Ultra thin Pd membrane on α-Al2O3 hollow fiber by electroless plating: High permeance and selectivity

Journal of Membrane Science 284 (2006) 110–119

Ultra thin Pd membrane on ␣-Al2O3 hollow fiber by electroless plating: High permeance and selectivity G.B. Sun, K. Hidajat, S. Kawi ∗ Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 119260, Singapore Received 11 May 2006; received in revised form 3 July 2006; accepted 8 July 2006 Available online 14 July 2006

Abstract The morphology of Al2 O3 hollow fiber has been systematically tuned by varying both the internal and external coagulants as well as the calcination temperature in order to enhance the mechanical strength and improve the surface property of the Al2 O3 hollow fiber. The bending test and SEM results show that the finger-like void-free Al2 O3 hollow fiber calcined at 1500 ◦ C possesses not only very high bending strength but also uniform surface. The Al2 O3 hollow fiber has also been successfully employed as a substrate to form an ultra thin Pd membrane by electroless plating without any modification process. The permeation test shows that this ultra thin Pd membrane supported on the Al2 O3 hollow fiber substrate exhibits high permeance and selectivity for separation of hydrogen from a hydrogen/nitrogen mixture. © 2006 Elsevier B.V. All rights reserved. Keywords: Al2 O3 hollow fiber substrate; Morphology; Surface property; Pd membrane; Hydrogen

1. Introduction The ever-increasing demand for H2 as a green energy carrier or chemical in a variety of industries has promoted intense research interests in H2 production and purification [1]. However, the problem with H2 production is that H2 is generally produced along with other co-products or side products. For instance, methane reforming, which is an important route for H2 production, simultaneously generates carbon monoxide as a co-product. Hence, the separation or purification of H2 is an inevitable process for many applications where pure H2 is needed, such as in semi-conductor processing, fuel cell applications, etc. Among the numerous H2 separation techniques, separation with a Pd-based membrane has received growing attention due to its theoretically infinite selectivity and high permeability [2]. It is also applied in H2 related membrane reactors to boost reaction efficiency [3–8] or to generate pure hydrogen on-board for the operation of mobile fuel cell unit [9–13]. The Pd-based membrane can be in the form of a freestanding foil or tube [14–16]. However, these foils or tubes are usually quite thick in order to meet the mechanical requirement;

∗

Corresponding author. Tel.: +65 65166312; fax: +65 67791936. E-mail address: [email protected] (S. Kawi).

0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.07.015

hence not only the potential permeability of Pd is reduced but also the cost of membrane is prohibitively high. Therefore, the research for Pd-based composite membranes has focused on lowering the cost of Pd-based membrane while still achieving higher H2 permeation rate. The reported methods of preparing Pd composite membrane include chemical vapor deposition (CVD) [17,18], physical vapor deposition (PVD) [19], spray pyrolysis [20], sputtering [21] and electroless plating [22–25]. Among them, the electroless plating appears to be an attractive method as it requires very simple processing equipments and it produces uniform deposition on all kinds of substrates even with complex shapes. Traditionally, either porous glass disks or tubes (with diameter from 1 to 10 cm), or stainless steel or ceramic are often used as the substrates [26–29], however, they can only provide a low to medium separation area per unit packing volume (30–250 m2 /m3 ) [30]. Recently Pan et al. [31,32] managed to fabricate and employ Al2 O3 hollow fiber with much higher packing density (as high as 1000 m2 /m3 ). Although the high radius of curvature may impair the durability of ceramic composite materials to some extent when undertaking sustained high temperature cycling [33], the large packing density and excellent chemical resistance of the hollow fiber still make it one of the most promising substrates, especially suitable for a compact hydrogen generator required by the mobile fuel cell unit.

