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X-ray absorption spectroscopy in exhaust gas catalysis research

Jeudi 17 octobre 2013 10:30 - Duree : 1 heure
Lieu : Room 500 - 501, Central Building, ESRF - 6 Rue Jules Horowitz - Grenoble

Orateur : Alexey BOUBNOV (Karlsruhe Institute of Technology, Germany)

Increasingly stringent regulations for the automotive industry and pollution control authorities call for optimisation of exhaust after-treatment catalysts, which can be achieved when the structural properties and catalytic performance are correlated. In situ X-ray absorption spectroscopy (XAS) is regarded as a powerful catalyst characterisation tool since it provides structural information on the molecular level, obtained under operation conditions including realistic gas atmospheres and high temperatures. A common process for removing nitrogen oxides (NOx) from heavy-duty vehicle emissions is selective catalytic reduction by ammonia (NH3-SCR) over Fe-zeolite catalysts. The structure, nuclearity and redox properties of the Fe centres in these materials have been previously investigated by XAS [1, 2]. For understanding the impact of these properties but also of the carrier on the SCR activity, our group has recently compared [3] the highly active Fe/BEA zeolite system with a much less active Fe/Al2O3 catalyst. The results indicate that the reducibility of the Fe sites from Fe3+ to Fe2+ is directly related to the catalytic performance : the low-temperature SCR activity was correlated to the facile generation of Fe2+ sites at the ion exchange positions of the BEA zeolite. Significantly higher temperatures are required to produce Fe2+ in Fe/Al2O3. Furthermore, the coordination geometry of the Fe centres was followed by analysing the Fe K-pre-edge spectra. Fe K-pre-edge analysis was carried out using a MATLAB-based tool, which subtracts the main edge onset and automatically fits the extracted pre-edge feature to 1-5 pseudo-Voigt peak functions. This allows the quantification of Fe valence and coordination geometry based on the method of Wilke [4]. This empirical approach allows the position, width and Gaussian fraction of the model peaks to float and accounts for e.g. the peak broadening due to overlapping but not individually resolvable transitions. This method is well-applicable to standard-resolution XAS, especially under in situ conditions, where noise and poor data point sampling often occur due to low concentrations, sample inhomogeneities and rapid energy-scanning. To improve the instrumentation for in situ XAS measurements, a movable setup equipped with a gas dosing manifold including high-pressure-rated components, sample heating and online gas analysis by mass spectrometry and FTIR was built, tested and optimised at the ANKA XAS beamline for future applications at the new CAT-ACT wiggler beamline. Instruments and connections are accessible to the user, which makes the setup intuitively understandable and user-friendly. The gas mixing options are left flexible and can thus be adapted to the experimental requirements ; the entire setup can be remotely controlled by a computer. This setup allows transferring and building up in situ experiments at the beamline with higher time efficiency.

References [1] A. A. Battiston et al., Journal of Catalysis 213 (2003) 251-271 [2] M. Høj et al., Applied Catalysis B : Environmental 93 (2009) 166-176 [3] A. Boubnov et al., Journal of Physics : Conference Series 430 (2013) 012054 [4] M. Wilke et al., American Mineralogist 86 (2001) 714-730

Contact : eva.jahn@esrf.fr

Discipline évènement : (Physique)
Entité organisatrice : (ESRF)
Nature évènement : (Séminaire)
Evènement répétitif : (Séminaire ESRF)
Site de l'évènement : Polygone scientifique

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