J. Phys. Chem. 1982, 86, 2681-2690
2881
CH3CC13,CHC13, et^.^^^^ In the absence in our experimenta of information on the possibility of 18Freaction with (allyl)& to form (allyl)3Sn18F,no further direct comparisons seem possible. A reasonable correlation exists between increasing bond strength of the C-F bond being formed28and increasing yield of R'BF in the order allyl < n-propyl < ethyl < methyl < vinyl. While the per-group substitution yield of C6H,'8F from (CH3)3Sn(C6H5)is not small, it is not greater than for CH28F as might be expected from bond strength alone. The relative yield of n-C3H;8F might also be expected to be equivalent to that of C2H5'8F,and is probably somewhat less, although no direct intermolecular competitions were feasible with (n-C3H7),Sn. Since the yields for the produds involving the two largest substituents both seem to be lower, there is some indication that steric access to the C-Sn bond is partially limited in those cases. The bondstrength correlation implies that the successful formation of a new C-F bond is dependent upon the depth of the potential well, with deeper wells leading to higher yields of R18F. We have no information from our experiments about the corresponding unsuccessful collisions for the molecules with shallower potential wells. The competing reaction processes could certainly include inelastic scattering without reaction, and formation of an Sn-F bond and an R3SnF product. Acknowledgment. This research has been supported by Department of Energy Contract DE-AT03-76ER-70126 and has been completed during the tenure of an Alexander von Humboldt award to F.S.R.
therefore given by k15 = (2.1 f 0.3) X cm3molecule-' s-l, or about 6 times faster than the direct substitution reaction to form CHi8F from (CHJ,Sn. The rate constant summation for all other processes with (CH2=CH)4Sn is therefore about 1X lo9 cm3molecule-' s-'. Although this represents very rapid overall reaction, the rate constant is reasonable in comparison with estimates of its component rates from similar molecules. For example, the rate constant for addition to CH2=CH2is 1.5 X lO-'O cm3 molecule-' s-l, or 6 X 10-lo cm3 molecule-' s-' for four ethylenic double bonds in (CH2=CH),Sn, together with cm3 molecule-' for an additional 1 X 10-lo-2 X abstraction from 12 ethylenic C-H positions. The substitution to form C 2 H Pand n-C3H7Fas shown in Table I occurs in each case in competition with other possible reactions with the parent tin molecules, of which hydrogen abstraction is certainly the most important. Under our experimental conditions, intermolecular competitive mittures are difficult with the higher-boiling tin compounds since accurately measured ratios of the competitor molecules are required. If the plausible assumption is made that the abstraction of H from all positions in alkyl substituents on Sn is equally likely, then the substitution of F to form RF proceeds with the relative probabilities of 1.0 (=8.5% X 12 H atoms) for CH3, 0.4(2% X 20) for C2H6,and 0.2 (0.8% X 28) for n-C3H,, respectively. On this same scale, fluorine substitution to form CH2=CHF is approximately 6, while formation of CH2FCH=CH2 is 0.1. The relative rate of the substitution reaction to form C6H$ can be judged from the intramolecular competition in (CH3)3Sn(C6H5)and is approximately the same (0.9 f 0.1) as for CH3F. The total range of reactivity toward formation of RF covers at least a factor of 60. The absence of a substantial yield for CH28FCH=CH2 can be contrasted with the postulated high yields for QCH2CH=CH2 from Q reactions with R3Sn(CH2CH= CHJ, the Q coming from alkyl halides such as CC14,
(25) M.Kosugi, K. Kurino, K. Takayama, and T. Migita, J. Organomet. Chem., 56, C11 (1973).
(26) J. Grignon and M. Pereyre, J.Organomet. Chem., 61,C33 (1973). (27) J. Grignon, C. Servens, and M. Pereyre, J. Organomet. Chem.,96, 225 (1975). (28) S.W.Benson, 'Thermochemical Kinetics", 2nd ed., Wiley, New York, 1976.
