In situ XAS and XRD studies on the structural evolution of aMmonium paratungstate during thermal decomposition

Article abstract of DOI:10.1002/ejic.200400872

The bulk structural evolution during the decomposition of ammonium paratungstate (APT) under various oxidizing and reducing gases was elucidated by the complementary techniques, in situ XAS, in situ XRD, and TG/DSC, combined with mass spectrometry. In the temperature range from 300 to 650 K, the decomposition of APT proceeds nearly independently of the gas employed. At higher temperatures, an oxidizing gas results in the formation of crystalline triclinic WO₃ as the majority phase at 773 K, while mildly reducing gases (propene, propene and oxygen, and helium) result in the formation of partially reduced and highly disordered tungsten bronzes. No further reduction is observed under propene or helium, and this indicates a strongly hindered oxygen mobility in the tungsten oxide lattice in the temperature range employed. The decomposition of APT under hydrogen results in the formation of WO ₂ and, eventually, tungsten me tal. During the thermal treatment of APT, major changes occur at ca. 500 K where a complete structural rearrangement takes place that results in the destruction of the polyoxotungstate ion of APT and the formation of a tungsten oxide bronze. The rather low temperature for the formation of a three-dimensional lattice compared to the thermal treatment of common polyoxomolybdate precursors indicates the lower stability of the precursor and intermediates as ligands are removed. For tungsten to act as a potential structural or electronic promoter in molybdenum oxide based catalysts, tungsten needs to be incorporated in regular molybdenum oxide structures already at a very early stage of the catalyst preparation. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400872

FULL PAPER
In Situ XAS and XRD Studies on the Structural Evolution of Ammonium
Paratungstate During Thermal Decomposition
[a]
[a]
[a]
[a]
Olga Kirilenko, Frank Girgsdies, Rolf E. Jentoft, and Thorsten Ressler*
Keywords: In situ studies / X-ray absorption spectroscopy / X-ray diffraction / Tungsten / Oxidation / Decomposition /
Structure–activity relationships / Heterogeneous catalysis
The bulk structural evolution during the decomposition of
ammonium paratungstate (APT) under various oxidizing and
reducing gases was elucidated by the complementary tech-
niques, in situ XAS, in situ XRD, and TG/DSC, combined
with mass spectrometry. In the temperature range from 300
to 650 K, the decomposition of APT proceeds nearly indepen-
dently of the gas employed. At higher temperatures, an oxi-
tal. During the thermal treatment of APT, major changes oc-
cur at ca. 500 K where a complete structural rearrangement
takes place that results in the destruction of the polyoxo-
tungstate ion of APT and the formation of a tungsten oxide
bronze. The rather low temperature for the formation of a
three-dimensional lattice compared to the thermal treatment
of common polyoxomolybdate precursors indicates the lower
stability of the precursor and intermediates as ligands are
removed. For tungsten to act as a potential structural or elec-
tronic promoter in molybdenum oxide based catalysts, tung-
sten needs to be incorporated in regular molybdenum oxide
structures already at a very early stage of the catalyst prepa-
ration.
dizing gas results in the formation of crystalline triclinic WO
as the majority phase at 773 K, while mildly reducing gases
propene, propene and oxygen, and helium) result in the for-
3
(
mation of partially reduced and highly disordered tungsten
bronzes. No further reduction is observed under propene or
helium, and this indicates a strongly hindered oxygen mo-
bility in the tungsten oxide lattice in the temperature range
employed. The decomposition of APT under hydrogen re-
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
2
sults in the formation of WO and, eventually, tungsten me-
Introduction
W mixed oxides. Because they are less complex than the
industrially employed catalysts, these oxides constitute suit-
able model systems to reveal structure–activity relationships
of mixed oxides under the reaction conditions. Therefore,
investigations of the decomposition of APT are required
to identify and quantify tungsten oxide phases and their
formation under various gases, and to reveal correlations
between catalytic activity and the structural evolution of
APT.
Molybdenum oxide based catalysts are extensively em-
ployed for the partial oxidation of light alkanes and al-
[
1]
kenes. Binary molybdenum oxides have a limited catalytic
applicability, mostly because of their tendency to readily
transform into the less active molybdenum trioxide, MoO3,
under the reaction conditions. Thus, it would be desirable to
stabilize the active nanostructure of the molybdenum oxides
[2]
(
e.g. Mo O -type structures ) with the substitution of
5 14
The thermal decomposition of APT has been investi-
gated by numerous authors who employed a variety of
some of the molybdenum by additional metal centers such
as vanadium, niobium, or tungsten.
[
5–18]
[14]
methods.
situ Fourier transform infrared (FT-IR) spectroscopy
Thermal analysis studies combined with in
Mixed oxide catalysts are commonly prepared by chemi-
cally or physically mixing suitable catalyst precursors. In
order to prepare and stabilize the appropriate molybdenum
oxide phase, the behavior of the catalyst precursors during
thermal treatment needs to be elucidated. Previously, we re-
ported on the structural evolution of ammonium heptamo-
lybdate (AHM) and heteropolyoxomolybdates (HPOM)
during thermal activation.
Ammonium paratungstate [(NH ) H W O ·4H O,
APT] is a common precursor for the preparation of Mo/
[13]
identified WO as the product of decomposition at tempera-
3
tures above ca. 650 K. The mechanism for thermal decom-
position of APT in air has been studied by X-ray diffrac-
tion, thermogravimetry, and IR and UV/Vis diffuse reflec-
[
15]
tance spectroscopy.
