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A novel electrochemical proton pump |
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Journal of the Chemical Society, Perkin Transactions 2,
Volume 1,
Issue 11,
1995,
Page 1949-1951
Michio Matsumura,
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摘要:
J. CHEM. SOC. PERKIN TRANS. 2 1995 1949 A novel electrochemical proton pump Michio Matsumura,* Masahiro Nohara and Teruhisa Ohno Research Center for Photoenergeticsof Organic Materials, Osaka University, Toyonaka, Osaka 560, Japan Protons are transported electrochemically from one aqueous phase to the other phase via an oil phase containing a hydrophobic quinone compound (vitamin K3)using a pair of solid polymer electrodes set at the oil/water boundaries. In recent years the mechanism of photosynthesis has evoked great interest with a view to constructing efficient solar energy conversion and storage systems. In this respect, production of hydrogen and reduction of carbon dioxide on semiconductor photocatalysts and assembled molecules is called artificial photosynthesis and has been studied very intensively.' How- ever, the function of the proton pump of photosynthesis2q3 has not attracted much attention in the field of artificial light energy conversion systems, although it is one of the key functions of photosynthesis.Pioneering work on the proton pump mimicking the bio- .~logical system has been reported by Anderson et ~1 using a liquid membrane containing vitamin K, and coenzyme Qlo. More recently, similar systems have been reported by Kobuke and Hamache 'using a liposomal membrane and by Imanishi f NQH2 and co-workers6 using a liquid membrane. The membranes Organic phase contained electron carriers and separate aqueous solutions (Dichloroethane) having electron donors in one phase and electron acceptors in Aqueousphase NQH+-l ' r the other phase.As the electrons were passed from electron donors to electron acceptors through the electron carriers in the membranes, protons were co-transported. Therefore, in these systems the difference in the electrochemical potentials of two aqueous phases gave the driving force for the proton transport. Eschegoyen and co-workers ' have demonstrated through Nafion hembrane Pt electrochemistry the cation transport through the oil phase uia Fig. 1 Two-compartment cell for cyclic voltammetry the redox reactions of quinone compounds dissolved in the oil phase. The reactions were driven by two sets of electrochemical Anion exchange membrane systems. Our proton pumping system is unique in that the redox reaction of quinone compounds takes place at the oil/water I Iinterfaces using a pair of electrodes placed at the interfaces.In this system no net chemical change occurs, except the transport of protons and the counter ions. As a whole, therefore, the electric energy is converted to the chemical potential as acid concentration. The electrochemical experiments were carried out using NafionB membrane electrodes loaded with a porous 0platinum layer on one side of the membrane. They were Lprepared according to the literature. These electrodes have been applied to fuel cells and organic electrochemistry. The membrane electrodes with an apparent surface area of 0.79 cm2 were fixed to two-compartment and three-compartment H+ =-(Dichloroethane cells, as shown in Figs.1 and 2. The platinum deposited side of 'NQthe Nafion@ membrane electrodes faced the oil phase. As the I oil, we used dichloroethane containing no supporting electrolytes. Potassium chloride was added to the aqueous phases as the supporting electrolyte at a concentration of 0.05 mol dm-3. A platinum counter electrode with an apparent Nafion membrane Pt Pt Nafion membrane surface area of 1.O cm2 was used in the two-compartment cell. Before constructing the proton pump system, we searched for Fig. 2 Three-compartment cell for proton transfer experiments the quinone compounds to bt used in the system. For this purpose, electrochemical properties of quinones at the oil/water Hydrophobic quinone compounds were dissolved in the oil boundary were studied using the two-compartment cell.phase and their electrochemical reactions on the Pt-loaded J. CHEM. soc. PERKIN TRANS. 2 1995 (a) t t1 1.0 -nfi 250 300 350 400 Wavelength (nm) 250 300 350 400 Wavelength (nm) Fig. 3 (a) Spectral change of NQ (1.0 x mol dm-3) in dichloroethane by electrochemical reduction at -1 .O V vs. Ag/AgClfor 3 h under anaerobic conditions in the two-compartment cell. (b) Spectral change of the above solution by oxidation at 0.0 V vs. Ag/AgCl for 3 h. Trends of spectral changes with time are shown by arrows. Nafionm electrode were followed by cyclic voltammetry. From the experimental results, we found that vitamin K,, or 2-methyl- 1,4-naphthoquinone (NQ), shows redox reactions on the electrode surface, the redox potential being at ca.