Catalysts are indispensable for the large-scale production of many chemicals, as they accelerate chemical reactions. Some of them contain metal atoms such as iron as key building blocks. The electrons of each individual iron atom possess a tiny magnetic moment, the so-called spin. When several iron atoms meet in a catalyst, the way their spins are oriented relative to each other is crucial. A research team at the University of Vienna was able to decipher the mode of action of an iron-based catalyst using quantum mechanical calculations. They showed that the phenomenon of superposition stabilizes the catalyst and makes it more effective.
The synthesis of ammonia from nitrogen and hydrogen is a key chemical process, primarily used for fertilizer production, and currently accounts for around 2 % of global energy consumption. The search for new, more efficient catalysts for ammonia production is therefore an important factor in reducing global CO2 emissions.
One promising candidate is MIL-101(Fe), a metal-organic framework (MOF) in which three iron atoms are arranged in a triangular shape around a central oxygen atom. In parallel with experimental studies, researchers have been trying for years to understand the structure and properties of this material using computer simulations in order to unravel the mechanisms underlying its catalytic activity at the atomic level.
Until now, it was assumed that the spins of the three iron atoms are aligned in parallel to each other. The team led by Leticia González (Faculty of Chemistry) and Georg Kresse (Faculty of Physics), both members of the FWF-Excellence Cluster "Materials for Energy Conversion and Storage (MECS)" , has now shown that the spins are ideally aligned antiparallel to one another. However, since each iron atom has two neighbors, this condition cannot be satisfied for all three simultaneously: two of the three iron atoms can meet the requirement, but the third is always parallel to one neighbor and antiparallel to the other. Physicists refer to this as a spin-frustrated state.
"You can imagine this like three people trying to sit around a round table, with each wanting to sit directly opposite someone else. That’s not possible for all three at the same time, which leads to frustration for one of them," explains Patrick Lechner, first author of the study published in Angewandte Chemie International Edition.
Unlike in classical physics, such states can be described "satisfactorily for all parties" using quantum mechanics: all possible arrangements coexist simultaneously in the form of a so-called superposition. The various spin configurations exist all at the same time, and the overall system can only be accurately described if all these possibilities are considered. This principle is reminiscent of Schrödinger's famous thought experiment with the cat that is simultaneously alive and dead until its state is measured – a similar situation occurs with spin frustration. "This magnetic frustration, which can only be explained through a superposition of different quantum states, stabilizes the structure of the catalyst and enables a particularly efficient interaction with small gas molecules such as N2 and CO – which accounts for its catalytic activity," González explains.
The study on these triangular iron clusters, published in Angewandte Chemie International Edition, could in the long run help increase the efficiency and performance of such catalysts and thus pave the way toward more sustainable ammonia production.
Original publication:
Patrick Lechner, Gaurab Ganguly, Michael J. Sahre, Georg Kresse, Johannes C. B. Dietschreit, Leticia González. Spin Frustration Determines the Stability and Reactivity of Metal-Organic Frameworks with Triangular Iron(III)-oxo Clusters. In Angew. Chem. Int. Ed. (2025)
DOI: 10.1002/anie.202514014