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

Fe/ZSM-5 prepared via sublimation catalyzes the reduction of NOx to N-2 in the presence of excess O-2 and H2O. Propane, isobutane, and propene are active reductants; methane is inactive. The NOx reduction rate is negligible in the absence of O-2; it increases steeply with P-O2 and passes through a maximum. The equilibrium between NO + O-2 and NO2 is swiftly established over a clean catalyst, but deposits impede this reaction. NO + O-2 forms chemisorption complexes, NOy, with y greater than or equal to 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 N-2. N2O seems not to be a precursor of N-2. Isotopic labeling shows that one N atom in every N-2 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 NOx reduction depends on the nature of the hydrocarbon. Deposits are oxidized by O-2 and volatilized by H2O at high temperature; these processes contribute to the relative efficiency of different hydrocarbons in NOx reduction over Fe/ZSM-5. (C) 1998 Academic Press.