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Electrochemical study of niobium fluoride and oxyfluoride complexes in molten LiF–KF–K2NbF7 bath
www.elsevier.nl/locate/elecom Electrochemistry Communications 1 (1999) 295–300
Electrochemical study of niobium fluoride and oxyfluoride complexes in molten LiF–KF–K2NbF7 bath ´ ˇ 1, V. Danek ˇ ´ J. Hıves V. Van *, A. Silny, ´ ´ cesta 9, 842 36 Bratislava, Slovak Republic Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dubravska Received 1 June 1999; received in revised form 15 June 1999; accepted 16 June 1999
Abstract The reduction of niobium in an equimolar molten mixture of LiF and KF has been investigated using a cyclic voltammetric technique. A two-step mechanism of reduction of niobium in LiF–KF melt is proposed. The influence of oxide ions on the reduction has been studied and it was found to strongly influence the redox properties and structure of niobium ions in the melt. The presence of oxide ions caused the formation of the mono-oxyfluoride complex [Nb(V)OF5]2y. The formation of the mono-oxyfluoride complex was not complete at 7208C. Free oxide ions caused an appearance of di-oxyfluoride complex. In the redundant oxide ions the di-oxyfluoride complex was converted to [Nb(IV)O2F]y by a redox reaction. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Niobium; Molten fluorides; Electrodeposition; Cyclic voltammetry; Reduction mechanism
Nb(I)qeysNb(0)
1. Introduction Niobium is an excellent material for surface treatment of steel materials for use in the chemical industry due to its hardness and corrosion resistance in wet acidic conditions. Niobium is also used for preparation of superconducting tapes and in other branches of industry, for instance, in nuclear technology and metallurgy. For these applications it is necessary to prepare high purity niobium. Electrolytic deposition of metallic niobium from molten electrolytes provides niobium with the required purity. Despite research in this field starting in the early sixties, the mechanism of electrolytic deposition in the presence of complex niobium ions in the electrolyte is not definitely solved. Three basic types of reduction step have been suggested [1–3]. Niobium was found to be present in the melt in all oxidation states from Nb(V) to Nb(0). Senderoff and Mellors [1] confirmed a three-step mechanism of reduction of Nb(V) in a FLINAK melt: Nb(V)qeysNb(IV) Nb(IV)q3eysNb(I)
(1)
* Corresponding author. Tel.: q421-7-5941-0450; fax: q421-7-59410444; e-mail: [email protected] 1 Present address: Faculty of Chemical Technology, Slovak University of ´ 9, 812 37 Technology, Department of Inorganic Technology, Radlinskeho Bratislava, Slovak Republic.
Qiao and Taxil [2] proposed a two-step mechanism of Nb(V) reduction in a LiF–NaF melt: Nb(V)qeysNb(IV) Nb(IV)q4eysNb(0)
(2)
Los and Josiak [3] suggested another two-step mechanism of Nb(V) reduction in the same melt: Nb(V)q2eysNb(III) Nb(III)q3eysNb(0)
(3)
Similar mechanisms were proposed by other authors in chloride and fluoride–chloride melts [2–9]. In the case of chloride and fluoride–chloride melts differences in the reduction mechanism were probably due to a formation of niobium chloride complexes dependent on working temperature. In the case of fluoride melts the differences arose probably as a result of an oxyfluoride complex formation in the presence of oxide ions in the melts. The possibility of obtaining metallic niobium coatings has been demonstrated in fluoride, chloride and mixed fluoride– chloride electrolytes. The deposition of Nb from fluoride baths with O2y ions present has been investigated by Christensen et al. [9]. This is very important because it is extremely difficult to prepare a melt free of O2y ions, especially in industrial application. Therefore, a great part of the
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research effort is concentrated on the influence of O2y ions on the reduction of niobium and the formation of its oxyfluoride complexes in the melt [4,7–11]. The presence of [Nb(V)OF5]2y and [Nb(V)O2F4]3y ions was proved by Raman and infrared spectroscopy in a FLINAK melt [12]. Similar complexes are present in a KCl–NaCl–K2NbF7 melt containing a small amount of O2y ions [13]. The presence of O2y ions lowers the quality of niobium coatings or prevents their formation completely. Relatively pure Nb coatings can be obtained from FLINAK melts if the nO:nNb(V) ratio is less than unity. A small amount of O2y ions can entirely change the deposition mechanism [4,7] depending on different types of the niobium oxyfluoride which are formed in the melt. The purpose of this work is to study, mainly by the cyclic voltammetric technique, the reduction of niobium and its complex formation in a LiF–KF–K2NbF7 melt with a defined amount of O2y ions.
