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Preparation of asymmetric pure titania ceramic membranes with dual functions
Materials Science and Engineering A 445–446 (2007) 611–619
Preparation of asymmetric pure titania ceramic membranes with dual functions Yong Hong Wang ∗ , Xing Qin Liu, Guang Yao Meng Department of Materials Science and Engineering, University of Science & Technology of China, Hefei, Anhui 230026, PR China Received 21 April 2006; received in revised form 27 July 2006; accepted 29 September 2006
Abstract The asymmetric 100% titania membranes have been prepared by a reproducible sol-coated process assisting with a combined drying method and modified sintering technique. The pretreated rutile granules resulted in better green strength in extrudate and favorably sintered at a relative low temperature. Scanning electron microscopy (SEM) images show that the prepared titania supports possess uniform microstructures with good interconnectivity between grains due to the optimized preparation conditions. The supports exhibit narrower pore size distribution centered at pore diameter 2.10 m, excellent chemical stability and higher fluid permeation. Moreover, the results show that the asymmetric membranes with a pore size ca. 0.10 m and active layer thickness of 15.0–20.0 m have desired fluid permeation along with remarkable in situ photocatalysis. These primary measurements imply such ceramic membranes may be envisaged as a promising separation media for the treatment of industrial effluents. © 2006 Elsevier B.V. All rights reserved. Keywords: Titania; Inorganic membrane; Microstructure; Photoactivity; Porous ceramic
1. Introduction Ceramic membranes have been paid increasing attentions for their powerful applications in the fields such as gas/liquid separation [1,2], oxygen—enriched air production and methane reforming for syngas [3,4], etc., due to their intrinsic tolerance to high temperature, resistance against non-aqueous solvent, mechanical rigidity and chemical inertness in caustic environments. In fact, in many cases, the formidable problems in practical device with controlled geometric shape, density and dimension for the specific applications still remain being great challenges [2]. Today, different types of such ceramic membranes with a specific geometric shape and pore size standard from microfiltration to nanofiltration even pervaporation have been reported widely [5–9]. For instance, at present, TAMI is the only sole ceramic membrane corporation in the world range who devoted to the commercial production of pure titania membranes from support to separation membrane layer [9]. The asymmetric multilayer configuration is usually preferred for the manufacture
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Corresponding author. Tel.: +86 551 3519041; fax: +86 551 3607627. E-mail address: [email protected] (Y.H. Wang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.107
of inorganic membrane, which is often composed of macroporous support, thin intermediate layer and/or toplayer with a decreasing pore size [10]. As to ceramic membrane materials, mostly Al2 O3 , ZrO2 and TiO2 are considered due to their abundance and higher chemical stability. However, in view of the relative high fabrication costs and the limited chemical stability in caustic media for zirconia, alumina and their composite membranes, pure titania membranes provide competitive advantages with better fouling resistance hence higher fluid flux due to their amphiphilic surface properties [11] and lower fabrication costs because of their reduced sintering temperature [8]. Further, titania membranes have numerous potential applications in sensitive fields, e.g. food, biotechnology, pharmaceutical industries and environment markets, where the use of alumina membrane is widely controversial for the health care. Even more, it is very interesting and economical that titania membranes can be efficiently employed to integrate the two discrete processing units of separation filtration and photocatalysis into one simplified manipulation [12]. After all, the membrane separation of TiO2 particle photocatalyst from the treated water after detoxification is complicated and strongly affected by cross-flow velocity, transmembrane pressure, feed concentration, pH of the suspension and ionic strength [13]. Zhao et al. [14] have obtained some achievements in the preparation of mesoporous titania film
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on porous substrate with dual functions of photocatalysis and separation. In addition, the improvement of permeability (permeation flux) along with reliable membrane preparation technique is always pursued in the development of asymmetric inorganic membrane, which is close related to the quality of ceramic supports [10]. It is recognized that the toplayer membrane formation making up the multilayer configuration now can be readily realized by the colloidal or polymeric sol–gel routes [5–8,14]. In particular, as the polymer science develops, the skillful utilization of functional polymer acting as active additive (e.g. dispersant, template, structure directing agents, etc.) greatly contributes to the ceramic membrane preparation [8,14,15]. However, to the best of our knowledge, few are the systematical reports of preparation on pure titania membranes with dual functions up to now. In fact, there are many technological difficulties in massive preparation of tubular macroporous titania supports from commercially available rutile powder that was formed through spray drying methods. An outstanding drawback of titania granules during the forming stage is their poor compression resistance strength. Moreover, it is very difficult to avoid the defect formation such as warpage and cracks in the resultant green specimen during the drying and sintering [16]. Herein, we attempted to develop a reproducible sol-coated process using a combination method of controllable humidity drying and microwave drying to manage the microstructure development and properties of pure titania membranes under mild sintering conditions. A sintering method termed as twice heating treatment was also investigated with respect to the avoidance of sintering stress accumulation and the improvement of practical performances. Besides, for the prepared asymmetric TiO2 membranes, future works related to coupling membrane separation and photocatalytic reaction in a given process of purification or decontamination of effluents would be explored by the static characterization results of fluid permeation and degradation of organic solution (methylene blue, MB) under ultraviolet (UV) light irradiation.
