Journal of Physics and Chemistry of Solids 64 (2003) 833–839
www.elsevier.com/locate/jpcs
Structural characterization of the V2O5/TiO2 system obtained
by the sol –gel method
Cristiane B. Rodella, Valmor R. Mastelaro*
Departamento de Fı́sica e Ciência dos Materiais, Instituto de Fı́sica de São Carlos, Universidade de São Paulo,
C.P. 369, São Carlos, SP, 13560-970, Brazil
Received 9 May 2002; accepted 10 September 2002
Abstract
The influence of the vanadium load and calcination temperature on the structural characteristics of the V2O5/TiO2 system
was studied by X-ray diffraction and X-ray absorption spectroscopy (XAS) techniques. Samples of the V2O5/TiO2 system were
prepared by the sol – gel method under acid conditions and calcined at different temperatures. The rutile phase was found to
predominate in pure TiO2 calcined at 450 8C as a result of the reduction of phase transition temperature promoted by the sol– gel
method under acid conditions. The anatase phase became predominant at 450 8C as the amount of vanadium increased from 6 to
9 wt%. A structural change in the TiO2 phase from predominantly anatase to totally rutile with increased calcination
temperature was observed in 6 wt% samples. An analysis of the vanadium X-ray Absorption Near Edge Structure (XANES)
spectra showed that the oxidation state of vanadium atoms in the samples containing 6 and 9 wt% of vanadium and calcined at
450 8C was predominantly V4þ. However, the presence of V5þ atoms cannot be ruled out. A qualitative analysis of extended Xray absorption fine structure (EXAFS) spectra of the samples containing 6 and 9 wt% of vanadium calcined at 450 8C showed
that the local structure around vanadium atoms is comparable to that of VO2 crystalline phase, in which vanadium atoms are
fourfold coordinated in a distorted structure. For the sample after calcination at 600 8C, the EXAFS and XANES results showed
that a significant portion of vanadium atoms were incorporated in the rutile lattice with a VxTi(12x )O2 solid solution formation.
The conditions of sample preparation used here to prepare V2O5/TiO2 samples associated with different amounts of vanadium
and calcination temperatures proved to be useful to modifying the structure of the V2O5/TiO2 system.
q 2002 Elsevier Science Ltd. All rights reserved.
Keywords: V2O5/TiO2 system; Structure; X-ray diffraction; X-ray absorption spectroscopy
1. Introduction
Vanadium supported on titania is commonly used as a
catalyst for a number of industrially important reactions,
including the selective oxidation reactions of o-xylene
[1 –4], ammoxidation of hydrocarbons [2 –5], as well as
selective reduction of NOx with NH3 in the presence of O2
[6 –9]. From a scientific viewpoint, this catalytic system is
an interesting example of a strong interaction between the
support (TiO2) and the active phase (V2O5). In particular,
* Corresponding author. Tel.: þ55-16-273-9755; fax: þ 55-16273-9824.
E-mail address: [email protected] (V.R. Mastelaro).
the spreading of vanadium over the TiO2 support leads to a
modification of the chemical – physical peculiarities of
the former and to an enhancement of its catalytic properties
[1,10– 12].
The structural characteristics and the activity and
selectivity catalytic of vanadium/titania systems depends
mainly on the method and conditions of preparation, such as
crystallographic titania structures, vanadium load and
calcination temperature [1].
Several procedures have been proposed to prepare
V2O5/TiO2 catalytic systems: impregnation of V2O5
on TiO2 aerogels [8,9], co-geling of vanadia and titania
[13,14], and preparation of aerogels by two-step procedure [15]. In all these studies, V2O5 could be
0022-3697/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 4 1 4 - 6
834
C.B. Rodella, V.R. Mastelaro / Journal of Physics and Chemistry of Solids 64 (2003) 833–839
immobilized on pre-formed anatase or V2O5 and TiO2
oxides were allowed to gel by a procedure that favors the
anatase formation.
