Influence of Thickness on Catalytic Efficiency of Cobalt Corrin-Polyion

Colin J. Campbell, Christopher K. Njue, Bharathi Nuthakki, and. James F. Rusling*. Department of Chemistry, Box U-60, University of Connecticut, Storr...
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Langmuir 2001, 17, 3447-3453

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Influence of Thickness on Catalytic Efficiency of Cobalt Corrin-Polyion Scaffolds on Electrodes in Microemulsions Colin J. Campbell, Christopher K. Njue, Bharathi Nuthakki, and James F. Rusling* Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-3060 Received March 20, 2000. In Final Form: February 21, 2001 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 CoIIL/ CoIL 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 CoIL 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.

Introduction The need for environmentally friendly chemical syntheses has led to a renewed recognition of the advantages of catalysts immobilized on electrodes.1-4 These coated electrodes provide efficient use of catalysts and can facilitate catalyst recycling and downstream processing. Immobilization of catalysts on electrodes has found utility in analytical sensors,5-7for investigations of enzyme reactions,2 and in synthetic electrolyses.1,8,9 To avoid toxic, expensive organic solvents, we have been exploring microemulsions of oil, water, and surfactants as alternative fluid media.10,11 We found that catalyst films covalently bound to electrodes have sufficient stability for synthetic use in microemulsions but that simple adsorbed or ion exchanged films are unstable.4 * To whom correspondence should be addressed. E-mail: [email protected]. Tel: +353-1700-5913. Fax: +353-1700-5503. (1) (a) Osa, T.; Kashiwage, Y.; Bobbitt, J. M.; Ma, Z. In Electroorganic Synthesis; Little, R. D., Weinberg, N. L., Eds.; Marcel-Dekker: New York, 1991; pp 343-354. (b) Bedioui, F.; Devynck, J.; Bied-Charreton, C. Acc. Chem. Res. 1995, 28, 30-36. (2) Zu, X.; Lu, Z.; Zhang, Z.; Schenkman, J. B.; Rusling, J. F. Langmuir 1999, 15, 7372-7377. (3) Lvov, Y. M.; Kamau, G. N.; Zhou, D.-L.; Rusling, J. F. J. Colloid Interface Sci. 1999, 212, 570-575. (4) (a) Zhou, D.-L.; Njue, C. K.; Rusling, J. F. J. Am. Chem. Soc. 1999, 121, 2909-2914. (b) Mbindyo, J. K. N.; Rusling, J. F. Langmuir 1998, 14, 7027-7033. (5) (a) Biosensors; Turner, A., Karube, I., Wilson, G., Eds.; Oxford University Press: New York, 1987. (b) Kauffmann, J. M.; Guilbault, G. G. In Bioanalytical Applications of Enzymes, Methods of Biochemical Analysis; Wiley: New York, 1992; Vol. 36, pp 63-113. (6) Commerical Biosensors; Ramsay, G., Ed.; Wiley: New York, 1998. (7) (a) Bartlett, P. N.; Pletcher, D.; Zeng, J. J. Electrochem. Soc. 1997, 144, 3705-3710. (b) Chen, S.-M. Electrochim. Acta 1998, 43, 33593369. (c) Sun, C.; Li, W.; Sun, Y.; Zhang, X.; Shen, J. Electrochim. Acta 1999, 44, 3401-3407. (8) Ciszewski, A.; Milczarek, G. J. Electroanal. Chem. 1997, 426, 125-130. (9) Collman, J. P.; Hendricks, N. H.; Leidner, C. R.; Ngameni, E.; L Her, M. Inorg. Chem. 1988, 27, 287-393. (10) Rusling, J. F.; Zhou, D.-L. J. Electroanal. Chem. 1997, 439, 8996. (11) Rusling, J. F. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, in press.

