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Rapid synthesis of highly ordered Si-MCM-41
Journal of Colloid and Interface Science 319 (2008) 377–380 www.elsevier.com/locate/jcis
Letter to the Editor
Rapid synthesis of highly ordered Si-MCM-41 Xianbin Liu, Hui Sun, Yanhui Yang ∗ School of Chemical and Biomedical Engineering, Nanyang Technological University, 637459 Singapore Received 4 October 2007; accepted 18 November 2007 Available online 24 November 2007
Abstract A very short-time synthesis of highly ordered MCM-41 molecular sieve was formulated by using cetyltrimethylammonium bromide (CTAB) as the template and silica gel from SiliCycle as the silica source. The physical properties of MCM-41 samples were characterized by X-ray diffraction (XRD), nitrogen physisorption, and transmission electron microscopy (TEM). The MCM-41 sample prepared in this study exhibited well defined long-range order and good hydrothermal stability. It was demonstrated that reducing the time of self-assembly step to 2 h during the synthesis had no unfavorable effect on the quality of MCM-41 materials. © 2007 Elsevier Inc. All rights reserved. Keywords: Si-MCM-41; Rapid synthesis; Nitrogen physisorption; TEM; XRD
1. Introduction Mobil has patented a family of nanoporous materials (mesoporous molecular sieve) designated as M41S, in which MCM41 is considered to be the most promising structure [1,2]. Narrow pore size distribution with size controllable pores over micrometer length scales makes the MCM-41 materials attractive to many applications, e.g., the separation of proteins, selective adsorption of large molecules from effluents [3]. Most applications require MCM-41 with a narrow pore size distribution, large surface area and long-range ordered structure. Intensive efforts have been devoted to the synthesis and characterization of MCM-41 materials [4,5]. Generally, MCM-41 is prepared under alkaline conditions by using surfactant as the template under high temperature. The synthesis procedure usually takes about 2 days or even longer, it makes the practical application of this material severely hampered because it is quite unfavorable to the large scale production. It would be useful to develop an economically feasible method to synthesize MCM-41 for large scale, reducing the synthesis time, while maintaining the ordered mesoporous structure, will be the most applicable approach. Recently, the occurrence of self-assembly of surfactant and silica species has been found in a relatively short time * Corresponding author. Fax: +65 6794 7553.
E-mail address: [email protected] (Y. Yang). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.11.025
(about 2 h) using in situ synchrotron small angle X-ray scattering (SAXS), X-ray diffraction (XRD), time-resolved NMR, TEM, and EPR spectroscopy [6,7]. However, this work did not explore the feasibility of fabricating high-quality mesoporous materials within a shorter synthesis time. There are only few reports on the rapid synthesis of MCM-41 up to now; the very first result of the rapid synthesis of MCM-41 using successive two steps (aging for 30 min, then evaporation-to-dryness for 2 h) was reported by Inagaki et al. [8]. In this contribution, we formulated a facile method for the rapid synthesis of Si-MCM-41, the aging time was reduced to 2 h instead of 48 h usually applied [1,2]. The thermal and hydrothermal stabilities were tested as well. 2. Materials and methods Si-MCM-41 was synthesized using cetyltrimethylammonium bromide (CTAB, Sigma–Aldrich) as the surfactant source and silica gel from SiliCycle as the silica source. In a typical synthesis, CTAB and NaOH were dissolved in deionized water and the silica source was added to this solution and stirred for 30 min. The final reactant molar ratios were 1SiO2 :0.1CTAB:0.28NaOH:25H2 O. The synthesis suspension was transferred into an autoclave and placed in the oven at 373 K for 1–2 h under static condition. The resulting powder was recovered by filtration, washed with deionized water and