in the subsequent pugging stage. The resultant paste was covered with a plastic wrap for 24 h under controlled humidity to avoid premature drying and to ensure complete homogenization of wet paste. The 250 mm in length and 12 mm in external diameters tubular supports fabricated by extrusion method from this paste were set on rollers avoiding deformation or twisting and dried in controllable humidity at ambient temperature for some times then undergone microwave drying using a household microwave oven (EM-551S, SANYO brand, china, power supply 1150 W, microwave heating power 800 W). The green specimen embedded into a loose compaction of calcined corundum powder were heated on a programmable HT furnace (Nabertherm, Germany) in air at a ramping rate of 0.5 ◦ C/min to 400–600 ◦ C dwelling 0.5 h in this temperature range for the drainage of burning out of organic additives; followed by the same heating rate up to 850 ◦ C then 0–1.5 ◦ C/min to 1100–1200 ◦ C and sintered at the maximum temperature for 2 h. The later annealing temperature was designed by a ramping rate 1.0 ◦ C/min up to 1300 ◦ C/1400 ◦ C, where the sample was sintered for 2–8 h, respectively, then cooled naturally to room temperature. Fine anatase powder with median particle size D50 = 0.37 m was obtained from the calcined hydrolyzate of titanyl sulfate. TiO2 separation layers on the obtained rutile supports were fabricated by dip-coating techniques with the prepared anatase suspensions dispersed by polyacrylic acid (PAA), dried for at least 24 h in closed glove-box with controllable relative humidity under ambient air atmosphere and sintered at 500–700 ◦ C for 2 h by programmed temperature furnace. 2.2. Characterizations on TiO2 membrane microstructures
2. Experimental procedures
The thermal evolution of sample was studied by DT/TG-50 model differential thermal analysis and thermal gravimetric analyzer (Sahimadzu Co., Japan) using ␣-Al2 O3 powder as standard reference in air. The support microstructures were imaged by a X-650 model scanning electron microscope (SEM, HITACHI, Japan). The shrinkage measurement of TiO2 green bar during sintering was carried out on a horizontal dilatometer (NETZSCH DIL 402C, Germany) at a ramping rate 10 ◦ C/min.
2.1. Preparation of pure TiO2 membranes
2.3. Measurement of membrane properties
Rutile powder with a median particle size of 10.31 m (Hubei, China) was commercially available and used as support raw materials. Titanium sol was prepared from titanium sulfate hydrolyzed at ambient temperature under acidic conditions using triethanolamine as hydrolysis inhibitor and polyvinyl alcohol as dispersants. The titania powder mixtures containing some pore former (e.g. 1.0–3.0 wt.% corn starch) and organic binder (e.g. 5.0–10.0 wt.% methyl cellulose) were dry mixed for 2–6 h then appropriate amount of absolute alcohol was introduced into the powder mixtures under strongly mechanical blending. After blending for another several hours, precise weighed titanium sol was sprayed into the prepared paste and continued blending for several hours. Next, appropriate proportions of lubricant and plasticizer were dissolved in de-ionized water (80 ◦ C) and mixed
The mass loss and bending strength changes of samples were adopted to characterize the chemical stability of TiO2 membranes according to the National Standard GB/T1970–1996 of PR China [17]. Three point flexural strength measurements were executed on 5 mm × 5 mm × 60 mm rectangular bars with a mechanical testing machine (Sahimadzu Co., Japan). The porosity of sintered sample was measured according to the Archimedes’s principle taking the theoretical density of rutile as 4.25 g/cm3 . The active pore size and pore size distribution of membranes were measured by the gas/liquid displacement porometry (GLDP) method using water as impregnation media, N2 gas permeation and clean water flux through TiO2 membrane tube were dynamically measured on a pilot plant working permeation device at ambient temperature. Photocatalytic reactions of titania membranes were statically evaluated on a self-made catal-
Y.H. Wang et al. / Materials Science and Engineering A 445–446 (2007) 611–619
ysis reactor, which consisted of a 500 ml cylindrical Pyrex glass vessel equipped with plugging quartz tube containing a mercury lamp as ultraviolet (UV) light irradiation sources (UV-A type, irradiation power, 4 W). This quartz tube arrayed aside by four tubular anatase membranes (effective photocatalytic membrane area 0.0014 m2 ) had a Pyrex cylindrical jacket in which cooling water was circulated to avoid the heating of the solution. A volume of 100 ml aqueous solution of methylene blue (MB) with an initial concentration of 10 ppm was introduced into the reactor under the UV light illumination. At the same time, oxygen was continuously bubbled into the reaction mixture. A UV–vis spectrophotometer (Sahimadzu Corp., Japan, UV-2401PC) was used to determine the degradation of organic MB. 3. Results and discussion 3.1. Preparation of pure titania membrane supports Fig. 1 shows the morphology of raw rutile powder particle before and after titanium sol-coating treatment. It is found that a uniform coating layer of titanium sol strongly adhered onto the as-received flat titania entities surface and filled the cracks as well as the voids between agglomerates (Fig. 1c). The irregular shape of titania hollow granules with rough, even crack-like surfaces or void holes (Fig. 1a and b) implies the poor fluidity and low compression resistance strength of green extrudate thus leads to heterogeneously packing structures and low production quality. Ananthakumar and Warrier [18] reported that the alumina pastes using 18.5 vol.% of boehmite sol as dispersant and binder exhibited adequate plasticity and fluidity for extrusion and produced better green strength hence uniform sintered microstructures. Li et al. [19] obtained bulk TiO2 nanoceramics with an average grain size of less than 60 nm and relative density over 95% by a phase-transformation-assisted pressureless sintering at a relative low temperature. In this study, the titanium sol-coating layer is expected to not only produce good green strength as processing aids during extrusion processes but also act as sintering additives lowering the sintering temperature.
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However, the liquid removal of titania porous green extrudate is of significant importance due to the presence of higher water content ca. 20 wt.% during the plastication and sol-coated processes. As we know, the complexity of drying processes is often associated with both circumstance conditions like drying temperature, etc., and green body parameters such as thickness, geometric shape and pore structure properties accompanying with drying stress development and release as water evaporation proceeds [16]. Many fatal drying flaws are prone to occur in the obtained titania membrane tube due to the drying stress intensity exceeding over the intrinsic critical fracture strength of porous titania materials if adopting a unreasonable drying method. Therefore, it is indispensable to utilize a combined designation of different drying methods from a technical and economical point of view. Fig. 2 shows the green density as a function of drying time by the combined methods with isothermally controlled humidity drying and microwave drying. The density of green titania membrane tube was determined by the ratio of green body weight divided by the dimensional volume. During the drying processes of constant temperature with controllable humidity (RH 60%, 30 ◦ C) in air (Fig. 2a), the green compaction density initially decreases at a slower drying rate up to a recorded drying time of ca. 1 h attributed to the volume shrinkage equal to the volume of water removed by evaporation in a constant drying rate period then approaches to the drying critical point subsequently comes forth the first falling rate period in another prolonging 1 h. At the critical point, capillary pressure reaches a maximum stress value and most likely causes cracks in light of the drying theory [20]. In this situation, the precision control of drying rate as well as drying uniformity with respect to the drying stress distribution in green compactions is very important for the final microstructure development and the defect formation. The green density in later 6 h reduces little because the evaporation occurs inside the body through Knudsen diffusion transport mechanisms as the drying stage enters into the second falling period, where the drying rate becomes less sensitive to external conditions, e.g. temperature, humidity, draft rate, and so on. Therefore,
Fig. 1. Shapes of raw rutile powder particles: (a) ultrasonically dispersing with ethanol; (b) magnified image of picture (a); (c) after sol-coated treatment.
