Performance of alumina, zeolite, palladium, Pd–Ag alloy membranes for hydrogen separation from Towngas mixture

Journal of Membrane Science 204 (2002) 329–340

Performance of alumina, zeolite, palladium, Pd–Ag alloy membranes for hydrogen separation from Towngas mixture Y.S. Cheng a , M.A. Peña b , J.L. Fierro b , D.C.W. Hui a , K.L. Yeung a,∗ a

Department of Chemical Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR-PR China b Instituto de Catálisis y Petroleoqu´ımica, CSIC, Campus UAM, Cantoblanco, Madrid 28047, Spain Received 4 September 2001; received in revised form 30 January 2002; accepted 5 February 2002

Abstract The hydrogen permeation and separation properties of alumina, zeolite (ZSM-5), palladium and Pd–Ag alloy membranes were measured and compared. Hydrogen separation from a commercial Towngas mixture (49% H2 , 28.5% CH4 , 19.5% CO2 and 3% CO) was conducted. The commercial alumina membrane displayed excellent hydrogen permeation rate but was unable to separate hydrogen from the Towngas. Supported ZSM-5 membrane prepared by ex situ method was able to produce a product stream containing 60% H2 from Towngas, whereas, high purity hydrogen was generated using thin palladium and Pd–Ag alloy membranes prepared by electroless plating technique. Some of the constituent gases in Towngas mixture inhibit hydrogen flux through the palladium membrane, but the addition of silver serves to ameliorate the situation. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrogen separation; Pd–Ag alloy membrane; Zeolite membrane; Towngas mixture

1. Introduction There is an increasing demand for hydrogen in petroleum refining, petrochemical production and semiconductor processing [1], as well as in new energy-related applications such as clean fuel for vehicles and fuel cells [2]. This motivates research into new methods for hydrogen generation, separation and purification [3–6]. Membrane related processes are considered to be one of the most promising technologies for production of high purity hydrogen needed for semiconductor and fuel cell applications. This is particularly true for palladium-based membranes that are capable of separating ultra-high purity hydrogen ∗ Corresponding author. Tel.: +852-2358-7123; fax: +852-2358-0054. E-mail address: [email protected] (K.L. Yeung).

from mixtures [7–10]. When employed in membrane reactors, these membranes can produce pure hydrogen from partial oxidation of methane [11], steam reforming [12–14] and dehydrogenation reactions [15–18]. Supra-equilibrium conversion and higher product yield are the added benefits from the continuous and selective removal of hydrogen from these equilibrium-limited reactions [19–21]. Most commercial Pd and Pd-alloy membranes are made of free-standing foils and tubes [8–10]. However, it is necessary to prepare thinner membranes in order to lower the cost and to achieve a higher hydrogen permeation rate. Supporting the membrane layer on a porous substrate can significantly increase its mechanical strength and thermal stability, enabling the preparation of ultra-thin Pd and Pd-alloy membranes. Porous refractory ceramic, glass and stainless steel were successfully used as support for

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 0 5 9 - 5

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palladium membranes. Besides its excellent mechanical strength and high thermal conductivity, stainless steel support has the advantage of the industry’s familiarity in its fabrication, maintenance and repair. However, ceramic supports such as refractory alumina are more resistant to corrosion and can withstand significantly higher operating temperatures. There are a numerous ways of depositing thin layer of Pd and Pd-alloy films onto the porous support [19]. Electroless plating is a popular technique for preparing thin Pd [22–27] and Pd-alloy [28–30] supported membranes that have excellent hydrogen permselectivity. The addition of silver improves the membrane resistance to embrittlement and extent the membrane operation to lower temperatures [19]. Silver also improves the membrane permeance for hydrogen [28] and being cheaper than palladium, its use for alloying significantly lowers the cost of the membrane element. Besides dense palladium-based membranes, microporous membranes made of iridium, platinum, rhodium and ruthenium have been tested by Uemiya and co-workers [31,32] for hydrogen permeation. In this work, palladium and palladium–silver (Pd–Ag) alloy membranes were prepared using electroless plating. A hydrazine-based plating bath is employed for Pd deposition and using the established plating kinetics [26,30], membranes of desired thickness and morphology were obtained. Pd–Ag alloy membranes were prepared by simultaneous depositions of palladium and silver [30]. The performance of the thin supported Pd and Pd–Ag membranes were tested for hydrogen permeation and separation, and compared with a composite zeolite membrane and a commercial alumina membrane. For hydrogen separation, a commercial ‘Towngas’ mixture containing 49% H2 , 28.5% CH4 , 19.5% CO2 and 3% CO was used. This mixture is obtained by naphtha cracking [33] and an important heating fuel in Hong Kong, and a possible cheap local source of hydrogen for fuel cell. 2. Experimental 2.1. Membrane preparation 2.1.1. Alumina membrane The microstructure of the commercial alumina membrane purchased from US Filter is shown in

