J. Phys. Chem. 1994, 98, 13687-13694
13687
Zeolite-Modified Electrodes: Electrochemical Response as a Probe of Intracrystalline Cation-Exchange Dynamics in Zeolites X and Y Chandana Senaratne" and Mark D. Baker* Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Guelph, Guelph, Ontario, NIG 2WI Canada Received: July 20, 1994@
The electrochemical response of zeolite X- and Y-modified electrodes, in which the electroactive ions are exclusively located in the small-channel network (sodalite cages and hexagonal prisms), is used to gain insights into intracrystalline cation-exchange processes. The ability to reduce Ag+ depends on the properties of the electrolyte cation, the nature of the large-channel cations, and the framework charge of the zeolite. In contrast to the case where the electroactive ions are in the large-channel network (supercages), the initial electrochemical activity is small or negligible. A subsequent growth in current is observed as the electrode is repetitively cycled. This is controlled by the rate of ion exchange of Ag+ with electrolyte cations. For zeolite X-modified electrodes a faradaic current is observed only in NaNO3, indicating that ion exchange of small-channel silver ions (Ag+(sc)) is much slower with other cations. Reduction of Ag+(sc) in zeolite Y-modified electrodes is possible in the presence of all the cations used in the study. In the presence of Na+ and K+ electroactivity was observed in the first cycle. In other electrolytes (Li+, Cs+, Rb+, N&+, and alkaline earth cations) a redox current was observed only after several scans. The induction period observed for the appearance of redox activity due to Ag+(sc) depends on the nature of the electrolyte cation and indicates that the electroactivity observed in CsN03 and RbN03 is, most likely, due to a redistribution of cations which allows the Ag+(sc) ions to ingress the large-channel network and thereby diffuse to the electrode-solution interface. For the alkaline earth series the rate of exchange (proportional to the reciprocal of the length of the induction period) was highest for Sr2+ and varied according to Sr > Ca > Mg > Ba. The results are interpreted in terms of zeolite ion-exchange properties and the differences in framework charge in zeolites X and Y. We present evidence to indicate that redox current at zeolite-modified electrodes, in certain cases, is controlled by the ionic radii and hydration energies of extraframework cations, framework charge, and cation redistributions.
Introduction The electrochemical response of a zeolite-modified electrode is sensitive to the concentration and nature of electrolyte cations, 1-4 the zeolite coca ti on^,^ and the solvent present in the external ~olution!~~Cation-exchange properties have a large influence on the electrochemistry of zeolite-modified electrodes due to the intimate link between the reduction or oxidation of intrazeolite cations and the ability of the solution phase cations to ingress the zeolite framework. In zeolites with two distinct site groups: such as zeolite Y (and also X), the electrochemical response can be further affected by the location of the electroactive cation within the zeolite framework! Although a large number of publications have appeared on zeolite-modified electrodes over the past de~ade,~-lO only a few address the effects of ion exchange on the observed electrochemistry. In fact, in some cases, the apparent ignorance of ion exchange effects has led to erroneous interpretation of data. The effect of the size of the electrolyte cation on the response of zeolite-modified electrodes was first demonstrated by Gemborys and Shaw.' Their results showed that the cathodic current observed for zeolite Y-modified electrodes containing various electroactive cations was considerably attenuated in the presence of the tetrabutylammonium (TBA+) cation in comparison to that observed with smaller non-size-excluded cations. The current attenuation is due to the inability of TBA+ ions to ingress the zeolite framework, thus preventing ion exchange with electroactive ions, which must occur to facilitate redox processes of intrazeolite cations. Thus, the electroactivity of the intrazeolite @
Abstract published in Advance ACS Abstracts, November 1, 1994.
