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Cyclic Water Cleavage by Visible Light Drastic Improvement of Yield of H2 and O2 with Bifunctional Redox Catalysts.

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active for photochemical water cleavage in homogeneous solution require either strongly acid conditionsl2”1or stoichiometric amounts of a mild reducing agent such as triethanolamine[2b’.We now report the photoreduction of water proceeding in neutral solution in the presence of transition me= 800 nm)r’l.
tal dithiolene complexes (A,,
We irradiated (A2290 nm) one of the complexes of Table
It4] in tetrahydrofuran (THF)/H,O (11.5: 1 v/v), and detected H2 by gas chromatography of the gas phase; no H2 is
detected on exclusion of light. The first quantitative H2 determination~[~]
were performed with the nickel complex (I):
irradiation in THF/H20 at A s 290 nm for 185, 350, and 483
h, respectively, generates 46,100, and 225% of HZL61,
after irradiation for the same time in anhydrous THF, 9, 16, and 77%
of H2 were detected; a simultaneously irradiated blank
(THF/H,O) containing no compound ( I ) yielded traces of
hydrogen (ca. 0.5%) after 165 h. On irradiation (As254 nm)
in a quartz apparatus, H2 formation is much faster: 141 mg
(0.26 mmol) of (1) in 200 ml of THF/H20 (9: 1) gives 209
Nml (9.4 mmol) of hydrogen in 22 h.
Table 1 “Transition metal dithiolenes”
lZ
Cation:
Me4N *
Bu4N
Bu4N
Ph4P *
+
+
n
M
R
2
2
2
2
2
3
3
2
Ni
Pd
Pt
Fe
co
W
Mo
Ni
Ni
Ni
Ni
W
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Ph
Ph
CN
CN
2
2
2
3
z
0
(1)
0
0
(2)
(3)
0
0
(4)
0
0
0
-1
-2
-2
-2
(5)
(6)
I 7)
(8)
(9)
(10)
(11)
(12)
The fact that even the reaction in anhydrous THFI’1 gives
some H2 suggests that part of the H, could come from ( I )
and/or the solvent. The former appears highly unlikely since
irradiation of (1) in CCl, fails to produce H2. In order to elucidate the second possibility, several irradiation runs were
performed (A 3 290 nm) in the presence of D 2 0 and the composition of the gas phase was determined by mass spectrometry“’. At the same solvent/D20 ratio (12: I), the following
values were found for the HD/D2 ratio: THF, 13 and
CH’CN, 2.1; the D/H ratio is accordingly about 1 and 2.
These results confirm the cleavage of D 2 0 and H abstraction
from the solvent. The formation of hydrogen, though in
small amounts, on irradiation of (1) in heterogeneous aqueous solution demonstrates that the organic solvent is not an
essential component of the system.
The mechanism of this novel photochemical cleavage of
water is still unknown. It is uncertain whether electronically
excited (1) and/or its mono- or dianion, (9) and (lo),respectively, promote the reaction; since formation of (10) was observed in all reactions of
the last two possibilities cannot be ruled outr‘*].
Received: October 30. 1978:
supplemented: June 24. 1980 [Z 540 IE]
German version: Angew. Chem. 92, 664 (1980)
[ I ] Cf. N. Getof: Wasserstoff als Energietrager. Springer-Verlag. Berlin 1977,
p. 173; E Schumacher, Chimia 32, 193 (1978).
(21 Cf.. e . g . K. R Mann. N . S. Lewis. V. M. Miskowski. D. K. Erwin. G. S.
Hammond, H. B. Gray, J. Am. Chem. SOC.YY. 5525 (1977); W. C. Trogler,
D. K . Erwin, G L. Geoffrey, H. B. Gray, ibid. 100, 1160 (1978). b) J-M.
Lehn, J - P . Sauvage. Nouveau J. Chim. 1, 449 (1977)
131 In this contribution. An,,tx always refers to the lowest energy absorption maximum.
[4] Review: G. N. Schraurer, Adv. Chem. Ser. 110. 73 (1972); E. Hoyer, W.
Dierzsch, W. Schroth, Z. Chem. / I , 41 (1971); J. A. McCleverry. Prog. Inorg.
Chem. 10.49 (1968).
