Cyclic voltammetric behaviour of platinum in dried and wet nitrates melt

Elecrrochimica Acta, Vol. 38, No. 17. pp. 2611-2615, Printed in Great Britain.

CYCLIC

0013s4686/93

1993

VOLTAMMETRIC BEHAVIOUR OF PLATINUM DRIED AND WET NITRATES MELT I. B.

56.00 + 0.00

0 1993. Pergamon PressLtd.

SINGH, S. SULTAN

and K.

IN

BALAKFUSHNAN

Central Electrochemical Research Institute, Karaikudi-623 006, India (Received 25 November 1992; in revisedform 5 May 1993)

Abstract-Electrochemical investigations in equimolar NaNO,-KNO, melt and in NaNO, and KNO, melt were carried out separately on platinum surfaces at 300 and 340°C. respectively. A well defined cathodic peak and two anodic peaks related, respectively, to nitrate ion reduction and oxidation of alkali metal oxides were observed in mixed melts. Studies carried out in separate nitrate melt confirmed the respective alkali metal oxides formation. Evidence for the water reduction wave and the effect of moisture content on the solubility of alkali metal oxide has been elaborated. Key words: nitrates melt, platinum, cyclic voltammetry, oxides, water wave.

INTRODUCTION Numerous basic electrochemical investigations related to nitrate ion reduction in fused salts have been reported[l-6-J. However, there are still some unresolved issues[7-91 relating to the production of oxide (02-), peroxide (O:-) and superoxide (0;) ions in addition to nitrite (NO;) ions as a major product in the nitrate melts. An extensive literature survey reveals that nitrite and oxide ions exist as initial electrochemical reduction products in nitrate melts. The following reaction has been shown to explain the nitrate ion reduction. NO;

+2e-*NO;

+O’-

(1)

Inference drawn from various kinds of previous investigations[ lo-131 indicates the existence of unstable oxide ions in the nitrate melt. The following reversible reactions were put forward in favour of oxide ions transforming to superoxides via peroxide ions. 02- + NO;tiNO; O:-

+ 2NO; =2NO;

+ O;+ 20;

(2) (3)

Zambonin[14] has conducted a detailed study with respect to oxide, peroxide and superoxide formation during electrochemical reduction on a platinum surface in a (Na/K) NO, melt up to 280°C. Though a limiting effect on cathodic reduction due to formation of sodium oxide has been observed, surprisingly no mention has been made about the formation and subsequent effect of potassium oxide. Instead, highly soluble superoxide formation was associated with the electrochemical reduction of KNO,-NaNO, melt. The investigation NaNO, or KNO,

was not carried

soluble superoxide ions are the main cathodic reaction product. Moreover, thermodynamic data[16] predict the stability of mixed alkali metal oxide in (Na/K) NO, melt. Apart from this, a number of studies have been conducted on the reduction behaviour of nitrate melt on different metal surfaces, namely Pt, Au, Pd, Ni etc. in alkali metal nitrate melts in the presence of water[2, 15-J. A significant wave corresponding to water reduction prior to nitrate reduction, is common on many metal surfaces except on Ni. It has been widely reported that the presence of water catalyses nitrate ion reduction but as to how it affects the solubility of alkali metal oxide layers on the electrode surface is not yet clear and deserves further investigation. The present investigation has been undertaken in order to explore the existence and nature of alkali metal oxides on platinum surfaces in equimolar NaNO,-KNO, melts at 300°C by cyclic voltammetry. The influence of water on the electrochemical processes in the nitrate melt has also been examined.

out in the

melt alone. Miles and Fletcher[15] studied the cationic effect in nitrate melt. Incidently, they also did not emphasize the formation of KO, and its subsequent oxidation though a small anodic peak, supposedly related to KO, oxidation on reverse scan could be observed. According to them, in the case of KNO, melt, excessively

EXPERIMENTAL NaNO, and KNO,, the main constituents of the present electrolyte, were of analytical grade and were dried for 12h in a vacuum oven at 150°C. After drying, 5Og of their equimolar mixture was kept at 250°C (above their eutectic melting point) for 6h in vacuum. Oxygen free and well dried nitrogen gas was bubbled continuously for 3 h in this eutectic melt for removing traces of interstitial[2] water present in melt. the Undried equimolar NaNO,-KNO, was added to introduce moisture into the melt. The electrochemical set-up was similar to that used by other workersC14, 151. The equimolar melt was contained in a glass tube that served as the electrochemical cell. Pyrex glass was used for the cell assembly. The entire electrochemical cell was placed vertically in the tubular furnace and the temperature

2611

2612

I.B. SINGH et al.

