A Small Paramagnetic Platinum Cluster in an NaY Zeolite:  Characterization and Hydrogen Adsorption and Desorption

Abstract

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.

Structure and Magnetization of Small Monodisperse Platinum Clusters

“…The samples also give rise to an EPR spectrum that shows a multiplet due to a cluster of 12 equivalent Pt atoms with perhaps an invisible 13th atom [4]. The cluster size is compatible with the present EXAFS measurements, but this signal accounts for <1% of the total Pt in the sample.…”

“…It is the 5d band which is mainly engaged in bonds to chemisorbed molecules and when electrons are removed or added. Density functional calculations for Pt 13 found several distorted or even amorphous cluster geometries with energies lower by as much as 0.16 eV (1300 cm ÿ1 ) than those of an ideal icosahedron or cuboctahedron [3,8], also in the presence of chemisorbed hydrogen [4].The synthesis of small Pt clusters of 13-20 atoms in zeolite Y was reported previously [9], but there were no magnetic measurements. Here, Pt clusters were prepared in NaY zeolite by ion exchange with 3 mM aqueous Pt NH 3 4 Cl 2 solution, followed by stirring at 343 K for at least 48 h. Chemical analysis confirmed that this procedure leads to a Pt loading of ca.…”

“…3(a), [3]. For neutral clusters with adsorbed hydrogen, a ground state of 1 S 4, depending on the degree of coverage, was predicted [4]. Although within error the observed effective moments translate into integer values of S, the samples may contain mixtures of clusters with different spin states.…”

“…It is the 5d band which is mainly engaged in bonds to chemisorbed molecules and when electrons are removed or added. Density functional calculations for Pt 13 found several distorted or even amorphous cluster geometries with energies lower by as much as 0.16 eV (1300 cm ÿ1 ) than those of an ideal icosahedron or cuboctahedron [3,8], also in the presence of chemisorbed hydrogen [4].…”

“…The Debye-Waller factor of the surface atoms is more than twice the value of the bulklike central atom in the icosahedra. The high value for the mean-square displacement can partly also be due to high disorder caused by small structural deviations from the ideal icosahedral symmetry, which are obtained as multiple energy minima in atomistic calculations [4]. A three-atom oxygen shell is attributed to the zeolite matrix [14], as an oxidation of the clusters can be excluded from XANES considerations.…”