Comparison of the Behavior of Several Cobalt Porphyrins as

Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. .... Chan, R. J. H.; Su, Y. O.; Kuwana, T. Inorg. ... Bettelheim, A.; Chan, R. J. H.; Kuwan...
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Langmuir 1998, 14, 4315-4321

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Comparison of the Behavior of Several Cobalt Porphyrins as Electrocatalysts for the Reduction of O2 at Graphite Electrodes Euihwan Song, Chunnian Shi, and Fred C. Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena California 91125 Received January 21, 1998. In Final Form: May 18, 1998 Four cobalt porphyrins were adsorbed on graphite electrodes and used to catalyze the electroreduction of O2. The two porphyrins without substituent groups in the meso positions of the porphyrin ring operated at the most positive potentials and catalyzed the reduction of O2 to both H2O2 and H2O, but the H2O did not result from significant reduction of H2O2. The porphyrins containing meso substituents catalyzed only the reduction of O2 to H2O2. The catalysts that accomplish the four-electron reduction of O2 are argued to consist of dimeric (or higher oligomeric) forms of the adsorbed porphyrins. The present results and those of two recent related studies1,2 indicate that the presence of only hydrogen or small alkyl groups in the meso positions of porphyrin rings facilitates the spontaneous formation of van der Waals dimers with greater catalytic activity for the reduction of O2 by four electrons. Such cobalt porphyrins were also found to be unusually active catalysts for the electro-oxidation of H2O2.

In recent studies designed to establish how the identity and sites of attachment of substituent groups on porphyrin rings affect the electrocatalytic activity toward the reduction of O2 of cobalt porphyrins, we examined the unsubstituted cobalt porphine1 and the corresponding 5,10,15,20tetramethyl-substituted derivative.2 Both of these relatively simple porphyrins, when adsorbed on graphite electrodes, exhibited significant catalytic activity toward the four-electron reduction of O2 to H2O. This result was surprising because most mononuclear cobalt porphyrins serve as catalysts only for the two-electron reduction of O2 to H2O2. In an effort to obtain a better understanding of the reactivity patterns displayed by cobalt porphyrin catalysts we decided to compare the behavior of four, more familiar cobalt porphyrins whose electrocatalytic activities have been reported in various, scattered, previous studies.3-8 The idea was to determine if a correlation could be established between the substituents on the porphine ring and the catalytic activities of the corresponding cobalt porphyrins. The points of primary interest were the stoichiometry of the catalytic reduction of O2, the potential where the catalytic reduction proceeded compared with the formal potential of the Co(III/II) couple of the porphyrin adsorbed on graphite, and the extent to which possible oxidation of the porphyrin ring had an effect on the behavior of the catalysts. Such a comparison of a structurally related set of cobalt porphyrins has not previously been reported from a single laboratory under a constant set of experimental conditions. * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: (626) 395-6000. Fax: (626) 4050454. (1) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36, 4294. (2) Shi, C.; Anson, F. C. Inorg. Chem. 1998, 37, 1037. (3) Buttry, D. A.; Anson, F. C. J. Am. Chem. Soc. 1984, 106, 59. (4) Jiang, R.; Dong, S. J. Phys. Chem. 1990, 94, 7471. (5) Chan, R. J. H.; Su, Y. O.; Kuwana, T. Inorg. Chem. 1985, 24, 3777. (6) Bettelheim, A.; Chan, R. J. H.; Kuwana, T. J. Electroanal. Chem. 1979, 99, 391. (7) Ni, C. L.; Anson, F. C. Inorg. Chem. 1985, 24, 4754. (8) , Durand, R. R., Jr., Ph.D. Thesis, California Institute of Technology, Pasadena, CA, 1984.

