Electrochemical reactions activation parameters

The values of electron work function (see Section 9.2.1) have been adduced most often when correlating electrocatalytic activities of given metals. They are situated between 3 and 5 eV. Two points were considered when selecting the electron work function as the parameter of comparison (1) it characterizes the energy of the electrons as basic, independent components of aU electrochemical reactions, and (2) it is closely related to many other parameters of metals. 

The catalytic activity of an electrode is determined not only by the natnre of the electrode metal (its bulk properties) but also by the composition and stmcture of the snr-face on which the electrochemical reaction takes place. These parameters, in tnm, depend on factors such as the method of electrode preparation, the methods of snr-face pretreatment, conditions of storage, and others, all having little effect on the bulk properties. 

Figure 3.6 shows codimension-1 singularities in a parameter plane spanned by the activation energy of the anodic reaction yA and the activation energy of the electrolyte conductivity yK, when the total cell current I is used as the bifurcation parameter. Hysteresis and isola varieties are found. The nonlinear behavior becomes increasingly complicated for increasing values of yK. Steady-state multiplicities even exist for rather small values of yK, but vanish if the electrolyte s conductivity is temperature-independent, i.e., yK = 0. This confirms that the multiple steady states are caused not by the electrochemical reaction alone, but by the combination of reaction and varying electrical conductivity. 

Electrochemical rate constants and activation parameters for inorganic electrochemical reactions span wide ranges e.g., standard rate constants vary from ca. 10 to > 10 cm s. For transition-metal redox couples, values of AH vary - from 0 to 80 kJ mol". As for homogeneous redox processes, it is highly desirable to elucidate the underlying reasons that are responsible for the observed rate parameters. In the following sections, the theory of electrochemical electron-transfer kinetics is outlined, followed by a discussion of some pertinent experimental results. 

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. 

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