Chacterization of a 20 kHz sonoreactor: Part II: Analysis of chemical effects by classical and electrochemical methods
V. Sáeza, A. Frías-Ferrera, J. Iniestaa, J. González-Garcíaa1, A. Aldaza y E. Rierab
aGrupo de Electroquímica Aplicada y Electrocatálisis, Departamento de Química Física. Universidad de Alicante. Ap. Correos 99. 03080 Alicante (Spain).
bInstituto de Acústica, CSIC, Serrano 114, 28006 Madrid (Spain)
Keywords: sonoreactor, ultrasonic intensity, characterization, electrochemical probe, chemical dosimeter
Abstract
A new electrochemical redox probe has been investigated in order to characterize the local production of radicals during the cavitation events. The results have been compared with those obtained with Fe(CN)63-
/Fe(CN)64-
(electrochemical probe for local mechanical effects) and classical chemical methods such as iodide and Fricke dosimeters (chemical probes for global effects).
1. Introduction
Nowadays, it is a matter of interest to characterize the behaviour of sonoreactors in order to identify the actives zones (especially those related to the cavitation events) and so provide an accurate picture of the reaction environment in the sonoreactor [1]. In this context, attention is being paid on the subdivision of the sonochemical applications based on the "true" and "false" effects [2]: differences are established between the chemical effects (result of the cavitation event) from the effects caused by the mechanical actions (mainly consequence of the bubble collapse). So, from a chemical point of view, characterization methods based on chemical reactions must be more representative of the environment to achieve. Several chemical methods have been
1 Corresponding author: [email protected]
proposed in literature [3-7] consequence of the effects induced by cavitation. Normally these methods provide global measurements averaged through the reaction medium and only some of the electrochemical probes [8-15] can be considered local methods. In a large number of these last references, the well-characterised quasi-reversible system Fe(CN)63-
/Fe(CN)64-
is the most redox agent used. This system exhibits a fast quasi- reversible eletron transfer so the electrode kinetics is considered to be under mass transport control [16]. This electrochemical probe is based on the mechanical effects of cavitation, which provide convective-diffusion conditions [17] close to electrode surface, resulting in an enhancement of the mass transport to/from electrode. The mass transport coefficent km obtained from the experimental limiting current density, jL, allows one to characterize locally the mechanical effects of the cavitation near electrode. Recently, for the Fe(CN)63-
/Fe(CN)64-
system, several authors [13, 18] have presented another effects rather than improved mass transport induced by an ultrasonic irradiation. Other electrochemical approaching to the characterisation of cavitation has been proposed based on the electrochemical detection of hydrogen peroxide produced sonochemically [19].
However, to our best knowledge none of the papers found in the literature have used electrochemical probes based on the radical generation from the bubble cavitation.
In this paper we present data for the characterization of a sonoreactor Undatim 20kHz/100W using several chemical methods, and compare these with a new suggested electrochemical probe.
2. Experimental 2.1. Equipments
The sonoreactor consisted of a jacketed Sonoreactor (20 kHz, 100 W maximum power, 68 mm diameter, 84 mm depth) supplied by Undatim. This apparatus can operate at optimum frequency by means of an electronic device and so ensure a maximum power transmitted to the reactor. A sketch of the experimental cell, arranged as a sonoelectrochemical reactor, is shown in figure 1. In order to maintain the system at a constant temperature, an additional cooling glass coil system (60 mm outer diameter, 40 mm inner diameter, not shown in figure 1), connected in series to the jacket, was immersed in the solution, and the coil was fitted to the inner wall of the sonoreactor according to results obtained from part I of this work [20]. The sonicated volume (200 mL for each experiment) has a cylindrical tall shape with 68 mm of diameter and 54 mm of height. All the experiments in both silent and insonated conditions were maintained constant at 298±1 K and monitored with a thermistor (Pt100
Thermometer 638 Pt, Crison). A titanium horn (stepped, 7.07 cm2 emitter area) was used as ultrasound source and was fitted at the bottom of the cell.
