Electrochemical investigations of the Ce(m)/Ce(iv) couple related to a Ce(iv)-assisted process for SO2/NOx abatement

This paper presents the results of electrochemical investigations of the Ce(lii)/Ce(iv) couple, for use in a SO₂/NO_x abatement process. A voltammetric study was conducted at a platinum rotating disc electrode for the conditions corresponding to the gas scrubbing processes. The charge transfer coefficients were weakly affected by increasing the concentration of sulfuric acid up to 5 m. Conversely, the diffusion coefficients and the rate constants were decreasing functions of the concentration of sulfuric acid. The electrochemical regeneration of Ce(iv) was also studied at a platinized titanium surface and the oxidation appeared to be linked to other oxidative processes such as O₂ formation. Batch experiments, carried out in a filter press cell, showed good faradaic yields even in the potential domain corresponding to significant background current attributable to the supporting electrolyte.     

Mechanically stable flat anodic titania membranes for gas transport applications

Anodic titanium oxide (ATO) membranes were produced by two-step anodic oxidation of titanium foil in ethylene glycol electrolyte containing NH₄F at the anodization voltage of 60?V. To provide the mechanical strength necessary for applying tubular anodic films as gas membranes, we utilized the formation of protective continuous TiO₂ layer at the top film surface prior to second anodization. As compared to conventional two-step anodic oxidation this technique decreases dissolution rates of titanium oxide phases with oxidation states lower than +4 (Ti₂O₃, Ti₃O₅), which are forming between titania nanotubes during anodization. The structural parameters of anodic titania films were determined by small-angle X-ray scattering and scanning electron microscopy techniques. According to SEM the proposed method resulted in growth of ATO films with a flat surface without nanotube endings, which enabled to use the films as gas separation membranes. The permeance of individual gases through ATO membranes were found to depend on gas molecular weight (M^?0.5), with absolute values twice exceeding theoretical permeabilities as it was predicted by Knudsen diffusion (up to 63?m³/(m²?×?bar?×?h) for nitrogen at 298?K). Here we ascribe this phenomenon to diffusion according to Knudsen-Smoluchoski mechanism (diffusion with slip, involving specular reflections of molecules), which is appropriate for membranes with straight pores and smooth internal pore surfaces.     

Mechanistic and kinetic aspects of silver dissolution in cyanide solutions

The factors influencing the dissolution kinetics of pure silver in cyanide solution have been analysed in terms of an electrochemical mechanism. A kinetic model is presented which incorporates coupled diffusion and charge transfer for the anodic branch, and combined diffusion, adsorption and charge transfer for the cathodic branch. The anodic oxidation of silver has been investigated using a silver rotating-disc electrode for concentrations between 10^–3 and 10^–1 m NaCN. Oxygen reduction on silver has been studied at oxygen partial pressures between 0.104 and 1.00 atm. Mechanistic aspects of the oxygen discharge reaction are considered in explaining the kinetic differences between gold and silver dissolution in cyanide solution. It is shown that under conditions typical of conventional cyanidation gold dissolves measurably faster than silver.     

Electrosynthesis of a manganese(iii) aminopolycarboxylate complex in alkaline media

Mn(iii)CyDTA [(trans-cyclohexane-1,2-diamine-N,N,N,N-tetraacetato) manganate(iii)] was generated electrochemically from Mn(ii)CyDTA (6–54 mm) in 0.1 m NaHCO₃ at pH 9.0 and 10.5, respectively, 25 °C, for two CyDTA/Mn molar ratios (1/1 and 2/1). A divided batch electrochemical reactor was employed with anode current densities from 2.6 to 102 A m^–2. Separate cyclic voltammetry experiments of Mn(iii)CyDTA in alkaline media showed a prepeak behaviour, indicating the adsorption of Mn(ii) species. The visible anodic deposit, formed during the electrosynthesis of Mn(iii)CyDTA at pH 10.5 and 1/1 CyDTA/Mn molar ratio on stainless steel and PbO₂/Pb, reduces the current efficiency for Mn(iii). For a Mn(ii) concentration of 18 mm and at 13 A m^–2, the graphite and platinized titanium anodes gave a current efficiency for Mn(iii) of 78% and 66%, respectively, without a visible deposit. A 2/1 CyDTA/Mn molar ratio, avoided a visible anodic deposit formation, but gave lower current efficiencies for Mn(iii) than in the case of a 1/1 ligand to metal ratio. The electrosynthesis of Mn(iii)CyDTA is recommended for routine preparation of the complex and is also suitable for in situ electrochemically mediated oxidations in alkaline media (up to pH 11).     

