Liquid-Crystalline Phases with Liquid Ammonia: Synthesis of Porous Si3N4, TiN, VN, and H2-Sorption of Si3N4 and Pd@Si3N4

Abstract

Liquid-crystalline phases in liquid ammonia were used to obtain meso- and microporous Si3N4, TiN, and VN. The liquid-crystalline phase was established at -50 °C with liquid ammonia as the polar phase, heptane as the nonpolar dispersant phase, dimethyldioctylammonium iodide (DDAI) as the surfactant, and heptylamine as the cosurfactant. Silicon(IV) iodide, tetrakis(dimethylamino)titan, and vanadium(IV) chloride were added and ammonolyzed in the liquid-crystalline phase. After the mixture contents were separated and washed, ammonolysis was completed by slow heating to 600 °C (TiN, VN) and 800 °C (Si3N4) in vacuum or forming gas (N2/H2). The obtained high-surface nitrides are characterized by high purity (e.g., Si3N4 with carbon content <3 at %), great specific surface area (Si3N4: 610 m2/g; TiN: 203 m2/g; VN: 63 m2/g), and great total pore volume (Si3N4: 1.5 cm3/g; TiN: 0.3 cm3/g; VN: 0.4 cm3/g). Si3N4 and TiN show considerable meso- and microporosity, whereas the less-stable VN only shows mesoporosity. Electron energy loss spectroscopy (EELS) proves low oxygen contents. As a proof-of-concept, uniform one-pot modification of Si3N4 with Pd nanoparticles (Pd@Si3N4) as well as the reversible hydrogen sorption of differently treated Si3N4 were studied, which results in a maximum H2 uptake of 2.5 wt % (at 25 °C, 40 bar).