moiety is effectively suppressed by blocking the ion-exchange reaction. The deliberate addition of small cations to the electrolyte lifts the suppression by facilitating rapid ion exchange of the electroactive ions and allows the quantification of the smaller ~ a t i o n .It~has also been shown for alkali metal cations that the rate of ion exchange is controlled by their hydrated radii3 This behavior is typical for zeolites with open structures as well as resinous ion exchangers. Indeed, Marinsky and coworkers",'* successfully applied a model based on the osmotic pressure differences across a semipermeable membrane, previously used to describe the selectivity of organic ion exchangers,13-15 to describe ion-exchange properties of zeolite A. This lends credence to the approximation of the environment of intrazeolite cations to a concentrated electrolyte solution. In zeolites with less open structures, such as analcime, cations undergo near complete dehydration prior to exchange.16 In such cases the rate of ion exchange would be governed by the solvation energies and ionic radii of the cations. In this paper we demonstrate that the response of zeolitemodified electrodes can be used to gain information regarding intrazeolite cation-exchange processes. Hitherto published papers on zeolites X- and Y-modified electrodes have focused on the electrochemistry of cations located in the large-channel network.'** In an earlier paper we reported the preliminary data obtained for zeolite Y in which the Ag+ ions were predominantly located in the small-channel network! Here we describe the electrochemistry of Ag+ cations exclusively located in the smallchannel network of both zeolites X and Y and compare and contrast the data obtained for the two cases4 The effects of the large-channel cocation and the electrolyte cation on the
0022-3654/94/2098-13687$04.50/0 0 1994 American Chemical Society
13688 J. Phys. Chem., Vol. 98, No. 51, 1994
Senaratne and Baker
TABLE 1: Percentage (w/w)Composition of Zeolite Samples. Values in Parentheses Are the Corresponding Concentrations in Millimoles per Gram. The Numbers in the Last Three Columns Are Calculated Values Na79.5H6.0X Na57,lY
Ag5.4Na~3.5X
Ags.oN~s.2Y
Agi,I (N&)63.9X Ago.6(N&)48.6Y
Si
A1
Na
16.05 (5.88) 21.79 (7.76) 16.97 (6.04) 2 1.47 (7.65) 18.99 (6.76) 23.40 (8.33)
12.74 (4.72) 8.41 (3.12) 12.54 (4.65) 8.37 (3.10) 13.52 (5.01) 8.86 (3.28)
10.10 (4.39) 7.44 (3.24) 6.85 (2.98) 5.82 (2.53)
0.44 (0.19) 0.94 (0.41)
Ag
3.24 (0.30) 2.99 (0.28) 0.72 (0.067) 0.42 (0.039)
N
0”
Hb
H20C
0.03
5.50 (3.92) 4.12 (2.94)
33.92 (21.20) 34.82 (21.76) 34.21 (21.38) 34.40 (21.50) 37.66 (23.54) 37.15 (23.22)
27.16 (15.08) 27.54 (15.28) 26.05 (14.46) 26.92 (14.94) 21.50 (1 1.90) 23.92 (13.27)
(0.33) 0.14 (1.37) 0.03 (0.29) 0.08 (0.84)
+
2(mmol of Si mmol of Al). The difference between mmol of A1 and the total number of mmol of extraframeworkcations. The difference in weight after all the other components are accounted for.
electrochemical response are interpreted in terms of the different rates of ion exchange and cation redistributions that may take place. We show that the relative rates of ion exchange involving small-channel cations are controlled by ionic radii and hydration energies rather than the size of the cation-water complex. In addition, new data are provided on zeolite-modified electrodes where the electroactivityof the intrazeolite moiety is suppressed. These may have potential applications as chemical sensor^.^,^,^^ Experimental Section
rate of ion exchange of Ag+.20 This effectively removes all of the Ag+ from the large channels while leaving some of the small-channel Ag+ ions. Thus the Ag+ ions in Agl.l(NH4)63.9X and Ag0.6(Nb)48.6Y are, most likely, present in the small channels while the large channel is predominantly occupied by ions. There are large discrepancies among the values reported for the maximum level of exchange in zeolites X and Y.18*21The apparent inability to obtain fully N&+ exchanged faujasites led to the conclusion that ions cannot penetrate the six-ring pores of the sodalite cage. However, it has been shown that the discrepancies resulted from the incomplete exchange of Na+ by N&+ owing to the slow exchange of small-channel cations.18 All the Na+ ions in NaX can be removed by exchanging with while the fraction of Na+ removed in NaY depends on the SUA1 ratio. When the SUA1 ratio of NaY is 1.81,99% of the Na+ ions can be removed whereas at SUA1 = 2.44 only 87% of Na+ can be removed.18 However, in zeolite X and the less siliceous NaY sample significant levels of hydronium ions (17% and 7%, respectively) were present. The modified electrodes were fabricated according to a method described e l ~ e w h e r e . ~ Measurements ,~~*~ were carried out using a two-compartment three-electrode cell, and the potentials are referenced to a Pt wire quasi-reference electrode. Tetrabutylammonium perchlorate (TBAP) was purchased from Aldrich Chemical Co. and used without further purification. Cyclic voltammograms were obtained using a PAR 273A electrochemistry system (EG&G Instruments). In the figures anodic current is plotted positive in concert with IUPAC recommendations.
