Synthesis, Structural Characterization, and Photocatalytic

Aug 25, 2010 - Tel: 1-605-677-6189. Fax: 1-605-677-6397. E-mail: [email protected] (R.T.K.), [email protected] (D.Z.)., †. University of South D...
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J. Phys. Chem. C 2010, 114, 15728–15734

Synthesis, Structural Characterization, and Photocatalytic Performance of Mesoporous W-MCM-48 Dan Zhao,*,† Adrian Rodriguez,† Nada M. Dimitrijevic,‡ Tijana Rajh,‡ and Ranjit T. Koodali*,† Department of Chemistry, UniVersity of South Dakota, Vermillion, South Dakota 57069, and Center for Nanoscale Materials, and Chemical Sciences and Engineering DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: June 6, 2010; ReVised Manuscript ReceiVed: August 2, 2010

Tungsten-containing mesoporous MCM-48 was synthesized by a rapid and facile room-temperature procedure. The mesoporous structure and the local environment of tungsten species were studied by powder X-ray diffraction, transmission electron microscopy, nitrogen adsorption isotherms, UV-visible diffuse reflectance spectroscopy, and Raman spectrometry. The long-range ordered mesoporous structure of MCM-48 was well preserved after tungsten incorporation. Tungsten oxide species were highly dispersed in the MCM-48 matrix, and no bulk crystalline WO3 was formed. The as-prepared W-MCM-48 materials show notable photocatalytic activity for hydrogen evolution from a methanol-water mixture under UV irradiation though bulk WO3 is not active for the reaction. The photocatalytic mechanism was studied by electron spin resonance spectroscopy. Introduction Ordered mesoporous materials characterized by a large internal surface area and long-range ordered nanoscale pores have attracted extensive research interest in the past decades. The construction of an ordered mesoporous structure by using structure-directing templates provides new opportunities to improve the adsorption ability, catalytic activity, and catalytic selectivity without changing the chemical composition of materials.1-3 Transition-metal oxides are widely used as heterogeneous catalysts in acid catalysis, redox catalysis, and photocatalysis reactions. The catalytic activity can be greatly improved by forming the pure mesostructured transition-metal oxide or by incorporating transition-metal active centers into a mesoporous silica matrix. Because the surfactant control synthesis of many ordered mesoporous transition-metal oxides is unsuccessful, great efforts have been made to develop transition-metal-containing mesoporous silica materials. Various transition metals, including Ti, Zr, V, Cr, Mn, Fe, Mo, and W, have been doped into the hexagonal MCM-41 and cubic MCM-48.4-15 The resulting materials show a high catalytic activity in hydroxylation, epoxidation, and other selective oxidation. Transition-metal-containing mesoporous silica are also promising catalysts for photoreaction. The photocatalytic activity of single-metal-containing porous silica was studied by Anpo and some other researchers.16-26 The series research of Anpo et al. showed that Ti, V, and Mo oxide species dispersed in porous silica matrix are active under UV irradiation, whereas Cr-HMS and Cr-MCM-41 show photocatalytic reactivity under visible light irradiation. These transition-metal oxides incorporated in the porous silica exhibit a high and unique photocatalytic reactivity for various reactions, such as the decomposition of NOx into N2 and O2, the reduction of CO2 with H2O to produce CH3OH, and the preferential oxidation of CO in the presence of H2.16,17 The local environment of transition-metal species is * To whom correspondence should be addressed. Tel: 1-605-677-6189. Fax: 1-605-677-6397. E-mail: [email protected] (R.T.K.), [email protected] (D.Z.). † University of South Dakota. ‡ Argonne National Laboratory.

