Redox features of β-VOPO4 catalyst using 18O tracer and laser Raman spectroscopy

Article abstract of DOI:10.1016/S1381-1169(97)00217-3

The oxygen ions of the β-VOPO₄ catalyst were exchanged with an ¹⁸O tracer by a reduction-oxidation method and by a catalytic oxidation of but-1-ene using ¹⁸O₂. The bands at 992 and 900 cm^-1 were more shifted to lower frequencies than those at 1076 and 1002 cm^-1. Applying the correlation between the Raman bands and stretching vibrations in the literature, the exchanged oxygen species were estimated. The results suggest that the P-O-V vacancies corresponding to 992 and 900 cm^-1 were responsible for reoxidation and the V=O oxygen corresponding to the 1002 cm^-1 band of β-VOPO₄ was not. The (VO)₂P₂O₇ was oxidized to β-VOPO₄ by O₂ above 823 K. The insertion position of oxygen was determined at the bands at 992 and 900 cm^-1 of β-VOPO₄ using ¹⁸O₂, which is the same as the exchanged position.

Full text of DOI:10.1016/S1381-1169(97)00217-3

Ž
.
Journal of Molecular Catalysis A: Chemical 130 1998 261–269
Redox features of b-VOPO4 catalyst using 18O tracer and laser
Raman spectroscopy
)
Hideo Numata, Takehiko Ono
Department of Applied Chemistry, Osaka Prefecture UniÕersity, 1-1 Gakuen-cho, Sakai, Osaka 593, Japan
Received 24 June 1997; accepted 26 September 1997
Abstract
The oxygen ions of the b-VOPO4 catalyst were exchanged with an 18O tracer by a reduction–oxidation method and by a
18
1
catalytic oxidation of but-1-ene using O2. The bands at 992 and 900 cmy were more shifted to lower frequencies than
those at 1076 and 1002 cmy1. Applying the correlation between the Raman bands and stretching vibrations in the literature,
the exchanged oxygen species were estimated. The results suggest that the P–O–V vacancies corresponding to 992 and 900
cmy were responsible for reoxidation and the V5O oxygen corresponding to the 1002 cmy band of b-VOPO4 was not.
1
1
Ž
.
The VO 2P2O7 was oxidized to b-VOPO4 by O2 above 823 K. The insertion position of oxygen was determined at the
bands at 992 and 900 cmy of b-VOPO4 using 18O2, which is the same as the exchanged position. q 1998 Elsevier Science
1
B.V.
1. Introduction
The V–P–O catalysts has been used for oxi-
w
x
dation of n-butane to maleic anhydride 4–8 .
Previously, we have investigated the Raman
Ž
.
VO P2O7 has been specially indicated to be
2
w x
w x
band shifts of MoO3 1 , Mo mixed oxide 2
active phase for this oxidation. Schrader et al.
and V2O5 catalysts 3 exchanged with 18O tracer
w x
w
x
9–11 have studied these catalysts and the role
of the oxygen species in the n-butane oxidation
via oxidation reactions. With the V2O5 catalyst
y
1
18
w x
3 , the band at 700 cm
was more shifted to
w
x
using b-VOPO4 labeled with O 11 . Koyano
lower frequencies than the band at 998 cmy
.
1
w
x
et al. 12,13 have studied the oxidation and
reduction processes of the surface vanadyl pyro-
phosphate by means of Raman spectroscopy and
This suggested that the oxygen insertion from
the gas phase takes place at the anion vacancies
corresponding to the V–O band at 700 cmy in
1
w
x
XPS. Abdelouahab et al. 14,15 have studied
various VOPO4 catalysts using hydration and
Raman spectroscopy methods. Zhang-Lin et al.
the V–O layer rather than those of the V5O
band in the oxygen layer. With the MoO3 cata-
lyst, similar results were obtained 1 .
w x
w
x
16,17 have reported activated VPO catalysts
based vanadyl pyrophosphate. Assignments of
Raman spectra in accordance with normal coor-
dination analysis have not been reported for
)
Corresponding author. Fax: q81-722-593340; e-mail:
ono@chem.osakafu-u.ac.jp
1381-1169r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
Ž
.
PII S1381-1169 97 00217-3

