UV/Vis/near-IR spectroscopic characteristics of H4-xCsxPVMo11O40 (x = 0, 2) catalyst under different temperatures and gas atmospheres

Article abstract of DOI:10.1039/b109293f

In order to understand the transformations of the Keggin-type H_4-xCs_xPV-Mo₁₁O₄₀ (x = 0, 2) compounds with rising temperature over a long time in a stream of different gas atmospheres, in situ UV/Vis/near-IR spectroscopic studies were carried out. Diffuse reflectance spectra were recorded using an improved spectrometer and a suitable microreactor. Visible and near-IR peak intensities, peak positions and the band gap energies were determined from apparent absorption spectra. Propene, isopropanol, water and the oxidation products were analyzed by GC. The experimentally observed blue shift of the visible absorption band in the region of crystal water loss and the increase in the near-IR absorption were explained on the basis of quantum-mechanical calculations of the shapes and positions of the charge transfer and d-d bands arising from Mo⁵⁺-Mo⁶⁺ and V⁴⁺-Mo⁶⁺ pairs in intact and ill-defined fragments of the Keggin structure. It was concluded that with removal of crystal water during the action of He, He/H₂O, propene, O₂/propene and increasing temperature, reduced species with protons located at the bridging oxygens promote a blue shift of the visible band, while a large number of ill-defined species form the near-IR part of the spectra in the temperature range 326-600 K.

Full text of DOI:10.1039/b109293f

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UV/Vis/near-IR spectroscopic characteristics of H4ꢀxCsxPVMo11O40
(x ¼ 0, 2) catalyst under different temperatures and gas atmospheres
a
J. Melsheimer,* J. Krohnert, R. Ahmad, S. Klokishner, F. C. Jentoft, G. Mestl and
a
a
b
a
a
¨
a
¨
R. Schlogl
a
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Department of Inorganic Chemistry,
Faradayweg 4-6, D-14195 Berlin, Germany. E-mail: jm@FHI-Berlin.mpg.de
State University of Moldova, Mateevich str.60, 2009 Kishinev, Moldova.
E-mail: sophia@usm.md
b
Received 11th October 2001, Accepted 8th March 2002
First published as an Advance Article on the web 29th April 2002
In order to understand the transformations of the Keggin-type H4ꢀxCsxPV–Mo11O40 (x ¼ 0, 2) compounds
with rising temperature over a long time in a stream of different gas atmospheres, in situ UV/Vis/near-IR
spectroscopic studies were carried out. Diffuse reflectance spectra were recorded using an improved
spectrometer and a suitable microreactor. Visible and near-IR peak intensities, peak positions and the band gap
energies were determined from apparent absorption spectra. Propene, isopropanol, water and the oxidation
products were analyzed by GC. The experimentally observed blue shift of the visible absorption band in the
region of crystal water loss and the increase in the near-IR absorption were explained on the basis of quantum-
mechanical calculations of the shapes and positions of the charge transfer and d–d bands arising from Mo5+
–
Mo6+ and V4+–Mo6+ pairs in intact and ill-defined fragments of the Keggin structure. It was concluded that
with removal of crystal water during the action of He, He/H2O, propene, O2/propene and increasing
temperature, reduced species with protons located at the bridging oxygens promote a blue shift of the visible
band, while a large number of ill-defined species form the near-IR part of the spectra in the temperature range
326–600 K.
now, this spectroscopic method has been applied to the inves-
1. Introduction
tigation of heteropoly compounds by some groups, but all
spectra were recorded ex situ.17–21 The heteropoly compounds
alter the spectroscopic behaviour due to the loss of water.22,23
In certain compounds of this type, even a small water loss
leads to noticeable changes in the absorption band intensity.
The resulting strong alterations of light absorption can only
be observed with improved spectroscopic equipment.24 We
report here on a systematic in situ study of the influence of
both an inert gas (He) and other gas atmospheres (O2 , pro-
pene, O2/propene) on the spectroscopic feature at room tem-
perature and higher temperatures, respectively. The goal of
the paper is also to find a correlation between the spectro-
scopic characteristics and catalytic data for the oxidation of
propene on the title compounds.
In ref. 25 a model was worked out for the calculation of the
charge transfer and d–d bands arising from the homonuclear
Mo5+–Mo6+ and heteronuclear V4+–Mo6+ pairs in intact
and ill-defined fragments of the Keggin structure contributing
to the absorption spectra under different experimental condi-
tions. Electron transfer processes V4+–O–Mo6+ ! V5+–O–
Mo5+, Mo5+–O–Mo6+ ! Mo6+–O–Mo5+ involving two metal
centers were taken into consideration. The vibronic coupling
constants and crystal field splittings determining the positions
and the shapes of the charge transfer and d–d bands were cal-
culated in the exchange charge model26 of the crystal field. In
the present paper, on the basis of the regularities established in
ref. 25, a qualitative explanation of the temperature behaviour
of the visible (Vis) and near-IR (NIR) parts of the absorption
spectra of H4ꢀxCsx PVMo11O40 (x ¼ 0,2) will be given. Along
with this, the band gap energies that change with different gas
atmospheres will be determined as a function of temperature.
Cs salts of vanadiummolybdophosphoric acid, used because of
their good redox and acid/base properties, are, for example,
catalyst precursors for the oxidative dehydrogenation reaction
of isobutyric acid to methacrylic acid.1–3 Unfortunately, how-
ever, the title compound, in particular with x ¼ 0, is thermally
unstable, which makes it especially difficult to determine the
catalytically active center. Many efforts have been made to
investigate the thermally unstable state, for example, by
XRD,2,4 TG-DTA coupled with IMR-MS and DSC,2,5
IR,4,6–8 EPR9,10 and Raman.11–13 To improve the stability
and catalytic activity of such compounds, cations were intro-
duced in the secondary structure.14,15 Recently, the caesium-
containing compound with x ¼ 2 proved to be the best cata-
lyst among the different title compounds, for instance for the
partial oxidation of ethene and propene by molecular oxy-
gen.14–16 An important role in the structural dynamics of this
type of compound is played by the reducibility of the cationic
centres, which was investigated by EPR spectroscopy.10
Changes in the redox state of heteropoly compounds can also
be investigated by the UV/Vis/near-IR spectroscopy. In the
case of gas/solid reactions this method is restricted to the
reflectance mode because the absorption in the catalyst sam-
ples, which must have a certain necessary thickness, is too
high. During changes in the redox state the spectroscopic char-
acteristics, such as the number of the absorption bands, peak
positions and band gap energies, also change. Besides the oxy-
gen ligand to metal charge transfer band (LMCT) there are
absorption bands which can develop or disappear due to the
optical d–d transitions in V4+ and Mo5+ and intervalence
V4+–Mo6+ and/or Mo5+–Mo6+ optical transitions. Up to
2398
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This journal is # The Owner Societies 2002
DOI: 10.1039/b109293f

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2. Experimental
A Perkin-Elmer Lambda 9 spectrometer with an enlarged inte-
grating sphere and a flow-through microreactor was used for in
situ UV/Vis/NIR diffuse reflectance spectroscopy on different
dilute catalyst samples up to 663 K. In the home-made flow-
through microreactor, temperature gradients in the catalyst
bed of up to 30 K were registered at the highest temperatures.
Approximately 110 mg of the catalyst were mixed with SiO2
(Heraeus, 0.1–0.4 mm) (7–10 wt.%) and placed in a home-
made microreactor operating under continuous gas flow. All
spectroscopic measurements were carried out sequentially with
a scan speed of 240 nm minꢀ1, a slit width of 5.0 nm and
a response time of 0.5 s, as well as with a spectralon as a
reference.
