Ind. Eng. Chem. Res. 2009, 48, 2061–2065
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Preparation and Characterization of Ultrathin Palladium Membranes X. Li,*,† T. M. Liu,† D. Huang,‡ Y. Q. Fan,‡ and N. P. Xu‡ College of Chemical Engineering, Shenyang Institute of Chemical Technology, Shenyang 110142, China, and College of Chemistry and Chemical Engineering and Key Laboratory of Material-Oriented Chemical Engineering of Jiangsu ProVince and MOE, Nanjing UniVersity of Technology, Nanjing 210009, China
An ultrathin palladium membrane (1 µm) has been prepared by an improved photocatalytic deposition (PCD) pretreatment and the electroless modification method. SEM results demonstrated good adhesion of the plated palladium membrane on the porous composite TiO2 support and dense coalescence of the palladium membrane. The flux and H2/N2 selectivity of the palladium membrane were determined in the temperature range of 623-823 K and the pressure difference range of 0.02-0.15 MPa. At 673 K and 0.1 MPa pressure difference, the hydrogen permeation flux of the composite membrane was as high as 0.27 mol m-2 s-1, and the H2/N2 selectivity coefficient was 361. In this study, the activation energy of hydrogen permeation through the composite palladium membrane was 17 kJ/mol. The corrected pressure exponent for the palladium composite membrane was nearly 0.8 deviated from Sievert’s law. 1. Introduction In recent years, the demand for highly pure hydrogen has increased rapidly because of its use in many fields such as semiconductor manufacturing and petrochemical processing.1 Palladium composite membranes are receiving much attention because of their ideal permeability and permselectivity toward hydrogen. Some methods have been proposed and developed to prepare palladium membranes including chemical vapor deposition (CVD),2 sputter coating,3 electroless plating,4 and electrochemical plating,5 among others. The technique of electroless plating is one of the most popular methods of preparing palladium composite membranes because of its good performance. Prior to electroless plating, the membrane surface needs to be activated to provide catalysis centers for the plating process. A two-step process has been used with palladium salt and tin chloride salt. However, widespread use of electroless technologies is constrained primarily by cumbersome pretreatment process. In addition, the presence of tin compounds in the sensitization procedure can cause the formation of pinholes. The adhesion between the metallic membrane and the substrate is also questionable. Many improvements in the activation procedure have been developed. For example, Paglieri and Way6 reported a new activation procedure not only with osmosis pressure but also using palladium acetate instead of palladium chloride. Zhao et al.7 modified the electroless plating procedure with boehmite gel activation on a porous alumina substrate in which the boehmite gel coating must be treated with hydrogen at 773 K to reduce the palladium. By these methods, both the two-step activation process and the tin impurities can be avoided. In this study, a simple and valuable activation procedure called photocatalytic deposition (PCD) was established8 that involved a photocatalytic reaction at the interface between a TiO2 semiconductor tube and a Pd(II) liquid film initiated by direct irradiation. Conducting the reaction processes in a liquid film at room temperature, instead of in solution as in earlier methods, effectively avoided a homogeneous reaction and the introduction of impurities throughout the bath. Then, the tube support activated by PCD was further modified by electroless * To whom correspondence should be addressed. Tel.: +86-24-89383902. Fax: +86-24-8938-3760. E-mail:
[email protected]. † Shenyang Institute of Chemical Technology. ‡ Nanjing University of Technology.