the most important factor that influences the photocatalytic activity and selectivity of transition-metal-containing porous silica. The isolated and highly dispersed Ti oxide species exhibited a much higher selectivity than bulk TiO2 for CH3OH formation in CO2 reduction and for N2 formation in NO decomposition. Another bimetal incorporating photofunctional system based on metal-to-metal charge-transfer (MMCT) excitation was developed by Frei and Hashimoto.27-32 The oxobridged bimetallic charge-transfer units were covalently anchored on the surface of mesoporous silica and act as a visible light charge-transfer pump. Ti(IV)-O-Ce(III) and Zr(IV)O-Cu(I) anchored MCM-41 mesoporous silica were reported to be efficient photocatalysts for visible light CO2 reduction by H2O and visible light photooxidation of 2-propanol.27,32 Titanium-containing porous silica is widely studied because of the good photocatalytic performance of a TiO2 semiconductor. Ti oxide was dispersed on the surface of cavities or within the framework of porous silica by impregnation, grafting, and incorporating during the hydrothermal synthesis.4,33 The materials were used for photocatalytic decomposition of undesirable molecules in gas phase and for solar energy conversion.22,23,25 It was believed that a WO3 semiconductor is unsuitable for efficient oxidative decomposition of organic compounds in air though it possesses a small band gap of 2.7 eV and the ability to absorb visible light. The conduction band of WO3 is more positive than the reduction potential of O2; thus, the capture of photogenerated electrons by O2 is thermodynamically prohibited. WO3 is reported to be a good photocatalyst for O2 evolution in the presence of an electron acceptor, such as Ag+ and Fe3+, but it is not active for H2 evolution because its conduction band is more positive than the redox potential of H+/H2.34,35 Recently, the photocatalytic application of WO3 was reconsidered. Abe et al. demonstrated that WO3 loaded with Pt nanoparticles exhibits a high efficiency for the decomposition of organic compounds under visible irradiation due to the multielectron O2 reduction.36,37 The low conduction band limits the application of WO3 as a photocatalyst material. However, the good activity for O2 production and organic pollutant degradation in the presence of a Pt cocatalyst indicates the high efficiency of charge

10.1021/jp105190v  2010 American Chemical Society Published on Web 08/25/2010

Mesoporous W-MCM-48 separation in a WO3 semiconductor. Thus, tungsten-containing porous silica is expected to be a promising photocatalyst material, which may show the same or even better performance than titanium-containing porous silica. In the present work, tungsten-containing mesostructure designed W-MCM-48 was synthesized and the photocatalytic performance for hydrogen evolution from a methanol-water mixture under UV irradiation is reported for the first time. There are several reports about the incorporation of tungsten into mesoporous MCM-41 during hydrothermal synthesis for redox applications. However, the homogeneous distribution of tungsten oxide species remains a challenge. According to the results of Zhang et al., tungsten was incorporated in the tetrahedrally coordinated positions of the MCM-41 framework when its general content is lower than 5.6 wt %.38 A critical value for the Si/W ratio of 30 was also reported by other researchers for the synthesis of W-doped MCM-41.39,40 A higher tungsten ratio resulted in the formation of extraframework crystalline WO3 as detected by XRD and Raman spectroscopy. To improve the dispersion of tungsten oxide species in the MCM-41 matrix, an oxo-peroxo route was developed by Bregeault et al.41,42 The addition of H2O2 in an acid medium generated low condensed oxoperoxometalate species in the gel and avoided the formation of extraframework crystalline WO3 during calcination. However, the formation of extraframework crystalline WO3 could not be completely avoided by H2O2. Raman peaks due to crystalline WO3 were observed with a high W content. Compared with other mesoporous materials, MCM48 is believed to be a better support for catalytic applications due to its interwoven and continuous three-dimensional pore system. However, little research has been carried out on MCM48 due to the difficulty in synthesis. Recently, a rapid and facile synthesis of siliceous MCM-48 mesoporous material was reported by our group.43 In the present study, the facile roomtemperature procedure was extended to the synthesis of highquality tungsten-containing cubic MCM-48 material. The mesoporous structure and the local environment of tungsten species were studied by powder X-ray diffraction (XRD), nitrogen adsorption isotherms, transmission electron microscopy (TEM), UV-visible diffuse reflectance spectroscopy (DRS), Raman spectrometry, and electron spin resonance (EPR) spectroscopy. It is found that tungsten oxide species were dispersed evenly in the mesoporous MCM-48 matrix and no bulk crystalline WO3 was formed. Experimental Section Materials. Tetraethyl orthosilicate (TEOS, 98%) and tungsten(VI) chloride (99.9+%) were purchased from Acros. Cetyltrimethylammonium bromide (CTAB, 98%) was obtained from Alfa Aesar. Ammonium hydroxide and ethanol were obtained from Fisher and Pharmo-AAPER, respectively. Deionized water was used throughout this study. Commercial WO3 nanopowder ( 0.9 instead of leveling off, and this indicates the formation of macropores. However, because of the small amount and the complicated three-dimensional pore system of MCM-48, the macropores are not clearly observed by TEM in this work. Thus, the TEM image of W-MCM-48100 only exhibits a long-range ordered cubic-type mesoporous structure. The nitrogen adsorption-desorption isotherms of W-MCM48 samples are shown in Figure 3A. A typical reversible type

