Molecular structure and reactivity of titania-supported transition metal oxide catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane

Article abstract of DOI:10.1016/j.crci.2015.08.013

The molecular structure and catalytic performance of (MoO_x)_n/TiO₂, (WO_x)_n/TiO₂ and (VO_x)_n/TiO₂ catalysts (synthesized by the equilibrium–deposition–filtration/EDF method) for oxidative dehydrogenation (ODH) of ethane were studied by in situ Raman spectroscopy at 430?°C and catalytic measurements in the temperature range of 420–480?°C. The extent of association within the deposited oxometallic phase followed the sequence (VO_x)_n/TiO₂?>>?(MoO_x)_n/TiO₂?>?(WO_x)_n/TiO₂; a concurrent trend in reduction susceptibility was evidenced by exploiting the relative normalized Raman band intensities while monitoring the response of the vibrational properties of the catalytic samples under reactive (C₂H₆/O₂/He) and reducing (C₂H₆/He) conditions by in situ Raman spectroscopy. The catalyst reactivity tracks the corresponding trend in reduction susceptibility as evidenced by the in situ Raman spectra. Selective reaction pathways are favored at high coverage whilst combustion routes are activated at low coverages due to the involvement of carrier lattice oxygen sites. The observed apparent reaction rates and activation energies are discussed in relation to various structural and reactivity aspects.

Full text of DOI:10.1016/j.crci.2015.08.013

C. R. Chimie xxx (2016) 1e11
Contents lists available at ScienceDirect
Comptes Rendus Chimie
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Full paper/Memoire
Molecular structure and reactivity of titania-supported
transition metal oxide catalysts synthesized by equilibrium
deposition filtration for the oxidative dehydrogenation of
ethane
Antonios Tribalis, George Tsilomelekis 1, Soghomon Boghosian*
Department of Chemical Engineering, University of Patras and FORTH/ICE-HT, GR-26504 Patras, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
The molecular structure and catalytic performance of (MoOx)n/TiO2, (WOx)n/TiO2 and
(VOx)n/TiO2 catalysts (synthesized by the equilibriumedepositionefiltration/EDF method)
for oxidative dehydrogenation (ODH) of ethane were studied by in situ Raman spectros-
copy at 430 ꢀC and catalytic measurements in the temperature range of 420e480 ꢀC. The
extent of association within the deposited oxometallic phase followed the sequence
(VOx)n/TiO2 >> (MoOx)n/TiO2 > (WOx)n/TiO2; a concurrent trend in reduction susceptibility
was evidenced by exploiting the relative normalized Raman band intensities while
monitoring the response of the vibrational properties of the catalytic samples under
reactive (C2H6/O2/He) and reducing (C2H6/He) conditions by in situ Raman spectroscopy.
The catalyst reactivity tracks the corresponding trend in reduction susceptibility as evi-
denced by the in situ Raman spectra. Selective reaction pathways are favored at high
coverage whilst combustion routes are activated at low coverages due to the involvement
of carrier lattice oxygen sites. The observed apparent reaction rates and activation energies
are discussed in relation to various structural and reactivity aspects.
Received 24 June 2015
Accepted 27 August 2015
Available online xxxx
Keywords:
Molybdenum oxide
Tungsten oxide
Vanadium oxide
Titania-supported catalysts
ODH of ethane
In situ Raman spectroscopy
Equilibrium deposition filtration
ꢀ
© 2016 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
remains a formidable challenge in the design of efficient
catalysts for the ODH of light alkanes.
The catalytic dehydrogenation of light alkanes by se-
lective oxidation is an attractive route for the synthesis of
C2eC4 olefins [1e10]. In contrast to the direct dehydroge-
nation (DH) [6], the presence of O2 as an oxidant (ODH)
[4,5,7e10] contributes to an exo-thermicity that helps to
achieve a control over the carbon deposition problem,
albeit to the expense of the performance in selectivity
grounds. Therefore, the achievement of high selectivities at
significant conversions is of utmost importance and
Oxides of transition metals (e.g., V, Mo, W, etc.) in the
form of a deposited oxometallic phase on a typical oxidic
support (e.g., Al2O3, TiO2, etc.) constitute a well-established
class of catalytic materials that have been examined for the
ODH catalytic reaction [3e5,8,9,11,12e17]. The choice of a
particular transition metal oxide (as the active phase)
greatly affects the catalyst performance. For example, V2O5-
based catalysts are by far superior to their respective MoO3-
based counterparts for the ODH of light alkanes [17,18]. The
efficient activation of the CeH alkane bond is directly
related to inter alia: (a) the speciation of the oxometallic
species in the deposited phase; (b) the local structure at the
molecular level; (c) the active phase coverage; (d) the na-
ture of the support; (e) the applied synthesis route.
* Corresponding author.
E-mail addresses: bogosian@iceht.forth.gr, bogosian@chemeng.
upatras.gr (S. Boghosian).
1
Present address: Department of Chemical and Biochemical Engi-
neering, Rutgers School of Engineering, USA.
http://dx.doi.org/10.1016/j.crci.2015.08.013
ꢀ
1631-0748/© 2016 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

2
A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
Most commonly, the studied catalysts are synthesized
and initial concentrations lying on the plateau of the
adsorption isotherms of Mo(VI), W(VI) and V(V) [20e22].
The samples were collected by filtration, following a 24-h
equilibration, dried at 110 ꢀC for 4 h and then calcined in air
in a muffle furnace at 450 ꢀC for 5 h. The calcined samples
are denoted as xMTi, where x denotes the surface density
(M/nm2) and M denotes the particular transition metal
(M ¼ Mo, W, V). The catalyst loading (% wt M) was found by
determining spectrophotometrically the initial and equi-
librium concentrations of Mo(VI), W(VI) or V(V) in the
impregnation solutions [25]. The surface density (M/nm2)
was calculated for each sample by combining the % wt M
loading with the corresponding BET specific surface area
(SBET).
by wet impregnation involving a contact of the support
grains with a solution containing the oxometallic precursor
species at a specified pH and concentration and removal of
the solvent by evaporation. Typically, such a step is fol-
lowed by drying and calcination. However, during the
evaporation of the solvent, drastic changes occur in the
concentration and pH of the impregnating solution,
thereby greatly affecting the speciation of the oxometallic
solute species (resulting also in massive precipitation and
formation of overlayers).
