The methyl viologen incorporated zeolite modified carbon paste electrode—part 2. Ion exchange and electron transfer mechanism in aqueous medium

Ekctrochimica Acta,Vd. 38, No. 15.pp. 2267-2276.1993 Printedin GreatBritain.

0013~46s6j93 s6.00+ 0.00 0 1993.perpmcmplcrs Ltd.

THE METHYL VIOLOGEN INCORPORATED ZEOLITE MODIFIED CARBON PASTE ELECTRODE-PART 2. ION EXCHANGE AND ELECTRON TRANSFER MECHANISM IN AQUEOUS MEDIUM A. WALCARIUS*L. LAMBERTS*~and E. G. DEROUANE~ * Facultes Universitaires Notre-Dame de la Paix, Laboratoire d’Electrochimie et de Chimie Analytique, $ Laboratoire.de Catalyse, 61 rue de Bruxellcs, B-5000 Namur, Belgium (Receiued 20 January 1993; in revised$orm14 April 1993) Abstrati-An investigation of ion exchange between methyl viologen ions (MV+ +) and sodium ions (Na*) in zeolite Y has been completed in order to describe the electron transfer mechanism operative in a ze-olite modified carbon paste electrode (ZMCPE). Preliiinary work needed the characterization of tbe Linde Y-54 zeolite batch used and the search for equilibrium conditions. The ion exchange isotherm for the MV++ exchange for Na+ demonstrates the great selectivity for the large organic cation and a maximum capacity of 1.0 x lo-“mole MV++ per gram of zeolite, corresponding to an occupation of about 64% of the total exchanging sites. The use of zeolite containing different amounts of methyl viologen has emphasized that the selectivity is greatly influenced by the activity coefficients of the exchanging species within the zeolite framework. This property of the ion exchange equilibrium allowed to determine the electron transfer mechanism that is operative for the cathodic reduction of MV+ + initially exchanged in a ZMCPE. In aqueous solution, MV+ + is reduced after being exchanged for the electrolyte cation. Finally, a structural model of the electrode has been proposed. It is in good agreement with experimental results and takes into account all the electrochemical behaviour of the ZMCPE and the advocated electron transfer mechanism. Kev words: zeolike modified electrode, electron transfer mechanism, methyl viologen, ion exchange, struct&l model.

INTRODUCTION Modifie~I electrodes have been extensively used in the last decade by electrochemists attempting to control the direction and extent of electrode reactions[ 11. Many electrochemical and spectroscopic techniques have been used in order to describe the redox behaviour of the electroactive substance and to estimate the diffusion coefficients of reactants and products within the film[2-41. Zeolite modified electrodes have attracted the attention of many electrochemists because of the ion exchange properties of the zeolites[S] and the rigid lattice structure of this material allows shape and size selectivities[6]. Hopeful uses in the fields of electroanalytical chemistry[7-141 and electrocatalysis[lS 213 have been described. Zeolites were usually supported on platinum or vitreous carbon disks [ 13, 18-20, 22-251, embedded in carbon paste[ 11, 12, 14, 15, 17, 181 or carbon composite[26-281 materials, as well as pressed into pellets[8,25,29,30]. A variety of organic[14,15,18-20,28,31-343 and inorganic [S-12, 16, 17, 23, 24, 353 cations have been shown to be electroactive when incorporated into zeolite modified electrodes, as described in several recent reviews[7, 18,34,36,37-J. In the first paper of this series[14], it was pointed out that a study of ion exchange processes (between electroactive and non electroactive species) in zeolite(s) would afford a unique opportunity to understand electron transfer reactions occurring at t Author to whom correspondence should be addressed.

zeolite modified electrodes. Indeed, it has been emphasized that the electrochemical behaviour of an electroactive cation exchanged into a zeolite is greatly influenced by the nature of the electrolyte cation[l4, 18, 193. This cation compensates for the negative charges resulting in zeolite after the reduction of the electroactive species and/or replaces them by ion exchange in the bulk of the zeolite. The cation mobility also takes part in the electron transfer kinetics. Two mechanisms of the electron transfer operative in zeolite modified electrode have been proposed by Shaw et aI.[18]. Mechanism I involves the cathodic reduction of the zeolite bound electroactive probes followed by the neutralization of the generated charges in the aluminosilicate framework. The redox behaviour of the electroactive ion is similar to that observed in solution, the zeolite being considered as a preconcentration support. Mechanism II is concerned when the electron transfer occurs after the electroactive species being exchanged out of the zeolite and replaced by the electrolyte cations. Evidences were found to support mechanism II especially when ion exchange of the electroactive species is prevented due to the presence of an adsorbate layer located around the zeolite particlesC37-J In this paper, an approach is described for ion exchange and voltammetric measurements respectively based on zeolite Y and zeolite Y modified carbon paste electrode with methyl viologen (MV++) as electroactive probes and sodium ion (Na+) as exchanging species. An understanding of the ion exchange properties for MV+ + to Na+ ions

