Development of a Highly Efficient Ammonia Synthesis Using Base Metals in an Electric Field

Ammonia is an important chemical feed stock which is mainly utilized as a resource of fertilizers, fibers and resins. These days, it is also anticipated as a hydrogen carrier owing to its high weight-based hydrogen content. The main ammonia synthesis process to date, Haber-Bosch process, requires high temperature (673-873 K) and high pressure (20-40 MPa) because of its thermodynamic and kinetic limitations.

Recently, our group reported the activation of ammonia synthesis using Ruthenium (Ru)-based catalyst by applying the electric field. It is revealed that the surface protonics in the electric field promote cleavage of nitrogen triple bond, which is the rate-determining step in ammonia synthesis. Theoretical calculations and in-situ IR measurements revealed that the N₂ dissociation proceed through N₂H as a specific intermediate in the electric field.

Regarding conventional heterogeneous catalysts, Ru-based heterogeneous catalyst exhibits higher turn-over-frequency (TOF) than other metals catalyst like Fe, Co, Mo and Ni. However, the ammonia synthesis mechanism in the electric field is different from conventional ones. Therefore, the activity trend would change by applying the electric field. Herein, we investigated the ammonia synthesis in the electric field over various active metals (Ru, Fe, Co, Ni, Pd, Pt). As a result, Fe-supported or Ni-supported catalysts exhibited higher TOF than Ru-supported catalyst in the electric field at low temperature region. Although the trend without the electric field was Ru>Fe>Co>Ni>Pd=Pt, the trend in the electric field was Fe>Ni>Ru>Co>Pd>Pt. N₂ dissociation energy, N₂H formation energy and N₂H cleavage energy over each metal was calculated using DFT method to identify the reasons for the specific activity order in the electric field. Results show that ammonia synthesis rate order in the electric field is determined by the N₂H formation energy over active metals.