Structure of low concentrations of vanadium on TiO2 determined byXANES and ab initio calculations
Yasuo Izumi,*a Fumitaka Kiyotaki,a Hideaki Yoshitake,b Ken-ichi Aika,a Tae Sugihara,c Takashi
Tatsumi,c Yasuhiro Tanizawa,d Takafumi Shidod and Yasuhiro Iwasawada Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259
Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. E-mail: izumi@chemenv.titech.ac.jp
b Graduate School of Environment and Information Sciences, Yokohama National University, Tokiwadai,
Hodogaya, Yokohama 240-8501, Japanc Graduate School of Engineering, Yokohama National University, Tokiwadai, Hodogaya, Yokohama
240-8501, Japand Graduate School of Science, The University of Tokyo, Bunkyo-ku, Hongo 113-0033, Japan
Received (in Cambridge, UK) 24th July 2002, Accepted 6th September 2002
First published as an Advance Article on the web 20th September 2002

DOI: 10.1039/b207159b

The major local structure of low concentrations (1–3 wt% V)
of vanadium on TiO2 was determined to have two terminal
oxo groups and in total five oxygen coordination by means ofvanadium K-edge XANES and ab initio calculations ofXANES spectra.

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The optimal catalytic performance of vanadium catalysts has
been reported to correspond to low concentrations of V
dispersed on TiO2.1–3 X-Ray absorption fine structure (XAFS)
directly determines the local structure of non-crystalline andheterogeneous materials. As V and Ti are neighboring in theperiodic table, a high concentration of Ti interferes with thedetection of relatively weak photon absorption of V and/or Xray fluorescence derived from V.4 In the case of the V site,which has one/two terminal oxo group(s), precise determinationof the coordination numbers of VNO and V–O is difficult fromthe curve fit analysis of EXAFS because the bond distances
differ by only 0.1–0.3 Å.5 In this communication, X-ray
absorption near-edge structure (XANES) was measured for low
concentrations of V on TiO2 by utilizing a high-energyresolution fluorescence spectrometer.
TiO2 (Aerosil P-25, BET surface area 60 m2 g21) wasimpregnated with vanadium triisoproxide oxide (1) in 2-propanol solution (V/TiO2). An aqueous solution of 1 and titaniumtetraisopropoxide was kept at 333 K for six days, following
which the filtered powder was heated at 453 K for ten days (VTiO2). In both procedures, the dried powder was heated in air at473 K.
XAFS spectra were measured at KEK-PF 7C. A Si(111)
monochromator was used. The X-ray fluorescence from thesample was analyzed using a Rowland-type spectrometer.6,7
The V Ka1 emission spectrum was measured and the fluorescence spectrometer was tuned to the peak. Next, the V K-edge
XANES was measured. Normalized V K-edge XANES spectra
for V/TiO2 and V-TiO2 are shown in Fig. 1a–c. The energy
values of the pre-edge peak, absorption edge, and post-edge
peaks are summarized in Table 1. These values were within therange of 0.3, 0.5, and 2 eV, respectively, suggesting that the Vsite structure was essentially identical whether supported on
TiO2 (anatase) or prepared by the sol–gel method, independentof the V loading ratio. The energy differences compared to thespectrum of V2O5 were 20.9 to 21.2, 1.8–2.3, and 2–9 eV,respectively. Thus, the V site in these samples was not an
epitaxially-grown V2O5(010) layer.3
These experimental spectra were compared to XANES data
theoretically generated by ab initio calculations. A code
FEFF8.2 was applied to model clusters A–D based on anatase
TiO28 (Fig. 2) up to fourth coordination (26–49 atoms) from theabsorbing V atom and also to a model of V2O5 (Table 2)9 up to
third coordination (39 atoms) both in self-consistent field andmultiple scattering calculation modes.10,11 The calculated
XANES spectra are shown in Fig. 1d–g. The spectrum pattern
CHEM. COMMUN., 2002, 2402–2403

