Contribution of the Surface Reactions to the Overall Oxygen

and the surface reactions on the both sides of the membrane; therefore, it is ... The effect of applying a porous layer on the permeate side (low oxyg...
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Ind. Eng. Chem. Res. 2006, 45, 2824-2829

Contribution of the Surface Reactions to the Overall Oxygen Permeation of the Mixed Conducting Membranes Xianfeng Chang, Chun Zhang, Zhentao Wu, Wanqin Jin, and Nanping Xu* Membrane Science and Technology Research Center, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China

Oxygen permeation through a mixed conducting membrane is essentially controlled by both the bulk diffusion and the surface reactions on the both sides of the membrane; therefore, it is important to know the proportion of surface reactions in the overall oxygen permeation to improve the oxygen permeability of membranes effectively. In this study, the contribution of surface reactions to the overall oxygen permeation was investigated in detail through the calculation and the oxygen permeation measurement of the noncoated or coated SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) mixed conducting membranes. The contribution of the surface reactions to the overall oxygen permeation was observed to increase with decreases in the membrane thickness for both the noncoated and coated membranes, and that the contribution of the surface reactions was significantly reduced by coating a porous layer. The effect of applying a porous layer on the permeate side (low oxygen partial pressure) on the decrease of the contribution of the surface reactions was more remarkable than that on the feed side (high oxygen partial pressure). 1. Introduction Mixed conductive oxides have been found to exhibit substantial ionic and electronic conductivity under the oxygen chemical potential gradient at the elevated temperature, typically above 700 °C.1 This behavior results in the spontaneous occurrence of oxygen transportation through the dense membrane in the form of O ions without electrodes and external electrical circuits. In addition, perovskite-type ionic conductors, which are one type of mixed conductive oxides, have been of growing interest, because of their promising technological applications in oxygen separation from air,2,3 solid oxide fuel cells,4,5 and partial oxidation of methane to syngas.6-12 Generally, the overall oxygen permeation through the mixed conducting membrane consists of the following two processes: (i) bulk diffusion through the membrane, and (ii) surface reaction on either side of the membrane. When the membrane is thick enough, the overall oxygen permeation is considered to be determined by the bulk transport through the membrane, and the classical Wagner’s equation can be used to describe the oxygen permeation flux through the membrane as13

J O2 )

RT 16F2L

∫lnlnP′′P′

O2

O2

σiσe d ln PO2 σi + σe

(1)

where σi and σe (S/cm) are the ionic and electronic conductivity, respectively; P′O2 and P′′O2 are the upstream and downstream oxygen partial pressure, respectively (given in pascals); F is the Faraday constant (given in units of C/mol); R the gas constant (equal to 8.314 J/(mol K)); T (K) the temperature (given in Kelvin); L is the membrane thickness (given in centimeters); and PO2 is the oxygen partial pressure (also given in pascals). When the membrane thickness is less than the characteristic thickness Lc, which is the ratio of the oxygen self-diffusivity and surface exchange coefficient (Ds/ks),1 the surface reactions on the both sides of the membrane become the rate-limiting step. Any further increase in permeation rate can only result * To whom all correspondence should be addressed. Tel.: +86-258358-7171. Fax: +86-25-8330-0345. E-mail: [email protected].

from increasing the effective surface area for exchange of oxygen or coating materials of superior oxygen exchange properties on the membrane surfaces. Generally, the membrane thickness is between the above two conditions for the practical application;14-16 thus, oxygen permeation through the membrane is under the mixed control of bulk diffusion and surface reactions on the both sides of the membrane, which implies that surface reactions have an important role in the oxygen permeation. Therefore, it is more important to know the definite proportion of the surface reactions in the overall oxygen permeation to improve the rate of surface reactions. Some efforts17-21 have placed emphasis on the improvement of the surface reactions for the enhancement of the oxygen flux by coating a porous layer on the surface of the dense membrane. Kim et al.17 have reported that the oxygen flux of the La0.7Sr0.3Ga0.6Fe0.4O3-δ membrane is strongly influenced by the microstructure of La0.6SrCoO3-δ porous layers. Jacobson et al.18 have described the effect of porous layers and provided a model to predict the increase in the oxygen flux of the SrCo0.8Fe0.2O3-δ membrane. Teraoka et al.21 have investigated the effect of materials of porous layers on the oxygen permeability of the perovskite dense membranes, which has shown that the selection of the material of porous layers is based on the oxygen sorption property of the material. Although the coated porous layers have been demonstrated to have the strong effect on the oxygen permeability of the membrane, it might be more helpful for one to quantitatively know the effect of a porous layer on the proportion of surface reactions in the overall oxygen permeation to improve the oxygen permeability of mixed conducting membranes effectively. Therefore, the objectives of this work are to give an expression for quantitatively calculating the proportion of surface reactions in the overall oxygen permeation of membrane, and comprehensively investigate definite changes of the surface reactions in the overall oxygen permeation by coating a porous layer on either side of membrane. In our present work, a perovskite-type oxide of SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) was selected as the representative membrane material, which has

