Ind. Eng. Chem. Res. 1996, 35, 973-977
973
RESEARCH NOTES Thin Palladium Film Formation on Shot Peening Modified Porous Stainless Steel Substrates Naceur Jemaa, Jun Shu, Serge Kaliaguine, and Bernard P. A. Grandjean* Department of Chemical Engineering and CERPIC, Laval University, Ste-Foy, Quebec, Canada G1K 7P4
The rapid development of catalytic membrane reactors requires materials with a higher permeability and a better mechanical stability than the current thick membranes. Pd-based composite membranes supported on porous stainless steel offer such an alternative. However, commercially available porous stainless steel materials have to be further worked to reduce the surface pore sizes and ensure the formation of thin Pd films in an impervious form. In this work, a shot peening treatment was performed on the surface of a porous stainless steel to modify its surface pore size. Substrates with effectively reduced surface pore sizes were obtained under mild peening conditions. The permeation behavior of the substrates was examined using argon as the permeation gas. Impervious thin palladium films were deposited on the modified porous substrate surface by electroless plating. Hydrogen permeability through the resulting membranes was found to be comparable to that of pure palladium sheets, while the permeation flux was significantly enhanced due to the use of the thin Pd membrane. Introduction Palladium-based films are permeable to hydrogen. They are catalytically active toward many hydrogeninvolving reactions such as hydrogenation. Due to the combination of these functions, Pd-based catalytic membrane reactors have received much attention in the recent years. The removal of hydrogen from a reaction medium is proved to be effective in shifting equilibriumlimited dehydrogenation reactions toward the product side. For hydrogenation reactions, the controlled introduction of hydrogen often results in a high selectivity of target products. Currently, the development of permselective thin membranes with a high permeability, a good thermal stability, and a good mechanical strength is an intensely sought target. Generally speaking, massive palladium sheets are not suitable for the use in membrane reactors. On the one hand, hydrogen permeation flux is inversely proportional to the membrane thickness. Membranes must be thin enough (for example, less than 20 µm) to ensure a reasonable permeation flux. Such a thin membrane thickness does not have the mechanical strength to be self-standing. On the other hand, economic considerations do not allow the industrial use of the expensive Pd in the massive form. Composite palladium membranes meet to some extent these criteria and are extensively employed in membrane reactors to separate mainly the produced hydrogen (Armor, 1992; Shu et al., 1991; Uemiya et al., 1988; Collins and Way, 1993). Among various substrates studied, porous stainless steel is advantageous due to its thermal expansion coefficient close to that of Pd-based films, ease to process, corrosion resistance, high thermal stability, high mechanical strength, and so forth (Shu et al., 1993, 1994). However, commercially available sintered porous stainless steel substrates in forms of either sheets or tubes have too large pores to be directly useful as * Author to whom correspondence should be addressed.
0888-5885/96/2635-0973$12.00/0
supports for Pd deposition due to the limitation of powder metallurgical technology (ASM, 1982b). SEM studies indicated that pores as large as several microns exist in the porous stainless steel sheet with a nominal particle retention size of 0.5 µm (Shu et al., 1993). To obtain an impervious composite membrane, a relatively thick Pd film has to be deposited on the substrate surface by either electroplating or electroless plating. As mentioned above, a thick Pd film implies not only a low hydrogen permeation flux (thus a limited hydrogen separation capacity) but also a high investment level. One of solutions to solve this problem is the surface modification of the sintered stainless steel substrate aiming at reducing its surface pore size. In this work, shot peening was practiced on the porous stainless steel substrate surface. The shot peening treatment impinges a stream of shot in a high velocity at a metal surface. This action produces slight, rounded depressions in the surface, stretching it radially and causing plastic flow of surface metal at the instant of contact (ASM, 1982a). As a result, modified porous stainless steel substrates with reduced surface pore sizes would be obtained. Thinner palladium films were further deposited on the modified substrate surface in a dense form. Gas permeation behaviors through the prepared thin Pd/stainless steel membranes were also investigated. Experimental Section Shot Peening Treatment. A porous 316L stainless steel sheet with a thickness of 1 mm and a nominal particle retention size of 0.5 µm was commercially obtained from Mott Metallurgical. It was cut into disks of 36 mm in diameter. The disks first experienced a shot peening treatment for a few minutes. Iron shots with an average diameter of less than 125 µm were used to hit the metal surface. For comparison, iron shots with diameters larger than 125 µm were also tested. These shots were released under a peening pressure of © 1996 American Chemical Society
974
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
Results and Discussion
Figure 1. Experimental apparatus for gas permeation.
