Influence of metal oxide modification of alumina on the dispersion and activity of vanadia catalysts

Article abstract of DOI:10.1021/jp963450f

Alumina was modified with 10 wt % MO_x (MO_x = TiO₂, ZrO₂, La₂O₃, or MgO) prior to its impregnation with 12 wt % vanadia. The catalysts VTiAl, VZrAl, VLaAl, and VMgAl were characterized by X-ray diffraction (XRD), electron spin resonance (ESR), FT-Raman spectroscopy, ⁵¹V solid state nuclear magnetic resonance (⁵¹V NMR), and oxygen chemisorption. The activities of the catalysts were determined by methanol partial oxidation and their acid-base properties were evaluated for the decomposition of 2-propanol. XRD and FT-Raman spectroscopy indicated the formation of bulk TiO₂ and ZrO₂ on the titania and zirconia modified alumina. ⁵¹V solid state NMR results suggest the presence of both octahedrally and tertrahedrally coordinated vanadia species in the catalysts VTiAl, VZrAl, and VAl and the presence of tetrahedrally coordinated vanadia species in the catalysts VLaAl and VMgAl. ESR spectra recorded at ambient temperature showed the presence of V⁴⁺ ions having axial symmetry. Oxygen chemisorption results indicated an enhanced number of reducible vanadia sites, i.e., redox sites in the modified catalysts. Metal oxide modification is found to influence significantly the surface coverage and the methanol partial oxidation activity of vanadia supported on alumina. With proper MO_x modification enhanced reducibility of vanadia could be attained, which in turn makes the partial oxidation reaction of methanol more facile.

Full text of DOI:10.1021/jp963450f

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
3324
J. Phys. Chem. B 1997, 101, 3324-3328
Influence of Metal Oxide Modification of Alumina on the Dispersion and Activity of
Vanadia Catalysts
L. Jhansi Lakshmi* and Elmer C. Alyea
Department of Chemistry and Biochemistry, UniVersity of Guelph, Guelph, Ontario N1G 2W1, Canada
S. T. Srinivas and P. Kanta Rao
Physical and Inorganic Chemistry DiVision, Indian Institute of Chemical Technology,
Hyderabad-500 007, A. P., India
ReceiVed: NoVember 4, 1996; In Final Form: January 15, 1997X
Alumina was modified with 10 wt % MOx (MOx ) TiO2, ZrO2, La2O3, or MgO) prior to its impregnation
with 12 wt % vanadia. The catalysts VTiAl, VZrAl, VLaAl, and VMgAl were characterized by X-ray
diffraction (XRD), electron spin resonance (ESR), FT-Raman spectroscopy, 51V solid state nuclear magnetic
resonance (51V NMR), and oxygen chemisorption. The activities of the catalysts were determined by methanol
partial oxidation and their acid-base properties were evaluated for the decomposition of 2-propanol. XRD
and FT-Raman spectroscopy indicated the formation of bulk TiO2 and ZrO2 on the titania and zirconia modified
alumina. 51V solid state NMR results suggest the presence of both octahedrally and tertrahedrally coordinated
vanadia species in the catalysts VTiAl, VZrAl, and VAl and the presence of tetrahedrally coordinated vanadia
species in the catalysts VLaAl and VMgAl. ESR spectra recorded at ambient temperature showed the presence
of V4+ ions having axial symmetry. Oxygen chemisorption results indicated an enhanced number of reducible
vanadia sites, i.e., redox sites in the modified catalysts. Metal oxide modification is found to influence
significantly the surface coverage and the methanol partial oxidation activity of vanadia supported on alumina.
With proper MOx modification enhanced reducibility of vanadia could be attained, which in turn makes the
partial oxidation reaction of methanol more facile.
Introduction
these species formed on the support surface. They concluded
from this study that the presence of bulk TiO2 particles in the
catalyst enhances the methanol partial oxidation activity and
HCHO selectivity due to the predominance of redox sites.
