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Materials Letters 62 (2008) 861 – 864
www.elsevier.com/locate/matlet

Photo-oxidation over mesoporous V-TiO2 catalyst under visible lightmonitored by vanadium Kβ5,2-selecting XANES spectroscopy ☆
Yasuo Izumi a,⁎, Kazushi Konishi b , Tomohisa Miyajima c , Hideaki Yoshitake cab

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, JapancGraduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
Received 15 February 2007; accepted 30 April 2007
Available online 19 July 2007

Abstract
Visible light-excited ethanol dehydration reaction steps to adsorb ethanol and then to donate an electron to transform V(IV) to V(III) site
exposed on mesoporous V(IV)-TiO2 catalyst were successfully monitored by means of V Kβ5,2-selecting XANES spectroscopy. Spherically-bentGe(422) crystal was used for the X-ray fluorescence analyses and Xe arc lamp of 500 W was illuminated at 10 mm apart from catalyst samples
under on reaction conditions. Specific redox reaction mechanism between V(IV) and V(III) states was proposed for dehydration reaction overthe mesoporous V-TiO2 catalyst based on the V Kβ5,2-selecting XANES compared to general vanadium catalysts starting from V(V) state fordehydrogenation reaction.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Catalysts; Photo-catalysis; Characterization methods; X-ray techniques; Vanadium

1. Introduction
Utilization of solar energy is expected to be clean energy
source. One of the technological possibilities is photo-catalysis
over TiO2-based catalysts doped with V, Cr, Mn, Fe, Ni [1], C,N, or S atoms [2,3] under the illumination of visible light.
Recently, we reported that the energy gap between conduction
and valence bands became smaller for mesoporous TiO2 dopedwith 1–3 wt.% of V [4] compared to conventional catalysts
based on anatase-type TiO2 [5].
Promoting effects of V doping on/in TiO2 were reported forethanol photo-oxidation [6], degradation of crystal violet andmethylene blue [7,8], and nitric oxide [9]. Under theillumination of visible light (N 420 nm), mesoporous TiO2
exhibited poor activity [5]. The mesoporous V-TiO2 catalyst
with the specific surface area (SA) as much as 1000 m2g− 1
became active and proceeded dehydration of ethanol (1.0 kPa)
to produce acetaldehyde, in clear contrast to the ethanol☆

Part 21 of the state-sensitive X-ray absorption fine structure analysis series.
⁎ Corresponding author. Tel./fax: +81 43 290 3696.
E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2007.04.120

dehydrogenation over [V-doped] anatase-type TiO2 catalysts[5]. Because the V addition sometimes hampers photo-catalysis
of TiO2 [10], it is critical to understand the catalytic role of
V under the illumination of visible light. The role must becomplex [10] and still open for discussion, especially in the caseof mesoporous V-TiO2 catalysts [5].
The keys to understand the V catalytic role are (1) statesensitive monitoring of V site structure transformation by X-rayabsorption fine structure (XAFS) under on-reaction conditions[11–13] and (2) sample setting design to illuminate the catalyst
very near the exit window of visible light lamp during the XAFS
measurements under on-reaction conditions. To satisfy both
requisites, we employed X-ray fluorescence-detecting XAFS to
selectively detect V Kβ5,2 fluorescence associated with theelectronic transition from V 3d and oxo π levels to V K-level[14–16]. V Kβ5,2 fluorescence (M4,5→ K) was chosen for thefirst time in the state-sensitive XAFS measurements rather thanV Kα1 fluorescence (L3→ K) [11–13] because the Kβ5,2 isderived from the electronic transition from frontier levels e.g.
V 3d to V 1s and thus the electronic state information of frontierlevels will be emphasized in the V Kβ5,2-selecting XAFS,
closely correlated with the photo-catalysis originated from the862

Y. Izumi et al. / Materials Letters 62 (2008) 861–864

Fig. 1. The sample, UV-visible Xe arc lamp, UV-cut filter, analyzer crystal, anddetectors set for vanadium Kβ5,2-selecting XAFS measurements.

electron-hole pair formation at the frontier levels. Also, greaterchemical shifts were expected for V Kβ5,2 than for V Kα1.

