Consecutive Charge-transfer Steps Research Articles

Electrochemical oxidation mechanism of procarbazine at glassy carbon electrode

A voltammetric study of the redox behavior of procarbazine (PCZ), one of the most used chemotherapy agents to face Hodgkin’s lymphoma, was carried out to investigate its oxidation at glassy carbon electrode (GCE). Essays of cyclic, differential pulse and square-wave voltammetries, on a large pH interval, revealed the electrochemical oxidation behavior of PCZ as an irreversible adsorption-controlled oxidation process, occurring in four consecutive charge transfer steps. The first, second and third steps are pH-dependent and occur with the transfer of one electron and one proton, leading to the formation of an electroactive product in acid media, while the last one is pH-independent, involving the transfer of one electron. A reaction mechanism for the electrochemical oxidation of PCZ is proposed, where, in the first and second step the oxidation occurs on the hydrazine group, followed by isomerization and, after hydrolysis, conversion to benzaldehyde-procarbazine. The third and fourth steps are associated with the oxidation of the benzaldehyde-procarbazine with the production of the N-isopropylterephthalic acid.

Elementary Reaction Kinetics of the CO ∕ CO2 ∕ Ni ∕ YSZ Electrode

Results of combined experimental and theoretical investigations of elementary chemical reaction processes of gas mixtures at nickel/yttria-stabilized zirconia (Ni/YSZ) solid oxide fuel cell (SOFC) model anode systems are presented. Temperature-programmed desorption and reaction measurements were performed in order to determine adsorption/desorption kinetic data as well as thermodynamic parameters for the and /yttria-stabilized zirconia (YSZ) heterogeneous reaction systems. From these data, an elementary kinetic reaction mechanism of the electrochemical CO oxidation at Ni/YSZ anodes was developed. Numerical simulations were performed for three different spillover mechanisms. Steady-state polarization curves and electrochemical impedance spectra were calculated, allowing for a direct comparison with experiments performed by Lauvstad et al. [ J. Electrochem. Soc. , 149 , E506 (2002) ]. Best agreement with the experimental data was obtained when assuming two consecutive charge-transfer steps from via to Ni–O, the second step being accompanied by oxygen spillover over the three-phase boundary.

Influence of temperature on the reduction kinetics of Zn 2+ at a mercury electrode

The reduction of Zn 2+ at a mercury electrode has been studied by the a.c. impedance technique in the 263–318 K temperature range. Kinetic data have been analyzed in terms of a mechanism consisting of two consecutive charge transfer steps. The apparent activation enthalpies and charge transfer coefficients of both steps are found to be temperature independent. Differential capacity curves and potentials of zero charge of the NaClO 4 supporting electrolyte solutions were measured at each temperature of interest. Double layer effects on the Zn 2+ reduction rate constants were analyzed within the framework of the unequal distances of closest approach theory, and the corrected rate constants were found to be independent of supporting electrolyte concentration when allowance for changes in the Zn 2+ activity coefficient was made. The implications of the activation parameter values on the nature of the rate determining steps are also discussed.

On the theory of the coulostatic pulse technique as applied to redox electrodes with two consecutive charge transfer steps

Analysis of the coulostatic pulse technique, as applied to a redox electrode with two consecutive charge-transfer steps, Red-e−=X and X-e−=Ox, under conditions of non-stationary linear semi-infinite diffusion of the reagents Red, Ox and the intermediate X has been carried out. Relaxation equations for the linear current density–overpotential region and for different excitations (coulostatic pulse of a charge, trapezoidal and triangular pulses of current) have been derived in analytical form. Cases of the relaxation processes controlled by (a) consecutive charge-transfer and diffusion simultaneously and (b) pure charge-transfer kinetics have been considered. Simple analytical approximations of the relaxation equations have been obtained for limiting cases. These approximations can be used for the classical graphical analysis of experimental data to determine the kinetic parameters of partial charge-transfer steps. It is shown that the relaxation process immediately after coulostatic excitation is controlled by the fastest elementary charge-transfer step, and its kinetics can be determined from the initial portion of the transient decay curve.

