Scientists Peel Away Mystery Behind Gold's Catalytic Prowess

Tag : Agrochemicals

But for all of its popular uses – money, medals, jewelry andmore – gold's potential as a catalyst lay hidden until the1980s, when Masatake Haruta and Graham Hutchings independentlydiscovered that gold, which had long been considered inactive,could be an extraordinarily good catalyst. Haruta demonstrated thelow-temperature oxidation of CO and Hutchings the hydrochlorinationof acetylene to vinyl chloride.
Gold particles measuring less than 5 nanometers in diameter possessa high level of catalytic activity when they are deposited onmetal-oxide supports, Haruta learned. One nanometer (nm) is equalto one one-billionth of a meter, or about the width of five atoms.
In particular, Haruta found that gold nanoparticles are effectiveat catalyzing the critical conversion of toxic carbon monoxide (CO)into more benign carbon dioxide (CO 2 ) at room temperature and even at temperatures as low as -76degrees C. CO oxidation is vital to firefighters and others whomust enter burning buildings, and it is also critical to theprotection of hydrogen fuel cells from CO contamination.
In the two decades since Haruta's discovery, scientists have soughtto determine exactly how gold nanoparticles function as catalysts.
Now, researchers from Lehigh University in Bethlehem, Pa., andCardiff University in the UK believe they have pinpointed theactive species at which the critical oxidation reaction occurs whengold is supported on iron oxide.
In an article to be published in Science, researchers from LehighUniversity in Bethlehem, Pa.; Cardiff University in Wales, and theNational Institute of Standards and Technology (NIST) report thatbilayer clusters measuring about one-half nanometer in diameter andcontaining only about 10 gold atoms are responsible for triggeringthe CO oxidation reaction.
The researchers, using aberration-corrected transmission electronmicroscopy capable of resolving single gold atoms, also report thata simple change in preparation – the drying of the catalystin flowing rather than static air – helps impart to the goldits catalytic capability.
The authors are Christopher Kiely, director of theNanocharacterization Laboratory in Lehigh's Center for AdvancedMaterials and Nanotechnology; Graham Hutchings, Albert Carley andPhilip Landon of Cardiff's School of Chemistry; and Andrew Herzingof NIST's Surface and Microanalysis Science Division. Herzingearned a Ph.D. from Lehigh in 2006.
Hutchings and Kiely have collaborated since 1989 and have workedtogether on gold catalysts since 2000. In this project, Hutchings'group carried out the fabrication and catalytic testing of the goldnanoparticles, and the characterization of the catalyst using x-rayphotoelectron spectroscopy (XPS). Kiely's group then used Lehigh'saberration-corrected 2200 JEOL scanning transmission electronmicroscope (STEM) to examine the gold's nanostructure. Lehigh iscurrently the only university in the world with twoaberration-corrected electron microscopes, which are the world'smost powerful instruments for chemical analysis.
The researchers compared two groups of gold nanoparticles. One,dried in static air, was what scientists call a "dead"catalyst with little or no catalytic activity. The other group,dried with flowing air, was a 100-percent-active catalyst for COoxidation.
On the inactive catalyst, Herzing saw two types of gold species– particles larger than 1 nm in size and individual atomsscattered about on the iron-oxide support. On the100-percent-active catalyst, he found a third species –clusters of 8 to 12 gold atoms arranged in two layers measuringabout 0.5 nm in dimension.
"This was the clue that enabled us to identify the tinybilayer clusters as the important species in the catalyticreaction," said Kiely. "It turns out that only about 2percent of the gold deposited on the support ended up in thisparticular type of cluster.
"We then deactivated the catalyst by various heat treatmentsand found that we could correlate the loss of the clusters with theloss of activity. This gives us strong evidence that the activespecies in the catalyst are the tiny bilayer clusters.
"We believe we have obtained the first conclusive evidencethat bilayer clusters are occurring in a real gold catalyst, thatthey are the key species on that catalyst, and that their presenceor absence correlates with the ability or failure of the catalystto perform CO oxidation."
Before Lehigh's acquisition of the aberration-corrected electronmicroscopes in 2004, Kiely and Hutchings were able to see thelarger gold particles, but not the individual atoms or bilayerclusters of atoms.
"At that time, when we compared the dead catalyst and theactive catalyst," said Kiely, "both looked the same. Thenew microscopes have opened up a new window allowing us to see whatis really going on."
The aberration-corrected STEM enabled Herzing and Kiely to use amicroscopy technique called high-angle annular dark-field imaging,which requires an extremely fine, 1-angstrom-wide beam of electronsto obtain a scanned image of a specimen. An angstrom is equal toone-tenth of a nanometer.
Kiely said the gold catalysts could find a potential application inthe protective masks capable of converting CO to CO 2 that are worn by firefighters and others exposed to high levels ofCO. Another application is to fuel cells that are vulnerable topoisoning by the CO that is present in the hydrogen fuel stream.
Gold catalysts are also being explored for their effectiveness incatalyzing the reaction that is used to steam-reform methane intohydrogen.
The article by Kiely, Hutchings and their collaborators is thesecond by the group to be published by Science in the past twoyears. The first, published in 2006, reported on the use ofgold-palladium nanoparticles to catalyze the conversion of alcoholsinto aldehydes, the chemical process that is important in thesynthesis of some spices and perfumes.
In 2005, the group reported in Nature that the selective oxidationprocesses used to make compounds contained in agrochemicals,pharmaceuticals and other chemical products could be accomplishedmore cleanly and more efficiently with gold nanoparticle catalysts.
Journal reference : Christopher Kiely, Graham Hutchings, Albert Carley, Philip Landon,Andrew Herzing. Identification of Active Gold Nanoclusters on Iron Oxide Supportsfor CO Oxidation . Science , Sept. 5, 2008