Although the synthesis of V2O5/TiO2 systems has been
extensively studied, the role of the active phase in the
modification of the support’s structural properties and,
hence in the active phase itself, has received little attention.
The present work focuses on the study of the structural
properties of the V2O5/TiO2 catalytic system prepared by
the sol– gel method.
The literature contains some controversies concerning
the role of the TiO2 crystalline phase in determining the
characteristics of the active phase. It is generally agreed that
TiO2 as anatase phase gives rise to superior active catalysts
compared to TiO2 as rutile phase [8,9,11,12,16– 18]. This
improvement in catylitic properties has been associated with
different surfaces species of vanadium that are stabilized on
the TiO2 support: the V4þ species is found as an isolated
monomeric group on TiO2 anatase in a distorted octahedral
symmetry, while the V5þ species is found on TiO2 rutile in
the form of polymeric vanadates in a distorted pyramidal
symmetry [11].
It is well known that the transformation of part of anatase
into rutile worsens catalytic performance owing to a
decrease in surface area, a partial reduction of V2O5, and
the incorporation of some V4þ into rutile structure in the
form of VxTi(12x )O2 (rutile solid solution) [1,10,11].
According to the literature, the presence of vanadium
can promote a significant decrease in the threshold
temperature [1]. Busca et al. [19] and Balikdjian et al.
[20] have studied vanadium supported on titania in
anatase phase, corresponding to a V/Ti molar ratio of up
to 0.06. Despite the lower V/Ti ratio and the presence of
bulk V species, these samples were found to be preserved
mainly in their anatase phase after calcinations at 600
and 700 8C [19,20].
In this paper we studied the influence of the vanadium
loading and calcination temperature on the structural
characteristics of the V2O5/TiO2 system using X-ray
diffraction (XRD), X-ray absorption near edge structure
(XANES) and extended X-ray absorption fine structure
(EXAFS) techniques.
2. Experimental procedures
TiO2/V2O5 samples with different weight contents of
V2O5 (0, 6, and 9 wt%) were synthesized using the sol–
gel method. A solution (named A) was prepared by
dissolving the appropriate amount of ammonium metavanadate (Carlo Erba 99.5%) in an aqueous solution (pH
1.0) of nitric acid (Merck p.a.) and subjecting it to
ultrasonic vibration at 75 W for 1 min. The amount of
ammonium metavanadate was calculated to obtain
samples containing 6 and 9 wt% of V2O5 supported in
95 and 91 wt% of TiO2, respectively. Another solution
(called B) was prepared by diluting tetraisopropyl
orthotitanate (Fluka 99.9%) in isopropyl alcohol (Merck
99.7%) with a molar ratio of 0.25:1.0. Solution (A) was
then added to solution (B) for the hydrolysis of the
tetraisopropyl orthotitanate by water. The amount of
water in solution A was calculated for the complete
hydrolysis of the tetraisopropyl orthotitanate (solution B)
in a molar ratio of 1:10 alkoxide and water. This cause
the immediate formation of a gel (pH 1.0), which was
vigorously stirred for 5 min under ultrasonic vibration at
75 W and then allowed to rest at room temperature. For
the TiO2 sample (0 wt% of V2O5), the solution A,
composed only of an aqueous solution of acid nitric, was
then added to solution B.
X-ray powder diffraction patterns were obtained using
an automatic Rigaku Rotaflex diffractometer model RU
200B with Cu Ka radiation (50 kV/100 mA, 1.5405 Å)
selected by a graphite monochromator.
Ti and V K-edge X-ray absorption spectra were taken
at the LNLS (Laboratório Nacional de Luz Sincrotron,
Campinas, Brazil) storage ring, using the X-ray absorption spectroscopy (XAS) beam line facility [21]. The
storage ring was operated at 1.36 GeV and 100– 160 mA.
