Structural changes occurring at the surface of alumina-supported nanoscaled Mo-V-Nb-(Te)-O catalytic system during the selective oxidation of propane to acrylic acid

Article abstract of DOI:10.1016/j.apcata.2011.08.002

The effect of total coverage on alumina is evaluated for Mo ₅V₄Nb₁/Al₂O₃ catalysts by operando Raman-GC spectroscopy during reaction, demonstrating structural changes during reaction that do not occur during catalyst calcination. XRD detects show no appreciable change but Operando Raman-GC studies are sensitive to changes occurring in the nanometer scale, which modulate the performance during propane oxidation, delivering the structure-activity relationship for deactivation. Raman spectra confirm the formation of nanocrystalline MoO ₃-type structures during reaction up to 375 °C, accompanied by coke deposits, able to block the active sites, being detrimental for the acrylic acid formation. Three new catalysts were preparing by the addition of tellurium as dopant, in this case, the presence of MoO₃ is not detected under reaction conditions, since the formation of a distorted rutile structure under reaction conditions is observed, and in this case acrylic acid is obtained as the main reaction product with a yield of ca. 25% at 400 °C.

Full text of DOI:10.1016/j.apcata.2011.08.002

Applied Catalysis A: General 406 (2011) 34–42
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Structural changes occurring at the surface of alumina-supported nanoscaled
Mo–V–Nb–(Te)–O catalytic system during the selective oxidation of propane to
acrylic acid
a
b
c
a,∗
Ricardo López-Medina , J.L.G. Fierro , M. Olga Guerrero-Pérez , M.A. Ba n˜ ares
a
Catalytic Spectroscopy Laboratory, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie, 2, E-28049-Madrid, Spain
Energía y Química Sostenible, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie, 2, E-28049-Madrid, Spain
Departamento de Ingeniería Química, Universidad de Málaga, E-29071-Málaga, Spain
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2011
Received in revised form 27 July 2011
Accepted 1 August 2011
Available online 10 August 2011
5 4 1 2 3
The effect of total coverage on alumina is evaluated for Mo V Nb /Al O catalysts by operando Raman-
GC spectroscopy during reaction, demonstrating structural changes during reaction that do not occur
during catalyst calcination. XRD detects show no appreciable change but Operando Raman-GC studies
are sensitive to changes occurring in the nanometer scale, which modulate the performance during
propane oxidation, delivering the structure–activity relationship for deactivation. Raman spectra confirm
◦
the formation of nanocrystalline MoO3-type structures during reaction up to 375 C, accompanied by
Keywords:
Operando Raman
Mo–V–Te–O
Acrylic acid
Structure–activity relationship
Support-stabilized nanoparticle
Nanostructured catalysts
coke deposits, able to block the active sites, being detrimental for the acrylic acid formation. Three new
catalysts were preparing by the addition of tellurium as dopant, in this case, the presence of MoO3 is
not detected under reaction conditions, since the formation of a distorted rutile structure under reaction
conditions is observed, and in this case acrylic acid is obtained as the main reaction product with a yield
◦
of ca. 25% at 400 C.
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
cient to stabilize nanosized MoO3
and (VNbMo)5O14 crystals
−x
[
11].
Understanding the working state of a catalyst requires deter-
Industrial supported catalysts contain an active component,
typically nanometer-sized particles of transition metals and oxides,
which may be dispersed on a high-area support made of a rel-
atively inert oxide [1]. Supported vanadium and molybdenum
oxide catalysts represent an important class of catalysts which
are industrially applied in oxidation reactions [2–4]; in particu-
lar, the selective oxidation of light alkanes over MoVNbTe mixed
oxide catalysts has received much attention lately because of
their industrial importance in the direct oxidation of propane
to acrylic acid [5–9]; their performance is strongly affected by
structural features. The addition of niobium and/or tellurium
to vanadium and molybdenum oxides stabilizes the phases or
mixed MoVNb oxide systems with high-performing selective oxi-
dation catalytic properties, known as M1 and M2 phases [1–3].