Fig. 1 Spectroscopic screening for He/H2Cs2PVMo11O40 from RT
to 663 K. RT1 : RT, He flux : 6 min; RT2 : RT, He flux 105 min.
The free heteropoly acid (H4PVMo11O40), denoted as HPA,
was prepared by hydrothermal synthesis from a stoichiometric
mixture of metal oxides (MoO3 , V2O5) and phosphoric acid in
water. Cs2CO3 and the solution of heteropoly acid were used
for the preparation of the Cs2 salt (Cs2H2PVMo11O40),
denoted as Cs2A.
The gases used were helium (purity > 99.999 vol.%), helium
saturated with water at room temperature (RT) and oxygen
(purity > 99.998 vol.%). In addition, the feed mixture was 10
vol% propene (purity q 99.8 vol.%) in helium or 10 vol% pro-
pene plus 10 vol% oxygen in helium with a total gas flow of 71
or 74 ml minꢀ1, respectively. Oxygen, propene and the reaction
products were analyzed on-line with two gas chromatographs
(Perkin Elmer), equipped with heated automatic gas sampling
valves, an FFAP column (Macherey-Nagel) and a packed Car-
boxen-1000 column using an FID and a TCD.
Three different methods of gas atmosphere treatment were
applied for spectra recording. First, He treatment was carried
out for about 12 h at RT. Second, propene/He treatment was
carried out for about 2 h at RT, and then the temperature was
increased at a rate of 1 K minꢀ1 to 323 K, and the spectra were
recorded over a period of ca. 5 h. Third, an isochronic–isother-
mic temperature–time program was chosen for all gas atmo-
spheres as follows: gas atmosphere treatment for ca. 2 h at
RT, whereby RT1 denotes the initial spectrum and RT2 the
spectrum after 2 h on gas flow. Then the temperature was
increased from RT2 to 326 K with a heating rate of 1 K minꢀ1
and from this temperature to 663 K in steps of ꢁ50 K at the
same heating rate, with a holding time of 2 h for each gas at
each temperature.
interval was determined in correspondence with the results
obtained in ref. 25 which showed that the maximum of the
lower energy intervalence charge transfer band arising from
ill-defined dimeric clusters is about 2000 nm. For fixed tem-
peratures the relative spectrum intensity was determined as
n ¼ Ii/I0 where Ii and I0 are the intensities of spectra at tem-
peratures T and T ¼ 663 K. The peak positions of the absorp-
tion bands were evaluated after
a linear background
subtraction. The spectra of heteropoly compounds in the range
of light-induced LMCT are usually interpreted as interband
optical transitions or transitions between discrete molecular
orbitals. However, we employ the band model bearing in mind
two factors: Firstly, the disintegration of heteropoly com-
pounds examined is accompanied by water removal (crystal
or constitutional water). Independent of the stage of disinte-
gration of the Keggin units, the mean geometric distance
between them decreases. This facilitates polymerisation and
thus interunit electron transfer. Secondly, the final product
of disintegration is the semiconductor MoO3 . The filled
valence band and the conduction band are formed by the oxy-
gen 2p and metal d orbitals,27 respectively. At the beginning of
the experiment (RT1 , short time on gas flux), when the inter-
valent charge transfer (IVCT) and d–d bands are not yet devel-
oped, the absorption band edge is determined as a point at
which the slope of the apparent absorption coefficient for the
LMCT band is at its maximum. At higher temperatures in
the range of 400–600 nm the overlap of the LMCT band, d–
d and IVCT bands becomes appreciable, and this procedure
does not lead to reasonable results. Therefore, for tempera-
tures above RT1 we evaluate the shift of the absorption band
edge relative to its position under the initial experimental con-
ditions. Assuming that for direct interband optical transitions
the apparent absorption coefficient is a function f of ꢀho ꢀ Eg
(where ꢀho is the photon energy, and Eg is the optical band
gap energy) at different temperatures we estimate the values
ꢀho corresponding to a fixed value of f equal to f0 and taken
in the vicinity of the maximum of the LMCT band, i.e., in
the range within which the UV absorption is not affected by
the d–d and IVCT transitions. The change ꢀhDo in the photon
energy determined in this way is equal to the shift DEg of the
absorption band edge.
For all operations described in sections 3 and 4, the last
spectrum was always taken before the temperature was
increased. The holding time of 2 h was sufficient because no
spectroscopic changes were observed after this time. The
apparent absorption was evaluated from the diffuse reflectance
data using the formula 1 ꢀ Rmixture/RSiO , whereby at each
2
desired temperature the measured reflectance data of the mix-
ture were divided by the pure SiO2 diluent. For some spectra it
was necessary to eliminate the jumps close to the lamp (ca. 320
nm) and the detector change (ca. 860 nm). In addition, the
reflectance data in short wavelength ranges near these particu-
lar points were smoothed.
3. Determination of spectroscopic characteristics
4. Results and discussion
From Fig. 1 it is seen that above 326 K, the intensity of the
spectra increases significantly both in the Vis and near-IR
range. The investigation with Raman spectroscopy13 shows
that the formation of ill-defined clusters takes place at higher
temperatures. Therefore the transformations of the spectrain
the range 326 < T < 663 K are attributed to these clusters. Cor-
respondingly, the spectral intensities were determined by inte-
gration in the range of A < l < B, where A is the lowest point
of the band edge and B is 2000 nm. The upper limit of this
4.1. Peak intensity versus time on He stream at room
temperature
The loss of crystal water can be observed from the change in
the near-IR absorption bands at 1430 and 1930 nm. Such a
water loss, recognizable in the decreasing peak intensity, is
already observed at RT in dependence upon time on He
stream, as can be seen in Fig. 2. In addition, in the ultraviolet
spectral range HPA and Cs2A exhibit pronounced LMCT
Phys. Chem. Chem. Phys., 2002, 4, 2398–2408
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Fig. 3 Relative visible peak intensity of He/H4PVMo11O40 at room
temperature calculated from experimental and theoretical spectra vs.
peak intensity at 1930 nm.
Fig. 2 Experimental UV/Vis/near-IR diffuse reflectance spectra of
H4PVMo11O40 in dependence upon time on He stream at room tem-
perature. Time on stream (h) : 1.4 (1); 1.9 (2); 2.4 (3); 9.9 (6); 12.3 (7).
different in the presence of propene or the mixture of O2/pro-
pene instead of He. After a time on stream of 2 h the apparent
absorption already increases enormously at RT in the presence
of propene. No water band could be detected any longer in the
presence of propene for Cs2A after this time at RT and for
HPA after 2 h at 326 K (not shown). The reason for the very
strong increase in the apparent absorption in the presence of
propene is that propene apparently draws the crystal water
from the catalyst, and when that has disappeared the constitu-
tional water as well. The Vis peak position suffers a blue shift
from 740 nm to 700 nm after a time on stream of ca. 90 min
with a slight temperature increase (to 319 K) (not shown),
accompanied by the formation of isopropanol detected by
GC. In the presence of O2 the increase in the apparent absorp-
tion is not as strong as in the case of He and the removal of
water molecules proceeds more slowly with increasing time
on stream and temperature. In addition, for HPA treated in
O2 no successive increase in the apparent absorption is
observed with increasing temperature, so that, for instance,
the apparent absorption in a certain temperature range can
be greater than at higher temperatures. He saturated with
water appears to have a stabilizing action on these compounds.
