Electrochemical systems spectroelectrochemistry

I 3 In situ Structural and Spectroscopic Probes of Electrochemical Systems Tab. 1 Features of UV-visible spectroelectrochemistry... 

The term spectroelectrochemistry describes experimental techniques in which an electrochemical experiment is combined with a spectroscopic technique. While electrochemical experiments typically yield information on macroscopic properties like reaction rates, spectroscopic techniques usually are applied to yield information on a molecular level, i.e., the stmcture of molecules, their electronic configuration, etc. The combination of both electrochemical and spectroscopic approaches will thus unravel a more complete picture of the system under study. A broad variety of spectroscopic techniques has been coupled with electrochemistry, including UV-vis, Raman, EPR, and NMR [1 ]. The application of infrared spectroscopy to study electrochemical systems is labelled infrared spectroelectrochemistry and will be described in the following. 

IR spectroelectrochemistry has been the subject of a sizeable amount of early reviews, where the experimental details and applications have been described [5-7]. Regardless the fact that electrochemistry is an extremely broad field, the following discussion will be restricted to classical electrochemical systems where a solid electrode is in contact with a liquid electrolyte solution which may contain electroactive species. Since the typically used electrolyte solutions (mostly aqueous solutions) are strongly IR absorbing, it is not possible to use a standard laboratory electrochemical cell, but for spectroelectrochemical experiments, special cell designs and beam paths have to be employed. There are two general principles on how the IR beam is directed to the electrode surface called internal reflection and external reflection, respectively. 

Fortunately, the rewards of research are complementary, and whatever direction or thrust a particular spectroelectrochemical project may take, several fields of science ultimately may benefit. The interchange of knowledge and ideas in electrochemistry and surface science at the analytical-physical interface, for example, is notable. Such progress offers a better understanding of electrochemical processes through theoretical advances and experimental discovery or validation, by both pure and applied motivation. These new electroanalytical techniques permit reevaluation of important practical electrochemical systems such as corrosion mechanisms, biochemical redox intermediates, kinetic and catalytic processes of analytical and chemicals production importance, and optical devices. Spectroelectrochemistry continues to be an exciting and challenging field in which to work. 

The electrosynthesized (0EP)Ge(CgHs)C10, was characterized in situ by thin-layer spectroelectrochemistry. The final product of electrosynthesis was spectrally compared with the same compounds which were synthesized using chemical and photochemical methods(35). (0EP)Ge(C6H5)Ci and (0EP)Ge(CsHs)0H were also electrochemically generated by the use of specific solvent/supporting electrolyte systems(35). 

Electrochemically generated solutions of radical-cations will react with nucleophiles in an inert solvent to generate a radical intemiediate. Under these conditions the intermediate is oxidised to the carboniiim ion by a further radical-cation. Generally, an aromatic system is then reformed by loss of a proton. Reactions of 9,10-diphenylanthracene radical-cation nucleophiles in acetonitrile are conveniently followed either by stop flow techniques or by spectroelectrochemistry. Reaction with chloride ion follows the course shown in Scheme 6.2, where the termination... 

To a large extent, the discovery and application of adsorption phenomena for the modification of electrode surfaces has been an empirical process with few highly systematic or fundamental studies being employed until recent years. For example, successful efforts to quantitate the adsorption phenomena at electrodes have recently been published [1-3]. These efforts utilized both double potential step chronocoulometry and thin-layer spectroelectrochemistry to characterize the deposition of the product of an electrochemical reaction. For redox systems in which there is product deposition, the mathematical treatment described permits the calculation of various thermodynamic and transport properties. Of more recent origin is the approach whereby modifiers are selected on the basis of known and desired properties and deliberately immobilized on an electrode surface to convert the properties of the surface from those of the electrode material to those of the immobilized substance. 

