Electroreduction of hexavalent chromium by polyaniline

BACKGROUND: A plate‐gap model interpretation of enzymatic reaction kinetics and rotating disc voltammetry were applied for evaluation of the nature of the reaction of the electroreduction of Cr(VI) (as dichromate ions) on a polyaniline (PANI)‐modified glassy carbon (GC) electrode. RESULTS: The kinetic parameters (the maximal current (V_max) and Michaelis constant (K_M)) for electroreduction of Cr(VI) on the PANI‐modified GC electrode were determined as V_max = 0.34 × 10^?7 mol cm^?3 s^?1 and K_M = 0.47 × 10^?6 mol cm^?3. The reduction of dichromate is intensified by PANI film growth. CONCLUSION: To characterise the electroreduction of Cr(VI) on a PANI‐modified GC electrode, the kinetic parameters of the reaction were determined using a plate–gap model interpretation of enzymatic reaction kinetics and rotating disc voltammetry. The catalytic nature of Cr(VI) electroreduction on the PANI‐modified electrode has been shown. Copyright © 2009 Society of Chemical Industry     

Modulated Cr(III) oxidation in KOH solutions at a gold electrode: Competition between disproportionation and stepwise electron transfer

The electrochemical oxidation of aqueous Cr(III) was examined using cyclic voltammetry with a polycrystalline Au electrode in KOH solutions of varying pH and Cr(III) concentration. The mechanism and kinetics for the oxidation of Cr(III) is a quasi-reversible diffusion-controlled reaction and is largely dependent on the solution pH. The reaction mechanism is initiated by an irreversible electrochemical electron transfer to form Cr(IV) which is the rate-determining step (RDS). Following the RDS, subsequent oxidation of Cr to its hexavalent state occurs by the disproportionation of Cr(IV) at low KOH concentrations and electron transfer at high KOH concentrations due to the involvement of OH⁻ in the disproportionation reaction. As the solution pH increases, the Cr(III) oxidation peak potential shifts negatively owing to the involvement of OH⁻ in the RDS. The competitive adsorption of OH⁻ and CrO₂⁻ on the electrode surface also plays an important role in the oxidation behavior.     

Correlation of electrochemical and ellipsometric data in relation to the kinetics and mechanism of Cu2O electroformation in alkaline solutions

The kinetics of copper anodization were studied in the potential range of Cu₂O electroformation in different alkaline solutions by using voltammetric, rotating disc electrode, rotating ring-disc electrode and ellipsometric techniques. Thin layer and massive copper electrodes were employed. Results indicate the formation of soluble Cu(I) species at the early stage of copper anodization. Other soluble Cu(II) species are produced after the Cu₂O layer electroformation. Data are discussed on the basis of a reaction mechanism involving different reaction products and the influence of the copper surface on the initial electrochemical reaction.     

Electrochemical oxidation of formaldehyde on gold and silver

The electrochemical oxidation of formaldehyde has been studied by cyclic voltammetry on gold and silver rotating disc electrodes varying the sweep rate, the angular velocity, formaldehyde concentration and pH of the supporting electrolyte. The Tafel parameters and reaction orders as well as the rate constants have been determined. A reaction path is proposed which involves the release and oxidation of the first hydrogen atom under an interaction of formaldehyde molecule with the metal surface and with the OH⁻-ion, followed by a non-electrochemical reaction or a second charge transfer reaction, depending upon the electrode potential.     

Anodic dissolution and passivation of gold,particularly in presence of chloride

A study of the anodic behaviour of gold was performed on a rotating disc electrode by the potentiostatic method and cyclic voltammetry, in neutral and acidic sulphate solutions with and without addition of chloride. These measurements, combined with photopolarisation (performed on stationary electrode in K₂SO₄ solution) enabled an assumption of the general scheme of the mechanism of anodic processes on gold, involving:—in sulphate solutions: chemisorption of oxygen and OH^?; building of the first layer by “place exchange” mechanism; further growth of the anodic layer by ionic migration; presence of semiconducting compounds on electrode surface.—in sulphate solutions with chloride: anodic dissolution of gold over a primarily formed layer on the electrode surface, diffusion-activation control of the dissolution process, and probable precipitation reaction with blocking of the electrode surface (at higher cd and insufficient chloride flux).     

Voltammetric behaviour of the copper(II)—thiourea system in sulphuric acid medium at platinum and glassy carbon electrodes

Thiourea, a levelling agent used in copper electrorefining baths, is the focus of a cyclic and rotating disc or ring-disc electrode voltammetric study. Thiourea adsorption, its interaction with Cu²⁺ and formation of Cu(Tu) _n ⁺ complexes are part of the multistep electrode mechanism proposed. Results at platinum and glassy carbon electrodes are applicable to copper electrodes in electrorefining.     

