The words “”carbon dioxide”” usually describe an unwanted product that is generated when fossil fuels are burned to power cars or heat homes. But researchers at UCSD, with a little help from solar energy, are working on a device that could turn the greenhouse gas into a useful fuel. 

Professor of chemistry and biochemistry Clifford Kubiak and graduate student Aaron Sathrum are developing a method to use the sun’s energy to split carbon dioxide into oxygen and carbon monoxide, the latter of which is a valuable industrial fuel. While the researchers currently have to supply some of the energy necessary for the reaction, they hope to create a device that is entirely solar-powered. Their results, which they presented at the national meeting of the American Chemical Society in Chicago last month, show that such a construction is theoretically possible.

The device uses a semiconductor to convert energy from the sun into electrical energy that can be used to split carbon dioxide. When sunlight hits the semiconductor, the light’s energy is capable of pushing electrons into an “”excited”” state, generating electrical energy. A special catalyst is then required to convert the electrical energy into chemical energy. This catalyst takes the energy produced by free-flowing electrons and uses it to split carbon dioxide into carbon monoxide and oxygen.

Carbon monoxide is an important industrial gas, used as a reactant in a variety of chemical processes.

It can be used to make detergents as well as many types of liquid fuels. While there are many ways to generate carbon monoxide, the unique aspect of the device designed by Kubiak and Sathrum is that it utilizes solar energy while consuming a greenhouse gas.

The current model uses a silicon semiconductor because chemists are very familiar with the properties of silicon, making it an ideal material for Kubiak and Sathrum to use in testing their device.

However, a solar-powered silicon semiconductor can only provide about half the energy needed to split carbon dioxide. The researchers were able to get the device to work if they provided the other half of the energy needed for the reaction.

Kubiak and Sathrum are in the process of making a device that uses a gallium phosphide semiconductor. Gallium phosphide is a crystalline material that is mainly used for research purposes.

Since it has only been developed recently, scientists are still unfamiliar with its properties, and Kubiak’s group is currently working with engineers to figure out how this material can be used to optimize the device. Gallium phosphide are such that it can absorb more energy from sunlight and generate a larger amount of electrical energy than silicon, so under the right conditions, a gallium phosphide semiconductor should be able to produce enough energy to split carbon dioxide. 

There is much more research devoted to splitting water than to splitting carbon dioxide because people are interested in finding ways to generate hydrogen gas from water, since hydrogen gas is seen as a “”cleaner”” fuel.

However, Kubiak said that some methods for splitting water produce carbon dioxide as well as hydrogen gas.

So while using hydrogen gas for fuel does not directly release carbon dioxide into the atmosphere, the process of making hydrogen gas still results in carbon dioxide.

Thus, Kubiak believes that carbon dioxide splitting is just as important as water splitting because the former uses a greenhouse gas and makes a fuel that is just as useful as hydrogen gas.

 A system for capturing carbon dioxide from the air still needs to be worked out.

“”One of the big problems in schemes that would utilize atmospheric carbon dioxide is transporting a gas present at 0.03 percent of atmospheric pressure to the reactors where it is needed,”” Kubiak said. “”Much will need to be done to develop means of concentrating atmospheric carbon dioxide, or placing carbon dioxide conversion systems near large industrial sources of carbon dioxide, such as power plants.””

This work is particularly timely, with the state’s green agenda gaining prominence under Gov. Arnold Schwarzenegger.

And recently, billionaire Richard Branson offered a $25 million prize to the person who invents a way to remove at least one billion tons of carbon dioxide from the atmosphere over a 10-year period.