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Study on electroreduction of titanium tetrachloride in acetonitrile solutions
STUDY ON ELECTROREDUCTION OF TITANIUM TETRACHLORIDE IN ACETONITRILE SOLUTIONS S. BIALLOZOR
and A.
LISOWSKA
Institute of Chemical Engineering, Technical University Gdalisk, Poland (Received
3C Nmmber
1979)
Abstract-The reaction of electroreduction of TiCl, in acetonitrilic solution in the presence of Cl.1M LiBF4 has been investigated. It has been suggested that processes of reduction-oxidation Ti(IVkTi(II1) proceed relatively easy. The experimental values of and &,, have been calculated. The mechanism of overall reduction reaction : Ti(IV)-Ti(IIIkTi(II)-Ti(0) has been postulated. It has been found that in the presence of Li+ ions in solution the process of chemical Ti(III)-Ti(0) reduction can occur.
INTRODUCTION
A growing interest in electrochemistry of non-aqueous solutions has been observed recently. This is mainly connected with possibility of practical use of organic solvents in applied electrochemistry, eg in electrodeposition of metals being otherwise impossible to separate by electrolysis in aqueous solutions[ l-3). Among these metals titanium is worth mentioning due to its physical properties and high corrosion resistance. Numerous attempts to obtain metallic Ti by electrodeposition from organic solvents have been undertaken in the past[4-7). So far there is no practical method of producing titanium on industrial scale, and investigations concerning that matter are still under way. There are some patents regarding the subiectr8-101. Manv investigations on Ti deal with the pra&i&l asp&t of e&ctrodeiosition with no attention payed to electrochemistry of the process. Information on these investigations gives no clear explanation as regards the mechanism of elcctrodeposition of Ti or the function and influence of solvent nature on the rate and overpotential of the cathodic electrodeposition process. There is also lack of polarographic data for Ti in organic solvents. The exception is the work of Kolthoff and Thomas[ll] which concerns the studies on polarographic electroreduction of TiCI, in aceto&rile (AN). It might be of particular interest to investigate with the electrochemistry of Ti reduction process on solid electrodes. The aim of this work is to find some more information about this process. EXPERIMENTAL The solutions were prepared by dilution of saturated solution of TiCl, in AN. To obtain that solution the anhydrous TiC14 was added dropwise to AN cooled by dry ice[ 111. TiCl, was previously purified by vacuum distillation. Methods of purification and drying of this and other reagents have been presented elsewhere[12,13]. All operations on solutions were carried out in a dry box. The working electrode was Pt wire mounted in a glass tube. The exposed area of
* Cambridge, Micro-Scan 5.
working electrode was 3.14. 10m2 cm2. Some expcriments with deposition were carried out on Pt plate electrodes. The reference electrode was ferri-ferrocene (0.01 M in AN) electrode and all potentials wererecalculated us (see) aq without correction for liquid-junction potential. The auxiliary electrode was Pt with visible area about 100 times larger than working electrodes. Two kinds of cells were used. The first had cathodic and anodic arms separated by sintered glass Schott filter No. 3. The second had no separation. The choice ofcell depended upon concentration of TiCl,. All solutions were deoxygenated with pre-purified Ar that had been saturated with acetonitrile. Experiments were conducted tit 25 (+2)“C. Cyclic (triangular wave) voltammetry (cv), chronopotentiometry and galvanostatic methods were used. In order to obtain analytical data of the electrodeposit, X-ray microanalyser* was used.
RESULTS AND DISCUSSION In Figure 1 the shape of cv curves for Pt electrode 1.2mM TiCl.+ + 0.1 M LiBF, acetonitrile solution
in at
various rate of potential changes in time can be seen. In the cathodic portion of the cycle a sharp current peak lcp appears and also a peak of reverse current on anodic branch of triangular wave I, is visible. In Tables 1 and 2 there are listed cv quantitative data obtained for solutions of various TiCl, concentration. Chemical analysis carried out in accordance with the procedure given in[ll] showed that within the uotential range -0.3 to -0.8 V the reduction ofTi _ , io Ti(III) to&k place. Weakly expressed “hump” which may be seen in case of the cyclic polarization (Fig. 1B) within the potential range - 0.75 to 0.80 V is subject to evident increase in height and convented into a second peak. In Figure 2 there is shown the run of the cv curves obtained .from solution containing excessive amount of CI- ions being introduced in form of Et4NCl salt to solution in which one could expect the total Ti(IV) being bound into complexes ofhigher Cl- ion content TiCI;, TiCI;-[ll]. As could be seen there is only one distinct cathodic peak for that solution corresponding
1209
S. BIALLOZOR ANU A. LSOWSKA
1210
0
+0.5
0
Fig. l(a).
Cyclic voltammetry
v
curves of TiCI, (c = 0.0012 M) in 0.1 M LiBF, at various scan rates: l-0.03; 2-0.06; 3-0.08; 4-0.12 and 5-O.l8Vs-‘.
to E, = - 78 V. Thus one may presume that formation of the “hump” within the -0.75 to -0.8 V potential range without Cl- excess is connected with possibility of discharging on the electrode the Ti-complexes which contain more than 4Cl- ions.
