The Journal of
Physical Chemistry
0 Copyright, 1983, by the American Chemical Society
VOLUME 87, NUMBER 8
APRIL 14, 1983
LETTERS Visible-Light-Induced Cleavage of Water in Colloidal Clay Suspensions: A New Example of Oscillatory Reaction at Interfaced H. Nljs, J. J. Frlplat, and H. Van
Damme'
C.N.R.S. Centre de Recherche sur ies Solides 3 Organisation Cristaliine Imparfaite, 45045 Orleans, France (Received: January 3, 1983)
The visible-light-induced cleavage of water, via oxidative quenching of tris(2,2'-bipyridine)ruthenium(II) by E d n species,has been studied in aqueous suspensions of clays and related minerals. Separation of the primary electron transfer products and of the catalytic sites for H2and O2generation is achieved through the use of two different colloids. The fiit colloid is associated with the sensitizer and with the catalyst for oxygen generation. The second colloid is associated with the acceptor and with the catalyst for hydrogen generation. Gas evolution was found to display a damped oscillatory behavior. Although the turnover numbers with respect to the sensitizer and to the acceptor are low (20 and 5, respectively), the process seems to be catalytic.
Introduction The visible-light-induced decomposition of water into oxygen and hydrogen sensitized by organic or coordination compounds has attracted much attention in the past few years.' Among the possible photosensitizers, tris(2,2'bipyridine)ruthenium(II),Ru(bpy),2+,has certainly stimulated so far most of the experimental work, because of several attractive features. On the basis of its ground- and excited-state thermodynamic properties, two basic water-splitting cycles have been designed,2the more popular being *D + A Do, + Ared (1)
-+ -+
Do, + OH-
Catox
D
'/402
+ '/2Hz0
+
-
which, in most cases, very efficiently competes with reactions 2 and 3. Several options are possible to favor reactions 2 and 3, one obviously being to accelerate them catalytically. Supported and unsupported RuOz and Pt have been used in order to catalyze the half-reactions 2
(2)
(3) 'This paper is dedicated to the memory of Marible Cruz.
A is a suitable acceptor capable of oxidative quenching of the excited sensitizer (or donor), *D, and able to reduce H+ in its reduced state, Ared. The major problems in this reaction scheme are more of a kinetic than a thermodynamic nature, the most serious problem being the prevention of the energy-wasting "back"-electron transfer reaction Do, Ared D + A (4)
(1)For a recent review article on the subject, see for instance J. Kiwi, K. Kalyanasundaram, and M. Gratzel, Structure Bonding (Berlin),49, 37 (1982). (2)M.Kirch, J. M. Lehn, and J. P. Sauvage, Helo. Chim. Acta, 62, 1345 (1979).
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and 3, respectively,l with considerable success, but attempts to combine these two catalyzed “sacrificial”reactions in a single cyclic system have met with very few positive resu1ts.l In fact, it appears that systems based on reaction scheme 1-3 have little chance to be effective unless one designs their architecture, at the supramolecular level, in such a way as to (i) at least not hinder “forward”electron transfer between *D and A; (ii) separate Do, and Ared as soon as they are formed (this is the so-called “cage escape” or, more generally, “charge separation” problem); (111)separate at the microscopic level, the catalytic sites for H2 and O2 production; (IV) allow Do, and fired to migrate selectively toward these catalytic sites or, alternatively, couple A with Cat, and D with Cat,,. Finally, the catalysts used should themselves be as selective as possible in order to avoid regeneration of water from H2 and 02. Electron injection into semiconductors has been proposed as a way of achieving charge separation and has led to several molecularly sensitized cyclic water cleavage systems.l Our approach to the above described “architectural”problem is to use charged, but electronically insulating, solid surfaces in order to support the