Kinetic and Thermodynamic Enhancement of Low-Temperature Oxygen Release from Strontium Ferrite Perovskites Modified with Ag and CeO 2 .

The redox behavior of the nonstoichiometric perovskite oxide SrFeO 3-δ modified with Ag, CeO 2 , and Ce was assessed for chemical looping air separation (CLAS) via thermogravimetric analysis and by cyclic release and uptake of O 2 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 N 2 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 CeO 2 at the surface or into the bulk of SrFeO 3-δ resulted in more modest changes, with a decrease in temperature for O 2 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 CeO 2 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 % CeO 2 , 75.7 kJ mol -1 and 623 mol O 2 s -1 m -3 Pa -1 for SrFeO 3-δ mixed with 2.5 wt % CeO 2 in the bulk, 29.9 kJ mol -1 and 0.88 mol O 2 s -1 m -3 Pa -1 for Sr 0.95 Ce 0.05 FeO 3-δ , and 69.0 kJ mol -1 and 278 mol O 2 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 × 10 10 mol O 2 s -1 m -3 Pa -1 , and Sr 0.95 Ce 0.05 FeO 3-δ , giving the activation energy E a,oxidation = 64.0 kJ mol -1 , and pre-exponential factor A oxidation = 584 mol O 2 s -1 m -3 Pa -1 .