Ultrasound-Assisted Nanocasting Fabrication of Ordered Mesoporous

Jan 22, 2010 - With the assistance of ultrasonic waves, the metal precursors can readily diffuse from the bulk solution to the inner pores of the sili...
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Ultrasound-Assisted Nanocasting Fabrication of Ordered Mesoporous MnO2 and Co3O4 with High Surface Areas and Polycrystalline Walls Jiguang Deng, Lei Zhang, Hongxing Dai,* Yunsheng Xia, Haiyan Jiang, Huan Zhang, and Hong He Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of EnVironmental and Energy Engineering, Beijing UniVersity of Technology, Beijing 100124, China ReceiVed: October 24, 2009; ReVised Manuscript ReceiVed: January 2, 2010

Highly ordered mesoporous MnO2 and Co3O4 are prepared by adopting an SBA-16-based nanocasting strategy under ultrasonic irradiation and characterized by means of numerous techniques. It is shown that the asfabricated manganese and cobalt oxides possess well-ordered mesoporous architectures with polycrystalline pore walls. With the assistance of ultrasonic waves, the metal precursors can readily diffuse from the bulk solution to the inner pores of the silica template. The repeated four-step fabrication process, filling f filtration f washing f calcination, is beneficial for preventing the formation of manganese or cobalt oxide nanoparticles on the external surfaces of the template and facilitating more metal precursors to fill the mesopore channels of the template. After removal of the silica template by a 2 mol/L NaOH aqueous solution, the as-received highly ordered mesoporous MnO2 and Co3O4 exhibit a surface area of up to 266 and 313 m2/g, respectively, which is about 2-3 times higher than that reported in the literature. The mesoporous MnO2 and Co3O4 samples are more readily reduced at low temperatures and show much better catalytic performance for toluene complete oxidation than their bulk counterparts. The excellent performance of the mesoporous materials is ascribed to their ordered mesoporous structure, better reducibility, and high surface area. Introduction Because of their unique physicochemical properties and potential applications to various fields, ordered mesoporous transition-metal oxides have received great attention.1-18 Unlike the siliceous materials, the transitional metal oxides are more susceptible to hydrolysis, redox reactions, or phase transitions, and possess a number of different coordination numbers and multiple oxidation states. Hence, it is rather difficult to obtain their mesoporous structures.2,4,9,10,12,16 In recent years, the soft and hard templating methods for synthesis of well-ordered mesoporous transition-metal oxides have been reported.9 In terms of the former, soft organic materials, such as block copolymers, are used to structurally direct transition-metal oxides into organic-inorganic hybrids often with a hexagonal morphology. After removing the organics by calcination in air, metal oxides with a mesoporous structure can be obtained. However, because the temperature for thermal treatment without a structural collapse cannot exceed 400 °C, the resulting oxides are not highly crystalline,9,16 which may limit their wider application in nanotechnology.1,6,11,19 Thus, the recently developed nanocasting approach seems to be an attractive alternative.11,19-21 Typically, ordered mesoporous silica or replicated mesoporous carbon can be employed as a rigid template into which a solution-based precursor of the desired phase is introduced. After generating the desired phase by calcination, the silica template can be dissolved away with a suitable concentration of NaOH or HF solution, finally leaving a replica mesoporous structure of the target compound. Unfortunately, the inorganic precursors are inclined to be adsorbed on the external surface of templates, forming large particles outside the mesopores. Furthermore, the pores cannot be completely filled, even when the vinyl* To whom correspondence should be addressed. Tel: +86-10-67396588. Fax: +86-10-6739-1983. E-mail: [email protected].

