14884

J. Phys. Chem. B 2005, 109, 14884-14891

X-ray Absorption Fine Structure Combined with X-ray Fluorescence Spectrometry. Part 15.
Monitoring of Vanadium Site Transformations on Titania and in Mesoporous Titania by
Selective Detection of the Vanadium Kr1 Fluorescence
Yasuo Izumi,*,† Fumitaka Kiyotaki,† Nobuhiro Yagi,‡ Aurel-Mihai Vlaicu,‡ Atsushi Nisawa,‡
Sei Fukushima,‡ Hideaki Yoshitake,§ and Yasuhiro Iwasawa⊥
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, Japan, National Institute for Research in Inorganic
Materials, Kouto 1-1-1, Mikazuki-cho, Sayo, Hyogo, 679-5198, Japan, Graduate School of EnVironment andInformation Sciences, Yokohama National UniVersity, Tokiwadai, Hodogaya, Yokohama 240-8501, Japan, andGraduate School of Science, The UniVersity of Tokyo, Bunkyo-ku, Hongo 113-0033, Japan
ReceiVed: April 20, 2005; In Final Form: May 28, 2005

X-ray absorption fine structure combined with X-ray fluorescence spectrometry was applied to various V+TiO2
hybrid samples. Emitted V KR1 fluorescence from the sample was selectively counted by using a highenergy-resolution (0.4 eV) spectrometer equipped with a Ge(331) crystal. Two advantages of this method,
extremely high signal/background ratio and the compatibility of measurements in the atmosphere of reactiongas (in situ study in relation to heterogeneous catalysis), were effective at the V K-edge. Structure transformationof the V sites was spectroscopically followed for the V/TiO2 catalyst. The monooxo tetrahedral vanadate site
was demonstrated to exist at 473 K. It transformed into dispersed species of 5-fold coordination in ambient
air and further into polymeric VOx species in 0.85 kPa of water at 290 K. In the presence of 3.2 kPa of
2-propanol, dissociative adsorption of 2-propanol on the dispersed V species was strongly suggested at290-473 K. In situ structure changes of V sites on TiO2 were reported by means of XAFS for the first time.
The VV sites for the V/TiO2 catalysts were essentially identical with those for V supported on mesoporous(high-surface-area) TiO2 and V-TiO2 sample prepared by the sol-gel method. However, predominant VIV
sites were found for mesoporous V-TiO2. The VIV sites substituted on the Ti sites of TiO2. When the molar
ratio of V/Ti increased from 1/100 to 1/5.0, major octahedral V sites in the TiO2 matrix looked to transform
into tetrahedral ones.

Introduction
We have developed and applied a structural analysis technique, X-ray absorption fine structure (XAFS) combined with
X-ray fluorescence (XRF) spectrometry, to heterogeneous
catalysis and environmental chemistry.1-11 The fluorescence
spectrometer utilized was Rowland-type1 and the energy resolution (∆E) was 1.1 eV at 8 keV including the contribution of
beamline.4 Three major advantages have been demonstrated.
As adVantage (1), this method enables a chemical/spin statesensible structural (both geometric and electronic) study for
X-ray absorbing element.2-4,12,13 The chemical shifts of emitted
X-ray fluorescence were discriminated by high-energy-resolution
XRF spectrometry. AdVantage (2) of this method is a spectralfeature of ultimately high signal/background (S/B) counts
ratio.5-11 By suppressing the predominant background counts
originating from high concentrations of element(s) in sample,
weaker signals originating from lower concentrations of element
were observed selectively. AdVantage (3) is to eliminate thecore-hole lifetime broadening effects by detecting the X-ray* To whom correspondence should be addressed. E-mail: yizumi@
chemenv.titech.ac.jp.
† Tokyo Institute of Technology.
‡ National Institute for Research in Inorganic Materials.
§ Yokohama National University.
⊥ The University of Tokyo.

fluorescence with a smaller energy band than the core-hole
lifetime width of the X-ray absorbing K or L shell.2,6,10,13,14
In this paper, vanadium K-edge XAFS combined with highenergy-resolution XRF was applied to selectively monitor low
concentrations of V on/in titanium oxide. Structure transformations of the V sites were spectroscopically followed in thepresence of 2-propanol or water in relation to catalysis.
Advantage (2) was effective for this system7,8 because Ti KR1(4510.8 eV), Ti Kβ1,3 (4931.8 eV), Ti Kβ5 (4962.3 eV), and/or
diffracted X-ray from V+TiO2 (VOx-TiO2) sample wereeffectively suppressed by detecting the V KR1 (4952.2 eV) with
∆E of 0.4 eV using a fluorescence spectrometer.1,8 The
feasibility of advantage (3) at the V K-edge is discussed to study
the preedge peak feature originating from V 1s f 3d electronictransition in more detail. Recently by the measurements of ironKβ1,3-detected XANES (X-ray absorption near-edge structure),
the Fe site symmetry was discussed based on more precise
separation of Fe K preedge peaks for Fe-ZSM-5 zeolites.15 In
an atmosphere of reaction gas, the only way to selectively
monitor minor V sites on/in high concentrations of TiO2 matrix
is selective, high-energy-resolution detection of V X-ray
fluorescence associated with the V K-edge absorption.8
Low concentrations (less than monolayer) of VOx supported
on TiO2 are good catalysts for the nitric oxide reduction withammonia,16 selective oxidation of naphthalene/o-xylene to10.1021/jp052038+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/16/2005