2261

2268

A. WALCARR~S et al.

in zeolite Y is of great importance because of their intervention in electron transfer processes occurring with methyl viologen incorporated zeolite modified electrodes. Detailed studies of this ion exchange equilibrium should provide the mean to determine the type of electron transfer mechanism. Finally, taking into accounts the studies performed with zeolite modified carbon paste electrode@& 12, 14, 15, 17, 181, we will propose a general model for the electrode in accordance with all electrochemical experiments, allowing hopeful application in the field of electroanalysis. EXPERIMENTAL Materials

Methyl viologen (N,N’-dimethyl-4,4’-bipyridyl dichloride) was purchased from Aldrich and used without further purification. The methyl viologen form of the zeolite Y (MVY) was prepared according the previously reported procedures[14, 15, 18, 193. Non completely exchanged zeolites (NaMVY) were obtained from solutions of poor methyl viologen concentration (stipulated elsewhere). Zeolite Y was in the form of Linde Molecular Sieve Base L-Y54 powder (UOP, Molecular Sieve Division) and large crystals of the sodium form of zeolite X were prepared using the Charnell’s procedure[38]. Both zeolites were used in their wet form. Zeolite modified carbon paste electrodes were prepared from zeolite particles, high purity graphite powder (“Ultra F” 325 mesh, Johnson Matthey) and mineral oil (Janssen), as previously described[14]. The electrode surface was renewed prior to electrochemical scan. Supporting electrolytes (NaCl, NaBr, NH,Br,N(C,H,).,Br) were all analytical grade. All solutions were prepared with high purity water (18 M&m) from a Millipore Milli-Q water purification system. Procedure

Two types of experiment were performed. Ion exchange experiments were carried out from aqueous suspensions of zeolite being stirred continuously for 24 h. Mixtures of methyl viologen and/or sodium chloride were added to obtain a definite ratio of the two exchanging ions. The equilibrium concentrations were determined using atomic absorption spectrometry, for Na+, and spectrophotometry[39-411 or square wave voltammetry (SWV), for MV++. Methyl viologen is reduced by sodium dithionite in alkaline solution to the blue cation radical MV+[42]. Bulk analysis of methyl viologen containing zeolites (MVY or NaMVY) was performed by SWV in solutions of decomposed MV++ exchanged zeolites (procedure modified from Gemborys and Shaw[19]). The zeolite framework was decomposed on OSM aqueous acid solution (1OOmg/1OOml), releasing MV+ + in solution; a small volume was then transferred into 5.OOml 0.05 M NaCl solution and the SWV experiment was carried out with a square wave frequency of 1OOHz and a potential step of 2 mV. Typical voltammetric experiments were carried out in 0.05 M aqueous electrolyte solution, by using a zeolite modified carbon paste electrode (area:

0.17cm’) containing 10% zeolite NaMVY, 60% carbon and 30% mineral oil by mass. The voltammograms were recorded immediately 10s after immersion of the zeolite modified electrode into the electrolyte solution. Instrumentation The instrumentation used for electrochemical experiments was the EG & G Princeton Applied Research model 273 potentiostat/Galvanostat with a three electrode configuration. An undivided cell with 50ml volume was used for all voltammetric measurements, with a Pt gauze counter electrode and a saturated calomel reference electrode (see). The working electrode was the methyl viologen doped zeolite modified carbon paste electrode. The electrochemical cell is lllled with supporting electrolyte wherein the modified electrode is immersed. SWV experiments were carried out with the EG & G PAR 303 static mercury drop electrode (side) associated to the EG & G PAR model 273 potentiostat. The auxiliary electrode was a platinum wire and an Ag/AgCl wire was used as reference electrode (E=E,,-4OmV). Spectrophotometric measurements were performed with a Spectronic 20D spectrophotometer, and atomic absorption experiments with a PyeUnicam model PU 9200X spectrometer (Philips). All experiments were carried out at 20°C.

