Photocatalytic Reduction of CO2 with H2O

Yokohama Research Center, Mitsubishi Chemical Corporation 1000 Kamoshida-cho, Aoba-ku, .... Ti-β(F) (Si/Ti ) 60) was prepared by the same procedures ...
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J. Phys. Chem. B 2001, 105, 8350-8355

Photocatalytic Reduction of CO2 with H2O on Ti-β Zeolite Photocatalysts: Effect of the Hydrophobic and Hydrophilic Properties Keita Ikeue, Hiromi Yamashita, and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, Gakuen-cho, Sakai, Osaka 599-8531, Japan

Takahiko Takewaki Yokohama Research Center, Mitsubishi Chemical Corporation 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan ReceiVed: March 8, 2001; In Final Form: June 22, 2001

Two types of Ti-β zeolites synthesized by a hydrothermal synthesis method under different conditions using OH- and F- ion as anions of the structure-directing agents (SDA) exhibited photocatalytic reactivity for the reduction of CO2 with H2O at 323 K to produce CH4 and CH3OH. In situ photoluminescence, diffuse reflectance absorption, and XAFS (XANES and FT-EXAFS) investigations of these Ti-β zeolites indicate that the titanium oxide species are highly dispersed in their frameworks and exist in a tetrahedral coordination state. From the H2O adsorption isotherm on these Ti-β zeolites at 300 K, it was found that the Ti-β zeolites synthesized using OH- ions (Ti-β(OH)) exhibited hydrophilic properties and the Ti-β zeolites synthesized using Fions showed hydrophobic properties. With the addition of H2O, Ti-β(OH) exhibited a more efficient quenching of the photoluminescence of the highly dispersed tetrahedrally coordinated titanium oxide species and a more remarkable decrease in the preedge intensity of the XANES spectra of the Ti K-edge by the addition of H2O as compared with that of Ti-β(F) having hydrophobic properties. These results indicated that the H2O molecules added were easily able to gain access to the tetrahedrally coordinated titanium oxide species in the Tiβ(OH) zeolite. The differences in the H2O affinity to the zeolite surface led to a strong influence on the reactivity and selectivity for the photocatalytic reduction of CO2 with H2O. Therefore, the properties of the zeolite cavities were important factors controlling the reactivity and selectivity in the photocatalytic reduction of CO2 with H2O to produce CH4 and CH3OH on these Ti-β zeolite catalysts.

Introduction The photocatalytic reduction of CO2 with H2O, a significant reaction especially as a means of artificial photosynthesis, has been attempted in view of the importance of the storage of carbon sources. Inoue and Fujishima et al. first reported on the photocatalytic reduction of CO2 with H2O in the liquid phase to produce HCOOH, HCHO, and trace amounts of CH3OH and CH4 on semiconductor photocatalysts such as TiO2 and SrTiO31 powders. Recently, Saladin et al. have reported that CO, CH4, H2, and higher hydrocarbons could be produced in the photocatalytic reduction of CO2 on TiO2 surfaces in the presence of gaseous H2O.2,3 Although many researchers have focused on studies of the photocatalytic fixation of CO2,4-10 the efficiency of the photocatalytic reduction of CO2 with H2O on catalytic surfaces is still quite low. Great efforts must be made to understand the fundamental reaction mechanisms in order to improve the efficiency. We have previously reported that UV irradiation of tetrahedrally coordinated titanium oxide photocatalysts highly dispersed in a silica matrix led to the highly selective formation of CH3OH in the photocatalytic reduction of CO2 with gaseous H2O as compared to that of the octahedrally coordinated bulk TiO2 photocatalyst.11-16 With tetrahedrally coordinated highly dispersed titanium oxide photocatalysts, it is possible not only to achieve more active and selective * Corresponding author. E-mail: [email protected].

