Improvement in thermal stability of stainless steel supported silica membranes by the...

Journal of Membrane Science 236 (2004) 53–63

Improvement in thermal stability of stainless steel supported silicamembranes by the soaking–rolling method

Dong-Wook Leea,b, Yoon-Gyu Leea, Bongkuk Seaa, Son-Ki Ihmb, Kew-Ho Leea,∗a Membrane and Separation Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-606, South Korea

b Department of Chemical and Biomolecular Engineering, National Research Laboratory for Environmental Catalysis,Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, South Korea

Received 4 September 2003; accepted 15 January 2004

Abstract

The thermal stability of stainless steel supported silica membranes in the presence of hydrogen was observed by comparing nitrogenpermeance before hydrogen permeation test with that after hydrogen permeation test. The silica composite membranes significantly failedin the presence of hydrogen above 250◦C, resulting from the reduction of metal oxide such as iron oxide formed on the surface of stainlesssteel substrates. To improve the thermal stability of the membranes, the interface between the coating layer and the stainless steel, which isan important factor in the membrane failure, was minimized by introducing new technique of the soaking–rolling method. As a result, thethermal stability of the stainless steel supported silica membranes was substantially improved linked with high H2/N2 selectivity.© 2004 Elsevier B.V. All rights reserved.

Keywords: Stainless steel; Thermal stability; Composite membrane; Soaking–rolling method

1. Introduction

The silica membranes for hydrogen separation has at-tracted a great deal of interest. The gas separation mech-anisms of porous inorganic membranes such as the silicamembranes mainly consists of Knudsen diffusion, surfacediffusion and molecular sieving. Of these three main mech-anisms, only surface diffusion and molecular sieving canprovide selectivity high enough to be commercially used. Inorder to achieve high selectivity, pinhole-free silica mem-branes with high microporosity must be synthesized by ei-ther sol–gel or CVD processes. The sol–gel processes areparticularly attractive because they allow cheap and sim-ple membrane synthesis with controlled microstructure us-ing dip or spin coating procedures. Consequently, a num-ber of research groups have focused on the sol–gel derivedsilica composite membranes. Raman and Brinker[1] re-ported the use of an organic templated approach for silicamembrane preparation. The permeance of helium, carbon

∗ Corresponding author. Tel.:+82-42-860-7240;fax: +82-42-861-4151.

E-mail address: [email protected] (K.-H. Lee).

dioxide and methane for their membrane were 4.4 × 10−8,6.8×10−8 and 9.5×10−10 mol m−2 s−1 Pa−1, respectively.The microporous silica membranes with hydrogen perme-ance of (4.7–17.7) × 10−7 mol m−2 s−1 Pa−1 and carbondioxide permeance of(2.0–3.2) × 10−7 mol m−2 s−1 Pa−1

were fabricated by de Vos and Verweij[2]. Wu et al. [3]reported the production of silica membranes with similarqualities made by the CVD method. Kusakabe et al.[4]synthesized organic templated membranes with TEOS andalkyltriethoxysilanes resulting in hydrogen permeance of1.0×10−7 mol m−2 s−1 Pa−1, carbon dioxide permeance of8.0 × 10−9 mol m−2 s−1 Pa−1 and methane permeance of2.8 × 10−9 mol m−2 s−1 Pa−1. West et al.[5] prepared thesol–gel derived silica membranes governed by surface dif-fusion. da Costa et al.[6] reported the molecular sieve silicamembranes with the pore size in the region of 3 Å resultingfrom the two-step catalyzed hydrolysis sol–gel process.

Concerning the application of the inorganic compositemembranes to high temperature process such as the catalyticdehydrogenation of hydrocarbons, important aspects to con-sider in the application include the improvement of the ther-mal stability and good permeability. Drioli and Romano[7]and Li et al.[8] emphasized that thermally stable substrates

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2004.01.027

54 D.-W. Lee et al. / Journal of Membrane Science 236 (2004) 53–63

should be needed to maintain high permeability and selectiv-ity of a top coated layer. Our previous work[9] showed thatthe failure of the stainless steel supported composite mem-branes in the presence of hydrogen at high temperature wasattributed to the reduction of reducible metal oxides such asiron oxide.

