- Email: [email protected]
Synthesis and characterization of Pd membranes on alumina-modified porous stainless steel supports
Desalination 245 (2009) 508–515
Synthesis and characterization of Pd membranes on alumina-modified porous stainless steel supports M. Brogliaa, P. Pinaccia,*, M. Radaellia, A. Bottinob, G. Capannellib, A. Comiteb, G. Vanacorec, M. Zanic a
CESI RICERCA,Via Rubattino 39, 20134 Milano, Italy, Tel. +39 02 39925865; email: [email protected] b Dipartimento di Chimica e Chimica Industriale, Università di Genova, Italy c Dipartimento di Fisica, Politecnico di Milano, Italy Received 27 June 2008; revised 20 January 2009; accepted 20 January 2009
Abstract Palladium composite membranes have been obtained by deposition of Pd on alumina-modified stainless steel supports. The preparation procedure involves deposition of a double layer of γ-alumina (γ-Al2O3) on the support by dip coating, followed by drying and calcination, activation of the modified support, deposition of palladium by electroless plating. SEM/EDS and Auger spectroscopy analysis, coupled with sputtering allowed the identification of critical issues and a proper re-elaboration of the preparation procedure. It has been found that γ-Al2O3 completely covers the support surface determining a significant reduction of surface rugosity. Activation, however, even if performed inside the γ-Al2O3 pH stability range, determines the partial dissolution of alumina; this phenomenon is enhanced while decreasing pH value. Moreover, Pd seeds, which catalyze subsequent Pd deposition, are not distributed evenly on the surface. As a consequence of the above observations, membrane preparation has been modified by introducing a pre-activation of the γ-Al2O3 external layer, consisting of the addition of palladium chloride to the alumina gel and palladium reduction in sodium boron hydride solution after calcination. Keywords: Calcination; electroless plating; Sputtering; Surface rugosity
1. Background Palladium alloy membranes are widely studied for CO2 capture and hydrogen separation from synthesis gas in membrane reactors. Due to the *Corresponding author.
considerable cost of palladium, efforts have been made to obtain low-thickness deposits on cheaper materials, such as ceramics and macroporous metals, by various techniques, including electroplating [1], sputtering [2], chemical vapor deposition [3], and electroless plating [4,5]. Electroless plating, in particular, is an autocatalytic process which
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi: 10.1016/j.desal.2009.01.004
M. Broglia et al. / Desalination 245 (2009) 508–515
allows to obtain thin films with good adhesion characteristics; besides that, it does not require expensive set-up and is relatively easy to scale-up from laboratory to industrial scale. Porous ceramics substrates such as α-alumina have aroused a great attention since early times because of the relatively smooth surface and narrow pore distribution [6,7]. Ceramic substrates, however, present the problem — still unsolved — of obtaining leak tight seals at the ceramic– metal interface, as well as an intrinsic fragility which, in perspective, makes it difficult to assemble them into a module, affecting their potential industrial application as well [8,9]. Therefore, the research effort has been addressed to porous metals, which have higher robustness and mechanical stability, and can be easily welded and assembled in a module [10]. Commercially available porous metals, however, present high rugosity and large pore dimensions (micron range); this determines high thickness of the Pd deposit (up to 20–30 μm) in order to obtain dense membranes [11]. Moreover, high operating temperatures (up to 500°C) and long exposure times can determine the diffusion of substrate metals (such as iron) into the palladium layer and, subsequently, a significant decrease of hydrogen permeance and selectivity. To avoid this problem, the support should be modified, introducing a suitable barrier layer. Several types of barrier layers have been considered, including mixed oxides grown by oxidation of the support [12], a bi-metallic deposited multilayer [13], ceramic materials such as zirco-
Fig. 1. External profile of the porous AISI 316L support.
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nia oxide [14], zirconia-yttria [15], cerium oxide [16], and alumina [17,18]. Although a barrier layer in the microporous range does allow to obtain low thickness dense palladium deposits, it may cause an increase in mass transfer resistance and, ultimately, a decrease in hydrogen permeation through the membrane. From this point of view, alumina can be very useful, since it has a low water residual volume after dehydration at high temperatures and, therefore, minimizes resistance to gas diffusion through the barrier layer [19]. This paper deals with the preparation and characterization of palladium composite membranes obtained by deposition of Pd by electroless plating on alumina-modified stainless steel supports. More specifically, it focuses on the characterization of the barrier layer and modifications which can occur during the activation phase of the preparation procedure. 2. Experimental Investigated supports are 10 mm O.D. AISI 316L porous tubes, with a nominal pore size of 0.1 μm, supplied by Mott Metallurgical Corporation. Nominal pore size is determined by the manufacturer based on a 95% rejection of particles with size greater than 0.1 μm. The actual pore size, however is much larger — a mean and maximum value of about 2 and 5 μm, respectively, has been determined by mercury intrusion measurements [11].
