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1-1-1973

Electrochemical Reduction of Molybdenum(VI) Compounds in Molten Lithium ChloridePotassium Chloride Eutectic Branko N. Popov University of South Carolina - Columbia, [email protected]

H. A. Laitinen University of Illinois at Urbana

Follow this and additional works at: http://scholarcommons.sc.edu/eche_facpub Part of the Chemical Engineering Commons Publication Info Journal of the Electrochemical Society, 1973, pages 1346-1350. © The Electrochemical Society, Inc. 1973. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in the Journal of the Electrochemical Society. http://www.electrochem.org/ DOI: 10.1149/1.2403259

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1346

T

Ui Ul,o V Vss

Vt g

~m,n

K

Ai AD ~o

~ss r

r p

0 0' Och Vi

J. Electrochem. Soc.: E L E C T R O C H E M I C A L SCIENCE A N D TECHNOLOGY absolute temperature (~ eigenfunctions in series for ct value of Ui at the electrode surface electrode potential (V) steady-state part of the electrode potential (V) transient part of the electrode potential (V) distance from plane of disk (cm) parameters in the kinetic coefficient Nernst diffusion layer thickness (cm) Kronecker delta rotational elliptic coordinate surface overpotential (V) conductivity of the solution ( o h m - l - c m -1) dimensionless eigenvalue eigenvalue characteristic of diffusion potential in the solution (V) value of r at the electrode surface (V) steady-state part of potential in the solution (V) transient part of potential in the solution (V) potential in the solution adjacent to the disk corresponding to the p r i m a r y current distribution (V) rotational elliptic coordinate time constants for decay (sec) dimensionless time for the charging period dimensionless time for the decay period dimensionless total period of charging constants in the series for V t (normalized to unity} angular frequency of rotation (radians/sec)

REFERENCES 1. S. Glasstone, J. Chem. Soc., 123, 2926 (1923). 2. A. Hickling, Trans. Faraday Soc., 33, 1540 (1937). 3. S. Schuldinger and R. E. White, This Journal, 97, 433 (1950).

October 1973

4. J. D. E. McIntyre and W. F. Peck, Jr., ibid., 117, 747 (1970). 5. J. Newman, ibid., 113, 501 (1966). 6. J. Newman, ibid., 113, 1235 (1966). 7. J. Newman, ibid., 114, 239 (1967). 8. J. Newman, ibid., 117, 507 (1970). 9. L. Nanis and W. Kesselman, ibid., 116, 454 (1971); and ibid., 118, 1967 (1971). 10. J. Newman, ibid., 118, 1966 (1971). 11. W. Tiedemann, J. Newman, and D. N. Bennion, ibid., 120, 256 (1973). 12. B. Miller and M. I. Bellavance, ibid., 120, 42 (1973). 13. J. Newman, Intern. J. Heat Mass Transfer, 10, 983 (1967). 14. W. R. Parrish and J. Newman, This Journal, 116, 169 (1969). 15. W. R. Parish and J. Newman, ibid., 117, 43 (1970). 16. J. Newman, J. Phys. Chem., 73, 1843 (1969). 17. J. Newman, This Journal, 117, 198 (1970). 18. D. C. Grahame, Chem. Rev., 41, 441 (1947). 19. E. Levart and D. Schuhmann, J. ElectroanaL Chem., 24, 41 (1970). 20. A. M. Johnson and J. Newman, This Journal, 118, 510 (1971). 21. J. Newman, "Electrochemical Systems," p. 58, Prentice-Hall, Inc., Englewood Cliffs, N. J. (1973). 22. E. Mattsson and J. O'M. Bockris, Trans. Faraday Soc., 55, 1586 (1959). 23. D.-T. Chin, This Journal, 118, 1434 (1971). 24. R. Parsons, Advan. Electrochem. ELectrochem. Eng., 1, 1 (1961). 25. K. Nisancio~lu and J. Newman, This Journal, 120, 1356 41973). 26. J. R. Selman, Ph.D. Thesis (UCRL-20557), University of California, Berkeley, June, 1971. 27. L. Nanis and I. Klein, This Journal, 119, 1683 (1972).

