2008 The Chemical Society of Japan

Bull. Chem. Soc. Jpn. Vol. 81, No. 10, 1241–1249 (2008)

1241

Speciﬁc Oxidative Dehydrogenation Reaction Mechanism
over Vanadium(IV/III) Sites in TiO2 with
Uniform Mesopores under Visible Light1
Yasuo Izumi,1 Kazushi Konishi,1;2 and Hideaki Yoshitake3
1

Department of Chemistry, Graduate School of Science, Chiba University,
1-33 Yayoi, Inage-ku, Chiba 263-8522
2Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8502
3

Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501
Received March 10, 2008; E-mail: yizumi@faculty.chiba-u.jp

Selective oxidation under visible light is attractive for application in environment-benign societies. Oxidative
dehydrogenation of ethanol proceeds over mesoporous V-doped TiO2 under visible light in clear contrast to ethanol
dehydrogenation over crystalline TiO2 doped or undoped with V and inactive mesoporous TiO2 . The red-ox states of
VIV and VIII were detected using X-ray spectroscopies. In this paper, kinetic measurements of each catalytic step and
X-ray spectroscopic monitoring under catalytic reaction conditions are correlated. The ethanol conversion to acetaldehyde in the absence of O2 was monitored under visible light coupled with reduction of VIV to VIII species based on
V K5;2 emission and V K5;2 -selecting X-ray absorption ﬁne structure spectra. The oxidative dehydrogenation ofdissociatively adsorbed ethoxyl species proceeded in O2 under visible light with formation rates 38% of steadyphoto-catalysis in ethanol + O2 . The acetaldehyde desorption step by H subtraction from ethoxyl species on VIII
was found to be rate-limiting. The origin of oxidative dehydrogenation over mesoporous V–TiO2 was suggested tobe coordinatively unsaturated VIII species speciﬁcally activating O2 molecules. Further, a poisoning eﬀect of productwater was demonstrated to block the active V sites. UV light illumination was found to be eﬀective to re-activate
the mesoporous V–TiO2 catalyst.

Semiconductors are used for the oxidative decomposition ofvolatile organic compounds (VOC) to protect/improve theatmospheric environment and for decomposition of dyes andindustrial organic compounds to purify water environments.2,3
Titanium dioxide is a representative semiconductor photo-catalyst.4 To reduce the band gap (3.2 and 3.0 eV for anatase- andrutile-type TiO2 crystallines, respectively) to the energy regionof visible light and thus enable catalysis activated under visible
light, new preparation routes and morphologies of TiO2 havebeen intensively reported.5 Various hetero atoms, e.g. C,5N,6–8 F,9 Al,10 P, S,11 V,12,13 Cr,12 Mn,12 Fe,12,14 Ni,12 andI,15,16 have been doped to anatase- and/or rutile-type TiO2
crystallines to create impurity levels between the band gap.
Mesoporous frameworks of TiO2 , both unadulterated and those
doped with N,7 F,9 or V13 have been less extensively tested
for photo-catalysis. Mesoporous TiO2 prepared both withand without organic templates4,17 generally consists of smaller
particles than those for crystallines, e.g. Degussa P25 (particle
size 15–40 nm) and is aﬀected by quantum size eﬀects towiden the band gap.12,13
Oxidative detoxiﬁcation of VOC is practical if it proceeds
under activation by visible light. In this paper, ethanol waschosen as a representative VOC and tested for excitation undervisible light. Selective, oxidative dehydrogenation of ethanol

was reported over mesoporous V–TiO2 of narrow pore size
distribution centered at 3 nm.13 The speciﬁc surface area(SA) was 1000 m2 g1 and it catalytically produced acetaldehyde and water. Mesoporous TiO2 with a speciﬁc SA of
1200 m2 g1 was found to be essentially inactive. The dehydrogenation reaction proceeded to form acetaldehyde only
over V-doped or undoped crystalline TiO2 (P-25) under visible
light. The mesoporous V–TiO2 is advantageous for X-ray
spectroscopic study because the VIV sites substitute on theTi sites of TiO2 matrix and distribute homogeneously. When
the V/Ti atomic ratio was 1/21, only VIV species coordinated
to 4–5 oxygen atoms was detected.18 The V-doped mesoporous
TiO2 is expected to perform selective O2 molecule activation.
V K5;2 -selecting V K-edge X-ray absorption ﬁne structure(XAFS) detected VIV and VIII states independently under catalytic reaction conditions.19 The V K5;2 ﬂuorescence emitted
from the catalysts was analyzed using high energy-resolution
ﬂuorescence spectrometry and structural and electronic information of the V sites were given state-selectively. On-reaction
monitoring of valence and structure changes for active V sites
had not been performed yet.
In this paper, kinetic measurements for each reaction stepand on-reaction X-ray spectroscopic monitoring were correlated to clarify the reaction mechanism of ethanol oxidative

Published on the web October 10, 2008; doi:10.1246/bcsj.81.1241

1242 Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

Scheme 1. Photo-catalytic reaction cell and visible lightillumination.

