Uniform synthesis of palladium species confined in a small-pore zeolite via full ion-exchange investigated by cryogenic electron microscopy

Abstract

Finely dispersing noble metal species with high phase homogeneity in zeolites is crucial to develop an efficient catalyst. However, for palladium-impregnated zeolites, fully utilizing active palladium species in small-pore zeolites with high dispersion is challenging despite the establishment of a general synthetic approach of ion-exchange and subsequent thermal treatment to generate encapsulated nanoparticles. Herein, we achieve full dispersion of isolated Pd²⁺ ions in a small-pore SSZ-13 zeolite via a controlled ion-exchange process, and successfully generate uniformly dispersed nano-sized PdO clusters in SSZ-13 supported by mechanistic understanding of nanoparticle growth. Direct investigation via cryogenic electron microscopy and ultramicrotomy allows the successful artifact-free imaging of electron-beam-sensitive zeolite-based catalysts, and reveals that the formation of nano-sized PdO clusters during thermal treatment is governed by the rapid nucleation and suppressed particle growth in a confined space. Through fully utilizing active Pd species in SSZ-13 by controlled ion-exchange and rationalized thermal treatment, enhanced catalytic performances toward a passive NO_x adsorber and CH₄ combustion are achieved.