Perovskite-type Membranes Research Articles

Preparation and characterisation of BaxSr1–xCo0.8Fe0.2O3–δ perovskite-type membranes: Part I

This feature describes the preparation and characterisation of BaxSr1–xCo0.8Fe0.2O3–d perovskite-type membranes. The first instalment of this article [DOI: 10.1016/S0958-2118(09)70031-X], which was published in the February 2009 issue of Membrane Technology, provides an introduction to these materials and their use. It also looks at the effects of sintering dwell time and pressing pressure on membrane density. The second part, which appears here, discusses the effect that sintering temperature has on membrane density, briefly covers the X-ray diffraction results, and includes further analysis based on scanning electron microscopy.

Grain boundaries as barrier for oxygen transport in perovskite-type membranes

Perovskite-type membranes of (Ba 0.5Sr 0.5)(Co 0.8Fe 0.2)O 3− δ (BSCF) and (Ba 0.5Sr 0.5)(Fe 0.8Zn 0.2)O 3− δ (BSFZ) were successfully prepared via liquid-phase sintering using BN as sintering aid. The obtained membranes were examined via powder X-ray diffraction pattern (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and oxygen permeation experiments. It has emerged that the use of BN as sintering aid lowers sintering temperatures in order to obtain dense membranes with relative densities in the range of 93–96% as proven by the Archimedes method. It was further shown that the perovskite structure could be maintained after sintering with BN. Additionally, BN was completely removed from the sintered membranes. Investigation of the microstructure revealed that the average grain size of the membranes was influenced by the amount of BN added prior the sintering process. It was found that large amounts of BN effectively lower the average grain size. Oxygen permeation experiments have shown that the lower the average grain size the lower the oxygen permeation performance, particularly in the case of BSCF. Transmission electron microscopy revealed that no evidence for an amorphous layer or any other interfacial phase in the grain boundary is present.

Novel hollow fibre membrane reactor for the partial oxidation of methane

In recent years perovskite-type membranes (ABO 3) have been studied intensively as oxygen permeable materials. These perovskite ceramics very often have a disc or tube geometry. In this paper the first data of a membrane reactor for POM with hollow fibres as membrane are reported. The hollow fibres were spun from BCFZ (Ba(Co,Fe,Zr)O 3− δ ) powder. Diluted methane as well as methane with steam were investigated as reaction gases. The spent catalyst was investigated by high-resolution transmission electron microscopy (HR-TEM) after the reaction. Membrane failures were evaluated by field-emission scanning electron microscopy (FE-SEM).

In situ high temperature X-ray diffraction studies of mixed ionic and electronic conducting perovskite-type membranes

Phase structure and stability of three typical mixed ionic and electronic conducting perovskite-type membranes, SrCo0.8Fe0.2O3−δ (SCF), Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and BaCo0.4Fe0.4Zr0.2O3−δ (BCFZ) were studied by in situ high temperature X-ray diffraction at temperatures from 303 to 1273 K and under different atmospheres (air, 2% O2 in Ar and pure Ar) at 1173 K. By analyzing their lattice parameters the thermal expansion coefficients (TECs) of BSCF, SCF and BCZF are obtained to be 11.5×10−6 K−1, 17.9×10−6 K−1 and 10.3×10−6 K−1, respectively. A relationship between phase stability and TEC was proposed: the higher is the TEC, the lower is the operation stability of the perovskite materials.

Planar and tubular perovskite-type membrane reactors for the partial oxidation of methane to syngas

Dense planar and tubular oxygen separation membranes of La 0.6Ca0.4Fe0.75Co0.25O3-d were investigated as reactors for the partial oxidation (POX) of methane to syngas. Disk-shaped membranes were prepared by compaction and tubes by extrusion. Their permeation properties were measured in an air/argon pO2 gradient as a function of temperature. At 900°C, the oxygen flux through a 1.26 mm thick membrane was 0.075 micromol/cm2s and through a 0.25 mm thick tube, 0.24 micromol/cm2s. For the POX measurements, a catalyst was added to the membrane and methane was introduced on the argon side. This resulted in a gradual increase of the oxygen flux with increasing concentration of methane, reaching 2micromol/ cm2s at 900°C with pure methane. The catalytic properties of the POX reactors were characterised as a function of the methane content. For the planar reactor, the CO selectivity reached 99% and the CH4 conversion 75% at 918°C with pure methane. For the tubular reactor, the CO selectivity and CH4 conversion were respectively 83 and 99% for the same conditions. In all cases the H2/CO ratio was about 2. After 1’400 hours of operation in a tubular POX-reactor, the membrane was examined revealing phase demixing and local decomposition.

Preparation and performance of a perovskite-type tubular membrane for the partial oxidation of methane

Thin-walled (∼200 μm) tubular membranes intended for the catalytic partial oxidation of natural gas have been manufactured from La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) powders using viscous plastic processing technology. The perovskite-type powders were synthesised using various conventional and novel techniques. The tubes have been characterised using a custom-built gas analysis rig with on-line mass spectrometry.

