Ammonia is one of the most important chemicals globally, with annual demand of about 200 million tons. The Haber–Bosch process, while industrially dominant, operates under harsh conditions (400–500 °C, 100–200 atm), consumes about 2 % of global energy, and emits 300 million tons of carbon dioxide annually. A study published in ENG. Chem. Eng. reviews emerging catalytic routes for ammonia synthesis under mild conditions, offering potential pathways toward green, low‑carbon ammonia production.
One‑step thermal catalysis. In 2017, a dual‑active center catalyst (transition metal–lithium hydride) overcame scaling relationship limitations, achieving nitrogen fixation below 300 °C. In 2025, a BaC₂ support with C₂ defects enabled N₂ “horizontal adsorption”, achieving an ammonia production rate of 0.20 mmol·g⁻¹·h⁻¹ at just 100 °C and atmospheric pressure, with an activation energy of only 28.5 kJ·mol⁻¹. Additionally, machine learning‑assisted design of Ru‑based intermetallic compounds identified Sc₁/₈Nd₇/₈Ru₂, which achieved an activity of 9.03 mmol·g⁻¹·h⁻¹ at 0.9 MPa and 325–400 °C, with stable operation for 105 hours.
Electrocatalysis. Electrochemical N₂ reduction suffers from low N₂ solubility (∼0.65 mmol·L⁻¹) and the competing hydrogen evolution reaction (HER). The lithium‑mediated method in nonaqueous electrolytes achieves faradaic efficiencies (FE) up to ∼100 %, but lithium is stoichiometrically consumed, and lithium dendrite formation poses safety hazards. Nitrate (NO₃⁻) and nitric oxide (NO) reduction have emerged as promising alternatives. A Co₆Ni₄ heterostructure catalyst achieved 99.21 % FE, 5.50 mmol·cm⁻²·h⁻¹, and 120 h stability. A Cu₆Sn₅ alloy achieved an NH₃ production rate of 10 mmol·cm⁻²·h⁻¹ with FE >96 %, remaining stable for 135 h. In a scaled‑up electrolyzer, the NH₃ production rate reached ∼2.5 mol·h⁻¹.
Photocatalysis and plasma catalysis. Photocatalytic ammonia synthesis operates at room temperature and pressure, avoiding harsh conditions. However, solar energy conversion efficiency remains below 0.1 %, far from industrial requirements. Plasma catalysis can effectively activate N₂; a radiofrequency plasma with Ni‑MOF‑74 catalyst achieved an ammonia yield of 13.53 mmol·g⁻¹·kWh⁻¹, though further improvement in selectivity and energy efficiency is needed.
Two‑step N₂ oxidation–reduction route. This emerging strategy uses air and water as raw materials. Plasma first oxidizes N₂ to NOₓ (NO concentration up to 9710 ppm with 94.02 % selectivity), followed by electrocatalytic or photocatalytic reduction to NH₃. Coupled plasma–photocatalysis achieved 100 % NO conversion, 98.33 % NH₃ selectivity, and 240 h stable operation. However, the plasma step has low energy efficiency (0.25 %–7 %), and the levelized cost of ammonia (1.8–2.5 USD·kg⁻¹) remains far above the Haber–Bosch process (0.5–0.8 USD·kg⁻¹).
Detection challenges. Accurate quantification of low‑concentration ammonia is critical. The indophenol blue method is simple but prone to interferences; ion chromatography is more accurate but unsuitable for Li⁺/Na⁺‑containing electrolytes. The ¹⁵N₂ isotope‑labeling experiment is the only reliable method to verify the ammonia source and avoid false positives. The review calls for unified experimental practices, including three blank controls, ¹⁵N₂ verification, and cross‑validation between detection methods, to improve data credibility and advance the field toward industrialization.
ENGINEERING Chemical Engineering
Experimental study
Not applicable
Beyond Haber-Bosch: emerging pathways for sustainable ammonia synthesis
1-May-2026