Electrodeposition of lanthanum from ionic liquids at

Electrodeposition of lanthanum from ionic liquids at room temperature

Electrodeposition of lanthanum from ionic liquids at room temperature E.Bourbos1, I.Giannopoulou1, A.Karantonis2, D.Panias1 and I.Paspaliaris1 National Technical University of Athens 1

School of Mining and Metallurgical Engineering

2

School of Chemical Engineering Iroon Polytechniou 9

157 80 Athens, Greece

Abstract Production of rare earth metals and alloys is conventionally performed by high temperature molten salts electrolysis, which creates highly corrosive environment and demands high energy consumption. Recently, the use of organic solvents has been implemented; however they were considered inadequate, due to their high volatility and flammability. Ionic liquids (ILs) gain increasing attention in the recent years, as electrolytes for the recovery of metals more electropositive than hydrogen. The type of cations and anions adjusts the properties for ILs, such as low melting points, chemical and thermal stability, negligible vapor pressure, ability to dissolve a wide range of compounds, ionic conductivity and broad electrochemical window. In this paper, the reduction of lanthanum in the ionic liquids N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI) and N-Trimethyl-butylammonium bis(trifluoromethylsulfonyl)imide (Me3NBu-TFSI) at room temperature, is studied. The above mentioned ionic liquids were selected due to their electrochemical, air and water stability, as well as for their ionic conductivity. Cyclic voltammetry (CV) studies on a Pt working electrode revealed that lanthanum trivalent cations can be reduced to the metallic state in both selected ionic liquids. Furthermore, the electrodeposition of lanthanum from the systems BMP-TFSI / La(NO3)3 and Me3NBu-TFSI / La(NO3)3 was realised on a copper substrate, under potentiostatic conditions for 5 h at -3.1 V at 25 oC. The examination of the formed deposit with Scanning Electron Microscopy (SEM) and their analysis by Energy-dispersive Spectroscopy (EDS), revealed the electrodeposition of lanthanum.

1

Introduction

Production of rare earth metals and alloys is conventionally performed by high temperature molten salts electrolysis, which creates highly corrosive environment and demands high energy consumption. The alternative to high-temperature molten salts is to use an ionic substance that melts at a low Proceedings of EMC 2015

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temperature [1]. To this direction, significant efforts have been carried out from the middle of the 20th century, aiming at developing low temperature molten salts. The major achievement of these efforts was to synthesize a low temperature melt for aluminum deposition, which led to the formation of Li+/ K+/ AlCl3 eutectics that have melting points close to 100 ◦C [2,3]. The use of quaternary ammonium, pyrrolidinium and imidazolium salts has pushed the melting point of molten electrolytes down to ambient temperature. The term “room temperature ionic liquids” (RTILs) was coined to distinguish these low temperature compounds from the high temperature analogues, which are predominantly composed of inorganic ions [4, 5]. Several applications of RTILs are being investigated and these are as diverse as fuel desulfurization and precious metals processing, but few have yet come to practical fruition. Several other processes are also at the pilot plant scale, while some ionic liquids are commercially used as additive in different products e.g. binders in paints [1, 3]. Concerning the use of ionic liquids in non-ferrous metallurgy, it must be pointed out that even though the deposition of a wide range of non-ferrous metals has been studied and demonstrated from numerous ionic liquids [1, 2], the complex nature of process parameters limits any industrial application yet. An important disadvantage of RTILs is the low electrical conductivity, which is about 10 times lower than the currently used molten electrolytes in the production of nonferrous metals [1]. However, the possible operation of ionic liquids at temperatures above 100 ◦C, where electrical conductivity is increased, enhances their application as molten electrolytes [6]. The ongoing development of ionic liquids might lead to even better conducting materials; preliminary studies have proven that RTILs are capable solvents [7-10] and electrolytes of reactive metals, such as Li, Na, Al, Mg [11-14]. The electrodeposition of rare earth elements from ionic liquids has been mainly investigated with four different families [7] of ionic liquids based on their cation. It concerns for imidazolium-, pyrrolidinium-, ammonium- and the phosphonium- based ionic liquids with a variety of anions. The ionic liquids used in this research belong to the pyrrolidinium and to ammonium families and therefore, our interest is focused on these two categories of ionic liquids.