Fig. 1a. The membrane consists of a coarse ␣-Al2 O3 tube coated with finer layers of ␣-Al2 O3 (two layers, 10 ␮m each) and ␥-Al2 O3 (one layer, 0.7 ␮m). The topmost mesoporous ␥-Al2 O3 layer has a 5 nm pore size (Fig. 1a, inset). The graded structure of the membrane ensured low flow resistance and, thus, high permeation flux. For comparison purposes, this membrane was tested for hydrogen permeation and separation. 2.1.2. Zeolite membrane A second inorganic membrane was prepared for comparison with the Pd and Pd–Ag alloy membranes. The zeolite (ZSM-5) membrane (Fig. 1b) was grown onto the topmost ␥-Al2 O3 layer of the commercial alumina membrane using ex situ synthesis method [34]. The support was first seeded with a thin layer of colloidal zeolite seeds followed by hydrothermal treatment at 403 K in a synthesis solution containing 40:10:20,000 tetraethyl orthosilicate (TEOS):tetrapropylammonium hydroxide (TPAOH):H2 O, respectively. After synthesis, the zeolite was carefully rinsed with deionized distilled (DI) water and dried overnight in an oven at 333 K. Prior to the removal of the organic templates, the membrane was subjected to helium leak test at 3 barg. The templates were removed by air calcination at 823 K. Hydrogen permeation and separation were conducted on the calcined zeolite membrane. 2.1.3. Pd and Pd–Ag alloy membranes The commercial alumina membrane was modified for use as support for the Pd and Pd–Ag alloy membranes. The alumina membrane was calcined in air at 1373 K for 60 h to convert the topmost ␥-Al2 O3 layer into the more stable ␣-Al2 O3 . This step was necessary in order to create a more suitable support for the Pd-based membranes. Fig. 2a shows that significant sintering and surface roughening occurred as the alumina undergoes phase transformation. The average size of the alumina particles is about 0.2 ␮m. The support was seeded with palladium by immersing the support in 5 mM Sn(II) solution for 5 min, followed by rinsing in DI water. The tube was then placed in 5 mM Pd(II) solution for 5 min before it was again rinsed in DI water. This procedure was repeated 10 times to obtain a uniformly seeded support.

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Fig. 1. Cross-sectional SEM images of: (a) commercial alumina; (b) ZSM-5/porous alumina membranes (inset figures are the top view of membrane layer at the same magnification); (c) X-ray diffraction patterns of ␥-alumina (bottom) and ZSM-5 membranes (top).

Palladium and Pd–Ag alloy membranes were prepared from hydrazine-based baths. The starting composition of the plating solutions is summarized in Table 1. The metal precursors, Pd and Ag were dissolved in concentrated ammonium hydroxide solution and stabilized with sodium EDTA [30]. The plating was conducted in a constant temperature bath kept at 323 and 328 K for Pd and Pd–Ag membranes, respectively. The activated support was immersed in the plating solution and plating commences after the addition of the hydrazine reducer. The support tube was wrapped with Teflon tape to prevent deposition on the outer surface. The plating bath composition, temperature and time were adjusted using the plat-

ing kinetics derived in the previous works [26,30] to deposit membranes with the desired thickness and composition. 2.2. Membrane characterization The structure and composition of the membranes were analyzed by X-ray diffraction (XRD) Philips 1080, scanning electron microscopy (SEM) JEOL 6300 and X-ray photoelectron spectroscopy (XPS) Physical Electronics PHI 5600. The structure, crystallinity and orientation (i.e. texture) of the membrane were analyzed using X-ray diffraction. Also, the bulk composition of the miscible Pd–Ag alloy

