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Preparation of dense Pd composite membranes on porous Ti–Al alloy supports by electroless plating
Journal of Membrane Science 387–388 (2012) 24–29
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation of dense Pd composite membranes on porous Ti–Al alloy supports by electroless plating Dongqiang Zhang a , Shouyong Zhou a,b , Yiqun Fan a,∗ , Nanping Xu a , Yuehui He c a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China School of Chemistry and Chemical Engineering, Huaiyin Normal University, Key Lab for Chemistry of Low-Dimensional Materials of Jiangsu Province, No.111 West Changjiang Road, Huaian 223300, Jiangsu Province, China c State Key Laboratory for Powder Metallurgy, Central South University, Changsha, Hunan 410083, China b
a r t i c l e
i n f o
Article history: Received 30 May 2011 Received in revised form 9 September 2011 Accepted 2 October 2011 Available online 6 October 2011 Keywords: Palladium membrane Ti–Al alloy TiO2 membrane Hydrogen separation
a b s t r a c t Dense Pd/TiO2 /Ti–Al alloy composite membranes on porous Ti–Al alloy supports were prepared by electroless plating. The TiO2 membrane was applied as an inter-diffusion barrier between the Pd layer and the Ti–Al alloy support to prevent intermetallic diffusion. Electron probe microanalysis (EPMA) and EDX cross-sectional line scans showed that the intermediate layer was effective as the diffusion barrier for Pd membranes on Ti–Al alloy supports after 40 h of treatment at 973 K in pure hydrogen. The thickness of the resulting Pd composite membrane was approximately 14 m. The hydrogen permeance through the Pd composite membrane was 1.07 × 10−3 mol m−2 s−1 Pa−0.5 at 773 K. The permeance and selectivity of the Pd composite membrane were measured at different temperatures and pressures. The permeance tests showed that the Pd/TiO2 /Ti–Al alloy composite membranes have high H2 /N2 selectivity. In addition, heat cycles indicated that the structure of the Pd composite membranes was stable. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been renewed interest in developing methods for highly pure hydrogen production because of the increasing demand for hydrogen as a clean energy carrier for fuel cells [1]. Pd membranes are well known for their use in hydrogen separation and purification due to their high permeance and perfect selectivity for hydrogen. They are increasingly used in dehydrogenation membrane reactors where in situ removal of the produced hydrogen from the reaction atmosphere eliminates the equilibrium constraint [2,3]. Because the permeation rate through a Pd membrane is often inversely proportional to its thickness [4,5], Pd membranes are usually prepared as a composite consisting of a thin Pd layer (which provides high hydrogen permselectivity) on a porous support (which provides adequate mechanical strength to support the thin Pd layer) [6,7]. Ti–Al alloy was proposed as a promising support for Pd membranes based on the very desirable properties such as having low density, high specific strength, good oxidation resistance at high temperature [8–11] and similar thermal expansion coefficient to that of the Pd [12]. However, although the Ti–Al alloy effectively avoided the diffusion of metals at temperatures lower than 873 K, intermetallic diffusion between the Pd
∗ Corresponding author. Tel.: +86 25 83172277; fax: +86 25 83172292. E-mail address: [email protected] (Y. Fan). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.10.004
layer and the Ti–Al alloy also occurred after 40 h of treatment in hydrogen at 973 K [13]. An effective solution for preventing the intermetallic diffusion is to fabricate an inter-diffusion barrier layer between the Pd layer and the metal support. The intermediate layer should be stable at elevated temperatures under reducing and oxidizing atmospheres. Edlund and McCarthy [14] have reported the application of a 250 m thick porous aluminum oxide as diffusion barrier between Pd foil and vanadium support. The Pd composite membrane exhibited good stability over a period of 80 h at 973 K. Nam and Lee [15] have applied sol–gel techniques to form a colloidal silica sol layer, which was used as an inter-diffusion barrier layer between Pd–Cu alloy layer and the PSS support. Electron probe microanalysis indicated that the composite membranes were structurally stable. Huang and Dittmeyer [4] applied ZrO2 , YSZ, and TiO2 as porous barriers between the palladium membrane and the sinter-metal support to prevent intermetallic diffusion. They found YSZ was identified as the most promising barrier, followed by TiO2 and ZrO2 . Ma et al. [16] introduced an oxide layer on the porous stainless steel supports by in situ oxidation process at high temperatures. They found that forming an oxide layer above 873 K created an effective diffusion barrier between the Pd and the PSS. Yepes et al. [17] used ␥-Al2 O3 layer as an inter-diffusion barrier layer between Pd–Ag alloy layer and the PSS support. The Auger and EPMA depth profiles indicated that the ␥-Al2 O3 layer effectively blocked intermetallic diffusion. Zhang et al. [18] have reported that two different intermediate layers (in situ oxidized metal oxide and sol–gel derived
D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
YSZ) were used as a diffusion barrier between the Pd layer and the PSS. Stability tests performed over 100 h showed that both intermediate layers were effective as diffusion barrier for Pd membranes on PSS supports in the temperature range of 773–873 K. At temperatures above 873 K, only the YSZ intermediate layer was effective at preventing intermetallic diffusion. In this paper, TiO2 membranes were deposited on Ti–Al alloy supports to prevent intermetallic diffusion and smooth the support surface. Then dense Pd membranes were prepared on the TiO2 /Ti–Al supports by electroless plating. The resistance of the TiO2 membranes to intermetallic diffusion was investigated at high temperature by EPMA and EDX cross-sectional line scans. Permeation measurements and scanning electron microscopy (SEM) were used to characterize the properties of the prepared Pd/TiO2 /Ti–Al alloy composite membranes. The stability of the Pd composite membranes was also investigated. 