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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 Pt13Hm, 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.