Mediated electrochemical catalysis offers the opportunity to decrease energy requirements by lowering electrode potentials required, to cut down on the expense of using stoichiometric amounts of oxidizing or reducing agents, and to reduce pollution from side products.12 Transitionmetal-complex mediators such as vitamin B12 and its hexacarboxylic acid analogue [B12(COOH)6] contain cobalt ligated by a conjugated corrin macrocycle. Such complexes in the CoIIL form undergo electron transfer, forming CoIL species capable of reacting with alkyl halides to yield alkylCoIIIL complexes. Alkyl-CoIIIL bonds can be cleaved by light to yield radicals which can be trapped by activated olefins to form carbon-carbon bonds.12ab Cyclizations can be achieved if the activated olefin and alkyl halide moieties reside on the same reactant molecule. For example, 2-(4bromobutyl)cyclohexen-1-one is debrominated to the corresponding alkyl radical which adds to the double bond and gives predominantly trans-1-decalone in microemulsions.13 Reductions of vicinal dihalides such as trans-1,2dibromocyclohexane (DBCH) to the corresponding alkene can be effected by electrochemically generated CoIL without light at the CoIIL/CoIL redox potential (Scheme 1). The CoIL complex is oxidized back to a CoIIIL form which then undergoes reduction to CoIL to participate in subsequent catalytic cycles.14,15 This reaction proceeds in high yields using dissolved vitamin B12 as the catalyst in organic solvents or microemulsions.10ab,16 (12) (a) Scheffold, R.; Rytz, G.; Walder, L. In Modern Synthetic Methods; Scheffold, R., Ed.; Wiley: New York, 1983; Vol. 3, p 355. (b) Scheffold, R.; Abrecht, S.; Orlinski, R.; Ruf, H.-R.; Stamouli, P.; Tinembart, O.; Walder, L.; Weymuth, C. Pure Appl. Chem. 1987, 59, 363-372. (c) Pletcher, D.; Walsh, F. C. Industrial Electrochemistry, 2nd ed.; Blackie Academic: London, 1993. (13) Gao, J.; Njue, C. K.; Mbindyo, J. K. N.; Rusling, J. F. J. Electroanal. Chem. 1999, 464, 31-38. (14) Lexa, D.; Saveant, J.-M.; Schafer, H. J.; Su, K.-B.; Vering, B.; Wang, D. L. J. Am. Chem. Soc. 1990, 112 (2), 6164-6177. (15) Lexa, D.; Saveant, J.-M.; Su, K.-B.; Wang, D. L. J. Am. Chem. Soc. 1987, 109, 6464-6470. (16) Gao, Y.; Rusling, J. F.; Zhou, D.-L. J. Org. Chem. 1996, 61, 59725977.

10.1021/la000416q CCC: $20.00 © 2001 American Chemical Society Published on Web 05/03/2001

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Langmuir, Vol. 17, No. 11, 2001 Scheme 1. Reaction Pathway for Catalytic Debromination Using CoIIL

Vitamin B12 and its analogues have been used effectively to catalyze a number of such reactions involving organic substrates in microemulsions as solvents.17 Microemulsions are intimate mixtures of oil, water, surfactant (anionic, cationic, or nonionic), and in some cases cosurfactant.18 Although they are macroscopically homogeneous, microemulsions feature dynamic water-in-oil (w/ o), oil-in-water (o/w), or bicontinuous nanostructures. Bicontinuous and o/w microemulsions are most useful for electrolysis because water continuity and ionic surfactant or salt impart the necessary conductivity. An important feature for mediated electrochemical reactions is that microemulsions can dissolve substrates of widely diverse polarities and maintain them in relatively close proximity. We have identified a number of reactions in which microemulsions provide unique control over mediated reaction pathways.17,19 Given the success of synthetic electrolyses in microemulsions with dissolved mediators, we began to extend these systems to stable catalytic polymer networks on electrodes. Microemulsions are such excellent solvents that useful catalytic films require covalent bonding of all components to the electrode. Recently, we constructed films of poly-L-lysine (PLL) bound by amide linkages to vitamin B12 hexacarboxylate [B12(COOH)6], with this entire scaffold covalently linked to oxidatively treated graphite.4a The resulting catalytic electrodes are reasonably stable in microemulsions and in fact require these fluids for reactions of water-insoluble organic substrates because the films are inoperative in common organic solvents. Turnover numbers for the catalytic conversion of trans-1,2-dibromocyclohexane to cyclohexene utilizing the PLL-B12(COOH)6 network were 17-fold larger than for vitamin B12 dissolved in microemulsions. Catalytic efficiency of very thin films of this type on carbon cloth electrodes can be controlled and optimized by microemulsion composition, utilizing interactions of the protonated lysine groups with anionic surfactant in the microemulsions.20 Although turnover numbers for catalytic reactions could be controlled by microemulsion composition for very thin films (∼30 nm) under kinetic control of the reaction between CoIL and reactant, thicker films began to lose this ability because of limitations from transport of charge and reactant within the PLL-B12(COOH)6 network.20 We expected that excess unreacted carboxylates at the film surface after construction of one layer could be utilized to attach additional layers of PLL-B12(COOH)6 and increase catalyst loading and film thickness systematically. Theoretical models considering the influences of film thickness and mediator loading on electrochemical cata(17) Rusling, J. F.; Zhou, D.-L. J. Electroanal. Chem. 1997, 439, 8996. (18) (a) Bourrel, M.; Schechter, R. S. Microemulsions and Related Systems; Marcel Dekker: New York, 1988. (b) Rusling, J. F. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum Press: New York, 1994; No. 26, pp 49-104. (19) Carrero, H.; Gao, J.; Rusling, J. F.; Lee, C.-W.; Fry, A. J. Electrochim. Acta 1999, 45, 503-512. (20) Njue, C. K.; Rusling, J. F. J. Am. Chem. Soc. 2000, 122, 64596463.