NH4)0.33WO , and hexagonal WO were identified in the
Ammonium tungsten bronze,
[
3]
[4]
(
3
3
calcination reaction of APT at 673 K for 2 h under static
air. On heating up to 770 K, the tungsten bronze together
4
10
2
12 42
2
with hexagonal WO transformed completely into triclinic
3
[
a] Fritz-Haber-Institut of the MPG, Department of Inorganic tungsten trioxide.
Chemistry,
In more detail, the decomposition of APT in air has been
studied by in situ laser Raman spectroscopy (LRS) com-
bined with thermal gravimetric analysis (TGA), differential
Faradayweg 4–6, 14195 Berlin, Germany
Fax: +49-30-8413-4405
E-mail: ressler@fhi-berlin.mpg.de
2124
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400872
Eur. J. Inorg. Chem. 2005, 2124–2133

Structural Evolution of Ammonium Paratungstate During Thermal Decomposition
FULL PAPER
thermal analysis (DTA), and X-ray diffraction (XRD).[16] b = 14.46 Å, c = 10.95 Å, and β = 109.1°, which are in good
It was found that the decomposition in air proceeds agreement with the single-crystal data. The crystallite size
through three steps in the temperature ranges 300–473 K, calculated from XRD peak-broadening was about 600 nm.
4
73–623 K and 623–773 K. The formation of WO was de- No further crystalline phases were detectable in the experi-
3
tected at temperatures above 623 K. Besides studies in air, mental pattern of APT. Figure 2 shows the experimental
3
the thermal decomposition of APT was also investigated and theoretical WL -edge EXAFS FT[χ(k)·k ] of APT.
under hydrogen.
III
[
17,18]
In the temperature range 470–520 K, The corresponding distances and Debye–Waller factors ob-
APT was found to decompose into (NH4)0.33WO . This tained from the XAFS refinement are given in Table 1. The
3
compound is stable in the temperature range 520–840 K, model of the local structure around the W centers in the
and is reduced to a mixture of WO and tungsten metal at polyoxotungstate ion of APT is adequate to simulate the
2
870 K, while complete reduction to tungsten metal is WLIII EXAFS spectrum. The good agreement between the
achieved at about 1000 K. In total, although the decompo- theoretical structural data (XRD, XAS, TG) and the experi-
sition of APT has been investigated before, several ques- mental data confirms that the ammonium paratungstate
tions remain unanswered with regard to the sequence of tetrahydrate used does indeed correspond to (NH ) H -
4
10
2
formation of the intermediates, their characterization, and W O ·4H O (ICSD 15237) and excludes the presence of
12
42
2
the dependence of the decomposition pathways on the reac- major impurity phases, either crystalline or amorphous.
tion gases for catalytically relevant gas phases (e.g. propene,
and propene and oxygen).
The catalytic properties of tungsten trioxide has been in-
vestigated in a variety of reactions.[
19–25]
The selective oxi-
dation of propene to acrolein has been studied with two
[
24,25]
series of mixed molybdenum/tungsten oxide catalysts.
The first series of oxides exhibits the typical WO -type
3
structure, which consists of corner-sharing WO units and
6
the general formula Mo W O (x = 0.4–0.9). The second
x
1–x
3
series, with the general formula (Mo W ) O
(x = 0.4–
x
1–x n 3n–1
0.9; n = 8–14), comprises crystallographic shear structures.
Members of the latter series showed a higher conversion
and selectivity than the corresponding members of the first
series. Selectivity was found to decrease as the tungsten con-
tent increased and as the density of crystallographic shear
planes decreased. In situ oxidation of the reduced molybde-
num oxide series of catalysts (Mo W ) O
resulted in
x
1–x n 3n–1
little change in selectivity but a significant decrease in con-
version.
In this work, the decomposition of APT was studied with
Figure 1. Experimental (dotted) and simulated (solid) X-ray dif-
in situ X-ray diffraction (XRD) and in situ X-ray absorp- fraction pattern of ammonium paratungstate (APT), together with
tion spectroscopy (XAS) to reveal the influence of different a schematic structural representation.
treatment parameters on the decomposition process, and to
provide a detailed analysis of the short- and long-range
structure evolution during the treatments. The decomposi-
tion of APT was studied under various oxidative and re-
ductive gases (oxygen, propene and oxygen, helium, pro-
pene, and hydrogen), and the structural changes detected
are discussed and compared with the mechanism for the
decomposition of common polyoxomolybdates.
Results
Characterization of Ammonium Paratungstate
The experimental X-ray diffraction pattern of APT is de-
picted in Figure 1 together with a simulated pattern and a
schematic structural representation. The simulated pattern
of APT was obtained by using the corresponding single-
crystal structural data of APT [(NH ) H W O ·4H O,
4
10
2
12 42
2
Figure 2. Experimental (dotted) and theoretical (solid) WLIII
ICSD 15237, P2 /n, a = 15.08 Å, b = 14.45 Å, c = 11.00 Å,
3
1
XAFS FT[χ(k)·k ] of ammonium paratungstate. Structural param-
β = 109.4°]. The refined lattice constants are a = 15.04 Å, eters are given in Table 1.
Eur. J. Inorg. Chem. 2005, 2124–2133
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2125

O. Kirilenko, F. Girgsdies, R. E. Jentoft, T. Ressler
FULL PAPER
Table 1. Structural parameters [type of pairs and number (N) of
nearest neighbors at distance R] obtained from a refinement of the
ammonium paratungstate model structure (based on ICSD 15237)
to the experimental XAFS functions χ(k) of APT (Figure 2) at the
WLIII edge (Nind = 23, Nfree = 14, 12 single scattering paths and
2
1
9 multiple scattering paths, R range 0.8–4.4 Å, k range 1.8–
–1
2
3.4 Å , E
0
0
= 7.6 eV, S = 0.9).
