Kinetics of molecular oxygen electroreduction on platinum modified by tin underpotential deposition

The kinetics of molecular oxygen electroreduction were studied on platinum surfaces modified by tin underpotential deposition. The surface process was analysed by cyclic voltammetry in aqueous 10^–4 M tin(II)/1 M sulfuric acid in the 0.05 to 0.70 V vs RHE range. Platinum sites involving (1 1 0) planes are mainly related to tin underpotential deposition as observed in the hydrogen sorption region. Kinetic runs for oxygen reduction were performed with the rotating ring-disc electrode technique on tin-modified platinum surfaces. It was concluded that molecular oxygen reduction on tin-modified platinum takes place through bulk hydrogen peroxide and water formation. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electrochemical rate constants for oxygen reduction pathways were calculated as a function of deposition potential based on Damjanovi's reaction scheme.     

Kinetics and mechanism of the oxygen evolution reaction at oxide-coated Co-Ni amorphous alloy electrodes

Oxygen evolution reaction (o.e.r.) kinetics in NaOH solutions have been studied on both fresh and oxide covered Co₅₀Ni₂₅Si₁₅B₁₀ amorphous alloy (G-16) electrodes. Steady state polarization curves obtained in different aqueous xM NaOH (0.1 x 4) in the 30–80°C range fulfill Tafel relationships at low overpotentials; the Tafel slope is close to 2.3(RT/F) V dec^–1 for both G-16 and oxide coated G-16 electrodes. At high overpotentials, ohmic relationships with slopes becoming increasingly steep, regardless of the NaOH concentration, are observed. In the Tafel region, the reaction order with respect to OH^– is near 2. The apparent current density at constant potential, for oxide coated G-16 electrodes, is greater than that for uncoated G-16. The high catalytic activity of the oxide coated G-16 for the o.e.r. is attributed to its spinel-type structure. The kinetics of the o.e.r. at low overpotentials is explained through a mechanism involving a first electron transfer step followed by a rate-determining chemical step.     

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.     

On the electroreduction mechanism of 2-pyridinecarboxylic (picolinic) acid on mercury electrodes

This paper presents polarographic (direct current, dc, and differential pulse, DP) and voltammetric (linear-sweep cyclic voltammetry) studies on the electroreduction of picolinic acid (2-pyridinecarboxylic acid) on mercury electrodes. The studies were performed in the acidity range 2.25 M H₂SO₄ to pH 11. Above the last pH value no signals were obtained. Kinetic parameters such as Tafel slopes and electrochemical reaction orders have been determined at the potentials corresponding to the foot of the waves. From polarographic, voltammetric and kinetic results, reaction pathways are proposed. The reduction of the species protonated at the heterocyclic nitrogen occurs through a process in which the rate-determining step (r.d.s.), is a protonation placed after two reversible two-electron transfers. In this reaction, proton-donors other than the H⁺ ion can be involved. At pH>1, the recombination of the carboxylate anion with the H⁺ precedes the reduction process. The reduction of picolinate anion, protonated at the heterocyclic nitrogen, consisted of the uptake of one electron and two H⁺ ions, followed by an irreversible one-electron transfer, which is the r.d.s. In both cases, the hydrated aldehyde obtained is reduced only partially, due to the slow rate of the dehydration reaction. Above pH 5.3 the protonation of the pyridine ring precedes the reduction.     

