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Cyclic Cleavage of Water into H2 and O2 by Visible Light with Coupled Redox Catalysts.

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181 E. Clar: Polycyclic Hydrocarbons, Val. 2. Academic Press, New York/
Springer-Verlag, Berlin 1964. p. 97.
191 Concerning the ODMR method, see references in D. Schweifrer, K . H .
Hausser, V. Taglieber, H . A . Staab, Chem. Phys. 14, 183 (1976).
[lo] Regarding these results with ( I ) it is interesting that similar ODMR results
have been obtained recently with hexahenzocoronene and related aromatic
hydrocarbons of the “polyaryl type” with high molecular symmetry: J. Yoitlander, personal communication.
[ l I ] Attempted theoretical treatment of this hypothesis: H. Vogler, Symposium
on Aromaticity, Duhrovnik. September 1979.
tion of Ru(bpy):+ by TI3+have been described in the literaturel5’. An advantage of the T13+ system is the high quantum
yield of Ru(bpy):+ oxidation (4=2); however, the pH of the
solution has to be kept below 1.7 to avoid precipitation of hydroxide complexes. Higher O2 yields are obtained at pH between 4 and 5. It is noteworthy that even after long irradiation periods there is practically no depletion in sensitizer, indicating that the reverse reaction of Ru(bpy):
to
Ru(bpy): (Scheme 2) occurs faster than thermal decompositionI6l. (The turnover number of the sensitizer must be at
least 100.) Table 1 also shows that colloidal RuOz is much
more effective in mediating O2 evolution than the powdered
compound, since almost the same yields are obtained at 50100 times smaller concentrations. The colloidal dispersion
also has the advantage of being completely transparent.
The experiments for the simultaneous light-induced production of H2 and O2were performed with dimethylviologen
(N,N-dimethylbipyridine dication, MV’ +) as an acceptor.
MV2+ is reduced by photo-excited Ru(bpy):+ in a diffusion-controlled process["^'^. The reaction products, i. e.
Ru(bpy):+ and M V + , have the thermodynamic ability to
produce oxygen and hydrogen from water. However, to mediate these processes, very active redox catalysts have to be
employed since the back reaction can occur with a rate constant of k = 2.4 x lo9 1 mol- ‘ s - ‘[‘‘I. We recently found that a
finely dispersed, centrifuged Pt sol
mol/l) reoxidizes
MV’ within microsecondsl’l. This is therefore used to catalyze H2 formation: R u 0 2 , on the other hand, can mediate efficiently the O2 evolution step. When illuminating a solution
containing
mol/l of Ru(bpy):+ and 2 x
mol/l of
MV2+ at a p H of 4.7 in the presence of R u 0 2 powder (50
mg/150 ml) or colloidal R u 0 2 (1 mg/150 ml) and colloidal
Pt (4.5 mg/150 ml), simultaneous evolution of H2 and O2is indeed observed. Typically, we obtained 0.3 ml of O2 and 0.6
ml of H2 after 3 hours’ illumination of 150 ml of solution
with a 250-W projector lamp. Blank experiments showed
that the presence of both redox catalysts is mandatory for
production of both gases. The depletion of Ru(bpy):’ during 3 hours’ irradiation is less than 5%, illustrating the cyclic
nature of this cleavage of water. The two catalysts seem to be
sufficiently specific to avoid short-circuiting of the back reaction. Also, it appears that charge transfer from MV+ to Pt
can compete efficiently with O2 reduction according to
+
+
Cyclic Cleavage of Water into H2 and O2 by Visible
Light with Coupled Redox Catalysts[**’
By Kuppuswamy Kalyanasundararn and Michael Gratzel1‘I
Hydrogen evolution from water can be catalyzed by suitable noble metal (oxide) dispersions[’]. Recently, we succeeded in developing redox catalysts which are capable of
mediating oxygen production from water‘’]. R u 0 2 proved to
be a judicious choice of material since it has an extremely
low overpotential for O2 evolution131.This paper reports on a
system of two coupled redox catalysts permitting cyclic
cleavage of water by light.
The water contained Ru(bpy):+ as a sensitizer, which after light excitation reduces an electron acceptor (A). Initially
we added only R u 0 2 to the solution and chose the acceptor
such that it undergoes rapid subsequent reaction once it has
been reduced. Such a system is suitable for separate investigation of the light-induced evolution of oxygen from water
(Scheme 1).