catalysts and to control the position of the ionic species in the reaction medium. Clay minerals bearing a permanent negative charge are in this respect a particularly broad and potentially useful class of high surface area solids, but related minerals such as silica, magnesium, or aluminum hydroxides are equally interesting even though their surface charge is generally pH dependent. We have used hectorite and sepiolite clays in the sacrificial photooxidation of water with homogeneous or heterogeneous catalyst^,^^^ whereas Willner et al. have shown by fast spectroscopictechniques that efficient charge separation can be achieved in aqueous suspensions of colloidal silica, provided the charge of the silica surface and of the sensitizer-acceptor couple are carefully ~ h o s e n . ~ In the present work we have tried to go one step further by coupling two solid surfaces in such a way as to minimize, as far as possible, the electrochemical and catalytic “short circuits” of the system. Thus, the system that we used is a mixed aqueous suspension of two mineral colloids. The first colloid is associated with the donor (D)-Cat,, couple, and will be referred to hereafter as colloid,,. The second colloid is associated with the acceptor (A)-Catred couple, and will be refered to hereafter as COllOidred. Colloid,, is a negatively charged fibrous clay (sepiolite) on the surface of which D (the photosensitizer, R ~ ( b p y ~ ~is+cation ) ) exchanged. Colloidd is an ill-organized aluminum hydroxide in which the acceptor A is embedded. Eu3+was chosen as the acceptor because it is known to oxidatively quench the excited state of Ru(bpy)gZ+and because of its suitable redox potential for hydrogen evolution in acidic conditions (E,&Eu,~~+/~+) = -0.3m V).s Colloid,, is supporting Ru02 particles as an oxygen generation catalyst, whereas colloidrd is supporting Pt particles as a hydrogen generation catalyst. Thus, the system is compartimentalized in two subsystems, [colloid,,-D-Cat,,] , on the one hand, and [colloidred-A-Catred], on the other. Coupling of the two subsystems is achieved through interparticular association in the reaction medium. Spontaneous association of col(3) H. Nijs, M. I. Cruz, J. J. Fripiat, and H. Van Damme, J . Chem. SOC.,Chem. Commun., 1026 (1981); Nouu. J. Chin., in press.
(4) H. Nijs, M. I. Cruz, J. J. Fripiat, and H. Van Damme in “Photochemical,Photoelectrochemical and PhotobiologicalProcesses”, Vol. 1, D. 0. Hall and W. Palz, Ed., Reidel, New York, 1982, p 102. (5) I. Willner, J. M. Yand, C. Laane, J. W. Otvos, and M. Calvin, J . Phys. Chem., 85,3277 (1981). (6) N. Sutin and C. Creutz, Adu. Chem. Ser., No. 168, 1 (1978).
Letters
Flgure 1. Schematic representation of the structure of the mixed co”oida’ system described in the text.
loidal aluminum hydroxides with clay minerals is indeed occurring as well in artificial‘ as in naturals conditions. The expected structure of the system is sketched in Figure 1. It is clear that the mixed colloidal system just described is a naive approach to the various kinetic and “architectural”requirements outlined above (i-iv) for water cleavage. Nevertheless, the principles seem to be valuable since, as will be shown below, this system was found to be able to decompose water under visible light irradiation. In most cases however, an oscillatory behavior was observed. Recently, several examples of oscillatory reactions on solid surfaces have been reported in varied fields of catalysis.s11 These oscillatory reactions are usually discussed within the larger framework of other oscillatory phenomena observed in homogeneous and biological media.12-14 On the other hand, in photochemistry, periodic chemiluminescence has recently been reported by Bolletta and Balzani,15 with Ru(bpy)Qa+as luminophore. However, to our knowledge, no oscillatory photochemical production of a gas has been found yet. We therefore wish to report on our observations both for their fundamental interest and also for their potential applications.
