Oxygen reduction mechanism on corroded zinc

The mechanism of oxygen reduction on the as-polished and corroded zinc specimens has been studied using a rotating ring disc electrode (RRDE) system. On the as-polished surface, oxygen was reduced into two distinct steps. In the first step, about 44% of O₂ was reduced to H₂O₂ in a 2-electron reaction with the rest being reduced to OH^? in a 4-electron reaction. On the other hand, in the second step, with the increase of overpotential O₂ was almost exclusively reduced to OH^? in a 4-electron reaction. The first step reduction occurred on an air-formed oxide-covered surface at more positive potential than ?1.2 V vs. Ag/AgCl and the second step reduction (E < ?1.2 V) took place on a semi-uniformly active surface. On the corroded surface, the second step was not distinctly observed on the polarization curve, because reduction of the zinc corrosion products simultaneously took place around ?1.2 V. The O₂ reduction in the first step was inhibited by deposition of the corrosion products, though the ratio of amount of O₂ reduced to OH^? in a 4-electron reaction was larger than that on the as-polished surface. The mechanism of oxygen reduction is discussed on the basis of results obtained from the RRDE experiment.     

Study of the electrocatalytic reduction of nitrite with silicotungstic heteropolyanion

Three couples of reversible redox waves of the SiW₁₂O^4?₄₀ anion which are composed of two one-electron and one two-electron processes occur in the potential range +0.1 to ?0.7 V in aqueous solutions. The first (most positive) and third (most negative) redox waves exhibit good electrocatalytic activities for nitrite reduction in acid solutions with pH < 2 for the former and with $\text{pH}\text{? 4}$ for the latter. The behavior of the third catalytic wave, which is quite unusual, was studied in detail. The rate constant governing the reduction of NO^?₂ by the first wave of SiW₁₂O^4?₄₀ was measured by an ultramicrodisk electrode as 3.73 × 10³ M^?1 s^?1.     

DNA–Cu(II) complex as a novel electrocatalyst for a hydrogen peroxide sensor

A DNA–Cu(II) complex was effectively utilized as a novel electrocatalyst for an H₂O₂-sensing electrode. The DNA–Cu(II) complex was immobilized onto the surface of a glassy carbon (GC) electrode by polyion complex formation with poly(allylamine) (PAA). Cyclic voltammetry revealed that the copper ion embedded in the DNA/PAA layer exhibited a pair of well-defined redox peaks which is due to the Cu(II)/Cu(I) redox couple. The resulting DNA–Cu(II)/PAA/GC electrode showed excellent electrocatalytic activity for H₂O₂ reduction. The amperometric response to H₂O₂ obtained at ?0.2 V (vs. Ag∣AgCl) was rapid and highly sensitive. A calibration curve obtained in air-saturated buffer (pH 5) was linear in the concentration range from 0.1 to 135 μM with a detection limit of 0.05 μM (S/N = ca. 3). Moreover, this sensor showed long term-storage stability of more than 30 days under dry conditions 4 °C.     

Temperature-dependence of oxygen reduction activity at a platinum electrode in an acidic electrolyte solution investigated with a channel flow double electrode

The temperature dependence of the oxygen reduction reaction (orr) activity at a platinum electrode in 0.1 M HClO₄ electrolyte solution was investigated with a channel flow double electrode at 20–90 °C. We demonstrate that an apparent rate constant (k_app) for the orr can be evaluated from the hydrodynamic voltammograms by correcting the oxygen concentration in the electrolyte solution. The apparent activation energy for the k_app was found to be 38 kJ/mol at ?0.525 V vs. E⁰ (0.760 V vs. RHE at 30 °C). Production of H₂O₂ was not detected at potentials more positive than 0.3 V in the temperature region examined. One capacitive semicircle was observed in electrochemical impedance spectroscopy for the orr, suggesting the fast complete reduction of intermediates to H₂O following the rate determining step of the first electron-transfer.     

