Platinum is an important catalyst. But so far, no one knows exactly how platinum atoms behave during catalysis.
What happens when a cat climbs on a sunflower? The sunflower is unstable, will quickly bend, and the cat will fall to the ground. However, if the cat only needs a quick nudge to grab a bird from there, the sunflower can act as a “meta-stable mid-stage.” It is essentially the mechanism by which the individual atoms of a catalyst capture molecules in order to chemically transform them.
Several years ago, the surface physics group at the Vienna University of Technology discovered that platinum “single atom” catalysts could oxidize carbon monoxide at temperatures which, according to their theoretical models, would not should not have been possible. Now, using atomic-scale microscopic images and complex computer simulations, they have been able to show that the catalyst itself and the material it is anchored to assume energetically unfavorable “metastable” states for a short time to allow reaction. happen in a special way. The results were published in the journal Scientific advances.
Single atoms as catalysts
The research group of Professor Gareth Parkinson from the Institute of Applied Physics at TU Wien is investigating the smallest possible catalysts: Individual platinum atoms are placed on an iron oxide surface. They then come into contact with carbon monoxide and turn into carbon dioxide, as happens in a modern car exhaust.
“This process is technically very important, but what exactly happens when the size of the catalyst is reduced to the limit of a single atom was not clear until now,” explains Gareth Parkinson. “In our research group, we study these processes in several ways: on the one hand, we use a scanning tunneling microscope to produce very high resolution images on which you can study the movement of individual atoms. And on the other hand, we analyze the reaction process with spectroscopy and computer simulations. »
Whether platinum atoms are active as a catalyst is temperature dependent. In the experiment, the catalyst is slowly and evenly heated until the critical temperature is reached, and the carbon monoxide is converted into carbon dioxide. This threshold is around 550 Kelvin. “However, this did not match our original computer simulations,” says Matthias Meier, first author of the current publication. “According to density functional theory, which is normally used for such calculations, the process could only take place at 800 Kelvin. So we knew something important had been overlooked here so far. »
A metastable state: fleeting, but important
For several years, the team gained extensive experience with the same materials in other reactions, and as a result, a new picture emerged step by step. “With density functional theory, you normally calculate the state of the system that has the lowest energy,” explains Matthias Meier. “That makes sense, because that’s the state the system assumes most often. But in our case, there is a second state that plays a central role: a so-called metastable state. »
The platinum atoms and the iron oxide surface can switch between different quantum physical states. The ground state, with the lowest energy, is stable. When the system transitions to the metastable state, it inevitably returns to the ground state after a short time – like the cat trying to reach the top on an unstable climbing pole. But in the catalytic conversion of carbon monoxide, the system need only be in a metastable state for a very short time: just as a brief moment in a wonky climbing state may be enough for a cat to catch a bird with its paw, the catalyst can convert carbon monoxide to the metastable state.
When carbon monoxide is first introduced, two platinum atoms bond to form a dimer. When the temperature is high enough, the dimer can move to a less favorable position where the surface oxygen atoms are less loosely bound. In the metastable state, the iron oxide changes its atomic structure precisely at this point, releasing the oxygen atom that the carbon oxide needs to form carbon dioxide, which instantly flies away – completing the catalysis process. “If we include these previously unconsidered short-term states in our computer simulation, we get exactly the result that was also measured in the experiment,” says Matthias Meier.
“Our research results show that in surface physics you often need a lot of experience,” says Gareth Parkinson. “If we hadn’t studied very different chemical processes over the years, we probably would never have solved this puzzle. Recently, artificial intelligence has also been used with great success to analyze quantum chemical processes – but in this case, Parkinson is convinced, it probably would not have been successful. To find creative solutions outside of what was previously thought possible, you probably need humans after all.
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The enigma of platinum – Houssenia Writing News
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