Electrocatalytic oxidation of dopamine at a cobalt hexacyanoferrate modified glassy carbon electrode prepared by a new method

A stable electroactive thin film of cobalt hexacyanoferrate (CoHCF) was electrochemically deposited on the surface of a glassy carbon (GC) electrode with a new and simple method. The cyclic voltammograms of the CoHCF Film modified GC (CoHCF/GC) electrode prepared by this method exhibit two pairs of well-defined redox peaks, at scan rates up to 200 mV s(-1). The advantage of this method is that it is easy to manipulate and to control the surface coverage of CoHCF on the electrode surface. The modified electrode shows good electrocatalytic activity towards the electrochemical reaction of dopamine (DA) in a 0.1 mol dm (3) KNO3 + phosphate buffer solution (pH 7.0). The rate constant of the electrocatalytic oxidation of DA at the CoHCF/GC electrode is determined by employing rotating disk electrode measurements.

Fabrication of metalloporphyrin-polyoxometalyte hybrid film by layer-by-layer method and its catalysis for dioxygen reduction

Through layer-by-layer method [tetrakis(N-methylpyridyl)porphyrinato] cobalt (CoTMPyP) and polyoxometalyte were alternately deposited on 4-aminobenzoic acid (4-ABA) modified glassy carbon electrode. The resulting organic-inorganic hybrid films were characterized by cyclic voltammetry (CV), UV/visible absorption spectroscopy, and atomic force microscopy (AFM). It was proved that the multilayer films are uniform and stable. CoTMPyP-containing multilayer films exhibit remarkable electrocatalytic activity for the reduction of O-2. Rotating disk electrode (RDE) voltammetry and rotating ring-disk electrode (RRDE) voltammetry confirm that P2W18/CoTMPyP multilayer films can catalyze the four-electron almost reduction of O-2 to water in pH > 4.0 buffer solution, while SiW12/CoTMPyP multilayer films catalyze about two-electron reduction of O-2 to H2O2 in pH 1 - 6 buffer solutions. The kinetic constants for O-2 reduction were comparatively investigated at P2W18/CoTMPyP and SiW12/CoTMPyP multilayer films electrodes.

Kinetics and mechanism of Y(III) extraction with CA-100 using a constant interfacial cell with laminar flow

The Yttrium(III) extraction kinetics and mechanism with secnonylphonoxy acetic acid (CA-100) were investigated by a constant interfacial cell with laminar flow. The studies of interfacial tension and solubility of extractant and effects of the stirring rate, temperature, specific interfacial area and species concentration on the extraction rate showed that the extraction regime was dependent on the extraction conditions and the most probable reaction zone was at the liquid-liquid interface. The rate equation of extracting yttrium by CA-100 in heptane was Rf = k[Y3+]((a))[H(2)A(2)]((o))(0.88)[H+]((a))(-1.08).

Electrocatalytic reduction of O-2 and H2O2 at the glass carbon electrode modified with microperoxidase-11

Electrocatalytic reduction of O-2 and H2O2 at the glass carbon electrode modified with microperoxidase-11 immobilized with Nafion film has been studied by means of cyclic voltammetry and rotating disk electrode techniques. The modified electrode shows high catalytic activity toward the reduction of both O-2 and H2O2. The rate constants of Oz and H2O2 reduction at the modified electrode have been measured and compared. It is found that O-2 undergoes a four-electron reduction at the modified electrode and the catalytic activity for the reduction of O-2 is dependent on the pH of the solutions.

Orientation and electrocatalysis of riboflavin adsorbed on carbon substrate surfaces

Based on scanning tunnelling microscopy and electrochemical measurements, orientation and electrocatalytic function of riboflavin adsorbed on carbon substrates have been described for the first time. Scanning tunnelling micrographs show clearly that tip induction may result in an orientation change of the adsorbed riboflavin molecule on highly oriented pyrolytic graphite from the initially vertical orientation to the stable flat form. The adsorbed riboflavin as an effective mediator can accelerate the reduction of dioxygen which accepts two electrons from the reduced riboflavin to generate hydrogen peroxide. The rate constants of the electrocatalytic reaction in various pH solutions were determined using a rotating disc electrode modified with riboflavin. The pH effect and possible catalytic mechanism are discussed in detail.

