Preparation of palladium composite membranes by modified electroless plating procedure

Preparation of palladium composite membranes by modified electroless plating procedure

Journal of Membrane Science 142 (1998) 147±157 Preparation of palladium composite membranes by modi®ed electroless plating procedure H.-B. Zhaoa, K. ...

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Journal of Membrane Science 142 (1998) 147±157

Preparation of palladium composite membranes by modi®ed electroless plating procedure H.-B. Zhaoa, K. P¯anzb, J.-H. Gua, A.-W. Lia, N. Strohb, H. Brunnerb, G.-X. Xionga,* a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b Fraunhofer Institute of Interfacial Engineering and Biotechnology, Nobelstraûe 12, D-70569 Stuttgart, Germany Received 11 June 1997; received in revised form 16 September 1997; accepted 16 September 1997

Abstract A thin palladium composite membrane was produced by modi®ed electroless plating procedure. Compared with the conventional electroless plating procedure, the modi®ed electroless plating procedure consists of the activation of a ceramic substrate by the sol±gel process of a Pd(II)-modi®ed boehmite sol. Additionally, the in®ltration of an electroless plating solution to a porous substrate during the deposition of palladium was employed with the ®lter device to improve adherence of a palladium layer to a substrate. The resulting membrane with a thickness of about 1 mm has a high compactness. The membrane shows a hydrogen selectivity of 20±130 for H2/N2, and a hydrogen ¯ux of 1.8±87 m3/m2h, depending on operation conditions. # 1998 Elsevier Science B.V. Keywords: Pd membrane; Electroless plating; Sol±gel process; Hydrogen separation

1. Introduction It is well known that hydrogen selectively permeates through palladium membrane by the solution± diffusion mechanism [1]. Hydrogen embrittlement can be overcome by alloying of palladium with Group IB metals such as silver [2]. A commercial hydrogen puri®cation equipment utilizing tubes of palladium± silver (23 wt.%) alloy was developed by Johnson Matthey in the early 1960's [3]. Palladium-based membranes were used to extract tritium from liquid-metal tritium breeders [4]. Recently, palladium-based membranes were exploited as hydrogen

*Corresponding author. 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00287-1

separator in hydrogenation/dehydrogenation membrane reactors [5]. Recently many works [6,7] have been devoted to the fabrication of thin palladium-based membranes for increasing the permeation rate of hydrogen. This requirement was usually met by forming a thin palladium layer on a mechanically stable substrate. Gryaznov et al. [6] and Uemiya et al. [7] have made a big step towards thin palladium-based membranes. Using plasma sputtering, Gryaznov et al. [6] produced a 2 mm thick Pd±Ru±In alloy/SS membrane. The membrane showed 100% H2-selectivity, and could operate well to separate hydrogen from reforming gases on a bench scale over a period of one month. This threecomponent alloy membrane was resistant to temperature cycling in hydrogen atmosphere for 450 times.

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But XPS analysis showed that the concentration of indium in the surface of the membrane increased. Flame spraying technique was investigated to prepare Pd±Ag alloy/Al2O3 membranes with 1.5±2 mm thickness [8]. The membrane had a H2-selectivity of 24 at 5008C. This implied that there were some defects in the membranes. The more interesting result was reported by Uemiya et al. Using electroless plating procedure, a Pd±Ag alloy/Al2O3 membrane with 5 mm thickness was produced [9]. It was claimed that the membrane still exhibited 100% selectivity for hydrogen, and its hydrogen ¯ux was ten-times larger than a commercial 150 mm thick Pd±Ag alloy membrane. Collins et al. [10] deposited 1±5 mm thick palladium membranes on a g-Al2O3 substrate. The membranes could withstand a temperature cycle from room temperature to 6008C and then back without delamination. However, no evidence was given that the permeation through these membranes was still controlled by the deposited palladium layer. But later the same group [11] reported that the 20 mm thick palladium layer delaminated from the g-Al2O3 substrate (10 nm pore size) after hydrogen permeation experiments. In this paper, a thin palladium membrane was produced by modi®ed electroless plating procedure. The membrane was characterized by scanning electron microscopy and hydrogen permeation experiments. 2. Experimental

