Surface Physics

The Dance of the Atoms

Catalysts can fail when surface atoms start moving around.

17. 7. 2013 | Researchers at the Vienna University of Technology (TU Wien) were able to film this "dance of the atoms", and also provide an explanation for this curious phenomenon.
Lone people standing in a ballroom don't tend to move a lot. Only when they find a suitable dance partner rapid motion sets in. Atoms on iron-oxide surfaces behave in a similar way: Only with the right molecular partner do they dance across the surface. Scientists at the Vienna University of Technology now could prove that carbon monoxide is indeed the partner responsible for the quick motion. Their movies made from microscope images show that this motion leads to clustering – an effect that can do great harm to catalysts.

Prof. Ulrike Diebold
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Prof. Ulrike Diebold
"Metals such as gold or palladium are often used as catalysts to speed up certain chemical reactions", professor Ulrike Diebold from the Institute of Applied Physics explains. But as soon as the atoms ball together, most of them do not get into contact with the surrounding gas any more, and the catalytic effect diminishes drastically. For this reason, Ulrike Diebold's team investigates how atom clusters form on surfaces and searches for ways to inhibit the process.

Theories about this effect have been discussed for years, but the researchers in Vienna have now literally observed the clustering of the atoms in real time. "We are using palladium atoms on an extremely clean iron-oxide surfaces in an ultra high vacuum chamber. For several hours, we take pictures of the surface with a scanning tunneling microscope", Gareth Parkinson explains. These pictures are made into a movie that enables the researchers to track the paths of individual atoms.

Zbynek Novotny and Gareth Parkinson
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Zbynek Novotny and Gareth Parkinson
With this technique, the team discovered that the rapid atomic dance on the surface is indeed initiated by carbon monoxide molecules binding to individual palladium atoms. When this happens, the palladium is hardly connected to the surface anymore and can move around almost freely. "This is also known as the skyhook effect", Zbynek Novotny elaborates. The carbon monoxide and palladium move happily together across the surface, until they collide with other 'dancing couples'. Whenever this happens, they stick together, creating a small cluster that keeps growing.

With this new method of watching clustering in real time under the microscope, the mechanisms involved can be studied in detail: "We discovered that OH groups on the surface can suppress the clustering effect", Parkinson continues. If the carbon monoxide/palladium couples do not encounter each other, but instead find an OH group, they get trapped and cannot form a cluster. Therefore a future hydroxyl coating of surfaces could lead to a significant stability improvement in catalysts.

In 2013 alone, Prof. Diebold has not only received the Arthur W. Adamson Award for Distinguished Service in the Advancement of Surface Chemistry from the American Chemical Society - she is also the recipient of the 2013 Wittgenstein Award, the highest science award in Austria. Diebold had earned her doctorate at Vienna University of Technology in 1990, then she spent three years at Rutgers University in New Jersey. In 1993, she moved on to Tulane University in New Orleans, becoming a full professor. In 2010, Diebold was appointed to the TU's chair for surface science.   (© Vienna University of Technology, Austrian Science Fund FWF)
Florian Aigner

More information

Source

  • Gareth S. Parkinson, Zbynek Novotny, Giacomo Argentero, Michael Schmid, Jiří Pavelec, Rukan Kosak, Peter Blaha, Ulrike Diebold: "Carbon monoxide-induced adatom sintering in a Pd–Fe3O4 model catalyst", Nature Materials 2013, published online 9. June 2013, doi:10.1038/nmat3667

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