The structures of the four cobalt porphyrins examined in this study and their abbreviations are shown in Figure 1A. The results obtained with this set of common cobalt porphyrins are also compared with those obtained recently with the two, less familiar cobalt porphyrins shown in Figure 1B.1,2 All of these cobalt porphyrins are insoluble in water and have been shown to be retained in the adsorbed state on graphite electrodes for at least several hours with the electrodes immersed in aqueous solutions. The only exception is CoTPyP which is soluble in aqueous acid. As a result, this porphyrin is gradually desorbed from graphite electrodes immersed in acidic solutions. Experimental Section Materials. Co(III) protoporphyrin IX chloride, 5,10,15,20tetraphenyl porphine, 2,3,7,8,12,13,17,18-octaethylporphine, and 5,10,15,20-tetrapyridylporphine were commercial materials obtained from Porphyrin Products, Inc. (Logan, UT). Cobalt(II) was inserted into the porphine rings by a standard procedure.9 The isolated Co(II) porphyrins were purified by column chromatography on neutral alumina (Brockman I). CHCl3 (Aldrich) was passed through a column of basic alumina (Brockman I activated Basic; Aldrich) just before use to remove traces of an oxidizing impurity. Other chemicals were reagent grade and were used as received. Laboratory distilled water was further purified by passage through a purification train (MilliQ Plus). Pyrolytic graphite rods with the edges of the graphitic planes exposed (Union Carbide Co.) were mounted to stainless steel shafts with heat-shrinkable polyolefin tubing (Alpha Wire Co.). The exposed graphite disk had an area of 0.32 cm2. Apparatus and Procedures. Conventional, commercially available electrochemical instrumentation and two-compartment cells were employed. Just before it was coated with porphyrins, the edge plane graphite disk electrode was polished with moist, 600 grit SiC paper, sonicated in purified water, washed with acetone, and dried. Except for CoTPP, porphyrins were adsorbed on the electrode surface by means of a dip-coating procedure: The freshly polished electrode was dipped for 60 s in a 0.1 mM solution of the porphyrin in CHCl3, removed, washed immediately with pure CHCl3, and dried in air. This procedure, which produced coatings that exhibited the best catalytic activity, was (9) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem. 1970, 32, 2443.

S0743-7463(98)00084-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/24/1998

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Figure 1. (A) Structures of the four cobalt porphyrins examined in this study and the abbreviations used to represent them. (B) Structures of two related cobalt porphyrins examined in recent studies. ineffective with CoTPP. For this porphyrin 20-30 µL aliquots of 0.1 mM solutions in CHCl3 were transferred to the electrode surface and the solvent was allowed to evaporate in air. Potentials were measured and are reported with respect to a saturated calomel reference electrode. Experiments were conducted at ambient laboratory temperatures, 22 ( 2 °C. Levich currents for the reductions of O2 were calculated using the following parameters: [O2] ) 0.28 mM in air-saturated solutions. Diffusion coefficient of O2 ) 1.7 × 10-5 cm2 s-1, and kinematic viscosity of aqueous solutions ) 0.01 cm2 s-1.

Results Cyclic Voltammetric Responses of the Four Cobalt Porphyrins in the Absence of O2. The four porphyrins of Figure 1A all adsorb strongly and irreversibly on edge plane pyrolytic graphite (EPG) electrodes. However, the voltammetric responses obtained from the Co(III/II) couples of the adsorbed porphyrins vary widely among the set. Shown in Figure 2 are voltammograms for the four adsorbed porphyrins recorded in the absence of O2. Except for CoTPP, each porphyrin coating was prepared by dipping the freshly polished EPG electrode for 60 s in a 0.1 mM solution of the porphyrin in CHCl3, followed by washing with pure CHCl3 and transferring to aqueous 1 M HClO4 to record the voltammograms. Coatings of CoTPP were prepared by transferring 30 µL aliquots of a 0.1 mM solution in CHCl3 to the electrode and allowing the solvent to evaporate. Attempts to use the dip-coating procedure with CoTPP produced coatings which exhibited almost no electrochemical responses. It was necessary to prepare coatings that contained considerably more porphyrin than that obtained by dip coating to observe responses from CoTPP. The reasons for this atypical behavior of CoTPP are not clear. The voltammetric scans in Figure 2 were not extended to potentials more positive than 0.6 V in order to avoid oxidation of the porphyrin ring that occurs at more positive potentials, especially for CoOEP and CoPPIX.10

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Figure 2. Cyclic voltammograms for the porphyrins of Figure 1A adsorbed on freshly polished EPG electrodes by dip coating from 0.1 mM solutions of the porphyrins in CHCl3 (except for CoTPP for which the coating was prepared by transferring 30 µL of a 0.1 mM solution in CHCl3 to the electrode surface and allowing the solvent to evaporate). The dashed curves are the responses obtained when the coating process was repeated with no porphyrin in the coating solution. Supporting electrolyte: 1 M HClO4 under argon. Scan rate: 50 mV s-1.

