6104
J. Phys. Chem. 1993,97, 6104-6106
Simultaneous Oscillations of the Surface Mass and Potential in the Course of the Galvanostatic Oxidation of 2-Propanol Gyorgy Inzelt,' Vilmos Kerthz, and Gyozii Ldng Department of Physical Chemistry, Ebtvbs Lorhnd University, Budapest 112, P.O. Box 32, H-1518, Hungary Received: March 15, 1993; In Final Form: April 15, 1993
The results presented on simultaneous oscillations of the potential and surface mass during the galvanostatic oxidation of 2-propanol in acid media are the first experimental evidence that the oscillation arising in the course of galvanostatic oxidation of simple organic compounds is associated with the accumulation and consumption of the chemisorbed species. The results of the electrochemical quartz crystal microbalance experiment may open up new vistas in the quantitative description of the heterogeneous oscillation phenomena.
Introduction Oscillatory behavior in electrochemical systemshas been known to exist for a long time and has been the topic of many experimental and theoretical studies. l p 2 Periodical changes in the potential under galvanostaticor open-circuit condition^^-^ or in the current under potentiostatic conditionss-10 have been observed for very different systems. Typical examples are the potential oscillations in the case of oxidationof small organiccompounds on a platinum electrode in an acidic solution under galvanostatic conditionsM or at open-circuit conditions when the constant flux of electrons to or from the electrode is realized by appropriate redox systems in the solution^.^ All oscillation mechanisms which have been proposed so far involve the accumulation and consumption of adsorbed species or films at the s~rface.~-lOThere is, however, little direct information concerning the processes taking place on the surface of the electrode. Some efforts have been made to follow the changes of the surface composition by radiotracer" and ellipsometriclI methods. The electrochemical quartz crystal microbalance (EQCM)12 may supply direct evidencein this respect. It allows thecontinuous measurement of the surface mass changes without disturbing the primary experimental conditions. In this paper, we report experimental results on the periodic changes of the surface mass in the case of galvanostaticoxidation of 2-propanol in acid medium on platinized platinum. As far as we are aware, this is the first experimental evidence of this phenomenon. Experimental seftion
Ten megahertz AT-cut crystals were used. Each side of the crystals was coated with gold by the standard evaporation technique. A crystal coated with gold was mounted in a suitably formed part of a holder. This holder also contained the oscillator circuit which was isolated from the solution. Only one side of the crystal was exposed to the electrolyte solution. The connections to the gold coatings of both sides of the crystals were made with gold foil. This arrangement allowed us to use the crystal as a normal plate working electrode (geometrical area 0.3 cm2). (A detailed descriptionof this recently developed EQCM instrument will be published.) A Pt wire was used as a counter electrode. The referenceelectrodewas a saturated sodium calomel electrode (SCE). In order to obtain high, thus more reliable, frequency responses, platinized platinum working electrodes with surface roughness of ca. 400 were used. The platinum layer was electrodepositedgalvanostatically from 1% H2PtCls solution. The thickness of the layer was ca. 10 pm,
* To whom correspondence should be addressed. 0022-3654/93/2097-6 104$04.00/0
which value was calculated from the frequency decrease caused by the platinum layer and by using an integral sensitivity, Cf= 2.264 X lo8 Hz cm2g-I. The roughness factor was determined from the area of the cyclic voltammetric hydrogen waves and from charging curves. 2-Propanol (p.a, Reanal) was dissolved in 1 mol dm-3 aqueous HC104 (p.a, Merck) solutions. The concentrationof 2-propanol varied between 0.01 and 2 mol dm-'. The reaction temperature was 25 OC. All solutions were purged with oxygen-free nitrogen and stirred. An Electroflex 450 potentiostat and Universal Frequency Counter TR-5288 connected with an IBM personal computer were used for the control of the measurements and for the acquisition of the data.
Resde and Discussion Although the common kinetic principles for the Occurrence of electrochemicaloscillations1V2and the models for several known ~ y s t e m s + ~ have J ~ J ~been developed during the past two decades, it is still problematic to design a system of periodical behavior. In addition, if we want to follow the mass changes on the electrode surface, special experimental difficulties arise. The main problem is related to the reliable measurements of the frequency (mass) changes. No smaller value than 1 Hz can be measured accurately, and the frequency can be detected once every second, only. Therefore, we increased the real surface area by using a platinized platinum electrode and varied the concentration of the organic compound and the current densityuntil the time constant of the oscillation fell into the measurable region. The choice of 2-propanol is supportedby the fact that the product of the oxidation is acetone;i.e., there is no intensive gas evolution occurs, as e.g., in the case of formic acid which may make the frequency response noisy. A typical oscillation pattern is presented in Figure 1. This result was obtained when a constant current density of 7 X 10-4 A cm-2 (calculated by using the geometrical surface area) was applied to a platinized platinum electrode immersed in 1 mol dm-3 HClO, solutions containing 1 mol dm-3 2-propanol at 25 OC. It may be Seen from this figure that the rapid decrease of the potential is accompanied by a fast decrease of the frequency, Le., by an increase of the mass on the surface. As the potential increases, again the frequency also increases. The latter changes are somewhat slower, which indicates that the rate of chemisorption is higher than the oxidation of the chemisorbed particles according to the model which will be discussed below. Another important feature of this oscillation is that the amplitudes of the potential and mass oscillations and the period time gradually increase. When the positive potential limit during the oscillation Q 1993 American Chemical Society
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Figwe 1. Typical oscillation patterns for galvanostatic oxidation of 2-propanol in 1 mol d d HClO4 at platinized platinum electrode: (a) potential oscillations,and (b) frequency oscillations. Current density is 7 X 1 W A cm-2, and concentration of 2-propanol is 1 mol dm-3. The potential is given on the SCE scale throughout this paper.