photocatalytic systems but also to obtain detailed information on the nature of the active sites and the reaction mechanisms at the molecular level. Ti-containing zeolites have attracted much attention due to their unique character and photocatalytic properties.17 Recently, Corma et al. reported that the incorporation of Ti ions into the framework of aluminum-free zeolite β could be achieved in Fmedium to produce hydrophobic selective oxidation catalysts.18,23 It has been found that two kinds of Ti-β zeolites synthesized using OH- ions and F- ions as anions of the structure-directing agents (SDA) exhibited hydrophilic and hydrophobic properties, respectively.18-20,22,23 There have been many reports on catalytic reactions in the liquid phase on zeolite catalysts having such hydrophilic and hydrophobic behaviors.21-23 In liquid-phase reactions, it has been found that the hydrophobichydrophilic nature of catalysts significantly influences the catalytic reactivity and selectivity. However, studies on catalytic reactions in the gas phase, especially photocatalytic reactions, have yet to be reported. In the present study, two types of Ti-β zeolites having hydrophobic and hydrophilic properties were synthesized, and their photocatalytic reduction of CO2 with H2O to produce CH4 and CH3OH at 323 K has been investigated. The effect of both the hydrophilic and hydrophobic properties of the zeolite cavities as well as the local structure of the titanium oxide species incorporated within the zeolite frameworks on the reactivity and

10.1021/jp010885g CCC: $20.00 © 2001 American Chemical Society Published on Web 08/08/2001

Ti-β Zeolite Photocatalysts

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selectivity for the photocatalytic reduction of CO2 with H2O to produce CH4 and CH3OH have been clarified. Experimental Section Catalysts. Two types of Ti-β zeolites were synthesized under hydrothermal conditions using OH- ions and F- ions as anions of the SDA. These Ti-β zeolites are denoted according to the kinds of SDA, i.e., Ti-β(OH) and Ti-β(F). The sources of the silica and titanium oxide are tetraethyl orthosilicate (TEOS) and tetraisopropylorthotitanate (TPOT), respectively. The composition of material mixture for Ti-β(OH) (Si/Ti ) 60) is as follows: SiO2:TiO2:N,N′-dibenzyl-4,4′-trimethylene bis(N-methylpiperidinium) dihydroxide:H2O ) 1:0.02:0.2:30. The gelation of the material mixture was carried out at room temperature, and these gel mixtures were placed in Teflon-lined stainless steel autoclaves and heated statically at 413 K for 5 days while being rotated at 60 rpm. The product was collected by centrifugal filtration, washed with distilled water, and dried in air at 353 K. To remove the occluded organic molecules, the samples were calcined under a flow of dry air at 823 K for 4 h. Ti-β(F) (Si/Ti ) 60) was prepared by the same procedures as those for Ti-β(OH) from a material mixture having the following composition: SiO2:TiO2:tetraethylammonium fluoride (TEAF):H2O ) 1:0.02:0.56:7. TS-1 (Si/Ti ) 60) was prepared from a material mixture having the following composition: SiO2:TiO2:tetrapropylammonium hydroxide (TPAOH):H2O ) 1:0.02:0.4:30. Ti/FSM-16 (1.0 wt % as TiO2) was prepared by impregnating FSM-16 with an aqueous titanium ammonium oxalate solution. The fluorination of the Ti/FSM-16 catalysts was carried out by a procedure previously reported.24 The FSM16 was immersed in 12.5% and 25.0% ammonium fluoride solution for 2 h, dried, heated at a rate of 10 K/min to 973 K, and kept at 973 K for 5 h. The TiO2 powder catalysts (JRCTIO-4: anatase 92%, rutile 8%) were supplied by the Catalysis Society of Japan. Photocatalytic Reactions. The photocatalytic reductions of CO2 with H2O were carried out with the catalysts (50 mg) in a quartz cell with a flat bottom (88 cm3) connected to a conventional vacuum system (1 × 10-4 Pa range). Prior to photoreactions and spectroscopic measurements, the catalysts were heated in O2 at 723 K for 8 h and evacuated at 473 K for 2 h to 1 × 10-4 Pa. UV irradiation of the catalysts in the presence of CO2 (36 µmol) and H2O (180 µmol) was carried out using a 100 W high-pressure Hg lamp (λ > 250 nm) at 323 K. The reaction products were analyzed by gas chromatography. Characterizations. The photoluminescence spectra of the catalysts were measured at 298 and 77 K using a SPEX FLUOLOG-3 spectrophotofluorometer. The diffuse reflectance UV-vis absorption spectra were measured at 298 K by a Shimadzu UV-2200A double-beam digital spectrophotometer equipped with an integrating sphere, where a BaSO4 was used as reference. The FT-IR spectra were recorded at 298 K with a JASCO FTIR-7300 spectrometer using the sample wafer. The XAFS spectra (XANES and EXAFS) of the catalysts were measured at the BL-7C facility of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba. The X-ray source from the 2.5 GeV electron storage ring was monochromatized using a Si(111) double-crystal monochromators. The Ti K-edge absorption spectra were recorded in the transmission and fluorescence mode at 298 K. The normalized spectra were obtained by a procedure described in previous literature,25 and Fourier transformations were performed on k3weighted EXAFS oscillations in the range 3-10 Å-1. The curvefitting of the EXAFS data were carried out by employing the

