Zn Promotes Chemical Looping Ammonia Synthesis Mediated by LiH-Li₂ NH Couple

Chemical looping ammonia synthesis (CLAS) is a promising alternative route to ammonia production because of its advantages of avoiding competitive adsorption of N₂ and hydrogen source (H₂ O or H₂ ) and intervening the scaling relations in the catalytic process. Our previous studies showed that NH₃ can be synthesized at low temperatures via a CLAS mediated by an alkali or alkaline earth metal hydride-imide couple with the aid of transition metal catalysts. Herein, we demonstrate that a group-IIB metal Zn, which has rarely been studied in the thermal-catalytic process, can significantly promote the performance of the lithium hydride-lithium imide (LiH-Li₂ NH)-mediated CLAS process (denoted as Zn-LiH-Li₂ NH). The addition of Zn dramatically changes the reaction pathway of the LiH-Li₂ NH mediated loop by forming a series of intermediates including Li₂ NH, lithium zinc intermetallic compounds (LiZn_x ), and a ternary metal nitride (LiZnN). LiZnN together with Li₂ NH functions as nitrogen carrier in the Zn-LiH-Li₂ NH-mediated CLAS. Because of these properties, the kinetics of N₂ fixation is significantly enhanced with a reduction in apparent activation energy from 102 kJ mol^-1 to 50 kJ mol^-1 . The ammonia production rate reaches 956 μmol g^-1  h^-1 at 350 °C, which is 19 times higher than that of the neat LiH-Li₂ NH-mediated CLAS.