Study of the mixed conducting SrFeCo0.5Oy ceramic membrane material by in-situ neutron powder diffraction

Dense ceramic tubes of the multi-phase mixed ionic/electronic conductor SrF eCo0.5Oy have been synthesized by solid-state reaction, and the stability o f its component phases studied using in-situ neutron diffraction over the o xygen partial pressure (pO(2)) range 10(-0.68) to 10(-15.0) atm at 900 degr ees C, This being the likely conditions to which membranes are exposed in o perational methane conversion reactors. This material is stable between air and log pO(2) = -2.3, with Sr-2(Fe,Co)(3)O-y (236) being the majority phas e. When the oxygen partial pressure is lowered, 236 starts decomposing into perovskite and rocksalt phases. The small fraction of remaining 236 rapidl y undergoes decomposition at log pO(2) = -12.2, leaving only SrFeO2.5 and C oO. Rietveld analysis indicates that the oxygen content of the perovskite p hase does not drop below O = 2.5 when log pO(2) = -15.0, while there is an indication that CoO is reduced to cobalt metal. This decomposition at low p artial pressures may be reversed when the gas is switched back to air, i.e. , SrFeO2.5 + CoO re-form Sr-2(Fe,Co)(3)O-y, to an extent that is seemingly dependent on sample and thermal history. (C) 2000 Elsevier Science Ltd. All rights reserved.