dehydrogenation over mesoporous V–TiO2 under visible light.
The V states were monitored by on-reaction V K5;2 emission
and V K5;2 -selecting V K-edge XAFS spectroscopies. In addition, the poisoning eﬀect of product water was kineticallyconﬁrmed and structurally monitored by the VIII /VIV -state
selective XAFS spectroscopy. Then, repeated catalysis tests
were performed over mesoporous V–TiO2 by illuminating
UV–visible light between reaction batches.
Experimental
Sample Preparation.
Vanadium metal (25 mm thickness,
99.7%; Aldrich) and V2 O3 (99.99%, Aldrich) were used asreceived. The V2 O3 powder was thoroughly mixed with boron
nitride (Wako) under argon to 3.0 wt % of V and pressed into a
20 mm-diameter disk.
The preparation route of mesoporous V–TiO2 catalysts wasfollowed from literature.20 In brief, water was added dropwise
to a mixture of vanadium triisopropoxide oxide, titanium tetraisopropoxide, and dodecylamine at 273 K. 0.1 M HCl was added. The
suspension was maintained at 333 K for 4 days. Then, the solution
was ﬁltered and washed with methanol and diethyl ether. The
obtained powder was heated under vacuum at 453 K for 2 h andsealed. The sample was kept at 453 K for 10 days and then washedwith p-toluenesulfonic acid in ethanol. The V content was ﬁxed to
3.0 wt %, corresponding to a V/Ti atomic ratio of 1/21.
Kinetic Tests. A hundred milligrams of catalyst was evacuated in a branched, ﬂat pillbox shaped quartz reaction cell (bottom
area 23.8 cm2 ; Scheme 1) at 106 Pa for 2 h at 290 K. Ethanoloxidation was performed in a closed circulating system (internal
volume 100 mL) connected to the reaction cell and illuminatedusing a xenon short arc lamp operated at 400 W (UXL-500D,
Ushio; Scheme 1). A UV-cut ﬁlter L42 (>420 nm; Kenko) was
set at the light exit, 2 mm from the bottom of quartz reaction cell.
UV-cut ﬁlters (Kenko) L-37 (>370 nm), Y-48 (>480 nm), Y-52(>520 nm), and O-58 (>580 nm) were also used to observe thecatalytic reactivity dependence on wavenumber. The products
and reactants were analyzed using an online gas chromatographequipped with a thermal conductivity detector (Shimadzu GC8AT) and Porapak-Q column (GL Sciences). The change of oxygen gas pressure was monitored using a capacitance manometer(CCMT-1000A, ULVAC).
1.33 kPa of ethanol and 2.67 kPa of oxygen were introduced to
the reaction system. Control photo-catalytic kinetic measurements

Photooxidation Mechanism on Mesoporous V–TiO2
were performed under 1.33 kPa of ethanol over fresh mesoporous
V–TiO2 or under 1.33 or 2.67 kPa of oxygen over mesoporous
V–TiO2 , ethanol-pre-adsorbed. The pre-adsorption was performed
under 1.50 kPa of ethanol for 15 min followed by evacuated at
106 Pa for 15 min at 290 K. Separately, 1.73 kPa of water was
introduced to the catalyst for 15 min, evacuated at 106 Pa for
15 min, and then 1.33 kPa of ethanol and 2.67 kPa of O2 wereintroduced for kinetic tests all at 290 K.
Reactivation of ethanol oxidation catalyst was tested by repeating reaction batches for 5 h. Between each reaction batch, mesoporous V–TiO2 was evacuated at 106 Pa under the illuminationof a xenon short arc lamp without a UV-cut ﬁlter.
X-ray Measurements. The X-ray measurements were performed in the Photon Factory at the High-Energy Accelerator
Research Organization (Tsukuba) on a bending-magnet beamline
7C at 290 K. The storage-ring energy was 2.5 GeV and the ring
current was between 449 and 238 mA. A Si(111) double crystal
monochromator and a higher-harmonics-rejection mirror of
fused silica were inserted into the X-ray beam path. The X-ray
beam was focused at the sample position, 490 mm from the
X-ray beam exit of the vacuum beam duct, using the Sagittalfocusing mechanism and fully tuned using a Piezo translator.
The X-ray ﬂuorescence emitted from samples was analyzed
with a homemade ﬂuorescence spectrometer.18,19,21,22 A slit 0
was set in front of an ion chamber (S1194-B1, Oken) for the I0
signal count. The ion chamber was purged with a mixture ofhelium and nitrogen (7:3). The sample was set on a plane tilted
from the horizontal plane toward the incident X-ray beam by
6.0 and toward a spherically bent Johann-type Ge(422) analyzer
crystal (Saint Gobain) by 7.0 . A slit 1 was set at 100 mm from thesample, between the sample and the Ge analyzer crystal. A slit 2
was set in front of the NaI(Tl) scintillation counter (SP10, Oken)
for the If signal count. The slit opening size was 1:0  1:0 mm2
for slit 0 and 8:0  8:0 mm2 for slit 1 and slit 2. The sample, slit
2, and Ge analyzer crystal were vertically set in the Rowland
conﬁguration. The crystal’s curvature radius was 450 mm. The
spectrometer was purged with helium. The scintillation counter
was purged with nitrogen gas to avoid discharging of the preampliﬁer. The sample section was covered with a 1 mm-thick leadplate housing, except for the openings for the incident X-ray beamand the exit of X-ray ﬂuorescence from the sample to suppress the
background of the If signal.
A hundred milligrams of mesoporous V–TiO2 powder werepressed into a 20 mm-diameter disk. Each sample disk was set
in an on-reaction cell equipped with 16 mm-thick polyethylene
naphthalate windows (Q51-16, Teijin) and evacuated at 106 Pa
for 3 h. Then, 77 kPa of argon or 0.37 kPa of water was introduced.
Separately, the catalyst was in 4.3 kPa (constant) of ethanol bymaking a loop between the on-reaction cell and liquid ethanol
storage (0.5 mL). The gas in the loop was mechanically circulated
at a rate of 1 mLgas min1 . The catalyst in constant ﬂow of ethanol
gas was illuminated 10 mm from the exit window of a Xe short arc
lamp equipped with a L42 ﬁlter.
The V K5;2 emission spectra (X-ray ﬂuorescence, XRF)
were measured for V metal and mesoporous V–TiO2 by scanning
the ﬂuorescence spectrometer with the excitation energy ﬁxed at5483.6 eV. The emission energy for the V metal was calibrated
to 5462.9 eV (Bragg angle B ¼ 79:335 ).23,24 The scan step ofthe emission energy was 0:36 eV and the accumulation timewas 60–90 s per point. The count rates of emitted photons
measured by scintillation counting were 102 –101 and 101 –100
counts s1 for V metal and mesoporous V–TiO2 , respectively.

