Comparative study of natural and synthetic clinoptilolites containing silver in different states

Clinoptilolite is a widely used natural zeolite due to its abundance in nature. The present work discusses the characteristics of silver modified clinoptilolites (natural and synthetic) containing silver in different states. The results obtained by the combined use of XRD, UV-Visible spectroscopy, and SAXS are described. These analyses were carried out to determine the features of silver in the zeolitic structure (natural and synthetic) and the particle size distribution of the Ag conglomerates produced in the reduced samples. The crystalline structure of the Ag-synthetic clinoptilolite was not affected by the thermal reduction of Ag; however, the Ag-natural clinoptilolite samples were less thermally stable. In synthetic and natural clinoptilolites after reduction at 100 degrees C, small clusters (radius of <0.5 nm) located inside the channels along with metal particles (with radius <7 nm) on the external surface were observed. In Ag-natural clinoptilolite reduced at 100 degrees C, along with the small Ag-2(+) and Ag-4(delta+) clusters stabilized inside clinoptilolite pores, large 2 Ag-8(0) and Ag-8(delta+) clusters can be formed inside large mordenite channel or in the interstitial spaces of the particles that compose this mineral. Reduction at 400 degrees C leads to disappearance of silver clusters and large particle formation. In both supports the particles cannot be located in the zeolite cavities, which are much smaller. (c) 2005 Elsevier Inc. All rights reserved.