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

Abstract

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.

Development and Recent Progress on Ammonia Synthesis Catalysts for Haber–Bosch Process

“…They both exhibited a significant activity increase over the best performing ternary nitride catalyst, which was Co 3 Mo 3 N, with activities of 11 mmol g −1 h −1 for K 2 [Mn(NH 2 ) 4 ], 6 mmol g −1 h −1 for Rb 2 [Mn(NH 2 ) 4 ], and 5.3 mmol g −1 h −1 for Co 3 Mo 3 N, all of which were measured at 400 °C and 1 MPa. [ 145,149 ] This increase in activity was also reported at a much‐reduced pressure, with the ternary amides tested at 1 MPa compared to 10 MPa for Co 3 Mo 3 N. Along with the results highlighted in the previous section where an increase in activity is noted when the support material for a Ru catalyst is changed from hydride to amide (CaH 2 to Ca(NH 2 ) 2 ), [ 105,116 ] this move from ternary nitrides to ternary amides may be the key next step in this class of catalysts.…”

“…Alongside the ternary nitrides that are highlighted in Table 6, Cao et al [ 149 ] have reported on ternary amides of transition and alkali metals. Both Rb 2 [Mn(NH 2 ) 4 ] and K 2 [Mn(NH 2 ) 4 ] were synthesized through ball milling the respective metals at room temperature under 0.7 MPa of NH 3 .…”

Complex Hydrides for Energy Storage, Conversion, and Utilization

“…Although the active phases are composed of TM and hydrides, amide and/or imide were often observed as intermediate species during the catalysis. Ternary amides of Rb 2 Mn(NH 2 ) 4 and K 2 Mn(NH 2 ) 4 , which contain transition metal Mn and Rb/K in one phase, were also demonstrated highly catalytically functional for ammonia synthesis …”

Compositional flexibility in Li–N–H materials: implications for ammonia catalysis and hydrogen storage

“…[12][13][14] Lithium imide based catalysts have shown comparable or superior performance to state-of-the-art ruthenium catalysts. [15][16][17][18] In this context, in situ neutron powder diffraction experiments indicated that the lithium imide becomes nonstoichiometric under operating conditions, with a mixed amide-imide composition dependent on the temperature and conversion. 15 Isotope exchange experiments have confirmed that this variable composition allows for bulk nitrogen and hydrogen exchange between the catalyst and ammonia.…”