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Glukhov et al. [15] have stated the reduction of Y, Gd and Yb in the ionic liquid BMP-CF3SO3 on both, platinum and copper electrodes. On the voltammetric curves performed with Pt electrodes in the systems of this IL with Y and Gd, the observed cathodic peak with a maximum value of -2.6 V (vs Ag/0.1 M Cl-) was attributed to the reduction of the metals trivalent cations to the metallic state. The electrodeposition of these metals was performed under potentiostatic or galvanostatic conditions and a tenuous black precipitate was formed on the electrode’s surface. It was also shown that the reduction of Yb(III) to the metallic form occurs step by step via Yb(II) formation and the limiting stage of the cathodic process is the adsorption of the metal cation on the electrode. Legeai et al. [16] have shown that the reduction of La in the ionic liquid OMP-TFSI is an irreversible reaction and that La electrodeposition in this ionic liquid under potentiostatic polarisation conditions at -1.5 V (vs Ag/AgCl) for 120 min, at 298 K, resulted in the cathodic deposit of a thick La film of 350 nm. In addition, Yamagata et al. [17] studied by cyclic voltammetry the redox reactions of the Sm(III)/Sm(II) couple in the ionic liquid BMP-TFSI, using a glassy carbon electrode and determined the redox reactions occurred as quasi-reversible or irreversible. They also studied the electrochemistry of Yb(III) in BMP-TFSI by cyclic voltammetry and concluded to a cathodic and an anodic peak at around -0.95 V (vs Ag/Ag+), which considered attributable to the reduction of Yb(III) and the oxidation of Yb(II), respectively. The electrochemical behaviour of Eu(III) in BMP-TFSI was investigated by Rao et al. [18] with glassy carbon and stainless steel electrodes, at various temperatures. The cyclic voltammograms revealed a quasi-reversible behaviour of the redox couple Eu(III)/Eu(II) and an irreversible behaviour of the couple Eu(II)/Eu(0), which presented a cathodic peak attributed to the reduction of Eu(II) to the metallic state. In the same ionic liquid (BMP-TFSI) that contained also chloride anions, Hussey et al. [19] studied the electrochemical and spectroscopic behavior of Nd(III) and Pr(III). According to their results, the electrolytic dissolution of both metals in this ionic liquid’s system produces only the respective trivalent cations, which can be reduced to the Ln2+(II) state, but the resulting divalent species exhibit only transient stability, undergoing rapid disproportionation to Ln3+(III) and Ln0 states. Glukhov et al. [15] have investigated also the reduction of Y, Gd, Yb in the ammonium-based ionic liquid Bu3MeN-CF3SO3 and concluded that the deposition of the above metals is only possible on a copper substrate and does not occur on a platinum one. Bhatt et al. [20] reported the reduction of selected lanthanide cations (La, Sm and Eu) to the zero-valent state in the ionic liquid Me3NBuTFSI. In this research, the lanthanide cations were introduced to the ionic liquid as the TFSI hydrate salts. Cyclic voltammograms revealed a cathodic peak at -2.4 V vs Fc+/Fc for the lanthanum system attributed to the reduction to metallic state, whereas two peaks were observed for Sm- and Eu- systems, the less negative were associated with (III)/(II) reduction reaction and the more negative with the reduction to the metallic state. The present work aims at investigating the electrorecovery of metallic lanthanum at ambient temperature, from pyrrolidinium- and ammonium- based ionic liquids. Taking into account the literature data and preliminary screening experiments, electrodeposition of lanthanum was researched in Proceedings of EMC 2015

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the ionic liquids BMP-TFSI and Me3NBu-TFSI. These hydrophobic ionic liquids present good properties for the electrodeposition of light rare earth elements: wide electrochemical stability, low hygroscopy, low viscosity and significant ionic conductivity.

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Experimental

2.1

Materials and apparatus

The ionic liquids BMP-TFSI and Me3NBu-TFSI were supplied by Solvionic. Lanthanum nitrate hexahydrate was purchased by Alfa Aesar. The nitrate salt was heated at 100 oC under vacuum for 72 h and then, was dissolved in pure acetone, where subsequently the proper amount of electrolyte was added to form a lanthanum concentration of 0.06 M. The solution was placed at 60 oC under vacuum for approximately 2 h to remove acetone and then, at 100 oC under vacuum for 24 h to remove residual water. Cyclic voltammetry tests were performed in a three milielectrodes cell (PAR) connected to a VersaSTAT3 potentiostat (PAR); the obtained experimental data were analysed with the VersaStudio software (PAR). The working electrode was a platinum disk of 1.98 mm diameter; as a counter electrode a Pt wire was used, while the redox couple Ag/0.1 M AgNO3 in acetonitrile was employed as reference electrode. The working electrode was polished with alumina paste 1 μm on a velvet pad and cleaned electrochemically in 1 M sulfuric acid, before use. Although BMPTFSI and Me3NBu-TFSI exhibit a hydrophobic character and are stable under abient atmospheric conditions, the cyclic voltammetry and potentiostatic tests were performed in a glove box, providing an inert atmosphere by purging nitrogen gas to minimise oxygen and moisture contamination. The morphology of electrodeposits was examined by a Scanning Electron Microscope (JEOL6380LV) provided with an Energy Dispersive Spectrometer.

