Gradual reduction of molybdosilicates and related compounds

~o~rnaio~the Less-Common Metals, 36(1974) 139 - 150 @ Efsevier Sequoia %A., Lausanne - Printed in The Nether~nds

GRADUAL REDUCTION OF MOLYBDOSILICATES COMPOUNDS*

139

AND RELATED

JEAN PIERRE LAUNAY, RENE MASSART and PIERRE SOUCHAY Laboratoire de Chimie IV, Associe au C.N.R.S., Universitkde Paris VI, Paris (Francej

SUMMARY

For SiMo i204e4- and related compounds, the products obtained by the gradual reduction of Mo(V1) atoms form redox-reversible, very mobile systems. Their redox and chemical behaviour (stability and disproportionation reactions) and the relations between (Yand 0 isomers have been studied. For higher reduction stages, the reduced derivatives form a new series with different properties. The members of each series are in redoxreversible equilibrium with each other, but not with members of the other series.

INTRODUCTION

The reduction of 12-molybdos~licates, SiMo, a04e4; and analogous compounds has been known for a long time. The deep blue colour obtained is used for the colorimetric determination of the central element (P, Si, Ge, As) [l] . As the number of MO(W) atoms reduced to the MO(V) state is variable, there are many resulting species. Hence, the behavlour of reduced compounds varies with the experimental conditions and the nature of the reducing agent. Furthermore, since Strickland’s work [2 1, it has been known that two molybdosilicic isomers exist. The P-derivative is formed first; then it becomes the more stable a-isomer. The coexistence of CY-and D-isomers was not always taken into account in the reduction studies. This paper deals with both pure isomers. The pure P-isomer could be studied by stabilising it by means of a hydra-organic solvent [3]. In order to avoid any transformation into the cr form, all experiments starting from the @form were carried out in water-alcohol (1: 1) medium. EXPERIMENTAL

Electrolysis was carried out at controlled potentials in order to obtain progressively reduced compounds and to avoid introducing foreign reagents. The choice of potential was determined by a preliminary polarographic study. The compounds being reduced by mercury, a platinum electrode was used. Either of these metals could be employed for *Presented at the Conference on “The Chemistry and Uses of MoIy~enum”, at the University of Reading, England, September 17 - 21,19?3, sponsored by Climax MoIybdenum Co. Ltd., and the Chemical Society (Dalton Division).

140

the sufficiently reduced derivatives. Almost all the redox systems involved were remarkably fast and reversible. In some cases, the primary reduction product rapidly altered after its formation. Therefore, electrolysis did not lead to the species expected from the polarogram. The Si concentrations were about 10m4 M for polarography and varied from 5 to 100 X low3 M for electrolysis in the presence of a supporting electrolyte (0.5 - 1.O M NaCl). The half-wave potentials, Es, are given against the saturated calomel electrode. In the following, the products obtained will be designated by a roman numeral equal to the number, IZ, of electrons fixed by the starting product (itself designated by 0). RESULTS AND DISCUSSION

12-Molybdosilicates:

States with n < 6.

(a) The a-isomer: even states /4 - 6] In an acidic medium (0.5 M HCl), polarograms exhibit three waves, each of two electrons, corresponding to the formation of products (II), (IV) and (VI) (Es = 0.25, 0.13, -0.06 V). The reversibility of systems (@-(II) and (II)is proved by the polarograms of(U), (II) and (IV) which can be superimposed, and by the potentials of graphs E = f(n). The acids (II) and (IV) can be prepared from sufficiently concentrated solutions, electrolysed at potentials corresponding to the respective tops of waves, and precipitated by concentrated HCl. This method cannot be applied to prepare (VI) which is unstable in acidic medium; it can be obtained in solution at a pH greater than 4.8 and at 0 ‘C [7]. In this case the reduction is also reversible, since the polarogram of (VI) gives an anodic wave (VI)-(IV) with the same Es as the cathodic wave (VI)-(IV). A number of weakly acid groups appear with the reduction. Thus, while (0) shows four strongly acid groups, (IV) shows, in addition, four weakly acid groups @K’s 3.7, 5.7,8.4, 10.9). These compounds probably conserve the initial structure by adding H to the free apices of octahedra and forming OH groups, according to n.m.r. studies. [B] . Titration of (VI) cannot be carried out completely because of its instability in acidic medium. However, it probably has ten acid groups. It is worth noticing that stability in basic medium is favoured by reduction. Thus, while (0) decomposes into silica and molybdates at pH values greater than 4, (II) decomposes only at pH values greater than 7, because of the disproportionation reaction 2(11) + (IV) + (0). The equilibrium is shifted by the decomposition (IV), silica and molybdates are finally obtained. and (VI) is unstable in acidic medium.

