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 N 2 , an Fe/SSZ-13 catalyst active in NH 3-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 extraframework cations. The likely monomeric and dimeric ferric ions in hydrated form are [Fe(OH) 2 ] + and [HO-Fe-O-Fe-OH] 2+ , based on Mössbauer measurements. During the harsh hydrothermal aging (HTA) applied in this study, a majority of cationic Fe species convert to FeAlO x and clustered FeO x species, accompanied by dealumination of the SSZ-13 framework. The clustered FeO x 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/NH 3 oxidation reaction tests reveal that dehydrated cationic Fe are substantially more active in catalyzing oxidation reactions than the hydrated ones. For NH 3-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 NH 3 oxidation under "dry" conditions, however, becomes detrimental to NO x 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 FeAlO x and FeO x 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.

The effect of iron loading and hydrothermal aging on one-pot synthesized Fe/SAPO-34 for ammonia SCR

“…For potential practical applications, the authors suggested that Fe/SSZ-13 may be used as a cocatalyst together with Cu/CHA as integral aftertreatment SCR catalysts on the basis of the stable high temperature activity after hydrothermal aging. 37 Further, there exist a few studies in the literature addressing the hydrothermal synthesis of Fe/SAPO-34 catalysts, directly incorporating Fe into the structure, for the applications of chloromethane transformation to light olefins and of methanol conversion. 34,35 In a recent patent application, NH 3 SCR of Fe/SAPO-34, with Fe incorporated into the structure, was studied, but only up to 500 C.…”

“…In a very recent work, an active Fe/SSZ-13 catalyst for NH 3 -SCR was successfully synthesized by traditional aqueous solution ion-exchange method under the protection of N 2 . 37 The method used in the study was applied to prevent Fe 2+ ions from being oxidized to bulky Fe 3+ moieties (e.g., FeO(OH)) that can hardly penetrate into zeolite pores. It was demonstrated that the small pore CHA structure does not impede Fe 2+ ion exchange and the formed Fe species become active at temperature above 400 o C after aging of the Fe/SSZ-13 catalyst.…”

Effects of Alkali and Alkaline Earth Cocations on the Activity and Hydrothermal Stability of Cu/SSZ-13 NH3–SCR Catalysts

“…Details about the chemicals used and the synthesis procedure can be found elsewhere. 25 The assynthesized material was calcined in air at 550 °C for SDA combustion to obtain fresh 4 Na/SSZ-13. This material was further ion-exchanged with a 0.1 M NH 4 NO 3 solution at 80 °C to form NH 4 /SSZ-13; again detailed procedures can be found elsewhere.…”

Activity enhancement of WO3 modified Fe2O3 catalyst for the selective catalytic reduction of NO by NH3

“…Fe-containing catalysts, such as Fe/SSZ-13, are active for the NH 3 -SCR of NO x [5,6]. Recently, Fe-Ti spinel has been developed as an environmentally benign SCR catalyst, which showed excellent activity and selectivity at 300-400°C [7].…”