Data were collected at the Ti (4966 eV) and V (5465 eV)
K-edges in transmission mode, using a Si (111) channelcut monochromator. Ionization chambers were used to
detect the incident and transmitted flux. An energy step
equal to 0.4 eV was used to collect the data at the near
edge in the region of the absorption spectra, while a
1.0 eV step was used to collect the EXAFS data at the
vanadium’s K edge. The maximum of the pre-edge
structure in metallic titanium XANES spectra was used
in to calibrate the monochromator energy. To monitor the
energy calibration, XANES spectra of a metallic foil
were recorded simultaneously to the XANES spectra of
the samples, using a third ion chamber. Due to the low
critical energy of the machine, harmonic contamination
from the monochromator was negligible at the energy at
which the measurements were taken [22].
The vanadium K-edge EXAFS oscillation curves were
analyzed by a standard procedure [23], normalized,
background removed, and Fourier transformed using the
available programs for Macintosh computers [24]. To
allow for a direct comparison between the various data
sets we collected, the EXAFS spectra were K3 weighted,
a Kaiser apodization function being applied for unknown
and reference spectra over similar K ranges (2.8 –
11.7 Å21) Crystalline samples were taken as structural
references: TiO2 (anatase and rutile phases), V2O5 and
VO2 compounds.
For purposes of comparison between different samples,
all the XANES spectra were background removed and
normalized, using as unity the inflexion point of the first
EXAFS oscillation.
C.B. Rodella, V.R. Mastelaro / Journal of Physics and Chemistry of Solids 64 (2003) 833–839
835
Fig. 1 shows the typical XRD peaks attributed to the
anatase, rutile, and brookite phases for the samples obtained
by the sol – gel method and calcined at 450 8C. Peaks
associated with V2O5 crystalline phase were not observed in
all the samples. The XRD results indicated a predominance
of the rutile phase in pure TiO2 calcined at a lower
temperature (450 8C).
According to the literature, the transformation of
anatase into rutile is considered an efficient process for
TiO2 samples calcined at temperatures above 700 8C [25].
Balikdjian et al. [20] have studied the thermal behavior
of the V/Ti system with different vanadium loading
obtained by the sol– gel method, but under different
conditions of preparation. They found that, in the V-free
titania sample, the anatase-rutile phase transformation
began in samples calcined at around 700 8C [20]. In the
vanadium free sample we prepared by the sol – gel
method, we observed the predominance of the rutile
phase after a calcination at 450 8C. We attribute this
decrease in the phase transition temperature of anatase to
rutile that was observed principally owing to the
conditions of sample preparation. In acidic preparations,
hydrolysis of the titanium alkoxide is faster than
condensation, producing samples containing many
hydroxyl radicals. The thermal treatment of these samples
induces dehydroxylation and forms VacþO22 anionic
vacancies [26]. The high concentration of defects in
anatase crystals thus contributed to the lower phase
transformation temperature of anatase into rutile. In short,
our first results indicate that a TiO2 rutile phase can be
obtained at a very low temperature in a pure TiO2
sample.
The effect of the vanadium load on the pure TiO2 support
is also shown in Fig. 1: a substantial increase in the intensity
of the anatase peak can be observed in the samples
containing 6 and 9 wt% of vanadium after calcinations at
450 8C. We observed that the anatase phase became
predominant with the highest vanadium load (9 wt%),
whereas with a lower load (6 wt%), titania predominated
in the rutile phase. A shift of the anatase-rutile transformation to higher temperatures through the addition of
vanadium to titania was also observed by Balikdjian et al.
[20]. They found the presence of only the anatase form at a
lower V/Ti molar ratio (0.02 and 0.06) in samples calcined
at 500 8C. In the work reported on herein, we analyzed
samples with a higher V/Ti molar ratio, i.e. 6 and 9 wt% of
V2O5, corresponding, respectively, to a molar ratio of
approximately 2.7 and 4.2 after calcination at 450 8C. Our
results are similar to those obtained by Balikdjian et al. [20]:
for the same calcination temperature, the amount of anatase
phase was highest in the sample containing the highest V/Ti
ratio.