-0.68 V us. Ag/AgCl. In order to determine the reduced form of NQ and the reversibility of the redox reaction in our system, NQ was electrochemically reduced at -1.O V us. Ag/AgCl in the two- compartment cell under argon atmosphere. As the reduction proceeded, new absorption bands peaking at 245 and 322 nm grew as shown in Fig. 3(a). The absorption bands agreed with those of 2-methyl- 1,4-naphthohydroquinone (NQH,). By applying 0.0 V us. Ag/AgCl (oxidative potential) to the Nafions membrane electrode after the electrochemical reduction of NQ, the absorption peaks due to NQH, decreased and the bands due to NQ reappeared as shown in Fig.3(b). These results confirm the two-electron and two-proton redox reactions between NQ and NQH, (Scheme 1) on the Nafion@ membrane electrode at the water/oil interface. Protons involved in the reactions are expected to be supplied from and released to the aqueous phase through the NafionB membrane, which separates the water/oil boundary and has good affinity to water. When other quinone compounds were studied, we sometimes 0 OH Scheme 1 observed degradation products of quinone compounds on the platinum layer of the NafionB membrane electrodes during electrolysis and poor reversibility of the reactions. On the basis of the above results, we carried out the proton transfer experiments using the redox couple of vitamin K, in the three-compartment cell.Ca. 2.5 x lo4 dm3 of dichloroethane solution containing NQ and NQH, (5.0 x mol drn-,, respectively) was added to the middle compartment. Each of the two compartments at both ends contained 8.0 x lo-, dm3 aqueous sodium chloride (5.0 x lo-, mol dmp3). The pH of the solution was adjusted to pH 3.0 by adding hydrochloric acid. A bias voltage of 0.4 V was applied between the two electrodes and maintained using a potentiostat during electrolysis. In the absence of NQ and NQH, in the organic phase, the current was less than 0.2 pA and no pH change was observed in the aqueous phases even after application of the voltage over several hours. No current flowed when the aqueous liquid junction was dis- connected. On the other hand, when NQ and NQH, were dissolved in the organic phase and the aqueous liquid junction was connected, ca.13 pA current kept flowing. After an electric charge of 100 mC had passed, the pH changes of both aqueous phases were measured. They were in good agreement with those expected from the electric charge passed in the system; the amount of protons transported between the aqueous phases corresponded to 98% of the electrons passed as current. These results prove that protons are carried through the NQ-NQH, redox system added to the organic phase. The neutrality of the solutions is maintained by the transfer of chloride ions through the anion exchange membrane in the cell. By means of the novel proton pump system, we have demonstrated the electrochemical concentration of acid using a suitable quinone compound and platinum loaded Nafionm membrane electrodes.It is important that no chemical changes occur in the system. A new solar energy conversion system mimicking the photosynthesis could be constructed by utili- zing the proton pump if it is operated photoelectrochemically or photocatalytically using semiconductor electrodes or semi- conductor particles in place of the two porous platinum layers. Acknowledgements The authors thank Dr M. Mizuhata of Osaka National Research Institute for his help in the fabrication of the Pt-loaded NafionB electrodes. Anionic ion exchange membranes (NEOCEPTA ACM) were kindly supplied by TOKUYAMA co. References 1 Photochemical Conversion and Storage of Solar Energy, eds.E. Pelizzetti and M. Schiavello, Kluwer, Amsterdam, 1991. 2 J. Darnell, H. Lodish and D. Boltimore, Molecular Cell Biology, Scientific American Books, 2nd edn., 1990. 3 J. Amesz, Biochem. Biophys. Acta, 1973,301, 35. 4 S. S. Anderson, I. G. Lyle and R. Paterson, Nature, 1976,259, 147. 5 Y. Kobuke and I. Hamachi, J. Chem. SOC., Chem. Commun., 1989, 1300. 6 E. Ozeki, S. Kimura and Y. Imanishi, J. Chem. SOC., Chem. Commun., 1988, 1353. 7 L. Echeverria, M. Delgado, V. J. Gatto, G. W. Coke1 and L. Eschegoyen, J. Am. Chem. Snc., 1986, 108, 6826; J. CHEM. SOC. PERKIN TRANS. 2 1995 L. E. Eschegoyen, H. K. Yoo, V. J. Gatto, G. W. Gokel and 11 M. Inaba, J. T. Hinatsu, Z. OgumiandZ. Takehara, J. Electrochem. L. Eschegoyen, J. Am. Chem. SOC.,1989,111,2440. SOC.,1993,140,706. 8 H. Takenaka, E. Torikai, Y. Kawami and N. Wakabayashi, Znt. Hydrogen Energy, 1982,7, 397. 9 Z. Poltarzewski, P. Staiti, V. Alderucci, W. Wieczorek and N. Giordano, J. Electrochem. SOC., 1992, 139,761. Paper 5/04602E 10 M. Inaba, Z. Ogumi and 2. Takehara, J. Electrochem. SOC., 1994, Received 13th July 1995 141,2579. Accepted 21st August 1995
ISSN:1472-779X
DOI:10.1039/P29950001949
出版商:RSC
年代:1995
数据来源: RSC
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