In-Situ Observation of Reaction Intermediate in the Selective Catalytic Reduction of N₂O with CH₄ over Fe Ion-Exchanged BEA Zeolite Catalyst for the Elucidation of Its Reaction Mechanism Using FTIR

Abstract

The selective catalytic reduction (SCR) of N2O with CH4 in the absence and presence of excess O2 has been studied over ion-exchanged Fe−BEA catalyst by the combination of an activity test with in-situ infrared spectroscopy to understand the nature of the surface species involved in the SCR of N2O with CH4. From the results of flow reaction studies, the Fe−BEA catalyst exhibited high activity in the SCR of N2O with CH4, even in the presence of excess oxygen, which demonstrates that the active Fe species for the SCR are formed on the ion-exchanged Fe−BEA catalyst. In the FTIR spectra of fresh catalysts, the OH band on Fe ion species (Fe−OH) was observed on the Fe−BEA catalyst, and the Fe−OH species can be involved in the SCR of N2O with CH4. Furthermore, the reaction intermediates of methoxy and formate species over the Fe−BEA catalyst were observed during the reaction. We measured the oxidation rates of these surface species with N2O and O2, and found that the methoxy species were oxidized with N2O more rapidly than O2, while the formate species were oxidized with both N2O and O2 at almost the same rate. On the basis of these results, we discuss the reaction mechanism of the SCR of N2O with CH4.

A comprehensive investigation of influences of NO and O2 on N2O-SCR by CH4 over Fe-USY zeolite

“…Although (R7) is not favored as (R2), (R8) is very much competitive to (R3), as observed by Nobukawa et al [28]. The occurrence of (R8) and (R7) has also been supported by our own observations.…”

“…Nobukawa et al [25,26,28] found that the simultaneous presence of CH 4 and N 2 O is essential for the high SCR activity. They proposed that some oxygen species dissociated from N 2 O are highly reactive (nascent) but some non-reactive (thermally accommodated).…”

“…They proposed that some oxygen species dissociated from N 2 O are highly reactive (nascent) but some non-reactive (thermally accommodated). The highly reactive (nascent) oxygen species can be formed only in the simultaneous presence of CH 4 and N 2 O, which subsequently oxidizes CH 4 (R1) [25,26,28,30]. The intermediate Fe-OCH 3 species is then oxidized preferentially by N 2 O to form Fe-OOCH (R2).…”

“…The intermediate Fe-OCH 3 species is then oxidized preferentially by N 2 O to form Fe-OOCH (R2). Finally, the oxidation of Fe-OOCH by N 2 O (R3) continues in a much easier way, producing CO 2 and regenerating the Fe-OH active sites [28]. In addition, Wood et al [31] observed that during oxidation of CH 4 to CH 3 OH over Fe/Al-FMI by N 2 O, a minor amount of CO is detected.…”

“…However, to our knowledge, recent studies about N 2 O-CH 4 SCR are mostly focused on Fe-Beta and Fe-MFI catalysts to understand the CH 4 activation mechanism in N 2 O-SCR [25][26][27][28], and there is no report about how SCR of N 2 O by CH 4 is affected by the co-existence NO, O 2 and/or H 2 O that are inevitably present in the N 2 O emission site, such as the nitric acid and adipic acid factories. Although most zeolite catalysts suffer the activity loss during the operation under these practical conditions, we have luckily found that Fe-USY zeolite shows a high activity and a high hydrothermal stability for N 2 O decomposition in our previous studies under the similar conditions [11,12].…”