The newly developed iron-based catalyst shows significantly better performance in ammonia (NH3) production compared to the traditional catalyst that has been in use for a century. By creating this innovative catalyst with an inverse design, researchers have successfully enhanced the NH3 production rate per volume, surpassing the capabilities of conventional catalysts. These advancements may pave the way for more efficient ammonia synthesis, which is vital for the agricultural and chemical sectors.
NH3 is a crucial chemical in modern society, primarily used for manufacturing fertilizers that increase agricultural productivity and support the growing global population. For over 100 years, ammonia production has depended on the Haber-Bosch (HB) process, which involves the combination of nitrogen (N2) and hydrogen with a catalyst. Surprisingly, a century-old iron-based catalyst known as ‘Promoted-Fe’ remains dominant in large-scale NH3 production, despite numerous attempts to identify more energy-efficient alternatives.
In the HB process, ammonia is generated in a reactor that contains a limited amount of catalyst. The output of ammonia from this reactor is influenced by the production rate based on catalyst volume rather than weight. While both measurements may seem similar at first glance, they are fundamentally different. No known catalyst has managed to exceed the NH3 production rate per unit of catalyst volume compared to Promoted-Fe throughout various temperature and pressure conditions. This situation has led many researchers to overlook the distinction, instead reporting NH3 production rates based on catalyst weight in academic literature, a practice that does not significantly improve the HB process.
In response to this challenge, a research group from the Institute of Science Tokyo (Science Tokyo), Japan, has made a significant advancement by rethinking catalyst design. In their recent publication in the journal Advanced Science on January 23, 2025, Professor Michikazu Hara and his team applied the design principles of conventional iron-based catalysts to create innovative solutions, yielding impressive outcomes.
Typical supported metal catalysts for NH3 production consist of transition metal particles placed on a support with a high surface area and low density. This design is intended to maximize the active surface area and enhance the NH3 production rate relative to catalyst weight. However, this leads to a lower NH3 production rate per catalyst volume due to the low density.
To resolve this dilemma, the Science Tokyo team engineered and evaluated metal catalysts with an inverse structure. Their model featured large iron particles combined with an appropriate ‘promoter.’ Professor Hara explains, “In the design of our inverse catalysts, highly active sites can radiate outward from the center of a deposited promoter.” Yet, until now, it was unclear which structure was more effective in boosting the NH3 production rate per catalyst volume.
After testing various material compositions, the researchers landed on a catalyst formed from aluminum hydride (AlH) and potassium on relatively larger iron particles (AlH-K+/Fe). This new catalyst demonstrated outstanding performance across several metrics, achieving an NH3 production rate per volume that was approximately three times that of Promoted-Fe. Furthermore, this catalyst is capable of producing NH3 at temperatures below 200 °C, a feat that Promoted-Fe cannot accomplish. Hara notes, “The new catalyst has not only outperformed Promoted-Fe, which has yet to be matched by any other catalyst, but it can also synthesize NH3 at temperatures as low as 50 °C. Importantly, our catalyst remains stable, showing no decline in activity after 2,000 hours of operation.”
The researchers conducted mechanistic studies to uncover why the AlH-K+/Fe catalyst performs so well. Their findings indicated that the inverse structure enhances electron donation at the iron particles’ surface, increasing the number of active sites available. This improvement results in a more effective breakdown of N2, the step that limits the overall reaction rate.
In summary, this study underscores the promise of iron-based catalysts with an inverse design for NH3 production. Given that these catalysts can be easily manufactured from materials that are abundant on Earth, they may contribute significantly to more efficient industrial NH3 production, ultimately aiding in efforts to combat climate change.
