An investigation of the structural and electronic origins of enhanced chemical looping air separation performance of B-site substituted SrFe1-xCoxO3-δ perovskites

Chemical looping air separation (CLAS) is a promising process intensification technology for extracting oxygen from air for oxygen enrichment in process streams. Co-doped strontium ferrites (SrFe1-xCoxO3-δ) have been found to have outstanding activities for CLAS processes. In this study, we explore the underlying factors driving the enhancement in oxygen uptake and release performance of perovskite structured SrFe1-xCoxO3-δ oxygen carriers for CLAS. Phase-pure perovskites, with B site substituted by up to 75 mol% Co, were prepared by a sol-gel method and systematically investigated through a wide range of well controlled experimental and computational approaches. While all SrFe1-xCoxO3-δ oxygen carriers showed excellent cyclic stability and structural reversibility over CLAS cycles, increased B site occupancy by Co resulted in monotonic decrease in onset temperature for oxygen release and increase in oxygen carrying capacity. These experimental trends can be fundamentally explained by an increase in the structural tolerance factor, an elevation in transition metal d-band, as well as an increased degree of hybridization between the metal d-band and the O p band. Therefore, these ab initio structural and electronic descriptors are useful design rationales for the hypothesis-driven synthesis of high-performing oxygen carriers for CLAS.

Consistent interpretation of isotope and chemical oxygen exchange relaxation kinetics in SrFe0.85Mo0.15O3-δ ferrite

This paper is devoted to the study of phase composition and kinetic and thermodynamic characteristics of Mo-doped strontium ferrite SrFe0.85Mo0.15O3-δ (SFM15) under oxygen-conducting membrane working conditions. Single-phase SFM15 with a cubic Pm3̄m structure was synthesized using a ceramic method. It was shown that the molybdenum introduction stabilizes the perovskite cubic structure over a wide range of oxygen pressures and temperatures, preventing the bulk phase transition at high temperatures. Oxygen exchange constants, diffusion coefficients and activation energy of oxygen exchange were obtained using oxygen relaxation and isotopic exchange techniques, and the obtained values are consistent with known literature data. It was shown that the surface reaction rates obtained using chemical and tracer relaxation methods are quantitatively comparable with each other, despite significantly different experimental conditions. This result not only confirms the reliability of the data obtained by independent methods, but also allows one to expand the area of physical conditions for studying the kinetics of oxygen transfer where another method has technical or methodological limitations.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

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.

Thermodynamics of Formation and Disordering of YBaCo2O6-δ Double Perovskite as a Base for Novel Dense Ceramic Membrane Materials

Differential scanning calorimetry studies of the complex oxide YBaCo₂O_6-δ (YBC), combined with the literature data, allowed outlining the phase behavior of YBC depending on the oxygen content and temperature between 298 K and 773 K. The oxygen nonstoichiometry of single-phase tetragonal YBC was measured at different temperatures and oxygen partial pressures by both thermogravimetric and flow reactor methods. The defect structure of YBC was analyzed. As a result, the thermodynamic functions (∆Hi○, ∆Si○) of the defect reactions in YBC were determined. Experimental data on the oxygen content and those calculated based on the theoretical model were shown to be in good agreement. Standard enthalpies of formation at 298.15 K (∆Hf○) were obtained for YBC depending on its oxygen content using solution calorimetry. It was found that ∆Hf○ = f(6-δ) function is linear in the range of (6-δ) from 5.018 to 5.406 and that its slope is close to the value of the enthalpy of the quasichemical reaction describing oxygen exchange between the oxide and ambient atmosphere, which confirms the reliability of the suggested defect structure model.

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.

The Brønsted-Evans-Polanyi relationship in oxygen exchange of fuel cell cathode material SrCo0.9Ta0.1O3-δ with the gas phase

Perovskite related oxides ABO3-δ exhibiting mixed ionic-electronic conductivity (MIEC) possess large deviations from the oxygen stoichiometry. When providing excellent application potential, this feature also makes it very difficult to study the reaction mechanism between such oxides and molecular oxygen, also known as the oxygen reduction reaction. The complexity of the theoretical interpretation of kinetic experiments originates from the significant dependence of the kinetic and equilibrium properties of MIEC oxides on δ. It is proposed to consider such grossly nonstoichiometric oxides having different oxygen nonstoichiometry as chemical homologues participating in the oxygen exchange reaction and forming a series continuous in δ. The continuous homologous series approach is considered using the example of SrCo0.9Ta0.1O3-δ, an SOFC cathode material. The equilibrium and kinetic properties of the oxide were studied by new methods of oxygen partial pressure relaxation and oxygen release. Linear free-energy relationships have been discovered in the homologous series: thermodynamic and kinetic enthalpy-entropy compensations, as well as the Brønsted-Evans-Polanyi relation. A relationship has been established between the change in the observed LFERs and the morphotropic phase transition in the oxide.

Nonstoichiometric oxides as a continuous homologous series: linear free-energy relationship in oxygen exchange

A novel methodology based on the continuous homologous series is suggested for the analysis of oxygen exchange in practically important non-stoichiometric oxides. Linear free-energy relationship is established analogous to Brønsted equation or Taffel plot.

Investigation into the Effect of Sulfate and Borate Incorporation on the Structure and Properties of SrFeO3-δ

In this paper, we demonstrate the successful incorporation of sulfate and borate into SrFeO3-δ, and characterise the effect on the structure and conductivity, with a view to possible utilisation as a cathode material in Solid Oxide Fuel Cells. The incorporation of low levels of sulfate/borate is sufficient to cause a change from a tetragonal to a cubic cell. Moreover, whereas heat treatment of undoped SrFeO3-δ under N2 leads to a transformation to brownmillerite Sr2Fe2O5 with oxygen vacancy ordering, the sulfate/borate-doped samples remain cubic under the same conditions. Thus, sulfate/borate doping appears to be successful in introducing oxide ion vacancy disorder in this system.

Relation between oxygen stoichiometry and thermodynamic properties and the electronic structure of nonstoichiometric perovskite La0.6Sr0.4CoO3−δ

Within the framework of the itinerant electron model, the dependence of the oxide nonstoichiometry on the oxygen activity was related to the density of electronic states near the Fermi level.

Ferroelasticity of SrCo0.8Fe0.2O3–δperovskite-related oxide with mixed ion–electron conductivity

A group-theoretical analysis was carried out to determine the possible orientation states of domains formed as a result of the `perovskite–brownmillerite' phase transition in SrCo0.8Fe0.2O2.5oxide with mixed ion–electron conductivity (MIEC). The results of the theoretical analysis agree with the experimental data obtained in the study of the SrCo0.8Fe0.2O2.5microstructure by means of transmission electron microscopy. Brownmillerite SrCo0.8Fe0.2O2.5(BM) has a lamellar texture composed of 90° twins 60–260 nm in size; the 〈010〉BMand 〈101〉BMdirections are linked through twinning in accordance with the predictions of the group-theoretical analysis. The presence of twins and their switching under mechanical load provide evidence that the perovskite–brownmillerite phase transition in SrCo0.8Fe0.2O2.5is ferroelastic. Comparative analysis of the phenomena observed for ferroelectrics and MIEC oxides indicates their similarity based on the common nature of ferroelectricity and ferroelasticity, and allows us to suppose that nonstoichiometric SrCo0.8Fe0.2O3−δwith compositional disorder may be considered (in terms of its microstructural features) a `relaxor ferroelastic'.