RESULTS

AND DISCUSSION

Ion exchenge In the case of zeolite Y, the structure of the anionic framework and the positions of some of the counterions are known[6]. The unit cell is a cube 24.7A on edge. The framework consists of sodalite cages linked tetrahedrally through D6Rs in arrangement like carbon atoms in diamond. The unit cell contains eight cavities _ 13 A in diameter and typical contents may be expressed as follow: Na,, [Al&),, @iWIJ61. N 250 H,O. The large cavities called supercages are connected to the sodalite cages (B cages) by rings of 6 tetrahedra with free diameters of about 2.5 A. Entrance to these supercages must be via the channels through 1Zring apertures of about 7.4A. The cations have been located in two types of sites in zeolite Y. 16 univalent ions are present in the ia,,t;eFfes I) while the supercages (sites II) contain The first objective of this work is to characterize the ion exchange between Na+ and MV++ in zeolite Y. Considering Na+ initially present in the anionic framework and MV+ + in solution, the ion exchange process may be represented by the following equation : 2Na& + MV&+

+

2Na& + MV&+

(1)

where the subscripts z and s refer to the zeolite and solution, respectively. The thermodynamic equilibrium constant, K., for this reaction is defined by: K, =

(ahC.,+)*ahbVg,+ + (aN.,,,+YawC,,++

(2)

Ion exchange and electron transfer mechanism aMVg,+ are the respective activities in aqueous solution of the ions Na+ and MV+ +, and where aN_+ and aMVg,++are the activities in the solid state. The relation (2) shows at first glance that the numerical value of the equilibrium constant depends inherently on the definition of the activities and, hence, on the choice of the standard states. Zeolite can be treated as a solid solution of the swollen particulates NaY and MVY and the ion content is then expressed in terms of equivalent fractions of the exchanging cations[5]. However, we will use the molal scale for both ion exchanger and the external solution. The reference state (activity coeffcients equal to unity) is a solution or a (fictitious) pore liquid of infinite dilution, and the standard state (activity equal to unity) is an approximately one molal solution or pore liquid (the deviation from 1 M is given by the activity coefficient, which is defined by the choice of the reference state). The equilibrium constant obtained with these choices of standard and reference states will be called the molaf equilibrium constant and relation (2) may be written as follow :

whereaNa(

K

=

a

9

(YN~(.)+)2(mN~(.)+)~~,,)+ mod

+

+

YMVg)+ + mMVc,)+ + &=~+)2(mNq,~+)2

Yap,.,+ + are the respective activity coefficients in aqueous solution of the ions Na+ and MV+ +, as defined by the Debye-Huckel theory[431, and where fNaC,,+and fW ++ are the activity coefficients in the solid state. +hese values are dependent on the selectivity of the ion exchanger and, thus, not easily determinedC61. mu, + , mm,,++, mNa,,+ and mMV ++ represent the molaities of Na+ and kV+ + in so“iutron . and in zeolite, respectively. Equation (3) makes clear that the equilibrium constant K, is connected to the ratio of the activity coefficients of the exchanging ions within the zeolite, and hence that the extent of ion exchange strongly depends on experimental conditions. Figure 1 shows that for mp,,,++ < mNqs,+the absorption of MV’ + becomes directly proportional to mm,,,++as a good approximation. For mm,,,+ + B MN.,,,+the absorption

0.5

0.0

t&W++

of MV++ becomes constant and independent of MV+ + concentration in solution and corresponds to the maximum exchange capacity of the zeolite Y for MV+ + ions. The maximum loading is observed whenever 1 g of zeolite Y is suspended in a solution containing at least 2 x lo-‘moles MV++ without This other cations. maximum value any when 0.974mmoleg-‘[19]) (1 mmoleg-‘[14], MV++ exchanged zcolite Y is prepared from solutions containing a great excess of MV+ + with regards to the MV+ + quantity incorporated into zeolite after ion exchange reaction (1). Further informations about ion exchange selectivity can be obtained studying the ion exchange isotherm for the MV-Na-Y system. Referring to equation (l), the equivalent fractions of the exchanging cation in the solution and zeolite are defined by: MV, =

MV, =

2m,, ++ 2mMV,,)+ + + mNq,)+

(4)

number equivalents of MV+ + total equivalents of cations in the zcolite’

(5) (3)

where P$,,+ ,

E

2269

The ion exchange isotherm is a plot of MV, as a function of MV, at a given total concentration in the equilibrium solution and at a constant temperature. Figure 2 presents the ion exchange isotherm for the exchange of MV+ + for Na+ in the Linde Y-54 zeolite batch. There is a marked MV++ selectivity and only 64% of the Na+ ions are replaced by MV+ + ions. This 64% replacement value is the maximum value of MV+ + loading in zeolite Y: it is also obtained with any MV+ + concentration higher than 0.02 N. This is consistent with the description of the zeolite given above. In zeolite Y, the entrances of the small cavities are rings of 6 tetrahedra with free diameters ranging from 2.2 to 2.5 A. Methyl viologen has dimensions of 6.3 A x 13.4 A in its planar conformation. This cation cannot enter the hexagonal prisms to jI cages, explaining why MV+ + cannot be exchanged in type A zeolite[ 18, 193. This would limit exchange to 70% for zeolite Y, considering that