Electrodics of methanol oxidation on Pt-f-multiwalled carbon nanotube composite, prepared by γ-radiolysis

We report the electrodics of methanol oxidation on Pt-multiwalled carbon nanotube composites (Pt-f-MWCNTs), prepared by γ-radiolysis of K₂PtCl₆ in the presence of HOOC-functionalized multiwalled carbon nanotubes. The electrocatalytic activity for the methanol oxidation was studied using cyclic and linear sweep voltammetric techniques on the stationary indium tin oxide and rotating gold disc electrodes, respectively. Higher values of oxidative (anodic) current were obtained using Pt-f-MWCNTs compared to the polycrystalline Pt electrode. This phenomenon is attributed to the synergistic effect of oxy groups on MWCNTs, which alleviate CO poisoning. The electrodics of the reaction at various temperatures was studied using linear sweep voltammetry (LSV) on a rotating disc gold electrode, modified with the composite. From the Koutecky-Levich plots, the standard rate constant (k⁰) was determined to be 7.9 ± 1.9 × 10⁻⁸ cm s⁻¹.     

Electrocatalytic reduction of chlorophenoxy acids at palladium-modified glassy carbon electrodes

Palladium species can be immobilized on a glassy carbon electrode by voltage cycling between 0.0 and −0.4 V versus SCE in solutions containing 0.5 mM Na₂PdCl₆ in order to facilitate the electrocatalytic reduction of chlorophenoxycarboxylic acids in solutions buffered at pH 7. Cyclic voltammetry, measurements at the rotating disc electrode (RDE) and chronoamperometric techniques were used in order to investigate the electrochemical behaviour of the modified electrodes (GC/Pd) towards the catalytic reduction of chlorophenoxycarboxylic acids. A reaction mechanism is proposed and discussed. A probable scheme for the electroreduction of chlorophenoxycarboxylic species in neutral medium involves a simultaneous and competitive adsorption of the organic molecules and hydrogen atoms on the catalytic sites, followed by an irreversible hydrodechlorination reaction. 2,4-Chlorophenoxyacetic acid can be dehalogenated to a chlorine-free product in neutral aqueous solutions at relatively low cathodic polarizations and at ambient temperature using a GC/Pd electrode.     

The electrochemical behaviour of copper in alkaline solutions containing sodium sulphide

The electrochemical behaviour of copper in NaHCO₃ solution (pH 9) and NaOH solutions (pH 14) in the presence of sodium sulphide (10^–3 to 5×10^–2m) was investigated by using rotating disc electrode and rotating ring-disc techniques, triangular potential scanning voltammetry and potentiostatic steps. When the potential increases from –1.2V upwards, copper sulphide layers are firstly formed at potentials close to the equilibrium potentials of the Cu/Cu₂S and Cu/CuS reversible electrodes. When the potential exceeds 0.0 V (NHE), the copper oxide layer is electroformed. Pitting corrosion of copper is observed at potentials greater than –0.3 V. The charateristics of copper pitting are also determined through SEM optical microscopy and EDAX analysis. There are two main effects in the presence of sodium sulphide, namely, the delay in the cuprous oxide formation by the presence of the previously formed copper sulphide layer and the remarkable increase of copper electrodissolution when the potential exceeds the cuprous oxide electroformation threshold potential. These results are interpreted on the basis of a complex structured anodic sulphide layer and on a weakening of the metal-metal bond by the presence of adsorbed sulphur on the copper surface.     

Indole and 5-chloroindole as inhibitors of anodic dissolution and cathodic deposition of copper in acidic chloride solutions

The effect of indole, IN, and 5-chloroindole, Cl-IN, on the anodic dissolution of copper in acidic sodium chloride solutions was studied using voltammetry on a rotating disc electrode (RDE). Both compounds used at 10^–3 M concentration act as strong inhibitors on the copper dissolution, but indole exhibits better inhibiting properties. The inhibitory action substantially increases with decreasing solution pH. The influence of these organic additives on the electrodeposition of copper on platinum was also investigated using RDE and electrochemical quartz crystal microbalance (EQCM) techniques. The EQCM measurements show that a sparingly soluble layer of the inhibitor is responsible for the protective effects observed in chloride solutions.     