Table 1. Some cyclic voltammetry
Kolthoff and Thomas have proved[ll] solutions of TiCl, the following reaction 2TiC1, - 2AN P TiCI; Themagnitude
E .w
0.31 0.32 0.34 0.34 0.32
-0.44 -0.33 -0.36 -0.34 -0.38
Ti(lI1) in
I @P
(mA cm-‘)
* 10%; (V) -0.05 -0.09 - 0.02 - 0.03 - 0.07
0.39 0.24 0.34 0.31 0.31
1.75 2.61 3.82 4.49 4.62
I./~,
1.114 1.37 2.55 2.96 3.44
0.64 0.69 0.66 0.66 0.75
Table 2. Experimental values ofE,,, cathodic and anodic portion ofpolarization curves obtained by cv method, sweep rate of 0.12 V s- ’ CTICI. lo3 (M dcm-“) 1.20 1.74 4.34 8.74 9.60
E Will
*J%,l”
*Ec,,“’ f 10%
- 0.22 -0.21 -0.16 -0.13 -0.18
-
1.38 1.35 1.12 1.21 1.30
* No peak, only wave was observed. t The ill wave appeared.
t( - 2.50) - 2.25 -2.32 - 2.38
EPP121 E.p,lll
E42 ”
; 07 - 0.29 -0.28 -021 - 0.22 -0.26
-
1.30 1.32 1.23 1.12 1.25
(1)
to 7. 10m4 (degree.
process of Ti(IV)+
I CP
A-%
that in AN takes place :
+ TiCI:(
of K, of (1) amounts
characteristics of the electroreduction acetonitrile, sweep rate of 0.12 V s-l E cm
El CTiCL4 10’ (M dcm-3) 1.20 1.74 4.34 8.74 9.60
-1.0
-0.5
E.
t( - 2.50) (-2.10) -2.05
Study on electroreductionof titanium tetrachloridein acetonitrile solutions
E.
Fig. l(b). Series ofcyclic voltammetry
1211
V
curves of TiCI, in 0.1 M LiBF&, cT1c,4 = 0.001 M, scan rate 0.12 V s- ‘,
or
(3)
TiCI
+ e--* TiCI: -
The increase of the “hump” height with successive cycles on the cv curve (Fig. lb) seems to indicate that Cl- ions adsorbed on the electrode surface participate in (3). This is far more probable since TiCI; ion in AN is labile and is subject to dissociation[17] with formation of a stable complex TiC13 ‘3AN TiCl; + TiCI, + ClAccording to diagnostic criteria[14,15] the electrode process under investigation belongs to irreversible electrode reactions. This results from
4
E
E Pi2 -I&>--
0.058 V (4)
n
In this case the value of an, can be estimated by basing on following relation[ 141 E P/2-E,z-
Fig. 2. Cyclic voltammetrycurves of TiCI, (c = 0.008 M) in 0.1 M Et,NCI
at three scan rates: 3-0.09vs-1.
l-0.03;
2-0.06
and
0.048
(5)
an,
If the values of peak current is limited by linear diffusion of the depolarizer ions, the relation between I,, and cTic4 is as follows: I, = 2.99 ’ 105AnJ(an,VDo)c
of dissociation cr, = 0.026), thus in a solution without Cl- ion excess, one can assume that cT,cI, > cric13, CTiCl i. One may therefore suppose on the above basis that the 1st cathodic peak corresponds to reduction reaction* TiCl., + e -
TiCI,
(2)
while the 2nd peak may result from the reaction: TiCI;
+ e*TiClg-
+ For convenience the participation reactions has been nealected.
of AN molecules
on
(6)
where: A = electrode area (cm’), and other symbols have their usual meaning. Value k can be additionally calculated from the relation : Ig k, = 1.1141 +
$ lg(an,Du) + s(E,
- E,)
(7)
Straight line 2 in Fig. 3 represents dependence I,, = f/cric,,) within the concentration range of 1.2 to 9.6 mM. The value of DTiC,a.2ANcalculated from the slopeof the straight line in accordance with (6) is equal to 3.4 aiO_” cm2 s- ’ while values of DWL and &XXZ
S. BIALLOZOR
AND
A. LISOWSKA
I
I
0
A:.\ 0
I
02
v
055
4
2
i,
mA
6
cm-’
Fig. 5. Chronopotentiometry curve of TiCl, (c = 0.0096 M) in 0.1 M LiBF,; i, = 3.06 mA cm-’ (1) and i,r”’ us i, in this same solution (2).