2. Experimental 2.1. The cell The cell (see Fig. 1) consisted of a platinum crucible, which served as a counter electrode, and a platinum wire (OD 1 mm) used as a working electrode. The vertical position (immersion) of the working electrode was adjusted by a micrometer screw. As a quasi-reference electrode a platinum wire with a diameter of 1 mm was used. The platinum reference electrode is not a true reference electrode, but it is extremely difficult to construct a stable reference electrode in these systems. However, it is stable enough for potential control during the recording of the voltammograms and the reference potential is also stable as long as the melt composition is constant. The crucible containing 25 g of the melt was placed in a vertical furnace with an argon atmosphere and heated up to the required temperature range of 700 to 7508C. A Tacussel PRT 20-2 potentiostat was used to run the cyclic voltammetry measurements with a scanning potential rate ranging from 0.1 to 36 V sy1. 2.2. Chemicals The electrolytic bath consisted of an equimolar mixture LiF and KF with additions of K2NbF7 in a concentration range between 20 and 475 mol my3. The melt was prepared from KFØ2H2O (p.a. grade, Lachema) and LiF (pure grade, Ubichem). KF was initially dehydrated under vacuum for 500 h at ambient temperature in the presence of P2O5, and was additionally dehydrated by keeping it for 48 h at 2008C under vacuum. The equimolar mixture of LiF–KF was dehydrated in a vacuum dryer for 48 h at 2008C, then in the furnace for 24 h at 4508C under an argon atmosphere. The niobium oxyfluorides were prepared in situ by means of controlled addi-
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Fig. 1. Experimental apparatus: 1, micrometric screw; 2, movable upper lid; 3, stationary upper lid; 4, thermocouple; 5, lead for counter electrode; 6, platinum crucible; 7 furnace tube; 8, heating elements; 9, lower lid; R, reference electrode; W, working electrode; C, counter electrode.
tions of K2CO3 (p.a., Lachema) to adjust the nO:nNb(V) ratio in the range 0–1.5. K2NbF7 was prepared from Nb2O5 (99.9%, Fluka) according to the method described by Sakawa and Kuroda [14]. The purity of the K2NbF7 was better than 98 mass %.
3. Results and discussion Fig. 2 shows a cyclic voltammogram obtained in the melt with an initial Nb(V) concentration of 250 mol my3 at 7008C, and a scanning rate of 360 mV sy1. Two reduction waves R1 and R2 appeared at potentials Ec1sy0.26 V and Ec2sy0.96 V versus the platinum quasi-reference electrode. While the current intensity of the first wave is much smaller than the second one, the first wave is not clearly defined in this voltammogram. For better visualisation of the first wave the switching potential was reduced and the voltammograms as shown in Fig. 3 were recorded. The peak potentials linearly shift in a negative direction with increasing scanning rate. The shift of the peak potential, when the scanning rate increased tenfold, gave a value of 0.039 V. This value is smaller in comparison with the theoretical value for the irreversible wave (0.048 and 0.193 V for the number of electrons Ns4 and 1, respectively, and working temperature ts7008C [15]) . In fact the potential of the platinum quasi-reference electrode due to the residual amount of oxide ions in the melt may change which causes an apparent potential shift of the
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of electrons involved in the process can be obtained from [15]: E ayE cs2.303RT/nF
(4)
where Ea and Ec are the peak potentials of the anodic and the cathodic waves and n is the number of electrons involved. In this case EayEcs0.15 V resulting in the number of electrons close to one. Two reduction waves observed on the voltammogram of the melt (Fig. 2) indicate a two-step reduction mechanism of [Nb(V)F7]2y. While only [Nb(V)F7]2y was present in the initial melt and a number of electrons in the first reduction is one, the number of electrons in the second reduction must be four. The reduction mechanism can be schemed as Nb(V)qeysNb(VI) Nb(VI)q4eysNb(0) Fig. 2. Cyclic voltammogram of niobium in LiF–KF–K2NbF7 melt at 7008C. cNb(V)s250 mol my3, potential scanning rate 0.36 V sy1. Platinum quasireference electrode.