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composition was also analyzed by energy dispersive X-ray spectroscopy (EDXS), Oxford Inca 200. Post-mortem analysis of the membrane microstructure was conducted after the permeation and separation experiments had been carried out. The membranes were sectioned using a diamond wafering saw and the pieces of samples were rinsed with DI water to remove dirt and contaminants. The samples were dried and mounted onto aluminum specimen stubs using conducting silver paste. Sample charging was reduced by sputter coating with Au. To facilitate the measurements of film thickness and grain size, the SEM pictures were scanned into computer and analyzed using a standard imaging software NIH Image 1.55 (National Institute of Health). The surface composition of the membrane was determined by X-ray photoelectron spectroscopy. 2.3. Membrane permeation and separation experiments

Fig. 2. SEM images of: (a) thermally treated porous alumina support; (b) palladium layer deposited by electroless plating onto the porous support; (c) palladium membrane after high temperature (923 K) annealing in hydrogen.

could be determined from the X-ray diffraction pattern. Yamamura et al. [15] have shown that there is a direct relationship between the lattice constant and the silver content in the Pd–Ag alloy. The membrane

A stainless steel shell-and-tube membrane module was used for membrane permeation and separation experiments. The membrane was sealed using either Viton or graphite O-rings. For low temperature measurements, Viton O-ring gave a leak-free seal even at 3 barg trans-membrane pressure difference. Graphite O-ring is needed for high temperature operation and the helium leak rate through the graphite seal is usually between 0.08 and 0.10 cm3 min−1 bar−1 . The test module can be uniformly heated at the rate of 1 K min−1 using a cloth heater (Thermodyne) controlled by a temperature programming unit (Omega, CN2000). The operating temperature was measured using a K-type thermocouple inserted in the membrane unit. A gas delivery system metered hydrogen, helium and ‘Towngas’ mixture to the membrane module. The membrane pressure was adjusted using back-pressure regulators and metering valves and monitored with a pressure gauge (Ashcroft). The pressure of the permeate-side was maintained at ambient conditions and no sweep gas was used. The permeate flux was measured using a bubble flowmeter maintained at ambient conditions (i.e. 1 bar, 298 K), and analyzed using an on-line gas chromatograph (HP 6890) equipped with thermal conductivity and flame ionization detectors in-series and a CTR I column. The flammable exhausts from membrane

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Table 1 Composition of hydrazine-based Pd and Pd–Ag plating bath Plating bath composition

1 l Pd plating bath

1 l Pd–Ag plating bath

Palladium precursor Silver precursor

3.6 g PdCl2 –

2.6 g (NH3 )4 Pd(NO3 )2 0.2 g AgNO3

Complexing agent

76.0 g Na2 EDTA 650 ml NH4 OH (16 M)

60.0 g Na2 EDTA 200 ml NH4 OH (16 M)

Reducing agent

10 ml N2 H4 (1 M)

2.5–40 ml N2 H4 (1 M)

Plating temperature

323 K

328 K

module were mixed and burned using a Bunsen burner. 2.3.1. Single gas permeation experiments The permeance of hydrogen and helium across the commercial alumina and ZSM-5 membranes were measured at 323 K for different trans-membrane pressure difference. The membranes were purged with helium at 343 K at the start of each permeation test to remove moisture and adsorbed gases. More than five measurements were made at each pressure and the average value was recorded. For the Pd and Pd–Ag alloy membranes, the permeation experiments must be conducted at elevated temperatures. The membrane was heated from room temperature to 723 K at 1 K min−1 in flowing helium. It was then annealed in hydrogen at 723, 823 and 923 K for 12 h each. Annealing was used to activate the Pd membrane and also create the alloy in Pd–Ag membrane. After annealing at each temperature, the membrane was cooled down to 723 K where the permeation of hydrogen was measured at different feed pressures (i.e. 0.1–1.1 barg). This enabled us to determine the effects of annealing on the membrane performance. After annealing at 923 K for 12 h, the hydrogen permeance at 723, 823 and 923 K was measured with respect to feed pressure. From this experiment, the activation energy for hydrogen transport across the membrane can be calculated assuming an Arrhenius relation. 2.3.2. Hydrogen separation from Towngas After the single gas permeation experiment, the membranes were tested for hydrogen separation from Towngas mixture (49% H2 , 28.5% CH4 , 19.5% CO2

