Microemulsions electrochemical reduction

Hernandez J, Solla-Gullon J, Herrero E. 2004. Gold nanoparticles synthesized in a water-in-oil microemulsion Electrochemical characterization and effect of the surface structure on the oxygen reduction reaction. J Electroanal Chem 574 185-196. 

Vaze, A. and Rusling, J.F. (2006) Microemulsion-controlled reaction sites in biocatalytic films for electrochemical reduction of vicinal dibromides. Langmuir 22, 10788-10795. 

Zhou, D.-L., Carrero, H. and Rusling, J.F. (1996) Radical vs anionic pathway in mediated electrochemical reduction of benzyl bromide in a bicontinuous microemulsion. Langmuir 12, 3067-3074. 

Only a few other direct electrochemical reductions have been studied in microemulsions. Reductions of naphthalene and biphenyl resulted in selective reduction of a single benzene ring in the polyaromatic hydrocarbon [40], as in 8 to 12. Products 8 and 9, in addition to biphenyl, were also found from catalytic reduction of polychlorinated biphenyls in microemulsions [46]. 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

The ITIES with an adsorbed monolayer of surfactant has been studied as a model system of the interface between microphases in a bicontinuous microemulsion [39]. This latter system has important applications in electrochemical synthesis and catalysis [88-92]. Quantitative measurements of the kinetics of electrochemical processes in microemulsions are difficult to perform directly, due to uncertainties in the area over which the organic and aqueous reactants contact. The SECM feedback mode allowed the rate of catalytic reduction of tra 5-l,2-dibromocyclohexane in benzonitrile by the Co(I) form of vitamin B12, generated electrochemically in an aqueous phase to be measured as a function of interfacial potential drop and adsorbed surfactants [39]. It was found that the reaction at the ITIES could not be interpreted as a simple second-order process. In the absence of surfactant at the ITIES the overall rate of the interfacial reaction was virtually independent of the potential drop across the interface and a similar rate constant was obtained when a cationic surfactant (didodecyldimethylammonium bromide) was adsorbed at the ITIES. In contrast a threefold decrease in the rate constant was observed when an anionic surfactant (dihexadecyl phosphate) was used. 

The reductive elimination of vicinal dihalides has been accomplished by using many reagents, including the use of aqueous media.16 An interesting method is the reductive elimination of vicinal dihalides by an electrochemical method using vitamin Bi2 in a water-in-oil microemulsion (Eq. 6.8).17... 

The use of electrochemical methods for the destruction of aromatic organo-chlorine wastes has been reviewed [157]. Rusling, Zhang and associates [166, 167] have examined a stable, conductive, bicontinuous surfactant/soil/water microemulsion as a medium for the catalytic reduction of different pollutants. In soils contaminated with Arochlor 1260, 94% dechlorination was achieved by [Zn(pc)] (H2pc=phthalocyanine) as a mediator with a current efficiency of 50% during a 12-h electrolysis. Conductive microemulsions have also been employed for the destruction of aliphatic halides and DDT in the presence of [Co(bpy)3]2+ (bpy=2,2 -bipyridine) [168] or metal phthalocyanine tetrasulfonates [169]. 

In activated sludge, 80.6% degraded after a 47-h time period (Pal et al., 1980). Chemical/Physical. Zhang and Rusling (1993) evaluated the bicontinuous microemulsion of surfactant/oil/water as a medium for the dechlorination of polychlorinated biphenyls by electrochemical catalytic reduction. The microemulsion (20 mL) contained didodecyldi-methylammonium bromide, dodecane, and water at 21, 57, and 22 wt %, respectively. The catalyst used was zinc phthalocyanine (2.5 nM). When PCB-1221 (72 mg), the emulsion and catalyst were subjected to a current of mA/cm on 11.2 cm lead electrode for 10 h, a dechlorination yield of 99% was achieved. Reaction products included a monochlorobiphenyl (0.9 mg), biphenyl, and reduced alkylbenzene derivatives. 

A wide variety of reactions other than substitutions and hydrolyses have been performed in microemulsions. Examples include alkylations [29], Knoevenagel condensations [13], oxidations [30,31], reductions [32], formation and decomposition of Meisenheimer complexes [33], aromatic substitution reactions such as nitration and bromination [34-36], nitrosation [37] and lactone formation, i.e. esterification [38-40]. Microemulsions have also been used for photochemical and electrochemical reactions [41-45]. 