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dried under ambient conditions. The pre-dried solid was heated at a constant rate from room temperature to 813.2 K over 20 h under He and held for 1 h under the same condition, then the sample was held at 813.2 K for 5 h in air to remove the residual surfactant. The thermal and hydrothermal stabilities were tested by calcining at 1073 K for 12 h in air flow and boiling for 2 h in 50 ml of distilled water for 1 g as-synthesized MCM-41, respectively. Powder X-ray diffraction patterns were recorded with a Bruker AXS D8 diffractometer (under ambient conditions) using filtered CuKα radiation. Diffraction data were recorded between 0.5◦ and 8◦ (2θ ) with a resolution of 0.02◦ . Nitrogen physisorption isotherms were measured at 77 K with a static volumetric instrument Autosorb-6b (Quanta Chrome). Prior to each measurement, the sample was outgassed at 473 K to a residual pressure below 10−4 Torr. A Baratron pressure transducer (0.001–10 Torr) was used for low-pressure measurements. The specific surface area was estimated by the Brunauer–Emmett–Teller (BET) method [9]. The pore size distribution (PSD) was calculated from the adsorption branch using the Barrett–Joyner–Halenda (BJH) method [10]. This method underestimates the PSD to some extent [11]. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010, operated at 200 kV. The samples were suspended in ethanol and dried on holey carbon-coated Cu grids. 3. Results and discussion It is well known that MCM-41 sample is typically characterized by an intense X-ray diffraction in the vicinity of 2θ = 2◦ and several other diffraction peaks in the 2θ range from 3◦ to 8◦ . The XRD patterns of the parent silica gel and resultant MCM-41 products synthesized with our formulation are shown in Fig. 1. The absence of XRD diffraction peaks from the silica gel means the lack of long-range order structure. The MCM-41 material aged for 1 h shows only one weak (100) diffraction peak, indicating the poor mesostructure. When the aging time increases to 2 h, the sample exhibits the characteristic diffraction peaks assignable to a hexagonal symmetry of mesoporous MCM-41; one intense (100) peak with four well-resolved (110), (200), (210), and (300) peaks between 2θ = 3.0◦ –8.0◦ , exhibiting the long-range order and good textural uniformity of mesoporous structure. The FWHM (fullwidth-at-half-maximum) of the (100) peak is about 0.2◦ , it demonstrates that the grains were particularly well developed within 2 h. The distinct difference of XRD patterns between parent silica gel and resultant powders results from the successful transformation from silica gel to highly ordered mesoporous MCM-41 structure. TEM image of the calcined sample is depicted in Fig. 1 inset. Ordered channel arrays of parallel lines are observed clearly along the direction of the pore arrangement. This shows that the sample has highly ordered long-range structure. The distance between two center of adjacent black lines are 3 nm, it is in good agreement with the value (2.5 nm shown in Fig. 2c) determined from the nitrogen adsorption measurement.
Fig. 1. XRD patterns of parent silica and MCM-41 at different aging times: (a) parent silica, (b) aging time of 1 h and (c) aging time of 2 h (inset: TEM image of this sample).
The nitrogen physisorption isotherms and corresponding pore size distributions of parent silica and calcined MCM-41 materials are shown in Fig. 2. The parent silica exhibits type IV adsorption–desorption isotherms with H2 hysteresis loop according to the IUPAC classification, the isotherms present a capillary condensation step which can be attributed to the interparticles pores [12]. The BET surface area is about 480 m2 /g; it shows a broad pore size distribution centered at 6.0 nm. For the sample aged for only 1 h, it exhibits a capillary condensation step but not very clear, although the porosity analysis indicates a weak presence of porous structure with an average pore diameter of 2.7 nm. For the sample of 2 h aging, it shows a sharp step increase at P /P0 = 0.3–0.37 due to the capillary condensation of nitrogen in the regular mesopores. A narrow pore size distribution with the average pore diameter of 2.5 nm is obtained. The pore wall thickness is about 2.2 nm calculated by subtracting √ the pore diameter from the lattice unit parameter (2d100 / 3 − pore diameter). The textural parameters of various samples are summarized in Table 1. As the aging time increases, the surface areas increase from 480 to 1270 m2 /g and the pore volumes increase from 1.1 to 2.0 cm3 /g, respectively. Combine with XRD results, we confirm that the parent silica gel has been transformed into the mesoporous MCM-41 in 2 h. In this synthesis, the MCM-41 grains are formed by recondensation of silica monomers dissolved originally from the parent silica gel. The proposed formation mechanism here is that the particles dissolve themselves first, once the virtual concentration of silica species has been reached, and then these intermediates evolve to the ordered MCM-41 at shorter treat-
