J . Phys. Chem. 1984, 88, 1302-1307
1302
all three cases, whereas in the IR spectrum the dominant peaks correspond to the C H oscillator at positions associated with conformational energy minima.36 Cavagnat and L a ~ c o m b edeveloped ~~ a quantum description which explicitly introduced the frequency dependence of the CH-stretching mode of CHD2C6D,on the dihedral angle between the CH bond and the ring plane. Their treatment predicted that the Raman spectrum should be dominated by transitions involving states where the rotation of the CHDz group is free. Such transitions occur with a frequency corresponding to the average for the C H vibration during rotation. The treatment also leads to the existence of such a peak in the IR spectrum thus accounting for the observations of McKean et a1.35336 Because of the correlation between B and rcH5,8,14 and since rCH is a function of the dihedral angle 8, one might anticipate that a distribution of frequencies over all possible rCHshould be observed for the freely rotating methyl group. However, the central peak which we have assigned to the freely rotating methyl group is relatively narrow. This situation could well be analogous to that which occurs in N M R couplings37 and ESR hyperfine s p l i t t i n g ~also , ~ ~involving freely rotating methyl groups. In both cases, a quantum-mechanical treatment demonstrates that the ensemble average (sinz %) must be computed before the value of the observable can be predicted. This latter result will correspond to the average over the distribution and not to the whole range of values in the entire distribution. This value of the observable can also be approximated by the classical average. The methyl C H bond lengths from the optimized geometries13 follow the (37) W. J. E. Parr and T. Schaefer, Acc. Chem. Res., 13, 400 (1980). (38) P. B. Ayscough, M. C. BriceTand R. E. D. McClung, Mol. Phys., 20, 41 (1971).
expected sin2 % dependence. The classically averaged value of the bond length is 1.0944A, which is virtually identical with the value calculated on the basis of the overtone frequency shifts (1.0946 A). Thus, it appears that, for toluene, m-xylene, and p-xylene, the most reasonable assignment of the central peak in the methyl region is to transitions originating from rotational levels with energies above the rotational barrier.
Summary The overtone spectra of toluene and the xylenes in the gas phase are found to ,provide direct experimental evidence of subtle asymmetry in the ring, with variations in rcH(aryl) of up to 0.003 A. The two inequivalent CH bonds in the methyl group of oxylene lead to well-resolved spectral peaks which are useful in assigning the more complex toluene and m- and p-xylene spectra. A possible interpretation of the methyl regions of these latter spectra involves "free-rotor'' states. Nonbonded interactions, as determined from ab initio 4-21G calculations, account for the observed variations in methyl C H bond length. Both the aryl and methyl C y bond lengths determined from overtone frequency shifts are in better agreement with 4-21 calculation^^^ than those from any other technique. Acknowledgment. We are grateful to Professor T. Schaefer and to Drs. W. Siebrand and T. Wildman for helpful discussions. We are indebted to Dr. J. D. Goddard for a modified version of the GAUSSIAN80 program. K.M.G. is grateful to the University of Manitoba for a graduate fellowship. Lastly, we are grateful to the National Sciences and Engineering Research Council for financial support. Registry No. Toluene, 108-88-3; o-xylene, 95-47-6; m-xylene, 10838-3; p-xylene, 106-42-3.
Optimization of Conditions for Photochemical Water Cleavage. Aqueous Pt/TiO, (Anatase) Dispersions under Ultraviolet Light J. Kiwi* and M. Gratzel Institut de Chimie Physique, Ecole Polytechnique F2dZrale. CH- 1015 Lausanne, Switzerland (Received: May 26, 1983; In Final Form: August 22, 1983)
This paper describes the preparation and optimization of a water-cleavage catalyst consisting of Pt/TiOz (P-25 Degussa). Two different methods were used to prepare the catalyst: impregnation followed by reduction (calcination in certain cases) and exchange. HzPtC16and Pt(NH,),(OH), were used as base materials. The efficiency of the catalyst was tested in relation to its ability to mediate Hz evolution for water under UV light. The influence of pH, temperature, concentration of Pt, as well as preparation temperature and time of reduction are described. Characterization by electron microscopy (EM), surface area measurements (BET), diffuse reflectance spectroscopy (DRS), hydrodynamic particle radius Rh,and electrophoresis is carried out. It is shown that metal islands with a particle size below 8 A deposited onto TiOz agglomerates of 3000 < Rh < 6000 A are the most active catalytic species. The fate of oxygen produced by conduction band holes is also investigated.
