Articles Anal. Chem. 1997, 69, 5-15
Measurement of Heterogeneous Electron Transfer Rate Constants for Fe(H2O)62+/3+ during Sonication of Suspensions of Alumina Particles Nanette A. Madigan and Louis A. Coury, Jr.*
Department of Chemistry, Box 90346, Duke University, Durham, North Carolina 27708-0346
Heterogeneous electron transfer kinetics for reduction of Fe(H2O)63+ at Pt electrodes in the presence of equimolar Fe(H2O)62+ in noncoordinating 1.0 M HClO4 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 Al2O3. 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 Al2O3 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 Al2O3) 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)63reduction in the presence of equimolar Fe(CN)64-. Nonsonication, variable-temperature studies of Fe(H2O)62+/3+ gave activation energies for the kinematic viscosity, Fe(H2O)63+ 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 Al2O3 and (410 ( 10) K with 16.7 g of Al2O3 per liter of solution. The addition of larger quantities of Al2O3 produced even greater effects.
(1) Suslick, K. S., Ed. Ultrasound: Its Chemical, Physical, and Biological Effects; VCH Publishers: New York, 1988. (2) Mason, T. J.; Lorimer, J. P. Sonochemistry: Theory, Applications and Uses of Ultrasound in Chemistry; Ellis Horwood-Wiley: New York, 1988. S0003-2700(96)00742-1 CCC: $14.00
© 1996 American Chemical Society
The use of high-intensity ultrasound in chemical research has become widespread in recent years.1,2 Ultrasound does not couple directly to chemical systems but instead may cause thermal activation of molecules through the process known as acoustic cavitation. This phenomenon is characterized by the nucleated formation of vapor-filled bubbles in a condensed medium undergoing ultrasonic irradiation. These bubbles grow during the rarefaction half-cycle of acoustic excitation due to rectified diffusion, in which gas dissolved in the surrounding medium partitions into the bubble interior.3 As a bubble grows in size and its radius exceeds the critical value for resonant stability, the bubble may rapidly implode, generating extreme local temperatures and pressures on a submicrosecond time scale. The most widely accepted chemical estimates of the conditions produced at the site of bubble collapse indicate temperatures4 of about 5000 K and pressures5 approaching 1000 atm. Because bubble formation is a nucleated process occurring most efficiently at solid-liquid interfaces, electrochemical techniques are well-suited for fundamental investigations of sonochemical phenomena. First, the turbulence generated during sonication leads to high mass transport rates, thereby increasing faradaic signals. Second, because the measurement is electrical in nature rather than optical, light scattering by bubbles is of little consequence. Finally, the electrode surface itself may serve as a nucleation site for bubble formation. For these reasons as well as others, reports of the combination of electrochemistry with (3) Atchley, A. A.; Crum, L. A. Acoustic Cavitation and Bubble Dynamics. In Ultrasound: Its Chemical, Physical, and Biological Effects; Suslick, K. S., Ed.; VCH Publishers: New York, 1988. (4) Flint, E. B.; Suslick, K. S. Science 1991, 253, 1397-1399. (5) Brennen, C. E. Cavitation and Bubble Dynamics; Oxford University Press: New York, 1995; Chapter 3. (6) Bard, A. J. Anal. Chem. 1963, 35, 1125-1128. (7) Connors, T. F.; Rusling, J. F. Chemosphere 1984, 13, 415-420. (8) Huck, H. Ber. Bunsenges. Phys. Chem. 1987, 91, 648-654. (9) Dewald, H. D.; Peterson, B. A. Anal. Chem. 1990, 62, 779-782. (10) Mohammad, M. Bull. Electrochem. 1990, 6, 806-807. (11) Cataldo, F. J. Electroanal. Chem. 1992, 332, 325-331. (12) Hagan, C. R. S.; Coury, L. A., Jr. Anal. Chem. 1994, 66, 399-405. (13) Klı´ma, J.; Bernard, C.; Degrand, C. J. Electroanal. Chem. 1994, 367, 297300. (14) Marken, F.; Eklund, J. C.; Compton, R. G. J. Electroanal. Chem. 1995, 395, 335-339. (15) Bockris, J. O’M.; Reddy, A. K. N. Modern Electrochemistry; Plenum-Rosetta: New York, 1970; Vol. 2, p 1170. (16) Nakayama, T.; Sasa, K. Corrosion 1976, 32, 283-285.
Analytical Chemistry, Vol. 69, No. 1, January 1, 1997 5
ultrasound have appeared sporadically over the past 30 years. Effects such as enhanced mass transport,6-14electrode surface activation,8,15-19 polymer film ablation,20 metal deposition,21 and unusual homogeneous solution reactivity22,23 have been reported. In this paper, we report the results of heterogeneous kinetic studies of the Fe(H2O)62+/3+ redox system at a Pt electrode in noncoordinating perchloric acid solutions. By comparing results obtained during sonication with data from another hydrodynamic system, the rotating disk electrode, we will show that measurable increases in electron transfer rates occur during sonication upon the addition of small refractory particles to the electrolyte. Data will be presented to show that this procedure does not significantly alter the Pt electrode surface but instead amounts to physically focusing ultrasonic energy at the electrode to generate heat and accelerate redox reactions. EXPERIMENTAL SECTION Samples of Fe(H2O)62+ and Fe(H2O)63+ of equimolar concentrations at pH ) 0 were prepared from solid, low chloride content (18.1 MΩ-cm was then passed through the column until Cl- no longer eluted (as determined by lack of a precipitate upon addition of the eluent to concentrated AgNO3 solution). Ion-exchange of Na+ for K+ was necessary due to the low solubility of KClO4. The alumina particles used (16.7 g/L of solution unless otherwise indicated) were 1-µm-diameter deagglomerated R-Al2O3 (Buehler). The temperature-controlled, three-compartment cell used for voltammetric measurements during sonication has been described previously.12 Electrochemical data were collected with either a PAR-253, PAR-263, or BAS-100B potentiostat and are unfiltered. The reference electrode used was a 3.0 M KCl-filled Ag/AgCl minielectrode (AAI-Abtech RE 803), which was chosen because its solution junction is a ceramic frit with a low flow rate (
