Structure of vanadium oxosulfato complexes in V2O5-M2S2O7-M2 SO4 (M = K, Cs) melts. A high temperature spectroscopic study

Article abstract of DOI:10.1021/jp0126715

The V^V and V^IV oxosulfato Complexes formed in V₂O₅-M₂S₂O₇-M₂ SO₄ (M = K, Cs) melts under SO₂(g) or O₂(g) atmosphere have been studied by electronic absorption (VIS/NIR) and Raman spectroscopy at 450 ?°C. VIS/NIR spectra have been obtained at 450 ?°C for V₂O₅-K₂S₂O₇ molten mixtures in SO₂ atmosphere (P_SO2 = 0-1.2 atm). The data are in agreement with the V^V a?? V^IV equilibrium: (VO)₂O(SO₄)₄^4-(l) + SO₂(g) a?? 2VO(SO₄)₂^2-(l) + SO₃(g). SO₂ does not coordinate to the V^V complex but starts significantly to coordinate to V^IV for P_SO2 > 0.4 atm according to VO(SO₄)₂^2-(l) + SO₂(g) a?? VO(SO₄)₂SO₂^2-(l). The Raman spectral features and the exploitation of the relative Raman intensities indicate that the (VO)₂O(SO₄)₄^4- dimeric complex unit, possessing a V-O-V bridge, is formed in the V₂O₅-M₂S₂O₇ binary mixtures. The spectral changes occurring upon interaction of the binary V₂O₅-K₂S₂O₇ mixtures with SO₂ or addition of M₂SO₄ to the binary V₂O₅-M₂S₂O₇ mixtures indicate a cleavage of the V-O-V bridge and formation of the V_IVO(SO₄)₂^2- or V^VO₂(SO₄)₂^3- monomeric complex units, respectively. The most characteristic bands due to the various complexes in the melts have been assigned. The spectral data are discussed in terms of possible structures. For the first time, high-temperature vibrational spectroscopy has been used to study the structural and vibrational properties of V₂O₅-K₂S₂O₇ and V₂O₅-K₂S₂O₇-K₂ SO₄ melts. The results are valuable for the mechanistic understanding of SO₂ oxidation at the molecular level.

Full text of DOI:10.1021/jp0126715

J. Phys. Chem. B 2002, 106, 49-56
49
Structure of Vanadium Oxosulfato Complexes in V2O5-M2S2O7-M2SO4 (M ) K, Cs)
Melts. A High Temperature Spectroscopic Study
,†
†
‡
Soghomon Boghosian,* Athanassios Chrissanthopoulos, and Rasmus Fehrmann
Department of Chemical Engineering, UniVersity of Patras and Institute of Chemical
Engineering and High-Temperature Chemical Processes (FORTH/ICE-HT), P.O. Box 1414,
GR-26500 Patras, Greece and Department of Chemistry and ICAT (Interdisciplinary Center for Catalysis),
The Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark
ReceiVed: July 11, 2001; In Final Form: September 27, 2001
V
IV
The V and V oxosulfato complexes formed in V
O
2 5
-M S
2 2
O -M
7 2
SO
4
(M ) K, Cs) melts under SO
2
(g) or
O
2
(g) atmosphere have been studied by electronic absorption (VIS/NIR) and Raman spectroscopy at 450 °C.
VIS/NIR spectra have been obtained at 450 °C for V -K molten mixtures in SO atmosphere (PSO2
2
V
O
5
2
IV
S
2
O
7
2
4-
)
0-1.2 atm). The data are in agreement with the V T V equilibrium: (VO)
2
O(SO
4
)
4
(l) + SO
2
(g) T
2
-
V
2
VO(SO
4
)
2
(l) + SO
to V for PSO2 > 0.4 atm according to VO(SO
features and the exploitation of the relative Raman intensities indicate that the (VO)
unit, possessing a V-O-V bridge, is formed in the V -M binary mixtures. The spectral changes
occurring upon interaction of the binary V -K mixtures with SO or addition of M SO to the binary
-M mixtures indicate a cleavage of the V-O-V bridge and formation of the V O(SO
(SO
3 2
(g). SO does not coordinate to the V complex but starts significantly to coordinate
IV
2-
2-
4
)
2
(l) + SO
2
(g) T VO(SO
4
)
2
SO
2
(l). The Raman spectral
4-
2
4
O(SO )
4
dimeric complex
2
O
5
2 2 7
S O
O
2 5
2
S
2
O
7
2
2
4
IV
2-
V
2
V
O
5
2
S
2
3-
O
7
4
)
2
or
V O
2
4 2
)
monomeric complex units, respectively. The most characteristic bands due to the various
complexes in the melts have been assigned. The spectral data are discussed in terms of possible structures.
For the first time, high-temperature vibrational spectroscopy has been used to study the structural and vibrational
properties of V
understanding of SO
2
O
5
-K
2
2
S
2
O
7
and V
2
O
5
-K
2
S
2
O
7
2 4
-K SO melts. The results are valuable for the mechanistic
oxidation at the molecular level.
Introduction
compounds isolated from the V2O5-M2S2O7 molten systems
and characterized by single-crystal X-ray study are the Cs4-
The catalyst used for sulfuric acid production, catalyzing the
reaction SO2 + /2O2 f SO3, is a supported liquid phase (SLP)
catalyst, usually made by calcination of diatomaceous earth,
V2O5 (or other V salts), and alkali salt promoters (usually in
the form of their sulfates) with the alkali-to-vanadium ratio
ranging from 2 to 5. During the activation process, the catalyst
takes up SO3, forming molten alkali pyrosulfates, which dissolve
16
15,17
13,17
(
VO)2O(SO4)4, Rb4(VO)2O(SO4)4,
and K4(VO)2O(SO4)4.
1
The complex formation of V and the equilibrium VV
T
IV
VIV in the molten salt-gas system V2O5-K2S2O7/SO2-SO3-
N2 in controlled SO2-SO3 atmospheres have been studied by
3
high-temperature VIS/NIR and ESR spectroscopy. The impor-
tance of the lower valence states of vanadium (V and V ) is
well recognized, because precipitation of V and V com-
pounds has been shown to cause deactivation of catalyst
IV
III
IV
III
1
the vanadium salts. Thus, the molten salt/gas system V2O5-
M2S2O7-M2SO4/SO2-O2-SO3-N2 (M ) K, Na, Cs or mix-
tures of these) at 400-600 °C is considered to be a realistic
model of the working industrial catalyst. Recently, the catalyst
1
8,19
melts.