Introduction The problem of H z evolution in sacrificial' as well as cyclic photoinduced processesZis a topic of relevant research in several laboratories working in the area of energy-conversion processes. ~~~~
~
~
~~
~~
~~
~
~
~
~~
~
Lately, a need has arisen to improve deposition techniques of small metal islands of platinum over the existing procedure^.^,^ Pt5,6 deposited on a photosensitive support like TiOz has been shown to be a suitable material to use in photocleavage processes. TiOa has also recently received attention as a noncoventional support
~
(1) (a) B. Koryakin, S. Dzhabiev, and E. Shilov, Dokl. Akad. Nauk. SSSR, 233,359 (1977); (b) J.-M. Lehn and P. Sauvage, Nouu. J . Chim., 1, 449 (1977); (c) K. Kalyanasundaram, J. Kiwi, and M. Gratzel, Helu. Chim. Acta, 61, 2720 (1978); (d) J. Kiwi and M. Gratzel, J . Am. Chem. SOC.,103, 2939 (1981). (2) (a) J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca, and M. Gratzel, Angew. Chem., In?. Ed. Engl., 19, 647 (1980); (b) J.-M. Lehn, P. Sauvage, R. Ziessel, and L. Hilaire, Isr. J . Chem., 22, 168 (1982); (c) E. Borgarello, J. Kiwi, E. Pelizzetti, M. Visca, and M. Gratzel, J. Am. Chem. SOC.,103,6324 (1981); (d) J. Kiwi, K. Kalyanasundaram, and M. Gratzel, Struct. Bonding (Berlin), 49, 37 (1982); (e) J. Kiwi, E. Borgarello, E. Pelizzetti, M. Visca, and M. Gratzel in 'Photogeneration of H2", Academic Press, London, 1982, p 119.
0022-3654/84/2088-1302$01.50/0
(3) (a) J. Turkevich, K. Aika, L. Ban, J. Okura, and S. Namba, J . Res. Inst. Catal., Hokkaido Uniu., 24, 54 (1976); (b) J. Kiwi and M. Gratzel, Nature (London), 281 657 (1979); (c) E. Borgarello, K. Kalyanasundaram, Y . Okuno, and M. Gratzel, Helu. Chim. Acta, 64, 1937 (1981); (d) J.-M. Lehn, J. Sauvage, and R. Ziessel, Nouu. J . Chim.,5, 291 (1981); (e) P. Keller and A. Moradpour, J . Am. Chem. SOC.,102, 7193 (1980); (f) A. Mills and G. Porter, J. Chem. Soc., Faraday Trans. 1,78,3659 (1982); (g) A. Harriman and G. Porter, J . Chem. SOC.,Faraday Trans. 2, 78, 1937 (1982). (4) H. Miles and A. Thomason, J . Electrochem. SOC.,123, 1459 (1976). (5) A. Heller, "Semiconductor Liquid Junction Solar Cells", The Electrochemical Society, Princeton, NJ, 1977, pp 195-208. (6) H. Gerischer, Pure Appl. Chem., 52, 2649 (1980).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1303
Photochemical Water Cleavage
45
0.5
0
1.0
'~ 1.5 2.0 w t % Pt on T i 0 2 - ~
Figure 1. Media rate of evolution of H2 as a function of Pt loading for
methanol-impregnated samples of Tiqz. The reported H2 evolution rate was a medium value for a 24-h irradiation with a 450-W Xe lamp. Solutions were 25 cm3of 0.1 N NaOH and contained 40 mg of catalyst.
-
0.09
t .-ON F
E" .
0.06
-
U
0
P, vi 0
--0.03 I!
