J. Phys. Chem. C 2008, 112, 17809–17813
17809
Ordered Mesoporous CeO2 Synthesized by Nanocasting from Cubic Ia3d Mesoporous MCM-48 Silica: Formation, Characterization and Photocatalytic Activity Pengfei Ji,† Jinlong Zhang,*,† Feng Chen,† and Masakazu Anpo§ Laboratory for AdVanced Materials and Institute of Fine Chemicals, East China UniVersity of Science and Technology, 130 Meilong Road, Shanghai 200237, P.R. China, and Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan ReceiVed: June 19, 2008; ReVised Manuscript ReceiVed: August 19, 2008
Ordered nanocrystalline mesoporous cerium oxide was successfully synthesized using MCM-48 molecular sieves as a hard template. The long-range ordered mesostructure was characterized by low-angle and wideangle X-ray diffraction (XRD), N2 adsorption-desorption technique, and transmission electron microscopy (TEM), and the obtained cerium oxide exhibited high similarity to the cubic Ia3d symmetry of the silica template. Due to smaller crystal size, the mesoporous materials have a blue shift in light absorption as compared with nonporous materials. X-ray photoelectron spectrum (XPS) and energy-dispersive X-ray spectrum (EDS) patterns confirm that mesoporous materials possess more surface vacancies than bulk CeO2. Photocatalytic degradation of azodye acid orange 7 under visible light illumination was used as a catalytic test reaction. The activity of mesoporous product in dye decolorization is substantially higher than those for a nonporous sample and TiO2 P25.
Cerium oxide is one of the most reactive rare earth metal oxides, which has been extensively studied and employed in various applications including fast ion conductors, oxygen storage capacitors, catalysts, UV blockers, polishing materials, and electrolytes for solid oxide fuel cells (SOFC).1-7 CeO2 or CeO2-based materials have also been found to be very important in environmental protection. In particular, supported CeO2 and CeO2-based mixed oxides are effective catalysts for the oxidation of different hydrocarbons and for the removal of organics from polluted water from different sources.8-10 The incorporation of CeO2 in the formulation of oxidation catalysts promotes various catalytic reactions such as CO2 activation,11 CO oxidation,8 CO/NO removal,12 and combustion of hydrocarbons.13 In all of these applications, two features are mainly responsible for making CeO2 a promising material for use as an active catalyst: (1) the redox couple Ce3+/Ce4+, with the ability of ceria to shift between CeO2 and Ce2O3 under oxidizing and reducing conditions, respectively, and (2) the ease of formation of labile oxygen vacancies and the relatively high mobility of bulk oxygen species.14 Nanocrytalline CeO2 materials benefit not only those applications, but they also possess some other unique properties, such as the Raman-allowed modes shifting and broadening,15 lattice expansion,16 transition from boundary diffusion to lattice diffusion,17 and a blue shift in ultraviolet absorption spectra.18 Mesoporous cerium dioxide with high surface area has been prepared by different template-assisted methods.19,20 Such materials by virtue of their large surface area exhibit greater catalytic activity. With the preparation in 1991 of mesoporous silica, a new area of chemistry, allowing the exploitation of high surface area materials, was opened up.21 The use of surfactants
as liquid-crystal templating agents so as to create a regular threedimensional micellar array about which an inorganic precursor could form a framework gives a reliable method to produce ordered mesoporous solids. The nanocasting method for CeO2 which employed uniform mesoporous silica as a hard template, pioneered by the group of Ryoo, allowed synthesis of highly ordered thermally stable mesoporous CeO2 with nanocrystalline walls for the first time.22 This method gives us the opportunities to create new ordered mesoporous metal oxides which were hardly achieved through surfactant-templating strategy. Additionally, the silica matrix serves as a rigid skeleton, allowing for the metal oxide to crystallize without growing to larger size. Therefore the nanocrystal size could be controlled in this way. Finally, the silica/metal oxide composite was washed with concentrated sodium hydroxide (NaOH) or hydrofluoric acid (HF) to remove the silica template. According to Fallah et al., CeO2 could be photoactivated by near-UV-vis range irradiation.23 These characteristics suggest that ceria could be potentially used as a visible light responsive photocatalyst. Recently, some research groups reported that ceria could decompose organics in aqueous phase under solar irradiation.24,25 Herein are reported a hard-templating method employed to form ordered mesoporous ceria with nanocrystalline frameworks using Ia3d MCM-48 as the silica template and the detailed characterizations. The silica template MCM-48 was synthesized with improved long-range ordered structure and does not need post treatment to modify pore surface. The obtained ceria materials showed well-defined ordered mesoporous structure and the same space group as that for the silica template. Also we employed acid orange 7 (AO7), a nonbiodegradable azodye, as target contaminant to test photocatalytic activities of the obtained mesoporous and nonporous materials under visible light irradiation.
