Steady-State Isotopic Transient Oxygen Exchange

Steady-State Isotopic Transient Oxygen Exchange 

Case Study Prediction of the Oxygen Permeation Flux through a Thin Ceramic Membrane from Powder Measurements 

Mixed ionic and electronic conductors (MIECs) are an important class of materials that include oxides of different structures such as perovskites, brownmillerites, and fluorites. They can potentially be employed in high-temperature applications for which simultaneous reaction and separation is advantageous [29,30]. These 

803 g (LSCF) was processed in the same manner as for membrane preparation in order to generate an identical microstructure. The powder was dry pressed into a sintered pellet and was crushed and sieved for TGA and oxygen exchange studies. 

The degree of oxygen nonstoichiometry (S) of the perovskite was measmed as a function of temperature by TGA. The sample was pretreated in situ in an oxy-gen/helium mixture for 2h at 1173K to remove all traces of water and carbonates. It was then cooled down to room temperature. The structure of the perovskite was verified by XRD after TGA. The initial oxygen nonstoichiometry (d) of LSCF was measiu-ed by iodometric titration, as described by Manthiram et al. [10]. The TGA profiles showed a constant fraction of vacancies till approximately 700 K. The vacancy fractions increased linearly from 823 K imder Pq = 1 kPa and from 1050 K imder P02 =21 kPa. The data fell within the typical range of oxygen nonstoichiometry generally found for LSCF [34,35]. A value of 39 3 kj/mol for the enthalpy of adsorption was measured. 

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Steady-State Isotopic Transient Oxygen Exchange... 

The following sections provide a kinetic analysis of the transient responses based on an atomic state for the chemisorbed oxygen, 0(s). We show that this approach allows us to account for the qualitative features of the results described above, the temperature dependence of the rate of isotope scrambling under steady-state conditions, and results ftom temperature programmed desorption (TPD) experiments performed at very low pressure. The steady-state exchange and TPD experiments are described in Sec. 3.1.3.. The kinetics of isotope exchange of O2 (gas) with oxide materials have been reviewed by Ceilings and Bouwmeester. Readers are referred to this work and references therein for a more comprehensive discussions of the mechanisms and kinetics involved in more complex systems. 

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