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

Abstract

Using a traditional aqueous solution ion-exchange method under a protecting atmosphere of N2, an Fe/SSZ-13 catalyst active in NH3-SCR was synthesized. Mössbauer 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 [HOFeOFeOH]2+, based on Mössbauer 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 Mössbauer spectrum, indicating that these are highly disordered. However, some Fe species in cationic positions remain after aging as determined from Mössbauer 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 ∼300°C. This is due mainly to contribution from the “fast” SCR channel. Above ∼300°C, enhancement of NH3 oxidation under ‘dry’ conditions, however, becomes detrimental to NOx conversions. The HTA sample loses much of the SCR activity below ∼300°C; however, above ∼400°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.