Incorporating platinum precursors into a NaA-zeolite synthesis mixture promoting the formation of nanosized zeolite

Abstract

Sodium A-zeolite with different platinum contents were prepared by directly incorporating platinum precursor (Pt(NH 3) 4Cl 2) into the zeolite during synthesis. Pt/KA-zeolite was then obtained by ion-exchange with KCl solution. The effect of platinum concentration on the crystal morphology and platinum dispersion was investigated by hydrogen chemisorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), and time-of-flight secondary ion mass spectrometer (TOF-SIMS). SEM results revealed a bimodal crystal size distribution for NaA-zeolite with low platinum concentration (<1 wt%). Chamfered edges were found for all the larger cubic crystals (∼4 μm). Two types of cubic crystals, smooth surface cubes with chamfered edges and rough surface cubes, were observed for the smaller crystals (∼400 nm). The proportion of the rough surface nanosized cubes increased as the platinum content increased. At about 4 wt% platinum, only nanosized rough surface cubic crystals were obtained, which were transformed into nanocrystals of KF-zeolite after ion-exchange with KCl, as indicated by X-ray diffraction results. TOF-SIMS data taken before and after sputtering the surface layers revealed that platinum was distributed homogeneously inside of the zeolite, which was supported by hydrogen chemisorption results, indicating that platinum particles were confined in the zeolitic cages for both the microsized and nanosized cubic crystals. A mechanism was proposed to elucidate the role of platinum precursor on the nucleation and growth of nanosized zeolite, which is consistent with all of the characterization results. Crown Copyright © 2008.