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J. Phys. Chem. 1996, 100, 11098-11100
Activity Analysis of Electrochemical Water Oxidation Catalyst Confined in a Coated-Polymer Membrane Masayuki Yagi,† Kosato Kinoshita,‡ and Masao Kaneko*,‡ Faculty of Science, Ibaraki UniVersity, Mito 310, Japan, and Faculty of Engineering, Saitama UniVersity, Urawa 338, Japan ReceiVed: February 8, 1996; In Final Form: April 23, 1996X
The activity of the electrochemical water oxidation catalyst based on Ru-red incorporated in an electrodecoated Nafion membrane was studied. The complex worked as an active catalyst, and an optimum concentration for the catalyst turnover number (TN) was exhibited. The TN increased with the concentration in low concentrations because of facilitated charge transfer between the catalysts, and the increase in the concentration brought about decreased TN due to bimolecular decomposition of the catalyst. An activity model for the electrochemical catalyst in a membrane was proposed based on intermolecular distance distribution, and the activity was analyzed in terms of a charge transfer distance (r0/nm) and a critical decomposition distance (rd/nm). Their values were obtained as 1.28 and 1.21 nm, respectively.
Introduction The activity of molecule-based catalysts immobilized in various heterogeneous matrices such as ion exchange regions,1-3 clays,4 intercalation compounds,5 inorganic particles,6 and polymer membranes7-10 has been extensively investigated in order to apply them to photochemical and electrochemical systems. For applications of a heterogeneous catalyst to electrochemical devices, charge transport among the moleculebased catalysts is important in addition to their intrinsic activity. To optimize an electrochemical catalysis in a heterogeneous matrix, it is of importance to derive an activity model involving the charge transport factor. In our previous papers,11,12 charge transfer between redox centers incorporated in a polymer membrane has been studied. The degree of the electrochemical reaction increased with the redox center concentration, which was ascribed to a facilitated charge hopping between the redox centers by the concentration increase. The relationship between the ratio of the electron transport and the redox center concentration was analyzed in terms of charge transfer distance based on an intermolecular distance distribution of the redox center. Water oxidation is one of the most important and fundamental catalyses in nature.13 It is important to study water oxidation catalysis for biological activity as well as for designing an artificial photosynthetic model. We have already studied7 the activity of a water oxidation catalyst based on a trinuclear ruthenium complex, Ru-red ([(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+) in a heterogeneous Nafion membrane using a CeIV oxidant. A bimolecular decomposition of the catalyst was found in high concentrations in a homogeneous aqueous solution. However, the bimolecular decomposition was remarkably suppressed by incorporating the catalyst into a Nafion membrane. It was also found that the catalyst is stabilized against decomposition when its concentration decreased. The catalytic activity was analyzed in terms of a critical decomposition distance based on an intermolecular distance distribution of the catalysts.7 This result of stabilizing the catalyst in a heterogeneous membrane has thrown light on the hitherto reported difficulty in establishing an active and stable artificial water †
Saitama University. Ibaraki University. X Abstract published in AdVance ACS Abstracts, June 1, 1996. ‡
S0022-3654(96)00402-9 CCC: $12.00
oxidation catalyst. It should be noted that the concentration conditions for stabilizing the catalyst (lower concentration is better) are in conflict with charge hopping between the redox centers (higher concentration is better). In the present paper, the activity of an electrochemical water oxidation catalyst based on Ru-red incorporated in an electrodecoated Nafion membrane was studied. An optimum concentration for its activity was found, and the activity was for the first time analyzed in terms of both critical bimolecular decomposition distance between the catalysts and charge transfer distance between them based on its intermolecular distance distribution. Experimental Section Material. Ru-red of commercially available purest grade was purchased from Wako Pure Chemical Industries Ltd. and used as received. Nafion 117 solution (5 wt % alcoholic solution) was purchased from Aldrich Chemical Co. Inc. and diluted to 2.5 wt % with methanol