Metal surfaces play a role as catalysts for many important applications - from fuel cells to the purification of car exhaust gases. However, their behaviour is decisively determined by oxygen atoms that adhere to the surface.
The phenomenon has been known for a long time, but until now it was not possible to precisely investigate the influence of oxygen in complex surfaces point by point in order to understand the chemical background at the atomic level. This has now been achieved at TU Wien in cooperation with a team from the Elettra Synchrotron in Trieste. This made it possible to explain why in previous studies partly contradictory results had been obtained: the oxygen atoms do not distribute themselves evenly, but settle down particularly easily in very specific places.
Precision measurements instead of average values
"Examining a metal surface directly during catalysis is a great challenge," says Prof. Günther Rupprechter from the Institut für Materialchemie der TU Wien, öffnet eine externe URL in einem neuen Fenster, opens an external URL in a new window.
"Of course, you can put the whole catalyst into a reactor and measure exactly which chemical products are produced - but you only ever get an average value. You can't know which points on the catalyst contributed to the chemical reaction and in what way."
Another possibility is not to use a real catalyst, but a simple, highly clean, idealised piece of it - such as a tiny single crystal, with exactly known properties, which you can then study under the microscope. In this case, you get precise, reproducible results, but they don't have much to do with practical applications.
The research group led by Günther Rupprechter and Yuri Suchorski therefore wanted to combine the best of both approaches. They use thin foils made of rhodium, which consist of small grains. On each grain, the surface atoms can be arranged differently. In one grain, they form a smooth, regular surface with the outer atoms all in exactly the same plane; next to it, the atoms may arrange themselves to form a more complicated arrangement consisting of many atomic steps.
The favourite places of oxygen
It is precisely these steps that turn out to be crucial. "For the catalytic activity, the oxidation state of the catalyst plays a central role - i.e. whether oxygen attaches itself to the metal atoms or not," explains Philipp Winkler, the first author of the study. In earlier experiments, we found that we were often dealing with a state between "oxidised" and "not oxidised" - a result that is difficult to interpret.
However, this becomes understandable when one realises that not every grain of the rhodium foil is oxidised to the same degree. The oxidation starts where there are corners, edges and steps - there it is particularly easy for the oxygen atoms to bind to the surface. Therefore, different grains with different surface structures are oxidised to different degrees.
Electron microscope and synchrotron in Trieste
This could be determined with a combination of highly developed technologies: "In a special electron microscope, the sample is irradiated with UV light during the chemical reaction and the resulting electron emission is measured with spatial resolution down to the micrometre," explains Yuri Suchorski, "this allows us to determine exactly which grains of the film are particularly catalytically active. The same sample is then examined again with a completely different microscope: grain by grain with X-rays at the synchrotron, which provides very precise information about the surface oxidation of the sample."
If you combine both results, you can determine exactly which structures show which chemical behaviour. The decisive advantage: you can examine the entire film in a single experiment, with hundreds of different grains. Instead of studying tiny single crystals separately, you can use a single sample containing several structures used for catalysis under real conditions, and get information about the properties of these structures all at once.
"This is an important step for catalysis research," Rupprechter emphasises. "We now no longer have to be satisfied with measuring an average value that inadequately describes the entire sample, but can really understand in detail which atomic structures have which effects. This will also make it possible to specifically improve important catalysts that are needed for many applications in energy and environmental technology."
The work was carried out as part of the FWF-funded project "Spatial-temporal Phenomena".
Original publication: P. Winkler et al., How the anisotropy of surface oxide formation influences the transient activity of a surface reaction, Nature Communications 12, 69 (2021)., opens an external URL in a new window
Prof. Günther Rupprechter
Institut für Materialchemie
Technische Universität Wien
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Dr. Florian Aigner
Technische Universität Wien
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