Phase transformation of titanium dioxide in the preparation of titanium oxide-Vanadium oxide electrodes for complexometric titration

Journal of the Less-Common

Metals,

137 (1988)

353 - 365

353

PHASE TRANSFORMATION OF TITANIUM DIOXIDE IN THE PREPARATION OF TITANIUM OXIDE-VANADIUM OXIDE ELECTRODES FOR COMPLEXOMETRIC TITRATION*

R. BEHRENS

and F. UMLAND

Anorganisch-Chemisches Znstitut der Westfiilischen-Wilhelms-Universitiit, Wilhelm Klemm Strasse 8, 4400 Miinster i. W. (F.R.G.) (Received

June

15,1987)

Summary Vanadium(V) oxide and vanadium-titanium oxides supported on titanium were examined by electrochemical measurement, Raman spectroscopy, scanning electron microscopy and X-ray diffraction to assess their use for amperometric end point indication of complexometric titrations. The electrodes were prepared by coating titanium sheets or wires with mixed solutions of vanadium(V) and titanium(IV) and heating. The decomposition is accompanied by a phase transformation of the titanium dioxide from the anatase to the rutile lattice. The electrode has high activity for the electro-oxidation of ethylenediaminotetraacetic acid (EDTA) in alkaline buffered solution. It also performs better than other electrodes made by anodic deposition of transition metal oxides on platinum.

1. Introduction Conducting metal oxides have received increasing interest because of their various technological and analytical applications. They are applied to saturated brine electrolysis [ 11, seawater electrolysis, organic electrosynthesis [2], electrochemical storage of energy in batteries [3], photothermal conversion [ 41, optoelectronic devices [ 51 and gas sensors [ 61 and they are used in analytical chemistry as materials for electrodes in end point indication of complexometric and precipitation titrations [ 71. Many techniques such as evaporation, anodic deposition [8], sputtering, chemical vapour deposition (CVD) [9] and others may be used to deposit conducting metallic oxides on various base materials. In analytical chemistry the anodic deposition of non-noble and noble metal oxides on platinum was chosen by Kainz et al. to produce electrodes suitable for amperometric end point indication of complexometric titrations of *Dedicated OQ22-5088/88/$3.50

to Professor

Harald

Schiifer on the occasion 0 Elsevier

of his 75th

Sequoia/Printed

birthday. in The Netherlands

354

ethylendi~ino~~~cetic acid (EDTA) or other chelating reagents such as cupferron diethyldithiocarbamate as titration reagent [lo]. In our group, similar metal oxide electrodes have been developed for the adsorption polarized electrode (APE) indication of precipitation titrations [ 111. In contrast with the electrodes used for analytical chemistry, the electrodes used for technical purposes, such as brine electrolysis, consist of an electrode core of a conducting metal (e.g. tantalum, titanium, zirconium, niobium or their alloys} which is coated with a solid solution ~content greater than 50 mol.%) of the ~o~esponding oxide and with a non-film-producing noble or non-noble metal such as nickel, iron, lead, copper etc. These types of electrodes are called dimensionally stable anodes (DSA) and were originally proposed in a patent (Beer [ 121). ‘Although titanium as well as other valve metals produce a passive film on anodic polarization (consisting of an n-semiconduc~g layer) such films become metallic or quasimetallic conducting on the addition of ruthenium dioxide and other metal oxides. The advantages of this type of electrode are their durability in practical use. They are chemically and mech~ic~y stable in corrosive media and allow high current densities and low overpotentials for the electrode process for durations of up to 15 years [ 131. The preparation technique of thermal decomposition used for these electrodes allows a wide variation in the composition of the active layer in contrast with oxide electrodes made by anodic deposition. The stability of these DSAs seems to make them promising for use in analytical chemistry. Therefore we investigated a thermally prepared vanadium oxidetitanium oxide electrode. This electrode cannot be made by anodic deposition from vandal solutions. Vanadium was chosen because vanadium oxides are commonly used as good catalysts in oxygen transferring reactions, for example in the gas phase oxidation of sulphur dioxide, carbon monoxide and hydrocarbons. Information on vanadium oxides or titanium-vanadium oxides as electrode materials is rare [ 14 - 16 ] in the literature. The use of metal oxide electrodes for the end point indication of complexometric titration depends on the fact that chelated EDTA and analogous reagents are not oxidized at potentials around +1 V on the electrode surface. In contrast, if an excess of free EDTA is present this can be electro-oxidized. The oxidation of EDTA on oxide layers is much better than on bare metal electrodes such as platinum. Titration is possible in very dilute solutions. The titration of calcium described in this paper is only one example. Many other cations can be titrated with EDTA in the same way. 2. Expe~ent~