5.1 × 105 Pa from an ordinary sand blasting machine. During shot peening, specimens were fixed 0.25 m below the shot release nozzle. After shot peening, specimens were purged with air under a pressure of 5.1 × 105 Pa for 5 min to remove residual iron shots. Electroless Pd Plating. After the shot peening treatment, the modified substrates were washed in an ultrasonic bath of carbon tetrachloride for 1 h to remove possible contaminants. For the electroless plating, the membrane surfaces were first sensitized and activated by successive dipping in two solutions containing tin chloride and tetraammine palladium nitrate (Pd(NH3)4(NO3)2), respectively. The dipping sequence was repeated 10 times. The substrates were then air-dried at 393 K overnight. The detailed electroless Pd plating procedures were previously described by Shu et al. (1993). The bath was composed of 4.5 g/L of Pd(NH3)4(NO3)2, 50 g/L of Na2EDTA‚2H2O, 198 mL/L of 28% NH3, and 5 mL/L of 1 M N2H4. The deposition was conducted at 323 K. Under these conditions, palladium was deposited at a rate of about 1.5 µm/h. After deposition, the substrate was washed with deionized water and dried at 393 K. The deposited Pd thickness was calculated based on the weight gain of the specimen during the deposition assuming a continuous uniform film. Scanning Electron Microscopy (SEM). The surface morphology of the specimens was characterized using a scanning electron microscope (JEOL JSM-840A). The deposit composition was analyzed with the aid of an energy-dispersive X-ray analyzer (EDX; Tracor Northern). Gas Permeation Measurements. Gas permeation behaviors through the modified substrates and the prepared Pd membranes were tested in the setup shown in Figure 1. The membrane disk was installed in the membrane cell with a copper O-ring for sealing. The exposed membrane surface area was about 8 × 10-4 m2. Argon was used as the permeation gas to investigate the permeation behavior of the shot peening treated substrates. Hydrogen permeation was performed only in the case of Pd membranes. Pure hydrogen was introduced into the upstream compartment of the membrane cell at a pressure of p1. The downstream side (permeation side) was maintained at atmospheric pressure (p2). The flowrate of the permeated hydrogen was measured from the exit of the downstream side using a bubble flowmeter. An electric oven was used to heat the membrane cell. The temperature was measured using a thermocouple placed close to the membrane.
Surface Morphology of Substrate upon Shot Peening. Shot peening is originally a method of cold working that has a major purpose to increase the fatigue strength of the treated surface (ASM, 1982a). Small shots with high velocity strike the designated area for a period of time. In the present work, the purpose behind shot peening is to reduce pore openings on the surface of the porous stainless steel substrate. Various shot peening conditions, such as shot size, shot-releasing pressure, and peening time, were examined in order to obtain a substrate with an adequately reduced surface pore size. The shot-releasing pressure effect was first examined. The peening treatments were carried out at shotreleasing pressures of 5.1 × 105 and 6.5 × 105 Pa for 5 min. The surface morphology of the original and treated substrates was examined using SEM, as shown in Figure 2. As can be seen from the original stainless steel surface, surface pore openings as large as 5-6 µm exist (Figure 2a). These pores were significantly reduced upon the shot peening action at 5.1 × 105 Pa. The resultant pore size was uniform and was estimated to be around 1 µm (Figure 2b). This image also shows a relatively flat modified surface. A further strong peening action at 6.5 × 105 Pa gave an overpeened surface morphology (Figure 2c). In this case, the high shotreleasing pressure exerted a high velocity on the iron shots. Obvious defects can be seen from the damaged surface, which is undesired for the purpose of Pd deposition. Shots larger than 125 µm were also tested. Upon shot peening at 5.1 × 105 Pa for 5 min, the surface morphology was observed using SEM, as shown in Figure 2d. It can be seen that the use of larger shots resulted in a stronger impingement against the substrate surface and thus a rough surface. To verify if there are some