Fe species over wet and solid ion exchanged Fe/ZSM-5 catalysts

In-sik Nam
Dept. of Chem. Eng./School of Envi. Sci. & Eng., Pohang University of Science and Technology

Joo-Hyoung Park
School of Envi. Sci. & Eng., Pohang University of Science and Technology

Jong-Hyun Choung

Sung-Won Ham
Dept. of Chem. Eng., Kyungil University

Alexis T. Bell
Dept. of Chem. Eng., University of California

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     Last modified: February 18, 2006

Abstract
FeZSM5 is recognized to well catalyze the decomposition of N2O into nitrogen and oxygen [1, 2]. The state of iron on the surface of FeZSM5 catalyst strongly depends on the iron content and the method of iron exchange such as wet ion exchange (WIE) and solid state ion exchange (SSIE) [3, 4]. A variety of Fe species including the isolated Fe ions, Fe4O4 nanocluster, oxygen bridged Fe and large Fe3O4 cluster exist on the surface of FeZSM5 according to the method of ion exchange, Fe loading and pretreatment condition [3]. Lobree et al. [4] identified the presence of isolated Fe ions at low loading of Fe and the formation of small FeOx particles at higher loading of iron on the surface of FeZSM5 catalyst. El-Malki et al. [2] observed that N2O decomposition rate exponentially increased at the high content of Fe (Fe/Al=0.4) exchanged ZSM5 catalyst. They suggested that binuclear and large cluster of iron were more active for N2O decomposition reaction than mononuclear type of iron including isolated iron on the surface of ZSM5 catalyst. However, the extensive nature of Fe species responsible for N2O decomposition over FeZSM5 catalyst has been hardly identified yet.
The present study has examined the state of Fe species formed on the surface of the series of FeZSM5 catalysts containing a variety of Fe content with respect to the preparation method and their role for N2O decomposition activity. The catalytic decomposition of nitrous oxide in the presence of excess oxygen and in the presence of and absence of water vapor was carried out over FeZSM5-WIE and SSIE catalysts. The decomposition activity of FeZSM5-SSIE is basically higher than that of FeZSM5-WIE. As the content of Fe over the FeZSM5-SSIE catalysts increases up to the ratio of Fe/Al = 0.49, the catalytic activity steeply increases. When Fe/Al > 0.49, the activity gradually increases and then levels off to about 80% of N2O conversion to N2. The decomposition activity of N2O over FeZSM5-WIE catalysts decreases if Fe/Al > 0.46, mainly due to the formation of Fe2O3 particles on the catalyst surface confirmed by XRD, UV-Vis and XAFS.
Based upon the results from the theoretical EXAFS function chi(k) for Fe2O3 and the imaginary parts of Fe-Fe, Fe-Al, and Fe-Si backscatterings calculated by an ab initio EXAFS code FEFF8, the second neighboring atom to Fe at ca. 2.7 Å in the RSF spectra for the FeZSM5-SSIE catalysts may be attributed to the Fe-Fe backscattering, neither Fe-Al, nor Fe-Si. EXAFS analyses reveal that an unique Fe ion over FeZSM5-SSIE catalysts prepared in the present study may be Fe binuclear species such as oxygen bridged binuclear Fe complex, not larger multinuclear Fe clusters, even with the high Fe loadings on the catalyst surface. It can be concluded that binuclear Fe species is an active reaction site for N2O decomposition and is responsible for the high catalytic performance of FeZSM5-SSIE over -WIE catalysts mainly containing mononuclear Fe or small Fe2O3 particles.

References
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