In this paper, the thermal stability of the sol–gel derivedsilica composite membranes was observed, when the porousstainless steel was used as a support. Scanning electron mi-croscopy (SEM) and energy dispersive X-ray spectrometer(EDS) were used to observe the variation of morphologyand characteristics of the membrane surface after permeationtest at high temperature. Thermal stability of the stainlesssteel supported silica membrane was investigated by com-paring nitrogen permeance before hydrogen permeation testwith that after hydrogen permeation test, or comparing thegas permeability and the H2/N2 selectivity for single gaspermeation test with those for mixture gas permeation test.From the results, the reason why the stainless steel supportedmembranes are unstable at high temperature was discussed.In order to improve the thermal stability of the stainless steelsupported silica membranes, new technique of the membranepreparation was introduced. The new technique, which isdifferent from the general method of membrane preparationsuch as dip-coating method, gives rise to penetration of thecoating layer into porous stainless steel support, resultingin minimization of interface area between coating layer andthermally unstable surface of the stainless steel support.

2. Experimental

2.1. Sol preparation

Colloidal silica sols and a boehmite sol as materials forintermediate layer were prepared to modify the pore struc-ture of stainless steel supports. The colloidal silica sols weresynthesized from base catalyzed hydrolysis–condensationreactions of tetraethyl orthosilicate (TEOS) purchased fromAldrich. The compositions of the reactants for colloidal sil-ica sols are shown inTable 1 [10]. Prior to the addition of theNH3/H2O mixture, the TEOS/ethanol mixture was stirredvigorously in an oil bath of 50◦C. The addition of NH3/H2Owas carried out dropwise followed by refluxing the mixturefor 3 h at vigorous stirring resulting in the stable colloidalsilica sols. The boehmite sol was produced as suggested byKusakabe et al.[11]. Polymeric silica sol was prepared un-der acid-catalyzed conditions via hydrolysis of TEOS andconcentration reactions. A molar ratio of TEOS, water and

Table 1Compositions of the reactants for the preparation of colloidal silica sols

Code Particle size of silica (nm) Water/TEOS molar ratio (r) Ethanol/TEOS molar ratio NH3/TEOS molar ratio

SiO2(500 nm) 500 35.7 32.9 11.8SiO2(100 nm) 100 53.6 40.1 0.64

nitric acid was 0.096/0.56/0.008. A mixture of TEOS, waterand nitric acid was stirred at room temperature for 20 min.The reaction mixture was diluted with additional water toadjust the volume to 500 ml, and refluxed for 8 h at 80◦C[12]. From these conditions, the weakly branched clusterswhich allow the formation of dense films were obtained.

2.2. Preparation of SiO2(polymeric)/γ-Al2O3/SiO2(colloidal)/SUS membranes by dip-coating method

Disks of 316L stainless steel (SUS) used as a porous sub-strate were purchased from Matt Metallurgical. The stain-less steel support has a thickness of 1 mm, a surface areaof 5 cm2, and an average pore size of 0.5�m. The supportas purchased has wide pore size distribution, rough surfaceand too many macropores above 10�m to be used directly.Some modifications of the porous stainless steel substratewere needed using boehmite sol and colloidal silica sols,so that the pore size and surface roughness of the substratecould be gradually reduced. The first modification of thestainless steel support was conducted by dip-coating the sup-port into silica (500 nm) sol. The second modification ofthe support with boehmite sol was carried out to reduce thepore size of the support into the region of Knudsen diffu-sion, followed by drying process of the modified support at25◦C and RH 60% and calcination at 650◦C with ramp-ing rate of 1◦C/min. The silica top layer was synthesizedby dip-coating procedure with polymeric silica sol. The toplayer was dried at same temperature and humidity as modifi-cation procedure with�-Al2O3 and calcined at 500◦C withramping rate of 1◦C/min.

2.3. Preparation of SiO2(polymeric)/γ-Al2O3/SiO2(colloidal)/SUS membranes by the soaking–rolling method

The porous stainless steel support was modified by sil-ica xerogels with particle size of 500 or 100 nm obtainedby drying colloidal silica sols with a rotary evaporator. Thesilica xerogel was pressed into the macropores of one sideof the stainless steel by a press under 10 MPa. The mod-ified stainless steel was calcined at 650◦C with rampingrate of 1◦C/min. The second modification of the stainlesssteel support was conducted by the soaking–rolling methodwith boehmite sol. The back side of the support loaded onthe o-ring sealed-cell was vacuumed by a rotary vacuumpump. Boehmite sol was poured onto the front side of thesupport, then, maintaining vacuum of the back side of thesupport for 3 min so that the boehmite sol could penetrateinto inner pores of the support. After the soaking process, the