of (0), unstable at this pH. As a result, (IV) and (VI) are stable at higher pH’s,

(b) The cr-isomer: odd states (0) breaks down at pH greater than four. Hence polarographic studies at higher pH’s have been carried out on (IV). At pH greater than four, the splitting of (II) + (0) is observed with the formation of (I), and at a pH greater than ten the splitting of (IV) + (II) with the formation of (III) (see Fig. 1). The formation of these species having increasing values of n

141 I(A) PH 0

Fig. I. Polarogram

4.8

of ~[SiMo,,O,,]‘-

13.3

at different

pH values at a rotating

platinum

electrode.

(Ref. 14).

has been confirmed by spectrophotometry and the conditions required for their existence have been specified. In more basic media, the (IV) + (VI) wave splits, and in 1 M LiOH it was possible to obtain (V). However, no splitting occured in 1 A4 NaOH. The behaviour of these highly charged anions is, in fact, influenced by the nature of the cation present in the solution because of the formation of ion pairs. (c) The fl-isomer..properties Comparison of the first two waves of the a-isomer with those of the P-isomer shows that the latter is the more reducible. Thus in water-alcohol (1: 1) medium containing 0.5 M HCl, Ex takes the following positive values: cr-form p-form

first wave 0.30 v 0.42 V

second wave 0.19 v 0.32 V

Hence, a mixture of (Yand fl shows three waves [9]. The first corresponds to the first wave of p, the last to the second wave of (II,and the middle one to the second wave of p and the first wave of QI.Therefore, by measuring the heights hi and h3 of the first and the third waves, it is possible to determine the relative proportions of cxand /3 in the mixture (see Fig. 2 showing the change, with time, of /3 into cr in aqueous medium). Consequently, it is not necessary to use less direct methods (see ref. 10, where our own earlier work has not been mentioned). Not only the kinetics of the transformation /3+ (Yhave been studied, but also the purity of solid samples of /3 [9]. These samples were precipitated from solutions of the fl compound in a water-alcohol medium [9] by means of concentrated HC104 (work also not mentioned in ref. IO). We shall not discuss the behaviour of reduced derivatives of the P-series, described elsewhere [4,7, 111, and analogous to that of a-series derivatives. It should, however, be noticed that it has not been possible to adduce evidence for (V)p, and that disproportionation of (II)/3 occurs not only in basic but in acidic medium, probably because the equilibrium 2(II)P = (IV)0 + (0)s

142

E (V)

Fig. 2. Evolution p -+ afSiMo,,O,,] 4: Each curve is shifted 0.3 V with respect to the preceding one. Times, I, are in min. T = 20 *C. (Ref. 9.).

is completely displaced by the irreversible transformation (WP -+ (ONSo far as 12-molybdogermanic acid is concerned [ 121, the kinetics of this reaction is of second order, with k = 0.021 min-’ in 0.5 M HCl, 0.03 in 1 M HCl and 0.0075 at pH 4.6 for a MO(V) concentration 1.2 X 10e3 M and a temperature of 40 “C. fd) Conditions for the existence of isomers

In the presence of a non-aqueous solvent in sufficiently high proportions (aqueous alcohol or dioxan 1: l), (~)~ does not change [3] , But in aqueous media, it is completely transformed into (Ok. (IV)@in an aqueous medium is not transformed into (IV)cc. On the other hand, at 70 ‘C (IV@ is transformed into its isomer within a few hours. As far as (II)0 is concerned, it is worth recalling its disproportionation into (IV)0 and (Ok. Stabilisation of (IV)/3is still more remarkable with 12-molybdogermanic derivatives. In aqueous medium, (IV)o isomerises into (IV)/3 [ 121 according to a first-order reaction with a half life of 176 min at 40 ‘C and 60 min at 60 “C (0.5 M HCl solution). This reaction is much slowed down in less acidic media. In the same way, in the As(V) series, chemical or electrochemical reduction of acidic aqueous solutions of molybdates and arsenates easily leads to the (IV)@derivative [ 131, while in such media unreduced o! or fi derivatives cannot occur. In this As(V) series, the evolution of (II) likewise occurs in the direction (Y+ 0, but (II&3disproportionates in turn into (IV)@and an arsenate-molybdate mixture. Thus, reduction stabilises reduced derivatives in alkaline media, and the P-series versus the o-series. (e) Eg -pH patterns