before use. Preparation of a Nafion Membrane Incorporating RuRed. A Nafion membrane (thickness ca. 3 µm) was first prepared by casting 30 µL of a 2.5 wt % Nafion solution onto a platinum plate electrode (1 × 1 cm2). Before the membrane was air-dried completely, it was soaked in pure water for ca. 30 min to enhance the adsorptivity of the complex. The Nafion membrane coated on a Pt electrode was immersed in a 1.0 × 10-4 M Ru-red aqueous solution (5 mL) to adsorb the complex. The amount of the complex in the membrane was estimated from the visible absorption spectral change of the aqueous solution before and after the adsorption. The complex concentration in the membrane was obtained from the amount of the complex in the membrane and the membrane volume. Measurements. A BAS CV-27 cyclic voltammograph and a Rikadenki RW-21 recorder were used for electrochemical experiments. A conventional single-compartment cell was equipped with a modified working Ag/AgCl reference and platinum wire counter electrodes. A supporting electrolyte solution (pH 5.4) of 0.1 M potassium nitrate was deaerated by bubbling argon gas for 1 h. The dioxygen (O2) evolved in a potentiostatic electrolysis was analyzed by a Hitachi 163 gas chromatograph equipped with a 5 Å molecular sieve column using argon carrier gas. The amount of the O2 evolved was © 1996 American Chemical Society
Water Oxidation Catalyst
Figure 1. Relationship between the amount of O2 evolved and applied potential in a potentiostatic electrolysis for 1 h using (a) a Pt electrode coated with Ru-red/Nafion membrane and (b) a Nafion-coated Pt electrode dipped in 0.1 M KNO3 (pH 5.5).
obtained by subtracting the amount of the O2 detected for a blank experiment without electrolysis.
J. Phys. Chem., Vol. 100, No. 26, 1996 11099
Figure 2. Dependence of turnover number (TN) of the catalyst for O2 evolution on the catalyst concentration in a potentiostatic electrolysis at 1.4 V (Ag/AgCl) for 1 h. The solid line is the calculated curve based on eq 5.
Results and Discussion Ru-red (charged with 6+) was incorporated electrostatically in a Nafion membrane by cation exchange. The complex is supposed to be present in hydrophilic regions formed by the sulfonate groups. The maximum complex concentration in the membrane was 0.23 M. The concentration of the sulfonate group in the membrane was estimated by elemental analysis as 1.4 M. From the ratio of these concentrations (Ru-red/sulfonate group ) 1/6.1), the charges are almost neutralized between cationic Ru-red (6+) and the anionic sulfonate groups (1-) under the maximum concentration conditions. In the present Nafion membrane, counteranions (Cl-) for the complex do not exist. The activity of the electrochemical water oxidation catalyst based on Ru-red incorporated in the membrane is shown in Figure 1 as a function of the applied potentials. The amount of O2 evolved was much higher in the presence of the catalyst than that without it in the whole potential range, showing that at least two-thirds of the O2 is evolved Via the catalyst. The potential to evolve O2 decreased significantly by incorporating the Ru-red. The plot of the catalyst turnover number (TN) for the O2 evolution vs the catalyst concentration in the membrane is shown in Figure 2. Interestingly, an optimum concentration for TN was exhibited around 0.1 M. The TN increased with the concentration in low concentrations, which is ascribed to a facilitated charge transfer between the catalysts by the concentration increase. Charge transfer between redox centers is important to provide charges to the catalyst. We have reported11 that the degree of electrochemical reaction of incorporated redox centers increased with the concentration in a Ru(bpy)32+/Nafion membrane system. The TN in Figure 2 decreased in concentrations over 0.1 M. However, the amount of the charge passed increased monotonically with the concentration as shown in Figure 3, indicating that charges are transported to the catalyst. It has been reported in our earlier paper7 that dinitrogen (N2), which results from oxidation of the ammine ligands of Ru-red, is evolved by a bimolecular decomposition in a Ru-red aqueous solution with a CeIV oxidant. To investigate the bimolecular decomposition of Ru-red, potentiostatic electrolysis was carried out with a high Ru-red concentration, and the gas evolution data are shown in Table 1. The N2 evolution shows that decomposition takes place also in the membrane. The decreas-
Figure 3. Dependence of the amount of charge passed on the catalyst concentration in a potentiostatic electrolysis at 1.4 V (Ag/AgCl) for 1 h.