Electrochemical copolymerization of aniline and para-phenylenediamine on IrO2-coated titanium electrode

Copolymerization of aniline (AN) and para-phenylenediamine (PPDA) was electrochemically performed by cyclic voltammetry on IrO₂-coated titanium electrodes in 0.5m H₂SO₄. The cyclic voltammograms, with and without a middle peak at about 580 mV, can be produced by controlling the PPDA concentration in the aniline solution during polymer preparation. The peak at about 580 mV corresponds to the para-aminophenol/benzoquinoneimine (PAP/QI) redox couple and crosslinking sites. The mass of polymer deposited on an IrO₂-coated titanium electrode is correlated with the polymer anodic peak current, which allows the rates of polymer deposition to be monitored by increases in the anodic peak current at various PPDA concentrations. SEM photographs show that the morphology of the polymer film depends dramatically on PPDA concentration. Stability test information can be used in generating an effective index to discern between the crosslinking and the PAP/QI reactions induced by PPDA. The linear relationship between the second redox process potential (E _1/2), corresponding to the oxidation and reduction between polaronic emeraldine and pernigraniline in PPDA-modified PANI films, and pH, possesses a slope of about -120 mV/pH.     

Influence of flow pattern on the anodic mass transport of hypochlorite in a chlorate cell

Electrolysis was conducted at 40°C, pH 6 and circulation rate of 14 ml/s in two different chlorate cells with graphite electrodes of industrial dimensions. Large differences in the values of mass transfer coefficient for hypochlorite discharge were observed in the two cells in which the circulation patterns were different. The result is explained on the basis of the Ibl and Landolt mechanism for anodic chlorate formation in dilute NaCl solutions.     

The mechanism of the chlorine evolution on different types of graphite anodes during the electrolysis of an acidic NaCl solution

The behaviour of the electrode and the formation of chlorine were studied practically for porous graphite used in the industrial electrolysis of an aqueous NaCl solution only.In this investigation this was done for Acheson graphite electrodes and for highly orientated pyrolytic graphite electrodes of which the edge plane or the basal plane served as electrode surface. Voltammograms were determined by continuous scanning with different rates of the potential between a minimum and maximum value. From the experimental results it follows that two different surface compounds, the lp oxygen and the hp oxygen are formed on all the graphite electrodes investigated. In the stationary state lp oxygen is present at low potential and the hp oxygen at high potentials. The formation and the removal of each compound depends on the electrolytic conditions, especially on the maximum potential of the sweep. The conditions of the sweep determine whether these oxides are really present on the electrode surface during the anodic or the cathodic sweep. Chlorine is formed according to the Volmer-Heyrovsky mechanism on all the three types of electrodes. The current density, the transfer coefficient and the activation energy for the edge plane electrode and the Acheson graphite electrode are of the same order of magnitude, while for the basal plane electrode they deviate strongly.     

Mechanism of anodic oxidation of chlorate to perchlorate on platinum electrodes

The kinetics and mechanism of the anodic oxidation of chlorate to perchlorate on platinum electrodes have been investigated. The current efficiency for perchlorate formation and the electrode potential have been determined as a function of current density for various solution compositions, flow rates and pHs at 50°C. The results have been compared with theoretical relations between the ratio of the current efficiencies for perchlorate and oxygen formation, the electrode potential, the concentration of chlorate at the electrode surface and the current density for various possible mechanisms. It is concluded that the formation of an adsorbed hydroxyl radical is the first step in the overall electrode reaction. The mechanism proposed for the C10₄ perchlorate and oxygen formation is:

Formation of hypochlorite,chlorate and oxygen during NaCl electrolysis from alkaline solutions at an RuO2/TiO2 anode

On-site electrolysis of a weak alkaline solution of NaCl has been applied to an increasing extent for disinfection. To optimize the electrolytic cell and the electrolysis conditions, the current efficiency for hypochlorite, chlorate and oxygen formation at a commercial RuO₂/TiO₂ anode were determined under various conditions. It was found that for solution with low NaCl concentrations, (lower than 200 mol m^?3), and at 298 K, solution flow velocity of 0.075 ms^?1 and high current density, (higher than 2 kA m^?2), hypochlorite formation is determined by mass transfer of chloride. The formation of chlorate in weakly alkaline media at a chlorine and oxygen-evolving anode is ascribed to two reactions, namely, the direct oxidation of chloride to chlorate and the conversion of hypochlorite. This is suggested to split the well-known electrochemical Foerster reaction into a chemical reaction for the conversion of hypochlorite in chlorate and the electrochemical oxidation reaction of water. It is proposed that in an acidic reaction layer at the anode the mechanism of chlorate formation may be given by the following:      

