Andrieux and Saveant

ORR catalysis by Fe or Co porphyrins in Nation [Shi and Anson, 1990 Anson et al., 1985 Buttry and Anson, 1984], polyp5rrolidone [Wan et al., 1984], a surfactant [Shi et al., 1995] or lipid films [CoUman and Boulatov, 2002] on electrode surfaces has been studied. The major advantages of diluting a metalloporphyrin in an inert film include the abUity to study the catalytic properties of isolated molecules and the potentially higher surface loading of the catalyst without mass transport Umit-ations. StabUity of catalysts may also improve upon incorporating them into a polymer. However, this setup requires that the catalyst have a reasonable mobUity in the matrix, and/or that a mobile electron carrier be incorporated in the film [Andrieux and Saveant, 1992]. The latter limits the accessible electrochemical potentials to that of the electron carrier. 

It follows that the value of the electrochemical transfer coefficient may allow the distinction between stepwise and concerted electron-transfer-bond-breaking reactions when a chemical bond of normal strength is involved (Andrieux and Saveant, 1986b Andrieux et al., 1990b). If the reduction wave possesses the characteristics of a process controlled by slow electron transfer rather than controlled by a follow-up reaction, and if a is significantly larger than 0.5, then one can conclude that the reaction proceeds in a stepwise manner. The same is true when the wave exhibits the characteristics of a process controlled by a follow-up reaction, electron transfer remaining at equilibrium. 

Fig. 7 Representation of (64) showing the procedure for determining the standard potential, Jx/rx ". the intrinsic barrier, AGJ (AGJ = RT In k. (Adapted from Andrieux and Saveant, 1986b.)...

For faster reactions, the decay of RX- within the diffusion layer, i.e. within the solvent cage, should be taken into account (Andrieux and Saveant, 1986b Grimshaw and Thompson, 1986). An extreme situation, opposite to that just discussed, is when the follow-up reaction is so fast that RX- collapses before having time to diffuse away from the donor molecule. This is the homogeneous equivalent of the surface follow-up reaction case in electrochemistry. Equation (64) is then replaced by (69) (Andrieux and... 

In their work, Andrieux and Saveant analyse the behaviour in terms of the characteristic current densities for each process. This has the advantage that, in general, the expressions for these characteristic current densities are fairly straightforward and obvious and, therefore, easily derived. In the case where the different processes involved are strictly first-order, the reciprocal of total current density can be obtained by combining the reciprocals of the current densities for the individual steps. For example, the limiting current density, //., for a reaction at the surface of a rotating disc coated by a permeable membrane is given by... 

Polarographic catalytic currents" are well known, especially in inorganic electrochemistry, with classic examples, such as the reoxidation of ferrous ions—fonned cath-odically from ferric ions—by hydrogen peroxide or hydroxylamine. However, it appears obvious that the term catalysis is used too often quite ambiguously. Therefore it use in organic electrochemistry needs to be clarified a distinction should be made between the two main kinds of catalysis. This differentiation has been emphasized by Andrieux and Saveant [1,2]. 

Description of electrocatalytic processes in such modified electrodes can be derived from the intersection between the theory of Andrieux and Saveant (1980, 1988) for mediated electrocatalysis in redox polymers and those for metal oxide electrocatalysis (Lyons et al., 1992,1994 Attard, 2001 Pleus and Schulte, 2001) and the recent models for the voltammetry of microparticles given by Lovric and Scholz (1997, 1999) and Oldham (1998) and combined by Schroder et al. (2000). 

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