ECE Reactions Research Articles

Transient chronoamperometric current at rotating disc electrode for second-order ECE reactions

The approximate transient chronoamperometric limiting current generated from the electrochemical reaction in a rotating disc electrode for second-order ECE reactions is derived when the chemical step is irreversible. This model is based on nonlinear reaction–diffusion equations containing a nonlinear term related to homogeneous reaction. By solving the nonlinear equations applying a new approach of homotopy perturbation methods, simple and closed-form expressions for the reactant concentration and the current response as a function of the rotation rate and diffusion coefficients are obtained. The impact of various physical parameters on concentration and current has been analyzed and graphically displayed. Sensitivity analysis of parameters such as rate constant, the ratio of bulk concentration, the thickness of diffusion layer on current is discussed. In addition, the concentration/current expressions match well with the numerical and limiting case results.

New analytical method for solving nonlinear equation in rotating disk electrodes for second-order ECE reactions

The analytical and numerical solution of nonlinear diffusion equations are performed to study the chronoamperometric limiting current generated from the electrochemical reaction in a rotating disk electrode for second-order ECE reactions when the chemical step is irreversible. Simple and closed-form of expressions for the concentration of reactant and the current response are obtained as a function of the rotation rate and diffusion coefficients. The effect of various physical parameters on concentration and current have been computed and shown graphically. Also, the concentration/current expressions here derived using hyperbolic function and padé approximation technique are in satisfactory agreement with numerical results and limiting case results.

The theory of steady state current for chronoamperometric and cyclic voltammetry on rotating disk electrodes for EC’ and ECE reactions

A theory is presented for the shape of current curves measured at a rotating-disc electrode for the cases where the substrate reacts via for the E, EC′ and ECE reaction mechanisms respectively. An analytical expression of concentration and current is presented by solving convection-diffusion equations for the E and EC’ reaction mechanism. The closed-form of an analytical expression of concentration and current for ECE reaction mechanisms are also reported by solving the system of nonlinear reaction-convection-diffusion equations using homotopy-perturbation method. The solutions enable us to obtain the current response in chronoamperometry and steady-state voltammetry. Analytical results are also compared with the simulation result. An excellent agreement is observed between the series of solutions obtained by simulation solution. The approximate analytical expression derived here is useful to predict the dynamical behaviour of the system. The effect of the parameters on concentration and current are discussed and presented graphically.

Laccase-catalyzed oxidation and intramolecular cyclization of dopamine: A new method for selective determination of dopamine with laccase/carbon nanotube-based electrochemical biosensors

This study demonstrates a new electrochemical method for the selective determination of dopamine (DA) with the coexistence of ascorbic acid (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC) with laccase/multi-walled carbon nanotube (MWNT)-based biosensors prepared by cross-linking laccase into MWNT layer confined onto glassy carbon electrodes. The method described here is essentially based on the chemical reaction properties of DA including oxidation, intramolecular cyclization and disproportionation reactions to finally give 5,6-dihydroxyindoline quinone and on the uses of the two-electron and two-proton reduction of the formed 5,6-dihydroxyindoline quinone to constitute a method for the selective determination of DA at a negative potential that is totally separated from those for the redox processes of AA and DOPAC. Instead of the ECE reactions of DA with the first oxidation of DA being driven electrochemically, laccase is used here as the biocatalyst to drive the first oxidation of DA into its quinone form and thus initialize the sequential reactions of DA finally into 5,6-dihydroxyindoline quinone. In addition, laccase also catalyzes the oxidation of AA and DOPAC into electroinactive species with the concomitant reduction of O 2. As a consequence, a combinational exploitation of the chemical properties inherent in DA and the multifunctional catalytic properties of laccase as well as the excellent electrochemical properties of carbon nanotubes substantially enables the prepared laccase/MWNT-based biosensors to be well competent for the selective determination of DA with the coexistence of physiological levels of AA and DOPAC. This demonstration offers a new method for the selective determination of DA, which could be potentially employed for the determination of DA in biological systems.

