Catalytic films were constructed by covalently binding poly-L-lysine (PLL) onto oxidized carbon electrodes and then forming covalent amide linkages from PLL to the cobalt corrin vitamin B 12 hexacarboxylic acid [B 12 (COOH) 6 ]. Covalent bonds from electrode to PLL and PLL to B 12 (COOH) 6 provided good stability in microemulsions. PLL-B 12 (COOH) 6 films gave reversible electron transfer for the Co(II)/Co(I) redox couple and exhibited characteristic voltammetric features of surface-confined electrochemistry. Formal potentials of the Co(II)/Co(I) couple in the films were controlled by the concentration of electrolyte in the fluid and by Coulombic interactions with surfactant. PLL-B 12 (COOH) 6 films demonstrated excellent catalytic activity in microemulsions for the reduction of vicinal dihalides to olefins, for dechlorination of trichloroacetic acid, and for alkylation of an activated olefin. Turnover numbers for conversion of dibromocyclohexane to cyclohexene in microemulsions were 3-fold larger for PLL-B 12 (COOH) 6 on carbon cloth cathodes than those for the same cobalt catalyst chemisorbed onto nanocrystalline TiO 2 cathodes.
Catalytic electrodes were prepared by covalently binding poly-L-lysine (PLL) onto oxidized carbon electrodes and then linking the cobalt corrin vitamin B12 hexacarboxylic acid [B12(COOH)6] to this surface. Additional layers of PLL-B12(COOH)6 were attached in a similar way. Quartz crystal microbalance studies showed regular and reproducible layer formation. Electrochemical and catalytic properties of the Co II L/ Co I L redox couple in these films were investigated using voltammetry and preparative electrolysis in an sodium dodecyl sulfate microemulsion. These films obeyed theoretical predictions of a maximum in voltammetric catalytic efficiency as thickness increased for the reduction of 1,2-dibromocyclohexane (DBCH) in a microemulsion. In films with less than optimum thickness, kinetic control of the chemical reaction between Co I L and DBCH predominated. As film thickness was increased beyond that found for maximum efficiency, electron and reactant mass transport within the films became limiting factors. Under synthetic electrolysis conditions, optimal turnover numbers were found for very thin films on porous electrodes, and best yields and current efficiencies were obtained with the relatively small catalyst coverage of about 2 nmol cm -2 .
Electronic absorption spectroscopy was used to measure the molecular association of copper phthalocyanine tetrasulfonate in micellar solutions, a microemulsion made with cationic surfactant, and homogeneous solvents. Analysis of absorbance versus concentration data using a multiple-aggregation model and non-linear regression analysis gave values of association constants, molar absorptivities and estimates of average aggregation number. Microemulsions and aqueous micellar solutions made with alkylammonium surfactants inhibited aggregation, probably because of interactions between the phthalocyanine sulfonate groups and the cationic surfactant head groups at interfacial surfaces. Similar aggregation behavior was observed previously in multiple-bilayer films of cationic surfactants. Water and aqueous solutions containing tetraethylammonium bromide or anionic SDS micelles provide environments facilitating extensive aggregation of Cu II PcTS 4−. The major species are dimers in water and acetonitrile/water, but the formation of higher aggregates is promoted by addition of SDS or TEAB. Aprotic organic solvents provide environments intermediate between these two extremes, giving relatively large aggregation numbers (i.e. five to seven) but smaller association constants than aqueous media not containing cationic surfactants.
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