1042
Langmuir 1992,8, 1042-1044
Rate Enhancement and Control in Electrochemical Catalysis Using a Bicontinuous Microemulsion? Geoffrey N. Kamau* Department of Chemistry, University of Nairobi, P.O. Box 30197, Nairobi, Kenya
Naifei Hut and James F. Rusling* Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-3060 Received January 6, 1992. I n Final Form: February 3, 1992 Reductions of 1,2-dibromobutane (DBB), trans-1,2-dibromocyclohexane (t-DBCH),and trichloroacetic acid (TCA) mediated by nickel and copper phthalocyaninetetrasulfonates (MPcTS) were compared in a bicontinuous microemulsion and in isotropic acetonitrilelwater. MPcTS mediators were adsorbed to glassy carbon cathodes from the didodecyldimethylammonium bromide (DDAB)/dodecane/water microemulsion and from acetonitrilelwater. Catalytic efficiencies measured by voltammetry showed that TCA was more reactive in isotropic solvent than in the microemulsion, but DBB and t-DBCH were more reactive in the microemulsion. Results suggest kinetic control by an adsorbed film of DDAB on the electrode surface.
Introduction Electrochemical catalysis uses mediators to shuttle electrons between substrate molecules and electrodes. This lowers the applied potential needed to reduce or oxidize recalcitrant substrates. Catalytic reductions usually feature a bimolecular rate-determining reaction between reduced mediator and substrate.' The rates of such reactions can be greatly enhanced by a preconcentration effect if they can be made to occur in a film of surfactant on an electrodes2 Thus, the rate of reaction of 9-phenylanthracene anion radical with 4-bromobiphenyl on a Hg electrode coated with a thick film of adsorbed cetyltrimethylammonium bromide (CTAB) in micellar solutions was >1000-fold larger than in isotropic organic ~ o l v e n t . ~ Unfortunately, large rate enhancementswere only achieved at potentials negative of about -2.1 V vs SCE, where the thick film of CTAB was formed. Using other mediators with more positive formal potentials in micellar CTAB resulted in much smaller rate enhancements.2 Recent work in our laboratory showed that aqueous dispersions and microemulsions of didodecyldimethylammonium bromide (DDAB) gave much better yields of hydrocarbon products than micellar CTAB in catalytic dechlorination of polychlorinated biphenyls on carbon felt cathode^.^ These reactions also occurred in surface films. In this report, we describe preliminary voltammetric studies on the reactivity of several aliphatic organohalides toward reductive dehalogenation mediated by metal phthalocyaninetetrasulfonates in bicontinuous microemulsions of water/dodecane/DDAB. Comparisons of catalytic efficiencies with those in isotropic solution demonstrated rate enhancement and control by the microemulsion at potentials of -1.15 to -1.5 V vs SCE. Significantly +Part 9 of the series Electrochemical Catalysis in Organized Assemblies. On leave from Beijing Normal University, Beijing, China. (1)Bard, A.J.;Faulkner, L. R. ElectrochemicalMethods; Wiley: New York, 1980. (2)Rusling, J. F.Acc. Chem. Res. 1991,24,75-81. (3)Rusling, J. F.; Shi, C. N.; Gosser, D. K.; Shukla, S. S. J. Electroanal. Chem. Interfacial Electrochem. 1988,240,201-216. (4)(a) Rusling, J. F.; Iwunze, M. 0.J.Electroanal. Chem. Interfacial Electrochem. 1989, 266, 197-201. (b) Couture, E. C. Ph.D. Thesis, University of Connecticut, Storrs, CT, 1991.
*
enhanced reactivities in microemulsions were found for alkyl vicinal dihalides and decreased reactivity for trichloroacetate.
