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High-frequency sonoelectrochemical processes: mass transport, thermal and surface effects induced by cavitation in a 500 kHz reactor
Ultrasonics Sonochemistry 6 (1999) 189–197 www.elsevier.nl/locate/ultsonch
High-frequency sonoelectrochemical processes: mass transport, thermal and surface effects induced by cavitation in a 500 kHz reactor F. Javier Del Campo a, B.A. Coles a, F. Marken a, R.G. Compton a, *, E. Cordemans b a Physical & Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK b Undatim Ultrasonics S.A., Zoning Industriel, Rue de l’Industrie 3, B-1400 Nivelles, Belgium Received 29 March 1999
Abstract The use of high frequency ultrasound in electrochemical systems is of major interest for the optimisation of electrosynthetic and electroanalytical procedures, especially when the strong mechanical effects of 20 kHz ultrasound are detrimental. The characterisation of a 500 kHz ultrasound reactor for sonoelectrochemical experiments by voltammetric and potentiometric measurements revealed the presence of considerable thermal, as well as mass transport, effects depending on geometric parameters and the material used for the construction of the working electrode. Micromixing and cavitation processes govern the mass transport to and from the electrode surface and are shown by atomic force microscopy (AFM ) to cause erosion on the electrode surface. Electrochemically active films of Prussian blue are shown to be gradually removed by cavitation erosion. Degassing the solution prior to sonication increases the efficiency of cavitation processes. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Catalyst; Cavitation; Frequency effect; Mass transport; Micromixing; Modified electrode; Prussian blue; Sonoelectrochemistry
1. Introduction Over the recent years a considerable amount of work has been published on the use of power ultrasound in electrochemical systems [1] with the aim of improving existing electroanalytical [2,3] and electrosynthetic [4– 7] techniques and further identifying new processes, in which activation of the electrode process in the presence of ultrasound )e.g. in emulsion environments [8]) allows the enhancement or suppression of processes in a selective manner. Most studies to date have employed horn-type ultrasound emitters operating in the frequency range of 20– 40 kHz due to their ease of implementation and use. In particular, the highly localised formation of a macroscopic liquid jet and a cloud of cavitation bubbles near to the horn tip allows the working electrode of the electrochemical cell to be positioned in the high intensity region with maximum effect on the electrochemical process [9]. The main effects observed in this type of sonoelectrochemical experiment are: (i) an extremely * Corresponding author. Tel.: +44-1865-275448; fax: +44-1865-275410. E-mail address: [email protected] (R.G. Compton)
high rate of mass transport beneficial to both analytical and synthetic electrochemical processes; (ii) a surface erosion process which may be attributed to both interfacial cavitation and mechanical damage of the electrode material caused by shock waves; (iii) rapid removal of gas bubbles and solid particles from the electrode surface; and (iv) in-situ formation of suspensions and emulsions. Although measurements of Faradaic currents based on the electrochemical oxidation or reduction of redox active solution species have been employed for the characterisation of high frequency sonochemical systems [10,11] there is very little known about the physical nature of the effects of high frequency ultrasound at the surface of the electrode in electrochemical systems. Faradaic currents in the presence of ultrasound are commonly used as a measure of sound adsorption induced mass transport which may be expressed conveniently in terms of the diffusion layer thickness, or the Sherwood number [10]. This parameter has been shown by Trabelsi and co-workers [10], in the case of a 560 kHz ultrasound reactor, to correlate with the cavitation activity and the cavitation induced emission of light from the luminol chemiluminescence process. High frequency ultrasound has been studied by micro-
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electrode voltammetry [12] and used in electrosynthetic applications [13] and for modifying the properties and growth of polymer films formed electrochemically [14]. Although interesting effects have been observed, there has been little attention to why they occur and what the physical nature of the effects is. In this work, a first step is taken to further investigate the key factors dominating electrochemical and interfacial chemical reactivity in aqueous media in the presence of 500 kHz ultrasound. It is shown that sonoelectrochemistry at high frequency is based on distinctively different processes compared to those which govern sonoelectrochemical processes at 20–40 kHz ultrasound and that processes involving slowly diffusing species are especially relatively enhanced. 2. Experimental 2.1. Reagents and instrumentation Chemical reagents used were Ru(NH ) Cl , 36 3 K Fe(CN ) , Fe (SO ) , H SO , HNO (all Aldrich), 3 6 2 43 2 4 3 NaOH, KCl, KI, K Fe(CN ) , Na SO (BDH, AnalaR) 4 6 2 4 obtained commercially and used without further purification. Electrolyte solutions were prepared from ultra high quality water of a resistivity not less than 18 MV cm obtained from an Elgastat water purification system (High Wycombe, Bucks, UK ). As working electrodes, different diameter Pt or Au disc electrodes mounted in a Teflon or glass holder were used. Electrodes were carefully polished prior to measurements using alumina lapping compounds (Aldrich or Microglass Instruments, Melbourne, Australia) of decreasing size down to 0.05 mm. A gold or platinum wire served as the counter electrode and a saturated calomel electrode (SCE ) (Radiometer Kopenhagen) was used as a reference electrode. For electrochemical measurements an Autolab PGSTAT 20 system (Eco Chemie, Netherlands) and an Oxford Electrodes Bipotentiostat were used. If not otherwise stated, experiments were conducted at 20±2°C. Analyses of the electrode surface by atomic force microscopy (AFM ) were carried out using a TopoMetrix TMX 2010 Discoverer system in contact mode (typically 3 Hz scan rate) with TopoMetrix standard AFM probes (no. 5200). 2.2. The sonochemical reactor A 500 kHz 25 W D-type reactor ( Undatim Ultrasonics S.A., Belgium) was used, fitted with a water cooled jacketed glass cell and air cooling of the transducer (see Fig. 1). The size of the reactor was 4.5 cm inner diameter with a 3.6 cm diameter stainless steel transducer and a fill height of 7.5 cm and was kept constant in all experiments. The power output of the ultrasound system was calibrated calorimetrically by
Fig. 1. Schematic drawing of the 500 kHz sonoelectrochemical cell.
