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

The selective catalytic reduction (SCR) of NO with NH(3) has been investigated on a portion of the Fe-ZSMS which contains five T-atoms by using density functional theory. The iron was represented as a mononuclear species. For the fast SCR and NO(2) SCR, it is most likely that ammonia adsorbs on Z(-)[FeO](+) and a proton transfer leads to Z(-)[NH(2)FeOH](+). A subsequent reaction with NO or NO(2) 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(-)[FeNH(2)](+) and water, and a final reaction of the amino group with NO(2) to nitrosamine restores the initial site. The intermediates nitrosamine and nitramide can be assumed to decompose on Bronsted acids to nitrogen and nitrous oxide, respectively, together with water. For the increase in selectivity of the NO(2) SCR to nitrogen with temperature, a decomposition of both intermediately formed N(2)O and NO(2) 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 NO(2) followed by the reaction with ammonia to Z(-)[NH(2)FeOH](+), a mechanistic explanation for the new "enhanced" SCR is also outlined. Finally, the reaction of oxygen with Z(-)[FeNH(2)](+), leading first to the radical H(2)NO 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.