Reduction of no on smooth,porous platinum and platinum-carbon electrodes in acidic solutions

Rate determining step of one-electron reduction of NO to N₂O and the rate constant of this reaction k° = 1.7 · 10^?4 on smooth platinum electrode were determined. It was shown that PtC electrode activity increases with increasing platinum content of the catalyst and depends largely on the kind of carbon carrier. The most active electrodes prepared of powder platinum black reduce NO to NH⁺₄ even at E ? 0.4 V.     

Étude de la reduction électrochimique de AlCl3 dans le dimethylsulfoxyde à 25°C

The electrochemical reduction of AlCl₃ in dimethylsulphoxide at 25°C was studied by triangular voltammetry on platinum electrodes. The reduction mechanism corresponds to the kinetic scheme: $$Ox(ads) + ne \to Red$$ where Ox is irreversibly adsorbed on the cathode. The reduction peak of the aluminium ions, Al(III), appears at ?1.96±0.05 V versus the reference electrode which is the half cell Ag/AgCl ₂ ^? 10^?3 M, LiCl 0.1 M. It is preceded by another badly defined peak around ?1.74 V, very probably due to the reduction of the protons present in traces in the electrolyte solution. The value of the quantityan _a (wherea is the transfer coefficient andn _a is the number of electrons involved in the rate determining step) was determined as 0.45±0.05 from measurements of all the experimental curves.     

Flow-through electrodes composed of parallel screens

The electrolysis of 2.0 × 10^?3 M K₃Fe(CN)₆ in 1 M KCl in water at room temperature (22°C) has been investigated on flow-through porous electrodes composed of parallel platinum screens. The effect of changing the number of screens on the limiting current and the limiting degree of conversion of substrate has been studied. The results obtained confirm the previously derived model of limiting current on porous electrodes under conditions of solution flow. The experimental results have also been presented in form of mass transfer correlations for Sherwood number and j_D-factor. The potential drop in the electrode composed of six screens has been measured and compared with a theoretical equation. The effect of error in the determination of flow rate on the accuracy of presentation of experimental data is discussed.     

Kinetics measurements on a stationary disk electrode in a uniformly rotating fluid (SDERF). II. The electron transfer reaction in the Fe(CN)4−6/Fe(CN)3−6-system

The operation of the SDERF-cell in the study of the electron transfer kinetics of the Fe(CN)^4?₆/Fe(CN)^3?₆-system in 1 M KCl and 1 M KNO₃-solutions at a stationary Pt-disk electrode is reported. The experimental current—overpotential curves are recorded by linear sweep voltammetry and analysed by two different methods using the theoretical relationship derived for a stationary disk electrode placed in a free rotating fluid. Both methods give the same value for the experimental rate constant k*. The effects of the temperature (0° to 40°C) and of the ratio of the rotor radius (r_r) to the electrode radius (r_e)(r_r/r_e = 0.50 to 0.81) have been studied. The activation energy for the redox process in 1 M KCl and 1 M KNO₃ are: E_a = 3.4 ± 0.6 kcal/mol and E_a = 3.7 ± 0.7 kcal/mol respectively, while the k*-values at 25°C are: k* = (5.67 ± 0.41) × 10^?3 cm.s^?1 and k* = (4.53 ± 0.29) × 10^?3 cm.s^?1 respectively. The difference from the standard rate constant k₀ ? 0.100 cm.s^?1 is explained by the effect of the cell-geometry characterized by the G-factor, so that k° = Gk*, where G ? 19 for our cell.     

Potential of the Pb,PbSO4/H2SO4 electrode at temperatures up to 240°C

Standard lead—lead sulphate electrode potential was determined over the temperature range 20–240°C from emf measurements of the Pb, PbSO₄H₂SO₄ (0.05M)K₂SO₄KClHCl(0.1M)/AgCl, Ag and Pb, PbSO₄H₂SO₄(m)K₂SO₄H₂SO₄(0.05M)PbSO₄, Pb cells where m = 0.005, 0.01, 0.1 and 0.5 M. To this effect lead—lead sulphate electrode potential was calculated using the temperature relationship of the standard silver—silver chloride electrode potential and activity coefficients of hydrochloric acid determined by Greeley et al. at temperatures up to 260°C. Diffusion potentials occurring at the phase boundaries in the cells under investigation were calculated using the Henderson's equation. Values of the standard lead—lead sulphate electrode potential were determined by extrapolation of the E°′ function to the zero ionic strength which was calculated using the second sulphuric acid dissociation constant determined by Lietzke et al. at temperatures up to 300°C. The standard electrode potential was described in the temperature range 20–240°C by the following relationship: E°_{Pb, PbSO4/SO^2?4}(V) = 0.040-0.00126T. A change in entropy ΔS° of the electrode reaction Pb + SO^2?₄ = PbSO₄ + 2e^? is constant in this temperature range and is ?243 JK^?1 mol^?1 (?1018 cal K^?1 mol^?1).     

