Characterization of vanadium and titanium oxide supported SBA-15

Article abstract of DOI:10.1021/jp0507750

Supported vanadium and titanium oxide catalysts were prepared by adsorption and subsequent calcination of the vanadyl and titanyl acetylacetonate complexes, respectively, on mesoporous SB A-15 by the molecular designed dispersion (MDD) method. Liquid and gas phase depositions at different temperatures were carried out with vanadyl acetylacetonate, and the different results together with those of titanyl acetylacetonate in the liquid phase deposition were discussed. The bonding mechanism, the influence of the metal interaction with the support material, and differences due to the way of deposition and the temperature were investigated by TGA, chemical analysis, FTIR, and Raman spectroscopy. Elevated dissolving temperatures in the liquid phase led to higher final loadings on the SBA-15 without the formation of clusters, even at high loadings. The decomposition of the anchored vanadium and titanium complexes, their thermal stability, and the conversion to the covalently bound VO_X and TiO_x species on SBA-15 were studied and investigated by in situ transmission IR spectroscopy. In general, the titanium complex is more reactive than the vanadium complex toward the surface of SBA-15 and has a higher thermal stability. The MDD method of the VO(acac)₂ and TiO(acac)₂ enables to create a dispersed surface of supported VO_x and TiO_x, respectively. The structure configurations of VO_x and TiO_x oxide catalysts obtained at different metal loadings were studied by Raman spectroscopy. Pore size distributions, XRD, and N₂ sorption confirmed the structural stability of these materials after grafting. VO_x/SBA-15 and TiO _x/SBA-15 samples, with different metal loadings, were also catalytically tested for the selective catalytic reduction (SCR) of NO with ammonia. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp0507750

J. Phys. Chem. B 2005, 109, 12071-12079
12071
Characterization of Vanadium and Titanium Oxide Supported SBA-15
Y. Segura,*,† P. Cool,† P. Kustrowski,‡ L. Chmielarz,‡ R. Dziembaj,‡ and E. F. Vansant†
Department of Chemistry, Laboratory of Adsorption and Catalysis, UniVersity of Antwerp (UA),
UniVersiteitsplein 1, B-2610 Wilrijk, Belgium, and Faculty of Chemistry, Jagiellonian UniVersity,
Ingardena 3, 30-060 Krakow, Poland
ReceiVed: February 14, 2005; In Final Form: April 13, 2005
Supported vanadium and titanium oxide catalysts were prepared by adsorption and subsequent calcination of
the vanadyl and titanyl acetylacetonate complexes, respectively, on mesoporous SBA-15 by the molecular
designed dispersion (MDD) method. Liquid and gas phase depositions at different temperatures were carried
out with vanadyl acetylacetonate, and the different results together with those of titanyl acetylacetonate in
the liquid phase deposition were discussed. The bonding mechanism, the influence of the metal interaction
with the support material, and differences due to the way of deposition and the temperature were investigated
by TGA, chemical analysis, FTIR, and Raman spectroscopy. Elevated dissolving temperatures in the liquid
phase led to higher final loadings on the SBA-15 without the formation of clusters, even at high loadings.
The decomposition of the anchored vanadium and titanium complexes, their thermal stability, and the conversion
to the covalently bound VOx and TiOx species on SBA-15 were studied and investigated by in situ transmission
IR spectroscopy. In general, the titanium complex is more reactive than the vanadium complex toward the
surface of SBA-15 and has a higher thermal stability. The MDD method of the VO(acac)2 and TiO(acac)2
enables to create a dispersed surface of supported VOx and TiOx, respectively. The structure configurations
of VOx and TiOx oxide catalysts obtained at different metal loadings were studied by Raman spectroscopy.
Pore size distributions, XRD, and N2 sorption confirmed the structural stability of these materials after grafting.
VOx/SBA-15 and TiOx/SBA-15 samples, with different metal loadings, were also catalytically tested for the
selective catalytic reduction (SCR) of NO with ammonia.
1. Introduction
Materials containing vanadium and titanium oxides are known
as excellent catalysts in several redox reactions. Vanadium oxide
Metal oxide dispersions in mesoporous materials have been
of great interest by means of the incorporation of transition
metals in the framework of the support material. The first
publication related to this domain was the incorporation of Fe2O3
particles in MCM-41.1 Since then, much more work has been
done in the field of transition metal oxides supported on different
porous materials. One of the most common methods for the
preparation of metal oxide dispersions on mesoporous materials
is the impregnation with solutions of precursor salts.2,3 The
materials described in this work are no longer prepared by the
conventional methods but are the result of a molecular designed
dispersion (MDD)4 method of active elements on carefully
prepared micelle templated structures (MTS)5 using their
acetylacetonate complexes.
