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Na2S dispersions
0360-~19990 $300 ~ 0 00 Pergamon Press plc ,C 1990InternationalAssociationfor H.~drogenEnergy
Int J. H)'drogen Energy, V o l 15, N o 5, pp M9-323. 1990. Printed in Great Britain.
PHOTOCHEMICAL HYDROGEN PRODUCTION FROM CdS/RhO~/Na2S DISPERSIONS M. M. KOSANI(" and A. S. TOPALOV Institute of Chemistry. Faculty of Sciences, University of Novi Sad, YU-21000 Novi Sad. Yugosla~,ia (Received for publication 14 November 1989)
A~traet--Hydrogen evolution from aqueous cadmium sulfide dispersions with rhodium oxide as catalyst in the presence of sulfide ions as electron donor was followed. The effect of the amount of CdS and RhO,, as well as the temperature dependence, were studied in detail. Results obtained indicate that the observed decline in the rate of the hydrogen production is due to the loss of the catalyst activity. Regeneration of hydrogen production can be achieved by removing hydrogen from illuminated solution.
[20]. Besides, S2 ions in solution shift the flatband potential of CdS particles toward the cathodic direction [20, 21]. This would allow an efficient hydrogen formation. Taking into account that sulfides occur widely in nature and are produced in large amounts as a waste product, their degradation is a process of potential significance. In this paper we report on the generation of hydrogen over CdS/RhO~/Na:S system. It is known that the rate of H, production slows during prolonged illumination and for this reason the possibility of regeneration of studied system has been examined. In order to determine the optimum conditions for H 2 evolution, the effect of the amount of semiconductor, the influence of RhO< loading, as well as the temperature effect have been also investigated.
INTRODUCTION Cadmium sulfide (CdS) particles absorb in the visible spectrum and are of potential use in solar energy conversion technology, Dispersions of the CdS particles have been used in various laboratories [l-12] to catalyse the photoproduction of hydrogen from aqueous solutions under illumination. However, CdS is not stable and undergoes photocorrosion [13-15]. This undesired process may be prevented by the presence of suitable electron donors and catalysts [16. 17]. Photochemical water splitting is the result of band gap excitation of CdS particles producing electrons in the conduction band and holes in the valence band, The electrons migrate to the interface where reduction of water to hydrogen occurs 2e + 2 H ~ ~ H ,
(1) EXPERIMENTAL
while the holes react with the electron donor D, which irreversibly decomposes: h+ + D ~ D
~
D* --, products.
(2) (3)
The reported efficiencies of hydrogen production on a pure semiconductor are very low. However, hydrogen evolution can be enhanced when particles are covered with metals of the platinum group. The majority of studies has been clone with CdS particles covered with Pt and RuO 2, although rhodium oxide (RhOx) was also found to be very efficient for hydrogen production [9]. Previously [18, 19] we reported results concerning the hydrogen evolution from aqueous cadmium sulfide suspensions with RhO~ as the catalyst in the presence of S2ions as the reducing species which suppress the photocorrosion process of the chalcogenide semiconductor
We have used CdS prepared in our laboratory. The CdS powder was prepared by adding 1.3g of CdSO4.8H~O to 100 cm 3 of a 1.2 tool dm ~ solution of acetate buffer at pH 4.5 and mixing with H2S. After that. the CdS powder in buffered solution of acetic acid was stirred for 2 h at 60~C, filtered, washed with triply distilled water and dried al 60cC. This pretreatment [15] gave stoichiometrically pure CdS powder without CdO fraction. In order to load CdS particles with RhOx, a photochemical procedure [9, 22] with minor changes was applied. The time needed for photochemically depositing of RhO~ was determined on the model system with known initial Rh(Ill) concentration. Determination of the Rh(III) concentration in the photochemically treated solutions was performed by measuring the absorbance of the complex formed with SnCI2 [23]. Solutions were
319
320
M. M. KOSANIC AND A. S. TOPALOV
prepared with triply distilled water using reagents which were at least reagent grade commercial products. The dispersion of CdS/RhO~ powder in the solution was achieved by sonication and before exposure to light was purged for 10 min with argon. For illumination a 150W halogen lamp with an appropriate 400 nm cut-off filter and a concave mirror of selective reflection with spectral distribution close to solar spectrum were used. Geometry of the irradiation cell and the lamp position was given elsewhere [24]. In that position the absorbed light intensity was 5.10 -7 einst m i n - J c m -3 in potassium ferrioxalate actinometric solution. Illumination of the solution was performed at room temperature (20:C) and at higher temperatures (30 and 40-C) with variations of + 2°C. During illumination the suspension was stirred with a magnetic stirrer. The hydrogen evolved under illumination was measured on a Jeol type 80 gas chromatograph with TC detection using a silica gel column at 40°C and argon as the carrier gas. The absorbances were recorded on \'arian Cary 219 spectrophotometer.
