The cluster Ir₄ and its interaction with a hydrogen impurity. A density functional study

To contribute to the understanding of how iridium particles act as catalysts for hydrogenation and dehydrogenation of hydrocarbons, we have determined structures and binding energies of various isomers of Ir₄ as well as HIr₄ on the basis of relativistic density functional theory. The most stable isomer of Ir₄ showed a square planar structure with eight unpaired electrons. The tetrahedral structure, experimentally suggested for supported species, was calculated 49 kJ mol ^-1 less stable. Hydrogen coordinates preferentially to a single Ir center of the planar cluster with a binding energy of up to 88 kJ mol ^-1 with respect to the atom in the H₂ molecule. Terminal interaction of hydrogen with an Ir₄ tetrahedron causes the cluster to open to a butterfly structure. We calculated terminal binding of hydrogen at different Ir₄ isomers to be more stable than bridge coordination, at variance with earlier studies.