2. Experimental 2.1. Preparation of Pd composite membranes Porous Ti–Al alloy discs (provided by He’s group, Central South University, China), with a diameter of 34 mm and a thickness of 2.5 mm, were used as supports for the thin Pd composite membranes. The most frequent pore size of the support is 6 m. Before use, the supports were first cleaned ultrasonically with acetone at 323 K and deionized water at 333 K, respectively, and then dried overnight at 423 K. The coating of a TiO2 layer on the Ti–Al alloy supports was done through the following published methods [19]. A porous Ti–Al alloy disc was used as the cathode, and a parallel stainless steel disc was used as the anode. They faced each other across a distance of 1.5 cm. The applied voltage was kept constant between the two electrodes using a DC power supply. The coated membranes were dried at room temperature, 343 K and 383 K for 12 h, respectively, and then was sintered in an argon atmosphere at 1323 K for 3 h in an electric furnace with a heating and cooling rate of 1 K/min. The Ti–Al alloy support was activated before electroless plating to seed the surface layer with nuclei of Pd that can initiate the autocatalytic process at the start of the electroless plating [7]. The Pd membrane was deposited at 313 K. The plating solution (PdCl2 , 4 g/L; EDTA·2Na, 70 g/L; N2 H4 (85%), 0.2 mol/L; pH = 11) was changed every 60 min until a thickness of dense Pd layer was achieved. After completing the electroless plating, the Pd membranes were immediately cleaned with deionized water to remove salts adsorbed in pores of the support or entrapped in the Pd coating layer. The composite membranes were dried overnight in an oven at 393 K. 2.2. Membrane characterization and gas permeation The morphology of the membrane was examined by scanning electron microscopy (SEM, Quanta200, USA). Mean pore size and pore size distributions of the TiO2 /Ti–Al alloy composite membranes were obtained using the gas bubble pressure method, which was performed according to the guidelines in the American Society for Testing and Materials Publication (ASTM F316-80). Electron probe microanalysis (EPMA-8705QH2, Shimadzu, Japan) was used to analyze intermetallic diffusion of the elements between the Ti–Al support and the Pd thin film. Gas permeation through the Pd membrane at high temperatures was measured on the permeation apparatus [6]. The Pd composite membrane was placed inside a stainless steel permeator. Permeate side was kept at atmospheric pressure with H2 or N2 being fed on the retentate side. The upstream pressure was controlled by back pressure regulator. After stabilization of temperature and pressure, single gas fluxes were
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measured with mass flow controllers (Models D08-4D/ZM, Beijing Sevenstar Electronics Co., Ltd., Beijing, China). 3. Results and discussion 3.1. Characterization of TiO2 /Ti–Al composite membranes The surface and the cross-section of the supported TiO2 /Ti–Al composite membranes are shown in Fig. 1. Fig. 1a indicates that the surface of the membranes is compact with some surface defects. In the cross-sectional image of a supported TiO2 /Ti–Al composite membrane (Fig. 1b), the homogeneous membrane layer can be seen on the right side. This membrane layer has a thickness of 40 m, which is almost the same as that measured by weighing the membrane disc before and after the membrane was formed. Attempts to physically remove the TiO2 membranes from the porous Ti–Al alloy supports suggest that the bond between the membrane and the support is adequate. Gas bubble pressure measurements for the supported TiO2 /Ti–Al composite membranes were performed at 293 K. As shown in Fig. 2, the average pore size of the supported TiO2 /Ti–Al composite membranes is 0.28 m, and the pore size distribution is very narrow. The narrow pore size distribution indicates that there are no large defects in the membranes. That is to say, the surface defects observed by SEM do not appear to continue through the membrane layer. 3.2. Intermetallic diffusion BSE was used to analyze the inter-diffusion of the metals. The BSE images not only provide topographical information but also provide qualitative information on the composition. For example, areas with a larger average atomic number (e.g., the Pd layer) appear brighter in BSE. This phenomenon allows for easy identification of the Pd from the sinter-metal components [4,20]. Fig. 3a shows the BSE images of the cross-sections of the Pd/Ti–Al alloy composite membranes after hydrogen exposure at 973 K for 40 h [13]. The inter-diffusion of the Pd/Ti–Al alloy clearly becomes more severe after 40 h of treatment at 973 K in pure hydrogen; the palladium layer turned slightly darker, while the adjacent Ti–Al alloy region became brighter, which clearly indicates the diffusion of Pd into the support and the diffusion of metals from the support into the Pd layer. Fig. 3b shows the BSE images for the Pd/TiO2 /Ti–Al composite membranes after 40 h of treatment at 973 K in pure hydrogen. In this case, the boundary of the Pd layer and the TiO2 /Ti–Al composite membranes remained clear; neither the diffusion of Pd nor the diffusion of Ti and Al was observed. The clear boundary demonstrates that the TiO2 membranes can effectively prevent the diffusion of metals. To further confirm the intermetallic diffusion and metal distribution, EDX cross-sectional line scans were performed on the Pd/TiO2 /Ti–Al composite membranes. Fig. 4a shows the crosssectional line scans of the Pd/Ti–Al composite membranes after hydrogen exposure at 973 K for 40 h [13]. It is clear that intermetallic diffusion occurs between Pd and the Ti–Al alloy. The Ti and Al atoms diffuse from the metal-support interface to the membrane surface. Pd also diffuses into the Ti–Al alloy support. Fig. 4b shows the cross-sectional line scans of Pd/TiO2 /Ti–Al composite membranes after hydrogen exposure at 973 K for 40 h. Compared to Fig. 4a, the rather abrupt interface between the Pd membrane and the TiO2 /Ti–Al composite membrane shows no inter-diffusion between the Pd layer and the TiO2 /Ti–Al composite membrane. This result suggests that the TiO2 membrane effectively impedes intermetallic diffusion. The EDX results are good agreement with the BSE results presented in Fig. 3.
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D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
Fig. 1. The SEM micrographs of the supported TiO2 /Ti–Al alloy composite membrane, sintered at 1323 K for 3 h.