V Site Transformation on TiO2 Monitored by V KR1-XAFS
TABLE 1: The V Contents, Molar Ratio of V/Ti, andSurface VOx Density for Various V+TiO2 Samples
entry
a
b
c
d
e
f

sampleV/TiO2
mesoporous V-TiO2
V/mesoporous TiO2
sol-gel V-TiO2

V content(wt %)

V/Ti molarratio

VOx density(nm-2)

3.0
1.0
0.6
10.4
1.0
2.0

1/20
1/63
1/100
1/5.0
1/63
1/31

7.1
2.4
0.059a
5.4a
0.18
3.9a

a
Estimated values assuming all the V species dispersed on TiO2
surface. Judging from the synthetic routes, V may be buried inside the
TiO2 matrix for these samples.

phthalic anhydride,17,18 and selective oxidation of butane tomaleic anhydride.19 The V site structures under these catalytic
reaction conditions are still controversial.20 The spectroscopic(technical) difficulties in obtaining information of V sites on
TiO2 compared to on SiO2 or Al2O3 were reported for XAFS,21
Raman (strong scattering from TiO2),21 and ultraviolet (UV)visible (strong photon absorption by TiO2) measurements.22
Despite the technical difficulties, site structures of monomericmonoxo21-25 or dioxo23,24,26 vanadate, polyvanadates such as
decavanadate ([V10O28]6-),27 and the change of presence ratio
when these V species coexist have been reported on TiO2 by
Raman,21,22,27 51V nuclear magnetic resonance (NMR),25 andUV-visible spectroscopies21,22 and density functional theory(DFT) calculations.23,24 The phase similar to an epitaxial V2O5(010) layer was reported to prevail over TiO2 when the Vloading was greater than 4 wt %.28 The V K-edge XAFS was
measured for 3.9 wt % of V on TiO2 in the transmission
mode.29-31
Among various spectroscopic techniques, XAFS gives direct
information on geometric and electronic structures of the V
center. Thus in this paper, 0.6-10.4 wt % of V site transformations on/in TiO2 are studied in relation to catalysis by using
modern highly brilliant synchrotron X-rays and V KR1 XRFcombined K-edge XAFS.
Experimental Section
Sample Preparation. TiO2 (Degussa P25; specific surface
area 60 m2 g-1, anatase/rutile phase ≈ 95/5) was impregnatedwith V triisopropoxide oxide (1) in the 2-propanol solution.
These catalysts are denoted as impregnated V/TiO2. The V
contents were varied between 1.0 and 3.0 wt % on the V metal
basis (1.8-5.4 wt % on the V2O5 basis).
Mesoporous V-TiO2 samples were prepared from compound
1, Ti tetraisopropoxide (2), and dodecylamine (3). An aqueous
solution was maintained at 333 K for 6 days and filtered. The
obtained powder was heated at 453 K for 10 days, and thenwashed with p-toluenesulfonic acid in ethanol. The V contents
were varied between 0.6 and 10.4 wt %. The molar ratio of
V/Ti was listed in Table 1 for each sample. The specific surface
area was 1200 m2 g-1 for the sample of 0.6 wt % V. Based on
the X-ray diffraction (XRD) analysis, the spacing of its
mesopores was ≈30 Å and wormhole-like structure prevailed
rather than ordered mesoporous crystalline structure.32 Mesoporous TiO2 prepared from compounds 2 and 3 in a similar
procedure was impregnated with compound 1 in the 2-propanol
solution. This sample is denoted as V/mesoporous TiO2. Anotherreference sample was prepared from compounds 1 and 2 in theabsence of compound 3, in a similar procedure to that formesoporous V-TiO2. This sample is denoted as sol-gel
V-TiO2. All of dried powders were calcined in air at 473 K
and pressed into a disk of diameter 20 mm in ambient air.