Experimental Section Ru(bpy)?+ was obtained from Ventron and used as such. Mineralogically pure sepiolite from Spain was used as received. Its cation exchange capacity was 15 mequiv/ 100 g, and its nitrogen BET surface area was 250 m2/g. Eu3+was incorporated in an Al(OH)3gel by coprecipitation by NaOH (1N) from a 2% solution of E u ( N O ~ ) ~ * H20 and A12(S04)3*6H20 (Merck) at pH 9.5. After washing by centrifugation (5000 g, 20 min, 3 times), a co-gel containing 3% of dry matter was obtained. The [sepiolite-Ru02-Ru( bpy)2’3 colloid was prepared by ion exchanging sepiolite with Ru(NH3)&13(Strem) and oxidizing the resulting material in air at 300 “C overnight. The clay was then crushed and reexchanged with Ru(bPY)32+. (7) N. Lahav and V. Shani, Clays Clay Mineral., 26,107 (1978). (8) S. Yariv and H. Cross, “Geochemistry of Colloid Systems”, Springer Verlag, Berlin, 1979, pp 164, 300, 359, and 371. (9) R. Dagonnier, M. Dumont, and J. Nu@, J . Catal., 66,130 (1980). (10) M. Nitta, S. Kanefusa, Y. Taketa, and M. Haradome, Appl. Phys. Lett., 32, 590 (1978). (11) A. Schwartz, L. Holbrook, and J. Wise, J . Catal., 21,199 (1971). (12) P. Rehmus, Y. Termonia, and J. Ross, Kinam, 3, 123 (1981). (13) P. Glansdorff and I. Prigogine, “Thermodynamic Theory of Structure, Stability and Fluctuations”, Wiley-Interscience,New York, 1971. (14) G. Nicolis and I. Prigogine, “Self Organization in Non-Equilibrium Systems”, Wiley-Interscience,New York, 1977. (15) F. Bolletta and V. Balzani, J. Am. Chem. SOC.,104,4250 (1982).
Letters
The [A~,Eu~-,(OH)~-P~] colloid was prepared by dissolving K2PtC1, (Ventron) in a suspension of the supporting colloid and reduction by a few drops of NzH4-H20 (Prolabo). The gels were then washed several times by centrifugation in order to eliminate any excess of hydrazine. The two colloids were mixed in the dark in a closed Pyrex reactor (100 mL) containing 50 mL of phosphate buffer (pH 3.5). The mixed suspension was found to be considerably less stable than the suspensions of the individual colloids. The mixture was therefore continuously stirred. Before starting the reaction, the cell was outgassed with a fore-vacuum pump for 30 min. Illumination was carried out with a 1000-W tungsten-halogen lamp equipped with a UV cutoff filter and an IR reflector. The gas pressure in the cell was measured with a conventional mercury manometer or, later, continuously monitored by a Bell and Howell pressure transducer. Gas analysis was performed with a Girdel gas chromatograph equipped with a TCD detector and a molecular sieve 5A column using helium, argon, or methane as a carrier gas.
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Results As stated in the Introduction, irradiation of a mixed suspension of the [~epiolite-RuO~-Ru(bpy)~~+] and [A~,Eu~-,(OH)~-P~] colloids with visible light (>360 nm) was found to generate a gas which was analyzed as a mixture of O2 and H2. However, in the large majority of cases, the pressure increase in our static photoreactor was not continuous but displayed a pronounced oscillatory character. A very strong dependence of the oscillatory nature of the photoreaction on the temperature was found. Thermostating the reaction cell at room temperature or below was found to restrict the oscillations. In the absence of external temperature control, irradiation slightly heats the suspension and thermal equilibrium is attained at 39-40 "C. These conditions turned out to be almost optimal for observing the oscillations. The production of gas (in excess of the water vapor pressure) became generally oscillatory just before attaining thermal equilibrium, i.e., after 30 min. A typical example of the oscillatory behavior is displayed in Figure 2. The concentrations of the components involved are given in the figure caption. Only the oscillatory part of the reaction is shown. The following features can be derived from Figure 2: (i) the