The electrochemical behaviour of cobalt in alkaline nitrate melts at different temperatures

The electrochemical behaviour of cobalt in molten alkali nitrates in the temperature range from 141 to 321°C has been investigated. At lower temperatures the metal dissolves as Co(II); the oxidation product at higher temperatures is Co₃O₄. Nitrogen oxides are also formed. Passivation and localized corrosion occur under definite anode potential, Co(II) concentration and temperature conditions. These effects have been studied by non-stationary measurements. Co(II) dissolved in the melt can be electrodeposited either on cobalt or platinum cathodes.     

Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRu0.2 catalysts: A quantitative DEMS study

The oxidation of acetaldehyde on carbon supported Pt/Vulcan, PtRu/Vulcan and Pt₃Sn/Vulcan nanoparticle catalysts and, for comparison, on polycrystalline Pt and on an unsupported PtRu_0.2 catalyst, was investigated under continuous reaction and continuous electrolyte flow conditions, employing electrochemical and quantitative differential electrochemical mass spectroscopy (DEMS) measurements. Product distribution and the effects of reaction potential and reactant concentration were investigated by potentiodynamic and potentiostatic measurements. Reaction transients, following both the Faradaic current as well as the CO₂ related mass spectrometric intensity, revealed a very small current efficiency for CO₂ formation of a few percent for 0.1 m acetaldehyde bulk oxidation under steady-state conditions on all three catalysts, the dominant oxidation product being acetic acid. Pt alloy catalysts showed a higher activity than Pt/Vulcan at lower potential (0.51 V), but do not lead to a better selectivity for complete oxidation to CO₂. C–C bond breaking is rate limiting for complete oxidation at potentials with significant oxidation rates for all three catalysts. The data agree with a parallel pathway reaction mechanism, with formation and subsequent oxidation of CO_ad and CH _{x, ad} species in the one pathway and partial oxidation to acetic acid in the other pathway, with the latter pathway being, by far, dominant under present reaction conditions.     

The mechanism of oxidation of copper in alkaline solutions

The formation of Cu₂O by the oxidation of Cu in alkaline solutions under various controlled potential conditions has been studied by potentiodynamic methods, the rotating ring disc technique and by employing colloidal Cu(OH)₂ electrodes supported on vitreous carbon.The kinetics of the electrochemical reactions, both anodic and cathodic, are interpreted in terms of a complex reaction mechanism involving various intermediates participating in the phase oxide formation, (e.g. adsorbed OH, soluble Cu(I) and metal sites of different activity).Besides the electrochemical reactions the model includes various ageing and surface restructuring processes. The growth mechanism is envisaged to depend on the conditions of oxidation.     

Cobalt(III)-mediated oxidative destruction of phenol using divided electrochemical cell

Mediated electrochemical oxidation is one of the suitable processes for the destruction of hazardous organic compounds and the dissolution of nuclear wastes at ambient temperature and pressure. The electrochemical oxidation of Co(II) was carried out in an undivided and divided electrochemical cell. The formation of Co(III) was studied in an divided electrochemical cell by varying conditions such as temperature and concentration of nitric acid in a batch type electrochemical reactor in recirculation mode. It was found that the formation of Co(III) increased with increasing nitric acid concentration and decreased with increasing temperatures. The produced Co(III) oxidant was then used for the destruction of phenol. It was noted that phenol could be mineralized to CO₂ and water by Co(III) in nitric acid under different nitric acid concentrations and temperatures. The evolved CO₂ was continuously measured and used for the calculation of destruction efficiency. The destruction was increased with increasing nitric acid concentration as well as the temperature. The maximum efficiency was observed to be 78% based on CO₂ evolution for 5,000 ppm phenol solution at 60 °C in a continuous feed mode. The destruction efficiency was increased 28% by addition of silver at 25 °C.     

Electrocatalytic carboxylation of chloroacetonitrile at a silver cathode for the synthesis of cyanoacetic acid

The electrocatalytic carboxylation of chloroacetonitrile to cyanoacetic acid performed at silver cathodes was investigated both theoretically and experimentally. Silver exhibits powerful electrocatalytic activities towards the reduction of chloroacetonitrile. In CO₂-saturated CH₃CN, reduction of NCCH₂Cl occurs at potentials that are about 0.7 V more positive than those observed at glassy carbon and gives cyanoacetic acid in good yields. Theoretical considerations on the effect of operative parameters on the performances of the process were confirmed by electrocarboxylation experiments performed in undivided cells equipped with sacrificial anodes both in a bench-scale electrochemical batch reactor and in a continuous batch recirculation reaction system equipped with a parallel plate electrochemical cell. Selectivities and Faradic efficiencies higher than 80% were obtained by working under anhydrous conditions both under amperostatic and potentiostatic alimentation at proper values of either current density or applied potential.     

