Thin Defect-Free Pd Membrane Deposited on Asymmetric Porous

Research Institute of Innovative Technology for the Earth (RITE), Kizu-cho, Soraku-gun, Kyoto 619-0292,. Japan, Research Institute for Innovation in S...
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Ind. Eng. Chem. Res. 2005, 44, 8025-8032

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Thin Defect-Free Pd Membrane Deposited on Asymmetric Porous Stainless Steel Substrate Jianhua Tong,*,†,‡ Yukari Kashima,† Ryuichi Shirai,† Hiroyuki Suda,‡ and Yasuyuki Matsumura§ Research Institute of Innovative Technology for the Earth (RITE), Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan, Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba 305-8565, Japan, and Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Institute Science and Technology (AIST), Mirorigaoka, Ikeda, Osaka 536-8577, Japan

Commercially available asymmetric porous stainless steel (APSS) tube filters with a smooth top layer were employed as substrates for preparing thin Pd-based composite membranes. The simple electroless plating and electroplating methods cannot successfully prepare defect-free thin (less than 6 µm) Pd-based membranes on the APSS substrates. Using the improved electroless plating technique based on a multidimensional plating mechanism, it is easy to prepare thin defect-free (less than 6 µm) Pd/APSS composite membranes. The heat treatment of the APSS substrates has an important effect on membrane performance. The as-prepared 3-µm Pd/APSS composite membrane under optimum conditions has a hydrogen flux as high as 0.409 mol/(m2 s) at 773 K with a pressure difference of 100 kPa while keeping the complete hydrogen selectivity versus helium. The stability of the hydrogen flux and the hydrogen selectivity were also proven by operating times as long as 200 h and gas-exchanging cycles as many as 40. 1. Introduction Compared with other hydrogen separation techniques, the Pd-based membrane has an obvious superiority due to its infinite theoretical hydrogen selectivity, higher hydrogen permeability, and higher resistivity to be oxidized.1-6 Besides preparation of ultrapure hydrogen from hydrogen mixtures, it is extensively used in dehydrogenation and hydrogenation chemical reactions, and some meaningful results have already been achieved.7-18 It can also supply special hydrogen or oxygen active species for improving the selectivity of intended products in some fine chemical reactions.19-21 In recent years, the in situ supplying of pure hydrogen for fuel cell operation through the Pd-based membrane has also attracted more attention.22-24 However, the lower hydrogen permeance, the poorer stability, and the higher cost are big obstacles in the face of its versatile applications. The composite membrane configuration with a thin Pd or Pd alloy membrane supported on an inert porous substrate can well solve the above problems. Surface chemistry, porosity, and roughness of the substrate will determine the amount of Pd required to form a hydrogen permselective membrane without defects. Porous glass, alumina, and stainless steel disks/tubes are the most prevalent substrates used for preparing thin Pd-based composite membranes.25-39 Due to the relatively smaller * To whom correspondence should be addressed at Membrane Separation Processes Group, Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan. Tel./fax: +81-29861-4675. E-mail: [email protected]. † RITE. ‡ Research Institute for Innovation in Sustainable Chemistry, AIST. § Research Institute for Ubiquitous Energy Devices, AIST.

