Hydrogen Evolution from Water under Visible Light

J. Am. Chem. SOC. 1981, 103,2923-2921 2923
Ultrafine and Specific Catalysts Affording Efficient
Hydrogen Evolution from Water under Visible Light
Illumination
Pierre- Alain Brugger, Pierre Cuendet, and Michael Gratzel*
Contribution from the Institut de Chimie Physique, Ecole Polytechnique Fgdgrale de Lausanne,
IO1 5 Lausanne. Switzerland. Received March 12, 1980
Abstract: Platinum particles of 32-A diameter were produced in aqueous solution by citrate reduction of hexachloroplatinate.
A variety of synthetic polymers were tested with respect to their protective action and activity to catalyze hydrogen evolution
from reduced methylviologen (MV’) and water according to 2MVt + 2H20 3 2MV2+ + 20H- + H2. MV+ is produced
in a light-induced redox reaction of methylviologen with Ru(bpy)?’. Carbowax-20M-protected Pt particles were found to
give outstanding stability and achieve high hydrogen generation rates even at concentrations as low as 1.4-mg Pt/L. An even
higher activity is obtained when the microparticles are protected by the cationic polysoap PVP-CI6. In a photoredox system
containing Ru(bpy)32+ or zinc tetrakis(N-methylpyridy1)porphyrin as a sensitizer and N-tetradecyl-N’-methyl-4,4’-bipyridine
as an electron relay, the latter catalyst can intercept the thermal back reaction by specific interaction with the reduced relay.
The access of the oxidized sensitizer to the PVP-CI6 protected microparticles is impaired by electrostatic and hydrophobic
forces.
Introduction
Previously, a number of homogeneous or microheterogeneous
solution systems were reported which under illumination with
visible light produce hydrogen from water.’ The essential ingredients
of such a system are a sensitizer (S), an electron relay
(R), and a catalyst. Excitation of the sensitizer induces electron
transfer
S + R (1) - S+ + R- hu
which is followed by a catalytic step
catalyst R- + H20 - 1/2H2 + R + OHleading
to H2 generation. The back conversion of S+ into S may
be achieved by sacrificing a donor added to the solution through
irreversible oxidation:
D + S++ D++ S
Alternatively, the recycling of the sensitizer can be coupled to
water oxidation
(4)
via catalysis by noble metal oxides such as PtOz, Ir02,2 and RuO2.,
For the successful operation of such a cyclic water decomposition
system, it is mandatory that both catalysts operate selectively
and at a high rate.”
(3)
catalyst
S+ + 1/2H20 - S + 1/40+2 H +
(1) (a) B. V. Koryakin, T. S. Dzhabiev, and A. E. Shilov, Dokl. Akad.
Nauk. SSSR, 238,620 (1977); (b) J. M. Lehn and J. P. Sauvage, Noun J.
Chim., 1, 441 (1977); (c) K. Kalyanasundaram, J. Kiwi, and M. Gratzel,
Helu. Chim. Acra, 61, 2720 (1978); (d) A. Moradpour, E. Amouyal, P.
Keller, and H. Kagan, Nouu. J. Chim., 2,547 (1978); (e) B. 0. Durham, W.
J. Dressick, and T. J. Meyer, J . Chem. SOC.C, hem. Commun., 381 (1979);
(f) P. J. Delaive, B. P. Sullivan, T. J. Meyer, and D. G. Whitten, J. Am. Chem.
Soc., 101, 4007 (1979); (8) A. I. Krasna, Photochem. Photobiol., 29, 267
(1979); (h) T. Kawai, K. Tanimura, and T. Sakada, Chem. Lett., 137 (1979);
(i) M. Kirsch, J. M. Lehn, and J. P. Sauvage, Helu. Chim. Acta, 62, 1345
(1979); 6) K. Kalyanasundaram and M. Gratzel, J. Chem. SOC., Chem.
Commun., 1138 (1979); (k) A. I. Krasna, Photochem. Photobiol., 31, 75
(1980); (1) G. M. Brown, S. F. Chan, C. Creutz, H. A. Schwarz, and N.
Sutin, J. Am. Chem. Soc., 101, 7638 (1979); (m) G. M. Brown, B. S.
