Metal clusters of a given element can have chemical and electronic properties resembling those of a different atom. Such clusters, or 'superatoms', can be described using shell-filling rules that result from quantum confinement of the valence electrons, which parallel the shell-filling rules of atoms. Aluminium clusters can have superatom properties, and these shell-filling rules have been able to account for their reactivity with oxygen, but not with water. Welford Castleman and co-workers1 at Pennsylvania State University and Virginia Commonwealth University have now discovered that the reactions of aluminium anion clusters with water depend on geometric structure rather than electronic shell structure.

Castleman and colleagues reacted water with anionic clusters of between 7 and 60 aluminium atoms in a fast-flow reactor. They found those clusters that adsorbed water molecules contained active surface-sites comprising of two aluminium atoms. One of the atoms acts as an acid whereas the other acts as a base, binding OH or H from the water respectively.

These active sites are most likely to be found on clusters that have an irregular distribution of charge due to defects in their surface structure. In particular, clusters of 16, 17 or 18 aluminium atoms have two active sites, meaning that H from two water molecules can combine and escape as molecular hydrogen.

These results highlight the extraordinary affect minute variations in shape and size can have on reactivity, and could help in the design of materials with a high proportion of active sites.