Hydrodynamic cryoelectrochemical ESR: The reduction of ortho-bromonitrobenzene in acetonitrile

Novel methodology permitting cryoelectrochemical ESR measurements under hydrodynamic conditions allows new insights into the mechanism of multistep electron transfer processes in general. Specifically the electroreduction of ortho- and para-bromonitrobenzene in acetonitrile is investigated at low temperatures, employing a tubular electrode under conditions of hydrodynamic flow. At room temperature, a full two electrons per molecule are transferred in the ortho-case due to an ECE reaction in which the nitrobenzene radical anion is generated, but at 233 K only a single electron transfer takes place (as is the case for the para-derivative at room temperature). The analytical theory for ECE reactions at a tubular electrode is developed under the assumption of a thin diffusion layer and applied to the above system in order to extract rate constants from the limiting current data. The variation of these kinetic parameters with temperature is used to then determine an activation energy for the homogeneous chemical reaction. A newly designed coaxial-tube electrochemical ESR cell is utilised for the detection of paramagnetic species electrogenerated in situ at variable temperatures. At room temperature, the para-bromonitrobenzene reduction yields a spectrum indicative of the corresponding radical anion which is found to be stable on the experimental timescale, but in the ortho-case the nitrobenzene radical anion spectrum is observed as a result of the ECE process. Upon sufficient cooling, however, we observe a spectral change to that of the ortho-bromonitrobenzene radical anion, as the kinetics of the chemical reaction are slowed.

Non-aqueous electrochemical studies at a high temperature channel flow cell heated by radio frequency radiation

The effect of applied temperature was studied on various non-aqueous one- and two-electron systems of ferrocene, tris(4-bromophenyl)amine and N, N, N′, N′-tetramethyl-1,4-phenylenediamine in acetonitrile solvent at both radio frequency channel cell (RFCC) and microdisk electrode apparatus. Experimentally obtained diffusion coefficients ( D c) and activation energies ( E A) for both RFCC and microelectrode experiments are in excellent agreement with each other and published data. Analysis of the data in terms of the Stokes–Einstein equation is also presented. This expands on previous work done in this area to incorporate not only solvent-based systems in addition to simple aqueous electrochemistry, but also more mechanistically complex electrode processes with coupled homogeneous kinetics such as ECE reactions. The application of the RFCC as a tool for mechanistic studies is discussed with the investigation of the well characterised ECE reaction of m-iodo-nitrobenzene in acetonitrile, giving a value of 80±5 kJ mol −1 for the activation energy of the rate constant for the decomposition of the m-iodo-nitrobenzene radical anion. This represents, to the authors' knowledge, the first example of a quantitative, fully modelled investigation of a mechanistically complex electrochemical process at a locally heated electrode.

Padé approximation of ECE and DISP processes at channel electrodes

A two-point Padé approximation is derived for ECE and DISP1 reactions occurring at channel electrodes for all rate constants. These are shown to be in excellent agreement with previous studies.

Computational Electrochemistry. Simulations of Homogeneous Chemical Reactions in the Confluence Reactor and Channel Flow Cell

The finite element method (FEM) is employed to simulate steady-state convection diffusion problems involving electrode processes and coupled homogeneous kinetic reactions in the channel cell and the confluence reactor. Initial FEM simulations are reported for EC, ECE, and DISP reactions in the channel cell, and the results are shown to agree with previous art. Calculations reveal the FEM extends the kinetic range accessible for such mechanisms beyond those reported previously using the backward implicit finite difference method. The FEM is then applied to simulate, for the first time, the quantitative effects of coupled homogeneous reactivity on the voltammetric response of the confluence reactor. Specifically, a working surface is presented for the application of the confluence reactor to the investigation of the CE type reactions. This surface provides a quantitative relationship between the chemical reaction rate, volume flow rate, and inlet concentrations using this new device. In all simulations performed, the FEM is found to be a highly efficient and accurate alternative to the finite difference method when applied to hydrodynamic voltammetric measurements.

