Saturation of small supported metal clusters by adsorbed hydrogen. a computational study on tetrahedral models of Rh₄, Ir₄, and Pt₄

With density functional calculations, we explored the successive adsorption of hydrogen on tetrahedrally shaped zeolite-supported M₄ clusters (M = Rh, Ir, and Pt). Similarly to our earlier results for models of Ir ₄, hydrogen adsorption on Rh₄ and Pt₄ causes an increase of the metal-metal distances. The type of metal strongly affects the adsorption energy of hydrogen, and the optimum H/M ratio varies with the metal, being ∼2 for Rh and ∼3 for Ir and Pt. As judged by the core-level shifts and atomic charges, Rh and Pt clusters are oxidized through interactions with the support and the hydrogen ligands, similarly to our earlier findings for Ir₄.