PULSED EPR FOR STUDYING SILVER CLUSTERS

The cationic silver clusters of different nuclearity have been produce d by radiolysis of zeolite A and SAPO molecular sieves containing Agas exchangeable cations. The pulsed EPR spectroscopy has been applied for studying the local environment of silver cluster in order to under stand the mechanism of cluster formation and stabilization. The electr on spin echo modulation (ESEM) results on Ag-6(n+) cluster in dehydrat ion zeolite A indicate that the hexameric silver is stabilized only in sodalite cages which are surrounded by alpha-cages containing no wate r molecules. Trimeric silver clusters formed in hydrated A zeolites st rongly interact with water, thus the paramagnetic center can be consid ered as a cluster-water adduct. In SAPO-molecular sieves, silver clust ers are formed only in the presence of adsorbed alcohol molecules. Fro m ESEM it is determined that Ag-4(n+) in SAPO-42 is stabilized in alph a-cages, where it is directly coordinated by two methanol molecules. D imeric silver, Ag-2(+) in SAPO-5 and SAPO-II is located in 6-ring chan nels and interacts with three CH3OH molecules, each in different 10-ri ng or 12-ring channels. The differences of Ag-2(+) stability in SAPO-5 and SAPO-II are also discussed.