Electroreduction of molecular oxygen on preferentially oriented platinum electrodes in acid solution

The oxygen electroreduction reaction was studied on two different preferentially oriented ((111)-type and (100)-type and on a conventional polycrystalline (PC) platinum rotating disc electrodes in acid solutions at 30 °C. At low overpotentials, Tafel lines of –0.060 V decade^–1 were obtained on the three electrodes in oxygen-saturated 1.0m H₂SO₄ and 1.0m H₂SO₄ + y m K₂SO₄ (0 y 1). At high over-potentials the usual Tafel slope of -0.120V decade^–1 was observed on both (111)-type and PC platinum electrodes in 1.0m H₂SO₄, whereas a slope of –0.165V decade^–1 was found on (100)-type platinum. In oxygen-saturated 1.0m H₂SO₄ the surface coverage by O-containing adsorbates on (100)-type platinum was greater than on both (111)-type and PC platinum. Rotating ring-disc electrode data showed that a higher amount of H₂O₂ was produced on (100)-type platinum than on the other platinum surfaces. The overpotential against current density plots are influenced by the anion concentration depending on the type of preferentially oriented platinum.     

Kinetics and mechanism of the hydrogen-evolution reaction on mercury in HCl-dimethylsulphoxide solutions

Rapid galvanostatic and potentiostatic stationary E/I curves, E/time characteristics at current interruption and electrode capacitance measurements were made to investigate hydrogen-ion discharge on mercury in HCl-DMSO solutions, with and without the addition of LiCl as supporting electrolyte. Kinetic parameters and their temperature and concentration dependences are discussed in terms of the initial electron-trnsfer reaction as rate-determining.     

Kinetics and mechanism of reduction of oxide film on smooth platinum electrodes with hydrogen

The kinetics and mechanism of the reaction of chemisorbed oxide (Pt-O) layer on a smooth Pt electrode with H₂ dissolved in 1 M H₂SO₄ solution were investigated under the open circuit condition.It was found that a monolayer of Pt-O on the electrode surface is reduced first at a slow rate which is of second order in chemisorbed oxygen in the range 1 ≧ θ ≥ 0·65 and then at a rapid rate which is proportional to (1 – nθ)² in the range 0·6 ≥ θ > 0, where θ is the fraction of the surface covered by oxygen. The factor n, which was constant for the electrode oxidized under a given condition, was assumed to be the number of Pt sites deactivated by each one of the chemisorbed oxygen atoms. For the transiently formed oxide, n was estimated to be 1·64. It was also observed that all over the coverage range the reaction rate was proportional to the partial pressure of H₂. The variation in reduction rate with the decrease of the coverage was interpreted in terms of change in reduction mechanism from the chemical reaction to the electrochemical reaction.     

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO₄ and 0.05 M H₂SO₄ solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25-20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O₂ reduction was higher in HClO₄ and decreased with decreasing film thickness. In H₂SO₄, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec⁻¹ in high current density range and −60 mV dec⁻¹ in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O₂ reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O₂ molecule was close to four for all thin Pt films studied.     

Kinetics of oxygen reduction reaction at tin-adatoms-modified gold electrodes in acidic media

In the present report, oxygen reduction reaction (ORR) at polycrystalline gold (Au (poly)) electrode in situ modified by the underpotential deposition (upd) of Sn-adatoms is addressed. The ORR was investigated at the Sn-adatoms-modified Au (poly) electrode by the hydrodynamic voltammetric technique with a view to evaluating the various related kinetic parameters. The results demonstrated that the underpotential deposited Sn-adatoms on the Au (poly) electrode substantially promoted the activity of the electrode towards an exclusive one-step four-electron ORR forming H₂O as the final product.     

The oxygen evolution reaction on platinum,iridium, ruthenium and their alloys at 80°C in acid solutions

Kinetic parametes were determined for the oxygen evolution reaction on 50–50 atom percent alloys of RuIr, RuPt, and IrPt and compared with results obtained using ruthenium, iridium, platinum, and RuO₂/TiO₂ electrodes. The potentiostatic studies were made on oxide covered electrodes at 80°C in both 1.0 M H₂SO₄ and 1.0 M CF₃SO₃H. Cyclic voltammetric studies showed that while these noble metals and alloys are about equally effective as electrocatalysts for the hydrogen evolution reaction, striking differences in activity are found for the oxygen evolution reaction. The order of electrocatalytic activity towards oxygen evolution in H₂SO₄ is Ru > RuIr alloy ～ RuO₂/TiO₂ ～ Ir > IrPt alloy > RuPt alloy > Pt. The type of acid used had very little effect on the kinetic parameters. The lower electrocatalytic activities when platinum is present is probably due to the formation of a platinum oxide film. The dual barrier model is used to interpret the results for the electrodes containing platinum. The best electrocatalysts for oxygen evolution in acid solutions consist of noble metals which form oxide films (RuO₂, IrO₂) possessing metallic characteristics.     

