Molecular dinitrogen anions pave the way for low-temperature ammonia synthesis

Molecular dinitrogen anions pave the way for low-temperature ammonia synthesis Sadie Harley Scientific Editor Andrew Zinin Chief Editor While ammonia (NH₃) is best known for its role in fertilizer production, it is attracting attention as a carbon-free hydrogen carrier, potentially offering a practical way to store and transport clean fuel. Despite this promise, NH₃ synthesis still depends on high-temperature, high-pressure processes that are difficult to scale up sustainably. The reason behind this is the extremely strong bond between nitrogen atoms in nitrogen gas (N₂), which requires a lot of energy to break. Many scientists are trying to develop catalysts to make NH₃ synthesis possible under milder conditions. Although some high-performance catalysts have been reported, they often rely on scarce noble metals, such as ruthenium. Barium (Ba)-based materials could be a cost-effective alternative, but highly active compounds like barium subnitride (Ba₂N) are unstable in hydrogen and degrade during NH₃ synthesis. How can we design a stable barium-based catalyst that activates nitrogen efficiently? A stable shell with activated nitrogen A research team led by Dr. Yihao Jiang, Professor Hideo Hosono and Professor Masaaki Kitano from the MDX Research Center for Element Strategy, Institute of Science Tokyo (Science Tokyo), Japan, set out to answer this question. Their latest study, published in the Journal of the American Chemical Society, introduces a hierarchically structured antiperovskite-supported cobalt for NH₃ synthesis. The researchers synthesized a new material with the formula Ba₃AlO₄₋ₓNᵧHz, which features an anti-perovskite structure. This flexible crystal arrangement accommodates several types of negatively charged species, including oxide, nitride, hydride and molecular dinitrogen anions (N₂)⁻. In this form, the nitrogen atoms are in a highly activated state and have a weakened nitrogen–nitrogen bond, which makes it easier to cleave them. To boost performance, the team grew this material as an ultrathin shell on aluminum oxide (Al₂O₃) nanorods, greatly increasing the catalyst's accessible surface area. They also added cobalt (Co), which self-organized during synthesis into small clusters on the outer shell of the material. This structure was not only stable but also exhibited an NH₃ synthesis rate of 15.9 mmol per gram of catalyst per hour under mild conditions of 300 °C and 0.9 MPa. "The optimized Co/Ba₃AlO₄₋ₓNᵧHz/Al₂O₃ catalyst achieved NH₃ synthesis rates surpassing benchmark ruthenium-based and industrial iron catalysts," highlights Kitano. Coordinated elements drive the reaction To better understand these results, the team conducted isotope labeling experiments, spectroscopy measurements and computer simulations. They found that activated (N₂)⁻ ions at the interface sites between Co, Ba and aluminum are central to the reaction. Simply put, Ba-containing sites weaken the nitrogen–nitrogen bond, enabling both easy dissociation of (N₂)⁻ and quick replenishment with N₂. Meanwhile, Co and aluminum sites assist with the subsequent reaction steps that ultimately lead to NH₃. Overall, this study demonstrates a new strategy for designing stable materials that incorporate activated nitrogen and coordinate the roles of different elements to improve catalytic performance. The study also uncovers a previously unknown NH₃ synthesis method mediated by (N₂)⁻ ions. It could guide the development of NH₃ production technologies and support future low-carbon energy systems in an emerging hydrogen economy, as Kitano concludes, "The findings of our work open new opportunities for the development of highly active NH₃ synthesis catalysts capable of operating efficiently under mild conditions without the use of precious metals." Publication details Yihao Jiang et al., Hierarchically Structured Anti-Perovskite Oxynitride-Hydride Enabling Ammonia Synthesis via a Cooperative Mechanism Mediated by (N2)− Ions, Journal of the American Chemical Society (2026). DOI: 10.1021/jacs.6c06380 Journal information: Journal of the American Chemical Society Provided by Institute of Science Tokyo