Fe/SSZ-13 as an NH3-SCR catalyst: A reaction kinetics and FTIR/Mossbauer spectroscopic study

Using a traditional aqueous solution ion-exchange method under a protecting atmosphere of N-2, an Fe/SSZ-13 catalyst active in NH3-SCR was synthesized. Mossbauer and FTIR spectroscopies were used to probe the nature of the Fe sites. In the fresh sample, the majority of Fe species are extra-framework cations. The likely monomeric and dimeric ferric ions in hydrated form are [Fe(OH)(2)](+) and [HO-Fe-O-Fe-OH](2+), based on Mossbauer measurements. During the harsh hydrothermal aging (HTA) applied in this study, a majority of cationic Fe species convert to FeAlOx and clustered FeOx species, accompanied by dealumination of the SSZ-13 framework. The clustered FeOx species do not give a sextet Mossbauer spectrum, indicating that these are highly disordered. However, some Fe species in cationic positions remain after aging as determined from Mossbauer measurements and CO/NO FTIR titrations. NO/NH3 oxidation reaction tests reveal that dehydrated cationic Fe is substantially more active in catalyzing oxidation reactions than the hydrated ones. For NH3-SCR, enhancement of NO oxidation under 'dry' conditions promotes SCR rates below similar to 300 degrees C. This is due mainly to contribution from the fast SCR channel. Above similar to 300 degrees C, enhancement of NH3 oxidation under 'dry' conditions, however, becomes detrimental to NO conversions. The HTA sample loses much of the SCR activity below similar to 300 degrees C; however, above similar to 400 degrees C much of the activity remains. This may suggest that the FeAlOx and FeOx species become active at such elevated temperatures. Alternatively, the high-temperature activity may be maintained by the remaining extra-framework cationic species. For potential practical applications, Fe/SSZ-13 may be used as a co-catalyst for Cu/CHA as integral aftertreatment SCR catalysts on the basis of the stable high temperature activity after hydrothermal aging. Published by Elsevier B.V.