ammoxidation of propane over v-sb and v-sb-nb mixed oxides
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Applied Catalysis A: General 298 (2006) 1–7
Ammoxidation of propane over V-Sb and V-Sb-Nb mixed oxides
M.O. Guerrero-Perez 1, M.V. Martınez-Huerta 2, J.L.G. Fierro, M.A. Banares *
Instituto de Catalisis y Petroleoquimica, CSIC, Marie Curie 2, E-28049-Madrid, Spain
Received 15 July 2005; received in revised form 7 September 2005; accepted 15 September 2005
Available online 3 November 2005
Abstract
Pure VSbO4 phase is not active for propane ammoxidation, unless some segregated a-Sb2O4 is present on its surface. Surface vanadium may be
necessary to activate propane. For the ternary Sb-V-Nb oxide catalysts: Sb-V-O, Nb-V-O and Sb-Nb-O phases may form. SbNbO4 is inert and forms
at the expense of efficient Sb-V-O and V-Nb-O phases. The extent of formation of these phases strongly depends on the preparation method. The Sb
precursor determines if V-Sb or Nb-Sb is the preferred interaction. Tuning the interaction of Sb with the other elements affects the catalyst structure
and performance.
# 2005 Elsevier B.V. All rights reserved.
Keywords: V-Sb-Nb-O; V-Sb-O; Oxidation; Ammoxidation; Propane; Acrylonitrile; Structure–activity relationship; In situ Raman; XRD
1. Introduction
Acrylonitrile has been industrially produced from propylene
as an important intermediate for the preparation of fibers,
important polymers as styrene–acrylonitrile, acrylonitrile–
butadiene–styrene, and other valuables chemicals [1]. The
direct conversion of propane into acrylonitrile by reaction with
oxygen and ammonia is an alternative route to the conventional
propylene ammoxidation since propylene is typically more
expensive than propane. The economic implications of this new
route are very important, but the activation of propane is the
limiting step [2]. Since the adsorption rate of propane is near ten
times smaller than that of propylene [3] the conversion of
propane is at least ten times smaller than propylene [4]. In
addition, the reaction conditions to activate the C–H bond in
propane are more energy demanding, which has a negative
effect on selectivity.
There are several studies about the catalysts used for
propane ammoxidation, but most of the works concentrate on
two types of catalysts [1,5]: the antimonates with rutile
* Corresponding author. Tel.: +34 91 585 4788; fax: +34 91 585 4760.
E-mail address: banares@icp.csic.es (M.A. Banares).1 Actual address: Departamento de Ingenierıa Quımica, Universidad de
Malaga, Campus de Teatinos, E-29071-Malaga, Spain.2 Actual address: Departamento de Quımica Fısica, Universidad de La
Laguna, c/Astrofısico Francisco Sanchez s/n, E-38071 La Laguna, Tenerife,
Spain.
0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2005.09.013
structure [6,7] and the molybdates [1–18]. Both catalyst
systems usually incorporate vanadium as an essential ingre-
dient. Mo-V catalysts modified with Nb and Te may afford near
50% yield to acrylonitrile [2] and acrylonitrile yields of 60%
have been reported for the antimonates system [9]. Despite the
large relevance of these catalysts, the nature of the active site is
still not fully understood. Sb-V-O catalysts with an excess of V
are highly active and selective for propane oxidative
dehydrogenation (ODH) while an excess of Sb affords Sb-V-
O catalysts that are more efficient for propane ammoxidation
[19].
Several preparation methods have been reported for the Sb-V-
O-based catalysts and it is concluded that the preparation method
has an important effect on the structure and on the performance
[12,20,21]. The methods most usually studied correspond to
patents filed in 1988 by Standard Oil Co. [9–11] and it consists on
keeping in reflux during several hours an aqueous suspension of
NH4VO3 or V2O5 and Sb2O3. Others studies use solutions of V4+
in oxalic acid to which antimonic acid is added [22] or even
prepare catalysts with a solution of VCl3 and SbCl5 on HCI
[8,23]. Brazdil et al. have also developed a sol–gel method for
this kind of catalysts [21]. The aim of all these preparation
methods is to provide an intimate contact between Sb5+ and V4+
in order to improve their redox reaction to form VSbO4 phase
[24], which is directly related to acrylonitrile formation
[2,13,19–24], and provides the catalyst with a high specific
area. More recently, mechanical activation has also proved to be
an interesting method to prepare VSbO4 phases [25].