1.0

Initial concantration

Fig. 1. Ion exchange of Na+ for MV++ in zeolite Nay. solution of different MVCl,

1.5

2.0

2.5

(mmoler/100ml) 1 g of zeolite suspended

concentrations.

in 1OOml aqueous

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Fig. 2. The ion exchange isotherm for the MV-Na-Y system at 0.02 total nommlity and 25°C. (0) experimental data; (0) normalized data. entering ions are only located in the supercages. For

MV++ ions however the observed limit in zeolite Y is only 64%. This value corresponds to about 90% level of supercage occupancy. This incomplete exchange of sites II can be attributed to the presence of “hided” sites by the ions themselves or the volume required by the supercage cations determines the maximum degree of exchange. Steric factors based on cation size relative to zeolite aperture diameter and to void volume of supercages may thus explain the incomplete exchange observed for both sites I and II with MV+ + in zeolite Y. Because of the ion-sieve effect with MV+ + ions, ion exchange isotherm has to be normalized before describing other parameters[5]. The normalized curve (Fig. 2) is obtained by setting the maximum equal to unity and then limit of exchange, x,,, multiplying each point of the exchange isotherm by a normalization factor, fN, such that fN = l/x_. The rational selectivity coefficient, Kg++‘, are calculated for some of the points collected (Table 1). This quantity is defined by the chemical reaction (1) and following equations : pv++

Na+

_

MV,Naf=K?.++ Nat MV,

CNa

Yrbtv,s,+ +

X=

(6)

where K,iJ++’ is defined as the corrected selectivity coefficient (including a correction for the activity coefficients of the ions in the equilibrium solution), MV,, Na,, MV, and Na, are the equivalent fractions of MV++ and Na+ ions, respectively, in the zeolite and in the solution phases. Table 1 shows clearly the preference of the zeolite for the large organic cation MV++, suggesting stronger interactions with the Table 1. Data relative to the ion exchange isotherm of Fig. 2

MY*

YNwt+

0.969

0.765 0.773 0.781 0.785 0.789

0.890 0.828 0.719 0.547

YbIv*,++

1og#;++ ‘)

0.586 0.598 0.612 0.617 0.622

2.49 1.87 1.97 1.82 1.88

zeolite framework than those with Na+ cations. Such interactions have been evidenced by Raman spectroscopy[44]. They are attributed to the same diffused character of charges for both the aluminosilicate framework and MV++ ion relative to the more localized charge of Na+ ion. The thermodynamic equilibrium constant cannot be calculated directly but the rational equilibrium constant, K,, is evaluated using the following relation[45-473 : 1

lnK,=

-1

In K,g++‘dMV, (7) f0 A K, value of about 30 is obtained from a plot of In K&'++' against MV, which is referred to as the Kielland plot[48]. The establishment of the ion exchange equilibrium has been found to be very rapid[19, 333. When methyl viologen containing zeolite particles are suspended in an aqueous 0.05M NaCl solution, a steady state value of MV, is obtained in less than one second. The time scale of the experiment should be extended in order to evaluate the integral diffusion coefficient @‘++‘[477. It is thus necessary to use zeolite particles of larger size. Since large crystals of zeolite Y do not exist, zeolite X was used. Zeolites X and Y have the same structural properties (faujasite) with a Si/Al ratio greater for zeolite Y (1.5-3) than for zeolite X (l-1.5)[6]. Large crystals of zeolite X have been obtained following a published preparation procedure[38]. Zeolite particles are assumed to be spherical, the evolution of mu,,,, ++ with time is followed and an experimental &‘++’ value of 2 x lo-scm’s-’ is obtained. This value can be considered as a good approximation of the order of magnitude of BE+++ in zeolite Y. Immersion of the methyl viologen containing zeolite modified carbon paste electrode into an electrolyte solution (eg NaCl 0.05 M) will promote the ion exchange of MV+ + for the electrolyte cation, Na+. This back exchange reaction [equation (1) reversal] can be considered by studying the behaviour of zeolites of different amounts of MV++. Figure 3 shows the relationships that have been found to hold between the amounts of MV+ + +

