J. Phys. Chem. 1985, 89, 1902-1905
1902
Investlgatlon of the Mechanlrm of Hydrogen Evolution during Photocatalytic Water Decomposltion on MetaCLoaded Semiconductor Powders Ryo Baba, Seiichiro Nakabayashi, Akira Fujishima,* Department of Synthetic Chemistry, Faculty of Engineering, University of Tokyo, Hongo, Bunkyo- ku, Tokyo 1 1 3, Japan
and Kenichi Honda Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Sakyo- ku, Kyoto 606, Japan (Received: September 20, 1984)
The active sites for photocatalytic hydrogen evolution are identified for some noble metals deposited on TiOz particles by measuring the hydrogen isotope effect of the gases produced by the photodecomposition of water using a series of metal-loaded TiOz suspensions (separation factor analysis). The mechanism of the hydrogen evolution process on metal-loaded TiOZparticles is discussed in relation to the hydrogen electrode reaction of the same metals. By means of separation factor analysis the hydrogen evolution process on Pt/TiOz, Pd/TiOz, and Rh/TiOZ is assigned to a slow recombination mechanism.
Introduction Since the first report1 on the photosensitized decomposition of water into hydrogen and oxygen by a photoelectrochemical cell using a Ti02 single-crystal anode, photoelectrochemical processes using semiconductor materials under band-gap illumination have been given much attention? partly because of the interest in converting solar energy into electrical and/or chemical energy. However, most semiconductor materials have disadvantages,z3 such as low activity (low quantum yield), instability (photocorrosion), and wide band-gaps compared with the solar spectrum. To overcome some of these disadvantages, a noble metal such as Pt is loaded on the semiconductor surfaces.," Electrocatalytic effects have been observed for semiconductor electrodes with deposits of certain noble metals. To simplify such macrocell systems, semiconductor powders can be employed instead of electrodes to carry out heterogeneous photocatalytic and photosynthetic reactions. For example, when a noble metal is deposited on semiconductor powders some specific photochemical reactions occurred6and the photocatalytic activity was i m p r ~ v e d . ~Such a metal-loaded semiconductor powder system is regarded as a model of a photochemical diode*J1or a short-circuited photoelectrochemical m i c r ~ ~ e l lfor , ~which ~ * ~ the deposited metal serves as a redox catalyst for surface reactions. The proposed energetics based on such a model can explain the increased rates of redox reactions in spite of the apparent cont r a d i ~ t i o nthat ~ ~ the Schottky barrier reduces the photopromoted electron flow to the deposited metal where the reduction process is supposed to take place. (1) A. Fujishima and K. Honda, Nature (London), 238, 37 (1972). (2) See those reviews and referenccs cited in (a) A. J. Bard, J. Photochem., 10, 59 (1979); (b) M. GrBtzel, Eer. Bunsenges. Phys. Chem., 84,981 (1980); (c) A. J. Nozik, Annu. Rev. Phys. Chem., 29, 189 (1978). (3) R. N. Dominey, N. S. Lewis, J. A. Bruce, D. C. Bookbinder, and M. S. Wrighton, J . Am. Chem. Soc., 104,467 (1982). (4) A. Heller and R. G. Vadimsky, Phys. Reu. Lett., 46,1153 (1981); A. Heller, E. A . Shalom, W. A. Bonner, and B. Miller, J. Am. Chem. Soc., 104, 6942 (1982). (5) L. R. Faulkner, Chem. Eng. News, Feb. 27, 28 (1984). (6) B. Kraeutler and A. J. Bard, J. Am. Chem. Soc., 100,2239,4903,5985 (1978); H. Reiche and A. J. Bard, fbid., 101, 3127 (1979). (7) J. M. Lehn, J. P. Sauvage, and R. Ziessel, Nouu. J . Chim., 4, 623 (1983). ( 8 j A . J. Nozik, Appl. Phys. Lett., 30, 567 (1977). (9) A. J. Bard, Science, 207, 139 (1980). (10) T. Sakata, T. Kawai, and K. Hashimoto, Chem. Phys. Lett., 88, 50 (1982). (1 1) (a) M. Fujihira, Y.Sato, and T. Osa, Bull. Chem. Soc. Jpn., 55,666 (1982); (b) C. D. Jaeger and A . J. Bard, J . Phys. Chem., 83, 3146 (1979). \ -
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0022-3654/85/2089-1902$01.50/0