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Fig. 2. Green body density change curves by methods: (a) isothermal drying; (b) microwave drying.
the further removal of last traces of liquid potentially risking oxidization corrosion to the key elements of high temperature furnaces requires rather drastic drying conditions [20]. Microwave drying method provides an alternative with fast and uniform heating. As it can be seen from Fig. 2b, a 4.12% reduction in green density at expense of only 9 min drying time (top time scale of Fig. 2) is observed, indicating the advantages of microwave drying method for porous ceramic fabrication. Fig. 3a illustrates the thermal evolution diagrams of pure titania green compactions up to 1200 ◦ C at heating rate 10 ◦ C/min. The initial endothermic loss in the range of 30–160 ◦ C is attributed to the evaporation of free water/alcohol and bound water from the wet paste. The major weight loss in TGA curve occurring in 240–450 ◦ C accompanying with slightly exothermal effects in DTA curve is assigned to the decomposition of organic pore forming agent along with the anatase crystallization from the titanium sol-coating layer. The large exothermal peak in DTA curve without weight loss shown in TGA curve appears at 1000 ◦ C resulting from the structural transformation of anatase to rutile and the crystallization. The thermal shrinkage measurements offering obvious evidence on the densification processes of titania membrane support are present in Fig. 3b. Below 200 ◦ C, a slight volume expansion of green titania compactions is related to the vaporization of water/alcohol and the thermalinduced dilatation. Shrinkages initiating in 200–400 ◦ C indicate the evaporation of water and the decomposition of organic additives, which is in good accordance with the results of DTA–TGA characterization. Whereas a little shrinkage in 400–800 ◦ C is associated with the phase transformation of anatase to rutile involving the decrease of cell volume and increase of density. Kim and Kim [21] experimentally and theoretically found that the relative density in a nanocrystalline titania powder compact under pressureless sintering increased rapidly between 600 ◦ C and 800 ◦ C because the increasing mobility of atoms
Fig. 3. (a) DTA–TGA curves of pure titania green specimen; (b) dilatometeric curves of pure titania support during sintering.
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Fig. 4. Comparison with the microstructures: (a) obtained pure titania support sintered at 1200 ◦ C; (b) sample of TAMI.
during the phase transformation enhanced the sintering rate of nanocrystalline titania powder compacts. The shrinkage proceeds in 950–1200 ◦ C due to the nucleation and growth of rutile phase during the sintering processes as stated in the DTA–TGA analysis, where a maximum shrinkage rate peak appears at 1100 ◦ C. Voigt et al. [8] pointed out that the sintering of titania support occurred at 1300 ◦ C, thus, the slower shrinkage rate subsequently in 1200–1400 ◦ C is attributed to the coarsening processes of rutile grains. It was established by La et al. [22] that the initial sintering of rutile powder occurred in 750–1000 ◦ C complying with the plastic flow kinetic model. Therefore, the sintering of titania in 1000–1200 ◦ C should be fallen into the intermediate sintering stage accompanying with a drastic volume shrinkage, which is most likely to cause the defect formation. To solve this difficulty, twice heating treatment technology was adopted. The pre-calcination of sample prior to sintering densification could effectively promote the sintering microstructure uniformity and improve the production quality due to the avoidance of sintering stress concentration during the densification processes. Sato and Carry [23] confirmed that the pre-treatment of submicron-grained ␣-alumina could create a more uniform sintering microstructure causing little change in apparent activation energy in the intermediate sintering stage. Moreover, the applied external forces during the heating processes could significantly affect the specimen sintering kinetics and the microstructure development. For example, Moritz et al. [24] produced quite smooth and plane titania graded layers employing alumina disc cover to suppress the layer warpage and deformation. Kinemuchi et al. [25] have synthesized graded porous structures in ceramics by applying centrifugal acceleration during sintering, which has been demonstrated according to the kinetic model of liquidphase sintering driven by both capillary and centrifugal forces. In this work, the special embedding disposal with auxiliary corundum powder towards the dry green titania tube effectively allows the homogeneous stress development at different locations of sintered specimen.
3.2. Structural properties of pure titania membranes Fig. 4 shows the SEM pictures of pure titania supports. For comparison, the photograph of commercially available production made by TAMI [9] under the same test conditions was also displayed (Fig. 4b). It is apparent that the uniform sintered microstructures in our sample (Fig. 4a) have average grain size in the range of 2.0–3.0 m with good interconnectivity while that in TAMI compose of many irregular/rod-like grains with an obvious aspect ratio. The distinct porous structures between our specimen and that of TAMI should be owed to the sol-coated processes and subsequent drying and sintering technology modifications in our cases. Thus, the better mechanical strength as well as desired fluid permeation as a consequence would be well expected. Fig. 5 illustrates the mechanical properties and porosity of pure titania bar as a function of sintering temperature. In Fig. 5,
Fig. 5. Mechanical properties of porous titania bar sintered at different temperature by cold uniaxial pressing methods.
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Fig. 7. Pore size and pore size distribution of pure titania membranes.
