Thermodynamic and kinetic considerations of nitrogen carriers for chemical looping ammonia synthesis

Abstract

Ammonia (NH ₃) is a promising clean energy carrier, provided that its production is driven by renewable energy rather than fossil fuel-based Haber–Bosch (H–B) process. Chemical looping ammonia synthesis (denoted as CLAS) can intervene in the ubiquitous scaling relations in catalytic ammonia synthesis by separately feeding reactants to a nitrogen carrier to achieve atmospheric operation, which provides an alternative synthetic route to the H-B process. The key of CLAS is to develop efficient N carrier materials with suitable thermodynamic and kinetic properties. Metal nitrides and metal imides are two kinds of N carrier materials for the CLAS process, and H ₂ and H ₂O are commonly used as the hydrogen sources of NH ₃. Here, we first analyze the thermodynamic properties of the reactions of various metal nitrides and imides with water or hydrogen to produce NH ₃, N ₂ fixation on metals or metal hydrides, and the regeneration of metals from metal oxides, respectively. The thermodynamic calculation results display the reduction of main group metal hydroxide, early transition metal oxides, and rare earth metal oxides to the corresponding metallic state or hydrides, the nitridation of late transition metals to the corresponding nitrides, are the thermodynamic limiting steps for the metal nitride carriers. The metal imides, such as lithium imide and barium imide, have the relatively proper thermodynamics for two-step chemical looping reactions, however, their performance is limited by the thermodynamics of hydrogenation reaction. Moreover, for the thermodynamically unfavorable steps in the CLAS, we propose potential electrochemical processes to run the loop, such as molten salt electrolytic cell and solid electrolyte electrolytic cell. Finally, we put forward some strategies, such as controllable synthesis of N carriers and adding efficient catalysts, to improve the kinetics of chemical looping reactions.