m+
m+
m+
The zeolites were donated in their sodium form by UOP Corp (Whistler, Alabama), and the elemental analyses (Table 1) were carried out at Galbraith Laboratories (Knoxville, TN). All ion exchanges were done at room temperature for 1 day. Silver ion exchange was carried out in the absence of light as silver ion reduction is known to occur in the presence of light and water.17 Prior to silver ion exchange, the zeolites were ion exchanged in a 1 M NaCl solution and carefully washed with a minimum of deionized water (resistivity 18 MQ cm) until the filtrate was free of chloride ions. The composition of these two samples, labeled Na79.5H6.0X and Na57.1Y in Table 1, is and Na57.1[(A102)55(Si02)137]*269H20. The degree of hydrolysis in zeolite X is known to be higher than that in zeolite Y,18J9and the level of H30+ present in NagoHa is not unusual under the experimental conditions emp10yed.l~The cation/Al ratio in zeolite Y is 1.04, and the samples for which total extraframework cation concentration was available from chemical analysis indicate a twocation excess above the A1 content. Although the source of the high cation exchange capacity (CEC) is unclear, a similar value has been reported for zeolite Y.18 The zeolites were then Results and Discussion stored over a solution of N&C1 until required for use. Silverexchanged zeolites X and Y used as starting material for the Most of the electrochemistry discussed in this paper involves syntheses of AgN&X and AgN&Y samples were prepared by zeolite X- and Y-modified electrodes with electroactive cations ion exchanging Na8oHa and Nas7Y 3 times with equivalent exclusively present in the small-channel network. It is, thereamounts of Ag+ (as Nos-) in 200 mL/g of zeolite. The Ag5.4fore, necessary to briefly describe the zeolite framework Na53.5X and Ags.oN~5.2Ysample (composition Ag5.aa53.5structure as well as the relevant ion-exchange properties. As shown in Figure 1, the framework of zeolites X and Y consists (H30)24.6[(A102)s4(Si02)1081’235H20 and Ag5.0Na45.2(H30)5.2of two types of sublattices or site groups6 which are (1) the [(A102)55(Si02)1371.262H20, respectively) were synthesized by ion exchanging Na8oH6X and Nas7Y with Ag+ equivalent to large-channel network formed by the interconnection of the 10% of the CEC. The samples labeled Agl.l(N&)63.9X and supercages which are linked tetrahedrally to form zig-zag Ag0.6(NH4)48.6Y (composition Agl,lNa3,1(NH4)63.9(H30)13.7- channels and (2) the small-channel network formed by the tetrahedral interconnection of sodalite cages through hexagonal [(A102)8z(Si0~)1101’180H20and Ago,sNas.~(NH4)4~.6[(A10~)54(Si02)1381’219H20, respectively) were prepared by back exprisms. Both networks contain extraframework cation sites. The changing maximally Ag+-exchanged zeolites X and Y twice access to the large-channel network is through the oxygen-12with an excess of NbSCN. ring pores (diameter 7.4 A; highlighted in Figure l), and thus Ammonium thiocyanate was used to back exchange silver fully hydrated cations can ingress the large-channel network. owing to (a) the large difference in cation exchange rates for Indeed it has been shown that intrazeolite cation counterdiffusion the exchange of small-channel and large-channel cations by coefficients can approach those observed in the solution phase.23 NH4+ and (b) the ability of SCN- to significantly enhance the Access tofthe small-channel network, on the other hand, is
m+
J. Phys. Chem., Vol. 98, No. 51, 1994 13689
Zeolite-Modified Electrodes SCHEME 2
M+(z)
+ C+(s)
M+(esi)
+ C+(z)
f MO
Figure 1. The structure of zeolites X and Y showing the large- and small-channel systems.
SCHEME 1 A!J+(sc)
+
intracrystalline
M+(W
* Ag+(lc) + M+(sc)
I
counterdiffusion
CM+@) M+(lc) As+@)
through oxygen-6-rings (diameter for univalent ion exchange between 2.66 and 2.88 8, at 25 0C24925),and thus cations must partially desolvate in order to access these channel^.^^-^^ Entry into hexagonal prisms from the sodalite cages requires further cation desolvation due to volume constraints. Cations within the hexagonal prisms are fully dehydrated and usually are coordinated to the framework oxygens in a pseudo-octahedral ge~metry.~~.~~ The presence of two types of channel networks in zeolites X and Y lends the extraframework cations distinct ion-exchange properties. The polyfunctional nature of zeolites X and Y has been described, and the resultant selectivity reversals have been experimentally o b ~ e r v e d . ~For ~ * example ~~ in the K-Na-Y system, over the first 70% of K+ loading (which corresponds to the replacement of supercage Na+ ions) K+ is preferred to Na+. However, during the exchange of small-channel Na+ ions (above 70% loading) Na+ is preferred over K+.23 Thus, in addition to ion-exchange properties typical for zeolites with open structures, zeolites X and Y also exhibit ion-exchange properties characteristic of zeolites with less open or dense structures. Cations, as well as the water molecules, present in the sodalite cages and hexagonal prisms are bound to the zeolite framework (or sited) owing to the full or partial desolvation of cations and size restrictions within the cages. In contrast, the cations in the supercages are relatively mobile due to the larger free diameter (13 A) and the pore size of the supercage which can sustain fully hydrated cations. Indeed, there is much physical evidence to support the notion that cations and water in open zeolites, such as faujasites, behave as a concentrated aqueous salt solution.23 The long-range transport of cations, exclusively through the small-channel network, would require many cycles of complete desolvation and partial resolvation of cations as well as their intersite transfer or hopping. Such a process would be extremely slow compared to ion exchange via the largechannel network, which is controlled by relatively fast counterdiffusion of mobile, aquated cations. The most likely pathway for the exchange of small-channel network sited cations, therefore, is for them to first become mobile through an intracrystalline exchange process as shown in Scheme 1,33-34 where the descriptors (sc) and IC)denote the small channel and large channel, respectively.
