Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

10B/2004G339

XAFS Study on Mo/SiO2 and Mo/Al2O3 Photocatalysts for the Selective Oxidationof CO with O2 in the Presence of H2 at 293 K
Takashi KAMEGAWA1, Masaya MATSUOKA*1, Masakazu ANPO1
1
Department of Applied Chemistry, Graduate School of Engineering
Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531

Results and Discussion
The photocatalytic selective oxidation of CO into CO2
with O2 in the presence of H2 was investigated onMo/SiO2 and Mo/Al2O3 under UV light irradiation at 293
K. After UV irradiation for 180 min, concentration of CO
gas reached below the detection limit of GC analysis (lessthan 8 ppm), while the amount of H2 remained constant[1]. The CO conversion and selectivity reached ~100 %and 99 %, respectively, on Mo/SiO2, while no reaction
proceeded on Mo/Al2O3. Moreover, Mo/Al2O3 exhibited
quite weak photoluminescence as compared to that ofMo/SiO2.
The XANES spectra of Mo/SiO2 and Mo/Al2O3 exhibit
well-defined preedge peaks due to the 1s-4d transition of6+Mo atoms, indicating that Mo -oxide species exist in atetrahedral symmetry for both samples (Fig. 1 (A), (B)).
The Fourier transforme of EXAFS spectra of Mo/SiO2and Mo/Al2O3 showed peaks due to the existence ofneighboring oxygen atoms (Mo-O) at around 0.8-2.0 Å,
while the additional peak due to the Mo-O-Mo bond was
not observed between 3.0-4.0 Å (Fig. 1 (a), (b)). These

Mo=O

(A)

19960

(B)

(a)

Mo-O
R
CN
1.68 2.17
1.90 2.15

Magnitude / a. u.

Experimental
Mo/SiO2 and Mo/Al2O3 (0.60 wt% molybdenum asmetal) were prepared by an impregnation method of SiO2(aerosil 300; Degussa) or Al2O3 (Aluminium Oxide C;Degussa), respectively, using an aqueous solution of(NH4)6Mo7O24•4H2O. Photocatalytic reactions were carriedout in a closed system using quartz reactor under UV
irradiation at 293 K using a 100 W high-pressure mercury
lamp.
XAFS spectra at Mo K-edge were measured in atransmission mode at room temperature at the BL-10B
station. Curve fitting analysis of the EXAFS spectra was3-1
conducted on k χ (k) in k-space (k range = 3-12 Å ) witha REX2000J program (Rigaku).

6+results suggest that Mo -oxide species are highly
dispersed on SiO2 and Al2O3.

Normalized Absorbance / a. u.

Introduction
The preferential oxidation (PROX) of CO is one of themost straightforward method to reduce the COcontamination in H2-rich gas. For now, noble metals
supported catalysts generally have been tested andproposed for thermal PROX reaction. To the best of ourknowledge, no attempt has been made to apply
photocatalysis for the PROX reaction. Therefore, We
have applied the Mo/SiO2 and Mo/Al2O3 photocatalysts
for PROX reaction at 293 K.

Mo=O(b)

Mo-ORCN
1.72 2.17
1.79 2.16

20040
Energy / eV024
Distance / Å

6Moreover, the results of curve fitting analysis of Mo-O
6+bonds suggest that Mo -oxide species on SiO2 and Al2O3
have two short Mo=O double bonds and two long Mo-O
6+
single bonds, while Mo -oxide species on Al2O3 haslonger M=O double bonds and shorter Mo-O single bond
6+
than that on SiO2. The short Mo-O single bond of Mo oxide species on Al2O3 can be ascribed to the large ioniccharacter of framework metal-oxygen bond (Al-O) ofAl2O3 as compared to that (SiO) of SiO2 [2]. The shorter
Mo-O single bond also indicates the strong interaction
6+
between Mo -oxide species and Al2O3.
From the results of XAFS measurements, the weak
photoluminescence intensity of Mo/Al2O3 can beexplained by the efficient thermal deactivation from6+
photoexcited Mo -oxide species to support, which is
6+enhanced by the strong interaction between Mo -oxide
species and Al2O3 as suggested by XAFS results. Theefficient thermal deactivation process of photoexcited6+
state of Mo -oxide species on Al2O3 dramaticallydecrease catalytic reaction rate on Mo/Al2O3.
References[1] T. Kamegawa, R. Takeuchi, M. Matsuoka, and M.
Anpo, Catal. Today., 111, 248 (2006).
[2] Y. Iwasawa, Adv. Catal., 35, 187 (1987).
* matsumac@chem.osakafu-u.ac.jp

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