Y. Izumi et al.

Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

1243

Figure 1. Time course of ethanol oxidation/decomposition reactions over mesoporous V–TiO2 (3.0 wt % V). Visible light(>420 nm) was illuminated at 290 K. Initial concentrations were ethanol (1.33 kPa) + O2 (2.67 kPa) (A and D), ethanol(1.33 kPa) (B), and O2 (2.67 kPa) (C) in a closed circulating system (100 mL). Fresh catalyst (A and B) and catalyst in ethanol(1.50 kPa) (C) or water (1.73 kPa) (D) for 15 min and in vacuum for 15 min (C and D) were used (100 mg). The O2 pressure change
was monitored starting from initial 1.33 (dotted line) or 2.67 kPa of O2 (solid line) and a liquid nitrogen trap was inserted in thereaction loop during catalytic test (C).
The V K5;2 -selecting V K-edge XAFS spectrum was measured
by tuning the ﬂuorescence spectrometer to ﬁxed emission energies
around the V K5;2 emission peak. The scan step of photon energy
was 0:25 eV and the accumulation time was 60–200 s per point.
The V K-edge energy for V metal was calibrated to 5463.9 eV.23,24
The energy positions of the monochromator and the ﬂuorescence
spectrometer were reproduced within the errors of 0:1 and0:2 eV, respectively. Energy resolution of the ﬂuorescence spectrometer was evaluated to be 1.1 eV at 5.5 keV.
Analysis. The XAFS data was analyzed with the software
package XDAP version 2.2.7 (XAFS Services International) basedon the works of M. Vaarkamp, H. Linders, and D. Koningsberger.
The pre-edge background was approximated by the modiﬁed
Victoreen function C2 =E2 þ C1 =E þ C0 . The background of
post-edge oscillations was approximated by a smoothing-spline
function, calculated by the equation for the number of data
points N.
NXðxi  BGi Þ2
i¼1

expð0:075ki 2 Þ smoothing factor

ð1Þ

The similarity of XANES spectra (spectral ﬁt) was evaluatedbased on the R-factor.
Z
Rf ¼ jobserved data ðkÞ  reference data ðkÞj2 dkð2Þ.Zjobserved data ðkÞj2 dk

Results
Photo-Kinetics over Mesoporous V–TiO2 .
The timecourse of photo-oxidation of ethanol over mesoporous V–TiO2
catalyst under visible light (>420 nm) is shown in Figure 1A.
The oxidative dehydrogenation proceeded over mesoporous
V–TiO2 to produce acetaldehyde and water. Initially water
formation was quicker however after one hour acetaldehyde
and water formation reached a steady state (Table 1, Entry
A). The molar quantity of water formed in the ﬁrst 1 h corresponded to 36% of the molar amount of V in the mesoporous
V–TiO2 . Because the sample was evacuated for 2 h before thephoto-catalysis and water formation in Figure 1A was repeated
when the photo-catalysis batches were repeated, the water
formation (initial, steady) was catalytic.
Comparable photo-catalytic tests in ethanol were performedin the absence of O2 under visible light. Ethanol dehydrogenation proceeded to form acetaldehyde as a single product(Figure 1B). The formation rate was 10% of the oxidative
dehydrogenation rate in ethanol + O2 (Table 1, Entries A
and B). This control reaction may consist of the ﬁrst two stepsof oxidative hydrogenation: dissociative adsorption of ethanol
and H atom subtraction from the adsorbed ethoxyl species to
evolve acetaldehyde.
Another control reaction was performed in oxygen (initialpressure 2.67 kPa) over ethanol pre-adsorbed mesoporous1244 Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

Photooxidation Mechanism on Mesoporous V–TiO2Table 1. Product Formation and Decomposition Rates in the Ethanol Photo-oxidation/Photo-decomposition over Mesoporous V–TiO2
CatalystaÞ under Visible Light (>420 nm)
Entry
A

Catalyst
Fresh

B
C
C0
D

Ethanol adsorbedeÞ
Water adsorbedf ÞReaction gas
Ethanol (1.33 kPa)
+ O2 (2.67 kPa)
Ethanol (1.33 kPa)
O2 (2.67 kPa)O2 (1.33 kPa)
Ethanol (1.33 kPa)
+ O2 (2.67 kPa)

Decomposition rates/mmol h1 gcat 1 bÞ
O2
—
—
15
13
—

Formation rates/mmol h1 gcat 1 cÞ
CH3 CHOH2 O
CO223212 (16)dÞ
0.3
2.4
8.8
7.4
12

0
6.1
4.1
0

0
0
0
0

a) 3.0 wt % of V. b) Monitored using capacitance manometer. c) Monitored using gas chromatograph. d) Steady-state rate after 1 hof reaction. e) 1.50 kPa of ethanol was introduced for 15 min and catalyst placed under reduced pressure of 106 Pa for 15 min.
f) 1.73 kPa of water was introduced for 15 min and catalyst placed under reduced pressure of 106 Pa for 15 min.

Figure 2. Repeated reaction tests of ethanol oxidation under visible light over mesoporous V–TiO2 (3.0 wt % V). The reaction
conditions were similar to those for Figure 1A. A liquid nitrogen trap was inserted in the reaction loop during catalysis. Aftereach reaction batch, the gas phase was evacuated and the catalyst was illuminated with UV–visible light from a xenon arc lamp
operated at 400 W.