The surface and cross-sectional SEM images of the Pd/TiO2 /Ti–Al alloy composite membranes are exhibited in Fig. 5. Fig. 5a shows that the membrane surface is homogeneous and smooth. Although the images correspond to very small areas of the membranes, gas leak tests further indicate that defect-free Pd membranes are obtained. Fig. 5b shows that the Pd membranes are uniform in thickness, and the thickness of the membranes is approximately 12–14 m, which is in good agreement with the thickness determined from the gravimetric method. 3.4. Hydrogen permeation of Pd/TiO2 /Ti–Al alloy composite membranes
Fig. 2. Pore size distribution ( ) of the supported TiO2 /Ti–Al alloy composite membrane, sintered at 1323 K for 3 h, ( ), the average pore size.
3.3. Characterization of Pd/TiO2 /Ti–Al alloy composite membranes The average thickness of the deposited Pd layer was approximately 14 m as measured by the gravimetric method (the weight gain of the sample divided by the product of the plated surface area and the metal density).
Single gas permeation is a simple but effective method to measure the permeation properties of Pd-based membranes. After activation under a hydrogen atmosphere for 10 h, no nitrogen leakage was detected at a pressure difference of 0.3 MPa, which corresponds to a nitrogen permeance smaller than 10−10 mol m−2 s−1 Pa−1 based on the permeation equipment limit. The hydrogen permeation flux at different operating temperatures under various pressure differences was measured while maintaining the atmospheric pressure on the permeate side (Fig. 6). The hydrogen permeation flux increases with an increasing pressure difference (Ph 0.5 − Pl 0.5 ). The operating temperature also has a positive effect on the permeation flux. These trends can be attributed to the solution-diffusion mechanism. In addition, according to Fig. 6,
Fig. 3. BSE images of the composite membranes after treatment at 973 K in pure hydrogen for 40 h.
D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
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Fig. 4. Cross-sectional line scans of the composite membrane after treatment at 973 K in pure hydrogen for 40 h.
the n value was approximately 0.5 under all experimental conditions, indicating that hydrogen permeation was mainly governed by the bulk-diffusion process. A comparison of the Pd membrane thickness and permeance performance for this study and the studies of other researchers is shown in Table 1. It can be concluded that hydrogen permeance ranges on the order of 10−3 to 10−4 mol m−2 s−1 Pa−0.5 . In this work, the value was on the order of 10−3 mol m−2 s−1 Pa−0.5 .
Experimental data of the hydrogen permeance at different temperatures were used to estimate the activation energy for hydrogen permeation through the membrane. The relationship between temperature and gas permeance can be expressed by an Arrhenius equation as F=
Q0 L
exp
−Ea RT
Fig. 5. SEM photograph of the Pd/TiO2 /Ti–Al alloy composite membrane.
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D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
Table 1 Comparison of the membrane prepared in this study and similar studies in the literature. Membrane
Pore size of support (m)
Thickness (m)
Temperature (K)
n
F (mol m−2 s−1 Pa−0.5 )
Separation factor
Reference
PSS/CeO2 /Pd PSS/oxide/Pd PSS/WO3 /Pd PSS/NaAZ80/Pd PSS/Al2 O3 /Pd Ti–Al/TiO2 /Pd
0.2 0.2 0.2 0.2 0.2 0.28
13 21.3 12 19 5 14
773 723 773 723 723 773
0.5 0.5 0.5 0.5 0.5 0.5
1.27 × 10−3 1.43 × 10−4 2 × 10−3 1.1 × 10−3 2.48 × 10−3 1.07 × 10−3
∞ ∞ 10,000 608 ∞ ∞
[21] [22] [23] [24] [7] Our work
Fig. 6. The dependence of hydrogen fluxes on Ph 0.5 − Pl 0.5 at various temperatures. Fig. 7. Arrhenius relation between the hydrogen permeance and temperature.
The Arrhenius relation between the hydrogen permeance and the temperature is shown in Fig. 7. The average activation energy for hydrogen permeation of the resulting Pd composite membranes was calculated to be 13.65 kJ/mol in the temperature range of 623–773 K, which is consistent with the value presented in the literature [25]. 3.5. Hydrogen permeation stability Pd membranes should demonstrate long-term stability if they are to be viable for industrial applications. To examine the influence of temperature fluctuations on hydrogen permeation, temperature cycling was performed for the Pd/TiO2 /Ti–Al composite membranes. Fig. 8 shows the hydrogen flux data of the Pd/TiO2 /Ti–Al composite membranes operating in three heat cycles of 623 K → 673 K → 723 K → 773 K (at P = 0.1 MPa). Under the
same conditions, the hydrogen flux showed no observable changes during the thermal cycles from 623 K to 773 K, which indicates that the Pd composite membranes have good thermal stability. After the stability tests, the Pd composite membranes were checked for pinholes and cracks by means of nitrogen permeation, and no leakage was observed. 4. Conclusions Pinhole-free Pd/TiO2 /Ti–Al alloy composite membranes on porous Ti–Al alloy discs were fabricated by electroless plating. The TiO2 membrane was applied as a barrier layer between the Pd layer and Ti–Al alloy support to prevent intermetallic diffusion. The resistance to this diffusion was characterized by EPMA and EDX
Fig. 8. Variation of the hydrogen flux during thermal cycling (P = 0.1 MPa).
