Electrocatalytic reactions in organized assemblies

201

Chem., 240 (1988) 201-216 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

J. Electraanal.

ELECTROCATALYTIC

REACTIONS

IN ORGANIZED

PART I. YUKON OF 4-BROMOBIP~~ NON-IONIC MICELLES

JAMES F. RUSLING Department

ASSEMBLIES

IN CATIONIC

AND

l. CHUN-NIAN SHI, DAVID K. GOSSER ** and SHYAM S. SHUKLA ***

of Chemistry (U-60), Unruersrty of Connecticut,

Storrs, CT 06268

(U.S.A)

(Received 14th August 1986; in revised form 20th July 1987)

ABSTRACT Electrocatalysis in micelles enables difficult redox reactions of non-polar substrates to be done in aqueous media, and may lead to altered kinetics compared to isotropic solvents. Herein we report quantitative reduction of 4bromobiphenyl (4-BB) to biphenyl in cationic micelles (0.1 M cetyltrimethylammonium bromide, CTAB) by electrcchemically generated anion radicals of 9-phenylanthracene (9-PA). In addition to electrocatalysis of the reduction of 4-BB, the reaction involves micellar catalysis of the forward electron transfer between 4-BB and the anion radical of g-PA. Compartmentalization of reactants in a hydrophobic aggregate phase at the electrode increases the effective rate constant for the forward electron transfer from 300 M-’ s-r m DMF to about 10’ M-’ s-i in 0.1 M CTAB. Contrary to DMF, in which the above reaction is rate determining, decomposition of the 4-BB anion radical product of this step exerts kinetic influence on voltammetric curves in CTAB. The positive surface potential of the aggregate and hydrophobic interactions stabilize the 9-PA anion radical against side reactions so that it can react with aggregate-bound 4-BB. Conversely, in non-ionic micelies of Igepal, competition between debromination of substrate and decomposition of the 9-PA anion radical diminished yields of biphenyl greatly.

INTRODUCTION

The use of surfactant water for electrochemical Studies of adsorption of of micelles were reviewed

micelles to solubilize non-polar organic compounds in measurements was first described by Proske [l] in 1952. surfactants at electrodes and electrochemical applications in 1979 [2]. Recent work has addressed mass transport of

* To whom correspondence should be addressed. ** Present address: City College of New York, City University, New York, NY 10031, U.S.A. *** Present address: Lamar University, Beaumont, TX 77710, U.S.A. 0022-0?28/88/$03.50

0 1988 Elsevier Sequoia S.A.

202

reactants and stability of products in electrochemical reactions in aqueous micelles [3-131. Surfactant assemblies have also been used to suppress unwanted electrode reactions and to form catalytic films on platinum anodes [14,15]. We were particularly intrigued by reports that ion radicals produced at electrodes could be stabilized by aqueous micelles. McIntire, Blount, and co-workers [3-51 found that micelles of sodium dodecylsulfate (SDS) stabilized both lo-methylphenothiazine cation radical and nitrobenzene anion radical. Saveant and co-workers [6] claimed stabilization of phth~o~t~le anion radical by micelles of cetylt~methylammonium bromide (CTAB). Kaifer and Bard [13] reported that methyl viologen cation radical was stabilized by coulombic and hydrophobic interactions with SDS micelles. Indeed, electrochemical [3-6,11,13] and photochemical 116-201 studies show that in general the stability of organic ion radicals in micelles depends on interactions of the ion with the surface charge and with hydrophobic sites of the micelle. Electrochemically generated anion radicals sufficiently stable in a micelle’s hydrophobic region should be able to transfer an electron rapidly to a suitable organic substrate resident in the same micelle. Such a reaction regenerates the parent of the anion radical to compiete a catalytic cycle. A related concept was first used by Kuwana and co-workers [21,22], who formed ferricinium ion at an electrode in non-ionic micelles for redox titrations of cytochrome c and cytochrome c oxidase. We recently began to explore such micellar el~trocatalytic systems because: (i) they may make possible in aqueous media difficult organic redox reactions now requiring toxic, expensive, organic solvents; (ii) the approach may lead to micellar enhancement of kinetics and improved control of reactions over that in isotropic, homogeneous media; and (iii) such studies could point the way to the design of electrocatalytic organized assemblies which mimic redox events in natural membrane-bound systems [19]. Our interest was initially directed to micelles as possible aqueous media for dehalogenation of organohalide pollutants. Electrocatalytic dehalogenation of halobiphenyls in dry N, N-dimethylformamide (DMF) proceeds by the pathway in Scheme 1 [23,24], similar to that for halobenzenes and pyridines [25]. The first step is fast transfer of an electron from electrode to catalyst A (eqn. 11, and the rate determining step (rds) in DMF is homogeneous electron transfer from the anion SCHEME 1 A-be-=A’-

