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Chromium oxide intermetallic diffusion barrier for palladium membrane supported on porous stainless steel
Journal of Membrane Science 347 (2010) 8–16
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Chromium oxide intermetallic diffusion barrier for palladium membrane supported on porous stainless steel Sutheerawat Samingprai a , Supawan Tantayanon b,c,∗ , Yi Hua Ma d a
Graduate Program of Petrochemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Green Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand National Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand d Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, USA b c
a r t i c l e
i n f o
Article history: Received 16 May 2009 Received in revised form 28 September 2009 Accepted 30 September 2009 Available online 7 October 2009 Keywords: Intermetallic diffusion barrier Electroless plating Palladium membrane Chromium oxide
a b s t r a c t Dense palladium membrane on oxidized porous stainless steel (oxPSS) tube was prepared. Its hydrogen permeance was observed to decline at the temperature higher than 400 ◦ C. SEM–EDX analysis of the cross-section of the annealed tube at 500 ◦ C indicated the occurrence of the intermetallic diffusion. The heat treatment study of the palladium membrane on oxPSS disks in hydrogen atmosphere at various temperatures was essentially carried out. Their SEM–EDX analysis results confirmed that the in situ metal oxide barrier could not inhibit the intermetallic diffusion in hydrogen atmosphere. The chromium oxide layers at different thicknesses were then developed on oxPSS disks before palladium plating by controlled chromium elctrodeposition followed by oxidation in air at 700 ◦ C. The similar heat treatment study and SEM–EDX analysis of these disks revealed that the presence of chromium oxide layer could suppress the intermetallic diffusion. Then, the dense palladium membrane tube with chromium oxide layer was prepared and its heat treatment in hydrogen atmosphere was studied. The result showed the steady increase in hydrogen permeance with increasing temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has long been used in several industrial applications including food, pharmaceuticals, chemical processing, electronics, metal production and fabrication, and petroleum recovery and refinery. Currently it has been of great interest as a clean alternative energy. Steam reforming of hydrocarbons is the major source of hydrogen production. Many reports on methane steam reforming using the catalytic palladium membrane reactor were published [1–6]. A few reports on methane dry reforming in the catalytic palladium membrane reactor were recently published as well [7–9]. Although the process was not optimized, the feasibility of achieving the enhancement of the CH4 conversion and CO and H2 selectivity beyond the limits stipulated by thermodynamic equilibrium during reforming of methane to synthesis gas was convincingly demonstrated by the use of a palladium membrane reactor comparing to a conventional reactor as reported by Galuszka et al. [7]. Among several types of membranes [10–14], the palladium and palladium alloy membranes were tested to have high selectivity
∗ Corresponding author at: Green Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. Tel.: +66 2 218 7641; fax: +66 2 218 7598. E-mail address: [email protected] (S. Tantayanon). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.09.058
[15–17]. Although palladium supported on porous stainless steel had several advantages comparing to other supported materials, its use at high temperature would cause the long-term stability problem due to the intermetallic diffusion of the metal elements from the support into palladium layer. Accordingly, much attention had been paid on preventing the intermetallic diffusion by creating an additional layer between the stainless steel support and palladium layer. These include tungsten and a tantalum oxide layer [18], aluminium oxide [19], zirconia and titania [20], zeolite [21] and tungsten oxide [22]. Silica [23] was also used as a diffusion barrier between the palladium–copper active layer and nickel–porous stainless steel composite membranes. In 1998, Ma and his co-workers had introduced an oxide-layer diffusion barrier by controlled in situ oxidation of the porous stainless steel supports prior to palladium and palladium/silver metal deposition [24–28]. They also reported that the oxidation had little effect on the mean pore size of the porous stainless steel indicating that oxidation did not constrict the internal pore system even at 800 ◦ C [27]. They further explained that the oxide layers formed at temperatures equal or higher than 500 ◦ C was effective in terms of preventing intermetallic diffusion on hydrogen permeance. They also showed that the structure of the oxide layer formed at 600 ◦ C in air was similar to the one formed at 800 ◦ C, which composed of an iron oxide rich layer on top of a chromium oxide layer [26]. However, the suitable material as the intermetallic diffu-
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Fig. 1. Experimental setup for permeation measurement.
sion barrier should at least have the Tamman temperature higher than palladium (640 ◦ C or 913 K) and stainless steel (550–560 ◦ C or 823–833 K), as the rate of diffusion of metal elements at the interface between two metallic layers would become greatest when they were at or above their Tamman temperature [24]. Therefore, chromium oxide, with Tamman temperature of 1081 ◦ C (1354 K), can be one of the potential candidates. As no literature reports on using chromium oxide as the intermetallic diffusion barrier, it is interesting to investigate its performance. In this study, the incremental formation of chromium oxide layer on top of the in situ mixed oxides on porous stainless steel was described and its performance as an intermetallic diffusion barrier between palladium membrane and porous stainless steel was discussed. 2. Experimental 2.1. Palladium membrane preparation A porous 316L stainless steel (PSS) tube, welded to non-porous stainless steel tube at both ends, was purchased from Mott Metallurgical Corporation (Farmington, CT). It had 9 mm in outer diameter, 70 mm long, 1 mm wall thickness, 0.1 m average pore size and 17% porosity. The PSS disks were prepared by cutting a 1 mm thick media-grade 0.1 m 316L PSS sheet into 1 cm2 pieces and then a small hole with 1 mm diameter was drilled at one
corner of the disk for hanging during metal deposition. All PSS supports were cleaned before activation and metal deposition, with the alkaline solution containing 5 ml/l of a household detergent. PSS supports were then thoroughly washed with deionized water (DI water) until the rinsing water had pH 7 and finally immersed in iso-propanol. All cleaning steps were carried out in ultrasonic bath at 60 ◦ C. The supports were dried at 120 ◦ C for 4 h. The cleaned PSS tubes and disks were oxidized at 450 ◦ C with heating rate 4 ◦ C/min in air in the muffle furnace. The weight of each oxidized PSS (oxPSS) support was recorded. Prior to palladium plating, the surface of oxPSS supports was activated by the consecutive immersion in SnCl2 (0.1 g/l in 0.01 M HCl), PdCl2 (0.1 g/l in 0.01 M HCl) with intermediate rinsing in DI water between both solutions. Then the activated PSS supports were rinsed with 0.01 M HCl. A two-step immersion sequence in SnCl2 and PdCl2 solution was generally repeated 3 times. A perfectly activated layer had a dark-brown color and smooth surface. Then the activated supports were deposited with palladium using the typical palladium plating solution composed of 4.0 g/l of Pd(NH3 )4 Cl2 ·H2 O, 198 ml/l of 28% NH4 OH, 40.1 g/l of Na2 EDTA, and 5.6–7.6 ml/l of 1 M N2 H4 at 60 ◦ C for 90 min. It was then cleaned with the warm DI water and let it at room temperature. Plating and cleaning were repeated for 3 times more. Finally, they were dried at 120 ◦ C for 4 h. In case of the plated tube, it was subjected to the helium permeation flux measurement. If it was not dense, it would be submitted to another plating cycle. The palladium layer thickness
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of the plated tube and disks was determined from the weight increase. 2.2. Hydrogen permeance measurement The hydrogen permeation flux measurements were carried out with the experimental setup as shown in Fig. 1. The feed gas flowed upward through the shell side, and the permeant gas was collected on the tube side. The pressure at the shell side was monitored by a capacitance pressure transducer and the pressure at the tube side was kept atmospheric. A soap-bubble capillary flowmeter was used for flow rate less than 1 ml/min: a digital flowmeter (Alltech), for flow rate in the range of 1–200 ml/min; a wet-test meter for flow rate greater than 200 ml/min. Temperature controller (Omega CN9000) was used to control the temperature inside the furnace and inside the tube side. The temperature variation along the membrane was controlled not to exceed 3 ◦ C. Prior to the hydrogen permeation tests, the Pd membrane was heated in helium at a rate of about 1 ◦ C/min. The temperature of the inside tube was varied from 350 to 500 ◦ C. The hydrogen permeation flux and hydrogen permeance of the dense palladium membrane tube at each pressure were then calculated according to the following equations: hydrogen permeation flux (m3 /m2 h)
=
flow rate of hydrogen gas in tube side (ml/ min) active surface area (cm2 )
hydrogen permeance (m3 /m2 h atm0.5 )
=
hydrogen permeation flux (m3 /m2 h)
× 0.6
0.5 0.5 PH − PH (atm0.5 ) at shell side at tube side 2
Princeton Gamma-Tech Avalon Energy Dispersive X-ray (EDX) spectrophotometer light element detector for the qualitative and quantitative analysis. At an accelerating voltage of 20 kV, the spatial resolution for the SEM–EDX lies between 0.8 and 1.2 m for the samples investigated. For the SEM cross-section analysis, the samples were cut by using a SiC saw blade and mounted with phenolic powder in a Smithells II mounting press. The mounting samples were ground with SiC papers with increasing grain fineness from 80 to 400 grit. Grinding was performed by using Metaserv 2000 grinder-polisher. Prior to SEM cross-section analysis, samples were painted with carbon ink and gold-coated to avoid charging. 2.6. X-ray diffraction Phase identification analyses were carried out by Rigaku Geigerflex X-ray diffractometer equipped with a Cu K␣ ( = 1542 × 10−10 m (1.542 Å)) radiation source and a nickel filter. The voltage was set at 37.5 kV and current at 27 mA. The scan rate was 1–2◦ /min in the range 2 = 30–80◦ . 2.7. Temperature-programmed reduction Two samples, Cr/oxPSS and Cr2 O3 /oxPSS disks, were used in this study. The temperature-programmed reduction (TPR) experiments were carried out on AutoChem 2910 (Micromeritics, USA) instrument. Prior to TPR, each sample was pretreated by passing ultra high pure (99.999%) helium (30 ml/min) at 700 ◦ C for 2 h. After pretreatment, it was cooled to room temperature. The carrier gas consisting of 5% hydrogen and balance helium (30 ml/min) was purified by oxy-trap and molecular sieves. The data were recorded while the temperature was ramped from 100 to 900 ◦ C at a heating rate of 10 ◦ C/min.