The water bands disappear only at 473 K (not shown).
bands at ca. 400 and 360 nm,respectively. The intensity and the
shape of this band do not change during the action of the He
stream. In the Vis/near-IR range an asymmetrical broad band
is observed, whose peak intensity increases with increasing
time on He stream. The peak maximum of this band is located
in the range 740–780 nm and a shoulder can be distinguished in
the range 900–1000 nm. A semiempirical model that considers
a dimeric cluster V4+–Mo6+ was employed for the simulation
of the spectra.28 At 300 K under He the spectra of HPA in the
Vis/near-IR range were shown to be determined by the optical
d–d transitions in V4+ and Mo5+ ions with intact oxygen sur-
roundings (MO6) and the intervalence transition V4+–Mo6+
.
The maximum observed in the range 740–780 nm (Fig. 2)
was assigned to the intense b2–e (d–d) transitions in V4+ and
Mo5+. The shoulder in the range 900–1000 nm was assigned
to an IVCT peak of the heteronuclear pair V–Mo. Appreciable
spectroscopic changes between 600 and 1300 nm were attribu-
ted to the increase in the parameter characterising electron
transfer between the V4+ and Mo6+ ions (electron transfer
parameter) due to water loss. By varying this parameter a good
fit of the calculated spectra to the experimental ones could be
obtained.28 Besides the electron transfer parameter all other
characteristic system parameters were determined from experi-
mental data. The theoretical calculations28 make it possible to
separate the contributions of the d–d transitions and the
charge transfer band from the total spectrum intensity. At
RT the relative contribution of the d–d transitions decreases
appreciably with time on He stream, while the intensity of
the charge transfer band grows with the increase in the transfer
parameter. For example, for the spectrum recorded at RT after
12.2 h on He stream these contributions are 34% (d–d) and
66% (charge transfer). In addition, it was determined that
the peak intensity of the near-IR H2O absorption bands corre-
lates with the peak intensity in the range indicated above. Fig.
3 shows this connection for HPA treated in He selecting the
greater near-IR H2O peak intensity at 1930 nm.
4.3. Influence of gas atmospheres on the visible and near-IR
peak intensity between RT and 663 K
With the exception of He/H2O, the Vis and near-IR peak
intensities increase strongly in the presence of the gas atmo-
spheres applied up to 371 K, as can be seen in Table 1. The
initial dramatic change in the Vis and near-IR peak intensity
points to the crystal water loss which already begins at RT
(see section 4.1) and continues up to 400–420 K. A lower
change in peak intensity is observed in the range 520–540 K
(Table 1), wherein the constitutional water loss begins.14,15
As already mentioned in section 4.2., the strongest increase
can be recognized for both catalysts in the presence of propene
and the mixture of O2/propene up to 371 K. In contrast, the
peak intensity decreases rapidly for HPA in the presence of
O2 above 371 K, and above 467 K no peak intensity was
observed. For Cs2A the peak intensity increases in the presence
of O2 similarly to the increase in the presence of He. However,
the peak intensity is higher than that of He in the range 326–
608 K. Another peak intensity vs. temperature curve is
observed in the presence of He/H2O. A clear increase in this
intensity can only be detected above 371 K.
4.2. Spectroscopic screening in dependence upon the gas
atmosphere and temperature
For Cs2A treated in He from RT to 663 K the apparent
absorption increases strongly with the temperature (Fig. 1).
Such an increase in UV/Vis was already shown and discussed
in ref. 22. Simultaneously, the near-IR absorption bands of the
crystal water disappear completely, with the consequence that
a Vis broad absorption band, and at higher temperatures addi-
tionally a near-IR absorption band, can be clearly detected. A
similar feature is observed for HPA in the presence of He. In
contrast, the spectroscopic feature for HPA and Cs2A is
For HPA the decrease in the peak intensity of the Vis and
near-IR absorption band above 371 K (Table 1) in the pre-
sence of O2 may be attributed to the oxidation of Mo5+ and
V4+ centers by this gas phase. For HPA the O2 produces such
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Table 1 Influence of gas atmospheres and temperature on the relative Vis and near-IR peak intensities and TG of H4ꢀxCsxPVMo11O40 (x ¼ 0, 2).
TG data recorded under He (50 ml minꢀ1) with 5 K minꢀ1
Relative Vis and near-IR peak intensity
Temperature/K
H4PVMo11O40
He
He/H2O
Propene
O2
O2/propene
TG weight loss of water (%)
RT1
RT2
0.1212
0.1180
0.2873
0.7106
0.8126
0.8865
0.8991
0.8517
0.9237
1.0
0.0111
0.1480
0.1284
0.2328
0.6569
0.9082
0.9412
0.9679
0.9981
1.0
0.0468
0.4732
0.6291
0.7323
0.7115
0.8133
0.8533
0.8514
0.9926
1.0
0.0107
0.0117
0.4009
1.0
0.2291
0.4609
0.7523
0.7951
0.7396
0.6925
0.7459
0.7805
0.8430
1.0
0
0.26
325.9
371.3
422.1
466.8
511.5
561.1
607.5
663.1
0.69
4.90
0.4960
0.2491
—
10.40
10.90
11.00
11.30
12.00
12.60
—
—
—
Cs2H2PVMo11O40
RT1
RT2
0.0088
0.0724
0.3219
0.6148
0.7132
0.8420
0.9120
0.9316
0.9595
1.0
0.0445
0.0766
0.0864
0.1297
0.7223
0.8364
0.8711
0.8954
0.9417
1.0
0.6159
—
0.0230
0.1521
0.4311
0.5133
0.6119
0.7358
0.8682
0.9537
0.9813
1.0
0.0088
0.8119
0.8438
0.9056
0.8329
0.8429
0.8799
0.9243
0.9671
1.0
0
0.15
0.38
2.00
2.70
2.87
3.05
3.27
3.45
3.45
325.9
371.3
422.1
466.8
511.5
561.1
607.5
663.1
0.8325
0.8773
0.9309
0.9447
0.9614
0.9737
0.9855
1.0
a strong oxidation that no more absorption bands could be
identified. In contrast, the Cs+ ions play a stabilizing role in
the Cs2A salt and hinder significant oxidation. Above 512 K,
for HPA and Cs2A the Vis and near-IR peak intensity increase
under reaction conditions (O2/propene).
paired (MO6) or ill-defined (M((Ot)1ꢀs (Op)1ꢀy (Ob)4ꢀx Haz )),
i
i
i
i
where Ob , Ot are the bridging and terminal oxygens, Op stands
for the oxygen in a phosphorus tetrahedral environment, si , xi
and yi are the numbers of oxygen ions extracted in the process
of constitutional water or oxygen evolution, zi is the number of
protons localized on the moiety, a ¼ b or t for protons resid-
ing on the oxygens Ob and Ot , respectively. As in the Piepho–
Krausz–Schatz model29 the extra electron localized on a cer-
tain site (metal ion) is supposed to interact with the full sym-
metric displacement (‘‘breathing’’ mode) of the local
surrounding. The full-symmetric dimensionless coordinates
for the twelve (M((Ot)1ꢀs (Op)1ꢀy (Ob)4ꢀx Haz )) units will be
4.4. Influence of gas atmospheres on the visible peak position
between RT and 663 K
Table 2 shows that the peak position of the Vis absorption
band is blue shifted for HPA in the presence of He, the mixture
of O2/propene, or propene (strong shift) with increasing tem-
perature. For Cs2A in the presence of these gas atmospheres,
the spectroscopic feature is the same up to 371 K (propene),
422 K (He) and ca. 467 K (O2/propene), as can be seen in
Table 2. Above these temperatures the Vis peak positions are
nearly constant (He, propene) or increase slightly (O2/pro-
pene). In contrast, a Vis peak position for HPA treated in
O2 can only be observed up to 371 K, and for Cs2A treated
in the same gas an increase is detected after a decrease up to
371 K. In the case of O2/Cs2A two data sets show the same
spectroscopic trend (Table 2). He/H2O appears to have a sta-
bilizing effect up to ca. 371 K and 422 K for HPA and Cs2A,
respectively (Table 2). If the crystal water is removed with
increasing temperature one observes a blue shift of the Vis
peak position that is lower in the case of Cs2A than of HPA.
