The Journal of
Physical Chemistry
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0 Copyright, 1985, by the American Chemical Society
VOLUME 89, NUMBER 8
APRIL 11,1985
LETTERS Cadmium Sulfide Photocatalyzed Hydrogen Productlon from Aqueous Solutions of Sulfite: Effect of Crystal Structure and Preparation Method of the Catalyst Michio Matsumura, Satoru Furukawa, Yukinari Saho, and Hiroshi TsubOmura* Laboratory for Chemical Conversion of Solar Energy and Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: July 6, 1984; In Final Form: February 5, 1985)
Platinum-loaded CdS powder having a hexagonal crystal structure has been found to be much more efficient as a photocatalyst in hydrogen production from aqueous solutions of sulfite than that having a cubic crystal structure. It has also beem found that the activity of the photocatalyst is lowered by the mechanical damage caused by grinding, which increases the number of electron-hole recombination centers. On the basis of these results, the method of preparation of the photocatalyst has been improved and the hydrogen production quantum efficiency reached 28%at 30 O C and 35% at 60 “C,more than 3 times that reported previously.
Introduction Hydrogen production by use of semiconductor photocatalysts has recently received much attention in view of solar energy utilization. Photocatalysis has been studied for TiO2?-’ SrTiOj,’ and CdS.&l0 Hyrogen was produced in the presence of sacrificial ) Matsumura, M; Saho, Y; Tsubomura, H. J . Phys. Chem. 1983,87,
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!) (a) Kawai, T.; Sakata, T. J . Chem. Soc., Chem. Commun. 1980,694. iakata, T.; Kawai, T. Chem. Phys. Lett. 1981, 80, 341. I) Kawai, T.; Sakata, T. J . Chem. Sa.,Chem. Commun. 1919, 1047. I) Kawai, T.; Sahta, T. Nuture (London) 1980, 286, 474. 1) Kawai, T.; Sakata, T. Chem. Lett. 1981, 81. I) John, M. R. St.; Furgala, A. J.; Sammells, A. F. J. Phys. Chem. 1983. - 01. -(7) Mills, A.; Porter, G. J. Chem. Sa.,Faraday Trans. I , 1982.78, 3659.
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(8) Thewissen, D. H.M. W.; Tinnema~,A. H. A.; Ecuwhorst-Reinten, M.; T i m e r , K.; Mackor, A. N o w . J. Chim. 1983, 7, 191. (9) Borgarcllo, E.; Erbs, W.; Graetzel, M. Nouu. J . Chim. 1983, 7, 195. (10) Matsumura, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H. J . Phys. Chem. 198488, 248.
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reductants such as alcohols,2J0 carbohydorates$ solid carbon,3 et^.^ We have reported, independent of some other group,11J2hydrogen production under visible light from aqueous solutions of sulfite by using platinum-loaded CdS powder, hydrogen being produced by the sacrificial oxidation of sulfite ion to sulfate and dithionate ions.’ Hydrogen production coupled with dithionate formation is an endergonic reaction, while that coupled with the formation of sulfate is exergonic. As sulfate and dithionate are produced with a 4:3 molar ratio as reported previously,’ the energy stored in the whole reaction is calculated to be approximately 20 kJ/mol of sulfite consumed. Though the energy stored is not large, the result is significant in that useful hydrogen, sulfate, and dithionate are produced from a polluting (11) (a) Buhler, N.; Meier, K.;Rekr, J.-F. J. Phys. Chem. 1984,88,3261; (b) Reber, J.-F.;Meier, K.; Buhler, N. ‘4th International Conference on Photochemical Conversion and Storage of Solor Energy, Book of Abstracts”, Jerusalem, 1982, p 252. (12) Aruga, T.; Jhmen, K.; Naito, S.;Onishi, T.; Tamaru, K. Chem. Lcrr. 1983, 1037.
