Hydride ions (H−) are attracting increasing interest as alternative charge carriers to protons in electrochemical devices; however, the development of H–/e– mixed conductors remains challenging. Herein, we demonstrate a novel design strategy for hydride incorporation using oxide‐ion‐conducting transition metal oxides as precursors. Using Ba3V2O8 as the precursor, we successfully synthesized the first palmierite‐type oxyhydride, Ba3V2H0.5O7.5 (BVHO), via a topochemical ion exchange reaction. Structural analyses revealed that BVHO exhibits a unique crystal structure featuring a charge ordering of V5+ or V4+ at tetrahedral sites and V3+ or V2+ at octahedral sites, accompanied by a random distribution of H and O at the anion sites. Electrochemical impedance spectroscopy of a symmetric BVHO|Ba1.75LiH2.7O0.9|BVHO cell confirmed that BVHO functioned as a hydrogen reversible electrode. These results highlight the potential of oxide‐ion‐conducting oxides as effective precursors for creating novel oxyhydrides with distinct framework structures, thereby expanding the exploration space for H–·e− mixed conductors.

Currently, most commercial environmental catalysts depend on large amounts of platinum-group metals (PGMs) to achieve high purification efficiency against environmental pollutants such as CO and NO. However, such catalysts are not readily available and have high cost, while PGMs exhibit significant price volatility. Toward the development of PGM-free environmental catalysts, we focus on the Mars-van Krevelen mechanism in which lattice oxygen constituting transition metal oxides participates in the catalytic reaction. This study demonstrates the contribution of the lattice oxygen constituting the SrTi1-xCoxO3 perovskite to the CO oxidation activity. The catalytic activity of SrTi1-xCoxO3 increased with increasing Co substitution and reached a maximum at x = 0.2. By contrast, Co substitution at x ≥ 0.4 resulted in decreased CO oxidation activity. H2 temperature-programmed reduction and in situ X-ray diffraction (XRD) under a H2 atmosphere suggested that the catalytic activity of SrTi1-xCoxO3 corresponded well with the lattice oxygen release behavior; that is, among the SrTi1-xCoxO3 samples, Ti-rich SrTi0.8Co0.2O3 released lattice oxygen at the lowest temperature under the H2 atmosphere. As the amount of substituted Co increased, the lattice oxygen release temperature increased, indicating that the Co-O-Ti bond was more reactive than the Co-O-Co bond. The states of the lattice oxygen in SrTi1-xCoxO3 were investigated in detail through synchrotron XRD and O K-edge X-ray absorption spectroscopy. Notably, in the samples with x ≥ 0.4, which had a large number of Co-O-Co bonds, partial electron donation from lattice oxygen to Co species occurred, suggesting the strengthening of the Co-O-Co bonds. These results indicate that lattice oxygen, which forms relatively weak Co-O-Ti bonds in SrTi1-xCoxO3, acts as an effective active site for CO oxidation, providing valuable guidance for the rational design of PGM-free oxide catalysts.

Here, we report the synthesis and characterization of melilite-type A 2MnC 2O7 (A = Sr, Ba; C = Si, Ge) and the discovery of its unconventional oxygen storage and release properties. Unlike conventional Mn-containing oxygen storage materials driven by the Mn2+/Mn3+ redox couple, Ba2MnGe2O7+delta (BMG) does not require highly reducing atmospheres for oxygen release and exhibits reversible oxygen storage/release under oxygen-rich conditions at moderate temperatures (200-500 degrees C). Comprehensive compositional and structural analyses utilizing X-ray absorption spectroscopy, synchrotron in situ powder X-ray diffraction, single-crystal X-ray diffraction, and powder neutron diffraction revealed that the oxygen storage/release processes involve changes in the local coordination environment. Specifically, MnO4 tetrahedra in the reduced phase change into MnO5 trigonal bipyramids in the oxidized phase, accompanied by a distinct transformation from the fundamental melilite-type structure of Ba2MnGe2O7 (tetragonal, space group P421 m) to a 5a x 5a x 1c superstructure of Ba2MnGe2O7.455(4) (tetragonal, P4). BMG exhibits a maximum oxygen storage capacity of delta approximate to 0.45 and, notably, develops a distinctive blue color upon oxygen storage. This characteristic response suggests promising potential for various oxygen-related applications, such as oxygen sensors and oxygen-sensitive inorganic pigments.