Partial or total exclusion of electrolyte cations from zeolite cages can be due to steric effects, unfavorable charge distributions on the zeolite framework, or thermodynamic and electrostatic effects. As equilibrium ion-exchange conditions do not prevail on the time scale of electrochemical experiments, the effects of exclusion and sieving of cations can be even more pronounced. That is, from an electrochemical viewpoint ionexchange kinetics are more important. Kinetic studies have shown that the cation-exchange rates are also affected by the above-mentioned factors. The rates and activation energies for ion exchange (taking place via counterdiffusion as well as other mechanisms) have been shown to be influenced by cation hydrated and ionic radii, ion charge, and thermodynamic factors.6v23*27-32*35*36 A fast process and a slow process have been observed in ion-exchange experiments carried out with zeolites X and The fast process is associated with the exchange of large-channel cations while the slow one involves small-channel cations. We too have recently shown that the activation energy for the latter is higher.39 Dyer and get tin^^^.^^ have shown that the rate of exchange of largechannel cations is controlled by the size of the cation-solvent complex. Studies of Na exchange with alkaline earth cations in NaX and NaY have demonstrated that the rate of exchange of small-channel cations depends on hydration energies, the bare ion size, the valence of the cation, and/or the framework ~ h a r g e . ~ ~ The , ~ ~slow - ~ exchange ~ - ~ ~ of La3+ ions with smallchannel Na+ has been attributed to the large dehydration energy of the La3+ ion.28 A more general discussion on the influence of cation-exchange properties on the response of zeolitemodified electrodes as well as some specific examples of partial cation sieving is available el~ewhere.~ In this paper we adopt the following model to deal with charge-transport phenomena at a zeolite-modified electrode: when an electroactive extraframework cation undergoes reduction there are several pathways for the overall electron transfer reaction to occur.2 When the electroactive ion can undergo a fast ion-exchange reaction with the electrolyte cation, as in the case for monovalent ion exchange, the dominant pathway is given by Scheme 2, where M is an electroactive cation orginally present in the zeolite (z) and C is a cation present in the electrolyte solution (s). Descriptor esi denotes zeolite-electrodesolution interface. That is, ion exchange occurs before electron transfer. Contributions from intrazeolite electron transfer processes are therefore considered to be negligible. In a majority of cases the electron transfer to intrazeolite cations has been shown to take place at the zeolite-solution-electrode interface following their ion e ~ c h a n g e . ~The - ~ electrochemical oxidation or reduction of species in their sites within the zeolite cages has been achieved when the ion exchange and/or diffusion of intrazeolite moieties are hi~~dered!~?~~ Small inorganic extraframework cations (in our case Ag+ ions) can freely exchange with solution phase cations, when the pores are not blocked. Thus the reduction of Ag+ ions takes place following their ion exchange with electrolyte cations, which can be conveniently tracked using cyclic voltammetry. It is clear from the foregoing discussion that when the electroactive cation is located in the large-channel network, the rate of ion exchange and hence the current is largely controlled by the hydrated radii of the exchanging cations. However, when the electroactive cation is exclusively located in the smallY.23927-32*34937938
Senaratne and Baker
13690 J. Phys. Chem., Vol. 98, No. 51, 1994
of TBA+ from the zeolite interior.'g3 The background current observed before the addition of non-size-excluded cations (indicated by the nonzero intercepts in Figure 2) could be due to the exchange of Ag+ ions residing on the exterior surface of the zeolite, non-size-excluded impurities, or h y d r o l y ~ i s . ~ , ~ J ~ , ~ ~ Addition of trace amounts of a smaller cation increases the observed current. For Ags.oN~5.2Y,at a given concentration of the alkali metal cation, the highest current is observed for Cs+ and decreases according to Cs > K > Na > Li. Although the differences are small, especially between K and Na, it is clear that the exchange of Ag+(lc) with Cs is fastest. A similar trend has been observed both in studies of zeolite-modified electrodes2s3and zeolite-dispersion ele~trolysis.4~ In the case of AgNaX the approximate trend was K > Na > Cs Li. The differences in the slopes are small, indicating, for the purposes of the present discussion, that the rate of ion exchange of Ag+(IC) with alkali metal cations is similar. In view of this contrast between the X and Y zeolites it is worth considering the factors controlling the penetration of the supercage cavities by alkali metal cations. Dyer and get tin^^^.^^ have shown that although the cation-exchange rates are dependent on the size of the cation-solvent complex the interactions are not totally steric in nature. A plot of ion-exchange activation energy vs the number of aluminum atoms per 12 ring resulted in a small energy barrier upon extrapolation to zero framework charge, indicating diffusion comparable to that in an electrolyte solution. Thus the unusual behavior observed in Cs+ addition experiments is most likely due to the higher framework charge on zeolite X. The results indicate that when the electroactive ions are located in the large-channel network the counterdiffusion rates, in general, are determined by the hydrated radii of the electrolyte and intrazeolite cations. In hydrous zeolites X and Y the activation energy associated with cation counterdiffusion has been correlated with the size of the hydration shell and shown to be dependent on the solvent.6-4 This trend is the one expected for most ion exchangers including zeolites with open channels. However, it is observed only when the electroactive ions are present in the large-channel network of zeolites X and Y. In Figures 3 and 4 the cyclic voltammetry of AgX- and AgYmodified electrodes in 0.1 M N d 0 3 are compared. Each figure shows the voltammograms of electrodes with (Figures 3a and 4a) and without (Figures 3b and 4b) electroactive Ag+(lc). In the latter case silver ions are exclusively located in the smallchannel network (Le., only Ag+(sc) is present). There are differences in both the magnitude of the currents observed at the electrodes and their variation with time, when Ag+ ions are exclusively located in the small-channel network. The silver content in Ag5.4Na53.5Xis about 5 times larger than that in Agl.1(N&)63.