Ag WIRE

ti-

IN EOUIMOLAR

A - FURNACE G - ELECTROCHEMICAL C-

INNER

GLASS

CELL

TUBE

0-

GAS INET

E-

REFERENCE

F-

WORKING

ELECTRODE

I PI 1

G - COUNTER

ELECTRODE

I Pt 1

ELECTRODE

H - THERMOCOUPLE

Fig. 1. Electrochemical set-up. of the melt was maintained within +OS”C by a variable transformer. A nitrogen environment above the melt was maintained during the experiment. Figure 1

depicts the electrochemical set up. The conventional three electrode system, namely platinum foil as the counter electrode, platinum wire as the working electrode and Ag/Ag’ as the reference electrode was employed in the present study. The platinum foil (lcm* area) and platinum wire were fused in a glass tube. The reference electrode was made of pure silver wire immersed in the NaNO,-KNO, melt containing 0.07 M AgNO,[14] in a separate glass tube as shown in Fig. 1. Prior to the experiments, the exposed area of the working electrode (0.01 cm’) was polished well using various grades of emery paper and degreased by trichloroethylene solution. All cyclic voltammetric results were obtained with a PAR model 173 potentiostat and a PAR model 175 programmer. A X-Y recorder (model Rikadenki 0363) was used for recording all the cyclic voltammograms.

RESULTS

-10001 - 2.5

I - 2.0 E/V

I -1.5

-1

(Ag/Ag+)

Fig. 2. Cyclic voltammetric behaviour of platinum (area, 0.01cm’) in dried NaNO,-KNO, melt at 300°C at various scanrate[(1)20mVs-‘,(2)50mVs-l,(3)

100mVs-‘1.

was also invariably observed. An interesting feature is the increase of the cathodic current around - 1.55 V which is not affected by the scan rate. The process responsible of giving a rise to such a cathodic current at this potential is well studied and is known to be the result of nitrate ion reduction as per equation (1). The cathodic limiting current related to the peak is attributed to the alkali metal oxide formation at the electrode surface. A plot of i, vs. vl/’ for peak 1 of Fig. 2 is shown in Fig. 3. This diffusion controlled behaviour for alkali metal oxide formation is quite the case with fused salt electrochemistry. A closer examination of Fig. 2 reveals another cathodic peak near -2.OV (at the 20mV s- ’ scan rate) which is likely to correspond to the formation of a different oxide, possibly K,O, or its converted form of KO,. K,O has been reported[l4, 151 to be rapidly converted into KO, . The small anodic peak (peak II) noticed around - 1.78 V, (at the 20mV s- ’ scan rate) which gets shifted towards more positive potentials accompanied by increased peak height at higher scan rates

AND DISCUSSION

Cyclic voltammetric investigation was carried out in dried and wet conditions of equimolar (Na/K)NO, melt. The results obtained are reported and discussed separately to evolve a clear understanding of the process involved. Dried melt

Cyclic voltammetric curves were recorded at various scan rates on the platinum surface at 300°C (Fig. 2). The voltage range was limited to -1.0 to -2.5 V vs. Ag/Ag+. The reverse scan in the anodic direction was limited to - l.OV as it was thought interesting to extensively explore this potential region in order to clarify earlier speculations. A well defined cathodic peak (peak I) was found at the potential . _. of _ . _- 1.88 _ _ V at the 20mVs-’ scan rate. A cathodic shift of the same with increasing scan rate

Fig. 3. i, vs. u”* relationship for Na,O reduction on platinum surface in dried NaNO,-KNO,

melt at 300°C.