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)
262
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
these V–P–O oxides, but some empirical as-
2.2. Procedures
signments have been attempted on b-VOPO4
w
x
11,18 . It is interesting to study which oxygen
The exchange of lattice oxygen of b-VOPO4
catalysts with 18O were performed by two meth-
ods. The first method is as follows: the mixture
species are responsible for the b-VOPO4 cata-
lyst for the oxidation reactions though the selec-
tivity to maleic anhydride is lower.
of 1-butene at ca. 4 kPa and 18O2 98.1 at%,
Ž
In this work the oxide ions of b-VOPO4
were exchanged with an 18O tracer via 1-butene
oxidation. The Raman spectra of b-VOPO4
partly exchanged with 18O were registered and
preferential shifts of the bands were compared.
.
Isotec at ca. 1 kPa was reacted on the catalyst
at 713 K using a circulation system ca. 360
cm . The second method, a reduction–oxida-
Ž
3
.
tion method, is as follows: the reduction of
b-VOPO4 catalysts by 1-butene was carried out
at ca. 4 kPa and 700–750 K and the reoxidation
by 18O2 was carried out at the same tempera-
ture. The reaction products were buta-1,3-diene,
CO, and CO2, whose 18O% were determined
The oxidation of VO P2O7 by 16O2 and 18O2
Ž
.
2
was carried out and the Raman spectral changes
with O2 uptake were examined. The redox fea-
tures of these catalysts were studied and the
active sites for oxidation and reoxidation for
b-VOPO4 were discussed.
Ž
using a mass spectrometer Shimadzu GCMS
QP 2000A . The amount of 18O exchanged in
.
the b-VOPO4 catalyst was assumed as those of
16O in the products for the first method. For the
second method, those were assumed to be the
oxygen amount of the products in the reduction
2. Methods
experiments. VO P2O7 was oxidized by 16O2
2.1. Preparation of catalysts
Ž
.
2
and 18O2 at 823–873 K using a circulation
system. The percentage oxidized with O2 was
w
x
The b-VOPO4 was prepared as follows 19 :
0.1 mol of H3PO4 was added to 0.1 mol
NH4VO3 solution at 333 K. After evaporation
on a water bath, the yellowish precursor was
heated at 873 K for 10 h and then the b-VOPO4
was obtained.
calculated using the following reaction:
1
Ž
.
VO P2O7 q 2 O2 ™2b-VOPO4.
2
The structure of catalysts was determined by
the X-ray diffraction method using Cu K a radi-
ation and by laser Raman spectroscopy. The
laser Raman spectra of the catalyst samples
exchanged with 18O were recorded on a JASCO
NR-1000 laser Raman spectrometer. An Ar-ion
laser was tuned to 514.5 nm for excitation. The
laser power was set at 150–200 mW. The data
were stored on a computer and the peak-shape
analysis was carried out using the technique
Ž
.
The VO P2O7 was prepared by two steps
2 w
x
as follows 20,21 : firstly, the precursor
Ž
.
VO HPO4 P0.5H2O was prepared. The desired
amount of H3PO4 and V2O5 is mixed in the
ŽŽ
.
.
hydroxyl ammonium chloride NH3OH Cl so-
lution at 353 K for 1 h. The solution was kept at
403 K for 24 h. It changed to a paste state. Then
it was boiled in water for 10 min. After filtra-
tion and washing, the greenish powder, i.e. the
w
x
reported by Miyata et al. 22,23 .
Ž
.
precursor VO HPO4 P0.5H2O, was obtained.
This phase was confirmed by X-ray diffraction
as JPDSC 38-0291 and 37-0269. Secondly, the
precursor was heated step by step to 823 K in a
stream of nitrogen. After cooling, it was heated
at 773 K in the presence of air for 2 h. The
sample was washed by water and dried after
3. Results and discussion
3.1. Structure and Raman spectra of b-VOPO4
The structure of b-VOPO4 was investigated
Ž
.
w x
filtration. Then VO P2O7 was obtained.
by some workers 24,25 . The structure model is
2