10.1021/ie051162c CCC: $33.50 © 2006 American Chemical Society Published on Web 03/09/2006

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been found to be structurally stabile with higher oxygen permeability in our previous study.22 2. Theory 2.1. Contribution of the Surface Reactions to the Overall Oxygen Permeation through the Membrane. For both membrane bulk diffusion and surface reactions, the oxygen permeation flux through the membrane can be described as23 -m

JO2 )

A(P′′O2s

-m

- P′O2s

(2)

where the parameters A and m are constants, Dv is the oxygen vacancy diffusivity of the membrane (given in units of cm2/s), and the other parameters have the same meaning as stated previously. The surface exchange coefficient, ks (given in units of cm/s), is given by the relation

kakd ka + kd

(3)

A(P′′O2s-m - P′O2s-m)

(4)

2/Dv

substitution of eq 4 into eq 2 gives the oxygen permeation flux:

J O2 )

(

1/ks k 1L 1/ks + 2L/Dv

)

(5)

We define the contribution of the surface reactions to the overall oxygen permeation, φ, as

φ)

1/ks 1/ks + 2L/Dv

(6)

Substituting eq 6 into eq 5 then produces the oxygen permeation flux:

k JO2 ) (1 - φ) L

(7)

The procedure for the solution of φ is given as follows. First of all, taking the reciprocal of eq 2 yields the following expression:

1 ks

1 1 ) L+ JO2 k k(2/Dv)

()

(9)

where teltionσtotal is the average value of the product of the electronic transference number tel, the ionic transference number tion, and the total conductivity σtotal (σtotal ) σel + σion) over the applied oxygen partial pressure differential. The total drop in ) is defined as chemical potential across the membrane (∆µOtotal 2 /P′ ). ∆µOtotal ) RT ln(P′′ O O 2 2 2 Based on the relationship between the oxygen flux and the membrane thickness, oxygen flux through the membrane can also be obtained by1

JO2 ) λLγ

where ka and kd (each given in terms of cm/s) are the absorption and desorption rate constants, respectively. By defining k (in units of mol/(cm s)) as follows,

k)

total

teltionσtotal ∆µO2 1 JO2 ) L 1 + (2LC/L) 42F2

)

1/ks + 2L/Dv

ks )

predominant control by bulk diffusion to that by surface reactions. Hence, more attention should be given to the estimation of Lc. Assuming linear kinetics for bulk diffusion and surface reactions, the oxygen flux through the membrane can be written as1

(8)

Then, 1/JO2 is plotted against the membrane thickness L, and k is calculated from the reciprocal of the slope of the regressed straight line. Finally, φ is calculated from the dependence of the oxygen flux on the membrane thickness on the basis of eq 7. 2.2. Estimation of the Characteristic Membrane Thickness through the Oxygen Permeation Measurement. The characteristic membrane thickness (Lc) is not only a valuable criterion for the selection of candidate membrane material, but also a convenient parameter for determining the transition from

(10)

where the parameter λ is constant, and the value of γ, at a given L, corresponds with the negative slope in the double logarithmic plot of the oxygen flux versus the membrane thickness. Meanwhile, taking the logarithm of eq 9, γ can be given by partial differentiation with respect to log L as

(

γ)- 1+

)

2LC L

-1

(11)

Substitution of eq 11 into eq 10 yields eq 12 for the oxygen flux through the membrane:

