Low-Temperature Nitridation of Manganese and Iron Oxides Using NaNH2 Molten Salt

Nitrogen-Rich Molybdenum Nitride Synthesized in a Crucible under Air

The triple bond in N2 is significantly stronger than the double bond in O2, meaning that synthesizing nitrogen-rich nitrides typically requires activated nitrogen precursors, such as ammonia, plasma-cracked atomic nitrogen, or high-pressure N2. Here, we report a synthesis of nitrogen-rich nitrides under ambient pressure and atmosphere. Using Na2MoO4 and dicyandiamide precursors, we synthesized nitrogen-rich γ-Mo2N3 in an alumina crucible under an ambient atmosphere, heated in a box furnace between 500 and 600 °C. Byproducts of this metathesis reaction include volatile gases and solid Na(OCN), which can be washed away with water. X-ray diffraction and neutron diffraction showed Mo2N3 with a rock salt structure having cation vacancies, with no oxygen incorporation, in contrast to the more common nitrogen-poor rock salt Mo2N with anion vacancies. Moreover, an increase in temperature to 700 °C resulted in molybdenum oxynitride, Mo0.84N0.72O0.27. This work illustrates the potential for dicyandiamide as an ambient-temperature metathesis precursor for an increased effective nitrogen chemical potential under ambient conditions. The classical experimental setting often used for solid-state oxide synthesis, therefore, has the potential to expand the nitride chemistry.

Combustion Reactions between Transition-Metal Chlorides and Sodium Amide and Their Ignition Temperature

Combustion reactions between metal chlorides and sodium amide proceed in a short time; however, these reactions must be carried out with appropriate safety measures. Investigating their ignition temperatures would facilitate safe handling and give kinetic insights about the reaction between powders. Here, we investigated the products of the reactions between metal chlorides and sodium amide and measured their ignition temperatures. The products were mainly composed of nitrides, metals, and sodium chloride. The reactions of 4d and 5d metal chlorides initiated the reaction below room temperature, while 3d metal chlorides, except copper chloride, initiated the reaction upon heating. We found the correlation between the ignition temperatures and the reaction energies of the combustion reaction.

Combination of theoretical and in situ experimental investigations of the role of lithium dopant in manganese nitride: a two-stage reagent for ammonia synthesis

Manganese nitride related materials are of interest as two-stage reagents for ammonia synthesis via nitrogen chemical looping. However, unless they are doped with a co-cation, manganese nitrides are thermochemically stable and a high temperature is required to produce ammonia under reducing conditions, thereby hindering their use as nitrogen transfer materials. Nevertheless, when lithium is used as dopant, ammonia generation can be observed at a reaction temperature as low as 300 °C. In order to develop strategies for the improvement of the reactivity of nitride materials in the context of two-stage reagents, it is necessary to understand the intrinsic role of the dopant in the mechanism of ammonia synthesis. To this end, we have investigated the role of lithium in increasing the manganese nitride reactivity by in situ neutron diffraction studies and N₂ and H₂ isotopic exchange reactions supplemented by DFT calculations.

Low-Temperature Nitridation of Fe3O4 by Reaction with NaNH2

Low-temperature soft chemical synthesis routes to transition-metal nitrides are of interest as an alternative to conventional high-temperature ammonolysis reactions involving large volumes of chemotoxic NH₃ gas. One such method is the reaction between metal oxides and NaNH₂ at ca. 200 °C to yield the counterpart nitrides; however, there remains uncertainty regarding the reaction mechanism and product phase assemblage (in particular, noncrystalline components). Here, we extend the chemical tool box and mechanistic understanding of such reactions, demonstrating the nitridation of Fe₃O₄ by reaction with NaNH₂ at 170-190 °C, via a pseudomorphic reaction. The more reduced Fe₃O₄ precursor enabled nitride formation at lower temperatures than the previously reported equivalent reaction with Fe₂O₃. The product phase assemblage, characterized by X-ray diffraction, thermogravimetric analysis, and ⁵⁷Fe Mössbauer spectroscopy, comprised 49-59 mol % ε-Fe_2+xN, accompanied by 29-39 mol % FeO_1-xN_x and 8-14 mol % γ″-FeN. The oxynitride phase was apparently noncrystalline in the recovered product but could be crystallized by heating at 180 °C. Although synthesis of transition-metal nitrides is achieved by reaction of the counterpart oxide with NaNH₂, it is evident from this investigation that the product phase assemblage may be complex, which could prove a limitation if the objective is to produce a single-phase product with well-defined electrical, magnetic, or other physical properties for applications. However, the significant yield of the FeO_1-xN_x oxynitride phase identified in this study opens the possibility for the synthesis of metastable oxynitride phases in high yield, by reaction of a metal oxide substrate with NaNH₂, with either careful control of H₂O concentration in the system or postsynthetic hydrolysis and crystallization.

Highly Coordinated Iron and Cobalt Nitrides Synthesized at High Pressures and High Temperatures

Highly coordinated iron and cobalt nitrides were successfully synthesized via direct chemical reaction between a transition metal and molecular nitrogen at pressures above approximately 30 GPa using a laser-heated diamond anvil cell. The synthesized novel transition metal nitrides were found to crystallize into the NiAs-type or marcasite-type structure. NiAs-type FeN could be quenched at ambient pressure, although it was gradually converted to the ZnS-type structure after the pressure was released. On the other hand, CoN was recovered with ZnS-type structure through a phase transition from NiAs-type structure at approximately a few gigapascals during decompression. Marcasite-type CoN₂ was also synthesized at pressures above approximately 30 GPa. High-pressure in situ X-ray diffraction measurement showed that the zero-pressure bulk modulus of marcasite-type CoN₂ is 216(18) GPa, which is comparable to that of RhN₂. This indicates that the interatomic distance of the N-N dimer in marcasite-type CoN₂ is short because of weak orbital interaction between cobalt and nitrogen atoms, as in RhN₂. Surprisingly, a first-principles electronic band calculation suggests that the NiAs-type FeN and CoN and marcasite-type CoN₂ exhibit metallic characteristics with magnetic moments of 3.4, 0.6, and 1.2 μ_B, respectively. The ferromagnetic NiAs-type structure originates from the anisotropic arrangement of transition atoms stacked along the c axis.

An efficient low-temperature route to nitrogen-doping and activation of mesoporous carbons for CO2 capture

Nitrogen-doping of soft-templated mesoporous carbons is achieved at low temperatures (230–380 °C) and high yields (>90%) by using sodium amide.