- Email: [email protected]
Au)n nanolayers
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
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Technical Communication
High-flux H2 separation membranes from (Pd/Au)n nanolayers Lei Shi, Andreas Goldbach*, Hengyong Xu Dalian Institute of Chemical Physics, Chinese Academy of Science, Zhongshan Road 457, Dalian 116023, China
article info
abstract
Article history:
A novel strategy for the preparation of supported PdAu alloy layers allows the facile and fast
Received 7 August 2010
fabrication of highly permeable and selective H2 separation membranes from refractory
Received in revised form
metals via electroless plating and low-temperature alloying. Homogenous alloying of multiple,
10 November 2010
separately deposited Pd and Au layers with thickness in the nm range required less than one
Accepted 11 November 2010
week at 773 K under atmospheric H2 as evidenced by X-ray diffraction and H2 permeation
Available online 18 December 2010
measurements. The H2 permeation rate JH2 became stable within a day even, reaching 0.62 mol m2 s1 at 773 K and DPH2 ¼ 100 kPa. The corresponding N2 leak rate remained
Keywords:
constant during a 350 h experiment, resulting in an ideal H2/N2 selectivity of 1400 and
PdAu membranes
demonstrating that such membranes tolerate extended operation at that temperature well.
PdAu alloying
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Nanolayers Electroless plating Hydrogen permeation
1.
Introduction
Ultrathin Pd membranes attract widespread interest [1,2] since, unlike industrial H2 purification technologies, they can be readily scaled to the demands of residential power generation units [3] or even fuel cells for portable devices [4]. Their integration into catalytic reactors is gaining momentum too, not only as a means to compact and highly efficient processes for H2 production [5,6], but also opening new synthetic routes for, e.g. oxidation reactions with intrinsically higher selectivity [7,8]. However, pure Pd is prone to embrittlement below 573 K and H2 pressures PH2 2 MPa [1]. This can be alleviated by alloying with other metals, which can improve H2 permeability, mechanical properties and chemical resistance too. Also, recent computational studies predict novel binary and ternary Pd alloys as promising membrane materials [9,10].
Electroless plating (ELP) is the technically most facile and versatile method for preparation of ultrathin Pd membranes supported on ceramic or metallic substrates. Alloy membranes have to be prepared by sequential metal deposition in this way as co-plating of different metals is unfeasible with few exceptions [1]. However, high alloying temperatures are prohibitive due to divergent metal and ceramic thermal expansion and detrimental diffusion of support metals into the Pd alloy layer, even though the latter can be slowed through introduction of diffusion barriers [11,12]. Thus, alloying of the typical metallic bilayers has to be accomplished far below the melting points, which is very time-consuming even for low-melting group 11 metals (Fig. 1, route A) [13,14]. Indeed, supported state-of-theart Pd alloy membranes often exhibit a considerable degree of inhomogeneity [15,16]. Using PdAu layers as an example we show that this obstacle can be overcome via nanostructuring of the precursor metal layers enabling Pd alloy membrane
* Corresponding author. Tel.: þ86 8437 9229; fax: þ86 411 8469 1570. E-mail address: [email protected] (A. Goldbach). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.056
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
3.
Fig. 1 e Alloying of (A) a conventional Pd/Au bilayer and (B) a (Pd/Au)n multilayer with equivalent overall thickness.
preparation from refractory metals via cost-efficient electroless plating. This novel preparation strategy is outlined as route B in the schematic Fig. 1. Instead of depositing all Pd first and then Au to produce a bilayer at the beginning (Fig. 1, route A), an alloy precursor is prepared consisting of multiple Au nanolayers embedded in a Pd matrix.
2.
