A DFT Study toward Understanding the High Activity of Fe-Exchanged Zeolites for the “Fast” Selective Catalytic Reduction of Nitrogen Oxides with Ammonia

The possible mechanisms for the “fast” selective catalytic reduction (SCR) of nitrogen oxides with ammonia over Fe-exchanged zeolites have been investigated with density functional theory calculations. Three mechanisms studied in this work include (1) the reaction of (NO + NO 2 ) with NH 3 in gas phase (via N 2 O 3 ), (2) the decomposition of NH 4 NO 2, and (3) the reaction of (NO + NO 2 ) with NH 3 catalyzed by the Fe-exchanged zeolites (modeled by the Z − [FeO] + site). For the mechanisms 1 and 2, our calculations show that the activation barriers of the rate-limiting steps are 22.5 and 24.0 kcal/mol, respectively. In these two mechanisms, the key intermediate is NH 2 NO, which decomposes into the final products N 2 and H 2 O. For mechanism 3, we find that the key intermediate NH 2 NO can be formed from the reaction of NO with NH 3 on the [FeO] + site, and the resulting reduced active site [FeOH] + can be reoxidized by NO 2 and NH 3 to regenerate the active site. The formation of NH 2 NO is calculated to be exergonic by 34.2 kcal/mol and with an activation barrier of 3.0 kcal/mol. The regeneration of the active site involves an activation barrier of 32.0 kcal/mol. Mechanisms 1 and 2 may be responsible for the “fast SCR” of NO x with NH 3 observed at lower temperatures (below 373 K), and mechanism 3 may provide a possible explanation on the high activity of the “fast SCR” of NO x with NH 3 over the Fe-exchanged zeolites at higher temperatures.