IV adsorption isotherm was observed for all the samples. At relative pressures (P/P0) between 0.2 and 0.4, a sharp inflection due to capillary condensation within the mesopores is observed, which is characteristic of cubic MCM-48 mesoporous materials. The sharpness of this inflection indicates a uniform pore size distribution. The position of the inflection, which is related to the diameter of the mesopore, shifts to high relative pressures after tungsten incorporation compared with siliceous MCM48. The pore size distribution patterns in Figure 3B show the gradual change of pore diameter with an increase in tungsten content. The siliceous MCM-48 exhibits a unimodal pore size distribution with a maximum at 20 Å. For the W-MCM-48100 sample, a bimodal pore size distribution is observed and a new peak appears at 24 Å. The continuous increase of tungsten content leads to the complete disappearance of the peak at 20 Å. The W-MCM-48-25 sample shows a maximum pore size distribution at 24 Å and a small shoulder peak at about 37 Å. The pore diameters were also calculated from the BarrettJoyner-Halenda (BJH) equation using the desorption isotherm and are listed in Table 1. Consistent with the pore size distribution pattern, a higher value of 24.3 Å was obtained for W-MCM-48-25 and W-MCM-48-10 compared with 19.9 Å for siliceous MCM-48. The increase in pore size as mentioned above for W-MCM-48 materials is probably due to the larger ionic radius of W6+ (56 pm) compared with Si4+ (40 pm). Such increases have been observed previously.44,45 In addition, the isotherms of the two W-MCM-48 samples (W-MCM-48-100 and W-MCM-48-25) show a strong increase of nitrogen adsorption at P/P0 > 0.9. According to the theory of nitrogen adsorption, the sharp increase of adsorption at P/P0 > 0.9 is due to the macropore filling, and this is consistent with an increase in the unit cell volume too (see Table 1). These results indicate that a small amount of macropores is formed after the incorporation of tungsten into the MCM-48 matrix at high ratios due to the coalescence of some of mesopores to form larger macropores. This is also evident from the fact that the sharpness of the inflection point is decreased in W-MCM-48-25, indicating

TABLE 1: Textural Properties of W-MCM-48 Materials material

Si/Wa (mol)

Si/Wb (mol)

surface areac (m2 g-1)

pore volumed (cm3 g-1)

pore diametere (Å)

unit cell parameterf (Å)

W-MCM-48-200 W-MCM-48-100 W-MCM-48-50 W-MCM-48-25 W-MCM-48-10

200 100 50 25 10

1220 516 177 81 45

1230 1092 1131 970 925

1.01 0.92 0.99 0.99 1.04

19.8 20.0 24.2 24.3 24.3

78.9 80.4 82.0 83.8 83.1

a Molar ratio in the gel. b Molar ratio in the product measured by ICP. c Determined by applying the Brunauer-Emmett-Teller (BET) equation to a relative pressure (P/P0) range of 0.05-0.35 in the adsorption isotherm. d Calculated from the amount of N2 adsorbed at the highest relative pressure of 0.99. e Calculated from the Barrett-Joyner-Halenda (BJH) equation using the desorption isotherm. f Calculated using the formula a ) 6d, where d represents the d211 reflection.

