Isothermal relaxation kinetics for the reduction and oxidation of SrFeO3 based perovskites

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.

Thermochemical process and compact apparatus for concentrating oxygen in extraterrestrial atmospheres: a feasibility study

The Martian atmosphere contains 0.16% oxygen, which is an example of an in-situ resource that can be used as precursor or oxidant for propellants, for life support systems and potentially for scientific experiments. Thus, the present work is related to the invention of a process to concentrate oxygen in the oxygen-deficient extraterrestrial atmosphere by means of a thermochemical process and the determination of a suitable best-case apparatus design to carry out the process. The perovskite oxygen pumping (POP) system uses the underlying chemical process, which is based on the temperature-dependent chemical potential of oxygen on multivalent metal oxide, to release and absorb oxygen in response to temperature swings. The primary goal of this work is therefore to identify suitable materials for the oxygen pumping system and to optimize the oxidation–reduction temperature and time, required to operate the system, to produce 2.25 kg of oxygen per hour under the Martian most-extreme environmental conditions and based on the thermochemical process concept. Radioactive materials such as 244Cm, 238Pu and 90Sr are analyzed as a heating source for the operation of the POP system, and critical aspects of the technology as well as weaknesses and uncertainties related to the operational concept are identified.

Impact of the Sr content on the redox thermodynamics and kinetics of Ca1-xSrxMnO3-δ for tailored properties

CaMnO_3-δ-based perovskites find application in a variety of thermochemical cycles, e.g. oxygen partial pressure adjustment, chemical looping processes, and thermochemical energy storage. The applicability of these materials is governed by their thermodynamic and kinetic properties. Therefore, tunability of these properties is desirable to adapt the material to the required conditions. In this study, the effect of Sr content in Ca_1-xSr_xMnO_3-δ on thermodynamics and kinetics is investigated by thermogravimetric analysis. The thermodynamics are measured in the temperature range of 873 K to 1473 K with oxygen partial pressures of 1 × 10^-4 bar to 0.8 bar. The oxidation kinetics were characterized in the temperature range from 473 K to 673 K with oxygen partial pressures of 0.01 bar to 1 bar. The reduction kinetics were very rapid in the temperature range of 873 K to 1023 K, with the measured rates limited by the constraints of the measurement device. The results show that with increasing Sr content the structural changes of the material decrease the reduction enthalpy and the oxidation activation energy. This not only leads to a tunability of material properties, but can also be used to predict changes of these properties when only the structural changes are known.

Selective formation of propan-1-ol from propylene via a chemical looping approach

A novel chemical looping approach for propan-1-ol production from propylene.

Effect of Mn and Cu Substitution on the SrFeO3 Perovskite for Potential Thermochemical Energy Storage Applications

Perovskites are well-known oxides for thermochemical energy storage applications (TCES) since they show a great potential for spontaneous O2 release due to their non-stoichiometry. Transition-metal-based perovskites are particularly promising candidates for TCES owing to their different oxidation states. It is important to test the thermal behavior of the perovskites for TCES applications; however, the amount of sample that can be used in thermal analyses is limited. The use of redox cycles in fluidized bed tests can offer a more realistic approach, since a larger amount of sample can be used to test the cyclic behavior of the perovskites. In this study, the oxygen release/consumption behavior of Mn- or Cu-substituted SrFeO3 (SrFe0.5M0.5O3; M: Mn or Cu) under redox cycling was investigated via thermal analysis and fluidized bed tests. The reaction enthalpies of the perovskites were also calculated via differential scanning calorimetry (DSC). Cu substitution in SrFeO3 increased the performance significantly for both cyclic stability and oxygen release/uptake capacity. Mn substitution also increased the cyclic stability; however, the presence of Mn as a substitute for Fe did not improve the oxygen release/uptake performance of the perovskite.

