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Low-temperature sonoelectrochemical processes
www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 477 (1999) 71 – 78
Low-temperature sonoelectrochemical processes Part 1. Mass transport and cavitation effects of 20 kHz ultrasound in liquid ammonia F. Javier Del Campo, Andreas Neudeck, Richard G. Compton, Frank Marken * Physical and Theoretical Chemistry Laboratory, Oxford Uni6ersity, South Parks Road, Oxford OX1 3QZ, UK Received 3 August 1999; received in revised form 17 September 1999; accepted 17 September 1999
Abstract Sonoelectrochemical processes in liquid ammonia in a temperature range between − 70 and −35°C in the presence of 20 kHz power ultrasound are studied with the aim of improving low temperature electrosynthetic procedures. The one and two electron reductions of nitrobenzene and para-chloronitrobenzene are investigated as model systems. Placing an immersed ultrasonic horn emitter ‘face-on’ to a platinum disc electrode in liquid ammonia is shown to result in extreme mass transport enhancements with a resulting diffusion layer thickness of approximately d= 2 mm. This limit of the diffusion layer thickness is shown to be essentially temperature independent and correspondingly, the highest limiting currents can be observed near the boiling point of liquid ammonia. Cavitation processes are detected even at − 70°C and result in a considerable fluctuation in the observed mass transport controlled limiting current. Further, the deposition of ionic products formed in the second reduction step for both nitrobenzene and para-chloronitrobenzene reduction and the associated drop in current, can be shown to be affected by sonication. Ultrasound has been found to be beneficial by (i) causing extremely fast mass transport; (ii) enhancing the mixing and dissolution kinetics at low temperature; and (iii) affecting the formation of solid products at the electrode surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Ultrasound; Cavitation; Mass transport; Sonoelectrochemistry; Low temperature electrochemistry; Voltammetry; Liquid ammonia; Nitrobenzene
1. Introduction Liquid ammonia is a very interesting solvent for chemical and electrochemical processes at low temperatures and under mild conditions. The characteristic properties of liquid ammonia are a relatively low viscosity, high dielectric constant, good cation solvation properties, and the ability to produce stable solutions of solvated electrons [1,2]. Problems in the use of liquid ammonia as a low temperature reaction environment may arise due to the slow heterogeneous kinetics associated with low temperature dissolution processes, e.g. of the starting material, and the formation of ionic precipitates on the electrode surface. Introducing a source of power ultrasound in the electrochemical cell may in * Corresponding author. Tel.: +44-1865-275443; fax: +44-1865275410. E-mail address: [email protected] (F. Marken)
principle solve both of these problems as well as improve the efficiency of the process. The use of power ultrasound in electrochemistry, or sonoelectrochemistry (for modern reviews see Refs. [3,4]), was proposed in pioneering work by Moriguchi [5] and further developed by Yeager in the 1960’s. Only over recent years have the fundamentals and the potential of combining high intensity ultrasound horn systems with electrochemical processes been explored in more detail. Both the development of novel electroanalytical and electrosynthetic procedures have been proposed and the processes involved studied quantitatively. In the present study experimental evidence is given for the beneficial effects of 20 kHz power ultrasound in a low temperature electrochemical system and a model for the ultrasound enhanced mass transport at various temperatures is sought. The reduction of nitrobenzene and para-chloronitrobenzene have been chosen as
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model systems and a comparison with earlier studies on the electrochemical properties and reactivity of these compounds is made. The mass transport dependent formation of non-conducting and blocking deposits is considered in detail.
2. Experimental
2.1. Reagents and instrumentation Chemical reagents used were KI (AnalaR, 99.8%), lithium wire (Aldrich, 99.9%), nitrobenzene (Aldrich,
99 + %), 1-chloro-4-nitrobenzene (Aldrich, 99%), and ammonia (BOC). Argon (Pureshield, BOC) was used to maintain a dry and inert atmosphere. In electrochemical experiments 1 mm diameter Pt disc electrodes mounted in a glass holder (lime-soda) were used as the working electrode. Teflon was avoided as an electrode shrouding material due to its large coefficient of thermal expansion, which caused the Pt electrode to protrude. Electrodes were polished carefully prior to measurements using alumina lapping compounds (BDH or Microglass Instruments, Greensborough, Victoria 3088, Australia) of decreasing size down to 0.05 mm. A gold wire served as the counter electrode and a silver wire (Aldrich, 99.9% 0.5 mm diameter) was used as a pseudo-reference electrode. For electrochemical measurements an Autolab PGSTAT 20 system (Eco Chemie, Netherlands) was used. The three electrode electrochemical cell (see Fig. 1) allowed experiments to be conducted at temperatures ranging from − 70 to − 359 1°C. An ultrasound horn transducer system (Sonics & Materials, VCX400) with a stepped 3 mm diameter microtip horn (titanium alloy) was employed. The horn probe was electrically insulated and positioned ‘face-on’ [6] towards the working electrode with a distance kept constant at 10 mm.
2.2. Experimental procedure
Fig. 1. Schematic drawing of the experimental sonoelectrochemical cell. Gas in- and out-let are used for transferring ammonia and for keeping the system under an inert atmosphere of argon. The horn was sealed into the glass joint with Parafilm.
Fig. 2. Plot of the ultrasound intensity emitted from the horn probe versus the amplitude setting used for water at room temperature (2) and for liquid ammonia at − 35°C (). The ultrasound intensity was determined from the power input of the transducer system (see Section 2).