Brunschwig, C. Creutz, J. F. Endicott, and N. Sutin, ibid., 101, 7638 (1979).
(2) J. Kiwi and M. Gritzel, Angew. Chem., Int. Ed. Engl., 17,860 (1978).
(3) (a) J. Kiwi and M. Gritzel, Angew. Chem., Int. Ed. Engl., 17, 860
(1978); (b) Chimia, 33,289 (1979); (c) M. Graitzel in “Dahlem Conferences
1978 on Light-Induced Charge Separation”, H. Gerischer and J. J. Katz, Eds.,
Verlag Chemie, Weinheim, Germany, 1979, p 299; (d) J. Kiwi and M.
Gritzel, Angew. Chem., 91,659 (1979); (e) J. M. Lehn, J. P. Sauvage, and
R. Ziessel, Nouu. J. Chim., 3,423 (1979); (f) K. Kalyanasundaram and M.
GrBtzel, Angew. Chem., Inr. Ed. Engl., 18, 701 (1979).
0002-7863/8l/lS03-2923$01.25/0
Recently, we investigated light-induced hydrogen evolution from
a photochemical system in which Ru(bpy)?’ was used as a
sensitizer, methylviologen (MV2t) as an electron relay and a
centrifuged Pt sol stabilized by a polymeric material as a catalyst!
These studies established a trend to higher activity as the radius
of the Pt particles decreased. The results obtained encouraged
us to search for Pt aggregates having minimal size and low polydispersity.
We report here on the preparation, stabilization, and
performance of such a sol in the photoinduced H2 generation from
water. Furthermore, pathways are exploited to achieve specificity
of the Pt microelectrodes with respect to their intervention in the
water reduction process.
Experimental Section
Preparation and Characterization of the Catalysts. The colloidal
platinum was obtained via reduction of hexachloroplatinate solutions by
sodium citrate. The reduction procedure was similar to the one described
by Turkevich et aL5 A solution of 255 mL of H20 containing 15 mg
of Pt (in the form of H,PtCl,) was brought to boiling temperature; 30
mL of sodium citrate (1% weight aqueous solution) were added and the
mixture refluxed for 4 h. Thereafter the solution was cooled in an ice
bath. For excess citrate and electrolyte removal, the solution was stirred
with an Amberlite-MB-1 exchange resin in its Ht and OH- form until
the conductivity of the solution was smaller than 5 pS/cm. After filtration
the protective agent was added and allowed to equilibrate with
the Pt sol for at least 1 h. A schematic summary of this preparation
mode is given in Figure 1. The platinum content of the solution was
determined by atomic absorption spectroscopy using a Pye Unicam-SP
191 spectrophotometer. The size of the platinum particles was determined
by transmission electron microscopy (TEM). Samples were prepared
by spraying the colloidal solutions in small droplets (I-10-pm
diameter) with a nebulizer on Formvar membranes coated with carbon.
Observations were made with the Hitachi HU-12 electron microscope of
the Centre de Microscopie Electronique de I’Universitt de Lausanne.
Apparatus: Continuous photolysis experiments were carried out with
an XBO 450-W xenon lamp. Experimental details are given below. The
volume of the irradiated solution was always 35 mL. Prior to photolysis,
the samples were flushed with highly purified argon for removal of oxygen.
A ferrioxalate chemical dosimeter was employed to measure the
photon flux. Hydrogen was analyzed by gas chromatography, using a
Carbosieve 5-8, column at 35 “C and a Gow-Mac thermal conductivity
detector with a detection limit of 1 nmol. Laser photolysis experiments
(4) (a) J. Kiwi and M. Grltzel, Nature (London) 281, 657 (1979); (b)
J. Kiwi and M. Gratzel, J. Am. Chem. Soc., 101, 7214 (1979).
(5) (a) J. Turkevich, J. Chem. Phys., 13, 235 (1945); (b) J. Turkevich,
P. C. Stevenson, and J. Hillier, Discuss. Faraday Soc., 11, 55 (1951); (c) J.
Turkevich and G. Kim., Science, (Washington, D.C.) 169, 873 (1970); (d)
J. Turkevich, K. Aika, L. L. Ban, I. Okura, and S. Namba, J. Res. Insf. Carol.,
Hokkaido Uniu., 24,54 (1976); (e) J. Turkevich, Proceedings of a Symposium
on Electrocatalysis of Fuel Cell Reactions, Brookhaven, NY, p 123.
