Abstract
A number of theoretical models describing the transport and kinetic processes involved in heterogeneous redox catalysis of solution phase reactants at electrode surfaces coated with redox active monolayers (such as self assembled alkane thiols containing a ferrocene group) is presented. These models are : simple bimolecular outer-sphere, generalized Koutecky-Levich, and substrate binding/ adduct formation . For each model a general expression for the steady state reaction flux is derived. Simplified analytical expressions for the net reaction flux under steady state conditions, are derived for experimentally reasonable situations, and kinetic case diagrams are constructed outlining the relationships between the various approximate solutions. The theory developed enables simple diagnostic plots to be constructed which can be used to analyse experimental data.
Highlights
The chemistry of molecularly designed electrode surfaces has been a major aspect of interfacial electrochemistry for over twenty years
We review a simple analysis of mediated electron transfer in the context of steady state rotating disc voltammetry
We examine the situation where substrate diffusion to the redox monolayer is slow and rate determining, and where mediator generation and direct substrate reaction at the unmodified electrode surface are greater than the heterogeneous cross exchange reaction kinetics between the immobilized mediator and the substrate species
Summary
The chemistry of molecularly designed electrode surfaces has been a major aspect of interfacial electrochemistry for over twenty years. In figure 4 we examine the situation where substrate diffusion to the redox monolayer is slow and rate determining, and where mediator generation and direct substrate reaction at the unmodified electrode surface are greater than the heterogeneous cross exchange reaction kinetics between the immobilized mediator and the substrate species. In the previous sections of the paper we developed models in which the steady state current response varies in a linear manner with the bulk concentration of the substrate species in solution This result will pertain when either simple bimolecular reaction between substrate and immobilized mediator species is assumed, or if an activated complex involving the substrate species undergoes surface electron transfer to yield product. It is clear from eqn. that by neglecting substrate diffusion effects in solution , we can obtain a transparent expression in which the important kinetically limiting steps may be cleanly separated from one another
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