Because of the rapidity of electronic exchanges, these patterns have to be very close to those giving the apparent redox potential. An advantage of polarography is that it makes it possible to measure the latter at pH values at which one of the two redox partners is not stable. This is the case of (0) at pH values greater than five, or of (II) at pH values greater than seven. Their destruction is slow enough, (as compared with the electrochemi~l reaction) to obtain potentials of couples (O)-(II) and (II)from anodic reoxidation waves of (IV).

143

Fig. 3. .E’g = f@H) for cu[SiMo, 2040] ‘;

0

Fig. 4. Es=f(pH)

5

PH

10

for ~~[AsMo,~O~~]~T (Ref. 13.)

The patterns of Figs. 3 and 4 are with reference to the a-series for Si(IV) and the /3 for As(V). As Es depends on the number of protons involved in redox processes, the curves are straight lines, but change slope as the pH approaches the pK value of the oxidised or the reduced form, unless the pK’s of both forms happen to be close together. The curves enclose the regions in which several derivatives are capable of existence. As two curves come together, the species corresponding to the region between the two curves disappears by disproportionation. Thus it can be explained that (I) and (III) occur only beyond some pH values. These results can only be applied to the pure oxidised or reduced forms which

i(A)

j

Fig. 5. Evotution of polarogram (Hg electrode) during the reduction (IV) -+ (XII’) and the oxidation (XII’) + (X’) in 0.5 M NaOH.

have not undergone any transformation, for (0) cannot exist at upper pH’s, nor (II) in acidic PH. States with n > 6 Above a definite value of n, a structural change occurs for the reduced compounds. The species obtained are in equilibrium with each other, but not with the less-reduced species. Both classes behave independently of one another. Before dealing with the 12.molybosilicic series, some results on the 12-molybdoarsenic series are reported for comparison. (a) IZmolybdoarsenates We shall confine our attention to the reduction of (IV)& which is particularly stable [13]. The splitting of the (IV) -+ (VI) wave is observed at a pH greater than 11 (Fig. 4) and that of (VI) + (VIII) at a pH greater than 12.5. (VIII) has been obtained by electrolytic reduction of (IV) at about pH 10 (carbonate buffer) and with a potential close to -0.9 V. The progress of the reaction is followed by coulometry and checked by polarography in the electrolysis cell itself, the products being very oxidisable. For the reduction state n = 8, the waves (IV) + (VI) and (VI) + (VIII) of the starting product have undergone a translation into the anodic range without