For the chemical dosimeters experiments, absorbance measurements were carried out using a UV/VIS spectrophotometer (SHIMADZU UV-120-02) provided with 1 cm path light quartz cells.
The chronoamperometric curves were obtained using a Voltalab electrochemical system with a DEA 332 potentiostat and an IMT 102 electrochemical interface, connected to a PC for data acquisition and control. A glassy carbon rod CV25 (0.07 cm2 electrode area) from Sofacel (Le Carbone-Lorraine) was used as a working electrode. The glassy carbon rod was sheathed by two cylinders of Teflon. The first cylinder was fitted thermally whereas the second one was fitted by pressing, providing a
wide sheath. The counter electrode was a spiral wound platinum wire and the reference electrode was a saturated calomel electrode (SCE) (Radiometer, Copenhagen) connected to the electrochemical cell via a Luggin capillary. The working electrode was placed “face on” at known different distances from the ultrasound horn surface.
The distance d between the emitting surface and the electrode surface was a variable under study.
2.2 Chemical dosimeters
Two of the most chemical dosimeters studied [21], Fricke [6, 22-27] and iodide [27-31] dosimeters, were used for the chemical characterization of the sonoreactor.
During these studies, the solutions were saturated with an argon stream, before and during each experiment. Further details for the experimental procedure can be obtained from the respective references mentioned above.
2.3 Electrochemical methods
Before each experiment, the glassy carbon electrode was polished with decreasing size alumina powder (1, 0.3 and 0.05 µm) until a mirror finish was obtained.
After that, the electrode was thoroughly rinsed with ultrapure water. All solutions were prepared using ultrapure water from a Millipore Mill-Q system. Solutions were degassed and saturated with a stream of Ar in order to keep the same amount of gas in the electrolytic cell during the experiments, and avoid possible interference caused by oxygen. A stream of Ar was also maintained over the surface of the electrolyte during the experiments. During insonation of the glassy carbon electrode, no damage was detected by visual inspection and SEM micrographs.
-Fe(CN)63-/Fe(CN)64- redox agent: The concentration of the solution employed for the chronoamperometry study was 12.5 × 10-3 mol dm-3 K4Fe(CN)6 + 5 × 10-3 mol dm-3 K3Fe(CN)6 + 1 mol dm-3 KOH (Merck a.r.).
-PbO2/Pb2+ redox agent: A final concentration of Pb(II) of 0.1 mol dm-3 was obtained from dissolving lead (II) oxide in 1 mol dm-3 perchloric acid (Merck a.r.) to perfrom the chronoamperometry study. After each experiment, the lead dioxide deposit was stripped off the surface using a mixture of acetic acid and hydrogen peroxide (50:50 v/v) followed by a thoroughly rising of the electrode with water.
3. Results
3.1 Chemical dosimeter measurements -Fricke and iodide dosimeters
Figure 2 shows the energy yields, G (in mol J-1) [32], for Fe(III) and I2 tests (at 35 min of sonolysis) for different ultrasonic intensities with a maximum for ultrasonic intensities ranging between 2.7 and 3.4 W cm-2. This figure summarizes the results obtained by these two classical chemical methods. The global behaviour were in agreement with those obtained by other authors [33], with a intensity threshold located in the range [0.8-1] W cm-2, close to others found in literature [24]
3.2 Electrochemical methods
The previous characterization either by using physical [20] or chemical methods has provided the specific experimental conditions where a comparative analysis of the both electrochemical probes must be done.
-Fe(CN)63-/Fe(CN)64- redox agent
Figure 3 shows the cyclic voltammograms for the electrochemical behaviour of Fe(CN)63-
anion on a carbon vitreous electrode. The curves present the typical characteristics for a quasi-reversible electron transfer process (Table 1). From this figure, potential values were chosen to perform the potential step experiments under pure mass transport control. Figure 4 shows a family of j vs t transients for the reduction of Fe(CN)63-
anion under convective-diffusion control and at an ultrasonic intensity of 5.1 W cm-2 as a function of the electrode-tip surface distance, d. The limiting current density, jL, obtained from the current plateau, is given for the following equation [34]:
bulk m
L zFk c
j = (eq. 2)
where z is the number of electrons exchanged, F is the Faraday constant (96485 cul mol-
1), cbulk is the electroactive species concentration in the solution bulk and km is the mass transport constant which characterizes the mass transport conditions from and/or towards electrode surface and, therefore, denotes a local measurement of the hydrodynamic conditions.
