Location of silver clusters confined in FAU skeleton of dehydrated bi-metallic AgxM96−x-LSX (M = Na+, Li+) zeolite and resultant influences on N2 and O2 adsorption

Abstract

Abstract The location of Ag+-clusters in well-defined dielectric cavities of bi-metallic AgxNa96−x-LSX, AgxLiyNa96−x−y-LSX, and LiyAgxNa96−x−y-LSX zeolites was investigated by XRD, SEM, XPS, ESR, PESA and TG-DTG characterizations. The results showed that color of AgxM96−x-LSX zeolites changes to yellow, brown and even to black by heating up to 623 K, which may be related to the formation of Ag-Ag clusters produced by autoreduction or interaction of Ag+ ions with framework O2. During heat treatment the Ag+-clusters are migrated towards the two distinct possible cation sites (SII’ and SIII). Their ESR silent spectra indicated the diamagnetic clusters Ag3n+ formation. The ionization potentials of AgxM96−x-LSX further suggested that the electronic properties of Ag+-clusters depend strongly on space confinement and cation content. Moreover, N2 and O2 adsorption capacity, selectivity and isosteric heat show strong dependence on the location, nature, extent and order of cations exchange. Particularly, a marked decrease in isosteric heats of adsorption with an increasing N2 loading suggests heterogeneous interactions between N2 and Ag+-cluster. The strong influence of Ag+ loading and host environment on the cluster formation in correlation with structural parameters is utilized in understanding to find the most influential sites for N2 and O2 adsorption.