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

10B/1999G067

Fe K-edge XAFS Study of Fe-MFI Metallosilicates for NO Decomposition
Catalysts with CH4
Hirofumi ARITANI,*1 Shin-ichi, KOYAMA,1 Atsushi NAKAHIRA,2 and Tsunehiro TANAKA3
1Saitama Institute of Technology, Okabe, Saitama 369-0293, Japan
2
Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
3
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
Introduction
Fe-modified zeolites have been reported for application ofdeNOx catalysts. For example, Fe/H-MFI shows a highcatalytic activity for selective catalytic reduction (SCR)
of NO or N2O with hydrocarbons. In this case,catalytically active species are Fe ions, and the ions
supported on the zeolites are not stable structurally for thereaction. In case of metallosilicate-type materials, Fe ionscan be stabilized in MFI-zeolite framework. The MFI3+
type metallosilicates with Fe in the framework havebeen reported by several workers, and it is applied to thecatalyst with red-ox activity. Recently, Wu et al. reported
3+
that Fe3+-substituted Al -mordenites are synthesized1
under template-free condition. We recently synthesized3+
the Fe -substituted Al-mordenites with high crystallinity.
In our previous study, Fe-incorporated MOR catalysts
synthesized by hydrothermal synthesis shows highactuvuty for NO-SCR. The local structure of Fe ions in
the Fe-substituted mordenite (FeMOR) catalysts wereevaluated by K-edge XAFS. In this report, we applied Fe
K-edge XAFS for characterization of Fe-substituted MFI
metallisilicates (synthesized as decribed below), whichshows higher activity for NO-SCR than FeMOR. In3+addition, the stability of the local Fe structure for NOCH4 reaction is also evaluated.
Experimental
The metallosilicates of Fe-substituted MFI zeolites wereprepared by the hydrothermal synthesis. In brief, Femordenites with several ratio of Fe/(Fe+Al) weresynthesized at 343 K for 7 days from the Na2OFe2O3(and/or Al2O3, Ga2O3)-SiO2-H2O system withthe tetrapropylammonium bromide as a template. Detailof the synthesis is similar to that of FeMOR, described inother paper.2 Each Fe content is denoted as FeNMFI (N =Fe/(Fe+Al) [%]), and molecular ratio of silica/alumina (=Si/(Fe+Ga+Al)2) is 72 in whole samples. Microporous
properties of the synthesized samples with highcrystallinity have been confirmed by means of XRD,Raman, and N2-isotherm (at 77 K). The XAFS
measurement was carried out at the BL-10B station of theKEK-PF in a transmission mode at room temperature.
Data analysis of EXAFS was carried out by the method
described elsewhere.3
Results and discussion
Fig. 1 shows Fe K-edge XANES of FeMFI andFeGaMFI before/after the NO-CH4 reaction. In case of178

Users' ReportFigure 1 Fe K-edge XANES of FeMFI samples
before/after NO-CH4 reaction at 873 K.
FeMFI, XANES spectra in all Fe ratios are almost due totetrahedral Fe3+ with Td symmetry. This result indicatesthe incorporation of Fe ions in MFI-zeolite framework.
By co-existence of Ga in FeMFI (FeGaMFI), mixtute oftetrahedra and FeOOH-like octahedra may be present
from XANES spectra. It indicates the distortion of Fe
symmetry by co-substitution of Ga3+. On these samples,order of NO-SCR activity is; Fe75Ga25MFI > Fe100MFI> Fe75MFI > Fe50MFI. These result suggests thatincorpotated Fe3+ ions act as active species, and Ga cosubstitution enhance the activity of Fe3+ species. AfterNO-CH4 reaction at 873 K, structural change can beshown by XANES in both FeMFI and FeGaMFI. ForFeMFI, distortion of Fe3+-tetrahed is brought about toform C3v-like octahedra. On the other hand, Fe species inFeGaMFI shows nearly Td-symmetric tetrahedra after thereaction. The feature of structural change during thereaction is definitely different from FeMFI. In addition,
result of the tendency of the structural change is
supported by that of FT-EXAFS. From these results, Tdsymmetric Fe3+ species in MFI framework act as theactive species, and its stability relates to the catalytic
activity for NO-SCR.
1
23P. Wu et al. Microporous and Mesoporous Materials, 20,
139 (1998).
A. Nakahira et al., J. Mater. Sci., in revision.
T. Tanaka et al., J. Chem. Soc. Faraday Trans. 1, 84, 2987
(1988).

* aritani@sit.ac.jp