The application of the equilibrium deposition filtration
(EDF) synthesis method (also called equilibrium adsorption
method) gives rise to catalyst materials with high disper-
sion of the deposited species [19]. By applying this tech-
nique, certain physicochemical properties can be tailored to
a significant extent and thus a suitable catalytic behavior
for a given reaction may be obtained through a molecular-
level-understanding of the processes taking place at each
preparation step. For example, the deposition step takes
place at equilibrium with precise control of the deposition
pH and the solideliquid separation takes place by filtration
leading to the removal of the non-adsorbed oxometallic
precursors with the filtrate, thereby resulting in a high
dispersion of oxometallic species on the support. To this
end, we have synthesized three series of titania-supported
transition metal oxide catalysts [(MoOx)n/TiO2, (WOx)n/TiO2
and (VOx)n/TiO2] and tested them for the ODH of ethane.
The catalyst synthesis was done by applying the EDF
method in a wide range of deposition pH 9.5e4 and solu-
tion concentrations of Mo(VI), W(VI) and V(V) lying on the
plateau of the respective adsorption isotherms of Mo(VI),
W(VI) and V(V) on TiO2 [20e22]. In situ Raman spectra
were recorded at 430 ꢀC under O2(g) and the response of
the catalyst molecular structure was examined by applying
reaction conditions (C2H6/O2/He) and reducing conditions
(C2H6/He). Catalytic measurements were undertaken in the
temperature range of 420e480 ꢀC at various residence
times. The data are used to discuss structure/activi-
tyeselectivity relationships for the ODH of ethane.
The specific surface area was measured by N2 adsorp-
tion (following a 2 h evacuation) and standard multipoint
BET methods in a Micromeritics Gemini II 2370 analyser. A
Philips PW 1830 diffractometer with Cu Ka radiation was
used to obtain the X-ray diffraction patterns of the studied
catalysts and the only crystalline phase detected was
anatase.
Table 1 compiles important synthesis parameters and
characteristics of the catalyst samples.
2.2. In situ Raman spectroscopy under oxidizing (O2), ODH
reaction and reducing (C2H6/He) conditions
A homemade Raman cell (previously described in e.g.,
[10,26]) was used to record the in situ Raman spectra of the
catalysts. A detailed description of the apparatus and of the
procedures used to record in situ Raman spectra from
catalyst materials can be found in previous articles [10,26].
Raman spectra were excited using the 488.0 nm line of a
Spectra Physics Stabilite 2017 Arþ ion laser. In order to
reduce sample irradiance, the incident laser light (adjusted
at a 40 mW power level) was slightly defocused.
In situ Raman spectra were obtained for the studied
catalysts under flowing (15 cm3 minꢁ1) O2 gas (99.999%) at
430 ꢀC following a 1 h treatment. Subsequently, for moni-
toring the response of the catalysts' molecular structure to
changes in the reactor gas atmosphere, each sample was
subjected to a 20 cm3 minꢁ1 flow of 5.6% C2H6/5.6% O2/He
reaction mixture for 1 h and thereby the in situ Raman
spectrum was recorded under ethane ODH reaction con-
ditions. Finally, each catalyst was reoxidized under flowing
O2 for 1 h and was exposed to a 20 cm3 minꢁ1 flow of 11.2%
2. Experimental
2.1. Synthesis and textural characterization of catalyst
samples
The catalyst samples were prepared by the Equilibrium
Deposition Filtration (EDF) method. The support material
used was industrial titania (Degussa P25, 80% anatasee20%
rutile) with a surface composition of 90% anatase and 10%
rutile [23,24]. The precursors used for preparing the
impregnation solutions were (NH4)6Mo7O24$4H2O,
Table 1
Synthesis parameters and properties of the MOx/TiO2 (M ¼ Mo, W, and V)
catalysts.
Catalyst
pH/initial M
concentration (M) (wt% M)
Loading, SBET
Surface density
(m2/g) (M/nm2)
(NH4)10W12O41$5H2O and NH4VO3. The ionic strength of
0.3MoTi 9/10ꢁ2
0.26
2.21
3.18
2.77
3.03
5.89
0.84
1.22
1.69
53.7
53.5
51.8
52.2
52.7
51.5
51.7
51.7
50.6
0.3
2.6
3.8
1.7
1.9
3.8
1.9
2.8
4.0
the solutions was adjusted with 0.1 M NH4NO3. For each
synthesis, 3.5 g of titania were added to 175 mL of the
2.6MoTi 6/1.5 ꢂ 10ꢁ2
3.8MoTi 4/2.5 ꢂ 10ꢁ2
respective solution in
a
25oC-operated thermostatted
1.7WTi
1.9WTi
3.8WTi
1.9VTi
2.8VTi
4.0VTi
9/5 ꢂ 10ꢁ3
7/4.2 ꢂ 10ꢁ3
5/2 ꢂ 10ꢁ2
9.5/6 ꢂ 10ꢁ3
7/5 ꢂ 10ꢁ3
5/2 ꢂ 10ꢁ2
double-wall pyrex vessel equipped with a magnetic stirrer
and a Perspex lid with appropriate fittings for electrodes
and N2 admission (necessary to prevent CO2 from inter-
fering with the solution pH). The samples were prepared at
different pH (in the range 9.5e4, automatically controlled)
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
3
C2H6/He for 1 h and its in situ Raman spectrum was
recorded under reducing conditions. In order to be able to
perform mutual comparisons among spectra of different
samples, all Raman spectra were normalized. The normal-
ization was performed by dividing each entire spectrum by
the total area under the spectrum in the wavenumber re-
gion of interest (550e1100 cmꢁ1); prior to this process the
baselines of the spectra were zeroed by subtracting the
minimum intensity (at ca. 1100 cmꢁ1) from the intensities
at each point.