Type
N
Rmodel [Å]
R [Å]
σ2 [Å]2
W–O
W–O
W–O
W–O
W–O
W–W
W–W
W–W
1
2
1
1
1
2
2
1
1.72
1.84
1.92
2.02
2.30
3.33
3.70
3.84
1.70
1.83
1.96
2.13
2.26
3.27
3.67
3.84
0.0020
0.0021
0.0021
0.0022
0.0023
0.0035
0.0035
0.0036
Thermal Decomposition of Ammonium Paratungstate (TG/
DSC)
The evolution of the mass loss during thermogravimetric
measurements of APT under various gases (helium, 5% hy-
drogen in helium, 20% oxygen in helium, 10% propene in
helium, and 10% propene, and 10% oxygen in helium) is
shown in Figure 3. The measurements were conducted at a
–
1
heating rate of 6 Kmin
1
and a total gas flow of
–
1
00 mLmin . Additionally, the evolution of the MS signals
for water (m/z = 18) and ammonia (m/z = 15) measured
during the decomposition of APT under 20% oxygen are
depicted in Figure 3. The gas-phase product composition is
nearly identical for decomposition under oxygen, helium
and hydrogen, even during the last step of the decomposi-
tion. In addition to the signals for water and ammonia, sig-
nals at m/z = 30 and m/z = 44 were also detected. These
Figure 3. Evolution of relative weight during decomposition of am-
signals could be assigned to NO and N O; however, the monium paratungstate under (a) 20% oxygen in helium, (b) 10%
2
propene and 10% oxygen in helium, (c) helium, (d) 10% propene
intensity of these signals follows the intensity of the signal
–
1
in helium, and (e) 5% hydrogen in helium (flow rate 100 mLmin ,
for ammonia throughout the decomposition, and they may
be artifacts formed by the mass spectrometer. Additionally,
mass signals m/z = 30 and m/z = 44 were also detected dur-
–
1
heating rate 6 Kmin ), together with the MS signals for H
m/z = 18) and NH (m/z = 15) measured for the decomposition of
APT in 20% oxygen in helium.
2
O
(
3
ing thermal decomposition of ammonium iodide (NH I) in
4
an inert gas, and this corroborates the assumption that nals in the water and ammonia MS traces. The total weight
these masses cannot be assigned unambiguously to the evol- loss after the forth decomposition step at 680 K depends on
ution of NO or NO at particular reaction temperatures the gas used and is 9.9% under oxygen, 10.1% under pro-
2
during the decomposition of APT.
pene and oxygen, 10.4% under helium, 10.5% under pro-
In the temperature range 300–650 K, the evolution of the pene, and 10.8% under hydrogen. Because the decomposi-
mass loss from APT during thermal decomposition is ne- tion under oxygen resulted in WO , the other weight losses
3
arly independent of the gas employed. Four decomposition indicate the formation of partially reduced tungsten oxide
steps at 360, 470, 520, and 680 K can be distinguished in species under reducing gases. Assuming that the initial ma-
the TG and MS traces measured. The decomposition steps terial is identical, or alternatively, that the intermediates at
at 470, 520, and 680 K are accompanied by the evolution of 400 K are identical in composition, then the extent of re-
water and ammonia, whereas for the decomposition steps at duction of the tungsten oxide at the end of the decomposi-
360 K, only water is detectable in the gas phase. The weight tion can be calculated. For the hydrogen-containing gas,
loss after the first decomposition step at 360 K is 1.2% and this partial reduction results in an oxide formula of about
corresponds to a loss of 2 equiv. of water of crystallization WO2.85. The difference between a theoretical mass change
(
1.1%). Apparently, the treatment under helium performed when decomposing APT to WO of 11.2% and the loss of
3
prior to the TG/DSC measurements to remove adsorbed 9.9% measured here during the decomposition under oxy-
water already reduced the number of equiv. of water of gen is accounted for by the above-mentioned loss of 2 equiv.
crystallization from the initial 4 to 2. The major mass loss of water of crystallization before the start of the TG experi-
of 8.8% at about 520 K is accompanied by the largest sig- ments.
2126
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 2124–2133

Structural Evolution of Ammonium Paratungstate During Thermal Decomposition
FULL PAPER
The differential scanning calorimetry traces measured
during the decomposition of APT under various gases are
depicted in Figure 4. Independent of the gas-phase compo-
sition, each of the first three mass losses shows one or more
endothermic signals; the strongest endothermic signal ac-
companies the third decomposition step at about 520 K.
The DSC measurements indicate an endothermic decompo-
sition step at 700 K under reducing gases. However, a
strong exothermic signal is observed at 700 K during the
decomposition of APT under oxygen. Because the gas-
phase products are nearly identical for both the oxygen and
helium decomposition, the exothermic signal must be due
either to a phase change, or to a reaction such as the re-
oxidation of a somewhat reduced tungsten oxide, which
would give no gas-phase products.
Figure 5. Evolution of X-ray diffraction patterns measured during
decomposition of ammonium paratungstate under 20% oxygen in
helium in the temperature range 300–770 K (effective heating rate
–
1
–1
0.1 Kmin , flow rate 80 mLmin ). Three major stages of the de-
composition are indicated.