Rulbpyli’
+
A
hv
Rulbpyl;’
A-
+
1
\1,
irreversible p r o d u c t s
Scheme I
Table 1 . Light induced evolution of O2 from water with Ru(hpy):+ as sensitizer and R u 0 2 ascatalyst. t=irradiation time. Variant A: 50 ml ofsolution with 20
mg of R u 0 2 powder or 0.3 mg of RuOl colloid. Variant B: 150 ml of solution
with 50 mg of RuO, powder or 1.0 mg of RuO: colloid.
MV’
Acceptor
Formula
c [mol/ll
pH
TICI,
8x
1.6
[CO(NH,)~CI]’+
10
4.8 [a]
[CO(NH,)~B~]”
CO(C204)1
RuO,
form
10.’
10-2
4.8
4.8
Powder
Colloid
Powder
Colloid
Powder
Colloid
Powder
Colloid
Variant
f [h]
A
0.5
0.33
1.5
3
9
9
1.5
1.5
A
A
B
B
B
A
A
+ O2 + 0;+ MV”
O2
Wl
0.3
0.4
From these findings we deduce the following reaction sequence shown in Scheme 2.
1.2
12
3
3.2
0.3
0.4
[a] The pH value of the solutions with cobalt complexes increases during irradiation, leading to formation of a brown precipitate. Precipitation can he avoided by
buffering with acetate; cf. also 191.
Results are shown in Table 1. Oxygen is obtained with all
the acceptors employed. The pathway of decomposition of
the cobalt(m) complexes141and the irreversible photooxida[‘I
Prof. Dr. M. Gratzel, Dr. K. Kalyanasundaram
Institut de chimie physique, Ecole Polytechnique Federale
CH-1015 Lausanne (Switzerland)
[**I
Acknowledgment is made to the Swiss National Foundation for supporting
this work under grant No. 4.061.076.04, and to Ciha-Geigy Ltd., Basel, Switzerland.
Angcw Chem. Inl. Ed. Engl. 18 (1979) No. 9
LO2 +
H’
Scheme 2
According to these findings, the cleavage of water by visible light is undoubtedly possible in a four-quantum process.
0 Verlag Chcmte, GmbH, 6940 Wernheim. 1979
05 70-0X33/79/0909-0701
S 02.50/0
701
In the development of such systems we see a possibility of
achieving solar energy conversion by use of homogeneous or
microheterogeneous aqueous solutions.
Experimental
The Ru02 catalysts used were R u 0 2 .HzO, (Alfa Inorganics) and a colloidal suspension stabilized by a copolymer of
styrene and maleic anhydride1101.
The colloid was prepared
by treating a neutral solution of Ru04 in water (50 mg) with
a 0.5% aqueous solution (25 ml) of the copolymer. The pH is
adjusted to 8 and the solution stirred for one hour. A Ru02
sol is, formed spontaneously with a mean particle radius of
300 A.
Prior to illumination, the whole system is carefully deaerated by flushing with nitrogen. The two stopcocks are then
closed and the flask illuminated by a 250-W slide projector
lamp. After irradiation the gas evolved in the solution is
flushed out and transferred to the oxygen meter by a stream
of nitrogen. In order to check hydrogen production a sample
of gas is taken through the septum by an air-tight syringe
and subjected to gas chromatography.
M Ce4+ solution
A test run was first made with a 8 x
in I N HzS04 which, on contact with the hydrated Ru02
powder, gave oxygen in the darkt2’under simultaneous reduction of Ce4+ to Ce3+.This reaction produces Or with a
> 80% efficiency. Unhydrated RuOz failed to give oxygen.
I
Received: July 13, 1979 [ Z 291 IE]
German version: Angew. Chem. 91, 759 (1979)
-5
I
Fig. 1. Schematic illustration of the instrumental set-up used for light-induced
cleavage ofwaler. I , 250-W lamp; 2, catalyst; 3. septum; 4, three-way stopcock; 5,
to oxygen measuring instrument.
The solution is placed in a flask containing the solid catalyst in a side arm. This is also fitted with a septum allowing
for injection of colloidal catalyst and removal of gas samples.
The flask is connected to a trap and an End-0-Mess oxygen
meter (Friedrichsfeld GmbH, Mannheim, Germany) (Fig.
1).