TABLE 1: Data of N2 and O2 Evolution after Potentiostatic Electrolysis for 1 h at 1.4 V (vs Ag/AgCl) in 0.1 M KNO3 (pH 5.3)a system
N2 (µmol)
O2 (µmol)
Pt/Nf Pt/Nf[Ru-red]b
0 1.1
1.3 1.9
a Surface area of the electrode is ca. 30 cm2. b Complex concentration in the membrane is 0.23 M.
ing TN in Figure 2 at high concentrations is therefore ascribable to bimolecular decomposition. It is most probable that the charge transfer between the catalysts as well as their bimolecular decomposition determines the catalyst activity. We have analyzed the electrochemical catalyst activity in the membrane as follows. Both the charge transfer and the bimolecular decomposition should depend on the intermolecular distance, since the catalyst is immobilized in the membrane. Intermolecular distance distribution rather than average intermolecular distance has been taken into account to analyze the activity. In a random dispersion, the distance distribution between the nearest-neighbor molecules is represented by eq 1:7,11,12,14,15
P(r))4πr2NARc × 10-24exp[-4π(r3- s3)NARc × 10-24/3 ] (1) where P(r)/nm-1, NA/mol-1, c/mol dm-3, and r/nm are the
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Yagi et al.
TN ) kO2[exp{-4π(rd3- s3)RcNA × 10-24/3} exp{-4π(r03- s3)RcNA × 10-24/3}] (5)
Figure 4. Intermolecular distance distribution between the nearestneighbor molecules for various concentrations. s is the contact distance between the catalysts, r0 is the charge transfer distance, and rd is the critical decomposition distance between the catalysts.
probability density, Avogadro’s number, apparent average complex concentration in a membrane, and distance between the nearest-neighbor molecules, respectively. The parameter s/nm is the contact distance between the catalysts. For the s value, the diameter (1.06 nm)7 of a sphere, which has the same volume (0.62 nm3) as the Ru-red molecule (approximated as a cylinder with 0.75 nm diameter and 1.40 nm length), was adopted. In the present system where the complex is adsorbed from its aqueous solution into a preformed membrane, the complex is present only in a hydrophilic region to result in localization of the catalyst. The degree (R) of the localization in the present Nafion membrane was estimated as 5.1 in our earlier work.12 The preexponential factor of eq 1, 4πr2NARc × 10-24, indicates the probability density of finding the complex on the surface of a sphere with r nm radius. The exponential term, exp[-4π(r3 - s3)NARc × 10-24/3], expresses the probability density that no complex center exists inside the same sphere. According to eq 1, the probability distribution curves as a function of the nearest intermolecular distance are depicted in Figure 4. We assumed that the charges on molecules can be transferred only by hopping between the molecules present within a charge transfer distance (r0/nm) and also that bimolecular decomposition takes place between molecules present within a critical decomposition distance (rd/nm). The fraction RCT of the catalyst that can accept charges and the fraction Rdec of the catalyst that undergoes a bimolecular decomposition are expressed by the following equations.
RCT ) ∫s P(r) dr ) 1 - exp(-4π(r03- s3)RcNA × 10-24/3) (2) r0
Rdec ) ∫s P(r) dr ) rd
1 - exp(-4π(rd3- s3)RcNA × 10-24/3) (3) The fraction of the effective catalyst is given by (RCT - Rdec), and the TN, which should be proportional to (RCT - Rdec), is therefore given by eq 4:
TN ) kO2(RCT - Rdec) Equation 5 is derived from eqs 2-4.