D.-W. Lee et al. / Journal of Membrane Science 236 (2004) 53–63 55

boehmite sol on the front side of the support was rolled outwith an urethane rolling pin in order to remove the boehmitesol on the surface of the stainless steel support. The mod-ified support was dried overnight at 25◦C and calcined at650◦C. The preparation of top layer using polymeric silicasol was carried out by the same manner as the support modi-fication with boehmite sol, and the silica top layer was driedat 25◦C and calcined at 500◦C. Relative humidity duringthe drying process was 60 or 30%.

2.4. Permeation measurement

Permeation measurements for single gas were made withgases of nitrogen and hydrogen between room temperatureand 450◦C. The permeation apparatus is shown schemat-ically in Fig. 1. The system consists of a membrane cell,furnace, temperature controller, pressure gauge, pressuretransducer, and mass flow controller (meter). The perme-ation area was 4.52 cm2. A single gas test was conductedafter valve of the retentate side and inlet valve of the per-meate side were closed. The feed side of the membrane waspressurized by pure hydrogen or nitrogen with no flow whilethe permeate side of the membrane was under atmosphericpressure without sweeping gas. The single gas permeationtest was conducted with permeation time changing the feedgas in turns of nitrogen, hydrogen and nitrogen. The trans-membrane pressure difference was 0.042 MPa and the fluxof permeated gas was measured by mass flow meter or soapfilm flow meter. The selectivity for single gas permeationtest is defined as the ratio of hydrogen permeance to nitrogen

Fig. 1. Schematic diagram of the permeation apparatus.

permeance measured before hydrogen permeation test underthe same transmembrane pressure difference and tempera-ture. For a permeation test of a mixture gas, H2 (99%)/N2(1%) was used as a feed gas. The flow rates of the feed andsweeping gas were controlled by mass flow controller aftervalve of the retentate side and inlet valve of the permeateside were open. The sweeping gas of argon was fed to thepermeate side of the membrane and diluted the permeatedgas. The mole fraction of the permeate side was measuredusing gas chromatography (GC-14B, SHIMADZU). Theparameter to describe the separation efficiency for a binarymixture is the separation factorα, which is a measurementof the enrichment of a gas component after it has passedthe membrane. Separation factorα is defined as

α = y

1 − y

1 − x

x

wherex andy are the mole fraction of the feed and perme-ate, respectively.

2.5. Characterization

The particle sizes of the prepared sols and the membranemorphology were determined using scanning electron mi-croscopy (XL 30S FEG, Philips). Energy dispersive X-rayspectrometer (EDAX, Phoenix) was used to analyze thechange of surface elements of the stainless steel support withtemperature and atmosphere.

3. Results and discussion

3.1. The failure of the stainless steel supported silicamembranes prepared by dip-coating method

To investigate the thermal stability of SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by dip-coat-ing method, the single gas permeation test was conductedwith permeation time changing the feed gas in turns of nitro-gen, hydrogen and nitrogen at 350◦C. Because we can esti-mate the thermal stability of the composite membrane in thepresence of hydrogen by comparing the nitrogen permeancemeasured before hydrogen permeation test with that mea-sured after hydrogen permeation test. That is, when the com-posite membrane is failed in the presence of hydrogen at hightemperature, the nitrogen permeance after hydrogen perme-ation test becomes higher than that before hydrogen perme-ation test[9]. Fig. 2shows the variation of permeance of thesilica composite membrane prepared by dip-coating method.As shown inFig. 2, nitrogen permeance before hydrogenpermeation test was about 6.8 × 10−9 mol m−2 s−1 Pa−1.After changing the feed gas into hydrogen, hydrogen per-meance increased rapidly for 30 min and reached steadystate. The ratio of hydrogen permeance to nitrogen perme-ance measured before hydrogen gas test is about 56. How-ever, when nitrogen permeance was measured directly after


Fig. 2. The variation of permeance of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by dip-coating method (dryingcondition: 25◦C, RH 60%) with permeation time changing the feed gas(N2 → H2 → N2) at 350◦C.

hydrogen permeation test, unexpected increase in nitrogenpermeance was observed and the ratio of hydrogen perme-ance to nitrogen permeance measured after hydrogen per-meation test was about 4.0.Fig. 3 presents the surface ofthe stainless steel supported silica composite membrane pre-pared by dip-coating method. While the silica membrane asprepared by dip-coating method showed pinhole-free sur-face, needle-like cracks were formed on the membrane sur-face after hydrogen permeation test at 350◦C. Consequently,it means that the stainless steel supported silica compositemembrane is failed in the presence of hydrogen at high tem-perature above 350◦C.