%
I
D
E
I 0
I
I
2
4
I
I
6 8 exchange pH-
25 cm3. Prior to photolysis, the samples were flushed with highly purified N 2 for the removal of oxygen. Hydrogen was analyzed by gas chromatography using a Carbosieve 5-8, column at 40 OC and a Gow-Mac conductivity detector. The irradiation flasks were so modified that no grease or rubber part was exposed to solution vapor during irradiation, which is especially important when irradiations at pHs above pH 8 were carried out. The impregnated Pt/TiO, catalysts were prepared by wetting the TiO, (P-25 Degussa) with the minimum amount of alcohol containing H,PtCl, in order to obtain catalysts with different loadings. Methanol was used as dispersing media. Such dispersions have been shown to produce smaller size platinum clusters upon reduction of the catalyst as compared with similar catalysts prepared in water.l2,l3 The slurry having the consistency of a thin paste was dried in a vacuum oven (lo-, P) at room temperature for 48 h and stirred regularly during the drying process to retain ~ n i f o r m i t y . ' ~The resulting powder was then placed in a silica tube flushed with N 2 while raising the temperature and subsequently reduced in a H2 flow (80 cm3/min) at 425 "C for 18 h. In this way, the platinum ions on the surface of TiO, were reduced to the metallic state. Ion-exchanged Pt/Ti02 catalysts were prepared by dropwise addition of an appropriate amount of H2PtC16to a stirred aqueous suspension of TiO,. The pH of such additions was 4.5 since it was shown to be the pH for maximum adsorption. The solution was stirred for 24 h, when the equilibrium had been established, and the equilibrium concentration of the acid in the solution was determined spectrophotometrically (in centrifuged and washed samples) at 280 nm. The adsorption isotherm was expressed as the dependence of the adsorbed quantity of H2PtC16 per unit weight of the support on the equilibrium concentration in the solution. In separate cation-exchange experiments, Pt(NH3)4(OH), (Johnson Matthey, Chem. Ltd.) was adsorbed from dilute solution of TiOz at pH 8.5. The most efficient catalyst containing 0.05 wt. % Pt on TiO, was prepared in the following way: 40 KL of Pt(NH,),(OH), was dropped into 150 cm3 of distilled water containing 900 mg of TiO,-(P25) and stirring was continued for 24 h. The solution was centrifuged 30 min at 20000 rpm and washed twice with 150 cm3 of 0.01 M ",OH solution to eliminate the egcesp of Pt(NH,),,+ residing on the surface of the catalyst. The supernatant of solutions contacted for a few seconds was examined. The spectrum of Pt(NH3)42+decreased, indicating impregnation by Pt(NH,)42+. This spectrum of Pt(NH3)42+was further decreased after a 24-h exchange had taken place with TiOz ( 50% more). That the platinum cations are strongly absorbed by TiO, is shown by their resistance to removal by washing as shown in the Experimental Section. Washing was performed with 0.01 M ",OH to remove occluded solution. N o ammonia is left on the catalyst in the preparation and whatever Pt(NH3)42f may be present is destroyed by ammonia volatilization. Our electron microscopy studies (see Figure 8b) show that ion exchange with Pt(NH3)?+ renders a catalyst with a more even distribution of platinum particles than does impregnation and exchange with H,PtCl6. The slurry was dried 3 h at 120 "C in air and reduced at 425 O C in a H, stream at 80 mL/min as mentioned for the impregnated samples. Electron microscopy (TEM) was carried out with a Phillips300s instrument. The limit of resolution for such an instrument is 3.5 8,. Diffuse reflectance spectroscopy (DRS) used a Perkin Elmer-Hitachi Model 340, equipped with an integrating sphere. The base line was determined relative to the response of the instrument to a calibrated sample of MgC03.1S The electrophoretic mobility was measured with a commercial Mark I1 microelectrophoresis apparatus (Rank Bros., Cambridge, England). A cylindrical cell was used and the velocity of 20 particles was determined at each stationary level, reversing the polarity N
Figure 2. Adsorption from solution containing H2PtCl6onto TiO: P25)
as a function of pH at room temperature. for noble metals such as R u , Ni,* ~ and Pt.9 The objective of the present study is to determine the respective role of Pt and TiO, in photocatalytic water decomposition. Impregnation and exchange of Pt have been extensively studied on S O 2 , A1203,and zeolites for their well-known catalytic applications.lOqll A detailed study is here undertaken for Pt-loaded Ti0,. Improved catalysts described in this work will prove to be useful in accelerating reactions leading to water decomposition. The Pt-TiO, catalyst consists of fused spherical particles of 140-A diameter with a hydrodynamic radius of ca. 6000 8, (0.6 r m ) .