High‐entropy oxides are attracting attention for catalysis, but there are relatively few detailed studies on their precise structure, hampering true detailed studies on fundamental properties affecting their activities. In addition, diffusion has been often characterized as generally slow in high‐entropy systems. Here, we determine the precise oxygen content and structure of the fluorite‐like high‐entropy oxide (La, Ce, Pr, Nd, Y)O1.68 and have identified a large oxygen storage capacity based on efficient Ce/Pr redox due to facile oxide diffusion pathways and suppression of sintering. The structure and composition were identified through a combined Rietveld refinement of X‐ray and neutron diffraction data, and the oxidation state of Ce and Pr was investigated by high energy resolution fluorescence detected–X‐ray absorption near edge spectra (HERFD–XANES). (La, Ce, Pr, Nd, Y)O1.68 utilizes the full redox range of Ce/Pr, resulting in a high oxygen storage cumulative capacity despite the lower content of Ce/Pr compared to other well‐known ceria derivatives. Diffusion pathway analysis by bond valence site energy mapping shows decreased barriers for oxide anion diffusion through the bulk, also benefiting redox reactions. The high‐entropy nature also suppresses sintering, resulting in better cycling performance. This results in a higher performance as a methane oxidation catalyst support. We also investigate its use as a NOx reduction catalyst support.

This study examines how the exposed crystal facets of SrTiO3 (STO) nanosheets affect the catalytic behavior of supported Pd nanoparticles for automotive exhaust purification. The Pd/STO(111)-facet nanosheet shows lower light-off temperatures for CO-C3H6 oxidation and NO reduction than the Pd/STO(100)-facet nanosheet, even though its surface area is much smaller. Structural analyses reveal that the STO(111) facet creates a much stronger metal-support interaction (MSI), stabilizing highly dispersed Pd nanoparticles (approximate to 4.6 nm) and maintaining both PdO and Pd0 during the reaction. This strong MSI suppresses sintering and supports dual oxidation states, leading to superior catalytic performance. In contrast, the weaker MSI on the STO(100) facet allows Pd to grow into large particles (approximate to 60.2 nm) with limited redox flexibility and fewer active sites, resulting in poorer activity. These results demonstrate that the facet-dependent MSI is the key to controlling Pd dispersion and oxidation-state dynamics, highlighting crystal facet engineering as an effective strategy for designing high-performance oxide-supported catalysts.

We investigated the photoelectrical conversion properties of various rare-earth ferrites (REFeO3) as photoelectrochemical capacitor cathode materials. Among electrodes of LaFeO3 with different crystallinities, the electrode using it prepared by solid-state synthesis showed the highest photovoltage. Its highest crystallinity probably suppressed the electron-hole recombination to promote the hole accumulation. In various REFeO3 (LaFeO3, NdFeO3, GdFeO3, and ErFeO3), electrical conductivity and photovoltage were increased with increasing rare-earth ion size. We consider that hole accumulation was facilitated because overlapping between Fe 3d and O 2p orbitals became weaker by broadening of the Fe-O-Fe bonding angle with increasing rare-earth ion size in the orthorhombic perovskite structure.

Precise control of metal-metal oxide interfaces is crucial for developing efficient heterogeneous catalysts, yet strategies for tuning these junctions remain limited. This study shows that the heterojunction interface between Pd metal and a metal oxide can be controlled by adjusting the composition of Mg-Al-Mn spinel oxides. H2 reduction at 900 °C induces the aggregation of Pd metal particles (>20 nm) on γ-Al2O3 with a defective spinel structure, while MgAl2O4 suppresses aggregation, yielding Pd metal nanoparticles with a size of around 10 nm. Interestingly, when the Pd catalyst is supported on MgAlMnO4, where parts of the Al3+ ions are replaced by Mn3+ ions, a core-shell structure forms, consisting of Pd-Mn alloy particles (≈10 nm) encapsulated by a thin Mn oxide layer. In NO reduction with C3H6 and CO, the reduced Pd/MgAl2O4 and Pd/MgAlMnO4 catalysts exhibit considerably higher activities compared with that of the reduced Pd/Al2O3. Among these catalysts, the reduced Pd/MgAlMnO4 shows superior N2 selectivity compared with that of the reduced Pd/MgAl2O4. The core-shell structure in the reduced Pd/MgAlMnO4 disassembles during the catalytic reaction; that is, a spontaneous migration of Mn ions slightly exposes the Pd metal surface. This unique structure, formed via the core-shell structure, contributes to high catalytic activity and N2 selectivity. These results demonstrate that precise composition control of spinel oxides enables the rational design of Pd-oxide interfaces, offering a versatile strategy for developing thermally stable and highly selective catalysts for NO reduction and related reactions.

