The electrochemical behaviour of synthetic zeolites—1. The NaA zeolite and the NaA. NaNO3 inclusion complex

The electrochemical behaviour of synthetic zeolites—1. The NaA zeolite and the NaA. NaNO3 inclusion complex

THE ELECTROCHEMICAL BEHAVIOUR OF SYNTHETIC ZEOLITES-1. THE NaA ZEOLITE AND THE NaA, NaNO, INCLUSION COMPLEX M. MUSIC and N. PEI-RANOVIC Institute of P...

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THE ELECTROCHEMICAL BEHAVIOUR OF SYNTHETIC ZEOLITES-1. THE NaA ZEOLITE AND THE NaA, NaNO, INCLUSION COMPLEX M. MUSIC and N. PEI-RANOVIC Institute of PhysicaiChemistry,Faculty of Sciences.Belgrade, SFR,Yugoslavia (Receiwd12 October 1977) A&@act-

The electrochemical

behaviourof a 4A-zcolite,lo NaA-zdotiteand its

inclusion

complex

NsA . NaNO,, wasstudiedby cyclicvoltammetry and chronoamperometry in the tcrnperaturerangeup to

450°C.The specimensdemonstrateda typicalelectrolyticbehaviour.It wss e&ablisitcdthat the electrode processeson Pt and Ag electrodesare irrexrsible.Dependingon temprrature and pok’ii%ion time,the overallchargetransportprocessis controlledby diffusion.resctionrate or by both.

NaNOs as described previously[lfil. A chemical analysis has shown that the obtained zeolite has the

INTRODUCTlON

In recent years considerable progress has been made in

the study of solid ionic conductors. New methods for their electrochemical investigation have been introduced. A useful survey of ceramic ionic conductors has been given by Worrell[l] while a more extensivereview of recent achievements in this field has been published in Physics oj EJectroJytes[2]. This has led to some new types of battery[3] and electrochemical system sensors[4], the potentials of which are sensitive to the composition of gas mixtures and to the presence of gases in liquid metals[5]. Of special interest are the “superionic conductors” of the type MAg&, stabilized A&O, and ZrO, as aationic and anionic conductors[l, 6,7]. So far a number of interesting electrochemical results on these systems have been obtained by electrochemical methods such as chronoamperometry, chronopotentiometry and cyclic voltammetry. Hladik[8,9,10] has investigated electrode processesin the KCI-LiCl eutectic by sweepchronoamperometry. Karpachov and Obrosov[ll] have studied the i-t relation in the NaCI-LiCI eutectic, while Hladik[l2] has shown that KNO, is reduced in a KCI-LiCI system at 320°C. However, in the literature there are almost no data oh zeolites as ionic conductors. In our preliminary investigations[13,14] we have pointed out ,the mobility of ions in zeolites and their typical electrolytic behaviour. Bearing in mind the structure of zeolite and the fact that they may be modified by a number of methods, it is to be expected that their electrochemicalproperties will also be modified,hence the present study.

composition Na,,A. IONaNO, per unit cell. The polyctystalline powder was pressed under a pressure of

2.5 ton/cm* into pellets 2-3 mm thick and about 8 mm in dia. Pellets were placed in a three- or twoelectrode system (Fig. l), so that the surface area of the Pt counter electrode was huger than that of the pellets. whereas the surface area of the working Pt or Ag electrodes was smaller than that of the counter electrode. A Pt win served as refexcnccektrode~ The resultant cell was placed in an electric-furnace and investigated by cyclic voltammetric and double pulse chronoamperomctrig methods in air t@noaphere at different temperatures Me~uremanta were made with a PAR-170 electrochemical system. RESULTS

NaA-zeolite Pellets placed between Pt-electrodes already at 314°Cgive well defined cyclic voltammograms similar to those observed with electrolyte soltttions containing dissolved ekctro-active material (Fig. 2). The height and potential of the peaks are dependent on the extent

W

Rf

ctr

EXPERIMENTAL

A polycrystalline powder of a 4A-zeolitewas used in the sodium form NaA(Linde 4A) and as its inclusion Fig. 1. Sample.plaoiag: S-sample; G-institor @ass phcomplex NaA . NaNoj (where A is the ahmosilicate tes); W-work&g electr* Ag or Pt tin; CV-countex anionic residue of the xeolite). This complex was ekctrode, platinum tin; Rj-rrfaaar electred%Pt wire; SC-scrtw; R-meta& frame. prepared by equilibrating the NaA-zeolite in molten 1271

Fii 3. Cyclic vohammogmm and chronoampcrograms of NaAzeoliteonplatinum(thra~ekctrodas)at355”C;(1)from +2.0 to -2.OV, sweep speed 500mV/s; (2, 3 and 4) g, = +ZOV, fi,= -2.OV. Pulse time is 2.5, 5.0 and LOS, raspectively.

