Synthesis and monitoring of α-Bi2Mo3O 12 catalyst formation using thermo-Raman spectroscopy

Article abstract of DOI:10.1002/ejic.200300604

Thermo-Raman spectroscopy was used to monitor the dehydration and phase transformations of Bi₂Mo₃O₁₂·5H ₂O. The hydrated forms Bi₂Mo₃O ₁₂·5H₂O, Bi₂Mo₃O ₁₂·4.75H₂O, Bi₂Mo₃O ₁₂·3H₂O, Bi₂Mo₃O ₁₂·2H₂O, and anhydrous Bi₂Mo ₃O₁₂ were observed during dehydration in the wave-length range from 200 to 1400 cm^-1. Representative Raman spectra of these compounds are reported for the first time. The thermo-Raman intensity thermogram showed a systematic dehydration in four steps, and the differential thermo-Raman intensity thermogram confirmed this. Thermogravimetry, differential thermogravimetry, and differential scanning calorimetry results were in harmony with the results of the thermo-Raman spectroscopy. Additionally, the dehydration resulting in formation of anhydrous Bi₂Mo ₃O₁₂ (amorphous Bi₂Mo₃O₁₂ phase) and the final transformation into the α-Bi₂Mo ₃O₁₂ phase was observed to be a dynamic thermal process. The slow, controlled heating rate produced α-Bi₂Mo ₃O₁₂ catalyst with a particle size averaging 200 nm. The catalyst formed was further characterized by Fourier transform infrared spectroscopy, X-ray diffraction, time of flight SIMS, transmission electron microscopy, and energy-dispersive X-ray analysis. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200300604

FULL PAPER
Synthesis and Monitoring of α-Bi2Mo3O12 Catalyst Formation using
Thermo-Raman Spectroscopy
Anil Ghule,[a] Shin-Hwa Tzing,[a] Jia-Yaw Chang,[a] Kalyani Ghule,[a] Hua Chang,[a] and
Yong Chien Ling*[a]
Keywords: Thermo-Raman spectroscopy / Thermochemistry / Dehydration / Phase transformations / Bismuth /
α-Bi2Mo3O12 catalyst
Thermo-Raman spectroscopy was used to monitor the dehyd-
ration and phase transformations of Bi2Mo3O12·5H2O. The
the thermo-Raman spectroscopy. Additionally, the dehydra-
tion resulting in formation of anhydrous Bi2Mo3O12 (amorph-
hydrated forms Bi2Mo3O12·5H2O, Bi2Mo3O12·4.75H2O, ous Bi2Mo3O12 phase) and the final transformation into the
Bi2Mo3O12·3H2O, Bi2Mo3O12·2H2O, and anhydrous Bi2- α-Bi2Mo3O12 phase was observed to be a dynamic thermal
Mo3O12 were observed during dehydration in the wave- process. The slow, controlled heating rate produced α-Bi2-
length range from 200 to 1400 cm−1. Representative Raman
spectra of these compounds are reported for the first time.
Mo3O12 catalyst with a particle size averaging 200 nm. The
catalyst formed was further characterized by Fourier trans-
The thermo-Raman intensity thermogram showed a system- form infrared spectroscopy, X-ray diffraction, time of flight
atic dehydration in four steps, and the differential thermo- SIMS, transmission electron microscopy, and energy-dispers-
Raman intensity thermogram confirmed this. Thermogravi- ive X-ray analysis.
metry, differential thermogravimetry, and differential scan-
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
ning calorimetry results were in harmony with the results of Germany, 2004)
Introduction
techniques such as X-ray absorption,[13] X-ray diffrac-
tion,[14] neutron diffraction,[15] time-of-flight SIMS,[16] elec-
tron diffraction techniques,[17] high-resolution electron mi-
croscopy and vibrational spectroscopy.[18Ϫ20] Raman spec-
troscopy has proved to be an effective tool for the charac-
terization of the structures of transition metal oxides in the
bulk and two-dimensional surface phases.[21] Thus, a sys-
tematic study has been made of Raman stretching fre-
quency, bond length, and bond strength empirical corre-
lations. However, all these studies concentrated on the α-
Bi2Mo3O12 phase and little attention has been paid to
Bi2Mo3O12·5H2O and the other hydrated species, which
could be important in optics and acoustic applications. The
catalytic α-Bi2Mo3O12 phase used in earlier studies was usu-
ally prepared by co-precipitation,[22] sol-gel methods,[23]
solid-state reactions,[20] spray drying,[24] high-temperature
calcination of solid mixtures or several other methods.[25]
However, co-precipitation techniques result in co-precipi-
tation of pure hydroxides and also precipitate bismuth with
polymerized Mo ions. Solid-state reactions yield byproducts
(phases) formed as a result of non-stoichiometric compo-
sitions,[26] and thus require several high-temperature calci-
nation steps and regrinding to give a single, selective phase
of the catalyst. Attempts to use mechanical mixtures have
been made because of co-operative catalytic effects in multi-
phase systems where individual catalytic phases were pre-
pared separately before mixing. Occasionally, pure forms,
often contaminated with small amounts (ca. 2%) of other
phases, have been reported.[27] Thus, monitoring of the
Bismuth molybdates (α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-
Bi2MoO6 phases) are very popular and widely studied cata-
lysts because of their commercial importance.[1] Extensive
investigation of their ability to catalyze the selective oxi-
dation and ammoxidation of alkenes[1Ϫ4] and hydrogen sul-
fide[5] has been carried out. The importance of bismuth
molybdates is not limited to catalysis since they also show
interesting physical properties with potential technological
applications. Vannier et al.[6] reported Bi26Mo10O69 as being
