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

A new complex ternary amide, Rb [Mn(NH ) ], 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 [Mn(NH ) ] 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 [Mn(NH ) ] decomposes to N , H , NH , Mn N , and RbNH under inert atmosphere; whereas it releases NH at a temperature as low as 80 °C under H atmosphere. Those unique behaviors enable Rb [Mn(NH ) ], and its analogue K [Mn(NH ) ], to be excellent catalytic materials for ammonia decomposition and synthesis. Experimental results show both ammonia decomposition onset temperatures and conversion rates over Rb [Mn(NH ) ] and K [Mn(NH ) ] are similar to those of noble metal Ru-based catalysts. More importantly, these ternary amides exhibit superior capabilities in catalyzing NH 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(NH ) H , are likely the active sites. The chemistry of Rb /K [Mn(NH ) ] and Rb/K(NH ) H with H /N and NH correlates closely with the catalytic performance.