Effect of WO3 in mixed V2O5-WO3/ZrO2 catalysts on their surface structures and decomposition of propan-2-ol

Article abstract of DOI:10.1039/a809536a

V₂O₅-WO₃/ZrO₂ catalysts prepared by a two-step impregnation method have been studied by FTIR and laser-Raman spectrometry together with a pulse method. Addition of WO₃ to 2 wt.% V₂O₅/ZrO₂ catalyst increased the activity of propan-2-ol decomposition and the selectivity for propene. On 5 wt.% V₂O₅/ZrO₂ catalyst the activity was hardly affected but the selectivity for propene was increased by the additive. From the FTIR bands in the region of 1100-900 cm^-1, when WO₃ was added to V₂O₅/ZrO₂ catalysts, the tungsten oxide species was well dispersed on the zirconia support and directly interacted with zirconia to form the bridging layer(s) between the support and vanadate species. Thus, the addition of WO₃ caused the reconstruction of the surface vanadate species. As a result, the surface acidity as well as the catalytic activity changed with the amount of WO₃.

Full text of DOI:10.1039/a809536a

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E†ect of WO in mixed V O –WO /ZrO catalysts on their surface
2 5
structures and decomposition of propan-2-ol
3
3
2
Yoshio Toda,*a Takashi Ohno,b Fumikazu Hatayamab and Hisashi Miyatac¤
a Department of Industrial Chemistry, Osaka Prefectural College of T echnology, Neyagawa,
Osaka 572-8572, Japan. E-mail: toda=ipc.osaka-pct.ac.jp
b Department of Biofunctional Chemistry, Kobe University, Nada, Kobe 657-8501, Japan
c Department of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka 593-8231,
Japan
Received 7th December 1998, Accepted 19th January 1999
V O ÈWO /ZrO catalysts prepared by a two-step impregnation method have been studied by FTIR and
2
5
3
2
laser-Raman spectrometry together with a pulse method. Addition of WO to 2 wt.% V O /ZrO catalyst
2 5
increased the activity of propan-2-ol decomposition and the selectivity for propene. On 5 wt.% V O /ZrO
3
2
2
5
2
catalyst the activity was hardly a†ected but the selectivity for propene was increased by the additive. From the
FTIR bands in the region of 1100È900 cm~1, when WO was added to V O /ZrO catalysts, the tungsten
3
2 5
2
oxide species was well dispersed on the zirconia support and directly interacted with zirconia to form the
bridging layer(s) between the support and vanadate species. Thus, the addition of WO caused the
3
reconstruction of the surface vanadate species. As a result, the surface acidity as well as the catalytic activity
changed with the amount of WO .
3
Supported vanadium-containing catalysts are widely used
as selective catalysts for the oxidation of various hydrocar-
bons and in the selective catalytic reduction (SCR) of nitrogen
oxides (NOx).1,2 Although V O is highly active for the SCR
We have previously studied the reduction of nitric oxide
with ethylene over V O /ZrO catalysts prepared from a gas
2
5
2
phase method by using FTIR spectrometry.13 From a quanti-
tative analysis of the VxO band in the region of 1100È900
cm~1, it has been concluded that the surface VxO species in
the top layer of the catalyst interact with adsorbed species. We
have also characterised the di†erence of the surface VxO
species between the monolayer and multi-layer catalysts.
However, little work has been carried out on
V O ÈWO /ZrO systems.
2
5
reaction, the commercial SCR catalysts contain mixed oxides
of V O with WO as the major additive. The role of WO is
2
5
3
3
to provide thermal stability to the catalyst and to promote the
SCR of NOx. Investigations on the reactivity, structural and
physico-chemical characteristics of V O ÈWO /TiO catalysts
2
5
3
2
have been reported by many workers.3h10
2 5
3
2
More recently, we have studied the acidic properties of
Vuurman et al.3 investigated the molecular structures of
V O ÈWO /ZrO catalysts using the adsorption of pyridine
V O ÈWO /TiO mixed samples. They concluded that under
2 5
3
2
2
5
3
2
by FTIR.14 In 2 wt.% V O /ZrO , the number of both
dehydrating conditions, both the highly distorted vanadium
oxide species and octahedrally coordinated tungsten oxide
species are found on the titania surface at a metal oxide
loading up to 10% and seem not to be inÑuenced by one
another. Paganini et al.4 reported that a mutual (structure and
electronic) interaction exists between the V O and WO in
2 5
2
BrÔnsted and Lewis acidic sites increased with the addition of
WO , while in 5 wt.% V O /ZrO , only BrÔnsted acidic sites
3
2 5
2
increased and Lewis acidic sites did not change.