(1)

ArX+A’-2ArX’-+A k,

(2)

AS--tjAr.+X-

(3)

Ar’+A’Ar-+(H+)

-+ A+Ar-+ ArH

(41 (5)

203

radical of the catalyst to halobiphenyl ArX (eqn. 2). This rds is thermodynamically unfavorable [25,26], and the overall reaction is driven by rapid cleavage of the anion radical of the halobiphenyl in eqn. (3). Subsequently, fast electron transfer and/or chemical steps [25], represented by eqns. (4) and (5), yield biphenyl as the ultimate product [23,27]. In reactions following Scheme 1, an increase in cathodic current for the reduction of the catalyst occurs upon addition of substrate because of recycling of A at the electrode. Kinetics for the reaction can be derived from voltammetric experiments, usually varying expe~ental parameters such as scan rate and concentration 124,253. A variety of non-polar compounds catalyze electroreduction of halobiphenyls in DMF [26]. In this paper, we report electrocatalysis of the reduction of 4-bromobiphenyl to biphenyl with 9-phenylanthracene in aqueous micelles. The reaction occurs without extensive decomposition of catalyst in 0.1 M hexacetyltrimethylammonium bromide (CTAB). EXPERIMENTAL

Chemicals 4-Bromobiphenyl, 9-phenyl~t~acene (98%), and Igepal CO-720 (polyoxyethylene( 1l)nonylphenol) were obtained from Aldrich Chem. Co. 4-Bromobiphenyl was crystallized from methanol before use; the other compounds were used as received. Igepal CO-630 (polyoxyethylene(9)nonylphenol) was obtained from GAF Corp., CTAB (99.8%) was Fisher certified reagent, tetraethylammonium bromide (TEAB) was obtained from Eastman and these materials were used as received. Distilled water purified by a Sybron/Barnstead NANOpure system and with specific resistance > 12 MO cm was used in all studies. Apparatus and procedures A Bioanalytical Systems BAS-100 electrochemistry system, three-electrode cells, and procedures similar to those described previously [24,28] were used for cyclic voltammetry (CV) and potential-step chron~oulomet~. The working electrode was a PARC Model 9323 hanging drop mercury electrode (HDME, A = 0.019 cm2), with a fresh drop for each experiment. The inner surface of the HDME’s capillary was treated with 25% dichlorodimethylsilane (Alpha) in spectrograde dichloromethane before each day’s use. At the end of a series of experiments, the inside of the capillary was washed immediately with pure water and spectrograde methanol to remove traces of surfactant solutions. Severely contaminated capillaries were cleaned with 30% nitric acid or chromic acid cleaning solution. Frequent cleaning and resilanization were necessary to keep the HDME operating properly in rnicellar solutions. Reference electrodes were a saturated calomel (SCE) or a Ag/AgCl electrode, but all potentials are reported vs. SCE. A 0.5 mM solution of Cd(I1) in 1 M nitric

204

acid was used as a standard for peak potential measurements. Cd(I1) gave reversible characteristics at all scan rates by CV when the cell’s ohmic drop was fully compensated. However, in micellar solutions, it was only possible to compensate a portion of the ohmic drop. Uncompensated resistances of 45-190 Q in micellar solutions influenced CVs significantly when currents in excess of 50 PA were generated. The magnitude of this influence was estimated from CVs of the Cd(I1) standard obtained with purposely uncompensated resistance. Peak currents were reproducible within + 5%. All CV results were confirmed by at least two replicate experiments. Micellar solutions were prepared by heating surfactant and water to about 50 o C until all surfactant dissolved, then equilibrating the solution at 30-35 o C for several days with stirring. Micellar solutions containing electrolyte (0.1 M TEAB), catalyst, and substrate were prepared in a similar manner, with ultrasonication used to speed solubilization. Typically, surfactant solutions containing catalyst were allowed to equilibrate for several days at the temperature of the experiment, and equilibration was judged complete when no further increases in voltammetric currents were obtained. Concentrations of all solutes were confirmed by UV-VIS spectroscopy or high-pressure liquid chromatography after filtering samples through 0.22 pm Millipore filters. Experiments using 0.1 M CTAB were thermostated at 30°C to avoid precipitation of surfactant. All other experiments were done at ambient temperature (24 + 2” C). A two-compartment, three-electrode cell and a McKee-Pederson MP1026A potentiostat were used for controlled-potential electrolyses. The working electrode was a stirred mercury pool (A = 17.3 cm2), the reference was Ag/AgCl, and the counter electrode was a spectroscopic-grade carbon rod isolated from the reaction chamber by a glass frit. Resistance of the assembled cell was 40-90 Q in surfactant + 0.1 M TEAB. Cell potentials and currents were monitored with a Keithley Model 177 digital multimeter. Analysis of electrolyzed solutions by HPLC was by the method described previously [27]. UV-VIS spectra were obtained with a Cary 17D spectrophotometer. Non-linear regression of chronocoulometric Q-t data was done with a general program as described previously [28], with modifications for background charge as described in the text. RESULTS

