High Oxygen Permeation Rate in La0.6Sr0.4Ti0.3Fe0.7O3 Thin

May 24, 2010 - On the other hand, partial oxidation of CH4 into CO and H2 is dominant on the LSTF membrane. The oxygen permeation rate increased as th...
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Ind. Eng. Chem. Res. 2010, 49, 5511–5516

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High Oxygen Permeation Rate in La0.6Sr0.4Ti0.3Fe0.7O3 Thin Membrane on a Porous Support with Multichannel Structure for CH4 Partial Oxidation Akihiro Kawahara,*,†,‡ Yousuke Takahashi,‡ Yuji Hirano,‡ Masayoshi Hirano,§ and Tatsumi Ishihara† Department of Applied Chemistry, Faculty of Engineering, Kyushyu UniVersity, Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan, R&D Center, Noritake Company, LTD., Miyoshi, Aichi, Japan, and Chubu Electric Power, Nagoya, Aichi, Japan

A preparation of La0.6Sr0.4Ti0.3Fe0.7O3 (LSTF) perovskite oxide on a support with a unique multichannel structure achieved a high oxygen permeation rate. A LSTF thin membrane with 60 µm thickness was successfully prepared on the multichannel support with 30% porosity. The oxygen permeation rate was much improved by using the multichannel structured support, as the permeated oxygen was removed rapidly. On the other hand, partial oxidation of CH4 into CO and H2 is dominant on the LSTF membrane. The oxygen permeation rate increased as the thickness of the membrane and the porous support decreased, and a value of 13.3 cm3/ (min cm2) was achieved at 1273 K under a CH4 partial oxidation condition. 1. Introduction A mixed electronic and oxide ionic conductor (MIEC or mixed conductor) shows oxide ion conductivity and electronic conductivity simultaneously, and no external circuit is required for the oxygen permeation, as electronic neutrality is maintained within the MIEC materials. Therefore, various applications are now considered for mixed conductors. In particular, the application for oxygen separation from air is one of the most important areas for mixed conductors. Up to now, a number of mixed conductors such as perovskite oxide of La0.7Sr0.3Ga0.6Fe0.4O3 and La0.1Sr0.9Co0.9Fe0.1O3, or a defect perovskite oxide of Pr2NiO4 were reported for oxygen separation.1-8 However, reported oxygen permeation rates in those conventional mixed conducting materials are still not high enough to satisfy the oxygen demand in various processes. In addition, when the mixed conductor is used as the oxygen permeating membrane for CH4 partial oxidation, high chemical stability is required; however, the stability of the conventional mixed conducting oxide is still not obtained for those applications. In our previous study, we investigated the oxygen permeation property of an LSTF membrane prepared on the LSTF porous support. It is generally considered that the bulk diffusion step of oxide ion generally determines the oxygen permeation rate in the case of a thick membrane; however, with decreasing thickness, the surface reaction tends to be the rate determining step. Under those surface reaction limiting conditions, it is reported that the oxygen permeation rate is independent of the membrane thickness.9-13 However, our previous study revealed that the oxygen permeation rate in the LSTF membrane was also strongly influenced by oxygen diffusion in the porous support, and the control of pore structure of the porous support was effective for improving the oxygen permeation rate. Therefore, the oxygen permeation rate of the LSTF membrane was improved by increasing porosity of the support. As a result, the use of the support with straight pores, which were made by a laser cutting procedure, led to a much higher oxygen * To whom correspondence should be addressed. Tel.: +81-9-28022870. Fax: +81-9-2802-2871. E-mail: [email protected]. Address: Motooka 744, Nishi-ku, Fukuoka, 819-0395, Japan, Kyushu University. † Kyushyu University. ‡ Noritake Company, LTD. § Chubu Electric Power.