Fig. 6. Effect of sintering conditions on the chemical stability of titania support: (a) mass loss ratios; (b) mechanical properties of specimen sintered at 1300 ◦ C before and after corrosion test.
as estimated by the microstructure observations, the over 32 MPa mechanical strength of the obtained titania support sintered at 1150–1450 ◦ C is high enough for membrane applications and increases with the elevated temperature while reduces inversely with the open porosity. Moreover, the three distinct portions of open porosity curve corresponding to a certain of sintering temperature scope are in good agreement with the analysis results in Fig. 3b, indicating a slight increase in open porosity at 1450 ◦ C due to rutile grain growth. The effects of sintering conditions on the chemical stability of pure titania membranes are depicted in Fig. 6. Seen from Fig. 6a, the rutile supports show superior resistance to alkaline corrosion to acid corrosion and deteriorate with the prolonged sintering time and elevated sintering temperature. In Fig. 6b, the maximum mechanical strength 120 MPa with the lowest open porosity 9% was obtained in the sintered titania specimen at 1300 ◦ C for 4 h, but further increase in sintering time leaded to a drastic reduction in mechanical properties. And the variation trends of mechanical properties after corrosion test reflecting the chemical stability of support
were similar to the cases observed in Fig. 6a. Indeed, it has been confirmed that the titania membrane demonstrates more stable in concentrated acid or alkaline solutions at high temperature than that case of ␣-alumina [8]. Nevertheless, the remarkable variation of pure titania in chemical stability with the sintering conditions is mostly ascribed to the microstructure change, indicating the importance of sintering condition optimization. Too high sintering temperature and overlong sintering period are not accepted for the preparation of pure titania supports with better performances. The pore size of pure titania supports sintered at 1200 ◦ C for 2 h and that of TAMI under the identical measuring conditions were synchronously plotted in Fig. 7. As shown in Fig. 7, our specimen with an average pore diameter 2.10 m has narrower pore size distribution within 1.50–3.0 m than that 1.0–3.75 m of TAMI centered at 1.70 m in diameters. The fluid flux as a function of transmembrane pressure difference in Fig. 8 shows a well linear dependency relationship according to the Hagen–Poisseuille equation. The nitrogen gas flux (Fig. 8a) and the pure water permeation (Fig. 8b) of titania support was determined as 1641, 10.43 m3 m−2 h−1 bar−1 , respectively. As stated, the nitrogen gas permeability of ours is far higher than that value 1416 m3 m−2 h−1 bar−1 of TAMI, which is in accordance with the open porosity 43.09% of our support to that 32.35% of TAMI, implying the significant reduction of fluid permeation resistance inside the prepared membranes. 3.3. Dual roles of membrane separation and photocatalysis Bosc et al. [12] have reported some experimental results of anatase coating for coupling membrane separation and photocatalyzed reactions. However, in their experiments, the integrated properties such as chemical stability and thermal compatibility between the titania membranes and the metastable ␥-alumina substrate must be discounted. Fig. 9 illustrates the SEM photographs of asymmetric 100% titania membranes and
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Fig. 8. Fluid penetration of obtained pure titania membranes:(a) nitrogen gas flux; (b) pure water permeation.
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the monochannel tubular pure TiO2 membranes sintered at 1200 ◦ C. A smooth and uniform anatase membrane layer with a pore size ca.0.10 m (Fig. 7) closely coated onto the prepared rutile support. The thinner membrane thickness (15.0–20.0 m, Fig. 9b) and the narrower pore size distribution (0.10–0.35 m, Fig. 7) greatly contribute to the improvements of nitrogen gas flux (676.2 m3 m−2 h−1 bar−1 , Fig. 8a) and pure water permeation (0.74 m3 m−2 h−1 bar−1 , Fig. 8b) of the asymmetric membranes. Bosc et al. [12] further pointed out that the higher membrane porosity could favor the transport of oxygen to the titania membrane surface and the extraction of the degradation products thus would enhance the photocatalytic efficiency of the coating. An elementary concept for the further work of coupling membrane separation and photocatalysis is described in Fig. 10a, which means such a membrane process providing retention of colloids or macromolecules whereas degradation of small molecules on the anatase membrane side under the UV light irradiation. Fig. 10b offers the static characterization results of methylene blue (MB) solution photocatalyzed by the prepared anatase membranes. Adsorption of MB by the membranes was confirmed to be very small in the dark (without UV irradiation). It appears that the initial absorbance 1.78% of MB solution at 670 nm wavelength, which varied little with UV irradiation time in absence of anatase membrane, decreased to 0.83% after a period of irradiation times. Therefore, the observed decrease in absorbance of the solution was deduced to be due to the decomposition of MB by photocatalytic reactions of anatase membranes. This preliminary experiment suggests such functional membranes might be envisaged as an alternative for the detoxification treatment of industrial waste water. Of course, further improvement of photocatalysis efficiency in such membrane configuration should be done in the future works dealing with the modifications of membrane reactor designation and membrane material preparation, i.e. the improved activity of catalyst [26]. Fortunately, many attempts are under progressing towards the enhanced photocatalytic activity of titania [27,28].
Fig. 9. SEM photographs of prepared 100% titania membrane: (a) surface; (b) cross-section morphology; (c) prepared 100% titania tubular membrane elements.
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chemical stability. The prepared pure titania support have an average pore diameter 2.10 m with 1.5–3.0 m narrower pore size distribution, excellent nitrogen gas flux and pure water permeation. 3. The characterizations of fluid permeation and degradation of methylene blue (MB) solution under ultraviolet (UV) light irradiation for the prepared membranes show that the multilayer membranes have higher fluid permeation as well as obvious in situ photocatalysis. These primary results display such functional membranes may be efficiently employed in such fields as the treatment of industrial effluents. Acknowledgements Great acknowledgements should be given for their financial supports from the Natural Science Foundation of China and the Ministry of Science and Technology of China (under contract nos. 2003CB615700 and 2001AA323090). References
Fig. 10. (a) Schematic diagrams of coupling membrane separation and photocatalysis; (b) UV–vis adsorption spectra of methylene blue solution with 100% anatase membrane.