In the absence of M1 and M2 phases, synergistic effects between
mining its structure and activity in a simultaneous fashion;
operando analyses are particularly efficient to tackle this approach
[12–18]. It is thus possible to characterize catalytic materi-
als under reaction conditions, even at high temperatures, and
assess structure–activity relationships [19,20]. Such informa-
tion is the base for further progress and improvements on
catalyst formulation and also to develop real-time spectro-
scopic control [21]. In a previous paper [22] we have reported
how alumina-stabilized nanosized rutile Mo–V–Nb–(Te)–O oxide
phases can be prepared and how such nanosized phases are
efficient for the conversion of propane into acrylic acid. It
has also been described [23] how the structure of these cat-
alytic materials depends on calcination and reaction conditions,
determining the catalytic behaviour. Thus, with the aim to
uncover the changes occurring in the nanometer scale at the
catalysts during reaction, the role of tellurium species, and
to establish the relationships between the structure of cata-
lysts and their catalytic performance, present paper describes an
operando Raman-GC study of alumina-supported MoVNbO and
MoVNbTeO catalysts during partial oxidation of propane to acrylic
acid.
nanocrystaline Mo5O14 and an oxygen-defective MoO
are proposed to account for the high catalytic performance of the
multicomponent Mo–V–Nb oxide system [10]. Niobium is also effi-
phase
3−x
∗ Corresponding author.
E-mail address: miguel.banares@csic.es (M.A. Ba n˜ ares).
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.08.002

R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
35
3
3
2
2
1
1
50
00
50
00
50
00
a
b
c
d
5
0
0
,0
0,2
0,4
0,6
0,8
1,0
0
Relative Pressure (p/p )
Fig. 1. Nitrogen adsorption–desorption isotherms of the (a) ␥-Al2O3, (b)
Mo5V4Nb1, (c) 8Mo5V4Nb1 and (d) 12Mo5V4Nb1 catalysts. Corresponding pore size
distribution plots are show in the inset.
4
2
. Experimental
2.1. Catalysts preparation
Alumina-supported catalysts containing Mo–V–Nb–(Te)–O
Fig. 2. XRD patters of 4Mo5V4Nb1, 8Mo5V4Nb1 and 12Mo5V4Nb1 catalysts.
were prepared by impregnation of ␥-Al O (Sasol Puralox SCCa
2
3
2
5
/200, 193 m /g) with surface coverages varying from 4 to 12 metal
atoms/nm with aqueous solutions of ammonium heptamolyb-
◦
Bragg’s angles 2ꢀ from 5 to 70 . Analysis of the diffraction peaks
was done with the computer program X’Pert HighScore Plus ver-
sion 2.2a (2.2.1). Surface areas of the catalysts were measured from
2
date tetrahydrate (NH ) Mo7O 4H O; (Sigma–Aldrich), NH VO
4
6
24
2
4
3
(
Sigma–Aldrich), an aqueous solution of ammonium niobium solu-
◦
the adsorption isotherms of N2 at −196 C using the BET method
ble complex (Niobium Products) and telluric acid (Sigma–Aldrich).
(
0.05 < P/Po < 0.27) in a Micromeritics ASAP-2000 apparatus. The
◦
The slurry was introduced into a rotary evaporator at 80 C and a
◦
samples were previously degassed under helium flow at 140 C for
reduced pressure of 10–40 mm Hg. The excess of water was evap-
2
h.
Raman spectra were run with a Renishaw Micro-Raman Sys-
◦
orated while stirring at 80 C; the remaining solid was then dried
◦
◦
at 120 C for 24 h and finally calcined at 600 C for 2 h in air. The
calcination was performed with a heating rate of 5 C/min. The
tem 1000 equipped with a cooled CCD detector and a holographic
super-Notch filter to remove the elastic scattering. Wavenumber
calibration of the spectrograph was checked daily using the Si line
◦
samples are named as xMo5V Nb1 or xMo5V Nb0.5Te0.5, where “x”
4
4
2
indicates the number of metal atoms per nm of alumina support.
