Electrocatalysis of the first and second kind: Theoretical and experimental study in conditions of square-wave voltammetry

Abstract

A comparative analysis of two common electrocatalytic mechanisms in conditions of square-wave voltammetry (SWV) is presented from both theoretical and experimental points of view. The catalytic mechanism of the first kind is represented by a simple regenerative EC′ reaction scheme, where C′ is a follow-up irreversible chemical reaction through which the substrate is transformed and the catalyst regenerated. The second mechanism, known as electrocatalysis of the second kind, assumes the formation of an electroactive complex YS between the catalyst Y and the electroinactive substrate S in a chemically reversible reaction, followed by an irreversible electrochemical reaction of YS that yields the catalyst Y and the final electroinactive product P. The latter mechanism is mathematically modelled based on the semi-infinite diffusion model, deriving a general solution in the form of an integral equation that may be used for any voltammetric technique. A specific recurrent formula is derived based on the step-function method, enabling the simulation of SW voltammetric experiments with a wide range of experimental parameters. The properties of the response are thoroughly analyzed in terms of the kinetics and thermodynamics of the preceding chemical reaction, as well as the kinetics of the electrode reaction. The comparative analysis of the two reaction schemes reveals that both mechanisms are rather similar, exhibiting comparable sensitivity toward the kinetics of the catalytic reaction. Theoretical analysis is experimentally illustrated with the electrocatalytic reduction of bromate and nitrite ions catalyzed by Fe(II) (catalysis of the first kind) and Co(II) ions (catalysis of the second kind), respectively.