Cardiopulmonary exercise testing and cardiopulmonary morbidity in patients undergoing major head and neck surgery

Cardiopulmonary exercise testing (CPET) is used as a risk stratification tool for patients undergoing major surgery. In this study, we investigated the role of CPET in predicting day five cardiopulmonary morbidity in patients undergoing head and neck surgery. This observational cohort study included 230 adults. We recorded preoperative CPET variables and day five postoperative cardiopulmonary morbidity. Full data from 187 patients were analysed; 43 patients either had incomplete data sets or declined surgery/CPET. One hundred and nineteen patients (63.6%) developed cardiopulmonary morbidity at day five. Increased preoperative heart rate and duration of surgery were independently associated with day five cardiopulmonary morbidity. Those with such morbidity also had lower peak V?O₂ 11.4 (IQR 8.4-18.0) vs 16.0 (IQR 14.0-19.7) ml.kg^-1.min^-1, P<0.0001 and V?O₂ at AT 10.6 (IQR 9.1-13.1) vs 11.5 (IQR 10.5-13.0) ml.kg^-1.min^-1, p=0.03. Logistic regression model containing peak V?O₂ and duration of surgery demonstrated that increased peak V?O2 was associated with a reduction in the likelihood of cardiopulmonary complications OR 0.92 (95%CI 0.87 to 0.96), p=0.001. The area under the receiver operating characteristic curve for this model was 0.75(95%CI 0.68 to 0.82), p<0.0001, 64% sensitivity, 81% specificity. CPET can help to predict day five cardiopulmonary morbidity in the patients undergoing head and neck surgery. A model containing peak V?O₂ allowed identification of those with such complications.     

Electroreduction of peroxodisulfate anion at a Cd(0001) single-crystal plane electrode

Electroreduction of peroxodisulfate anion on an electrochemically polished Cd(0001) plane in aqueous NaF solution with different additions of Na₂S₂O₈ was studied by rotating disc voltammetry. The rate of electroreduction of S₂O^2?₈ depends on the electrode potential and base electrolyte concentration, i.e. on the thickness of the diffuse layer. The kinetic current densities at E=const have been obtained by the Koutecký–Levich method and used for the determination of the apparent rate constant values of electroreduction of S₂O₈^2? anion. The ψ₀ potential values obtained according to the Gouy–Chapman–Grahame (GCG) model have been used for the construction of corrected Tafel plots, which were linear at E?ψ₀<?1.1 V (SCE). The value of the charge transfer coefficient α decreases somewhat with the concentration of the base electrolyte solution (0.18≤α≤0.22), but the value of α is practically independent of the concentration of S₂O₈^2? ions if c_NaF=const.     

Preparation,characterization, and electrocatalytic properties of copper hexacyanoferrate film and bilayer film modified electrodes

Copper(II)hexacyanoferrate films have been prepared from various electrolyte aqueous solutions using consecutive cyclic voltammetry. The cyclic voltammograms recorded the direct deposition of copper(II)hexacyanoferrate films from the mixing of Cu²⁺ and Fe(CN)₆^3? ions from solutions of ten cations: Li⁺, Na⁺, K⁺, Rb⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, H⁺ and Al³⁺. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the copper(II)hexacyanoferrate films. The copper(II)hexacyanoferrate film showed a single redox couple that exhibited a cation effect (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and an anion effect (F^?, Cl^? and Br^?) in the cyclic voltammograms and formal potential of the redox couple. The electrochemical and EQCM properties of the film indicate that the redox process was confined to the immobilized copper(II)hexacyanoferrate, and the interaction between the copper(II)hexacyanoferrate film with K⁺ (monovalent cation) and Ca²⁺ (divalent cation). The electrocatalytic oxidation properties of NADH, NH₂OH, N₂H₄, SO₃^2? and S₂O₃^2? were also determined. The electrocatalytic reduction properties of SO₅^2? and S₂O₈^2? by monolayered iron, nickel, and cobalt hexacyanoferrate films, and by bilayered metal–copper hexacyanoferrate films were determined. Two-layered modified electrodes and hybrid films composed of a copper(II)hexacyanoferrate film with iron(II)hexacyanoferrate, cobalt(II)hexacyanoferrate, or nickel(II)hexacyanoferrate film were prepared, and these films caused the electrocatalytic reduction of SO₅^2? and S₂O₈^2?.     