Experimental
Deposition of Pd from commercial (PALLA TOP, Okuno Chemical Industries) and Au from home-made plating baths on ceramic support tubes (OD 12 mm, ID ¼ 8 mm) from Nanjing University of Technology by ELP has been previously described in detail including the pretreatment and activation of the support with commercial Pd seeding solutions (OPC-50 inducer and OPC-150 cryster, Okuno) [14,17]. Here, two 50 mm long (Pd/Au)9/Pd multilayers M1 and M2 were prepared by alternating deposition of 10 Pd and 9 Au layers. Au was directly deposited on Pd layers after rinsing those with water, but Au layers had to be activated again prior to each Pd deposition using the Pd seeding solutions. The plating times for Pd and Au layers were kept constant at 9 and 3 min, respectively. The (Pd/Au)9/Pd multilayers were dried at 393 K in air overnight after the final Pd deposition. The thickness and composition of the metal layers were determined before and after alloying, employing scanning electron microscopy (SEM, QUANTA 200F, FEI), energy-dispersive X-ray spectroscopic analysis (EDS) and X-ray diffraction (XRD, Philips PANalytical, Cu Ka ¼ 0.15406 nm at 40 mA and 40 kV). Multilayer M1 was broken to follow the progress of alloy formation at 773 K by XRD. A fragment was heated to 623 K in N2 before admitting H2 (both 99.999% purity) at atmospheric pressure and further heating to 773 K and then maintaining that temperature. Periodically the M1 fragment was cooled down to room temperature while switching back to N2 atmosphere at 623 K to monitor the progress of alloy formation via XRD. Alloying was also monitored in real-time by measurement of the H2 permeation rates JH2 during annealing of multilayer M2 at 773 K under atmospheric H2 pressure in a previously described permeation test setup [18]. Periodically PH2 was raised to 200 kPa and JH2 was determined using a bubble flowmeter while the permeate side was kept at atmospheric pressure. No sweep gas was used. After JH2 had stabilized at 773 K, the single gas H2 and N2 permeation characteristics of membrane M2 were determined over a wider temperature range at DP ¼ 100 kPa.
Results and discussion
Fig. 2a displays an SEM image of the freshly prepared, ca. 2.5 mm thick multilayer M1. The brighter, less than 30 nm wide Au layers and the 250 nm wide Pd layers can be readily discerned. The regular spacing of the Au veins within the Pd matrix can be also recognized in an EDS line scan of the multilayer cross section where they appear as spikes and dips, respectively, in the Au and Pd traces (Fig. 2b). The line scan insinuates already an evenly distributed Au content of ca. 9e10 at.%, which is owed to the width of the EDS analyzing beam, i.e. 1 mm. Thus approximately 3 Pd and Au layers each are sampled for every data point. Fig. 2b demonstrates that the spacing of the much narrower Au layers still can be mapped, because the sampled Au area is larger whenever the beam spot is centered on such a layer (vice versa for Pd). The veined multilayer pattern had disappeared and the Pd and Au EDS traces had become flat after one day of annealing at 773 K under atmospheric H2 pressure, indicating that homogenization of the multilayer proceeded rapidly under those circumstances.
Fig. 2 e (a) SEM image of freshly prepared M1 (Pd/Au)9/Pd multilayer cross section; (b) Pd and Au traces of EDS scan along the line marked in the SEM image.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
Fig. 3 e Evolution of the 111 XRD reflections from the M1 (Pd/Au)9/Pd multilayer during alloying at 773 K and PH2 [ 100 kPa.
The XRD patterns shown in Fig. 3 track the alloy formation during annealing of an M1 fragment at 773 K more accurately. The Au(111) reflection had disappeared within 6 h while the Pd (111) peak shifted to lower angles due to alloying. It took 150 h for this PdAu(111) peak to become completely symmetric. The lattice constant corresponded to a Au content of 8.5 at.% according to a literature relationship [19] in good agreement with the EDS line scan. Hence a fully homogenized PdAu alloy had been formed within less than a week at 773 K. In contrast, the far Pd surface of a previously studied, 3 mm thick Pd/Au bilayer M3 [20] had remained almost pure Pd following 3 weeks of annealing under the same conditions [14], despite that the eventual, completely alloyed 3 mm thick Pd90Au10 membrane had a comparable composition and thickness as M1 [14]. Fig. 4 shows H2 fluxes measured during alloying of membrane M2 at 773 K and atmospheric PH2 , which contained 7e8 at.% Au according to posterior SEM and XRD analyses. The permeation rate became stable within a day at