Adsorbed CoOEP exhibits a clear, reversible response for the Co(III/II) couple at 0.41 V (Figure 2). The corresponding response from CoPPIX is less prominent, but it can be discerned at 0.45 V (Figure 2). The response from adsorbed CoTPP, which could be obtained only by depositing multiple layers of the porphyrin on the electrode, has a magnitude that is much smaller than expected for the quantity of CoTPP present on the surface. It is unclear why most of the CoTPP is apparently not electroactive. The adsorbed CoTPyP exhibits weak responses at several potentials. The response near 0.05 V has been ascribed to a form of the adsorbed porphyrin that interacts strongly with functional groups on the electrode surface.6 The more prominent response near 0.6 V is believed to arise from oxidation and reduction of the porphyrin ring because the formal potential of the Co(III/II) couple for CoTPyP dissolved in 0.05 M H2SO4 has been reported to be near 0.18 V. The responses from the Co(III/II) couple of the major portion of the adsorbed CoTPyP are too indistinct to be identified with certainty. One reason for the relatively weak voltammetric responses from the Co(III/II) couples in most cobalt porphyrins is the requirement for axial ligation of the Co(III) oxidation state. In the absence of good axial ligands the interconversion between the Co(II) and Co(III) oxidation states tends to be sluggish.10 In an attempt to verify that the proposed assignments of the voltammetric responses in Figure 2 to the Co(III/II) couple were correct, we examined the effect of the addition of pyrazine to the (10) Kadish, K. M. Progress in Inorganic Chemistry; Cotton, F. A., Ed.; John Wiley: New York, 1986; Vol. 34.

Cobalt Porphyrins as Electrocatalysts

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Figure 4. Variation in the quantity of CoOEP adsorbed on an EPG electrode with the concentration of CoOEP in the CHCl3 dip-coating solution. The electrode was freshly polished before being dipped for 60 s in each coating solution. The values of ΓCoOEP were evaluated from the areas under cathodic peaks such as the one in Figure 2. Figure 3. Effect of the presence of pyrazine on the voltammetry of the four cobalt porphyrins adsorbed on EPG electrodes. Supporting electrolyte: solid curves, 1 M HClO4; dashed curves, 1 M HClO4 + 2 mM pyrazine. Other conditions as in Figure 2.

supporting electrolyte on the responses (Figure 3). Pyrazine has a reasonably high affinity for the cobalt center of cobalt porphyrins,2 and the weak Bronsted acidity of the ligand allows its coordination to proceed even in acidic solutions. As is shown in Figure 3, the reversible responses assigned to the Co(III/II) couples of the adsorbed porphyrin are shifted to more negative potentials in the presence of pyrazine. The sensitivity of the voltammetric features to the addition of pyrazine provides strong support for the assignment of the responses to the Co(III/II) couples. The direction of the potential shift indicates that the pyrazine ligand is bound more strongly to the oxidized, Co(III) forms of the porphyrins. The diminished responses from the adsorbed porphyrins, especially CoTPP, in the presence of pyrazine suggest that coordination of the ligand may lead to desorption of the porphyrins from the surface. Having verified that the prominent voltammetric response exhibited by CoOEP in Figure 2 corresponds to the Co(III/II) couple, we measured the area encompassed by the cathodic peak to estimate the quantities of the porphyrin on the electrode surface. The dependence of the resulting values on the concentration of CoOEP in the dip-coating solution is shown in Figure 4. The electrode surface becomes essentially saturated at CoOEP concentrations above 0.1 mM. Similar behavior was also observed with CoPPIX. Susceptibility to Oxidation of Cobalt Porphyrins Lacking Substituents in the meso Positions of the Ring. That metalloporphyrins having only hydrogen atoms in the meso positions of the porphyrin ring are more readily oxidized by chemical oxidations than are mesosubstituted porphyrins is well-known.11-13 The set of four porphyrins examined in this study fit the same behavioral (11) Bonnet, R.; Dimsdale, M. J. L. J. Chem. Soc., Perkin Trans. 1 1972, 2540.