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Fig. 2. (Left) Nitrogen adsorption/desorption isotherms. (Right) Pore size distributions calculated from the adsorption branch by BJH method: (a) parent silica, (b) aging time of 1 h, (c) aging time of 2 h, (d) calcined at 1073 K for 12 h in air and (e) hydrothermal treatment in boiling water for 2 h. Table 1 Textural parameters of various Si-MCM-41 samples Sample
d spacing (nm)
SBET (m2 /g)
Pore size (nm)
Vp (cm3 /g)
Parent silica 1h 2h 1073 K for 12 h Boiling water for 2 h
– 4.0 4.2 3.0 3.0
480 610 1270 450 760
6.0 2.7 2.5 2.0 2.0, 4.2
1.1 1.4 2.0 0.4 1.1
ment times by locally internal reorganization directed by the surfactant templates. One-hour synthesis does not result in good MCM-41 structure, it is probably because of low content of dissolved silica in the synthesis suspension which the selfassembly cannot be fully achieved. Thermal stability and hydrothermal stability tests of resultant MCM-41 materials were also tested in this study, respectively. For the sample treated at 1073 K for 12 h under air, the pore diameter becomes smaller and the pore size distribution is broader than that of the fresh MCM-41 sample. Obviously, the collapse of pore structures occurs under this treatment, indicating the lack of high thermal stability of this MCM-41 sample due to uncompleted condensation of silanol groups. However, the sample treated in boiling water for 2 h still shows clear capillary condensation in N2 physisorption, it means that the pore structure has been preserved during the hydrothermal treatment. It exhibits bimodal pore diameter distribution, which ascribes to the hydrolysis of surface silanol groups in the channel lead to the expansion of pore diameter to some extent. The pore diameter distribution centered at 2.0 nm becomes broad, which indicates the reduction of mesostructured order. Low thermal stability means that the sample prepared within 2 h in this study is not suitable for those reactions under very harsh conditions, e.g., high temperature and high pressure. However, the sample in this study shows high hydrothermal stability, it makes this sample a very
good candidate for those liquid phase reactions under mild conditions, or the separation of large organic molecules, such as proteins, and selective adsorption of large molecules from effluents, etc. 4. Summary In summary, the high-quality mesoporous MCM-41 is rapidly synthesized in a very short preparation time of 2 h. The sample has the high BET specific surface area and pore volume of 1270 and 2.0 cm3 /g. Experimental results show that the sample prepared in this rapid synthesis has fairly good hydrothermal stability. This rapid and facile method opens the way to the large scale industrial production for synthesis of MCM-41. Acknowledgments We are grateful to the Start-up Grant of College of Engineering, Nanyang Technological University, Singapore, for financial support. We also thank to AcRF Grant RG45/06 and AcRF Grant RG118/06 for partial financial support. References [1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmidt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [3] Y. Ma, L. Qi, J. Ma, Y. Wu, O. Liu, H. Cheng, Colloids Surf. A 229 (2003) 1. [4] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56. [5] A. Corma, Chem. Rev. 97 (1997) 2373. [6] K. Flodstrom, C.V. Teixeira, H. Amenistsch, V. Alfressm, M. Linden, Langmuir 20 (2004) 4885. [7] K. Flodstrom, H. Wennerstrom, V. Alfressm, Langmuir 20 (2004) 680.
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[8] S. Inagani, Y. Aratani, E. Kikuchi, M. Matsukata, Stud. Surf. Sci. Catal. 158 (2005) 533. [9] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [10] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373.
[11] J. Choma, M. Jaroniec, W. Burakiewicz-Mortka, M. Kloske, Appl. Surf. Sci. 196 (2002) 216. [12] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169.