Gibbs free energy of formation of Bi2O3 from EMF cells with δ-Bi2O3 solid electrolyte

Determination of thermodynamic data on the formation of Bi₂O₃ was established by emf measurements as a function of temperature in the range 660–820°C on the system Metal, Bi(1)|Bi₂O₃|Pt, O₂. From the results using a tungsten electrode the following relation was found: ΔG⁰ = ?(134.7 ± 1.2) + (64.0 ± 1.2) × 10^?3 T kcal mole^?1 (validity range: 940–1080 K). Standard thermodynamic data at 298 K were calculated as ΔG⁰ = ?119.2 ± 2.1 kcal mole^?, ΔH⁰ = ?139.0 ± 1.2 kcal mole^?1, and ΔS⁰ = ?66.3 ± 1.2 eu.     

A reversible oxygen electrode in an equimolar KNO3-NaNO3 melt saturated with sodium peroxide—I. An equilibrium study

A reversible graphite oxygen electrode has been found in molten (K-Na)NO₃ eutectic, saturated with sodium peroxide. The measured emf of ?1150 ± 5 mV at 503 K vs Ag|Ag⁺ (0.1 m) and its temperature dependence ?0.44 mV K^?1 are compared with theoretical values, obtained with thermodynamic data.An independent value of the superoxide disproportionation constant K_p = (5.3 ± 0.3) × 10^?2 kg atm mol^?1 at 503 K has been obtained. Our equilibrium experiments confirm the results of Zambonin et al.     

Electron transfer and homogeneous redox catalysis on glassy carbon electrode—I. Linear sweep and cyclic voltammetric studies on Ti(IV) oxalate-NH2 OH system in oxalic acid media

The electron transfer rate of titanic oxalate on glassy carbon is found to be slower when compared to the same on Hg electrode. The estimated rate constant using Nicholson's method was found to be 1.2 ± 0.1 × 10^?3 cm s^?1. The effect of pH shows similarity with the reports on Hg electrode. Fractional reaction orders were obtained in the Tafel region. These observations may be due to the fact that the electrode surface under experimental conditions is partially covered. The estimated reduction rate of NH₂OH by the electrogenerated titanous oxalate agrees very well with the literature values at lower NH₂OH concentrations. However, deviations are noticed when higher concentrations of Ti (IV) oxalate as well as NH₂OH are employed. The possibility of existence of more complicated redox-catalytic mechanism is indicated.     

EFFECT OF ANIONS ON THERMODYNAMICS OF EXTRACTION OF Am(III) AND Eu(III) WITH OCTYL(PHENYL)-N,N-DIISOBUTYL-CARBAMOYLMETHYLPHOSPHINE OXIDE

The extraction of Am(III) and Eu(III) by octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in xylene from aqueous media containing 1.0 M NH₄NO₃, NH₄SCN, NH₄ClO₄ or a mixture of 0.3 M NH₄NO₃ + 0.7 M NH₄ClO₄ at pH 2.70 and at the temperatures of 15, 25, 35 and 45±0.1 °C has been studied. At all the temperatures, the species extracted were ML₃ · 3CMPO (M = Am(III) or Eu(III), L = NO₃ ^?, SCN^? or ClO₄ ^?) and M(NO₃)(ClO₄)₂ · 3CMPO. The thermodynamic parameters of the extraction reactions have been evaluated using the temperature coefficient method. The ?ΔG values follow the order SCN^? > NO₃ ^? + ClO₄ ^? > NO₃ ^? ≈ ClO₄ ^?, whereas the ?ΔH values an order NO₃ ^? + ClO₄ ^? > SCN^? > NO₃ ^? ≈ ClO₄ ^?. The effect of these anions on the thermodynamic parameters have been discussed employing compensation effects and also on the basis of the energy associated with the transfer of these anions from aqueous to the extractant phase.     

The diffusivity of oxygen in dilute alkaline solution from 0° to 65°C

Diffusion coefficients for oxygen in 0·1 M sodium hydroxide solution have been determined from 0° to 65°C using a rotating platinum electrode. The results may be represented with a standard deviation of approximately 1% by the expression:

where D_o = 8·03 × 10^?3cm²s^?1 and the apparent activation energy, Q_D, is 3·49 kcal mol^?1. Between 25° and 65°C the product of the diffusivity and the solution viscosity is essentially constant:

Values for diffusion coefficients for oxygen in pure water have been derived from these data with an absolute error probably less than 4%.Difficulties in obtaining reproducible results at a rotating platinum electrode are attributed to deactivation of the electrode by reduction of a surface oxygen phase. Results at the higher temperatures indicate that methods of determining diffusivities by diffusion through a stagnant layer of solution involve an increasing indeterminate error as the temperature rises, due to convective mass transport.     