Kinetics and mechanism of the electrochemical oxidation of nitrite ion dissolved as sodium nitrite in dimethylsulphoxide solutions on platinum electrodes

The kinetics of the electrochemical oxidation of NO₂⁻ ion dissolved in DMSO has been studied on Pt electrodes at temperature ranging from 25 to 44°C, by means of potentiostatic E/I curves and by relaxation techniques.     

Electrochemical and XPS studies of the surface oxidation of synthetic heazlewoodite (Ni3S2)

X-ray photoelectron spectroscopic (XPS) and electrochemical techniques have been applied to the investigation of the surface oxidation of synthetic heazlewoodite (Ni₃S₂). The XPS data showed that exposure of the sulphide to air resulted in nickel atoms migrating to the surface to form an overlayer of a hydrated nickel oxide and leave a sulphur-rich heazlewoodite. A hydrated nickel oxide was also produced on immersion of heazlewoodite in acetic acid solution in equilibrium with air, despite nickel being soluble under these conditions. After the acetic acid treatment, the S(2p) spectrum had a component at the binding energy of NiS and a small contribution due to sulphur-oxygen species. Voltammetry with bulk heazlewoodite electrodes, and the ground sulphide in a carbon paste electrode, indicated that, at pH 4.6, the initial anodic product was a sulphur-rich heazlewoodite and that oxidation was inhibited when NiS was formed on the surface. Further oxidation to higher nickel sulphides and elemental sulphur occurred at high potentials. In basic solutions, oxidation was restricted due to the formation of nickel oxide.     

Chloride behaviour in electromembrane treatment of brine issued from desalination plants

In this work, the chloride behavior through an electrochemical treatment of brines is examined using ion exchange membranes like in electrodialysis. All experiments have been performed using solutions of NaCl before an application on real brines issued from an Algerian desalination plant. After checking oxidation parameters of chloride oxidation by electrolysis, ion exchange membrane have been introduced to control both the pH and the species migrated. The effect of current density and the membrane nature has been studied. The electrochemical treatment described in this work allows transforming the brines in useful products as NaOH, HCl and Cl₂. The pH and the salt concentration are varied and the products obtained at the electrodes were identified and analyzed. It was shown that we can get chlorates according to the current density applied and the fixed pH. This fact gives rise to an economical process where valuable products can be obtained using only the chloride oxidation current. Results were linked to the Pourbaix diagram and allow the prediction of the process efficiency.     

Synthesis gas production in methane conversion using the P|yttria-stabilized zirconia|Ag electrochemical membrane system

The electrochemical membrane reactor of YSZ (yttria-stabilized zirconia) solid electrolyte coated with Pd and Ag as anode and cathode, respectively, has been applied to the partial oxidation of methane to synthesis gas (CO + H₂). The Pd|YSZ|Ag catalytic system has shown a remarkable activity for CO production at 773 K, and the selectivity to CO was quite high (96.3%) under oxygen pumping condition at 5 mA. The H₂ production strongly depended on the oxidation state of the Pd anode surface. Namely, the H₂ treatment of the Pd anode at 773 K for 1 h drastically reduced the rate of H₂ production, while air treatment enhanced the H₂ production rate. From the results of the partial oxidation of CH₄ with molecular oxygen, it is considered that the reaction site of the electrochemical oxidation of CH₄ to synthesis gas was the Pd–YSZ–gas-phase boundary (triple-phase boundary). In addition, it is found that the oxygen species pumped electrochemically over the Pd surface demonstrated similar activity to adsorbed oxygen over Pd, PdO_ad, for the selective oxidation of CH₄ to CO, when the Pd supported on YSZ was used as a fixed-bed catalyst for CH₄ oxidation with the adsorbed oxygen. The difference with respect to the H₂ formation between the electrochemical membrane system and the fixed-bed catalyst reactor results from differences in the average particle size of Pd and the way of the oxygen supply to the Pd surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Focused microwaves in electrochemical processes

