Few materials have had more impact on human history than gold. But it was not until the 1980s that two chemists, Masatake Haruta and Graham Hutchings, discovered that the Noble element, long considered inactive, could be an extraordinarily good catalyst.
Haruta learned that gold particles measuring less than 5 nm across possess a high level of catalytic activity when deposited on metal-oxide supports and that the particles effectively catalyze the conversion of CO into CO2 at room temperature.
Scientists have sought since then to determine exactly how gold nanoparticles function as catalysts.
Now, researchers from Lehigh and the UK believe they have pinpointed the active species at which the oxidation reaction occurs when gold is supported on iron oxide.
Writing in 2008 in Science, the team said bilayer clusters measuring about 0.5 nm in diameter and containing about 10 gold atoms were responsible for triggering the CO oxidation reaction. The researchers also reported that a simple change in preparation – drying the catalyst in flowing rather than static air – helps impart to the gold its catalytic capability.
Gold catalysts could find an application in the protective masks capable of converting CO to CO2 that are worn by persons exposed to high levels of CO. Another application is to fuel cells, which are vulnerable to damage by CO present in the hydrogen fuel stream.
The Science article was coauthored by Christopher Kiely, director of the Nanocharacterization Laboratory in Lehigh’s Center for Advanced Materials and Nanotechnology; Hutchings and two colleagues at the UK’s Cardiff Catalysis Institute; and Andrew Herzing of the U.S. National Institute of Standards and Technology. Herzing earned a Ph.D. from Lehigh in 2006.
Hutchings’ group carried out the fabrication and catalytic testing of the gold nanoparticles, and characterized the catalyst using x-ray photoelectron spectroscopy. Kiely’s group then used Lehigh’s aberration-corrected 2200 JEOL scanning transmission electron microscope to examine the gold’s nanostructure.
The researchers compared two groups of gold nanoparticles. One, dried in static air, was a “dead” catalyst with little or no catalytic activity. The other, dried with flowing air, was a 100-percent-active catalyst for CO oxidation.
On the inactive catalyst, Herzing saw two types of gold species – particles larger than 1 nm in size and individual atoms scattered about on the iron-oxide support. On the 100-percent-active catalyst, he found a third species – clusters of eight to 12 gold atoms arranged in two layers measuring about 0.5 nm in dimension.
“This was the clue that enabled us to identify the tiny bilayer clusters as the important species in the catalytic reaction,” says Kiely. “We deactivated the catalyst and found we could correlate the loss of the clusters with the loss of activity.
“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.”
Lehigh’s two aberration-corrected electron microscopes, acquired in 2004, enabled the researchers to see the individual atoms and bilayer clusters of atoms. They also made it possible to use high-angle annular dark-field imaging, a technique that requires a 1-angstrom-wide beam of electrons to obtain a scanned image of a specimen.
Hutchings and Kiely have studied the catalytic potential of gold nanoparticles for 15 years, publishing four papers in the past four years on the topic for Science and Nature.
In 2005, they reported that the selective oxidation processes used to make compounds contained in agrochemicals, pharmaceuticals and other chemical products could be accomplished more cleanly and efficiently with gold. In 2006, they reported the potential of gold-palladium nanoparticles to oxidize primary alcohols to aldehydes in a more environmentally friendly manner. That reaction is important to the production of spices and perfumes.
In February 2009, they wrote in Science that gold-palladium nanoparticles, properly tailored, could lead to a cleaner, safer method of producing hydrogen peroxide (H2O2). The method, which requires the pretreatment of a carbon support with nitric acid, could also enable the direct production of H2O2 from hydrogen and oxygen in smaller quantities and more desirable concentrations than is possible with current processing techniques.