Electrochemical experiments were carried out under dry conditions in an inert atmosphere of argon. The supporting electrolyte, KI, was pre-treated by grinding it into a fine powder and drying in an oven at 130 – 140°C overnight. The electrodes were kept in a dessicator. The ammonia was dried by first condensing it into a flask containing pieces of lithium. Then 509 2 cm3 of it was distilled under argon into the electrochemical cell containing the supporting electrolyte, KI. The time consuming process of dissolving the reagents in liquid ammonia at − 70°C was considerably enhanced by sonication. For the ultrasound intensity calibration in liquid ammonia conventional calorimetric methods [7,8] are not applicable and a method based on the measurement of the electrical power consumed by the transducer was developed. The power consumed by the transducer at various ultrasound intensity settings was monitored. Assuming that (i) the energy emitted in air is negligible compared to that emitted in a solvent and (ii) the energy lost due to heat dissipation in the transducer, including all electronic components, is constant when a properly tuned tip is used, the power dissipation in the solvent can be measured from the difference of the total consumed power in air and in the solvent under study. A linear dependence of the power emitted
F.J. Del Campo et al. / Journal of Electroanalytical Chemistry 477 (1999) 71–78 Table 1 Ultrasound intensity determined as energy dissipated in liquid ammonia from an immersed 3 mm diameter horn tip with an amplitude fixed at 20% as a function of temperature (see Section 2) T/K
Ultrasound intensity/W cm−2
208 213 218 223 228 233
500 9 100 430 9 90 370 9 70 3009 60 230 9 50 1709 30
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3. Results and discussion
3.1. The effect of 20 kHz ultrasound on the 6oltammetry in liquid ammonia The electrochemical reduction of nitrobenzene has been studied extremely well in various media [11–13] and is of interest both as a model system and for the electrochemical synthesis of aniline [14]. In aprotic media the reduction process is characterised by two consecutive one electron reduction steps. In the presence of protons the reduction may follow various pathways and the six electron reduction to aniline as well as the formation of more complex dimeric products has been reported [15]. In liquid ammonia, and under anhydrous conditions, nitrobenzene undergoes two reversible one electron reduction processes [13]. In the first reduction step the radical mono-anion is formed and the product of the second reduction step is the dianion (Eq. 1).
(1)
Fig. 3. Cyclic and sono-voltammograms for the reduction of 0.5 mM nitrobenzene in liquid ammonia + 0.1 M KI at − 70°C at a 1 mm diameter Pt disc electrode in the absence (a) and in the presence (b – e) of 20 kHz ultrasound (scan rate 0.2 V s − 1, ultrasound intensities b: 70, c: 140, d: 220, e: 300 W cm − 2, 10 mm electrode to horn distance).
into the liquid versus the amplitude (amplitude setting of the instrument) used was found (Fig. 2). The data obtained for the energy emitted into water are in good agreement with earlier calorimetric measurements [9]. The electrically insulated horn system used for the electrochemical experiments are difficult to tune and an offset at minimum power level (not shown) is attributed to the energy lost or dissipated internally due to a mismatch in the tuning. The variation of the energy emitted from the horn probe in water at room temperature and in liquid ammonia at −35°C is shown in Fig. 2. The energy emitted into the liquid ammonia is considerably lower probably due to, in part, the low viscosity and the formation of bubbles. A similar effect has been noted previously in dichloromethane solutions [10]. The power dissipated in liquid ammonia at a given ultrasound amplitude increases as the temperature is lowered. An approximately linear relationship between the power dissipated and temperature exists with liquid ammonia at −60°C adsorbing ultrasound energy to a similar extent compared to water at room temperature (see Table 1).
One of the main advantages of liquid ammonia compared to other solvent systems is the relatively long lifetime, which can be observed for radical ions or other high-energy intermediate species. This unusual stability may be attributed to the low working temperatures and the stabilisation of charged intermediates [16]. A cyclic voltammogram obtained at a 1 mm diameter Pt disc electrode for the first reduction process for 0.5 mM nitrobenzene dissolved in liquid ammonia (0.1 M KI) at − 70°C is shown in Fig. 3a. A well-defined voltammetric response at E1/2 = − 0.32 V versus Ag consistent with literature reports [17,18] is detected. In the presence of ultrasound emitted from a horn probe at a 10 mm distance from the working electrode a considerable increase in current can be detected (Fig. 3b–d). The observed effect is dependent on the applied ultrasound intensity and in general is of the same order of magnitude compared to those observed in other solvents at room temperature [10]. Superimposed on the mass transport controlled limiting current is a current fluctuation which is believed to be dominated by interfacial cavitation events. In agreement with earlier studies [19] with increasing ultrasound intensity current spikes are found to increase associated with the cavitation process becoming more violent. The reduction of para-chloronitrobenzene at − 70°C in liquid ammonia is known [20] to follow a similar reaction path in the absence of protons (Eq. 2).
(2)
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However, the dianionic product has been shown to be kinetically labile and even at very low temperature a slow cleavage of the carbonchloride bond followed by protonation of the carbanion has been shown to occur [20]. The cyclic voltammetric study of the first reduction process for 0.5 mM para-chloronitrobenzene in liquid ammonia (0.1 M KI) at − 70°C gave a well-defined reversible response at E1/2 = − 0.21 V versus Ag. The characteristics of the sonovoltammetric curves (not shown) are very similar to those observed for the reduction of nitrobenzene.
3.2. The determination of the diffusion coefficients In order to achieve a quantitative description of the sonovoltammetric reduction process the diffusion coefficients for nitrobenzene and para-chloronitrobenzene have to be determined at low temperature. Several different techniques, e.g. based on microelectrode voltammetry [21,22] or chronoamperometry [23– 26] are known but problems arise due to the rather high contribution of convection to the mass transport in a low viscosity solvent such as liquid ammonia. Therefore a technique with a shorter time domain, insensitive to convection effects, based on cyclic voltammetry was chosen. The analysis of the peak currents observed at a range of different scan rates is possible with a suitable form of the Randles-Sevcˇik equation for cyclic voltammetry at a macrodisc electrode [27]. Scan rates ranging from 0.1 V s − 1 up to 10 V s − 1 and a 1 mm diameter Pt disc electrode have been used. For the analysis of the peak current data it was ensured that the relationship between peak current, Ip, and the square root of the scan rate, n 1/2, gave a straight line going through zero. This indicated Table 2 Diffusion coefficient data from the literature [17] and obtained from cyclic voltammetric experiments (see text) for the reduction of 0.5 mM nitrobenzene and 0.5 mM para-chloronitrobenzene at a 1 mm diameter Pt disc electrode in liquid ammonia+0.1 M KI at various temperatures Nitrobenzene
Para-chloronitrobenzene
105 Dlit. a/cm2 s−1
105 Dexp/cm2 s−1
203 208 213 218 223 228 233 238
1.08 1.28 1.51 1.76 2.05 2.36 2.70 3.08
1.02 1.18 1.41 1.38 1.53 2.02 2.16 2.51
Ea/kJ mol−1
12 a
109 1 b
T/K
a b
Taken from Ref. [17]. Error calculated from the standard error for the slope (see Fig. 4).
