistent with the fact that the electronic equilibrium was then governed by O₂ + e^– O ₂ ^– . OIE over ZrTiO₄ occurred predominantly via the overall mechanism which involves the exchange of two surface oxygen atoms for each exchange act. It was very slow as compared with OIE over photocatalytically active anatase samples which, in addition, occurs via another mechanism. These results allow one to predict that this ZrTiO₄ sample is a poorly active photocatalyst for oxidations involving gaseous oxygen, and further illustrate the interest of and OIE measurements to evaluate the photosensitivity of semiconductor oxide samples.