New Triclinic Perovskite-Type Oxide Ba5CaFe4O12 for Low-Temperature Operated Chemical Looping Air Separation.

We present the discovery of Ba5CaFe4O12, a new iron-based oxide with remarkable properties as a low-temperature driven oxygen storage material (OSM). OSMs, which exhibit selective and rapid oxygen intake and release capabilities, have attracted considerable attention in chemical looping technologies. Specifically, chemical looping air separation (CLAS) has the potential to revolutionize oxygen production as it is one of the most crucial industrial gases. However, the challenge lies in utilizing OSMs for energy-efficient CLAS at lower temperatures. Ba5CaFe4O12, a cost-competitive material, possesses an unprecedented 5-fold perovskite-type A5B5O15-δ structure, where both Fe and Ca occupy the B sites. This distinctive structure enables excellent oxygen intake/release properties below 400 °C. This oxide demonstrates the theoretical daily oxygen production rate of 2.41 mO23 kgOSM-1 at 370 °C, surpassing the performance of the previously reported material, Sr0.76Ca0.24FeO3-δ (0.81 mO23 kgOSM-1 at 550 °C). This discovery holds great potential for reducing costs and enhancing the energy efficiency in CLAS.

Tin(II)-Based Metal-Organic Frameworks Enabling Efficient, Selective Reduction of CO2 to Formate under Visible Light.

Certain metal complexes are known as high-performance CO2 reduction photocatalysts driven by visible light. However, most of them rely on rare, precious metals as principal components, and integrating the functions of light absorption and catalysis into a single molecular unit based on abundant metals remains a challenge. Metal-organic frameworks (MOFs), which can be regarded as intermediate compounds between molecules and inorganic solids, are potential platforms for the construction of a simple photocatalytic system composed only of Earth-abundant nontoxic elements. In this work, we report that a tin-based MOF enables the conversion of CO2 into formic acid with a record high apparent quantum yield (9.8 % at 400 nm) and >99 % selectivity without the need for any additional photosensitizer or catalyst. This work highlights a new MOF with strong potential for photocatalytic CO2 reduction driven by solar energy.

Tunable Oxygen Intake/Release Characteristics of Brownmillerite-Type Ca2AlMnO5+δ Involving Atomic Defect Formations.

The oxygen intake/release characteristics were systematically studied for Ca2AlMnO5+δ samples synthesized under precisely controlled oxygen pressures. Both the oxygen storage capacity (OSC) and operating temperature were systematically lowered as the oxygen pressure in the firing atmosphere increased. Notably, the sample fired under a 1% O2 atmosphere exhibited sufficiently large OSC and superior oxygen intake/release kinetics to the pristine sample synthesized in an anaerobic condition. The high-angle annular dark-field scanning TEM observation revealed that the samples contain defects in their atomic arrangement when fired in oxygen-rich atmospheres. This result indicates that the oxygen intake/release characteristics of Ca2AlMnO5+δ are sensitive to the synthesis condition and widely tunable even without chemical substitutions.

Improvement of the Oxygen Storage/Release Speed of YBaCo4O7+δ Synthesized by a Glycine-Complex Decomposition Method.

The present study explores the oxygen storage capacity of YBaCo4O7+δ prepared by a glycine-complex decomposition method. We reported for the first time that the YBaCo4O7+δ sample was successfully synthesized at such a low temperature of 800 °C by this method. The YBCO-800 N sample exhibited a faster oxygen absorption/desorption speed than that of high calcination temperature samples, and the time required for complete oxygen storage/release was 5 and 6 min at 360 °C, respectively. Moreover, the superior performance observed for this product in the temperature swing adsorption process makes it a promising candidate in oxygen production technologies. This research demonstrated that the glycine-complex decomposition method is an effective method for improving the oxygen storage property of YBaCo4O7+δ and provides a new insight into designing other novel oxygen storage materials.

Electrochemical Crystal Growth of Titanium Oxyfluorides-A Strategy for Development of Electron-Doped Materials.

We report on the growth of single crystals of an electron-doped titanium oxyfluoride, Li2Ti(O,F)3, employing high-temperature electrolysis of TiO2 with a eutectic Li2MoO4-LiF melt. Greenish octahedral-shaped crystals (∼30 µm in size) with a cubic rocksalt-type structure were successfully obtained by precisely tuning the applied voltage. The temperature-dependent magnetic susceptibility data revealed a paramagnetic behavior at low temperatures, ensuring the presence of Ti3+ ions (mean valence number of +3.78; F/Ti ∼ 0.15). The crystals exhibited clear visible-light absorption and produced H2 from water in the presence of a sacrificial reagent under UV-light irradiation. Li2Ti(O,F)3 more efficiently produced H2 compared with a nondoped oxyfluoride Li5Ti2O6F, likely due to the doped electrons for the former. This work highlights a promising electrochemical approach toward growing electron-doped oxyfluoride crystals.