2613

Cyclic voltammetric behaviour of platinum

may be due to the oxidation of the potassium oxide formed. At this stage, this inference can, at best, be a guess, as no report in the literature could be found pertaining to this particular anodic peak. In confirmation of the point just made, cyclic voltammetric investigations on platinum sufaces were carried out separately in NaNO, and KNO, melts at 340°C. Figure 4 depicts the small anodic peak, observed in the case of the KN03 melt. This probably indicates the electro-oxidation of KOr in the KNO, melt as well as in the mixed melt. The important point to be noted here is the similar anodic peak (peak II) height for the (Na/K)NO, as well as the KNO, melt alone (Figs 2 and 4). Further, in the (Na/K)NO, melt, the current begins to diminish (peak II) significantly with decreasing scan rate. It seems that the alkali oxide formed is soluble in nature and gets partially dissolved if the approach of the requisite oxidation potential is delayed. Surprisingly, a slight anodic peak has also been cursorily observed by Miles and Fletcher[15] during their study in KNO, melt alone at - 1.6V, but they did not pursue it further to confirm the formation of KO, and its subsequent oxidation. In the (Na/K)NOJ melt, the reverse scan gives rise to a well defined second anodic peak (peak III) which shows a substantial current increase with scan rate. Confirmation of this particular oxidation reaction was obtained by carrying out a similar study in NaNO, melt at 340°C on the same electrode (Pt) surface (Fig. 4). The position of the anodic peak and its nature correspond to a great extent to the second anodic peak (peak III) observed in the (Na/K)NO, melt. This confirms the formation of Na,O and its subsequent oxidation[14]. In order to confirm the oxide formation its and soluble nature, Zambonin[14] adopted a “Waiting Time* technique by stopping the linear scan on the reverse cycle after the cathodic peak. Figure 4 clearly shows the large difference in cathodic current for the nitrate reduction in KNO, and NaNO, separately, the for-

mation of Na,O and its lower solubility compared to KO,. Therefore, the limiting cathodic current is reached at a less cathodic potential (- 1.8 V) in the case of the NaNOs melt. This viewpoint gets support when the anodic current peak for these two melts are compared. Highly soluble superoxide ions[2] which are likely to be formed in the case of KN03 (as per equation (3)) is a responsible for the continuous increase of the cathodic current. The observed limitation in the cathodic current was due to the probable formation of K,O and its rapid conversion to KO,. In the case of the NaNO, melt, limitation in cathodic current is due to the oxide and peroxide ion formation[14]. The peroxide formed (equation (2)) is less soluble than the superoxide (0;) ion. The strength of the positive electric field of the alkali cations dictate the preferential formation of oxide, peroxide and superoxide[ 15, 173. Considering anodic peaks in Figs 2 and 4 of mixed KOr and Na,O as well as KNO, and NaNO, melts, separately, the peak height clearly related to the difference in solubility of KO, and Na,O. KO,, because of its higher solubility, does not contribute to the time dependent oxidation current and the less soluble Na,O gives rise to much higher anodic current. Miles and Fletcher[lS] have done a similar study in mixed as well as separate alkali metal nitrates and report the order of stability of alkali metal oxide formation. According to them, solubility of alkali metal oxides increases in the order of K > Na > Li. A probable oxidation reaction in the light of previous study can be given as follows (peak IL) KOz + K+ + O2 + e(peak III) Na,O -+ 2Na+ + l/20;

+ 1.5 e-.

(4) (5)

Superoxide ions (0;) are formed when Na,O gets electro-oxidized. Oxygen as an ultimate electrooxidation product in the case of Na,O has been reported[14], when the reverse scan was further extended to positive potentials, ie -0.70 vs. Ag/Ag+. Wet melt

1.25.

O-1.25 -

- 2.50 -

.

Q E

-3.75 /I - soo-

1" 'I

- 6.25 -

- 7.50 - &75-10.00. -2.5

/j

(I

i

/

'1 :' L' -2.0

I -1.5

-1.0

ElV(Ag/Ag+l

Fig. 4. Cyclic voltammetric behaviour of platinum (area 0.01 cm*) in dried NaNO, (-) and KNO, (-- --) melt seperately at 340°C (scan rate = 50 mV s- I).