Electrocatalysis of O2 reduction at polyaniline+molybdenum-doped ruthenium selenide composite electrodes

Composite layers consisting of a polyaniline (PANI) matrix and a molybdenum-doped ruthenium selenide (MRS) dispersed phase have been tested as cathodes for O₂ reduction in 0.5 M H₂SO₄. The investigation, carried out on rotating disc electrodes using steady-state techniques, shows that a 4-electron reduction of O₂ is efficiently carried out on PANI+MRS composites. The onset potential is close to 0.5 V (SCE) and a current plateau is observed at E<0.1 V (SCE). The overall process is under mixed control. Besides the charge transfer and mass transport in solution, either O₂ diffusion in the film or partial blockage of the electrode are possible limiting factors. The performance of PANI+MRS electrodes becomes better than that of Pt in 0.5 M H₂SO₄ in the presence of methanol. Diminished current densities are obtained at higher pH due to a decreased O₂ solubility, the conductivity of the PANI matrix being always good at pH<3.     

A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.     

Electrode reactions and accumulation of hydrogen at carbon paste electrodes in the presence of tetrachloropalladate

The electrochemical behaviour of the unmodified carbon paste electrode (CPE) in the presence of [PdCl₄]^2? in chloride solutions of different pH (in the range from 3 to 7) is studied by cyclic voltammetry (CV). During cathodic treatment or cycling, Pd(0) is deposited in situ on the electrode surface. Results obtained at negative potentials as a function of the concentrations of Pd(II) and protons in the solution are presented. It is concluded that hydrogen ions are reduced in different ways: first, the reduction at deposited Pd(0) or the simultaneous reduction of H⁺ and [PdCl₄]^2?, respectively, which occurs during the whole cathodic cycle; second, the reduction in the potential range from about ?0.5 to ?1.0 V versus SCE, where surface groups are reduced at the unmodified CPE. The second pathway is the prevailing reduction at lower pH, which occurs at the carbon surface probably via protonated chloropalladate complexes adsorbed at the electrode surface. Potentiostatic pretreatment in the voltammetric range between 0 and +0.2 V leads in solutions of pH 5 to a pronounced increase of the H⁺ reduction current caused by an increase of the sorption capacity of the electrode or by the accumulation of protons at the electrode surface. Protons could be accumulated at the electrode surface by formation of protonated chloropalladate complexes at the CPE.     

Theoretical and experimental aspects of electrodeposition under hydrodynamic conditions

The utility of a recently derived model [M.E. Hyde, O.V. Klymenko, R.G. Compton, J. Electroanal. Chem. 534 (2002) 13] for the potentiostatic nucleation and three dimensional growth of deposits on an electrode surface under hydrodynamic conditions is demonstrated. 2.1 mM Pb²⁺ solution is used as the depositing species, and a wall-tube and stirred cell are used to generate conditions in which the diffusion layer thickness is in the range 10–40 μm. It is shown that the model provides excellent fits to the experimental data. The nucleation parameters N₀, the number of active sites, and A, the nucleation rate are derived from analysis of the transients. These values are compared to those extracted from experiments under quiescent conditions, and also to previously reported data in which ultrasound was used to induce hydrodynamic flow.     

Anodic electrodeposition of copper oxide/hydroxide films by alkaline solutions containing cuprous cyanide ions

A novel procedure of anodic electrodeposition of copper oxides and hydroxides on glassy carbon and noble metals electrodes in an alkaline medium is described. The deposited films have been investigated by electrochemical and by X-ray photoelectron spectroscopy (XPS) techniques. The copper films are deposited on the electrode surface as Cu(OH)₂ and CuO after the dissociation of the Cu(CN)_n^1?n complex caused probably by the formation of Cu^III species at relatively high applied potentials (i.e. 0.5 V). The resulting active copper films, independently of the electrochemical procedure adopted (i.e. potentiostatic or potentiodynamic conditions), appear to be uniform, smooth and with a good degree of adhesion on the electrode surface The electrocatalytic properties of the copper deposited films were investigated in an alkaline medium for the electrooxidation of several classes of organic compounds.     

