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MATERIALS AND INTERFACES Thin Palladium Film Supported on SiO2-Modified Porous Stainless Steel for a High-Hydrogen-Flux Membrane Caili Su,* Tetsuro Jin, and Koji Kuraoka† Green Life Materials Research Group, Special Division of Green Life Technology, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorrigaoka, Ikeda City, Osaka 563-8577, Japan
Yasuyuki Matsumura Chemical Research Group, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizukawadai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan
Tetsuo Yazawa Department of Material Science and Chemistry, Graduate School of Engineering, Hemiji Institute of Technology, 2167 Shosha, Hemeji 671-2201, Japan
A thin palladium film supported on SiO2-modified porous stainless steel has been fabricated by a novel preparation procedure. SiO2 colloid suspensions with different particle sizes were applied to modify the pore size of the substrate and form an intermediate SiO2 layer to support the palladium layer as well. Palladium nuclei were seeded inside the pores of the modified substrate besides some particles deposited over the surface by a chemical vapor deposition process using Pd(F6acac)2 as the metal precursor, and then a palladium layer of 2-6 µm was prepared by an electroless plating process. The membrane had a H2 permeance of 2.7 × 10-6 mol/m2‚s‚Pa and a permselectivity of PH2/PN2 in the range of 300-450 measured using single pure gases at a pressure difference of 0.5 × 105 Pa and at 773 K. The surface morphology and the components of the different layers of the membrane have been studied by scanning electron microscopy and energy-dispersive X-ray analysis. The permeance and permselectivity of the membranes with different thicknesses have bean measured at different temperatures and pressures. The stability of the membrane during the heat cycles indicated that a stable structure of the composite membrane was fabricated. The transportation mechanism is elucidated based on the analysis. Introduction Hydrogen (H2) has been considered to be clean energy, and much effort has been put on the subjects related to H2 production, separation, purification, storage, transportation, and end-use technologies, e.g., fuel cell processes, in which chemical energy is converted into electrical energy. Steam reforming of natural gas has proven to be one of the least expensive ways to produce H2 (CH4 + 2H2O ) 4H2 + CO2).1 However, the reaction is endothermic (∆H°298 ) 165 kJ/mol) and needs a reaction temperature of as high as 1073-1273 K in the case where Ni-based catalysts are used. If a high-H2-permeance membrane reactor is applied in the process, H2 can be separated * To whom correspondence should be addressed. Present address: Membrane Reactor Technologies, Ltd., NRC Innovation Center, 170E-3250 East Mall, Vancouver, British Columbia V6T 1W5, Canada. Tel.: (604) 827-5166. Fax: (604) 8275165. E-mail:
[email protected]. † Present address: Faculty of Maritime Sciences, Kobe University, Fukaeminami, Higashinada, Kobe 658-0022, Japan. Tel.: 81-78-431-6332. Fax: 81-78-431-6365. E-mail:
[email protected].
from the reaction system and the thermodynamic equilibrium of the reaction will be pushed toward the direction of producing H2. This is a possible way to obtain high conversion of methane at lower temperature.2 Dense Pd and its alloy membranes are attractive for their nearly perfect permselectivity to H2. However, thicknesses of at least several tens or hundreds of micrometers for structure integrity limit their application because of their low permeance and high cost. Metal-ceramics composite membranes in which a thin Pd or its alloy films supported on porous substrates, for instance, porous glass,3 ceramics,4-10 and stainless steel11,12 have been reported and are expected to be feasible ways to solve the low-permeance and high-cost problems. Among all of these reported porous substrates, porous stainless steel (PSS) has shown advantages in the membrane module as well as its close thermal expansion coefficient to that of Pd. A shortcoming is that the commercially available PSS usually has macropores and high surface roughness. It still needs a thick layer of Pd to span all of the pores and get a leak-free membrane if the Pd layer is fabricated directly on the PSS. Our experiments have shown that a Pd
10.1021/ie049349b CCC: $30.25 © 2005 American Chemical Society Published on Web 03/19/2005
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Figure 1. SEM of the PSS and its pore-size distribution measured by using mercury porosimetry.
