Ind. Eng. Chem. Res. 1999, 38, 2659-2665
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Activity of Cadmium Sulfide Photocatalysts for Hydrogen Production from Water: Role of Support Manjit K. Arora, Namita Sahu, S. N. Upadhyay, and A. S. K. Sinha* Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India
Alumina-supported cadmium sulfide photocatalysts for reduction of water to hydrogen were prepared by different techniques and characterized. The band gap, the dispersion of cadmium sulfide, and the contact between cadmium sulfide and alumina are affected by the preparation technique. Those preparation techniques that promote better dispersion of cadmium sulfide and give rise to a chemical interaction between cadmium sulfide and alumina impart a superior activity to the catalyst. The chemical interaction leads to decoration of cadmium sulfide surface by alumina which results in an intimate contact between them. Catalyst prepared by coprecipitation shows the intimate contact but has a poor dispersion of cadmium sulfide, whereas impregnation of alumina hydrogel results in a catalyst having both intimate contact between alumina and cadmium sulfide and better dispersion of cadmium sulfide; consequently, this results in enhanced activity. Introduction Chalcogenides have been reported as active photocatalysts for the reduction of H2O to H2. Among chalcogenides, cadmium sulfide has been extensively studied.1-5 Because of its narrow band gap (∼2.4 eV), it can be activated by visible light and thus has a potential to harvest solar energy. In addition, its sufficiently negative flat band position (-0.66 V at pH 7) is suitable for the photogenerated electrons to carry out the reduction of H2O to H2. The essential steps of the reaction include the separation of electrons and holes by absorption of light by CdS and the transfer of electrons to its conduction band. Absorption of light by a photocatalyst increases by increasing the surface area of CdS per unit volume of catalyst; however, in fine dispersions, recombination of photogenerated electrons and holes also takes place at an enhanced rate due to inadequate bending of bands.6 Thus, considerable attention has been paid to prevent charge recombination in order to improve the activity of CdS. The steps reported in the literature to prevent charge recombination include incorporation of a redox catalyst, e.g., Pt, to the semiconductor so that the photogenerated electrons are transferred from the conduction band of CdS to the redox catalyst. Incorporation of another semiconductor has been also reported to improve the activity. The increase in activity depends on the method of incorporation of the second semiconductor; as discussed below, various hypotheses have been proposed for this increase in activity. Serpone et al.7 have proposed an interparticle electrontransfer mechanism to account for the observed improvement in activity when CdS is mixed with RuO2 supported on TiO2. They have postulated that the conduction band electron of CdS is either directly trapped by RuO2 or is first transferred to the conduction band of TiO2 then to RuO2. Therefore, the displacement of electrons from the CdS reduces the rate of electron* Telephone: 091-0542-317192 (Off) 091-0542-317179 (Res). Fax: 091-0542-316925.