Mo/V molar ratio was fixed at 5/4 whereas that for Mo/Nb, at 5. As a
reference, the dispersion limit (“monolayer” coverage) of molybde-
num, vanadium, niobium and tellurium oxide species on alumina
−1
at 520 cm . The samples were excited with the 488 nm line. The
in situ laser Raman spectroscopy studies were carried out using an
operando cell (Linkam) in which samples are heated in synthetic air
flow (50 ml/min). The operando Raman experiments were obtained
under reaction conditions in a home-made reaction cell [24]. It was
made using quartz tubing connected to optical quality quartz. So,
it was possible to make a fixed-bed catalytic reactor with walls
that are optically appropriate for Raman spectroscopy. The laser
power on the sample was kept very low (below 9 mW) to pre-
vent local heating at the spot of spectral acquisition, which would
have made the spectra not representative of the catalyst bed. As a
consequence, the signal-to-noise ratio is low and acquisition time
was adjusted to compensate (30 scans of 5 s). The activity in the
operando reaction cell was measured with an on-line gas chromato-
2
is reached at ca. 6, 8 and 8 atoms/nm , respectively [19,20].
2.2. Characterization and operando experiments
The bulk elemental composition of catalysts was examined by
inductively coupled plasma (ICP). The chemical analyses of the
catalysts were carried out by inductively coupled plasma atomic
emission spectroscopy (ICP-AES) using a Perkin–Elmer Optima
3
300 DV spectrometer. The X-ray photoelectron spectra (XPS) were
obtained with a VG Escalab 200R spectrometer equipped with
a hemispherical electron analyzer and a MgK␣ (h.␯ = 1254.6 eV,
TM
graph, a Varian CP-3800 system equipped with a Porapak Q and
−
19
1
eV = 1.6302 × 10
J) X-ray source, powered at 120 W. The con-
molecular sieve 5 A˚ column, fitted to TCD and FID detectors using an
tamination Cs line was selected as the kinetic energies reference, at
a value of 284.6 eV. Wagner sensibility factors were used in order
to quantify the different elements on the surface. Peaks were con-
sidered to be combinations of Gaussian and Lorentzian functions
in a 80/20 ratio, working with a Shirley type baseline background
subtraction by using XPS Peak Fit software. An estimated error of
automatic sampling valve (VALCO). The correctness of the analyti-
cal 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%). To prevent
participation of homogeneous reactivity, the reactor was designed
to minimize gas-phase activation of propane by minimizing void
volume. Tests were made using 0.2 g of sample with particle dimen-
sions in the 0.25–0.125 mm range. The feed flow rate was fixed at
±
0.1 eV can be assumed for all measurements.
Powder X-ray diffraction (XRD) patterns of the samples were
−
1
recorded on a Seifert 3000 diffractometer using Ni-filtered CuK␣
radiation (( = 0.15418 nm) and a graphite monochromator. Work-
4800 h (STP) gas hourly space velocity (GHSV). The reaction mix-
ture consisted of 12.5 mol% C H , 20.4 mol% O , 15.9 mol% steam
3
8
2
◦
ing conditions were 40 kV, 30 mA, and scanning rate of 2 /min for
and 51.2 mol% He, and the flow rate was held constant at 40 ml/min.

3
6
R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
Fig. 3. In situ Raman spectra of (A) 4Mo5V4Nb1 and (B) 12Mo5V4Nb1 precursors under synthetic air atmosphere.
Table 1
Textural properties of catalysts.
2
((Mo + V)/Al) ICPa
((Mo + V)/Al)) XPSb
Catalyst
Loading Mo + V + Nb (atoms/nm)
BET surface area (m /g)
Pore diameter (nm)
4Mo5V4Nb1
8Mo5V4Nb1
12Mo5V4Nb1
4
8
12
165
110
54
11
16
18
0.34
0.48
0.76
0.075
0.586
2.16
a
Determined by ICP.
Surface atomic ratios calculated from XPS data.
b
Table 2
Binding energies (eV) obtained by XPS.
