Theoretical Investigation of the Mechanism of the Oxidation of Nitrogen Oxide on Iron-Form Zeolites in the Presence of Water

The oxidation of NO to NO2 by oxygen and the influence of water have been investigated on a portion of the Fe-ZSM5 which contains 5 T atoms by using the density functional theory. The iron was considered to be a mononuclear species. For the main reaction pathway it is most likely that NO initially adsorbs on Z(-)[FeO](+), forming the nitrite Z(-)[FeON](+) which includes a spin change from the septet to the quintet potential energy surfaces (PES). The adsorption of oxygen and the desorption of NO2 leads to Z(-)[FeO2](+) which is attacked by NO from the gas phase. Within another spin change from the septet to the quintet PES, the ligand with the sequence -OONO is formed of which NO2 desorbs. The latter step is rate-limiting for the overall mechanism. A final spin change restores the initial active site on the sextet PES. At the same time, the nitrite Z(-)[FeO2N](+) and the nitrate Z(-)[FeO2NO](+) are rapidly formed and block the active site. Water leads to the formation of, first, a dihydroxylated iron Z(-)[HOFeOH](+) and after the reaction with NO or NO2 to nitrous and nitric acid, respectively, together with monohydroxylated iron Z(-)[FeOH](+). The dihydroxylated iron is inactive for the oxidation of NO, and the monohydroxylated exhibits a reduced activity as compared to Z(-)[FeO](+). Further removal of hydroxyl groups by reaction with NOx is also accessible, restoring nonhydroxylated iron sites. Furthermore, water also potentially reacts directly with the surface nitrite and nitrate, leading to nitrous and nitric acid. This explains the phenomena of a short-term release of significant amounts of NO2 upon water addition in transient experiments. The results of this work are in agreement with the experimental literature and account for many observed phenomena.