surface pore size and roughness of the asymmetric porous glass and alumina substrates, some thin and defect-free Pd-based composite membranes have been successfully prepared.25,28-30,32,35 However, the great difference of the thermal coefficients between these nonmetal substrates and the Pd-based membranes makes it difficult to operate these composite membranes steadily over a thermal cycle. Porous stainless steel has a thermal coefficient similar to that of Pd or Pd alloy, which can well improve the thermal stability of the composite membranes. The ease of sealing and constructing module makes it more possible industrial applications. However, the big pores and roughness on the surface of substrates prohibit the preparation of thin defect-free Pd-based membrane on general porous stainless steel substrates. For example, Shu et al. found that symmetric porous stainless steel (SPSS) with nominal 0.2 µm pores required at least a 15 µm thickness of Pd membrane deposited by electroless plating to form a dense, impervious membrane.26,27 Mardilovich et al. reported a Pd membrane thickness between 19 and 28 µm electroless plated on SPSS substrate that had a nominal 0.5 µm pore size.31 Some techniques, such as shot peening of iron particles,28 mechanical or chemical polishing treatment,31 treating under oxidation atmosphere,33 and depositing thin nickel/copper/silica layers,34 were used to smooth the surface of the SPSS substrates. However, the decrease in the pore size resulted in increasing the resistance of the substrate against hydrogen permeation. In our previous work, a new technique employing zirconium oxide, cerium hydroxide, and aluminum hydroxide suspension was developed to in situ electroless plate thin Pd membranes on SPSS substrates based on a multidimensional plating mechanism.18,37-39 Although some of these techniques can decrease the Pd membrane thickness without greatly impairing hydrogen permeance, it is still very difficult to obtain a defect-

10.1021/ie050534e CCC: $30.25 © 2005 American Chemical Society Published on Web 09/10/2005

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free Pd-based membrane thinner than 6 µm. In this work, commercially available asymmetric porous stainless steel (APSS) tube filters with a relatively smooth top layer consisting of stainless steel fibers (micrometer size) were used as the substrates. Using the improved electroless plating method based on the multidimensional plating mechanism, a thin defect-free Pd composite membrane with a thickness of 3 µm was successfully prepared. A superior hydrogen permeation performance was obtained for the as-prepared composite membrane. 2. Experimental Section Asymmetric porous stainless steel (APSS) tube substrate (SUS316L, 10 mm in diameter and 50 mm in length) having a top layer of micron stainless steel fibers was purchased from Nippon Seisen Co. One end of the tube was blind, and the other end was welded to an SUS316L plain tube. Pd membranes were deposited using three different methods, such as simple electroless plating, simple electroplating, and improved electroless plating based on a multidimensional plating mechanism. In the case of electroless plating, the as-cleaned APSS substrate without any special pretreatment was activated by dipping in activating solution (Okuno, OPC-50) and reducing solution (Okuno, OPC-150) in turn three times. Then the Pd membrane plating was carried out in a commercial Pd plating solution (Okuno, Palla-Top) at 313-343 K with pH value of 5-7. The asprepared composite membrane was denoted ELP10. In the case of electroplating, an APSS substrate was also activated by the same process for electroless plating. Then the electroplating was carried out in a solution containing Pd ammine complex with nitrate and sulfonate anions at room temperature. The as-resulted composite membrane was denoted EP4. In the case of improved electroless plating based on the multidimensional plating mechanism, APSS substrate heated at 1373 K for 0.5 h and at 773 K for 2 h with vacuum was used. After cleaning, a thin Ag protective layer of 250 nm was electroplated on the tube surface with a plating solution provided by Okuno (Muden-Silver) at room temperature. Then the Pd/aluminum hydroxide sol prepared from Pd activating solution (Okuno, OPC-50), aluminum nitrate, and sodium carbonate was introduced to the surface pores at room temperature by suction from the interior side of the tube for 3 h.39 After complete removal of the particles stuck on the substrate surface, the as-modified APSS tube substrate was dipped in the reducing solution (Okuno, OPC-150) for 0.5 h at 313 K. Electroless plating of Pd on the tube was carried out in the same plating bath under similar conditions. Two composite membranes with thicknesses of 3 and 8 µm were prepared by this process, which were denoted H-MD-ELP3 and H-MD-ELP8, respectively. Another 3-µm composite membrane was also prepared using the same process except for a different heating procedure (1303 K for 2 h and 773 K for 2 h with vacuum), and was denoted L-MD-ELP3. The Pd concentration of Pd plating solutions before and after Pd deposition was measured by ICP (ICPS7000, SHIMADZU), which was used to evaluate the thickness of our Pd composite membranes. The morphology of the composite membranes was characterized using a Keyence laser-scanning confocal microscope (LSCM). The permeation tests of the thin Pd/APSS composite membranes were performed by a single gas method in