(5)
The difference EayEcs0.45 V suggests that the second step is irreversible, while the ratio Ia/Ic being close to unity indicates a reversibility of the step. It is known that in the case of deposition of an insoluble product the value of the stripping peak current, Ia, and also the value of the potential, Ea, depend on the amount of the metal deposited during the cathodic scan. The nonclassical behaviour of the reduction wave is due to deposition of the metal. A plot of the reduction peak current density (R2) versus square root of the scanning rate (Fig. 4) shows the transition from a reversible to an irreversible process with an increasing sweep rate. The irreversibility of the reduction is probably due to an adsorption of an insoluble product on the cathode, which can explain the disturbance on the linearity of the jpsf(v1/2) plot in Fig. 4.
Fig. 3. Cyclic voltammograms of niobium in LiF–KF–K2NbF7 melt at 7008C. cNb(V)s250 mol my3, potential scanning rate [V sy1]: (a) 0.36; (b) 1; (c) 4; (d) 9; (e) 16. Platinum quasi-reference electrode.
peaks in the positive direction. This shift is similar to the one caused by the irreversibility of the electrochemical reaction. On the other hand an anode wave was still observed in the backward direction which implies that the above shift is not caused by the irreversibility of the reduction. The shift may also be caused by an uncompensated resistance of the electrolyte. When the irreversibility is excepted the linearity of the relationship between the [Nb(V)F7]2y reduction peak current density and the square root of the scanning rate implies the reversibility of this reduction step, so the number
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Fig. 4. Linear relationship of Nb(IV) reduction peak current density vs. square root of potential scanning rate in LiF–KF–K2NbF7 melt at 7008C. cNb(V)s250 mol my3. Platinum quasi-reference electrode; r, reversible process; i, irreversible process.
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Fig. 5 shows cyclic voltammograms of K2NbF7 in the LiF– KF melt with cNb(V)s57.1 mol my3 and various concentrations of oxide ions added, recorded at 7008C with a scanning rate of 360 mV sy1. When oxide ions were added a shift to more positive values of the voltammogram was registered. The reason for this could be a slight change of the platinum quasi-reference electrode potential with oxide content in the melt. A new reduction peak (R3) was observed at a potential about 100 mV more negative than R2. The occurrence of another reduction peak, together with a decreasing intensity of the reduction peaks R1 and R2, signifies a ligand change and the formation of a new complex. This transformation is complete when the ratio nO:nNb(V)s1, when a new reduction peak R4 occurs as well. The solidified melt had a white colour and X-ray diffraction analysis showed the presence of a new
unknown substance. A light-grey surface layer was obtained on the working electrode when electrolysis of the system was carried out under potentiostatic conditions. This layer is most probably made of NbO(s) as a reduction product of the mono-oxyfluoride complex [Nb(V)OF5]2y. On the other hand, von Barner et al. [12] identified the [Nb(V)OF5]2y complex by Raman spectroscopy in a FLINAK melt with the ratio nO:nNb(V)s1. The new complex can be ascribed to the mono-oxyfluoride complex [Nb(V)OF5]2y which appeared from a ligand displacement reaction: [Nb(V)F7]2yq2O2ys[Nb(V)OF5]2yq2Fy
(6)
If the nO:nNb(V) ratio is above unity, the reduction peak current R3 decreases and R4 increases. When the molar ratio nO:nNb(V)s1.5, only the reduction peak R4 is left in the
Fig. 5. Cyclic voltammograms of niobium in LiF–KF–K2NbF7–K2O melt at 7008C. cNb(V)s57.1 mol my3, potential scanning rate 0.36 V sy1, nO:nNb(V)s(a) 0; (b) 0.2; (c) 0.6; (d) 1; (e) 1.2; (f) 1.5. Platinum quasi-reference electrode.