Preparation,characterization and electrocatalytic properties of polynuclear mixed-valent ruthenium oxide/hexacyanoruthenate film modified electrodes

Polynuclear mixed-valent ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate or mvRuO/RuCN) films were prepared using consecutive cyclic voltammetry directly from the mixing of Ru³⁺ and Ru(CN)₆^4? ions from solutions of two divalent cations (Ba²⁺ and Ca²⁺), and seven monovalent cations (H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, and Ga⁺). The films exhibited three redox couples with Ba(NO₃)₂ or BaCl₂ aqueous solutions, and the formal potentials of the redox couples showed a cation and pH effect. An electrochemical quartz crystal microbalance (EQCM), cyclic voltammetry, UV–visible spectroscopy, and the stopped-flow method (SFM) were used to study the growth mechanism of the mvRuO/RuCN films. The results indicated that the redox process was confined to the immobilized ruthenium oxide/ruthenocyanide. The EQCM results showed a Ba²⁺ ion exchange reaction for the two most negative redox couples. The electrocatalytic reduction properties of SO₅^2?, and S₂O₈^2? by the ruthenium oxide/ruthenocyanide films were determined. The electrocatalytic oxidation of NADH and dopamine were also determined, and revealed two different types of properties. The electrocatalytic oxidations of SO₃^2?, S₂O₃^2?, and N₂H₄ were also investigated. The electrocatalytic reactions of the ruthenium oxide/ruthenocyanide films were investigated using the rotating ring-disk electrode method.     

Speciation of a water-soluble chromium porphyrin by spectral and electrochemical methods

A water-soluble chromium porphyrin, [Cr^IIITF₄TMAP]⁵⁺ (chromium(III) meso-tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-anilinium)yl) porphyrin) was synthesized and the spectral and electrochemical properties were examined. In aqueous solution, [Cr^IIITF₄TMAP]⁵⁺ exhibits two pK_as at 7.04 and 10.07. The cyclic voltammogram showed that a reversible reduction wave appeared at ?0.84 V in pH 5.0 buffer solution, which is assigned as the Cr^III/II redox couple. The oxidation formal potentials were obtained by spectroelectrochemistry. With suitable control of the oxidation potentials and solution conditions, the high valent oxo-Cr^IV and oxo-Cr^V porphyrins were obtained. The ligand trans to the oxygen atom is either H₂O or OH^?, depending on the pH of the solution. In this paper, oxo-Cr^VTF ₄TMAP is thus the first reported water-soluble Cr^V porphyrin that can be generated electrochemically and chemically. Electrogenerated oxo-Cr^V porphyrin species catalyzed the oxidation of cyclopent-2-ene-1-acetic acid to cyclopent-2-ene-4-one-1-acetic acid in the presence of dioxygen and returned to oxo-Cr^IV porphyrin, which is not reactive toward the substrate.     

Potential-dependent electrochemiluminescence of luminol in alkaline solution at a gold electrode

The behavior of luminol electrochemiluminescence (ECL) at a polycrystalline gold electrode was studied under conventional cyclic voltammetric (CV) conditions. At least six ECL peaks were observed at 0.28 (ECL-1), 0.56 (ECL-2), 0.95 (ECL-3), 1.37 (ECL-4), ?0.43 (ECL-5) and 1.00 (ECL-6, a broad wave after the reverse scan from +1.66) V (vs. SCE), respectively, on the curve of ECL intensity versus the potential. These ECL peaks were found to depend on the presence of O₂ and N₂, the pH of the solution, KCl concentration, scan rate, and potential scan ranges. The emitter of all ECL peaks was identified as 3-aminophthalate by analyzing the CL spectra. It is believed that ECL-1 at 0.28 V was correlated to luminol radicals produced by the electro-oxidation of luminol anion and ECL-2 at 0.56 V was caused by the reaction of luminol radical anions with gold oxide formed on the electrode surface. ECL-1 and ECL-2 could be strongly enhanced by O₂ and O₂^?. ECL-3 at 0.95 V was likely to be due to the reaction of luminol radical anions with O₂ oxidized by OH^?. ECL-4 at 1.37 V suggested that OH^? was electro-oxidized to HO₂^? at this potential and then to O₂^?, which reacted with luminol radical anions to produce light emission. ECL-5 at ?0.43 V seems to be due to the reaction of luminol with ClO^? electrogenerated at higher positive potential and HO₂^? electrogenerated at negative potential. ECL-6 was attributed to the reaction of luminol radical anions and ClO^? electrogenerated at higher positive potential. The results indicated that luminol ECL can be readily initiated by various oxygen-containing species electrogenerated at different potentials, leading to multi-channel light emissions. Furthermore, the present work also reveals that ECL-2 is a predominant ECL reaction route at a gold electrode with higher potential scan rates under CV conditions.     