D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
cross-sectional line scans. The results showed that the intermediate layer was effective as a diffusion barrier for Pd membranes on Ti–Al alloy supports after 40 h of treatment at 973 K in pure hydrogen. The thickness of the resulting Pd composite membrane was approximately 14 m. The hydrogen permeance through the Pd composite membrane was 1.07 × 10−3 mol m−2 s−1 Pa−0.5 at 773 K. No detectable nitrogen flux at a pressure difference up to 0.3 MPa indicated that the Pd composite membrane had high H2 /N2 selectivity in the experimental temperature range. The activation energy for hydrogen permeation was 13.65 kJ/mol in the temperature range of 623–773 K. Under the same conditions, the hydrogen flux showed no observable changes during thermal cycling, which indicates that the Pd/TiO2 /Ti–Al alloy composite membranes had good thermal stability. Acknowledgements This work was supported by the National Basic Research Program of China (No. 2009CB623400), the National Nature Science Foundation of China (No. 20636020) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] H.-J. Neef, International overview of hydrogen and fuel cell research, Energy 34 (2009) 327–333. [2] R. Dittmeyer, J. Caro, Catalytic membrane reactors, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, 2nd ed., Wiley-VCH, Weinheim, 2008, pp. 2198–2248. [3] J. Zhang, H.Y. Xu, W.Z. Li, High-purity COx -free H2 generation from NH3 via the ultra permeable and highly selective Pd membranes, J. Membr. Sci. 277 (2006) 85–93. [4] Y. Huang, R. Dittmeyer, Preparation and characterization of composite palladium membranes on sinter-metal supports with a ceramic barrier against intermetallic diffusion, J. Membr. Sci. 282 (2006) 296–310. [5] T.L. Ward, T. Dao, Model of hydrogen permeation behavior in palladium membranes, J. Membr. Sci. 153 (1999) 211–231. [6] X. Li, Y.Q. Fan, W.Q. Jin, Y. Huang, N.P. Xu, Improved photocatalytic deposition of palladium membranes, J. Membr. Sci. 282 (2006) 1–6. [7] A.W. Li, J.R. Grace, C.J. Lima, Preparation of thin Pd-based composite membrane on planar metallic substrate: Part II. Preparation of membranes by electroless plating and characterization, J. Membr. Sci. 306 (2007) 159–165.
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[8] Y.H. He, Y. Jiang, N.P. Xu, J. Zou, B.Y. Huang, C.T. Liu, P.K. Liaw, Fabrication of Ti–Al micro/nanometer-sized porous alloys through the kirkendall effect, Adv. Mater. 19 (2007) 2102–2106. [9] H.R. Gong, Y.H. He, B.Y. Huang, Bond strength and interface energy between Pd membranes and Ti–Al supports, Appl. Phys. Lett. 93 (2008) 101907. [10] B.Y. Huang, Y.H. He, J.N. Wang, Improvement in mechanical and oxidation properties of TiAl alloys with Sb additions, Intermetallics 7 (1999) 881–888. [11] K. Zhang, Z.W. Li, W. Gao, Hot corrosion behaviour of Ti–Al based intermetallics, Mater. Lett. 57 (2002) 834–843. [12] M.E. Ayturk, I.P. Mardilovich, E.E. Engwall, Y.H. Ma, Synthesis of composite Pdporous stainless steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier, J. Membr. Sci. 285 (2006) 385–394. [13] D.Q. Zhang, Y.Q. Fan, N.P. Xu, Y.H. He, Effect of intermetallic diffusion between Pd and Ti–Al alloy on the performance of Pd/Ti–Al alloy composite membranes, J. Membr. Sci. 377 (2011) 221–230. [14] D.J. Edlund, J. McCarthy, The relationship between intermetallic diffusion and flux decline in composite-metal membranes: implication for achieving long membrane lifetime, J. Membr. Sci. 107 (1995) 147–153. [15] S.-E. Nam, K.-H. Lee, Hydrogen separation by Pd alloy composite membranes: introduction of diffusion barrier, J. Membr. Sci. 192 (2001) 177–185. [16] Y.H. Ma, B.C. Akis, M.E. Ayturk, F. Guazzone, E.E. Engwall, I.P. Mardilovich, Characterization of intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes, Ind. Eng. Chem. Res. 43 (2004) 2936–2945. [17] D. Yepes, L.M. Cornaglia, S. Irusta, E.A. Lombardo, Different oxides used as diffusion barriers in composite hydrogen permeable membranes, J. Membr. Sci. 274 (2006) 92–101. [18] K. Zhang, H.Y. Gao, Z.B. Rui, P. Liu, Y.D. Li, Y.S. Lin, High-temperature stability of palladium membranes on porous metal supports with different intermediate layers, Ind. Eng. Chem. Res. 48 (2009) 1880–1886. [19] S.Y. Zhou, Y.Q. Fan, Y.H. He, N.P. Xu, Preparation of titania microfiltration membranes supported on porous Ti–Al alloys, J. Membr. Sci. 325 (2008) 546–552. [20] A. Konkol, G.R. Booker, P.R. Wilshaw, Backscattered electron contrast on cross sections of interfaces and multilayers in the scanning electron microscope, Ultramicroscopy 58 (1995) 233–237. [21] J.H. Tong, Y. Matsumura, H. Suda, K. Haraya, Thin and dense Pd/CeO2 /MPSS composite membrane for hydrogen separation and steam reforming of methane, Sep. Purif. Technol. 46 (2005) 1–10. [22] K.S. Rothenberger, A.V. Cugini, B.H. Howard, R.P. Killmeyer, M.V. Ciocco, B.D. Morreale, R.M. Enick, F. Bustamante, I.P. Mardilovich, Y.H. Ma, High pressure hydrogen permeance of porous stainless steel coated with a thin palladium film via electroless plating, J. Membr. Sci. 244 (2004) 55–68. [23] M. Zahedi, B. Afra, M. Dehghani-Mobarake, M. Bahmani, Preparation of a Pd membrane on a WO3 modified porous stainless steel for hydrogen separation, J. Membr. Sci. 333 (2009) 45–49. [24] M.L. Bosko, F. Ojeda, E.A. Lombardo, L.M. Cornaglia, NaA zeolite as an effective diffusion barrier in composite Pd/PSS membranes, J. Membr. Sci. 331 (2009) 57–65. [25] Y. Huang, X. Li, Y.Q. Fan, N.P. Xu, Progress in palladium-based composite membranes: principle, preparation and characterization, Prog. Chem. 18 (2006) 230–238.
