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

Abstract

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 H1-x , are likely the active sites. The chemistry of Rb2 /K2 [Mn(NH2 )x ] and Rb/K(NH2 )x H1-x with H2 /N2 and NH3 correlates closely with the catalytic performance.