* Corresponding author. Tel: +86-21-64252062; fax: +86-21-64252062; e-mail:
[email protected]. † East China University of Science and Technology. § Osaka Prefecture University.
2. Experimental Section 2.1. Materials. Tetraethylorthosilicate (TEOS) was AR grade for the silica source of MCM-48 hard template. Cetyltrimethy-
1. Introduction
10.1021/jp8054087 CCC: $40.75 2008 American Chemical Society Published on Web 10/29/2008
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Figure 1. (A) Powder low-angle XRD patterns of (a) the MSP CeO2 sample obtained after the removal of its template, (b) calcined sample of silica template MCM-48 (×1/50), inset) wide-angle XRD of the obtained MSP CeO2 sample. (B) EDS pattern of the MSP CeO2 sample.
lammonium bromide (CTAB), sodium hydroxide (NaOH), and sodium fluoride (NaF) were all CP grade. Cerium nitrate (Ce(NO3)3 · 6H2O) was AR grade and used for precursor of CeO2. Absolute ethanol serving as solvent was AR grade. All the above chemicals were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd., China. Acid orange 7 was purchased from Acros. Double distilled water was used throughout in this experiment. 2.2. Experimental. The silica molecular sieves MCM-48 were prepared in aqueous solution using CTAB and TEOS, following the recently developed procedure using NaF to improve mesoporous structure.26 The typical synthesis of crystalline mesoporous CeO2 was as follows: 2.5 g of Ce(NO3)3 · 6H2O was dissolved in 20 mL of absolute ethanol. To this solution, 0.5 g MCM-48 was dispersed and heated at 333 K under vigorous stirring. After the ethanol was evaporated, the cerium precursor/silica composite was subjected to calcinations at 723 K and gave ceria inside the silica template. The process was repeated one more time with an ethanol solution of 1.2 g Ce(NO3)3 · 6H2O and calcined at 873 K to prepare crystalline ceria networks inside the silica template. The silica template was then removed by treating three times with 2 M NaOH solution at 60 °C for 10 min each time. The obtained mesoporous product was denoted as MSP CeO2. The nonporous CeO2 was prepared by direct calcinations of cerium nitrate as reference material. 2.3. Characterization. X-ray powder diffraction (XRD) patterns of all samples were recorded on Rigaku D/MAX-2550 diffractometer using Cu KR radiation of wavelength 1.541 Å, typically run at a voltage of 40 kV and current of 100 mA. Energy dispersive X-ray spectrum (EDX) was recorded on FALCON (EDAX) instrument. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) analyses of the samples were done using a JEM 2000-EX (JEOL) instrument, and the electron beam accelerating voltage was 200 kV. N2 adsorption and desorption isotherms were carried out by using Micromeritics ASAP 2010. For BET (Brunauer-Emmett-Teller method), the as-synthesized samples were calcined following the procedures described in the experimental section and the samples were then outgassed to remove moisture and impurities at 623 K for 5 h before measurement. UV-visible absorbance spectra were obtained for the dry-pressed disk samples using a Scan UV-visible spectrophotometer (Varian, Cary 500) equipped with an intergrating sphere assembly, using BaSO4 as reflectance sample. The spectra were recorded at room temperature in air within the range 200-800 nm. The instrument employed for XPS studies was a Perkin-Elmer PHI 5000C ESCA System with Al KR radiation operated at 250 W.