functionalized silica templates and/or multi-impregnation methods are used,16,21,22 all of which may cause the framework formed inside the pores to be lacking in sufficient internal crosslinkage.21 Therefore, it remains a major challenge to fully fill the pores of templates and obtain highly ordered mesoporous transition-metal oxides with large surface areas and crystalline pore walls. Owing to its special cavitation effect, which generates hot spots possessing a temperature of about 5000 °C, a pressure of about 500 atm, and lifetimes of a few microseconds,23 the ultrasonic technique has been proven to be an excellent method for the preparation of mesoporous materials.22,24-27 The ultrasound is beneficial for creating a clean, highly reactive surface on metals,22,23 promoting liquid-solid mass transfer and dispersion of precursors,25,28 and improving the mesoporosity of the as-fabricated materials.27 By combining two techniques of ultrasonic irradiation and nanocasting method, we herein report the facile fabrication and catalytic activity for toluene combustion of highly ordered mesoporous MnO2 and Co3O4 with extremely high surface areas and crystalline pore walls. Experimental Section Sample Preparation. The three-dimensional (3D) mesoporous silica (SBA-16) template was synthesized according to the procedure described elsewhere.29 In a typical synthesis of mesoporous MnO2, 0.5-1.0 g of SBA-16 was added to 10-15 mL of a Mn(NO3)2 aqueous solution (concentration, 0.12-0.40 mol/L), and then the mixture was ultrasonically irradiated (100 kHz ultrasonic waves produced by the 300 W output power) at room temperature (RT). After irradiation for 60-100 min, the Mn-containing SBA-16 was filtered, washed thoroughly with deionized water, and dried at 60 °C for 24 h. The so-obtained solid was thermally treated in a muffle furnace with a heating

10.1021/jp910159b  2010 American Chemical Society Published on Web 01/22/2010

Nanocasting Fabrication of Mesoporous MnO2 and Co3O4 rate of 1 °C/min from RT to 260 °C and then maintained at this temperature for 1.5 h. The above procedure was repeated several times (depending upon the metal precursor concentration employed) until the weight of the dried solid did not change. The resulting material was calcined in air at a ramp of 1 °C/ min from RT to 450 °C and kept at this temperature for 2.5 h. For fabrication of ordered mesoporous Co3O4, the cobalt precursor and the first thermal treatment temperature was Co(NO3)2 and 300 °C, respectively. Such derived corresponding samples were denoted as MnO2/SBA-16 and Co3O4/SBA-16. By using a sodium hydroxide aqueous solution (2 mol/L), the silica template was removed under ultrasonic irradiation at RT for 2 h. After filtration, washing with deionized water, and drying at 60 °C for 24 h, one can obtain the well-ordered mesoporous MnO2 and Co3O4 materials. All of the chemicals were of analytical grade and purchased from Beijing Chemical Reagent Company. Sample Characterization. Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker/AXS D8 Advance diffractometer operated at 35 kV and 35 mA using Cu KR radiation and a Ni filter (λ ) 0.15406 nm). Crystal phases were identified by referring the diffraction lines to those of the powder diffraction files, 1998 ICDD PDF database. Surface areas, pore size distributions, and N2 adsorption-desorption isotherms of the samples were measured via N2 adsorption at -196 °C on an ASAP 2020 adsorption analyzer (Micromeritics) with the samples being outgassed at 250 °C for 2 h under vacuum prior to measurements; by using the data of desorption branches of the isotherms, surface areas and pore size distributions were calculated according to the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) method, respectively. The energy-dispersive X-ray spectroscopic (EDXS) spectra of the samples were recorded by means of a Gemini Zeiss Supra 55 apparatus operating at 20 kV. By means of a JEOL-2010 instrument (operated at 200 kV), high-resolution transmission electron microscopic (HRTEM) images and selected area electron diffraction (SAED) patterns of the samples were obtained. Hydrogen temperature-programmed reduction (H2-TPR) was carried out in the 50-700 °C range on a Micromeritics AutoChem II 2920 apparatus. A 0.1 g portion of the sample was treated in a helium flow of 25 mL/min at 120 °C for 1 h and then cooled in the same atmosphere to 50 °C. The sample was then reduced in a flow of 5% H2-95% He at a ramp of 10 °C/min. The outlet gases were analyzed online using a thermal conductive detector (TCD). The TCD responses were calibrated against that of the complete reduction of a known powder of standard CuO (Aldrich, 99.995%). Catalytic Test. The catalytic activity was evaluated with the sample charged in a continuous flow fixed-bed quartz microreactor (i.d. ) 4 mm). To minimize the effect of hot spots, the catalyst (0.1 mL, 40-60 mesh) was diluted with an equal amount of quartz sands (40-60 mesh). The reactant feed (flow rate ) 33.3 mL/min) was 1000 ppm toluene + O2 + N2 (balance) with a toluene/O2 molar ratio of 1/200, and the space velocity (SV) was 20 000 h-1. The outlet gases were analyzed online by a gas chromatograph (Shimadzu GC-2010) equipped with a flame ionization detector and a TCD, using a Chromosorb 101 column for toluene and a Carboxen 1000 column for permanent gas separation. The balance of carbon throughout the investigation was estimated to be 99.5%. For comparison purposes, we also measured the catalytic performance of bulk MnO2 (surface area, 10.1 m2/g) and Co3O4 (surface area, 6.7 m2/g) samples (Beijing Chemical Reagent Company, A.R., 99.9%).