RESULTS AND DISCUSSION It was observed [19] that the rate of hydrogen generation increases with Na2S concentration up to 0.2 tool dm -3. Higher concentrations do not affect H, generation. Because of that, effects of CdS amount, *Various values for pK2 of H2S have been reported in the literature [25-28].
RhO,-Ioading and temperature on the rate of H~ production in 0.21 moldm -3 solution of Na,S have been investigated. The pH of this solution was 13.6. Using 11.96 as the pK2 value of H,S* [26], predominantly the S2- form was present in solution and, according to the reaction (4), 2Sz + 2h ~ --* S~-.
(4)
only S~- ions were formed as the final product of the oxidation of sulfide ions. The effect of the rate of H z evolution with increasing the amount of the semiconductor is shown in Fig. 1. Up to 10mgcm -3 of CdS, rate of H z evolution increases linearly with increasing amount of CdS. Further increase of CdS amount influences only slightly the evolution rate. Darwent [1] and Furlong et al. [12] also studied the effect of increasing CdS amount in CdS/Pt dispersion. The observed effect of increasing rate of H 2 evolution to a certain value could be ascribed to the increase in the number of photons being absorbed by semiconductor. Figure 2 shows that the amount of RhO~ loaded on CdS particles also influences the rate of H: evolution. Thus, for dispersions with less than 5% Rh in relation to the amount of CdS, hydrogen evolution rate linearly increases with Rh content, while with higher contents it remains constant. The increase of the rate of hydrogen evolution with the content of catalyst has been studied by Borgarello et al. [5, 6] using RuO 2 and Biihler et al. [10] using Pt. The catalytic effect of platinum group metals and metal oxides has also been investigated using other semiconductors [29-33]. Their role can be ascribed not only to
2.5
o •
l
1,5
i 20
Q; 1.0
0
05.
I
5
1~0
~__
CdS {mg cm -3)
Fig. I. Dependence of initial rate of hydrogen evolution on amount of CdS loaded with RhO, (5% Rh) in 0.21 tool dm -3 Na~S.9H20 solution.
H2 FROM Cds/RhO, Na~S DISPERSIONS
321
25 I rE
20
-e
E o
E 15 ::z..