Electrochemical Reduction of Molybdenum(VI) Compounds in Molten Lithium Chloride-Potassium Chloride Eutectic B. N. Popov Faculty of Technology and Metallurgy, University Kiril and Metodij, Skopje, 91000, Yugoslavia

and H. A. Laitinen* Roger Adams Laboratory, School of Chemical Sciences, University o3 Illinois, Urbana, Illinois 61801 ABSTRACT Molybdenum(VI) oxide reacts with molten LiC1-KCI eutectie at 450 ~ to form MOO2C12, which probably is present as an anion MoO2Cl4 =, and p y romolybdate, Mo207 =. Both of these species are electrochemically reduced to MOO2, which can be reoxidized to MoO2CI2 by current reversal. A second r e duction step, observed whether MoO3 or Mo2072- is added to the melt, can be attributed to the reduction of MoO4 =, formed as a secondary reaction product in the first step. The reduction of molybdate proceeds in two steps, the first at --0.85V and the second at --1.75V vs. the IM P t ( I I ) / P t reference electrode. The first step shows an abnormally short transition time, attributable to a slow equilibrium. The second step corresponds to a diffusion-controlled reduction with n = 0.5, yielding a product of the empirical formula LisMo2Os. The literature contains v e r y little fundamental information on the behavior of molybdenum compounds in molten systems. Stavenhagen and Engels (1) described a sodium molybdenum bronze in the form of a dark bluish-grayish powder formed by electrolytic reduction of fused sodium molybdate. Cannery (2) reported the preparation of lithium, sodium, and potassium molybdenum bronzes using the same method. However, the next year Burgers and van Liempt (3) showed that the bronzes mentioned by Cannery were * E l e c t r o c h e m i c a l Society Active M e m b e r . K e y w o r d s : m o l y b d e n u m , m o l y b d e n u m ( V I ) oxide, m o l y b d e n u m o x y c h l o r i d e , l i t h i u m pyromolybdate, lithium molybdate, molten salts, electrochemistry.

mixtures consisting of molybdenum(IV) oxide and molybdenum blue. According to Magneli, only molybdenum oxides were obtained by Hagg in similar experiments (4, 5). Wold, Kunmann, Arnot, and Ferretti (6) prepared pure MoO2 crystals by electrolytic reduction of MoO3-Na2MoO4 mixtures at 675~ In addition, both sodium molybdenum bronze and potassium molybdenum bronze crystals were grown from m o l y b denum(IV) oxide-alkali molybdate melts under carefully controlled conditions. Senderoff and Brenner (7, 8) have investigated the reduction of molybdate dissolved in molten alkali halides. They found that the bulk of the reaction product was dispersed throughout

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Vol. 120, No. 10

1347

ELECTROCHEMICAL REDUCTION OF Mo(VI)

the melt as a black, water insoluble powder c o n t a i n i n g about 77% m o l y d e n u m , corresponding to MoO2 (75% Mo). The present investigation was u n d e r t a k e n in order to study the reduction of m o l y b d e n u m ( V I ) compounds and to characterize a n y reduction products observed. It was also considered possible that some aspects of m o l y b d e n u m chemistry and interactions b e t w e e n m o l y b d e n u m ( V I ) compounds and chloride melt could be deduced. Experimental Apparatus.--The i n s t r u m e n t a t i o n a n d e q u i p m e n t used i n this study have b e e n previously described (911). E l e c t r o d e s . - - P l a t i n u m microelectrodes for v o l t a m m e t r y were prepared from 26B and S gauge p l a t i n u m wire as described earlier (10). Indicator electrodes for chronopotentiometric m e a s u r e m e n t s were m a d e by sealing 26 gauge p l a t i n u m wire into P y r e x glass and t h e n spot welding a piece of 0.5 cm 2 p l a t i n u m foil to the end of the wire extending from the p l a t i n u m glass seal. The cathodes used for the preparation of the electrode deposits w e r e constructed from 52-mesh p l a t i n u m gauze. The counterelectrode was a u diameter graphite rod. All potentials are referred to the 1M P t ( I I ) / P t reference electrode. Details on the generation a n d use of this reference electrode i n LiCl-KCl have been described previously (11, 12). Chemicals.--All chemicals used in this study were reagent grade. Those chemicals containing water of h y d r a t i o n were v a c u u m dried at l l 0 ~ before being added to the melt. The LiC1-KCI eutectic was obtained from A n d e r s o n Physics Laboratories, Inc., Urbana, Illinois. The method of purification has been described (11). All experim e n t s in the melt were carried out at 450~ unless otherwise noted. Solid chemicals were added to t h e melt b y m e a n s of a small glass spoon. A b l a n k e t of argon was kept over the melt at all times to exclude oxygen and w a t e r vapor. The purification t r a i n used i n p u r i f y i n g the argon has b e e n described (13).

