Convolution Potential Sweep Voltammetry Research Articles

Heterogeneous Kinetics of Thiourea Electro-Catalytic Oxidation Reactions on Palladium Surface in Aqueous Medium.

The electrochemical behaviors of thiourea (TU) oxidation have been studied at Palladium (Pd) electrode in the acidic medium by recording cyclic voltammograms (CVs). The influence of pH was investigated in the pH range of 1.0 to 9.0. Facilitated adsorption of TU on electrode surface results in enhanced catalytic response in acidic medium and maximum electro-catalytic response was found at pH∼3.0. Chronoamperometric (CA) experiment determined this oxidation as 1e- transfer process and the variation of TU concentration reveals a 1st order kinetics. In the CV responses, the large value of peak separation (▵Ep >380 mV) calculated by the variation of scan rate indicates that oxidation of TU is an irreversible process. With the aid of convolution potential sweep voltammetry (CPSV), the standard rate constant (k°) for the reaction was found to be 7.1×10-4 cm/s and the formal potential constant (E°' ) was evaluated to be ∼0.37 V vs Ag/AgCl (sat. KCl). The value of transfer coefficient (α) was found to vary from 0.74 to 0.40 with applied potential (E). From the potential dependent variation of transfer coefficient (α) and activation energy (▵G≠ ), it was concluded that the overall electrochemical oxidation of TU follows a stepwise mechanism at lower potential (<0.40) V and a concerted one at relatively higher potential (>0.40) V. The FTIR analysis of the product after oxidation of TU molecules confirmed the appearance of a new sharp band near 530 cm-1 due to the formation of S-S bonds suggesting formation of formamidine disulfide (FD) ions.

PH dependent kinetic insights of electrocatalytic arsenite oxidation reactions at Pt surface

Kinetics of electrocatalytic oxidation of arsenite ions has been investigated at a Pt disk electrode using cyclic voltammetry, convolution potential sweep voltammetry and electrochemical impedance spectroscopy. A complimentary environment pertaining to oxidation reactions of arsenite ions is attained in the acidic medium compared to a neutral or basic medium. It is suggested that in the neutral and basic media, direct electron transfer from the solution to electrode instigate the oxidation process without any pre adsorption. Meanwhile, in the acidic medium, prior to oxidation, arsenite ions are adsorbed on the Pt surface and a stepwise reaction mechanism is involved. Using impedance analysis, it is suggested that different forms of surface oxides at various pH values control the oxidation kinetics.

Dissociative electron transfer to diphenyl-substituted bicyclic endoperoxides: the effect of molecular structure on the reactivity of distonic radical anions and determination of thermochemical parameters.

The heterogeneous electron transfer reduction of the bicyclic endoperoxide 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]hept-5-ene (4) was investigated in N,N-dimethylformamide at a glassy carbon electrode. The endoperoxide reacts by a concerted dissociative ET mechanism resulting in reduction of the O-O bond with an observed peak potential of −1.4 V at 0.2 V s−1. The major product (90% yield) resulting from the heterogeneous bulk electrolysis of 4 at −1.4 V with a rotating disk glassy carbon electrode is 1,4-diphenyl-cyclopent-2-ene-cis-1,3-diol with a consumption of 1.73 electrons per mole. In contrast, 1,4-diphenyl-2,3-dioxabicyclo[2.2.2]oct-5-ene (1), undergoes a two-electron reduction mechanism in quantitative yield. This difference in product yield between 1 and 4 is suggestive of a radical-anion mechanism, as observed with 1,4-diphenyl-2,3-dioxabicyclo-[2.2.2] octane (2) and 1,4-diphenyl-2,3-dioxabicyclo[2.2.1]heptane (3). Convolution potential sweep voltammetry is used to determine unknown thermochemical parameters of 4, including the O-O bond dissociation energy and the standard reduction potential and a comparison is made to the previously studied bicyclic endoperoxides 1–3 with respect to the effect of molecular structure on the reactivity of distonic radical anions.

