J . Phys. Chem. 1990. 94, 8703-8708
8703
Silver Ion Exchanged Zeolite Modified Electrodes: Observation of Electrochemically Distinct Silver Ions M. D. Baker* and J . Zhang Guelph Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, Unicersity of Guelph, Guelph, Ontario N l G 2 W I , Canada (Receiced: February 23, 1990; I n Final Form: May 8, 1990)
I n this paper the electrochemical response of electrodes modified with silver ion exchanged Y type zeolites is examined. The cyclic voltammograms for the fully exchanged materials resemble closely those observed previously for silver mordenite zeolites. At low exchange levels ( < I O % , i.e.. light11 ground together w i t h an cqual weight of graphite p w d c r for ;ibou\ 10 niin. Thir was then dispersed in a solution of T H F containing I O mg of polystyrene and stirred vigorously. With ;I iiiicropipct. 20-pL aliquots of this solution wcrc then applied to I T 0 blanks (donated by Donnely Meirs Corp.. Michigan. IL). which had already been made into working clcctrodcz ( s e e bclow) and allowed to dry in air. The weight of the electrode coating typically was about 1.5 mg. The base IT0 clcctrodcs were prepared in exactly the same manner as described b j Mallouk et al.? A nickel wire was contacted to the conductive surI',ice of the I T 0 electrodes uith silver print (GC Electronics, Rockford. IL). and then this was covered with 5-min epoxy. The hire w a s then fcd through a glass tube and sealed onto it with cpox!. Lind finally the electrical contact was coated with RTV silicone sealant. The clectrodes were fabricated in this manner bcforc the 7eolitc coating was applied. The coated electrodes were storcd in the dttrk until needed for use. Elcctrochcmical measurements were carried out in aqueous solutiolis of 0.1 M supporting electrolyte which were either perchlorate or nitrate salts of the cation. Prior to cyclic voltammetry. thc electrolyte solution was well purged with oxygen-free nitrogen. Current densities in this paper are reported per square centimeter. l o attempt is made to correct for the real electrochemical area of the electrodes. The water used in electrochemistry and ion exchange was from a Barnstead purification system consisting of a n organic removal column and two ultrapure mixed-bed filters. Results and Discussion
(..!die volt;immograms of fully silver ion exchanged zeolite Y. .AY( 100). in 0.1 M LiC104 are shown in Figure 1 . The data are vcr? similar to those reported previously26 by Pereira-Ramos et ai. for niordcnitc 7colites. The diffusionally shaped cathodic wave h a s :I half-width of close to 200 mV, which is broad for a oneclcclron reduction process.' Broadening of the voltammetric wave can bc cnurcd bq :I number of effects including inhomogeneous cffccis. repulsive interactions between the silver I R losses. and coupled chemical reactions. The contribution to the peak uidth from inhomogeneous effects. i.e.. silver ions occupying __~ (26) Pcrcir;i-Rnmo.;. .I - P : Vessina. R.: Perichon. J . J . Elerrroanal. Chem. 1983. 46. 157 ( 2 7 ) Murray. R . W.: Daum. P. J . Ph.v.7. Chem. 1981. 85. 389, ( 2 8 ) Smith. D. F.: Kuo. W . K.: Murray, R . W . J . Elerrroanal. Chem. 1979, 9.5, 217. ( 2 9 ) Pccrw. P. J : Burd. A . J. J . Electroanal. Cheni. 1980. 114, 8 9 . ( 3 0 ) U u r r . E I. : Kerkeni. M.:Sellami. A,: Ben Tarrit. Y . J . Elertroanal. ( ' h ~ n r 1988. 246. 461 ( 3 1 ) S h a u . B. R.; Crcasj, K . E.: Lznczjcki, C. L.. SargeJni. J . A . J . E / C YrrtrhCwt. SOC 1988. 135. 869. ( 3 2 ) Li. Z ,\I',ing. C M ; Pcrsaud. L.: Mallouk. T. J . Phj,c. Chem. 1988. 92. 2592
Baker and Zhang
P
1 1 -200
\ \'
CATHODIC
I
I
0
I
h/ 200
I
I
400
I
600
POTENTIAL / m V
Figure I . Cyclic voltammetry of fully exchanged silver zeolite Y. recorded in 0.1 M LiC104. Scan speed 5 m V / s . T h e decrease in current lor each scan i b duc to ion cxchangc with thc supporting electrolyte ion.
