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

Abstract

AbstractChemical 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 (H2O 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−Li2NH)‐mediated CLAS process (denoted as Zn−LiH−Li2NH). The addition of Zn dramatically changes the reaction pathway of the LiH−Li2NH mediated loop by forming a series of intermediates including Li2NH, lithium zinc intermetallic compounds (LiZnx), and a ternary metal nitride (LiZnN). LiZnN together with Li2NH functions as nitrogen carrier in the Zn−LiH−Li2NH‐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−Li2NH‐mediated CLAS.