Electroreduction of Hexaamminecobalt(III) Cation on Bi(hkl) Electrodes from Weakly Acidified LiClO[sub 4] Solutions

Abstract

The cyclic voltammetry and rotating disk electrode methods have been used for investigation of electroreduction kinetics of the hexaamminecobalt͑III͒ cations on the electrochemically polished Bi planes. The rate constant for the heterogeneous electroreduction reaction of the ͓Co͑NH 3 ͒ 6 ͔ 3+ cation on the Bi͑001͒ and Bi͑011͒ planes in x M LiClO 4 + 0.001 M HClO 4 aqueous base electrolyte solutions has been established and the results have been compared with the data for the Ag͑110͒, Au͑hkl͒, and Hg electrodes as well as with the data for Bi͑hkl͒ in x M HClO 4 solutions. In the region of zero charge potential, the value of the experimental transfer coefficient ␣ exp is slightly higher than 0.5 and depends on the crystallographic structure of the Bi plane. For more concentrated base electrolyte solutions, the values of apparent transfer coefficient ␣ app , corrected for the double-layer effect, are practically independent of the base electrolyte concentration and slightly depend on the electrode polarization. Noticeably different values of the double-layer-corrected rate constant k het 0 and ␣ app for the Bi͑001͒, Bi͑011͒, Bi͑111͒, Hg, and Ag͑110͒ electrodes from the data for Au͑hkl͒ planes have been explained by the different water and ClO 4 − adsorption energies at the metals under discussion.The electroreduction of ͓Co͑NH 3 ͒ 6 ͔ 3+ complex cation is observed as a model reaction for theoretical analysis of so-called outersphere reaction. 1-12 However, the rate constant, corrected for the electrical double-layer effect, depends noticeably on the chemical nature of the electrode. 4,5,10,11 It was found that the rate constant of ͓Co͑NH 3 ͒ 6 ͔ 3+ reduction at Au single-crystal planes at fixed electrode potential increases in the order Au͑100͒ Ͻ Au͑111͒ Ͻ Au͑110͒ Ͻ Au͑210͒, i.e., with increasing the reticular density of the surface studied. 7 After correction for the electrical double-layer effect, the kinetic data fall on one common plot for all Au electrodes studied, and a single value of the apparent transfer coefficient ͑␣ app ͒ is obtained for this electrode reaction. The electric double-layercorrected rate constant depends on some specific properties of the electrode and the redox species, such as the zero charge potential or the electronic work function of the single-crystal plane and its relation to the standard potential. [3][4][5]7,[9][10][11] The reduction kinetics of hexa-aqua-iron ͑III͒ ion was studied at Au͑hkl͒ ͑at various concentrations of HClO 4 as the base electrolyte 8 ͒ but differently from ͓Co͑NH 3 ͒ 6 ͔ 3+ reduction; 7 the electron transfer coefficient depends on the crystallographic structure of the electrode surface. It was found that ␣ app increases with reticular density of the Au͑hkl͒ planes. 8 Our preliminary studies of ͓Co͑NH 3 ͒ 6 ͔ 3+ electroreduction at Bi͑hkl͒ ͑Ref. 11͒ show that ␣ app obtained depends on the crystallographic structure and increases slightly with the reticular density of the plane. Differently from the Au͑hkl͉͓͒Co͑NH 3 ͒ 6 ͔ 3+ + HClO 4 + H 2 O system, 7 the heterog...

Investigation of Electroreduction Kinetics of the Hexaamminecobalt(III) Cations on the Electrochemically Polished Bi Planes by the Electrochemical Impedance Spectroscopy Method

“…Electrochemical impedance data were obtained using Autolab PGSTAT30 FRA2 system at ac frequency from 5•10 -2 to 5•10 4 Hz within the region of electrode potential from -0.85 to -0.4 V vs Ag| AgCl. For better comparison with the data of work (11) some cyclic voltammograms (CVs) (scan rate from 5 to 200 mV•s -1 ) and rotating disc voltammograms (scan rate from 5 to 20 mV•s -1 ) were obtained by using the "Pine" rotating ring-disc electrode system and the data measured were in a good agreement with the results published in (11,14) (Fig. 1).…”

“…Dependence of current density (j) on the electrode potential (E) for the reduction of [Co(NH 3 ) 6 ](ClO 4 ) 3 in 1×10 -3 M … 1×10 -1 M solution of HClO 4, is presented in Figure 1. According to these data, the current of electroreduction of the [Co(NH 3 ) 6 ] 3+ cation depends noticeably on the electrode potential and on the rotation velocity (11,14), as well as on the base electrolyte and [Co(NH 3 ) 6 ] 3+ cation concentration (Fig. 1).…”

“…Previous studies based on cyclic voltammetry and rotating disk electrode data of [Co(NH 3 ) 6 ] 3+ electroreduction at Bi(hkl) (11,14) indicate that the rate constant of this reaction as well as α app values obtained depend on the crystallographic structure of Bi(hkl) studied and α app increased slightly with the reticular density of the plane.…”

“…), clearly visible current plateaus were observed ( -1 (0-5000) rev min ) caused by the limiting rate of the diffusion step (11,14).…”

“…Analysis of experimental data published shows that there is a noticeable dependence of 0 het k on the chemical nature of the base electrolyte used (4,5,7,(9)(10)(11)14,15), i.e. on the effective solvation radius (or crystallographic radius) of the cation as well as on the anion studied.…”