Issue Date: January 26, 2009
Atom Pairs In Metal Clusters Are Key To Reactivity
Pairs of complementary atoms in small aluminum cluster anions are responsible for the clusters’ reactivity with water, according to a research team led by A. Welford Castleman Jr. of Pennsylvania State University, University Park, and Shiv N. Khanna of Virginia Commonwealth University (Science 2009, 323, 492). Using a combination of experimental and computational methods, the researchers found that a lone pair of electrons on the oxygen atom of a water molecule can serve as a nucleophile in an attack on a specific metal site of an Al12– cluster, forming an Al12H2O– intermediate. An adjacent Al atom then pulls a hydrogen atom away from the water, forming an Al–H bond. For slightly larger clusters—Al16–, Al17–, and Al18–—the researchers observed that the complexes evolve H2. The researchers propose that water is dissociatively adsorbed at two sites on these clusters so that the intermediate species (one shown) have enough thermal energy to recombine the surface-bound hydrogens and release H2. A better understanding of how specific sizes and shapes of metal clusters affect their reactivity could facilitate the design of new materials for targeted applications, the researchers note.
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