Electrochemical reduction of TiO2 in molten LiCl-Li2O

Article abstract of DOI:10.1246/cl.2007.1028

An investigation into the electrochemical reduction of TiO₂ to Ti in molten LiCI-Li₂O has been performed. Analysis of the time-dependent changes of a phase composition shows that the reduction proceeds through lithium-containing intermediates consisting of LiTi₂O ₄ and LiTiO₂. The reduction of TiO₂ in molten LiCl-Li₂O allows a much lower reaction temperature compared to the conventional reduction process in molten CaCl₂. Copyright

Full text of DOI:10.1246/cl.2007.1028

1028
Chemistry Letters Vol.36, No.8 (2007)
Electrochemical Reduction of TiO2 in Molten LiCl–Li2O
Jin-Mok Hur,ꢀ Su-Chul Lee, Sang-Mun Jeong, and Chung-Seok Seo
Korea Atomic Energy Research Institute, Daejeon 305-353, Korea
(Received May 7, 2007; CL-070486; E-mail: jmhur@kaeri.re.kr)
An investigation into the electrochemical reduction of TiO2
plied. Pt rods were used for an anode and a pseudo-reference
electrode. In order to produce partially reduced titanium oxides,
the electrochemical reduction was interrupted after different
reaction times. The recovered samples were thoroughly rinsed
with distilled water and then methanol to remove the residual
salts, and vacuum drying was applied at around 25 ꢁC. Phase
composition of the various specimens was determined through
X-ray powder diffraction (XRD). XRD patternes were collected
on a Rigaku MiniFlex diffractometer with a monochromatic
Cu Kꢀ radiation (ꢁ ¼ 0:15405 nm), at a scan rate of 4 deg/
min. Micrographic analysis was performed with a SEM system
(Jeol, JXA 8600). During the electrochemical reduction experi-
ments, the samples of the electrolyte were collected and titrated
by using 0.1 M HCl and a phenolphthalein indicator to measure
the Li2O concentration in LiCl.
Figure 1 shows typical potential–time curves, which were
recorded during the chronopontentiometry experiments. The
operation voltage is lower than the thermodynamic decomposi-
tion potential of LiCl, ꢂ3:46 V but higher than the decomposi-
tion potential of Li2O, ꢂ2:47 V. It should be emphasized that
the nonconducting magnesia holder in the cathode assembly
can confine the in situ generated Li inside the magnesia holder.
In the earlier stages of the electrochemical reduction, the
decrease of the Li2O concentration was in accordance with
Faraday’s law of an electrolysis. On the other, the decrease
rate of the Li2O concentration was slowed in the later stages
of the reaction.
to Ti in molten LiCl–Li2O has been performed. Analysis of the
time-dependent changes of a phase composition shows that the
reduction proceeds through lithium-containing intermediates
consisting of LiTi2O4 and LiTiO2. The reduction of TiO2 in mol-
ten LiCl–Li2O allows a much lower reaction temperature com-
pared to the conventional reduction process in molten CaCl2.
Over the recent past, the development of an electrochemical
reduction technique using molten salts has been pursued inten-
sively, particularly for the reduction of TiO2 to Ti.1–8 In an elec-
trochemical reduction of TiO2, usually molten CaCl2 at 850 ꢁC is
employed as the electrolyte. Also, there is a report for the elec-
trochemical reduction of TiO2 to Ti in LiCl–KCl–CaCl2 eutectic
melt at 450 ꢁC though the slow diffusivity of oxide species in the
solid phase at lower temperatures may be disadvantages for mass
production of Ti metal.8 As a part of research efforts for the treat-
ment of spent nuclear fuel, the electrochemical reduction of
uranium oxides in molten LiCl–Li2O at 650 ꢁC has been studied,
and the reduction of U3O8 to U metal was achieved with a more
than 99% conversion.9 The molten LiCl–Li2O system can be
applied to the electrochemical reduction of TiO2 since LiCl is
stable over the voltage range for the electrochemical reduction
of TiO2 and has a high oxide ion solubility, which was measured
as 8.8 wt % of Li2O in LiCl at 650 ꢁC.10 The report for the
electrochemical synthesis of lithium titanate showed that the
constant voltage electrolysis (1.2–3.2 V) of TiO2 in pure molten
LiCl only resulted in the formation of lithium titanate com-
pounds, even at 700 ꢁC.11 Here, we report on Ti production in
molten LiCl–Li2O, with the expectation of the promotion of
TiO2 reduction by in situ electrolysis of Li2O. This study pro-
vides a valuable insight into the reaction mechanism and
its dependency on the Li2O in the system. The experimental
approach consists of the investigation of partially reduced
specimens and measurement of the Li2O concentration during
the reduction reaction.
