Fabrication of Mesoporous Silica Coating by Electrophoretic

Castro , Y.; Ferrari , B.; Moreno , R.; Duran , A. Coatings Produced by Electrophoretic Deposition from Nano-Particulate Silica Sol−Gel Suspensions ...
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Ind. Eng. Chem. Res. 2008, 47, 7236–7241

Fabrication of Mesoporous Silica Coating by Electrophoretic Deposition Hideyuki Negishi, Akira Endo,* Takao Ohmori, and Keiji Sakaki National Institute of AdVanced Industrial Science and Technology (AIST), AIST Central 5-2, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

The technological feasibility of applying electrophoretic deposition (EPD) to ordered mesoporous silicate (MPS) powder has been studied. The EPD technique was investigated for the fabrication of an MPS thick film using MPS synthesized by the spray-drying method. It was found that the MPS powder deposition rate was low with an alcohol EPD bath and very high with an acetone EPD bath. The deposition amount could be easily controlled by changing the MPS powder concentration, the applied dc voltage, and the deposition time. An MPS particle/acetone suspension in which the particles were negatively charged showed the highest deposition rate among the tested organic solvents. MPS powder was deposited by EPD at 50 V for 10 min onto a tubular stainless steel substrate, and a uniform coating approximately 250 µm in thickness was obtained. After heat treatment at 573 K, the MPS powder could be fixed on the substrate without the addition of binders. The obtained MPS thick film had the same porous structure and nitrogen and water vapor adsorption properties as the parent MPS powder. 1. Introduction Recently, highly ordered mesoporous materials templated by the self-assembly of amphiphilic molecules have attracted increasing attention.1 These ordered mesoporous materials have the potential for use in catalysis, for example, and as adsorbents and membranes, based on their uniformly arranged mesopores. These mesopores range in size from 1.5 to 30 nm and have a large specific surface area and pore volume. If these ordered mesoporous materials are to find practical applications, it is important to establish a method for fixing them onto substrates to improve ease of handling. It is well-known that thin films of mesoporous silica can be formed by spin- or dip-coating using an acidic silicate/surfactant solution on a substrate.2-5 With this method, as the solvent evaporates, surfactant molecules are concentrated and induced to self-assemble and form an ordered mesostructure cooperatively with silicate oligomers, resulting in an ordered hexagonal, cubic, or lamellar mesostructure. However, the thickness of the obtained films is limited to less than several micrometers in most cases, because of the incidence of cracks or peeling that occurs with thicker films during the solvent evaporation process. In addition, the compactness of spin- or dip-coated films might be a disadvantage if the films are to be used as catalysts or adsorbents because of the slow diffusion of reactants or absorbates in nanosized pores. On the other hand, in terms of a fixation method, the formation of films from mesoporous silica powders by wet coating can be considered. In general, sintering or calcination by the addition of a binder is used as the fixing process for ceramic powders. However, there are certain difficulties when these methods are applied to ordered mesoporous materials, because the ordered porous structure can collapse at the high temperatures employed for the sintering process6 and/or the addition of binder can lead to a deterioration in the adsorption/ desorption properties. For example, Shim et al. reported the adsorption properties of volatile organic compounds and water vapor for a wash-coated MCM-48 layer on a cordierite honeycomb.7 The thickness of the MCM-48 in the corners of the * To whom correspondence should be addressed. E-mail: [email protected].