Mechanism and Regioselectivity of the Electrochemical Reduction in Polychlorobiphenyls (PCBs): Kinetic Analysis for the Successive Reduction of Chlorines from Dichlorobiphenyls

The regioselective electrochemical reduction of three different dichlorobiphenyls is analyzed using quantum chemical calculations and convolution potential sweep voltammetry. The heterogeneous rate constants of C–Cl bond reductions of each of the dichlorobiphenyls are estimated. The mechanism of the electrochemical reduction is confirmed by estimating the intrinsic barrier from the experimental data on transfer coefficients as well as theoretical calculations involving bond length and potential energy diagrams. The reductions follow the Marcus–Hush quadratic activation driving force relation barring the meta-Cl of the 3,4-dichlorobiphenyl which obeys Butler–Volmer kinetics. The first electron transfer step is rapid in comparison with the bond-breaking, implying the stepwise reduction for all the dichlorobiphenyls studied here. High performance liquid chromotagraphy analysis of the products of the bulk electrolysis confirms the order of the reduction in dichlorobiphenyls viz. the ease of reduction follows ...

Electrocatalytic oxidation of methanol to soluble products on polycrystalline platinum: Application of convolution potential sweep voltammetry in the estimation of kinetic parameters

Irreversible oxidation of methanol on polycrystalline platinum leading to soluble products has been carried out by fast scan voltammetry, and the reaction has been studied under diffusion controlled process. The conventional analysis of current–potential data, viz. dependence of peak potential on scan rate and peak width measurements, resulted in the estimation of apparent diffusion coefficient of methanol and the anodic transfer coefficient of the electrode reaction. However, from the convolution potential sweep voltammetry, a more accurate and reliable kinetic data were obtained. Under the above conditions, methanol oxidation follows Butler–Volmer rate law with a linear variation of logarithmic heterogeneous rate constant with electrode potential. A constant apparent anodic transfer coefficient independent of electrode potential was observed pointing to the fact that the standard potential of the reaction cannot be determined from the voltammetric experiments. The experimental current–potential curve was compared with a theoretical voltammogram and further oxidation of products at the electrode surface has also been analyzed using limiting convolution current.

ChemInform Abstract: Kinetic Studies on the Chemical and Electrochemical Reduction of Some 4-Arylidene-5-pyrazolone Compounds.

Kinetics of chemical and electrochemical reduction of some 4-arylidene-5-pyrazolones were studied. The chemical reduction was performed using aluminum hydride. The electrochemical reduction in 0.1 mol dm−3 TEAP/DMF was studied by cyclic and convolution potential sweep voltammetry at HDME. The title compounds exhibit two diffusion-controlled irreversible mono electronic cv waves. Both waves follow EC kinetics and they are attributed to the reduction of the pyrazolin carbonyl group. The kinetics of the chemical and electrochemical reduction were proposed and discussed in terms of the molecular structure of the subject arylidenes.

Regioselective electrochemical reduction of 2,4-dichlorobiphenyl – Distinct standard reduction potentials for carbon–chlorine bonds using convolution potential sweep voltammetry

The reductive cleavage of carbon–chlorine bonds in 2,4-dichlorobiphenyl (PCB-7) is investigated using the convolution potential sweep voltammetry and quantum chemical calculations. The potential dependence of the logarithmic rate constant is non-linear which indicates the validity of Marcus–Hush theory of quadratic activation-driving force relationship. The ortho-chlorine of the 2,4-dichlorobiphenyl gets reduced first as inferred from the quantum chemical calculations and bulk electrolysis. The standard reduction potentials pertaining to the ortho-chlorine of 2,4-dichlorobiphenyl and that corresponding to para chlorine of the 4-chlorobiphenyl have been estimated.

