Nanotubes could reduce CO2 emissions

Sept. 16, 2002 at 5:30 PM
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PITTSBURGH, Sept. 16 (UPI) -- A team led by scientists at Carnegie Mellon University said Monday that carbon nanotubes, which are straw-like structures with walls a single atom thick, could filter gases much more quickly than current systems.

The atoms of carbon nanotubes are arranged so that they offer practically no friction to passing gas molecules, said David Sholl, a professor of chemical engineering at Carnegie Mellon.

Such smooth surfaces mean the tubes theoretically can transport gas through a membrane at rates that are orders of magnitude greater than current microporous substances used in gas separation, Sholl told United Press International.

"What I'm hoping with our paper is that we'll spur some of the experimental people into seeing these things could be really exciting and worth (study)," Sholl said.

Sholl led the team, which also involved the University of Pittsburgh and the National Energy Technology Laboratory, in computer-simulation research soon to be published in the journal Physical Review Letters. Two possible applications for nanotubes' gas transport qualities involve carbon dioxide and hydrogen, Sholl said.

Since carbon dioxide helps trap heat in Earth's atmosphere, possibly contributing to global warming, governments worldwide are trying to reduce emissions of the gas from internal-combustion engines and power plants.

Properly sized and assembled nanotubes could separate out the gas with lower power requirements and without increasing the pressure in exhaust systems, Sholl said.

Hydrogen is the key to fuel cells, which generate electricity as hydrogen and oxygen combine to make water. The nanotubes could speed up the process of purifying hydrogen created by breaking down natural gas and other methods, Sholl said.

Nanotubes are among the most-investigated structures in nanotechnology, although practically all work to date has dealt with the tubes' other physical and electrical properties.

Nanoscience, which involves manipulating matter at the atomic or molecular scale, works with objects that have dimensions ranging from 1 to 100 nanometers, or billionths of a meter. A nanometer is to an inch what an inch is to 400 miles.

Sholl's team has created very good simulations of a property with possible future uses, said Kevin Ausman, executive director for Rice University's Center for Biological and Environmental Nanotechnology in Houston. The trick now is transferring those predictions into real-world data, and there are hurdles to doing so, he told UPI.

Nanotubes form with closed, rounded ends, Ausman said, and the simulations don't seem to account for this. Very little research has been done to determine how nanotubes can be aligned perpendicular to whatever membrane material, such as a polymer, is used, he said.

For hydrogen production, the tubes' closed ends might be useful, Sholl said. Most nanotubes form in multi-walled clumps, and the spaces between them could be an even better transport mechanism for hydrogen, he said.

Taking advantage of multi-wall tubes' interstitial spaces to deal with hydrogen does make sense, according to Tom Chapman, acting director of the Chemical and Transport Systems Division at the National Science Foundation, which partially funded Sholl's work. Nanotubes' frictionless quality also could play a role in efficiently storing hydrogen, Chapman told UPI.

Sholl's team lends credence to their simulation's accuracy by comparing their work to experiments done with zeolites, crystal structures known to be effective gas filters, Chapman said.

Still to be dealt with are questions of how selective the tubes can be in admitting desired gases, he said. If that issue is resolved, nanotube membranes might even prove a more economical way to pull nitrogen and oxygen from Earth's atmosphere, he said.

(Reported by UPI Technology Correspondent Scott R. Burnell in Washington.)

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