2.4. Photocatalytic Activity Test. In a typical photocatalytic experiment, aqueous suspension of AO7 (50 mL, 70 mg/l) and 0.05 g photocatalyst powder were placed in a quartz tube with vigorous agitation. The used photoreactor was homemade in which a 1000 W halogen lamp was used as an illuminated light source. A cutoff filter was place outside the lamp to completely remove all wavelengths less than 420 nm to secure irradiation with visible light only. During the reaction, a water-cooling system cooled the water-jacketed photochemical reactor to maintain the solution at room temperature. The distance between the lamp and the center of quartz tube was 10 cm. The above suspension during the course of irradiation was extracted from the mixture solution every hour after centrifugation. A Cary 100 UV-vis spectrometer was used to record the change of concentration of AO7 during visible light irradiation. The starting point of the concentration-time plot was collected after the suspension was stirred for 1 h in darkness to reach the adsorption equilibrium. 3. Results and Discussion Figure 1A, traces a and b, shows the powder X-ray diffraction (XRD) patterns of the MSP cerium oxide product and cubic Ia3d MCM-48 silica matrix. The cerium oxide product obtained from the MCM-48 exhibts XRD peaks characteristic of the same space group as for the silica template. The CeO2 sample is the negative replica of the MCM-48 silica materials. A slight broadening of the peaks indicates a certain degree of loss in structural order. The wide-angle XRD (inset) of the obtained CeO2 sample is broad and low in intensity, demonstrating that the mesoporous walls of the sample are composed of nanocrystalline CeO2 frameworks which are also observed by transmission electron microscopy (TEM); the reflections correspond to the fluorite-type structure of CeO2. Application of Scherrer’s formula suggests that crystal domain size is 5.1 nm. Energy dispersive X-ray analysis (EDS) (Figure 1B) confirms almost complete removal of the silica template from the washed cerium oxide samples, which has also been described by the pioneering work of Ryoo to synthesize mesoporous CeO2 through nanocasting strategy employing a silica hard template. The silica composition could be dissolved after repeated wash in heated NaOH solution (60 °C, 2 M). According to the result of EDS, the molar ratio of the remaining Si is about 1.1% whereas the ratio of Ce:O is 1.22, much lower than the stoichiometic ratio. Transmission electron microscopy (TEM) confirms the mesostructural properties of the MSP CeO2 products. Figure 2 shows some representative images taken from distinct specimens. Figure 2A and 2B are taken along the [110] and [100] directions of cubic Ia3d MSP CeO2. They showed a high
Ordered Mesoporous CeO2
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17811
Figure 2. (A, B) TEM images of the template-free CeO2 sample calcined for 2 h at 823 K, after increasing the temperature to 823 K at 1 K/min of heating rate. (C) The HRTEM image of the CeO2 sample that shows its nanocrystalline nature. (D) The SAED pattern confirms the atomic-scale crystallinity of the porous materials.
similarity to the cubic Ia3d symmetry of MCM-41 over a long range. Figure 2C is a high resolution TEM image from which the polycrystalline nature and crystal size could be clearly observed. HRTEM reveals that the MSP CeO2 is crystalline and the crystal size is approximately 5 nm, which is in good agreement with XRD observations. The lattice fringes could be observed in magnified images of different crystallites (Figure 2C inset) and measured to be 0.28 and 0.31 nm, corresponding to the interplane distances between the (100) and (111) lattice planes of cubic CeO2. The selected-area electron diffraction (SAED) patterns consist of single spots superimposed on diffuse rings (Figure 2D), indicating that the crystal domains within the pore walls are quite small, which is in agreement with the XRD data. It is worthy to note that the TEM exhibit the images of template-free samples after subjecting to 823 K posttreatment. Therefore, we conclude that the obtained MSP CeO2 has high thermal stability and maintains ordered mesostructures upon calcination at 823 K. N2-sorption isotherms and the corresponding pore size distribution curve (inset) are shown in Figure 3. The isotherms show the adsorption jump at P/P0 ≈ 0.5-0.8, characteristic of capillary condensation in the mesopore, and the pore size distribution is narrow and centers at 3.7 nm. The template-free mesostructured cerium oxide has BET surface area of 145 m2 g-1 and total pore volume of 0.26 cm3 g-1. Since the specific BET surface area and the total pore volume of the employed hard template MCM-48 are 1223 m2 g-1 and 1.11 cm3 g-1, the negative copy CeO2 seems to have much lower values. Tiemann et al. proposed that if the different densities of CeO2 and SiO2 are taken into account, the specific surface areas of the MSP CeO2 are reasonably close to those for silica.27 Additionally,
Figure 3. Adsorption-desorption isotherms of nitrogen at 77 K for the CeO2 sample and pore size distribution (inset) calculated with the adsorption branch using the BJH method.
the complete copy of the hard template at mesoscale is impossible to achieve and there must be some precursor not going into template matrix, leading to crystal growing upon calcinations, thus decreasing surface area. It should be noted that Figure 3 exhibits a type IV gas adsorption isotherm, but the hysteresis loop is an intermediate between type H3 and H4, which indicates that the mesostructure has a uniform slit-like mesopore.28 This also could be observed in TEM images. In the etching process, the mesoporous structure of CeO2 emerged after the silica template was washed off from the ceria/silica composite. Therefore, the mesopores in the CeO2 matrix resembles the same narrow form of silica walls which are slitlike.