layer of 15-20 µm prepared directly over the PSS (separation grade, 0.2 µm), which has an average pore size of 2.7 µm, still had huge voids and possessed no selectivity to H2 compared to N2. Grandjean et al.13 modified the commercial PSS using a process of shot peening at 5.1 × 105 Pa. The results showed that the H2 flux of the membrane with a thickness of 6 µm of the Pd layer was just slightly higher than that of the membrane of 20 µm supported on the PSS, which was not modified.14 It is probable that the porosity of the membrane was decreased greatly by the modifying process. In this work, a process to modify the pore size of the PSS and decrease the surface roughness as well by coating with SiO2 colloids was applied because the SiO2 membrane has been reported to possess high H2 flux and permselectivity.15 The thin SiO2 coating is also expected to be a barrier16 to inhibit the penetration of iron into the Pd layer, which will deteriorate the performance of the Pd-based membrane. The macropores of the substrate could be modified as mesopores by coating with the SiO2 layer. It was found that the morphology of the intermediate layer influenced the formation of the metal film and the performance of the membrane greatly. Excellent work had been done by Koros et al.17,18 on modifying the macroporous ceramic substrate using a silica colloid for the formation of a polymer-ceramics composite membrane, but modifying the PSS using a silica colloid has not been found to be reported for the Pd-ceramics composite membrane for H2 separation. Dip coating used in this work is believed to be a more controllable way than simple deposition for the distribution of silica particles in the pore of the substrate. Different ceramics colloids had been tried in our work; it is interesting that silica was the best one for Pd adhesion, but we are not sure of the reason so far. The thin Pd film was fabricated by electroless plating over the modified substrate to activate H2 and release the resistance of SiO2 to H2. To increase the adhesion of the Pd layer to the modified substrate, Pd nuclei were induced into the pore by a chemical vapor deposition (CVD) process before electroless plating. Experimental Section A PSS tube (Mott Corp.; separation grade, 0.2 µm) with 7-mm i.d. and 9.5-mm o.d. was used as the substrate. The morphology and pore-size distribution of the PSS are shown in Figure 1. SiO2 colloid suspensions
(Nissan Chemical Industries, Ltd., Snowtex ZL and OL) with different particle sizes (70-100 and 20-50 nm) were coated over the PSS in a clean room maintained at a temperature of 295 K and 50% relative humidity. Each of the two suspensions was prepared by mixing the Snowtex and ethanol at a weight ratio of 1:5 and was coated over the outer surface of the substrate four times at a dipping and withdrawal speed of 1 mm/s. The modified substrate was dried at 573 K. The permeances of He and N2 were measured, and the ratio of PHe/PN2 was calculated to estimate the degree of pore-size modification. A metal-organic CVD process using palladium(II) hexafluoroacetylacetonate [Pd(F6acac)2] as the metal precursor was applied to form some Pd nuclei in the pores. It was found in our work that this CVD process could seed well-distributed Pd particles in the substrate compared with the conventional seeding process, in which the CVD took a longer time. The seeding amount can be well controlled by the vacuum conditions, temperature, and time. Large agglomerated Pd particles could be avoided by deposition for a relatively short time at our CVD conditions. N2 was flowing first to exchange air in the reactor. The reactor was evacuated to 300 Pa before increasing the temperature. The reactor was heated by two ovens. One was set at 333 K to vaporize the metal precursor, and the other one was set at 473 K to heat the membrane. A total of 100 mg of Pd(F6acac)2 was used for one time of the CVD process. The deposition time was 3 h, and then the sample was treated at 773 K for 2 h in a H2 environment. The thin Pd film was prepared over the outer surface of the modified substrate by electroless plating at 333 K using a bath supplied by Okuno Chemical Industries Co., Ltd.; one side of the membrane was evacuated slightly using a water pump after 30 min of plating. Bath contained mixtures of commercial solutions of “pallatop A” containing a Pd salt and “pallatop B” containing the reductant. The concentration of both of the A and B solutions was 100 mL/L (2.0 g/L of Pd), respectively. The pH of bath was 6.0. The formation speed of the Pd layer was 1 µm per 30 min. The thickness of the membrane was either estimated by weight gain or observed by scanning electron microscopy (SEM). H2 and N2 permeances were measured at 773 K at a pressure difference of 0.5 × 105 Pa using the equipment described in the work of Kuraoka et al.19 The down-
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Figure 2. SEM of the SiO2 layer on the PSS at different magnifications.