permeation rate compared with the use of a conventional porous support. However, because of the cost production, this type of support is not suitable for commercialization. In this study, we prepared a porous support with a unique multichannel structure by an extruded method which is more suitable for commercial products. The LSTF membrane was prepared on the support with a slip-coating method and as the oxygen permeation membrane for the CH4 partial oxidation reaction. 2. Experimental Section 2.1. Disk-Shape LSTF Membrane. LSTF thin film membranes with a disk shape were prepared by the so-called dipcoating method. A commercial powder of La0.6Sr0.4Ti0.3Fe0.7O3 (LSTF) prepared by a conventional solid state reaction method using metal oxide for starting materials was used in this study at all times. The powder was mixed with a ball-mill mixer with a ZrO2 ball and pot before use. After precalcination at 1273 K, the obtained powder was mixed with an organic binder in xylene with the ball-mill mixer to obtain a slip for coating film (LSTF/ methacrylic acid polymer(binder)/xylene ) 100:10:60 in weight ratio). For adjustment of the thermal expansion coefficient, we used LSTF with the same composition with a membrane for a porous substrate. The prepared LSTF powder was mixed with an organic compound to make a porous structure with a disk shape 20 mm in diameter, and after being pressed at 100 MPa the porous LSTF substrate was obtained by calcination at 1673 K for 1 h in air. We also prepared a 2 mm thick disk-shape LSTF membrane by sintering the green LSTF disk obtained by pressing isostatically at ca. 100 MPa. The thickness of the LSTF self-supporting membrane was always set to 2 mm by a diamond grinder. To enhance the surface activity, NiO (Wako) and La0.5Sr0.5CoO3(LSC) solutions were also coated by dip coating on the CH4 and air sides, respectively. The diameter of the catalyst was 18 mm, and so the effective oxygen permeation area was estimated to be 254 mm2. 2.2. Multichannel LSTF Porous Support. Multichannel porous support was prepared by the extrusion method by using the LSTF paste. Figure 1 shows a schematic view of the designed multichannel porous support in this study. The LSTF membrane was deposited on the outer surface of the tube, and permeated oxygen was removed by gas flow inside of the tube.

10.1021/ie901874j  2010 American Chemical Society Published on Web 05/24/2010

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Figure 1. Schematic view of the LSTF membrane on a multichannel support.

Figure 2. X-ray diffraction (XRD) pattern of La0.6Sr0.4Ti0.3Fe0.7O3 oxide powder (top) obtained by calcination at 1273 K and LSTF membrane (bottom) after sintering.

An advantage of this structure is that the thickness of the outer tube support can be made thinner while keeping a high mechanical strength. To adjust the thermal expansion property, we used La0.6Sr0.4Ti0.3Fe0.7O3 (LSTF) as the porous support, which is the same composition as the membrane. LSTF powder was prepared by a conventional solid state reaction method using metal oxide; the starting powder was first mixed with a ballmill mixer using ZrO2 balls and pot. After precalcination at 1673 K, the powder was mixed again with the ZrO2 ball-mill mixer. The average particle diameter was estimated to be ca. 10 µm. The thus obtained LSTF powder was mixed with organic binder with the kneader in the following ratio: LSTF powder (87 wt %), a methycellulose binder (2 wt %), a butyl benzyl phthalate (1 wt %) and water (10 wt %). After extruding the slurry consisting of the powder at ca. 50 MPa, the obtained tube was dried followed by calcination at 1623 K for 6 h in air. 2.3. Multichannel LSTF Thin Membrane. A LSTF thin membrane was prepared by the so-called dip coating method on the prepared multichannel LSTF support. La0.6Sr0.4Ti0.3Fe0.7O3 (LSTF) powder was also prepared by the conventional solid state reaction method. After precalcination at 1273 K, the obtained powder was mixed with an organic binder in xylene with the ball-mill mixer to obtain a slip for coating the membrane (LSTF/binder/xylene ) 100:10:60 in a weight ratio). After being dip coated, the obtained membrane was calcined at 1673 K. It was noted that the thickness of the membrane was between ca. 50 and 100 µm. Furthermore, it was also noted that no gas leakage was observed at 0.2 MPa pressure applied with nitrogen. To enhance the surface activity, NiO (Wako) and La0.5Sr0.5CoO3 (LSC) solution were also coated by dip coating on the CH4 and air sides, respectively. The catalyst powder made by the solid state reaction was mixed with an organic binder in xylene with the ball-mill mixer (catalyst/ binder/xylene ) 100:10:100 by weight). The catalyst slurry obtained was dropped into the porous support and calcined at 1273 K for 1 h. In the multichannel structured support, a CH4 partial oxidation experiment was performed by feeding CH4 and air at 1000 cc/