Vanadia supported on high surface area carriers like Al2O3,
SiO2, etc., has been widely studied for catalyzing a great variety
of reactions such as partial oxidation, selective catalytic reduc-
tion of NOx, and oxidative dehydrogenation of light alkanes.1-5
The oxides like TiO2 and ZrO2 when employed as supports for
partial oxidation and hydroprocessing catalysts were found to
exhibit enhanced activities and selectivities in comparison with
Al2O3 supported catalysts.2,6 The disadvantages with these
carriers are their low surface area, high surface acidity, low
volume activity, and phase transition at high temperatures,
making them unsuitable for industrial applications. By addition
of small quantities of these oxides to relatively high surface
area, thermally stable supports such as alumina the aforesaid
problems can be obviated. Support modification is said to
influence the nature of active sites by way of increasing the
reducibility of the transition metal oxide, resulting in enhanced
activity and selectivity of the catalysts. The addition of Zn2+
and Sn2+ was found to increase the dispersion of platinum on
alumina.7 The acidification of γ-Al2O3 with titania resulted in
enhanced dispersion of copper.8 Jehng and Wachs9 have
investigated a series of V2O5 catalysts supported on TiO2
modified SiO2 by employing the in situ Raman spectroscopic
technique and the methanol partial oxidation reaction to
determine the structure-activity relationships of the catalysts.
They observed from these Raman studies the formation of small
TiOx species on the SiO2 support surface at low titania loadings
while at higher TiO2 concentration they identified bulk TiO2
particles; vanadia was assumed to be interacting with both of
Mastikhin et al.10,11 have employed 51V and H NMR to
1
characterize V2O5/TiO2/SiO2 catalysts with various titania
contents. They showed that there is a nonuniform surface
coverage of SiO2 with TiO2 and that only a small part of vanadia
1
interacts with TiOx. They also observed in their H NMR
studies that supported vanadia interacts with the terminal Si-
OH groups as well as with Ti-OH groups. Similar observations
were made earlier by Rajadhyaksha et al.12 in their studies on
titania modified silica supported vanadia catalysts. They
suggested the presence of two types of surface vanadates,
namely, a polyvanadate species anchored on the silica surface
which was uncovered and a second vanadia species interacting
with the dispersed titania. According to Reardon and Datye,13
the addition of titania to γ-alumina prevents the formation of
an inactive Al2(MoO4)3 phase due to the titration of unwanted
hydroxyl groups bound to tetrahedrally coordinated alumina.
They also observed that R-alumina supported molybdena is more
active than γ-alumina supported MoO3 because of the more
efficient utilization of active phase which was attributed to the
absence of tetrahedrally coordinated Al3+ at its surface. γ-Al2O3
was found to have both octahedral and tetrahedrally coordinated
Al3+, and therefore on reaction with molybdena was found to
form an inactive Al2(MoO4)3 phase. Wei et al.14 also observed
increased reducibility of molybdena and higher hydrodesulfu-
rization activity on titania modified γ-Al2O3 supported hydro-
processing catalysts. Thus, with the addition of specific
molecules or dopants as overlayers on Al2O3 with desirable
properties such as high surface area and pore structure, one can
design more efficient supports in order to achieve high disper-
* To whom correspondence should be addressed. Fax (519) 766-1499;
E-mail jhansi@uoguelph.ca.
X Abstract published in AdVance ACS Abstracts, April 1, 1997.
S1089-5647(96)03450-5 CCC: $14.00 © 1997 American Chemical Society

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Dispersion and Activity of Vanadia Catalysts
J. Phys. Chem. B, Vol. 101, No. 17, 1997 3325
TABLE 1: Physicochemical Properties and the Product Distribution during the IPA Decomposition on Vanadia Catalysts
active
bulk anal
V2O5/MOx,
wt %
O2
uptake,
µmol/g
site
surface
% surface
coveragea
densityb
% selectivity
ACE/PROP
catalyst
area, m2/g
(nm-2
)
% conv
VAl
12.3/-
8.9/5.1
10.5/5.7
10.7/6.6
7.9/7.2
168
122
129
153
115
56
133
89
58
22
9.9
32.3
20.4
10.8
5.7
0.40
1.21
0.83
0.44
0.28
7
23
19
12
4
100/-
74/26
83/17
98/2
VTiAl
VZrAl
VLaAl
VMgAl
99/1
a Surface coverage is defined as 100× active surface area/BET SA of reduced catalyst. b Active site density is the number of “O” atoms chemisorbed
per unit area of catalyst.
sion and activity. However, the influence of support modifica-
tion on the redox and acid-base properties of V2O5/Al2O3 is
not yet thoroughly understood.