stream pressure was atmospheric pressure, and the upstream pressure was 0.5 × 105 Pa higher than the downstream pressure. The downstream side did not contain a sweep gas or pull by a pump because of the high permeance of the membrane. During the thermal stability measurement, the membrane was heated to 773 K at a speed of 3 K/min with flowing N2 at 100 sccm/ min. The membrane was kept at 773 K for 2 h, and then the permeances of the membrane to N2 and H2 were measured at the same conditions. After this measurement, N2 was flowed until the temperature of the membrane decreased to room temperature. The following heat cycles were repeated in this procedure. SEM and energy-dispersive X-ray (EDX) analysis were performed on a JSM-5900LVS. Cross-sectional EDX analysis was carried out over an EDS2000 system. The pore-size distributions of the samples were measured using a Micromeritics Auto Pore IV (Shimadzu).
Figure 3. SEM over the surface of the membrane after a CVD process to seed Pd particles over SiO2/PSS. Table 1. Permeances of PSS, SiO2/PSS, and Pd(CVD)/ SiO2/PSS to N2 and He at 298 K (Knudsen Flow of PHe/PN2 ) 2.6)
Results and Discussion Surface Morphology and the Structure of the Membrane. The morphology of the PSS is shown in Figure 1a. The particles of stainless steel are in the range of 10-50 µm. Its pore-size distribution is shown in Figure 1b. Although some of the pores have an opening size of 10-20 µm on the surface, most of them become narrow inside, and the average pore size is about 2.7 µm measured by using mercury porosimetry. The porosity of the substrate is about 40%. The surface morphology and SEM of the intermediate layer of SiO2 are shown in Figure 2. It is clear that the voids among the particles of stainless steel were filled by SiO2 and the surface roughness of the substrate was greatly improved. The particle size of the upper layer of SiO2 was about several tens to hundreds of nanometers. The SiO2 particles entered into the pores of the substrate, which are not deeper than 50 µm from the surface, as shown in the cross-sectional image of the SEM. The Si mapping is put together with the Pd mapping for convenient comparison, as shown in Figure 4d. The Si mapping does not seem to show a continuous Si layer, which is different from the images of Figure 2a,b. This is due to the roughness of the PSS. The silica layer was very thin at the top of the wave, and also the silica particles were probably “trapped” by Pd, so the mapping image could not show the existence of Si at these positions exactly. Because the size of the particles of a SiO2 colloid used for coating decreased gradually, the structure of the membrane turned out to be stable
substrate and intermediate layers substrate (PSS) SiO2/PSS (CVD) SiO2/PSS
permeance/ mol/m2‚s‚Pa × 106 N2 He 19.5 4.63 4.50
21.6 7.84 8.86
PHe/PN2 1.10 1.69 1.96
even after long-time treatment at 773 K. The ratio of permeances of H2 to N2 at 773 K was about 4.5 after coating with SiO2, which was slightly higher than the Knudsen separation factors of H2 and N2. Thus, the pore size of the intermediate layer was estimated to be in the range of mesopores. The seeding process by CVD of Pd(F6acac)2 as the metal precursor was applied to form some Pd nuclei over the intermediate layer, as shown in Figure 3, and EDX analysis showed that Pd was present over the substrate after the CVD process. Pd particles with the size of several tens of nanometers were observed over the surface, and agglomeration of the fine Pd particles to bigger particles in the range of several hundreds of nanometers as well on some spots was observed. The gas permeance data in Table 1 show that the permeances of He and N2 decreased and the ratio of PHe/ PN2 increased after the CVD process, which implied that Pd particles were put into the pores of the SiO2 layer during the CVD process. SEM of the membrane with a Pd layer fabricated by electroless plating is shown in Figure 4. The size of the Pd particles of the film is about 0.5 µm. It is supposed
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Figure 4. SEM of the Pd/SiO2/PSS membrane: (a) surface; (b) cross section; (c and d) cross section, Pd and Si mapping measured by using EDX analysis at the magnification for part b.
