Reversible Transformation of Pt Nanoparticles into Single Atoms inside High-Silica Chabazite Zeolite

Manuel Moliner(1), Jadeene E. Gabay(2), Chris E. Kliewer(3), Robert T. Carr(3), Javier Guzman(4), Gary L. Casty(3), Pedro Serna(3), and Avelino Corma(1), 2016

Image courtesy of JACS

Abstract

We report the encapsulation of platinum species in highly siliceous chabazite (CHA) crystallized in the presence of N,N,N-trimethyl-1-adamantammonium and a thiol-stabilized Pt complex. When compared to Pt/SiO2 or Pt-containing Al-rich zeolites, the materials in this work show enhanced stability toward metal sintering in a variety of industrial conditions, including H2, O2, and H2O. Remarkably, temperatures in the range 650–750 °C can be reached without significant sintering of the noble metal. Detailed structural determinations by X-ray absorption spectroscopy and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy demonstrate subtle control of the supported metal structures from ∼1 nm nanoparticles to site-isolated single Pt atoms via reversible interconversion of one species into another in reducing and oxidizing atmospheres. The combined used of microscopy and spectroscopy is critical to understand these surface-mediated transformations. When tested in hydrogenation reactions, Pt/CHA converts ethylene (∼80%) but not propylene under identical conditions, in contrast to Pt/SiO2, which converts both at similar rates. These differences are attributed to the negligible diffusivity of propylene through the small-pore zeolite and provide final evidence of the metal encapsulation.

Impact Statement

Here, the authors encapsulated platinum in CHA to protect the interior noble metals from sintering. By controling the activation treatment, they were also able to control the morphology of the PT species within. This was all made possible by environmental control of the material in Atmosphere.