In the present work, the e†ects of the addition of WO to
3
V O /ZrO catalysts have been investigated by using FTIR
2 5
2
2
5
3
and laser-Raman techniques as well as by the analysis of
decomposition products of propan-2-ol. Zirconia shows no IR
absorption band above 900 cm~1 which makes it possible to
observe the surface species in the region of 1100È900 cm~1 on
the V O ÈWO /ZrO catalysts. Thus, the correlation between
the V O ÈWO /TiO system. Ramis and Busca5 concluded
2
5
3
2
that addition of WO has small e†ect on the l(VxO) of
3
V O /TiO catalysts. Thus, there are some contradictions
2
5
2
about the e†ect of the WO additive.
3
TiO has been widely used as a support in those studies.
Interest has recently focused on zirconium oxide as a support
2 5
3
2
2
the surface structures of V O ÈWO /ZrO catalysts and the
activity for propan-2-ol decomposition is discussed in detail.
2 5
3
2
because of its amphoteric properties and high thermal stabil-
ity.11h13 Gazzoli et al.11 studied the characterisation of the
zirconia-supported tungsten oxide system and concluded that
the di†erence in their structure, such as monomeric tungstate
or polytungstates, is related not to the pH of the contacting
solution but to the surface concentration of tungstate.
Indovina12 investigated the preparation and the character-
isation of VO /ZrO and reported that the pH of solutions
Experimental
Catalysts preparation
The support used in this study was ZrO (Nippon Shokubai
2
Co., Ltd., 28 m2 g~1). The V O /ZrO catalysts were pre-
2
5
2
pared by a dry impregnation method. In the case of 2 wt.%
V O /ZrO catalyst, a small amount (0.525 g) of ammonium
metavanadate was dissolved in 5 cm3 of 1 mol dm~3 oxalic
acid solution. The solution was added a little at a time onto
x
2
used in the equilibrium adsorption did not a†ect the nature of
surface vanadate species.
2
5
2
¤
Current address: Department of Information Science, Hiroshima
the powdered ZrO (20.0 g) heated at ca. 373 K with stirring.
Then the sample was dried at 383 K for 12 h and calcined at
2
Prefectural University, Shobara, Hiroshima 727-0023, Japan.
Phys. Chem. Chem. Phys., 1999, 1, 1615È1621
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7
23 K for 5 h in air. The 5 wt.% V O /ZrO sample was also
Raman spectra were recorded on a JASCO NR-1000 laser
Raman spectrometer under ambient conditions. Details of the
procedures have been described previously.15
X-Ray di†raction patterns of the catalysts were obtained on
a Rigaku RINT 2000 di†ractometer using CuKa radiation.
2
5
2
prepared by a similar manner.
In the case of 2 wt.% WO /ZrO catalysts, a small amount
3
2
(
0.580 g) of ammonium metatungstate was dissolved in 5 cm3
of distilled water. This solution was added dropwise onto the
powdered ZrO2 (20.0 g) heated at ca. 373 K with stirring.
Then the sample was dried and calcined using the same condi-
tions as mentioned above. The WO
prepared was 2.0, 5.0 and 8.0 wt.%.
3 content in the samples
Results
The V O ÈWO /ZrO catalysts were prepared by a two-
Decomposition of propan-2-ol on V O –WO /ZrO
2
2
5
3
2
2 5
3
step impregnation method. First the V O /ZrO catalysts
2
5
2
The decomposition of propan-2-ol has been examined at 443
K with a pulse method. The results obtained from the Ðrst
pulse injection onto the catalysts are shown in Fig. 1. Propan-
were prepared as described above. Then a small volume (5È10
cm3) of ammonium metatungstate solution was added drop-
wise onto 20.0 g of Ðnely pulverised V O /ZrO catalyst
2
5
2
2-ol decomposes over catalysts to form mainly propene,
heated at 373 K with stirring. These catalysts were dried and
calcined using the same conditions as mentioned above.