In the present work, high-temperature VIS/NIR spectroscopy
V
IV
is applied to study the complex formation of V and V in the
2
has been used also for desulfurization of power plant flue gas.
molten salt-gas system V2O5-K2S2O7/SO2 at 450 °C in order
Despite persistent research efforts, conclusions concerning the
molecular structure of the VV and especially V complexes
participating in the catalytic cycle are still lacking due to the
V
to investigate the possible coordination of SO2 to the V and
IV
IV
V
complexes formed in the catalyst. The VIS/NIR study is
IV
limited to dilute solutions of V , i.e., 0.5 mol/L (corresponding
difficulties encountered in spectroscopic studies of supported
to XV O ) 0.03) due to the very high absorbance of the studied
molten salt catalysts.3
2 5
melts in the visible and UV regions. The composition of the
So far, various methods including potentiometry, cryoscopy,
spectrophotometry, conductometry, calorimetry, NMR, cyclic
voltammetry, and Raman spectroscopy have been used to study
industrial catalysts lies in the range of 3-4.5 mol V/L
(
corresponding to XV O in the range of 0.17-0.22) for the
2
5
V2O5-K2S2O7 binary system.
V
the V complexes in V2O5-M2S2O7-M2SO4 (M ) K, Cs)
Furthermore, the present work is concerned with the structural
4
-11
melts.
The phase diagrams of the V2O5-M2S2O7 (M ) 80%
V
IV
and vibrational properties of (i) the V and V oxosulfato
12
13
14
K + 20% Na), V2O5-K2S2O7, V2O5-Cs2S2O7, and V2O5-
complexes formed in binary V2O5-M2S2O7 (M ) K or Cs)
Rb2S2O71 systems have been constructed. The only V
5
V
V
melts under SO2-O2(g) atmosphere and (ii) the V oxosulfato
complexes formed in the ternary V2O5-M2S2O7-M2SO4 (M
*
To whom correspondence should be addressed.
University of Patras and Institute of Chemical, Engineering and High-
)
K or Cs) melts under O2(g) atmosphere. High-temperature
†
Raman spectroscopy is applied at 450 °C and in the following
V2O5 mole fraction ranges: (a) 0-0.25 for V2O5-K2S2O7 (up
Temperature Chemical Processes.
‡
The Technical University of Denmark.
1
0.1021/jp0126715 CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/13/2001

50
J. Phys. Chem. B, Vol. 106, No. 1, 2002
Boghosian et al.
sample compartment of a Hitachi U-3000 spectrophotometer.
A period of several days was necessary to equilibrate the
K2S2O7-V2O5(l)/SO2(g) mixtures at 450 °C, and progress
toward equilibrium was followed by recording the spectrum of
the molten phase and measuring the absorbance at 730 nm due
IV
to the V complexes. After equilibrium in the melt was
established, the partial pressure of SO2 was found to decrease,
and the spectrum of the gas phase was recorded for determining
(i) the final equilibrium pressure of SO2 and (ii) the number of
SO2 moles that were consumed.
The quantitative interpretation of the spectra is based on
3
V
Beer’s law. As explained previously, the redox equilibrium V
IV
V
T V involves only two absorbing species (one V and one
V ), and the measured optical density, A (absorbance), is related
IV
to the concentration of the absorbing species according to
A
l
)
ꢀV
cV
V V
+ ꢀVIVcVIV
(1)
Figure 1. Cell assembly connecting two rectangular optical cells (a)
cell containing molten salt mixture and (b) cell used for measurements
in the gas phase.
V
IV
where l is the path length (cm), cV and cV are the equilibrium
V
IV
V
IV
concentrations of the V and V species (mol/L), ꢀV and ꢀV
are the molar absorptivities (L mol cm ) of the absorbing
species, and cV + cV ) c (c denotes the initial concentra-
tion of vanadium). The values of ꢀV and ꢀV determined
-
1
-1
0
V
0
V
to 4.8 mol V/L), (b) 0-0.25 for V2O5-K2S2O7-K2SO4, (c)
V
IV
0-0.33 for V2O5-Cs2S2O7 (up to 8.6 mol V/L), and (d) 0-0.25
V
IV
for V2O5-Cs2S2O7-Cs2SO4. Thus, the Raman study covers the
very dark and viscous melts characteristic of the industrial
catalyst.
3,20
V
-1
-1
IV
previously
9.61 ( 0.08 L mol cm at 730 nm and 450 °C.
The decrease in the number of moles of SO2 can be due to
are ꢀV ) 1.57 ( 0.02 L mol cm and ꢀV )
-1
-1
1
V
IV
1
(
i) reduction of V to V ( /2 mol SO2 needed per mol of
vanadium) and (ii) possible coordination of SO2 to V or to
complexes. The solubility of SO2 in molten K2S2O7 is small
and can be neglected. The number of moles of SO2 coordinated
Experimental Section
V
IV
V
Chemicals and Sample Preparation. The hygroscopic
K2S2O7 and Cs2S2O7 were made by thermal decomposition of
K2S2O8 (Merck p.a.) and Cs2S2O8 (synthesized as discussed
21
V
IV
c
SO2
to vanadium (V or V ), n , is defined by the mole balance
1
4
previously ). V2O5 (Cerac, pure 99.9%), K2SO4 (Fluka p.a),
c
SO2
0
SO2
eq
SO2
n
) n - 1/2nVIV - n
(2)
and Cs2SO4 (Alfa, 99.999%) were dried in vacuo at 300 °C for
4
h and stored in the glovebox until use. The gases used were
SO2 (Matheson, Union-Carbide, 99.98%), N2 (L’Air Liquide,
9.999%), and O2 (L’Air Liquide, 99.99%). All handling of the
0
SO2
where n
/
is the number of moles of SO2 initially added,
2nV is the number of moles of SO2 used for reducing
vanadium, and n is the number of moles of SO2 in the gas
1
IV
9
eq
SO2
chemicals and filling of the VIS/NIR and Raman optical cells
took place in a nitrogen filled glovebox. Preweighed amounts
of the chemicals were transferred into clean, dry, flamed, and
degassed optical cells. The cells were then attached to a gas-
addition/vacuum line equipped with a mercury manometer for
introducing the gases at the desired pressures. The VIS/NIR
cell assembly (Figure 1) was then closed with a Teflon stopcock,
whereas the Raman cells were immersed in liquid nitrogen for
trapping the gases and were finally sealed under vacuum.
phase at equilibrium. The densities of the V2O5-K2S2O7 melts
7
are taken from previous work, and the melt volume is assumed
unchanged, regardless of the gas atmosphere applied.
Raman Spectroscopy. The Raman cells were made of
cylindrical fused silica tubing (4 ( 0.1 mm o.d., 3 ( 0.1 mm
i.d., and ∼3 cm long for the part containing the melts). The
spectra were excited with the 647.1 and 676.4 nm lines of a
Spectra Physics Stabilite model 2017 krypton ion laser and the
5
14.5 nm line of a Spectra Physics model 164 argon ion laser.
0
The symbol Xi denotes the initial mole fraction of the
The scattered light was collected at an angle of 90°, analyzed
with a 0.85 m Spex 1403 double monochromator, and detected
with a -20 °C cooled RCA photomultiplier. The Raman optical
components of the V2O5-M2S2O7 binary mixture (weighed-in
amounts). The composition of the V2O5-M2S2O7-M2SO4
0
V O
2
2
ternary mixture is defined by combining X
(neglecting M2-
furnace has been described previously.