Ion exchange and electron transfer mechanism t

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0.5

( M)

Na+ equilibrium concentration ( M ) Fig. 3. Ion exchange of MV+ + for Na+ in zeolite NaMVY. 40mg of zeolite suspended in 40ml aqueous solution of different NaCl concentrations; (a) evolution of MV+ + equilibrium concentration in solution; (b) evolution of MV+ + equilibrium concentration in zeolite.

leached into the external solution (Fig. 3a) or exchanged in bulk zeolite (Fig. 3b) and the equilibrium concentration of added electrolyte. It appears at first sight that MV++ is distributed homogeneously in the bulk of the zeoiite: the curves of Fig. 3b tend to join together as MV++ content in zeolite decreases for the highest values of rnN,,,,)+. This also indicates that leaching of MV+ + proportionally to its initial amount in zeolite seems to be more important for zeolite of high MV+ + content than for zeolite of low MV++ content. Despite the fact that the ratio of equivalent fractions of ions in solution MV$Na, is greater when using zeolite of high MV’+ content than for low MV++ content (for the same added quantity of Na+), the equilibrium (1) is proportionally less displaced to the left in the case of low MV+ + content. This demonstrates the great effect of the activity coefficients of the ions within the zeolite indicating that the ion exchange equilibrium (1) not only obeys to the mass action law. These peculiar properties of ion exchange processes will provide the mean to precise the electron transfer mechanism that is operative with a zeolite

modified carbon paste electrode, using methyl viologen as the electroactive probe. Electron transfer mechanism Reduction of methyl viologen in zeolite Y embedded in a carbon paste electrode is quite different than in solutionC14, 151. It has been demonstrated that the thickness of paste involved in the electrochemical reaction of MV++ increases as the experiment evolves and that MV+ + leaching out of the electrode into external solution is responsible for only a small part of the sampled currents. However, this observation does not exclude the fact that MV+ + may be leached out of zeolite particles without leaching out of the bulk of the electrode into external solution. This is consistent with the values of the diffusion coeficients of MV+ + respectively measured into the paste (8 x lo-” cm* s-‘)[14] and estimated within zeolite Y (-2 x lo-* cm’ s-‘). MV+ + ions leave more rapidly the bulk of the zeolite particles than they are leached from the modified carbon paste into the solution. Moreover, by using a carbon paste electrode modified with native zeolite alone (Nay), it has

A. WALCARIUS et al.

2212

been found that MV+ + ions can also diffuse from the external solution into the bulk of the electrode and/or the zeolite particles. When the electrode modified with NaY alone is exposed to solutions of methyl viologen, the resulting voltammetric currents were found to increase to a steady-state value depending on the solution MV+ + concentration (Fig. 4). In this case the amount of MV+ + in the bulk of the zeolite is defined by the ratio aMVt.)+ + lath,,,+ - as described above. Two electron transport mechanisms have been proposed for this complex media [equations (8), (9a) and (9b)][18]. MVi$-“‘+ + nNa& MV,:,+ + ne- + nNa(:, =

(8) MV&+ + 2Na(:, MV&+ + ne-

G= =

MV&+ + 2Na& MVi;-“‘+ s

.

(9a) C’W

In the first case [mechanism I, equation (8)], MV++ should be reduced in the bulk of the zeolite, via electron hopping among MV+ + ions as zeolite is a non conductive material. The resulting negative charge of the aluminosilicate framework should be counterbalanced by the electrolyte cation (Na+). Mechanism II [equations (9a) and (9b)] involves the ion exchange between MV+ + and Na+ prior to the cathodic reduction of the electroactive species. It is possible to distinguish between these two mechanisms by using zeolites of different amounts of MV+ +. If mechanism I is operative, the cathodic peak current for MV++ reduction would increase linearly with the MV+ + content of the zeolite under well defined experimental conditions and this linearity must be verified in any case (eg for other scan rates or other Na+ ions concentrations). In the second case (mechanism II), currents would be related to free MV+ + (leached from zeolite), depending on experimental conditions because the ion exchange equilibrium [equation (l)] does not strictly obey to the mass action law. These two mechanisms were checked using four zeolites containing MV+ + at different levels of the maximum capacity of zeolite Y (C%), corresponding

Fig. 4. Cyclic voltammetric peak currents for the first reduction of MV+ ” as a function of the immersion time of the modified electrode. Zeolite NaY modified carbon paste electrode respectively in 1 x 10m4M, 3 x 10e4M and 5 x lo-*M MVCI, aqueous solutions. Supporting electrolyte, 0.05 M Nail. Scan rate, 0.1 V s-l.