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Figure 3. (A) Nitrogen adsorption-desorption isotherms of (a) siliceous MCM-48, (b) W-MCM-48-100, and (c) W-MCM-48-25. (B) Pore size distribution of (a) siliceous MCM-48, (b) W-MCM-48-100, and (c) W-MCM-48-25.

that there is some loss of uniformity in the pores. The isotherms of W-MCM-48-200 and W-MCM-48-50 are not shown because they overlap with other samples. Table 1 shows the textural properties of the W-MCM-48 materials obtained from nitrogen adsorption-desorption and low-angle XRD analysis. The W-MCM-48 materials exhibit a large surface area (900-1100 m2 g-1) and a large pore volume (>0.8 cm3 g-1), which are comparable to that of siliceous MCM48 and indicate the maintenance of the integrity of mesoporous structure. The increase in tungsten loading led to a progressive decrease in the surface area in general. The relatively lower surface area observed for W-MCM-48-25 and W-MCM-48-10 is attributed primarily to the presence of some amount of macropores in these two samples. On the other hand, there seems to be no general trend in the variation of the pore volumes. The pore volumes increase in the order W-MCM-48-10 > W-MCM-48-25, W-MCM-48-50 > W-MCM-48-100. This trend is consistent with an increase in the pore diameter in the same order. However, sample W-MCM-48-200 exhibited the higher surface area and a pore volume greater than that of W-MCM48-50 or W-MCM-48-25, but with a lower pore diameter of 19.8 Å. The lower value in the pore diameter is understandable because of the relatively low W content, and the higher surface area is due to the high uniformity of the pores. We are not entirely sure at this moment why the pore volume of this sample is higher compared with W-MCM-48-100, W-MCM-48-50, and W-MCM-48-25. Table 1 also lists the Si/W ratios in the gel and the actual Si/W ratios in the products measured by ICPMS. The actual Si/W ratios in the samples are higher than the initial values in the gel, which is due to the formation of soluble tungsten hydroxide species under basic synthesis conditions and the subsequent leaching of W during the filtration and washing steps. Similar results were also reported for the basic synthesis of tungsten-containing MCM-41 materials.46 UV-vis diffuse reflectance spectra (DRS) have been widely used to explore the local environment of transition metals dispersed in a porous silica matrix. It is very sensitive to distinguish between incorporated metal species and extraframework metal oxides.20,47 Figure 4 shows the UV-vis DRS of commercial WO3 nanopowder and W-MCM-48 materials with different tungsten contents. No absorption band was observed for the siliceous MCM-48. After the introduction of tungsten into MCM-48, a broad absorption band appeared in the range of 200-380 nm. A shift of about 100 nm to the lower wavelength with respect to the absorption edges of bulk WO3

Figure 4. Diffuse reflectance spectra of (a) WO3, (b) W-MCM-48200, (c) W-MCM-48-100, (d) W-MCM-48-50, (e) W-MCM-48-25, and (f) W-MCM-48-10. The inset shows the plots of the transformed Kubelka-Munk function versus the light energy.

is observed. As reported previously, the low-energy absorption edge shift toward lower wavelengths arises when the nuclearity of molybdenum or tungsten entities decreases due to the quantum size effects.48 Therefore, this broad absorption band should be attributed to the isolated tungsten species or low condensed tungsten oxide species. Similar results were also reported for other tungsten-containing mesoporous materials.44,46 The band-gap energy of W-MCM-48 varied between 3.7 and 3.4 eV for W-MCM-48-200 to W-MCM-48-10. The absence of an absorption band in the range of 380-480 nm indicates that tungsten oxide species were highly dispersed and no bulk crystalline WO3 was formed. Raman spectra and wide-range XRD spectra of the W-MCM48 materials were also collected to further explore the structure of the incorporated tungsten oxide species. As shown in Figure 5, the commercial WO3 nanopowder exhibits four lattice vibration bands at 803, 712, 325, and 271 cm-1. However, no Raman band attributed to the crystalline WO3 is observed for all the three W-MCM-48 samples. Consistent with the Raman analysis, no XRD reflection due to the crystalline WO3 was detected in the wide-range XRD patterns for the W-MCM-48 samples (Figure 6), confirming the absence of extraframework

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Figure 5. Raman spectra of (a) WO3, (b) W-MCM-48-100, (c) W-MCM-48-25, and (d) W-MCM-48-10.