151 Compound ( l j (0.024 mmol) in 8 ml of solution was irradiated (Philips
HPK 125 W) in a Solidex reaction vessel ( A > 290 nm) at room temperature
in a water bath. The total amount of H1was then determined by gas chroF. Weeke, E. Bastian, G. Schommatography 10.8 m molecular sieve 5
burg. Chromatographia 7, 163 (1974)I.
[6] Assuming that 1 mol of (I) leads to the formation of 1 mol of H2.
171 Distilled over LiAIHI
181 NO H2 was found we thank Dr. P Potzinger, Mulheim, for these analyses.
[9] Detected by isolation of (10) after addition of Bu,NI to the irradiated reaction solution.
1101 Nore addedinproo/(June 19, 1980): Catalysts of type ( 1 ) (M=Zn, R = C N .
R-R = trithiocarbonate I l l ] ) meanwhile produce more than two liters of
hydrogen in 24 h [in 120 ml of 2,5-dihydrofuran/D20 ( 1 :I), immersion well
apparatus, Philips HPK 125 W lamp, Aa254 nm. turnover number 1600
mmol hydrogen/mmol catalyst. gas composition: 76% D2, 21% HD, 3%
A;
H21.
11 11 G. Steinmecke, H:J. Sieler, R. Kirmse, E. Hoyer, Phosphorus Sulfur 7, 49
(1979).
1121 Cf. P. G. Jeffery, P. J. Kipping: Gas Analysis by Gas-Chromatography. Pergamon Press, London 1964.
Cyclic Water Cleavage by Visible Light: Drastic
Improvement of Yield of H2 and O2with Bifunctional
Redox Catalysts[**’
By John Kiwi, Enrico Borgarelio, Ezio Pelizzetti, Mario
Visca, and Michael GratzeI[*l
The photoinduced reduction of methylviologen (N,N-dimethylbipyridine dication, MV2+) by a suitable sensitizer,
such as Ru(bpy):’, can be exploited to achieve cleavage of
water into hydrogen and oxygen1”.The photoredox process
is performed in the presence of two redox catalysts, one of
which, i. e. colloidal platinum, effects reduction of water[’],
and the other, e.g. RuO,, oxidation of water‘’!
Scheme 1
0 2
Prof. Dr. M.Gratzel, Dr. J. Kiwi, E. Borgarello
Institut de Chimie Physique
Ecole Polytechnique Federale
CH-1015 Lausanne (Switzerland)
Procedure
Depending upon their solubility, up to 0.1 mmol of the
“metal dithiolene” is dissolved in 25 ml of a THF/H,O mixture (11.5: 1, v/v). The solution is flushed with argon for ca.
30 min in a screw-cap vials with a magnetic stirrer and gastight rubber seal; the vials are then closed and irradiated in a
merry-go-round apparatus at room temperature in a water
bath for 5-7 d with an Hg high pressure lamp (Philips HPK
646
125 W). GC analysis of the gaseous products was performed
on a molecular sieve column ( 5 A>[i21.
0 Verlag Chemie, CmbH, 6940 Weinheim, 1980
Prof. Dr. Pelizzetti
Istituto di Chimica Analitica, Universita di Torino
Torino (Italy)
Dr. M. Visca
Centro Richerche SIBIT (Montedison)
Spinetta Marengo (Italy)
[“I This work was supported by the Swiss National Foundation for Scientific
Research as well as Ciba-Geigy and Engelhard Industries Corporation.
0570-0833/80/0808-0646
S 02 SO/O
Angew. Chem. Int. Ed. Engl. 19 (1980) No. 8
In order to render the water decomposition by light effective, one has to employ an extremely active platinum catalyst. At moderately high Pt concentrations ( % 2 x 1 0 - 4 ~ )
this has to react with the reduced electron relay (MV') in
the microsecond time domain. Use of a catalyst system comprising an ultra fine Pt sol protected by a copolymer of styrene and maleic anhydride, and colloidal or macrodispersed
This is 100
Ru0, gave quantum yields of ca. 1.5 x
times smaller than the H2-yield obtained with a sacrificial
systernl4]were EDTA instead of water served as the reductant of Ru(bpy):+. Undoubtedly, the decreased quantum
yield arises from undesirable cross reactions, in particular the
reaction of oxygen with the reducing MV . This report describes the performance of a new cofunctional redox catalyst
which achieves astonishingly high quantum yields in the water splitting process.
