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Modifications of porous stainless steel previous to the synthesis of Pd membranes
10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.
Modifications of porous stainless steel previous to the synthesis of Pd membranes C. Mateos-Pedreroa,*, M. A. Soriaa, I. Rodríguez-Ramosa, A. Guerrero-Ruizb. a
Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie, 2, Cantoblanco, 28049 Madrid b Dpto. Química Inorgánica yTécnica, Facultad de Ciencias, UNED, Senda del Rey, 9. 28040 Madrid
Abstract Two Pd composite membranes were prepared by electroless plating over Porous Stainless Steel (PSS) support. In order to avoid the intermetallic diffusion between the Pd film and the PSS and also to permit the Pd layer to be thinner an intermediate layer was required. With this aim, the surface of PSS was modified by oxidation or coating with different refractory metal oxides. The modified-PSS substrates and the Pd composite membranes were characterized by SEM/EDX, XRD, Hg porosimetry, gravimetric analysis and permeation measurements. The formed Pd film was thinner when oxide coating was used to create the intermediate layer (in particular ZrO2-coating) in comparison with the oxidized-PSS. In addition for ZrO2-coated PSS a lower number of plating cycles were necessary to get a dense Pd membrane. Keywords: palladium membrane, hydrogen separation, surface modification
1. Introduction H2 separation technologies are of great importance due to the role of H2 as an alternative, clean, energy-efficient carrier. Membrane related processes are considered to be one of the most promising routes in the production of high purity H2. Pd membranes are well known for their application in H2 separation and purification due to their high chemical permeability and perfect selectivity to hydrogen [1]. Since the permeation rate through a Pd membrane is often inversely proportional to its thickness [2], thin membranes are always preferable. In addition, thin membranes save expensive Pd. To form a thin continuous Pd membrane without defects, the support surface should be smooth and the external pore size small. PSS are promising substrates due to their good mechanical strength, operation at high pressures, etc [1]. However, the pore size of conventional PSS tubes can be very large, rendering the coverage of all pores very difficult. Moreover, direct deposition of Pd onto PSS would cause intermetallic diffusion at high temperature, decreasing the stability of the membrane. To reduce the surface roughness and the external pore size gradually intermediate layers are necessary prior to Pd deposition. The creation of different intermetallic diffusion barriers: metallic [3] or ceramic [1, 4] has been reported in the past. The present contribution is aimed at preparing sufficiently thin and stable Pd membranes on modified PSS supports. In this sense, two techniques, oxidation or coating, to produce barrier layers against intermetallic diffusion are studied.
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2. Experimental 2.1. Preparation of PSS modified supports In this work, Pd composite membranes were prepared by deposition of Pd onto a porous substrate. Media grade 0.5 μm 316L PSS tubes from Mott Corporation were used as substrates. Two types of oxides were used as intermetallic diffusion barriers: those obtained after heating PSS at different temperatures (metallic oxides by surface oxidation) and those obtained after coating PSS with different oxides (ceramic oxides by coating). All PSS supports were first cleaned ultrasonically (at 60°C) with an alkaline solution (45 g/l NaOH, 65 g/l Na2CO3, 45g/l Na3PO4.12H2O, 5ml/l of an industrial detergent), deionised water and isopropanol and then dried at 120°C overnight. After cleaning, the supports were submitted to oxidation or coating. 2.1.1. Oxidation The supports were oxidized in stagnant air at the desired temperature (600, 700 or 800ºC; with heating ramp rate of 2°C/min) for 12h. Table 1. List of the modified-PSS substrates studied in this work. Sample name OX-600 OX-700 OX-800 SiO2 Al2O3 ZrO2
Modification of PSS support
He permeance* (m3/m2⋅h)
Oxidation at 600°C for 12h Oxidation at 700°C for 12h Oxidation at 800°C for 12h Coating with SiO2 from TEOS Coating with Al2O3 from Al isopropoxide
470.8 372.3 272.1 187.6 457.3 280.3
Coating with ZrO2 from Zr tetrabutoxide
*He flux at 25ºC and ΔP= 1 bar. He flux of the cleaned PSS at 25ºC and ΔP of 1 bar was 587.6 (m3/m2⋅h).