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Fig. 6. X-ray diffraction pattern for sample obtained in solidified LiF–KF–K2NbF7–K2O melt near the working electrode after electrolysis under potentiostatic conditions (1600 A cmy2; 20 s; 7008C). cNb(V)s570.46 mol my3, nO:nNb(V)s1.5.
and X-ray diffraction analysis showed the presence of KNbO2F (Fig. 6). The appearance of KNbO2F is most probably due to redox reaction: 4[Nb(V)O2F4]3yq2O2ys4[Nb(IV)O2F]yqO2q12Fy (8)
Fig. 7. Cyclic voltammograms of niobium in LiF–KF–K2NbF7–K2O at 7208C. cNb(V)s475 mol my3, potential scanning rate 1.0 V sy1; nO:nNb(V)s0.5, switching potential El [V]: (a) 0.7; (b) 0.9; (c) 1.1; (d) 1.2; (e) 1.3; (f) 1.5; (g) 1.7. Platinum quasi-reference electrode.
voltammogram. This is caused by another ligand displacement reaction leading to formation of a di-oxyfluoride complex of niobium (R4): [Nb(V)OF5]2yq2O2ys[Nb(V)O2F4]3yq2Fy
(7)
Under potentiostatic conditions no deposit was obtained on the working electrode. It seems that reduction of the dioxyfluoride complex does not lead to any deposit. The solidified melt near the working electrode had a dark-grey colour,
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Fig. 7 shows cyclic voltammograms of the LiF–KF– K2NbF7–K2O system with an initial concentration of K2NbF7 cNb(V)s475 mol my3 and the ratio nO:nNb(V)s0.5 recorded at 7208C, with the scanning rate of 1 V sy1, and different switching potentials. The voltammograms show all four reduction peaks. The appearance of R4 while R2 is still present could be explained by the fact that the reaction (6) did not proceed to completion and free oxide ions led to the conversion of mono-oxyfluoride complex to di-oxyfluoride complex. This is supported also by Matthiesen et al. [10] who found that the transformation of the fluoride complex to the mono-oxyfluoride complex was not complete, and it was temperature dependent. In contrast to this work they did not identify the reduction peak R4, possibly because their voltammograms were recorded at lower temperature, and also with a lower amount of free oxide ions in the melt. It is very interesting to observe the part of the curves between the reduction peaks R1 and R2 where the reduction is accelerated. This region disappears at the ratio nO:nNb(V)s1.5 when only R4 is observed on the voltammogram (Fig. 5(f)). The region is caused by the adsorption phenomenon [16] giving a so-called ‘prepeak’ which is visible only at low scanning rate. Since the reduction R4 does not produce any deposited product, no adsorption is observed.
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4. Conclusions
Acknowledgements
The reduction of niobium in the LiF–KF–K2NbF7 melt at 7008C can be described as
The present work was financially supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences under No. 2/ 4032/97.
Nb(V)qeysNb(IV) References
Nb(IV)q4eysNb(0) at potentials of y0.29 and y0.96 V, respectively, versus a platinum quasi-reference electrode. The suggested mechanism of the reduction of Nb(V) complexes in pure fluoride melts can be compared only with the work of Senderoff and Mellors [1] and some newer works [8–10], because most of the authors used melts without Kq ions. The present oxide ions cause the formation of the oxyfluoride complexes according to the reaction: [Nb(V)F7]2yqO2ys[Nb(V)OF5]2yq2Fy
[6] [7] [8] [9] [10]
and at 7208C this ligand displacement reaction is not complete. Free oxide ions cause a further conversion of monooxyfluoride complex to di-oxyfluoride complex. These complexes coexist in the melts. At higher concentration of oxide ions in the melt di-oxyfluoride complex is reduced by a redox reaction: 4[Nb(V)O2F4]3yq2O2ys4[Nb(IV)O2F]yqO2q12Fy
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[1] [2] [3] [4] [5]
[11] [12] [13] [14] [15] [16]
S. Senderoff, G.W. Mellors, J. Electrochem. Soc. 113 (1966) 66. Z. Qiao, P. Taxil, J. Appl. Electrochem. 15 (1985) 250. P. Los, J. Josiak, Bull. Electrochem. 5 (1989) 820. M. Chemla, V. Grinevitch, Bull. Soc. Chim. France N3 (1973) 853. A. Khalidi, P. Taxil, B. Lafage, A.P. Lamaze, Mater. Sci. Forum 73– 75 (1991) 421. A. Barhoun, Y. Berghoute, F. Lantelme, J. Alloys Comp. 179 (1992) 241. V.I. Konstantinov, E.G. Polyakov, P.T. Stangrit, Electrochim. Acta 26 (1981) 445. P. Chamelot, B. Lafage, P. Taxil, Electrochim. Acta 43 (1997) 607. E. Christensen, X. Wang, J.H. von Barner, T. Østvold, N.J. Bjerrum, J. Electrochem. Soc. 141 (1994) 1212. F. Matthiesen, E. Christensen, J.H. von Barner, N.J. Bjerrum, J. Electrochem. Soc. 143 (1996) 1793. F. Lantelme, Y. Berghoute, J.H. von Barner, G.S. Picard, J. Electrochem. Soc. 142 (1995) 4097. J.H. von Barner, E. Christensen, N.J. Bjerrum, B. Gilbert, Inorg. Chem. 30 (1991) 561. J.H. von Barner, R.W. Berg, Y. Berhoute, F. Lantelme Proc. Molten Salt Forum, 1-2 (93/94), 1993, p. 121. M. Sakawa, T. Kuroda, Denki Kagaku 36 (1968) 653. D.D. McDonald, Transient Techniques in Electrochemistry, Plenum, New York, 1977, p. 191. R.H. Wopschall, I. Shain, Anal. Chem. 39 (1967) 1514.