According to our previous work[9], the failure of thestainless steel supported composite membranes in the pres-ence of hydrogen at high temperature is attributed to the re-duction of reducible metal oxides such as iron oxide formedon the surface of the stainless steel support. That is, the stain-less steel support can contribute to the failure of the compos-ite membranes due to surface oxide layers. Shieu et al.[13]reported that an oxide film of 70 nm thick was produced onsurface of a type 316L stainless steel by an oxidation. Theoxide film on the surface of the 316L stainless steel has amultilayered microstructure in which the top layer is com-posed of nanoscale�-Fe2O3 grains, followed by a mixtureof �-Fe2O3 and Fe3O4 phases.Figs. 4 and 5show the vari-ation of the surface of the stainless steel in the sequence ofthe single gas permeation test(N2 → H2 → N2) at hightemperature above 350◦C. After nitrogen permeation test,a porous iron oxide layer was formed on the surface of thestainless steel with an increase in oxygen content comparingwith that of the stainless steel as purchased. The oxidationof stainless steel surface was induced by oxygen adsorbedbefore the permeation test at room temperature and oxygendiffused from the permeate side of the membrane duringnitrogen permeation test. During hydrogen permeation test,hydrogen penetrates into the iron oxide layer and quickly

reaches the metal oxide interface followed by the reductionof the oxide layer. As shown inFig. 4(c), the reduction ofthe iron oxide by hydrogen destroyed the oxide layer onthe stainless steel surface.Fig. 5(c) shows that the oxygencontent decreased due to the reduction of the oxide layerresulting in the failure of the support. The deformation ofsurface of the porous stainless steel support contributed tocrack formation of silica skin layer supported on the porousstainless steel as shown inFig. 3(b). After nitrogen per-meation following hydrogen permeation, the surface of thestainless steel was reoxidized with three-fold increase in theoxygen content. As seen inFigs. 4(d) and 5(d), the stain-less steel surface reduced by hydrogen was reoxidized byoxygen diffused from the permeate side of the membraneduring nitrogen permeation test following hydrogen perme-ation. Therefore, it can be suggested that the sequence ofreduction and oxidation of the iron oxide on the surface ofthe stainless steel support is attributed to the failure of thecomposite membranes.

3.2. Improvement in the thermal stability of the stainlesssteel supported silica membrane prepared by thesoaking–rolling method

3.2.1. Morphology of the silica composite membraneIn order to improve the thermal stability of the stainless

steel supported membranes, new technique for synthesis ofthe composite membranes was suggested so that the contactarea between the coating materials and the surface of stain-less steel, which has potentially strong possibility of themembrane failure, is minimized. The silica composite mem-brane prepared by the new technique of the soaking–rollingmethod is structurally different from that prepared bydip-coating method as schematically shown inFig. 6. Forthe composite membrane prepared by dip-coating method,the coating layers such as polymeric silica or�-aluminaconsist of vertically only one layer, and the contact area be-tween the coating layer and the stainless steel is much largerthan that of the membrane prepared by the soaking–rollingmethod. For the soaking–rolling method, however, the coat-ing materials deeply penetrate into the inner pores of thestainless steel and the interstitial voids among colloidal sil-ica particles, resulting in minimization of interface betweenthe coating layer and the stainless steel. The coating layerhas vertically multilayered structure divided by the wallsof the stainless steel, and even top layer includes colloidalsilica particles.

