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 (LiZnx ), 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.
Keywords: ammonia synthesis; chemical looping; hydrides; ternary metal nitrides; zinc.
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