Green chemical pathway of plasma synthesis of ammonia from nitrogen and water: a comparative kinetic study with a N2/H2 system

Abstract

Sustainable ammonia synthesis under mild conditions that relies on renewable energy sources and feedstocks is globally sought to replace the current Haber–Bosch process. Electricity-driven plasma catalysis is receiving increasing attention as a sustainable technology for the efficient synthesis of ammonia. However, the current implementation of nonthermal plasma technology still faces several hurdles, particularly the extensive usage of energy-intensive H₂, low energy efficiency and the lack of understanding of the underlying mechanisms of the plasma catalysis process. Here we propose a green chemical pathway of plasma catalysis from nitrogen and water for sustainable ammonia production. In order to understand the different characteristics of the N₂/H₂O plasma system and seek further improvements in ammonia synthesis, detailed plasma kinetics modelling is presented in comparison with the N₂/H₂ system at atmospheric pressure. The model includes both electron and vibrational kinetics and updated surface reactions adapted from DFT calculation results on the SiO₂ surface. The facile dissociative adsorption of H₂O and the Eley–Rideal mechanisms are demonstrated to be important to enable efficient NH₃ production as the experimental observation. The model reproduced the measured trends of ammonia production on the different concentrations of H₂O input and power density with reasonable accuracy and provided an explanation of the critical influence of temperature and catalyst packing in the N₂/H₂O system in comparison to the N₂/H₂ system in relation to the quenching effect of H₂O by vibrational–translational relaxation.