Combined ESR-DRS spectroscopies of transition metal ions and metal ion clusters in zeolites

Abstract

The results of ESR and DRS spectroscopies on zeolites are discussed for three themes: (1) the coordination of Cu 2+ to oxygen six-rings; (2) the formation of Ni +; and (3) of charged Ag-clusters. Cu 2+ trigonally coordinated to oxygen six-rings, is subjected to a dynamical Jahn-Teller effect. Two types of six-rings can be distinguished: the type II six-ring, connecting the supercage with the cubo-octahedron and the type I' six-ring of the hexagonal prism. The spectral properties of Cu 2+ on these two six-rings are independent of the structure type and the chemical composition of the zeolites. Five Ni + species have been identified by ESR in zeolites X and Y. The corresponding DRS spectra have not yet been obtained. The green colour of the partially reduced Ni-zeolites is probably due to diamagnetic Mi-clusters. Paramagnetic Ag-clusters with a nuclearity of one, two and three have been found in Ag-exchanged zeolites A, X and Y after irradiation and—to a lesser extent—after mild reduction with H 2. The Ag 6 + cluster is, up until now, only obtained on zeolite A. In irradiated fully Ag-exchanged samples the cluster is associated with six, seven or eight Ag +, dependent on the residual water content. Upon reduction with H 2, the cluster occurs as an isolated species. The colour changes of Ag-zeolites during activation and reduction have been described, but the assignment of the DRS bands to specific clusters needs independent confirmation.