V–TiO2 (Figure 1C). The steady formation rates of acetaldehyde and water were both 38% of the corresponding rates overfresh catalyst in ethanol + O2 (Table 1, Entries A and C). The
rate of reverse reaction to form reactant ethanol was faster:
51 mmol h1 gcatalyst 1 . The O2 decomposition rates did notcorrespond to formation rates of acetaldehyde and water
because reverse reaction from ethoxyl adsorbed to ethanol proceeded and O2 molecules may be consumed in other reaction(s).
The reaction for ethanol pre-adsorbed mesoporous V–TiO2
was also performed in 1.33 kPa of O2 (initial pressure). The
consumption of oxygen is plotted in Figure 1C (dotted line)
and consumption/formation rates are summarized in Table 1,
Entry C0 . The rates were 67–84% of the corresponding rates
starting from 2.67 kPa of O2 (Table 1, Entry C). Therefore,
this reaction rate dependence on the O2 pressure was less thanone. The oxidation of ethoxyl species consists of more than
two steps. The ﬁrst step is H atom subtraction from the adsorbed ethoxyl species to evolve acetaldehyde and the next steps
are the reaction of gaseous O2 with remaining H atoms overthe catalyst and the dissociation of O–O bonds.
Next, photo-oxidation tests were performed in ethanol(1.33 kPa) + O2 (2.67 kPa) over water pre-adsorbed mesoporous V–TiO2 (Figure 1D). The rate of O2 consumption was
slower than that observed with fresh catalyst (panel A). The

product was acetaldehyde only and the formation rate was
52% of the steady formation rate over untreated mesoporous
V–TiO2 (Table 1, Entries A and D). Thus, eﬀective V sites
for the O2 adsorption/activation were partially blocked withwater molecules.
Finally, photo-catalytic batch reactions were repeated(Figure 2). Between each batch, mesoporous V–TiO2 catalyst
was placed under vacuum (106 Pa) under UV–visible light.
In the time course proﬁle for the second and third batches,water was produced quicker in the ﬁrst 1 h and then the formation rates of acetaldehyde and water reached a steady state,similar to the proﬁle for the ﬁrst batch. The formation ratesof acetaldehyde and water (initial, steady) were 108, 52, and23%, respectively, in the second batch and 93, 90, and 31%,respectively, in the third batch compared to corresponding
rates in the ﬁrst batch.
V K5;2 Emission Spectra. The V K5;2 emission spectra
were measured for mesoporous V–TiO2 (Figure 3). A major
peak for the fresh sample at 5462.1 eV (spectrum-a) shiftedby 0:8 eV in 0.37 kPa of water (spectrum-b). A similar peakshift to lower energy (0:5 eV) was reported for mesoporous
V–TiO2 in 2.1 kPa of ethanol (Table 2A).19
For the mesoporous V–TiO2 in 4.3 kPa of ethanol (constant)under visible light, the time course change of V K5;2 emission
spectra was monitored (Figures 3c–3f). Compared to the major

Y. Izumi et al.

Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

peak energy before visible light illumination (5461.6 eV19),
the peak energy shifted by 0.6 eV toward lower energy for
1.2–11.9 h (spectra c–e) and then further by 0.8 eV toward
lower energy for 22.4 h (spectrum-f; Table 2A). The shoulder
on the lower energy side was not as well resolved for mesoporous V–TiO2 as for fresh catalyst (spectrum-a) or in thepresence of 0.37 kPa of water (spectrum-b). The shoulder peak
(5457.8–5458.2 eV) was resolved from the major peak andthe peak intensity progressively grew over 1.2–22.4 h (spectrac–f).
The K5;2 emission spectra a and f were deconvoluted witha minimum number of Lorentz functions. The spectrum-a for
fresh sample was ﬁtted with two Lorentz functions (Figure 3a,
dotted lines) centered at 5462.1 (main) and 5458.6 eV (side)(Table 2B). Spectrum-f required four Lorentz functions forﬁtting (Figure 3f, dotted lines). The four components may beconsidered to be main peaks at 5461.2 and 5459.5 eV accompanied with side peaks at 5457.4 and 5455.7 eV, respectively(Table 2B).
V K5;2 -Selecting V K-Edge XANES Tuned to V K5;2
Main Peak. V K5;2 -selecting XANES (X-ray absorption
near-edge structure) spectra were measured for the mesoporous V–TiO2 . The tune energy was ﬁrst ﬁxed to the main peaks(short/red arrows in Figure 3). First, the V K5;2 -selecting
XANES spectrum for fresh sample was compared to that measured in transmission mode (Figures 4a and 4b). The enhanced
peak feature at 5486 and 5504 eV for spectrum-a was due to aresonance excitation eﬀect reported for Fe K1;3 -selecting,25
Cu K1 -selecting,21 Sn K1 -selecting,21 and Pb L1 -selecting
XANES spectra.21

Figure 3. V K5;2 emission spectra for mesoporous V–
TiO2 as (a) untreated in argon, (b) in 0.37 kPa of water,and (c–f) in 4.3 kPa (constant) of ethanol under visible
light (>420 nm) for 1.2 (c), 3.3 (d), 11.9 (e), and 22.4 h(f). The excitation energy was set to 5483.6 eV. Spectrum
deconvolutions are depicted as dotted lines (each peakcomponent and the sum) with Lorentz functions forspectra a and f. The longer/black and shorter/red arrows
indicate peak top and tune energy (denoted as ‘‘TE’’) forthe measurements in Figure 4, respectively.