Fig. 2 Cyclic voltammograms of NaA zeolite on platinum (three ekctrodca) at 355°C; (a) @aatial polarization between +2.OV and -2.5 V: (b) poiarization potential from: (1) +3.ov,(2) +2.5v,(3) +2.o.v,(4) t LSV,(5) +1.ov to - 2.0 V. Sweep speed = YX mV/s.

and rate of polarization and on temperature. The peak separation and its dependence on sweep rate indicates that the efeetrode processes are irreversible. An analysis of chronoamperograms from the point ofview ofthediffusion rate of electmacbvespeciea and the rate of electron transport between ions and electrode shows that the processes are complex. From the relations i =f(tl’*) and i =f(c-‘I*) it follows that at lower temperatures and short polarization times the current is controlled by rate processes at the electrode, whereas as the temperature and polarization time increase diffusion of eleetroactive species through electrolyte becomes a predominant process (Figs3 and 4). Storing of NaA zeolite at a temperature of about 450°C and repeated heating and cooling do not affect its electrochemical properties. If the Pt working electrode was replaced by Ag e&&ode the behaviour of NaA zeolite completely changes. The system then behaves as an electrolyte with a rather lower decomposition potential. Figure 5 shows cyclic voltammograms obtained with a Ag working electrode. The electrode processes are again irreversible.

00

1.0

2.0

3.0

t”Z IS1

Fig. 4. Analysis of cbronoamperogrsnu of NaA (two Pt electrodes): 37OT, i us t-l” (a) $= -2.OV (0) E = l.OVar~diu~r”~(@)E, = -2.OV;43OT,iust -v,k = -2.5V. (a) E, = LOV and i us t”’ (A) Er = -2SV (righthand side).

amperometry at a number of temperatures show that, depending on pulse duration, the overall process is NaA NaNOs. Pt-electrodes controlled by either diffusion rate or electrode process This form of zeolite has a betterconductivity than rate, or by both processes at the tie time, as in NaA the NaA zeolite, and already at a temperature of 2OCPC zeohte. At shorter polarization times the process is gives well defined cyclic voltammograms with ca- controlled by the reaction at the electrode, whereas for thodic and anodic peaks the height and position of longer polarizations the charge transport is diffusionwhich are depedent on temperature, potential polaricontrolktd (Fig. 6). zation and polarization rate characteristic of an Although nitrates decompose appreciably only at irreversible electrode process. Double pulse chronotemperatures above 450°C long storing of the

The electrochemical behaviour of synthetic zealites

a

1273

I.5

E

Fig. 5. Cyclic voltammograms of NsA zeolite on silver at 355°C (counterelectrodeisPttin andreferenceelatrodcis Pt wire) Polariastion potential &am: (1) fO.0 V, (2) +0.5 V, (3) +0.75v, (4) +1.ov, (5) t1.25v to -3.ov. sweep speed = 500mV/s.

F

t‘% 1.5

0.5

.

Fig. 7. Cyclic voltammograms of NaA. NaNO, heated at T 3lO’C(two Pt electrodes).The rates of polarization for 1 and 2 are 200mV/s and 100mV/s, rwpectively (T = 374°C).

n 0

.o

5 4 + 2 0

I.5

Fig. 6. Analysis of chronoamperograms of NaA. NaNOs (twoPtelectrodes):210°C,iwt-1~2 (A)E, = -2SV(&E, = 3.0 V (right hand side); 3OWCi 0s tsl” (Q) E, = -2.5 V (W) E,= 2.5V. and i us=tl:i$o) E, = -2SV (0) E,

NaA .NaNO, zeolite at a temperature T B 300°C may lead tocertain changes. A sample kept for 15-20 h at 320°C showed in repeated measurement a higher conductivity and qualitatively different cyclic voltammograms;(Fig. 7) in which two peaks appear in cathodic and in anodic region, depending on the observation temperature and the duration of thermal treatment.

DISCUSSION These results suggest that the two zeolites investigated here are solid electrolytes of a special type. NaA has an alumosilicate framework in which the immobilii anion behaves as a “solvent” for mobile Na+ counter ions, which behave as a “solute”. From Fig. 2a it is seen that the cathodic peak potential remains constant when the polarisation is carried out at a constant positive potential, and that a shift of the limiting pofarisation potential leads to a reduction in the anodic peak. On the other hand (Fig. 2b), by maintaining the negative polarisation potential at a constant value and by shifting the original positive potential to lower values the anodic peak remains unchanged, whereas the cathodic peak decreases and disappears. From this it is apparent that the cathodic peak corresponds to the reduction of the product formed by oxidation on the Pt working electrode as it was anodically polarized. It follows that the zeolite is decomposed at the anode most likely to form an oxide film on Pt. The cathodic peak is spread and the current does not fall to zero, which indicates that at the platinum electrode also a reduction process occurs, most probably the reduction of Na* to Na and formation of Na,O, since the process proceeds in air atmosphere. It is also possible that the cathode is passivated chemically by this layer of alkali metal oxide, and thus the current does not increase markedly with increasing potential, The anodic peak, which increases as does the negative polarization potential, is