14.5

change in their shape, which proves their reversibility. (The electrolysis cannot be carried further, for the structure is destroyed with the precipitation of molybdenum oxides.) (VII)0 has also been obtained by the electrolytic reduction of (IV)/3 in 1 M NaOH atO”Cand-1.1 V. (b) cu-molybdosilicate: existence of two types of reduced compounds (IV)0 has been used as the starting material. In classical polarography (mercury electrode), the (IV) -+ (VI) wave described above is observed (Fig. 5, a). It is followed by three waves (b, c, g). The best resolution of waves (b) and (c) is obtained at pH > 10 where they correspond to n = 12 (b) and 18 (c). In 0.5 M NaOH, the electrolysis of (IV) at - 1.15 V (plateau of b wave) does give the corresponding product with n = 12. If the process were reversible, the new polarogram could be subtracted from that of (IV) by a translation towards the anodic range (Fig. 5, waves c’, b’ and a’). Actually the (c’), (d) and (e) waves are obtained and also a more positive wave (f) which is not represented on Fig. 5. This compound thus has a different nature from the species with n < 6. It will be denoted (XII’); the heights of the anodic (d) and (e) waves correspond to the oxidation of (XII’) into (X’) (d) and (VIII’) (e). Thus, while the polarographic reduction of (IV) or (VI) to (XII’) is straightforward, the reoxidation of (XII’) occurs through (X’) and (VIII’). In fact, the electrolytic oxidation of (XII’) at the top of the (d) wave does give (X’), the potassium salt of which has been isolated; rt is noticeable that (XII’) and (X’) are in reversible equilibrium because (X’) can be reduced again into (XII’), and its cathodic wave has the same Ey2 value as the anodic wave (dotted polarogram). We also tried to obtain (VIII’) by the electrolytic oxidation of (XII’) or (X’) at about -0.65 V (top of the e wave). In fact, (VIII’) is not very stable; its life time is long enough to make it appear during a mercury drop period, but on the other hand, it must undergo some transformation during an electrolysis period, and the product finally obtained is (IV). This will be explained below. (c) Use of linear sweep voltammetry In order to supplement the preceding observations, we have used linear sweep voltammetry with glassy/carbon electrodes. At pH 5, when starting from (IV), two cathodic and reversible waves, corresponding to n = 6 and n = 8, are observed (Fig. 6 (a)). In fact, in classical polarography, for 5 < pH < 8, beyond the (a) wave, (b) and (c) waves are well resolved as for pH > 10, but the (b) wave corresponds to n = 8. The species which forms for n = 8 is of the same type as the less-reduced products (and thus will be denoted (VIII)) because a new cathodic scan displays the (II)and (IV)-(VI) waves at their usual places for this pH. On the other hand, a trapezoidal scan with a 20 s stop time at the potential where (VIII) forms, shows the instability of this product, because three new anodic waves appear (indicated by arrows, Fig. 6 (b)) corresponding to (d, e and f) polarographic waves of (XII’) at this pH. With the same conditions, except for pH 7.5, the evolution of (VIII) is not appreciable. This proves that its transformation is accelerated by acidity. In 0.5 M NaOH, the (b) wave, which is now a 6-electron-wave, is irreversible. No evidence can be found for (VIII), whatever the scan rate. Conversely we have studied the oxidation of (XII’). The best conditions are achieved at pH > 10 and with a stationary mercury electrode. It has been found that the

LL_1l0.2

0

-02

-0

4

- 0.6

E (V)

Fig. 6. Reduction of (IV) (Si = 2 X 10m4AI) on glassy carbon at pH 5 (a) v = 0.2 V s-l ; triple scan; (b) v = 0.2 V s-l ; double scan with 20 s stop at -0.6 V. New anodic waves are indicated by arrows.

(XII’)-(X’) and (X’)-(VIII’) waves (corresponding to (d) and (e) in classical polarography) are equal and reversible (Fig. 7 (a)). Scanning towards less negative potentials (Fig. 7 (b)) feads to an important oxidation wave of (VIII’) analogous to (0; the products thus formed are not very stable, for a further cathodic scan, after a IO s stop, gives rise to the characteristic (b) wave of the usuaf series. However, for higher scan rates (10 V/s) at pIi 1 I, instead of the preceding wave a reversible system is observed (Fig. 81, corresponding to (VIII’) * (VI’) + 2 e: (VI’) is very unstable. tion has occurred.

If the cathodic scan is carried out after a 10 s stop, its decomposi-

(d) Interpretation of the preceding reuc tions The gradual reduction of ff-molybdo-situate thus gives rise, for n G 12, to two series of compounds. The stable products are (I) to (VI) for the first series; (X’) and (XIf) for the second. Evidence has been given for both (VIII) and (VIII’) but their life times are short; it is still shorter for (VI’).

147

,

i

(b)

f

B

-06

-06

-04

-I

-1.2

E (V)

Fig. 7. Oxidation of (XII’) (Si = 10.’ M) on Hg in 0.5 M NaOH. Scan rate 0.1 V s-’ (a) triangular scan; (b) trapezoidal scan with 10 s stop.

_~~_

f / i-__

* -04

-0.6

-or

-1

-1.2

E(V)

Fig. 8. Oxidation of (XII’) (Si = 10e4 M) on Hg at pH 11. Triangular scan 10 V s“.

