- Email: [email protected]
Effects of synthesis methods on oxygen permeability of BaCe0.15Fe0.85O3−δ ceramic membranes
Journal of Membrane Science 283 (2006) 158–163
Effects of synthesis methods on oxygen permeability of BaCe0.15Fe0.85O3−δ ceramic membranes Xuefeng Zhu a,b , You Cong a , Weishen Yang a,∗ a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China b Graduate School of the Chinese Academy of Sciences, China
Received 6 April 2006; received in revised form 13 June 2006; accepted 14 June 2006 Available online 18 June 2006
Abstract Dense BaCe0.15 Fe0.85 O3−δ (BCF1585) ceramic membranes synthesized by the solid-state reaction (SSR) method and EDTA-citric acid (EC) process were investigated by X-ray powder diffraction, total conductivity, oxygen permeation, etc. XRD results revealed the perovskite structure of the powders prepared by the EC process was easier to be developed than that of prepared by SSR method. Membranes derived from EC had higher density, pure phase structure and fewer defects comparing to those derived from SSR method. However, membranes derived from SSR method had higher oxygen permeability. Thickness experiments revealed that the oxygen permeation fluxes of the membranes synthesized by both methods are all jointly controlled by surface exchange and bulk diffusion in the range of 0.7–2.0 mm. The long-term oxygen permeation operation revealed that the membranes derived from both methods exhibit good oxygen permeation stability. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceramic membrane; Synthesis methods; Oxygen separation; Mixed conducting; Perovskite oxides
1. Introduction Oxygen permeable ceramic membranes with mixed ionic and electronic conductivity are receiving considerable attention due to their potential applications in oxygen separation from air [1–3], partial oxidation of natural gas in membrane reactors [4–9] and as SOFC cathode materials [10]. For industrial application of mixed conducting membranes, the membrane materials should have high oxygen permeability, high oxygen permeation stability and excellent structural stability under reducing environments. Oxygen permeable membranes with high oxygen permeability are usually in perovskite structure and contain Co2+/3+ in the materials. However, Co2+/3+ are easy to be reduced in reducing atmosphere and result in the failure of membrane structure in operation. Recently, we have developed a series of cobalt-free oxygen permeable membranes (BaCex Fe1−x O3−δ ) [11] with
∗
Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://www.anggroup.dicp.ac.cn.
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.020
perovskite structure. The membranes possess not only considerable oxygen permeability, but also high structural stability under reducing environments at elevated temperatures. The materials can sustain their perovskite structure under 10% H2 –Ar mixed gas at 900 ◦ C only with 5% cerium in the B-site. At the same time, the membranes have oxygen permeation fluxes of over 0.5 ml/cm2 min under air/He gradient at 900 ◦ C for 1.0 mm thickness membranes if the amount of cerium doped in B-site is no more than 15%. There are many methods to prepare perovskite powders, such as solid-state reaction (SSR) method, EDTA-citric acid (EC) process, glycine-nitrate combustion process (GNP) and chemical co-precipitation method, etc. Powders synthesized by different processes have different particle sizes, sintering activities and chemical compositions as expected. These factors will lead to different microstructures of as obtained ceramic membranes, thus result in different oxygen permeation fluxes. Kharton et al. [12] reported that SrCo0.6 Fe0.25 Cu0.15 O3−δ synthesized by SSR method had larger particle size and higher oxygen permeability than that of synthesized by cellulose precursor process. Tan et al. [13] reported that oxygen permeability of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ synthesized by SSR method was
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
159
Table 1 Sintering conditions and properties of the BCF1585 ceramic membranes Synthesis method
EC method SSR method a b
Sintering conditions T (◦ C)
Time (h)
Heating/cooling rate (◦ C/min)
1320 1370
3 3
1–2 1–2
Relatively density (%)
Cell parameters (nm)
δa
Fen+b
93 88
0.4096 0.4083
0.254 0.265
3.402 3.376
Determined by iodometry method. Supposing the oxidation state of cerium in the perovskite oxides is 4+ under the synthesis conditions.
higher than that of synthesized by EC process. Both of them attributed this effect to the higher oxygen ions transport in the bulk than along the grain boundary. Furthermore, it was reported that mixed conducting oxides synthesized by different methods had different phase compositions, for example, the phase composition of SrFeCo0.5 Oy is largely determined by the synthesis method [14,15]. In this paper, we will continue our previous investigation and focus on the effects of synthesis methods on ceramic microstructure, phase composition, total conductivity and oxygen permeation of BaCe0.15 Fe0.85 O3−δ membranes. 2. Experimental BaCe0.15 Fe0.85 O3−δ (BCF1585) powders were synthesized by solid-state reaction (SSR) method and EDTA-citric acid (EC) process. For the SSR method, stoichiometric amounts of BaCO3 (99.9%), CeO2 (99.99%) and Fe2 O3 (99.9%) were mixed and ball milled for 5 h. The mixture was then calcined at 900, 1000, 1100, 1200, 1300 ◦ C for 10 h, respectively, with interval ball milling. After calcined at 1300 ◦ C, the powder was ball milled for 50 h, and the powder less than 400 mesh was used to prepare disk-shaped membrane. For the EC method, stoichiometric amounts of the nitrate solutions were mixed with EDTA and citric acid with a ratio to total metal ions of 1.0 and 1.5, respectively. Ammonia was used to adjust the pH value around 6. After water evaporated on a hot plate, the resulting gel was calcined at about 600 ◦ C to remove organic compounds, and then calcined at high temperatures for 5 h in air. The powders were pressed into disks with a stainless steel module under 200–300 MPa. Green disks were sintered at 1320 ◦ C for EC method and 1370 ◦ C for SSR method, respectively, and maintained at the temperatures for 3 h, as shown in Table 1. To obtain gas-tight membranes, the sintering temperature (1370 ◦ C) of membranes derived from SSR method was very close to the melting point (1380 ◦ C). If membranes derived from EC method also sintering at 1370 ◦ C, the disks would become bowl-like. Actually, the EC derived membranes can obtain relative density of >90% just sintered at 1150 ◦ C for 5 h [11]. Silver rings were used as sealant and sealed at 960 ◦ C. The oxygen permeation experiments were performed in a vertical high temperature gas permeation cell. Effective membrane areas of the membrane discs were controlled around 0.8 cm2 . Permeation tests were performed between 750 and 940 ◦ C. Dried synthesized air was used as the feed at a flow rate of 100 ml/min. High purity helium flowed on the other side of the membrane as sweep gas at a flow rate of 30 ml/min. The effluents were ana-