that the edge effect at the 1 mm diameter electrode was negligible in the appropriate scan rate range. However, the peak to peak separation of up to 150 mV in voltammograms obtained at scan rates of 10 V s − 1 shows that the data were recorded under quasireversible conditions. This is known to decrease the peak current up to 20%. Therefore the modified Randles-Sevcˇik equation approach [28,29] Ip = cp(L)nFAc
'
nF6D RT
(3)
had to be used to determine the diffusion coefficients. In this expression the peak current, Ip, is related to the stoichiometric number, n, the Faraday constant, F, the electrode area, A, the concentration, c, the scan rate, 6, and the diffusion coefficient, D. The factor cp(L) depends on the dimensionless rate constant for the heterogeneous charge transfer, L= ks/ nF6D/(RT), and can be approximated by 1 cp(L)= (tanh(0.5245+ 1.43 log10 (L))+ 1) 2 ×(0.446− 0.351)+ 0.351
(4)
This yields values of 0.446]cp(L)] 0.351 where the limits correspond to the reversible and irreversible case, respectively. The logarithm of the dimensionless rate constant L can be evaluated directly from the peak to peak separation for the forward and reverse scan by using the following approximation function (Eq. (5)).
log10L= − 0.454 ln sinh 9.423 DEp − 2.22× − 0.364
RT nF
(5)
Therefore the correction factor in Eq. (3), cp(L), can be determined directly from the experimental peak-topeak separation, DEp. This method was used to calculate the diffusion coefficients from the peak current and the peak separation of cyclic voltammograms. The evaluated diffusion coefficients at constant temperature did not show a dependence on the scan rate, indicating that the voltammograms in the time domain used are not affected by convection processes in the cell. Therefore the average value of the diffusion coefficients from cyclic voltammograms obtained at different scan rates was used to determine D at different temperatures from 203 to 238 K. The results obtained for nitrobenzene were found to be in good agreement with literature data [13,17]. Diffusion coefficient data for different temperatures are summarised in Table 2. A plot of the logarithm of the diffusion coefficient versus the inverse temperature (Fig. 4) clearly demonstrates the Arrhenius type dependence with activation energies of Ea = 109 1 kJ mol − 1 for para-chloro-
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3.3. The effect of ultrasound power on the diffusion layer thickness Applying ultrasound has been found to enhance mass transport to the electrode surface massively. An almost 20-fold increase in current can be observed, as shown in Fig. 3. In order to describe the effect a simplistic mass transport model for sonovoltammetric limiting currents may be based on the model of a stagnant diffusion layer at a planar electrode surface (Eq. (6)) Ilim = nFDAc/d Fig. 4. Arrhenius-type plot of the logarithm of the experimental diffusion coefficient, D, taken from the literature [17] for nitrobenzene ( ) and determined from cyclic voltammetric data for parachloronitrobenzene (2) versus the inverse temperature.
Fig. 5. Plot of the diffusion layer thickness, d, determined from the limiting current of sonovoltammograms for the reduction of 0.5 mM para-chloronitrobenzene in liquid ammonia at − 70°C at a 1 mm diameter Pt disc electrode versus the ultrasound intensity applied at a electrode to horn distance of 10 mm.
Fig. 6. Plot of the diffusion layer thickness, d, determined from the limiting current of sonovoltammograms for the reduction of 0.5 mM nitrobenzene at a 1 mm diameter Pt disc electrode at different ultrasound intensities (amplitude 5% 2, 10% , 15% , 20% x) versus the temperature.
nitrobenzene. In the case of nitrobenzene values taken from the literature [17] are shown. The error of the activation energies was calculated from the standard error for the slope of the linear fit of the log10 (D) versus T − 1 plot.
(6)
In this expression the average limiting current, Ilim, is related to the stoichiometric number, n, the Faraday constant, F, the electrode area, A, the concentration of the redox active species, c, and the diffusion layer thickness, d. In agreement with this expression the increase in concentration from 0.5 to 2.5 mM and for changes in electrode area (0.5–2 mm diameter) a proportional increase in the sonovoltammetric limiting current was observed. With the diffusion coefficient known the diffusion layer thickness may be calculated directly from the observed average limiting current. An approximately linear increase in the detected limiting current or mass transport coefficient respectively, has been observed with increasing ultrasound intensity over a range from 70 to 200 W cm − 2. To calibrate the diffusion layer thickness for each applied power, five sonovoltammograms were recorded and averaged. A diffusion layer thickness ranging from ca. 9 mm for the minimum power applied, down to ca. 2 mm for the highest applied power was observed. In Fig. 5 a plot of the diffusion layer thickness, d, versus the applied ultrasound intensity with 10 mm electrode to horn distance is shown. It can be seen that with increasing intensity the diffusion layer thickness decreases. Further, a limit of diffusion layer thinning with ca. d=2 mm is observed in agreement with an earlier study on sonovoltammetry in various other solvents at room temperature [10]. This suggests that (i) increasing the ultrasonic power beyond ca. 300 W cm − 2 does not improve the mass transport any further although more violent conditions may change the nature of the process [19] and (ii) results observed at liquid ammonia temperatures are comparable with those obtained in other solvent systems and measurements at other temperatures.
3.4. The effect of temperature on the diffusion layer thinning in liquid ammonia Increasing the temperature in sonovoltammetric experiments results in higher limiting currents. Perhaps surprisingly, in spite of the increase in the limiting current, the diffusion layer thickness d is not affected by the change in temperature within experimental error. In Fig. 6 the calculated diffusion layer thickness for
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different ultrasound intensities and at various temperatures is shown. Clearly, a compensation of temperature effects appears to occur, which causes the effect of power ultrasound on the mass transport to become, to a good approximation, temperature independent at sufficiently high intensities. The details of the mass transport mechanism are complex and possibly involve contributions from cavitation as well as turbulent acoustic streaming. However a plausible explanation may be based on the opposite effects of viscosity changes and changes in the energy dissipated in the liquid ammonia system (see Section 2). Although at
higher temperature less energy is converted into kinetic and thermal energy, the drop in viscosity causes the mass transport to increase simultaneously. It is interesting to note that the possibly related observation of a temperature independent diffusion layer thickness at a heated microwire electrode has been made recently [30]. The simplistic diffusion layer model used here is based on a diffusion layer thickness, d, independent of the diffusion coefficient. However, results in water [9] have shown that the model may be inaccurate when systems with a wide range of diffusion coefficients are considered. Indeed, if data for the sonoelectrochemical mass transport limited current for the reduction of nitrobenzene in liquid ammonia are plotted versus the diffusion coefficient in a double logarithmic plot (not shown) a relationship I8 D 2/3 can be inferred. This effect is too small to affect data in Fig. 6 but suggests that Eq. (6) has to be used carefully in cases with redox systems of very dissimilar diffusion coefficients.