Experimental Section Chemicals and Solutions. Didodecyldimethylammonium bromide (DDAB,99+ % ) was from Eastman Kodak, and n-dodecane was ACS certified from Fisher Scientific. Acetonitrile was HPLC grade Baker Analyzed. Nickel and copper phthalocyaninetetrasulfonic acids, tetrasodium salts, were from Aldrich. All other chemicals were reagent grade. The bicontinuousmicroemulsion was prepared with reference to the phase diagram5as described previously.6 Specific conf2-l cm-' for composition by weight: 21 % ductancewas about DDAB; 39% water; 40% dodecane. The isotropic solution was 1:l acetonitrile/water (v/v) containing 0.1 M tetraethylammonium bromide. Apparatus and Procedures. A BioanalyticalSystemsBAS100 electrochemistry system was used for cyclic voltammetry (CV) and Osteryoung-type square wave voltammetry (SWV). The working electrode was a polished glassy carbon disk ( A = 0.072 cm2). A platinum wire was the counter electrode, and the reference was a Ag/Ag+ (0.01 M AgN03, 0.1 M tetraethylammonium tetrafluoroborate)connected to the microemulsion with a microemulsion-filledsalt bridge or a saturated calomel electrode (SCE)for the isotropic solution. The reference potential of the Ag/Ag+ electrode was approximately -150 mV vs SCE, and this value was added to E vs Ag/Ag+ to convert to E vs SCE. All potentials are reported vs SCE. Surface preparation methods described previously7were used for the working electrode, with polishing repeated before each voltammetric scan. Ohmic drop of the cell was fully compensated by the BAS-100. All solutions were thermostated at 25.0 f 0.1 "C and purged with purified nitrogen t o remove oxygen before voltammetry.
Results Voltammetry of nickel and copper phthalocyaninetetrasulfonates (NiPcTS and CuPcTS) was investigated first in the absence of substrates. Cyclic (CV) and square wave voltammetry (SWV)revealed two redox reactions involving diffusion and adsorption on the glassy carbon electrode. (5)Blum, F. D.; Pickup, S.; Ninham, B. W.; Chen, S. J.; Evans, D. F.
J.Phys. Chem. 1985,89,711-713.
(6)Iwunze, M. 0.; Sucheta, A.; Rusling, J. F. Anal. Chem. 1990,62, 644-649. (7)Kamau, G.N.;Willis, W. S.;Rusling, J. F. Anal. Chem. 1985,57, 545-551.
0743-7463/92/2408-lO42$03.00/00 1992 American Chemical Society
Langmuir, Vol. 8,No. 4, 1992 1043
Letters
- 0.50
0.30
E,
-0.50
-1.00
-1.50
-2.00
E, V vs. AgfAg+
12
2 1
0
0.30
0
-1.30
V vs. A g l A g +
-0.50
-1.30
E, V vs. Ag/Ag+
Figure 1. Forward square wave voltammograms at pulse height 25 mV of 0.4 mM NiPcTS in the DDAB microemulsion: (a)step 2 mV, 2 Hz;(b) step 4 mV, 15 Hz.
Peaks were better resolved with SWV. At low frequency 0 ,two diffusion-controlled peaks (Figure la) were observed for NiPcTS in the microemulsions a t -0.51 and -1.15 V (all vs SCE). As f was increased, two new peaks appeared a t potentials more positive of the main diffusion peaks (Figure lb). These peaks increased linearly in height with increasing f , characteristic of strong adsorption of the reduction products on the electrode.8 Similar results were found for CuPcTS in the microemulsions, with the two diffusion peaks at -0.52 and -1.4 V cf = 5 Hz), respectively. No reverse peaks in CVs up to scan rates ( u ) of 2 V s-1 were observed for either mediator in the microemulsion, suggesting deactivation of the products of the reductions.' CV and SWV of the mediators in isotropic acetonitrilewater showed a similar pattern of diffusion and adsorption peaks. The two diffusion peaks appeared a t about -0.7 and -1.36 V for both mediators. However, these peaks were about 5 0 4 0 % smaller than in the microemulsions for equivalent concentrations and voltammetric conditions. This may reflect a more highly aggregated state for the mediators in acetonitrile-water than in the microemulsions, as was also shown by UV-vis spectroscopy. Catalytic reductions of 1,Zdibromobutane (DBB), trans1,2-dibromocyclohexane (t-DBCH), and trichloroacetic acid (TCA)were examined by CV and SWV. For example, when a 10-fold molar excess of t-DBCH was added to a microemulsioncontaining NiPcTS, a large sigmoid-shaped wave was observed by SWV at a half-wave potential of about -1.25 V vs SCE (Figure 2). This wave is much larger than the second peak for the mediator alone; it is about 0.4 V positive of the peak for direct reduction of t-DBCH. Similar results were found by CV. The current increase for the second peak of NiPcTS in the presence of t-DBCH suggests catalytic reduction of the alkyl dibromide by the reduced form of the m e d i a t ~ r . ~ This bimolecular electron exchange regenerates the oxidized form of the mediator which can be reduced again at the electrode, increasing the current. (8)Osteryoung, J.; O'Dea, J. J. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1986;Vol. 14,pp 209-308. (9)(a) Andrieux, C.P.; Blocman, C.; Dumas-Bouchiat, J. M.; M'Halla, F.; Saveant, J. M. J.Electroanal. Chem. Interfacial Electrochem. 1980, 113,19-46. (b)Zeng, J.; Osteryoung, R. A. Anal. Chem. 1986,58,27662771.