A structural change in the TiO2 phase was clearly visible
with the increase in calcination temperature (Fig. 2). As can
be observed, both the anatase and the rutile phase were
present at 350 8C. As the temperature rose, the amount of
anatase phase decreased and, at 600 8C, the titania was
totally transformed into the rutile phase.
In summary, comparing our results with those published
by Balikdjian et al. [20], we found that the negative or
positive influence of vanadium on the structure of titania is
far more closely related to the conditions of preparation (pH
and temperature) of the V2O5/TiO2 samples than the
vanadium/titanium ratio.
Fig. 1. XRD patterns of pure TiO2 and V2O5/TiO2 samples with
different contents of vanadium.
Fig. 2. XRD patterns of 6 wt% V2O5/TiO2 heat-treated at different
calcination temperatures.
3. Results and discussion
3.1. X-ray diffraction results
836
C.B. Rodella, V.R. Mastelaro / Journal of Physics and Chemistry of Solids 64 (2003) 833–839
3.2. XANES results
Fig. 3 presents the XANES spectra of V2O5/TiO2
catalyst samples with different vanadium contents, calcined
at 450 8C. As can be seen, the introduction of V2O5 caused
some changes in the Ti pre-edge and post-edge features. The
pre-edge features located at approximately 4970 eV are
attributed to transitions from 1 s energy levels of Ti to the
Ti3d/O2p molecular orbital [27]. A 1s ! 3d transition is
Laporte forbidden by dipole selection rules, but admissible
when p – d orbital mixing occurs, such as that located in a
TiO4 tetrahedron or a ([5]TiO)O4 site (i.e. without a center of
symmetry). The height and position of this pre-edge feature
are direct functions of the degree of p– d mixing, site
distortion, oxidation state and experimental resolution. We
observed that the Ti XANES spectra of V2O5/TiO2 catalysts
could be well reproduced by a linear combination of TiO2
anatase and rutile XANES spectra. The results shown in
Table 1 are in good agreement with those obtained by XRD
measurements.
Fig. 4 presents the XANES spectra of catalysts heat
treated at different temperatures. No significant changes
were observed in the pre- or post-edge regions. In good
agreement with the XRD results, the XANES spectra of the
samples after calcinations at 350 and 450 8C resemble the
XANES spectra of the anatase phase, whereas the XANES
spectra of the sample after calcinations at 600 8C resemble
the XANES spectra of the rutile phase.
Fig. 5 shows the XANES spectra of the 6 and 9 wt%
V2O5/TiO2 samples, V2O5 and VO2 reference samples
obtained at the vanadium K edge. These XANES spectra
exhibit a pre-edge absorption feature, named A, that was
assigned as the 1s ! 4p dipole-allowed transition [28]. The
intensity of the pre-edge peak and edge energy position were
found to be related, respectively, with the symmetry and
Fig. 3. XANES spectra at the titanium K-edge of V2O5/TiO2
samples with different amounts of vanadium compared to TiO2
crystalline phases.
Table 1
Percentage of rutile and anatase phases obtained from Ti XANES
analysis for the samples after calcinations at 450 8C
Samples
Rutile phase
(%)a
(^5%)
Anatase phase
(%)a
(^5%)
0% V2O5/TiO2
6% V2O5/TiO2
9% V2O5/TiO2
100
50
10
0
50
90
a
Error estimated in ^5%.
oxidation state of V in the oxides [28]. In V2O5, the
oxidation state of vanadium is V5þ and vanadium is fivefold
coordinated in a distorted tetragonal pyramid of oxygen.