J. Phys. Chem. B, Vol. 109, No. 31, 2005 14885
V metal foil (25 µm-thickness, Aldrich), CrVO4, NH4VO3,
V2O5, VOSO4‚3H2O, and V2O4 (Special or First Grade, Wako)
were used as received. Mn0.9V1.8Mo0.2O6 and Mn0.67V1.33Mo0.67O6 crystallines were synthesized by the solid-state reactionof MnV2O6 with MoO3 at 873 K.33 The powders of thesechemicals were diluted with boron nitride to 1.0 wt % V, mixed
thoroughly using mortar and pestle, and pressed into a disk ofdiameter 20 mm.
V Kr1 Emission and K-Edge XAFS Spectra Measurements. For the series of V+TiO2 samples in each fresh state,
the V KR1 emission spectrum was measured with the excitation energy set at 5484.1 eV using a homemade highenergy-resolution fluorescence spectrometer1 at beamline 7C of
KEK-PF. The storage ring energy was 2.5 GeV and the ring
current was 390-270 mA. A Si(111) double-crystal monochromator was used. The X-ray beam was focused and fully tuned.
The X-ray fluorescence from the sample was analyzed by using
a spectrometer of Rowland radius set to 180 mm equipped witha Johansson-type Ge(331) crystal (50 × 30 mm2, Saint-Gobain)
and a scintillation counter (SP-10, Oken). Their positions were
controlled by PC on the micrometer level. After the exit ofbeam duct, the entire beam path was purged with helium gas
except for the inside of the I0 ion chamber (Oken; N2:He )3:7).1,4
The fluorescence spectrometer was tuned to the peak top foreach sample and the V K-edge XAFS spectrum was measured
at 290 K. The scan step of the monochromator was ∼6.9, ∼0.25,
and ∼2.75 eV in the energy range 5130-5440, 5440-5545,
and 5545-6015 eV, respectively. The dwell time of each data
point was 60-200 s. The energy positions of the monochromatorand the fluorescence spectrometer were reproduced within (0.1
and (0.2 eV, respectively. The V KR1 and V K rising-edgeenergies of the V metal were calibrated to 4952.20 and 5463.9
eV, respectively.34 The XAFS spectra for standard inorganic V
compounds were measured in transmission mode at 290 K using
an It ion chamber purged with N2 instead of the high-energyresolution fluorescence spectrometer.
In situ V KR1-detecting XAFS measurements for V/TiO2
catalyst were performed at beamline 15XU of SPring-8.35 The
storage ring energy was 8.0 eV and the ring current was
89-53 mA. A Si(111) double-crystal monochromator was used.
The planer undulator gap and the ∆θ of the second crystal ofthe monochromator (rocking curve) were optimized to maximize
the beam flux at each data point of the XAFS spectrum scan.
The scan steps and the setup and measurement conditions of
the fluorescence spectrometer were essentially the same as thosefor the measurements at KEK-PF. The fluorescence spectrometer
was tuned to the top of the V KR1 peak and then the XAFS
spectrum was measured. The dwell time of each data point was
10-60 s. The pressed disk of V/TiO2 (1.0 wt % V) was
introduced into a reaction cell made of stainless steel, equippedwith two Be windows for incident and emitted X-rays, ceramicheater, water cooling path, gas supply, and a connection to a
vacuum pump. The two Be windows of 100 µm thickness and
10 mm diameter were located to make an angle of π/2 projectedfrom the sample at the center of the reaction cell. The sample
temperature can be controlled between 290 and 673 K. The
reaction cell was accommodated in our high-energy-resolution
fluorescence spectrometer for spectral measurements. After thereaction cell was evacuated, 3.2 kPa of 2-propanol or 0.85 kPa
of water was introduced.
XAFS Data Analysis. The XAFS data analysis was performed with the package XDAP (XAFS Services International)
based on the works of M. Vaarkamp, H. Linders, and D.

14886 J. Phys. Chem. B, Vol. 109, No. 31, 2005

Izumi et al.

TABLE 2: Local Structure Information of the V Sites for Standard Inorganic V Compounds
no. of bonds

bond distance (Å)

ref

CrVO4
NH4VO3
V2O5
Mn1-xV2-2xMo2xO6(x ) 0.53)
VOSO4‚3H2O

formal valence
5
5
5
5

V-O
V-O
V-O
V-O

bond type4
4
5
5

1.72 × 2, 1.80 × 2
1.66 × 2, 1.81 × 2
1.585, 1.780, 1.878 × Z2, 2.021
1.661, 1.693, 1.913, 1.930, 2.151

37
37
37
33

4
4
5

1
2
2
6

1.56
2.02 × 2
2.05, 2.08
1.76, 1.86, 1.87, 2.01, 2.03, 2.05

37

V2O4
[V10O28]6-

V-O
V-OSO3
V-OH2
V-O(eight V atoms)
V-O(two V atoms)
V-O

6

1.61, (1.83-2.05) × 4, 2.23-2.34

6

1.70× 2, 1.93 × 2, 2.12 × 2

Koningsberger. The preedge background was approximated by
the modified Victoreen function

C2
E

2

+

C1
+ C0
E

The preedge peak energy position, width, and intensity were
evaluated by the fit of the pseudo-Voigt function36 using
KaleidaGraph version 3.6.4 (Synergy Software). The intensity
ratio of the Lorentzian and Gaussian components was fixed
to 1:1. The background of EXAFS (extended X-ray absorption
fine structure) oscillations was approximated by a smoothing
spline function calculated by the equation for the number ofdata points N
N

∑
i)1

(µxi - BGi)2exp(- 0.075ki2)

e smoothing parameter

Multiple shell curve-fit analyses were performed for the
Fourier-filtered EXAFS data using empirical parameters for
V-O extracted from EXAFS for CrVO4. The bond distanceand coordination number were based on the crystal structure
for CrVO4 (Table 2).37 The σ2 values are listed relative to thatfor this model. The many-body reduction factor (S02) was
assumed to be equal for sample and reference. The goodnessof fit was given as requested by the Committee on Standards
and Criteria in X-ray Absorption Spectroscopy.

