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 + NO2) with NH3 in gas phase (via N2O3), (2) the decomposition of NH4NO2 and (3) the reaction of (NO + NO2) with NH3 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-lirniting steps are 22.5 and 24.0 kcal/mol, respectively. In these two mechanisms, the key intermediate is NH2NO, which decomposes into the final products N-2 and HO. For mechanism 3. we find that the key intermediate NH2NO can be formed from the reaction of NO with NH3 on the [FeO](+) site, and the resulting reduced active site [FeO](+) can be reoxidized by NO2 and NH3 to regenerate the active site. The formation of NHAO 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 NOx with NH3 observed at lower temperatures (below 373 K), and mechanism 3 may provide a possible explanation on the high activity of the "fast SCR" of NOx with NH3 over the Fe-exchanged zeolites at higher temperatures.