3
The WL -edge XAFS FT[χ(k)·k ] of APT measured
III
during decomposition under 20% oxygen in helium in the
temperature range 300–770 K are shown in Figure 6. Three
temperature regions in the evolution of the local structure
around the W centers can be distinguished. At 500 K, the
initial APT spectrum exhibits changes, particularly, in the
region between 2 and 4 Å (not phase-shift-corrected). The
decreasing amplitude in this region indicates a significant
loss of structural order. Conversely, the increasing ampli-
tude of the first shell at ca. 1.5 Å indicates a decreasing
degree of distortion in the first W–O coordination. At ca.
Figure 4. Comparison of the DSC signal measured during decom-
position of ammonium paratungstate under helium (solid), 20%
oxygen in helium (dashed), 5% hydrogen in helium (dotted), and
650 K, a decrease in the amplitude of the first shell and
1
0% propene in helium (dash-dotted) (for the corresponding distinct changes in the region between 2 and 4 Å can be
3
weight loss evolution see Figure 3).
observed in the experimental FT[χ(k)·k ], which corre-
sponds to the last step in the decomposition of APT.
Thermal Decomposition of Ammonium Paratungstate (in
Situ XRD and XAS)
Decomposition of APT under 20% Oxygen
The evolution of X-ray diffraction patterns measured
during decomposition of APT under 20% oxygen in helium
in the temperature range 300–770 K is depicted in Figure 5.
Three major stages of the decomposition are indicated. The
first series of patterns up to ca. 450 K corresponds to
APT – a decreasing amount of water of crystallization ac-
counts for the changes in peak intensities and phase compo-
sition observed. In the region from 450 K to about 620 K,
a pronounced loss of crystallinity is visible from the XRD
patterns. The few reflections detectable could not be as-
signed to any particular tungsten phase on the basis of the
ICDD-PDF database. The last step in the structural evol-
ution during decomposition of APT under oxygen at 620 K
is accompanied by the occurrence of distinct XRD reflec-
3
Figure 6. Evolution of WLIII-edge XAFS FT[χ(k)·k ] of ammo-
nium paratungstate during decomposition under 20% oxygen in
helium (heating rate 5 Kmin , flow rate 40 mLmin ). Three prin-
ciple regions can be distinguished: (300–500 K) APT, (500–650 K)
–1
–1
tions, which can be assigned to triclinic WO as the ma-
3
jority phase.
4 x 3 3
(NH ) WO , (650–770 K) triclinic WO .
Eur. J. Inorg. Chem. 2005, 2124–2133
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O. Kirilenko, F. Girgsdies, R. E. Jentoft, T. Ressler
FULL PAPER
Decomposition of APT under Reducing Gases
the decomposition of APT to the amorphous phase is in-
creased by about 25 K, while the formation temperature of
The evolution of X-ray diffraction patterns measured
during the decomposition of APT under 5% hydrogen in
helium in the temperature range 300–770 K is depicted in
Figure 7. Similar to the decomposition of APT under 20%
oxygen, the first series of patterns up to about 450 K corre-
sponds to APT with a decreasing amount of water of
crystallization. In the region from 450 K to about 620 K, a
complete loss of crystallinity is visible resulting in an X-
ray-amorphous phase. At 620 K, the formation of highly
WO is lowered by about 25 K.
3
disordered hexagonal and triclinic WO and tungsten oxide
3
bronzes (NH ) WO is observed. The WO formed exhibits
4
x
3
3
a much lower crystallinity than that obtained during de-
composition of APT under oxygen. Eventually, at 770 K
under 5% hydrogen, the onset of the reduction of tungsten
oxides to tungsten metal is detected in the XRD patterns.
3
The evolution of the WL -edge FT[χ(k)·k ] measured dur-
III
ing decomposition of APT under the reducing gases studied Figure 8. Evolution of X-ray diffraction patterns measured during
(
i.e. propene, hydrogen, helium, propene and oxygen) is very decomposition of ammonium paratungstate in static air in the tem-
–1
perature range 300–770 K (effective heating rate 0.1 Kmin , flow
similar to that of the decomposition under oxygen.
–
1
rate 80 mLmin ). Three major stages of the decomposition are
indicated.
Characterization of the Decomposition Intermediates and
Products
The XRD pattern of the crystalline product of the de-
composition of APT under 20% oxygen in helium can be
simulated very well by a mixture of hexagonal and triclinic
WO (Figure 9). A Rietveld refinement of the correspond-
3
ing model structures to the experimental pattern resulted in
a phase composition of ca. 90% triclinic WO and ca. 10%
3
hexagonal WO , with crystallite sizes of 60 nm and 120 nm,
3
respectively, and lattice constants that are in good agree-
Figure 7. Evolution of X-ray diffraction patterns measured during
decomposition of ammonium paratungstate under 5% hydrogen in
helium in the temperature range 300–770 K (effective heating rate
–1
–1
0
.1 Kmin , flow rate 80 mLmin ). Three major stages of the de-
composition are indicated.
Decomposition of APT in Air (Static Conditions)
The evolution of X-ray diffraction patterns measured
during the decomposition of APT in static air in the tem-
perature range 300–770 K is shown in Figure 8. Similar to
the decomposition of APT under oxygen, three major
stages can be distinguished. The first series of patterns up
to about 470 K corresponds to APT with a decreasing
amount of crystal water. In the region from 470 K to about
550 K, the formation of an X-ray-amorphous phase is de-
tected. Eventually, at 570 K, the formation of well-ordered
and phase-pure triclinic WO is observed. The temperature
3
range over which no crystalline phase is detected is smaller
in static air than under flowing oxygen and hydrogen, and
an increased stability of APT in the temperature region be-
Figure 9. Experimental (dotted) and simulated (solid) X-ray dif-
fraction pattern of the product of the decomposition of ammonium
paratungstate under 20% oxygen in helium (300–770 K), together
with a schematic structural representation of the corresponding tri-
low 470 K and an accelerated formation of crystalline WO3
at 570 K already are observed. The onset temperature for clinic WO
3
phase.