VIII2O3, VIVO(SO4)·nH2O and Na3(VVO4), respectively. Thus, vanadium was valence state IV for fresh sample and was partially reduced
toward III state in ethanol and/or with visible light illumination. Thevalence state IV for fresh sample was in agreement with conclusion
based on V Kα1 emission spectra and V Kα1-detecting V K-edge
XANES (X-ray absorption near-edge structure) [11,13].
The fluorescence spectrometer was set to each V Kβ5,2 peak top
under the four conditions, and V K-edge XANES spectra were measured for mesoporous V-TiO2 sample (Fig. 2B). The data scan step was≈0.25 eV and the accumulation time was 60–100 s for a point.
The rising edge and pre-edge peak energy values were 5479.7 and5468.2 eV, respectively, for fresh sample. These were very similar
to corresponding values (5479.4–5479.8 and 5467.9–5468.5 eV,respectively) obtained for VIV standard compound (Fig. 2A—b), VKα1-selecting XANES of mesoporous V-TiO2 (0.6–10.4 wt.% V), andTi K-edge XANES for mesoporous TiO2 shifted by + 499.8 eV [11].
Post-edge peak positions were also similar. Thus, V(IV) sites substituted on the Ti sites of amorphous TiO2 matrix (Fig. 3a). This structure
was supported by the curve fit analyses for V Kα1-selecting V K-edge
EXAFS (extended XAFS) [11].

2. Experimental
The X-ray measurements were performed at beamline 7C or9C in KEK-PF (Tsukuba) at 290 K. The storage ring energy was
2.5 GeV and the ring current was between 449 and 281 mA. Si(111) monochromator and focusing/higher-harmonics-rejection
mirror were inserted. The beam was focused at sample position
and fully tuned. In the setup (Fig. 1), catalyst disk positioned at
10 mm apart from the exit window of Xe arc lamp (500 W,UXL-500D, Ushio) and UV-cut filter L42 (cut-off energy
420 nm, Kenko) was set in between. The detectors 1 and 2 wereionization chamber purged with the mixture of helium andnitrogen gas (7:3) and scintillation counter, respectively.
Sample, detector 2, and spherically-bent Johann-type Ge(422)crystal (Saint Gobain) were set in Rowland configuration ofradius 225 mm. The curvature (radius) of the crystal was
450 mm. The inside of spectrometer was purged with helium
and the spectrometer was set as close as possible to beam-ductconnected to monochromator.
Energy resolution of the fluorescence spectrometer was1.1–1.9 eV for the V Kβ5,2 measurements [13]. The V Kβ5,2emission peak and V K absorption edge energy values werecalibrated to 5462.9 (θBragg = 79.335°) and 5463.9 eV, respectively [17]. Synthesis procedure of mesoporous V-TiO2 catalyst
was already described in Refs. [11,18]. The V content was fixed
to 3.0 wt.%, corresponding to atomic ratio of V/Ti to be 1/21.
3. Results and discussion
V Kβ5,2 emission spectra were measured for mesoporous V-TiO2
samples in argon (fresh catalyst), after the introduction of ethanol(2.1 kPa), in the ethanol with illumination of visible light (N 420 nm,illuminated for 7 h), and in ethanol after the arc lamp was turned off.
The peak top for fresh sample at 5462.1 eV shifted progressively
toward lower-energy side to 5461.6 eV in ethanol in dark, then to5461.0 eV in ethanol under visible light. In ethanol after the visible
light lamp was turned off, the peak top increased back to 5461.6 eV.
The V Kβ5,2 peak top was at 5458.5, 5462.0, and 5462.9 eV forFig. 2. (A) V K-edge XANES spectra for (a) VIII2O3, (b) VIVO(SO4)·nH2O, and(c) Na3VVO4 measured in transmission mode. (B) V Kβ5,2-detecting V K-edge
XANES spectra for mesoporous V-TiO2 catalyst (3 wt.% V). (a) Fresh sample inargon, (b) in 2.1 kPa of ethanol, (c) in ethanol with the illumination of visible
light, and (d) in ethanol after the visible light was turned off.

Y. Izumi et al. / Materials Letters 62 (2008) 861–864

Fig. 3. Proposed reaction mechanism of ethanol dehydration over mesoporousV-TiO2 based on V Kβ5,2-selecting XANES.

