Oxygen sorption/desorption behavior and crystal structural change for SrFeO3−

Fe-Doped SrCoOx FET Sensors for Extreme Alkaline pH Sensing

The accurate measurement of pH in highly alkaline environments is critical for various industrial applications but remains a complex task. This paper discusses the development of novel Fe-doped SrCoOx-based FET sensors for the detection of extreme alkaline pH levels. Through a comprehensive investigation of the effects of Fe doping on the structure, electrical properties, and sensing performance of SrCoOx, we have identified the optimal doping level that significantly enhances the sensor's performance in highly alkaline conditions. With a Fe doping level of 5 mol %, the sensitivity of the sensor improves to 0.86 lg(Ω)/pH while maintaining the response rate. Further increasing the Fe doping to 10 mol % results in a sensor that demonstrates favorable response time, a suitable pH range, and a linear correlation between lg(R) and pH. The combination of X-ray photoelectron spectroscopy and X-ray diffraction analysis provides insight into the regulation mechanisms of Fe doping on the crystal structure, electronic structure, and oxygen vacancy concentration of SrCoOx. Our findings indicate that Fe doping leads to an increase in oxygen vacancy concentration and a decrease in the energy barrier for oxygen ion migration, which contributes to the improved sensing performance of the Fe-doped SrCoOx sensors. Additionally, the study highlights the influence of oxygen vacancy concentration on the electrical properties of SrCoOx. Precise control over the concentration of oxygen vacancies is crucial for optimizing the sensitivity and response speed of SrCoOx FET sensors under extreme alkalinity conditions.

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.

Sr1-xKxFeO3 Perovskite Catalysts with Enhanced RWGS Reactivity for CO2 Hydrogenation to Light Olefins

The catalytic hydrogenation of CO2 to light olefins (C2–C4) is among the most practical approaches to CO2 utilization as an essential industrial feedstock. To achieve a highly dispersed active site and enhance the reactivity of the reverse water–gas shift (RWGS) reaction, ABO3-type perovskite catalysts Sr1-xKxFeO3 with favorable thermal stability and redox activity are reported in this work. The role of K-substitution in the structure–performance relationship of the catalysts was investigated. It indicated that K-substitution expedited the oxygen-releasing process of the SrFeO3 and facilitated the synchronous formation of active-phase Fe3O4 for the reverse water–gas shift (RWGS) reaction and Fe5C2 for the Fischer–Tropsch synthesis (FTS). At the optimal substitution amount, the conversion of CO2 and the selectivity of light olefins achieved 30.82% and 29.61%, respectively. Moreover, the selectivity of CO was up to 45.57% even when H2/CO2=4 due to CO2-splitting reactions over the reduced Sr2Fe2O5. In addition, the reversibility of perovskite catalysts ensured the high dispersion of the active-phase Fe3O4 and Fe5C2 in the SrCO3 phase. As the rate-determining step of the CO2 hydrogenation reaction to light olefins over Sr1-xKxFeO3 perovskite catalysts, FTS should be further tailored by partial substitution of the B site. In sum, the perovskite-derived catalyst investigated in this work provided a new idea for the rational design of a catalyst for CO2 hydrogenation to produce light olefins.

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.

The optimal co-doping of SrFe1-xCoxO3-δ oxygen carriers in redox applications

Although the oxygen carrier SrCoO₃ has higher redox activity than SrFeO₃, cobalt is both more expensive and scarcer than iron, which would hinder the wide implementation of SrCoO₃. For these reasons, doping SrFeO₃ with Co is a potential compromise, benefitting the redox properties of SrFeO₃, while still limiting the overall amount of cobalt being used. To find the optimal level of Co-doping, density functional theory calculations were performed to investigate the Co-doping effect on the oxygen vacancy formation and oxygen migration in SrFe_1-xCo_xO_3-δ (x = 0, 0.125, 0.25, 0.375, 0.5). Our findings show that the oxygen vacancy formation energies (E_f) decrease with the increase of Co content resulting from the increased composition of the O-2p band at the Fermi level upon Co doping. In particular, the E_f decreases nearly 0.5 eV between the x = 0 and x = 0.25 samples while E_f only decreases 0.1 eV further as Co content is increased to x = 0.5. We obtain that x = 0.25 is an optimal cost/benefit ratio for Co doping, which is preserved at both low oxygen vacancy concentrations (δ = 0.0625 values listed above) and at high concentrations of δ = 0.1875 and 0.375. Kinetically, the oxygen migration barrier has slight change upon Co doping due to the similar size of Co and Fe. Therefore, considering both redox activity and economics in reversible oxygen storage applications, x = 0.25 is suggested as the optimal Co-doping value in SrFe_1-xCo_xO_3-δ.