employing the method by Margulis and Mason [15,16 ]. The energy emitted into the solution phase at 100% output power was determined to be 13.2 W. Based on the area of the transducer of 9.6 cm2, this corresponds to an ultrasound intensity of 1.4 W cm−2. Electrical insulation of the stainless steel transducer exposed to the solution phase was possible by covering the floor of the reactor with lamination foil (250 mm, Muromail Ltd., Weston-super-Mare, UK ) but in most experiments no electrical interference from the transducer was observed and no modification of the reactor was required. For electrochemical experiments, counter and reference electrodes were placed outside close to the reactor wall to minimise the effect of these on the pattern of sound waves in the cell. The visualisation of the flow pattern in the reactor was possible with a suspension of Mearlmaid OL ( The Mearl Cooperation, 41 East 42nd street, New York NY 10017). A degassing procedure, devised to reduce the gas content of the aqueous solution before transfer into the sonoreactor, consisted of connecting a vacuum pump system to a flask with a submerged capillary gas inlet. Argon (Pureshield, BOC ) was allowed to bubble through the solution at a pressure of approximately 10 Torr for at least 10 min. 2.3. Electrode modification Films of Prussian blue were grown electrochemically [17] by immersion of a freshly polished Au disc electrode
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in a solution containing 1 mM Fe3+ and 1 mM Fe(CN )3− in aqueous 1 M KCl and cycling the electrode 6 potential between 0.0 and 0.8 V vs. SCE over 20 cycles with a scan rate of 0.1 V s−1. After removal from the modified electrode and rinsing in pure water, sonoelectrochemical experiments were conducted in aqueous 1 M KCl.
3. Results and discussion 3.1. Mass transport effects in the presence of 500 kHz ultrasound At high ultrasound frequencies the physical nature of effects at the electrode–solution interface is considerably different to those observed at 20 kHz and some of the benefits observed in sonoelectrochemistry at lower frequencies change dramatically. The strong macroscopic acoustic streaming effect observed with a 20 kHz horn probe of 60 W cm−2 power is absent predominantly due to the lower intensity, 1.4 W cm−2, and different geometry used in the 500 kHz system (see Fig. 1). An experiment in which an aqueous suspension of Mearlmaid OL is used to visualise the flow pattern in the liquid phase at 1.4 W cm−2 ultrasound shows that a macroscopic torroidal and turbulent flow of liquid away from the transducer in the centre of the reactor and back at the walls of the reactor is present. However, the flow rate is rather low, being of the order of 5 cm s−1 and therefore is only a minor factor in contributing to mass transport effects. Because of the much shorter wavelength at 500 kHz compared to 20 kHz ultrasound, the sound propagation is affected by reflection and interference phenomena which give rise to a characteristic patttern of sonochemical activity. Recently Trabelsi and co-workers have shown [10] that a ring-shaped zone of activity with approximately half the cell diameter exists with 560 kHz ultrasound, detected by the chemiluminescence of luminol which is induced by the cavitation process. This chemiluminescence phenomenon indicating a ring shaped reaction zone was also observed in the 500 kHz reactor used in this study. In comparison to systems employing low frequency ultrasound, 500 kHz power ultrasound is known to give better efficiency in sonochemical processes due to the number and time scale of cavitation events which, according to Petrier and co-workers [18], reaches an optimum around 0.5 MHz. This has been exploited, for example, for processes in which organic waste products are mineralised sonochemically. The macroscopic flow of liquid in the 500 kHz reactor shown in Fig. 1 is much less vigorous compared to that observed at 20 kHz horn probes. The absence of strong convective flow results in the formation of gas bubbles
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at the electrode surface which, from a gas saturated solution phase, readily nucleate at the electrode surface and affect the flow of current in a very non-reproducible manner. Therefore a degassing procedure was devised in which argon was passed through the solution phase at approximately 10 Torr vacuum (see Section 2). The lower partial pressure of gas in the solution phase not only considerably improved the reproducibility of results but also strongly affected the violence of cavitation events at the electrode–solution interface (vide infra). In Fig. 2 typical cyclic voltammetric responses are shown for the reduction of a solution of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 1 mm diameter 36 Pt disc electrode (mounted in glass) in the absence (a) and in the presence (b) of 500 kHz ultrasound. It can be seen that 500 kHz ultrasound has a considerable effect in enhancing the mass transport to the electrode although the effect is much smaller compared to the observation made at 20 kHz horn probes. In order to quantify the rate of mass transport, the effective diffusion layer thickness, d, may be calculated from Eq. (1), I
lim
=
nFDAc d
bulk .
(1)
In this expression the average limiting current detected at the electrode, I , is related to a stochiometlim ric index describing the number of electrons transferred per mole of reactant, n; the Faraday constant, F; the diffusion coefficient, D; the electrode area, A; and the concentration in the bulk liquid, c . The magnitude bulk of d calculated for the average limiting current shown in Fig. 2(b) is d=11 mm. The ‘noise’ superimposed on the sonovoltammetric current is characteristic for fluctuations in the mass transport induced by phenomena such as cavitation or turbulent flow. The positioning of the electrode in the reactor can be shown, in agreement with data from Trabelsi and co-workers [10], to be a crucial factor in determining the mass transport effect. The profile of the average
Fig. 2. Cyclic voltammograms observed for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 1 mm Pt disc electrode 36 mounted in glass: (a) silent conditions; and (b) with 1.4 W cm−2 500 kHz ultrasound applied and a scan rate of 100 mV s−1.
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mass transport controlled limiting current observed at a 1 mm diameter Pt disc electrode is shown in Fig. 3 as a function of immersion depth and horizontal position. Data from a total of 35 points was collected as the average of at least four voltammograms. It was found that it was not the centre of the cell where the current enhancement is most significant. With an electrode mounted in glass, the intensity pattern attributed to high cavitational activity which peaks in a ring-shaped pattern approximately half way between the centre and the reactor walls is reproduced (Fig. 3(a)). This clearly suggests that cavitation is an important factor in inducing mass transport at the electrode solution interface. If macroscopic acoustic streaming is important then a
maximum effect would be anticipated in the centre of the reactor. An important observation was that of the effect of the type of the electrode mounting material. Data shown in Fig. 3(b) has been recorded under identical conditions compared to those in Fig. 3(a), except that a 1 mm Pt disc electrode mounted in Teflon was used. In this case the limiting currents observed are substantially higher and the pattern observed in the reactor is totally different. In order to further study the effect responsible for this dramatic difference observed in limiting currents, the temperature at the 1 mm diameter Pt disc electrodes was monitored by an in-situ potentiometric method. The equilibrium potential of the Fe(CN )3−/4− redox 6 couple is known to be very sensitive to temperature changes due to entropy effects [19]. A calibration plot measured in a non-isothermal cell containing 4 mM Fe(CN )3− and 4 mM Fe(CN )4− in aqueous 1 M KCl 6 6 is shown in Fig. 4(a). The equilibrium potential drift of −1.52 mV K−1 is in good agreement with literature data [20]. Then the effect of 500 kHz ultrasound on the equilibrium potential of the Teflon mounted 1 mm diameter Pt disc electrode placed in the centre of the sonoreac-
(a)
(a)
(b) (b)
Fig. 3. 3-dimensional plots of the limiting current observed for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 1 mm 36 diameter Pt disc electrode: (a) mounted in glass; and (b) mounted in Teflon as a function of horizontal and vertical positioning in the sonoelectrochemical reactor.
