WO3–TiO2 monolithic catalysts for high temperature SCR of NO by NH3: Influence of preparation method on structural and physico-chemical properties, activity and durability

The WO3–TiO2 catalysts with different WO3 loadings prepared by the coprecipitation method were investigated in comparison with those prepared by the conventional impregnation method for the activity and durability in the high temperature SCR of NO by NH3 and the structural and physico-chemical properties which were characterized by BET and XRD measurements, IR, Raman and XPS spectroscopies. The catalyst prepared by coprecipitation, as compared with that prepared by impregnation, was found to exhibit a higher SCR activity at high temperatures and also to possess a larger surface area, higher Brønsted acidity and larger monolayer capacity of the support with WO3. Increasing the WO3 loading of the catalysts enhances the SCR activity and simultaneously increases the Brønsted acidity. The observed improvement of SCR activity for the catalyst prepared by coprecipitation is mainly attributed to the higher Brønsted acidity and the presence of the more highly dispersed WO3 species which is suggested by the larger monolayer capacity of ca. 13μmol(W)/m2 and no crystalline WO3 on TiO2 detected with XRD at the high WO3 loading up to 40wt.%. The catalyst with 20wt.% WO3, as compared with that prepared by impregnation, was found to exhibit a better thermal durability at high temperatures from 550 to 600°C. The better durability is attributed to that the reduction of the surface area and the formation and subsequent growth of crystalline WO3 upon aging are more remarkably inhibited.