Promotion of Ammonia Synthesis By Surface Protonics at Low Temperature

Ammonia is an important feedstock used as a raw material for fertilizers, resins, and fibers. Lately, much attention has been paid to the ammonia utilization as a hydrogen carrier due to its high hydrogen content and handling faculty. However, there are still a lot of challenges in terms of harsh reaction conditions because of rigid N₂ bonding. To date, Haber Bosch process using Fe-based catalyst has been utilized under extreme condition of 20-40 MPa and 673-873 K owing to its thermodynamic and kinetic limitation. Thus a more efficient process is required for ammonia synthesis.

Recently, our group reported the activation of ammonia synthesis using Ru-based catalyst by applying the electric field. It is revealed that the surface protonics in the electric field promote ammonia synthesis, especially the cleavage of N₂ bonding, rate determining step of this reaction. By means of in-situ IR measurements and first principle calculation, it is clarified that N₂cleavage is considerably enhanced by proton in the electric field, proceeding via novel intermediate (N₂H⁺).

Herein, we conducted kinetic analysis for ammonia synthesis in the electric field to investigate the contribution of proton to the N₂ dissociation in detail. Results showed that the apparent activation energy decreased gradually as the temperature decreased. We inferred this unique phenomenon was induced by the increment of proton species on the surface. For the elucidation of the assumption, the temperature dependency for the conductivity of CeO₂ pellets under dry condition (N₂ + H₂) was measured using electrical impedance spectroscopy. Results clarified that the proton conductivity increased at lower temperature, which is the same tendency as the activity. These results suggest that N₂ cleavage in the electric field proceeds through N₂H⁺ with proton species produced from lattice oxygen and supplied H₂.