Selective Preparation of Macroporous Monoliths of Conductive Titanium Oxides Ti_<i>n</i>O_{2<i>n</i>–1} (<i>n</i> = 2, 3, 4, 6)

Monolithic conductive titanium oxides Ti_<i>n</i>O_{2<i>n</i>–1} (<i>n</i> = 2, 3, 4, 6) with well-defined macropores have been successfully prepared as a single phase, via reduction of a macroporous TiO₂ precursor monolith using zirconium getter. Despite substantial removal of oxide ions, all the reduced monoliths retain the macropore properties of the precursor, i.e., uniform pore size distribution and pore volume. Furthermore, compared to commercial porous Ebonex (shaped conductive Ti_<i>n</i>O_{2<i>n</i>–1}), the bulk densities (1.8 g cm^–3) are half, and the porosities (60%) are about 3 times higher. The obtained Ti_<i>n</i>O_{2<i>n</i>–1} (<i>n</i> = 2, 3, 4, 6) macroporous monoliths could find applications as electrodes for many electrochemical reactions

A Nearly Ideal One-Dimensional <i>S</i> = 5/2 Antiferromagnet FeF₃(4,4′-bpy) (4,4′-bpy =4,4′-bipyridyl) with Strong Intrachain Interactions

An ideal one-dimensional (1D) magnet is expected to show exotic quantum phenomena. For compounds with larger <i>S</i> (<i>S</i> = 3/2, 2, 5/2, ...), however, a small interchain interaction <i>J</i>′ tends to drive a conventional long-range ordered (LRO) state. Here, a new layered structure of FeF₃(4,4′-bpy) (4,4′-bpy = 4,4′-bipyridyl) with novel <i>S</i> = 5/2 (Fe³⁺) chains has been hydrothermally synthesized by using 4,4′-bpy to separate chains. The temperature-dependent susceptibility exhibits a broad maximum at high as 164 K, suggesting a fairly strong Fe–F–Fe intrachain interaction <i>J</i>. However, no anomaly associated with a LRO is seen in both magnetic susceptibility and specific heat even down to 2 K. This indicates an extremely small <i>J</i>′ with <i>J</i>′/<i>J</i> < 3.2 × 10^–5, making this new material a nearly ideal 1D antiferromagnet. Mössbauer spectroscopy at 2.7 K reveals a critical slowing down of the 1D fluctuations toward a possible LRO at lower temperatures

Sr₂FeO₃ with Stacked Infinite Chains of FeO₄ Square Planes

The synthesis of Sr₂FeO₃ through a hydride reduction of the Ruddlesden–Popper layered perovskite Sr₂FeO₄ is reported. Rietveld refinements using synchrotron and neutron powder diffraction data revealed that the structure contains corner-shared FeO₄ square-planar chains running along the [010] axis, being isostructural with Sr₂CuO₃ (<i>Immm</i> space group). Fairly strong Fe–O–Fe and Fe–Fe interactions along [010] and [100], respectively, make it an <i>S</i> = 2 quasi two-dimensional (2D) rectangular lattice antiferromagnet. This compound represents the end-member (<i>n</i> = 1) of the serial system Sr_<i>n</i>+1Fe_<i>n</i>O_2<i>n</i>+1, together with previously reported Sr₃Fe₂O₅ (<i>n</i> = 2) and SrFeO₂ (<i>n</i> = ∞), thus giving an opportunity to study the 2D-to-3D dimensional crossover. Neutron diffraction and Mössbauer spectroscopy show the occurrence of <i>G</i>-type antiferromagnetic order below 179 K, which is, because of dimensional reduction, significantly lower than those of the other members, 296 K in Sr₃Fe₂O₅ and 468 K in SrFeO₂. However, the temperature dependence of magnetic moment shows a universal behavior

Titanium-Based Hydrides as Heterogeneous Catalysts for Ammonia Synthesis

The problem of activating N₂ and its subsequent hydrogenation to form NH₃ has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N₂, but without catalytic formation of NH₃. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH₂ and BaTiO_2.5H_0.5) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH₃ under H₂/N₂ flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g·^–1·h^–1) almost comparable to conventional supported Ru catalysts such as Cs–Ru/MgO or Ru/BaTiO₃ that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activity for Ti, making this a previously overlooked element, but our results show that by introducing hydride, the repertoire of heterogeneous catalysts can be expanded to include formerly unexamined compositions without resorting to precious metals

High-Level Doping of Nitrogen, Phosphorus, and Sulfur into Activated Carbon Monoliths and Their Electrochemical Capacitances

The present report demonstrates a new technique for doping heteroatoms (nitrogen, phosphorus, and sulfur) into carbon materials via a versatile post-treatment. The heat-treatment of carbon materials with a reagent, which is stable at ambient temperatures and evolves reactive gases on heating, in a vacuum-closed tube allows the introduction of various heteroatom-containing functional groups into a carbon matrix. In addition, the sequential doping reactions give rise to dual- and triple-heteroatom-doped carbons. The pore properties of the precursor carbon materials are preserved through each heteroatom doping process, which indicates that independent tuning of heteroatom doping and nanostructural morphology can be achieved in various carbon materials. The electrochemical investigation on the undoped and doped carbon monolithic electrodes applied to supercapacitors provides insights into the effects of heteroatom doping on electrochemical capacitance

Promoted Hydride/Oxide Exchange in SrTiO₃ by Introduction of Anion Vacancy via Aliovalent Cation Substitution