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Rugosity of the surface is also rather high, as shown in Fig. 1; the maximum peak of the profile is Ry = 31 μm, while the mean rugosity is Rm = 7.2 μm. Due to the morphological and topographic characteristics of the support, a multi-graded barrier has been deposited in order to reduce surface rugosity and porosity. Porous AISI 316L tubes have been cut to obtain 4-cm long samples. Composite membranes have been prepared from these samples according to the following subsequent steps: 1. Cleaning of the porous stainless steel support; 2. Deposition of a double layer of gammaalumina (γ-Al2O3) on the external surface of the support by dip coating, followed by drying and calcination; 3. Activation of the modified support; 4. Deposition of palladium by electroless plating. Cleaning of the support has been performed in an ultrasonic bath with acetone. Afterwards, the support was rinsed with water, diluted hydrochloric acid, deionized water up to a neutral pH, and acetone. The deposition of both γ-alumina layers has been obtained by dip coating of a colloidal dispersion of γ-alumina particles. The first layer has been prepared by a commercial colloidal dispersion (Alfa Aesar) of coarse particles (size 50 nm); polyvinyl alcohol (PVA) has been added to the dispersion (0.2 % by weight), in order to improve the film homogeneity. The second layer has been prepared in laboratory by hydrolysis of a metal-organic alumina precursor, with the addition of the 0.2% PVA. Both coatings were left to dry at room temperature and, successively, in an oven at 75°C; finally, calcination has been performed in static air at 500°C for 6 hours. The activation of the modified support is made by dipping into a stannous chloride solution and
a palladium chloride solution, alternately, for many times. The corresponding reaction is: SnCl2 + PdCl2 → SnCl4 + Pd Activation is usually performed at pH < 1 in order to maintain a good salt solubility [5,9]; in this case, however, pH has been increased up to 4, in order to operate inside the γ-Al2O3 stability range. (3 < pH < 9). Activation is performed at room temperature and the number of dipping cycles depends on the support characteristics. Stannous solution, indeed, enters the pores of stainless steel and reduces palladium ions, thus creating auto-catalytic sites where palladium plating can occur. Pd seeds must be in sufficient number and homogeneously distributed on the surface, in order to allow the subsequent palladium deposition. This step is the most critical, since it influences the subsequent deposition of palladium, dimension of grains, and ultimately film morphology and stability. Deposition of palladium by electroless plating is performed in a solution consisting of palladium chloride, ammonia (to control pH), EDTANa2 (complexing agent) and hydrazine (reducing agent). Deposition occurs according to the following reaction: 2 Pd [ NH 3 ]42 + + N 2 H 4 + 4OH − → 2 Pd 0 + 8 NH 3 + N 2 + 4 H 2O Pd plating is obtained by circulating the solution in a reactor where the membrane is immersed and kept in rotation in order to obtain a homogeneous deposit on the external surface and ease nitrogen evacuation from the reaction zone; temperature is kept constant (45–50°C) by a thermostatic bath. Deposition is performed at pH = 11, e.g. outside the pH stability range of γ-Al2O3. However, dissolution of γ-Al2O3, can be considered negligible since Pd deposition is very fast, provided that activation is performed correctly and palladium seeds are distributed evenly on the surface.
M. Broglia et al. / Desalination 245 (2009) 508–515
Fig. 2. SEM images of the AISI 316L surface: (A top, top view) and (A down, perspective view) as received; (B top, top view) and (B down, perspective view) after γ−Al2O3 deposition.
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Fig. 4. SEM image of the section of a sample after γ−Al2O3 deposition.
Membrane samples from each step of the preparation procedure have been characterized by SEM/EDS analysis. Fig. 2 compares the SEM images of the surface of the porous AISI 316 support, before and after γ-Al2O3 deposition. It can be noted that deposition of the γ-Al2O3 layer determines a strong reduction of the surface rugosity.
