A well-defined cluster containing 12 equivalent platinum atoms was prepared by ion exchange of an NaY zeolite, followed by hydrogen reduction. It was characterized by electron paramagnetic resonance (EPR) spectroscopy, hyperfine sublevel correlation (HYSCORE), and theoretical calculations. Combing the results of the experiments with density functional calculations, the likely structure of this cluster is icosahedral Pt 13 H m , possibly with a low positive charge. The adsorbed H/D on the Pt cluster surface can be exchanged reversibly at room temperature. From H/D desorption experiments, an H 2 binding energy of 1.36 eV is derived, in reasonable agreement with the calculated value but clearly larger than that for a (111) Pt single-crystal surface, revealing a finite size effect. While the hydrogen-covered cluster should clearly be regarded as a molecule, it is conceivable that the cluster adopts metallic character upon hydrogen desorption. It is likely that up to m ) 30 H atoms bind to this cluster with 12 surface atoms, which has important implications for the determination of the dispersion of small Pt catalyst particles by hydrogen chemisorption. Calculations as well as experiments give evidence of an interesting magnetic behavior with high-spin states playing a prominent role. There are strong indications that a reservoir of EPR silent but structurally similar clusters exists which can partly be converted to EPR visible species by H/D exchange or by gas adsorption.

CO and NO react on hydrogen-covered 13-atom Pt clusters, O2 does not, and the hydrogenation of ethene shows structure sensitivity.

A Small Paramagnetic Platinum Cluster in an NaY Zeolite: Characterization and Hydrogen Absorption and Desorption. -A well-defined cluster containing 12 equivalent Pt atoms is prepared by ion-exchange of NaY zeolite with aqueous [Pt(NH3)4]Cl2 solution at 343 K (about 48 h) followed by reduction in flowing H2 at 473 K (1 h). The samples are characterized by EPR, hyperfine sublevel correlation, and DFT calculations. The likely structure of the cluster is icosahedral Pt 13 H m (m ≤ 30). The adsorbed H on the Pt cluster surface can be exchanged reversibly at room temperature. The H2 binding energy of 1.36 eV is clearly larger than that for a (111) Pt single crystal surface, revealing a finite size effect. Evidence of an interesting magnetic behavior with high-spin states playing a prominent role is given. Pt clusters dispersed in zeolites are highly active catalysts for the oxidation of CO and residual hydrocarbons in automotive exhaust catalysts and for hydrogenation in petrochemical reactions and other commercial applications. -(LIU, X.; DILGER, H.; EICHEL, R. A.; KUNSTMANN, J.; RODUNER*, E.; J.