Reduction of NO_x 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 NO_x is much faster with NH₃ than with isobutane. A 100% NO_x 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 NO_x 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 N₂O_x 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.