pattern: CoOEP and CoPPIX, which lack meso substituents, are oxidized by exposure to H2O2 while CoTPP and CoTPyP are not. Evidence of the oxidation is the shift of the formal potentials of the Co(III/II) couple to more negative potentials as shown in Figure 5. The site of the oxidation is believed to be the porphyrin ring, and the resulting Co(III) oxyphlorin complex requires a more negative potential for its reduction. The oxidation can be carried out electrochemically as well as chemically. For CoOEP and CoPPIX the electrochemical oxidation begins to occur at potentials more positive than approximately 0.7 V. That is why the initial potentials for these porphyrins in Figure 5 were restricted to less positive values. Neither chemical or electrochemical oxidations of CoTPP or CoTPyP were observed under conditions where CoOEP and CoPPIX were readily oxidized (Figure 5). In recent studies of the cobalt porphyrins shown in Figure 1B, we observed that CoP is more readily oxidized both electrochemically and by H2O2 than is CoTMP.1,2 The oxidation of CoP, like that of CoOEP and CoPPIX, was irreversible; the original porphyrin was not regenerated by electrochemical reduction in the accessible potential range. CoTMP stands out as an unusual, contrasting case. Its oxidation is reversible; the original porphyrin can be regenerated by electroreduction at the same potential where the Co(III) center of the oxidized product is reduced.2 Electrocatalytic Activity of the Four Porphyrins toward the Reduction of O2. A primary objective of this study was to compare the catalytic behavior of the four porphyrins in Figure 1A toward the electroreduction of O2. We utilized a rotating EPG disk electrode to determine which, if any, of the porphyrins was able to achieve the four-electron reduction of O2 to H2O. (Experiments were also conducted with a commercial rotating (12) Clezy, P. S. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. II. (13) Balch, A. L.; Mazzanti, M.; St. Claire, T. N.; Olmstead, M. M. Inorg. Chem. 1995, 34, 2194.

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Figure 5. Effect of oxidation of adsorbed porphyrins on their voltammetric responses. Solid curves, before oxidation; dotted curves, after the adsorbed porphyrins were immersed in a 50 mM H2O2 solution for 30 s before the voltammograms were recorded. Other conditions as in Figure 2.

platinum ring-graphite disk electrode.14 Qualitatively similar results were obtained, but more reproducible data were obtained with homemade rotating EPG electrodes. We attribute this behavioral difference to our inability to polish reproducibly the graphite disk of the ring-disk electrode because of the need to avoid damage to the fragile platinum ring electrode and the fact that the dip-coating procedure could not be applied to the ring-disk electrode because of the contamination of the platinum ring that sometimes occurred. For these reasons, we relied on data obtained with the simple rotating EPG electrode in this study.) Shown in Figure 6 are sets of current-potential curves for the reduction of O2 at a rotating graphite disk electrode coated with one of the cobalt porphyrins from Figure 1A. As was reported recently for CoP and CoTMP,1,2 the porphyrins without meso substituents (CoOEP and CoPPIX) exhibit a two-step reduction of O2 while the two porphyrins with meso substituents catalyze the reduction of O2 in a single step. The catalytic reduction currents are notably larger at electrodes coated with CoOEP and CoPPIX than with CoTPP or CoTPyP coatings. The very small currents obtained with the CoTPyP coating probably reflect the instability of the adsorbed coating in the acidic supporting electrolyte in which CoTPyP is much more soluble than are the other three porphyrins. For this reason, we did not examine the CoTPyP coatings in greater detail. Levich plots15 for the plateau currents in Figure 6A are shown in Figure 6B. For CoOEP and CoPPIX, plots corresponding to both steps of the reduction are included. (14) Pine Instrument Co., Grove City, PA. (15) Levich, V. G. Physicochemical Hydrodynamics; Prentice Hall: Englewood Cliffs, NJ, 1962.

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Figure 6. (A) Reduction of O2 at rotating EPG disk electrodes coated with one of the four porphyrins of Figure 1A. Supporting electrolyte: 1 M HClO4 saturated with air. The electrode rotation rates from bottom to top in each set of curves were 100, 400, 900, 1600, 2500, and 3600 rpm. Scan rate: 5 mV s-1. The electrode was coated as in Figure 2. (B) Levich plots of the plateau currents from (A) (except for CoTPyP). (O) First current plateau for CoOEP and CoPPIX; (b) final current plateau. (C) Koutecky-Levich plots of the data from (B). The broken lines in (B) and (C) are the calculated responses expected for the diffusion-convection limited reduction of O2 by two or four electrons.