The apex-oxygen distance is only 1.585 Å, whereas the
basal V – O distances vary from 1.78 to 2.02 Å. In VO2, the
oxidation state of vanadium is V4þ and vanadium atoms are
fourfold coordinated in a distorted structure of two
vanadium sites: a V1 site with distances ranging from 1.66
to 2.18 Å and a V2 site with distances ranging from 1.82 to
2.15 Å [29].
As shown in Fig. 5, the pre-edge feature in all the
samples is located around 5470 eV. Although the
intensity of this pre-edge is lower in the samples
analyzed here, an analysis of the position of the preedge feature does not provide information about the
oxidation state of V2O5/TiO2 samples. However, an
analysis of the post-edge features reveals an interesting
similarity between the V2O5/TiO2 and VO2 samples. As
the oxidation state of vanadium in the VO2 sample is
V4þ, one can state that in the samples analyzed, the
oxidation state of vanadium atoms is predominantly V4þ
Fig. 4. XANES spectra at the titanium K-edge of the 6 wt%
V2 O5 /TiO2 samples heat-treated at different calcination
temperatures.
C.B. Rodella, V.R. Mastelaro / Journal of Physics and Chemistry of Solids 64 (2003) 833–839
Fig. 5. XANES spectra at the vanadium K-edge of V2O5/TiO2
samples with different amounts of vanadium compared to VO2 and
V2O5 crystalline phases.
in a distorted octahedral symmetry. However, the
presence of V5þ atoms in our V2O5/TiO2 samples cannot
be ruled out.
Fig. 6 shows the XANES spectra at the vanadium K-edge
of the 6 wt% V2O5/TiO2 samples calcined at different
temperatures. Significant changes were observed in the
sample calcined at 600 8C. According to the literature, at
temperatures close to 600 8C, vanadium V4þ ions are
incorporated within the rutile structure [30]. To verify this
hypothesis, a comparison is made in Fig. 7 of the XANES
spectra obtained at the titanium and vanadium K-edges in
the 6% V2O5/TiO2 sample calcined at 600 8C. As can be
seen, the XANES spectra obtained at the vanadium and the
titanium K edges are similar, indicating that a substantial
amount of vanadium atoms are occupying the rutile lattice
substitutionally as V4þ ions.
Fig. 6. XANES spectra at the vanadium K-edge of the 6 wt%
V2 O5 /TiO2 samples heat-treated at different calcination
temperatures.
837
Fig. 7. XANES spectra at the vanadium and titanium K edges for the
6 wt% V2O5/TiO2 sample treated at 600 8C.
3.3. EXAFS results
This region of the X-ray absorption spectrum provides
information about the type, number and bond distances of
the absorber atom from its nearest neighbors [31]. However,
due to the complexity of the local structure in our samples,
we made only a qualitative comparison with VO2 and V2O5
crystalline samples, whose local structure is well known.
Fig. 8 shows the EXAFS spectra of 6 and 9 wt% V2O5/
TiO2 samples compared to those of the VO2 and V2O5
reference samples. The Fourier transform moduli of these
EXAFS spectra are presented in Fig. 9. The Fourier
transforms are incorrect for the phase shift parameters, so
that the maximum peak positions are shifted to values below
the real one. As can be observed in Fig. 8, the EXAFS
spectra of 6 and 9 wt% V2O5/TiO2 samples are basically the
same. A complex short-range order structure is observed for
Fig. 8. EXAFS spectra obtained at the vanadium K-edge of
V2O5/TiO2 samples with different amounts of vanadium compared
to VO2 and V2O5 crystalline phases.
838
C.B. Rodella, V.R. Mastelaro / Journal of Physics and Chemistry of Solids 64 (2003) 833–839
Fig. 9. Fourier transform of EXAFS spectra shown in Fig. 8.
