Chemical and electrochemical behavior of gallium in the room-temperature ionic liquid of the composition [C6H11N 2][N(SO2CF3)2]

Article abstract of DOI:10.1023/A:1026220211491

The behavior of gallium trichloride in the room-temperature ionic liquid of the composition [C₆H₁₁N₂][N(SO ₂CF₃)₂] at 298-308 K in an inert gas atmosphere was studied by linear and cyclic v

Full text of DOI:10.1023/A:1026220211491

Radiochemistry, Vol. 45, No. 5, 2003, pp. 499 502. Translated from Radiokhimiya, Vol. 45, No. 5, 2003, pp. 449 452.
Original Russian Text Copyright 2003 by Smolenskii, Bove, Khokhryakov, Osipenko.
Chemical and Electrochemical Behavior of Gallium
in the Room-Temperature Ionic Liquid of the Composition
[C6H11N2][N(SO2CF3)2]
V. V. Smolenskii*, A. L. Bove*, A. A. Khokhryakov**, and A. G. Osipenko***
* Institute of High-Temperature Electrochemistry, Ural Division, Russian Academy of Sciences,
Yekaterinburg, Russia
** Institute of Metallurgy, Ural Division, Russian Academy of Sciences, Yekaterinburg, Russia
*** Research Institute of Atomic Reactors, Russian Federation State Scientific Center, Dimitrovgrad,
Ul’yanovsk oblast, Russia
Received September 26, 2002
Abstract The behavior of gallium trichloride in the room-temperature ionic liquid of the composition
[C6H11N2][N(SO2CF3)2] at 298 308 K in an inert gas atmosphere was studied by linear and cyclic voltam-
metry and spectrophotometry. The reduction and oxidation potentials of gallium were determined. Assump-
tions concerning the composition of the gallium complex species were made.
Weapons plutonium contains up to 5% of gallium
which is an undesirable impurity in energy reactor
fuel. Therefore, conversion of weapons plutonium into
MOX fuel implies exhaustive removal of gallium [1].
studied by linear and cyclic voltammetry and IR spec-
troscopy, as well as by registration of changes in
the potential of the working electrode after its brief
polarization.
Room-temperature ionic liquids are promising
media for preparation, separation, and decontamina-
tion of a number of elements and compounds. The
reason is that a low temperature lifts many limitations
due to, above all, corrosivity of melts.
All the manipulations, including assembling the
electrochemical and spectrophotometric cells, were
carried out in a dry glove box in an argon atmosphere.
Argon was continuously freed from oxygen-contain-
ing impurities by forced circulation through a column
packed with zirconium metal chips heated to 1000 K.
Argon circulation was effected by a membrane pump
placed directly inside the glove box.
In this work, we used as solvent the [C H N ]
6
11 2
[N(SO CF ) ] (EtMeIm Tf N) ionic liquid offering
2
3 2
2
all the advantages characteristic of this class of
compounds. In particular, it is thermally stable to
670 K, melts at 270 K, and exhibits certain hydro-
phobicity, enhanced (for its class) conductivity ( 1.3
As solvent served the room-temperature ionic liquid
[C H N ][N(SO CF ) ] (EtMeIm Tf N) which was
6
11
2
2
3 2
2
synthesized, purified, and supplied by the Los Alamos
National Laboratory (the United States). Prior to use,
it was treated additionally for 12 h at the residual
2
1
10 S cm ), and a wide range of electrochemical
inertness ( 5 V) [2 7].
3
There are published data on the electrochemical
behavior of gallium compounds in room-temperature
ionic liquids, e.g., in 1-butylpyridinium chloride and
1-methyl-3-ethylimidazolinium chloride [7 9]. How-
ever, the results of these studies do not suggest un-
ambiguously the reduction mechanism for gallium.
pressure of 10 atm.
Gallium trichloride was prepared by chlorination of
the metal (99.99%) in a flow of chlorine prepared by
electrolysis of lead chloride. Chlorination was carried
out at 480 K in a Pyrex reactor. The GaCl vapor was
3
transported by a chlorine flow to the water-cooled sec-
tion of the reactor, and excess chlorine left the reactor
via a hydroseal filled with concentrated sulfuric acid.
The product was stored in sealed ampules. Gallium
trichloride solutions of required concentration were
prepared by mixing the preliminarily prepared equi-
molar EtMeIm Tf N GaCl solution with the straight
In this study, we comprehensively studied the
behavior of gallium compounds in a room-temperature
ionic liquid, 1-ethyl-3-methylimidazolinium bis[(tri-
fluoromethyl)sulfonyl]amide (EtMeIm Tf N).
2
EXPERIMENTAL
2
3
solvent.
The chemical and electrochemical behavior of gal-
lium in the room-temperature ionic liquid (RTIL) was
Electrochemical studies were carried out in a quartz
1066-3622/03/4505-0499$25.00 2003 MAIK Nauka/Interperiodica

