Brownmillerite Structure Research Articles

Phase transition behaviors of Sr0.72Ca0.28Fe1-xNixO3-δ perovskite oxygen sorbents in a fixed bed reactor

Perovskite oxides have emerged as promising materials for oxygen storage applications, with their phase transitions being a key area of research. This study reports on the unique oxygen absorption and desorption breakthrough curves observed in the Sr0.72Ca0.28Fe1-xNixO3-δ series (x = 0.05) during fixed-bed experiments, featuring a distinct plateau. Through in situ X-ray diffraction (XRD) analysis, we demonstrated the reversible phase transition between the perovskite and brownmillerite (BM) structures. High Resolution Transmission electron microscopy (HR-TEM) further revealed the coexistence of these two phases within a granule. Focusing on the Sr0.72Ca0.28Fe0.95Ni0.05O3-δ sample at 550 °C, we conducted a detailed analysis of the oxygen absorption curve. Our findings indicate that the plateau arises from the oxygen-deficient BM phase absorbs oxygen to form the oxygen-saturated BM phase, followed by further oxygen absorption, leading to the formation of the perovskite phase. The inflection point in the oxygen absorption curve marks a critical oxygen concentration, signifying the threshold for effective oxygen absorption. This critical concentration is influenced by both material composition and operation temperature. Based on our observations, a phase diagram for x = 0.05 was drawn, encompassing both oxygen concentration and temperature. This diagram allows for the rational selection of operation temperatures and oxygen concentration swing ranges to enhance the performance of the oxygen sorbent. Our study highlights the crucial role of phase transition in oxygen storage, offering insights into the design and development of high-performance perovskite-based materials for various applications.

Advanced Electrode Materials Based on Brownmillerite Calcium Ferrite for Li‐Ion Batteries

AbstractIron‐based materials are considered potential anode materials for lithium‐ion batteries thanks to their low cost, abundancy, non‐flammability, good safety, environmental benignity, and high specific capacity. Here, a series of calcium iron oxides materials having brownmillerite structure (i. e., Ca2Fe2‐xMxO5, where M=Mn, Ni and Cu and x=0 and 0.1) has been extensively studied for their use as conversion anodes in lithium cell. In particular, a mechanochemical approach has been used either to synthesize the samples and to prepare electrodes for the tests in lithium cell. Ca2Fe2O5 based electrodes proved excellent performance in lithium cell, approaching the theoretical capacity and being stable upon prolonged cycling (529 mAh g−1 at C/10 and a capacity retention of 81 % after 100 cycles). Through the use of ex‐situ diffraction measurements, we have analyzed the conversion mechanism and proved the partial reversibility of its electrochemical reaction. Also, the incorporation of dopants into the structure of calcium iron oxide resulted in further improvement of its electrochemical performance as is the case of Mn doped sample that show a considerable specific capacity of 567 mAh g−1 and the capacity retention is almost 99 % after 100 cycles.

The electronic structure and magnetic properties of stable Sr2BB′O5 (B = Fe, Co, Ni and B′ = Co, Ni, Mn) with the brownmillerite structure

First-principles calculations have been performed to investigate the structural, electronic, and magnetic properties of Sr2BB′O5 (B = Fe, Co, Ni; B′ = Co, Ni, Mn) brownmillerite materials. By considering the atomic arrangements and the tetrahedral rotated modes, including typical magnetic configurations, the ground states are found by comparing their total energy. The thermodynamic and dynamic stability studies revealed that two compounds, Sr2FeCoO5 and Sr2NiMnO5, could be stable in the brownmillerite structure. Further electronic structure calculations show that the ground state of Sr2FeCoO5 is a G-AFM insulator, while the ground state of Sr2NiMnO5 is an A-AFM half-metal with a half-metallic bandgap of about 1 eV, exhibiting excellent half-metallic properties. In addition, the magnetic interactions are analyzed, which suggest that Sr2FeCoO5 could suffer from the influence of the B-site atomic disorder to maintain the G-AFM state, while Sr2NiMnO5 is more sensitive to atomic disorder, so its A-AFM state would be easily destroyed by the B-site disorder. These results would be helpful for further research.