The catalysts are designated as VxWyZ, where
x \ vanadium oxide percentage (0, 2.0, 5.0 wt.%); and
y \ tungsten oxide percentage (0, 2.0, 5.0, 8.0 wt.%). The com-
position and BET surface area of catalysts are listed in Table
acetone, water and a small amount of diisopropyl ether. On
V2 series catalysts, the addition of WO caused an increase in
3
the amount of propene and a decrease of acetone. The
C H /C H O ratio was 7.2 on V2W2Z and 1.9 on V2W0Z,
3
6
3 6
respectively. This ratio increased slightly with increasing the
tungsten oxide loading over V2 series catalysts.
1
together with the number of acidic sites.14
On V5 series catalysts, the addition of WO led to the
3
Propan-2-ol decomposition
increase of the C H /C H O ratio slightly while the total
3
6
3 6
amount of products hardly changed. In the case of WO /ZrO
3
2
Decomposition of propan-2-ol was studied at 443 K by a
pulse method, using helium as carrier gas. The catalysts (ca. 30
mg) were heated at 723 K for 30 min in a stream of oxygen
catalysts, V0W2Z catalyst showed a low catalytic activity,
whereas V0W5Z or V0W8Z catalyst showed a high catalytic
activity to propene alone.
(
ca. 101 kPa), followed by heating at 723 K for 30 min in a
The catalytic activity of V O or WO without the support
2
5
3
stream of helium before the reaction. The propan-2-ol pulse (2
ll) was injected only once and the amount of products were
ascertained. The effluent gases were analysed with a Shimadzu
GC-4BPT gas chromatograph (GC) with thermal conductivity
detection and using a 3 m column of 15% PEG6000 at 333 K.
was also examined in separate experiments. On bulk V O ,
2
5
the ratio of C H /C H O was ca. 8.9 and a large amount of
3
6
3 6
diisopropyl ether was formed, while on bulk WO , mainly
3
propene was formed together with a small amount of diisop-
ropyl ether. Considering that V2 and V5 series catalysts have
produced acetone, vanadia species play an important role in
acetone formation in the decomposition of propan-2-ol.
It was noticed that the activity and selectivity for decompo-
sition of propan-2-ol were more a†ected by the addition of
FTIR and laser-Raman studies
The apparatus used was a conventional closed-circulation
system equipped with an IR cell in the circulation loop. A disc
of catalyst (20 mm diameter, ca. 100 mg) was placed in the IR
cell and the disc temperature was slowly increased from room
temperature to 723 K under evacuation and kept at that tem-
perature under oxygen circulation (ca. 10 kPa). This treatment
was repeated several times in order to obtain the same surface
states of the catalysts as those reported in the experiments of
propan-2-ol decomposition.
Fourier-transform infrared (FTIR) spectra were recorded on
a Shimadzu FTIR-8100M equipped with a DLATGS detector
operating in a single beam mode at 2 cm~1 resolution in the
region of 4000È400 cm~1. Some spectra were obtained after
subtraction of the catalyst itself by using a personal computer
WO on V2 series catalysts.
3
FTIR spectra of propan-2-ol on V O –WO /ZrO catalysts
2 5
3
2
A small amount of propan-2-ol was introduced onto the
V O ÈWO /ZrO samples. The resulting spectra are shown in
2
5
3
2
Figs. 2 and 3. The spectra below 1800 cm~1 show the di†er-
ence between the original and the catalyst itself.
Bands due to adsorbed isopropoxide are observed in the
spectra of V2W0Z sample [Fig. 2A(a)]. A band at 1110
cm~1 is assigned to l(CwO) of surface isopropoxide species
interacted with Lewis acidic sites. The temperature of the
catalyst was raised in stages under evacuation. The alkoxyl
band reduced in intensity at 373 K [Fig. 2A(b)]. Simulta-
(
COMPAQ PROLINEA 4/66) with data acquisition and
analysis software (Shimadzu Hyper IR).