XPS study of the electrochemical surface oxidation of Platinum in N H2SO4 acid electrolyte

The surface of a platinum electrode was anodically polarized at 3 V vs standard hydrogen electrode in 1 N H₂SO₄at 300 K for 16 h so that a thick uniform oxidation layer was formed. Analysis by X-ray photoelectron spectroscopy (XPS) showed that the stoichiometry O-to-Pt of this layer was equal to four, with O1s and Pt4f$\text{7}\text{2}$ binding energies at 53.1 and 74.4 eV respectively. The oxidation layer is therefore proposed to consist of Pt(OH)₄. Cyclic voltammograms showed a single cathodic oxygen peak in support of a single uniform oxidation compound.The comparison with spectra of thinner layers, grown at potentials between 1.2 and V vs she for only 15min, led to the conclusion that those are also made up predominantly of platinum hydroxide. Surface Pt(OH)₄ thermally decomposes at 405 ± 5 K into a mixture of Pt⁴/Pt²⁺ hydroxides and oxides and Pt-metal with a measured ratio Pt:O = 1. At about 700 ± 50 K only a monolayer of adsorbed oxygen is left and at 1070 K all oxygen is desorbed.     

Determination of the charge-transfer kinetics of ferrocene at platinum and vitreous carbon electrodes by potential steps chronocoulometry

Potential step chronocoulometric methods were used to measure kinetic parameters for the oxidation of ferrocene at platinum vitreous carbon electrodes in acetonitrile. The formal rate constants observed, k^o_f ～ 0.02 cm s^?1, were independent of electrode material and of the supporting electrolyte chosen (LiClO₄, TEAP or TEABF₄). After correcting for the effects of double layer structure, the experimental activation energy for electron transfer was found to be ΔG≠ (exp) = 0.03 ±0.01 eV molecule^?1 which agreed within 20% with a theoretical value ΔG≠ (theor) = 0.26 eV molecule^?1 calculated for activation by solvent repolarization alone from the Born equation.     

Electrolyte effects on oxygen reduction kinetics at platinum: A rotating ring-disc electrode analysis

A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H₂SO₄ ～ CF₃SO₃H > H₃PO₄ > HClO₄. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH^? ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.     

The effect of surface roughness on the hydrogen evolution reaction kinetics with mild steel and nickel cathodes

The effect of electrode surface roughness on the kinetics of the hydrogen evolution reaction has been studied for mild steel and nickel cathodes in 10 M NaOH at 348°K. With the nickel cathodes, the Tafel slope was independent of whether the electrode surface was smooth or roughened, but the exchange current density was about twice that for the roughened electrode. With the mild steel cathodes, the roughened electrode showed a considerably lower Tafel slope than the smooth electrode with b = 51 mV for the rough electrode and b = 110 mV for the smooth one. The exchange current density was also lower for the roughened mild steel cathode than for the smooth electrode: 1.0 × 10^t-5 A/cm²vs 2.3 × 10^?5 A/cm².     

Voltammetric studies of lithium salt-acetonitrile solutions containing traces of water

Cathodic scans of purged lithium salt-acetonitrile solutions containing trace amounts of water were carried out at platinum electrode. A single peak was observed at ?1.6 V vs the sce when a slow scan rate (< 10 mV/s) was employed. This peak was attributed to the electrochemical reduction of water at the platinum electrode to hydroxide and hydrogen followed by the subsequent precipitation of a passivating LiOH coat onto the platinum surface. The presence of oxygen in the lithium salt-acetonitrile solutions inhibited the reduction of water. Evidently, oxygen was reduced to superoxide, O^?₂, and the superoxide subsequently reacted with lithium ions to form a passive LiO₂ coating on the platinum surface. It was found that with fast sweep voltammograms of 0.1 M LiClO₄-distilled CH₃CN solutions at platinum, a plot of peak current, ip (?1.6 V), vs water concentration over the 150–1300 μg/ml range yielded a fairly straight line with a slope equal to about 0.5.     

