Journal of Membrane Science 172 (2000) 177–188
Investigation of the permeation behavior and stability of a Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ oxygen membrane Zongping Shao, Weishen Yang∗ , You Cong, Hui Dong, Jianhua Tong, Guoxing Xiong1 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 20 August 1999; received in revised form 24 January 2000; accepted 24 January 2000
Abstract SrCo0.8 Fe0.2 O3−δ (SCFO) and Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ (BSCFO) oxides were successfully prepared, using a combined citrate–EDTA complexing method. The results of O2 -TPD and XRD showed that the introduction of barium into SCFO could effectively suppress the oxidation of Co3+ and Fe3+ to higher valence states of Co4+ and Fe4+ in the lattice, and stabilize the perovskite structure under lower oxygen partial pressures. Oxygen permeation experiment showed that BSCFO membrane also had higher oxygen permeation flux than that of SCFO under air/He oxygen partial pressure gradient. At 950◦ C, the permeation flux through 1.80 mm BSCFO membrane exposed to flowing predried air (PO0 2 =0.21 atm) and helium (PO002 =0.037 atm) is ca. 1.4 ml/cm2 min and the activation energy for oxygen transportation is 40.9 kJ/mol within the temperature range of 775–950◦ C. The permeation flux of BSCFO was less sensitive to minor amounts of CO2 and water vapor presented in the air than that of SCFO. Long-term oxygen permeation study of more than 1000 h at 850◦ C indicated that the BSCFO membrane could operate stably as an oxygen generator at that temperature. A very slow exponential decay in the measured oxygen permeation flux occurred at temperatures lower than 825◦ C, which was caused by a phase transition. The phase transition was found to be reversible at higher temperatures, but a long time was needed for the equilibration. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Inorganic membrane; Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ ; Perovskite; Oxygen separation; Phase stability
1. Introduction Dense ceramic membranes exhibiting mixed oxygen-ionic and electronic conductivity (MIECM) provide a new way for oxygen production. At high temperatures, with the presence of an oxygen partial pressure gradient across the membrane, oxygen can permeate through MIECM with infinite permselectiv∗ Corresponding author. Fax: +86-411-469-4447. E-mail addresses:
[email protected] (W. Yang),
[email protected] (G. Xiong) 1 Co-corresponding author.
ity [1,2]. Comparison with industrial scale methods of pressure swing adsorption and cryogenic separation, ceramic membranes have the advantage of continuous oxygen production. MIECM also find promising application in the partial oxidation of light hydrocarbons [3–6]. For example, ceramic oxygen membrane reactors can combine the separation of oxygen from air and the conversion of natural gas into syngas (POM) in one operation, thereby eliminating a costly oxygen separation plant that is needed in the POM units [7,8]. Mixed conducting perovskite-type oxides represent one type of the most promising materials satisfying such purposes [2,9]. When the A-site of perovskite is
0376-7388/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 0 ) 0 0 3 3 7 - 9
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doped with lower valence state metal ions (such as partial substitution of La3+ by Sr2+ in LaCoO3 ), oxygen vacancies as well as a change in the valence state of the B ions in the lattice will occur in order to maintain the electrical neutrality, which is the reason for oxygen ionic and electronic conductivity [10,11]. Some of the perovskites have considerably high oxygen ionic conductivity with overwhelming electronic conductivity at elevated temperatures, and usually high permeation fluxes are found for such materials [12,13]. For practical applications, MIECM must possess sufficiently high oxygen permeability and sustainable structural stability to withstand harsh conditions (synthetic gas atmosphere, carbon dioxide, H2 O vapor etc., for example). Teraoka et al. were the first to report very high oxygen permeation flux through a SrCo0.8 Fe0.2 O3−δ (SCFO) membrane [14]. The high permeation flux is attributed to the high concentration of oxygen vacancy in the lattice due to the totally substitution of A3+ metal ion by Sr2+ in the A-site of perovskite. Attracted by the unusual high permeability of this material, the