1388-2481/00/$ - see front matter q2000 Elsevier Science S.A. All rights reserved. PII S1388-2481 ( 00 ) 00054-0 Thursday May 04 10:25 AM StyleTag -- Journal: ELECOM (Electrochemistry Communications) Article: 231 www.elsevier.nl/locate/elecom Electrochemistry Communications 2 (2000) 431–435 Sonoelectrochemistry — cyclohexanoate electrooxidation at 38 kHz and 850 kHz insonation frequencies compared D.J. Walton *, S.S. Phull, U. Geissler, A. Chyla 1 , A. Durham, S. Ryley, T.J. Mason, J.P. Lorimer School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK Received 8 March 2000; received in revised form 20 March 2000; accepted 20 March 2000 Abstract Insonation of cyclohexanoate electrooxidation at platinum in methanol produces a similar switch in product distribution from one-electron per-molecule dimers towards products from the two-electron route via the carbocation, irrespective of whether the ultrasonic bath employs 38 or 850 kHz frequencies. However, 850 kHz produces a cleaner reaction and higher yields, with more than two-fold greater yield of total extracted products over the silent system, and one-and-a-half times yield enhancement over insonation at 38 kHz. q2000 Elsevier Science S.A. All rights reserved. Keywords: Sonoelectrochemistry; Carboxylate electrooxidation; Ultrasound; Frequency dependence 1. Introduction The improvement of electrosynthetic reaction systems is an important aim, and one approach is to expose the elec- trolysis cell to simultaneous ultrasonic irradiation. Early reports were sporadic, although a system for high-speed cou- lometry was described [1]. Recent enhancements in both electrochemical and ultrasonic methodologies have greatly increased the scope and usefulness of this dual-activation technique, and the current status has been reviewed [2,3]. In sonoelectroanalysis there is increase in voltammetric limiting currents due to improved mass transport [4,5], with specific improvements to electroanalytical systems that are affected by electrode surface phenomena, including analyses for vita- min C (absorbic acid)[6], DNA bases [7] and also anodic [8] and absorptive [9] stripping voltammetries. Sonoelec- troanalysis is useful in real media such as blood, beer and wine that are affected by electrode fouling, and new electrode materials such as boron-doped diamond offer further benefit [10]. In sonoelectrochemiluminescence, there is increased light output accompanying diminution of electrode fouling [11]. * Corresponding author. Fax: q44-1023-838282; e-mail: d.walton@ coventry.ac.uk 1 Present address: Physical and Theoretical Chemistry Department, Tech- nical University, Wroclaw, Poland. The majority of these studies have been performed in the ‘power’ ultrasound region (20–60 kHz frequency), using ultrasonic apparatus including cleaning baths or ultrasonic horn systems. Cavitation, which is the rapid growth and decay of microbubbles in a medium and which is responsible for the mechanical abrasive power of ultrasound, is widely recog- nised and well-described in this frequency regime [12]. However, the role of cavitation in all sonoelectrochemical phenomena is not certain. The use of microelectrodes of diameter smaller than a cavitation bubble produces voltam- mograms showing sporadic events that may accompany cav- itation [13,14]. However, a recent study of different electrode/sonic source geometries (‘face-on’, side-on’ or ‘sonoelectrode’ (use of the horn itself as electrode)) [15] at a constant 20 kHz frequency in electrolytes of different kin- ematic viscosities suggests the importance of acoustic stream- ing [16]. Electroanalytical studies require fine control of system par- ameters such as electrode/sonic source distance and attitude, important when employing a geometry-dependent waveform subject to reflection, damping and other modulations. How- ever, electrosynthetic reactions perforce require larger elec- trode areas and greater cell volumes; thus, it is more difficult to quantify sonoelectrochemical phenomena. Nevertheless, a number of sonoelectroorganic syntheses have shown impor- tant benefits: the diminution of applied cell voltage, improved cell energetics and lessening of necessary electrolyte salt con-