Partial oxidation of methane reaction in Ba0.9Co0.7Fe0.2Nb0.1O3-δ oxygen permeation membrane with three-layer structure

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Partial oxidation of methane reaction in Ba0.9Co0.7Fe0.2Nb0.1O3-d oxygen permeation membrane with three-layer structure Shidong Song a,*, Peng Zhang b, Xin Zhang b, Minfang Han b,** a

School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China School of Chemical and Environmental Engineering, China University of Mining & Technology, Beijing 100083, China b

article info

abstract

Article history:

Perovskite oxides exhibiting mixed ionic and electronic conductivities can be used as the

Received 16 February 2015

membrane material in the partial oxidation of methane (POM) to syngas. Currently, one of

Received in revised form

the most promising membrane materials is Ba0.9Co0.7Fe0.2Nb0.1O3-d (BCFN9721) perovskite

23 June 2015

with A-site deficiency which possesses the high oxygen permeation flux and stability. In

Accepted 25 June 2015

this work, for further enhancing the performance and stability of Ba0.9Co0.7Fe0.2Nb0.1O3-

Available online 26 July 2015

d

membrane reactor in POM and reducing the loading of catalyst, a novel three-layer

structure consisting of a dense thin film and two porous support layers is applied. By Keywords:

using a thin BCFN9721 dense layer and a low loading of Ni catalyst deposited in the

Oxygen permeation membrane

BCFN9721 porous support layer at reaction side, an enhanced oxygen permeation flux and

Three-layer structure

CH4 conversion efficiency can be achieved. Carbon deposition can be effectively alleviated

Perovskite

as well, which is possibly attributed to the in-situ deep oxidation of the dissociated carbon

Mixed conductor

by the absorbed oxygen and active oxygen species produced by the oxygen exchange re-

Syngas

action on the surface of dense layer and the wall of porous support layer. A stability test is

Partial oxidation of methane

conducted and the performance of Ba0.9Co0.7Fe0.2Nb0.1O3-d catalytic membrane with threelayer structure shows no obvious degradation. The morphology and element composition of three-layer membrane are almost unchanged after 100 h operation in POM reaction. However, the conventional dense membrane shows obvious change in the morphology and element composition. The results indicate that the Ba0.9Co0.7Fe0.2Nb0.1O3-d membrane with three-layer structure possesses a higher performance and stability than the conventional dense membrane and can be very promising for the application in POM to syngas. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. Tel./fax: þ86 22 83955451. ** Corresponding author. E-mail addresses: [email protected] (S. Song), [email protected] (M. Han). http://dx.doi.org/10.1016/j.ijhydene.2015.06.134 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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Introduction Mixed ionic-electronic conducting (MIEC) oxygen permeation membrane is a very attractive technology because the membrane is energy- and cost-efficient for separating oxygen from air. Driven by an oxygen partial pressure gradient across the membrane, the ionic conductivity allows oxygen to diffuse through the membrane, while the electronic conductivity ensures an internal partial short-circuit for oxygen exchange reaction. At present, the most widely studied MIEC materials are ABO3-type perovskite oxides where A-site and B-site are generally occupied by Ln (Ln ¼ rare earth elements) cations in 12-fold coordination and by transition cations in 6-fold coordination, respectively. Generally, Co [1,2], Fe [3,4], Ti [5,6], Ce [7] and Mn [8] are favorably used in the B-site of perovskites for their mixed charge valence that allows easy formation of oxygen defects. Nearly all of the cobalt-containing perovskite oxides, such as La1xSrxCo1yFeyO3d (LSCFO) oxides [9,10], can exhibit very high oxygen permeability, which is attributed to the weak bonding energy between CoeO. La is widely used in the A-site due to its appropriate ionic radius that enables easy construction of the cubic ABO3 lattice. As trivalent La cation is replaced by divalent alkaline earth elements, such as Ca [6,11,12], Sr [9,12] and Ba [12e14] on A-site gradually, the electron holes and oxygen vacancies are generated by charge compensation, leading to an increase in the oxygen vacancy concentration. Perovskite oxygen permeation membranes have been extensively applied in the partial oxidation of methane (POM) to syngas [15,16]. Harada et al. [17,18] first reported BaCo0.7Fe0.2Nb0.1O3d oxide (BCFNO) as the material for oxygen permeation membrane. BCFNO membrane exhibited a high oxygen permeation flux of 20 mL min1 cm2 and maintained a 300 h consecutive operation in POM reaction. Oxygen permeability of BCFNO can be further increased by introducing A-site deficiency in lattice, which can create additional oxygen vacancies to facilitate oxygen exchange reaction and promote oxygen ion diffusion [19]. Thus, A-site deficient Ba0.9Co0.7Fe0.2Nb0.1O3d (BCFN9721) has been systematically evaluated in our prior work, as oxygen permeation membranes [19,20] and cathode for SOFCs [21e24]. The results showed BCFN9721 membrane had a higher oxygen permeation performance and stability compared with BCFNO membrane. The thermogravimetric analysis (TGA) in CO2 indicated that by introducing deficiency at A-site of perovskite, BCFN9721 could achieve a better chemical stability than BCFNO in CO2 at high temperature [19]. To date, different kinds of reforming catalysts for POM reaction with high CH4 conversion efficiencies have been investigated [25e27]. Ni-based catalysts have been regarded as very promising catalysts for POM reaction due to their high catalytic activities and low costs. Generally, the Ni-containing active component needs to be deposited on the stable supports to form the applicable reforming catalysts, such as NiO/ MgO [28], NiO/Al2O3 [29] and LiNiREOx/g-Al2O3 (RE ¼ La or Ce) [29]. In a fixed bed reactor, a sufficient quantity of catalysts have to be packed or prepared on the surface of membrane since only Ni0 phase is highly active for the POM reaction, leading to a high catalyst loading as well as the poor reaction interfaces. If the Ni-containing active component is deposited