Electrochemistry

of 9-PA in micelles

Cathodic residual currents in CTAB in the absence of 9-phenylanthracene (9-PA) were small up to about -2.3 V (vs. SCE) and no anodic peaks were observed. Cyclic voltammetric (CV) data of 9-PA in 0.02 and 0.1 M CTAB were consistent with nearly reversible reduction of 9-PA (Fig. la). CVs at scan rates (u) above 0.1 V S -I showed equal anodic (i,,) and cathodic (i,) peak currents, and a difference between anodic and cathodic peak potentials (AE) at u < 10 V s-i only slightly larger than the expected value [29] of 59/n mV. Peak potentials were independent

205

-E/V

T

100 rA

-E/V

vs. SCE

v

Fig. 1. CVs at 0.5 V s-l m M 4-bromobiphenyl.

in 0.1 M CTAB+O.l

Fig. 2. CVs in 0.1 M CTAB+O.l (c) 0.2 mM 9-PA, 10 mV s-‘.

M TEAB: (a) 0.34 mM 9-PA, (b) 0.34 mM 9-PA+0.79

M TEAB: (a) 0.2 mM 9-PA. 1 mV s-l, (b) 0.0 mM 9-PA, 1 mV s-‘,

of u. Increases in AE and decreases in current functions (i,,v-‘I*) as v increased (Table 1) can be attributed mostly to uncompensated ohmic drop, as shown by comparison with data for Cd(I1) obtained with uncompensated resistance (R,)of

TABLE I Voltammetric data for 0.2 mM

V/

vs-’

-*,2

‘pcV PAS

l/2

/

v-‘/2

9-PA in 0.1 M CTAB+O.l

M TEAB

'Pa1%

- EpJ mV

at 30° C

nAE/ mV

lpcRU/ mV

0.10

40.0

1.07

2208

74

1.2

0.50

35.6

1.06

2190

72

2.3 3.7

1 .oo

31.1

1.10

2190

73

5.12

30.4

1.08

2183

71

7.7

10.24

26.3

1.07

2190

75

10.3

51.20

23.8

1.13

2190

90

20.1

1.4

0.5 mM Cd(II) m I M HNO,

(R, = 250 a)