JO2 ) λL-(1+2LC/L)-1

(12)

Fitting the experimental data of the dependence of the oxygen flux on the membrane thickness to eq 12 produces the value of the characteristic membrane thickness, Lc. 3. Experimental Section 3.1. Powder and Membrane Preparation. SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) powders were synthesized by the conventional solid-state reaction method. Appropriate amounts of SrCO3, Co2O3, Fe2O3, and monoclinic ZrO2 were mixed and ball-milled in deionized water for 24 h, followed by calcinations in air at 1223 K for 5 h with heating and cooling rates of 2 K/min. The calcined powders were pressed into disks 16 mm in diameter, using an oil pressure of 200 MPa, and then were sintered in air at 1473 K for 5 h to form dense membranes. The sintered membranes without a porous layer were polished to the thickness of interest (0.06, 0.08, or 0.10 cm) before being used in experiments, and the coated membranes were obtained by coating either surface of the pretreated membrane with a porous layer (of SCFZ material) of ∼50 mg/cm2, which was applied either on the feed side or on the permeate side, and subsequently sintering at a temperature of 1313 K for 4 h. In the experiments, the thickness and structure of the porous layer was considered constant in all cases. The densities of the sintered membranes were determined using the Archimedes method. These densities exceeded 90% of the theoretical density in all cases. 3.2. Characterization. The crystal structure of the synthesized powder was characterized via X-ray diffraction (XRD),

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Figure 1. Schematic diagram of this work, showing the membranes of different thickness (a) without a porous layer, (b) with a porous layer on the feed side, and (c) with a porous layer on the permeate side. The darkershaded regions represent the dense membrane, and the lighter-shaded regions represent the porous layer.

using Cu KR radiation (Bruker, model D8 Advance). The diffraction patterns were collected at room temperature by step scanning at an increment of 0.05° in the range of 20° e 2θ e 80°. The morphology of the coated membrane was examined by environmental scanning electron microscopy (ESEM) (model QUANTA-2000). 3.3. Oxygen Permeation Measurement. Oxygen permeation measurements were performed using a permeation apparatus that has been reported previously.24 A prepared membrane was mounted on a quartz tube (6 mm inner diameter (ID), 12 mm outer diameter (OD)), using a gold ring seal. A tubular furnace whose temperature could be controlled within (1 K by a programmable temperature controller (Model AI-708PA, Xiamen Yuguang Electronics Technology Research Institute, PRC) was used for heating. Before the oxygen permeation measurement was begun, the assembly was heated to 1313 K with a heating rate of 2 K/min and held for 4 h to form the bonding. The furnace was then cooled to the temperature of interest at a cooling rate of 2 K/min. Gas leakage, if present, could be detected by monitoring the nitrogen concentration in the effluent from the oxygen-lean side of the membrane. In the oxygen permeation experiments, the flow rates of the inlet gas were controlled by mass flow controllers (model D07-7A/ZM, Beijing Jianzhong Machine Factory, PRC), which were calibrated by a bubble flowmeter. One side of the membrane was exposed to air at a flow rate of 100 mL(STP)/min, while the other side was exposed to a lower oxygen partial pressure that was controlled by regulating the helium flow rate. The effluent gases were analyzed by gas chromatography (GC) (Shimadzu, model GC-8A), which was equipped with a 2 m 5 Å molecular sieve operated at 323 K with helium as the carrier gas. The oxygen permeation flux through the membrane was calculated from the flow rates and the oxygen concentrations of the effluents. The arrangement for studying the contribution of surface reactions to the overall oxygen permeation was illustrated in Figure 1 (Figure 1a for Case I, Figure 1b for Case II, Figure 1c for Case III). In Case I, the contribution of the surface reactions was first investigated as a function of the membrane thickness for the noncoated membrane. Then, in Case II and Case III, the dependence of the contribution of the surface reactions on the membrane thickness was examined for the membrane with a porous layer on the feed side and on the permeate side, respectively. Finally, the detailed study on the contribution of the surface reactions to the overall oxygen permeation of the SCFZ membrane was performed by the combination of the aforementioned three cases. For convenience, the membrane without a porous layer was denoted as “M-Blank”, the mem-

Figure 2. X-ray diffraction (XRD) pattern of SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) oxide powder calcined at 1223 K.

