1154

Chemistry Letters 2002

0.6–3.0 wt% of Vanadium on/in Titania Monitored by X-ray Absorption Fine Structure
Combined with Fluorescence Spectrometry
Yasuo Izumi, Fumitaka Kiyotaki, Hideaki Yoshitake,y; Ken-ichi Aika, Tae Sugihara,yy Takashi Tatsumi,yy
Yasuhiro Tanizawa,yyy Takafumi Shido,yyy and Yasuhiro Iwasawayyy
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta, Midori-ku, Yokohama 226-8502
yGraduate School of Environment and Information Sciences,
Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240-8501
yy
Graduate School of Engineering, Yokohama National University, Tokiwadai, Hodogaya, Yokohama 240-8501
yyy
Graduate School of Science, The University of Tokyo, Bunkyo-ku, Hongo 113-0033(Received August 26, 2002; CL-020724)

Vanadium catalysts are used for the reduction of nitric oxidewith ammonia,1 the oxidation of naphthalene/o-xylene to phthalicanhydride, and the oxidation of butane to maleic anhydride.2 The
optimal catalytic performance in these applications has beenreported to correspond to concentrations of V of less than amonolayer, dispersed on TiO2 . When the concentration of V
corresponds to monolayer levels, major V species has been
proposed to be a monomeric monooxo3 or dioxo vanadate,4polyvanadates3 such as decavanadate ([V10 O28 ]6 ),5 or to an
epitaxial V2 O5 (010) layer over TiO2 6 by Raman, 51 V NMR, UVvisible, etc.
XAFS directly determines the local structure of non-crystalline
materials. However, exceptionally it is very diﬃcult to measure
XAFS data for low concentrations of V in the TiO2 matrix. When
0.6 wt% of V (1.1 wt% V2 O5 ) is mixed with TiO2 , the V K-edge
jump is only 0.037 compared to a total absorption of 4 intransmission mode. The photon number ratio of V K1 /Ti K1 isonly 0.012 in ﬂuorescence mode. When a solid-state detector (SSD;E  100 eV) is used, the V K1 (4952.2 eV) peak can beseparated from the Ti K1 (4510.8 eV) peak. However, the Ti K1;3(4931.8 eV) still overlaps the V K1 . The ratio of V K1 /Ti K1;3 is0.058. The selective detection of V K1;3 (5427.3 eV) is diﬃcultusing the SSD because scattered X-rays overlap. Auger orsecondary photoelectrons derived from V are selectively monitored
using an electron energy analyzer. However, these experiments
need ultra-high vacuum, and in-situ measurements are impossible.
In this Letter, XAFS combined with ﬂuorescence spectrometry7;8
was applied to selectively monitor low concentrations of V on/in
TiO2 .
TiO2 (P25, 60 m2 g1 ) was impregnated with V triisopropoxide
oxide (1) in isopropanol solution (V/TiO2 ). A high surface area
(HSA; 1200 m2 g1 ) V-TiO2 was prepared from compound 1, Ti
tetraisopropoxide (2), and dodecylamine.9 An aqueous solution waskept at 333 K for six days, and ﬁltered. The obtained powder washeated at 453 K for ten days, and then washed by p-toluenesulfonicacid/ethanol. HSA-TiO2 prepared in a similar procedure wasimpregnated with compound 1 (V/HSA-TiO2 ). V-TiO2 wasprepared from compounds 1 and 2 in the absence of docecylamine
in a similar manner to the case of HSA V-TiO2 . In all cases, the driedpowder was heated in air at 473 K and pressed into a disk in ambientair.
The XAFS spectra were measured at KEK-PF 7C. The storage
ring energy was 2.5 GeV, and the current was 390–270 mA. A
Si(111) double crystal monochromator was used. The beam wasfocused and fully tuned. The X-ray ﬂuorescence from the sample
was analyzed using a Rowland-type spectrometer (R ¼ 180 mm)
equipped with a Johansson-type Ge(331) crystal and scintillation
counter.7 The entire beam path was in helium, except for the I0 ionchamber (N2 : He ¼ 3 : 7).
The V K1 emission spectrum was measured with theexcitation energy set at 5484.1 eV. Next, the V K-edge XANES
was measured. The ﬂuorescence spectrometer was tuned to theemission peak energy (1000 cps). The Ge(331) crystal received asolid angle of 0.007 sr for the X-ray ﬂuorescence from the sample.
The step scan was 0:25 eV. The dwell time of each data point was60–200 s. The energy position was reproduced within 0:1 eV. TheV K1 and V K rising edge energy of the V metal were calibrated to4952.2 and 5463.9 eV, respectively.
The V K1 emission spectra for the V/TiO2 catalysts appeared
at 4951.5–4952.0 eV (Figure 1). The chemical shifts with respect toV metal were reported to 0:1, 0:3–0:4, and 0:5–0:6 eV for
VIII , VIV , and VV compounds, respectively.10 Hence, the V sites inthe catalysts are in the oxidation state of IV/V. The FWHM