To modify the porous stainless steel support, the silica xe-rogel(500 nm) was pressed into the pores of the substrate un-der 10 MPa followed by calcination at 650◦C. Fig. 7showsthe surface morphology of well-modified stainless steel sup-port with silica xerogel(500 nm). The pore size of the stain-less steel substrate, which has very rough surface and widepore size distribution, was successfully diminished by thewell-packed silica xerogel. The second modification of thesupport by the soaking–rolling method was carried out to de-


Fig. 3. SEM images of the surface of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by dip-coating method (drying condition:25◦C, RH 60%): (a) as prepared, (b) after hydrogen permeation at 350◦C.

crease the pore size of the support into the region of Knudsendiffusion.Fig. 8shows the SEM micrographs of the surfaceand the cross-section of�-Al2O3/SiO2(500 nm)/SUS sup-port. The pore sizes of such�-Al2O3 layers are known tobe in the 3–4 nm range. Micro-sized�-Al2O3 layers are iso-lated and surrounded by the walls of the stainless steel sup-port, and formed several micrometer lower than the surfaceof the stainless steel. The silica top layer was also preparedby the soaking–rolling method.Fig. 9 shows the SEM im-ages of cross-section of the silica composite membranes pre-pared by the soaking–rolling and dip-coating method. Themorphologies of the membranes are consistent with the ex-pected structures are shown inFig. 6. Comparing with themembrane prepared by dip-coating method, the skin layerfabricated by the soaking–rolling method penetrated into the

stainless steel substrate and included the colloidal silica par-ticles. It is well-known that, for dip-coating method, capil-lary filtration occurs when the dry support comes into contactwith a sol and the pore surface is wetted by the sol, followedby capillary suction of the support. For the soaking–rollingmethod, forced suction combined with capillary suction isapplied by a vacuum pump during the soaking procedure,and concentrated gel layer formed on surface of the supportis removed by the rolling process. That is the reason why thesoaking–rolling method makes the skin layer deeply pene-trate into the inner pores of the support.

3.2.2. Thermal stability of the silica composite membraneThe thermal stability of SiO2(polymeric)/�-Al2O3/SiO2

(500 nm)/SUS membrane prepared by the soaking–rolling


Fig. 4. SEM images of the surface of the stainless steel support after different treatment: (a) as purchased, (b) nitrogen permeation at 450◦C, (c) hydrogenpermeation at 450◦C, (d) nitrogen permeation for 24 h after hydrogen permeation at 450◦C.

method was also observed by single gas permeation test withpermeation time changing the feed gas in turns of nitrogen,hydrogen and nitrogen at 350◦C. As shown inFig. 10, thenitrogen permeance measured before hydrogen permeationtest was about 5.7×10−9 mol m−2 s−1 Pa−1. After changingthe feed gas into hydrogen, the hydrogen permeance reachedsteady state at 6.7× 10−8 mol m−2 s−1 Pa−1 in 10 min. Dif-ferently from the permeation result of the silica membraneprepared by dip-coating method shown inFig. 2, the nitrogenpermeance was rapidly decreased after changing hydrogenfeed gas into nitrogen. The nitrogen permeance measureddirectly after hydrogen permeation test was almost consis-tent with that measured before hydrogen permeation test.Fig. 11 presents the surface of the silica composite mem-brane prepared by the soaking–rolling method. Interestingly,the islands of stainless steel still appeared in the micrographof the surface of the silica composite membrane preparedby the soaking–rolling method despite several coating pro-cedure. In addition, although morphology of the islands ofstainless steel was changed by the reduction of iron oxideon the stainless steel surface after hydrogen permeation test,silica skin layer was not affected by deformation of the sur-face of the stainless steel support. From the results shown inFigs. 10 and 11, it can be concluded that the stainless steed

supported silica membrane prepared by the soaking–rollingmethod was not failed in the presence of hydrogen at 350◦C.

Presumably, it is postulated that the improvement ofthe thermal stability of the stainless steel supported mem-brane is attributed to the minimization of the interfacebetween coating materials and stainless steel induced bythe soaking–rolling method, and colloidal silica particles,included even in top layer, as a barrier of propagation ofthe cracks derived from the remaining interface betweenthe coating layer and the wall of stainless steel. The soak-ing process leads to the penetration of the coating layerinto pores of porous stainless steel support, and the rollingprocess following the soaking process contributes to theremoval of concentrated surface gel layer formed duringthe soaking process. Therefore the interface between theporous stainless steel and the coating layer can be reduced,when the stainless steel supported silica membranes arefabricated by the soaking–rolling method.