Fig. 4. (a) Plot of the equilibrium potential measured potentiometrically at a 1 mm diameter Pt disc electrode in a solution containing 4 mM Fe(CN )3−, 4 mM Fe(CN )4−, and 1 M KCl. (b) Temperature 6 6 drift observed at a Teflon mounted 1 mm diameter Pt disc electrode immersed in a solution containing 4 mM Fe(CN )3−, 4 mM 6 Fe(CN )4−, and 1 M KCl in a 500 kHz ultrasound reactor. The periods 6 in which 1.4 W cm−2 ultrasound was switched off and on are indicated.
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tor was measured and the temperature versus time plot is shown in Fig. 4(b). A surprisingly big temperature change occurs at the electrode surface which stabilises over a period of approximately 120 s reaching a temperature of 340 K. At the same time the temperature of the bulk solution undergoes no significant temperature change. This effect is not observed at 20 kHz ultrasound nor at 500 kHz ultrasound with a glass mounted Pt electrode. However, a Teflon rod with a thermocouple inserted in the centre also gives a similar temperature effect so that the absorption of sound waves in the Teflon is shown to be responsible for the dramatic change in electrochemical activity. In conclusion, temperature effects can be significant at high frequency ultrasound and have to be taken into account. The effect of temperature on the diffusion coefficient, D, is usually expressed in terms of an Arrhenius-type law [21] which allows the magnitude of the change in limiting current for a given change in temperature to be calculated based on Eq. (2),
G H G C
E D x lim,1 = 1 = exp − A I D R lim,2 2
I
A
1 T 1
−
1 T 2
BDH
x
.
(2)
In this expression D and D correspond to the 1 2 diffusion coefficients at T and T , and the exponent 1 2 x=1 for I 3 D (see Eq. (1)). The activation energy, lim E =13.6 kJ mol−1, for the diffusion process of A Ru(NH )3+ in aqueous 0.1 M KCl has been determined 36 independently and the limiting current is predicted by Eq. (2) to increase by a factor of 2.3 for going from T =290 K to T =340 K. The experimentally observed 1 2 effect is approximately a factor of four which leads to the conclusion that the mass transport mechanism itself is also affected by the temperature change, presumably due to a change in viscosity. It has been shown above that cavitation events govern the mass transport at the electrode solution interface and the number of cavitation events is known to increase with temperature [22]. The results reported below concerning the exponent, x#0.24, allow the expected increase in limiting current by a factor of only 1.2 to be calculated using Eq. (2) and this further amplifies the conclusion reached here. The magnitude of the average limiting currents observed at a glass mounted Pt disc electrode is proportional to the concentration of reactant, c , and to the bulk area of the electrode, A, for a range of electrode diameters from 0.25 to 6.0 mm and in full agreement with the expression given in Eq (1). However, the effect of the diffusion coefficient, D, on the observed limiting current has been shown in previous work [23,24] to reveal the subtleties of the nature of mass transport observed. A linear relationship, I 3D, is an indication lim of a truly stagnant diffusion layer and in the presence of 20 kHz always I 3D2/3 has been observed in aquelim ous media in agreement with hydrodynamic flow or the
193
‘streaming case’. In Fig. 5 data for the experimentally observed diffusion layer thickness, d, as a function of diffusion coefficient, D, is shown for five aqueous redox couples: (a) the reduction of Fe3+, (b) the reduction of Fe(CN )3−, (c) the reduction of Ru(NH )3+, (d) the 6 36 oxidation of I −, and (e) the reduction of H+. Data for the diffusion coefficients and experimental conditions are summarised in Table 1. The plot in Fig. 5, which contains data averaged over 4–5 experiments and for several concentrations of redox reagent, reveals that the effect of the diffusion coefficient on the limiting current is much smaller than anticipated on the basis of existing mass transport models for ultrasonically enhanced processes. The plot suggests that, approximately, d 3D0.76 and therefore I 3D0.24 and, although slightly different lim results were obtained at higher temperature or at different positions in the reactor there is always a significant trend to very low exponents, corresponding to processes other than diffusion dominating the mass transport. A simplistic model, which allows the observation to be rationalised, has to account for a mechanism other than diffusion being important in the mass transport process. If at a rate of 500 kHz microjetting or micromixing phenomena occur at the electrode–solution interface, then the time scale for diffusion becomes short and the distance which molecules can travel, d , diff becomes short (Eq. (3)) d
diff
=
S
D 500 kHz
#0.05 mm.
(3)
Therefore at high ultrasound frequencies and at electrodes of slow time constant ( large diameter), transport due to mixing on a microscopic scale may effectively compete with transport due to diffusion. The rate constants for the two limiting cases of mass transport are k for the stagnant diffusion layer, and k for the 1 2
Fig. 5. Plot of the calculated diffusion layer thickness, d, measured for the redox couples: (a) Fe3+/2+; (b) Fe(CN )3−/4−; (c) 6 Ru(NH )3+/2+; (d ) I −/I ; and (e) H+/H versus the diffusion coeffi36 2 2 cient D (see Table 1).
