Combination of V2O5/WO3−TiO2, Fe−ZSM5, and Cu−ZSM5 Catalysts for the Selective Catalytic Reduction of Nitric Oxide with Ammonia

Abstract

Cordierite monoliths coated with V2O5/WO3−TiO2, Fe−ZSM5, and Cu−ZSM5 catalysts were combined to investigate the possibility of combining the advantages of the different single catalysts for the selective catalytic reduction of nitric oxide with ammonia (NO SCR) while minimizing their drawbacks. Selected combinations of two of the three above-mentioned catalysts were connected in series such that the volume of each catalyst was halved in order to maintain the total space velocity constant. The combinations V2O5/WO3−TiO2 followed by Fe−ZSM5 and the reversed catalyst order achieved markedly lower NOx reduction efficiencies (DeNOx) than the pure vanadia-based catalyst. Fe−ZSM5 applied downstream of V2O5/WO3−TiO2 did not reduce the N2O formed over the V-based catalyst at temperatures above 450 °C, as expected from the known N2O decomposition and N2O SCR activities of Fe−ZSM5. When V2O5/WO3−TiO2 was mounted downstream of Fe−ZSM5, ammonia slip was observed even at small NOx reduction efficiencies. The combination of Fe−ZSM5 and Cu−ZSM5 showed high activities at temperatures from 200 to 700 °C with small N2O formation, similar to pure Fe−ZSM5. V2O5/WO3−TiO2 combined with Cu−ZSM5 showed slightly higher activity at lower temperatures than V2O5/WO3−TiO2 only. At temperatures above 350 °C potential ammonia excess was selectively oxidized to nitrogen; in other words, Cu−ZSM5 functions as a selective catalyst for ammonia oxidation.