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directly on the MIEC materials rather than on the inert supports, the reaction interfaces for POM reaction can thereby be largely extended. Thus, high CH4 conversion efficiencies can be achieved by employing a low loading of catalysts. In our prior work, we developed a BCFN9721 membrane with threelayer structure consisting of a thin BCFN9721 dense layer and two BCFN9721 porous support layers [20]. The membrane showed very high and stable oxygen permeation fluxes at 900  C for 100 h. In this work, for further improving the performance and stability of Ba0.9Co0.7Fe0.2Nb0.1O3-d membrane reactor and reducing the catalyst loading for POM reaction, a novel oxygen permeation membrane with three-layer structure is applied. By impregnating Ni into the BCFN9721 porous support layer at reaction side, a three-layer BCFN9721 catalytic membrane is prepared and evaluated. It is expected that a high performance can be achieved for POM by employing three-layer catalytic membrane with only a small amount of Ni catalysts. The oxygen permeation flux, CH4 conversion efficiency, CO selectivity and the membrane stability under the reactive conditions are systematically studied. Though the research on BCFN9721 oxygen permeation membranes have been extensively carried out, to the best of our knowledge, no investigation on BCFN9721 catalytic membrane with threelayer structure in POM to syngas has been reported yet.

Experimental methods Preparation of Ba0.9Co0.7Fe0.2Nb0.1O3d catalytic membrane All reagents used in this work were obtained from Beijing Chemical reagent Company. Powder samples of Ba0.9Co0.7Fe0.2Nb0.1O3d (BCFN9721) were prepared by the conventional solid state reaction process according to our previously reported paper [19e24]. The BCFN9721 membrane with three-layer structure was fabricated using tape casting technique. The details can be found in our prior work [20]. The slurry of porous support for tape casting was prepared by ballmilling BCFN9721 powder mixing with graphite in a weight ratio of 50:50 for 24 h, along with appropriate amounts of dispersant, binder, plasticizer and solvent. The resulting homogeneous slurry was then cast using a tabletop caster (DR150, Japan). The dense layer was fabricated by casting BCFN9721 slurry which was prepared in the same way as the porous support except that no graphite was added. The threelayer tape was obtained by stack the tape of dense film between two tapes of porous support layers under a pressure of 20 MPa at 80  C by a thermal isostatic pressing machine (30T, Shanxi, China). The laminate was punched to disks of 19 mm in diameter, and then fired in air at 1130  C for 6 h. Ni(NO3)2 was impregnated into one of the porous support layers by a solution infiltration process [21]. In order to introduce sufficient amount of Ni into the porous substrate, multiple infiltrations were used, followed by firing at 450  C for 1 h after each infiltration. Finally, the as-prepared catalytic membrane was sintered at 800  C for 6 h. NiO was reduced to Ni0 phase in 30% H2/Ar at a flow rate of 40 mL min1 at 800  C for 1 h before testing. The final Ni loading was about 32.7 mg cm2. A conventional dense membrane was fabricated as a reference sample according to our previously reported paper

