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Gold cure for heavy industry's hangover
05 March 2005
From New Scientist Print Edition
Kurt Kleiner Toronto

Gold may hold the key to cleaning up a persistent toxic pollutant from groundwater much faster and cheaper than is possible now. Scientists in the US have hit upon a promising way to use the precious metal to clean up the carcinogen trichloroethane (TCE).

TCE is a colourless organic liquid that smells a little like chloroform. It is mostly used as an industrial degreaser in the manufacture of metal engineering parts such as steel tubes and pipes. TCE can cause liver damage and cancer in humans, but until regulations came into effect in the US in 1989, industry dumped hundreds of thousands of tonnes of the stuff on land or into water courses, according to the US Environmental Protection Agency. And because TCE breaks down slowly in the environment, a lot of it still contaminates groundwater. Estimates for cleaning it up in America's brownfield sites run to billions of dollars.

One promising solution is to pump contaminated groundwater out of aquifers, mix it with hydrogen gas and send it through a filter containing a palladium metal catalyst. This converts the TCE molecule into non-hazardous ethane and chloride ions, and the water can then be returned to the aquifer.

Now researchers have found that bonding palladium atoms to particles of gold creates a catalyst that is 100 times more efficient than particles of palladium on their own. The material could make it far more cost-effective to clean up TCE, according to Michael Wong and colleagues at Rice University in Texas and the Georgia Institute of Technology in Atlanta.

In trying to boost the efficiency of the catalyst, Wong's team initially investigated making the particles as small as possible to increase the catalytic surface area. But even in palladium particles measuring just 6 nanometres (or 15 hydrogen atoms) in diameter, only a quarter of the atoms are exposed on the surface. So the team tried a different approach.

They created particles of gold 5 nanometres in diameter and coated them with a single layer of palladium atoms. This means that every single palladium atom is available to help the catalysis.

The gold-centred particles proved a great success, but the research threw up a puzzle: partially coated gold particles were more efficient catalysts than particles covered entirely, even though fewer palladium atoms were available. The most efficient catalyst was a gold particle with about a third of its surface covered, yielding a hundredfold improvement on pure palladium.

Wong is baffled, since gold by itself isn't a catalyst. Somehow the gold is making the reaction more efficient, he says. However it works, Wong estimates the new catalyst would be cheaper than today's commercial palladium version. Even though it uses gold, so little catalyst is needed that there would be an overall saving.

From issue 2489 of New Scientist magazine, 05 March 2005, page 26

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