Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

12C/2003G255

Structural Analysis of the Active Sites of Fe-MFI Catalysts for
Selective Catalytic Removal of Nitrous OxideTakeshi NOBUKAWA*1, Masanori YOSHIDA1, Kou SUGAWARA1,2
1
1Kazu OKUMURA , Keiichi TOMISHIGE , Kimio Kunimori
1
Institute of Materials Science, Univ. of Tsukuba, Tsukuba Ibaraki 305-8573, Japan2
Department of Materials Science, Tottori Univ., Koyama-cho, Minami, Tottori 680-8552, Japan

18Users' Report

In the case of 2.8 wt% Fe-MFI, the oscillation after thetreatment with N2O is different from that after O2
treatment, especially, higher k region. From the fitting
results of EXAFS data [2], we observed the contributionsof the bridged oxygen species such as Fe-O-Fe and Fe-Fe
bond after N2O treatment. This behavior can beinterpreted the presence of vacancy sites, which can beonly fulfilled by bridged oxygen species after N2Otreatment. These are in good agreement with the results ofO2-TPD study [1]. Considering also other characterization
results from H2-TPR and O2-TPD [1], it is strongly
suggested that binuclear Fe species are formed on 2.8
wt% Fe-MFI. The coordination number of Fe-Fe is lessthan unity. This means that mononuclear Fe species alsopresent over Fe-MFI. From the results of EXAFS analysis
combined with other characterization, it is found that 2.8
wt% Fe-MFI contains binuclear and mononuclear Fespecies, and 0.70 wt% Fe-MFI contains only amononuclear Fe species.
(a)3k χ(k)O2 treatment

N 2O treatment

2468k / 10 nm

1012-1(b)O2 treatment

3k χ(k)Introduction
Nitrous oxide (N2O) is a strong green-house effect gas
and can contribute to catalytic stratospheric ozone
destruction. Recently we reported that the catalytic
activity in N2O reduction with CH4 on Fe-MFI wasdependent on the exchange level of Fe ion [1]. From theresults of activity test and catalyst characterization, wesuggested the presence of two types of Fe ion sites:mononuclear and binuclear Fe species. In this study, wecharacterized the structure of Fe species on Fe-MFI anddiscuss the nature of active sites of N2O reduction on thebasis of the relation between catalyst structure andperformance.
Experimental
The zeolite-supported Fe-MFI catalyst was prepared by
wet ion-exchange method [1]. The precursor of iron wasFeSO4•7H2O (Wako). The Na form MFI zeolite wasprovided by TOSOH Co. (Si/Al2 = 23.8). The loading
amount of Fe is 0.35 and 2.8 wt%. The sample disk (0.5mm, 10 mmφ) for EXAFS measurements was pretreated
at 773 K with 11 kPa O2 or N2O for 0.5 h in a closed
circulation reactor, respectively. After treatment, thesample disk was transformed to the measurement cellwithout exposing the sample disk to air. Iron K-edge
EXAFS spectra were measured by transmission mode atliquid nitrogen temperature. After back ground3
subtraction, k weighted EXAFS functions were Fouriertransformed into R space and the three or four-shell fitting
were analyzed by curve fitting.
Results and Discussion
3
Figure 1(a) shows k -weighted Fe K-edge EXAFS
oscillations for 0.70 wt% Fe-MFI measured after
treatment with O2 or N2O. It seems that the oscillation on
0.70 wt% Fe-MFI treated with O2 is similar to that with
N2O. From the fitting results of EXAFS data [2], wecould fit these spectra with the Fe-O1 (OH species
coordinated with Fe ion sites), Fe-O2 (zeolite lattice), andFe-Si shells. Therefore, it is expected that the Fe species
over 0.70 wt% Fe-MFI can be isolated. Furthermore, thepossibility of Fe-Fe bond over 0.70 wt% Fe-MFI can beruled out because this catalyst contains almost exclusively
Fe ions with lower reducibility from H2-TPR and O2-TPD
results [1]. The comparison of the results between O2 andN2O treatments shows that the coordination number andthe distance were almost the same values. This also
indicates that only mononuclear Fe species are present
over 0.70 wt% Fe-MFI catalyst.

N2O treatment

2468k / 10 nm

1012-1

3Fig. 1. k -weighted Fe K-edge EXAFS for (a) 0.70 wt%Fe-MFI and (b) 2.8 wt% Fe-MFI.
References[1] M. Yoshida et al., J. Catal. 223 (2004) 454.
[2] T. Nobukawa et al., J. Catal. 229 (2005) 374.
*nobukawa@ims.tsukuba.ac.jp