Preparation of Highly Dispersed Pt Particles in Zeolite Y with a Narrow Particle Size Distribution : Characterization by Hydrogen Chemisorption, TEM, EXAFS Spectroscopy, and Particle Modeling

Abstract

Research on metal-support interaction calls for highly dispersed metal particles inside the zeolite with a narrow particle size distribution. Pt particles thus dispersed in zeolite Y are obtained in this work using both ion exchange and dry impregnation techniques. For both preparation techniques a very low heating rate during calcination turns out to be essential. This low heating rate of 0.2}o{C/min leads to particles with an EXAFS Pt-Pt coordination number of 5-6, i.e., particles of 13-20 atoms, i.e., smaller than 1.1 nm. A rate of 1}o{C/min as used in the literature already leads to a bimodal particle size distribution. About 5 wt% large particles of 4-9 nm is (partially) occluded in the zeolite. Ninety-five weight percent of the Pt particles have an average size of 1-1.2 nm. The necessity to use a very low heating rate is explained by the slow desorption of water and ammonia from the microporous zeolite in combination with the stabilization of mobile Pt species by the cavity walls of the zeolite. The particle size distribution reported could be obtained only by combining the results of hydrogen chemisorption, TEM, EXAFS, and modeling of Pt particles.