A period of 5-8 days was needed to equilibrate the K S O -
2
5
SO4) with the ratio n(SO42 )/n(V) of the number of sulfate
groups added per mole of vanadium. This ratio was varied
between 0 and 2.5.
-
2
2
7
V O (l)/SO (g) mixtures at 450 °C due to slow diffusion of SO ,
2
5
2
2
whereas overnight heating at 450 °C was sufficient for
equilibrating the M2S2O7-V2O5(l)/O2(g) (M ) K or Cs)
mixtures. The M2S2O7-V2O5-M2SO4 melts had to be heated
for up to 20 days to attain equilibrium due to slow dissolution
of the sulfate.
Spectrophotometry. A cell assembly (Figure 1) connecting
two rectangular fused-silica optical cells (Ultrasil from Helma,
Germany) was used for the spectrophotometric study. An optical
cell with a 0.2 cm path length was used for obtaining the VIS/
NIR spectra of the V and VV complexes in the melt. For
determining the equilibrium partial pressure of SO2 in the gas
phase, optical cells with 0.05 or 0.2 cm path lengths were used
IV
Obtaining Raman spectra at elevated temperatures for these
very dark-colored, viscous, and hygroscopic melts was very
difficult due to strong absorption of the laser light. Attempts to
V
(depending on the SO2 pressure), and the absorbance at 300.2
detect scattered light from the V containing melts using the
nm was measured in the UV/vis spectrum of the gas. Appropri-
ate linear PSO2 vs A300.2 calibration lines were constructed.
The cell part containing the melt was mounted in an upright
position inside an optical furnace, which was placed in the
514.5, 488.0, and 457.0 nm argon lines as excitation sources
failed. This is because a strong charge-transfer band occurs in
the UV/vis spectra of M2S2O7-V2O5 and M2S2O7-M2SO4-
5
,23
V2O5 melts, and Raman spectra could only be obtained with

Structure of Vanadium Oxosulfato Complexes
J. Phys. Chem. B, Vol. 106, No. 1, 2002 51
0
Figure 2. VIS/NIR spectra of the V
mol V /L) at 450 °C equilibrated with (a) PO2 ) 0.2 atm, (b) PSO2
0.08 atm, (c) PSO2 ) 0.20 atm, (d) PSO2 ) 0.25 atm, and (e) PSO2
.30 atm.
2
O
5
-K
2
S
2
O
7
mixture (cV ) 0.25
Figure 3. Plots of the degree of vanadium reduction, nVIV/n (O), and
V
c
2 5
O
of the coordination number, n /n
V
(∆), as a function of the partial
SO2
)
0
)
pressure of SO , PSO2, at equilibrium.
2
TABLE 1: V TV Model Redox Equilibriaa
V
IV
the 647.1 and 676.4 nm krypton lines. Better results were
obtained with the 647.1 nm line, which overlaps with the tail
of the electronic charge-transfer band of V , and therefore, the
obtained spectra are of preresonance character, leading to
enhancement of the band intensities. In certain cases, light
reflected from the walls of the quartz cells had to be masked
by proper adjustment of the monochromator entrance slit height.
model
No.
equilibrium
constant
model equilibrium
σ
K/PSO3
V
c(VIV)2 SO3
P
(VV)2 + SO2 T (V )2 + SO3 K )
IV
0.325 33.7
1
2
3
1
c(VV)2
PSO2
2
PSO3
)
(cVIV
2VV + SO2 T 2VIV + SO3 K )
369.9
330
2
2
PSO2
)
(
cV
V
2
P
SO3
(
cVIV
)
Results and Discussion
V
IV
(V )2 + SO2 T 2V + SO3 K )
0.239 29.3
3
(
cVV)2 PSO2
VIS/NIR Spectra of the V2O5-K2S2O7/SO2(g) System at
c(VIV)2
PSO3
4
5
50 °C. The VIS/NIR spectra were recorded in the range of
50-900 nm at 450 °C for 0.25 mol/L V2O5 (i.e. 0.5 mol/L V)
V
IV
4
2V + SO2 T (V )2 + SO3 K )
428.2
376
4
2
P
(cVV
)
SO2
solutions in K2S2O7 under 20 different SO2 partial pressures in
a
Equilibrium constants (K/PSO3) and variances (σ) at 450 °C.
the range 0 < PSO < 1.5 atm. For convenience, only some of
2
the spectra obtained are shown in Figure 2. The band at 730
of the sulfuric acid catalyst.18,19 The powder IR and Raman
spectra of these crystals were indeed the same as the known
spectra of KV(SO4)2.
Previous studies
molten solutions, V is present in the form of monomeric or
dimeric complexes and V presumably behaves in a similar
way. The possible combinations of these proposed complexes
give rise to four model equilibria, which are presented in Table
-
1
IV
nm (13 700 cm ) is due to V complexes and is assigned to
2
2
the Lapporte-forbidden d-d B2(dxy) f E(dxz, dyz) electronic
24
IV
transition of V in the distorted octahedral field of a complex
5-9
suggest that, in dilute V2O5-K2S2O7
2+
of the vanadyl ion (VO ) with equatorially coordinated
bidentate SO4 ligands in a C2υ configuration. The isosbestic
point found at around 620 nm and 16 L mol cm compares
well with the values found previously.3 A graphical summary
of the results is shown in Figure 3, where for each experiment,
the degree of vanadium reduction, nV /nV, and the coordination
number, n /nV, i.e., the number of moles of SO2 coordinated
V
3
IV
-
1
-1
,23
1
.
IV
Now, for each measurement, the partial pressure of SO2 is
and ꢀVIV values, the
known, and by using eq 1 and the ꢀV
c
SO2
V
per mole of vanadium, are plotted against PSO at equilibrium.