details

2.1. Apparatus For the electrochemical measurements the Metrohm potentiograph E436 coupled with a Metrohm 10 ml burette was used. Titrations were performed in a 50 ml cell with magnetic stirring.

Electrode surfaces were examined by scanning electron microscopy (SEM) using a Stereoscan S180 (Cambridge Instruments) equipped with a PGT Delta 100 (Princeton Gamma Tech) energy dispersive X-ray (EDX) analyser for the determination of the elemental identity of the particles. The laser Raman spectra of the samples were recorded with a Raman T 800 (Coder-y) spectrometer. The green (514.5 nm) emission line from an argon laser Mod 52 (coherent) was used for excitation. The scanning speed was 50 cm min- ’ . Spectra were recorded from 100 cm-’ to 1100 cm-‘. X-ray diffraction patterns were collected using a Siemens D 500 goniometer and a Siemens Kristalloflex 810 generator with nickel filtered Cu Ka radiation (L = 154.18 pm; 40 kV; 20 mA). The resulting X-ray powder patterns were identified by comparing their reflections with those given by the ASTM X-ray powder data file. Spectra were recorded from 20 = 10” to 90” with a goniometer rate of 1 deg min-’ . All patterns were taken at the same detector sensitivity of 400 counts s-’ . 2.2. Materials Chemicals and solutions used were as follows: 0.01 M calcium chloride (Merck), ethylenediaminotetraacetic acid (disodium salt) (Merck), 25% ammonia solution and ammonium chloride buffer (pH 10) (Merck), ammonium metavanadate (Merck), titanium dioxide as anatase (Fluka), titanium tetrabutoxide (Dynamit Nobel AG), titanium tetraisopropoxide (Kronos Titan), oxalic acid and 35% hydrogen peroxide solution (Riedel), titanium as sheets, 0.1 mm in thickness and titanium wire 0.8 mm in diameter (both Ventron), vanadium pentoxide (Merck). 3. Electrochemical measurement The electrodes were prepared by the thermal decomposition method. Titanium sheets (5 mm X 10 mm X 0.1 mm) or titanium wires (40 mm X 0.8 mm) were coated by the following standard procedure involving 4 to 5 activation cycles. (i) The titanium was degreased with acetone; (ii) etched with 10% oxalic acid at 90 “C for 10 min and rinsed with distilled water; (iii) immersed in activation solution; (iv) dried at 130 “C; (v) annealed in an electric furnace at 600 “C (in air) for 10 min; (vi) steps (iii) - (v) were repeated five times; (vii) a final heat treatment was carried out at 600 “C for 1 h. The activation solution contained titanium and vanadium in the molar ratio 1:2 (0.1 M for titanium and 0.2 M for vanadium). The solution was prepared by hydrolysing titanium tetrabutoxide or titanium tetraisopropoxide in hot 5% oxalic acid to titanium dioxide hydrate which dissolves slowly to the oxodioxalatotitanium complex. Then, the equivalent (by weight) of ammonium metavanadate and 2 g ammonium nitrate were added. Vanadium pentoxide first precipitates and then dissolves under reduction and gas evolution (CO,) to a blue vanadium(IV) oxalato complex. During the