[I] a) B. K Koryakin, T. S. Dzhabieu, A . E. Shiloc, Dokl. Akad. Nauk SSSR
238, 620 (1977); b) J. M. Lehn, J. P. Sauvage, Nouveau J. Chim. I . 449
(1977); c) K. Kalyanasundaram, J. Kiwi, M. Cratzel, Helv. Chim. Acta 61,
2720 (1978); d) A. Moradpour, E. Amouyal, P. Keller, H. Kagan, Nouveau J .
Chim. 2, 547 (1978); e) B. 0. Durham, W J. Dressick, T. J. Meyer, J .
Chem. Soc. Chem. Commun. 1979, 381
121 J. Kiwi. M. Grutrel, Angew. Chem. 90, 900 (1978); 91, 659 (1979); Angew.
Chem. Int. Ed. Engl. 17. 860 (1978); 18, 624 (1979); Chimia 33, 289 (1979).
(31 H. H. Miles, J. Electroanal. Chem. Interfacial Electrochem. 60,84 (1975).
[4] H.D. Gafney, A. W. Adamson, J . Am. Chem. Soc. 94, 8238 (1972); G. Navan, N. Sutin, Inorg. Chem. 13, 2159 (1974); J. N. Demas, A . W Adamson, J.
Am. Chem. Soc. 95, 5159 (1973).
151 C. S. Laurence, V. Balzani, Inorg. Chem. 13, 2976 (1974).
[6] C. Creutz, N . Surin, Proc. Nat. Acad. Sci. USA 72, 2858 (1975).
[71 R. C. Young, T. J. Meyer, D. G. Whitten, J. Am. Chem. Soc. 98, 286
(1976).
[R] J. Kiwi, M. Cratrel, J. Am. Chem. Soc.. in press.
[9] We thank Prof. Lehn, for providing us with a manuscript in which evidence
for O 2 evolution with the same system was presented.Note added in
proof: cf. J:M. Lehn, J:P. Sauvage, R. Ziessel, Nouveau J . Chim. 3, 423
(1979).
[lo] We thank Unilever, Port Sunlight (England). for the gift of the copolymer.
BOOK REVIEWS
Hummel/Scholl: Atlas of Polymer and Plastics Analysis.
2nd, revised edition. Vol. 1. Polymers: Structures and
Spectra. By D. 0. Hummel. Verlag Chemie, Weinheim/
Carl Hanser Verlag, Munchen 1978. xxxii, 671 pp., 1903
IR spectra, DM 475.00.
IR spectroscopists working with polymers will welcome
publication of this new edition of ‘‘Hummer’-ten years after the appearance of the first edition. Subdivision and aim
of the compilation of spectra have changed: Volume 1 now
bears the title “Structures and Spectra“, and the subsequent
Volumes 2 (Plastics, Fibres, Rubbers, Resins) and 3 (Additives and Processing Acid) are subtitled “Spectra and Methods of Identification”. The textual volume has been dropped.
New features include the use of an improved decimal classification system, adoption of names recommended by
IUPAC, and consistent use of a linear wave number scale.
The 1903 spectra depict a cross-section through the polymer chemistry of the past decade, ranging from materials of
construction to polymers newly developed, e. g., as ion exchangers, semiconductors, or carriers for enzymes. Most of
the polymers have been described in the literature; the respective samples reached the author from laboratories
throughout the world.
702
A problem attending such a work is always that of the purity of the substances concerned. Thus, a glance at page 2 of
the book already shows absorption bands of ubiquitous silicone oil (spectrum no. 4) or gives an indication of oxidative
degradation (spectra nos. 5 and 6). Whenever recognized,
any impurities are mentioned in the accompanying text.
Searches are greatly facilitated by a list of contents, decimal classification, subject index, molecular formula index,
and author index.
The spectra cover the range of 4000 to 400 cm ’ . The scale
of the two formats adopted agree in the region between 2000
and 400 cm- Comparison and reading off of wave numbers
is often made difficult by the failure to emphasize the round
hundreds on the wave number grid. There is no wavelength
scale for those still accustomed thereto. (Perhaps a transparent foil bearing a wave number and wavelength grid could
be supplied with futures volumes.)
In spite of its high price, the book can be recommended as
a valuable tool to all concerned with the IR spectroscopy of
polymers.
~
’.
Hans H. Suhr
[NB 474 IE]
Angew. Chem. Ini. Ed. Engl. 18 (1979) No. 9
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