Figure 6. Wide-range XRD patterns of (a) WO3, (b) W-MCM-48100, (c) W-MCM-48-25, and (d) W-MCM-48-10.

crystalline WO3 and the highly uniform and homogeneous distribution of tungsten oxide species in the MCM-48 matrix. The synthesis of tungsten-containing mesoporous silica has been studied by several groups. It is meaningful to compare the present work with other studies. Tungsten-containing MCM41 was synthesized by Zhang et al. with hydrothermal synthesis in a strong acidic medium.38 The Raman bands assigned to crystalline WO3 were observed for the sample with a Si/W ratio of 35.4. Dai et al. also studied the synthesis of W-doped MCM41 and reported the formation of crystalline WO3 with a Si/W ratio lower than 30.39,40 The basic synthesis of W-MCM-41 was developed by Klepel et al.46 It is found that the washing step is very essential, and it led to the loss of a large amount of tungsten. Extraframework crystalline WO3 was detected with a Si/W ratio of 30 without a prior washing step. In the case of hydrothermally synthesized W-MCM-48, the formation of crystalline WO3 was observed by UV-vis DRS in the samples with a Si/W ratio lower than 20.44 In the present study, the W-containing MCM-48 mesoporous materials were synthesized by a rapid and facile room-temperature procedure. No crystalline tungsten oxide was observed for the material with a Si/W ratio

Zhao et al.

Figure 7. Photocatalytic H2 evolution kinetics of (a) W-MCM-48200, (b) W-MCM-48-100, (c) W-MCM-48-50, (d) W-MCM-48-25, and (e) W-MCM-48-10.

of 45 (actual Si/W ratio in the product) as revealed by UV-vis DRS, Raman, and wide-range XRD analysis. The roomtemperature synthesis realized the high dispersion of tungsten oxide species in the silica matrix and the maintenance of a cubic mesoporous structure of MCM-48 at the same time. The photocatalytic process, which converts the abundant solar energy into useful chemical energy, has shown great potential applications in environmental remediation and alternative energy supply. Hundreds of novel semiconductor photocatalysts have been synthesized to further improve the photoelectric conversion efficiency. Transition-metal-containing mesoporous materials are reported to show a higher photocatalytic activity than its corresponding bulk semiconductor due to the high porosity and large surface area, which facilitate the absorption and transfer of reactant molecules. Moreover, it is also reported that the transition-metal oxides incorporated in the framework of porous silica exhibit a unique photocatalytic reactivity for various reactions due to the different local environment and coordination state in the porous silica matrix.16,17 The photocatalytic mechanism of transition metals dispersed on silica supports is based on the ligand-to-metal charge-transfer excitation. The isolated metal oxide chromophores are excited by UV or visible light with appropriate energy and form a charge-transfer excited state [Mn+-O-]*. The photogenerated electrons and holes have stronger reduction and oxidation abilities compared with the charges generated in the corresponding bulk semiconductor photocatalyst. As the best-studied metal-containing mesoporous material, the photoexcitation process of Ti-MCM-41 was monitored by photoluminescence spectra and phosphorescence lifetime analysis.23 It was reported that the isolated Ti oxide species dispersed in silica matrixes exhibited a typical photoluminescence peak at 450-550 nm and a phosphorescence lifetime of 0.1 ms, indicating the long lifetime of photogenerated charges and high charge separation efficiency. Tungsten-containing mesoporous materials were used as catalysts for selective oxidation and dehydrogenation.39,40,49 This work is the first report to study the photocatalytic activity of tungsten-containing mesoporous material for hydrogen evolution. The photocatalytic H2 evolution was performed in a methanol-water mixture under UV irradiation. Figure 7 shows the hydrogen evolution kinetics of W-MCM-48 with different tungsten contents. No H2 was produced by siliceous MCM-48

Mesoporous W-MCM-48

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15733 Conclusion In this work, tungsten oxide species were incorporated into the MCM-48 matrix during the rapid and facile room-temperature synthesis. XRD, TEM, and nitrogen adsorption isotherms demonstrated that the incorporation of W oxide species did not destroy the ordered mesoporous structure of MCM-48. The W-MCM-48 materials show good dispersion of tungsten oxide species in mesoporous silica. The tungsten oxide species that are highly dispersed in the MCM-48 matrix possess a suitable band gap and sufficient reduction potential for the photocatalytic reduction of H2O to generate H2.