that the flexibility of the layer (wave) of Pd is due to its small separate particle structure. The thickness of the Pd layer is 2-3 µm as observed from SEM after one occurrence of the electroless plating process. The thickness of the Pd layer showed a slight variation with the surface wave of the substrate at higher magnifications of SEM images (not shown), but most part of the layer had a 2-3-µm thickness. Even a very clear solid Pd layer with a definite thickness was observed; the exact thickness of the separation layer is difficult to describe because some of the Pd particles were put in the pores. As we have mentioned, it is difficult to obtain a defectfree thin Pd film without coating with the SiO2 layer over the PSS. Figure 5 shows the surface morphology of the membrane, which was prepared directly over the substrate and had a thickness of at least 15 µm. We can see that the particles of Pd became great and the voids between the particles were kept huge. It can be concluded that a high quality of thin Pd film can only be obtained over the modified PSS, which has a small, uniform pore size and low roughness. The particle sizes of the SiO2 colloids also greatly influence the structural stability and performance of the membrane. The layer starting from tetraethyl orthosilicate had also been coated over the substrate in our experiments; the permeance of the membrane became very low because of the filling depth of the small particles in the substrate. Permeance and Permselectivity of the Membrane to H2 at 773 K. Permeances of the membranes with different thicknesses to H2 and N2 at 773 K are
Figure 5. SEM of the surface of the membrane Pd/PSS (without the SiO2 coating).
shown in Figure 6. Only H2 could pass through the membrane, which had a 2-3-µm Pd layer, at the first two heat cycles. It was observed that the H2 flux increased greatly when the membrane was treated at 773 K for a few heat cycles. This was probably due to the sintering of the Pd crystallites, and the microvoids decreased in most of areas of the Pd film. It has been supposed that lattice defects, grain boundaries, and microvoids probably act as traps, slowing H2 permeation and increasing the activation energy for diffusion.20 The increase of the H2 flux in the membrane means a decrease of the lattice defects, grain boundaries, and microvoids. Gradually, the membrane became perme-
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Figure 6. Permeances of the membrane of Pd/SiO2/PSS to H2 and N2 at 773 K.
able to N2 at a permeance of 3 × 10-9 mol/m2‚s‚Pa and increased slightly with the heat cycles. This was presumably caused by the formation of a few pinholes due to the impurities incorporated into the Pd film during the electroless plating. The permeance of the membrane to H2 with a thickness of 2-3 µm was 2.3 × 10-6 mol/ m2‚s‚Pa, and the permselectity of the membrane to H2 compared with N2 was in the range of 300-450 during the heat cycles that we have measured. The thermal stability of the membrane increased with the thickness of the membrane. Microcracks were produced after 10 heat cycles in the membrane with a thickness of 2-3 µm. The membrane with a thickness of 5-6 µm had a H2 permeance of (2.5-2.7) × 10-6 mol/m2‚s‚Pa and was stable for a further 20 heat cycles. The slight increase of H2 permeance indicated that if the thickness of the membrane was thinner than 6 µm, the flux of H2 was not influenced by the thickness of the membrane. The permselectivity to H2 was kept above 300 during 28 heat cycles and then decreased slightly because of the increase of permeance to N2. When the thickness of the membrane increased to 11.0 µm, the permeance of the membrane decreased to 1.28 × 10-6 mol/m2‚s‚Pa, which indicated that when the thickness of the membrane exceeds some range (g10 µm),20 the permeance of the membrane to H2 would be inversely proportional to the thickness of the membrane as described in Sievert’s law [F ) Q(P11/2 - P21/2)/l],20-22 where F is the flux of H2, Q is the permeability, l is the thickness of the Pd layer, P1 and P2 are the H2 partial pressures in the feed and permeate sides of the membrane, respectively. We know that the transportation of H2 through the conventional Pd membrane following the solutiondiffusion mechanism includes the following steps: H2 diffusion through the boundary layer to the metal surface from the gas, dissociative chemisorption [H2(g) f 2Hchemsorbed], absorption into the bulk metal, diffusion to the opposite face through the metal lattice, passage from the bulk to the surface, associative desorption (recombination into molecular H2), and diffusion away from the surface into the gas. The diffusion of atomic H2 is the rate-limiting step in the thick Pd membrane. The flux of H2 is inversely proportional to the thickness with an approximately square root dependence on the H2 partial pressure. As shown in Figure 7, the H2 flux through the membrane with a thickness of 6 µm was proportional to the difference of the pressure between the feed and permeate sides. This suggests that the H2 transfer through the membrane was not just controlled by the atom-diffusion step. Yan et al.4 considered that
Figure 7. Dependence of the H2 flux on the pressure difference between the feed and retentate flows (the thickness of the Pd layer was 6 µm, and temperature was 773 K).
