Fe/Mn-based Perovskite-Type Oxides with Excellent Oxygen Permeability and Reduction Tolerance
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Fe/Mn-based Perovskite-Type Oxides with Excellent Oxygen Permeability and Reduction Tolerance Yasutake Teraoka, Hironobu Shimokawa1, Hajime Kusaba and Kazunari Sasaki Department of Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 1 Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan ABSTRACT A family of Co-free, Fe/Mn-based perovskite-type oxides, (Sr, A’)(Fe, Mn)O3-δ (A’=La, Ba, Ca), was synthesized, and their oxygen permeability and phase stability in reducing atmosphere were investigated. The substitution of Mn at B site caused the decrease in oxygen permeability. As for the effect of A-site substitution, prominent promotion was observed by the substitution of Ba for 30% of Sr, and Ba0.3Sr0.7FeO3-δ was found to be one of most excellent oxygen permeable materials with the permeation flux of 3.0 cm3(STP) cm-2 min-1 at 900 ºC. Reduction tolerance was evaluated by TG measurements in a 5%H2/N2 stream up to 1000 ºC. After the TG measurements, crystal structures of La-Sr-Co-Fe-O and Sr-Fe-(Mn)-O perovskites were decomposed or transformed into low oxygen permeable phases, but the perovskite-type structure of Ba-Sr-Fe-(Mn)-O survived. The Fe/Mn-based perovskites with high oxygen permeability and exceeding reduction tolerance could be used as stable membrane materials for membrane reactors catalyzing NO-CH4 reaction and the partial oxidation of CH4 into synthesis gas. INTRODUCTION Mixed ionic-electronic conductive (MIEC) perovskites have been attracting a great deal of attention as dense membrane materials of a pressure-driven oxygen separator [1,2], and for this purpose, La1-x SrxCo1-yFeyO3-δ (LSCFs) perovskites, which were first reported by Teraoka et al. in the late 1980s [3,4], are one of most excellent membrane materials. The other possible application of the MIEC membrane system is membrane reactors for catalyzing the partial oxidation of methane to synthesis gas (POM) [5,6], oxidative coupling of methane [7], NOx removal [8] and so on. For the membrane reactor application, higher oxygen permeability is required to increase the rate of the target chemical reaction. Another requirement is the reduction tolerance because one side of the membrane is usually exposed to a reducing gas such as CH4. Although LSCFs with the Sr/Co-rich compositions are inherently excellent oxygen-permeable materials, the lack of the stability against thermal and chemical reduction of LSCFs makes it difficult to construct stable membrane reactors and even oxygen separation [7,9-11]. Therefore, development of oxygen-permeable and reduction-tolerant materials without easily reductive Co ion [6,12-15] is important in the field of MIEC science and technology. As a candidate of Co-free, oxygen-permeable material, we focused on the Fe/Mn-based perovskites, (Sr, A’)(Fe, Mn)O3-δ (A’=La, Ba, Ca) in this study. It has turned out that they are excellent oxygen permeable materials with excee
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