J. Phys. Chem. B 1997, 101, 3957-3960
3957
Enhancing Effect of an Amino Acid Residue Model for the Electrochemical Water Oxidation Catalyst Confined in a Polymer Membrane Masayuki Yagi,† Kosato Kinoshita, and Masao Kaneko* Faculty of Science, Ibaraki UniVersity, 2-1-1 Bunkyo, Mito 310, Japan ReceiVed: NoVember 18, 1996; In Final Form: March 12, 1997X
The activity of an electrocatalytic oxygen evolving center (OEC) molecule based on the trinuclear Ru complex, Ru-red ([(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+) codispersed with an amino acid residue model compound in a Nafion membrane was studied. The activity of the OEC molecule increased remarkably in the presence of p-cresol (p-Cre), a Tyr model compound. The activity was analyzed based on the intermolecular distance distribution in terms of charge transfer distances between the OEC molecules mediated by p-Cre, their critical decomposition distance, and the intrinsic activity. The charge transfer distance increased remarkably from 1.28 to 2.25 nm by the presence of p-Cre, suggesting that p-Cre works as a mediator for the charge transfer.
Introduction
SCHEME 1: Electrocatalytic Water Oxidation
Water oxidation at the oxygen evolving center (OEC), which provides electrons to the whole photosynthetic system, is one of the most important fundamental catalyses in nature.1,2 This process is important not only in biological activity but also in a photochemical solar energy conversion system, attracting much attention as a renewable energy resource.2 It has been difficult to construct an active and stable artificial OEC model, whose reason has remained unsolved. In our finding, bimolecular decomposition of artificial OEC molecules (Ru-red; [(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]6+) takes place in a high oxidation state. This fact has led to successful construction of an efficient and stable artificial OEC model by isolating the OEC molecule in a solid matrix.3 However, in an electrochemical OEC model as shown in Scheme 1, a difficult problem arises concerning charge transport by hopping between the OEC molecules. There exists contradictory conditions as to the distance between the OEC molecules for the charge transport and bimolecular decomposition, the former requiring a short distance between the OEC molecules and the latter a long distance. There has been considerable interest in the long-range electron transfer in biological systems;4-7 aromatic amino acid residues have been suggested to serve as a pathway for a long-range electron transfer.5,6 In a photosynthetic system, it has been reported that Tyr residue mediates electron transfer between OEC and P680 in the photosystem II.7 For a long-range electron transfer in a synthetic system, we have reported mediation effects of 3-methylindole, a Trp residue model compound, on the photoinduced electron transfer from excited Ru(bpy)32+ to methyl viologen in a polymer membrane.8 A mediated longrange electron transfer gave us an important clue to overcome the above problem for designing an artificial OEC model. In the present paper, it was found that a Tyr residue model, p-cresol (p-Cre), enhances remarkably the electrocatalytic activity of Ru-red OEC molecules in a Nafion membrane. The enhancing effect of p-Cre on the activity will be analyzed and discussed based on the intermolecular distance distribution. † Present address: Faculty of Education, Niigata University, 8050 Igarashi-2, Niigata 950-21, Japan. X Abstract published in AdVance ACS Abstracts, May 1, 1997.