The aim of the present investigation is to elucidate the role
of modification of the γ-Al2O3 support with TiO2, ZrO2, La2O3,
or MgO on the nature and dispersion of vanadia species and
methanol partial oxidation activity of vanadia catalysts.
acquired for these samples. Chemical shifts were referenced
to external VOCl3. An all-glass high-vacuum apparatus with a
facility to reduce (at 500 °C, in a flow of H2) the samples in
situ was used to carry out the low-temperature (-78 °C) oxygen
chemisorption experiments. The details of the experimental
procedure are described elsewhere.15 BET surface areas of the
catalysts were determined by N2 physisorption using the same
equipment.
Experimental Section
Activity studies for the partial oxidation of methanol were
carried out in the temperature range 175-250 °C, taking 200
mg of the catalyst (0.5 mm size) packed in a fixed bed tubular
glass reactor of 6 mm i.d. Purified air at a flow rate of 60 mL/
min, saturated with methanol (by passing through a saturator
maintained at 25 °C), was introduced into the reactor. The low
amount of catalyst and low feed rate were chosen to keep the
conversions below 20%. Comparisons of activities and selec-
tivities were made at a reaction temperature of 200 °C, at which
both were moderate. After a steady state period of 30 min the
products were analyzed on-line with a 10% Carbowax 20M
column (2 m long) using a flame ionization detector. The
product stream comprised mainly formaldehyde and dimethyl
ether, with some traces of methyl formate, CO, and CO2.
Oxidative decomposition of 2-propanol was carried out in the
same reactor under similar reaction and analysis conditions at
a constant temperature of 175 °C and at atmospheric pressure,
using about 200 mg of catalyst. The products observed
consisted mainly of propene and acetone with traces of
diisopropyl ether.
Four different modified supports, namely, TiAl, ZrAl, LaAl,
and MgAl, were prepared by impregnating the γ-Al2O3 (Har-
shaw, Gamma phase, S.A. 196 m2/g) support with 10 wt %
each of MOx (MOx ) TiO2, ZrO2, La2O3, MgO). The precursors
used were Fluka A.R. grade (C12H28O4)Ti and (C12H28O4)Zr
for TiO2 and ZrO2, respectively, and Loba Chemie, A.R. grade
Mg(NO3)2‚6H2O and La(NO3)3‚6H2O for MgO and La2O3,
respectively. The metalloorganic precursors of titania and
zirconia were dissolved in methanol prior to impregnation on
Al2O3. Lanthanum and magnesium hydroxides were precipi-
tated by ammonia hydrolysis of their aqueous nitrate solutions.
The excess solutions were evaporated to dryness on a water
bath, and the catalyst masses were further dried in an air oven
at 110 °C for 12 h. The modified supports were calcined at
550 °C for 5 h. The amount of metal oxides in the modified
supports was determined by inductively coupled plasma (ICP)
analysis on a Varian Liberty 100-OES spectrometer after
calibrating the instrument with NIST traceable standards. A
weighed sample was digested in an acid mixture of nitric acid,
sulfuric acid, and perchloric acid until the dissolution was
complete, and then the solution was diluted to a specific volume
prior to analysis. VMAl catalysts (M ) Ti, Zr, La, Mg) were
synthesized by impregnation with an aqueous solution contain-
ing a calculated amount of NH4VO3 (Fluka, A.R. grade),
corresponding to 12 wt % V2O5 of the modified supports.
Drying and calcination procedures were similar to those
described above. Following the same procedure, a 12 wt %
V2O5/Al2O3 (VAl) was prepared as a reference. The vanadia
contents of the calcined catalysts were estimated by ICP
analysis.