37
38

of a Brannerite-type crystal structure.40 The V site wascoordinated by two nearer O atoms (1.661-1.693 Å) and threefarther O atoms (1.913-2.151 Å) when the x value was 0.53
(Table 2).33 A shoulder peak appeared at 5486 eV in Figure
1b,g. The postedge patterns for CrVVO4 (h), NH4VVO3 (i), andVV2O5 (j) were qualitatively different from those of spectra a
and b.
A 1s f 3d preedge peak appeared at 5468.3-5468.6 eV for
V/TiO2 (Figure 1 and Table 3) at similar energy positions tothose for Mn0.9V1.8Mo0.2O6, CrVO4, and NH4VO3 (5468.25468.6 eV). However, the peak intensity for spectra a and b
was only 62-70% of that for CrVO4 (h). This difference
suggests that the V site symmetry for V/TiO2 (1.0-3.0 wt %V) was higher than tetrahedral for the V sites of CrVO4 and
NH4VO3. The preedge peak intensity was comparable between
V/TiO2 (Table 3, entries a and b) and Mn0.9V1.8Mo0.2O6 (Table
3, entry g) within the difference of 20%.
XANES of Mesoporous V-TiO2, V/Mesoporous TiO2, and
Sol-Gel V-TiO2. For mesoporous V-TiO2 (0.6 wt % V), therising edge position shifted by 1.0-1.3 eV toward the lower
energy side compared to data for V/TiO2 (Table 3, entries a-c).
This shift suggested the dominance of VIV and VV sites formesoporous V-TiO2 and impregnated V/TiO2, respectively,
based on rising edge energy positions for standard VIV (Table
3, entries k and l) and VV (Table 3, entries g-j) compounds

Results
V Kr1 Emission Spectra. The V KR1 emission peaks for
all V+TiO2 samples appeared at 4951.5-4952.0 eV.8 The
chemical shifts with respect to V metal were reported at -0.1,-0.3 to -0.4, and -0.5 to -0.6 eV for VIII, VIV, and VV
compounds, respectively.39 Hence, the V valence states in
V+TiO2 samples studied in this paper were IV and/or V.
XANES of V/TiO2. Normalized V K-edge XANES spectra
are shown in Figure 1. The intensity and energy positions forthe preedge peak and rising edge were summarized in Table 3.
For impregnated V/TiO2 (Figure 1a,b), the rising edge appeared
at 5480.8-5481.1 eV. Among data for reference inorganic V
compounds (spectra g-l), the rising edge position for Mn0.9VV1.8Mo0.2O6 (g) or NH4VO3 (i) was most close to the values for
V/TiO2. The spectrum for Mn0.67VV1.33Mo0.67O6 (not shown)was essentially the same as that for Mn0.9V1.8Mo0.2O6 (g). Two
broad peak features were observed in the postedge region (5489
and 5502 eV) at nearly equal intensity in spectra a and b. The
two peak energy positions were similar to 5489 and 5503 eV
for V/TiO2 (3.9 wt % V; f)31 and 5490 and 5504 eV for
Mn0.9V1.8Mo0.2O6 (g). Mn1-xV2-2xMo2xO6 (0 < x < 1) consists

Figure 1. V K-edge XANES spectra for several V+TiO2 samples
combined with high-energy-resolution XRF (a-e). Impregnated V/TiO2of 3.0 (a) and 1.0 wt % V (b), mesoporous V-TiO2 (0.6 wt % V; c),V/mesoporous TiO2 (1.0 wt % V; d), and sol-gel V-TiO2 (2.0 wt %;e). Reference V K-edge XANES spectra (f-l) measured in transmission
mode for V/TiO2 (3.9 wt % V; f),30 Mn0.9V1.8Mo0.2O6 (g), CrVO4 (h),NH4VO3 (i), V2O5 (j), VOSO4‚3H2O (k), and V2O4 (l). Spectrum l′(dotted line) was measured in fluorescence detection mode with a Lytle
detector.

V Site Transformation on TiO2 Monitored by V KR1-XAFS

J. Phys. Chem. B, Vol. 109, No. 31, 2005 14887

TABLE 3: The Intensity and Energy Position of Preedge Peaks and the Energy Position of Rising Edge for Various V+TiO2
Samples and Standard Inorganic V Compoundsa
entry
a
b
c
d
e
f
g
h
i
j
k
l
l′
m

sampleV/TiO2
mesoporous V-TiO2
V/mesoporous TiO2
sol-gel V-TiO2
Mn0.9V1.8Mo0.2O6
CrVO4
NH4VO3
V2O5
VOSO4‚3H2O
V2O4
V2O4b
mesoporous TiO2

V content(wt % V)intensity

preedge peakfwhm (eV)energy (eV)rising edgeenergy (eV)

0.47
0.53
0.32
0.80
0.59
0.39
0.57
0.76
0.73
0.57
0.46
0.47
(0.49)
0.25

2.5
2.5
2.3
2.4
2.4
2.5
2.4
2.3
2.3
2.5
2.2
3.4
(3.4)
3.0

5468.65468.35467.95468.25468.55468.55468.25468.65468.65469.55468.85469.4(5469.4)
4968.15480.85481.15479.85479.75481.15481.35480.55482.25481.35480.05479.85477.5(5477.5)
4978.4

3.0
1.0
0.6
10.4
1.0
2.0

a Entries a-f were for XAFS spectra combined with high-energy-resolution XRF. Entries g-m were measured in transmission mode. b Entry l′
was measured in fluorescence detection mode with a Lytle detector.