2
3. Results and discussion 2.3. Preparation of Pd/Cr2 O3 /oxPSS Prior to chromium plating, the PSS tube and disks were cleaned with an alkali solution as described in Section 2.1 and oxidized by heating at 450 ◦ C for 6 h. Chromium was then deposited by electroplating technique, using lead as the anode and the oxPSS as the cathode immersed in the solution of chromic acid (250 g/l) and sulfuric acid (1.25 g/l) in a ratio of 200:1. The chromium plating was performed at room temperature with current density 100–150 A/ft2 for 2 and 5 min. Finally, the chromium layer was oxidized in air at 700 ◦ C for 6 h. The thickness of chromium and chromium oxide layers was determined from the weight increase. Both oxPSS tube and disks containing the chromium oxide layer were then subjected to palladium electroless plating as described in Section 2.1. 2.4. Intermetallic diffusion tests Pd/Cr2 O3 /oxPSS or Pd/oxPSS disks were placed in the tube side of the experimental setup (Fig. 1) with quartz wools on top and the bottom. The tube side was heated under helium flow (20 ml/min) at a ramp of 1 ◦ C/min. When the temperature reached 500 ◦ C, helium was replaced with pure hydrogen at a flow rate of 20 ml/min. After treatment at 500 ◦ C in hydrogen for 24 h, the pure hydrogen was replaced with helium before cooling the disks down to room temperature. SEM–EDX of these disks was then performed. 2.5. Scanning electron microscopy Surface characterization was performed using an Amray 1610 turbo Scanning Electron Microscope (SEM) equipped with a
3.1. Effect of temperature on the intermetallic diffusion of Pd/oxPSS The PSS tube was oxidized in air at 450 ◦ C (oxPSS), prior to palladium plating, to give the in situ metal oxide on the substrate surface which could function as the intermetallic diffusion barrier described by Ma et al. [24,26]. The dense palladium membrane supported on oxPSS tube (Pd/oxPSS) was prepared with palladium thickness of 17.5 m. It was then characterized by firstly measuring the helium permeation flux which was found that no helium was detected. This result indicated that it was a dense palladium membrane. According to the data obtained from the measurement of the hydrogen permeation flux in the temperature range from 350 to 450 ◦ C, the Sievert law plots for this membrane exhibited all straight lines with regression coefficient around 0.999 as revealed in Fig. 2. The results illustrated that the dependence of hydrogen solubility in palladium followed Sievert law and the hydrogen diffused through this palladium membrane only via solution-diffusion mechanism [29]. When the hydrogen permeance was plotted against time at the constant pressure difference of 1 atm between the tube and shell sides of palladium membrane tube, it was found that the hydrogen permeance increased when the temperature was raised as exhibited in Fig. 3. At 350 and 400 ◦ C, the hydrogen permeances were stable over 35 h at 1.12 and 1.36 m3 (STP)/m2 h atm0.5 , respectively. Although the hydrogen permeances at 450 and 500 ◦ C were higher than at 350 and 400 ◦ C, the decline of hydrogen permeances was observed which started to decrease from after the first 6 h of heating. Both became stable after 20 h of heating, from 2.04 to 1.72 m3 (STP)/m2 h atm0.5 at 450 ◦ C and from 2.08 to 1.64 m3 (STP)/m2 h atm0.5 at 500 ◦ C. These results illustrated the
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Fig. 2. Sievert law plots for palladium membrane of Pd/oxPSS tube.
Fig. 3. Hydrogen permeance of Pd/oxPSS tube at constant pressure difference of 1 atm with various temperatures.
possible diffusion of the metal elements from the substrate into the palladium membrane which would cause the suppression of the hydrogen permeation through the palladium membrane. The lower hydrogen permeance at 500 ◦ C, comparing to the one at 450 ◦ C, indicated that the palladium layer might contain higher content of other metal elements. In other words, more deterioration of the membrane was resulted at higher temperature. Fig. 4 showed the SEM micrograph and the corresponding EDX compositional depth profile of the cross-section of Pd/oxPSS tube after hydrogen exposure at 500 ◦ C for 35 h. It should be notified that the mixed metal oxide layer, as depicted to be generated by in situ oxidation of PSS at 450 ◦ C for 6 h, could not be seen in this
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SEM micrograph or detected by the corresponding EDX line scan. The similar observation was noted by Ma et al. [26] in the case of the oxidation of the PSS disk at 400 ◦ C that the mixed metal oxide layer might be too thin to be seen. From Fig. 4, it exhibited clearly that the diffusion of metal elements, Fe, Cr, and Ni from PSS into palladium layer, was taken place. The explanation to this result could be that the metal oxides were reduced to their metallic state when they were exposed to hydrogen [30] and the intermetallic diffusion barrier was therefore destroyed. Similarly, Ayturk et al. [31] used Pd/PSS cylindrical cup (oxidized at 400 ◦ C) and observed a steady decline of hydrogen permeance at 450 ◦ C within the first 24 h of testing. They concluded that the deterioration in hydrogen permeance, about 20 wt.% of the original permeance, of the membrane was attributed to the intermetallic diffusion of the support elements (Fe, Cr, and Ni) into dense palladium layer along with the formation of undesired surface [32]. The similar result was clearly observed in this study. To confirm this observation, another experiment was performed using three Pd/oxPSS disks with palladium thickness of 10 m. Two of them, Disks 2 and 3, were annealed under hydrogen atmosphere at 450 and 500 ◦ C, respectively. After hydrogen exposure for 24 h, the SEM micrographs and EDX composition depth profiles of these two disks were recorded comparing to the one without annealing, Disk 1. The higher content of Fe, Cr, and Ni in palladium layer was detected after hydrogen exposure at 500 ◦ C comparing at 450 ◦ C and no annealing, as revealed in Fig. 5. The metal distribution of Pd, Fe, Cr, and Ni in the palladium layer of each disk in percent by weight, as tabulated in Table 1, was recorded at the position in the plateau region of Pd composition away from the interface of Pd/oxPSS. When the disk was annealed, each metal element could diffuse into the palladium layer and the individual content depended on the nature of the metal element and annealing temperature. It was found that when the annealing temperature increased, the higher content of each of the metal elements was resulted which would arbitrarily lower the Pd content in the palladium layer. Among three metals, Fe was always found to be the highest content in palladium layer. These results were in consistent with the original metal composition of PSS in which the content of Fe was the highest, i.e., Fe 68–74 wt.%, Cr 16–18 wt.%, and Ni 10–14 wt.% Moreover, the palladium layer composition of Pd/oxPSS tube appeared to be very similar to the Pd/oxPSS disk annealed at the same temperature of 500 ◦ C. Accordingly, this study showed that the intermetallic diffusion of Pd/oxPSS could take place at the temperature as low as 450 ◦ C. It may be concluded that the in situ metal oxide layer on porous stainless steel created at 450 ◦ C (Pd/oxPSS) could probably be an effective intermetallic
Fig. 4. (a) SEM micrograph (2500×) of the cross-section of Pd/oxPSS tube after hydrogen exposure at 500 ◦ C for 35 h (b) and the compositional depth profile along the length of the arrow indicated in (a).
