Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitrogen Oxide with Ammonia on Fe-Form Zeolites

Abstract

The selective catalytic reduction (SCR) of NO with NH3 has been investigated on a portion of the Fe-ZSM5 which contains five T-atoms by using density functional theory. The iron was represented as a mononuclear species. For the fast SCR and NO2 SCR, it is most likely that ammonia adsorbs on Z–[FeO]+ and a proton transfer leads to Z–[NH2FeOH]+. A subsequent reaction with NO or NO2 forms nitrosamine or nitramide together with Z–[FeOH]+, which is probably the most abundant surface species. The reduction of monohydroxylated iron with ammonia leads to Z–[FeNH2]+ and water, and a final reaction of the amino group with NO2 to nitrosamine restores the initial site. The intermediates nitrosamine and nitramide can be assumed to decompose on Brønsted acids to nitrogen and nitrous oxide, respectively, together with water. For the increase in selectivity of the NO2 SCR to nitrogen with temperature, a decomposition of both intermediately formed N2O and NO2 to NO and oxygen was concluded to be responsible, rather than an additional high-temperature pathway. With respect to the decomposition of nitric acid on Z–[FeOH]+ to dihydroxylated iron and NO2 followed by the reaction with ammonia to Z–[NH2FeOH]+, a mechanistic explanation for the new “enhanced” SCR is also outlined. Finally, the reaction of oxygen with Z–[FeNH2]+, leading first to the radical H2NO and then via nitroxyl to NO, is capable to explain the mechanism of the selective oxidation of ammonia. The results of this work account for many observed phenomena of the experimental literature.