plating. The performances of the resulting palladium membranes were also investigated. The technique can be characterized by both the easy control of the uniformly packed palladium membrane and the simplicity of the process. 2. Experimental Section 2.1. Palladium Membrane Preparation. The surface of both ends of a ZrO2-Al2O3 tubular support was coated with glaze, and the porous part left for the palladium membrane was fixed at 5 cm in length. One end of the tubular support was also closed with glaze. The coating of the TiO2 layer on the ZrO2-Al2O3 tubular support was done by a method reported elsewhere.9 The specifications of TiO2 supports are listed in Table 1. The PCD process on the outer tube surface was carried out on a photocatalytic setup (see Figure 1).8 A 160-W ultraviolet lamp (254.6 nm) was located about 50 mm away from the tubular TiO2 membrane. The PCD bath contained PdCl2, deionized water, HCl, and ethylene diamine tetraacetate (EDTA). In addition, the pH value of the reaction bath ranged between 2 and 3 in order to prohibit the hydrolysis of PdCl2. Prior to irradiation, the TiO2 support was immersed into the PCD bath and maintained for 0.5 h to allow sufficient adsorption of the PCD bath onto the titania surface. Then, the TiO2 membrane was lifted out of the reaction solution, and a thin liquid film formed on the TiO2 surface. The liquid film coating was directly irradiated at room temperature for 1 h. At the same time, the support was rotated slowly with electric motor [0.00833 Hz (0.5 rpm)] for uniform incidence of the radiation. After photocatalytic pretreatment, the derived TiO2 membrane with palladium nuclei was removed and cleaned with distilled water. The palladium was deposited from an ammine solution stabilized with EDTA using hydrazine as the reducing agent. The composition and conditions of the electroless bath are listed in Table 2. After a given time, the deposits were washed with deionized water and dried. Table 1. Specification of Supports support
tube diameter (mm)
thickness (mm)
pore size (nm)
length (mm)
ZrO2-Al2O3 support TiO2 membrane
12 12
3 0.007
200 70
330 50
10.1021/ie8004644 CCC: $40.75 2009 American Chemical Society Published on Web 12/29/2008
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Figure 3. Pore size distribution of supported TiO2 membrane. Figure 1. Schematic diagram of the experimental setup for the PCD reaction: 1, light-tight box; 2, lamp bracket; 3, ultraviolet lamp; 4, quartz reactor; 5, condenser; 6, thermocouple; 7, electric motor; 8, supported TiO2 tube membrane; 9, stirring rod; 10,11, sample connection. Table 2. Composition and Conditions of Electroless Reaction Bath component
concentration
time
temperature
PdCl2 EDTA · 2Na N2H4 (85%) NH4OH (28%)
6 g/L 80 g/L 0.2 mol/L 290 mL/L
2h
room temperature
2.2. Palladium Membrane Characterization. The morphology and elemental composition of the palladium membrane and the pore size distribution of the support were examined with a field-emission scanning electron microscope (LEO 1530 VP system, with an energy-dispersive detector; LEO Elektronenmikroskopie GmbH, Oberkochen, Germany) and a gas-bubble pressure apparatus, respectively. 2.3. High-Temperature Gas Permeation Measurements. The gas permeation through the palladium membranes was measured on the apparatus illustrated in Figure 2. One end of the obtained membrane was closed, and the other was fitted with Teflon gaskets; the membrane was then was placed inside a stainless steel permeation cell. The separation area was 18.84 cm2, the permeate side was kept at atmospheric pressure with H2 or N2 (99.999% each), and the seal was kept outside the furnace. The upstream pressure was controlled with a backpressure regulator. After stabilization of the temperature and pressure, single gas fluxes were measured with mass flow controllers (model D08-4D/ZM, Beijing Sevenstar Huachuang Electonic Co., Ltd., Beijing, China).
3. Results and Discussion 3.1. Supports. ZrO2-Al2O3 microfiltration membranes were used as supports for the TiO2 membrane. The pore size distribution (PSD) of the support characterized by the gasbubble pressure (GBP) method is shown in Figure 3. The PSD of the TiO2/ZrO2-Al2O3 support was obtained using the GBP approach, which was performed following the American Society for Testing and Materials (ASTM) test method (F316-80). All samples were dipped into isobutyl alcohol (22.783 mN/m, 295.15 K) for 2 h under a pressure of 999.918 × 102 Pa (750 mmHg). During the measurement, the flow rate and transmembrane pressure of nitrogen were measured. Both the average pore size and the most frequent pore size of the membrane layer were about 0.08 µm, and the narrow distribution of pores was expected, considering that the support itself was composed of particles with a narrow distribution of sizes. 3.2. Characterization Results of the Palladium Membrane. The quality of the palladium membrane is dependent on the plating rate. A lower plating rate is vital to form a dense layer. If the plating rate is too high, the coating shows poor selectivity characteristics. The high hydrazine concentrations, high temperatures, and low EDTA concentrations would cause higher plating rates. The scanning electron microscopy (SEM) results are shown in Figure 4. For the resulting palladium membrane, the surface seemed dense in both the cross-sectional view and the top view. There were no continuous defects in this structure, although a very small area of the membrane is depicted in Figure 4a. The thickness of the membrane was about 1 µm (Figure 4b), which would probably favor improved hydrogen permanence. 3.3. Gas Permeability of the Palladium Membrane. The hydrogen flux through the palladium layer by the solutiondiffusion mechanism can be expressed in terms of Fick’s first law as10 J ) F(Phn - Pln)
Figure 2. Schematic diagram of hydrogen measurements for the tube-type Pd/TiO2 membrane: 1,3, gas cylinders; 2,4, stage regulators; 5-7, flow control valves; 8,14, pressure gauges; 9, membrane permeator; 10, backpressure regulator; 11, flow meter; 12, temperature regulator; 13, heating furnace; 15, mass flow controllers.