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Fig. 5. SEM micrographs (2500×) of cross-section of Pd membrane disks with EDX compositional depth profiles along the length of the arrows indicated in the corresponding micrographs, Pd/oxPSS disks (left) and Pd/Cr2 O3 /oxPSS disks (right): before (Disks 1 and 15) and after annealing in hydrogen atmosphere for 24 h at 450 ◦ C (Disks 2) and 500 ◦ C (Disks 3, 16 and 17).
diffusion barrier in other systems, but not in hydrogen at 450 ◦ C and higher. 3.2. Preparation of chromium oxide layer on oxPSS Chromium layer was generated on five pieces of the oxPSS disks as described in Section 2.3. The electroplating time of 5 min was applied for every disk. The chromium thickness of each disk was calculated from the gained weight. The average chromium thickness of 10 m with the relative standard deviation (RSD%) of 0.96
was obtained. The deposition of chromium was visually observed as gray color. The surface morphology of these disks became rough and different from the ones before chromium deposition as shown in Fig. 6(a) and (b). In addition, the EDX spectrum of Disk 4, as shown in Fig. 6(d), confirmed the composition of the disk surface to be only chromium element. Then the chromium oxide layer was generated by oxidizing in air at 700 ◦ C for 6 h. The average thickness of chromium oxide was 9.9 m with the RSD% of 0.99. The chromium oxide was obviously seen as the color was changed from gray to green. The chromium
Table 1 Preparation condition and elemental distribution in palladium layers of Pd/oxPSS and Pd/Cr2 O3 /oxPSS disks and tubes. Disk no.
Cr oxidation temperature (◦ C)
Cr2 O3 thickness (m)
Pd thickness (m)
Pd/oxPSS 1 2 3 Pd/oxPSS tube
– – – –
– – – –
9.9 9.9 10.0 17.5
Pd/Cr2 O3 /oxPSS 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Pd/Cr2 O3 /oxPSStube
700 700 700 700 700 – 400 500 600 700 800 700 700 700 700
9.9 10.0 9.8 10.0 9.9 None None None 3.0 10.0 10.0 11.1 10.1 2.0 2.0
10.0 10.0 9.9 10.0 9.9 9.9 10.0 9.9 9.8 9.9 10.0 9.7 9.8 9.7 32.0
Annealing temperature in H2 (◦ C)
Pd layer composition (wt.%) Pd
Fe
Cr
Ni
– 450 500 500
95.0 86.0 79.0 80.0
3.0 7.0 13.0 12.0
1.0 4.0 5.0 5.0
1.0 3.0 4.0 3.0
– – – – – – – – – – – – 500 500 500
– – – – – – – – – – – 96.0 96.3 95.0 –
– – – – – – – – – – – 2.0 2.3 3.0 –
– – – – – – – – – – – 1.0 1.0 1.0 –
– – – – – – – – – – – 1.0 0.6 1.0 –
Remark: Elemental distribution in palladium layer of all disks was measured after 24 h of hydrogen exposure at 500 ◦ C.
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Fig. 6. SEM micrographs (2500×) of Disk 4, before (a) and after chromium plating (b), after air oxidation at 700 ◦ C for 6 h (c), and EDX spectra of Disk 4 before (d) and after oxidation of chromium (e).
oxide disks were taken for further investigation on its morphology and composition by SEM equipped with EDX area scan mode, which were revealed in Fig. 6(c) and (e). The surface morphology of the top layer on the oxPSS disk was changed to a fluffier surface. From EDX spectrum, it indicated the presence of Cr atoms and O atoms. The XPS spectra of Disk 4 showed the O1S peak with the binding energy at 530.1 eV (Fig. 7(a)) indicating the presence of the oxide oxygen, whereas the binding energy at 576.9 eV in Fig. 7(b) was assigned to Cr2P3/2 peak of Cr2 O3 . According to the binding energy and peak shape, it was confirmed that the oxide of chromium was in the form of Cr2 O3 [33–35]. These results illustrated that the chromium and chromium oxide layers were easily generated by electroplating followed by oxidation in air, and their thicknesses
Fig. 7. XPS spectra of Disk 4 deposited with chromium and oxidized in air at 700 ◦ C for 6 h O1S (a) and Cr2P3/2 (b).
Fig. 8. XRD patterns of chromium layer on oxPSS disks before (Disk 9) and after oxidation at 400 ◦ C (Disk 10), 500 ◦ C (Disk 11), 600 ◦ C (Disk 12), 700 ◦ C (Disk 13) and 800 ◦ C (Disk 14).
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could be well controlled. The formation of chromium oxide at various oxidation temperatures 400–800 ◦ C, was also investigated. The XRD spectra, as shown in Fig. 8, displayed that no chromium oxide formation could be detected at the oxidation temperature lower than 600 ◦ C. The increase in intensity of chromium oxide pattern was observed when higher oxidation temperature was applied. Diffraction lines appeared at 2 = 33.6, 36.2, 39.8, 41.5, 50.3, 54.6, 63.6, 65.2, and 73.2, respectively (ASTM 6-0504) [36], indicated the existence of chromium oxide. Although chromium oxide formed at 800 ◦ C seemed to be a little better than at 700 ◦ C, it was postulated that the calcinations at higher temperature might cause the collapse of the internal adjacent pores of porous stainless steel which could later create a partial blockage of the pore system of support [37]. The suitable oxidation temperature was therefore considered to be at 700 ◦ C for 6 h. 3.3. Intermetallic diffusion of Pd/Cr2 O3 /oxPSS disks Another three oxPSS disks, Disks 15, 16 and 17, were deposited with chromium by electroplating technique as described earlier. Disks 15 and 16 were electroplated with chromium for 5 min while Disk 17 was subjected to chromium deposition for 2 min. After oxidation in air at 700 ◦ C for 6 h, chromium oxide was yielded with the average thickness of 11.1, 10.0 and 2.0 m, respectively. All three disks were then plated with palladium. Disks 16 and 17 were annealed under hydrogen atmosphere at 500 ◦ C for 24 h. The SEM micrographs of their cross-sections comparing to Disk 15, which was not annealed, were taken as shown in Fig. 5. The corresponding compositional depth profiles of all three disks were also recorded using line scan mode of SEM equipped with EDX. The line scan data were taken from inside the PSS substrate, which was the darker part in the picture, to the outer surface of the palladium layer, the lighter one, as indicated by the arrow in each SEM micrograph. It should be pointed here that SEM–EDX profile of Disk 15 showed the distance between oxPSS/Cr2 O3 and Cr2 O3 /Pd interfaces to be 16 m. However, it could not represent its Cr2 O3 thickness since it was taken at only one area of the whole disk. The average thickness of Disk 15 was determined by the gravimetric method to be 11.1 m. Unfortunately, the SEM–EDX profile of Disk 17 with Cr2 O3 thickness of 2 m exhibited the unclear interface of Cr2 O3 /Pd due to the limitation of the spatial resolution for the SEM–EDX which was 0.8–1.2 m. However, the elemental distribution, Pd, Fe, Cr, and Ni, in the palladium layer of each disk in percent by weight was able to be determined and compared by recording at the position in the plateau region of Pd composition away from the interface of Cr2 O3 /Pd as exhibited in Table 1. It was found that Disks 15 and 16, containing Cr2 O3 approximately 10 m in thickness before and after hydrogen exposure, respectively, had the same elemental distribution in their palladium layers which were very similar to Disk 17, containing Cr2 O3 thickness of 2 m after hydrogen exposure. These results were also comparable to the elemental distribution in palladium layer of Disk 1 with no Cr2 O3 and no hydrogen annealing. As revealed in Fig. 5 and Table 1, the SEM–EDX profile of Disk 3, Pd/oxPSS, after 24 h of hydrogen exposure at 500 ◦ C, showed lower Pd composition but higher Fe, Cr and Ni composition in its palladium layer, i.e., 79, 13, 5 and 4 wt.%, respectively, than those of Disks 15, 16 and 17, Pd/Cr2 O3 /oxPSS. From all these data, it could be concluded that the intermetallic diffusion from stainless steel to the palladium layer was suppressed by the presence of Cr2 O3 . In addition, the temperature-programmed reduction (TPR) analysis, a common technique for evaluating the reducibility of metal oxides, was applied to investigate the stability of Cr2 O3 . For example, MoO3 was reduced in two steps which exhibited as two major reduction peaks at 767 and 997 ◦ C in its TPR profile [38]. In this study, TPR profile of Cr2 O3 was obtained by
Table 2 Helium permeation flux and permeance of Pd membrane tube with various Pd thicknesses.