Letter to the Editor
Rapid synthesis of highly ordered Si-MCM-41 Xianbin Liu, Hui Sun, Yanhui Yang ∗ School of Chemical and Biomedical Engineering, Nanyang Technological University, 637459 Singapore Received 4 October 2007; accepted 18 November 2007 Available online 24 November 2007
Abstract A very short-time synthesis of highly ordered MCM-41 molecular sieve was formulated by using cetyltrimethylammonium bromide (CTAB) as the template and silica gel from SiliCycle as the silica source. The physical properties of MCM-41 samples were characterized by X-ray diffraction (XRD), nitrogen physisorption, and transmission electron microscopy (TEM). The MCM-41 sample prepared in this study exhibited well defined long-range order and good hydrothermal stability. It was demonstrated that reducing the time of self-assembly step to 2 h during the synthesis had no unfavorable effect on the quality of MCM-41 materials. © 2007 Elsevier Inc. All rights reserved. Keywords: Si-MCM-41; Rapid synthesis; Nitrogen physisorption; TEM; XRD
1. Introduction Mobil has patented a family of nanoporous materials (mesoporous molecular sieve) designated as M41S, in which MCM41 is considered to be the most promising structure [1,2]. Narrow pore size distribution with size controllable pores over micrometer length scales makes the MCM-41 materials attractive to many applications, e.g., the separation of proteins, selective adsorption of large molecules from effluents [3]. Most applications require MCM-41 with a narrow pore size distribution, large surface area and long-range ordered structure. Intensive efforts have been devoted to the synthesis and characterization of MCM-41 materials [4,5]. Generally, MCM-41 is prepared under alkaline conditions by using surfactant as the template under high temperature. The synthesis procedure usually takes about 2 days or even longer, it makes the practical application of this material severely hampered because it is quite unfavorable to the large scale production. It would be useful to develop an economically feasible method to synthesize MCM-41 for large scale, reducing the synthesis time, while maintaining the ordered mesoporous structure, will be the most applicable approach. Recently, the occurrence of self-assembly of surfactant and silica species has been found in a relatively short time * Corresponding author. Fax: +65 6794 7553.
E-mail address: [email protected] (Y. Yang). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.11.025
(about 2 h) using in situ synchrotron small angle X-ray scattering (SAXS), X-ray diffraction (XRD), time-resolved NMR, TEM, and EPR spectroscopy [6,7]. However, this work did not explore the feasibility of fabricating high-quality mesoporous materials within a shorter synthesis time. There are only few reports on the rapid synthesis of MCM-41 up to now; the very first result of the rapid synthesis of MCM-41 using successive two steps (aging for 30 min, then evaporation-to-dryness for 2 h) was reported by Inagaki et al. [8]. In this contribution, we formulated a facile method for the rapid synthesis of Si-MCM-41, the aging time was reduced to 2 h instead of 48 h usually applied [1,2]. The thermal and hydrothermal stabilities were tested as well. 2. Materials and methods Si-MCM-41 was synthesized using cetyltrimethylammonium bromide (CTAB, Sigma–Aldrich) as the surfactant source and silica gel from SiliCycle as the silica source. In a typical synthesis, CTAB and NaOH were dissolved in deionized water and the silica source was added to this solution and stirred for 30 min. The final reactant molar ratios were 1SiO2 :0.1CTAB:0.28NaOH:25H2 O. The synthesis suspension was transferred into an autoclave and placed in the oven at 373 K for 1–2 h under static condition. The resulting powder was recovered by filtration, washed with deionized water and
