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 , Mn₃ N₂ , 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 )_x H_1-x , are likely the active sites. The chemistry of Rb₂ /K₂ [Mn(NH₂ )_x ] and Rb/K(NH₂ )_x H_1-x with H₂ /N₂ and NH₃ correlates closely with the catalytic performance.