Figure 3. SEM image of the cross section of a coated membrane on a porous support sintered at 1673 K.

Figure 4. Temperature dependence of the oxygen permeation rate in the obtained LSTF membrane. The 2 mm thick membrane is self-supported and the 0.05 mm thick membrane is supported on a 2.5 mm thick porous LSTF substrate.

min, and the formed CO and H2 were analyzed by using a TCDtype gas chromatograph. The amount of oxygen permeation was estimated by the formation rate of CO, CO2, and water of which the formation rate was estimated by the amount of hydrogen atoms before and after the membrane reactor. It is also noted that no O2 and N2 was detected in the exhaust gas at the CH4 side. At each temperature, a CH4 partial oxidation measurement was performed for more than a few hours, and the amount of oxygen permeation was estimated by averaging the measured values at each 30 min interval. The crystal structure of the membrane was measured by a XRD diffractometer with Cu KR line (Rigaku, Ultrax18-TTR3300). Thickness and also surface morphology of the membrane were measured with a SEM (Hitachi, JSM-6490LA).

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a

Table 1. The Oxygen Permeation of Thin Support Thickness at 1273 K (Ni/LSTF/LSC)

oxygen permeation rate H2 production rate membrane thickness porous support thickness (cc/min/cm2) (cc/min) CO selectivity (%) (mm) (mm) temperature (K) CH4 conversion (%)

a

0.05

2.5

2

none

1173 1223 1273 1173 1223 1273

5.3 8.9 16.0 2.8 4.2 8.7

0.9 2.3 4.8 0.5 1.3 2.6

8.0 16.2 31.4 4.8 8.0 17.2

99.9 99.9 99.8 99.4 99.4 99.6

Effective area ) 254 mm2.

Figure 5. Photograph of multichannel porous support.

3. Results and Discussion 3.1. Oxygen Permeation Property of LSTF Membrane on the Support with Disk Shape. Figure 2a shows the XRD pattern of the prepared La0.6Sr0.4Ti0.3Fe0.7O3 powders used for the membranes and supports. Evidently, all diffraction peaks were assigned to those from La0.5Sr0.5TiO3, and single phase of La0.6Sr0.4Ti0.3Fe0.7O3 was obtained in this study. On the other hand, in Figure 2b, the XRD pattern of the membrane after sintering is also shown. Since the membrane composition is the same as that of the support, the single phase of La0.5Sr0.5TiO3 is also obtained for the membrane after dip coating and sintering. Therefore, Fe-doped La0.6Sr0.4TiO3 membrane is successfully obtained in this study. It is also noted that the quantitative analysis of the sample composition is performed, and the result corresponded to that of La0.6Sr0.4Ti0.3Fe0.7O3. For the purpose of studying the fundamental oxygen permeation property of the LSTF film on a porous LSTF substrate, we first studied the oxygen permeation under CH4 partial oxidation by using a 20 mm diameter disk-shape substrate, which is a much smaller area than that of the film on a multichannel tubular substrate. Figure 3a shows the SEM photographs of the LSTF membrane obtained on the disk shape support. Obviously, dense LSTF membrane was successfully prepared on the porous LSTF support. The thickness of the membrane is estimated to be ca. 60 µm. The porosity and the average pore size of the support are estimated to be ca. 30% by the Archimedes method and