4. Conclusions 1. The asymmetric pure titania membrane was prepared by a reproducible sol-coated route with the helps of combination methods of controlled humidity drying and microwave drying. The rutile granules coated by the titanium sol produce good green strength during extrusion and low sintering temperature. The major technique modifications in drying methods and sintering steps for the TiO2 membrane preparation significantly promote the homogeneous stress development and effectively avoid the defect formation. 2. SEM observations of the pure titania membrane sintered at 1200 ◦ C reveal that the prepared tubular titania support have uniform microstructures with good interconnectivity between grains. The sintering conditions obviously affect the open pore structures of pure titania membranes hence the
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Preparation of asymmetric pure titania ceramic membranes with dual functions Yong Hong Wang ∗ , Xing Qin Liu, Guang Yao Meng Department of Materials Science and Engineering, University of Science & Technology of China, Hefei, Anhui 230026, PR China Received 21 April 2006; received in revised form 27 July 2006; accepted 29 September 2006
Abstract The asymmetric 100% titania membranes have been prepared by a reproducible sol-coated process assisting with a combined drying method and modified sintering technique. The pretreated rutile granules resulted in better green strength in extrudate and favorably sintered at a relative low temperature. Scanning electron microscopy (SEM) images show that the prepared titania supports possess uniform microstructures with good interconnectivity between grains due to the optimized preparation conditions. The supports exhibit narrower pore size distribution centered at pore diameter 2.10 m, excellent chemical stability and higher fluid permeation. Moreover, the results show that the asymmetric membranes with a pore size ca. 0.10 m and active layer thickness of 15.0–20.0 m have desired fluid permeation along with remarkable in situ photocatalysis. These primary measurements imply such ceramic membranes may be envisaged as a promising separation media for the treatment of industrial effluents. © 2006 Elsevier B.V. All rights reserved. Keywords: Titania; Inorganic membrane; Microstructure; Photoactivity; Porous ceramic
1. Introduction Ceramic membranes have been paid increasing attentions for their powerful applications in the fields such as gas/liquid separation [1,2], oxygen—enriched air production and methane reforming for syngas [3,4], etc., due to their intrinsic tolerance to high temperature, resistance against non-aqueous solvent, mechanical rigidity and chemical inertness in caustic environments. In fact, in many cases, the formidable problems in practical device with controlled geometric shape, density and dimension for the specific applications still remain being great challenges [2]. Today, different types of such ceramic membranes with a specific geometric shape and pore size standard from microfiltration to nanofiltration even pervaporation have been reported widely [5–9]. For instance, at present, TAMI is the only sole ceramic membrane corporation in the world range who devoted to the commercial production of pure titania membranes from support to separation membrane layer [9]. The asymmetric multilayer configuration is usually preferred for the manufacture
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Corresponding author. Tel.: +86 551 3519041; fax: +86 551 3607627. E-mail address: [email protected] (Y.H. Wang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.107
of inorganic membrane, which is often composed of macroporous support, thin intermediate layer and/or toplayer with a decreasing pore size [10]. As to ceramic membrane materials, mostly Al2 O3 , ZrO2 and TiO2 are considered due to their abundance and higher chemical stability. However, in view of the relative high fabrication costs and the limited chemical stability in caustic media for zirconia, alumina and their composite membranes, pure titania membranes provide competitive advantages with better fouling resistance hence higher fluid flux due to their amphiphilic surface properties [11] and lower fabrication costs because of their reduced sintering temperature [8]. Further, titania membranes have numerous potential applications in sensitive fields, e.g. food, biotechnology, pharmaceutical industries and environment markets, where the use of alumina membrane is widely controversial for the health care. Even more, it is very interesting and economical that titania membranes can be efficiently employed to integrate the two discrete processing units of separation filtration and photocatalysis into one simplified manipulation [12]. After all, the membrane separation of TiO2 particle photocatalyst from the treated water after detoxification is complicated and strongly affected by cross-flow velocity, transmembrane pressure, feed concentration, pH of the suspension and ionic strength [13]. Zhao et al. [14] have obtained some achievements in the preparation of mesoporous titania film
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on porous substrate with dual functions of photocatalysis and separation. In addition, the improvement of permeability (permeation flux) along with reliable membrane preparation technique is always pursued in the development of asymmetric inorganic membrane, which is close related to the quality of ceramic supports [10]. It is recognized that the toplayer membrane formation making up the multilayer configuration now can be readily realized by the colloidal or polymeric sol–gel routes [5–8,14]. In particular, as the polymer science develops, the skillful utilization of functional polymer acting as active additive (e.g. dispersant, template, structure directing agents, etc.) greatly contributes to the ceramic membrane preparation [8,14,15]. However, to the best of our knowledge, few are the systematical reports of preparation on pure titania membranes with dual functions up to now. In fact, there are many technological difficulties in massive preparation of tubular macroporous titania supports from commercially available rutile powder that was formed through spray drying methods. An outstanding drawback of titania granules during the forming stage is their poor compression resistance strength. Moreover, it is very difficult to avoid the defect formation such as warpage and cracks in the resultant green specimen during the drying and sintering [16]. Herein, we attempted to develop a reproducible sol-coated process using a combination method of controllable humidity drying and microwave drying to manage the microstructure development and properties of pure titania membranes under mild sintering conditions. A sintering method termed as twice heating treatment was also investigated with respect to the avoidance of sintering stress accumulation and the improvement of practical performances. Besides, for the prepared asymmetric TiO2 membranes, future works related to coupling membrane separation and photocatalytic reaction in a given process of purification or decontamination of effluents would be explored by the static characterization results of fluid permeation and degradation of organic solution (methylene blue, MB) under ultraviolet (UV) light irradiation.