3.5. The effect of ultrasound on the second reduction process
Fig. 7. Silent voltammogram (a) and sonovoltammograms (b,c) for the two electron reduction of 2.5 mM para-chloronitrobenzene in liquid ammonia +0.1 M KI at − 70°C at a 1 mm diameter Pt disc electrode (scan rate 0.2 V s − 1, ultrasound intensity b:220, c: 300 W cm − 2).
Fig. 8. Cyclic voltammogram (a) and sonovoltammograms (b,c) for the two electron reduction of 5 mM nitrobenzene in liquid ammonia+0.1 M KI at − 70°C at a 1 mm diameter Pt disc electrode (scan rate 0.01 V s − 1, ultrasound intensity b: 200, c: 300 W cm − 2).
In liquid ammonia at − 70°C the second reduction process leading to the formation of the dianionic products can be observed at potentials of E1/2 = −1.0 V versus Ag for para-chloronitrobenzene and at E1/2 = − 1.2 V versus Ag for nitrobenzene. In Fig. 7 a cyclic voltammogram and sonovoltammograms for the reduction of 2.5 mM para-chloronitrobenzene in liquid ammonia at − 70°C with a scan rate of 200 mV s − 1 are shown. It can be seen that the voltammetric responses for both reduction processes are well defined although a shoulder and a small peak appearing in the back scan (Fig. 7a) have been proposed to be associated with a deposit of the dianion salt formed in the second reduction step on the electrode surface [16]. Such deposits can actually modify or completely block the electrode surface with a dramatic effect on the limiting current. These important effects in electrosynthetic work are dependent on many experimental parameters and are most readily detected at slow scan rates, under fast mass transport conditions, and in the presence of a higher concentration of redox reagent. In Fig. 8 voltammograms obtained under silent conditions (a) and in the presence of (b) 200 W cm − 2 and (c) 300 W cm − 2 power ultrasound for the reduction of 5 mM nitrobenzene are shown. Under silent conditions a sigmoidially shaped response is detected due to the slow scan rate used and convection present in the system. In the presence of ultrasound the first reduction response is detected but for the response associated with the second reduction step only the ‘foot’ of the response is detected. Then the limiting current rapidly decreases to a level below that observed in the first reduction process. However, the blocking process is not complete and
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the signal recovers and stabilizes at a value close to that observed for the first reduction process. These observations are very closely related to those reported recently for the sonoelectrochemical two electron reduction of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone [31]. Based on the analogous voltammetric characteristics the same mechanism for the blocking process may be proposed. The formation of the deposit is time and flux dependent. Fast mass transport causes the formation of many nucleation sites (small overlapping diffusion spheres) and the high flux of material causes the volume of deposited material to increase rapidly. The process is believed to be based on a solid deposit growing directly at the electrode surface rather than on precipitation of solid from solution because power ultrasound would be expected to stop the precipitation process. Further, the fact that the limiting current stabilizes on the level corresponding to approximately a one electron reduction suggests a ‘steady state’ situation in which formation of more dianion causes more blocking and a drop in current. In this situation the actual amount of deposit present at the electrode surface may be very small and the continuous formation of the mono-anionic product may be based on a conproportionation step (Eq. (7)). 2− [K+ ]solid +NB ? 2K+ +2NB − 2 NB
(7)
The effect of changing the electrode to horn separation to less than 10 mm is to increase considerably the ‘violence’ of cavitation processes [19] and can be predicted to increase further the mass transport and cleaning effects caused by ultrasound [32,33]. The implications of a thin film deposit of, for example, para-chloronitrobenzene dianion salt formed under high flux conditions in the presence of power ultrasound on chemical kinetics, e.g. cleavage of the carbonhalide bond and coupling processes, remains to be explored.
4. Conclusions It has been shown to be extremely beneficial to employ power ultrasound to assist electrochemical processes in liquid ammonia at low temperatures. Considerable mass transport and in-situ electrode modification effects have been demonstrated. In particular, the details of the temperature dependence of the diffusion layer thickness are intriguing and the results suggest that the effect of power ultrasound on electrochemical systems under a wide range of conditions is predictable. Further studies on the effects of cavitation at low temperatures, smaller electrode to .
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horn separation, and on the details of the mass transport model are in progress.
Acknowledgements F.M. thanks the Royal Society for a University Research Fellowship and New College (Oxford) for a Stipendiary Lectureship. Dr R.K. Thomas is gratefully acknowledged for providing liquid ammonia and the EPSRC for financial support (GR/L81123).
References [1] C.K. Mann, K.K. Barnes, Electrochemical Reactions in NonAqueous Solvents, Dekker, New York, 1970. [2] H. Smith, Organic Reactions in Liquid Ammonia, Wiley, New York, 1963. [3] D.J. Walton, S.S. Phull, Adv. Sonochem. 4 (1996) 205. [4] R.G. Compton, J.C. Eklund, F. Marken, Electroanalysis 9 (1997) 509. [5] For a review see E. Yeager, F. Hovorka, J. Acoust. Soc. Am. 25 (1953) 443 and Refs. therein. [6] R.G. Compton, J.C. Eklund, F. Marken, T.O. Rebbitt, R.P. Akkermans, D.N. Waller, Electrochim. Acta 42 (1997) 2919. [7] M.A. Margulis, N.A. Mal’tsev, Russ. J. Phys. Chem. 43 (1969) 4. [8] T.J. Mason, J.P. Lorimer, D.M. Bates, Ultrasonics 30 (1992) 40. [9] H.A.O. Hill, Y. Nakagawa, F. Marken, R.G. Compton, J. Phys. Chem. 100 (1996) 17395. [10] F. Marken, R.P. Akkermans, R.G. Compton, J. Electroanal. Chem. 415 (1996) 55. [11] H. Sadek, A.B. Abd-el-Nabey, Electrochim. Acta 17 (1972) 2065. [12] E. Steudel, J. Posdorfer, R.N. Schindler, Electrochim. Acta 40 (1995) 11. [13] R.M. Crooks, F.F. Fan, A.J. Bard, J. Am. Chem. Soc. 106 (1984) 6852. [14] A. Cyr, E. Laviron, J. Lessard, J. Electroanal. Chem. 263 (1989) 69 and references cited therein. [15] U. Schindewolf, J. Phys. Chem. 88 (1984) 3820. [16] W.H. Smith, A.J. Bard, J. Am. Chem. Soc. 97 (1975) 5203. [17] R.M. Crooks, A.J. Bard, J. Phys. Chem. 91 (1987) 1274. [18] J. Mortensen, J. Heinze, Angew. Chem. Int. Ed. Engl. 23 (1984) 84. [19] J.L. Hardcastle, J.C. Ball, Q. Hong, F. Marken, R.G. Compton, S.D. Bull, S.G. Davies, Ultrasonics Sonochem., in press. [20] T. Teherani, A.J. Bard, Acta Chem. Scand. B 37 (1983) 413. [21] K. Aoki, Electroanalysis 5 (1993) 627. [22] J. Heinze, Ber. Bunsenges. Phys. Chem. 85 (1981) 1096. [23] G. Denuault, M.V. Mirkin, A.J. Bard, J. Electroanal. Chem. 308 (1991) 27. [24] K. Aoki, J. Osteryoung, J. Electroanal. Chem. 125 (1981) 315. [25] M. Kakihana, H. Ikeuchi, G.P. Sato, K. Tokuda, J. Electroanal. Chem. 117 (1981) 201. [26] C. Amatore, in: I. Rubinstein (Ed.), Physical Electrochemistry: Principles, Methods and Applications, Marcel Dekker, Chapter 4, New York, 1995. [27] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and Applications, Wiley, New York, 1980.