Campbell et al. Scheme 2. Conceptual Diagram of Electrode Preparation Procedure

lytic efficiency predict that under kinetic control of the chemical reaction, for example, between CoIL and DBCH, increasing thickness of the polymer layer will increase its voltammetric catalytic efficiency.21 As thickness increases, however, limitations from transport of charge and reactant within the polymer network will begin to limit the catalytic efficiency. Thus, plots of catalytic limiting current versus film thickness are expected to show a maximum.21 This theory has been supported by experimental examples in homogeneous solutions but not in microemulsions. In the present work, we investigated catalytic efficiencies of electrodes with up to eight layers of PLL-B12(COOH)6 (Scheme 2) in a microemulsion with a view to testing the above predictions. We found an optimum in voltammetric catalytic efficiency for reduction of DBCH to olefin as the number of catalytic layers is increased, but optima in turnover and yields during preparative electrolysis occur at a much smaller number of layers. Experimental Section Chemicals. Pentanol, trans-1,2-dibromocyclohexane, and 1-[3-(dimethylamino)propyl]-3-ethyl carbodiimide (EDC) were from Aldrich. Sodium dodecyl sulfate (SDS) was from Kodak, tetradecane was from Acros, and poly-L-lysine (MW 150 000300 000) was from Sigma. Vitamin B12 hexacarboxylate was synthesized by a previously reported method.22 Water was purified by a Hydro Nanopure system to a specific resistance of >15 MΩ cm-2. The SDS microemulsion, characterized as bicontinuous,23 was made by mixing SDS/pentanol/0.1 M NaCl/ tetradecane in a weight ratio of 13.3/26.7/52/8. Electrode Preparation. For voltammetry, basal-plane pyrolytic graphite (PG) disks (diameter of 0.4 cm) were attached to steel rods using heat-shrink Teflon tubing and sequentially polished on wet silicon carbide paper (600 grit from Buehler) and a slurry of 0.05 µm alumina (Buehler) and water. After rinsing, electrodes were sonicated in water for 30 s. Electrodes were then oxidized4a using aqueous potassium dichromate (2.5% w/v) and nitric acid (10% v/v) by scanning once between 1.5 and 1.7 V versus saturated calomel electrode (SCE) at 5 mV s-1. Oxidized electrodes were rinsed thoroughly and dried, and a solution of EDC (5 µL, 24 mM) was evenly spread on the surfaces to activate the carboxyl groups. After 20 min, a solution of PLL (5 µL, 4 mM as lysines) was applied. After 4 h, the electrodes were rinsed and dried using a stream of nitrogen before application of solutions of B12(COOH)6 (5 µL, 2 mM) and EDC (5 µL, 24 mM), which were left overnight, and then rinsed in water. One cycle of addition of PLL + B12(COOH)6 to the surface is arbitrarily defined in this paper as a single layer of catalyst. Less than single layer coverage was achieved by the same (21) Andrieux, C. P.; Saveant, J. M. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; Techniques of Chemistry Series; WileyInterscience: New York, 1992; Vol. 22, pp 207-270. (22) Miaw, C.-L.; Hu, N.; Bobbit, J. M.; Ma, Z.; Ahmadi, M. F.; Rusling, J. F. Langmuir 1993, 9, 315-322. (23) Mackay, R. A.; Myers, S. A.; Brajter-Toth, A. Electroanalysis 1996, 8, 759-764.