channels reached about 200 µm and that along the side walls appeared to be several microns. Although the samples still showed the characteristic adsorption properties of MCM-48 even after wash-coating, a reduction in pore diameter and pore volume was observed, as well as a steep increase in the adsorption amount around a relative pressure of 0.3 as a result of the capillary condensation of nitrogen. Thus, it is meaningful to establish other methods for fixing ordered mesoporous materials onto substrates that do not degrade their unique porous structure and adsorption properties. We focused on the electrophoretic deposition (EPD) method, which is a colloidal process in which a dc electric field is applied across a suspension of charged particles, attracting them to an oppositely charged electrode. The EPD method has some advantages over other fabrication techniques, especially with regard to reducing the fabrication costs and the structural flexibility of the substrates. Therefore, the EPD technique has been widely used as a ceramic processing technique for a variety of technical applications, e.g., electrodes, solid oxide fuel cells, and nanostructured materials and coatings for electronic, biomedical, optical, catalytic, and electrochemical applications.16-21 As examples of silica coating by EPD, Castro et al. and Hasegawa et al. reported the fabrication of silica coatings with thicknesses of 2-5 µm by electrophoretic sol-gel deposition.22,23 In addition, we reported the formation of thick films of ordered mesoporous silica on stainless steel by EPD.24 The feasibility of the EPD technique has been established based on these reports. In this study, we investigated the usefulness of the preparation of mesoporous silicate (MPS) thick films on a stainless steel substrate using the electrophoretic deposition (EPD) technique without the addition of binders. We measured the influence of the EPD conditions, specifically, the kind of organic solvent used for the EPD bath, the applied voltages, and the powder concentration, on the rate of the deposition and prepared MPS coatings of various sizes. Moreover, the deposited MPS sample was heat-treated, and then the morphology and the adsorption/ desorption isotherms of nitrogen and water vapor were measured.

10.1021/ie071473i CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008

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Figure 1. Experimental setup for EPD.

2. Experimental Section 2.1. Synthesis of Mesoporous Silica Powder. The particle size of mesoporous silica powder is very important in the preparation of a stable suspension for the EPD process. Thus, we used the spray-drying method to prepare the mesoporous silica powder. The synthesis procedure is the same as that described in our previous report.25 First, tetraethoxysilane (TEOS) and cetyltrimethylammonium chloride (C16TAC) were dissolved in ethanol (EtOH). After an HCl aqueous solution (10-3 M) had been added to the mixture, we stirred the obtained solution at room temperature for 1 h to hydrolyze the TEOS. The TEOS/C16TAC/EtOH/HCl/H2O molar ratio of the starting solution was 1:0.2:10:1.8 × 10-4:10. The solution was then transferred to a round-bottom flask and evaporated using a vacuum rotary evaporator at 70 hPa for 40-60 min. Then, the solution was spray dried using a spray dryer (Yamato Kagaku Co. Ltd. GS310). The inlet temperature was 433 K, and the gas pressure was 0.075 MPa. The resulting solid, a silica-surfactant composite, was calcined at 873 K for 5 h to remove the surfactant. 2.2. Electrophoretic Deposition (EPD). The EPD of MPS was carried out as follows: First, an EPD bath was prepared for the MPS powder by adding the powder to organic solvents, namely, methanol (MeOH), ethanol, 1-propanol (1-PrOH), 2-propanol (2-PrOH), and acetone, at a concentration of 3-10 g/L. Then, the MPS powder was dispersed by ultrasonic vibration for 10 min. The EPD for fabricating the MPS coating was performed using the cell configuration shown in Figure 1. Three types of stainless steel substrates were used as deposition electrodes, namely, a stainless steel wire (diameter ) 0.8 mm, length ) 20 mm), a small stainless steel support (outer diameter ) 3 mm, length ) 40 mm), and a large stainless steel tube (outer diameter ) 10 mm, length ) 50 mm). A stainless steel mesh was used as the counter electrode. The electrode distance was about 10 mm. A dc voltage of 25-100 V was applied for 3-10 min using a dc power supply (Takasago Ltd., TP0360022D) between the substrate and the counter electrode. The obtained MPS coatings were calcined at 573 K for 8 h to eliminate the solvent. 2.3. Characterization of Mesoporous Silica Powder. X-ray diffraction (XRD) and nitrogen adsorption/desorption measurements were carried out to characterize the mesoporous silica powder and the obtained MPS coating. The XRD measurements were performed using a Bruker AXS D8 Advance diffractometer (Cu KR radiation, operated at 40 kV and 30 mA). The nitrogen adsorption/desorption isotherms were measured at 77 K using fully automatic adsorption isotherm measuring equipment (BEL Japan, Inc., Belsorp-mini). The pretreatment was carried out at 573 K for 5 h in a nitrogen flow. The particle size distribution was measured using a laser diffraction particle size analyzer (Horiba Ltd., LA-950). The surface morphology of the MPS coating was observed using a

Figure 2. Particle size distribution of MPS powder synthesized by the spraydrying method.

