Photon Factory Activity Report 2019 #37 (2020)

BL-9C/2015G545

XAFS Analysis on Vanadium Species Dispersed in Transparent Silica GlassYusaku YAMAMOTO1, Yuki KAJIYAMA1, Misaki KATAYAMA2, and Yasuhiro INADA1,2,*
1
Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University,
1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan2
SR Center, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu 525-8577, Japan
1 Introduction
The vanadium(V) oxide highly dispersed on silica
shows the photocatalytic activity for partial oxidation ofalkane and alkene, because the local structure of the V(V)center is different from that in the bulk crystal [1]. It is
important for improving the photocatalytic performance tounderstand the electronic state and the local structure of thephotoexcited state. The pump and probe DXAFS method
is a useful technique for analysis of photoexcited states.
However, the excitation laser is scattered at the powder
surface and it excites only the V(V) species located near
the particle surface. Although it is not a problem for thephotocatalysis reaction, the selective observation of suchthe active species is impossible for the DXAFS techniquein the transmission mode. An efficient excitation method
for the entire V(V) species is necessary to apply theDXAFS technique. One solution is to prepare a transparent
glassy plate, in which the active V(V) species is dispersed.
The purpose of this research is the chemical state analysisof V(V) species supported in a transparent silica glass
during its preparation process.
2 Experiment
The V(V) oxide photocatalyst was prepared by the solgel method. The tetraethyl orthosilicate (TEOS), nitric acid,deionized water, and N,N-dimethylformamide were added
into ethanol, and the solution was stirred for 1.5 h at 50 °C.
Then, V(V) oxytriisopropoxide (VOTIP) and ammoniumhydroxide were added into the solution to get the wet gel.
It was dried for a week at 80 °C and for 3 h at 200 °C. The
sample was then heated at 500 °C for 1.5 h in air.
The obtained sample was transparent and colorless justafter the treatment at 500 °C, but the color was changed to
reddish brown after cooling down. The in-situ XAFS
measurement was performed at BL-9C of PF to analyze thechemical state change of the V species. The glassy sample
plate with the thickness of 0.18 mm was set in the flowtype in-situ XAFS cell, and it was heated up to 500 °C
under 10 vol% O2/He gas flow as the heating treatment.
3 Results and Discussion
Only a broad diffraction line at 25° assigned toamorphous silica was appeared in the XRD pattern of theprepared sample. It indicates that supporting silica isformed by the hydrolysis and the condensation of TEOS
and that the V species was highly dispersed in silica.
Figure 1 shows the XANES spectrum of the prepared Vphotocatalyst supported in the transparent silica glass. Theabsorption edge energy of the sample after the heating
treatment at 500 °C agrees with that of V2O5, however thespectral shape was apparently different. Especially, thepre-edge peak intensity of the V(V) photocatalyst is
stronger than that of V2O5. It is known that the pre-edge
peak is assigned to the transition from 1s to 3d which ismixed with 2p of oxygen [2]. Therefore, the peak intensity
is dependent on the geometry of binding O atoms aroundthe V(V) center. A similar intense peak is observed for
VOTIP and NH4VO3 with the similar VO4 unit, which iscontrast to the V2O5 with the tetragonal VO5 unit. The
present study revealed that the V(V) center with the VO 4unit was dispersed in the transparent silica glass. After itwas cooled down in air, the pre-edge peak intensity of
XANES spectrum was drastically decreased. It isconsidered that the structure change from VO4 to VO5 wascaused by the water molecule absorbed from air [3]. Theseresults indicate that the protection against the water
absorption is necessary to sustain the active VO4 unit tocarry out the pump and probe DXAFS experiment.

Fig. 1: XANES spectrum of the V photocatalyst supported
in the transparent silica glass (A). For (a), the solid anddashed line shows the spectrum of the photocatalyst at
500 °C and at room temperature, respectively. The
standard samples are V2O5 (b), VOTIP (c), and NH4VO3(d). The pictures show the sample just after the heating
treatment (B) and after the water absorption (C).
References[1] S. Yoshida, Y. Matsumura, S. Noda, and T. Funabiki, J.
Chem. Soc. Faraday Trans. 1 77, 2237 (1981).
[2] T. Tanaka, H. Yamashita, R. Tsuchitani, T. Funabiki,and S. Yoshida, J. Chem. Soc., Faradaytrans. 84, 2987
(1988).
[3] S. Yoshida, T. Tanaka, Y. Nishimura, H. Mizutani, andT. Funabiki, Proc. Int. Congr. Catal. 3, 1473 (1988).
* yinada@fc.ritsumei.ac.jp