48Intensity (cps)Selective XAFS measurements of 0.6–3.0 wt% V sites on/inTiO2 were enabled utilizing a ﬂuorescence spectrometer. The Vsites were VIV species in the case of high-surface-area V-TiO2 incontrast to VV species in the case of conventional V/TiO2 catalysts,
V supported on HSA-TiO2 , and V-TiO2 prepared in the absence ofdodecylamine (general sol-gel method).

Kα132
Kα2
1604940
49504960
Emisson Energy (eV)Figure 1. V K1 and 2 emission spectrum for V/TiO2 of 3.0 wt%-V. Bestenergy resolution of the ﬂuorescence spectrometer (narrower peak of dotted line)
and of beamline (wider peak of dotted line) is also drawn.

Copyright Ó 2002 The Chemical Society of Japan

Normalized intensity

Chemistry Letters 2002

11554lk
3 jih2 gfe
1 dcba0546054805500
Energy (eV)Figure 2. V K-edge XANES spectra for V+TiO2 catalysts measured using aﬂuorescence spectrometer (a–c). V/TiO2 of 3.0 (a) and 1.0 wt%-V (b). HSA VTiO2 (0.6 wt%-V, c). V/HSA-TiO2 (1.0 wt%, d). V-TiO2 (2.0 wt%, e). ReferenceV (f–k) and Ti K-edge (l) XANES measured in transmission mode for V/TiO2(3.9 wt%-V, f, above 5484 eV),11 Mn0:90 VV 1:80 Mo0:20 O6 (g), CrVV O4 (h),12VV 2 O5 (i),12 VIV OSO4 3H2 O (j),12 VIV 2 O4 (k),12 and HSA-TiO2 (l).9

progressively decreased (3.7–2.4 eV) as the length of two slits in theRowland circle changed from 8.0 to 2.0 mm.7
Figure 2 shows the normalized V K-edge XANES spectra. In
the case of V/TiO2 (a and b), the rising edge appeared at 5480.8–5481.1 eV. Two broad post-edge peak features were observed at5489 and 5502 eV at nearly equivalent intensity. The two peak
positions were similar to the cases of V/TiO2 (3.9 wt%-V, f; 5489
and 5503 eV)11 and Mn0:90 V1:80 Mo0:20 O6 (3) (g; 5490 and5504 eV). The V site in compound 3 is coordinated by two nearer and three farther O atoms (1.913–O atoms (1.661–1.693 A)
 13 A weaker shoulder at 5486 eV appeared in (b) and (g).
2.151 A).
The post-edge pattern for CrVV O4 (h) and VV 2 O5 (i) was entirely
diﬀerent from that of (a) and (b).
A 1s–3d pre-edge peak appeared at 5468.3–5468.6 eV for V/
TiO2 (a and b, Table 1). The FWHM values of the pre-edge peaks
were 2.5 eV, independent of the V loadings (3–1 wt%) and of theE of the ﬂuorescence spectrometer. The pre-edge peak width isrelated to the core-hole lifetime width of K+M (1:01 eV) and theE of the beamline, whereas the emission peak width is related tothe width of K+L3 (1.26 eV) and the E of the beamline+theﬂuorescence spectrometer. Therefore, the E of the beamline wasestimated to be 2.0 eV (Figure 1) and the E of the ﬂuorescence
Table 1. The Intensity and Energy Position of Pre-edge Peaks