Fresh
Used
Catalysts
V (2p3/2)
Mo (3d5/2)
Nb (3d5/2)
V (2p3/2)
Mo (3d5/2)
Nb (3d5/2)
207.1
517.4 (89)[
a]
b]
232.8 (85)[c]
516.7 (100)[a]
232.7 (65)[c]
4
8
1
Mo5V4Nb1
Mo5V4Nb1
2Mo5V4Nb1
16.1 (11)[
[d]
207.1
207.1
207.3
[d]
5
231.7 (15)
231.7 (35)
517.6 (94)[
a]
232.9 (90)[c]
517.5 (100)[a]
517.5 (100)[a]
232.8 (78)[c]
16.3 (6)[
b]
[d]
[d]
207.1
207.1
5
231.7 (10)
231.7 (22)
517.5 (100)[a]
232.9 (90)
2
[c]
232.8 (84)[c]
31.8 (10)[
d]
[d]
231.7 (16)
In parenthesis are peak percentages, [a] V5+ 2p3/2, [b] V4+ 2p3/2, [c] Mo6+ 3d5/2, [d] Mo5+ 3d5/2, [e] Nb5+ 3d5/2.
The operando reactor performance was benchmarked vs. conven-
tional fixed-bed reactor, the reactants were feed with the same
reaction feed system and the reaction products were analysed in
the same GC system that was used for the operando Raman cell. The
conventional cylindrical fixed-bed reactor (7 mm ID) was made of
quartz too. The axial temperature profile was monitored by a ther-
mocouple sliding inside the catalytic bed. In this case we used the
same amount of catalyst with the same mesh and flows than those
used during the operando experiments.
3
. Results and discussion
3.1. MoVNbO supported catalysts
The BET surface area values of the samples are listed in Table 1.
As expected, BET area decreases with total Mo + V + Nb coverage. In
line with most supported oxides, BET area values drop dramatically
2
above dispersion limit loading (ca. 8 atoms/nm ), 12Mo5V4Nb1
2
BET area decreases to 54 m /g.
Fig. 1 shows the nitrogen adsorption–desorption isotherms, that
represent a type IV isotherm according to the BDDT classifica-
tion [25], with a hysteresis loop of type H1, indicating that these
materials present a high mesoporosity. The hysteresis loop shifted
Fig. 4. Yield to acrylic acid during operando Raman-GC study during the selective
oxidation of propane on 4Mo5V4Nb1, 8Mo5V4Nb1 and 12Mo5V4Nb1 catalysts. Reac-
−
1
tion conditions: C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of catalyst.

R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
37
Fig. 5. Propane conversion vs. temperature and vs. selectivity to acrylic acid obtained for several catalysts in the conventional fixed bed reactor and in the operand reactor.
−
1
Reaction conditions: C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of catalyst.
towards higher relative pressure, which reflects an increase in aver-
age pore size (see insert in Fig. 1 and Table 1). In order to have an
idea about the composition at the surface, the (Mo + V)/Al ratios
have been calculated by ICP and by XPS, and the results are included
in Table 1. The Mo + V coverage on the surface, as determined by
XPS, remains linear with bulk ICP loading trend, however, the XPS
signal grows significantly faster than the bulk ratio, which indi-
cates a high degree of coverage of alumina support. This makes
sense since the dispersion limits for MoOx and VOx species on alu-
mina support, understood as the maximum surface loading of oxide
increases; thus, alumina support interacts with the Mo–V–Nb–O
phases. No XRD lines due to crystalline MoO3 or V O5 oxides are
2
detected for any sample. No significant changes are observed in
these patterns before and after the catalytic tests.
The binding energies of Mo 3d5/2, V 2p3/2 and Nb 3d5/2 peaks
for 4Mo4V4Nb1 sample are shown in Table 2. For Mo region, the
spin-orbit splitting between the Mo3d5/2 and Mo3d3/2 signals was
fixed to 3.1 eV and the intensity ratio was set to 0.67. Deconvolu-
tion of the spectra of fresh and used catalysts indicate the presence
of two different Mo species, whose binding energies correspond
5+
6+
units that remain dispersed, with no crystalline V O5 and/or MoO ,
to Mo and Mo species [29–31]. The population of the reduced
2
3
5+
have been calculated to be close to 8 atoms of Mo and/or V [26].