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[28] A. Neudeck, J. Dittrich, Z. Chem. 29 (1989) 35. [29] A. Neudeck, J. Dittrich, J. Electroanal. Chem. 313 (1991) 37. [30] A. Beckmann, B.A. Coles, R.G. Compton, P. Gru¨ndler, F. Marken, A. Neudeck, J. Phys. Chem., submitted for publication.
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[31] M. Oyama, F. Marken, R.D. Webster, J.A. Cooper, R.G. Compton, S. Okazaki, J. Electroanal. Chem. 451 (1998) 193. [32] H.H. Zhang, L.A. Coury, Anal. Chem. 65 (1993) 1552. [33] P.R. Birkin, S. Silva-Martinez, J. Electroanal. Chem. 416 (1996) 127.
Low-temperature sonoelectrochemical processes Part 1. Mass transport and cavitation effects of 20 kHz ultrasound in liquid ammonia F. Javier Del Campo, Andreas Neudeck, Richard G. Compton, Frank Marken * Physical and Theoretical Chemistry Laboratory, Oxford Uni6ersity, South Parks Road, Oxford OX1 3QZ, UK Received 3 August 1999; received in revised form 17 September 1999; accepted 17 September 1999
Abstract Sonoelectrochemical processes in liquid ammonia in a temperature range between − 70 and −35°C in the presence of 20 kHz power ultrasound are studied with the aim of improving low temperature electrosynthetic procedures. The one and two electron reductions of nitrobenzene and para-chloronitrobenzene are investigated as model systems. Placing an immersed ultrasonic horn emitter ‘face-on’ to a platinum disc electrode in liquid ammonia is shown to result in extreme mass transport enhancements with a resulting diffusion layer thickness of approximately d= 2 mm. This limit of the diffusion layer thickness is shown to be essentially temperature independent and correspondingly, the highest limiting currents can be observed near the boiling point of liquid ammonia. Cavitation processes are detected even at − 70°C and result in a considerable fluctuation in the observed mass transport controlled limiting current. Further, the deposition of ionic products formed in the second reduction step for both nitrobenzene and para-chloronitrobenzene reduction and the associated drop in current, can be shown to be affected by sonication. Ultrasound has been found to be beneficial by (i) causing extremely fast mass transport; (ii) enhancing the mixing and dissolution kinetics at low temperature; and (iii) affecting the formation of solid products at the electrode surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Ultrasound; Cavitation; Mass transport; Sonoelectrochemistry; Low temperature electrochemistry; Voltammetry; Liquid ammonia; Nitrobenzene
1. Introduction Liquid ammonia is a very interesting solvent for chemical and electrochemical processes at low temperatures and under mild conditions. The characteristic properties of liquid ammonia are a relatively low viscosity, high dielectric constant, good cation solvation properties, and the ability to produce stable solutions of solvated electrons [1,2]. Problems in the use of liquid ammonia as a low temperature reaction environment may arise due to the slow heterogeneous kinetics associated with low temperature dissolution processes, e.g. of the starting material, and the formation of ionic precipitates on the electrode surface. Introducing a source of power ultrasound in the electrochemical cell may in * Corresponding author. Tel.: +44-1865-275443; fax: +44-1865275410. E-mail address: [email protected] (F. Marken)
principle solve both of these problems as well as improve the efficiency of the process. The use of power ultrasound in electrochemistry, or sonoelectrochemistry (for modern reviews see Refs. [3,4]), was proposed in pioneering work by Moriguchi [5] and further developed by Yeager in the 1960’s. Only over recent years have the fundamentals and the potential of combining high intensity ultrasound horn systems with electrochemical processes been explored in more detail. Both the development of novel electroanalytical and electrosynthetic procedures have been proposed and the processes involved studied quantitatively. In the present study experimental evidence is given for the beneficial effects of 20 kHz power ultrasound in a low temperature electrochemical system and a model for the ultrasound enhanced mass transport at various temperatures is sought. The reduction of nitrobenzene and para-chloronitrobenzene have been chosen as
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model systems and a comparison with earlier studies on the electrochemical properties and reactivity of these compounds is made. The mass transport dependent formation of non-conducting and blocking deposits is considered in detail.
2. Experimental
2.1. Reagents and instrumentation Chemical reagents used were KI (AnalaR, 99.8%), lithium wire (Aldrich, 99.9%), nitrobenzene (Aldrich,
99 + %), 1-chloro-4-nitrobenzene (Aldrich, 99%), and ammonia (BOC). Argon (Pureshield, BOC) was used to maintain a dry and inert atmosphere. In electrochemical experiments 1 mm diameter Pt disc electrodes mounted in a glass holder (lime-soda) were used as the working electrode. Teflon was avoided as an electrode shrouding material due to its large coefficient of thermal expansion, which caused the Pt electrode to protrude. Electrodes were polished carefully prior to measurements using alumina lapping compounds (BDH or Microglass Instruments, Greensborough, Victoria 3088, Australia) of decreasing size down to 0.05 mm. A gold wire served as the counter electrode and a silver wire (Aldrich, 99.9% 0.5 mm diameter) was used as a pseudo-reference electrode. For electrochemical measurements an Autolab PGSTAT 20 system (Eco Chemie, Netherlands) was used. The three electrode electrochemical cell (see Fig. 1) allowed experiments to be conducted at temperatures ranging from − 70 to − 359 1°C. An ultrasound horn transducer system (Sonics & Materials, VCX400) with a stepped 3 mm diameter microtip horn (titanium alloy) was employed. The horn probe was electrically insulated and positioned ‘face-on’ [6] towards the working electrode with a distance kept constant at 10 mm.