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Figure 1. (A) Wide-angle and (B) small-angle XRD patterns of the ordered mesoporous (a) MnO2/SBA-16, (b) MnO2, (c) Co3O4/SBA16, and (d) Co3O4 samples.

Results and Discussion According to the wide-angle XRD patterns (Figure 1A), we found that the introduction of transition-metal oxides to the mesopores of SBA-16 led to a decline in intensity of the broad peak ascribable to the mesopore structure, a clear indication of MnO2 and Co3O4 presence in the mesoporous channels.30 Because of the presence of a large mass loss during decomposition of the metal nitrate precursors, however, it is hard to achieve a complete filling of the pores with MnO2 and Co3O4.2 To increase the loadings as much as possible, we repeated the fourstep fabrication process (filling f filtration f washing f calcination) 4-5 times. By referring to the standard data of MnO2 (JCPDS PDF No. 24-0735) and Co3O4 (JCPDS PDF No. 78-1970), one can realize that the XRD line positions and relative intensities of the silica-free manganese and cobalt oxides were consistent with those of their standard counterparts.18,31 In other words, the as-prepared MnO2 and Co3O4 samples exhibited good crystallinity. The EDXS analyses on various

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Figure 2. N2 adsorption-desorption isotherms and pore size distributions (insets) of the ordered mesoporous (a) MnO2/SBA-16, (b) MnO2, (c) Co3O4/SBA-16, and (d) Co3O4 samples.

entities of the silica-removed samples indicate complete elimination of the silica template after the digestion of the samples in the NaOH solution. The Mn:O and Co:O atomic ratios were close to 1:2 and 3:4, respectively (Table S1 of the Supporting Information). Thus, the corresponding as-fabricated samples could be attributed to MnO2 and Co3O4. Figure 1B shows the small-angle XRD patterns of the samples before and after template removal. Although the intensities of the XRD lines due to the (110) and (200) planes in the Mn- and Co-containing samples (Figure 1B) were much lower than those in the SBA16 sample (Figure S1a of the Supporting Information), these XRD lines at 2θ ) 0.90 and 1.56°, characteristics of an ordered mesoporous structure, could be observed clearly, demonstrating the formation of mesopores in the as-received MnO2 and Co3O4 samples. The discrepancy in intensity of the small-angle signals indicates the difference in quality of ordered mesoporosity. The SBA-16 sample displayed a type-IV sorption isotherm, with a hysteresis loop characteristics of mesoporous materials, and the pore size distribution of SBA-16 was quite narrow (Figure S1b,c of the Supporting Information). Shown in Figure 2 are the N2 adsorption-desorption isotherms and pore size distributions of the as-fabricated MnO2/SBA-16, Co3O4/SBA16, MnO2, and Co3O4 samples. Clearly, the sorption curves in the relative pressure range of 0.4-0.8 of those samples exhibited