The thickness of the alumina layer, however, is not constant; in fact, as shown in Fig. 3, in the clear/bright areas (SP2), usually located in correspondence of surface edges, the alumina is so thin that the elements of the support (Fe, Ni, Cr,) are detected by the EDS probe, while in the dark areas (SP1) only alumina and oxygen can be detected. Fig. 4 shows a detail of the double layer of γAl2O3 deposited on the AISI 316L support; the first layer is 2–3 μm thick, while the external layer is much less than 1μm; moreover, EDS analysis has detected γ-Al2O3 spots in the support porosity up to a depth of 20 μm. Fig. 5 shows the sample covered with γ-Al2O3 before and after activation. Despite the fact that
Fig. 3. SEM image in back scattering modality of the AISI 316L surface after γAl2O3 deposition: points where EDS spectra has been measured are indicated.
Fig. 5. Secondary electron detection (SED) images of the surface of samples covered with γ- Al2O3, non-activated (left) and activated (right).
3. Results and discussion
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Fig. 6. SEM images of a sample covered with Pd, surface (left); section (right).
activation has been performed inside the γ-Al2O3 pH stability range, a strong increase in the rugosity of the activated sample can be observed, due to alumina dissolution. Furthermore, the presence of cracks, usually near surface edges, is evident on non-activated sample. Finally, SEM images of a sample covered with palladium are shown in Fig. 6. It can be observed that, although alumina has been partially dissolved during activation, a continuous, crack-free Pd layer about 11 μm thick, well anchored to the surface, has been obtained. The typical “cauliflower-like” morphology of palladium grain agglomerates can be noticed from the surface image.
Samples covered with γ-Al2O3, before and after activation, have been analyzed further by Auger spectroscopy coupled with sputtering, to check if γ-Al2O3 c overed all the surface, detect if Pd seeds were uniformly distributed on the surface, and, finally identify impurities such as chlorides and tin which could contaminate palladium [20]. Sputtering has been performed by using a 0.5 mm beam of Ar+ ions, at an energy of 4 keV and a current of 1 μA, with a 30-degree angle of incidence. All the measured thickness are in terms of SiO2 equivalent, since alumina reference samples for the calculation of the erosion velocity were not available. The chemical analysis has been performed at different locations on the surface of each sample. Analyses have been performed at different sputtering times, on areas of 100 μm × 100 μm, where the Auger peaks of the elements of interests have been detected. Typical Auger spectra of the two samples are shown in Fig. 7(a) and (b), respectively. Figures show that, on the non-activated sample, after 63.5 min of sputtering, corresponding to an equivalent thickness of SiO2 of about 4 μm, only alumina and oxygen can be detected. On the contrary, on
Fig. 7. Auger spectra measured at different sputtering times on of samples covered with γ-Al2O3, non-activated (left) and activated (right).
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Fig. 8. SEM image of the surface of a sample activated at pH < 1, white circles indicate the zones where EDS analysis have been performed.
Fig. 9. SEM image of a sample after Pd deposition without activation. Rectangles indicate the zones where EDS analysis have been performed.
the activated sample, iron peaks can be detected after the first sputtering cycle (30 sec), corresponding to an equivalent thickness of SiO2 of about 10 nm. A slight carbon superficial contamination, probably arising from EDTA, can also be detected on the surface. Similar Auger spectra have been obtained on the other areas of the samples. It can be assumed, therefore, that γ-Al2O3 completely covers the support surface, even along edges and surface depressions, where deposit thickness is lower; activation, even if performed in mild conditions (pH = 4), determines a strong reduction of this layer, of about 10 nm. This means that activation completely removes the outer mesoporous γ-Al2O3 layer, which is essential to obtain a thin (less than 10 μm), pinholefree palladium layer by electroless plating. Furthermore, chemical analysis performed at different locations on a surface area of 58 μm × 39.5 μm of the sample have not detected the presence of palladium (detection limit 1%). This observation is consistent with the fact that at pH = 4 the reduction reaction of palladium is in very slow progress. On the other hand, if activation is performed at a proper pH value, the γ-Al2O3 layer is almost completely dissolved by the solution, as shown
by SEM micrographs in Fig. 8. In fact, EDS spectra of P1, P2, and P3 areas stressed the fact that alumina is present only in the P2 area and palladium deposited mostly in the P3 area, where alumina is absent. Similarly, if activation is not performed, deposition occurs very slowly and not evenly. Fig. 9 shows the SEM micrograph of a non-activated sample. EDS analyses have been performed on different locations, as indicated on the image. Pd can be detected only in areas 2 and 3, while completely absent in area 1; alumina is present in all the areas, although at different concentrations, thus indicating a partial dissolution due to the long exposure to the deposition bath. The above observations suggest that, in order to obtain a thin adherent double γ-Al2O3 layer, membrane preparation can be re-addressed as follows: – reduce the support roughness, due to a combined mechanical and chemical treatment; – pre-activate the γ-Al2O3 external layer by adding Pd seeds to the alumina gel. Experimental tests are now in progress, and in particular, pre-activation has been performed by adding PdCl2 to the γ-Al2O3 laboratory-made gel; reduction of Pd has been obtained after calcina-
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Fig. 10. SEM image of a Pd deposit on the surface of a pre-activated sample.