In the generally accepted model, the deposited noble metal is believed to promote the cathodic process by lowering the reaction overpotential and providing catalytically active sites for the reduction reactions, but there seems to be no experimental proof for this scheme. The deposited R on Ti02 particles was first shown to provide cathodic sites for hydrogen evolution in the photodecomposition of water in the authors' previous paper.12 However, the detailed mechanism of heterogeneous photocatalytic reaction processes on the deposited metal is not well established.lOJ1 The mechanism of hydrogen ion reduction on metal electrodes has been discussed by many r e s e a r ~ h e r s l ~in- ~terms ~ of the hydrogen electrode reaction (HER). For these studies some researchers have utilized hydrogen separation factor analysis which is based on the fact that the hydrogen gas electrolytically evolved from water containing D 2 0 (and/or HDO) has a H / D ratio that is characteristic of the cathode material for a given solution and electrolytic conditions. In the present study we employed hydrogen separation factor analysis to ascertain whether the mechanism of hydrogen evolution in the photocatalytic decomposition of water on metal-loaded Ti02 particles depends on the deposited metal. Experimental Section Preparation of Catalyst. T i 0 2 powders (P-25, Nihon-Aerosil) had an average particle diameter of 300 A and an average surface area of 50 m2/g (BET). The TiOz powders were reduced in flowing hydrogen at 600 OC for 6 h and then cooled to room temperature in hydrogen. Metal deposition on the reduced Ti02 was done photoelectrochemically in almost the same manner as Bard et al.?, using metal chlorides (H2PtC16,PdC12,RhCl,, RuCl,, (12) S.Nakabayashi, A. Fujishima, and K. Honda, Chem. Phys. Lett., 102, 464 (1983). (13) G. A. Hope and A. J. Bard, J. Phys. Chem., 87, 1979 (1983). (14) D. E. Asones and A. Heller. J . Phvs. Chem., 87. 4919 (1983). ( l 5 j B. E. Coiway and J. O'M. Bockris;J. Chem.Phis., 26,532 (i957). (16) R. R. Dogonadze, A. M. Kuznetsov, and V. G. Levich, Electrochim. Acta, 13, 1025 (1968). (17) L. I. Krishtalik, J . Electroanal. Chem., 130, 8 (1981). (18) H. Kita, J. Electrochem. Soc., 113, 1095 (1966). (19) J. O M . Bockris and S.Srinivasan, J. Electrochem. Soc., 111, 844, 853, 858 (1964). (20) H. F. Walton and J. H. Wolfenden, Trans. Faraday SOC.,34, 436 (1938). (21) L. I. Krishtalik and V. M. Tsionsky, J. Electroanal. Chem., 31, 363 (1 97 1). (22) J. Horiuti and G. Okamoto, Sci. Pap. fnst. Phys. Chem. Res. (Jpn.), 28, 231 (1936).
0 1985 American Chemical Society
Photocatalytic H 2 0 Decomposition on Semiconductor Powders The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1903 NiC12, and SnC12). The sacrificial reagent used for metal reduction and deposition was 50 vol 5% EtOH(aq). The resultant slurry, which was grayish in color, was washed with distilled water and centrifuged repeatedly more than three times. The precipitated catalyst was dried overnight in an oven a t 80 OC and crushed. The amount of loaded metal was 2 wt 5% Ti02. Samples of Hg-Pt or Hg-Pd d e p o s i t e d on T i 0 2were prepared by loading mercury by photoelectrochemical decomposition of Hg2C12in the presence of Pt/Ti02 or Pd/Ti02. Ru02-loaded Ti02 powders were prepared by grinding a mixture of R u 0 2 and T i 0 2 powders. Doubly distilled water ( H 2 0 ) was used throughout the experiment. Procedure. The metal/Ti02 catalyst (0.08 g) was dispersed in 8 mL of an equimolar solution of D 2 0 (>99.85% pure) and H 2 0in a 150-mL glass tube with a quartz tubular part for illumination. The sample in the tube was stirred magnetically while being deaerated by evacuation through a cold trap, and the tube was sealed. The sample was illuminated with a 500-W highpressure Hg lamp (Ushio) through a 12-cm water jacket with quartz windows to remove infrared light. After irradiation, the evolved gas in the cell was transferred by means of a vacuum line through a liquid nitrogen trap to remove water vapor, and the residual gas was measured by a pressure gauge (MKS Baratron-227A) and analyzed by a quadrupole mass analyzer (Anelva AGA-100) which was connected to the vacuum line. A 250-W high-pressure mercury lamp and a 250-W xenon lamp were also used in limited studies as other sources for illumination of the sample suspensions. Gas Analysis and Calculations. The mass spectra were recorded in the m / e range of 1 to 4 which includes H+, H2+,HD+, and D2+. Under the present analytical condition (total pressure