a double-tube permeation cell composed of an outside dense stainless steel tube and an inside Pd-based composite membrane tube. The temperature of the permeation cell was increased to 773 K at a rate of 0.5 K/min under argon atmosphere. After that, pure hydrogen gas was introduced to the annulus space and the Pd composite membranes were treated with a small hydrogen difference for 2-10 h to activate the substrate pores. Then, the hydrogen permeation flux and the hydrogen selectivity versus argon or helium were measured in detail at different operating temperatures with different pressure differences. The stabilities of the hydrogen flux and the hydrogen selectivity versus the operating time were studied at 773 K with a pressure difference of 100 kPa. The gas-exchanging experiment between hydrogen and helium for characterizing the membrane stability was carried out at 773 K. After measurement of the hydrogen flux, the hydrogen pressure difference was decreased to about zero. Then pure helium was switched on line. After stabilizing for 1 h, the helium pressure difference was increased to 100 kPa and the helium leak was measured. After measurement of the helium leak, the helium pressure difference was decreased to about zero and pure hydrogen was switched on line. The permeation cell was also stabilized for 1 h. Then the hydrogen pressure difference was increased to 100 kPa and the hydrogen flux was measured. The hydrogen selectivity was defined as the ratio between hydrogen flux and argon or helium flux at the same temperature with the same pressure difference. The minimum flux measured by our system is 5 × 10-4 mL/min; leaks lower than this value are undetectable by our system. If the helium leak is undetectable until the pressure difference of 300 kPa, the membrane will be thought to have infinite selectivity. Moreover, the permeation flux of the substrate after the modification by the Pd/aluminum hydroxide gel and higher temperature recovering was compared with that of the Pd composite membrane. Due to the great decrease of the volume (87%) from water containing Pd/aluminum hydroxide gel to Pd/Al2O3, the pressure difference for obtaining the same hydrogen permeation through the modified substrate is less than 5% of that through the Pd composite membranes. 3. Results and Discussion 3.1. Substrate and Membrane Morphology. The morphology of the substrate and membrane is an important factor for preparation of defect-free membranes and hydrogen permeation behavior. Figure 1a shows that the top layer of the fresh APSS tube substrate is composed of short stainless steel fibers having diameters of 2-3 µm. Short stainless steel fibers were sintered three-dimensionally and formed the characteristic porous structure, which is completely different from that of SPSS tube substrates shown in Figure 1d. The average surface pore size is less than 10 µm, much smaller than that of SPSS substrates (20-100 µm). Obviously, the pore population is much larger than that of the SPSS substrate. Moreover, the three-dimensional calculation of the substrate surface indicates that the biggest roughness of this substrate is just 14 µm, which is much smaller than that (80 µm) of the SPSS substrate. The thickness of the top layer of the APSS substrate is about 250 µm (Figure 2c). From Figure 1b it can be easily seen that some Ag particles were covered on the stainless steel fibers or the pores of the APSS

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Figure 1. LSCM images of substrate surfaces. (a) Fresh APSS substrate; (b) Ag modified APSS substrate; (c) Pd/aluminum hydroxide gel filled APSS substrate; (d) fresh SPSS substrate.