2
concentrations of the vanadium species participating in the four
model redox equilibria can be calculated. The partial pressure
of SO3 above the dilute V2O5-K2S2O7 mixtures studied is
unknown but presumably constant, i.e., equal to the partial
pressure of SO over molten K S O at 450 °C. Therefore, the
V
The results show that SO2 does not coordinate to V complexes
IV
eq
SO2
(
which are reduced to V even at very low P ) but only to
IV
eq
V , apparently at P > 0.4 atm, increasing to a coordination
SO2
eq
number of 1 at P ≈ 1.2 atm. The scattering of the data in
SO2
3
2
2
7
eq
Figure 3 reflects the experimental difficulties. At P g 1.4
relation between PSO and the appropriate concentration quotient
SO
2
2
atm, precipitation of a crystalline compound was observed. The
green (V ) hexagonal crystals (isolated after cooling and
flushing the solidified melt with water) were reminiscent of KV-
for the correct model using the 7 different measurements in the
III
range 0 < PSO < 0.4 atm, i.e., before SO2 starts to coordinate
2
IV
significantly to the V complex, should be linear with a slope
(SO4)2, which is known to be one of the deactivation products
corresponding to PSO /K. The variances given in Table 1 are
3

52
J. Phys. Chem. B, Vol. 106, No. 1, 2002
Boghosian et al.
the results of least-squares fits of the concentration quotient for
each particular model vs the partial pressure of SO2 and suggest
the exclusion of models 2 and 4. This result combined with the
ESR evidence3 for monomeric V complexes in the studied
melts, a fact which excludes model 1, points to model 3 as the
most plausible model. Thus, the redox equilibrium could be
formulated as
,23
IV
4
-
2-
(
VO) O(SO ) (l) + SO (g) T 2VO(SO ) (l) + SO (g)
2 4 4 2 4 2 3
(3)
-1
-1
PSO3/K ) 29.3 L mol atm
The above complex formulas are deduced from previous
knowledge of the complex chemistry of vanadium in pyrosulfate
3
-9,11
melts
and are in agreement with a recent potentiometric
and ESR study.23 The results from the Raman study discussed
V
IV
below confirm that the V and V complexes shown in (3) are
indeed the most plausible ones. Moreover, the equilibrium
constant of reaction 3 has previously been determined: K )
-
1
3
0
.0145 L mol at 450 °C. Thus, by taking into account the
above PSO /K value, one can calculate the equilibrium partial
3
pressure of SO3 over the V2O5-K2S2O7 melt studied at 450 °C
-
4
as PSO ) (4.9 ( 0.3) × 10 atm.
3
IV
Finally, the coordination of SO2 to the V complex assuming
a SO2 coordination number of 1 (according to the results shown
in Figure 3) could be formulated as
VO(SO4)2 -(l) + SO (g) T VO(SO ) SO 2-(l) (4)
2
Figure 4. Raman spectra of the V
2
O
5
-K
2
S
2
O
7
molten mixtures at 450
2
4 2
2
0
0
°
0
)
C in oxygen atmosphere (PO2 ) 0.2 atm) (a) X
) 0, (b) X
)
V O
V O
2
5
2
5
0
0
0
0
.03, (c) X
0.100, (g) X
) 0.054, (d) X
) 0.063, (e) X
) 0.087, (f) X
V O
V O
V O
V O
2
5
2
5
2
5
2
5
Note that in the previous study of the V2O5-K2S2O7 system at
equilibrium in controlled SO2/SO3 atmospheres it was not
possible to exclude model 2, which however could be excluded
in the present study.
0
0
0
V O
) 0.150, (h) X
) 0.200, (i) X
) 0.228, and
V O
V O
3
2
5
2
5
2
5
0
(j) X
) 0.249. Spectral conditions: λ ) 647.1 nm; laser power,
0
V O
2
5
-1
-1
-1
-1
1
75 mW; scan speed, 60 cm min (a-g), 18 cm min (h-j);
1
-
time constant, 0.3 s (a), 1 s (b-d), 3s (e-j); spectral slit width, 6 cm
.
Raman Spectra of the V2O5-M2S2O7/O2(g) Systems (M
0
where C denotes the complex, can be determined by exploiting
the relative Raman band intensities of the spectra, shown in
Figure 4, by a recently derived method based on the definition
)
0
K or Cs). Several V2O5-M2S2O7 mixtures with X
)
V O
2
5
0
-0.25 for the K system and X
) 0-0.33 for the Cs
V O
5
2
system were placed in cells which were then sealed under
/
25
of the following parameters, I and I°:
oxygen (PO ) 0.2 atm) in order to stabilize vanadium in the
2
+
4
5 state. By dissolution of V2O5 in M2S2O7 (M ) K or Cs) at
0
I/ ) (
IS2O72-,1085)/X
2-
(6)
(7)
50 °C, dark brown melts were obtained, the colors of which
S2O7
were much darker for the K containing melts. Therefore, Raman
0
(
Icomplex,1050)/X
0
V O
V O
2 5
spectra could not be obtained for the K melts at X
> 0.25,
5
2
because of very strong absorption of the excitation laser lines
used. However, spectra obtained from the two systems were
similar.
(
IS2O72-,1085)/N
eq,S O
2-
7
2
I° )
(Icomplex,1050)/Neq,complex
Figure 4 shows Raman spectra obtained from V2O5-K2S2O7
molten mixtures at nine different compositions. The Raman
spectra of molten K2S2O7 are included in Figure 4 for
comparison. Addition of V2O5 gives rise to the appearance of
new bands the assignments and polarization characteristics of
which are summarized in Table 2. The most prominent among
0
V O
I* is plotted vs X
in Figure 5a and extrapolates to 0.36
5
2
pointing to a stoichiometry near 1:2 (n ) 2).
I° is plotted vs X
values of n. The numerator and denominator of eq 7 should
each be independent of X ; the scattering power per mol-
ecule of S2O7 (numerator) or complex ion (denominator) are
universal constants, apart from an instrumental factor which
cancels in eq 7. The number of moles at equilibrium for each
species, Neq,i, can be calculated by assuming a value for n in
reaction 5 and considering the appropriate mass balances. From
Figure 5b, we conclude that n ) 2, leading to a constant value
for I°. Thus
0
in Figure 5b for different assumed
V O
2
5
0
V
V2O5
these bands, which are attributed to the V complex formed,
2
-
-
1
are observed at 1050, 1000, 830, 780, 676, and 392 cm . The
2-
intensities of these bands increase relative to the bands of S2O7
with increasing amounts of V2O5 added and dominate the
0
V O
spectrum of the sample with X
) 0.25, indicating that the
25
2
5
reaction taking place leads to a vanadium oxosulfato complex