356

heat treatment in an electric furnace the adsorbed vanadium complex is decomposed and the vanadium(IV) is oxidized to vanadium(V). Surface tension of the solution was lowered by the addition of a surfactant. With this activation solution good adherent mouse-grey coatings can be achieved. Electrodes were prepared by connecting the tit~ium wires or sheets to a copper wire with soldering metal and pasting in a glass shaft with 2-component epoxide glue. The standard composition of the titration solutions was as follows: 30 ml distilled water; 5 ml buffer (pH 10); 50 ,uI 35% hydrogen peroxide; variable amounts of calcium solution from 0.1 to 10 ml. The current densities were calculated from the geometric area of the electrodes used. The working electrode was the oxide electrode. The counter electrode was a platinum sheet (Metrohm EA 211). A typical titration curve is shown in Fig. 1 [ 171. The end point of the titration is reached when the current increases, which indicates the oxidation of EDTA. In order to obtain a good evaluation of the curve a sharp increasing current at the equivalence point is required. This can be assisted by a slow rate of titration. In our experiments a rate of 10 ml of reagent added (EDTA 0.1 M solution) per 18 mm was used. Figure 2 shows the calibration function for variable amounts of calcium solution. The function is exactly linear for solutions from 2.5 X 10m5 to 2.5 X 10U3 M calcium in the titration volume. More concentrated solutions provide no problems and more dilute solutions can also be used. A similar accuracy can be obtained down to lo-’ M [ 181. To avoid a change in adsorption only fresh reagent solutions should be used and burettes and cells must be rinsed very carefully. Figure 3 shows the influence of a variation in indication voltage on the resulting difference in height between ground current and limiting current. Increasing indication voltage also produces increasing signals in the titration, but at potentials larger than 1.1 V the titration curves are disturbed by large fluctuating currents produced by the detachment of gas bubbles (oxygen) from the electrode surface. The electrode is stable for more than 30 titrations. Defect electrode wires can be easily exchanged because they are very cheap and they can be prepared quickly in large numbers. This is an advantage over most of the other electrodes produced by the anodic deposition of tr~sition metal oxides. In acid medium (pH of 2 - 3) the electrode is irreversibly blocked. This can be explained by a dissolution of the vanadium out of the layer. The vanadium oxide in the layer is essential for the function of the electrode; a pure TiO? electrode does not work as shown by experiment. Such electrodes, prepared either by heating a titanium sheet in air or by coating a titanium sheet with titanium dioxide in the anatase form (from a titanium tetrabutylate solution in n-butanol) and heating under the same conditions as the mixed oxide electrodes, show fluctuating currents and h-reproducible potentials during the titrations.

357 -z 0 7;

P

J

,”

0 30 4 0

+/

/

1

2

3

4

5

6

7 8 added

9 1Oml Ca+*lO%

added

1.0 Ca’*lO 4

b

.lO

50%

Fig. 1. Titration electrode. Fig. 2. Calibration EDTA.

P

10036 per cent

titrated

of calcium

with

function

05

0.1 M EDTA

for calcium

using a titanium

of various

1

oxide-vanadium

oxide

titrated

0.1 M

concentrations

with

EP0l-O Fig. 3. Influence

of indication

voltage

on the maximum

current

I

density

on the electrode.

358

4. Scanning electron microscopy

studies

The micrographs were taken using only secondary electrons (SE) and are shown in Fig. 4. A scale is marked on the micrographs. Prior to surface analysis the electrodes used for the titrations were washed with a stream of disti~ed water followed by acetone and then air dried. Micrographs 4(a) and 4(b) both show a titanium oxide-vanadium oxide electrode before use, supported on a titanium sheet. The thickness of the layer is approximately 1 pm. The micrographs show a good adherent dense coating of the mixed oxide without any cracks in the surface; the small crystallites on the surface are segregated crystals of vanadium pentoxide (see X-ray and Raman investigation). Micro~aph 4(b) shows an edge of the same sheet as a micrograph 4(a). The grooves shown on the left were produced by the cutting of the sheet. The E%X analysis of this part of the micrograph shows titanium only.

(a)

Ib)

(d)

Fig. 4. Micrographs of the electrode surfaces: (a) titanium sheet with active layer (electrode before use); (b) same as (a), an edge of the sheet is shown; (c) electrode sheet after use; (d) same as (c).