Entryabcdeghijkl

SampleV/TiO2 (3.0 wt%)
V/TiO2 (1.0 wt%)
HSA V-TiO2
V/HSA-TiO2
V-TiO2
Mn0:90 V1:80 Mo0:20 O6
CrVO4
V2 O5
VOSO4 3H2 O
V2 O4
HSA-TiO2

Intensity
0.47
0.53
0.32
0.59
0.39
0.57
0.82
0.79
0.48
0.35
0.25

Energy Position (eV)5468.65468.35467.95468.55468.55468.25468.75469.55468.7
5469.34967.7

Only the post-edge region was reported for Fig. 2f.11

spectrometer decreased from 2.8 to 0.3 eV (Figure 1) as the slit
width was decreased from 8.0 to 2.0 mm.7 No contribution of Ti
K1;3 appeared with these energy resolution values, demonstrating
entirely no contribution of Ti in Figure 2a–e.
The pre-edge peak in (a) and (b) appeared at a similar energy to
the case of compound 3 (g) and CrVO4 (h). The peak intensity for (a)and (b) was 57–65% of that for (h), suggesting that the V site
symmetry of V/TiO2 was higher than Td .
For HSA V-TiO2 , the V K1 emission peak position(4951.7 eV) corresponds to VIV /VV state. The rising edge positionof the XANES (5479.8 eV, c) shifted by 1.0–1.3 eV toward lower
energy compared to the case of V/TiO2 . Two broad peak features
were observed at 5487 and 5505 eV in the post-edge region, shiftedby 2 eV from the case of V/TiO2 (a and b). The post-edge patternof VOSO4 3H2 O (j) was entirely diﬀerent from (c). The post-edge
pattern of V2 O4 (k) was relatively similar to (c), but the pre-edge
peak position was at higher energy by 1.4 eV. The Ti K-edge data forHSA-TiO2 was compared to the V K-edge by shifting the energy byþ499:8 eV (l), since the phase parameters are similar between Tiand V. Both the post-edge pattern and pre-edge peak were similar tothe case of (c).
The FWHM value of the pre-edge peak at 5467.9 eV (Table 1)for HSA V-TiO2 (c) was 2.3 eV, a value smaller than the case of V/TiO2 (2.5 eV, a and b). This implies that HSA V-TiO2 primarily
consists of VIV sites rather than a mixture of VIV and VV sites. The
VIV site of V2 O4 is coordinated by three nearer O atoms (1.76– and three farther O atoms (2.01–2.05 A).
 The pre-edge peak
1.87 A)intensity of HSA V-TiO2 was 91% for the case of V2 O4 , suggestinga slightly higher symmetry than for the case of V2 O4 . EXAFS
combined with ﬂuorescence spectrometry until the wavenumber of 1 gave an average V-O distance of 1.74 A
 in the case of HSA
11 AV-TiO2 .
The pre-edge, rising edge, and two post-edge peaks appeared at5468.5, 5481.1–5481.3, 5487–5488, and 5501–5504 eV and theFWHM of pre-edge peak was 2.4–2.5 eV for V/HSA-TiO2 (d) andV-TiO2 (e), very similar to the case of V/TiO2 (a and b).
The VIV sites may be substituted on the Ti sites only in the caseof HSA V-TiO2 , prepared from compounds 1 and 2 anddodecylamine. VV species were dominant for the other samplesand major V site structure may be common due to the similarity ofspectrum. Eight of ten V atoms of [V10 O28 ]6 are coordinated by and four farther O atoms (1.83–one nearer O atom (1.61 A)
 14 similar to the case of V site in V2 O5 . Relatively low
2.05 A),calcination temperature (473 K) and ambient condition may be thereasons why monovanadate, decavanadate, and V2 O5 were notobserved.3{6References
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