A link between palladium cluster size, the electronic properties of the cluster, and catalytic activity is being reported by a research group led by the University of Utah’s Scott L. Anderson (Science 2009, 326, 826). The activity of heterogeneous catalysts is known to depend on catalyst cluster size, but a lack of experimental probes for looking at sample structure has prevented a systematic study of catalytic trends. Anderson and colleagues deposited clusters of up to 25 palladium atoms on titania and then used X-ray photoemission spectroscopy to study palladium’s electronic structure. The team also evaluated the palladium clusters’ ability to catalyze carbon monoxide oxidation reactions. They found that low catalytic activity for Pd1, Pd7, and Pd25 correlated to relatively high 3d electron binding energies. Although palladium’s core 3d electrons are not involved in bonding, they indirectly reflect the element’s valence electronic structure. That allowed the researchers to interpret the low catalytic activity as due to a particularly stable valence electronic structure for those three clusters. The researchers suggest that understanding factors such as electronic structure that impact catalytic activity will enable chemists to target only the most reactive cluster sizes to design better and less expensive catalysts.