Heterogeneous ECE Processes at Channel Electrodes: Voltammetric Waveshape Theory. Application to the Reduction of Nitromethane at Platinum Electrodes

Theory is developed for heterogeneous ECE reactions occurring at channel electrode that predicts the voltammetric waveshape, as characterized by the half-wave potential and the Tafel slope as a fun...

Channel Microband Electrode Arrays for Mechanistic Electrochemistry. Two-Dimensional Voltammetry: Transport-Limited Currents

A channel electrode array, with electrodes ranging in size from the millimeter to the submicrometer scale, is used for the amperometric interrogation of mechanistically complex electrode processes. In this way, the transport-limited current, measured as a function of both electrode size and electrolyte flow rate (convection), is shown to provide a highly sensitive probe of mechanism and kinetics. The application of "two-dimensional voltammetry" to diverse electrode processes, including E, ECE, ECEE, EC', and DISP2 reactions, is reported.

Heterogeneous ECE Processes at Channel Electrodes: Analytical Theory. Distinguishing Hetero- and Homogeneous ECE Reactions

Heterogeneous ECE Processes at Channel Electrodes: Analytical Theory. Distinguishing Hetero- and Homogeneous ECE Reactions

The application of the concept of reaction layer to the study of electrode processes coupled with the second-order chemical reactions at microelectrodes under steady-state conditions

Under steady-state conditions, the current equations of the second-order EC, ECE and DISPI reactions at microdisk, microspherical and microring electrodes are derived with the aid of the concept of the reaction layer. The conditions under which these equations would be valid are also discussed. Using these equations, methods to determine the kinetic parameters for the second-order EC, ECE and DISPI reactions are presented. The reduction of 2,6-diphenyl-pyrylium cation and oxidation of triphenylamine were investigated as examples of the second-order EC and ECE reactions.

ECE and DISP Processes at Channel Electrodes: Analytical Theory

Analytical theory is developed for ECE and DISP1 reactions occurring at channel electrodes. Simple expressions are presented which allow the ready mechanistic interpretation of experimental data and the deduction of corresponding rate constants. These are shown to be in excellent agreement with numerical simulations and with experiments conducted on the reductions of o-bromonitrobenzene in dimethylformamide solution and of p-bromonitrobenzene in acetonitrile in the presence of light of wavelength 325 nm.

The application of the concept of the steady-state reaction—diffusion layer to a study of the electrode processes with multistep reactions at microelectrodes under steady-state conditions

pUnder steady-state conditions, the general current equations for the first-order or pseudo-first-order EC, EC′, ECE and DISP1 reactions and the second-order EC′ reactions at a spherical microelectrode are derived with the aid of the concept of the steady-state reaction—diffusion layer. The approach relies on the analogy between a spherical microelectrode and a rotating-disk electrode. The characteristics of the E rC, E qC, E iC, E rC′, E qC′, E iC′, E rCE r, E rCE q, E rCE i, E qCE r and DISP1 mechanisms, where the electron transfer reaction is reversible, quasi-reversible or irreversible, are also discussed. Using these equations, some methods of determining kinetic parameters for the EC, EC′, ECE and DISP1 reactions are presented.

A general computational method for mass-transport problems involving wall-jet electrodes and its application to simple electron-transfer, ECE and DISP1 reactions

A computational procedure based on the Backwards Implicit Method is shown to be a powerful and general method of solving problems of mass transport to a wall-jet electrode. Conventional numerical methods based on a Cartesian grid are unsatisfactory because the electrode is very non-uniformly accessible. An expanding grid which increases in size in proportion to the diffusion layer thickness across the electrode surface, is shown to be effective in computing the limiting current-flow rate behaviour of (a) a simple electron-transfer reaction, (b) ECE, and (c) DISP1 processes.