Techniques for preparation and analysis of electrode deposits.--Before their insertion in the melt solution, the gauze electrodes were cleaned in boiling, concentrated HNO3, rinsed with distilled water, and dried at 130~ for 20 hr. After the material had been deposited on the electrode, the electrode was allowed to cool, washed with deionized water, and dried at 120~ The deposits were then dissolved i n 5 m l of concentrated nitric acid b y h e a t i n g o n a hot plate. The m o l y b d e n u m content of the deposit was d e t e r m i n e d by addition of an excess of P b 2+ which was b a c k - t i t r a t e d with EDTA using xylenol orange as the indicator. The total m o l y b d e n u m was also obtained by amperometric titration with lead, a procedure developed by A y l w a r d (14). The a m o u n t of reduced m o l y b d e n u m in the sample was d e t e r m i n e d b y the method developed by Bourret (15). L i t h i u m was q u a n t i t a t i v e l y d e t e r m i n e d b y flame photometry. Chloride was determined b y the Volhard method.

"•10 o

2

i / f l 0,2

i 0,6

i

I 1,0

I

I 1.~

I

I 1.B

I

I 2.2

- E (v)

Fig. 1. Current-voltage curve for the reduction of MOO3. Electrode area ~- 2.2"10 -3 cm2; MoO3 = 3.63"i0 -3 M.

jMg(CIO&) z acetone

~ de~ L~e

Cu {&00~

~::~

~

Ar

N2

Fig. 2. Apparatus used to study acld-base reaction of MoO3 and chloride melt.

using four to six c u r r e n t densities at three different MoOs concentrations (Table I). Constancy of the q u a n tity I~I/e/C indicated diffusion-controlled reaction. However, it was observed during the m e a s u r e m e n t s that a volatile product was slowly evaporating from the melt. Therefore, owing to the u n c e r t a i n concent r a t i o n it was not possible to calculate a valid diffusion coefficient from the data. To establish the n a t u r e of the volatile product, the apparatus shown i n Fig. 2 was used. Previously dried M003 was added to the melt, air was excluded, and the volatile product was displaced by inert gas into a weighed trap cooled with acetone and dry ice. The product was yellow in color, v e r y volatile, and soluble in water. Analysis for Mo and C1- b y the methods described above showed that the sample weight could be completely accounted for in terms of the compound M002C12. To determine t h e product formed from the Table I. Chronopotentiometric data for the first reduction step of MoO3

Results and Discussion

MoO3-LiCI-KCI system.~Preliminary

steady-state voltammetric e x p e r i m e n t s (Fig. 1) showed that MoO3 exhibits two reduction waves in LiC1-KC1 at 450 ~ The h a l f - w a v e potentials are approximately --0.3 a n d --1.75V. The voltammetric curves were not highly r e producible because of the formation of a n insoluble product which rapidly increases the surface area of the electrode. Upon prolonged electrolysis at the limiting plateau, a b r o w n - v i o l e t film formed on the electrode, a n d the c u r r e n t increased with time. Chronopotentiometric experiments with proper choice of conc e n t r a t i o n a n d c u r r e n t density c i r c u m v e n t e d the difficulty due to film formation. Two transitions, at --0.35 and --1.75V, respectively, were observed, i n agreement with the v o l t a m m e t r i c observations. The Sand equation was checked for the first reduction step

CMoos (M)

I (mA)

1"/'1/3 (A-secl/D

( A - s e e 1/2 c m 3 r e a l -1)

