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

Abstract

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 (Mirc ˇeski, 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 ] 3or [Fe(CN) 6 ] 4-.

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

“…The more general case, including a quasireversible electrode reaction, was briefly addressed in a recently published monograph [18]. Later on, the theory was extended to encompass surface electrode processes, where both components of the redox couple are firmly immobilized on the electrode surface [21], as well as complex electrocatalytic mechanisms encountered with thin electroactive films [22].…”

Thiol anchoring and catalysis of gold nanoparticles at the liquid interface of thin-organic film-modified electrodes

“…For these reasons, the voltammetric response of TFE is particularly sensitive to all phenomena affecting the properties of the L/L interface. In the presence of another redox species in the aqueous phase, the overall electrochemical reaction could additionally involve a heterogeneous electron-exchange reaction across the L/L interface [1,6]. In the course of the repetitive potential cycling the metal deposition is manifested as a decreasing of the voltammetric response (inset of Fig.…”

“…the three-phase electrodes) [2], are a useful tool for studying complex electrochemical processes where the electrode reaction is coupled with an ion-transfer across a L/L interface. Their application ranges from determination of thermodynamic [2,3] and kinetic parameters of ion [4,5] and/or electron transfers [1,6], mechanistic [7] and biomimetic studies of membrane processes [8], to bioelectroanalysis [9,10]. Besides, it has been demonstrated that TFE are an excellent scaffold for studying metal nanoparticle deposition at the L/L interfaces [11], which is a unique environment due to its molecular smoothness [12].…”

Studying the ion transfer across liquid interface of thin organic-film-modified electrodes in the presence of glucose oxidase

“…Their application spans over analysis of ion [11][12][13][14] or electron-transfer [15,16] reactions across liquid|liquid (L|L) interface, study of the mechanism of redox transformation in the organic film [17][18][19], nanoparticle preparation [20], electrocatalysis [21], and bioelectrochemical studies of redox-inactive proteins [22,23]. The thin-film electrode is a rather simple system that consists of graphite electrode (GE) covered with a thin, micrometer-dimension film, of a water immiscible organic solvent (O).…”