The catalytic material consists again of a combination of
Pt and Ru02. However, in contrast to the previous experiments['] where individual particles of the two catalysts were
suspended in solution, the new preparation contains Pt and
Ru02 codeposited on a common carrier, i. e. colloidal Ti02.
A mixed oxide of Ru02 and TiOz was first prepared from a
solution of the respective chlorides. The Ti02 powder produced has a grain size of 1000 to 2000 A and contains ca.
0.1% R u 0 2 . The Ti02 is made n-type conducting through
doping with Nb. The powder is subsequently charged with
finely divided Pt by stimng it in solution containing a Ptsol (mean particle diameter 30 A). Thereby colloidal particles are produced which can act simultaneously as oxidation and reduction catalysts (cofunctionality). Results obtained from the photolysis of aqueous solutions containing
this catalyst are shown in Table 1. Irradiation of a solution
,
containing Ru(bpy):+ ( 1 0 - 4 ~ ) ,MV2+ (5 x I O p 3 ~ ) and
) visible light illustrates the high effiEDTA (5 x 1 0 - 2 ~with
ciency of this catalyst with respect to hydrogen generation.
In this system no oxygen is produced. Instead of water,
EDTA is irreversibly ~ x i d i z e d ~ One
~~.~
notes
] . that for a given Pt concentration (40 mg/l) the H2-evolution rate is influenced by the Ti02 content of the solution. Optimal yields
are obtained with a Ti02 concentration of 50 mg/100 ml. Interestingly, with such a system one obtains a hydrogen generation rate which exceeds that obtained with colloidal platinum alone (particle diameter 30 A, protected by Carbowax2 0 ~ by
) a factor of 4.
+
a
Table 1 . Evolution of hydrogen from aqueous solutions with various redox catalytic systems under the action of visible light.
Catalyst
Ial
Amount
(mg/l00 ml)
System
pH
ml
H2/l/h
Pt/Ti02/Ru02
100
Ru(bpy):'/
MV'+/EDTA
4.7
50
300
50
25
10
260
250
200
80
5
Carbowax/Pt
80
Pt/Tr02/Ru0?
50
Ru(bpy)i /
MV' +
47
3.0
10
45
3
2
tum yield for hydrogen generation is astonishingly high. If
one accounts for the fact that EDTA doubles the quantum
yield for MVZ+reduction, the efficiency of the cyclic system
is only a factor of three lower than that of the sacrificial one.
This may be further improved by optimization. It is noted
from Table 1 that the rate of hydrogen evolution decreases
with the solution pH. This is attributed to the decrease in the
driving force for water oxidation by Ru( bpy) .
A further important feature of the cyclic system is that the
H2 evolution can be sustained over long irradiation times.
Thus after two days of irradiation there was no noticeable
decrease in the hydrogen generation rate.
Oxygen production occurs simultaneously with the formation of hydrogen under illumination. Thus in the experiment
where 45 ml of H2 was generated during one hour of photolysis the quantity of O2 produced was 16 ml. This value is
distinctly below the stoichiometric proportion. Presumably
part of the O2 generated in the photolysis is absorbed on the
surface of Ti02 which under our conditions may be more
than 100 m2/1 solution.
A series of blank experiments was performed to confirm
the above results: There is no H2 formation in the absence of
light. Both the electron relay and sensitizer have to be present to obtain light-induced water decomposition: If the
Ru02,Pt-dopedTiOz particles are illuminated alone in aqueous suspensions there is neither hydrogen nor oxygen formation. Presumably visible light is inefficient in directly exciting the Ti02 semiconducting particles which have a band
gap of 3 eV. When excited by UV-light Ti02 can mediate a
number of photosynthetic processes[51and has been claimed
to cleave water[6'.
The surprisingly high activity of our cofunctional redox
catalyst may be rationalized in terms of adsorption of the redox species in solution on the Ti02 surface. This favors the
catalytic processes and eliminates rate limits imposed by
three-dimensional diffusion of the reactants. It is feasible
that the whole process of water cleavage occurs on a single
TiO, particle via adsorbed species. Whether electronic levels
of the Ti02 participate in this reaction remains to be investigated.