Experimental Section Results shown in Figures 1 and 2 were obtained by mixing appropriate amounts of H,PtCl, with T i 0 2 (P-25 Degussa) (50 m2/g surface area and a negligible pore volume). Continuous photolysis experiments were carried out with an XBO-450-W xenon lamp. The volume of the irradiated solutions was always (7) A. Vannice and L. Garten, J . Catal., 63, 255 (1980). (8) A. Vannice and L. Garten, J . Catal., 56, 236 (1980). (9) J. Tauster, C. Fung, and L. Garten, J . Am. Chem. Soc., 56, 170 (1980). (10) (a) R. Ferrauto, AICE Symp. Ser., 70, 9 (1979); (b) H. Sinfelt, Catal. Rev., 3, 175 (1969); (c) M. Boudart, W. Aldag, L. Ptak, and H. Benson, J. Catal., 11, 35 (1968); (d) E. Drauglis and I. Jaffe, "The Physical Basis of Heterogeneous Catalysis", Plenum Press, London, 1975. (11) P. Grange, P. Jacobs, and G. Poncelet, "Scientific Basis for the Preparation of Heterogeneous Catalysts, 3". Elsevier, London, 1983.
(12) R. Zsigmondy, "Kolloidchemie, Verlag Otto Spamer, Leipzig, 1927. (13) (a) G. Blanchard, H. Charcosset, H. Dexpret, E. Freund, C. Leclercq, and G. Martino, J . Catal., 70, 168 (1981); (b) L. Tournayan, G. Blanchard, and H. Charcosset in "Adsorption at the Gas-Solid-Liquid Interface", 3. Rouquerol and K. Sing, Eds., Elsevier, Amsterdam, 1982 (14) H. Muira and R. Gonzalez, J . Phys. Chem., 86, 1577 (1982). (15) A Tseung and H. Bevan, J . Mater S c i , 5 , 604 (1976)
1304
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Kiwi and Gratzel
for successive timings. The average velocity was calculated by dividing the average velocity by the field strength (potential/ distance between the electrodes), All measurements were made at 25 f 0.1 ‘C. Quasielastic light scattering measurements were carried out by means of a Malvern scattering unit equipped with a BI 2020 correlator and the angle of observation was carried up to 14O0l6to derive diffusion coefficients for TiOz agglomerates.
Results and Discussion Studies with Impregnated Samples of TiO,. Figure 1 presents the results of the H2 evolution rate for UV irradiation of a platinized catalyst of TiOz loaded with different amounts of platinum. In all cases, 35 mg of the Pt/TiQ2 catalyst was used under stirring and a flux of 200 mW/cm2 of the 450-W Xe lamp was used. If light is adsorbed within the band gap of TiO,, an electron-hole pair is created near the surface: Ti02
hu
hf
+ e-
(1)
The photoproduced electron is subsequently channeled to Pt sites where an ohmic contact develops and hydrogen evolution occ u r ~ . ~ ’ -It~has ~ been shown that the efficiency of H, production (and other photolysis products) is related to the possibility of the created charge to transfer through catalytic sites (Pt) to the solution.20 Hydrogen production at Pt sites is expressed by eq 2. Pt-
+ H,O
-
Pt
+ OH- + ‘/,HZ
w t % Pt on Ti0,-
Figure 3. Media rate of Hz evolution for exchanged-impregnated Pt(NH3)4(0H)z/water/Ti02 samples as a function of Pt loading. Other irradiation conditions as in Figure 1. Crystallite size of Pt on TiO, is
shown on the right-hand side of the ordinate. For more details about crystallite size, see Figure 8.