JH2 ¼ 0.62 mol m2 s1. The corresponding N2 leak rate remained constant too at JN2 ¼ 4.4 104 mol m2 s1 during the 350 h experiment, resulting in an ideal H2/N2 selectivity of 1400. This shows that these membranes tolerate extended operation at that temperature well. For comparison, Fig. 4 shows also H2 and N2 fluxes during alloying of another previously studied Pd/Au bilayer at 823 K and PH2 ¼ 100 kPa, which yielded a 3 mm thick membrane M4 with average stoichiometry Pd90.5Au9.5 [21]. Clearly, it took around 5 days until JH2 had become stable despite of the higher alloying temperature. Furthermore, JN2 increased slowly, but steadily from 1.14 103 mol m2 s1 to 1.36 103 mol m2 s1 within 300 h corroborating that the integrity of these composite membranes begins to deteriorate at that higher temperature. Evidently H2 permeation through membrane M2 had stabilized within a few hours and thus several days before the alloy was fully homogenized according to XRD (Fig. 3). This phenomenon has been also observed for membrane M4, though time scales were much longer there [21], and can be ascribed to the weak variation of the H2 permeability of PdAu alloys with Au content up to 15 at.% [22]. Above 573 K the H2 permeation rates measured at DPH2 ¼ 100 kPa could be represented by an Arrhenius law, i.e. JH2 ¼ (3.0 0.2) mol m2 s1 exp [(9.9 0.2) kJ mol1 K1/RT] yielding JH2 ¼ 0.51 mol m2 s1 at 673 K for example. Note that H2 fluxes of membranes M2 are in excellent agreement with those of similar membrane M4 [21], deviating by less than 3% in the experimental range 573e773 K. The pressure dependence of the H2 flux deviated slightly from Sieverts’ law with a pressure exponent n ¼ 0.62 0.10 in the pressure range 20 kPa PH2 100 kPa. The permeability of membrane M2 is among the highest reported for supported PdAu membranes up to now. For comparison, a H2 flux of ca. 0.18 mol m2 s1 can be derived for 673 K and DPH2 ¼ 100 kPa from permeation data reported for a w5 mm thick Pd94Au6 layer on a ceramic capillary [23], whereas JH2 amounted to ca. 0.20 mol m2 s1 for a 9 mm thick Pd91.3Au8.7 layer on a stainless steel support at 673 K and slightly higher DPH2 ¼ 138 kPa [24]. The facile alloying of nanostructured (Pd/Au)n multilayers can be primarily attributed to the much expanded Pd/Au interface, which is 18-fold enlarged in case of the here studied multilayers in comparison to a bilayer. Furthermore, Au has to diffuse only 125 nm to fully infiltrate the ca. 250 nm wide Pd sections of the M1 multilayer. In contrast, it has to penetrate 20 times as deep into the 2.5 mm thick Pd of a bilayer with equivalent thickness of the ten M1 Pd nanolayers. Obviously, the alloying time will remain the same for (Pd/Au)n precursors if the number of alternating nanolayers with given thickness is increased in order to prepare much thicker PdAu membranes than investigated here. It is also clear that efficient alloying could be achieved at even lower temperatures if the thickness of the separate metal layers was further reduced.
4.
Fig. 4 e H2 and N2 fluxes (at DP [ 100 kPa) during alloying of the M2 (Pd/Au)9/Pd multilayer at 773 K and M4 Pd/Au bilayer at 823 K under atmospheric PH2 .
2283
Conclusion
Nanostructuring of the precursor metal layers enables the fast preparation of supported PdAu alloy membranes through multiple sequential ELP steps and subsequent alloying. Fully functional membranes are obtained within a few hours of annealing at moderate temperatures and independent of final
2284
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
separation layer thickness. This strategy is not limited to PdAu, but allows also the fabrication of supported Pd alloy membranes from even more refractory metals, which have been rarely studied up to now. At the same time this approach could facilitate alloy preparation if that is complicated by discontinuous phase diagrams. For example, alloying temperatures have to be limited to less than 723 K in case of high-flux, body-centered cubic PdCu membranes because of a hightemperature phase transition with concomitant stoichiometric segregation that diminishes the membranes H2 permeability and stability [13]. This fabrication strategy opens avenues for research on new multi-component Pd alloy membranes and could advance the utilization of these promising separation tools in purification as well as chemical reaction technology.
