— 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
"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."
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
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."
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