Electrochemical study of the reduction of Tm(III) ions in a molten NaCl-2CsCl eutectic

Article abstract of DOI:10.1134/S1070427212020097

Electrochemical behavior of TmCl₃ solutions in a NaCl-2CsCl eutectic melt at temperatures of 823-973 K on an inert Mo electrode was studied by cyclic and square-wave voltammetry and the method of electromotive forces. The mechanism of the cathodic process was determined, kinetic parameters of the Tm(III) + e ? Tm(II) reaction were found, and the temperature dependence of the diffusion coefficients of Tm(III) ions was calculated. The conditional standard redox potential for the Tm(III)/Tm(II) pair was measured in the temperature range under study. The basic thermodynamic characteristics of the Tm(III) + e ? Tm(II) redox reaction in a molten NaCl-2CsCl eutectic mixture were determined. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S1070427212020097

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 2, pp. 218−224. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © A.V. Novoselova, V.V. Smolenskii, 2012, published in Zhurnal Prikladnoi Khimii, 2012, Vol. 85, No. 2, pp. 229−235.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Electrochemical Study of the Reduction of Tm(III) Ions
in a Molten NaCl–2CsCl Eutectic
A. V. Novoselova and V. V. Smolenskii
Institute of High-Temperature Electrochemistry, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia
e-mail: A.Novoselova@ihte.uran.ru
Received July 21, 2011
Abstract—Electrochemical behavior of TmCl3 solutions in a NaCl–2CsCl eutectic melt at temperatures of 823–973
K on an inert Mo electrode was studied by cyclic and square-wave voltammetry and the method of electromotive
forces. The mechanism of the cathodic process was determined, kinetic parameters of the Tm(III) + e ' Tm(II)
reaction were found, and the temperature dependence of the diffusion coefficients of Tm(III) ions was calculated.
The conditional standard redox potential for the Tm(III)/Tm(II) pair was measured in the temperature range under
study. The basic thermodynamic characteristics of the Tm(III) + e 'Tm(II) redox reaction in a molten NaCl–2CsCl
eutectic mixture were determined.
DOI: 10.1134/S1070427212020097
The concept of partitioning and transmutation (P&T)
of fission products, presently considered an efficient
way to reduce the activity of radioactive waste, consists
in isolation of long-lived fission products from spent
nuclear fuel (SNF) and their incorporation into an inert
matrix (free of uranium) for subsequent transmutation in
appropriate reactors. Pyrochemical partitioning methods
have a number of advantages over hydrometallurgical
techniques, the main of which are the high radiation
hardness of salt solutions and possible regeneration of
SNF with short cooling time [1–3].
the formation thermodynamics of TmCl3 compound in
a melt were presented. In [19, 20], the redox potentials
of the Tm(III)/Tm(II) pair in a NaCl–KCl–CsCl eutectic
melt and CsCl relative to a chlorine reference electrode
and temperature dependences of E*Tm(III)/Tm(II) were
measured and thermodynamic characteristics of the
Tm(III) + e ' Tm(II) reaction were calculated.
In this study, we examined the electrochemical
properties of thulium compounds in a molten NaCl–
2CsCl eutectic.
The goal of our study was to optimize the efficiency
of partitioning of minor actinides and to minimize
their content of fission products, primarily rare-earth
elements, which are neutron poisons. To tackle with this
problem, it is important to know the electrochemical and
thermodynamic properties of lanthanides and actinides
[4–17].
EXPERIMENTAL
Sodium and cesium chlorides of chemically pure
grade were used. Preliminarily, these compounds were
subjected to prolonged drying in a vacuum at gradually
elevated temperature, purified by directed crystallization
[21], and then fused in a prescribed ratio [22].
There is only scarce published evidence about
properties of thulium compounds in chloride melt.