Investigation of ideal zirconium-doped perovskite-type ceramic membrane materials for oxygen separation

Zirconium-doped perovskite-type membrane materials of BaCo 0.4Fe 0.6− x Zr x O 3− δ ( x=0–0.4) with mixed oxygen ion and electron conductivity were synthesized through a method of combining citric and EDTA acid complexes. The results of X-ray diffraction (XRD), oxygen temperature-programmed desorption (O 2-TPD) and hydrogen temperature-programmed reduction (H 2-TPR) showed that the incorporation of proper amount of zirconium into BaCo 0.4Fe 0.6O 3− δ could stabilize the ideal and cubic structure of perovskite. Studies on the oxygen permeability of the as-synthesized membrane disks under air/He gradient indicated that the content of zirconium in these materials had great effects on oxygen permeation flux, activation energy for oxygen permeation and operation stability. The high oxygen permeation flux of 0.90 ml cm −2 min −1 at 950 °C, the single activation energy for oxygen permeation in the range of 600–950 °C and the long-term operation stability at a relatively lower operational temperature of 800 °C under air/He gradient were achieved for the BaCo 0.4Fe 0.4Zr 0.2O 3− δ material. Meanwhile, the effect of carbon dioxide on structural stability and oxygen permeability of this material was also studied in detail, which revealed that the reversible stability could be attained for it.

Preparation of an asymmetric perovskite-type membrane and its oxygen permeability

A crack-free asymmetric membrane of perovskite-type oxide (La 0.6Sr 0.4Co 0.2Fe 0.8O 3− δ ) was successfully prepared by coating a slurry containing La 0.6Sr 0.4Co 0.2Fe 0.8O 3− δ powders directly on the surface of a green support of the same composition, followed by sintering. It was found that crack-free asymmetric membranes could be obtained by controlling the powder concentration of the slurry in the range of 15–25 wt.%. After sintering, the crystal phase of the top layer of asymmetric membranes prepared was the same as that of powders, which were of the cubic perovskite phase. The nitrogen permeability and SEM photograph of the support showed that the support was porous, and the gas-tight test and SEM demonstrated that the top layer of asymmetric membrane was dense and crack-free. The asymmetric membrane prepared, whose dense top layer was 200 μm thick, exhibited about three to four times as high an oxygen flux as a 2 mm dense sintered disc.

Experimental research on partial oxidation of methane to syngas in tubular perovskite-type membrane reactor

Experimental research on partial oxidation of methane to syngas in tubular perovskite-type membrane reactor

Perovskite-type B-site Bi-doped ceramic membranes for oxygen separation

Novel mixed conducting oxides, B-site Bi-doped perovskites were exploited and synthesized. Cubic perovskite structures were formed for BaBi0.2CoyFe0.8-yO3-δ (y⩽S0.4) and BaBixCo0.2Fe0.8-xO3-δ(x=0.1-0.5). The materials exhibited considerable high oxygen permeability at high temperature. The oxygen permeation flux of BaBi0.2Co0.35Fe0.45O3-δ membrane reached about 0.77×10-6 mol/cm2 · s under an air/helium oxygen partial pressure gradient at 900°C, which was much higher than that of other bismuth-contained mixed conducting membranes. The permeation fluxes of the materials increased with the increase of cobalt content, but no apparent simple relationship was found with the bismuth content. The materials also demonstrated excellent reversibility of oxygen adsorption and desorption. Stable time-related oxygen permeation fluxes were found for BaBi0.22O0.35Co0.35Fe0.45O33-δ and BaBi0.3Co0.2Fe0.5O3-δ membranes at 875°C.

Experimental and modeling study on tubular dense membranes for oxygen permeation

Tubular La0.6Sr0.4Co0.2Fe0.8O3–δ perovskite-type membranes were prepared by isostatic pressing. A mathematical model was developed to simulate the performance of the tubular perovskite-type dense membranes for oxygen permeation. The experimental oxygen permeation fluxes of tubular La0.6Sr0.4Co0.2Fe0.8O3–δ perovskite-type membrane increased with decreasing downstream oxygen partial pressure and increasing helium flow rate, which coincided with the results of the oxygen permeation modeling study. Parametric study indicated that air should be supplied sufficiently during the oxygen permeation operation. Modeling oxygen permeation fluxes as a function of the tubular membrane length and tubular membrane thickness are discussed in detail. The oxygen flux slightly decreased after long-term operation over 110 h. EDS and XRD analysis indicated that SrSO4, CoSO4, SrO, Co2O3, and La2O3 were formed on the surfaces of the tubular membrane due to the interaction with trace SO2 in the air and the helium, and segregation of surface elements. The oxygen permeation of tubular La0.6Sr0.4Co0.2Fe0.8O3–δ membrane was stable after SO2 in the air and the helium has been removed.

Comparison of Oxygen Permeability and Stability of Perovskite Type La0.2A0.8Co0.2Fe0.8O3-δ (A = Sr, Ba, Ca) Membranes

The oxygen permeation and stability of La0.2A0.8Co0.2Fe0.8O3-δ (A = Sr, Ba, Ca) perovskite-type membranes were studied at high temperatures and low oxygen partial pressures. The oxygen vacancy diffusivity and concentration gradient calculated from the unsteady-state and steady-state oxygen data for the three membranes decrease in the order of Sr > Ba > Ca. The activation energies for oxygen permeation increase in the order of Sr < Ba < Ca. The oxygen permeation data of the three membranes can be explained by the average bond energy, free volume, and critical radius of the materials. After exposure to air at 1173 K for 10 h, part of La0.2Sr0.8Co0.2Fe0.8O3-δ transforms to La2O3, SrO, CoO, and Fe and the crystallite size of the perovskite phase decreases. These changes were not observed for La0.2Ba0.8Co0.2Fe0.8O3-δ under the same conditions, indicating that La0.2Ba0.8Co0.2Fe0.8O3-δ membrane is much more stable than La0.2Sr0.8Co0.2Fe0.8O3-δ at high temperatures and low oxygen partial pressures.