Fig. 11. Fourier transform spectra of V2O5/TiO2 samples heattreated at different calcination temperatures.
the two catalyst samples (Fig. 9). When the amount of
vanadium increased from 6 to 9%, we observed that the
intensity of Fourier transform first peak increases and the
second peak (located at around 1.5 Å) becomes well
defined. Comparing the data of the 6 and 9 wt% V2O5/
TiO2 samples with that of the reference samples, we can
state that there is a certain similarity with the data from the
VO2 sample, in agreement with the XANES analysis
presented previously. It can also be observed that, at high
K values, the spectra of the analyzed samples have a poor
structure, indicating a higher degree of disorder. Although
quantitative structural data is not given here, the essential
agreement between the experimentally measured spectrum
and the VO2 reference sample indicates that the characteristic EXAFS spectrum indeed reflects the characteristic
local structure around vanadium atoms.
Fig. 10 presents the EXAFS spectra of 6 wt% V2O5/TiO2
samples after calcination at different temperatures, while
Fig. 11 illustrates the Fourier transform moduli of these
EXAFS spectra. The EXAFS and Fourier transform spectra
of the sample after calcinations at 350 8C are similar to those
of the sample after calcinations at 450 8C. In agreement with
what was observed in the XANES spectra, significant
changes were observed in the sample after calcination at
600 8C. Fig. 10 also shows the Ti K-edge EXAFS spectra of
TiO2 rutile phase. As can be observed, the EXAFS spectra
of the 6 wt% V2O5/TiO2 sample after calcination at 650 8C
is comparable to the EXAFS spectra of TiO2 rutile phase,
thus corroborating the results obtained from the XANES
spectra regarding the incorporation of vanadium atoms in
the rutile lattice.
Fig. 10. EXAFS spectra obtained at the vanadium K-edge of
V2O5/TiO2 samples heat-treated at different calcination temperatures compared to the EXAFS spectra of TiO2 rutile phase obtained
at the titanium K edge.
4. Conclusions
The rutile phase was predominant in the pure TiO2
sample after calcination at 450 8C as a result of the decrease
in phase transition temperature promoted by the use of the
sol– gel method under acid conditions. The addition of
vanadium to the TiO2 caused the phase transition temperature to increase and the percentage of anatase phase was
found to be higher in the sample containing 9 wt% of
vanadium. Our analysis of the vanadium XANES spectra
revealed that, in the samples containing 6 and 9 wt% of
vanadium and calcined at 450 8C, the oxidation state of
vanadium atoms was predominantly V4þ. However, the
presence of V5þ atoms in V2O5/TiO2 samples could not be
ruled out. The qualitative analysis of EXAFS spectra of the
samples with 6 and 9 wt% of vanadium calcined at 450 8C
showed that the local structure around vanadium atoms was
comparable to that of VO2 crystalline phase, in which
vanadium atoms are fourfold coordinated in a distorted
structure. For the 6 wt% V2O5/TiO2 sample after calcination
at 600 8C, our XANES and EXAFS results corroborated
earlier results published in the literature to the effect that, at
C.B. Rodella, V.R. Mastelaro / Journal of Physics and Chemistry of Solids 64 (2003) 833–839
temperatures close to 600 8C, a significant amount of
vanadium atoms are incorporated into the rutile lattice.
Finally, we concluded that the conditions of sample
preparation used here to prepare our V2O5/TiO2 samples
associated with different amounts of vanadium and calcination temperatures proved to be useful to modifying the
structure of the V2O5/TiO2 system.
Acknowledgements
The authors gratefully acknowledge the financial support
of the Brazilian research funding agencies FAPESP and
CNPq. This research work was partially performed at the
LNLS-National Synchrotron Light Laboratory, Campinas,
SP, Brazil.
References
[1] B. Grzybowska-Swierkosz, Appl. Catal. A 157 (1997) 263.
[2] K.V. Narayana, A. Venugopal, K.S. Rama Rao, S. Khaja
Masthan, V. Venkat Rao, P. Kanta Rao, Appl. Catal. A 167
(1998) 11.
[3] C.R. Dias, M.F. Portela, G.C. Bond, J. Catal. 157 (1995) 344.