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Figure 4. UV-vis diffuse reflectance spectra of mesoporous (a) and nonporous (b) cerium dioxide.
TABLE 1: Element Molar Ratio, Crystal Size, and Surface Area of Mesoporous and Nonporous CeO2 sample
Ce:Oa
crystal sizeb (nm)
surface areac (m2/g)
MSP CeO2 bulk CeO2
1:1.22 1:1.88
5.1 57.2
145 9
TABLE 2: XPS O1s Data Measured for the Mesoporous and Bulk CeO2 E (eV)
a Measured by EDS analysis. b Measured by XRD (111) pattern according to Scherrer equation. c Measured by N2-sorption technique.
Figure 4a and 4b, respectively, show the UV/vis absorption spectra of the mesoporous CeO2 and the bulk ceria material. The spectra in Figure 4 show that both of the samples have intense absorption in the UV that trails into the visible region of the spectrum. The absorption edge of mesoporous CeO2 is blue-shifted compared to that of the bulk material. The optical band gap Eg was calculated based on the absorbance spectrum of the powders, using eq 1.
EBG ) 1240/λAbsorp.Edge
Figure 5. XPS spectra of the Ce 3d core level of mesoporous and bulk CeO2 (A), O (1s) of mesoporous (B), and bulk (C) CeO2.
(1)
The onset of absorption of mesoporous and bulk CeO2 were at ∼440 and 410 nm, which corresponds to the bandgap energy of ca. 2.81 and 3.02 eV, respectively. These values are consistent with the characteristic values given in literature for CeO2 materials, i.e. (EBGCeO2 ) 2.8-3.2 eV).29,30 The blue shift of the absorbance and subsequent increase of bandgap of mesoporous material result from the small crystal size, which is in agreement to the literature.18 Some of the physical properties of the two samples are listed in Table 1. Mesoporous material shows much smaller crystal size and much bigger surface area. This is because the frameworks of silica template MCM-48 inhibit the crystal growth of cerium dioxide upon calcination. Additionally, the nanocrystal obtained in the silica matrix exhibits a large extent of nonstoichiometry according to the results of EDS analysis. The molar ratio of O to Ce is 1.22, less than the 1.88 for that of the bulk material and stoichiometric value of 2, which indicates there are large amounts of oxygen vacancies in the surface of nanocrytalline mesoporous CeO2. To further confirm the result of EDS analysis, preliminary X-ray photoelectron spectrum (XPS) measurements were carried out to investigate the surface states. Figure 5A gives the Ce 3d core-level spectra for two samples, MSP and bulk CeO2. The series of V and U peaks are from the 3d5/2 and 3d3/2 states, respectively. The peak of V and V′ could be assigned to a mixing configuration of 3d94f2(O2p4) and 3d94f1(O2p5) Ce4+ states, and V′′ to the 3d94f0(O2p6) Ce4+ state. The valley between V and V′ is attributed to 3d94f1(O2p6) Ce3+ final state. The series of U structures can be explained in the same way.31 Since the
sample
O1s
MSP CeO2 bulk CeO2
530 529.4
O2 529.65 529.2
O/OT (%) OH 531.56 531.77
O2–
/OT
57.9 87.2
OH/OT 42.1 12.8
two valleys between V and V′ and U and U′ are due to photoemission from Ce3+ cations, a qualitative estimation of the degree of reduction of Ce4+ oxide can be made based on this two valley features of the spectrum. If Ce oxide contains only a small amount of Ce3+, then the valleys are very well defined, but if the degree of reduction of Ce4+ to Ce3+ is high, then Ce3+ becomes more concentrated, the valleys between V and V′ and U and U′ start to vanish.32-36 It is clearly that the valleys between V and V′ and U and U′ are much less well defined for mesoporous CeO2. This indicates nanocrystal of mesoporous material has more concentrated Ce3+ and thus more oxygen vacancies. According to J. Z. Shyu,37 another way to determine the Ce4+/Ce3+ ratio is the ratio of intensity of the U′′ peak to the total intensity of Ce3d. Therefore, we also quantitatively measured the ratios of U′′/Ce3d of both samples. For bulk CeO2, the ratio of the intensity of the U′′ peak to the total intensity of Ce3d is about 10.5%, and is larger than that of MSP sample, which is about 7.9%. This result also confirmed that the MSP sample has