Electrochemical regeneration of ceric sulphate in an undivided cell

Ceric sulphate (0–0.5 m) was generated electrochemically from cerous sulphate slurries (0.5–0.8 m total cerium) in 1.61 m sulphuric acid, at 50 °C, using a bench scale differential area undivided electrochemical cell with an anode to cathode ratio of eleven. A cell current efficiency for Ce(IV) of 90% was obtained at an anode current density of 0.25 A cm^–2. An empirical model illustrates an increase in overall current efficiency for Ce(IV) with an increase in electrolyte velocity, an increase in total cerium concentration, and a decrease in the cell current. From separate kinetic studies on rotating electrodes, both, anode and cathode kinetics were found to be affected by cerium sulphate adsorption processes. Anode adsorption of cerous sulphate species leads to inhibited mass transfer and negatively affected current efficiencies for Ce(IV). Cathode adsorption of cerium sulphate is thought to be responsible for high cathode current efficiencies for hydrogen (93–100%). The dissolved cerous sulphate concentration increased with increasing ceric sulphate and total cerium sulphate concentrations resulting in slurries with a stable dissolved cerous sulphate concentration of as high as 0.851 m in 1.6 m H₂SO₄ at room temperature.     

Electrolysis of simulated flue gas solutions in an undivided cell

The electrolysis of simulated flue gas solution based on sulphite/sulphuric acid is reported. Two undivided cells were employed, a small batch cell and a sieve plate electrochemical reactor (SPER). With the batch cell, two anode materials were tested for the oxidation of sulphite to sulphate, platinum and graphite. Ebonex and zirconium cathodes were used in connection with both the anodes. High current efficiencies, above 85%, were obtained in all cases, without the formation of elemental sulphur. Comparable results, 85–95%, were obtained with the sieve plate electrochemical reactor, for a current density range of 10–25 mA cm⁻² at low sulphite concentration (0·005 M ) in 2–6 M sulphuric acid. ©1997 SCI     

Electrogenemtion of Mn(III) in an undivided cell

The electrochemical oxidation of manganous ion to manganic ion in acetic acid may be efficiently carried out in a parallel plate undivided cell. Reduction of manganic ion to manganous ion at the cathode is a relatively inefficient reaction, allowing the formation of solutions of manganic ion as high as 0.05 mol dm^?3, at greater than 80% current efficiency. The effects of the major variables have been evaluated.     

Paired electro-oxidation Part II: Production of anthraquinone in an undivided cell

Oxidation of anthracene to anthraquinone by the anodically generated oxidant V⁵⁺ and both V⁵⁺ and · OH free radicals generated cathodically was carried out simultaneously in an undivided cell. Both the yield and selectivity of anthraquinone by paired electro-oxidation are higher than that by anodic oxidation only. The maximum total current efficiency for anthraquinone production in the paired electro-oxidation was 151%. A process for anthraquinone production using an undivided cell electrolyser is proposed.     

无隔膜法电解制备次氯酸钠及其稳定性研究

消毒是饮用水处理工艺中必不可少的一个环节,而化学消毒法中的次氯酸钠消毒法因具有很好的消毒效果及安全性而拥有较好的发展前景。无隔膜法电解制备次氯酸钠具有反应装置简单经济、电解原料简单易得、能耗低等优点,但其机理仍需探讨,其较差的稳定性也限制更广泛的应用。探究了温度、pH、电解质类型、电解质浓度等多种因素对无隔膜电解制备次氯酸钠的电解效率的影响,得到了优化的电解条件。用微分法确定电解反应级数n=4.38,计算电解的反应速率常数,建立动力学方程,对电解机理和步骤进行了深入的探讨。同时探究了次氯酸钠溶液中有效氯浓度随时间的变化,并建立了有效氯衰减一级、二级动力学模型,以便对其稳定性进行调控。     

Dissolved oxygen concentration in an undivided rotating cylinder electrode reactor