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Nevertheless, the preparation of Al2 O3 hollow fiber as the membrane substrate is still facing two critical problems. One of the critical issues is how to maximize the mechanical strength of Al2 O3 hollow fiber in order to overcome the mechanical weakness inherent to its small dimension. The other critical issue is how to achieve a smooth surface of Al2 O3 hollow fiber in order to ensure the formation of a leak-free Pd membrane. To the best of our knowledge, so far the methods for solving these problems are very limited. One of the reported methods in enhancing the mechanical strength of Al2 O3 hollow fiber is to increase the solid content of the spinning dope and the calcination temperature [34]. Others [35–37] reported the method of improving the surface property of Al2 O3 hollow fiber by post modification in order to reduce the amount of defects on the original surface of the Al2 O3 hollow fiber before Pd deposition. In the present study, we report a different preparation method which requires no post modification of the base hollow fiber at all but yet still improves the surface property and enhances the mechanical strength of the Al2 O3 hollow fiber. The resultant Al2 O3 hollow fiber is then directly used as a substrate to form ultra thin Pd membrane of high permeance and selectivity. 2. Experimental 2.1. Membrane preparation 2.1.1. Preparation of Al2 O3 hollow fiber substrate Polyethersulfone (PESf) (Radel A-300, Ameco Performance, USA) and N-methyl-2-pyrrolidone (NMP) (Synthesis Grade, Merck) were used to prepare a polymer solution. Commercial alpha aluminum oxide (␣-Al2 O3 ) powder with an average particle diameter of ∼0.2 ␮m (Alfa AESAR) was then slowly added to this polymer solution at room temperature under strong stirring until a uniform dope was formed. In this study, a dope consisting of ∼50 wt.% of Al2 O3 , ∼8 wt.% of PESf and ∼42 wt.% of NMP was used to prepare the hollow fiber samples. The dope was finally extruded under N2 pressure (∼2 bar) through a tubein-orifice spinneret with tube/orifice diameter of 0.72/2.0 mm. Either deionized (DI) water or NMP aqueous solution was used as the internal and external coagulants. The nascent hollow fiber passed through an air-gap distance of 1 cm before free falling into the external coagulant without any extension by external drawing and the hollow fiber was left for 30 min. The hollow fiber precursors spun by this method were as long as a few meters; however, after thoroughly washed with DI water and dried in static air at room temperature they were cut into ∼15 cm segments for the convenience of calcination in a tube furnace which has a uniform heating zone of about 30 cm as well as for Pd deposition. These ∼15 cm long segments were heated in a tube furnace at 500 ◦ C for 2 h (to remove polymer binder), followed by calcination at temperature of either 1300, or 1400 or 1500 ◦ C for 8 h. To annotate the different samples, the hollow fibers coagulated in H2 O/H2 O (internal/external) are designated as batch A, in 60NMP:40H2 O/60NMP:40H2 O as batch B and in 90NMP:10H2 O/60NMP:40H2 O as batch C, respectively. Associated with the calcination temperature, sample A13,

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Table 1 Pd plating bath composition Component

Quantity (g/l)

PdCl2 Na2 EDTA NH3 N2 H4 ·H2 O

4.7 37.2 300.0 0.3

for example, stands for a hollow fiber of batch A calcined at 1300 ◦ C. 2.1.2. Electroless plating of Pd membrane The resultant hollow fiber substrate with sealed ends was dipped into DI water for a few minutes to wash away the unwanted substances attached to the hollow fibers. The cleaned hollow fiber was initially immersed in a 5 mM SnCl2 solution at 50 ◦ C for 5 min, followed by rinsing with a small amount of DI water. This hollow fiber was then immersed in a 5 mM Pd(II) solution at 50 ◦ C for 5 min, also followed by gentle rinsing with DI water. This procedure was repeated for five times to obtain a well-seeded substrate. After the activation step, the seeded substrate was placed in the plating bath solution – as described in Table 1 – at 50 ◦ C for certain periods in order to form the Pd layer on the outer surface of the hollow fiber. Finally, the annealing process was carried out at 450 ◦ C for 2 h after the Pd coated hollow fiber had been washed and dried at 120 ◦ C for 1 h. 2.2. Membrane characterizations 2.2.1. (Field emission) scanning electron microscopy Structures and morphologies of the fresh Al2 O3 hollow fiber substrates and Pd/Al2 O3 composite membranes were visually observed using scanning electron microscopy (Jeol JSM5600LV) and field emission scanning electron microscopy (Jeol JSM-6700F). The fresh hollow fiber was snapped up at room temperature to obtain a clear cross-sectional fracture. However, the Pd/Al2 O3 hollow fiber composite membranes were first immersed in liquid nitrogen for about 10 min and the frozen membranes were slowly flexed in the liquid nitrogen until a clear cross-sectional fracture was obtained. 2.2.2. Three-point bending test The mechanical strength of the fresh Al2 O3 hollow fiber was characterized by the values of three-point bending strength. The measurements were performed with an Instron tensile tester (Model 5544) provided with a load cell of 5 kN. A 6 cm length of hollow fiber sample was fixed on the sample holders. The bending strength, σ F , was calculated from the following equation [34]: σF =