significantly changed depending upon whether the V site had
two (Fig. 1d–f) or one (Fig. 1g) oxo group(s). In the case ofmodels which contained a single oxo group, the dominant single
post-edge peak energy gradually shifted from 5497 to 5491 eV
on going from total coordination number five (Fig. 1g) to three(model CA; Table 2), and then to six (model D; Table 2) (Table1). A pre-edge peak was not resolved from the absorption edge
for the calculated spectra of models CA and D (spectra notshown). As the pre-edge peak was well resolved and two broadpeaks (5488 ± 1 and 5502 ± 1 eV) of nearly equivalent intensity
were observed in the post-edge region for experimental spectra(Fig. 1a–c), single oxo models were rejected.
In the case of models which contained two terminal oxogroups, the post-edge peak position of XANES for model A offive coordination (Table 2) was most similar to Fig. 1a–c within
the range of 1–5 eV (Table 1) and the two peak intensity was
nearly equivalent (Fig. 1d). The XANES for four coordination
models B and C (Fig 1e and f) were similar to each other despite
the difference of the distance to fifth and sixth oxygen atoms(2.791 and 2.875 Å, respectively). Therefore, bonding distanceand coordination number of the first shell primarily determine
the V K-edge XANES pattern. The intensity of the second postedge peak at 5500–5501 eV was stronger than that of the firstpeak at 5487–5488 eV in Fig 1(e) and (f). A six coordination V
site which has two terminal oxo groups is not plausible.5
The typical errors in the estimate of density of state of V 3d(pre-edge peak) and Fermi level (absorption edge) by FEFF are
1 eV.10 In this work, energy values of pre-edge peak andabsorption edge for V2O5 were reproduced within the range of
0.2–0.6 eV (Table 1). The differences of these values between

Fig. 1 Normalized V K-edge XANES for V/TiO2, 3.0 (a) and 1.0 wt% (b)and V-TiO2, 2.0 wt% (c). V K-edge XANES generated by FEFF for model
A on anatase TiO2(001) (d), model B on (110) (e), model C on (011) (f), andmodel of V2O5 (g).

This journal is © The Royal Society of Chemistry 2002

Table 1 Energy values (eV) of the pre-edge peak, absorption edge, and post-edge peaks of V K-edge XANES for V/TiO2 and V-TiO2Sample V

wt%

Pre-edge peak

Absorption edge

Post-edge peaks

V/TiO2

3.0
1.0
2.0

5468.65468.35468.55469.55480.85481.15481.35479.0

5489
5487
5487
5485

5502
5502
5501
5493

5468.85469.55469.85471.2a5468.8a5468.9

5479.65479.95481.65483.65478.65478.85492
5487sh5488sh5496
5491
5488sh

5503
5501
5500

V-TiO2V2O5ModelA
BCCA
D
V2O5No. of oxogroups
222
1
1
1

5497

sh: unresolved shoulder peak.a Peak was not well resolved from the absorption edge.

Fig. 2 V site models supported on TiO2 (anatase) on (001) (A), (110) (B), (011) (C), and (010) face (D). Vanadate in models A–C and D has the coordinationof two and one terminal oxo group(s), respectively. Model C that has one terminal oxo group was also considered (CA). All the smaller balls are oxygen atoms
and all the larger balls without the notation are titanium atoms.