Figure 6. A photograph of the oxygen permeation reactor (a) and the schematic view of the measurement system for the CH4 partial oxidation reactor (b) used in the experiments.

ca.10 µm by SEM, respectively. The thickness of the support was 2.5 mm. Figure 3b shows the top view of the obtained membrane. Evidently, the obtained membrane is highly dense, uniform, and flat. There is no crack or pinhole observed on the surface. It is also noted that the grain size of the membrane is estimated to be ca. 1 µm. Therefore, the highly dense LSTF membrane was successfully obtained on the porous LSTF support in this study. Figure 4 shows the temperature dependence of the oxygen permeation rate of the LSTF membranes which are are 2.0 and 0.05 mm thick. The 0.05 mm thick LSTF film was prepared on a porous substrate, and the 2 mm planar film was “self supporting”. It was found that the LSTF membrane exhibited a fairly large oxygen permeation rate, and the oxygen permeation rate of 2.6 cc/(min cm2) at 1273 K was achieved when the thickness of LSTF membrane was 2.0 mm. The oxygen permeation rate was significantly improved by decreasing the membrane thickness to 0.05 mm: ca. 5 cc/ (min cm2). Corresponding to the theoretical equation for oxygen permeation under a bulk diffusion limitation, the oxygen permeation rate will be 40 times higher by decreasing the thickness of the membrane to 0.05 mm from 2 mm. The dependence of the improved oxygen permeation on the decreasing membrane thickness obeyed the theoretical equation; however, the increase in the oxygen permeation rate

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Table 2. Effects of Outer Porous Support Thickness and Membrane Thickness on the Oxygen Permeation Rate and Product Distribution under CH4 Partial Oxidation Condition at 1273 K (Ni/LSTF/LSC)a Membrane thickness

Porous support thickness

CH4 Conversion

Oxygen Permeation rate

H2 production rate

CO selectivity

CO2 selectivity

µm

mm

%

cc/min/cm2

cc/min

%

%

100 100 60

0.42 0.26 0.26

61.1 51.6 40.1

13.1 15.3 19.2

831.7 863.0 413.0

86.6 67.1 34.8

13.4 32.9 63.2

a

Effective area ) 3391 mm2.

Figure 7. Time dependence of the oxygen permeation rate of LSTF membrane on a multichannel substrate under CH4 oxidation conditions at 1273 K.

was only doubled despite the use of a 40-fold thinner membrane thickness. The insufficient improvement in oxygen permeation rate can generally be explained by the limitation of the surface reaction for oxygen dissociation. However, in our previous study, the removal of permeated oxygen in a porous support was not sufficient and may be the ratedetermining step when the pore structure is not well controlled. Table 1 shows the effect of the support thickness on the oxygen permeation rate and the product distribution in CH4 partial oxidation when the obtained LSTF membrane is used as the oxygen permeation membrane. The formation rate of CO2 was almost negligible, and so partial oxidation of CH4 into only CO and H2 proceeded on both cells. However, the oxygen permeation rate is strongly influenced by the support thickness. By decreasing the thickness of the porous support, the oxygen permeation rate was much improved for the sample with the same membrane thickness. This could be explained by the insufficient gas removal rate in the thicker porous support. As a result, further improvement in the oxygen permeation rate could be achieved by using a thinner support with a larger porosity. However, considering the mechanical strength and thickness of the support as well as the porosity has some limitations. To achieve the high mechanical strength and a high gas diffusion property simultaneously, we recently adopted a multichannel structure design for the porous support as shown in Figure 1, and the oxygen permeation property of the LSTF membrane under CH4 partial oxidation conditions was further studied in detail. 3.2. Oxygen Permeation Rate of LSTF Membrane on Multichannel Support. Figure 5 shows the photographs of (top) the prepared multichannel porous support tube and (bottom) after the LSTF membrane was deposited. The prepared multichannel porous support tube had a diameter of 18 mm and a length of 60 mm, and the membrane was 60 µm thick. By using a double tubular design, a thinner but highly strong porous support could