oxPSS Cr/oxPSS (Cr thickness, 2 m) Cr2 O3 /oxPSS (Cr2 O3 thickness, 2 m) Pd/Cr2 O3 /oxPSS Cycle 1 (total Pd thickness, 5 m) Cycle 2 (total Pd thickness, 10 m) Cycle 3 (total Pd thickness, 15 m) Cycle 4 (total Pd thickness, 22 m) Cycle 5 (total Pd thickness, 32 m)
He permeation flux (m3 /m2 h)
He permeance (m3 /m2 h atm0.5 )
1.77 × 102 1.55 × 102
7.08 × 101 6.20 × 101
1.62 × 102
6.48 × 101
9.96 × 101
3.98 × 101
3.59 × 10−2
1.44 × 10−2
3.00 × 10−4
1.20 × 10−4
1.00 × 10−4
4.00 × 10−5
1.00 × 10−6 (dense)
4.00 × 10−7 (dense)
annealing Cr2 O3 /oxPSS disk in hydrogen atmosphere from 100 to 900 ◦ C. No hydrogen was consumed as no reduction peak was observed. The similar experiment with Cr/oxPSS was carried out as a controlled test which showed no reduction peak as well. It can therefore be interpreted that Cr2 O3 is stable and will not be reduced when it is exposed to hydrogen at 100–900 ◦ C. 3.4. Hydrogen permeance of palladium membrane of Pd/Cr2 O3 /oxPSS tube The dense palladium membrane with 32 m thickness and containing 2 m Cr2 O3 layer on a porous stainless steel (Pd/Cr2 O3 /oxPSS) tube was prepared as described in Sections 2.3 and then 2.1. The helium permeation flux of the tube was measured after each step of the preparation and the corresponding helium permeance was calculated as shown in Table 2. Helium permeation flux of Cr2 O3 /oxPSS tube was about the same as those of Cr/oxPSS and oxPSS tubes. These data indicated that chromium deposition and chromium oxide formation did not affect the helium permeation flux. It was thus reasonable to assume that the pore properties of PSS were not drastically changed. After each palladium plating, the helium permeation flux of the tube was measured as well. It was observed that lower helium permeation flux was detected. After the fifth palladium plating, the dense palladium membrane tube with 32 m palladium thickness was obtained. The Sievert law plots for this membrane were all straight lines with regression coefficient around 0.999 as revealed in Fig. 9. The
Fig. 9. Sievert law plots for palladium membrane of Pd/Cr2 O3 /oxPSS tube.
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Acknowledgements The authors would like to thank Bangkok Industrial Gas Co., Ltd., Bangkok, Thailand, for the support of specialty gases used in this research. One of the authors is grateful to Worcester Polytechnic Institute for providing him the scholarship during his one-year research in the institute. The courtesy of Mr. Leenawat Kanda, Ms. Thitinat Sukonket and Mr. Matthieu Simon Fleys on some sample measurements is acknowledged. References
Fig. 10. Hydrogen permeance of Pd/Cr2 O3 /oxPSS tube at constant pressure difference of 1 atm after 24 h of hydrogen exposure at 350–500 ◦ C comparing to Pd/oxPSS tube.
results illustrated that the hydrogen diffused through the palladium membrane of Pd/Cr2 O3 /oxPSS tube via the solution-diffusion mechanism [29]. The hydrogen permeance of the palladium membrane of Pd/Cr2 O3 /oxPSS tube was measured to be 0.84, 1.11, 1.34, and 1.51 m3 /m2 h atm0.5 at 350, 400, 450 and 500 ◦ C, respectively. It increased steadily with increasing temperature as shown in Fig. 10. However, Pd/Cr2 O3 /oxPSS tube had lower hydrogen permeance than Pd/oxPSS tube at every temperature. This could be attributed to the higher palladium membrane thickness of the first tube. Interestingly, no decline of the hydrogen permeance for the first tube was observed, whereas the latter one showed a drop in hydrogen permeance at 500 ◦ C. This observation conformed with the intermetallic diffusion tests of the Pd/Cr2 O3 /oxPSS disks that the Cr2 O3 layer could act as an intermetallic diffusion barrier for the palladium membrane supported on porous stainless steel. Although the decline of hydrogen permeance was also reported by other research groups and the cause due to the intermetallic diffusion was proposed [32], this study had been the first report to demonstrate that Cr2 O3 layer was the effective intermetallic diffusion barrier for palladium membrane supported on porous stainless steel tube in hydrogen exposure.
4. Conclusion It was found that the mixed metal oxide generated on porous stainless steel by in situ air oxidation at 450 ◦ C for 6 h could not function as the intermetallic diffusion barrier after annealing in hydrogen atmosphere. The intermetallic diffusion from porous stainless steel support to palladium membrane had occurred at 450 ◦ C and higher temperature. In this study the chromium oxide layer, in the form of Cr2 O3 was firstly introduced as the incremental layer before palladium plating. It was discovered that the presence of Cr2 O3 layer with 2 m thickness could suppress the intermetallic diffusion. In addition, the stability of Cr2 O3 in hydrogen atmosphere was demonstrated with its TPR profile and its formation did not affect the helium permeation flux of the oxPSS tube. Moreover, the hydrogen permeance of Pd/Cr2 O3 /oxPSS tube was observed to increase steadily with increasing temperature up to 500 ◦ C. All these results indicated that Cr2 O3 was the promising intermetallic diffusion barrier for palladium membrane supported on PSS which could be used in hydrogen atmosphere. However, the optimum thickness of the chromium oxide layer should be investigated in order to balance the hydrogen permeance of the membrane. The long-term stability of hydrogen permeance of palladium membrane containing chromium oxide is also required. Both investigations are currently in progress.