The effects of high intensity microwave radiation in electrochemistry are summarized and discussed. In situ microwave activation of electrochemical processes has been introduced recently and is possible by placing a carefully designed electrochemical cell directly into a microwave cavity. Self-focusing of intense microwave radiation occurs into a region close to the electrode | solution (electrolyte) interface of a microelectrode placed into the electrochemical cell. The electrode diameter and the electrode material strongly affect the observed mass transport enhancement and temperature effects. Experiments have been conducted to determine the temperature at the electrode surface electrochemically and to quantify the rate of chemical processes which occur in the vicinity of the electrode under high intensity microwave conditions. The effects of microwaves in a wide range of solvent systems from aqueous solutions to organic solvents (DMSO, acetonitrile, DMF, formamide) and in an ionic liquid (BMIM⁺PF₆⁻) have been investigated. Considerable current and temperature enhancements are observed in all solvents and are explained based on the interaction of microwaves with the liquid (electrolyte) and the physical properties of the liquids or solutions.     

Electrochemical oxidation of 2-naphthol with in situ electrogenerated active chlorine

The electrochemical oxidation of 2-naphthol mediated by active chlorine electrogenerated in situ on a Ti-Ru-Sn ternary oxide has been studied by galvanostatic electrolysis under different experimental conditions. Measurements of chemical oxygen demand (COD), HPLC and GC analyses have been used to follow the oxidation. In the absence of NaCl, only a small fraction of 2-naphthol was oxidized by direct electrolysis, while its complete mineralization was obtained in the case of chlorine-mediated electrolysis. In particular, the rate of naphthol oxidation was found to increase with chloride concentration, pH and to be independent of current density. Analysis of oxidation product has shown that initially organochlorinated compounds have been formed which have been further completely oxidized.     

Oxidation at boron doped diamond electrodes: an effective method to mineralise triazines

The electrochemical oxidation of cyanuric acid (CA) is examined by bulk electrolyses at boron doped diamond (BDD) electrode. The influence of operating conditions on the reaction trend is investigated and suitable conditions for oxidative demolition of CA are individuated. In particular a strong effect of current density and pH is evidenced, being neutral pH and high current the most favourable conditions to achieve CA mineralisation. The paper also presents the results from atrazine (2-Cl-4-ethylamino-6-isopropylamino-1,3,5-triazine) and 2-Cl-4,6-diammino-1,3,5-triazine oxidation at BDD anode. Triazine compounds are scarcely degraded even with powerful methods such as photocatalytic TiO₂-mediated processes, being CA generally obtained as final product of their oxidation. Individuation of favourable conditions for CA demolition has allowed to find suitable conditions to mineralise also triazine compounds.     

Influence of Zn on the oxide layer on AISI 316L(NG) stainless steel in simulated pressurised water reactor coolant

The oxidation of AISI 316L(NG) stainless steel in simulated pressurised water reactor (PWR) coolant with or without addition of 1 ppm Zn at 280 °C for up to 96 h has been characterised in situ by electrochemical impedance spectroscopy (EIS), both at the corrosion potential and under anodic polarisation up to 0.5 V vs. the reversible hydrogen electrode (RHE). Additional tests were performed in simulated PWR coolant with the addition of 0.01 M Na₂B₄O₇ to exclude the effect of pH excursions probably due to Zn hydrolysis reactions. The thickness and in-depth composition of the oxide films formed at open circuit and at 0.5 V vs. RHE in the investigated electrolytes have been estimated from X-ray photoelectron spectroscopy (XPS) depth profiles. The kinetic and transport parameters characterising the oxide layer growth have been estimated using a calculational procedure based on the mixed conduction model for oxide films. Successful simulations of both the EIS and XPS data have been obtained. The parameter estimates are discussed in terms of the effect of Zn on the oxide layers on stainless steel in PWR conditions.     

