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.

Ru-promoted perovskites as effective redox catalysts for CO2 splitting and methane partial oxidation in a cyclic redox scheme

The current study reports A_xA'_1-xB_yB'_1-yO_3-δ perovskite redox catalysts (RCs) for CO₂-splitting and methane partial oxidation (POx) in a cyclic redox scheme. Strontium (Sr) and iron (Fe) were chosen as A and B site elements with A' being lanthanum (La), samarium (Sm) or yttrium (Y), and B' being manganese (Mn) or titanium (Ti) to tailor their equilibrium oxygen partial pressures (P_O2s) for CO₂-splitting and methane partial oxidation. DFT calculations were performed for predictive optimization of the oxide materials whereas experimental investigation confirmed the DFT-predicted redox performance. The redox kinetics of the RCs improved significantly by 1 wt% ruthenium (Ru) impregnation without affecting their redox thermodynamics. Ru-impregnated LaFe_0.375Mn_0.625O₃ (A = 0, A' = La, B = Fe, and B' = Mn) was the most promising RC in terms of its superior redox performance (CH₄/CO₂ conversion >90% and CO selectivity ∼95%) at 800 °C. Long-term redox testing over Ru-impregnated LaFe_0.375Mn_0.625O₃ indicated a stable performance during the first 30 cycles followed by an ∼25% decrease in the activity during the last 70 cycles. Air treatment was effective to reactivate the redox catalyst. Detailed characterizations revealed the underlying mechanism of the redox catalyst deactivation and reactivation. This study not only validated a DFT-guided mixed oxide design strategy for CO₂ utilization but also provides potentially effective approaches to enhance redox kinetics and long-term redox catalyst performance.