Study of Thermal Insulation Nature of A2Fe2O6-δ (A and A’ = Ca, Sr)

Materials with low thermal conductivity have been used in thermoelectrics, electronics, solar devices, and land base and aerospace gas turbines to prevent heat dissipation and provide safety and longevity of equipment. We report Ca2Fe2O6-δ, and Sr2Fe2O6-δ for their low thermal conductivities. These compounds are vacancy-ordered oxygen-deficient perovskites but with different vacancy arrangements. Ca2Fe2O6-δ has a brownmillerite type structure while Sr2Fe2O6-δ has a different structure.

Enhanced chemical looping CO2 conversion activity and thermal stability of perovskite LaCo1-xAlxO3 by Al substitution.

The reverse water-gas shift chemical looping (RWGS-CL) process that utilizes redox reactions of metal oxides is promising for converting CO2 to CO at low temperatures. Metal oxides with perovskite structures, particularly, perovskite LaCoO3 are promising frameworks for designing RWGS-CL materials as they can often release oxygen atoms topotactically to form oxygen vacancies. In this study, solid solutions of perovskite LaCo1-xAlxO3 (0 ≤ x ≤ 1), which exhibited high CO production capability and thermal stability under the RWGS-CL process, were developed. Al-substituted LaCo0.5Al0.5O3 (x = 0.5) exhibited a 4.1 times higher CO production rate (2.97 × 10-4 CO mol g-1 min-1) than that of LaCoO3 (x = 0; 0.73 × 10-4 CO mol g-1 min-1). Diffuse reflectance infrared Fourier transform spectroscopy studies suggested that an increase in CO2 adsorption sites produced by the coexistence of Al and Co was responsible for the enhancement of CO production rate. Furthermore, LaCo0.5Al0.5O3 maintained its perovskite structure during the RWGS-CL process at 500 °C without significant decomposition, whereas LaCoO3 decomposed into La2O3 and Co0. In situ X-ray diffraction study revealed that the high thermal stability was attributed to the suppression of phase transition into a brownmillerite structure with ordered oxygen vacancies. These findings provide a critical design approach for the industrial application of perovskite oxides in the RWGS-CL processes.

Functions of local structural surrounding in activity of Ca-containing catalysts for vapor upgrading during biomass thermal decomposition

The catalytic activity of calcium-containing complex oxides with perovskite structure (CaTiO3), brownmillerite structure (Ca2Fe2O5 and Ca2FeAlO5), spinel structure (CaAl2O4), and pseudowollastonite structure (CaSiO3) was compared to CaO for the upgrading of Oakwood fast pyrolysis vapors. Initial catalyst particle sizes range between 0.61 and 3.21 μm, while the BET surface area is between 0.2 and 8.7 m2/g. In contrast to CaO, the complex oxides compounds demonstrated negligible CO2 and H2O chemisorption during catalytic fast pyrolysis, thereby a low deactivation of catalysts, good structural and morphological stability and thus high reusage feasibility. Due to their unique features, Ca-containing catalysts were found to promote specific reaction pathways, such as the conversion of guaiacol to 3-methyl phenol, 4-ethenyl-2-methoxyphenol to 4-ethyl-2-methoxyphenol, 1-(4-hydroxy-3,5-dimethoxyphenyl)ethanone to 3,5-dimethoxy acetophenone and long chain carboxylic acids to acetic acid through a multitude of reaction routes including demethylation/demethoxylation, hydrogenation and hydrodeoxygenation, oxidative cleavage, and dehydration reactions. CaO and Ca2Fe2O5 promoted the alkylation reactions within the methoxy phenolics, whilst CaSiO3 promoted hydrogenation reactions. The formation of acetic acid was promoted over Ca2FeAlO5, CaAl2O4, and CaSiO3, while acetone, 2-butanone, new cyclic C5 ketones were revealed over CaO. The resulting ketonic fraction was noticeably affected by the use of Ca2Fe2O5 and CaAl2O4 catalysts.