Figure 2. Forward square wave voltammograms at 4 mV step and 25 mV pulse height: (a) 0.4 mM NiPcTS + 4 mM t-DCBH, 15 Hz;(b) 4 mM t-DBCH, 5 Hz;(c) 0.4 mM NiPcTS, 15 Hz. Table I. Catalytic Efficiencies for Reductions of Aliphatic Organohalides in DDAB Microemulsion and Isotropic Solvent. microemulsion* MeCN/wat& mediator substrate -El/*$ V iJid -E~/z,dV idid Square Wave Voltammetry NiPcTS TCA 0.99 17 0.89 48 DBB 1.19 97 1.19 43 t-DBCH 1.20 120 1.12 61 CuPcTS TCA 1.17 2.5 1.05 48 DBB 1.51 195 1.21 17 t-DBCH 1.15 100 1.08 6 Cyclic Voltammetryf NiPcTS TCA 0.99 12 0.90 67 DBB 1.20 136 1.24 53 t-DBCH 1.16 90 1.12 74 CuPcTS TCA 1.15 4.4 1.07 55 DBB 1.52 161 1.33 30 t-DBCH 1.16 108 1.09 6 0.4mM catalyst, 4 mM DBB and t-DBCH, 6 mM TCA. * 21 % DDAB, 40% dodecane, 39% water. 0.1 M tetraethylammonium bromide. d V vs SCE. e 15 Hz,2 mV step, 25 mV pulse height. f 100
mV s-1.
The catalytic efficiency of a mediated electrochemical reaction is the ratio of the catalytic current (i,) for mediator in the presence of substrate to the diffusion current (id) of the mediator alone.98 Catalytic efficiencies were measured for the three organohalides and two mediators by CV and SWV (Table I). Sigmoid-shaped current-potential curves as in Figure l a for catalytic reactions that are pseudo , k is the first order in mediator imply a large k ~ where pseudo-first-order rate constant for reaction of reduced mediator with substrate and T is pulse width in SWV or u-1 in CV. Under these conditions, k is proportional to (idid)*for both CV and SWV.g Catalytic current potential curves were all recorded with at least a 10-fold molar excess of substrate and were sigmoid shaped or nearly so. Thus, the catalytic efficiencies measured under the same experimental conditions can be used to compare relative rates of reaction. Catalytic efficiencies were different in microemulsions and isotropic solutions in all cases (Table I). Trichloroacetic acid reacted faster in isotropic solvent than in the microemulsion. DBB and t-DBCH reacted much faster in the microemulsion than in isotropic solvent. To a first approximation, the potentials at which these reactions are observed are similar in microemulsions and isotropic solvent.