100 ml and about 10ÿ4 mol/l Pd(II). The pH values of the samples were adjusted to the pH range of 1±10 with diluted HNO3 and NaOH, and the pH difference between two neighboring samples remained a pH unit of 0.5 or so. The samples were shaken at room temperature until the chemical equilibrium of the adsorption of Pd(II) on the g-Al2O3 particles was reached. The pH values of the samples were measured. The solutions were separated by centrifugation, and the concentration of Pd(II) left was analyzed by spectrophotometric method. The amount of the Pd(II) adsorbed was calculated by subtraction of the concentration determined from the total concentration of Pd(II) added. Adsorption percentage is de®ned as the ratio of the amount of Pd(II) adsorbed to the total Pd(II) added. Finally the adsorption percentages of Pd(II) on the g-Al2O3 particles were plotted against the pH values of the solutions. 2.2. Membrane preparation The synthesis of the palladium composite membrane mainly consisted of the activation of porous alumina substrate and the deposition of palladium on the activated substrate, and is shown in Fig. 1. The activation of porous alumina substrate: The activation of porous alumina substrate was performed by the sol±gel process of a Pd(II)-modi®ed boehmite sol. Boehmite sol was produced by peptization of the suspension of a boehmite powder at 808C using nitric acid as a peptization agent (H‡/Alˆ0.03±0.09 in mol), followed by 5 h aging at the same temperature. Then a

2.1. Determination of the amount of adsorption of metal ions as a function of pH 2ÿ The adsorption of Pd…NH3 †2‡ 4 , PdEDTA , and on g-Al O particles as a function of pH PdCl2ÿ 2 3 4 was determined in the following way. g-Al2O3 powder was prepared by calcining PURAL SB powder (Condea Chemice GmbH) at 6008C for 5 h. The g-Al2O3 particles of 0.3000 g were weighed and put into conical ¯asks. Thus, ®fteen samples were obtained. The given volume of distilled water was added into the above samples, respectively. These samples were aged in order to hydrate the surface of the g-Al2O3 particles. The standard solution containing Pd(II) was added, resulting in each of the samples having a volume of

Fig. 1. The synthesis route to the palladium composite membrane.

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suitable Pd(II) complex was added to the boehmite sol, and adsorption of the Pd(II) complex on the boehmite sol particles took place, resulting in a Pd(II)-modi®ed boehmite sol. A commercial ¯at-shaped micro®ltration membrane (Kerafol GmbH) was used as a substrate for the coating of the Pd(II)-modi®ed boehmite sol. The substrate had a 1.6 mm average pore size and 48% porosity. The casting sol was composed of 0.5± 1.0 mol/l AlOOH, 2±4 wt.% Pd(II) (Pd/g±Al2O3), 2± 5 wt.% poly(vinyl alcohol) (PVA) plus polyethylenglycol (PEG) (Merck-Schuchardt) with PVA/PEGˆ 2/1. According to the product instruction of the PVA and PEG, their molecular weights were 72000 and 400, respectively. Casting of the sol was carried out by spin-coating. The gel-coatings were dried at 58C and 65% relative humidity for two days, followed by calcination at 6008C for three hours with a heating and cooling rate of less than 0.58C/min. The sol±gel derived substrate was treated with hydrogen at 5008C to reduce palladium. The deposition of palladium on the activated substrate: A plastic ®lter device was employed to perform the electroless plating of palladium. The activated substrate was connected to the ®lter by rubber Orings, and the Pd/g-Al2O3 layer of the activated substrate faced up to the electroless plating solution. Using the device, the in®ltration of the electroless plating solution to the substrate during the deposition of palladium was enhanced by vacuum-®ltration. The composition and state of the standard electroless plating bath is listed in Table 1. The palladium composite membranes obtained were washed with deionized water and acetone in sequence, then dried at ambient temperature.

