Kinetic and Thermodynamic Enhancement of Low-Temperature Oxygen Release from Strontium Ferrite Perovskites Modified with Ag and CeO₂

The redox behavior of the nonstoichiometric perovskite oxide SrFeO_3−δ modified with Ag, CeO₂, and Ce was assessed for chemical looping air separation (CLAS) via thermogravimetric analysis and by cyclic release and uptake of O₂ in a packed bed reactor. The results demonstrated that the addition of ∼15 wt % Ag at the surface of SrFeO_3−δ lowers the temperature of oxygen release in N₂ by ∼60 °C (i.e., from 370 °C for bare SrFeO_3−δ to 310 °C) and more than triples the amount of oxygen released per CLAS cycle at 500 °C. Impregnation of SrFeO_3−δ with Ag increased the concentration of oxygen vacancies at equilibrium, lowering (3 – δ) under all investigated oxygen partial pressures. The addition of CeO₂ at the surface or into the bulk of SrFeO_3−δ resulted in more modest changes, with a decrease in temperature for O₂ release of 20–25 °C as compared to SrFeO_3−δ and a moderate increase in oxygen yield per reduction cycle. The apparent kinetic parameters for reduction of SrFeO_3−δ, with Ag and CeO₂ additives, were determined from the CLAS experiments in a packed bed reactor, giving activation energies and pre-exponential factors of E_a,reduction = 66.3 kJ mol^–1 and A_reduction = 152 mol s^–1 m^–3 Pa^–1 for SrFeO_3−δ impregnated with 10.7 wt % CeO₂, 75.7 kJ mol^–1 and 623 mol_O2 s^–1 m ^–3 Pa^–1 for SrFeO_3−δ mixed with 2.5 wt % CeO₂ in the bulk, 29.9 kJ mol^–1 and 0.88 mol_O2 s^–1 m^–3 Pa^–1 for Sr_0.95Ce_0.05FeO_3−δ, and 69.0 kJ mol^–1 and 278 mol_O2 s^–1 m^–3 Pa^–1 for SrFeO_3−δ impregnated with 12.7 wt % Ag, respectively. Kinetics for reoxidation were much faster and were assessed for two materials with the slowest oxygen uptake, SrFeO_3−δ, giving the activation energy E_a,oxidation = 177.1 kJ mol^–1 and pre-exponential factor A_oxidation = 3.40 × 10¹⁰ mol_O2 s^–1 m^–3 Pa^–1, and Sr_0.95Ce_0.05FeO_3−δ, giving the activation energy E_a,oxidation = 64.0 kJ mol^–1, and pre-exponential factor A_oxidation = 584 mol_O2 s^–1 m^–3 Pa^–1.