2128 © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjic.org
Eur. J. Inorg. Chem. 2005, 2124–2133

Structural Evolution of Ammonium Paratungstate During Thermal Decomposition
ment with the single-crystal data [triclinic WO3 (ICSD
FULL PAPER
1620, a = 7.31 Å, b = 7.52 Å, c = 7.68 Å, α = 88.8°, β = γ
=
90.9°) a = 7.34 Å, b = 7.53 Å, c = 7.69 Å, α = 89.6°, β =
γ = 90.5°]. A schematic representation of the structure of
triclinic WO is depicted in Figure 9. The nearest-neighbor
3
distances in triclinic WO are given in Table 2. The structure
3
is related to the ReO structure and consists of edge-sharing
3
WO units with W–O distances ranging from 1.8 to 2.2 Å.
6
In comparison with the ReO structure, the WO units in
3
6
triclinic WO are strongly tilted with respect to each other
3
resulting in a triclinic unit cell.
Table 2. Structural parameters [type of pairs and number (N) of
nearest neighbors at distance R] obtained from a refinement of a
triclinic WO model structure (based on ICSD 1620) and a hexago-
4 3
nal (NH )0.25WO model structure (ICSD 23537) to the experimen-
3
tal XAFS functions χ(k) of the product (at 300 K; Figure 10B) and
of an intermediate (at 545 K; Figure 10A) of the decomposition of
APT under oxygen at the WLIII edge (Nind = 28, Nfree = 16, 12
single-scattering paths and 29-multiple scattering paths, R range
–1
2
0
0 0
.8–4.2 Å, k range 1.7–13.1 Å , E = 6.2 eV, S = 0.9).
Type
N
WO
Model Product (300 K)
3
N
(NH
4 3
)0.25WO
Model Intermediate (545 K)
2
2
2
2
R [Å]
R [Å]
σ [Å ]
R [Å]
R [Å]
σ [Å ]
W–O
W–O
W–O
W–O
W–W
W–W
W–W
3
1
1
1
2
3
1
1.78
1.95
2.06
2.21
3.71
3.80
3.86
1.78
1.95
2.07
2.19
3.71
3.81
3.90
0.0014
0.0015
0.0015
0.0016
0.0060
0.0060
0.0060
2
2
2
–
4
2
–
1.88
1.89
1.96
–
3.70
3.78
–
1.75
1.90
2.13
–
3.65
3.74
–
0.0040
0.0041
0.0042
–
0.0084
0.0084
–
Figure 10. Experimental (dotted) and theoretical (solid) WLIII
3
XAFS FT[χ(k)·k ] of the intermediate of the thermal decomposi-
3
tion of ammonium paratungstate at 523 K (A) and of WO as the
product of the decomposition at 773 K (B) measured at 300 K. The
corresponding structural parameters are given in Table 2.
A detailed XAFS analysis of the APT decomposition
product under oxygen and of a representative intermediate
of the decomposition from the amorphous region in the
XRD series was performed to elucidate the local structure
around the W centers in the two stages of decomposition.
3
The experimental and theoretical WLIII FT[χ(k)·k ] of the
intermediate of the thermal decomposition of APT at
5
7
23 K and of WO as the product of the decomposition at
73 K are depicted in Figure 10, A and B, respectively. The
3
corresponding structural parameters are given in Table 2. It
can be seen that the local structure around the W centers
in the product WO is in good agreement with that in the
3
triclinic WO model structure. Similarly, the local structure
3
around the W center in the intermediate of the decomposi-
tion at 523 K can also be simulated by the triclinic WO3
model structure. However, characteristic deviation in the
first W–O distances (i.e. absence of neighbors at 2.2 Å) indi-
cates a higher degree of regularity in the WO units of the
6
Figure 11. Schematic structural representation of hexagonal
intermediate phase relative to those of triclinic WO . More- (NH ) WO (ICSD 23537).
3
4 0.25
3
over, the W–W distances obtained are slightly smaller than
those determined for triclinic WO . It emerges that the local
structure around the W centers in the intermediate phase
at 523 K can be better explained by assuming a hexagonal
3
Discussion
Thermal Decomposition of Ammonium Paratungstate
tungsten bronze such as (NH4)0.25WO (ICSD 23537). A
3
schematic structural representation of (NH4)0.25WO is de-
picted in Figure 11.
The thermal analysis data shown in Figures 3 and 4 for
the decomposition of APT under the various gases em-
3
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O. Kirilenko, F. Girgsdies, R. E. Jentoft, T. Ressler
FULL PAPER
ployed indicate that up to a temperature of ca. 650 K, the sulting in more regular WO units similar to those charac-
6
decomposition proceeds largely independently of the gas. teristic of tungsten bronzes. With increasing temperature,
This is corroborated by the XRD data presented in Fig- the last decomposition stage and formation of triclinic WO3
ures 5, 7, and 8, which exhibit a series of patterns corre- at ca. 650 K results in an increased distortion of the WO6
sponding to a loss of water of crystallization up to 470 K, units characteristic of triclinic WO3.
and transformation into a poorly crystalline or amorphous
phase that persists up to ca. 650 K. At temperatures above
6
50 K, the product of decomposition depends on the gas- Decomposition of APT under Partial Oxidation Reaction
phase composition. This holds for both the phase composi- Conditions
tion and the crystallinity of the products obtained.