[4] T. Mongkhonsi, L. Kershenbaun, Appl. Catal. A 170 (1998)
33.
[5] G. Centi, Appl. Catal. A 147 (1996) 267.
[6] M.D. Amiridis, I.E. Wachs, G. Deo, J.-M. Jehng, D.S. Kim,
J. Catal. 161 (1996) 247.
[7] L. Casagrand, L. Lietti, I. Nova, P. Forzatti, A. Baiber, Appl.
Catal. B 22 (1999) 63.
[8] O. Zegaoui, C. Hoang-Van, M. Karroua, Appl. Catal. B 9
(1996) 211.
[9] M.A. Reiche, E. Ortelli, A. Baiker, Appl. Catal. B: Environ. 23
(1999) 187.
[10] I.E. Wachs, B.M. Weckhuysen, Appl. Catal. A 157 (1997)
67–90.
[11] P. Courtine, E. Bordes, Appl. Catal. A 157 (1997) 45.
839
[12] G.C. Bond, Appl. Catal. A 157 (1997) 91.
[13] M.A. Cauqui, J.M. Rodrı́guez-Izquierdo, J. Non-Cryst. Solids
147 –148 (1992) 724.
[14] M. Schneider, M. Maciejewski, S. Tschudin, A. Wokaun, A.
Baiker, J. Catal. 149 (1994) 326.
[15] C. Hoang-Van, O. Zegaoui, P. Richat, J. Non-Cryst. Solids
225 (1998) 157.
[16] F. Cavani, G. Centi, E. Foresti, F. Trifirò, G. Busca, J. Chem.
Soc., Faraday Trans. 84 (1988) 237.
[17] B.M. Reddy, I. Ganash, E.P. Reddy, J. Phys. Chem. B 101
(1997) 1769.
[18] G. Centi, E. Giamello, D. Pinelli, F. Trifirò, J. Catal. 130
(1991) 220.
[19] G. Busca, P. Tittarelli, E. Tronconi, P. Forzatti, J. Solid State
Chem. 67 (1987) 91.
[20] J.P. Balikdjian, A. Davidson, S. Launay, H. Eckert, M. Che,
J. Phys. Chem. B 104 (2000) 8931–8939.
[21] H.C.N. Tolentino, A.Y. Ramos, M.C.M. Alves, R.A. Barrear,
E. Tamura, J.C. Cezar, N. Watanabe, J. Synchrotron Rad. 8
(2001) 1040–1046.
[22] A.Y. Ramos, H. Tolentino, R. Barrea, M.C.M. Alves.
Laboratório Nacional de Luz Sincrotron-LNLS, Internal
Report 02/99.22, 1999.
[23] D.E. Sayers, B.A. Bunker, in: D.C. Konisberger, R. Prins
(Eds.), X-Ray Absorption Techniques of EXAFS, SEXAFS
and XANES, Wiley, New York, 1988.
[24] A. Michalowicz, J. Phys. IV, France 7 (C2) (1992)
235–236.
[25] L. Lietti, P. Forzatti, G. Ramis, G. Busca, F. Bregani, Appl.
Catal. B 3 (1993) 13.
[26] B. Xia, H. Huang, Y. Xie, Mater. Sci. Engng B 57 (1999) 150.
[27] F. Farges, G.E. Brown Jr., J.J. Rehr, Geoch. Cosmoch. Acta 60
(16) (1996) 3023.
[28] Z.Y. Wu, G. Ouvrard, P. Gressier, C.R. Natoli, Phys. Rev. B
55 (1997) 10382.
[29] J.P. Nogier, A.M. Kerbsabiec, J. Fraissard, J. Appl. Catal. 185
(1999) 109.
[30] J. Wong, F.W. Lytle, R.P. Messmer, Phys. Rev. B 10 (1984)
5596–5609.
[31] J.J. Rehr, R.C. Alberts, Rev. Mod. Phys. 72 (3) (2000)
621 –629.