lyzed by a gas chromatograph (GC, Agilent 6890) equipped with a 3 m 13 X column. Oxygen concentration was calculated by an external standard method. Oxygen permeation fluxes through membranes were calculated based on helium flow rate and oxygen concentration in the effluents. Due to slight leakages of the seal, some nitrogen was detected in the effluents by gas chromatograph, and the leakage amounts of oxygen were subtracted when calculating oxygen permeation fluxes. The leakages were usually less than 3%. Total conductivity was measured using bar-shaped samples (25 mm × 4 mm × 2 mm) in air with a four-probe dc method. A constant direct current of 10 mA was supplied to the outside two probes by potentiostat (Princeton Applied Research Galvanostat Model 263), and the voltages between the inside two probes were measured by multimeter (Keithley 2000). Phase structure of the membrane materials was determined by X-ray diffraction (XRD, Rigaku D/Max-RB, Cu K␣) in a 2θ range of 20–80◦ with a step width of 0.02◦ . Crystal parameters were calculated by Rietveld method. Surface morphologies of membranes were observed on a Philips XL-30 scanning electron microscopy (SEM). 3. Results and discussion 3.1. Structures and morphologies of the membranes Fig. 1 shows the X-ray diffraction patterns of the BaCe0.15 Fe0.85 O3−δ (BCF1585) powders synthesized by EC process (Fig. 1A) and SSR method (Fig. 1B) calcined at different temperatures for 5 and 10 h, respectively. For powders synthesized by EC process, the carbonate was completely decomposed after the powders calcined at 800 ◦ C, but CeO2 still could be detected, as shown in Fig. 1A. With the increase of the calcined temperatures, diffraction peaks of CeO2 become weak gradually and perovskite structure was fully developed until the temperature is up to 900 ◦ C. So we calcined all the series of oxides at 950 ◦ C for 5 h to insure the completely building of perovskite structure for powders synthesized by EC method. For powders synthesized by SSR method, though the main phase appeared at 1000 ◦ C, there is still a little BaCeO3 phase being detected even at the temperature as high as 1300 ◦ C. Actually, BaCeO3 phase could still be detected after last sintering of membranes at 1370 ◦ C for 3 h. With the increase of the calcined temperature, the diffraction peaks of perovskite shift to lower diffraction angles gradually while the diffraction peaks of BaCeO3 become weaker, that means the slight amounts cerium coming into B-site can lead to a remarkable shift of the diffraction peaks, as shown in Fig. 1B.
160
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
Fig. 2A). For powder synthesized by SSR method, the resultant membranes possess larger grain size of 20–50 m, as shown in Fig. 2C. Due to the higher synthesis temperature of the SSR method, the powder was prepared with partially sintered. After the powder ball milled and meshed, the disks pressed from the resultant powder can be sintered only at temperatures higher than 1370 ◦ C, which are close to the melting point (1380 ◦ C) of the materials. Though it was found that the membranes are dense from the top view, as shown in Fig. 2A and C, there are many gas pores occurring in the cross-section of the both methods derived membranes, as shown in Fig. 2B and D. The pores in the EC membranes are smaller than those of in the SSR membranes, and it can be seen that the density of EC samples is higher than that of SSR samples from the SEM pictures, which is in accordance with the relative density shown in Table 1. Although there are many gas pores occurring in the cross-sections, the gas permeation tests revealed that there were no open pores in both membranes. 3.2. Conductivity and oxygen permeability of the membranes
Fig. 1. XRD patterns of BaCe0.15 Fe0.85 O3−δ oxide powders synthesized by EC method (A), and SSR method (B) at different temperatures. () Perovskite oxides; () BaCO3 ; (䊉) CeO2 ; (×) BaCeO3 .
The cell parameter of EC process derived sample is a little larger than that of SSR method derived sample, as shown in Table 1, which is due to a little difference in cerium amount in the B-sites as shown in Fig. 1B, and in oxidation state of iron as shown in Table 1. Based on the results mentioned above, the formation of perovskite structure for SSR synthesis process followed two steps, i.e., first, the mixed oxides react each other and produce a perovskite phase with little amount cerium in the lattice at lower temperatures, then cerium ions merge into the perovskite lattice at higher temperatures. It means that the incorporating of Ce4+ into the BaFeO3 lattice is difficult, which may be due to the larger ionic radii difference between Ce4+ (101 pm) and Fe3+ (78.5 pm) or Fe4+ (72.5 pm). Islam [16] reported that the solution energies are low if the ionic radii of substituting elements are similar to that of the substituted ions in perovskite oxides. Therefore, it seems that the high solution energy is the main reason of the occurrence of minor BaCeO3 phase after the powder calcined at 1300 ◦ C for 10 h. Morphologies of sintered disks were observed on SEM. Fig. 2 shows the surface morphologies of fresh BCF1585 membranes synthesized by EC and SSR methods. As showed in Fig. 1A and C, ceramic grains with clear grain boundaries were closely packed together. The grains are in the range of 5–20 m for the membranes synthesized by EC process (as shown in
It was reported that different ceramics microstructures will lead to different electric conductivities [17]. So effects of powder synthesis methods on total conductivities of as-obtained materials were investigated, and the results are shown in Fig. 3. The conductivity of EC derived sample is about one order higher than that of SSR derived sample. The lower total conductivity may be related to the lower density and large oxygen defects in the lattice, as shown in Table 1. Additionally, the second phase BaCeO3 in SSR derived materials with low conductivity blocks the transport of carriers from one grain to another. All these factors will reduce the conductivity of the materials. Fig. 4 shows the dependence of oxygen permeation fluxes of membranes synthesized by different processes on the operation temperatures. As shown in Fig. 4, the oxygen permeation fluxes of the membrane derived from SSR method are higher than that of sample derived from EC method in the investigated temperature range. However, the total conductivity of SSR sample is lower than that of EC sample. For perovskite-typed mixed conducting materials, electronic conductivity usually is far higher than oxygen ionic conductivity, and the lower total conductivity is not necessary corresponding to lower oxygen ionic conductivity. So the lower total conductivity of sample derived from SSR method does not necessary leads to lower oxygen permeability. The difference in oxygen permeation fluxes of membranes derived from SSR and EC methods is determined by many factors, such as grain size, phase composition, defects in the materials, density, etc. The grain sizes are different for the membranes derived from the two methods, as shown in Fig. 2. It was suggested that the larger grain sizes are usually benefit to the oxygen permeability of mixed conductor materials [18,19]. The phase compositions of the membranes derived from SSR and EC methods are also different, which can be found in the XRD patterns shown in Fig. 1, i.e., the powder prepared by SSR method has less cerium ions in the perovskite lattice. Defects in the bulk and along the boundary are different. For instance, there has an
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
161
Fig. 2. SEM pictures of as-prepared membrane synthesized by EC (A and B) and SSR (C and D) method. (A and C) top view; (B and D) cross-section.