The electrochemical reduction of ruthenium(IV) in perchloric acid solutions on platinum electrodes

Two reversible waves are observed for the reduction of Ru(IV) on platinum electrodes. Each wave is the sum of two one-electron steps according to:

Potentials (vs nhe) and species are given for 1.0 M HClO₄ solutions; more hydrolysed products are formed in less acid solutions. The tetrameric Ru(III) ion slowly decomposes to a more stable Ru(III) species, probably a dimeric ion.     

Kinetics and mechanism of anodic chlorine and oxygen evolution reactions on transition metal oxide electrodes

A short résumé of the data on kinetics of chlorine reaction at RuO₂ and RuO₂ —TTiO₂ electrodes is given. The three step mechanism of the reaction with the rate determining transfer of the second electron and the formation of the nominally unipositive chlorine as an intermediate is substantiated. The applicability of an analogous mechanism to the oxygen evolution reaction is discussed as well as the possibility of participation in the process of higher oxidation states of ruthenium. Some analogies between anodic processes at RuO₂ and other transition metal oxides (perovskites MNi(Co)O₃, pyrochlores M₂Re₂O₇, spinels MCo₂O₄, MCr₂O₄) were pointed out.     

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.     

Reaction intermediates as a controlling factor in the kinetics and mechanism of oxygen reduction at platinum electrodes

A rotating platinum disk electrode was used to study oxygen reduction over the entire pH range and to compare the kinetics in acid and alkaline solutions. In both solutions two Tafel regions are confirmed. At low cd's ?V/?ln i = ?RT/F and at high cd's it is ?2RT/F. In the low cd region, the reaction order with respect to H₃O⁺ is $\text{3}\text{2}$ in acid, and $\text{1}\text{2}$ in alkaline solutions. For the high cd region the reaction orders are 1 and 0 in acid and alkaline solutions, respectively. Transition of the kinetics from the low to the high cd region, as well as the fractional reaction orders at low cd's, are interpreted in terms of the first charge transfer step as rate determining in both cd regions but under Temkin adsorption conditions in the low, and Lagmuirian conditions in the high cd region. Coverages, θ, with oxygen intermediates are also determined as a function of potential and pH. At all pH's, substantial coverages are observed only at potentials in the low cd region.The usual Tafel slope of ?120 mV for the first charge transfer step as rate determining is not observed at low cd's since it is modified by the dependence of activation energy on θ, and hence on V. At high cd, θ is low and ineffective and the usual slope for the first charge transfer step is observed. At all pH's the transition from low to high cd's occurs at potentials, V_T, that decrease 60 mV as pH increases one unit. Dependence of V_T and i_T on pH is discussed. Fractional reaction orders at low cd's are also related to θ. Real reaction orders with respect to H₃O⁺ are modified by the dependence of activation energy on θ, and hence on pH. The change of the kinetics from the acid to the alkaline region is discussed.The range of cd's accessible to measurements is limited at both high and low cd's. At high cd's it is limited by the supply of oxygen and/or H₃O⁺ from the solutions. At low cd's, V—log i relationship extends only to a rest potential which decreases 60 mV as pH increases one unit. The limits of measurements and the nature of the rest potential are discussed.     