& however, the peak current of the anodic wave of the first cycle is more than 2 orders magnitude larger. In zeolite Y, while the concentration ratio of silver in the 2 samples (Ag5.oNa45.2Y and Ag0.6(N&)48.6Y) is about 10, the peak currents differ by more than 3 orders of magnitude. The other major difference in these modified electrodes is in the behavior of the peak currents as a function of cycle number. In the case of Ag5.4Na53.5Xand Ag5.oN~5.2Ythe peak current observed in the first cycle is a maximum and then decays as the electrode is repetitively scanned. However, for Agl.l(NH4)63.gXand Ag0.6(NH4)48.6Y the peak current for the first cycle was a minimum, growing with time as the electrode was repetitively cycled. The large currents observed for Ag5.4Na53.5X- and Ag5.0Na45.2Y-modified electrodes reflect the presence of silver ions in the large-channel network. As the ion exchange of Ag+(lc) with solution phase cations is rapid owing to large counterdif-
-
ConcentrationImM 35 30 25
I 20
Pe L
15 10 5 1
Oolo
0.2
0.4
0.6
0.8
ConcentrationImM Figure 2. Results of addition of non-size-excluded cations to 0.1 M TBAP in 3:l methanollwater: (a, top) A ~ s . ~ N Qand ~ , ~(b, Y bottom) AgNaX. Temperature 23 & 1 "C.
channel network of zeolites X and Y the rate of ion exchange is largely controlled by the solvation energies and ionic radii of the cations. The rate of ion exchange, which depends on the cocation present in the large-channel network (vide supra), will be markedly slower in the latter case and thus will influence the reduction current of the cation. The magnitude of the cyclic voltametric currents as well as the differences in their time dependence should, therefore, reflect the dissimilarity in the rates for the processes involving the two species (viz., large- and small-channel electroactive moieties (see Scheme 1)). Figure 2 shows the results of addition experiments3 carried out with zeolite X- and Y-modified electrodes. Zeolites X and Y (Ag5.0Na45.2Y)samples were 15% and 10% silver exchanged, respectively (zeolite X sample was not analyzed). At these loadings there are silver ions present in the supercages of zeolites X and Y (vide infra). In all cases the supporting electrolyte was tetrabutylammonium perchlorate in 3: 1 methanollwater, in which the electrode response is attenuated due to size exclusion
Zeolite-Modified Electrodes
J. Phys. Chem., Vol. 98, No. 51, 1994 13691
0.08
0.06
4 0.04
E
% e L
5 0.02
I
0*0°
*
-1.5
-0.4
PotentialN
1.0 lS5
0.5
I
-1.5
'
-0.4
-0.2
0.0
0.2
I 0.4
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-0.2
0.0
0.2
I 0.4
0.2
4
PotentialN
4t
-3
'
6o
-0.4
-0.2
-A
0.0
PotentialN
Figure 3. Cyclic voltammograms in 0.1 M NaN03: (a, top) Ag5.4Na53.5X and (b, bottom) Agl.l(N&)63.& (Numerals refer to the cycle number). Scan rate 20 mV/s. Temperature 23 f 1 "C.
Figure 4. Cyclic voltammograms in 0.1 M NaN03: (a, top) Ag5.0Na45.2Y and (b, bottom) Ag0.6(NH&8.6Y. (Numerals refer to the cycle number). Scan rate 20 mV/s. Temperature 23 f 1 "C.
fusion rates, the concentration of Ag+(esi) will be high and will gradually decrease due to diffusion of electroactive ions into the bulk of the electrolyte solution. In addition, cation counterdiffusion rates can decrease owing to the change of cation composition in the large ~hannels.4~ The electrochemical results are in line with the distribution of silver ions in hydrated zeolite Y obtained by XRD,46which indicates that Ag+ ions locate in supercage sites when the loading exceeds 4 Ag+ ions per unit cell, although similar data are not available for zeolite X. However, behavior similar to that in NaN03 was observed for these modified electrodes in other electrolytes (viz, CsN03, NO3, Ca(N03)2, Sr(N03)2, etc.) indicating the presence of Ag+(IC). In accord with the model proposed by Sherry and cow o r k e r ~ , Ag+(sc) ~ ~ . ~ ~ in A ~ ~ . I ( N H ~ )and ~ ~Ago.6(NH&8.6Y .BX must first become mobile by exchanging with supercage cations in order to participate in the ion-exchange process. As the exchange of Ag+(sc) with N%+(lc) is slow at room tempera-
ture,18,21 this intracrystalline exchange will only occur when Na+(s) exchange with supercage ammonium ions (N&+(lc)). The process can be pictorially represented as in Scheme 3. In contrast to the case where Ag+(lc) is initially present, the concentration of Ag+(esi) is governed by the rate-determining intracrystalline exchange process which is substantially slower than the exchange of Nb+(lc) with Na+(s).1s,21Upon immersion of the modified electrode the intracrystalline exchange rate is slow or nonexistent as the concentration of Na+(lc) is close to zero. As the concentration of Na+(lc) increases the rate of intracrystalline exchange increases resulting in an increasing Ag+(lc) concentration and thus a larger redox current. Indeed, our data indicate that the induction period (time taken for the observation of redox activity of Ag+) depends on the nature of the solution phase cation in a predictable manner for a series of monovalent and divalent cations (vide infra). Cyclic voltammograms were also recorded for both Agl.1(N&)63.&- and Ag0.6(NH4)48,6Y-modified electrodes in aqueous
m-
13692 J. Phys. Chem., Vol. 98, No. 51, 1994
Senaratne and Baker 5
6 L 50
5 4
C
3
s c a
2
is
1
eL
0
-0.6
-0.2
-0.4
0.0
0.2
-0.4
PotentialN
-0.2
0.0
0.2
0.4
PotentialN 1.5
2 1.0
4
0
4 0.5
i
Pe 2
U
-0.8
-0.6
-0.4
-0.2
0.0
0.2
‘
-1.0 -0.8
-0.6
-0.4
-0.2
0.0
I
.2
PotentialN
PotentialN
Figure 5. cyclic voltammetry of Ag0.6(NH4)48.6Y in (a, top left) L a o s , (b, top right) K N 0 3 , (c, bottom left) CsN03, and (d, bottom right) NI&NO,. All electrolytes were 0.1 M. (Numerals refer to the cycle number). Scan rate 20 mV/s. Temperature 23 f 1 ‘C.