Figure 3. Amount deposited using various EPD baths on stainless steel wire. EPD bath: 3 g/L. EPD conditions: 50 V for 3 min.

confocal laser scanning microscope (Keyence Corporation, VK9500). The water vapor adsorption isotherms were measured using a Belsorp-18PLUS or Belsorp-Aqua instrument. 3. Results and Discussion 3.1. Characterization of Mesoporous Silica Powder. The XRD pattern and the N2 adsorption/desorption isotherm revealed that the obtained MPS had a highly ordered structure with a hexagonal pore arrangement.25 The average MPS particle diameter was about 10 µm, as shown in Figure 2. The BET specific surface area was 889.9 m2/g, and the apparent density estimated from the true density of silica (about 2.3 g/cm3, measured by He picnometer) and the porosity (about 0.5) was approximately 1.15 g/cm3. 3.2. EPD Bath Selection. To select the organic solvent for EPD, a stainless steel wire (diameter ) 0.8 mm, length ) 20 mm) was used as the both the substrate and counter electrode. In these experiments, the concentration of MPS powder, dc voltage, and deposition time were kept constant at 3 g/L, 50 V, and 3 min, respectively. Figure 3 shows the amount deposited for each solvent. When using alcohol solvents, the MPS powder deposition rate was very low. With 1-PrOH and 2-Pr-OH baths, the MPS powder was deposited on both electrodes. For example, the amount deposited was about 0.36 mg/cm2 with the 1-PrOH bath. With MeOH and EtOH baths, the amount deposited was approximately zero. Therefore, the use of an alcohol suspension is unsuitable for fabricating thick coatings of the MPS powder. Generally, the surface of silica particles is negatively charged in alcohols. However, as described above, a small amount of silica deposition on the cathode was also observed. We consider that the deposition on the cathode is due to the adherence of

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Figure 4. Relationship between deposition amount and deposition time with various applied voltages using acetone. EPD bath: 3 g/L. Substrate: small stainless steel tube (outer diameter ) 3 mm, length ) 40 mm). Applied voltage: (a) 25, (b) 50, and (c) 100 V.

silica particles during the pulling up process from the EPD bath and the drying process of liquid films after the deposition. Actually, the amount deposited on the cathode was about 0.5 layer of silica particles for the alcohol bath and 2.8 layers for the acetone bath. On the other hand, a thick coating of MPS powder was easily obtained with an acetone bath. In particular, the amount deposited on the anode was much larger than that on the cathode. This means that the MPS particles are mainly negatively charged in an acetone bath. In this case, 39 mg/cm2 of the MPS powder was deposited using acetone. Here, we do not discuss the details of the differences in deposition rate among the tested organic solvents. The physical properties of the solvent, such as the relative dielectric constant, viscosity, and water content, can affect the surface charge and the state of the aggregation of the MPS powders, resulting in a difference in deposition rate. Alcohols are protic solvents, and acetone is an aprotic solvent. Thus, the structure of the solvent can cause the difference of charging state, although further investigation is necessary to elucidate the reason for the difference of amounts deposited between alcohols and acetone. As a result, MPS particles dispersed in acetone were used as the EPD bath in the subsequent experiments. 3.3. EPD Behavior of MPS in an Acetone Bath. A small stainless steel tube (outer diameter ) 3 mm, length ) 40 mm) was used as the substrate, and acetone was selected as the EPD bath medium. The other EPD conditions were the same as described in the previous section. Figure 4 shows the relationship between the deposition amount and the deposition time for different dc voltages (25, 50, and 100 V). The deposition amount increased with increasing deposition time and applied voltage. At 50 V for 600 s, the amount deposited was about 6.5 mg/ cm2, and at 100 V for 600 s, it was about 11 mg/cm2. The deposition amount was almost proportional to the applied voltage, and it saturated. It is considered that this saturation is caused by the reduction in the MPS powder concentration in the EPD bath as EPD progresses. Figure 5 shows the relationship between deposition amount and deposition time for EPD baths with various MPS powder concentrations. The deposition amounts are almost proportional to the MPS powder concentration. These results suggest that the deposition amount can be easily controlled by controlling the MPS powder concentration, the applied voltage, and the deposition time. In addition, we measured the current density during EPD. Figure 6 shows the relationship between the current density and the deposition time at 50 V. Figure 6a shows the current density when a voltage was applied to a powder-free bath (acetone only), and Figure 6b shows the current density for a 3 g/L EPD bath.