and 3% CO). Hydrogen separation was conducted at 323 K for the commercial alumina and zeolite membranes and at 723 K for the Pd and Pd–Ag alloy membranes. For each case, the membrane was allowed to equilibrate in flowing Towngas for about 1 h before the separation. The permeate flux was measured for trans-membrane pressures between 1.1 and 2.2 barg. The composition of the permeate stream was analyzed with an on-line gas chromatograph to determine the selectivity of the membrane for hydrogen. 2.4. Materials The tubular alumina membrane (5 nm pores) was purchased from US Filter and cut into the desired length using a wafering saw. The commercial alumina, ZSM-5, palladium and Pd–Ag alloy membranes are all 7.5 cm in length with an i.d. 0.63 cm, giving a membrane area of 14.4 cm2 . The ends of the membrane tube were sealed with glass enamel (Aremco 617) that can withstand temperatures of 1100 K. The chemicals used in preparing the zeolite membranes were tetraethyl orthosilicate (98%, Aldrich) and tetrapropylammonium hydroxide (1 M, Aldrich). Palladium and Pd–Ag alloy membranes were prepared from electroless plating solutions containing palladium chloride (99%, Aldrich), tetraaminepalladium nitrate salt (10%, Aldrich), silver nitrate (99%, Aldrich), Na2 EDTA (Reagent, Fishers), ammonium hydroxide (Reagent, Fishers), hydrazine (30%, Aldrich) and Sn(II)Cl2 (Reagent, Aldrich). Hydrogen (UHP) and helium (UHP) gases used in the permeation experiments were purchased from HKO Company Ltd., whereas, HSG Company Ltd. supplied the Towngas mixture.

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3. Results and discussion 3.1. Mesoporous alumina and zeolite membranes The topmost ␥-Al2 O3 layer of the commercial alumina membrane shown in Fig. 1a has a nominal pore size of 5 nm. In the absence of macroscopic defects, the flow of hydrogen and helium through the membrane was expected to be primarily Knudsen diffusion [35]. Fig. 3a displays the permeance (cm3 cm−2 min−1 bar−1 ) of hydrogen and helium as a function of average pressure (P ave = (P f + P p )/2). The permeance indicates that Knudsen diffusion is the dominant transport mechanism across the membrane. The ideal H2 /He separation ratio obtained by taking

Fig. 3. Permeance of hydrogen, helium and Towngas through: (a) commercial alumina; (b) ZSM-5/porous alumina membranes.

the ratio of the H2 /He permeances is 1.5, which is comparable to the Knudsen separation value of 1.41. Fig. 1b displays the microstructure of the ZSM-5 membrane deposited onto the seeded surface of the commercial alumina membrane. The polycrystalline zeolite membrane consists of inter-grown zeolite crystals (Fig. 1b, inset). The individual zeolite crystals have an inverted pyramidal shape with the rectangular base forming the surface structure of the membrane. The membrane is 7.6 ␮m thick with a preferred (1 0 1) crystallographic orientation that aligned the tortuous channel parallel to the flow through the zeolite layer. The Si/Al ratio is uniform across the thickness of zeolite layer and has an average value of 45. The size of the pore mouth along the zigzag channel is about 0.5 nm. The single gas permeance for helium and hydrogen is displayed in Fig. 3b. The separation behavior is characterized as mainly Knudsen. It has been predicted by model [36,37] that the diffusion of molecules and atoms smaller than 0.3 nm through the ZSM-5 pore channel will be mainly Knudsen. In ZSM-5, Xiao and Wei [36,37] demonstrated that configurational diffusion only becomes important for molecules larger than 0.3 nm. The X-ray diffraction patterns for both membranes are also shown in Fig. 1c. The single gas permeation results indicate that for hydrogen and helium, Knudsen diffusion is the main transport mechanism in both mesoporous alumina and microporous ZSM-5 membranes. The ideal H2 /He separation ratio is comparable for the two membranes, but the permeance of the ZSM-5 membrane is an order of magnitude smaller than that of the commercial alumina membrane. The performance of these two membranes for hydrogen separation from Towngas mixture is displayed in Fig. 3 and Table 2. There is no hydrogen separation for the commercial alumina membrane, but the ZSM-5 membrane was able to concentrate the hydrogen content from 49 to 59%. This is mainly due to the difference in the transport of bulkier components of Towngas (i.e. CH4 , CO and CO2 ) through the ␥-alumina and ZSM-5 membranes. In the former, Knudsen diffusion is the main transport mechanism while in the latter, the contributions from the configurational diffusion and chemical interaction with the pore wall could be significant. Indeed, the concentration of carbon monoxide in the permeate stream of the zeolite membrane was drastically reduced from 3.0 to 1.2%. The permeance through the