Figure 1. Diffuse reflectance UV-vis absorption spectra of Ti-β (OH) (a), Ti-β(F) (b), and TS-1 (c) catalysts.

iterative nonlinear least-squares method and the empirical backscattering parameter sets extracted from the shell features of the titanium compounds. SEM images were collected on a Camscan Series Model 2-LV electron microscope operating at an acceleration voltage of 15 kV. From the SEM images of Ti-β(OH) and Ti-β(F) catalysts, the Ti-β(OH) (0.25-0.50 µm ca.) catalyst has a much smaller particle size as compared to the Ti-β(F) catalyst (5-7.5 µm ca.). From the BET surface area measurements, the surface area of Ti-β(OH) and Ti-β(F) was 630 and 443 m2 g-1, respectively. Results and Discussion Local Structure of the Titanium Oxide Species. Figure 1 shows the diffuse reflectance UV-vis absorption spectra of the Ti-β and TS-1 zeolite catalysts. These catalysts exhibit an absorption band in the wavelength region of 200-260 nm, attributed to the LMCT (ligand-to-metal charge transfer) band of the highly dispersed tetrahedrally coordinated titanium oxide species. 26-28 Figure 2 shows the XANES spectra of the Ti-β zeolites. The XANES spectra of the titanium oxide catalysts at the Ti K-edge show several well-defined preedge peaks, which are related to the local structures surrounding the Ti atoms. The number, position, and relative intensity of these preedge peaks provide vital information on the coordination environment of Ti.29-31 In a tetrahedral symmetry, one intense single preedge peak corresponding to a dipolar-allowed transition from the 1s to t2 molecular levels built from the 3d and 4p metal orbital and from a neighboring orbital could be observed.32 As shown in Figure 2 a-c, the Ti-β(OH), Ti-β(F), and TS-1 zeolite catalysts exhibit one intense preedge peak. It was found that the local structure of the titanium oxide species of these zeolites is in tetrahedral coordination. Figure 2 also shows the FT-EXAFS spectra of the zeolite catalysts, and all data are given without corrections for phase shifts. All of the zeolite catalysts exhibit a strong peak at around 1.6 Å, which could be assigned to the neighboring oxygen atoms (a Ti-O bond). Ti-β(OH) (Figure 2-A) and Ti-β(F) (Figure 2-B) as well as TS-1 (Figure 2-C) exhibit only a Ti-O peak, indicating the presence of an isolated titanium oxide species on these zeolite catalysts. From the results obtained by the curvefitting analysis of the EXAFS spectra, it was found that the titanium oxide species incorporated within these zeolite frameworks exists in tetrahedral coordination with a Ti-O bond distance of 1.84 Å for Ti-β(OH) and 1.83 Å for Ti-β(F). These

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Figure 4. H2O adsorption isotherms at 298 K of Ti-β(OH) (a), TS-1 (b), and Ti-β(F) (c) catalysts.

Figure 2. Ti K-edge XANES (a-c) and FT-EXAFS (A-C) spectra of Ti-β(OH) (a, A), Ti-β(F) (b, B), and TS-1 (c, C). CN, coordination number; R, Ti-O bond distance (Å).

Figure 5. Effect of the addition of H2O molecules on the preedge peak of the XANES spectra. (a) Ti-β(OH) and (b) Ti-β(F). The amount of added H2O molecules: 0, 1.4, 3.0, 4.6 mmol/g-cat (from top to bottom). Figure 3. IR peaks attributed the surface hydroxyl groups on the Tiβ(OH) (a), TS-1(b), and Ti-β(F) (c) catalysts.

