Przemysław T. Sanecki

The problem of the accuracy of electrochemical kinetic parameter determination for the ECE reaction mechanism

Abstract The theoretical and practical possibilities of the determination of elementary kinetic parameters k 1 , α 1 , k 2 , α 2 for sequence A → k 1 ,α 1 B → k f C → k 2 ,α 2 D were examined for various k 2 / k 1 ratios on the basis of electrochemical simulations. The errors of the kinetic parameters evaluated from theoretical CV curves or from those with artificial noise added were calculated and discussed. The subject is important for the modelling of the ECE mechanism and other consecutive reaction steps. The work demonstrates quantitatively the relations that are qualitatively familiar to chemists: a precise determination of second stage kinetic parameters in a consecutive process is impossible if their magnitude is much higher than that in the first stage.

The cyclic voltammetry simulation of a competition between stepwise and concerted dissociative electron transfer, the modeling of alpha apparent variability, the relationship between apparent and elementary kinetic parameters

A comparison of elementary (input) and apparent (output) values of the transfer coefficient was carried out for simulated kinetic cyclic voltammetry spectrum from stepwise to concerted mechanism. The resulting patterns of alphaapp versus Ep plots do not provide sufficient information to discriminate between the two mechanisms. It has been demonstrated that non-linear alphaapp patterns are characteristic for the pure stepwise mechanism obeying the Butler-Volmer kinetics. The change in alphaapp is not a sufficient condition for the transition from stepwise to concerted mechanism. This paper contains visualizations of the relationship between the apparent and elementary transfer coefficient. The difference between the potential variabilities of alphaapp and alphaelem (a frequent source of confusion) is also discussed.

A numerical modelling of voltammetric reduction of substituted iodobenzenes reaction series. A relationship between reductions in the consecutive-mode multistep system and a multicomponent system. Determination of the potential variation of the elementary charge transfer coefficient.

The cyclic voltammetry reduction process of the reaction series of substituted iodobenzenes X-C6H4-I where X = H, p-Cl, p-Br, p-I, p-CH3, m-CF3 was investigated in 0.3 M TBAP in DMF. A numerical model of the process consistent with the ECE mechanism of mono-iodobenzenes reduction and consecutive ECE-ECE reduction of p-diiodobenzene was applied. On the basis of alpha(i) vs. E(p,i) dependence, the value of delta a(i)/delta E was found to be 0.37 +/- 0.07 for first electron transfer (Eq. (10)). The ECE-ECE system seems to be an another example of elementary alpha kinetic discrimination between two identical two-electron processes analogous to that described in a previous paper (Sanecki, P., Kaczmarski, K. (1999). J. Electroanal. Chem. 471, 14 and erratum to it (2001)). A method of simultaneous treatment of the substrate and all electroactive intermediates, i.e. the transformation of any experimental consecutive CV reduction curves (e.g. ECE or ECE-ECE) into curves corresponding to reduction of the multi-component systems is presented and discussed.

The voltammetric reduction of some benzenesulfonyl fluorides, simulation of the ECE mechanism and determination of the potential variation of the transfer coefficient by using the compounds with two reducible groups

Abstract The cyclic voltammetry reduction process of benzenesulfonyl fluoride (BSF) and m -benzenedisulfonyl difluoride ( m -BDF) was investigated in 0.3 M TBAP in DMF. A numerical model of the process was developed consistent with the ECE mechanism of BSF reduction and consecutive ECE–ECE reduction of m -BDF. The values of elementary kinetic parameters were determined. The value of ∂ α 1 /∂ E for –SO 2 F reduction in m -BDF was found to be 0.44±0.02 V −1 . The ECE–ECE system is probably the first recognized example of elementary α kinetic discrimination between two identical two-electron processes.

Application of EC, ECE, and ECE-ECE Models with Potential Dependent Transfer Coefficient to Selected Electrode Processes

The electrochemical-chemical (EC), electrochemical, chemical, electrochemical (ECE), and electrochemical, chemical, electrochemical-electrochemical, chemical, electrochemical (ECE-ECE) models with and without an included α variability were applied to the reduction of 2-methyl-2-nitropropane, benzenesulfonyl fluoride, and p-toluenesulfonyl fluoride. The three cases where a variability may occur are noted and discussed. It is shown that a variability, if present, may be of importance for ECE sequence in relation to k f determination.The presented cyclic voltammetry data obtained on glassy carbon electrode in the wide scan rate region were fully suitable for determination of kinetic parameters by estimation.

A comparison of the multistep consecutive reduction mode with the multicomponent system reduction mode in cyclic voltammetry

The multistep consecutive ECE-ECE reduction process A(e)-->B(k(f2))-->C(e)-->D(e)-->E(k(f2))-->F(e)-->G has been compared with reduction in multicomponent system A(e)-->B, C(e)-->D, D(e)-->E, F(e)-->G. A simple method of transformation has been devised to disclose the subtle structure of the complex cyclic voltammetry (CV) responses and illustrated by the ECE-ECE process modeled earlier. The method can be applied to any multi-electron CV experimental curve for which a numerical modeling has been done. Electroreduction processes similar to those considered here are often met in practice. An attempt of unification of consecutive electroreduction and electroreduction of multicomponent system has been made. Interrelation between research and analytical voltammetry aspects of the problem is also discussed.

The experimental verification of mathematical two-plate model describing the metal deposition/dissolution process

The advanced two-plate model with BET adsorption equation has been verified by the use of the experimental Cu2+/Cu0 system in 1 M NaClO4/pH 3 solution at different scan rates and at 5, 10, 25 mM concentrations of Cu2+. A change of electrode properties during electrode process was included into the applied model. The model reflects a characteristic loop on cathodic part of CV response. The values of elementary kinetic parameters have been determined and discussed.

Mathematical Modeling of Electrode Processes – Potential Dependent Transfer Coefficient in Electrochemical Kinetics

The connection between experimental results and mathematical modeling of electrode processes may become an inspiration for new results and deeper understanding of the nature of electrochemical processes and its kinetic description. Sometimes experiment is preceded by a theory, sometimes it is the other way round. To avoid over discovering of phenomena, uncertainties and even mistakes, a responsible validation of model results is required. The analysis of complex, multi-electron electrode processes with chemical step(s) provides the respective examples.

The Application of Metal Deposition in Optical Sensor Technique. The Microscale Cu Deposition as “Electrochemical Welding”

The problem of how to attach an optical glass fiber to a copper plate was solved with the use of microscale electrolyser of special design. The respective laboratory scale technology was presented. The results were documented by photos of ready elements. The matter of relation between deposition by means of constant current electrolysis without potential control and the process realized with the use of cyclic voltammetry and chronopotentiometry techniques was raised and discussed.