The electrochemical experiments were performed in a
vertical tubular stainless steel reactor with a magnesia crucible
that served as the reaction vessel. The upper end of the reactor
was equipped with feedthroughs for the electrode leads, gas inlet
and outlet, and a thermocouple as well as a sampling port. The
interior of the reactor was continuously purged with argon.
In this experimental set-up, the TiO2 powder (Showa, rutile,
99%, 5.0 g) kept in a porous magnesia holder was made the
cathode in a electrolytic cell and charged into molten LiCl (Alpa,
99%)–Li2O (Cerac, 99.5%) at 650 ꢁC. The porous magnesia
holder enables the molten salt to readily migrate through the
cathode assembly. Electric contact to the cathode was through
a stainless steel rod. Each experiment was performed with a
computer-controlled potentiostat/galvanostat (Won-A-tech,
WPG100), and a constant current (1.2 A) versus time was ap-
The samples for the XRD and SEM analyses were prepared
by withdrawing the cathode from the electrolytic cell when the
value of the electric charge that passed through the cell was
about 10378 C, 22518 C, and 33552 C. Samples quenched after
10378 C of current passage were found to consist predominantly
1
0.5
0
5
4.5
4
(A)
(D)
(E)
3.5
3
-0.5
-1
2.5
2
-1.5
-2
1.5
1
(B)
(C)
-2.5
-3
0.5
0
-3.5
0
1
2
3
4
5
6
7
8
9
10
Time/h
Figure 1. Variations of the anode (A), cathode (B), and cell (C)
potential during a chronopotentiometry experiment in molten
LiCl–Li2O. The observed Li2O concentration in LiCl was denot-
ed by (D). (E) means the calculated Li2O concentration in LiCl
values according to supplied charge.
Copyright ꢀ 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.8 (2007)
1029
of LiTiO2 and a small amount of LiTi2O4. Samples recovered
after 22518 C of current passage were primarily composed of
LiTiO2. After about 33552 C of current passage, this value
was about 139% of the theoretical electric charge necessary
for reducing 5 g of TiO2, the X-ray diffraction spectra obtained
were those of Ti metal. The significant evolution of the hydrogen
gas by hydration of this sample with the molten salt in water
suggests that a large amount of Li accumulation occurs inside
the magnesia holder.
The experimental results obtained in the present study
provide strong evidence that the electrochemical reduction of
TiO2 to Ti metal in molten LiCl–Li2O proceeds through
lithium-containing intermediates, and we could propose the
following mechanism. This electrochemical reaction commen-
ces with the formation of Li metal and then incorporation of
Li metal into the titanium compounds with a change of the Ti
oxidation states from +4 finally to +0. It should be noted that
except for reaction (1), the other cathode reactions are chemical-
ly driven. And except for reaction (4), the cathode reactions do
not involve the liberation of an oxide ion. From the proposed
mechanism, the change of the Li2O concentration during the
electrochemical reduction reaction can be explained. While the
formation of lithium titanates with Li metal was ongoing, the
liberation of oxide ion from the cathode could not occur. There-
fore, the decrease of the Li2O concentration in molten LiCl was
in accordance with Faraday’s law of an electrolysis in the earlier
stages of the electrochemical reduction. On the other, when the
compound formation had ceased in the later stages of the electro-
chemical reduction, the generation of oxide ion through reaction
(4) becomes the predominant process causing the slowing down
of the decrease rate of the Li2O concentration. After stopping a
supply of the electric energy, the increase of the Li2O concentra-
tion in a bulk electrolyte was observed suggesting the accumula-
tion of oxide ions inside the magnesia holder. The anodic
reaction is the generation of oxygen gas (reaction (5)). Under
theses conditions, the overall reaction is the reduction of Ti4þ
to Ti at the cathode and the evolution of oxygen gas at the anode.