in the subsequent pugging stage. The resultant paste was covered with a plastic wrap for 24 h under controlled humidity to avoid premature drying and to ensure complete homogenization of wet paste. The 250 mm in length and 12 mm in external diameters tubular supports fabricated by extrusion method from this paste were set on rollers avoiding deformation or twisting and dried in controllable humidity at ambient temperature for some times then undergone microwave drying using a household microwave oven (EM-551S, SANYO brand, china, power supply 1150 W, microwave heating power 800 W). The green specimen embedded into a loose compaction of calcined corundum powder were heated on a programmable HT furnace (Nabertherm, Germany) in air at a ramping rate of 0.5 ◦ C/min to 400–600 ◦ C dwelling 0.5 h in this temperature range for the drainage of burning out of organic additives; followed by the same heating rate up to 850 ◦ C then 0–1.5 ◦ C/min to 1100–1200 ◦ C and sintered at the maximum temperature for 2 h. The later annealing temperature was designed by a ramping rate 1.0 ◦ C/min up to 1300 ◦ C/1400 ◦ C, where the sample was sintered for 2–8 h, respectively, then cooled naturally to room temperature. Fine anatase powder with median particle size D50 = 0.37 m was obtained from the calcined hydrolyzate of titanyl sulfate. TiO2 separation layers on the obtained rutile supports were fabricated by dip-coating techniques with the prepared anatase suspensions dispersed by polyacrylic acid (PAA), dried for at least 24 h in closed glove-box with controllable relative humidity under ambient air atmosphere and sintered at 500–700 ◦ C for 2 h by programmed temperature furnace. 2.2. Characterizations on TiO2 membrane microstructures
2. Experimental procedures
The thermal evolution of sample was studied by DT/TG-50 model differential thermal analysis and thermal gravimetric analyzer (Sahimadzu Co., Japan) using ␣-Al2 O3 powder as standard reference in air. The support microstructures were imaged by a X-650 model scanning electron microscope (SEM, HITACHI, Japan). The shrinkage measurement of TiO2 green bar during sintering was carried out on a horizontal dilatometer (NETZSCH DIL 402C, Germany) at a ramping rate 10 ◦ C/min.
2.1. Preparation of pure TiO2 membranes
2.3. Measurement of membrane properties
Rutile powder with a median particle size of 10.31 m (Hubei, China) was commercially available and used as support raw materials. Titanium sol was prepared from titanium sulfate hydrolyzed at ambient temperature under acidic conditions using triethanolamine as hydrolysis inhibitor and polyvinyl alcohol as dispersants. The titania powder mixtures containing some pore former (e.g. 1.0–3.0 wt.% corn starch) and organic binder (e.g. 5.0–10.0 wt.% methyl cellulose) were dry mixed for 2–6 h then appropriate amount of absolute alcohol was introduced into the powder mixtures under strongly mechanical blending. After blending for another several hours, precise weighed titanium sol was sprayed into the prepared paste and continued blending for several hours. Next, appropriate proportions of lubricant and plasticizer were dissolved in de-ionized water (80 ◦ C) and mixed
The mass loss and bending strength changes of samples were adopted to characterize the chemical stability of TiO2 membranes according to the National Standard GB/T1970–1996 of PR China [17]. Three point flexural strength measurements were executed on 5 mm × 5 mm × 60 mm rectangular bars with a mechanical testing machine (Sahimadzu Co., Japan). The porosity of sintered sample was measured according to the Archimedes’s principle taking the theoretical density of rutile as 4.25 g/cm3 . The active pore size and pore size distribution of membranes were measured by the gas/liquid displacement porometry (GLDP) method using water as impregnation media, N2 gas permeation and clean water flux through TiO2 membrane tube were dynamically measured on a pilot plant working permeation device at ambient temperature. Photocatalytic reactions of titania membranes were statically evaluated on a self-made catal-
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ysis reactor, which consisted of a 500 ml cylindrical Pyrex glass vessel equipped with plugging quartz tube containing a mercury lamp as ultraviolet (UV) light irradiation sources (UV-A type, irradiation power, 4 W). This quartz tube arrayed aside by four tubular anatase membranes (effective photocatalytic membrane area 0.0014 m2 ) had a Pyrex cylindrical jacket in which cooling water was circulated to avoid the heating of the solution. A volume of 100 ml aqueous solution of methylene blue (MB) with an initial concentration of 10 ppm was introduced into the reactor under the UV light illumination. At the same time, oxygen was continuously bubbled into the reaction mixture. A UV–vis spectrophotometer (Sahimadzu Corp., Japan, UV-2401PC) was used to determine the degradation of organic MB. 3. Results and discussion 3.1. Preparation of pure titania membrane supports Fig. 1 shows the morphology of raw rutile powder particle before and after titanium sol-coating treatment. It is found that a uniform coating layer of titanium sol strongly adhered onto the as-received flat titania entities surface and filled the cracks as well as the voids between agglomerates (Fig. 1c). The irregular shape of titania hollow granules with rough, even crack-like surfaces or void holes (Fig. 1a and b) implies the poor fluidity and low compression resistance strength of green extrudate thus leads to heterogeneously packing structures and low production quality. Ananthakumar and Warrier [18] reported that the alumina pastes using 18.5 vol.% of boehmite sol as dispersant and binder exhibited adequate plasticity and fluidity for extrusion and produced better green strength hence uniform sintered microstructures. Li et al. [19] obtained bulk TiO2 nanoceramics with an average grain size of less than 60 nm and relative density over 95% by a phase-transformation-assisted pressureless sintering at a relative low temperature. In this study, the titanium sol-coating layer is expected to not only produce good green strength as processing aids during extrusion processes but also act as sintering additives lowering the sintering temperature.
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However, the liquid removal of titania porous green extrudate is of significant importance due to the presence of higher water content ca. 20 wt.% during the plastication and sol-coated processes. As we know, the complexity of drying processes is often associated with both circumstance conditions like drying temperature, etc., and green body parameters such as thickness, geometric shape and pore structure properties accompanying with drying stress development and release as water evaporation proceeds [16]. Many fatal drying flaws are prone to occur in the obtained titania membrane tube due to the drying stress intensity exceeding over the intrinsic critical fracture strength of porous titania materials if adopting a unreasonable drying method. Therefore, it is indispensable to utilize a combined designation of different drying methods from a technical and economical point of view. Fig. 2 shows the green density as a function of drying time by the combined methods with isothermally controlled humidity drying and microwave drying. The density of green titania membrane tube was determined by the ratio of green body weight divided by the dimensional volume. During the drying processes of constant temperature with controllable humidity (RH 60%, 30 ◦ C) in air (Fig. 2a), the green compaction density initially decreases at a slower drying rate up to a recorded drying time of ca. 1 h attributed to the volume shrinkage equal to the volume of water removed by evaporation in a constant drying rate period then approaches to the drying critical point subsequently comes forth the first falling rate period in another prolonging 1 h. At the critical point, capillary pressure reaches a maximum stress value and most likely causes cracks in light of the drying theory [20]. In this situation, the precision control of drying rate as well as drying uniformity with respect to the drying stress distribution in green compactions is very important for the final microstructure development and the defect formation. The green density in later 6 h reduces little because the evaporation occurs inside the body through Knudsen diffusion transport mechanisms as the drying stage enters into the second falling period, where the drying rate becomes less sensitive to external conditions, e.g. temperature, humidity, draft rate, and so on. Therefore,
Fig. 1. Shapes of raw rutile powder particles: (a) ultrasonically dispersing with ethanol; (b) magnified image of picture (a); (c) after sol-coated treatment.