Contents lists available at SciVerse ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Preparation of dense Pd composite membranes on porous Ti–Al alloy supports by electroless plating Dongqiang Zhang a , Shouyong Zhou a,b , Yiqun Fan a,∗ , Nanping Xu a , Yuehui He c a
State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China School of Chemistry and Chemical Engineering, Huaiyin Normal University, Key Lab for Chemistry of Low-Dimensional Materials of Jiangsu Province, No.111 West Changjiang Road, Huaian 223300, Jiangsu Province, China c State Key Laboratory for Powder Metallurgy, Central South University, Changsha, Hunan 410083, China b
a r t i c l e
i n f o
Article history: Received 30 May 2011 Received in revised form 9 September 2011 Accepted 2 October 2011 Available online 6 October 2011 Keywords: Palladium membrane Ti–Al alloy TiO2 membrane Hydrogen separation
a b s t r a c t Dense Pd/TiO2 /Ti–Al alloy composite membranes on porous Ti–Al alloy supports were prepared by electroless plating. The TiO2 membrane was applied as an inter-diffusion barrier between the Pd layer and the Ti–Al alloy support to prevent intermetallic diffusion. Electron probe microanalysis (EPMA) and EDX cross-sectional line scans showed that the intermediate layer was effective as the diffusion barrier for Pd membranes on Ti–Al alloy supports after 40 h of treatment at 973 K in pure hydrogen. The thickness of the resulting Pd composite membrane was approximately 14 m. The hydrogen permeance through the Pd composite membrane was 1.07 × 10−3 mol m−2 s−1 Pa−0.5 at 773 K. The permeance and selectivity of the Pd composite membrane were measured at different temperatures and pressures. The permeance tests showed that the Pd/TiO2 /Ti–Al alloy composite membranes have high H2 /N2 selectivity. In addition, heat cycles indicated that the structure of the Pd composite membranes was stable. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, there has been renewed interest in developing methods for highly pure hydrogen production because of the increasing demand for hydrogen as a clean energy carrier for fuel cells [1]. Pd membranes are well known for their use in hydrogen separation and purification due to their high permeance and perfect selectivity for hydrogen. They are increasingly used in dehydrogenation membrane reactors where in situ removal of the produced hydrogen from the reaction atmosphere eliminates the equilibrium constraint [2,3]. Because the permeation rate through a Pd membrane is often inversely proportional to its thickness [4,5], Pd membranes are usually prepared as a composite consisting of a thin Pd layer (which provides high hydrogen permselectivity) on a porous support (which provides adequate mechanical strength to support the thin Pd layer) [6,7]. Ti–Al alloy was proposed as a promising support for Pd membranes based on the very desirable properties such as having low density, high specific strength, good oxidation resistance at high temperature [8–11] and similar thermal expansion coefficient to that of the Pd [12]. However, although the Ti–Al alloy effectively avoided the diffusion of metals at temperatures lower than 873 K, intermetallic diffusion between the Pd
∗ Corresponding author. Tel.: +86 25 83172277; fax: +86 25 83172292. E-mail address: [email protected] (Y. Fan). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.10.004
layer and the Ti–Al alloy also occurred after 40 h of treatment in hydrogen at 973 K [13]. An effective solution for preventing the intermetallic diffusion is to fabricate an inter-diffusion barrier layer between the Pd layer and the metal support. The intermediate layer should be stable at elevated temperatures under reducing and oxidizing atmospheres. Edlund and McCarthy [14] have reported the application of a 250 m thick porous aluminum oxide as diffusion barrier between Pd foil and vanadium support. The Pd composite membrane exhibited good stability over a period of 80 h at 973 K. Nam and Lee [15] have applied sol–gel techniques to form a colloidal silica sol layer, which was used as an inter-diffusion barrier layer between Pd–Cu alloy layer and the PSS support. Electron probe microanalysis indicated that the composite membranes were structurally stable. Huang and Dittmeyer [4] applied ZrO2 , YSZ, and TiO2 as porous barriers between the palladium membrane and the sinter-metal support to prevent intermetallic diffusion. They found YSZ was identified as the most promising barrier, followed by TiO2 and ZrO2 . Ma et al. [16] introduced an oxide layer on the porous stainless steel supports by in situ oxidation process at high temperatures. They found that forming an oxide layer above 873 K created an effective diffusion barrier between the Pd and the PSS. Yepes et al. [17] used ␥-Al2 O3 layer as an inter-diffusion barrier layer between Pd–Ag alloy layer and the PSS support. The Auger and EPMA depth profiles indicated that the ␥-Al2 O3 layer effectively blocked intermetallic diffusion. Zhang et al. [18] have reported that two different intermediate layers (in situ oxidized metal oxide and sol–gel derived
D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
YSZ) were used as a diffusion barrier between the Pd layer and the PSS. Stability tests performed over 100 h showed that both intermediate layers were effective as diffusion barrier for Pd membranes on PSS supports in the temperature range of 773–873 K. At temperatures above 873 K, only the YSZ intermediate layer was effective at preventing intermetallic diffusion. In this paper, TiO2 membranes were deposited on Ti–Al alloy supports to prevent intermetallic diffusion and smooth the support surface. Then dense Pd membranes were prepared on the TiO2 /Ti–Al supports by electroless plating. The resistance of the TiO2 membranes to intermetallic diffusion was investigated at high temperature by EPMA and EDX cross-sectional line scans. Permeation measurements and scanning electron microscopy (SEM) were used to characterize the properties of the prepared Pd/TiO2 /Ti–Al alloy composite membranes. The stability of the Pd composite membranes was also investigated. 