-PbO2/Pb2+ redox agent
The electrodeposition of lead dioxide from aqueous solutions of Pb (II) has been extensively studied and its thermodynamic and kinetic behaviour was analyzed [35-38]
in static [39], pure convection-diffusion [40, 41] and ultrasonic [42, 43] conditions. At the initial stage, the process proceeds through the three-dimensional progressive nucleation and crystal growth mechanism under crystallization overpotential. In the literature [44-46], we can find theoretical models to analyze curves of current density, j, vs time obtained by chronoamperometry technique which provide quantitative values of
the kinetics parameters of the process: three dimensional nucleation constant N0A and the growth constant k. In the case of the lead dioxide electrodeposition on an inert substrate like a vitreous carbon electrode, the typical j vs t curve obtained under crystallization overpotential is shown in figure 5, where two new kinetic parameters have to be added, the induction time t0 and the current density at induction time j0. Different electrodeposition mechanisms have been proposed [35, 47, 48] but, in spite of that, the first step in all mechanisms corresponds to
e H OH O
H2 → ads+ + + (5)
where the generation of OHads is widely accepted. Taking into account that OH· radicals are generated by the ultrasound in aqueous solutions [49], the electrodeposition of lead dioxide is strongly influenced by the presence of an ultrasonic field, thereby modifying the induction time value from potential step experiments [50]. By considering the hypothesis that during the induction time, the limiting rate step of the kinetics is associated with the first mechanism step (adsorption of OH on the substrate), the measurement of this kinetic parameter would be correlated with the local OH· concentration produced by ultrasound.
In order to analyze the feasibility of this electrochemical system such as electrochemical probe for the local production of radicals during the cavitation events, the influence of the variables such as ultrasonic intensity and electrode-emitter surface have been analyzed. Figure 6 shows the chronoamperometric curves for PbO2 from Pb(II) solution on a glassy carbon electrodes at different global ultrasonic intensities at a distance electrode-emitter surface of 1 cm. It can be seen a decrease in the induction time with global ultrasonic intensity increase. This behaviour is in agreement with the hypothesis presented in the above paragraph, the OH· generation is increase with higher
global ultrasonic intensities and a decrease in the induction time is observed when we use these higher values.
Figure 7 shows the chronoamperometric curves for PbO2 deposition from Pb(II) solution on a glassy carbon electrode at different distances electrode-emitter surface at an ultrasonic field of 5.1 W cm-2. These results are in agreement with those obtained by thermal probe method [20] using the same sonoreactor (local maxima located to 2.5 cm in the axial direction) and therefore, with the widely accepted relationship between high temperatures and radicals formation.
In order to compare the different results obtained from the two electrochemical probes, the dependence on the Sherwood number (characterizing hydrodynamics conditions obtained from Fe(CN)63-
/Fe(CN)64-
redox agent) and t0-1
(characterizing the OH· production rate obtained with PbO2/Pb2+ redox agent) is shown in figure 8. From the former, Sherwood number is defined as Sh =kmr/D, being r the radius of the electrode and D the diffusion coefficient of the electroactive species. The electrochemical study has been carried out for the first centimetres near the emitter where the ultrasonic field (“near field”) is known to be very heterogeneous. The decrease in the sherwood number with the electrode-tip distance is in agreement with other results reported elsewhere [9, 14, 15] as well as with the results obtained in part I [20], where the cavitation events have not been considered.
Nonetheless, a different behaviour has been obtained using the PbO2/Pb2+ redox agent, monitored by the t0-1
vs distance relationship, and therefore the PbO2/Pb2+ system could be use as method for the local discrimination of chemical (related to radical
formation) and physical (related to microstreaming and shock waves) effect from the cavitation events.
