Using just light and tiny nanoparticles of the rare metal rhodium, researchers at Duke University have found a way to help turn carbon dioxide into one of the building blocks of many fuels. The newly discovered chemical reaction could use natural sunlight to reduce growing levels of CO2 in the atmosphere and lead to the development of alternative energies without the creation of unwanted byproducts like toxic carbon monoxide.

Ultraviolet light fuels the reaction using the nanoparticles of the silvery element rhodium, which is related to platinum. Using light instead of heat is not only more efficient, but critically it more heavily favors the formation of methane over other undesirable byproducts.

This is potentially good news for space travelers, since methane is one of SpaceX's fuels of choice for getting to Mars.

"We discovered that when we shine light on rhodium nanostructures, we can force the chemical reaction to go in one direction more than another," said Henry Everitt, an adjunct professor of physics at Duke and senior research scientist at the Army's Aviation and Missile RD&E Center in Alabama. "So we get to choose how the reaction goes with light in a way that we can't do with heat."

A powdery material consisting of synthesized rhodium nanocubes was placed in a reaction chamber and mixtures of carbon dioxide and hydrogen were then passed through the material. When the nanoparticles were illuminated with a high-powered ultraviolet LED, the reaction took place at room temperature and yielded almost exclusively methane, compared to creating an equal mix of methane and poisonous carbon monoxide when 300-degree Celsius (576° F) heat was used to generate the reaction instead.

Scientists refer to this ability to be able to control the output of a reaction as its selectivity.

"If the reaction has only 50 percent selectivity, then the cost will be double what it would be if the selectively is nearly 100 percent," said Xiao Zhang, a graduate student in Professor Jie Liu's lab at Duke. "And if the selectivity is very high, you can also save time and energy by not having to purify the product."

The advance comes out of the field of plasmonics, and involves using light to add energy to tiny bits of metal at the nanoscale. We've already seen the approach lead to results for new solar cells and data storage, among numerous applications. Rhodium is a candidate for use in experiments because, although rare, small amounts of it are already used as a catalyst to accelerate industrial processes involved in the production of fertilizers, detergents and pharmaceuticals. It can also be found in catalytic converters in automobile exhaust systems.

"Effectively, plasmonic metal nanoparticles act like little antennas that absorb visible or ultraviolet light very efficiently and can do a number of things like generate strong electric fields," said Everitt. "For the last few years there has been a recognition that this property might be applied to catalysis."

Next, the researchers will see if light can drive other reactions in which heated rhodium is used as a catalyst. They also hope to tweak the results of this study to work specifically with sunlight in the hopes that a solar-powered reaction can become part of renewable energy systems.

A paper on the research appears in the online version of Nature Communications.

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