Ammonia, one of the most important synthetic feedstocks, is mainly produced by the Haber–Bosch process at 400–500 °C and above 100 bar. The process cannot be performed under ambient conditions for kinetic reasons. Here, we demonstrate that ammonia can be synthesized at 45 °C and 1 bar via a mechanochemical method using an iron-based catalyst. With this process the ammonia final concentration reached 82.5 vol%, which is higher than state-of-the-art ammonia synthesis under high temperature and pressure (25 vol%, 450 °C, 200 bar).
The mechanochemically induced high defect density and violent impact on the iron catalyst were responsible for the mild synthesis conditions.
Mechanochemical method for ammonia synthesis
The process is illustrated schematically in Fig. 1 and Supplementary Video 1. Ammonia is synthesized via ball milling using iron powder as the catalyst. The entire synthesis process can be divided into two
steps: nitrogen dissociation and subsequent hydrogenation. In the first step, stable N2 is adsorbed and dissociated into atomic nitrogen on the defects of the iron particles [Fe(N*)], where Fe(N*) denotes
nitrogenated iron particles. The low-coordinated defects, which are created by repeated collisions during ball milling, are considered highly active for nitrogen dissociation16,3