2. Experimental 2.1. Preparation of Pd composite membranes Porous Ti–Al alloy discs (provided by He’s group, Central South University, China), with a diameter of 34 mm and a thickness of 2.5 mm, were used as supports for the thin Pd composite membranes. The most frequent pore size of the support is 6 m. Before use, the supports were first cleaned ultrasonically with acetone at 323 K and deionized water at 333 K, respectively, and then dried overnight at 423 K. The coating of a TiO2 layer on the Ti–Al alloy supports was done through the following published methods [19]. A porous Ti–Al alloy disc was used as the cathode, and a parallel stainless steel disc was used as the anode. They faced each other across a distance of 1.5 cm. The applied voltage was kept constant between the two electrodes using a DC power supply. The coated membranes were dried at room temperature, 343 K and 383 K for 12 h, respectively, and then was sintered in an argon atmosphere at 1323 K for 3 h in an electric furnace with a heating and cooling rate of 1 K/min. The Ti–Al alloy support was activated before electroless plating to seed the surface layer with nuclei of Pd that can initiate the autocatalytic process at the start of the electroless plating [7]. The Pd membrane was deposited at 313 K. The plating solution (PdCl2 , 4 g/L; EDTA·2Na, 70 g/L; N2 H4 (85%), 0.2 mol/L; pH = 11) was changed every 60 min until a thickness of dense Pd layer was achieved. After completing the electroless plating, the Pd membranes were immediately cleaned with deionized water to remove salts adsorbed in pores of the support or entrapped in the Pd coating layer. The composite membranes were dried overnight in an oven at 393 K. 2.2. Membrane characterization and gas permeation The morphology of the membrane was examined by scanning electron microscopy (SEM, Quanta200, USA). Mean pore size and pore size distributions of the TiO2 /Ti–Al alloy composite membranes were obtained using the gas bubble pressure method, which was performed according to the guidelines in the American Society for Testing and Materials Publication (ASTM F316-80). Electron probe microanalysis (EPMA-8705QH2, Shimadzu, Japan) was used to analyze intermetallic diffusion of the elements between the Ti–Al support and the Pd thin film. Gas permeation through the Pd membrane at high temperatures was measured on the permeation apparatus [6]. The Pd composite membrane was placed inside a stainless steel permeator. Permeate side was kept at atmospheric pressure with H2 or N2 being fed on the retentate side. The upstream pressure was controlled by back pressure regulator. After stabilization of temperature and pressure, single gas fluxes were
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measured with mass flow controllers (Models D08-4D/ZM, Beijing Sevenstar Electronics Co., Ltd., Beijing, China). 3. Results and discussion 3.1. Characterization of TiO2 /Ti–Al composite membranes The surface and the cross-section of the supported TiO2 /Ti–Al composite membranes are shown in Fig. 1. Fig. 1a indicates that the surface of the membranes is compact with some surface defects. In the cross-sectional image of a supported TiO2 /Ti–Al composite membrane (Fig. 1b), the homogeneous membrane layer can be seen on the right side. This membrane layer has a thickness of 40 m, which is almost the same as that measured by weighing the membrane disc before and after the membrane was formed. Attempts to physically remove the TiO2 membranes from the porous Ti–Al alloy supports suggest that the bond between the membrane and the support is adequate. Gas bubble pressure measurements for the supported TiO2 /Ti–Al composite membranes were performed at 293 K. As shown in Fig. 2, the average pore size of the supported TiO2 /Ti–Al composite membranes is 0.28 m, and the pore size distribution is very narrow. The narrow pore size distribution indicates that there are no large defects in the membranes. That is to say, the surface defects observed by SEM do not appear to continue through the membrane layer. 3.2. Intermetallic diffusion BSE was used to analyze the inter-diffusion of the metals. The BSE images not only provide topographical information but also provide qualitative information on the composition. For example, areas with a larger average atomic number (e.g., the Pd layer) appear brighter in BSE. This phenomenon allows for easy identification of the Pd from the sinter-metal components [4,20]. Fig. 3a shows the BSE images of the cross-sections of the Pd/Ti–Al alloy composite membranes after hydrogen exposure at 973 K for 40 h [13]. The inter-diffusion of the Pd/Ti–Al alloy clearly becomes more severe after 40 h of treatment at 973 K in pure hydrogen; the palladium layer turned slightly darker, while the adjacent Ti–Al alloy region became brighter, which clearly indicates the diffusion of Pd into the support and the diffusion of metals from the support into the Pd layer. Fig. 3b shows the BSE images for the Pd/TiO2 /Ti–Al composite membranes after 40 h of treatment at 973 K in pure hydrogen. In this case, the boundary of the Pd layer and the TiO2 /Ti–Al composite membranes remained clear; neither the diffusion of Pd nor the diffusion of Ti and Al was observed. The clear boundary demonstrates that the TiO2 membranes can effectively prevent the diffusion of metals. To further confirm the intermetallic diffusion and metal distribution, EDX cross-sectional line scans were performed on the Pd/TiO2 /Ti–Al composite membranes. Fig. 4a shows the crosssectional line scans of the Pd/Ti–Al composite membranes after hydrogen exposure at 973 K for 40 h [13]. It is clear that intermetallic diffusion occurs between Pd and the Ti–Al alloy. The Ti and Al atoms diffuse from the metal-support interface to the membrane surface. Pd also diffuses into the Ti–Al alloy support. Fig. 4b shows the cross-sectional line scans of Pd/TiO2 /Ti–Al composite membranes after hydrogen exposure at 973 K for 40 h. Compared to Fig. 4a, the rather abrupt interface between the Pd membrane and the TiO2 /Ti–Al composite membrane shows no inter-diffusion between the Pd layer and the TiO2 /Ti–Al composite membrane. This result suggests that the TiO2 membrane effectively impedes intermetallic diffusion. The EDX results are good agreement with the BSE results presented in Fig. 3.
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D. Zhang et al. / Journal of Membrane Science 387–388 (2012) 24–29
Fig. 1. The SEM micrographs of the supported TiO2 /Ti–Al alloy composite membrane, sintered at 1323 K for 3 h.
