Homogeneous versus Heterogeneous Catalysis at Electrodes Modified with a Thin Organic Layer: Theoretical and Experimental Study under Conditions of Square-Wave Voltammetry

A catalytic mechanism at thin film-modified electrodes is studied both theoretically and experimentally under conditions of square-wave voltammetry (SWV). The electrode system considered consists of a solid electrode covered with an electrochemically inactive thin film (e.g., a layer of a water-immiscible organic solvent) containing a neutral redox probe (R-1). The modified electrode is immersed in an electrolyte solution (typically an aqueous electrolyte solution) that contains a second redox probe (R-2) present in large excess compared to R-1. The product of the assumed one-electron electrode reaction O-1(+), confined within the film, is transformed back to initial reactant R-1 by means of a catalytic redox reaction with R-2. If the permeation of R-2 in the thin film is significant, then the catalytic reaction is a homogeneous process confined within the boundaries of the thin film. Otherwise, the catalytic reaction is a heterogeneous process involving electron transfer across the interface between the thin film and the electrolyte solution. Simple numerical solutions for both mechanisms have been derived with the aid of the step-function-modified method (Mirceski, V. J. Electroanal. Chem. 2003, 545, 29), and a comparative analysis of their theoretical voltammetric responses has been performed. SWV provides clear criterions for discriminating between two mechanisms. The studied catalytic mechanisms are of particular importance for assessing the electron transfer across the interface between two immiscible liquids by means of thin-organic-film-modified electrodes. Theoretical predictions have been confirmed by experiments with lutetium bis(tetra-t-butylphthalocyaninato) as a redox probe dissolved in a thin nitrobenzene film deposited on the surface of an edge-plane pyrolytic graphite electrode in contact with an aqueous solution of [Fe(CN)(6)](3-) or [Fe(CN)(6)](4-).