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Electrochemical study of niobium fluoride and oxyfluoride complexes in molten LiF–KF–K2NbF7 bath ´ ˇ 1, V. Danek ˇ ´ J. Hıves V. Van *, A. Silny, ´ ´ cesta 9, 842 36 Bratislava, Slovak Republic Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dubravska Received 1 June 1999; received in revised form 15 June 1999; accepted 16 June 1999
Abstract The reduction of niobium in an equimolar molten mixture of LiF and KF has been investigated using a cyclic voltammetric technique. A two-step mechanism of reduction of niobium in LiF–KF melt is proposed. The influence of oxide ions on the reduction has been studied and it was found to strongly influence the redox properties and structure of niobium ions in the melt. The presence of oxide ions caused the formation of the mono-oxyfluoride complex [Nb(V)OF5]2y. The formation of the mono-oxyfluoride complex was not complete at 7208C. Free oxide ions caused an appearance of di-oxyfluoride complex. In the redundant oxide ions the di-oxyfluoride complex was converted to [Nb(IV)O2F]y by a redox reaction. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Niobium; Molten fluorides; Electrodeposition; Cyclic voltammetry; Reduction mechanism
Nb(I)qeysNb(0)
1. Introduction Niobium is an excellent material for surface treatment of steel materials for use in the chemical industry due to its hardness and corrosion resistance in wet acidic conditions. Niobium is also used for preparation of superconducting tapes and in other branches of industry, for instance, in nuclear technology and metallurgy. For these applications it is necessary to prepare high purity niobium. Electrolytic deposition of metallic niobium from molten electrolytes provides niobium with the required purity. Despite research in this field starting in the early sixties, the mechanism of electrolytic deposition in the presence of complex niobium ions in the electrolyte is not definitely solved. Three basic types of reduction step have been suggested [1–3]. Niobium was found to be present in the melt in all oxidation states from Nb(V) to Nb(0). Senderoff and Mellors [1] confirmed a three-step mechanism of reduction of Nb(V) in a FLINAK melt: Nb(V)qeysNb(IV) Nb(IV)q3eysNb(I)
(1)
* Corresponding author. Tel.: q421-7-5941-0450; fax: q421-7-59410444; e-mail: [email protected] 1 Present address: Faculty of Chemical Technology, Slovak University of ´ 9, 812 37 Technology, Department of Inorganic Technology, Radlinskeho Bratislava, Slovak Republic.