Figure 5. Relationship between deposition amount and deposition time with various MPS powder concentrations using acetone. EPD bath: (a) 1, (b) 3, and (c) 5 g/L. Substrate: small stainless steel tube (outer diameter ) 3 mm, length ) 40 mm). Applied voltage: 50 V.

Figure 6. Relationship between current density and deposition time at 50 V: (a) acetone without MPS powder and (b) acetone containing 3 g/L MPS powder.

These current densities are almost the same regardless of the presence of the MPS powder. The current density was almost constant during the change in the EPD process. This means that the deposited MPS particles do not contribute to the current. Therefore, MPS particle migration is caused only by the electric field. We have reported two types of EPD mechanisms: (i) The deposition layer acts as a resistance layer, as in the EPD of yttria-stabilized zirconia (YSZ), and the current density decreases with the progress of EPD under a constant applied voltage.13 (ii) The deposition layer does not act as a resistance layer, as in the EPD of zeolite, and the current density is approximately constant under a constant applied voltage.26 The current density profile during the EPD of MPS powder is similar to that of zeolite, indicating that the deposited MPS layer does not act as a resistance layer. This means the electrical field gradient between the substrate and the counter electrode is constant during the progress of EPD. Therefore, the MPS particle migration rate is constant regardless of the deposition layer. In such a case, we can expect to fabricate a thick coating. 3.4. Preparation of MPS Thick Films. We investigated the fabrication of MPS coatings for larger substrates. The EPD was carried out at an applied voltage of 50 V for deposition times of up to 600 s on a 15.7 cm2 tubular stainless steel support (outer diameter ) 10 mm, length ) 50 mm). The EPD bath concentration was 10 g/L (1.4 g/140 mL). The deposition amount increased rapidly up to 200 s and then slowly increased up to 600 s, as shown in Figure 7. Here, a deposition of 18 mg/cm2 is equivalent to a deposition of 0.275 g on a 15.7 cm2 substrate. This means that 20% of the MPS powder in the EPD bath was deposited. The obtained MPS coating was calcined at 573 K in air for 8 h to eliminate the acetone and possibly

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Figure 7. Relationship between MPS deposition amount and deposition time at 50 V. EPD bath: 10 g/L. Substrate: large stainless steel tube (outer diameter ) 10 mm, length ) 50 mm).

Figure 9. 3D image and thickness profile of MPS coating made by the EPD method at 50 V for 10 min. EPD bath: 10 g/L. Substrate: large stainless steel tube (outer diameter ) 10 mm, length ) 50 mm).

Figure 8. Photograph of MPS coating made with the EPD method on (a) stainless steel wire (diameter ) 0.8 mm, length ) 20 mm), (b) small stainless steel tube (outer diameter ) 3 mm, length ) 40 mm), and (c) large stainless steel tube (outer diameter ) 10 mm, length ) 50 mm).

enhance the strength of the coating. After the thermal treatment, the MPS powder was fixed on the substrate without the addition of binders. The MPS powder was deposited only on the outside of the tubular substrate, and a uniform coating was obtained as shown in Figure 8c. Parts b and a, respectively, of Figure 8 show photographs of the deposited samples on a small tube (outer diameter ) 3 mm, length ) 40 mm) and a stainless wire (diameter ) 0.8 mm, length ) 20 mm). A thick coating can be seen in these figures, and the surfaces of the tubular samples are smooth. Moreover, it is clear that it is also easy to increase the dimensions. The surface morphology was observed using a confocal laser scanning microscope. Figure 9 shows a 3D image and a thickness profile of the deposited MPS layer. The lower level (dark area) is the substrate, and the upper level is the surface of the deposited MPS coating. The surface is almost flat, and the layer thickness is about 250 µm. In this case, the deposition area is 15.7 cm2, and the amount deposited is 0.275 g. Therefore, the deposition layer has a volume of about 0.39 cm3 and a density of 0.70 g/cm3. Here, the density estimated from the true density of silica (about 2.3 g/cm3, measured by He picnometer) and the porosity of MPS was approximately 0.5. Therefore, the density of MPS was about 1.15 g/cm3. As a result, the porosity of the deposited layer (not including that of the mesopores of the MPS particles) was estimated to be 39%. Generally, EPD films are connected to the substrates through van der Waals interaction.27 Therefore, we examined the mechanical strength of the films by the three-point bending test. The films did not peel off after the application of a 20-N bending force. However, it is necessary to investigate the possibility of adding the minimum amount of binder to the EPD bath in order