0 198 1 American Chemical Society
LYL4 J. Am. Lnem. JUC., 101. ivJ. 1"". 1 1 , 1.101
:heme for the preparation of ultrafine Pt :le$.
Figure 2. Fraction of R precipitated after difTerent times as a function
of the Carbowax 20 M-platinum ratio: NaCl = 1% (wt/v). Pt = 45
mg/L.
were performed with a J. K. frequency doubled nwdymium laser. The
Qswitched pulse had a duration of 20 ns. Details of the fast kinetic
detection method haw ken reported elsewhere?
Materink Hexachlomplatinic acid hexahydrate (Merck, pa.). sadium
citrate (Fluka, PA.). Ru(bpy),CI, (Strem), and methylnologen (BDH)
wen used as supplied. The synthesis of N-tctradayl-N'-methylviologen
has ken prNiOUS!y described.' The polymer protective agent Carbowax-
20M was supplied by Union Carbide. The poly(ethylene glycols)
(M, 5oM). 10000, 20oM)) were generous gifts from Dr. Tadros. IC1
Corporation. The cationic polysoap was synthesized by Dr. Humphrey-
Baker in our laboratory according to the following procedure: 0.5
g of poly(4-vinylpyridine) (M,68 700). supplied by Ciba-Geigy Cop. was
dissolved together with 0.16 g of bromohenadecane in 25 mL of dimethylformamide.
The solution was heated at 50 "C for 1 h and the
solvent rapidly evaporated under vacuum. The solid residue was taken
up in a mixture of IO-mL methanal and 4 0 " diethyl ether. After
decanting it was refluxed for 2 h in ether, filtered. and dried. NMR
analysis shows that approximately 50% of the pyridine groups are quaternized
by hcxadecyl chains.
Results and Discussion
Effect of Polymers on the Stability of the Pt Sol Against
Flocculation by Salts. In these experiments the Pt sol prepared
according to the procedure given above was added to an aqueous
solution of NaCl and the concentration of platinum measured as
a function of time. The platinum content of the solution was
determined either by atomic absorption spectroscopy or more
simply from optical absorbance measurements. (This sol shows
a characteristic absorption rising steadily toward the UV?
Precipitation diminishes the intensity of the band without changing
its shape. Hence, at a given value, the absorbance of the solution
is proportional to the R concentration.) Amongst a variety of
polymers tested as protective agents for the ultrafine particles,
(6) G. Rothcnkgm, P. P. Infclta. and M. Gritzel. 1. Phys. Chcm., 83,
(7) M.-P. F'ileni, A. M. Braun, and M. Gratzcl, Photochem. Photobiol..
(8) The UV-sbsorplion spcctrum of the sol has been published prcViously.
1871 (1979).
31,423 (19x0).
cf. ref 5d.
Brugger. Cuender, and Gratzel
Fipre3. Eleclron micrographs from different Pt sols. (a. lap) Unprotected
platinum, magnification 3400OOX, Pi = 0.4 mg/L in the original
solution. (b, middle) Carbowax/Pt, magnification 18OOOOX. Pi = 4
mg/L in the original solution, R = 2.5. (c. bottom) Carbowai/Pt.
magnification 300000X, Pt = 40 mg/L in the original solution, R = 2.5.
Carbowax-20M9 and PVP-C16 were found to be particularly effective.
Figure 2 illustrates the flocculation behavior in 1% NaCl
solution as a function of polymer concentration. In the case of
Carbowax-2OM stability is achieved when the mass ratio of
polymer to platinum is at least 1. A somewhat higher value, Le.,
R = 2 is required for efficient protection by PVP-Ct6.