Finally, the reaction rates expressed in terms of the
turnover frequency (TOF, expressed as moles of C2H6 con-
verted per sec and per mole of M) were determined by:
in
exit
_
_
NC ꢁ NC
2H6
2H6
TOF ¼
;
S
BET$W$ns
where
in
exit
_
_
2
NC H ; NC H : the molar flow rates of C2H6 at the reactor
inl2et and e6xit, respectively (moles ethane/s);
SBET: the catalyst BET specific surface area (m2/g);
W: the catalyst weight (g); and
6
2.3. Catalytic measurements
ns: the surface density (mol M/m2)
The catalytic measurements were performed in a dif-
ferential fixed-bed reactor, described previously [10]. The
reactor cell comprised a quartz frit for holding in place the
3. Results and discussion
catalyst powder as a fixed-bed (particle size, 125e180 mm).
The reactant gas mixture consisted of 5.6% C2H6 and 5.6% O2
balanced in He and was admitted after 1 h of treatment
under flowing O2 at 480 ꢀC. The samples were first exam-
ined at a residence time (expressed as the W/F ratio; W:
catalyst weight; F: total flow) of W/F ¼ 1.25 g s cmꢁ3 for the
Mo/TiO2 and W/TiO2 series; and 0.06 g s cmꢁ3 for the V/
TiO2 series in the temperature range 420e480 ꢀC. The
above W/F values were selected in such a way that the
conversion measured for the most reactive sample in each
series is below 10% at the highest measured temperature
(480 ꢀC). For studying the temperature dependence of
catalytic activity, measurements started at 480 ꢀC and were
repeated for each system at fixed temperature intervals (in
the range 480e420 ꢀC). Finally, the effect of residence time
was checked by varying the W/F in the range of
0.03e1.25 g s cmꢁ3 (by varying either the catalyst weight or
the total gas flow) at a fixed temperature of 480 ꢀC. All gas
flow rates were controlled by using electronic mass
flowmeters.
The gas stream exiting the reactor was analyzed by
using an on line Schimadzu 14B GC using Porapak Q and
Molecular Sieve 5A packed columns in a series or bypass
configuration and a TCD detector. The catalytic measure-
ments as well as their temperature dependence were per-
formed under conditions where conversions were below
10% in order that differential reactor conditions could be
safely assumed and apparent activation energies could
thereby be inferred. Runs with an empty reactor were done
in order to check that the possible contribution of gas phase
reactions and the pertinent ethane conversion at 480 ꢀC
was negligible.
3.1. In situ Raman spectra of MOx/TiO2 (M ¼ Mo, W and V)
catalysts
3.1.1. In situ Raman spectra under O2(g)
Figs. 1,2 and 3 show the in situ normalized Raman
spectra obtained under flowing O2(g) at 430 ꢀC for the
(MoOx)n/TiO2 (Fig. 1), (WOx)n/TiO2 (Fig. 2) and (VOx)n/TiO2
TiO2
ν1 Eg
T=430 °C
630
(
)
(MoOx)n/TiO2
under O2(g)
Mo/nm2
(c)
(b)
Mo=O
995
Mo-O-Mo
900-925
3.8
(a)
994
992
2.6
0.3
The selectivity, SN, with reference to a specific product of
the reaction (N ¼ C2H4, CO, and CO2) was determined by:
TiO2
2
ν
4(B1g)
exit
_
n$NN
SNð%Þ ¼
ꢂ 100
in
exit
_
_
N
ꢁ N
C2H6
C2H6
1100 1000
900
800
700
600
500
where
Raman Shift, cm-1
NeNxit: the molar flow rate of component N at the reactor
exit, with n ¼ 1 for C2H4 and n ¼ 2 for CO and CO2;
_
Fig. 1. In situ normalized Raman spectra of (MoOx)n/TiO2 catalysts under
flowing O2(g) at 430 ꢀC: (a) 0.3 Mo/nm2; (b) 2.6 Mo/nm2; (c) 3.8 Mo/nm2.
Laser wavelength, l0 ¼ 488.0 nm; laser power, w ¼ 40 mW; spectral slit
in
exit
_
_
NC H ; NC H : the molar flow rates of C2H6 at the reactor
inl2et6 and2 e6xit, respectively (moles ethane/s);
width, ssw ¼ 7 cmꢁ1
.
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

4
A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
TiO2
TiO2
ν1 Eg
T=430 °C
T=430 °C
630
630
ν1(
Eg)
( )
(WOx)n/TiO2
under O2(g)
(VOx)n/TiO2
under O2(g)
W/nm2
V/nm2
(c)
(b)
(a)
W=O
(c)
(b)
(a)
V=O
1029
V-O-V
880-940
W-O-W
900-925
800
1008
3.8
4.0
1028
1005
1005
2.8
1.9
1.7
1026
1.9
TiO2
2ν4(B1g)
1100 1000
900
800
700
600
500
1100 1000
900
800
700
600
500
Raman Shift, cm-1
Raman Shift, cm-1
Fig. 2. In situ normalized Raman spectra of (WOx)n/TiO2 catalysts under
flowing O2(g) at 430 ꢀC: (a) 1.7 W/nm2; (b) 1.9 W/nm2; (c) 3.8 W/nm2.
Recording parameters: see Fig. 1 caption.
Fig. 3. In situ normalized Raman spectra of (VOx)n/TiO2 catalysts under
flowing O2(g) at 430 ꢀC: (a) 1.9 V/nm2; (b) 2.8 V/nm2; (c) 4.0 V/nm2.
Recording parameters: see Fig. 1 caption.
