Everything is in near constant movement even if it cannot be seen by the naked eye. Just the right movement can cause things like chemical reactions to happen. Catalysts can stop working when atoms on the surface start moving. At the Vienna University of Technology, this dance of the atoms could now be observed and explained.
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other by chemical bonds based on the same force, forming a molecule.
Atoms on iron-oxide surfaces behave in a similar way to people finding a dance partner: Only with the right molecular partner do they move across the surface and find other suitable partners.
Scientists at the Vienna University of Technology have now filmed the actual atoms, proving that carbon monoxide is the atomic/molecule partner responsible for the motion. Their movies show that the motion leads directly to clustering — an effect that can do great harm in catalysts. The findings have now been published in the journal Nature Materials.
"Metals such as gold or palladium are often used as catalysts to speed up certain chemical reactions", says Professor Ulrike Diebold (Institute of Applied Physics, Vienna University of Technology). When 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 clusters form from single atoms on a surface, and search for ways to inhibit the process.
Theories about this clustering effect have been discussed for years, but the researchers at the Vienna University of Technology have now directly observed the clustering of the atoms. "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", says Gareth Parkinson (Vienna University of Technology). These pictures were then made into a movie, in which the paths of the individual atoms could be tracked.
Using this technique, the research team discovered that the rapid atomic dance on the surface is initiated by carbon monoxide molecules, which bind to individual palladium atoms. As soon as this happens, the palladium is hardly connected to the ground and can move around almost freely, as if it had been lifted out by the carbon monoxide.
With the new possibility of watching clustering in real-time under the microscope, the mechanisms can now be studied in detail by actually watching the atoms move and react: "We discovered that OH groups on the surface can suppress the clustering effect", says Gareth Parkinson. If the carbon monoxide-palladium couples do not encounter each other, but instead find an OH group, they get trapped there and cannot form a cluster. A hydroxyl coating of the surface could therefore lead to a significant improvement of the stability of catalysts.
For further information see Atoms.
Molecule image via Wikipedia.