Acknowledgement The authors gratefully acknowledge financial support from the Ministry of Science and Technology of China (Grant No. 2005CB221401).
references
[1] Paglieri S, Way JD. Innovations in palladium membrane research. Sep Purif Methods 2002;31:1e169. [2] Pan XL, Xiong GX, Sheng SS, Stroh N, Brunner H. Thin dense Pd membranes supported on a-Al2O3 hollow fibers. Chem Commun; 2001:2536e7. [3] Han J, Kim IS, Choi KS. High purity hydrogen generator for on-site hydrogen production. Int J Hydrogen Energy 2002;27: 1043e7. [4] Mahlon WS. Methanol decomposition fuel processor for portable power applications. Int J Hydrogen Energy 2009;34: 2955e64. [5] Chen YZ, Wang YZ, Xu HY, Xiong GX. Hydrogen production capacity of membrane reformer for methane steam reforming near practical working conditions. J Membr Sci 2008;322:453e9. [6] Bi YD, Xu HY, Li WZ, Goldbach A. Water-gas shift reaction in a Pd membrane reactor over Pt/Ce0.6Zr0.4O2 catalyst. Int J Hydrogen Energy 2009;34:2965e71. [7] Niwa S, Eswaramoorthy M, Nair J, Raj A, Itoh N, Shoji H, et al. A one-step conversion of benzene to phenol with a palladium membrane. Science 2002;295:105e7. [8] Shi L, Goldbach A, Zeng GF, Xu HY. Direct H2O2 synthesis over Pd membranes at elevated temperatures. J Membr Sci 2010;348:160e6.
[9] Kamakoti P, Sholl DS. Towards first principles-based identification of ternary alloys for hydrogen purification membranes. J Membr Sci 2006;279:94e9. [10] Nanu DE, Bo¨ttger AJ. Towards designing stable Pd-based membranes for hydrogen gas separation: a statistical thermodynamics approach. Adv Funct Mater 2008;18: 898e906. [11] Shu J, Adnot A, Grandjean BPA, Kaliaguine S. Structurally stable composite PdeAg alloy membranes: introduction of a diffusion barrier. Thin Solid Films 1996;286:72e9. [12] Huang Y, Dittmeyer R. Characterization of intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes. J Membr Sci 2006;282:296e310. [13] Yuan LX, Goldbach A, Xu HY. Real-time monitoring of metal deposition and segregation phenomena during preparation of PdCu membranes. J Membr Sci 2008;322:39e45. [14] Shi L, Goldbach A, Zeng GF, Xu HY. Preparation and performance of thin-layered PdAu/ceramic composite membranes. Int J Hydrogen Energy 2010;35:4201e8. [15] Roa F, Way JD, McCormick RL, Paglieri SN. Preparation and characterization of PdeCu composite membranes for hydrogen separation. Chem Eng J 2003;93:11e22. [16] Ma YH, Akis BC, Ayturk ME, Guazzone F, Engwall EE, Mardilovich IP. Characterization of intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes. Ind Eng Chem Res 2004;43:2936e45. [17] Zeng GF, Goldbach A, Xu HY. Impact of support mass flow resistance on low-temperature H2 permeation characteristics of a Pd95Ag5/Al2O3 composite membrane. J Membr Sci 2009;326:681e7. [18] Li H, Goldbach A, Li WZ, Xu HY. PdC formation in ultra-thin Pd membranes during separation of H2/CO mixtures. J Membr Sci 2007;299:130e7. [19] Maeland A, Flanagan TB. Lattice spacings of gold-palladium alloys. Can J Phys 1964;42:2364e6. [20] The bilayer consisted of a ca. 2.65 mm thick Pd and a ca. 0. 35 mm thick Au layer based on thickness and composition of the alloyed Pd90Au10 layer. [21] Shi L, Goldbach A, Zeng GF, Xu HY. H2O2synthesis over PdAu membranes. Catal Today 2010;156:118e23. [22] Shu J, Grandjean BPA, Neste AV, Kaliaguine S. Catalytic palladium-based membrane reactors: a review. Can J Chem Eng 1991;69:1036e60. [23] Okazaki J, Pacheco Tanaka DA, Llosa Tanco MA, Wakui Y, Ikeda T, Mizukami F, et al. Preparation and hydrogen permeation properties of thin PdeAu alloy membranes supported on porous alpha-alumina tube. Mater Trans 2008; 49:449e52. [24] Roa F, Thoen PM, Gade SK, Way JD, DeVoss S, Alptekin GO. Palladium-copper and Palladiumegold alloy composite membranes for hydrogen separations. In: Bose AC, editor. Inorganic membranes for energy and environmental applications. New York: Springer; 2009. p. 221e39.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