XRD patterns were recorded on a Phillips PW 1051 diffrac-
tometer using Ni filtered Cu KR radiation. ESR spectra were
recorded on a Bruker ER 200 D SRC X band spectrometer with
100 kHz modulation at ambient temperature. The microwave
frequency was 9.71 GHz. Raman spectra were recorded on a
Bruker FRA 106 FT-Raman module interfaced to a Bruker IFS-
66 FTIR bench. All Raman spectra were recorded at room
temperature and under ambient conditions using a 80 mW power
setting for the incident radiation at 943.4 nm from a Nd:YAG
laser; samples were held in glass vials. Wide-line 51V NMR
spectra were obtained on a Bruker ASX 200 MHz spectrometer,
operating at 52.6 MHz for vanadium, equipped with a wide-
line probe and a 10 mm insert. A 2 µs pulse was applied
following a 2 s relaxation delay; typically 1400 scans were
Results and Discussion
The metal oxide and vanadia contents of the calcined supports
and catalysts, BET surface areas, oxygen uptake, active site
density and surface coverage values, and product distribution
during the IPA decomposition are shown in Table 1. It can be
seen that the surface areas of the vanadia catalysts supported
on MOx modified alumina (VMAl) are significantly lower than
vanadia on unmodified alumina (VAl), which in turn is lower
than the alumina support itself. This observation is expected
due to the blockage of pores in the alumina as a result of the
addition of the metal oxide as well as the active component.
Except in the case of VMgAl, oxygen uptakes of the VMAl
catalysts are more when compared to the VAl catalyst, indicating
an increase in the number of redox sites. Among the four
catalysts, VTiAl shows the highest oxygen uptake, which may
be due to a greater number of reducible vanadia sites. Both
active site density and the surface coverage are high in VTiAl,
indicating that the V2O5 is highly dispersed on the TiO2 modified
Al2O3 support. Generally, the strength of interaction between
the support and the active component governs the dispersion
and hence oxygen uptake capacity.16 The increased oxygen
uptakes of VTiAl, VZrAl, and VLaAl catalysts may be due to
an increase in the number of labile oxygen atoms on the catalyst
surface, in other words an increase in the number of redox sites.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
3326 J. Phys. Chem. B, Vol. 101, No. 17, 1997
Lakshmi et al.
TABLE 2: Spin-Hamiltonian Parameters of V4+ in the VAl
and VMAl Catalysts
B
catalyst
g|
g
A|
A
Aiso
(∆g|/∆g ) â2*2
VAl
1.942 1.988 185 67 106
1.938 1.987 185 70 108
1.936 1.984 185 69 108
1.934 1.986 184 66 105
4.2
4.3
4.1
4.1
4.7
0.69
0.64
0.66
0.67
0.68
VLaAl
VZrAl
VTiAl
VMgAl 1.934 1.988 185 61 102
modifier oxide, showing clear hfs of isolated V4+ sites, were
observed. Inomata and co-workers23 have already indicated that
at most only a few percent of reduced vanadium sites are
expected for supported vanadia catalysts. Following earlier
workers,18-23 we attribute the observed spectra to the presence
of an axially symmetric ligand field. The derived ESR
parameters, g| and g , and hyperfine coupling constants, A| and
A , are given in Table 2. The values compare very closely to
those reported in the earlier studies18-23 for surface vanadia
species; usually C4V symmetry arising from either square-
pyramidal or axially distorted octahedral geometry has been
involved for the V4+ ion environment. However, the presence
of a tetrahedral geometry is not easily distinguished since g >
g| and A| > A in all the cases, as Davidson and Che19 indicate
in their tabulation of much literature data, though Aiso is smaller
(80-100 G) in the tetrahedral case. Some authors have changed
their assignment of geometry after reducing the same V2O5/
Al2O3 samples at a lower temperature, though the ESR
parameters were virtually identical.21,22 In the present catalysts,
since the 51V and Raman data (vide infra) verify that only
tetrahedral sites are common to all the species, we deduce that
these sites give rise to the observed axially symmetric V4+
environments. We also measured the ESR spectra after evacu-
ation of the samples at 250 °C in vacuum. Since the patterns
remained identical to those observed at ambient temperature,
though more intense due to the presence of more V4+ sites as
a result of the reducing conditions, we conclude that the
observed V4+ sites at ambient temperature are not due to
hydration of tetrahedral environments. Since the ESR spectra