inside the pH stability range of γ-Al2O3, determines the complete dissolution of the outer layer and a partial dissolution of the inner layer; this phenomenon is enhanced by decreasing the pH value. Moreover, Pd seeds, which catalyze subsequent Pd deposition, are not well distributed on the surface. As a consequence of the above observations, membrane preparation has been re-addressed as follows: – reduction of the support roughness thanks to a combined mechanical and chemical treatment; – pre-activation of the γ-Al2O3 external layer, consisting in the addition of palladium chloride to the alumina gel and palladium reduction in sodium boron hydride solution after calcination. Aknowledgments
tion by the addition of sodium boron hydride solution. Preliminary results (Fig. 10) show that, following this procedure, a uniform deposit of Pd crystallites of a few hundred nanometers in size can be obtained. 4. Conclusions Palladium composite membranes have been obtained by deposition of Pd on alumina modified stainless steel supports. The preparation procedure involves cleaning of the macroporous stainless steel support, deposition of a double layer of γ-alumina (γ-Al2O3) on the external surface of the support by dip coating, followed by drying and calcination, activation of the modified support, and deposition of palladium by electroless plating. SEM/EDS and Auger spectroscopy analysis allowed the identification of critical issues and a proper re-elaboration of preparation procedure. It has been found that γ-Al2O3 determines a strong reduction of the surface rugosity and completely covers the support surface, even along edges and surface depressions, where deposit thickness is lower. The activation step, although performed
This work has been financed by the Research Fund for the Italian Electrical System under the Contract Agreement between CESI RICERCA and the Ministry of Economic Development — General Directorate for Energy and Mining Resources, stipulated on June 21, 2007 in compliance with Law Decree no. 73 of June 18th, 2007. References [1] S.E. Nam, K.H. Lee, Hydrogen separation by Pd alloy composite membranes: introduction of diffusion barrier, J. Membr. Sci., 192 (2001) 177. [2] H.B. Zhao, G.X. Xiong, G.V. Baron, Preparation and characterization of palladium-based composite membranes by electroless plating and magnetron sputtering, Catal. Today, 56 (2000) 89. [3] N. Itoh, T. Akiha, T. Sato, Preparation of thin palladium composite membrane tube by a CVD technique and its hydrogen permselectivity, Catal. Today, 104 (2005) 231. [4] A. Li, W. Liang, R. Hughes, Fabrication of dense palladium composite membranes for hydrogen separation, Catal. Today, 56 (2000) 45. [5] Y.S. Cheng, K.L. Yeung, Effects of electroless plating chemistry on the synthesis of palladium membranes, J. Membr. Sci., 182 (2001) 195.