High-flux H2 separation membranes from (Pd/Au)n nanolayers Lei Shi, Andreas Goldbach*, Hengyong Xu Dalian Institute of Chemical Physics, Chinese Academy of Science, Zhongshan Road 457, Dalian 116023, China
article info
abstract
Article history:
A novel strategy for the preparation of supported PdAu alloy layers allows the facile and fast
Received 7 August 2010
fabrication of highly permeable and selective H2 separation membranes from refractory
Received in revised form
metals via electroless plating and low-temperature alloying. Homogenous alloying of multiple,
10 November 2010
separately deposited Pd and Au layers with thickness in the nm range required less than one
Accepted 11 November 2010
week at 773 K under atmospheric H2 as evidenced by X-ray diffraction and H2 permeation
Available online 18 December 2010
measurements. The H2 permeation rate JH2 became stable within a day even, reaching 0.62 mol m2 s1 at 773 K and DPH2 ¼ 100 kPa. The corresponding N2 leak rate remained
Keywords:
constant during a 350 h experiment, resulting in an ideal H2/N2 selectivity of 1400 and
PdAu membranes
demonstrating that such membranes tolerate extended operation at that temperature well.
PdAu alloying
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Nanolayers Electroless plating Hydrogen permeation
1.
Introduction
Ultrathin Pd membranes attract widespread interest [1,2] since, unlike industrial H2 purification technologies, they can be readily scaled to the demands of residential power generation units [3] or even fuel cells for portable devices [4]. Their integration into catalytic reactors is gaining momentum too, not only as a means to compact and highly efficient processes for H2 production [5,6], but also opening new synthetic routes for, e.g. oxidation reactions with intrinsically higher selectivity [7,8]. However, pure Pd is prone to embrittlement below 573 K and H2 pressures PH2 2 MPa [1]. This can be alleviated by alloying with other metals, which can improve H2 permeability, mechanical properties and chemical resistance too. Also, recent computational studies predict novel binary and ternary Pd alloys as promising membrane materials [9,10].
Electroless plating (ELP) is the technically most facile and versatile method for preparation of ultrathin Pd membranes supported on ceramic or metallic substrates. Alloy membranes have to be prepared by sequential metal deposition in this way as co-plating of different metals is unfeasible with few exceptions [1]. However, high alloying temperatures are prohibitive due to divergent metal and ceramic thermal expansion and detrimental diffusion of support metals into the Pd alloy layer, even though the latter can be slowed through introduction of diffusion barriers [11,12]. Thus, alloying of the typical metallic bilayers has to be accomplished far below the melting points, which is very time-consuming even for low-melting group 11 metals (Fig. 1, route A) [13,14]. Indeed, supported state-of-theart Pd alloy membranes often exhibit a considerable degree of inhomogeneity [15,16]. Using PdAu layers as an example we show that this obstacle can be overcome via nanostructuring of the precursor metal layers enabling Pd alloy membrane
* Corresponding author. Tel.: þ86 8437 9229; fax: þ86 411 8469 1570. E-mail address: [email protected] (A. Goldbach). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.056
2282
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
3.
Fig. 1 e Alloying of (A) a conventional Pd/Au bilayer and (B) a (Pd/Au)n multilayer with equivalent overall thickness.
preparation from refractory metals via cost-efficient electroless plating. This novel preparation strategy is outlined as route B in the schematic Fig. 1. Instead of depositing all Pd first and then Au to produce a bilayer at the beginning (Fig. 1, route A), an alloy precursor is prepared consisting of multiple Au nanolayers embedded in a Pd matrix.
2.