Oxygen storage materials (OSMs) play a significant role in automotive catalysts and oxygen separation systems. Here, we examine the oxygen storage ability of Ruddlesden-Popper-type layered perovskites A n+1Fe n O3n+1-delta (A = Sr, La) using temperature-programmed reduction with H2 (H2-TPR) and thermogravimetry (TG) measurements. As a result, we found that the oxygen storage capacity exhibited an increasing trend with the number of perovskite layers. Furthermore, time-resolved X-ray diffraction measurements revealed reaction dynamics during the redox reactions, visualizing the process of phase changes and reaction times. Among the compounds we examined, LaSr3Fe3O10-delta with triple perovskite layers (n = 3) has high structural stability and favorable oxygen storage characteristics. From this perspective, we propose a perovskite-based OSM design guideline to combine oxygen storage perovskite layers and robust rock-salt layers. These findings provide valuable insights into the design of oxygen storage materials.

To strategically realize the epitaxial growth of heterogeneous catalysts, it is necessary to use highly crystalline ceramic materials, which is a major disadvantage of catalyst materials in terms of surface area. Therefore, it is difficult to intentionally apply epitaxial growth in the design of catalyst materials. Against this background, this study reveals that a distorted PdO is epitaxially bonded to the ab surface of Sr3Ti2O7, which is a ceramic material synthesized by calcination at 1273 K, by loading a very small amount of a Pd species on it. Density functional theory calculations suggest that the charge-transfer from Sr3Ti2O7 to PdO induces an epitaxial junction to produce a structural fluctuation of PdO. Pd/Sr3Ti2O7 catalyst with a unique heterojunction exhibits an excellent catalytic activity for the purification of automotive exhaust gases. In addition, the Pd/Sr3Ti2O7 catalyst is more active than the benchmark Pd/Al2O3 catalyst despite its extremely small surface area. This study demonstrates that ceramic materials can be used as catalyst materials by precisely designing their heterojunctions.

Electrochemically inserting and extracting hydrogen into and from solids are promising ways to explore materials’ phases and properties. However, it is still challenging to identify the structural factors that promote hydrogen insertion and extraction and to develop materials whose functional properties can be largely modulated by inserting and extracting hydrogen through solid-state reactions at room temperature. In this study, guided by theoretical calculations on the energies of oxygen reduction and hydrogen insertion reactions with oxygen-deficient perovskite oxides, we demonstrated that the oxygen vacancy ordering in Sr(Fe1−xCox)Oy (SFCO) epitaxial films can be stabilized by increasing the Co content (x ≥ 0.3) and revealed that it plays a key role in promoting proton accommodation into the SFCO lattice. We also show that the electrical resistance of SFCO films can be reversibly modulated by electrochemical proton insertion and extraction, and the modulation exceeds three orders of magnitude for Sr(Fe0.5Co0.5)O2.5 epitaxial films. Our results provide guidelines for controlling material properties through the insertion and extraction of hydrogen and for designing and exploring hydrogen-insertion materials. Hydrogen insertion and extraction in solids is a way to explore materials’ phases and properties. Here authors investigate the stabilisation of oxygen vacancy ordering in strontium iron cobalt oxides epitaxial films and their impact on proton accommodation and electrical resistance modulation.

Platinum group metal-free catalysts can be designed by maximizing the reactivity of structural oxygen in metal oxide catalysts. This study indicates that the symmetry of the perovskite oxide affects the CO oxidation activity and the release behavior of oxygen in the perovskite lattice. Specifically, cubic-type SrTi0.8Co0.2O3 and hexagonal-type BaTi0.8Co0.2O3 showed much higher CO oxidation activities than orthorhombic-type CaTi0.8Co0.2O3. The oxygen-release properties evaluated under H2 flow show that SrTi0.8Co0.2O3 and BaTi0.8Co0.2O3 exhibit better performance and structural stability than CaTi0.8Co0.2O3. Density functional theory calculations of the energy required for the oxygen desorption reveal that the ΔR value, which denotes the difference in the Co-O bond lengths between the oxidized and reduced states of Co-doped ATiO3, is a valid indicator of the oxygen reactivity and structural stability. The ΔR value primarily depends on the symmetry of the crystal structure and the magnetic moment of the Co ion. The structural oxygen in SrTi0.8Co0.2O3 or BaTi0.8Co0.2O3, which have smaller ΔR values than that of CaTi0.8Co0.2O3, can be desorbed at lower energy. This study clearly demonstrates that the reactivity of structural oxygens in a metal oxide catalyst can be controlled by adjusting the symmetry of the perovskite unit.

