Bug enzyme generates fuel from water
By Colin Barras, New Scientist, 13 January 2009
Light-powered, bacterial enzyme-containing nanoparticles that release hydrogen from water could lead the way to new strategies for generating the energy-rich gas.
The lack of low-cost ways to create hydrogen gas is one of the main barriers to the dream of economies fuelled by hydrogen not oil.
A class of enzymes called hydrogenases are used by organisms to convert hydrogen ions to hydrogen gas during anaerobic - without oxygen - respiration. These enzymes have long interested chemists searching for alternatives to existing, expensive, platinum-catalysed hydrogen generation.
The metal-containing enzymes are all crippled in varying degrees by the presence of oxygen and are also damaged by the very hydrogen they produce. That makes them difficult and expensive to use on industrial scales, says chemist Erwin Reisner, at Oxford University in the UK.
Bacterial gift
Now Reisner and colleague Fraser Armstrong have discovered that one bacterial hydrogenase is much more resistant to both gases.
The nickel, iron and selenium-rich enzyme, first isolated by Juan Fontecilla-Camps at the University of Joseph Fourier in Grenoble, France, is produced by a sulphate-reducing bacterium.
Its efficiency is relatively unaffected by the presence of hydrogen gas, and it continues to work even if the surrounding air contains 1% oxygen by volume - ordinarily even a few parts per million of oxygen would block hydrogenase activity.
The new enzyme also binds strongly to titanium dioxide nanoparticles, making it easy to produce a kind of light-powered, hydrogen-generating dust.
'Promising trial'
The dust particles are each attached both to the enzyme and to light-absorbing dye molecules that are used in some solar cells.
In the presence of an electron-donating buffer solution, the dye absorbs light and releases excited electrons, which then pass to the enzyme. Suitably energised, the hydrogenase then converts hydrogen ions from water molecules into hydrogen gas - just as they would during the bacteria's respiration.
After a small sample of the nanoparticles spent 8 hours in a buffer solution under a tungsten-halogen lamp, the headspace gas above the solution was 4.6% hydrogen by volume - a result Armstrong calls "promising for a first trial". In a control experiment without the enzyme only trace quantities of hydrogen were present.
The reaction falls short of "true water splitting", says Armstrong. This would require another catalyst to release oxygen gas from the water molecules and provide the electrons to fuel the dye, making the buffer solution redundant. Developing that complete system is Reisner and Armstrong's next goal, they say.
Catalyst inspiration
The turnover rates for hydrogen gas cycling are comparable with platinum catalysts under certain conditions, so studying the new hydrogenase might inform the "design" of simpler catalysts that are as effective as platinum, but considerably cheaper.
Marc Fontecave at the Atomic Energy Commission in Grenoble, France, is working on such catalytic molecules. "What we need is catalysts working under aerobic conditions and not just 1% oxygen," he says. "Obviously, I'm more confident at the present stage in this type of catalyst than in hydrogenases."
Chris Pickett at the University of East Anglia in the UK agrees, but is still impressed with what has been achieved with the natural hydrogenase. "It is the enzyme systems which are showing what can be done and which are currently making the pace," he says.
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