3.2.3. Effect of drying condition on thermal stability of thesilica membrane

To observe the effect of drying condition on improvementof the thermal stability of the silica composite membranesprepared by the soaking–rolling method, the composite


Fig. 5. EDS results of the stainless steel support after different treat-ment: (a) as purchased, (b) nitrogen permeation at 450◦C, (c) hydrogenpermeation at 450◦C, (d) nitrogen permeation for 24 h after hydrogenpermeation at 450◦C.

membranes were prepared with different drying condition(lower relative humidity), and compared with the mem-brane are shown inFigs. 10 and 11. Fig. 12shows the silicacomposite membranes prepared by the soaking–rollingmethod (drying condition: 25◦C, RH 30%). Although thecomposite membrane seen inFig. 12 was prepared by thesoaking–rolling method, the islands of stainless steel supportwere partially covered by the skin layer at drying condition

Fig. 6. The schematic comparison of the soaking–rolling and dip-coating method.

of low relative humidity where the flow of liquid–vapormeniscus into the inner pores due to gravitation may besuppressed by faster drying rate. The thermal stability ofthe membrane was estimated as comparison of the nitrogenpermeance between before and after hydrogen permeationtest.Fig. 13 shows the thermal stability of the silica com-posite membrane shown inFig. 12 in the presence ofhydrogen at 350◦C. For the composite membrane preparedby the soaking–rolling method (drying condition: 25◦C,RH 60%) shown inFig. 10, the nitrogen permeance afterhydrogen permeation test was consistent with that beforehydrogen permeation test, which imply that the membranewas thermally stable in the presence of hydrogen at 350◦C.For the membrane shown inFig. 12, however, the nitrogenpermeance was increased into the region of pure Knudsensystem after hydrogen permeation test, and the hydrogenpermeance was much higher than that appeared inFig. 10.

The failure of the membrane shown inFig. 13is derivedfrom an increase in the interface between the coating layerand porous stainless steel. The increase of the interface isattributed to inhibition of the penetration of coating layerinto pores of the stainless steel induced by rapid drying atlow relative humidity. Consequently, it is considered thatthe improvement of the thermal stability of stainless steelsupported membranes could not be expected when relativehumidity during drying procedure is low even if the mem-brane is prepared by the soaking–rolling method. Because,as shown inFig. 12, the skin layer is partially built on theislands of stainless steel substrates, resulting in the increasein possibility of the membrane failure derived from reduc-tion of the iron oxide at the interface between the coatinglayer and the surface of stainless steel.

3.2.4. Effect of the colloidal silica intermediate layer onthermal stability of the silica membrane

The interface between the coating layer and the wall ofstainless steel remains even though composite membranesare prepared by the soaking–rolling method and the coatinglayers diffuse enough into the pores. As mentioned before,it is speculated that the colloidal silica particles (500 nm)included in skin layer play an important role in a barriersuppressing the propagation of micro-cracks derived fromthe interface between the coating layer and the wall of stain-less steel. Therefore the interstitial volume among colloidalsilica particles should be diminished using smaller colloidal


Fig. 7. A SEM image of the surface of the stainless steel supports after modification with the silica xerogel(500 nm).

Fig. 8. SEM images of the�-Al2O3/SiO2(500 nm)/SUS support preparedby the soaking–rolling method: (a) surface, (b) cross-section.

Fig. 9. SEM images of the cross-section of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by different method (dry-ing condition: 25◦C, RH 60%): (a) the soaking–rolling method, (b)dip-coating method.


Fig. 10. The variation of permeance of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by the soaking–rolling method(drying condition: 25◦C, RH 60%) with permeation time changing thefeed gas(N2 → H2 → N2) at 350◦C.

Fig. 11. SEM images of the surface of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by the soaking–rolling method(drying condition: 25◦C, RH 60%): (a) as prepared, (b) after hydrogenpermeation at 350◦C.

Fig. 12. A SEM image of the surface of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membranes prepared by the soaking–rolling method(drying condition: 25◦C, RH 30%).

silica xerogel so that micro-cracks, which are induced by thevariation in morphology of the stainless steel surface duringpreparation step or permeation test as shown inFig. 4, arereduced more.

In first modification step of the porous stainless steel,the silica xerogel (100 nm) was pressed into the pores ofthe support under 10 MPa. Second modification was carriedout using boehmite sol by the soaking–rolling method. Thesilica top layer was also fabricated with polymeric silicasol by the soaking–rolling method. As appeared inFig. 14,crack-free silica skin layer was obtained among the islandsof the stainless steel substrate. The gas permeation test wasconducted to observe the effect of modification using silicaxerogel (100 nm) on the performance of the membrane.Tables 2 and 3show the results of single and mixture gas testfor the silica composite membrane, respectively. At 450◦C,the nitrogen permeance and H2/N2 selectivity for single gas

Fig. 13. The variation of permeance of the SiO2(polymeric)/�-Al2O3/SiO2(500 nm)/SUS membrane prepared by the soaking–rolling method(drying condition: 25◦C, RH 30%) with permeation time changing thefeed gas(N2 → H2 → N2) at 350◦C.


