Study of the selective catalytic reduction of NOx on an efficient Fe/HBEA zeolite catalyst for heavy duty diesel engines

The present paper addresses the removal of NO, from the exhaust of heavy duty vehicles using the SCR technique. The studies were conducted with a highly active H-BEA zeolite exhibiting a molar Si/Al ratio of 12.5 and a Fe load of 1.0 wt.% (1Fe/HBEA). The pronounced efficiency of 1Fe/HBEA is reflected by the apparent turnover frequency being superior to traditional V2O5/WO3/TiO2. The nature of the Fe sites was investigated with high resolution transmission electron microscopy (HRTEM), Fe-57 Mossbauer spectroscopy and powder X-ray diffraction (PXRD). In connection with previous examinations it is deduced that the iron sites represent octahedrally coordinated high spin Fe3+ cations. Furthermore, highly dispersed species, which are the most active sites, are supposed to be paramagnetic, while oligomeric and particulate structures indicate superparamagnetic behaviour. The practical evaluation of the 1Fe/HBEA catalyst was systematically carried out including laboratory studies of granulated powder and honeycomb samples as well as engine bench tests. For the latter studies a coated honeycomb prototype was employed showing very similar efficiency as referred to a commercial V2O5/WO3/TiO2 pattern. Furthermore, 1Fe/HBEA exhibits pronounced hydrothermal stability after aging at 550 degrees C which represents an elevated exhaust temperature of heavy duty vehicles. The aging caused no change in fast SCR, i.e. when a c(NO2)/c(NOx) ratio of 0.5 was used, and only minor decline in standard SCR. The slight aging effect is mainly referred to little decrease in BET surface area and NH3 uptake, respectively. PXRD indicated maintenance of the BEA structure, whereas Al-27 nuclear magnetic resonance spectroscopy showed removal of some Al from the zeolite framework. Contrary, UV-vis spectroscopy evidenced no effect of hydrothermal aging on the composition of the Fe sites. Finally, the catalyst also maintained its efficiency after SO2 aging at 300 degrees C. Diffuse reflectance infrared Fourier transform spectroscopic studies showed adsorption of molecular SO2 on the zeolite substrate releasing already at about 400 degrees C. (C) 2009 Elsevier B.V. All rights reserved.