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[19]. In brief, BCFN9721 oxide powder was pressed into green pellets (20 mm in diameter) under a uniaxial pressure at 200 MPa. Then the green samples were sintered at 1150  C in air for 6 h to obtain a dense BCFN9721 membrane.

Physical-chemical characterization and oxygen permeation measurements The crystallographic phases of the before- and after-tested BCFN9721 oxygen permeation membranes were verified using XRD analysis on a PANalytical X'pert PRO diffractometer using Cu-Ka radiation. The morphology and element composition of the membrane before and after POM reaction were investigated by scanning electron microscopy (SEM) with a ZEISS-EVO 18 Special Edition instrument alongside EDS (Bruker X-Flash Detector 5010). The permeation properties of the disk-shaped oxygen permeation membranes were investigated by gas chromatography (GC) using a homemade vertical high-temperature oxygen permeation apparatus [19,20]. All the permeation studies were performed under an ambient air/30% CH4 (with balance Ar) oxygen gradient in the temperature range of 800e875  C. The flow rates of feed gas were controlled by mass flow controllers, and measured accurately by a soap flow meter. The temperature of the membrane was monitored using a K-type thermocouple located very close to the membrane. BCFN9721 membrane pellet was loaded between a quartz tube and an alumina tube. Ceramic paste (Aremco-552 high temperature ceramic adhesive paste) was used as the sealant. No nitrogen leakage was detected during the measurements. 30% CH4 was fed into the Ni-loaded porous support layer at a flow rate of 40 mL min1. Another porous support layer without catalyst was exposed to ambient air. The conversion of CH4 (XCH4) and the selectivity of CO (SCO) are defined as follows: XCH4 ¼

SCO ¼

in out fCH  fCH 4 4

in fCH 4

in fCH 4

 100%

out fCO  100% out  fCH 4

(1)

(2)

where fiin and fiout are the mole flow rates of the fed and the permeated gas i, respectively.

Results and discussion Physical-chemical characterization of three-layer BCFN9721 membrane As an example, the XRD pattern of the pristine BCFN9721 oxygen permeation membrane with three-layer structure before loading Ni catalyst is shown in Fig. 1. Fig. 1a and b shows the XRD pattern of the porous support layer and dense layer of the pristine membrane, respectively. Both of them show that all the peaks match a cubic perovskite phase. The absence of other peaks indicates that any other additional phases if present are below the detection limit of the XRD experiment. The membrane material is therefore of high

Fig. 1 e XRD patterns of (a) the porous support of the pristine three-layer membrane, (b) the dense layer of the pristine three-layer membrane, (c) the porous support of the three-layer catalytic membrane at reaction side after stability test.

phase-purity. BCFN9721 samples were prepared with nominal A-site deficiency. Further refined characterization of the Asite deficient can be carried out by some more powerful tools, such as neutron diffraction [30], which will be conducted subsequently and published elsewhere. The XRD pattern (c) is discussed in Section 3.2. Fig. 2a and b present the SEM images of the cross-section of pristine three-layer membrane before loading Ni catalyst. The dense layer looks dense and has a mean thickness of 120 mm, as shown in Fig. 2a. Both the porous support layers exhibit highly porous microstructures. The pore size of the porous layer is about 5e10 mm and the intercommunicating pores are uniformly distributed in the whole porous support layers, as shown in Fig. 2b. The interfaces between the dense layer and porous support layers show no separation, indicating good contact and adhesion between these layers. The thickness of each porous support layer is approximately 250 mm. Estimated porosity for the porous support is about 57%, measured by the Archimedes method. In Fig. 2c and d, NiO particles are uniformly and sufficiently deposited in the porous support layer, which form a large number of Ni/ BCFN9721 interfaces. The EDS results of three-layer catalytic membrane are shown in Table 1. The compositions of the dense layer and the porous support layer at air side are close. The SEM images in Fig. 2e and f are discussed in Section 3.2.

