Kinetics and mechanism of sulphite oxidation on a rotating platinum disc electrode in an alkaline solution

The reaction of sulphite at high pH and room temperature has been examined by cyclic voltammetric methods on a rotating platinum disc electrode. The mechanism of the oxidation reaction is studied by varying the sweep rate, the rotation rate and the concentration. The oxidation of sulphite on a platinum electrode under alkaline conditions appears to follow a catalytic reaction mechanism where weakly adsorbed sulphite is first oxidised to a strongly adsorbed sulphite radical in the rate-determining step. In the next step two sulphite radicals combine and form dithionate, which disproportionate into sulphite and sulphate.     

Effect of cations in solution on the oxidation of methanol on the surface of platinum electrode

The effect of metal cations in solution on the oxidation of methanol on the electrode surface of platinum is a neglected aspect to direct methanol fuel cell (DMFC). In this paper, a smooth platinum electrode absorbing metal cations as the working electrode was applied to investigate the methanol oxidation with the cyclic voltammetry (CV) in 1.0 mol L⁻¹ H₂SO₄. From the analysis of experiment, it is found that the cations, Li⁺, Ce⁴⁺, Mn²⁺, Ni²⁺, Cu²⁺, have some negative effect on the catalytic oxidation of methanol on the surface of platinum. The degree of the effect from different cations was analyzed.     

Voltammetric response of iron hydroxide layers precipitated on platinum electrodes in alkaline solution

Voltammograms of thin and thick iron hydroxide layers chemically precipitated on platinum were obtained. The influence of the precipitation conditions of iron hydroxide was studied. Different types of combined potential/time perturbation programmes were employed to correlate the anodic and cathodic voltammetric peaks and to detect possible ageing-like effects. The electrochemical response of the iron hydroxide layer depends considerably on the layer thickness and precipitation conditions. The voltammetric behaviour of the precipitated iron hydroxide layer is interpreted in terms of the complex structure involving an inner poorly hydrated layer and an outer porous layer recently proposed from in situ ellipsometry data.     

The surface oxidation of pyrite in alkaline solution

The collector-less flotation of pyrite has been studied by conventional techniques and is correlated to the electrochemical behaviour of pyrite in alkaline solution (1m NaClO₄, pH 11). It was concluded that the initial oxidation of pyrite produces a hydrophobic sulphur rich surface together with hydrophilic iron hydroxide species. Also upon grinding, the surface is covered by hydrophilic species and therefore no significant flotation was obtained in the absence of a collector. However, collectorless flotation was readily obtained in an iron complexing solution like EDTA. This indicates that the remaining sulphur-rich layer is responsible for the floatability of pyrite under these conditions.     

Kinetics and mechanism of oxygen reduction at hydrous oxide film-covered platinum electrode in alkaline solution

This study uses rotating ring-disk electrode (RRDE) and linear sweep voltammetry (LSV) to characterize oxygen reduction kinetics in alkaline solution on platinum electrodes with various thickness of hydrous oxide (oxyhydroxy) film. Oxyhydroxy films are created on Pt electrodes by pretreatment in 1.0 mol dm⁻³ KOH at a constant voltage. The pretreatment voltage ranges from −1.2 to 1.0 V and is increased stepwise before each new experimental run to produce seven discreet films. LSV plots show oxyhydroxy film thickness strongly inhibits oxygen reduction and is inversely proportional to RRDE oxygen reduction current I_D for LSV voltages E_D from −0.1 to −0.46 V, but this trend reverses at E_D more negative than −0.46 V so that the worst-performing electrode becomes the best. However, this improvement disappears at around −0.8 V, suggesting this change involves a negatively charged ion, possibly embedded into the metal in the top few atomic layers either interstitially or substitutionally. The 1.0 V-pretreated electrode in the E_D range from −0.46 to −0.9 V of highest oxygen reduction current also exhibits the lowest hydrogen peroxide production, with zero H₂O₂ produced at −0.6 V, indicating the brief presence of the oxyhydroxy film on the Pt surface has strong lingering effects. The post-oxyhydroxy Pt surface is very different than the native Pt for oxygen reduction pathway and efficiency. Reaction order with respect to oxygen is close to 1. The rate constants of the direct O₂ to H₂O electroreduction reaction are increased with decreasing the potential from −0.2 to −0.6 V, but the O₂ to H₂O₂ electroreduction is contrary to this expectation. The rate constants of H₂O₂ decomposition on the oxyhydroxy film-covered Pt electrode are near constant around 1 × 10⁻⁴ cm s⁻¹ at E_D > −0.5 V.     