Photoelectrochemical Cells for the Production of Hydrogen and Hydrogen Peroxide via Photoredox Reactions M. Neumann-Spallart and K. Kalyanasundaram Institut de Chimle Physique, Emle Po!v-techniqueFMrale de Lausanne, CH-1015 Lausanne. Switzerland (Received: November 17, 1981; In Final Form: March 5, 1982)
The performance of various dye-sensitizedphotoredox systems leading to either water reduction or Br- oxidation has been examined in photoelectrochemicalcells. In the fist part, we consider visible-light-inducedHzevolution sensitized by dyes such as Ru(bpy),2+,water soluble zinc porphyrin (zinc tetrakis(4-N-methylpyridyl)porphyrin, ZnTMPyP), proflavin, and phenosafranine. Various factors such as the oxidative vs. reductive cycle, the role of relay and its concentration, etc., in relation to photoelectrochemical cells are examined. A quantitative analysis and a kinetic model for the current-potential curves in cells involving photoredox reactions (systems involving oxidative quenching) is presented. Later, the cell system C I R U ( ~ ~ ~ ) ~ ~ + , M V is~ examined + , ~ ~ I in ~ Hdetail, B~~C where photoinduced oxygen reduction to H20zi s coupled to bromide oxidation. For all the cell systems examined,
there is good agreement between the observed photocurrents and photopotentials with those deduced from the intersection of the individual i-e curves.
Introduction Formation of strong oxidants Or reductants in the electron-transfer quenching of excited states has been amply demonstrated in recent years for a wide variety of 0022-3654/82/2006-2681$01.25/0
inorganic metal complexes and organic dyes.14 One major goal of recent photochemical investigations is to use these (1) C. Creutz and N. Sutin, Pure Appl. Chem., 52, 2717 (1980).
0 1982 American Chemical Society
2682
The Journal of Physical Chemistry, Vol. 86, No. 14, 1982
Neumann-Spallart and Kalyanasundaram
Scheme I
intermediate species and suitable redox catalysts to achieve useful net chemical conversions. Schemes based on this are of special interest in photochemical conversion and storage of solar energy. A case widely studied is the photodecomposition of water into H2and 02.&' The catalytic step is usually carried out "in situ" with catalyst either homogeneously present or dispersed in colloidal or powder form. An alternate, more elegant approach is to use catalytic electrodes and to carry out these reactions in electrochemical cells. The advantage of the cell system is that the ultimate products are produced at different sites which makes subsequent separation unnecessary and also prevents possible reaction of the products with each other or with the reactive intermediates. In addition, a twocompartment cell system allows a more in-depth analysis of the reaction sequences. The photochemical and catalytic steps are separated in space and can therefore be analyzed separately. Optimization is thus facilitated. While the principles and applications of photoelectrochemical cells with semiconductor electrodes are well established and are in a very advanced stageas similar studies incorporating visible-light-driven photoredox systems are very sparse. In recent years several dye-sensitized photoredox systems which evolve either