The interest of using these complexes is the requirement of
obtaining a high dispersion of catalytic active sites on support
materials. Their bulky acetylacetonate ()acac) ligands are able
to isolate the central metal ion on the surface, allowing high
dispersions. Following the MDD process, metal acac complexes
are reacted with the surface hydroxyls of the support material
and converted into the metal oxide form after calcination. Both
liquid and gas phase depositions have been performed to deposit
metal acac complexes on different supports.6 A great variety
of acac complexes on different supports have already been
studied and reported in the literature.7-10
is a powerful redox catalyst in many industrial processes, and
it is used in oxidation reactions for the manufacturing of
important chemicals.11 Titanium oxide, when it is on a silica
support, is mainly used as a photocatalyst, acid catalyst, and
redox catalyst. TiO2 is widely used as well for many oxidation
reactions of organic molecules requiring a high disperse layer
on the support.12 Catalysts based on V2O5/TiO2 are mainly used
in the reduction of nitrogen oxides. The selective catalytic
reduction (SCR) of NOx with ammonia is important to reduce
emissions of NO from waste gases of stationary sources.
However, the use of a silica support instead of titania offers
advantages because of the higher surface area and higher
resistance to sintering. The V2O5/TiO2 system has already been
studied widely well.13,14 In this paper, the systems VOx/
SBA-15 and TiOx/SBA-15 are fully characterized and studied
separately for the SCR reaction.
For the SCR reaction, the vanadia content in the catalyst is
preferably lower than the amount required for a monolayer
coverage,15,16 but for other types of oxidation reactions, a higher
loading is required with a high dispersion.17-19 Therefore, the
importance of tuning and controlling the final loading of metals
and their dispersion on the support material by different
deposition methods and temperatures is obvious.
VO(acac)2 and TiO(acac)2 have already been deposited on
different supports such as silica,20 alumina,21 MCM-48,22 or
zirconia.23 Recently, the oxidation of bulkier molecules has
required the synthesis of a material like SBA-15, which is a
* Corresponding author. E-mail: yolanda.segura@ua.ac.be.
† University of Antwerp (UA).
‡ Jagiellonian University.
10.1021/jp0507750 CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/27/2005

12072 J. Phys. Chem. B, Vol. 109, No. 24, 2005
Segura et al.
mesoporous silica molecular sieve with uniform pores. It is
characterized by a very high pore volume, thick pore walls, and
intrinsically combined micro- and mesopores with a high surface
area.24
In this study, we present different methods to prepare TiOx
and VOx on SBA-15 following the MDD method. A detailed
characterization of the catalyst precursors (before calcination)
and the final VOx/SBA-15 and TiOx/SBA-15 samples (after
calcination) is performed using IR transmission spectroscopy
following their conversion to the covalently bound VOx and
TiOx species on the SBA-15 surface. Additional characterization
is obtained by FTIR-PAS, TGA, chemical analysis, FT-Raman,
and N2 sorption measurements.
VOx/SBA-15 and TiOx/SBA-15 were catalytically tested in
the SCR of NO with ammonia. The catalytic performance of
the samples depends on the metal loading. VOx/SBA-15
catalysts are active in a low temperature range; however, an
increase of their activity is observed in the high temperature
range, while the TiOx/SBA-15 catalysts are not active below
250 °C.
temperature, because of decomposition of the complex (not
sublimation).22 The SBA-15 materials with adsorbed complexes,
obtained from the liquid and gas phases were called the
precursors. The acac ligands were removed by calcination, which
was performed in a programmable oven at 550 °C in ambient
air, resulting in a final vanadium or titanium oxide catalyst.
2.2. Characterization Methods. The concentration of the
metals on the support was determined by UV-vis spectro-
photometry after destruction of the samples and measured
colorimetrically. The measurements were performed on a
Unicam 8700 UV-vis instrument. The samples were stirred
for 20 min in hot sulfuric acid (2.5 M). After filtration, H2O2
was added, and the vanadium and titanium concentrations were
measured at 450 and 407 nm, respectively.25
Photoacoustic infrared spectra (FTIR-PAS) were recorded on
a Nicolet 20 SX spectrometer, equipped with a McClelland
photoacoustic cell, to ensure IR measurements under dry
conditions. About 1000 scans were taken with a resolution of
8 cm-1. The PAS spectrometer was placed in an isolated bench,
which was constantly purged with dry nitrogen to ensure
complete dry conditions.
2. Experimental Section
Transmission spectra were measured on a Nicolet20SX
spectrometer equipped with a vacuum system cell and a DTGS
detector and using a home-built in situ vacuum IR transmission
cell. Self-supporting disks with a thickness of 20 mg/cm2 were
used. One hundred scans were measured with a spectral
2.1. Sample Preparation and Treatment. 2.1.1. Support.
SBA-15 was prepared by using 4 g of Pluronic P123 triblock
copolymer surfactant (EO20-PO70-EO20) dissolved in a 2 M
water/HCl solution. Subsequently, an amount of tetraethyl
orthosilicate (TEOS) was added. The resulting mixture was
stirred for 8 h at 45 °C and then aged for 15 h at 80 °C. The
white product was filtered, washed, and dried. The sample was
subsequently calcined at 550 °C at 1 °C/min for 6 h in air
atmosphere.