Effect of moisture introduced through the addition of undried 50% equimolar NaNO, and KNO, in the meticulously dried melt, has been studied on a platinum surface by linear sweep voltammetry. Figure 5 shows the two linear sweeps in dried and wet (moisture) melts. It is evident that moisture facilitates nitrate reduction as the same occurs at slightly lower cathodic potentials in comparison to the electrochemical reduction of the melt under dried conditions. The value of the limiting cathodic current also seems to be affected by the presence of moisture. While alkali metal oxide formation and its inhibition of nitrate ion reduction are quite effective in dried melts, the same is not the case in the wet melt. Difference in the magnitude of the after-peak current is well stipulated (Fig. 5). The wave around - 1.5 V observed prior to the reduction of nitrate ions in the case of the wet melt was attributed to the presence of moisture[ 151. In the present investigation, when the potential sweep was reversed at - 1.8V, no corresponding anodic peak was observed (Fig. 5). This clearly indicates that the reduction of water molecules precedes

2614

I.B. SINOH

al.

able KO, oxidation in our case), in his investigation carried out in (Na/K)NO, melts, was only related to highly soluble superoxide formation. Interestingly, Miles and Fletcher[lS] cursorily referred to the occurrence of a small anodic peak in the KNO, melt. They also did not co&m the existence and identity of the small anodic peak by way of further studies in wet melts. Different routes for the reduction of water in nitrate melts have been discussed[21-231. Of course, mere electrolysis of water resulting in the formation of hydrogen and oxygen gas, responsible for the water reduction wave is ruled out. Different explanations, including complex formation with nitrite ions, have been put forth, but nothing absolute could be confirmed. The complex formation reaction of water and nitrate ion is as follows[23].

250 -

125 -

-125d ‘;;-250-

-500

-675

-750 I -6751 -2.5

et

H,O + NO; + HONO, I -2.0

I -1.5

HONO,

-1.0

E/'J(Ag/Ag+)

Fig. 5. Cyclic voltammetric behaviour of platinum (area, O.Olcm’) in dried (-- --) and wet (-) NaNO,-KNO,

melt at 300°C at lOOmVs_’ scan rate.

ip = (4.64 x 106/T”z)n3’2AD”2Co”2.

(6)

The value of the diffusion coefficient, D, was taken from the earlier reportC20-J. The water concentration present in the melt at 300°C was of the order of 10m4 Molar. The present study has been instrumental in confirming not only the established idea but could also clarify, to some extent, prevailing controversy. Although the order of soluble nature of alkali metal oxides was found to be similar to earlier studies, the observation of the limiting of the cathodic current by way of well defined peak formation in the case of molten KNO, was a new phenomenon. The soluble nature of potassium oxide (KO,)[12] was confirmed when a cyclic voltammetric study was carried out in a wet (Na/K) NO3 melt. Zambonin[14] only discussed the superoxide formation in the case of the (Na/K)NO, melt in addition to the oxide and the peroxide formed. He, however, did not extend his investigation to the NaNO, and KNO, melts alone. The absence of the anodic peak (related to the prob-

(7)

+ 2 e- + NO; + OH-

(g)

The overall reaction for nitrate ion reduction in the presence of water suggested by Bombi and coworkers[4,22] is as follows. H,O+NO;

nitrate ion reduction. Another interesting feature is the reduced height of the prominant anodic peak (peak III, Na,O oxidation) in the case of the wet melt. In addition, the prominant anodic peak appears earlier on the reverse scan in the presence of moisture. Obviously, Na,O formed on the electrode gets partially dissolved in the wet melt. A noticeable point on further scrutiny of Fig. 5 pertains to the virtual absence of the small anodic peak (peak II, KO, oxidation) in the wet melt. This is in good agreement with the order of solubility of the oxides of alkali metals in nitrate melts: K > Na > Li as reported earlierC5-J. To estimate the moisture content quantitatively, cyclic voltammograms at various scan rates were obtained and calculations were made using the following Randles-Sevick equation[ 18, 191:

