Measurement of Heterogeneous Electron Transfer Rate Constants for Fe(H₂O)₆^2+/3+during Sonication of Suspensions of Alumina Particles

Abstract

Heterogeneous electron transfer kinetics for reduction of Fe(H 2 O) 6 3+ at Pt electrodes in the presence of equimolarFe(H 2 O) 6 2+ in noncoordinating 1.0 M HClO 4 have been studied during irradiation with high-intensity ultrasound at 20 kHz both in the presence and in the absence of small solid particles. Increases in heterogeneous rate constants have been measured during sonication in the presence of 1-µm particles of Al 2 O 3 . During sonication, particles collide with the electrode surface, generating heat and enhancing kinetic rates relative to those determined for sonication in the absence of particles. Control experiments (in the absence of ultrasound) were conducted with a rotating disk electrode (RDE). Rate constants from charge transfer resistance measurements within (0.025 V of the equilibrium potential obtained by RDE, sonication, and sonication with 16.7 g of Al 2 O 3 per liter of solution were (3.4 ( 0.1) × 10 -4 , (4.2 ( 0.5) × 10 -4 , and (20 ( 2) × 10 -4 cm/s, respectively. By comparison, Tafel plots corrected for mass transport effects gave rate constants (RDE, sonication, and sonication with Al 2 O 3 ) of (4.4 ( 0.2) × 10 -4 , (6 ( 1) × 10 -4 , and (15 ( 5) × 10 -4 cm/s, respectively. Additional studies indicated that no increases in electrode area or rate constants were measurable postsonication; thus, rate enhancements are attributed to thermal effects (cavitation and collisions). Similar enhancements were also found for Fe(CN) 6 3-reduction in the presence of equimolar Fe(CN) 6 4-. Nonsonication, variable-temperature studies of Fe(H 2 O) 6 2+/3+ gave activation energies for the kinematic viscosity, Fe(H 2 O) 6 3+ diffusion coefficient, and standard heterogeneous rate constant of 15.8, 14.9, and 12.1 kJ/mol, respectively. From the Arrhenius relationship, effective temperatures at the electrode surface during sonication of a solution held at 273 K were found to be (282 ( 5) K without Al 2 O 3 and (410 ( 10) K with 16.7 g of Al 2 O 3 per liter of solution. The addition of larger quantities of Al 2 O 3 produced even greater effects.

Sonoelectrochemically Enhanced Electrocatalytic Processes: The Pb(IV) Catalyzed Cleavage of 1,2-cis-Cyclopentanediol at Graphite and Glassy Carbon Electrodes

“…In Figure 5 a plot of the logarithm of the limiting current versus the concentration of cis-1,2-cyclopentanediol with numerically generated as well as experimental data are shown and, although there is no perfect match between the experimental and the theoretical curve, an excellent agreement of the rate constant determined sonoelectrochemically and by conventional electrochemical methods is obtained. For very close electrode to horn separation (< 4 mm) a slight increase of the value for k was observed which possibly indicates a thermal effect under extreme conditions [25]. But in principle the result validates the use of the model of a planar diffusion layer which appears to be robust, at least for the analysis of homogeneous chemical reaction steps coupled to an ultrasonically enhanced electrode process.…”

Ultrasonically Enhanced Voltammetric Analysis and Applications: An Overview

“…Madigan and Coury measured heterogeneous rates constants during insonation of alumina particles in which a change in the rate constants was observed. The temperature of the solution at the electrode surface had been found to raise by approximately 100 K by the impact of alumina particles added to the solution which are −thrown× onto the electrode surface by cavitational processes [31]. Huck quantified the typical sonovoltammetric response obtained by the insonation of aqueous ferricyanide redox under insonation.…”

Optimization of Mercury Thin Film Electrodes for Sono-ASV Studies

“…One major advantage is the extremely enhanced mass transport to and from the electrode surface achieved under insonation [10,16]. Other key features include activation and cleaning of electrode surfaces [10, 17 ±19], thermal effects [10,20] and the production of highly reactive intermediate species [21]. In particular, the applicability of ultrasound to ASV has been investigated.…”