In view of the excellent stabilization achieved with Carbcwax-
20M, it is interesting to note that simple poly(ethylene glycols)
(M, 10000 or 20000) are very poor protective agents. A subtle
difference in the structure of these compounds brings about this
opposing behavior. Carbowax-20M is a block copolymer containing
two poly(ethylene glycol) chains (each, M, 6000) linked
by a short hydrophobic epoxide chain.I0 The presence of the latter
is crucial in that it is anchored in the Pt surface, the remaining
chain segments forming loops that protrude into the aqueous
bulk."J2 These loops afford the repulsive forces required to
(9) Carbawar-20M was first noted by Horisberger to be the best stabilizer
of colloidal gold used as a marker for electron mienscopy (M. Horiskrgcr,
Bid. Cell. 36, 253 (1979).
(10) "Carbowax Polyethylene Glycols". Union Carbide Corporation
Chemicals and Plastics. New York, 1978.
(11) (a) E. Vincent, Ado. ColIoidInter/occSei.,4, 193 (1974): (b)Th.
F. Tadros, review to be published in Colloids Sur/. and references cited in
thee reviews.
Hydrogen Evolution from Water
prevent aggregation of colliding particles. The poor protective
effect exhibited by the simple poly(ethy1ene glycols) reflects weak
adsorption on the Pt particles.
Structure and Constitution of the Pt/Polymer Particles. The
extraordinary stability and activity (see below) of the Pt/Carbowax
catalyst prompted us to perform more detailed investigations on
the nature of the colloidal aggregates present in such a solution.
Electron microscopy was selected as the most suitable method
relative to other alternatives such as light scattering. This technique
was previously applied by Turkevich et al. to characterize
a variety of colloidal metal dispersions? Results are given in Figure
3 which shows electron micrographs of three different sols: (a)
was taken in the absence of protective agent. It reveals the
presence of spherical monodisperse particles having a diameter
of 30-35 A. When Carbowax is present as a protective agent the
dilute solution (4 mg of Pt/L) gives a similar result. Particle
diameters are between 30 and 35 A. At high concentrations (40
mg of Pt/L, Figure 3c), the formation of clusters becomes apparent.
A fraction of the Pt particles form loosely connected
assemblies extending over regions of several hundred nanometers.
From these data, it may be inferred that the unprotected as
well as the protected Pt sol in dilute aqueous solution consists of
uniformly sized particles having a diameter of 30-35 A. This
finding is in agreement with the results obtained by Turkevich
et alSs who obtained 32-A Pt aggregates with a mean deviation
of 12% in particle size.
Catalysis of Photoinduced H2 Production from Water. (a)
Sacrificial Systems. In these experiments the 32-A Pt particles
were tested for their activity in Hz-generating photolytic systems.
The aqueous solution contained R ~ ( b p y ) , ~(+4 X M) as
sensitizer and methylviologen (MV2+) as an electron relay.
Kinetics and mechanism of the processes occurring under illumination
have been investigated earlier.1d,4,13 The excited state
of the ruthenium complex transfers an electron to MV2+
*Ru(bpy)t+ + MV2+ - MV+ + Ru(bpy),,+ (5)
which in the presence of Pt catalyst is reoxidized by water under
hydrogen generation:
(6)
We shall first describe experiments in which R~ ( b p y ) , ~i+s reconverted
to the 2+ oxidation state by ethylenediaminetetraacetic
acid (EDTA) ( M). The latter is used as a sacrificial donor
which undergoes irreversible oxidation. The aim of these studies
was to establish optimum conditions for Hz production. The
procedure is now described in detail for the Pt/Carbowax catalyst.
Irradiations were carried out in the cell device consisting of an
Osram XBO 450-W lamp in conjunction with a cutoff filter (400
nm) and a 15-cm water cell to absorb the infrared radiation. The
solution (35 mL) is contained in a cylindrical cell (path length
15 mm) which is equipped with side arms for deoxygenation prior
to illumination and for volumetric determination of hydrogen.
Both the H2 burette and the cell are imbedded in a water jacket
which was thermostated at 25 OC. The light beam after traversing
the cell impinges on a monochromator connected to a photodiode
(Hamamatsu R 314), allowing for the measurement of optical
density changes in the solution during illumination.
We first examined, for a fixed Carbowax/Pt ratio (R = 2.7),
the effect of catalyst concentration on the hydrogen evolution rate.