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Fig. 2. Green body density change curves by methods: (a) isothermal drying; (b) microwave drying.
the further removal of last traces of liquid potentially risking oxidization corrosion to the key elements of high temperature furnaces requires rather drastic drying conditions [20]. Microwave drying method provides an alternative with fast and uniform heating. As it can be seen from Fig. 2b, a 4.12% reduction in green density at expense of only 9 min drying time (top time scale of Fig. 2) is observed, indicating the advantages of microwave drying method for porous ceramic fabrication. Fig. 3a illustrates the thermal evolution diagrams of pure titania green compactions up to 1200 ◦ C at heating rate 10 ◦ C/min. The initial endothermic loss in the range of 30–160 ◦ C is attributed to the evaporation of free water/alcohol and bound water from the wet paste. The major weight loss in TGA curve occurring in 240–450 ◦ C accompanying with slightly exothermal effects in DTA curve is assigned to the decomposition of organic pore forming agent along with the anatase crystallization from the titanium sol-coating layer. The large exothermal peak in DTA curve without weight loss shown in TGA curve appears at 1000 ◦ C resulting from the structural transformation of anatase to rutile and the crystallization. The thermal shrinkage measurements offering obvious evidence on the densification processes of titania membrane support are present in Fig. 3b. Below 200 ◦ C, a slight volume expansion of green titania compactions is related to the vaporization of water/alcohol and the thermalinduced dilatation. Shrinkages initiating in 200–400 ◦ C indicate the evaporation of water and the decomposition of organic additives, which is in good accordance with the results of DTA–TGA characterization. Whereas a little shrinkage in 400–800 ◦ C is associated with the phase transformation of anatase to rutile involving the decrease of cell volume and increase of density. Kim and Kim [21] experimentally and theoretically found that the relative density in a nanocrystalline titania powder compact under pressureless sintering increased rapidly between 600 ◦ C and 800 ◦ C because the increasing mobility of atoms
Fig. 3. (a) DTA–TGA curves of pure titania green specimen; (b) dilatometeric curves of pure titania support during sintering.
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Fig. 4. Comparison with the microstructures: (a) obtained pure titania support sintered at 1200 ◦ C; (b) sample of TAMI.
during the phase transformation enhanced the sintering rate of nanocrystalline titania powder compacts. The shrinkage proceeds in 950–1200 ◦ C due to the nucleation and growth of rutile phase during the sintering processes as stated in the DTA–TGA analysis, where a maximum shrinkage rate peak appears at 1100 ◦ C. Voigt et al. [8] pointed out that the sintering of titania support occurred at 1300 ◦ C, thus, the slower shrinkage rate subsequently in 1200–1400 ◦ C is attributed to the coarsening processes of rutile grains. It was established by La et al. [22] that the initial sintering of rutile powder occurred in 750–1000 ◦ C complying with the plastic flow kinetic model. Therefore, the sintering of titania in 1000–1200 ◦ C should be fallen into the intermediate sintering stage accompanying with a drastic volume shrinkage, which is most likely to cause the defect formation. To solve this difficulty, twice heating treatment technology was adopted. The pre-calcination of sample prior to sintering densification could effectively promote the sintering microstructure uniformity and improve the production quality due to the avoidance of sintering stress concentration during the densification processes. Sato and Carry [23] confirmed that the pre-treatment of submicron-grained ␣-alumina could create a more uniform sintering microstructure causing little change in apparent activation energy in the intermediate sintering stage. Moreover, the applied external forces during the heating processes could significantly affect the specimen sintering kinetics and the microstructure development. For example, Moritz et al. [24] produced quite smooth and plane titania graded layers employing alumina disc cover to suppress the layer warpage and deformation. Kinemuchi et al. [25] have synthesized graded porous structures in ceramics by applying centrifugal acceleration during sintering, which has been demonstrated according to the kinetic model of liquidphase sintering driven by both capillary and centrifugal forces. In this work, the special embedding disposal with auxiliary corundum powder towards the dry green titania tube effectively allows the homogeneous stress development at different locations of sintered specimen.
3.2. Structural properties of pure titania membranes Fig. 4 shows the SEM pictures of pure titania supports. For comparison, the photograph of commercially available production made by TAMI [9] under the same test conditions was also displayed (Fig. 4b). It is apparent that the uniform sintered microstructures in our sample (Fig. 4a) have average grain size in the range of 2.0–3.0 m with good interconnectivity while that in TAMI compose of many irregular/rod-like grains with an obvious aspect ratio. The distinct porous structures between our specimen and that of TAMI should be owed to the sol-coated processes and subsequent drying and sintering technology modifications in our cases. Thus, the better mechanical strength as well as desired fluid permeation as a consequence would be well expected. Fig. 5 illustrates the mechanical properties and porosity of pure titania bar as a function of sintering temperature. In Fig. 5,
Fig. 5. Mechanical properties of porous titania bar sintered at different temperature by cold uniaxial pressing methods.
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Fig. 7. Pore size and pore size distribution of pure titania membranes.