However, such a shift is not detected for Cs2A with the excep-
tion of He/H2O and O2 in the temperature range in which the
constitutional water is removed (Table 2).
i
i
i
i
denoted as qi (i ¼ 1–12). The full Hamiltonian of the system
can be presented as
H ¼ He þ Hv þ Hev;
ð1Þ
where He is the electronic Hamiltonian determining the wave
functions and the eigenvalues of a Keggin molecule in the fixed
nuclear configuration (qi ¼ 0), HL is the Hamiltonian of free
lattice vibrations, and Hev is the vibronic interaction Hamilto-
nian. The electronic Hamiltonian He includes two parts:
He ¼ h0 þ g;
ð2Þ
where the one electronic operator h0 involves the kinetic and
potential energies of all d-electrons, g is the two-particle (inter-
electronic) repulsion. The free harmonic vibrations (with the
frequencies o) of the twelve M((Ot)1ꢀs (Op)1ꢀy (Ob)4ꢀx Haz
units are assumed to be vibronically independent, and the
)
i
i
i
i
Hamiltonian Hv has the form:
The results obtained can be explained on the basis of calcu-
lations carried out in ref. 25. For a clearer understanding of the
interpretation presented below we first give a short description
of the background of the model employed.25 We consider a
reduced Keggin unit. Depending upon time on gas flux and
temperature the nearest oxygen surrounding of the i-th
(i ¼ 1–12) metal ion (M) in the Keggin unit may be unim-
ꢀ
ꢁ
12
X
2
ꢀho
2
@
Hv ¼
q2i ꢀ
ð1Þ
@q2i
i¼1
The vibronic interaction Hamiltonian in our model is sup-
posed to be linear with respect to the atomic displacements
Phys. Chem. Chem. Phys., 2002, 4, 2398–2408
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Table 2 Vis peak positions of H4ꢀxCsxPVMo11O40 (x ¼ 0, 2) in dependence on some gas atmospheres and temperature
Vis peak position/nm
Temperature /K
H4PVMo11O40
He
He/H2O
Propene
O2
O2/propene
RT1
RT2
740
750
739
731
740
715
698
680
670
668
773
765
788
770
766
750
729
694
669
677
737
737
743
691
689
660
665
641
636
644
740
750
761
747
—
742
748
738
737
733
711
704
697
679
—
325.9
371.3
422.1
466.8
511.5
561.1
607.5
663.1
—
—
—
—
—
Cs2H2PVMo11O40
RT1
RT2
760
771
748
724
710
709
711
710
712
709
750
750
740
740
742
736
733
732
732
714
770
717
695
685
687
695
693
692
—
727.5 ꢃ 2.5
748 ꢃ 8
752
—
325.9
371.3
422.1
466.8
511.5
561.1
607.5
663.1
751 ꢃ 3
730
716
709
705
710
718
718
726
730.5 ꢃ 0.5
734 ꢃ 0
752.5 ꢃ 2
754 ꢃ 3
750.5 ꢃ 3
749 ꢃ 6
—
742 ꢃ 4
and can be represented in the following form:
where Gp (p ¼ 2,4) are dimensionless parameters of the model
obtained from a comparison of the calculated and measured
crystal-field splittings of the d-levels of V4+ and Mo5+ ions,
X X
~
Hev
¼
viðrkÞqi
ð4Þ
i
k
2
2
2
SpðRaÞ ¼ ðSsðRaÞÞ þ ðSsðRaÞÞ þ gpðSpðRaÞÞ ;
ð9Þ
~
where the functions vi(rk) of the electronic coordinates rk are
determined as the derivatives of the electron-nuclear potential
~
g2 ¼ 1, g4 ¼ ꢀ4/3, the overlap integrals Ss(Ra) ¼ hnd,m ¼ 0|2si,
Ss(Ra) ¼ hnd,m ¼ 0|2p,m ¼ 0i, Sp(Ra) ¼ hnd,m ¼ 1|2p,m ¼ 1i
have been calculated using the radial 3d and 4d wave functions
of the V4+ and Mo5+ and the 2s, 2p wave functions of the
O2ꢀ ion given in ref. 30. The symmetry of the unimpaired
units MO6 comprising the Keggin molecule was assumed
~
~
energy W(|rk ꢀ Ria|) in the points qi ¼ 0:
!
ꢂ
ꢂ
ꢂ
ꢂ
~
X
~
@Wð rk ꢀ Ria
@qi
Þ
við~rkÞ ¼
a
qi¼0
to be C4v
M((O ) (O ) (O )
the scheme of reduction of the C4v group on the subgroups.
,
the symmetry of the ill-defined moieties
H ) was taken in accordance with
t 1ꢀsi p 1ꢀyi b 4ꢀxi azi
!
ꢂ
ꢂ
ꢂ
ꢂ
~
X
~
@Wð rk ꢀ Ria
Þ
1
pffiffi
¼
ð5Þ
@Ria
l
0
~
a
~
Rie¼Ria
The one-electron vibronic coupling parameters
D
E
viG
¼
jGgðrkÞjviðrkÞjjGgðrkÞ ;
ð10Þ
~
where Ria are the coordinates of the ligand surrounding of the
~
~
~
0
~
metal site i and Ria are their equilibrium values, a numbers the
ligands in the first coordination sphere, l is their total number.
In the exchange–charge model of the crystal field26
~
were calculated with the aid of eqn. (4)–(8). In eqn. (10) jGg(rk)
is the wave function of the electron occupying the orbital G, G
is the irreducible representation of the point group of the
M((Ot)1ꢀs (Op)1ꢀy (Ob)4ꢀx Haz ) unit, g enumerates the basis
X
X
Bmp CpmðWk; jkÞ
ð6Þ
i
i
i
i
~
Vc ¼
Wðjrk ꢀ RiajÞ ¼
functions of G.
k;a
k;p;m
The shape and the intensity of the IVCT bands are deter-
mined not only by the vibronic coupling parameters but also
by the parameters characterizing electron transfer between dif-
ferent metal sites of the Keggin molecule. It is known28 that the
intensity of an IVCT transition is proportional to the square of
the electron transfer parameter. Hence, the most intense
charge transfer bands arise from the transitions in which,
under the action of light, the only electron jumps from its ori-
ginal site to a neighbouring one. Therefore, while calculating
the absorption spectrum of a reduced Keggin unit we only take
into account single-electron transfers between adjacent edge-
sharing and corner-sharing metal sites. Confining ourselves
to the consideration of the IVCT transitions in the visible
and near-IR range, we only allow for the transfer of the excess
electron between the ground states of neighbouring metal sites.