The steady state mass balance for dissolved oxygen in an undivided rotating cylinder electrode reactor was established. Cases involving the absence or presence of diffusion controlled oxygen reduction reaction were treated. The results show that in the absence of the oxygen reduction reaction and under low electrolyte velocities employed industrially, the rotating cylinder electrode reactor, exhibiting a continuous stirred tank reactor behaviour, yields lower dissolved gas concentrations than a plug flow reactor. Therefore, the rotating cylinder electrode reactor leads to lower redissolution rates of the detached metallic particles by oxygen corrosion. The effect of the dissolved gas concentration in the inlet electrolyte also demonstrates the benefits of a prior degassing step in the flow sheet. The simpler equation derived in the absence of oxygen reduction may be used in its presence for inlet dissolved oxygen concentrations below saturation and industrially relevant electrolyte velocities ranging from approx. 0 to 0.3 m s⁻¹ if metal deposition current efficiency is higher than 87% or, conversely, if the dissolved metal concentration to dissolved oxygen concentration ratio is greater than 27. An even simpler equation based on the added hypothesis of total desorption of the gas produced can also be used with electrolyte velocities below 0.025 m s⁻¹. Finally, the derived equations allow optimization of the rotating cylinder electrode reactor.     

Electrosynthesis of adiponitrile in undivided cells

It has been shown that adiponitrile can be synthesized in undivided cells under favourable conditions. Because of strong decomposition of the quaternary ammonium salt support electrolyte, its concentration must be kept at a very low level. To avoid excessive resistance of the cell, the distance between the electrodes must then be smaller than 0·5 mm. We have developed various types of such cells. The capillary gap cell is simple in design and can be readily scaled up. Continuous operation is possible. Yields of about 90% of adiponitrile have been obtained. The energy consumption is less than 3 kWh/kg product. Work up of the solutions is greatly facilitated by the low salt concentrations. It has been shown that the anodic oxidation of isopropanol, which is used as cosolvent, is strongly influenced by the acrylonitrile present. A substantial portion of the isopropanol is oxidized to CO₂.     

Electrochemical synthesis in an undivided cell of aromatic mono and dichalcogeno derivatives by SRN1 substitution reactions in MeCN

The two-step electrochemical synthesis of chalcogeno derivatives ArEPh (1) and ArEEAr (3) resulting from the oxidation of ArE^– (2) (AR = 4 – NCC₆H₄, 1-naphthyl, 2-quinolyl; E = S, Se, Te) can be conveniently carried out in MeCN, by reduction of an aryl halide (ArX) in the presence of electrogenerated PhE^– or E₂ ^2– anions (S_RN1 substitution reaction). To circumvent the disadvantages connected with the use of a divided cell equipped with degradable membranes, a method based on a beaker-type cell has been developed. Thus, the synthesis of substituted compounds1 and3 can be carried out in an undivided cell equipped with a magnesium anode, if a fluoride salt, such as Et₄NF.2H₂O is purposely added before electrolysis (for3) or after the first step (for1). Under such conditions, the yields are competitive with those obtained in a divided cell.     

Electrochemical destruction of thiourea dioxide in an undivided parallel plate electrodes batch reactor

The paper presents the results of a study on the electrochemical destruction of pollutants present in a spent reducing bath of a textile factory. The investigations comprised the electro-oxidation of thiourea dioxide (TUD) (the main component of the reducing bath), sulphites and urea, which are formed during oxidation of TUD. The study performed in an undivided cell parallel plate electrodes reactor, using eight different anode materials under various hydrodynamic conditions, proved that electro-oxidation can be successfully applied for treatment of spent reducing baths. The best results of TUD and SO₃²⁻ electro-oxidation were obtained with a Ti/Pt electrode, which showed electrocatalytic effect for both the compounds, indicating a possibility of their direct electro-oxidation on the anode. Destruction of TUD and SO₃²⁻ proceeded also via indirect electro-oxidation, mediated by chlorine evolved on the anode. The process kinetics was mass transport controlled till Re=5000. No electrocatalytic effects were observed for urea with any of the tested anode materials. The elimination of urea resulted to proceed only by indirect electro-oxidation, mediated by chlorine. For elimination of urea a Ti/Pt–Ir electrode proved to be the best anode, probably due to its high efficiency in electro-oxidation of chlorides into chlorine.     

2. Nitrogen transformations in flooded rice soils

A review is made of the recent literature pertaining to the reactions and processes that soil and fertilizer N undergo in lowland rice soils in relation to the improved N management and overal N economy of lowland rice soils. Topics discussed include: nitrogen leaching, ammonium fixation and release, ammonia volatilization, N₂ fixation, mineralization-immobilization, nitrification-denitrification, dissimilatory nitrate reduction, urea hydrolysis, critical pathways for control of nitrogen loss.     