Comparison of the direct electrochemistry of myoglobin and hemoglobin films and their bioelectrocatalytic properties

The electrocatalytic properties of two related proteins, myoglobin (Mb) and hemoglobin (Hb), immobilized in a surfactant film (didodecyldimethylammonium bromide, DDAB) on an electrode were investigated. The direct electrochemistry of the myoglobin/DDAB and hemoglobin/DDAB films was compared and showed one redox couple, two redox couples, and three redox couples, when transferred to strong acidic, weak acidic and basic, and pH 13 aqueous solutions, respectively. The redox couples and their formal potentials are pH dependent. An electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of both DDAB deposition on gold disk electrodes and myoglobin deposition on DDAB film-modified electrodes. The electrocatalytic properties were investigated and showed that the myoglobin/DDAB and hemoglobin/DDAB film modified electrodes are both electrocatalytically active for oxygen reduction, and more H₂O₂ was produced during electrocatalytic reduction using a myoglobin/DDAB film than when using a hemoglobin/DDAB film. The electrocatalysis that was active for l-cystine, N₂O, and 2,2′-dithiosalicylic acid reductions showed an electrocatalytic current developed from the cathodic peak of the redox couple at about ?0.9 V (Fe(II)/Fe(I) redox couple) in neutral and weak basic aqueous solutions. The electrocatalysis that was active for l-cysteine oxidation, showed an electrocatalytic current developed from the cathodic peak of the redox couple at about +0.2 V (Fe(III)/Fe(IV) redox couple). Mb/DDAB and Hb/DDAB film modified electrodes are electrocatalytically active for trichloroacetic acid reduction in weak acidic and basic buffered aqueous solutions through the Fe(II)/Fe(I) redox couple. However, the electrocatalytic current developed from the cathodic peak of the redox couple at a potential of about ?0.1 V (from the Fe(III)/Fe(II) redox couple) in strong acidic aqueous solutions occurs only with higher concentrations of reactant.     

Effect of pH and temperature on carbon-supported manganese oxide oxygen reduction electrocatalysts

Manganese oxides nanoparticles were chemically deposited on a high area (ca. 300 m² g⁻¹) carbon black substrate to act as electrocatalysts for oxygen reduction. The morphology and chemistry of the carbon-supported MnO_x nanoparticles was characterised by Transmission Electron Microscopy), X-ray Diffraction, and chemical analysis. The oxygen reduction reaction (ORR) catalytic activity was studied in the 7–10 pH range using a rotating disk electrode (RDE). High activity towards oxygen reduction and very good stability in neutral and slightly basic solution were obtained. At low current densities, at 25 °C, MnO_x/C displayed a reaction order with respect to OH⁻ ions of −0.5 and Tafel slopes of −0.153 and −0.167 V dec⁻¹ at pH 7 and 10 respectively; showing that the ORR mechanism on MnO_x/C is unchanged in the 7–10 pH range. From the data, we propose that the first electrochemical step of the 4-electron ORR mechanism, in the 7–10 pH range, is the quasi equilibrium proton insertion process in MnO₂ yielding MnOOH (insoluble in neutral or slightly basic solution). The ORR activity of the MnO_x/C materials increased with increasing temperatures from 5 to 40 °C. The 2-electron pathway of oxygen reduction, yielding hydrogen peroxides as intermediates, may however be favoured over the 4-electron O₂ reduction at higher temperatures.     

