Facile Uptake and Release of Ammonia by Nickel Halide Ammines

Abstract

Although major difficulties are experienced for hydrogen‐ storage materials to meet performance requirements for mobile applications, alternative fuel cell feedstocks such as ammonia can be stored in the solid state safely at high capacity. We herein describe the NiX2‐NH3 (X=Cl, Br, I) systems and demonstrate their exceptional suitability for NH3 storage (up to 43 wt % NH3 with desorption that begins at 400 K). The structural effects that result from the uptake of NH3 were studied by powder X‐ray diffraction (PXD), FTIR spectroscopy and SEM. NH3 release at elevated temperatures was followed by in situ PXD. The cycling capabilities and air stability of the systems were also explored. NH3 is released from the hexaammines in a three‐step process to yield the diammine, monoammine and NiX2 dihalides respectively and (re)ammoniation occurs readily at room temperature. The hexaammines do not react with air after several hours of exposure.

Ni(NH3)2(NO3)2—A 3-D Network through Bridging Nitrate Units Isolated from the Thermal Decomposition of Nickel Hexammine Dinitrate

“…The highest capacity (gravimetric density of ammonia) materials-typically hexammines-can be synthesised simply by the ambient temperature and pressure ammoniation of the respective transition metal salts. In an earlier study, we identified nickel salts as a promising group of ammonia storage materials [4]. Associated with this work, we were able to demonstrate that the reaction of ammonia with nickel nitrate hexahydrate induces ligand exchange and the growth of nickel nitrate hexammine crystals [5].…”

“…Unlike these copper and platinum compounds, which achieve three-dimensional connectivity through hydrogen bonds, Ni(NH3)2(NO3)2 is connected in all three dimensions via bridging nitrate anions. 4 ] octahedra connected through four bridging nitrate anions per metal centre (and hence, each individual nitrate ligand bridges two metal centres, Figure 4). The nitrate ligands are located in the equatorial positions with the ammonia ligands trans-to one-another in the axial positions.…”

“…This is consistent with the symmetry inequivalence of the two ammonia ligands in Ni(NH3)2(NO3)2 and hence supports the crystallographic model (where the two ammonia nitrogen positions are inequivalent). [4] given in blue and Ni(NH3)6(NO3)2 [5] given in black.…”

“…This can be attributed to the different bonding environments of the species. While the nitrate anion in hexammine nickel nitrate is not coordinated directly to the complex and [4] given in blue and Ni(NH 3 ) 6 (NO 3 ) 2 [5] given in black.…”

“…Assignment of the infrared bands in Ni(NH 3 ) 2 (NO 3 ) 2 in comparison to the documented values of Ni(en) 2 (NO 3 ) 2 (en = ethylenediamine) [17], Ni(NH 3 ) 2 Cl 2 [4], and Ni(NH 3 ) 6 (NO 3 ) 2 [5].…”