Figure 8. Temperature dependence of the oxygen permeation rate in the multichannel substrate with a porous outside tube thickness of 0.26 mm and using La0.6Sr0.4Ti0.3Ni0.1Fe0.6O3 as catalyst.

be obtained. The estimated porosity of the support was ca. 30%, and the two types of multichannel support were prepared: 26 mm thick tube and 0.42 mm thick tube. Figure 5b shows the LSTF membrane prepared on the porous LSTF multichannel support. Comparing the porous support (Figure 5a), it is seen that the surface of the tube became smooth and dense. Therefore, the evidently, uniform and flat LSTF membrane was also successfully deposited on the multichannel porous tube, similar to that on the LSTF disk. It is also noted that the effective surface area of the membrane was 3391 mm2. The oxygen permeation rate was measured by using the oxygen gas concentration cell as shown in Figure 6a. In this setup, the end part of the multichannel tube was sealed with a LSTF disk with ceramic cements. The multichanneled LSTF tube was connected to an Al2O3 tube. Air and CH4 were fed into the inside and outside of the multichanneled tube, respectively. A schematic view for the gas flow is shown in Figure 6b, where it can be seen that air is turned at the end part of the inner tube to flow back along the outer channel. Therefore, air flow in the outer tube is parallel to that of CH4 flow. Table 2 shows the effect of porous support and the membrane thickness on the oxygen permeation rate and the product distribution. As the thickness of the porous support of the outer tube decreases, the oxygen permeation rate is increased. However, the H2 formation rate hardly changed, while CO2 selectivity was much improved. Although we did not analyze H2O, it seems that H2O selectivity was also increased with decreasing thickness of the outer tube, considering the hydrogen balance before and after the membrane reactor. Therefore, when the thickness of the porous support decreased, the amount of permeated oxygen increased, resulting in the shift to the deep oxidation reaction (CH4 + 2O2 f CO2 + 2H2O) from partial oxidation (CH4 + 1/2O2 f CO + 2H2). On the other hand, by decreasing the membrane thickness, the oxygen permeation rate was further improved, and a value of 13.3 cc/(min cm2) was achieved. However, the amount of H2 and CO decreased, while CO2 increased. Therefore, it was thought that the deep

Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010 Table 3. Temperature Dependence of the Oxygen Permeation Rate and Product Distribution Under CH4 Partial Oxidation Condition temperature (K) 1173 1223 1273

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oxygen permeation rate H2 production rate porous support thickness (cc/min/cm2) (cc/min) CO selectivity (%) CO2 selectivity (%) (mm) CH4 conversion (%) 0.26 0.26 0.26

61.6 64.9 67.0

16.1 18.1 19.5

938.0 947.0 922.0

62.7 63.0 66.7

37.3 37.0 33.3

a Effective area ) 3391 mm2. Thicknesses of the outer support tube and LSTF membrane were 0.26 mm and 60 µm, respectively. La0.6Sr0.4Ti0.3Ni0.1Fe0.6O3 was used for the surface catalyst.