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dried under ambient conditions. The pre-dried solid was heated at a constant rate from room temperature to 813.2 K over 20 h under He and held for 1 h under the same condition, then the sample was held at 813.2 K for 5 h in air to remove the residual surfactant. The thermal and hydrothermal stabilities were tested by calcining at 1073 K for 12 h in air flow and boiling for 2 h in 50 ml of distilled water for 1 g as-synthesized MCM-41, respectively. Powder X-ray diffraction patterns were recorded with a Bruker AXS D8 diffractometer (under ambient conditions) using filtered CuKα radiation. Diffraction data were recorded between 0.5◦ and 8◦ (2θ ) with a resolution of 0.02◦ . Nitrogen physisorption isotherms were measured at 77 K with a static volumetric instrument Autosorb-6b (Quanta Chrome). Prior to each measurement, the sample was outgassed at 473 K to a residual pressure below 10−4 Torr. A Baratron pressure transducer (0.001–10 Torr) was used for low-pressure measurements. The specific surface area was estimated by the Brunauer–Emmett–Teller (BET) method [9]. The pore size distribution (PSD) was calculated from the adsorption branch using the Barrett–Joyner–Halenda (BJH) method [10]. This method underestimates the PSD to some extent [11]. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010, operated at 200 kV. The samples were suspended in ethanol and dried on holey carbon-coated Cu grids. 3. Results and discussion It is well known that MCM-41 sample is typically characterized by an intense X-ray diffraction in the vicinity of 2θ = 2◦ and several other diffraction peaks in the 2θ range from 3◦ to 8◦ . The XRD patterns of the parent silica gel and resultant MCM-41 products synthesized with our formulation are shown in Fig. 1. The absence of XRD diffraction peaks from the silica gel means the lack of long-range order structure. The MCM-41 material aged for 1 h shows only one weak (100) diffraction peak, indicating the poor mesostructure. When the aging time increases to 2 h, the sample exhibits the characteristic diffraction peaks assignable to a hexagonal symmetry of mesoporous MCM-41; one intense (100) peak with four well-resolved (110), (200), (210), and (300) peaks between 2θ = 3.0◦ –8.0◦ , exhibiting the long-range order and good textural uniformity of mesoporous structure. The FWHM (fullwidth-at-half-maximum) of the (100) peak is about 0.2◦ , it demonstrates that the grains were particularly well developed within 2 h. The distinct difference of XRD patterns between parent silica gel and resultant powders results from the successful transformation from silica gel to highly ordered mesoporous MCM-41 structure. TEM image of the calcined sample is depicted in Fig. 1 inset. Ordered channel arrays of parallel lines are observed clearly along the direction of the pore arrangement. This shows that the sample has highly ordered long-range structure. The distance between two center of adjacent black lines are 3 nm, it is in good agreement with the value (2.5 nm shown in Fig. 2c) determined from the nitrogen adsorption measurement.
Fig. 1. XRD patterns of parent silica and MCM-41 at different aging times: (a) parent silica, (b) aging time of 1 h and (c) aging time of 2 h (inset: TEM image of this sample).
The nitrogen physisorption isotherms and corresponding pore size distributions of parent silica and calcined MCM-41 materials are shown in Fig. 2. The parent silica exhibits type IV adsorption–desorption isotherms with H2 hysteresis loop according to the IUPAC classification, the isotherms present a capillary condensation step which can be attributed to the interparticles pores [12]. The BET surface area is about 480 m2 /g; it shows a broad pore size distribution centered at 6.0 nm. For the sample aged for only 1 h, it exhibits a capillary condensation step but not very clear, although the porosity analysis indicates a weak presence of porous structure with an average pore diameter of 2.7 nm. For the sample of 2 h aging, it shows a sharp step increase at P /P0 = 0.3–0.37 due to the capillary condensation of nitrogen in the regular mesopores. A narrow pore size distribution with the average pore diameter of 2.5 nm is obtained. The pore wall thickness is about 2.2 nm calculated by subtracting √ the pore diameter from the lattice unit parameter (2d100 / 3 − pore diameter). The textural parameters of various samples are summarized in Table 1. As the aging time increases, the surface areas increase from 480 to 1270 m2 /g and the pore volumes increase from 1.1 to 2.0 cm3 /g, respectively. Combine with XRD results, we confirm that the parent silica gel has been transformed into the mesoporous MCM-41 in 2 h. In this synthesis, the MCM-41 grains are formed by recondensation of silica monomers dissolved originally from the parent silica gel. The proposed formation mechanism here is that the particles dissolve themselves first, once the virtual concentration of silica species has been reached, and then these intermediates evolve to the ordered MCM-41 at shorter treat-