0.10

17.1

1.00

606

66

1.00

17.9

0.95

607

68

25.6

15.3

1.00

618

88

19

51.2

14.4

1.00

620

110

26

4.5

206 TABLE 2 Voltammetnc characteristics of 0.5 mM 9-PA in 0.09 M Igepal CO-720+0.1 L’/ VS-’

pAs

0.10 0.20 0.50 1.00 5.12 10.24 25.6 51.2

15.0 15.2 12.6 11.1 10.7 9.5 10.0 8.2

’ pco l/2

/ v-l/2

‘Pa /‘F

0.4 0.5 0.7 0.75 0.89 1.03 0.96 1.05

M TEAB at 24OC

mV

nAE/ mV

Q&J mV

2170 2114 2172 2162 2155 2155 2162 2168

15 79 71 74 73 73 83 88

3.0 3.7 6.2 7.0

-l/2 -f&d

250 9. Changes in voltammograms with increasing u for 9-PA in 0.1 M CTAB were similar to those for Cd(I1) when experiments with nearly the same values of i,,R, are compared (Table 1). Peak currents increased linearly with concentration of 9-PA in the submillimolar range. CV data were similar in 0.02 M CTAB, although solubility of 9-PA was only about 0.2 m M. At a scan rate of 1 mV s-‘, CV in either 0.02 or 0.1 M CTAB did not reflect diffusion-controlled electrochemistry. At this low scan rate the final current rise partly obscured the cathodic peak for reduction of 9-PA, but on reverse scans a sharp, symmetric peak was observed (Fig. Za,b). The shape of this anodic peak indicates thin-layer electro~he~st~ [29]. As the scan rate is increased the anodic peak shape changes until at 10 mV s -’ (Fig. 2~) it is the same as that at higher U. These results suggest that the anion radical of 9-PA is oxidized in a thick film of surfactant aggregate on the electrode. CVs of 9-PA in 0.09 M Igepal CO-720 or 0.03 M Igepal CO-630 were somewhat different from those in CTAB. Although effects of ohmic drop were small, the cathodic residual current in the absence of 9-PA was much larger than in CTAB and required up to a 50% correction to obtain the current of 9-PA. Furthermore, although peak potentials remained constant, current functions decreased with increasing scan rate to a greater extent than can be expiained by ohmic drop (Table 2). Peak current ratios were smaller than one at lower u (Fig. 3), but increased to about one at u 2 10 V s-l. Solubility of 9-PA in 0.02 M Igepal CO-720 was too low to obtain useful data. Apparent diffusion coefficients (D’) estimated from electrochemical data are often used to assess the binding of the electroactive species to the micelle (7-131. However, it is important to realize that these are apparent values only, and that they depend on surfactant and solute concentration in most cases [2]. D’ values computed from the Randles-SevEik equation [29] by using current functions at low scan rates for 9-PA were surprisingly large in the micellar solutions (Table 3). To confirm this we also used chronocoulometry for estimating D’. Data were obtained after a single-potential pulse from an initial potential of - 1.7 V to a final value of

Fig. 3. CVs at 0.5 V s-’ in 0.09 M Igepal 9-PA + 8 mM Cbromobiphenyl. Fig. 4. Chronocoulometry calculated data; ( -)

CO-720+0.1

M TEAB:

of 0.34 mM 9-PA in 0.1 M CTAB + 0.1 M TEAB. (0) from regression onto eqn. (8).

-2.25 V. Initially, data were tested for adherence reaction involving diffusion to a spherical electrode, Q(t)

(a) 0.5 mM

= 2nFAD”‘2c*r-“2

[ t1’2 + s’/2D’1’2t/2r,]

to the model eqn. (6) [28]:

9-PA,

Selected

(b) 0.5 mM

experimental

for a reversible

(6)

+ Qor

where Q(f) is the charge at time t after the pulse, c * is the concentration of 9-PA, r,, is the radius of the electrode, Qm_ is the double-layer charge, and the other terms have their usual meanings. Using non-linear regression [30] onto eqn. (6) with parameters 2nFAD’1’2c*n-‘/2, QDL, and 7r’/2D’1’2/2r0, reasonable fits resulted

TABLE Apparent

3 diffusion

coefficients

of 9-PA in micellar

c/mM

Surfactant

0.33 0.20 0.20 0.34 0.20 0.20 0.50 0.34 0.40

CTAB 0.02 CTAB 0.02 CTAB 0.02 CTAB 0.10 CTAB 0.10 CTAB 0.10 Igepal-CO-630 0.02 Igepal-CO-720 0.09 Igepal-CO-720 0.09 N, N-dimethylformamide (0.1 M TBAI)

es/M

solutions

a

Method

103D’/cm2

cv

5.6 4.3 2.3 1.9 1.1 1.0 1.3 0.76 0.49

cv Chronocoul. CV’ cv Chronocoul. cv cv Chronocoul. cv

a Micellar solutions contained 0.1 M TEAB. b Avg. of 6-9 trials, avg. std. dev. *lo%. ’ Avg. of 3 sets of rD vs. I?” data, std. dev. *lo’%.

b

b

b

s-i

106D”/cm2 0.15 0.12 0.062 1.28 0.76 0.68

9.0

s-i

208

with data for 9-PA in CTAB and Igepal solutions. However, residuals were distinctly non-random and computed values of D’ varied when data over different time ranges were used. Thus, we modified eqn. (6) to include the small contribution of a background reduction process in the surfactant solutions. The charge, Q,, from a slow process controlled by electrode kinetics can be described approximately by: Q, = S [ (2t”2/H+2)

- H-‘1

(7)

which has the form of the expression for quasi-reversible electrode kinetics [29]. Equation (7) provided a good description of the background charge. Using S and H as additional regression parameters, and the modified model: Q(t)

= QF+

QDL+

Q,

(8)

where Qr is the faradaic charge represented by the bracketed term and its multiplier on the rhs of eqn. (6), we obtained significantly better fits to the data (Fig. 4). Computed parameters did not depend on their starting values in the regression, and the computed D’ was independent of the time range used (25-100 ms). Although some non-randomness in residual plots remained, average standard deviations of regression (SD) decreased from 0.11 PC for eqn. (6) to 0.028 PC when data were regressed onto eqn. (8), identifying the latter equation as the better model. Non-linear regression onto eqn. (8) provided estimates of D’ confirming those obtained by CV (Table 3). Note that D’ is smaller at the higher surfactant concentrations. Table 3 also lists values of D”, computed by using as c * the concentration of 9-PA in the hydrophobic volume [33] of the CTAB solutions. Electrocatalytic

