Ammonia is a cornerstone of modern fertilizer production and has increasingly attracted extensive attention as a promising hydrogen carrier. Over a century, ammonia has been predominantly produced via the Haber–Bosch process, which stands as one of the most crucial catalytic reactions in modern industry. Nevertheless, despite decades of research efforts, a key aspect of the reaction mechanism may have long been overlooked: the catalyst support itself.
A new study conducted by researchers from the University of Sydney and Beijing University of Technology demonstrates that the support plays a far more critical role than merely immobilizing metal particles or donating electrons. In the process of ammonia synthesis, oxygen defects present on the support surface are capable of capturing and transferring active nitrogen species, thereby inducing surface nitrogen spillover. This mechanism establishes an additional reaction pathway independent of metal particles, mitigates the reconversion of reactive nitrogen into nitrogen gas, and liberates metal active sites to facilitate sustained nitrogen activation. These findings uncover a novel mechanism, through which catalyst supports facilitate ammonia formation and provide new insights for the design of higher-efficiency ammonia synthesis catalysts.
Looking beyond the metal particles
For years, research studies on ammonia synthesis catalysts have primarily focused on the metal component, where nitrogen and hydrogen molecules undergo activation and dissociation. In contrast, the support material has often been regarded merely as a passive scaffold, or at most, as an electronic promoter capable of modifying the metal surface.
However, certain experimental observations have indicated that the support material may exert a more direct influence on the catalytic reaction. To explore this hypothesis, the research team selected two model catalysts, Ru/MgO and Ru/Al₂O₃, and monitored their catalytic behavior during the ammonia synthesis process under realistic reaction conditions.
Through the integrated application of in situ environmental transmission electron microscopy, electron energy-loss spectroscopy, infrared spectroscopy, and theoretical calculations, the team systematically characterized the dynamic evolution of the catalyst surfaces during the reaction. The findings demonstrated that nitrogen and hydrogen molecules were initially activated and dissociated on Ru particles, generating reactive nitrogen and hydrogen intermediates.
A support that captures and transfers nitrogen
In MgO-supported catalysts, oxygen defects generated in situ on the surface of the support can effectively capture active nitrogen species and serve as a reservoir for such species. Instead of confining reactive nitrogen exclusively to the ruthenium surface, a portion of the reactive nitrogen is able to migrate to and diffuse across the support surface, thereby inducing surface nitrogen spillover.
This process exerts a significant regulatory effect on the reaction pathway. The accumulation of reactive nitrogen on the metal surface may lead to the recombination of partial nitrogen species and subsequent loss in the form of nitrogen gas. However, the capture of partial nitrogen by the support suppresses this non-productive reaction pathway. Concurrently, the active sites on the metal surface are regenerated more rapidly, facilitating the activation and dissociation of incoming nitrogen molecules.
In summary, the support does not merely play a secondary auxiliary role in the reaction. Instead, it directly participates in the storage, transfer and modulation of key reaction intermediates.
Watching a hidden pathway emerge
To elucidate the underlying mechanism, the team conducted theoretical calculations, which demonstrated that the migration of active nitrogen into oxygen defect sites on the MgO surface is energetically advantageous. This finding is consistent with experimental observations and clarifies the spontaneous occurrence of nitrogen spillover under reaction conditions.
Collectively, experimental and theoretical results indicate a synergistic mechanism: Ru particles facilitate nitrogen activation, whereas the support captures and transfers the generated reactive nitrogen intermediates. Such synergy between the metal and the support enhances the efficiency of nitrogen activation and ammonia synthesis.
A fresh perspective on a classic industrial reaction
This study provides an innovative perspective for catalyst design in ammonia synthesis. Rather than focusing solely on the modification of metal active sites, researchers can further engineer support surfaces to stabilize and mediate the transport of critical reaction intermediates. Accordingly, the support functions not merely as a structural component, but as an active participant in the reaction pathway.
By uncovering the role of oxygen defects on the support surface in nitrogen transformation, this research advances the mechanistic understanding of one of the oldest and most vital catalytic processes in industry. It also creates new avenues for the development of high-efficiency catalysts applicable to ammonia production and related energy technologies.
National Science Review
Experimental study