OXYGEN-TRANSPORT THROUGH LA1-XSRXFEO3-DELTA MEMBRANES .2. PERMEATION IN AIR CO, CO2 GRADIENTS/

Oxygen permeation measurements were performed on dense La1-xSrxFeO3-de lta (n = 0.1-0.4) membranes in large oxygen activity gradients, i.e. a ir/CO, CO2, in the temperature range 1173-1323 K, yielding fluxes up t o 25 mmol m(-2) s(-1) at 1273 K. The oxygen semipermeability was linea rly proportional with the CO partial pressure and the strontium conten t. It was concluded that the fluxes are limited by the surface exchang e kinetics. Two simple models are proposed for the oxygen exchange rea ction in the presence of CO. In both models oxygen vacancies at the ph ase boundary play a definite role in the exchange process. XPS and SEM analysis of the perovskite/CO, CO2 interfaces indicated segregation o f strontium, being present at the surface mostly in the form of SrCO3 and/or SrO. The enhanced oxygen fluxes in air/He gradients as they wer e observed after exposure of the membrane surface to CO are probably ( indirectly) related to the level of strontium segregation. Based on SE M analysis and surface profile measurements, it is suggested that the segregation process is accompanied by an enlargement of the specific s urface area, which may promote the speed of the overall permeation pro cess under exchange-controlled conditions.