It is interesting to underline that pore diameter increases as total
Mo species increases in the catalyst after use in catalytic tests.
The V 2p spectra for the fresh sample exhibit two components:
one at higher binding energy, near 517.4 eV, associated with V5+
species [32], and another at lower energy, near 516.1 eV, related
with the presence of V4+ species [33]. After reaction, only one
component associated with V5+ species is detected, thus, during
propane oxidation some changes in the overall oxidation states of
vanadium and molybdenum species are detected by the changes in
the binding energies. Binding energies of Nb3d levels for fresh and
used samples are characteristic of Nb5+ species [34,35].
oxide coverage is increased (Table 1), from 12 nm in the 4Mo5V Nb1
4
catalyst to 18 nm for 12Mo5V Nb1 sample. Such increase in meso-
4
porosity must be related with the supported oxide Mo–V–Nb–O
structures, As has been described before [27,28] it is possible to
prepare mesoporous bulk Mo–V–O oxides, in this case, a layer of
mesoporous oxide structure has been obtained over an alumina
support.
The XRD patterns for fresh and used catalysts are shown in Fig. 2.
All the samples exhibit the diffraction pattern of alumina support
Raman spectra provide additional information about the struc-
ture of the MoVNbO-based catalysts due to its sensitivity to
nanosized crystalline domains that do not generate diffraction
patterns. Fig. 3 shows the Raman spectra during calcination in
(
JPCDS file 37-1462). The sample with lowest coverage (fresh and
aged) exhibits no other diffraction pattern. Crystalline Al–Mo–O
and Al–Mo–V–O phases become increasingly visible as coverage

3
8
R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
Fig. 6. Operando Raman-GC spectra during the selective oxidation of propane on 4Mo5V4Nb1; (left), Raman spectra during reaction at the temperature indicated; (right),
−
1
simultaneous activity/selectivity data obtained during Raman spectra acquisition. Reaction conditions: C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of catalyst.
[37–39]. The Raman band near 883 cm 1 is characteristic of Mo O
−
synthetic air for samples with the lowest (Fig. 3A) and the high-
est (Fig. 3B) coverages. In all the cases, a strong signal near
5
14
structures [40,41]. This band is more intense for the sample with
higher coverage.
−1
9
90–1030 cm
is detected. Such band is characteristic of the
Raman bands near 815 and 380 cm 1 are more intense in Fig. 3B,
for the sample with the highest coverage; such Raman bands have
been reported for Mo–V-oxide based catalysts [37,42] and belong
−
stretching modes of Mo O and/or V O bonds, suggesting the pres-
ence of surface MoOx and/or VOx species [20,36]. Also, in that
region, some bands can be assigned to vibrations of Mo–O–V bonds
Fig. 7. Operando Raman-GC spectra during the selective oxidation of propane on 8Mo5V4Nb1; (left), Raman spectra during reaction at the temperature indicated; (right),
−
1
simultaneous activity/selectivity data obtained during Raman spectra acquisition. C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of catalyst.

R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
39
Fig. 8. Operando Raman-GC spectra during the selective oxidation of propane on 12Mo5V4Nb1; (left), Raman spectra during reaction at the temperature indicated; (right),
−
1
simultaneous activity/selectivity data obtained during Raman spectra acquisition. C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of catalyst.
to a Mo–V–O phase. It makes sense that by increasing the coverage,
the presence of some interactions between molybdenum and vana-
dium species is detected. On the contrary, Raman bands near 770
and 230 cm , are more intense for the sample with lower coverage,
such bands are related to an AlVMoO7 structure [42]. The intensity
of these bands increases with temperature, indicating that the ther-
mal treatment enhances the interactions of the supported Mo and
V species with the alumina support.
tions [24,42]. Thus, the catalytic and structural study will be based
on the operando reactor data.
