Ammonia Synthesis and the Role of Promoters in Designing Efficient Transition Metal Catalysts

Ammonia synthesis is crucial for global fertilizer production, traditionally relying on iron-based catalysts in the Haber–Bosch process [1,2]. However, the limitations of iron catalysts—such as low reaction rates and susceptibility to deactivation—have driven interest in alternative catalysts, particularly Ru-, Co- and high-entropy-based transition metal systems. The use of promoters, both in single and double settings, has become key to enhancing the efficiency and stability of these transition metal catalysts [3]. This presentation focuses on the role of various promoters in optimizing Ru-, Co- and Fe-based catalysts, exploring their mechanisms and potential to advance ammonia synthesis.

Results:
The inclusion of promoters such as K₂O, Ba, Ce, or BaO, among others, significantly enhances the catalytic performance of Ru-, Co- and Fe-based catalysts. For iron, these promoters improve the adsorption of N₂ and its dissociation, leading to higher ammonia yields and longer catalyst lifespans. In cobalt-based systems, these promoters stabilize the active metal sites and promote efficient nitrogen dissociation within a temperature range of below 500 ⁰C. The promoters modify the electronic properties of the catalysts, improving their overall efficiency. The nature of the rate-determining step cannot always be limited to N₂ decomposition; it can also involve other hydrogenation steps.

Conclusion:
Promoters are crucial in optimizing the performance of transition metal catalysts in ammonia synthesis. These promoters enhance catalyst stability, increase reaction rates, and suppress deactivation, offering a promising pathway for more efficient and sustainable ammonia production. Future research should focus on refining promoter–metal interactions to further improve catalyst performance and reduce energy consumption.

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2. Haber, F. The Synthesis of Ammonia from Its Elements. Nobel Lecture 1920.
3. Huang, J.; Yuan, M.; Li, X.; Wang, Y.; Li, M.; Li, J.; You, Z. Inhibited Hydrogen Poisoning for Enhanced Activity of Promoters-Ru/Sr2Ta2O7 Nanowires for Ammonia Synthesis. Journal of Catalysis 2020, 389, 556–565, doi:10.1016/j.jcat.2020.06.037.