2541
KINETICSOF FASTELECTRODE PROCESSES WITH ESR
Study on the Kinetics of Fast Electrode Processes with Pulse Technique and Electron Spin Resonance Methods by Ryo Hirasawa, Takashi Mukaibo, Department of Industrial Chemistry, the University of T o k y o , Bunlcyo, T o k y o , J a p a n
Hideo Hasegawa, Noboru Odan, and Tetsuo Maruyama J a p a n Electron Optics Laboratory, A k i s h i m a , J a p a n
(Received J a n u a r y 16, 1968)
Experiments have been carried out demonstrating a technique by which it is possible to observe, with esr methods, in a direct fashion the complete curve of the fast formation and the decay (in milliseconds) of radicals produced by pulse electrolysis. A loss of the signal-to-noise ratio for a rapid esr observation is compensated for by means of average accumulation with a spectrum computer. Some organic compounds such as dibutyl phthalate, anthracene, nitrobenzene, stilbene, diphenylacetylene, and tetraphenylethylene were treated with this method in acetonitrile solution with tetra-n-propylammonium perchlorate or bromide as the supporting electrolyte, There are three types of reactions in which the rate-determining step is in one of the following three consecutive processes: (i) in the electron transfer between the electrode and a depolarizer; (ii) in the desorption from the electrode; and (iii) following the reaction in the solution. The technique is useful not only in the investigation of kinetics and mechanisms of electrode processes but also for the study of the reactivity of paramagnetic species produced by the electrolysis.
Introduction
Experimental Section
Techniques to determine the complete curve of the fast formation and the decay of radicals produced by pulse radiolysis’ or the pulsed electron beam2 were reported. A computer of average transients (C.A.T.)l or a “boxcar” integrator2 in conjunction with an esr spectrometer for the enhancement of the signal-to-noise ratio of the curve was used. This paper is concerned with a technique for the study of the kinetics and mechanisms of short-lived paramagnetic intermediates (the lifetimes of which are longer than a few milliseconds) produced by pulse electyolysis. Thjs technique is the combination of the recording of the esr signal intensity vs. time (8-t curve) a t a fixed magnetic field and the use of a spectrum computer in treating the recordings of esr and the improvement of the signal-to-noise ratio. The pulse electrolysis technique, using a constant-current pulse, provided a powerful tool for the formation of organic intermediatesa and was used in the measurement of the electric potential-time curve (E-t curve) for the study of mechanism of the electrode processes, the same as the galvanostatic-transient method. Some organic compounds such as dibutyl phthalate, nitrobenzene, anthracene, stilbene, diphenylacetylene, and tetraphenglethylene were treated with this method. It was possible to clarify the mechanism of the electrochemical formation of radicals generated from these substances and also to investigate the kinetics of decay behaviors of these radicals. The mechanisms, in all cases studied, are classified into three types according to the difference in the rate-determining step in the formation process of radicals.
Equipment and Procedure. The block diagram of this method is given in Figure 1. Paramagnetic species are generated by the constant-current pulse electrolysis in an electrolytic cell placed directly into a microwave resonance cavity, so that it is possible to observe fast electrode processes involving adsorption and desorption on the electrode surface. The solution is pumped out through the electrolytic cell by a stroke pump or is circulated if the circulation of the solution does not affect any results. The electrode surface is renewed by the flow of the solution before the next pulse is charged. However, it was confirmed that the relation of the esr signal intensity to time was not influenced by the flow of solution during a certain time, which was determined by the flow rate of the solution. Only during the time, e.g., 580 msec at 20 cm/sec, was it possible to observe a significant relation between the concentration of the paramagnetic substance and the time. The 8-t curve was observed at a magnetic field fixed to the maximum or minimum intensity of a first-derivative signal under overmodulation. Because of the small amount of electricity in one shot pulse and the weak esr signal intensity of the rapid observation, which requires a spectrometer of short response time, (1) L. H. Piette, presented a t the Sixth International Symposium on Free Radicals, Cambridge, England, July 1963. (2) R. W. Fessenden, J . P h y s . Chem., 68, 1508 (1964). (3) M. Fleischmann, I. N . Petrov, and W. F. K. Wynne-Jones, Proceedings of the First Australian Conference on Electrochemistry, Sydney, Australia, Feb 1963. (4) P. Delahay, “New Instrumental Methods in Electrochemistry,” Interscience Publishers, Inc., New York, N. Y., 1954, pp 179-216. V o l u m e 72,N u m b e r 7
J u l y 1968
R. HIRASAWA, T. MUKAIBO, H. HASEGAWA, N. ODAN,AND T. MARUYAMA
2542 ESR Spectrometer
I
I
4-1
Generator
Figure 1. Schematic block diagram for the study of fast electrode processes.