The performance and stability of three-layer BCFN9721 catalytic membrane in POM reaction Fig. 3 shows the temperature dependence of oxygen permeation flux, CH4 conversion efficiency and CO selectivity of three-layer BCFN9721 catalytic membrane under ambient air/30% CH4 oxygen gradient. The flow rate of 30% CH4 is kept at 40 mL min1. When the temperature increases from 800 to 875  C, the oxygen permeation flux and CH4 conversion

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Fig. 2 e Typical SEM images of (a) and (b) the cross-section of the pristine three-layer membrane before loading Ni catalyst at different magnifications; (c) the cross-section of three-layer catalytic membrane at reaction side before test; (d) the surface of three-layer catalytic membrane at reaction side before test; (e) the surface of three-layer catalytic membrane at reaction side after stability test; (f) the cross-section of three-layer catalytic membrane at reaction side after stability test.

Table 1 e EDS results of the before- and the after-tested three-layer BCFN9721 catalytic membrane. Membrane Before test

After test

Position

Ba (mol%)

Co (mol%)

Fe (mol%)

Nb (mol%)

Ni (mol%)

C (mol%)

Surface of porous support at CH4 side Cross section of porous support at CH4 side Cross section of dense layer Cross section of porous support at air side Surface of porous support at CH4 side Cross section of porous support at CH4 side Cross section of porous support at air side

43.43 44.76 47.72 46.65 34.36 35.89 46.51

33.78 34.81 36.85 36.85 26.72 27.92 36.85

9.65 9.95 10.18 11.32 7.63 7.98 11.41

4.83 4.97 5.25 5.18 3.82 3.99 5.23

8.31 5.51 0 0 8.39 4.16 0

0 0 0 0 19.08 20.06 0

Fig. 3 e Effect of temperature on the performance of threelayer BCFN9721 catalytic membrane in POM reaction.

efficiency increases monotonically with temperature from 11.6 to 15.7 mL min1 cm2 and from 78.7% to 96.6%, respectively. Thus, the corresponding ratio of CH4/O2 in the membrane reactor decreased from 1.1 to 0.8, which leads to a slight decrease in the selectivity of CO, from 86.9% to 78.7%. In our prior work, 1 mm thick BCFN9721 dense membrane exhibited an oxygen permeation flux of 3.6 mL min1 cm2 and a CH4 conversion of 45.4% at 800  C under synthetic air (200 mL min1)/30% CH4 (40 mL min1) oxygen gradient [19]. The three-layer catalytic membrane achieves much higher oxygen permeation flux and CH4 conversion efficiency compared with the conventional dense membrane. For MIEC ceramic membranes, assuming the bulk diffusion of oxygen ions is the rate-limiting step, the permeation flux through the membrane can be expressed by the Wagner equation [31], as shown in Eqn. (3).

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JO2

RT ¼ 16F2 L

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ln PhO2

Z

ln PlO2

sion sel d½ln PO2  sion þ sel

(3)