10 o,,-
o
5
2
4
6
8
~0
*/. Rh Fig. 2. Dependence of initial rate of H 2evolution on the content of RhO, loaded on CdS particles. Suspensions ol lmgcm -~CdS in the solution of 0.21moldrn : Na.S.9H.O the catalysis of H 2 evolution, but also to some other effects such as the lowering of the H 2 overvohage, more efficient separation of electron-hole pairs and the hindrance of reverse reaction [34, 35]. The observed increase of photoactivity in the present work is probably due to the increased RhO~ content at the CdS surface. An unchanged catalytic effect at higher concentrations of RhO~ indicated that the content of 5% Rh was enough so that all electrons from semiconductor's surface. reacting according relation (1), were scavenged. Effect of screening of CdS surface at higher concentrations of RhOx, accompanied by lowering the extent of adsorption of S2- ions is not significant in the studied range of RhOx contents. Temperature effect on the rate of hydrogen production was investigated by Bfihler et al. [10] and by The~issent et al. [7]. Thewissen et al. found that the process of hydrogen generation is only slightly temperature dependent, while Biihler's results show that the efficiency of H 2 production increases remarkably with temperature. Our results (Table 1) show that in the Table 1. Temperature dependence of the rate of hydrogen formation* Temperature (°C)
H2 (#tool cm-3 rain i)
5 16 20 27 40
0.038 0.054 0.058 0.064 0.076
*Suspensionsof 1 mg cm -~ CdS/RhO, (5% Rh) in solution of 0.21 mol dm - ' Na2S-9H20.
0.21moldm ' N a 2 S t h e a m o u n t o f H 2 released at 40:C is about 30°0 larger than at room temperature More pronounced effect of temperature we have found in dispersion of commercial CdS product (Koch Eight, 99.999%) [19] The enhancement of photoactivity with temperature can be attributed to the increase in the exchange rate of the species at the CdS surface [10, 36], as well as to the increase of efficiency of recombination of hydrogen atoms competing with side reactions of H atoms (or electrons) with intermediate products of sulfide oxidation. A number of reports [1,2, 6, 8, 10, 12, 37-39] have shown that H: production slows with illumination time. The reasons for the decrease in the rate of H 2 production were attributed to the changes in pH [8], decomposition of semiconductor [1.2], light absorption by semiconductor and noble metal catalyst [40], depletion of the electron donor [6, 8, 12] and deactivation of the catalyst [12]. Furlong et al. [12] described experiments in which they attempted to regenerate H 2 production by addition of CdS/Pt catalyst and 'or Na 2S. Significant regeneration (70%) was only attained with renewal of both Na:S and CdS/Pt showing that the "death" of the system (at 80 min for this catalystl results from catalyst inactivity and lack of the donor. We also found that H, formation slows with illumination time. Figure 3 shows that after illumination of about 80 rain the saturation of system is attained and the cessation of H_, production is observed. We attempted to regenerate the system by removing H2 from the illuminated solution with argon. After removing H 2 the process of H 2 evolution starts again, as Fig. 3 shows, so that almost complete regeneration of the system can be achieved. The above procedure can be repeated several times without noticeable change of the efficiency of H2
322
M. M. KOSANI(~ AND A. S. TOPALOV
Removing of H2 with argon I
25
E
g~5 O
E .<
0.5
20
,o
8o
Illuminotion time (rain) Fig. 3. Regeneration of H2 production by removing the evolved H 2 with argon. Suspension of 1 mg cm- ~CdS/RhO~ (5% Rh) in 0.21 tool dm -3 Na2S. 9H20 solution. production. These results indicate that during prolonged photolysis, a major reason for the decline in the rate of H2 production is probably due to the deactivation of the catalyst by adsorption of H2 at the active RhOx sites. This conclusion we reached having in mind the fact that by bubbling with argon after irradiation, the composition of the system remains unchanged, with exception of H, elimination. Furlong et al. [33] also inferred that, for P t / T i O : / E D T A system, decrease in the rate of H2 formation during irradiation is caused by deactivation of the catalyst by H2. CONCLUSION Obtained results have shown that the CdS/RhOx dispersion in an aqueous solution of sulfide ions works as a suitable H:-evolution system. It was established that the rate of H, evolution increases with increasing both the a m o u n t of CdS up to 10 mg cm -3 and the content of RhO,. up to 5%. The rate of H, formation is temperature dependent. The obtained cessation of H 2 formation with prolonged illumination can be prevented by removing H2 from the system. Although the investigated system can be regenerated, long-term stability needs to be improved. Studies in that direction are in progress. REFERENCES [. J. Darwent, J. Chem. Soc. Faraday Trans. H 77, 1703 (1981). 2. J. Darwent and G. Porter, J. Chem. Soc. Chem. Commun., 145 (1981).