slightly distorted type-IV isotherms, indicating the formation of mesopores9,18 with the quality of pores inferior to those of the SBA-16 template. In addition, one can also observe a small hysteresis loop in the relative pressure range of 0.8-1.0 of each sample, implying the presence of a small amount of macropores that might originate from the voids between the ordered mesoporous entities.32 There are two peaks observed in the pore size distribution profiles of the Co3O4/SBA-16 and Co3O4 samples: one in the 3.6-3.8 nm range and the other in the 5.5-6.4 nm range. It indicates the presence of two kinds of mesopores5,15 in the two Co3O4-containing samples. The mesoporous MnO2/SBA-16 and MnO2 displayed a narrow pore size distribution, with the pore size peaked at 7.0-7.6 nm. The textural properties of these samples are summarized in Table 1. Due to inclusion of manganese oxide and cobalt oxide precursors in the mesopores of the silica template, the surface areas and pore volumes of MnO2/SBA-16 and Co3O4/SBA-16 decreased greatly as compared with those of SBA-16. The pore size distributions of either MnO2 and MnO2/SBA-16 or Co3O4 and Co3O4/SBA-16 obtained were similar (Figure 2 and Table 1), a result possibly due to incomplete filling of the pores in SBA-16 by MnO2 or Co3O4. It is confirmed by the fact that the surface area of the MnO2/SBA-16 or Co3O4/SBA-16 composite was larger than that of the mesoporous MnO2 or Co3O4 product

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Figure 3. TEM images and SAED patterns (insets) of the ordered mesoporous (a) MnO2/SBA-16, (b, c) mesoporous MnO2, (d) Co3O4/SBA-16, and (e, f) mesoporous Co3O4 samples.

TABLE 1: Physical Properties of the As-Fabricated Samples sample

surface area (m2/g)

average pore diameter (nm)

pore volume (cm3/g)

SBA-16 Co3O4/SBA-16 mesoporous Co3O4 MnO2/SBA-16 mesoporous MnO2

1011 456 313 490 266

3.6 5.0 5.1 6.1 6.4

1.0 0.6 0.4 0.7 0.5

because a complete filling of the mesopores in SBA-16 would cause the composite material to show a low surface area. In other words, the higher surface area of the MnO2/SBA-16 or Co3O4/SBA-16 composite was mainly from the incompletely filled mesopores in SBA-16. Furthermore, the MnO2 or Co3O4 nanoparticles initially formed in the mesopores of SBA-16 grew gradually to form distorted nanorods during calcination, causing the mesoporous MnO2 or Co3O4 generated after silica template removal to display a pore structure different from that of the silica template. The presence of a small amount of macropores might contribute to the increase in average pore size of MnO2 or Co3O4. It is worth noticing that, although the surface area and pore volume of ordered mesoporous MnO2 and Co3O4 were lower than those of the corresponding MnO2/SBA-16 and Co3O4/SBA-16, the silica-free manganese and cobalt oxides still possessed a very high surface area of 266 and 313 m2/g, respectively. Despite that the surface area of our mesoporous MnO2 sample was less than that (316 m2/g) of mesoporous MnO2 prepared by a surfactant-assisted wet-chemistry route,3,14 it was much higher than that (84-127 m2/g) of mesoporous MnO2 fabricated using other silica templates (KIT-6 and SBA-15).8,15,22,31 To the best of our knowledge, porous Co3O4 materials reported in the literature so far have surface areas of not larger than 160 m2/g,2,12,17,18,33 which is only about half of

