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Molecular machines split water

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3:31pm, September 7, 2004
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From Philadelphia, at a meeting of the American Chemical Society

Instead of running fuel cells on hydrogen derived from fossil fuels, a future hydrogen economy might be driven by water and sunlight. Inspired by natural photosynthesis, chemists at Virginia Polytechnic Institute and State University in Blacksburg have devised complex molecular machines that use the sun's energy to split water and form hydrogen.

Led by Karen Brewer, the researchers made the tiny machines, called supramolecular complexes, out of three building blocks. Each block, made of a metal surrounded by organic compounds, performs its own task within the molecular machine. Two of the building blocks contain light-absorbing ruthenium atoms. When solar energy excites a single electron in each of these units, the electrons are shuttled toward the third building block. There, the metallic element rhodium collects the two electrons and delivers them to water molecules.

The collection of electrons at the core of the molecular machine is critical for splitting water efficiently, says Brewer. That's because the reaction that produces hydrogen gas (H2) from water (H2O) requires two electrons.

Other approaches to splitting water are limited in their ability to efficiently collect electrons. Either they can generate only one electron at a time, or they require a large amount of energy to collect two electrons and therefore can't be driven by the low energy carried in visible light, says Brewer.

To test their new scheme, the researchers placed the molecular machines in a glass vial containing water. When they exposed the setup to visible light, hydrogen gas formed in abundance. Brewer envisions incorporating her molecular complexes into solid materials that could look like typical solar panels.

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