Results are presented in Figure 4. The hydrogen output under
photostationary conditions rises steeply with Pt concentration up
to 1.4 mg of Pt/L from where on further augmentation is relatively
s10w.I~ At the break point, the rate is already astonishingly high
in view of the very small Pt concentration (-7 X lod M) present
MV+ + HzO - MV2+ + 1/2Hz + OHJ.
Am. Chem. SOC., Vol. 103, No. 11, 1981 2925
(12) W. Heller and W. Tanaka, Phys. Rev., 82,301 (1951); W. Heller and
T. C. Pugh, J. Chem. Phys., 22, 1778 (1954); W. Heller and T. C. Pugh, J.
Polynt. Sci., 97, 203 (1960).
(13) C. R. Bock, J. J. Meyer, and D. G. Whitten, J. Am. Chem. Soc., 96,
4710 (1974).
(14) At higher Pt levels the H, evolution rate decreases again. This effect
is caused by absorption of light by the Pt particles. The extinction coefficient
of the Pt solution is 2.3 X lo3 M-' cm-' at 450 nm.
Figure 4. Effect of Pt Carbowax concentration on the H2 generation rate
(R = 2.7), Ru(bpy),j* = 4 X M, MV2+ = 2 X lo-) M, EDTA =
M, potassium hydrogen phthalate buffer 5 X M, pH 4.5.
0,
IN 2
L
0
QJ
LL
% O
An
T>- A
R, [mg Carbowax 20-MImg Pt]
Figure 5. Effect of mass ratio of Carbowax 20 M-platinum on (a) the
rate of H2 generation and (b) the photostationary concentration of MV'
measured 30 s after beginning of photolysis. Pt = 6 mg/L, other conditions
as in Figure 4.
in solution. In fact, this figure can even be further improved, since
at the Ru(bpy)?+ concentration employed only a fraction of the
incident light-the maximum percentage is 86 for X = 452 nm-is
absorbed by the solution. This condition was selected intentionally
for our kinetic studies in order to avoid inhomogeneities due to
complete light absorption over a small path length. Given these
facts, the hydrogen evolution obtained with 1.4 mg of Pt/
Carbowax per L becomes comparable with that observed4 for
Pt/PVA at ca. 100 mg of Pt/L.
Secondly, it was attempted to check the effect of Carbowax
concentration on the rate of light-induced H2 evolution. Figure
5 shows that upon increasing the mass ratio of polymer to platinum
(R)f rom 0 to 2, the hydrogen output augments by a factor of
more than 4. At the same time the MV+ level present under
photostationary conditions decreases. This effect is particularly
pronounced between R values of 1 and 2 where the MV' concentration
diminishes abruptly by a factor of 8. At R values above
2, one notes a decrease of the hydrogen output concomitant with
a rise of the MV+ level present in the photostationary state.
Apparently, at very high Carbowax concentrations, the coating
of the Pt surface by the polymer blocks the access of the electron
relay to the active sites. At R values below 1 the Pt sol is unstable
as was shown above. Larger aggregates are formed which leads
to a decreased activity under these conditions.
A final noteworthy point concerns the surprisingly stable o p
eration of this catalytic system. Thus under optimum conditions
(R = 2, Pt = 1-10 mg/L), the Hz evolution rate establishes itself
at 7-8 mL/h in the presence of M EDTA. Hz formation
continues at this rate until more than 90% of the EDTA is consumed,
yielding one Hz per EDTA molecule. At this time the
turnover numbers for the sensitizer and electron relay are 200 and
4, respectively. Interested in the performance of the Carbowax
system at longer times, we photolyzed a solution containing lo-'
instead of M EDTA over a longer period. After an induction
time of several minutes, H2 is generated here at 9 mL/h. This
can be sustained for several hours without any noticeable degradation
of the sensitizer, turnover numbers being in excess of
1000. The process of hydrogen generation slows down once the
2926 J. Am. Chem. Soc., Val. 103, No. I / . 1981
solution pH raises due to exhaustion of the buffer. Higher pH
conditions are detrimental" for both the sensitizer and the electron
relay.I6
Apart from Carbowax-20M. we tested a series of other polymen
with respect to their Hz generating activity. Only polymers that
prevent effectively aggregation of the 32-A Pt particles were found
to be satisfactory. Amongst those one notices still a surprisingly
large difference in catalytic activity. Thus, the protective agents
PVP-C,,, Carbowax-ZOM, and PVA M, 60000 all give stable sols
at a polymer/Pt ratio of ca. 2. However, the PVP-C,, protected
catalyst is 4 times more active than PVA and 1.5 as active as
Carbowax/Pt. Evidently, these agents differ in their degree of
interaction with active sites while maintaining the same size and
shape of the Pt aggregates.