(2)
Figure 1 presents the observed H, evolution as a function of loading of the catalyst obtained by impregnating TiO, with H2PtC16in alcoholic solutions as mentioned in the Experimental Section. From Figure 1 it is readily seen that, at 0.5% loading of Pt on TiO,, the maximum activity for Hz generation is obtained and that the dispersion of the metallic deposit of Pto(as later shown in the electron micrograph in Figure Sa for the impregnated sample) gives crystallites with a size of -34 A. The size of these crystallites is independent of the amount of Pt loading of the catalyst up to 2%. This experimental observation is in agreement with size effects observed for crystallites of platinum on polymers which are active in H2 evolution induced by light?1,22 The factors thought to be essential in controlling catalyst activity, stability, and selectivity in the case of light-induced H2 evolution are temperature and time of reduction by H,. Optimal yield of H2 was attained in the present studies by reduction at 425 ‘C for a period of 18-24 h. It seems that a high temperature of reduction is important to attain the crystallographic form of Pt active on Ti02 for photoinduced H, evolution processes. Tauster et al.23reported the formation of separate intermetallic phases (SMSI) when Ti0, and H2PtC1, were heated under H2 at temperatures close to 500 ‘C. This SMSI effect has already been reported to be responsible in photocatalytic processes involving irradiated Pt/TiQ2 surfaces in organic reaction^.^"^^ Spectroscopic measurements25have (16) B. Chu, “Laser Light Scattering”, Academic Press, New York, 1974. (17) W. Latimer, “The Oxidation States of the Elements and Their Potentials in Aqueous Solution”, Prentice-Hall, Englewood Cliffs, NJ, 1949. (18) H. Hardee and A. Bard, J . Electrochem. SOC.,123, 1024 (1976). (19) Since anatase already possesses excess free electrons, the photoproduction of holes (h+ in eq 1) is the limiting process involved in the photocatalytic reaction. (20) B. Kreutler and A. Bard. J . Phvs. Chem.. 24. 3146 (1979). (21j P. Keller and A. Moradpour, J . k m . Chem. Sbc., 102, 7193 (1980). (22) R. Anderson, Adv. Catal.,23, 1 (1973). (23) S. Tauster, S. Fung, and L. Garten, J . Am. Chem. Soc., 100, 170 (1978). (24) H.Courbon, J.-M. Herrmann, and P. Pichat, J . Cat& 72, 129
(1981).
t PtlAI
(25) Y. Chung and W. Weissbard, Phys. Rev., Sect. E , 21, 1344 (1980). (26) J. Tauster, S. Fung, R. Baker, and J. Horsely, Science, 211, 1121 (1981). (27) E. Rideal, “An Introduction to Surface Chemistry”, Cambridge University Press, Cambridge, 1946. (28) J. Anderson, “The Structure of Metallic Catalysts”, Academic Press, New York, 1975. (29) “Handbook of Chemistry and Physics”, Chemical Rubber Publishing Co., Cleveland, OH, 1982.
0
40
20
60
80
rng c a t a l y s t / 2 5 cc-
Figure 4. Media rate of H2evolution under UV irradiation for 25-cm3 solutions of 0.1 M NaOH containing 0.05% Pt by weight on TiOz. Catalysts were added in the amounts shown; a 450-W lamp was used.
shown that partial electron transfer from Ti3+to Pt in an SMSI type of interaction may occur. Figure 2 represents the adsorption from solution of H,PtC16 onto TiO,-(P25). Clearly a marked pH effect controls this process. Equilibration for the observed process requires 24 h. It is known that ion exchange in acid media takes place between inorganic oxo acids30and hydroxyl groups3’ on TiO, by an anion-exchange mechanism. Optical absorption at 280 nm was used to follow this process. A maximum absorption at pH 5 is observed in Figure 2. This absorption may be indicative of the hydrolytic species Pt(C14(OH),(H20)2-,.28~3z For Ti02-(P25), B ~ e h mdemon~~ strated that half of the O H is acidic in character and the other half is mainly basic. Ionization of hydroxyl groups takes place in either of the following two ways: TiOH TiOH
+ H+
+ OH-
-
-
TiOH,+
TiO-
+ H,O
(3a) (3b)
(30) J. Beukenkamp and D. Herrington, J . Am. Chem. Soc., 82, 3025 (1960). (31) A. Boonstra and C. Mutsaers, J . Phys. Chem., 79, 1694 (1975). (32) G. Parfitt, Pure Appl. Chem., 48, 415 (1976). (33) P. Boehm, Discuss. Faraday SOC.,52, 264 (1971).
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984
Photochemical Water Cleavage
1305
t 0
.
v)
cv
100
15
45
30 hours-
2
4
6
8
10
12
14
I
I
Figure 6. Amount of peroxotitanate formation as a function of time for 0.1 N NaOH solution under UV light, using a 0.05% Pt/Ti02 exchan-
ged-impregnated catalyst. H2 production as function of time is shown on the right-hand side.