Table 1 The composition, BET surface area and the number of Lewis- and BrÔnsted-acidic sites on the V O ÈWO /ZrO
2
5
3
2
V O ÈWO /ZrO
Acidic sitesa/1017 m~2
2
5
3
2
Surface area
Catalyst
V O
5 (%)
WO3 (%)
/m2 g~1
Lewis
BrÔnsted
2
V0W0Z
V0W2Z
V0W5Z
V0W8Z
V2W0Z
V2W2Z
V2W5Z
V2W8Z
V5W0Z
V5W2Z
V5W5Z
V5W8Z
0
0
0
0
2
2
2
2
5
5
5
5
0
2
5
8
0
2
5
8
0
2
5
8
28
29
29
26
31
30
28
29
25
27
22
25
6.1
10.0
4.7
5.9
4.5
5.3
5.5
4.7
6.0
4.8
5.8
5.3
0.0
0.7
4.2
4.9
3.4
5.4
5.5
4.6
3.4
5.1
5.6
4.9
a After pyridine adsorption at room temperature followed by evacuation at 373 K for 30 min.14
1616
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Fig. 1 Products and the C H /C H O ratio of propan-2-ol decomposition on V O ÈWO /ZrO catalysts at 443 K.
3
6
3
6
2
5
3
2
neously, new bands due to acetone coordinated on Lewis-
acidic sites appeared at 1676 and 1252 cm~1.
The FTIR spectrum of V2W2Z sample [Fig. 2B(a)] exhibits
basically the same bands as those of V2W0Z [Fig. 2A(a)],
except for the l(CwO) band at 1105 cm~1. At 373 K the spec-
tral behaviour of V2W2Z [Fig. 2B(b)] is almost the same as
that of V2W0Z.
Fig. 2C(a) shows the spectra of propan-2-ol adsorbed on
V0W2Z. The l(CwO) band of isopropoxide appeared at 1168
and 1117 cm~1. The intensity of the band at 1168 cm~1 is
stronger than that of V2W0Z. In a separate experiment using
ZrO alone, the band at 1173 cm~1 appeared after an intro-
2
duction of propan-2-ol. Two types of isopropoxide are present
on the V0W2Z: one interacts with tungsten oxides (1117
cm~1), the other (1168 cm~1) with zirconia. After evacuation
at 373 K [Fig. 2C(b)], the bands remained in almost the same
position as those in V0W2Z at room temperature. At this tem-
perature, the band at 1676 cm~1 due to acetone appeared on
the V2 series catalysts and was completely absent on V0W2Z.
The above results show that the isopropoxide species strongly
Fig. 2 FTIR spectra of propan-2-ol on V2W0Z (A), V2W2Z (B) and
V0W2Z (C). (a) After introduction of propan-2-ol at 298 K (ca. 0.1
kPa for 30 min) followed by 30 min evacuation, (b) followed by 30
min evacuation at 373 K.
Fig. 3 FTIR spectra of propan-2-ol on V5W0Z (A), V5W5Z (B) and
V0W5Z (C). (a) After introduction of propan-2-ol at 298 K (ca. 0.1
kPa for 30 min) followed by 30 min evacuation, (b) followed by 30
min evacuation at 373 K.
Phys. Chem. Chem. Phys., 1999, 1, 1615È1621
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interacts with the surface of V0W2Z to form neither propene
nor acetone. Thus V0W2Z is less active for decomposition of
propan-2-ol.
due to the VxO species of the second or third layer of vana-
dium oxides and those at 1037 and 1030 cm~1 were due to the
VxO species in the top layer. Similar situations would be
expected in the present study, i.e.; on the V2W0Z sample the
bands at 1052, 1042 and 1035 cm~1 were due to the VxO
species in the top layer and that at 1020 cm~1 due to similar
species in the bulk of vanadia. These bands show the presence
of several VxO species which will be discussed below in
detail.
Similar experiments were carried out with V5W0Z, V5W5Z
and V0W5Z samples (Fig. 3). Bands due to adsorbed isopro-
poxide are observed in the spectra of V5 series samples. These
bands reduced in intensity at 373 K and bands due to acetone
appeared in these samples. In the case of V0W5Z, the bands
due to isopropoxide species reduced drastically in intensity by
evacuation at 373 K, corresponding with the formation of
large amounts of propene in the pulse experiment. Bands due
to acetone did not appear in this sample. The l(CwO) band
appeared at 1107, 1105 and 1107 cm~1 on V5W0Z, V5W5Z
and V0W5Z, respectively.
The spectrum of the V2W2Z sample [Fig. 4B(a)] shows the
VxO band at 1037 cm~1 together with a weak band at 1020
cm~1. It was noticed that the addition of WO changed the
3
doublet bands at 1042 and 1035 cm~1 on V2W0Z into the
single band at 1037 cm~1 on V2W2Z. When propan-2-ol
adsorbed on V2W2Z the band at 1037 cm~1 reduced in inten-
sity and a band at 1014 cm~1 appeared [Fig. 4B(b)].