2.2. Experimental procedure
Fig. 1. Schematic drawing of the experimental sonoelectrochemical cell. Gas in- and out-let are used for transferring ammonia and for keeping the system under an inert atmosphere of argon. The horn was sealed into the glass joint with Parafilm.
Fig. 2. Plot of the ultrasound intensity emitted from the horn probe versus the amplitude setting used for water at room temperature (2) and for liquid ammonia at − 35°C (). The ultrasound intensity was determined from the power input of the transducer system (see Section 2).
Electrochemical experiments were carried out under dry conditions in an inert atmosphere of argon. The supporting electrolyte, KI, was pre-treated by grinding it into a fine powder and drying in an oven at 130 – 140°C overnight. The electrodes were kept in a dessicator. The ammonia was dried by first condensing it into a flask containing pieces of lithium. Then 509 2 cm3 of it was distilled under argon into the electrochemical cell containing the supporting electrolyte, KI. The time consuming process of dissolving the reagents in liquid ammonia at − 70°C was considerably enhanced by sonication. For the ultrasound intensity calibration in liquid ammonia conventional calorimetric methods [7,8] are not applicable and a method based on the measurement of the electrical power consumed by the transducer was developed. The power consumed by the transducer at various ultrasound intensity settings was monitored. Assuming that (i) the energy emitted in air is negligible compared to that emitted in a solvent and (ii) the energy lost due to heat dissipation in the transducer, including all electronic components, is constant when a properly tuned tip is used, the power dissipation in the solvent can be measured from the difference of the total consumed power in air and in the solvent under study. A linear dependence of the power emitted
F.J. Del Campo et al. / Journal of Electroanalytical Chemistry 477 (1999) 71–78 Table 1 Ultrasound intensity determined as energy dissipated in liquid ammonia from an immersed 3 mm diameter horn tip with an amplitude fixed at 20% as a function of temperature (see Section 2) T/K
Ultrasound intensity/W cm−2
208 213 218 223 228 233
500 9 100 430 9 90 370 9 70 3009 60 230 9 50 1709 30
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3. Results and discussion
3.1. The effect of 20 kHz ultrasound on the 6oltammetry in liquid ammonia The electrochemical reduction of nitrobenzene has been studied extremely well in various media [11–13] and is of interest both as a model system and for the electrochemical synthesis of aniline [14]. In aprotic media the reduction process is characterised by two consecutive one electron reduction steps. In the presence of protons the reduction may follow various pathways and the six electron reduction to aniline as well as the formation of more complex dimeric products has been reported [15]. In liquid ammonia, and under anhydrous conditions, nitrobenzene undergoes two reversible one electron reduction processes [13]. In the first reduction step the radical mono-anion is formed and the product of the second reduction step is the dianion (Eq. 1).
(1)
Fig. 3. Cyclic and sono-voltammograms for the reduction of 0.5 mM nitrobenzene in liquid ammonia + 0.1 M KI at − 70°C at a 1 mm diameter Pt disc electrode in the absence (a) and in the presence (b – e) of 20 kHz ultrasound (scan rate 0.2 V s − 1, ultrasound intensities b: 70, c: 140, d: 220, e: 300 W cm − 2, 10 mm electrode to horn distance).
into the liquid versus the amplitude (amplitude setting of the instrument) used was found (Fig. 2). The data obtained for the energy emitted into water are in good agreement with earlier calorimetric measurements [9]. The electrically insulated horn system used for the electrochemical experiments are difficult to tune and an offset at minimum power level (not shown) is attributed to the energy lost or dissipated internally due to a mismatch in the tuning. The variation of the energy emitted from the horn probe in water at room temperature and in liquid ammonia at −35°C is shown in Fig. 2. The energy emitted into the liquid ammonia is considerably lower probably due to, in part, the low viscosity and the formation of bubbles. A similar effect has been noted previously in dichloromethane solutions [10]. The power dissipated in liquid ammonia at a given ultrasound amplitude increases as the temperature is lowered. An approximately linear relationship between the power dissipated and temperature exists with liquid ammonia at −60°C adsorbing ultrasound energy to a similar extent compared to water at room temperature (see Table 1).
One of the main advantages of liquid ammonia compared to other solvent systems is the relatively long lifetime, which can be observed for radical ions or other high-energy intermediate species. This unusual stability may be attributed to the low working temperatures and the stabilisation of charged intermediates [16]. A cyclic voltammogram obtained at a 1 mm diameter Pt disc electrode for the first reduction process for 0.5 mM nitrobenzene dissolved in liquid ammonia (0.1 M KI) at − 70°C is shown in Fig. 3a. A well-defined voltammetric response at E1/2 = − 0.32 V versus Ag consistent with literature reports [17,18] is detected. In the presence of ultrasound emitted from a horn probe at a 10 mm distance from the working electrode a considerable increase in current can be detected (Fig. 3b–d). The observed effect is dependent on the applied ultrasound intensity and in general is of the same order of magnitude compared to those observed in other solvents at room temperature [10]. Superimposed on the mass transport controlled limiting current is a current fluctuation which is believed to be dominated by interfacial cavitation events. In agreement with earlier studies [19] with increasing ultrasound intensity current spikes are found to increase associated with the cavitation process becoming more violent. The reduction of para-chloronitrobenzene at − 70°C in liquid ammonia is known [20] to follow a similar reaction path in the absence of protons (Eq. 2).
(2)
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However, the dianionic product has been shown to be kinetically labile and even at very low temperature a slow cleavage of the carbonchloride bond followed by protonation of the carbanion has been shown to occur [20]. The cyclic voltammetric study of the first reduction process for 0.5 mM para-chloronitrobenzene in liquid ammonia (0.1 M KI) at − 70°C gave a well-defined reversible response at E1/2 = − 0.21 V versus Ag. The characteristics of the sonovoltammetric curves (not shown) are very similar to those observed for the reduction of nitrobenzene.