Direct electrorefining of copper scrap using an anode-support system in a bipolar cell

This paper describes the application of an anode-support system (AS) to the direct electrorefining of chopped copper scrap using the bipolar connection (series system). The authors have determined that it is possible to utilize a specially designed basket made with sheet (cathodic face) and mesh (anodic side) titanium, under the same operating conditions used for the industrial electrorefining of copper with conventional cast anodes. Titanium material remains passive and acts only as electrical contact between the anodic copper pieces and the external current source. Electrolysis of 72 h duration could be carried out using the AS system loaded with scrap copper pieces, with a current efficiency in the range 91–92% and a power requirement of 0.14 kWh (kg Cu)⁻¹. The AS system involves less copper inventory in the cell room in the order of 45–55% compared with the traditional bipolar electrorefining process. In addition, no pyrometallurgical operations are necessary.     

Experimental and theoretical analysis of a thermogalvanic undivided flow cell with two aqueous electrolytes at different temperatures

The electrochemical and thermal performance of a new thermogalvanic undivided flow cell using two aqueous electrolytes maintained at different temperatures between platinum electrodes is described. Measurements on the Fe(CN) ₆ ^3– /Fe(CN) ₆ ^4– redox system in NaOH medium yield a thermoelectric effect between –1.1 and –1.4mV/degree depending on the composition of the electrolytes. The influence of different parameters (temperature gradient, concentration of the electroactive species, fluid velocities and channel thickness) on the power output are determined. The experimental results show that electrical power is largely limited by charge and mass transfer overvoltages, but that it is possible to maintain the temperature gradient between the electrodes, i.e. the thermal boundary layers developing at the interface between the electrolytes do not reach the electrodes. A model based on the generalized Butler-Volmer kinetic equation is found to be in very good agreement with the experiments in terms of the current-voltage relationship and power output. This model is used for the prediction of the maximum electrochemical performance. From a practical point of view the efficiency of these devices remains very low due to the high thermal flux between the hot and cold electrolytes.     

A new approach to electrochemical production of benzaldehyde from toluene in an undivided cell in the presence of the couple V5+/V4+

Oxidation of toluene to benzaldehyde in the presence of the redox couple V⁵⁺/V⁴⁺ was carried out in an undivided cell where oxygen gas was continuously bubbled over the cathodic surface and the electrolyte was a mixture of aqueous H₂SO₄ solution containing V⁵⁺ and toluene. Some experimental conditions affecting the current efficiency for benzaldehyde production, such as H₂SO₄ concentration, current density, V⁵⁺ concentration and surfactants, were determined. The maximum current efficiency for benzaldehyde production at ambient temperature was 156.3% under the conditions of 11 M H₂SO₄, 2.7 × 10^–4 M CTAB, current density 1.25 mA cm^–2 and 0.0128 M V⁵⁺.     

Electrochemical conversion of 2,3-butanediol to 2-butanone in undivided flow cells: a paired synthesis

A procedure has been developed for converting 2,3-butanediol inca. 10% aqueous solution to 2-butanone by passing it through a porous anode at which it is selectively oxidized to acetoin by electrogenerated NaBrO and then pumping to a porous cathode at which it is reduced to 2-butanone. The not fully optimized yields and current efficiencies are 75% and 60%, respectively. The procedure employs: Pb/Hg or Zn/Hg cathodes, graphite anodes, pHca. 7, ambient temperature, current density of 2 mA cm^–2, five minute residence time outside the cell, packed bed electrodes, and parallel electrolyte and current flow.     

Electrochemical treatment of waste solutions containing ferrous sulfate by anodic oxidation using an undivided reactor

This paper describes an analysis of the performance of an electrochemical undivided reactor for the recycling of waste solutions containing ferrous sulfate. The effect of oxygen evolution as side anodic reaction on the figures of merit of the reactor is studied. The results suggest that the anode potential must represent a compromise between the increase in space time yield and the increase in the specific energy consumption. Experimental data are correlated with a mathematical treatment based on the stirred tank model.     

