An international team under the leadership of Hamburg scientists has observed a catalyst in action on the molecular level with the world's strongest X-ray laser. The study shows surprising details of a chemical reaction and opens up the possibility to see live pictures of these ultrafast processes. For the first time, scientists directly verified a state of transition in which the molecules hover above the catalyst for a short time before finally flying away. This investigation method provides new insights into the surface chemistry world and may contribute to improve a large number of catalysts. The group headed by Martina Dell'Angela and Wilfried Wurth from the Advanced Study Group of the University of Hamburg at the Center for Free-Electron Laser Science CFEL presents their work in the scientific journal "Science".
Catalysts are materials which accelerate or even enable a chemical reaction without being consumed by the reaction itself. They are indispensable in numerous industrial processes including environmental technology, the production of fuel and the fabrication of agricultural fertilizers. The best known example is the automobile catalytic exhaust converter. It is made, among others, of the noble metal platinum which on its surface transforms carbon monoxide (CO) into carbon dioxide (CO2).
At the X-ray laser LCLS (Linac Coherent Light Source) at the accelerator centre SLAC in California, the scientists examined the accumulation of carbon monoxide on a catalyst surface made of the metal ruthenium. They were able to trace in detail how carbon monoxide molecules disconnect from the ruthenium surface. "The molecule does not just fly away. It remains above the surface for a moment in a weakly bound state of transition in which it still interacts with the surface," said lead author Dell'Angela from the University of Hamburg. This is important, for example, to understand, how new molecules find a place on an almost full catalyst surface."
"This short-lived state of transition was already postulated more than half a century ago. Now, for the first time, it could actually be observed," explained co-author Wurth, speaker of the Advanced Study Group of the University of Hamburg at the Center for Free-Electron Laser Science CFEL. "We never expected to see this state; this was a surprise," said co-author Anders Nilsson, deputy director of the Center for Interface Science and Catalysis of SLAC and Stanford University. Among other things, the scientists detected that a surprisingly large amount of molecules remain in this kind of state for an unexpected long period of time.
Although it still requires experimental work to trace a complete catalytic reaction with an X-ray laser at a large-scale industrial catalyst; according to Wurth, the observation of carbon monoxide desorption at ruthenium is an important step to explore the ultrafast dynamics of surface reactions. "With this study we verified that the observation of these processes is possible with X-ray lasers," Wurth emphasizes. "This also opens up the possibility to investigate much more complex reactions." The scientists believe that these studies will bring to light several surprises. "To see a reaction like this one in real-time is the dream of a chemist," said Nilsson. "It is really a leap into the unknown."
Studies like these are also in the focus of the Cluster of Excellence "The Hamburg Centre for Ultrafast Imaging" (CUI). Apart from the University of Hamburg, DESY, the Max Planck Society, the European XFEL GmbH and the European Molecular Biology Laboratory (EMBL) participate in CUI. For these investigations, X-ray sources like the European X-ray laser European XFEL are essential. This facility is currently being built in Hamburg, with DESY as its main contractor. When it is completed, it will be the world's best instrument of its kind.