substrate. The biggest roughness decreased to 12 µm from originally 14 µm after Ag modification. Continuously filling the Pd/aluminum hydroxide gel in the pores of the silver-modified APSS substrate did not change the surface morphology (Figure 1c). The biggest roughness is still 12 µm. Combining with the undetectable water flux with a pressure difference of 100 kPa, it can be said that the Pd/aluminum hydroxide gel filled the deep positions of the substrate pores. Figure 2a shows the surface morphology of fresh membrane of L-MD-ELP3, demonstrating that the Pd membrane was continuously deposited on the substrate and there are not obvious defects. The biggest roughness of this membrane calculated from this three-dimensional LSCM image is 11 µm, which is 1 µm lower than that of the Pd/aluminum hydroxide gel filled APSS substrate. The decrease of the roughness shows that the Pd membrane is preferentially deposited into the surface pore because of the larger amount of Pd seeds introduced by filling Pd/aluminum hydroxide gel. Figure 2b displays the surface morphology of the used L-MDELP3, showing that the long-term and versatile gas permeation tests did not greatly affect the membrane morphology. The membrane is still continuously covered on the substrate surface and there are no obvious defects. The biggest roughness also does not change. The cross-sectional image of L-MD-ELP3 (Figure 2c) indicates that the homogeneous Pd membrane thickness of

3 µm was deposited on the top layer of the APSS substrate. The value is in accordance with that determined by measuring the Pd concentration of the plating solutions. 3.2. Performance of ELP10 Membrane. The asprepared Pd/APSS composite membrane of ELP10 (10 µm) by the simple electroless plating method still has some defects due to the detectable water leak. The LSCM characterization result indicates that there is still a large amount of pores uncovered on the substrate surface. The Pd metal was mainly deposited on stainless steel fibers rather than into surface pores, which is consistent with the increase of the biggest roughness of membrane compared to that of the original substrate. ELP10 shows a hydrogen flux as high as 0.488 mol/(m2 s) at 773 K with a pressure difference of 57 kPa, while the hydrogen separation factor (H2/Ar) is just 4.51 under the same condition. The lower separation factor indicates there exists a large amount of defects on ELP10. Therefore, the big amount of Pd (10 µm in thickness) and the lower separation factor make it impossible to apply the simple electroless plating method to deposit thin defect-free Pd membrane on the commercially available APSS tube filter substrates. 3.3. Performance of EP4 Membrane. The asprepared Pd/APSS composite membrane of EP4 (4 µm) by the simple electroplating method has no obvious big defects at room temperature due to the un-detectable

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Figure 3. H2 permeation flux versus H2 pressure difference (PH - PL, kPa) at different temperatures for Pd/APSS composite membrane of EP4.

Figure 4. Arrhenius plot for Pd/APSS composite membrane of EP4.

Figure 5. H2 permeation flux and separation factor versus operating time at 773 K with a pressure difference of 100 kPa for Pd/APSS composite membrane of EP4.

Figure 2. LSCM images of L-MD-ELP3. (a) Fresh membrane surface; (b) used membrane surface; (c) cross section of membrane.

water leakage. The hydrogen flux at different temperature with different pressure differences is shown in Figure 3, appearing that both temperature and pressure difference have positive effect on the hydrogen flux. A hydrogen flux as high as 0.125 mol/(m2 s) was obtained at 773 K with a pressure difference of 100 kPa. The n value of hydrogen partial pressure in the hydrogen equation of J ) (Q/l)(PHn - PLn) is about 1 rather than 0.5 (bulk diffusion). This unit value implies that surface reactions and/or defect diffusion have an important role in the hydrogen permeation process through EP4. The argon leak at 773 K with a pressure difference of 100 kPa shows there still exist some defects for EP4, and the hydrogen separation factor (H2/Ar) of 29.2 was determined. The existence of some defects is in agree-

ment with the unit n value. The activation energy of hydrogen permeation through EP4 was also investigated based on the Arrhenius equation of Q/l ) (Q0/l)[exp(-EA/RT)]. The linearity of the Arrhenius plot (Figure 4) is so good that there exists only one activation energy within our investigated range. The value was determined to be 15.9 kJ/mol, which is a little larger than that for the Pd/ceramic membrane but a little smaller than that for the Pd/SPSS membrane.31 The membrane stability was also studied in detail. The hydrogen flux and separation factor versus operating time are shown in Figure 5, demonstrating that the hydrogen flux decreases rapidly with the increase of the operating time. After operation for 200 h, the hydrogen flux has decreased about 27% of its original value, which can be ascribed to the reaction between short stainless steel fibers and Pd metal through the decrease of the inside surface area. This means that the inside surface reaction has an effect on the hydrogen permeation flux, which can interpret the higher activation energy with existing defects. The hydrogen separation factor has decreased to 11.9 from 29.2; it appears that some new