The chemical stability and electrochemical reversibility of PVF films makes them potentially useful in a variety of applications. These include electrocatalysis of organic reductions [20] and oxidations [21], sensors [22], secondary batteries [23], electrochemical diodes [24] and non-aqueous reference electrodes [25]. These same characteristics also make PVF attractive as a model system for mechanistic studies. Classical electrochemical methods, such as voltammetry [26-28] chronoamperometry [26], chronopotentiometry [27], and electrochemical impedance [29], and in situ methods, such as spectroelectrochemistry [30], the SECM [26] and the EQCM [31-38] have been employed to this end. Of particular relevance here are the insights they have provided on anion exchange [31, 32], permselectivity [32, 33] and the kinetics of ion and solvent transfer [34-... 

For the first reduction the IR shifts point to a porphyrin-centred electron transfer. This is supported by further spectroscopy on the anion radical complexes [(Por)Ru(CO)(L)]. The observed EPR lines are narrow, unstructured, with g values around 2. The UV-Vis-NIR spectra of the radical anions are characterised by redshifted Soret bands of reduced intensity, a weak structured band system around 600 nm and weak broad absorptions around 800 or 900 nm (see Figure 4.15). Further support comes from resonance Raman investigations on [(OEP)Ru(CO)(THF)] for which the observed Raman bands fit perfectly to those of the [(OEP)VO] radical anion. There is some evidence that if the spectroelectrochemistry is not carried out in very aprotic and unpolar solvents or traces of water are present, the radical anionic complexes are readily transformed. This has been investigated for the [(OEP)Ru(CO)(L)] system, where the use of solvents like MeOH or nitriles for the electrochemical reduction leads to altered species with unreduced porphyrin ligands (see Figure 4.15)." ... 

Spectroelectrochemistry [99] Is a hybrid technique resulting from the association of electrochemistry with spectroscopy via the use of cells with optically transparent electrodes [100-103]. The potential of this technique lies in the possibility of Identifying both the type and the amount of the species generated In an electrochemical step. The Intrinsic characteristics of spectroelectrochemistry require the use of fast measuring systems —spectroscopic image detectors in most cases [104-107]— and the consequent acquisition of the large number of data provided by the detection system In a short time by means of an oscilloscope or, even better, of a computer also allowing the subsequent exhaustive treatment of the raw data. 

Applications of the OTTLSET to other systems were reviewed by Heineman et al in 1984. This chapter mainly emphasizes the development of the in situ UV-VIS spectroelectrochemistry, especially the OTTLSET, for investigation of direct electrochemical oxidation and reduction reactions of redox proteins. 

Since the establishment of spectroelectrochemistry very little effort has been devoted to the direct electrochemistry of redox proteins. Although many thermodynamic and kinetic parameters can be determined by UV-VIS spectroelectrochemistry, the electrochemical reaction mechanisms for redox proteins are not well understood. New techniques md new theoretical treatments are needed to address this issue. Moreover, most attention has been placed on relatively simple electron transfer proteins to date no one has reported the direct electrochemistry of a more complex system (e.g., a redox enzyme system) which unequivocally undergoes electron transfer to (or from) its active site. Considerable experimental work is needed to develop more fully spectroelectrochemical methods for biological systems. 

Past efforts allow us to formulate three objectives for the present work. First we would like a technique that is roughly 2 to 4 (or more) orders of magnitude more sensitive than existing spectro-electrochemical methods. If this were achieved, the techniques could be applied to high-sensitivity analysis where one has a complex mixture and one makes use of the selectivity of spectroelectrochemis-try. Second, it would be valuable to lower the usable time scale of spectroelectrochemistry down into the microsecond region for a variety of chemical systems. With an optically transparent electrode and virtually all spectroelectrochemical methods, the response is limited by an effective path length which decreases with the time scale. Therefore, it is very difficult to monitor species on a microsecond time scale simply due to the low sensitivity of the techniques. The third objective is spatial.resolution of the diffusion layer. It would be very informative from both fundamental and practical standpoints to be able to accurately observe concentration vs. distance profiles. 

In the case of channel electrodes, the solution containing the electroactive species flows in a channel such as that shown in Figure 8.3 where a rectangular electrode of length Xe and width w is placed on the channel floor. The mass transport by convection can be controlled through the channel design, the electrode size and the flow rate. Moreover, this setup enables the incorporation of electrochemical measurements to flow systems as well as its use in spectroelectrochemistry and photoelectrochemistry [4]. 

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