Electrochemical sensor-based gold nanoparticle/poly(aniline-co-o-toluidine)/graphene oxide nanocomposite modified electrode for hexavalent chromium detection: a real test sample

This paper presents the fabrication of poly(aniline-co-o-toluidine)/graphene oxide nanocomposite with a general abbreviation [PANI-co-PoT/GO_a–e] by well-known in-situ oxidative polymerization method with ultrasonic assistance. These materials were based on variable loading of GO when prepared. The chemical structures of the composite materials were confirmed by characterization technique. The FE-SEM and TEM micrographs were used to investigate the morphological features. Furthermore, FT-IR, XRD, TGA, and electrical conductivity measurements were utilized to estimate its complete performance. All nanocomposites showed CDT_max values in the range of 287.25–463.51 ºC which is significantly higher than that observed for pure copolymer (204.79 ºC). The main focus of this paper is to study the electroselective application using gold nanoparticle as a coating. A steady electroactive modified electrode [AuNPs/PANI-co-PoT/GO] was effectively prepared on a gold electrode (Au) surface using an electroadsorption process for the determination of Cr(VI). The electrochemical attitude of the modified sensor toward the reduction of Cr(VI) was studied by a square wave voltammetry (SWV) and a cyclic voltammetry (CV) technique. The AuNPs/PANI-co-PoT/GO modified electrode displayed a perfect electrochemical activity toward the reduction of Cr(VI). Using an SWV method, the modified electrode gave a linear response to Cr(VI) through the concentration range 5–500 µM with a limit of detection 0.0215 µM. The suggested sensor displayed good stability, sensitivity and selectivity and has exhibited potential for the detection of Cr(VI) in real samples.     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Preparation of catalytic films of platinum on Au substrates modified by self-assembled PAMAM dendrimer monolayers

In this work we demonstrate the preparation of highly catalytically active Pt formed by the galvanic replacement of the copper adlayer on Au substrates, modified by the self-assembly of fourth generation amine terminated PAMAM dendrimer (G4NH₂). The copper adlayer was formed on the dendrimer-modified gold substrate by chemical preconcentration of copper ions followed by electrochemical reduction. The Pt overlayer was characterized by SEM, XPS and by cyclic voltammetry. The catalytic efficiency of the modified film thus prepared through soft route was evaluated by the electro catalytic oxidation of methanol using cyclic voltammetry, chronoamperometry and AC impedance techniques. This work also demonstrates that the copper adlayer formed on the dendrimer-modified electrode can undergo galvanic replacement by nobler metals like Au and Ag, besides Pt. An elegant soft route involving a new three-step protocol to build the concentration of active Pt on the Au surface has been developed. Concentration of the same metal (Pt) or two different metals (Pt–Au) can be built at the interface in a stepwise manner at ambient temperature.     

Impedance spectroscopy data for S2O8 anion electroreduction at Bi(1 1 1) plane

Electroreduction kinetics of the peroxodisulfate anions on the electrochemically polished Bi(1 1 1) single crystal electrode has been studied by impedance spectroscopy. Influence of the electrode potential, reaction intermediates, base electrolyte and reactant concentrations on the kinetic parameters of electroreduction has been established. Systematic analysis of the fitting results demonstrates the noticeable influence of adsorption of the reaction intermediate or reactant on the electroreduction rate of the S₂O₈²⁻ anions at the Bi(1 1 1) electrode. In the region of so-called “current pits” in the cyclic voltammetry curves, obtained by rotating disc electrode method, the mixed kinetics, i.e. the adsorption and “true” charge transfer limited steps have been established by impedance spectroscopy.     

Rate of the MnO−4/MnO2−4 and MnO2−4/MnO3−4 electrode reactions in alkaline solutions at solid electrodes

The influence of the ionic strength, pH, nature of the background electrolyte and its concentration as well as the influence of the electrode material (Pt, glassy carbon, carbon paste) on the electrode kinetics of the MnO^?₄/MnO^?2₄ and MnO^2?₄/MnO^3?₄ systems has been investigated by means of cyclic voltammetry and the rotating disc electrode method. Rate constants determined with the use of the carbon paste electrode agreed well with those which were in some instances available in the literature. At the Pt. electrodes the rate could be only estimated.The rate constants obtained for both electrode reactions were considered in terms of the Marcus theory. Also the charge of rate constants and formal potentials with studied parameters was discussed.     

Approaches for multicopper oxidases in the design of electrochemical sensors for analytical applications

We report an effective approach for the construction of a biomimetic sensor of multicopper oxidases by immobilizing a cyclic-tetrameric copper(II) species, containing the ligand (4-imidazolyl)ethylene-2-amino-1-ethylpyridine (apyhist), in the Nafion^® membrane on a vitreous carbon electrode surface. This complex provides a tetranuclear arrangement of copper ions that allows an effective reduction of oxygen to water, in a catalytic cycle involving four electrons. The electrochemical reduction of oxygen was studied at pH 9.0 buffer solution by using cyclic voltammetry, chronoamperometry, rotating disk electrode voltammetry and scanning electrochemical microscopy techniques. The mediator shows good electrocatalytic ability for the reduction of O₂ at pH 9.0, with reduction of overpotential (350 mV) and increased current response in comparison with results obtained with a bare glassy carbon electrode. The heterogeneous rate constant (k^′ME) for the reduction of O₂ at the modified electrode was determined by using a Koutecky-Levich plot. In addition, the charge transport rate through the coating and the apparent diffusion coefficient of O₂ into the modifier film were also evaluated. The overall process was found to be governed by the charge transport through the coating, occurring at the interface or at a finite layer at the electrode/coating interface. The proposed study opens up the way for the development of bioelectronic devices based on molecular recognition and self-organization.     

