High-throughput screening of suitable nitrogen carriers for chemical looping ammonia synthesis

Chemical looping ammonia synthesis is a promising pathway to synthesise ammonia with lower energy and economic costs than the current Haber-Bosch process, for example, by lowering the operating temperature and/or pressure, whilst maintaining comparable NH₃ yield. A solid nitrogen carrier is used to mediate the reaction and form ammonia in consecutive steps. In contrast to the Haber-Bosch process, the thermodynamics and kinetics of the reaction are dependent on the properties of the nitrogen carrier. This introduces additional opportunities for novel designs, though finding suitable materials becomes the challenge. Herein, nitrides from the Materials Project database are screened based on their equilibrium nitrogen pressures and nitrogen exchange capacities. The equilibrium nitrogen pressure was used as a proxy to determine the equilibrium ammonia mole fraction of the reaction, a key performance indicator for chemical looping ammonia synthesis. Using high-throughput gas-solid equilibrium calculations, 2515 nitrides were screened, and 111 nitrides were found to achieve sufficiently high ammonia mole fraction at equilibrium to compete with the Haber-Bosch process. Due to large inaccuracies associated with theoretical material properties, the viability of screened candidates cannot be adequately ascertained. The quality and quantity of theoretical and experimental data for nitrogen carriers, respectively, must be improved further to identify suitable materials for chemical looping ammonia synthesis.