Figure 8. EPR spectra of siliceous (a) MCM-48 and (b) W-MCM48-10. The spectra were recorded at 5 K after illuminating the samples under UV irradiation at 77 K for 30 min.

or commercial WO3 nanopowder. Notable photocatalytic activity for hydrogen evolution was observed for all the W-MCM-48 materials. The hydrogen evolution rates were calculated to be 2683 µmol/h · gW for W-MCM-48-200, 6015 µmol/h · gW for W-MCM-48-100, and 5480 µmol/h · gW for W-MCM-50, which are much higher than 13 µmol/h · gTi for P25 TiO2 measured under the same conditions and are also higher than 3000 µmol/ h · g reported recently for the TiO2 and a heteropolyacidincorporated zeolite photocatalyst.50 The best performance was observed for W-MCM-48-100. A further increase in tungsten content led to the decrease of hydrogen amount produced by each gram of tungsten, which may be due to the formation of relatively larger tungsten oxide clusters with low activity. However, they are unable to be identified by XRD because of the high dispersion and low crystallinity. EPR spectra allowed us to directly monitor the formation of photogenerated charges (electrons and holes) in W-MCM-48 materials. Figure 8 shows the EPR spectra of siliceous MCM48 and W-MCM-48-10 after 30 min of photoirradiation at 77 K. The samples were EPR silent prior to photoirradiation. A strong signal near g ) 2.003 due to the formation of V centers in silica was observed for both siliceous MCM-48 and W-MCM48-10 after photoirradiation.51 W-MCM-48-10 shows another EPR axially asymmetric resonance signal with g| ) 1.695 and g⊥ ) 1.776. This signal corresponds to W(V) in tungsten-oxo species, formed in the reaction of W(VI) with photogenerated electrons.52,53 The small signal at g ) 1.845 is due to a polytungstate impurity in the sample, according to the literature.54 In this study, the resonance of photogenerated holes trapped at lattice oxygen was submerged by the strong signals from V centers.55 As mentioned above, small tungsten oxide species were formed in the W-MCM-48 materials and a significant quantum size effect was observed for these tungsten oxides species dispersed in MCM-48 matrix. These tungsten oxides species were excited by UV irradiation and generated electron-hole pairs. Water was reduced by the trapped photogenerated electrons (W(V)), and hydrogen was produced. The small size and high dispersion of tungsten oxide species in MCM-48 matrix enable it to act as an efficient photocatalyst for photocatalytic hydrogen evolution.