XAFS (XANES and FT-EXAFS) and diffuse reflectance UVvis absorption investigations indicated that the local structure of the titanium oxide species are the highly dispersed and exist in tetrahedral coordination, while differences in the coordination geometry of these species for Ti-β(OH) and Ti-β(F) could scarcely be observed. FT-IR and Adsorption Isotherm of H2O. Figure 3 shows the FT-IR spectra of Ti-β zeolites in the range of 2800-4000 cm-1. In the spectra of Ti-β(OH) (Figure 3a) and TS-1 (Figure 3b), an intense band at 3740 cm-1 and a weak and broad band in the 3400-3700 cm-1 region are observed, and these are assigned to the isolated silanol groups and silanol groups having strong hydrogen bonds, respectively.33-35 In the case of Tiβ(F) (Figure 3c), weak bands at 3740 cm-1 as well as at 3731 and 3692 cm-1 could be observed. The latter two bands are assigned to silanol groups having weak hydrogen bonds.23 In general, high-silica zeolites synthesized in an OH- medium show a large concentration of defects, typically exceeding the

concentration needed to compensate the charge of the templates.36 The strong intensity of the band at 3740 cm-1 for Tiβ(OH) and TS-1 indicates the presence of a large amount of defect sites in these zeolite catalysts. Figure 4 shows the adsorption isotherm of H2O molecules at 298 K. A larger amount of adsorbed H2O molecules can be observed with Ti-β(OH) and TS-1 than for Ti-β(F). In the XPS measurements, the presence of a trace amount of F atoms can be observed on the surface of Ti-β(F). The presence of F atoms on the catalyst surface tends to prevent the adsorption of H2O on the catalyst. The difference in the amount of adsorbed H2O molecules can be ascribed to the high concentration of defects with Ti-β(OH) and TS-1 and the presence of F atoms for Ti-β(F). These different characteristics of Ti-β(OH) and Ti-β(F) can readily explain their respective hydrophilic and hydrophobic properties. The hydrophilicity of Ti-β(OH) was found to be the highest among these zeolite catalysts. In Situ XAFS. Figure 5 shows the effect of the addition of H2O molecules on the intensity of the preedge peak and the shift in the XANES spectra. In both Ti-β zeolites, the addition of H2O molecules leads to a decrease in the preedge peak

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Figure 6. Photoluminescence spectra of the Ti-β(OH) (a), TS-1 (b), and Ti-β(F) (c) catalysts at 77 K. Excitation at 260 nm.

intensity. The peak intensity decreased more significantly on Ti-β(OH) with the addition of H2O than that for Ti-β(F). Moreover, on Ti-β(OH), the position of the preedge peak shifts to the high-energy region by the addition of excess amounts of H2O molecules (4.6 mmol/g-cat). The decrease in the peak intensity and the shift in the peak position of the preedge peak intensity are attributed to a modification in the local geometry of the Ti atoms from the tetrahedral coordination to a 5- or 6-fold coordination. Since the amount of H2O molecules for the photocatalytic reaction was to be about 3.6 mmol/g, it can be supposed that most of the titanium oxide species retain a tetrahedral coordination and a part of them probably have a 5or 6-coordinated geometry. Although a decrease in the preedge peak intensity and a shift in the position of the preedge peak could be observed with Ti-β(F), it was not as remarkable as that of Ti-β(OH). These results indicate that H2O molecules easily access to the tetrahedrally coordinated titanium oxide species of Ti-β(OH), which is in agreement with the results obtained from measurements of the adsorption isotherms of the H2O molecules. Photoluminescence. Figure 6 shows the photoluminescence spectra of Ti-β(OH) and Ti-β(F). These zeolite catalysts exhibit a photoluminescence spectra at around 480 nm by excitation at 260 nm at 77 K. Although the photoluminescence spectra of these catalysts at 298 K shifted to a lower wavelength (460 nm), the relative intensity of the photoluminescence with Ti-β(OH) and Ti-β(F) maintained the same ratio. The observed photoluminescence spectra were attributed to the radiative decay process from the charge-transfer excited state to the ground state of the highly dispersed tetrahedrally coordinated titanium oxide species.37 As shown in Figure 6, the photoluminescence yield of the Ti-β(OH) and TS-1 catalyst is higher than that of the Ti-β(F) catalyst. The photoluminescence yields can be said to be related to the concentration of the charge-transfer excited complexes, [ Ti3+-O- ]*. A difference in the environment around the Ti sites in these zeolites and the presence of F atoms on Ti-β(F) may be related to the low intensity of the photoluminescence. Additionally, as shown in Figures 7 and 8, it was found that the addition of CO2 and H2O molecules on Ti-β(OH) and Ti-β(F) leads to a quenching of the photoluminescence at 298 K. At the same time, it was also observed that the lifetime of the charge-transfer excited state was shortened by the addition of CO2 and H2O molecules. The lifetime of Ti-β(OH) was 173 µs under vacuum and 147 µs in the presence of 0.53 mmol/g of H2O (153 µs in the presence of 1.1 mmol/g of CO2), whereas the lifetime of Ti-

Figure 7. Effect of the addition of CO2 molecules on the photoluminescence spectra of Ti-β(OH) (a) and Ti-β(F) (b) catalysts at 298 K. Excitation at 260 nm. The amounts of the added CO2: (a) 0, 0.05, 0.21, and 1.1 mmol/g; (b) 0, 5.3, 10.5, and 15.8 mmol/g (top to bottom, respectively).