Fig. 6. Effect of sintering conditions on the chemical stability of titania support: (a) mass loss ratios; (b) mechanical properties of specimen sintered at 1300 ◦ C before and after corrosion test.
as estimated by the microstructure observations, the over 32 MPa mechanical strength of the obtained titania support sintered at 1150–1450 ◦ C is high enough for membrane applications and increases with the elevated temperature while reduces inversely with the open porosity. Moreover, the three distinct portions of open porosity curve corresponding to a certain of sintering temperature scope are in good agreement with the analysis results in Fig. 3b, indicating a slight increase in open porosity at 1450 ◦ C due to rutile grain growth. The effects of sintering conditions on the chemical stability of pure titania membranes are depicted in Fig. 6. Seen from Fig. 6a, the rutile supports show superior resistance to alkaline corrosion to acid corrosion and deteriorate with the prolonged sintering time and elevated sintering temperature. In Fig. 6b, the maximum mechanical strength 120 MPa with the lowest open porosity 9% was obtained in the sintered titania specimen at 1300 ◦ C for 4 h, but further increase in sintering time leaded to a drastic reduction in mechanical properties. And the variation trends of mechanical properties after corrosion test reflecting the chemical stability of support
were similar to the cases observed in Fig. 6a. Indeed, it has been confirmed that the titania membrane demonstrates more stable in concentrated acid or alkaline solutions at high temperature than that case of ␣-alumina [8]. Nevertheless, the remarkable variation of pure titania in chemical stability with the sintering conditions is mostly ascribed to the microstructure change, indicating the importance of sintering condition optimization. Too high sintering temperature and overlong sintering period are not accepted for the preparation of pure titania supports with better performances. The pore size of pure titania supports sintered at 1200 ◦ C for 2 h and that of TAMI under the identical measuring conditions were synchronously plotted in Fig. 7. As shown in Fig. 7, our specimen with an average pore diameter 2.10 m has narrower pore size distribution within 1.50–3.0 m than that 1.0–3.75 m of TAMI centered at 1.70 m in diameters. The fluid flux as a function of transmembrane pressure difference in Fig. 8 shows a well linear dependency relationship according to the Hagen–Poisseuille equation. The nitrogen gas flux (Fig. 8a) and the pure water permeation (Fig. 8b) of titania support was determined as 1641, 10.43 m3 m−2 h−1 bar−1 , respectively. As stated, the nitrogen gas permeability of ours is far higher than that value 1416 m3 m−2 h−1 bar−1 of TAMI, which is in accordance with the open porosity 43.09% of our support to that 32.35% of TAMI, implying the significant reduction of fluid permeation resistance inside the prepared membranes. 3.3. Dual roles of membrane separation and photocatalysis Bosc et al. [12] have reported some experimental results of anatase coating for coupling membrane separation and photocatalyzed reactions. However, in their experiments, the integrated properties such as chemical stability and thermal compatibility between the titania membranes and the metastable ␥-alumina substrate must be discounted. Fig. 9 illustrates the SEM photographs of asymmetric 100% titania membranes and
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Fig. 8. Fluid penetration of obtained pure titania membranes:(a) nitrogen gas flux; (b) pure water permeation.
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the monochannel tubular pure TiO2 membranes sintered at 1200 ◦ C. A smooth and uniform anatase membrane layer with a pore size ca.0.10 m (Fig. 7) closely coated onto the prepared rutile support. The thinner membrane thickness (15.0–20.0 m, Fig. 9b) and the narrower pore size distribution (0.10–0.35 m, Fig. 7) greatly contribute to the improvements of nitrogen gas flux (676.2 m3 m−2 h−1 bar−1 , Fig. 8a) and pure water permeation (0.74 m3 m−2 h−1 bar−1 , Fig. 8b) of the asymmetric membranes. Bosc et al. [12] further pointed out that the higher membrane porosity could favor the transport of oxygen to the titania membrane surface and the extraction of the degradation products thus would enhance the photocatalytic efficiency of the coating. An elementary concept for the further work of coupling membrane separation and photocatalysis is described in Fig. 10a, which means such a membrane process providing retention of colloids or macromolecules whereas degradation of small molecules on the anatase membrane side under the UV light irradiation. Fig. 10b offers the static characterization results of methylene blue (MB) solution photocatalyzed by the prepared anatase membranes. Adsorption of MB by the membranes was confirmed to be very small in the dark (without UV irradiation). It appears that the initial absorbance 1.78% of MB solution at 670 nm wavelength, which varied little with UV irradiation time in absence of anatase membrane, decreased to 0.83% after a period of irradiation times. Therefore, the observed decrease in absorbance of the solution was deduced to be due to the decomposition of MB by photocatalytic reactions of anatase membranes. This preliminary experiment suggests such functional membranes might be envisaged as an alternative for the detoxification treatment of industrial waste water. Of course, further improvement of photocatalysis efficiency in such membrane configuration should be done in the future works dealing with the modifications of membrane reactor designation and membrane material preparation, i.e. the improved activity of catalyst [26]. Fortunately, many attempts are under progressing towards the enhanced photocatalytic activity of titania [27,28].
Fig. 9. SEM photographs of prepared 100% titania membrane: (a) surface; (b) cross-section morphology; (c) prepared 100% titania tubular membrane elements.
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chemical stability. The prepared pure titania support have an average pore diameter 2.10 m with 1.5–3.0 m narrower pore size distribution, excellent nitrogen gas flux and pure water permeation. 3. The characterizations of fluid permeation and degradation of methylene blue (MB) solution under ultraviolet (UV) light irradiation for the prepared membranes show that the multilayer membranes have higher fluid permeation as well as obvious in situ photocatalysis. These primary results display such functional membranes may be efficiently employed in such fields as the treatment of industrial effluents. Acknowledgements Great acknowledgements should be given for their financial supports from the Natural Science Foundation of China and the Ministry of Science and Technology of China (under contract nos. 2003CB615700 and 2001AA323090). References
Fig. 10. (a) Schematic diagrams of coupling membrane separation and photocatalysis; (b) UV–vis adsorption spectra of methylene blue solution with 100% anatase membrane.
4. Conclusions 1. The asymmetric pure titania membrane was prepared by a reproducible sol-coated route with the helps of combination methods of controlled humidity drying and microwave drying. The rutile granules coated by the titanium sol produce good green strength during extrusion and low sintering temperature. The major technique modifications in drying methods and sintering steps for the TiO2 membrane preparation significantly promote the homogeneous stress development and effectively avoid the defect formation. 2. SEM observations of the pure titania membrane sintered at 1200 ◦ C reveal that the prepared tubular titania support have uniform microstructures with good interconnectivity between grains. The sintering conditions obviously affect the open pore structures of pure titania membranes hence the
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