M.O. Guerrero-Perez et al. / Applied Catalysis A: General 298 (2006) 1–72
Grasselli [1,5] reports on ternary mixed metal oxide
catalysts with the formula VSbxMyOz, where M stands for
W [13,26], Te [9,13], Ni [13,27], Mg [13], Nb [13], Sn [28,29],
Bi [30], Al [26,32,33], Fe [34], and Ti [35], among others.
Usually the metal M forms a solid solution with the rutile
structure of VSbO4 forming mixed phases with Sb and V, like
Al1�xSbVxOx [32] or Sb0.9V0.9�xWxO4 [26] or V-Sb-Ti-O [58].
Niobium compounds and materials such as niobium pentoxide,
niobic acid, niobium phosphate, and mixed oxides containing
niobia are shown to be useful on catalysis as supports and as
promoters for different reactions [37–41]. Specially, catalysts
containing niobium have been found active in dehydrogenation
reactions [42–44] and the dehydrogenation of propane is
commonly proposed as the first step in propane ammoxidation
[23,45–47], so Nb seems to be a key element on propane
ammoxidation to acrylonitrile, specially mixed oxides contain-
ing Nb (Mo-V-Nb-Te catalysts) have been found active and
selective propane ammoxidation [13,48–50].
A third element may also be present as the cation of an oxide
support. The use of an oxide to support/promote VSbO4 is
particularly interesting, for instance, alumina-supported VSbO4
[31]; it performs like bulk VSbO4 with a more economic
composition [51]. Other supports, like niobia, significantly
promote the performance of VSbO4 [52]. These catalysts are
prepared by impregnation of the oxide support. It would be
desirable to assess direct synthesis of efficient V-Sb-Nb oxide
catalysts for propane ammoxidation. However, the use of
niobia, vanadia and antimony supports may lead to efficient
Nb-promoted Sb-V-O phases or to inert SbNbO4 phases. This
manuscript evaluates the relevance of different precursors on
the preparation of V-Sb-Nb oxide catalysts.
2. Experimental
Bulk Sb-V-O catalysts (SV series) were prepared drying in a
rotatory evaporator at 80 8C NH4VO3 (Sigma) and antimony that
was kept in solution according to a method developed in our
laboratory [49]. The resulting solid was dried at 115 8C for 24 h
and then calcined in air at 660 8C for 12 h and at 750 8C for 24 h.
Four catalysts were prepared SV1, SV3, SV5, and SV7 with Sb/V
molar ratios of 1, 3, 5, and 7, respectively.
Table 1
Precursors used during synthesis, composition and BET area of catalysts
Catalysts Compositiona BET surface area (m2 g�
Sb-V-O catalysts
SV1 VSb 0.6
SV3 VSb3 0.5
SV5 VSb5 0.8
SV7 VSb7 0.6
Sb-V-Nb-O catalysts
SVN1A V0.1Sb0.3Nb0.3 72
SVN1B V0.1Sb0.3Nb0.1 19
SVN2 V0.2Sb0.3Nb0.5 8
SVN3 V0.2Sb0.1Nb0.4 12
a Based on the ICP element analyses.
Sb-V-Nb-O catalysts (SVN series) were prepared with three
different methods. The Method 1 uses an aqueous solution with
SbCb, NH4VO3, and Nb2(C2O4)5 and two catalysts were
prepared SVN1A and SVN1B (Table 1), with different molar
ratios. In this method a solution of Nb2(C2O4)5 [50] was added
to an aqueous solution of an appropriate amount of NH4VO3
and SbCl3 at 60 8C. The solution was kept under stirring for 2 h
at this temperature, and then was separated by centrifugation.
The samples were dried at 120 8C for 12 h and calcined in air at
400 8C for 5 h. The Method 2 uses an organic solution and was
used for the preparation of SVN2 catalyst, a suspension of
Nb2(C2O4)5 in CH2Cl2 was added to a solution of SbCl5 and
(C5O2H8)2VO in CH2Cl2. The solution was stirred at 60 8C for
2 h and the solvent was evaporated. The catalyst powder was
dried at 120 8C for 16 h and calcined in air at 500 8C for 5 h.
Method 3 uses the same molar ratio and methodology than 2,
but Nb2O5 is used as the precursor of Nb, catalyst SVN3 was
prepared according this method.
