Make methane while the sun shines

Researchers have used sunlight to convert carbon dioxide and water vapour into a range of fuels faster than ever before, thanks to  nanotube catalyst. Materials scientists Craig Grimes and his colleagues at Pennsylvania State University Park have used hollow titania (titanium dioxide) nanotubes around 135 nanometers wide and a tenth of a millimetre long to catalyze the reaction. Scientist have used titania nanoparticles to speed up this process before, but Grimes and his colleagues were able to generate hydrocarbons around 20 times faster  than that achieved in previous studies The researchers added a little nitrogen to their nanotubes and loaded copper and platinum nanoparticles onto the surfaces. On its own, titania works best as a catalyst for this reaction in ultraviolet light. But adding nitrogen and copper to the mix shifts the preference of the titania tubes to visible light, Grimes says. And the copper and platinum nanoparticles are thought to speed up the latter stages of the reaction.The reaction itself also takes place inside the nanotubes, which are hollow and have a large internal surface area thanks to their thin 20 nanometer-thick walls. The researchers filled steel tubes with carbon dioxide and water vapour, covered the end of the chambers with a film of their nanotubes, and capped the containers with a quartz window to let light in.The closed chambers were then set outside on the university campus in the university campus on sunny days from July to September 2008. When light falls on the nanotubes, they release energetic charge carriers, which split the water molecules inside them into two reactive components-hydroxide radicals and hydrogen ions.The hydrogen ions combine to form hydrogen gas. The researchers don't yet understand exactly what happens next, but they think that the carbon dioxide also splits to form oxygen and carbon monoxide, which then reacts with gaseous hydrogen to form methane and water. The devices generated roughly 160 microliters of the hydrocarbons per hour pergram of their titania nanotube, a rate at least 20 times higher than in previous studies done with ultraviolet light. The findings are published online in the journal Nano Letters.But the researchers argue that their process could be made continuous if carbon dioxide and water vapour could be passed though the nanotube film and the methane fuel collected form the other side.Even with their current nanotubes, Grimes calculated that a reflector that concentrates sunlight on a square meter of the nanotube film could yield 500 liters of methane over the course of eight hours.Grimes, however, agrees that the production rates are still quite low. But he hopes that depositing copper nanoparticles more evenly onto the surfaces of the nanotubes and making other improvements will help boost their conversion rates by a factor of several thousand.

The Hindu, 27th February 2009