The surface and cross-sectional SEM images of the Pd/TiO2 /Ti–Al alloy composite membranes are exhibited in Fig. 5. Fig. 5a shows that the membrane surface is homogeneous and smooth. Although the images correspond to very small areas of the membranes, gas leak tests further indicate that defect-free Pd membranes are obtained. Fig. 5b shows that the Pd membranes are uniform in thickness, and the thickness of the membranes is approximately 12–14 m, which is in good agreement with the thickness determined from the gravimetric method. 3.4. Hydrogen permeation of Pd/TiO2 /Ti–Al alloy composite membranes
Fig. 2. Pore size distribution ( ) of the supported TiO2 /Ti–Al alloy composite membrane, sintered at 1323 K for 3 h, ( ), the average pore size.
3.3. Characterization of Pd/TiO2 /Ti–Al alloy composite membranes The average thickness of the deposited Pd layer was approximately 14 m as measured by the gravimetric method (the weight gain of the sample divided by the product of the plated surface area and the metal density).
Single gas permeation is a simple but effective method to measure the permeation properties of Pd-based membranes. After activation under a hydrogen atmosphere for 10 h, no nitrogen leakage was detected at a pressure difference of 0.3 MPa, which corresponds to a nitrogen permeance smaller than 10−10 mol m−2 s−1 Pa−1 based on the permeation equipment limit. The hydrogen permeation flux at different operating temperatures under various pressure differences was measured while maintaining the atmospheric pressure on the permeate side (Fig. 6). The hydrogen permeation flux increases with an increasing pressure difference (Ph 0.5 − Pl 0.5 ). The operating temperature also has a positive effect on the permeation flux. These trends can be attributed to the solution-diffusion mechanism. In addition, according to Fig. 6,
Fig. 3. BSE images of the composite membranes after treatment at 973 K in pure hydrogen for 40 h.
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Fig. 4. Cross-sectional line scans of the composite membrane after treatment at 973 K in pure hydrogen for 40 h.
the n value was approximately 0.5 under all experimental conditions, indicating that hydrogen permeation was mainly governed by the bulk-diffusion process. A comparison of the Pd membrane thickness and permeance performance for this study and the studies of other researchers is shown in Table 1. It can be concluded that hydrogen permeance ranges on the order of 10−3 to 10−4 mol m−2 s−1 Pa−0.5 . In this work, the value was on the order of 10−3 mol m−2 s−1 Pa−0.5 .
Experimental data of the hydrogen permeance at different temperatures were used to estimate the activation energy for hydrogen permeation through the membrane. The relationship between temperature and gas permeance can be expressed by an Arrhenius equation as F=
Q0 L
exp
−Ea RT
Fig. 5. SEM photograph of the Pd/TiO2 /Ti–Al alloy composite membrane.
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Table 1 Comparison of the membrane prepared in this study and similar studies in the literature. Membrane
Pore size of support (m)
Thickness (m)
Temperature (K)
n
F (mol m−2 s−1 Pa−0.5 )
Separation factor
Reference
PSS/CeO2 /Pd PSS/oxide/Pd PSS/WO3 /Pd PSS/NaAZ80/Pd PSS/Al2 O3 /Pd Ti–Al/TiO2 /Pd
0.2 0.2 0.2 0.2 0.2 0.28
13 21.3 12 19 5 14
773 723 773 723 723 773
0.5 0.5 0.5 0.5 0.5 0.5
1.27 × 10−3 1.43 × 10−4 2 × 10−3 1.1 × 10−3 2.48 × 10−3 1.07 × 10−3
∞ ∞ 10,000 608 ∞ ∞
[21] [22] [23] [24] [7] Our work
Fig. 6. The dependence of hydrogen fluxes on Ph 0.5 − Pl 0.5 at various temperatures. Fig. 7. Arrhenius relation between the hydrogen permeance and temperature.
The Arrhenius relation between the hydrogen permeance and the temperature is shown in Fig. 7. The average activation energy for hydrogen permeation of the resulting Pd composite membranes was calculated to be 13.65 kJ/mol in the temperature range of 623–773 K, which is consistent with the value presented in the literature [25]. 3.5. Hydrogen permeation stability Pd membranes should demonstrate long-term stability if they are to be viable for industrial applications. To examine the influence of temperature fluctuations on hydrogen permeation, temperature cycling was performed for the Pd/TiO2 /Ti–Al composite membranes. Fig. 8 shows the hydrogen flux data of the Pd/TiO2 /Ti–Al composite membranes operating in three heat cycles of 623 K → 673 K → 723 K → 773 K (at P = 0.1 MPa). Under the
same conditions, the hydrogen flux showed no observable changes during the thermal cycles from 623 K to 773 K, which indicates that the Pd composite membranes have good thermal stability. After the stability tests, the Pd composite membranes were checked for pinholes and cracks by means of nitrogen permeation, and no leakage was observed. 4. Conclusions Pinhole-free Pd/TiO2 /Ti–Al alloy composite membranes on porous Ti–Al alloy discs were fabricated by electroless plating. The TiO2 membrane was applied as a barrier layer between the Pd layer and Ti–Al alloy support to prevent intermetallic diffusion. The resistance to this diffusion was characterized by EPMA and EDX
Fig. 8. Variation of the hydrogen flux during thermal cycling (P = 0.1 MPa).