8FLDo π(Do4 − Di4 )

where F is the recorded force at which fracture takes place; L, Do and Di are the length, the outer diameter and the inner diameter, respectively.

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Fig. 1. Schematic diagram of apparatus for gas permeation.

2.2.3. Determination of volumetric porosity of Al2 O3 hollow fiber substrate The volumetric porosities of the fresh Al2 O3 hollow fiber substrates were determined based on the gravimetric analysis of water entrapped in the pores of the fiber walls. The weights of fully wetted and dried (in vacuum oven at 250 ◦ C for 5 h) Al2 O3 hollow fibers were recorded. The volumetric porosity, εv , can then be calculated according to the following equation: εv =

(wwet − wdry )/ρH2 O (1/4)πL(Do2 − Di2 )

where wwet and wdry are respectively the weights of the wet and dry fibers, and ρH2 O is the density of DI water. 2.2.4. Gas permeation test In order to identify the integrity of the hollow fiber substrate, the permeabilities of nitrogen and hydrogen through the fresh hollow fiber were measured at room temperature in a permeation testing apparatus shown schematically in Fig. 1. The permeation module used in this study consists of a 10 cm long hollow fiber with one end sealed that was placed in a stainless steel tube for collecting the permeating gas. The other end of the hollow fiber was connected to feed gas. The gap between the hollow fiber and the stainless steel tube was carefully sealed by high temperature ceramic glue to obtain a leak-free connection. The permeate side was set at atmospheric pressure, while a pressure regulator mounted on the gas cylinder was used to control the transmembrane pressure difference. The permeated flow was measured with a soap bubble flow meter. The permeability of a single gas (N2 or H2 ) through the Pd/Al2 O3 hollow fiber composite membrane was measured at temperatures from 350 to 500 ◦ C using a similar set-up. 3. Results and discussion 3.1. Effect of coagulant on morphology and property of Al2 O3 hollow fiber substrate In order to investigate the effect of coagulant on the morphology and property of the Al2 O3 hollow fiber, three batches of Al2 O3 hollow fiber samples have been prepared with different

