Transition and Alkali Metal Complex Ternary Amides for Ammonia Synthesis and Decomposition

A new complex ternary amide, Rb-2[Mn(NH2)(4)], which simultaneously contains both transition and alkali metal catalytic sites, is developed. This is in line with the recently reported TM-LiH composite catalysts, which have been shown to effectively break the scaling relations and achieve ammonia synthesis under mild conditions. Rb-2[Mn(NH2)(4)] can be facilely synthesized by mechanochemical reaction at room temperature. It exhibits two temperature-dependent polymorphs, that is, a low-temperature orthorhombic and a high-temperature monoclinic structure. Rb-2[Mn(NH2)(4)] decomposes to N-2, H-2, NH3, Mn3N2, and RbNH2 under inert atmosphere; whereas it releases NH3 at a temperature as low as 80 degrees C under H-2 atmosphere. Those unique behaviors enable Rb-2[Mn(NH2)(4)], and its analogue K-2[Mn(NH2)(4)], to be excellent catalytic materials for ammonia decomposition and synthesis. Experimental results show both ammonia decomposition onset temperatures and conversion rates over Rb-2[Mn(NH2)(4)] and K-2[Mn(NH2)(4)] are similar to those of noble metal Ru-based catalysts. More importantly, these ternary amides exhibit superior capabilities in catalyzing NH3 synthesis, which are more than 3 orders of magnitude higher than that of Mn nitride and twice of that of Ru/MgO. The in situ SR-PXD measurement shows that manganese nitride, synergistic with Rb/KH or Rb/K(NH2)(x)H1-x, are likely the active sites. The chemistry of Rb-2/K-2[Mn(NH2)(x)] and Rb/K(NH2)(x)H1-x with H-2/N-2 and NH3 correlates closely with the catalytic performance.