the first unidimensional bismuth-based oxide ion conduc-
tor. Bismuth molybdates also show good optical quality
with a wide transmission window between 0.415 and 5.2
µm.[7] Additionally, single crystals with large photon-
phonon scattering cross-sections have attracted increasing
interest as acousto-optical materials. They possess low optic
and acoustic absorptions, good mechanical properties, and
chemical stability (e.g. insolubility in water).[8,9] The tetrag-
onal phases of thin-film bismuth-derived compounds are
efficient photoconductors[10] and thin films of bismuth mol-
ybdates are effective gas sensors showing potential appli-
cations in breathalyzer devices.[11]
Bismuth molybdate catalyst systems have been structur-
ally characterized,[12] and studied extensively by different
[a]
Department of Chemistry, National Tsing Hua University,
Hsinchu, Taiwan 30043, Republic of China
Fax: (internat.) ϩ 886-35711082
E-mail: ycling@mx.nthu.edu.tw
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A. Ghule, S.-H. Tzing, J.-Y. Chang, K. Ghule, H. Chang, Y. C. Ling
FULL PAPER
preparation process is important to obtain selectively a sin- thermogram shows weight losses of 0.5% or 0.25 water mol-
gle phase. This should enable a clarification of the roles and ecules (T ϭ 90Ϫ100 °C to form Bi2Mo3O12·4.75H2O), 3.5%
stabilities of individual bismuth molybdate phases during or 1.9 water molecules (T ϭ 100Ϫ165 °C to form
selective oxidations.
Bi2Mo3O12·3H2O), 5.4% or 2.93 molecules (T ϭ 165Ϫ200
Thermogravimetry (TG), differential thermal analysis °C to form Bi2Mo3O12·2H2O), and 9% or 4.94 water mol-
(DTA), and differential scanning calorimetry (DSC) are ecules (T ϭ 200Ϫ290 °C to form anhydrous Bi2Mo3O12)
techniques generally used to study the thermal properties in the first, second, third, and fourth steps of dehydration,
of solid samples in dynamic and static thermal processes.[28] respectively. The entire dehydration process extended up to
However, these techniques fail to provide direct information 290 °C. The observed weight losses were in close agreement
about the phases and their compositions during a dynamic with the theoretical weight losses of 0.5, 3.6, 5.4, and 9.1%,
process. Thermo-Raman spectroscopy (TRS) is a useful in the respective dehydration steps. Further heating gave a
technique in which successive collection of Raman spectra slight loss in weight upon transformation into the α-Bi2-
yields many details about a dynamic thermal process. The Mo3O12 phase. This might be due to the diffusion of lattice
compositional changes of all the intermediates and phase oxygen atoms to the surface and its subsequent loss, as the
transformations can be easily monitored and identified on bulk is known to act as an oxygen carrier.[1]
a time scale of seconds and in temperature increments of
one degree. The Raman intensity and the derivative of the
Raman intensity of the characteristic band are plotted as a
function of temperature and are the thermo-Raman inten-
sity (TRI) and differential thermo-Raman intensity (DTRI)
thermograms, respectively. These provide detailed quanti-
tative information about the compositional changes and
phase transformations[29] and can be compared with ther-
mograms from TG, DTA, and DSC. Thus, thermo-Raman
spectroscopy should be an ideal technique for monitoring
and study of the morphological and compositional changes
occurring during heating and calcinations.
In this work, the compositional variations of
Bi2Mo3O12·5H2O were observed by thermo-Raman spec-
troscopy in the temperature range from 25 to 600 °C. Spec-
2Ϫ
tral variation monitored in the H2O and MoO4 regions
revealed four dehydrations resulting in formation of
Bi2Mo3O12·4.75H2O, Bi2Mo3O12·3H2O, Bi2Mo3O12·2H2O,
and Bi2Mo3O12, respectively. Characteristic Raman spectra
representing all the intermediates were obtained. The TRI
thermogram and DTRI thermogram for the H2O bands
were plotted revealing four steps of dehydration with a pro-
portionate loss of water in each step. The TG thermogram
showed loss of 9% weight for the entire dehydration process
suggesting loss of five H2O molecules, with differential ther-
mogravimetry (DTG) thermograms showing dips at 95, 148,
190, and 221 °C. Formation of anhydrous Bi2Mo3O12
(amorphous Bi2Mo3O12) in the temperature range from 290
to 320 °C was followed by transformation to the α-Bi2-
Mo3O12 phase, which was identified by its Raman spectrum
in the temperature range from 320 to 345 °C. This resulting
Figure 1. (a) TG, (b) DTG, and (c) DSC thermograms from 25 to
600 °C starting from Bi2Mo3O12·5H2O
The DTG thermogram, which is the derivative of the TG
α-Bi2Mo3O12 phase was subjected to morphological and thermogram, is shown in Figure 1(b). It shows dips at 95,
phase purity characterization.
148, 190, and 221 °C indicating maximum weight losses
during the first, second, third, and fourth dehydration steps,
respectively. A slight shoulder at 250 °C to the dip at 221
°C is due to the prolonged fourth step of dehydration. The
dip at 319 °C is assigned to the transformation from the
anhydrous form to the α-Bi2Mo3O12 phase. No further in-
Results and Discussion
Thermograms
The TG thermogram of Bi2Mo3O12·5H2O in the tem- formation could be extracted at higher temperatures.
perature range from 25 to 600 °C is shown in Figure 1(a).