(Fig. 3) catalyst samples. A well-defined band observed at
992e995 cmꢁ1 for (MoOx)n/TiO2, 1005e1008 cmꢁ1 for
(WOx)n/TiO2 and 1026e1029 cmꢁ1 for (VOx)n/TiO2 is due to
the terminal M]O (M ¼ Mo, W, and V) stretching mode,
nM]O, of deposited mono-oxo metallates [26e28]. The band
due to the nM]O mode evolves with gradually increasing
relative intensity with increasing metal surface density (M
atoms/nm2). At the same time, its position undergoes a
concurrent progressive slight blue shift, i.e. the nMo]O shifts
from 992 cmꢁ1 for 0.3 Mo/nm2 to 995 cmꢁ1 for 3.8 Mo/
nm2; the nW]O shifts from 1005 cmꢁ1 for 1.7 W/nm2 to
1008 cmꢁ1 for 3.8 W/nm2; and the nV]O shifts from
1026 cmꢁ1 for 1.9 V/nm2 to 1029 cmꢁ1 for 4.0 V/nm2. The
nM]O band asymmetry (i.e. tailing at its low-frequency side,
especially for samples with coverage higher than 2 M/nm2)
is due to the occurrence of a number of structurally related
(O]MOx)n configurations in sites with local variation in the
coordination number of the metal atom (CNM) and/or with
varying extent of polymerization. Notably, the signal-to-
noise ratio for the 4.0VTi catalyst (Fig. 3) is inherently
lower due to the very dark color exhibited by this sample at
elevated temperatures, thereby leading to partial absorp-
tion of the incident laser radiation.
nMꢁOꢁM, is observed in all three systems, albeit at different
relative intensities. The relative intensity of the nMꢁOꢁM
mode flags the extent of association/polymerization of the
deposited oxometallic phase. In particular, nMοꢁOꢁMο is
observed at 900e925 cmꢁ1 only for the samples with
medium (2.6 Mo/nm2) and high (3.8 Mo/nm2) surface
density (Fig. 1). A nWꢁOꢁW weak/broad feature is observed
only for the high loaded 3.8 W/nm2 WOx/TiO2 sample
(Fig. 2), evidenced from the elevated background in the
900e925 cmꢁ1 range. The relative intensity of the nVꢁOꢁV
mode (clearly observed in the 890e940 cmꢁ1 range) for the
VOx/TiO2 samples (Fig. 3) is substantially higher (compared
to that of its nMοꢁOꢁMο and nWꢁOꢁW counterparts), indi-
cating a higher extent of association/polymerization for the
deposited (VOx)n phase compared to the extent of associ-
ation for the (MoOx)n and (WOx)n deposited phases. Like-
wise, the progressive blue shift of the nM]O mode with
increasing loading is attributed to the formation of associ-
ated (polymeric) species with a domain size that increases
with increasing coverage [26e28]. The prevalence of
associated/polymeric species in the deposited oxo-V(V)
phase of V2O5/TiO2 catalysts is well established at surface
densities exceeding 2 V/nm2 [28]. The broad band in the
880e940 cmꢁ1 region (Fig. 3) as well as the weaker
~800 cmꢁ1 feature can be assigned to VeOeV
An inherently weak/broad feature at 900e940 cmꢁ1
assigned to bridging MeOeM functionalities [26,28],
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
5
functionalities [28e30]. This can be explained by observing
that the intensity of the, e.g., 880e940 cmꢁ1 feature, IVꢁOꢁV
increases (with increasing surface density, from 1.9 to 4.0 V/
nm2) relative to the IV]O intensity of the terminal
stretching mode and this can only be explained if one
contemplates the increase at the extent of incorporation of
V atoms in VeOeV bridges with increasing loading. Since
there is only one V]O site per V, the progressive increase
in the number of VeOeV bridges per V can account for the
relative increase in IVꢁOꢁV. Previously [31], for a very low
loaded (0.1 V/nm2) VOx/Al2O3 catalyst a band at ~910 cmꢁ1
has been assigned to a VeOeAl mode instead of conven-
tionally assigning the band to a VeOeV mode.
630
TiO2
T=430 °C
ν1(Eg)
(WOx)n/TiO2
3.8 W/nm2
(c)
TiO2
ν4 B1g
2
(
)
(b)
(a)
One of the governing factors for the observed variations,
in the extent of polymerization in the individual catalyst
systems is the interfacial speciation of the deposited oxo-
metallic phase right after the equilibration stage of the
catalyst preparation by the EDF method. The interfacial
speciation is controlled inter allia by the acidebase prop-
erties of the support, the pH and the precursor concentra-
tion of the impregnation solutions [20,27]. The speciation
of oxo-Mo(VI) and oxo-W(VI) species in the precursor so-
lutions used [20, 27] as well as the interfacial speciation of
the deposited oxometallic phases after the equilibrium
deposition and filtration catalyst preparation steps [20, 27]
are suggestive of a strong prevalence of monomeric spe-
cies for the preparation conditions used (Table 1). The
monitoring of the temperature dependent evolution of the
molecular structures and configurations by in situ Raman
snapshots has provided sound evidence suggesting a
prevalence of monomeric MoOx and WOx species after
calcination [26,27]. However, one should be rather careful
when suggesting a low extent of association (polymeriza-
tion) of oxometallic units on anatase, based solely on e.g.,
the low relative intensity of the ~925 cmꢁ1 MoeOeMo
band, as compared to the intensity of the 995 cmꢁ1 Mo]O
band. Pertinent concern on the possibility for laser wave-
length dependence of the relative intensities of bands due
to the deposited species resulted in the discovery that
Raman bands due to out-of-plane symmetric stretching
vibrations of bridging oxygen units (MoeOeMo) appear
much stronger with UV excitation compared to the corre-
sponding band intensity when visible laser excitation was
used [32].