where F is the Faraday constant, L is the membrane thickness, sion is the oxygen ionic conductivity, sel is the electronic conductivity, and PhO2 and PlO2 represent the oxygen partial pressures on the oxygen rich-side and the oxygen lean-side, respectively. According to Eqn. (3), if the temperature and oxygen partial pressure gradient are kept constant, the oxygen permeation flux will be directly proportional to the reciprocal of membrane thickness. Ascribe to the very thin dense-layer, the bulk diffusion process of oxygen ions can be remarkably promoted. Additionally, the fluxes of oxygen permeation are thought to be influenced by both bulk diffusion process and oxygen surface exchange reaction for BCFN9721 membrane with the thickness less than 1 mm, as reported in our prior work [19]. For the three-layer membrane, the surfaces of dense layer are modified by coating the porous support layers, which may also lead to an increase in the available surface area for the oxygen surface exchange as well as for the POM reaction. As a result, the oxygen permeation membrane with three-layer structure can achieve very high oxygen permeability and CH4 conversion efficiency. Generally, in a fixed bed reactor for POM, the loading of Nibased catalysts is relatively high. Yang et al. [29] employed BCFNO membrane reactor for POM in coke oven gas (COG) to syngas using 1 g LiNiREOx/g-Al2O3 (RE ¼ La or Ce) catalyst. 81.9% CH4 conversion can be achieved at 875  C. Subsequently, they applied 1 g NiO/MgO catalyst for the same reaction and 95% CH4 conversion was achieved at 875  C [29]. By employing a BCFN9721 catalytic membrane with three-layer structure, high CH4 conversion efficiencies can be achieved by using a low loading of Ni catalyst. The extensive distribution of Ni0 phase in the porous support layer of three-layer catalytic membrane provides abundant Ni/BCFN9721 interfaces, which may contribute to the high CH4 conversion efficiency for POM reaction. The oxygen permeation fluxes of three-layer catalytic membrane in POM reaction appear to be not as extremely high as we expect though the thickness of dense layer is only about 120 mm. One of the reasons may be the relatively low oxygen partial pressure gradient produced by ambient air and 30% CH4. Yang et al. [32] reported that the oxygen permeation under COG condition is much higher than that under CH4. The oxygen permeation flux of BCFNO membrane was 17.2 mL min1 cm2 under air/COG gradient, nearly 5 times higher than the value (3.8 mL min1 cm2) under air/CH4 gradient. It is also generally observed that the oxygen permeation flux of the membrane is related to the activity of reforming catalyst, besides the properties of the membrane and operation parameters, such as the temperature and oxygen gradient. If employing highly active reforming catalysts, such as the noble metal [26], the oxygen permeation flux should be further enhanced. The CO selectivity is not high. The low ratio of CH4/O2 should be responsible for the deep oxidation of methane which causes the low CO selectivity. Two mechanisms of POM reaction have been proposed: (1) the direct partial oxidation in which the methane is directly dissociated into adsorbed carbon and hydrogen at the Ni0

sites; the H radicals recombine to molecular H2 which desorbs from the catalyst and the remaining carbon is oxidized by the adsorbed oxygen [33]; and (2) the combustion-reforming mechanism, in which CH4 and O2 first form CO2 and water following a complete combustion pathway; and then CO2 and H2O reform with the residual CH4 to produce CO and H2 [34]. Yang et al. [32] systematically studied the mechanism of POM reaction on Ni catalyst in a BCFNO membrane reactor and found the reaction may follow the direct partial oxidation mechanism. In a three-layer catalytic membrane, the porous support layers are made of BCFN9721, as the same MIEC material as that of the dense layer. Ni catalysts are directly deposited on the wall of BCFN9721 porous support layer at reaction side. The oxygen permeated from the dense layer needs to diffuse through the whole reaction zone of POM, which is formed by the porous support layer and Ni catalyst. Thus, the dissociated carbon may be in-situ oxidized by the absorbed oxygen and active oxygen species which are produced by the oxygen exchange reaction on the surface of dense layer and the wall of BCFN9721 porous support layer. In this case, the complete oxidation of the dissociated carbon to form CO2 may carry out more easily, leading to the decrease in CO selectivity. In the meanwhile, the carbon deposition on catalysts or membrane surface could be alleviated due to the in-situ deep oxidation of the dissociated carbon. Being applied as a reactor for POM, the oxygen permeation membrane should have adequate structural and chemical stability, besides the high oxygen permeability. In order to investigate the stability of three-layer BCFN9721 catalytic membrane, an 100 h stability test in POM reaction at 875  C was conducted under ambient air/30% CH4 oxygen gradient. The flow rate of 30% CH4 was kept at 40 mL min1. Fig. 4 shows the stability of three-layer catalytic membrane. It can be seen that there is no obvious degradation during 100 h operation. The oxygen permeation flux and conversion of CH4 maintains a relatively stable value of about 16 mL min1 cm2 and 96.6%, respectively. The selectivity of CO changed slightly from 78.7% to 74.7%. A conventional BCFN9721 dense membrane was tested under the same operational condition for 100 h in POM reaction. 1 g of NiO/ MgO solid solution was placed on top of the membrane and

Fig. 4 e The oxygen permeation flux, CH4 conversion efficiency and selectivity of CO for three-layer BCFN9721 catalytic membrane in the 100 h stability test for POM.