(1)
where J is the gas flux (mol m-2 s-1), F is the permeance (mol m-2 s-1 Pa-n), and n is a constant indicating the exponential dependency of the permeation rate on pressure. Hydrogen permeation generally is a rate-limiting step, which can be either a surface process (n ) 1) or a bulk-diffusion process (n ) 0.5, Sievert’s law). Thus, the n values range from 0.5 to 1.0, depending on the relative mass-transfer resistance of the layers. The conventional starting point for the description of gas permeation through porous media is the dust gas model.11 The permeance through a porous medium can be expressed as the
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Figure 5. Hydrogen flux through Pd composite membrane as a function of the pressure difference at different permeation temperatures: (∆) 823, (1) 773, (2) 723, (b) 673, and (9) 623 K.
Figure 4. SEM images of palladium membrane: (a) surface, (b) cross section.
sum of the Poiseille-flow part, FvPave, and the Knudsen-flow part, Fk F ) Fk + FvPave
(2)
Ph + Pl (3) 2 where F is the permeance (mol m-2 s-1 Pa-1), Pave is the mean pressure across the membrane (Pa), Ph is the pressure at the upstream side in the permeation measurement (Pa), and Pl is the pressure at the downstream side in the permeation measurement (Pa). Figure 5 summarizes the results of the hydrogen flux experiments performed with the prepared membrane. The hydrogen fluxes increased with increasing pressure difference and temperature. The solution-diffusion enhanced with increasing temperature, whereas the Knudsen diffusion exhibited the opposite trend. Thus, the tendency of the hydrogen flux can be attributed to the solution-diffusion mechanism. Hydrogen permeation generally is a rate-limiting step, which can be either a surface process (n ) 1) or a bulk-diffusion process (n ) 0.5, Sievert’s law). Thus, the n values range from 0.5 to 1.0, depending on the relative mass-transfer resistance of the layers. Nam et al.5 concluded that, as the thickness decreases (e.g., to 0.8 µm), the exponent increases, approaching 1. According to eq 1 and Figure 6, the n value was approximately 0.8 under all experimental conditions. This indicates that the hydrogen permeation performance diverged from Sievert’s law. Simultaneously, the nitrogen gas flux as a function of the pressure difference across the membrane during heating and cooling is given in Figure 7a,b, respectively. Figure 7a shows that the nitrogen flux increased with increasing pressure Pave )
Figure 6. Hydrogen flux through Pd composite membrane as a function of Ph. n ) 0.8 (solid line), 0.5 (dashed line), and 1 (dotted line).
difference and temperature. This is consistent with the results reported by the Xomeritakis group,12 who inferred that insufficient time was allowed for thermal equilibration at the intermediate-temperature points. The behavior in our work was speculated to be due to the presence of a few defects or pinholes in the skin layer at high temperature. Nitrogen leaking through membrane defects was quite dependent on temperature, indicating that the defects were not in the mesoporous or macroporous range. In Figure 7b, during the cooling stage of the experiment, the nitrogen flux exhibited the opposite trend as the temperature decreased, which can be attributed to the Knudsen diffusion mechanism. The mechanism suggests that, during heating, the occurrence of microstructure rearrangements results in a decrease of the size or number of pinholes and intercrystalline spaces that form diffusion pathways for nitrogen gas. In Figure 8a,b, the H2/N2 selectivity coefficients can be seen to decrease with increasing mean pressure, mainly due to Poiseuille flow. Furthermore, the selectivity coefficients increase with increasing temperature. In fact, nitrogen gas can permeate through the pinholes only by Knudsen and surface diffusion, whereas hydrogen gas permeates through the dense metal layer by solution-diffusion and the pinholes by Knudsen and surface diffusion. As the temperature increases, solution-diffusion and Knudsen diffusion exhibit opposite trends, which leads to an increase in hydrogen permeation and a decrease in nitrogen permeation. Figure 8b shows that the H2/N2 separation factors for the palladium composite membrane obviously decreased,
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Figure 7. N2 permeation flux as a function of the pressure difference at different temperatures: (a) during the heating process at (∆) 823, (2) 723, (b) 673, and (9) 623 K; (b) during the cooling process at (∆) 823, (1) 773, (2) 723, (b) 673, and (9) 623 K.