The possibte occurrence of two forms with the same n value has also been proved for the

tungsten compounds (tungsto-silicates and borates [ 151 and metat~gstates [ I.61> and for n = 6. In this case, however, (VI’) is the more stable form. The compounds of each series are in redox equ~ibrium with each other, but not with those of the other series. It is worth noticing that the redox potentials of the “ ’ ” series are either of the same order of magnitude or less negative than the potentials of the initial series, &though their n values are higher. (See Fig. 5 for (Iv)-(W) (a) and {VIII’~-~~f~ (e), and Figure 9-j For the n = 8 stage, the stabilities of (VIII) and (VIII’) seem to be low but comparable, which allows an interpretation of preceding reactions. Thus, the formation of (XII’) for 5 < pH < 8 at the top of the (b) wave [(VI)-(VIII) wave] can be explained by a disproportionation for the n = 8 state, the relative positions of redox potentials in the two series allowing oxidation of (VIII) by (VIII’) or (X’)

YI’/ mr’

XllI’yx’

-0.4

-06

X'/XlI' -0.8

-1

E (V)

Fig. 9. Relative positions of redox potentials for the two series at pH 11.1.

Fig. 10. Reduction of (Land p molybdosilicic acids by Sn(II) and evolution of products. The bordered species are stabilised in the presence of dioxan.

The reduction of (VI) to (XII’) at pH > 10, also at the plateau of(b) wave] , would involve the two first steps of this mechanism, followed by (VIII’) + 4e- ----+ The oxidation explained by

[(VI)-(XII’)

(XII’).

of (X’) to (IV) at the plateau of the (X’)-(VIII’)

(X’)

=+ (VIII’) t 2e-

(VIII’)

= (VIII).

The relative positions of redox potentials into (IV) at the imposed potential.

(e) wave could be

in the two series allow reoxidation

of (VIII)

Action of metallic reducing agents The reduction of heteropolyanions can also be carried out by means of reducing metallic ions [Sn(II), Ti(III), Cr(II)] [2, 17, 181. The reduced products obtained are not always the same as those described above because they can include the metallic element in their structure. Indeed, the existence of numerous heteropolymolybdates (tungstates) in which one or two MO or W atoms are substituted by metallic atoms, is now established

P91.

149 r--------i

Fig. 11. Electrochemical

reduction scheme for OL molybdosilicate.

The framed products are unstable.

When the cr-molybdosilicic ion is reduced by SnC12 in 0.5 M HCl, the species Si[(Mo6+)6(Mo5+)4(Sn4+)203,] 6 -, (A), is obtained, which is transformed rapidly, but can be stabilised in the presence of dioxan. In the absence of dioxan, (A) changes within 9 h. During this evolution, the spectra run through two isobestic points and the polarogram is also transformed. A new product, (X), is thus obtained while there is precipitation of tin(IV) hydroxide. After 9 h, the change becomes slower but can be accelerated by heating at 55 “C. The reaction A + X now requires 30 min, and a further change gives a new species, (B), characterised by a new spectrum and a new polarogram. Finally, the last step being very slow at 55 ‘C, the temperature is raised to 70 “C. The transformation of (B) into (IV)0 is then observed after several hours (recall that at 70 ‘C, (IV)ol turns into (IVM). (X) and (B) contain only one tin atom (which explains the precipitation of tin(IV) hydroxide by evolution of (A)). This can be checked by the action of SnClz on (II)& in equimolar proportions, which instantaneously gives (B). The relations between these different species are displayed in Fig. 10 [20]. In the case of Ti(III), the reduction of cr-molybdosilicic ion in 1: 1 water-dioxan medium and 0.5 M HCl leads directly to (IV)ol. Then (IV@ reacts upon the Ti(IV) formed and gives a compound analogous to (A). This compound is transformed afterwards in the same way as (A) but faster. In the case of Cr(II), the reduction is very slow and the occurrence of (II)(w and (IV)cz is proved by polarography. The Cr(II1) liberated reacts only very slowly after heating for several days at 70 “C. We never obtained, quantitatively, the (A) analogue, but a mixture of it and of (IV)p [21]. CONCLUSION

It is possible to obtain, specifically, the various reduced heteropolymolybdic species. Their stability ranges have been specified and evidence has been given for two series of derivatives with different chemical and redox behaviour. The different species thus obtained are collected in the general reduction scheme, Fig. 11. REFERENCES 1 M. Jean, Chim. anal. (Paris), 37 (1955) 125, 163. 2 J. D. H. Strickland, J. Am. Chem. Sot., 74 (1952) 802,876. 3 T. Skerlak, Bull. Sot. Chim. Rep. Pop. Bosnie-Herzegovini, 5 (1956) 27; T. Skerlak, T. Ribar and B. Skundric, Glasnik Drustva Hemicara Technol, N.R. Bosne Hercegovine, 9 (1960) 19, 23; 11 (1962) 31.

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