+
Experimental
For the preparation of the Pt-doped Ti02/RuOz particles
was
a platinum sol with a mean particle diameter of 30
first produced[71.(100 mg H2PtCI, was dissolved in 250 ml
H 2 0 and subsequently reduced with 35 ml of a 1% solution
of citrate. The mixture was heated during ca. 4 h at 90°C
and the excess of citrate was removed with an Amberlite ion
exchange resin.) To disperse the Pt on TiO, it suffices to stir
the mixture containing the required amount of TiOz for approximately 1 h.
Figure 1 shows results obtained from applying the quasielastic light-scattering technique to the particle dispersions.
Correlation functions were measured with a Chromatix''] instrument. The Pt-doped RuOz/Ti02 particles show a mean
hydrodynamic radius of 470 A which lies distinctly below the
grain size of the powder. Apparently a higher degree of dispersion is achieved in solution.
A
670A
[a] Concentration of platinum 40 mgfl. [b] No illumination
In the second part of Table 1 are listed results obtained
with a cyclic system, i. e. in the absence of EDTA. The quanAngew. Chem. Inr. Ed. Engl. 19 (1980) No. 8
Fig. 1 . Correlation function obtained from aqueous solutions of the Pt/YiO,/
RuOz particles (10 rng/l). Scattering angle f3=4.5"
0 Verlag Chemie, GmbH, 6940 Weinheim, 1980
0570-0833/80/0808-0647
$ 02.50/0
647
Illuminations were carried out with a XBO 450-W Xe
lamp. The IR and UV content of the light was removed by a
16 cm water jacket and a 450 nm cut-off filter. Hydrogen and
oxygen were analyzed by gas chromatography. Alternatively
O2 detection was carried out with an END-0-MESS instrument[91.
In principle, the bonds marked in Scheme 1 can be cleaved
(a'-,
a-,and @cleavage) on phot9ysis ofosulfonamide8 and
sulfonylureas. The radicals R'XS02, RS02, and RS02X
should eliminate SO2 and form simple alkyl or aryl radicals,
which then stabilize by recombination or by abstraction of
hydrogen from the solventI2].
Received May 29, 1980;
revised: June 18, 1980 [Z 539 IE]
German version: Angew. Chem. 92, 663 (1980)
ff'
ff
P - R
D
+
R'X
SO2
CAS Registry numbers:
Pt, 7440-06-4; Ti02, 13463-67-7; RuOz. 12036-10-1; Ru(bpy): ' , 15158-62-0; H2,
1333-74-0; carbowax, 25322-68-3; MV", 4685-14-7; HzO, 7732-18-5
X = 0, NH, NR"
Scheme 1
[ l ] K. Kalyanasundaram, M . Grbfzel. Angew. Chem. 91, 759 (1979); Angew.
Chem. Int. Ed. Engl. 18, 701 (1979).
121 a) B. V. Koryakin, T. S. Dzhabiev, A. E. Shilou, Dokl. Akad. Nauk SSSR 238,
620 (1977); K. Ka[ynnasundaram, J. Kiwi, M. Grafzel, Helv. Chim. Acta 61.
2720 (1978); A . Moradpour, E. Amouyal, P. Keller, H. Kagan, Nouveau J.
Chim. 2,547 (1978); b) B. 0. Durham, W. J. Dressick, T J. Meyer, J. Chem.
Soc.Chem. Commun. 1979, 381; c) P. J. Delaiue, B. P. Sullivan, T. J. Meyer,
D. G. Whiffen,J. Am. Chem. Soc. 101. 4007 (1979); T. Kawar, K. Tanimura,
T Sakada, Chem. Lett. 1979, 137; M. Kirsch, J. M. Lehn, J. P Sauvage,
Helv. Chim. Acta 62, 1345 (1979); K. Kalyanasundaram, M . Grafzel,J. Chem.
Soc. Chem. Commun. 1979, 1138; A . I. Krasna, Photochem. Photobiol. 29,
267 (1979); 31, 75 (1980).
131 a) J. Kiwi, M. CrEtzel, Angew. Chem. 90,900 (1978); 91,659 (1979); Angew.
Chem. Int. Ed. Engl. 17, 860 (1978); 18, 624 (1979); Chimia 33, 289 (1979);
M. Grafzelin H. Gerischer, J. J. KQ[z: Dahlem Conferences 1978 on Light-Induced Charge Separation. Verlag Chemie, Weinheim 1979, p. 299: b) J. M .
