Bernd Speiser

Combinatorial microelectrochemistry: Development and evaluation of an electrochemical robotic system

An electrochemical robotic system using standard microtiter plates as reaction wells for potentiostatic and galvanostatic electrosynthesis and high-throughput electroanalysis was conceived and realized using stepmotor driven positioning stages in combination with a flexible software. Electrode bundles specifically adapted to the experimental needs are accurately positioned in the wells of a microtiter plate followed by the automatic performance of sequences of electrosynthetic or electroanalytical techniques. The system allows us to work under inert-gas atmosphere, in aqueous and organic solvents, and to add or remove solutions by means of integrated syringe pumps. A specifically developed script language permits the user to perform very complex experimental sequences in the different wells of the microtiter plate. The hardware and software features of the developed electrochemical robotic system, the design of suitable electrode arrangements for electrosynthesis and electroanalytical techniques, as well ...

Stöber silica particles as basis for redox modifications: particle shape, size, polydispersity, and porosity.

The synthesis of Stöber silica particles as basis for redox modifications is optimized for desired properties, in particular diameter in a wide sub-micrometer range, spherical shape, monodispersity, the absence of porosity, and aggregation free isolability for characterization and later covalent modification. The materials are characterized by SEM, DLS, nitrogen sorption isotherms, helium as well as Gay-Lussac (water) pycnometry, and DRIFT spectroscopy. Particles with diameters between approximately 50 and 800 nm are obtained by varying the concentrations of the reagents and reactants, the type of solvent as well as the temperature. The use of high water concentrations and post-synthetic calcination at 600 °C results in silica particles that can be considered as nonporous with respect to the size of the active molecules to be immobilized. The effect of reaction temperature on size distribution is identified. Low polydispersity is achieved by performing the reaction in a temperature range in which a change in temperature has only a weak or no effect on the final particle diameter. Upon optimization of the sol-gel process, the shape of the particles is still spherical. The agreement between experimental and geometric data is within the expected precision of the characterization techniques.

Electroanalytical investigations: Part II. Application of the orthogonal collocation technique to the simulation of cyclic voltammograms*

Abstract The orthogonal collocation technique-introduced into electrochemistry by Whiting and Carr[1] — has, been applied to the simulation of cyclic voltammograms. Simulations of the totally reversible redox couple and the ECirr scheme give values which are nearly identical with those obtained by the numerical integration technique of Nicholson and Shain [2]. It is possible to simulate even very complicated mechanistic problems in justifiable cpu-time. An example of a reaction sequence consisting of five coupled chemical and electrochemical steps is presented. It demonstrates the applicability of the method and the simplicity of implementing new problems. The value and optimization of the dimensionless parameter β=D/aL2 is discussed.

Electroanalytical investigations: Part VI. The simulation of fast chemical equilibrium reactions in cyclic voltammetric reaction-diffusion models with spline collocation

Abstract Theoretical cyclic voltammograms resulting from EC and CE reaction-diffusion models have been generated using a spline collocation procedure. Dimensionless rate constants up to 106 imposed no numerical problems when a backward diffentiation algorithm was used to integrate the systems of ordinary differential equations resulting from spatial discretization of the model equations. The boundary conditions are calcultated as unknowns of a system of linear equations, thus reducing the complexity of the derivations. The calculation results are verified by comparison to limiting cases and to results from classical simulation techniques.

Electrochemistry of anilines: Part I. Oxidation of 2,6-DI-tert-butyl-anilines to radical cations: General considerations, electroanalytical experiments and spectroscopy

Abstract A general scheme for the electro-oxidation of anilines is outlined. The oxidation of 2,6-di-tert-butyl-4-R-aninlines (R=phenyl, p -methoxyphenyl and p -dimethylaminophenyl) in the first oxidation wave is investigated with electroanalytical techniques (chronoamperometry, chronopotentiometry and cyclic voltammetry). The anilines are oxidized in a reversible one-electron trasfer to the corresponding stable radical cations, which are examined by cyclic voltammetry and ESR spectroscopy. Modulated specular reflectance spectroscopy (MSRS) is performed during the oxidation of these anilines as well as 2,4,6-tritert-butyl-aniline, to yield the UV/VIS spectra of the radical cations. Recording of transient reflectance signals and open circuit relaxation experiments further substantiate the conclusion that a reversible electron transfer without a chemical follow-up reaction occurs in the case of the former three anilines. The 2,4,6-tri-tert-butyl-aniline radical cation, however, decays in a slow (pseudo) first-order reaction.

