| Popis: |
In zeolites 50 to 80% of the total volume is available for hosting other organic or inorganic phases, and this free volume is highly structured according to the crystallographically defined pore system. Small chalcogenide particles can be incorporated into the pore systems of molecular sieves either during the crystallization process or, in a manner much easier to control, by post-synthesis modification aiming at new functionalities. For example, alkali or alkaline earth metal oxides have been introduced into zeolites to create catalytically active base sites. The entrapment of transition metal oxides aims at the formation of active and selective catalysts for oxidation or ammoxidation reactions, selective catalytic reduction of NOx, or photocatalysis. The formation of isolated, zeolite-anchored oxidic species, which are often less easily reduced than bulk oxides, is desired to favor mild and selective oxidation, i. e., to avoid deep or even total oxidation. Supported metal oxide clusters of nanometer size are also studied with respect to a possible application in redox sensor devices, i. e., for the rapid and selective detection of low concentrations of various gases. The regular pore system of zeolite supports is expected to favor the stabilization of highly dispersed metal oxides of uniform particle size and, thus, uniform properties. The host charge, its hydrophobicity, the pore size relative to the guest species, their concentration and three-dimensional arrangement, the guest polarity and its structural properties can all be adjusted to a large extent and in many instances independently from each other. |