Table 2. (A) Energy Values for Vanadium K5;2 Emission Peaks, Pre-Edge Peaks, and Vanadium K Absorption
Edges and (B) Peak Deconvolution Results to Vanadium K5;2 Emission for Mesoporous V–TiO2 CatalystaÞ
(A)
Sample, conditions
mesoporous V–TiO2
Incipient (fresh)
In 0.37 kPa of waterK5;2 emission
5462.15461.3

Energy/eVPre-edge peak5468.85468.5

K absorption
5479.75479.2

Refs.
This work
This work5468.05479.35468.05468.45468.15479.05477.75476.919
This work
This work
This work
This work

In 4.3 kPa of ethanol under visible light0h5461.6
1.2 h5461.0, 5457.8(w)3.3 hbÞ
5461.0, 5458.0(w)5461.0, 5458.2(m)11.9 hcÞ
22.4 hdÞ
5460.2, 5457.8(m)(B)
In 4.3 kPa of ethanol under visible light0h
22.4 hIn 4.3 kPa of ethanol under visible light
22.4 h1245

Side-25455.7; 0.98

Peak energy/eV; Area/105 eVeÞSide-1Main-25458.6; 1.1
5457.4; 1.1
5459.5; 3.2

Main-15462.1; 5.0
5461.2; 2.6

Peak height at 5458.0 eV (tune energy for Figure 4f)/105Side-2Side-1Main-2Main-1
0.11
0.33
0.50
0.19

a) 3.0 wt % of V. b) At 7.0 h for X-ray absorption measurements. c) At 17.3 h for X-ray absorption measurements.
d) At 27.7 h for X-ray absorption measurements. e) Approximated to (peak height)  (full width at the half
maximum).

1246 Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

Photooxidation Mechanism on Mesoporous V–TiO2

ly shifted by 0.3 eV toward lower energy over 7 h (spectrumd), and further by 0.3 eV by 17.3 h (spectrum-e). The reaction
time is the average for each spectrum measurement because
it took several hours to measure one V K5;2 -selecting
XANES spectrum. The pre-edge peak intensity appearing at
5468.0–5468.8 eV gradually became weaker as the photoreaction proceeded (Figures 4a, 4d, and 4e).
V K5;2 -Selecting V K-Edge XANES Tuned to V K5;2
Side Peak. The V K-edge XANES spectrum was measured
for mesoporous V–TiO2 under visible light tuning the spectrometer to 5458.0 eV at the end of photo-catalytic monitoringin ethanol (27.7 h). A lower energy shoulder peak was chosen
as tune energy (Figure 3f, short/red arrow) to extract photoreduced V species selectively.
The obtained vanadium K-edge XANES spectrum (Figure4f) was compared to the weighted sum of spectrum-a measured
for fresh mesoporous VIV –TiO2 and spectrum-g measured
for V2 O3 crystalline (VIII sites). The best ﬁt was with a mixing
ratio of 25:75 (Rf ¼ 0:54%; Figure 4, inset).
Discussion

Figure 4. V K5;2 -selecting V K-edge XANES spectra formesoporous V–TiO2 as (a, solid line) untreated, kept under argon, (c) in 0.37 kPa of water, and (d–f) in 4.3 kPa(constant) of ethanol under visible light (>420 nm) for
7.0 (d), 17.3 (e), and 27.7 h (f) and for V2 O3 diluted withboron nitride (3.0 wt % V; g). The tune energy values were5462.4, 5461.7, 5460.9, 5460.9, 5458.0, and 5458.7 eV, respectively. Spectrum-b (dotted line) is the corresponding
data for spectrum-a, measured in transmission mode.

With the introduction of 0.37 kPa of water, the vanadium K
absorption edge shifted by 0.5 eV toward lower energy(Figure 4c), consistent with the negative energy shift of the
V K5;2 main peak (Figures 3a and 3b and Table 2A). The
spectral pattern in which a dominant peak appeared at
5487 eV resembled that for V K1 -selecting XAFS measured
for impregnated V/TiO2 (Degussa P25; anatase:rutile = 7:3)
in 0.85 kPa of water at 290 K.18 The growth of a post-edge
peak at 5487 eV was also reported for hydrated V species supported on SiO2 and Al2 O3 .26 However, absorption edge energy
shifted toward higher energy (+0.4 eV) with water adsorption
on V/TiO2 (crystalline). This diﬀerence may be due to thestructural diﬀerence of V sites. The polymerization suggested
to occur for surface V species on V/TiO2 (crystalline)18 should
not take place for V sites substituting for Ti atoms in the mesoporous TiO2 matrix used in this work.18,19
Next, time course changes of V K5;2 -selecting XAFS weremonitored for fresh mesoporous V–TiO2 in 4.3 kPa (constant)of ethanol under visible light. The vanadium K absorption
edge energy for one ethanol-adsorbed (Table 2A) progressive-

Kinetics vis-a-vis X-ray Spectroscopies. Under visible
light, in clear contrast to inactive mesoporous TiO2 (uniformly
distributed pores centered at 3 nm27), mesoporous V-doped
TiO2 speciﬁcally catalyzed oxidative dehydrogenation of
ethanol (Figure 1A). A similar doping eﬀect of VIV in sol–gel synthesized TiO2 has also been reported for the decomposition of crystal violet and methylene blue under visible light.28
The reaction was found to require molecular oxygen because
the ethanol decomposition rate to acetaldehyde in the absenceof O2 on mesoporous V–TiO2 was only 10% of that inethanol + O2 (Table 1, Entries A and B).
Because the energy shifts of the V K5;2 emission peak andV K-edge absorption edge were signiﬁcant (0:5 and 1:7 eV,
respectively) upon ethanol adsorption for incipient mesoporous V–TiO2 ,19 ethoxyl groups should be the major surface
species bound to V. The V K5;2 emission main peak for catalyst ethanol-adsorbed17 shifted by 0:6 eV under visible lightover 1.2–11.9 h (Figures 3c–3e and Table 2). The emission
spectrum in ethanol under visible light for 22.4 h required aminimum of four Lorentz functions for ﬁtting (Figure 3f andTable 2B) compared to two functions for the spectrum of untreated material (Figure 3a). The new main peak at 5459.5 eV
was in the energy range of VIII species (5458.8–5460.5 eV),separated by 1.7 eV from another main peak assignable to
CH3 CH2 O–VIV species (Figure 5b). The methanol dehydrogenation mechanism was theoretically studied for V/TiO2
catalyst using density functional theory calculations.29 Dissociative adsorption of methoxy intermediate followed by H
atom subtraction from the substrate to surface V=O species
was suggested as the mechanism. In the case of VIV substituted
sites in the TiO2 matrix in this study,18,19 the presence of V=O
double bonds is unlikely. Based on kinetic measurements inethanol (Figure 1B), the reduced species should be H–VIII asa result of H subtraction from ethoxyl followed by desorptionof acetaldehyde (Figures 5b and 5c). The two side peaks
at 5455.7 and 5457.4 eV are associated with main peaks at5459.5 and 5461.2 eV, respectively, separated by 3.8 eV fromthe main peaks.