Figs. 6–8 show the catalytic results obtained during propane
oxidation and the Raman spectra obtained during the operando
study. In all the cases, when the temperature reaction is increased
−1
◦
up to 375 C, the shape of the spectra changes drastically. Bellow
−
1
that temperature, Raman bands near 815 and 380 cm assigned to
−
1
Mo–V–O structures and bands near 1000 cm assigned to MoOx
and/or VOx dominate the spectra. In addition, Raman bands near
760 and 230, assigned to AlVMoO7 structure can be detected and
Fig. 4 shows yields to acrylic acid at different tempera-
tures vs. coverage on alumina; coverage close to the monolayer
2
−1
(
8 atoms/nm ) affords better performances. Sample with lower
the Raman band near 880 cm assigned to Mo5O14-type structure.
−
1
coverage performs worse than the others, this sample do not reach
dispersion limit coverage, and thus do not form bulk mixed oxide
phases that appear necessary for this process. Only, at the highest
temperature, it outperforms the other two catalysts, but the yield
is significantly lower than that achieved by the other catalysts at
significantly lower temperatures. It should be noted that the high-
Conversely, Raman bands near 960, 780 and 237 cm dominate the
◦
spectra when reaction temperature reaches 375 C; such Raman
◦
est yields are obtained at the lowest temperature (350 C) whereas
◦
there is a significant drop in performance at 375 C and above.
3.2. Operando study
The operando Raman-GC study during propane ammoxida-
tion was performed under reaction conditions in a home-made
operando reaction cell that was described previously [24], which
was designed in our laboratory [43]. Essentially, it is a fixed-
bed catalytic reactor with walls that are optically appropriate for
in situ Raman spectroscopy. Thus, such reactor allows obtaining
the Raman spectrum of the catalysts and genuine catalytic data.
Fig. 5 shows the results obtained for several catalysts at different
temperatures in the operando-reactor and also in the conventional
fixed bed reactor; it can be observed how the activity/selectivity
obtained in both reactors is essentially the same. These results are
in accordance with previous papers in which was described that
the activity values obtained in the operando reactor for other reac-
Fig. 9. Yield to acrylic acid during the selective oxidation of propane on
4
Mo5V4Nb0.5Te0.5, 8Mo5V4Nb0.5Te0.5 and 12Mo5V4Nb0.5Te0.5 catalysts. Reaction
−
1
conditions: C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of catalyst.

4
0
R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
Fig. 10. Operando Raman-GC spectra during the selective oxidation of propane on 4Mo5V4Nb0.5Te0.5; (left), Raman spectra during reaction at the temperature indicated;
−
1
(
right), simultaneous activity/selectivity data obtained during Raman spectra acquisition. Reaction conditions: C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of
catalyst.
bands are indicative of a ␣-MoO3 with some minor amounts of
vanadium species [20,44]. The amount of such MoO3 structure is
not very high since no diffraction pattern for a similar structure has
been detected, but its bands dominate the Raman spectra since this
phase possesses very high Raman section. XRD analyses uncover
the presence of Mo–V–O and Mo–V–Al–O phases, which are not
seen by Raman spectra, probably due to their lower Raman scat-
tering section; thus, the spectra are dominated by the distorted
oxide-like structure. Raman spectra uncover important changes at
the nanometer scale during reaction, which are not told by XRD.
whereas vanadium species are oxidized. These results are in line
with previous paper with similar catalytic systems. The selectivity
to partial oxidation products is higher when mixed phases form
(e.g., Mo–V–O, V–P–O or Sb–V–O), typically in their rutile phase;
on the contrary, the distribution of product shifts to non-selective
oxidation products (COx) if segregated V O5, Sb O and/or MoO3
2
2
3
oxides form.