Qiao and Taxil [2] proposed a two-step mechanism of Nb(V) reduction in a LiF–NaF melt: Nb(V)qeysNb(IV) Nb(IV)q4eysNb(0)
(2)
Los and Josiak [3] suggested another two-step mechanism of Nb(V) reduction in the same melt: Nb(V)q2eysNb(III) Nb(III)q3eysNb(0)
(3)
Similar mechanisms were proposed by other authors in chloride and fluoride–chloride melts [2–9]. In the case of chloride and fluoride–chloride melts differences in the reduction mechanism were probably due to a formation of niobium chloride complexes dependent on working temperature. In the case of fluoride melts the differences arose probably as a result of an oxyfluoride complex formation in the presence of oxide ions in the melts. The possibility of obtaining metallic niobium coatings has been demonstrated in fluoride, chloride and mixed fluoride– chloride electrolytes. The deposition of Nb from fluoride baths with O2y ions present has been investigated by Christensen et al. [9]. This is very important because it is extremely difficult to prepare a melt free of O2y ions, especially in industrial application. Therefore, a great part of the
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research effort is concentrated on the influence of O2y ions on the reduction of niobium and the formation of its oxyfluoride complexes in the melt [4,7–11]. The presence of [Nb(V)OF5]2y and [Nb(V)O2F4]3y ions was proved by Raman and infrared spectroscopy in a FLINAK melt [12]. Similar complexes are present in a KCl–NaCl–K2NbF7 melt containing a small amount of O2y ions [13]. The presence of O2y ions lowers the quality of niobium coatings or prevents their formation completely. Relatively pure Nb coatings can be obtained from FLINAK melts if the nO:nNb(V) ratio is less than unity. A small amount of O2y ions can entirely change the deposition mechanism [4,7] depending on different types of the niobium oxyfluoride which are formed in the melt. The purpose of this work is to study, mainly by the cyclic voltammetric technique, the reduction of niobium and its complex formation in a LiF–KF–K2NbF7 melt with a defined amount of O2y ions.
2. Experimental 2.1. The cell The cell (see Fig. 1) consisted of a platinum crucible, which served as a counter electrode, and a platinum wire (OD 1 mm) used as a working electrode. The vertical position (immersion) of the working electrode was adjusted by a micrometer screw. As a quasi-reference electrode a platinum wire with a diameter of 1 mm was used. The platinum reference electrode is not a true reference electrode, but it is extremely difficult to construct a stable reference electrode in these systems. However, it is stable enough for potential control during the recording of the voltammograms and the reference potential is also stable as long as the melt composition is constant. The crucible containing 25 g of the melt was placed in a vertical furnace with an argon atmosphere and heated up to the required temperature range of 700 to 7508C. A Tacussel PRT 20-2 potentiostat was used to run the cyclic voltammetry measurements with a scanning potential rate ranging from 0.1 to 36 V sy1. 2.2. Chemicals The electrolytic bath consisted of an equimolar mixture LiF and KF with additions of K2NbF7 in a concentration range between 20 and 475 mol my3. The melt was prepared from KFØ2H2O (p.a. grade, Lachema) and LiF (pure grade, Ubichem). KF was initially dehydrated under vacuum for 500 h at ambient temperature in the presence of P2O5, and was additionally dehydrated by keeping it for 48 h at 2008C under vacuum. The equimolar mixture of LiF–KF was dehydrated in a vacuum dryer for 48 h at 2008C, then in the furnace for 24 h at 4508C under an argon atmosphere. The niobium oxyfluorides were prepared in situ by means of controlled addi-
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Fig. 1. Experimental apparatus: 1, micrometric screw; 2, movable upper lid; 3, stationary upper lid; 4, thermocouple; 5, lead for counter electrode; 6, platinum crucible; 7 furnace tube; 8, heating elements; 9, lower lid; R, reference electrode; W, working electrode; C, counter electrode.
tions of K2CO3 (p.a., Lachema) to adjust the nO:nNb(V) ratio in the range 0–1.5. K2NbF7 was prepared from Nb2O5 (99.9%, Fluka) according to the method described by Sakawa and Kuroda [14]. The purity of the K2NbF7 was better than 98 mass %.
3. Results and discussion Fig. 2 shows a cyclic voltammogram obtained in the melt with an initial Nb(V) concentration of 250 mol my3 at 7008C, and a scanning rate of 360 mV sy1. Two reduction waves R1 and R2 appeared at potentials Ec1sy0.26 V and Ec2sy0.96 V versus the platinum quasi-reference electrode. While the current intensity of the first wave is much smaller than the second one, the first wave is not clearly defined in this voltammogram. For better visualisation of the first wave the switching potential was reduced and the voltammograms as shown in Fig. 3 were recorded. The peak potentials linearly shift in a negative direction with increasing scanning rate. The shift of the peak potential, when the scanning rate increased tenfold, gave a value of 0.039 V. This value is smaller in comparison with the theoretical value for the irreversible wave (0.048 and 0.193 V for the number of electrons Ns4 and 1, respectively, and working temperature ts7008C [15]) . In fact the potential of the platinum quasi-reference electrode due to the residual amount of oxide ions in the melt may change which causes an apparent potential shift of the
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of electrons involved in the process can be obtained from [15]: E ayE cs2.303RT/nF
(4)
where Ea and Ec are the peak potentials of the anodic and the cathodic waves and n is the number of electrons involved. In this case EayEcs0.15 V resulting in the number of electrons close to one. Two reduction waves observed on the voltammogram of the melt (Fig. 2) indicate a two-step reduction mechanism of [Nb(V)F7]2y. While only [Nb(V)F7]2y was present in the initial melt and a number of electrons in the first reduction is one, the number of electrons in the second reduction must be four. The reduction mechanism can be schemed as Nb(V)qeysNb(VI) Nb(VI)q4eysNb(0) Fig. 2. Cyclic voltammogram of niobium in LiF–KF–K2NbF7 melt at 7008C. cNb(V)s250 mol my3, potential scanning rate 0.36 V sy1. Platinum quasireference electrode.