at the expense of S2O72 . Table 2 lists also the Raman bands,
polarization characteristics, and assignments for the V2O5-
Cs2S2O7 melts, of which the spectra are similar to those shown
in Figure 4. The stoichiometry of the reaction
-
V O + 2S O f (VO) O(SO ) 4-(l)
2-
(8)
2
5
2
7
2
4 4
in accordance with eq 3 and the previous evidence for the
dimeric V(V) complex mentioned above. The following discus-
V O + nS O f C2n-
2
7
-
(5)
2
5
2

Structure of Vanadium Oxosulfato Complexes
J. Phys. Chem. B, Vol. 106, No. 1, 2002 53
SO /O (g), and V -K /SO (g) Molten
-K
TABLE 2: Raman Frequencies (cm-1) for V
O
-M
S
2 2
O
/O
(g), V
-M
O
-M
-M SO
S
O
-M
O
O
2
5
7
2
2
5
2
2
7
2
4
2
2
5
2
S
2
7
2
Mixtures at 450 °C (M ) K or Cs)a
V
2
O -M S
5 2 2
O /O
7 2
(g)
V
2
O
5
2
S
2
O
7
2
4
/O
2
(g)
V
2
O
5
S
2 2
O
7
/SO
2
(g)
assignments for
assignments for
assignments for
V
4-
V
3-
IV
2-
2
M ) K
M ) Cs
(V O)
2
O(SO
4
)
4
M ) K
1168 w
M ) Cs
1166 w
V O
2
(SO )
4 2
M ) K
V O(SO )
4 2
SO
1
1
1
8
7
180 m,p
1175 m,p
1046 s,p
996 m,p
825 s,br,p
765 m,p
ν
3
(SO
4
)
ν
3
(SO
4
)
1170 w,p
135 w,p
1038 m,p
ν
ν
3
(SO
4
)
1
s
2
(SO )
050 s,p
000 m,p
30 s,br,p
70 m,p
ν(VdO)
1042 s,p
1037 s,p
ν(VdO)
of free SO
ν(VdO)
b
b
2-
4
9
68 s,p
961 s,p
ν
1
ν(S-Oterminal
)
940 s,p
938 s,p
ν(S-Oterminal
)
990 w,sh,p
65 m,p
935 m,sh,p
9
ν(S-Oterminal
)
ν(S-Obridging
)
∼880 sh,br,p
825 br
∼880 sh,br,p
ν(S-Obridging
)
ν(S-Obridging)
∼
ν(V-O-V)
6
90 w,p
668 m,p
618 vw,dp
600 w,dp
534 w,p
486 w,p
408 s,p
665 m,p
611 w,dp
598 w,dp
533 w,p
485 w,p
406 s,p
686 m,p
4 4
ν (SO )
ν(V-Oequat)
6
6
5
4
3
3
1
76 w,p
08 w,p
86 m,p
88 m,p
92 dp
05 s,p
95 sh, p
669 w,p
611 w,p
582 m,p
483 m,p
390 dp
302 s,p
194 sh,p
ν
4
(SO
4
)
ν
4
(SO
4
)
∼600 w,dp}
ν(V-Oequat
)
ν(V-Oequat)
ν
2
(SO
4
)
ν
2 4
(SO )
}
}
282 sh,dp
228 s,dp
279 sh,dp
227 s,dp
ν[(OdV)-Otr
]
ν[(OdV)-Otr]
a
b
Abbreviations: s ) strong; m ) medium; w ) weak; br ) broad; sh ) shoulder; v ) very; p ) polarized; dp ) depolarized. Seen in melts
2 4
saturated with M SO .
1
6
S-O distances and the structures of the K and Rb salts are
1
3,15,17
similar.
The S-O bonds involving oxygen not coordinated
to vanadium (terminal S-O bonds) are in the range 1.43-1.44
Å, whereas the S-O bonds involving oxygen coordinated to
vanadium (bridging S-O bonds) are unusually long (in the range
1
.52-1.58 Å) far from the usual value of 1.47 Å found for
-
1
sulfate groups. Furthermore, values of 968 and 961 cm are
2
-
found for the ν1(SO4 ) mode of free sulfate in K2S2O7 and
Cs2S2O7 melts, respectively (see Table 2 and Figure 6). It is
4
-
therefore evident that if the (VO)2O(SO4)4 ion is the dimer
complex formed in the melts then one shouldbe able to observe
at least two bands due to S-O modes in the Raman spectra
accounting for terminal and bridging S-O modes, which relative
-
1
2-
to the above value of 968 and 961 cm found for ν1(SO4 ),
should lie at higher and lower wavenumbers, respectively. Thus,
-
1
the band at 1000 cm in the K containing melts in Figure 4
-
1
(
seen at 996 cm in the Cs melts) is assigned to terminal S-O
-
1
/
0
V O
stretching, whereas the broad band centered at 830 cm (seen
at 825 cm in the Cs melts) is assigned to bridging S-O modes.
Figure 5. (a) Plot of I vs X . An approximate value of n ) 2 is
2
5
-1
0
found, corresponding to crossing at 0.33; (b) plot of I° vs X
various values of n. A value of n ) 2 is found.
for
V O
2
5
A large number of related configurations of bridging oxygen
can account for the broadness of the latter band. This variation
in S-O stretching frequencies is similar to that observed in
sion on band assignments and the results from the Raman study
of the V2O5-M2S2O7-M2SO4 molten mixtures supports this
interpretation.
-
3-
Raman spectra of VO2SO4 and VO2(SO4)2 complexes in
27
molten M2SO4-V2O5 (M ) K or Cs) mixtures and is
3
+
One should expect to observe the group vibrations of VO
analogous to the differences observed between terminal and
and SO42 units of the (VO)2O(SO4)4 dimer V complex
-
4-
V
26
bridging metal-halogen stretching frequencies. Moreover, the
-
1
formed according to reaction 8. The four sulfate fundamentals
band at 1180 and several bands in the 400-680 cm range
are assigned to split components of the ν2-ν4 fundamentals of
coordinated SO4 ligands, and similar assignments are made for
the Cs melts (see Table 2).
(
ν1-ν4) for a tetrahedral Td configuration span the following
representation:
Γvib ) A (ν ) + E(ν ) + 2F (ν + ν )
Turning our attention to V-O stretching modes and assuming
1
1
2
2
3
4
4-
octahedral coordination around V within (VO)2O(SO4)4 (as
26
16
-1
and are well-known from Raman work on aqueous solutions:
in the crystalline Cs4(VO)2O(SO4)4 ), we assign the 1050 cm
-
1
-1
-1
V
26
-1
ν1(A1) ≈ 980 cm , ν2(E) ≈ 450 cm , ν3(F2) ≈ 1100 cm ,
band to the V dO terminal mode and the 770 cm band to
V-O-V bridging. Indeed, according to an empirical correlation
-
1
and ν4(F2) ≈ 615 cm . However, coordination of the sulfate
ion and interactions with other ions are expected to shift the
bands, reduce the symmetry, and lift the degeneracies of the
ν2, ν3, and ν4 modes. This behavior has already been observed
in the Raman spectra of molten M2SO4-V2O5 (M ) K or Cs)
28
between V-O stretching frequencies and bond lengths, a value
-
1
of 765 cm (found in the Cs melts, see Table 2) corresponds
to a bond length of ca.1.74 Å, which is very close to the reported
value of 1.77 Å for V-O bond lengths along the V-O-V
2
7
16
mixtures.
bridge in crystalline Cs4(VO)2O(SO4)4. The discussion of the
Within Cs4(VO)2O(SO4)4, there are four crystallographically
different bidentate chelating SO4 groups with greatly different
Raman spectra of the V2O5-M2S2O7-M2SO4 melt, where
cleavage of the V-O-V bridge caused by alterations in sulfate

54
J. Phys. Chem. B, Vol. 106, No. 1, 2002
Boghosian et al.
