The problem of activating N and its subsequent hydrogenation to form NH 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 N, but without catalytic formation of NH. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH and BaTiOH) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH under H/N flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g··h) almost comparable to conventional supported Ru catalysts such as Cs-Ru/MgO or Ru/BaTiO 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.

produced by the Haber-Bosch process, typically using fused iron as the catalyst. However, the iron-based catalyst requires harsh reaction conditions (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) and catalysts that work under mild conditions are therefore much needed. This may become more relevant in the future because as the needs for ammonia shift from fertilizer to energy, new plants operating under different conditions (lower pressure, alternate N 2 / H 2 ratios, etc.) may become necessary.In terms of the catalyst metal, ruthenium (Ru) is one of the most promising candidate elements for ammonia synthesis under mild conditions. Ru-based catalysts on carbon [3] and MgO [4] have also been extensively reported. Regarding support materials, over the most recent past few years, a number of new studies have focused on new catalysts incorporating metal hydride or electride in the support, such as 12CaO·7Al 2 O 3 :e − (C12A7:e − ), [5] [Ca 2 N]:e − , [6] CaH 2 , [7] Ca 2 NH, [8] Y 5 Si 3 , [9] LaScSi, [10] LiH, [11] and BaH 2 , [12] where all of these catalysts show high activities and unusual mechanisms. As related materials, we have recently examined BaTiO 2.5 H 0.5 and TiH 2 as a catalyst for NH 3 synthesis under Haber-Bosch conditions (400 °C, 5 MPa). [13] Titanium, being an early transition metal, was traditionally viewed as an inactive metal for catalytic NH 3 Ammonia is an attractive energy carrier for the hydrogen economy, given its high hydrogen density and ease of liquefaction. A titanate oxyhydride has recently been demonstrated that can catalyze ammonia synthesis without Ru or Fe metal, despite titanium being regarded as an inert element. Here, the synthesis activity of ammonia is examined when Ru, Fe, and Co particles are supported onto the oxyhydride BaTiO 2.5 H 0.5 . The activity of BaTiO 2.5 H 0.5 as support is significantly higher than BaTiO 3 . For example, the activity for Fe and Co increases by a factor of 70-400, making them more active than Ru/ MgO, one conventional Ru catalyst. In terms of mechanism, for Ru, H/D isotope studies show participation of lattice hydride in the catalytic cycle, while kinetic analysis shows reduced H 2 poisoning probably due to spillover. For Fe (and Co), the presence of hydride results in significantly lower activation energy and N 2 reaction order, likely due to strong electron donation from the oxyhydride. This metal-dependent support effect is further verified by N 2 isotopic exchange experiments. These perovskite-type oxyhydrides can be easily modified in terms of A-and B-site (A = Ba, B = Ti); the high potential for compositional variation and morphologies will expand the search for efficient catalysts for ammonia synthesis.

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 (E a) for H/D exchange and H2 release in BaTiO3–x H x (x = 0.35–0.60) and LaSrCoO3H0.70. It is revealed that, for each BaTiO3–x H x at a given hydride concentration (x), both H/D exchange and H2 release experiments provide similar E a values. For BaTiO3–x H x with different x, the obtained E a values significantly decrease with increasing x 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 (x < 0.4) includes oxide as well as hydride diffusion, whereas, for 0.4 < x (<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 LaSrCoO3H0.70 yielded a similar E 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.

Lithium–sulfur batteries are strongly expected to be the next-generation energy storage technology due to their superior theoretical specific capacity and energy density.

We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO3-xHx (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO3-xHx, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N(3-)/H(-)-exchanged product EuTi(4+)O2.82N0.12□0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N(3-)/O(2-) exchange, yielding a fully oxidized Eu(3+)Ti(4+)O2N with the GdFeO3-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO3 proceeded through a 1:1 exchange of N(3-) with O(2-) only above 600 °C and resulted in incomplete nitridation to EuTiO2.25N0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO2.82H0.18 play a crucial role in promoting anion (N(3-)/O(2-)) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.

The high-pressure synthesis of a manganese oxyhydride LaSrMnO3.3 H0.7 is reported. Neutron and X-ray Rietveld analyses showed that this compound adopts the K2 NiF4 structure with hydride ions positioned exclusively at the equatorial site. This result makes a striking contrast to topochemical reductions of LaSrMnO4 that result in only oxygen-deficient phases down to LaSrMnO3.5 . This suggests that high H2 pressure plays a key role in stabilizing the oxyhydride phase, offering an opportunity to synthesize other transition-metal oxyhydrides. Magnetic susceptibility revealed a spin-glass transition at 24 K that is due to competing ferromagnetic (Mn(2+) -Mn(3+) ) and antiferromagnetic (Mn(2+) -Mn(2) , Mn(3+) -Mn(3+) ) interactions.

Substitution of oxide anions (O2–) in a metal oxide for nitrogen (N3–) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden–Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N·2H2O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N·1.6H2O. Topochemical ammonolysis reaction of a Dion–Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N·2H2O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N·2H2O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N·2H2O modified with a Pt cocatalyst generated H2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (λ > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N·2H2O was an order of magnitude lower than that of K2LaTa2O6N·1.6H2O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N·2H2O was shorter than that in K2LaTa2O6N·1.6H2O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N·2H2O.

Nanozymes are a kind of nanomaterial mimicking enzyme catalytic activity, which has aroused extensive interest in the fields of biosensors, biomedicine, and climate and ecosystems management. However, due to the complexity of structures and composition of nanozymes, atomic scale active centers have been extensively investigated, which helps with in-depth understanding of the nature of the biocatalysis. Single atom nanozymes (SANs) cannot only significantly enhance the activity of nanozymes but also effectively improve the selectivity of nanozymes owing to the characteristics of simple and adjustable coordination environment and have been becoming the brightest star in the nanozyme spectrum. The SANs based sensors have also been widely investigated due to their definite structural features, which can be helpful to study the catalytic mechanism and provide ways to improve catalytic activity. This perspective presents a comprehensive understanding on the advances and challenges on SANs based sensors. The catalytic mechanisms of SANs and then the sensing application from the perspectives of sensing technology and sensor construction are thoroughly analyzed. Finally, the major challenges, potential future research directions, and prospects for further research on SANs based sensors are also proposed.