The epitaxial growth of heterogeneous catalysts requires the use of highly crystalline metal oxide-based ceramics as catalyst supports. However, the catalytic applications of these ceramics are limited owing to their inherently low surface area. This study demonstrates that loading a very small amount of Pd species onto Sr3Ti2O7, a ceramic material synthesized via calcination of a precursor at 1273 K, leads to the formation of epitaxially bonded Pd oxide on the ab surface of Sr3Ti2O7. The Pd oxide particles with sizes ranging from 2 to 5 nm exhibit a superperiodic structure based on Fm (3) over barm space group, in which ordered and disordered Pd atoms are cyclically repeated along the a-axis of Sr3Ti2O7. Density functional theory calculations revealed that the charge transfer from Sr3Ti2O7 to Pd oxide facilitated the formation of an epitaxial junction and induced structural fluctuations within the Pd oxide. In addition, the Pd/Sr3Ti2O7 catalyst with a unique heterojunction exhibited excellent catalytic activity for the purification of automotive exhaust gases, despite its low surface area of only 2 m2 g-1. This study demonstrates that the catalytic potential of metal oxide-based ceramics can be realized through the precise engineering of the heterojunction interface.

Electrochemical protonation provides ways to control physical properties and even explore unprecedented phases of solid-state materials. While how proton accumulation changes materials' properties is investigated, how protonation of solids can be controlled and promoted remains an enigmatic puzzle. In the work reported here, the influence of electrochemical proton injection duration (t(Vg)) is investigated on the protonation of SrCoO2.5 (SCO) films in electric-field-effect transistor structures with gate layers of the proton-conducting electrolyte Nafion. The proton concentration accumulated in SCO films varies depending on the duration of the proton injection. When protons are injected in a relatively short t(Vg) (<= 600 s), the hydrogen concentration accumulated in SCO film increases with increasing t(Vg), reaching the maximum proton concentration of approximate to 1.9 per formula unit of SCO for the t(Vg) = 600 s case. On the other hand, when t(Vg) is longer than 900 s, the proton concentration decreases with t(Vg), implying the occurrence of counterreactions that extract protons from protonated SCO and oxidize the channel. These observations indicate that protons accumulated at the Nafion/SCO interface play a role in the protonation of SCO films and that suppressing the interfacial proton accumulation is the key to maximizing the proton concentration accumulated in SCO films.

The crystal structure of Co-based perovskite oxides (ACoO(3)) can be controlled by adjusting the A-site elements. In this study, we synthesized Y1-xBaxCoO3-delta (x = 0, 0.5, and 1.0) via a coprecipitation method and investigated their CO oxidation performances. YCoO3 (x = 0; cubic perovskite oxide; Pbnm) shows a higher catalytic performance than Y0.5Ba0.5CoO2.72 (x = 0.5; A-site-ordered double perovskite oxide; P4/nmm), which exhibits high oxygen nonstoichiometric properties, and BaCoO3 (x = 1.0; hexagonal perovskite oxide; P6(3)/mmc), which contains high-valent Co4+ species. To elucidate the reaction mechanism, we conducted isotopic experiments with CO and O-18(2). The CO oxidation reaction on YCoO3 proceeds via the Langmuir-Hinshelwood mechanism, which is a surface reaction of CO and O-2 gas that does not utilize lattice oxygen. Because of the significantly smaller specific surface area of YCoO3 compared with that of the reference Pt/Al2O3, the bulk features of the crystal structures affect the catalytic reaction. When density functional theory is applied, YCoO3 clearly exhibits semiconducting properties in the ground state with the diamagnetic t(2g)(6)e(g)(0) states, which can translate to a magnetic t(2g)(5)e(g)(1) configuration upon excitation by a relatively low energy of 0.64 eV. We propose that the unique nature of YCoO3 activates oxygen in the gas phase, thereby enabling the smooth oxidation of CO. This study demonstrates that the bulk properties originating from the crystal structure contribute to the catalytic activity and reaction mechanism.

The production of high-value-added chemicals and their raw materials by partial oxidation of methane (POM) is advantageous. The screening of 31 simple oxide catalysts for direct POM showed that ZrO2...