Figure 7. (a-c) Raman spectra of the V
2
O
5
2 2 7
-Cs S O molten mixtures
saturated with Cs
2
SO
) 0.027, (b) X
4
at 450 °C in oxygen atmosphere (PO2 ) 0.2
0
0
0
V O
Figure 6. Titration-like series of Raman spectra of the V
2
O
5
-
atm); (a) X
) 0.066, (c) X
) 0.147. (d)
V O
V O
2
5
2
5
2
5
0
Cs
oxygen atmosphere (P 2
nm; laser power, 175 mW; scan speed, 60 cm min ; time constant,
S
2 2
O -Cs SO
7 2 4
molten mixtures (X
) 0.066) at 450 °C in
Raman spectra of molten Cs
Raman spectra of the V ‚Cs
conditions: λ ) 647.1 nm; laser power, 175 mW; scan speed, 60 cm
4
(VO)
2
O(SO
)
4 4
saturated with Cs
2 4
SO . (e)
V O
2
5
O
) 0.2 atm). Spectral conditions: λ
0
) 647.1
O
2 5
2
S O
2 7
‚2Cs
2
SO molten mixture. Spectral
4
-
1
-1
-1
0
-
1
-1
-1
0
.3 s (a); spectral slit width, 6 cm
.
min ; time constant, 0.3 s; spectral slit width, 6 cm .
coordination leads to elimination of this band, will provide
further confirmation to this assignment. The 586 and 488 cm-
bands are assigned to V-O modes along V-O-S chains in
the equatorial plane of the VO6 octahedra, in agreement with
the relative V-O distances along the V-O-S chains of Cs4-
-1
9
96 (S-Oterminal), 825 (S-Obridging), and 765 (V-O-V) cm
1
-1
bands and the appearance of the 938/961 cm doublet. Thus,
sulfate addition results in cleavage of the V-O-V bridge and
in formation of a V complex with sulfate ligands exhibiting
V
much less distortion as judged from the relative departure of
(
VO)2O(SO4)416 and with the empirical correlation between
-1
-1
the 938 cm band from the 961 cm “ideal” value.
28
V-O Raman stretching frequencies and bond lengths. Similar
assignments are done for the respective bands observed in the
Cs melts.
To examine the reaction of molten Cs4(VO)2O(SO4)4 with
Cs2SO4, the following procedure was adopted. Cs4(VO)2O(SO4)4
was synthesized by slow cooling of a V2O5-Cs2S2O7 melt with
Raman Spectra of the V2O5-M2S2O7-M2SO4/O2(g) Sys-
tems (M ) K or Cs). Addition of M2SO4 to the V2O5-M2S2O7
melts results, as judged from the Raman spectra, in a reaction
leading to alterations in SO4 coordination and in the structure
0
X
) 0.33 of which the Raman spectrum at 450 °C showed
V O
2
5
2
-
no bands due to S2O7 , indicating that the reaction was
completed. This compound was then mixed with excess Cs2-
SO4 and equilibrated at 450 °C. The Raman spectrum of the
resulting melt (Figure 7d) shows that the reaction proceeds with
the formation of S2O7 as indicated by the appearance of the
characteristic 1078, 725, and 313 cm pyrosulfate bands. The
V
of the V complexes. The reaction was followed by recording
0
V O
Raman spectra for V2O5-M2S2O7 samples with fixed X
in
2
5
2-
which various amounts of M2SO4 were added. A careful study
-
1
of the titration-like series of Raman spectra obtained (shown in
2
-
V
0
V O
above observations of the (i) 1:1 SO4 /V ratio of the number
of added SO4 moles reacting vs number of extant VV atoms
and (ii) cleavage of the V-O-V bridge and production of
Figure 6 for the V2O5-Cs2S2O7-Cs2SO4 system with X
)
2
5
4-
0
.066) shows that the (VO)2O(SO4)4 dimer reacts with the
added sulfate up to a SO4 /V ratio (ratio of number of added
SO4 moles vs the number of extant V atoms, denoted by Y in
2-
V
2
-
V
S2O7
upon sulfate addition can be accounted for by the
following reaction scheme only:
Figure 6) equal to 1. Actually, the spectra remain unchanged
beyond Y ) 1.40, and the extra 0.4 SO4 ions per vanadium
above Y ) 1 correspond to the solubility of Cs2SO4 in molten
4
-
2-
3-
2-
(VO) O(SO ) + 2SO4 f 2VO (SO ) + S O7
2
4 4
2
4 2
2
8
-1
Cs2S2O7, and the 961 cm band, seen for Y g 1.40, is due to
the ν1(A1) mode of free dissolved SO42 . Excess Cs2SO4 added
above Y ) 1.40 remains as a precipitate. The dependence of
Raman spectra for V2O5-Cs2S2O7 melts saturated with Cs2-
SO4 on the V2O5 content is shown in Figure 7a-c. The band at
(9)
-
Reactions 8 and 9 indicate that if all of the conclusions reached
and the hypotheses made up to this point are correct then the
constituents of V2O5‚M2S2O7‚2M2SO4 molten mixtures would
react at 450 °C stoichiometrically to produce molten M3VO2-
(SO4)2 without leaving excess SO4 or S2O7 ions. This is indeed
the case as shown for the Cs system in Figure 7e depicting the
Raman spectrum of the Cs3VO2(SO4)2 molten complex which
is formed according to the following reaction:
-
1
9
38 cm is due to coordinated SO4 and its relative intensity
increases with increasing V2O5 content. Similar results are
obtained for the V2O5-K2S2O7-K2SO4 system. The changes
observed upon addition of sulfate (i.e., with increasing values
of Y in Figure 6) are associated with the disappearance of the

Structure of Vanadium Oxosulfato Complexes
J. Phys. Chem. B, Vol. 106, No. 1, 2002 55
2
7
-
2-
3-
V O + S O + 2SO4 f 2VO (SO )
4 2
(10)
2
5
2
2
which is merely a combination of reactions 8 and 9.
Raman Spectra of the V2O5-K2S2O7/SO2(g) System. To
IV
gain insight into the structural properties of the V complexes,
Raman spectra for oxidized and reduced samples of the same
initial composition were obtained at 450 °C. Figure 8a shows
0
V O
Raman spectra for a 0.25 mol V2O5/L (i.e., X
) 0.03)
2
5
solution in K2S2O7 under oxygen (PO ) 0.2 atm). Figure 8b
shows spectra recorded for the same melt which has been
2
equilibrated under SO2 (PSO ) 0.8 atm) for 20 days, and its
2
color has gradually turned from dark brown (characteristic of
V
IV
V ) to dark green (characteristic for high content of V ). The
Raman spectra of molten K2S2O7 are also included in Figure
8
c.
