Oxygen permeation and stability of La0.4Ca0.6Fe1 - xCoxO3-delta (x=0, 0.25, 0.5) membranes
Three perovskite-type compounds of composition La0.4Ca0.6Fe1-xCoxO3-d (x=o, o.25 and 0.5) were investigated for use as oxygen separation membranes for the partial oxidation (POX) of methane to syngas. Special attention was given to the question of their stability in real operating conditions. A permeation set-up was specially designed to measure oxygen fluxes through these materials when placed in a strong pO2 gradient. It also facilitated testing the long-term stability of the specimen. Permeation measurements performed in an air/argon gradient between 800 and 1000 °C showed that the highest fluxes were obtained with the highest content of cobalt (La0.4Ca0.6Fe0.5Co0.5 O3-d =La0.4Ca0.6Fe0.75Co0.25 O3-d >La0.4Ca0.6FeO3-d ). In addition, comparison between the fluxes of samples of different thickness gave clear evidence of surface limitations in the oxygen transport. The long-term stability test showed opposite trends: only the two lowest Co containing compounds (x = 0 and 0.25) sustained and air/(Ar + H2) gradient over more than 600 h. The other (x = 0.5) broke shortly after the introduction of H2. In the presence of H2, the oxygen flux was increased by a factor 10 compared to Ar and reached 0.83 mmol/cm2s for La0.4Ca0.6Fe0.75Co0.25O3-d at 900°C. Post-operation SEM examination of the cross-section and both surfaces revealed that the surface exposed to H2 had started to decompose resulting in the formation of a thin porous layer but the bulk of the material remained unchanged.