hole recombination on the excited particles. Borgarello et al.8 studied photodecomposition of H2S by cadmium sulfide in the presence of RuO2-loaded alumina. They have concluded that the transfer of electrons from the conduction band of CdS to alumina is possible. The electrons transferred to alumina can further move to the redox catalyst or can be used for the reaction. On the other hand, Sobez´ynski et al.,9 who studied the visible light-photoassisted production of hydrogen from methanol-water solutions containing mixtures of small particles of CdS/SiO2 and a platinized wide band gap semiconductor (TiO2, ZnO, SnO2, or WO3) separately supported on silica, concluded that electron injection from the conduction band of CdS to the conduction band of the metal oxide was not an important pathway for charge transport to the Pt particles. Rather, an agglomeration of the particles of CdS/SiO2 and of separately supported platinum has been proposed to account for the relatively efficient charge transfer. Kakuta et al.10 studied ZnS and CdS coprecipitated into Nafion films and onto SiO2. They have proposed that, in addition to the interparticle electron transfer from CdS to ZnS, tuning of the band gap of the cofabricated system is also likely. Although they have not explained the mechanism of interaction of CdS and ZnS, they concluded that an intimate contact between CdS and ZnS is essential for a better activity. They did not observe any solid solution between CdS and ZnS; instead, the coprecipitated system consisted of particles with CdS-rich cores coated with a ZnS-rich exterior. On the basis of the sizes of coprecipitated CdS and ZnS particles, it was concluded that the above morphology resulted in a large interfacial area and a better activity. Similar observations have been reported for ZnS-CdS/ SiO211 and ZnS-CdS/Al2O312 catalysts. Catalysts prepared by sequential deposition of ZnS followed by CdS were much less active for hydrogen production than those prepared by either sequential deposition in the opposite order or coprecipitation. Kobayashi et al.12 have further argued that, although the interaction between ZnS and CdS particles at their boundaries plays an important role in the hydrogen generation, the inter-
10.1021/ie980400j CCC: $18.00 © 1999 American Chemical Society Published on Web 06/18/1999
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particle electron-transfer mechanism probably cannot explain the enhanced rate of hydrogen generation. Rather, the key to the enhanced rate of hydrogen generation lies in the large area of contact between the ZnS and CdS particles. Finlayson et al.13 have concluded that the improvement in activity of ZnS-CdS/SiO2 might be due to the reduction in the number of recombination sites on the surface of CdS, and this would be effective when ZnS coats the surface of CdS and not the reverse. They also have observed a shift in the flat band of CdS by 0.7 V to more negative potentials in the ZnSCdS/SiO2 catalysts. Surface modification of CdS by coprecipitation with Ag2S and improved activity have been reported by Reber and Rusek.14 Similar views have been expressed by Youn et al.,15 who studied ZnS-CdS catalysts generated in situ and stabilized by dihexadecyl phosphate (DHP) vesicles. They also observed that the mechanical mixtures of preformed CdS particles and ZnS particles (both incorporated on the outer surface of DHP vesicles) showed no improvement in efficiency. Precipitation of ZnS on the CdS particle surface has been suggested to remove or repair low-energy surface states. Blocking these low-lying surface states would increase the fraction of conduction band electrons available at a more negative potential for H2 production. From the above-reported works, it can be concluded that the activity of CdS photocatalyst improves when a wide band gap semiconductor, e.g., ZnS, is incorporated. An intimate contact between CdS and the other semiconductor and a large interfacial area are necessary for a better activity. However, the nature of the “intimate contact” remains to be explained; in addition, the effect of the support, e.g., silica and alumina (which are also wide-band semiconductors) on the photocatalytic activity of CdS has not been studied. These supports are commonly used for achieving better dispersion of active ingredients in a catalyst. The semiconductor on an oxide support may have a bonding interaction between the interface and consequently affect the activity. Subrahmanyam et al.16 have reported the photocatalytic activity of CdS supported on MgO, Al2O3 or Al2O3, modified with CaO/BaO. The catalysts were prepared by the conventional impregnation technique. The Al2O3 support modified with CaO/BaO was prepared by impregnation of the respective nitrate salts and decomposition in air at 723 K for 5 h. They observed that, in comparison with MgO-supported systems, the hydrogen evolution on Al2O3-supported catalysts was less by a factor of more than three. Surface basicity of MgO was considered to promote a better dispersion of CdS as well as an intimate contact between the support MgO and CdS. Modification of the surface acidity of Al2O3 by BaO/ CaO also resulted in a better activity. In the present work, alumina-supported CdS catalysts were prepared by different techniques. It has been observed that the catalyst structure, the dispersion of CdS, the contact between CdS and alumina, and consequently the activity are affected by the preparation technique. The nature of intimate contact has been also discussed. Experimental Section Catalyst Preparation. Predried alumina is widely used as a support for catalyst preparation, and in the present work, alumina was used as a support. However, a departure was made, and a hydrogel of alumina, instead of predried alumina, was mixed with the