W=O
1008
C2H6(g)
C2H6/O2(g)
O2(g)
W-O-W
800
1100 1000
900
700
600
500
Raman Shift, cm-1
Fig. 4. In situ normalized Raman spectra obtained for the (MoOx)n/TiO2
catalyst with 3.8 Mo/nm2 at 430 ꢀC under: (a) flowing O2(g); (b) flowing 5.6%
C2H6(g)/5.6% O2(g)/He(g); (c) 11.2% C2H6(g)/He. Recording parameters: see
caption of Fig. 1.
of the reactor atmosphere on the in situ Raman spectra
obtained at 430 ꢀC for the high-loaded 3.8 Mo/nm2 (Fig. 4),
3.8 W/nm2 (Fig. 5) and 4.0 V/nm2 (Fig. 6) catalyst samples.
Typically, the following effects are observed:
(a) a decrease in the relative Raman band intensity due to
the nM]O mode; the decrease follows the sequence
(VOx)n >> (MoOx)n > (WOx)n and is furthermore mono-
tonically larger with increasing coverage. A quantitative
exploitation of the normalized band intensities and plots of
Finally, it is noteworthy that no bands due to crystalline
transition metal oxides (MoO3, WO3, and V2O5) are
observed in Figs. 1e3, indicating a good dispersion for the
deposited oxometallic phases, achieved by the EDF prepa-
ration method.
the relative normalized intensity of the nM]O mode, IM]O
,
for each sample are compiled in Fig. 7. It is seen that the
intensity decrease for IV]O is rather abrupt (reaching ~65%)
for the high loaded 4.0VTi sample and is barely ~6% for the
WTi samples.
3.1.2. Spectral response to catalyst exposure to reacting
(C2H6(g)/O2(g)) and reducing (C2H6(g)) atmospheres
The response of the vibrational properties of the
deposited (MOx)n amorphous phase to changes in the gas
atmosphere was studied by switching the feed gas from
O2(g) to 5.6% C2H6/5.6% O2/He (i.e. ethane ODH reaction
conditions) and to 11.2% C2H6/He (i.e. reducing conditions)
and obtaining the pertinent in situ Raman spectra at 430 ꢀC
under the respective flowing gas mixtures after 1 h of
treatment. The protocol used has been outlined in Section
2.2. Figs. 4e6 show (as representative examples) the effect
(b) a progressive red shift is observed for the band position
of the nM]O mode upon exposure of the (MoOx)n/TiO2 and
(VOx)n/TiO2 samples to C2H6/O2 and C2H6 containing gases;
no such effect is observed for the (WOx)n/TiO2 catalysts, for
which the effect of the reducing atmosphere is generally
low. Table 2 compiles the M]O band wavenumbers for the
high loaded catalysts measured under O2(g) and C2H6/
He(g) conditions as well as the corresponding measure of
the red shift observed in the nM]O
.
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
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6
A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
TiO2
TiO2
T=430 °C
T=430 °C
630
630
ν1(Eg)
ν1(Eg)
(WOx)n/TiO2
3.8 W/nm2
(VOx)n/TiO2
4.0 V/nm2
(c)
(b)
(c)
V=O
TiO2
ν4 B1g
1018
2
(
)
(b)
(a)
C2H6(g)
W=O
1008
C2H6(g)
C2H6/O2(g)
(a)
C2H6/O2(g)
V-O-V
1029
O2(g)
O2(g)
W-O-W
800
1100 1000
900
800
700
600
500
1100 1000
900
700
600
500
Raman Shift, cm-1
Raman Shift, cm-1
Fig. 6. In situ normalized Raman spectra obtained for the (VOx)n/TiO2
catalyst with 4.0 V/nm2 at 430 ꢀC under: (a) flowing O2(g); (b) flowing 5.6%
C2H6(g)/5.6% O2(g)/He(g); (c) 11.2% C2H6(g)/He. Recording parameters: see
caption of Fig. 1.
Fig. 5. In situ normalized Raman spectra obtained for the (WOx)n/TiO2
catalyst with 3.8 W/nm2 at 430 ꢀC under: (a) flowing O2(g); (b) flowing 5.6%
C2H6(g)/5.6% O2(g)/He(g); (c) 11.2% C2H6(g)/He. Recording parameters: see
caption of Fig. 1.
Previously, it has been demonstrated by in situ Raman
spectroscopy that under reducing conditions the band due
to nM]O undergoes an intensity loss along with broadening
and a slight red shift and that the extent of these obser-
vations is commensurate with the extent of reduction
[7e10,28]. Therefore, the data in Fig. 7 and Table 2 are
suggestive of an extent of reduction that follows the
sequence (VOx)n >> (MoOx)n > (WOx)n and is furthermore
increasing with increasing coverage within each series.
Evidently, a large extent of reduction is related to a corre-
spondingly high extent of association/polymerization evi-
denced by the in situ Raman spectra (Figs.1e6). Therefore, a
higher susceptibility to reduction is evidenced for the
associated/polymeric species (MOx)n, compared to their
isolated counterparts, MOx. Moreover, as will be discussed
below (Section 3.2.3), the extent of the reduction suscep-
tibility exhibited by the deposited (MOx)n phase is
commensurate with its reactivity during ethane ODH,
albeit the reaction pathways (i.e. ethylene selectivity, etc.)
are affected by a number of additional parameters. Finally,
whereas the intensity decrease of the nM]O mode is inter-
preted to indicate oxygen removal (especially for the nV]O
case suggesting a partial loss of the vanadyl character for
the V(IV) species) and decrease in the abundance of M]O
1.1
1.0
0.9
M
3.8MoTi
0.8
0.7
0.6
0.5
0.4
0.3
2.6MoTi
0.3MoTi
3.8WTi
1.9WTi
1.7WTi
4.0VTi
2.8VTi
1.9VTi
C2H6/O2
Feed
C2H6
O2
Fig. 7. Relative normalized intensity of the M]O band (M ¼ Mo, W, and V)
under the indicated gas feed conditions (see text) for the catalyst samples as
indicated.
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
7
Table 2
(A)
Vibrational Raman wavenumbers of terminal M]O bond vibrations
(M ¼ Mo, W, and V) at 430 ꢀC.