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Table 1 Data for diffusion coefficients and experimental conditions used for the determination of the effect of the diffusion coefficient on the observed average mass transport controlled limiting current in the presence of 1.4 W cm−2 500 kHz ultrasound. Degassed solutions at 20±2°C and a 1 mm diameter Pt disc electrode mounted in glass were used Redox system
Conditions
Fe3+/2+ Fe(CN )3−/4− 6 Ru(NH )3+/2+ 36 I −/I 2 H+/H 2
1–5 mM 1–5 mM 1–5 mM 1–5 mM 1–5 mM
in in in in in
1 M H SO 2 4 1 M KCl 0.1 M KCl 0.125 M H SO 2 4 0.1 M Na SO 2 4
transport purely by jetting or micromixing, n V D k = and k = jet jet . 2 1 d A
(4)
In these expressions n denotes the number of jets jet per second hitting the electrode surface A and V is the jet volume replaced by a single jet. Combining these expressions for the case of competing pathways of mass transport gives Eq. (5), I
lim
=nFAc
A
B
D n V + jet jet . bulk d A
(5)
This expression may be used to fit the data shown in Fig. 5 and to explain the small effect of the diffusion coefficient on the limiting current. The line fitted through the data points in Fig. 5 may be represented by the following expression, I
D lim = +2.8×10−5 m s−1. (6) nFAc 227 mm bulk However, little is currently known about the details of the mechanism for mass transport and for the parameters n and V at high ultrasound frequencies, and jet jet meaningful comparison and interpretation is not warranted at the present stage. Both the size and the time scale of cavitation events at high ultrasound frequencies are different from the processes at lower ultrasound frequencies and this switches the mode of mass transport from diffusion to a micromixing model. It is interesting to note that for the mode of transport dominated by the microjetting, k , changes in the pathway of chemical processes could 2 occur depending on how the mixing process perturbs the concentration gradient at the electrode–solution interface. 3.2. Surface effects induced by 500 kHz ultrasound It is well documented that cavitation near phase boundaries results in an asymmetric collapse followed by the formation of a strong liquid jet [27]. At solid– liquid interfaces the resulting process is commonly referred to as cavitation erosion and has been extensively
Diffusion coefficient D / m2 s−1
Reference
0.30×10−9 0.76×10−9 0.91×10−9 2.0×10−9 10.0×10-9
[25] [25] [26 ] [25] [25]
studied for sonoelectrochemical applications at lower frequencies [28]. Although the mass transport has been shown to be dominated by cavitation and micro-streaming processes for the 500 kHz ultrasound reactor. there appears to be little damage to conventional electrodes made of platinum, gold or carbon even after prolonged periods of sonication. This can be an advantage if mechanically less robust electrode materials such as lead, graphite or metal oxide have to be used. In Fig. 6 a gold surface freshly polished (a) and after a period of 30 min sonication (b) in the high intensity region of the 500 kHz reactor are shown. The erosion process can be seen to result in only subtle effects indicated by the loss of sharp edges of the scratch lines. However, the surface effects of the cavitation process is significant as may be shown for a film of Prussian blue deposited electrochemically onto the gold electrode surface. Prussian blue [29] is one of a large family of electro-active materials with zeolitic structure which have been used in the form of films deposited on electrode surfaces for catalytic processes, e.g. the hydrogen peroxide [30] and hydrazine [31] detection. A typical voltammogram of a Prussian blue film grown electrochemically (see Section 2) on a 1 mm diameter Au disc electrode is shown in Fig. 7(a). Oxidation (P ) and reduction (P ) correspond to a ox red one electron conversion of the Fe(III/II ) redox couple bound in the lattice of the solid, accompanied by the expulsion or uptake of potassium ions from the solution phase. The growth results in films which, after removal from the solution, rinsing and immersion in aqueous 1 M KCl, give voltammetric responses stable over many potential cycles. This response is used to monitor the effect of 500 kHz ultrasound and results are shown in Fig. 7(a)–(c). It can clearly be seen that the film is affected. The diminishing area under the voltammetric signal observed after several minutes of sonication suggests that the film is gradually removed and the change in the shape of the voltammetric response suggests also that the rate of transport of ions into the film, or electrons out of the film, becomes slower. A plot of the change of the peak current, I red , for the reduction of p the Prussian blue film with sonication time suggests a continuous degradation of the film ( Fig. 8). A compari-
F. Javier Del Campo et al. / Ultrasonics Sonochemistry 6 (1999) 189–197
(a)
(b)
Fig. 6. AFM images of a polished gold electrode surface (a) before and (b) after 30 min treatment with 1.4 W cm−2 500 kHz ultrasound.
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Fig. 7. Cyclic voltammograms obtained for Prussian blue deposited onto a 1 mm diameter Au disc electrode and immersed in aqueous 1 M KCl (a) before and after: (b) 5 min; (c) 10 min treatment with 500 kHz ultrasound.
relatively small dependence of the mass transport controlled limiting current on the diffusion coefficient suggests that at even higher ultrasound frequencies electrochemical processes could become independent of the diffusion coefficient, thereby relatively favouring processes involving very slowly diffusing species such as proteins [23]. The implications of this kind of characteristics for analytical methodolgy has been noted in the literature [32]. On the basis of the results described, it may be concluded that: 1. cavitation processes are very important in 500 kHz sonoelectrochemistry but the observed currents may not be a simple measure of cavitational activity; 2. thermal and degassing effects are of importance as well as the reactor geometry and electrode materials; 3. the effects of the unusual mass transport characteristics in the presence of 500 kHz ultrasound on heterogeneous and homogeneous chemical processes may be significant and have yet to be explored. Acknowledgements
Fig. 8. Plot of the peak current for the reduction process, I red , observed p in cyclic voltammograms for Prussian blue deposited on a 1 mm Au disc electrode immersed in 1 M KCl. The effect of 1.4 W cm−2 500 kHz ultrasound in degassed and non-degassed solution are compared.
son of film degradation observed in the presence of 500 kHz ultrasound with and without degassing of the solution prior to the experiments reveals a dramatic increase of the cavitation effect after degassing. A plausible interpretation of this effect comes from the increase in violence of cavitation processes after degassing. Gas and gas bubbles present in the solution phase act to cushion the violence of the implosion process. In the degassed solution, the coloured Prussian blue film is completely removed from the electrode surface after approximately 15 min application of ultrasound.
4. Conclusions It has been shown that electrochemical processes in the presence of 500 kHz ultrasound are governed by processes considerably different to those which are important at lower frequencies. In particular, thermal effects and surface erosion due to cavitation, which may find applications in electrochemical processes, have been studied. The mass transport model, which has to be applied for high frequency ultrasound, is suggested to be best based on microjetting or micromixing. The
Financial support from the EPSRC (grant GR/ L81123) is gratefully acknowledged. Shelley Wilkins, Marco Fidel Suarez and Sarah Roberts are acknowledged for assistance in AFM and thermoelectrochemical measurements. F.M. thanks the Royal Society for a University Research Fellowship and New College (Oxford ) for a Stipendiary Lectureship.
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[24] F. Marken, R.G. Compton, J.E.H. Buston, M.G. Moloney, Electroanalysis 10 (1998) 1188. [25] R.N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969. [26 ] F. Marken, J.C. Eklund, R.G. Compton, J. Electroanal. Chem. 395 (1995) 335. [27] N.A. Madigan, C.R.S. Hagan, H.H. Zhang, L.A. Coury, Ultrasonics Sonochemistry 3 (1996) S239. [28] R.G. Compton, J.C. Eklund, S.D. Page, G.H.W. Sanders, J. Booth, J. Phys. Chem. 98 (1994) 12410. [29] K. Itaya, I. Uchida, V.D. Neff, Acc. Chem. Res. 19 (1986) 162. [30] A.A. Karyakin, E.E. Karyakina, L. Gorton, J. Electroanal. Chem. 456 (1998) 97. [31] U. Scharf, E.W. Grabner, Electrochim. Acta. 41 (1996) 233. [32] A.V. Sokirko, K.B. Oldham, J. Electroanal. Chem. 1430 (1997) 15.
