Angewandte Chemie International Edition 2007, 46, 7770–7774
Splitting Water with Sunlight
Semiconductor acts as photocatalyst, storage, and separator for hydrogen and oxygen from water
Contact: Martin Demuth, Max-Planck-Institut für Bioanorganische Chemie, Mülheim an der Ruhr (Germany)
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A Titanium Disilicide Derived Semiconducting Catalyst for Water Splitting under Solar Radiation—Reversible Storage of Oxygen and Hydrogen
Hydrogen is one of the most important fuels of the future, and the sun
will be one of our most important sources of energy. Why not combine the
two to produce hydrogen directly from solar energy without any detours
involving electrical current? Why not use a process similar to the
photosynthesis used by plants to convert sunlight directly into chemical
energy? Researchers from the German Max Planck Institute have now
developed a catalyst that may do just that. As they report in the
journal Angewandte Chemie, titanium disilicide splits water into
hydrogen and oxygen. And the semiconductor doesn’t just act as a
photocatalyst, it also stores the gases produced, which allows an
elegant separation of hydrogen and oxygen.
“The generation of hydrogen and oxygen from water by means of
semiconductors is an important contribution to the use of solar energy,”
explains Martin Demuth (of the Max Planck Institute for Bioinorganic
Chemistry in Mülheim an der Ruhr). “Semiconductors suitable for use as
photocatalysts have been difficult to obtain, have unfavorable
light-absorption characteristics, or decompose during the reaction.”
Demuth and his team have now proposed a class of semiconductors that
have not been used for this purpose before: Silicides. For a
semiconductor, titanium disilicide (TiSi2) has very unusual
optoelectronic properties that are ideal for use in solar technology. In
addition, this material absorbs light over a wide range of the solar
spectrum, is easily obtained, and is inexpensive.
At the start of the reaction, a slight formation of oxide on the
titanium disilicide results in the formation of the requisite
catalytically active centers. “Our catalyst splits water with a higher
efficiency than most of the other semiconductor systems that also
operate using visible light,” says Demuth.
One aspect of this system that is particularly interesting is the
simultaneous reversible storage of hydrogen. The storage capacity of
titanium disilicide is smaller than the usual storage materials, but it
is technically simpler. Most importantly, significantly lower
temperatures are sufficient to release the stored hydrogen.
The oxygen is stored as well, but is released under different conditions
than the hydrogen. It requires temperatures over 100 °C and darkness.
“This gives us an elegant method for the easy and clean separation of
the gases,” explains Demuth. He and his German, American, and Norwegian
partners have founded a company in Lörrach, Germany, for the
further development and marketing of the proprietary processes.