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If hydrogen gas is to be the fuel of the future, then water will be the source from which it is derived. Although the raw material costs nothing, extracting hydrogen from it is energy intensive. It can be done by electrolytic means, but the electricity has to be generated and indeed that source of power might be better used in other ways. The cheapest source of energy on planet Earth is of course sunlight, and now paper #6 shows how this too might be used to generate hydrogen gas from water. Thirty years ago it was discovered that water could be decomposed to oxygen and hydrogen by means of ultraviolet light using a titanium dioxide catalyst, but this type of radiation only accounts for 4% of the incoming solar radiation. Now, scientists at the Photoreaction Control Research Center, which is part of Japan’s National Institute of Advanced Industrial Science and Technology based at Tsukuba Science City, have discovered a catalyst that can bring about the same decomposition using visible light, which accounts for more than 40% of the incoming solar energy. The group is led by Hironori Arakawa, and the work was done in conjunction with Jinhau Ye of the National Institute for Materials Science, which is also at Tsukuba. The catalyst they have discovered is indium-tantalum-oxide (InTaO4) doped with nickel. When a suspension of 0.5 g of this in 250 ml of pure water was exposed to blue light from a xenon arc light (wavelength 420 nm), bubbles of hydrogen and oxygen were produced. When the light was switched off there was no evolution of gas, but when it was again switched on the evolution began again. Indeed, Arakawa carried out all kinds of tests to prove the effect was not only genuine, but that the new catalyst did not lose its ability with time, even after 400 hours. The catalyst was made from very pure In2O3, Ta2O5, and NiO by a solid-state reaction at 1100 degrees Celsius. The group have also developed a two-catalyst system that mimics natural photosynthesis (see K. Sayama, et al., Chem.Commun., 23: 2416-2417, 2001), and recently they have published further details of the catalyst (see Z.G. Zhou, et al., J. Phys.Chem. B, 106[51]: 13098-101, 2002). The new catalysts may not yet guarantee lots of cheap hydrogen, as they have an energy efficiency of only 0.7% (for light of wavelength 420 nm), but they do show that the process is viable. Speaking to Science Watch, Arakawa admits that they have a long way to go: "We need to improve the activity about 100 times for commercial application, but the research holds out great potential for the future. This is a very simple system and a relatively economical way of directly splitting water into hydrogen and oxygen. What we need to develop is a highly efficient oxide semiconductor photocatalyst, and to that end we are designing new mixed oxide semiconductor materials." Also in the current Hot Ten at #7 is another paper from Japan and also on visible light photocatalysis. In this work, a team headed by Ryoji Asahi and based at the Toyota Central R&D Laboratories at Nagakute, used nitrogen-doped titanium dioxide to decompose simple organic molecules such as methylene blue and acetaldehyde. And while this work does not offer hope of solving the world’s energy needs, it may well have important applications in enabling the world’s chemical industry to become greener. 
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.
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Thomson Corporation. |
n 1996 the Royal National Theatre in London staged a play by Stephen
Poliakoff called Blinded by the Sun in which a chemist claimed to
have discovered a catalyst that would produce copious quantities of
hydrogen gas from water, simply by exposing it to sunlight. Of course
the claim was false, not that it prevented the chemist from becoming
famous—and infamous. Much as the world might long for such a method of
solving its energy problems, the idea seemed intrinsically implausible.
A mere five years later, however, a group of Japanese scientists was
within reach of such a goal.
.