359

Unfortunately the Kfl line of titanium and the strongest vanadium Ko line overlap. Therefore only the vanadium Kfl line and lines of less intensity can be used for identification. Additional disturbances result from the very thin layers so that the electron beam also releases X-rays from the base material. The strongest lines in the EDX spectra are always the titanium lines. Micrographs 4(c) and 4(d) show the electrodes after use. The small crystallites of vanadium pentoxide have completely disappeared. They were dissolved in the alkaline buffer solution during the first immersion of the electrode in this solution. The remaining layer now consists of a relatively homogeneous mixed oxide or solid solution of vanadium oxide and titanium dioxide. This is the actual active principle of the electrode. Vanadium is still present in the layer and is randomly distributed as shown by the EDX examination. The texture of the layer is very fissured, so that the real active surface is much greater than the geometric surface. Using titanium as the base material has the advantage that if the cracks in the layer expose the titanium sheet, a passive layer of n-semiconducting titanium dioxide is produced under anodic polarization. Therefore the charge transport can only occur through the mixed oxide layer. This is valid for thick layers and for low voltages. In very thin layers electron transfer can occur by tunnelling and at high voltages the passive layer breaks down and titanium is dissolved anodically. A model of the electrode surface is shown in Fig. 5. Figure 5(a) shows an electrode before use. On the surface of the layer there are particles of segregated vanadium pentoxide. The reason for the segregation is probably a limited solubility of the vanadium oxide in the titanium dioxide matrix. Figure 5(b) shows the electrode after use for titration. The vanadium oxide particles which were on the surface of the mixed oxide coating have been dissolved and a thin layer of anodically insulating TiOz now grows on the surface of the base titanium sheet. These parts of the electrodes are not active for the electro-oxidation of EDTA. A large number of these insulating parts can lead to a loss in sensitivity of the electrode.

(a)

(b)

Fig. 5. Model for the active layer on a titanium trode, with a cracked layer, after use.

sheet:

(a) electrode

before

use; (b) elec-

5. Raman scattering studies Mixed oxide powder samples for the structural analysis were prepared as follows. Weighed amounts of TiOz (anatase) and V,O, were ground in an

360

agate mortar and heated to 600 “C for 9 h. The products were then reground and heated for another 12 h. Other samples were prepared by evaporation to dryness of the complex coating solution at 130 “C, followed by heat treatment at 600 “C for 1 h in air (analogous to electrode preparation). From the Raman spectrum in Fig. 6(c) it can be seen that, in the heated samples of the titanium-vanadium oxalato complex, crystalline

b)

a)

1 ’ 1000

’

cm-’

’

’

’

’

1

’

’

100

cm-1

Fig. 6. Raman spectra: (a) Vz05 from NHdV03 at 600 “C; (b) TiO2 in anatase form (Fluka); (c) mixture of V205 and TiO2 from solution of oxalates heated for 4 h at 600 “C; (d) V205 on titanium wire from NHdV03 solution.

361

vanadium pentoxide is present. (The Raman lines of V,O, are approximately 285, 306, 406, 483, 530, 703 and 996 cm-’ [19].) Crystalline vanadium pentoxide is also found on a titanium wire as seen in spectrum 6(d). For comparison the spectrum 6(a) is of pure crystalline V,O, prepared by the thermal decomposition of ammonium metavanadate. Raman lines in this spectrum are more intense and sharper than in spectrum 6(c) and 6(d) but the positions of the lines are identical. Different crystal shapes and orientations seem to be responsible for the changes in intensities of the Raman lines. Spectrum 6(b) shows the Raman lines of pure anatase centred at 400, 515 and 640 cm-‘. None of these lines can be detected in the spectrum of the titanium-vanadium mixed oxide sample. The two small and sharp additional lines in this spectrum indicate the presence of rutile. Anatase and brookite are metastable modifications of TiOz, but the phase transformation to the stable TiO, (r-utile form) only occurs at temperatures between 850 and 1100 “C. This is because of the high activation energy of 775 kJ mol-’ for this transformation [ 201. Cole et al. [21] studied the physicochemical properties of V,O,anatase mixtures as commercial catalysts for the mild oxidation of o-xylene to phthalic anhydride at increasing temperatures. These workers and others [22] observed the evolution of oxygen from the mixture at temperatures in the range 600 - 700 “C accompanied by a gradual phase transformation from anatase to r-utile. Vanadium pentoxide loses oxygen when it is heated in the presence of TiO, as anatase or brookite and is reduced to vanadium(IV). This vanadium(IV) forms a solid solution in the TiO, matrix. VO, is isostructural with TiO, in the rutile lattice. This phenomenon contradicts the established bulk thermodynamic properties of TiO, and V,O, [23]. Bulk vanadium pentoxide is not reduced by simple heating at 615 “C in air. However, reduction is observed in the presence of anatase due to its activating property. In addition, vanadium pentoxide activates the phase transformation of anatase to r-utile. Vanadium(IV) is not active [ 24, 251. The anatase (or brookite) structure is essential for the reduction of vanadium pentoxide. The reduction does not occur on TiOz in the rutile form.