Studies of Oxygen Reduction on Polycobaltprotoporphyrin Film with RRDE

The mechanism of oxygen reduction on polycobaltprotoporphyrin IX dimethyl ester (PolyCoPP) film has been studied by using the rotating ring(Au)-disk(pyrolytic graphite, PG) electrode (RRDE) technique. The PolyCoPP/PG electrode promotes the oxygen reduction via two-electron process which produces peroxide as a main product in O-2-saturated 0.1 mol.dm(-3) NaOH. Once HO2- has been formed, no further reduction to OH- takes place at the disk. When the disk potential shifts to more negative, either the direct reduction of O-2 to OH- or the further reduction of HO2- to OH- occurs.

A comparison of electrocatalytic ability of various mediators adsorbed onto paraffin impregnated graphite electrodes for oxidation of reduced nicotinamide coenzymes

Twelve mediators have been modified by adsorption onto the paraffin impregnated graphite electrodes (IGE). The resulting electrodes exhibit electrocatalytic activity of different degrees towards oxidation of 1,4-dihydronicotinamide adenine dinucleotide (NADH). The electrocatalytic ability of the chemically modified electrode (CME) depends mainly on the formal potential and molecular structure of mediator. The formation of the charge transfer complex between NADH and adsorbed mediator has been demonstrated by the experiments using a rotating disk electrode. An electrocatalytic scheme obeying Michaelis-Menten kinetics has been confirmed, and some kinetic parameters were estimated. The solution pH influences markedly the electrocatalytic activity of the modified electrode. Various possible reasons are discussed.

KINETIC-STUDY OF ELECTROCATALYTIC REDUCTION OF HYDROGEN-PEROXIDE AT IRON(III) PORPHYRIN MODIFIED GLASSY-CARBON ELECTRODE IN ALKALINE-MEDIUM USING THE ROTATION-SCAN METHOD

Reduction of hydrogen peroxide at a glassy carbon (GC) electrode modified with sigma-bonded pyrrole iron(III) octaethylporphyrin complex, (OEP)Fe(Pyr), was studied by cyclic voltammetry and a rotating disk electrode. In 0.1N NaOH solution, it is shown that such an (OEP)Fe(Pyr)/GC electrode has a significant catalytic activity towards hydrogen peroxide reduction (E(D) = -0.80 V, k = 0.066 cm s(-1)); however, the electrode stability is low. The deactivation is observed when the reaction charge (Q) is passing through the (OEP)Fe(Pyr)/GC disk electrode. A linear rotation scan method is applied to study the kinetic process by determining the disk electrochemical response (i(D)) to rotation rate (omega) at a definite disk potential (E(D)). Considering that the number of adsorbed electroreduced catalyst molecules (Red) varies according to the disk potential, a factor theta(= Gamma(Red)/(Gamma(Red) + Gamma(Ox))) is introduced to describe the electrode surface area fraction for electroreduced species. The obtained Koutecky-Levich equation is applicable whatever the potential is.

ELECTROCATALYTIC OXIDATION OF REDUCED NICOTINAMIDE COENZYMES AT ORGANIC DYE-MODIFIED ELECTRODES

Chemically modified electrodes (CMEs) were prepared by adsorbing different dyes, including methylene blue (MB), toluidine blue (TB) and brilliant cresyl blue (BCB), onto glassy carbon electrodes (GCE) with anodic pretreatment. The electrochemical reactions of adsorbed dyes are fairly reversible at low coverages. The CMEs are more stable in acid solutions than in alkaline ones, which is mainly due to decomposition of the dyes in the latter media. They exhibit an excellent catalytic ability for the oxidation of nicotinamide coenzymes (NADH and NADPH). The formation of a charge transfer complex between the coenzyme and the adsorbed mediator has been demonstrated using a rotating disk electrode. The charge transfer complex decomposition is a slow step in the overall electrode reaction process. Some kinetic parameters are estimated. Dependence of the electrocatalytic activity of the CMEs on the solution pH is discussed.

ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODES IN ACETONITRILE

The anodic oxidation kinetics of hydrazine on glassy carbon electrodes in acetonitrile were examined by cyclic voltammetry, a rotating ring-disc electrode technique and chronoamperometry. The experimental results of the rotating ring-disc electrode prove that hydrazine is oxidized to HN=NH, which cannot be oxidized further in acetonitrile. Hydrazine molecules are adsorbed on the electrode surface. One-third of the adsorbed hydrazine molecules are oxidized to HN=NH and the other two thirds act as proton acceptors. A possible mechanism of hydrazine oxidation is proposed.