Performance optimization of Ca2Fe2O5 oxygen carrier by doping different metals for coproduction syngas and hydrogen with chemical looping gasification and water splitting

This work focused on the potential effects of different metals (Co/Al/Ni/Ce) on the chemical looping performance of brownmillerite-structured Ca2Fe2O5 for syngas and hydrogen production. Oxygen carriers (OC) were prepared by the sol-gel method. The presence of NiO and CeO2 phases in fresh Ca2Fe2O5 samples indicated that Ni and Ce were not fully doped into the brownmillerite structure. The oxides of Al and Co were not detected in the fresh Ca2Fe2O5 sample, indicating that the Al and Co were doped into the brownmillerite structure. The introduction of Ni and Co decreased the reduction temperature and increased the reduction rate of OC. However, the increase in reactivity with Ni-modified OC is mainly due to the high reactivity of the NiO phase. The presence of Al and Ce inhibited the reduction of Ca2Fe2O5 and decreased the oxygen transport capacity, which is beneficial for syngas production but leads to a lower H2 production in the steam reactor (SR). The Co-modified Ca2Fe2O5 exhibited better resistance to carbon deposition and increased the H2 production in SR. When the ratio of Ca:Fe:Co equals 2:1.5:0.5, the formation of Ca2Co0.5Fe1.5O5 exhibited the highest hydrogen yield and purity of 10.92 mmol/g OC and 99.32%, respectively. In addition, the Ca2Co0.5Fe1.5O5 showed almost stable reactivity over the 5 reaction cycles.

Spray-Flame Synthesis of LaFexCo1-xO3 (x = 0.2, 0.3) Perovskite Nanoparticles for Oxygen Evolution Reaction in Water Splitting: Effect of Precursor Chemistry (Acetates and Nitrates).

The product properties of mixed oxide nanoparticles generated via spray-flame synthesis depend on an intricate interplay of solvent and precursor chemistries in the processed solution. The effect of two different sets of metal precursors, acetates and nitrates, dissolved in a mixture of ethanol (35 Vol.%) and 2-ethylhexanoic acid (2-EHA, 65 Vol.%) was investigated for the synthesis of LaFexCo1-xO3 (x = 0.2, 0.3) perovskites. Regardless of the set of precursors, similar particle-size distributions (dp = 8-11 nm) were obtained and a few particles with sizes above 20 nm were identified with transmission electron microscopy (TEM) measurements. Using acetates as precursors, inhomogeneous La, Fe, and Co elemental distributions were obtained for all particle sizes according to energy dispersive X-ray (EDX) mappings, connected to the formation of multiple secondary phases such as oxygen-deficient La3(FexCo1-x)3O8 brownmillerite or La4(FexCo1-x)3O10 Ruddlesden-Popper (RP) structures besides the main trigonal perovskite phase. For samples synthesized from nitrates, inhomogeneous elemental distributions were observed for large particles only where La and Fe enrichment occurred in combination with the formation of a secondary La2(FexCo1-x)O4 RP phase. Such variations can be attributed to reactions in the solution prior to injection in the flame as well as precursor-dependent variations in in-flame reactions. Therefore, the precursor solutions were analyzed by temperature-dependent attenuated total reflection Fourier-transform infrared (ATR-FTIR) measurements. The acetate-based precursor solutions indicated the partial conversion of, mainly La and Fe, acetates to metal 2-ethylhexanoates. In the nitrate-based solutions, esterification of ethanol and 2-EHA played the most important role. The synthesized nanoparticle samples were characterized by BET (Brunauer, Emmett, Teller), FTIR, Mössbauer, and X-ray photoelectron spectroscopy (XPS). All samples were tested as oxygen evolution reaction (OER) catalysts, and similar electrocatalytic activities were recorded when evaluating the potential required to reach 10 mA/cm2 current density (∼1.61 V vs reversible hydrogen electrode (RHE)).