Fig. 3. The total conductivity of the materials synthesized by EC and SSR method. Atmosphere: static air.
Fig. 5. Arrhenius plots of oxygen permeation fluxes of membranes prepared by SSR method and EC process. The oxygen partial pressure of the sweeping side was controlled at 0.01 atm by adjusting the He flow rate. Air flow rate, 100 ml/min; thickness, 1.0 mm.
Fig. 4. Dependence of oxygen permeation fluxes of the membranes synthesized by different processes on temperature. Thickness, 1.0 mm; air flow rate, 100 ml/min; He flow rate, 30 ml/min.
oxygen nonstoichiometric difference in the as prepared samples, as shown in Table 1. Furthermore, the surface oxygen exchange rates are different for the membrane derived from SSR and EC process, which will be discussed in the following section. All the factors are related to the oxygen permeability of materials. So it is difficult to conclude that which factor predominate the oxygen permeability. Fig. 5 shows the Arrhenius plots of oxygen permeation fluxes of membranes prepared by SSR method and EC process. The oxygen partial pressure of the sweeping side was controlled at 0.01 atm by adjusting the He flow rate. Under the constant oxygen partial pressure gradient, the SSR and EC method derived membranes reached oxygen permeation fluxes of 0.92 and 0.71 ml/cm2 min at 940 ◦ C, respectively. As shown in Fig. 5, there occurs little difference in the apparent activation
162
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
Fig. 6. The dependence of the oxygen permeation fluxes on the thicknesses of membranes at 900 ◦ C. Air flow rate, 100 ml/min; He flow rate, 30 ml/min.
energies (Ea ), and lower Ea value is related to higher oxygen permeation flux. 3.3. Thickness dependence on oxygen permeation of the membranes To further understand the differences in the oxygen permeability of materials prepared by SSR and EC methods, the dependence of the oxygen permeation fluxes on the thicknesses of the membranes at 900 ◦ C was investigated. As shown in Fig. 6, for membranes derived from EC method, the oxygen permeation flux linearly increases with the reciprocal of the thickness if membranes are thicker than 1.4 mm, however, the oxygen permeation flux is independent of the thickness if membranes are thinner than 0.78 mm. For a certain dense oxygen permeable membrane, the permeation rate is controlled by two factors: one is the oxygen ions diffusion within the bulk and the other is the interfacial oxygen exchange on the both sides of membrane. If the oxygen permeation rate is controlled by bulk diffusion, according to the Wagner’s equation [20], the oxygen permeation fluxes can be expressed: ln p O2 σ e σi RT JO2 = − d ln pO2 (1) 2 16F L ln pO σe + σi 2
where, σ e and σ i are electronic and oxygen ionic conductivity, respectively; pO2 and pO2 are high and low oxygen partial pressure of the each side of the membrane; L the thickness of the membrane; the other parameters have the usual meaning. Therefore, the oxygen permeation flux should increase along the dash line, as shown in Fig. 6. If the oxygen permeation rate is only controlled by surface exchange, the oxygen permeation flux is independent of the thickness. Fig. 6 reveals the oxygen permeation determining steps are changed from the bulk diffusion limitation to the surface exchange limitation if we reduce the thickness. The characteristic membrane thickness, Lc , is the membrane thickness at which point the transition from predominant control by bulk diffusion to that by surface exchange [21]. Reducing the membrane thickness thinner than Lc , the oxygen permeation flux can only marginally be increased. For
Fig. 7. The 100 h operation of the SSR and EC membranes at 850 ◦ C. Thickness, 1.0 mm; air flow rate, 100 ml/min; He flow rate, 30 ml/min.