M. Broglia et al. / Desalination 245 (2009) 508–515 [6] S. Uemiya, T. Matsuda, E. Kikuchi, Hydrogen permeable palladium-silver alloy membrane supported on porous ceramics, J. Membr. Sci., 56 (1991) 315. [7] S. Uemiya, State-of-the-Art of Supported Metal Membranes for Gas Separation, Sep. and Pur. Meth., 28 (1999) 51. [8] A. Bottino, G. Capannelli, A. Comite, R. Di Felice, M. Broglia, P. Pinacci, Pd and Pd/Ag composite membranes prepared by the electroless plating technique: preparation, characterization and interpretation of the H2 transport mechanism, Proc. of ICIM 9, Lillehammer (NO), June 25–29, 2006. [9] J.V. Dijkstra, Y.C. van Delft, D. Jansen, P. Pex, Development of a hydrogen membrane reactor for power production with pre-combustion decarbonisation, Proc. of GHGT 8 Trondheim (No), June 18– 23, 2006. [10] Y.H. Ma, Pd-Based Hydrogen Separation Membranes – Status and Prospective, Proc. of ICIM 9, Lillehammer (No), June 25–29, 2006. [11] I.P. Mardilovich, E. Engwall, Y. H. Ma, Dependence of hydrogen flux on the pore size and plating surface topology of asymmetric Pd-porous stainless steel membranes, Desalination, 144 (2002) 85. [12] Y.H. Ma, B. Akis, M. Engin Ayturk, F. Guazzone, 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. [13] M. Engin Ayturk, I.P. Mardilovich, E. Engwall, Y. H. Ma, Synthesis of composite Pd-porous stainless
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steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier, J. Membr. Sci., 285 (2006) 385. H. Gao, Y. Li, J. Y.S. Lin, B. Zhang, Characterization of zirconia modified porous stainless steel supports for Pd membranes, J. Porous Mater., 13 (2006) 419. R.P. Singh, T. Powers, K. Rekciz, S. Opkins, C. Love, Supports for high temperature gas separation membranes, Proc. of ICIM9, Lillehammer (No), June 25–29, 2006. J. Tong, C. Su, K. Kuraoka, H. Suda, Y. Matsumura, Preparation of thin Pd membrane on CeO2-modified porous metal by a combined method of electroless plating and chemical vapor deposition, J. Membr. Sci., 269 (2006) 101. Y.H. Ma, F. Guazzone, Composite gas separation modules having a layer of particles with a uniform binder distribution, US patent application, PA US2006/0016332. D.A. Pacheco Tanaka, M.A. Llosa Tanco, S. Niwa, Y. Wakui, F. Mizukami, T. Namba, T.M. Suzuki, Preparation of palladium and silver alloy membrane on a porous α-alumina tube via simultaneous electroless plating, J. Membr. Sci., 247 (2005) 21. J.Tong, H. Suda, K. Haraya, Y. Matsumura, A novel method for the preparation of thin dense Pd membrane on macroporous stainless steel tube filter, J. Membr. Sci., 260 (2005) 10. J.K. Ali, P. Hasler, E.J. Newson, D.W.T. Trippin, Irreversible poisoning of Pd-Ag membranes, Int. J. Hydrogen Energy, 19 (1994) 877.
Synthesis and characterization of Pd membranes on alumina-modified porous stainless steel supports M. Brogliaa, P. Pinaccia,*, M. Radaellia, A. Bottinob, G. Capannellib, A. Comiteb, G. Vanacorec, M. Zanic a
CESI RICERCA,Via Rubattino 39, 20134 Milano, Italy, Tel. +39 02 39925865; email: [email protected] b Dipartimento di Chimica e Chimica Industriale, Università di Genova, Italy c Dipartimento di Fisica, Politecnico di Milano, Italy Received 27 June 2008; revised 20 January 2009; accepted 20 January 2009
Abstract Palladium composite membranes have been obtained by deposition of Pd on alumina-modified stainless steel supports. The preparation procedure involves deposition of a double layer of γ-alumina (γ-Al2O3) on the support by dip coating, followed by drying and calcination, activation of the modified support, deposition of palladium by electroless plating. SEM/EDS and Auger spectroscopy analysis, coupled with sputtering allowed the identification of critical issues and a proper re-elaboration of the preparation procedure. It has been found that γ-Al2O3 completely covers the support surface determining a significant reduction of surface rugosity. Activation, however, even if performed inside the γ-Al2O3 pH stability range, determines the partial dissolution of alumina; this phenomenon is enhanced while decreasing pH value. Moreover, Pd seeds, which catalyze subsequent Pd deposition, are not distributed evenly on the surface. As a consequence of the above observations, membrane preparation has been modified by introducing a pre-activation of the γ-Al2O3 external layer, consisting of the addition of palladium chloride to the alumina gel and palladium reduction in sodium boron hydride solution after calcination. Keywords: Calcination; electroless plating; Sputtering; Surface rugosity
1. Background Palladium alloy membranes are widely studied for CO2 capture and hydrogen separation from synthesis gas in membrane reactors. Due to the *Corresponding author.