2.1.2. Metal Deposition. Single depositions of the vanadium
and titanium metal oxides on the ordered mesoporous material
SBA-15 were done by the MDD method of their acetylacetonate
complexes. The TiO(acac)2 and VO(acac)2 complexes both have
the same structure, except for the fact that TiO(acac)2 is present
as a bidentate complex ([TiO(acac)2]2), which consists of a
cyclic dimer with the Ti atoms linked through oxygen atoms.
Liquid phase deposition was performed by the deposition of
the metal acetylacetonate complex (VO(acac)2 or TiO(acac)2)
onto the SBA-15 surface by the designed dispersion method
using toluene as a solvent. An amount of TiO(acac)2 or
VO(acac)2 was dissolved in 100 mL of zeolite dried toluene.
The dried SBA-15 was then added, and the solution was stirred
for 1 h. After reaction, the modified support was filtered off,
washed a few times with toluene, and dried under vacuum. The
reaction was carried out at RT and in the absence of air. Liquid
phase deposition was also carried out by dissolving the
acetylacetonate complex at 45 °C. After adding the SBA-15
support at 45 °C, the temperature was dropped to room
temperature in an hour while the suspension was stirred. After
reaction, the modified support was filtered off, washed a few
times with toluene, and dried under vacuum.
resolution of 4 cm-1
.
FT-Raman spectra were recorded on a Nicolet Nexus
FT-Raman spectrometer with a Ge detector. All samples were
measured at room temperature in a 180° reflective sampling
configuration, with a 1064 nm Nd:YAG excitation laser. For
each spectrum, 2.000 scans were averaged and the laser power
was set between 1 and 2 W.
Thermogravimetric (TG) measurements were performed on
a Mettler TG50 thermobalance, equipped with a M3 micro-
balance and connected to a TC10A processor. Samples were
heated from 30 to 50 °C and from 50 to 550 °C at a rate of
5 °C/min.
XRD diffractograms were taken on a Philips PW1840 powder
diffractometer using Ni-filtered Cu KR radiation (λ )
1.542 Å)
Porosity and surface area studies were performed on a
Quantachrome Autosorb 1 MP instrument. N2 adsorption/
desorption isotherms were recorded at -196 °C. The Brunauer-
Emmett-Teller (BET) model was used to calculate the specific
surface area. Micropore volume was calculated by the t-plot
analysis, and the total pore volume was calculated by means of
the total amount of adsorbed gas at P/P0 ) 0.98. The samples
were outgassed overnight at room temperature for the precursors
or at 200 °C for the supported oxides.
The catalytic experiments were performed at atmospheric
pressure in a fixed-bed flow reactor (inside diameter, 7.0 mm;
length, 240 mm). The reactant concentrations were continuously
measured using a quadrupole mass spectrometer (VG QUARTZ)
connected to the reactor via a heated line. Prior to the reaction,
each sample of the catalyst was outgassed in pure helium at
200 °C for 30 min. Samples of 100 mg of the catalyst were
used. At the reactor inlet, the composition of the gas mixture
was [NO] ) [NH3] ) 2500 ppm, [O2] ) 2.5%, and He as a
balancing gas at a total flow rate of 40 mL/min. The reaction
was studied at temperatures ranging from 100 to 550 °C. The
intensities of the mass lines corresponding to all reactants and
products were measured at a given temperature for 15-30 min
SBA-15 supported vanadium oxides were also obtained by a
gas phase deposition avoiding the use of the organic solvents.
The gas phase modification of the SBA-15 with VO(acac)2 was
carried out by placing both components in a glass holder initially
under vacuum at RT. The temperature was increased, and the
sublimed vanadium complex reacted with the surface of
SBA-15. The excess of VO(acac)2 was removed, by taking out
the modifier holder inside of the glass holder. Gas phase
deposition using the TiO(acac)2 complex was not possible. It
was found in previous research that a very small amount of
titanium and acac ligands were detected after a reaction at high

Vanadium and Titanium Oxide Supported SBA-15
J. Phys. Chem. B, Vol. 109, No. 24, 2005 12073
Figure 1. Schematic representation of the “molecular designed dispersion” method of acetylacetonates.
Figure 2. FT-IR spectra of (a) pure SBA-15 and SBA-15 supported vanadium oxides: (b) precursor deposited by the liquid phase method;
(c) precursor deposited by the gas phase method (at 140 °C); (d) liquid phase deposition calcined sample; (e) gas phase deposition calcined sample.
