"This was the clue that enabled us to identify the tiny bilayer clusters as the important species in the catalytic reaction," said Kiely. "It turns out that only about 2 percent of the gold deposited on the support ended up in this particular type of cluster.
"We then deactivated the catalyst by various heat treatments and found that we could correlate the loss of the clusters with the loss of activity. This gives us strong evidence that the active species in the catalyst are the tiny bilayer clusters.
"We believe we have obtained the first conclusive evidence that bilayer clusters are occurring in a real gold catalyst, that they are the key species on that catalyst, and that their presence or absence correlates with the ability or failure of the catalyst to perform CO oxidation."
Before Lehigh's acquisition of the aberration-corrected electron microscopes in 2004, Kiely and Hutchings were able to see the larger gold particles, but not the individual atoms or bilayer clusters of atoms.
"At that time, when we compared the dead catalyst and the active catalyst," said Kiely, "both looked the same. The new microscopes have opened up a new window allowing us to see what is really going on."
The aberration-corrected STEM enabled Herzing and Kiely to use a microscopy technique called high-angle annular dark-field imaging, which requires an extremely fine, 1-angstrom-wide beam of electrons to obtain a scanned image of a specimen. An angstrom is equal to one-tenth of a nanometer.
Kiely said the gold catalysts could find a potential application in the protective masks capable of converting CO to CO2 that are worn by firefighters and others exposed to high levels of CO. Another application is to fuel cells that are vulnerable to poisoning by the CO that is present in the hydrogen fuel stream.
|Contact: Kurt Pfitzer|