Stabilization of Ultra-Small Zeolite-Encapsulated Sub-Nanometre Platinum
Dianwei Hou, Lukáš Grajciar, and Christopher J. Heard
Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague
Noble metals are used extensively in the chemical industry, [1] where due to limited reserves and high prices it is desirable to use every atom efficiently. Nowadays, it is possible to produce single atom noble metal catalysts, [2] despite their low resistance against sintering. It has recently been observed experimentally that single Pt atoms (and small clusters) encapsulated by zeolites may show impressive sintering resistance and reversible re-dispersion. [3-5] However, the mechanism by which atoms are stabilized against migration and growth is not well understood. Here, we investigate, via basin-hopping global geometry optimization (GO) method as implemented in the Atomic Simulation Environment (ASE) code, the stabilization and migration properties of single noble metal atoms and small clusters confined within zeolite LTA. We consider the role of isomorphous framework heteroatom (Al, B) substitution, metal cluster size on the energetics and mechanisms of trapping and migration. We find that the purely silicious zeolite exhibits impressive stabilization of single Pt atoms, due to the optimal local geometry of the binding site. In addition, framework heteroatoms significantly enhance the kinetic trapping of Pt This work moves us closer to understanding in mechanistic detail, the surprisingly high degree of kinetic stability of ultra-small noble metal particles encapsulated in zeolites.
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