0 1985 American Chemical Society
Letters
1328 The Journal of Physical Chemistry, Vol. 89, No. 8, 1985
TABLE I: Rate of Hydrogen Production from an Aqueous Solution of Na2S03at pH 8.7 by Using CdS Photocatalysts Obtained from Various Commercial Sourcs and Their Main Crystal Structure rate of Hz production, 1 0-5 main crystal CdS powder“ mol m i d structureb Furuuchi Chem. 4.0 hexagonal Mitsuwa Chem. (A) 3.8 hexagonal Mitsuwa Chem. (B) 0.4 cubic Kojundo Chem. Lab. 0.4 cubic Katayama Chem. 0.3 cubic
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‘CdS powder was loaded with 2 wt % Pt powder. bThe crystal structure existing in more than 80% of the specimen. material, sulfur dioxide. The quantum efficiency for hydrogen production previously reported by us1was 8.2% a t 436 nm, and the rate of hydrogen production under illumination by a solar simulator (AMI, 1.00 kW m-2) was ca. 0.09 mol m-2 h-l. Reber et al.” have reported nearly the same rate, Le., 0.04 mol m-2 h-l, under natural sun light (0.55 kW m-2). In the search for higher reaction efficiencies, we have studied the effect of crystal structure of the CdS powder and other factors on the photocatalytic activity. Experimental Section CdS powder specimens were obtained from several commercial sources. X-ray diffraction measurements showed that they were a mixture of cubic and hexagonal crystal structures with good cxrystallinity. The ratio of the two crystal structures in the various specimens differs. They were used as obtained or after heat treatment in a nitrogen atmosphere. The platinum loading of CdS powder was carried out by shaking the CdS powder with platinum powder (Japan-Engelhard) in a glass vessel for 1 h, unless otherwise stated. The Pt-loaded CdS powder prepared according to this method showed a high efficiency and good reproducibility as a photocatalyst. The deviation in the efficiency was within 10% when the photocatalyst was prepared from the same CdS and Pt powder. The photocatalytic reaction was performed in a 100-mL flask, to which the Pt-loaded CdS (250 mg) and an aqueous solution of 1 mol dm-3 Na2S03buffered at pH 8.7 (20 mL) were added. After the air in the flask was removed by repeated freeze-pumpthaw cycles, the flask was illuminated from the bottom by a 500-W high-pressure mercury lamp combined with a UV cutoff filter (Toshiba L39), whose transmittance at 390 nm was 50%. The amount of hydrogen produced was measured by introducing it into a vacuum line provided with an oil manometer. X-ray powder diffraction data were obtained on a Shimadzu VD- 1 diffractometer using the Cu Ka radiation. ESCA spectra were obtained with a Shimadzu ESCA 750 spectrometer. Diffuse reflection spectra were measured by use of a Shimadzu UV-360 spectrophotometer equipped with an integrating sphere. The photoluminescence spectra of the CdS powder were obtained by monitoring the intensity of light emitted by excitation at 365 nm through a Jovin Yvon H-20-IR monochromator with a Hamamatsu Photonics R316 photomultiplier.
Results and Discussion It was found that the rate of hydrogen production varied enormously for the CdS powder specimens, as shown in Table I, if they were used as obtained. X-ray diffraction measurements showed that the hexagonal crystal structure prevails in the materials which exhibited high efficiency, while the cubic crystal structure prevails in the materials which are not efficient. In order to make the situation clearer, the CdS powder obtained from Kojundo Chem. Lab., whose crystal structure was mostly cubic, was heated at various temperatures in a nitrogen atmosphere for 1 h and the photocatalytic activity was examined. The CdS powder was washed in a 1 mol dm-’ acetic acid solution buffered at pH 4.5 before the platinum loading, because it was reported that the surface oxide layer was removed by this treatment,Ila although x-ray and ESCA measurements showed that there was no detectable amount of CdO on our specimens even before
Temperature I
OC
Figure 1. Abundance of the hexagonal structure in the CdS powder (Kojundo Chem. Lab.) as a function of the temperature of the heat treatment in a nitrogen atmosphere for 1 h (O), and the rate of hydrogen production from 1.0 mol dm-’ NazS03using the CdS powder loaded with 2 wt % Pt powder ( 0 ) . 3
.-c
% 3 -
BO
5 Time I m i n
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Figure 2. Rate of photocatalytic hydrogen production from 1.O mol dm-) Na2S03as a function of the time of grinding of CdS (Furuuchi Chem.) and 2 wt % Pt powder. The deviation in the efficiency was within 20%.
