Zn Promotes Chemical Looping Ammonia Synthesis Mediated by LiH−Li2NH Couple

Chemical looping ammonia synthesis (CLAS) is a promising alternative route to ammonia production because of its advantages of avoiding competitive adsorption of N 2 and hydrogen source (H 2 O or H 2 ) and intervening the scaling relations in the catalytic process. Our previous studies showed that NH 3 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−Li 2 NH)-mediated CLAS process (denoted as Zn−LiH−Li 2 NH). The addition of Zn dramatically changes the reaction pathway of the LiH−Li 2 NH mediated loop by forming a series of intermediates including Li 2 NH, lithium zinc intermetallic compounds (LiZn x ), and a ternary metal nitride (LiZnN). LiZnN together with Li 2 NH functions as nitrogen carrier in the Zn−LiH−Li 2 NH-mediated CLAS. Because of these properties, the kinetics of N 2 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−Li 2 NH-mediated CLAS. Graphical Ammonia (NH 3 ) synthesis promoted by Zn : Zn is rarely used in NH 3 synthesis reaction. Here, it is found that Zn increases the NH 3 production rate 19 times compared to the only LiH−Li 2 NH-mediated chemical looping ammonia synthesis (CLAS) at 350 °C. It is mainly because Zn leads to new pathways for CLAS by forming a series of lithium zinc intermetallic compounds (LiZn x ) and a ternary nitride LiZnN.