3.63.10-s

1.540 1.050 0.650 0.680 0.596 0.630

9.84-104

2.500 2.000 1.670 1.430

2 . 3 " 1 0 -=

10.000 6.670 5.550 4.540

0 . 7 8 4 ' 1 0 -~ 0.715"10 4 0 . 6 7 1 ' 1 0 -8 0 . 6 6 4 ' 10 4 0 . 6 6 0 ' 10 4 0.680" 10 -3 A v g 0.679" 10 -3 1.802" 10 "~ 1.780"10 4 1.734"10 4 1 . 7 7 0 ' 10 4 A v g 1.770"10 -3 4 . 2 5 0 " 1 0 -~ 4 . 2 0 0 ' 10 -8 4 . 3 5 0 ' 1 0 "8 4 . 1 4 0 ' 10 -3 A v g 4 . 2 3 0 , 1 0 -8

210 197 185 183 183 187 A v g 187 183 181 176 181 A v g 181 165 183 189 160 A v g 184


1348

October 1973

J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y

reaction of the oxide ion, the melt was extracted from the product with ethanol and acetone to yield Li2MO2OT, which accounted completely for the sample weight. Accordingly the reaction b e t w e e n MoO8 and the melt can be w r i t t e n

o 0,48

0.4/. 0.4C

3MOO3 -I-2CI- ~ MOO2C12 + MO207 = A phase diagram study of M o O 3 and Li2MoO4 was carried out to verify further the existence of Li2M02OT. Phase transition temperatures were determined for eight samples ranging in composition from 42 to 55 m / o (mole per cent) MOO3. The experiments were done by the visual polythermal method in which the temperature of the first crystals was measured by a Pt-Rh thermocouple. F r o m Table II it is evident that a local m a x i m u m exists for the molar ratio M o O J Li2MoO4 = 1, implying the formation of a solid compound Li2M02OT. A n x-ray powder pattern of the 1:1 sample is described in Table III. N o lines for M o O 3 or Li2MoO4 were found in the diffraction pattern, thus precluding the possibility that the material is a mixture of M o O 3 and Li2MoO4. To determine whether the acid-base reaction occurs completely before the electrolysis experiments, the data of Table I were examined for a chemical reaction preceding charge t r a n s f e r by plotting Io~I/2/C vs. Io and Io/C. The plots showed no discernible d o w n w a r d slope, b u t only a scatter that could be a t t r i b u t e d to u n c e r tainties in concentration owing to volatility of the oxychloride, a n d perhaps to variation i n electrode area with the formation of solid deposit. Plots of E vs. log (~1/2 _ tl/2) were linear as shown i n Fig. 3. This function would be expected to yield linear plots either for a totally irreversible charge t r a n s f e r process or for a reversible process with the formation of a solid product of constant activity. C u r r e n t reversal chronopotentiometry showed a n anodic t r a n s i t i o n upon reversal of the c u r r e n t at the inflection point (Fig. 4) : ratios of the coulombs of charge in the forward to b a c k w a r d electrolysis varied b e t w e e n 0.985 and 1.025, indicating the q u a n t i t a t i v e formation of a solid product that can be reoxidized. If the slope of the log plot is equated to 2.3 RT/~n~F, we calculate Table II. Melting points of MoO3-Li2MoO4 system Mole per cent M o O .

Mole per cent LisMoO4

Melting point, ~

42 45 48 49 50 51 63 55

58 55 52 61 50 49 47 45

520 495 526 534 515 535 555

550

Table III. X-ray powder diffraction pattern of Li2Mo207 d (A)

6.4241 5.3679 4.1385 3.7824 3.6820 3.5199 2.9809 2.9780 2.BS23 2.8331 2.7136 2.5074 2.3659 2.1917 2.1396 2.0064 1,9012 1.8398 1.8294 1.8030 1.7697 1,7323 1,6937 1.8161 1.6900 1.6522 1.5043

I/Io I0 I0 30 I0 I0 80 50 60 10 10 50 10 10 100 50 6O 80 5 5 5 5 5 70 10 20 90 40

0.3~ >~ 0.32 o

o

0.2~ ,/ O.2,'.

