Flask-based photosynthesis
If we could emulate photosynthesis we would have a way to harness the endless supply of energy from the sun and perhaps solve many of our environmental problems. In a flash of inspiration, MIT researchers have taken a leaf out of nature's book to come up with a compound that produces hydrogen gas with the help of a catalyst and a burst of light.
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According to James McCusker, writing in Science*, "It has been estimated that the amount of solar energy reaching Earth's surface every day is more than mankind could use in 30 years". It makes environmental sense, therefore, to find the ways and means to cash in on this energy currency.
Daniel Nocera and former graduate student Alan Heyduk reckon they have come up with something close to "photosynthesis in a beaker", which could provide the raw fuel needed for fuel cells, which combine hydrogen with oxygen from the air to produce electricity, and water as a by-product.
Ford, Mercedes, General Motors, and other car manufacturers are looking to fuel cells as the greener alternative to the fossil-fuel guzzling internal combustion engine. The problem has remained in producing the hydrogen on which a fuel cell runs. If this can be generated catalytically using sunlight, then the reliance on fossil fuels for hydrogen production would also be removed from the overall energy-environment equation.
The MIT team have essentially trapped photon energy in a structurally well-defined molecule. They can then control the subsequent reactions that convert light into hydrogen. They used a two-electron mixed-valence dirhodium compound to photocatalyse the reduction of hydrohalic acid to hydrogen. In this cycle, photons break two RhII-X bonds of a LRh0-RhIIX2 core in the presence of a halogen trap to regenerate the active LRh0-Rh0 catalyst, which reacts with hydrohalic acid to produce hydrogen.
The closest researchers have come previously to achieving this goal is to use solid photocatalysts but these require massive surface areas to be effective. A homogenous catalyst circumvents this problem. McCusker points out that the MIT success is tempered by certain problems such as the need to close the catalytic loop and remove the halogen without it simply accumulating in a waste product trap. The conversion efficiency of this particular catalyst, which also degrades too quickly to be practical in the long-term, are not important at this stage. "The importance of this work...lies in the new opportunities it offers," explains McCusker, "Heyduk and Nocera have taken fundamental ideas of photochemistry and harnessed them to achieve a long sought-after but elusive goal, the molecular-based photocatalytic production of a usable fuel." Nocera concedes that the new process is not perfect, but he suggests this the work may re-ignite solar energy research.
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