New Iron-Based Catalyst Outperforms Traditional Ammonia Synthesis

Scientists at Science Tokyo have developed an iron-based catalyst that outperforms the century-old conventional catalyst used for ammonia (NH₃) production. The team designed the catalyst with an inverse structure, significantly increasing the NH₃ generation rate per volume of catalyst beyond what traditional catalysts can achieve. These results have the potential to enhance the efficiency of NH₃ production, a vital process for the agricultural and chemical industries.

Innovative Catalyst Design for Efficient Ammonia Synthesis

NH₃ is a critical compound used in fertilizers to enhance agricultural yields, supporting the growing global population. For nearly a century, NH₃ production has relied on the Haber-Bosch (HB) process, which combines nitrogen (N₂) and hydrogen in the presence of a catalyst.

An iron-based catalyst developed over 100 years ago, known as "Promoted-Fe," remains the standard for large-scale NH₃ production despite numerous efforts to develop more energy-efficient alternatives.

In the HB process, NH₃ production within a reactor is limited by the rate of NH₃ generation per catalyst volume, not per catalyst weight. While these metrics are often conflated, they represent fundamentally different measures. To date, no catalyst has surpassed Promoted-Fe in NH₃ generation rate per volume across all operating temperatures and pressures. As a result, much academic research has focused on improving NH₃ production rates per catalyst weight, a metric less relevant to optimizing the HB process.

To address this challenge, researchers from Japan's Institute of Science Tokyo (Science Tokyo), led by Professor Michikazu Hara, developed a new catalyst design strategy. Their approach uses design principles from existing iron-based catalysts to achieve notable advancements in performance.

Conventional supported metal catalysts for NH₃ synthesis typically consist of transition metal particles dispersed on a low-density, high-surface-area support material. This configuration increases the active surface area and improves NH₃ production rates per catalyst weight. However, the low density of these catalysts limits NH₃ generation rates per catalyst volume.

The Science Tokyo team addressed this limitation by creating and testing metal catalysts with an inverse structure. These catalysts feature large iron particles combined with appropriate "promoters," enhancing NH₃ production rates per catalyst volume.

In the inverse catalyst design, highly active sites can spread outwards concentrically on the metal surface from the center of a deposited promoter. Nonetheless, it had not been verified which structure is more effective in increasing the NH₃ production rate per catalyst volume—until now.

Michikazu Hara, Professor, Institute of Science Tokyo

The research team tested various compositions before selecting a catalyst composed of potassium and aluminum hydride (AlH) applied to larger iron particles (AlH-K⁺/Fe). This catalyst exhibited remarkable performance, achieving an NH₃ generation rate per volume nearly three times higher than that of Promoted-Fe.

The proposed catalyst also demonstrated activity below 200 °C, a temperature range where Promoted-Fe is ineffective, and produced NH₃ at temperatures as low as 50 °C.

Hara added, “The new catalyst did not only exhibit much higher catalytic performance than Promoted-Fe that has never been surpassed by any catalyst developed so far but also synthesized NH₃ even at 50 °C. Needless to say, the catalyst is stable. We have confirmed that the catalyst produces NH₃ without any decrease in activity over 2,000 hours.”

Mechanistic experiments revealed the reasons behind the improved performance of the AlH-K⁺/Fe catalyst. The inverse structure increases the density of active sites per unit area. It enhances electron donation on the iron surface, enabling more efficient cleavage of N₂ during the rate-limiting step of the reaction.

The findings highlight the potential of inversely structured iron-based catalysts for industrial NH₃ synthesis. These catalysts, made from earth-abundant materials, could enhance efficiency and contribute to more sustainable NH₃ production, aligning with efforts to mitigate climate change.

Journal Reference:

‌Hattori, M., et al. (2025) Ammonia Synthesis Over an Iron Catalyst with an Inverse Structure. Advanced Science. doi.org/10.1002/advs.202410313.