3. K. Kalyanasundaram, E. Borgarello and M. Gr/itzel, Helvet Chim. Acta 64, 362 (1981). 4, K. Kalyanasundaram, E. Borgarello, D. Duonghong and M. Gratzel, Angew. Chem. Int. Ed. Engl. 20, 987 (1981). 5, E. Borgarello, K. Kalyanasundaram and M. Gr~itzel. Helvet. Chim. Acta 65, 243 (1982). 6. E. Borgarello, W. Erbs, M. Gr/itzel and E. Pelizetti, Nouv J. Chim. 7, 195 (1983). 7. D. H. M. W. Thewissen, A H. A. Tinnemans, M Eeuwhorst-Reinten, K. Timmer and A. Mackor, Nouv. J Chim. 7, 191 (1983). 8. M. Matsumura, Y. Saho and H. Tsubomura, J. Phys. Chem. 87, 3807 (1983). 9. T. Rajh and O. I. Mi6i6, J. Seth. Chem. Soe. 48, 335 (1983). 10. N. Biihler, K. Meier and J. F. Rebel J. Phys. Chem. 88, 3261 (1984). 11. Z. A. Sinitsyna, Yu. L. Kiryukhin, I. D. Belova, Yu. E. Roginskaya and Kh. S. Bagdasar'yan, Dokl. Akad. Nauk S S S R 291, 405 (1986). 12. D. N. Furlong, F. Gieser, D. Hayes, W. Sasse and D. Wells, J. Phys. Chem. 90, 2388 (1986) 13. H. Gerischer and E Meyer, Z Phys. Chem. (Wiesbaden) 74, 302 (1971). 14. H. Gerischer, J. Electroanal. Chem. lnterfaeial Electrochem. 58, 263 (1975). 15. M. Vu6emilovi~, N. Vukeli~ and T. Rajh, J. Photochem. Photobiol. 42, 157 (1988). 16. G. Hodes, J. Manassen and D. Cahen, Nature, Lond 261~ 403 (1976). 17. B. Miller and A. Heller, Nature, Lond 262, 680 (1976). 18. A. S. Topalov and M. M. Kosani6, Review o f Research Faculty of Science, University o f Novi Sad 14, 25 (1984). 19. M. Kosani6 and A. Topalov, J. Serb. Chem. Soc. 52, 537 (1987). 20. G. Hodes, In Energy Resources through Photochemistry and Catalysis, p. 421. Academic Press, New York (1983).
H 2 FROM Cds/RhO~/Na,S DISPERSIONS 21. T. lnoue, T. Watanabe, A. Fujishima and K. Honda, Bull. Chem. Soc. Jpn. 52, 1243 (1979). 22. J. M. Lehn, J. P. Sauvage, R. Ziessel and L. Hilaize, Israel J. Chem. 22, 168 (1982). 23. Analytical Chemistry o f the Elements. In the series 1. M. Kotthoff, P. J. Elving and E. B. Sandell (eds), Treatise on Analytical Chemistry, part I1, Vol. 8, p. 511. Interscience, New York (1963). 24. M. Kosanic, A. Topalov, I. Janic and 1. Guth, Yugoslar Appl. Sei. 13[14, 19 (1988). 25. L. Khodakovskii, V. V. Zhogina and B. N Byzhenko, Geokhimiva, p. 827. Mir, Moskva (1965). 26. Handbook o f Chemistry and Physics, 58th edn, p. D- 15 I. CRC Press. Cleveland (1977). 27. R. J. Myers, J. Chem. Educ. 63, 687 (1986). 28. S. Light and J. Manassen, J. Eleetrochem. Soc. 134, 918
( 1987). 29. J. Kiwi and (1987).