High-frequency sonoelectrochemical processes: mass transport, thermal and surface effects induced by cavitation in a 500 kHz reactor F. Javier Del Campo a, B.A. Coles a, F. Marken a, R.G. Compton a, *, E. Cordemans b a Physical & Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, UK b Undatim Ultrasonics S.A., Zoning Industriel, Rue de l’Industrie 3, B-1400 Nivelles, Belgium Received 29 March 1999
Abstract The use of high frequency ultrasound in electrochemical systems is of major interest for the optimisation of electrosynthetic and electroanalytical procedures, especially when the strong mechanical effects of 20 kHz ultrasound are detrimental. The characterisation of a 500 kHz ultrasound reactor for sonoelectrochemical experiments by voltammetric and potentiometric measurements revealed the presence of considerable thermal, as well as mass transport, effects depending on geometric parameters and the material used for the construction of the working electrode. Micromixing and cavitation processes govern the mass transport to and from the electrode surface and are shown by atomic force microscopy (AFM ) to cause erosion on the electrode surface. Electrochemically active films of Prussian blue are shown to be gradually removed by cavitation erosion. Degassing the solution prior to sonication increases the efficiency of cavitation processes. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Catalyst; Cavitation; Frequency effect; Mass transport; Micromixing; Modified electrode; Prussian blue; Sonoelectrochemistry
1. Introduction Over the recent years a considerable amount of work has been published on the use of power ultrasound in electrochemical systems [1] with the aim of improving existing electroanalytical [2,3] and electrosynthetic [4– 7] techniques and further identifying new processes, in which activation of the electrode process in the presence of ultrasound )e.g. in emulsion environments [8]) allows the enhancement or suppression of processes in a selective manner. Most studies to date have employed horn-type ultrasound emitters operating in the frequency range of 20– 40 kHz due to their ease of implementation and use. In particular, the highly localised formation of a macroscopic liquid jet and a cloud of cavitation bubbles near to the horn tip allows the working electrode of the electrochemical cell to be positioned in the high intensity region with maximum effect on the electrochemical process [9]. The main effects observed in this type of sonoelectrochemical experiment are: (i) an extremely * Corresponding author. Tel.: +44-1865-275448; fax: +44-1865-275410. E-mail address: [email protected] (R.G. Compton)
high rate of mass transport beneficial to both analytical and synthetic electrochemical processes; (ii) a surface erosion process which may be attributed to both interfacial cavitation and mechanical damage of the electrode material caused by shock waves; (iii) rapid removal of gas bubbles and solid particles from the electrode surface; and (iv) in-situ formation of suspensions and emulsions. Although measurements of Faradaic currents based on the electrochemical oxidation or reduction of redox active solution species have been employed for the characterisation of high frequency sonochemical systems [10,11] there is very little known about the physical nature of the effects of high frequency ultrasound at the surface of the electrode in electrochemical systems. Faradaic currents in the presence of ultrasound are commonly used as a measure of sound adsorption induced mass transport which may be expressed conveniently in terms of the diffusion layer thickness, or the Sherwood number [10]. This parameter has been shown by Trabelsi and co-workers [10], in the case of a 560 kHz ultrasound reactor, to correlate with the cavitation activity and the cavitation induced emission of light from the luminol chemiluminescence process. High frequency ultrasound has been studied by micro-
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electrode voltammetry [12] and used in electrosynthetic applications [13] and for modifying the properties and growth of polymer films formed electrochemically [14]. Although interesting effects have been observed, there has been little attention to why they occur and what the physical nature of the effects is. In this work, a first step is taken to further investigate the key factors dominating electrochemical and interfacial chemical reactivity in aqueous media in the presence of 500 kHz ultrasound. It is shown that sonoelectrochemistry at high frequency is based on distinctively different processes compared to those which govern sonoelectrochemical processes at 20–40 kHz ultrasound and that processes involving slowly diffusing species are especially relatively enhanced. 2. Experimental 2.1. Reagents and instrumentation Chemical reagents used were Ru(NH ) Cl , 36 3 K Fe(CN ) , Fe (SO ) , H SO , HNO (all Aldrich), 3 6 2 43 2 4 3 NaOH, KCl, KI, K Fe(CN ) , Na SO (BDH, AnalaR) 4 6 2 4 obtained commercially and used without further purification. Electrolyte solutions were prepared from ultra high quality water of a resistivity not less than 18 MV cm obtained from an Elgastat water purification system (High Wycombe, Bucks, UK ). As working electrodes, different diameter Pt or Au disc electrodes mounted in a Teflon or glass holder were used. Electrodes were carefully polished prior to measurements using alumina lapping compounds (Aldrich or Microglass Instruments, Melbourne, Australia) of decreasing size down to 0.05 mm. A gold or platinum wire served as the counter electrode and a saturated calomel electrode (SCE ) (Radiometer Kopenhagen) was used as a reference electrode. For electrochemical measurements an Autolab PGSTAT 20 system (Eco Chemie, Netherlands) and an Oxford Electrodes Bipotentiostat were used. If not otherwise stated, experiments were conducted at 20±2°C. Analyses of the electrode surface by atomic force microscopy (AFM ) were carried out using a TopoMetrix TMX 2010 Discoverer system in contact mode (typically 3 Hz scan rate) with TopoMetrix standard AFM probes (no. 5200). 2.2. The sonochemical reactor A 500 kHz 25 W D-type reactor ( Undatim Ultrasonics S.A., Belgium) was used, fitted with a water cooled jacketed glass cell and air cooling of the transducer (see Fig. 1). The size of the reactor was 4.5 cm inner diameter with a 3.6 cm diameter stainless steel transducer and a fill height of 7.5 cm and was kept constant in all experiments. The power output of the ultrasound system was calibrated calorimetrically by
Fig. 1. Schematic drawing of the 500 kHz sonoelectrochemical cell.
employing the method by Margulis and Mason [15,16 ]. The energy emitted into the solution phase at 100% output power was determined to be 13.2 W. Based on the area of the transducer of 9.6 cm2, this corresponds to an ultrasound intensity of 1.4 W cm−2. Electrical insulation of the stainless steel transducer exposed to the solution phase was possible by covering the floor of the reactor with lamination foil (250 mm, Muromail Ltd., Weston-super-Mare, UK ) but in most experiments no electrical interference from the transducer was observed and no modification of the reactor was required. For electrochemical experiments, counter and reference electrodes were placed outside close to the reactor wall to minimise the effect of these on the pattern of sound waves in the cell. The visualisation of the flow pattern in the reactor was possible with a suspension of Mearlmaid OL ( The Mearl Cooperation, 41 East 42nd street, New York NY 10017). A degassing procedure, devised to reduce the gas content of the aqueous solution before transfer into the sonoreactor, consisted of connecting a vacuum pump system to a flask with a submerged capillary gas inlet. Argon (Pureshield, BOC ) was allowed to bubble through the solution at a pressure of approximately 10 Torr for at least 10 min. 2.3. Electrode modification Films of Prussian blue were grown electrochemically [17] by immersion of a freshly polished Au disc electrode
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in a solution containing 1 mM Fe3+ and 1 mM Fe(CN )3− in aqueous 1 M KCl and cycling the electrode 6 potential between 0.0 and 0.8 V vs. SCE over 20 cycles with a scan rate of 0.1 V s−1. After removal from the modified electrode and rinsing in pure water, sonoelectrochemical experiments were conducted in aqueous 1 M KCl.