Electrochemical incineration of chloranilic acid using Ti/IrO2, Pb/PbO2 and Si/BDD electrodes

The electrochemical oxidation of chloranilic acid (CAA) has been studied in acidic media at Pb/PbO₂, boron-doped diamond (Si/BDD) and Ti/IrO₂ electrodes by bulk electrolysis experiments under galvanostatic control. The obtained results have clearly shown that the electrode material is an important parameter for the optimization of such processes, deciding of their mechanism and of the oxidation products. It has been observed that the oxidation of CAA generates several intermediates eventually leading to its complete mineralization. Different current efficiencies were obtained at Pb/PbO₂ and BDD, depending on the applied current density in the range from 6.3 to 50 mA cm⁻². Also the effect of the temperature on Pb/PbO₂ and BDD electrodes was studied.UV spectrometric measurements were carried out at all anodic materials, with applied current density of 25 and 50 mA cm⁻². These results showed a faster CAA elimination at the BDD electrode. Finally, a mechanism for the electrochemical oxidation of CAA has been proposed according to the results obtained with the HPLC technique.     

Capacitance properties of electrochemically deposited polyazulene films

Polyazulene films formed by electrochemical oxidation of azulene have been studied as active components in electrochemical capacitors. The film shows reversible electrochemical behavior in the positive potential range and exhibits p-doping properties. The influence of film formation conditions on the films electrochemical properties has been investigated. A strong effect of solvent on the polyazulene deposition has been observed. The highest yield of film deposition was found for dichloromethane. Polyazulene films also exhibit stable voltammetric properties in aprotic solvents. The voltammetric response of the film is affected by the size of the anion of the supporting electrolyte. In solutions containing tetra(alkyl)ammonium perchlorates, tetrafluoroborates or hexafluorophosphates, reversible oxidation of polyazulene is obtained. In the presence of large tetra(phenyl)borate anions, polyazulene is irreversibly oxidized upon electrochemical oxidation. The capacitance properties of these materials have been investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The polyazulene film displays a relatively high specific capacitance close to 400 F g⁻¹. Such high value of C_s locates this material among very good polymeric redox pseudo-capacitors.     

Pt-WO3 supported on carbon nanotubes as possible anodes for direct methanol fuel cells

The carbon nanotube (CNT) synthesised by the template carbonisation of polypyrrole on alumina membrane has been used as the support for Pt-WO₃, Pt-Ru, and Pt. These materials have been used as the electrodes for methanol oxidation in acid medium in comparison with E-TEK 20 wt% Pt and Pt-Ru on Vulcan XC72R carbon. The higher electrochemical surface of the carbon nanotube (as evaluated by cyclic voltammetry) has been effectively used to disperse the catalytic particles. The morphology of the supported and unsupported CNT has been characterised by scanning electron micrograph and high-resolution transmission electron micrograph. The particle size of Pt, Pt-Ru, and Pt-WO₃ loaded CNT was found to be 1.2, 2, and 5 nm, respectively. The X-ray photoelectron spectra indicated that Pt and Ru are in the metallic state and W is in the +VI oxidation state. The electrochemical activity of the methanol oxidation electrode has been evaluated using cyclic voltammetry. The activity and stability (evaluated from chronoamperometric response) of the electrodes for methanol oxidation follows the order: GC/CNT-Pt-WO₃-Nafion>GC/E-TEK 20% Pt-Ru/Vulcan Carbon-Nafion>GC/CNT-Pt-Nafion>GC/E-TEK 20% Pt/Vulcan carbon-Nafion>Bulk Pt. The amount of nitrogen in the CNT plays an important role as observed by the increase in activity and stability of methanol oxidation with N₂ content, probably due to the hydrophilic nature of the CNT.     

Electrochemical and peroxidase-catalyzed oxidation of epinephrine

Electrochemical and peroxidase-catalyzed oxidation of epinephrine (EPI) has been studied. In the electrochemical studies a single well-defined, 4e, 4H⁺, pH-dependent oxidation peak was observed in square wave and cyclic sweep voltammetry at edge plane pyrolytic graphite electrode. In the reverse sweep a redox couple was observed. The decay of the UV-absorbing intermediate generated and the first-order rate constants were calculated at different pH and were found to be ～6.3 × 10^?3 s^?1. The detection limit and sensitivity are found to be 17 × 10^?8 M and 2.325 μA μM^?1 respectively. At pH 7.2, the electro-oxidation product was characterized using NMR and DEPT studies as leucoadrenochrome. The peroxidase-catalyzed oxidation was carried out using horseradish peroxidase and initiated by adding H₂O₂. The identical spectral changes, rate constants and product formed during electrochemical and enzymatic oxidation suggest that the same intermediate species is generated during both the oxidations. A tentative pathway for the oxidation of EPI has been suggested. It is concluded that the electrochemical and peroxidase-catalyzed oxidation of EPI proceed by an identical pathway.     