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Fig. 5 e (a) The oxygen permeation flux, CH4 conversion efficiency, the corresponding CH4/O2 ratio; and (b) the selectivity of CO, for BCFN9721 dense membrane in the 100 h stability test for POM reaction.

applied as the catalyst for POM. As shown in Fig. 5, there is no obvious degradation in the oxygen permeation flux, conversion of CH4 and the selectivity of CO during 100 h operation. The oxygen permeation flux and CH4 conversion efficiency is about 6.6 mL min1 cm2 and 91.8%, respectively, which is much lower than the value for three-layer catalytic membrane. The selectivity of CO is relatively higher, about 92.1%, possibly ascribed to the higher ratio of CH4/O2. After 100 h stability test, the morphology of the aftertested three-layer catalytic membrane is characterized by SEM, EDS and XRD. Fig. 2e and f show the SEM images of the three-layer catalytic membrane after stability test. Compared with the image of three-layer membrane before test, as shown in Fig. 2c and d, no obvious change in morphology is observed. Fig. 6 shows the SEM images of conventional BCFN9721 dense membrane at reaction side after test. In Fig. 6a the membrane surface at the reaction side show the formation of porous regions, which are possibly composed of CoO, barium carbonate and carbon caused by carbonation and carbon deposition according to the EDS results in Table 2 and the reports in literature [20,28]. In Fig. 6b, the cross-section image of the dense membrane at reaction side shows that the corrosion layer is about 8 mm in

Table 2 e EDS results of conventional BCFN9721 dense membrane after 100 h stability test for POM. Position Cross section at air side Reaction side surface Cross section at reaction side

Ba Co Fe Nb C (mol%) (mol%) (mol%) (mol%) (mol%) 53.6

30.7

8.9

6.8

0

75.6

10.2

0.2

0.1

13.9

25.6

56.2

11.2

7.4

0

thickness after 100 h operation in the POM reaction. By the contrast analysis, Fig. 2e and f show no evident change in the morphology for both the dense layer and porous support layer of the three-layer catalytic membrane. The results suggest that the BCFN9721 catalytic membrane with threelayer structure processes better chemical stability than the conventional dense membrane and is very promising for the application in POM reaction. Further characterizations by EDS and XRD reveal the formation of a few BaCO3 and carbon deposits on three-layer membrane, as shown in Table 1 and Fig. 1c. However, the element composition of BCFN9721 membrane is almost

Fig. 6 e Typical SEM images of (a) the surface of the conventional BCFN9721 dense membrane at reaction side after 100 h test, and (b) the cross-section of conventional BCFN9721 dense membrane at reaction side after 100 h test.

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unchanged after stability test since the ratio of Ba:Co:Fe:Nb for the after-tested membrane is very close to that of the original membrane before test. The results imply that by using the three-layer structure, the element segregation of the threelayer membrane caused by CO2 corrosion and low oxygen partial pressure is not as severe as that of the conventional dense membrane, which is in agreement with our prior study [20]. BCFN9721 membrane with three-layer structure can maintain the perovskite phase very well for a consecutive operation in POM operation.

Conclusions Ba0.9Co0.7Fe0.2Nb0.1O3-d catalytic membrane with three-layer structure was investigated as the reactor in partial oxidation of methane (POM) to syngas. The three-layer catalytic membrane could achieve very high oxygen permeation flux and CH4 conversion efficiency. The abundant interfaces of Ni/ BCFN9721 could be beneficial for in-situ deep oxidation of the dissociated carbon, which resulted in a relatively low CO selectivity as well. The results indicated that by employing the three-layer structure with a dense thin film and two porous support layers, and depositing the Ni catalyst in the porous support layer, not only the high oxygen permeation flux and CH4 conversion efficiency for POM reaction could be achieved, but also the carbon deposition and element segregation caused by CO2 corrosion and low oxygen partial pressure could be alleviated. During the 100 h operation in POM reaction, the performance of Ba0.9Co0.7Fe0.2Nb0.1O3-d catalytic membrane with three-layer structure showed no obvious degradation and the pervoskite structure remained unchanged. The Ba0.9Co0.7Fe0.2Nb0.1O3-d membrane with threelayer structure possesses a higher performance and stability than the conventional dense membrane and can be very promising for the application in POM to syngas.