It was noted that the l(CwO) band shifted to a lower posi-
tion by adding WO . The wavenumber shifted lower by 5
3
cm~1 in the V2 series and 2 cm~1 in the V5 series, respec-
The spectral features of V2W5Z and V2W8Z samples in the
region above 1000 cm~1 were almost the same compared with
that of the V2W2Z sample. The absorption below 950 cm~1
was strong on V2W5Z and became stronger on V2W8Z.
Surface coverage has been calculated assuming a monolayer
tively.
FTIR spectra of the VxO and WxO region
Fig. 4 shows the FTIR spectra in the 1100È900 cm~1 region
of the V2 series samples. After oxidation treatment for the
V2W0Z sample [Fig. 4A(a)], the VxO bands were observed
at 1042 and 1035 cm~1 together with weak shoulders at 1052
and 1020 cm~1. When a small amount of propan-2-ol was
introduced onto V2W0Z at 298 K [Fig. 4A(b)], the bands at
capacity of 4.2 molecules nm~2 for WO .8 As a result, a 5
3
wt.% WO /ZrO sample is slightly above monolayer cover-
3
2
age. Therefore, the strong absorption below 950 cm~1, which
is directly corresponding to WO loading, supports the forma-
tion of bulk WO . Thus, bulk WO is generated on V2W5Z
3
3
3
and V2W8Z samples.
1052, 1042 and 1035 cm~1 reduced in intensity. Simulta-
Fig. 5 shows the FTIR spectra in the region of 1100È900
cm~1 of the V5 series samples. The VxO bands appeared at
neously a new broad band appeared at 1012 cm~1. The
reduction in the intensities of the 1052, 1042 and 1035 cm~1
bands seemed to be accompanied by the appearance of the
band at around 1012 cm~1.
1
5
023 cm~1 together with a shoulder band at 1036 cm~1 [Fig.
A(a)]. This band increased with increasing V O loading.
2 5
After adsorption of propan-2-ol at 298 K, the shoulder band
at 1036 cm~1 disappeared, while the band at 1023 cm~1
remained almost constant. For V5W2Z, V5W5Z and V5W8Z
samples, as shown in Fig. 5BÈ5D, the spectral behaviour in
Previously, we have reported similar observations using the
V O /ZrO samples prepared by a gas phase method.13 In
2
5
2
that report, we concluded that the band at 1020 cm~1 was
Fig. 4 FTIR spectra of the VxO or WxO stretching region of
propan-2-ol adsorbed on V2W0Z (A), V2W2Z (B), V2W5Z (C) and
V2W8Z (D). (a) Background, (b) after introduction of propan-2-ol at
Fig. 5 FTIR spectra of the VxO or WxO stretching region of
propan-2-ol adsorbed on V5W0Z (A), V5W2Z (B), V5W5Z (C) and
V5W8Z (D). (a) Background, (b) after introduction of propan-2-ol at
298 K (ca. 0.1 kPa for 30 min) followed by 30 min evacuation.
2
98 K (ca. 0.1 kPa for 30 min) followed by 30 min evacuation.
1618 Phys. Chem. Chem. Phys., 1999, 1, 1615È1621

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the region above 980 cm~1 was almost the same compared
with that of the V5W0Z sample. For V5W5Z and V5W8Z
samples, strong absorption below 950 cm~1 was observed,
suggesting the formation of bulk WO , as mentioned in the
3
cases of V2W5Z and V2W8Z samples.
The FTIR spectra of WO /ZrO catalysts are shown in Fig.
3
2
6
. In the case of V0W2Z a band appeared at 1011 cm~1
together with shoulder bands at 1007 and 996 cm~1 after the
oxidation treatment. The spectrum of V0W5Z catalyst shows
the bands at 1019 and 1013 cm~1. For V0W8Z the bands
appeared at 1018 and 1009 cm~1 followed by the strong
absorption below 960 cm~1. These bands at 1019È996 cm~1
can be assigned to the WxO stretching of wolframyl species.3
After adsorption of propan-2-ol at room temperature on
WO /ZrO catalysts, these bands drastically reduced in inten-
3
2
sity and broad bands around 1010È980 cm~1 appeared in
V0W5Z and V0W8Z samples. Since this behaviour could not
be seen in V2 and V5 series samples which contained tungsten
oxide, wolframyl species were not present on the surface of V2
and V5 series catalysts.