With the introduction of 2.1 kPa of ethanol, rising edge shape forFig. 2B—a split into two at 5476.7 and 5479.3 eV (Fig. 2B—b).
This suggests two major vanadium states present. Because the risingedge positions were observed at 5471.0–5474.7, 5477.5–5479.8, and5480.0–5482.2 eV for several V(III), V(IV), and V(V) standard inorganic
compounds, respectively (Fig. 2A) [11,19], partially reduced V(IV-δ)states (Fig. 3b) should be formed due to the dissociative adsorption ofethanol on V(IV). The V(IV-δ) site was mixed with V(IV) site in thiscondition. Pre-edge and post-edge peak positions were relatively similar
to those for fresh sample (Fig. 2B—a). Dissociative adsorption of2-propanol was reported over mesoporous V-TiO2 at 290 K [11].
Next, visible light was illuminated on the sample for Fig. 2B—b. The
rising edge shape in the obtained data (Fig. 2B—c) was with inflection
point at 5477.8 eV, shifted by −1.9 eV from native mesoporous V(IV)TiO2 catalyst. Hence, created electrons excited by visible light were
distributed to the V(IV) to form V(III) (Fig. 3c). The ratio of V(III) state
was quite small under the reaction equilibrium due to the fast electron
excitation/distribution process [i.e. short lifetime of V(III) state], so thatfully reduced state to V(III) may not be detected in comparison with data
for standard V(III) compounds (Fig. 2A—a) [11,19].
Then, the visible light was turned off after 7 h illumination. Obtained
data (Fig. 2B—d) was most similar to Fig. 2B—b except that the preedge peak intensity at 5468.0 eV became fairly weak.
The trend of valence state change in four sample conditions above
was consistent in the V Kβ5,2 emission spectra and V K absorption
edge shifts: progressive shift from V(IV) (fresh) to partially reduced V(IV-δ) state in argon, ethanol, and then under the illumination of visible
light. A shift back toward V(IV) was detected after the visible light off.
The energy shifts were reproduced in repeated tests both for V Kβ5,2emission and V K-edge XANES spectra measurements. However, thefinal V(IV)-like state for Fig. 2B—d was not identical with native V(IV) state. The pre-edge peak remained essentially at the same position:
5468.0–5468.4 eV (Fig. 2B—a–d), but the intensity progressively
decreased to 26%. Among mesoporous V-TiO2 samples, systematicincrease of the pre-edge peak was observed as the V content increased
from 0.6 to 10.4 wt.% V due to the drop of site symmetry, while
keeping the rising edge and post-edge feature unchanged [11]. Thus, Vsite symmetry was distorted as in Fig. 3a in fresh sample and became863

higher due to ethoxyl group adsorption. The pre-edge peak intensity
was still smaller in the presence of O2 and subsequent visible lightillumination [20].
Based on above spectroscopic observation, dehydration reaction
mechanism was proposed in Fig. 3 in the presence of ethanol. Theredox pair of V(IV) and V(III) is reasonable because the redox energy
level near the conduction band was proposed for the V(IV)-substituting
model in TiO2 matrix to easily receive excited electron to form V(III)[10]. V(V) species were reported for the majority of impregnatedcatalysts over TiO2. The difference of vanadium valence states may be
primary reason of different catalysis: dehydration [V(IV)⇄V(III)] anddehydrogenation [native V(V)] of ethanol under visible light. The V(IV) sites were effectively exposed to surface in our mesoporous
samples due to the high specific SA compared to V(IV)-TiO2 samples
prepared via sol–gel method [7,8,20,21].
The advantage of V Kβ5,2-selecting XANES spectra should be
mentioned compared to conventional XANES taken in transmission
mode [18] or V Kα1-selecting XANES [11–13]. The pre-edge peakintensity gradually increased from 0.32, to 0.46, and then to 0.80 for
mesoporous V-TiO2 samples with 0.6 [11], 5.0 [18], and 10.4 wt.% V[11], respectively. The peak intensity was 0.92 for mesoporous V-TiO2samples with 3.0 wt.% V in this work selecting V Kβ5,2. Thus, the preedge peak was enhanced due to resonance excitation effect similar toreports for Fe Kβ1,3-selecting [21,22], Cu Kα1-selecting [22,23], and Sn
Kα1-selecting XANES [22,23]. In this work, the enhanced peak wasderived from levels of V 3d character. Thus, responsible valence V sites
excited by visible light illumination should be emphasized and thequantitative discussion of 1s→ 3d pre-edge peak was enabled by selecting V Kβ5,2 fluorescence peak. Another advantage of V Kβ5,2-selecting
XANES was greater chemical shifts between V(V) and V(IV) andbetween V(IV) and V(III) states (1.0–3.2 eV) compared to correspondingvalues (0.2–0.5 eV) for V Kα1 emission spectra [11] for state-sensitive
XAFS measurements of V(III), V(IV), and V(V).

4. Conclusions
Vanadium Kβ5,2-selecting XANES spectroscopy was successfully applied to mesoporous V-TiO2 catalyst under onreaction conditions of ethanol dehydration under the illumination of visible light. The pre-edge peak due to electronictransition K→ M4,5 was enhanced and the chemical shifts weregreater compared V Kα1 emission. The initial state was V(IV)and ethanol dissociatively adsorbed to partially reduce the V(IV) site. When visible light was illuminated, electron should beexcited and distributed to the V(IV) sites associated withethoxyl group. However, fully reduced V(III) state was notdetected probably because the reaction equilibrium due toelectron excitation/distribution process was fast and the lifetimeof V(III) state was short. The partially reduced V(IV-δ) sites
oxidized back to V(IV) after the visible arc lamp was turned off.
Acknowledgements
X-ray measurements were performed under the approvalof the Photon Factory Proposal Committee (2005P014,2006G097) and technically supported by Professors M. Nomuraand Y. Inada. This work was financially supported from theGrant-in-aid on the Priority Area “Molecular Nano-dynamics”(Y.I. 432-17034013).

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