Elaboration of La0.8Sr0.2Fe0.7Ga0.3O3−δ/La0.8M0.2FeO3−δ (M = Ca, Sr and Ba) asymmetric membranes by tape-casting and co-firing
Introduction
For the last decades, conversion of natural gas into synthesis gas, i.e. H2 and CO mixture, has been of great interest for hydrogen or fuel production via GTL process. Catalytic membrane reactors were found to be an economically interesting alternative for syngas production, as they allow oxygen separation from air and the catalytic partial oxidation of methane in a single step. The membrane material, based on perovskite-type oxides (ABO3), may exhibit both ionic and electronic conductivity [1], [2], [3], [4]. Such material allows oxygen diffusion through the membrane thanks to the gradient of oxygen partial pressure between its opposite faces. Moreover, high oxygen semi-permeation rates, long-term chemical and thermal stabilities in the working conditions, and suitable mechanical properties are required for the membrane material in view of an industrial use [5], [6], [7].
The reactors studied in this paper basically consist of a two-layer system with a thin dense membrane supported on a thick porous support. This way, mechanical properties and oxygen semi-permeation fluxes are optimized. The porous layer ensures suitable mechanical properties and gas permeability, whereas the dense membrane ensures high oxygen semi-permeation flux. A catalytic layer could be deposited on the surface of the dense membrane in order to enhance the partial oxidation of methane to produce synthesis gas.
Promising materials for ceramic membrane reactors are La0.8Sr0.2Fe0.7Ga0.3O3−δ perovskites because of their excellent chemical stability over a wide range of oxygen partial pressure (pO2) and of a high oxygen diffusion [8]. Using this material for both the dense membrane and the porous support constitutes the easiest solution to prevent cracking due to expansion mismatch during co-firing. However, gallium containing material cost is prohibitive for an industrial use. In this respect, the challenge is to choose an adequate material for the porous layer that presents a good chemical compatibility with the La0.8Sr0.2Fe0.7Ga0.3O3−δ membrane material, similar sintering conditions and thermal expansion behaviour in order to co-sinter the porous and dense layers while restricting internal stresses. Some authors have processed La0.5Sr0.5FeO3−δ-supported Sr1−xFe(Al)O3–SrAl2O4 composite membranes by co-pressing and co-firing [9]. In this work, reactors are elaborated by tape-casting and co-firing. The tape-casting process allows a better control of final microstructure and an easy control of shrinkage in the co-firing process.
The solution presented in this paper is the elaboration of porous supports in the La0.8M0.2FeO3−δ (M = Ca, Sr, Ba) system exhibiting a good chemical and physical compatibility with the La0.8Sr0.2Fe0.7Ga0.3O3−δ dense membrane.
The nature of M cation was chosen and the sintering ability of La0.8M0.2Fe0.7O3−δ powder was adjusted, by means of the grain size distribution, in order to reach similar sintering shrinkage and thermal expansion behaviours to the dense layer. The purpose of this study is to select the best material for the elaboration of supported membranes. Then, the oxygen permeation fluxes through La0.8Sr0.2Fe0.7Ga0.3O3−δ self-supported and supported membranes were compared.
Section snippets
Powder synthesis
The submicronic powder of La0.8Sr0.2Fe0.7Ga0.3O3−δ (LSFG8273), synthesized by spray pyrolysis, is provided by Pharmacie Centrale de France (PCF, France). The as-synthesized powder is agglomerated and has a specific surface area (BET) of 15 m2/g.
La0.8M0.2FeO3−δ (M = Ca, Sr, Ba) perovskite powders were synthesized using a citrate method. La(NO3)3·6H2O, Ca(NO3)·4H2O, Sr(NO3)2, Ba(NO3)2, Fe(NO3)3·9H2O (Alfa Aesar, France) with high purity (>99.9%), citric acid (Prolabo, France) and ammonia solution
Chemical and structural analysis
Calcination of the synthesized powders, La0.8M0.2FeO3−δ (M = Ca, Sr, Ba), at 1000 °C during 12 h leads to uniform black agglomerated powders (d50 = 14 μm) with a low specific surface area (2–3 m2/g). Chemical compositions of synthesized powders are close to theoretical compositions (Table 1).
XRD patterns indicate that a perovskite-type structure is confirmed for all the compositions (Fig. 2), and second phases or impurities were not detected within the limit of the XRD technique.
Diffraction peaks of
Conclusions
A catalytic membrane reactor (CMR) consisting of a La0.8Sr0.2Fe0.7Ga0.3O3−δ (LSFG8273) dense thin film supported by a porous layer, was elaborated by tape-casting lamination, and co-firing. The thicknesses of the dense thin film and of the porous layer were respectively 80 μm and 1 mm. The porosity of the porous layer was achieved by addition of corn starch particles in the tape-casting suspension.
The material of the porous layer, without gallium for cost reasons, was chosen and adapted to have
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