The Raman spectrum of the reduced melt shown in Figure
b exhibits new (i.e., other than due to S2O72 ) bands at ∼600,
-
8
6
-
1
86, 935, 965, 990, 1038, 1135, and 1170 cm (see Table 2
for polarization characteristics and assignments). It is evident
from Figure 3 that, for PSO ) 0.8 atm, vanadium is almost
2
V
IV
completely reduced from V to V and furthermore that SO2
is partly coordinated to the VIV complex formed. A comparison
between spectra in Figure 8a of the oxidized (V ) melt and those
of Figure 8b of the reduced (V ) melt indicates a drastic change
V
Figure 8. Raman spectra of the molten mixture K
2
S
2
O
7
-V
2 5 0
O (c )
IV
-
3
0
.25 mol dm
V O ) at 450 °C equilibrated under partial PSO2 ) 0.2
2 5
of the structural properties of the complexes involved. The bands
2 2 7
atm (a) and PSO2 ) 0.8 atm (b). The spectrum of the pure molten K S O
-
1
at 830 and 1000 cm in spectrum a of Figure 8, which are due
at 450 °C is included for comparison (c). Spectral conditions: λ )
0
to bridging and terminal S-O stretches, are replaced by a triplet
647.1 nm (a and c), 514.5 nm (b); laser power, 175 mW (a and c), 160
-
1
mW (b); scan speed, 60 cm-1 min-1 (a and c), 90 cm-1 min-1 (b);
feature centered at 965 cm with shoulders at 935 and 990
-
1
-
1
time constant, 1 s; spectral slit width, 6 cm .
cm in spectrum b of Figure 8, thus indicating a much lower
IV
distortion of SO4 ligands participating in the V complex.
-
1
Furthermore the 770 cm band due to V-O-V bridging of
the (VO)2O(SO4)44 dimer V complex is no longer present,
-
V
IV
indicating that the V complex formed is monomeric in
agreement with the results of the VIS/NIR study.
IV
The SO4 modes of the V complexes (Figure 8b) are
identified at 600 and 686 (ν4 split components), 935 (bridging
S-O stretch along S-O-V chain), 965 (terminal S-O stretch),
-
1
IV
and 1170 cm (ν3). The V dO stretching is expected at 985
-
1 25,29
IV
(
50 cm .
One could assign to V dO stretching either
-
1
the 965 or the 1038 cm band. These two bands correspond
according to the V-O stretching frequency vs bond length
28
correlation to V-O distances of 1.61 and 1.58 Å, respectively.
Significantly, the V-O distances between vanadium and the
short bonded oxygen in the apical direction within VO6
octahedra reported for VIV oxosulfato compounds isolated from
V2O5-M2S2O7 (M ) Na or K) melts are in the range 1.580-
.598 Å.3
0-32
Therefore, it is likely that the 1038 cm band is
-1
1
IV
-1
due to V dO stretching and the 965 cm due to terminal S-O
-
1
stretch. Finally, the weak band at 1135 cm can be assigned
to the symmetric stretching mode of SO2 coordinated to the
IV
26
V
2 4 4
Figure 9. Possible structural models for (a) (V O) O(SO )
4-, (b)
V
through an oxygen atom, probably trans to the short-
V
3-
IV
2-
2
-
2 4 2 4 2
V O (SO ) in monomeric and polymeric form, and (c) V O(SO )
bonded apical vanadyl oxygen of the VO(SO4)2SO2 complex
formed according to reaction 4. Such a configuration is found
also in crystalline VOSO4SO2-type compounds isolated from
catalyst melts of relevant composition.19
IV
2-
and V O(SO
4
)
2
SO
2
.
with high V O content, whereby the viscosity and the glass-
2
5
forming ability of the melts increases, pointing to the existence
of chainlike or network-like groups of ions.
Structural Models. The preceding discussion has revealed
that the complex units formed are (i) the V dimer (VO)2O-
V
Figure 9 depicts the possible structures of the above-
-
IV
4-
(
SO4)44 in V2O5-M2S2O7 (M ) K, Cs) melts, (ii) the V
mentioned vanadium oxosulfato complexes. (VO)2O(SO4)4 is
2-
2-
monomers VO(SO4)2 and VO(SO4)2SO2 in V2O5-K2S2O7
melts under SO2 atmosphere, and (iii) the V monomer
VO2(SO4)2 in V2O5-M2S2O7-M2SO4 (M ) K, Cs) melts
under O2 atmosphere. These units do not occur necessarily in
isolated “free” form but may participate in oligomeric or
polymeric chainlike complex ions. This is pronounced in melts
shown in Figure 9a with the same configuration found for the
V
16,17
M4(VO)2O(SO4)4 (M ) Cs, K, Rb) compounds.
The
3
-
IV
structural models for the V complexes are shown in Figure
2-
2+
9c. VO(SO4)2 can be formulated from the vanadyl ion, VO ,
and two bidentate chelating sulfate groups, giving rise to an
apparent 5-fold coordination for vanadium. However, it is

56
J. Phys. Chem. B, Vol. 106, No. 1, 2002
Boghosian et al.
likely that one of the terminal sulfate oxygens lies trans to the
vanadyl oxygen (in loose contact) by satisfying partly the
preferential 6-fold coordination for vanadium. SO2 may coor-
dinate in this oxygen-deficient site through an O atom leading
to VO(SO4)2SO22 as shown in Figure 9c. Figure 9b shows the
of Research and Technology of the Greek Ministry of Develop-
ment, the Danish Natural Science Research Council and ICAT
(Interdisciplinary Research Center for Catalysis) is gratefully
acknowledged. Flemming Borup, Andreas Paulsen and Morten
Jorgensen (visiting scientists in FORTH/ICE-HT) are thanked
for their help in some of the experiments. The authors are
indebted to Professor G. N. Papatheodorou for his support and
valuable comments during the course of the investigation.
-
3-
structural model for the VO2(SO4)2 complex, formulated from
the dioxovandium ion and two bidentate chelating SO4 groups.
Figure 9b shows also, as an example, an oligomeric chain
-
consisting of this unit, in which VO2SO4 units are linked by
bidentate bridging SO4 groups and the six-coordinated vanadium
is found in the usual octahedral environment.
References and Notes
(
1) Villadsen, J.; Livbjerg, H. Catal. ReV.-Sci. Eng. 1978, 17, 203.
(
2) Masters, S. G.; Chrissanthopoulos, A.; Eriksen, K. M.; Boghosian,
Conclusions
S.; Fehrmann, R. J. Catal. 1997, 166, 16.
3) Karydis, D. A.; Eriksen, K. M.; Fehrmann, R.; Boghosian, S. J.
Chem. Soc., Dalton Trans. 1994, 2151.
(4) Lapina, O. B.; Bal’zhinimaev, B. S.; Boghosian, S.; Eriksen, K.
M.; Fehrmann, R. Catal. Today 1999, 51, 469.
(
The combination of VIS/NIR and Raman spectroscopic
investigations at 450 °C show that a redox equilibrium between
V
4-
IV
2-
dimeric V , (VO)2O(SO4)4 , and monomeric V , VO(SO4)2
,
(
5) Hansen, N. H.; Fehrmann, R.; Bjerrum, N. J. Inorg. Chem. 1982,
1, 744.