Figure 10. (a) Nitrogen adsorption/desorption isotherms at 77 K: (O) MPS powder, (0) MPS film obtained by EPD (before drying), (]) MPS film obtained by EPD (after drying). (b) Pore size distribution curves calculated using the Broekhoff and de Boer equation with the D-H algorithm employing the cylindrical pore model.

to increase the mechanical strength. We will report the effects of binder addition in a subsequent work. 3.5. Adsorption Properties of Water Vapor. Figure 10a shows the adsorption/desorption isotherms of nitrogen for the parent MPS powder and the MPS thick films prepared by EPD at 77 K, both before and after heat treatment at 573 K. All of the isotherms were similar to type IV in the IUPAC classification, clearly indicating a mesoporous structure. The BET surface areas were 889.9, 855.8, and 837.3 m2/g, respectively. The pore

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Figure 12. XRD patterns of MPS before and after EPD.

However, certain aspects of the EPD process remain to be investigated with regard to the preparation of adsorption modules using MPS materials. These include (i) controlling the dispersed MPS particle charge, (ii) preparing a uniform coating on various kinds of metal substrates, (iii) controlling the apparent porosity of the MPS coating, and (iv) increasing the strength of the MPS coating. For these purposes, it is important to clarify the EPD behavior and mechanisms in detail. Figure 11. Water vapor adsorption/desorption isotherms of parent MPS powder (triangles) and MPS coating obtained by EPD (circles). Solid symbols denote the adsorption branch, and open symbols denote the desorption branch. (a) First adsorption/desorption and (b) second adsorption/ desorption.

size distribution (PSD) curves were calculated using the Broekhoff and de Boer equation28 with the Dollimore-Heal algorithm.29 employing the cylindrical pore model, as shown in Figure 10b. All of the samples had almost the same pore size of 2.9 nm. Although the BET surface area had decreased slightly after the EPD process, the pore diameter, pore volume, and steep increase in the adsorption amount around a relative pressure of 0.3 caused by the capillary condensation of nitrogen were almost the same as those of the parent MPS powder, suggesting that the porous structure of MPS does not change during the EPD and sintering process. Figure 11a shows the first adsorption/desorption isotherms of water vapor for the original MPS powder and MPS film made by EPD. The measurement for MPS powder was carried out just after calcination at 873 K followed by preheating at 573 K for 8 h in vacuum. The measurement for the MPS coating was carried out after preheating at 573 K for 8 h in vacuum. All of the isotherms showed typical type V behavior (IUPAC classification) with a hysteresis loop. After the first measurement, the surfaces of the silica walls were rehydrolyzed, and the isotherms for the later measurements were shifted to a lower relative pressure because of the increase in the density of surface silanol groups on the pore surface. Figure 12 shows the XRD patterns of MPS before and after the EPD process. Both of the patterns have well-resolved three peaks that can be indexed as (100), (110), and (200) diffraction peaks associated with p6mm hexagonal symmetry. The XRD data, in addition to the nitrogen and the water vapor adsorption/desorption isotherms, confirmed that the obtained thick films of MPS had the same porous structure and unique adsorption properties with respect to water vapor as the parent MPS powder owing to their ordered porous structure.

4. Conclusion We investigated the usefulness of the electrophoretic deposition (EPD) technique for preparing mesoporous silica films on stainless steel substrates. We found that the deposition rate of MPS powder was low with an alcohol EPD bath and very high with an acetone EPD bath. The deposition amount could be easily controlled by changing the MPS powder concentration, the applied dc voltage, and the deposition time. The MPS powder/acetone suspension has excellent potential for the fabrication of an MPS thick coating. An MPS powder was coated on a substrate by EPD at 50 V for 10 min. The thickness of the coating was 250 µm, and the apparent density of the film was 39%. After heat treatment at 573 K in air, the MPS powder was fixed on the substrate without the addition of any binders. The obtained thick films of MPS had the same porous structure and adsorption properties with respect to nitrogen and water vapor as the parent MPS powder, suggesting that no significant change in the porous structure of mesoporous silica occurred during the EPD process. Finally, we confirmed that the EPD process is a promising method for fabricating MPS coatings, which could find applications in various industrial fields including adsorption, catalysis, and separation. Acknowledgment This study was supported by the Industrial Technology Research Grant Program in 2006 of the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Literature Cited (1) Zhao, D.; Wan, Y. The Synthesis of Mesoporous Molecular Sieves. Stud. Surf. Sci. Catal. 2007, 168, 241. (2) Ogawa, M.; Ishikawa, H.; Kikuchi, T. Preparation of Transparent Mesoporous Silica Films by a Rapid Solvent Evaporation Method. J. Mater. Chem. 1998, 8, 1783. (3) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I.

