Sunlight, Gold, Water, Carbon Dioxide Make Renewable Fuel

“Many of these chemical reactions are multiproton, multielectron reactions,” Jain said.

But instead of relying on biodegradable plant pigments to convert light energy into chemical energy, scientists are turning to something better: electron-rich metal catalysts like gold, which at specific light intensities and wavelengths can transfer photoexcited electrons and protons to reactants without being degraded or used up.

“In our study, we used spherical gold particles that are 13 to 14 nanometers in size,” Jain said. “The nanoparticles have unique optical properties, depending on their size and shape.”

When coated with a polymer and suspended in water, for example, the nanoparticles absorb green light and reflect a deep red color. Under light excitation, the nanoparticles transfer electrons to probe molecules, which then change color. This allows scientists to measure how efficiently the electron-transfer reactions are taking place.

“Researchers have managed in the past to use photochemistry and these light-absorbing materials to transfer one electron at a time,” Jain said. “But in the new study, we’ve identified the principles and rules and conditions under which a metal nanoparticle catalyst can transfer two electrons at a time.”

By varying the intensity of laser light used in the experiments, Jain and his colleagues discovered that at four to five times the intensity of solar energy, the gold nanoparticles in the system could transfer up to two electrons at a time from ethanol to an electron-hungry probe.

Two-electron reactions are far preferable to one-electron reactions, Jain said.

“You need a pair of electrons to make a bond between atoms,” he said. “When you don’t provide a pair of electrons – and a pair of protons to neutralize the loss of electrons – you end up making free radicals, which are highly reactive and can back-react, wasting the energy you used to create them. They also can react with other chemicals or destroy your catalyst.”

University of Illinois chemistry professor Prashant Jain and his colleagues report they can use light to drive two-electron chemical reactions, enhancing a method for converting CO2 into hydrocarbons.

Credit:  L. Brian Stauffer