membranes derived from EC method, the Lc should be around 0.78 mm. However, for membranes derived from SSR method, the dependence of oxygen permeation flux on thickness is complex. If oxygen permeation process follows Wagner’s equation, the data line should pass coordinate origin; if the oxygen permeation process is controlled by surface exchange, the data line should be independence of thickness. Bearing the two cases in mind, we suggest that the oxygen permeation process is limited by both oxygen surface exchange and oxygen ion bulk diffusion in the investigated thickness range (0.7–1.9 mm) for the SSR derived membranes. 3.4. Long-term oxygen permeation operation of the membranes For an oxygen permeable membrane, besides high permeability, it should also possess long-term oxygen permeation stability. Tsai et al. [5] reported that it took about 100 h to reach an oxygen permeation steady state at 990 ◦ C for La0.4 Ca0.6 Fe0.8 Co0.2 O3−δ , and the oxygen flux decreased from 0.5 ml/cm2 min to the steady state value of 0.3 ml/cm2 min. Fig. 7 shows the 100 h operation of the SSR and EC membranes at 850 ◦ C. As shown in Fig. 7, there was no visible degradation of oxygen permeation flux at the investigated temperature for the both methods derived membranes. So membranes prepared by the two methods all have good oxygen permeation stability. 4. Conclusions BaCe0.15 Fe0.85 O3−δ powders were synthesized by two methods, i.e., solid-state reaction (SSR) method and EDTA-citric acid (EC) process. Oxygen permeation fluxes of membranes derived from SSR method are higher than those of membranes derived from EC method in the investigated temperature range. However, it is difficult to prepare pure phase and high density membranes if the powder is synthesized from SSR method. Thickness dependence of oxygen permeation fluxes revealed that the oxygen permeation process is controlled by surface exchange and bulk diffusion when the thickness is in the range of 0.78–1.4 mm for
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
the EC derived membranes, and in the range of 0.7–1.9 mm for the SSR derived membranes, respectively. Acknowledgements This work was supported financially by the Ministry of Science and Technology, China (Grant no. 2005CB221404), and National Science Foundation of China (50332040). References [1] W.S. Yang, H.H. Wang, X.F. Zhu, L.W. Lin, Development and application of oxygen permeable membrane in selective oxidation of light alkanes, Top. Catal. 155 (2005) 35. [2] P.N. Dyer, R.E. Richards, S.L. Russek, D.M. Taylor, Ion transport membrane technology for oxygen separation and syngas production, Solid State Ionics 134 (2002) 21. [3] Z.P. Shao, W.S. Yang, Y. Cong, H. Dong, J.H. Tong, G.X. Xiong, Investigation of the permeation behavior and stability of a Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ oxygen membrane, J. Membr. Sci. 172 (2000) 177. [4] H.J.M. Bouwmeester, Dense ceramic membranes for methane conversion, Catal. Today 82 (2003) 141. [5] C.Y. Tsai, A.G. Dixon, W.R. Moser, Y.H. Ma, Dense perovskite membrane reactors for the partial oxidation of methane to syngas, AIChE J. 43 (1997) 2741. [6] U. Balachandran, J.T. Dusek, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, A.C. Bose, Dense ceramic membranes for partial oxidation of methane to syngas, Appl. Catal. A 133 (1995) 19. [7] Z.P. Shao, H. Dong, G.X. Xiong, Y. Cong, W.S. Yang, Performance of a mixed-conducting ceramic membrane reactor with high oxygen permeability for methane conversion, J. Membr. Sci. 183 (2001) 181. [8] J.H. Tong, W.S. Yang, R. Cai, B.C. Zhu, L.W. Lin, Novel and ideal zirconium-based dense membrane reactors for partial oxidation of methane to syngas, Catal. Lett. 78 (1–4) (2002) 129. [9] H.H. Wang, Y. Cong, W.S. Yang, Partial oxidation of ethane to syngas in an oxygen-permeable membrane reactor, J. Membr. Sci. 209 (2002) 143.
163
[10] O. Yamamoto, Low temperature electrolytes and catalysts, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cell-Fundamentals, Technology and Applications, Volume 4: Fuel Cell Technology and Applications, John Wiley & Sons Ltd., 2003, pp. 1002–1014, ISBN: 0-47149926-9. [11] X.F. Zhu, H.H. Wang, W.S. Yang, Novel cobalt-free oxygen permeable membrane, Chem. Commun. (2004) 1130. [12] V.V. Kharton, V.N. Tikhonovich, L. Shuangbao, E.N. Naumovich, A.V. Kovalevsky, A.P. Viskup, I.A. Bashmakov, A.A. Yaremchenko, Ceramic microstructure and oxygen permeability of SrCo(Fe, M)O3−δ (M = Cu or Cr) perovskite membranes, J. Electrochem. Soc. 145 (1998) 1363. [13] L. Tan, X. Gu, L. Yang, W. Jin, L. Xiong, N. Xu, Influence of powder synthesis methods on microstructure and oxygen permeation performance of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ perovskite-type membranes, J. Membr. Sci. 212 (2003) 157. [14] Y. Xia, T. Armstrong, F. Prado, A. Manthiram, Sol–gel synthesis, phase relationship, and oxygen permeation properties of Sr4 Fe6−x Cox O13+δ (0 ≤ x ≤ 3), Solid State Ionics 130 (2000) 81. [15] Z.Q. Deng, W. Liu, D.K. Peng, C.S. Chen, W.S. Yang, Combustion synthesis, annealing, and oxygen permeation properties of SrFeCo0.5 Oy membranes, Mater. Res. Bull. 39 (2004) 963. [16] M.S. Islam, Ionic transport in ABO3 perovskite oxides: a computer modeling tour, J. Mater. Chem. 10 (2000) 1027. [17] V.V. Kharton, F.M.B. Marques, Mixed ionic-electronic conductors: effects of ceramic microstructure on transport properties, Curr. Opin. Solid State Mater. Sci. 6 (2002) 261. [18] K. Zhang, Y.L. Yang, D. Ponnusamy, A.J. Jacobson, K. Salama, Effect of microstructure on oxygen permeation in SrCo0.8 Fe0.2 O3−δ , J. Mater. Sci. 34 (1999) 1367. [19] H.H. Wang, C. Tablet, A. Feldhoff, J. Caro, Investigation of phase structure, sintering, and permeability of perovskote-type Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ membranes, J. Membr. Sci. 262 (2005) 20. [20] C. Wagner, Equation for transport in solid oxides and sulfides of transition metal, Prog. Solid State Chem. 10 (1975) 3. [21] H.J.M. Bouwmeester, A.J. Burggraaf, Dense ceramic membranes for oxygen separation, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, FL, 1997 (Chapter 14).