26
after the reaction reached a steady state. The signal of the helium
line served as the internal standard to compensate for small
fluctuations of the operating pressure. The sensitivity factors
of the analyzed lines were calibrated using commercial mixtures
of gases. The possible changes in the molar flow caused by the
NO and NH3 conversion were negligible in the diluted reaction
mixtures. The conversion (expressed in percent) was calculated
by the difference between inlet and outlet molar flow of the
reactants in the reactor.
associated with one metal (vanadium or titanium) atom.
mmol(acac)/gtotal
R )
mmol(TiorV)/gtotal
A ligand exchange results in an R value of 1, because only one
acetylacetonate ligand remains bonded to the metal atom. For
the hydrogen bonding, an R value of 2 can be obtained, since
there is no loss of any of the acac ligands.20,22
3.1.1. VO(acac)2/SBA-15 Support. According to this MDD
method, the surface of SBA-15 was modified with different
amounts of V, using vanadyl acetylacetonate as the V source,
under liquid phase and gas phase conditions and at different
temperatures.
In Figure 2, the infrared spectra are shown for the pure
SBA-15 and the SBA-15 supported vanadium oxide precursors
and final VOx catalyst obtained by gas phase and liquid phase
deposition.
The same initial concentration (0.75 g of VO(acac)2) was used
by liquid and gas phase deposition of the vanadium complex.
However, a higher vanadium loading was obtained by the gas
phase deposition at 180 °C (1.5 mmol/g) compared to the
loading obtained on the surface of SBA-15 by the liquid phase
deposition at RT (0.4 mmol/g).
3. Results and Characterization
3.1. Characterization of the Precursors. The TiO(acac)2
and VO(acac)2 acetylacetonate complexes were grafted on the
surface of the SBA-15 following the MDD method. A schematic
representation of this MDD mechanism is depicted in
Figure 1.
The adsorption of the complexes on the support follows two
different mechanisms. One is by hydrogen bonding between
an acetylacetonate ligand and the surface hydroxyls, and the
other one is by a ligand exchange mechanism in which a
covalent metal-oxygen support bond is formed while an
acetylacetonate ligand (Hacac) is lost.
The calculation of R value allows us to elucidate the reaction
mechanism, since it reproduces the number of acac ligands

12074 J. Phys. Chem. B, Vol. 109, No. 24, 2005
Segura et al.
TABLE 1: Structural Properties, Vanadium Loadings, and R Values of Blanks, Precursors, and Final Vanadium Oxide
Catalysts on the SBA-15 Support
SBET
micropore
vol (cc/g)
total pore
vol (cc/g)
V loading
(mmol/g)
sample
(m2/g) (loss %)
V atoms/nm2
0.33
R value
2.0
Liquid Phase (RT)
SBA-15 blank
SBA-15/VO(acac)2 prec.
SBA-15/VOx
735
319
557 (24%)
0.146
0.001
0.094
0.76
0.46
0.62
0.4
1.3
1.5
Gas Phase (140 °C)
SBA-15 blank
735
171
463 (37%)
0.146
0.004
0.048
0.76
0.24
0.56
SBA-15/VO(acac)2 prec.
SBA-15/VOx
1.06
1.2
Gas Phase (180 °C)
SBA-15 blank
886
215
376 (58%)
0.150
0.000
0.022
0.91
0.28
0.48
SBA-15/VO(acac)2 prec.
SBA-15/VOx
1.20
1.0
TABLE 2: Structural Properties and Metal Loadings on SBA-15 Obtained under Liquid Phase Conditions at RT and at 45 °C
(Initial Concentration 1 g of acac Complex per 1 g of SBA-15)
SBET
micropore vol
(cc/g)
total pore vol
(cc/g)
metal loading
(mmol/g)
sample
SBA-15 blank
SBA-15/VOx (liquid phase, RT)
SBA-15 blank
SBA-15/VOx (liquid phase, 45 °C)
SBA-15 blank
SBA-15/VOx (gas phase, 180 °C)
SBA-15 blank
(m2/g) (loss %)
metal atoms/nm2
R value
722
623 (14%)
848
630 (26%)
717
392 (45%)
848
628 (26%)
759
612 (20%)
0.125
0.091
0.162
0.078
0.135
0.027
0.162
0.083
0.116
0.083
0.80
0.71
0.99
0.81
0.79
0.57
0.99
0.80
0.88
0.72
0.45
0.76
1.65
0.92
1.10
0.37
0.54
1.40
0.65
0.90
2.0
1.8
1.1
1.9
1.8
SBA-15/TiOx (liquid phase, RT)
SBA-15 blank
SBA-15/TiOx (liquid phase, 45 °C)
It is shown in Figure 2c that the concentration of vanadium
deposited on the surface by gas phase reaction was enough to
cover most of the free hydroxyl groups of the silica surface.
The silanol band at 3675 cm-1 has almost completely disap-
peared, while in the case of liquid phase deposition (Figure 2b)
a noticeable silanol band is still visible although less intense
compared to the pure SBA-15 support.
exchange mechanism, while in the liquid phase the complex
interacts with the SBA-15 surface by hydrogen bonding (the R
value is 2). This is in accordance with the IR spectrum (Figure
2b), which shows a significant absorption in the region between
3600 and 3200 cm-1 attributed to the presence of hydrogen
bonded silanols.
