Making Gasoline from Carbon Dioxide

A solar-powered reaction turns a greenhouse gas into a valuable raw material.

By Kevin Bullis

Solar splitter: An amber-colored semiconductor (gallium phosphide), together with metal contacts, is part of a new device that uses solar energy to split carbon dioxide to make carbon monoxide.

Chemists have shown that it is possible to use solar energy, paired with the right catalyst, to convert carbon dioxide into a raw material for making a wide range of products, including plastics and gasoline.

Researchers at the University of California, San Diego (UCSD), recently demonstrated that light absorbed and converted into electricity by a silicon electrode can help drive a reaction that converts carbon dioxide into carbon monoxide and oxygen. Carbon monoxide is a valuable commodity chemical that is widely used to make plastics and other products, says Clifford Kubiak, professor of chemistry at UCSD. It is also a key ingredient in a process for making synthetic fuels, including syngas (a mixture largely of carbon monoxide and hydrogen), methanol, and gasoline.

The work is part of a growing effort to find practical uses for carbon dioxide, a leading greenhouse gas, says Philip Jessop, professor of chemistry at Queen's University, in Ontario, Canada. Converting carbon dioxide into carbon monoxide is difficult to do, which Jessop says makes the UCSD work impressive and exciting.

At least at first, such a process will not make a significant impact on reducing greenhouse gases in the atmosphere--that would take quite large-scale operations, Kubiak says. But "any chemical process that you can develop that uses CO₂ as a feedstock, rather than having it be an end product, is probably worth doing." He adds that "if chemical manufacturers are going to make millions of pounds of plastics anyway, why not make them from greenhouse gases rather than making tons of greenhouse gases in the process?"

The system may also be part of a solution to a continuing problem with solar energy. For solar panels to be useful when the sun isn't shining, the electricity they produce has to be stored. A potentially practical way of doing that is by converting the electrical energy into chemical energy. One popular approach is to use solar cells to produce hydrogen, which could then be used in fuel cells. But hydrogen gas is much more difficult to transport and store than are liquid fuels, such as gasoline, which contain far more energy by volume than hydrogen does. The UCSD system shows that it is possible to use solar energy to make carbon monoxide that then, together with hydrogen, can be converted into gasoline. Currently, carbon monoxide is made from natural gas and coal. But carbon dioxide is a more attractive raw material in part because it's very cheap--indeed, it's something industrial companies will pay to get off their hands, Jessop says. "There are very few chemicals which are cheaper than free, and carbon dioxide is one of them," he says.