to 1.0 x 10-3moleg-1 (C% = lOO%), 7.2 x 1O-4 moleg-’ (C% = 72%), 5.0 x 10-4moleg-1 (C% = 50%) and 3.0 x 10-4moleg-’ (C% = 30%), respectively. The zeolite Y is then written as NaMVY. At first time, NaMVY zeolite suspensions are brought in solutions of increasing NaCl concentration, and measurements were made from suspensions of 40mg of the solid material in 40ml aqueous solution. Figure 5 shows the evolution of the molalities of MV ++ in the external solution with initial MV++ content in zeolite (C%) for various NaCl concentrations in the external solution. Results are normalized to the 100% MV++ exchange level by multiplying each current values by a normalization factor, L,, such that fN,= 100/C%. Figure 5 shows that the extent of ion exchange strongly depends on the experimental conditions via the differences observed for partially MV+ + exchanged zeolite particles suspended in the same electrolyte solution. The evolutions of Fig. 5 become slightly less pronounced when higher zeolite quantities are used for the same solution volume (or smaller solution volumes for an identical solid content). The relative increase of the entity “mMv,,,++x fN,n with C% is higher in Fig. 5 than that observed with suspensions of higher zeolite content. In a second time, these four zeolites NaMVY are incorporated into carbon paste and the resulting zeolite modified electrode is immersed in aqueous solutions of N(C,H,),Br (0.05M) with added NaCl electrolyte to obtain solutions of increasing Na+ ions concentrations. It has been demonstrated earlier for the cathodic reduction of MV+ + that the use of N(C,H,),Br and NaCl in mixture gives rise to the same electrochemical response as those in NaCl or NaBr alone[14]. Figure 6 illustrates the evolution of the observed currents (normalized to the 100% MV++ level of exchange) for the electrochemical reduction of MV++ with initial MV+ + content in zeolite (C%) for several NaCl concentrations in the external solution. An horizontal line should be observed if currents are strictly proportional to the initial MV++ content (Co/). Figure 6 shows that this is not the case. Comparing with Figs 5 and 6 it makes clear that currents are more related to free MV+ + rather than to bulk MV+ +. The neighbouring of the zeolite particles incorporated in a carbon paste is made of carbon particles, paraffin oil and void volume. The existence of this latter is evidenced by the fact that the value of the experimentally determined paste density (D,, = 1.35) is smaller than the calculated value from densities of each component of the paste (Cc,,,c= 1.80), taking into account the paste composition. The void volume can perhaps be attributed to the preparation of the paste itself, but the use of an automated mortar, at atmospheric pressure, is essential to homogenize the paste. When the modified carbon paste is immersed in an electrolyte solution, this solution is allowed to diffuse into the paste (in zeolite particles as well as in the void volume). This peculiar environment experienced by the zeolite particles is confined and resembles that of concentrated aqueous suspensions of zeolite. These observations and the dynamic character of the electrochemical experiment explain the small differences between the evolutions

Ion exchange and electron transfer mechanism

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Fig. 5. Ion exchange of MV+ + for Na+ in xeolite NaMVY. 4Omg of xeolite suspended in 4Oml aqueous solution of different Nail concentrations: 2 x lo-“, 5 x lOTa, 1 x lo-‘, 2 x lo-‘, 4 x lo-‘, 6 x lo-‘, 8 x lo-‘, 1 x lo-‘, 2 x lo-‘, 3 x 10-i and 4 x lo-‘M. Zeolites used contain 0.30, 0.50, 0.72 and l.OOmmole MV+ + per gram, respectively; evolution of the normalized MV++ equilibrium concentration in solution as a function of the initial MV+ + content in the molite.

for currents with zeolite modified carbon paste electrode (Fig. 5) and for mu,,,++ with zeolite suspensions (Fig. 6). These results lead to the conclusion that mechanism II is operative for the cathodic reduction of MV++ exchanged zeolite incorporated in a carbon paste electrode. They do not specify if mechanism I is active or not, they just demonstrate that at least an undoubtedly great part of the electron transfer occurs via mechanism II. This electron transfer mechanism is here demonstrated for a zeolite modified carbon paste electrode. However, evidences were found to support one of the two proposed mechanisms, using various zeolite modified electrodes and electroactive probes. Mechanism II has often been proposed to explain the electron transport occurring at a zeolite modified electrode. From chronoamperometric measurements, obtained

