Issue Date: August 25, 2008
Nanogold Oxidizes Alone
SHRINK GOLD nanoparticles down to a mere 1.4 nm in diameter and they can catalyze selective oxidation reactions without the help of support materials or additives that are normally required, according to a new study.
Gold has long been known to have size-dependent catalytic activity: The metal is inert in bulk, whereas gold nanoparticles 3 to 5 nm in diameter can catalyze a variety of reactions. But those larger nanoparticles require the addition of H2 or peroxide species to drive the reaction, or an electronic interaction with a support material such as titanium dioxide.
A group led by Richard M. Lambert, a chemistry professor at Cambridge University, has now reported that 55-atom gold clusters supported on inert material can catalyze the reaction of O2 with styrene to produce styrene oxide without the need for additives (Nature 2008, 454, 981).
Getting rid of additives makes the system more environmentally friendly. The particles likely adsorb and dissociate O2 into individual O atoms on the surface of the clusters. Then the O atoms initiate the reaction with styrene, Lambert says. Oxygenated hydrocarbons, in particular epoxides such as styrene oxide, are used in a variety of commercial chemical applications.
The use of cluster chemistry to prepare the catalysts seems to be "a convenient and direct route to produce particles too small to be easily synthesized by conventional preparation methods," says D. Wayne Goodman, a chemistry professor at Texas A&M University, in a commentary accompanying the report. He adds that the cluster particles conform to a very narrow and reproducible size distribution, unlike the products of traditional catalyst preparation.
The cluster-based system could also be extended to other metals or other classes of reactions, including the industrially important epoxidation of propylene, Lambert says. "Going down to such small particle sizes could also trigger other kinds of unexpected chemistry," he adds, referencing single-crystal studies in which reactions are sensitive to the geometry of crystal surfaces.
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