Spatial and Mechanistic Resolution

Apart from the detection and characterisation of reaction intermediates it is often desirable to gain quantitative mechanistic information in transient or 

Steady-State electrochemical experiments. From voltammetric experiments, it is known that in some cases one type of voltammetric current response maybe interpreted by more than one mechanism [53]. Depending on the type of experiment, coupling a spectroscopic method to a voltammetric experiment may provide complementary data, which then allows a definite decision on which reaction pathway is followed. 

Therefore, spectroelectrochemical experiments may be used to obtain more than just data to identify the nature of the reaction intermediate. The spatial distribution and concentration gradients of reactants and products give direct insight into the reaction mechanism of a particular electrode process [54]. A spectroelectrochemical technique with high spatial resolution can yield very valuable data. However, the experimental approach shown, for example, in Fig. II.6.2 can be seen to give an absorbance measurement equivalent to the integral value of all the material produced in the diffusion layer without spatial resolution. 

A very elegant approach overcoming this problem has been proposed based on a channel flow cell geometry with downstream detection (Fig. II.6.2D). The potential of the electrode is stepped during steady-state flow of the solution across the electrode. A downstream UV/Vis detector system is then employed to measure the time dependence of the concentration profile formation at the electrode surface. A computer program is employed to relate the time-dependent absorbance signal to the concentration profile of reactant and product at the electrode surface. Alternatively, direct measurement of the concentration profiles at the electrode surface has also been reported based on confocal Raman spectroelectrochemistry [55]. 

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