The influence of nitrate concentration and acidity on the electrocatalytic reduction of nitrate on platinum

A study was performed to determine the influence of nitrate concentration and acidity on the reaction rate and selectivity of the electrocatalytic nitrate reduction on platinum. There are two different nitrate reduction mechanisms on platinum: a direct mechanism (0.4–0.1 V vs. SHE) and an indirect mechanism (0.9–0.5 V vs. SHE). In the direct mechanism the dependence of the reaction rate on the nitrate concentration changes with increasing nitrate concentration. Whereas at low concentrations (<0.1 M) the reaction order in nitrate is positive, at high concentrations (>0.1 M) the reaction order is negative. This suggests that at high concentrations the amount of free surface sites determines the reaction rate. These free surface sites are needed either for the adsorption of a second species necessary for the reaction (water or hydrogen) or for the dissociation of nitrate to nitrite. Both at low and high nitrate concentrations the direct reduction is mainly selective towards ammonia, although small amounts of N₂O and N₂ were observed using differential electrochemical mass spectrometry (DEMS) at potentials between 0.4 and 0.2 V at high concentrations of nitrate. This N₂ and N₂O formation seems to be related to the NO_ads coverage on the electrode. The indirect reduction mechanism is autocatalytic as is illustrated by its unusual stirring behavior. Large amounts of NO were observed using DEMS. This suggests that not nitrate but NO⁺ (? HNO₂) is involved in the actual electron transfer. NO⁺ is reduced to NO, which then reacts with HNO₃ to reproduce NO⁺, resulting in an overall reaction of nitrate to nitrite. Both nitrite and a high acidity are needed for this mechanism to develop, but addition of nitrite is not necessary since nitrite is present in small amounts in HNO₃ solutions of concentrations over 4 M. The autocatalytic reduction mechanism slows down and eventually terminates when NO starts to react to N₂O, as was observed using DEMS.     

Origin and features of the electrochemiluminescence of luminol – Experimental and theoretical investigations

The electrochemiluminescence accompanying the oxidation of luminol on platinum or graphite anodes was investigated under potentiostatic and potentiodynamic conditions in order to find out how it varies in time and with electrode potential, pH of the medium, and substrate concentration. The single-stage process observed in the range between 0 and 1 V can be attributed to the oxidation of the anion occurring as a result of H⁺ abstraction by OH^? from one of the NH sites of luminol. The radical species thus formed undergo either disproportionation or, more probably, electrochemical oxidation (preceded by the abstraction of H⁺ by OH^? from the remaining NH sites) to yield 5-aminophthalazine-1,4-dione. Electrochemical oxidation of the addition products of OH^? to one of the carbonyl carbon atoms of this molecule, accompanied by N₂ elimination, leads to radical intermediates which, after the addition of successive OH^? ions, are converted to the radical anion of 3-aminophthalic acid. Electrochemical oxidation of the radical anion thereby formed – accompanied by the abstraction of H⁺ by OH^? from one of carboxylic groups – generates electronically excited mono-anions of 3-aminophthalic acid capable of emitting light. The proposed reaction scheme is confirmed by thermodynamic data obtained at the PM3, PM3(COSMO), DFT and DFT(PCM) levels of theory. The prospective analytical applications of the direct electrochemical oxidation of luminol are mentioned.     

Oxygen reduction in alkaline medium at thin MnxCo3−xO4 (0≤x≤1) spinel films prepared by spray pyrolysis. Effect of oxide cation composition on the reaction kinetics

We have studied the relationships between the cation distributions in bulk and on the surface and the electrocatalytical reactivity towards the oxygen reduction reaction (orr) on thin film electrodes of cobalt and manganese spinel type oxides Mn_xCo_3?xO₄ (0≤x≤1). The cationic distributions were computed from XRD, XPS, magnetism and oxidation power measurements showing the presence of the Co³⁺/Co²⁺ and Mn⁴⁺/Mn³⁺ solid state redox couples and also vacancies in octahedral sites. The electrocatalytic activity and kinetic parameters for the orr were determined using the RRDE technique in 1 M KOH solution at 25 °C. Two Tafel zones of slopes b_c1 and b_c2 around ?60 and ?120 mV dec^?1 were observed. The disk current/ring current ratio, (I_D/I_R), increases with x and decreases with the applied overpotential. At constant overpotential η the real electrocatalytical current densities j increase when x increases. The orr can be characterized by an intrinsic kinetic parameter J′ obtained from this current ratio at various electrode rotational frequencies. This J′ increases with the redox couple Co²⁺/Co³⁺ ratio, increasing more in the bulk than at the surface of the electrode.