A recent SECM study of electrochemical catalysis at the ITIES was based on a similar concept (23). The ITIES was used as a model system to study catalytic electrochemical reactions in microemulsions. Microemulsions, i.e., microheterogeneous mixtures of oil, water, and surfactant, appear attractive for electrochemical synthesis and other applications (63). The ITIES with a monolayer of adsorbed surfactant is of the same nature as the boundary between microphases in a microemulsion. The latter interface is not, however, directly accessible to electrochemical measurements. While interfacial area in a microemulsion can be uncertain, the ITIES is well defined. A better control of the ITIES was achieved by using the SECM to study kinetics of electrochemical catalytic reduction of //zms-l, 2-dibromo-cyclohexane (DBCH) by Co(I)L (the Co(I) form of vitamin B12) ... 

Recently, taking advantage of the very narrow size distribution of the metal particles obtained, microemulsion has been used to prepare electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) Catalysts containing 40 % Pt Ru (1 1) and 40% Pt Pd (1 1) on charcoal were prepared by mixing aqueous solutions of chloroplatinic acid, ruthenium chloride and palladium chloride with Berol 050 as surfactant in iso-octane. Reduction of the metal salts was complete after addition of hydrazine. In order to support the particles, the microemulsion was destabilised with tetrahydrofurane in the presence of charcoal. Both isolated particles in the range of 2-5 nm and aggregates of about 20 nm were detected by transmission electron microscopy. The electrochemical performance of membrane electrode assemblies, MEAs, prepared using this catalyst was comparable to that of the MEAs prepared with a commercial catalyst. 

Garcia et al. [77,78] reported an electron transfer percolation threshold in highly resistive oil-continuous microemulsions. The Faradaic electron transfer is modulated by the amount of cosurfactant present in AOT-toluene-water microemulsions. Below a certain threshold concentration of the cosurfactant, the electron transfer between electroactive solutes in the water droplets and ultramicroelectrode is retarded or blocked. Electron transfer becomes facilitated, and a sharp increase in Faradaic current is observed above the threshold concentration. This effect was demonstrated for ruthenium hexamine reduction [77,78], ferrocyanide oxidation [77,78], acrylamide oxidation [77], and allQ lamide oxidation [77,79] with acrylamide, alkylamides, and acetonitrile as cosurfactants in AOT microemulsions. NMR results [80] suggest that there is an interfacial packing transition of the surfactant (AOT) at about the same cosurfactant concentration as the threshold transition observed electrochemically. 

Electrochemical catalysis constitutes a general synthesis route that is amenable to rate control and enhancement in microemulsions [5,6]. Owlia et al. [36] were the first to investigate electrochemical catalysis in oil-continuous microemulsions. The kinetics of reduction of several allQ l vicinal dibromides was studied in the presence of vitamin B12 as catalyst. The following reaction mechanism was suggested ... 

An attempt was made to model the interfacial chemistry in microemulsions by using the interface between two immiscible electrolyte solutions [95]. The reaction between the electrochemically generated Co(I) form of vitamin B12 in the aqueous phase and /-DBCH in benzonitrile was probed directly at the interface by using scanning electrochemical microscopy. The kinetics of /-DBCH reduction by vitamin B12 was observed to be more complex at the liquid/liquid interface than in a homogeneous solution [95]. 

Raghuveer V, Ferreira PJ, Manthiram A (2006) Comparison of Pd-Co-Au electrocatalysts prepared by conventional borohydride and microemulsion methods for oxygen reduction in fuel cells. Electrochem Commun 8(5) 807-814... 

Nonconductive w/o microemulsions were used for electrochemically induced polymerization at a specially designed solid polymer working electrode. Using a microemulsion of water, toluene, and sodium bis(2-ethylhexyl) sulfosucci-nate (AOT), acrylamide was polymerized via persulfate reduction [51]. An applied potential initiated polymerization, which continued for several hours after the power was off. This resulted in a latex suspension with comparable molecular weight and particle size to those obtained by thermal or UV initiation. The degree of stirring controlled the particle size (7-130 nm). 

Reductive Dehalogenations Microemulsions are usually more useful than micelles for electrochemical synthetic applications because larger amounts of polar and nonpolar reactants can be solubilized. Electrochemical catalysis has been used in microemulsions for the electrolytic conversion of organohalide pollutants to hydrocarbons [53] using mediators such as metal phthalocyanines and cobalt complexes. Microemulsions were used for the complete electrochemical catalytic... 

Shao, Y., Mirkin, M. V., Rusling, J. F. Liquid/hquid interface as a model system for studying electrochemical catalysis in microemulsions—Reduction of trans-l,2-dibromocyclohexane with vitamin-B-12. J. Phys. Chem. B 1997,101, 3202-3208. 

Y. H. Shao, M. V. Mirkin,andJ. F. Rusling,/. Phys. Chem. B, 101,3202 (1997).Liquid/Liquid Interface as a Model System for Studying Electrochemical Catalysis in Microemulsions. Reduction of Trans-l,2-Dibromocyclohexane with Vitamin B-12. 

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