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Results

3.1

Electrodeposition of La from BMP-TFSI ionic liquid

The electrochemical window was determined at room temperature by cyclic voltammetry and the cathodic limit of the electrolyte was determined at -3.5 V vs Ag/Ag+. Figure 1 shows the cyclic voltammogram recorded in the system BMP-TFSI/La. As it is obvious, a generation of an intense cathodic peak begins at -2.2 V vs Ag/Ag+, attributed to the reduction of trivalent lanthanum cation (La3+) to the metallic state. The absence of corresponding anodic peak on the reverse scan allows the assumption that lanthanum reduction is irreversible [15, 16]. Moreover, the cathodic peak at -1 V vs Ag/Ag+ (Fig. 1) is ascribed to the limited reduction of hydrogen cations, due to slight moisture [1] that inevitably was present and the narrow shoulder at -1.5 V vs Ag/Ag+ is attributed either to an

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Electrodeposition of lanthanum from ionic liquids at room temperature adsorption La3+ reaction occurring on the electrode, prior to the reduction of lanthanum [15], or to an under potential deposition phenomenon that is taking place over this range of potentials. 2

j(mA/cm2)

1 0 -4

1

-3,5

-3

-2,5

-2

-1,5

-1

-0,5

-1

0

-2

2

-3 -4

E(V) vs Ag/Ag+

Figure 1:

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CVs of (1) BMP-TFSI and (2) the system BMP-TFSI / 0.06 M La (WE = Pt, scan rate = 20mV/s, temperature = 25oC).

Based on the cathodic limit of BMP-TFSI, the electrodeposition of lanthanum was attempted under potentiostatic conditions on a copper substrate at -3.1 V vs Ag/Ag+, for 5 h at 25 oC. The formed electrodeposit was examined with a Scanning Electron Microscope (Fig. 2a) and the Energy Dispersive Spectrometry analysis (Fig. 2b) revealed the presence of lanthanum, thus confirming its reduction. The electrodeposit of lanthanum was of granular shape, while the cracking of the deposit surface was evident. The elevated concentrations of sulfur and fluorine on the lanthanum deposit (Fig. 2b) indicate rather inadequate remove of the electrolyte from the deposit’s surface.

a

Figure 2:

Element

Weight%

OK

11.50

FK

16.19

SK

12.52

Cu K

29.48

La L

30.32

Totals

100.00

b

Lanthanum deposition from BMP-TFSI (a) SEM image and (b) EDS analysis.


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3.2

Electrodeposition of La from Me3NBu-TFSI ionic liquid

The electrochemical window of the ionic liquid Me3NBu-TFSI was determined at room temperature by cyclic voltammetry and the cathodic limit of the electrolyte was found to be -3.6 V vs Ag/Ag+ (figure 3). In figure 3, the cyclic voltammogram recorded in the system Me3NBu-TFSI/La is also presented (solid line); in this voltammogram, the cathodic peak with the form of a shoulder that begins to shape after -2.2 V vs Ag/Ag+ is attributed to the reduction of trivalent lanthanum cation (La3+) to the zerovalent state. The absence of corresponding anodic peak on the reverse scan allows the assumption that lanthanum reduction is irreversible in this ionic liquid, similarly to the ionic liquid BMP-TFSI. This may suggest that the bistriflimide anion (TFSI) that is common in the ionic liquids studied in this work is mainly responsible for the hindering of the oxidation of the electrodeposited metal.

3 2 1

j(mA/cm2)

0 -4

-3,5

-3

-2,5

-2

-1,5

1

-1

-0,5

-1

0

-2 -3 -4

2

-5

E(V) vs Ag/Ag+

Figure 3:

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CVs of (1) Me3NBu-TFSI and (2) the system Me3NBu-TFSI /0.06 M La (WE = Pt, scan rate = 20mV/s vs Ag/Ag+, temperature =25oC).

The electrodeposition of lanthanum was performed on a copper electrode, at the same conditions with the previously mentioned system of BMP-TFSI / La. More specifically the potentiostatic polarization of the copper electrode took place at -3.1V vs Ag/Ag+ for 5h, at 25oC. The formed electrodeposit was examined in SEM (figure 4a), while the EDS analysis of the deposit proved the existence of lanthanum. The lanthanum deposition was mainly accumulated on the edges of the electrode surface. The morphology of the deposit revealed either small nuclei or flat clusters which were produced by the coalition of lanthanum crystals.

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a a

Element

Weight%

SK

5.77

La L

94.23

Totals

100.00

bb

Figure 4: Lanthanum deposition from Me3NBu-TFSI (a) SEM image of electrodeposits (b) the EDS analysis.

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Conclusions

It is derived from the bibliographic inspection that ionic liquids are promising electrolytes, suitable for the electrodeposition of rare earth metals. The research in this area has proven their ability to reduce drastic metals and to permit their electrodeposition, without the drawbacks present in the currently used technology. The preliminary electrochemical investigation of using the ionic liquids BMP-TFSI and Me3NBu-TFSI as electrolytic media for lanthanum electrodeposition enhances these arguments, since the reduction of lanthanum was possible in both ionic liquids under research. The electrodeposition was successfully performed from both systems at room temperature on a copper substrate. The lanthanum electrodeposit in the case of BMP-TFSI was dispersed all over the electrode surface, where in the case of Me3NBu-TFSI, it was mainly observed on the edges of the electrode.

Acknowledgments The research leading to these results has received funding from the European Community’s Seventh Framework Programme ([FP7/2007-2013]) under grant agreement n°309373. This publication reflects only the author’s view, exempting the Community from any liability. Project web site: www.eurare.eu


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