Conductivity, NMR, Thermal Measurements, and Phase Diagram of the K
2
S
2
O
7
-KHSO
4
System
K. M. Eriksen,
²
R. Fehrmann,*
,²
G. Hatem,
‡
M. Gaune-Escard,
‡
O. B. Lapina,
§
and
V. M. Mastikhin
§
Chemistry Department A, Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark, Institut UniVersitaire
des Syste ` mes Thermiques Industriels, UniVersite ´ de ProVence, Centre de Saint Je ´ ro ˆ me, AVenue Escadrille
Normandie Niemen, 13397 Marseille Cedex 20, France, and BoreskoV Institute of Catalysis,
630090 NoVosibirsk, Russia
ReceiVed: December 18, 1995; In Final Form: April 1, 1996
X
The phase diagram of the catalytically important K
2
S
2
O
7
-KHSO
4
solvent system has been investigated by
means of electrochemical, thermal, and spectroscopic methods. The phase diagram exhibits a eutectic at
KHSO
4
) 0.94(1) with a temperature of fusion of 205 °C. No compound is formed in the system, but the
strong R f solid-solid transition of K
2
S
2
O
7
, found at 318 °C with ∆H
tr
) 21.8 kJ/mol, gives rise to a
marked change in the slope of the liquidus curve at this temperature. The experimental phase diagram is in
very good accordance with a calculated diagram based on the assumption of an ideal liquid mixture.
39
K,
1
H,
17
O, and
33
S NMR measurements on the molten K
2
S
2
O
7
-KHSO
4
mixtures up to 540 °C show that a fast
ionic exchange takes place in the melt at all compositions. The conductivities of the solid and molten K
2
S
2
O
7
-
KHSO
4
systems were measured at 13 different compositions in the whole composition range,
KHSO
4
) 0-1.
For each composition in the temperature range examined , the conductivity of the molten mixtures has been
expressed by equations of the form κ ) A() + B()(T - 600) + C()(T - 600)
2
. The measurements
indicate an enhanced molar conductivity of the binary system, probably due to delocalization of the conducting
ions compared to the pure molten components.
Introduction
The molten salt-gas system K
2
S
2
O
7
/KHSO
4
/V
2
O
5
-SO
2
/O
2
/
SO
3
/CO
2
/H
2
O/N
2
at around 400 °C is considered a realistic
model of the catalyst used for the oxidation of SO
2
to SO
3
by
O
2
in a novel “wet” process, developed for the purification of
flue gases. The chemistry of the “dry” system, i.e., K
2
S
2
O
7
/
V
2
O
5
-SO
2
/O
2
/SO
3
/N
2
, reflecting the traditional sulfuric acid
catalyst, has previously been explored in detail by us.
1-5
In
both catalytic processes, the active component is the vanadium
complex formed in the molten K
2
S
2
O
7
or in the molten K
2
S
2
O
7
-
KHSO
4
system, respectively. Thus, detailed information about
the molten K
2
S
2
O
7
-KHSO
4
solvent, including the species
formed, their structure, the equilibria governing the melt, and
fundamental physicochemical data, seems important for the
understanding of the working catalyst for flue gas cleaning. Such
investigations have been undertaken by us by means of
spectroscopic, thermal, and electrochemical methods. Previous
6
Raman and NIR spectroscopic investigations on the molten
K
2
S
2
O
7
-KHSO
4
have shown that the dominant species most
probably present are S
2
O
7
2-
, HSO
4
-
, and H
2
O. The H
2
O
molecules seem to be strongly associated by hydrogen bonds
to the other species of the melt, even at 450 °C. The vapor
pressure of H
2
O appears to control the position of the water
sensitive equilibrium 2HSO
4
-
a S
2
O
7
2-
+ H
2
O and, thus, the
composition of the melt.
The work presented here concerns a reinvestigation of the
equilibrium phase diagram of the binary system K
2
S
2
O
7
-
KHSO
4
. Our work has proven marked discrepancies from the
diagrams previously published 60-70 years ago.
7,8
Knowledge
of this phase diagram may be useful for the design of new low-
melting catalysts that are able to operate in the desired
temperature range below 400 °C. Four different methods of
investigation have been applied for this study, i.e., electrical
conductivity, differential enthalpic analysis (DEA), multinuclear
NMR, and NIR spectrophotometry at temperatures up to 540°C.
These investigations were also carried out to give additional
information about the species present in the molten catalyst
solvent.
Experimental Section
Chemicals. The hygroscopic K
2
S
2
O
7
used was obtained by
thermal decomposition of K
2
S
2
O
8
(Merck, pa) and was kept in
sealed ampules until use in the dryboxes, as previously
described.
9
The KHSO
4
used for additions (Merck, Suprapur,
99%) was dried at 110 °C and stored in the drybox. By this
procedure, the hygroscopic KHSO
4
typically lost mass corre-
sponding to 15 mol % of H
2
O.
Conductivity Measurements. The borosilicate glass cell,
with gold electrodes used for measuring the electrical conduc-
tivity, has been described in detail previously.
10
The cell was
filled in the drybox, sealed, placed in the measuring furnace,
and regulated to within (0.1 °C, as previously described.
11
The
composition of the melt was varied by the addition of chemicals
(KHSO
4
or K
2
S
2
O
7
) to the cell in the drybox by cutting the
stem open and resealing it again under nitrogen or in a vacuum.
The mixture was mixed well manually by shaking the cell holder
for a short while outside the furnace. The resistance of the cell
was measured when it was constant. The temperature was
lowered in steps of 2-10 °C, and in several cases subcooling
was observed, indicated by a sudden jump in the resistance to
a much higher value when crystallization occurred. Thereafter,
the temperature was gradually raised until the resistance
coincided with the previous measurements in the liquid region.
The temperature was measured by a calibrated Pt(100) platinum
²
Technical University of Denmark.
‡
Universite ´ de Provence.
§
Boreskov Institute of Catalysis.
X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
10771 J. Phys. Chem. 1996, 100, 10771-10778
S0022-3654(95)03744-0 CCC: $12.00 © 1996 American Chemical Society