Air separation via two-step solar thermochemical cycles based on SrFeO3-δ and (Ba,La)0.15Sr0.85FeO3-δ perovskite reduction/oxidation reactions to produce N2: rate limiting mechanism(s) determination

Two-step solar thermochemical cycles based on reversible reactions of SrFeO_3-δ and (Ba,La)_0.15Sr_0.85FeO_3-δ perovskites were considered for air separation. The cycle steps encompass (1) the thermal reduction of SrFeO_3-δ or (Ba,La)_0.15Sr_0.85FeO_3-δ perovskites driven by concentrated solar irradiation and (2) oxidation in air to remove O₂ and produce N₂. Rate limiting mechanisms were examined for both reactions using a combination of isothermal and non-isothermal thermogravimetry for temperature-swings between 673 and 1373 K, heating rates of 10, 20, and 50 K min^-1, and O₂ pressure-swings between 20% O₂/Ar and 100% Ar at atmospheric pressure. Evolved O₂ and associated lag due to transport behavior were measured with gas chromatography and used with measured sample temperatures to predict equilibrium compositions from a compound energy formalism thermodynamic model. Measured and predicted chemical equilibrium changes in deviation from stoichiometry were compared. Rapid chemical kinetics were observed as the samples equilibrated rapidly for all conditions, indicative that heat and mass transfer were the rate limiting mechanisms. The effects of bulk diffusion (or gas diffusion through the bed or pellet) were examined using pelletized and loose powdered samples and determined to have no discernable impact.

Investigation of Sr0.7 Ca0.3 FeO3 Oxygen Carriers with Variable Cobalt B-Site Substitution

A-site and B-site substitutions are effective methods towards improving well-studied oxygen carrier materials that are vital for emerging gasification technologies. Such materials include SrFeO₃ , which greatly benefits from the inclusion of calcium and/or cobalt, and Sr_0.8 Ca_0.2 Fe_0.4 Co_0.6 O₃ has been regarded as the best-performing composition. In this study, systems with higher calcium and lower cobalt contents are investigated with a view to lessening the societal and economic burdens of these dual-doped carriers. Density functional theory calculations are performed to illustrate the Fe-O bonding and relaxation contributions to the oxygen vacancy formation energy in Sr_1-x Ca_x Fe_1-y Co_y O₃ systems (x=0.1875, 0.25, 0.3125; y=0.125, 0.25, 0.375, 0.5) and determine that increased calcium A-site substitution requires the use of less cobalt B-site doping to reach the same oxygen vacancy formation. These findings are experimentally validated in situ and ex situ characterization of bulk Sr_0.7 Ca_0.3 Fe_1-y Co_y O₃ materials. Sr_0.7 Ca_0.3 Fe_0.7 Co_0.3 O₃ is found to have similar O₂ adsorption/desorption rates and storage capacity to Sr_0.8 Ca_0.2 Fe_0.4 Co_0.6 O₃ in air/N₂ cycling experiments. Additionally, both materials are outperformed by Sr_0.7 Ca_0.3 Fe_1-y Co_y O₃ systems with y=0-0.10 at 400-500 °C, which cycle 1.5 wt% O₂ in under ten minutes.

An A- and B-Site Substitutional Study of SrFeO3-δ Perovskites for Solar Thermochemical Air Separation

An A‑ and B‑site substitutional study of SrFeO_3−δ perovskites (A’_xA_1−xB’_yB_1−yO_3−δ, where A = Sr and B = Fe) was performed for a two‑step solar thermochemical air separation cycle. The cycle steps encompass (1) the thermal reduction of A’_xSr_1−xB’_yFe_1−yO_3−δ driven by concentrated solar irradiation and (2) the oxidation of A’_xSr_1−xB’_yFe_1−yO_3−δ in air to remove O₂, leaving N₂. The oxidized A’_xSr_1−xB’_yFe_1−yO_3−δ is recycled back to the first step to complete the cycle, resulting in the separation of N₂ from air and concentrated solar irradiation. A-site substitution fractions between 0 ≤ x ≤ 0.2 were examined for A’ = Ba, Ca, and La. B-site substitution fractions between 0 ≤ y ≤ 0.2 were examined for B’ = Cr, Cu, Co, and Mn. Samples were prepared with a modified Pechini method and characterized with X-ray diffractometry. The mass changes and deviations from stoichiometry were evaluated with thermogravimetry in three screenings with temperature- and O₂ pressure-swings between 573 and 1473 K and 20% O₂/Ar and 100% Ar at 1 bar, respectively. A’ = Ba or La and B’ = Co resulted in the most improved redox capacities amongst temperature- and O₂ pressure-swing experiments.