reactions

The ratio of cathodic peak current for the catalyst in the presence of substrate (i,) to that in the absence of substrate (id) is an indicator of catalytic efficiency at a given scan rate [31]. Addition of 4-bromobiphenyl(4-BB) to solutions of 9-PA in 0.1 M CTAB, 0.03 M Igepal CO-630, or 0.09 M Igepal CO-720 caused an increase in cathodic peak current of 9-PA (Figs. 1 and 3). For both types of surfactant, i,/i, increased slightly with increasing scan rate (Table 4) but i,.Ji, showed little change from values for the catalyst alone. In 0.1 M CTAB, i,/i, increased with increasing concentration ratio (y = [4-BB]/[9-PA]) up to about y = 1, but remained nearly the same at higher y. Up to 8 mM 4-BB alone gave no significant current above that of the background in the surfactant solutions. Addition of a 20-fold or more excess of biphenyl to surfactant solutions of 9-PA gave no significant increase in cathodic current over the entire range of scan rates. This latter experiment rules out a large co-surfactant effect in the current increases observed with 9-PA and 4-BB, since the physical properties of biphenyl are similar to those of 4-BB. In 0.02 M CTAB, addition of a 4-fold excess of 4-BB to 0.2 mM 9-PA caused a decrease in the cathodic current and disappearance of the anodic peak. Since catalytic efficiency and reactant solubilities were poor at this concentration of CTAB, it was not studied further.

209 TABLE Catalytic

4 efficiencies

for reduction

of 4-BB with 9-PA in micelles v/V s-1

Solution

k/j, y =

0.1 M CTAB+O.l

M TBAB

0.10 0.20 0.50 1.00 5.12 10.24 25.6 51.2

2.3 a

3.20 3.24 3.32 3.42 3.39 3.34 3.37 3.58

y =l.O

b

y = 0.2 b

y=20b

3.35

1.09

2.80

3.33 3.49 3.42 3.90

1.22 1.21 1.17 1.31

2.90 3.16 3.04 3.37

3.60

1.17

3.33

y=l6= 0.03 M IgepaI CO-630 +O.l M TEAB

0.10 0.20 0.50 1.00 10.24

2.2 2.0 2.8 3.1 3.9

0.09 M Igepal CO-720 +O.l M TEAB

0.10 0.50 1.00 5.12 10.24 51.2

2.4 3.5 4.6 3.7 4.2 4.2

a [9-PA] = 0.34 mM. b [g-PA] = 0.20 mM. ’ [9-PA] = 0.50 mM.

TABLE

5

Analysis

of electrolysis

Surfactant

r/h

products

by HPLC

a

10e6 Amount/m01 initial

final

9-PA

4-BB

9-PA

2.3

8.5

19.8

6.8

3.0

0.09 M IgepaI CO-720

2.0

8.0

20.3

2.0

19.1

2.2

7.8 12.5

89 200

2.4 1.9

Q “/C

17.3 (103%) 0.0

3.0

4-BB

0.1 M CTAB

2.2

Biphenyl (yield)

84

(0%) 2.5

142

(50%) 39

1.9 8

(63%) a Conditions described in experimental section; 25 cm3 used for each electrolysis. Applied potentials were -2.145 V for CTAB and -2.125 V for IgepaI CO-720 (vs. SCE). b Corrected by subtracting the charge obtained for a blank experiment with micelle/electrolyte.

210

Electrolysis

at stirred mercury pool electrodes

Electrolyses in solutions of cationic and non-ionic micelles were done at applied potentials corresponding to half-peak potentials of cathodic voltammograms. In 0.1 M CTAB with 0.34 mM 9-PA and a 2.3-fold excess of 4-BB, the concentration of 4-BB decreased 85% after 2.3 h of electrolysis. All of the reacting 4-BB was converted to biphenyl (Table 5), and the charge passed was 1.8 electrons per molecule of 4-BB converted. The concentration of 9-PA decreased by about 20%. Results of electrolyses in 0.09 M Igepal CO-720 depended on the relative amounts of catalyst and substrate. About 30% of a 16-fold excess of 4-BB reacted after 2 h, but the concentration of 9-PA decreased by 85% (Table 5). With a lo-fold excess of substrate, less 4-BB was converted to biphenyl and 70% of the 9-PA decomposed. With a 2.5-fold excess of 4-BB, no biphenyl was formed, and only electrolysis of 9-PA occurred. Uncertainty in the amount of charge passed was large because of a large background correction. No products of these electrolyses other than biphenyl were found by HPLC, but mass balances suggest that small amounts of side products may have formed. DISCUSSION