H;m or 02- from water upon (2)D. G. Whitten, Acc. Chem. Res., 13,83 (1980). (3)N. Sutin, J. Photochem., 10, 19 (1979). (4)V. Balzani, F. Boletta, M. T. Gandolfi, and M. Maestri, Top. Curr. Chem., 75,1 (1978). (5)J. Bolten, Ed., "Solar Power and Fuels", Academic Press, New York, 1977. (6)J. S. Connolly, Ed., "Plenary Lectures of I11 International Conference on Photochemical Conversion and Storage of Solar Energy", Academic Press, New York, 1981. (7)J. Kiwi, K. Kalyanasundaram, and M. Gritzel, Struct. Bonding, 49,39 (1982). (8) (a) F. Cardon, W. P. Gomes, and W. Dekeyser, Ed., "Photovoltaic and Photoelectrochemical Solar Energy Conversion", NATO AS1 Series B, Vol. 69,Plenum Press, New York, 1981. (b) "Faraday Society Discussion No. 70 on Photochemistry", Chemical Society, London, 1980. (9)M. Neumann-Spallart and K.Kalyanasundaram; Ber. Bunsenges. Phys. Chem., 85,1111 (1981). (10)B. V. Koryakin, T. S. Dzhabiev, and A. E. Shilov, Dokl. Akad. Nauk. SSSR,238,620 (1977). (11)J.-M. Lehn and J.-P. Sauvage, Nouu. J . Chim., 1, 441 (1977). (12)K. Kalyanasundaram, J. Kiwi, and M. Gratzel, Helu. Chim. Acta, 61,2720 (1978). (13)A. Moradpour, E. Amouyal, P. Keller, and H. Kagan, Nouu. J . Chim., 2, 547 (1978). (14)A. I. Krasna, Photochem. Photobiol., 29,267 (1979). (15) M. Kirsch, J.-M. Lehn, and J.-P. Sauvage, Helu. Chin. Acta, 62 (1979). 101,7214 (1979). (16)J. Kiwi and M. Gratzel, J. Am. Chem. SOC., (17)K. Kalyanasundaram and M. Grltzel, Helu. Chim. Acta, 63,478 (1980). (18)G. M. Brown, S. F. Chan, C. Creutz, H. A. Schwarz, and N. Sutin, J. Am. Chem. SOC.,101,7638(1979). (19)G. M. Brown, B. S. Brunschwig, C. Creutz, J. F. Endicott, and N. Sutin, J. Am. Chem. SOC.,101,1298 (1979). (20)C. V. Krishnan and N. Sutin, J.Am. Chem. SOC.,103,2141(1981). (21)K. Kalyanasundaram and D. Dung, J. Phys. Chem., 84, 2551 (1980). (22)K. Kalyanasundaram and M. Gratzel, J. Chem. SOC.,Chem. Commun., 1137 (1979). (23)A. Harriman, G. Porter, and M. C. Richoux, J. Chem. SOC.,Faraday Tram. 2,77,833(1981). (24)S. F. Chan, M. Chou, C. Creutz, T. Matsubara, and N. Sutin, J . Am. Chem. SOC., 103,369 (1981). (25)M. Gohn and N. Getoff, 2.Naturforsch. A, 34,1135 (1979). (26)I. Okura and N. Kim-Thuan, J . Mol. Catal., 5,311 (1979). (27)P. Keller, A. Moradpour, E. Amouyal, and H. B. Kagan, Nouu. J . Chim., 4,377 (1980). (28)P. Keller, A. Moradpour, E. Amouyal, and H.B. Kagan, J. Mol. Catal., 7, 539 (1980). (29)A. I. Krasna, Photochem. Photobiol., 31,75 (1980). (30)E.Amouyal, P. Keller, and A. Moradpour, J. Chem. SOC.,Chem. Commun., 1019 (1980).
Cell la
e-.-,
Iht' EDTAy:x
oxidn. products
Cell I b
-
MV
Nafion-
e-
--.
Nafion
Cell l l
--e-
Nafion Ru stands for trisbipyridylruthenium complex
exposure to visible light have been identified. Earlier, we have examinedws3 the feasibility of carrying out some of (31)Y. Okuno and 0. Yonemitau, Chem. Lett., 959 (1980). (32)0.Johaneen, A. Launikonis, A. W. H.Mau, and W. H.F. Sasse, Aust. J. Chem., 33, 1643 (1980). (33)P. Keller and A. Moradpour, J.Am. Chem. SOC.,102,7193 (1980). (34)D. S. Miller and G. McLendon, Inorg. Chem., 20, 950 (1981). (35)D.S. Miller, A. J. Bard, G. McLendon, and J. Ferguson, J. Am. Chem. SOC..103.5336 11981). (36)I. Okura'and N.' Kim-Thuan, J. Chem. SOC.,Faraday Trans. 