(b) Development of Ultrafine and Selective Catalysts for Cyclic
Water Decomposition Systems. In the preceding chapters, the
salient features of catalytic H, generation with ultrafine platinum
particles were explored. The results obtained serve now as a basis
for the development of catalysts which, apart from their high
activity, are operating selectively on one redox species. Only such
mediators are suitable for cyclic water decomposition systems.
The problem which has to be solved in this case is lo make the
Pt particle selectively interact with the reduced electron relay
(MV+). Contact of the cathodically tuned particle with the
oxidized donor [Ru(bpy),'+] has to be avoided, since this could
lead to short circuiting of the back reaction: reduction of Ru-
(bpy),'+ would occur instead of hydrogen production. Such a
photocatalytic system performs the transformation
"20
R- t S* (7)
The cycle of light-induced water decomposition can then be closed
either by addition of Ru02 catalyst3f or by coupling to an oxygen-
producing half-cell."
In the search for such a system, we based our strategy on the
use of the methylviologen derivative
n. S + R
m 3 * W - , r H 2 A - c P 3
2c1-
C,,MV2*
as an electron relay. Due to its strongly hydrophilic nature, this
surfactant shows little tendency to form micellesI8 or interact with
amphiphilic agents. By contrast, the monoreduced form (C,,MV+)
exhibits pronounced hydrophobicity and hence is prone to solubilization
by surfactant assemblies. This effect has been exploited
to achieve charge separation in the photoinduced redox reaction
between Ru(bpy)?+ and C,,MV2+. The CI4MV+ produced is
rapidly entrapped into a cationic micelle to which Ru(bpy)F,
for reasons of electrostatic repulsion, has no access.'8 Hence, a
drastic retardation of the back reaction is observed.
A similar principle was applied in the development of a platinum
catalyst that achieves both charge separation and hydrogen formation.
In this case, it is desirable to employ the cationic plysoap
PVP-C16 as the protective agent. As shown above by the flocculation
studies, the latter is an excellent stabilizing agent for the
Brugger. Cuender. and Grurzel
(IS) M. Gohn and N. Gctaff, Z. Nofurlorsch. A 34.. I135 (1979).
(16) MV" can be reduced by H2 to MV+ in the presence of Pi/Carbowar
catalyst. In nentral aqueous solution the latter was found lo decay aver a
pcricd of 2 h. probably due to catalytic hydrogenation (W. F. Sasse. private
communieation). The rateof this pr-s decreased drasticly an lowering the
oH. Interstinz. C,.MV+ men under neutral conditions a.v. w n t o be resistant
io hydrogenat& '.
(17) M. Neumann-Spallart. K. Kdyanasundaram, C. Gritzcl. and M.
GrHlzel, Hclu. Chim. Am. 63, I I I I (1980).
(18) P.-A. Bruggerand M. Gratzel. J. Am. Chem.Soe.. 102,2462 (1980).
P.-A. Bruggcr. P. P. Infelta, A. M. Braun, and M. Gratzcl. ibid., 103. 320
(1981).
Laser puke
Figure 6. Oscilloscope traces obtained from a deaerated solution of
Ru(bpy)p (10- M). C,,MV2+2CI- (5 X IO4 M), PVP-C,, (40 mg/L),
and colloidal Pt (20 mg/L). Upper trace. transient absorbance at 602
nm; lower tract, transient absorbance at 470 nm.