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[27] B.C. Akis, E.E. Engwall, I.P. Mardilovich, Y.H. Ma, Effect of the in situ formation of an intermetallic diffusion barrier layer on the properties of composite palladium membranes, ACS Fuel Chem. Div. Preprints 48 (1) (2003) 337. [28] F. Guazzone, M.E. Ayturk, Y.H. Ma, Effect of intermetallic diffusion barrier on the stability of composite Pd/PSS membranes at high temperatures, in: Proceedings of the 21st Annual International Pittsburgh Coal Conference Proceedings, September 13–17, Osaka, Japan, 2004. [29] J. Shu, B.P.A. Grandjean, A.V. Neste, S. Kaliaguine, Catalytic palladium-based membrane reactors: review, Can. J. Chem. Eng. 69 (1991) 1036–1060. [30] F. Guazzone, Engineering of substrate surface for the synthesis of ultra thin composite Pd and Pd–Cu membranes for hydrogen separation, Ph.D. Thesis, Worcester Polytechnic Institute, 2005. [31] M.E. Ayturk, I.P. Mardilovich, E.E. Engwall, Y.H. Ma, Synthesis of composite Pdporous stainless steel (PSS) membranes with a Pd/Ag intermetallic diffusion barrier, J. Membr. Sci. 285 (1–2) (2006) 385. [32] Y.H. Ma, I.P. Mardilovich, E.E. Engwall, Composite gas separation modules having intermediate porous metal layers, US Patent 7,175,694 (2007).
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Chromium oxide intermetallic diffusion barrier for palladium membrane supported on porous stainless steel Sutheerawat Samingprai a , Supawan Tantayanon b,c,∗ , Yi Hua Ma d a
Graduate Program of Petrochemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Green Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand National Center of Excellence for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand d Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609, USA b c
a r t i c l e
i n f o
Article history: Received 16 May 2009 Received in revised form 28 September 2009 Accepted 30 September 2009 Available online 7 October 2009 Keywords: Intermetallic diffusion barrier Electroless plating Palladium membrane Chromium oxide
a b s t r a c t Dense palladium membrane on oxidized porous stainless steel (oxPSS) tube was prepared. Its hydrogen permeance was observed to decline at the temperature higher than 400 ◦ C. SEM–EDX analysis of the cross-section of the annealed tube at 500 ◦ C indicated the occurrence of the intermetallic diffusion. The heat treatment study of the palladium membrane on oxPSS disks in hydrogen atmosphere at various temperatures was essentially carried out. Their SEM–EDX analysis results confirmed that the in situ metal oxide barrier could not inhibit the intermetallic diffusion in hydrogen atmosphere. The chromium oxide layers at different thicknesses were then developed on oxPSS disks before palladium plating by controlled chromium elctrodeposition followed by oxidation in air at 700 ◦ C. The similar heat treatment study and SEM–EDX analysis of these disks revealed that the presence of chromium oxide layer could suppress the intermetallic diffusion. Then, the dense palladium membrane tube with chromium oxide layer was prepared and its heat treatment in hydrogen atmosphere was studied. The result showed the steady increase in hydrogen permeance with increasing temperature. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has long been used in several industrial applications including food, pharmaceuticals, chemical processing, electronics, metal production and fabrication, and petroleum recovery and refinery. Currently it has been of great interest as a clean alternative energy. Steam reforming of hydrocarbons is the major source of hydrogen production. Many reports on methane steam reforming using the catalytic palladium membrane reactor were published [1–6]. A few reports on methane dry reforming in the catalytic palladium membrane reactor were recently published as well [7–9]. Although the process was not optimized, the feasibility of achieving the enhancement of the CH4 conversion and CO and H2 selectivity beyond the limits stipulated by thermodynamic equilibrium during reforming of methane to synthesis gas was convincingly demonstrated by the use of a palladium membrane reactor comparing to a conventional reactor as reported by Galuszka et al. [7]. Among several types of membranes [10–14], the palladium and palladium alloy membranes were tested to have high selectivity
∗ Corresponding author at: Green Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. Tel.: +66 2 218 7641; fax: +66 2 218 7598. E-mail address: [email protected] (S. Tantayanon). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.09.058
[15–17]. Although palladium supported on porous stainless steel had several advantages comparing to other supported materials, its use at high temperature would cause the long-term stability problem due to the intermetallic diffusion of the metal elements from the support into palladium layer. Accordingly, much attention had been paid on preventing the intermetallic diffusion by creating an additional layer between the stainless steel support and palladium layer. These include tungsten and a tantalum oxide layer [18], aluminium oxide [19], zirconia and titania [20], zeolite [21] and tungsten oxide [22]. Silica [23] was also used as a diffusion barrier between the palladium–copper active layer and nickel–porous stainless steel composite membranes. In 1998, Ma and his co-workers had introduced an oxide-layer diffusion barrier by controlled in situ oxidation of the porous stainless steel supports prior to palladium and palladium/silver metal deposition [24–28]. They also reported that the oxidation had little effect on the mean pore size of the porous stainless steel indicating that oxidation did not constrict the internal pore system even at 800 ◦ C [27]. They further explained that the oxide layers formed at temperatures equal or higher than 500 ◦ C was effective in terms of preventing intermetallic diffusion on hydrogen permeance. They also showed that the structure of the oxide layer formed at 600 ◦ C in air was similar to the one formed at 800 ◦ C, which composed of an iron oxide rich layer on top of a chromium oxide layer [26]. However, the suitable material as the intermetallic diffu-
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9
Fig. 1. Experimental setup for permeation measurement.
sion barrier should at least have the Tamman temperature higher than palladium (640 ◦ C or 913 K) and stainless steel (550–560 ◦ C or 823–833 K), as the rate of diffusion of metal elements at the interface between two metallic layers would become greatest when they were at or above their Tamman temperature [24]. Therefore, chromium oxide, with Tamman temperature of 1081 ◦ C (1354 K), can be one of the potential candidates. As no literature reports on using chromium oxide as the intermetallic diffusion barrier, it is interesting to investigate its performance. In this study, the incremental formation of chromium oxide layer on top of the in situ mixed oxides on porous stainless steel was described and its performance as an intermetallic diffusion barrier between palladium membrane and porous stainless steel was discussed. 2. Experimental 2.1. Palladium membrane preparation A porous 316L stainless steel (PSS) tube, welded to non-porous stainless steel tube at both ends, was purchased from Mott Metallurgical Corporation (Farmington, CT). It had 9 mm in outer diameter, 70 mm long, 1 mm wall thickness, 0.1 m average pore size and 17% porosity. The PSS disks were prepared by cutting a 1 mm thick media-grade 0.1 m 316L PSS sheet into 1 cm2 pieces and then a small hole with 1 mm diameter was drilled at one