Pereira-Ramos et aL[35] have shown that electrodeposition of the silver ions in mordenite is diffusion controlled and proposed then a mechanism involving the ion exchange of the electroactive species before reduction. Shaw et aI.[18], El Murr et aI.[lfl and our group[ 141 have found that the voltammetric peak current for various electroactive probes (Ru(NH,)i+, Cu+ + and MV+ + is proportional to the square root of scan rate. By using electrolytes of different hydrated cation size, peak currents have been found to decrease with the use of cations of great hydrated radii[14, 18, 191. Moreover, the evolution of cathodic currents for electroreduction of MV+ + exchanged in zeolite Y and the evolution of MV+ + concentration in the equilibrium solution for zeolite MVY suspensions, with supporting electrolyte concentration, are similar (Fig. 7). These results corroborate mechanism II. However, the mobility

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Fig. 6. Data from cyclic voltammetric experiments using xeolite NaMVY moditied carbon paste electrode; Zeolites used contain 0.30, 0.30, 0.72 and 1.00mmole MV+ + per gram, respectively; evolution of the normalixed peak current for the monoelectronic reduction of MV+ + as a function of the initial MV++ content in the xeolite; electrolyte solution, 0.05M N(C,H,),Br with dilferent NaCl concentrations: 5 x lo-“, 1 x lo-*, 2 x lo-‘, 4 x 10e2, 6 x lo-‘, 2 x 10-i and 3 x lo-’ M.

A. WALCANVS et al.

2274

=

1.25

1.0 lo-*

5

8.0 10-3

f

d m= ii r $ 0

l.OLl -

o.

. 0.75

-

0

l

0

5 Z. -

6.0 1W3

-

4.0

-

2.0 10-y

;

. .

.

NaCI

concbntratlon

10-J

0 g 3 =t B 2

(MN)

Fig. 7. Evolution, as a function of the NaCl concentration, of: (0) the current for MV+ ’ reduction using zeolite MVY modified carbon paste electrode in 0.05M N(C,H,),Br solution; (0) MV++ equilibrium concentration in 40ml of a suspension of 40mg zeolite MVY. (and hence, the diffusion) of the electrolyte cation also influences the reaction (8). This emphasizes the

need to use zeolites of different amounts of electroactive species to determine if mechanism I or mechanism II is operative for the reduction of a selected electroactive probe with a zeolite modified electrode. Another approach to describe the electron transfer occurring at a zeolite modified carbon paste electrode was made by Mallouk’s group[31-33, 373. They have considered a more complex system made of spatially defined electron transfer chains. For example, a viologen located in the bulk of a zeolite particle may be reduced via a mediation process by a located around the zeolite porphyrin ring particle[33]. The solvent used plays also an important role. Gemborys and Shaw[19] have studied the electrochemical reduction of MV+ + incorporated in a zeolite Urn supported on a solid electrode immersed in an acetonitrile solution. The resulting voltammograms are quite different than those recorded in aqueous solution and mechanism I has to be considered as the electron transfer mechanism. While mechanism I is of interest for electrocatalytic purposes[36], mechanism II is of interest in designing the electroanalytical determinations around zeolites, especially using zeolite modified carbon paste electrodes for which the surface is easily renewed[ll-141. It is thus not surprising that most applications of the zeolite modified electrodes consist in analytical determination of electroactive cations that can be (pre)concentrated within a zeolite framework. More recently, Baker and SenaratneC 131 have proposed an amperometric detection of alkalimetal cations using a zeolite modified electrode with Cu + + ions as electroactive species. Model of the zeolite carbon paste electrode The zeolite modified carbon paste electrode is a complex system made of solid particles distributed in the three dimensions of the space hold together by a viscous liquid. Previous[14] and the above described results let us to consider the structural model of the electrode as a porous medium in which the electrolyte solution is allowed to diffuse. Pores and channels of the zeolite as well as void volume of the