Thermal Decomposition of Diammonium Tetrachloroplatinate to form Platinum Nanoparticles and its Application as Electrodes

Pt nanoparticles were obtained via the thermal decomposition of (NH₄)₂[PtCl₄] (diammonium tetrachloroplatinate) by heating from room temperature to 760 °C. The thermal decomposition process was analyzed using thermogravimetric analysis (TGA) and differential thermal analysis (DTA), X-ray thermodiffraction and infrared spectroscopy. The size and structure of the platinum particles were analyzed using transmission electron microscopy (TEM). The electrochemical activity of Pt particles was assessed by cyclic voltammetry in 0.5 M H₂SO₄. The TGA and DTA results suggested that the thermal decomposition of the precursor proceeded in two stages: loss of NH₄Cl at ~300 °C, followed by loss of NH₄Cl and Cl₂ at ~372 °C. Metallic Pt particles were then produced at temperatures of 372 °C and above. At 760 °C, the mean ± SD size of the Pt particles was (4.1 ± 1.6) nm, as determined from TEM measurements. In cyclic voltammetry (CV) measurements, an electrode comprised of glassy carbon and Pt particles in 0.5 M H₂SO₄ exhibited behavior similar to that observed using a polycrystalline Pt electrode.     

Potentiometric and polarographic studies of cadmium(II) complexes with tetraethylenepentamineheptaacetic acid

Stability constants for cadmium(II) complexes with tetraethylenepentamineheptaacetic acid (TPHA, H₇L) were determined by the pH titration method. In an aqueous solution (μ = 0.1), three complex species, CdH₂L, CdHL and CdL are confirmed. The structure of uninuclear complexes are discussed. The formation constants of the complexes stated above have been calculated as follows (at 25 ± 0.1°C): log K_CdL = 15.35, log K_CdHL = 13.33 and log K_CdH2L = 7.89. The polarographic behaviour of the cadmium(II) in the presence of TPHA was studied over the pH range 3–5. Mechanisms of the electrode processes were elucidated and electrochemical kinetic parameters were evaluated from dependence of the half-wave potentials on the hydrogen ion and TPHA concentration. In the presence of an excess of TPHA, the wave B is assigned to the irreversible reductions of the complex, CdH₂L^3? (pH range 3–4) or CdHL^4? (pH range 4–5). The electrode reaction can be written:

and

Where k_e (the rate constant) = 2.3 × 10^?2 cm s^?1 and k′_e = 1.59 × 10^?4 cm s^?1. The other polarographic methods were also used in the elucidation of the electrode process.     

Oxydation electrocatalitique du glycerol sur electrodes d'or et de platine en milieu aqueux

The electrocatalytic oxidation of glycerol has been investigated at platinum and gold electrodes, both in acid and in alkaline medium. In acid medium only platinum electrodes are electroactive, whereas in alkaline medium both electrodes, particularly gold electrodes, give relatively high current densities (about 20 mA cm^?2 for 0.1 M glycerol at 25°C).By varying the reaction parameters (electrode potential, concentrations of glycerol and of hydroxylions), it is possible to determine the transfer coefficient and the reaction orders. The fractional values obtained may be interpreted by formulating a reaction mechanism involving adsorbed intermediates.     

The influence of the tunnel probability on the anodic oxygen evolution and other redox reactions at oxide covered platinum electrodes

Potentiostatic and galvanostatic pulse measurements were carried out to investigate the anodic oxygen evolution at platinum electrodes in 1N H₂SO₄ in dependence on the oxide layer thickness d and the electrode potential ε. The thickness d (1·5–10 Å) was obtained from cathodic charging curves. Further, the temperature dependence (0°–81°C) was evaluated from Bowden's measurements. Summarizing, the current i_o2 follows the relation, log i = A - (E⁰_a - αFη)/2·3 RT - d/d_o. The experimental activation energy E_o = E^o_a = αFη decreases linearly with increasing overvoltage η. The linear decrease of log i with increasing d, which is given by the term d/d_o, is correlated to the probability of the quantum mechanical tunnel transition of the electron from adsorbed ions, OH^?_ad or O^2?_ad respectively, through the oxide layer to the metal. Similar effects of the oxide layer thickness on the current density were observed in the case of the oxygen evolution at iridium, the CO-oxidation on platinum, and the reduction of Cl^? and Ce⁴⁺ at platinum. In these cases a rate determining electron transfer through the oxide layer is also assumed.     

Kinetic parameters of the electrode reaction of chloropenta-amminecobalt(III) ions at a gold electrode

The cathodic reduction of chloropenta-amminecobalt(III) ions at active gold electrodes has been studied in 0.1 M HClO₄ + 0.1 M NaClO₄ by means of the potentiostatic method. Chloropentaamminecobalt(III) ions gave one irreversible reduction wave in the double layer potential region of gold electrodes. The kinetic parameters of the electrode reaction were measured at 5, 15, 25, 35 and 45°C, and the activation energy was calculated to be 19 kcal mol^?1 for the electron transfer process and 4.5 kcal mol^?1 for the diffusion process.