LIGHT INDUCED ELECTRON TRANSFER
COUPLED TO HYDROGEN EVOLUTION
OVERALL REACTION
S + H,O S' + ;H, + OH
& CIIT1ONIC POLISOIIP
C,.M"" C " , . W N ~ICII,',: C",
* C I ~
Figure 7. Scheme for the specific intervention of PVP-C,,-protected PI
microspheres in the photoredox process.
32-A particles. Mass ratios of surfactant platinum of 2.0 are
sufficient to produce stable sols. However, the Pt particles protected
by PVP-C,, are different from the Carbowax catalyst in
that they are positively charged and their surface has amphiphilic
properties.
Photolysis experiments were carried out with solutions containing
IO-] M CI,MV", IO4 M Ru(bpy),'+, IO4 M of colloidal
Pt, and 40 mg of PVP-C,,. The excited state of Ru(bpy),'+
reduces Cl,MV2+ with a specific rate of 8 X IO8 M-' s? . The
back reaction
C,,MV+ + Ru(bpy),'+ - C14MVz+ + Ru(bpy),'+ (8)
has a rate constant of 2 X IO9 M-' s ~ u' n der the conditions
employed. We shall now investigate the fate of ClIMV+ and
Ru(bpy)? in solutions containing PVP-C,,-protected platinum
particles. Laser photolysis results are presented in Figure 6. The
temporal behavior of the two transient species was monitored by
following the absorbance of the solution at 602 and 470 nm,
respectively. The initial rise of the signal at 602 nm is due to the
Hydrogen Evolution from Water
formation of C14MV+a fter laser excitation of R~ ( b p y ) , ~+T.h e
absorption decays sharply N 30 ps) back to the zero level,
indicating rapid consumption of CI4MV+. The formation of
R ~ ( b p y ) , ~is+ a pparent from the bleaching at 470 nm. There is
a fractional recovery of the negative signal to a plateau constituting
4030% of the initial value from where on no further changes are
noted. This indicates that a major part of R ~ ( b p y ) , ~f+or med
in the photoredox reaction is preserved and does not undergo back
reaction with CI4MV+.
The rapid disappearance of C14MV+ may be interpreted by a
mechanism involving first scavenging by the Pt particles through
hydrophobic interaction with the protective agent (Figure 7).
Charge transfer and water reduction occur simultaneously on the
Pt surface. The formation of hydrogen is readily seen in continuous
photolysis. Selectivity is achieved by making use of
electrostatic and hydrophobic interaction: Ru(bpy),,+ has no
access to the surface of the Pt aggregates. Hence, neither the
reduced relay nor the particle itself can interact with Ru(bpy)d+
which explains its astonishingly long lifetime in such a system.
Figure 6 demonstrates that the electron transfer from the reduced
viologen to the Pt particles is fast enough at lo4 M Pt to
intercept efficiently the back reaction. Under typical conditions
employed in our laser experiments, the latter proceeds with a first
half-lifetime of ca. 65 ps (initial concentration of Ru(bpy)d+ and
CI4MV+ E 4 X 10" M, k8 = 2 X lo9 M-' s-'). In Figure 7, the
first half-lifetime of the C14MV+ decay is 35 ps. From the difference
in the decay times obtained in the absence and presence
of catalyst one calculates a pseudo-first-order rate constant for
the electron transfer from CI4MV+ to the Pt particles of N lo4
s-l. This corresponds to a second-order rate constant of ca. lo8
M-' s-l if the analytical Pt concentration is used as a reference.
However, on the basis of particle concentration (the aggregation
number of Pt is 1200), the specific rate is 10" M-' s-l. This shows
that the reaction with the colloidal particles occurs at a very high
rate essentially controlled by the diffusion of the reactants. Similar
rates have been obtained for the discharge of ketyl radicals on
ultrafine Pt particles in a water/alcohol mixture.19
Selective intervention of the Pt microelectrodes is not restricted
to the case where Ru(bpy)32+ serves as a sensitizer. A similar
effect can be achieved, for example, also with zinc tetrakis(Nmethylpyridy1)
porphyrin (ZnTMPyp+) as the photoactive donor.