Figure 3. Plots of both the reciprocal of the oxygen flux (1/JO2) and the oxygen flux (JO2), each as a function of membrane thickness at 1223 K and the oxygen partial pressure gradient of 0.21 × 105/1 × 102 Pa.

brane with a porous layer on the feed side was denoted as “FPorous”, and the membrane with a porous layer on the permeate side was denoted as “P-Porous”. 4. Results and Discussion 4.1. Characterization of the Membrane Material. Figure 2 shows the XRD spectrum of SCFZ oxide powder calcined at 1223 K. As can be seen from the figure, the sample has a cubic perovskite structure with a trace of the SrZrO3 phase, which is a solid-state reaction product between ZrO2 and perovskite at elevated temperature. The crystal phase of SCFZ used in our experiments is similar to that previously reported by our group.22 4.2. Contribution of the Surface Reactions to the Overall Oxygen Permeation of the SCFZ Membrane without a Porous Layer. First, we were devoted to investigating Case I and performed the measurements for oxygen permeation at 1223 K. When the bulk diffusion through the membrane is the ratelimiting step, the oxygen permeation flux should vary inversely to the membrane thickness according to eq 1; that is, the reciprocal of the oxygen flux (1/JO2) is directly proportional to the membrane thickness (L). In our experiments, however, we found that the plot of 1/JO2 vs L did not pass through the origin of the coordinate, as shown in Figure 3. The observation indicated that the oxygen permeation was partially governed

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by the surface reactions. To further ascertain the rate-limiting step in the oxygen permeation, the characteristic thickness of the SCFZ membrane is calculated as follows. From the plot of the oxygen flux against the membrane thickness in Figure 3, it is found that oxygen permeation flux decreases as the thickness of the membrane increases. Fitting the experimental data to eq 12 yields the characteristic thickness of the SCFZ membrane of Lc ≈ 59 µm, which indicates that the oxygen permeation is controlled by the surface reactions for the membrane thickness less than 59 µm. Therefore, for the membranes in Case I, the oxygen permeation was governed by both the bulk diffusion and surface reactions. Based on the dependence of the reciprocal of the oxygen flux on the membrane thickness shown in Figure 3, regression of eq 8 gives k ) 8.2225 × 10-8 mol/(cm s). Therefore, φ for various membrane thicknesses at 1223 K can be calculated by eq 7, as listed in Table 2 (presented later in this work). From the calculation results, it is clearly demonstrated that the contribution of surface reactions increases as the membrane thickness decreases. 4.3. Contribution of the Surface Reactions to the Overall Oxygen Permeation of the SCFZ Membrane Coated by a SCFZ Porous Layer. In this work, the material that comprised the porous layer was the same as the membrane material, so that the match of thermal expansion between the membrane and the porous layer could be realized for the good interfacial bonding between each other. Figure 4 shows the morphology of the coated SCFZ membrane sintered at the temperature of 1313 K. As seen from the scanning electron microscopy (SEM) micrograph of the top surface of the porous layer in Figure 4a, the specimen exhibits a porous microstructure. A porous coating layer ∼10 µm thick can be identified in Figure 4b, and the good interfacial bonding between the membrane and the porous layer is obtained. The oxygen permeation data for Case II and Case III are given in Figure 5. To compare, the oxygen permeation data for Case I are also presented in it. It can be observed from the figure that the oxygen flux is enhanced by coating either side of the membrane, which is attributed to the increase in the effective specific surface area for the exchange of oxygen. This suggests that coating a porous layer on the surface of dense membrane should be an effective way to improve the oxygen permeation performance of the membrane. Furthermore, the degree of the enhancement of oxygen flux by coating a porous layer on the permeate side (low oxygen partial pressure) is larger than that on the feed side (high oxygen partial pressure), as shown in Figure 5. This is in agreement with the results of Jacobson et al.18 and Teraoka et al.21 The enhancement factor (η) of the oxygen flux by coating a porous layer can be calculated using the following equation:

η)

