Hydrogen dissociation on small aluminum clusters

Abstract

Transition states and reaction paths for a hydrogen molecule dissociating on small aluminum clusters have been calculated using density functional theory. The two lowest spin states have been taken into account for all the Al n clusters considered, with n=2-6. The aluminum dimer, which shows a Π3u electronic ground state, has also been studied at the coupled cluster and configuration interaction level for comparison and to check the accuracy of single determinant calculations in this special case, where two degenerate configurations should be taken into account. The calculated reaction barriers give an explanation of the experimentally observed reactivity of hydrogen on Al clusters of different size [Cox, J. Chem. Phys. 84, 4651 (1986)] and reproduce the high observed reactivity of the Al6 cluster. The electronic structure of the Aln - H2 systems was also systematically investigated in order to determine the role played by interactions of specific molecular orbitals for different nuclear arrangements. Singlet Aln clusters (with n even) exhibit the lowest barriers to H2 dissociation because their highest doubly occupied molecular orbitals allow for a more favorable interaction with the antibonding σu molecular orbital of H2. © 2010 American Institute of Physics.