A lanthanum silicate La9.33Si6O26 (LSO) crystallizes in an apatite-type structure and has been known as a promising oxide-ion conductor. Here, we report the activity of LSO for catalytic partial oxidation of methane (CPOX) to synthesis gas. The LSO catalyst demonstrated relatively high catalytic activity from 500 to 700 degrees C, with CH4 conversion reaching 22.1% at 700 degrees C while retaining moderate CO and H2 selectivities of 20-60%. Notably, LSO exhibited higher CPOX activity than non-apatite-type La2SiO5 despite their similar specific surface areas. The higher CPOX activity of LSO is likely attributed to its structural superiority involving mobile oxide ions in the crystal structure. The reaction kinetic study showed that the reaction orders for methane and oxygen in the CPOX reaction over the LSO catalyst were 0.69-0.73 and 0.08-0.21, respectively. Furthermore, the small contribution of adsorbed O species generated from gas-phase O2 molecules indicated that the lattice oxygen may be involved in the reaction mechanism. The kinetic isotope effect (KIE) study using a CD4 suggested that C-H bond breaking is the rate-determining step of CPOX over LSO. The oxide-ion-conductive lanthanum silicate La9.33Si6O26 with an apatite-type structure exhibits high activity for the catalytic partial oxidation of methane (CPOX).

The metal-support interaction (MSI) between platinum-group metals, such as Pd or Rh, and metal oxides significantly affects the catalytic performance. This study shows the catalytic activity for purifying exhaust gases on a Pd catalyst supported on Y1-xBaxCoO3-delta, which is a perovskite structure with different symmetries depending on the ratio of Y and Ba. For the NO-CO-C3H6-O-2 reaction, the catalytic activity of Pd/YCoO3 is considerably higher than that of Pd/Y0.5Ba0.5CoO3-delta or Pd/BaCoO3 and comparable with that of Pd/Al2O3 even though the surface area of the Pd/YCoO3 catalyst is extremely small. In Pd/YCoO3, a unique MSI occurs between Pd oxide and YCoO3, and Pd oxide is thinly dispersed on YCoO3. By contrast, an aggregated PdO species is observed on BaCoO3 and Y0.5Ba0.5CoO3-delta. Density functional theory calculations indicate that charge transfer from Pd oxide to YCoO3 induces the formation of Pd-O-Co bonds and suppresses the aggregation of the Pd species. These findings highlight the importance of MSIs and offer valuable insights into the development of advanced catalysts.

This study demonstrates the reaction behavior during the purification of model automotive exhaust gases over Pd catalysts before and after thermal degradation. In particular, to investigate the relationship between the Pd state and the reaction behavior of Pd/Al2O3 and Pd/CeO2-ZrO2 (CZ), operando X-ray absorption spectroscopy measurements were performed during purifying exhaust gases over real and model catalysts mimicking the degradation of Pd particles and CZ supports after accelerated aging tests. The NO reduction activity of the aggregated Pd metal species was as high as that of the highly dispersed Pd species, but hydrocarbon (HC) poisoning was significantly enhanced by the aggregation of Pd metal particles caused by thermal aging. The existence of a three-phase boundary (TPB) between the CZ, the Pd particles, and the gas phase strongly affected the catalytic activity at low temperatures, and the presence of a sufficient TPB facilitated the combustion of unburned HCs owing to the oxygen storage performance of CZ. Thus, the TPB reduced the poisoning of the precious metal surface by HC species at low temperatures. Therefore, the findings of this study will facilitate the development of next-generation gas purification catalysts with high activity and durability. Operando X-ray Absorption Spectroscopy revealed the importance of the three-phase boundary between Pd nanoparticles, CeO2, and the gas phase for efficient conversion of toxic CO, hydrocarbons, and NOx from simulated automobile exhaust to CO2, H2O, and N2. image

Oxygen storage materials (OSMs) have potential applications in various fields, including catalysis, wherein a high oxygen storage capacity is desired because lattice oxygen and oxygen vacancies can act as reactants in catalytic reactions. In this study, we demonstrated that orthorhombic FeNbO4 has a high oxygen storage capacity, making it an effective catalyst for NO reduction and CO oxidation. FeNbO4 could release and store lattice oxygen via reversible topotactic transformations while maintaining cation ordering in its crystal structure. The lattice oxygen release properties were significantly enhanced by doping FeNbO4 with Ti at the Nb sites. The oxygen storage capacity of Ti-doped FeNbO4 was further improved by reduction and reoxidation pretreatments at 773 K, in which sufficient lattice oxygen was released under H-2 atmosphere and reoxidized under O-2 atmosphere. In NO reduction by CO in the presence of O-2, Ti-doped FeNbO4 having high oxygen storage performance exhibited an excellent catalytic activity comparable to that of Pt/Al2O3, despite the absence of platinum group metals (PGMs). This could be explained by the participation of oxygen vacancies in the reaction via the Mars-van Krevelen mechanism, as revealed by temperature-programmed reactions with NO and CO. We propose that a catalyst design utilizing the highly reactive lattice oxygen and oxygen vacancies in OSMs can circumvent the need for expensive PGMs.