Analysis of C–F bond cleavages in methylfluorobenzoates—Fragmentation and dimerization of anion radicals using convolution potential sweep voltammetry

The electrochemical reduction of methylfluorobenzoates at glassy carbon electrodes is analyzed using the convolution potential sweep voltammetry (CPSV). The stabilization of the radical anion due to the electron-withdrawing group is shown to lead to intra-molecular stepwise dissociative electron transfer. While methyl 2-fluorobenzoate ( ortho isomer) follows EC mechanism, the methyl 4-fluorobenzoate ( para-isomer) undergoes electro-dimerization prior to C–F bond cleavage. The first order rate constant for the EC mechanism and the dimerization rate constant for the electro-dimerization are deduced from the classical as well as convolution potential sweep voltammetry. A plausible mechanism of dimerization is suggested. The influence of the electron-withdrawing groups is illustrated by comparing the reduction behaviour of 4-fluorobenzonitrile. The potential energy surfaces and electron density mapping employing Gaussian 03 calculations provide further support for the validation of the mechanism pertaining to C–F bond cleavages.

Mechanistic Analysis of the Reductive Cleavage of Carbon–Halogen Bonds in Halopentafluorobenzenes

The mechanistic analysis pertaining to the reductive cleavage of C–X (, Br, I) bonds in halopentafluorobenzenes has been investigated at glassy carbon electrodes using convolution potential sweep voltammetry. The quadratic activation-driving force relation has been found valid, with potential-dependent transfer coefficients. The charge density analysis using density functional calculations, estimation of intrinsic barriers, and standard reduction potentials indicate that the mechanism cannot be classified unambiguously either as stepwise or concerted in these compounds. From various diagnostic criteria, the C–X bond cleavage mechanism appears to be borderline between stepwise and concerted.

Electrochemical reduction of carbon–fluorine bond in 4-fluorobenzonitrile – Mechanistic analysis employing Marcus–Hush quadratic activation-driving force relation

The reduction of carbon–fluorine bond in 4-fluorobenzonitrile in acetonitrile as the solvent, is analyzed using convolution potential sweep voltammetry and the dependence of the transfer coefficient on potential is investigated within the framework of Marcus–Hush quadratic activation-driving force theory. The validity of stepwise mechanism is inferred from solvent reorganization energy estimates as well as bond length calculations using B3LYP/6-31g(d) method. A novel method of estimating the standard reduction potential of the 4-fluorobenzonitrile in acetonitrile is proposed.

Effect of the vapor-deposited Au nanoparticles on the rate of the redox reaction at the highly oriented pyrolytic graphite electrode

Abstract Effect of the vapor-deposited Au nanoparticles on the rate of the Fe ( CN ) 6 - 3 / Fe ( CN ) 6 - 4 redox reaction at the highly oriented pyrolytic graphite (HOPG) electrode is investigated by cyclic and convolution potential sweep voltammetry. Surface morphology and electronic structure of Au deposits is studied by AFM and XP spectra measurements of Au 4f core electrons and of valence electrons, respectively. Au/HOPG surfaces are shown to have the morphology, which is characterized by the preferential deposition of Au nanoparticles along step edges and the formation of larger Au islands on HOPG planes. Au deposits after annealing have the electronic structure close to that of the bulk metal, they remain stable during voltammetric measurements of the Fe ( CN ) 6 - 3 / Fe ( CN ) 6 - 4 electron transfer, but they dissolve upon the surface oxide formation at far positive potentials. Dissolution does not allow using the usual electrochemical pretreatment, i.e., cycling the electrode potential between limits at which gold surface is oxidized and reduced, prior to kinetic measurements. The rate of the Fe ( CN ) 6 - 3 / Fe ( CN ) 6 - 4 electron transfer on the Au/HOPG electrodes increases with the increasing Au coverage. Apparent standard rate constant related the Au coverage attains a maximum of approximately 10% of the value for a polycrystalline Au electrode. It is proposed that slower kinetics is related to the presence of the adsorbed impurities on the Au/HOPG surface, or to a double layer effect. Linear diffusion and the Marcus kinetic model provide a plausible description of the electrode process at both the HOPG and Au/HOPG electrodes.

Distinction between stepwise and concerted mechanisms in reductive cleavage reactions—use of voltammetric current function in the analysis of non-linear kinetic laws

A systematic way of distinguishing stepwise and concerted mechanisms in reductive cleavage reactions has been formulated involving current function analysis of the voltammetric data. The electrochemical reductive cleavage of the carbon–iodine bond in 1,3-dichloro-2-iodobenzene has been analyzed from a mechanistic point of view to illustrate the methodology. 1,3-Dichloro-2-iodobenzene undergoes an initial stepwise electron transfer obeying quadratic activation-driving force relationship. The current function analysis yields the reorganization energy for the reduction of 1,3-dichloro-2-iodobenzene and the results have been verified independently using convolution potential sweep voltammetry.

