Density functional study of hydrogen adsorption on tetrairidium supported on hydroxylated and dehydroxylated zeolite surfaces

We explored computationally successive adsorption of hydrogen on Ir ₄ clusters both in the gas phase and on hydroxylated or dehydroxylated zeolite support, the latter described by cluster models. Free and supported Ir₄ clusters studied were calculated to adsorb dissociatively large amounts of hydrogen, up to three hydrogen atoms per metal atom. In the range covered, the energy for dissociative adsorption of hydrogen on Ir₄H_x clusters (x = 0, 3, 6, 9, 12) in the gas phase is almost independent of x, ∼70 kJ/mol per adsorbed atom. The corresponding energy gain is smaller for zeolite-supported Ir₄. The average Ir-Ir distance of free and supported clusters increases with hydrogen loading at very similar rates, by ∼1.9 pm per adsorbate. Charged Ir₄H_x complexes adsorbed on a dehydroxylated zeolite surface are always more stable than complexes of the same chemical composition in gas phase or supported on a hydroxylated surface. Therefore, reverse hydrogen spillover from OH groups of the support onto the metal cluster is calculated to be energetically favorable not only for bare clusters but also after partial loading of hydrogen. From the calculated structural and energetic parameters we suggest that the tetrairidium species in faujasite-type zeolites, produced experimentally and investigated by extended X-ray absorption fine structure (EXAFS), correspond to hydrogenated moieties Ir₄H_x (x = 9-12) supported on a dehydroxylated framework.