Raman spectra
Fig. 7 shows the Raman spectra in the 1200È200 cm~1 region
of various V O ÈWO /ZrO catalysts. The spectra of the
2
5
3
2
V2W0Z sample exhibits bands due to the monoclinic phase of
the zirconia (645, 620, 485, 386 and 340 cm~1). In the case of
the V2W2Z sample, these bands reduced in intensity by WO
addition. The spectrum of the V5W0Z sample shows a
3
number of bands, which are attributable to crystalline
Fig. 7 Raman spectra of V2W0Z (A), V2W2Z (B), V5W0Z (C),
V5W5Z (D) and V0W5Z (E and E@).
V O (1000, 708, 535, 485, 412, 310 and 290 cm~1). In the case
2
5
of the V5W5Z sample, these bands due to crystalline V O
2
5
are reduced in intensity.
Considering that the crystalline vanadia phase is more
Raman-active than the surface vanadia phase,16 the above
results show that the V2 and V5 series catalysts are composed
mainly of surface vanadate species, although the crystalline
V O partially exists on the V5 series catalysts.
2
5
The spectrum of the V0W5Z sample exhibits bands due
to ZrO together with very weak bands at ca. 960 and 810
2
cm~1. In order to clarify the existence of these weak bands,
the signal accumulations were increased by eight times (Fig.
7
E@). The bands appearing at 967 and 810 cm~1 indicate the
terminal WxO of tungsten oxide and crystalline WO ,
respectively. In the same experimental conditions, these bands
3
could not be seen in the V5W5Z sample, since Raman scat-
terers depend on the colour of the samples; the WO /ZrO
3
2
2
without V O samples are white, the V O ÈWO /ZrO being
2 5
2
5
3
dark yellow.
In separate XRD experiments, the results (not shown in the
Figure) showed that the peak at 23¡ (2h) due to crystalline
WO appeared in V2W5Z, V2W8Z, V5W5Z and V5W8Z
3
samples. Therefore, bulk tungsten oxide species exist in these
samples.
Discussion
Acidity and catalytic activity
Recently we have studied the acidic properties of the same
catalysts used in this study.14 As shown in Table 1, V2W0Z
and V5W0Z catalysts exhibited both Lewis and BrÔnsted
acidities. The addition of WO to V2 series catalysts led to an
3
increase of the number of both Lewis and BrÔnsted acidic
sites, while only the number of BrÔnsted acidic sites increased
on V5 series catalysts.
Generally, the alcohol decomposes by two parallel reac-
tions: dehydration to propene and/or diisopropyl ether on
acidic-type centres and dehydrogenation to acetone on basic
redox centres.16,17 In the case of V2 series catalysts, the
increase of conversion and selectivity to propene are due to
Fig. 6 FTIR spectra of WxO stretching region of propan-2-ol
adsorbed on V0W2Z (A), V0W5Z (B) and V0W8Z (C). (a) Back-
ground, (b) after introduction of propan-2-ol at 298 K (ca. 0.1 kPa for
3
0 min) followed by 30 min evacuation.
Phys. Chem. Chem. Phys., 1999, 1, 1615È1621
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the increase of the number of both acidic sites. For V5 series
catalysts, the increase of the selectivity to propene seems to
depend on the number of BrÔnsted acidic sites. As shown in
Fig. 1, the V0W2Z sample showed less reactivity for propan-2-
ol decomposition because it has only Lewis acidic sites. Thus,
the propene formation is correlated with not only the quantity
of acidic sites but also the quality of them.
1, 1052; peak 2, 1042; peak 3, 1035; and peak 4; 1020 cm~1),
although the curve Ðtting is insufficient around 1000 cm~1.
Surface coverage has been calculated assuming a monolayer
capacity of 2.4 nm~2 for V O .17 On the V2W0Z sample, the
2
5
value of 2 wt.% is estimated to be the V O content for which
2
5
the theoretical monolayer is formed on the ZrO support.
