RUTHENIUM CRYSTALS: Two separate groups of researchers working on two separate parts of the photosynthetic reaction happened to be using the same class of catalyst--ones with an atom of the metal ruthenium (seen here) surrounded by organic molecules. Image: Courtesy of Periodictableru, via Wikimedia Commons
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Sunlight can provide more than enough energy to meet our needs—in theory. In practice, the skies are sometimes covered with clouds—and whether fair or overcast, the sun daily disappears behind the horizon. To get around these limitations, scientists have worked for years on new ways of converting sunlight into chemical energy, artificial forms of photosynthesis that would store solar energy in liquid or gaseous form—a "solar fuel." For years, they have sought a chemical catalyst that can perform this complex feat of chemical processing. Now some researchers think they may have found it.
Thomas Meyer came upon the solution almost by accident. Meyer, a chemist at University of North Carolina at Chapel Hill and director of its Energy Frontier Research Center in Solar Fuels, noticed that two separate groups of researchers working on two separate parts of the photosynthetic reaction happened to be using the same class of catalyst—ones with an atom of the metal ruthenium surrounded by organic molecules. One group used this type of catalyst to split water into hydrogen and oxygen; the other one was splitting carbon dioxide into carbon monoxide and oxygen. "Finding a single catalyst that does both was a big surprise," Meyer says.
By combining the two steps and using the same catalyst, Meyer realized that they could reproduce photosynthesis in its entirety. Whereas natural photosynthesis, after multiple reactions, converts water, carbon dioxide and sunlight into oxygen and energy-rich fuels such as sugar, Meyer's version converts water and carbon dioxide into oxygen, hydrogen and carbon monoxide—and the latter can be combined with hydrogen to eventually make a fuel such as methanol.
These findings suggest that it may be feasible to take carbon emitted from, say, a coal plant and use it to make a liquid fuel such as methanol that replaces or supplements fossil fuels for transportation or electricity generation. How would it work? Carbon dioxide-laden water from a fossil-fuel plant would pass across ruthenium catalyst membranes, which would trigger artificial photosynthesis, breaking it down into oxygen as well as constituents that can be converted to fuel. Electrical energy to drive the catalytic reaction would come from solar-power cells—although eventually researchers might be able to modify the catalyst to absorb sunlight directly. "That really would make it like photosynthesis," Meyer says. He and his colleagues detailed their findings online June 4 in Proceedings of the National Academy of Sciences.
The work is "a very interesting result from the point of view of fundamental science," says Princeton University chemist Andrew Bocarsly, who did not take part in this research. "It could be a dramatic simplification of a natural system, which could be very useful from a pragmatic point of view." (Bocarsly is working on ways of splitting carbon dioxide to create carbon monoxide, which can then be used to manufacture methanol and other fuels. Massachusetts Institute of Technology chemist Dan Nocera is developing cobalt-based catalysts that split water.)
Meyer's finding, however, needs further development before it is ready for commercial-scale use. For instance, Meyer's ruthenium catalyst is not very energy efficient when it comes to splitting carbon dioxide. It requires 1.65 volts to drive the reaction. By contrast, Bocarsly's system can turn carbon dioxide into methanol, "an arguably much more useful molecule, with 0.2 volts." Meyer acknowledges this drawback, and he's working on improving the process so that it works faster and needs less voltage. "We're continuing the process of iteration to make things better," he says.
Nevertheless, Meyer's experiment with the ruthenium catalyst, the first to do photosynthesis in its entirety, is a potentially big practical breakthrough.