oscillations are not perfectly periodic but they are not chaotic either. (ii) The oscillatory process is damped. The amplitude of the oscillations diminishes in the course of the reaction, becoming zero after 4 h. The exact cause of the rapid deactivation of the system is not known. (iii) Interestingly, the amplitude of the first oscillations corresponds, within experimental error, to a turnover number close to 1for each Eu ion, assuming (see below) that the gas composition is H2/02= 2/1, Le., that, on the average, 2.66 electrons have to be transferred per gas molecule. With respect to Ru(bpy)?, the same oscillations correspond to a significantly larger turnover number, up to 5 in some cases. Although gas chromatographic analysis of the residual gases showed the presence of hydrogen and oxygen, we have to recognize that we cannot safely conclude that they are present in a 2 / 1 ratio. On the other hand, it is clear that recombination of H2and O2 occurs in our closed reactor. Indeed, all the attempts to bring the gaseous products to atmospheric pressure by injecting a liquid into the reactor led to fast and total recombination of the gas phase. Several control experiments were performed with one of the components missing. No gas evolution was observed
t (min)
Figure 2. Two typical examples of oscillatory behavior during lightinduced gas evolution, at thermal equilibrium. I n A, the pressure was monitored with a mercury manometer. I n B, it was monitored with a pressure transducer. The composition of the system in B was as follows: 50 mL of H 0, pH 3.1 (phosphate buffer); 500 mg of [seploIite-RuO,-R~bpy,~'] with 4 % w/w RuO, and 5 mequiv of Ru(bpy),'+/100 g of clay; 5 g of hydrated AIo,,,E~o,o,(OH),-R gel containing 97.3% water and 0.2% w/w Pt.
when either Ru(bpy)32+,Eu3+,Pt, or Ru02 were lacking. Adding Pt to the system in a nonmetallic form (in the form of K2PtC14for instance), instead of prereducing it on the colloidal A1-Eu hydroxide, was found to be uneffective. Since hydrazine was used as a reducing agent for the reduction of platinum on the Al,Eu,,(OH), particles, a possible participation of traces of hydrazine or of Eu2+was suspected. However, increasing the amount of NzH4 used for the reduction step did not modify the behavior of the system. Similar results were also obtained with citrate as reducing agent. In several runs, the reaction mixture was centrifuged after completion of the reaction in order to examine the supernatant spectroscopically. Only negligible desorption of Ru(bpy),2+was found to occur, showing that the reaction is indeed driven by adsorbed R ~ ( b p y ) ~This ~ + . is further supported by the fact that no gas production was observed when R ~ ( b p y ) was ~ ~ +added to the reaction mixture in excess (-100%) of the cation exchange capacity of the clay. Thus, any excess of sensitizer in the aqueous phase is not only useless but, in fact, detrimental to the reaction. One of the most critical parameters to evaluate in this system is the real gas production and, related to this, the total turnover numbers for Ru(bpy)gl+and Eu3+. Indeed, as pointed out above, recombination of O2 and H2 is very fast in our closed reactor and this leads to an underestimation of the true gas production. In addition, the occurrence of oscillations suggests that the production of gases is not continuous. A reasonable estimate of the minimum amount of gases produced may therefore be made by simply adding the amplitudes of the successive oscillations, as shown in Figure 2. Assuming again that on the average the gas composition is H 2 / 0 2= 2/1, the total apparent turnover numbers that we observed are 19 for R ~ ( b p y ) ~and l + 6 for E d + . Although these values are
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too small to conclude unambiguously that the process is truly catalytic, they clearly show that it is more than simply stoichiometric.