Figure 8. Effect of the addition of H2O molecules on the photoluminescence spectra of Ti-β(OH) (A) and Ti-β(F) (B) catalysts at 298 K. Excitation at 260 nm. The amounts of the added H2O: (A) 0, 0.11, 0.27, and 0.53 mmol/g; (B) 0, 0.27, 0.52, and 1.04 mmol/g (top to bottom, respectively).

β(F) was 159 µs under vacuum and 150 µs in the presence of 1.04 mmol/g of H2O (152 µs in the presence of 15.8 mmol/g of CO2). Such a quenching of the photoluminescence with the addition of CO2 and H2O molecules suggests that the added CO2 and H2O interacts with the titanium oxide species incorporated within the Ti-β zeolite in its excited state. As also shown in Figures 7 and 8, the quenching of the photoluminescence of these Ti-β(OH) and Ti-β(F) with CO2 molecules shows almost the same quenching efficiency. On the other hand, the quenching of the photoluminescence of the Tiβ(OH) catalyst with H2O molecules is much more efficient than that with the Ti-β(F) catalyst. From these results, it is clear that an efficient quenching by the addition of H2O observed

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Figure 9. Yields of CH4 and CH3OH in the photocatalytic reduction of CO2 with H2O at 323 K on the Ti-β(OH), TS-1, Ti-β(F), and TiO2 (P-25) catalysts. Intensity of light is 265 µW cm-2. Reaction time is 6 h.

with Ti-β(OH) can be attributed to its hydrophilic properties due to the easy accessibility and interaction of H2O molecules with the excited state of the tetrahedrally coordinated titanium oxide species. Photocatalytic Reaction. Figure 9 shows the yields of the main products in the photocatalytic reduction of CO2 with H2O 323 K. UV irradiation of Ti-β zeolite photocatalysts in the presence of CO2 and H2O led to the formation of CH4 and CH3OH as well as trace amounts of CO, C2H4, and O2. As shown in Figure 9, Ti-β(OH) exhibits higher reactivity compared to Ti-β(F). On the other hand, the selectivity for the formation of CH3OH from Ti-β(F) (41%) is higher than for that from Ti-β(OH) (11%). Moreover, the selectivity for the formation of CH3OH from Ti-β(F) is higher than that of the other Ti-containing zeolites and molecular sieve catalysts (TS-1 (23%), Ti-MCM-41 (31%), Ti-MCM-48 (29%)). The higher reactivity of Ti-β(OH) over Ti-β(F) is attributed to the higher concentration of the charge-transfer excited complexes for Ti-β(OH) as observed by the photoluminescence. In our previous studies, it has been found that the competitive interaction of CO2 and H2O molecules with the charge-transfer excited state of the tetrahedral titanium oxide species results in the formation of C radicals, H atoms, and OH radicals on the surface, while CH4 and CH3OH are formed by the reaction of C radicals with H atoms and OH radicals.11,12 On Ti-β(F) which has hydrophobic properties, the relative concentration of H2O is much lower than that for Ti-β(OH), resulting in the lower reactivity yet high selectivity for the formation of CH3OH. The effect of the F atoms on the photocatalytic reduction of CO2 with H2O on the other catalysts was also investigated. As shown in Figure 10, the partially fluorinated catalysts exhibit higher selectivity for the formation of CH3OH than the original catalysts without fluorination. Although a significant increase in CH3OH selectivity could be observed with the fluorinated catalysts, the CH3OH selectivity is still less than 30%. This is ascribed to the fact that the fluorinated Ti/FSM-16 still exhibits slight hydrophilic properties associated with the presence of hydroxyl groups. However, as shown in Figure 11, it was confirmed that fluorinated catalysts have smaller amounts of hydroxyl groups than unfluorinated catalyst. These results indicate the same tendency as the FT-IR results for the Ti-β catalysts and do support the view that the hydrophobic property allows a higher selectivity for the formation of CH3OH. The

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Figure 10. Effect of the fluorination of FSM-16 on the yields of CH4 and CH3OH in the photocatalytic reduction of CO2 with H2O on Ti/ FSM-16 catalysts: (a) no treatment, (b) 12.5% NH4F treatment, and (c) 25.0% NH4F treatment.