considerable cost of palladium, efforts have been made to obtain low-thickness deposits on cheaper materials, such as ceramics and macroporous metals, by various techniques, including electroplating [1], sputtering [2], chemical vapor deposition [3], and electroless plating [4,5]. Electroless plating, in particular, is an autocatalytic process which
Presented at the conference Engineering with Membranes 2008; Membrane Processes: Development, Monitoring and Modelling – From the Nano to the Macro Scale – (EWM 2008), May 25–28, 2008, Vale do Lobo, Algarve, Portugal. 0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved. doi: 10.1016/j.desal.2009.01.004
M. Broglia et al. / Desalination 245 (2009) 508–515
allows to obtain thin films with good adhesion characteristics; besides that, it does not require expensive set-up and is relatively easy to scale-up from laboratory to industrial scale. Porous ceramics substrates such as α-alumina have aroused a great attention since early times because of the relatively smooth surface and narrow pore distribution [6,7]. Ceramic substrates, however, present the problem — still unsolved — of obtaining leak tight seals at the ceramic– metal interface, as well as an intrinsic fragility which, in perspective, makes it difficult to assemble them into a module, affecting their potential industrial application as well [8,9]. Therefore, the research effort has been addressed to porous metals, which have higher robustness and mechanical stability, and can be easily welded and assembled in a module [10]. Commercially available porous metals, however, present high rugosity and large pore dimensions (micron range); this determines high thickness of the Pd deposit (up to 20–30 μm) in order to obtain dense membranes [11]. Moreover, high operating temperatures (up to 500°C) and long exposure times can determine the diffusion of substrate metals (such as iron) into the palladium layer and, subsequently, a significant decrease of hydrogen permeance and selectivity. To avoid this problem, the support should be modified, introducing a suitable barrier layer. Several types of barrier layers have been considered, including mixed oxides grown by oxidation of the support [12], a bi-metallic deposited multilayer [13], ceramic materials such as zirco-
Fig. 1. External profile of the porous AISI 316L support.
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nia oxide [14], zirconia-yttria [15], cerium oxide [16], and alumina [17,18]. Although a barrier layer in the microporous range does allow to obtain low thickness dense palladium deposits, it may cause an increase in mass transfer resistance and, ultimately, a decrease in hydrogen permeation through the membrane. From this point of view, alumina can be very useful, since it has a low water residual volume after dehydration at high temperatures and, therefore, minimizes resistance to gas diffusion through the barrier layer [19]. This paper deals with the preparation and characterization of palladium composite membranes obtained by deposition of Pd by electroless plating on alumina-modified stainless steel supports. More specifically, it focuses on the characterization of the barrier layer and modifications which can occur during the activation phase of the preparation procedure. 2. Experimental Investigated supports are 10 mm O.D. AISI 316L porous tubes, with a nominal pore size of 0.1 μm, supplied by Mott Metallurgical Corporation. Nominal pore size is determined by the manufacturer based on a 95% rejection of particles with size greater than 0.1 μm. The actual pore size, however is much larger — a mean and maximum value of about 2 and 5 μm, respectively, has been determined by mercury intrusion measurements [11].
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Rugosity of the surface is also rather high, as shown in Fig. 1; the maximum peak of the profile is Ry = 31 μm, while the mean rugosity is Rm = 7.2 μm. Due to the morphological and topographic characteristics of the support, a multi-graded barrier has been deposited in order to reduce surface rugosity and porosity. Porous AISI 316L tubes have been cut to obtain 4-cm long samples. Composite membranes have been prepared from these samples according to the following subsequent steps: 1. Cleaning of the porous stainless steel support; 2. Deposition of a double layer of gammaalumina (γ-Al2O3) on the external surface of the support by dip coating, followed by drying and calcination; 3. Activation of the modified support; 4. Deposition of palladium by electroless plating. Cleaning of the support has been performed in an ultrasonic bath with acetone. Afterwards, the support was rinsed with water, diluted hydrochloric acid, deionized water up to a neutral pH, and acetone. The deposition of both γ-alumina layers has been obtained by dip coating of a colloidal dispersion of γ-alumina particles. The first layer has been prepared by a commercial colloidal dispersion (Alfa Aesar) of coarse particles (size 50 nm); polyvinyl alcohol (PVA) has been added to the dispersion (0.2 % by weight), in order to improve the film homogeneity. The second layer has been prepared in laboratory by hydrolysis of a metal-organic alumina precursor, with the addition of the 0.2% PVA. Both coatings were left to dry at room temperature and, successively, in an oven at 75°C; finally, calcination has been performed in static air at 500°C for 6 hours. The activation of the modified support is made by dipping into a stannous chloride solution and
