Ammonia decomposition mediated at nitrogen vacancies on NaCl-type binary metal nitrides supporting transition metal nanoparticles.

The ability of NaCl-type binary transition metal nitrides (incorporating La, Ce, Y, Zr or Hf) to act as catalytic supports facilitating ammonia decomposition was examined. The effect of nitrogen vacancies formed on nitrides can be understood in terms of the ionic radii of the metal cations. A clear correlation between the N2 desorption temperature and catalytic activity was found.

BaTiO3-xNy: Highly Basic Oxide Catalyst Exhibiting Coupling of Electrons at Oxygen Vacancies with Substituted Nitride Ions.

The base strength of oxide catalysts is controlled by the electron charge distribution between cations and anions, with unsaturated oxygen ions that have lone pair electrons typically acting as basic sites. Substitution of oxide ions with anions that have different valences, such as nitride and hydride ions, can often generate basic sites. It is plausible that electrons trapped at oxygen vacancy sites could provide increased electron density and shift the highest occupied molecular orbital energy levels of anions upward in the case that the oxygen vacancies couple with surface-substituted anions. The present work demonstrates that high catalytic basicity can be obtained via site-selective doping of anions at face-sharing Ti2O9 dimer sites with oxygen vacancies in BaTiO3-x. This improved basicity stems from the coupling of substituted nitride ions to electrons at oxygen vacancies. The oxynitride BaTiO3-xNy was found to contain nitride ions that have increased electronic charge density on the basis of such interactions. Enhanced surface basicity following doping with nitride ion was also confirmed by CO2 temperature-programmed desorption and infrared spectroscopy in conjunction with the adsorption of CHCl3. The strong Lewis base sites resulting from the formation of the oxynitride evidently facilitated the catalytic activation of C-H bonds to promote Knoevenagel condensation reactions between aldehydes and active methylene compounds with pKa values of up to 28.9.

Ammonia Synthesis over Fe-Supported Catalysts Mediated by Face-Sharing Nitrogen Sites in BaTiO3-x Ny Oxynitride.

Invited for this Issue's cover is the group of Professor Hideo Hosono at Tokyo Institute of Technology. The Cover image explores the question which activation dominates N2 activation for ammonia synthesis. The Research Article itself is available at 10.1002/cssc.202300551.

Ammonia Synthesis over Fe-Supported Catalysts Mediated by Face-Sharing Nitrogen Sites in BaTiO3-x Ny Oxynitride.

Nitride and hydride materials have been proposed as active supports for the loading of transition metal catalysts in thermal catalytic ammonia synthesis. However, the contribution of nitrogen or hydride anions in the support to the catalytic activity for supported transition-metal catalysts is not well understood, especially for Fe-based catalysts. Here, we report that hexagonal-BaTiO3-x Ny with nitrogen vacancies at face-sharing sites acts as a more efficient support for Fe catalysts for ammonia synthesis than BaTiO3 or BaTiO3-x Hx at 260 °C to 400 °C. Isotopic experiments, in situ measurements, and a small inverse isotopic effect in ammonia synthesis have revealed that nitrogen molecules are activated at nitrogen vacancies formed at the interface between Fe nanoparticles and the support. Nitrogen vacancies on BaTiO3-x Ny can promote the activity of Fe and Ni catalysts, while electron donation and suppression of hydrogen poisoning by BaTiO3-x Hx are significant in the Ru and Co systems.

Hexagonal BaTiO(3-x)Hx Oxyhydride as a Water-Durable Catalyst Support for Chemoselective Hydrogenation.

We present heavily H--doped BaTiO(3-x)Hx (x ≈ 1) as an efficient and water-durable catalyst support for Pd nanoparticles applicable to liquid-phase hydrogenation reactions. The BaTiO(3-x)Hx oxyhydride with a hexagonal crystal structure (P63/mmc) was synthesized by the direct reaction of BaH2 and TiO2 at 800 °C under a stream of hydrogen, and the estimated chemical composition was BaTiO2.01H0.96. Density functional theory calculations and magnetic measurements indicated that such heavy H- doping results in a metallic nature with delocalized electrons and a low work function. The potential of BaTiO(3-x)Hx as a catalyst support was examined for the selective hydrogenation of unsaturated C-C bonds by Pd nanoparticles deposited on BaTiO(3-x)Hx. We found that the turnover frequency for phenylacetylene hydrogenation per total amount of Pd in Pd/BaTiO(3-x)Hx was the highest among the supported Pd catalysts reported to date. The strong electronic charge transfer between Pd and the support, as confirmed by X-ray photoelectron spectroscopy measurements, can be attributed to be responsible for such high catalytic activity. The combination of the BaTiO(3-x)Hx support and Pd nanoparticles provides for the selective hydrogenation of unsaturated C-C bonds and highlights the validity of catalyst design that integrates H- in support materials.

Ship-in-a-Bottle Synthesis of High Concentration of N2 Molecules in a Cage-Structured Electride.

We report the formation of neutral nitrogen molecules in the cages of [Ca12Al14O32]2+ (C12A7) framework compensated by extra-framework anions. NH3 treatment of C12A7 electride (C12A7:e-) at 800 °C leads to the formation of N2 and NH2- species in the C12A7 cages. N2 and NHx species in the cages are identified using the Raman spectroscopy of 14NH3 and 15NH3-treated C12A7:e-. The concentration of H and N in the C12A7 cages after NH3 treatment is ∼1021 cm-3. We propose a two-step mechanism, supported by density functional theory (DFT) modeling, of N2 incorporation into the C12A7 cages, i.e., incorporation of NH2- formed from decomposition of NH3 at C12A7:e- surface followed by the NH2- species reacting to form N2 molecules. Encapsulation of neutral molecules, as opposed to negatively charged species reported in C12A7 previously, offers new opportunities for trapping and storing gaseous substances in nanoporous materials.

Low-Temperature Synthesis of Perovskite Oxynitride-Hydrides as Ammonia Synthesis Catalysts.

Mixed anionic materials such as oxyhydrides and oxynitrides have recently attracted significant attention due to their unique properties, such as fast hydride ion conduction, enhanced ferroelectrics, and catalytic activity. However, high temperature (≥800 °C) and/or complicated processes are required for the synthesis of these compounds. Here we report that a novel perovskite oxynitride-hydride, BaCeO3-xNyHz, can be directly synthesized by the reaction of CeO2 with Ba(NH2)2 at low temperatures (300-600 °C). BaCeO3-xNyHz, with and without transition metal nanoparticles, functions as an efficient catalyst for ammonia synthesis through the lattice N3- and H- ion-mediated Mars-van Krevelen mechanism, while ammonia synthesis occurs over conventional catalysts through a Langmuir-Hinshelwood mechanism with high energy barriers (85-121 kJ mol-1). As a consequence, the unique reaction mechanism leads to enhancement of the activity of BaCeO3-based catalysts by a factor of 8-218 and lowers the activation energy (46-62 kJ mol-1) for ammonia synthesis. Furthermore, isotopic experiments reveal that this catalyst shifts the rate-determining step for ammonia synthesis from N2 dissociation to N-H bond formation.