1, 77,1411 (1981). (37)I. Okura, N. Kim-Thuan, and M. Takeuchi, Inorg. Chim. Acta, 53,L149 (1981). (38)H. Agershi, T.Endo, and M. Okawara, J. Polym. Sci., Polym. Chem.. 19.1085 (1981). (39)T. Nishijimia, T. Nagakura, and T. Matauo, J. Polym. Sci., Polym. Lett., 19,65 (1981). (40)M. S.Tanulli and J. H.Fendler, J. Am. Chem. SOC.,103,2507 (1981). (41)N. Toshima, M. Kuriyama, Y. Yamada, and H. Hirai, Chem. Lett., 793 (1981). (42)J.-M. Lehn and J.-P. Sauvage, N o w . J. Chim., 5, 291 (1981). (43)A. J. Frank and K. L. Stevenson, J.Chem. SOC., Chem. Commun., 593 (1981). (44)J.-M. Lehn, J.-P. Sauvage, and Ziessel, N o w . J. Chim., 3, 423 (1979). (45)J.-M. Lehn, J.-P. Sauvage, and R. Ziessel, N o w . J. Chim., 4,355 (1980). ' (46)K. Kalyanasundaram and M. Gritzel, Angew. Chem., Int. Ed. End.. 18. --,701 -~ (1979). ~~- -, (47)K. Kalyanasundaram, 0.Micic, E. Pramauro, and M. Gritzel, Helu. Chim. Acta, 62,2432 (1979). (48)V. Ya. Shafirovich, N. K. Khannanov, and V. V. Streleta, N o w . J. Chim., 4,81 (1980). (49)K. Chandrasekaran and D. G. Whitten. N o w . J. Chim.,. 5.. 275 (1981). (50)M. Neumann-Spallart, K. Kalyanasundaram, C. Gratzel, and M. Gratzel, Helu. Chim. Acta, 63, 1111 (1980). (51)M. Neumann-Spallart and K. Kalyanasundaram, J.Chem. SOC., Chem. Commun., 437 (1981). (52)M. Neumann-Spallart and K. Kalyanasundaram, Ber. Bunsenges. Phys. Chem., 85,704 (1981). (53)B. Durham, W. J. Dressick, and T. J. Meyer, J. Chem. Soc., Chem. Commun., 381 (1979). ~
I
The Journal of Physical Chemistty, Vol. 86, No. 14, 1982 2683
PECs for H, and H,02 Production
the reactions leading to 02,Br2, or C12evolution in electrochemical cells with Ru02 as the catalytic electrode. Herein we examine similar systems for light-induced H2 evolution from water with various sensitizers such as Ru( b ~ y ) (tris(2,2'-bipyridyl)ruthenium(II)), ~~+ ZnTMPyP (zinc tetrakis(4-N-methylpyridyl)porphyrin),proflavin (3,6-diaminoacridine),and phenosafranine (3,7-diamino5-phenylphenazine). The role of several factors such as
H
Ph
PROFLAVINE
PHENOSAFRANINE
the oxidative or reductive cycle for the regeneration of the sensitizer, the role of the relay and its concentration, added support electrolytes, etc., as applied to the cell systems are examined. The observed cell currents and potentials under light are correlated to the values derived from the intersections of the individual current-potential curves of the two electrodes. We also examine in some detail a photoelectrosynthetic cell using Ru(bpy)gP+as a dye where the photoinduced reduction of O2 to Hz02 is coupled to bromide oxidation on a carbon anode. There has been a growing interest in photoelectrochemical cells (PEC) which utilize photoredox reactions in the
Experimental Section Materials. R~(bpy)~Cl~.GH~0 (Strem), proflavin (Fluka), and phenosafranine (Fluka) were all commercial p.a. grade chemicals. The chloride salt of zinc tetrakis(4-Nmethylpyridy1)porphyrin (ZnTMPyP) was synthesized from TPyP according to the procedures reported earlier.17 Other chemicals (methylviologen (MV2+)(BDH), EDTA (Fluka), HBr (BDH), and Na2S04(Fluka)) were also p.a. grade chemicals and were used as such. Viologen-550 was a generous gift from Dr. J. Kiwi of our laboratory.