2.1.2. Coating The supports were modified through the deposition of a layer of a ceramic oxide (Al2O3, SiO2 or ZrO2) by the coating technique. The washcoating was performed with: Al-isopropoxide, TEOS and Zr-tetrabutoxide for Al2O3, SiO2 and ZrO2, respectively. The coating solution was prepared by the sol-gel method. The non-porous parts of the PSS support were covered with a Teflon tape and both sides of the PSS tube were sealed with Teflon caps in order to avoid the coating of the inner part of the PSS tube. Then the PSS was placed vertically in a vessel containing the synthesis gel for a few min. After coating, the substrate was calcined in air at 650ºC for 5h (for SiO2 and Al2O3) or 500ºC for 2h (for ZrO2). Table 1 summarizes the modified PSS substrates studied in this work.
2.2. Preparation of Pd dense membranes The Electroless Plating technique (EPD) [5] was used to obtain dense Pd layers on various PSS substrates. From the characterization results obtained for modified PSS supports, two of them were set as substrates for the synthesis of Pd composite membranes by EPD. The first support was obtained after oxidation at 700ºC for 12h and the second one was obtained by coating a thin layer of ZrO2. The PSS substrates were activated prior to Pd deposition. The activation process was performed using sequential dipping in SnCl2-HCl (1 g/l, pH 2) and PdCl2-HCl (0.1 g/l, pH 2) solutions. The plating bath used for EPD contained a solution of Pd(NH3)4Cl2·H2O (4 g/l), NH4OH (28%, 198 ml/l), Na2EDTA.2H2O (40 g/l) and N2H4 (1M, 5.6 ml/l). The nonporous stainless steel parts were covered with a Teflon tape, and then the PSS substrate
Modifications of porous stainless steel previous to the synthesis of Pd membranes
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was immersed in the plating bath for 90 min in an ultrasonic bath at 50ºC. The activation-plating procedure was repeated until the composite Pd membrane became dense (He permeance <10-4 m3/m2⋅h⋅bar; 25ºC; ΔP= 1 bar). Once the plating was finished the membrane was thoroughly rinsed with deionised water and dried overnight at 120ºC. Table 2 summarizes the Pd composite membranes prepared for this study. Table 2. List of the Pd composite membranes studied in this work. Sample name
He permeance* (m3/m2⋅h)
Modification of PSS support
Pd thickness Gravimetric (μm)
MB-003
dense
Coating with ZrO2
18
MB-004
dense
Oxidation-700°C; 12h
20
*He flux measured at 25ºC and ΔP= 1 bar
2.3. Characterization The samples were characterized by SEM/EDX, XRD, Hg porosimetry, gravimetric analysis and He permeation measurements.
3. Results and discussion For the modified PSS carriers, He permeance decreased after oxidation or coating. For the oxidized PSS this decrease is more marked at higher temperature. In the case of the coated PSS, the permeation depends on the kind of oxide, decreasing in the order: γAl2O3> ZrO2>> SiO2. It is observed that, in general, coating leads to a larger reduction of the He permeance (Table 1), which is in good agreement with results reported by other authors [1, 4] For the Pd composite membranes, the He flux gradually decreases while increasing the amount of plated Pd, as expected. Moreover a lower number of plating steps was necessary in the case of the ZrO2-coated membrane in order to get a dense Pd membrane. This indicates that the coating with ZrO2 leads to an increase in the plating effectiveness. This information has not been published before to the best of our knowledge. Hg porosimetry measurements for the coated PSS substrate shown the maximum pore size (external porosity) greatly decreases after coating whereas the mean pore size remains unchanged. It is important to mention that, the decrease in the external porosity follows the same trend as permeation. For the oxidized PSS substrates the maximum and mean pore size tends to decrease while increasing the calcination temperature, which matches previous studies [4]. After Pd deposition, a dramatic decrease in porosity was observed but both Pd composite membranes (MB-003 and MB-004) present similar values. The sample oxidized at 600ºC shows the same XRD pattern as for the original PSS and only the main peaks of PSS (43.8º, 51.0º and 74.9º) are observed. However, for the samples oxidized at higher temperatures together with the PSS lines additional features are observable. These new reflection lines, even though they are difficult to be assigned (little is said in literature about the application of XRD to the characterization of PSS and modified PSS substrates), could be attributed to oxides of the other elements of the stainless steel (Fe, Ni, Cr, etc.). In the case of the coated samples, no significant differences are observed with respect to the original PSS. The XRD patterns of these samples show the characteristic lines of PSS. For the MB-004 and MB-003 membranes only the reflection features of metallic Pd (40.1º, 46.7º, 68.1° and 82.1°) are observed, and no peaks from the modified PSS support were detected.
