Direct observation of polytungstate cluster and monolayer tungstate on zirconium oxide by high-resolution TEM

Article abstract of DOI:10.1246/cl.2004.1250

Direct observation of WO₃/ZrO₂ by using high-resolution TEM images revealed the formation of polytungstate cluster with a narrow size distribution ranging in 0.4-0.7 nm, and the agglomeration into monolayer tungstate after the aging at 1073 K for 100 h.

Full text of DOI:10.1246/cl.2004.1250

1250
Chemistry Letters Vol.33, No.10 (2004)
Direct Observation of Polytungstate Cluster and Monolayer Tungstate
on Zirconium Oxide by High-resolution TEM
Atsushi Satsuma,ꢀ Hitoshi Yokoi,y Hiroyuki Nishiyama,y Shiro Kakimoto,y Satoshi Sugaya,y
Takafumi Oshima,y and Tadashi Hattori
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University,
Furo-cho, Chikusa-ku, Nagoya 464-8603
yR&D Center, NGK Spark Plug Co. Ltd., 2808, Iwasaki, Komaki, Aichi 485-8510
(Received July 20, 2004; CL-040845)
Direct observation of WO3/ZrO2 by using high-resolution
345, 378, 437, 617, and 637 cmꢁ1) were observed, and also a
broad band at 900–1100 cmꢁ1 was observed. Since the band at
880 cmꢁ1 is assigned to polytungstate,9,11 and that at 1019
cmꢁ1 to symmetic stretching mode of terminal W=O of mono-
tungstate and polytungstate,10,11 the bands indicate that the sur-
face tungstate is well spread on the zirconia surface. On the other
hand, after the aging treatment, a small diffraction line attribut-
able to WO3 phase was observed at 23.3 degree in the XRD pat-
tern, and the relative intensity of the line of tetragonal phase in-
creased greater than those of monoclinic phase. The relative in-
tensities of the diffraction line of tetragonal phase at 30.4 degree
and WO3 line at 23.3 degree were 29 and 2.1% relative to the
monoclinic line at 28.3 degree as 100% after the aging at
1073 K for 100 h. Such structural change was also detected in
Raman spectra, i.e., the bands at 804 and 714 cmꢁ1, which are
attributable to crystalline WO3, were observed.
TEM images revealed the formation of polytungstate cluster
with a narrow size distribution ranging in 0.4–0.7 nm, and the
agglomeration into monolayer tungstate after the aging at
1073 K for 100 h.
From the environmental points of view, replacement of the
liquid acid catalyzed process, such as isomerization and alkyla-
tion of hydrocarbons, to solid acid catalysis is desired.1,2 WO3/
ZrO2 is one of the promising candidates of the key material for
‘‘greener’’ solid acid process.3–5 Since the first report of the gen-
eration of strong acid sites on WO3/ZrO2 by Hino and Arata,6–8
a great deal of attention has been attracted. Since the catalytic
performance of WO3/ZrO2 strongly depends on the preparation
method, tungsten content, and calcination temperatures, many
attempts have been carried out on the characterization of sup-
ported tungsten oxide on zirconia by means of various physico-
chemical techniques, i.e., Raman,9–11 UV–vis,11,12 XAFS,12,13
NMR,14 and so on.15–17 Aiming the better understanding of the
origin of acidity of WO3/ZrO2, we examined morphology of
surface tungsten oxide on zirconia by high-resolution TEM,
and report the first direct observation of polytungstate cluster
and layered tungsten oxide on zirconia surface.
Table 1. BET surface area and W density of 10 wt %WO3/
ZrO2
BET Surface Area W Density
Treatment
/m2gꢁ1
/nmꢁ2
Before the aging
After the aging at 1073 K
51.4
30.5
5.1
8.5
ZrO2 support was prepared by hydrolysis of zirconium oxy-
nitrate 2-hydrate (Kishida, 99%) in distilled water by gradual
adding 1.0 mol dmꢁ3 an aq NH4OH solution, filtration of precip-
itate, washing with distilled water for three times, drying at
383 K, and calcination at 673 K for 24 h. WO3/ZrO2 was pre-
pared by impregnation of the obtained ZrO2 with an aqueous so-
lution of ammonium paratungstate (Mitsuwa) at pH 10 with an
aq NH4OH solution. The suspension was evaporated in rotary-
evaporator. The solid thus obtained was dried and then calcined
at 973 K for 5 h. The loading of WO3 was 10 wt %. For the aging
treatment, the WO3/ZrO2 was heated at 1073 K for 100 h in air.
High-resolution TEM images were obtained by a HITACHI HF-
2000 at HV ¼ 200 kV. BET surface areas were measured by a
flow apparatus according to the one-point method. Raman spec-
tra were measured by a JASCO NRS-1000 spectrophotometer
with a laser at 532 nm. XRD diffraction patterns were obtained