4. Conclusions
The 20kHz-100 W Sonoreactor from Undatim have been fully characterized by classical and new methods in order to be used in sonoelectrochemical experiments. As a general conclusions, results from part I (physical effects) and part II (chemical effects) provide an accurate picture which identify a good location for the several tools needed in the experiments: cooling coil, thermometer, reference, auxiliar and working electrodes, etc. There is a gradual decrease in stirring effect from the emitter surface in contrast to the chemical effect “pseudo” sequence. In order to obtain reproducible responses from electrochemical experiments, the influence of the electrode-emitter surface distance in the axial direction has been carefully analyzed.
In addtion, this preliminary study presents the electrodeposition of lead dioxide from Pb(II) solutions such as a promising probe to detect locally chemical effects (OH· radicals formation) opposite Fe(CN)63-/Fe(CN)64- probe related to pure mass transport events. Further work will be done with larger solution volumes and other sonoreactors in order to carry out a fully complete validation.
In the same way, it must be stressed that glassy carbon is not the best carbonaceous electrode material due to its lacks of reproducibility, and so currently a forthcoming paper is carrying out with boron-doped diamond (BDD) electrodes whose surfaces provides potentially better reliability and reproducibility for analytical studies.
Acknowledgements
The authors would like to thank Universidad de Alicante for financial support under project GR03-05.
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Figure captions
Figure 1.- Diagram of experimental set-up: (1) ultrasonic probe: (2) transductor: (3) working electrode: (4) counter electrode: (5) reference electrode/Luggin system: (6) gas passing: (7) electrolyte: (8) cooling jacket: (9) Teflon adapter: (10) O-ring joints.
Figure 2.- Yields (G) of Fe3+ () and I2 () as a function of the global ultrasonic intensity.
Figure 3. Cyclic voltammetry for the electrochemical behaviour of 5 × 10-3 mol dm-3 K3Fe(CN)6 in 1 mol dm-3 KOH using a glassy carbon electrode at silent conditions.
T=298 K. Sweep rate: (a) 10, (b) 20, (c) 50, (d) 100, (e) 200 mV s-1.
Figure 4.- Potential step experiments (j vs t) for the reduction of 5 × 10-3 mol dm-3 K3Fe(CN)6 in 1 mol dm-3 KOH at a glassy carbon electrode as a function of the electrode-emitter surface distance: (a) 1 cm, (b) 1.5 cm, (c) 2 cm, (d) 2.5 cm, (e) 3 cm, (f) 3.5 cm, (g) 4 cm. Ef = +130 mV vs SCE. T = 298 K. I = 5.09 W cm-2.
Figure 5.- Typical j vs t curve for the electrodeposition of lead dioxide under a crystallization overpotential.
Figure 6.- j vs t curves for the electrodeposition of lead dioxide as a function of the global ultrasonic intensity. Inlet box shows induction time vs global ultrasonic intensity (values obtained from the numerical simulation of j vs t curves. 0.1 mol dm-3 lead (II) + 1 mol dm-3 perchloric acid. T=298 K, Ef = 1480 mV vs SCE.
Figure 7.- j vs t curves for the electrodeposition of lead dioxide as a function of the electrode-emitter surface distance: (a) 1 cm, (b) 1.5 cm, (c) 2 cm, (d) 2.5 cm, (e) 3 cm, (f) 3.5 cm, (g) 4 cm. Electrolyte 0.1 mol dm-3 lead (II) + 1 mol dm-3 perchloric acid.
T=298 K, I = 5.09 W cm-2. Ef = 1420 mV vs SCE.
Figure 8.- Sherwood number () and inverse of the induction time () vs distance electrode-emitter surface. Diffusion coefficient for Fe(CN)63-
ion taken from reference 51.
Sweep rate/ mV s-1 ∆∆∆∆Ep / mV -jcp/jap
10 65 0.92
20 69 0.94
50 76 0.93
100 75 0.94
200 108 0.97
Table 1.- Experimental values for the reversibility test obtained from cyclic voltammograms for the electrochemical behaviour of Fe(CN)63-
/Fe(CN)64-
redox couple.