Figure 2. V K-edge XANES spectra for mesoporous V-TiO2 samples
combined with high-energy-resolution XRF. V contents: 0.6 (a) and10.4 wt % (b). Reference Ti K-edge XANES measured in transmission
mode for mesoporous TiO2 (c). The energy of spectrum c was shiftedby +499.8 eV for comparison to V K-edge spectra.

and data in the literature.37 Two broad peak features wereobserved at 5487 and 5505 eV in the postedge region, shiftedby (2 eV from the values for V/TiO2. Although the rising edge
position for VIVOSO4‚3H2O was identical with that for mesoporous V-TiO2, the postedge pattern was totally different
between the two (Figure 1c,k). The postedge pattern for VIV2O4(l) was relatively similar to that for spectrum c. The Ti K-edge
spectrum for mesoporous TiO2 was compared to V K-edge
spectra by shifting the former energy scale by +499.8 eV toadjust the preedge peak energy. The phase parameters are similar
between Ti and V. The rising edge energy position and postedge
pattern were very alike those of V K-edge for mesoporous
V-TiO2 (Figure 2a,c).
The preedge peak intensity and energy position for VOSO4‚
3H2O and V2O4 were not comparable to those for mesoporous
V-TiO2 (Table 3, entries c, k, and l). The Ti K preedge peakintensity for mesoporous TiO2 was comparable to that for theV K preedge peak for mesoporous V-TiO2. These preedge peakintensities (0.32 and 0.25) were 53-68% of that for V2O4. The
VIV sites of V2O4 are coordinated by three nearer O atoms(1.76-1.87 Å) and three further O atoms (2.01-2.05 Å) (Table2).37 Thus, the V site symmetry was suggested to be evenhigher for mesoporous V-TiO2 than that for V2O4. The fullwidth at half-maximum (fwhm) value of the preedge peak for

Figure 3. In situ V K-edge XANES spectra for V/TiO2 (1.0 wt % V)
combined with high-energy-resolution XRF: as prepared (a), in 3.2
kPa of 2-propanol at 290 (b) and 473 K (c), and mesoporous V-TiO2(0.6 wt % V) for comparison (d).

mesoporous V-TiO2 (2.3 eV) was smaller than those for
V/TiO2 (2.5 eV). This implies that the V sites in mesoporous
V-TiO2 are more statistically uniform compared to those in
V/TiO2 because the spectral measurement conditions for thesesamples were essentially the same. In summary, the local structure of predominant VIV species in mesoporous V-TiO2 wasmore uniform and similar to that of Ti sites in mesoporous TiO2.
The XANES spectrum for mesoporous V-TiO2 that contains
more V content (10.4 wt % V) by 20 times is depicted in Figure
2b. The spectrum was very similar to those for mesoporous
V-TiO2 (0.6 wt % V) and mesoporous TiO2 (spectra a and c)
except for a more intense preedge peak by 2.5-3.2 times (Table
3, entry d). For V/mesoporous TiO2 and sol-gel V-TiO2, thepreedge, rising edge, and two postedge-region peaks appeared
at 5468.5, 5481.1-5481.3, 5487-5488, and 5501-5504 eV,very close to those for impregnated V/TiO2 (5468.3-5468.6,
5480.8-5481.1, 5489, and 5502 eV). Preedge peak intensities
for V/mesoporous TiO2 and sol-gel V-TiO2 were 111-126%
and 74-83%, respectively, compared to values for V/TiO2(Table 3). Thus, the V sites of these two reference samples were
essentially the same as those of V/TiO2.
In Situ XANES of V/TiO2. In situ V K-edge XANES spectrain 3.2 kPa of 2-propanol were measured by selectively detecting the V KR1 signals (Figure 3 and Table 4). The spectra
measured at 290 and 473 K in the reaction cell were similar to14888 J. Phys. Chem. B, Vol. 109, No. 31, 2005

Izumi et al.

TABLE 4: Changes of Intensity and Energy Position ofPreedge Peaks and the Energy Position of Rising Edge for
V/TiO2 (1.0 wt % V, a-c) in the Presence of 2-Propanol (3.2kPa)a and Reference Spectrum Data (d)preedge peakrising edge

preedge peakrising edge

fwhm energy
(eV)
(eV)

energy(eV)

entry

conditions

intensity

fwhm
(eV)

energy(eV)energy
(eV)

0.53

2.5

5468.35481.1

a

0.53

2.5

5468.35481.1

0.20
0.18
0.32

2.5
2.6
2.3

5467.75467.75467.95479.95479.55479.8bcdonce exposed in air,
290 K
in water, 290 K
in water, 473 K
NH4VO3

0.59
0.47
0.73

2.5
2.5
2.3

5468.75468.45468.65480.25482.15481.3entry

conditions

intensityaonce exposed in air,
290 K
in 2-propanol, 290 K
in 2-propanol, 473 K
mesoporous V-TiO2(0.6 wt % V)bcd

TABLE 5: Changes of Intensity and Energy Position ofPreedge Peaks and the Energy Position of Rising Edge for
V/TiO2 (1.0 wt % V, a-c) in the Presence of Water (0.85kPa)a and Reference Spectrum Data (d)aEntries a-d were for XAFS spectra combined with high-energyresolution XRF.

a Entries a-c were for XAFS spectra combined with high-energyresolution XRF. Entry d was measured in transmission mode.

