Nanogold is very different from its bulk counterpart – the former is an excellent catalyst, whilst the latter is extremely inert. Nanogold is very good for "alkene epoxidation" – an important industrial process in which an oxygen atom is inserted into the carbon–carbon double bond of the alkene. However, the problem is that such catalysis requires oxygen to be supplied in an activated form (as a peroxide or co-fed with hydrogen gas, for example).

Now, Mark Turner and colleagues have shown that if the particles are made sufficiently small (around 1.4 nm), they can carry out epoxidation using molecular oxygen alone. "Besides the environmental and industrial implications of this finding, the results are fundamentally interesting, especially for anyone involved in the field of gold catalysis," Turner told nanotechweb.org.

The researchers obtained their results by synthesizing 55-atom clusters of gold and immobilizing these on inert supports. They tested the supported gold as a catalyst for alkene epoxidation in a simple liquid-phase reactor using only oxygen as the feedstock.

High-resolution transmission electron microscopy was used to directly image the gold clusters, which allowed the team to measure the size of many nanoparticles. High-resolution X-ray photoelectron spectroscopy at the UK Daresbury Lab also revealed how small particle sizes directly affect the electronic properties of gold.

Special particles
So what is so special about such tiny gold particles? "We have begun to answer this question using X-ray photoelectron spectroscopy to show that the electronic structure of gold is perturbed in this size regime," explained Turner. "This is a result of quantum confinement effects that occur in extremely small metal clusters." The small gold nanoparticles also contain a number of gold atoms in "edge" or corner sites, a low-coordination environment that favours catalysis.

"Although we are some way off an industrial catalyst, our work points to the development of heterogeneous gold catalysts capable of carrying out industrially important transformations using only clean oxygen," added Turner.

The team will now test their nanogold catalysts for the epoxidation of propene and other target alkenes in the gas phase.

The results were reported in Nature.