A Comparative Study of In- and Post-Plasma Catalysis Configurations for Ammonia Synthesis: Effect of Temperature

Abstract

As an important chemical, ammonia plays an important role in human life and society. Nevertheless, there is a substantial release of CO2 for the traditional industrial ammonia synthesis process. Plasma catalysis can activate gas molecules and lower activation energy, making it a promising avenue for this reaction. The current study reveals that ammonia decomposition within the plasma discharge region significantly limits ammonia production for in-plasma catalysis (IPC) configuration. On the other hand, for post-plasma catalysis (PPC) configuration, the densities of reactive species will decrease when migrating to the downstream catalyst region, which is unfavorable for plasma catalytic ammonia synthesis. Therefore, the performances of ammonia synthesis under different plasma catalytic configurations have been investigated with the catalyst of Ni/Al2O3. In experiments with a N2:H2 flow rate ratio of 3:1, the ammonia concentration under PPC configuration was $224.53~\mu $ mol/h, while the synthesis rate under IPC configuration reached $592.30~\mu $ mol/h. This indicates that the catalyst located in the discharge zone accelerates ammonia synthesis under ambient temperature. Furthermore, ammonia synthesis experiments were conducted under IPC and PPC configurations at different temperatures. Experimental results show higher ammonia concentrations with IPC configuration at temperatures below 200 °C. Beyond 200 °C, the ammonia concentrations between the two configurations become closer. Finally, the ammonia synthesis reaction mechanisms under IPC and PPC configurations are analyzed, which help to clarify the difference in ammonia synthesis performance under IPC and PPC configurations.