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Selective Catalytic Reduction of NOx by Hydrocarbons over Fe/ZSM5 Prepared by Sublimation of FeCl3. Characterization and Catalysis Nitrogen oxides (NOx) are unwanted by-products of combustion. They are generated primarily from motor vehicles and stationary sources, like power stations and industrial heaters. New catalytic materials are constantly developed in order to improve the efficiency of the cleaning-up technologies for NOx. With this respect an important breakthrough has recently been obtained from the synthesis a new catalytic material: over-exchanged Fe/ZSM5. This material consists of a crystalline microporous aluminum-silicate structure (zeolite) with iron embodied in its micropores. By introducing iron via a gas-phase synthesis route (FeCl3 sublimation) a high concentration of iron can be loaded in the zeolite's micropores. The resulting material is extremely promising since it displays high activity and durability under conditions close to those encountered under practical applications (diesel cars and industrial emissions). In this PhD project we have tried to answer a variety of questions concerning this catalytic material. Firstly we have studied and improved the synthesis procedure. Secondly, we have extensively characterized the material to determine the structure of the catalytically active phase. This has been achieved using highly specialized techniques like X-Ray Absorption Spectroscopy (XAFS). This technique is highly powerful since it allows the determination of local structures at atomic scale. Furthermore it can be applied at high temperature and at atmospheric pressure, i.e. on catalytic materials under real working conditions. Finally, we have studied the accessibility of the catalytically active phase for the reactants during catalysis. This is a very important factor since in some cases it can determine the success of a catalyst. The results have shown that the catalytically active phase in Fe/ZSM5 consist of binuclear Fe-oxo/hydroxo complexes. Their presence can be maximized by carefully controlling washing and heat treatments during the synthesis procedure. These binuclear complexes possess a Fe-O-Fe core structure. The high reactivity of the oxygen bridging the two iron atoms appears to be the key-step for the peculiar catalytic performance of these complexes towards the cleaning-up of NOx. Due to high concentration of iron in the micropores of the zeolite not all the Fe-binuclear complexes are easily accessible during reaction. In particular, only the Fe-species located close to the entrance of the zeolite's micropore appear to be promptly available for the reactants during catalysis. This result indicates that a possibility for further improving the catalytic properties of Fe-based catalysts would be the use of zeolitic materials with higher micropore diameters or analogous mesoporous supports. |