SCHEME 3 Ag+(sc)
+
NH4+(lc)
-
intraciystalline exchange
slow at RT
0.1 M LiNo3, m03,CsN03, RbN03, and “03. The trends observed during repetitive cycling were quite distinct for the two zeolites and will be discussed separately. Agl.l(NH4)63.9Xmodified electrodes did not show a growth of current in the presence of any of the electrolytes listed above. The absence of electrochemical activity in KNo3 and LiN03 reflects slow intracrystalline ion exchange between Ag+(sc) and the electrolyte cations. Thus the exchange of Agf(sc) ions with Na+(lc) is much faster than with either Li+(lc) or K+(lc) in zeolite X. Voltammetry recorded from 50 to 80 “C indicates that the exchange of Ag+(sc) with Kf(lc) is faster than that with Li+-
(IC). As stated earlier this trend is different from that observed for the exchange of supercage cation^.^-^ The control of the intracrystallineexchange rate mainly by the dehydration energies would lead to faster exchange with K+(lc) ions while exchange with Li+(lc) ions would be faster if the rate is controlled solely by the ionic radius of the large-cage cation. The faster exchange rate observed for Na+, therefore, indicates that both ionic radii and dehydration energies contribute to the activation energy of the rate-determining step. As discussed later the trend is similar to that observed for alkaline earth cations. Figure 5 shows the voltammograms obtained for Ago.6(NH4)48.6Y in aqueous 0.1 M CsN03, KNO3, LiN03, and NH4NO3 (see also Table 2). A growth of current was observed in all electrolytes, albeit at different rates. In the presence of Na+ and K+, redox activity of Ag+ was observed in the first cycle and the current increased in the subsequent cycles. However, in LiN03, CsN03, and NI€+N03redox activity due to Agf was observed only after the electrodes were repetitively cycled. Electroactivity was observed for Lif, Cs+, and N&+ following
J. Phys. Chem., Vol. 98, No. 51, 1994 13693
Zeolite-Modified Electrodes
TABLE 2: Induction Periods and Average Current Growth Rates of A ~ O . ~ ( N I & as ) ~ a. ~Function Y of the Electrolyte Cation. Scan Rate 20 mV/s. Temperature 23 & 1 "C. Relevant Cation Properties Are also Listed ionic radius" (A)
hydrated radiusb (A)
hydration energy (kJ mol-')
time (cycles)
current growth rate @A/10 cycles)
0.6 0.95 1.33 1.69 0.65 0.99 1.13 1.35
3.82 3.58 3.31 3.29 4.28 4.12 4.12 4.04
511 41 1 337 284 1906 1593 1447 1318
7 1 1 10 40 20 15 110
1 1 1 0.5 0.01 0.05 0.4
Li+ NaC
K+ cs+ Mg2' Ca2+ Sr2+ BaZ+ a
0.007
Rosseinsky, D. R. Chem. Rev. 1965, 65, 467. Nightingale, E. R. J . Phys. Chem. 1959, 63, 1381.
0'4
0.2
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L
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u
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ua
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C
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ua
0.2
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-0.4
-0.4
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,
I
-0.6
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0.2
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I
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z
3
0.2
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-0.4
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-0.2
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Figure 6. Cyclic voltammetry of A ~ o . ~ ( N H & . i~nY(a, top left) Mg(NO&, (b, top right) Ca(NO&, (c, bottom left) Sr(NO&, and (d, bottom right) Ba(N03)~. All electrolytes were 0.1 M. (Numerals refer to the cycle number). Scan rate 20 mV/s. Temperature 23 f 1 "C.
