Stabilization of metal ensembles at room temperature. Palladium clusters in zeolites

Abstract

Palladium clusters of low nuclearity have been prepared in the cavities of X zeolites via ion exchange with Pd(NH,),Cl,, oxidative dehydration, and subsequent reduction of the dry Pd(I1) zeolites with hydrogen at 295 and 425 K. Detailed EXAFS analysis of Pd K-edge data shows that the average first-shell Pd-Pd coordination number of the reduced system is only about N = 1.5 at a distance of 2.78 A. A long, relatively disordered coordination of the reduced Pd to framework oxygen and silicon indicates the stabilizing, templating function of the zeolite matrix. Geometric models based upon the EXAFS results show that intrazeolite Pdz, Pd,, and Pd4 clusters are formed by partial occupation of SI and SII positions of the sodalite subunits of the zeolite. This study presents evidence for the formation of molecular metal ensembles stabilized in an open-framework matrix to at least 425 K