At 300 degrees Celsius, a new catalyst developed at the Institute of Science Tokyo produced ammonia at a rate of 15.9 millimoles per gram of catalyst per hour. It worked at 0.9 megapascals of pressure, using common elements rather than relying on scarce precious metals.
Ammonia is widely used to make fertilizer, and it is also being studied as a way to store and move hydrogen without carbon emissions. But making ammonia from nitrogen gas usually takes high heat and pressure. Nitrogen atoms are bound tightly together, so breaking that bond takes a great deal of energy. Some catalysts can speed up the process, but high-performing options often use scarce metals such as ruthenium. Barium-based materials could be a less costly choice, though active compounds such as barium subnitride break down in hydrogen.
A team led by Dr. Yihao Jiang, Professor Hideo Hosono and Professor Masaaki Kitano set out to make a barium catalyst that would stay stable while activating nitrogen. Their study, published in the Journal of the American Chemical Society, describes a material with the formula Ba3AlO4-xNyHz. It has an antiperovskite structure, a crystal arrangement that can hold several kinds of negatively charged particles, including oxide, nitride, hydride and molecular nitrogen ions called (N2)-.
Those nitrogen ions are in an activated state: the bond between their two nitrogen atoms is weakened, making it easier to break. To give the reaction more working surface, the researchers formed the barium material as an ultrathin layer around aluminum oxide nanorods. They added cobalt, which formed small clusters on the outside of that layer during synthesis.
The resulting structure remained stable during ammonia production. At 300 degrees Celsius and 0.9 megapascals, it reached a rate of 15.9 millimoles of ammonia per gram of catalyst per hour. The researchers reported that the optimized cobalt, barium, aluminum and oxygen catalyst surpassed benchmark ruthenium-based catalysts and industrial iron catalysts in synthesis rate under the tested conditions.
Isotope labeling, spectroscopy and computer simulations helped the team trace how the different parts work together. They found that activated (N2)- ions at points where cobalt, barium and aluminum meet are central to the reaction. Barium-containing sites weaken the nitrogen bond and help the material take up new nitrogen gas. Cobalt and aluminum sites help with later steps that turn nitrogen into ammonia.
The work offers a design strategy for catalysts that both keep activated nitrogen and coordinate the jobs of several elements. It also identifies a previously unknown route to ammonia production mediated by (N2)- ions. The researchers say the approach could inform future ammonia technologies and low-carbon energy systems, though further work will be needed to develop practical production methods. For now, the study shows how a carefully arranged set of abundant elements can tackle one of ammonia-making's hardest steps without depending on precious metals.