Y. Izumi et al.

Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

1247

Figure 5. Proposed reaction mechanism of ethanol oxidative dehydrogenation based on several kinetic studies, XRF, and
VIII /VIV -state selective XAFS.

Based on the main XRF peak area (Table 2B), 55% of
VIV sites were reduced to VIII (Figure 5c) under visible lightover 22.4 h. This ratio corresponds to a reduction rate of
14 mmol-V h1 gcatalyst 1 from VIV to VIII , 5.8 times greaterthan acetaldehyde formation in ethanol under visible light(Table 1B). One of the possibilities of this discrepancy is
re-adsorption of product acetaldehyde on the catalyst surface.
Another possibility is that polymerization took place and the
product was not detected by GC. However, the reactivity ofphoto-catalysis was repeated fairly well as shown in Figure 2.
Thus, the polymerization should be a minor path if it proceeds.
Active VIV species cannot be recreated by oxidation in ethanol
gas only. The estimated reduction rate from VIV to VIII was
comparable to the steady acetaldehyde synthesis rate (23mmol h1 gcatalyst 1 ) in ethanol + O2 measured for 5 h (Table
1A) taking the gradual catalytic deactivation (Figure 1A) for
22.4 h into account. The acetaldehyde desorption step was
found to be rate-limiting (Figures 5b and 5c) in the oxidative
dehydrogenation of ethanol followed by quicker photo-oxidation steps.
It is evident that the rate-limiting step requires light because
the photo-catalytic activity gradually decreased when the UVcut was progressively changed from 370 to 580 nm (Figure 6).
The red-ox potential of methanol reduction to formaldehyde is

Figure 6. The acetaldehyde formation rate dependence on
UV cut-oﬀ wavelength. The reaction conditions weresame as in Figure 1A. UV-cut ﬁlters L37, L42, Y48,
Y52, and O58 were used at the exit of the xenon arc lamp.

+0.04 V. The conduction and valence band levels for mesoporous V–TiO2 should be smaller/greater than 0:52 and+2.53 V, respectively,3 for rutile-type TiO2 because of theband gap growth due to quantum size eﬀect.12 The band gap

