Activity and durability of Fe/ZSM-5 catalysts for lean burn NO_x reduction in the presence of water vapor

A novel method for the preparation of superior Fe/ZSM-5 catalysts is described. FeCl₃ is sublimed into the cavities of H/ZSM-5, where it reacts chemically with the acid sites of the zeolite. With this method, Fe/ZSM-5 with an iron loading as high as Fe/Al=1 can be easily achieved. This preparation method is less sensitive to the origin of the zeolite and its synthesis history than the conventional ion-exchange method. HRAEM, FTIR, H₂-TPR and CO-TPR results show that iron is located at the exchange sites of the zeolite. An oxygen bridged binuclear iron complex is a plausible prototype for the active iron species. The iron is reduced from +3 to +2 below 800°C. These catalysts are very active and selective in the catalytic reduction of NO_x with iso-butane; they have a high durability under conditions simulating vehicular emission. A conversion of NO to N₂ of about 76% was observed near 350°C. Remarkably and in contrast to most other M/ZSM-5 catalysts, the performance of the new catalysts was not impaired when 10% H₂O was added to the feed. H₂O actually enhances the NO_x conversion at low temperatures. CO and CO₂ are both true primary products of NO_x SCR over Fe/ZSM-5 catalysts.