3. Results and discussion 3.1. Mass transport effects in the presence of 500 kHz ultrasound At high ultrasound frequencies the physical nature of effects at the electrode–solution interface is considerably different to those observed at 20 kHz and some of the benefits observed in sonoelectrochemistry at lower frequencies change dramatically. The strong macroscopic acoustic streaming effect observed with a 20 kHz horn probe of 60 W cm−2 power is absent predominantly due to the lower intensity, 1.4 W cm−2, and different geometry used in the 500 kHz system (see Fig. 1). An experiment in which an aqueous suspension of Mearlmaid OL is used to visualise the flow pattern in the liquid phase at 1.4 W cm−2 ultrasound shows that a macroscopic torroidal and turbulent flow of liquid away from the transducer in the centre of the reactor and back at the walls of the reactor is present. However, the flow rate is rather low, being of the order of 5 cm s−1 and therefore is only a minor factor in contributing to mass transport effects. Because of the much shorter wavelength at 500 kHz compared to 20 kHz ultrasound, the sound propagation is affected by reflection and interference phenomena which give rise to a characteristic patttern of sonochemical activity. Recently Trabelsi and co-workers have shown [10] that a ring-shaped zone of activity with approximately half the cell diameter exists with 560 kHz ultrasound, detected by the chemiluminescence of luminol which is induced by the cavitation process. This chemiluminescence phenomenon indicating a ring shaped reaction zone was also observed in the 500 kHz reactor used in this study. In comparison to systems employing low frequency ultrasound, 500 kHz power ultrasound is known to give better efficiency in sonochemical processes due to the number and time scale of cavitation events which, according to Petrier and co-workers [18], reaches an optimum around 0.5 MHz. This has been exploited, for example, for processes in which organic waste products are mineralised sonochemically. The macroscopic flow of liquid in the 500 kHz reactor shown in Fig. 1 is much less vigorous compared to that observed at 20 kHz horn probes. The absence of strong convective flow results in the formation of gas bubbles
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at the electrode surface which, from a gas saturated solution phase, readily nucleate at the electrode surface and affect the flow of current in a very non-reproducible manner. Therefore a degassing procedure was devised in which argon was passed through the solution phase at approximately 10 Torr vacuum (see Section 2). The lower partial pressure of gas in the solution phase not only considerably improved the reproducibility of results but also strongly affected the violence of cavitation events at the electrode–solution interface (vide infra). In Fig. 2 typical cyclic voltammetric responses are shown for the reduction of a solution of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 1 mm diameter 36 Pt disc electrode (mounted in glass) in the absence (a) and in the presence (b) of 500 kHz ultrasound. It can be seen that 500 kHz ultrasound has a considerable effect in enhancing the mass transport to the electrode although the effect is much smaller compared to the observation made at 20 kHz horn probes. In order to quantify the rate of mass transport, the effective diffusion layer thickness, d, may be calculated from Eq. (1), I
lim
=
nFDAc d
bulk .
(1)
In this expression the average limiting current detected at the electrode, I , is related to a stochiometlim ric index describing the number of electrons transferred per mole of reactant, n; the Faraday constant, F; the diffusion coefficient, D; the electrode area, A; and the concentration in the bulk liquid, c . The magnitude bulk of d calculated for the average limiting current shown in Fig. 2(b) is d=11 mm. The ‘noise’ superimposed on the sonovoltammetric current is characteristic for fluctuations in the mass transport induced by phenomena such as cavitation or turbulent flow. The positioning of the electrode in the reactor can be shown, in agreement with data from Trabelsi and co-workers [10], to be a crucial factor in determining the mass transport effect. The profile of the average
Fig. 2. Cyclic voltammograms observed for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 1 mm Pt disc electrode 36 mounted in glass: (a) silent conditions; and (b) with 1.4 W cm−2 500 kHz ultrasound applied and a scan rate of 100 mV s−1.
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mass transport controlled limiting current observed at a 1 mm diameter Pt disc electrode is shown in Fig. 3 as a function of immersion depth and horizontal position. Data from a total of 35 points was collected as the average of at least four voltammograms. It was found that it was not the centre of the cell where the current enhancement is most significant. With an electrode mounted in glass, the intensity pattern attributed to high cavitational activity which peaks in a ring-shaped pattern approximately half way between the centre and the reactor walls is reproduced (Fig. 3(a)). This clearly suggests that cavitation is an important factor in inducing mass transport at the electrode solution interface. If macroscopic acoustic streaming is important then a
maximum effect would be anticipated in the centre of the reactor. An important observation was that of the effect of the type of the electrode mounting material. Data shown in Fig. 3(b) has been recorded under identical conditions compared to those in Fig. 3(a), except that a 1 mm Pt disc electrode mounted in Teflon was used. In this case the limiting currents observed are substantially higher and the pattern observed in the reactor is totally different. In order to further study the effect responsible for this dramatic difference observed in limiting currents, the temperature at the 1 mm diameter Pt disc electrodes was monitored by an in-situ potentiometric method. The equilibrium potential of the Fe(CN )3−/4− redox 6 couple is known to be very sensitive to temperature changes due to entropy effects [19]. A calibration plot measured in a non-isothermal cell containing 4 mM Fe(CN )3− and 4 mM Fe(CN )4− in aqueous 1 M KCl 6 6 is shown in Fig. 4(a). The equilibrium potential drift of −1.52 mV K−1 is in good agreement with literature data [20]. Then the effect of 500 kHz ultrasound on the equilibrium potential of the Teflon mounted 1 mm diameter Pt disc electrode placed in the centre of the sonoreac-
(a)
(a)
(b) (b)
Fig. 3. 3-dimensional plots of the limiting current observed for the reduction of 1 mM Ru(NH )3+ in aqueous 0.1 M KCl at a 1 mm 36 diameter Pt disc electrode: (a) mounted in glass; and (b) mounted in Teflon as a function of horizontal and vertical positioning in the sonoelectrochemical reactor.
