Researchers at the University of Vienna and TU Wien discovered that atomic magnetism dictates why iron-based catalysts can produce ammonia using sunlight while nearly identical chromium-based catalysts fail. Published in ACS Energy Letters, the findings show that magnetic frustration in iron triangles allows nitrogen to bind and release efficiently.
Research teams studying catalysts designed to use sunlight to produce ammonia found that replacing three iron atoms with chromium atoms at the nodes of a metal-organic framework stops the chemical reaction entirely.
Industrial production typically requires high temperatures and high pressures, making it extremely energy-intensive.
Crystal Structure of MOF MIL-101 and the Role of Metal Triangles
The research teams studied a structure called MOF MIL-101. At each node of this crystal, three metal atoms are arranged in a triangle around a central oxygen atom. Metal-organic frameworks are crystals containing small cavities. Their framework consists of nodes with metal atoms connected by organic molecules, according to Jana Bischoff of TU Wien.
When built with iron atoms, the structure produces ammonia under light irradiation. When built with chromium, the reaction does not take place. Both research teams are part of the FWF Cluster of Excellence MECS, known as Materials for Energy Conversion and Storage, which develops new materials for the energy transition.
Magnetic Frustration in Iron Versus Chromium Triangles
Instead, the team discovered that the magnetic orientation of the atoms—known as spin—plays the decisive role, determining how the spins interact.
In this crystal structure, neighboring atoms prefer opposite spin directions, such as one pointing up and the other down.
In our catalyst, however, we are dealing with a triangle of three iron atoms. This means the first two iron atoms can adopt opposite magnetic directions, but the third can no longer fulfill this requirement. It is as if three people at a table all wanted to sit directly opposite each other. The first two can look each other straight in the eye. But for all three at once, that is impossible.
Patrick Lechner, University of Vienna
In contrast, a triangle of chromium atoms behaves fundamentally differently. Two of the chromium atoms couple strongly to form a pair, while the third couples only weakly, similar to two people turning toward each other while a third person stands aside. This phenomenon is called magnetic frustration.
Magnetic Coupling Affects Nitrogen Binding Strength
These distinct magnetic couplings directly affect how strongly nitrogen binds to the metal atoms from outside. For iron atoms influenced by magnetic frustration, the bond strength between iron and nitrogen is moderate. It is strong enough to hold and convert nitrogen into ammonia, yet weak enough to release the nitrogen atoms afterward, allowing continuous ammonia formation.
Chromium atoms, however, bind nitrogen much more tightly. The nitrogen cannot be released quickly enough, leaving the metal site occupied and preventing the structure from holding additional nitrogen. Consequently, the chromium-based structure is unsuitable as a catalyst.
Leticia González of the University of Vienna emphasized that this magnetic fingerprint serves as an important key to understanding catalyst behavior. Dominik Eder of TU Wien noted that observing how metal atoms at crystal nodes interact magnetically represents an important step toward energy-saving ammonia production. Ammonia is indispensable for humanity, helping feed a large part of the world’s population through fertilizers.