Y.-L. Liu et al. / Electrochimica Acta 147 (2014) 104–113
105
therefore the separation of actinides from lanthanides by using a
2.3. Preparation and characterization of La-Al intermetallic
compounds
solid Al electrode was more efficient [19,20]. In another context, the
electrochemical behaviors of lanthanum on the Al electrode were
studied by Nagarajan et al. [14], they found that the reduction of
La(III) on a Al cathode occurs at a more anodic potential than that
on an inert W electrode, due to the formation of intermetallic com-
highly relevant to the separation efficiency, depends on the forma-
tion of intermetallic compounds. The La-Al phase diagram shows
the presence of six intermetallic compounds (AlLa3, AlLa, Al2La,
AlxLa, Al3La, Al11La3) [21], but only the formation of Al11La3 was
observed by Nagarajan et al. Therefore, further investigations on
the intermetallic compound formation between La and Al are still
necessary for potential technological and industrial applications.
Typically, two processes could be employed to obtain Al-La inter-
ions on a reactive Al cathode; (ii) co-reduction of La(III) ions with
aluminium ions using an inert electrode. In our previous work, we
found that more kinds of Al-Th, Al-Gd, Al-Er, Al-Sm, Sm-Zn inter-
metallic compounds were formed by the co-reduction process than
that by the UPD process [22–26]. In addition, Gibilaro et al. also
observed the formation of more than one kind of Ce-Al and Sm-
Al intermetallic compounds through similar depositions. Hence, in
the present work, the co-reduction behavior of La(III) and Al(III) on
the inert Mo electrode is investigated to get more insights of the La-
Al intermetallic compounds. To the best of our knowledge, similar
work has not been reported so far.
The samples of La-Al intermetallic compounds were prepared by
potentiostatic or galvanostatic electrolysis on a molybdenum elec-
trode, or on an active solid aluminum electrode. The active solid
aluminum electrode was an Al plate working electrode (length,
2 cm; width, 1 cm; thickness, 2 mm). After electrolysis, the alloys
were extracted from the melt under an argon atmosphere box. All
the samples were washed in hexane (99.8% purity) in an ultra-
sonic bath to remove salts and stored in the glove box. And the
samples were washed by 1, 2-ethanediol using ultrasound before
analysis. The microstructure and micro-zone chemical analysis of
these alloys were measured with SEM (Hitachi S-4800) and the
composition of the samples were analyzed by XRD (Bruker D8).
3. Results and Discussion
3.1. Co-reduction behaviors of La(III) and Al(III) on the Mo
electrode
Co-reduction means a simultaneous reduction of two or more
at least two metals. The theoretical principles and practical details
of a co-reduction process in aqueous media had been described by
Brenner [28]. Recently, the co-reduction process in molten salts was
also proposed by Taxil et al. and Gibilaro et al. for nuclear wastes
reprocessing [1,29]. By the principle of Taxil and Gibilaro, the co-
reduction process with Al(III) and La(III) ions can be expressed
through the following reactions I, II and III [1,29]:
2. Experimental
xAl3+ + 3xe− = xAl
yLa3+ + 3ye− = yLa
(I)
(II)
2.1. Preparation and purification of the melt
Anhydrous LiCl (Alfa Aesar, AR grade), anhydrous KCl (Alfa Aesar,
AR grade), LaCl3, AlCl3 (Alfa Aesar, AR grade), were used for the
preparation of the electrolyte. A mixture of 45 g LiCl and 55 g KCl
was dried under vacuum for more than 100 h at 473 K to remove the
excess water. Then the salt mixture was melted in a 200 cm3 alu-
mina crucible placed in a quartz cell located in an electric furnace.
The temperature of the melt was measured with a nickel-chromium
thermocouple sheathed with an alumina tube. When the salt mix-
ture was completely melted, the melt was then pre-electrolysis at
-2.2 V (vs. AgCl/Ag) for several hours (usually more than 4 hours) to
remove the metal ion impurities. Lanthanum, aluminum elements
were introduced into the bath in the form of dehydrated LaCl3 and
AlCl3 powder, respectively. Due to its volatility, the exact concen-
tration of AlCl3 in the melts could not be measured. Hence, the
concentration of AlCl3 we present in this work is the concentration
of AlCl3 initially added into the melts. Dehydrated LaCl3 was pre-
pared from La2O3 and NH4Cl as follows: 3 g La2O3 and 6 g NH4Cl was
heated at 573 K for 3 h in a vacuum furnace [27], and the product
was confirmed to be LaCl3 by XRD (Bruker D8).
xAl + yLa = AlxLay
(III)
The overall process can be expressed as:
xAl3+ + yLa3+ + 3(x + y)e− = AlxLay
In this case, the equilibrium potential of the La3+/AlxLay system
can be described by the following equation:
(1)
aLa
RT
nF
3+
ELa
= EL0a
= ELa
+
ln[
]
(2)
(3)
3+
3+
/Al La
x
y
/La
/La
aLa(inAlxLay)
RT
nF
ELa
−
ln[aLa(inAlxLay)]
3+
3+
/Al La
x
y
Where ELa
is the equilibrium potential of the pure La ele-
3+
/La
ment, T denotes the absolute temperature in K, n presents the
number of exchanged electron, F corresponds the Faraday con-
stant (96485C) and aLa(in AlxLay) is the activity of La in the AlxLay
intermetallic compounds.
pounds, the reported La-Al phase diagram is presented in Fig. 1. In
the La-Al system, the intermetallic compounds AlLa3, AlLa, Al2La,
AlxLa, Al3La, Al11La3 were treated as stoichiometric phases. The
composition of AlxLa was described as Al7La3 [21].
2.2. Electrodes and electrochemical apparatus
The inert working electrode consisted of 1 mm molybdenum
wire (Alfa Aesar 99.99%), and the active electrode surface area
was determined after each experiment by measuring the immer-
sion depth of the electrode in the molten salts. A 6 mm diameter
graphite rod (Alfa Aesar > 99.99%) was used as the counter elec-
trode. The reference electrode is a pyrex tube containing a silver
wire (Alfa Aesar, 99.99%, d = 1 mm) dipped into a solution of
1.0 wt.% AgCl in LiCl-KCl melt. All potentials were referred to the
AgCl/Ag reference electrode. PGSTAT302N electrochemical work-
station (Autolab, Metrohm) controlled with the Nova 1.9 software
package was used to obtain electrochemical data.
3.1.1. Electrochemical evidence for Al-La intermetallic
In order to study the co-reduction behavior of La(III) and
Al(III) ions, the experiment was carried out in both LiCl-KCl-
LaCl3(9.74 × 10−5 mol/cm3) and LiCl-KCl-LaCl3(9.74 × 10−5)-AlCl3
(6.04 × 10−5 mol/cm3) melts, the recorded voltammograms are dis-
palyed in Fig. 2a. The black curve corresponds to the CV obtained in
the LiCl-KCl-LaCl3 melt, a cathodic peak B at around -2.16 V and its
corresponding anodic peak Bꢀ at about -2.03 V can be observed. It is