Research progress of control of physical properties of topological phase change oxide films by external field

Perovskite transition-metal oxides can undergo significant structural topological phase transition between perovskite structure, brownmillerite structure, and infinite-layer structure under the external field through the gain and loss of the oxygen ions, accompanied with significant changes in physical properties such as transportation, magnetism, and optics. Topotactic phase transformation allows structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for utilizing the redox reaction and oxygen diffusion within transition metal oxides, and establishing great potential applications in solid oxide fuel cells, oxygen sensors, catalysis, intelligent optical windows, and neuromorphic devices. In this work, we review the recent research progress of manipulating the topological phase transition of the perovskite-type oxide films and regulating their physical properties, mainly focusing on tuning the novel physical properties of these typical films through strong interaction between the lattice and electronic degrees of freedom by the action of external fields such as strain, electric field, optical field, and temperature field. For example, a giant photoinduced structure distortion in SrCoO<sub>2.5</sub> thin film excited by photons is observed to be higher than any previously reported results in the other transition metal oxide films. The SrFeO<sub>2</sub> films undergo an insulator-to-metal transition when the strain state changes from compressive state to tensile state. It is directly observed that perovskite SrFeO<sub>3</sub> nanofilament is formed under the action of electric field and extends almost through the brownmillerite SrFeO<sub>2.5</sub> matrix in the ON state and is ruptured in the OFF state, unambiguously revealing a filamentary resistance switching mechanism. Utilizing <i>in situ</i> electrical scanning transmission electron microscopy, the transformation from brownmillerite SrFeO<sub>2.5</sub> to infinite-layer SrFeO<sub>2</sub> under electric field can be directly visualized with atomic resolution. We also clarify the relationship between the microscopic coupling mechanism and the macroscopic quantum properties of charges, lattices, orbits, spin, etc. Relevant research is expected to provide a platform for new materials, new approaches and new ideas for developing high-sensitivity and weak-field response electronic devices based on functional oxides. These findings about the topological phase transition in perovskite oxide films can expand the research scope of material science, and have important significance in exploring new states of matters and studying quantum critical phenomena.

In-situ investigation on the oxygen vacancy-driven topotactic phase transition in charge-orbital ordered Nd0.5Sr0.5MnO3 films

The topotactic phase transition from perovskite to oxygen vacancies ordered brownmillerite structure in transition metal oxides intrigues various new application possibilities. However, the in-situ observation of the oxygen vacancies formation process accompanying the phase transition is still rare. Here in this article, we report the topotactic phase transition from the perovskite to a brownmillerite phase transition in the intermediate bandwidth- (101)-Nd0.5Sr0.5MnO3-δ films. The oxygen vacancies migration process in the films was identified by the TEM technique during the in-situ high vacuum annealing process. The TEM results demonstrate a vertical brownmillerite structure with the oxygen vacancies channel perpendicular to the film plane in the (101)-oriented films. Moreover, the in-situ investigation reveals that oxygen atoms in the {010}PV plane of the perovskite phase migrate out of the film along the out-of-plane <101>PV direction at first, and the formed oxygen vacancies become ordered by elongating the high-vacuum annealing process to 42 h, transforming to the brownmillerite structure. Further studies reveal that the associated physical properties of the topotactic phases can be controlled by the phase transition. The charge-orbital ordered antiferromagnetic insulating ground state transfers to a highly insulating state with a novel magnetic phase that is distinct from that of brownmillerite structure of other manganites. The present studies thus pioneered topotactic phase transition in a new manganite system and provide useful information for developing perovskite-based functional materials by defect engineering.

Atomic-scale observation of strain-dependent reversible topotactic transition in La0.7Sr0.3MnOx films under an ultra-high vacuum environment

Reversible topotactic phase transition between perovskite (PV) and oxygen-vacancy-ordered brownmillerite (BM) structures provides an effective platform for realizing the control of physical properties in complex transition metal oxides. However, such reversibility always requires extreme external conditions, that is, a high temperature and vacuum environment during the PV-BM transition, while an oxidizing atmosphere and relatively low temperature vice versa. Here, we experimentally observe the reversible process in strained La0.7Sr0.3MnOx films at atomic scale by using in-situ heating aberration-corrected scanning transmission electron microscopy (STEM). Apart from the conventional reduction reaction of creating oxygen-vacancy-ordered frameworks after heating in the TEM, the inverse process of BM-to-PV transition is unexpectedly discovered under such an ultra-high vacuum atmosphere (∼10−9 Torr) at room temperature. Moreover, this abnormal behavior is strain-dependent. The large compressive strain is found to be detrimental to the inverse phase transition. The density functional theory (DFT) calculations show that the high oxygen affinity of La0.7Sr0.3MnO2.5 is responsible for the reversible transitions. Our findings provide a new insight into the redox reactions of manganite and might be further utilized for potential applications in solid fuel cells, oxygen sensors or resistive switching memories.