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Experimental Section Materials. Ru-red was purchased from Wako Pure Chemical Industries Ltd. and used as received. p-Cre and toluene (Tol) were purified before use by distillation under reduced pressure. Nafion 117 solution (5 wt % alcoholic solution) was purchased from Aldrich Chemical Co. Inc. All the materials are of commercially available purest grade. Preparation of Nafion Membrane Incorporating Ru-Red Codispersed with Amino Acid Model Compound. A 4.8 × 10-4 to 1.9 × 10-3 mol dm-3 (M) p-Cre methanol solution was prepared, and then 0.300 g of the solution was mixed with 0.300 g of a 5 wt % Nafion solution to prepare a mixture solution (the density is 0.83 g cm-3) containing 2.5 × 10-4 for 1.9 × 10-5 M p-Cre and 2.5 wt % Nafion. A Nafion membrane (thickness of 3 µm) was prepared by casting a 30 µL of this mixture solution onto a platinum plate electrode (1 cm2). The Nafion-coated electrode was immersed in 2 mL of 1.0 × 10-4 M aqueous Ru-red solution to adsorb the complex into the membrane. The amount of the complex incorporated into the membrane was estimated from the visible absorption spectral change of the aqueous solution before and after the adsorption of the complex. The concentrations of the complex and p-Cre in the membrane were calculated from the amount of each compound 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, a Ag/AgCl reference, and a platinum wire counter electrode. A supporting electrolyte solution (pH 5.4) of 0.1 M potassium nitrate was deaerated by bubbling argon gas for 1 h. The 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 at 40 mL min-1 flow rate. The amount of the evolved O2 © 1997 American Chemical Society
3958 J. Phys. Chem. B, Vol. 101, No. 20, 1997
Figure 1. Relationships between turnover number (TN) of the OEC molecule for O2 evolution and the catalyst concentration in the absence of amino acid model compound (a), in the presence of 5.0 × 10-2 M p-cresol (b), and with toluene (c). The solid lines are the calculated curves based on eq 8, and the dashed line is the calculated curve based on eq 5. The dash-dotted curve (I) is a simulated curve in the presence of p-Cre when r0 and rd are the same as those in the absence of p-Cre, and curve II is a simulated curve in the presence of p-Cre when r0 and kO2 are the same as those in the absence of p-Cre.
was obtained by subtracting the amount of the O2 detected for a blank experiment without electrolysis. Results and Discussion In the absence of p-Cre, the relationship between the turnover number (TN) of the OEC molecule for the O2 evolution and the OEC concentration in the membrane is shown in Figure 1a. At low concentrations (below 0.1 M), TN increased with the concentration, which is ascribed to facilitated charge transfer between the OEC molecules by hopping.9,10 A maximum TN was exhibited (around 0.1 M), and then it decreased with the concentration. This TN decrease is ascribed to a bimolecular decomposition of the OEC molecules to yield N2.3,9 Thus, TN depends on the charge transfer and bimolecular decomposition as well as on the intrinsic activity. This is supported by activity analysis described later. In the presence of p-Cre (Figure 1b), the TN increased remarkably at low complex concentrations compared with that without p-Cre. Instead of p-Cre, toluene, which is structurally similar to p-Cre, and a model compound of Phe were used (Figure 1c). Almost no change of TN was observed, showing that the hydroxyl group of p-Cre is important for the O2 evolution. The dependence of the TN on the p-Cre concentration is shown in Figure 2. The TN increased steeply at low p-Cre concentrations and then is almost saturated over 0.05 M, at which the molar ratio of Ru-red/p-Cre is 1. The electrocatalytic activity of the OEC molecule in the membrane is determined by three factors (Vide supra), i.e., the intrinsic activity of the OEC molecule, charge transfer between the OEC molecules, and their bimolecular decomposition. In order to investigate the intrinsic activity, the chemical O2 evolution in the Ru-red/Nafion membrane system was carried out using a CeIV oxidant.3 In this reaction system, contribution of charge transfer to the activity is excluded, since CeIV is also adsorbed into the Nafion membrane and oxidizes directly the catalyst. The O2 evolution data are summarized in Table 1. The amount of O2 evolved increased only slightly by p-Cre, which is almost negligible in comparison with that in the electrochemical system (compare parts a and b in Figure 1).
Yagi et al.
Figure 2. Relationship between turnover number (TN) of the OEC molecule for O2 evolution and the p-Cre concentration. The complex concentration is 5.0 × 10-2 M.
