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

Chemistry - a European journal 23(41), 9766 - 9771 (2017). doi:10.1002/chem.201702728
A new complex ternary amide, Rb$_2$[Mn(NH$_2$)$_4$], 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(NH$_2$)$_4$] 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(NH$_2$)$_4$] decomposes to N$_2$, H$_2$, NH$_3$, Mn$_3$N$_2$, and RbNH$_2$ under inert atmosphere; whereas it releases NH3 at a temperature as low as 80 °C under H2 atmosphere. Those unique behaviors enable Rb$_2$[Mn(NH$_2$)$_4$], and its analogue K$_2$[Mn(NH$_2$)$_4$], 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(NH$_2$)$_4$] and K$_2$[Mn(NH$_2$)$_4$] 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(NH$_2$)$_x$H$_{1−x}$, are likely the active sites. The chemistry of Rb$_2$/K$_2$[Mn(NH$_2$)$_x$] and Rb/K(NH$_2$)$_x$H$_{1−x}$ with H$_2$/N$_2$ and NH$_3$correlates closely with the catalytic performance.
Published by Wiley-VCH, Weinheim