electrode are progressively occupied by the solution as the experiment evolves. First, only the surface located zeolite particles are involved with the ion exchange reaction (1) and reduced methyl viologen only provides from these particles. Later, MV+ + is reduced in the bulk of the electrode and it has been demonstrated[14] that only a small part of the eleo troactive MV+ + is leached out of the electrode into the external solution. The zeolite particle located at the electrode surface is in contact with both phases paste and solution, while the vicinity of the bulk zeolite is made entirely of modified paste. In this case, MV+ + becomes electroactive when the electrolyte solution diffusing through the paste has reached the bulk of the electrode. The simple model of the electrode consists in a porous medium containing quasi spherical zeolite particles which are considered as reservoirs of electroactive ions (MV+ +). This model is perhaps a very simplified vision of a more complex reality, but it corroborates all electrochemical experiments performed in aqueous electrolyte solution with MV++ incorporated zeolite modified carbon paste electrode[ 14, 15-J. For short immersion times of the electrode into the solution, all processes are referred to as the “surface mode”. The ion exchange reaction (1) occurs at the surface bound zeolite particles and MV+ + is reduced in the diffusion layer located at the electrode surface. So, cyclic voltammetric curves recorded at high scan rates (> 1 Vs-‘) show that the monoelectronic reduction of MV+ + with a zeolite modified carbon paste electrode is a fast reversible reaction[14], as reported for the same species dissolved in solution[42]. When the electrode is immersed into the electrolyte solution, the MV++ ion is exchanged for the electrolyte cation. The amount of MV+ + leaching out of the electrode into the solution comes essentially from the surface bound zeolite particles. Figure 8 shows that this amount does not vary linearly in the initial steps with the square root of time. The low values recorded at the beginning of the experiment (Fig. 8) correspond to the homogenization period of the medium which was disturbed by the immersion of the electrode into the electrolyte solution. The following

Ion exchange and electron transfer mechanism

2215

6.50 1O-9 - 0’

0

10

20

30

40

SO

60

70

80

Jtime (2.‘)

Fig. 8. Ion exchange of MV++ for Na+ in zeolite MW modified carbon paste electrode immersed in lOm1 aqueous 0.05 M NaCl; evolution of MV++ equilibrium concentration in solution with the square root of the immersion tune of the electrode into the electrolyte solution. rapid increase (Fig. 8) is due to the ion exchange of the surface bound zeolite particles. These latter behave as particles suspended in an aqueous electrolyte solution. A similar evolution is observed for chronocoulometric measurements performed at constant potential (E~pp,icd= -0.825 V). At the beginning of the experiment, a blue color characteristic of the cation radical MV’+ is observed at the electrode surface. For greater immersion times, however, the results are different. The ion exchange always occurs at the spherical zeolite particles but the amount of MV+ + leached out of the electrode into the external solution is now proportional to the square root of time (Fig. 8), as describing by the equation (10) characteristic of linear diffusion. This is due to the great value of the diffusion coefficient in the zeolite (-2 x 10-scmz s- ‘) relative to that measured within the electrode (N 8 x lo- l1 cm2 s-‘)[14]. o.5

x

n-o.5+2

{

f(-I)%@-nxl

“=I

JiiG

1

(lo)

where M, denotes the amount of diffusing substance leached from paste into solution at time t and M, the initial quantity which corresponds to the leached quantity after infinite time. I represents past thickness and ierfc(x) is a tabulatedC49) associate error function. The ion exchange equilibrium around the zeolite particles is attained more rapidly than the MV++ or Na+ ions diffuse into the paste. The diffusion and electrochemical processes are now limited by the ion transport within the zeolite modified electrode. This fully explains why the chronoamperometric currents are linear with the square root of time (elapsing during the potential step, not the immersion time) even if tetrabutylammonium ion is used as the electrolyte cation. In this case, MV + + exchanged on the external surface of the zeolite particle is responsible for the observed currents which are controlled by the electrolyte solution into the

paste. Similarly, voltammograms recorded at slow scan rates (< 100 mV s- ‘[4]) reveal the stronger interactions experienced by the methyl viologen ion in the bulk of the paste than those occurring in the diffusion layer at the electrode surface.

CONCLUSIONS This study emphasizes the great influence of the ion exchange equilibrium (1) on the electrochemical processes occurring with a zeolite modified carbon paste electrode. The zeolite Y has been shown to prefer the large organic ion MV++, relative to the small Na+ ion. Most electrochemical studies of zeolite modified electrodes involve the choice of an electroactive probe that exhibits a good selectivity for the used zeolite, as this is useful for preconcentration purposes. It is the case of various organic ions in the zeolite Y[14, 15, 18-20, 22, 28, 31-343 and of the silver ion in zeolite A[12,35]. The cathodic reaction of MV+ + incorporated in a zeolite modified carbon paste electrode immersed in an aqueous electrolyte solution has been studied. MV++ is reduced after being exchanged for the electrolyte cation (mechanism II), while the same reduction in acetonitrile solutions seems to take place in the bulk of the zeolite (mechanism 1)[19]. As the experiment time evolves, bulk zeolite particles are progessively reached by the external solution and the great part of free MV+ + is reduced within the electrode. Such mechanism will promote the possibilities of electroanalytical applications of the zeolite modified electrodes. Walcarius was supported by a specialization grant from the I.R.S.I.A.

Acknowledgement-A.

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