Four single transfer parameters p1 , p2 , p3 , p4 are involved in
the model. The parameters p1 and p2 describe single-electron
is the crystal field potential acting on the electronic shell of the
metal ion. In eqn. (6) Cmp (Wk ,jk) ¼ (4p/(2p + 1))1/2Ypm) are
spherical operators, the parameters Bmp are represented in the
form
Bmp ¼ Bmp ðpcÞ þ BpmðecÞ
;
ð7Þ
here the indices pc and ec identify the partial contributions to
the crystal field from the point charges of lattice ions and the
exchange charges.The component Bmp ðpcÞ is determined in the
usual manner.26 The parameter Bmp ðlsÞ is given by the following
relation:
2e2ð2p þ 1Þ
SpðRaÞ
ꢂ
X
Bmp ðesÞ
¼
Gp
Cpm ðWa; jaÞ
ð8Þ
5
Ra
a
2402
Phys. Chem. Chem. Phys., 2002, 4, 2398–2408

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transfer between neighbouring edge-sharing and corner-shar-
ing V and Mo sites, respectively. The parameters p3 and p4
relate to the pair Mo–Mo. For example, the transfer parameter
p3 can be expressed as
nating from all possible single-electron transfers between
neighbouring corner- and edge-sharing sites and d–d transi-
tions in Mo5+ and V4+ ions, taking into account in each case
the thermal population of the initial adiabatic potential sheet
participating in the optical transition. The considerations given
above led us to a study of optical electron transfer and d–d
bands of binuclear clusters consisting of metal ions joined
through an edge or an apex and containing a single electron
over the filled shells of these ions (clusters Mo5+–Mo6+ or
V4+–Mo6+).
Further the structure of reduced binuclear mixed-valence
species responsible for the spectral transformations on differ-
ent stages of the experiment will be denoted as
((Ot)1ꢀs1(Op)1ꢀy1(Ob)3ꢀx1Haz1)d1–Ob–d0((Ot)1ꢀs2(Op)1ꢀy2
(Ob)3ꢀx2Hbz2), where d1 designates the 3d or 4d-electron, and
the meaning of all other symbols remains the same as above.
The composition of very likely species was chosen on the basis
of the thermal gravimetric, differential thermal and IMR-MS
analysis,2,23 geometric structure of MoO3 and quantum-
mechanical calculations of proton affinities.34 The intervalent
transitions in species ((Ot)1ꢀs1(Op)1ꢀy1(Ob)3ꢀx1 Haz1 )d1–Ob–
d0((Ot)1ꢀs2(Op)1ꢀy2(Ob)3ꢀx2Hbz2) have one maximum at the
point25
p3 ¼ hjG ðiÞ j h j jG ðjÞi;
ð11Þ
gr
gr
here jGgr(i) and jG (j) are the ground orbitals of the Mo5+
gr
ions occupying the edge-sharing sites i and j in one of the four
triads M3O13 composing the Keggin structure. It should be
mentioned that the number of the transfer parameters was
reduced to four by neglecting the differences arising from the
two-electron contributions.31 Microscopic estimations testify
that the transfer parameters p1 , p2 , p3 and p4 are small in com-
parison with the vibronic coupling constants viG for the
gr
ground orbitals calculated as described above, see eqn. (5)–
(10). This allows us to consider the tunnel interaction as a per-
turbation and to identify the stationary states of the system
with those in which the excess electrons are localized. Then
we employ the adiabatic approximation, neglecting the nuclear
kinetic energy in the Hamiltonian eqn. (3). The energies of a
Keggin molecule thus obtained represent the adiabatic poten-
tials. This approach was used in ref. 32 and 33 and was shown
to give good results when calculating spectroscopic and ther-
modynamic characteristics of mixed-valence systems. Taking
the above into account we reduce the problem of calculation
of the IVCT band positions of the whole Keggin unit (intact
or defective) to that of light-induced electron transfer between
two adjacent metal ions (Mo5+, Mo6+ or V4+, Mo6+). Besides
the IVCT transitions for V4+ and Mo5+ ions the d–d transi-
tions from the ground states to the excited ones are taken into
account. The full spectrum (i.e. the observed spectrum) of the
Keggin unit can be obtained by summation over spectra origi-
"
#
1=2
ꢀ
ꢁ
v22
D
2
v22
D
2
Tðv21 þ v22Þ
2ꢀho
ðꢀhOÞmax
¼
þ
þ
þ
2ꢀ
ð12Þ
2ꢀho
2ꢀho
here n1 and n2 are the vibronic coupling constants for the
moieties d1((Ot)1ꢀs1 (Op)1ꢀy1(Ob)4ꢀx1Haz1) and d1((Ot)1ꢀs2
(Op)1ꢀy2(Ob)4ꢀx2Hbz2), respectively, and D is the energy gap
between the ground orbitals of these moieties. The calculated
maxima of charge transfer bands arising from intact and some
ill-defined bi-nuclear species are listed in Tables 3 and 4.
Table 3 Calculated positions of the charge-transfer (CT) and d–d bands arising from different types of species in the Vis range
Species
Band
Temperature /K
Position /nm
(OtOp(Ob)3)Hb1)V4+–Ob–Mo6+(OtOp(Ob)3)
(OtOp(Ob)3)V4+–Ob–Mo6+(Ot(Ob)3)
(Ot(Ob)2)Mo5+–Ob–V5+(OtOp(Ob)3)
(Ot(Ob)2)Mo5+–Ob–Mo6+(OtOp(Ob)3)
(OtOp(Ob)2)Mo5+–Ob–Mo6+(OtOp(Ob)2)
(Ot (Ob)2)V4+–Ob–Mo6+(OtOp(Ob)3)
(OtOp(Ob)2)Mo5+–Ob–V5+(OtOp(Ob)3)
(OtOp(Ob)2)V4+–Ob–Mo6+(OtOp(Ob)2)
(OtOp(Ob)2)Mo5+–Ob–Mo6+(OtOp(Ob)3)
(OtOp(Ob)2)V4+–Ob–Mo6+(OtOp(Ob)3)
(OtOp(Ob)4)V4+
CT
CT
CT
CT
CT
CT
CT
CT
CT
CT
d–d
d–d
d–d
d–d
330
500
500
500
500
500
500
500
500
500
830a
755
750
722
664
600
552
543
537
457
769, 625
743, 720, 519
712, 444
701, 505, 498
(OtOp(Ob)4Hb1)V4+
(OtOp(Ob)4)Mo5+
(OtOp(Ob)4Hb1)Mo5+
a
This band is on the border of the Vis and near-IR range in the temperature interval of crystal water loss.
Table 4 Calculated positions of the charge-transfer bands in the near-IR range
Species
Temperature/K
Band position/nm
(OtOp(Ob)3Hb1)V4+Ob–Mo6+(OtOp(Ob)3Hb1
(OtOp(Ob)3)V4+–Ob–Mo6+(OtOp(Ob)3)
)
340
340
340
340
340
340
560
560
560
560
922
1012
1011
1068
1142
1224
1091
1000
980
(OtOp(Ob)3Hb1)Mo5+–Ob–Mo6+(OtOp(Ob)3)
(OtOp(Ob)3Hb1)Mo5+–Ob–V5+(OtOp(Ob)3)
(OtOp(Ob)3Hb1)Mo5+–Mo6+(OtOp(Ob)3Hb1
(OtOp(Ob)3)Mo5+–Mo6+(OtOp(Ob)3)
(OtOp(Ob)3)V4+–Ob–Mo6+(Op(Ob)3)
(OtOp(Ob)3)Mo5+–Ob–V5+(Ot(Ob)3)
(Op(Ob)3)V4+–Ob–Mo6+(Op(Ob)3)
(OtOp(Ob)3)Mo5+–Ob–Mo6+(Ot(Ob)3)
)
873
Phys. Chem. Chem. Phys., 2002, 4, 2398–2408
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Quantum-mechanical calculations25 show that reduced
binuclear species with protons localized on the bridging oxy-
gens (Tables 3 and 4) or with vacancies of the bridging oxygens
(Table 3) give blue shifted charge transfer bands relative
to those arising from intact ones; a proton on the terminal
oxygen leads to a red shift. The Vis part of the spectra
in heteropoly acids and Cs-salts is revealed to be formed by
course of constitutional water release is approximately two
times smaller than in HPA. The propene atmosphere leads to
the same effects in the Vis part of HPA and Cs2A spectra.