There are several reports about the presence of the hy-
It indicates a weight loss of 9% for the entire dehydration drated species. Murakami and Imai[30] reported the pres-
process suggesting the loss of five water molecules (calcu- ence of Bi2Mo3O12·4.75H2O and anhydrous Bi2Mo3O12
lated weight loss for five water molecules ϭ 9.1%). The TG (amorphous) based on thermogravimetric (TG) analysis.
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α-Bi2Mo3O12 Catalyst Formation
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The amorphous phase formation occurred at 150 °C based phases, but also about the size, shape, and state of aggre-
on the X-ray diffraction pattern, contradicting their TGA gation of the particles that constitute the powder.[32] As-
data, which indicated a transition at 250 °C. Yinjie and Ai- suming the internal and external modes of the sample to be
min[31] estimated the presence of Bi2Mo3O12·3.86H2O based independent of one another, the internal modes of metal
on their TGA data. Thus, it was of interest to determine oxide molecules within the unit cell of a crystal occur in the
the different hydrated species of Bi2Mo3O12·5H2O.
medium- and high-frequency regions (Ͼ 400 cmϪ1 for the
2Ϫ
The DSC thermogram of Bi2Mo3O12·5H2O in the tem- MoO4 species). External modes, including translational
perature range from 25 to 600 °C is shown in Figure 1(c). and librational modes, occur at lower frequencies (Ͻ 400
It has a broad endotherm at 190 °C accounting for the first cmϪ1). This helps in identification and assignment of spec-
three dehydration steps and an endotherm at 222 °C for the ies based on their characteristic Raman spectra. The in-
fourth dehydration step . The sharp exotherm at 336 °C ternal modes in the range from 400 to 1200 cmϪ1 provide
2Ϫ
(enthalpy ϭ Ϫ22.6 J/g) is as a result of the phase transform- information about the environment around the MoO4
ation from the amorphous phase to the α-Bi2Mo3O12 phase. ions. The free molybdate ion, MoO42Ϫ, is tetrahedral with
The DSC thermogram also shows that the entire dehy- point group Td symmetry. It has four vibrational modes
dration process extends up to 290 °C, in contrast to the ν1(A1), ν2(E), ν3(T2), and ν4(T2). The stretching modes con-
earlier report by Murakami and Imai.[30]
nect ν1(A1) and ν3(T2), while the bending modes connect
ν2(E) and ν4(T2). In addition, the α-Bi2Mo3O12 phase con-
tains three different tetrahedra which might result in differ-
ent band positions. Matsuura et al.[18] reported six bands
Thermo-Raman Studies
Extensive theoretical considerations and experimental around 900 and 800 cmϪ1 attributed to the stretching
studies have shown that vibrational spectroscopy of pow- modes of each tetrahedral species. Theobald et al.,[15] in
dered samples provides information not only about the their XANES experiments, observed that the Mo6ϩ ions in
Table 1. Raman bands for Bi2Mo3O12·5H2O, Bi2Mo3O12·4.75H2O, Bi2Mo3O12·3H2O Bi2Mo3O12·2H2O, anhydrous Bi2Mo3O12, and α-Bi2-
Mo3O12
Bi2Mo3O12
5H2O
25 °C
Bi2Mo3O12
4.75H2O
100 °C
Bi2Mo3O12
3H2O
165 °C
Bi2Mo3O12
2H2O
200 °C
Bi2Mo3O12
(anhydrous)
290 °C
α-Bi2Mo3O12
450 °C
Assignment[a]
3549 w[b]
3502 w
3462 s
3305 w
3273 w
3233 w
3542 w
3502 w
3458 w
3521 w
3220 w
3453 s
3451 s
H2O
(stre.vib.)
3265 m
3285 w
3220 w
996 s
959 w
920 s
866 s
920 s
866 s
920 s
866 s
920 s
864 s
820 w
928 m
905 m
899 s
MoϪO
stre.vib.
844 w
681 w
824 s
669 s
684 w
649 s
525 s
684 w
649 s
525 s
684 w
649 s
525 s
657 m
525 m
542 w
504 m
495 w
BiϪO stre.
vib.
Mo2O2 dioxo
499 w
499 w
501 w
467 w
418 s
467 w
418 s
467 w
418 s
420 m
374 m
342 w
374 s
374 s
352 m
374 s
374 w
370 s
352 w
342 w
324 w
292 w
267 w
352 w
342 w
322 w
291 w
267 w
MoϪO
340 w
324 w
289 s
bend./wag.
and external
vib. modes
322 w
289 w
267 w
[a]
Refs.[12,21] [b] s ϭ strong, m ϭ medium, w ϭ weak, and stre. vib. ϭ stretching vibrations.