We investigated topochemical anion exchange reactions for a Sc^III-substituted SrTi^IVO₃ perovskite, Sr­(Ti_1–<i>y</i>Sc_<i>y</i>)­O_{3–<i>y</i>/2}□_<i>y</i>/2 (<i>y</i> ≤ 0.1), using CaH₂. It was found that the initial introduction of a small amount of anion vacancies (<i>y</i>/2) is crucial to enhance the anion (H^–/O^2–) exchangeability. For example, hydride reduction of Sr­(Ti_0.95Sc_0.05)­O_2.975 yielded the oxyhydride SrTi_0.95Sc_0.05O_2.56H_0.41 in which the hydride concentration is increased by 33% with respect to pristine SrTiO₃ (leading to SrTiO_2.76H_0.24). This observation highlights the importance of anion vacancies to improve anion (H^–/O^2–) diffusion, which is a well-known strategy for improving oxide anion conductivity, and suggests that such a vacancy-assisted reaction could be applied to other anion exchange reactions (e.g., F^–/O^2– and N^3–/O^2–) to extend the solubility range

Hierarchically Porous Monoliths Based on N‑Doped Reduced Titanium Oxides and Their Electric and Electrochemical Properties

In this report, we demonstrate a novel synthesis method to obtain reduced titanium oxides with monolithic shape and with a well-defined hierarchically porous structure from the titanium-based network bridged with ethylenediamine. The hierarchically porous monoliths are fabricated by the nonhydrolytic sol–gel reaction accompanied by phase separation. This method allows a low-temperature crystallization into Ti₄O₇ and Ti₃O₅ at 800 and 900 °C, respectively, with N-doped carbon. These reduced titanium oxides are well-doped with N atoms even under argon atmosphere without NH₃, which accounts for the low-temperature reduction. The resultant monolithic materials possess controllable macropores and high specific surface area together with excellent electric conductivity up to 230 S cm^–1, indicating promise as a conductive substrate that can substitute carbon electrodes

Effect of Calcination Conditions on Porous Reduced Titanium Oxides and Oxynitrides via a Preceramic Polymer Route

A preceramic polymer route from Ti-based inorganic–organic hybrid networks provides electroconductive N-doped reduced titanium oxides (Ti_<i>n</i>O_{2<i>n</i>–1}) and titanium oxynitrides (TiO_<i>x</i>N_<i>y</i>) with a monolithic shape as well as well-defined porous structures. This methodology demonstrates an advantageously lower temperature of the crystal phase transition compared to the reduction of TiO₂ by carbon or hydrogen. In this study, the effect of calcination conditions on various features of the products has been explored by adopting three different atmospheric conditions and varying the calcination temperature. The detailed crystallographic and elemental analyses disclose the distinguished difference in the phase transition behavior with respect to the calcination atmosphere. The correlation between the crystallization and nitridation behaviors, porous properties, and electric conductivities in the final products is discussed

An Antiferro-to-Ferromagnetic Transition in EuTiO_3–<i>x</i>H_<i>x</i> Induced by Hydride Substitution

We have prepared the oxyhydride perovskite EuTiO_3–<i>x</i>H_<i>x</i> (<i>x</i> ≤ 0.3) by a low temperature CaH₂ reduction of pyrochlore Eu₂Ti₂O₇ and perovskite EuTiO₃. The reduced EuTiO_3–<i>x</i>H_<i>x</i> crystallizes in the ideal cubic perovskite (<i>Pm</i>3̅<i>m</i>), where O/H anions are randomly distributed. As a result of electron doping by the aliovalent anion exchange, the resistivity of EuTiO_3–<i>x</i>H_<i>x</i> shows metallic temperature dependence. Moreover, an antiferromagnetic-to-ferromagnetic transition is observed even when a small amount of hydride (<i>x</i> ∼ 0.07) is introduced. The Curie temperature <i>T</i>_C of 12 K is higher than those of any other EuTiO₃-derived ferromagnets. The ferromagnetism can be explained by the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction between the Eu²⁺ spins mediated by the itinerant Ti 3d electrons. The present study shows that controlling the oxide/hydride ratio is a versatile method to tune magnetic and transport properties

On Hydride Diffusion in Transition Metal Perovskite Oxyhydrides Investigated via Deuterium Exchange

Perovskite oxyhydrides may find diverse applications, ranging from catalysis, topochemical synthesis to solid state ionics, but the understanding of their hydride transport behavior has remained limited. Here, gaseous hydrogen exchange and release experiments were analyzed using the Kissinger method to estimate the activation energy (<i>E</i>_a) for H/D exchange and H₂ release in BaTiO_3–<i>x</i>H_<i>x</i> (<i>x</i> = 0.35–0.60) and LaSrCoO₃H_0.70. It is revealed that, for each BaTiO_3–<i>x</i>H_<i>x</i> at a given hydride concentration (<i>x</i>), both H/D exchange and H₂ release experiments provide similar <i>E</i>_a values. For BaTiO_3–<i>x</i>H_<i>x</i> with different <i>x</i>, the obtained <i>E</i>_a values significantly decrease with increasing <i>x</i> until around 0.4; beyond 0.4, it becomes nearly constant (200–220 kJ mol^–1). This observation suggests that the diffusion process in the low hydride concentration (<i>x</i> < 0.4) includes oxide as well as hydride diffusion, whereas, for 0.4 < <i>x</i> (<0.75), only hydride migrates, with second-nearest-neighbor (2NN) jumps as a rate-determining process, which is supported by DFT calculations. The Kissinger analysis of LaSrCoO₃H_0.70 yielded a similar <i>E</i>_a of 170–190 kJ mol^–1, consistent with the 2NN hopping scenario. The presented method provides a facile tool for designing and improving hydride conductivity in oxyhydrides regardless of the presence of electronic conductivity