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The surface and cross-sectional morphological characteristics of the modified PSS and Pd composite membranes were examined by SEM (Fig 1). The SEM picture of the original PSS is also shown for comparison. It appears that the surface becomes smoother after oxidation or coating. This effect is even more marked in the case of the coated samples (Fig. 1), which indicates that the bigger pores are being closed and the surface becomes smoother after coating, whatever the oxide. For these samples, it is clear that the surface morphology and roughness change with the kind of oxide. For example the coating with SiO2 allows to obtain the higher surface coverage, and although some cracks are appreciable in the micrograph of this sample (Fig.1 G). The composition on the outer surface of the various samples was checked by means of EDX. For the oxidized samples the amount of Fe on the surface decreases as the calcination temperature increases from 600 to 700ºC. On the contrary, for the substrate calcined at 800ºC the Fe/Cr at. ratio (EDX) increased dramatically indicating the formation of an Fe-rich oxide layer (it is about 6 μm thick) on the outermost layer. This is in line with studies reported by other authors [4]. The major component on the external surface of the samples after coating was the corresponding oxide; anyway Fe and Cr were still observable, which is likely due to the partial coverage of PSS or to the thinness of the deposited oxide layer. The SEM image of the cross section of the fresh membrane (MB003) is shown in Fig. 1. The mean thickness of the Pd layer is around 17 and 20 µm for MB003 and MB004 respectively. This is consistent with the layer thickness assessed by gravimetric analysis for both membranes (Table 2), and indicates that slightly thinner Pd layer is formed for the ZrO2-coated PSS than for the oxidized one. It is also observed that a dense Pd film is present without the appearance of cracks. Leak test confirmed the absence of detectable defects in both membranes.
A 100 μm E 100 μm
B 100 μm F 100 μm
C 100 μm G 100 μm
D 100 μm H Pd layer B C PSS substrate 100 μm
Fig. 1. SEM images of: (A) PSS, (B) oxidation at 600ºC, (C) oxid. at 700ºC, (D) oxid. at 800ºC, (E) γ-Al2O3 coating, (F) ZrO2 coating, (G) SiO2 coating and (H) cross section after Pd plating.
4. Conclusions The modification of PSS by coating results in a better substrate for Pd deposition by EPD. So the ZrO2-coated PSS allows a more effective plating process as well as the deposition of thinner Pd layers. In order to achieve more stable and thinner Pd membranes, further research will be focused on the synthesis conditions to be used for ZrO2 coating.
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Acknowledgments The authors wish to acknowledge the financial support received from MICINN (CTQ 2008-03068-E/PPQ). This work has been realized in the frame of the ACENET project (ACE.07.012) “Methane activation as a route to CO2 remediation: the integration of dry reforming into Fischer-Tropsch fuel production plants”
References [1] M. L. Bosko, F. Ojeda, E. A. Lombardo, L. M. Cornaglia, 2009, NaA zeolite as an effective barrier in composite Pd/PSS membranes, J. Membr. Sci., 331, 57. [2] T. L. Ward, T. Dao, 1999, Model of hydrogen permeation behavior in palladium membranes, J. Membr. Sci., 153, 211. [3] S. Nam, K. Lee, 2001, Hydrogen separation by Pd alloy composite membranes: Introduction of diffusion barrier, J. Membr. Sci., 192, 177. [4] Y.H. Ma, B. Ceylan Akis, M. Engin Ayturk, F. Guazzone, E.E. Engwall, I.P. Mardilovich, 2004, Characterization of Intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes, Ind. Eng. Chem. Res., 43, 2936. [5] US Patent 6152987, (28 November, 2000).