washing. The ESCA spectra in the S(2p) region of the CdS powder before washing showed a main peak at a binding energy of 161.6 eV and a small peak at ca. 169 eV. The latter peak has been attributed to After the samples were washed, however, no peak due to oxidized sulfur was detected. X-ray diffraction measurements revealed that the content of the hexagonal structure increased with the temperature of the heat treatment in a nitrogen atmosphere for 1 h, as shown by the solid line in Figure 1. The rate of hydrogen production observed by using the same CdS powder specimens also increased as shown by the broken line. Both show a good correlation with each other. This result strongly suggests that the CdS powder having a hexagonal crystal structure is much more efficient as a photocatalyst than the cubic one. The photoluminescence spectra of the CdS powder spread on a flat plate were measured in air. The CdS specimens having a hexagonal crystal structure showed luminescence with peaks a t 510, 770, and 950 nm. The band a t 510 nm is attributable to the edge emission. The latter two bands nearly agree with those which have been attributed to the sulfur vacancyI4 and cadmium ~acancy,’~ respectively. On the other hand, the CdS powder having mostly a cubic crystal structure showed no luminescence or very weak luminescence. The most efficient photocatalyst we have obtained so far is that prepared from Furuuchi Chem. CdS powder by annealing for 1 h a t 800 OC in a nitrogen atmosphere and loaded with 2 wt % platinum powder. The quantum efficiency for hydrogen production, i.e., twice the number of hydrogen molecules produced (13) Lichtensteiger, M.;Webb, C. J. Appl. Phys. 1983,54,2127. Appl. Phys. Lett. 1981, 38, 323. (14) Kulp, B. A.; Kelley, R. H. J. Appl. Phys. 1960, 31, 1057. (15) Kulp, B. A. Phys. Reo. 1962, 125, 1965.
J. Phys. Chem. 1985,89, 1329-1330
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500
550 Wavelength I nm
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Figure 3. Diffuse reflectance spectra of CdS powder (Furuuchi Chem.) as obtained (-); those obtained after grinding in an agate mortar for 5 (---) and 10 min (---).
per incident photon, observed at 436 nm is 28% at 30 O C and 35% at 60 OC, no correction being made for reflections from the flask and the photocatalyst. It is nearly constant at wavelengths shorter than 480 nm but drops to zero at wavelengths longer than 550 nm. The rate of hydrogen production under illumination by a solar simulator (AM1, 1.00 kW m-2) is 0.32 mol m-2 h-' a t 30 O C and 0.50 mol m-2 h-' at 70 OC. The quantum efficiency and the rate of hydrogen production are more than three times as high as those reported previously.' In the previous report,' we prepared the photocatlayst by grinding the CdS powder and platinum powder together in an
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agate mortar. It was later found that the efficiency of the photocatalyst decreases with increasing grinding time as shown in Figure 2. The reflectance spectrum of the well-ground CdS powder shows enhanced photoabsorption at wavelengths longer than that of band-gap excitation (520 nm) as shown in Figure 3. The photoluminescence of the CdS powder having a hexagonal crystal structure is weakened by grinding. These results show that the efficiency is decreased by grinding the CdS powder due to the formation of recombination centers. The high efficiency for hydrogen production observed with the annealed CdS powder may, therefore, be partly attributed to the removal of the defects formed during the manufacturing of the CdS powder. Generally, the grinding method is known to be useful for a Pt-loaded T i 0 2 p h o t ~ c a t a l y s t .For ~ ~ ~CdS, ~ however, the grinding process gives rise to an undesirable effect as shown in Figure 2. Both luminescence and absorption spectroscopic results show increased defects from grinding. Photodeposition of Pt on CdS powder from aqueous solutions of platinum(1V) hexachloride was also camed out in order to make damage-free photocatalysts. However, the efficiencies of the photocatalysts prepared by this method under various conditions were a little lower than those obtained by the photocatalysts prepared by the shaking method. The platinum powder (platinum black) used for the loading by the latter method has been manufactured for use as a hydrogenation catalyst and has a large surface area, ca. 50 mz g-'. The good results obtained may be attributed to this quality. Registry No. Hl,1333-74-0; CdS, 1306-23-6;Pt,7440-06-4; Na2S0,, 7757-83-1.