Raman spectra were run with a single-monochromator
Renishaw System 1000 equipped with a thermoelectrically
cooled CCD detector (�73 8C) and holographic super-Notch
filter. The holographic Notch filter rejects the elastic scattering.
The samples were excited with the 514 nm Ar line; spectral
resolution was ca. 3 cm�1 and the spectrum acquisition time was
300 s. The spectra were obtained under dehydrated conditions
(ca. 390 K) in a hot stage (Linkam TS-1500). Raman spectro-
scopy provides information about the bulk of the material.
X-ray diffraction patterns were recorded on a Siemens
Krystalloflex D-500 diffractometer using Cu Ka radiation
(l = 0.15418 nm) and a graphite monochromator. Working
conditions were 40 kV, 30 mA, and scanning rate of 28 min�1
for Bragg’s angles (2u) from 5 to 708. Nitrogen adsorption
isotherms (�196 8C) were recorded on an automatic Micro-
meritics ASAP-2000 apparatus. Prior to the adsorption
experiments, samples were outgassed at 413 K for 2 h. BET
areas were computed from the adsorption isotherms (0.05 < P/
Po < 0.27), taking a value of 0.164 nm2 for the cross-section of
the adsorbed N2 molecule at �196 8C. ICP analyses were
performed in a 3300 Perkin-Elmer apparatus. Around 100 mg
of sample were dissolved with nitric, chloride, and fluoride
acids in a microware oven.
1) Method Precursors
– NH4VO3, Sb soluble complex
– NH4VO3, Sb soluble complex
– NH4VO3, Sb soluble complex
– NH4VO3, Sb soluble complex
1 SbCl3, NH4VO3, Nb2(C2O4)5
1 SbCl3, NH4VO3, Nb2(C2O4)5
2 SbCl5, (C5O2H8)2VO, Nb2(C2O4)
3 SbCl5, (C5O2H8)2VO, Nb2O5
5
M.O. Guerrero-Perez et al. / Applied Catalysis A: General 298 (2006) 1–7 3
Fig. 1. XRD patterns of fresh Sb-V-O catalysts.
Activity measurements were performed using a conven-
tional microreactor with on-line gas chromatograph equipped
with a flame ionization and thermal conductivity detector. The
correctness of the analytical determinations was checked for
each test by verification that the carbon balance (based on the
propane converted) was within the cumulative mean error of the
determinations (�10%). Tests were made using 0.2 g of sample
with particle dimensions in the 0.25–0.125 mm range. The axial
temperature profile was monitored by a thermocouple sliding
inside a tube inserted into the catalytic bed. Tests were made
using the following feedstock: 25% O2, 9.8% propane, 8.6%
ammonia, and helium balance. The total flow rate was
20 ml min�1 corresponding to a gas-space velocity (GHSV)
near 3000 h�1. The quantity of catalyst and total flow were
determined in order to avoid internal and external diffusion
contributions. Yield and selectivity values were determined on
the basis of the moles of propane feed and products, considering
the number of carbon atoms in each molecule.
3. Results
3.1. Sb-V-O system
Table 1 represents the composition and BET area values for
the Sb-V-O catalysts. The BET area values are quite low for
these catalysts, probably due to the high calcination tempera-
ture. The XRD patterns of the fresh catalyst are displayed in
Fig. 1. SV1 pattern (Sb/V = 1) corresponds to the VSbO4 phase
(JCPDS file 16-0600), the patterns of SV3, SV5, and SV7
catalysts (Sb/V = 3, 5, and 7, respectively) correspond to
VSbO4 and a-Sb2O4 phases (JCPDS file 11-0694 and 32-0042).
The Raman spectra of the fresh and used catalyst are rep-
resented in Fig. 2. The spectrum of the fresh SV1 catalyst pre-
sents a band centered at 880 cm�1, which has been assigned to the
Sb0.92V0.92O4 phase [19]. Theweak Raman band near 1020 cm�1
is not sensitive to humidity, so this band must correspond to
Fig. 2. (A) Raman spectra of fresh and used Sb-V-O catalysts. (B) Sp
the VSbO4 phase and not to surface vanadium oxide species.
However, this band is quite broad, and it is not possible to provide
a conclusive absence of surface vanadium oxide species.