However, propene is more efficient in the extraction of water
than He, due to the additional reaction leading to the product
isopropanol. Therefore, for propene–HPA the Vis band shift is
larger, and in Cs2A the Vis peak position stabilizes at lower
temperatures.
Under action of O2 the band in the Vis part of the HPA
spectrum disappears after the crystal water loss.At the same
time under O2 flux in Cs2A salt the Vis peak can be observed
from RT to 663 K, and this is due to the presence of Cs+ ions
which prevent oxidation.
d–d and IVCT transitions V4+–O–Mo6+ ! V5+–O–Mo5+
,
Mo5+–O–Mo6+ ! Mo6+–O–Mo5+ (Table 3). Depending on
temperature, the IVCT transitions arise from species
(OtOp(Ob)3Hb1)V4+–Ob–Mo6+(OtOp (Ob)3)with a proton loca-
lized on the bridging oxygen or reduced species of the type of
VMoO10 , VMoO9 , Mo2O10 , Mo2O9 with one or two oxygen
vacancies25 (Table 3).
The spectroscopic changes in the UV/Vis range at the high-
est temperature (607 K, 663 K) are shown in Figs. 4 and 5. HPA
treated in He develops a clear absorption band at 665 nm
(directly observed in the spectra, deconvoluted on two bands
at 660 and 740 nm, see section 3). For Cs2A treated in He as
well a broader absorption band at 715 nm (directly observed
in the spectra deconvoluted on two bands at 680 nm and 770
nm, see section 3) is detected (not shown). At 663 K the Vis
peak position for Cs2A treated in He (Table 2 ) is close to that
of HPA, permitting the conclusion that MoO3 and Cs3A salt
are formed from Cs2A in good agreement with a Raman shift
of the main Keggin band from 988 cmꢀ1 for Cs2A to 992
cmꢀ1 for Cs3A.13 This spectroscopic feature indicates an expul-
sion of vanadyl species based on the calculated peak maximum
at 660/740 nm for HPA and 680/770 nm for Cs2A. These con-
clusions can also be drawn on the basis of observed absorption
bands of VOSO4ꢄ5H2O at 625 and 766 nm.36 However, corre-
sponding Raman bands indicating vanadyl species could not
be observed for these catalysts treated in He.13 The absorption
band at 665 nm that is observed in the presence of He disap-
pears in the presence of O2 for HPA (Fig. 4). It also suffers a
red shift and is weaker for Cs2A treated in O2 . As for the Vis
peak position in Cs2A, it can be recognized that this position
regains its RT2 value at higher temperatures. Hence, the pre-
sence of molecular O2 could stabilize the Cs2A structure. In
HPA, He/H2O effects an even stronger Vis absorption band
at ca. 665 nm than pure He. In this case the expulsion of vana-
dium species seems to be clearer, still in contrast to the Raman
observation (no corresponding bands). For Cs2A treated in
He/H2O a weaker Vis band, which is red shifted, could indicate
a smaller number of expelled vanadyl species. HPA treated in
propene shows a very broad weak absorption band with an
observed peak maximum at ca. 635 nm and 2 deconvoluted
spectra whose peak maxima are around 520 and 650 nm (not
shown). In contrast, for Cs2A treated in propene as for HPA
At RT after a certain time on gas flux (for example, He (Fig.
2)) in HPA the loss of crystal water begins. At this initial stage
there is no reduction, the HPA is partly in the hydrated phase,
the protons are not localized and reside on the bridging water
moieties H5O2+. However, the quantitative determination of
the V4+ content by EPR10 shows that even at this stage vana-
dium is present as V4+, its amount is about 0.5–2 mol%. At the
same time a small amount of Mo5+ is also detected. Thus initi-
ally, the Vis part of the spectra originates mainly from the d–d
transitions in the contained V4+ and Mo5+ ions.28 The total
spectrum intensity increases due to the increase in the intensity
of the charge transfer band arising from the V4+–Mo6+ inter-
valent transition in intact species VMoO11 28 which contributes
to the first near-IR band (Table 4 and section 4.5.). At tem-
peratures 326–422 K the removal of crystal water (see thermo-
gravimetric (TG) data in Table 1) is accompanied by the
localization of acidic protons, the most energetically favour-
able sites of which have been shown to be the bridging oxy-
gens34,35 All reduced species with protons residing on the
bridging oxygens listed in Tables 3 and 4 give rise to new
d–d transitions blue shifted in comparison with those arising
from intact VMoO11 clusters (Table 3). Besides this, at tem-
peratures 326–422 K on the border of the Vis and near-IR
range a new charge transfer band originating from the
(OtOp(Ob)3Hb1)V4+–Ob–Mo6+(OtOp(Ob)3) species appears
(Table 3). On the other hand, with temperature rise in the
region of crystal water loss the concentration of reduced pro-
tonated species increases as fast as the protons localize on
the oxygens of the reduced intact species. Both mentioned fac-
tors lead to the observed gradual blue shift of the Vis peak
position in HPA and Cs2A (Table 2). At the same time, the
spectrum intensity increases due to increase in the transfer
parameter with water loss. However, in spite of the smaller
number of localized protons for Cs2A in the region of crystal
water loss, the blue shift of the Vis band is of the same order
for HPA and Cs2A. This can be explained by the stronger crys-
tal field and electron–vibrational coupling in Cs2A.
As the temperature continues to rise, constitutional water
evolves (see TG data in Table 1). This water is formed by
the extraction of an oxygen by two protons and leads to the
formation of defective clusters in which bridging oxygens are
removed. However, the constitutional water evolution itself
is not accompanied by the appearance of new reduced clusters.
Therefore, at this stage the transformations of the spectra may
occur due to the reduced species of the type of VMoO10
,
VMoO9 , Mo2O10 , Mo2O9 enumerated in Table 3 and formed
from reduced protonated species. Meanwhile, the evolution of
molecular oxygen may lead to the formation of the same
clusters VMoO10 , VMoO9 , Mo2O10 , Mo2O9 from intact
non-reduced VMo11 , Mo2O11 ones. The indicated ill-defined
species promote a further blue shift of the Vis band.Their con-
centration increases with temperature rise, and the total inten-
sity of the Vis part of the spectra grows. After crystal water
removal for Cs2A the position of the Vis band remains nearly
unchanged insofar as, due to the structure of this compound,
the number of species with oxygen vacancies formed in the
Fig. 4 Influence of gas atmospheres on the in situ UV/Vis spectra of
H4PVMo11O40 at 607 and 663 K. a,b: He; c,d: He/H2O; e,f: O2 ; g,h:
propene; i,k: O2/propene. For better visualization the spectra were ver-
tically shifted.
2404
Phys. Chem. Chem. Phys., 2002, 4, 2398–2408

View Article Online
of Cs2A indicating the lower conversion of O2 (Figs. 4 and 5
and section 4.3.).16 One can conclude that Cs2A is structurally
and electronically much more stable than HPA.
4.5 Influence of gas atmospheres on the near-IR peak position
between RT and 663 K
The first near-IR peak position (that is the near-IR peak posi-
tion with the lower wavelength in comparison with the second
near-IR peak position) in the presence of He, He/H2O or the
mixture of O2/propene shows a maximum for HPA around
512 K, as can be seen in Table 5. However, in HPA above
512 K a blue shift is detected in the presence of He, He/
H2O and the mixture of O2/propene. A maximum at ca. 467
K is observed for HPA in the presence of propene and then
a further increase in the peak position above 561 K. Using
O2 , peak positions are only observed up to 467 K and those
show a blue shift up to 422 K. Only He/H2O shows a stabiliz-
ing effect up to ca. 422 K for HPA (Table 5). A decrease of the
near-IR peak position is detected for Cs2A after 467 K in the
presence of He/H2O and over the whole temperature range
for the mixture of O2/propene whereas under He, after a
decrease the peak position again reaches a maximum above
467 K (Table 5). The propene near-IR peak position increases
up to 608 K, and O2 yields a broad maximum around 422 K.
