Reversible Surface Two-Electron Transfer Reactions in Square Wave Voltcoulommetry: Application to the Study of the Reduction of Polyoxometalate [PMo₁₂O₄₀]^3– Immobilized at a Boron Doped Diamond Electrode

Abstract

Reversible surface two-electrons transfer reactions (stepwise processes) are analyzed using square wave voltcoulommetry (SWVC), which is a variety of square wave techniques based on the measurement of the transferred charge. Such reversible surface redox processes are exhibited by many two-redox center and multicenter biomolecules (proteins, enzymes, ...) and inorganic molecules like polyoxometalates (POMs), which have very interesting applications, mainly as electrocatalysts. Because of the stationary character of the response obtained, the key parameters that govern the cooperativity degree of the two reversible electron transfers (ETs) are the difference between their formal potentials, ΔE(0), and the square wave amplitude, |E(SW)|, whose combined effect sets the two peaks → one peak transition in the response. Working curves based on the variation of the peak parameters (peak potentials, half-peak widths, and peak heights) with ΔE(0) and |E(SW)| are given, from which the formal potentials and the total surface excess can be accurately determined. SWVC has been applied to the study of the reduction of polyoxometalate [PMo12O40](3-) adsorbed at a boron doped diamond electrode (BDD), for which three stable and well-defined reversible charge peaks, corresponding to three cooperative EE processes, are obtained in the interval (0.6, -0.2) V by using low square wave frequencies. From the analysis of these peaks, the values of the total surface excess and the formal potentials of the six ETs have been obtained in aqueous media for two electrolytes: HClO4 and LiClO4.

Integration of Heuristic and Automated Parametrization of Three Unresolved Two‐Electron Surface‐Confined Polyoxometalate Reduction Processes by AC Voltammetry

“…The initial guess was that all k 0 values are sufficiently large that with a low frequency of 9.02 Hz that all one electron processes could initially be assumed to be reversible. This is consistent with the square wave studies of Molina et al . who assumed complete reversibility applies at boron doped diamond electrodes with frequencies in the range of 5 to 10 Hz.…”

“…Further points for noting which can be gained from perusal of Figure and which need to be accommodated in simulation‐experiment comparisons are that the first two processes partially overlap, Process II has a larger peak current magnitude than either Processes I or III and the shapes of all three process differ. Importantly, for each of the three reduction steps it has been proposed,, that two unresolved reversible one electron transfer steps of purely adsorbed material occur as in Eq. 5– …”

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box

“…Each two-electron reduction step of 12-MPA is accompanied by the addition of two protons [38,39], and increasing solution acidity shifts its reduction potential to more positive values [38,39] since the overall negative charge of the complex is decreased as it protonates. In fact, the correlation between the anionic charge of 12-MPA and its first reduction potential is linear [38,43].…”

Models and Their Limitations in the Voltammmetric Parameterization of the Six‐Electron Surface‐Confined Reduction of [PMo12O40]3− at Glassy Carbon and Boron‐Doped Diamond Electrodes

“…Each process, despite commonly having a significantly different shape, corresponds to the overall two electron‐two proton processes represented by Eq. 1–3 . Clearly, the background non‐faradaic current also contributes more extensively to the total current (faradaic plus non‐faradaic) in the DC and AC voltammetry derived from the GC electrode than for the BDD one (compare Figures b and 1d and 1c and 1e), and this also is expected to have an impact in the degree of difficulty and indeed level of success encountered in the the modelling. …”

“…In this particular example, integration of experimenter‐controlled heuristic and automated data optimization was employed to estimate up to 17 of the parameters employed in the model. spontaneously adsorbs onto many electrode surfaces and has been used widely in this state in many practical applications (see for example). In 1 M HClO 4 , the electrolyte of interest in this study, three well‐defined surface confined reduction processes are found under conditions of DC cyclic voltammetry at glassy carbon (GC) and boron doped diamond (BDD) electrodes (Figures b and 1c respectively) within the potential range employed in these experiments.…”