Powered only by natural sunlight, an array of nanotubes is able to convert a mixture of carbon dioxide and water vapour into natural gas at unprecedented rates.
Such devices offer a new way to take carbon dioxide from the atmosphere and convert it into fuel or other chemicals to cut the effect of fossil fuel emissions on global climate, says Craig Grimes, from Pennsylvania State University, whose team came up with the device.
Although other research groups have developed methods for converting carbon dioxide into organic compounds like methane, often using titanium-dioxide nanoparticles as catalysts, they have needed ultraviolet light to power the reactions.
The researchers' breakthrough has been to develop a method that works with the wider range of visible frequencies within sunlight.
Enhanced activity
The team found it could enhance the catalytic abilities of titanium dioxide by forming it into nanotubes each around 135 nanometres wide and 40 microns long to increase surface area. Coating the nanotubes with catalytic copper and platinum particles also boosted their activity.
The researchers housed a 2-centimetre-square section of material bristling with the tubes inside a metal chamber with a quartz window. They then pumped in a mixture of carbon dioxide and water vapour and placed it in sunlight for three hours.
The energy provided by the sunlight transformed the carbon dioxide and water vapour into methane and related organic compounds, such as ethane and propane, at rates as high as 160 microlitres an hour per gram of nanotubes. This is 20 times higher than published results achieved using any previous method, but still too low to be immediately practical.
If the reaction is halted early the device produces a mixture of carbon monoxide and hydrogen known as syngas, which can be converted into diesel.
Copper boost
"If you tried to build a commercial system using what we have accomplished to date, you'd go broke," admits Grimes. But he is confident that commercially viable results are possible.
"We are now working on uniformly sensitising the entire nanotube array surface with copper nanoparticles, which should dramatically increase conversion rates," says Grimes, by at least two orders of magnitude for a given area of tubes.
This work suggests a "potentially very exciting" application for titanium-dioxide nanotubes, says Milo Shaffer, a nanotube researcher at Imperial College, London. "The high surface area, small critical dimensions, and open structure [of these nanotubes] apparently provide a relatively high activity," he says.
Abstract in Nano Letters
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