Revealing Resistive Switching Mechanism in CaFeOx Perovskite System with Electroforming-Free and Reset Voltage-Controlled Multilevel Resistance Characteristics.

Recently, perovskite (PV) oxides with ABO3 structures have attracted considerable interest from scientists owing to their functionality. In this study, CaFeOx is introduced to reveal the resistive switching properties and mechanism of oxygen vacancy transition in PV and brownmillerite (BM) structures. BM-CaFeO2.5 is grown on an Nb-STO conductive substrate epitaxially. CaFeOx exhibits excellent endurance and reliability. In addition, the CaFeOx also demonstrates an electroforming-free characteristic and multilevel resistance properties. To construct the switching mechanism, high-resolution transmission electron microscopy is used to observe the topotactic phase change in CaFeOx . In addition, scanning TEM and electron energy loss spectroscopy show the structural evolution and valence state variation of CaFeOx after the switching behavior. This study not only reveals the switching mechanism of CaFeOx , but also providesa PV oxide option for the dielectric material in resistive random-access memory (RRAM) devices.

Crystal structures, dielectric properties, and phase transitions in Sr2ScGaO5-based brownmillerite ceramics

In the present work, the crystal structures and dielectric properties of the Sr2Sc1-xInxGaO5 (x = 0.0, 0.1, 0.2, 0.3) ceramics with the brownmillerite structures were investigated. The ceramics belong to the I2mb polar phases with fully ordered oxygen tetrahedral chains based on the X-ray diffraction and transmission electron microscope results. A typical second-order phase transition from the polar I2mb to non-polar Imma phase is identified by the dielectric properties and Raman analysis. The phase transition temperature increases linearly with the x value in Sr2Sc1-xInxGaO5 ceramics, which is induced by the increased ordering degree of GaO4 tetrahedral chains. The fact that no thermal hysteresis was found in dielectric response during the heating and cooling process along with the Curie-Weiss fitting result confirmed the second-order phase transition in the ceramics. The present work identifies the phase transition temperature in Sr2ScGaO5-based brownmillerite ceramics, and it can be easily adjusted by substituting the cations at the octahedral center in the brownmillerite ceramics.

Self-Assembled Multiphase Nanocomposite SrCo1–xFexO3-δ Thin Films with Voltage-Controlled Magnetism for Spintronic Applications

SrCo1–xFexO3-δ (SCFO) grown on SrTiO3 substrates using pulsed laser deposition forms either a single-phase film or a two-phase self-assembled nanocomposite film depending on the oxygen partial pressure, PO2. A high PO2 of 150 mTorr during growth promotes phase separation of SCFO into a nanocomposite comprising Co oxide pillars embedded epitaxially in a Fe-rich SCFO matrix made up of brownmillerite and oxygen-deficient perovskite, despite the average composition of Sr/(Co + Fe) = 1. The SCFO matrix in the nanocomposite consists of a brownmillerite structure at a high Co content and an oxygen vacancy ordered perovskite at a high Fe content. In contrast, single-phase SCFO films grow at 20 mTorr. Ionic liquid gating of the nanocomposites oxidizes brownmillerite into perovskite with interlayer defects and renders the matrix phase magnetic with a saturation magnetization of 200 emu cm–3 at 173 K when x = 0.30.