Figure 8. Description of the structure of the membrane.
a combination of diffusion of H2 in the membrane and adsorption of H2 to the surface of the membrane was responsible for the transportation of H2 through the Pd/ R-alumina membrane. It is supposed that the same transportation mechanism might be followed in our membrane because the solid-state diffusion would become fast through the thin Pd film and the flux of H2 increased with the pressure difference. The H2 flux of our membrane at 773 K and 0.5 × 105 Pa pressure difference was about 10.8 m3/h‚m2, and the flux of the 20-µm Pd membrane deposited directly on stainless steel measured at the same temperature and 1.0 × 105 Pa pressure difference was about 4.0 m3/h‚m2.14 The 6-µm Pd membrane on shot-peening-modified PSS possessed a H2 flux of 4.7m3/h‚m2 13,24 at 673 K and 1.0 × 105 Pa pressure difference. The higher flux and high thermal stability of this Pd/SiO2/SS membrane should be due to its structure, as described in Figure 8, in which particles of the intermediate layer were put into the pores of the substrate and the particles of Pd were induced into the pores of the intermediate layer by evacuation during both the CVD and electroless plating processes. It has been found in our experiments that the evacuation during electroless plating was an effective way to fabricate a dense membrane. We know that one of the difficulties of the Pd and its alloy membranes is their structure stability, especially in the case when different layers are applied, we apply the stable structure of this composite Pd membrane using the same fabricating process to prepare the novel Pd-alloy membranes. The permeance of the membrane to N2 and H2 at different temperatures is shown in Figure 9. The permeance of H2 increased with temperature, while the permeance of N2 decreased with temperature. The transportation of H2 through a Pd film has been known to be an activated process, while the transportation of
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Figure 9. Permeances of the membrane to N2 and H2 at different temperatures (the thickness of the Pd layer was 6 µm).
N2 through the defects of the Pd layer and the porous substrate should follow Knudsen flow. As discussed above, the difference of the transportation of H2 through this thin supported Pd membrane from that of the thick Pd membrane would be the different rate-limiting steps.23,25-27 It is not clear whether diffusion or surface processes are the rate-limiting steps in our composite membrane even though non-Sievert’s law behavior was observed, and we supposed that H2 transfer through the membrane was not controlled by the atom-diffusion step. The non-Sievert’s law behavior of this supported thin membrane is probably also related to mass-transfer resistance on the lower pressure side of the membrane, the nuclear thickness of the Pd, and the impurities in the Pa layer. Conclusion A defect-free and structure-stable thin Pd membrane (2-6 µm) has been fabricated successfully over a SiO2 intermediate layer using PSS as the substrate. The membrane possesses a H2 permeance of 2.7 × 10-6 mol/ m2‚s‚Pa (∆P ) 5 × 104 Pa) and a permselectivity of PH2/ PN2 in the range of 300-450. The H2 flux was not influenced by the thickness of the membrane when the Pd layer was thinner than 5-6 µm. Atomic diffusion of H2 was no longer theorized to be the rate-limiting step of H2 transportation. An exact description for the transportation mechanism needs further study. Literature Cited (1) Barreto, L.; Makihira, A.; Riahi, K. The hydrogen economy in 21th century: a sustainable development scenario. Int. J. Hydrogen Energy 2003, 28, 267. (2) Kikuchi, E. Membrane reactor application to hydrogen production. Catal Today 2000, 56, 97. (3) Uemiya, S.; Sato, N.; Ando, H.; Kude, Y.; Matsuda, T.; Kikuchi, E. Separation of hydrogen through palladium thin film supported on a porous glass tube. J. Membr. Sci. 1991, 56, 303. (4) Yan, S.; Maeda, H.; Kusakabe, K.; Morooka, S. Thin palladium membrane formed in supported pores by metal-organic chemical vapor deposition method and application to hydrogen separation. Ind. Eng. Chem. Res. 1994, 33, 616. (5) Xomeritakis, G.; Lin, Y. S. Fabrication of a thin palladium membrane supported in a porous ceramics substrate by chemical vapor deposition. J. Membr. Sci. 1996, 120, 261.