which is consistent with the results presented in Figure 7. As shown, at 673 K and 0.1 MPa pressure difference, the hydrogen flux and H2/N2 selectivity through the palladium composite membrane were 0.27 mol m-2 s-1 and 361, respectively. Table 3 provides a comparison of the palladium membrane thickness and permeance performance between this study and those of other researchers. We can conclude that hydrogen permeability ranges on the order of 10-6-10-7 mol m-2 s-1 Pa-1. In our work, the value was on the order of 10-6 mol m-2 s-1 Pa-1, and the higher permselectivity and smaller thickness of the palladium membrane would reduce the cost. Palladium membranes should demonstrate long-term stability to be viable for industrial applications. Shown in Figure 9 are the hydrogen flux data of a palladium composite membrane operated in two heating cycles (at 0.1 MPa). As can be seen, the hydrogen flux kept a steady value under the same conditions, which indicates that the resulting palladium membrane has a better thermal stability. At 673 K and 0.1 MPa differential pressure, however, for the third heating cycle (see Figure 10), the nitrogen flux rapidly increased from 7.5 × 10-4 to 2.9 × 10-2 mol m-2 s-1, and the hydrogen flux increased slightly from 0.27 to 0.31 mol m-2 s-1. Moreover, the H2/N2 selectivity coefficient decreased from 361 to 10. Because the permeance data were measured during the heating and cooling stages of the experiment, rearrangements of the microstructure of the palladium membrane took place that resulted in the change in the palladium membrane performance. Moreover, the membrane permeation area changed slightly due to O-ring deformation during the heating stage. In addition, the change in results was presumably caused by the formation of microcracks in the palladium membrane, probably because of its ultrathinness. Su
Figure 8. H2/N2 selectivity as a function of mean pressure at different temperatures: (a) during the heating process and (b) during the cooling process at (∆) 823, (1) 773, (2) 723, (b) 673, and (9) 623 K. Table 3. Hydrogen Permeance through Pd Membranes Prepared by Different Methods D permeance selectivity method membrane type (µm) (mol m-2 s-1 Pa-1) (H2/N2) electroless Pd-Cu/alumina plating Pd/alumina CVD Pd/SUS-SiO2 PCD Pd/TiO2-Al2O3ZrO2-
11 1 2 1
2.3 × 10-6 (723 K) -7
2.2 × 10 (723 K) 2.7 × 10-6 (773 K) 2.7 × 10-6 (673 K)
1150
ref 4
20-130 7 300 13 361 this work
et al.13 reported permeances to H2 and N2 of membranes with different thicknesses at 773 K. They observed that the thermal stability of the membrane increased with the thickness of the membrane. Accordingly, there are complicated reasons for the change in the palladium membrane performance during the third cycle. Proper preparation conditions and permeation operation will improve the long-term stability of palladium membranes. The relationship between the temperature and the gas premeance can be expressed by an Arrhenius equation as
Figure 9. Variation of the hydrogen flux with heat cycles (∆P ) 0.1 MPa).
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F ) F0 exp(-Ea/RT )
(4)
where F0 is the pre-exponential factor and Ea is the apparent activation energy. Figure 11 shows the Arrhenius relation between the rate of hydrogen permeance and the temperature. The average activation energy of the resulting palladium composite membrane was calculated to be 17 KJ/mol in the temperature range of 623-823 K, which includes the energy barriers for dissolution and diffusion of hydrogen in the palladium layer and hydrogen permeation in the porous support. As shown, a thinner metallic film would result in a relatively higher activation energy value, indicating that hydrogen transport in submicron-thick palladium membranes is dominated by surface phenomena such as adsorption of H2 on metallic surfaces and dissociation to atomic hydrogen. According to eqs 1 and 4 and Figure 11, the permeation equation of the resulting palladium membrane in the temperature range of 623-823 K is given by
(
J ) 4.88 × 10-5 exp -
2.045 (Ph0.8 - Pl0.8) T
)
Figure 10. Comparison of three heating cycles (∆P ) 0.1 MPa, T ) 673 K).