pairs of internal/external coagulant as mentioned in experimental Section 2.1. All the hollow fiber samples described in this section are calcined at the same temperature 1300 ◦ C in order to avoid the effect of calcination temperature. SEM micrographs of the resultant hollow fibers, as shown in Fig. 2, illustrate the obvious difference in the morphology between different batches of hollow fiber samples. It can be seen that when water is used as both the internal and external coagulants, two layers of fingerlike voids form near both the outer and inner walls of the hollow fiber (Figs. 2a and b). Tan et al. [38] also observed similar morphology on the cross-section of their Al2 O3 hollow fibers and they attributed the formation of the finger-like voids to the rapid precipitation of the solid content occurring near both the inner and outer fiber walls. The formation of these finger-like voids has also been commonly encountered during the preparation of polymeric hollow fiber membranes when water was used as the coagulant [39,40]. Chung et al. [40] clearly explained the formation mechanism of the finger-like voids based on thermodynamic principles, whereby the significant solubility-parameter difference between the nascent fiber and the coagulants made the Gibbs free energy of mixing very large, disfavoring a stable solution mixture and hence prompting the phase separation within the nascent fiber. It is believed that the same thermodynamic principles could be applied to the inorganic hollow fiber membrane system. Therefore, the dope would not be stable when it interfaces with the coagulants (H2 O in this case) and may undertake the phase separation into two discontinuous phases, i.e. NMP solvent phase and PESf/Al2 O3 solid phase. After NMP solvent has been exchanged or evaporated during washing and drying processes, the spaces occupied previously by the NMP solvent are emptied and then turn into the finger-like voids. It can be noticed from Fig. 2b that the finger-like voids near the inner wall are larger than those near the outer wall, indicating that the phase separation at the inner edge is more severe than at the outer edge. The difference in phase separation could be attributed to the change of the NMP content in the nascent fiber during the spinning process. Some NMP solvent molecules could evaporate to the surrounding air from the external surface and some NMP molecules could diffuse to the internal coagulant, so that the nascent fiber contains less NMP when it interfaces with the external coagulant. This reduces the solubility-parameter difference between the nascent fiber and the external coagulant and hence reduces the extent of phase separation at the outer edge. Finger-like voids are usually not desired for both polymeric and ceramic hollow fiber membranes because they tend to cause significant reduction of mechanical strength. Hence elimination of the finger-like structure is very important, especially for ceramic hollow fiber whose mechanical strength is of a concern for the real application due to its very small wall thickness. In the present study, by varying both the internal and external coagulants from water to 60 wt.% NMP aqueous solution, the finger-like structure can be effectively controlled. As shown in Fig. 2c and d, the content and volume of the finger-like voids in hollow fiber B13 are obviously reduced in comparison with those in hollow fiber A13. However, there is still a layer of finger-like voids forming near the inner wall, indicating that the

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Fig. 2. SEM micrographs of Al2 O3 hollow fibers: (a) A13, overview; (b) A13, fiber wall; (c) B13, overview; (d) B13, fiber wall; (e) C13, overview; (f) C13, fiber wall.

solubility-parameter difference between the nascent fiber and internal coagulant is still not small enough to prevent the occurrence of phase separation. When the NMP concentration of the internal coagulant is further increased to 90 wt.% while keeping the external coagulant at 60 wt.% of NMP, the resultant hollow fiber C13 is uniform and free of finger-like void (see Fig. 2e and f). The coagulants are found to influence not only the morphology but also the surface property of the hollow fiber. As shown in the first three FESEM micrographs of hollow fibers A13, B13 and C13, which were calcined at 1300 ◦ C (Fig. 3), the Al2 O3 particles can be clearly seen on the surface of these hollow fibers without any clear sintering between the particles. A comparison of these three FESEM micrographs shows that the surface of hollow fiber A13 is quite rough with many big holes/defects. Most of these big holes could be attributed to the finger-like voids formed at the wall of the fiber. Although the finger-like voids of the polymeric hollow fibers have been reported not to pene-

trate the entire cross-section of the polymeric hollow fibers [39], however a thin skin layer protecting the finger-like voids could form on the surface of the polymeric hollow fibers [40,41]. The formation of a thin skin layer protecting the finger-like voids has not been seen for the ceramic hollow fiber A13. The probable reason is that the thin skin layer covering the finger-like voids is very likely to be destroyed during the high temperature calcination process, especially if the skin consists of a high content of polymer (PESf in this case), hence causing the surface of the hollow fiber A13 to be quite rough with many holes/defects. In contrast, the surfaces of hollow fibers B13 and C13 possess much less amount of big holes/defects, probably because these hollow fibers are free of the finger-like voids near the outer surface. These results show that the morphology tuning can effectively control the formation of big holes/defects on the surface of the Al2 O3 hollow fiber. The porosity of the hollow fiber is also found to be affected by the variation of coagulants. As listed in Table 2, the porosity

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Fig. 3. FESEM micrographs of hollow fiber surfaces: (a) A13; (b) B13; (c) C13; (d) C14; (e) C15.