The Raman spectra of catalysts with antimony excess (SV3,
SV5, and SV7) present modes at 190, 255, 372, 451, and
716 cm�1 that correspond to the a-Sb2O4 phase [51], these
bands are very strong while VSbO4 possesses a low Raman
section; therefore, Fig. 2B blows-up the 530–1030 cm�1
window. The weak bands at 610, 650, 710, 749, and
820 cm�1 that correspond to the a-Sb2O4 phase are evident.
Raman spectra of all catalysts with antimony excess show the
bands that correspond to the a-Sb2O4 phase. The SV1, SV3,
and SV5 catalysts exhibit the bands of VSbO4 phase; these
bands are particularly weak for catalyst SV7. The Raman
spectra show no appreciable changes for the used catalysts.
These catalysts exhibit a steady-state operation for at least
20 h. Fig. 3 represents the yields to different products for
ectra between 530 and 1030 cm�1 with a-Sb2O4 reference phase.
M.O. Guerrero-Perez et al. / Applied Catalysis A: General 298 (2006) 1–74
Fig. 3. Yields (%) to different products at 500 8C for catalysts SV1, SV3, SV5,
and SV7. Reaction conditions: 20 h time-on-stream, total flow 20 ml min�1,
feed composition (vol.%): C3H8/O2/NH3 (9.8:25:8.6), 200 mg of catalysts.
Fig. 5. Raman spectra of fresh and used Sb-V-Nb-O catalysts.
catalysts SV1, SV3, SV5, and SV7 at 500 8C. The catalyst with
the highest vanadium content (SV1, with Sb/V = 1) is the most
active and presents the highest yield to acrylonitrile; however,
selectivity to acrylonitrile is rather low. Sb excess leads to less
active, but more acrylonitrile selective, catalysts. The catalysts
SV5 is most selective towards acrylonitrile.
3.2. Sb-V-Nb-O system
XRD patterns of fresh and used Sb-V-Nb-O catalysts are
represented in Fig. 4. The catalyst SVN2 presents a diffraction
pattern that corresponds to the phases Nb2O5 (JCPDS 30-0873),
a-Sb2O4 (JCPDS 11-0694), and VSbO4 (JCPDS file 16-0600).
After reaction, the diffraction peaks of VSbO4 become more
intense. Catalyst SVN3 exhibits a pattern quite similar to that of
SVN2, it presents the pattern of Nb2O5 (JCPDS file 7-0061),
while after reaction it presents those of Nb2O5 and a-Sb2O4.
SVN1A is amorphous but it exhibits the pattern of SbNbO4
phase after reaction (Fig. 4) (JCPDS 16-0907). Fresh and used
SVN1B catalyst present the pattern of SbNbO4 and a-Sb2O4
phases, and the diffraction peaks of SbNbO4 become more
intense for the used sample.
Fig. 4. XRD patterns of fresh and used Sb-V-Nb-O catalysts.
Fig. 5 shows the Raman spectra of fresh and used dehydrated
Sb-V-Nb-O catalysts. The Raman spectra of fresh and used
SVN2 and SVN3 are very similar with broad bands near 900,
700, and 200 cm�1, typical of Nb2O5 TT crystalline phase
[52,53]. The Raman spectrum of the fresh SVN1A catalyst
(Fig. 5) shows broad Raman bands that correspond to Nb2O5
phase but after reaction it exhibits Raman bands at 190, 255,
372, and 451 cm�1, typical of a-Sb2O4 phase [51] and features
near 845, 686, and 621 cm�1 that correspond to SbNbO4 phase
[52,53]. Spectra of fresh and used SVN1B catalyst show Raman
bands that correspond to a-Sb2O4 and SbNbO4 phases. Mixed
V-Nb-O phases, if present, could not be observed. These
possess weak Raman bands [54].
Fig. 6. Yields (%) to different products at 500 8C for catalysts SVN2, SVN1A,
SVN1B, and SVN3. Reaction conditions: total flow 20 ml min�1, feed com-
position (vol.%): C3H8/O2/NH3 (9.8:25:8.6), 200 mg of catalysts.
M.O. Guerrero-Perez et al. / Applied Catalysis A: General 298 (2006) 1–7 5
Fig. 6 represents the yields to the different products for
catalysts SVN2, SVN1A, SVN1B, and SVN3, at 500 8C.
Catalyst SVN3 is much more active than SVN2, SVN1A, and
SVN1B, but at 500 8C it is very selective towards propylene
while catalysts SVN2 is the most selective towards acrylonitrile.