In this way it is confirmed that for temperatures lower than 570
K the trend in behaviour of the near-IR position for O2/Cs2A
can be reproduced.
Fig. 5 Influence of gas atmospheres on the in situ UV/Vis spectra of
H2Cs2PVMo11O40 at 607 and 663 K. a,b: He; c,d: He/H2O; e,f: O2 ,
one of the two O2/H2Cs2PVMo11O40 data sets was used; g,h: propene;
i,k: O2/propene. For a better visualization the spectra were vertically
shifted.
in the presence of O2/propene at 665 K no peak positions could
be definitely detected. An observed blue shifted peak position
does not exist in the presence of the mixture of O2/propene,
either for HPA or Cs2A. At ca. 605 K for HPA and Cs2A, peaks
can be recognized at ca. 670 nm and 720 nm, respectively. How-
ever, at the highest temperature of 665 K only a very broad
weak absorption band is detected for Cs2A. This weak absorp-
tion band has a nearly symmetrical shape with the peak max-
ima of ca. 720 nm (Cs2A) and can be fitted exactly by one
Gauss curve. The bands at 670 nm (HPA, 605 K) and 720
nm (Cs2A, 605 K, 665 K) point once again to vanadyl species.
Raman spectra show bands at 1030 cmꢀ1 and 1008/1002 cmꢀ1
which were assigned to both vanadyl (HPA) and to molybdenyl
species (HPA, Cs2A).13 In addition, the peak intensities of the
Vis absorption band for HPA are obviously lower than that
The second near-IR peak position has not been plotted and
discussed here since it is difficult to determine definitely the
position of this band due to its strong overlap with the first
near-IR band.
On the basis of quantum mechanical calculations25 it can be
concluded that independent of the gas atmosphere (O2/pro-
pene, He or He/H2O), the main trend in the temperature beha-
viour of the first near-IR band of the HPA consists in the
following: At RT the first near-IR band originates from the
heteronuclear intervalent transitions V4+–Mo6+. In the range
Table 5 Near-IR peak positions of H4PVMo11O40 and H2Cs2PVMo11O40 in dependence on some gas atmospheres and temperature
Near-IR peak position/nm
Temperature /K
H4PVMo11O40
He
He/H2O
Propene
O2
O2/propene
RT1
RT2
—
—
989
980
901
—
—
—
924
976
325.9
371.3
422.1
466.8
511.5
561.1
607.5
663.1
887
992
980
1046
1014
1058
1060
990
978
1025
1074
1092
1119
1148
1136
1099
1069
1991
1053
1093
1137
1121
1122
895
938
918
1073
—
999
1051
1120
1119
1063
1069
972
—
—
1044
1165
—
Cs2H2PVMo11O40
RT1
RT2
970
959
967
954
947
969
982
984
949
986
990
990
—
981
—
970
966
1036
—
325.9
371.3
422.1
466.8
511.5
561.1
607.5
663.1
1049
1022
1015
1024
1040
1076
1084
1039
991
997
1011
966
974
—
980
983
1028
985
990
963
953
1019
1029
1020
992
957
934
967
952
959
940
Phys. Chem. Chem. Phys., 2002, 4, 2398–2408
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of temperatures in which crystal water is removed (T ꢁ 326–
422 K) the main contribution to the near-IR band is provided
–
by the homonuclear Mo5+–Mo6+ and heteronuclearV4+
Mo6+ intervalent transitions in reduced species (OtOp-
(Ob)3Hb1)V4+–Ob–Mo6+(OtOp(Ob)3Hb1),
(Ot–Op(Ob)3Hb1)-
Mo5+–Ob–Mo6+(OtOp(Ob)3), (OtOp(Ob)3Hb1)Mo5+–Ob–V5+
(OtOp(Ob)3), (OtOp(Ob)3Hb1)Mo5+–Ob–Mo6+(OtOp(Ob)3Hb1
-
)
with acidic protons localized on the bridging oxygens (Table
4). In ref. 25 it was shown that among these species the latter
three are the most probable ones.The maxima of the charge
transfer bands arising from the species (OtOp(Ob)3Hb1)-
Mo5+–Ob–V5+(OtOp(Ob)3), (OtOp(Ob)3Hb1)Mo5+–Ob–Mo6+
(OtOp(Ob)3Hb1) are red shifted in comparison with those aris-
ing from VMoO11 species. However, the maximum of the band
originating from the species (OtOp(Ob)3Hb1)Mo5+–Ob–
Mo6+(OtOp(Ob)3) is close to that of the VMoO11 species. As
a result a red shift of the near-IR band is observed in the range
of crystal water loss. At the same time the increase in the trans-
fer parameter with water loss promotes the increase in the
band intensity. Then at higher temperatures, before constitu-
tional water release and oxygen evolution, a further red shift
of the maximum of the near-IR band takes place, and this is
in agreement with the usual behaviour of charge transfer bands
with increasing temperature.28 Further temperature increase
leads to the evolution of constitutional water and molecular
oxygen. The newly appeared reduced species, for instance of
the type (OtOp(Ob)3)V4+–Ob–Mo6+(Op(Ob)3), (OtOp(Ob)3)-
Mo5+–Ob–V5+(Ot(Ob)3), (Op(Ob)3)V4+–Ob–Mo6+(Op–(Ob)3),
(OtOp(Ob)3)Mo5+–Ob–Mo6+(Ot(Ob)3) (Table 4) cause a blue
shift of the near-IR band. With increasing reduction, the inten-
sity of the near-IR band increases.
Fig. 6 Influence of gas atmospheres and temperature on the band
gap energy of H4PVMo11O40 (a) and H2Cs2PVMo11O40(b).1 : He; 2 :
He/H2O; 3 : O2 ; 4 : O2/ propene.
Under He the Cs2A salt demonstrates a different behaviour
from the HPA behaviour of the near-IR peak position. The
maxima of the bands arising from protonated clusters are
higher in energy than those in acids. This may result in a slight
blue shift of the near-IR peak position in the range of 326–422
K. Then, for temperatures 422–561 K, the red shift of the band
may be explained by the usual temperature dependence of the
charge transfer bands. The stabilization of the peak position at
higher temperatures points to the fact that in Cs salts the num-
ber of ill-defined species formed under constitutional water
removal is much smaller than in acids.
high apparent absorption in the relevant wavelength range.
For the temperature behaviour of the absorption band edge
under a He stream, the following qualitative explanations
may be given: Due to the peculiarities of the d electrons the
conduction band formed by their orbitals can be described
by the approximation of strong coupling. In this approxima-
tion the width of the conduction band is proportional to the
hopping integral. With loss of water the mean distance
between neighbouring units diminishes, and the overlap of
their wave functions increases. This in turn leads to an increase
in both the hopping integral and the width of the conduction
band. In consequence, the width of the forbidden band and
the absorption band edge decrease. In the case of He/H2O
flux, the following interpretation may be proposed for the ther-
mal behaviour of the band gap energy in HPA. The increase in
the optical band gap at the initial stage evidences the decrease
in the width of the conduction band due to the additional
external hydration of the system. It is clear that at this stage,
the water of the flux prevents crystal water loss. The action
of the water flux turns out to be stronger than that of He
and temperature. Further temperature increase in common
with a He stream suppresses the hydration of the system by
flux (H2O from flux) and facilitates the release of water. There-
fore, a further decrease in band gap energy is observed for
HPA. Under He/H2O the Cs2A salt does not show an initial
increase in the optical band gap because the water coordina-
tion to Cs+ ions is obstructive. The results obtained also show
that over a wide temperature range for both catalysts under O2
flux the ‘‘drying’’ effect of temperature exceeds that of O2 ,
which consists in oxidation of the reduced ions. At higher tem-
peratures the number of electrons in the d band decreases
appreciably due to oxidation. This promotes an apparent
increase in the band gap energy in HPA. The action of propene
removing the water proves to be stronger than that of oxygen.