(5)
The difference EayEcs0.45 V suggests that the second step is irreversible, while the ratio Ia/Ic being close to unity indicates a reversibility of the step. It is known that in the case of deposition of an insoluble product the value of the stripping peak current, Ia, and also the value of the potential, Ea, depend on the amount of the metal deposited during the cathodic scan. The nonclassical behaviour of the reduction wave is due to deposition of the metal. A plot of the reduction peak current density (R2) versus square root of the scanning rate (Fig. 4) shows the transition from a reversible to an irreversible process with an increasing sweep rate. The irreversibility of the reduction is probably due to an adsorption of an insoluble product on the cathode, which can explain the disturbance on the linearity of the jpsf(v1/2) plot in Fig. 4.
Fig. 3. Cyclic voltammograms of niobium in LiF–KF–K2NbF7 melt at 7008C. cNb(V)s250 mol my3, potential scanning rate [V sy1]: (a) 0.36; (b) 1; (c) 4; (d) 9; (e) 16. Platinum quasi-reference electrode.
peaks in the positive direction. This shift is similar to the one caused by the irreversibility of the electrochemical reaction. On the other hand an anode wave was still observed in the backward direction which implies that the above shift is not caused by the irreversibility of the reduction. The shift may also be caused by an uncompensated resistance of the electrolyte. When the irreversibility is excepted the linearity of the relationship between the [Nb(V)F7]2y reduction peak current density and the square root of the scanning rate implies the reversibility of this reduction step, so the number
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Fig. 4. Linear relationship of Nb(IV) reduction peak current density vs. square root of potential scanning rate in LiF–KF–K2NbF7 melt at 7008C. cNb(V)s250 mol my3. Platinum quasi-reference electrode; r, reversible process; i, irreversible process.
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Fig. 5 shows cyclic voltammograms of K2NbF7 in the LiF– KF melt with cNb(V)s57.1 mol my3 and various concentrations of oxide ions added, recorded at 7008C with a scanning rate of 360 mV sy1. When oxide ions were added a shift to more positive values of the voltammogram was registered. The reason for this could be a slight change of the platinum quasi-reference electrode potential with oxide content in the melt. A new reduction peak (R3) was observed at a potential about 100 mV more negative than R2. The occurrence of another reduction peak, together with a decreasing intensity of the reduction peaks R1 and R2, signifies a ligand change and the formation of a new complex. This transformation is complete when the ratio nO:nNb(V)s1, when a new reduction peak R4 occurs as well. The solidified melt had a white colour and X-ray diffraction analysis showed the presence of a new
unknown substance. A light-grey surface layer was obtained on the working electrode when electrolysis of the system was carried out under potentiostatic conditions. This layer is most probably made of NbO(s) as a reduction product of the mono-oxyfluoride complex [Nb(V)OF5]2y. On the other hand, von Barner et al. [12] identified the [Nb(V)OF5]2y complex by Raman spectroscopy in a FLINAK melt with the ratio nO:nNb(V)s1. The new complex can be ascribed to the mono-oxyfluoride complex [Nb(V)OF5]2y which appeared from a ligand displacement reaction: [Nb(V)F7]2yq2O2ys[Nb(V)OF5]2yq2Fy
(6)
If the nO:nNb(V) ratio is above unity, the reduction peak current R3 decreases and R4 increases. When the molar ratio nO:nNb(V)s1.5, only the reduction peak R4 is left in the
Fig. 5. Cyclic voltammograms of niobium in LiF–KF–K2NbF7–K2O melt at 7008C. cNb(V)s57.1 mol my3, potential scanning rate 0.36 V sy1, nO:nNb(V)s(a) 0; (b) 0.2; (c) 0.6; (d) 1; (e) 1.2; (f) 1.5. Platinum quasi-reference electrode.
