Reduction of NOx with Ammonia over Fe/MFI: Reaction Mechanism Based on Isotopic Labeling

The selective catalytic reduction of nitrogen oxides has been studied in the presence of O2 over Fe/MFI catalysts with Fe/Al∼1, prepared by sublimation of FeCl3 vapor onto HMFI. Reduction of NOx is much faster with NH3 than with isobutane. A 100% NOx reduction to N2 is observed for an NH3/NO ratio above unity and 90% for a 1/1 dosage. The catalyst is very active and stable in the presence of water vapor. Isotopic labeling with 15NO shows that the preferred path for the reduction of NOx with NH3 is via NH4NO2. This nitrite decomposes to N2+2H2O. A 100% pure 15N14N product is obtained by circulating a 1/1 mixture of 15NO and 15NO2 over a catalyst covered with adsorbed 14NH3. In contrast, 14N2 is formed by oxidation of 14NH3 with O2 or N2Ox with x>3. The high efficiency of this catalyst is due to the fact that ammonia swiftly intercepts the oxidation of NO when the value of x=3 is reached. At high temperature when little adsorbed water is present, water vapor promotes activity because it is needed for the formation of ammonium nitrite. At low temperature it acts kinetically as a mere competitor for the active sites.