In [18], the behavior of TmCl3 solutions in a molten
LiCl–KCl eutectic was studied. The mechanism by
which trivalent thulium is reduced to the metal on
inert and active electrodes was described and data on
the diffusion coefficient of thulium(III) ions and on
Anhydrous thulium trichloride was prepared by
chlorination of TmCl3·nH2O (n = 4.5–5.0) crystal
hydrate, under heating in carbon tetrachloride vapor by
the method reported in [23]. The crystal hydrate was
produced by dissolution of Tm2O3 of OST (All-Union
Standard) 46-205–81 brand in hydrochloric acid to
pH 1–1.5, with subsequent evaporation of the solution
218

ELECTROCHEMICAL STUDY OF THE REDUCTION OF Tm(III) IONS
219
on a water bath to dryness. The resulting TmCl3 was
stored, ground, and weighed in a box in the atmosphere
of nitrogen.
liminarily impregnated with a solvent-salt in a vacuum.
Particular attention was given to purification of argon.
The assembled cell was evacuated to a residual pressure
of 1.3–2.7 Pa, monitored with a VIT-2 ionization-ther-
mocouple vacuum gage. Purified argon was delivered
into the cell. The procedure was repeated several times.
The inert gas in the measuring cell was subjected to ad-
ditional purification before an experiment.
A melt under study was prepared immediately
before an experiment by fusing a prescribed amount
of preliminarily prepared NaCl–2CsCl eutectic with
thulium trichloride. In all experiments, the concentration
of TmCl3 in the salt case did not exceed 4.5 mol %.
Samples for chemical analysis (~200 mg) were taken
from a salt melt and dissolved in a dilute solution of nitric
acid. The total concentration of thulium was analyzed
by an ICP-MS instrument. The concentration of divalent
thulium in the samples under study was determined by
volumetrically.
Voltammetric studies were carried out in a standard
three-electrode quartz cell in a glassy carbon crucible in
the atmosphere of dry argon at a temperature of 823–
973 K. As the working electrode served a molybdenum
wire1mmindiameter,submergedintoamelttoadepthof
3–10 mm. The cathode surface area was experimentally
determined after each experiment. A glassy carbon
3 mm in diameter was used as a counter electrode.
The measurements were made relative to the standard
chlorine reference electrode [24]. Electrochemical
processes were analyzed by the following methods:
cyclic and square-wave voltammetry, coulometry,
and zero-current potentiometry. Measurements were
made with an Eco Chemie AUTOLAB PGSTAT 30
potentiostat-galvanostat software.
A cyclic voltammogram of a NaCl–2CsCl melt,
measured on an inert molybdenum electrode at 823 K,
is shown in Fig. 1, curve 1. It has a wide electrochemical
window delimited by reduction of the alkali metal on
one side, and by evolution of gaseous chlorine on the
other.
Upon introduction of thulium trichloride into the
supporting electrolyte, the cyclic voltammogram shows
two new cathodic peaks at potentials of –2.865 (A)
and –3.158 V (B) and the corresponding two anodic
current peaks at potentials of –2.906 (B1) and –2.562 V
(A1) relative to the chlorine reference electrode (Fig. 1,
curve 2). A potentiostatic electrolysis at the potentials
of cathodic peak A did not yield a solid phase on the
electrode surface (Fig. 2, curve 1), the working electrode
remained visually unchanged, whereas polarization of
the working electrode at the potentials of cathodic peak
Potentiometric studies were carried out in an elec-
tro-chemical cell whose design and experimental pro-
cedure have been described in detail previously [25].
As the indifferent material for the working electrode
served a molybdenum wire 1 mm in diameter. The elec-
trode was submerged in a melt under study, placed in a
crucible made of sintered beryllium oxide. To remove
air from pores of the material, the crucible was pre-
I, A
E, V
E, V
τ, s
Fig. 1. Cyclic voltammogram obtained for (1) NaCl–2CsCl
and (2) NaCl–2CsCl–TmCl3 melts on a Mo working electrode
(S = 0.27 cm2). TmCl3 concentration 9.83 × 10–2 mol kg–1,
sweep rate 0.2 V s–1, T = 823 K. (I) Peak current and (E)
potential; the same for Fig. 5.
Fig. 2. Dependence of the potential E on time τ upon short-
time polarization of the Mo working electrode (S = 1.2 cm2) in
a NaCl–2CsCl–TmCl3 melt (mTmCl3 = 9.83 × 10–2 mol kg–1) at
823 K. Deposition potential Ed (V) and τd (s), respectively: (1)
2.906, 15; and (2) 2.158, 8.