Discussion Voltammetry of the Mediators. Comparisons of diffusion peak potentials of CuPcTS and NiPcTS in the microemulsion and isotropic solvent with data in isotropic
1044 Langmuir, Vol. 8, No. 4, 1992
Letters
organic solvents1° provide the identities of the species reduced. For Ni(I1)and Cu(I1)phthalocyanines, electronic and electron spin resonance spectroscopy showed that reductions occur on the macrocyclic ligand rather than the metal. Initial peaks in this work are not likely to involve reduction of MIIPcTS3-, since oxidations of MIIPcTS4occur at potentials close to +1 V vs SCE.'O Thus, the electrode reactions are M"PcTS4- + e- = M"PcTS5M"PcTS5-
+ e- = M"PcTS'-
(-0.5 V)
(1)
(-1.1 to -1.4 V) (2)
The adsorption peaks at more positive potentials than the diffusion peaks confirm strong adsorption of reduced forms of these phthalocyanines on the glassy carbon electrode. Catalytic Reductions. Alkyl vicinal dibromides are reduced to olefinsl1J2 and trichloroacetic acid is reduced to acetic acid13 in reactions mediated by metal macrocyclic complexes. Trichloroacetic acid loses chloride in a stepwise fashion and catalytic voltammograms reflect loss of the first ch10ride.l~ These reactions take place a t the potentials of the second peak for both mediators (Table I). Thus, two electrons are added to the mediators to form MI1PcTS6-,which is the active form that reduces the substrates. The MPcTS4- are highly water soluble; in the microemulsion they either reside in the water phase or associated with DDAB in interfacial regions. Trichloroacetic acid (TCA) is a strong acid present mainly as the acetate in the water phase of the microemulsion. Yet TCA reacts more slowly in the microemulsion, where it resides close to the mediators, than in isotropic solution. DBB and t-DBCH have low solubilities in water and reside mainly in the oil phase.14 In electrochemical catalytic reduction of these species in a microemulsion where catalyst was in the water phase and the reaction did not occur in an adsorbed surface mode, reactivity was decreased by several orders of
magnitude.14 However, in the present case reactivity is enhanced for DBB and t-DBCH in the microemulsion, even though they are in a different phase from the mediators. A possible explanation for the observed catalytic efficiencies is that reactions in the microemulsion occur with substrate, mediator, and DDAB coadsorbed on the glassy carbon surface. Similar rate enhancement were identified in aryl halide dechlorinations in CTAB and DDAB media. DDAB has been shown to adsorb strongly to carbon cathode^.^ In the present case, voltammetric data clearly indicate adsorption of reactive mediator species. Thus, a film of DDAB and mediator on the electrode could exert selectivity via the binding properties of the s01ute.l~ Neutral alkyl vicinal dibromides would easily enter such a film and perhaps preconcentrate in it, enhancing reactivity by creating high concentrations of reactants on the electrode in the microemulsion. The reason for decreased reactivity of TCA in the microemulsions is more elusive. TCA present as the acetate may compete to enter the film with Br- ions in the DDAB microemulsions. However, recent experiments showed that up to 10-fold preconcentration of TCA occurred from aqueous salt solutions in DDAB films cast onto pyrolytic graphite.16 Clearly, further experiments are needed to elucidate mechanistic factors controlling reactivities in the microemulsion. In summary, preliminary results demonstrate enhancement and control of reactivity in microemulsions for substrates of different solubility types. Our working hypothesis is that kinetic control occurs via a film of adsorbed surfactant on the electrode. This mode of rate enhancement has been observed p r e v i o u ~ l y ~in- ~CTAB and DDAB systems a t potentials negative of -2.1 V vs SCE. The present systems are the first in which significant rate enhancement has been demonstrated a t such positive potentials.
Acknowledgment. This work was supported by US. PHS Grant No. ES03154 awarded by the National Institute (10)Lever,A.B.P.;Licoccia,S.;MagneU,K.;Minor,P.C.;Ramaswamy, of Environmental Health Sciences. Participation of B.S. Adv. Chem. Ser. 1982, No. 201, 237-251. G.N.K. was supported by NSF Grant No. INT-9002223. (11) Connors, T. F.;Arena, J. V.;Ruslinn. J. F. J. Phvs. Chem. 1988, 92, 2810-2816. Registry No. DDAB, 3282-73-3; NiPcTS+4Na,27835-99-0; (12)Sheffold, R.; Abrecht, S.; Orlinski, R.; Ruf, H.-R.; Stamouli, P.;
Tinembart, 0.; Walder, L.; Weymuth, C. Pure Appl. Chem. 1987, 59,
363-372. (13)Rusling, J. F.;Miaw, C. L.; Couture, E. C. Znorg. Chem. 1990,29, 2025-2027. (14)Owlia, A.;Wang, Z.; Rusling, J. F. J. Am. Chem. SOC.1989, 1 1 1 , 5091-5098.
CuPcTS.4Na, 27360-85-6; DBB, 533-98-2; t-DBCH, 7429-37-0; TCA, 76-03-9. (15) Zhang, H.; Rusling, J. F. Langmuir 1991, 7, 1791-1796.
(16)Zhang, H.; Rusling, J. F., University of Connecticut, unpublished results.