corner of the disk for hanging during metal deposition. All PSS supports were cleaned before activation and metal deposition, with the alkaline solution containing 5 ml/l of a household detergent. PSS supports were then thoroughly washed with deionized water (DI water) until the rinsing water had pH 7 and finally immersed in iso-propanol. All cleaning steps were carried out in ultrasonic bath at 60 ◦ C. The supports were dried at 120 ◦ C for 4 h. The cleaned PSS tubes and disks were oxidized at 450 ◦ C with heating rate 4 ◦ C/min in air in the muffle furnace. The weight of each oxidized PSS (oxPSS) support was recorded. Prior to palladium plating, the surface of oxPSS supports was activated by the consecutive immersion in SnCl2 (0.1 g/l in 0.01 M HCl), PdCl2 (0.1 g/l in 0.01 M HCl) with intermediate rinsing in DI water between both solutions. Then the activated PSS supports were rinsed with 0.01 M HCl. A two-step immersion sequence in SnCl2 and PdCl2 solution was generally repeated 3 times. A perfectly activated layer had a dark-brown color and smooth surface. Then the activated supports were deposited with palladium using the typical palladium plating solution composed of 4.0 g/l of Pd(NH3 )4 Cl2 ·H2 O, 198 ml/l of 28% NH4 OH, 40.1 g/l of Na2 EDTA, and 5.6–7.6 ml/l of 1 M N2 H4 at 60 ◦ C for 90 min. It was then cleaned with the warm DI water and let it at room temperature. Plating and cleaning were repeated for 3 times more. Finally, they were dried at 120 ◦ C for 4 h. In case of the plated tube, it was subjected to the helium permeation flux measurement. If it was not dense, it would be submitted to another plating cycle. The palladium layer thickness
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of the plated tube and disks was determined from the weight increase. 2.2. Hydrogen permeance measurement The hydrogen permeation flux measurements were carried out with the experimental setup as shown in Fig. 1. The feed gas flowed upward through the shell side, and the permeant gas was collected on the tube side. The pressure at the shell side was monitored by a capacitance pressure transducer and the pressure at the tube side was kept atmospheric. A soap-bubble capillary flowmeter was used for flow rate less than 1 ml/min: a digital flowmeter (Alltech), for flow rate in the range of 1–200 ml/min; a wet-test meter for flow rate greater than 200 ml/min. Temperature controller (Omega CN9000) was used to control the temperature inside the furnace and inside the tube side. The temperature variation along the membrane was controlled not to exceed 3 ◦ C. Prior to the hydrogen permeation tests, the Pd membrane was heated in helium at a rate of about 1 ◦ C/min. The temperature of the inside tube was varied from 350 to 500 ◦ C. The hydrogen permeation flux and hydrogen permeance of the dense palladium membrane tube at each pressure were then calculated according to the following equations: hydrogen permeation flux (m3 /m2 h)
=
flow rate of hydrogen gas in tube side (ml/ min) active surface area (cm2 )
hydrogen permeance (m3 /m2 h atm0.5 )
=
hydrogen permeation flux (m3 /m2 h)
× 0.6
0.5 0.5 PH − PH (atm0.5 ) at shell side at tube side 2
Princeton Gamma-Tech Avalon Energy Dispersive X-ray (EDX) spectrophotometer light element detector for the qualitative and quantitative analysis. At an accelerating voltage of 20 kV, the spatial resolution for the SEM–EDX lies between 0.8 and 1.2 m for the samples investigated. For the SEM cross-section analysis, the samples were cut by using a SiC saw blade and mounted with phenolic powder in a Smithells II mounting press. The mounting samples were ground with SiC papers with increasing grain fineness from 80 to 400 grit. Grinding was performed by using Metaserv 2000 grinder-polisher. Prior to SEM cross-section analysis, samples were painted with carbon ink and gold-coated to avoid charging. 2.6. X-ray diffraction Phase identification analyses were carried out by Rigaku Geigerflex X-ray diffractometer equipped with a Cu K␣ ( = 1542 × 10−10 m (1.542 Å)) radiation source and a nickel filter. The voltage was set at 37.5 kV and current at 27 mA. The scan rate was 1–2◦ /min in the range 2 = 30–80◦ . 2.7. Temperature-programmed reduction Two samples, Cr/oxPSS and Cr2 O3 /oxPSS disks, were used in this study. The temperature-programmed reduction (TPR) experiments were carried out on AutoChem 2910 (Micromeritics, USA) instrument. Prior to TPR, each sample was pretreated by passing ultra high pure (99.999%) helium (30 ml/min) at 700 ◦ C for 2 h. After pretreatment, it was cooled to room temperature. The carrier gas consisting of 5% hydrogen and balance helium (30 ml/min) was purified by oxy-trap and molecular sieves. The data were recorded while the temperature was ramped from 100 to 900 ◦ C at a heating rate of 10 ◦ C/min.
2
3. Results and discussion 2.3. Preparation of Pd/Cr2 O3 /oxPSS Prior to chromium plating, the PSS tube and disks were cleaned with an alkali solution as described in Section 2.1 and oxidized by heating at 450 ◦ C for 6 h. Chromium was then deposited by electroplating technique, using lead as the anode and the oxPSS as the cathode immersed in the solution of chromic acid (250 g/l) and sulfuric acid (1.25 g/l) in a ratio of 200:1. The chromium plating was performed at room temperature with current density 100–150 A/ft2 for 2 and 5 min. Finally, the chromium layer was oxidized in air at 700 ◦ C for 6 h. The thickness of chromium and chromium oxide layers was determined from the weight increase. Both oxPSS tube and disks containing the chromium oxide layer were then subjected to palladium electroless plating as described in Section 2.1. 2.4. Intermetallic diffusion tests Pd/Cr2 O3 /oxPSS or Pd/oxPSS disks were placed in the tube side of the experimental setup (Fig. 1) with quartz wools on top and the bottom. The tube side was heated under helium flow (20 ml/min) at a ramp of 1 ◦ C/min. When the temperature reached 500 ◦ C, helium was replaced with pure hydrogen at a flow rate of 20 ml/min. After treatment at 500 ◦ C in hydrogen for 24 h, the pure hydrogen was replaced with helium before cooling the disks down to room temperature. SEM–EDX of these disks was then performed. 2.5. Scanning electron microscopy Surface characterization was performed using an Amray 1610 turbo Scanning Electron Microscope (SEM) equipped with a
3.1. Effect of temperature on the intermetallic diffusion of Pd/oxPSS The PSS tube was oxidized in air at 450 ◦ C (oxPSS), prior to palladium plating, to give the in situ metal oxide on the substrate surface which could function as the intermetallic diffusion barrier described by Ma et al. [24,26]. The dense palladium membrane supported on oxPSS tube (Pd/oxPSS) was prepared with palladium thickness of 17.5 m. It was then characterized by firstly measuring the helium permeation flux which was found that no helium was detected. This result indicated that it was a dense palladium membrane. According to the data obtained from the measurement of the hydrogen permeation flux in the temperature range from 350 to 450 ◦ C, the Sievert law plots for this membrane exhibited all straight lines with regression coefficient around 0.999 as revealed in Fig. 2. The results illustrated that the dependence of hydrogen solubility in palladium followed Sievert law and the hydrogen diffused through this palladium membrane only via solution-diffusion mechanism [29]. When the hydrogen permeance was plotted against time at the constant pressure difference of 1 atm between the tube and shell sides of palladium membrane tube, it was found that the hydrogen permeance increased when the temperature was raised as exhibited in Fig. 3. At 350 and 400 ◦ C, the hydrogen permeances were stable over 35 h at 1.12 and 1.36 m3 (STP)/m2 h atm0.5 , respectively. Although the hydrogen permeances at 450 and 500 ◦ C were higher than at 350 and 400 ◦ C, the decline of hydrogen permeances was observed which started to decrease from after the first 6 h of heating. Both became stable after 20 h of heating, from 2.04 to 1.72 m3 (STP)/m2 h atm0.5 at 450 ◦ C and from 2.08 to 1.64 m3 (STP)/m2 h atm0.5 at 500 ◦ C. These results illustrated the
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Fig. 2. Sievert law plots for palladium membrane of Pd/oxPSS tube.
Fig. 3. Hydrogen permeance of Pd/oxPSS tube at constant pressure difference of 1 atm with various temperatures.