Figure 4. In situ V K-edge XANES spectra for impregnated V/TiO2(1.0 wt % V) combined with high-energy-resolution XRF: as prepared(a) and in 0.85 kPa of water at 290 (b) and 473 K (c). Data for
NH4VO3 were measured in transmission mode for comparison (d).

each other (Figure 3b,c). The preedge peaks became weaker
34-38% and shifted toward the lower energy side by 0.6 eVwith respect to the peak for fresh sample (Table 4). The risingedge energy position also shifted toward the lower energy sideby 1.2-1.6 eV. The postedge broad peaks at 5489 and 5502
eV for fresh sample shifted to 5486 and 5501 eV in 2-propanol.
The former peak became more intense than the latter (Figure3b,c). The values of intensity, width, and energy position of
preedge peak, rising edge energy position, and the two postedge
broader peak positions for spectra of V/TiO2 in 2-propanol(Table 4, entries b and c) were similar to corresponding values
for mesoporous V-TiO2 of 0.60 wt % V (Table 4, entry d, andFigure 3d).
In situ V K-edge XANES spectra for V/TiO2 measured in
0.85 kPa of water are depicted in Figure 4. Representative
data values are summarized in Table 5. At 290 K, the preedge
peak negligibly changed compared to the fresh one, but therising edge shifted toward the lower energy side by 0.9 eV
and the intense postedge peak appeared at 5486 eV (Figure4b). This V species is not identical with that postulated toexist in 2-propanol (Figure 3b,c) because the preedge peakintensities were different by 3.0-3.3 times (Table 4, entries band c, and Table 5, entry b). At 473 K, the preedge peak
was still negligibly changed. On the contrary, the postedge
region qualitatively changed. The broad peak at 5506 eV became more intense compared to that at 5489 eV (Figure 4c).
The most similar spectrum profile to this was found for
NH4VO3 (d). The transformation of V species corresponding
to Figure 4a into 4c was reversible. A very similar spectrum

Figure 5. V K-edge EXAFS. (A) χ functions for fresh mesoporous
V-TiO2 (10.4 wt % V) (a), V/TiO2 (1.0 wt % V) in 0.85 kPa of waterat 473 K (b), NH4VO3 (c), VOSO4‚3H2O (d), and Mn0.9V1.8Mo0.2O6(e). Best fits for filtered k3-weighted χ-functions of V/TiO2 (B) andmesoporous V-TiO2 (C) both in k-space (1) and R-space (2; themagnitude and imaginary parts): (s) experiment and (- - -) theory.

to spectrum a was obtained when the sample for spectrum
c was cooled to 290 K, evacuated, and exposed to ambient
air.
EXAFS. V KR1-detecting EXAFS spectra were measured for
V/TiO2 (1.0 wt % V) in 0.85 kPa of water at 473 K andmesoporous V-TiO2 (10.4 wt % V). The background-subtracted EXAFS χ-functions are depicted in Figure 5A (band a, respectively) along with those for reference compounds,
NH4VO3 (c), VOSO4‚3H2O (d), and Mn0.9V1.8Mo0.2O6 (e). The
χ-functions a-c were relatively similar in this k-range. The bestfit results for filtered k3-weighted χ-functions in both k-spaceand R-space are depicted in Figure 5, parts B and C, and thedata are listed in Table 6. Only one-wave fits were performed
due to the limited available k-range of data. For both samples,the coordination number of V-O bonds was found to 3-4. The

V Site Transformation on TiO2 Monitored by V KR1-XAFS

J. Phys. Chem. B, Vol. 109, No. 31, 2005 14889

TABLE 6: Best-Fit Results of V K-edge EXAFS Combined with V Kr1-Detecting Fluorescence Spectrometry for V/TiO2 (1.0
wt % V) and Mesoporous V-TiO2 (10.4 wt % V)aV-OasampleNR (Å)∆(σ)2 (Å2)

goodness of fit

V/TiO2 in water, 473 K
Mesoporous V-TiO2,
once exposed in air

3.4((0.7)
4.1((0.6)

1.863((0.018)
1.667((0.017)-0.0016((0.0019)
0((0.0014)

674
360

Values in parentheses are estimated fit errors.

average V-O bond distance was shorter for mesoporous
V-TiO2 (1.67 Å) than that for V/TiO2 in water at 473 K(1.86 Å).
Discussion
Advantages of V Kr1-Detecting XAFS. V KR1-detecting
V K-edge XAFS spectra were obtained for various fresh
V+TiO2 samples (Figure 1). The application of this technique
was extended to V/TiO2 catalyst in two kinds of reaction gas atelevated temperature (Figures 3-5). In addition to advantage(2) (see the Introduction section), selective V KR1 detection isadvantageous compared to selective Auger or secondary photoelectron detection derived from V sites by an electron energy
analyzer because X-ray fluorescence-detecting XAFS is compatible with the presence of reaction gases.
In V K-edge XANES spectra, the preedge peak width isrelated to the core-hole lifetime width of K + M4or5 (≈1.01eV),41 the ∆E of the beamline, the energy splitting of theunoccupied V 3d multiplet to which the 1s electron can transit,
and the peak energy variation due to the presence of some kindsof V sites in a sample. The last factor is negligible in the caseof VOSO4‚3H2O crystalline. A general trend exists that thewidth of the preedge peak decreased as the coordination number
to V decreased (accordingly smaller 10Dq values) for standard
inorganic V compounds (Tables 2 and 3). Thus, the latter two
factors should be minimal for the spectra of VOSO4‚3H2O,
CrVO4, and NH4VO3. Their preedge peak widths were smallest(2.2-2.3 eV, Table 3). Therefore, the total energy width due
to ∆E of the beamline and the latter two additional factors wasestimated to 2.0 eV.
The V KR1 emission peak width is related to the core-hole
lifetime width of K + L3 (1.26 eV),41 the ∆E of the beamline
and of fluorescence spectrometer, and the peak energy variation
due to the presence of some kinds of V sites in sample. The
smallest fwhm values for the V KR1 peak were 2.4 eV for V
metal and impregnated V/TiO2.8 If we assume the V 3d multiplet
splitting effect (for vacant levels to which the 1s electron transits) on the V preedge peak width was negligible, the ∆E of
our fluorescence spectrometer is estimated to 0.4 eV at 5 keV.
To apply advantage (3) (the Introduction section) at the V
K-edge, the convolution of ∆E of the beamline and fluorescence
spectrometer should be less than the core-hole lifetime widthof vanadium K (1.01 eV). To satisfy this condition, the ∆E ofthe beamline should be less than 0.9 eV. Thus, advantage (3)was not effective in this paper. The fact is evident in Table 3that the preedge peak widths for V KR1-detecting XAFS using
a fluorescence spectrometer (2.3-2.5 eV, a-e) were comparable
to those for spectra measured in transmission mode (2.2-3.4
eV, g-l).
V Site Structure Transformations. Olthof et al.21 reported
XANES for supported V samples over SiO2, Al2O3, ZrO2, andHfO2 under hydrated and dehydrated conditions. The V K-edge
data were measured in conventional fluorescence detection
mode.42 Reported XANES profile changes before and after
dehydration in dry air at 723 K for V/SiO2 (8.4 wt % V) and