Acknowledgments This project was sponsored by financial supports from Major State Basic Research Development Program of China (973 Program, No. 2012CB215406), Program for New Century Excellent Talents in University (No. 80051803), and Fundamental Research Funds of Tianjin Polytechnic University (No. 030371).

references

[1] Sunarso J, Motuzas J, Liu S, Diniz da Costa JC. Bi-doping effects on the structure and oxygen permeation properties of BaSc0.1Co0.9O3-d perovskite membranes. J Memb Sci 2010;361:120e5. [2] Yang N, Kathiraser Y, Kawi S. La0.6Sr0.4Co0.8Ni0.2O3d hollow fiber membrane reactor: Integrated oxygen separation e CO2 reforming of methane reaction for hydrogen production. Int J Hydrogen Energy 2013;38:4483e91.

[3] Kida T, Yamasaki A, Watanabe K, Yamazoe N, Shimanoe K. Oxygen-permeable membranes based on partially B-site substituted BaFe1-yMyO3-d(M ¼ Cu or Ni). J Solid State Chem 2010;183:2426e31. [4] Park CY, Lee TH, Dorris SE, Balachandran U. A cobalt-free oxygen transport membrane, BaFe0.9Zr0.1O3d, and its application for producing hydrogen. Int J Hydrogen Energy 2013;38:6450e9. [5] Takahashi Y, Kawahara A, Suzuki T, Hirano M, Shin W. Perovskite membrane of La1xSrxTi1yFeyO3d for partial oxidation of methane to syngas. Solid State Ionics 2010;181:300e5. [6] Murashkina A, Pikalova E, Medvedev D, Demin A, Tsiakaras P. Hydrogen production aided by new (1-x) SrTi0.5Fe0.5O3dxCe0.8(Sm0.8Sr0.2)0.2O2d (MIEC) composite membranes. Int J Hydrogen Energy 2014;39:12472e9. [7] Song J, Li L, Tan X, Li K. BaCe0.85Tb0.05Co0.1O3-d perovskite hollow fibre membranes for hydrogen/oxygen permeation. Int J Hydrogen Energy 2013;38:7904e12. [8] Kharton VV, Naumovich EN, Nikolaev AV. Materials of hightemperature electrochemical oxygen membranes. J Memb Sci 1996;111:149e57. [9] Teraoka Y, Nobunaga T, Okamoto K, Miur N, Yamazoe N. Perovskite membrane of La1xSrxTi1yFeyO3d for partial oxidation of methane to syngas. Solid State Ionics 1991;48:207e12. [10] Morales M, Espiell F, Segarra M. Performance and stability of La0.5Sr0.5CoO3d perovskite as catalyst precursor for syngas production by partial oxidation of methane. Int J Hydrogen Energy 2014;39:6454e61. [11] Diethelm S, Van herle J, Middleton PH, Favrat D. Oxygen permeation and stability of La0.4Ca0.6Fe1-xCoxO3-d (x ¼ 0,0.25, 0.5) membranes. J Power Sources 2003;118:270e5. [12] Khine MSS, Chen L, Zhang S, Lin J, Jiang SP. Syngas production by catalytic partial oxidation of methane over (La0.7A0.3)BO3 (A ¼ Ba, Ca, Mg, Sr, and B ¼ Cr or Fe) perovskite oxides for portable fuel cell applications. Int J Hydrogen Energy 2013;38:13300e8. [13] Shao Z, Yang W, Cong Y, Dong H, Tong J, Xiong G. Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8 Fe0.2O3-d oxygen membrane. J Memb Sci 2000;172(1e2):177e88. [14] Liu J, Cheng H, Jiang B, Lu X, Ding W. Effects of tantalum content on the structure stability and oxygen permeability of BaCo0.7Fe0.3-xTaxO3d ceramic membrane. Int J Hydrogen Energy 2013;38:11090e6. [15] Dashliborun AM, Fatemi S, Najafabadi AT. Hydrogen production through partial oxidation of methane in a new reactor configuration. Int J Hydrogen Energy 2013;38:1901e9. [16] Zhang Y, Su K, Zeng F, Ding W, Lu X. A novel tubular oxygenpermeable membrane reactor for partial oxidation of CH4 in coke oven gas to syngas. Int J Hydrogen Energy 2013;38:8783e9. [17] Harada M, Domen K, Hara M, Tatsumi T. Oxygen-permeable membranes of Ba1.0Co0.7Fe0.2Nb0.1O3-d for preparation of synthesis gas from methane by partial oxidation. Chem Lett 2006;35(8):968e9. [18] Harada M, Domen K, Hara M, Tatsumi T. Ba1.0Co0.7Fe0.2Nb0.1O3-d dense ceramic as an oxygen permeable membrane for partial oxidation of methane to synthesis Gas. Chem Lett 2006;35(12):1326e7. [19] Song Shidong, Zhang Peng, Han Minfang, Singhal Subhash C. Oxygen permeation and partial oxidation of methane reaction in Ba0.9Co0.7Fe0.2Nb0.1O3-d oxygen permeation membrane. J Memb Sci 2012;415e416:654e62. [20] Zhang Peng, Song Shidong, Han Minfang. Oxygen permeation and stability of Ba0.9Co0.7Fe0.2Nb0.1O3d