shots retained in the substrate surface upon the shot peening treatment, an energy-dispersive X-ray analysis (EDX) was performed to analyze the surface composition of the original stainless steel and the specimen shot-peened at 5.1 × 105 Pa for 5 min (after CCl4 cleaning). The composition results are listed in Table 1. The composition of the original substrate was found to be a little different from the nominal composition of 316L stainless steel. In particular, a high level of argon was detected, which resulted from the exposure of the substrate to argon for gas permeation measurements. In comparison with the original stainless steel substrate, the surface composition after the shot peening had only a minor increase in iron percentage due to the shot peening action of iron particles. Shot peening time is the most important factor in modifying the surface pore sizes of the substrate surface. This can be reflected by the measurement of argon permeation through specimen disks. Such results are presented in Figure 3. The argon permeation flux depends strongly on the shot peening time. Disks upon shot peening for 1 and 5 min exhibited a significantly reduced argon permeation flux compared to the original substrate. A treatment for 10 min under a shotreleasing pressure of 5.1 × 105 Pa was found to almost block the passage of argon through the membrane under the studied pressure range (for clearness, the curve was not drawn on the figure). SEM observations of the peened surface morphology indicated that a long shot peening time damaged the substrate surface rather than
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 975
Figure 2. Scanning electron micrographs: (a, top left) original stainless steel; (b, top right) shot peened at 5.1 × 105 Pa; (c, bottom left) shot peened at 6.5 × 105 Pa; (d, bottom right) with shots >125 µm. Table 1. EDX Data of Surface Compositions (at %) specimen
Mo
C
Ar
O
Cr
Fe
Ni
original shot peened
1.6 ( 0.2 1.4 ( 0.2
10.4 ( 1.1 4.4 ( 0.8
3.0 ( 0.4 3.6 ( 0.4
10.2 ( 0.5 8.5 ( 0.6
10.3 ( 0.4 10.3 ( 0.4
61.0 ( 1.0 67.0 ( 1.0
3.5 ( 0.4 4.7 ( 0.6
reduced the pore size. Therefore, the shot peening with shots of less than 125 µm under a releasing pressure of 5.1 × 105 Pa for 5 min was chosen for further experiments. The resultant modified surface structure was found to be reproducible under this treatment. Pd Deposition on Modified Porous Stainless Steel Substrate. Palladium deposition was performed on the shot-peened stainless steel substrate by electroless plating in the hydrazine solution bath. After 1 h of deposition, the treated stainless steel surface was covered with a thin and bright Pd film. The deposited Pd was estimated to be 2 µm thick based on the weight gain of the substrate. Figure 4 shows SEM observations of the deposited Pd film. The membrane surface was relatively uniform. An EDX analysis confirmed the formation of Pd film. However, this specimen was still permeable to argon. Thus, a relatively thicker film was required to completely cover the substrate, giving a hydrogen permselective membrane. A Pd membrane of about 6 µm thick was further prepared after 4 h of deposition. No argon permeation through this sample was detected at 673 K under the studied pressure range, indicating no pinholes or cracks developed during preparation.
Figure 3. Argon permeation flux before and after shot peening at 5.1 × 105 Pa: (1) original stainless steel; (2) 1 min; (3) 5 min.
Hydrogen Permeation through Pd/Stainless Steel Membranes. It is known that hydrogen dissolves into
976
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996
Figure 4. Scanning electron micrograph of the Pd deposited on porous stainless steel modified with shot peening at 5.1 × 105 Pa: (a, left) ×1000; (b, right) ×10000. Table 2. Comparison of Hydrogen Permeation Fluxes (F) through Pd Membranes
a
thickness, µm
∆P, Pa
T, K
F, m3/s‚m2
Fequiv,a m3/s‚m2
ref
6 20 20 25
1 × 105 1 × 105 2 × 105 3.4 × 105
673 773 673 813
1.3 × 10-3 1.1 × 10-3 1.8 × 10-3 1.9 × 10-3
1.3 × 10-3 1.1 × 10-3 1.0 × 10-3 0.95 × 10-3
this work Shu (1994) Uemiya et al. (1991) Grashoff et al. (1983)
Value equivalent to the permeation flux at p10.5 - p20.5 of 132 Pa0.5.
Figure 5. Hydrogen permeability after Pd deposition on porous stainless steel modified with shot peening at 5.1 × 105 Pa.