Figure 11. IR spectra in OH stretching region of the fluorinated Ti/ FSM-16 catalysts: (a) no treatment, (b) 12.5% NH4F treatment, and (c) 25.0% NH4F treatment.

chemical nature of being either hydrophilic or hydrophobic seems to affect on the product selectivity in the photocatalytic reaction. Conclusions Ti-β zeolites synthesized under different conditions, i.e., Tiβ(OH) and Ti-β(F), and their photocatalytic reactions for CO2 reduction with H2O at 323 K have been investigated. From the UV-vis absorption and XAFS (XANES and FT-EXAFS) spectra, the titanium oxide species of Ti-β(OH) and Ti-β(F) were found to be highly dispersed in tetrahedral coordination in the zeolite frameworks. No large difference existed in the coordination geometry for the titanium oxide species in both zeolites. Measurements of the FT-IR spectra and the adsorption isotherm of H2O indicated that the surface chemical properties of these Ti-β were hydrophilic for Ti-β(OH) and hydrophobic for Ti-β(F). The hydrophilic properties of Ti-β(OH) caused a higher efficiency in the quenching of the photoluminescence with the addition of H2O and a large change in the Ti coordination in the rehydration process, indicating that H2O molecules can more easily gain access to the tetrahedrally coordinated titanium oxide species on Ti-β(OH) than on Tiβ(F). UV irradiation of these Ti-β zeolites in the presence of CO2 and H2O led to the formation of CH4 and CH3OH and

Ti-β Zeolite Photocatalysts trace amounts of CO, C2H4, and O2. Ti-β(OH) catalyst having hydrophilic properties exhibited higher reactivity than Ti-β(F), attributed to the higher concentration of the charge-transfer excited complexes, (Ti3+ - O-)*. On the other hand, a highly selectivity for the formation of CH3OH was observed on the Ti-β(F) catalyst having hydrophobic properties. Therefore, the hydrophilic-hydrophobic properties of these zeolites were found to be the controlling factor in the reactivity and selectivity for the photocatalytic reduction of CO2 with H2O. Acknowledgment. The present work has been supported in part by the Grant-in-Aid on Priority-Area-Research on “Molecular Physical Chemistry” (12042271). The kind guidance on zeolite synthesis by Prof. M. E. Davis and people in his group of Caltech are gratefully acknowledged. References and Notes (1) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637. (2) Saladin, F.; Forss, L.; Kamber, I. J. Chem. Soc., Chem. Commun. 1995, 533. (3) Saladin, F. and Alxneit, I. J. Chem. Soc., Faraday Trans. 1997, 93, 4159. (4) Sayama, K.; Arakawa, H. J. Phys. Chem. 1993, 97, 531. (5) Ishitani, O.; Inoue, C.; Suzuki, Y.; Ibusuki, T. J. Photochem. Photobiol., A: Chem. 1993, 72, 269. (6) Kuwabata, S.; Uchida, H.; Ogawa, A.; Hirao, S.; Yoneyama, H. J. Chem. Soc., Chem. Commun. 1995, 829. (7) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. Chem. Commun. 1997, 841. (8) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Faraday Trans. 1998, 94, 1875. (9) Kohno, Y.; Tanaka, T.; Funabiki, T.; Yoshida, S. Phys. Chem. Chem. Phys. 2000, 2, 5302 (10) Ulagappan, N.; Frei, H. J. Phys. Chem. A 2000, 104, 7834. (11) Anpo, M.; Chiba, K. J. Mol. Catal. 1992, 74, 207. (12) Anpo, M.; Yamashita, H.; Ichihashi, Y.; Fujii, Y.; Honda, M. J. Phys. Chem. B 1997, 101, 2632. (13) Anpo, M.; Yamashita, H.; Ikeue, K.; Fujii, Y.; Ichihashi, Y.; Zhang, S. G.; Park, D. R.; Ehara, S.; Park, S. E.; Chang, J. S.; Yoo, J. W. Stud. Surf. Sci. Catal. 1998, 114, 177. (14) Yamashita, H.; Fujii, Y.; Ichihashi, Y.; Zhang, S. G.; Ikeue, K.; Park, D. R.; Koyano, K. Tatsumi, T.; Anpo, M. Catal. Today, 1998, 45, 221.

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