Fig. 4. (a) Plot of the equilibrium potential measured potentiometrically at a 1 mm diameter Pt disc electrode in a solution containing 4 mM Fe(CN )3−, 4 mM Fe(CN )4−, and 1 M KCl. (b) Temperature 6 6 drift observed at a Teflon mounted 1 mm diameter Pt disc electrode immersed in a solution containing 4 mM Fe(CN )3−, 4 mM 6 Fe(CN )4−, and 1 M KCl in a 500 kHz ultrasound reactor. The periods 6 in which 1.4 W cm−2 ultrasound was switched off and on are indicated.
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tor was measured and the temperature versus time plot is shown in Fig. 4(b). A surprisingly big temperature change occurs at the electrode surface which stabilises over a period of approximately 120 s reaching a temperature of 340 K. At the same time the temperature of the bulk solution undergoes no significant temperature change. This effect is not observed at 20 kHz ultrasound nor at 500 kHz ultrasound with a glass mounted Pt electrode. However, a Teflon rod with a thermocouple inserted in the centre also gives a similar temperature effect so that the absorption of sound waves in the Teflon is shown to be responsible for the dramatic change in electrochemical activity. In conclusion, temperature effects can be significant at high frequency ultrasound and have to be taken into account. The effect of temperature on the diffusion coefficient, D, is usually expressed in terms of an Arrhenius-type law [21] which allows the magnitude of the change in limiting current for a given change in temperature to be calculated based on Eq. (2),
G H G C
E D x lim,1 = 1 = exp − A I D R lim,2 2
I
A
1 T 1
−
1 T 2
BDH
x
.
(2)
In this expression D and D correspond to the 1 2 diffusion coefficients at T and T , and the exponent 1 2 x=1 for I 3 D (see Eq. (1)). The activation energy, lim E =13.6 kJ mol−1, for the diffusion process of A Ru(NH )3+ in aqueous 0.1 M KCl has been determined 36 independently and the limiting current is predicted by Eq. (2) to increase by a factor of 2.3 for going from T =290 K to T =340 K. The experimentally observed 1 2 effect is approximately a factor of four which leads to the conclusion that the mass transport mechanism itself is also affected by the temperature change, presumably due to a change in viscosity. It has been shown above that cavitation events govern the mass transport at the electrode solution interface and the number of cavitation events is known to increase with temperature [22]. The results reported below concerning the exponent, x#0.24, allow the expected increase in limiting current by a factor of only 1.2 to be calculated using Eq. (2) and this further amplifies the conclusion reached here. The magnitude of the average limiting currents observed at a glass mounted Pt disc electrode is proportional to the concentration of reactant, c , and to the bulk area of the electrode, A, for a range of electrode diameters from 0.25 to 6.0 mm and in full agreement with the expression given in Eq (1). However, the effect of the diffusion coefficient, D, on the observed limiting current has been shown in previous work [23,24] to reveal the subtleties of the nature of mass transport observed. A linear relationship, I 3D, is an indication lim of a truly stagnant diffusion layer and in the presence of 20 kHz always I 3D2/3 has been observed in aquelim ous media in agreement with hydrodynamic flow or the
193
‘streaming case’. In Fig. 5 data for the experimentally observed diffusion layer thickness, d, as a function of diffusion coefficient, D, is shown for five aqueous redox couples: (a) the reduction of Fe3+, (b) the reduction of Fe(CN )3−, (c) the reduction of Ru(NH )3+, (d) the 6 36 oxidation of I −, and (e) the reduction of H+. Data for the diffusion coefficients and experimental conditions are summarised in Table 1. The plot in Fig. 5, which contains data averaged over 4–5 experiments and for several concentrations of redox reagent, reveals that the effect of the diffusion coefficient on the limiting current is much smaller than anticipated on the basis of existing mass transport models for ultrasonically enhanced processes. The plot suggests that, approximately, d 3D0.76 and therefore I 3D0.24 and, although slightly different lim results were obtained at higher temperature or at different positions in the reactor there is always a significant trend to very low exponents, corresponding to processes other than diffusion dominating the mass transport. A simplistic model, which allows the observation to be rationalised, has to account for a mechanism other than diffusion being important in the mass transport process. If at a rate of 500 kHz microjetting or micromixing phenomena occur at the electrode–solution interface, then the time scale for diffusion becomes short and the distance which molecules can travel, d , diff becomes short (Eq. (3)) d
diff
=
S
D 500 kHz
#0.05 mm.
(3)
Therefore at high ultrasound frequencies and at electrodes of slow time constant ( large diameter), transport due to mixing on a microscopic scale may effectively compete with transport due to diffusion. The rate constants for the two limiting cases of mass transport are k for the stagnant diffusion layer, and k for the 1 2
Fig. 5. Plot of the calculated diffusion layer thickness, d, measured for the redox couples: (a) Fe3+/2+; (b) Fe(CN )3−/4−; (c) 6 Ru(NH )3+/2+; (d ) I −/I ; and (e) H+/H versus the diffusion coeffi36 2 2 cient D (see Table 1).