Modifications of porous stainless steel previous to the synthesis of Pd membranes C. Mateos-Pedreroa,*, M. A. Soriaa, I. Rodríguez-Ramosa, A. Guerrero-Ruizb. a
Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie, 2, Cantoblanco, 28049 Madrid b Dpto. Química Inorgánica yTécnica, Facultad de Ciencias, UNED, Senda del Rey, 9. 28040 Madrid
Abstract Two Pd composite membranes were prepared by electroless plating over Porous Stainless Steel (PSS) support. In order to avoid the intermetallic diffusion between the Pd film and the PSS and also to permit the Pd layer to be thinner an intermediate layer was required. With this aim, the surface of PSS was modified by oxidation or coating with different refractory metal oxides. The modified-PSS substrates and the Pd composite membranes were characterized by SEM/EDX, XRD, Hg porosimetry, gravimetric analysis and permeation measurements. The formed Pd film was thinner when oxide coating was used to create the intermediate layer (in particular ZrO2-coating) in comparison with the oxidized-PSS. In addition for ZrO2-coated PSS a lower number of plating cycles were necessary to get a dense Pd membrane. Keywords: palladium membrane, hydrogen separation, surface modification
1. Introduction H2 separation technologies are of great importance due to the role of H2 as an alternative, clean, energy-efficient carrier. Membrane related processes are considered to be one of the most promising routes in the production of high purity H2. Pd membranes are well known for their application in H2 separation and purification due to their high chemical permeability and perfect selectivity to hydrogen [1]. Since the permeation rate through a Pd membrane is often inversely proportional to its thickness [2], thin membranes are always preferable. In addition, thin membranes save expensive Pd. To form a thin continuous Pd membrane without defects, the support surface should be smooth and the external pore size small. PSS are promising substrates due to their good mechanical strength, operation at high pressures, etc [1]. However, the pore size of conventional PSS tubes can be very large, rendering the coverage of all pores very difficult. Moreover, direct deposition of Pd onto PSS would cause intermetallic diffusion at high temperature, decreasing the stability of the membrane. To reduce the surface roughness and the external pore size gradually intermediate layers are necessary prior to Pd deposition. The creation of different intermetallic diffusion barriers: metallic [3] or ceramic [1, 4] has been reported in the past. The present contribution is aimed at preparing sufficiently thin and stable Pd membranes on modified PSS supports. In this sense, two techniques, oxidation or coating, to produce barrier layers against intermetallic diffusion are studied.
C. Mateos-Pedrero et al.
780
2. Experimental 2.1. Preparation of PSS modified supports In this work, Pd composite membranes were prepared by deposition of Pd onto a porous substrate. Media grade 0.5 μm 316L PSS tubes from Mott Corporation were used as substrates. Two types of oxides were used as intermetallic diffusion barriers: those obtained after heating PSS at different temperatures (metallic oxides by surface oxidation) and those obtained after coating PSS with different oxides (ceramic oxides by coating). All PSS supports were first cleaned ultrasonically (at 60°C) with an alkaline solution (45 g/l NaOH, 65 g/l Na2CO3, 45g/l Na3PO4.12H2O, 5ml/l of an industrial detergent), deionised water and isopropanol and then dried at 120°C overnight. After cleaning, the supports were submitted to oxidation or coating. 2.1.1. Oxidation The supports were oxidized in stagnant air at the desired temperature (600, 700 or 800ºC; with heating ramp rate of 2°C/min) for 12h. Table 1. List of the modified-PSS substrates studied in this work. Sample name OX-600 OX-700 OX-800 SiO2 Al2O3 ZrO2
Modification of PSS support
He permeance* (m3/m2⋅h)
Oxidation at 600°C for 12h Oxidation at 700°C for 12h Oxidation at 800°C for 12h Coating with SiO2 from TEOS Coating with Al2O3 from Al isopropoxide
470.8 372.3 272.1 187.6 457.3 280.3
Coating with ZrO2 from Zr tetrabutoxide
*He flux at 25ºC and ΔP= 1 bar. He flux of the cleaned PSS at 25ºC and ΔP of 1 bar was 587.6 (m3/m2⋅h).
