Migration of zeolite-encapsulated Pt and Au under reducing environments

Abstract

The encapsulation of noble metal atoms into zeolites is a promising route to generate controlled size distributions of stable metal catalysts. Pinning of single metal atoms to particular binding sites represents the optimal atom-efficiency and is a desirous outcome, despite the propensity of metal clusters to sinter. Currently, sintering resistance of noble metals in siliceous and high-silica frameworks is incompletely understood, while the role of influencing factors such as adsorbates and metal element identity, have not been ascertained. Here, we investigate the nature of metal–zeolite interactions, via density functional global structure optimisation and kinetic Monte Carlo simulations of the binding and migration of Pt and Au in a siliceous zeolite with framework topology LTA. We show that strong binding of Pt atoms to the framework severely hinders migration, even in the absence of framework heteroatoms, while Au diffuses freely through the pore. Reducing agents CO and H₂ change the preferred binding site of Pt and flatten the potential energy surface, which reduces migration barriers and thereby promotes particle growth. PtCO is found to represent a compromise between strongly framework-bound Pt₁, and bulky, volatile Pt(CO)_x clusters, exhibiting fast diffusion. This work provides an atomistic picture of single metal atom kinetics inside high-silica zeolites, which represent a fundamental basis for understanding nano-catalyst deactivation.