Medium Effects for Very Fast Electron Transfer Reactions at Electrodes:  the [Ru(NH₃)₆]³⁺/²⁺ System in Water

Abstract

The reduction kinetics of [Ru(NH(3))(6)](3+) was studied at Au(111) and Au(100) single-crystal ultramicroelectrodes in dilute perchloric acid electrolytes. Both heterogeneous rate constants and experimental transfer coefficients varied with the crystallographic orientation of the gold surface. The value of the heterogeneous rate constant at Au(111) was significantly larger than that at Au(100). The experimental transfer coefficients also increased but in the opposite order. Standard rate constants at both electrodes increased with an increase in electrolyte concentration. Using double-layer data obtained in 0.01 M HClO(4), it is shown that the true transfer coefficient for this reaction is 0.5 within experimental error. The effective charge on the reactant which has a nominal charge of +3 is close to +1. The latter result reflects the distribution of charge within the polyatomic reactant.

Fifty years of studies of double layer effects in electrode kinetics—a personal view

“…Then experiments were carried out at Au(111) and Au(100) scumes. The rate constant for a given HClO 4 concentration was higher at Au(111) than at Au(100) [81]. Furthermore, it increased with increase in HClO 4 concentration, the highest value of k s being 0.55 cm The standard potential for the reaction is negative of the pzc at both electrodes, but it occurs at more negative potentials at Au(111).…”

Kinetics of Reduction of Aqueous Hexaammineruthenium(III) Ion at Pt and Au Microelectrodes: Electrolyte, Temperature, and Pressure Effects

“…where R ct is the charge transfer resistance. 22,23,58 Extrapolation of Z f ′ to infinite frequency gives R ct , from which the standard rate constant k el for the electrode reaction can be calculated from eq 3, in which n is the number of electrons transferred per mol (here, n = 1), a is the area of the UME disk, C is the bulk concentration of the electroactive species, and the other symbols have their usual meaning in the SI.…”

“…S4; the correction is negative since α < z. The effective charge on Ru(NH 3 ) 6 3+ with respect to double-layer calculations has been estimated at about +1, 3 in which case (α -z) ≈ -0.5, but for maximum effect it is taken in Fig. S4 as +3, or (α -z) = -2.5.…”

“…In many of the early reports, however, it is unclear whether the uncompensated resistance of the solutions had been adequately allowed for in deriving k el values. 46 For convenient adaptation to high pressure experiments, we chose to study the Ru(NH 3 ) 6 3+/2+ electrode kinetics by ac impedance measurements at Pt and Au ultramicroelectrodes, as used by Muzikaŕ̌and Fawcett 22,23 in their studies of this system in dilute HClO 4 media at 25 °C. Oxidizing anions such as perchlorate and nitrate, however, are usually avoided in Ru ammine chemistry because of the risk of oxidation of Ru(NH 3 ) 6 3+ and especially Ru(NH 3 ) 6…”

Modeling and Quantification of Electrochemical Reactions in RDE (Rotating Disk Electrode) and IRDE (Inverted Rotating Disk Electrode) Based Reactors

“…The reduction of hexaammineruthenium (III) to hexaammineruthenium (II) is extensively described in literature (Beriet & Pletcher, 1994;Deakin et al, 1985;Elson et al, 1975;Khoshtariya et al, 2003;Marken et al, 1995;Muzikar & Fawcett, 2006) as a one electron transfer reaction, i.e., Ru(NH 3 )…”