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NOVOSELOVA, SMOLENSKII
B resulted in that a plateau appeared on the potential–
time dependence (Fig. 2, curve 2), which indicated that
a solid phase was formed on the cathode surface.
voltammetry (Fig. 5) by measuring the W1/2 half-peak
width (V) in the range in which the peak current is
directly proportional to the square root of the signal
frequency by the equation
It was found that the cathodic and anodic potentials
of the current peak for the A–A1 recharging process are
independent of the logarithm of the sweep rate up to
0.1 V s–1 (Fig. 3). Further increase in the potential sweep
rate leads to a change of the reduction mechanism. It
was shown that the cathodic and anodic peak currents
are directly proportional to the square root of the
polarization rate in the entire range of potentials under
study (Fig. 4).
(1)
where W1/2 is the half-peak width (V), and R, T, n, and F
are common designations.
Our calculations demonstrated that the number of
electrons involved in reduction of Tm(III) ions to Tm(II)
is 1.01 ± 0.03.
The number n of electrons involved in the electrode
reaction A–A1 was determined by square-wave
According to the theory of cyclic voltammetry
[26, 27], electrode processes in which trivalent ions of
thulium are reduced to its divalent state and oxidation of
Tm(II) ions to Tm(III) are reversible and are controlled
by mass-transfer processes at low sweep rates. At v >
0.1 V s–1, the process is irreversible and is controlled by
the charge-transfer rate.
E, V
Analysis of the results obtained suggests the
following reduction mechanism of thulium(III) ions to
the metal:
'
Tm(III) + е Tm(II),
(2)
(3)
lnV [V]
'
Tm(II) + 2е Tm(0).
Fig. 3. Dependence of the potential E of the current peak on
the logarithm of the sweep rate, log V, in a NaCl–2CsCl–
The diffusion coefficients of Tm(III) ions in a molten
NaCl–2CsCl eutectic were calculated by the Randles–
Ševčik equation for the case of a reversible process (v <
0.1 V s–1):
TmCl3 melt (mTmCl = 9.83 × 10–2 mol kg–1) for the reaction
3
Tm(III) + e ' Tm(II) on the Mo working electrode at 823 K.
Dependence: (1) cathodic and (2) anodic; the same for Fig. 4.
I, A
I, A
V1/2, V1/2
E, V
Fig. 4. Dependence of the peak current I on the square root of
Fig. 5. Square-wave voltammogram of a NaCl–2CsCl–TmCl3
melt (mTmCl3 = 9.83 × 10–2 mol kg–1) at ν = 12 Hz on the Mo
working electrode at 823 K.
the sweep rate, V1/2, in a NaCl–2CsCl–TmCl3 melt (mTmCl3
=
9.83 × 10–2 mol kg–1) for the reaction Tm(III) + e ' Tm(II) on
the Mo working electrode at 823 K.
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ELECTROCHEMICAL STUDY OF THE REDUCTION OF Tm(III) IONS
221
10–2, it can be assumed that the activity coefficients in
molten salts remain nearly constant. Having introduced
the conditional standard potential E*Tm(III)/Tm(II), we can
write the following equation [24]:
(4)
where Ip is the peak current (A); S, cathode area (cm2);
c0, concentration of electrically active species in the
electrolyte bulk (mol cm–3); D, diffusion coefficient
(cm2 s–1); and v, sweep rate (V s–1).
(10)
The empirical temperature dependence of the
diffusion coefficients of Tm(III) ions is described by the
Arrhenius equation
Assuming that the diffusion coefficients of tri- and
divalent thulium ions are close to each other, we can
take that RT/nF ln(DTm(II)/DTm(III))
1/2 ≈ 0.
In view of the aforesaid, we calculated the
conditional standard redox potential E*Tm(III)/Tm(II) from
data furnished by cyclic voltammetry for the reversible
process (v < 0.1 V s–1) by the equations
(5)
The activation energy Ea of the diffusion process is
44.0 kJ mol–1. Comparison of the diffusion coefficients in
the NaCl–2CsCl eutectic melt at 823 K (DTm(III) = 0.58 ×
10–5 cm2 s–1), found in this study, with the coefficients
reported in [18] for a 2LiCl–3KCl eutectic melt at 823 K
(DTm(III) = 1.22 × 10–5 cm2 s–1) demonstrates that the
diffusion coefficients steadily grow as the inverse radius
of the solvent–salt cation becomes larger on passing
from LiCl to CsCl because of the increase in the strength
of electrically active [TmCl6]3– complex ions.