XPS and UPS investigation of the diamond surface oxidation by UV irradiation

In this work we describe the effect of oxidation on undoped nanocrystalline H-terminated CVD diamond films. Surface oxidation was performed using UV photons in air and in pure dry oxygen atmosphere. The samples were then thermally treated to study the effect of the UV-induced oxidation on the electronic properties of diamond. Different annealing temperatures were applied to induce a controlled oxygen desorption. Both UV- and X-ray photoelectron spectroscopies were performed in situ in order to correlate the electron affinity changes to the oxygen atomic abundances detected on the diamond surfaces. Our results show that UV oxidation is less invasive if compared to other conventional processes like chemical or plasma oxidations. This enables a recovering of the surface electronic properties with the thermal desorption of oxygen from the diamond surfaces.     

Anodic oxidation of potassium cyanide on platinum electrode

The mechanism of anodic oxidation of cyanide on platinum electrode was studied electrochemically and chemically. Cyanide was oxidized at a first order with respect to hydroxyl ion and at zero with respect to cyanide ion on the rate determining step. On the other hand, the potential controlled coulometry at 0·45 V vs N.H.E.) indicated that two moles of hydroxyl ion and two faradays electricity were consumed during anodic oxidation of one mole of cyanide.Several consecutive reaction paths for the oxidation of cyanide were proposed on the basis of the above diagnostic criteria. Among them, the mechanism invoking metal peroxide would be available in view of the chemical properties of several intermediate species.     

Reactions of hydrogen peroxide on a silver electrode in alkaline solution

Reactions with hydrogen peroxide on silver in alkaline solutions with H₂O₂ concentration 5 × 10^?7 mol/ml have been studied with the ring-disk electrode. The amount of oxygen formed on the disk as the result of catalytic decomposition of hydrogen peroxide and its oxidation was established on the ring-electrode made from pyrographite. The rate constants of H₂O₂ electrochemical reduction (k₃), its oxidation (k′₂) and catalytic decomposition (k₄) and their dependence on potential have been evaluated. The constant k₄ scarcely depends on potential; it is ca 10^?2 cm/s.     

Electrocatalytic oxidation of hydrazine on platinum electrodes in alkaline solutions

The mechanism and structure sensitivity of the electrocatalytic oxidation of hydrazine on platinum in alkaline solutions were investigated using cyclic voltammetry, steady-state current measurements, and on-line electrochemical mass spectrometry. The voltammetry of hydrazine oxidation on platinum in alkaline media is characterized by a single diffusion-controlled wave. The on-line electrochemical mass spectrometry measurements of hydrazine oxidation on Pt(1 1 1), Pt(1 0 0), and Pt(1 1 0) surfaces indicated the formation of molecular nitrogen. No oxygen-containing nitrogen compounds were detected under the given experimental conditions. A comparative analysis of voltammetric data for hydrazine oxidation in alkaline solutions on the three surfaces at low overpotentials points to a structure sensitivity of the reaction. The electrocatalytic activity of basal planes increases in the order Pt(1 1 0) > Pt(1 0 0) > Pt(1 1 1), as deduced from the onset of the oxidation wave. The structure of the electrocatalyst surface affects the mechanism of the reaction, although without affecting the selectivity. At low overpotentials the hydrazine oxidation on the Pt(1 1 0) and Pt(1 1 1) surfaces is limited by the rate of electrochemical steps, whereas on Pt(1 0 0) a chemical step that probably involves N₂H₂ adsorbed intermediate is the rate-determining step. Platinum is more active in hydrazine oxidation in alkaline solution than in acidic solutions. In contrast to hydrazine oxidation on platinum in acidic media, the stabilization of chemisorbed hydrazine does not occur to a significant extent in alkaline media.     

Mössbauer spectroscopy investigation of the iron electrode during cycling in alkaline solution

The processes which take place in the iron electrode during cycling in alkaline solution were investigated by Mössbauer spectroscopy and the reaction products were determined quantitatively.It was proved that-FeOOH is invariably formed at the end of the second anodic reaction in 5 N KOH. At the end of charging there is always a certain amount of residual Fe(OH)₂.After three to five cycles in 5 N KOH containing 9 gl^–1 LiOH,-FeOOH is converted into Fe₃O₄. At the same time a drop is observed in the capacity of the electrode. At the end of charging under these conditions a certain amount of Fe₃O₄ was always found.     