That is why the band gap energy decreases under propene/O2
atmosphere.
4.6 Influence of gas atmospheres on the band gap energy
between RT and 663 K
The band gap energy values for HPA and Cs2A under different
gas atmospheres are presented in Fig. 6 as a function of tem-
perature. For HPA treated in He, the band gap energy
decreases in the temperature range RT1–607 K. For Cs2A
under He flow a decrease in the band gap energy is observed
up to 561 K, after which it remains unchanged. HPA and
Cs2A treated in He/H2O show quite different behaviour. For
HPA after an increase in the band gap energy up to 371 K,
a steep decrease in this energy is detected. For the Cs2A salt
under He/H2O flux only a decrease in the band gap energy
takes place. Both catalysts show a largely similar gap feature
in pure O2 : in the range RT1–467 K a decrease in the band
gap energy is observed. At higher temperatures an increase
occurs for HPA, whereas, in contrast, the band gap energy
remains approximately constant for Cs2A. For HPA and
Cs2A under the propene/O2 atmosphere the band gap energy
decreases with temperature increase up to 561 K for HPA
and Cs2A. At higher temperatures it is impossible to indicate
a straight trend in temperature dependence for Cs2A. It should
also be mentioned that for both catalysts the band gap energy
undergoes the maximum change under action of propene/O2
flux. In addition, for both catalysts the band gap energies were
not determined for a propene atmosphere because of the very
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4.7 In situ catalytic and spectroscopic experiments of O2/
propene on HPA and Cs2A
for the acid. While comparing experimental data on Cs2A
and HPA with the results of quantum-mechanical calcula-
tions25 the types of reduced dimeric V–Mo, Mo–Mo clusters
contributing to the formation of the Vis and NIR bands at dif-
ferent temperatures were identified. With the aid of the calcu-
lated positions of the d–d and charge transfer bands
originating from intact and ill-defined reduced species the
behaviour of the Vis and near-IR bands in the examined com-
pounds with increasing temperature was qualitatively inter-
preted. In this connection, it should be mentioned that at
high temperatures (520 K < T < 667 K ), especially for HPA
treated in some gas atmospheres (He, He/H2O) there is no
clear trend, for example, in the temperature dependence of
the Vis peak position. The following reasons can be enumer-
ated. At high temperatures the latter cannot be singled out
due to the large broadening and significant overlap of the
bands. The identification of the absorption bands is also ren-
dered more difficult because the signal/noise ratio drops with
increasing temperature. If the required temperature is rela-
tively high under stoichiometric and catalytic conditions, the
broadening and overlap of the bands can also be increased
by deposits and redox processes.
In the case of propene oxidation the determination of the
desired quantities from the spectroscopic feature is impeded
by the following circumstances: On the one hand, the reaction
must be carried out at temperatures of at least 513 K for HPA
and 617 K for Cs2A in order to achieve an appreciable conver-
sion. On the other hand, structural and electronic changes in
the catalysts accompanied, for instance, by an enormous
increase in the apparent absorption, already appear at low
temperatures owing to the crystal water loss, which leads to
the formation of isopropanol. Consequently, a correlation
between the spectroscopic and analytic data could not be
found. From this one can conclude that for a possible correla-
tion between the electronic structure of the working catalyst
and its performance, the optimal catalyst temperature should
be T < 623 K for a clear observation of the spectroscopic char-
acteristics. For this reason reactants that are not reduced too
strongly by the suitable working catalysts under catalytic con-
ditions are necessary for the investigation of the redox states.
At 513 K on HPA a reaction and at 613 K on Cs2A a catalytic
reaction occurs with a very low product concentration of acro-
lein and acetic acid in the reaction gas stream. At still higher
temperatures, the conversion increases and in addition to these
products propanal, acetone and CO are detected.16 Table 6
shows that the conversion of propene on HPA at 513 K and
613 K is ca. 3% (no conversion change) and that of O2 ca.
0.3% and 2%, respectively. On Cs2A at 613 K (first evidence
of oxidation products) the conversion of propene is ca. 3%
and that of O2 ca. 12%, and the greatest selectivities were
determined for acrolein and CO as may be seen in Table 7.
It can be clearly recognized that for the propene oxidation
on HPA the lattice oxygen is selective and the gas phase oxy-
gen is unselective. In addition, it may be concluded from the
catalytic data of the Table 6 that for HPA a catalytic reaction
begins only above 573 K. The transition between the lattice
and gas phase oxygen reaction cannot be recognized because
a small reduction of the HPA catalyst also continues above
573 K (Table 6). For O2/propene on Cs2A one can determine
the desired spectroscopic quantities (Vis peak position and
band gap energy) only at 617 K (lowest reaction temperature).
Above this temperature the low signal/noise ratio, large
broadening and significant overlap of the bands prevent the
determination of the mentioned spectroscopic characteristics.
In contrast to the HPA investigations, gas phase oxygen
appears only to be responsible for the O2/propene reaction
on Cs2A in the temperature range of 617 to 723 K because
of the much higher O2 conversion compared to that on HPA.
5. Conclusions
Spectroscopic characteristics represented as peak intensities,
peak positions and band gap energies were determined for het-
eropoly compounds of the type of H4ꢀxCsxPVMo11O40 (with
x ¼ 0,2) in the presence of different gas atmospheres over a
wide range of temperatures. The obtained characteristics
demonstrate an essential difference in the structural dynamics
of HPA and Cs2A under the action of different gas atmo-
spheres. The nearly constant position of the Vis band in the
range of constitutional water loss, the smaller values of the
shifts of the near-IR band and sometimes opposite trends in
the temperature behaviour of the band positions for Cs2A in
comparison with HPA clearly evidence that the Cs2A com-
pound remains stable up to higher temperatures than those
Acknowledgement
The work was supported financially by the BMBF through its
catalysis program (BMBF-Vorhaben 03D0058B). The authors
are indebted to O. Timpe for preparing the heteropoly com-
pounds and to M. Thiede for experimental assistance in UV/
Table 6 In situ catalytic and spectroscopic results in the propene oxidation on H4PVMo11O40 after 1.5 h reaction time
Conversion (%) Selectivity (mol%) Spectroscopic quantities
Acrolein Propanal Acetone CO Acetic acid By-product Visible peak position /nm Band gap energy /eV
T/K Propene
O2
513
573
613
3
3
3
0.3
0.3
2
89
40
34
24
—
—
30
13
—
—
6
—
51
25
29
9
8
2
1
1
2
706
692
679
—
2.05
2.0
4
26
1.96
—
665 12
6
6
Table 7 In situ catalytic and spectroscopic results in the propene oxidation on H2Cs2PVMo11O40 after 1.5 h reaction time
Conversion (%) Selectivity (mol%) Spectroscopic quantities
Acrolein Propanal Acetone CO Acetic acid By-product Visible peak position /nm Band gap energy /eV
T/K Propene
O2
617
673
723
3
4
8
12
12
18
28
30
42
16
17
10
7
3
4
39
34
31
9
9
7
1
7
6
722
—
1.96
—
—
—
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