Fabrication of copper coated polymer foam and their application for hexavalent chromium removal

The polymer foam coated with zero-valent copper (Cu⁰) was designed and prepared for the removal of hexavalent chromium (Cr(VI)) in water. Firstly, porous poly(tert-butyl acrylate) was fabricated by concentrated emulsion polymerization and then acrylic acid groups were generated on the surface of foam by hydrolysis reaction. Secondly, with the help of the large amount reactive carboxylic acid groups, polyethyleneimine (PEI) were chemically grafted onto the surface by the reaction between amine group and acrylic acid group. Finally, zero-valent copper was reduced by sodium borohydride (NaBH₄) and coated on the surface of polymer foam. Thus the copper functionalized porous adsorbent (Cu⁰–PEI–PAA) was constructed, and then applied for removing Cr(VI) from aqueous solution. The removal mechanism of Cr(VI) involved redox reaction by zero-valent copper and adsorption by amine groups, simultaneously. As a result, 99.5% of Cr(VI) could be removed within 2 h, and the maximum removal capacity for Cr(VI) of Cu⁰–PEI(1800)–PAA was 9.16 mg/g. Furthermore, the effect of initial concentration of Cr(VI), pH value, and temperature on the Cr(VI) removal was investigated. Therefore, the as-prepared zero-valent copper-loaded polymer foam could be an efficient and promising remediation material to remove Cr(VI) from wastewater.     

Electrochemistry of pyrylium compounds—II[1]. Structural effects on the voltammetric behavior in acetonitrile

The electrochemical behavior of 25 pyrylium perchlorates and 2 benzopyrylium perchlorates in acetonitrile was investigated using voltammetry and oscillopolarography at a rotating disc electrode, voltammetry at a rotating ring-disc electrode, cyclic voltammetry and coulometry of the potentiostatic reduction. All compounds exhibit two cathodic waves corresponding to the formation of pyranyl radicals P^. and pyranyl anions P^? respectively. The dimerization of P^. leading to bipyranes P₂ decisively influences the voltammetric behavior. This process occurs more rapidly at the carbon atom 4 than at the carbon atom 2 of the pyrylium cation and is reversible in the case of 2,4,6-triaryl-compounds and irreversible in the case of pyrylium cations having H, methyl, benzyl or styryl in position 2 or 4. Some dimerization rate constants were determined and the equilibrium between P^. and the isomers of P₂ is discussed in structural terms.     

Cathodic codeposition of alloyed materials for use in lithium battery negative electrodes

This work aimed at studying the possibility of preparing Wood alloy structures by cathodic codeposition of Bi, Pb, Sn and Cd from aqueous solutions. The metal ions were dissolved in a fluoborate medium. The reduction mechanisms of the single ions, studied using a rotating disc copper electrode or by linear sweep voltammetry, were simple with a single fast electron transfer step, except for the case of Bi, where the reaction was found to be irreversible. By potentiostatic codeposition of the four metals at a copper rotating disc electrode, metallic powders, having a composition close to that of the Wood alloy, with Bi, Sn, Cd and Pb₇Bi₃ phases, were prepared. These powders had homogeneous grains with micrometric sizes, and a specific surface of 1.5 m² g^–1. Their performances as lithium host structures in negative electrodes will be assessed.Nomenclature transfer coefficient - k ⁰ standard rate constant - E _i=0 thermodynamic potential - E _p, E _p/2 peak and half-peak potentials - C _ox concentration of the oxidized species at the interface - C _ox ⁰ bulk concentration of the oxidized species - rotation rate (r.p.m.) - i ₁ limiting current density - i _p peak current density - kinematic viscosity     

Electrochemical reduction of nitrate in weakly alkaline solutions

The electrocatalytic activity of several materials for the nitrate reduction reaction was studied by cyclic voltammetry on a rotating ring disc electrode in solutions with different concentrations of sodium bicarbonate. Copper exhibited highest catalytic activity among the materials studied. Nitrate reduction on copper was characterized by two cathodic shoulders on the polarization curve in the potential region of the commencement of hydrogen evolution. In this potential range an anodic current response was observed on the Pt ring electrode identified as nitrite to nitrate oxidation. This indicates that nitrite is an intermediate product during nitrate reduction. These conclusions were verified by batch electrolysis using a plate electrode electrochemical cell. Copper and nickel, materials representing the opposite ends of the electrocatalytic activity spectra, were used in batch electrolysis testing.