Fig. 3. Plots of log i, against log cr,,-,~(l) and I,, against c,,,,,(2) in 0.1 M LiBF,. *
calculated from equation Zcr =f(v”‘) (see Fig. 4) for fluoroboride and chloride solutions amount to 2.1 . lo-’ and 3.05. 10e5 cm3 s-l respectively. Kolthoff and Thomas[ll] by applying the polarographic method, obtained similar results for D values, 4ic,+~ = 1.0x 10-5cmz s-l and &,,-,s- =2.1 x 10m5 cm2 s-l. Dependence of log I, = f(log cTiclr) for LiBF, and Et,NCl solutions is shown in Fig. 4.
The Semerano coefficients calculated from the slope of these straight lines are equal to xcgF-,) = 0.4 and xcc,-, = 0.38 respectively, thus indicating that kineticdiffusion processes decide of the I, height. This is also evidenced by graphical presentation of the dependence i,/r =f(iJ(Fig. 5), obtained by the chronopotentiometric method. On the basis of the relation I,,/V”2 =f(V) (Fig. 4) we can conclude that reduction reaction Ti(IV) -+ Ti(III) is a catalytic process[14]. In the electrochemical literature the possibility of catalytic effect of ligands in case of reduction of complex Ti(IV) compounds in aqueous solutions has been quoted several times[l8]. On the above basis one could presume that in the
Fig. 4. log I_ uslog V plots in 0.1 M UBF, ( I-c,,~,, = 0 .0015 M)and 0.1 M Et,NCI (2-c,,,,, 3--I&’ us V plot (0.0015 M TiCI, + 0.1 M LiBF,).
= 0.008 M)
Study on electroreduction
1213
of titanium tetrachloride in acetonitrile solutions
Fig. 6. Effect of triangular waves amplitude on cyclic voitammetry
cuves
of TiCI, (c = 0.0043 M), scan rate
0.18 vs-‘.
system being subject to investigation, the catalytic effect may be exhibited by Cl- ions adsorbed on Pt surface. However results obtained with Et,NCl solutions do not confirm this hypothesis in an explicit manner. The magnitude of transfer coefficients (r calculated from Tafel’s equation for E, < E,iz and from (5) by assuming that the latter may be applied for the system investigated, in order to obtain approximate values of electrode reaction kinetic parameters in LiBF, and Et,NCl solutions, an, amount to 0.3 kO.05 and 0.20+0.02, whereas kCEplljvalues to 1.58 . 10m3 cm s-l cm s-’ (c,,,, = (CTiW = 5mM) and 1.9.10e3 8.4mM) respectively (7).* Unfortunately in the systems investigated a simultaneous change ofcation and anion had to take place while passing from boron fluoride to chloride solutions. This results from the fact that LiCl is not sufficiently soluble in AN that it could be used as the supporting electrolyte, and on the other hand, cation Et4N+ having higher adsorption ability on Pt surface may exert inhibitive action in respect to the electrode reaction examined. The distinct flattening of I,, curve observed in the course of successive, rapid cathodic polarizations proves that such a possibility should be taken into account. Nevertheless, the principle of the catalytic reaction associated with reduction process Ti(IV) -* Ti(II1) in AN solutions cannot be, for the time being, defined. The order of the cathodic reaction calculated from Vetter’s equation log i, ~~-
( > 1%Cl
B = oonst
=
PC
is equal to 1.2 (see Fig. 3, curve 1). In Figs 6 and 7 there are shown cv curves for high cathodic polarization. The third poorly defined cathodic peak I,,(III) appears on these curves at Epi2 = - 1.3 V. The height of this peak current is almost independent of TiC14 concentration. On the anodic
* Assuming that in case of sufficiently high polarization rate when the magnitude of I,,current is mainly determined by the diffusion process, the WZ,and k values have been calculated by applying (7).
-2
-1
E,
v
Fig. 7. Cyclic tration
voltammetry curves of TiCI., (lPconcenO.OO!X M) and of supporting electrolyte (2 -concentration 0.01 M LiBF&), scan rate 0.12 V SK’.
portion of the cycle a sharp current peak 1.&l) is observed. There is a slight dependence of direct proportional character between height of I&L) and the scanning amplitude. Taking into account the data from literature[ll] it is possible to suggest that the process of electroreduction of Ti(ITI) to Ti(II) is defined by the second cathodic wave. In this electrode reduction process the salt of TiCl, slightly soluble in AN is formed. According to[17] that salt is unstable in AN and undergoes disproportionation which gives TiCI, and Ti(0). One may assume that in effect of above electrode reaction the surface of Pt electrode is covered by layer of insoluble TiCl,, which is oxidized in the reverse run of polarization cycle. In consequence of this latter reaction TiCl, is removed from the electrode surface. This is reflected by the sharp peak current I,, on the anodic cycle of curve. The values of potential Q2 for the individual I, are listed in Table 2. In Fig. 7 there are shown cv curves obtained for the case when E > - 3 V. As can be seen electrodecomposition of the supporting electrolyte
S.
1214
BIALLOZORAND
which results in electrodecomposition of Li is taking place when potential of the working electrode exceeds the value of - 3.1 V. On the growing cathodic portion of the curve a step in the potential value of y -2.2 occurs
which is not observed
in solutions
d
I
\
,
.
