Clusters of gold

High-resolution microscopy reveals that bilayer clusters of ∼10 gold atoms are responsible for most of the activity of a supported gold catalyst

Credit: © 2008 AAAS

Catalysis of the low-temperature oxidation of carbon monoxide is frequently performed using gold nanoparticles on oxide supports. However, so far it has not been clear which active species is responsible for this process. Andrew Herzing and Christopher Kiely from Lehigh University in the USA, working with Graham Hutchings and colleagues at Cardiff University, UK, have now used various microscopy techniques to investigate Au/FeO_x catalysts¹ that have been prepared in the same way, but with different final steps.

Following previous work relating these catalysts' performance to the drying conditions used during their preparation, the researchers dried one Au/FeO_x sample in static air and a second sample in flowing air. The second sample catalysed carbon monoxide oxidation significantly better than the first sample, with 100% conversion at 25 °C, compared with <1%. Kiely and colleagues used atomic-resolution scanning tunnelling electron microscopy to determine the particle sizes of each sample, and observed subnanometre clusters of gold in the sample dried under flowing air, but not in the one dried under static air. Further investigation determined that the active species responsible for the catalysis are 0.5-nm-sized bilayer clusters containing about 10 gold atoms.

The key to the activity of Au/FeO_x catalysts therefore seems to lie in the way they are prepared. X-ray photoelectron spectroscopy showed that the first sample contained hydroxylated species, indicating that the lack of air flow inhibited their removal. These species seem to increase the sintering of the subnanometre clusters into larger particles, thus deactivating the catalyst. Drying the catalyst in flowing air removes the hydroxylated species, resulting in the subnanometre clusters retaining their small size and catalytic activity.