The Levich plots are nonlinear, but (at most rotation rates) the measured currents on the second plateau for CoOEP and CoPPIX exceeded the values calculated for the diffusion-convection-limited two-electron reduction of O2. The slopes of lines obtained by linear regression of the data to Koutecky-Levich (K-L) plots,16 shown in Figure 6C, were used to estimate roughly the average number of electrons, nav, involved in the reduction reactions. For CoTPP the slope of the K-L plot corresponded to nav ) 2.0 ( 0.04, as expected for a porphyrin that catalyzes the reduction of O2 to H2O2 but not beyond. K-L plots were prepared for both the first and second steps of the reduction catalyzed by CoOEP (Figure 6C). The values of nav calculated from the slopes of the lines in Figure 6C were 2.8 ( 0.1 on both the first and the second current plateau. These values require that a portion of the O2 that reaches the electrode surface be reduced by four electrons to H2O on both current plateaus. For CoPPIX the currents on the first current plateau were too weakly dependent on the electrode rotation rate for a reliable K-L slope to be evaluated. The slope of the K-L plot for the second plateau corresponded to nav ) 2.6 ( 0.3, indicating that CoPPIX also catalyzes the fourelectron reduction of a portion of the O2 that reaches the electrode surface. Electroreduction of H2O2. Adsorbed porphyrins can catalyze the electroreduction of O2 by more than two electrons if they are also catalytically active toward the reduction of H2O2. We therefore tested the four porphyrins of Figure 1A in solutions containing H2O2 but no O2. Coatings of CoTPP and CoTPyP were essentially inert, (16) Koutecky, J.; Levich, V. G. Zh. Fiz. Khim. 1956, 32, 1565.

Cobalt Porphyrins as Electrocatalysts

Figure 7. (A) Current-potential curve recorded with a rotated EPG disk electrode coated with CoOEP in a 0.28 mM solution of H2O2. The dashed curve was recorded in pure supporting electrolyte: 1 M HClO4 deaerated with argon. Rotation rate: 100 rpm. Scan rate: 5 mV s-1. (B) Repeat of (A) after the solution was saturated with air. The dotted curve was recorded in the same solution with the uncoated electrode.

but both CoOEP and CoPPIX exhibited weak catalytic activity toward the reduction of H2O2. In addition, both of these porphyrins were very active as catalysts for the oxidation of H2O2 to O2. For example, shown in Figure 7A is a current potential curve with a CoOEP-coated rotated disk electrode in a 0.28 mM solution of H2O2. The cathodic current for the reduction of the H2O2 is quite small, much smaller than the reduction current obtained under the same conditions with O2 as the reactant (Figure 6A). The cathodic current in Figure 7A is so small that the value of nav that exceeded 2.0 (nav ) 2.8 ( 0.1) obtained from the slope of the K-L plot in Figure 6C (for the second reduction step) cannot be attributed to the co-reduction of O2 and H2O2. It follows that the component of the electrode process that carries the reduction of O2 beyond the H2O2 stage must occur without the release of H2O2 as an intermediate reaction product. Electro-oxidation of H2O2. The anodic plateau current in Figure 7A, which corresponds to the oxidation of H2O2, is much larger than the cathodic current resulting from the reduction of the H2O2. The magnitude of the anodic current is comparable to the cathodic current obtained for the reduction of O2 under the same conditions (Figure 6A). (Similar catalytic activity toward the oxidation of H2O2 was also exhibited by CoPPIX (not shown) but not by CoTPP or CoTPyP.) Neither the anodic nor the cathodic plateau current in Figure 7A is diffusionconvection controlled (nonlinear Levich plots result from both plateau currents), but the chemical step that limits the anodic current is clearly much more rapid than the chemical step that controls the cathodic current. The potential where the oxidation of H2O2 to O2 proceeds is not very different from that where the reverse process occurs. In Figure 7B is shown the composite wave obtained

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Figure 8. Reduction of O2 at a rotating EPG disk electrode coated with CoOEP. (A) Freshly coated electrode. (B) Repeat of (A) after the electrode was immersed for 60 s in 50 mM H2O2. Supporting electrolyte: 1 M HClO4 saturated with air. Rotation rate: 2500 rpm. Scan rate: 5 mV s-1.