Li(0) + Ti(II1) -
_
in results
Li ’ + Ti(0)
This process may be also favoured by high chemical activity of Ti freshly deposited on the electrode surface. Owing to their complexity, the questions raised in the present article will be further investigated.
containing
only supporting electrolyte (LiBF4). Most likely the step observed is caused by electroreduction of the layer of TiCl,, salt to Ti(0). Though there is no clear visible cathodic wave corresponding to the process mentioned above, one can observe a sharp peak of current I,,(III) after the anodic peak related to the dissolution of Li(I,,-IV). The peak current I&III) appears neither in case of the supporting electrolyte only is in AN solution nor when the cathodic polarization is lower (see Fig. 6). The most probable explanation of this fact seems to be the presumption that the third anodic peak on the current curve corresponds to the anodic oxidation of freshlv deoosited TilOl. One may oresume that Ti(0) appears on the electrode primarily secondary chemical reaction :
A. LI~OWSKA
oi a
SUMMARY
1. It has been found that electroreduction of TiCI.+ in AN is multi-step process. The first is electroreduction of Ti(IV) to Ti(II1). Some kinetic parameters of this reaction have been calculated. 2. The electroreduction to Ti(0) demanded so high cathodic polarization that it was proceeded by electrodeposition of Li(0). 3. Basing on the experimental data the mechanism of examined process has been proposed. It has been confirmed that reduction to Ti(0) proceeds mainly by chemica1 reaction
with Li(ol-
Acknowledgement - This work was supported by the Coordination Committee of MR J-11 Problem.
(9)
This suggestion is confirmed also by the investigation that has been conducted to check the possibility of electrodeposition of Ti from AN on Pt electrode. It has been found that during electrolysis of 0.078 M TiCl, + 0.1 M LiBF, under cathodic potentials from - 3.5 to -3.7V (i, = 0.8 A .dcrn-‘) a dark metallic deposit was formed on the platinum electrode. The X-ray microanalysis of the deposit composition showed that it contained up to 37% Ti and the thickness of the layer was about 15 pm. In addition to Ti the deposits showed very high content of carbon (about 60 per cent) that remains in agreement with observations of other authors[7]. Presumably this is the result ofconducting the reaction under very high cathodic polarization conditions, which is associated with decomposition of organic electrolyte. It should be however, stressed that in solutions not containing TiCL, or in solutions of the latter in Et,NBF& as a supporting electrolyte, formation of deposits containing neither metallic Ti nor carbon has been observed. One can, therefore, suppose that Ti(II1) and Ti(I1) compounds formed during reduction of Ti(IV) salts in organic solvent, exhibit so strong a reducing action that may cause, under conditions ofadvanced cathodic polarization, reduction of solvent molecules up to formation of C deposits on the electrode.
REFERENCES 1. A. J. Parker, Proc. R. Austral. Chew Inst. 39,163 (1972). and Elec2. A. Brenner, Advances in Electrochemistry trochem. Engng Vol. 5, p. 205. New York (1967). 3. A. Bartecki in Pierwiastki Rzadkie i Metnlurgia Chemiczna, Vol. 2, p. 17. Wroclaw (1973). 4. W. E. Reid, J. M. Bish and A. Brenner, J. electrochem. Sot. lolr, 21 (1967). 5. E. Santos and F. Dyment, Plating 60, 821 (1973). 6. A. S. Avaliani, Trudy in-ta prikl. khimij i elektrokh. AN Grur. SSSR 3, 67 (1962). I. A. M. Levinskiene and L. E. Simanavicius, V. Ja. Akimov, Trudy AN LiQt. SSE, ser. 3 5, 37 (1977). 8. Patent RFN Nr 169074. 26 Jan. (1968). 9. Patent lap. Nr 9202, 30’Oct. (1927). ’ 10. V. V. Kaznetsov, V. P. Grigoriev, 0. A. Osipov, V. A. Kogan, V. K. Chikhirkin and E. P. Borschenko, Ockryr. Izobr. Prom. Obrazcy, Too. Znaki 51, 104 (1974). 11. J. M. Kolthoffand F. G. Thomas, .I. efectrochem. Sot. 111, 1065 (1964). 12. S. Bia‘llozor, 2. Nouk. 215, Chemia 24 (1974). 13. S. Biallozor, Electrochim. Acta 17, 1243 (1972). 14. P. Delahay, New Instrumental Methods in Ele&chemistry, p. 143. Interscience, New YorkkLondon (1954). 15. R. S. Nicholson, I. Shain, Analyt. Chem. 36, 706 (1964). 16. A. G. Stromberzand A. J. Kartushinskaia. _ Zh. Fir. Khim. 34, 1684 (i96Oj: 17. T. C. Franklin and H. V. Seklemian. J. inorg. nucl. Chem. 12, 181 (1959). 18. 0. Je. Ruvinskij, in PolarograJja, Problemy i Perespektyvy, p. 196. Riga (1977).
and A.