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procedures with all reagent concentrations decreased to 0.1 or 0.4 of the above. Subsequent layers of catalyst were added on top of the first single layers by sequentially repeating the singlelayer procedure, that is, applying EDC, leaving for 20 min, applying PLL, leaving for 4 h, applying B12(COOH)6 and EDC, and leaving overnight as described above. For preparative electrolyses, carbon cloth (Zoltek Corp., 2 cm × 2 cm) was rinsed thoroughly with water, sonicated, rinsed again, and dried with a stream of nitrogen. This electrode was then electrochemically oxidized as described above, rinsed thoroughly, and dried again in nitrogen. The cloth was covered with a solution of EDC (24 mM) for 20 min, which was then decanted and replaced with PLL solution (4 mM in lysine residues). After 4 h, the cloth was drained and thoroughly rinsed. B12(COOH)6 (2 mM) and EDC (24 mM) were then added in equal volumes to cover the cloth and left overnight. Further layers were added in a similar fashion, by rinsing, adding EDC, leaving for 20 min, adding PLL, leaving for 4 h, adding EDC and B12 hexacarboxylate, and leaving overnight. For electrodes with a low surface coverage of catalyst, the same procedure was used but the reactant concentrations were 5 mM EDC, 2 or 1 mM PLL, and 0.5 or 0.17 mM B12 hexacarboxylate. Electrolytic Turnover Numbers. A two-compartment electrochemical cell was used at 20 °C and separated by an agarKCl bridge (prepared by dissolving 1 g of agar + 7 g of KCl in 23 mL of H2O) behind a glass frit. Reaction mixtures were thoroughly degassed with purified nitrogen in advance, and a blanket of nitrogen was maintained during electrolysis. The working electrode (coated as described above) was 1.5 cm × 2 cm; the remainder of the original 2 cm × 2 cm piece was used to voltammetrically determine surface concentration, as detailed below. The working electrode chamber contained DBCH (0.012 M) dissolved in SDS microemulsion (10 mL). The counter electrode was a graphite rod, and the electrolyte in the counter electrode chamber was 0.1 M KBr. All electrolyses were run for 30 min and performed in duplicate using the same working electrode and fresh solution. Reaction mixtures were eluted on a silica column and then analyzed using a HP6890 gas chromatograph as previously described.13 Turnover numbers are reported as moles product/moles catalyst/hour. Current efficiency (CE) is reported as

Figure 1. CVs for electrodes with multiple layers of catalyst at 50 mV s-1 in SDS microemulsion. The CVs are labeled with the number of layers; 0.4 represents a film formed by decreasing the concentrations of all solutions by 0.4 (see Experimental Section).

%CE ) (100%)2F (mol cyclohexene found)/coulombs passed Voltammetry. Voltammetry was done in a three-electrode cell at 25 °C. The working electrodes were coated PG as described above, and the counter electrode was a platinum wire. The reference electrode was an SCE. A coulometric assay was used to determine surface coverage of active catalyst. In an electrolyte solution of Tris (5 mM) and NaCl (45 mM) at pH 7.1, each electrode was scanned at 10 mV s-1, and the area under the catalyst peak was used with Faraday’s law to estimate surface coverage. For carbon cloth electrodes, a 0.5 cm × 0.5 cm square was used for this assay. Catalytic efficiency as ICat/ID was determined by cyclic and rotating disk (1800 rpm) voltammetry (RDV)21,20 in the microemulsion. ID is the CoIIL reduction current at the peak potential in the absence of DBCH, and ICat is the reduction current at the same potential in the presence of DBCH. Quartz Crystal Microbalance (QCM). QCM (USI System, Japan) was used to monitor film assembly. The long-term stability (several hours) of frequency was (2 Hz. QCM resonators were covered by evaporated gold electrodes (0.13 cm2), and the resonance frequency was 9 MHz (AT-cut). To mimic oxidized carbon surfaces, gold surfaces were coated with mercaptopropanecarboxylic acid (MPA) before films were prepared on them.20 The Saurbrey equation provides a relation between adsorbed mass and frequency shift ∆F (Hz). Taking into account characteristics of the 9 MHz quartz resonators, the film mass per unit area M/A (g cm-2) is given by