Figure 6. Proposed V site transformations for V/TiO2 in reactant/product gas (A) and for mesoporous V-TiO2 by changing the molar
ratio of V/Ti (B). [H] for models a and c indicates the presence/absenceof hydrogen cannot be determined.

V/Al2O3 (5.6 wt % V) in ref 21 were similar to the change
observed in this paper before and after the water (0.85 kPa)
treatment at 473 K for V/TiO2 (1.0 wt % V) (Figure 4a,c).
Postedge profile changes were remarkable in both cases.
Structural change by dehydration of V/TiO2 (0.6-2.2 wt %V) was studied using 51V NMR43 and Raman.27 Upon dehydration, transformation from 6-fold coordination polymeric vanadate (e.g. metavanadate) into dispersed tetrahedral monooxo
vanadate was suggested. The V site transformation from higher
symmetry to tetrahedral was supported by DFT calculations23,24,44 and ab initio calculations of V K-edge XANES
spectra.7 Spectrum profiles of Figure 4a and 4c were moresimilar to data for surface V site models of 5-fold and 4-foldcoordination, respectively, calculated by ab initio calculationsof XANES.7 V KR1-detecting EXAFS analysis demonstrated
V-O bonds at 1.86 Å (average) with the coordination numberof 3-4 rather than 5 (Table 6) for V/TiO2 in water at 473 K.
Thus, the V sites of V/TiO2 in water at 473 K (Figures 4c,
5A-b, and 5B) are determined for monooxo (distorted) tetrahedral species (Figure 6b).
Monooxo tetrahedral V species were reported over SiO2,
Al2O3, and TiO2 based on a VdO stretching Raman peak at≈1030 cm-1 in comparison with data for standard compounds(OdV)X3 (X ) halogens and alkoxides).21,45 Jehng et al.
reported the effects of water on the V site structure for V/TiO2(0.56 wt % V) as a function of temperature using Raman
spectroscopy.46 In moisture, the peak at 1024 cm-1 was observed
between 623 and 723 K but it diminished between 393 and 443
K. This trend does not contradict with our results in Figure 4.
Namely, (distorted) monooxo tetrahedral V species in spectrum
c measured at 473 K in water transformed into the V species ofhigher symmetry until 290 K (spectrum a). Proposed V sitetransformation was illustrated in Figure 6 (b f a f d).
The XANES spectrum for fresh V/TiO2 (Figure 1b) was
measured inside the acrylic chamber of the fluorescence
spectrometer purged with He gas; however, the sample was once
exposed to ambient air before the measurement. The XANES
was most similar to that for Mn1-xV2-2xMo2xO6 (Figure 1g and14890 J. Phys. Chem. B, Vol. 109, No. 31, 2005
Table 3, entry g) in which the V sites are 5-fold coordinated(Table 2).33 The V-OH bond length is reported between 1.75
and 1.85 Å23,44,47 versus V-O bond lengths for standard V
compounds listed in Table 2. Therefore, dispersed vanadate
species of 5-fold coordination are proposed for fresh, exposed
V/TiO2 transformed from monooxo V species (Figure 6, b fa). The extent of hydration for the V species of Figure 1b should
be intermediate. When the V/TiO2 sample was in 0.85 kPa ofwater at 290 K (Figure 4b), the dispersed vanadate species(Figure 6, a and/or b) was thoroughly hydrated to transform
into surface polymerized species (d).21,22,27,46,48 For V/TiO2 inambient condition, a higher ratio of polymeric vanadate versus
dispersed tetrahedral monooxo vanadate was reported compared to the cases of V/SiO2 and V/Al2O3 in ambient condition.21 Hence, species a and d illustrated in Figure 6 may coexist
for ambient V/TiO2 (1.0-3.0 wt % V) in this study. Even if
the V sites were the mixture, the V sites of 5-fold coordination should be dominant because the XANES spectra (Figure 1a,b) were most similar to that for Mn0.9V1.8Mo0.2O6(Figure 1g).
In situ V K-edge XANES for V/TiO2 in 2-propanol (3.2 kPa)
both at 290 and 473 K suggested higher coordination (higher
symmetry) V sites (Figure 3b,c). Hence, dissociative adsorptionof the 2-propanol molecule on dispersed vanadate was suggested
to form octahedral vanadate species (Figure 6c). Went et al.
observed the reaction of V/SiO2, V/Al2O3, and V/TiO2 withethanol by Raman. They discussed that with the addition of
ethanol the cleavage of the V-O-M bond proceeded when M) Si but simple adsorption of ethanol was proposed when M) Al and Ti.49 The transformation proposed in Figure 6a,c isconsistent with their discussion.
Varieties of V Sites On/Inside TiO2. In the series of V+TiO2
samples, only the V sites of mesoporous V-TiO2 were foundto be at the VIV state and the local site structure was similar tothat for Ti sites of mesoporous TiO2 (Figure 2a,c). The VIV
sites should be substituted on the Ti sites (Figure 6e). The reason
is unclear why only the preedge peak intensity grew by 2.5 times(Table 3, entry d) when the V content increased from 0.6 to
10.4 wt % (molar ratio of V/Ti increased from 1/100 to 1/5.0)
in spite that other spectral profiles were essentially retained(Figure 2a,b). The preedge peak intensity for mesoporous
V-TiO2 (10.4 wt % V) was close (105-110%) to those forthe tetrahedral V sites of CrVO4 and NH4VO3 (Table 3, entriesd, h, and i). One of the plausible explanations for this preedge
peak intensity change is that major octahedral V sites in thecase of 0.6 wt % V in the TiO2 matrix switched to tetrahedral
ones for mesoporous V-TiO2 of 10.4 wt % V (Figure 6f). Thispossibility was supported by V KR1-detecting EXAFS analysis
for the 10.4 wt % V sample (Figure 5A-a and C). The
coordination number was evaluated to four for the V-O bonds(Table 6). The bonding distance was relatively short (1.67 Å).
V site incorporation within the TiO2 (anatase) matrix was
reported for sol-gel V-TiO2 based on XRD, X-ray photoelectron spectroscopy, electron spin resonance, and 51V NMR
measurements.50 The molar ratio V/Ti was varied between 1/50
and 1/17 in ref 50. The molar ratio of V/Ti for sol-gel V-TiO2synthesized in the absence of template molecule in this paper
was comparable (1/31, Table 1). However, dispersed V species
over TiO2 surface were proposed based on V KR1-detecting
XAFS, similar to the V species for impregnated V/TiO2 andV/mesoporous TiO2 (Figures 1 and 6A). The difference in the
calcination temperatures (723-1173 K in ref 50 and 473 K in
this work) may be the reason of different distribution of V on/
inside TiO2.