On the theory of the galvanostatic relaxation technique as applied to redox electrodes with two consecutive charge-transfer steps

Analysis of the galvanostatic single-pulse (GSP) method as applied to a redox electrode with two consecutive charge-transfer steps Red - e − → X and X - e − → Ox under conditions of non-stationary linear semi-infinite diffusion of the reagents Red and Ox and the intermediate X has been carried out. Relaxation equations for the linear region of the current density-overpotential relations and for exciting current pulses of step and quasi-trapezoidal form have been derived in analytical form. Methods of extracting kinetic data using these equations are discussed. Simple analytical approximations of the relaxation equations have been obtained for limiting cases. These approximations can be used for the classical graphical analysis of experimental data to determine the kinetic parameters of partial charge-transfer steps and diffusion coefficients of the reagents Red and Ox and the intermediate X. The GSP and impedance methods are compared.

Kinetics of the Second Charge Transfer Step in an EE Mechanism by Rotating Ring‐Disk Electrode Voltammetry

Theoretical rotating ring and disk electrodes voltammograms are discussed for a process consisting of two consecutive charge transfer steps under steady‐state conditions. The relationship between total disk current, , and ring current,, is examined. Plots of vs. the reciprocal of the square root of the rotation rate, , are classified into three cases depending on the reversibility of the second charge transfer step. Although the voltammograms of the disk electrode may be those of a single wave, a distorted s‐shape, or a well‐separated two step, the ratio is not affected by the kinetic parameters of the first charge transfer step, except for the number of electrons.

The galvanostatic double-pulse method in the study of the kinetics of stepwise electrode processes involving the adsorption of intermediates

The kinetics of the overall electrode reaction Me 0−3 e − = Me 3+ proceeding through three consecutive charge-transfer steps involving the intermediate ions Me + and Me 2+ adsorbing on the electrode according to the Langmuir. Temkin and Frumkin isotherms under transient single- and double-pulse galvanostatic conditions, have been analysed. The curves of η− t, θ 1− t, θ 2− t have been calculated by computer for different ratios of the exchange current densities of the charge transfer steps and different adsorption isotherms, taking account of the absence of interaction, repulsion and attraction between the intermediates adsorbed. The general equation for the stationary polarization curve of the three-step electrode process with two steps of adsorption of intermediates obeying the Langmuir and Temkin isothersm, has been derived and analyzed. It is shown that when the equilibrium surface coverage with Me + (Me 2+) ions, θ 1° (θ 2°), exceeds a certain critical value, θ 1cr° (θ 2cr°), determined by the ratio of the kinetic parameters of the partial charge transfer steps, the anode (cathode) stationary polarization curve has no linear region corresponding to the limiting step of the first electron transfer, as demanded by the classical theory of stepwise processes without adsorption of intermediates. In this case, the evaluation of the kinetic parameters of the fast first step from stationary polarization data and the data obtained by the galyanostatic single-pulse method is impossible. It is shown that when the equilibrium coverage of the electrode surface with Me + ions exceeds a certain critical value, θ 1cr°, kinetic parameters of the fastest step, Me 0 − e − = Me + ads, can be determined by the double-pulse method; the standard value of the rate constant of this step should not exceed 180 s −1 (direct mechanism of insertion of adatoms into the metal lattice, assuming that surface concentrations of adatoms and ad-ions in k s calculations are expressed as mol/cm 2). On the basis of the theory worked out, the galvanostatic double-pulse method (GDP) is used for studying the kinetics of the overall electrode process Bi 0(solid) − 3 e − = Bi 3+ The GDP study is carried out on a polycrystalline Bi electrode in solutions containing 10 −3 −10 −1 M Bi and 2 M perchloric acid at 25°C. It is shown that the overall electrode reaction proceeds through three partial steps of charge transfer involving the adsorption of intermediates: Bi 0(solid) − e − = Bi + ads (a) Bi + ads − e − = Bi + ads (b) Bi 2+ ads − e − = Bi 3+ (c) The kinetic parameters of the partial charge transfer steps (b) and (C) are found to be i 02 = 8.9 × 10 −6 A/cm 2; i 03 = 8.9 × 10 −5 A/cm 2 at [Bi 3+] = 10 −3 M; β 2 = α 3 ≅ 0.5. The equilibrium surface coverage with Bi + ads exceeds a critical value (θ 1° > θ 1cr°). In this case, in conformity with the theory developed, it is impossible to study the kinetics of the fastest step (a) of Bi + ion transfer using stationary polarization curves. The analysis of transient η− t curves for an extremely short period of time (2–8 μs) after the beginning of the polarization yields detailed information on the kinetics of the fastest step (a) ( i 01 = 4.1 × 10 −3 A/cm 2 at [Bi 3+] = 10 −3 M; α 1 = β 1 = 0.5 ± 0.01). It is shown that the kinetics of step (a) obeys the Butler-Volmer-Frumkin theory of slow charge transfer quantitatively. The adsorption of Bi + intermediate is shown to proceed on the energetically inhomogeneous surface of solid bismuth electrode and to obey the Temkin isotherm with a high parameter of inhomogeneity.