1248 Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

Photooxidation Mechanism on Mesoporous V–TiO2

for mesoporous V–TiO2 was estimated to be 2.67 eV.13 Thus,
the impurity energy level for V substitution at the matrix
Ti sites is 0:14 eV or greater. The red-ox potential for ethanol
reduction to acetaldehyde may be greater than the V impurity
energy level and the reaction mechanism illustrated in
Figure 5 was enabled.
The V sites were also monitored using state-selective
V K-edge XAFS under several catalytic conditions (Figure 4).
The tune energies between 5462.4 and 5460.9 eV for Figures
4a, 4c, 4d, and 4e were in the energy region for standard
inorganic VIV compounds.19 The main peak at 5462.1 eV
accompanied a lower energy side peak, separated by 3.5 eV
for untreated mesoporous V–TiO2 (Table 2B). The chemical
shift on going from VIV to VIII states is also toward lower
energy, e.g. 3:2 eV on going from VIV O(SO4 ) nH2 O to
VIII 2 O3 .19 Thus, the tune energies between 5462.4 and5460.9 eV include smaller contribution from VIII sites andthe VIV states in the samples were preferably monitored inthe V K5;2 -selecting XAFS measurements (Figures 4a, 4c,
4d, and 4e) compared to conventional XAFS data (Figure 4b).
The V K absorption edge shifted toward lower energy inethanol under visible light over 0–17.3 h (0:4– 2:0 eV,
Table 2A) within the energy range for VIV (5476.9–5479.8
eV). Progressive photo-reduction from VIV to VIII was suggested (Figure 5c). VIII sites should co-exist in samples forspectra d and e, but due to VIV -selective measurements tuned
to 5460.9 eV, the rising edge positions were in the energy
range for VIV . Decrease of pre-edge peak intensity appeared
at 5468.0–5468.8 eV on going from Figure 4a (untreated)to d (7.0 h) and then to e (17.3 h) suggesting an increase of
V site symmetry due to the coordination of intermediate
species (Figures 5b and 5c).
The selection of VIII was tried in V K5;2 -selecting XANES
tuned to 5458.0 eV in ethanol under visible light for 27.7 h(Figure 4f). Deconvoluted side peak-1 was at 5457.4 eV(Table 2B) near the tune energy. Based on the V K5;2 emission peak intensity of main peak-1 (VIV ) and main peak-2(VIII ), the population of VIII sites was evaluated to be 55% under those conditions. If we assume that the side peaks at
5457.4 and 5455.7 eV were accompanied by main peaks-1
and -2, respectively, the sum of contributions of main peak-2
and side peak-2 (VIII state) was 54% at 5458.0 eV (Table 2B).
Thus, unfortunately site selection was not eﬀective in the
XANES measurement tuned to 5458.0 eV.
The selection of VIII in the XANES spectrum-f (Figure 4)
was evaluated to 75% (Figure 4, inset) based on comparison
to the weighted sum of V K5;2 -selecting XANES data for untreated VIV (Figure 4a) and reference VIII state (g). The higher
VIII site ratio in XANES (75%) compared to 55% in XRF may
be a result of the diﬀerence in photo-reaction time (27.7 and22.4 h, respectively). Thus, the VIII site structure in the catalyst
was similar to that in V2 O3 except for the diﬀerence of thecoordination number of V–O (4–5 vs. 6, respectively).
The oxidative dehydrogenation also proceeded for ethanol
pre-adsorbed mesoporous V–TiO2 (Figure 1C). This is the reaction of dissociatively adsorbed ethoxyl (Figure 5b) with gaseous O2 . The formation rates of acetaldehyde and water wereboth 38% of the corresponding rates in ethanol + O2 (Table 1,
Entries A and C). The consumption/formation rate depen-dence on O2 gas pressure was less than one order, probably
a half order. The reverse reaction rate to form ethanol was
220% of the acetaldehyde formation rate in ethanol + O2 . A
step of H subtraction from ethoxyl species (Figures 5b and5c) and subsequent O2 adsorption (d) were suggested for thiskinetic measurement. In the steps from states d to e, reactive
OH radical species may be also formed with holes.17,30,31
A proposed reaction cycle of ethanol oxidative dehydrogenation is illustrated in Figure 5. For mesoporous V–TiO2 inethanol gas only (Figure 1B for kinetics, Figures 3a and 3c–
3f for XRF, and Figures 4a and 4d–4f for XANES), transformation from Figure 5a to 5b and then 5c was monitored. In
the oxidation of ethanol pre-adsorbed catalyst (Figure 1C),transformation from Figure 5b to 5c, 5d, 5e, and thenﬁnally to 5a should take place, but the reverse reaction from
Figure 5b to 5a was also observed in the absence of gas-phaseethanol. Because the formation of water is typical for mesoporous V–TiO2 and it is not formed on crystalline TiO2 either
doped or not doped with vanadium,13 the structures d and eshould be speciﬁc for mesoporous V–TiO2 for selective activation of O2 .
Amorphous TiO2 and/or VIV sites substitution for the Ti
atoms generally leads to electron–hole recombination.2,17,30
Improved oxidative dehydrogenation activity found formesoporous V–TiO2 (Table 1, Entry A) may be due to unsaturated coordination of V and extremely high speciﬁc SA(1000 m2 g1 ).
Poisoning with Water and Re-activation in UV Light. In
the ethanol oxidation over mesoporous V–TiO2 , quicker water
formation was observed in the ﬁrst 1 h, and then water andacetaldehyde formation reached a steady state (Figure 1A).
After the catalyst was in 1.73 kPa of water, water was notproduced in ethanol + O2 under visible light (Figure 1B)
and the formation rate of acetaldehyde was 52% of that overuntreated mesoporous V–TiO2 (Table 1, Entries A and D).
Partial reduction of VIV sites was suggested based on
the peak shifts toward lower energy in the V K5;2 emission
spectra (0:8 eV, Figures 3a and 3b) and V K-edge absorption
edge (0:5 eV, Figure 4c) in moisture. The V K5;2 peak energy (5461.3 eV), V K absorption edge energy (5479.2 eV),post-edge intense peak energy (5487 eV), and XANES proﬁlein the range of 5465–5515 eV (Figures 3b and 4c) weremost similar to those for the catalyst in 2.1 kPa of ethanol.19
Therefore, dissociatively adsorbed water blocked active VIV
sites and induced partial negative charge donation to the VIV
sites. The surface agglomeration of VOx species on crystalline
TiO2 18 did not take place.
Based on kinetic and X-ray spectroscopic implication,
mesoporous V–TiO2 was treated under UV–visible lightbetween catalytic reaction batches (Figure 2). The initial formation rates of acetaldehyde and water in the second and third
batches were 108–52% of the corresponding rates in the ﬁrst
batch. Time course proﬁles of initial water formation andsteady state formation for both products were repeated. Thus,a major portion of water molecules blocking the V sites wasdesorbed under UV light. The water desorption under UV light
can be understood from the energy diagram.3 The red-ox potential between dissociatively adsorbed two hydroxyl species
and water may be similar to that for H2 /H2 O (0:41 V).

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Bull. Chem. Soc. Jpn. Vol. 81, No. 10 (2008)

