CU ION SITING IN HIGH-SILICA ZEOLITES - SPECTROSCOPY AND REDOX PROPERTIES

It has been shown that two main Cu sites present in ion exchanged Cu-Z SM-5 which differ in positive charges on the Cu2+ ion, exhibit substan tially different redox properties and reactivity towards NO molecules investigated by IR spectra of adsorbed NO and CO, and Cu+ emission spe ctra. The Cu ion with low positive charge (tested by electron density transfer from NO molecule to the cation) is assumed to be balanced by a low negative charge, i.e., a single framework Al atom. This Cu site preferably forms Cu+ dinitrosyl complexes and exhibits high stability of Cu+ even in oxygen at high temperature (720 K) compared to the othe r Cu ions with higher positive charges, which are suggested to be bala nced by two framework Al atoms. A decisive role of local negative fram ework charge adjacent to the cation as well as total negative framewor k charge given by Si/Al ratio for the redox properties and reactivity of the Cu ions has been indicated.