M.
Gr~itzel, J.
Molec.
Catal. 39, 63
323
30. A. J. Frank, I. Willner. Z Goren and Y Degani. J . . 4 m Chem. Soc. 109, 3568 II987). 31. Y. Yu. Graifer, A. P. Koshchee~ and I A Myasniko~, Kinet. Katal, 28, 1137 (1987). 32. S. U. M. Khan. R. C. Kainthla and J. O'M Bockris. Int. J. Hydrogen Energ 3 13, 225 (1988) 33. D. N. Furlong, D. \Veils and W H Sassc. d Phrs. Chem 89, 1922 (1985). 34. H Gerischer, J. Ph~s Chem. 88, 6096 (1994) 35. T. Sakata, K. Hashimoto and T. Kat~ai. J PI~ ~ (-hem 88, 5214 (1984). 36. J. F. Reber and K. Meier, J. Phys Chem 88, 5903 (1984). 37. N. Serpone, E. Borgarello and M Gr~itzel, J Chem. Soc., Chem. Comman.. 342 11984). 38. M. Gutierrez and A. Henglein, Ber. Bun.wngcs Phys. Chern 87, 474 (1983). 39. H. Gerischer, Top. Appl. Phys. 31, 115 (lqTC~). 40. D. E. Aspnes and A J Heller. J Phv~ Clwm. 87, 4919 (1983).
Int J. H)'drogen Energy, V o l 15, N o 5, pp M9-323. 1990. Printed in Great Britain.
PHOTOCHEMICAL HYDROGEN PRODUCTION FROM CdS/RhO~/Na2S DISPERSIONS M. M. KOSANI(" and A. S. TOPALOV Institute of Chemistry. Faculty of Sciences, University of Novi Sad, YU-21000 Novi Sad. Yugosla~,ia (Received for publication 14 November 1989)
A~traet--Hydrogen evolution from aqueous cadmium sulfide dispersions with rhodium oxide as catalyst in the presence of sulfide ions as electron donor was followed. The effect of the amount of CdS and RhO,, as well as the temperature dependence, were studied in detail. Results obtained indicate that the observed decline in the rate of the hydrogen production is due to the loss of the catalyst activity. Regeneration of hydrogen production can be achieved by removing hydrogen from illuminated solution.
[20]. Besides, S2 ions in solution shift the flatband potential of CdS particles toward the cathodic direction [20, 21]. This would allow an efficient hydrogen formation. Taking into account that sulfides occur widely in nature and are produced in large amounts as a waste product, their degradation is a process of potential significance. In this paper we report on the generation of hydrogen over CdS/RhO~/Na:S system. It is known that the rate of H, production slows during prolonged illumination and for this reason the possibility of regeneration of studied system has been examined. In order to determine the optimum conditions for H 2 evolution, the effect of the amount of semiconductor, the influence of RhO< loading, as well as the temperature effect have been also investigated.
INTRODUCTION Cadmium sulfide (CdS) particles absorb in the visible spectrum and are of potential use in solar energy conversion technology, Dispersions of the CdS particles have been used in various laboratories [l-12] to catalyse the photoproduction of hydrogen from aqueous solutions under illumination. However, CdS is not stable and undergoes photocorrosion [13-15]. This undesired process may be prevented by the presence of suitable electron donors and catalysts [16. 17]. Photochemical water splitting is the result of band gap excitation of CdS particles producing electrons in the conduction band and holes in the valence band, The electrons migrate to the interface where reduction of water to hydrogen occurs 2e + 2 H ~ ~ H ,
(1) EXPERIMENTAL
while the holes react with the electron donor D, which irreversibly decomposes: h+ + D ~ D
~
D* --, products.