Experimental
Deposition of Pd from commercial (PALLA TOP, Okuno Chemical Industries) and Au from home-made plating baths on ceramic support tubes (OD 12 mm, ID ¼ 8 mm) from Nanjing University of Technology by ELP has been previously described in detail including the pretreatment and activation of the support with commercial Pd seeding solutions (OPC-50 inducer and OPC-150 cryster, Okuno) [14,17]. Here, two 50 mm long (Pd/Au)9/Pd multilayers M1 and M2 were prepared by alternating deposition of 10 Pd and 9 Au layers. Au was directly deposited on Pd layers after rinsing those with water, but Au layers had to be activated again prior to each Pd deposition using the Pd seeding solutions. The plating times for Pd and Au layers were kept constant at 9 and 3 min, respectively. The (Pd/Au)9/Pd multilayers were dried at 393 K in air overnight after the final Pd deposition. The thickness and composition of the metal layers were determined before and after alloying, employing scanning electron microscopy (SEM, QUANTA 200F, FEI), energy-dispersive X-ray spectroscopic analysis (EDS) and X-ray diffraction (XRD, Philips PANalytical, Cu Ka ¼ 0.15406 nm at 40 mA and 40 kV). Multilayer M1 was broken to follow the progress of alloy formation at 773 K by XRD. A fragment was heated to 623 K in N2 before admitting H2 (both 99.999% purity) at atmospheric pressure and further heating to 773 K and then maintaining that temperature. Periodically the M1 fragment was cooled down to room temperature while switching back to N2 atmosphere at 623 K to monitor the progress of alloy formation via XRD. Alloying was also monitored in real-time by measurement of the H2 permeation rates JH2 during annealing of multilayer M2 at 773 K under atmospheric H2 pressure in a previously described permeation test setup [18]. Periodically PH2 was raised to 200 kPa and JH2 was determined using a bubble flowmeter while the permeate side was kept at atmospheric pressure. No sweep gas was used. After JH2 had stabilized at 773 K, the single gas H2 and N2 permeation characteristics of membrane M2 were determined over a wider temperature range at DP ¼ 100 kPa.
Results and discussion
Fig. 2a displays an SEM image of the freshly prepared, ca. 2.5 mm thick multilayer M1. The brighter, less than 30 nm wide Au layers and the 250 nm wide Pd layers can be readily discerned. The regular spacing of the Au veins within the Pd matrix can be also recognized in an EDS line scan of the multilayer cross section where they appear as spikes and dips, respectively, in the Au and Pd traces (Fig. 2b). The line scan insinuates already an evenly distributed Au content of ca. 9e10 at.%, which is owed to the width of the EDS analyzing beam, i.e. 1 mm. Thus approximately 3 Pd and Au layers each are sampled for every data point. Fig. 2b demonstrates that the spacing of the much narrower Au layers still can be mapped, because the sampled Au area is larger whenever the beam spot is centered on such a layer (vice versa for Pd). The veined multilayer pattern had disappeared and the Pd and Au EDS traces had become flat after one day of annealing at 773 K under atmospheric H2 pressure, indicating that homogenization of the multilayer proceeded rapidly under those circumstances.
Fig. 2 e (a) SEM image of freshly prepared M1 (Pd/Au)9/Pd multilayer cross section; (b) Pd and Au traces of EDS scan along the line marked in the SEM image.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
Fig. 3 e Evolution of the 111 XRD reflections from the M1 (Pd/Au)9/Pd multilayer during alloying at 773 K and PH2 [ 100 kPa.
The XRD patterns shown in Fig. 3 track the alloy formation during annealing of an M1 fragment at 773 K more accurately. The Au(111) reflection had disappeared within 6 h while the Pd (111) peak shifted to lower angles due to alloying. It took 150 h for this PdAu(111) peak to become completely symmetric. The lattice constant corresponded to a Au content of 8.5 at.% according to a literature relationship [19] in good agreement with the EDS line scan. Hence a fully homogenized PdAu alloy had been formed within less than a week at 773 K. In contrast, the far Pd surface of a previously studied, 3 mm thick Pd/Au bilayer M3 [20] had remained almost pure Pd following 3 weeks of annealing under the same conditions [14], despite that the eventual, completely alloyed 3 mm thick Pd90Au10 membrane had a comparable composition and thickness as M1 [14]. Fig. 4 shows H2 fluxes measured during alloying of membrane M2 at 773 K and atmospheric PH2 , which contained 7e8 at.% Au according to posterior SEM and XRD analyses. The permeation rate became stable within a day at