JOC 2 JO2

(13)

where JOC 2 is the flux of coated membrane and JO2 is the flux of noncoated membrane. Table 1 summarizes the enhancement factors of membranes with various thickness by coating a porous layer at 1123 K. From Table 1, it can be obtained that the enhancement factor η of the oxygen flux, whether the porous layer is located on the feed side or on the permeate side, increases as the membrane thickness decreases. This is due to the increase in contribution of surface reactions to the overall oxygen permeation with decreasing membrane thickness, as discussed in Case I. Moreover, this indicates that, with the

Figure 4. Scanning electron microscopy (SEM) micrographs of (a) the top surface of the porous layer on the dense membrane and (b) the cross section of the coated membrane sintered at 1313 K. The dense layer is to the left, and the porous layer on the surface of membrane is to the right.

decrease of the membrane thickness, the effect of surface modification by coating a porous layer on the enhancement of oxygen permeation is more remarkable. Based on the oxygen permeation data in Figure 5, the φ values of three cases at 1223 K are calculated, as listed in Table 2. This table shows that the contribution of surface reactions to the overall oxygen permeation increases with decreasing membrane thickness in all cases. In addition, for the same membrane thickness, the contribution of surface reactions in both Case II and Case III is much lower than that in Case I. Table 2 also shows that the contribution of surface reactions in Case III is less than that in Case II. This is because the surface

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Figure 5. Temperature dependence of oxygen flux through SCFZ membranes of different thickness without or with a porous layer at the oxygen partial pressure gradient of 0.21 × 105/1 × 102 Pa: (a) L ) 0.10 cm, (b) L ) 0.08 cm, (c) L ) 0.06 cm. Table 1. Oxygen Permeability without or with a Porous Layer on the Surface of Dense Membrane at 1223 K JOC 2 × 107, mol/cm2‚s

Enhancement Factor, η

L (cm)

JO2 × 107, mol/cm2‚s

feed side

permeate side

feed side

permeate side

0.10 0.08 0.06

5.14 6.03 6.85

5.37 6.62 8.09

7.05 8.76 10.92

1.04 1.10 1.18

1.37 1.45 1.59

Table 2. Contribution of Surface Reactions to the Overall Oxygen Permeation for Both the Coated and Noncoated Membrane s at 1223 K φ (%) coated L (cm)

noncoated

feed side

permeate side

0.10 0.08 0.06

37.5 41.3 50.0

16.3 17.2 24.7

12.8 14.1 19.6

reaction kinetics may be different at both sides of the disk. The difference of the surface reaction kinetics can be caused by the following two reasons. First, the compositional and structural variations between the two surfaces lead to the difference of the surface reaction kinetics at both sides of the disk. Because the SCFZ membrane has been demonstrated to exhibit remarkable structural stability in a low-oxygen-partial-pressure atmosphere at high temperature,22 this is not responsible for giving rise to the phenomena. Second, the difference of atmospheres around the membrane causes the difference of the surface reaction kinetics at both sides of the disk. The oxygen partial pressure at one side of the membrane is high, and that at the other side is low. The surface exchange coefficient (ks) is a function of oxygen partial pressure (PO2) and temperature. The parameter ks can be related to PO2 at a given temperature as25

ks ) aPO2n

(14)

where the parameters a and n are positive constants. It can be inferred from eq 14 that ks increases as the oxygen partial pressure in the gas phase increases, and ks on the feed side (high oxygen partial pressure) is larger than that on the permeate side (low oxygen partial pressure). In other words, the resistance of the surface reaction on the permeate side is greater than that on the feed side. Therefore, when such a method as coating a porous