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zeolite membrane was 42.8 cm3 min−1 cm−2 bar−1 . Although the separation is low, the throughput is high and a cascade of zeolite membranes could be an attractive means of concentrating hydrogen in mixtures at room temperature. 3.2. Palladium and Pd–Ag alloy membranes A 3 ␮m thick palladium film (PdM1) was deposited onto the modified alumina support (cf. Fig. 2a) from hydrazine-based palladium plating bath. Before annealing the Pd membrane consists of sub-micron grains as shown in Fig. 2b. A significant microstructural transformation due to grain sintering and hydride phase formation was observed after the step-wise annealing of the membrane in hydrogen (Fig. 2c). Both XRD and XPS analyses indicated that the membrane is pure palladium with most of the surface impurities originating from the ambient conditions (i.e. adsorbed water, carbon dioxide and hydrocarbons). Hydrogen transport through the palladium membrane involves molecular hydrogen adsorption and dissociation at the membrane surface, followed by the dissolution of atomic hydrogen and its transport through the interstices of the metal matrix. The atomic hydrogen emerging on the other side of the membrane then re-associates to form molecular hydrogen and desorbed from the surface. The hydrogen diffusion is sustained by chemical potential gradient across the membrane. The dissolution and transport of atomic hydrogen through the dense metal matrix are usually the rate-limiting step in the process. Under this condition, it is expected that Sievert’s law should govern the transport of hydrogen across the Pd membrane [7,38] and the hydrogen flux should display a linear dependence on the square-root of pressure (Fig. 4a). The hydrogen permeation rate (723 K) was observed to increase after each stages of annealing (Fig. 4a). The Pd membrane displayed 2.5 times increase in hydrogen flux after annealing at 823 K compared to 723 K. An additional 2.5 times improvement was observed when the membrane was further annealed at 923 K. This activation process has also been observed by other authors [26,39]. It has been suggested that this enhancement was due to the removal of surface contaminants and formation of hydride phases during the annealing process. Fig. 4b shows the effects of temperature on the hydrogen perme-

ation flux across the annealed Pd membrane. It can be seen that the hydrogen permeation rate increases with temperature giving an activation energy (Ea ) of 10.3 kJ mol−1 . PdM2 membrane was prepared from a hydrazinebased plating bath containing both palladium and silver metal precursors (Table 1). A thinner 1.6 ␮m thick layer containing 3.2 at.% silver was deposited onto the alumina support. Fig. 4c shows that hydrogen permeation flux is five times higher after the membrane was annealed at 823 K compared to 723 K. However, the membrane suffered a 50% drop in hydrogen flux when it was further annealed at 923 K. Although the initial co-deposited palladium and silver layer had a spatially uniform composition, XPS analysis indicated that there is a significant silver enrichment at the membrane surface following the last annealing step. The silver content at the surface of the annealed PdM2 (923 K) is 29 at.%, which is higher than its bulk composition (i.e. 3.3 at.%). This surface enrichment is due to the lower surface free energy of silver as compared to palladium [40]. More silver at the surface means less palladium sites for adsorption and dissociation of hydrogen. This may explain the observed drop in the hydrogen permeation. Yamamura et al. [15] also reported silver enrichment at the surface of their Pd–Ag films prepared by RF-sputter coating after annealing in hydrogen. Fig. 4d displays the hydrogen permeation at different temperatures for the annealed PdM2 membrane. The activation energy (Ea ) for hydrogen permeation through the PdM2 is 35.20 kJ mol−1 . The third membrane PdM3, was also prepared by simultaneous deposition of palladium and silver by electroless plating (Table 1). The membrane is 1.8 ␮m thick and has a higher bulk silver content of 16.5 at.% as compared to PdM2. Fig. 4e displays the effects of annealing on the permeation properties of PdM3 membrane. The improvement in the hydrogen flux after annealing at 823 K is slight, however, the membrane exhibits about four-fold increase in hydrogen permeation after it was annealed at 923 K. The effects of annealing on PdM1, PdM2 and PdM3 membranes are unique and may be due to several factors that include alloying process, hydride formation and hydride phases that are dependent on the membrane composition. Once the membrane was stabilized at 923 K, the temperature dependency of the hydrogen flux (Fig. 4f) gave an activation energy (Ea ) of 11.36 kJ mol−1 ,

