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

7C/2003G071

Structural changes in supported
manganese oxides
during ozone decomposition
121
1Hisahiro EINAGA* , Masafumi HARADA , Shigeru FUTAMURA
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan
2
Nara Women’s University, Nara 630-8506, Japan

Introduction
We herein report the X-ray absorption fine structure(XAFS) spectroscopic studies to investigate the structuralchanges in alumina-supported manganese oxide catalysts
during ozone decomposition reactions. The presence ofwater, ubiquitous constituent in effluent gas stream andformed in VOC decomposition, causes the structuralchanges in Mn oxide structures during the ozone
decomposition. The used catalyst can be regenerated by
the heat treatment in an O2 flow at 723 K.
Experimental
Alumina-supported manganese oxides were preparedby the impregnation of γ-Al2O3 (Catalysis Society ofJapan, JRC ALO-4, SBET 170 m2 g-1) with the aqueous
solutioncontaining
appropriate
amountofMn(CH3COO)2•4H2O. Catalyst samples were dried at383 K and then calcined at 773 K for 3 h in air.
XAFS measurements were carried out on the Photon
Factory beam line BL-7C. Catalyst samples were pressed
into thin self-supporting wafers with 20 mmφ in diameterand set in an in situ cell with polyimide film (Du PontToray Co., Ltd. Kapton 200H), which was connected to aflow-type reaction system. XAFS spectra were recorded
at a temperature of 296 K. Reaction gases were preparedby N2 and O2 in cylinders by using the sets of thermal
mass flow controllers. Ozone was synthesized from O2
by a silent discharge ozone generator.
Results and discussion
Fig. 1 shows the Mn-K edge XANES spectra of theMnOx/Al2O3 catalyst. The absorption edge of XANES
spectra was gradually shifted to higher energy with timeon stream and a preedge peak newly appeared at 6540.6
eV, when ozone was fed into the cell with water vapor(water concentration: 0.55%). No such spectral changes
occurred during ozone decomposition when water vapor
was absent.
Linear relationship is observed between the energy
shift of the Mn-K absorption edge and the oxidation stateof the Mn-oxide samples. The absorption edge ofMnOx/Al2O3 sample is close to that of Mn3O4, which
consists of MnII and MnIII species with the averageoxidation state of 2.67. The shift of absorption edge tohigher energy indicates that Mn oxidation state waschanged to higher oxidation state. The appearance of apreedge band at 6540.6 eV, which is characteristic of thatfor MnO2, implies the formation of MnIV species. TheMn-K edge EXAFS Fourier transforms of the usedsample (line c) is compared with fresh catalyst with (lineb) and without water vapor (line a) in Fig. 2. The band at

10Users' Report

R=1.5 Å increased in its intensity after the ozone
decomposition with water vapor. The increased band wasascribed to the coordination of water to Mn sites.
The coordination of water to Mn site was caused by thebond cleavage of Mn-O-Al bond. Although the bondcleavage of Mn-O-Al was not reflected in XAFS spectra,
it is evidence by the fact that Mn ions were extracted
from the catalyst surface by washing the used catalyst
with water. This finding indicates that Mn was isolated
from catalyst support to become a free Mn ion to whichwater is coordinated. The catalyst color was also changedslightly from brown to brownish pink. Heat treatment inO2 flow at 723 K gave the same spectrum with that for
fresh catalyst sample (line d), indicating that the catalyst
was regenerated by the treatment.

Fig. 1 Changes in Mn-K edge XANES spectraof Mn/Al2O3 sample during ozone decomposition.

Fig. 2 Mn-K edge EXAFS
transforms of Mn/Al2O3 sample.
*h-einaga@aist.go.jp

Fourier