R values around 1 are obtained for the samples prepared under
gas phase conditions by a ligand exchange mechanism. In fact,
the broad band at 3600-3200 cm-1 is less pronounced in
spectrum c of Figure 2. As the treatment temperature decreases
(from 180 to 140 °C), the reaction mechanism shows a
combination of the two mechanisms (from R ) 1 to R ) 1,2).
Experiments reveal that gas phase deposition at higher
reaction temperatures leads to an increase in V loading in the
final catalyst. As a consequence, the porosity of the calcined
sample is slightly reduced as a function of the vanadium loading,
as shown in Table 1.
For the deposited sample at 140 and 180 °C, a loss of 37
and 58% in SBET, respectively, can be observed. These decreases
in surface area (expressed in percent) are in agreement with
the differences in V content (1.3 and 1.5 mmol, respectively).
In the case of the gas phase deposition of V, the surface area
and mesopore volume are restored to some extent after calcina-
tion, but the micropore volume barely increases after removing
the organic ligands by a thermal treatment. This can be explained
by the high concentration of vanadium deposited on the surface
of SBA-15 and a possible migration of the V toward the
micropore walls during the calcination process, especially for
the 180 °C reaction (Table 1).
Parts A and B of Figure 3 show the N2 adsorption/desorption
isotherms at -196 °C for the blank SBA-15 and samples
obtained under liquid and gas phase conditions, respectively.
All adsorption/desorption isotherms are type IV according to
IUPAC classification and exhibit an H1 hysteresis loop, which
is typical for mesoporous materials.28
The intensity of the silanol band at 3745 cm-1 decreases by
increasing the V loading. At the same time, the band at
∼930 cm-1 assigned to Si-O-V appears and increases as the
silanol band decreases in intensity. This band roughly indicates
the content of V on the SBA-15 and shifts toward a higher
wavenumber with an increase of the V concentration. This can
be observed in spectrum d in Figure 2 for a sample deposited
under liquid phase conditions where Si-O-V appears at
922 cm-1. The same band appears at 936 cm-1, as is shown in
spectrum e of Figure 2 for a sample deposited by gas phase
deposition containing a higher V loading. The bands between
1200 and 1700 cm-1 are assigned to acac ligands26 and disappear
after thermal conversion toward VOx. The free silanol band at
3745 cm-1 is restored again after calcination, and one new band
at 3660 cm-1 appears which is attributed to the presence of
V-OH surface species.27 A broad contribution between 3600
and 3400 cm-1 assigned to hydrogen bonding interaction is also
present. This band appears due to the interaction between vicinal
OH groups, as Si-OH and V-OH groups are present.
The structural parameters specific surface area, SBET, meso-
pore and micropore volume, final V loading, and R value for
the different types of synthesized samples are summarized in
Table 1. Higher loadings can be achieved by a deposition of
vanadium under gas phase conditions. This is in accordance
with previous studies using other types of supports, such as silica
or MCM-48.4
Adsorption of the vanadium complex by the gas phase
deposition process follows almost exclusively the ligand

Vanadium and Titanium Oxide Supported SBA-15
J. Phys. Chem. B, Vol. 109, No. 24, 2005 12075
Figure 3. (A) N2 adsorption/desorption isotherms at -196 °C of SBA-15 supported vanadium oxide samples: (a) pure SBA-15 support;
(b) precursor synthesized by the liquid phase; (c) calcined sample synthesized by the liquid phase. The inset shows the corresponding pore size
distributions. (B) N2 adsorption/desorption isotherms at -196 °C of SBA-15 supported vanadium oxide samples: (a) pure SBA-15 support;
(b) precursor synthesized by the gas phase; (c) calcined sample synthesized by the gas phase. The inset shows the corresponding pore size distributions
.
The isotherms of the calcined SBA-15 and their grafted
derivatives exhibit a sharp increase in the adsorbed N2 volume
at P/P0 ∼ 0.65, characteristic for capillary condensation within
the uniform mesopores of the materials.
deposition (Figure 3B). Pore size distributions of precursor
samples (inset Figure 3A-b and B-b) shift to lower values
(especially the sample with a high V content deposited under
gas phase conditions, Figure 3B-b). After the thermal conversion
of the adsorbed complex, the porosity features are restored to a
certain extent, due to the removal of the acetylacetonate ligands.
The sharp pore size distributions of blank SBA-15 and calcined
VOx/SBA-15 indicate very uniform pores. No peak broadening
can be observed, indicating that the deposited layer is highly
homogeneous, as will be also confirmed by the absence of bands
assigned to cluster VOx in Raman spectroscopy.
