The Quasicatalytic Mechanism: A Variation of the Catalytic (EC′) Mechanism

The classic electrochemical catalytic mechanism, often referred to as the EC' mechanism, is traditionally represented by the two reactions A + e = B (Es, and B + P reversible arrow A + Q (K(eq), k(f), k(b)). Implicit in this mechanism is the additional heterogeneous electron transfer P + e reversible arrow (E(P/Q)(0), k(P/Q)(0), alpha(P/Q)). To observe EC' behavior, the following conditions must be met (we focus on cyclic voltammetric responses): (1) E(P/Q)(0) > E(A/B)(0) (ensuring that K(eq) > 1), (2) k(P/Q)(0)c(P) exp[ -alpha(P/Q)(F/RT)(E - E(P/Q)(0))]/(0.446cA(FD(A)vertical bar v vertical bar/RT)(1/2)) << 1 over the potential range of interest (ensuring that the reaction P + e = Q does not occur to any significant extent relative to the peak current for reaction A + e = B alone), (3) k(f)c(P)RT/F vertical bar v vertical bar > 1 (ensuring that the catalytic effect is significant). We offer arguments based on Marcus theory that when condition 2 is met, fulfilling condition 3 will be difficult. This could explain why EC' behavior is rare. In the present work we show that EC'-like cyclic voltammetric responses can be obtained even when P + e reversible arrow Q is facile if D(P,Q) (the diffusion coefficient for the substrate-couple species P and Q) is much smaller than D(A,B) (the diffusion coefficient for the mediator-couple species A and B). When D(P,Q)/D(A,B) is sufficiently small, the system behavior becomes identical to that seen for the classical EC' system. We suggest that this "quasicatalytic" behavior should be considered when EC'-like behavior is observed and when the electrochemical system involves a substrate couple whose diffusion coefficients are much smaller than those of the mediator couple. As has been known for some time, when the diffusion coefficients of species A, B, P, and Q are identical (an assumption commonly made to simplify theoretical analysis) and when both heterogeneous electron transfers are reversible, the homogeneous kinetics have no effect on the cyclic voltammetric response even though the distribution of species in the diffusion layer is dramatically altered.