LISOWSKA
Institute of Chemical Engineering, Technical University Gdalisk, Poland (Received
3C Nmmber
1979)
Abstract-The reaction of electroreduction of TiCl, in acetonitrilic solution in the presence of Cl.1M LiBF4 has been investigated. It has been suggested that processes of reduction-oxidation Ti(IVkTi(II1) proceed relatively easy. The experimental values of and &,, have been calculated. The mechanism of overall reduction reaction : Ti(IV)-Ti(IIIkTi(II)-Ti(0) has been postulated. It has been found that in the presence of Li+ ions in solution the process of chemical Ti(III)-Ti(0) reduction can occur.
INTRODUCTION
A growing interest in electrochemistry of non-aqueous solutions has been observed recently. This is mainly connected with possibility of practical use of organic solvents in applied electrochemistry, eg in electrodeposition of metals being otherwise impossible to separate by electrolysis in aqueous solutions[ l-3). Among these metals titanium is worth mentioning due to its physical properties and high corrosion resistance. Numerous attempts to obtain metallic Ti by electrodeposition from organic solvents have been undertaken in the past[4-7). So far there is no practical method of producing titanium on industrial scale, and investigations concerning that matter are still under way. There are some patents regarding the subiectr8-101. Manv investigations on Ti deal with the pra&i&l asp&t of e&ctrodeiosition with no attention payed to electrochemistry of the process. Information on these investigations gives no clear explanation as regards the mechanism of elcctrodeposition of Ti or the function and influence of solvent nature on the rate and overpotential of the cathodic electrodeposition process. There is also lack of polarographic data for Ti in organic solvents. The exception is the work of Kolthoff and Thomas[ll] which concerns the studies on polarographic electroreduction of TiCI, in aceto&rile (AN). It might be of particular interest to investigate with the electrochemistry of Ti reduction process on solid electrodes. The aim of this work is to find some more information about this process. EXPERIMENTAL The solutions were prepared by dilution of saturated solution of TiCl, in AN. To obtain that solution the anhydrous TiC14 was added dropwise to AN cooled by dry ice[ 111. TiCl, was previously purified by vacuum distillation. Methods of purification and drying of this and other reagents have been presented elsewhere[12,13]. All operations on solutions were carried out in a dry box. The working electrode was Pt wire mounted in a glass tube. The exposed area of
* Cambridge, Micro-Scan 5.
working electrode was 3.14. 10m2 cm2. Some expcriments with deposition were carried out on Pt plate electrodes. The reference electrode was ferri-ferrocene (0.01 M in AN) electrode and all potentials wererecalculated us (see) aq without correction for liquid-junction potential. The auxiliary electrode was Pt with visible area about 100 times larger than working electrodes. Two kinds of cells were used. The first had cathodic and anodic arms separated by sintered glass Schott filter No. 3. The second had no separation. The choice ofcell depended upon concentration of TiCl,. All solutions were deoxygenated with pre-purified Ar that had been saturated with acetonitrile. Experiments were conducted tit 25 (+2)“C. Cyclic (triangular wave) voltammetry (cv), chronopotentiometry and galvanostatic methods were used. In order to obtain analytical data of the electrodeposit, X-ray microanalyser* was used.
RESULTS AND DISCUSSION In Figure 1 the shape of cv curves for Pt electrode 1.2mM TiCl.+ + 0.1 M LiBF, acetonitrile solution
in at
various rate of potential changes in time can be seen. In the cathodic portion of the cycle a sharp current peak lcp appears and also a peak of reverse current on anodic branch of triangular wave I, is visible. In Tables 1 and 2 there are listed cv quantitative data obtained for solutions of various TiCl, concentration. Chemical analysis carried out in accordance with the procedure given in[ll] showed that within the uotential range -0.3 to -0.8 V the reduction ofTi _ , io Ti(III) to&k place. Weakly expressed “hump” which may be seen in case of the cyclic polarization (Fig. 1B) within the potential range - 0.75 to 0.80 V is subject to evident increase in height and convented into a second peak. In Figure 2 there is shown the run of the cv curves obtained .from solution containing excessive amount of CI- ions being introduced in form of Et4NCl salt to solution in which one could expect the total Ti(IV) being bound into complexes ofhigher Cl- ion content TiCI;, TiCI;-[ll]. As could be seen there is only one distinct cathodic peak for that solution corresponding
1209
S. BIALLOZOR ANU A. LSOWSKA
1210
0
+0.5
0
Fig. l(a).
Cyclic voltammetry
v
curves of TiCI, (c = 0.0012 M) in 0.1 M LiBF, at various scan rates: l-0.03; 2-0.06; 3-0.08; 4-0.12 and 5-O.l8Vs-‘.
to E, = - 78 V. Thus one may presume that formation of the “hump” within the -0.75 to -0.8 V potential range without Cl- excess is connected with possibility of discharging on the electrode the Ti-complexes which contain more than 4Cl- ions.
