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

A new complex ternary amide, Rb2[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. Rb2[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. Rb2[Mn(NH2)4] decomposes to N2, H2, NH3, Mn3N2, and RbNH2 under inert atmosphere; whereas it releases NH3 at a temperature as low as 80 °C under H2 atmosphere. Those unique behaviors enable Rb2[Mn(NH2)4], and its analogue K2[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 Rb2[Mn(NH2)4] and K2[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)xH1−x, are likely the active sites. The chemistry of Rb2/K2[Mn(NH2)x] and Rb/K(NH2)xH1−x with H2/N2 and NH3 correlates closely with the catalytic performance. Ammonia: The first case of using A2[Mn(NH2)4] (A=Rb and K) for low temperature NH3 synthesis and decomposition is presented. The ternary amides were synthesized through ball‐milling under NH3, before thorough structural characterization and evaluation of their ammonia synthesis properties.