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Table 1 Data for diffusion coefficients and experimental conditions used for the determination of the effect of the diffusion coefficient on the observed average mass transport controlled limiting current in the presence of 1.4 W cm−2 500 kHz ultrasound. Degassed solutions at 20±2°C and a 1 mm diameter Pt disc electrode mounted in glass were used Redox system
Conditions
Fe3+/2+ Fe(CN )3−/4− 6 Ru(NH )3+/2+ 36 I −/I 2 H+/H 2
1–5 mM 1–5 mM 1–5 mM 1–5 mM 1–5 mM
in in in in in
1 M H SO 2 4 1 M KCl 0.1 M KCl 0.125 M H SO 2 4 0.1 M Na SO 2 4
transport purely by jetting or micromixing, n V D k = and k = jet jet . 2 1 d A
(4)
In these expressions n denotes the number of jets jet per second hitting the electrode surface A and V is the jet volume replaced by a single jet. Combining these expressions for the case of competing pathways of mass transport gives Eq. (5), I
lim
=nFAc
A
B
D n V + jet jet . bulk d A
(5)
This expression may be used to fit the data shown in Fig. 5 and to explain the small effect of the diffusion coefficient on the limiting current. The line fitted through the data points in Fig. 5 may be represented by the following expression, I
D lim = +2.8×10−5 m s−1. (6) nFAc 227 mm bulk However, little is currently known about the details of the mechanism for mass transport and for the parameters n and V at high ultrasound frequencies, and jet jet meaningful comparison and interpretation is not warranted at the present stage. Both the size and the time scale of cavitation events at high ultrasound frequencies are different from the processes at lower ultrasound frequencies and this switches the mode of mass transport from diffusion to a micromixing model. It is interesting to note that for the mode of transport dominated by the microjetting, k , changes in the pathway of chemical processes could 2 occur depending on how the mixing process perturbs the concentration gradient at the electrode–solution interface. 3.2. Surface effects induced by 500 kHz ultrasound It is well documented that cavitation near phase boundaries results in an asymmetric collapse followed by the formation of a strong liquid jet [27]. At solid– liquid interfaces the resulting process is commonly referred to as cavitation erosion and has been extensively
Diffusion coefficient D / m2 s−1
Reference
0.30×10−9 0.76×10−9 0.91×10−9 2.0×10−9 10.0×10-9
[25] [25] [26 ] [25] [25]
studied for sonoelectrochemical applications at lower frequencies [28]. Although the mass transport has been shown to be dominated by cavitation and micro-streaming processes for the 500 kHz ultrasound reactor. there appears to be little damage to conventional electrodes made of platinum, gold or carbon even after prolonged periods of sonication. This can be an advantage if mechanically less robust electrode materials such as lead, graphite or metal oxide have to be used. In Fig. 6 a gold surface freshly polished (a) and after a period of 30 min sonication (b) in the high intensity region of the 500 kHz reactor are shown. The erosion process can be seen to result in only subtle effects indicated by the loss of sharp edges of the scratch lines. However, the surface effects of the cavitation process is significant as may be shown for a film of Prussian blue deposited electrochemically onto the gold electrode surface. Prussian blue [29] is one of a large family of electro-active materials with zeolitic structure which have been used in the form of films deposited on electrode surfaces for catalytic processes, e.g. the hydrogen peroxide [30] and hydrazine [31] detection. A typical voltammogram of a Prussian blue film grown electrochemically (see Section 2) on a 1 mm diameter Au disc electrode is shown in Fig. 7(a). Oxidation (P ) and reduction (P ) correspond to a ox red one electron conversion of the Fe(III/II ) redox couple bound in the lattice of the solid, accompanied by the expulsion or uptake of potassium ions from the solution phase. The growth results in films which, after removal from the solution, rinsing and immersion in aqueous 1 M KCl, give voltammetric responses stable over many potential cycles. This response is used to monitor the effect of 500 kHz ultrasound and results are shown in Fig. 7(a)–(c). It can clearly be seen that the film is affected. The diminishing area under the voltammetric signal observed after several minutes of sonication suggests that the film is gradually removed and the change in the shape of the voltammetric response suggests also that the rate of transport of ions into the film, or electrons out of the film, becomes slower. A plot of the change of the peak current, I red , for the reduction of p the Prussian blue film with sonication time suggests a continuous degradation of the film ( Fig. 8). A compari-
F. Javier Del Campo et al. / Ultrasonics Sonochemistry 6 (1999) 189–197
(a)
(b)
Fig. 6. AFM images of a polished gold electrode surface (a) before and (b) after 30 min treatment with 1.4 W cm−2 500 kHz ultrasound.
195
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Fig. 7. Cyclic voltammograms obtained for Prussian blue deposited onto a 1 mm diameter Au disc electrode and immersed in aqueous 1 M KCl (a) before and after: (b) 5 min; (c) 10 min treatment with 500 kHz ultrasound.
relatively small dependence of the mass transport controlled limiting current on the diffusion coefficient suggests that at even higher ultrasound frequencies electrochemical processes could become independent of the diffusion coefficient, thereby relatively favouring processes involving very slowly diffusing species such as proteins [23]. The implications of this kind of characteristics for analytical methodolgy has been noted in the literature [32]. On the basis of the results described, it may be concluded that: 1. cavitation processes are very important in 500 kHz sonoelectrochemistry but the observed currents may not be a simple measure of cavitational activity; 2. thermal and degassing effects are of importance as well as the reactor geometry and electrode materials; 3. the effects of the unusual mass transport characteristics in the presence of 500 kHz ultrasound on heterogeneous and homogeneous chemical processes may be significant and have yet to be explored. Acknowledgements
Fig. 8. Plot of the peak current for the reduction process, I red , observed p in cyclic voltammograms for Prussian blue deposited on a 1 mm Au disc electrode immersed in 1 M KCl. The effect of 1.4 W cm−2 500 kHz ultrasound in degassed and non-degassed solution are compared.
son of film degradation observed in the presence of 500 kHz ultrasound with and without degassing of the solution prior to the experiments reveals a dramatic increase of the cavitation effect after degassing. A plausible interpretation of this effect comes from the increase in violence of cavitation processes after degassing. Gas and gas bubbles present in the solution phase act to cushion the violence of the implosion process. In the degassed solution, the coloured Prussian blue film is completely removed from the electrode surface after approximately 15 min application of ultrasound.
4. Conclusions It has been shown that electrochemical processes in the presence of 500 kHz ultrasound are governed by processes considerably different to those which are important at lower frequencies. In particular, thermal effects and surface erosion due to cavitation, which may find applications in electrochemical processes, have been studied. The mass transport model, which has to be applied for high frequency ultrasound, is suggested to be best based on microjetting or micromixing. The
Financial support from the EPSRC (grant GR/ L81123) is gratefully acknowledged. Shelley Wilkins, Marco Fidel Suarez and Sarah Roberts are acknowledged for assistance in AFM and thermoelectrochemical measurements. F.M. thanks the Royal Society for a University Research Fellowship and New College (Oxford ) for a Stipendiary Lectureship.
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[24] F. Marken, R.G. Compton, J.E.H. Buston, M.G. Moloney, Electroanalysis 10 (1998) 1188. [25] R.N. Adams, Electrochemistry at Solid Electrodes, Marcel Dekker, New York, 1969. [26 ] F. Marken, J.C. Eklund, R.G. Compton, J. Electroanal. Chem. 395 (1995) 335. [27] N.A. Madigan, C.R.S. Hagan, H.H. Zhang, L.A. Coury, Ultrasonics Sonochemistry 3 (1996) S239. [28] R.G. Compton, J.C. Eklund, S.D. Page, G.H.W. Sanders, J. Booth, J. Phys. Chem. 98 (1994) 12410. [29] K. Itaya, I. Uchida, V.D. Neff, Acc. Chem. Res. 19 (1986) 162. [30] A.A. Karyakin, E.E. Karyakina, L. Gorton, J. Electroanal. Chem. 456 (1998) 97. [31] U. Scharf, E.W. Grabner, Electrochim. Acta. 41 (1996) 233. [32] A.V. Sokirko, K.B. Oldham, J. Electroanal. Chem. 1430 (1997) 15.