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With the goal of efficient fuel cell operation in
mind, researchers first need to turn their attention to hydrogen,
which is one of the leading energy sources being investigated by
scientists sponsored by the DOE as part of its mission to ensure the
nation’s future energy needs. A major problem facing today’s most
promising fuel-cell technologies is that the same hydrogen-rich
materials feeding the reaction often contain high levels of carbon
monoxide (CO), which is formed during hydrogen production. Within a
fuel cell, CO "poisons" the expensive platinum catalysts that convert
hydrogen into electricity, deteriorating their efficiency over time
and requiring their replacement.
"We’re trying to find a catalyst that achieves two
things: produces hydrogen while removing a large amount of CO,"
Rodriguez said.
One way to eliminate the CO byproduct is to combine
it with water to produce hydrogen gas and carbon dioxide in a process
known as the "water-gas shift" reaction. With the assistance of proper
catalysts, the water-shift reaction can convert nearly 100 percent of
the CO into carbon dioxide. Using catalyst characterization techniques
at Brookhaven’s National Synchrotron Light Source (NSLS), Rodriguez
and coworkers Jonathan Hanson and Jan Hrbek found that nanoparticles
of either gold or copper, supported on a metal, can perform this
catalytic role. In particular, they found that the greatest catalytic
activity is achieved with extremely small nanoparticles – less than 4
nanometers (4 billionths of a meter) – supported on the metal cerium
oxide, or ceria.
"Metal nanoparticles alone are not able to do the
catalysis," Rodriguez said. "But when you put them on the ceria, you
see tremendous catalytic activity."
At the nanoscale, gold has long been known to
exhibit chemical reactivity that makes it a potent catalyst. The
problem, however, comes with its hefty price tag. "We wanted a
material that was less expensive," Rodriguez said. "We wanted to see
if we could replace the gold with copper." Using x-ray diffraction,
absorption, and spectroscopy studies at the NSLS, Rodriguez’s group
showed that the substitution is indeed possible. Although gold
nanoparticles continue to show the greatest catalytic activity, copper
is almost as reactive and its cost is much lower. |