a spectrum computer is employed for the enhancement of the signal-to-noise ratio as an average-response c~mputer.~ The output from the esr spectrometer is transformed into digital numbers and is stored in a memory core of the spectrum computer triggered with a pulse generator. The summation of many transits is recorded, and the accumulated data are displayed on a recorder after the transformation from digitaI to analog. The signal-to-noise ratio is proportional to the square root of the response time and the square root of the number of the repetition of the experiments.6 The effect of the accumulation is shown in Figure 2, which was obtained from the electrolytic pulse reduction of dibutyl phthalate in acetonitrile. A usual esr spectrum is composed by plotting S-t curves obtained at each point of different magnetic fields. Furthermore, for obtaining an esr spectrum, the rapid scanning of the magnetic field was combined with the technique described above successfully.6 The relation of electric potential near the working electrode to time is observed with a cathode-ray oscilloscope and is recorded by a camera. Varying the pulse width and the current density, one may select any electrochemical condition corresponding to a potential plateau in the E-t curve. The Pulse Generator. The pulse generator consists of two circuits of a monostable oscillator similar to a usual pulse generator7 and supplies a rectangular constantcurrent pulse of 500 mA at the maximum. The pulse width and the pulse interval vary from 1 msec to 16 sec, independently. T h e Electrolytic Cell. The electrolytic cell and its position in the microwave cavity are given in Figure 3. Both wire electrodes, platinum anode and silver cathode, are located in a quartz tube of which the inner and the outer diameter are 1.7 and 5 mm, respectively, and are kept strictly parallel with polyethylene resin, even outside of the resonance cavity with the same configuration as inside.* The working electrode, 0.5 mm in diameter and 8 cm in length, of which the lower 4.5 cm is placed in the resonance cavity, has an area of ca. The Journal of Physical Chemistry
-
m
? I 4 rn sec
Time
-+
Figure 2. Esr signal intensity us. lime curves of the dibutyl phthalate anion radical, which was generated by the electrochemical reduction of 1.0 X mol/l. of the parent molecule in acetonitrile with 0.1 mol/l. of tetra-n-propylammonium perchlorate: upper trace, one traverse; middle and lower trace, the results of the accumulation of 256 and 4096 traverses, respectively. The pulse width was 4 msec.
1.2 em2. The counter electrode has a radius of 0.3 mm. An aqueous saturated calomel electrode makes an electrolytic contact with the working electrode through a sintered-glass disk and a Luggin capillary. The resistance of the cell circuit was 12-15 ohms in the case of the solutions studied. The stroke-pump empIoyed covers the Aow-rate range from 1 to 250 cm/sec at the cell. T h e Electron S p i n Resonance Spectrometer. The esr spectrometer used in this study is partly different from the Japan Electron Optics Model 3BS-X, which has a Hall element to control the magnetic field and a vernier scale to set it. The output of the spectrometer is directly introduced from the end of the phasesensitive detection unit, in which the capacitance is exchanged, so that the response time of the spectrometer becomes, theoretically, 0.5 msec. The Spectrum Coinputer. The spectrum computer, Japan Electron Optics Model JRA-5, was used which has 4096 memories for the operation program and the ( 5 ) M. P. Klein and G. W. Barton, Jr., Rev. Sci. Instrum., 34, 754
(1963). (6) R. Hirasawa, T. Mukaibo, H. Hasegawa, Y. Kanda, and T. Maruyama, {bid., in press. (7) W. Jaenicke and H. Hoffman, Ber. Bunsenges. Phys. Chem., 66, 803 (1962). (8) If the species generated a t the counter electrode is observed with esr spectroscopy together with these a t the working electrode, the counterion of the supporting electrolyte which is more d e polarizable than the sample is required.