Effects of synthesis methods on oxygen permeability of BaCe0.15Fe0.85O3−δ ceramic membranes Xuefeng Zhu a,b , You Cong a , Weishen Yang a,∗ a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China b Graduate School of the Chinese Academy of Sciences, China
Received 6 April 2006; received in revised form 13 June 2006; accepted 14 June 2006 Available online 18 June 2006
Abstract Dense BaCe0.15 Fe0.85 O3−δ (BCF1585) ceramic membranes synthesized by the solid-state reaction (SSR) method and EDTA-citric acid (EC) process were investigated by X-ray powder diffraction, total conductivity, oxygen permeation, etc. XRD results revealed the perovskite structure of the powders prepared by the EC process was easier to be developed than that of prepared by SSR method. Membranes derived from EC had higher density, pure phase structure and fewer defects comparing to those derived from SSR method. However, membranes derived from SSR method had higher oxygen permeability. Thickness experiments revealed that the oxygen permeation fluxes of the membranes synthesized by both methods are all jointly controlled by surface exchange and bulk diffusion in the range of 0.7–2.0 mm. The long-term oxygen permeation operation revealed that the membranes derived from both methods exhibit good oxygen permeation stability. © 2006 Elsevier B.V. All rights reserved. Keywords: Ceramic membrane; Synthesis methods; Oxygen separation; Mixed conducting; Perovskite oxides
1. Introduction Oxygen permeable ceramic membranes with mixed ionic and electronic conductivity are receiving considerable attention due to their potential applications in oxygen separation from air [1–3], partial oxidation of natural gas in membrane reactors [4–9] and as SOFC cathode materials [10]. For industrial application of mixed conducting membranes, the membrane materials should have high oxygen permeability, high oxygen permeation stability and excellent structural stability under reducing environments. Oxygen permeable membranes with high oxygen permeability are usually in perovskite structure and contain Co2+/3+ in the materials. However, Co2+/3+ are easy to be reduced in reducing atmosphere and result in the failure of membrane structure in operation. Recently, we have developed a series of cobalt-free oxygen permeable membranes (BaCex Fe1−x O3−δ ) [11] with
∗
Corresponding author. Tel.: +86 411 84379073; fax: +86 411 84694447. E-mail address: [email protected] (W. Yang). URL: http://www.anggroup.dicp.ac.cn.
0376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2006.06.020
perovskite structure. The membranes possess not only considerable oxygen permeability, but also high structural stability under reducing environments at elevated temperatures. The materials can sustain their perovskite structure under 10% H2 –Ar mixed gas at 900 ◦ C only with 5% cerium in the B-site. At the same time, the membranes have oxygen permeation fluxes of over 0.5 ml/cm2 min under air/He gradient at 900 ◦ C for 1.0 mm thickness membranes if the amount of cerium doped in B-site is no more than 15%. There are many methods to prepare perovskite powders, such as solid-state reaction (SSR) method, EDTA-citric acid (EC) process, glycine-nitrate combustion process (GNP) and chemical co-precipitation method, etc. Powders synthesized by different processes have different particle sizes, sintering activities and chemical compositions as expected. These factors will lead to different microstructures of as obtained ceramic membranes, thus result in different oxygen permeation fluxes. Kharton et al. [12] reported that SrCo0.6 Fe0.25 Cu0.15 O3−δ synthesized by SSR method had larger particle size and higher oxygen permeability than that of synthesized by cellulose precursor process. Tan et al. [13] reported that oxygen permeability of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ synthesized by SSR method was
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
159
Table 1 Sintering conditions and properties of the BCF1585 ceramic membranes Synthesis method
EC method SSR method a b
Sintering conditions T (◦ C)
Time (h)
Heating/cooling rate (◦ C/min)
1320 1370
3 3
1–2 1–2
Relatively density (%)
Cell parameters (nm)
δa
Fen+b
93 88
0.4096 0.4083
0.254 0.265
3.402 3.376
Determined by iodometry method. Supposing the oxidation state of cerium in the perovskite oxides is 4+ under the synthesis conditions.
higher than that of synthesized by EC process. Both of them attributed this effect to the higher oxygen ions transport in the bulk than along the grain boundary. Furthermore, it was reported that mixed conducting oxides synthesized by different methods had different phase compositions, for example, the phase composition of SrFeCo0.5 Oy is largely determined by the synthesis method [14,15]. In this paper, we will continue our previous investigation and focus on the effects of synthesis methods on ceramic microstructure, phase composition, total conductivity and oxygen permeation of BaCe0.15 Fe0.85 O3−δ membranes. 2. Experimental BaCe0.15 Fe0.85 O3−δ (BCF1585) powders were synthesized by solid-state reaction (SSR) method and EDTA-citric acid (EC) process. For the SSR method, stoichiometric amounts of BaCO3 (99.9%), CeO2 (99.99%) and Fe2 O3 (99.9%) were mixed and ball milled for 5 h. The mixture was then calcined at 900, 1000, 1100, 1200, 1300 ◦ C for 10 h, respectively, with interval ball milling. After calcined at 1300 ◦ C, the powder was ball milled for 50 h, and the powder less than 400 mesh was used to prepare disk-shaped membrane. For the EC method, stoichiometric amounts of the nitrate solutions were mixed with EDTA and citric acid with a ratio to total metal ions of 1.0 and 1.5, respectively. Ammonia was used to adjust the pH value around 6. After water evaporated on a hot plate, the resulting gel was calcined at about 600 ◦ C to remove organic compounds, and then calcined at high temperatures for 5 h in air. The powders were pressed into disks with a stainless steel module under 200–300 MPa. Green disks were sintered at 1320 ◦ C for EC method and 1370 ◦ C for SSR method, respectively, and maintained at the temperatures for 3 h, as shown in Table 1. To obtain gas-tight membranes, the sintering temperature (1370 ◦ C) of membranes derived from SSR method was very close to the melting point (1380 ◦ C). If membranes derived from EC method also sintering at 1370 ◦ C, the disks would become bowl-like. Actually, the EC derived membranes can obtain relative density of >90% just sintered at 1150 ◦ C for 5 h [11]. Silver rings were used as sealant and sealed at 960 ◦ C. The oxygen permeation experiments were performed in a vertical high temperature gas permeation cell. Effective membrane areas of the membrane discs were controlled around 0.8 cm2 . Permeation tests were performed between 750 and 940 ◦ C. Dried synthesized air was used as the feed at a flow rate of 100 ml/min. High purity helium flowed on the other side of the membrane as sweep gas at a flow rate of 30 ml/min. The effluents were ana-