possible diffusion of the metal elements from the substrate into the palladium membrane which would cause the suppression of the hydrogen permeation through the palladium membrane. The lower hydrogen permeance at 500 ◦ C, comparing to the one at 450 ◦ C, indicated that the palladium layer might contain higher content of other metal elements. In other words, more deterioration of the membrane was resulted at higher temperature. Fig. 4 showed the SEM micrograph and the corresponding EDX compositional depth profile of the cross-section of Pd/oxPSS tube after hydrogen exposure at 500 ◦ C for 35 h. It should be notified that the mixed metal oxide layer, as depicted to be generated by in situ oxidation of PSS at 450 ◦ C for 6 h, could not be seen in this
11
SEM micrograph or detected by the corresponding EDX line scan. The similar observation was noted by Ma et al. [26] in the case of the oxidation of the PSS disk at 400 ◦ C that the mixed metal oxide layer might be too thin to be seen. From Fig. 4, it exhibited clearly that the diffusion of metal elements, Fe, Cr, and Ni from PSS into palladium layer, was taken place. The explanation to this result could be that the metal oxides were reduced to their metallic state when they were exposed to hydrogen [30] and the intermetallic diffusion barrier was therefore destroyed. Similarly, Ayturk et al. [31] used Pd/PSS cylindrical cup (oxidized at 400 ◦ C) and observed a steady decline of hydrogen permeance at 450 ◦ C within the first 24 h of testing. They concluded that the deterioration in hydrogen permeance, about 20 wt.% of the original permeance, of the membrane was attributed to the intermetallic diffusion of the support elements (Fe, Cr, and Ni) into dense palladium layer along with the formation of undesired surface [32]. The similar result was clearly observed in this study. To confirm this observation, another experiment was performed using three Pd/oxPSS disks with palladium thickness of 10 m. Two of them, Disks 2 and 3, were annealed under hydrogen atmosphere at 450 and 500 ◦ C, respectively. After hydrogen exposure for 24 h, the SEM micrographs and EDX composition depth profiles of these two disks were recorded comparing to the one without annealing, Disk 1. The higher content of Fe, Cr, and Ni in palladium layer was detected after hydrogen exposure at 500 ◦ C comparing at 450 ◦ C and no annealing, as revealed in Fig. 5. The metal distribution of Pd, Fe, Cr, and Ni in the palladium layer of each disk in percent by weight, as tabulated in Table 1, was recorded at the position in the plateau region of Pd composition away from the interface of Pd/oxPSS. When the disk was annealed, each metal element could diffuse into the palladium layer and the individual content depended on the nature of the metal element and annealing temperature. It was found that when the annealing temperature increased, the higher content of each of the metal elements was resulted which would arbitrarily lower the Pd content in the palladium layer. Among three metals, Fe was always found to be the highest content in palladium layer. These results were in consistent with the original metal composition of PSS in which the content of Fe was the highest, i.e., Fe 68–74 wt.%, Cr 16–18 wt.%, and Ni 10–14 wt.% Moreover, the palladium layer composition of Pd/oxPSS tube appeared to be very similar to the Pd/oxPSS disk annealed at the same temperature of 500 ◦ C. Accordingly, this study showed that the intermetallic diffusion of Pd/oxPSS could take place at the temperature as low as 450 ◦ C. It may be concluded that the in situ metal oxide layer on porous stainless steel created at 450 ◦ C (Pd/oxPSS) could probably be an effective intermetallic
Fig. 4. (a) SEM micrograph (2500×) of the cross-section of Pd/oxPSS tube after hydrogen exposure at 500 ◦ C for 35 h (b) and the compositional depth profile along the length of the arrow indicated in (a).
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Fig. 5. SEM micrographs (2500×) of cross-section of Pd membrane disks with EDX compositional depth profiles along the length of the arrows indicated in the corresponding micrographs, Pd/oxPSS disks (left) and Pd/Cr2 O3 /oxPSS disks (right): before (Disks 1 and 15) and after annealing in hydrogen atmosphere for 24 h at 450 ◦ C (Disks 2) and 500 ◦ C (Disks 3, 16 and 17).
diffusion barrier in other systems, but not in hydrogen at 450 ◦ C and higher. 3.2. Preparation of chromium oxide layer on oxPSS Chromium layer was generated on five pieces of the oxPSS disks as described in Section 2.3. The electroplating time of 5 min was applied for every disk. The chromium thickness of each disk was calculated from the gained weight. The average chromium thickness of 10 m with the relative standard deviation (RSD%) of 0.96
was obtained. The deposition of chromium was visually observed as gray color. The surface morphology of these disks became rough and different from the ones before chromium deposition as shown in Fig. 6(a) and (b). In addition, the EDX spectrum of Disk 4, as shown in Fig. 6(d), confirmed the composition of the disk surface to be only chromium element. Then the chromium oxide layer was generated by oxidizing in air at 700 ◦ C for 6 h. The average thickness of chromium oxide was 9.9 m with the RSD% of 0.99. The chromium oxide was obviously seen as the color was changed from gray to green. The chromium
Table 1 Preparation condition and elemental distribution in palladium layers of Pd/oxPSS and Pd/Cr2 O3 /oxPSS disks and tubes. Disk no.
Cr oxidation temperature (◦ C)
Cr2 O3 thickness (m)
Pd thickness (m)
Pd/oxPSS 1 2 3 Pd/oxPSS tube
– – – –
– – – –
9.9 9.9 10.0 17.5
Pd/Cr2 O3 /oxPSS 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Pd/Cr2 O3 /oxPSStube
700 700 700 700 700 – 400 500 600 700 800 700 700 700 700
9.9 10.0 9.8 10.0 9.9 None None None 3.0 10.0 10.0 11.1 10.1 2.0 2.0
10.0 10.0 9.9 10.0 9.9 9.9 10.0 9.9 9.8 9.9 10.0 9.7 9.8 9.7 32.0
Annealing temperature in H2 (◦ C)
Pd layer composition (wt.%) Pd
Fe
Cr
Ni
– 450 500 500
95.0 86.0 79.0 80.0
3.0 7.0 13.0 12.0
1.0 4.0 5.0 5.0
1.0 3.0 4.0 3.0
– – – – – – – – – – – – 500 500 500
– – – – – – – – – – – 96.0 96.3 95.0 –
– – – – – – – – – – – 2.0 2.3 3.0 –
– – – – – – – – – – – 1.0 1.0 1.0 –
– – – – – – – – – – – 1.0 0.6 1.0 –
Remark: Elemental distribution in palladium layer of all disks was measured after 24 h of hydrogen exposure at 500 ◦ C.
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Fig. 6. SEM micrographs (2500×) of Disk 4, before (a) and after chromium plating (b), after air oxidation at 700 ◦ C for 6 h (c), and EDX spectra of Disk 4 before (d) and after oxidation of chromium (e).
oxide disks were taken for further investigation on its morphology and composition by SEM equipped with EDX area scan mode, which were revealed in Fig. 6(c) and (e). The surface morphology of the top layer on the oxPSS disk was changed to a fluffier surface. From EDX spectrum, it indicated the presence of Cr atoms and O atoms. The XPS spectra of Disk 4 showed the O1S peak with the binding energy at 530.1 eV (Fig. 7(a)) indicating the presence of the oxide oxygen, whereas the binding energy at 576.9 eV in Fig. 7(b) was assigned to Cr2P3/2 peak of Cr2 O3 . According to the binding energy and peak shape, it was confirmed that the oxide of chromium was in the form of Cr2 O3 [33–35]. These results illustrated that the chromium and chromium oxide layers were easily generated by electroplating followed by oxidation in air, and their thicknesses
Fig. 7. XPS spectra of Disk 4 deposited with chromium and oxidized in air at 700 ◦ C for 6 h O1S (a) and Cr2P3/2 (b).
Fig. 8. XRD patterns of chromium layer on oxPSS disks before (Disk 9) and after oxidation at 400 ◦ C (Disk 10), 500 ◦ C (Disk 11), 600 ◦ C (Disk 12), 700 ◦ C (Disk 13) and 800 ◦ C (Disk 14).