Low temperature synthesis of barium oxynitridosilicates using BaCN2 and SiO2.

Barium oxynitridosilicates, Ba3Si6O12N2 and Ba3Si6O9N4, were obtained from a mixture of BaCN2 and SiO2 at 800 °C, which is several hundred degrees lower than the temperature required in solid state reactions using BaCO3, SiO2 and Si3N4. The low-temperature formation mechanism was investigated by thermogravimetry analysis in conjunction with gas chromatography and mass spectroscopy. The phase ratio between the oxynitridosilicates was controlled by tuning the reaction temperature, duration, and atmosphere. Almost single-phase Ba3Si6O12N2 was obtained by reaction at 800 °C for 15 h under a N2 atmosphere, but the product changed to Ba3Si6O9N4 after 50 h at 800 °C or by heating at 950 °C for 15 h. The photoluminescence properties of Eu-doped products obtained at 800 °C using a mixture of BaCN2 : Eu and SiO2 were investigated.

Amorphous High-Surface-Area Aluminum Hydroxide-Bicarbonates for Highly Efficient Methyl Orange Removal from Water.

Amorphous high-surface-area aluminum hydroxide-bicarbonates were synthesized starting from AlCl3, base, and bicarbonate in water. Composites with a chemical formulas of [Al13O4(µ-OH)24(H2O)6.5(OH)5.5](HCO3)1.5 (I-NaOH) and [Al13O4(µ-OH)24(H2O)6(OH)6](HCO3) (I-NH3) were obtained by the use of NaOH/NaHCO3 and NH3/NH4HCO3 as base/bicarbonate, respectively. The surface area of the composites was highly dependent on the pH level of the synthetic solution, and composites with high surface areas (ca. 200 m2 g-1) were obtained around pH 7-8. Pore-size distributions determined from the N2 adsorption isotherms showed that I-NH3 and I-NaOH possess mainly large (pore radius rp > 3 nm) and small (rp < 3 nm) pores, respectively, despite similar surface areas. While SEM images showed that both I-NH3 and I-NaOH were aggregates of nanoparticles, the particles were more fused in I-NaOH, which is in line with the existence of small pores and the use of a stronger base (NaOH), which would facilitate the dehydration condensation reaction. The composites were applied as adsorbents to remove methyl orange (MO) from water. The time course of MO adsorption was readily fitted with a pseudo-second-order model, and over 90% MO removal was attained within 10 min with I-NH3, while I-NaOH showed much less MO removal (26%). The MO adsorption isotherm of I-NH3 was reproduced with a Langmuir model with an adsorption capacity of 154 mg g-1. Notably, the aluminum hydroxide-bicarbonates could not absorb methylene blue, which is a cationic dye, while anions (MO and PO43-) were readily absorbed. Solid-state 27Al MAS NMR spectra showed that the concentration of 5-coordinated aluminum species, which may serve as guest binding sites, was higher for I-NH3. These results show that electrostatic interaction between anionic MO and coordinatively unsaturated 5-coordinated cationic aluminum species and the large external surface area of I-NH3 contribute to the highly efficient MO adsorption.

Transition-Metal Distribution in Brownmillerite Ca2FeCoO5.

Ca2Fe2-xCoxO5 (0 ≤ x ≤ 1) with higher Co content, which crystallizes in a brownmillerite-type structure, is currently one of the best oxygen-evolution-reaction (OER) catalysts. Identifying the Fe/Co occupancies at the octahedral (Oh) and tetrahedral (Td) sites in the structure is the foundation for the understanding of the role of cobalt in each site and the exploration of further improvement in the OER activity. Here, we investigate the Fe/Co distribution in Ca2FeCoO5 by means of atomic-resolution energy dispersive X-ray spectroscopy in scanning transmission electron microscopy and dynamical image simulations combined with systematic density functional theory calculations. Our careful microscopic study reveals the absence of long-range Fe/Co order within the transition-metal (TM) layers, but cobalt is slightly enriched at the Td and Oh sites in the as-synthesized (1100 °C) and 800 °C annealed for a month samples, respectively. The observed Co site preferences are interpretable from the viewpoints of TM ionic size effect and ligand field effect, which are competitive around a crossover point at a certain temperature between 800 and 1100 °C. We also elucidate that the as-synthesized sample with Co enrichment at the Td site shows the better OER activity, and the optimum annealing temperature for more OER active Ca2FeCoO5 should be higher than the crossover temperature.

Activation of Catalytically Active Edge-Sharing Domains in Ca2FeCoO5 for Oxygen Evolution Reaction in Highly Alkaline Media.