(6) Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. Inorg. Chem. 1986,
5, 1132.
(7) Hatem, G.; Fehrmann, R.; Gaune-Escard, M.; Bjerrum, N. J. J. Phys.
Chem. 1987, 91, 195.
8) Folkmann, G.; Hatem, G.; Fehrmann, R.; Gaune-Escard, M.;
Bjerrum, N. J. Inorg. Chem. 1993, 32, 1559.
9) Lapina, O. B.; Mastikhin, V. M.; Shubin, A. A.; Eriksen, K. M.;
Fehrmann, R. J. Mol. Catal. A: Chem. 1995, 99, 123.
10) Petrushina, I.; Bjerrum, N. J.; Cappeln, F. J. Electrochem. Soc. 1998,
145, 3721.
(11) Boghosian, S.; Borup, F.; Chrissanthopoulos, A. Catal. Lett. 1997,
48, 145.
is established in V2O5-K2S2O7/SO2(g) melts. A quantitative
treatment of the Raman band intensities and the Raman spectral
features obtained from V2O5-M2S2O7/O2(g) (M ) K or Cs)
2
2
point to (VO)2O(SO4)44 as the V complex formed. Bands due
-
V
to VdO, V-O-V, and coordinated SO4 groups were observed
and assigned in the Raman spectra. The study covers the
composition of industrial SO2 oxidation catalysts used for
sulfuric acid production and SO2 removal from flue gases. Thus,
it can be concluded that the active molten phase of these
(
(
(
4-
catalysts is dominated by the (VO)2O(SO4)4 complex.
V
The VIS/NIR results show that the V complex, (VO)2O-
(
12.
12) Karydis, D. A.; Boghosian, S.; Fehrmann, R. J. Catal. 1994, 145,
-
(
SO4)44 , does not coordinate SO2 under the studied conditions
3
IV
but is reduced to monomeric V . It seems therefore likely that,
as discussed recently, O2 and not SO2 is activated in the
coordination sphere of the dimeric V complex prior to reaction
with SO2 as the initial step of the catalytic cycle for the reaction
(13) Folkmann, G.; Eriksen, K. M.; Fehrmann, R.; Gaune-Escard, M.;
4
Hatem, G.; Lapina, O. B.; Terskikh, V. J. Phys. Chem B 1998, 102, 24.
(14) Folkmann, G.; Hatem, G.; Fehrmann, R.; Gaune-Escard, M.;
Bjerrum, N. J. Inorg. Chem. 1991, 30, 4057.
V
(15) Abdoun, F.; Hatem, G.; Gaune-Escard, M.; Eriksen, K. M.;
4
2
SO2 + O2 f 2SO3. The proposed catalytic cycle involves
Fehrmann, R. J. Phys. Chem B 1999, 103, 3559.
V
(16) Nielsen, K.; Fehrmann, R.; Eriksen, K. M. Inorg. Chem. 1993, 32,
only V species (dimeric or binuclear fragments of larger
oligomers). The bidentate sulfate ligands found within the
4
825.
17) Nielsen, K.; Eriksen, K. M.; Fehrmann, R. Manuscript in prepara-
tion.
(18) Boghosian, S.; Fehrmann, R.; Bjerrum, N. J.; Papatheodorou, G.
N. J. Catal. 1989, 119, 121.
19) Eriksen, K. M.; Karydis, D. A.; Boghosian, S.; Fehrmann, R. J.
(
4-
dimeric (VO)2O(SO4)4 complex, Figure 9a, are strained due
to their unusually large distortion and some of them may open
up, making the complex coordinatively unsaturated in the
horizontal positions and leading to the suggested coordination
of O2 as the first step of the catalytic cycle [(VO)2O(SO4)4
O2 T (VO)2O(SO4)4O2 ]. Coordination of SO2 to form the
(
Catal. 1995, 155, 32.
4
-
+
(20) Karydis, D. A., Ph.D Thesis, University of Patras, Greece, 1994.
(21) Zavida, D.; Munk, N.; Marcussen, E. S.; Livbjerg, H. Fluid Phase
Equil. 1990, 55, 319.
4
- 4
IV
2-
monomeric V complex, VO(SO4)2SO2 , is only important
at rather high partial pressures of SO2, much above the usual
range for industrial SO2 oxidation. The changes in the Raman
spectra upon interaction of V2O5-K2S2O7 with SO2 or upon
addition of M2SO4 to V2O5-M2S2O7 mixtures point to cleavage
(22) Papatheodorou, G. N. In Proceedings of the 10th Materials Research
Symposium on Characterization of High-Temperature Vapors and Gases;
Hastie, J. W., Ed.; NBS Publication 561, National Bureau of Standards:
Washington, DC, 1979; p 647.
(
23) Rasmussen, S. B.; Eriksen, K. M.; Fehrmann, R. J. Phys. Chem. B
1
999, 103, 11282.
IV
2-
of the V-O-V bridge and formation of the V O(SO4)2 or
(24) Fehrmann, R.; Krebs, B.; Papatheodorou, G. N.; Berg, R. W.;
Bjerrum, N. J. Inorg. Chem. 1986, 25, 1571.
V
3-
V O2(SO4)2 monomeric complexes, respectively. Significant
presence of sulfate can be encountered in industrial catalysts
under certain conditions (e.g., at the inlet to the first bed where
the high temperature, exceeding sometimes 600 °C, and the low
(
(
25) Boghosian, S.; Berg, R. W. Appl. Spectrosc. 1999, 53, 565.
26) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th ed.; Wiley-Interscience: New York, 1986;
pp 140, 248, 329, 344.
(27) Boghosian, S. J. Chem. Soc., Faraday Trans. 1998, 94, 3463.
(28) Hardcastle, F. D.; Wachs, I. E. J. Phys. Chem. 1991, 95, 5031.
2-
conversion of SO2 may shift the equilibrium SO4 + SO3 T
S2O72- to the left). The reported structural properties of the
(
29) Selbin, J. Coord. Chem. ReV. 1966, 1, 93; Chem. ReV. 1965, 65,
53.
(30) Fehrmann, R.; Boghosian, S.; Papatheodorou, G. N.; Nielsen, K.;
Berg, R. W.; Bjerrum, N. J. Inorg. Chem. 1989, 28, 1847.
31) Fehrmann, R.; Boghosian, S.; Papatheodorou, G. N.; Nielsen, K.;
Berg, R. W.; Bjerrum, N. J. Inorg. Chem. 1990, 29, 3294.
32) Boghosian, S.; Eriksen, K. M.; Fehrmann, R.; Nielsen, K. Acta
Chem. Scand. 1995, 49, 703.
vanadium oxosulfato complexes are very useful for understand-
ing the mechanism of the SO2 oxidation at the molecular level.
1
(
Acknowledgment. NATOs Scientific Affairs Division in the
framework of the Science for Peace Program (SfP 971984) has
sponsored this research. Support from the General Secretariat
(