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Because their device is not yet optimized, they
still need to input additional energy for the process to work. However,
they hope that their results, which they presented at last month's
meeting of the American Chemical Society, will draw attention to the
promise of the approach.
"For every mention of CO2 splitting,
there are more than 100 articles on splitting water to produce
hydrogen, yet CO2 splitting uses up more of what you want
to put a dent into," explained Kubiak. "It also produces CO, an
important industrial chemical, which is normally produced from natural
gas. So with CO2 splitting you can save fuel, produce a
useful chemical and reduce a greenhouse gas."
Although carbon monoxide is poisonous, it is highly
sought after. Millions of pounds of it are used each year to
manufacture chemicals including detergents and plastics. It can also
be converted into liquid fuel.
"The technology to convert carbon monoxide into
liquid fuel has been around a long time," said Kubiak. "It was
invented in Germany in the 1920s. The U.S. was very interested in the
technology during the 1970s energy crisis, but when the energy crisis
ended people lost interest. Now things have come full circle because
rising fuel prices make it economically competitive to convert CO into
fuel."
The device designed by Kubiak and Sathrum to split
carbon dioxide utilizes a semiconductor and two thin layers of
catalysts. It splits carbon dioxide to generate carbon monoxide and
oxygen in a three-step process. The first step is the capture of solar
energy photons by the semiconductor. The second step is the conversion
of optical energy into electrical energy by the semiconductor. The
third step is the deployment of electrical energy to the catalysts.
The catalysts convert carbon dioxide to carbon monoxide on one side of
the device and to oxygen on the other side.
Because electrons are passed around in these
reactions, a special type of catalyst that can convert electrical
energy to chemical energy is required Researchers in Kubiak's
laboratory have created a large molecule with three nickel atoms at
its heart that has proven to be an effective catalyst for this process.
Choosing the right semiconductor is also critical
to making carbon dioxide splitting practical say the researchers.
Semiconductors have bands of energy to which electrons are confined.
Sunlight causes the electrons to leap from one band to the next
creating an electrical energy potential The energy difference between
the bands - the band gap - determines how much solar energy will be
absorbed and how much electrical energy is generated.
Kubiak and Sathrum initially used a silicon
semiconductor to test the merits of their device because silicon is
well-studied. However, silicon absorbs in the infrared range and the
researchers say it is "too wimpy" to supply enough energy. The
conversion of sunlight by silicon supplied about half of the energy
needed to split carbon dioxide, and the reaction worked if the
researchers supplied the other half of the energy needed.
They are now building the device using a
gallium-phosphide semiconductor. It has twice the band gap of silicon
and absorbs more energetic visible light. Therefore, they predict that
it will absorb the optimal amount of energy from the sun to drive the
catalytic splitting of carbon dioxide.
"This project brings together many scientific
puzzle pieces," said Sathrum. "Quite a bit of work has been done on
each piece, but it takes more science to mesh them all together.
Bringing all the pieces together is the part of the problem we are
focused on." |