Eur. J. Inorg. Chem. 2004, 1753Ϫ1762
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A. Ghule, S.-H. Tzing, J.-Y. Chang, K. Ghule, H. Chang, Y. C. Ling
FULL PAPER
α-Bi2Mo3O12 are surrounded by five oxygen atoms in an
asymmetric MoϪO5 coordination environment, with an av-
˚
erage MoϪO interatomic distance of 1.91 A. However, the
Fourier transform EXAFS data revealed a substantial
structural complexity. The irregular oxygen coordination
about the Mo6ϩ ions results in three peaks at 1.26, 1.53,
˚
and 1.97A. The more distant Bi and Mo cations around
this molybdenum give rise to three peaks at 2.86, 3.30, and
˚
3.65 A, which are due to Mo···Mo, Mo···Bi, and Mo···Mo/
Bi back scatterings, respectively.[13] Generally, in dynamic
thermal process within crystals, structural distortions may
occur, which lower the symmetry causing shifting and split-
ting of the bands. Raman bands for Bi2Mo3O12 fall between
200 and 1200 cmϪ1 and are assigned as in Table 1. The
broad band in the range from 3200 to 3600 cmϪ1 is due to
lattice H2O (antisymmetric and symmetric OH stretch-
ing).[21]
Dehydration
First Dehydration
Figure 2(A)(a) shows the characteristic Raman spectrum
of Bi2Mo3O12·5H2O at 25 °C in the H2O region from 3000
to 3700 cmϪ1. The two broad bands at 3453 and 3265 cmϪ1
at 25 °C represent the lattice H2O. As the temperature rises
above 60 °C, these bands shift to 3451 and 3285 cmϪ1
,
respectively, as Bi2Mo3O12·4.75H2O forms during the first
dehydration step. The characteristic Raman spectrum of
Bi2Mo3O12·4.75H2O in the water region is shown in Fig-
ure 2(A)(b) (100 °C). The actual transformation occurs in the
temperature range from 60 to 75 °C, where the weakly bound
water molecule (water of crystallization) is easily lost.
Figure
2.
Characteristic
thermo-Raman
spectra
of
Bi2Mo3O12·5H2O during the dehydration and of the intermediate
hydrated species observed: (a) Bi2Mo3O12·5H2O (25 °C; intensity
reduced 4 times); (b) Bi2Mo3O12·4.75H2O (100 °C; intensity re-
duced 3 times); (c) Bi2Mo3O12·3H2O (165 °C; intensity reduced 2
times); (d) Bi2Mo3O12·2H2O (200 °C); (e) Bi2Mo3O12 (290 °C); (f)
Bi2Mo3O12 amorphous phase (350 °C); (g) α-Bi2Mo3O12 (450 °C)
The corresponding characteristic Raman spectrum of
Bi2Mo3O12·5H2O at 25 °C in the MoO42Ϫ region (from 200
to 1200 cmϪ1) is shown in Figure 2(B)(a). A slight decrease
in intensity of the two bands at 920 and 866 cmϪ1 is ob-
served with the rise in temperature to 100 °C. The bands at
684, 649, 525, 499, 467, 418, 374, 352, 342, 292, and 267
2Ϫ
in (A) the H2O region from 3000 to 3700 cmϪ1 and (B) the MoO4
region from 200 to 1200 cmϪ1
Second Dehydration
cmϪ1 show a slight broadening. No distinct changes in
The spectral variations in the H2O region for the second
2Ϫ
spectrum and symmetry of MoO4 were observed since dehydration in the temperature range from 100 to 165 °C
only a small amount of water is lost in this step. Among were observed and the characteristic Raman spectrum of
the noticeable changes, the weak band at 499 cmϪ1 appears Bi2Mo3O12·3H2O is shown in Figure 2(A)(c). The broad
as a shoulder to the band at 525 cmϪ1 and the doublet at band at 3451 cmϪ1 further broadens and shifts to 3462
352 and 342 cmϪ1 disappears as the temperature rises above cmϪ1 as the temperature rises above 128 °C. Two broad
60 °C. These spectral changes were especially evident in the bands at 3549 and 3502 cmϪ1 emerge and become distinct
temperature range from 60 to 75 °C and are represented in with the rise in temperature to 165 °C. The broad band at
the characteristic spectrum of Bi2Mo3O12·4.75H2O shown 3285 cmϪ1splits into bands at 3305, 3273, and 3233 cmϪ1
.
in Figure 2(B)(b). The first dehydration is the transform- Overall, a decrease in intensity of the H2O bands was ob-
ation from Bi2Mo3O12·5H2O to Bi2Mo3O12·4.75H2O. To served with major spectral changes in the temperature range
the best of our knowledge, no crystal data are available from 128 to 132 °C.
2Ϫ
for Bi2Mo3O12·5H2O. However, the structure of
Similar changes were observed in the MoO4 region for
Bi2Mo3O12·5H2O could be similar to that of the second dehydration in the temperature range from 100
Bi2Mo3O12·4.75H2O (reported by Murakami and Imai,[30]
)
to 165 °C, and the characteristic Raman spectrum of
based on the similarity of their Raman spectra. The crystal Bi2Mo3O12·3H2O is shown in Figure 2(B)(c). The bands ob-
structure of Bi2Mo3O12·4.75H2O has a monoclinic unit cell, served at 920 and 866 cmϪ1 show a notable decrease in
with lattice parameters calculated to be a ϭ 6.334, b ϭ intensity as the temperature rises to 165 °C. The singlet at
11.593, c ϭ 5.777 A and β ϭ 113.166° as indexed from the 351 cmϪ1, observed at 100 °C, splits into two bands at 352
˚
X-ray diffraction pattern.