2.1.2. Coating The supports were modified through the deposition of a layer of a ceramic oxide (Al2O3, SiO2 or ZrO2) by the coating technique. The washcoating was performed with: Al-isopropoxide, TEOS and Zr-tetrabutoxide for Al2O3, SiO2 and ZrO2, respectively. The coating solution was prepared by the sol-gel method. The non-porous parts of the PSS support were covered with a Teflon tape and both sides of the PSS tube were sealed with Teflon caps in order to avoid the coating of the inner part of the PSS tube. Then the PSS was placed vertically in a vessel containing the synthesis gel for a few min. After coating, the substrate was calcined in air at 650ºC for 5h (for SiO2 and Al2O3) or 500ºC for 2h (for ZrO2). Table 1 summarizes the modified PSS substrates studied in this work.
2.2. Preparation of Pd dense membranes The Electroless Plating technique (EPD) [5] was used to obtain dense Pd layers on various PSS substrates. From the characterization results obtained for modified PSS supports, two of them were set as substrates for the synthesis of Pd composite membranes by EPD. The first support was obtained after oxidation at 700ºC for 12h and the second one was obtained by coating a thin layer of ZrO2. The PSS substrates were activated prior to Pd deposition. The activation process was performed using sequential dipping in SnCl2-HCl (1 g/l, pH 2) and PdCl2-HCl (0.1 g/l, pH 2) solutions. The plating bath used for EPD contained a solution of Pd(NH3)4Cl2·H2O (4 g/l), NH4OH (28%, 198 ml/l), Na2EDTA.2H2O (40 g/l) and N2H4 (1M, 5.6 ml/l). The nonporous stainless steel parts were covered with a Teflon tape, and then the PSS substrate
Modifications of porous stainless steel previous to the synthesis of Pd membranes
781
was immersed in the plating bath for 90 min in an ultrasonic bath at 50ºC. The activation-plating procedure was repeated until the composite Pd membrane became dense (He permeance <10-4 m3/m2⋅h⋅bar; 25ºC; ΔP= 1 bar). Once the plating was finished the membrane was thoroughly rinsed with deionised water and dried overnight at 120ºC. Table 2 summarizes the Pd composite membranes prepared for this study. Table 2. List of the Pd composite membranes studied in this work. Sample name
He permeance* (m3/m2⋅h)
Modification of PSS support
Pd thickness Gravimetric (μm)
MB-003
dense
Coating with ZrO2
18
MB-004
dense
Oxidation-700°C; 12h
20
*He flux measured at 25ºC and ΔP= 1 bar
2.3. Characterization The samples were characterized by SEM/EDX, XRD, Hg porosimetry, gravimetric analysis and He permeation measurements.
3. Results and discussion For the modified PSS carriers, He permeance decreased after oxidation or coating. For the oxidized PSS this decrease is more marked at higher temperature. In the case of the coated PSS, the permeation depends on the kind of oxide, decreasing in the order: γAl2O3> ZrO2>> SiO2. It is observed that, in general, coating leads to a larger reduction of the He permeance (Table 1), which is in good agreement with results reported by other authors [1, 4] For the Pd composite membranes, the He flux gradually decreases while increasing the amount of plated Pd, as expected. Moreover a lower number of plating steps was necessary in the case of the ZrO2-coated membrane in order to get a dense Pd membrane. This indicates that the coating with ZrO2 leads to an increase in the plating effectiveness. This information has not been published before to the best of our knowledge. Hg porosimetry measurements for the coated PSS substrate shown the maximum pore size (external porosity) greatly decreases after coating whereas the mean pore size remains unchanged. It is important to mention that, the decrease in the external porosity follows the same trend as permeation. For the oxidized PSS substrates the maximum and mean pore size tends to decrease while increasing the calcination temperature, which matches previous studies [4]. After Pd deposition, a dramatic decrease in porosity was observed but both Pd composite membranes (MB-003 and MB-004) present similar values. The sample oxidized at 600ºC shows the same XRD pattern as for the original PSS and only the main peaks of PSS (43.8º, 51.0º and 74.9º) are observed. However, for the samples oxidized at higher temperatures together with the PSS lines additional features are observable. These new reflection lines, even though they are difficult to be assigned (little is said in literature about the application of XRD to the characterization of PSS and modified PSS substrates), could be attributed to oxides of the other elements of the stainless steel (Fe, Ni, Cr, etc.). In the case of the coated samples, no significant differences are observed with respect to the original PSS. The XRD patterns of these samples show the characteristic lines of PSS. For the MB-004 and MB-003 membranes only the reflection features of metallic Pd (40.1º, 46.7º, 68.1° and 82.1°) are observed, and no peaks from the modified PSS support were detected.