Structural effects in electrocatalysis: oxidation of formaldehyde on gold and platinum single crystal electrodes in alkaline solution

The oxidation of formaldehyde in sodium hydroxide solution has been studied on platinum and gold single crystal electrodes with the (111), (110) and (100) orientations. There is apparently no structural sensitivity of this reaction, since minor differences have been found between the three low index faces. This is valid for both platinum and gold electrodes. The hydrogen adsorption on platinum and the AuOH formation on gold electrodes exhibit structural sensitivity in the same solution. Similar activity of platinum and gold electrodes is noteworthy. A weak adsorption of gem-diol, formed in the interaction of formaldehyde with H₂O or OH^? appears as the origin of the structural insensitivity of this reaction.     

Electrocatalytic oxidation of glyoxalate in alkaline medium on platinum adatom electrodes

The influence of the foreign metal adatoms deposited at underpotentials on the oxidation of glyoxalate on platinum was studied in alkaline medium. Pronounced catalytic effects caused by underpotential submonolayers of Pb, Tl, Cd, Cu and Cr were observed. The enhancement of the oxidation processes by underpotential submonolayers has been interpreted according to the bifunctional theory of the electrocatalysis.     

The anodic evolution of oxygen on platinum oxide electrode in alkaline solutions

The anodic evolution of oxygen was investigated on the platinum oxide electrode, prepared by a thermal decomposition method, in alkaline solutions; the overvoltage data were reproducible on this electrode. On the basis of reliable mechanistic observations, the most probable path under Langmuir conditions was proposed, which is the same as that suggested by Krasil'shchikov for a nickel electrode in alkaline solution. The rate-determining step changes from the 2nd to the 1st step, depending on the overpotentials. This was also supported by the corresponding change in the values of activation energy. The similarity of this platinum oxide to oxide films produced by prolonged anodization of platinum in the anodic polarization characteristics was suggested.     

Electrochemical reduction of thick oxide film on platinum electrode in alkaline solutions

The electrochemical reduction of the thick oxide film formed on Pt electrode by severe preanodization has been studied in LiOH, NaOH and KOH solutions of different concentrations (0.001 ～ 1.0 M) using a galvanostatic technique.An outermost monolayer oxide and an inner multilayer oxide of the thick oxide film exhibit different potential behaviors in the cathodic reduction. In dilute solution, both the oxides are completely reduced in a potential range of 0.6-0.4 V (vs rhe in the test solution) in a single step. As the concentration is increased, however, the reduction potential of the inner oxide only shifts rapidly into a H-electrosorption potential region and the amount of the oxide reduced at this potential decreases. The remaining oxide is slowly reduced at H₂-evolution potential. The retardation of the reduction of the inner oxide is related to cations adsorbed on Pt electrode. This retardation effect increases in the order, K⁺ < Na⁺ < Li⁺.     

Electrocatalytic reduction of nitrate on copper electrode in alkaline solution

This work is devoted to the study of the kinetics and reaction mechanism of nitrate reduction on a copper electrode in 0.1 M NaOH, which acts as the supporting electrolyte. The experimental methods include cyclic voltammetry (CV), cronoamperometry (CA), controlled-potential electrolysis (CPE), and coulometry. In CV, there are three potential regions where charge transfer reactions take place, reactions which are associated with NO₃⁻ and/or intermediates reduction. Two isopotential points observed in CV indicate the existence of some competitive adsorption processes at the electrode surface.The three charge transfer steps were also made evident in the CA, CPE and coulometry studies. The correlation of the experimental results with the literature data led to the conclusion that NO₃⁻ reduction on a copper electrode in 0.1 M NaOH has an intermediate (N₂O₂²⁻) species, which reduces to N₂ at a potential of about −1.3 V and to NH₄OH at potential values lower than −1.4 V (both values are vs. SCE).     