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Figure 6. H2 permeation flux versus H2 pressure difference (PH0.64 - PL0.64, kPa0.64) at different temperatures for Pd/APSS composite membrane of H-MD-ELP8.

Figure 8. H2 permeation flux versus H2 pressure difference (PH - PL, kPa) at different temperatures for Pd/APSS composite membrane of H-MD-ELP3.

Figure 7. Arrhenius plot for Pd/APSS composite membrane H-MD-ELP8.

Figure 9. Arrhenius plot for Pd/APSS composite membrane of H-MD-ELP3.

defects have been formed or the defect size has been increased during the long-time operation. Therefore, the Pd membrane with a thickness of 4 µm is not enough to cover the APSS tube substrate well by simple electroplating. Also, the short stainless steel fibers are easier to react with Pd metal than the SPSS substrate. 3.4. Performance of H-MD-ELP8 Membrane. The as-prepared Pd/APSS composite membrane of H-MDELP8 by the improved electroless method consisting of heating substrate at high temperature (1373 K), modifying the substrate by silver plating, filling the substrate with Pd/aluminum hydroxide, and electroless plating. This membrane surface has no obvious defects according to the LSCM characterization. The same positive effect of both temperature and pressure difference was found for H-MD-ELP8 (Figure 6). The hydrogen flux of 0.116 mol/(m2 s) was obtained at 773 K with a pressure difference of 100 kPa. However, the n value in the hydrogen permeation equation is 0.64 rather than 0.5 and 1, showing that the hydrogen permeation process is more complex. The leak of helium at 773 K with a pressure difference of 300 kPa is negligibly small, showing that the hydrogen selectivity is infinite and there really are no defects on H-MD-ELP8. The contribution of defects to the hydrogen permeation process is almost zero. Therefore, besides the bulk diffusion, the surface reactions have a smaller contribution to the hydrogen permeation process. The activation energy through H-MD-ELP8 is determined from the Arrhenius plot (Figure 7). The value of 24.2 kJ/mol was obtained, which is larger than that through EP4. This activation energy difference can be explained by the fact that the existing defects in EP4 can decrease the activation energy to some degree. Also, the isolated Ag surface between the Pd membrane and the APSS substrate may contribute to the higher activation energy through H-MD-ELP8.

3.5. Performance of H-MD-ELP3 Membrane. The performance of H-MD-ELP8 indicates that the Pd membrane thickness of 8 µm is thick enough to obtain a defect-free Pd composite membrane, and the decrease of Pd membrane thickness is possible. Therefore, another composite membrane of H-MD-ELP3 (3 µm) prepared using the same procedure as that for H-MDELP8 was also investigated. Figure 8 shows that both temperature and pressure differences also have a positive effect on the hydrogen flux. However, hydrogen flux as high as 0.278 mol/(m2 s) was obtained under the same condition (at 773 K with a pressure difference of 100 kPa), 2.4 times that of H-MD-ELP8. The n value of hydrogen pressure in the hydrogen permeation equation also increased to 1 from 0.64. Combining the undetectable helium leak with a pressure difference of 300 kPa, it can be concluded that the hydrogen permeation process for H-MD-ELP3 may be controlled by surface reaction instead of the combination of bulk diffusion and surface reaction for H-MD-ELP8. The activation energy through H-MD-ELP3 was also determined from an Arrhenius plot (Figure 9): the value is 28.3 kJ/mol, a little higher than that (24.2) through H-MD-ELP8. It is reasonable because the surface controlling permeation always has a higher activation energy than the bulk diffusion permeation. According to our previous work, the inside contact effective area may be the more important factor to control the hydrogen surface reaction.37,39 Therefore, on the high temperature treated APSS, the thin (3 µm) defect-free Pd membrane can be prepared. The increase of the inner contact effective area may be the better way to increase the hydrogen flux more. 3.6. Performance of L-MD-ELP3 Membrane. The composite membrane of L-MD-ELP3 (3 µm) was prepared using a procedure similar to that for H-MD-ELP3 except for heating of the substrate at 1303 K for 2 h