Rechargeable zinc-air batteries are considered as one of the possible candidates to replace conventional lithium-ion batteries. One of the requirements for effective battery operation is an oxygen evolution reaction (OER) that needs to be generated in a highly alkaline electrolyte. The A2BB'O5 brownmillerite-type Ca2FeCoO5 electrocatalyst having a 57 Pbcm symmetry exhibits very high electrocatalytic activity toward OER in 4 mol dm-3 KOH. Our studies show that the electrocatalyst undergoes bulk amorphization upon OER and adequately activates catalytically active domains. The synchrotron radiation studies using the extended X-ray absorption fine structure (EXAFS) technique show that the central structural unit found in the polarized Ca2FeCoO5 is a cluster of edge-sharing CoO6 octahedra. The electrochemical data indicate that OER preferentially takes place on the edge-sharing CoO6 octahedra catalytic centers reconstructed in the brownmillerite-type electrocatalyst. The EXAFS second shell peaks at an interatomic distance of 2.8 Å are the fingerprints of the catalytically active domains.

Brownmillerite-type Ca2 FeCoO5 as a Practicable Oxygen Evolution Reaction Catalyst.

Here, we report remarkable oxygen evolution reaction (OER) catalytic activity of brownmillerite (BM)-type Ca2 FeCoO5 . The OER activity of this oxide is comparable to or beyond those of the state-of-the-art perovskite (PV)-catalyst Ba0.5 Sr0.5 Co0.8 Fe0.2 O3-δ (BSCF) and a precious-metal catalyst RuO2 , emphasizing the importance of the characteristic BM structure with multiple coordination environments of transition metal (TM) species. Also, Ca2 FeCoO5 is clearly advantageous in terms of expense/laboriousness of the material synthesis. These facts make this oxide a promising OER catalyst used in many energy conversion technologies such as metal-air secondary batteries and hydrogen production from electrochemical/photocatalytic water splitting.

Redox characteristics variations in the cation-ordered perovskite oxides BaLnMn2O5+δ (Ln = Y, Gd, Nd, and La) and Ca2Al1-xGaxMnO5+δ (0 ≤x≤ 1).

Two series of manganese-based oxygen storage materials, BaLnMn(2)O(5+δ) (Ln = Y, Gd, Nd, and La) and Ca(2)Al(1-x)GaxMnO(5+δ) (0 ≤x≤ 1), were synthesized and characterized to clarify cationic substitution effects on the oxygen intake/release behaviors of these materials. The thermogravimetric data revealed that the isovalent substitutions neighboring the active sites for oxygen intake/release are very effective. For BaLnMn(2)O(5+δ), fully-reduced δ≈ 0 products with larger Ln ions showed oxygen intake starting at lower temperatures in flowing O(2) gas, resulting in a systematic relationship between the onset temperature and the ionic radius of Ln(3+). Furthermore, the δ vs. P(O(2)) plots at 700 °C indicated a systematic trend: the larger the ionic size of Ln(3+) is, the larger oxygen contents the Ln-products exhibit. For Ca(2)Al(1-x)GaxMnO(5+δ), on the other hand, the temperature-induced oxygen intake/release characteristics appeared to be influenced by Ga-for-Al substitution, where the onset temperatures of oxygen release (upon heating) and oxygen intake (upon cooling) are decreased with the increasing Ga content (x).

Exfoliation of one-dimensional TiO5 chain in K2TiO3.

Potassium titanate, K2TiO3, with a chain structure of TiO5 polyhedra was exfoliated in an aqueous nitric acid solution of pH ≤ 1 and Tyndall scattering due to the colloid formed by the exfoliation was observed. The colloidal particle size measured by dynamic light scattering was approximately 300 nm. The stripe pattern observed by transmission electron microscopy for the aggregated grains after their drying suggested that the TiO5 chains were exfoliated in acid solution. X-ray absorption showed that the coordination polyhedron around Ti(4+) changed from a square pyramid in K2TiO3 powder to a distorted octahedron formed with the additional hydronium ions in the acidic solution.

Crystal structure and superconducting properties of hexagonal lithium-niobium oxynitride.

A hexagonal oxynitride (Li(0.88)□(0.12))Nb(3.0)(O(0.13)N(0.87))(4) was synthesized through ammonia nitridation of LiNb(3)O(8). The structural analysis revealed that this oxynitride consists of alternate stacking of octahedral and prismatic layers with different Li/Nb ratios: significant amounts of Li and Nb atoms (Li/Nb = 43/57) coexist in the octahedral layer, while the prismatic site is preferentially occupied by Nb (Li/Nb = 3/97). A metallic behavior was accompanied by an abrupt drop of electrical resistivity at about 3 K. Furthermore, large diamagnetism and specific-heat anomaly were observed below this temperature, suggesting the appearance of superconductivity in the Li-Nb oxynitride.

Positive and negative magnetodielectric effects in A-site ordered (BiMn3)Mn4O12 perovskite.

A-site ordered perovskite (BiMn3)Mn4O12 was synthesized through a high-pressure synthesis route at 5 GPa and found to exhibit two magnetic transitions and to show either a positive or a negative magnetodielectric effect depending on the temperature range/magnetic state.