lyzed by a gas chromatograph (GC, Agilent 6890) equipped with a 3 m 13 X column. Oxygen concentration was calculated by an external standard method. Oxygen permeation fluxes through membranes were calculated based on helium flow rate and oxygen concentration in the effluents. Due to slight leakages of the seal, some nitrogen was detected in the effluents by gas chromatograph, and the leakage amounts of oxygen were subtracted when calculating oxygen permeation fluxes. The leakages were usually less than 3%. Total conductivity was measured using bar-shaped samples (25 mm × 4 mm × 2 mm) in air with a four-probe dc method. A constant direct current of 10 mA was supplied to the outside two probes by potentiostat (Princeton Applied Research Galvanostat Model 263), and the voltages between the inside two probes were measured by multimeter (Keithley 2000). Phase structure of the membrane materials was determined by X-ray diffraction (XRD, Rigaku D/Max-RB, Cu K␣) in a 2θ range of 20–80◦ with a step width of 0.02◦ . Crystal parameters were calculated by Rietveld method. Surface morphologies of membranes were observed on a Philips XL-30 scanning electron microscopy (SEM). 3. Results and discussion 3.1. Structures and morphologies of the membranes Fig. 1 shows the X-ray diffraction patterns of the BaCe0.15 Fe0.85 O3−δ (BCF1585) powders synthesized by EC process (Fig. 1A) and SSR method (Fig. 1B) calcined at different temperatures for 5 and 10 h, respectively. For powders synthesized by EC process, the carbonate was completely decomposed after the powders calcined at 800 ◦ C, but CeO2 still could be detected, as shown in Fig. 1A. With the increase of the calcined temperatures, diffraction peaks of CeO2 become weak gradually and perovskite structure was fully developed until the temperature is up to 900 ◦ C. So we calcined all the series of oxides at 950 ◦ C for 5 h to insure the completely building of perovskite structure for powders synthesized by EC method. For powders synthesized by SSR method, though the main phase appeared at 1000 ◦ C, there is still a little BaCeO3 phase being detected even at the temperature as high as 1300 ◦ C. Actually, BaCeO3 phase could still be detected after last sintering of membranes at 1370 ◦ C for 3 h. With the increase of the calcined temperature, the diffraction peaks of perovskite shift to lower diffraction angles gradually while the diffraction peaks of BaCeO3 become weaker, that means the slight amounts cerium coming into B-site can lead to a remarkable shift of the diffraction peaks, as shown in Fig. 1B.
160
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
Fig. 2A). For powder synthesized by SSR method, the resultant membranes possess larger grain size of 20–50 m, as shown in Fig. 2C. Due to the higher synthesis temperature of the SSR method, the powder was prepared with partially sintered. After the powder ball milled and meshed, the disks pressed from the resultant powder can be sintered only at temperatures higher than 1370 ◦ C, which are close to the melting point (1380 ◦ C) of the materials. Though it was found that the membranes are dense from the top view, as shown in Fig. 2A and C, there are many gas pores occurring in the cross-section of the both methods derived membranes, as shown in Fig. 2B and D. The pores in the EC membranes are smaller than those of in the SSR membranes, and it can be seen that the density of EC samples is higher than that of SSR samples from the SEM pictures, which is in accordance with the relative density shown in Table 1. Although there are many gas pores occurring in the cross-sections, the gas permeation tests revealed that there were no open pores in both membranes. 3.2. Conductivity and oxygen permeability of the membranes
Fig. 1. XRD patterns of BaCe0.15 Fe0.85 O3−δ oxide powders synthesized by EC method (A), and SSR method (B) at different temperatures. () Perovskite oxides; () BaCO3 ; (䊉) CeO2 ; (×) BaCeO3 .
The cell parameter of EC process derived sample is a little larger than that of SSR method derived sample, as shown in Table 1, which is due to a little difference in cerium amount in the B-sites as shown in Fig. 1B, and in oxidation state of iron as shown in Table 1. Based on the results mentioned above, the formation of perovskite structure for SSR synthesis process followed two steps, i.e., first, the mixed oxides react each other and produce a perovskite phase with little amount cerium in the lattice at lower temperatures, then cerium ions merge into the perovskite lattice at higher temperatures. It means that the incorporating of Ce4+ into the BaFeO3 lattice is difficult, which may be due to the larger ionic radii difference between Ce4+ (101 pm) and Fe3+ (78.5 pm) or Fe4+ (72.5 pm). Islam [16] reported that the solution energies are low if the ionic radii of substituting elements are similar to that of the substituted ions in perovskite oxides. Therefore, it seems that the high solution energy is the main reason of the occurrence of minor BaCeO3 phase after the powder calcined at 1300 ◦ C for 10 h. Morphologies of sintered disks were observed on SEM. Fig. 2 shows the surface morphologies of fresh BCF1585 membranes synthesized by EC and SSR methods. As showed in Fig. 1A and C, ceramic grains with clear grain boundaries were closely packed together. The grains are in the range of 5–20 m for the membranes synthesized by EC process (as shown in
It was reported that different ceramics microstructures will lead to different electric conductivities [17]. So effects of powder synthesis methods on total conductivities of as-obtained materials were investigated, and the results are shown in Fig. 3. The conductivity of EC derived sample is about one order higher than that of SSR derived sample. The lower total conductivity may be related to the lower density and large oxygen defects in the lattice, as shown in Table 1. Additionally, the second phase BaCeO3 in SSR derived materials with low conductivity blocks the transport of carriers from one grain to another. All these factors will reduce the conductivity of the materials. Fig. 4 shows the dependence of oxygen permeation fluxes of membranes synthesized by different processes on the operation temperatures. As shown in Fig. 4, the oxygen permeation fluxes of the membrane derived from SSR method are higher than that of sample derived from EC method in the investigated temperature range. However, the total conductivity of SSR sample is lower than that of EC sample. For perovskite-typed mixed conducting materials, electronic conductivity usually is far higher than oxygen ionic conductivity, and the lower total conductivity is not necessary corresponding to lower oxygen ionic conductivity. So the lower total conductivity of sample derived from SSR method does not necessary leads to lower oxygen permeability. The difference in oxygen permeation fluxes of membranes derived from SSR and EC methods is determined by many factors, such as grain size, phase composition, defects in the materials, density, etc. The grain sizes are different for the membranes derived from the two methods, as shown in Fig. 2. It was suggested that the larger grain sizes are usually benefit to the oxygen permeability of mixed conductor materials [18,19]. The phase compositions of the membranes derived from SSR and EC methods are also different, which can be found in the XRD patterns shown in Fig. 1, i.e., the powder prepared by SSR method has less cerium ions in the perovskite lattice. Defects in the bulk and along the boundary are different. For instance, there has an
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
161
Fig. 2. SEM pictures of as-prepared membrane synthesized by EC (A and B) and SSR (C and D) method. (A and C) top view; (B and D) cross-section.