Figure 11. Arrhenius plot of hydrogen permeability through the membrane (∆P ) 0.05 MPa).
4. Conclusions An improved electroless plating method was proposed to produce palladium composite membranes. A new activation technique by photocatalytic deposition (PCD) of palladium was presented that is attractive for its avoidance of codeposited impurities on the surface of the resulting composite membrane. The thickness of the palladium composite membrane was 1 µm. The hydrogen flux and selectivity through the palladium composite membrane were 0.27 mol m-2 s-1 and 361, respectively, at 673 K and 0.1 MPa differential pressure. Hydrogen flux data of the palladium composite membrane indicated steady operation during two thermal cycles. The average activation energy of this ultrathin membrane for hydrogen permeation was 17 kJ/mol, and the n value was approximately 0.8 under all experimental conditions tested. The hydrogen permeation performance diverged from Sievert’s law. The permeation equation of the resulting palladium membrane in the temperature range of 623-823 K is given by J ) 4.88 × 10-5 exp(-2.045/T)(Ph0.8 - Pl0.8). Literature Cited (1) Paglieri, J. D.; Way, J. D. Innovations in palladium membrane research. Sep. Purif. methods 2002, 31, 1. (2) Huang, I.; Chen, C. S.; He, Z. D. Pd membranes supported on porous ceramics prepared by chemical vapor deposition. Thin Solid Films 1997, 302, 98. (3) Brault, P.; Thomann, A. L.; Vignolle, C. A. Percolative growth of Pd ultrathin films deposited by plasma sputtering. Surf. Sci. 1998, 406, 597. (4) Roa, F.; Way, D.; McCormick, R.; Paglieri, S. Preparation and characterization of Pd-Cu composite membranes for hydrogen separation. Chem. Eng. J. 2003, 93, 11. (5) Nam, S. E.; Lee, S. H.; Lee, K. H. Preparation of a Pd alloy composite membrane supported in a porous stainless steel by vacuum electrodeposition. J. Membr. Sci. 2001, 15, 163. (6) Paglieri, S. N.; Way, J. D. A new preparation technique for Pd/ alumina membranes with enhanced high-temperature stability. Ind. Eng. Chem. Res. 1999, 38, 1925. (7) Zhao, H. B.; Pflanz, J. H.; Gu, A. W.; Li, N. Preparation of palladium composite membranes by modified electroless plating procedure. J. Membr. Sci. 1998, 14, 147. (8) Li, X.; Fan, Y. Q.; Jin, W. Q.; Xu, N. P. Improved photocatalytic deposition of palladium membranes. J. Membr. Sci. 2006, 282, 1. (9) Ding, X. B.; Fan, Y. Q.; Xu, N. P. A new route for the fabrication of TiO2 ultrafiltration membranes with suspension derived from a wet chemical synthesis. J. Membr. Sci. 2006, 270, 179. (10) Huang, Y.; Li, X.; Fan, Y. Q.; Xu, N. P. Progress in palladiumbased composite membranes: Principle, preparation and characterization. Prog. Chem. 2006, 18, 230. (11) Tuchlenski, A.; Uchytil, P.; Morgenstern, A. S. An experimental study of combined gas phase and surface diffusion in porous glass. J. Membr. Sci. 1998, 140, 165. (12) Xomeritakis, G.; Lin, Y. S. CVD synthesis and gas permeation properties of thin palladium/alumina membranes. AIChE J. 1998, 1, 174. (13) Su, C. L.; Jin, T.; Kuraoka, K. J. Thin palladium film supported on SiO2-modified porous stainless steel for a high-hydrogen-flux membrane. Ind. Eng. Chem. Res. 2005, 44, 3053.
ReceiVed for reView March 22, 2008 ReVised manuscript receiVed October 29, 2008 Accepted November 13, 2008 IE8004644