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cross-sectional line scans. The results showed that the intermediate layer was effective as a diffusion barrier for Pd membranes on Ti–Al alloy supports after 40 h of treatment at 973 K in pure hydrogen. The thickness of the resulting Pd composite membrane was approximately 14 m. The hydrogen permeance through the Pd composite membrane was 1.07 × 10−3 mol m−2 s−1 Pa−0.5 at 773 K. No detectable nitrogen flux at a pressure difference up to 0.3 MPa indicated that the Pd composite membrane had high H2 /N2 selectivity in the experimental temperature range. The activation energy for hydrogen permeation was 13.65 kJ/mol in the temperature range of 623–773 K. Under the same conditions, the hydrogen flux showed no observable changes during thermal cycling, which indicates that the Pd/TiO2 /Ti–Al alloy composite membranes had good thermal stability. Acknowledgements This work was supported by the National Basic Research Program of China (No. 2009CB623400), the National Nature Science Foundation of China (No. 20636020) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References [1] H.-J. Neef, International overview of hydrogen and fuel cell research, Energy 34 (2009) 327–333. [2] R. Dittmeyer, J. Caro, Catalytic membrane reactors, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, 2nd ed., Wiley-VCH, Weinheim, 2008, pp. 2198–2248. [3] J. Zhang, H.Y. Xu, W.Z. Li, High-purity COx -free H2 generation from NH3 via the ultra permeable and highly selective Pd membranes, J. Membr. Sci. 277 (2006) 85–93. [4] Y. Huang, R. Dittmeyer, Preparation and characterization of composite palladium membranes on sinter-metal supports with a ceramic barrier against intermetallic diffusion, J. Membr. Sci. 282 (2006) 296–310. [5] T.L. Ward, T. Dao, Model of hydrogen permeation behavior in palladium membranes, J. Membr. Sci. 153 (1999) 211–231. [6] X. Li, Y.Q. Fan, W.Q. Jin, Y. Huang, N.P. Xu, Improved photocatalytic deposition of palladium membranes, J. Membr. Sci. 282 (2006) 1–6. [7] A.W. Li, J.R. Grace, C.J. Lima, Preparation of thin Pd-based composite membrane on planar metallic substrate: Part II. Preparation of membranes by electroless plating and characterization, J. Membr. Sci. 306 (2007) 159–165.
29
[8] Y.H. He, Y. Jiang, N.P. Xu, J. Zou, B.Y. Huang, C.T. Liu, P.K. Liaw, Fabrication of Ti–Al micro/nanometer-sized porous alloys through the kirkendall effect, Adv. Mater. 19 (2007) 2102–2106. [9] H.R. Gong, Y.H. He, B.Y. Huang, Bond strength and interface energy between Pd membranes and Ti–Al supports, Appl. Phys. Lett. 93 (2008) 101907. [10] B.Y. Huang, Y.H. He, J.N. Wang, Improvement in mechanical and oxidation properties of TiAl alloys with Sb additions, Intermetallics 7 (1999) 881–888. [11] K. Zhang, Z.W. Li, W. Gao, Hot corrosion behaviour of Ti–Al based intermetallics, Mater. Lett. 57 (2002) 834–843. [12] M.E. Ayturk, I.P. Mardilovich, E.E. Engwall, Y.H. Ma, Synthesis of composite Pdporous stainless steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier, J. Membr. Sci. 285 (2006) 385–394. [13] D.Q. Zhang, Y.Q. Fan, N.P. Xu, Y.H. He, Effect of intermetallic diffusion between Pd and Ti–Al alloy on the performance of Pd/Ti–Al alloy composite membranes, J. Membr. Sci. 377 (2011) 221–230. [14] D.J. Edlund, J. McCarthy, The relationship between intermetallic diffusion and flux decline in composite-metal membranes: implication for achieving long membrane lifetime, J. Membr. Sci. 107 (1995) 147–153. [15] S.-E. Nam, K.-H. Lee, Hydrogen separation by Pd alloy composite membranes: introduction of diffusion barrier, J. Membr. Sci. 192 (2001) 177–185. [16] Y.H. Ma, B.C. Akis, M.E. Ayturk, F. Guazzone, E.E. Engwall, I.P. Mardilovich, Characterization of intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes, Ind. Eng. Chem. Res. 43 (2004) 2936–2945. [17] D. Yepes, L.M. Cornaglia, S. Irusta, E.A. Lombardo, Different oxides used as diffusion barriers in composite hydrogen permeable membranes, J. Membr. Sci. 274 (2006) 92–101. [18] K. Zhang, H.Y. Gao, Z.B. Rui, P. Liu, Y.D. Li, Y.S. Lin, High-temperature stability of palladium membranes on porous metal supports with different intermediate layers, Ind. Eng. Chem. Res. 48 (2009) 1880–1886. [19] S.Y. Zhou, Y.Q. Fan, Y.H. He, N.P. Xu, Preparation of titania microfiltration membranes supported on porous Ti–Al alloys, J. Membr. Sci. 325 (2008) 546–552. [20] A. Konkol, G.R. Booker, P.R. Wilshaw, Backscattered electron contrast on cross sections of interfaces and multilayers in the scanning electron microscope, Ultramicroscopy 58 (1995) 233–237. [21] J.H. Tong, Y. Matsumura, H. Suda, K. Haraya, Thin and dense Pd/CeO2 /MPSS composite membrane for hydrogen separation and steam reforming of methane, Sep. Purif. Technol. 46 (2005) 1–10. [22] K.S. Rothenberger, A.V. Cugini, B.H. Howard, R.P. Killmeyer, M.V. Ciocco, B.D. Morreale, R.M. Enick, F. Bustamante, I.P. Mardilovich, Y.H. Ma, High pressure hydrogen permeance of porous stainless steel coated with a thin palladium film via electroless plating, J. Membr. Sci. 244 (2004) 55–68. [23] M. Zahedi, B. Afra, M. Dehghani-Mobarake, M. Bahmani, Preparation of a Pd membrane on a WO3 modified porous stainless steel for hydrogen separation, J. Membr. Sci. 333 (2009) 45–49. [24] M.L. Bosko, F. Ojeda, E.A. Lombardo, L.M. Cornaglia, NaA zeolite as an effective diffusion barrier in composite Pd/PSS membranes, J. Membr. Sci. 331 (2009) 57–65. [25] Y. Huang, X. Li, Y.Q. Fan, N.P. Xu, Progress in palladium-based composite membranes: principle, preparation and characterization, Prog. Chem. 18 (2006) 230–238.