at a CoOEP-coated electrode with a solution that contained equal concentrations of H2O2 and O2. The curve passes through the potential axis at 0.52 V with only a small inflection, indicating that the electrode process is proceeding nearly reversibly. Indeed, the open circuit potential of 0.52 V is very close to the equilibrium potential of 0.54 V calculated with the Nernst equation from the formal potential of the O2/H2O2 couple, Ef ) 0.54 V versus SCE (with 1 M as the standard state for O2), using [O2] ) 0.28 mM in air-saturated solutions, [H2O2] ) 0.28 mM, and [H+] ) 1 M. Thus, coating the EPG electrode with CoOEP converts the highly irreversible current-potential response obtained at the uncoated electrode (dotted curve in Figure 7B) into the nearly reversible response shown by the solid curve. Somewhat similar behavior has been reported previously with other porphyrins having unsubstituted meso positions, but the O2/H2O2 couple remained less reversible at coated electrodes in acidic supporting electrolytes.4,17 To the best of our knowledge, behavior such as that shown in Figure 7B has not been previously reported in acidic electrolytes. Effect of Oxidation of CoOEP on Its Electrocatalytic Behavior. As shown earlier, CoOEP and other porphyrins having empty meso positions exhibit a more negative formal potential for the Co(III/II) couple if they are oxidized before the voltammetric responses are recorded (Figure 4). The electrocatalytic behavior of the oxidized porphyrins is also altered. For example, in Figure 8 is shown the reduction of O2 at a rotating graphite disk electrode coated with CoOEP before and after the coated electrode was exposed to a 1 mM H2O2 solution for 5 min. The reduction continues to occur in two steps, but the first step is shifted to a more negative potential which is close to that for the Co(III/II) couple of the oxidized porphyrin (compare Figures 8 and 4). The same effect can be produced if the coated electrode is exposed to (17) Durand, R. R., Jr.; Anson, F. C. J. Electroanal. Chem. 1982, 134, 273.

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potentials more positive than approximately 0.8 V where a similar oxidation of the porphyrin occurs. The catalyzed reduction of O2 is shifted to somewhat (50-100 mV) more negative potentials, but neither the plateau currents nor the nav values are changed significantly. Of course, the reduction and oxidation of H2O2 can be examined only with oxidized catalysts, so the responses shown in Figure 7 were necessarily obtained with an oxidized coating of CoOEP. Discussion Overall Observations. The results obtained in this study combined with those in two recent related reports1,2 provide clear empirical evidence that the electrocatalytic behavior of cobalt porphyrins toward the reduction of O2 can be substantially different depending on whether the meso ring positions are occupied by aryl, pyridyl, or similar substituents. When the meso positions are occupied by bulky substituents, the adsorbed cobalt porphyrins catalyze the two-electron reduction of O2 to H2O2 but they are ineffective in catalyzing reactions that involve breakage of the O-O bond. By contrast, when the meso substituents are hydrogen or methyl, dimers and higher oligomers are believed to be formed spontaneously by the porphyrins adsorbed on graphite.1,2 Independent evidence for such van der Waals-driven association among unsubstituted cobalt (and other) porphyrins has been reported.18-20 The effect of such dimerization on the electrocatalytic properties of the adsorbed cobalt porphyrins has been argued to result from the interaction of both cobalt(II) centers in a dimer with the oxygen atoms in O2 to produce a transition state in which the bridging O2 molecule is activated toward accepting four electrons from the graphite electrode which causes the O-O bond to break. This mechanistic proposal is based on the analogous behavior of dimeric cobalt porphyrins in which the two porphyrin rings are held in a cofacial configuration by covalent bonds.21,22 The covalently linked, dimeric porphyrins are capable of directing the reduction of O2 almost exclusively along a four-electron reduction pathway.22 Dimeric species formed on electrode surfaces from monomeric precursors such as CoP, CoTMP, or CoOEP would not be expected to be the only form of the porphyrin present in the adsorbed coatings. Any monomeric porphyrins also present on the surface would catalyze the two-electron reduction of O2 to H2O2. As a result, the products of the reduction of O2 would consist of a mixture of H2O2 and H2O to produce values of nav between 2 and 4, which matches the behavior observed with CoOEP and CoPPIX in the present study and with CoTMP and CoP in our previous studies.1,2 We believe the contrasting behavior of cobalt porphyrins that catalyze the electroreduction of O2 exclusively by a twoelectron pathway and those that direct the reactions along a mixture of two- and four-electron pathways can be accounted for in general on the basis of this mechanistic proposal. However, some additional details of the behavior reported in this study remain to be explained. Origin of the Two-Step Catalytic Reduction of O2 by CoOEP and CoPPIX. We have suggested1,2 that spontaneous dimerization (or oligomerization) on graphite (18) Fleischer, E. G.; Webb, L. E. J. Chem. Phys. 1965, 43, 3100. (19) Sudhindra, B. S.; Fuhrhop, J. H. Int. J. Quantum Chem. 1981, 20, 747. (20) Jentzen, W.; Turowska-Tyrk, J.; Scheidt, W. R.; Schelnutt, J. A. Inorg. Chem. 1996, 35, 3559. (21) Collman, J. P.; Marrocco, M.; Denisevich, P.; Koval, C.; Anson, F. C. J. Electroanal. Chem. 1979, 101, 117. (22) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 1537.