Electroanalytical investigations: Part VIII. The use of an expanding simulation space in the simulation of electrochemical reaction-diffusion models with orthogonal collocation1

Abstract A method for the simulation of electroanalytical experiments is described by which the concentrations of species involved in the electrochemical reaction sequence are calculated in an expanding simulation space defined by the diffusion process. The theory is derived and the implementation into two simulation programs is discussed. Several mechanistic models are solved using orthogonal collocation for the spatial discretization of the partial differential equations and integration of the resulting ordinary differential equations. The results show that considerable improvements in terms of cpu-time usage and accuracy compared to the use of a constant simulation space are possible when simulating cyclic voltammetric and chronoamperometric experiments.

Electroanalytical investigations—IV. use of orthogonal collocation for the simulation of quasireversible electrode processes under potential scan conditions

Abstract The method of orthogonal collocation is used to discretize the differential equations describing quasireversible electrode processes under conditions of potential scanning. The resulting set of ordinary differential equations is solved to give numerical simulations. The numerical values are compared to literature data. A method for optimization of the dimensionless parameter β is given.

Electroanalytical investigations: Part III. Optimization of the dimensionless parameter β in orthogonal collocation simulations of cyclic voltammograms⋆

Abstract A method for the determination of an optimal value of the dimensionless parameter β in using the orthogonal collocation technique is described. The results are compared to our former empirical approach [Part II] and discussed.

Use of sensitivity analysis methods in the modelling of electrochemical transients Part 1. Gaining more insight into the behaviour of kinetic models

Abstract Sensitivity analysis is a commonly accepted methodology of studying the effect of parameter variations on the behaviour of mathematical models in various branches of chemical kinetics. However, it has not yet found wider application in electrochemical kinetics. In this paper we perform an introductory investigation of the utility of formal methods of sensitivity analysis for the modelling of electrochemical transients. We calculate and analyse first-order parametric sensitivity coefficients for the cyclic voltammetric transient corresponding to a quasi-reversible charge-transfer reaction E qrev , assuming potential-dependent charge-transfer coefficients, and for the chronopotentiometric transient corresponding to a C rev E reaction mechanism, taken as example problems. We show that this local sensitivity analysis provides straightforward quantitative answers to a number of fundamental questions about the dependencies of the transients on model parameters. Therefore, we recommend that sensitivity analysis be used in conjunction with traditional digital simulations, resulting in an added insight into the behaviour of kinetic models.

High-Temperature Chlorination-Reduction Sequence for the Preparation of Silicon Hydride Modified Silica Surfaces†

A general method for the functionalization of silica surfaces with silicon hydride (Si-H) groups is described for four different preparations of silica. The silica surface is reduced in a two-step chlorination-reduction procedure within a simple gas-flow system at high temperatures. After initial dehydroxylation of the silica surface, silicon chloride groups are formed by the reaction with thionyl chloride. The chlorination activates otherwise inaccessible surface siloxane moieties. A high silicon-hydride surface concentration results from the subsequent reduction of the chlorinated surface with hydrogen. The physical properties of the resulting silica are analyzed using scanning electron microscopy, as well as dynamic light scattering and Brunauer-Emmet-Teller measurements. The chlorination-reduction sequence has no significant impact on the structure, surface area and mesopore size of the silica materials used. The surface of the materials is characterized by diffuse reflectance infrared Fourier transform (DRIFT) and (29)Si CP/MAS NMR spectroscopy. The silicon-hydride groups are mostly of the