2
However, vanadium oxide may aggregate to form a partial
The addition of WO onto the catalysts suppressed the for-
mation of acetone, especially in V2W2Z (Fig. 1). However, the
amount of acetone did not reduce with increasing amount of
multilayer on the support during the calcination procedure.
3
In a previous study13 of V O /ZrO it was shown that the
2
5
2
band at 1030 cm~1 was due to the VxO species in the single
WO . Surface vanadate species still exist on the surface
layer of vanadium oxide on ZrO and that at 1037 cm~1 was
the same species on the top layer on multilayered V O .
Therefore, the similar assignments are applicable in the case of
the V2W0Z sample, i.e., peaks 2 and 3 are assigned to the
VxO species of the top layer on multilayered (double or
triple layers) vanadium oxide and similar species in the single
3
2
irrespective of the impregnation sequence of tungstate
2
5
species.3
As described above, Figs. 2 and 3 show that the l(CwO)
band of the isopropoxide species shifted slightly to a lower
wavenumber with increasing amounts of WO . Since the
3
wavenumber of the stretching vibration corresponds to the
layer vanadate on ZrO , respectively. The integrated peak
2
bond energy, the lower shift of l(CwO) indicates the weaken-
ing of the CwO bond of the isopropoxide species as well as
the strengthening of the Lewis acidic sites on which the iso-
propoxide species adsorbs. In comparison with both cases of
V2W0Z and V5W0Z, the shift of l(CwO) was 5 cm~1 on
V2W2Z and 2 cm~1 on V5W2Z. As a result, propene forma-
tion increased through the easier cleavage of the CwO bond
of the isopropoxide species on V2 series samples.
intensities of peaks 2 and 3 are almost the same, suggesting
that the amounts of multi- and single-layer vanadate are com-
parable on the catalyst surface. Peak 1 may be assigned to the
VxO species of an another surface vanadate. The heter-
ogeneity of the surface results in these several VxO peaks.
Peak 4 is due to the VxO species in the second or third
vanadia layer.
In the case of the V2W2Z sample, these bands were also
separated into four peaks (Fig. 8B). Addition of WO3 to
V2W0Z leads to the reduction of peak 2 and the increase of
peak 3@. The integrated intensity of the reduction of peak 2
corresponds to that of the increase of peak 3@, showing that
peak 2 in V2W0Z converts into peak 3@ in V2W2Z. Conse-
quently, it appears that the doublet bands at 1042 and 1035
cm~1 on V2W0Z changed into the single one at 1037 cm~1
on V2W2Z.
Structure characterisation of catalysts via the VxO and WxO
bands
As described above, there are several bands in the 1100È900
cm~1 spectral region, suggesting the presence of di†erent
surface vanadate species. Therefore, we will discuss the surface
species in detail by using band separation techniques.18
Fig. 8A shows the original and separate peaks of the VxO
band in the V2W0Z sample after the oxidation treatment. The
bands at 970È1070 cm~1 are separated into four peaks (peak
The results suggest that the added WO goes under the
3
vanadia species and interacts directly with the ZrO support.
2
Thus, the vanadia species were reconstructed by the addition
of WO and the new VxO species (1037 cm~1) formed over
3
the tungsten oxide species. Consequently, the shifts of the
VxO band are explicable.
V2W5Z and V2W8Z samples showed no appreciable
change in the VxO band in the top layer and no terminal
WxO band irrespective of the amount of WO . In addition,
3
the increasing loading of WO led to the appearance of the
3
band below 950 cm~1 due to crystalline WO . When WO is
3
3
incorporated onto the V O /ZrO catalysts, the majority of
2
5
2
WO seems to be intercalated between vanadia and zirconia.
3
In the case of the V5 series, the band at 1023 cm~1 is in
agreement with the VxO band of bulk V O (or crystalline
2
5
V O ), while the 1036 cm~1 band is attributable to the VxO
2
5
band in the top layer of vanadium oxides in the catalyst, as
considered above. The addition of WO hardly a†ected both
3
the VxO bands in the top layer and of bulk V O . Although
2
5
the band due to terminal WxO species appeared at 1019
cm~1 on V0W5Z (Fig. 6B), a similar band could not be
observed on V5W5Z (Fig. 5C). Thus, as described in the V2
series, the structure of the vanadia species is also reformed by
the added WO , being present under the vanadia layers. The
3
inÑuence of WO on the VxO band in the V5 series is little,
3
since the concentration of vanadia is large enough to form
multilayer vanadia species in the case of V5 series samples.