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 1 0 8 9 4 e1 0 9 0 1

[21]

[22]

[23]

[24]

[25]

[26]

[27]

membrane with three-layer structure. Mater Lett 2013;104:1e4. Song Shidong, Han Minfang, Zhang Jianqiang, Fan Hui. NiCuZr0.1Ce0.9O2-d anode materials for intermediate temperature solid oxide fuel cells using hydrocarbon fuels. J Power Sources 2013;233(1):62e8. Yang Z, Yang C, Jin C, Han M, Chen F. Ba0.9Co0.7Fe0.2Nb0.1O3d as cathode material for intermediate temperature solid oxide fuel cells. Electrochem Commun 2011;13:882e5. Yang Z, Han M, Zhu P, Zhao F, Chen F. Ba1-xCo0.9-yFeyNb0.1O3d (x ¼ 0e0.15, y ¼ 0e0.9) as cathode materials for solid oxide fuel cells. Int J Hydrogen Energy 2011;36:9162e8. Liu Z, Cheng L, Han M. A-site deficient Ba1xCo0.7Fe0.2Nb0.1O3d cathode for intermediate temperature SOFC. J Power Sources 2011;196:868e71. Aschcroft AT, Cheetham AK, Foord JS, Green MLH, Grey CP, Murrell AJ, et al. Selective oxidation of methane to synthesis gas using transition metal catalysts. Nature 1990;344:319e21. Hickman DA, Schmidt LD. Steps in CH4 oxidation on Pt and Rh surface: high-temperature reactor simulations. AIChE J 1993;39:1164e77. Vermerien WJM, Blomsma E, Jacobs PA. Catalytic and thermodynamic approach of the oxyreforming reaction of methane. Catal Today 1992;13:427e36.

10901

[28] Yang Z, Ding W, Zhang Y, Lu X, Zhang Y, Shen P. Catalytic partial oxidation of coke oven gas to syngas in an oxygen permeation membrane reactor combined with NiO/MgO catalyst. Int J Hydrogen Energy 2010;35:6239e47. [29] Yang Z, Zhang Y, Ding W, Zhang Y, Shen P, Zhou Y, et al. Hydrogen production from coke oven gas over LiNi/g-Al2O3 catalyst modified by rare earth metal oxide in a membrane reactor. J Nat Gas Chem 2009;18:407e14. [30] Tao S, Irvine JTS. Phase transition in perovskite oxide La0.75Sr0.25Cr0.5Mn0.5O3d observed by in situ hightemperature neutron powder diffraction. Chem Mater 2006;18:5453e60. [31] Wagner C. Equations for transport in solid oxides and sulfides of transition metals. Prog Solid State Chem 1975;10:3e16. [32] Yang Z, Zhang Y, Ding W. Investigation on the reforming reactions of coke-oven-gas to H2 and CO in oxygenpermeable membrane reactor. J Memb Sci 2014;470:197e204. [33] Hickman DA, Schmidt LD. Production of syngas by direct catalytic oxidation of methane. Science 1993;259:343e6. [34] Zhu JJ, van Ommen JG, Lefferts L. Reaction scheme of partial oxidation of methane to synthesis gas over yttriumstabilized zirconia. J Catal 2004;225:388e97.