JH2 ¼ 0.62 mol m2 s1. The corresponding N2 leak rate remained constant too at JN2 ¼ 4.4 104 mol m2 s1 during the 350 h experiment, resulting in an ideal H2/N2 selectivity of 1400. This shows that these membranes tolerate extended operation at that temperature well. For comparison, Fig. 4 shows also H2 and N2 fluxes during alloying of another previously studied Pd/Au bilayer at 823 K and PH2 ¼ 100 kPa, which yielded a 3 mm thick membrane M4 with average stoichiometry Pd90.5Au9.5 [21]. Clearly, it took around 5 days until JH2 had become stable despite of the higher alloying temperature. Furthermore, JN2 increased slowly, but steadily from 1.14 103 mol m2 s1 to 1.36 103 mol m2 s1 within 300 h corroborating that the integrity of these composite membranes begins to deteriorate at that higher temperature. Evidently H2 permeation through membrane M2 had stabilized within a few hours and thus several days before the alloy was fully homogenized according to XRD (Fig. 3). This phenomenon has been also observed for membrane M4, though time scales were much longer there [21], and can be ascribed to the weak variation of the H2 permeability of PdAu alloys with Au content up to 15 at.% [22]. Above 573 K the H2 permeation rates measured at DPH2 ¼ 100 kPa could be represented by an Arrhenius law, i.e. JH2 ¼ (3.0 0.2) mol m2 s1 exp [(9.9 0.2) kJ mol1 K1/RT] yielding JH2 ¼ 0.51 mol m2 s1 at 673 K for example. Note that H2 fluxes of membranes M2 are in excellent agreement with those of similar membrane M4 [21], deviating by less than 3% in the experimental range 573e773 K. The pressure dependence of the H2 flux deviated slightly from Sieverts’ law with a pressure exponent n ¼ 0.62 0.10 in the pressure range 20 kPa PH2 100 kPa. The permeability of membrane M2 is among the highest reported for supported PdAu membranes up to now. For comparison, a H2 flux of ca. 0.18 mol m2 s1 can be derived for 673 K and DPH2 ¼ 100 kPa from permeation data reported for a w5 mm thick Pd94Au6 layer on a ceramic capillary [23], whereas JH2 amounted to ca. 0.20 mol m2 s1 for a 9 mm thick Pd91.3Au8.7 layer on a stainless steel support at 673 K and slightly higher DPH2 ¼ 138 kPa [24]. The facile alloying of nanostructured (Pd/Au)n multilayers can be primarily attributed to the much expanded Pd/Au interface, which is 18-fold enlarged in case of the here studied multilayers in comparison to a bilayer. Furthermore, Au has to diffuse only 125 nm to fully infiltrate the ca. 250 nm wide Pd sections of the M1 multilayer. In contrast, it has to penetrate 20 times as deep into the 2.5 mm thick Pd of a bilayer with equivalent thickness of the ten M1 Pd nanolayers. Obviously, the alloying time will remain the same for (Pd/Au)n precursors if the number of alternating nanolayers with given thickness is increased in order to prepare much thicker PdAu membranes than investigated here. It is also clear that efficient alloying could be achieved at even lower temperatures if the thickness of the separate metal layers was further reduced.
4.
Fig. 4 e H2 and N2 fluxes (at DP [ 100 kPa) during alloying of the M2 (Pd/Au)9/Pd multilayer at 773 K and M4 Pd/Au bilayer at 823 K under atmospheric PH2 .
2283
Conclusion
Nanostructuring of the precursor metal layers enables the fast preparation of supported PdAu alloy membranes through multiple sequential ELP steps and subsequent alloying. Fully functional membranes are obtained within a few hours of annealing at moderate temperatures and independent of final
2284
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 2 2 8 1 e2 2 8 4
separation layer thickness. This strategy is not limited to PdAu, but allows also the fabrication of supported Pd alloy membranes from even more refractory metals, which have been rarely studied up to now. At the same time this approach could facilitate alloy preparation if that is complicated by discontinuous phase diagrams. For example, alloying temperatures have to be limited to less than 723 K in case of high-flux, body-centered cubic PdCu membranes because of a hightemperature phase transition with concomitant stoichiometric segregation that diminishes the membranes H2 permeability and stability [13]. This fabrication strategy opens avenues for research on new multi-component Pd alloy membranes and could advance the utilization of these promising separation tools in purification as well as chemical reaction technology.
Acknowledgement The authors gratefully acknowledge financial support from the Ministry of Science and Technology of China (Grant No. 2005CB221401).
references
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