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Title Alkali-resistant DeNOx Catalysts : Vanadia-, Copper-, and Iron-based Catalysts Supported on Sulfated and Tungstated Zirconia
Author Due-Hansen, Johannes (Department of Chemistry, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Supervisor Fehrmann, Rasmus (Department of Chemistry, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Christensen, Claus H. (Department of Chemistry, Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark)
Institution Technical University of Denmark, DTU, DK-2800 Kgs. Lyngby, Denmark
Thesis level Master's thesis
Year 2006
Abstract Vanadia based catalyst supported on sulfated and tungstated zirconia have been prepared and tested in the NO selective catalytic reduction (SCR) with ammonia. Resulting catalysts were characterized using N2-BET, XRD, Raman spectroscopy and NH3-TPD methods. The influence of the calcination temperature of zirconia (modified with tungsten oxide and sulfate) on the textural characteristics, acidity and catalytic performance has been studied. Resistance of the catalysts towards model poisoning with potassium was found to depend dramatically on the crystallinity and surface acidity. Vanadia supported on the tungstated zirconia calcined at 700°C revealed good catalytic performance and resistance towards alkali poisoning in comparison with traditional V2O5-WO3/TiO2 catalyst, which was used as reference catalyst. In general, vanadia-impregnated sulfated zirconia showed higher resistance towards potassium doping than tungstated samples, which was attributed to the stronger electron withdrawing effect of surface sulfate species. Vanadia-impregnated commercial sulfated 1.7V_3.4SZ_NP obtained from Saint-Gobain Norpro, NP (1.7 wt% vanadium, 3.4 wt% as SO3) exhibited excellent SCR activity, and good resistance towards poisoning with potassium. The support possessed very high surface acidity (537 μmol/g) and 73.4% crystallinity of which 76.8 vol% consisted of tetragonal zirconia (t-ZrO2). The superior ability of sulfates to host basic potassium dopants over tungsten oxides or zirconia resulted in significantly lower deactivations after poisoning. A possible mechanism to explain the effect of potassium poisoning on the degree of vanadium polymerization was suggested based on the results from the Raman spectroscopy. Catalytic studies of the iron and copper-impregnated commercial tungstated and sulfated zirconia were also performed. In case of the tungstated zirconia as support for the iron- and copper-based catalysts, rather poor catalytic activities in the NO-SCR were observed. Doping with a potassium-metal molar ratio 0.4 resulted in severe deactivation of the catalysts. Even though copper showed highest resistance towards potassium (deactivation 39% at 300°C) the initial activity was much lower at all temperatures compared with the reference V2O5-WO3/TiO2 catalyst. Copper and iron supported on the commercial sulfated zirconia displayed somewhat better initial SCR-activities, but still lower than the reference catalyst. The deactivation upon potassium poisoning was in all cases similar or higher than the reference. Poor SCR-activities before and after poisoning with potassium were in general observed for all copper- and iron-based catalysts, which could be connected with the fact that lower surface acidities were measured with NH3-TPD than for the corresponding vanadia-based samples. Based on the good catalytic properties of 1.7V_3.4SZ_NP in the SCR process, an optimization of vanadium loading was performed in order to study catalysts’ resistance toward a constant amount of potassium. Optimization of the vanadia content on the commercial support 3.4SZ_NP proved that the catalysts with 1.0 wt% vanadium loading (196 μmol/g) revealed highest turn-over frequency (TOF) of around 1 s – 1 at 300°C.Although optimal TOF activity was observed for lower loadings of vanadia, catalyst deactivated 55% at 300°C upon poisoning with 130 μmol/g potassium. Increasing the vanadium content to 3.0 wt% (589 μmol/g V) resulted in a very high overall activity and good resistance towards poisoning, deactivating less than 10% at 300°C. Better resistance of the samples based on sulfated and tungstated zirconia can be connected with the fact that a significant part potassium on the surface of the catalyst preferentially interacts with strong acid sites of the support, thus preventing vanadium species from deactivation and leaving them available for the catalytic cycle.
Pages 132
Fulltext
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Admin Creation date: 2007-03-08    Update date: 2007-03-08    Source: dtu    ID: 196074    Original MXD