Researchers have combined gold nanoparticles (in light red) with copper nanoparticles (in light green) to form hybrid nanoparticles (dark red), which they turned into powder (foreground) to catalyze carbon dioxide reduction. Photo: Zhichuan Xu |
Copper—the
stuff of pennies and tea kettles—is also one of the few metals that can
turn carbon dioxide into hydrocarbon fuels with relatively little
energy. When fashioned into an electrode and stimulated with voltage,
copper acts as a strong catalyst, setting off an electrochemical
reaction with carbon dioxide that reduces the greenhouse gas to methane
or methanol.
Various
researchers around the world have studied copper’s potential as an
energy-efficient means of recycling carbon dioxide emissions in
powerplants: Instead of being released into the atmosphere, carbon
dioxide would be circulated through a copper catalyst and turned into
methane—which could then power the rest of the plant. Such a
self-energizing system could vastly reduce greenhouse gas emissions from
coal-fired and natural-gas-powered plants.
But
copper is temperamental: easily oxidized, as when an old penny turns
green. As a result, the metal is unstable, which can significantly slow
its reaction with carbon dioxide and produce unwanted byproducts such as
carbon monoxide and formic acid.
Now
researchers at MIT have come up with a solution that may further reduce
the energy needed for copper to convert carbon dioxide, while also
making the metal much more stable. The group has engineered tiny
nanoparticles of copper mixed with gold, which is resistant to corrosion
and oxidation. The researchers observed that just a touch of gold makes
copper much more stable. In experiments, they coated electrodes with
the hybrid nanoparticles and found that much less energy was needed for
these engineered nanoparticles to react with carbon dioxide, compared to
nanoparticles of pure copper.
A paper detailing the results will appear in the journal Chemical Communications;
the research was funded by the National Science Foundation. Co-author
Kimberly Hamad-Schifferli of MIT says the findings point to a
potentially energy-efficient means of reducing carbon dioxide emissions
from powerplants.
“You
normally have to put a lot of energy into converting carbon dioxide
into something useful,” says Hamad-Schifferli, an associate professor of
mechanical engineering and biological engineering. “We demonstrated
hybrid copper-gold nanoparticles are much more stable, and have the
potential to lower the energy you need for the reaction.”
Going small
The
team chose to engineer particles at the nanoscale in order to “get more
bang for their buck,” Hamad-Schifferli says: The smaller the particles,
the larger the surface area available for interaction with carbon
dioxide molecules. “You could have more sites for the CO2 to come and
stick down and get turned into something else,” she says.
Hamad-Schifferli
worked with Yang Shao-Horn, the Gail E. Kendall Associate Professor of
Mechanical Engineering at MIT, postdoc Zichuan Xu and Erica Lai ’14. The
team settled on gold as a suitable metal to combine with copper mainly
because of its known properties. (Researchers have previously combined
gold and copper at much larger scales, noting that the combination
prevented copper from oxidizing.)
To
make the nanoparticles, Hamad-Schifferli and her colleagues mixed salts
containing gold into a solution of copper salts. They heated the
solution, creating nanoparticles that fused copper with gold. Xu then
put the nanoparticles through a series of reactions, turning the
solution into a powder that was used to coat a small electrode.
To
test the nanoparticles’ reactivity, Xu placed the electrode in a beaker
of solution and bubbled carbon dioxide into it. He applied a small
voltage to the electrode, and measured the resulting current in the
solution. The team reasoned that the resulting current would indicate
how efficiently the nanoparticles were reacting with the gas: If CO2
molecules were reacting with sites on the electrode—and then releasing
to allow other CO2 molecules to react with the same sites—the current
would appear as a certain potential was reached, indicating regular
“turnover.” If the molecules monopolized sites on the electrode, the
reaction would slow down, delaying the appearance of the current at the
same potential.
The
team ultimately found that the potential applied to reach a steady
current was much smaller for hybrid copper-gold nanoparticles than for
pure copper and gold—an indication that the amount of energy required to
run the reaction was much lower than that required when using
nanoparticles made of pure copper.
Going
forward, Hamad-Schifferli says she hopes to look more closely at the
structure of the gold-copper nanoparticles to find an optimal
configuration for converting carbon dioxide. So far, the team has
demonstrated the effectiveness of nanoparticles composed of one-third
gold and two-thirds copper, as well as two-thirds gold and one-third
copper.
Hamad-Schifferli
acknowledges that coating industrial-scale electrodes partly with gold
can get expensive. However, she says, the energy savings and the reuse
potential for such electrodes may balance the initial costs.
“It’s
a tradeoff,” Hamad-Schifferli says. “Gold is obviously more expensive
than copper. But if it helps you get a product that’s more attractive
like methane instead of carbon dioxide, and at a lower energy
consumption, then it may be worth it. If you could reuse it over and
over again, and the durability is higher because of the gold, that’s a
check in the plus column.”