and 342 cmϪ1 as the temperature rises above 128 °C. The
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α-Bi2Mo3O12 Catalyst Formation
FULL PAPER
band at 322 cmϪ1 becomes distinct and the bands at 289 Fourth Dehydration
and 263 cmϪ1 shift to 291 and 267 cmϪ1, respectively. The
The spectral variations in the H2O region for the fourth
dehydration in the temperature range from 200 to 290 °C
were monitored and the characteristic Raman spectrum for
anhydrous Bi2Mo3O12 is shown in Figure 2(A)(e). The
weak, broad bands observed at 3542, 3502, 3458, and 3220
cmϪ1 gradually disappear when the temperature reaches
290 °C. This indicates a complete loss of water to form an-
hydrous Bi2Mo3O12. The Raman spectral profile appears
almost flat.
spectral changes in the temperature range from 128 to 132
°C correspond to second dehydration to form
a
Bi2Mo3O12·3H2O from Bi2Mo3O12·4.75H2O. The two water
molecules lost below 165 °C are attributed to the water of
crystallization. To the best of our knowledge, the Raman
spectrum of Bi2Mo3O12·3H2O is reported here for the first
time and crystal structure data for Bi2Mo3O12·3H2O are yet
to be reported. However, based on the similarity of their
Raman spectra, its structure is expected to be similar to
Bi2Mo3O12·4.75H2O with a slight variation in lattice par-
ameters. Yinjie and Aimin,[31] reported the crystal structure
of Bi2Mo3O12·3.86H2O as monoclinic, with lattice param-
Similarly, spectral changes were clearly observed in the
2Ϫ
MoO4 region for the fourth dehydration in the tempera-
ture range from 200 to 290 °C with a transformation tem-
perature observed at 220 °C. The characteristic Raman
spectra of the anhydrous Bi2Mo3O12 is shown in Fig-
ure 2(B)(e). The broad band at 920 cmϪ1 gradually shifts to
928 cmϪ1 and a new band at 905 cmϪ1 starts to appear as
the temperature rises above 220 °C. The band at 864 cmϪ1
and its shoulder at 820 cmϪ1 disappear and a broad band
at 844 cmϪ1 appears as the temperature approaches 290 °C.
The band at 657 cmϪ1 decreases in intensity and shifts to
681 cmϪ1. The weak bands at 525 and 420 cmϪ1 disappear
and the weak bands at 374 and 342 cmϪ1 broaden and de-
crease in intensity as the temperature rises to 290 °C. Major
spectral changes observed in the temperature range from
220 to 240 °C are the result of the fourth dehydration where
Bi2Mo3O12·2H2O transforms to anhydrous Bi2Mo3O12. The
crystal structure of anhydrous Bi2Mo3O12 has not been re-
ported. However, an X-ray diffraction pattern recorded
contained no measurable peaks suggesting that the powder
is amorphous (data not shown).
˚
eters calculated to be a ϭ 6.338, b ϭ 11.658, c ϭ 5.787 A
and β ϭ 113.168° as indexed from the X-ray diffraction pat-
tern.
Third Dehydration
Spectral variations were observed in the H2O region for
the third dehydration in the temperature range from 165 to
200 °C with a transformation temperature at 192 °C. The
characteristic Raman spectrum of Bi2Mo3O12·2H2O ob-
tained after the third dehydration is shown in Fig-
ure 2(A)(d). The spectrum at 165 °C shows broad bands at
3549 and 3462 cmϪ1. These bands gradually shift to 3542
and 3458 cmϪ1, respectively, with the rise in temperature
above 190 °C. The band at 3502 cmϪ1 further broadens and
the weak bands at 3305 and 3273 cmϪ1 disappear as the
temperature approaches 200 °C. The weak band at 3233
cmϪ1 shifts to 3220 cmϪ1
.
Spectral variations were also observed in the MoO42Ϫ re-
gion for the third dehydration step in the temperature range
from 165 to 200 °C and the characteristic spectrum of
Phase Transformations
Figure 2(B)(f) shows the representative spectral varia-
2Ϫ
Bi2Mo3O12·2H2O is shown in Figure 2(B)(d). The band at tions in the MoO4 region from 200 to 1200 cmϪ1 for the
920 cmϪ1 broadens and drastically decreases in intensity. A onset of crystallization of the α-Bi2Mo3O12 phase from the
shoulder at 820 cmϪ1 to the band at 864 cmϪ1 starts ap- Bi2Mo3O12 amorphous phase. The crystallization process
pearing at 193 °C and gradually resolves as the temperature began at 312 °C. The spectrum at 350 °C shows traces of
rises to 200 °C. The band at 649 cmϪ1 broadens and shifts the amorphous phase as well as the α-Bi2Mo3O12 phase.
to 657 cmϪ1 and the shoulder at 684 cmϪ1 gradually disap- The weak bands at 996, 959, 504, 374, and 289 cmϪ1 are
pears as the temperature rises to 200 °C. Similarly, the band due to the α-Bi2Mo3O12 phase whereas the sharp bands at
at 525 cmϪ1 becomes weak and the shoulder at 501 cmϪ1 905, 672, and 328 cmϪ1 represent the amorphous phase.