Table 1. Some cyclic voltammetry
Kolthoff and Thomas have proved[ll] solutions of TiCl, the following reaction 2TiC1, - 2AN P TiCI; Themagnitude
E .w
0.31 0.32 0.34 0.34 0.32
-0.44 -0.33 -0.36 -0.34 -0.38
Ti(lI1) in
I @P
(mA cm-‘)
* 10%; (V) -0.05 -0.09 - 0.02 - 0.03 - 0.07
0.39 0.24 0.34 0.31 0.31
1.75 2.61 3.82 4.49 4.62
I./~,
1.114 1.37 2.55 2.96 3.44
0.64 0.69 0.66 0.66 0.75
Table 2. Experimental values ofE,,, cathodic and anodic portion ofpolarization curves obtained by cv method, sweep rate of 0.12 V s- ’ CTICI. lo3 (M dcm-“) 1.20 1.74 4.34 8.74 9.60
E Will
*J%,l”
*Ec,,“’ f 10%
- 0.22 -0.21 -0.16 -0.13 -0.18
-
1.38 1.35 1.12 1.21 1.30
* No peak, only wave was observed. t The ill wave appeared.
t( - 2.50) - 2.25 -2.32 - 2.38
EPP121 E.p,lll
E42 ”
; 07 - 0.29 -0.28 -021 - 0.22 -0.26
-
1.30 1.32 1.23 1.12 1.25
(1)
to 7. 10m4 (degree.
process of Ti(IV)+
I CP
A-%
that in AN takes place :
+ TiCI:(
of K, of (1) amounts
characteristics of the electroreduction acetonitrile, sweep rate of 0.12 V s-l E cm
El CTiCL4 10’ (M dcm-3) 1.20 1.74 4.34 8.74 9.60
-1.0
-0.5
E.
t( - 2.50) (-2.10) -2.05
Study on electroreductionof titanium tetrachloridein acetonitrile solutions
E.
Fig. l(b). Series ofcyclic voltammetry
1211
V
curves of TiCI, in 0.1 M LiBF&, cT1c,4 = 0.001 M, scan rate 0.12 V s- ‘,
or
(3)
TiCI
+ e--* TiCI: -
The increase of the “hump” height with successive cycles on the cv curve (Fig. lb) seems to indicate that Cl- ions adsorbed on the electrode surface participate in (3). This is far more probable since TiCI; ion in AN is labile and is subject to dissociation[17] with formation of a stable complex TiC13 ‘3AN TiCl; + TiCI, + ClAccording to diagnostic criteria[14,15] the electrode process under investigation belongs to irreversible electrode reactions. This results from
4
E
E Pi2 -I&>--
0.058 V (4)
n
In this case the value of an, can be estimated by basing on following relation[ 141 E P/2-E,z-
Fig. 2. Cyclic voltammetrycurves of TiCI, (c = 0.008 M) in 0.1 M Et,NCI
at three scan rates: 3-0.09vs-1.
l-0.03;
2-0.06
and
0.048
(5)
an,
If the values of peak current is limited by linear diffusion of the depolarizer ions, the relation between I,, and cTic4 is as follows: I, = 2.99 ’ 105AnJ(an,VDo)c
of dissociation cr, = 0.026), thus in a solution without Cl- ion excess, one can assume that cT,cI, > cric13, CTiCl i. One may therefore suppose on the above basis that the 1st cathodic peak corresponds to reduction reaction* TiCl., + e -
TiCI,
(2)
while the 2nd peak may result from the reaction: TiCI;
+ e*TiClg-
+ For convenience the participation reactions has been nealected.
of AN molecules
on
(6)
where: A = electrode area (cm’), and other symbols have their usual meaning. Value k can be additionally calculated from the relation : Ig k, = 1.1141 +
$ lg(an,Du) + s(E,
- E,)
(7)
Straight line 2 in Fig. 3 represents dependence I,, = f/cric,,) within the concentration range of 1.2 to 9.6 mM. The value of DTiC,a.2ANcalculated from the slopeof the straight line in accordance with (6) is equal to 3.4 aiO_” cm2 s- ’ while values of DWL and &XXZ
S. BIALLOZOR
AND
A. LISOWSKA
I
I
0
A:.\ 0
I
02
v
055
4
2
i,
mA
6
cm-’
Fig. 5. Chronopotentiometry curve of TiCl, (c = 0.0096 M) in 0.1 M LiBF,; i, = 3.06 mA cm-’ (1) and i,r”’ us i, in this same solution (2).
Fig. 3. Plots of log i, against log cr,,-,~(l) and I,, against c,,,,,(2) in 0.1 M LiBF,. *
calculated from equation Zcr =f(v”‘) (see Fig. 4) for fluoroboride and chloride solutions amount to 2.1 . lo-’ and 3.05. 10e5 cm3 s-l respectively. Kolthoff and Thomas[ll] by applying the polarographic method, obtained similar results for D values, 4ic,+~ = 1.0x 10-5cmz s-l and &,,-,s- =2.1 x 10m5 cm2 s-l. Dependence of log I, = f(log cTiclr) for LiBF, and Et,NCl solutions is shown in Fig. 4.