(6) Uemiya, S.; Kajiwara, M.; Kojima, T. Composite membranes of group VIII metal supported on porous alumina. AIChE J. 1997, 43, 2715. (7) Zhao, H.; Li, A.; Gu, J.; Xiong, G. A novel preparation method for porous noble metal/ceramic catalytic membranes. J. Mater. Sci. 1999, 34, 1. (8) Yeung, K. L.; Varma, A. Novel preparation techniques for thin metal-ceramic composite membrane. AIChE J. 1995, 41, 9. (9) Roa, F.; Way, J. D.; Mccormick, R. L.; Paglieri, S. N. Preparation and characterization of Pd-Cu composite membranes for hydrogen separation. Chem. Eng. J. 2003, 93, 11. (10) Hoang, H. T.; Tong, H. D.; Gielens, F. C.; Jansen, H. V.; Elwenspoek, M. C. Fabrication and characterization of dual sputtered Pd-Cu alloy films for hydrogen separation membranes. Mater. Lett. 2003, 4697, 525. (11) Shu, J.; Grandjean, B. P. A.; Ghali, E.; Kaliaguine, S. Simultaneous deposition of Pd and Ag on porous stainless steel by electroless plating. J. Membr. Sci. 1993, 77, 181. (12) Mardilovich, P. P.; She, Y.; Ma, Y. H.; Rei, M. H. Defectfree palladium membranes on porous stainless-steel support. AIChE J. 1998, 44, 310. (13) Jemaa, N.; Shu, J.; Kaliaguine, S.; Grandjean, B. P. A. Thin palladium film formation on shot peening modified porous stainless steel substrates. Ind. Eng. Chem. Res. 1996, 35, 973. (14) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Methane steam reforming in asymmetric Pd and Pd-Ag/porous SS membrane reactors. Appl. Catal. A 1994, 119, 305. (15) Renate, M. D. V.; Verweiji, H. High selectivity, high flux silica membranes for gas separation. Science 1998, 279, 1710. (16) Nam, S. E.; Lee, K. H. Hydrogen separation by Pd alloy composite membranes: introduction of diffusion barrier. J. Membr. Sci. 2001, 192, 177. (17) Trocha, M.; Koros, W. J. A diffusion-controlled procedure to close pores in ceramic membrane. J. Membr. Sci. 1994, 95, 259. (18) Moaddeb, M.; Koros, W. J. Silica-treated ceramic substrates for formation of polymer-ceramic composite membrane. Ind. Eng. Chem. Res. 1995, 34, 263. (19) Kuraoka, K.; Chujo, Y.; Yazawa, T. Hydrocarbon separation via porous glass membranes surface-modified using organosilane compounds. J. Membr. Sci. 2001, 182, 139. (20) Roa, F.; Way, J. D. Influence of Alloy Composition and Membrane Fabrication on the Pressure Dependence of the Hydrogen Flux of Palladium-Copper Membranes. Ind. Eng. Chem. Res. 2003, 42, 5827. (21) Paglieri, S. N.; Way, J. D. Innovations in palladium membrane research. Sep. Purif. Methods 2002, 31, 1. (22) Hwang, S. T.; Kammermeyer, K. Membrane in Separations; John Wiley & Sons: New York, 1975; p 31. (23) Ward, T. L.; Dao, T. Model of hydrogen permeation behavior in palladium membranes. J. Membr. Sci. 1999, 153, 211. (24) Dittmeyer, R.; Ho¨llein, V.; Daub, K. Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium. J. Mol. Catal. A: Chem. 2001, 173, 135. (25) Weyten, H.; Luyten, J.; Keizer, K.; Willems, L.; Leysen, R. Membrane performance: the key issues for dehydrogenation reactions in a catalytic membrane reactor. Catal. Today 2000, 56, 3. (26) Ho¨llein, V.; Thornton, M.; Quicker, P.; Dittmeyer, R. Preparation and characterization of palladium composite membranes for hydrogen removal in hydrocarbon dehyogenation membrane reactor. Catal. Today 2001, 67, 33. (27) Li, A.; Liang, W.; Hughes, R. The effect of carbon monoxide and steam on the hydrogen permeability of a Pd/stainless steel membrane. J. Membr. Sci. 2000, 165, 135.
Received for review July 23, 2004 Revised manuscript received February 2, 2005 Accepted February 9, 2005 IE049349B