Electron transfer from the porphyrin triplet to C14MV2+ produces
CI4MV+ and ZnTMPyP5+.I8 In the presence of 32-A Pt particles
(lo4 M Pt) protected by PvP-c16, the CI4MV+ dissappears within
200 ps effecting HzO reduction. By contrast, ZnTMPyP5+, which
is readily identified by its characteristic absorption between 600
and 750 nm, is stable over many milliseconds. This is due to
repulsion of the porphyrin cation from the surface of the Pt
particles by electrostatic and hydrophobic interactions. The case
of ZnTMPyP4+ is particularly interesting as the photoredox reaction
occurs here with a solvent cage escape yield of practically
100% compared to 30% for the Ru(bpy):+/viologen couple.l* The
choice of the protective agent is crucial in order to obtain Pt
microelectrodes capable of operating specifically. Thus, selectivity
is lost when PvP-cl6 is replaced by Carbowax-20M. If the latter
J. Am. Chem. SOC., Vol. 103, No. 11, 1981 2927
catalyst is employed in a system which does not contain a sacrificial
agent, there is no hydrogen formation. Instead, it was noted from
laser photolysis experiments that the rate of the back reaction
between oxidized sensitizer and reduced relay was increased
significantly in the presence of this catalyst: both the rate of decay
of the viologen radical as well as the bleaching recovery of Ru-
(bpy),*+ are enhanced. Apparently, in the case of Carbowax
protection, the Pt particles simply short circuit the back electron
transfer from reduced viologen to R~(bpy),~+.
These data are corroborated by results obtained from continuous
photolysis experiments. Carbowax-20M-protected Pt catalyst
when coupled to Ru02 fails to split water under illumination of
a cyclic system containing Ru(bpy),*+ as a sensitizer and methylviologen
as an electron relay. By contrast, if the Pt microspheres
are protected by PvP-cl6 and C14MV2+ is used as relay,
simultaneous H2 and O2 production are observed. The lack of
specificity of the Carbowax-20M-protected particles is attributed
to the strongly hydrophylic nature of this protective polymer,
providing facile access of the Ru(bpy)?+ cation to the platinum
surface. Uncharged hydrophylic polymers are thus unsuitable
for Pt protection in cyclic water decomposition systems.20
Conclusion
An important fact emerging from the present study concerns
the activity of colloidal redox catalyst operating in H2-producing
systems. The trends established earlier4 relating high activity to
small particle size are unambiguously confirmed. This finding
corroborates a fundamental principle of electrocatalysis: a small
particle size is advantageous, both from the viewpoint of mass
transport of the electroactive species and from the viewpoint of
surface area per gram of catalyst employed. Further optimization
is possible by employing a particle size below 30 A. A separate
paper2' will deal with the synthesis and catalytic action in the
photolytic water cleavage of such ultrafine Pt aggregates.
High activity is only one prerequisite to be satisfied if such
catalysts are to be used in cyclic water decomposition systems.
Equally important but more difficult to obtain is the specificity
of intervention in the water reduction step. The present study
for the first time establishes pathways to achieve selective performance
of the Pt microspheres. The development of such
catalysts is of primordial importance to improve the performance
of cyclic water decomposition systems.
Acknowledgment. We are grateful for support of this work by
the Swiss National Foundation (2.168.0.78), Ciba-Geigy AG,
Basel, Switzerland, and Engelhard Industries, Metro Park, NJ.
We also thank Drs. J. Kiwi, M. Horisberger, and A. Gautier for
stimulating discussions and B. Demarchi and B. Muller for their
assistance in the experimental work. Furthermore, the generous
gift of the polymer samples by Dr. Th. F. Tadros of IC1 Ltd.,
England, is greatly appreciated.
(19) C. K. Grltzel and M. Grltzel, J. Am. Chem. Soc., 101,7741 (1979).
(20) The use of protective polymers can be avoided by employing mineral
supports which yield excellent activity and also selectivity. Thus, water
decomposition using a bifunctional Pt/RuO, redox catalyst and Ru(bpy)?+
as a sensitizer has recently been achieved; e.g., E. Borgarello et al., Angew.
Chem., 92, 663 (1980).
(21) E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visa, and M. Grltzel, " w e
(London), 289, 158 (1981