The Semerano coefficients calculated from the slope of these straight lines are equal to xcgF-,) = 0.4 and xcc,-, = 0.38 respectively, thus indicating that kineticdiffusion processes decide of the I, height. This is also evidenced by graphical presentation of the dependence i,/r =f(iJ(Fig. 5), obtained by the chronopotentiometric method. On the basis of the relation I,,/V”2 =f(V) (Fig. 4) we can conclude that reduction reaction Ti(IV) -+ Ti(III) is a catalytic process[14]. In the electrochemical literature the possibility of catalytic effect of ligands in case of reduction of complex Ti(IV) compounds in aqueous solutions has been quoted several times[l8]. On the above basis one could presume that in the
Fig. 4. log I_ uslog V plots in 0.1 M UBF, ( I-c,,~,, = 0 .0015 M)and 0.1 M Et,NCI (2-c,,,,, 3--I&’ us V plot (0.0015 M TiCI, + 0.1 M LiBF,).
= 0.008 M)
Study on electroreduction
1213
of titanium tetrachloride in acetonitrile solutions
Fig. 6. Effect of triangular waves amplitude on cyclic voitammetry
cuves
of TiCI, (c = 0.0043 M), scan rate
0.18 vs-‘.
system being subject to investigation, the catalytic effect may be exhibited by Cl- ions adsorbed on Pt surface. However results obtained with Et,NCl solutions do not confirm this hypothesis in an explicit manner. The magnitude of transfer coefficients (r calculated from Tafel’s equation for E, < E,iz and from (5) by assuming that the latter may be applied for the system investigated, in order to obtain approximate values of electrode reaction kinetic parameters in LiBF, and Et,NCl solutions, an, amount to 0.3 kO.05 and 0.20+0.02, whereas kCEplljvalues to 1.58 . 10m3 cm s-l cm s-’ (c,,,, = (CTiW = 5mM) and 1.9.10e3 8.4mM) respectively (7).* Unfortunately in the systems investigated a simultaneous change ofcation and anion had to take place while passing from boron fluoride to chloride solutions. This results from the fact that LiCl is not sufficiently soluble in AN that it could be used as the supporting electrolyte, and on the other hand, cation Et4N+ having higher adsorption ability on Pt surface may exert inhibitive action in respect to the electrode reaction examined. The distinct flattening of I,, curve observed in the course of successive, rapid cathodic polarizations proves that such a possibility should be taken into account. Nevertheless, the principle of the catalytic reaction associated with reduction process Ti(IV) -* Ti(II1) in AN solutions cannot be, for the time being, defined. The order of the cathodic reaction calculated from Vetter’s equation log i, ~~-
( > 1%Cl
B = oonst
=
PC
is equal to 1.2 (see Fig. 3, curve 1). In Figs 6 and 7 there are shown cv curves for high cathodic polarization. The third poorly defined cathodic peak I,,(III) appears on these curves at Epi2 = - 1.3 V. The height of this peak current is almost independent of TiC14 concentration. On the anodic
* Assuming that in case of sufficiently high polarization rate when the magnitude of I,,current is mainly determined by the diffusion process, the WZ,and k values have been calculated by applying (7).
-2
-1
E,
v
Fig. 7. Cyclic tration
voltammetry curves of TiCI., (lPconcenO.OO!X M) and of supporting electrolyte (2 -concentration 0.01 M LiBF&), scan rate 0.12 V SK’.
portion of the cycle a sharp current peak 1.&l) is observed. There is a slight dependence of direct proportional character between height of I&L) and the scanning amplitude. Taking into account the data from literature[ll] it is possible to suggest that the process of electroreduction of Ti(ITI) to Ti(II) is defined by the second cathodic wave. In this electrode reduction process the salt of TiCl, slightly soluble in AN is formed. According to[17] that salt is unstable in AN and undergoes disproportionation which gives TiCI, and Ti(0). One may assume that in effect of above electrode reaction the surface of Pt electrode is covered by layer of insoluble TiCl,, which is oxidized in the reverse run of polarization cycle. In consequence of this latter reaction TiCl, is removed from the electrode surface. This is reflected by the sharp peak current I,, on the anodic cycle of curve. The values of potential Q2 for the individual I, are listed in Table 2. In Fig. 7 there are shown cv curves obtained for the case when E > - 3 V. As can be seen electrodecomposition of the supporting electrolyte
S.
1214
BIALLOZORAND
which results in electrodecomposition of Li is taking place when potential of the working electrode exceeds the value of - 3.1 V. On the growing cathodic portion of the curve a step in the potential value of y -2.2 occurs
which is not observed
in solutions
d
I
\
,
.