Stable Stationary States of Coupled Chemical Oscillators. Experlmental Evidence K. Bar-Eli* and S . Reuveni Department of Chemistry, Tel-Aviv University, Tel-Aviv, Israel 69978 (Received: July 19, 1984; In Final Form: February 5, 1985)
When chemical oscillators are coupled by mass transfer, they can, under appropriate conditions, stop oscillating and arrive at an inhomogeneous stable, stationary, steady state. This prediction is verified experimentally by coupling two Belousov-Zhabotinskii oscillators.
Introduction In previous it was shown that coupling chemical oscillators in a diffusionlike manner, Le., by a mass transfer which is proportional to the concentration difference between the coupled oscillators, may bring the whole system to an inhomogeneous stable, stationary steady state. This is a rather unexpected phenomenon, since intuitively we tend to think that the coupling or diffusion will tend to equalize and wash out the differences among the various parts of the system. Nevertheless, all tested mechanisms and models show that, when coupled under appropriate conditions, the oscillations will stop and a stable dissipative structure4 will be formed. The tested model oscillators were (1) the Brusselator due to prigogine and L e f e ~ e r(2) ; ~ an autocatalytic first-order decomposition due to K ~ m a r (3) ; ~ the Loth-Volterra prey-predator model;6 (4) the Noyes-Field-Thompson (NFT) (1) K. Bar-Eli, J . Phys. Chem., 88, 3636 (1984). (2) K. Bar-Eli, J . Phys. Chem., in press. (3) K. Bar-Eli, Physica D, in press. (4) I. Prigogine and R. Lefever, J. Chem. Phys., 48, 1695 (1968). ( 5 ) V. R. Kumar, V. K. Jayaraman, B. D. Kulkarni, and L. K. Doraiswamy, Chem. Eng. Sci., 38,673 (1983).
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model' for the oxidation of cerous by bromate ions; (5) the Oregonator;8 and (6) the Field-Koros-Noyes (FKN) m e ~ h a n i s m . ~ The last two model the famous Belousov-Zhabotinskii (BZ)l0 oscillating reaction. It is striking that such diverse models which differ in their type of nonlinearity (e.g., Oregonator vs. Brusselator) and may be conservative or not (eg., Lotka-Volterra vs. the other mechanisms) all may be stabilized when coupled. The coupled oscillators may also work under the same constraints and still become stable. This was first shown by Prigogine and Lefever4 for the case of two coupled identical Brusselators. Bar-Eli3 has shown that this ( 6 ) (a) A. Lotka, J . Am. Chem. SOC.,42, 1595 (1920); (b) A. Lotka, J . Phys. Chem., 14, 271 (1910); (c) V. Volterra, 'Lecons sur la Theorie Mathematique de la Lutte pour la Vie", Gauthier-Villars, Paris, 1931. (7) R. M. Noyes, R. J. Field, and R. C. Thompson, J . Am. Chem. SOC., 93, 7315 (1971). (8) R. J. Field, and R. M. Noyes, J . Chem. Phys., 60,1877 (1974). (9) R. J. Field, E. Koros, and R. M. Noyes, J . Am. Chem. SOC.,94, 8649 (1972). (10) (a) B. P. Belousov, Sb. ReJ Radiat. Med., 1958, 145 (1959). (b) A. M. Zhabotinskii, Dokl. Akad. Nauk. SSSR, 157, 392 (1969); (c) A. M. Zhabotinskii, Biofirica, 9, 306 (1964); (d) A. N. Zaikin and A. M. Zhabotinskii, Nature (London), 225, 535 (1970).
0 1985 American Chemical Society