Table 1 The composition and state of the standard electroless plating bath Components

Content

Pd(OOCCH3)2 (NH4)2EDTA Aqueous ammonia (25 wt.%) N2H4 H2O

3.37 g/l 35.2 g/l 182 g/l 1.03 g/l

pH Deposition temperature

10.5 Ambient temperature

149

2.3. Characterization techniques The particle sizes of the sols were measured by photo-correlation spectroscopy (Malvern Zetasizer 3, Malvern Instruments). The as-prepared sols were diluted with distilled water to obtain the samples with identical spheres. The morphologies of the activated substrates and the as-deposited palladium membranes were observed with a scanning electron microscopy (SEM) (Stereoscan 120, Cambridge Instruments). The distribution of palladium in the activated substrate was analyzed by a scanning electron microscopy-wave dispersive X-ray analyzer (SEM-WDX) (WDX Mapping S200, Cambridge Instruments). The palladium particle sizes of the Pd/g-Al2O3 materials were determined by transmission electron microscopy (TEM) (JEM-100C, JEOL). The Pd/gAl2O3 material was ground, and then was dispersed in ethanol. One drop of the suspension was transferred onto a carbon coated copper grid. The pure gas permeation experiments were carried out in a home-made set-up by the variable-volume method. The set-up consisted of a feed ¯ow system, a permeation cell, and data acquisition equipment. The side edge of the membrane was sealed by commercially available ceramic glass (UHLIG Kera-Dekor), which can resist to elevated temperatures up to 8008C. The membrane was connected with the stainless steel body of a home-made permeation cell using graphite gaskets. The temperature of feed side near the membrane was determined with a NiCr±Ni thermocouple. The downstream ¯ow rate was measured with a soapbubble ¯owmeter. Before measuring hydrogen permeation, the palladium membranes were activated in hydrogen atmosphere. 3. Results and discussion 3.1. The Pd(II) complexes for modification of boehmite sol Boehmite sol maintains its dynamic stability in a pH range of 3.5±4. Hence, adsorption of Pd(II) salt on the boehmite sol particles can only be performed in the above pH range, but the adsorption can still be optimized by proper choice of the ligands. Therefore, the

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Fig. 3. Particle size distribution of Pd(II)-modified boehmite sol (2 wt.% Pd/g-Al2O3). Fig. 2. Adsorption percentage of Pd(II) on g-Al2O3 particles as a function of pH in presence of different ligands (Clÿ: chloride, NH3: ammonium, EDTA: ethylene diamine tetra-acetic acid).

adsorption of Pd(II) on the g-Al2O3 particles as a function of pH was determined, and the effect of the ligands including Clÿ, NH3 and EDTA on the adsorption was addressed. The adsorption of 2ÿ 2ÿ on the gPd…NH3 †2‡ 4 , PdEDTA , and PdCl4 Al2O3 particles as a function of pH was shown in Fig. 2. From Fig. 2, the adsorption percentages of and PdEDTA2ÿ over the pH range of Pd…NH3 †2‡ 4 the stable boehmite sol were 85 and 95%, respectively. Hence, the two Pd(II) complexes can be used to modify the surface of the boehmite sol by surface complexation at the liquid/solid interface. It was shown that the ligands can become one of the effective means by which adsorption of the metal salt will be tailored to a speci®c application. A lyogel layer on g-Al2O3 particles can be formed when the surface of g-Al2O3 particles is hydrated, and the properties of the hydroxy groups on the hydrated g-Al2O3 particles are comparable to those of the boehmite sol particles [12]. Therefore, the interaction between the Pd(II) complexes and the boehmite sol particles could be similar to that between the Pd(II) complexes and the g-Al2O3 parti2ÿ can cles. As a consequence, Pd…NH3 †2‡ 4 or PdEDTA directly be used to prepare a Pd(II)-modi®ed boehmite sol.