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Fig. 2. (Left) Nitrogen adsorption/desorption isotherms. (Right) Pore size distributions calculated from the adsorption branch by BJH method: (a) parent silica, (b) aging time of 1 h, (c) aging time of 2 h, (d) calcined at 1073 K for 12 h in air and (e) hydrothermal treatment in boiling water for 2 h. Table 1 Textural parameters of various Si-MCM-41 samples Sample
d spacing (nm)
SBET (m2 /g)
Pore size (nm)
Vp (cm3 /g)
Parent silica 1h 2h 1073 K for 12 h Boiling water for 2 h
– 4.0 4.2 3.0 3.0
480 610 1270 450 760
6.0 2.7 2.5 2.0 2.0, 4.2
1.1 1.4 2.0 0.4 1.1
ment times by locally internal reorganization directed by the surfactant templates. One-hour synthesis does not result in good MCM-41 structure, it is probably because of low content of dissolved silica in the synthesis suspension which the selfassembly cannot be fully achieved. Thermal stability and hydrothermal stability tests of resultant MCM-41 materials were also tested in this study, respectively. For the sample treated at 1073 K for 12 h under air, the pore diameter becomes smaller and the pore size distribution is broader than that of the fresh MCM-41 sample. Obviously, the collapse of pore structures occurs under this treatment, indicating the lack of high thermal stability of this MCM-41 sample due to uncompleted condensation of silanol groups. However, the sample treated in boiling water for 2 h still shows clear capillary condensation in N2 physisorption, it means that the pore structure has been preserved during the hydrothermal treatment. It exhibits bimodal pore diameter distribution, which ascribes to the hydrolysis of surface silanol groups in the channel lead to the expansion of pore diameter to some extent. The pore diameter distribution centered at 2.0 nm becomes broad, which indicates the reduction of mesostructured order. Low thermal stability means that the sample prepared within 2 h in this study is not suitable for those reactions under very harsh conditions, e.g., high temperature and high pressure. However, the sample in this study shows high hydrothermal stability, it makes this sample a very
good candidate for those liquid phase reactions under mild conditions, or the separation of large organic molecules, such as proteins, and selective adsorption of large molecules from effluents, etc. 4. Summary In summary, the high-quality mesoporous MCM-41 is rapidly synthesized in a very short preparation time of 2 h. The sample has the high BET specific surface area and pore volume of 1270 and 2.0 cm3 /g. Experimental results show that the sample prepared in this rapid synthesis has fairly good hydrothermal stability. This rapid and facile method opens the way to the large scale industrial production for synthesis of MCM-41. Acknowledgments We are grateful to the Start-up Grant of College of Engineering, Nanyang Technological University, Singapore, for financial support. We also thank to AcRF Grant RG45/06 and AcRF Grant RG118/06 for partial financial support. References [1] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmidt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [2] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [3] Y. Ma, L. Qi, J. Ma, Y. Wu, O. Liu, H. Cheng, Colloids Surf. A 229 (2003) 1. [4] J.Y. Ying, C.P. Mehnert, M.S. Wong, Angew. Chem. Int. Ed. 38 (1999) 56. [5] A. Corma, Chem. Rev. 97 (1997) 2373. [6] K. Flodstrom, C.V. Teixeira, H. Amenistsch, V. Alfressm, M. Linden, Langmuir 20 (2004) 4885. [7] K. Flodstrom, H. Wennerstrom, V. Alfressm, Langmuir 20 (2004) 680.
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[8] S. Inagani, Y. Aratani, E. Kikuchi, M. Matsukata, Stud. Surf. Sci. Catal. 158 (2005) 533. [9] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60 (1938) 309. [10] E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373.
[11] J. Choma, M. Jaroniec, W. Burakiewicz-Mortka, M. Kloske, Appl. Surf. Sci. 196 (2002) 216. [12] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169.