Discussion The most striking feature of the results reported here is undoubtedly the oscillating character of gas evolution. Recombination of H2 and O2 is most probably involved in the falling part of the oscillations. Other “recombination” reactions such as the reduction of Eu3+by H2 or the oxidation of Ru(bpy)?+ by O2 can definitely be ruled out on thermodynamic grounds since a simple calculation based on the Nernst equation shows that H2 or O2 pressures of several atmospheres would be necessary. This is far beyond the maximum pressure reached in our reactor (a few tens of torr). Recombination of H2 and O2 does, however, not alone explain why oscillations occur. The necessary condition for the occurrence of instabilities in chemical systems is the presence of an autocatalytic-or more generally nonlinear-step.I3 In catalytic surface reactions a t a gas-solid interface, such as the oxidation of carbon monoxide or of hydrogen,8 this nonlinear step is likely to be of a thermochemical nature: the exothermic heat of reaction increases the local surface temperature, which in turn increases the reaction rate. Considering the very low reaction rates that we observed here, even in the initial ascending part of the oscillations (-lo-’ M s-l), such surface thermochemical effects are unlikely to be of importance in our liquid reaction medium. Much more likely seem to be the cross-catalytic effects which might arise from the presence of H, and 02, on the one hand, and from the presence of an oxidized (RuO,) and a reduced (Pt) catalyst, on the other. Each of these catalysts is indeed able to catalyze the oxidation or the reduction of the other. We also have to consider the possible effects related to the nature of the colloids that we used. Particularly significant in this respect is the above-mentioned fact that the amplitude of the first oscillations corresponds to a turnover number close to 1for the Eu3+ions. This suggests that an autocatalytic step might be involved in the reduction of the A1,Eul-,(OH),-Pt particles, and that the regeneration of this colloid in its initial form might be a slow step. As far as the primary photochemical events are concerned, it may seem surprising that light-induced electron transfer from a molecule adsorbed on a first particle to an ion embedded in another particle might be fast enough to compete efficiently with the radiative and the nonradiative deactivation pathways. Undoubtedly, the interparticular
Letters
association between colloid,, and colloidredis of primary importance to avoid diffusion of the particles. Once such an association is achieved, electron transfer may be quite efficient. It has been shown for instance that light-induced electron transfer from R ~ ( b p y ) adsorbed ~~+ on swelling clays to Fe3+ions is considerably (approximately two orders of magnitude) more efficient when the acceptor is embedded in the clay lattice than when it is coadsorbed with the Ru complex on the clay surface.I6 Preliminary luminescence lifetime measurements performed with Eu3+ ions as acceptor show that, for equivalent total concentrations, the quenching effect may be more than one order of magnitude larger in a [ ~epiolite-Ru(bpy),~+][Al,Eu,-,(OH),] colloidal system than in a purely homogeneous R ~ ( b p y ) ~ ~ + - E3+u ,solution.” It should be pointed out that the embeAding of the acceptor ions within a host lattice does not only affect the structure of the system (and thereby the local acceptor concentrations) but may also modify the thermodynamic and kinetic parameters (self-exchangerates) for the electron transfer reaction at a fundamental level. Turning now to the efficiency of the system described here for water splitting, one finds that several points have to be enlightened before a valuable conclusion could be drawn about the catalytic nature of the process. In particular, the reason for the short lifetime of the system, possibly related to the occurrence of oscillations, has to be established. Comparison with sacrificial water photooxidation systems based on clay-supported RuO, as oxygen generation catalyst4 suggests that the poisoning of this catalyst might be a lifetime-limiting factor. Finally, we would like to point out that, beside its fundamental interest, the occurrence of oscillations is not necessarily an undesirable feature for the achievement of a practical solar energy conversion device based on the type of photochemistry described here. Indeed, provided that their periodicity could be satisfactorily controlled and that hydrogen and oxygen generation could be separated in time (through the accumulation of intermediates), they might well represent the ultimate way of separating the gaseous products. Further work on the structure of our system, on the redox properties of the A~,Eu~-,(OH)~ colloid, and on the instantaneous nature of the gases is in progress. Registry No. H20, 7732-18-5; R u ( b p y ) P , 15158-62-0;Eu, 7440-53-1. (16) D. Krenske, S. Abdo, H. Van Damme, M. I. Cruz, and J. J. Fripiat, J. Phys. Chem., 84, 2447 (1980). (17) H. Nijs, F. Bergaya, A. Habti, J. J. Fripiat, and H. Van Damme, N o w . J . Chim., submitted for publication.
