Credit: © 2009 AAAS

Many useful catalysts can be made from clusters of metal atoms bound to a metal oxide surface, and it is well-known that the size of the clusters can affect the catalyst's reactive performance. Now, Scott Anderson and co-workers at the University of Utah1 have shown that the catalyst performance also depends on the electronic structure of the metal clusters, which is harder to predict.

Anderson and colleagues selected various sizes of palladium clusters in the gas phase, and deposited them onto titanium dioxide surfaces. This produced catalysts capable of inducing the oxidation of carbon monoxide into carbon dioxide — a simple reaction often used for pollution control.

The researchers made catalysts with palladium cluster sizes ranging from 1 to 25 atoms, and used X-ray photoemission spectroscopy to assess the electronic binding energies of core electrons in the clusters. Clusters containing 1, 7 or 25 atoms showed higher binding energies than expected and, interestingly, produced less carbon dioxide. According to the researchers, these clusters may have particularly stable valence electron structures that make them less reactive.

The study also showed that clusters up to around 10 atoms in size form single-layer 'islands' on the catalyst surface, whereas larger clusters pile up in more than one layer. Both findings could have complex implications for future catalyst design.