Electro-Catalytic Oxidation of Methanol on a Ni–Cu Alloy in Alkaline Medium

The electro-catalytic oxidation of methanol on a Ni–Cu alloy (NCA) with atomic ratio of 60/40 having previously undergone 50 potential sweep cycles in the range 0–600 mV vs. (Ag/AgCl) in 1 m NaOH was studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (EIS). The electro-oxidation was observed as large anodic peaks both in the anodic and early stages of the cathodic direction of potential sweep around 420 mV vs. (Ag/AgCl). The electro-catalytic surface was at least an order of magnitude superior to a pure nickel electrode for methanol oxidation. The diffusion coefficient and apparent rate constant of methanol oxidation were found to be 2.16 × 10⁻⁴ cm² s⁻¹ and 1979.01 cm³ mol⁻¹ s⁻¹, respectively. EIS studies were employed to unveil the charge transfer rate as well as the electrical characteristics of the catalytic surface. For the electrochemical oxidation of methanol at 5.0 m concentration, charge transfer resistance of nearly 111 Ω was obtained while the resistance of the electro-catalyst layer was ca. 329 Ω.     

Electrochemical regeneration of Ce(iv) for oxidation of p-methoxytoluene

The development of the electrochemical oxidation of aromatic compounds on an industrial scale is hindered by the deactivation of the electrode occurring under anodic polarization. This paper presents some results on the indirect oxidation of p-methoxy toluene by ceric sulfate electrochemically regenerated on a platinized titanium electrode. A technique that reactivates the fouled electrode on the one hand and avoids the passivation of the electrode during electrolysis on the other hand is proposed. Methylene chloride is used to extract the organic products from the aqueous phase before electrolysis. The preparative electrolysis of anisaldehyde is performed on the laboratory scale and the results show an overall (chemical and electrochemical) yield close to 80%.     

The behaviour of platinized platinum electrodes in acid solutions: correlation between voltammetric data and electrode structure

The electrochemical behaviour of platinized platinum electrodes prepared by different methods was studied in the H-adatoms and O-adatoms electroadsorption-electrodesorption potential range in 1m HClO₄ at 30°C. Porous platinized platinum electrodes of different degrees of platinization prepared at a constant potential show an anomalous voltammetric behaviour. The magnitude of the anomalous effect depends on the electroplating conditions and it becomes more remarkable at the lowest voltammetric sweep rate. The anomalous behaviour is explained on the basis of absorption of hydrogen into the porous structure and its influence on the different stages of the hydrogen electrode reaction. The absorption of hydrogen occurs either during electroplating and/or during the potentiodynamic stabilization of the porous electrode.     

Effect of additives on the anodic codeposition of lead dioxide and polypyrrole

The influence of acetate, Pb²⁺ species and pyrrole on the electrodeposition of lead dioxide and polypyrrole on a glassy carbon electrode in 1.0m potassium nitrate medium was studied using cyclic voltammetry, double potential step chronoamperometry and controlled potential electrolysis techniques. In the presence of acetate ion the deposition of lead dioxide initiates at least 200 mV less positive compared to nitrate ions alone. At lower acetate ion concentration (< 0.4m) two anodic peaks are observed due to the oxidation of acetate and nitrate complexed Pb²⁺ species. Pyrrole totally inhibits the lead dioxide formation at more anodic potentials due to an insulating polypyrrole film formation. At higher concentrations (> 20 mm) acetate ions are found to inhibit polypyrrole formation. The following is the decreasing order of inhibition of polypyrrole formation. Sodium acetate > lead nitrate > lead acetate. Acetate in the form of lead acetate is found to favour simultaneous deposition of lead dioxide and polypyrrole with minimum inhibition on polypyrrole formation.     

A simplified model to predict the current–voltage relationship of an electro-chlorination cell

A simplified model is described to predict the Current–Voltage (I–V) relationship of a parallel plate electro-chlorination cell containing aqueous NaCl solution as electrolyte. The simplifications allowed obtaining an analytical solution without recourse to computationally intensive numerical solutions like finite element method. The anodic and cathodic exchange current densities and symmetry factors for the model were obtained using linear sweep voltammetry experiments for two different electrodes, viz. graphite and mixed metal oxide coated titanium. Using them, anodic and cathodic overpotential values (for a particular device current I) were predicted using the Butler–Volmer equation. The solution potential drop for the same device current was determined using a modified Nernst–Plank equation. The predicted device voltage (for the device current I), which is the sum of equilibrium electrode potentials, electrode overpotentials and solution potential drop, was compared with experimental (I–V) data for the two electrochemical cells as mentioned above. Results showed that the simplified model could predict the I–V data well, when the electrode surface area was assumed to be twice the superficial area.     