r
I
I
12+ VIOLOGEN-550
v-550
Water was distilled once from KMn04 and twice from a quartz still. Photolysis studies employed a 250-W halogen-tungsten lamp (intensities varied with neutral density filters) used in conjunction with a 10-cm water filter and appropriate UV cutoff filters. Methods. Photoelectrochemicalstudies were carried out in an electrochemical cell described earliera51P2The cell consists of three compartments interconnected by a Ndion (Dupont) or a dialysis membrane. An Ag-AgC1 reference electrode and a Pt gauze or carbon cloth (Union Carbide) electrode (40 cm2 area) were employed. Potentiostat and other electrochemical equipment used have also been described earlier. All potentials are quoted vs. NHE. The (54) D. P. Rillemma, W. J. Dressick, and T. J. Meyer, J.Chem. SOC., Chem. Commun., 247 (1980). (55) K. Chandrasekaran and D. G. Whitten, J.Am. Chem. SOC.,102, 5119 (1980). (56) K. Chandrasekaran and D. G. Whitten, J. Am. Chem. SOC.,103, 7270 (1981). (57) T. Kawai, K. Tanimura, and T. Sakata, Chem. Lett., 137 (1979).
PEC was thermostated at 25 "C during all the experiments. Analysis of Products. H2 produced in the cathode compartment was analyzed quantitatively by an HWD on a Gow-Mac gas chromatograph after separation on a Carbosieve column. Br2 produced was analyzed with an ion selective electrode as described earlier.52 For analysis of H202,the catholyte solution after photolysis was passed through a cation-exchange column (to remove Ru(bpy)QP+, MV2+,etc.) and the eluant analyzed for H202after reaction with I-. The resultant 1, was measured spectrophotometrically.
Results and Discussion I . Photoelectrochemical Cells for H2 Evolution from Water. The various photoelectrochemical cells under study are represented in Scheme I. In cell Ia, the sensitized reduction of the relay, MV2+, to its radical ion, MV+, is achieved via an oxidative cycle (initial oxidation of the excited sensitizer S and subsequent reduction of S+ by the donor) while in cell Ib the same overall reduction occurs via a reductive cycle. An interesting feature of the reductive cycle is that most often the S- radical itself is a strong reductant and this provides a situation where one can eliminate the need for the relay, MV2+. Present studies with phenosafranine and proflavin demonstrate this feasibility. In both cell versions the photogenerated A- radicals are oxidized at the carbon anode with concomitant reduction of protons to H2 at the Pt counterelectrode. Hence the overall cell reactions are at the anode D
hu
+ MV2+sensitizer D+ + MV+
D+
--
irreversible products
MV+ a t the cathode
MV2++ e-
H+ + e-
-
'/2H2
(bulk) (bulk)
(electrode) (electrode)
(1)
(2)
(3) (4)
Various parameters (such as the light absorption threshold, quenching efficiencies of donor, acceptor relays, quantum yield for H2generation with colloidal Pt catalysts added in situ) which characterize the photoredox systems under study are collected in Table I along with their literature references. We now examine the results obtained in the cell system individually for various sensitizer-relay combinations. Table I1 summarizes experimental results for the various cell systems. ( i ) Ru(bpy),2+-Viologen-EDTA System (Cell l a ) . The Ru(bpy)z+-MV2+ system has been extensively studied by various a ~ t h o r s . l ~ - ' ~Figure J ~ l a presents the currentpotential curves both for the anode compartment (in the dark and upon illumination with visible light X 1 400 nm) and for the cathode compartments measured under potentiostated conditions. The anodic photocurrent (corresponding to the oxidation of MV+ on the cathode) has an onset around -0.4 V and increases sharply thereafter, reaching a plateau value of 3.0 mA at potentials 10.2 V. A t potentials ido),saturation occurs and the current does not increase any more with light intensity. ih = fidoCo
(23)
(3) kb # 0. In most of the experiments in this work, however, the falloff in current at potentials negative to the plateau region is much more drastic than the one computed with eq 21 due to back electron transfer between A- and S+ and if=0.6is less than one-tenth of the plateau current. Back-reactions become increasingly important at high light intensities (conditions where the concentrations of A- and S+ are high and k small) where S+ cannot be sufficiently fast removed by the sacrificial donor. (3.1) [A-]/Co