Electrical leakage currents in bipolar cell stacks

A brief review of the origins of bipolar leakage currents, the means used for its measurement and calculation and the importance of minimizing the phenomenon, is followed by a novel means of computing leakage currents more accurately than hitherto possible, using a commercially available simulation program. This has the further advantage that the reali-V characteristics of an electrochemical cell can be used, there being no need to represent the cell itself as a zener diode which is a poor representation of reali-V cell characteristics.     

Concentrating,refining and purification by means of rotating bipolar electrode cell

The use of a ‘rotating bipolar electrode’ (RBE) cell to achieve a transfer of a substance (copper) from solutions of low concentration to high concentration is described. The RBE cell employed consists of a central, rotating, cylindrical electrode, surrounded by axial wiper blades which form three compartments through which the electrode surface passes during rotation. An outer electrode in each compartment governs the potential of the rotating electrode, and copper is deposited on the electrode surface in two of the compartments, and stripped in the other. Stripping can be achieved by either electrochemical or chemical means. Very high compartment concentration ratios (>250) can be achieved and sustained by using cells with more than two compartments. The process can be operated with conditions thermo-dynamically more advantageous than possible with conventional cells. Typical costs for a RBE cell process are given.     

Electro-oxidation rate ofp-tert-butyltoluene in a bipolar packed-bed electrode cell

The side chain methoxylation ofp-tert-butyltoluene was carried out in a bipolar packed-bed electrode cell in which graphite pellets of diameter 4.74 mm and length 5mm were randomly packed in nine layers separated by inert mesh spacers. The reaction consisted of main consecutive reactions leading top-tert-butylbenzyl methyl ether andp-tert-butylbenzaldehyde dimethyl acetal (TBDA) and polymerization reactions. Overall reaction rate coefficients for each reaction step, current efficiency and energy consumption were determined on the basis of a reactor model. The selectivity and the current efficiency for TBDA increased with increasing current, but the energy consumption began to rise when the current exceeded 0.8 A. An addition of supporting electrolyte suppressed the overall reaction rate coefficients, although it decreased the energy consumption.     

Reduction of oxygen in an acidic methanol/oxygen (air) fuel cell: an online MS study

The reduction of oxygen was studied with online mass spectroscopy on 5% Pt/Norit BRX and on 20% FePP(NO₂)₄/RB PTFE-bonded gas diffusion electrodes. The performance of the catalysts was analysed under conditions of an acidic methanol/oxygen (air) fuel cell cathode in the presence of methanol in solution. Following the mass signalm/e = 32 for oxygen it can be shown that FePP(NO₂)₄ catalyses the oxygen reduction selectively. In contrast, on the Pt-carbon catalyst methanol is oxidized to CO₂ (m/e = 44) thus establishing a mixed potential and an additional consumption of methanol. Furthermore, it can be demonstrated that on a platinum-carbon electrode the methanol oxidation is catalysed in the presence of oxygen, shifting the onset potential of CO₂ production 100 mV more cathodic. This paper is dedicated to Professor Brian E. Conway on the occasion of his 65th birthday, and in recognition of his outstanding contribution to electrochemistry.     

Potential distribution and electrode stability in a bipolar electrolysis cell

A computational model is presented, which enables the identification of those zones endangered by corrosion in a bipolar electrolysis cell stack. The method consists of two steps: first the potential profile in the electrolyser is computed by numerical solution of the Laplace equation using the finite difference method; then, making use of the Criss-Cobble correspondence principle, this profile is related to the potential-dependent thermodynamic stabilities of the respective metals. This may be a useful tool in the design of intermittently operating electrolysers (for example those powered by solar energy).Nomenclature A metal phase - A _i single A-phase point - B electrolyte phase - B _i single B-phase point - F Faraday constant - h mesh interval (m) - i local current density (A m^–2) - i ₀ exchange current density (A m^–2) - j local current across the double layer (A) - j _iA,j _iB tangential or normal component of the double layer current (A) - K A, B phase conductivity ratio - m molality mol kg^–1 - R gas constant - T absolute temperature (K) - U potential (V) - U ₀ water decomposition voltage (V) - U _tot end plate potential (V) - x, y cartesian coordinates - overrelaxation factor - _a, _c anodic or cathodic overpotential (V) - _A, _B electrical conductivity (^–1 m^–1) - potential (V) - _m local double layer potential, electrode end (V) - _s local double layer potential, electrolyte end (V)