Selective catalytic reduction of nitric oxide by ammonia over iron exchanged yttrium zeolites

The selective catalytic reduction of nitric oxide by ammonia over iron exchanged Y zeolites with different Si/Al ratios was investigated in the presence and the absence of oxygen.; Kinetic studies performed in the temperature range between 500 and 620 K, revealed differences in the activity of the three FeY zeolites tested. Such differences are attributable to differences in the redox properties of Fe in these zeolites, as indicated by the results of Mossbauer spectroscopy. A NaHY zeolite examined under the same conditions was found to be inactive, indicating that the presence of Fe ions in the zeolite is necessary to sustain a steady-state catalytic activity for the NO-NH{dollar}sb3{dollar} reaction.; In the absence of O{dollar}sb2,{dollar} a slow deactivation was observed, with the steady-state rates being one to two orders of magnitude lower than the rates in the presence of O{dollar}sb2.{dollar} In situ Mossbauer spectroscopy studies demonstrated that during the deactivation a slow reduction of the Fe ions occurred from the ferric to the ferrous state. In addition, at steady-state the catalysts were confirmed to operate close to their fully oxidized and reduced states, in the presence and absence of O{dollar}sb2{dollar} respectively. Such results indicate that the ferric ions present in the FeY zeolites help sustain a higher level of catalytic activity for the NO-NH{dollar}sb3{dollar} reaction than the ferrous ions.; The results of in situ infrared spectroscopic studies indicated the presence of ammonia as NH{dollar}sb3{dollar} and NH{dollar}sb4sp+{dollar} on both Lewis and Bronsted acid sites respectively, and no significant amounts of nitrosyl species, on the catalyst surface under reaction conditions. Both ammonia species, as well as NH{dollar}sb4sp+{dollar} species adsorbed on a NaY zeolite, were found to be reactive with gas phase NO. Such a result indicates that the ferric ions are not involved in activating the adsorbed ammonia species, but they participate in regenerating the surface following the NO-NH{dollar}sb3{dollar} reaction.; Finally, the oxidation of NH{dollar}sb3{dollar} by O{dollar}sb2{dollar} was also found to take place over the FeY zeolite under the conditions studied. Infrared results indicate that this reaction proceeds via the formation of NH{dollar}sb2{dollar} and/or NH species on the catalyst.