(11)
(12)
(13)
According to the theory of cyclic voltammetry, the
c
following relationships between the cathodic (Ep )
The temperature dependence of the conditional
standard redox potential for the Tm(III)/Tm(II) pair
in a molten NaCl–2CsCl eutectic is described by the
empirical equation
and anodic (Epa) potentials of the current peak and the
half-wave potential E1/2 are valid for the reversible
electrochemical process:
*
E
Tm(III)/Tm(II) = –3.745 + 1.06 × 10–3T ± 0.003, V. (14)
(6)
(7)
We also measured the electromotive force of the
galvanic cell
(8)
Mo(solid)|[TmCl3, TmCl2, (NaCl–2CsCl)eut](solution)
|| [(NaCl–2CsCl)eut](solution) | Cl2(g), С(solid)
where
in relation to the ratio between the tri- and divalent
thulium concentrations in the solvent–electrolyte and
temperature.
The measured value of the electromotive force
includes, together with the sought-for difference of the
electrode potentials, also the thermoelectromotive force
between the molybdenum and carbon current leads to
the cell electrodes. It has a sign opposite to the sought-
for potential difference:
(9)
In the range of concentrations at which the molar
fractions N of depolarizer ions do not exceed (3–5) ×
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NOVOSELOVA, SMOLENSKII
0.99 ± 0.01.
εexp = ε – εT.
The temperature dependence of the conditional
standard redox potential E*Tm 2+, obtained on
the molybdenum electrode, is shown in Fig. 7. This
dependence is linear in the temperature range under
study. The data processed by the least-squares method
are described by the equation
The equation for the temperature dependence for the
molybdenum–carbon pair was taken from [24]:
/
3+
Tm
εT = –0.0076 + 1.74 × 10–5T ± 0.0008, V.
(16)
As the temperature is raised, the correction for
the thermoelectromotive force becomes increasingly
noticeable. All the experimental values of the
electromotive forces are given with consideration for
the thermoelectromotive force.
*
3+
2+
Е
= –(3.763 ± 0.013)
Tm /Tm
+ (1.06 ± 0.01) × 10–3T ± 0.003, V.
(18).
The experimentally measured values of the
conditional standard redox potentials E*Tm
Theexperimentalvaluesobtainedonthemolybdenum
indifferent indicator electrode in the NaCl–2CsCl
eutectic melt are presented in Fig. 6. In the coordinate
/
3+
2+
Tm
make it possible to calculate the change in the apparent
standard Gibbs energy of the redox reaction
system under study, the redox potential ETm
/
3+
2+
Tm
linearly depends on the logarithm of the ratio between
the trichloride and dichloride concentrations, ln([Ln3+]/
[Ln2+]), and is described by the Nernst equation. The
data processed by the least-squares method for the
temperature of 823 K have the form
'
TmCl2(solution) + 1/2Cl2(g) TmCl3(solution)
.
(19)
The data obtained for different temperatures are
listed in Table 1.
The empirical equation for ΔG* (kJ mol–1), derived
by the least-squares method, has the form
3+
2+
ЕTm
= –(2.920 ± 0.002) + (0.072 ± 0.002)
/Tm
× ln([Tm3+]/[Tm2+]) ± 0.003, V,
(17)
*
ΔG = –(364.2 ± 2.2) + (103.2 ± 2.4) × 10–3T ± 0.3. (20)
The first and second coefficients in expression (20)
are, respectively, the enthalpy and entropy of the redox
reaction (19). It can be seen that ΔG* decreases with
increasing temperature.
where [Tm3+] and [Tm2+] are the molar fraction
concentrations of thulium ions in different oxidation
states.
