Mechanochemical ammonia synthesis (McAmmonia)

Abstract

Conventional reactions are mostly driven by heat, light, and electricity. They are named as thermochemistry, photochemistry, and electrochemistry, respectively. Likewise, chemical reaction, caused by mechanical actions, is defined as mechanochemistry, which can deliver energies required to overcome reaction barriers via abrasion, friction, cracking, colliding, and so on. The most representative tool for operating mechanochemistry is ball-milling, which can offer a new tool for ammonia synthesis. To date, ammonia has mainly been produced by the Haber-Bosch process over 110 years. However, it cannot be performed under mild conditions, because of thermodynamic reasons, requiring a large centralized manufacturing facility. Ammonia as one of most efficient hydrogen carriers, a decentralized manufacturing system is urgently developed. We have discovered a new method for the synthesis of ammonia under mild conditions (45 °C and 1 bar) via mechanochemical ball-milling iron (Fe) catalyst in the presence of nitrogen and hydrogen. With this new process, the final concentration of ammonia with potassium (K) promoter has reached as high as 94.5 vol%, which is nearly 4 times higher than the state-of-art Haber-Bosch process (~25 vol%) under harsh conditions (450 °C and 200 bar). Stable nitrogen dissociation at the mild conditions is associated with mechanochemically induced high defect density and violent impact on the Fe catalyst.