1
20001
TiOz will, therefore, behave as an anion exchanger in acid and neutral solution and a cation exchanger in basic solution. These electrostatic effects also play a role in PtC16'- interaction with the TiO, ~ u r f a c e . ~ Adsorption ~-~~ of this type will be favored in a positively charge TiO, surface. The IEP for TiO,-(P25) is 6.5 as shown in Figure 10. Similar effects as reported in Figure 2 for H,PtC16-Ti0, interaction have been observed for H,PtC16 adsorption and subsequent deposition on A1203.37 Hydrogen Generation with Pt(NH3)r2+ Exchanged TiO? Figure 3 presents rates of H2generation as a function of Pt loading for Pt(NH3)42+exchanged catalysts on Ti02-(P25) at pH 8.5. The most efficient catalyst contained 0.05% Pt on Ti02 by weight, the initial rate in 0.1 N N a O H being (75 p L of H,/h)/25 cm3. The decreasing portion of the curve above this optimal value is probably due to catalysis of the H,/O, back-reaction which occurs simultaneously with H2generation. A similar favorable effect for Pt(NH3)42+vs. H,PtC16 has been reported for SiOz impregnat i o n ~ . ~Optical * ~ ~ absorption curves were run with this material at pII 8.5. Figure 4 shows the effect of catalyst concentration on the hydrogen generation in 0.1 N N a O H solutions irradiated under UV light. The increase in catalyst concentration increases the light absorption of the solution and hence the H2 Details regarding the effect of the amount of catalyst intervening in a reaction have previously been r e p ~ r t e d . ~ * ~ ~ - ~ ~ Figure 5 presents the results for the rate of H, evolution as a function of pH. An optimum value is found around pH 14. The shape of the H2 evolution as a function of pH resembles the shape of the surface titration curve for Ti02.45 It is important to note (34) R. Hoekstra, S.Siegel, and X. Gallagher, Adu. Chem. Ser., No. 98, 39 (1971). (35) P. Boehm and M. Herrman, Anorg. Allg. Chem., 352, 156 (1967). (36) G. Bond, Platinum Met. Reu., 19, 126 (1975). (37) R. van Hardeveld and F. Hartog, Adu. Catal., 22, 75 (1972). (38) H. Benesi, M. Curtis, and P. Studer, J. Caral., 10, 328 (1968). (39) R. Dalla Betta and M. Boudart in "Proceedings of the 5th International Congress on Catalysis", J. W. Hightower, Ed., North-Holland, Amsterdam, 1973. (40) D.Cormack and R. Moss, J . Catal., 13, 1 (1969). (41) J. Kiwi, Chem. Phys. Lett., 83, 594 (1981). (42) R. Smith, "Semiconductors", Cambridge University Press, Cambridge, 1978. (43)N . Hannay, "Semiconductors", Reinhold, New York, 1959. (44) R. Blickley and S . Stone, J. Caral., 31, 389 (1973). (45) S.R. Morrison, 'The Chemical Physics of Surfaces", Plenum Press, New York, 1977.
hours
-
Figure 7. UV irradiation of 0.05 Pt/TiO, catalyst, 40 mg in 25 cm3 of
lo-' M N a O H as a function of time of irradiation.
that, of the hydroxyls present on the surface, approximately half react fairly acidic with a pK of 2.933 (eq 3a) and the remainder are weakly acidic with a pK of 12.7. From the present results it is seen that these O H groups at the surface play a dominant role in the yields of Hz obtained from photolysis. The role of surface OH groups is to intervene in 0, uptake by the TiO, particles and hole scavenging to produce 0- or ~ x y g e n . ~ ~ - ~ * Fate of Oxygen Produced in Water Photolysis. Analysis of the gas produced during illumination showed that only hydrogen is generated and no oxygen appears in the gas phase. It was, therefore, interesting to examine the final reaction product resulting from the valence band process with water. Analysis of irradiated solutions by titration with K M n 0 4 showed that peroxides were produced. Figure 6 presents peroxide formation as a function of time in a 0.1 N NaOH solution containing 40 mg of 0.05% Pt/TiOz catalyst. At the right-hand side the concomitant H, production during this process is presented. Peroxotitanates were titrated with M K M n 0 4 by following the decrease in the optical absorption at 525 nm of an acidified solution at pH 0. Centrifugation of the irradiated slurry allows one to distinguish the (46) J. Harbour and M. Hair, J . Phys. Chem., 83, 652 (1979). (47) K.Hauffe, H. Raveling, and D. Rein, Naturwissenschaften, 64,91 (1977). (48) P. Jackson and G.Parfitt, Trans. Faraday SOC.,67, 2469 (1971).