Figure 2. SEM image TiO2 (A), the starting material, as well as
those of samples collected after the value of the electric charge
that passed through the cell was about 10378 C (B), 22518 C (C),
and 33552 C (D).
analysis. Therefore, small amounts of Ti in the sample for
Figure 2C might not be detected by XRD. The unique coral-like
structure with the surfaces covered with Ti metal flakes
(Figure 2D) is different from the Ti sponge produced by a direct
electrochemical reduction in molten CaCl.4
In summary, we were able to reduce TiO2 to Ti by using,
for the first time, a molten LiCl–Li2O electrolyte. Analysis of
the reaction mechanism proved that the reaction advances
through a chemically driven reduction by Li metal uptake
inside the magnesia holder. This approach using in situ genera-
tion of Li can be applied to produce other metals from their
respective oxides in molten LiCl–Li2O.
This work has been funded by the National Mid- and Long-
term Atomic Energy R&D Program supported by the Ministry of
Science and Technology of Korea.
References and Notes
1
2
3
R. O. Suzuki, M. Aizawa, K. Ono, J. Alloys Compd. 2000,
407, 361.
G. Z. Chen, D. J. Fray, T. W. Farthing, Nature 2000, 407,
361.
I. Park, T. Abiko, T. H. Okabe, J. Phys. Chem. Solids 2005,
66, 410.
Liþ þ eꢂ ¼ Li
ð1Þ
ð2Þ
ð3Þ
ð4Þ
ð5Þ
Li þ 2TiO2 ¼ LiTi2O4
LiTi2O4 þ Li ¼ 2LiTiO2
LiTiO2 þ 3Li ¼ 4Liþ þ Ti þ 2O2ꢂ
1/2O2 þ 2eꢂ ¼ O2ꢂ
4
5
C. Schwandt, D. J. Fray, Electrochim. Acta 2005, 51, 66.
K. Jiang, X. Hu, M. Ma, D. Wang, G. Qiu, X. Jin, G. Z. Chen,
Angew. Chem., Int. Ed. 2006, 45, 428.
6
7
P. Kar, J. W. Evans, Electrochem. Commun. 2006, 8, 1397.
K. Dring, R. Dashwood, D. Inman, J. Electrochem. Soc.
2005, 152, E104.
The samples analyzed by XRD were also subjected to SEM
analysis (Figure 2). As is visible in Figure 2A, the particle size of
the TiO2 powder, the starting material, is around 0.2 mm and
rather uniform. Figure 2B illustrates the microstructure of the
sample which is composed of LiTi2O4 and LiTiO2. Particles
are coarse and heterogeneous with an average particle size above
0.2 mm. The LiTiO2 shown in Figure 2C emerges in the form of
sintered grains. Figure 2D shows the microstructure of the Ti
metal. The grain of the particles reduces in size, and the overall
appearance is similar to coral and a little similar to that of
Figure 2C. At this point, it should be mentioned that only the
major phases of the sample could be identified in the XRD
8
9
Y. Katayama, B. Friedrich, Electrochemical Society
Proceedings Volume 2004-24 2006, 1046.
C. S. Seo, S. B. Park, B. H. Park, K. J. Jung, S. W. Park,
S. H. Kim, J. Nucl. Sci. Technol. 2006, 43, 587.
10 T. Usami, M. Kurata, T. Inoue, H. E. Sims, S. A. Beetham,
J. A. Jenkins, J. Nucl. Mater. 2002, 300, 15.
11 K. Jiang, X. Hu, H. Sun, D. Wang, X. Jin, Y. Ren, G. Z.
Chen, Chem. Mater. 2004, 16, 4324.
12 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Published on the web (Advance View) July 14, 2007; doi:10.1246/cl.2007.1028