The isotherm of the precursor obtained under gas phase
conditions containing a high concentration of vanadium is shown
in Figure 3B-b. The precursor shows an ink-bottle type of
hysteresis, which indicates the presence of pores that are very
narrow at the entrances. This will be the result of the dimensions
of the metal complexes inside the channels of SBA-15. The
precursor obtained under liquid phase conditions (Figure
3A-b) does not show this type of hysteresis. Indeed, the
concentration of vanadium is lower so that the steric hindrances
are limited. After calcination, both deposition conditions show
the same type of isotherm (Figure 3A-c and B-c).
3.1.2. Comparison between TiO(acac)2/SBA-15 and
VO(acac)2/SBA-15. TiOx/SBA-15 catalysts were also prepared
by the molecular designed dispersion (MDD) method using a
titanyl acetylacetonate complex. They were obtained under the
same liquid phase conditions as those for VOx/SBA-15 samples,
followed by a calcination process.
The insets of Figure 3 show a typical BJH pore size
distribution for SBA-15, V-acac/SBA-15, and VOx/SBA-15
prepared by liquid phase deposition (Figure 3A) and gas phase

12076 J. Phys. Chem. B, Vol. 109, No. 24, 2005
Segura et al.
Figure 5. Relation between the initial concentration and the final
loading on SBA-15 for VO(acac)2 (a) at RT and (b) at a 45 °C gradient
and for TiO(acac)2 (c) at RT and (d) at a 45 °C gradient as a function
of final loading (mmol of metal/g) deposited on SBA-15.
final loading on the SBA-15 surface. Increasing the initial
concentration of Ti and V, the final loading also increases until
a maximum coverage is reached. From the loadings and the IR
data, it is obvious that the titanium complex is more reactive
toward the surface of the SBA-15 compared to the VO(acac)2.
Although Ti is more reactive than V, the adsorption of their
complexes in both cases is mainly controlled by hydrogen
bonding with the surface hydroxyl groups, since for all samples
R values around 2 are found.
To equal the final loading of both metals on the SBA-15
surface under liquid phase conditions, a few parameters were
investigated in this work in order to increase the final loading
of vanadium on the SBA-15 surface. Experiments show that
the amount of solvent used for the liquid phase deposition does
not affect the final loading of the metal but that the temperature
plays an important role. In fact, when dissolving at high
temperature (45 °C), the final loading of VOx on the SBA-15
is higher than that at RT (Figure 5b). During the reaction, the
dissolving temperature is 45 °C and it is followed by a
decreasing temperature gradient, the vanadium and titanium
loadings on the support are higher compared to RT. This effect
is caused by an increase of the solubility of the complexes at
higher temperature. Then, during the subsequent decreasing
temperature gradient, the solubility of the complex becomes
smaller and the excess of complex in the solution is forced to
react with the support surface.29
The decomposition of the anchored complexes on the
SBA-15 support and the conversion of the physisorbed species
toward covalently bond VOx and TiOx surface groups were
studied by using a home-built in situ vacuum IR transmission
cell.30 Transmission IR seems to be the only appropriate
spectroscopic technique to follow the calcination process in situ,
which is required to understand the reaction mechanism.
The detailed in situ transmission IR study of the single
decomposition behavior of the TiO(acac)2 and VO(acac)2
complexes on the SBA-15 surface under liquid phase conditions
at RT is presented in parts A and B of Figure 6, respectively.
The interesting region to evaluate is located between 1600
and 1400 cm-1. TiO(acac)2 has a minor absorption at 1440 cm-1
and stronger bands at 1580 and 1530 cm-1, whereas VO(acac)2
shows absorptions bands at 1660, 1560, and 1530 cm-1 (Figure
6A and B). These bands assigned to V-acac disappear at
200 °C. However, the TiO(acac)2 spectra show the bands of
acetylacetonate decrease in intensity only at 300 °C. From these
data, it can be concluded that TiO(acac)2 has a higher thermal
stability compared to VO(acac)2.
Figure 4. (A) FTIR photoacoustic spectra of VOx/SBA-15 precursors
with a final concentration of (a) 0.17, (b) 0.25, (c) 0.29, (d) 0.36, and
(e) 0.43 mmol of V/g. (B) FTIR photoacoustic spectra of TiOx/SBA-
15 precursors with a final concentration of (a) 0.27, (b) 0.45, (c) 0.60,
(d) 0.76, and (e) 0.8 mmol of Ti/g.
Parts A and B of Figure 4 show the IR-PAS spectra (range
3500-3900 cm-1) for the precursors of the VO(acac)2 and
TiO(acac)2 on the SBA-15 support, respectively. The IR-PAS
spectra are given as a function of the final metal loading on the
SBA-15 support. The initial concentrations of both metal
complexes are the same ((a) 0.1, (b) 0.2, (c) 0.3, (d) 0.5, and
(e) 0.8 g of acac complex).
Figure 4B-e evidences almost completely the disappearance
of the free silanol band, which means that the Ti complexes
are attached to most of the OH on the SBA-15 surface at this
concentration (final concentration 0.8 mmol of Ti/g). However,
with the same initial concentration, Figure 4A-e shows a clear
band of the free silanols, indicating that not all of the surface
OH groups have reacted with the V complexes (final concentra-
tion 0.43 mmol of V/g).