782
C. Mateos-Pedrero et al.
The surface and cross-sectional morphological characteristics of the modified PSS and Pd composite membranes were examined by SEM (Fig 1). The SEM picture of the original PSS is also shown for comparison. It appears that the surface becomes smoother after oxidation or coating. This effect is even more marked in the case of the coated samples (Fig. 1), which indicates that the bigger pores are being closed and the surface becomes smoother after coating, whatever the oxide. For these samples, it is clear that the surface morphology and roughness change with the kind of oxide. For example the coating with SiO2 allows to obtain the higher surface coverage, and although some cracks are appreciable in the micrograph of this sample (Fig.1 G). The composition on the outer surface of the various samples was checked by means of EDX. For the oxidized samples the amount of Fe on the surface decreases as the calcination temperature increases from 600 to 700ºC. On the contrary, for the substrate calcined at 800ºC the Fe/Cr at. ratio (EDX) increased dramatically indicating the formation of an Fe-rich oxide layer (it is about 6 μm thick) on the outermost layer. This is in line with studies reported by other authors [4]. The major component on the external surface of the samples after coating was the corresponding oxide; anyway Fe and Cr were still observable, which is likely due to the partial coverage of PSS or to the thinness of the deposited oxide layer. The SEM image of the cross section of the fresh membrane (MB003) is shown in Fig. 1. The mean thickness of the Pd layer is around 17 and 20 µm for MB003 and MB004 respectively. This is consistent with the layer thickness assessed by gravimetric analysis for both membranes (Table 2), and indicates that slightly thinner Pd layer is formed for the ZrO2-coated PSS than for the oxidized one. It is also observed that a dense Pd film is present without the appearance of cracks. Leak test confirmed the absence of detectable defects in both membranes.
A 100 μm E 100 μm
B 100 μm F 100 μm
C 100 μm G 100 μm
D 100 μm H Pd layer B C PSS substrate 100 μm
Fig. 1. SEM images of: (A) PSS, (B) oxidation at 600ºC, (C) oxid. at 700ºC, (D) oxid. at 800ºC, (E) γ-Al2O3 coating, (F) ZrO2 coating, (G) SiO2 coating and (H) cross section after Pd plating.
4. Conclusions The modification of PSS by coating results in a better substrate for Pd deposition by EPD. So the ZrO2-coated PSS allows a more effective plating process as well as the deposition of thinner Pd layers. In order to achieve more stable and thinner Pd membranes, further research will be focused on the synthesis conditions to be used for ZrO2 coating.
Modifications of porous stainless steel previous to the synthesis of Pd membranes
783
Acknowledgments The authors wish to acknowledge the financial support received from MICINN (CTQ 2008-03068-E/PPQ). This work has been realized in the frame of the ACENET project (ACE.07.012) “Methane activation as a route to CO2 remediation: the integration of dry reforming into Fischer-Tropsch fuel production plants”
References [1] M. L. Bosko, F. Ojeda, E. A. Lombardo, L. M. Cornaglia, 2009, NaA zeolite as an effective barrier in composite Pd/PSS membranes, J. Membr. Sci., 331, 57. [2] T. L. Ward, T. Dao, 1999, Model of hydrogen permeation behavior in palladium membranes, J. Membr. Sci., 153, 211. [3] S. Nam, K. Lee, 2001, Hydrogen separation by Pd alloy composite membranes: Introduction of diffusion barrier, J. Membr. Sci., 192, 177. [4] Y.H. Ma, B. Ceylan Akis, M. Engin Ayturk, F. Guazzone, E.E. Engwall, I.P. Mardilovich, 2004, Characterization of Intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes, Ind. Eng. Chem. Res., 43, 2936. [5] US Patent 6152987, (28 November, 2000).