2543
KINETICSOF FAST ELECTRODE PROCESSES WITH ESR
fied by the method of Coetaee, et aL9 After distillation with phosphorus pentoxide, the solvent had a boiling point of 81.5” at 760 mm. Tetra-n-propylammonium perchlorate, tetramethylammonium perchlorate, and tetra-n-butylammonium bromide, for use as supporting electrolytes, were prepared by neutralizing an aqueous solution of tetraalkylammonium hydroxide, obtained as a 10% aqueous solution from Tokyo Kasei Kogyo. These salts were purified by the method of Geske and Maki.lo Dibutyl phthalate, nitrobenzene, anthracene, stilbene, diphenylacetylene, and tetraphenylethylene (all reagent grade) were used without further purification. Discussion of the Experimental Method. The electrode process under study is expressed as O + n e i - ~ k+ ~
(1)
where 0 is the oxidant, R is the reductant, and P is the product. The flow of the solution is laminar in the method described above and is parallel to the electrodes, and the measuring time is so short as to allow one to suppose that the wire electrode employed is plane. l 1 Thus the phenomenological equation involving diffusion, flow, and chemical reaction is
B
A
E
Figure 3. Electrolytic cell and its placement in the microwave resonance cavity: A, working electrode; B, counter electrode; C, Luggin capillary; D, inlet of solution; E, outlet; F, Teflon holder; G, polyethylene; H, quartz tube; I, resonance cavity.
data. The computer has a high speed A-D converter of successive comparison system and a D-A converter. The operating conditions are given in Table I. The solution consists of a 1-10-mmol/l. depolarizer and a 0.1-mol/l. supporting electrolyte, both dissolved in acetonitrile. Reagent grade acetonitrile was puri-
where CR is the concentration of the reductant R, u is the flow rate of the solution parallel to the electrode, r is the perpendicular distance from the axis of the wire electrode (the y axis), DR is the diffusion coefficient of the reductant, k is the rate constant of the first-order (or pseudo-first-order) reaction, and t is the time since the initiation of the pulse electrolysis. Since both wire electrodes are held parallel and only these lower parts are observed, one may assume that (3) during the time before no edge effect appears at the part of the electrode within the resonance cavity. Thus (4)
-
Table I : The Operating Conditions of the Spectrum Computer
Program
A
B C
Sampling period, pseo/point
20 100 360nb
Maximum numbera of sampling points
Maximum number of accumulation times
2500 760 3100
1,024 66 ,000 1,024
a The measuring time and the minimum experimental period are equal to the product of the sampling period and the number n = 1, 2, . . ., 256. of sampling points employed.
The initial and boundary conditions are: CR(1”, 0) = 0; C R ( m , t ) = 0; and -DRCR(~C,t ) / r = i/nF, where ro is the radius of the electrode and i is current density. The esr signal intensity is given by solving eq 4 (9) J. I?. Coetzee, G. P. Cunningham, D. K. McGuire, and G. R. Padmanabhan, Anal. Chew., 34, 1139 (1962). (10) D. H. Geske and A. H. .Maki, J . Amer. Chem. Soc:, 82, 2671 (1960). (11) P. Delahay, “New Instrumental Methods in Electrqchemistry,” Interscience Publishers, Inc., New York, N. Y., 1954, pp 68-70. Volume 72, Number 7
July 1068
R. HIRASAWA, T.MTJKNBO,H. HASE~AWA, N. OVAN,AND T.MARWYAIUA
2544
S
where
T
i JmmCndr = -(1
nFk
- e-*)
(t
5
T)
(5s)
is the pulse width. For
kt
T ) (7b) Relation 7 is also derived directly, under the wndition that the current pulse is short compared with the
flow of solution through the cell. Nitrobenzene in acetonitrile solution undergoes an initial one-electron reduction near the half-wave potential of the first wave in the polarogram, as shown in reaction 1." Since nitrobenzene anion radical is very stable in acetonitrile, relation 6 \vi11 be satisfied. Figure 4 is representative of the S-t curve of the nitmbenzene anion radical produced by the pulse electrolysis of nitrobenzene in a potential range corresponding to the first plateau in the galvanostatic transient. The signal intensity of the radical increases proportionally to the time elapsed since the beginning of the pulse electrolysis during the charging at a constant current and remains wnstant after the current is cut off. These behaviors agree with eq 7. However, this agreement is affected 580 msec after beginning to charge the pulse at the flow rate of 20 cm/sec. This suggests that the concentration profile along the y axis in the resonance cavity begins to change and eq 3 is not wrrect after this time. It may be noted that if the additional magnetic field generated by the current through electrodes is large in wmparison with the sharpness of the signal shape, the S-t curve is not available for kinetic studies so long as the pulse current is charged. For instance, the influence appeared in a measurement on the dibutyl phthalate anion radical overmodulated at 15 G in the case of the pulse current more than 3-50mA.