(4)
where kO2/h-1 is a constant to express the intrinsic activity (TN) of the catalyst. The theoretical model (eq 5) for TN involving kO2, rd, and r0 was applied to the data of Figure 2 using a nonlinear least-squares method, and the best fitting was obtained when kO2 ) 54.8 h-1, r0 ) 1.28 nm, and rd ) 1.21 nm. The r0 value (1.28 nm) is reasonable considering the reported electron transfer distance of various biological16 and our synthetic14 systems. The rd (1.21 nm) is almost the same as the critical decomposition distance (1.23 nm)7 in the chemical water oxidation system using a CeIV oxidant and is also reasonable considering the molecular size (cylinder with 0.75 nm diameter and 1.40 nm length) of the catalyst. The critical decomposition distance rd is close to the r0 value, showing that a very delicate condition should be satisfied as to the intermolecular distance between the catalysts in order to establish an efficient and stable artificial photosynthetic oxygen-evolving center model. In conclusion, the activity of an electrochemical water oxidation catalyst incorporated in a membrane was analyzed in terms of charge transfer distance and bimolecular decomposition distance between the catalysts. The proposed activity model equation (5) turned out to be useful for the analysis. The present result reveals that the design of an artificial water oxidation catalysis system is a difficult problem, as long as we stick to a conventional homogeneous or heterogeneous catalyst, compared with the photosynthetic oxygen-evolving center, which is isolated in a molecular level from each other and located in a charge transfer channel in photosystem II. A design of longrange charge transfer between the catalysts can be one of the important approaches to establish an active and stable water oxidation catalysis system. References and Notes (1) Thornton, A. T.; Laurence, G. S. J. Chem. Soc., Chem. Commun. 1987, 408. (2) Slamo-Schwok, A.; Feitelson, Y.; Rabani, J. J. Phys. Chem. 1981, 85, 2222. (3) Kurimura, Y.; Nagashima, M.; Takato, K.; Tsuchida, E.; Kaneko, M.; Yamada, A. J. Phys. Chem. 1982, 86, 2432. (4) Ramaraj, R.; Kira, A.; Kaneko, M. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1539. (5) Makoto, O.; Kazuyuki, K. J. Chem. ReV. 1995, 95, 399. (6) Yagi, M.; Ogino, I.; Miura, A.; Kurimura, Y.; Kaneko, M. Chem. Lett. 1995, 863. (7) Yagi, M.; Tokita, S.; Nagoshi, K.; Ogino, I.; Kaneko, M. J. Chem. Soc., Faraday Trans., in press. (8) Butty, D. A.; Anson, F. C. J. Am. Chem. Soc. 1984, 106, 59. (9) Sharp, M.; Montgomery, D. D.; Anson, F. C. J. Electroanal. Chem. 1985, 194, 247. (10) Compton, R. G.; Day, M. J.; Ledwith, A.; Abdour, I. I. A. J. Chem. Soc., Chem. Commun. 1986, 328. (11) Yagi, M.; Nagai, K.; Onikubo, T.; Kaneko, M. J. Electroanal. Chem. 1995, 383, 61. (12) Yagi, M.; Nagai, K.; Kira, A.; Kaneko, M. J. Electroanal. Chem. 1995, 394, 169. (13) Kaneko, M. Macromolecular Complexes: Dynamic Interaction and Electronic Processes; Tsuchida, E., Ed.; VCH Publishers, Inc.: New York, 1991; pp 353-377. (14) Nagai, K.; Tsukamoto, J.; Takamiya, N.; Kaneko, M. J. Phys. Chem. 1995, 99, 6648. (15) Nagai, K.; Takamiya, N.; Kaneko, M. J. Photochem. Photobiol. A 1994, 84, 271. (16) McLendon, G. Acc. Chem. Res. 1988, 21, 160-167.
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