impregnating solution. Catalysts were also prepared by impregnating predried alumina and by coprecipitating alumina and cadmium sulfide. A high-surface-area hydrogel was prepared in the laboratory using the method described by Lippens and Steggerda.17 A solution of aluminum nitrate (20 g in 100 mL of distilled water) was neutralized by dropwise addition of an aqueous solution of ammonia until the pH of the reaction mixture reached 9. The reaction was carried out at room temperature, and the mixture was continuously agitated throughout the duration of reaction. The hydrogel was aged for 48 h at room temperature and then washed with distilled water until it was free of nitrate ions. Catalysts used in the present study were prepared in the laboratory as described below. Catalyst 1. Predried alumina, i.e., the hydrogel prepared in the laboratory and dried at 473 K for 12 h, was impregnated with a requisite amount of a dilute aqueous solution (0.1 M) of 3CdSO4‚8H2O to yield the final composition by weight as CdS/Al2O3 ) 1:2. The mixture was kept stirred for 20 h. It was then dried over a water bath with continuous stirring and finally in an air oven at 383 K for 12 h. The dried granules (∼3 mm) were then reacted with pure H2S gas flowing at a very low rate in a packed-bed reactor at 473 K for 5 h. The resulting catalyst was washed with distilled water to remove any trace of unreacted CdSO4 and finally dried at 373 K in an air oven. It is appropriate to mention that preliminary studies were carried out to measure the extent of reaction of CdSO4 and H2S gas gravimetrically in a Stanton Redcroft thermal analyzer (STA 700 series). It was observed that the time required for the completion of the reaction at 473 K was less than 30 min. Catalyst 2. This was prepared by exactly the same method as catalyst 1. The only difference was that instead of predried alumina its hydrogel, prepared in the laboratory as described above, was impregnated. The weight of the alumina in the hydrogel was determined by heating a known weight of hydrogel at 600 K in air for 6 h. Catalyst 3. It was prepared by coprecipitation of CdS and Al(OH)3 at room temperature by dropwise addition of an ammoniacal solution of Na2S (5.00 g in 50.0 mL) into 100.0 mL of an aqueous solution containing 3.52 g of 3CdSO4‚8H2O and 14.68 g of Al(NO3)3‚9H2O. The final pH of the reaction mixture was 9, and the mixture contained an excess of Na2S. The precipitate was filtered and washed several times with distilled water. The precipitate was then dried over a water bath and finally in an air oven at 383 K for 12 h. Activity Measurement. The photocatalytic activity of various catalysts for hydrogen production from water was studied in a batch reactor. The photoreactor was a 250 mL flat-bottomed flask with one of its sides made flat to permit the entry of light through a plane wall. The reactor had a provision for measurement and control of pH and temperature. A 150 W Philips tungsten-halogen lamp was used as the light source. The choice of the light source was based on the availability as well as on the spectral characteristics of the emitted light. No UV or IR filter was used because the spectrum of the light source used showed a very negligible emission in the UV range and the use of a Pyrex glass reactor further prevented this radiation from reaching the catalyst. IR radiation can only be absorbed in the form of heat and could increase the
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temperature of the solution. During experimentation, the temperature could be maintained at the desired value; therefore, an IR filter was not used. Catalyst of size -200 mesh (2 g) was suspended in 250 mL of well-stirred aqueous solution of concentration 0.01 and 0.004 M with respect to Na2S and Na2SO3, respectively. The pH was maintained at 8.6 during the experimentation by adding the requisite quantities of NaOH and acetic acid. The temperature was maintained at 333 K, and the solution was deaerated by sparging nitrogen for 2 h prior to irradiation. The gas evolved was collected by the water displacement technique and analyzed by an online gas chromatograph using a 5 Å molecular sieve column, a thermal conductivity detector, and nitrogen gas as the carrier. Before entering the chromatograph, the evolved gas was passed through a cold trap to remove entrained moisture. Comparison of the retention time of the only peak that appeared on the chromatogram with the standard confirmed that the gas was only hydrogen. The reproducibility of data was tested through three separate runs for each catalyst. No significant deviation (