TABLE 1: O2 Evolution Data in OEC Molecule/p-Cre Polymer Membrane System after 30 min Using CeIV Oxidant OEC molecule/10-2 M
p-Cre 10-2 M
O2 evolution/10-6 mol
2.5
0 5.0 0 5.0
12.8 14.9 5.6 5.9
1.0
This result indicates that p-Cre does not bring about the increase of the intrinsic activity of the catalyst. In the low catalyst concentration region where p-Cre enhances remarkably the activity (Figure 1b), the degree of activity suppression by bimolecular decomposition is low (Figure 1a) so that the effect of p-Cre hindering bimolecular decomposition would be excluded. We have already reported that 3-methylindole, a model compound of tryptophan (Trp), increases the photoinduced electron transfer distance in a Ru(bpy)32+/methylviologen polymer membrane system.8 On the basis of these results, it is most probable that p-Cre acts as a mediator for charge transfer between the complexes in the present system. The activity of the present system was analyzed as follows. In a system where OEC molecules are immobilized in a membrane, the intermolecular distance between the OEC molecules is important for both charge transfer and bimolecular decomposition. Intermolecular distance distribution rather than an average distance should be considered. We have studied the charge transfer distance in Ru(bpy)32+/Nafion membrane system based on randomly dispersed statistics and obtained results consistent with the homogeneous random dispersion.10 Generally, distance distribution between the nearest-neighbor molecules is given by eq 1,3,8-10
P(r) ) 4πr2NARc × 10-24 exp[-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 probability density, Avogadro’s number, concentration in the membrane, and distance between the nearest-neighbor molecules, respectively. s/nm is the contact distance (1.06 nm)3 between molecules. R is the localization (5.1)11 of the molecule in the present Nafion membrane. According to the eq 1, probability distribution curves are depicted as a function of the nearest intermolecular distance in Figure 3. We reported that the charge on the OEC molecule is transferred by hopping only between the OEC molecules present within a charge transfer distance (r0/nm)9,10 and that bimolecular decomposition takes place among the catalysts present within a critical decomposition distance (rd/nm).3,9 The fraction (RCT) of the OEC molecule that can accept charge and
Amino Acid Residue Model
J. Phys. Chem. B, Vol. 101, No. 20, 1997 3959 TABLE 2: Parameters for the Best Fit in Figure 1 p-Cre
kO2/h-1
r0/nm
rd/nm
r0′/nm
absent present
54.8 60.1
1.28 1.29
1.21 1.24
2.25
probability density that the mediator center exists in the Vm. The term of ∫r0′r0 P(r) dr in eq 6 represents the probability density that the nearest-neighbor OEC molecule center is present in the distance between r0 and r0′. The term RCT′ is added to the fraction (RCT - Rdec) to give the TN expressed by eq 7.
TN ) kO2(RCT - Rdec + RCT′) Figure 3. Intermolecular distance distribution between the nearestneighbor molecules in the membrane for various concentrations. s is the contact distance of the catalyst, rd is the critical decomposition distance, r0 and r0′ are the charge transfer distance in the absence and in the presence of p-Cre, respectively.
the fraction (Rdec) of the OEC molecule that undergoes a bimolecular decomposition are expressed by the following eqs, respectively:
RCT )
∫sr
0
P(r) dr ) 1 - exp(-4π(r03 - s3)RcNA × 10-24/3) (2)
Rdec )
∫sr
d
P(r) dr ) 1 - exp(-4π(rd3 - s3)RcNA × 10-24/3) (3)
The fraction of the effective catalyst for the water oxidation is given by (RCT - Rdec). The TN that should be proportional to (RCT - Rdec) is therefore given by eq 4:
TN ) kO2(RCT - Rdec)
(4)
where kO2/h-1 is a constant to express an intrinsic OEC activity (TN). Then eq 5 is derived by eqs 2-4:
TN ) kO2[exp{-4π(rd3 - s3)RcNA × 10-24/3} - exp{-4π(r03 - s3)RcNA × 10-24/3}] (5) This eq 5 was applied to the experimental TN data in Figure 1a using nonlinear least-squares method, and the best fit was obtained when kO2 ) 54.8 h-1, rd ) 1.21 nm, and r0 ) 1.28 nm. In the presence of p-Cre, it is assumed that the charge on the OEC molecule is transferred Via p-Cre between OEC molecules present within a mediated charge transfer distance (r0′/nm) in addition to charge transfer between them present within r0. We also assumes a volume Vm (sphere with radius rm) in which p-cresol works as a mediator.8 When the p-cresol is present in the Vm, it can work to mediate charge transfer. The fraction (RCT′) of the OEC molecule that can accept charge Via p-Cre is represented by eq 6.
