On the nature of active sites in Fe/ZSM-5 catalysts for NOx abatement

Fe/ZSM-5 catalysts with high Fe loading (Fe/Al similar to 1)]have been prepared by sublimation of FeCl3 onto H-ZSM-5 samples of different Si/Al ratios. They catalyze NO, reduction with hydrocarbons in an excess of O-2 and H2O. TPR shows that the Fe in the zeolite cavities is different from Fe2O3 particles. Naked Fe3+ ions are absent; ore-ions, which are equally well reducible by CO and Hz, prevail. A minority of the Fe complexes lose oxygen upon mere heating to similar to 500 degrees C; some of the reduced sites are reoxidized only by N2O. The population of ore-complexes that lose oxygen by heating depends on the Si/Al ratio, this dependence is in qualitative agreement with the model of (2+) charged binuclear ions [HO-Fe-O-Fe-OH](2+). Upon reacting with NO, the bridging O atom is transferred and NO2 is formed. This step is not rate limiting for active catalysts with high Al/ Si ratio and high Fe loading, but it becomes critical with zeolites of low Al/Si ratio. (C) 1998 Elsevier science B.V. All rights reserved.