Electrochemical mediators for total oxidation of chlorinated hydrocarbons: formation kinetics of Ag(II), Co(III), and Ce(IV)

Mediated electrochemical oxidation (MEO) with Ag(n) and Co(iii) in 7 m HNO₃ and 3.5 M H₂SO₄ has been studied. High degradation rates were found for superchlorinated organic substances such as pentachlorophenol (PCP), Lindane and polychlorinated biphenyls (PCBs) using Ag(ii) in HNO₃ as mediator. An investigation on the formation kinetics of Ag(ii), Co(iii), and Ce(iv) by means of a rotating disc electrode (RDE) led to a quantitative determination of limiting currents at various mediator concentrations. Reaction rate constants of water oxidation by Ag(ii) were determined at various temperatures and compared to reaction rates of Co(iii).     

The electrochemical formation of graphitebisulphate intercalation compounds

The intercalation of bisulphate ion in graphite has been studied. Oxidation occurs in two stages, at 0.9 V and 1.8 V versus NHE, but the formation of thermally expandable compounds is associated with the second oxidation process. The first oxidation process is associated with the formation of a lower order of intercalation compound. The transport of HSO ₄ ^? between the graphite planes is diffusion controlled and the diffusion coefficient value estimated is in the range of 0.3 to 2.8×10^?6 cm² s^?1 which indicates that the intercalated ions can move quite freely between the graphite planes. The oxidation of the graphite flakes in a fluidized bed electrode was studied by AC and voltammetric techniques. Good interparticle contact was achieved when a slight compressive load was applied and fluidization resulted in a large decrease of the solid phase resistance of the bed electrode.     

Electrochemical behaviour of a magnesium alloy containing rare earth elements

The corrosion of a magnesium alloy containing rare earth elements (WE43 type alloy) was studied in 0.05 and 0.5 M Na₂SO₄ or 0.1 and 1 M NaCl solutions using electrochemical techniques: linear polarization resistance, potentiodynamic polarization, impedance measurements. The electrolytes favoured anodic magnesium oxidation but the presence of rare earth elements improved the tendency of magnesium to passivation. The dissolution rates in chlorides were higher than in sulphates because chlorides, in contrast to sulphates, interfered with the formation and maintenance of a protective layer of corrosion products which decreased the severity of the attack. The effects of galvanic corrosion due to cathodic intermetallic precipitates at grain boundaries were particularly evident in chloride media at long testing times.     

Carbon electrodes: Part 2. The anodic oxidation of N-propylamine

The anodic oxidation of a primary aliphatic amine, n-propylamine, has been studied at electrodes of vitreous carbon and pyrolytic graphite. The electrode reaction has been found to differ greatly for the two materials, as evidenced by the voltammetric behaviour at rotating disc electrodes and the results of preparative electrolyses. The observations at the vitreous carbon electrode are in agreement with the mechanism proposed by Mann and Barnes for the oxidation of propylamine at a stationary platinum electrode. A novel mechanism is proposed for the oxidation at a pyrolytic graphite anode.Part 1 M. P. J. Brennan and O. R. Brown,J. Appl. Electrochem.,2 (1972) 43.     

Mechanism of the chlorine evolution on a ruthenium oxide/titanium oxide electrode and on a ruthenium electrode

The mechanism of the chlorine evolution on titanium electrodes coated with a layer of ruthenium oxide and titanium oxide under different experimental conditions, and on a ruthenium electrode, both in acidic chloride solution, has been investigated. Potentiodynamic current density—potential curves were recorded as a function of the time anodic pre-polarisation, the composition of the solution and the temperature. Moreover, potential decay curves were determined. Theoretical potential decay curves were deduced for both the Tafel reaction (2 Cl_ad→Cl₂) and the Heyrovsky reaction (Cl^? + Cl_ad → Cl₂ + e^?) as a rate determining step in the formation of molecular chlorine. They were compared with those found experimentally. The influence of possible diffusion of atomic chlorine out of the electrode was also taken into consideration. It was found that for all the electrodes investigated, molecular chlorine is formed both at anodic polarisation and on open circuit according to the Volmer—Heyrovsky mechanism, where the Heyrovsky reaction is the rate-determining step. The transfer coefficient is 0.5 for the chlorine evolution at an “ideal” ruthenium oxide titanium oxide electrode and at a ruthenium electrode.