a larger Ce3+ content thus more surface vacancies. Figure 5B and 5C shows the O(1s) spectra of both samples. The O(1s) peak of MSP CeO2 is broader than that of bulk CeO2, indicating that the nanocrystalline powder of MSP CeO2 has more affluent oxygen species on the surface. The O1s peaks could be fitted into two peaks referred to as the lattice oxygen O2- and the chemisorbed OH group.38,39 The fitted curves are shown in Figure 5. The O1s peaks at about 529-530 eV could be attributed to the lattice oxygen (O2-) for CeO2, and 531-532 eV belongs likely to the chemisorbed OH species.38,40 Both of the spectra were fitted using a 50:50 Gaussian:Lorentzian peak shape,41 and satisfactory fitting results were obtained as shown in Table 2. As shown in Table 2, for mesoporous material, the percentage of the chemisorbed OH groups out of the total oxygen (OT) is much larger than that for the bulk. With the existence of Ce3+ on the surface of mesoporous CeO2 catalyst, it could create a charge imbalance, the vacancies and unsaturated chemical bonds being on the catalyst surface. These factors
Ordered Mesoporous CeO2
J. Phys. Chem. C, Vol. 112, No. 46, 2008 17813 Acknowledgment. This work has been supported by Science andTechnologyCommissionofShanghaiMunicipality(07JC14015); Shanghai Nanotechnology Promotion Centre (0752nm001), National Nature Science Foundation of China (20773039, 20577009); National Basic Research Program of China (973 Program, 2007CB613306, 2004CB719500); and the Ministry ofScienceandTechnologyofChina(2006AA06Z379,2006DFA52710). References and Notes
Figure 6. Temporal course of the photodegradation of AO7 in CeO2 aqueous dispersions under visible light irradiation. Experimental conditions: [AO7] ) 2 × 10-4 M (70 mg/L), [catalyst] ) 1 g/L, initial pH 6.8 (natural), T ) 298 K.
enable a certain amount of chemisorbed OH to exist on the surface. The increase of Ce3+ content facilitates the increase of chemisorbed oxygen on the catalyst surface. In order to test the photocatalytic performance of these two cerium samples, photodegradation experiments of acid orange 7 were carried out under visible light illumination at wavelengths longer than 420 nm in aqueous CeO2 dispersions. The data displayed in Figure 6 clearly indicate that under otherwise identical conditions MSP CeO2 exhibits a much higher activity than that for bulk CeO2. The experimental results showed that AO7 solution was stable under visible light irradiation in the absence of CeO2. For the MSP material, the total degradation rate is 95% within 7 h irradiation, much higher than that for the bulk sample which only reaches 65% of total decomposition. Also, the catalyst CeO2 exhibits better photocatalytic activity than the reference catalyst TiO2 P25 (31%) which acts as a benchmark material in photocatalyst study. However, the small crystal size of MSP CeO2 increases scattering effect of the catalyst in aqueous suspension, leading to an increase in the turbidity and hence a decrease in visible light penetration. Therefore, the photoactivated volume of the suspension decreases and the photoactivities do not show a proportional relationship to the surface area. After all, MSP material has larger surface area, and more surface oxygen vacancies result from smaller crystal size, making it a more favored candidate in wastewater treatment. 4. Conclusion In summary, the utilization of mesoporous MCM-48 silica as a structure matrix has turned out to be a versatile tool for the synthesis of mesoporous CeO2 in a replication process. The silica matrix not only serves as hard template, its rigid wall also effectively holds back the crystal growth during calcinations. The mesoporous product exhibits the same space group as for the silica template. Because of the controlled nanocrystal size, mesoporous CeO2 shows a blue shift in UV-vis absorbance and has many more surface vacancies. As a photocatalyst, the mesoporous material shows significantly increased catalytic activity in acid orange 7 decomposition in comparison with the corresponding nonporous analogues and standard reference TiO2 materials.
