Silver and copper clusters and small particles stabilized within nanoporous silicate-based materials

Abstract

Silver and copper ions were incorporated into mordenites with variable SiO2/Al2O3 molar ratio (MR) and silica xerogels. Heat treatment of the samples in hydrogen flow leads to the formation of reduced silver and copper species (nanoparticles and clusters). Their properties, influenced by variation of matrix structure, MR in mordenite and reduction conditions were studied by UV–vis optical spectroscopy. Investigation into optical spectra allows to follow the variation of electronic properties of reduced copper and silver small metal clusters and nanoparticles supported in zeolites and porous silica xerogel matrices. Metal nanoparticles exhibiting the plasmon resonance band are formed under the higher reduction temperatures. Their appearance has the complicated dependence on MR in mordenites, but similarity is observed for conditions leading to the formation of Cu and Ag nanoparticles. Small metal clusters were proposed to be responsible for the absorption bands in the short-wavelength range both for mordenites and silica xerogels. The more pronounced peaks for silver are assigned to Ag8 in mordenite and Ag19–Ag20 in silica xerogels. Structure and composition of matrix dominate over temperature dependence of metal ions reduction; similar reduced species appears at different temperatures for mordenite and xerogels. In the case of mordenites their chemical composition, that is, the SiO2/Al2O3 molar ratio, is the main parameter that permits to vary obtained metal species under the same condition of reduction.