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

Chemical looping ammonia synthesis (CLAS) is a promising alternative route to ammonia production because of its advantages of avoiding competitive adsorption of N and hydrogen source (H O or H ) and intervening the scaling relations in the catalytic process. Our previous studies showed that NH 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 NH)-mediated CLAS process (denoted as Zn-LiH-Li NH). The addition of Zn dramatically changes the reaction pathway of the LiH-Li NH mediated loop by forming a series of intermediates including Li NH, lithium zinc intermetallic compounds (LiZn ), and a ternary metal nitride (LiZnN). LiZnN together with Li NH functions as nitrogen carrier in the Zn-LiH-Li NH-mediated CLAS. Because of these properties, the kinetics of N fixation is significantly enhanced with a reduction in apparent activation energy from 102 kJ mol to 50 kJ mol . The ammonia production rate reaches 956 μmol g  h at 350 °C, which is 19 times higher than that of the neat LiH-Li NH-mediated CLAS.