From the prelogarithmic coefficient in Eq. (17),
we found the number n of electrons involved in the
electrochemical process of thulium trichloride reduc-
tion by reaction (2). The value of n was found to be
We used the apparent standard Gibbs energies of
the chemical reaction (19) to calculate conditional
equilibriumconstantsk*,whosetemperaturedependence
E, V
T, °C
Fig. 7. Dependence of the conditional standard redox potential
Fig. 6. Dependence of the redox potential E of thulium on
the logarithm of the ratio between the concentrations [Tm3+]
and [Tm2+] at 833 K in a molten NaCl–2CsCl eutectic. Initial
concentration [Tm3+] = 4.32 mol %.
E*Tm3+ 2+ in a NaCl–2CsCl eutectic melt on temperature T.
/Tm
Initial concentration [Tm3+] = 4.32 mol %.
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ELECTROCHEMICAL STUDY OF THE REDUCTION OF Tm(III) IONS
223
is described by the equation
The values measured for E*Tm
/
3+
were used
2+
Tm
to calculate the equilibrium chlorine pressure pCl2
over the NaCl–2CsCl melt containing thulium tri- and
dichlorides. Calculations of this kind were made for
equal concentrations of the tri- and dichlorides. The
results of these calculations are listed in Table 1, whence
it can be seen that the stability of thulium trichloride
increases with decreasing temperature.
*
ln k = –12.38 + (43799 ± 27)/T ± 0.03.
(21)
It can be seen in Table 1 that the equilibrium
constant decreases with increasing temperature. This
indicates that, as the temperature is lowered, reaction
(19) shifts toward formation of TmCl3. Moreover, the
fact that the constants substantially exceed unity (k* >>
1) means that the process of thulium chloride oxidation
by chlorine occurs nearly fully to give the final product,
i.e., the trichloride.
Table 2 presents the basic thermodynamic data for
reactions (19) and (22) in chloride melts of various
compositions. An increase in the cation radius of the
solvent-salt on passing from NaCl to CsCl results in
that ΔG* and k* for reaction (19) steadily grow and
the partial chlorine pressure over the melt decreases
because electrically active [TmCl6]3– complex ions
become more stable.
The equilibrium of redox reactions can be studied
in different ways. The activity of rare earth metal ions
in different oxidation states can be varied by setting
different partial pressures of chlorine over chloride
melts containing these ions. Therefore, there exist
a phenomenological relationship between conditional
standard redox potentials and the partial pressure of
chlorine, governed by the equilibrium of the reaction
CONCLUSIONS
(1) Cathodic processes occurring in reduction of
Tm(III) ions to the metal in a molten NaCl–2CsCl
eutectic in the temperature range 823–973 K were for
the first time studied by electrochemical methods. The
mechanism of the reaction occurring in two stages was
determined: the first stage of the process [Tm(III) →
Tm(II)] is reversible at low sweep rates and is controlled
'
LnCl3(l) LnCl2(l) + 1/2Cl2(g)
;
(22)
(23)
Table 1. Conditional standard Gibbs energy changes, equilibrium constants for reaction (19), and equilibrium partial pressure
of chlorine over a molten NaCl–2CsCl eutectic mixture at different temperatures
*
*
ΔG
ΔH
*
ΔS ,
*
*
Т, K
Е , V
k
p
, Pa
Cl2
J mol–1 K–1
kJ mol–1
823
873
923
973
–2.882
–2.842
–2.790
–2.733
–278.1
–274.2
–269.2
–263.6
2.79 × 1017
2.61 × 1016
1.75 × 1015
1.44 × 1014
1.28 × 10–36
1.47 × 10–33
2.34 × 10–31
3.50 × 10–29
–364.2
103.2
Table 2. Conditional standard Gibbs energy changes, equilibrium constants for reaction (19), and equilibrium partial pressure
of chlorine over chloride melts at 973 K
*
*
ΔG
ΔH
*
ΔS ,
*
*
Chloride melt
Е , V
k
p
, Pa
Cl2
J mol–1 K–1
kJ mol–1
NaCl–KCl–CsCl [20]
–2.721
–2.733
–262.6
–263.6
–354.1
–364.2
94.5
1.31 × 1014
1.44 × 1014
5.86 × 10–29
3.50 × 10–29
NaCl–2CsCl [present study]
CsCl [19]
103.2
–2.822
–272.3
–388.8
119.7
4.40 × 1014
5.61 × 10–30
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NOVOSELOVA, SMOLENSKII
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