The idea is simple. Find a way of removing an oxygen atom from a CO2 molecule and you are left with carbon monoxide (CO). From there it is but a short step to hydrocarbon riches. Mix CO with hydrogen, pass the mixture over a catalyst, and out comes liquid hydrocarbon fuel. This reaction, called the Fischer-Tropsch process, was invented as long ago as the 1920s.

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The idea of using solar energy to convert CO2 into a carbon-based fuel is being taken a step further by Gabriele Centi at the Department of Industrial Chemistry and Engineering of Materials at the University of Messina, Italy. Rather than producing CO with a view to turning that into something more useful, he is building an electrochemical cell that produces hydrocarbon molecules such as nonane and ethylene - important chemical building blocks for plastics and other materials currently derived from oil.

Centi's cell is a distant cousin of the fuel cells that generate electricity by reacting hydrogen or methanol with oxygen, but with the chemical reaction running in reverse. On one side of the cell is a titanium dioxide catalyst that encourages water molecules to split when hit by photons of sunlight, producing hydrogen ions and oxygen gas. The hydrogen ions migrate through a proton exchange membrane to the other side of the cell, where a catalyst containing platinum nanotubes facilitates the reaction with CO2 to produce hydrocarbons.

From March, 2008. This would create a closed carbon cycle. The more time goes by, the more I think this is what we actually need. A biotech answer is fungus that makes diesel.