M/A ) -∆F/(1.83 × 108)

(1)

on one side of the resonator. The thickness of a film was estimated from its mass using film density, assumed to be 1.3 g cm-3 for

Figure 2. Influence of the number of catalyst layers on electroactive surface coverage on PG electrodes in SDS microemulsions from integrating CVs at 10 mV s-1. the organic materials used, giving the following relation confirmed by scanning electron microscopy24 for nominal thickness (d) of dry films:

d (nm) = -(0.016 ( 0.002)∆F (Hz)

(2)

Results Film Properties. We arbitrarily define a single layer of catalyst as one resulting from one cycle of PLL + B12(COOH)6 attachment to the surface using standard reagent concentrations (see Experimental Section). Figure 1 shows cyclic voltammograms (CVs) for 0.4, 2, 3, 4, and 6 layers of catalyst deposited on an oxidized pyrolytic graphite electrode. Films with e3 layers gave voltammograms characteristic of the one-electron, quasireversible CoIIL/ CoIL redox couple.4a There is no clearly identifiable trend in midpoint potential for these electrodes. However, with 4-6 layers of catalyst, the CVs broaden considerably and peak current begins to decrease. Figure 2 shows that the electroactive surface concentration obtained from integrals (24) (a) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (b) Lvov, Y. In Protein Architecture: Interfacing Molecular Assemblies and Immobilization Biotechnology; Lvov, Y., Mo¨hwald, H., Eds.; Marcel Dekker: New York, 2000; pp 125-167.

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Figure 3. Average QCM frequency change for dry PLL-B12(COOH)6 films on four gold-coated resonators. The first layer is MPA. Subsequent layers are PLL and B12(COOH)6, respectively.

of slow scan CVs increases initially and reaches an optimum at 3 layers before decreasing. QCM on MPA-Au resonators provided a measure of dry mass of polymer and catalyst achieved at each surface reaction cycle. MPA chemisorbs via gold-thiol bonds, leaving carboxylates facing the solution, as in oxidized carbon.25 Figure 3 shows the average frequency shift versus cycle number of the film-forming surface reactions and indicates a reproducible formation of each layer. The first two layers of PLL and B12(COOH)6 have considerably smaller -∆F values than subsequent layers. Using eqs 1 and 2, we find that the first PLL + B12(COOH)6 layer contains about 4 µg cm-2 catalyst and is 18 nm thick. The MPA layer gave ∆F ) -12 Hz. Compared to a monolayer coverage giving 30 Hz for similar thiols,26 this represents 30-40% of a monolayer. The data suggest that less PLL binds to this MPA layer than to subsequent catalyst layers and that there are fewer available MPA sites for B12(COOH)6 coupling compared to successive layers. Additional PLL + B12(COOH)6 layers after the first one contain an average of 10 ( 4 µg cm-2 catalyst and are 70 nm thick. Comparison of the electroactive and total masses showed that about 22% of the first catalyst layer is electroactive and that this fraction drops steadily to about 10% electroactive for 2 layers and 1.5% electroactive for 6 layers. Catalytic Voltammetry. Figure 4 shows voltammograms of electrodes with catalytic layers in the SDS microemulsion with and without reactant DBCH. All CVs of coated electrodes with DBCH present show the characteristics of electrochemical catalytic reduction, that is, an increased current on the forward scan at the CoIIL reduction potential and minimal current for the back reaction for CoIL oxidation. This is a consequence of the reaction between DBCH and CoIL, using up the latter and cyclically regenerating CoIIL to provide this catalytic reduction current (ICat). DBCH is reduced directly on a noncatalytic PLL electrode (Figure 4a) at a potential about 0.7 V more negative than the CoIIL reduction potential. Figure 4b compares catalytic currents for 2-layer elec(25) McCreery, R. L. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1991; Vol. 17, pp 221-374. (26) Lvov, Y. M.; Lu, Z.; Schenkman, J. B.; Zu, Z.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073-4080.