6. X-ray diffraction

studies

The X-ray diffraction (XRD) studies verify the results of the Raman scattering studies. X-ray patterns of good quality could not be obtained from a direct examination of the electrode sheets. The active layers on the base titanium sheet yield only X-ray reflections of low intensity. So all X-ray patterns were obtained by using powder samples. The solid state reactions occurring on the electrode surfaces are expected to be exactly the same as those in the powder samples. Observed reflections (Fig. 7) were in good agreement with those listed in the ASTM card file (Fig. 8). Figure 7(a) shows the lines of single phase

al

I

2e=

TO0

,

60'

I

50"

I

40°

8 30"

I

2o"

10"

Fig. 7. XRD patterns: (a) VzO5 (Merck); (b) TiO* in anatase form (Fluka); (c) heated anatase (1040 “C for 48 h); (d) heated mixture of VzO5-TiO2 (anatase) at 350 “C for 48 h; (e) heated mixture of V205-TiO2 at 600 ‘C for 4 h; (f) ‘Pi02 (rutile) from sample (e) treated with hydrochloric acid.

Vz05. These strong lines and all the other less intense lines can also be found in Figs. 7(d) and 7(e). Figure 7(d) was obtained from a sample of V20, and anatase heated to 350 “C for 48 h while Fig. 7(e) was obtained from a sample that was heated to 600 “C for 4 h. The lines are in the same positions as those of vanadium pentoxide, with the exception of TiOz.

363

Cd

1

I

1.5

Fig. 8. X-ray patterns (ef VZ&..

2

from

I

2.5

-3

ASTM file: (a) rutile;

1

3.5

(b) anat::;

d[A] (c) brookite;

(d) TiV206;

Figure 7(b) shows the reflections for anatase. These reflections can also be found in Fig. 7(d) for the sample heated to 350 “C, but not in Fig. 7(e). The anatase lines have completely disappeared in this sample. Figure 7(e) shows additional rutile lines. These lines are also present in Fig. 7(f). The rutile remains in the mixed oxide sample prepared at 600 “C if the vanadium is dissolved by treatment with hydrochloric acid. Figure 7(e) indicates the phase tr~sformation of anatase to rutile. This agrees well with the results from the Raman scattering studies. A sample of pure anatase heated for 48 h to 1040 “C shows only a partial phase transformation. This fact is shown in Fig. 7(c). Most of the sample remained in the anatase lattice. In the presence of vanadium(V) the high activation energy for the transformation of pure anatase to rutile is lowered from 775 kJ mol’ to 42 kJ mol-’ [ 201. Lower vanadium oxides are present in the active layer, but could not be detected in the X-ray patterns. The studies of Vejux and Courtine [24] prove the formation of lower vanadium oxides. The maximum content of V,O, in their mixtures was 25 mol.%. In our experiments the VzO, content was always 200 mol.% with respect to TiOz. Only a small amount of the

364

vanadium(V) is reduced and so the X-ray patterns show only the reflections of the excess VZO,. In addition, phases between vanadium(IV) oxide and titanium dioxide such as TiV,06 (see Fig. 8(d)) could not be detected [16, 261.