(2) (3)
The reported efficiencies of hydrogen production on a pure semiconductor are very low. However, hydrogen evolution can be enhanced when particles are covered with metals of the platinum group. The majority of studies has been clone with CdS particles covered with Pt and RuO 2, although rhodium oxide (RhOx) was also found to be very efficient for hydrogen production [9]. Previously [18, 19] we reported results concerning the hydrogen evolution from aqueous cadmium sulfide suspensions with RhO~ as the catalyst in the presence of S2ions as the reducing species which suppress the photocorrosion process of the chalcogenide semiconductor
We have used CdS prepared in our laboratory. The CdS powder was prepared by adding 1.3g of CdSO4.8H~O to 100 cm 3 of a 1.2 tool dm ~ solution of acetate buffer at pH 4.5 and mixing with H2S. After that. the CdS powder in buffered solution of acetic acid was stirred for 2 h at 60~C, filtered, washed with triply distilled water and dried al 60cC. This pretreatment [15] gave stoichiometrically pure CdS powder without CdO fraction. In order to load CdS particles with RhOx, a photochemical procedure [9, 22] with minor changes was applied. The time needed for photochemically depositing of RhO~ was determined on the model system with known initial Rh(Ill) concentration. Determination of the Rh(III) concentration in the photochemically treated solutions was performed by measuring the absorbance of the complex formed with SnCI2 [23]. Solutions were
319
320
M. M. KOSANIC AND A. S. TOPALOV
prepared with triply distilled water using reagents which were at least reagent grade commercial products. The dispersion of CdS/RhO~ powder in the solution was achieved by sonication and before exposure to light was purged for 10 min with argon. For illumination a 150W halogen lamp with an appropriate 400 nm cut-off filter and a concave mirror of selective reflection with spectral distribution close to solar spectrum were used. Geometry of the irradiation cell and the lamp position was given elsewhere [24]. In that position the absorbed light intensity was 5.10 -7 einst m i n - J c m -3 in potassium ferrioxalate actinometric solution. Illumination of the solution was performed at room temperature (20:C) and at higher temperatures (30 and 40-C) with variations of + 2°C. During illumination the suspension was stirred with a magnetic stirrer. The hydrogen evolved under illumination was measured on a Jeol type 80 gas chromatograph with TC detection using a silica gel column at 40°C and argon as the carrier gas. The absorbances were recorded on \'arian Cary 219 spectrophotometer.
RESULTS AND DISCUSSION It was observed [19] that the rate of hydrogen generation increases with Na2S concentration up to 0.2 tool dm -3. Higher concentrations do not affect H, generation. Because of that, effects of CdS amount, *Various values for pK2 of H2S have been reported in the literature [25-28].
RhO,-Ioading and temperature on the rate of H~ production in 0.21 moldm -3 solution of Na,S have been investigated. The pH of this solution was 13.6. Using 11.96 as the pK2 value of H,S* [26], predominantly the S2- form was present in solution and, according to the reaction (4), 2Sz + 2h ~ --* S~-.
(4)
only S~- ions were formed as the final product of the oxidation of sulfide ions. The effect of the rate of H z evolution with increasing the amount of the semiconductor is shown in Fig. 1. Up to 10mgcm -3 of CdS, rate of H z evolution increases linearly with increasing amount of CdS. Further increase of CdS amount influences only slightly the evolution rate. Darwent [1] and Furlong et al. [12] also studied the effect of increasing CdS amount in CdS/Pt dispersion. The observed effect of increasing rate of H 2 evolution to a certain value could be ascribed to the increase in the number of photons being absorbed by semiconductor. Figure 2 shows that the amount of RhO~ loaded on CdS particles also influences the rate of H: evolution. Thus, for dispersions with less than 5% Rh in relation to the amount of CdS, hydrogen evolution rate linearly increases with Rh content, while with higher contents it remains constant. The increase of the rate of hydrogen evolution with the content of catalyst has been studied by Borgarello et al. [5, 6] using RuO 2 and Biihler et al. [10] using Pt. The catalytic effect of platinum group metals and metal oxides has also been investigated using other semiconductors [29-33]. Their role can be ascribed not only to
2.5
o •
l
1,5
i 20
Q; 1.0
0
05.
