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Transition and Alkali Metal Complex Ternary Amides for Ammonia Synthesis and Decomposition
Author(s) -
Cao Hujun,
Guo Jianping,
Chang Fei,
Pistidda Claudio,
Zhou Wei,
Zhang Xilun,
Santoru Antonio,
Wu Hui,
Schell Norbert,
Niewa Rainer,
Chen Ping,
Klassen Thomas,
Dornheim Martin
Publication year - 2017
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201702728
Subject(s) - catalysis , monoclinic crystal system , chemistry , alkali metal , ternary operation , inorganic chemistry , ammonia , ammonia production , transition metal , orthorhombic crystal system , nitride , decomposition , crystallography , crystal structure , organic chemistry , layer (electronics) , computer science , programming language
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 NH 3 at a temperature as low as 80 °C under H 2 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 NH 3 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.