500
SMOLENSKII et al.
stainless-steel beaker hermetically mounted into the
glove box bottom, which was the external part of the
temperature-control system.
The electrolyte under study was placed into a
container made of SU-2000 glassy carbon which
simultaneously served as counterelectrode. As work-
ing electrode served a face electrode made of VRN
2
metallic tungsten. It had an area of 0.1237 cm . As
reference electrode served VRN metallic tungsten.
The inert gas inside the cell was additionally freed
from oxygen-containing impurities by circulation for
several hours through the zirconium-packed gas-puri-
fication system using a ZALIMP PP-10-5A peristaltic
pump.
Fig. 1. Cyclic voltammogram of the C H
melt. Temperature 308 K; tungsten as working electrode,
N N(SO CF )
2 2 3 2
6
11
metallic tungsten as reference electrode, and glassy carbon
We used a PI-50-1 standard potentiostat equipped
with a PR-8 programmer and recorded the electro-
chemical responses from the system using an S9-8
digital storage oscillograph linked to a PC by a com-
munication line. The scanning rate of the potential
1
as counterelectrode; potential scanning rate 0.1 V s
C H N /N(SO CF ) = 1.
;
6
11
2
2
3 2
1
was varied from 0.040 to 10 V s . The experiments
were run at 298 308 K.
The IR spectra of gallium trichloride solutions in
the EtMeIm Tf N ionic liquid were recorded on a
2
1
Specord M-80 spectrophotometer at 4000 200 cm
at room temperature. The resolution of the spectro-
1
photometer was no less than 1 cm . The IR spectra
were recorded in cells with CsI windows. The liquid
layer was 0.1 mm thick.
Fig. 2. Cyclic voltammogram of the [C H
N N(SO
2 2
6
11
CF ) ] GaCl melt. GaCl concentration 0.95 mol %; all
3 2
3
3
RESULTS AND DISCUSSION
other conditions, as in Fig. 1.
Voltammetric Studies
Figure 1 shows the cyclic voltammogram of the
EtMeIm Tf N room-temperature ionic liquid at
2
308 K. At potentials from 3.0 to 3.0 V vs. tungsten
reference electrode, there were no visible peaks, which
suggests a wide range of electrochemical inertness and
makes this electrolyte promising as solvent. Upon
introducing gallium trichloride into solution (Fig. 2),
new peaks of current appeared at 1.7 V in the cath-
odic branch of the curve and at 0.2 V in the anodic
branch, which can be due to gallium reduction and
oxidation. Anodic polarization of the gallium-contain-
ing melt (Fig. 3) at potentials from 0.5 to 1.5 V
gives no visible peaks, which suggests that gallium in
the melt occurs in the trivalent state only.
Fig. 3. Anodic polarization of the [C H
N N(SO CF ) ]
2 2 3 2
6
11
1
GaCl melt. Potential scanning rate 0.025 V s ; all other
3
conditions, as in Fig. 2.
cell with a ground quartz cup equipped with tubes for
introducing the electrodes. The cell was hermetically
isolated by passing the electrodes through vacuum
rubber plugs preliminarily fitted over the tubes. Also,
the cell had two tubes for connection to an additional
gas purification system. The cell was placed into a
The potential time dependences recorded after
brief polarization of the working electrode showed
(Fig. 4) that, at cathodic polarizations more negative
than 1.7 V, a plateau appeared at a potential of
0.75 V. At the same time, special experiments
RADIOCHEMISTRY Vol. 45 No. 5 2003