RCT′ ) {1 - exp(-4πrm3NAcm × 10-24/3}
∫rr ′ P(r) dr 0
0
) {1 - exp(-4πrm3NAcm × 10-24/3)} [exp{-4π(r03 - s3)RcNA × 10-24/3} exp{-4π(r0′3 - s3)RcNA × 10-24/3}] (6) where cm is the mediator concentration in the membrane. The term of {1 - exp(-4πrm3NAcm × 10-24)} expresses the
(7)
When the eqs 2, 3, and 6 are substituted in the eq 7, eq 8 is obtained:
TN ) kO2[exp{-4π(rd3 - s3)RcNA × 10-24/3} exp{-4π(r03 - s3)RcNA × 10-24/3} + {1 exp(-4πrm3NAcm × 10-24/3)}[exp{-4π(r03 - s3)RcNA × 10-24/3} - exp{-4π(r0′3 - s3)RcNA × 10-24/3}]] (8) In the eq 8, which expresses the dependence of TN on the mediator concentration, TN increases with cm and is saturated when cm is large enough. This eq 8 was applied to the experimental TN data in the presence of p-Cre (Figure 1b). The parameters in the best fit were summarized in Table 2 together with the data without p-Cre. The kO2, r0, and rd values do not appreciably change by the presence of p-Cre. The r0′ value is remarkably longer than r0 without p-Cre. This result would suggest strongly a charge mediation effect of p-Cre. In order to test other possible roles of p-Cre, a simulation curve based on eq 5 was depicted when r0 and rd are the same both in the absence and in the presence of p-Cre but with a different kO2 value so that the maximum TN of the simulation curve becomes the same as the data of Figure 1b. The simulated curve thus obtained is shown in Figure 1I when kO2 ) 250 h-1. Furthermore, when kO2 and r0 are the same in the absence and the presence of the p-Cre but with a different rd value, the simulation curve is as in Figure 1II when rd ) 1.01 nm. For both the cases (curves I and II), the simulation curves are entirely different from the experimental data, showing that the charge mediation mechanism of p-Cre is most reasonable. One effect of the Tyr residue in OEC of photosystem II, namely, a hydrogen-atom transfer function of a Tyr-Z residue involved in water oxidation has been proposed by C. Tommos and S. Styring et al.12 However, in the present OEC model system, enhancement of the water oxidation kinetics by p-Cre is not likely from the above results, and the most probable role of the Tyr model for charge transport could be concluded. Conclusion The electrocatalytic activity of the oxygen evolving center model (OEC) molecule based on Ru-red confined in an electrode-coated Nafion membrane increased by the presence of p-cresol (p-Cre), a Tyr model compound in the membrane. The turnover number (TN) of the OEC molecule for O2 evolution was analyzed based on the intermolecular distance distribution in terms of charge transfer distances between the OEC molecules, their critical decomposition distance, and the intrinsic activity. The charge transfer distance increased from 1.28 to 2.25 nm by the Tyr model, showing that p-Cre works as a mediator for the charge transport to increase the OEC activity.
3960 J. Phys. Chem. B, Vol. 101, No. 20, 1997 References and Notes (1) Ort, D.; Yocum, C. F. Oxygenic Photosynthesis; Kluwer: Dordrecht, 1996. (2) Kaneko, M. Macromolecular Complexes: Dynamic Interaction and Electronic Processes; Tsuchida, E., Ed.; VCH Publishers: New York, 1991; p 353. (3) Yagi, M.; Tokita, S.; Nagoshi, K.; Ogino, I.; Kaneko, M. J. Chem. Soc., Faraday Trans. 1996, 92, 2457. (4) McLendon, G. Acc. Chem. Res. 1988, 21, 160. (5) Farver, O.; Pecht, I. J. Am. Chem. Soc. 1992, 114, 5764. (6) Hurley, J. K.; Cheng, H.; Xia, B.; Markley, J. L.; Medina, M.; Moreno, C. G.; Tollin, G. J. Am. Chem. Soc. 1993, 115, 11698.
Yagi et al. (7) Barry, B. A. Photochem. Photobiol. 1993, 57, 179. (8) Nagai, K.; Tsukamoto, J.; Takamiya, N.; Kaneko, M. J. Phys. Chem. 1995, 99, 6648. (9) Yagi, M.; Kinoshita, K; Kaneko, M. J. Phys. Chem. 1996, 100, 11098. (10) Yagi, M.; Nagai, K.; Onikubo, T.; Kaneko, M. J. Electroanal. Chem. 1995, 383, 61. (11) Yagi, M.; Nagai, K.; Kira, A.; Kaneko, M. J. Electroanal. Chem. 1995, 394, 169. (12) Tommos, C.; Tang, X.-S.; Warncke, K.; Hoganson, C. W.; Styring, S.; McCracken, J.; Diner, B. A.; Babcock, G. T. J. Am. Chem. Soc. 1995, 117, 10325.