3.2. The activation of ceramic substrates for postdeposition of palladium In this work, Pd…NH3 †2‡ 4 was used to produce the Pd(II)-modi®ed boehmite sol. The Pd(II)-modi®ed boehmite sols with up to 4 wt.% palladium (in Pd/ g-Al2O3) displayed a dynamic stability in a few weeks. Fig. 3 shows that the Pd(II)-modi®ed boehmite sol had a narrow particle size distribution and an average diameter of 95 nm. According to the packing model of particulates, the pore size and pore size distribution of a membrane is determined by the particle size and particle size distribution of its precursor sol [13]. It can be seen that the synthesis of a stable sol with narrow particle size distribution is a primary step to the Pd/gAl2O3 membranes prepared by the sol±gel method. The Pd/g-Al2O3 substrates were produced by a multiple sol±gel process. The SEM photograph of the cross-section of the Pd/g-Al2O3 substrate (4 wt.% Pd/g-Al2O3) prepared by three-time dipping is shown in Fig. 4. From Fig. 4, a uniform Pd/g-Al2O3 layer with a thickness of 9 mm was formed on the micro®ltration membrane. It was not found that there were any pinholes and microcracks in the activated layer. The activated substrate displayed a smooth surface. Fig. 5 shows the distribution of palladium in the Pd/ g-Al2O3 substrate obtained by three-time dipping. The right side is a SEM micrograph of the activated substrate, and the left side is the WDX result of the

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Fig. 4. SEM micrograph of the cross-section of Pd/g-Al2O3 substrate produced by three-time dipping.

Fig. 5. SEM-WDX micrograph of the Pd/g-Al2O3 substrate produced by three-time dipping.

corresponding substrate sampled in the picture. The density of the white points qualitatively represents the concentration of palladium in the activated substrate. As shown in Fig. 5, the palladium was uniformly dispersed in the toplayer of the activated substrate along the surface of the activated substrate. The TEM micrograph of the Pd/g-Al2O3 material (4 wt.% Pd/g-Al2O3) is given in Fig. 6. From the TEM

micrograph, the palladium particle size was about 10± 20 nm. The quality of an activated substrate includes the surface roughness, the amount of palladium particles and the distribution of palladium which have a strong effect on the quality of a palladium composite membrane. The defects such as microcracks and nonuniform distribution of metallic palladium particles in the

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Fig. 6. TEM micrograph of Pd/g-Al2O3 material (4 wt.% Pd/g-Al2O3).

activated substrate were transferred to the palladium deposit [11]. Based on the above results, the activated substrate derived by the sol±gel process had a smooth surface and a uniform distribution of palladium particles with high-dispersion. Therefore, the sol±gel process of a Pd(II)-modi®ed boehmite sol can be one of the activation procedures for electroless plating. 3.3. The deposition of palladium and the structure of palladium membranes The effect of the deposition rate of palladium on the compactness of palladium deposits was studied. The deposition rate of palladium was adjusted by the concentration of hydrazine in the electroless plating bath. Three kinds of electroless plating baths with a hydrazine concentrations of 0.51, 1.03 and 2.06 g/l were employed, respectively. Thus three types of the palladium membranes were produced with a deposition time of 1±2 h using vacuum-®ltration. The SEM micrographs of the membranes are shown in Fig. 7(a), (b) and (c). From Fig. 7, the palladium particle size decreased as the concentration of hydrazine increased. The membrane obtained with the electroless plating bath of 0.51 g/l hydrazine had a helium permeation rate of 0.54 m3/m2h atm. The other two types of the membranes were gas-tight for helium. When the

morphologies of the membranes were correlated with their gas-tightness, the palladium membranes composed of ®ner palladium particles produced at higher deposition rate of palladium were more compact. Initially, the in®ltration of the electroless plating solution to a porous substrate was employed to improve adherence of a palladium layer to a porous substrate. The in®ltration was enhanced by vacuum®ltration. Furthermore, the effect of vacuum-®ltration on the structure of palladium deposits was investigated. Using the electroless plating bath with a hydrazine concentration of 2.06 g/l, two kinds of palladium membranes were prepared with 1 h deposition time, one with vacuum-®ltration, the other without vacuum®ltration. The SEM micrographs of the membranes are shown in Fig. 8(a) and (b). From Fig. 8, no texture difference between the two types of the palladium deposits were seen by SEM. However, the helium permeation measurements indicated that the membrane prepared by vacuum-®ltration was gas-tight, and the membrane prepared without vacuum-®ltration had a helium permeation rate of 2.910ÿ2 m3/ m2hatm. Therefore, the palladium deposit could be made even more compact by vacuum-®ltration during the deposition of palladium. Electroless plating is de®ned as deposition of a metallic coating by a controlled chemical reduction which is catalyzed by the metal or alloy being depos-

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Fig. 7. SEM micrographs of the top-surface of palladium membranes prepared with different concentration of hydrazine: (a) 0.5 ml/l, (b) 1.0 ml/l, (c) 2.0 ml/l.

