Sunlight helps power high-energy hydrocarbon mill

Generating energy from sunlight comes with problems—such as how to store and distribute that energy—that are challenging some very smart people. That’s the message chemist  Frederick M. MacDonnell of the University of Texas, Arlington, has been opening his talks with lately.

Then he shows a photo of the comedy duo, Cheech & Chong, siphoning gasoline from a car to make a point: Any idiot can handle liquid fuels.

That’s why MacDonnell and his team have developed a process that creates liquid hydrocarbons and oxygen using sunlight, steam, and carbon dioxide (Proc. Natl. Acad. Sci. USA 2016, DOI: 10.1073/pnas.1516945113).

Their process—dubbed solar photothermochemical alkane reverse combustion or SPARC—generates fuel components, such as octane, that people worldwide already rely on, MacDonnell tells C&EN. “People can keep their cars, their jets, their lawnmowers.”

Researchers have previously produced hydrocarbons using the same starting ingredients, but it was rare for them to generate anything with more than one carbon, such as methane or methanol, MacDonnell says. Working with UT Arlington engineer Brian H. Dennis, the researchers developed a reactor that can create products with up to 13 carbon atoms.

Inside the reactor, sunlight strikes a bed of photocatalysts, made from cobalt-coated titanium dioxide beads, and liberates charge carriers. These carriers travel to a bead’s surface and oxidize water into oxygen and electrons. The cobalt gobbles up the electrons, which then react with protons and CO₂ to generate hydrocarbons in a process similar to Fischer-Tropsch synthesis, MacDonnell explains.

“This is an interesting and important demonstration,” says Nathan S. Lewis, who is researching fuel-from-sunlight techniques at the California Institute of Technology. But he adds, “further work will be required to improve the yields.”

MacDonnell agrees, noting the system is nowhere near optimized with less than 1% quantum yield—a measure of how many hydrocarbons come out for every photon that goes in. But he stresses that, “the real discovery is the ability to directly produce heavy hydrocarbons in quantity from carbon dioxide and water in a single step.”