(
)
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
263
w
x
Fig. 1. Structure model of b-VOPO4 11 .
Fig. 2. Raman spectra of b-VOPO4 exchanged with 18 O by a
Ž
Ž .
reduction–oxidation method see Table 2 . a no exchange; b 5
Ž .
at% of oxygen exchanged;
c
d well-exchanged
b-VOPO4, i.e., the reduction by 1-C4 H8 and reoxidation by 18 O2
shown in Fig. 1. The VO6 octahedra are linked
with each other through V5O by a corner-shar-
ing. The PO4 tetrahedra are also linked with
each V square oxygen by a corner-sharing. There
are two kinds of P–Ob–V present and they have
were repeated 5 times.
Ž
.
w
x
different distances Table 1 . There is one kind
of P–Oa–V present across VO6 chains as shown
in Fig. 1 and Table 1.
tively. On the other hand, Schrader et al. 11,26
have proposed that the vibration of phosphate
ions around V octahedra is similar in magnitude
and that the interactions between groups are
probably comparable to those within groups.
Then, they proposed the P–Oa–V and P–Ob–V
stretching vibrations as shown in Table 1. The
Fig. 2a shows the Raman spectrum of b-
VOPO4. The bands at 1076, 1002, 992, and 900
1
cmy are obtained. The assignments were re-
w
x
ported by Bhargava et al. 18 and Schrader et
al. 11 as listed in Table 1. Bhargava et al. have
assigned empirically the Raman bands at 1075
band at 1002 cmy is attributed to the V5O
1
w
x
vibration as the same as that by Bhargava et al.
The assignments by Schrader et al. have been
adopted here. The discussion will be added later
using our 18O data in the oxidation of
and 988 cmy to two asymmetric stretching
1
P–O vibrations of phosphate ions. The bands at
1
1
1000 cmy and 900 cmy were assigned to the
Ž
.
V5O and symmetric P–O stretches, respec-
VO P2O7 in 3.5.
2
Table 1
Raman spectra of b-VOPO4 and their assignments
y1
w
x
w
x
Ž
.
Bhargava and Condrate 18
Lashier and Schrader 11
This work cm
y1
y1
Ž
.
Ž
.
mode
Raman band cm
mode
Raman band cm
Ž
.
.
nas P–O
Ž
1075
999
988
896
P–Oa–V
V5O
P–Ob–V
P–Ob–V
1072
998
987
896
1076
1002
992
n V5O
Ž
.
.
nas P–O
Ž
ns P–O
900
˚
w
x
According to Gopal et al. 24 the distances of P–Oa and Oa–V for P–Oa–V are 1.528 and 1.886 A, respectively. Two cases are present for
˚
P–Ob–V. In the first one P–Ob1 and Ob1–V are 1.519 and 1.902 A, respectively. In the second one P–Ob2 and Ob2–V are 1.540 and 1.849
˚
˚
A. That of V5O is 1.566 A.

