High-energy electron beam driven N₂/H₂ ammonia synthesis under ambient temperature and pressure

Professor Jun Ma's research group at the University of Science and Technology of China reported a novel green ammonia synthesis strategy based on high-energy electron beam dissociation collisions. This work utilizes a MeV-level electron beam to directly act on an N₂/H₂ gas-phase system, achieving ammonia generation at room temperature, atmospheric pressure, and without an external heating source. The study shows that the high-energy electron beam can activate stable N≡N triple bonds through ionization, excitation, and secondary electron induction processes, driving the conversion of nitrogen and hydrogen into NH₃. In the catalyst-free system, an ammonia generation rate of 0.14 μmol·L⁻¹·s⁻¹ can be obtained by optimizing the electron beam parameters; further, the introduction of Ru nanocatalysts increases the ammonia yield by up to 2.8 times compared to the catalyst-free system. This work provides a new research approach for achieving small molecule activation and green ammonia synthesis using high-energy electron beams. The article was published as an open access Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Ammonia (NH₃) is a crucial raw material for fertilizers, nitrogen-containing chemicals, and potential zero-carbon energy carriers. Currently, industrial ammonia synthesis primarily relies on the Haber–Bosch process, which typically requires high-temperature and high-pressure conditions of 15–25 MPa and 400–500 °C, resulting in high energy consumption and stringent equipment requirements. Developing novel ammonia synthesis methods under milder conditions is an important direction in energy chemistry, green chemistry, and nitrogen cycle research. In recent years, mild nitrogen fixation methods such as photocatalysis, electrocatalysis, plasma, and mechanochemistry have made positive progress, but most systems are still limited by low ammonia yields, insufficient energy efficiency, catalyst stability, or difficulties in system scale-up. Unlike traditional catalytic methods, high-energy electron beams can deposit energy into the reaction system in a very short time, inducing molecular ionization, excitation, and dissociation, providing non-equilibrium energy input for the activation of inert small molecules. Therefore, exploring electron beam-driven direct N₂/H₂ ammonia synthesis not only helps expand the application of radiation chemistry in energy conversion but may also provide a new technological pathway for green distributed ammonia production.

Highlights of this article:

1. A novel strategy for ammonia synthesis at room temperature and pressure driven by a high-energy electron beam is proposed. NH₃ is generated by directly irradiating a N₂/H₂ gas mixture with a 2.0 MeV electron beam under catalyst-free, room temperature, and atmospheric pressure conditions. By controlling the H₂ ratio, electron beam energy, beam current intensity, and irradiation time, it was found that H₂ can act as a reducing agent to promote ammonia generation at an appropriate ratio, but excessive H₂ weakens the effective activation of N₂. In the catalyst-free system, 20% H₂ is the optimal condition, with the highest ammonia generation rate reaching 0.14 μmol·L⁻¹·s⁻¹.

2. Revealing that low-energy secondary electrons are the key driving force for nitrogen activation. Through multi-scale Monte Carlo simulations using Geant4-DNA, FLUKA, and TOPAS, the macroscopic electron beam energy deposition and microscopic molecular activation processes were linked. The results show that high-energy electrons entering the gas phase system generate a large number of low-energy secondary electrons, among which secondary electrons in the 150–250 eV range are most effective for the ionization and excitation of N₂. This indicates that the activation of the N≡N bond is not truly triggered by a single high-energy electron collision, but rather by a cascade of low-energy secondary electrons induced by high-energy electrons.

3. Ru nanocatalysts achieve synergistic enhancement of electron beam irradiation and surface catalysis. In catalyst-free systems, excessively high H₂ content consumes some electron energy and reduces N₂ activation efficiency. To overcome this limitation, carbon-supported Ru nanocatalysts were introduced, transforming the random collision process of gas-phase free radicals into a surface-mediated hydrogenation process. Results showed that the Ru/C catalyst changed the optimal H₂ ratio from 20% to a stoichiometric ratio of 50%, and increased ammonia yield by up to 2.8 times. XPS results indicated that electron beam irradiation can induce the Ru surface to transition to a more electron-rich state, which is beneficial for N₂ activation and subsequent hydrogenation reactions.

4. Isotope experiments and DFT calculations elucidate the reaction origin and pathway. Experiments using ¹⁵N₂ and D₂ isotope labeling confirmed that the nitrogen and hydrogen in the generated NH₃ originate from introduced N₂ and H₂, respectively, rather than from environmental pollution or background pollution from the equipment. DFT calculations show that in the gas phase, the radical recombination pathway of N· and H· is the most thermodynamically feasible; however, on the Ru surface, the N₂ dissociation energy barrier is approximately 32.7 kcal·mol⁻¹ lower than in the gas phase, and the subsequent stepwise hydrogenation process from NH·, NH₂· to NH₃ becomes thermodynamically increasingly favorable. This indicates that the Ru surface can act as an "energy ladder," reducing the difficulty of N₂ dissociation and stabilizing key intermediates.

Summary and Outlook:

In summary, this work develops a room-temperature and ambient-pressure ammonia synthesis strategy based on high-energy electron beam dissociation collisions, achieving a combination of radiation chemistry and heterogeneous catalysis. The study not only demonstrates that MeV-level electron beams can directly activate the N₂/H₂ gas-phase system and drive NH₃ generation, but also reveals the low-energy secondary electron-dominated nitrogen molecule activation mechanism and the crucial role of Ru catalysts in lowering the N₂ dissociation energy barrier and stabilizing reaction intermediates. Compared to traditional high-temperature and high-pressure ammonia synthesis routes, this method offers advantages such as milder conditions, electro-driven operation, easy coupling with renewable energy sources, and potential modular scale-up. In the future, by further optimizing electron beam energy deposition, reactor structure, and continuous operation, this strategy is expected to provide a new technological option for green, distributed ammonia production.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/.