Fig. 3. The total conductivity of the materials synthesized by EC and SSR method. Atmosphere: static air.
Fig. 5. Arrhenius plots of oxygen permeation fluxes of membranes prepared by SSR method and EC process. The oxygen partial pressure of the sweeping side was controlled at 0.01 atm by adjusting the He flow rate. Air flow rate, 100 ml/min; thickness, 1.0 mm.
Fig. 4. Dependence of oxygen permeation fluxes of the membranes synthesized by different processes on temperature. Thickness, 1.0 mm; air flow rate, 100 ml/min; He flow rate, 30 ml/min.
oxygen nonstoichiometric difference in the as prepared samples, as shown in Table 1. Furthermore, the surface oxygen exchange rates are different for the membrane derived from SSR and EC process, which will be discussed in the following section. All the factors are related to the oxygen permeability of materials. So it is difficult to conclude that which factor predominate the oxygen permeability. Fig. 5 shows the Arrhenius plots of oxygen permeation fluxes of membranes prepared by SSR method and EC process. The oxygen partial pressure of the sweeping side was controlled at 0.01 atm by adjusting the He flow rate. Under the constant oxygen partial pressure gradient, the SSR and EC method derived membranes reached oxygen permeation fluxes of 0.92 and 0.71 ml/cm2 min at 940 ◦ C, respectively. As shown in Fig. 5, there occurs little difference in the apparent activation
162
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
Fig. 6. The dependence of the oxygen permeation fluxes on the thicknesses of membranes at 900 ◦ C. Air flow rate, 100 ml/min; He flow rate, 30 ml/min.
energies (Ea ), and lower Ea value is related to higher oxygen permeation flux. 3.3. Thickness dependence on oxygen permeation of the membranes To further understand the differences in the oxygen permeability of materials prepared by SSR and EC methods, the dependence of the oxygen permeation fluxes on the thicknesses of the membranes at 900 ◦ C was investigated. As shown in Fig. 6, for membranes derived from EC method, the oxygen permeation flux linearly increases with the reciprocal of the thickness if membranes are thicker than 1.4 mm, however, the oxygen permeation flux is independent of the thickness if membranes are thinner than 0.78 mm. For a certain dense oxygen permeable membrane, the permeation rate is controlled by two factors: one is the oxygen ions diffusion within the bulk and the other is the interfacial oxygen exchange on the both sides of membrane. If the oxygen permeation rate is controlled by bulk diffusion, according to the Wagner’s equation [20], the oxygen permeation fluxes can be expressed: ln p O2 σ e σi RT JO2 = − d ln pO2 (1) 2 16F L ln pO σe + σi 2
where, σ e and σ i are electronic and oxygen ionic conductivity, respectively; pO2 and pO2 are high and low oxygen partial pressure of the each side of the membrane; L the thickness of the membrane; the other parameters have the usual meaning. Therefore, the oxygen permeation flux should increase along the dash line, as shown in Fig. 6. If the oxygen permeation rate is only controlled by surface exchange, the oxygen permeation flux is independent of the thickness. Fig. 6 reveals the oxygen permeation determining steps are changed from the bulk diffusion limitation to the surface exchange limitation if we reduce the thickness. The characteristic membrane thickness, Lc , is the membrane thickness at which point the transition from predominant control by bulk diffusion to that by surface exchange [21]. Reducing the membrane thickness thinner than Lc , the oxygen permeation flux can only marginally be increased. For
Fig. 7. The 100 h operation of the SSR and EC membranes at 850 ◦ C. Thickness, 1.0 mm; air flow rate, 100 ml/min; He flow rate, 30 ml/min.