(
)
264
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
Table 2
Conversions of 1-butene, product selectivities and average 18 O exchange per cent over b-VOPO4 catalyst at 713 K
Average 18 O exchange %
Ž
.
Ž .
product selectivity %
Conversion of 1-C4 H8
%
C4 H6
CO2
CO
5.2a
3.8
7.4
73
61
54
48
51
59
47
10
21
31
33
35
9
17
11
15
14
14
32
26
5
5
12
14
15
2
8.2
9.0
9.4b
22.8
27
5
a
Ž
.
Ž
.
By a reduction–oxidation method at p 1-C4 H8 s30 Torr 1 Torrs133.3 Pa , reaction time 15–35 min and 0.05 g of catalyst.
Ž
.
Ž18
.
b
By a catalytic oxidation method, p 1-C4 H8 s16 Torr, p O2 s9 Torr, and reaction time 22–36 min.
The 18 O% in CO2 is 35 and 29% for the conversions of 9.4% and 22.8%, respectively. The 18 O% in CO is 29 and 15% for conversions of
9.4% and 22.8%, respectively. The amounts of furan and maleic anhydride were not determined. The average, 18 O%, however, will be
nearly the same as described above.
18
3.2. Oxygen exchange with 18O oÕer b-VOPO4
catalysts
the increase in O exchange Fig. 2a–c , the
Ž
.
intensity of the band at 992 cmy decreases
1
while that of the new 965 cmy band increases.
1
The intensity of 900 cmy band decreases while
The catalysts were reduced with but-1-ene
and were reoxidized with 18O2 at the same
temperature. The average exchange 18O% in
this case was listed in Table 2, which was
calculated from the amounts of 16O in the prod-
ucts, assuming that reoxidation of b-VOPO4 by
18O2 takes place completely. The catalyst oxy-
gen ions were also exchanged with 18O via
catalytic oxidation using but-1-ene and 18O2
over b-VOPO4 catalysts. The conversion, prod-
uct selectivity, and 18O% in the products, and
average exchange 18O% in the b-VOPO4 are
also shown in Table 2. According to previous
1
that of the new ca. 880 cmy band increases.
1
On the other hand, the bands at 1076 and 1002
cmy do not change so much in their intensities
1
at low 18O exchange. In this work, we could not
observe the Raman spectrum after the reduction
w
x
by but-1-ene. Moser et al. 10 have reported
that the intensities of the bands at 987 992 and
896 902, this work cm decreased as com-
Ž
.
y
1
Ž
.
Ž
.
pared to the intensity of the band at 1067 1075
cmy in the butene oxidation at ca. 773 K, i.e.
in the reduction conditions for the catalyst.
In order to know the shift change in details,
the peak shape analysis was attempted with the
spectra in Fig. 2 using Lorenzian function
1
w
x
workers 9,10 furan as well as maleic anhy-
dride were produced on this catalyst in butane
oxidation. We have not determined the products
here though they may be less produced. The
average 18O% of catalysts were calculated from
the 16O% of products such as CO, CO2, and
H2O. The selectivity to buta-1,3-diene ranges at
around 50–60% in both cases.
w
x
22,23 . Fig. 3 shows the analysts for Fig. 2c.
Good curve fittings are obtained when the spec-
trum consisted of 1075, 1050, 1002, 992, 977,
965, 900, and 888 cmy1. It is thought that the
bands at 888 are shifted from 900, 965 from
992, and 977 from 1002 cmy1. The position of
977 cmy is not determined experimentally but
1
3.3. Raman band shifts of b-VOPO4 catalysts
exchanged with 18O by a reduction–oxidation
method
is estimated in this fitting analysis. A new band
shifted from 1076 cmy at low 18O exchange is
1
Ž
.
not detected Fig. 2b . The shifted fractions with
1
Fig. 2 shows the spectra of b-VOPO4 before
the 1002, 992 and 900 cmy bands are shown
and after they were exchanged with 18O. With
in Table 3.

(
)
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
265
Fig. 4. Peak shape analysis of the spectra of well-exchanged
Ž
.
catalyst Fig. 2d and separated peaks.
Fig. 3. Peak-shape analysis of the spectrum of Fig. 2c.
Fig. 2d shows the spectrum of b-VOPO4
sufficiently exchanged with 18O by repeating
reduction by but-1-ene and reoxidation by 18O2
for 5 times. All bands tend to exhibit line-
broadening. Fig. 4 shows the peak shape analy-
sis for the spectrum of Fig. 2d. The spectrum
consists of the bands at 1065, 1050, 999, 987,
976, 966, 902, and 880 cmy1. The band at 900
w
x
As described above, Schrader et al. 11 pro-
1
posed that the bands at 992 and 900 cmy are
attributed to the stretching vibrations of P–Ob–
1
V. That at 1002 cmy is to the V5O vibration
1
and that at 1075 cmy to P–Oa–V. The results
in Fig. 2 and Table 3 indicate that the oxygen
ions of P–Ob–V species exchange more prefer-
entially than those of V5O and P–Oa–V. These
indicate that oxygen insertion seems to take
place on the vacancies corresponding to P–Ob–
1
cmy shifts to 880 cmy1. Those at 978 and 966
1
cmy are shifted from 999 1002 and 988
Ž
.
y
1
Ž
.
Ž
.
992 cm respectively. The bands at 988 992
and 902 have exchanged remarkably, which
corresponds to P–Ob–V. The band at 999 cmy
1
1
V positions. The bands at 1002 and 1076 cmy
is the same as the one at 1002 cmy as for 18O
exchange in Fig. 3 and in Table 3 which
1
Ž
corresponding to V5O and P–Oa–V, res-
pectively, are not exchanged at low 18O ex-
change. This indicates that these are not the
sites for reoxidation. Furthermore, the P–Ob–V
oxygen ions seem to participate in the oxidation
reactions since the 18O exchange is very selec-
tive at the positions.
.
corresponds to V5O. Its intensity is not changed
much in spite of the high 18O exchange. The
band at 1065 seems to be the same band, ob-
1
served at 1075 cmy in Fig. 3 and Table 3,
which is affected by replacement other oxygen
Table 3
Fractions obtained from peak-shape analysis over b-VOPO4 catalyst exchanged with 18 O via redox reactions
Average 18 O% in b-VOPO4
Exchange %
Ž
.
Ž
.
Ž
.
P–Ob–V
V5O
P–Oa–V
Ž
.
Ž
.
Ž
.
I888r I902 qI888
I965r I992 qI965
I977r I1002 qI977
5a
12
14
2b
5
8
7
0
6
8
0
0
0c
0
5
0
0
27
39
8
13
25
5
0–10
6
a,b
have the same meaning as in Table 2.
cA very small peak was observed at around 1050 cmy1 at low exchange region.