Cyclic and convolution potential sweep voltammetry of reversible ion transfer across a liquid membrane

The theory of the cyclic and convolution potential sweep voltammetry of the reversible ion transfer across a liquid membrane is developed. The model system consists of the planar layer of an organic solvent separating two aqueous phases, each containing a supporting electrolyte. The transferred ion is supplied from one of the aqueous phases. It is shown that the coupling between the ion transfer processes at the two membrane interfaces has an effect on the shape and position of the voltammogram on the potential scale. The theory is used to evaluate the diffusion coefficient and the standard ion transfer potential of a semi-hydrophobic ion, i.e. protonated tetracaine, from voltammetric measurements in a supported o -nitrophenyl octyl ether ( o -NPOE) membrane cell. These results agree well with the data inferred from measurements of the ion transfer across a single water ∣ o -NPOE interface.

Reduction mechanism of benzoin on a mercury electrode

The electrochemical reduction of benzoin was studied in acid media at DME. Convolution potential sweep voltammetry was applied to obtain the charge transfer rate constants. The overall reduction corresponds formally to an irreversible two-electron, two-proton transfer. Deoxybenzoin was identified as the final product of the benzoin reduction. Low-temperature bulk electrolysis experiments showed the presence of an unstable enol intermediate. This compound undergoes an enol-keto transformation, whose acid catalyzed rate constants were determined by chronoamperometry. The kinetic results indicate a stabilization of the intermediate carbocation in the presence of perchlorate.

An electrochemical study of solvent-complex interactions in trans-1,2-diaminocyclohexane- N, N, N', N'-tetraacetatochromate(III) and 1,2-diaminoethane- n-hydroxyethyl- N, N', N'-triacetatochromium(III)

The electroreduction of trans-1,2-diaminocyclohexane- N, N, N', N'-Tetraacetatochromate(III) and 1,2-diaminoethane- N-hydroxyethyl- N, N', N'-triacetatochromium(III) has been investigated in different solvents on a mercury electrode by convolution potential sweep voltammetry. The dependence of the observed diffusion coefficients, standard potentials and double layer corrected electron transfer rates on solvent properties has been interpreted. It has been shown that the formal potentials for the reduction of both complexes are linear functions of the solvent acceptor numbers. The logarithm of the corrected standard rate constant for the trans-1,2-diaminocyclohexane- N,N,N',N'-tetraacetato complex depends on both the acceptor number and the logarithm of the solvent's longitudinal relaxation time. On the other hand in case of the 1,2-diaminoethane- N-hydroxyethyl- N, N', N'-triacetato complex, this quantity depends on the solvent's acceptor and donor number. It is concluded that the 1,2-diaminoethane- N-hydroxyethyl- N, N', N'-triacetatochromium(III) complex undergoes substitution of the coordinated water molecule originally present as a ligand. However, for the trans-1,2-diaminocyclohexane- N, N, N', N'-tetraacetato chromium( III) complex there is no indication of the incorporation of solvent molecules in the first coordination sphere as a ligand.

Electrochemistry of [V(III)EDTA]-in Ethylene Glycol−Water Mixtures. 2. Kinetic Aspects: Solvation of the Transition State

The effect of the solvent composition for water−ethylene glycol (EG) mixtures in the mole fraction range XEG = 0−1 on the kinetics of the ethylenediamine-N,N,N‘,N‘-tetraacetatovanadate(III/II) redox reaction has been investigated at a mercury electrode using convolution potential sweep voltammetry. Capacitance data for the mercury/solvent mixture interface have been obtained at two different concentrations of NaClO4 at each solvent composition. Observed electron transfer rates and transfer coefficients have been corrected for double-layer effects using these data. It has been shown that the change of the corrected electron transfer rates with solvent composition is due to differences in the solvation of the complex and can be described by a two-step solvent exchange mechanism. In addition, Gibbs energies of reorganization have been estimated using the experimental results. The logarithm of the corrected electron transfer rate constant also corrected for variation in the outer sphere contribution to the Gibbs reorganization energy is linearly correlated to the logarithm of the solvent's longitudinal relaxation time (τL). The solvent composition of the solvation shell of the transition state can be calculated from the estimated values of τL in each mixture. As expected, the composition of the solvation shell of the activated complex is intermediate between that of the reactant and product.