Electrochemical oxidation of manganese(II) at a platinum electrode

Electrochemical oxidation of Mn²⁺ in sulphuric acid to form MnO₂ was studied using stationary and rotating platinum/platinum ring-disc electrodes. It appears that nucleation of MnO₂ is governed by an equilibrium involving a Mn(III) intermediate. Growth of MnO₂ involves the reduction of MnO₂ surfaces by Mn²⁺ ions in the solution to form MnOOH intermediates. The subsequent electrochemical oxidation of MnOOH releases a hydrogen ion and results in the formation of MnO₂. The rate constant of MnOOH oxidation to MnO₂ was estimated to be 40 s^–1. With a sufficient supply of Mn²⁺ ions, a layer of MnOOH is built up and the in-solid diffusion of hydrogen ions becomes the ratedetermining-step. With a low Mn²⁺ concentration, diffusion of Mn²⁺ ions from bulk electrolyte to the MnO₂/electrolyte interface is a factor controlling the growth of MnO₂. The activation energy and the pre-exponential term of the diffusion coefficient of Mn²⁺ in 0.5m sulphuric acid were determined to be 44.8 kJ mol^–1 and 100 cm² s^–1, respectively.     

Electrochemical investigation of platinum electrode in solid electrolyte cell

Electro-oxidation of platinum film electrode deposited on yttria-stabilized zirconia (YSZ) is examined in situ using potential programmed voltammetry at 450 °C in oxygen containing atmosphere. Under prolonged anodic oxidation different sorts of oxidized species are formed which are consumed subsequently during a linear cathodic potential scan resulting in three distinct reduction peaks, one of them being fairly reversible while the two others strongly irreversible. Higher oxidation potential and longer time of polarization favor the irreversible processes. The coexistence of three electrochemical processes is explained with the extension of the triple phase boundary. The rapid first process is identified as formation of PtO_x at the electrode/metal interface, the second process - a much slower, parallel one - is related to the phenomenon of oxygen backspillover at the metal/gas interface, and the slowest third process - consecutive to the first one - is attributed to growth of the PtO_x layer at the electrode/metal interface toward the bulk of the metal by analogy to electro-oxidation of platinum in aqueous liquid electrochemistry.     

Electrocatalytic oxidation of cyclohexanol on a nickel oxyhydroxide modified nickel electrode in alkaline solutions

Electrocatalytic oxidation of cyclohexanol was investigated with cyclic voltammograms, linear galvanic voltammograms and chronoamperometric responses on a nickel oxyhydroxide modified nickel (NOMN) electrode prepared by cycling the potential of a nickel electrode in the potential range of 0.1 V to 0.6 V (vs SCE) in alkaline solutions. It was found that cyclohexanol was oxidized by NiOOH generated with further electrochemical oxidation of nickel hydroxide during the anodic potential sweep. One of the products of the reaction between cyclohexanol and NiOOH was Ni(OH)₂ which was subsequently oxidized to NiOOH on the anode. This resulted in the appearance of a new anodic peak in cyclic voltammograms compared with the absence of cyclohexanol and this anodic peak strongly depends upon potential scan rates and cyclohexanol concentrations. In addition, the presence of cyclohexanol in NaOH solutions also lead to the decrease of anodic potentials in linear galvanic voltammetric responses and the increase of current densities in chronoamperometric curves. Results showed that the oxidation of cyclohexanol on the NOMN electrode follows the catalytic reaction mechanism.     

Redox processes at iron hydroxide layers formed on platinum substrates in alkaline solutions

Redox processes taking place at precipitated hydrous iron hydroxide layers chemically formed on platinum substrates in sodium hydroxide solutions are investigated at 25°C. The electrochemical behaviour of these layers depends considerably on the electroreduction level reached in chargedischarge cycles. The accumulation of Fe₃O₄ during oxidation-reduction cycles can produce an increasing irreversibility of the Fe(II)/Fe(III) redox couple. Data are compared to results obtained with iron hydroxide layers formed by electrochemical and chemical methods on massive iron electrodes. Results are discussed in terms of a composite structure for the iron hydroxide layers.     

The anodic oxidation of nickel in alkaline solution

The anodic oxidation of nickel in alkaline solution was studied by cyclovoltammetric and optical techniques. The range of the scanning potential effects the resulting voltammograms. A constant I-E diagram with anodic peaks at 130 and 270 mV (at scan rate 10 mV · s^?1) is obtained after multiple scanning from ?800 to + 1200mV. The layer of Ni(OH)₂ that is formed in the anodic cycle, is only partially reduced by cathodic polarisation. Growth of the Ni(OH)₂ film on Ni occurs by repeated oxidation and reduction. This occurs via oxidation of Ni to α Ni(OH)₂ and conversion of α Ni(OH)₂ to β Ni(OH)₂.