Izumi et al.
Conclusions
V KR1-detecting K-edge XAFS spectra were measured for
V/TiO2 (1.0-3.0 wt % V), mesoporous V-TiO2 (0.6-10.4 wt% V), V/mesoporous TiO2 (1.0 wt % V), and sol-gel V-TiO2(2.0 wt % V). Owing to the selective detection of V KR1
fluorescence associated with the V K-edge absorption with theenergy resolution of 0.4 eV, spectra of extremely high S/B ratio
were obtained. The data were free from the effects of highconcentrations of Ti and measurements in the presence of 3.2
kPa of 2-propanol or 0.85 kPa of water were also enabled. These
gases are reactant and product in the catalysis (selective
reduction of NO and selective oxidation to phthalic hydride andmaleic anhydride).
Dispersed 5-fold coordination vanadate and/or surface polymeric VOx species of 5-fold coordination were found for V/TiO2
once exposed to ambient air. In the presence of 0.85 kPa ofwater, the species should be transformed thoroughly intopolymerized species at 290 K whereas reverse transformation
to monooxo (distorted) tetrahedral vanadate was observed at
473 K. This temperature dependence of V site structure inmoisture was consistent with literature based on Raman
spectroscopy.46 Dissociative adsorption of 2-propanol to thedispersed vanadate was suggested to constitute (nearly) octahedral V sites at 290-473 K. This was consistent with literature
where different reactivities of the V-O-M (M ) Si, Al, andTi) bond with ethanol were discussed based on Raman
spectroscopy.49 As far as our V+TiO2 samples were preheatedto 473 K in air, the VV site structures seem to be common for
V/TiO2, V/mesoporous TiO2, and sol-gel V-TiO2. An exception was the VIV species dominant for mesoporous V-TiO2.
The V sites substituted on the Ti sites of high-surface-area TiO2.
Acknowledgment. The XAFS experiments were performed
under the approval of the Photon Factory Proposal Review
Committee (2001G308 and 2002G285) and under the approvalof the SPring-8 Program Review Committee and the National
Institute for Materials Research as Nanotechnology Support
Project of the Ministry of Education, Culture, Sports, Science,
and Technology (MEXT) (2003A0146-NX-np and 2002B0739NX-np). The authors are thankful for financial support fromthe Grant-in-Aid for Scientific Research from MEXT (B1355
5230), Yamada Science Foundation (2000), and Toray Science
Foundation (98-3901).
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