Electrochemistry of [V(III)EDTA]-in Ethylene Glycol−Water Mixtures. 1. Thermodynamic and Transport Properties: Solvation of the Reactant and Product

The effect of the solvent composition for water−ethylene glycol (EG) mixtures in the mole fraction range XEG = 0−1 on the diffusion coefficients and formal potentials of the ethylenediamine-N,N,N‘,N‘-tetraacetatovanadate(III/II) redox reaction has been investigated at a mercury electrode using convolution potential sweep voltammetry. It has been shown that the formal potential of the redox reaction, measured on the ferrocene/ferricinium scale, depends on the solvent composition. This dependence has been characterized by a two-step solvent exchange mechanism, which describes the solvation of the reactant and product. Distribution diagrams for the differently solvated species have been calculated as a function of the mole fraction of EG. Preferential solvation of the more negatively charged vanadium(II) complex relative to the vanadium(III) complex by water has been found. Acceptor numbers for the mixtures have been proposed.

Solvent effects in the electroreduction of [diamine-N,N′-polycar☐ylato]chromate(III) complexes at a mercury electrode

The effect of the solvent on the electroreduction of ethylenediaminetetraacetatochromate(III) ([CrEDTA]−), cyclohexanediamine-tetraacetatochromate(III) ([CrCYDTA]−) and (2-hydroxyethyl)ethylenediaminetriacetato-chromium(III) ([CrHEDTRA]) has been investigated in different solvents on a mercury electrode using convolution potential sweep voltammetry. The observed electron transfer rate constant and the transfer coefficient have been corrected for double-layer effects. It has been shown that the formal potential for the reduction of all three complexes in a series of solvents is a linear function of the solvent acceptor number. The logarithm of the corrected standard rate constant for the EDTA and CYDTA complexes depends on the solvent acceptor number and the logarithm of the solvent's longitudinal relaxation time, whereas for [CrHEDTRA] it depends on the solvent's acceptor and donor numbers. It is concluded that, in the case of the EDTA and CYDTA complexes, no solvent molecules are incorporated as ligands in the first coordination sphere. However, in the HEDTRA complex the coordinated water molecule originally present is substituted by the solvent in which the reaction takes place.

Use of convolutive potential sweep voltammetry in the calculation of hydration equilibrium constants of α-dicarbonyl compounds

The applicability of the convolution potential sweep voltammetry to the calculation of hydration equilibrium constants K H for a series of α-dicarbonyl compounds is demonstrated. K H values for glyoxal, methylglyoxal, acetylbenzoyl, 1,2-cyclohexanedione, diacetyl and 3,4-hexanedione are obtained as functions of the temperature. The values of apparent Δ H and Δ S are calculated from these constants. The consistency of the K H values is tested by showing the occurrence of an enthalpy—entropy compensation effect. Hydration reactions are shown to be enthalpy controlled.

Investigation of the influence of residual uncompensated resistance and incomplete charging current correction on the calculation of electrode kinetics when global and convolution analysis methods are used

The global analysis and convolution potential sweep voltammetry (CPSV) procedures for the analysis of electrode kinetic parameters are compared under the influence of residual uncompensated resistance and incomplete charging current correction. An initial examination of simulated data containing these known artifacts indicates that both procedures are similarly susceptible to the effects of uncompensated resistance. CPSV may be unaffected by the effects of incomplete charging current correction, whereas the global analysis procedure does experience some inaccuracies and a combination of both procedures is recommended in this circumstance. The effect of unequal diffusion coefficients is also examined.