1306 The Journal of Physical Chemistry, Vola88, No. 7, 1984
peroxocomplex in the supernatant from that adsorbed on the powder. As the irradiation proceeds in time, more peroxo compound was found to be adsorbed on the precipitate and less in the supernatant liquid. A second centrifugation was necessary for irradiation beyond 30 h to desorb and titrate the total peroxide formed. We suggest from available literature data49the peroxo compound to be a r-peroxo-bridged titanium multimer and shall elaborate on this in a forthcoming paper. At 46-h irradiation 7.5 X mol of H2/(25 cm3 of solution) are produced while only 1.1 X mol of peroxide complex is formed. A ratio of 6.6:l for both species indicates that the titrated complex is only a fraction of the possible oxygenated species produced in the reaction. The remaining part must be absorbed oxygen in the form of either physisorbed, chemisorbed, or reduced 02-specie^.^^,^' Surface-bonded oxygen species can be eliminated by N 2 bubbling and this is shown in Figure 7. This figure shows Hz formation in a truly cyclic nature. The same initial rates were obtained for hydrogen production whenever irradiations were not carried out for more than 24 h. Bubbling in the first three recyclings will eliminate hydrogen and oxygen species adsorbed to Ti02. Therefore, reactivation of the hydrogen-generating process is possible when the solution is degassed with N2. Only 0.19% C was found on the Pt/Ti02 samples used. Since over 20 cm3 was made by recycling the sample 15 times, the photoproduced Hzcannot be ascribed to the oxidative C present in the catalyst. (Note Added in ProoF These experiments show that the amount of H2 obtained exceeds the concentration of Ti in solution indicating that the reaction is catalytic with respect to TiOz and is not due to the reaction Ti3+ H 2 0 Ti4+ OH- '/zH2 as recently proposed by K r a ~ n a In . ~ this ~ way, Hz generation and the formation of an oxidative species has been achieved in separate phases in the photochemical water cleavage process.) If irradiations are carried out up to saturation (46 h), the peroxide formed seems to irreversibly affect the catalyst. Subsequent runs show only 40% hydrogen generation efficiency (( -35 pL/h)/(25 cm3 of 0.1 N NaOH solution)) over a 24-h period. The present results illustrate the capability of the particles to split water into hydrogen in a cyclic fashion using Pt-loaded Ti02 catalysts. The initial O2formed in water dissociates due to hole interaction at the T i 0 2 surface46
-
+
4ht
+ 2Hz0
-+
4H'
+
+0 2
+
(4)
is rapidly followed by electron attachment to O2on the Ti0z18*20
Ti02
O2
+
Kiwi and Gratzel
O2-
Salvador has recently given some evidenceSothat basic hydroxyl groups (band gap surface states) mediate the transfer of electrons from the conduction band to adsorbed oxygen molecules. The 02presumably formed gives peroxo-bridge Ti dimers with subsequent reaction on the particle surface. Characterization of Pt(NH3)42+Exchanged-Zmpregnated Catalysts. Figure 8a represents the results of electron microscopy studies on 0.5% Pt/TiOz. Samples were obtained by impregnation (shown in Figure 1); 60% of the Pt islands were 34 8, in size. The size distribution for Pt was determined by counting only spherically shaped particles which exhibited higher contrast against the light areas of the support. Work was carried out at 100000 V to obtain highest resolution in the electron microscope. Figure 8b presents results for 0.05% Pt/Ti02 samples prepared by exchange-impregnation (as cited in Figure 3). More than 65% of the Pt particles were 510 a in size, a value close to the resolution of the instrument. For 34-8, Pt particles about 1000 Pt atoms are in a cluster36having a dispersion >33%; 10-8, Pt islands contain fewer than 100 atoms and a dispersion >90% on the support. This partly explains the higher H2 evolution rate obtained with this catalyst. If each surface site is a surface hydroxyl45 and if one (49) (a) J. Muhlebach, K. Muller, and G. Schwarzenbach, Inorg. Chem., 9,2381 (1970);(b) M. Mori, M. Skibata, E. Kyuno, and S . Ito, Bull. Chem. SOC.Jpn., 29, 904 (1956). (50) P. Salvador and C. Gutirrez, Chem. Phys. Lett., 86, 131 (1982).