I
5
1~0
~__
CdS {mg cm -3)
Fig. I. Dependence of initial rate of hydrogen evolution on amount of CdS loaded with RhO, (5% Rh) in 0.21 tool dm -3 Na~S.9H20 solution.
H2 FROM Cds/RhO, Na~S DISPERSIONS
321
25 I rE
20
-e
E o
E 15 ::z..
10 o,,-
o
5
2
4
6
8
~0
*/. Rh Fig. 2. Dependence of initial rate of H 2evolution on the content of RhO, loaded on CdS particles. Suspensions ol lmgcm -~CdS in the solution of 0.21moldrn : Na.S.9H.O the catalysis of H 2 evolution, but also to some other effects such as the lowering of the H 2 overvohage, more efficient separation of electron-hole pairs and the hindrance of reverse reaction [34, 35]. The observed increase of photoactivity in the present work is probably due to the increased RhO~ content at the CdS surface. An unchanged catalytic effect at higher concentrations of RhO~ indicated that the content of 5% Rh was enough so that all electrons from semiconductor's surface. reacting according relation (1), were scavenged. Effect of screening of CdS surface at higher concentrations of RhOx, accompanied by lowering the extent of adsorption of S2- ions is not significant in the studied range of RhOx contents. Temperature effect on the rate of hydrogen production was investigated by Bfihler et al. [10] and by The~issent et al. [7]. Thewissen et al. found that the process of hydrogen generation is only slightly temperature dependent, while Biihler's results show that the efficiency of H 2 production increases remarkably with temperature. Our results (Table 1) show that in the Table 1. Temperature dependence of the rate of hydrogen formation* Temperature (°C)
H2 (#tool cm-3 rain i)
5 16 20 27 40
0.038 0.054 0.058 0.064 0.076
*Suspensionsof 1 mg cm -~ CdS/RhO, (5% Rh) in solution of 0.21 mol dm - ' Na2S-9H20.
0.21moldm ' N a 2 S t h e a m o u n t o f H 2 released at 40:C is about 30°0 larger than at room temperature More pronounced effect of temperature we have found in dispersion of commercial CdS product (Koch Eight, 99.999%) [19] The enhancement of photoactivity with temperature can be attributed to the increase in the exchange rate of the species at the CdS surface [10, 36], as well as to the increase of efficiency of recombination of hydrogen atoms competing with side reactions of H atoms (or electrons) with intermediate products of sulfide oxidation. A number of reports [1,2, 6, 8, 10, 12, 37-39] have shown that H: production slows with illumination time. The reasons for the decrease in the rate of H 2 production were attributed to the changes in pH [8], decomposition of semiconductor [1.2], light absorption by semiconductor and noble metal catalyst [40], depletion of the electron donor [6, 8, 12] and deactivation of the catalyst [12]. Furlong et al. [12] described experiments in which they attempted to regenerate H 2 production by addition of CdS/Pt catalyst and 'or Na 2S. Significant regeneration (70%) was only attained with renewal of both Na:S and CdS/Pt showing that the "death" of the system (at 80 min for this catalystl results from catalyst inactivity and lack of the donor. We also found that H, formation slows with illumination time. Figure 3 shows that after illumination of about 80 rain the saturation of system is attained and the cessation of H_, production is observed. We attempted to regenerate the system by removing H2 from the illuminated solution with argon. After removing H 2 the process of H 2 evolution starts again, as Fig. 3 shows, so that almost complete regeneration of the system can be achieved. The above procedure can be repeated several times without noticeable change of the efficiency of H2
