Origin of the Unusual Stability of Zeolite-Encapsulated Sub-Nanometer Platinum

The mechanism by
which single metal atoms and small, zeolite-encapsulated
metal particles are stabilized against migration and growth is not
currently well understood. In this work, we employ an unbiased density
functional global optimization strategy to identify the locations
and energetic barriers for migration pathways between sites for platinum
(Pt) confined within the microporous volume of a purely silicious
zeolite with Linde type A topology and its aluminosilicate and borosilicate
variants. We observe an impressive stabilization of single Pt atoms
caused by a hitherto unreported binding mode, in which the six rings
in the framework are broken, leading to trapped, highly accessible
metal centers. In addition, heteroatom substituents are found to significantly
enhance the incorporation of Pt via an unexpected
insertion into framework SiO–H bonds. Migration of Pt is hindered
by high barriers, which are predicted to vary significantly with Si:X
(X = Al and B) ratios. It is proposed that an optimal Si:X ratio exists
for a given zeolite topology, in which the barriers will reach the
maximum value. The energetic preference for Pt clustering (via Ostwald ripening) remains but is significantly reduced
with respect to isolated clusters because of the strong interactions
between Pt atoms and the framework. Our findings suggest a means to
control noble-metal particle sintering, despite a thermodynamic driving
force toward Pt clustering. This work provides an explanation for
the surprisingly high degree of kinetic stability of ultrasmall, encapsulated
metal particles observed experiment.