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

[EN] 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 H-2, O-2, and H2O. Remarkably, temperatures in the range 650-750 degrees 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 similar to 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 (similar to 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. This work has been supported by the European Union through the SYNCATMATCH project (Grant Agreement No. 671093) and the Spanish Government through "Severo Ochoa Program" (SEV 2012-0267) and MAT2015-71261-R. This research used beamline 9-BM of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The Electron Microscopy Service of the UPV is acknowledged for their help in sample characterization. We thank Isabel Millet and Paul Stevens for technical assistance, and Randall Meyer, Aaron Sattler, and Dave Marler for review of the manuscript. We appreciate the support of ExxonMobil Research and Engineering for our efforts in fundamental catalytic research. Moliner Marin, M.; Gabay, JE.; Kliewer, CE.; Carr, RT.; Guzman, J.; Casty, GL.; Serna Merino, PM... (2016). Reversible Transformation of Pt Nanoparticles into Single Atoms inside High-Silica Chabazite Zeolite. Journal of the American Chemical Society. 138(48):15743-15750. https://doi.org/10.1021/jacs.6b10169