our mesoporous Co3O4 sample. In addition, the so-obtained transition-metal oxides exhibited ordered mesoporous architectures and large pore volumes, in favor of enhancing catalytic performance due to the easy diffusion of the reactants and facile accessibility of the active phases.17 The ordered mesoporosity of the as-obtained samples is further confirmed by the TEM observations. As shown in Figure S1d of the Supporting Information, the SBA-16 template displayed arrays of well-ordered mesopores. Such an ordered porous structure of SBA-16 was well-retained after the loading of the metal precursors. It is noted that no MnO2 and Co3O4 particles were seeable in the corresponding MnO2/SBA-16 and Co3O4/SBA-16 samples (Figure 3a,d). This could be attributed to (i) the enhancement of liquid-solid mass transfer and good dispersion of the precursor species resulting from the ultrasonic irradiation,25,28,30 and (ii) the effectiveness of the filtration and thorough washing before silica removal.28 With the elimination of the silica template, the as-prepared MnO2 and Co3O4 samples still retained a 3D ordered mesoporous structure. Theoretically, via such a nanocasting process, the pore diameter and wall thickness of the fabricated transition-metal oxides should be equal to the pore wall thickness and pore size of the SBA-16 template, respectively. Owing to incomplete filling of SBA-16 pores by the metal precursors and shrinking of the MnO2 and

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Figure 5. H2-TPR profiles of the bulk and ordered mesoporous MnO2 and Co3O4 samples.

Figure 4. HRTEM images of the as-fabricated ordered mesoporous (a) MnO2 and (b) Co3O4 samples.

Co3O4 aggregates during the calcination process, a slight deviation would be inevitable.2,12 The recording of multiple bright electron diffraction rings in the SAED patterns (insets of Figure 3c,f) reveals the formation of polycrystalline materials. In other words, the as-fabricated MnO2 and Co3O4 samples had crystalline pore walls.7,11 Another piece of supporting evidence for the generation of crystalline walls is from the HRTEM observations, as shown in Figure 4. The ordered mesoporous MnO2 and Co3O4 samples displayed clear lattice fringes. The d spacing of the mesoporous MnO2 particles was estimated to be 0.307 nm, which is rather close to the d spacing (0.311 nm) of the (110) lattice plane of MnO2 (JCPDS PDF No. 24-0735).31 For the ordered mesoporous Co3O4 sample, the d value was 0.287 nm, in good agreement with that (0.286 nm) of the (220) lattice plane of Co3O4 (JCPDS PDF No. 78-1970).12 Hence, the results of HRTEM analysis substantiate the deductions derived from the EDXS and wideangle XRD investigations. Figure 5 shows the H2-TPR profiles of the mesoporous and bulk MnO2 and Co3O4 samples. For the mesoporous MnO2 sample, there are two main reduction bands at 324 and 446 °C (with a shoulder at 493 °C); for the bulk MnO2 sample, however, a weak reduction band at 390 °C and a strong reduction band at 510 °C appear. In the case of the Co3O4 samples, there is a strong reduction band at 348 °C with a small shoulder at 273 °C for the mesoporous material and a strong reduction band at 490 °C with a weak shoulder at 415 °C for the bulk one. A quantitative analysis on the H2-TPR profiles reveals that the total H2 consumption is 11.0, 10.9, 16.2, and 16.0 mmol/g for the mesoporous MnO2, bulk MnO2, mesoporous Co3O4, and bulk Co3O4 samples, respectively. According to the results reported