Figure 4. Formation curve (esr signal intensity va. time) of the nitrobenzene anion radical which was generated in the potential range oi -0.5 to -0.0 V "a. sce (the first plateau in the E 4 curve). The pulse width was 154 msec. The . l a r d o/-Phy&d
Chimry
501 0
0
LOO
ma
Time [ rn see I
Figure 5. E 4 and S-l curves in the pulse electrolysis of 1.6 X lo-' molA. of nitrobenzene in amtonitrile with 0.1 molA. oi tetra-n-propylammonium perchlorate at various current densities. The pulse width was 500 m e .
Results and Discussion Typical S-t and E-t curves for the reduction of organic compounds are given in Figures 5 and 6 at several current densities and in Figure 7 for different pulse widths. The galvanostatic transient gives rise to several distinct plateaus in the E-t curves for various current densities and pulse widths. The behavior of the 8-1 curves wrresponds to the difference of the plateaus. As shown in Figure 5, for the reduction of nitrobenzene when the electric potential is on the first lowest plateau, the signal intensity of this radical increases linearly with time at several current densities, while in the sewnd-plateau range, the velocity of the increase of the signal intensity becomes smaller than in the first, and if the potential is raised to the third stage, it discontinuously increases immediately after the current is cut off.l2 Similarly for antbracene, in the first plateau the signal intensity of the anthracene anion radical increases linearly with time, but in the second i t decreases (Figure 6). I n Figure 7, the S-t curves and the E-t curves for tetraphenylethylene are shown at various pulse widths. The signal intensity in the third plateau increases continuously even after the current is cut off. Similar observations were made for the reduction of dibutyl phthalate, stilbene, and diphenylacetylene. (12) In the electrochemical reduction of nitrobenzene with tetranbutylammonium bromide. the =me results 8s with tetm-n-propylammonium perchlorate wem obtained. Even if the deotrochemieal resetion is made with a high current density and long pulse width, an esr signal was not observed for the reaction of tetra-nhutylammonium bromide only. while radienls were generated from perchlorate snion on an anode. a8 shown in Figure IO. Then we find that the supporting electrolyte does not nReet the esr signal intensity under the condition employed for the reduction of nitrobenzene.
2545
KINETICSOF FASTELECTRODE PROCESSES WITH ESR
-
I
W
'
Time I m sec) 15 30
I
0
.-
I
!
Current
1
.
45
I
I
5r c
c
c .*
a.
ii
w
0 Current density
.-2 e 0v)
;
I
7l
.-
.LI
j
300 Time (rn secl
c) +s
.-c -
: I
I
I
150
0
-LO
-
I
i
w
g
C
i
ml
is0
Figure 8. Three types of the esr signal intensity-time relation.
\\
.-m
-80
v, W
400
0
800
Tirne(rn s e c )
Figure 6 . E-t and S-t curves in the pulse electrolysis of 1.0 x 10-2 mol/l. of anthracene in the same solution as nitrobenzene. The magnetic field was fixed a t the field of minimum esr intensity of a first-derivative signal of the anion overmodulated a t 5 G. The pulse width was 400 msec.
-
.-
L
E
u
.-E 20
-b
10
. I -
..,....
O
$ 0 K
o
IO
20
Cur te n t .--a
20 m sac
Time
--i
Figure 7 . E-t and 8-t curves in the pulse electrolysis of 6.0
30
10
d e n 4 t y ( m A/cm2
Figure 9. Rate of formation of the nitrobenzene anion radical vs. current density in the first-potential plateau.
X lod3mol/l. of tetraphenylethylene in the same solution as nitrobenzene for various pulse widths, indicated by a, b, e, and d.