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Table 1. Hydrogen Separation Efficiency of Pd-Based Membranesa membrane

ref

prepn method

Pd/SPSS Pd/SPSS Pd/SPSS Pd/PG Pd/Al2O3 Pd/HF Pd-Ag/PG Pd-Ag/SPSS Pd-Cu/Al2O3 Pd-Cu/Al2O3 Pd-Ag foil Pd/SPSS Pd/Ag foil Pd/APSS

30 31 14 25 14 32 25 15 35 35 17 37 40 this work

ELP/O ELP ELP ELP ELP ELP ELP ELP ELP/O ELP/O cold rolling MD-ELP sputtering MD-ELP

thickness, µm 10 19-28 5 13 7-15 3 21.6 15 3.5 1.5 50 6 0.8-5 3

temp, K 753 773 673 773 673 703 673 773 623 623 773 773 573 773

H2 permeance, mol/(m2 s kPa)

sepn factor, H2/N2

0.089 0.015-0.030 0.155 0.095 0.086-0.134 0.136 0.034 0.052 0.056 0.499 0.01 0.260 0.3-2 0.409

1000 up to 5000 100-200 ∞ 100-1000 1000 ∞ H2/Ar, ∞ g7000 93 ∞ H2/He, ∞ not given H2/He, ∞

a SPSS, symmetric porous stainless steel; APSS, asymmetric porous stainless steel; HF, alumina hollow fiber; PG, porous glass; ELP, electroless plating; O, osmotic pressure method; MD, multidimensional plating mechanism.

Figure 10. H2 permeation flux versus H2 pressure difference (PH - PL, kPa) at different temperatures for Pd/APSS composite membrane of L-MD-ELP3.

and at 773 K for 2 h with vacuum instead of at 1373 K for 0.5 h and at 773 K for 2 h with vacuum. The relatively lower pretreatment temperature was supposed to increase the inside effective area to an extent without an effect on the stability. Figure 10 shows that both the temperature and the pressure difference have a positive effect on the hydrogen flux for L-MD-ELP3, the same as for some other membranes. Hydrogen flux as high as 0.409 mol/(m2 s) was obtained at 773 K with a pressure difference of 100 kPa. The hydrogen permeation performance of this membrane is compared with some other published results as shown in Table 1, revealing that it is relatively good. The n value in the hydrogen permeation equation is 1. Combining an undetectable helium leak at 773 K with a pressure difference of 300 kPa shows that the hydrogen permeation process may also be controlled by surface reaction. The Arrhenius plot of L-MD-ELP3 was also determined, and the result is shown in Figure 11. From the slope of this linear curve, the activation energy was determined to be 38.1 kJ/mol, which is greatly larger than that for the above-mentioned Pd/APSS membranes and some other published Pd/SPSS membranes.18,30,31,39 In a normal case, the contaminant or the dopant of other metal would increase the activation energy greatly, while the hydrogen permeation process was controlled by the surface reactions.5 In our case, the silver in contact position has a possibility to form an isolated Ag surface, which will increase the activation energy. Moreover, the uncovered (by silver) stainless steel fibers will react with Pd, which not only can greatly increase the activation energy but also can decrease the hydrogen permeation flux. This is why we try to deposit a 250 nm Ag layer on the APSS substrate before our Pd

Figure 11. Arrhenius plot for Pd/APSS composite membrane of L-MD-ELP3.