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(17) Uchikoshi, T.; Suzuki, T. S.; Okuyama, H.; Sakka, Y.; Nicholson, P. S. Electrophoretic Deposition of Alumina Suspension in a Strong Magnetic Field. J. Eur. Ceram. Soc. 2004, 24, 225. (18) Dokou, E.; Barteau, M. A.; Wagner, N. J.; Lenhoff, A. M. Effect of Gravity on Colloidal Deposition Studied by Atomic Force Microscopy. J. Colloid Interface Sci. 2001, 240, 9. (19) Zhitomirsky, I.; Petric, A. Electrophoretic Deposition of Ceramic Materials for Fuel Cell Applications. J. Eur. Ceram. Soc. 2000, 20, 2055. (20) Negishi, H.; Sakai, N.; Yamaji, K.; Horita, T.; Yokokawa, H. Application of the Electrophoretic Deposition Technique to Solid Oxide Fuel Cells. J. Electrochem. Soc. 2000, 147, 1682. (21) Negishi, H.; Yamaji, K.; Imura, T.; Kitamoto, D.; Ikegami, T.; Yanagishita, H. Electrophoretic Deposition Mechanism of YSZ/n-Propanol Suspension. J. Electrochem. Soc. 2005, 152, J16. (22) Castro, Y.; Ferrari, B.; Moreno, R.; Duran, A. Coatings Produced by Electrophoretic Deposition from Nano-Particulate Silica Sol-Gel Suspensions. Surf. Coat. Technol. 2004, 182, 199. (23) Hasegawa, K.; Tatsumisago, M.; Minami, T. Preparation of Thick Silica Films by the Electrophoretic Sol-Gel Deposition Using a Cationic Polymer Surfactant. J. Ceram. Soc. Jpn. 1997, 105, 569. (24) Negishi, H.; Endo, A.; Nakaiwa, M.; Yanagishita, H. Preparation of Mesoporous Silicate Thick Films by Electrophoretic Deposition and Their Adsorption Properties of Water Vapor. Key Eng. Mater. 2006, 314, 147. (25) Endo, A.; Inagi, Y.; Fujisaki, S.; Yamamoto, T.; Ohmori, T.; Nakaiwa, M. Synthesis of Metal-Doped Mesoporous Silica by Spray Drying and Their Adsorption Properties of Water Vapor. Stud. Surf. Sci. Catal. 2007, 165, 157. (26) Negishi, H.; Imura, T.; Kitamoto, D.; Ikegami, T.; Yanagishita, H. Electrophoretic Deposition Mechanism of Zeolite Powder in n-Propanol under Constant Voltages. J. Ceram. Soc. Jpn., Suppl. 2004, 112, S11. (27) Tseng, W. J.; Wu, C. H. Aggregation, Rheology and Electrophoretic Packing Structure of Aqueous A12O3 Nanoparticle Suspensions. Acta Mater. 2002, 50, 3757. (28) Broekhoff, J.; C, P.; de Boer, J. H. Studies on Pore Systems in Catalysts: XIII. Pore Distributions from the Desorption Branch of a Nitrogen Sorption Isotherm in the Case of Cylindrical Pores B. Applications. J. Catal. 1968, 10, 377. (29) Dollimore, D.; Heal, G. R. An Improved Method for the Calculation of Pore Size Distribution from Adsorption Data. J. Appl. Chem. 1964, 14, 109.

ReceiVed for reView October 30, 2007 ReVised manuscript receiVed July 9, 2008 Accepted July 9, 2008 IE071473I