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The gold catalysts can also carry out partial
oxidations under solvent-free conditions, the researchers said, making
them more environmentally friendly than oxidation processes that use
chlorine, and less costly than those employing organic peroxides.
The team, led by Graham Hutchings, professor of
physical chemistry at Cardiff University in Wales, included eight
other Cardiff chemists, four scientists from the Johnson Matthey
chemical company in the United Kingdom, and a materials scientist from
Lehigh University in Bethlehem, Pennsylvania.
Their article was titled "Tuneable gold catalysts
for selective hydrocarbon oxidation under mild conditions."
Masatake Haruta, a catalyst chemist at Tokyo
Metropolitan University who has been at the forefront of gold
nanoparticles research for more than a decade, said in a commentary
accompanying the Nature article that the breakthrough by Hutchings's
group had the potential to "transform" the chemical industry.
Noting that most industrial oxidation processes use
chlorine or organic peroxides, Haruta said, "the chemical industry
would be transformed if selective oxidation of hydrocarbons could be
achieved efficiently using cheap and clean oxygen from the air. The
advancement by Hutchings and colleagues of 'greener' methods for
oxidation catalysis using gold is therefore invaluable."
The industrial selective oxidation processes that
Hutchings's team catalyzed with gold nanoparticles are used to convert
unsaturated hydrocarbons to oxygen-containing organic compounds (e.g.,
epoxides, ketones), which in turn serve as higher-value compounds that
form the basis for many chemical products.
The challenge, says Chris Kiely, professor of
materials science and engineering at Lehigh University, is to
selectively insert an oxygen atom at specific positions into
long-chain or cyclic-ring hydrocarbon carbon molecules, something
which nanoparticulate gold achieves effectively.
The gold nanoparticles, which measure 2 to 15
nanometers in width (1 nm equals one one-billionth of a meter) must be
distributed evenly over a large surface area support and prevented
from coalescing and forming larger particles with weaker catalytic
properties.
"The nano-gold catalyst can effectively aid the
insertion of an oxygen atom into the unsaturated hydrocarbon," says
Kiely, who has co-authored several dozen papers with Hutchings. "Activated
carbon provides a viable support for the nanoparticles. The gold
catalyst can also be fine-tuned and made more effective, giving a
higher yield of epoxides and ketones, with the addition of occasional
atoms of bismuth.
"We're trying to determine the size, distribution
and shape of the gold nanoparticles, and to see how these parameters
relate to the measured catalytic properties. We are also interested in
the interaction of gold with other promoter elements, such as bismuth,
and we're trying to identify exactly where the bismuth atoms are going
and why they have a beneficial effect."
Kiely, who joined the Lehigh faculty in 2002 after
serving on the materials science and engineering and chemistry
faculties at Liverpool University, uses transmission electron
microscopy and various spectroscopic techniques to characterize the
gold nanoparticles.
The recent acquisition by Lehigh University of two
aberration-corrected electron microscopes, including a JEOL 2200FS
transmission electron microscope, will shed more light on future work
in the area of gold catalysis, he said.
"Before, we were able to see nanoparticles and
achieve atomic resolution, but not with the same degree of clarity
that the new JEOL microscope provides. The new instrument also gives
us the capability of doing chemical composition analysis with close to
atomic column precision, which will be a big boon."
Gold in recent years has drawn more attention from
researchers as a potential catalyst in chemical processing, pollution
control and fuel cell applications. Haruta, a pioneer in this area,
demonstrated a decade ago that gold nanoparticles could be used,
amongst other things, as catalysts to de-odorize restrooms and to
convert carbon monoxide to carbon dioxide at low temperatures.
But much remains to be learned for nano-gold to
realize its full potential, says Kiely, who directs the
Nanocharacterization Laboratory in Lehigh's Center for Advanced
Materials and Nanotechnology.
"Gold is a very useful catalyst for many chemical
reactions," says Kiely, "but we're still not sure what happens at the
molecular scale during the catalysis process. The more we learn, the
better we can fine-tune gold nanoparticle catalysts." |