a palladium chloride solution, alternately, for many times. The corresponding reaction is: SnCl2 + PdCl2 → SnCl4 + Pd Activation is usually performed at pH < 1 in order to maintain a good salt solubility [5,9]; in this case, however, pH has been increased up to 4, in order to operate inside the γ-Al2O3 stability range. (3 < pH < 9). Activation is performed at room temperature and the number of dipping cycles depends on the support characteristics. Stannous solution, indeed, enters the pores of stainless steel and reduces palladium ions, thus creating auto-catalytic sites where palladium plating can occur. Pd seeds must be in sufficient number and homogeneously distributed on the surface, in order to allow the subsequent palladium deposition. This step is the most critical, since it influences the subsequent deposition of palladium, dimension of grains, and ultimately film morphology and stability. Deposition of palladium by electroless plating is performed in a solution consisting of palladium chloride, ammonia (to control pH), EDTANa2 (complexing agent) and hydrazine (reducing agent). Deposition occurs according to the following reaction: 2 Pd [ NH 3 ]42 + + N 2 H 4 + 4OH − → 2 Pd 0 + 8 NH 3 + N 2 + 4 H 2O Pd plating is obtained by circulating the solution in a reactor where the membrane is immersed and kept in rotation in order to obtain a homogeneous deposit on the external surface and ease nitrogen evacuation from the reaction zone; temperature is kept constant (45–50°C) by a thermostatic bath. Deposition is performed at pH = 11, e.g. outside the pH stability range of γ-Al2O3. However, dissolution of γ-Al2O3, can be considered negligible since Pd deposition is very fast, provided that activation is performed correctly and palladium seeds are distributed evenly on the surface.
M. Broglia et al. / Desalination 245 (2009) 508–515
Fig. 2. SEM images of the AISI 316L surface: (A top, top view) and (A down, perspective view) as received; (B top, top view) and (B down, perspective view) after γ−Al2O3 deposition.
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Fig. 4. SEM image of the section of a sample after γ−Al2O3 deposition.
Membrane samples from each step of the preparation procedure have been characterized by SEM/EDS analysis. Fig. 2 compares the SEM images of the surface of the porous AISI 316 support, before and after γ-Al2O3 deposition. It can be noted that deposition of the γ-Al2O3 layer determines a strong reduction of the surface rugosity.
The thickness of the alumina layer, however, is not constant; in fact, as shown in Fig. 3, in the clear/bright areas (SP2), usually located in correspondence of surface edges, the alumina is so thin that the elements of the support (Fe, Ni, Cr,) are detected by the EDS probe, while in the dark areas (SP1) only alumina and oxygen can be detected. Fig. 4 shows a detail of the double layer of γAl2O3 deposited on the AISI 316L support; the first layer is 2–3 μm thick, while the external layer is much less than 1μm; moreover, EDS analysis has detected γ-Al2O3 spots in the support porosity up to a depth of 20 μm. Fig. 5 shows the sample covered with γ-Al2O3 before and after activation. Despite the fact that
Fig. 3. SEM image in back scattering modality of the AISI 316L surface after γAl2O3 deposition: points where EDS spectra has been measured are indicated.
Fig. 5. Secondary electron detection (SED) images of the surface of samples covered with γ- Al2O3, non-activated (left) and activated (right).
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Fig. 6. SEM images of a sample covered with Pd, surface (left); section (right).
activation has been performed inside the γ-Al2O3 pH stability range, a strong increase in the rugosity of the activated sample can be observed, due to alumina dissolution. Furthermore, the presence of cracks, usually near surface edges, is evident on non-activated sample. Finally, SEM images of a sample covered with palladium are shown in Fig. 6. It can be observed that, although alumina has been partially dissolved during activation, a continuous, crack-free Pd layer about 11 μm thick, well anchored to the surface, has been obtained. The typical “cauliflower-like” morphology of palladium grain agglomerates can be noticed from the surface image.
Samples covered with γ-Al2O3, before and after activation, have been analyzed further by Auger spectroscopy coupled with sputtering, to check if γ-Al2O3 c overed all the surface, detect if Pd seeds were uniformly distributed on the surface, and, finally identify impurities such as chlorides and tin which could contaminate palladium [20]. Sputtering has been performed by using a 0.5 mm beam of Ar+ ions, at an energy of 4 keV and a current of 1 μA, with a 30-degree angle of incidence. All the measured thickness are in terms of SiO2 equivalent, since alumina reference samples for the calculation of the erosion velocity were not available. The chemical analysis has been performed at different locations on the surface of each sample. Analyses have been performed at different sputtering times, on areas of 100 μm × 100 μm, where the Auger peaks of the elements of interests have been detected. Typical Auger spectra of the two samples are shown in Fig. 7(a) and (b), respectively. Figures show that, on the non-activated sample, after 63.5 min of sputtering, corresponding to an equivalent thickness of SiO2 of about 4 μm, only alumina and oxygen can be detected. On the contrary, on
Fig. 7. Auger spectra measured at different sputtering times on of samples covered with γ-Al2O3, non-activated (left) and activated (right).