Song et al.

surfaces of cobalt porphyrins with no ring substituents or with only small alkyl groups in the meso positions of the porphyrin ring could account for their ability to act as electrocatalysts for the four-electron reduction of O2. If CoOEP and CoPPIX, with unencumbered meso ring positions, were also able to form dimers on the electrode surface, the similarities in their behavior to those of CoP and CoTMP could be understood. The spontaneous dimerization of CoOEP and CoPPIX might be expected to be less extensive because of steric interactions among the substituents in the β-pyrrole positions of the ring which could account for their lower efficiencies as catalysts for the four-electron reduction of O2. This line of reasoning might lead one to attribute the two distinct steps in the current-potential curves for the reduction of O2 in Figure 6A to the reduction to H2O2 in the first step catalyzed by the monomeric porphyrin in the coating, followed by the reduction to H2O in the second step catalyzed by porphyrin dimers in the coating. However, this interpretation is inadequate for two reasons: (i) The K-L plots for the first step (Figure 6) have slopes that correspond to values of nav larger than 2 which is not possible unless some of the O2 is reduced beyond H2O2; (ii) The slopes of the K-L plots for the second reduction step correspond to nav values smaller than 4, indicating that the reduction of some of the O2 proceeds only to H2O2 even at potentials on the plateau of the second step. To account for the observed behavior, it appears necessary to invoke more than one form of a four-electron catalyst in the coating: The two (presumably dimeric) forms would differ in the potentials where their adducts with O2 were reducible to H2O. Parallel to the fourelectron reduction pathways associated with the oligomerized porphyrin, the reduction would also proceed along a two-electron pathway catalyzed by the monomeric form of porphyrin in the coating. The potential where the O2 adduct of the monomeric porphyrin is reduced is apparently close to that where at least one of the dimeric porphyrin-O2 adducts is reduced and the result is that both two- and four-electron reductions occur at the first step. At the second step, where additional four-electron pathways become available, higher plateau currents result but the two-electron pathway continues to contribute significantly to the overall reduction process. The nature of the different forms of adsorbed fourelectron catalysts that our results seem to require are not clear on the basis of the presently available data. One might imagine adsorption sites with differing properties that are reflected in the catalysts adsorbed on them. Alternatively, it is possible that porphyrin aggregates formed on the electrode surface could have a range of compositions/structures that yield porphyrin-O2 adducts with differing reduction potentials. Whatever their origin, the presence of differing forms of the adsorbed catalyst seems to be necessary to account for the two-step reduction of O2 evident in Figure 6A. Catalysis of the Oxidation of H2O2. The currentpotential response shown in Figure 7A is mechanistically revealing. There is no reason to doubt that coordination to the Co(II) center of the adsorbed CoOEP is the first step in the catalytic cycle for both the reduction and oxidation of the H2O2 substrate. However, the large difference in the magnitudes of the anodic and cathodic currents in Figure 7A shows that the coordinated H2O2 is activated much more toward oxidation than toward reduction. The oxidation of coordinated H2O2 can proceed by the transfer of electrons to the underlying electrode. The role of the cobalt center could be merely to couple the