Summarising the above considerations, a model of the V2
series catalysts illustrated in Fig. 9 can be proposed. In the
absence of tungsten oxide, two types of VxO bond are
present on the surface (1042 and 1035 cm~1). The addition of
WO led to the reconstruction of vanadia species, resulting in
3
the single VxO bond (1037 cm~1). The VxO species (1042
and 1037 cm~1) possibly convert into BrÔnsted sites and coor-
dinately unsaturated vanadium ions are Lewis sites in Fig. 9.
Thus, both Lewis- and BrÔnsted-acidic sites increased. The
increase of the acidic sites and their strength may be con-
Fig. 8 Band shape analysis of FTIR spectra of VxO bands of
V2W0Z (A) and V2W2Z (B) after oxidation treatment. (a) Original
peak, (b) separated peak.
1620
Phys. Chem. Chem. Phys., 1999, 1, 1615È1621

View Article Online
process of intercalation may take place during the calcination
of the catalysts. As a result, the surface acidity as well as the
catalytic activity changed with the amount of WO .
3
The authors would like to thank Mr. S. Konishi for carrying
out some of the experiments.
References
1
2
3
J. N. Armor, Appl. Catal. B, 1992, 1, 221.
K. C. Taylor, Catal. Rev. Sci. Eng., 1993, 35, 457.
M. A. Vuurman, I. E. Wachs and A. M. Hirt, J. Phys. Chem.,
1
991, 95, 9928.
4
M. C. Paganini, L. DallÏAcqua, E. Giamello, L. Lietti, P. Forzatti
and G. Busca, J. Catal., 1997, 166, 195.
5
6
7
8
G. Ramis and G. Busca, Catal. L ett., 1993, 18, 299.
G. Deo and I. E. Wachs, J. Catal., 1994, 146, 335.
J. P. Chen and R. T. Young, Appl. Catal. A, 1992, 80, 135.
L. J. Alemany, L. Lietti, N. Ferlazzo, P. Forzatti, G. Busca, E.
Giamello and F. Bregani, J. Catal., 1995, 155, 117.
L. Lietti, L. J. Alemany, P. Forzatti, G. Busca, G. Ramis, E. Gia-
mello and F. Bregani, Catal. T oday, 1996, 29, 143.
L. Lietti, P. Forzatti and F. Berti, Catal. L ett., 1996, 41, 35.
D. Gazzoli, M. Valigi, R. Dragone, A. Marucci and G. Mattei, J.
Phys. Chem. B, 1997, 101, 11129.
9
1
1
0
1
1
2
V. Indovina, Catal. T oday, 1998, 41, 95.
Fig. 9 A model of surface structures of V2W0Z (A) and V2W2Z (B).
The numbers in the Ðgure show wavenumber in cm~1.
13 T. Ohno, F. Hatayama, Y. Toda, S. Konishi and H. Miyata,
Appl. Catal. B, 1994, 5, 89.
1
4
Y. Toda, T. Ohno, F. Hatayama and M. Miyata, React. Kinet.
Catal. L ett., 1998, 65, 213.
sidered to be attributable to an electronic interaction between
15 H. Miyata, K. Fujii and T. Ono, J. Chem. Soc., Faraday T rans.,
1
988, 84, 3121.
the vanadia and tungstate species.4 Tungsten oxide, having a
1
6
7
H. Miyata, M. Kohno, T. Ono, T. Ohno and F. Hatayama, J.
cationic nature,5 is distributed over the basic sites of ZrO and
2
Mol. Catal., 1990, 63, 181.
repel each other. At low tungstate concentration tungsten is
1
B. Grzybowska-Swierkosz, G. Coudurier, J. C. Vedrine and I.
Gressel, Catal. T oday, 1994, 20, 165.
well dispersed on the support owing to repulsion.
In this paper, we focused on the e†ect of the addition of
WO to V O /ZrO catalysts. The strong interaction of tung-
18 H. Miyata, S. Tokuda and T. Yoshida, Appl. Spectrosc., 1989, 43,
522.
3
2 5
2
sten oxide with the support reconstructs the structure of
vanadia species. Considering our preparation methods, this
Paper 8/09536A
Phys. Chem. Chem. Phys., 1999, 1, 1615È1621
1621