W/F = 1.25 g s cm-3
3.8MoTi
3.8WTi
1.9WTi
1.7WTi
2.8x10-4
2.1x10-4
1.4x10-4
Catalyst
nM]O(cmꢁ1),
under O2(g)
nM]O(cmꢁ1),
under C2H6/He(g)
DnM]O(cmꢁ1
)
M
0.3MoTi
2.6MoTi
3.8MoTi
1.7WTi
1.9WTi
3.8WTi
1.9VTi
992
994
995
1005
1005
1008
1026
1028
1029
992
993
992
1005
1005
1008
1023
1024
1018
0
1
3
0
0
0
3
4
11
7.0x10-5
0.0
2.8VTi
4.0VTi
420 430 440 450 460 470 480
Temperature, oC
2.0x10-2
1.5x10-2
1.0x10-2
5.0x10-3
(B)
sites, the slight red shift and nM]O band broadening under
reduction conditions indicate a lower bond order for
certain M]O terminal bonds (also resulting in a wider
distribution of bond orders, as evidenced by the band
broadening). This can be a result of cascade effects (i.e.
events including alterations in local geometry, coordina-
tion, etc. that affect through the valence sum rule the bond
order of the M]O site) that result in weakening of a
number of M]O bonds under reducing conditions.
W/F = 0.06 g s cm-3
3.9VTi
2.8VTi
1.9VTi
3.2. Catalytic measurements
420 430 440 450 460 470 480
Temperature, oC
3.2.1. Effect of coverage and residence time on catalytic
performance
The dependence of the ethane TOF, expressed as moles
C2H6 converted per sec and per mol of M (M ¼ Mo, W, and
V) on temperature is shown in Fig. 8. The upper part
(Fig. 8(A)) pertains to the (MoOx)n/TiO2 and (WOx)n/TiO2
samples at a residence time, expressed by the ratio W/F, of
1.25 g s cmꢁ3. Notably, ethane conversions were nearly zero
for the 0.3MoTi and 2.6MoTi catalysts below 480 ꢀC and the
pertinent TOF data are not included in Fig. 8. Ethane TOF's
are generally low and exhibit an exponential-like depen-
dence on temperature.
The 3.8WTi sample, although of high coverage
(approximate monolayer), exhibits significantly lower
reactivity compared to the 1.7WTi and 1.9WTi samples of
mediumelow coverage. This is due to the inherent reac-
tivity of the exposed TiO2 phase, which for the samples
with mediumelow coverage (1.7WTi and 1.9WTi) con-
tributes very significantly to the apparent reactivity. Thus,
the deposition of (WOx)n species on TiO2 up to the
approximate monolayer (e.g., for the 3.8WTi sample)
merely consumes(blocks) the reactive carrier sites. Notably,
ethane ODH tests over pure TiO2 (at T ¼ 480 ꢀC and W/
F ¼ 1.25 g s cmꢁ3) resulted in ~3% conversion (compared to
1,1.4 and 0.5% for 1.7WTi, 1.9WTi and 3.8WTi, respectively),
albeit at negligible selectivity for C2H4 (~99% selectivity for
CO2). A comparison among the 3.8MoTi and 3.8WTi with
coverages approaching the approximate monolayer sug-
gest an almost triple reactivity for the deposited (MoOx)n
phase, compared to its (WOx)n counterpart at 480 ꢀC.
Meticulous analysis of the structural properties of (MoOx)n/
TiO2 and (WOx)n/TiO2 catalysts prepared by the
Fig. 8. Ethane conversion as a function of temperature for all catalysts:
(A) MoTi and WTi catalysts; W/F
¼
1.25 g s ; (B) VTi catalysts;
cmꢁ3
W/F ¼ 0.06 g s cmꢁ3
.
equilibriumedepositionefiltration (EDF) method is re-
ported elsewhere [20,26,27].
Fig. 8(B) displays the ethane TOFs measured over
(VOx)n/TiO2 catalysts in the 420e480 ꢀC range which were
measured at much lower residence times (corresponding to
W/F ¼ 0.06 g s cmꢁ3) due to the much higher reactivity of
the deposited vanadia phase. A consistent monotonic in-
crease in the achieved conversions with increasing
coverage is observed within the (VOx)n/TiO2 series. The
effect of the exposed TiO2 sites is negligible due to the
much higher reactivities of the deposited (VOx)n phases.
The discussion on catalyst reactivity is complemented
by also addressing the selectivity achieved at various con-
version levels by varying the residence time (see below).
3.2.2. Reaction pathways for catalysts with low and high
surface coverage
The ODH of ethane over supported transition metal
oxide catalysts follows diverse reaction pathways that
depend strongly also on the surface coverage of the
deposited oxometallic phase. A general scheme for alkane
dehydrogenation reaction pathways is shown in Fig. 9. The
uncovered TiO2 surface contains lattice and adsorbed ox-
ygen species that constitute total oxidation sites for C2H6 as
well as of C2H4 [33]. For shedding light on this mechanistic
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
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8
A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
(A)
100
90
80
70
60
50
40
30
35
30
25
20
15
10
5
Ethylene
CO
CO2
1.9VTi
Fig. 9. Reaction scheme for alkane dehydrogenation pathways.
20
10
0
issue we will examine the catalytic behavior of samples
with both low and high coverage.
0
0.30
0.00
0.05
0.10
0.15
0.20
0.25
For brevity, the discussion will be confined to the
(MoOx)n/TiO2 and (VOx)n/TiO2 catalysts; the (WOx)n/TiO2
catalysts exhibit limited reactivity for the ethane ODH
reaction.
Fig. 10 shows the selectivities for C2H4, CO and CO2 as
well as the C2H6 conversions achieved with increasing
residence time, expressed as the W/F ratio in the range of
0.28e1.25 g s cmꢁ3, for the 0.3MoTi and 3.8MoTi samples at
480 ꢀC. Fig. 11 displays the corresponding catalytic behavior
W/F, g s cm-3
(B)
100
90
80
70
60
50
40
30
20
10
0
35
30
25
20
15
10
5
Ethylene
4.0VTi
CO
CO2
(A)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
-3
W/F, g s cm
Fig. 11. Catalytic data (conversion and selectivities) as a function of W/F for:
(A) the 1.9VTi; (B) the 4.0VTi catalyst samples. T ¼ 480 ꢀC
of the 1.9VTi and 4.0VTi catalysts as a function of W/F,
which is varied between 0.04 and 0.28 g s cmꢁ3. The much
lower residence times for the VTi samples were necessary
in order to ensure the differential operation of the reactor
(conversion lower than 10%). Although conversions excee-
ded 10% for the 4.0VTi sample at W/F > 0.10 g s cmꢁ3
(Fig. 11(B)), the conversion vs W/F behavior is suggestive of
lack of mass transfer limitations at W/F higher than
(B)
0.10 g s cmꢁ3
.