(
)
266
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
with 18O in P tetrahedra. The band at 1050
1
cmy seems to be the shifted one such as
P–18Oa–V.
The shift intervals are determined as 26–22
1
cmy experimentally in the range from 1000 to
900 cmy1. The V5O oxygen is not very active
for oxidation reaction since the shift extent is
low as described above.
3.4. Raman band shifts of b-VOPO4 catalyst
exchanged with 18O by a catalytic oxidation
The catalyst oxygen ions were exchanged
with 18O via catalytic oxidation using but-1-ene
and 18O2. The average 18O% are shown in
Table 2. Fig. 5 shows the Raman spectra and
the results of peak-shape analysis for the 2%
sample. A small band at 967 and shoulder at ca.
Fig. 6. Raman spectra of VO 2 P2O7 oxidized by 18 O2 and 16O2.
Ž
.
18
Ž .
Ž
.
Ž .
Ž .
a No oxidation, i.e. VO 2 P2O7, b 23% oxidized by O2 , c
32% by O2 , d 81% by O2 , and e approx. 100% by 16 O2 ,
18
18
Ž .
Ž .
1
880 cmy appear. The shift fractions for these
i.e., b-VOPO4 at 823–873 K and for 14–30 h.
bands are also shown in Table 3. It is concluded
that the exchange, i.e., oxygen insertion, takes
place on the P–Ob–V vacancies rather than on
V5O and P–Oa–V. The characteristic feature in
this catalytic oxidation is the same as in the
reduction–oxidation method as described above.
The average exchange % in the catalytic
oxidation exhibits a smaller value though the
conversions of but-1-ene are higher than those
oxidation takes place at the surface regions. As
the product selectivities are nearly the same
between in the reduction and catalytic oxida-
tion, a redox mechanism should be operated
here.
in the reduction–oxidation method Table 3 .
The shift fractions of the 967 and 880 are
somewhat smaller in the case of catalytic reac-
tions. This seems to be because the catalytic
3.5. Oxidation of VO 2 P2O7 by 18O2 and 16O2
and their Raman spectra
Ž
.
(
)
Ž
.
2
It is well known that VO P2O7 is oxidized
Ž
.
to b-VOPO4 by O2 above 823 K. VO P2O7
2
Ž
.
Fig. 6a has one strong Raman band at 930
cmy which is attributed to the vibration of
1
w x
P–O–P 9 . Fig. 6 shows the spectra after the
oxidation of VO P2O7 with 18O2 and 16O2.
Ž
.
2
The catalyst which has been oxidized by 16O2
Ž
.
Fig. 6e exhibits bands at 1076, 1002, 992, and
900 cmy1. This is the same spectrum as shown
in Fig. 2a and characteristic for b-VOPO4. The
catalyst well-oxidized by 18O2 Fig. 6d exhibits
Ž
.
1
new bands at 965 and 888 cmy which were
previously well observed and identified after
Fig. 5. Raman spectra of b-VOPO4 exchanged with 18 O by a
reduction of b-VOPO4 and reoxidation by 18O2.
Ž
.
catalytic oxidation see Table 2, 2% of but-1-ene and its peak-
shape analysis.
The bands at 1076, 1002, 992, and 900 cmy
1