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Fig. 8. SEM micrographs of the cross-section of palladium membranes: (a) without vacuum-filtration, (b) with vacuum-filtration.

ited [14]. However, not all substrates are spontaneously plated in electroless plating solutions and some steps have to be taken to initiate the deposition. These steps are called sensitization and nucleation. In general, sensitization means the adsorption of a reducing agent, often a stannous compound, on the surface of the substrate. Nucleation is the preplating step in which a catalytic material, often a Pd compound, is adsorbed on the surface of the substrate. Using glass [7], alumina [9] or stainless steel [15] micro®ltration membranes as substrates for palladium-based deposits, the sensitization and nucleation were achieved by impregnation of Sn(II) solution and Pd(II) solution, respectively, and the procedure was usually repeated ten times to produce enough palladium particles for

post-deposition of palladium. The typical palladium particle size obtained on stainless steel was about 50 nm, and a 3 h deposition resulted in a relatively uniform palladium layer in which there were still some holes uncovered [15]. In this work, the activation of substrate was realized by the sol±gel process. Compared with the impregnation-activated substrate, the sol±gel-activated substrate had a more smooth surface and a more uniform distribution of palladium particles with high dispersion. As a result, the dense palladium membrane on the sol±gel-activated substrate was produced using a deposition time of 1±2 h. In addition, compared with the electroless plating baths used by Uemiya et al. [16] and Shu et al. [15], the electroless plating bath used in this work had a higher concentra-

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tion of hydrazine. Thus, ®ner palladium particles were produced at higher deposition rate of palladium, resulting in a more compact palladium membrane. 3.4. The hydrogen permeation of palladium composite membranes The palladium membranes prepared with vacuum®ltration peeled off when annealed in an inert gas atmosphere. This was because some stress was developed in the membranes during the deposition of palladium, and then a de-adherence was produced during the relaxation of the stresses by annealing. Therefore, the palladium membranes were prepared without vacuum-®ltration for hydrogen permeation experiments. The effects of transmembrane pressure and temperature on hydrogen transport through the palladium membranes prepared were determined, and the membranes used were also examined by nitrogen permeation measurements. The nitrogen permeation measurements of the membranes used are shown in Fig. 11. From Fig. 11, the used membranes became less and less gas-tight as the temperature increased. Accordingly, the hydrogen ¯ow consisted of two parts, one from the microholes of the membrane, the other from the dense part. The net hydrogen ¯ux through the dense part was calculated by subtracting the hydrogen ¯ux of the microholes from the total hydrogen ¯ux. According to Knudsen diffusion theory, the hydrogen ¯ux of the microholes was estimated using the corresponding nitrogen ¯ux. In addition, the hydrogen ¯uxes of the palladium membranes were comparable to those of the Pd/g-Al2O3 substrates (184.5± 164.5 m3/m2hatm in the temperature range from 300±4508C). Therefore, the mass transfer resistance of the substrate was corrected. In consideration of the above two points, Fig. 9 was obtained. Hydrogen permeates through a palladium ®lm via a solution±diffusion transport mechanism, the following transport equation of hydrogen is applied [2]. F ˆ DS…Pnh ÿ Pnl †=L where D and S are the hydrogen diffusion coef®cient and its solubility constant, respectively, Ph and Pl are hydrogen pressure on the feed side and the permeation side, respectively, n is the power of pressure, and L is the membrane thickness.

Fig. 9. Hydrogen flux as a function of the square root of pressure difference at different temperatures.

