Photodecomposition of Water Catalyzed by Zr- and Ti-MCM-41

ESO 25(6) #7759 Energy Sources, 25:591 596, 2003 Copyright 2003 Taylor & Francis 0090-8312/03 $12.00 +.00 DOI: 10.1080/00908310390195651 Photodecomposition of Water Catalyzed by Zr- and Ti-MCM-41 S.-H. LIU H. PAUL WANG Department of Environmental Engineering Cheng-Kung University Tainan, Taiwan Y.-J. HUANG Y. M. SUN Department of Industrial Saftey and Hygiene Chung Hwa College of Medical Technology Tainan, Taiwan K.-S. LIN Department of Chemical Engineering Yuan-Ze University Cung-Li, Taiwan M.-C. HSIAO Y. S. CHEN Department of Environmental Engineering Kun Shan University of Technology Tainan, Taiwan Experimentally incorporated Zr or Ti into the framework of MCM-41 could enhance the photocatalytic decomposition of H 2 OtoH 2. The hydrogen yield, for instance, on Zr-MCM-41 was about 7 mmol H 2 /hr-gzro 2. The enhancement of Zr-MCM-41 was over 80 times if compared to the bulk ZrO 2. The Ti-MCM-41 also possessed an enhancement of about 17 times for the H 2 yield (over TiO 2 ). Keywords photocatalysts, Zr-MCM-41, ZrO 2, MCM-41, TiO 2 Received 1 February 2002; accepted 12 November 2002. Address correspondence to H. Paul Wang, Department of Environmental Engineering, Cheng- Kung University, Tainan, 701, Taiwan. E-mail: wanghp@mail.ncku.edu.tw 591

592 S.-H. Liu et al. Generation of hydrogen via photocatalytic decomposition of water has attracted much attention from the viewpoint of conversion of solar energy into chemical fuels. Photocatalytic formation of hydrogen on semiconductors such as TiO 2 (Kudo et al., 1987; Sayama and Arakawa, 1992), SrTiO 3 (Domen et al., 1986), and ZrO 2 (Sayama and Arakawa, 1993) has been widely investigated. However, these semiconductor photocatalysts possess a very low efficiency of photoexcited charge formation and transfer to the surface reactants. It is generally known that incorporation of transition metal ions into molecular sieves may exhibit a high photocatalytic activity because of the high dispersion state of the photoactive metals in the zeolite framework that may cause an effective separation of electrons and holes. Photocatalytic decomposition of H 2 OonTiO 2 catalysts is not very effective because the reduction potential of electrons in the TiO 2 conduction band is almost the same as that of H 2 /H 2 O(E= 0 ev NHE, ph = 0) (Sayama and Arakawa, 1993). In a preliminary study, we found that Ti-substituted zeolites possessing high dispersion of Ti-O moieties in the framework highly enhanced the photocatalytic activity if compared to the bulk TiO 2. Anpo et al. (1998) indicated that Ti-substitued zeolites can initiate decomposition of NO under UV irradiation. In addition, Ti-substituted zeolites have a high reactivity and selectivity for production of CH 3 OH in a photocatalytic reduction of CO 2 with H 2 O (Zhang et al., 1997). The behavior of ZrO 2 catalysts is very different from that of TiO 2 because of its highly negative flat-band potential (E fb = 1.0 ev NHE, ph = 0) and the wide band gap (E bg = 5.0 ev). MCM-41 has a hexagonal crystalline structure with an adjustable mesopore size of 20 100 Å. Incorporation of Zr into the framework of MCM-41 may promote the photocatalytic activity for decomposition of H 2 O. Thus the main objective of the present work was to study the enhancement of photocatalytic decomposition of water affected by Zr-MCM-41 and Ti-MCM-41. The photocatalytic reactions were conducted in a homemade total reflection reactor. Experimental Fumed silica (Sigma) and sodium silicate solution (14% NaOH and 27% SiO 2 ) were used as silica sources in the synthesis of MCM-41. Hexadecyltrimethylammonium bromide (CTABr) and tetramethylammonium hydroxide (TMAOH, 25%) were used as a template and a mineralizer, respectively; 9.6 grams of TMAOH solution and 23.4 grams of CTABr were well mixed in 100 g of water with stirring until the solution was clear. Then 5.67 grams of sodium silicate and 4.52 grams of silica were added and stirred for 2 h. The ph value of the solution was adjusted in the range of 10.5 to 11.0 with diluted sulfuric acid (0.4 N) and stirred for an additional 30 min. The molar ratio of the main species in the mixture was 0.33(CTABr):0.95(TMAOH):41.9(H 2 O):1.0(SiO 2 ). About 1 5% of zirconium isopropoxide (Alfa) and titanium isopropoxide (Aldrich) were added to the MCM-41 mother solution for preparing the Zr- and Ti-MCM-41 photocatalyst, respectively. The mixture gels were heated in a Teflon-lined stainless-steel autoclave at 423 K for 48 h. The as synthesis solid materials were filtered, washed with distilled water, and dried in air at 373 K for 16 h. The photocatalysts were calcined at 823 K for 8 h to decompose the templating materials. Photocatalytic reactions were carried out in a quartz reactor (115 ml) with a total reflection mirror system (Liu and Wang, 2002). Typically, 0.1 g of the catalyst was suspended in H 2 O (90 ml) during magnetic stirring. A 500 W high-pressure Hg lamp (ORIEL, Model 66028) was used as the light source. Water was flushed with high-purity Argon prior to UV irradiation

