First principles investigation of H addition and abstraction reactions on doped aluminum clusters

We have investigated axial interactions of H2 with Al 12X(X = Mg, Al, and Si) clusters and found that homolytic dissociation leading to Al12XH and H atom proceeds without a barrier but is an extremely endothermic process. The calculated difference in energy of the addition and abstraction reactions indicates that any Al12X-based hydrogen storage technology that involves predissociation of H2 will be limited by the competing processes. We have also discovered that while there is a modest barrier for dissociation of H2 on a single Al12Mg cluster to give the dihydride, the process occurs spontaneously between two closely spaced Al12Mg clusters, resulting in the formation of two Al12MgH species. Doping of the cluster with an electropositive atom (Mg) enables the transfer of electron density to the Al cage, which enhances H2 dissociation. The information gained can contribute to the design of novel solid-state materials made of doped Al clusters, which may ultimately be suitable for catalytic processes.