Induction of titanium reduction using pyrrole and polypyrrole in the ionic liquid ethyl-methyl-imidazolium bis(trifluoromethanesulphonyl)amide

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Abstract

The electrowinning of titanium currently involves high temperature processing and quite extreme condition. As part of a project investigating the use of ionic liquids to refine titanium, we have investigated the use of a polymeric nucleating agent to assist the electro-deposition of the metal. Initial attempts focussed on polypyrrole coatings on the working electrode. These were unsuccessful due to the low conductivity of the conducting polymer in the IL at the reductive potentials required to deposit the titanium. However, it was found that pyrrole was a very successful additive able to induce deposition of titanium species from a TiCl4 containing ionic liquid electrolyte ([emim][Ntf2]). Well characterised titanium containing polypyrrole co-deposits have been achieved, when pyrrole was introduced, however bulk metallic titanium was not observed. The presence of titanium species in these deposits was confirmed by XPS. It is thought that the growing pyrrole oligomers form nucleation sites either in situ at the electrode–liquid interface or as a thin film on the electrode allowing co-precipitation of reduced Ti species from solution.

Introduction

Conventional production of titanium utilises the Kroll process which employs high temperatures and an expensive batch reduction of titanium tetrachloride, (produced by an earlier step in the process), by magnesium to yield titanium “sponge” from which magnesium chloride must be removed before further processing to useful metal [1]. Therefore alternative approaches particularly those that are more energy efficient are being researched. Room temperature ionic liquid (IL) electrolytes may hold some promise in this regard. Ionic liquids offer, in some cases, the benefit of very good solvent properties combined with wide electrochemical windows, giving them major advantages in electrowinning processes. The wide potential range gives access to redox chemistries not possible in conventional solvents. For this reason there has been much interest in the use of IL for electrochemical reduction processes. There has been to date a number of reports of new electrowinning or electro-deposition of a variety of metals, including magnesium [2], copper [3], zinc [4], [5] and germanium [6], [7]. Most of these reports have described only small scale studies and often involving other species to assist the nucleation sometimes resulting in a co-deposit.

To date reports of the direct electro-deposition of titanium from a range of ILs at room temperature have not been convincing or successful. Attempts have been hampered by difficulties of eliminating redox cycling of multivalent titanium ions and handing very reactive dendritic products [8]. It is believed that titanium is deposited in a very reactive form, which may be subsequently chemically attacked. The titanium electrodeposits can only be obtained by producing large nuclei, which affect either the nucleation kinetics or the chemical reactivity of the growing surface, and hence induce adherent titanium deposition [9]. Other work has reported that the titanium was electrodeposited [10] on Au(1 1 1) and the step edge [11] of HOPG (highly oriented pyrolytic graphite) from ionic liquids although only a trace amount of titanium metal was obtained, which was not fully characterised. Abbott et al. also reported that titanium electro-deposition was achieved using a foreign metal (such as silver) as an additive in several kinds of aromatic solvent, which nucleates readily, allowing growth of titanium on these nuclei [9]. Shinsaku has shown that the titanium electro-deposition has occurred on an aluminium cathode in propylene carbonate, with Ti forming an alloy with Al [12].

We have previously studied the ability of conducting polymers to electrochemically separate metals through polymer coated membranes facilitated by cycling the conducting polymer, polypyrrole, between its oxidized and reduced forms [13]. In this work we demonstrated that, for some metals, transport occurred through reducing the metal ion to the metal on the polymer surface and then re-oxidising it in a repetitive manner, such that pre-concentration of metal ions occurred at the membrane surface followed by net transport through the membrane. By this method we have separated copper from various matrices [14], [15] as well as other metals such as gold [16]. It has been shown by us and others that with some metals such as gold, the redox chemistry is such that it is possible reduce the metal from its ion in solution by either polypyrrole [17], [18], [19] or polyanline [20], [21] in an electroless fashion. In the case of gold, Au3+ is reduced to Au through the oxidation of the polymer. It is thus possible to use a conducting polymer coated substrate to nucleate metals such as gold to form good deposits. We have shown this can be done with a variety of substrates including reticulated vitreous carbon (RVC) [16] as well as conducting polymer coated textiles [22], [23].

In this study, we have used a similar approach by using pyrrole in the ionic liquid to first deposit polymer to provide a stable nucleation site for titanium to deposit on.

Section snippets

Chemicals

Ethyl-methyl-imidazolium bis(trifluoromethanesulphonyl)amide ([emim][Ntf2]) was prepared using an existing method [24]. Pyrrole was purchased from Sigma–Aldrich/Merck and freshly distilled prior to use. Titanium tetrachloride, titanium(III) and titanium(II) chloride were obtained from Sigma–Aldrich and used as purchased.

Apparatus and electrochemical measurements

All electrochemical experiments and liquid handling and transfers were performed in a nitrogen-filled glovebox. Cyclic voltammetry and electro-deposition were carried out using

Results and discussion

In an initial attempt to see if conducting polymers could facilitate the deposition of titanium from solution a preformed polypyrrole (PPy) coated electrode was used to induce Ti deposition. A PPy-TFSA coated electrode was used to record cyclic voltammograms (CVs) in the ionic liquid media. It was found that stable cyclic voltammograms with minimal currents (less than 50 A) could be obtained over the potential range –2.0 V  +1.5 V, indicating very good stability of the ionic liquid across the full

Conclusions

The present results indicate that pyrrole is able to effectively act as a nucleating agent for the electro-reduction of titanium species from TiCl4 solution in [emim][NTf2]. This is the first time such an effect has been observed. This is quite an unexpected result and may have practical use in electrochemical applications. The possibility of creating nucleation sites in situ in an electrochemical reduction using a polymeric redox active agent is attractive in that it enables a broader range of

Acknowledgements

This work was funded by an Australian Research Council Linkage Project (LP0454110). The financial and other support of Rio Tinto is also gratefully acknowledged.

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