Unusual desorption behavior on the surface of catalysts
Leaving the island
The reactions on the surface of a catalyst, for example in the catalytic converter of a car, can be roughly divided into three steps: First, the reactants adsorb on the surface of the catalyst, followed by the actual reaction and the desorption of the products formed. A group of scientists led by Professor Sebastian Günther of the TUM, Professor Joost Wintterlin of the LMU, and Dr. Andrea Locatelli from Syncrotron Elettra in Trieste, has now managed to make such a desorption process microscopically visible.
Unlike the complicated processes that occur on the surface during the catalytic reaction itself, the process of desorption was thought to be relatively simple: The reactants gain thermal energy from the solid surface, and desorb as soon as this energy exceeds the binding energy to the surface. This picture suggests that desorption is a purely statistical process that only depends on the number of molecules. “However, in many cases, the desorption rates calculated using this model do not agree with the experimentally determined values,” says Joost Wintterlin.
Imaging with nanometer resolution
The experiments carried out by Günther, Wintterlin and their colleagues reveal that not only the number of adsorbed molecules but their spatial distribution on the surface is important. In their study, the team employed low-energy electron microscopy (LEEM), which allows one to image solid surfaces with nanometer resolution, to follow what goes on during the course of temperature-induced desorption. LEEM works similarly as conventional electron microscopy. The only difference is that the energetic electrons that form the beam are decelerated just before they hit the surface of the sample. With the help of this technique, the researchers were able to monitor desorption of oxygen from a silver surface.
“It turned out that, during desorption, the layer of absorbed molecules breaks up into many isolated islands, and desorption takes place exclusively from the edges of these islands,” says Sebastian Günther, head of the Research Group Physical Chemistry with Focus on Catalysis and member of TUM's Catalysis Research Center (CRC). Moreover, the size distribution of the islands depends on the pretreatment of the silver surface. “Together, these effects account for the discrepancies between theoretical predictions and experimental measurements of desorption rates reported in earlier studies. They probably play a role in many other processes involving desorption from surfaces, and could change our concepts of the processes on catalytic surfaces,” Günther adds.
The work was funded by the German Research Foundation (DFG).
Sebastian Günther, Tevfik Onur Mentes, Miguel Angel Niño, Andrea Locatelli, Sebastian Böcklein, Joost Wintterlin
Desorption kinetics from a surface derived from direct imaging of the adsorbate layer
Nature Communications, May 2014; doi:10.1038/ncomms4853
LEEM movie of the decay of oxygen islands (dark) on a silver surface.