As seen in Fig. 10(A), primary steps of ethane activation
for the 0.3 Mo/nm2 catalyst (with a very high area of
exposed TiO2) follow mainly route r3 (combustion to CO2)
and partly also the selective route r1 (leading to C2H4).
Extrapolation of the CO2 and C2H4 selectivity plots to
W/F ¼ 0 points to ~70% and ~30% initial selectivities for CO2
and C2H4. Significantly, uncovered titania support contrib-
utes to the reactivity through the plethora of active lattice
oxygen as well as adsorbed oxygen sites favoring an easy
activation of ethane that, however, leads to CO2 formation
following combustion routes [33]. With increasing resi-
dence time the SCO increases at the expense of SC H
2
2
4
through activation of route r5 resulting in total oxidation of
part of the C2H4 produced over the exposed titania active
sites [33].
Fig. 10. Catalytic data (conversion and selectivities) as a function of W/F for:
(A) the 0.3MoTi; (B) the 3.8MoTi catalyst samples. T ¼ 480 ꢀC.
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
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A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
9
The behavior of the 3.8 Mo/nm2 sample, of which the
surface is mostly covered by the deposited (MoOx)n phase,
is markedly different (Fig. 10)(B). Primary steps appear to
follow route r1 over the (MoOx)n sites (leading to C2H4) and
(to a much lesser extent) route r3 over the exposed titania
sites (leading to CO2); this is evidenced by extrapolating the
corresponding selectivity plots to W/F ¼ 0, thereby pointing
to initial selectivities of ~85%, ~15% and 0% for C2H4, CO2 and
CO, respectively. The molybdena saturation capacity on
TiO2, determined by Raman spectroscopy, is ~5 Mo/nm2
[10,34,35]. Therefore, the partial activation of route r3 for
the 3.8 Mo/nm2 sample is ascribed to the partial occurrence
of exposed active oxygens of the carrier lattice [33].
Whereas the CO2 selectivity remains stable at a ~15% level,
a partial activation of route r4 (through which C2H4 is
converted to CO over the plethora of (MoOx)n sites) takes
place with increasing W/F. The general behavior of
decreasing C2H4 selectivity with increasing conversion
conforms to established trends for alkane ODH over sup-
ported transition metal oxide catalysts [7e9,12,36e42].
The behavior of the 1.9VTi sample with 1.9 V/nm2 is
quite unique (Fig. 11(A)). Primary steps of ethane activation
seemingly follow route r1 over (VOx)n sites leading to C2H4
and route r3 over the exposed titania sites leading to CO2.
Additionally, substantial amounts of CO are produced over
(VOx)n sites (presumably through the r1 / r4 sequence).
For a residence time corresponding to W/F ¼ 0.06 g s cmꢁ3
the selectivity values for C2H4, CO and CO2 are 40%, 29% and
31%. Notably, the monolayer capacity for the dispersion of
(VOx)n on TiO2(anatase) is ~7e8 V/nm2 [28]. Thus, a large
proportion of the surface offers exposed active oxygen sites
of the anatase that through route r3 result in a substantial
production of CO2. Although SC H appears to decline at the
(VOx)n/TiO2 catalysts with 4.0 V/nm2 prepared by
EDF (present work) and wet impregnation [7] exhibit for
W/F ¼ 0.06 g s cmꢁ3 alkane TOF values of 1.8 ꢂ 10ꢁ2 and
1.3 ꢂ 10ꢁ2 per mole of V and per sec. Thus, although (as
shown above) primary reaction paths of ODH of light al-
kanes over supported transition metal oxides follow highly
selective routes, the respective products (ethylene and
propene) are often converted to combustion products (CO
and CO2) through secondary reaction paths. Such non-
selective pathways are commonly activated at high con-
versions or low GHSV [7e9,17].
3.2.3. Apparent turnover rates and apparent activation energies
Comparative reactivity assessments among catalysts
can be made by comparing the apparent turnover fre-
quencies, TOF, expressed per M site (i.e. moles of C2H6
converted per sec and per M atom). The apparent TOF in-
corporates and reflects the site reactivity as well as aspects
related to site availability/accessibility. The eventual
inherent reactivity of the exposed carrier surface is also
unavoidably reflected by the apparent TOF. The compara-
tive picture is complemented by the pertinent selectivities
(see previous sub-section) as well as by the apparent acti-
vation energies, determined by the corresponding
Arrhenius-type plots (ln TOF vs 1/T).
Fig. 12 shows the Arrhenius-type plots of ln TOF vs 1/T
for all catalysts studied up to 480 ꢀC and the pertinent
apparent activation energies, determined from the slopes
of the plots shown in Fig. 12 (corresponding to ꢁEa/R), are
compiled in Table 3. A consistent monotonic increase of Ea
is found with increasing coverage of the carrier surface
from the deposited (MOx)n oxometallic phase. This
behavior implies a progressively more difficult activation of
ethane with increasing surface density (M/nm2). However,
the apparent ease of activation at low coverages is to be
ascribed to the active oxygen atoms of the exposed carrier
lattice (leading to non-selective pathways, as discussed
above) and not to the deposited (MOx)n phase. Conversely,
the progressive increase in the surface density of the
2
4
benefit of SCO for high W/F values (as expected) the overall
2
stability of SC H vs W/F is not completely understood.
