Nitrogen oxides, NOx (NO and NO2) and N2O, are noxious air pollutants in the atmosphere, contributing to ozone layer depletion, greenhouse effect, smog and acid rain [1-30]. The chief sources of NOx are automobiles, lean-burn diesel engines, fossil and fuel-fired power plants, etc [1-6], while N2O is mainly released from industrial processes such as nitric acid and caprolactam production, etc [7,8]. Therefore, many efforts have been made world wide to reduce NOx and N2O emissions from mobile and stationary sources.
For stationary sources, the selective catalytic reduction of NOx with NH3 (NH3-SCR) over vanadia-based catalysts is commercially applied since a long time [3,26-30]. However, the respective V2O5/TiO2 and V2O5-WO3/TiO2 catalysts used for this process work optimally only in a narrow temperature range (573–673 K), while they are not active enough at lower temperature and at higher temperature they oxidize the reduction agent NH3 and deactivate irreversibly. Therefore, much effort has been dedicated in recent years to develop catalysts that are suitable, too, for the low and high temperature range to gain more flexibility for the application of SCR devices in exhaust gas flows, whereby hydrocarbons have been frequently used as reducing agents (HC-SCR). For mobile sources, this has the advantage that no NH3 source has to be maintained on board.
For the abatement of N2O from tail gases, direct catalytic decomposition of N2O into N2 and O2 is an attractive and cost effective catalytic technology. Alternatively, SCR of N2O is also an effective method for N2O abatement. However, catalysts proposed in the literature so far (e.g. supported transition (Cu, Co, Ni) and noble (Rh, Ru, Pd) metal based catalysts) were frequently not active and stable enough under realistic conditions, i.e. in the presence of other components such as O2, NOx, and H2O in the flue gases .
Zeolites containing extra-framework Fe species, in particular those of the MFI type, revealed to be highly promising catalysts for the catalytic abatement of both NOx [31-36] and N2O [37-54]. Nevertheless, marked differences in their catalytic activity have been observed which are most likely due to the highly heterogeneous structure of Fe sites in these materials. This is mainly due to the fact that in most cases, depending on the preparation method and the Fe content, a variety of coexisting Fe species is created, ranging from isolated Fe ions via dimers and small oligonuclear Fe x O y clusters inside the pores to large Fe2O3 particles on the external surface. The inhomogeneous distribution of Fe species is a major problem for a doubtless identification of active sites. Detailed knowledge about the structure of Fe sites and their function in the catalytic NOx and N2O abatement process could open new ways for the development of highly active and stable catalysts.
The objective of this work comprises three main aspects:
Considerable research has been performed in recent years by several groups using a multitude of characterization techniques, among them XAS, TPR, voltammetry, Mössbauer and FT-IR spectroscopy. However, it turned out that a reliable identification of the different Fe species coexisting in these materials and, in particular, their relative quantities, is not straightforward because the sensitivity of the techniques for the various types of Fe species differs. Thus, it was found that EXAFS, being one of the most frequently used techniques, is highly sensitive for isolated Fe sites while it underestimates the presence of the Fe x O y clusters [43,55]. Moreover, the doubtless identification of particular species such as iron dimers [56,57] seems questionable in systems with different coexisting Fe species since EXAFS gives average values for coordination numbers and bond distances. In contrast, Mössbauer and TPR measurements indicated a much larger degree of clustering than was found by EXAFS [43,55].
Therefore, EPR and UV/VIS spectroscopy have been selected in this work to reach objective 1). EPR spectroscopy is a powerful tool to identify isolated Fe3+ species of different coordination geometry by the position of their signals [55,58-63] as well as Fe x O y cluster species of different degree of aggregation by analysis of the mutual magnetic interactions of the Fe sites [55,64,65]. UV/VIS spectroscopy, on the other hand, is especially sensitive to charge-transfer (CT) bands of Fe3+, the wavelength of which depends on the coordination number and the degree of aggregation [55,60,66]. By combining investigations with both techniques, progress in discriminating between different coexisting Fe species in the same matrix was expected.
The remarkable activity of Fe-MFI zeolites is motivating the research community to study the role of different Fe species in SCR of NO, N2O and N2O decomposition for further development of this catalyst. To this end, some efforts have been dedicated using in situ spectroscopic techniques under true reaction conditions. However, most of the studies were done by in situ FT-IR spectroscopy to derive reaction mechanism rather than active sites. A very few studies dealing with changes in oxidation and coordination state of Fe species under SCR of NO are reported. However, the findings are not consistent. Despite severe drawbacks of EXAFS, it being very sensitive to isolated Fe sites and giving only average values for coordination numbers and bond distances (see also above), Koningsberger et al. have used this technique in situ to study the changes in the oxidation and coordination states of Fe species under isobutane-SCR of NO . Based on these studies, the authors attributed the SCR activity of Fe-ZSM-5 merely to Fe-O-Fe dimers. In contrast, Kucherov et al. concluded from in situ EPR studies, performed during propene-SCR of NO, that coordinatively unsaturated isolated Fe sites reflected by EPR signals at g' ≈ 5.6 and 6.5 are the only active Fe sites in Fe-ZSM-5 . However, the authors did not discuss the role of other Fe sites reflected by EPR signals at g' ≈ 4.3 and 2.
Moreover, only little is known whether the different coordination state of Fe species in Fe-ZSM-5 give rise to alterations of the reduction and reoxidation behaviour which should be a crucial property in view of their participation in the catalytic redox cycle. Thus, it is an open question whether differently coordinated isolated Fe sites and iron oxide clusters of different nuclearity (including dimers) are participating equally in the catalytic cycle. Therefore, in the present study an effort is made to give more insights into the nature of active Fe sites for SCR of NO, N2O and N2O decomposition by in situ EPR, in situ UV/VIS-DR and in situ FT-IR spectroscopy. In situ EPR and in situ UV/VIS-DRS techniques are especially suitable since they provide simultaneous information on the oxidation and coordination state of Fe spices.
This thesis is part of a joint research project sponsored by Deutsche Forschungsgemeinschaft which was performed in cooperation with Prof. W. Grünert (Ruhr-University Bochum) and Prof. J. Pérez-Ramírez (Catalan Institution for Research and Advanced Studies, Tarragona). Parts of the work (synthesis of most catalysts, catalytic tests) have not been performed within this thesis but in the laboratories of our cooperation partners. This is denoted in the respective sections below. Based on the integrated evaluation of the characterization results obtained in this thesis including a comparative discussion of catalytic tests performed by the project partners, general features on the structure of Fe sites and their role in the catalytic reactions could be derived.
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