and the weak band at 467 cmϪ1 disappear. The doublet at The band at 928 cmϪ1 (290 °C) shifts to 905 cmϪ1 as the
352 and 342 cmϪ1 merges to form a broad singlet at 342 temperature approaches 350 °C. A further increase in tem-
cmϪ1. The lower modes at 322, 291, and 267 cmϪ1 also perature to 450 °C, as in Figure 2(B)(g), shows sharp bands
disappear as the temperature rises to 200 °C. The major at 996, 959, 899, 824, 669, 504 370, 340, 324, and 289 cmϪ1
changes observed in the spectra indicate the drastic change characteristic of the α-Bi2Mo3O12 phase. A major shift in
in symmetry of MoO42Ϫ, even though only one water mol- the band at 905 cmϪ1 to 899 cmϪ1 was observed. There
ecule is lost in this step, resulting in the transformation were no further spectral changes up to 600 °C indicating a
from Bi2Mo3O12·3H2O to Bi2Mo3O12·2H2O. This shows complete transformation into the pure α-Bi2Mo3O12 phase.
that the water molecule evolved in this third dehydration The crystal structure of the α-Bi2Mo3O12 phase (scheelite
must be different from the two water molecules evolved in structure) has been extensively studied and reported to be
the previous dehydrations and that it is associated directly monoclinic (trigonal-bipyramidal or distorted tetrahedral)
with the structural composition. It is assigned as a water of with space group P21/c and unit cell parameters a ϭ 7.685,
˚
constitution. The crystal structure of Bi2Mo3O12·2H2O is b ϭ 11.491, c ϭ 11.929 A with characteristic edge β ϭ
yet to be reported.
115.40°.[33] The crystal structure, reported by Cesari et
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A. Ghule, S.-H. Tzing, J.-Y. Chang, K. Ghule, H. Chang, Y. C. Ling
FULL PAPER
al.,[34] shows that the α-Bi2Mo3O12 phase consists of two
kinds of twin tetrahedra α1α1 and α2α3, one of which (α1α1)
contains a center of symmetry; α1 and α2 have Bi ions adjac-
ent to the tetrahedra, while there is a Bi ion vacancy in the
α3 tetrahedron. In all cases, there are three different and
distorted tetrahedra, resulting in complicated absorption
bands.
Intensity Variation
The thermo-Raman intensity (TRI) and the derivative of
the thermo-Raman intensity (DTRI) of the water bands in
the range from 3100 to 3700 cmϪ1 from 25 to 300 °C were
plotted as shown in Figure 3(a) and (b), respectively. Based
on the TGA information, the loss in intensity is equated
with loss of five molecules of water. Slopes in the TRI ther-
mogram indicate loss of water and plateaus imply inter-
mediate formation. The TRI thermogram shows four slopes
and four plateaus corresponding to four steps of dehy-
dration. The temperature range, percentage reduction in
intensity (between two intermediates), equivalent number of
molecules of water lost and dip observed in the DTRI dur-
ing each step of dehydration are tabulated in Table 2. The
entire dehydration process extended up to 290 °C and no
further decrease in intensity was observed, indicating com-
pletion of the dehydration. The slight temperature varia-
tions in the starting and end point of each of the dehy-
dration steps in the TRS and TGA can be attributed to the
different physical properties monitored by these techniques.
TRS monitors the surface of the sample whilst TGA moni-
tors the bulk. However, the end point of the completion of
the dehydration process was the same for both techniques.
Figure 3. (a) Thermo-Raman intensity (TRI) thermogram and (b)
differential thermo-Raman intensity (DTRI) thermogram for the
stretching bands of H2O during dehydration of Bi2Mo3O12·5H2O
in the temperature range from 25 to 300 °C
governs the activity and selectivity of bismuth molybdates,
which must provide sufficient lattice oxide ions during cata-
lytic redox cycles.[35]
Fourier Transform Infrared Spectroscopy
Band Shift
Figure 5 shows the FT-IR spectra of as-synthesized
Bi2Mo3O12·5H2O and the α-Bi2Mo3O12 phase (sample col-
lected at 450 °C). The IR spectra and the bands of the α-
Bi2Mo3O12 phase are in agreement with those observed by
Matsuura et al.[18] The major difference observed was the
disappearance of water bands and the bands at 1189 and
1879 cmϪ1. In addition, a drastic decrease in intensity was
noted in the spectra of the α-Bi2Mo3O12 phase with a major
Figure 4 shows the shifts of the bands at 920 and 866
cmϪ1 plotted as a function of temperature (from 25 to 600
°C). No noticeable shift in the band at 866 cmϪ1 was ob-
served until it gradually disappeared during the fourth step
of dehydration. A similar trend was observed for the
characteristic band at 920 cmϪ1, although it showed a grad-
ual shift to 928 cmϪ1 during the fourth step of dehydration.
This clearly indicates that a major structural transformation
occurs during the fourth dehydration, where two water mol-
ecules (water of constitution) are lost, distorting the struc-
splitting around 900 cmϪ1
.
X-ray Diffraction (XRD)
2Ϫ
ture and the environment around the MoO4 ions. Fur-
The XRD patterns of Bi2Mo3O12·5H2O and the α-Bi2-
Mo3O12 phase are shown in Figure 6. The XRD pattern of
the amorphous phase contains no diffraction peaks (data
not shown). The crystalline nature of Bi2Mo3O12·5H2O and
the α-Bi2Mo3O12 phase is demonstrated by the presence of
distinct diffraction peaks. However, notable changes in the
diffraction patterns between the two crystalline forms was
observed which agrees with the considerable spectral
changes observed in our thermo-Raman study. The diffrac-
thermore, a drastic shift of the band from 928 to 905 cmϪ1
was observed around 310 °C during the onset of crystalliza-
tion towards the formation of α-Bi2Mo3O12 phase. As the
temperature rose above 310 °C the band shifted gradually
to 899 cmϪ1 after which no further shift was noted marking
completion of transformation into the α-Bi2Mo3O12 phase.