Kinetics and mechanism of the reaction between ozone and hydrogen peroxide in aqueous solutions

A kinetic model has been developed, taking into account both decomposition of ozone molecules and interactions between ozone and hydrogen peroxide for formation of hydroxyl radical and subsequent reactions. Experiments were carried out at 25°C in the pH range of 3 to 13, indicating that the depletion rate of ozone increases with the concentrations of ozone, hydrogen peroxide and hydroxyl ion, as predicted by the kinetic model. Adverse scavenging reactions, however, also play significant roles at sufficient concentration ratios of hydrogen peroxide to ozone and high concentrations of hydroxyl ion in reducing the depletion rate. Results of this research suggest, that it is most desirable to conduct the peroxone oxidation for pollutant destruction by the hydroxyl radical reaction in alkaline solutions of pH below 11, while maintaining about the same concentration of ozone and hydrogen peroxide.     

Dynamic aspects of the electroreduction of chromic acid solutions

Potentiodynamic and impedance spectroscopy measurements were performed on ARMCO iron, steels (C30, C40, C50), cast iron and graphite electrodes in a bath of industrial composition: CrO₃ 250 g dm^–3, H₂SO₄ 2.5 g dm^–3. Attention focused on the electrochemical reactions occurring before the start of the chromium deposition. The electrochemical evidence is markedly affected by the chemical nature of the cathode. This is rationalized by proposing a reaction mechanism involving the formation, depending on the chemical nature of the substrate, of a passive adsorbed film consisting of mixed chromium and iron oxides. Moreover, the proposed overall scheme accounts for the presence of periodic current oscillations observed both in potentiodynamic and potentiostatic curves. In close agreement, impedance spectra show negative loops when measured at the corresponding potentials.     

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.     

The oxygen film on platinum electrodes and the electro-oxidation of acetate ions

The recent literature is reviewed and a consistent mechanistic and mathematical model is presented which can account for the varied phenomena observed experimentally during both steady-state and unsteady-state electrolysis of aqueous acetate solutions at Pt electrodes. In particular the following competing reactions are considered:

1.

complete oxidation of acetate ions to carbon dioxide,     

Kinetics of the electroreduction of anodically formed cadmium oxide layers in alkaline solutions

The kinetics of the potentiostatic electroreduction of cadmium oxide layers on polycrystalline cadmium electrodes have been investigated in 0.01–1.0m NaOH solutions. The study was undertaken to determine the influence of passive film formation conditions on the electroreduction process of the anodically produced hydrous cadmium hydroxide-oxide layers. By properly adjusting the electroreduction conditions the cathodic potentiostatic current transients can be satisfactorily described by a nucleation and growth mechanism involving the participation of soluble Cd(II) and passivating anodic species. Experimental data were analysed by the application of non linear least squares fit routines. From the parametric identification procedure coherent potential dependencies of the corresponding fitting parameters as well as reasonable values of the physicochemical constants included in the reaction model, have been obtained. In this way it is possible to correlate kinetic data of the electroreduction process of the anodic oxide layers to make a discrimination between the different mechanistic contributions to this complex reaction involving several steps.     

On the reduction of oxygen at platinum—oxygen alloy diaphragm electrodes

The steady-state reduction of oxygen in O₂-saturated 2 N H₂SO₄ solution was obtained at constant current on the front side of a Pt foil diaphragm before and after strong anodization of the back side. Strong anodization converts the Pt to a PtO alloy. In all cases, the overvoltage for O₂ reduction was less at the PtO alloy than at the Pt cathode. This enhanced catalytic activity of Pt containing dissolved oxygen is interpreted in terms of the band approximation model of metals. The dissolved oxygen in such high concentrations generates a 2p-band in the alloy which interacts with the 5d- and 6s-bands of the Pt. Electrons spill into the 5d-band causing a raising of the Fermi level of the metal with respect to the energy distribution of adsorbed species on the electrode surface. Consequently, it requires less cathodic polarization to transfer electrons from the metal to the species adsorbed on the metal surface by quantum mechanical tunnelling or by passing over the potential energy barrier.