4. Discussion
Sb-V-based oxide structures are directly related to propane
ammoxidation reaction. In particular, VSbO4 appears closely
related to efficient catalysts [55]. In excess of Sb, the VSbO4/a-
Sb2O4 system is more selective to acrylonitrile formation. SV1
is active (Fig. 3), to the production of carbon oxides, propylene
and acrylonitrile; the higher activity at lower Sb/Vatomic ratios
is probably related to the higher vanadium content [55–57].
Bulk Sb-V-O samples with Sb/V > 1 are more selective to the
formation of acrylonitrile [1,2,19,55]. Our results are consistent
with those works: samples possessing VSbO4 and a-Sb2O4
phases, like SV3, SV5, and SV7, are more selective to
acrylonitrile formation than the pure VSbO4 phase, as can be
observed in Fig. 3. The excess of Sb renders the Sb-V-O system
more selective; however, total conversion is much lower.
Surface vanadium oxide species, on the other hand, appear
critical to activate propane [49,58]. Surface vanadium oxide
species also activate conversion of propane towards oxidative
dehydrogenation reaction, even in the presence of Sb [58,59].
The effect of Sb/V atomic ratio is not clearly understood yet.
Nilsson et al. [19] have found that the best rate for formation of
acrylonitrile is achieved with catalysts with Sb/V = 2. Centi and
Mazzoli [8] reported catalysts with Sb/V > 1 as the most active
on acrylonitrile formation and Chiang and Lee [60] concluded
that the best activity results are achieved with catalysts with Sb/
V = 1. In general, mixed a-Sb2O4/VSbO4 phases are selective
to acrylonitrile formation; the results presented are in
accordance with this point. However, the origin of these
differences on the Sb/V values may be due to the fact that
surface composition does not usually correspond to the bulk
composition. Obviously, catalytic performance is directly
affected by the surface composition of the catalysts, where
the molecules react. There appears to be a delicate interaction
between segregated Sb oxide, surface vanadium oxide species,
and VSbO4 that renders the system efficient for propane
ammoxidation [49]. It may be due to a cooperative effect
between these phases, as it has been pointed out previously
[19,26]. Operando Raman-GC studies during propane ammox-
idation on supported Sb-V systems suggest that such
cooperation may be due to a migration cycle of Sb5+ species
that facilitate the redox cycle of vanadium species [49,58],
which involves surface V5+ species and lattice V3+ sites in
VSbO4 [49,55].
The interplay between Sb and V for propane ammoxidation
is strongly affected by the presence of a third element
[1,5,9,13,26–36]. Niobia is particularly interesting since many
possible interactions may take place, leading to V-Nb-O, V-Sb-
O, and Sb-Nb-O phases [52,53], some of which are present in
catalysts developed by Mitsubishi [17,18] and by bp [21]. The
V-Nb-O phases are efficient for propane ammoxidation in
niobia-supported vanadia catalysts [52] and SbNbO4 does not
appear active for propane ammoxidation [53].
The bulk V-Sb-Nb-O catalysts have been prepared follow-
ing different preparation methods to differently affect their
structures.
SVN2 and SVN3 are prepared by interaction between
vanadium acetyl acetonate and SbCl5 with different niobium
precursors. They possess similar structures; according to
combined XRD and Raman spectroscopy: VSbO4 and Nb2O5
phases are present, but not SbNbO4. The absence of the VSbO4
diffraction pattern for SVN3, visible to Raman spectroscopy,
must be due to the small size of its domains. Some antimony
remains segregated as a-Sb2O4; which becomes more evident
in used samples. Thus, the trend of antimony to migrate to the
catalysts surface observed with supported Sb-V-O catalysts
[45,51], is also observed in the present work for bulk Sb-V-Nb-
O catalysts. These catalysts perform propane ammoxidation
better that the SV series and the other SVN catalysts. This is
consistent with the beneficial effect of niobia on the
performance reported for VSbO4 [52]. Thus, the presence of
both VSbO4 and Nb2O5 account for the better performance,
compared to SV series.
