Selective catalytic reduction of NOx on combined Fe- and Cu-zeolite monolithic catalysts: Sequential and dual layer configurations

Iron and copper-based zeolites are effective catalysts for the lean selective catalytic reduction (SCR) of NOx with NH3. Cu-zeolites are more active at lower temperatures (350 degrees C) while Fe-zeolites are more active at higher temperatures (>= 400 degrees C). The effectiveness of a catalytic system comprising Fe- and Cu-based zeolites was examined for the standard (NO + O-2 + NH3) and fast (NO + NO2 + NH3) SCR reactions. Experiments carried out with in-house and commercial Fe- and Cu-zeolite monoliths of varying lengths quantified their relative SCR activities. The commercial Cu-zeolite achieved complete NOx conversion for the standard SCR at 250 degrees C while the commercial Fe-zeolite achieved high NOx conversion at higher temperatures (>= 400 degrees C) where it out-performed the Cu-zeolite. Subsequently, three configurations of combined Fe and Cu-zeolite catalysts were compared: Sequential brick catalyst comprising Fe-zeolite and Cu-zeolite monolith. Mixed washcoat catalyst comprising a washcoat layer having equal mass fractions of Fe- and Cu-zeolites. Dual layer catalyst comprising monolith coated with individual layers of Fe- and Cu-zeolites of different thicknesses and mass fractions. The sequential brick design with Fe-zeolite brick followed by a Cu-zeolite brick gave a higher conversion than the Cu/Fe sequence of equal loadings with the Fe(33%)/Cu(67%) achieving the highest NOx conversion over a wide range of temperatures. The mixed washcoat catalyst achieved NOx conversion that was nearly an average of the individual Fe-only and Cu-only catalysts. The dual layer catalyst with a thin Fe-zeolite (33% of the total washcoat loading) layer on top of a thicker Cu-zeolite layer (67%) resulted in very high NOx removal efficiencies over a wide temperature range for both the standard and fast SCR reactions. The performance of this dual-layer system was comparable to the series arrangement of Fe and Cu-bricks. The Cu-zeolite on Fe-zeolite dual layer catalyst was not nearly as effective for the same loadings. The Fe/Cu dual layer catalyst also exhibited superior performance for the fast SCR reaction. The results are interpreted in terms of the activities of each catalyst for SCR and ammonia oxidation. An assessment of the extent of washcoat diffusion limitations shows that the dual layer configuration is superior to the sequential brick configuration. The existence of an optimal loading distribution of the Fe- and Cu-zeolite catalysts as well as other intangible benefits of the dual layer SCR catalyst are discussed. (C) 2011 Elsevier B.V. All rights reserved.