membranes derived from EC method, the Lc should be around 0.78 mm. However, for membranes derived from SSR method, the dependence of oxygen permeation flux on thickness is complex. If oxygen permeation process follows Wagner’s equation, the data line should pass coordinate origin; if the oxygen permeation process is controlled by surface exchange, the data line should be independence of thickness. Bearing the two cases in mind, we suggest that the oxygen permeation process is limited by both oxygen surface exchange and oxygen ion bulk diffusion in the investigated thickness range (0.7–1.9 mm) for the SSR derived membranes. 3.4. Long-term oxygen permeation operation of the membranes For an oxygen permeable membrane, besides high permeability, it should also possess long-term oxygen permeation stability. Tsai et al. [5] reported that it took about 100 h to reach an oxygen permeation steady state at 990 ◦ C for La0.4 Ca0.6 Fe0.8 Co0.2 O3−δ , and the oxygen flux decreased from 0.5 ml/cm2 min to the steady state value of 0.3 ml/cm2 min. Fig. 7 shows the 100 h operation of the SSR and EC membranes at 850 ◦ C. As shown in Fig. 7, there was no visible degradation of oxygen permeation flux at the investigated temperature for the both methods derived membranes. So membranes prepared by the two methods all have good oxygen permeation stability. 4. Conclusions BaCe0.15 Fe0.85 O3−δ powders were synthesized by two methods, i.e., solid-state reaction (SSR) method and EDTA-citric acid (EC) process. Oxygen permeation fluxes of membranes derived from SSR method are higher than those of membranes derived from EC method in the investigated temperature range. However, it is difficult to prepare pure phase and high density membranes if the powder is synthesized from SSR method. Thickness dependence of oxygen permeation fluxes revealed that the oxygen permeation process is controlled by surface exchange and bulk diffusion when the thickness is in the range of 0.78–1.4 mm for
X. Zhu et al. / Journal of Membrane Science 283 (2006) 158–163
the EC derived membranes, and in the range of 0.7–1.9 mm for the SSR derived membranes, respectively. Acknowledgements This work was supported financially by the Ministry of Science and Technology, China (Grant no. 2005CB221404), and National Science Foundation of China (50332040). References [1] W.S. Yang, H.H. Wang, X.F. Zhu, L.W. Lin, Development and application of oxygen permeable membrane in selective oxidation of light alkanes, Top. Catal. 155 (2005) 35. [2] P.N. Dyer, R.E. Richards, S.L. Russek, D.M. Taylor, Ion transport membrane technology for oxygen separation and syngas production, Solid State Ionics 134 (2002) 21. [3] Z.P. Shao, W.S. Yang, Y. Cong, H. Dong, J.H. Tong, G.X. Xiong, Investigation of the permeation behavior and stability of a Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ oxygen membrane, J. Membr. Sci. 172 (2000) 177. [4] H.J.M. Bouwmeester, Dense ceramic membranes for methane conversion, Catal. Today 82 (2003) 141. [5] C.Y. Tsai, A.G. Dixon, W.R. Moser, Y.H. Ma, Dense perovskite membrane reactors for the partial oxidation of methane to syngas, AIChE J. 43 (1997) 2741. [6] U. Balachandran, J.T. Dusek, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, A.C. Bose, Dense ceramic membranes for partial oxidation of methane to syngas, Appl. Catal. A 133 (1995) 19. [7] Z.P. Shao, H. Dong, G.X. Xiong, Y. Cong, W.S. Yang, Performance of a mixed-conducting ceramic membrane reactor with high oxygen permeability for methane conversion, J. Membr. Sci. 183 (2001) 181. [8] J.H. Tong, W.S. Yang, R. Cai, B.C. Zhu, L.W. Lin, Novel and ideal zirconium-based dense membrane reactors for partial oxidation of methane to syngas, Catal. Lett. 78 (1–4) (2002) 129. [9] H.H. Wang, Y. Cong, W.S. Yang, Partial oxidation of ethane to syngas in an oxygen-permeable membrane reactor, J. Membr. Sci. 209 (2002) 143.
163
[10] O. Yamamoto, Low temperature electrolytes and catalysts, in: W. Vielstich, H.A. Gasteiger, A. Lamm (Eds.), Handbook of Fuel Cell-Fundamentals, Technology and Applications, Volume 4: Fuel Cell Technology and Applications, John Wiley & Sons Ltd., 2003, pp. 1002–1014, ISBN: 0-47149926-9. [11] X.F. Zhu, H.H. Wang, W.S. Yang, Novel cobalt-free oxygen permeable membrane, Chem. Commun. (2004) 1130. [12] V.V. Kharton, V.N. Tikhonovich, L. Shuangbao, E.N. Naumovich, A.V. Kovalevsky, A.P. Viskup, I.A. Bashmakov, A.A. Yaremchenko, Ceramic microstructure and oxygen permeability of SrCo(Fe, M)O3−δ (M = Cu or Cr) perovskite membranes, J. Electrochem. Soc. 145 (1998) 1363. [13] L. Tan, X. Gu, L. Yang, W. Jin, L. Xiong, N. Xu, Influence of powder synthesis methods on microstructure and oxygen permeation performance of Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ perovskite-type membranes, J. Membr. Sci. 212 (2003) 157. [14] Y. Xia, T. Armstrong, F. Prado, A. Manthiram, Sol–gel synthesis, phase relationship, and oxygen permeation properties of Sr4 Fe6−x Cox O13+δ (0 ≤ x ≤ 3), Solid State Ionics 130 (2000) 81. [15] Z.Q. Deng, W. Liu, D.K. Peng, C.S. Chen, W.S. Yang, Combustion synthesis, annealing, and oxygen permeation properties of SrFeCo0.5 Oy membranes, Mater. Res. Bull. 39 (2004) 963. [16] M.S. Islam, Ionic transport in ABO3 perovskite oxides: a computer modeling tour, J. Mater. Chem. 10 (2000) 1027. [17] V.V. Kharton, F.M.B. Marques, Mixed ionic-electronic conductors: effects of ceramic microstructure on transport properties, Curr. Opin. Solid State Mater. Sci. 6 (2002) 261. [18] K. Zhang, Y.L. Yang, D. Ponnusamy, A.J. Jacobson, K. Salama, Effect of microstructure on oxygen permeation in SrCo0.8 Fe0.2 O3−δ , J. Mater. Sci. 34 (1999) 1367. [19] H.H. Wang, C. Tablet, A. Feldhoff, J. Caro, Investigation of phase structure, sintering, and permeability of perovskote-type Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ membranes, J. Membr. Sci. 262 (2005) 20. [20] C. Wagner, Equation for transport in solid oxides and sulfides of transition metal, Prog. Solid State Chem. 10 (1975) 3. [21] H.J.M. Bouwmeester, A.J. Burggraaf, Dense ceramic membranes for oxygen separation, in: P.J. Gellings, H.J.M. Bouwmeester (Eds.), CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, FL, 1997 (Chapter 14).