Cobalt Porphyrins as Electrocatalysts

H2O2 to the electrode surface. That CoOEP is better at such coupling than, say, CoTPP (which does not catalyze the oxidation of H2O2) may be inferred from the large differences in the voltammetric responses exhibited by these two adsorbed porphyrins in Figure 2. After H2O2 coordinates to the adsorbed CoOEP in Figure 7A additional transformations are apparently required before the O-O bond can be reductively broken. It is tempting to speculate that interaction of both O atoms of the H2O2 with Co(II) centers is required to facilitate the reduction. The dimeric forms of the adsorbed CoOEP believed to be present in the coating (vide supra) might provide the structural feature needed to produce such a bridged transition state. The much larger catalytic currents for the reduction of O2 than of H2O2 might then be attributed to the availability of a more facile pathway to a bridged Co-O-O-Co transition state when the substrate is O2, which is unencumbered by attached protons that might inhibit the formation of a corresponding transition state when H2O2 is the substrate. The nearly reversible current-potential response shown in Figure 7B indicates that the adsorbed CoOEP serves as a platform on which H2O2 and O2 can sit and be readily interconverted by electron transfer to or from the electrode in a process that preserves the O-O bond. Comparison of CoOEP, CoPPIX, CoP and CoTMP as Catalysts for the Electroreduction of O2. A summary of the behavior of the six porphyrins in Figure 1 when they are used as electrocatalysts for the reduction of O2 is given in Table 1. Among all of the mononuclear cobalt porphyrins that have been investigated in the present and previous studies as catalysts for the electroreduction of O2, CoP stands out because of the unusually positive potential at which it operates and the high fraction of O2 molecules that is reduced by four electrons with fresh, unoxidized coatings.1 CoTMP operates at less positive potentials and yields somewhat more H2O2 on the first reduction plateau than does CoP.1,2 However, like CoP, CoTMP produces almost no H2O2 on the second reduction plateau and it has the advantage that, unlike CoP, the less active, oxidized porphyrin can be restored to its original activity by electrochemical reduction.2 Both CoP and CoTMP suffer from the lack of efficient, high-yield syntheses which makes them unattractively expensive. By contrast, CoOEP and CoPPIX are readily available either from commercial sources or by relatively straightforward syntheses. However, this appealing feature is off-set by the lower efficiencies of these catalysts for the direct reduction of O2 by four electrons and by their susceptibility to irreversible porphyrin ring oxidation

Langmuir, Vol. 14, No. 15, 1998 4321 Table 1. Behavioral Properties of Cobalt Porphyrins That Act as Electrocatalytsts for the Reduction of O2 porphyrina CoP CoTMP CoOEP CoPPIX CoTPP CoTPyP

E1/2b

navc

0.53/0.20 0.41/0.20 0.45/0.15 0.45/0.20 0.20 0.20

∼3.8/4.0e

3.3/4.0 f 2.8 ( 0.1/2.8 ( 0.1 2.6 ( 0.3/2.6 ( 0.3 2.0 ( 0.4 ∼2.0g

expensed high high low low low low

a Structures are given in Figure 1. b Half-wave potential(s) vs a SCE reference electrode for the reduction of O2 at a graphite disk electrode coated with the porphyrin and rotated at 100 rpm in 1 M HClO4 saturated with air. Where two values are listed, the current-potential curve contains two steps. c Average number of electrons consumed in the reduction of O2 as estimated from the slopes of K-L plots such as those in Figure 6C. Where two values are listed, the currents were measured on both the first and second plateaus in the current-potential curves. The uncertainties in the values correspond to the standard deviations in the linear regression of the data. d The costs in time and/or dollars required to obtain the porphyrin. e Based on data in ref 1 for the first scan with a freshly coated electrode. f Based on data in ref 2. g Based on data in refs 6 and 23.

which diminishes their catalytic activity (Figure 5). The results obtained thus far suggest that a catalyst with properties superior to all of the porphyrins in Table 1 might be found by further manipulation of small meso substituents in otherwise unsubstituted porphyrins. Conclusions The results of the present and two recent related studies1,2 have demonstrated that the catalytic behavior of cobalt porphyrins toward the electroreduction of O2 is strongly influenced by the nature of the substituents attached to the meso positions of the porphyrin ring. With unsubstituted or methyl-substituted meso positions, catalytic pathways leading to the four-electron reduction of O2 are available. The essential feature required to obtain the four-electron reduction is believed to be the presence of dimeric (or higher oligomeric) forms of the porphyrins in the coatings adsorbed on graphite electrodes. Some of the porphyrins that catalyze the (irreversible) reduction of O2 by four electrons are also active catalysts for the oxidation of H2O2 to O2 in a nearly reversible reaction. Acknowledgment. E.S. is grateful for a Fellowship provided by the Korea Science and Engineering Foundation. The work was also supported by the U. S. National Science Foundation. LA980084D