The current density was 10 mA/cm2.
A. Formation Process of Radicals. The formation processes of radicals observed by esr on each plateau of electric potentials are classified into three types, as shown in Figure 8 , and are discussed below. (i) Direct Radical Formation Mechanism o n the Electrode. The first type of S-t curve is shown in Figure Sa. The esr signal intensity increases in proportion to the time elapsed after beginning to charge the pulse current. The rate of increase of the esr signal intensity is proportional to the current density for the electrolysis, as shown in Figure 9, which was obtained in the electrochemical reduction of nitrobenzene at the first plateau of E-t curve. On the basis of the eq 7, this result indicates that the rate of formation of radicals is controlled with the current density. Similar observations were made for the reduction of anthracene, dibutyl phthalate, stilbene, diphenylacetylene, and tetraphenylethylene on each potential plateau, as summarized in Table 11.
(ii) Desorption Mechanism from the Electrode. I n the second type, the signal intensity begins to increase immediately after the current is cut off, and then the depolarization on the electrode decreases, as shown in Figure Sb. The amount of radicals generated in this type is related to the depolarization potential. The reduction of nitrobenzene and also tetraphenylethylene at the third and the fourth plateau, respectively, belongs to this type. More typically, the oxidation of perchlorate anion demonstrates a similar behavior, as given in Figure 10. Both the amount of increase in the esr signal intensity of this radical,13 AS, and the (13) The esr spectrum of this radical agrees with that assigned to the perchlorate radical generated electrochemically in acetonitrile by Maki and Geske (A. H. Malti and D. H. Geske, J . Chem. Phys., 30, 1356 (1959)). The lifetime of this radical is about 10 sec in acetonitrile. I t s hyperfine splitting of 17 G is small, in comparison with 50 and 72 G which were obtained from perchlorate radicals trapped in magnetically distinct sites in an irradiated crystal of KC104 (J. R. Morton, J . Chem. Phys., 45, 1800 (1966)), but is similar to the hyperfine splitting of ClOz radical in several solvents such as HzO, HzS04, and CCh, although the ClOz radical is stable in these solvents (N. Vanderkool, Jr., and T. R. Poole, Inorg. Chem., 5 , 1351 (1966)). Volume 73, Number 7
July 1968
R. HIRASAWA, T.MUKAIBO, H.HASEGAWA, N. ODAN,
2546
bCu'rrent den a . 3 3 0 ml b. 250 E.
.
.
b,
u)
w
T. n/lARUYAMA
/",
200
r,
c
t-
AND
-.-
LO
-
20
-
2.
I
I
I
I
0
200
LOO
600
I
Time(m sec)
Figure 10. E-t curves in the pulse electrolysis of 0.1 mol/l. of tetra-n-propylammonium perchlorate in acetonitrile a t various current densities.
velocity of increase immediately after the current is cut off, (dS/dt),=,, are proportional to the amount of the electricity for the oxidation at the first plateau in the potential range of 1.0-1.1 V us. sce (Figure 11). Assuming that the amount of the electricity supplied on the anode surface is proportional to the amount of adsorption, these behaviors are explained kinetically when the desorption from the electrode is the rate determining factor. (iii) Following Reaction Mechanism in Solution. The third type is shown in Figure 8c. As the signal intensity increases continuously even after the current is cut off, the rate-determining step is considered not to be directly related to the electron transfer. However, the maximum value of the S-t curve increases with amount of electricity for the electrolysis in the potential plateau related to this type. For example, tetraphenylethylene reacts at the electrode in the first plateau with the electron-transfer mechanism in the third plateau, which is in a more negative potential range than the first one. At the potential on the third plateau and at a current density of 10 mA/cm2, it is found that the concentration of radicals increases at two different rates even after the current is cut off, as given in Figure 7. At the higher current density or longer pulse width, it becomes impossible to distinguish these two processes in the S-t curve. However, it was observed by means of the rapid scanning of the magnetic field during the electrolysis where the potential is on the third plateau that two radicals are detected almost overlapping each other in the esr spectrum, observed under the condition of the exchange broadening. On the other hand, in the first plateau, a kind of radical is produced, of which the spectrum coincides with that of a kind of radicals in the third plateau in respect to the peak-to-peak value of 6.2 G, as shown in Figure 12. On the basis of the foregoing evidence, it is appropriate to assume that these two radicals are produced by means of the chemical reaction between the reduction products, e.g., the tetraphenylethylene dianion, generated electrochemically in the third plateau, and the surrounding molecules. This assumption is consistent with the experimental results of the large-scale electrochemical reduction of tetraphenylethylene (5.0 g) to The Journal of Physicml Chemistry
TI
VI
q
I
I
LO
60
10
Amount of clectrisityhq) Figure 11. A S and (dS/dt)t,~ao,,,, us. the amount of electricity for the oxidation of perchlorate anion.