In addition to monitoring changes occurring in the nanoscaled
Nb–Mo–V–O structures during propane oxidation, operando Raman
experiments evidence the formation of coke since bands appear in
◦
−1
All catalysts exhibit significant changes at 375 C that reflect sig-
the region 1200–1650 cm are attributed to graphite-like carbon
nificant changes on the structures of the supported oxides. Such
changes appear consistent with those underlined by the XPS BE
values of used catalysts. These indicate that Mo species reduce
deposits assigned to surface layers of sp2 carbon species [45–48],
indicative that coke structures are appearing on the surface of cat-
alysts.
Fig. 11. Operando Raman-GC spectra during the selective oxidation of propane on 12Mo5V4Nb0.5Te0.5; (left), Raman spectra during reaction at the temperature indicated;
−
1
(
right), simultaneous activity/selectivity data obtained during Raman spectra acquisition. Reaction conditions: C3H8/O2/H2O/He = 12.5/20.4/15.9/51.2; 4800 h ; 0.2 g of
catalyst.

R. López-Medina et al. / Applied Catalysis A: General 406 (2011) 34–42
41
3
.3. Effect of tellurium
catalysts. The entanglement between Mo and V ions, along with
those from alumina support, appears critical for an efficient oxi-
dation reaction. When reaction temperature reaches 375 C, the
◦
In order to improve the catalytic behaviour of these materi-
als three more catalysts were prepared by adding tellurium as
catalyst structure transforms, losing molybdenum oxide phases
rich in V and Al. These transform into ␣-MoO3 crystallites dis-
torted by a small fraction of V ions, which results in a less efficient
system. These data underline the direct involvement of mixed
Mo–V–(Al)–O phases on propane oxidation. In addition, incipient
coke formation becomes apparent when the selectivity to acrylic
acid decreases. Tellurium doping generates highly distorted rutile
structures during reaction, capable of inhibiting the formation of
␣-MoO3 crystallites under reaction conditions, and improving the
catalytic performance of these catalysts, reaching an acrylic acid
dopant, with the formulation Mo5V Nb0.5Te0.5. The XRD of fresh
4
and used Te-containing catalysts is shown in Additional supporting
information. Fig. 9 shows the yields to acrylic acid vs. reaction
temperature. The catalytic behaviour is markedly different to that
delivered by the tellurium-free series (Fig. 4). In this case the yield to
acrylic acid is higher, especially for the sample with the highest cov-
erage, which delivers ca. 25% acrylic acid yield. In order to assess the
nature of the structural changes that tellurium induces during reac-
tion, the operando study was performed with 12 Mo5V Nb0.5Te0.5
,
4
◦
that is the one that presented the highest acrylic acid yields. As a
reference, this is compared to the catalysts with lowest coverage.
Fig. 10 shows the operando study of the catalysts with lowest
yield of ca. 25% at 400 C.
Acknowledgements
coverage (4Mo5V Nb0.5Te0.5), which possess a BET surface area of
4
2
2
1
59 m /g, very similar to the 165 m /g of its tellurium-free counter-
The Ministry of Science and Innovation (Spain) funded this study
under project CTQ2008-04261/PPQ. R.L.M. thanks MAEC-AECID
(Spain) for his pre-doctoral fellowship and Elizabeth Rojas García
for her help with catalytic tests. The authors express their thanks to
Olaf Torno (Sasol Germany GmbH) for providing alumina support.
part (Table 1). The selectivity to acrylic acid for 4Mo5V Nb0.5Te0.5
4
(
Fig. 10) increases at all the temperatures with respect to the cata-
lyst without tellurium (Fig. 6); acrylic acid is not the main product,
though. This comparison shows that a small amount of tellurium
changes both the structure and the activity of the catalysts. Unlike
the tellurium-free series, the Raman bands of mixed Mo–V–O,
Mo–V–Al–O phases are not evident at low reaction temperature,
and the Raman bands of MoO3 phase at high reaction tempera-
ture are not apparent either. This suggests that tellurium doping
has a dramatic effect on the structures of mixed Mo–V oxides.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.08.002.
−
1
Which is evidenced by a broad Raman signal in the 800–1000 cm
15,49] range; this is indicative that tellurium induces the presence
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