Surface enhanced oxygen permeation in CaTi1−xFexO3−δ ceramic membranes

Powders of CaTi1−xFexO3−δ (x=0.15, 0.20) were obtained by ceramic route and used to prepare dense membranes covered with porous layers of the same material. The catalytic properties of some of these layers were enhanced by dispersed small particles of metallic Ag or a mixture of Pt and Pr oxide. Comparison of the oxygen permeability for membranes with different thickness indicated the existence of surface limitations to oxygen permeation. The increase of the membrane surface area with porous layers leads to an increase of the measured oxygen permeability. Maximum enhancements were attained with both sides modified (70%) and with the permeate side modified with Ag (86%). Smaller values were obtained when only the feed (13%) or the permeate (55%) sides were modified with porous layers. Results suggest that the effective electrochemically active area of the porous layer is restricted to the vicinity of the dense part of the membrane. Oxygen diffusion (DO) and surface exchange (Kex) coefficients in air were estimated to be DO=8.8×10−7 cm2 s−1 and Kex=6.7×10−6 cm s−1 at 950 °C.