reaches a high value (ca. 600-650 mV vs SCE), the oscillation comes to an end, a phenomenon which may be attributed to the formation of PtOH and RO species on the surface. In this region the simultaneous oxidation of the chemisorbed species and platinum occurs, and when the potential region of 900-950 mV is reached, a rapid frequency decrease can be observed which in effect is dlte to the formation of PtO phase on the surface. Oscillationsstart after an induction period. The pattern shown in Figure 2 was taken after the experiments shown in Figure 1. The oscillation behavior appeared again. In general, under the same conditions (current density, concentration) the oscillation experiment can be repeated many times, although the perfect reproduction in respect of the actual frequency and potential values is practically impossible, which may be in connection with thenonequilibrium conditions. The induction period is most likely in connection with the slow accumulation of adsorbed poisonous species on the surface. The duration of the induction period strongly depends on the current density applied and the potential immediately prior to current application. When the current was switched on after a delay time (1-10 min) at the rest potential (ca. 100 mV vs SCE), a fast increase of the potential up to ca. 400-420 mV can be observed. The potential increase slows down, and a simultaneous frequency decrease occurs, indicating the accumulation of the chemisorbed species on the surface. As the potential reaches a value of ca.4 5 0 4 7 0 mV vs SCE the system becomes unstable, and the oscillation starts. For a detailed analysis a part of the oscillation patterns is enlarged in Figure 3. It is seen that the frequency changes follow the changes of the potential. This phenomena can be explained as f0llows.3.~At least three processes take place simultaneously
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Figure4. Cyclic voltammogram obtained for 1 mol dm-3 HClO, solutions containing 0.5 mol dm-3 2-propanol at a platinized platinum electrode. The geometrical area is 0.3 cm2,the roughnessfactor is 400, and the scan rate is 10 mV s-1.
at the electrode surface: chemisorption, oxidation of the chemisorbed species, and oxidation of the substrate on the free sites (not occupied by chemisorbed species). The chemisorption involves dehydrogenation, which causes a decreaseof the potential, the rupture of bonds, possibly the splitting of the molecule, and considerable charge transfer. Essentially, this is an oxidation process resulting in an adsorption product of high degree of oxidation. The oxidation of the products of chemisorption starts
6106 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
only above certain potential values (ca. 400 mV vs SCE in our case), and the chemisorbed species can be completely removed in the potential interval 800-1000 mV vs SCE. When the chemisorbed species are removed by oxidation from the surface, Le., the number of free sites is increased,the rate of the oxidation of the substrate yielding some definite end product (acetone in our case) increases. The cyclicvoltammetricbehavior of the compounds mentioned above supports this mechanistic model. As can be seen in Figure 4, when the cycle is started at low potentials, the anodic current during the positive-going scan is much smaller than during the reverse scan, when the electrooxidationof 2-propanol takes place at the cleaned surface. In conclusion, it may be stated that the electrochemicalquartz crystal microbalance technique, which renders the continuous monitoring of the surface mass changes possible, is a powerful tool to gain a deeper understanding of the electrochemical oscillation phenomena. The results presented in this paper furnished experimental evidence on the crucial role of the chemisorbed species in the origin of potential oscillation in the course of galvanostatic oxidation of simple organic compounds, an idea which has been proposed in the literature.>' Further
Letters study is in progress in our laboratory in order to give a quantitative description of the oscillatory phenomenon observed.
Acknowledgment. Financial support from the National ScientificResearch Fund (OTKA-2110) isgratefully acknowledged. We thank Prof. G. HorBnyi for his useful advice. References and Notes (1) Wojtowicz, J. Oscillatory Bchaviour in Electrochemical Systems. In Modern Aspects of Electrochemistry; Bock& J. O'M., Conway, B. E., Fds.; Plenum Press: New York, 1972; Vol. 8. (2) Franck, U. F. Faraday Symp. Chem. Soc. 1974,9, 137. (3) Horhyi, G.; Inzelt, G.;Szctey, E. J. Electroad. Chem. 1977,81, 395. (4) Horhyi, G.; Inzelt, G. J. Electroanal. Chem. 1978, 87, 423. (5) Okamoto, H.Electrochim. Acta 1992, 37, 37. (6) Okamoto, H.; Tanaka, N. Electrochim. Acta 1993, 38, 503. (7) HorBnyi, G.; VCrtes, G.; Kdnig, P.2.Phys. Chem. (Munich) 1973, 254, 298. (8) Albahadily, F. N.; Ringland, J.; Schell, M.J. Chem.Phys. 1989,90, 813. (9) Schell, M.; Albahadily, F. N. J . Chem. Phys. 1989, 90,822. (10) Bassett, M.R.; Hudson, J. L. J. Phys. Chem. 1988,92,6963,1989, 93, 2731. (11) Conway, B. E.; Novak, D. M. J . Phys. Chem. 1977,81, 1459. (12) Buttry, D. A. Applications of the quartz crystal microbalance of electrochemistry. In E/ectroanalyticul Chemistry;Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17.