oxidation of CH4 was further improved by increasing the oxygen permeation by decreasing the membrane thickness. Since the reaction varied from partial oxidation to deep oxidation depending on the CH4/O2 ratio in the reactor, an increase in the CH4 feed amount would change the dominant reaction from deep oxidation to partial oxidation; the amount of H2 formation would further be improved by controlling the CH4 flow rate. Figure 7 shows the time dependence of the oxygen permeation rate of the LSTF membrane (60 µm thickness) on the outer LSTF porous tube support with 0.26 and 0.42 mm thicknesses. As discussed above, a much higher oxygen permeation rate was observed for the thinner support in spite of the same thickness of the LSTF dense membranes. Furthermore, there is no decrease in the permeation rate examined for over 3 h. Therefore, the chemical stability of the LSTF membrane under the CH4 partial oxidation condition is reasonably high. Figure 8 shows the temperature dependence of the oxygen permeation in the LSTF membrane (60 µm) prepared on the multichannel support (0.25 mm thickness) with La0.6Sr0.4Ti0.3Ni0.1Fe0.6O3 (LSTNF) as the surface catalyst, which is used for improving the surface activity of LSTF for the partial oxidation reaction of CH4. An oxygen permeation rate as high as 19.5 cc/(min cm2) at 1273 K is achieved; however, it decreases with decreasing temperatures, but the oxygen permeation rate is still maintained at a value of 16.1 cc/(min cm2) at 1073 K. Therefore, compared to the temperature dependence of the oxygen permeation rate in the conventional mixed conducting membrane, higher oxygen permeation rates could be maintained at a low temperature on a LSTF membrane. This could be explained by the small activation energy for the oxygen permeation in LSTF. Table 3 shows the effects of the addition of LSTNF to Ni on the oxygen permeation and product distribution in the CH4 partial oxidation. La(Sr)Ti(Fe)O3 is known as being relatively stable in a reducing atmosphere, and also XRD measurements show that peaks from LSTNF were observed after the reaction. Therefore, it seems that LSTNF stably exists under the CH4 oxidation conditions, and the Ni-LSTNF composite works as a surface catalyst. From Table 3, it is evident that there was no gas leakage observed in this experiment. Compared to the data shown in Table 2, the addition of LSTNF catalyst is effective in increasing CO selectivity, suggesting that the reaction shifts from the deep to partial oxidation. As discussed, the deep oxidation dominantly occurred on this reactor at a higher temperature because of the excess amount of oxygen permeation against the amount of CH4 fed. However, the application of the partial oxidation catalyst effectively prevents the deep oxidation, and the CO2 selectivity decreased from 64 to ca. 32% at all temperatures. On the other hand, with a decrease in the operating temperature, the oxygen permeation rate became smaller and the reaction was further shifted to the partial oxidation side. Therefore, the H2 formation rate was slightly increased despite decreasing temperatures from 1273 to 1223 K. However, because of the decreased surface activity, the CO2

selectivity increased, and H2 formation rate decreased by further decreasing the reaction temperature from 1273 to 1173 K. Consequently, the H2 formation rate showed complicated temperature dependence, that is, the CH4 conversion monotonically decreased when the H2 formation rate once increased, and then decreased with decreasing reaction temperature from 1273 to 1173 K. Considering the high CO2 yield observed in the studied temperature range, the deep oxidation still occurred with a reasonably high ratio, and this also suggests that the surface activity of the Ni-based catalyst is not high enough. Therefore, the formation rate of H2 could be further improved by improving the surface catalyst activity for the partial oxidation reaction or reforming the reaction of CH4 with CO2. This study revealed that control of the pore structure in the support of the oxygen permeation membrane is highly important for achieving a high permeation rate, and LSTF membrane is a promising oxygen permeation membrane for CH4 partial oxidation. 4. Conclusion CH4 partial oxidation was studied by using the LSTF membrane prepared on the multichannel tube. Under the CH4 partial oxidation condition, the LSTF membrane showed a high and stable oxygen permeation rate. By using a thinner porous LSTF support, the oxygen permeation rate was much improved, and an oxygen permeation rate of 19.5 cc/(min cm2) was achieved under CH4 partial oxidation conditions. However, when the amount of CH4 was insufficient, the deep oxidation reaction was favored resulting in a decreased H2 formation rate. The amount of oxygen decreased as the reaction temperature decreased, however, a fairly large oxygen permeation rate of 16.1 cc/(min cm2) was obtained at 1173 K. Consequently, this study reveals that the LSTF membrane on a thinner multichannel support, which is suitable for commercial and industrial application, is highly attractive as an oxygen permeation membrane for partial oxidation of CH4. Literature Cited (1) Balachandran, U.; Dusek, T.; Mieville, R. L.; Poeppel, R. B.; Kleefisch, M. S.; Pei, S.; Kobylinski, T. P.; Udovich, C. A.; Bose, A. C. Dense Ceramic Membranes for Partial Oxidation of Membrane to Syngas. Appl. Catal., A 1995, 133, 19–29. (2) Teraoka, Y.; Honbe, Y.; Ishii, J.; Fukukawa, H.; Moriguchi, I. Catalytic Effects in Oxygen Permeation through Mixed-Conductive LSCF Perovskite Membranes. Solid State Ionics 2002, 152-153, 681–687. (3) Ishihara, T.; Yamada, T.; Arikawa, H.; Nishiguchi, H.; Takita, Y. Mix Electronic-Oxide Ionic Conductivity and Oxygen Permeating Property of Fe-, Co-, or Ni-Doped LaGaO3 Perovskite Oxide. Solid State Ionics 2000, 135, 631–636. (4) Harada, M.; Demon, K.; Hara, M.; Tatsumi, T. Oxygen-Permeable Membranes of Ba1.0Co0.7Fe0.2Nb0.1O3-S for Preparation of Synthesis Gas from Methane by Partial Oxidation. Chem. Lett. 2006, 35, 968–969. (5) Araki, S.; Hoshi, Y.; Hamakawa, S.; Hikazudani, S.; Mizukami, F. Synthesis and Characterization of Mixed Ioni-Electronic Conducting Ca0.8Sr0.2Ti0.7Fe0.3O3-S Thin Membrane. Solid State Ionics 2008, 178, 1740– 1745.

