Two nearly identical materials with completely different chemical behavior. Research teams at the University of Vienna and TU Wien studied catalysts that use the energy of sunlight to produce ammonia. Surprisingly, when the three iron atoms that form tiny triangles in these catalysts are replaced by chromium atoms, the chemical reaction no longer takes place.
The teams have now explained why. Iron and chromium triangles have very different magnetic properties, and these magnetic properties also reveal whether the material works as a catalyst.
Both research teams belong to the FWF Cluster of Excellence MECS (Materials for Energy Conversion and Storage), which develops new materials for the energy transition.
Energy-intensive fertilizer production
Ammonia is indispensable for humanity. Fertilizers made from ammonia help feed a large part of the world's population. However, the industrial production of ammonia from nitrogen and hydrogen usually requires high temperature and high pressure, which makes it extremely energy-intensive. Researchers are therefore working on an alternative that needs less energy. Photocatalysts are special new materials that absorb sunlight and use its energy to turn nitrogen into ammonia.
"Metal-organic frameworks are promising candidates," says Jana Bischoff (TU Wien). "These are crystals that contain small cavities. Their framework consists of nodes with metal atoms, which are connected by organic molecules."
The team studied a structure called MOF MIL-101. At each node, three metal atoms are arranged in a triangle around a central oxygen atom. This structure can be made with different kinds of metal Atoms without changing the framework. Chemically, however, the difference is huge. With iron atoms, the structure produces ammonia under light. With chromium, the reaction does not occur.
Researchers originally tried to explain this difference by the different numbers of electrons. The team has now found that the magnetic orientation of the atoms, the so-called spin, plays the decisive role.
Three people cannot all look each other in the eye
The spin of such an atom can take on different states. In this crystal structure, neighboring atoms prefer opposite spin directions, for example one up and the other down. "In our catalyst, however, we are dealing with a triangle of three iron atoms," says Patrick Lechner (University of Vienna). "This means the first two iron atoms can adopt opposite magnetic directions, but the third can no longer fulfill this requirement." This phenomenon is called "magnetic frustration."
"It is as if three people at a table all wanted to sit directly opposite each other," explains Patrick Lechner. "The first two can look each other straight in the eye. But for all three at once, that is impossible."
A triangle of Chromium atoms, incontrast, behaves fundamentally differently. Two of the Atoms couple much more strongly and form a kind of pair. The third couples only weakly to them, as if two People turned toward each other and talked directly, while the third person stood aside.
Coupling strength makes the difference
It turned out that these different magnetic couplings are closely linked to how strongly nitrogen binds to the metal atoms from outside. For the iron atoms with their "magnetic frustration," the bond between iron and nitrogen has a moderate strength. It is strong enough to hold the nitrogen and turn it into ammonia, but weak enough to let the nitrogen atoms go again afterward. This is nearly optimal, so one ammonia molecule after another can form.
Chromium atoms, however, bind nitrogen much more strongly. The nitrogen cannot be released quickly enough, the metal site stays occupied, and the structure cannot hold any further nitrogen. The structure is therefore unsuitable as a catalyst.
The team's work shows that even when the crystal structure is otherwise identical, the choice of metal can determine how the spins interact. "This magnetic fingerprint is an important key to understanding how these materials work as catalysts," emphasizes Leticia González (University of Vienna).
"We can now, in a sense, watch how the metal atoms at a node of these crystals talk to each other magnetically, and how this affects the material's behavior as a catalyst," says Dominik Eder (TU Wien). "This is an important step toward energy-saving ammonia production."
About the University of Vienna:
For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering 188 disciplines, and approximately 11,000 employees, we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.
