IT IS the biggest contributor to climate change. Now chemists are hoping to convert carbon dioxide into a useful fuel, with a little help from the sun.

If they succeed, it will be possible to recycle the greenhouse gas produced by burning fossil fuels. The work could also lead to a way for future Mars missions to generate fuel for their return journey from carbon dioxide in the planet's atmosphere.

Chemists have long hoped to find a method of bringing the combustion of fuel full circle by turning CO₂ back into useful hydrocarbons. Now researchers at the University of Messina in Italy have developed an electro-catalytic technique they say could do the job. "The conversion of CO₂ to fuel is not a dream, but an effective possibility which requires further research," says team leader Gabriele Centi.

The researchers chemically reduced CO₂ to produce eight and nine-carbon hydrocarbons using a catalyst of particles of platinum and palladium confined in carbon nanotubes. These hydrocarbons can be made into petrol and diesel.

To begin with, the researchers used sunlight plus a thin film of titanium dioxide to act as a photocatalyst to split water into oxygen gas plus protons and electrons. These are then carried off separately, via a proton membrane and wire respectively, before being combined with CO₂ plus the nano-catalyst to produce the hydrocarbons.

Although the nano-catalysts produced two or three times more hydrocarbons than a commercially available catalyst, the process converted only about 1 per cent of the CO₂ at room temperature. Centi believes it will be possible to improve on that by using higher temperatures and a larger surface area of catalyst. It will also be necessary to boost the efficiency of the solar water-splitting, he says. With the right research, Centi believes that an efficient solar-powered reactor for converting CO₂ into fuel could be available "within a decade".

He presented his latest work, which is funded by the European Union, at a meeting of the American Chemical Society in San Francisco on 13 September. Other chemists reacted positively, but cautiously, to the findings. "It sounds feasible," says John Turner from the US National Renewable Energy Laboratory in Golden, Colorado. "The solar-to-hydrocarbon conversion efficiency is pretty small, but it sounds like they are just getting started."

Ian Plumb, who researches water-splitting reactions at the Australian national research institute CSIRO Industrial Physics, says that unless the efficiency is improved it will be too expensive to implement. "But there is no doubt that what they are trying to achieve is very worthwhile."