Figure 5 shows the relation between the different initial
concentrations of TiO(acac)2 and VO(acac)2 complexes and the
3.2. Characterization of TiOx and VOx Oxides Supported
on SBA-15 Catalysts. Calcination of the precursor at 550 °C

Vanadium and Titanium Oxide Supported SBA-15
J. Phys. Chem. B, Vol. 109, No. 24, 2005 12077
Figure 7. FT-Raman spectra of (a) TiOx/SBA-15 catalyst prepared
by liquid phase deposition (1.07 mmol of Ti/g), (b) VOx/SBA-15
catalyst prepared by liquid phase deposition (0.76 mmol of V/g), and
(c) deposited by the gas phase at 140 °C (1.3 mmol of V/g).
representing isolated vanadia centers in a tetrahedral coordina-
tion. Even for the highest V-concentration samples (0.76 mmol
of V/g for the liquid phase (Figure 7b) and 1.3 mmol of V/g
for the gas phase (Figure 7c)), the bands of VOx clusters
(994, 697, 286, and 147 cm-1) cannot be detected. Therefore,
high concentrations of TiOx and VOx are obtained on the
SBA-15 in the form of isolated titania and vanadia centers in a
well-dispersed way.
To study the structural changes of the SBA-15 after grafting
with the acac complexes, XRD patterns and N2 adsorption
isotherms at -196 °C were recorded. The XRD diffractograms
of blank SBA-15 and the SBA-15 after adsorption of acetyl-
acetonate complexes and its subsequent calcination exhibit one
strong reflection (100) at 2θ ∼ 1 and two weaker peaks (110)
and (200) at higher 2θ, associated with hexagonal symmetry
and likewise characteristic of the hexagonally ordered structure
of SBA-15. Very similar diffraction peaks are observed for
the pure and final loaded SBA-15 materials. Therefore, the
crystallinity and the ordering of the SBA-15 are retained
after the modification of the surface, for both metal oxide
samples.
The estimated structural parameters such as specific surface
area, SBET, and mesopore and micropore volume after the
deposition of the two metal complexes on the SBA-15 surface
are summarized in Table 2. The initial concentration for all
samples in Table 2 is 1 g of acetylacetonate complex per 1 g of
SBA-15. After grafting the surface OH groups with the metal
acac complexes, the pores are filled. Therefore, the porosity
and surface area are reduced, which increased again after
calcination of the precursor as the metal complexes are converted
into the vanadium and titanium oxides. The surface area and
pore volumes shift to lower values as a function of the metal
loading. This reduction of porosity in comparison to the blank
support is due to the narrowing of the pores after deposition of
the V and Ti acac complexes. However, XRD and SBET
characterizations reveal that the deposition of the V and Ti acac
complexes and their subsequent thermal conversion results in
a supported metal oxide catalyst with the same structure and
textural properties as the original SBA-15 material.
Figure 6. IR spectra of the thermal conversion process of single
deposition TiO(acac)2 (A) and VO(acac)2 (B) on SBA-15 as a function
of temperature.
in ambient air removes the ligands and yields the final TiOx
and VOx supported oxide catalysts. In the infrared spectra, all
the acac vibration modes disappear, while the band of free
silanols of the SBA-15 is partly restored. As shown in Figure
2d,e, the band at around 930 cm-1 is assigned to Si-O-V
stretch and represents the amount of V on the surface.
Furthermore, a band at 965 cm-1 can be attributed to Si-O-
Ti.
Raman spectroscopy is very sensitive to the presence of TiO2
and V2O5 crystals. TiOx supported SBA-15 materials obtained
by the MDD method do not show typical bands of anatase TiO2
clusters (638, 519, 399, and 147 cm-1), not even at high
concentrations (1.07 mmol of Ti/g) (Figure 7a). The Raman
spectra of the samples obtained under liquid phase conditions
at 45 °C and under gas phase conditions at 140 °C for the
3.3. Catalytic Tests in the Selective Catalytic Reduction
of NO with Ammonia. The catalytic results obtained for the
VOx/SBA-15 (Figure 7b and c) show a clear band at 1042 cm-1
,

12078 J. Phys. Chem. B, Vol. 109, No. 24, 2005
Segura et al.
Figure 8. Catalytic activity data of TiOx/SBA-15 catalysts having (1) 0.17, (2) 0.37, (3) 0.55, and (4) 1.00 mmol of Ti/g in terms of NO conversion
(I) and N2 selectivity (II).
Figure 9. Catalytic activity data of VOx/SBA-15 catalysts having (1) 0.11, (2) 0.40, (3) 0.45, and (4) 0.77 mmol of V/g in terms of NO conversion
(I) and N2 selectivity (II).
SCR of NO with ammonia over SBA-15 loaded with titanium
and vanadium oxide in terms of NO conversion (I) and N2
selectivity (II) are shown in Figures 8 and 9, respectively.
