WO3

1
Raman Spectroscopy evidence of 111 complex
formation during dissolution of WO3 K2SO4 in a
melt of K2S2O7 Rolf W. Berg 1 and Irene Maijó
Ferré 2 1Department of Chemistry, The Technical
University of Denmark, Kemitorvet, B. 207,
DK-2800 Lyngby, Denmark 2Facultat de Química,
Universitat Rovira i Virgili, Tarragona 43007,
Spain

Email irene.maijo_at_estudiants.urv.es
Emailrwb_at_kemi.dtu.dk
PURPOSE. The purpose of the present project was
to apply the Raman instrumentation to
characterize the products formed by the reaction
WO3 K2SO4 in melts of K2S2O7. Tungsten trioxide
(WO3, 1314-35-8) is known as a highly inert
solid, practically insoluble in acids. We have
discovered that WO3 can be dissolved in
considerable amounts in acidic sulfate melts at
high temperatures. We therefore decided to mix
WO3, K2SO4 and K2S2O7 in varying molar amounts
(Fig. 3) in sealed ampoules (K2S2O7 is
hygroscopic and needs to be handled in a dry
box). The ampoules were heated for equilibration
in a rocking furnace at 600 C for 1 hr. Raman
spectra of the molten solutions (Fig. 4) can be
interpreted to show that the WO22 ions formed
are solvated by SO42- ions. From this, the
stoichiometry of the reaction could be determined
fairly accurately as 111, or WO3 K2SO4
K2S2O7 ? products, by a method described
elsewhere 1. We have only indication of one
kind of complex, a dimeric K8?(WO2)2(?-SO4)2(SO4)4
?. Single crystals of this compound,
1WO31K2S2O71K2SO4, compound have recently been
obtained, and the X-ray structures solved 2, to
show that the dimeric compound exists from at
least 5 to 23 mol .
EXPERIMENTAL Raman equipment Raman spectra were
obtained by use of a DILOR XY 800 mm focal-length
multi-channel spectrometer (Fig.1) with macro-
and micro- entrance, Ar-ion laser excitation
(514.5 nm, ?400 mW, vertically polarized), and
140 K CCD detection. Filtration of Rayleigh
scattering was done with a Kaiser holographic
SuperNotch-Plus filter or with the double
pre-monochromator. The Raman spectral resolution
was within 2 to 10 cm-1. Temperature control was
achieved in a 4-window home-made electric furnace
with PID regulation. A sheet analyzer, permitting
vertically or horizontally polarized light to
pass, was used to obtain polarization data.
Fig. 2. Frozen melts in ampoules at 25 oC.
Furnace
Samples When samples were cooled slowly, melts
froze to microcrystalline lumps (for low WO3
contents) and clear glasses (for higher WO3
contents). Fig. 2 shows three samples, at the
top 3 WO3 3 K2SO4, in the middle 17 , at
the bottom 29 WO3 29 K2SO4.
Fig. 1. Schematics of the Raman DILOR-XY
spectrometer.
RESULS AND DISCUSSION
Vibrational Raman Spectra (Fig. 4) showed that
the characteristic bands of K2S2O7 gradually
disappeared when the tungsten trioxide content
was increased. At the same time, new bands
appeared monotonically, indicating that only one
reaction has occurred. The positions of the
fundamental bands of a regular SO42- ion of Td
symmetry are well known ?1 983, ?2 450,
?31105 and ?4 611 cm-1. The symmetry of the
SO42- ion is lowered by complex formation (Fig.
5), and two different approximate symmetries for
the sulfate ion are observed C3? (bound
unidentate) and C2? (bound bridged bidentate)
3. We see increasing bands of ?1(C3?) appearing
at 925 and ?1(C2v) at 965 cm-1. The WO22 ion
has three vibrational degrees of freedom. Under
C2v point group symmetry the fundamentals are
expected 4 at ?1 972, ?2 300 and ?3 928
cm-1. We observed an appearance of two of these
bands, ?1 1045 and ?2 390 cm-1, ?3 being
probably weak and broad.
111
Fig 5. Plot of the anion, showing 50
probability ellipsoids. The 8 K ions have been
omitted for clarity.
Fig 3. Ternary diagram of the molar fractions.
Percentage of the three reactants.
It is seen that the 1077 cm-1 band of K2S2O7
disappears and the new 965 cm-1 band of the
mixture (new compound) increases with the WO3
percentage. The area ratio between these two
bands (above the background) is shown for each
melt composition (Fig. 6). We can observe the
decrease of the area ratio with the increase of
the WO3 concentration
Structure. In Fig. 5 the WVI coordination sphere
is shown 2. Two independent WVIO2(SO4)3
"monomer" moieties by inversion symmetry result
in a "dimer" with two bridging sulfato ligands.
The unit cell has two such dimeric units 2. The
(?-SO4)2 way of linking of MO22 centers (M
metal) has previously been seen also for
molybdenum where polymeric strands occurred by
repetition of the linking in K2MoO2(SO4)2 5.
Here in Mo-containing melts, bands assignable to
?1 (C2v) in bridging sulfato groups were seen at
958 cm-1 5.
Fig 6. Plot of the area ratios of the two bands
(?disulfate 1086 cm-1, ?compound 965 cm-1)
versus composition.
Conclusions. By means of Raman spectroscopy and
previous X-rays work 2, it has been possible to
determine and confirm the stoichiometry of the
reaction as 111. The resulting complex is a
dimer with two bridging sulfate ligands and four
terminally bound sulfato groups.
Fig 4. Raman spectra of melts of different
compositions (X formal mole fraction of WO3
in the WO3-K2SO4-2S2O7 system at 600 oC.
References 1 Boghosian, S. Borup, F. Berg R.
W., "Stoichiometry, Vibrational Modes and
Structure of Molten Nb2O5-K2S2O7 Mixtures". Proc.
11th Intl. Symp. on Molten Salts, The
Electrochem. Soc., Pennington, N.J., 98-11,
536-543 (1998). 2 . Cline Schäffer, S. J. and
R.W. Berg, "Potassium bis(?-sulfato-1?O2?O)bis-
cis-dioxido-cis-bis(sulfato-?O)tungstate(VI)",
Acta Cryst. E 61, i49-i51 (2005). 3 Nakamoto,
K., "Infrared and Raman Spectra of Inorganic and
Coordination Coumpounds", Part B "Applications
in Coordination, Organometallic, and Bioinorganic
Chemistry", John Wiley Sons, Inc, New York,
1997. 4 Weltner, Jr., W. McLeod, Jr., D.
"Spectroscopy of Tungsten oxide molecules in neon
and argon matrices at 4 and 20K" , J. Mol.
Spectrosc., 17, 276-299 (1965). 5 Nørbygaard,
T. Berg, R.W. Nielsen, K., "Reaction between
MoO3 and Molten K2S2O7, Raman Spectra and Crystal
Structure". Proc. 11th Intl. Symp. on Molten
Salts, The Electrochem. Soc., Pennington, N.J.,
98-11, 553-573 (1998).
Poster presented at ICAVS-3, August 14-19, 2005,
Delavan, Wisconsin, USA