Electrocatalysis

in CTAB

micelles

Quantitative yields of biphenyl from electrolysis of 9-PA + 4-BB, and increases in cathodic CV currents for 9-PA upon addition of 4-BB clearly indicate reduction of 4-BB by 9-PA anion radicals in micelles of CTAB. D’ values about an order of magnitude smaller than those for the free molecule are expected if a micelle-bound electroactive molecule is the diffusing species [2-5,7-10,341. However, in the present system very large D’ values are found. This result and the symmetric anodic peaks for 9-PA at 1 mV s-l suggest that electrochemistry and electrocatalysis occur in a thick micellar layer at the electrode surface in 0.1 M CTAB. Diffusion-controlled behavior for 9-PA alone sets in as the scan rate is increased and the diffusion layer [29] becomes smaller than the thickness of the micellar layer on the electrode surface. A less likely possibility for the high currents for 9-PA in CTAB involves electrocatalysis by a mercury derivative [35-371 of reduced TEA+. However, reduction of 9-PA is reversible in organic solvents, and is not subject to catalysis. Furthermore, CV peaks of 9-PA are 300-500 mV more positive than reduction potentials of TEA+ [35] and no anodic peaks were observed for CTAB + TEAB alone, as reported for tetra-alkylammonium ions in aqueous media [36,37]. Thus, this possibility can be ruled out safely. Long chain tetra-alkylammonium ions are superequivalently adsorbed at the mercury/solution interface [38-411, but the interpretation at potentials more negative than - 1.8 V vs. SCE has been controversial. Differential capacitance and ellipsometric delta values for 3-4 mM decyltrimethylammonium chloride in aque-

211

ous KC1 at Hg electrodes were smaller than for surfactant-free KC1 solution at very negative potentials [38,39,41], suggesting that longer chain compounds at higher concentrations remain adsorbed at negative potentials. Hayter and Hunter [38] interpreted capacitance data at very negative potentials in terms of greater-thanmonolayer coverage of the mercury electrode by decylammonium ions. Preliminary surface-enhanced Raman studies on silver electrodes show evidence for molecular reorientation of a surface film of CTAB at E < - 1.5 V vs. SCE [42]. Also, thin layer reflectance FT-IR spectra on silver electrodes in CTAB solutions show a large increase in absorbance of C-H stretching bands as the potential is shifted from - 0.4 to - 2 V [43]. These findings support the proposal that surfactant aggregates, or multilayers of surfactant molecules, can exist on the electrode at the potentials where g-PA is reduced. Since non-polar aromatic molecules like 9-PA and 4-BB reside almost entirely in the micellar phase [32], the hydrophobic portion of the surface layer must contain a high concentration of g-PA. By using Tanford’s method [33], the hydrophobic volume of CTAB micelles was estimated at 2.6% of the total volume of the 0.1 M CTAB solution. From this, a concentration of 13 mM for 9-PA in the micelles is estimated for a nominal total concentration of 0.34 mM. The hydrophobic volume in 0.02 M CTAB is smaller, and the resulting larger surface concentration of 9-PA in a 0.34 mM solution yields the observed larger D' value. D" was computed from the concentration of g-PA in the hydrophobic volume, assuming that 9-PA is compartmentalized in a thick hydrophobic layer at the electrode. Reasonably constant average D" values (Table 3) were obtained for a given concentration of CTAB (1.1 x lo-’ cm* s-’ (0.02 M CTAB); 9 X lo-’ (0.1 M CTAB)}. These values are considerably smaller than the diffusion coefficient of 9-PA in DMF. Magnitudes of D" are reasonable considering the tenfold or larger microviscosities found for hydrophobic regions of aqueous micelles [19]. Differences in D" at different surfactant concentrations may reflect a difference in film structure, well known to depend on surfactant concentration [19]. CV results show that electron-transfer from the electrode to 9-PA is fast. It is likely that replacement of bromine in 4-BB with hydrogen is initiated by reactions in eqns. (l)-(3) in the hydrophobic regions of surfactant aggregates on the electrode. Interactions with the positive surface potential and hydrophobic sites of the CTAB aggregate stabilize the anion radical of 9-PA towards side reactions so that it remains in the aggregate long enough to react with 4-BB. The catalytic CV results can be interpreted within the framework of a general theoretical treatment based on Scheme 1 [31], which assumes fast heterogeneous electron transfer (eqn. l), and considers eqns. (2) and (3) as kinetic controlling steps. The theory was first elaborated for the one-electron process in eqns. (l)-(3). If reduction of Ar * is considered to occur predominantly by the fast second-order reaction in eqn. (4) current functions (JI z .-) at a given potential for the two-electron carbon-halogen cleavage were given by G2 .-(y) = $i .-(2y). That is, for a fixed set of experimental conditions, the current for the two-electron reduction is simply that of the one-electron reaction with twice the ratio of substrate to catalyst