Rotating-disc electrode voltammetry. Digital simulation of the current–voltage behaviour of electrode processes involving reversible electron-transfer and coupled homogeneous kinetics

A general computational strategy is presented for the calculation of steady-state current–voltage waveshapes at rotating-disc electrodes. The method is applicable to a wide range of electrode reaction mechanisms involving a reversible electron transfer, and theoretical results are given for EC, EC2, ECE, DISP1 and DISP2 reactions.

Rotating disc electrodes: the theory of chronoamperometry and its use in mechanistic investigations

A general computational strategy is presented for the calculation of the chronoamperometric responses arising from potential-step experiments at rotating disc electrodes. The method is applicable to a wide range of electrode reaction mechanisms and theoretical results are given for single- and double-potential-step experiments for ECE, DISP1, DISP2, EC' and CE reactions. For the last, the treatment is extended to cover the case where reactants have grossly unequal diffusion coefficients. Steadystate behaviour is also deduced. The extent to which the various mechanistic pathways can be distinguished is identified and the necessary experiments defined.

Recycle reactor models for complex electrochemical/chemical reaction systems

The performance of complex electrochemical reaction sequences in recycle plug flow reactors is mathematically modelled. The reactions include successive electron transfers (EE reactions), chemical reaction interposed between successive electron transfers (ECE reactions), simultaneous electron transfers and simultaneous electron transfer and chemical reaction. Both potentiostatic and galvanostatic operations are considered and the effects of important parameters such as mass transport coefficient, recycle ratio and chemical reaction rate in the recycle loop are highlighted. This is done by considering two important electro-organic synthesis reactions, the production ofp-aminophenol and the reduction of oxalic acid to glyoxylic acid.

Studies on electroreduction mechanisms of selected aromatic hydrocarbons in tetrahydrofuran

Detailed mechanisms of electrochemical reductions of seven polynuclear aromatic hydrocarbons (PAHs) including benzo[a]pyrene (BaP), benzo[e]pyrene (BeP), dibenz[a,h]anthracene, dibenz[a,c]anthracene, 3-methylcholanthrene, 7,12-dimethylbenz[a]anthracene, and benz[a]anthracene were studied using cyclic voltammetry, single potential step chronoamperometry, voltammetry at rotating disk electrodes, and controlled potential coulometry in tetrahydrofuran solution using 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte. Results indicate that these compounds undergo ECE reactions to a final product reduced at L-regions of each PAH with the exception of BaP and BeP. For these two PAHs, unreduced hydrocarbons were identified after exhaustive electrolysis. Heterogeneous electron-transfer rate constants, as well as homogeneous reaction rate constants, for the addition of the proton to electrogenerated anion radicals are also reported.

Studies on electroreduction mechanisms of selected aromatic hydrocarbons in tetrahydrofuran

Detailed mechanisms of electrochemical reductions of seven polynuclear aromatic hydrocarbons (PAHs) including benzo[a]pyrene (BaP), benzo[e]pyrene (BeP), dibenz[a,h]anthracene, dibenz[a,c]anthracene, 3-methylcholanthrene, 7,12-dimethylbenz[a]anthracene, and benz[a]anthracene were studied using cyclic voltammetry, single potential step chronoamperometry, voltammetry at rotating disk electrodes, and controlled potential coulometry in tetrahydrofuran solution using 0.1 M tetra-n-butylammonium perchlorate as a supporting electrolyte. Results indicate that these compounds undergo ECE reactions to a final product reduced at L-regions of each PAH with the exception of BaP and BeP. For these two PAHs, unreduced hydrocarbons were identified after exhaustive electrolysis. Heterogeneous electron-transfer rate constants, as well as homogeneous reaction rate constants, for the addition of the proton to electrogenerated anion radicals are also reported.