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Fig. 4. Permeation flux of hydrogen measured at 723 K after annealing: (a) PdM1; (c) PdM2; (e) PdM3 membranes at 723, 823 and 923 K. Hydrogen flux at different temperatures for the annealed: (b) PdM1; (d) PdM2; (f) PdM3 membranes (i.e. 923 K).

which is higher than the pure Pd membrane but smaller than PdM2. XPS analysis of the membrane indicates that the surface silver content of 29 at.% is comparable to that of PdM2 membrane. It is important to note that the thickness of the membranes has to be adjusted (i.e. 3 ␮m for PdM1 and ∼1.5 ␮m

for PdM2 and PdM3) in order to obtain comparable H2 /He permeation ratio (as shown in Table 1). Table 2 summarizes the thickness, bulk Ag content and surface Ag concentration for PdM1, PdM2 and PdM3 membranes, as well as the hydrogen flux after the membranes were annealed at 723 and 923 K.

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All three membranes exhibited improvement in hydrogen permeation after annealing at 923 K. The PdM1 membrane displayed a five-fold increase in hydrogen permeability, while PdM2 and PdM3 experienced three- and six-fold increase after annealing, respectively. The high concentration of silver on the surface (i.e. 29 at.%) of PdM2 membrane means that there are less Pd sites available for hydrogen adsorption and dissociation. Also, the low overall silver content in the bulk (3.3 at.%) does not provide significant expansion in the metal lattice (0.2237 versus 0.2240 nm) to speed-up the hydrogen transport and counteract this effect. This lead to the lower hydrogen permeability of the PdM2 Pd–Ag alloy membrane as compared to the PdM1 pure palladium membrane. The surface composition of Ag for PdM3 was maintained the same as PdM2, but the total amount of silver in the membrane was increased five-fold from 3.3 to 16.5 at.%. The resulting lattice spacing of the PdM3 alloy membrane (0.2255 nm) is larger than that of Pd metal. In terms of hydrogen permeability, the PdM3 membrane outperforms the PdM2 membrane by more than six times and PdM1 membrane by a factor of 3. Fig. 5a compares the performance of the three palladium-based membranes for hydrogen separation from ‘Towngas’ mixture at 723 K. For hydrogen separation from inert mixtures, the plots of separation and permeation fluxes as a function of the difference in the square-root of hydrogen partial pressures in the feed and permeate sides of the membrane are expected to be identical. Fig. 5a and b shows that the hydrogen flux (in cm3 cm−2 min−1 ) obtained from ‘Towngas’ mixture is less than a fifth of that measured from single gas permeation experiment. This suggests that the other component gases in the ‘Towngas’ mixture have an inhibiting effect on hydrogen separation. It is known that carbon monoxide can readily adsorb on palladium and, thus, effectively blocks the hydrogen adsorption and dissociation sites causing a significant drop in hydrogen permeation flux through palladium membrane [41]. Under the separation condition, it was observed that the carbon dioxide in the gas mixture reacts with hydrogen over the palladium membrane (i.e. PdM1) to produce carbon monoxide and water. This causes a decrease in the upstream hydrogen partial pressure, while simultaneously producing more carbon monoxide inhibitor. Table 2 clearly shows that the addition of silver can

Fig. 5. (a) Thickness corrected hydrogen flux through the PdM1, PdM2 and PdM3 membranes during hydrogen separation from Towngas at 723 K; (b) hydrogen permeation and separation flux through annealed PdM1 measured at 723 K.