ELECTROCHEMICAL CATALYSIS AT AN ULTRAMICROELECTRODE MODIFIED WITH REDOX SPECIES

General equations of the electrocatalytic reaction at an ultramicroelectrode modified with redox species have been described according to the Andrieux Saveant model. The electrocatalytic kinetic process has been discussed for the whole set of cases, ie (R), (R + S), (SR) (SR + E), (E), (R + E), (ER), (S), (ER + S) and (S + E) limiting situations. The effect of gamma on the catalytic steady state current shows that the higher the value of gamma, the lower the catalytic current. The kinetic process shifts rapidly from R to E with increasing values of gamma. It is favorable for catalysis only when gamma is very low. Therefore, the redox species modified ultramicroelectrode with thin film is utilized for electrocatalysis, and the larger the radius of ultramicroelectrode, the higher the catalytic efficiency.

THE ANODIC-OXIDATION OF HYDRAZINE ON GLASSY-CARBON ELECTRODE

The anodic oxidation kinetics of hydrazine (N2H4) on glassy carbon (GC) electrode was examined by cyclic voltammetry, rotating disk and ring-disc electrode techniques. The possible mechanisms of N2H4 oxidation in both aqueous and nonaqueous solutions are proposed.

ELECTROCHEMICAL-BEHAVIOR OF YTTRIUM ION IN LICL-KCL-NACL EUTECTIC MELT

The electrochemical reduction of yttrium ion on a molybdenum electrode in a LiCl-KCl-NaCl eutectic melt at 723 K was found to be almost reversible and to proceed by a one-step three electron reaction. The diffusion coefficient D of the Y(III) ion was measured to be (3.3 +/- 0.4) x 10(-6) cm2 s-1 by cyclic voltammetry, (5.0 +/- 0.9) x 10(-6) cm2 s-1 by the rotating disk electrode method, and (7.1 +/- 0.7) x 10(-6) cm2 s-1 by chronopotentiometry. The D values obtained by the latter two methods are in fairly good agreement with each other. The rather low D value obtained by cyclic voltammetry might be attributed to the fact that yttrium metal can dissolve slightly in the chloride melt. The standard potential of Y(III)/Y(0) couple was determined to be (-3.174 +/- 0.006) V (vs. Cl2/Cl-) by open-circuit potentiometry, (-3.15 +/- 0.02) V (vs. Cl2/Cl-) by the rotating disk electrode method and (-3.16 +/- 0.02) V (vs. Cl2/Cl) by chronopotentiometry. These three values are in good agreement with each other. Several types of Ni-Y intermetallic compounds were found to be formed on a nickel electrode.

INTERPRETATION OF THE PHOTOREDUCTION CURRENT RESPONSE ON POLYANILINE MODIFIED ELECTRODE

The photoreduction current response on a polyaniline modified electrode is interpreted as photo-assisted reduction of oxygen dissolved in the electrolyte solution but not due to the excited species PAn* and PAn* dagger. The effect of light is just the same as that of the electrode rotating or of stirring of the electrolyte which accelerates the oxygen dissolved in the electrolyte solution to react with leucoemeraldine (reduced polyaniline). The potentiostat is set to reduce the oxidized polyaniline at constant potential, thus producing a reduction current.

ELECTROCATALYTIC OXIDATION OF ASCORBIC ACID AT POLYANILINE FILM MODIFIED GLASSY CARBON ELECTRODES

A conducting polyaniline (PAn) film modified glassy carbon (GC) electrode was prepared by electrochemical polymerization. The electrochemical behavior of ascorbic acid (AH(2)) in aqueous solution at this PAn modified electrode was studied in detail. The experimental results show that PAn film modified electrode has good electrocatalytic activity on the oxidation of ascorbic acid in aqueous solution over a wide range of pH value, among which pH 4 is the optimum condition. The oxidation process of ascorbic acid at PAn film electrode can be regarded as an EC catalytic mechanism. The kinetic process of the catalytic reaction was investigated by rotating disk electrode (RDE) coated with PAn films. The rate constant of the catalytic reaction was evaluated. The catalytic peak currents are proportional to the concentrations tions of ascorbic acid in the range of 5 x 10(-2)-1 x 10(-6) mol . L-1. The PAn film elec trodes give very stable responce for the oxidation of ascorbic acid. The present investigation shows the posibility of using PAn film modified electrode for the determination of ascorbic acid.