The valence band level for mesoporous V–TiO2 should begreater than +2.53 V3 for rutile-type TiO2 due to quantum size
eﬀects.12 Thus, UV light shorter than 420 nm was required
for water desorption.
Conclusion
The kinetic measurements and on-reaction X-ray spectroscopic monitoring for speciﬁc oxidative dehydrogenation of
ethanol over mesoporous V/TiO2 under visible light werecorrelated. Because the transformation rate from VIV to VIIIin the catalyst monitored by V K5;2 emission spectra wascomparable to the catalytic acetaldehyde synthesis rate inethanol + O2 , the desorption step of acetaldehyde formed by
H subtraction from ethoxyl species bound to surface V sites
was found to be rate-limiting.
The synthesis rates of acetaldehyde and water in O2 forethanol pre-adsorbed mesoporous V–TiO2 were both 38% of
the corresponding rates in ethanol + O2 , but the reverse
reaction rate to form ethanol was 220% of the acetaldehyde
formation rate in ethanol + O2 . The consumption/synthesis
rate dependence on O2 pressure was less than one. The oxidation steps of H–VIII seem to be more quickly activated as
HOO–VIV and/or OH species under visible light.
Dissociative adsorption of water on V sites was conﬁrmed
based on the V K5;2 emission and V K5;2 -selecting XANES.
Water pre-adsorbed mesoporous V–TiO2 catalyst produced
only acetaldehyde at a rate 52% of that over untreated catalyst.
To overcome the problem of water poisoning, reactivation was
attempted by illuminating the catalyst under UV light between
catalytic batches. The formation rates of acetaldehyde andwater in the second and third batches were 108–52% ofcorresponding rates in the ﬁrst batch. The inconsistency of
the formed amounts of acetaldehyde and water may be
due to re-adsorption and/or polymerization of acetaldehyde.
Re-adsorbed acetaldehyde or polymerized species should bedesorbed during UV light illumination.
This work was ﬁnancially supported from Grant-in-Aid for
Basic Research (Y.I. C-19550134) and Grant-in-Aid in the priority area ‘‘Molecular Nano-Dynamics’’ (Y.I. 432-17034013).
X-ray measurements were performed under the approvalof the Photon Factory Proposal Committee (Nos. 2005P014,
2006G097, and 2007G576).
References
1 Part 28 of state-selective X-ray absorption ﬁne structure
series.
2 M. R. Hoﬀmann, S. T. Martin, W. Choi, D. W.
Bahnemann, Chem. Rev. 1995, 95, 69.
3 A. Fujishima, T. N. Rao, D. A. Tryk, J. Photochem. Photobiol., C 2000, 1, 1.
4 X. Chen, S. S. Mao, Chem. Rev. 2007, 107, 2891.
5 S. K. Mohapatra, M. Misra, V. K. Mahajan, K. S. Raja,

1249

J. Phys. Chem. C 2007, 111, 8677.
6 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga,
Science 2001, 293, 269.
7 A. R. Gandhe, S. P. Naik, J. B. Fernandes, Microporous
Mesoporous Mater. 2005, 87, 103.
8 Y. Cong, J. Zhang, F. Chen, M. Anpo, J. Phys. Chem. C
2007, 111, 6976.
9 J. G. Yu, J. C. Yu, B. Cheng, S. K. Hark, K. Iu, J. Solid
State Chem. 2003, 174, 372.
10 D. Zhao, C. Chen, Y. Wang, W. Ma, J. Zhao, T. Rajh,
L. Zang, Environ. Sci. Technol. 2008, 42, 308.
11 Y. Murakami, B. Kasahara, Y. Nosaka, Chem. Lett. 2007,
36, 330.
12 M. Anpo, Bull. Chem. Soc. Jpn. 2004, 77, 1427.
13 D. Masih, H. Yoshitake, Y. Izumi, Appl. Catal., A 2007,
325, 276.
14 M. S. Nahar, K. Hasegawa, S. Kagaya, S. Kuroda, Bull.
Chem. Soc. Jpn. 2007, 80, 1017.
15 G. Liu, Z. Chen, C. Dong, Y. Zhao, F. Li, G. Q. Lu, H. M.
Cheng, J. Phys. Chem. B 2006, 110, 20823.
16 S. Usseglio, A. Damin, D. Scarano, S. Bordiga, A.
Zecchina, C. Lamberti, J. Am. Chem. Soc. 2007, 129, 2822.
17 Y. Shiraishi, N. Saito, T. Hirai, J. Am. Chem. Soc. 2005,
127, 12820.
18 Y. Izumi, F. Kiyotaki, N. Yagi, A. M. Vlaicu, A. Nisawa,
S. Fukushima, H. Yoshitake, Y. Iwasawa, J. Phys. Chem. B 2005,
109, 14884.
19 Y. Izumi, K. Konishi, D. M. Obaid, T. Miyajima, H.
Yoshitake, Anal. Chem. 2007, 79, 6933.
20 H. Yoshitake, T. Tatsumi, Chem. Mater. 2003, 15, 1695.
21 Y. Izumi, H. Nagamori, F. Kiyotaki, D. Masih, T. Minato,
E. Roisin, J. P. Candy, H. Tanida, T. Uruga, Anal. Chem. 2005,
77, 6969.
22 Y. Izumi, F. Kiyotaki, H. Nagamori, T. Minato, J. Electron. Spectrosc. Relat. Phenom. 2001, 119, 193.
23 J. A. Bearden, Rev. Mod. Phys. 1967, 39, 78.
24 G. Zschornack, Handbook of X-ray Data, Springer, Berlin/
Heidelberg, 2007.
25 W. M. Heijboer, P. Glatzel, K. R. Sawant, R. F. Lobo,
U. Bergmann, R. A. Barrea, D. C. Koningsberger, B. M.
Weckhuysen, F. M. F. de Groot, J. Phys. Chem. B 2004, 108,8876.
26 B. Olthof, A. Khodakov, A. T. Bell, E. Iglesia, J. Phys.
Chem. B 2000, 104, 1516.
27 H. Yoshitake, T. Sugihara, T. Tatsumi, Chem. Mater. 2002,
14, 1023.
28 J. C. S. Wu, C. H. Chen, J. Photochem. Photobiol., A 2004,
163, 509.
29 R. Z. Khaliullin, A. T. Bell, J. Phys. Chem. B 2002, 106,7832.
30 S. T. Martin, C. L. Morrison, M. R. Hoﬀmann, J. Phys.
Chem. 1994, 98, 13695.
31 A. Lu, Y. Li, M. Lv, C. Wang, L. Yang, J. Liu, Y. Wang,
K. H. Wong, P. K. Wong, Sol. Energy Mater. Sol. Cells 2007, 91,
1849.