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M. &&e AM)N.

due to the re-oxidation of the product formed at the cathode. This peak is also spread, indicative of the slowness (and complexity) of the process. The model of NaA.NaNO, as an electrolyte is basically the same as that of NaA. The alumosilicate framework is in this case a “solvent” also for the included NaNO, salt. The processes at electrodes are the same as in NaA-zeolite, and it is likely that nitrates play a predominant role in the anodic process. The conductivity of this zeolite is higher than that of Na-zcolite by about two orders of magnitude. We assume that this enhancement results not only from an increase in the concentration of Na+ ions but also from a disturbance in the binding of Na+ ions, whereby they are easily transported. The electrode processes in the inclusion complex should be the same as those in the NaA zeolite so long as the system exists in the constant Nal,A. 10 NaNO, form. However, following prolonged use or even storage of the sample at T > 300°C a de&ration reaction may occur, NaNO, - Na,O, which may remain as such in the zeolite or react with the alumosilicate framework and change the zeolite structure. This leads to an increase in conductivity[2], and alters the mechanism of the overall process of charge transport. This is shown in the cyclic voltammograms in Fig. 7, where the double peaks are believed to indicate two kinds of depolarisator or two depolarization steps. It may be expected that beside the reduction of the oxide layer on platinum, the cathodic peak represents the reduction of one form of sodium in the zeolite. We assume, however, that the anodic peak is indicative of sodium transfer from the electrode to zeolite into two different energy sites. The replacement of the slightly active Pt working electrode by the considerably more electroactive Agelectrode leads to an alteration of the overall process. Figure 5 indicates that the cathodic peak is due to the reduction of Ag+ ions generated anodically in the previous half cycle. The peak does not occur without a previous anodic polarisation. As the negative polarization potential increases the current shows a tendency toincreaseand represents the reductionofNa+ ions to Na and its conversion into NaiO at the Ag-electrode. As the positive potential increases, so does the anodic

PETRANOVlt

current corresponding first to the oxidation of deposited Na into Na,O at silver and then to the oxidation process Ag + Ag*. This silver oxidation current is large and obscures the anodic peak corresponding to the re-oxidation of the sodium. The slight decrease of current with time in anodic chronoamperometric polarisations is indicative of a continual conversion of NaA zeolite into AgA zeolite at a rate which for given potential is determined by the rate of the process on the electrode. The NaA . NaNO, inclusion complex behaves like NaA zeolite at the Ag working electrode. Acknowledgement- This work is part of a research project of the Academy of Sciencesand Arts of SR Serbia.The authors are thankful to Professor Pavle Savic, the Chairman of the Academy, for his support of this work. REFERENCE?3 1. W. L. Worrell, Am. ceram. Sot. Bull. 53, 5,425(1974). 2. J. Hladik (Ed.), Physics of Electrolytes, Vola 1 and 2. Academic Press, L&don,New York (1972). 3. T. H. Estell and S. N. Flengaa,Gem. Rev. 70,339 (1970). 4. W. L. Worrell, Developing new electrocal seusors, Proc. Intl. Symp. on Metal-Slag-Gas Reaction and Processes, The Eleetrochamical Sot. Meeting, Toronto, Canada,

May 11-16 (1975). 5. R. J. Fruehan, L. J. Martonic and E. T. Turkdogan, Trans. Met. Sot. of AIME 2&X9 1501 (1969). 6. R. A. Rapp and D. A. Shores, PhysicochemicalMeasurementsin Metals Research, p. 123,part 2. Wiley,New York (1970). 7. H. Schmaizriad, MetclllurgiculChemistry (Edited by 0. Kubaachewski)p. 39. London (1972). 8. J. Hladik, Thesis, University of Paris (1964). 9. J. Hladik. Electrocbim. Acta 12. 245 119671. 10. J. Hladik; C. r. Acad. SC.265,i399 (i967): 11. S. V. Karpachov and V. P. Obrosov. Soviet.Electrochem. s,445 (1969). 12. J. Hladik and G. Morand, Bull. Sot. them. France 828 (1965). 13. M. SuliC,N. Petranovif and D. Mini&26 I.S.E. Meet& Baden near Wiena, Abstract p. 92 September 21-26 (1975). M. &lib and B.Jankovib,ibid.Abstract,p. 85. 14. D. VuEeliC, 15. M. %QC, N. Petranovii: and D. MioE, J. inorg. nucl. Chem.33,2647 (1971).