experimental data of Fig. 1a–c and calculated data for models
A–C (Fig. 1d–f), were in the range of 0.2–1.7 eV. Taking thetypical estimate errors of FEFF and data variation among the V/
TiO2 and V-TiO2 samples (0.2–0.5 eV, Table 1) into account,models A–C are acceptable with respect to the pre-edge andabsorption edge energies.
In summary, the calculated XANES spectrum for model A
reasonably reproduced the XANES for V/TiO2 and V-TiO2(Fig. 1a–c) in terms of the energy values of the pre-edge peak,absorption edge, and post-edge peaks and also the relative
intensity ratio of post-edge peaks.
The effects of the bonding distance of first coordination were
further evaluated for model A. The distance between V and two
terminal oxo groups was elongated on going from 1.620 (Table2) to 1.650, and then to 1.680 Å. The post-edge peak at 5492 eVgrew stronger whereas the peak at 5503 eV remained unchanged. The R factor (∫¡kexp 2 kfit¡2dE/∫ kexp 2dE) value of thefit to Fig. 1a increased from 2.3 to 2.5, then to 2.9%. Then, theV–O distance of 1.922 Å was elongated to 1.937 Å (shorterbonding distance of Ti–O in anatase). The post-edge two peaks
merged into a single peak at 5494 eV. The R factor value of thefit to Fig. 1a increased from 2.3 to 3.5%. As the oxidation stateof V supported on TiO2 is +5, a longer VV–O distance than theTiIV–O distance is unrealistic. The V site model for V/TiO2 wasproposed1,2 based on the V site structure of brannerite-type
Mn12xV222xMo2xO6 (x < 0.36).12 The V–O distances are
1.661, 1.693, 1.913, 1.939 and 2.151 Å. The VNO distance inmodel A may be shortened because two terminal oxo groups are
Table 2 Values of V–O distances and ONVNO angles used for XANES
calculations by FEFF8.2

ABCCA
DV2O5V–O distance/Å

ONVNO angle/deg

1.620 3 2, 1.922 3 2, 2.151
1.620 3 2, 1.922 3 21.620 3 2, 1.922 3 21.620, 1.922 3 21.800, 1.937 3 3, 1.964 3 2
1.54, 1.77, 1.88 3 2, 2.02

105.8
105.8
105.8
—
—
—

exposed to the surface. EXAFS analysis gave an average V–O
distance of 1.74 Å in the case of a sample prepared using asimilar procedure to V-TiO2, which is consistent with theaverage values of 1.620 and 1.922 Å obtained by XANES or of
1.65 and 1.90 Å given in ref. 2.
The above calculated data considered only dipole electronictransition from V 1s to 3d. Calculations which consider both
dipole and quadrupole transitions were also performed for eachmodel. The pre-edge peak energy position was unchanged andthe intensity increased only by 0–2.3%.
The models which contain a single oxo group e.g. epitaxial
V2O5(010) layer were found to be not plausible both in the casesof V/TiO2 and V-TiO2. The possibility of hydrated monomericmonovanadate or 2-dimensional polyvanadate cannot be excluded compared to Raman studies13 dependent upon ambient
gas and calcination temperature.

Notes and references
1 J. Haber, A. Kozlowska and R. Kozlowski, J. Catal., 1986, 102,52–63.
2 R. Kozlowski, R. F. Pettifer and J. M. Thomas, J. Phys. Chem., 1983, 87,5176–5181 and references therein.
3 A. Vejux and P. J. Courtine, Solid State Chem., 1978, 23, 93–103.
4 Y. Izumi, F. Kiyotaki and Y. Seida, J. Phys. Chem. B, 2002, 106,1518–1520.
5 A. F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford,
5th edn., 1984.
6 Y. Izumi, H. Oyanagi and H. Nagamori, Bull. Chem. Soc. Jpn., 2000, 73,
2017–2023.
7 The Rowland radius was 180 mm and the crystal was Ge(331). KEK-PF
approval no. 2001G308.
8 R. W. G. Wyckoff, Crystal Structure, John Wiley & Sons, New York,
1964, vol. 1, p.253.
9 R. W. G. Wyckoff, Crystal Structure, John Wiley & Sons, New York,
1964, vol. 2, p.184.
10 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, Phys. Rev.
B, 1998, 58, 7565–7576.
11 Y. Izumi, T. Shimizu, T. Kobayashi and K. Aika, Chem. Commun.,
2000, 1053–1054.
12 R. Kozlowski and K. Stadnicka, J. Solid State Chem., 1981, 39,271–276.
13 G. Deo and I. E. Wachs, J. Phys. Chem., 1991, 95, 5889–5895.
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