Figure 12. H2 permeation flux versus operating time at 773 K with a pressure difference of 100 kPa for Pd/APSS composite membrane of L-MD-ELP3.

deposition. In fact, the membrane of EP4 without a silver layer shows decreasing hydrogen permeation flux. The relatively lower pretreatment temperature for L-MD-ELP3 compared with H-MD-ELP3 increased the inside effective area to an extent, which not only increased the hydrogen flux but also increased the possibility to form Pd-Fe alloy. This larger amount of Pd-Fe alloy and some isolated Ag surface can well account for the higher activation energy of 38.1 kJ/mol. The hydrogen flux of L-MD-ELP3 versus operating time was measured at 773 K with a pressure difference of 100 kPa. The results are shown in Figure 12, demonstrating that the hydrogen flux is relatively stable within the long-time operation of 200 h. The helium leak after this long-time operation was measured again at 773 K with a pressure difference of 300 kPa, showing that the membrane is still defect-free and the hydrogen selectivity is still complete. Moreover, the stability of this membrane was also measured using a gas-exchanging cycle between hydrogen and helium, and the results

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Figure 13. H2 permeation flux versus gas-exchanging cycle between H2 and Ar at 773 K with a pressure difference of 100 kPa for Pd/APSS composite membrane of L-MD-ELP3.

are shown in Figure 13. It can be easily gas-exchanging cycles as many as 40 did not impair the hydrogen permeation flux. The helium leakage was still undetectable at 773 K with a pressure difference of 300 kPa. Finally, the surface morphology after all the measurements was compared to the fresh membrane in Figure 2, showing that the membrane also has stable surface morphology. 4. Conclusions Commercially available APSS tubes with a smooth top layer can be used as substrates for preparing a thin defect-free Pd-based composite membrane with a Pd layer about 3 µm by an improved electroless plating method based on a multidimensional plating mechanism. The simple electroless plating and electroplating cannot work well. The short stainless steel fibers (2-3 µm) of the top layer were a little easier to react with Pd membrane, which can result in the decrease of hydrogen flux stability. Modifying the APSS substrate with a silver layer of 250 nm can greatly improve the membrane stability by avoiding Pd-Fe alloy. Moreover, heat treatment at high temperature with vacuum also can stabilize the substrate and the resulting composite membranes. Under optimum conditions, the thin defectfree Pd-based composite membrane L-MD-ELP3 was obtained, which has a hydrogen flux as high as 0.409 mol/(m2 s) at 773 K with a pressure difference of 100 kPa. The helium leakage was undetectable at the same temperature of 773 K with a higher pressure difference of 300 kPa. The stability of the hydrogen flux and the hydrogen selectivity were also proven by the operating time as long as 200 h and gas-exchanging cycles as many as 40. Acknowledgment The financial support of the New Energy and Industrial Technology Development Organization (NEDO) and Japan Society for the Promotion of Science (JSPS) is gratefully acknowledged. Literature Cited (1) Athayde, A. L.; Baker, R. W.; Nguyen, P. Metal composite membranes for hydrogen separation. J. Membr. Sci. 1994, 94, 299. (2) Buxbaum, R. E.; Kinney, A. B. Hydrogen transport through tubular membranes of palladium-coated tantalum and niobium. Ind. Eng. Chem. Res. 1996, 35, 530. (3) Aasberg Petersen, K.; Nielsen, C. S.; Jørgensen, S. L. Membrane reforming for hydrogen. Catal. Today 1998, 46, 193. (4) Lin, Y. S. Microporous and dense inorganic membranes: current status and prospective. Sep. Purif. Technol. 2001, 25, 39.

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Received for review May 6, 2005 Revised manuscript received July 19, 2005 Accepted July 29, 2005 IE050534E