Elaboration and characterization of the brownmillerite Ca2Fe2O5 synthesized by citrate sol-gel method. Application for photocatalytic H2-Production under visible light over Ca2Fe2O5/ZnO hetero-system

The present work is devoted to the synthesis of the ferrite Ca2Fe2O5 as photocatalyst crystallizing in the brownmillerite structure. The ternary oxide is prepared by sol-gel auto combustion and characterized by physical and electrochemical methods. The thermal analysis (TG/DSC) shows that, the formation of the brownmillerite is observed above 660 °C. The X-ray diffraction and BET analysis show respectively a single phase with an active surface area of ∼6 m2 g−1. The SEM micrographs exhibit an inhomogeneous structure formed by agglomeration of irregular shaped grains, confirmed by the laser granulometry analysis. The forbidden band (∼2.3 eV) determined from the diffuse reflectance, permits to explore ∼ 30% of the sun spectrum into chemical energy. The p-type comportment of Ca2Fe2O5 is demonstrated by the capacitance-potential (C−2 - E) graph with a flat band potential (Efb = 0.93 VSCE), due to oxygen over-stoichiometry. The negative potential of the conduction band (−1.06 VSCE) predicts the feasibility of the H2 generation. Indeed, Ca2Fe2O5 is chemical stable in a wide pH domain and is positively experimented as photocatalyst for the H2-production under visible light. The best performance is obtained in alkaline medium (NaOH, 0.1 M) with a mean evolution rate of 18 μmol g−1 min−1. However, Ca2Fe2O5 coupled to ZnO sol-gel (ZnO-SG) improves the catalytic performance. The H2 evolution rate over (Ca2Fe2O5/ZnO-SG) reached 24 μmol g−1 min−1 after 60 min. It has also been shown that ZnO–P, prepared by precipitation, is more efficient than that synthetized by sol-gel method (ZnO-SG) and TiO2–P25.

Revealing the co-doping effects of fluorine and copper on the formation and hydration of cement clinker

Fluorine in fluorite mineralizer and copper were easily doped together into cement clinker during cement kiln co-disposing wastes bearing Cu, yet that co-doping effects on the formation and hydration of cement clinker get few attention. The findings in this work revealed that the co-doping CaF2 (fluorine source) and CuO (copper source) into cement clinker showed a synergistic effect on the burnability, the immobilization rates of fluorine and copper in clinker phased were promoted. Compared with doping CaF2 into cement clinker, Co-doping CaF2 and CuO further enhanced the formation of alite and brownmillerite, while hindered the formation of tricalcium aluminate, and also triggered the right shift of characteristic diffraction peaks for C3S polymorphisms. It co-doping not only changed the microstructures, but also induced the visible defects on the surfaces of clinker mineral grains. In addition, most of fluorine would likely enter into the alite structure by replacing O2– of silicate phases, while most of copper would possibly enter into the brownmillerite structure by replacing Fe3+. Compared with the hydration process of cement clinker doped with CaF2, Co-doping CaF2 and CuO further accelerated the initial hydration process, obviously prolonged the induction period, but promoted the later hydration of co-doped clinker. Although the mechanisms about co-doping on the hydration process of co-doped clinker need more investigation, the above findings could extend the value-added utilization of wastes bearing Cu to reduce the large CO2 emission and energy/resource consumption in cement industry.

Electronic-structure evolution of SrFeO3–x during topotactic phase transformation

Oxygen-vacancy-induced topotactic phase transformation between the ABO2.5 brownmillerite structure and the ABO3 perovskite structure attracts ever-increasing attention due to the perspective applications in catalysis, clean energy field, and memristors. However, a detailed investigation of the electronic-structure evolution during the topotactic phase transformation for understanding the underlying mechanism is highly desired. In this work, multiple analytical methods were used to explore evolution of the electronic structure of SrFeO3−x thin films during the topotactic phase transformation. The results indicate that the increase in oxygen content induces a new unoccupied state of O 2p character near the Fermi energy, inducing the insulator-to-metal transition. More importantly, the hole states are more likely constrained to the dx 2–y 2 orbital than to the d3z 2–r 2 orbital. Our results reveal an unambiguous evolution of the electronic structure of SrFeO3–x films during topotactic phase transformation, which is crucial not only for fundamental understanding but also for perspective applications such as solid-state oxide fuel cells, catalysts, and memristor devices.