During the decomposition of APT under propene, and
While under oxidizing gases triclinic WO is formed as
3
propene and oxygen no significant amounts of oxidation
products of propene (i.e. acrolein or carbon dioxide) are
detected in the gas phase. As mentioned above, no re-
[
14,18]
the majority phase,
the decomposition under reducing
gases results in the formation of partially reduced tungsten
[
16]
bronzes.
However, reduction of WO3–x formed at 650 K
duction of WO by propene is detected, indicating no avail-
3
under reducing gases (i.e. helium, propene, and hydrogen)
ability of lattice oxygen for propene oxidation. The absence
of propene oxidation activity in the temperature range
studied is in agreement with a generalized mechanism of
propene oxidation, which requires a certain weakening of
the metal–oxygen bonds to ensure the presence of charac-
teristic surface defects required for the activation of gas-
to WO or tungsten metal is detected during the decomposi-
2
tion of APT under hydrogen only.[
17,26–28]
Apparently, pro-
pene is not capable of further reducing WO in the tempera-
3
ture range employed. This difference between the reducing
powers of hydrogen and propene has already been observed
[
29]
for the reduction of MoO under propene and hydrogen.
3
phase oxygen and propene and the availability of oxygen
While hydrogen is capable of entering the lattice of MoO3
[30,31]
to the gas-phase reactants.
The invariance of the WO3
and WO , reduction of the oxide by propene requires the
3
structure under propene, and propene and oxygen at tem-
peratures above 600 K indicates that the W–O bonds in the
more regular WO building units of WO , relative to the
facilitated diffusion of oxygen to the surface to react with
the propene molecules adsorbed at the surface. On the one
6
3
hand, this is the case for the reduction of MoO under pro-
3
highly distorted MoO building units of α-MoO , are less
6
3
pene and helium, where oxygen can readily diffuse in the
susceptible of forming non-oxygen-terminated catalytically
layer structure of orthorhombic MoO . On the other hand,
3
active surface sites. Conversely, treatment of MoO under
3
diffusion of oxygen appears to be considerably hindered in
the ReO -type structure of WO . Thus, no lattice oxygen is
propene and oxygen clearly shows that the onset of catalytic
activity at ca. 600 K is correlated to the mobility of lattice
oxygen at this temperature. The latter is evident from the
3
3
made available at the surface of the WO3–x crystallites
formed in the decomposition of APT, and no significant
oxidation of propene by lattice oxygen with the correspond-
onset of reduction of MoO under propene at the same tem-
3
perature. In contrast with the characteristic layer structure
ing reduction of the WO lattice occurs.
3
of orthorhombic MoO , which is capable of accommodat-
3
The major mass loss (Figure 3) and thermal DSC event
ing oxygen vacancies by the formation of crystallographic
shear defects, the corner-sharing octahedrons in the triclinic
(
Figure 4), together with the major structural changes (e.g.
Figure 5) during the decomposition of APT proceeds at ca.
20 K. The amorphous or poorly crystalline phase detected
WO structure are less flexible, energetically more stable
3
5
and are thus not capable of permitting an increased amount
of oxygen vacancies in the tungsten oxide lattice. Under the
decomposition conditions investigated, which encompass
those used for the calcination of catalyst precursors in the
literature, no catalytically active tungsten oxide bronzes or
tungsten oxide shear-structures are obtained. The rather
by XRD at temperatures above 500 K can be identified by
XAS to correspond to a 3D network structure similar to
that of triclinic WO . While the local coordination of tung-
3
sten by the nearest-neighbor oxygen atoms in the intermedi-
ate phase at 500 K exhibits a characteristic distortion very
similar to that of triclinic WO (and different from that of
3
stable ReO -like three-dimensional tungsten oxides that al-
3
hexagonal WO ), the medium-range order appears to be
3
ready form at temperatures as low as 500 K exhibit no suf-
ficient oxygen mobility and, hence, no partial oxidation ac-
tivity in the temperature range employed.
more similar to that of a hexagonal WO3 structure,
(
NH4)0.25WO . A schematic representation of (NH )
-
3
4 0.25
WO is depicted in Figure 11. The rather short W–W dis-
3
tances found in the local structure around the W center
in the intermediate at ca. 500 K is characteristic for this
hexagonal tungsten oxide. Hence, the major decomposition
stage observed at ca. 500 K corresponds to a complete de-
struction of the polyoxo ions of APT, followed by a restruc-
Comparison of the Decomposition of APT and
Polyoxomolybdates
In previous works, we reported on the characteristic evol-
turing and formation of a three-dimensional network of ution of phases during the decomposition of ammonium
[
3]
corner-sharing WO units. During the decomposition in the heptamolybdate (AHM)
and heteropolyoxomolybdate
6
[
4]
temperature range 470–650 K prior to the formation of tri- (HPOM) under various reaction gases. In Figure 12 the
clinic WO , the evolution of FT[χ(k)·k ] shows that the dis- schematic pathway for the decomposition of APT is com-
3
3
tortion in the first oxygen coordination sphere decreases re- pared to those determined for the decomposition of AHM
2130
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Structural Evolution of Ammonium Paratungstate During Thermal Decomposition
FULL PAPER
Figure 12. Schematic representation of the structural evolution during decomposition of ammonium heptamolybdate (AHM), a Keggin-
3–
type heteropolyoxo molybdate ([PMo12
the temperature range 300–773 K.).
