Fuel from the air using the power of the Sun

Take carbon dioxide from the atmosphere and turn it into combustible fuel using the Sun in just one step. It sounds too good to be true – but a recipe published in the Proceedings of the National Academy of Sciences today claims to do just that.

Chemists and engineers from the University of Texas at Arlington made hydrocarbons by cooking carbon dioxide and water at 180 to 200 °C under high pressure, along with a catalyst and ultraviolet radiation from the Sun.

They say it's a “promising new path” to a carbon-neutral fuel cycle – important as we need to keep carbon dioxide emissions at zero or negative between 2030 and 2050 to limit global warming to 1.5 °C, according to the deal thrashed out at the Paris climate conference last year.

And while renewables such as solar and hydro-electricity have undoubtedly skyrocketed in the past decade, it will be harder to wean ourselves off hydrocarbons to keep cars and aeroplanes running.

Since the mid-1970s, researchers have tried to devise ways of making hydrocarbon fuels from carbon dioxide.

The first direct product was hydrocarbons containing just one carbon atom, such as methane and methanol. But while methane and methanol do produce energy when combusted, longer hydrocarbons containing more carbon atoms – such as octane, which has eight carbons – produce more energy bang for their buck. 

So researchers aimed to make hydrocarbons with five or more carbon atoms by adjusting temperature, pressure and radiation. Results so far have been inconsistent, and mostly yielded single-carbon products.

To try to maximise long hydrocarbons, the University of Texas at Arlington team embraced high heat, pressure and radiation. They pumped steam and carbon dioxide into a quartz tube containing glass beads coated in a cobalt and titanium oxide catalyst.

As they cranked up heat and pressure, and exposed the tube to ultraviolet light from the Sun, the water split into hydrogen and oxygen.

The cobalt catalyst snapped the carbon dioxide apart and reassembled it with hydrogen, producing water and a fuel mix. Some 13% of the hydrocarbons in the mix contained five or more carbon atoms. There was even some octane, and hydrocarbons with 13 carbon atoms.

The team admits the efficiency of the system is not commercially viable at the moment. But with optimisation, they write, “a sustainable and carbon-neutral liquid fuel cycle could be realised”.

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