properties of SCFO, such as the surface exchange kinetics, the conductivity, the nonstoichiometry, the phase structure and stability, and the oxygen permeation behavior etc., were later examined or re-examined [10,15–22] by several other investigators. Unfortunately, it was found that the material has very limited chemical and structural stability in reduced environments [15–17]. The perovskite phase is thermodynamically stable only at higher oxygen pressures (>0.1 atm) and at higher temperatures. A perovskite-brownmillerite two-phase region exists at lower oxygen partial pressures (Cu (910◦ C)>Fe (790◦ C). Kharton measured fluxes of SrCo0.8 B0.2 O3−δ (Cr, Mn, Ni, Cu, Ti) materials using an electrochemical method
[18,19]. It was found that the highest permeation flux was still with the composition of SrCo0.8 Fe0.2 O3−δ . The substitution of strontium in SCFO with higher valence state metal ions such as lanthanum usually leads to an increase in phase stability, but the oxygen permeability is lowered [14,23] due to the decrease of oxygen vacancy concentration. Recently, we have developed perovskite-type Ba1−x Srx (Co,Fe,Zr,Ti)y O3−δ membranes for oxygen separation [24]. By a proper substitution of strontium in SCFO by the barium ion with a larger radius, the phase stability of the material is greatly improved, while the oxygen permeation flux is not decreased. In the present work, we focus on the investigation of the stability and oxygen permeation properties of the Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ (BSCFO) membrane, one of the optimized composition of Bax Sr1−x Co0.8 Fe0.2 O3−δ . The objectives of this work were to synthesize dense BSCFO by a newly developed wet chemical method and to study the oxygen permeation properties and phase stability of such materials. Comparison was made with the SCFO membrane. Different preparation methods may lead to different membrane microstructures which might influence oxygen permeation behavior, so in this paper, SCFO was also synthesized by the same method for the preparation of BSCFO.
2. Experimental The material powders were synthesized using a new developed method named as combined citrate and EDTA complexing method. The preparation procedure of BSCFO for example, is shown in Fig. 1. The necessary amount of Ba(NO3 )2 was first dissolved in EDTA–NH3 ·H2 O solution under heating and stirring, then the calculated amounts of Sr(NO3 )2 , Co(NO3 )2 and Fe(NO3 )3 were added to the solution. After stirring for certain time, proper amount of citric acid was introduced, the mole ratio of EDTA acid:citric acid:total metal ions was controlled to be around 1:1.5:1. Precipitation might occur after citric acid addition, NH3 ·H2 O was then added to adjust the pH value to around 6, and the solution became transparent immediately. EDTA–NH3 and citrate formed a buffering solution, so the pH value of the system successfully sustained around 6 during the whole
Z. Shao et al. / Journal of Membrane Science 172 (2000) 177–188
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Fig. 1. Flow chart for the preparation of BSCFO powder.
process. With the evaporation of water, a dark purple gel was obtained. The gel was then heated at 120–150◦ C for several hours to make a primary powder, which was calcined at 950◦ C for 5 h to obtain the powder with final composition. The prepared powders were pressed into disks in a stainless steel mold (17 mm in diameter) under a hydraulic pressure of 3–4×104 kg based on an area of 2.3 cm2 . These green discs were sintered in a SiC muffle oven under a temperature between 1100 and 1150◦ C for 2–5 h, with a heating and cooling rate of 1 and 2◦ C/min, respectively. The densities of the sintered membranes were determined by the Archimedes method using ethanol. Only those membranes that had relative densities higher than 90% were used for permeation studies. The crystal structures of the as-synthesized powders or sintered membranes were characterized with an X-ray diffractometer (XRD, Rigaku D-Max/RB). Structural stability under conditions more reducing or oxidizing than the original in-air synthesis was studied by XRD. Samples (about 5–7 g) were loaded in an alumina boat and equilibrated at 850◦ C and different oxygen activities (1, 0.21, 0.01 and 10−5 atm) for 24 h
in a tube furnace. The samples were quenched to room temperature by pulling the crucible to the cool zone of the furnace (