Fig. 14. A SEM image of the surface of the SiO2(polymeric)/�-Al2O3/SiO2(100 nm)/SUS membrane prepared by the soaking–rolling method(drying condition: 25◦C, RH 60%).

permeation test was inconsistent with those for mixture gaspermeation test. The result means that the membrane includ-ing silica xerogel (100 nm) is still unstable in the presence ofhydrogen at 450◦C, because the nitrogen permeance shownin Table 2was measured before hydrogen permeation test.However, the membrane is thermally stable below 450◦C,and the nitrogen was not detected in the permeate sideby TCD (detection limit: 3.0 × 10−10 mol m−2 s−1 Pa−1 atP = 0.042 MPa). The silica composite membrane wasthermally stable below 450◦C, and the H2/N2 selectivity ofthat was improved due to the decrease in the micro-crackswhich could be formed during calcination or permeationtest. Consequently, it is concluded that the thermal stabilityand the performance of stainless steel supported mem-branes can be improved by penetration of the coating layerinto the pores of the support and dispersion of colloidalsilica particles in the coating layer, which contribute to theminimization of the interface between the coating layer

Table 2Permeances and selectivities of the SiO2(polymeric)/�-Al2O3/SiO2(100 nm)/SUS membrane for single gas permeation test at different permeationtemperatures using mass the mass flow meter (detection limit: 6.0 × 10−10 mol m−2 s−1 Pa−1 at P = 0.042 MPa)

Permeation temperature (◦C)

25 150 250 350 450

H2 permeance (mol m−2 s−1 Pa) 1.8× 10−8 2.5 × 10−8 3.2 × 10−8 3.8 × 10−8 7.8 × 10−8

N2 permeance (mol m−2 s−1 Pa) – – – – –H2/N2 selectivity – – – – –

Table 3Permeances and selectivities of the SiO2(polymeric)/�-Al2O3/SiO2(100 nm)/SUS membrane for mixture gas (H2, 99%/N2, 1%) permeation test at differentpermeation temperatures using the gas chromatography with TCD (detection limit: 3.0 × 10−10 mol m−2 s−1 Pa−1 at P = 0.042 MPa)

Permeation temperature (◦C)

25 150 250 350 450

H2 permeance (mol m−2 s−1 Pa) 1.7× 10−8 2.3 × 10−8 3.0 × 10−8 3.7 × 10−8 7.5 × 10−8

N2 permeance (mol m−2 s−1 Pa) – – – – 1.8× 10−8

H2/N2 selectivity – – – – 4.2

and the stainless steel and the decrease in micro-cracks,respectively.

4. Conclusions

The silica composite membrane supported on the porousstainless steel failed in the presence of hydrogen above250◦C, resulting from the reduction of iron oxide formedon the surface of stainless steel. The failure of the stainlesssteel supported silica membrane is derived from the inter-face between coating layer and surface of the stainless steelsupport. To improve the thermal stability of the membranes,the composite membranes were prepared by the new tech-nique of the soaking–rolling method. The silica compositemembrane prepared by the soaking–rolling method was ther-mally stable below 450◦C due to a decrease in the interface.Differently from dip-coating method, the coating materialsdeeply penetrated into the pores of the support, and the is-lands of stainless steel support appeared on the surface of themembrane despite several coating procedures of top layer.The soaking process leads to the penetration of the coatinglayer into pores of porous stainless steel support and the in-terstitial voids among colloidal silica particles of the inter-mediate layer, and the rolling process following the soakingprocess contributes to the removal of concentrated surfacegel layer formed during the soaking process. Therefore thesoaking–rolling method gives rise to the minimization of theinterface between coating layer and stainless steel surface.The improvement of thermal stability of the stainless steelsupported silica membrane by the soaking–rolling methodis attributed to the minimization of the interface, and col-loidal silica particles, included in coating layer, as a barrierof propagation of the cracks derived from the remaining in-terface between the coating layer and the wall of stainlesssteel.


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