3.2. The determination of the diffusion coefficients In order to achieve a quantitative description of the sonovoltammetric reduction process the diffusion coefficients for nitrobenzene and para-chloronitrobenzene have to be determined at low temperature. Several different techniques, e.g. based on microelectrode voltammetry [21,22] or chronoamperometry [23– 26] are known but problems arise due to the rather high contribution of convection to the mass transport in a low viscosity solvent such as liquid ammonia. Therefore a technique with a shorter time domain, insensitive to convection effects, based on cyclic voltammetry was chosen. The analysis of the peak currents observed at a range of different scan rates is possible with a suitable form of the Randles-Sevcˇik equation for cyclic voltammetry at a macrodisc electrode [27]. Scan rates ranging from 0.1 V s − 1 up to 10 V s − 1 and a 1 mm diameter Pt disc electrode have been used. For the analysis of the peak current data it was ensured that the relationship between peak current, Ip, and the square root of the scan rate, n 1/2, gave a straight line going through zero. This indicated Table 2 Diffusion coefficient data from the literature [17] and obtained from cyclic voltammetric experiments (see text) for the reduction of 0.5 mM nitrobenzene and 0.5 mM para-chloronitrobenzene at a 1 mm diameter Pt disc electrode in liquid ammonia+0.1 M KI at various temperatures Nitrobenzene
Para-chloronitrobenzene
105 Dlit. a/cm2 s−1
105 Dexp/cm2 s−1
203 208 213 218 223 228 233 238
1.08 1.28 1.51 1.76 2.05 2.36 2.70 3.08
1.02 1.18 1.41 1.38 1.53 2.02 2.16 2.51
Ea/kJ mol−1
12 a
109 1 b
T/K
a b
Taken from Ref. [17]. Error calculated from the standard error for the slope (see Fig. 4).
that the edge effect at the 1 mm diameter electrode was negligible in the appropriate scan rate range. However, the peak to peak separation of up to 150 mV in voltammograms obtained at scan rates of 10 V s − 1 shows that the data were recorded under quasireversible conditions. This is known to decrease the peak current up to 20%. Therefore the modified Randles-Sevcˇik equation approach [28,29] Ip = cp(L)nFAc
'
nF6D RT
(3)
had to be used to determine the diffusion coefficients. In this expression the peak current, Ip, is related to the stoichiometric number, n, the Faraday constant, F, the electrode area, A, the concentration, c, the scan rate, 6, and the diffusion coefficient, D. The factor cp(L) depends on the dimensionless rate constant for the heterogeneous charge transfer, L= ks/ nF6D/(RT), and can be approximated by 1 cp(L)= (tanh(0.5245+ 1.43 log10 (L))+ 1) 2 ×(0.446− 0.351)+ 0.351
(4)
This yields values of 0.446]cp(L)] 0.351 where the limits correspond to the reversible and irreversible case, respectively. The logarithm of the dimensionless rate constant L can be evaluated directly from the peak to peak separation for the forward and reverse scan by using the following approximation function (Eq. (5)).
log10L= − 0.454 ln sinh 9.423 DEp − 2.22× − 0.364
RT nF
(5)
Therefore the correction factor in Eq. (3), cp(L), can be determined directly from the experimental peak-topeak separation, DEp. This method was used to calculate the diffusion coefficients from the peak current and the peak separation of cyclic voltammograms. The evaluated diffusion coefficients at constant temperature did not show a dependence on the scan rate, indicating that the voltammograms in the time domain used are not affected by convection processes in the cell. Therefore the average value of the diffusion coefficients from cyclic voltammograms obtained at different scan rates was used to determine D at different temperatures from 203 to 238 K. The results obtained for nitrobenzene were found to be in good agreement with literature data [13,17]. Diffusion coefficient data for different temperatures are summarised in Table 2. A plot of the logarithm of the diffusion coefficient versus the inverse temperature (Fig. 4) clearly demonstrates the Arrhenius type dependence with activation energies of Ea = 109 1 kJ mol − 1 for para-chloro-
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3.3. The effect of ultrasound power on the diffusion layer thickness Applying ultrasound has been found to enhance mass transport to the electrode surface massively. An almost 20-fold increase in current can be observed, as shown in Fig. 3. In order to describe the effect a simplistic mass transport model for sonovoltammetric limiting currents may be based on the model of a stagnant diffusion layer at a planar electrode surface (Eq. (6)) Ilim = nFDAc/d Fig. 4. Arrhenius-type plot of the logarithm of the experimental diffusion coefficient, D, taken from the literature [17] for nitrobenzene ( ) and determined from cyclic voltammetric data for parachloronitrobenzene (2) versus the inverse temperature.
Fig. 5. Plot of the diffusion layer thickness, d, determined from the limiting current of sonovoltammograms for the reduction of 0.5 mM para-chloronitrobenzene in liquid ammonia at − 70°C at a 1 mm diameter Pt disc electrode versus the ultrasound intensity applied at a electrode to horn distance of 10 mm.
Fig. 6. Plot of the diffusion layer thickness, d, determined from the limiting current of sonovoltammograms for the reduction of 0.5 mM nitrobenzene at a 1 mm diameter Pt disc electrode at different ultrasound intensities (amplitude 5% 2, 10% , 15% , 20% x) versus the temperature.
nitrobenzene. In the case of nitrobenzene values taken from the literature [17] are shown. The error of the activation energies was calculated from the standard error for the slope of the linear fit of the log10 (D) versus T − 1 plot.
(6)
In this expression the average limiting current, Ilim, is related to the stoichiometric number, n, the Faraday constant, F, the electrode area, A, the concentration of the redox active species, c, and the diffusion layer thickness, d. In agreement with this expression the increase in concentration from 0.5 to 2.5 mM and for changes in electrode area (0.5–2 mm diameter) a proportional increase in the sonovoltammetric limiting current was observed. With the diffusion coefficient known the diffusion layer thickness may be calculated directly from the observed average limiting current. An approximately linear increase in the detected limiting current or mass transport coefficient respectively, has been observed with increasing ultrasound intensity over a range from 70 to 200 W cm − 2. To calibrate the diffusion layer thickness for each applied power, five sonovoltammograms were recorded and averaged. A diffusion layer thickness ranging from ca. 9 mm for the minimum power applied, down to ca. 2 mm for the highest applied power was observed. In Fig. 5 a plot of the diffusion layer thickness, d, versus the applied ultrasound intensity with 10 mm electrode to horn distance is shown. It can be seen that with increasing intensity the diffusion layer thickness decreases. Further, a limit of diffusion layer thinning with ca. d=2 mm is observed in agreement with an earlier study on sonovoltammetry in various other solvents at room temperature [10]. This suggests that (i) increasing the ultrasonic power beyond ca. 300 W cm − 2 does not improve the mass transport any further although more violent conditions may change the nature of the process [19] and (ii) results observed at liquid ammonia temperatures are comparable with those obtained in other solvent systems and measurements at other temperatures.
