Abstrakt
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.