A molecular-level strategy to boost the mass transport of perovskite electrocatalyst for enhanced oxygen evolution

Perovskite oxides are of particular interest for the oxygen evolution reaction (OER) due to their high intrinsic activity. However, low surface area and nonpores in bulk phase generally limit the mass transport and thereby result in unsatisfactory mass activity. Herein, we propose a “molecular-level strategy” with the simultaneous modulation of the ordered pores on the oxygen-deficient sites along with sulfur (S) substitution on oxygen sites at the molecular level to boost the mass transport behavior of perovskite electrocatalyst for enhanced mass activity. As a proof of concept, the elaborately designed brownmillerite oxide Sr2Co1.6Fe0.4O4.8S0.2 (S-BM-SCF) shows approximately fourfold mass activity enhancement in 1 M KOH compared with the pristine SrCo0.8Fe0.2O3-δ (SCF) perovskite. Comprehensive experimental results, in combination with theoretical calculations, demonstrate that the intrinsic molecular-level pores in the brownmillerite structure can facilitate reactive hydroxyl ion (OH−) uptake into the oxygen-vacant sites and that S doping further promotes OH− adsorption by electronic structure modulation, thus accelerating mass transport rate. Meanwhile, the S-BM-SCF can significantly weaken the resistance of O2 desorption on the catalyst surface, facilitating the O2 evolution. This work deepens the understanding of how mass transport impacts the kinetics of the OER process and opens up a new avenue to design high-performance catalysts on the molecular level.

Magnetism in Mixed Valence, Defect, Cubic Perovskites: BaIn1-x Fe x O2.5+δ, x = 0.25, 0.50, and 0.75. Local and Average Structures.

The series BaIn1–xFexO2.5+δ, x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba2In2O5 (BaInO2.5) crystallizes in an ordered brownmillerite structure, Ibm2, and Ba2Fe2O5 (BaFeO2.5) crystallizes in a complex monoclinic structure, P21/c, showing seven Fe3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm3̅m, with long-range disorder on the small cation and oxygen sites. 57Fe Mössbauer studies indicate a mixed valency, Fe4+/Fe3+, for both the air-fired and argon-fired samples. The increased Fe3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mössbauer fraction. It appears that the Pm3̅m phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at ∼6 K [5 K] for x = 0.50 and at ∼22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent χ′(max) and spin glass behavior, while for x = 0.75, χ′(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe3+–Fe3+ antiferromagnetic exchange and ferromagnetic Fe3+–Fe4+ exchange. The PDF and 57Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm3̅m model. Fe Mössbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe3+ sites are largely fourfold-coordinated and Fe4+ sites are fivefold- or sixfold-coordinated.

Highly Selective Production of Syngas from Chemical Looping Reforming of Methane with CO2 Utilization on MgO-supported Calcium Ferrite Redox Materials

Chemical looping reforming with CO2 splitting (CLRS) is an attractive process that can be used for conversion of hydrocarbons into syngas, an industrially important intermediate that serves as a building block for other value-added products. Under the chemical looping approach, the oxygen carrier that provides lattice oxygen, instead of molecular oxygen, is used for methane partial oxidation. This work focuses on MgO-supported Ca2Fe2O5 redox materials as the oxygen carriers for simultaneous syngas production and CO2 utilization through thermochemical CO2 splitting using a two-reactor chemical looping system. We experimentally achieve a near 100% CH4 conversion and a high syngas selectivity of >98%, which is by far the highest in chemical looping reforming systems. Complete regeneration of the reduced oxygen carriers is obtained using CO2 with ~78% conversion, thereby operating close to the thermodynamic limit. Density functional theory calculations reveal that the lattice oxygen in corner-sharing octahedra of brownmillerite structure possessed by Ca2Fe2O5 acts as the efficient active sites for CO and H2 production. The formed oxygen vacancy significantly reduces the energy barriers of C-H cleavage and CO formation, leading to the reactivity and selectivity enhancement. MgO assists in reactivity enhancement by enabling higher degree of Ca2Fe2O5 dispersion along with increasing Ca2Fe2O5′s tolerance towards sintering. These findings will contribute to the systematic design of high-performance redox materials and chemical looping processes for syngas production with CO2 utilization.