Phase transitions uphold catalytic activity

September 18, 2020

An international team of scientists around members of the Department of Inorganic Chemistry has now succeeded in microscopically observing continuous chemical processes on the surface of a working catalyst in real time with the help of state-of-the-art imaging methods. They found that continuous phase transitions are responsible for the functioning of catalysts.

Fundamental and pioneering work on oscillating surface reactions has already been carried out at the Fritz Haber Institute by Gerhard Ertl, who was awarded the Nobel Prize in Chemistry in 2007 for his work. Now Ertl's findings have been expanded. Researchers at the Fritz Haber Institute recently discovered that certain phase transitions on catalyst surfaces maintain catalytic activity.

The phase transitions, i.e. the physical or chemical change of substances and their properties, they found are of essential importance for catalysts in gas and liquid phase reactions. While it had been known for quite some time that catalytic reactions can be initiated by phase transitions, until recently, there was no data on the exact behavior of these phase transitions. Using the example of copper, a common catalyst for a large number of reactions, it has now been established that the phase transition from the metal to the stable oxide does not take place directly, but that several spatially separated phases are present simultaneously in a transition region and are constantly changing into each other.

Scanning electron microscopy (SEM) images showing different surface states. Source: Original Publication (see left colum on page).

In the science of catalysis, this is called 'frustrated phase transition'. Metallic and oxidic phases alternate periodically, always in the same pattern, like clockwork, because they are not stable under the applied conditions. This means that even a simple metal such as copper forms unexpected structures that are constantly changing into each other and thus also change chemical reactivity. In this case, the change is positive, because the catalytic activity is thus not prevented, but upheld.

The visual representation of such frustrated phase transitions on the microscale is new territory in catalysis research. The team from the Fritz Haber Institute around Marc Willinger, lead investigator of the study and former head of the electron microscopy group, succeeded in doing this with the help of a special electron microscope in which catalyst surfaces can be exposed to certain gas mixtures and temperatures. "This knowledge about the behavior of a working catalyst can now be used to create better catalyst models," says Prof. Schlögl, Director of the Department of Inorganic Chemistry. Last but not least, there could be indications of the catalyst's longevity. Since it is known that some substances change their reactivity over time after a large number of phase transitions, these results could also help in the design of more durable copper catalysts.

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