palladium at low temperatures to form R-hydride and further β-hydride. The formation of a β-hydride phase induces a significant lattice expansion of the Pd metal, the recycling of which often results in the membrane embrittlement. For the measurement of hydrogen permeability through Pd membrane, hydrogen has to be introduced at temperatures higher than 573 K. Therefore, the hydrogen permeability through the above 6-µm Pd membrane was measured at 673 K. The obtained hydrogen permeation flux is plotted versus the driving force (p10.5 - p20.5), as shown in Figure 5. With raising the permeation driving force, the measured permeation fluxes lie on a straight line under the
studied pressure range. This indicates essentially an atomic diffusion of hydrogen through the Pd membrane. The hydrogen permeation flux through the present 6-µm Pd membrane at 673 K and a pressure difference of 1 × 105 Pa was found to be 1.3 × 10-3 m3/s‚m2. This value is comparable to the literature data. Table 2 gives a comparison of hydrogen permeation fluxes. The hydrogen permeation flux through a 20-µm-thick Pd membrane deposited directly on stainless steel (no shot peening) at 773 K and 1 × 105 Pa pressure difference was about 1.1 × 10-3 m3/s‚m2 (Shu, 1994) and was 1.9 × 10-3 m3/s‚m2 through a 25-µm palladium membrane at 813 K and a pressure difference of 3.4 × 105 Pa (Grashoff et al., 1983). The increased permeation flux in this study is believed to result from the fact that the shot peening action reduced the surface pore sizes, thus requiring a thinner Pd film to cover the pores. At temperatures higher than 573 K, the composite Pd membrane was found to be mechanically stable. However, below 523 K, the Pd film was destroyed. This was attributed to the phase transformation from R-hydride to β-hydride. Thus, it is desired to operate Pd membranes always at elevated temperatures, for example, over 573 K. Another solution to this problem is the use of Pd-Ag alloy membranes which can be prepared by the simultaneous deposition of Pd and Ag on porous substrate (Shu et al., 1993) or a two-step deposition procedure (Shu et al., 1995). Conclusions The coating technology of thin palladium membranes on porous stainless steel substrate was examined in this study. A shot peening action was practiced on the porous stainless steel to reduce the surface pore sizes. Under mild peening conditions, the substrate surface pores were effectively reduced. Thin palladium films were deposited by electroless plating in a hydrazine solution bath. A 6-µm Pd membrane was prepared and found to be permeable to hydrogen. The measured
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 977
hydrogen permeation flux was comparable to the literature values. Acknowledgment This work was supported by the Natural Sciences and Engineering Research Council (NSERC) of Canada. The authors thank Professor Rock Anger for useful suggestions about the shot peening treatment. Literature Cited American Society for Metals. Metals HandbooksSurface Cleaning, Finishing and Coating, 9th ed; ASM: Metals Park, OH, 1982a; Vol. 5, p 138. American Society for Metals. Metals HandbooksPowder Metallurgy, 9th ed.; ASM: Metals Park, OH, 1982b; Vol. 7, p 697. Armor, J. N. Challenges in Membrane Catalysis. CHEMTECH 1992, 557. Collins, J. P.; Way, J. D. Preparation and Characterization of a Composite Palladium-Ceramic Membrane. Ind. Eng. Chem. Res. 1993, 32, 3006. Grashoff, G. J.; Pilkington, C. E.; Corti, C. W. The Purification of HydrogensA Review of the Technology Emphasing, the Current Status of Palladium Membrane Diffusion. Platinum Met. Rev. 1983, 27, 157. Shu, J. Development of Palladium-based Catalytic Membrane Reactors. Ph.D. Dissertation, Laval University, QC, Canada, 1994.
Shu, J.; Grandjean, B. P. A.; van Neste, A.; Kaliaguine, S. Catalytic Palladium-based Membrane Reactors: A Review. Can. J. Chem. Eng. 1991, 69, 1036. Shu, J.; Grandjean, B. P. A.; Ghali, E.; Kaliaguine, S. Simultaneous Deposition of Pd and Ag on Porous Stainless Steel by Electroless Plating. J. Membr. Sci. 1993, 77, 181. Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Methane Steam Reforming in Asymmetric Pd- and Pd-Ag/Porous SS Membrane Reactors. Appl. Catal. A 1994, 119, 305. Shu, J.; Adnot, A.; Grandjean, B. P. A.; Kaliaguine, S. Structurally Stable Composite Pd-Ag Membranes: Introduction of a Diffusion Barrier. Thin Solid Films 1995, in press. Uemiya, S.; Kude, Y.; Sugino, K.; Sato, N.; Matsuda, T.; Kikuchi, E. Palladium/Porous Glass Composite Membrane for Hydrogen Separation. Chem. Lett. 1988, 1687. Uemiya, S.; Sato, N.; Ando, H.; Matsuda, T.; Kikuchi, E. Steam Reforming of Methane in a Hydrogen-Permeable Membrane Reactor. Appl. Catal. 1991, 67, 223.
Received for review July 14, 1995 Revised manuscript received November 20, 1995 Accepted December 12, 1995X IE950437T
X Abstract published in Advance ACS Abstracts, February 1, 1996.