previously,34-37 the reduction process could be reasonably divided into two steps: (i) Mn4+ f Mn3+ (or Co3+ to Co2+) and (ii) Mn3+ f Mn2+ (or Co2+ to Co0). If the reduction of Mn or Co ions obeyed the above mechanism, the theoretical H2 consumption would be 11.5 and 16.6 mmol/g for the MnO2 and Co3O4 samples, respectively. Clearly, the theoretical H2 consumptions are only slightly higher than the corresponding experimental H2 consumptions. This result further confirms the formation of MnO2 and Co3O4 crystal phases in the as-prepared mesoporous samples. It should be pointed out that, although the total H2 consumptions of the bulk and mesoporous MnO2 and Co3O4 samples are almost the same, the onset reduction temperatures for the mesoporous materials are much lower than those for their bulk counterparts. It means that the mesoporous samples exhibit much better reducibility than their corresponding bulk ones. Such a result might be associated with the ordered mesoporosity and high surface area of the former.32 Figure 6 shows the catalytic performance of the bulk and mesoporous MnO2 and Co3O4 materials for the complete oxidation of toluene. Under the conditions of toluene concentration ) 1000 ppm, toluene/O2 molar ratio ) 1/200, and space velocity ) 20 000 h-1, toluene conversion increases monotonously with the rise in temperature. CO2 and H2O are the only products and the estimated carbon balance is around 99.5%, indicating that toluene can be completely oxidized over these materials. From Figure 6, one can observe that the temperature required for toluene complete oxidation is 270, 340, 240, and 310 °C over the mesoporous MnO2, bulk MnO2, mesoporous Co3O4, and bulk Co3O4 catalysts, respectively. Apparently, the mesoporous MnO2 and Co3O4 catalysts perform much better than their bulk counterparts, with the cobalt oxides outperforming the manganese oxides. Compared with the bulk materials, the ordered mesoporous samples show higher surface areas (Table 1) and better low-temperature reducibility (Figure 5). Both factors play an important role in enhancing the performance of the catalysts for toluene combustion. Therefore, it is understandable that the mesoporous materials outperformed their bulk counterparts. Similar results were also reported by other researchers who investigated the catalytic removal of volatile organic compounds over mesoporous chromia.10,32

Nanocasting Fabrication of Mesoporous MnO2 and Co3O4

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2699 liquid-solid mass transfer and dispersion of the metal precursors within the pores of the silica template and (ii) the effective multistep procedure (impregnation f filtration f washing f calcination) minimized the formation of manganese oxide and cobalt oxide nanoparticles outside the pores of the silica template and maximized the filling of the pore channels. The mesoporous MnO2 and Co3O4 samples are more reducible at lower temperatures and show much better catalytic performance than their bulk counterparts. The ordered mesoporous structure, better reducibility, and high surface area account for the excellent performance of the mesoporous materials for the combustion of toluene.

Figure 6. Toluene conversion as a function of reaction temperature over bulk MnO2 (∆), mesoporous MnO2 (2), bulk Co3O4 (0), and mesoporous Co3O4 (9) under the conditions of toluene concentration ) 1000 ppm, toluene/O2 molar ratio ) 1/200, and space velocity ) 20 000 h-1.

Acknowledgment. The work described above was supported by the projects granted by the National Natural Science Foundation of China (No. 20973017), the Beijing Municipal Commission of Education (No. PHR200907105), and the Natural Science Foundation of Beijing Municipal Commission of Education (Key Class B Project No. KZ200610005004). Supporting Information Available: Small-angle XRD pattern, N2 sorption isotherm, pore size distribution, TEM image of the SBA-16 template, and EDXS results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. Catalytic activity versus on-stream reaction time over mesoporous MnO2 at 270 °C (2) and mesoporous Co3O4 (0) at 240 °C under the conditions of toluene concentration ) 1000 ppm, toluene/ O2 molar ratio ) 1/200, and space velocity ) 20 000 h-1.

To examine the catalytic stability of the mesoporous MnO2 and Co3O4 samples, we measured their catalytic activities within 24 h of on-stream reaction, and the results are shown in Figure 7. It can be clearly seen that no significant losses in activity were observed in the reaction period of 24 h over the mesoporous MnO2 and Co3O4 catalysts. This result demonstrates that the two mesoporous materials are durable. Conclusions In summary, by adopting the ultrasound-assisted SBA-16nanocasting strategy, we have fabricated highly ordered mesoporous MnO2 and Co3O4 with polycrystalline walls. The surface area of the as-obtained MnO2 and Co3O4 samples was rather high, up to 266 and 313 m2/g, respectively. It might be a result due to the combined actions: (i) ultrasonic irradiation promoted

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