by a Rigaku RINT-1200 diffract meter using Cu Kꢀ radiation.
In the XRD pattern of WO3/ZrO2 before the aging treat-
ment, the lines attributable to tetragonal ZrO2 phase (ICSD-
68589) was observed at 2ꢁ ¼ 30:3 degree, together with weaker
lines at 28.3 and 31.6 degree, which are assignable to monoclinic
ZrO2 phase (ICSD-15983). The lines attributable to tungsten
oxides were not observed in the pattern. In Raman spectrum,
the scattering bands attributed to ZrO2 phase (176, 188, 330,
Table 1 shows BET surface area and W density of the WO3/
ZrO2 before and after the aging at 1073 K for 100 h. Before the
aging, the surface area of WO3/ZrO2 was initially 51.4 m2gꢁ1
,
and the surface W density was 5.1 W nmꢁ2, which is less than
11,12
the theoretical monolayer coverage of WO3, i.e., 7 W nmꢁ2
.
After the aging treatment, the surface area was significantly de-
creased to 30.5 m2gꢁ1. As a result, the surface W density became
8.5 W nmꢁ2 and exceeded the theoretical monolayer coverage.
As Iglesia et al. reported,11 the most essential determining factor
of the supported tungstate structure is the surface W density, and
the morphology of surface tungstate significantly changes
around the theoretical monolayer coverage (W density of
4–8 W nmꢁ2). Therefore, the detailed structure of the surface
tungstate was compared by high-resolution TEM images.
Figure 1 shows high-resolution TEM image of 10 wt
%WO3/ZrO2 before the aging treatment. The diameters of ZrO2
particles were around 13–23 nm, which is reasonable value com-
paring with surface area of 51.4 m2gꢁ1. Small darker dots, as-
signable to polytungstate clusters, were observed on the surface
of ZrO2 particles. In the figure, typical clusters were magnified in
the upper left corner and pointed by white arrows. The diameters
of these clusters are in the range of 0:59 ꢂ 0:16 nm. Interesting-
ly, the cluster size is distributed in very narrow range, although
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.10 (2004)
1251
the sample was prepared by a conventional impregnation meth-
od. Considering with the crystallographic parameters of tetrago-
nal WO3 (ICSD-27962), the diameter of the clusters indicates
that these oxide clusters contain 3–5-W atoms. The observed
surface coverage by the tungstate clusters was around 32%. Con-
sidering the surface W density of 5.1 W nmꢁ2, the theoretical
surface coverage of this sample is 73%. Since the ratio of theo-
retical surface coverage and the observed surface coverage is
2.3, it is reasonable to conclude that around 4-W atoms are
agglomerated in these polytungstate clusters.
diameter around 31 ꢂ 8 nm. This is in accordance with the de-
crease in the total surface area to 30.5 m2gꢁ1, indicating the ag-
ing treatment leads to the significant sintering and growth of
ZrO2 particles. Careful observation revealed that polytungstate
clusters with ca. 0.6 nm still remained on zirconia surface. As
magnified in the upper right corner, the size of a typical poly-
tungstate cluster was 0.76 nm, which is larger but not much ag-
glomerated than those before the aging treatment. Interestingly,
as pointed by the white bracket, the surface tungstate in layered
form was observed. The presence of layered tungstate indicates
that dispersed polytungstate clusters eventually formed W–O–W
bonds between neighboring polytungstate clusters, which leads
to the formation of 2-D tungsten oxide layer. Although the pres-
ence of WO3 crystallites was suggested by XRD pattern and
Raman spectrum, 3-D growth of tungstate was not clarified by
the images taken in this study. From the TEM images, at least,
it is clear that the three-dimensional growth of tungstate is less
significant than sintering of ZrO2 support.
These high-resolution TEM images clearly revealed the for-
mation of subnano-sized polytungstate cluster on zirconia sur-
face with uniform dispersion before the aging treatment. The
correlation between solid acid properties and the nano-structure
of surface tunstate is now under the examination and will be
reported in near future.
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Figure 1. TEM image of 10 wt %WO3/ZrO2 before the
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Figure 2. TEM image of 10 wt %WO3/ZrO2 after the aging
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Published on the web (Advance View) August 28, 2004; DOI 10.1246/cl.2004.1250