(
)
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
267
are also observed. With the catalyst oxidized by
18
1
32% with O2 , the bands at 963 and 880 cmy
appears as shown in Fig. 6c. Fig. 7 shows the
peak-shape analysis of it. The analysis indicates
1
that the bands at 965 and 888 cmy grows more
preferentially compared to other shifted bands.
This suggests that 18O oxygen is inserted prefer-
entially to the position at P–Ob–V sites in the
Ž
.
oxidation of VO P2O7. This feature is the
2
w
x
same as reported by Lashier and Schrader 11 .
They reported that the oxygen was inserted
mainly at the position corresponding to the band
at 900 cmy1, i.e. new bands at 880 cmy
1
appeared. The insertion of an oxygen seems to
q
take place at the pair V4 sites since the oxida-
q
tion of V4q™V5 will occur there. Fig. 8
Ž
.
shows a structure model of VO P2O7 and
2
b-VOPO4. Some predictions are given. The sep-
aration of octahedral pair sites seems to accom-
pany the oxygen insertion. The tetrahedral phos-
phate ions should be formed by the fission of
pyrophosphate species. The 18O does not enter
P–Oa–V. This suggests that the oxygen ions of
P–Oa–V come from those of square V and
pyrophosphate species. The oxygen ions in-
serted from gaseous phase enter the P–Ob–V
Ž
.
position as described above. When VO P2O7
2
is oxidized by gaseous oxygen, some VOPO4
phases will be formed prior to the stable b-
VOPO4 phase formation. This is discussed be-
low. Anyway, the Raman bands of b-VOPO4
Ž .
Ž . Ž
.
Fig. 8. Structure models of 1 b-VOPO4 and 2 VO 2 P2O7. In
Ž . Ž .
Ž .
Ž .
1 , a and b have the same meaning as those in Fig. 1. In 2 ,
the oxygen ions of V4q pair squares of VO 2 P2O7 are all linked
with pyrophosphate species, but some of them are omitted here.
Ž
.
are affected by V–O characters such as V–18O
and V–16O. In this sense, the assignments in
Table 1 by Schrader et al. will be better.
As shown in Fig. 6b, prior to b-VOPO4
phase formation the different bands appear when
the 23% of 18-oxygen uptake occurs over
Ž
.
VO P2O7 catalyst. The new small bands at
2
1
1045, 1000, 670, 580 and 540 cmy appear. In
using 16O2, the small bands at 1040, 1000, 570,
and 540 cmy were obtained at 17% of uptake.
1
w
x
According to Koyano et al. 12,13 , the bands at
1
1090, 937, and 590 cmy were obtained for
Fig. 7. Peak shape analysis of the spectrum of Fig. 6c.

(
)
268
H. Numata, T. OnorJournal of Molecular Catalysis A: Chemical 130 1998 261–269
Ž
.
Ž
As has been reported recently see e.g. Ref.
28 , the reduction of oxide surface and its
oxidation by gaseous oxygen occurs in different
regions on the oxide catalysts such as Bi–Mo
oxides. In this work, the oxidation takes place at
restrict sites over b-VOPO4 as far as the oxygen
release and oxygen insertion are concerned.
partially oxidized VO P2O7, which is named
2
w
x.
as a X1 phase which will be responsible for
w
x
maleic anhydride formation. In Ref. 14,15 the
bands for d-VOPO4 are observed at 1090, 1075,
1020, 936, and 590 cmy1. The bands by both
workers are partly resemble. Abdelouhab et al.
14,15 proposed the structure of d-VOPO4
phase which is different from aI, aII, and
w
x
g-phases. The Raman bands by these workers
w
x
12–15 are different from those in this work.
Acknowledgements
Ž
.
The surface oxidized compounds on VO P2O7
2
seem to depend on the experimental conditions
reported in the studies. The new band appear-
ance seems to originate from a new phase for-
We thank Prof. Hisashi Miyata for help in
computer peak-shape analysis. We also thank
Prof. Masakazu Anpo for using the Shimadzu
q
mation due to the oxygen insertion at V4 pair
Ž
.
Mass Spectrometer GCMS QP 2000A .
sites prior to the stable phase formation of
b-VOPO4.
References
3.6. Redox features on b-VOPO4 catalyst
w x
1
T. Ono, H. Numata, N. Ogata, J. Mol. Catal. A: Chemical
1
The bands at 992 and 902 cmy of b-VOPO4
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