EC mechanism of an adsorbed redox couple. Volume vs surface chemical reaction

Abstract

A theoretical model of a complex electrode mechanism coupled by adsorption of the redox couple and followed by two different types of irreversible chemical reactions is developed: Ox ( ads) +n e − ⇆ Red ( ads) → k s P ⇅ ⇅ ( Ox ( aq) ) x=0+n e − ⇆ ( Red ( aq) ) x=0 → k v P The theoretical consideration for this specific type of EC mechanism comprises all relevant phenomena such as diffusion mass transport, adsorption equilibria and kinetics of the following irreversible chemical reactions. The adsorbed electroactive product Red (ads) decays to the final inactive product P through a chemical reaction occurring exclusively on the electrode surface. It is designated as a surface chemical reaction associated by the rate constant k s, whereas the dissolved Red (aq) form transforms independently via a volume chemical reaction taking place in the solution layer adjacent to the electrode surface and is characterized by the rate constant k v. Adsorptions of both species of the redox couple are assumed to obey a linear adsorption isotherm law. The solutions obtained with the aid of Laplace transforms for the surface concentrations for both oxidized and reduced species are presented as integral equations, thus they are valid for any chronoamperometric technique. The numerical solution is adopted for square-wave voltammetry and both reversible and quasireversible cases were studied. Special attention is paid to revealing discrepancies between the surface and volume chemical reactions. It has been demonstrated that for a reversible case the influence of both chemical reactions is rather similar disabling a simple distinguishing of the reaction type. For the quasireversible case, clear and simple criteria for distinguishing the type of chemical reaction can be established utilizing the properties of split SW peaks and a quasireversible maximum.