layer is utilized to improve the performance of surface reactions of the membrane, the decrease in contribution of the surface reactions by coating a porous layer on the permeate side (low oxygen partial pressure) is more obvious. 5. Conclusions The contribution of surface reactions to the overall oxygen permeation of the SrCo0.4Fe0.5Zr0.1O3-δ (SCFZ) mixed conducting membrane was investigated by the calculation and the oxygen permeation measurement of both the noncoated and coated membranes in detail. It was found that the contribution of the surface reactions to the overall oxygen permeation increased as the thickness of the membrane was decreased for both the noncoated and coated membranes. For the noncoated SCFZ membranes, the contributions of the surface reactions are 37.5%, 41.3%, and 50.0% for the membrane thicknesses of 0.10, 0.08, and 0.06 cm, respectively. Meanwhile, experimental results have shown that coating a porous layer on the surface of a dense membrane is an effective way to decrease the contribution of the surface reactions. Furthermore, the effect of coating a porous layer on the permeate side (low oxygen partial pressure) of the membrane is more notable than that on the feed side (high oxygen partial pressure) in decreasing the contribution of the surface reactions. For the membranes with the thickness of 0.06 cm, the contribution of the surface reactions for the noncoated membrane is 50.0%, whereas the contributions of 24.7% and 19.6% are given for the coated membrane on the feed side and that on the permeate side, respectively. Our study demonstrates that the contribution of surface reactions in the overall oxygen permeation of a mixed conducting membrane can be quantitatively estimated, and the oxygen permeability can be improved effectively by coating a porous layer on either side of the membrane. Acknowledgment This work is sponsored by the National Basic Research Program of China (No. 2003CB615702), National Natural Science Foundation of China (NNSFC, No. 20125618, 20576051, and 20436030), Scientific Research Foundation for the Returned Overseas China Scholars from State Ministry of Education (No. 2004527), and Nanjing Ministry of Personnel, and the Key Laboratory of Material-Oriented Chemical Engineering of Jiangsu Province.

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Nomenclature F ) Faraday constant; F ) 96 485 C/mol R ) gas constant; R ) 8.314 J/(mol K) T ) temperature (K) PO2 ) oxygen partial pressure (Pa) P′O2 ) upstream oxygen partial pressure (Pa) P′′O2 ) downstream oxygen partial pressure (Pa) ) the total drop in chemical potential across the ∆µOtotal 2 membrane (J/mol) σi ) ionic conductivity (S/cm) σe ) electronic conductivity (S/cm) σtotal ) total conductivity (S/cm) tel ) electronic transference number, dimensionless tion ) ionic transference number, dimensionless Dv ) oxygen vacancy diffusivity (cm2/s) Ds ) oxygen self-diffusivity (cm2/s) ks ) surface exchange coefficient (cm/s) ka ) absorption rate constant (cm/s) kd ) desorption rate constant (cm/s) Lc ) characteristic membrane thickness (cm) L ) membrane thickness (cm) JO2 ) oxygen permeation flux (mol/(cm2 s)) φ ) contribution of the surface reactions to the overall oxygen permeation, dimensionless η ) enhancement factor of the oxygen flux, dimensionless Superscript C ) coated membrane Subscript S ) membrane surface Literature Cited (1) Bouwmeester, H. J. M.; Burggraaf, A. J. Dense Ceramic Membranes for Oxygen Separation. In Fundamentals of Inorganic Membrane Science and Technology; Burggraaf, A. J., Cot, L., Eds.; Elsevier: Amsterdam, The Netherlands, 1996; p 435. (2) Balachandran, U.; Ma, B.; Maiya, P. S.; Mieville, R. L.; Dusek, J. T.; Picciolo, J. J.; Guan, J.; Dorris, S. E.; Liu, M. Development of mixedconducting oxides for gas separation. Solid State Ionics 1998, 108, 363. (3) Steele, B. C. H. Oxygen ion conductors and their technological applications. Mater. Sci. Eng. 1992, 13, 79. (4) Yasumoto, K.; Inagaki, Y.; Shiono, M.; Dokiya, M. An (La,Sr)(Co,Cu)O3-δ cathode for reduced temperature SOFCs. Solid State Ionics 2002, 148, 545. (5) Wen, T.-L.; Wang, D.; Chen, M.; Tu, H.; Lu, Z.; Zhang, Z.; Nie, H.; Huang, W. Materials research for planar SOFC stack. Solid State Ionics 2002, 148, 513. (6) Balachandran, U.; Dusek, J. 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 methane to syngas. Appl. Catal., A 1995, 133, 19. (7) Thursfield, A.; Metcalfe, I. S. The use of dense mixed ionic and electronic conducting membranes for chemical production. J. Mater. Chem. 2004, 14, 2475.

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ReceiVed for reView October 18, 2005 ReVised manuscript receiVed January 1, 2006 Accepted January 24, 2006 IE051162C