(a) (b) Figure 12. Rapid-scan spectra observed in the first plateau (a) and in the third plateau (b).
1,1,2,2-tetraphenylethane(2.4 g) and diphenylmethane (2.0 g) in acetonitrile with tetra-n-butylammonium iodide as the supporting electr01yte.l~ Thus in this third type of S-t curve, the rate-determining step for the formation of radicals is regarded as a chemical reaction following the electron transfer and/or a charge transfer between reduced products and the surrounding molecules in the solution. All results of the pulse electrolysis for the compounds studied are summarized in Table 11. B. Decay Process of Radicals. In the case where the chemical reaction following the electron transfer is a first-order (or pseudo-first-order) reaction, the rate constant of the reaction for the radical produced in the direct radical formation mechanism is calculated from eq 5b. For the dibutyl phthalate anion radical produced by the electrochemical reduction in acetonitrile, it was found from the decay behavior of the S-t curve that the logarithm of the signal intensity is proportional to the time elapsed since the current is cut off. Because the reduction product is dihydrodibutyl phthalate, it seerns (14) S. Wawzonek, E. W. Blaha, R. Berkey, and &I. E. Runner, J. Electrochem. Soc., 102, 235 (19.55)
KINETICSOF FASTELECTRODE PROCESSES WITH ESR
2547 1
'
Table I1 : Results of Pulse Electrolysis"
Eri p b
Compound
Nitrobenzene
Anthracene Dibutylphthalate Stilbene
Diphenylacetylene
Tetraphenylethylene
Potential plateau
(V
U8.
see)
1st 2nd 3rd 1st 2nd 1st 2nd 1st 2nd 3rd 4th 1st 2nd 3rd 4th 1st 2nd 3rd
-0.6 -1.4 -2.0 -1.0 -1.6 -0.5 -1.2 -1.0 -1.4 -1.7 -2.0 -0.9 -1.1 -1.7 -2.0 -0.7 -1.2 -1.4
4th
-1.7
0
Mechanism for formation of radicals C
f d C
DBP k-25.5 se$
Half-life of radicala, maw
i i i i
9
h 27
C
h h C
120
9
i
9 C
180
f
180 -180 -10
e e
t
150
50
Timetrn sed
h h C
I '
d
'
a All experiments performed a t 22". Reference 4. Direct radical formation. Desorption. Following chemical reThe current efficiency for the formation of radicals action. is smaller than in the first potential plateau. Anion radicals produced in the more positive potential plateau disappear in this plateau. Independent of radicals. ' The half-life of the radical is too long to be measured.
'
that the radical disappears according to a pseudo-firstorder reaction between the solvent and the anion radical. The rate constant is 25.5 sec-l. Similarly, the rate constant found for the pseudo-first-order
Figure 13. Rate of the decay process of the dibutyl phthalate anion radical in acetonitrile. The concentration of dibutyl phthalate and phenol was 2.0 X 10-8 and 0.1 mol/l., respectively.
disappearance of dibutyl phthalate anion radical in acetonitrile with phenol is 42.5 sec-l (Figure 13). On the other hand, if the chemical reaction following the electron transfer is second order for radicals, we cannot investigate the kinetics of disappearance of these species in a direct fashion as described above. The conclusion is rather tentative regarding the "chemistry" as it is in most cases discussed in this article.
Acknowledgment. The authors wish to acknowledge many useful discussions with Dr. K. Fuelri. The authors are also grateful for the assistance of Mr. Y. Sugimoto and Alr. Y. Kanda in treating several of the electrical problems encountered in this work.
Volum 78, Number 7 July 1968