Characterization of the α-Bi2Mo3O12 Phase
Morphological characteristics depend on the rate of heat- tion pattern of pure α-Bi2Mo3O12 is in close agreement with
ing and calcination time. The particle size is important and that reported by Li and Cheng.[5] Measurement of the XRD
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α-Bi2Mo3O12 Catalyst Formation
FULL PAPER
Table 2. TRI and DTRI data obtained during dehydration of Bi2Mo3O12·5H2O
Dehydration
Temperature. range
[°C]
Intensity drop
(% of H2O)
H2O molecules
[equiv.]
DTRI endotherm
[°C]
First step
(Bi2Mo3O12·5H2O to Bi2Mo3O12·4.75H2O)
Second step
(Bi2Mo3O12·4.75H2O to Bi2Mo3O12·3H2O)
Third step
(Bi2Mo3O12·3H2O to Bi2Mo3O12·2H2O)
Fourth step
(Bi2Mo3O12·2H2O to anhyd. Bi2Mo3O12)
50Ϫ95
6.9
34.5
22.4
36.2
0.3
1.7
1.1
1.8
76
146
175
222
100Ϫ160
165Ϫ190
190Ϫ290
Figure 4. Band-shift measurements for the bands at 920 and 866
cmϪ1 of Bi2Mo3O12·5H2O observed in the temperature range from
25 to 600 °C
Figure 6. XRD pattern of as-synthesized Bi2Mo3O12·5H2O and α-
Bi2Mo3O12 (heated to 450 °C)
Time of Flight Secondary Ionization Mass Spectrometry
(TOF SIMS)
The positive ion static TOF-SIMS spectrum of the α-Bi2-
Mo3O12 phase was also recorded and is presented in Fig-
ure 7. The observed spectral pattern and the Bi3O4ϩ/Biϩ
ratio of 0.0004 are in close agreement with that previously
reported for the α-Bi2Mo3O12 phase.[16] Considering the
sensitivity of TOF-SIMS, the phase purity of α-Bi2Mo3O12
can be confirmed. However, the peaks at m/z ϭ 487 and
565 reported by Weng et al.[16] were not observed.
Electron Microscopic Study and EDS Analysis
Electron microscopy is extensively used for structural,
chemical (composition), and morphological studies (grain
size, crystallinity, and phase identification). Figure 8(a)
shows a TEM image of the α-Bi2Mo3O12 phase obtained
Figure 5. Fourier transform infrared spectra of as-synthesized
Bi2Mo3O12·5H2O and α-Bi2Mo3O12 in the range from 500 to
4000 cmϪ1
after controlled heating to 450 °C at a rate of 5 °C·minϪ1
.
patterns of the intermediate hydrated species should be of The distribution of the particle size was in the range of
interest for a better comparison with the data obtained several hundreds of nanometers (avg. 200 nm) as observed
from thermo-Raman spectroscopy. Ideally, an in situ meas- from the particle size distribution profile deduced from
urement of the temperature dependence of the XRD spec- TEM images [Figure 8,(b)]. However, in most of the earlier
trum over the temperature range of interest is required to reports where the calcination step probably involved fast
obtain accurate data.
heating rates, the particle size was reported to be in the
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A. Ghule, S.-H. Tzing, J.-Y. Chang, K. Ghule, H. Chang, Y. C. Ling
FULL PAPER
Figure 7. Positive ion TOF-SIMS spectra of α-Bi2Mo3O12 (heated to 450 °C)
Figure 8. Morphological and compositional characterization of α-Bi2Mo3O12 (heated to 450 °C): (a) TEM image indicating the particle
size and surface characteristics; (b) particle size distribution profile; (c) EDS spectra showing the composition of the α-Bi2Mo3O12 formed
(atomic ratio Bi/Mo ഠ 2:3)
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α-Bi2Mo3O12 Catalyst Formation
FULL PAPER
micrometer range (100Ϫ400 µm).[36] EDS is useful for tial future applications in monitoring catalyst formation if
quantitative analysis. EDS measurements were carried out a commercial instrument can be developed.
on Bi2Mo3O12·5H2O, α-Bi2Mo3O12 (heated at 450 °C), and
α-Bi2Mo3O12 (heated at 550 °C). The presence of bismuth,
molybdenum, and oxygen was examined and a representa-
tive EDS spectrum of α-Bi2Mo3O12 (heated at 550 °C) is
Experimental Section
shown in Figure 8(c). The bismuth and molybdenum con-
Sample Preparation: Bi2O3 (Aldrich Chem., WI, USA) and
tents, and the Bi/Mo ratios in all the samples were identical.
Na2MoO4·2H2O (RiedelϪde Hae¨n, Seelze, Germany) were used as
The atomic ratio of Bi/Mo was observed to be 43:57 which
is in close agreement with the molar ratios of Bi2O3/
Na2MoO4·2H2O in the starting solution.
received without further purification. Bi2O3 and Na2MoO4·2H2O
(1:3 molar ratio) were dissolved in 1  nitric acid and stirred vigor-
ously at 60 °C until a slurry was obtained. This slurry was cooled,
filtered, and the residue washed thoroughly with distilled water un-
til the washings reached pH ϭ 6. The sample was dried at room
temperature to ensure the retention of water of hydration. This
sample was lightly ground prior to analysis.