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trodes by CV and RDV. The latter current is larger because of the more efficient convective mass transport. Figure 4c presents the influence of scan rate on ICat, and Figure 4d shows the influence of the number of catalyst layers. The ratio ICat/ID is a voltammetric measure of catalytic efficiency. Figure 5 shows a peak-shaped dependence of ICat/ID on the number of catalyst layers. There is an optimal value from CV of ICat/ID at 0.4 layers. Rotation of the electrode at 1800 rpm (RDV) gave increased reduction currents from increased mass transport supplying reactant to the electrode-film interface. Plotting ICat/ID again gave a peak-shaped dependence but with the optimum moving up to 1 layer. Preparative Electrolysis. The results of a series of preparative electrolyses using electrodes with various numbers of catalyst layers are summarized in Table 1. It can be seen that the surface coverage of the electroactive catalyst on the electrode increases with the number of layers, before reaching a plateau, although there is a bit more scatter in these data than on the smaller electrodes. Decreasing turnover numbers for debromination of DBCH were found with increasing number of layers. The turnover numbers drop rapidly between the first 2 layers and level out around 5 layers. Over 8 layers, the current efficiency varies only slightly compared to the surface concentration and turnover number, but there is a decreasing trend from 100% as the number of layers increases above 4. To verify the trend in turnover numbers, electrodes were prepared using low concentrations of reactants so as to have submonolayer coverage. Electrolysis using these electrodes showed even higher turnover numbers although the actual amount of product formed was less than in the case of those electrodes with somewhat higher surface coverage, corresponding to our arbitrary definition of a single layer (cf. Table 1). The product yield in milligrams is relatively constant for films with g1 layer. We found little correlation between product yields and electroactive surface concentration. Discussion Catalytic layer-by-layer film architectures on electrodes have been, to a large extent, based on adsorption of alternate layers of polyions interspersed with electroactive species such as proteins or metal complexes.3,27,28 Although such techniques show great promise, they are probably unsuitable for use in microemulsions because of reliance on Coulombic forces as the main stabilizing interaction. For layered films of ionic metal complexes and polyanions, stability in microemulsions was rather poor because of the excellent solubilizing power of these fluids.28 The benefit of covalent attachment of each layer to the electrode surface is therefore realized in the greater stability of the films studied herein, which are relatively stable in microemulsions. Covalent anchoring allows the synthetic use of the electrode with good catalytic activity in microemulsions. Films prepared by layer-by-layer adsorption have had reported electrochemical responses which reach saturation as the number of layers and the thickness of the film is increased.3,27,28 We also observe this effect in our system with an optimal loading of electroactive molecules at around 3 layers (Figure 2). (27) Rusling, J. F. In Protein Architecture: Interfacing Molecular Assemblies and Immobilization Biotechnology; Lvov, Y., Mo¨hwald, H., Eds.; Marcel Dekker: New York, 2000; pp 337-354. (28) Lvov, Y. M.; Kamau, G.; Zhou, D.-L.; Rusling, J. F. J. Colloid Interface Sci. 1999, 212, 570-575.

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Figure 4. Voltammetry of PLL-B12(COOH)6 films in the SDS microemulsion: (a) CV at 50 mV s-1 with and without 12 mM DBCH for an electrode coated with 1 catalyst layer and 12 mM DBCH on a PLL-coated electrode, (b) CV and RDV at 50 mV s-1 with and without 12 mM DBCH for an electrode coated with 2 catalyst layers, (c) influence of scan rate on CV and RDV catalytic current for an electrode coated with 2 catalyst layers, and (d) the effect of increasing number of catalyst layers on CV catalytic current at 50 mV s-1 for reduction of 12 mM DBCH. Table 1. Results of Catalytic Electrolyses on Coated Carbon Cloth Electrodes no. of Γ (nmol cm-2) Γ (nmol cm-2) layersa geometic area actual area 0.1 0.4 1 2 3 4 5 6 7 8

Figure 5. Influence of number of catalyst layers on catalytic efficiency (Icat/ID) at 10 mV s-1 in SDS microemulsion. Each electrode was rotated at 1800 rpm for RDV.