2
4
A high reactivity with an initial selectivity for C2H4
exceeding 60% is evidenced for the 4.0VTi catalyst, albeit
with a selectivity for CO that increases markedly at the
expense of C2H4 selectivity with increasing conversion,
suggesting that an initial activation of the selective route r1
over (VOx)n sites is followed by route r4 (leading to CO) at
increasing residence times. A significant part of the carrier
remains exposed for the 4.0VTi sample (the monolayer
capacity is ~7e8 V/nm2), thereby justifying the partial
activation of route r3 resulting in CO2 through total oxida-
4.0VTi
2.8VTi
-4
1.9VTi
-5
-6
tion of C2H6. The observed slight increase of SCO at
2
increasing residence times is ascribed to total oxidation of
some of the extant C2H4 over exposed titania sites through
route r5.
-7
1.7WTi
1.9WTi
3.8WTi
-8
From the above discussion, it turns out that despite the
attempted tailoring of the deposited (MOx)n (M ¼ Mo, W,
and V) species on titania by means of the EDF method, the
synthesized catalysts exhibit mutatis mutandis a similar
behavior to the corresponding supported transition metal
oxides prepared by conventional methods (e.g., wet
impregnation) and tested for ODH reactions of light alkanes
[7e10,17]. Indicatively, for W/F ¼ 0.28 g s cmꢁ3, (MoOx)n/
TiO2 catalysts with a surface density of 3.8 Mo/nm2 pre-
pared by EDF(present work) and wet impregnation [10]
exhibit ethane TOF values of 1.5 ꢂ 10ꢁ4 and 3.0 ꢂ 10ꢁ4
per mole of Mo and per sec. Comparative data pertaining to
-9
3.8MoTi
-10
-11
-12
1.35
1.40
1.45
1.50
1000/T, K-1
Fig. 12. Arrhenius-type plots of ln TOF vs 1/T pertaining to ethane con-
sumption for the catalysts studied.
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

10
A. Tribalis et al. / C. R. Chimie xxx (2016) 1e11
Table 3
Apparent activation energies (Ea) of ethane conversion for the studied catalysts (kJ molꢁ1).
Catalyst
3.8MoTi
1.7WTi
1.9WTi
3.8WTi
1.9VTi
2.8VTi
4.0VTi
Ea (std. error)
106.2 (2.6)
109.6 (9.8)
116.5 (9.8)
127.8 (7.0)
94.1 (8.2)
122.5 (2.8)
131.3 (1.3)
(MOx)n deposited species activates selective reaction
pathways. Evidently, the possibility for the mobile inter-
mediate ethoxy species to “burn out” on exposed support
non-selective active center is diminished with increasing
coverage.
decrease and the red shift of the nM]O mode follow the
sequence (VOx)n >> (MoOx)n > (WOx)n and are further-
more much more pronounced with increasing coverage
within each catalyst series. Reaction pathways follow se-
lective routes at high coverages, albeit the selectivity is
partly controlled also by the achieved conversion, with
secondary routes to CO activated at high residence times.
The apparent activation energies (determined from dif-
ferential activity data) increase with increasing coverage,
revealing an ease of activation at low coverage where the
exposed carrier is involved by favoring non-selective
pathways. Preferences to selective reaction pathways
with increasing coverage occur concurrently with
increased susceptibility to reduction (evidenced by the in
situ Raman spectra) and are accounted for by changes in
the charge accommodation by the metal center that affect
the pre-exponential factor through the corresponding
electronic partition function.
Moreover, it is seen that the catalyst reactivity appar-
ently tracks the reducibility of the metal center evidenced
by the in situ Raman spectra (Section 3.1, Fig. 7) with the
relationship between reducibility and electron transfer
being of critical nature; the higher the density of states
susceptible to electron acceptance, the higher the reduc-
ibility of a substance. The increased activity with increasing
coverage (above 2.8 V/nm2) for the (VOx)n/TiO2 series can
be well understood under this viewing angle. With pro-
gressive increase in the extent of deposition, in parallel
with the structural and configurational evolution, changes
also occur in the electronic structure of the complexes. The
V atoms within the (VOx)n phase are linked to the surface
(VeOeTi anchors) and to each other through VeOeV
bridges and with increasing extent of association more
intermediate range interactions between V centers take
place. As a result, the electron levels form bands and the
density of states increases [43], thereby affecting the charge
accommodation by the V center. Therefore, whereas the
translational, rotational and vibrational partition functions
contributing to the pre-exponential factor (reflecting the
degrees of freedom and the structural properties of the
deposited amorphous phase at intermediate and high
coverage) can be considered as mutatis mutandis stable, it
remains to the electronic partition function of the pre-
exponential factor (as directly relevant to the density of
electronic states) to account for the observed increase in
the inherent activity of the deposited phase at high
coverage. The corresponding effect cannot be revealed for
the (WOx)n/TiO2 series, of which the deposited oxo-
tungsten phase exhibits very low reducibility (Fig. 7) and
limited reactivity that is thereby masked by the reactivity of
the exposed carrier sites.
Acknowledgment
George D. Panagiotou is thanked for assisting the cata-
lyst preparation. Financial support was provided by the
Research Committee of the University of Patras (C. Car-
atheodory program C.583). The article is dedicated to Pro-
fessor Edmond Payen, one of the luminaries in scientific
research at the interface of spectroscopy and catalysis.
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The molecular structural properties of the deposited
oxometallic phase in (MoOx)n/TiO2, (WOx)n/TiO2 and
(VOx)n/TiO2 catalysts prepared by the EDF (equili-
briumedepositionefiltration) method were studied by in
situ Raman spectroscopy under oxidizing (O2), reaction
(C2H6/O2/He) and reducing (C2H6/He) conditions. A well-
expected increase in the extent of association is evi-
denced with increasing coverage, following the sequence
(VOx)n >> (MoOx)n > (WOx)n. The response of the in situ
Raman spectra under reaction and reducing conditions
reflects (through intensity lowering and band position red
shift for the nM]O mode) the extent of reducibility
exhibited by the different catalysts; both the intensity
Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013

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Please cite this article in press as: A. Tribalis, et al., Molecular structure and reactivity of titania-supported transition metal oxide
catalysts synthesized by equilibrium deposition filtration for the oxidative dehydrogenation of ethane, Comptes Rendus Chimie
(2016), http://dx.doi.org/10.1016/j.crci.2015.08.013