J I 02

I

[ I I 0.6 1.0 - LOG ( ~bJ/2- t 1/21

I

I 1.4

[

Fig. 3. Potential-time curve for the first reduction step of MOO3.

1.0

as i i

0.2

0.4

0.6

0.0

1.0

1.2

1.4

1.6

i 1.0

I

2,0

~6rne ( s e e . )

Fig. 4. Typical reverse current chronopotentiogram for the first reduction step of MOO3. If = Ir = 2.0 mA/cm 2.

ana = 0.56 r a t h e r t h a n 2 for a reversible process. Neglecting the back-reaction, the value of kf,h at --0.208V, the starting potential, is calculated to be 3.18 • 10 -3 cm-sec-1. Analysis of the reduction products 05 MoO3.--Samples of the first reduction product, prepared coulometrically from 0.35M MoO3 solution using a p l a t i n u m gauze cathode, in the form of a homogeneous, b r o w n violet solid, were boiled i n distilled water to remove chlorides and soluble molybdates and dried at 130~ Analysis revealed only MOO2, and the x - r a y powder p a t t e r n agreed in d-spacings and approximate r e l a tive densities with the ASTM files for MOO2. Attempts were also made to prepare the second reduction product, by holding the potential of the w o r k ing electrode at --1.75V. A product showing x - r a y lines of MOO2, plus several weak lines, and containing Li but not K was formed, corresponding to a m i x t u r e of the first and second reduction products. The composition could not be completely accounted for in terms of MoO2 and Li20 or Mo205 and Li~:). It is possible that the second reduction product is partially soluble in the melt. MoO2Cl2-LiCI-KCl-system.--The solubility of MOO2C12 in the melt was d e t e r m i n e d at three t e m p e r a tures, 400 ~ 450 ~ and 500 ~ b y saturating an argon gas stream with the vapor and b u b b l i n g the dilute stream (heated to 120~ to avoid crystallization from the vapor) t h r o u g h the melt for at least 2 hr, and analyzing the melt for m o l y b d e n u m . The solubility was found to be 2.60 _ 0.12, 2.75 • 0.14, and 2.78 _ 0.09 X 10 -5 moles MoO2C12/cm8 at the three temperatures. The relatively high solubility a n d low vapor pressure over the melt indicates a strong interaction b e t w e e n MOO2C12, probably to form a n anion MoO~C14= with the chloride ion of the melt. Chronopotentiometry indicated a single wave, occ u r r i n g at a potential of --0.35V, in agreement with the postulated interaction b e t w e e n MoOs and melt. F r o m the v a l u e of Io~1/2/C = 560 _ 10 A-sec 1/2 cm mo1-1, n = 2, the diffraction coefficient was calculated to be 1.08 • 10 -5 cm2/sec.


Vat. 120, No. I0

ELECTROCHEMICAL REDUCTION OF Mo(VI)

Table IV. Chranopotentiometrlc data for the first reduction

1349

/

step of Li2Mo207

/ //

C=%%= (millimolar) 3.48

10.80

21.23

95.40

Io (mA)

Io.rl/~

lo~.zl=iC

(mA-sec112/cm~) (A-secl/2 c m S m o l e -~)