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Fig. 6. X-ray diffraction pattern for sample obtained in solidified LiF–KF–K2NbF7–K2O melt near the working electrode after electrolysis under potentiostatic conditions (1600 A cmy2; 20 s; 7008C). cNb(V)s570.46 mol my3, nO:nNb(V)s1.5.
and X-ray diffraction analysis showed the presence of KNbO2F (Fig. 6). The appearance of KNbO2F is most probably due to redox reaction: 4[Nb(V)O2F4]3yq2O2ys4[Nb(IV)O2F]yqO2q12Fy (8)
Fig. 7. Cyclic voltammograms of niobium in LiF–KF–K2NbF7–K2O at 7208C. cNb(V)s475 mol my3, potential scanning rate 1.0 V sy1; nO:nNb(V)s0.5, switching potential El [V]: (a) 0.7; (b) 0.9; (c) 1.1; (d) 1.2; (e) 1.3; (f) 1.5; (g) 1.7. Platinum quasi-reference electrode.
voltammogram. This is caused by another ligand displacement reaction leading to formation of a di-oxyfluoride complex of niobium (R4): [Nb(V)OF5]2yq2O2ys[Nb(V)O2F4]3yq2Fy
(7)
Under potentiostatic conditions no deposit was obtained on the working electrode. It seems that reduction of the dioxyfluoride complex does not lead to any deposit. The solidified melt near the working electrode had a dark-grey colour,
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Fig. 7 shows cyclic voltammograms of the LiF–KF– K2NbF7–K2O system with an initial concentration of K2NbF7 cNb(V)s475 mol my3 and the ratio nO:nNb(V)s0.5 recorded at 7208C, with the scanning rate of 1 V sy1, and different switching potentials. The voltammograms show all four reduction peaks. The appearance of R4 while R2 is still present could be explained by the fact that the reaction (6) did not proceed to completion and free oxide ions led to the conversion of mono-oxyfluoride complex to di-oxyfluoride complex. This is supported also by Matthiesen et al. [10] who found that the transformation of the fluoride complex to the mono-oxyfluoride complex was not complete, and it was temperature dependent. In contrast to this work they did not identify the reduction peak R4, possibly because their voltammograms were recorded at lower temperature, and also with a lower amount of free oxide ions in the melt. It is very interesting to observe the part of the curves between the reduction peaks R1 and R2 where the reduction is accelerated. This region disappears at the ratio nO:nNb(V)s1.5 when only R4 is observed on the voltammogram (Fig. 5(f)). The region is caused by the adsorption phenomenon [16] giving a so-called ‘prepeak’ which is visible only at low scanning rate. Since the reduction R4 does not produce any deposited product, no adsorption is observed.
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4. Conclusions
Acknowledgements
The reduction of niobium in the LiF–KF–K2NbF7 melt at 7008C can be described as
The present work was financially supported by the Scientific Grant Agency of the Ministry of Education of the Slovak Republic and the Slovak Academy of Sciences under No. 2/ 4032/97.
Nb(V)qeysNb(IV) References
Nb(IV)q4eysNb(0) at potentials of y0.29 and y0.96 V, respectively, versus a platinum quasi-reference electrode. The suggested mechanism of the reduction of Nb(V) complexes in pure fluoride melts can be compared only with the work of Senderoff and Mellors [1] and some newer works [8–10], because most of the authors used melts without Kq ions. The present oxide ions cause the formation of the oxyfluoride complexes according to the reaction: [Nb(V)F7]2yqO2ys[Nb(V)OF5]2yq2Fy
[6] [7] [8] [9] [10]
and at 7208C this ligand displacement reaction is not complete. Free oxide ions cause a further conversion of monooxyfluoride complex to di-oxyfluoride complex. These complexes coexist in the melts. At higher concentration of oxide ions in the melt di-oxyfluoride complex is reduced by a redox reaction: 4[Nb(V)O2F4]3yq2O2ys4[Nb(IV)O2F]yqO2q12Fy
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[1] [2] [3] [4] [5]
[11] [12] [13] [14] [15] [16]
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