CHEMICAL AND ELECTROCHEMICAL BEHAVIOR OF GALLIUM
501
showed that the quasiequilibrium potentials of metal-
lic gallium vs. tungsten reference electrode are also
approximately equal to 0.75 V.
Absorption band maxima of the EtMeIm Tf2N ionic liquid
1
Vibration frequencies, cm
Assignment
Our results showed that gallium is reduced in the
room-temperature ionic liquid of the composition
EtMeIm Tf N 0.95 mol % GaCl at 1.7 V, and its
EtMeIm
Tf2N
2
3
3160
3124
2992
1574
1465
1450
(CH) (ring)
(CH) (ring)
(CH)
s(ring)
s(ring)
quasiequilibrium potential is equal to 0.75 V.
Spectrophotometric Studies
s(ring)
The IR absorption spectra of the EtMeIm Tf N
2
1350
1330
1190
as(SO2)
as(SO2)
as(CF3)
room-temperature ionic liquid with and without gal-
lium trichloride additions are shown in Figs. 5 and 6,
respectively, and the absorption band maxima are
listed in the table.
1140
(ring)
s
1060
as(SNS)
(CH) (ring)
(CH) (ring)
as(ring)
as(ring)
as(SO2)
as(SO2)
as(CF3)
(SO2)
The vibrational spectra of EtMeIm (C H N ) and
6
11 2
840
790
650
628
Tf N [N(SO CF ) ] and their structural models were
2
2
3 2
discussed in detail in [10 13]. Figure 7 presents
schematically the structures of these ions. Comparison
of the experimental results reported in these works
with our data (see table) shows good agreement of the
vibration frequencies. The mutual influence of the
618
572
514
408
359
290
+
EtMeIm and Tf N fragments on the fundamental
2
(SO2)
(CF3)
Fig. 4. Potential time dependence recorded after brief
polarization of the working electrode in the [C H
N
6
11
2
N(SO CF ) ] GaCl melt. Polarization potential 2.7 V;
2
3 2
3
polarization time 180 s; all other conditions, as in Fig. 2.
Fig. 6. IR spectrum of gallium trichloride in [C H N ]
6
11 2
[N(SO CF ) ]. Temperature 298 K. GaCl concentration,
2
3 2
3
mol %: (1) 15.7, (2) 20.8, and (3) 50.2.
O
O
N
N
CF3
S
F3C
H3C
CH2CH3
S
N
O
O
Fig. 5. IR absorption spectrum of the [C H N ]
6
11 2
[N(SO CF ) ] ionic liquid. Temperature 298 K.
Fig. 7. Structural model of the EtMeIm Tf N ionic liquid.
2
2
3 2
RADIOCHEMISTRY Vol. 45 No. 5 2003

502
SMOLENSKII et al.
Thus, the IR absorption spectra of the solutions of
gallium trichloride in the EtMeIm Tf N ionic liquid
2
suggest that the Tf N ion is coordinated via one of
2
2
the oxygen atoms of the SO group, yielding GaOCl
2
3
heterocomplex with the C site symmetry.
3v
Our studies allowed certain suggestions concerning
the ionic composition and structure of RTIL and the
mechanism of the cathodic processes occurring during
electrolysis of the EtMeIm Tf N melt with gallium
2
additions.
Fig. 8. Difference spectrum obtained by subtracting the
ACKNOWLEDGMENTS
spectrum of the EtMeIm Tf N ionic liquid from that of the
2
EtMeIm Tf N GaCl solution. Temperature 298 K.
2
3
This work was supported by the US Department of
Energy according to the Joint Memorandum with the
Russian Academy of Sciences. The authors are grate-
ful to the Los Alamos National Laboratory for the
financial support and the interest in the work.
vibration frequencies seems to be weak and results in
insignificant departures from the known values. All
the absorption band maxima in the table were as-
signed according to [10 13].
Introduction of gallium trichloride into the
EtMeIm Tf N ionic liquid affects the vibration fre-
quencies lower than 500 cm . This holds for the
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2
1
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3
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3
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2
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band (Fig. 8). This band is split into two components,
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3v
Since the changes in the vibrational spectrum concern
primarily the libration modes of the SO group only,
2
it can be assumed that gallium trichloride, which is a
Lewis acid and interacts with Tf N , forms a hetero-
2
ligand gallium complex whose coordination sphere
contains, along with three chlorine atoms, one of the
oxygen atoms from the SO group of the Tf N anion.
2
2
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with the nitrogen atom (bearing the largest negative
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Tf N anion: (SNS), (SNS), (SO ), and (SO )
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2
2
2
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2
markedly affect the stretching vibrations of the SO
group and only affects the libration modes of the SO
2
2
and CF groups (Fig. 6).
3
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