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could be well controlled. The formation of chromium oxide at various oxidation temperatures 400–800 ◦ C, was also investigated. The XRD spectra, as shown in Fig. 8, displayed that no chromium oxide formation could be detected at the oxidation temperature lower than 600 ◦ C. The increase in intensity of chromium oxide pattern was observed when higher oxidation temperature was applied. Diffraction lines appeared at 2 = 33.6, 36.2, 39.8, 41.5, 50.3, 54.6, 63.6, 65.2, and 73.2, respectively (ASTM 6-0504) [36], indicated the existence of chromium oxide. Although chromium oxide formed at 800 ◦ C seemed to be a little better than at 700 ◦ C, it was postulated that the calcinations at higher temperature might cause the collapse of the internal adjacent pores of porous stainless steel which could later create a partial blockage of the pore system of support [37]. The suitable oxidation temperature was therefore considered to be at 700 ◦ C for 6 h. 3.3. Intermetallic diffusion of Pd/Cr2 O3 /oxPSS disks Another three oxPSS disks, Disks 15, 16 and 17, were deposited with chromium by electroplating technique as described earlier. Disks 15 and 16 were electroplated with chromium for 5 min while Disk 17 was subjected to chromium deposition for 2 min. After oxidation in air at 700 ◦ C for 6 h, chromium oxide was yielded with the average thickness of 11.1, 10.0 and 2.0 m, respectively. All three disks were then plated with palladium. Disks 16 and 17 were annealed under hydrogen atmosphere at 500 ◦ C for 24 h. The SEM micrographs of their cross-sections comparing to Disk 15, which was not annealed, were taken as shown in Fig. 5. The corresponding compositional depth profiles of all three disks were also recorded using line scan mode of SEM equipped with EDX. The line scan data were taken from inside the PSS substrate, which was the darker part in the picture, to the outer surface of the palladium layer, the lighter one, as indicated by the arrow in each SEM micrograph. It should be pointed here that SEM–EDX profile of Disk 15 showed the distance between oxPSS/Cr2 O3 and Cr2 O3 /Pd interfaces to be 16 m. However, it could not represent its Cr2 O3 thickness since it was taken at only one area of the whole disk. The average thickness of Disk 15 was determined by the gravimetric method to be 11.1 m. Unfortunately, the SEM–EDX profile of Disk 17 with Cr2 O3 thickness of 2 m exhibited the unclear interface of Cr2 O3 /Pd due to the limitation of the spatial resolution for the SEM–EDX which was 0.8–1.2 m. However, the elemental distribution, Pd, Fe, Cr, and Ni, in the palladium layer of each disk in percent by weight was able to be determined and compared by recording at the position in the plateau region of Pd composition away from the interface of Cr2 O3 /Pd as exhibited in Table 1. It was found that Disks 15 and 16, containing Cr2 O3 approximately 10 m in thickness before and after hydrogen exposure, respectively, had the same elemental distribution in their palladium layers which were very similar to Disk 17, containing Cr2 O3 thickness of 2 m after hydrogen exposure. These results were also comparable to the elemental distribution in palladium layer of Disk 1 with no Cr2 O3 and no hydrogen annealing. As revealed in Fig. 5 and Table 1, the SEM–EDX profile of Disk 3, Pd/oxPSS, after 24 h of hydrogen exposure at 500 ◦ C, showed lower Pd composition but higher Fe, Cr and Ni composition in its palladium layer, i.e., 79, 13, 5 and 4 wt.%, respectively, than those of Disks 15, 16 and 17, Pd/Cr2 O3 /oxPSS. From all these data, it could be concluded that the intermetallic diffusion from stainless steel to the palladium layer was suppressed by the presence of Cr2 O3 . In addition, the temperature-programmed reduction (TPR) analysis, a common technique for evaluating the reducibility of metal oxides, was applied to investigate the stability of Cr2 O3 . For example, MoO3 was reduced in two steps which exhibited as two major reduction peaks at 767 and 997 ◦ C in its TPR profile [38]. In this study, TPR profile of Cr2 O3 was obtained by
Table 2 Helium permeation flux and permeance of Pd membrane tube with various Pd thicknesses.
oxPSS Cr/oxPSS (Cr thickness, 2 m) Cr2 O3 /oxPSS (Cr2 O3 thickness, 2 m) Pd/Cr2 O3 /oxPSS Cycle 1 (total Pd thickness, 5 m) Cycle 2 (total Pd thickness, 10 m) Cycle 3 (total Pd thickness, 15 m) Cycle 4 (total Pd thickness, 22 m) Cycle 5 (total Pd thickness, 32 m)
He permeation flux (m3 /m2 h)
He permeance (m3 /m2 h atm0.5 )
1.77 × 102 1.55 × 102
7.08 × 101 6.20 × 101
1.62 × 102
6.48 × 101
9.96 × 101
3.98 × 101
3.59 × 10−2
1.44 × 10−2
3.00 × 10−4
1.20 × 10−4
1.00 × 10−4
4.00 × 10−5
1.00 × 10−6 (dense)
4.00 × 10−7 (dense)
annealing Cr2 O3 /oxPSS disk in hydrogen atmosphere from 100 to 900 ◦ C. No hydrogen was consumed as no reduction peak was observed. The similar experiment with Cr/oxPSS was carried out as a controlled test which showed no reduction peak as well. It can therefore be interpreted that Cr2 O3 is stable and will not be reduced when it is exposed to hydrogen at 100–900 ◦ C. 3.4. Hydrogen permeance of palladium membrane of Pd/Cr2 O3 /oxPSS tube The dense palladium membrane with 32 m thickness and containing 2 m Cr2 O3 layer on a porous stainless steel (Pd/Cr2 O3 /oxPSS) tube was prepared as described in Sections 2.3 and then 2.1. The helium permeation flux of the tube was measured after each step of the preparation and the corresponding helium permeance was calculated as shown in Table 2. Helium permeation flux of Cr2 O3 /oxPSS tube was about the same as those of Cr/oxPSS and oxPSS tubes. These data indicated that chromium deposition and chromium oxide formation did not affect the helium permeation flux. It was thus reasonable to assume that the pore properties of PSS were not drastically changed. After each palladium plating, the helium permeation flux of the tube was measured as well. It was observed that lower helium permeation flux was detected. After the fifth palladium plating, the dense palladium membrane tube with 32 m palladium thickness was obtained. The Sievert law plots for this membrane were all straight lines with regression coefficient around 0.999 as revealed in Fig. 9. The
Fig. 9. Sievert law plots for palladium membrane of Pd/Cr2 O3 /oxPSS tube.
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Acknowledgements The authors would like to thank Bangkok Industrial Gas Co., Ltd., Bangkok, Thailand, for the support of specialty gases used in this research. One of the authors is grateful to Worcester Polytechnic Institute for providing him the scholarship during his one-year research in the institute. The courtesy of Mr. Leenawat Kanda, Ms. Thitinat Sukonket and Mr. Matthieu Simon Fleys on some sample measurements is acknowledged. References
Fig. 10. Hydrogen permeance of Pd/Cr2 O3 /oxPSS tube at constant pressure difference of 1 atm after 24 h of hydrogen exposure at 350–500 ◦ C comparing to Pd/oxPSS tube.
results illustrated that the hydrogen diffused through the palladium membrane of Pd/Cr2 O3 /oxPSS tube via the solution-diffusion mechanism [29]. The hydrogen permeance of the palladium membrane of Pd/Cr2 O3 /oxPSS tube was measured to be 0.84, 1.11, 1.34, and 1.51 m3 /m2 h atm0.5 at 350, 400, 450 and 500 ◦ C, respectively. It increased steadily with increasing temperature as shown in Fig. 10. However, Pd/Cr2 O3 /oxPSS tube had lower hydrogen permeance than Pd/oxPSS tube at every temperature. This could be attributed to the higher palladium membrane thickness of the first tube. Interestingly, no decline of the hydrogen permeance for the first tube was observed, whereas the latter one showed a drop in hydrogen permeance at 500 ◦ C. This observation conformed with the intermetallic diffusion tests of the Pd/Cr2 O3 /oxPSS disks that the Cr2 O3 layer could act as an intermetallic diffusion barrier for the palladium membrane supported on porous stainless steel. Although the decline of hydrogen permeance was also reported by other research groups and the cause due to the intermetallic diffusion was proposed [32], this study had been the first report to demonstrate that Cr2 O3 layer was the effective intermetallic diffusion barrier for palladium membrane supported on porous stainless steel tube in hydrogen exposure.
4. Conclusion It was found that the mixed metal oxide generated on porous stainless steel by in situ air oxidation at 450 ◦ C for 6 h could not function as the intermetallic diffusion barrier after annealing in hydrogen atmosphere. The intermetallic diffusion from porous stainless steel support to palladium membrane had occurred at 450 ◦ C and higher temperature. In this study the chromium oxide layer, in the form of Cr2 O3 was firstly introduced as the incremental layer before palladium plating. It was discovered that the presence of Cr2 O3 layer with 2 m thickness could suppress the intermetallic diffusion. In addition, the stability of Cr2 O3 in hydrogen atmosphere was demonstrated with its TPR profile and its formation did not affect the helium permeation flux of the oxPSS tube. Moreover, the hydrogen permeance of Pd/Cr2 O3 /oxPSS tube was observed to increase steadily with increasing temperature up to 500 ◦ C. All these results indicated that Cr2 O3 was the promising intermetallic diffusion barrier for palladium membrane supported on PSS which could be used in hydrogen atmosphere. However, the optimum thickness of the chromium oxide layer should be investigated in order to balance the hydrogen permeance of the membrane. The long-term stability of hydrogen permeance of palladium membrane containing chromium oxide is also required. Both investigations are currently in progress.
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