remained virtually invariant as the modifying oxide changed,
in contrast to the differences in the observed activities for
methanol partial oxidation (vide infra), we also calculated two
ESR parameters that relate to the VdO bond strength. Baiker
and co-workers18 showed that B (∆g|/∆g ) and â2*2, which
indicates the degree of delocalization, are useful parameters to
monitor the effect of different supports on the strength of the
VdO bond. Our observed values of B are moderately large,
being more comparable to that observed (B ) 5.1)20 for an
evacuated (720 K) VO2+/H-ZSM-5 sample than those reported
by Baiker et al. for preparations of V2O5 on Al2O3, SiO2, and
MgO (B ) 2.1, 2.4, and 1.3, respectively). Since B decreases
with tetragonal distortion of the ligand field symmetry, the
estimated B values indicate that the VdO bond remains strong
and little changed from that in VAl itself (i.e., without modifying
oxides).Though â2*2 parameters for the catalysts signify sig-
nificant electron delocalization, their constancy again contrasts
with the observed appreciable differences in activity as MOx
varies. One possible conclusion is that V4+ sites only occur in
V-O-Al (not V-O-M) sites, whereas catalysis occurs at
V-O-M sites.
Figure 1. ESR patterns of the VAl and VMAl catalysts recorded at
ambient temperature.
Modification of alumina appears to have increased the reduc-
ibility of vanadia due to V-O-M bond formation. Similar
observations were made by Jehng and Wachs9 in their studies
on titania modified silica supported vanadia catalysts. They
attributed the increased activity of their catalysts to the increased
number of redox sites. Another reason for the observed
increased activity might be the expected improved dispersion
due to a thermal spreading of V precursors after calcination at
550 °C, as reported, for example, by Kno¨zinger and Taglauer.17
The XRD patterns of the VAl and VMAl catalysts show that
there are no lines corresponding to crystalline vanadia in all
the catalysts. Vanadia may thus be existing as a highly
dispersed amorphous phase. However, the presence of micro-
crystalline particles with size less than 40 Å cannot be ruled
out. The modified supports exhibited the peaks corresponding
to their respective oxides with low intensities. The anatase
phase of TiO2 was observed in TiAl, and lines corresponding
to tetragonal and monoclinic phases of ZrO2 were observed in
ZrAl. LaAl and MgAl were X-ray amorphous.
ESR spectroscopy is a very sensitive method for detecting
the presence of V4+ environments in vanadia surface species.
Hyperfine splitting (hfs) can arise for isolated V4+ sites because
1
the unpaired electron associated with V4+ (S ) /2) interacts
with the large nuclear magnetic moment of 51V (I ) 7/2) to give
rise to eight parallel and eight perpendicular components.
Several previous investigations18-23 showed that such hfs exists
for V2O5 dispersed on alumina and other supports, with the
occurrence of V4+ sites being effected by reduction methods
or simply calcination. Broad signals with unresolved fine
structure, as observed for V2O5 itself,24 are expected if exchange
of the electron can occur with neighboring V4+ sites, which is
the case at higher temperatures and/or higher vanadia concentra-
tions. Figure 1 shows the ESR spectra of the calcined catalysts
recorded at ambient temperature. DPPH was used as an external
reference. Well-resolved similar spectra, independent of the
Raman spectroscopy has been extensively used by researchers
to study the coordination structure of supported vanadia
catalysts.9,25-28 FT-Raman spectra of the various catalysts
recorded at ambient temperature are shown in Figure 2. Among
the modified supports, TiAl exhibited peaks corresponding to
the anatase phase of TiO2 at 640, 515, 396, 282, 198, and 144
cm-1 . The ZrAl, LaAl, and MgAl supports did not show any

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Dispersion and Activity of Vanadia Catalysts
J. Phys. Chem. B, Vol. 101, No. 17, 1997 3327
Figure 2. Raman spectra of the catalysts.
peaks. The catalysts VAl, VTiAl, and VZrAl showed a broad
peak at ∼994 cm-1 corresponding to the presence of octahedral
polyvanadate species.28 In the case of VMgAl and VLaAl no
such peak was present, however a broad band (also evident in
VZrAl and VTiAl) was observed at about 900 cm-1 corre-
sponding to tetrahedrally coordinated vanadia species in these
samples.9
Figure 3. 51V solid state NMR spectra of the catalysts.
