Solar-driven ammonia production

Without this chemical technology, feeding the world as we know it would be nearly impossible. The Haber–Bosch process, developed more than a century ago, converts nitrogen from the air into ammonia - the key ingredient in most synthetic fertilizers. Today, roughly half of the world's food production depends on fertilizers derived from ammonia, making the Haber–Bosch process one of the most important industrial innovations in human history.

Despite its enormous benefits, the Haber–Bosch process carries a significant environmental cost. The energy required to produce ammonia accounts for roughly 1.2% of global greenhouse gas emissions, motivating researchers worldwide to seek cleaner and more sustainable production methods. Using metal–organic frameworks (MOFs) as catalysts, scientists developed an alternative sustainable route to ammonia synthesis. Researchers at TU Wien have now demonstrated that MOF structures can be specifically tuned to modulate their catalytic performance, providing valuable insights into the design of more efficient and sustainable ammonia-production technologies.

The research project was carried out in collaboration with international teams: important measurement data came from Virginia Tech in the United States, while computer simulations were performed at the Technion – Israel Institute of Technology.

One of the Strongest Bonds in Chemistry

“We have to break one of the strongest bonds in chemistry,” says Jana Bischoff from the Institute of Materials Chemistry at TU Wien, first author of the current study. In air, nitrogen exists as N₂ molecules, where two nitrogen atoms are connected by an extremely stable triple bond. To produce ammonia (NH₃), this N₂ molecule must first be activated, allowing the nitrogen atoms to react with hydrogen.

In the Haber-Bosch process, which has been used for more than 100 years, this is achieved using pressures above 150 bar and temperatures of at least 400 °C. These extreme conditions make the process highly energy-intensive.

Nature Does It More Gently

In principle, nitrogen molecules can be converted in a different way—not by extreme pressures and temperatures, but with the help of carefully designed catalysts. Nature provides inspiration: certain bacteria use the enzyme nitrogenase, which contains iron and can bind nitrogen molecules and convert them under mild conditions.

Something similar can be achieved with metal-organic frameworks, or MOFs. These are porous materials in which metal ions are linked with specific organic compounds to form a larger structure. “As in natural nitrogenase, we also use iron in our metal-organic frameworks – a metal that is relatively inexpensive and readily available,” says Dr. Cornelia Baeckmann of TU Wien. “The key question in our research was: how can we adapt the organic ligands so that the material is able to produce ammonia?”

“When light is absorbed by a metal-organic framework, it generates an excited state in which electrical charge is redistributed, particularly toward the iron centers”, says Prof. Dominik Eder (TU Wien): “The surrounding organic linkers modulate the properties of the MOF and thus its catalytic performance. “ In this way, they influence electron-transfer kinetics, nitrogen binding strength and the accessibility of protons from the surrounding water to reach the active site. 

Once a nitrogen molecule attaches to a suitable iron site, its extremely stable triple bond is weakened and becomes more reactive. The molecule can then be gradually converted into ammonia through successive transfers of electrons and protons.

An Important Step Towards New Technologies

“We show that tiny changes in the organic ligands can strongly alter the catalyst activity,” says Jana Bischoff. “We investigated a series of metal-organic frameworks containing different organic ligands in order to understand how ammonia production activity can be tuned.”

The present work is not yet a starting signal for industrial ammonia production, but it is an important step in that direction. Metal-organic frameworks (MOFs) open up promising new routes toward tailor-made catalyst design for energetically challenging and globally important processes such as ammonia synthesis.