7. Conclusions It was shown that a mixed titanium oxide-vanadium oxide electrode is suitable for the indication of complexometric titrations in alkaline media. An excess of free EDTA and other chelating reagents can easily be electrooxidized by this electrode. The active principle is a mixed layer consisting of TiO, in the rutile lattice and vanadium oxide. At first metastable anatase is formed in the thermal decomposition step of the solution. In the presence of vanadium(V) and temperatures above 600 “C this anatase is completely transformed into rutile.

Acknowledgments The authors thank Mr. Gijcke for recording the Raman spectra and Mr. Stenner for performing the scanning electron microscopy. Thanks are also due to Dynamit Nobel AG and Kronos Titan who supplied samples of the titanium alkoxides and to Verband der Chemischen Industrie, Fonds der Chemie for financial support.

References 1 M. 0. Coulter,

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

in Modern Chloralkali Technology, Ellis Horwood, Chichester, 1980, chapters 8 - 12. D. Fleischmann, K. Korinek and D. Pletcher, J. Electroanal. Chem., 31 (1971) 39. R. Messina, J. Perichon, M. Broussely and G. J. Gerbier, Appl. Electrochem., 8 (1978) 87. J. C. Manifacier, Thin Solid Films, 90 (1982) 297. T. Ohzuku and T. Hirai, Electrochim. Acta, 27 (1982) 1263. M. Nitta, S. Kanefusa and M. Haredome, J. Electrochem. Sot., 125 (1978) 1676. H. Pink, I. Treilinger and L. Vite, Jpn. J. Appl. Phys., 19 (1980) 513. G. Kraft, 2. Anal. Chem., 238 (1968) 321. G. Kainz, H. A. Muller and G. Sontag, 2. Anal. Chem., 256 (1971) 345. J. Kane, H. P. Schweizer and W. Kern, Thin Solid Films, 29 (1975) 15. E. Schumacher and F. Umland, Microchim. Acta, 2 (1977) 449. H. B. Beer, G.D.R. Patent 55223, May 12,1965. S. Trasatti, in Electrodes of Conductive Metallic Oxides, Part B, Elsevier, Amsterdam, 1980. W. Gabriel, Organische Redoxkatalyse an Titandioxid und Vanadiumoxid modifizierten Kathoden, Dissertation, Duisburg, 1986. L. D. Burke, J. Electroannl. Chem., 111 (1980) 383. T. E. Phillips, K. Moojani, J. C. Murphy and T. 0. Poehler, J. Electrochem. Sot., 129 (1982) 1210.

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17 R. Behrens, Diplomarbeit, Universitlt Miinster, 1985. 18 G. Kraft and H. Dosch, 2. Anal. Chem., 260 (1972) 261. 19 F. Roozeboom, M. C. Mittelmeijer-Hazeleger, J. A. Moulijn, J. Medema, V. H. J. de Beer and P. J. Gellings, J. Phys. Chem., 84 (1980) 2783. 20 K. 0. Backhaus, R. Haase, U. Illgen, K. Jancke, J. Richter-Mendau, J. Scheve, I. Schulz and J. Vetter, Ber. Bunsenges, Phys. Chem., 8 7 (1983) 680. 21 D. J. Cole, C. F. Cullis and D. J. Hucknall, J. Chem. Sot. Faraday Trans. 1, 72 (1976) 2185.

D. J. Hucknall, in Selective Oxidations of Hydrocarbons, Academic Press, New York, 1974, pp. 152 - 158. 23 A. A. Fotievand V. L. Volkov, Russ. J. Phys. Chem., 45 (1971) 1516. 24 A. Vejux and P. Courtine, J. Solid State Chem., 23 (1978) 93. 25 G. Busca, P. Tittarelli, E. Tronconi and P. Forzatti, J. Solid State Chem., 67 (1987) 91. 26 Diffraction Card Nos. 21-1276 (rutile), 21-1272 (anatase), 29-1360 (brookite), 32. 1378 (TiV20,), 9-387 (VzOs), ASTM, Philadelphia, PA. 22