TABLE 3: 51V Solid State NMR Wide-Line Chemical Shifts
for the Peak Maxima Assigned to Tetrahedrally and
Octahedrally Coordinated Vanadia Species for Various
Catalysts
Solid state 51V NMR spectroscopy has been demonstrated to
be a powerful tool for determining the local vanadium coordina-
tion environment on various support surfaces.29-33 Major
factors influencing the type of supported vanadia surface species
are the pH of the impregnating solution, the concentration of
supported vanadia, and the treatment temperature. In general,
lower vanadia concentration results in broad often multicom-
ponent 51V NMR lines in the -300 to -800 ppm range,
assignable to tetrahedral environments. In contrast, higher
vanadium concentrations lead to the occurrence of a line with
an axial anisotropy of the chemical shift tensor with a major
peak near -300 ppm and a minor peak in the -900 to -1300
ppm range; V2O5 itself has -280 and -1250 ppm peaks.29
Species showing such 51V peaks can be assigned as having
octahedral vanadia sites if ∆δ is greater than 600 ppm.32,33
Wideline solid state 51V NMR spectra of the catalysts recorded
at 25 °C are shown in Figure 3. The chemical shifts are
tabulated in Table 3. The catalysts VAl, VTiAl, and VZrAl
with chemical shifts at -332 , -532; -335, -549; and -332,
-564 ppm, respectively, indicate the presence of both tetrahedral
and octahedral vanadia species. In the samples VLaAl and
VMgAl single peaks at -633 and -577 ppm, respectively,
correspond to the presence of only tetrahedrally coordinated
polymeric vanadia species on the support surface. In the case
of VLaAl the peak is sharper, indicating the presence of
vanadium in a more symmetric coordination environment.
Formation of polymeric clusters is indicated by an increase in
the width of the NMR peaks. The catalyst VAl was evacuated
at 10-3 Torr for 2 h at 200 °C, resulting in broadening of the
peak at -532 ppm, which can be explained on the basis of
removal of water molecules from the coordination sphere of
the surface species.34 51V and 1H MAS NMR studies by Lapina
et al.11 on titania modified silica supported vanadia catalysts
indicated that vanadia interacts with hydroxyl groups on both
chemical shift (ppm)
catalyst
tetrahedral
octahedrala
VAl
-532
-549
-564
-633
-577
-332
-335
-332
VTiAl
VZrAl
VLaAl
VMgAl
a At higher gain a weak absorption was also observed near -1200
pm.
titania and silica. Therefore, in the present study there is likely
an interaction of vanadia with both the modifier and the alumina
support, as implied by the ESR and activity data.
The activities of the catalysts were tested with methanol
partial oxidation as a model reaction because of its ability to
differentiate the nature of surface sites depending on the product
formed. The formation of dehydrated product dimethyl ether
is a measure of acid sites, whereas formaldehyde and methyl
formate formation indicates the presence of redox sites; basic
sites can be evaluated by CO and CO2 formation. The
conversions and selectivities toward formaldehyde and dimethyl
ether of the various catalysts in the partial oxidation of methanol
(compared at a reaction temperature of 200 °C) are plotted in
Figure 4. The activities of these VMAl catalysts are in the order
VTiAl > VZrAl > VLaAl > VMgAl. The O2 uptakes of the
catalysts also varied in a similar manner, indicating that the
active sites titrated by oxygen molecules are responsible for
the observed activities of the catalysts. Active sites are the
vacancies created by removal of labile oxygen atoms (by
reduction) upon which the dissociative chemisorption of oxygen
takes place. Selectivity toward the dehydrogenation product,