J. Indian Chern. Soc., Vol. 80, Apri12003, pp. 395-400 Microstructure and activity of platinized CdS-AI 2 0 3 photocatalysts for reduction of water to hydrogen by visible lightt A. S. K. Sinha, Namita Sahu and S. N. Upadhyay* Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221 005, India E-mail : asksinha@sify.com Fax: 91-0542-307070, 316589 Manuscript received 4 October 2002 Alumina supported Pt-CdS is an effective photocatalyst for the reduction of water to hydrogen using visible light. The activity is dependent on the method of incorporation of platinum on the catalyst. The catalysts were characterized by XRD, XPS, H 2 chemisorption, temperature programmed oxidation and optical spectroscopy. The results show that the band gap of CdS has no correlation with the activity. Crystallite size of CdS and the nature of contact between alumina and CdS, however, affect the activity. Platinum on the surface of the catalyst is in the form of Pt-PtS,. Alumina supported cadmium sulfide has been reported to be an active photocatalyst for reduction of water to hydrogen l-6 in presence of visible light. However, it is also likely that the photogenerated electrons and holes, instead of taking part into the reaction, may recombine and produce only heat. Therefore, the efficiency of a photocatalyst depends on the relative rates of generation and recombination of electrons and holes. The rate of recombination is substantially high on most of the semiconductors. We reported earlier 4 that the incorporation of alumina to CdS results into formation of a heterojunction that prevents electron- hole recombination. Incorporation of Group VIII metals, like platinum, also suppresses the rate of recombination. Uchihara eta/. 7 reported that when platinum is deposited on CdS, the Fermi level ofCdS moves towards a more positive potential, leading to a stronger band bending, and hence enhanced photocatalytic reaction. Apart from that, the Group VIII metals are also good hydrogenation - dehydrogenation catalysts. Therefore, these metals when present on the surface, would also catalyze the reaction, H+ + e- Y2 H 2 , and thereby increase the hydrogen evolu- tion rate and the quantum efficiency. Aspnes and Heller 8 on the other hand reported that the deposition of a metal on the surface on an n-type semiconductor results in the formation of a metal-semiconductor junction, or Schottky barrier, prevents electrons from reaching the catalytically active metal where proton reduction should be taking place. Thus, hydrogen generation is determined by the irradiance and the barrier height created between the semiconductor and the metal. They reported 8 that platinum reacts with CdS to form toedicated to Professor R. P. Rastogi. platinum sulfide. Thus the contact is not Pt/CdS, but a hy- drogen-insensitive contact (PtSx·Cd-CdS)/CdS. Many methods of platinum deposition have been reported in the literature and it has been observed that activity of the catalysts depends on the details of the preparation techniques9-l6_ However, a detailed study on the microstruc- ture of catalysts and its effect on their catalytic activities is not well-documented in the literature. In the present study, platinum was incorporated on CdS- Al203 catalysts by different techniques. Activities of these catalysts for reduction of water to hydrogen were measured. Catalysts were characterized to develop the inter-relation amongst preparation variables, catalysts microstructure and activity. Results and discussion ActiMy: Activities of the platinum incorporated catalysts are reported in Table I. The catalyst which was prepared by Table l. Activity and physical properties of catalysis Catalyst H 2 evolved* Mean Platinum Band gap no. in2 h crystallite exposed ofCdS (ml at NTP) size, nm % eV I 4.0 8.3 8.4 2.32 2 13.0 5.5 4.0 2.32 3 0.0 5.4 0.0 2.33 4 2.7 7.3 0.0 2.32 *per g ofCdS. 395