212

2.0

23 - E/V

Fig. 5. Simulated catalytic CV at 50 V s-’ s -‘, k =105 s-‘, E” = -2.2 V. Simulated same conditions was 2.68.

for Scheme 1 with y =I, k, =lO’ M-r ss’. k, =109 M-r catalytic reduction for the value for rc/rd for two-electron

Fig. 6. Catalytic efficiencies in 0.1 M CTAB vs. log of kinetic parameter for kk,/k, = 2.5 X lo4 ss’. (0) Total y =l; (0) total y = 2.3, but with catalytic efficiency computed using y = 1.2 estimated from the Working curves for Scheme 1 ( y = 1 and 2) from ref. 31. best fit. ( -)

concentrations. Thus, it is necessary to compute one-electron voltammograms only to predict trends in the two-electron current functions with variations in kinetic constants and u. We have used the same approach in digital simulations of CVs for Scheme 1, assuming that the catalytic reaction occurs in the thick surface layer. This assumption is justified for fast reactions [l&44]. It is first necessary to decide which reactions exert kinetic control. The presence of the reverse peak at all scan rates is a strong indication that decomposition of the 4-BB radical anion is a kinetically important factor, since if this step is fast with respect to the forward reaction (2) no contribution to the reverse peak is expected from the reverse reaction (2). This last statement was confirmed by finite-difference simulation of Scheme 1, using an expanded space-time grid [45]. The speed of this method made possible simulations of Scheme 1 with large rate parameters, although we were restricted to u 2 10 V s-l to attain practical computation times. A series of simulations were done for y = 1, IJ = 10-50 v s-1, lo6 $ k, $ lo9 M-’ s-l and a wide range of values of k and k,. Computed catalytic efficiencies and characteristic peak shapes were in excellent agreement with those derived previouly by an alternative method [31]. The simulations showed that significant catalytic efficiency (i.e. i,/i, > 1.2) and a reverse peak nearly as large as the cathodic one could be obtained only when eqn. (2) was not the sole rds. Specifically, a k, close to lo9 M-’ SC’ and k/k, between 0.01 and 0.1 reproduced experimentally observed features adequately over the range of u studied. Figure 5 is a simulated voltarmnogram for Scheme 1 at y = 1, k, = 10’ M-’ s-l, k, = lo9 M-’ s-l, and k = 10’ s-l, demonstrating that the observed voltammetric features can be reproduced as a consequence of the kinetics of eqns. (2) and (3). With the same parameters as in Fig. 5, except k, = lo8 M-’ s-l, a less reversible

213

simulated CV was obtained; with k, = 2 X lo6 M-’ s-’ and k = lo7 s-’ the CV is reversible but little catalysis is observed. These results illustrate the sensitivity of the shape of the CVs to the exact kinetic situation. Supporting evidence for eqn. (3) as an important kinetic factor also comes from observed catalytic efficiencies. The ratio i,/i, increases when y goes from 0.2 to 1, and decreases slightly at y > 1, suggesting that surface aggregates become saturated with substrate at high concentrations. This is similar to the situation for very fast reactions in micelles, i.e., the reactant concentration ratio at the reactive site is not simply the added molar ratio, but reflects the statistical ~st~bution of reactant molecules in the micelles [16-181. To a first appro~mation, the effective value of y in such reactive micelles can be obtained from Poisson distributions of reactants in the micelles. This was done by using an aggregation number of 80 in 0.1 M salt [33] and the reported CMC [46] of CTAB, and assuming no interaction between 9-PA and 4-BB before electrolysis. These statistical considerations showed that for 0.1 M CTAB containing 0.34 mM 9-PA and 0.8 mM 4-BB the estimated y in the “‘reactive” micelles is 1.2. Estimates for total y = 20 gave micellar y=2.6 in reactive micelles. This is qualitatively similar to the saturation effect that seems to pertain in the CTAB surface aggregates in equilibrium with micelles in the bulk aqueous phase. Thus, aty >, 1 catalytic efficiencies tic/id) do not vary much with concentration of substrate or catalyst over the entire range of u. Invariance of i,/i, when changing catalyst ~n~ntration is the predicted result [31] when radical decomposition, eqn. (3), is the rds with eqn. (2) as a rapid pre-equilibrium. Reasonably good fits of catalytic efficiencies were obtained with y close to one to working curves [31] of i,/2i,y vs. log{(RT/F)(kk,/k,v)}, applicable when reaction (3) is the rds (Fig. 6). Despite uncertainty in y at the reaction sites, predictions for CV in isotropic systems following Scheme 1 explain qualitatively observed trends in catalytic efficiency in 0.1 M CTAB as u is varied. Formal potentials of 9-PA and 4-BB in DMF [26] give a standard Gibbs energy of -0.42 eV for the reverse electron transfer in eqn. (2), suggesting [47] that this reaction occurs at a diffusion-limited rate. Assuming similar energetics in the micellar reaction, realizing that the viscosity of the hydrocarbon core is greater than the viscosity of the bulk solution 121,461, and conside~ng the average diffusionlimited rate constant for molecules of similar size to 9-PA and 4-BB was 5 x lo9 M-’ s-’ in DMF [48], a reasonable estimate for k, in CTAB is 109 M-’ s-‘, the same value which gave good simulations of experimental results. If eqn. (3) is the rds, the condition k2[9-PA] > k must be fulfilled [31]. Using the concentration of 9-PA in the hydrophobic reaction environment, and kk,/k, = 2.5 x lo4 s-’ from Fig. 6, the approximate values k, P 10’ M-’ s-’ and k 3 lo6 s-’ were estimated. These are considered reliable only within about an order of magnitude. Nonetheless, k, in CTAB is considerably larger than the 300 M-’ s-’ found in DMF [26], indicating a large micellar enhancement of the rate of the forward electron transfer in eqn. (2). The value for k in CTAB is in good agreement with observed rates of decomposition of unsubstituted halo~omatic anion radicals which are large (491, with lo5 s-’ to lo6 s-i as a lower limit. These results are in agreement with digital