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Fig. 8. SEM image of the surface of a sample activated at pH < 1, white circles indicate the zones where EDS analysis have been performed.
Fig. 9. SEM image of a sample after Pd deposition without activation. Rectangles indicate the zones where EDS analysis have been performed.
the activated sample, iron peaks can be detected after the first sputtering cycle (30 sec), corresponding to an equivalent thickness of SiO2 of about 10 nm. A slight carbon superficial contamination, probably arising from EDTA, can also be detected on the surface. Similar Auger spectra have been obtained on the other areas of the samples. It can be assumed, therefore, that γ-Al2O3 completely covers the support surface, even along edges and surface depressions, where deposit thickness is lower; activation, even if performed in mild conditions (pH = 4), determines a strong reduction of this layer, of about 10 nm. This means that activation completely removes the outer mesoporous γ-Al2O3 layer, which is essential to obtain a thin (less than 10 μm), pinholefree palladium layer by electroless plating. Furthermore, chemical analysis performed at different locations on a surface area of 58 μm × 39.5 μm of the sample have not detected the presence of palladium (detection limit 1%). This observation is consistent with the fact that at pH = 4 the reduction reaction of palladium is in very slow progress. On the other hand, if activation is performed at a proper pH value, the γ-Al2O3 layer is almost completely dissolved by the solution, as shown
by SEM micrographs in Fig. 8. In fact, EDS spectra of P1, P2, and P3 areas stressed the fact that alumina is present only in the P2 area and palladium deposited mostly in the P3 area, where alumina is absent. Similarly, if activation is not performed, deposition occurs very slowly and not evenly. Fig. 9 shows the SEM micrograph of a non-activated sample. EDS analyses have been performed on different locations, as indicated on the image. Pd can be detected only in areas 2 and 3, while completely absent in area 1; alumina is present in all the areas, although at different concentrations, thus indicating a partial dissolution due to the long exposure to the deposition bath. The above observations suggest that, in order to obtain a thin adherent double γ-Al2O3 layer, membrane preparation can be re-addressed as follows: – reduce the support roughness, due to a combined mechanical and chemical treatment; – pre-activate the γ-Al2O3 external layer by adding Pd seeds to the alumina gel. Experimental tests are now in progress, and in particular, pre-activation has been performed by adding PdCl2 to the γ-Al2O3 laboratory-made gel; reduction of Pd has been obtained after calcina-
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Fig. 10. SEM image of a Pd deposit on the surface of a pre-activated sample.
inside the pH stability range of γ-Al2O3, determines the complete dissolution of the outer layer and a partial dissolution of the inner layer; this phenomenon is enhanced by decreasing the pH value. Moreover, Pd seeds, which catalyze subsequent Pd deposition, are not well distributed on the surface. As a consequence of the above observations, membrane preparation has been re-addressed as follows: – reduction of the support roughness thanks to a combined mechanical and chemical treatment; – pre-activation of the γ-Al2O3 external layer, consisting in the addition of palladium chloride to the alumina gel and palladium reduction in sodium boron hydride solution after calcination. Aknowledgments
tion by the addition of sodium boron hydride solution. Preliminary results (Fig. 10) show that, following this procedure, a uniform deposit of Pd crystallites of a few hundred nanometers in size can be obtained. 4. Conclusions Palladium composite membranes have been obtained by deposition of Pd on alumina modified stainless steel supports. The preparation procedure involves cleaning of the macroporous stainless steel support, deposition of a double layer of γ-alumina (γ-Al2O3) on the external surface of the support by dip coating, followed by drying and calcination, activation of the modified support, and deposition of palladium by electroless plating. SEM/EDS and Auger spectroscopy analysis allowed the identification of critical issues and a proper re-elaboration of preparation procedure. It has been found that γ-Al2O3 determines a strong reduction of the surface rugosity and completely covers the support surface, even along edges and surface depressions, where deposit thickness is lower. The activation step, although performed
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