is one of the most important chemicals in today's world, as it is used in the production of fertilizers to boost agricultural yields and sustain the ever-growing global population. For over 100 years, NH3 production has relied on the Haber–Bosch (HB) process, which combines nitrogen (N2) and hydrogen in the presence of a catalyst. Interestingly, an iron-based catalyst developed a century ago (called 'Promoted-Fe') still remains at the forefront of mass NH3 production, despite countless efforts to find more energy-efficient alternatives. In the HB process, where NH3 is produced by a catalyst-filled reactor with a limited volume, the NH3 productivity in the reactor depends on the NH3 production rate, not per catalyst weight but per catalyst volume. While the former looks like the latter at a glance, these two are completely different. No catalyst has ever been able to surpass Promoted-Fe in NH3 production rate/catalyst volume over the whole temperature and pressure ranges. For this reason, most academic researchers have chosen to ignore the fact and have compared the NH3 production rates/catalyst weight of "new catalysts" in academic journals; this competition would not lead to a significant improvement in the HB process.
Against this backdrop, a research team from Institute of Science Tokyo (Science Tokyo), Japan, has taken a bold step forward with an innovative approach towards catalyst design. As reported in their latest paper, which was published in the journal Advanced Science on January 23, 2025, Professor Michikazu Hara and colleagues took the design principles of established iron-based catalysts and essentially turned them into an innovative tool, achieving remarkable results.
Supported metal catalysts used for NH3 production typically consist of transition metal particles deposited on a support material with a large specific surface area and low density, which ideally increases the active surface area and enhances NH3 production rate/catalyst weight. However, this design results in a small NH3 production rate/catalyst volume because of the low density.
To tackle this issue, the research team from Science Tokyo designed and tested metal catalysts with an inverse structure. In other words, the proposed catalysts consisted of large iron particles loaded with an appropriate 'promoter.' "In the inverse catalyst design, highly active sites can spread outwards concentrically on the metal surface from the center of a deposited promoter," explains Hara, "Nonetheless, it had not been verified which structure is more effective in increasing the NH3 production rate per catalyst volume—until now."
After experimenting with various compositions, the research team settled on a catalyst consisting of aluminum hydride (AlH) and potassium loaded onto relatively larger iron particles (AlH-K+/Fe). The catalytic performance of this new material was outstanding in various regards. The NH3 production rate/volume of the catalyst reached ca. 3 times that for Promoted-Fe. Moreover, the proposed catalyst could also produce NH3 below 200 °C at which Promoted-Fe cannot work for NH3 production, as Hara highlights, "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."
Through mechanistic studies, the researchers investigated the reason behind the enhanced performance of the AlH-K+/Fe catalyst. The results suggested that the inverse structure favors electron donation at the surface of the iron particles while increasing the number of active sites per unit area. This translates to more efficient cleaving of N2, which is the rate-limiting step.
Overall, the results of this study highlight the potential of iron-based catalysts with an inverse structure for NH3 production. Considering that such catalysts can be manufactured easily from earth-abundant materials, they could contribute to more efficient industrial NH3 production. This, in turn, would help us in our mission to halt climate change.
About Institute of Science Tokyo (Science Tokyo)
Institute of Science Tokyo (Science Tokyo) was established on October 1, 2024, following the merger between Tokyo Medical and Dental University (TMDU) and Tokyo Institute of Technology (Tokyo Tech), with the mission of "Advancing science and human wellbeing to create value for and with society."