Stepwise anodic dissolution of metals

Methods by means of which it can be decided whether the dissolution of a metal via several consecutive charge-transfer steps is a two or a three-step mechanism are surveyed. The information relevant to answer this question is furnished by the rate constants of the second reaction step. These rate constants can be calculated from polarization curves, or form data gathered using a rotating ring-disc electrode in the experiments. Using the methods described, we studied the anodic dissolution of indium in aqueous acidic pechlorate solution. It was found that the anodic dissolution of indium, under the conditions studied, takes place in three consecutive one-electron steps.

The galvanostatic double-pulse method in the study of the kinetics of stepwise electrode reactions

Analysis of the kinetics of the overall electrode reaction Me 0 − e − = Me 2+ proceeding through the three consecutive charge-transfer steps Me 0 − e − = Me + Me + − e − = Me 2+ Me 2+ − e − = Me 3+ involving non-adsorbed intermediates under transient single- and double-galvanostatic conditions has been made. Curves of η − t were plotted and change of intermediate concentrations with time were calculated numerically for different ratios of exchange current densities. It is shown that when the time of reaching the steady-state, caused by the rate levelling of single-electron steps considerably exceeds the time of double-layer charging by the short duration current impulse, the employment of the galvanostatic double-pulse method allows the stepwise electrode process under non-stationary conditions to be investigated and information about the kinetics of the fastest steps in the reaction sequence to be obtained. Comparison of the conclusions of the analysis and experiment has been carried out by the galvanostatic double-pulse method in the stepwise electrode reaction Bi 0 −3 e − = Bi(III) on an amalgam electrode in 2 M HClO 4 solutions.

Kinetics of copper dissolution and deposition in aqueous sulphate solution

Galvanostatic single-pulse, rotating-disc and chronopotentiometric measurements on etched polycrystalline copper electrodes in semimolar mixed solutions of CuSO 4, MgSO 4 and H 2SO 4 at 25°C have yielded kinetic parameters and specific rate data for either of the two consecutive charge-transfer steps of the Cu(I)/Cu(II) electrode. The ion-transfer step Cu/Cu(I), but not the electron-transfer step Cu/Cu(II), depends on a surface factor for crystal growth and demolition. Some information on this factor is obtained. Comparisons are made to the solid Ni/Ni(II) electrode and to the liquid Cu(Hg)/Cu(II) electrode.

Effect of ammonia on the Zn(Hg)/Zn(II) electrode reactions in aqueous chloride solutions

The title subject has been studied using single pulse and chronopotentiometric polarization measurements on the Zn(Hg)/Zn(II) electrode and equilibrium measurements on the same and the Zn/Zn(II) electrode, mainly in 2 M NH 4Cl with 0–0.3 M NH 3. At low ammonia concentrations, the Zn(Hg)/Zn(II) reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate, and with little or no participation of ammonia. At higher ammonia concentrations, however, nearly symmetric transfer by divalent zinc ion (α=0.5 and n=2) to and from diammine species appears to be the predominant charge-transfer step.