Acknowledgment. This work was supported by NSF-CHE 0722632, NSF-CHE 0532242, NSF-EPS 0554609, SD supported 2010 Center-CRDLM, and DOE-DE-FG02-08ER64624. The EPR experiments were performed at Argonne National Laboratory under DOE BES Contract No. DE-AC02-06CH11357. We are thankful to Sarah Chadima for help with powder XRD experiments. References and Notes (1) Meynen, V.; Cool, P.; Vansant, E. F. Microporous Mesoporous Mater. 2009, 125, 170. (2) Linssen, T.; Cassiers, K.; Cool, P.; Vansant, E. F. AdV. Colloid Interface Sci. 2003, 103, 121. (3) Yu, J. C.; Wang, X.; Fu, X. Chem. Mater. 2004, 16, 1523. (4) Morey, M. S.; O’Brien, S.; Schwarz, S.; Stucky, G. D. Chem. Mater. 2000, 12, 898. (5) Morey, M. S.; Stucky, G. D.; Schwarz, S.; Froba, M. J. Phys. Chem. B 1999, 103, 2037. (6) Mathieu, M.; Van Der Voort, P.; Weckhuysen, B. M.; Rao, R. R.; Catana, G.; Schoonheydt, R. A.; Vansant, E. F. J. Phys. Chem. B 2001, 105, 3393. (7) Guidotti, M.; Pirovano, C.; Ravasio, N.; La´zaro, B.; Fraile, J. M.; Mayoral, J. A.; Coq, B.; Galarneau, A. Green Chem. 2009, 11, 1421. (8) Chaudhari, K.; Bal, R.; Srinivas, D.; Chandwadkar, A. J.; Sivasanker, S. Microporous Mesoporous Mater. 2001, 50, 209. (9) Jha, R. K.; Shylesh, S.; Bhoware, S. S.; Singh, A. P. Microporous Mesoporous Mater. 2006, 95, 154. (10) Shylesh, S.; Srilakshmi, C.; Singh, A. P.; Anderson, B. G. Microporous Mesoporous Mater. 2007, 99, 334. (11) Parida, K. M.; Dash, S. S. J. Mol. Catal. A: Chem. 2009, 306, 54. (12) Wu, C.; Kong, Y.; Xu, X.; Wang, J.; Gao, F.; Dong, L.; Yan, Q. Stud. Surf. Sci. Catal. 2007, 165, 755. (13) Thitsartarn, W.; Gulari, E.; Wongkasemjit, S. Appl. Organomet. Chem. 2008, 22, 97. (14) Morey, M. S.; Bryan, J. D.; Schwarz, S.; Stucky, G. D. Chem. Mater. 2000, 12, 3435. (15) Chen, H.; Dai, W.-L.; Gao, R.; Cao, Y.; Li, H.; Fan, K. Appl. Catal., A 2007, 328, 226. (16) Anpo, M.; Kim, T.-H.; Matsuoka, M. Catal. Today 2009, 142, 114. (17) Anpo, M.; Thomas, J. M. Chem. Commun. 2006, 3273. (18) Hu, Y.; Nagai, Y.; Rahmawaty, D.; Wei, C.; Anpo, M. Catal. Lett. 2008, 124, 80. (19) Hu, Y.; Wada, N.; Tsujimaru, K.; Anpo, M. Catal. Today 2007, 120, 139. (20) Hu, Y.; Martra, G.; Zhang, J.; Higashimoto, S.; Coluccia, S.; Anpo, M. J. Phys. Chem. B 2006, 110, 1680. (21) Higashimoto, S.; Hu, Y.; Tsumura, R.; Iino, K.; Matsuoka, M.; Yamashita, H.; Shul, Y. G.; Che, M.; Anpo, M. J. Catal. 2005, 235, 272. (22) Anpo, M.; Yamashita, H.; Ikeue, K.; Fujii, Y.; Zhang, S. G.; Ichihashi, Y.; Park, D. R.; Suzuki, Y.; Koyano, K.; Tatsumi, T. Catal. Today 1998, 44, 327. (23) Hu, Y.; Higashimoto, S.; Martra, G.; Zhang, J.; Matsuoka, M.; Coluccia, S.; Anpo, M. Catal. Lett. 2003, 90, 161. (24) Yamashita, H.; Yoshizawa, K.; Ariyuki, M.; Higashimoto, S.; Anpo, M.; Che, M. Chem. Commun. 2001, 435. (25) Liu, S. H.; Wang, H. P.; Huang, Y. J.; Sun, Y. M.; Lin, K. S.; Hsiao, M. C.; Chen, Y. S. Energy Sources 2003, 25, 591. (26) Shen, S. H.; Guo, L. J. Catal. Today 2007, 129, 414. (27) Lin, W.; Frei, H. J. Am. Chem. Soc. 2005, 127, 1610. (28) Han, H.; Frei, H. J. Phys. Chem. C 2008, 112, 16156. (29) Han, H.; Frei, H. J. Phys. Chem. C 2008, 112, 8391. (30) Lin, W.; Frei, H. J. Phys. Chem. B 2005, 109, 4929.

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