4 10 2 12 2
O40] ), and ammonium paratungstate [(NH ) H W O42·4H O] under 20% oxygen in helium in
and HPOM under oxygen. In all three cases, decomposition under oxidizing gases, MoO under moderately reducing
2
starts with the loss of water of crystallization, present in gases, and Mo metal under hydrogen). Thus, two effects
varying amounts in the three materials. The decomposition result in a pronounced stability of the Keggin-type HPOM
of the relatively small Anderson-type polyoxomolybdate relative to APT. First, the stability of the three-dimensional
ions of AHM proceeds according to a series of polyconden- network of edge-sharing WO units in WO favors the de-
6
3
sation steps that result in an increasing dimensionality of composition of polyoxotungstate and the formation of
the structure of the decomposition intermediates.[ The X- WO . Second, the Keggin ions of heteropolyoxomolybdates
3]
3
ray-amorphous phase that forms at ca. 500 K during the are considerably stabilized by the phosphorus heteroatom.
decomposition of AHM under flowing reactants could be With respect to the preparation of tungsten-containing,
identified as ammonium tetramolybdate possessing a 1D mixed-metal oxide, the differences in the decomposition
chain structure. A three-dimensional lattice structure (hex- schemes revealed for ammonium paratungstate relative to
agonal MoO ) forms at ca. 600 K and is eventually trans- other common catalyst precursors such as ammonium hep-
3
formed into orthorhombic MoO at ca. 700 K. Evidently, tamolybdate and heteropolyoxomolybdates indicate certain
3
the thermal decomposition of both APT and AHM pro- prerequisites for suitable preparation routes. Because of the
ceeds through the formation of an ammonium- and water- low formation temperature of a stable 3D lattice structure
containing hexagonal phase, which eventually decomposes during decomposition of APT, precursor mixtures with
into mainly triclinic WO and orthorhombic MoO , respec- APT as a separate phase will most likely result in oxide
3
3
tively. In contrast to the decomposition of APT, the prod- mixtures containing catalytically less interesting triclinic or
ucts obtained from the decomposition of AHM depend hexagonal tungsten oxides. Hence, in order to use the struc-
strongly on the gas employed.[
3]
ture-promoting and stabilizing effect of the W centers in
Heteropolyoxomolybdates of the Keggin type, mixed molybdenum oxide catalysts, tungsten has to be in-
3
–
[
PMo O ] , exhibit a significantly increased stability corporated into the Mo precursors to ensure the presence
12 40
compared to APT, with a similar stoichiometry. Partial de- of W in the final catalyst.
composition and reduction of the Keggin ion starts at
about 600 K, which coincides with the formation of a three-
dimensional lattice during decomposition of AHM. At
Conclusions
600 K, the HPOM forms a lacunary Keggin ion with one
molybdenum center on an extra-Keggin framework posi-
The bulk structural evolution during the decomposition
tion. This arrangement is stable to about 700 K where a of APT under various oxidizing and reducing gases was elu-
complete structural rearrangement and formation of α- cidated by the complementary techniques in situ XAS, in
MoO is detected. While the extra-Keggin Mo center cer- situ XRD, and TG/DSC combined with mass spectrometry.
3
tainly acts as the first step towards “condensation” of the In the temperature range 300–650 K, the decomposition of
Keggin ions and formation of the extended MoO structure, APT proceeds nearly independently of the gas employed.
3
no further well-defined intermediates with an increasing de- Oxidizing gases result in the formation of crystalline tri-
gree of condensation are observed. Because the decomposi- clinic WO as the majority phase at 773 K, while mildly
3
tion of HPOM results eventually in the formation of MoO3 reducing gases (propene, propene and oxygen, and helium)
at temperatures above 700 K, the final product composition result in the formation of partially reduced and highly dis-
at 773 K is strongly dependent on the gas used (i.e. MoO3 ordered tungsten bronzes. No further reduction is observed
Eur. J. Inorg. Chem. 2005, 2124–2133
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2131

O. Kirilenko, F. Girgsdies, R. E. Jentoft, T. Ressler
FULL PAPER
under propene or helium, indicating strongly hindered oxy- fraction data) database and the ICSD (Inorganic crystal structure
database) were used for phase analysis and structure refinement.
gen mobility in the tungsten oxide lattice in the temperature
range employed. Because of the particular reducing capa- X-ray Absorption Spectroscopy (XAS): Transmission X-ray absorp-
bilities of hydrogen (i.e. incorporation in the lattice and for- tion spectra were measured in situ with the sample pellet in a flow
mation of bronzes), the decomposition of APT under hy- reactor under a controlled reactant gas.[ In situ XAS experiments
32]
were performed at the WLIII edge (10.204 keV) (Hamburger Syn-
chrotronstrahlungslabor, HASYLAB, beamline X1), by using an
drogen results in the formation of WO and, eventually,
tungsten metal.
2
Si (311) double crystal monochromator. Temperature-programmed
During the thermal treatment of APT major changes oc-
decomposition of APT was conducted at temperatures between 293
cur at ca. 520 K where a complete structural rearrangement
and 773 K under helium, 5% hydrogen in helium, 20% oxygen in
takes place which results in the destruction of the polyoxo-
helium, 10% propene in helium, and 10% propene and 10% oxygen
tungstate ion of APT and the formation of a tungsten oxide
in helium. For the in situ XAS measurements, 4 mg of APT was
bronze. This bronze already exhibits a three-dimensional
mixed with 30 mg of boron nitride and pressed into pellets of 5 mm
structure consisting of corner-shared WO units with a local in diameter (edge jump Δμx Ϸ 1.5 at the WLIII edge). The gas-
6
structure around the W centers similar to that in triclinic phase composition was continuously monitored by using a mass
WO . The rather low temperature for the formation of a spectrometer in a multiple ion detection mode (Omnistar, Pfeiffer).