Kinetics of Zn/Zn(II) reactions in acidified solutions of potassium chloride

The title subject has been studied using stationary, single-pulse, and chronopotentiometric polarization measurements on the Zn/Zn(II) electrode and equilibrium measurements on the same and the Ag/AgCl electrode in 0.5–4 M chloride solutions at 25°C. The Zn/Zn(II) electrode reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate. The ion-transfer step Zn/Zn(I) is too fast to exhibit its kinetics. The electron-transfer step Zn(I)/Zn(II) mostly occurs by the couple ZnCl2(H2O)y/ZnCl2(H2O)y, but species with one or no chloride ligand take over as the main electroactive ones at chloride (or salt) concentrations below 1 M. The value of y is not clearly revealed by the data. Some sluggishness in complex equilibration and some, double-layer effects are observed. A convenient scale for single-ion activities is described, used, and recommended.

Kinetics of Zn/Zn(II) reactions in acidified solutions of potassium chloride

The title subject has been studied using stationary, single-pulse, and chronopotentiometric polarization measurements on the Zn/Zn(II) electrode and equilibrium measurements on the same and the Ag/AgCl electrode in 0.5–4 M chloride solutions at 25°C. The Zn/Zn(II) electrode reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate. The ion-transfer step Zn/Zn(I) is too fast to exhibit its kinetics. The electron-transfer step Zn(I)/Zn(II) mostly occurs by the couple ZnCl 2(H 2O) y/ZnCl 2(H 2O) y, but species with one or no chloride ligand take over as the main electroactive ones at chloride (or salt) concentrations below 1 M. The value of y is not clearly revealed by the data. Some sluggishness in complex equilibration and some, double-layer effects are observed. A convenient scale for single-ion activities is described, used, and recommended.

Effect of ammonia on the Ni(Hg)/Ni(II) electrode reactions in aqueous chloride solutions

Abstract The title subject has been studied using single pulse and chronopotentiometric polarization measurements on the Zn(Hg)/Zn(II) electrode and equilibrium measurements on the same and the Zn/Zn(II) electrode, mainly in 2 M NH 4 Cl with 0–0.3 M NH 3 . At low ammonia concentrations, the Zn(Hg)/Zn(II) reactions are found to occur in two consecutive charge-transfer steps with Zn(I) as intermediate, and with little or no participation of ammonia. At higher ammonia concentrations, however, nearly symmetric transfer by divalent zinc ion (α=0.5 and n =2) to and from diammine species appears to be the predominant charge-transfer step.

Dreieckspannungsuntersuchungen der ameisensaureoxidation an blankem platin in schwefelsaurer losung

Subject of investigation is the anodic wave between +300 and +800 mV rhe in the voltage scan diagram of formic acid at smooth platinum in sulfuric acid solution. The experiments show that the formic acid oxidation in this potential region is hindred by an adsorbate formed at potentials less than +300 mV. Different experimental results prove that the formic acid oxidation is governed by catalytically active hydrogen dissolved in the platinum metal ( ie occluded hydrogen). Formic acid molecules are adsorbed at the occluded species by means of a bridge type bond; they are finally oxidized to carbondioxide in two consecutive charge transfer steps.

The Mechanism of the Cadmium Electrode

Measurements of current‐potential curves with solid cadmium in solutions of cadmium perchlorate and barium perchlorate under stationary and transient conditions indicate that the electrode reaction proceeds via two consecutive charge transfer steps with adsorbed monovalent cadmium as the intermediate. The rate constants and their dependence on potential and coverage were calculated from the experiments.

Kinetic studies of the electrolytic reduction of carbon dioxide on the mercury electrode

The reduction of CO 2 to HCOOH has been studied for the Hg electrode in neutral and acidic aqueous solutions in the potential range between −0.8 and −1.9 V(sce). In the neutral pH range all the current is consumed in the production of formic acid, while in acid solutions both HCOOH and H 2 are produced. Steady-state polarization curves, cathodic galvanostatic charging curves. current-efficiency measurements, reaction orders with respect to CO 2 partial pressure, and double-layer variation have been used to determine possible reaction pathways. In neutral solutions the mechanism may be described by a direct reduction of CO 2 in which two consecutive charge-transfer steps occur. In acidic solutions the above process occurs in parallel with the reduction of H 3O + ions. The H atoms formed react in a branching mechanism either with H 3 O + ions and electrons to yield H 2, or with CO 2 to yield eventually HCOOH.