Reduction of NOx over Fe/ZSM-5 catalysts:: mechanistic causes of activity differences between alkanes

Fe/ZSM-5 catalysts prepared by sublimation of FeCl3 onto H/ZSM-5 catalyze the selective reduction of NOx by hydrocarbons to N-2. The order of the relative rates and N-2 yields obtained with different alkanes reveals a non-trivial chemistry. The maximum yield is lower for propane than for n-butane but about the same for n- and iso-butane. However, at temperatures below this maximum, the N-2 yield is higher for propane and n-butane than for iso-butane. Deposits are formed on the catalyst that contain N atoms in a low-oxidation state which are able to react with NO2 to form N-2. TPO and FTIR results show that the amount and also the character of the deposits depend on the nature of alkanes. The change of the oxidation state of nitrogen from a high value in NO or NO2 to a lower value in nitrile and amino groups of the deposit is rationalized by applying mechanistic concepts of organic chemistry, including the Beckmann rearrangement and fragmentation. FTIR spectra and the observed oxygen- and nitrogen-containing compounds by GC-MS are potential clues to the reaction mechanism. (C)1999 Elsevier Science B.V. All rights reserved.