Photodecomposition of Water Catalyzed 593 for at least 30 min to reduce dissolved oxygen. Hydrogen yield was determined by gas chromatography (Carboxen 1000 column). Results and Discussion The relatively well-defined X-ray diffraction (XRD) pattern of the synthesized MCM-41 is shown in Figure 1a. The (100) peak with repetition distance of pores (d 100 ) can be indexed on a hexagonal lattice with a pore diameter of about 45 Å. The XRD patterns of Zr-MCM-41 and Ti-MCM-41 with the Si/Zr and Si/Ti atomic ratios of about 80 are also shown in Figures 1b and 1c. Four low-angle MCM-41 characteristic peaks ((100), (110), (200), (210)) with similar relative peak intensities were also found for the Zr-MCM-41 and Ti-MCM-41 photocatalysts. Furthermore, incorporation of a small amount of Zr or Ti into the MCM-41 framework might not cause a distortion of the hexagonal characteristic pore structure. However, the (100) peak of the Zr-MCM-41 or Ti-MCM-41 was, to some extent, shifted toward small 2θ angles, which suggested a slightly reduced regularity of the silicate framework of MCM-41. Dispersion of Zr or Ti in MCM-41 was also measured by diffuse reflectance UVvisible (DR UV-VIS) spectroscopy. The DR UV/VIS spectra were very sensitive for the presence of extra framework and coordination of Zr and Ti in MCM-41. In Figure 2, a single narrow band at 205 nm was observed, indicating that Zr might be highly dispersed in MCM-41. Because of low absorbance at 300 350 nm, the condensed crystalline TiO 2 might not exist in the synthesized photocatalysts. It is very likely that most of the Zr Figure 1. Powder X-ray diffraction patterns of (a) MCM-41, (b) Zr-MCM-41, and (c) Ti-MCM-41.

594 S.-H. Liu et al. Figure 2. DR UV-VIS spectra of (a) MCM-41, (b) Zr-MCM-41, and (c) Ti-MCM-41. and Ti were incorporated into the wall of the MCM-41 hexagonal pore structure. The absorption features at about 220 nm for Ti-MCM-41 may be due to ligand-to-metal charge transfer associated with Ti 4+ framework sites in tetrahedral coordination (Maschmeyer et al., 1995; Marchese et al., 1997; Rhee and Lee, 1997; Sung-Suh et al., 1997; Wu and Iwamoto, 1998). Yields of hydrogen from photocatalytic decomposition of H 2 O on Zr-MCM-41, Ti-MCM-41, and MCM-41 are shown in Figure 3. Photodecomposition of H 2 O may not be initiated without a catalyst or UV irradiation. As the photocatalytic time increased under UV irradiation, the hydrogen yields increased. On the Zr-MCM-41 catalyst, 7 mmol/g ZrO 2 of hydrogen was yielded accumulatively for a5hphotocatalytic reaction that was about 80 times of hydrogen yielded on the conventional ZrO 2 catalyst. In addition, an accumulated 2 mmol/g TiO 2 of hydrogen was yielded in the photocatalytic decomposition of H 2 O affected by Ti-MCM-41 for 5hoftheirradiation. Photocatalytic decomposition of H 2 OonTiO 2 could hardly proceed under UV irradiation, which is due to the fact that the reduction potential of electrons in the TiO 2 conduction band is almost the same as the potential of H 2 /H 2 O(E= 0 ev NHE, ph = 0) (Sayama and Arakawa, 1993). The band gap energy of Ti-MCM-41 estimated by extrapolating the adsorption edge was 0.7 ev, which is greater than that of TiO 2. Thus the conduction band of the Ti-MCM-41 is more negative than that of the bulk TiO 2. It is clear that the photodecomposition of H 2 O was enhanced by the Zr and Ti in the framework of MCM-41. The incorporated Zr and Ti species in the framework of MCM-41 might justify the band gap and prolong the recombination time of the photoinduced formation of electrons and holes (Anpo et al., 1998).

Photodecomposition of Water Catalyzed 595 Figure 3. Effect of UV irradiation time on the yield of H 2 for photocatalytic decomposition of H 2 O on (a) MCM-41, (b) Zr-MCM-41, and (c) Ti-MCM-41. In summary, we have demonstrated that incorporated Zr or Ti in the framework of MCM-41 could enhance the photocatalytic decomposition of H 2 OtoH 2. The enhancement of Zr-MCM-41 was over 80 times when compared to the conventional photocatalyst ZrO 2. The photodecomposition of H 2 O was initiated by Ti-MCM-41 mainly due to its highly negative conduction band. The Ti-MCM-41 also possessed an enhancement of about 17 times for the H 2 yield when compared to the conventional photocatalyst. References Anpo, M., H. Yamashita, K. Ikeue, Y. Fujii, S. G. Zhang, Y. Ichihashi, D. R. Park, Y. Suzuki, K. Koyano, and T. Tatsumi. 1998. Photocatalytic reduction of CO 2 with H 2 O on Ti-MCM-41 and Ti-MCM-48 mesoporous zeolite catalysts. Catalysis Today 44:327 332. Domen, K., A. Kudo, T. Onishi. 1986. Mechanism of photocatalytic decomposition of water into H 2 and O 2 over NiO-SrTiO 3. J. Catal. 102:92 98. Kudo, A., K. Domen, K. Maruya, and T. Onishi. 1987. Photocatalytic activities of TiO 2 loaded with NiO. Chem. Phys. Lett. 133:517 519. Liu, S.-H., and H. P. Wang. 2002. Photocatalytic production of hydrogen on Zr-MCM-41. Internation. J. Hydrogen Energy 27:859 862.

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