There have been many variations in the exponential dependency of the hydrogen permeation rate on pressure ranging from 0.5±1.0 [11,17±20]. The proposed mechanisms generally assumed a rate-determining step, which may be either surface processes involving adsorption and dissociation of molecular hydrogen or a bulk process involving transfer of atomic hydrogen through the palladium lattice. For a thick palladium ®lm, the bulk diffusion usually becomes the ratedetermining step for hydrogen permeation, thus n is equal to 0.5. However, for a thin palladium membrane, it was suggested that hydrogen transport could be controlled by a surface process of hydrogen. In this case, n approached one. In this work as seen in Fig. 9, the hydrogen ¯ux was correlated to the difference of the square root of pressure over the membrane. It was found that the hydrogen ¯ux as a function of transmembrane pressure over the temperature range investigated was not described by the law of the half power of pressure (nˆ0.5). According to the above mentioned, the surface processes of hydrogen on the palladium membranes used in this work were assumed to be mainly responsible for the rate-determining steps for hydrogen permeation. Hydrogen selectivity is de®ned as the ratio of hydrogen ¯ux to nitrogen ¯ux at the same conditions. The separation factors of H2/N2 are shown in Fig. 10.

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Fig. 10. The separation factor of hydrogen to nitrogen as a function of pressure difference at different temperatures.

Fig. 11. The nitrogen gas-tightness of the used membranes in hydrogen atmosphere as a function of temperature.

As shown in Fig. 10, the hydrogen selectivity increased with increasing the transmembrane pressure drop, but the increase in hydrogen selectivity became less and less as the temperature increased. It was assumed that the surface processes of hydrogen became the rate-determining step for hydrogen permeation. The surface coverage of hydrogen increased as the pressure increased, thus the hydrogen permeation rate increased, resulting in the improvements of the hydrogen selectivities. From Fig. 11, the membrane became less and less gas-tight as temperature increased, and thus Knudsen diffusion contributed

more and more to the hydrogen transport. Therefore, the increase in hydrogen selectivity with increasing pressure at elevated temperature became less and less. From Fig. 10, the palladium membrane had at least a hydrogen selectivity of 23 and the corresponding hydrogen ¯ux of 87 m3/m2h under the conditions of Phˆ2 atm, Plˆ1 atm and Tˆ4508C. In Table 2, other palladium-based composite membranes and their performances of hydrogen separation are summarized. Compared with other palladium-based composite membranes, the palladium membrane prepared by the modi®ed electroless plating had a high hydrogen

Table 2 Comparsion of the performances of palladium-based composite membranes Membrane

Thickness (mm)

Temperature (8C)

Permeation rate a (m3/m2 h atmn)

References

Pd±Ru/SS Pd±Ru-In/SS Pd±Ru/MgO Pd±Ag/glass Pd±Ag/alumina Pd±Ag/alumina Pd/alunina Pd/alumina Pd±Ag/alumina Pd/alumina Pd/alumina

10 2 10 5±8 0.5 2 8 4.5 5.8 11.4 1

800 400 700 366 250 500 500 400 400 550 450

33.5 (/) 13.7 (/) 46.6 (/) 1.2 (/) 1.2 (5.7) 4.8 (24) 41 (1000) 36.9 (/) 53.4 (/) 22.8 (380) 87 (23)

Gryaznov et al. [6] Gryaznov et al. [6] Gryaznov et al. [6] Gobina et al. [21] Jayaraman et al. [20] Li et al. [8] Yan et al. [19] Uemiya et al. [9] Uemiya et al. [9] Collins et al. [10,11] This work

a

The number in brackets is hydrogen selectivity, and the values of n can be obtained in their references.

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permeation rates, and meantime exhibited a good hydrogen selectivity. 4. Conclusions The modi®ed electroless plating was proposed to produce the palladium composite membranes. The best palladium membrane in this work was obtained using the activated substrate realized by the sol±gel process and the electroless plating bath with a higher concentration of hydrazine. The resulting membrane with a thickness of 1 mm was gas-tight for helium. The hydrogen permeation experiments showed that the resulting membrane can operate at elevated temperatures up to 4508C. The membrane had a hydrogen selectivity of 20±130 for H2/N2, and a hydrogen ¯ux of 1.8±87 m3/m2h, depending on the operation conditions.

[5] [6] [7] [8]

[9] [10] [11] [12]

Acknowledgements The authors are grateful to Miss M. Riedl for taking SEM photographs, Mr. S. Tudyka for helping with WDX analysis and Mr. S.-S. Sheng for building the gas permeation set-up.The project was supported by the National Sciences Foundation of China (grant number 29392003) and Chinese Academy of Sciences (grant number KMX85-23), German BMBF and FhGIGB.

[13] [14] [15] [16]

[17]

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