1.60

1.92

552

2.44 3.00

1.81

524 520

2.00

6.00 7.00 7.50 8.00

6.00

1.82

1.'17

509

4.97 4.85 4.7"/ 4,55

A v g 526 460 449 442 421 A v g 443

8.15

8.00 9.00 10.00

7.84 7//4 7.80

28.00 30.00 34.00 36.00

33.05 31.20

31.28

30,96

383

369 364

cl E o

7/._

~--~ "-~ ~.6

0.2 I

100

348

327

331 324 A v g 332

MoO2CI~ -b 2 e - ~=~MoO2 Jr 2C1or

MOO2C14= + 2 e - ~ MoO2 + 4C1Li2MO2OT-LiCI-KCI s y s t e m . - - T h e chronopotentiometric behavior of Li2Mo~O? was found to be quite similar to that of MOO3. Two waves were observed, one at --0.44V a n d the other at a p p r o x i m a t e l y --1.75V. The t r a n s i t i o n time data for the first reduction process (Table IV) were somewhat scattered, showing a t r e n d to decrease with concentration and with increasing c u r r e n t density. W h e n both MoO3 (3.55 mM) and Li2Mo207 (2.15 raM) were present, two waves were once again observed, one at --0.37V and t h e other at --1.75V. It is concluded that the first w a v e of MoO3 is actually the sum of t h e first waves of MoO2C12 and Mo207 =. The first reduction product of Li2Mo207 was characterized as before, and identified as MOO2. Accordingly, the first reduction process can be w r i t t e n Mo207 = "t- 4 e - -> 2MOO2 -t- 3 0 = with a following acid-base reaction, no doubt 3Mo207 = -t- 3 O = -> 6MOO4= The complexity of this process p r e s u m a b l y accounts for the lack of diffusion control i n the over-all process. MoO4 = - L ~ C ~ - K C t s y s t e m . - - C h r o n o p o t e n t i o g r a m s were recorded over a concentration range of 8.03 • 10-3M to 0.2725M Li2MoO4. At c u r r e n t densities below 2 m A / c m 2, two waves were discerned, one at --0.85V a n d the other at --1.75V. The first w a v e i n creased with concentration u n t i l the concentration reached 1.12 • 10-2M, and leveled off at higher concentrations (Fig. 5). The slope of the plot at low concentrations yielded Io~1/2/C = 9.87 _ 0.6 A-sec 1/2 cm r e a l - l , corresponding to nD 1/2 : 1.45 • 10 -4 cmo sec-~/% Such a n a b n o r m a l l y low transition time constant suggests a n e q u i l i b r i u m such as MoO4 = -~ 2C1- ~ M003C12 = + O = exists as a m i n o r i t y component first reduction step. If such an must be relatively slow, because to decrease w i t h increasing cur-

n

i

368 A v g 371

Constant c u r r e n t electrolysis carried out at potentials n e v e r more negative t h a n --0.35V, of a solution of MOO2C12 prepared b y b u b b l i n g t h e vapor t h r o u g h the melt for 2 h r produced a b r o w n - v i o l e t deposit. Comparison of the micromoles of m o l y b d e n u m with the microfaradays of c u r r e n t consumed indicated a consumption of two electrons per m o l y b d e n u m atom. The x - r a y powder p a t t e r n agreed with that of the first reduction product of MoOa and w i t h the ASTM patt e r n of MOO2. The electrode reaction at --0.35V can thus be described as

i n which M003C12 = responsible for the e q u i l i b r i u m exists, it Io~1/2 was not f o u n d

~"7E - u

I

I

200 300 CONC.(re,mole/tit.)

Fig. 5. Variation of Io~ V2 with Li2Mo04 concentration for the first reduction step. r e n t density as w o u l d be expected for a prior chemical step with appreciable interconversion d u r i n g the time of electrolysis. With a m o l y b d a t e concentration of 5.35 • 10-2M a n d with a c u r r e n t density of 16 m A / c m 2 only the second t r a n s i t i o n was observed. The Sand equation was tested for four to five c u r r e n t densities at five different molybdate concentrations (Table V). The average value of Io~t/2/C calculated from these data is 174 -~ 6 A-sec 1/2 cm mo1-1, corresponding to D = 1.64 • 10 -5 cm-sec -1 at 450 ~ taking n = 0.5 from the analytical data given below. Attempts to prepare the first reduction product by constant c u r r e n t electrolysis were unsuccessful. Using 0.85M Li~MoO4, even with c u r r e n t densities lower t h a n 100 ~A/cm 2, the electrode potential r a p i d l y rose beyond that corresponding to the first reduction step. Using c u r r e n t densities of 100 #A, 5 mA, a n d 10 m A / cm 2, deposits were prepared b y electrolyzing at --1.18 to --1.2V, at --1.5V, and at --1.75V, respectively. Exa m i n a t i o n of the cathodes showed onIy one type of solid product, an adherent dark b r o w n - b l a c k solid with a metallic luster. The samples were washed with distilled water, dried at 130 ~ and analyzed for Li and Mo w i t h the results given i n Table VI. The deposit was found to contain an average of 54.0% Mo and 9.85% Li but no K or C1. The ratio Li/Mo is 2.52. No appreciable t r e n d of composition w i t h c u r r e n t density was observed. If it is assumed that the deposit contains only Li, Mo, and O, the empirical f o r m u l a Li2.48 MoO3.96 is obtained indicating a n average oxidation state of 5.5 for m o l y b d e n u m . Additional preparations produced at --1.75V were analyzed for oxidation state, with the results listed in Table VII. The s u m of Li20, Mo205, and MoO8 is very close to 100%, confirming the absence of K and C1, and indicating once more that the product is Li2.sMoO4 or LisMo208. To ascertain whether the product is a single compound or a mixture, an x - r a y powder p a t t e r n was obtained for the products prepared at --1.2, --1.5, a n d --1.75V. The d spacing along with relative intensities are given in Table VIII. No lines for LiC1 or Li2MoO4 were found i n the diffraction pattern. The diffraction data do not correspond with a n y k n o w n m o l y b d e n u m compound listed i n the ASTM files or w i t h other deposits obtained b y reduction of Mo(VI) compounds. Table V. Chronopotentiometric data for the second reduction step of lithium molybdate Mo]ybdate cone. (millimolar)