7, 10, and 20 cycles, respectively. The response in R b N 0 3 (not shown) was similar to that in CsN03. The ammonium ion has
a kinetic diameter of 2.86 A which is between the diameters of K+ (2.66 A) and Csi (3.38 A) and exchanges slowly with smallchannel cations. Cesium ions are size excluded from the smallchannel network. Thus the electroactivity observed in C s N 0 3
must be due to a redistribution of Ag+(sc) triggered by the presence of cs+(lc). If the electroactivity of Ag0,6(N&)48,6Y resulted from residual Ag+(lc) the data would resemble that discussed earlier (see Figure 4a). Extraframework cation rearrangements that accompany ion exchange have been observed.47 It has also been shown that the site selectivity in the
13694 J. Phys. Chem., Vol. 98, No. 51, 1994
Senaratne and Baker
small-channel system depends on the occupancy and type of ions present in the large-channel s y ~ t e m . 4 ~For 9 ~ example, ~ the presence of Cs+ or NI&+ in the large-channel system decreased the affinity of silver for site I (inside the hexagonal prism) ~ignificantly.~~ The data obtained for Ag0,6(NH&g 6Y at room temperature indicate that the rate of exchange of Ag+(sc) with Na+(lc) and K+(lc) is similar yet higher than that with either Li+(lc) or (IC). Low temperature studies indicate that the rate of exchange with Na+(lc) is higher than that with K+(lc). Cyclic voltammograms recorded for Ago 6(NH4)48,6Yin NaNO3 and KNo3 at 1 "C showed distinct differences. In NaN03 electrochemical activity was observed in the first cycle while in KNO3 a current was observed only after about 20 scans. At 10 "C in CsN03 electrochemical activity was not observed even after 50 scans. Redox activity was observed in LiN03 at 10 "C after prolonged cycling (-40 cycles). The trend followed (viz., Na+ > K+ > Li+) is similar to that observed for Agl,l(N&)639X, and the same arguments apply for Ag0,6(Nfi)48,6Y. In general the ionexchange rates are lower in zeolite X, which is corroborated by earlier non-electrochemical s t ~ d i e s . ~ ~ , ~ ~ In zeolites X and Y, the rates of ion exchange of Na+(sc) with alkaline earth cations are smaller than those with alkali metal cation^?^,^^ Thus, it was anticipated that Agl 1(NH4)63.9xmodified electrodes would not yield a response in the presence of alkaline earth cations (viz., Mg2+,Ca2+,S?+, or Ba2+),which was experimentally verified. The behavior of Ago.6(NH4)48.6Ymodified electrodes in electrolytes with alkaline earth cations is different. As shown in Figure 6 (also see Table 2) a response is observed in the presence of all the alkaline earth catiops listed above. However, the induction periods are longer and the currents are smaller. In some cases the observed current is barely above the noise level. Nevertheless, replicate measurements in a given electrolyte indicate that the differences within the series are significant. The rate of ion exchange and the magnitude of the current followed the order Sr > Ca > Mg > Ba. A similar trend was observed in studies of ion exchange of Na+(sc) ions with alkaline earth cations in hydrous NaY and NaX.23*29It was shown that the rate of exchange increases from Mg2+ to Sr2+ and decreases from Sr2+ to Ba2+, which was attributed to the combined effects of bare-ion size and hydration energies.
m+-
Conclusions The response of zeolite X- and Y-modified electrodes, with silver ions exclusively present in the small-channels, is a minimum in the first scan and grows with time. This can be interpreted in terms of a silver-ion flux that is controlled by a slow intracrystalline ion-exchange process. The trends observed within the series of alkali metal cations and alkaline earth metal cations indicate that the faradaic current at these electrodes is governed by the ionic radii and dehydration energies of the electrolyte cations. A response was observed for zeolite Y-modified electrodes in electrolytes with cations that do not penetrate the smallchannel system (Cs+, Rb+, and This is interpreted in terms of small-channel-silver-ionredistributionsthat accompany cation-composition changes in the large-channel system.
m+).
Acknowledgment. The financial assistance from the Natural Sciences and Engineering Research Council (NSERC) and the Institute for Chemical Science and Technology (ICST) is gratefully acknowledged. References and Notes (1) Gemborys, H. A.; Shaw, B. R. J . Electroanal. Chem. 1986, 208, 95.