3.4. The effect of temperature on the diffusion layer thinning in liquid ammonia Increasing the temperature in sonovoltammetric experiments results in higher limiting currents. Perhaps surprisingly, in spite of the increase in the limiting current, the diffusion layer thickness d is not affected by the change in temperature within experimental error. In Fig. 6 the calculated diffusion layer thickness for
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different ultrasound intensities and at various temperatures is shown. Clearly, a compensation of temperature effects appears to occur, which causes the effect of power ultrasound on the mass transport to become, to a good approximation, temperature independent at sufficiently high intensities. The details of the mass transport mechanism are complex and possibly involve contributions from cavitation as well as turbulent acoustic streaming. However a plausible explanation may be based on the opposite effects of viscosity changes and changes in the energy dissipated in the liquid ammonia system (see Section 2). Although at
higher temperature less energy is converted into kinetic and thermal energy, the drop in viscosity causes the mass transport to increase simultaneously. It is interesting to note that the possibly related observation of a temperature independent diffusion layer thickness at a heated microwire electrode has been made recently [30]. The simplistic diffusion layer model used here is based on a diffusion layer thickness, d, independent of the diffusion coefficient. However, results in water [9] have shown that the model may be inaccurate when systems with a wide range of diffusion coefficients are considered. Indeed, if data for the sonoelectrochemical mass transport limited current for the reduction of nitrobenzene in liquid ammonia are plotted versus the diffusion coefficient in a double logarithmic plot (not shown) a relationship I8 D 2/3 can be inferred. This effect is too small to affect data in Fig. 6 but suggests that Eq. (6) has to be used carefully in cases with redox systems of very dissimilar diffusion coefficients.
3.5. The effect of ultrasound on the second reduction process
Fig. 7. Silent voltammogram (a) and sonovoltammograms (b,c) for the two electron reduction of 2.5 mM para-chloronitrobenzene in liquid ammonia +0.1 M KI at − 70°C at a 1 mm diameter Pt disc electrode (scan rate 0.2 V s − 1, ultrasound intensity b:220, c: 300 W cm − 2).
Fig. 8. Cyclic voltammogram (a) and sonovoltammograms (b,c) for the two electron reduction of 5 mM nitrobenzene in liquid ammonia+0.1 M KI at − 70°C at a 1 mm diameter Pt disc electrode (scan rate 0.01 V s − 1, ultrasound intensity b: 200, c: 300 W cm − 2).
In liquid ammonia at − 70°C the second reduction process leading to the formation of the dianionic products can be observed at potentials of E1/2 = −1.0 V versus Ag for para-chloronitrobenzene and at E1/2 = − 1.2 V versus Ag for nitrobenzene. In Fig. 7 a cyclic voltammogram and sonovoltammograms for the reduction of 2.5 mM para-chloronitrobenzene in liquid ammonia at − 70°C with a scan rate of 200 mV s − 1 are shown. It can be seen that the voltammetric responses for both reduction processes are well defined although a shoulder and a small peak appearing in the back scan (Fig. 7a) have been proposed to be associated with a deposit of the dianion salt formed in the second reduction step on the electrode surface [16]. Such deposits can actually modify or completely block the electrode surface with a dramatic effect on the limiting current. These important effects in electrosynthetic work are dependent on many experimental parameters and are most readily detected at slow scan rates, under fast mass transport conditions, and in the presence of a higher concentration of redox reagent. In Fig. 8 voltammograms obtained under silent conditions (a) and in the presence of (b) 200 W cm − 2 and (c) 300 W cm − 2 power ultrasound for the reduction of 5 mM nitrobenzene are shown. Under silent conditions a sigmoidially shaped response is detected due to the slow scan rate used and convection present in the system. In the presence of ultrasound the first reduction response is detected but for the response associated with the second reduction step only the ‘foot’ of the response is detected. Then the limiting current rapidly decreases to a level below that observed in the first reduction process. However, the blocking process is not complete and
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the signal recovers and stabilizes at a value close to that observed for the first reduction process. These observations are very closely related to those reported recently for the sonoelectrochemical two electron reduction of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone [31]. Based on the analogous voltammetric characteristics the same mechanism for the blocking process may be proposed. The formation of the deposit is time and flux dependent. Fast mass transport causes the formation of many nucleation sites (small overlapping diffusion spheres) and the high flux of material causes the volume of deposited material to increase rapidly. The process is believed to be based on a solid deposit growing directly at the electrode surface rather than on precipitation of solid from solution because power ultrasound would be expected to stop the precipitation process. Further, the fact that the limiting current stabilizes on the level corresponding to approximately a one electron reduction suggests a ‘steady state’ situation in which formation of more dianion causes more blocking and a drop in current. In this situation the actual amount of deposit present at the electrode surface may be very small and the continuous formation of the mono-anionic product may be based on a conproportionation step (Eq. (7)). 2− [K+ ]solid +NB ? 2K+ +2NB − 2 NB
(7)
The effect of changing the electrode to horn separation to less than 10 mm is to increase considerably the ‘violence’ of cavitation processes [19] and can be predicted to increase further the mass transport and cleaning effects caused by ultrasound [32,33]. The implications of a thin film deposit of, for example, para-chloronitrobenzene dianion salt formed under high flux conditions in the presence of power ultrasound on chemical kinetics, e.g. cleavage of the carbonhalide bond and coupling processes, remains to be explored.
4. Conclusions It has been shown to be extremely beneficial to employ power ultrasound to assist electrochemical processes in liquid ammonia at low temperatures. Considerable mass transport and in-situ electrode modification effects have been demonstrated. In particular, the details of the temperature dependence of the diffusion layer thickness are intriguing and the results suggest that the effect of power ultrasound on electrochemical systems under a wide range of conditions is predictable. Further studies on the effects of cavitation at low temperatures, smaller electrode to .
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horn separation, and on the details of the mass transport model are in progress.
Acknowledgements F.M. thanks the Royal Society for a University Research Fellowship and New College (Oxford) for a Stipendiary Lectureship. Dr R.K. Thomas is gratefully acknowledged for providing liquid ammonia and the EPSRC for financial support (GR/L81123).
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