INSITU MOSSBAUER-SPECTROSCOPY STUDIES OF FE-Y ZEOLITES FOR THE SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE BY AMMONIA

In situ Mossbauer spectroscopy was used to study iron-exchanged Y zeolites with Si/Al ratios of 2.4, 3.2, and 4.4 in H-2, O2, NO-NH3, and NO-NH3-O2 gas mixtures at temperatures from 473 to 623 K. Spectra collected in flowing H-2 shoW that Y zeolites with higher Si/Al ratios have a larger fraction of iron cations in accessible sites (I', II', and/or II). Migration of the iron cations from inaccessible (I) sites of the zeolite to more accessible sites takes place with increasing temperature in hydrogen. Spectra collected in flowing O2, as well as microbalance studies, show an increased difficulty in oxidizing ferrous cations to the ferric state in silicon-substituted zeolites. In situ Mossbauer spectra collected during the treatment of initially oxidized Fe-Y zeolites in NO-NH3 in He gas mixtures (typically 500 ppm NO and 500 ppm NH3) show reduction of the iron ions from the ferric to the ferrous state. At steady state, the iron cations are predominantly in the ferrous state. In NO-NH3-O2 in He gas mixtures (typically containing 1% O2), both ferric and ferrous ions are present at steady state, with the majority of the iron cations being in the ferric state. The relative quantities of ferric and ferrous cations are dependent on the Si/Al ratio of the zeolites, the treatment temperature, and the compositions of the NO-NH3 and the NO-NH3-O2 gas mixtures. These results, combined with catalytic activity measurements, suggest that ferric cations are associated with high activity for the selective catalytic reduction of nitric oxide by ammonia, since the catalytic activity is 2 orders of magnitude higher in the presence of oxygen.