Reduction of NO_x over Fe/ZSM-5 catalysts: Adsorption complexes and their reactivity toward hydrocarbons

Fe/ZSM-5 prepared via sublimation catalyzes the reduction of NO_x to N₂ in the presence of excess O₂ and H₂O. Propane, isobutane, and propene are active reductants; methane is inactive. The NO_x reduction rate is negligible in the absence of O₂; it increases steeply with P_O2 and passes through a maximum. The equilibrium between NO + O₂ and NO₂ is swiftly established over a clean catalyst, but deposits impede this reaction. NO + O₂ forms chemisorption complexes, NO_y, with y ≥ 2. Upon their reaction with hydrocarbon, a nitrogen-containing deposit is formed on the catalyst. It reacts with NO₂, but not with NO, releasing large quantities of N₂. N₂O seems not to be a precursor of N₂. Isotopic labeling shows that one N atom in every N₂ comes from the deposit; the other comes from NO₂. Less deposit is formed with propane, more with propene. As deposits also block catalyst sites, the rate limiting step in NO_x reduction depends on the nature of the hydrocarbon. Deposits are oxidized by O₂ and volatilized by H₂O at high temperature; these processes contribute to the relative efficiency of different hydrocarbons in NO_x reduction over Fe/ZSM-5.