Lehn, J. P. Sauvage, R Ziessel, Nouveau J . Chim. 3,423 (1979); c) K. Kalyanasundoram, 0. Micic, E. Pramauro, M . Grafzel, Helv. Chim. Acta 62, 2432
(1979).
[4] J. Kiwi, M . Grarzei. Nature (London) 657 (1979); J. Am. Chem. SOC. 101,
7214 (1979).
I S ] A. J. Bard, J. Photochem. 10, 59 (1979); T. Znone, A. Fujishima, J. Konishi, K.
Honda, Nature (London) 277,637 (1979).
[6] A . Y Bulafov, M. L. Khrdekel, Izv. Akad. Nauk SSSR Ser. Khim. 1976. 1902;
H. van Damme, W. K. Hall, J. Am. Chem. SOC. 101, 4373 (1979).
[7] J. Turkevich in "Electrocatalysis of Fuel Cell Reactions", Proc. Brookhaven
Symp. p. 123.
IS] Chromattx Application Note LS-8 (1978), 560 Oakmead Parkway, Sunnyvale, Calif. 94086 (USA).
[9] Details are given in [3c].
Photochemistry of Sulfonamides and Sulfonylureas:
A Contribution to the Problem of Light-Induced
Dermatoses
By Bernd Weiss, Heinz Diirr, and Hermann Josef Haas"'
Dedicated to Professor Matthias Seefelder on the occasion
of his 60th birthday
Sulfonamides and sulfonylureas have acquired great importance as pharmaceuticals, but they can elicit light-induced dermatoses or photosensitization of the human organism as side effect"]. Surprisingly, the photochemistry of the
sulfonamides and sulfonylureas has previously not been
studied in very great detail. The compounds are relatively
photostable, but give complex product mixtures on prolonged irradiation.
We report here on (i) model studies on the simple sulfonamides (la-d), (ii) the photochemistry of the chemotherapeutic sulfathiazole (lei and (iii) the photochemistry of the
antidiabetics tolbutamide (Rastinon) (7a) and carbutamide
(Invenol) (7b).
[*] Prof. Dr. H. Diirr, Dr. B. Weiss
Fachbereich 14 der Universitat
D-6600 Saarbriicken (Germany)
Prof. Dr. H. J. Haas
Fachbereich 3 der Universitat Saarbrucken
D-6650 Homburg (Germany)
648
0 Verlag Cfiemie, GmbH. 6940 Weinheim, 1980
The sulfonamides (la-e) were irradiated in a Grantzel
apparatus (Hg low-pressure lamp, 70 W) at room temperature in ether, methanol, or ethanol. Low-boiling components
were identified by GC analysis (comparison with authentic
samples), solids were separated by column or thin-layer
chromatography and identified by comparison with authentic material or spectra (Table 1).
R'
rHN'Rz
(2)
The sulfonylureas tolbutamide (Rastinon) (7a) and carbutamide (Invenol) (76) were photolyzed under conditions similar to those in the case of (1) (see Table 2). In the photolysis
of (7a), both in ether as well as in methanol, toluene @a),nbutylurea (9), and 4,4'-dimethylbiphenyl (1Oa) were formed.
In MeOH/02, however, no toluene or dimethylbiphenyl was
obtained thus indicating chemical rather than physical
quenching by 02.Prolonged photolysis (100 h) of (7b) in
methanol yielded aniline (Sb), n-butylurea (9), and three
products of unknown structure. Thus, a p-amino group drastically increases the photochemical stability of the sulfonamides (1) as well as the sulfonylureas (7).
In mechanistic studies on (7a) we obtained the following
results: 1) The excited state is likely to have n d or ~ F Tchar*
acter. 2) The effectiveness of the photofragmentation of (7a)
is moderate, i. e. the quantum yield is only 0.04 (disappearance of the educt). 3) Sensitization experiments with xanthone (ET = 73 kcaI/mol) proceeded negatively, while the
findings with MeOH/02 indicate chemical rather than physical quenching of the excited state. No luminescence is observed with (7a) [however, in the case of (7b) and (le) fluorescence was observed]. 4) A cage-effect participates in product formation.-All these findings indicate that the photo-
0570-0833/8010808-0648
$ &?50/0
Angew. Chem. In[. Ed. Engl. 19 (1980) No. 8
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