Use of potassium conductors in the electrochemical promotion of environmental catalysis

This study summarizes the effect of the electrochemical promotion of catalysis (EPOC) by using potassium conductors in environmental catalytic reactions applied to the removal of several automotive pollutants, such us CO, C₃H₆ and nitrogen oxides (N₂O and NO_x). It has been shown the extraordinarily potential of using Pt/K–βAl₂O₃ in a wide variety of environmental reactions (oxidation, reduction), activating the catalyst at lower reaction temperatures and decreasing the inhibitory effect of poisons such as water in the reaction atmosphere. In addition, a new application of potassium conductors-based electrochemical catalysts has been developed for the NO_x storage/reduction process (NSR). The idea of coupling catalysis and solid-state electrochemistry on this process would allow to improve and simplify the current NSR technology. Finally, the results have been obtained under reaction conditions compatible with the treatment of automotive exhaust emissions. This demonstrates the potential for the practical use of the phenomenon of electrochemical promotion by using potassium conductors on this kind of process.     

Potential and current oscillations during formaldehyde oxidation on platinum particles dispersed in three-dimensional pore networks of TiOx/Ti

Electrochemical oscillations during the anodic oxidation of formaldehyde (HCHO) were studied on a modified electrode of platinum particles highly dispersed in the three-dimensional pore networks of TiO_x/Ti (Pt-TiO_x/Ti). Under conditions of room temperature and stationary electrode, not only current oscillations under both cyclic voltammetric and potentiostatic conditions but also potential oscillations under galvanostatic conditions were obtained. The intensity of current oscillations strongly depends on the concentration of HCHO or H₂SO₄, upper potential limit (upl) of cyclic voltammetry, applied constant potential and duration of time (t_d) at constant potential. Potential oscillations exhibit various patterns such as periodic, quasi-periodic, mixed-mode oscillations and other different bifurcations, which are greatly effected by the applied constant current and the concentration of HCHO or H₂SO₄. Meanwhile, the oscillatory system has a bistable characteristic with stable states at both low and high potentials. The observed potential and current oscillations are caused by the cyclic formation/removal of intermediate poison CO from the electrode surface during HCHO oxidation. The highly dispersed Pt particles on the surface of Pt-TiO_x/Ti electrode improve the electrocatalytic activity of the electrode, which greatly facilitates the formation of CO by the oxidation of HCHO and the removal of CO by its reaction with hydroxyl radicals (OH). Furthermore, the three-dimensional pore networks of the electrode's TiO_x/Ti support are favorable to the adsorption/desorption of reactants or intermediate product and thus increase the rate of reactions giving rise to electrochemical oscillations.     

Some aspects of the electrochemistry of the flotation of pyrrhotite

The iron sulfide mineral, pyrrhotite (Fe_(1–x)S), has long been known to be more difficult to recover by flotation from alkaline slurries than many other base metal sulfide minerals. This paper summarizes the results of an electrochemical study of the surface reactions that occur during the flotation of nickeliferous pyrrhotite in the recovery of nickel and the platinum group metals. Mixed potential measurements conducted with natural pyrrhotite electrodes in various stages of an operating flotation plant showed that the mineral potential is positive to the equilibrium potential of the xanthate/dixanthogen couple. Similar results were obtained during batch flotation experiments and in synthetic solutions in the laboratory. Cyclic voltammetric and potentiostatic current/time transient experiments were used to investigate the oxidation of pyrrhotite under various conditions. In addition, the reduction of oxygen, the reaction of copper ions and the oxidation of xanthate ions at the mineral surface were investigated. The formation of dixanthogen on pyrrhotite surfaces is thermodynamically favourable in plant flotation slurries. However the interaction with xanthate at pH values above 7 is inhibited by a surface species formed during the conditioning prior to xanthate addition. In acidic solutions copper ions react readily with pyrrhotite to form a species, possibly CuS that can be oxidized at potentials above 0.4 V. At pH 9 this species does not form and there is no electrochemical reaction between pyrrhotite and copper ions. The beneficial effects of copper ions to flotation performance appear to be related to an enhancement of the oxidation of xanthate.