decreases from 58% of hollow fiber A13, to 41% of B13 and 37% of C13. This decrease probably results from the increase of shrinkage of the nascent hollow fiber during the process of coagulation with the increase of NMP content in coagulants. The coagulation is very fast when pure water is used as the coagulants, leaving very short time for the nascent hollow fiber

to shrink. But the increase of NMP content in coagulants slows down the coagulation rate, which consequently leaves longer time for shrinkage. Therefore, the more a hollow fiber shrinks the lower porosity it has. The mechanical strength of Al2 O3 hollow fiber is also significantly affected by the variation of coagulants. The mechanical

Table 2 Summary of effect of coagulant and calcination temperature Sample #

A13 B13 C13 C14 C15

Coagulant (NMP:H2 O) Internal

External

0:100 60:40 90:10 90:10 90:10

0:100 60:40 60:40 60:40 60:40

Calcination temperature (◦ C) 1300 1300 1300 1400 1500

Morphology Finger-like void

Sponge-like pore

Inner, outer Inner None None None

Middle Middle, outer All All All

Porosity (%)

Mechanical strength (MPa)

58 41 37 26 20

36.3 96.4 122.6 216.8 389.0

Surface property Feature

Defect amount

Rough Flat Flat Flatter Smooth

Many A few A few Few Negligible

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bending strength of the hollow fiber sharply increases from 36.3 MPa of hollow fiber A13 to 96.4 MPa of B13 and to 122.6 MPa of C13 (see Table 2). This sharp increase in mechanical strength can be attributed to the elimination of the finger-like voids within the fiber wall. 3.2. Effect of calcination temperature on morphology and property of Al2 O3 hollow fiber substrate For the clarity purpose, Al2 O3 hollow fibers of batch C are selected as examples to discuss the effect of calcination temperatures (1300, 1400, and 1500 ◦ C) on the morphology and property of Al2 O3 hollow fiber. Table 2 show that the mechanical bending strength of hollow fiber C15 reaches 389.0 MPa and is over three times as that of hollow fiber C13 (122.6 MPa). This result clearly shows that the mechanical strength of the Al2 O3 hollow fiber rapidly increases with the increase of calcination temperature. This improvement could be attributed to the aggravated sintering (induced by elevated temperature) between Al2 O3 particles, as observed from the FESEM micrographs in Fig. 3. Other researchers also found that the mechanical strength of Al2 O3 hollow fiber is closely related with the calcination temperature [34]. Liu et al. [34] reported that the highest bending strength of their Al2 O3 hollow fibers prepared with 1 ␮m Al2 O3 particles and calcined at 1500 ◦ C was only 196.64 MPa. It is also noted that the strength of Al2 O3 hollow fiber reported here reaches a comparable level to that of the commercial Al2 O3 tubes (around 300 MPa), suggesting that the hollow fibers prepared by this method have the potential of practical application. The significant improvement on the mechanical bending strength can be partly attributed to the finer Al2 O3 particles (0.2 ␮m) used, as finer Al2 O3 may be more prone to be sintered at high temperature, but attributed more significantly to the fact that all the finger-like voids have been eliminated from the hollow fibers. It is also found that the porosity of the hollow fiber is affected by the calcination temperature. The increase of the calcination temperature results in the decrease of the volumetric porosity of the hollow fiber from 37% of batch C13 to 20% of batch C15 (see Table 2). This decrease of the porosity is probably attributed to the increase of shrinkage of the hollow fiber with calcination temperature, as can be seen from SEM images (not listed in this report) that the average outer diameter/thickness decreases from 1130/161 ␮m of C13 to 1068/150 ␮m of C14 and to 1012/143 ␮m of C15. It can also be seen from Fig. 3 that Al2 O3 hollow fiber calcined at 1300 ◦ C possesses a rather flat surface with a few big holes. The surface of Al2 O3 hollow fiber calcined at 1400 ◦ C is even flatter and possesses fewer big holes/defects (see Fig. 3d), which could be attributed to the improved sintering between Al2 O3 particles at 1400 ◦ C. With the further increase of calcination temperature to 1500 ◦ C the surface of Al2 O3 hollow fiber is fairly smooth and almost free of big holes/defects (see Fig. 3e). The enlargement of grains with calcination temperature, as observed in Fig. 3, probably results from the sintering of the adjacent small grains, which clearly evidences the improvement of sintering with calcination temperature. This result suggests that the surface property of Al2 O3 hollow fiber can be