According to AFM, XRD, and XPS experiments carried
out by Yanina and Smith,[37] a loss of Mo was observed on
heating to temperatures higher than 500 °C in dry air or
air with 2.3% H2O due to volatilization. This resulted in
precipitation of traces of γ-Bi2MoO6 and β-Bi2Mo2O9 on
the surface of the α-Bi2Mo3O12 catalyst. However, no such
changes were observed in our experiments even though the
heating was performed in the presence of air. This might
be due to the slow and controlled heating rate used in our
experiment. Additionally, TOF-SIMS data also support the
presence of the α-Bi2Mo3O12 phase. This provides evidence
for the formation of the pure form of the α-Bi2Mo3O12 cata-
lyst under a controlled heating rate in the presence of air.
In addition, the particle size observed from TEM images
lies in the hundreds of nanometers range, suggesting that
the controlled heating rate is also important in governing
the particle size of the catalyst.
Thermo-Raman Study: The Raman spectra were obtained using
laser excitation at a wavelength of 514.5 nm (50 mW) from an ar-
gon ion laser (Coherent, Innova 100-15). A filter was used to re-
move the plasma lines. The scattered light was collected at right
angles, dispersed by a single spectrometer (Spex, 0.5 m) with a reso-
lution of 2 cmϪ1 and detected by a CCD camera (Princeton Instru-
ments, 1024 ϫ 1024 pixels). A notch filter was set in front of the
spectrometer to remove the Rayleigh scattered light. The sample
was placed in a sample holder and mounted on the thermocouple
in a homemade oven in stationary air. The temperature of the oven
was programmed to increase from 25 to 600 °C at a rate of 5 °C
minϪ1. Thermo-Raman spectra were collected continuously during
heating without disturbing the sample. The exposure time of the
CCD was set such that the temperature range covered by each spec-
trum was 1 °C. Overall, 575 spectra were collected and their repro-
ducibility checked in the spectral range from 200 to 1400 cmϪ1 for
2Ϫ
MoO4 whilst 275 spectra in the range from 2900 to 3900 cmϪ1
Conclusion
were collected for H2O. A BASIC computer program was written
to systematically determine the intensity variation for TRI thermo-
gram and the band-shift measurements.
A systematic study of the dehydration and phase-trans-
formation processes of Bi2Mo3O12·5H2O was carried out
using thermo-Raman spectroscopy in the temperature
range from 25 to 600 °C. Three different hydrated species,
Bi2Mo3O12·4.75H2O, Bi2Mo3O12·3H2O, Bi2Mo3O12·2H2O,
Characterization: A thermogravimetric analyzer (PerkinϪElmer
TGA6) was used to record the thermogram in the temperature
range from 25 to 600 °C with a heating rate of 5 °C·minϪ1 in a
and anhydrous Bi2Mo3O12 were observed as intermediates, flow of air at 20 mL·minϪ1. The DSC thermogram of the same
sample was recorded with a Setaram DSC131 in a crimped alumi-
num crucible in the temperature range from 25 to 550 °C with a
heating rate of 5 °C·minϪ1 in a flow of air at 30 mL·minϪ1. Fourier
transform infrared spectroscopy was carried out with a Bomem
Hartmann & Braun (MB-series) spectrometer. The X-ray diffrac-
tion patterns of the product were measured with a Material Analy-
sis and Characterization (MAC) advanced powder X-ray dif-
based on the spectral variations observed in the H2O and
MoO42Ϫ absorption regions during temperature increments
of 1 °C. Characteristic Raman spectra of these hydrated
species have been reported for the first time. The TRI ther-
mogram for the H2O bands supported the results revealing
four dehydration steps with a proportionate loss of water in
each step. The DTRI thermogram was also in harmony
with the TRI and TGA data. The phase transformation of
anhydrous Bi2Mo3O12 to the α-Bi2Mo3O12 phase was also
˚
fractometer using Cu-Kα1 radiation (1.54056 A). Positive TOF
SIMS (ION-TOF) measurements were performed using pulsed
Gaϩ ions (15 keV) and a post-acceleration of 10 kV. The analysis
observed and characterized by other techniques. Slow and area of 100 µm ϫ 100 µm with data acquisition time of 200 s was
used for static TOF-SIMS conditions. The best resolution obtained
was M/∆M ഠ 8000. The powders were pressed on to the carbon
tape supported on a clean Si wafer. Excess particles were removed
by blowing with compressed air such that the layer thickness was
minimal. The particle morphology was studied by transmission
electron microscopy (TEM) (Philips, Tecnai 20) working at a
200 kV accelerating voltage. A drop of sample suspension, which
had been previously dispersed in methanol by ultrasonication, was
placed on the microgrid and dried before microscopy study. The
elemental composition of the material was analyzed by energy-dis-
controlled heating resulted in the α-Bi2Mo3O12 phase with-
out precipitating γ-Bi2MoO6 and β-Bi2Mo2O9 phases and
the particle size thus obtained was in the range of a few
hundreds of nanometers as observed from the morphologi-
cal characterization. The characterization of the α-Bi2-
Mo3O12 phase by XRD, EDS, and FTIR for phase purity
was further supported by TOF-SIMS analysis. The fact that
the thermo-Raman spectroscopy setup could detect all the
dehydration steps involved and also the transformation to
the α-Bi2Mo3O12 phase indicates its sensitivity and poten- persive X-ray analysis (EDS) (Philips, Tecnai 20).
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FULL PAPER
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