The redox behavior
of the nonstoichiometric perovskite
oxide SrFeO3−δmodified with Ag, CeO2, and Ce
was assessed for chemical looping air separation (CLAS) via thermogravimetric
analysis and by cyclic release and uptake of O2in a packed
bed reactor. The results demonstrated that the addition of ∼15
wt % Ag at the surface of SrFeO3−δlowers
the temperature of oxygen release in N2by ∼60 °C
(i.e., from 370 °C for bare SrFeO3−δto
310 °C) and more than triples the amount of oxygen released per
CLAS cycle at 500 °C. Impregnation of SrFeO3−δwith Ag increased the concentration of oxygen vacancies at equilibrium,
lowering (3 – δ) under all investigated oxygen partial
pressures. The addition of CeO2at the surface or into
the bulk of SrFeO3−δresulted in more modest
changes, with a decrease in temperature for O2release
of 20–25 °C as compared to SrFeO3−δand a moderate increase in oxygen yield per reduction cycle. The
apparent kinetic parameters for reduction of SrFeO3−δ, with Ag and CeO2additives, were determined from the
CLAS experiments in a packed bed reactor, giving activation energies
and pre-exponential factors ofEa,reduction= 66.3 kJ mol–1andAreduction= 152 mol s–1m–3Pa–1for SrFeO3−δimpregnated with 10.7 wt %
CeO2, 75.7 kJ mol–1and 623 molO2s–1m–3Pa–1for SrFeO3−δmixed with 2.5
wt % CeO2in the bulk, 29.9 kJ mol–1and
0.88 molO2s–1m–3Pa–1for Sr0.95Ce0.05FeO3−δ, and 69.0 kJ mol–1and 278
molO2s–1m–3Pa–1for SrFeO3−δimpregnated
with 12.7 wt % Ag, respectively. Kinetics for reoxidation were much
faster and were assessed for two materials with the slowest oxygen
uptake, SrFeO3−δ, giving the activation energyEa,oxidation= 177.1 kJ mol–1and pre-exponential factorAoxidation= 3.40 × 1010molO2s–1m–3Pa–1, and
Sr0.95Ce0.05FeO3−δ,
giving the activation energyEa,oxidation= 64.0 kJ mol–1, and pre-exponential factorAoxidation= 584 molO2s–1m–3Pa–1.