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

Fe/ZSM-5 catalysts with high Fe loading (Fe/Al1) have been prepared by sublimation of FeCl₃ onto H-ZSM-5 samples of different Si/Al ratios. They catalyze NO_x reduction with hydrocarbons in an excess of O₂ and H₂O. TPR shows that the Fe in the zeolite cavities is different from Fe₂O₃ particles. Naked Fe³⁺ ions are absent; oxo-ions, which are equally well reducible by CO and H₂, prevail. A minority of the Fe complexes lose oxygen upon mere heating to 500°C; some of the reduced sites are reoxidized only by N₂O. The population of oxo-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.