(1) Zhang, J.; Ju, X.; Wu, Z. Y.; Liu, T.; Hu, T. D.; Xie, Y. N. Chem. Mater. 2001, 13, 4192. (2) Balducci, G.; Islam, M. S.; Kapsˇar, J.; Fornasiero, P.; Graziani, M. Chem. Mater. 2000, 12, 677. (3) Sayle, T.; Parkerb, C.; Sayle, D. C. Chem. Commun. 2004, 2438. (4) Wang, Z.; Feng, X. J. Phys. Chem. B 2003, 107, 13563. (5) Shchukin, D. G.; Caruso, R. A. Chem. Mater. 2004, 16, 2287. (6) Inaba, H.; Tagawa, H. Solid State Ionics 1996, 83, 1. (7) Tuller, H. L.; Nowick, A. S. J. Electrochem. Soc. 1975, 122 (2), 255. (8) Serre, C.; Garin, F.; Belot, G.; Marie, G. J. Catal. 1993, 141, 9. (9) Monteiro, R. S.; Dieguez, L. C.; Schmal, M. Catal. Today. 2001, 65, 77. (10) Imamura, S.; Faduda, I.; Ishida, S. Ind. Eng. Chem. Res. 1988, 27, 718. (11) Trovarelli, A.; Dolcetti, G.; De Leitenburg, C.; Kaspar, J.; Finetti, P.; Santoni, A. J. Chem.Soc., Faraday Trans. 1992, 88, 1311. (12) Zwinkels, M. F. M.; Jaras, S. G.; Menon, P. G. Catal. ReV.-Sci. Eng. 1993, 35, 319. (13) Paparazzo, E. Surf. Sci. Lett. 1990, 234, L253. (14) Mackrodt, W. C.; Fowles, M.; Morris, M. A. European Patent 91, 307, 165, 1991. (15) Weber, W. H.; Hass, K. C.; McBride, J. R. Phys. ReV. B. 1993, 50, 13297. (16) Zhou, X. D.; Huebner, W. Appl. Phys. Lett. 2001, 79, 3512. (17) Zhou, X. D.; Huebner, W.; Anderson, H. U. Appl. Phys. Lett. 2002, 80, 3814. (18) Tsunekawa, S.; Fukuda, T.; Kasuya, A. J. Appl. Phys. 2000, 87, 1318. (19) Lyons, D. M.; Harman, L. P.; Morris, M. A. J. Mater. Chem. 2004, 14, 1976. (20) Yada, M.; Kitamura, H.; Ichinose, A.; Machida, M.; Kijima, T. Angew. Chem., Int. Ed. 1999, 38, 3506. (21) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Bech, J. S. Nature 1992, 359, 710. (22) Laha, S. C.; Ryoo, R. Chem. Commun. 2003, 2138. (23) Fallah, J. E.; Hilaire, L.; Normand, F. Le. J. Electron Spectrosc. Relat. Phenom. 1995, 73, 89. (24) Zhai, Y.; Zhang, S.; Pang, H. Mater. Lett. 2007, 61, 1863. (25) Borker, P.; Salker, A. V. Mater. Chem. Phys. 2007, 103, 366. (26) Wang, L.; Shao, Y.; Zhang, J. Mater. Lett. 2005, 59, 3604. (27) Roggenbuck, J.; Scha¨fer, H.; Tsoncheva, T.; Minchev, C.; Hanss, J.; Tiemann, M. Microporous Mesoporous Mater. 2007, 101, 335. (28) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169. (29) Hogath, C. A.; Al-Dhhan, Z. T. Phys. Status Solidi B 1986, 137, K157. (30) Sundaram, K. B.; Wahid, P. Phys. Status Solidi B 1990, 161, K64. (31) Larsson, P.; ersson, A. J. Catal. 1998, 179, 72. (32) Mullins, D. R.; Overbury, S. H.; Huntley, D. R. Surf. Sci. 1998, 409, 307. (33) Romeo, M.; Bak, K.; El Fallah, J.; Le Normand, F.; Hilaire, L. Surf. Interface Anal. 1993, 20, 508. (34) Paulis, M.; Peyrard, H.; Montes, M. J. Catal. 2001, 199, 30. (35) Daturi, M.; Binet, C.; Lavalley, J. C.; Galtayries, A.; Sporken, R. Phys. Chem. Chem. Phys. 1999, 1, 5717. (36) Noronha, B.; Fendley, E. C.; Soares, R. R.; Alvarez, W. E.; Resosco, D. E. Chem. Eng. J. 2001, 82, 21. (37) Shyu, J. Z.; Weber, W. H.; Gandhi, H. S. J. Phys. Chem. 1988, 92, 4964. (38) Sinha, A. K.; Suzuki, K. J. Phys. Chem. B 2005, 109, 1708. (39) Larachi, F.; Pierre, J.; Adnot, A.; Bernis, A. Appl. Surf. Sci. 2002, 195, 236. (40) Chen, H.; Sayari, A.; Adnot, A.; Larachi, F. Appl. Catal., B 2001, 32, 195. (41) Nesbitt, H. W.; Canning, G. W.; Bancroft, G. M. Geochim. Cosmochim. Acta 1998, 62, 2097.
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