3
Data analysis of XAFS spectra was performed with the software
three-dimensional lattice relative to the thermal treatment
of common polyoxomolybdate precursors indicates the par-
WinXAS 3.0.[ The spectra were energy-calibrated with respect to
33]
a tungsten metal foil reference spectrum. For background subtrac-
ticular structural stability of the arrangement of corner-
tion and normalization, first-order polynomials were refined to the
sharing WO units in WO and tungsten oxide bronzes. For
6
3
pre-edge and EXAFS region. Spectra were converted to k space
tungsten to act as a potential structural or electronic pro-
moter in molybdenum oxide based catalysts, tungsten needs
to be incorporated in regular molybdenum oxide structures
using an E
Atomic absorption, μ
spline with 7 knots to minimize peaks at low R values (Ͻ1 Å) in the
0
defined as the first inflection point in the WLIII edge.
o
, fitting was performed by using a cubic
already at a very early stage of the catalyst preparation in- Fourier-transformed EXAFS χ(k). The pseudo radial distribution
3
3
stead of employing paratungstate-containing phase mix- function FT[χ(k)·k ] was calculated by Fourier transforming the k -
tures.
weighted experimental χ(k) function, multiplied by a Bessel win-
dow, into the R space. EXAFS data analysis was performed by
using theoretical backscattering phases and amplitudes calculated
with the ab initio multiple-scattering code FEFF7.[ Single-scat-
tering and multiple-scattering paths in monoclinic APT, hexagonal
34]
Experimental Section
4 3 3
(NH )0.25WO (intermediate), and triclinic WO (decomposition
Ammonium Paratungstate Tetrahydrate, (NH
4
)
10
H
2
W
12
O
42·4H
2
O:
product) were calculated up to 6.0 Å with a lower limit of 8.0% in
amplitude with respect to the strongest backscattering path. EX-
AFS refinements were performed in R space simultaneously to
Ammonium paratungstate tetrahydrate [APT, (NH
O] was used as purchased (Osram). For the investigations de-
scribed here, a sieved fraction with particles in the size range 100–
00 μm was employed (Figure 13).
4
)
10
H
2
W
12
O
42
·
4H
2
3
magnitude and imaginary part of a Fourier transformed k -
2
1
weighted and k -weighted experimental χ(k) by using the standard
EXAFS formula.[ Structural parameters that are determined by
34]
a least-squares EXAFS refinement of a model structure to the ex-
0
perimental spectra are (i) one overall E shift, (ii) Debye–Waller
factors for single-scattering paths, and (iii) distances of single-scat-
2
tering paths. Coordination numbers (CN) and S
0
were kept con-
stant in the refinement.
Thermal Analysis (TG and DSC): Thermal analysis [thermogravim-
etry (TG) and differential scanning calorimetry (DSC)] was per-
formed with a Netzsch STA 449 C TG/DSC instrument combined
with an Omnistar (Pfeiffer) mass spectrometer under pure helium,
Figure 13. Scanning electron micrographs of a sieved fraction of
as-purchased ammonium paratungstate [(NH ) H W O42·4H O]
4 10 2 12 2
with particles in the size range 100–200 μm.
5
% hydrogen in helium, 20% oxygen in helium, 10% propene in
helium, and 10% propene, and 10% oxygen in helium. Measure-
–
1
ments were carried out at heating rates of 6 Kmin , and at a total
–
1
flow of 100 mLmin .
X-ray Diffraction (XRD): In situ XRD studies were performed with
a STOE Bragg–Brentano diffractometer (Ge secondary monochro-
mator, Cu-K radiation) equipped with a Bühler HDK S1 high-
α
temperature cell. The product composition in the gas phase was
continuously monitored by using a mass spectrometer in a multiple
ion detection mode (Pfeiffer QMS 200). XRD measurements were
Scanning Electron Microscopy: Scanning electron microscopy
(
SEM) was conducted with an S 4000 FEG microscope (Hitachi).
The acceleration voltage was set at 10 kV, the objective aperture
was 30 mm, and the working distance was 10 mm.
conducted in the temperature range 323–773 K with an effective Acknowledgments
–1
heating rate of 0.1 Kmin . Diffraction patterns were recorded ev-
ery 25 K in a 2θ range of 5–50°. Analysis of experimental diffrac-
tion patterns was performed by using the software TOPAS (Bruker
AXS) Version 2.1. Crystallite sizes were estimated by using the
Scherrer equation. The ICDD-PDF (International center for dif-
The Hamburger Synchrotronstrahlungslabor, HASYLAB, is ac-
knowledged for providing beamtime for this work. G. Weinberg is
acknowledged for conducting the SEM measurements. We thank
B. Kniep, A. Szizybalski, E. Rödel, J. Osswald, and J. Wienold for
2132
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Eur. J. Inorg. Chem. 2005, 2124–2133

Structural Evolution of Ammonium Paratungstate During Thermal Decomposition
FULL PAPER
assistance at the beamline and for numerous valuable discussions.
The authors are grateful to Prof. R. Schlögl for his continuous
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Received: October 15, 2004
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2133