Fig. 4. Gas permeation through fresh hollow fiber substrates: (a) N2 ; (b) H2 .

improved by the increase of calcination temperature. Comparatively, the surfaces of Al2 O3 hollow fibers reported before [34,36,38,42] possessed quite a few big holes/defects and were far less smooth as reported here. This is because coarser Al2 O3 powder (0.3–1 ␮m) was used therein and the morphologies of these fibers were not tuned. Therefore, a well-tuned morphology and fine membrane materials coupled by high temperature calcination are necessary for preparing Al2 O3 hollow fibers with outstanding surface property. 3.3. The integrity test of the hollow fiber substrate Prior to plating of the Pd membrane on the hollow fiber substrates, the integrity of the hollow fiber substrates has to be verified through gas permeation test. The permeation results of the hollow fibers C13, C14, and C15 are shown in Fig. 4. It can be seen that both H2 and N2 permeations are almost linearly related to the mean pressure across the membranes, indicating that the total flow through these substrates is a combination of laminar flow and Knudsen diffusion as the intercept and slope of the gas permeation line correspond to the constants of Knudsen diffusion and laminar flow, respectively [36]. The slopes of N2 permeation curves shown in Fig. 4a are 44.8, 7.4, and 5.0, respectively, indicating that the contribution of the laminar flow

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to the total flow through hollow fiber C13 is much larger than that through hollow fibers C14 and C15. This result suggests that hollow fiber C13 has a larger amount of big pores than hollow fibers C14 and 15 because a significant laminar flow only takes place in big pores where pore size is much larger than the mean free path of the permeating gas molecules [43]. The separation factor (α) of H2 to N2 increases from ∼2.3 for hollow fiber C13 to ∼3.0 for C14 and to ∼3.4 for C15, showing that the laminar flow contributes less and less to the total flow through these three substrates. These results suggest that the rise of calcination temperature can effectively reduce the amount of big pores and improve the integrity of the hollow fiber. It is noted that the separation factor for hollow fiber C15 (α = 3.4) is quite close to the ideal value of Knudsen diffusion (α* = 3.7), indicating that hollow fiber C15 contains only a very small amount of defects.

3.4. Morphology and performance of Pd/Al2 O3 hollow fiber composite membrane All the three batches of hollow fibers C13, C14, and C15 have been used as the substrates to form Pd membranes without any further modification; however, different batches of hollow fiber substrates require different plating time in order to obtain leak-free Pd/Al2 O3 hollow fiber composite membranes. Substrate C13 typically requires ∼10 h of plating until a leak-free Pd layer forms, and the plating bath needs to be replenished whenever a very little release of N2 bubbles has been observed. Substrates C14 and C15 would typically take only ∼5 and ∼1 h, respectively, to acquire leak-free Pd membranes. Fig. 5 shows that, as expected, different plating durations lead to different thicknesses. The thickness of Pd membrane plated on hollow

Fig. 5. FESEM micrographs of Pd/Al2 O3 hollow fiber composite membranes: (a) Pd/C13, surface; (b) Pd/C13, cross-section; (c) Pd/C14, surface; (d) Pd/C14, cross-section; (e) Pd/C15, surface; (f) Pd/C15, cross-section.

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Table 3 Summary of Pd/Al2 O3 hollow fiber composite membrane’s property and performance Sample #

Pd thicknessa (␮m)

H2 flux (m3 /m2 h)b

Selectivityc

Pd/C13 Pd/C14 Pd/C15

∼5.0 ∼2.5 ∼1.5

10.8 15.9 19.1

340 1400 3115

a b c

Measured from FESEM. Measured at 450 ◦ C; P = 1 bar. Selectivity = H2 permeance/N2 permeance.