322
M. M. KOSANI(~ AND A. S. TOPALOV
Removing of H2 with argon I
25
E
g~5 O
E .<
0.5
20
,o
8o
Illuminotion time (rain) Fig. 3. Regeneration of H2 production by removing the evolved H 2 with argon. Suspension of 1 mg cm- ~CdS/RhO~ (5% Rh) in 0.21 tool dm -3 Na2S. 9H20 solution. production. These results indicate that during prolonged photolysis, a major reason for the decline in the rate of H2 production is probably due to the deactivation of the catalyst by adsorption of H2 at the active RhOx sites. This conclusion we reached having in mind the fact that by bubbling with argon after irradiation, the composition of the system remains unchanged, with exception of H, elimination. Furlong et al. [33] also inferred that, for P t / T i O : / E D T A system, decrease in the rate of H2 formation during irradiation is caused by deactivation of the catalyst by H2. CONCLUSION Obtained results have shown that the CdS/RhOx dispersion in an aqueous solution of sulfide ions works as a suitable H:-evolution system. It was established that the rate of H, evolution increases with increasing both the a m o u n t of CdS up to 10 mg cm -3 and the content of RhO,. up to 5%. The rate of H, formation is temperature dependent. The obtained cessation of H 2 formation with prolonged illumination can be prevented by removing H2 from the system. Although the investigated system can be regenerated, long-term stability needs to be improved. Studies in that direction are in progress. REFERENCES [. J. Darwent, J. Chem. Soc. Faraday Trans. H 77, 1703 (1981). 2. J. Darwent and G. Porter, J. Chem. Soc. Chem. Commun., 145 (1981).
3. K. Kalyanasundaram, E. Borgarello and M. Gr/itzel, Helvet Chim. Acta 64, 362 (1981). 4, K. Kalyanasundaram, E. Borgarello, D. Duonghong and M. Gratzel, Angew. Chem. Int. Ed. Engl. 20, 987 (1981). 5, E. Borgarello, K. Kalyanasundaram and M. Gr~itzel. Helvet. Chim. Acta 65, 243 (1982). 6. E. Borgarello, W. Erbs, M. Gr/itzel and E. Pelizetti, Nouv J. Chim. 7, 195 (1983). 7. D. H. M. W. Thewissen, A H. A. Tinnemans, M Eeuwhorst-Reinten, K. Timmer and A. Mackor, Nouv. J Chim. 7, 191 (1983). 8. M. Matsumura, Y. Saho and H. Tsubomura, J. Phys. Chem. 87, 3807 (1983). 9. T. Rajh and O. I. Mi6i6, J. Seth. Chem. Soe. 48, 335 (1983). 10. N. Biihler, K. Meier and J. F. Rebel J. Phys. Chem. 88, 3261 (1984). 11. Z. A. Sinitsyna, Yu. L. Kiryukhin, I. D. Belova, Yu. E. Roginskaya and Kh. S. Bagdasar'yan, Dokl. Akad. Nauk S S S R 291, 405 (1986). 12. D. N. Furlong, F. Gieser, D. Hayes, W. Sasse and D. Wells, J. Phys. Chem. 90, 2388 (1986) 13. H. Gerischer and E Meyer, Z Phys. Chem. (Wiesbaden) 74, 302 (1971). 14. H. Gerischer, J. Electroanal. Chem. lnterfaeial Electrochem. 58, 263 (1975). 15. M. Vu6emilovi~, N. Vukeli~ and T. Rajh, J. Photochem. Photobiol. 42, 157 (1988). 16. G. Hodes, J. Manassen and D. Cahen, Nature, Lond 261~ 403 (1976). 17. B. Miller and A. Heller, Nature, Lond 262, 680 (1976). 18. A. S. Topalov and M. M. Kosani6, Review o f Research Faculty of Science, University o f Novi Sad 14, 25 (1984). 19. M. Kosani6 and A. Topalov, J. Serb. Chem. Soc. 52, 537 (1987). 20. G. Hodes, In Energy Resources through Photochemistry and Catalysis, p. 421. Academic Press, New York (1983).
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