SVN1A and SVN1B are prepared by interaction between
NH4VO3 and SbCl3 and niobium oxalate. They possess similar
structures, both possess SbNbO4 but not VSbO4 or Nb2O5,
according to Raman spectroscopy (Fig. 5) and XRD patterns
(Fig. 4). The formation of VSbO4 is not observed for SVN1A or
SVN1B (from SbCl3); in these catalysts; antimony reacts
preferentially with niobium. SbNbO4 phase is inert for propane
ammoxidation [52,53]. The presence of SbNbO4 and absence
of VSbO4 accounts for their poor ammoxidation performance.
Apparently, niobia precursor is not critical for Sb-V-Nb
interactions since niobia oxalate may result in either Nb2O5 or
SbNbO4. However, the oxidation state of the antimony chloride
follows a trend: SbCl5 interacts preferentially with vanadia;
SbCl3, with niobia. Using SbCl5, Sb, and V interact forming
mixed phases like VSbO4 and Nb2O5 crystalline phase,
regardless of niobium precursor (SVN2, SVN3). Such interac-
tion is consistent with the redox reaction between Sb5+ and V4+ to
form VSbO4 [21,24]. In these catalysts, the VSbO4 phase
becomes more evident after propane ammoxidation, so this phase
is further formed during reaction. The in situ formation of this
phase during propane ammoxidation has been reported for
alumina-supported Sb-V-O catalysts using operando Raman-GC
methodology [24].
The interaction between Sb, V, and Nb can be tuned by the
oxidation state of antimony chloride. SbCl3 promotes the Sb-
Nb interaction, which leads to SbNbO4. This phase is not active
for propane ammoxidation; but, in addition, the interaction
between Sb and Nb prevents Sb-Vor Nb-V interactions. VSbO4
and V-Nb-O phases are efficient for propane ammoxidation.
Therefore, this system would not be efficient. On the other
hand, SbCl5 promotes the Sb-V interaction, forming VSbO4 and
segregating niobia as Nb2O5 on calcination. Niobia-promoted
VSbO4 performs better than VSbO4 [52]. The exact nature of
these catalysts if not fully understood, the promotion could be
due to the acidic sites of niobia and to the combination of Vand
M.O. Guerrero-Perez et al. / Applied Catalysis A: General 298 (2006) 1–76
Nb into V-Nb-O phases, which are efficient for propane
ammoxidation [52], along with VSbO4. Therefore, it is possible
to prepare Nb-promoted Sb-V-O catalysts in a single step by
appropriate control of the precursors.
SVN1 A, SVN1 B, and SVN2 use niobium oxalates, SVN3
uses Nb2O5 suspension. All the catalysts prepared with SbCl3possess SbNbO4. For a given niobium precursor (oxalate),
SVN1A and SVN1B possess SbNbO4, but SVN2 does not.
The interaction between Sb and V is stronger with SbCl5,
while the interaction between Nb and Sb appears stronger with
SbCl3.
5. Conclusions
Pure VSbO4 phase does not appear to be efficient for
propane ammoxidation to acrylonitrile. It appears that some
segregated Sb oxide is necessary for the catalytic cycle. The
combination of VSbO4 with a-Sb2O4 enhances the selectivity
to acrylonitrile, especially when Sb/V is near 5. The presence of
surface vanadium species cannot be determined in detail, but it
maybe important, since it would be at the surface, which is
directly exposed to the reactants.
For V-Sb-Nb oxide catalysts, the niobium precursor does not
appear to have a critical effect; however, the antimony
precursor shows a trend:
(a) S
bCl3 precursor results in preferential antimony–niobiuminteraction; the formation of the SbNbO4 phase is observed
and the catalyst is neither active nor selective to
acrylonitrile formation. Antimony interacts with niobium
at the expense of the VSbO4 and a-Sb2O4active phases.
(b) S
bCl5 precursor favors the redox reaction with vanadium toform the VSbO4 active phase. Nb improves the catalytic
behavior with respect to the Sb-V-O mixed oxide catalysts.
The presence of Nb-V-O phases could not be detected but its
presence may not be ruled out either.
Acknowledgments
This research was partially funded by the Ministry of
Education and Science, Spain (Project MAT-2002-04000-C02-
01). M.O.G.-P. thanks the Ministry of Science and Technology
of Spain, for a doctorate studies fellowship. M.V.M.-H.
acknowledges the Juan de la Cierva program of the Ministry
of Science and Technology of Spain for financial support. The
authors thank Dr. M.A. Vicente at the University of Salamanca
for his help with the XRD analysis and his helpful discussions.
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