IoT1/2 (rnA-secl/e/cm~)

R a n g e of Io (mA/cm2)

8.03 20.80 53.50 110.02 178.05

1.29 -~--0,08 3.46 • 0.06 9.72 -- 0.09 20.04 • 0.18 31.00 "~"0.90

2,5-4.0 6,0-10.0 24.0-20.0 24.0-44.0 26.0-34.0


1350

J. Electrochem. Soc.: E L E C T R O C H E M I C A L S C I E N C E A N D T E C H N O L O G Y

October 1973

Table Vh Typical analysis of Li2Mo04 deposit prepared at constant current Sample

A

C u r r e n t density, m A l c m ~ Sample weight, m g Li, m g f o u n d Per cent w e i g h t Li Mo, m g f o u n d Per cent w e i g h t Mo Per cent w e i g h t 0 = to 100% Li found, #moles Mo found, pxnoles O calculated, /~moles Empirical f o r m u l a

B

5 73.5 7.17 9,7 39.6 53.9

5 129.9 12,7 9.8 70.6 54.4

36.3

35.8

1035 412 1670 Lis.5~MoOs.e

1830 736 2920 Li2.4tMoOs.m

C

D

5 112.5 ii.0 9.8 61.1 54.1 36.1

1585 636 2520 Li2. ,~IVloOs.~

E

I0 150.6 15.2 I0.I 81.2 63.8

I0 148.2 14.7 9.g 60.23 54.0

36.1

36.1

2200 846 3380 Li~.~r

2120 835 3320 Lis.~IVIoOs.~

F I0 134.5 13.2 9.9 72.2 53.6 36.5

1910 753 3065 Li2.sAYloOt.o7

Table VII. Determination of the empirical formula of Li2MoO4 deposit prepared at constant potential Sample

A

Milliliters 0.05N Na2S2Os Milligrams NaIOs calculated Milligram total Mo calculated A v e r a g e valence of Mo Micromoles LJ Milligrams LisO Total Mo, m g Micromoles Mo Milligrams Mo (V) Milligrams Mo~O~ Milligrams Mo (VI) Milligrams MoOa Total m g found Sample weight, m g Per cent w e i g h t found Empirical f o r m u l a

6.29 25.9 50.7 5.50 1320 19.8 50.7 528 25.0 35.6 25.7 38.4 93.8 92.6 101.9 Li2.eoMoOi.0o

To s t u d y the s t a b i l i t y of t h e product, one s a m p l e was h e a t e d at 300 ~ u n d e r vacuum. No change in d i f fraction p a t t e r n was observed. The deposit was f u r t h e r boiled in distilled w a t e r 3 hr and d r i e d at 200 ~ The boiled p r o d u c t e x h i b i t e d a different x - r a y p a t t e r n as shown in Table IX. The a n a l y t i c a l d a t a indicated L i / Mo to be 4.43/2 and the sum of the weight p e r c e n t ages of Li20 -{- MoOs to be 97.66, indicating t h e p r e s ence of w a t e r in t h e s a m p l e or p a r t i a l oxidation of Mo205. T h e e l e c t r o d e reaction of m o l y b d a t e can b e w r i t t e n 2MOO4 = + e - -> Mo208 -5 The m e c h a n i s m of this r e a c t i o n is unclear. T h e r e is no evidence for d i m e r i z a t i o n of m o l y b d a t e p r i o r to reduction, a n d t h e diffusion coefficient is consistent Table VIII. X-ray powder diffraction pattern of Li5Mo20s d (A)