The catalytic activity of the film measured by the ratio ICat/ID for DBCH reduction follows a similar trend for PLLB12(COOH)6 (Figure 5), but the optimum number of layers is 1. Judging from the QCM results, film thickness is about 70 nm for a 1-layer film. This voltammetric behavior exhibited by varying the numbers of layers of catalyst attached to the electrode surface is in good accordance with theory for homogeneous solutions, which predicts that there are three factors capable of controlling the catalytic current at an electrode consisting of multiple layers of catalyst:21

0.3 1.3 1.8 7.5 6.0 11.6 9.8 20 19.2 15

0.02 0.07 0.1 0.42 0.33 0.64 0.54 1.11 1.07 0.83

product turnover current found numberb efficiency (h-1) (%) (mg) 2.8 3.1 5.4 4.4 3.1 5.5 2.7 3.8 4.2 5.4

75328 19384 25247 5432 4430 4484 1363 1735 1778 3333

100 100 100 100 100 97.0 91.0 76.6 94.2 83.2

a One layer is defined as one immobilization cycle of PLL (4 mM in lysine residues) + B12 hexacarboxylate (2 mM). Fractional layers were made using lower concentrations of PLL and B12 hexacarboxylate as in Experimental Section. b Turnover numbers as moles of product/moles of catalyst/hour.

1. The type of electrochemical catalysis, that is, redox or chemical catalysis; 2. The rate of electron transport through the polymer matrix; 3. The rate of diffusion of substrates through the polymer matrix. The reduction of DBCH with cobalt macrocylic complexes involves the chemical type of catalysis.14,15 This means that the catalyst undergoes activation by reduction from a CoIIL to a CoIL species prior to concerted electron transfer and bond breaking [Scheme 1, eqs 1 and 2].14 The

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chemical electron-transfer step involving CoIL and DBCH is inner sphere, as opposed to a simple outer-sphere electron-transfer event characteristic of redox catalysis. The concerted chemical process yields the olefin product and the inactive form of the catalyst which can undergo further electroreduction, leading to the characteristic current increase and lack of reverse peak in the presence of DBCH as seen in Figure 4. The strong chemical interaction between substrate and activated catalyst features inner-sphere electron transfer which can occur at rates up to 6 orders of magnitude faster than in outersphere redox catalysis.14 In redox catalysis, electron transfer occurs as the result of a physical, rather than chemical, interaction between substrate and catalyst. The acceleration of rate of reduction brought about by this kind of catalyst in solution relies on the greater threedimensional availability of electrons compared to a twodimensional electrode surface. As a result, the immobilization of a single layer of redox catalyst is not expected to lead to a significant increase in reaction rate because it changes the characteristics of the electrode little when viewed from the position of the substrate. Only when further layers create a three-dimensional network of catalyst at the surface will there be an appreciable acceleration in reaction rate. This is not necessarily the case with a chemical catalyst because there exists a strong chemical interaction with the substrate. Theory therefore predicts a modest amount of catalytic activity with monolayer coverage for chemical catalysis and few complications from factors such as permeation of substrate or electron movement through the layer. Increasing the number of layers increases the amount of catalyst available to react and also puts the catalyst in a three-dimensional array which may make it more physically available to the reactive substrate.21 (In our case, however, the single-layer films are about 18 nm thick and probably already form a three-dimensional network). An increase in ICat/ID is predicted as film thickness increases, as observed in Figure 5. Increasing the number of layers of catalyst also introduces the possibility that the layers of catalyst closest to the electrode will become less available to substrate molecules because of increased difficulty of permeation of the substrate and thus will be less efficient.21 In other words, as the layer gets thicker, mass transport to the interior and electron transport to the outermost, most substrate-accessible, catalyst layers become more difficult and may limit the rate of reaction.21 The competition of these factors leads to a relationship in which ICat/ID initially increases under the kinetic influence of the increased amount of catalyst, but as the film becomes thicker, mass and electron transport within the film become more important and ICat/ID starts to decrease.21 We observe the predicted increase in ICat/ID upon adding layers of catalyst to the electrode surface but only up to 0.4 to 1 layer. Addition of more layers may interfere with diffusion of the substrate to the more buried layers and also increases the distance of the furthest catalyst layer from the electrode, leading to the predicted decrease in ICat/ID. However, we also observe a maximum in the amount of electroactive catalyst in the film at about 3 layers. This suggests that electron transport to the outer layers of catalyst is beginning to become a limiting factor at this point. To increase the convection of the fluid in an attempt to improve the mass transport rate of the substrate to the film, we also did voltammetry with an electrode rotating at 1800 rpm. This RDV experiment is closer to the conditions of synthetic electrolyses. In addition to in-