Li(0) + Ti(II1) -
_
in results
Li ’ + Ti(0)
This process may be also favoured by high chemical activity of Ti freshly deposited on the electrode surface. Owing to their complexity, the questions raised in the present article will be further investigated.
containing
only supporting electrolyte (LiBF4). Most likely the step observed is caused by electroreduction of the layer of TiCl,, salt to Ti(0). Though there is no clear visible cathodic wave corresponding to the process mentioned above, one can observe a sharp peak of current I,,(III) after the anodic peak related to the dissolution of Li(I,,-IV). The peak current I&III) appears neither in case of the supporting electrolyte only is in AN solution nor when the cathodic polarization is lower (see Fig. 6). The most probable explanation of this fact seems to be the presumption that the third anodic peak on the current curve corresponds to the anodic oxidation of freshlv deoosited TilOl. One may oresume that Ti(0) appears on the electrode primarily secondary chemical reaction :
A. LI~OWSKA
oi a
SUMMARY
1. It has been found that electroreduction of TiCI.+ in AN is multi-step process. The first is electroreduction of Ti(IV) to Ti(II1). Some kinetic parameters of this reaction have been calculated. 2. The electroreduction to Ti(0) demanded so high cathodic polarization that it was proceeded by electrodeposition of Li(0). 3. Basing on the experimental data the mechanism of examined process has been proposed. It has been confirmed that reduction to Ti(0) proceeds mainly by chemica1 reaction
with Li(ol-
Acknowledgement - This work was supported by the Coordination Committee of MR J-11 Problem.
(9)
This suggestion is confirmed also by the investigation that has been conducted to check the possibility of electrodeposition of Ti from AN on Pt electrode. It has been found that during electrolysis of 0.078 M TiCl, + 0.1 M LiBF, under cathodic potentials from - 3.5 to -3.7V (i, = 0.8 A .dcrn-‘) a dark metallic deposit was formed on the platinum electrode. The X-ray microanalysis of the deposit composition showed that it contained up to 37% Ti and the thickness of the layer was about 15 pm. In addition to Ti the deposits showed very high content of carbon (about 60 per cent) that remains in agreement with observations of other authors[7]. Presumably this is the result ofconducting the reaction under very high cathodic polarization conditions, which is associated with decomposition of organic electrolyte. It should be however, stressed that in solutions not containing TiCL, or in solutions of the latter in Et,NBF& as a supporting electrolyte, formation of deposits containing neither metallic Ti nor carbon has been observed. One can, therefore, suppose that Ti(II1) and Ti(I1) compounds formed during reduction of Ti(IV) salts in organic solvent, exhibit so strong a reducing action that may cause, under conditions ofadvanced cathodic polarization, reduction of solvent molecules up to formation of C deposits on the electrode.
REFERENCES 1. A. J. Parker, Proc. R. Austral. Chew Inst. 39,163 (1972). and Elec2. A. Brenner, Advances in Electrochemistry trochem. Engng Vol. 5, p. 205. New York (1967). 3. A. Bartecki in Pierwiastki Rzadkie i Metnlurgia Chemiczna, Vol. 2, p. 17. Wroclaw (1973). 4. W. E. Reid, J. M. Bish and A. Brenner, J. electrochem. Sot. lolr, 21 (1967). 5. E. Santos and F. Dyment, Plating 60, 821 (1973). 6. A. S. Avaliani, Trudy in-ta prikl. khimij i elektrokh. AN Grur. SSSR 3, 67 (1962). I. A. M. Levinskiene and L. E. Simanavicius, V. Ja. Akimov, Trudy AN LiQt. SSE, ser. 3 5, 37 (1977). 8. Patent RFN Nr 169074. 26 Jan. (1968). 9. Patent lap. Nr 9202, 30’Oct. (1927). ’ 10. V. V. Kaznetsov, V. P. Grigoriev, 0. A. Osipov, V. A. Kogan, V. K. Chikhirkin and E. P. Borschenko, Ockryr. Izobr. Prom. Obrazcy, Too. Znaki 51, 104 (1974). 11. J. M. Kolthoffand F. G. Thomas, .I. efectrochem. Sot. 111, 1065 (1964). 12. S. Bia‘llozor, 2. Nouk. 215, Chemia 24 (1974). 13. S. Biallozor, Electrochim. Acta 17, 1243 (1972). 14. P. Delahay, New Instrumental Methods in Ele&chemistry, p. 143. Interscience, New YorkkLondon (1954). 15. R. S. Nicholson, I. Shain, Analyt. Chem. 36, 706 (1964). 16. A. G. Stromberzand A. J. Kartushinskaia. _ Zh. Fir. Khim. 34, 1684 (i96Oj: 17. T. C. Franklin and H. V. Seklemian. J. inorg. nucl. Chem. 12, 181 (1959). 18. 0. Je. Ruvinskij, in PolarograJja, Problemy i Perespektyvy, p. 196. Riga (1977).