fiber C13 is ∼5.0 and ∼2.5 ␮m on C14 and only ∼1.5 ␮m on C15. This result shows that the surface property of the substrate significantly affect the minimum thickness of a leak-free Pd membrane. This observation is consistent with the literature report that a support having a smoother surface requires a thinner metal film to be leak-free [44]. It is also noted from Fig. 5 that while the surface of the Pd membrane supported on hollow fiber C13 is rather rugged, which is probably caused from the relatively less smooth surface of the substrate C13, the surface of Pd membrane supported on hollow fiber C14 is relatively less rugged and that on C15 is fairly even. These results indicate that the surface properties of substrates greatly affect the surface morphologies of Pd membranes. Table 3 shows the results of the permeation tests of H2 and N2 at 450 ◦ C under a pressure difference of 1 bar. Pd/C13 offers the lowest selectivity despite the fact that it has the thickest Pd membrane. Interestingly, the thinnest Pd membrane supported on hollow fiber C15 has the highest selectivity. These results indicate that the Pd membrane supported on substrate C15 contains fewer defects than that on C13 due to the smooth surface of substrate C15. Therefore, it is reasonable to accept that the surface property of the substrate plays the most critical role in controlling the quality of the Pd membrane. Table 3 also shows that the H2 fluxes under the same pressure difference (1 bar) are 10.8, 15.9, and 19.1 m3 /m2 h for Pd/C13, Pd/C14, and Pd/C15, respectively. These interesting results show that the reduced thickness of Pd membrane increases not only the H2 selectivity but also the H2 flux of the composite membrane. Fig. 6 shows the effect of pressure difference on hydrogen flux on Pd/C15 (which is used as an example in this study) at different temperatures. The linear relationship between H2 flux and the pressure difference indicates that the pressure exponent (n) is close to 1 in the temperature range of 350–500 ◦ C. According to literatures, the exponent value could reach a value of ∼0.85 when the Pd thickness was close to ∼5 ␮m [37], and it could approach to 1 when Pd thickness was reduced to 2–3 ␮m [31]. Similarly, the Pd thickness on C15 of our study is around 1.5 ␮m and the exponent value is found to be close to 1. This result implies that resistance of the bulk diffusion of hydrogen is negligible in this case and this could be the reason why Pd/C15 membrane exhibits higher flux as compared with the thick membranes reported in the literatures [31,35]. In addition, based on the Arrhenius plot shown in Fig. 7, the activation energy for H2 transport through Pd/C15 is calculated to be 13.3 kJ/mol at the temperature range

Fig. 6. Relationship between pressure difference and H2 flux through Pd/C15 at temperatures from 350 to 500 ◦ C.

Fig. 7. Arrhenius plot of hydrogen permeance for Pd/C15.

of 350–500 ◦ C, which is similar to the value reported in the literature [35]. 4. Conclusions The morphology and the presence of finger-like voids of Al2 O3 hollow fiber can be systematically engineered by varying the internal and external coagulants. The tuned hollow fiber exhibits enhanced mechanical strength and smooth surface with negligible amount of defects compared to the un-tuned hollow fibers. Increasing calcination temperature further improves both

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the mechanical strength and surface property of the tuned hollow fiber. These tuned hollow fibers can be directly plated with an ultra thin layer of Pd membrane that exhibits very high H2 selectivity and permeation rate. Our preparation method can be used to avoid the traditional modification step for the preparation of Pd/hollow fiber composite membranes. It is expected that the Al2 O3 hollow fiber substrate prepared by this method can be extended to the preparation of thin alloy membranes of high permeance and selectivity such as Pd60 Cu40 in order to lower down the cost and increase the stability of the composite membrane, which is currently in the progress of our investigation. Acknowledgements The authors acknowledge financial support from National University of Singapore (Grants R-151-112 and R-179-112). They are also grateful to B. Wu and W.K. Teo for the technical assistance in spinning process.

Nomenclature Di Do F L n Ph Pl wwet wdry

inner diameter (m) outer diameter (m) force (N) membrane length (m) pressure exponent pressure at upstream side (bar) pressure at downstream side (bar) weight of the wet fiber (kg) weight of the dry fiber (kg)

Greek symbols α separation factor α* ideal value of Knudsen diffusion εv volumetric porosity of the hollow fiber membrane ρH2 O density of deionized water (kg/m3 ) σF bending strength (MPa)

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