ameliorate the decrease in the hydrogen permeance through the palladium membrane. It is important to note that pure hydrogen permeation test conducted after the Towngas separation gave reproducible results. Repeated separation and permeation tests gave similar results. This indicates that the membranes did not suffer irreversible changes during Towngas separation. Experimental test indicates that the main leakage originated from the graphite O-rings used to

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Table 3 Comparison of hydrogen permeance and permselectivity of Pd-based membranes prepared by different techniques Membrane

Preparation methods

Thickness (mm)

Pd/␣-Al2 O3

Electroless plating + osmosis to repair Electroless plating Electroless plating Electroless plating Chemical vapor deposition Electroless plating Magnetron sputtering

10.3

Pd/␣-Al2 O3 Pd/SS Pd/Vycor Pd/Al2 O3 Pd–Ag/␥-Al2 O3

11 20–25 0.75 3.3 8 1.0

seal the membrane in the module. The membrane itself was impervious to helium flow even at 3 barg when rubber O-rings were employed. A helium leak rate of 0.06–0.3 cm3 cm−2 min−1 was measured at 723 K and 1.5 bar pressure difference. This directly affects the membrane selectivity and hydrogen purity. A theoretical hydrogen purity of 99.8–99.99% was expected, but the experimental data in Table 2 shows that lower hydrogen purity was obtained for Towngas separation, which was mainly due to the lowered hydrogen flux. The CO concentrations in the permeate stream are 0.03, 0.3 and 0.08% for the PdM1, PdM2 and PdM3 membranes, respectively. 4. Concluding remarks This work demonstrates that commercial, mesoporous alumina membrane is unable to separate hydrogen from the Towngas mixture despite exhibiting good H2 /He ideal separation ratio. Zeolite, palladium and Pd–Ag alloy membranes were prepared using the commercial alumina membrane as a support. Although the zeolite membrane displayed similar H2 /He ideal separation ratio, it was able to produce a 60% H2 stream containing <1.2% CO from the Towngas. High purity hydrogen was obtained from the thin palladium and Pd–Ag alloy membranes prepared on porous alumina support by electroless plating technique. The results of this study compare well with those reported in the literature (Table 3). Hydrogen permeation and separation experiments demonstrate the importance of alloying with silver. Small quantity of silver (e.g. 3 at.%) can result in lower permeation flux due to the decrease in the Pd sites for H2 adsorption and dissociation caused by the surface en-

H2 permeance (cm3 cm−2 min−1 bar−1 ) 34 (after repairing) (740 K) 4 (773 K) 12 (623 K) 0.88 (623 K) 50 (773 K) 50 (773 K) 240 (735 K)

H2 /N2 970 >600 350 >900 13 ∞ 80

Reference [42] [27] [14] [26,30] [6] [6] [43]

richment of silver. However, the hydrogen permeance can be significant improved by further addition of silver (up to about 25 at.%) due to larger lattice spacing of the resulting alloy material. The component gases in Towngas inhibit the hydrogen flux across the palladium-based membranes by either blocking the Pd sites through competitive adsorption (i.e. CO) or decreasing the hydrogen partial pressure through reaction. The presence of silver tends to ameliorate this condition. Added advantages of alloying with silver are less embrittlement and significantly lowered cost of the membrane.

Acknowledgements We gratefully acknowledge the financial support from the Hong Kong Research Grant Council (HKUST 6029/98P). We also thank the Material Characterization and Preparation Facility (MCPF) of the Hong Kong University of Science and Technology (HKUST) for structure and surface analysis of the palladium and silver films. References [1] R. Ramachandran, R.K. Menon, An overview of industrial uses of hydrogen, Int. J. Hydrogen Energy 23 (1998) 593. [2] T.N. Veziroglu, Hydrogen energy system as a permanent solution to global energy–environmental problems, Chem. Ind. 53 (1999) 383. [3] M.A. Rosen, D.S. Scott, Comparative efficiency assessment for a range of hydrogen production processes, Int. J. Hydrogen Energy 23 (1998) 653. [4] H. Shiga, K. Shinda, K. Hagiwara, A. Tsutsumi, M. Sakurai, K. Yoshida, E. Balgen, Large scale hydrogen production form biogas, Int. J. Hydrogen Energy 23 (1998) 631.

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