214

presence of the reverse peak and simulations, which also explain the simultaneous catalytic activity. Taking k, = 10’ M-’ s-i and [9-PA] = [4-BB] = 0.2 mM as bulk concentrations, the specific rate of the forward electron transfer (eqn. 2) is 0.4 M s-‘. Dividing by reactant concentrations computed for the hydrophobic volume of 0.1 M CTAB gives an estimate of 600 M-’ s-’ for the actual rate constant in the surface aggregates. Considering the limited precision of k,, this value is reasonably close to the k, in DMF (300 M-’ s-‘f [26], suggesting that kinetic enhancement can be explained largely by high localized con~ntrations of reactants in the hydrophobic en~ronment. Reactions

in non-ionrc micelles

Electrolytic and CV data for Igepal micelles show that reduction of 4-BB competes with another reaction involving anion radicals of 9-PA. Large D’ values again suggest reaction in a surface aggregate layer, but we could not confirm this because of disappearance of the anodic peak for oxidation of 9-PA anion radical at low scan rates. Significant yields of biphenyl were found only when the ratio of substrate to catalyst was large (Table 5). The anodic CV peak for 9-PA in the presence or absence of 4-BB was si~ific~tly smaller than its cathodic counterpart at low scan rates, suggesting a fast follow-up decomposition of the 9-PA anion radical. A likely process competing with eqn. (2) is protonation of the anion radical by water followed by reduction of the resulting radical. This is an overall two-electron reduction of 9-PA in an ECE-type process [50-521. The decrease in current function and increase in peak current ratio with u in CV are consistent with an ECE pathway for catalyst decomposition [29], although additional work would be necessary to establish this point definitively.

CONCLUSIONS

Our results show that organic anion radicals generated electroche~cally in a thick surfactant phase on the electrode can efficiently reduce a non-polar substrate also resident at hydrophobic sites. The reaction occurs in 0.1 M CTAB with electrocatalysis of the reduction of 4-BB. as well as micellar enhancement of the rate of electron transfer between 4-BB and the anion radical of 9-PA. Reactant compartmentalization in the surface aggregates alters kinetics from those in DMF. Decomposition of the anion radical of 4-BB is kinetically important in CTAB, and the sole rds is not eqn. (2) as in DMF. The positive surface potential of CTAB aggregates probably stabilizes the catalyst anion radical against side reactions, allowing it to react with 4-BB to give excellent yields of biphenyl. Poor yields of biphenyl and catalyst decomposition in solutions of the non-ionic surfactant Igepal underscore the importance of the positive surface charge of the micelle for successful dehalogenation in aqueous media.

21s

Although at the present stage of our studies we can rationalize the observed CV features, a more quantitative model of micellar electrocatalysis is needed before all elementary details of these reactions can be understood. In this respect, a better understanding of surfactant films on highly charged electrodes would be helpful. Nevertheless, the successful aryl debromination reported here suggests interesting new possibilities for decomposing halogenated pollutants in aqueous systems, as well as extensions to other difficult organic reductions. Further aspects of these and related systems are under active study in our laboratory. ACKNOWLEDGEMENTS

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