Campbell et al.

creasing all the catalytic currents, electrode rotation had the effect of moving the catalytic current maximum from 0.4 to 1 layer. Results suggest that the increased rate of mass transport provides a better supply of substrate to the film-solution interface and thus better supplies inner layers of the electrodes than in the quiet solution CV experiment. In CV, solution mass transport relies only on diffusion, and a layer depleted of substrate builds up at the solution side of the film-solution interface during the forward potential scan. For the preparative electrolyses, carbon cloth electrodes with an average surface area 18 times larger than the geometric surface area were used. The surface concentration of catalyst based on geometric electrode area therefore appears much higher than in the voltammetric experiments. If the actual surface area is taken into account, the maximum surface coverage for preparative electrodes is roughly half that of voltammetric electrodes (cf. Table 1 and Figure 2). The scatter of surface concentration versus layer number is larger for these rougher electrodes, which may reflect variability of surface area or state in various regions of the cloth. In any case, the electroactive surface coverage increases to a maximum value at about 6-7 catalyst layers, as opposed to 3 on the voltammetric electrode. This is consistent with the smaller amount of electroactive catalyst per layer on the carbon cloth electrodes. The trend in catalytic turnover for the debromination of DBCH is different from that of ICat/ID, with the largest turnover numbers for the smallest electroactive surface coverage (Table 1). Given that DBCH reduction occurs via an inner-sphere chemical mechanism, it relies less on the three-dimensional structure of the film than a redox (outer-sphere) catalyst of similar potential.21 When viewed in isolation, a single immobilized catalyst site may see no disadvantage in being part of a low surface coverage system and may, in fact, achieve faster turnover by the lack of competing molecules around it as is seen in the electrodes with the lowest catalyst coverage. The lack of competing molecules allows the catalyst the maximum exposure to substrate with the minimum buildup of products in its vicinity. In other words, when further layers are added to the electrode surface there is more competition for substrate molecules, and because of the higher overall rate of reaction more reaction products build up in the film. In addition to these factors, the thickness of the film also slows down substrate mass transport and electron transport, slowing down the rate of reaction of individual catalyst molecules in the film. This may point to a higher sensitivity of overall reaction rate to reactant mass transport and electron transport within catalytic films on the longer time scale of the synthetic electrolysis compared to the relatively shorter voltammetric time scale. Practical implications for this system can be extrapolated from Table 1. The surface coverage for optimum product yield was about 2 nmol cm-2. Although this value does not give the highest turnover number, it leads to the largest amount of product formed in the reaction. The yield was quantitative, and current efficiency was 100%. Although similarly high yields can be attained at higher surface coverage, there is no benefit in adding more catalyst layers. In summary, we have shown that layered, covalently bound films of PLL-B12(COOH)6 in a microemulsion obey theoretical predictions21 of a maximum in voltammetric catalytic efficiency as film thickness increases. This suggests that in films with thickness up to the optimum, kinetic control of the reaction between the active CoIL

Efficiency of Cobalt Corrin-Polyion Scaffolds

and DBCH predominates. In the thicker films, both electron transport and reactant mass transport within the films become limiting. Under synthetic electrolysis conditions, optimal turnover numbers are found for very thin films, and the combined best yields and current efficiencies are obtained with the relatively small coverage of about 2 nmol cm-2. In the synthetic case, the experi-

Langmuir, Vol. 17, No. 11, 2001 3453

mental time scale may favor increased importance for mass and electron transport in the films. Acknowledgment. The authors are grateful to the National Science Foundation for financial support (Grant Nos. CTS-9632391 and 9982854). LA000416Q