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

The perovskite oxide SrFeO₃ has favourable redox properties for oxygen exchange applications, including oxygen separation and oxygen production chemical looping cycles. For such applications, lower temperature operation can improve the energy demand and feasibility of the process, but can also lead to kinetic limitations. Here we investigate the oxidation and reduction reaction kinetics of SrFeO₃ in the temperature range 450-750 K. Isothermal relaxation techniques are used to observe the reaction rates across this temperature range, using a thermogravimetric analysis system. Experimental data are analysed according to an isoconversional method and fit with a simple power law model to extract activation energies. The apparent activation energy of oxidation and reduction was found to be 92 ± 16 and 144 ± 17 kJ mol^-1 respectively. Comparison of oxidation and reduction kinetics together with considerations of particle size indicate that the oxidation reaction rate may be limited by diffusion in the bulk, while the reduction reaction rate is limited by the surface reaction. Furthermore, we also investigated the mixed perovskite Sr_0.93Ca_0.07Fe_0.9Co_0.1O₃, which exhibited a 4-fold increase in the oxidation rate.

Sucrose-Assisted Solution Combustion Synthesis of Doped Strontium Ferrate Perovskite-Type Electrocatalysts: Primary Role of the Secondary Fuel

The methodologies and experimental conditions used for the synthesis of cathode materials for electrochemical devices strongly influence their electrocatalytic performance. In particular, solution combustion synthesis is a convenient and versatile methodology allowing a fine-tuning of the properties of the material. In this work, we used for the first time a sucrose assisted-solution combustion synthesis for the preparation of Cerium and Cobalt-doped SrFeO3–δ electrocatalysts and we investigated the effect of polyethylene glycol (PEG) addition as a secondary fuel on their structural, microstructural, redox and electrochemical properties. The perovskite-type powders were characterized by X-ray diffraction coupled with Rietveld refinement, scanning, and high-resolution transmission electron microscopies, thermogravimetric analysis, nitrogen adsorption measurements, and temperature-programmed reduction. Electrical conductivity and overpotential measurements were performed after the deposition of the powders onto a Gd-doped ceria electrolyte pellet. Stable high-valence B-site cations were detected in the powders prepared from sucrose-PEG fuel mixtures, although a substantial improvement of the conductivity and a decrease of the overpotential values were obtained only with high molecular weight PEG. The superior electrochemical performance obtained using PEG with high molecular weight has been ascribed to a faster interaction of the powder with the oxygen gas phase favored by the nanometer-sized crystalline domains.

SrFe1−xSnxO3−δ nanoparticles with enhanced redox properties for catalytic combustion of benzene

A series of SrFe_1−xSn_xO_3−δ showed high catalytic activity for benzene combustion. The partial substitution of Fe with Sn increased specific surface area and accelerated redox rates of Fe, resulting in the improvement of the catalytic activity.

NOx Oxidation and Storage Properties of a Ruddlesden-Popper-Type Sr3Fe2O7-δ-Layered Perovskite Catalyst

The development of NO_x-trapping catalysts for automobiles is highly desired to meet the current strict exhaust emission regulations. This study demonstrates that NO_x oxidation and storage reactions proceed over Pt-free Sr₃Fe₂O_7-δ with a Ruddlesden-Popper-type layered perovskite structure. Two types of Sr-Fe perovskite with oxygen storage capacity, namely, SrFeO_3-δ and Sr₃Fe₂O_7-δ, are studied as NO_x-trapping catalysts. Sr₃Fe₂O_7-δ shows higher NO_x storage capacity than SrFeO_3-δ; its activity is comparable to that of Pt/Ba/Al₂O₃ calcined at 1273 K. NO_x temperature-programmed desorption and diffuse reflectance infrared Fourier transform experiments confirm the superior NO_x-trapping ability of Sr₃Fe₂O_7-δ over SrFeO_3-δ. In addition, NO temperature-programmed reactions and O₂ temperature-programmed desorption experiments reveal that these catalysts operate through a novel NO oxidation mechanism involving the consumption of their lattice oxygens and topotactic structural changes at a temperature of around 350-400 K. The reduction performance of trapped NO_x on Pd-modified Sr-Fe perovskites is investigated by lean-rich cycle experiments using H₂ as the reductant. Pd/Sr₃Fe₂O_7-δ shows significantly high NO_x removal efficiency over the entirety of each lean-rich period. Modifying Sr₃Fe₂O_7-δ with Pd is also effective for NO_x storage in the presence of H₂O and CO₂ and the regeneration of the catalyst following SO_x sorption. Sr₃Fe₂O_7-δ, with both NO_x adsorption and NO oxidation capabilities, acts as a Pt-free NO_x-trapping catalyst, exhibiting both high NO_x storage capacity and high thermal tolerance.