Figure 8. Electron micrographs of samples with magnification 260 000 X: (a) preparation by impregnation in alcohol of 0.5% Pt/TiO, as shown in Figure 1, (b) preparation by exchangeimpregnationin H,O for 0.05% Pt/TiO, as shown in Figure 3.
knows that 1015sites/cm2 (ref 33 and 51) are available and the BET area of P-25 Degussa is 50 m2/g (50 X l O I 9 sites/g), it follows that for 0.05% Pt/Ti02 loading, a coverage of 1.5 X lo'* atoms of Pt reside on 5 X lozosites/g of Ti02. Then about 0.3% of the available area is covered by platinum. Electron micrographs shown in Figure 8b roughly confirm this estimate. Figure 9a shows the diffuse reflectance spectra (DRS) for Ti02-(P25) loaded by impregnation (as shown for H2evolution results presented in Figure 1) with different percentages of Pt on the TiOz. The absorption increases when the loading increases up to 2%. The inflection in the curves at -400 nm is due to a band gap for Ti02 of 3.2 eV. The absorbance up to 900 nm shows the Pt loading. Trace 1 refers to Ti02-(P25). Curves 2-6 show T i 0 2 with Pt loadings of 0.1%, 0.2%, OS%, 1%, and 2%, respectively, on Ti02. Figure 9b presents Ti02(trace 1 and loadings of 0.025%, 0.05%, 0.1%, 0.2%, and 0.4% in traces 2-6, respectively. The Pt was deposited from Pt(NH3)42+by exchange-impregnation. It is interesting to note that a loading of 0.4% Pt gave an optical absorption of 0.75 in Figure 8b, while a loading of 2% Pt gave an optical density of 0.72 in Figure 9a. Then a more uniform dispersion of Pt on TiO, when exchange-impregnation was performed as compared to impregnation is reflected by these results. A higher number of smaller Pt islands of 10 8, (as shown in Figure 8b) takes place in Figure 9b. Therefore, the receptivity of the surface to form smaller agglomerates (Figure 9b) verifies the experimental observations by electron microscopy for both methods of Pt loading on TiO,. By impregnation, higher loadings of Pt on the surface formed bigger agglomerates. This is reflected in (51) T.Apple, P.Gajardo and C. Dybowski, J . Coral., 68, 103 (1981)
The Journal of Physical Chemistry, Vol. 88, No. 7, 1984 1307
Photochemical Water Cleavage nm-
r
300 I
600
900
I
I
a
0.9 '
Y \l
0.6
4
\ \\
t
\
0
0.3
'
+lD
t
.
5
3
-
\
0
+
1
I
1
b
-
0.9
Figure 10. Electrophoretic mobility as a function of pH for 0.01 N
+
0.6
(NaOH HCI) solutions of (1) Ti02-(P25),(2) exchanged-impregnated 0.05% Pt/TiO,, (3) 0.1% Pt/Ti02 (4) 0.2% Pt/Ti02, (5) 0.4% Pt/Ti02, (6) 0.8% Pt/Ti02, and (7) Pt alone.
-
t
\
0
0
0.3
t I
0
I
I
I
600
300
900
Figure 9. Diffuse reflectance spectra (DRS) for the following: (a)
samples prepared by impregnation (as shown in Figure 1) and loadings of (1) 0% Pt, (2) 0.1% Pt, (3) 0.2% Pt, (4) 0.5% Pt, (5) 1% Pt, and (6) 2% Pt on TiO,; (b) samples prepared by exchange-impregnation (as shown in Figure 3) and loadings of (1) 0% Pt, (2) 0.025% Pt, (3) 0.05% Pt, (4) 0.1% Pt, (5) 0.2% Pt, and (6) 0.4% Pt on TiO,. higher OD values and poorer catalytic results as shown in DRS studies. Figure 10 represents the results obtained for electrophoretic mobility at different pH values and constant ionic strength. The constant ionic strength was maintained by mixing the proper amounts of 0.01 N NaOH + 0.01 N HCl to yield the pH values shown in Figure 10. In this way the width of the double layer was kept constant throughout this study. The ordinate shows the mean electrophoretic velocity divided by the field strength yielding the reported mobility. Curve 1 shows an isoelectric point (IEP) of 6.5 for T i 0 2 and this value agrees well with the value of 6.6 reported by the m a n u f a c t ~ r e r . This ~ ~ value for the IEP also shows that chloride content in the TiO, used is low (