+ OH-

+2e-*NO;

+20H-

(9)

In our investigation, clear evidence for the reduction of water molecules and its catalytic effect on nitrate ion reduction has been observed. It is felt that further extensive studies are required to arrive at a particular reaction mechanism for the reduction of water. CONCLUSION Electrochemical investigation in nitrate melts has thrown light on the following aspects: l Nitrate ion reduction around - 1.55 V in mixed melts. l The limitation of the cathodic reaction due to the formation of oxides. l The formation of comparatively less soluble Na,O along with KO, formation. l The reduction of water molecules prior to nitrate reduction in wet melt. 0 The catalytic effect of water on nitrate ion reduction in the wet melt. l The increased solubility of the alkali metal oxides in the presence of water. Acknowledgement-The authors are grateful to Dr G. Venkatachari, Scientist, CECRI, Karaikudi for discussion.

REFERENCES 1. G. J. Hills and K. E. Johnson, in Advances of Polarography (Edited by I. S. Longmuir), Vol. 3, p. 974. Pergamon Press, New York (1960). 2. H. S. Swofford and H. A. Laitinen, J. electrochem. Sot. 110,814 (1963). 3. H. A. Laitinen, Talanta 12, 1237 (1965). 4. G. G. Bombi and M. Fiorani, Talanta 12, 1053 (1965). 5. H. E. Bartlett and K. E. Johnson, J. electrochem. Sot 114,64 (1967). 6. K. E. Johnson and W. Yerhoff, in Proceedings of the International Symposium of Molten Salts (Edited by G.

Cyclic voltammetric behaviour of platinum Mamantov et al.) Vol. 87-7, p. 613, The ElectrochemiCal Society, New Jersey (1987). 1. P. G. Zambonin and J. Jordon, .I. Am. them. Sot. 91, 2225 (1969). 8. D. H. Kerridge, in Inorganic Chemistry, Main Group Elements: Group V and VI, (Edited by C. C. Addion and D. B. Sowerby), series 1 Vol. 2, p. 30. Butterworths, London (1972). 9. P. G. Zambonin and J. Jordon, J. Am. them. Sot. 89, 6365 (1967). 10. J. Jordon, J. electroanal. Chem. Intdmial Electrochem. 29, 127 (1971). 11. P. G. Zambonin, J. electroanal. Chem. Interjbcial Electrochem. 45,45 l(1973). 12. J. M. Schlegel and D. Prirore, J. phys. Chem. 76, 2841 (1972). 13. S. Shibata and M. P. Sumino, Electrochim. Acta u), 605 (1976).

14. P. G. Zambonin, Electroanal. Chem. Intmfacial Electrathem. 24,365 (1970).

EA 38:17-J

2615

15. M. H. Miles and A. N. Fletcher, J. electrochem. Sot. 127, 1761(1980). 16. J. M. DeJong and G. H. J. Broers, Electrochim. Acta 21, 605 (1976).

17. R. B. Helsop and P. L. Robinson, Inorganic Chemistry p. 245. Elsevier, New York (1960). 18. Y. Kanzaki and M. Takahashi, J. electroanal. Chem. Intmfrrcial Electrochem. 58,349 (1975). 19. G. Mamantov. in Molten Salt Chemis~rw _ D. . 259. Reidel Dordrecht (1987). 20. S. M. White and W. M. Twaardoch, in Proceedings of 3rd International Symposium on Molten Salts (Edited by G. Mamantov et al.), Vol. 81-9, p. 284. The Electrochemical Society, New Jersey (1981). 21. D. G. Lovering, K. M. Oblath and A. K. Turner, Chem. Commun., 673 (1976). 22. G. A. Mazzocchin, G. G. Bombi and G. A. Sacchetto, J. electrochem. Sot. 21,305 (1969). 23. M. H. Miles, G. E. McManis and A. N. Fletcher, J. electrochem. Sot. 134,614 (1987).