(2) Shaw, B. R.; Creasy, K.; Lanczycki, C. L.; Sargeant, J.; Tirhado, M. J . Electrochem. Soc. 1988, 135, 869. (3) Baker, M. D.; Senaratne, C. Anal. Chem. 1992, 64, 697. (4) Baker, M. D.; Senaratne, C.; Zhang, J. J. Chem. Soc., Faraday Trans. 1992, 88, 3187. ( 5 ) Senaratne, C.; Baker, M. D. J. Electroanal. Chem. 1992,332,357. (6) Barrer, R. M. Pure Appl. Chem. 1980, 52, 2143. (7) Rolison, D. R. Chem. Rev. 1990, 90, 867. (8) Rolison, D. R.; Nowak, R. J.; Welsh, T. A,; Murray, C. G. Talanta 1991, 38, 27. (9) Bard, A. J.; Mallouk, T. E. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992. (10) Baker, M. D.; Senaratne, C. In Frontiers of Electrochemistry Vol. 3, Electrochemistry of Novel Materials; Ross, P. N., Lipkowski, J., Eds.; VCH: New York, 1994. (11) Platek, W. A.; Marinsky, J. A. J . Phys. Chem. 1961, 65, 2118. (12) Bukata, S.; Marinsky, J. A. J . Phys. Chem. 1964, 68, 994. (13) Gregor, H. P. J. Am. Chem. Soc. 1948, 70, 1923. (14) Gregor, H. P. J. Am. Chem. Soc. 1951, 73, 642. (15) Glueckauf, E. Proc. R. SOC. London 1952, A214, 207. (16) Barrer, R. M. J . Chem. Soc. 1950, 2342. (17) Leutwyler, S.; Schumacher, E. Chimia 1977, 31, 475. (18) Franklin, K. R.; Townsend, R. P.; Whelm, S. J.; Adams, C. J. Proceedings of the Seventh Intemational Conference on Zeolites, Tokyo, 1986; Murakami, Y.,Iijima, A., Ward, J. M., Eds.; Elsevier: Amsterdam, 1986. (19) Harjula, R.; Lehto, J.; Pothuis, J. H.; Dyer, A.; Townsend, R. P. J . Chem. SOC., Faraday Trans 1993, 89, 97 1. (20) Kerr, G. T. Zeolites 1983, 3, 295. (21) Fletcher, P.; Townsend, R. P. J. Chem. Soc., Faraday Trans. I 1982, 78, 1741. (22) Baker, M. D.; Zhang, J. J. Phys. Chem. 1990, 94, 8703. (23) Sherry, H. S. In Ion Exchange; Marinsky, J. A,, Ed.; Marcel1 Dekker: New York, 1969; Vol. 2. (24) Sherry, H. S. J . Phys. Chem. 1966, 70, 1158. (25) Theng, B. K. G.; Vansant, E.; Uytterhoeven, J. B. Trans. Faraday SOC. 1968.64, 3370. (26) Baur, W. H. Am. Minerol. 1964, 49, 697. (27) Barrer, R. M.; Rees, L. V. C.; Shamsuzzoha, M. J. Inorg. Nucl. Chem. 1969, 31, 2599. (28) Sherry, H. S. J . Colloid Interface Sci. 1968, 28, 288. (29) Sherry, H. S. J . Phys. Chem. 1968, 72, 4086. (30) Barrer, R. M.; Davies, J. A.; Rees, L. V. C. J. Inorg. Nucl. Chem. 1969, 31, 2599. (31) Dyer, A.; Gettins, R. B.; Brown, J. G. J. Inorg. Nucl. Chem. 1970, 32, 2389. (32) Constenoble, M. L.; Mortier, W. J.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1877. (33) Brown, L. M.; Sherry, H. S.; Krambeck, F. J. J. Phys. Chem. 1971, 75, 3846. (34) Brown, L. M.; Sherry, H. S. J . Phys. Chem. 1971, 75, 3855. (35) Dyer, A.; Gettins, R. B. Ion Exch. Process Ind. Pap. Conf. 1970, 357. (36) Dyer, A.; Gettins, R. B. J. Inorg. Nucl. Chem. 1970, 32, 2401. (37) Hoinkis, E.; Levi, H. W. Ion Exch. Process Ind. Pap. Conf. 1970, 339. (38) Sherry, H. S. In Advances in Chemistry Series 101; Flanigen, E. M., Sand, L. B., Eds.; American Chemical Society: Washington,DC, 1971. (39) Baker, M. D.; Senaratne, C.; Zhang, J. J. Phys. Chem. 1994, 98, 1668. (40) Li, Z.; Mallouk, T. E. J . Phys. Chem. 1987, 91, 643. (41) Li, Z.; Wang, C. M.; Persaud, L.; Mallouk, T. E. J . Phys. Chem. 1988, 92, 2592. (42) Harjula, R.; Dyer, A.; Townsend, R. P. J . Chem. Soc., Faraday Trans. 1993, 89, 977. (43) Rolison, D. R.; Stemple, J. Z.; Curran, D. J. Proceedingsfrom the Ninth Intemational Zeolite Conference, Montreal, 1992; von Ballmoos, R., Higgins, J. B., Treacy, M. M. J., Eds.; Buttenvorth-Heinemann: Boston, 1992. (44)Dyer, A.; Fawcett, J. M. J . Inorg. Nucl. Chem. 1966, 28, 615. (45) Brooke, N. M.; Rees. L. V. C. Trans. Faraday Soc. 1%8,64,3383. (46) Constenoble, M. L.; Maes, A. J. Chem. Soc., Faraday Trans. I 1978, 74, 131. (47) Cremers, A. Proceedings of the Fourth Intemational Conference on Molecular Sieves; Chicago, 1977; Katzer, J. R., Ed.; American Chemical Society: Washington, DC, 1977. (48) Maes, A.; Cremers, A. J . Chem. Soc., Faraday Trans. 1 1975, 71, 265. (49) Maes, A,; Cremers, A. J . Chem. Soc., Faraday Trans. I 1978, 74, 136.