I/Io

4.9130 2A661 2.3650 2.0?40 1.9125 1.4919 1.4443 1.3882

60 60 20 100 20 40 40 4O

Table IX. X-ray powder diffraction pattern of boiled deposit (A)

I/Io

3.6302 3.2783 2.9568 2,6727 2.5510 2~046 2.0279 1.8845 1,8345 1.7172 1.6752

60 30 I0 I00 1O 3O 30 i0 I0 20 20

B 7.42 29.8 60.7 5.51 1600 23.8 60.7 632 29.8 42.2 31.0 40.2 112.3 113.5 99.2 Li2.u~MoOt.~

C 8.54 35.2 67.7 5.49 1730 25.9 67.5 702 34.2 48.5 33.3 50.0 124.3 123.1 101.0 Li2. tsMoOs.*e

D 9.40 38.7 76.2 5.52 2010 30.1 78.2 815 37.8 53.6 40.4 60.6 144,4 143.3 100.5 Lis.47MoOa.w

w i t h our e x p e c t a t i o n for a monomer. The simplest m e c h a n i s m w o u l d be the f o r m a t i o n of t h e anion M0043-, w i t h t h e i n c o r p o r a t i o n of e q u i m o l a r amounts of M0042- w i t h l i t h i u m ions to f o r m t h e c r y s t a l l i n e compound Li5M0208. Acknowledgment One of the a u t h o r s (B. N. P.) is i n d e b t e d to the U n i v e r s i t y of Illinois for the s u p p o r t in the form of r e s e a r c h assistantship. F i n a n c i a l s u p p o r t of this r e search was p r o v i d e d b y the United States A r m y Research Office-Durham and b y the Center for A p p l i c a tion of Radioisotopes in I n d u s t r y , Skopje, Yugoslavia. M a n u s c r i p t s u b m i t t e d M a r c h 30, 1973; r e v i s e d m a n u script received M a y 18, 1973. A n y discussion of this p a p e r will a p p e a r in a Discussion Section to be p u b l i s h e d in t h e J u n e 1974 JOURNAL. REFERENCES 1. A. S t a v e n h a g e n and E. Engels, Bet., 28, 2281 (1895). 2. C. Cannery, Gazz. Chim. Itat., 60, 113 (1930). 3. W. G. B u r g e r s and J. A. M. Van Liempt, Z. Anorg. Allgem. Chem., 302, 325 (1931). 4. A. Magn~li, N. Acta Reg., Sc. Upsaliensis, 14, No. 816 (1939). 5. G. Hagg, Z. Physik. Chem., B29, 192 (1935). 6. A. Wold, W. K u n n m a n n , R. J. A r n o t t , and A. F e r retti, Inorganic Chem., 3, 545 (1964). 7. S. Senderoff and A. Brenner, This Journal, 101, 16 (1954). 8. S. Senderoff and A. Brenner, ibid., 101, 31 (1954). 9. H. A. L a i t i n e n and J. H. Propp, Anal. Chem., 38, 644 (1969). I0. H. A. L a i t i n e n a n d B. N. Popov, This Journal, 117, 482 (1970). 11. H. A. L a i t i n e n a n d K. W. Hanck, ibid., 118, 1123 (1971). 12. H. A. L a i t i n e n and K. R. Lucas, ibid., 12, 553 (1966). 13. H. A. L a i t i n e n and D. R. Rhodes, ibid., 10, 413 (1962). 14. G. H. A y l w a r d , Anal. Chim. Acta, 14, 386 (1956). 15. P. Bourret, J. M. Lecure, a n d K. Weis, Chim. Anal. ( P a r / s ) , 52, (1) 48 (1970).