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(6) Ishihara, T.; Tsuruta, Y.; Todaka, T.; Nishiguchi, H.; Takita, Y. Fe doped LaGaO3 Pervskite Oxide as an Oxygen Separating Membrane for CH4 Partial. Solid State Ionics 2002, 152-153, 709–714. (7) Park, J. H.; Kim, K. Y.; Park, S. D. Oxygen permeation and stability of La0.6Sr0.4TixFe1-xO3-δ (x ) 0.2 and 0.3) membrane. Desalination 2009, 245, 559–569. (8) Ishihara, I.; Nakashima, K.; Okada, S.; Enoki, M.; Matusmoto, H. Defect chemistry and oxygen permeation property of Pr2Ni0.75Cu0.25O4 oxide dope with Ga. Solid State Ionics 2008, 179, 1367–1371. (9) Xianfeng, C.; Chun, Z.; Xueliang, D.; Chao, Y.; Wanqin, J.; Nanping, X. Experimental and modeling study of oxygen permeation modes for asymmetric mixed-conducting membrane. J. Membr. Sci. 2008, 322, 429–435. (10) Watanabe, K.; Yuasa, M.; Kida, T.; Shimanoe, K.; Teraoka, Y.; Yamazoe, N. Oxygen permeation of a dense/porous asymmetric membrane using La0.6Ca0.4CoO3-δsBaFe0.975Zr0.025O3-δ system. Chem. Lett. 2009, 38, 94–95.

(11) Shaula, A. L.; Naumovich, E. N.; Viskup, A. P.; Pankov, V. V.; Kovalevsky, A. V.; Kharton, V. V. Oxygen transport in La2NiO4+δ: Assessment of surface limitations and multilayer membrane architectures. Solid State Ionics 2009, 180, 812–816. (12) Zydorczak, B.; Zhentao, W.; Li, K. Fabrication of ultrathin La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membranes for oxygen permeation. Chem. Eng. Sci. 2009, 64, 4383–4388. (13) Watnabe, K.; Yuasa, M.; Kida, T.; Shimanoe, K.; Teraoka, Y.; Yamazoe, N. Dense/Porous Asymmetric-Structured Oxygen Permeable Membrane Based on La0.6Ca0.4CoO3 Peroviskite-Type Oxide. Chem. Mater. 2008, 20, 6965–6973.

ReceiVed for reView November 28, 2009 ReVised manuscript receiVed March 3, 2010 Accepted April 19, 2010 IE901874J