Researchers find a way to enhance the particles’ catalytic activity
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Gold nanoparticles are becoming some of chemistry’s best diplomats.
They facilitate a wide range of reactions between molecules that would not normally interact or would do so only at much higher temperatures.
And in most cases, says Christopher Kiely, professor of materials science and engineering, they effect a single favorable outcome.
Conventional methods of preparing gold nanoparticles, however, alter their morphology and catalytic activity.
Researchers from Lehigh and from Cardiff University in Wales in the U.K. have developed a procedure that enhances the surface exposure of gold nanoparticles and their catalytic activity over a range of reactions. They reported their results in July in Nature Chemistry.
“In industry,” says Kiely, “the most common way of preparing gold nanocatalysts is to impregnate a nanocrystalline oxide support with chloroauric acid. A reduction reaction then converts the acid into metal nanoparticles.
“Unfortunately, this leads to a variety of gold species — isolated atoms, mono- and bilayer clusters and nanoparticles of various sizes — being dispersed on the support.”
An alternative technique that allows greater control over particle size and structure, is to pre-form the nanoparticles in a colloidal solution before depositing them onto the support.
The disadvantage to this method is that during fabrication the nanoparticles are coated with ligands that prevent them from clumping together. These ligands tend to impair the nanoparticles’ catalytic performance by blocking the approach of molecules to active sites.
Methods for stripping away these ligands involve heat treatments of up to 400 degrees C. This alters the morphology of the nanoparticles, causing them to coalesce and their catalytic activity to decrease.
Kiely’s team developed a milder way to remove ligands from polyvinyl alcohol-stabilized gold nanoparticles deposited on a titanium oxide support — a simple hot water wash.
Using aberration-corrected scanning transmission electron microscopy, the researchers compared catalysts that had been washed with those that underwent heat treatment.
“Hot water washing had very little effect on particle size,” says Kiely, “and while the particles retain their cub-octahedral morphology, their surfaces appear to become more distinctly faceted. This is presumably due to some surface reconstruction occurring after losing a significant fraction of the protective PVA ligands.”
“Heating the samples to 400 degrees C also effectively removed the ligands but average particle size increased from 3.7 to 10.4nm,” says Kiely.
“The particles tended to restructure and develop flatter, more extended interfaces with the TiO2 support.“
For the oxidation of CO to CO2, catalysts prepared by the hot water wash displayed more than double the activity of conventional gold/TiO2 catalysts. This reaction is crucial for removing CO from enclosed spaces and for prolonging the lifetimes of fuel cells and firefighter masks.