The catalytic tests were carried out on samples prepared by
the MDD method in the liquid phase at RT except for the highest
loading of both metals in which the MDD was carried out in
the liquid phase at 45 °C. All the catalysts show isolated V and
Ti species, as can be seen in the Raman spectra (Figure 7).
The catalytic role of TiOx is shown in Figure 8. The
conversions increase with an increase of Ti loading, to reach
their maximum activity at about 430 °C with a NO conversion
of ∼85% for the highest Ti content. For the catalysts with the
lowest transition metal loadings (samples 1 and 2), the NO
conversion increases up to about 430 °C and at higher
temperature decreases due to the oxidation of ammonia by
oxygen present in the reaction mixture. Samples with a high Ti
loading show this decrease at high temperatures. It could be
concluded that the surface concentration of titania species
determines the ratio of the ammonia conversion in the DeNOx
and NH3 oxidation processes. The N2 selectivity is quite high
in the whole temperature range studied.
increasing V loading on the SBA-15. As in the case of the
Ti-containing catalyst, the maximum activity is reached at about
430 °C with an ∼85% NO conversion. However, the
V-containing catalysts are active within a broader temperature
window. For the highest V-content sample, there is a clear
decrease at high temperatures most likely caused by the
oxidation of ammonia by oxygen. Its N2 selectivity also slightly
decreases, although it remains high. There is no strict correlation
between V content and the catalytic activity for SCR in this
high temperature region. Probably, textural parameters, loading,
and metal dispersion influence the activity of these catalysts at
high temperatures. In contrast to the Ti-containing catalysts, the
V-containing samples are also active in the low temperature
range for the SCR (T < 300 °C). At these temperatures, their
catalytic activity increases with increasing V loading. In the
low temperature range, ammonia is converted mainly in the
DeNOx process. An increase in the vanadium loading increases
NO conversion probably due to the formation of a larger amount
of surface VOx species, which play a role of active sites in this
process. At higher temperatures, such vanadia species are active
in the ammonia oxidation process. Thus, the highest decrease
in NO conversion was measured for the sample with the highest
V content.
VOx/SBA-15 catalysts with different V loadings are presented
in Figure 9. There is an increase in NO conversion with

Vanadium and Titanium Oxide Supported SBA-15
J. Phys. Chem. B, Vol. 109, No. 24, 2005 12079
4. Conclusions
Acknowledgment. P.C. acknowledges the FWO-Flanders
(Fund for Scientific Research-Flanders) for financial support.
The preparation of SBA-15 supported vanadium and titanium
oxide catalysts by the molecular designed dispersion of
VO(acac)2 and TiO(acac)2, respectively, has been studied and
compared. The bonding mechanisms, the influence of the two
metals, and the differences in material properties based on
different deposition procedures have been fully investigated. Gas
phase deposition leads to a higher vanadium oxide loading on
the surface of the SBA-15 compared to the liquid phase
procedure. In the latter deposition, high temperatures allow a
higher metal oxide loading to be deposited on the support with
respect to the room temperature deposition.
After thermolysis in air, supported VOx and TiOx on
SBA-15 are obtained. Raman spectroscopy measurements reveal
that catalysts containing a high metal loading can be prepared
with all V species in a strictly tetrahedral configuration and
isolated Ti species, without the presence of anatase phase. For
both metal oxide catalysts, the MDD method allows the
preparation of the samples without the formation of V2O5 and
TiO2 clusters, obtaining highly dispersed oxides on the silica
support.
Several techniques enable us to elucidate the reaction
mechanism. The adsorption of VO(acac)2 under gas phase
conditions occurs by a ligand exchange mechanism (R value
around 1), with the formation of a covalent metal-oxygen
support bonding and the loss of a ligand as acetylacetone
(Hacac). However, as the treatment temperature decreases, the
reaction mechanism shows a combination of the two mecha-
nisms. The adsorption of TiO(acac)2 and VO(acac)2 on the
SBA-15 under liquid phase conditions proceeds by hydrogen
bonding, as the R values for both metal complexes are around
2. For VO(acac)2 however, the maximum surface loading under
liquid phase conditions is lower than that in the case of titanium.
Ti is more reactive than V toward the surface of the SBA-15.
Transmission IR shows also the higher thermal stability of
TiO(acac)2 compared to VO(acac)2. According to the BET,
XRD, and FT-IR data of the supported vanadium and titanium
oxide catalysts, the unique structural properties of the SBA-15
are maintained.
TiOx/SBA-15 and VOx/SBA-15 catalysts were also catalyti-
cally tested for the SCR of NO with ammonia. Both types of
catalysts show about 85% NO conversion at around 400 °C.
Moreover, the VOx/SBA-15 catalysts also show catalytic activity
in the low temperature range (T < 300 °C). The sample with a
high V loading is found to be active at low temperatures, while
the catalysts with a lower V content exhibit high activity at
higher temperatures.
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