different sites in the zeolite, is a possible cause since it is known from X-ray diffraction experiment^^^,^^ that silver ions in hydrated zeolite Y locate in at least three distinct extraframework sites. The focus of this paper is indeed to explore the possibility that inhomogeneous effects due to occupancy of distinct sites by silver ions is operative in the broadening of thc waves shown in Figure I. The charge passed in the cathodic wave for a slow scan ranged from 4 X IO-? to 7 X C for a series of AY 100 electrodes, as estimated from the area under the cathodic waves. N o attempt was made to allow for charging current. The accuracy of our area determinations was a few percent, although there was some scatter in the result from one electrode to another. Nonetheless, even a 50% error would not change the conclusions drawn below. The weight of the zeolite used in this electrode was about I mg, and therefore the electrode contained a total of 3 X IOd mol of Ag'.' So about 20'70 of the silver ions in the electrode were electrochemically reduced. This is a far laiger proportion that one would expect on the external surface of the zeolite since in commercial molecular sieve zeolites such as LZY-52 the external surface area of the zeolite crystals is about 1%) of the total equivalent surface area.' This observation indicates that bulk silver ions (Le., those that are originally situated in the internal voids of the zeolite) are reduced during the cathodic scan. Whether these ions are reduced at the surface of the molecular sieve (following diffusion from the bulk) or in intrazeolite sites is unclear from these data. Similar results have been reported previously30 for copper ion exchanged mordenite zeolites. The separation of electrochemical responses originating from either surface or bulk silver ions is an important distinction not only in potential applications such as shape-selective electrocatalysis and size-selective electrochemical sensors but also in fundamentally understanding the operation of zeolitic electrodes. The possible operation of two general mechanisms describing the electrochemical processes occurring at zeolite electrodes has been recognized by several authors (see for example refs 2-6) and has recently been discussed by Shaw et al." Essentially these describe ( I ) electrochemical processes occurring at the surface of the zeolite electrode (ix., at the zeolite-solution interface) and (2) electrochemical processes occurring in the bulk of the zeolite (i.e., within the pore and cage system). These authors noted that all of their (33) Costcnoble. M.: Maes, A . J . Chem. Sor.. Faraday Trans. l 1978, 7 4 , 131. (34) Maes. A , : Cremers. A . J . Chem. Sor.. Farada)' Trans. I 1978. 7 4 , 136.
Modified Electrodes
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8705
/Fr3\
CURRENT
300
200
0
-200
-400
POTENTIAL1 m V
Figure 2. Cathodic linear sweep voltammogram of AY5 at 20 OC. Scan ratc 5 mV/s. Thc observation of two cathodic waves indicates the prcscncc of two clectrochcmically distinct silver ions. The delay time was 5
iiiin
(sx t e x t ) .
data were consistent with surface phenomena except for zeolites exchanged M i t h thc mcthylviologcn cation., Evidence for the electrochemical reduction of intrazeolite species has been presented by Mallouk et al.2.32in observing charge-trapping phenomena similar to those observed for bilayers formed in polymer films, and in viewing mediated charge transfer from surface to bulk species. Also noteworthy in this context are the reports by Percira-Ramosz6 and Rollison and co-workers4 that intrazeolite species are apparently electrochemically silent. I n what follows, the electrochemical reduction of AY5 is described a s n function of scan rate. temperature, and cocation. Evidence is presented for the existence of two distinct silver species that are undergoing slow site-to-site exchange. The location of these silver ions is probably at or close to the zeolite-graphite interface for reasons discussed later. Nevertheless, it will be shown that the concentration and cocation dependence of these ions follow closely that of bulk silver ions and that the silver ions observed in these experiments are related to occupancy of sites 1 and 1' (see Figure 3). In this paper we will mainly consider the cathodic linear sweep voltammetry of these electrodes. The anodic peak shapes were generally similar (nondiffusive) to that shown in Figure I . The sharp drop in anodic current, seen here on the anodic side of the wave, is reminiscent of a stripping peak and probably indicates a surface process. The anodic waves therefore appear to reflect the oxidation of surface silver produced during the reductive scan and its subsequent stripping both into solution and back into the zeolite. as silver ions. Further details on the appearance of the anodic waves are given later. The cathodic portion of the cyclic voltammogram for a silver ion exchanged type Y 7eolite. containing an upper limit of 5.6 Ag+ ions/unit cell (Le., 10%of the full cation exchange capacity) is shown in Figure 2: the electrolyte is 0.1 M KNO,. Following immersion of the electrode, a short setting-in period occurred whereupon the overall peak current decreased slightly and then stabilized. The initial loss of silver from the electrode is probably due to ion exchange of the silver and potassium ions, and the subsequent stabihation is due to either the difficulty of completely ion exchanging all of the silver for potassium or the establishment of an equilibrium between zeolite and solution-phase silver. The rapidity of the stabilization of the electrode was indeed facilitated by the controlled addition of small quantities of silver ion to the 0. I M KNO, electrolyte solution. This addition caused a cessation in the loss of current in the voltammograms. In some cases. for samples containing less than 5 Ag+/unit cell. the peak currents increased showing that the silver ions were concentrated in the films. in concert with the data of Shaw et aL3 It is impqrtant to notc that the peak currents for silver ion reduction at naked ITO-. plntinuin-. or graphite-coated IT0 electrodes were typically 20 timcs smiillcr than thosc observed for zeolite-coated electrodes (in thc stiiic clcctrolytc).
Figure 3. Three-dimensional perspective of part of the unit cell of zeolite Y . Silver ions in the hydrated zeolite preferentially occupy site I at the center of the hexagonal prisms up to a loading of 4 silver ions/unit cell.33 Reduction of the silver ions in the hydrated form is thought to induce a migration to site Charge balance is maintained in electrochemical reduction by thc incorporation of an electrolytc ion into the zeolite as depicted on the figure. Site I I is located on the surface wall of the supercage on a single six ring.
The technique used in this study to produce a stable electrode means that it is difficult to assess the exact concentration of silver in the zeolites. Nonetheless, the concentration of silver ions in the AY5 sample is at an upper limit 10% of the full cation exchange capacity (i.e., 5.6 silver ions/unit cell, which is readily determined from the concentration of the exchange solution), and since in all of the data presented in this paper on AY5 there was an initial decrease in peak current following immersion of the electrode, the actual concentration is somewhat less than this. Once the electrode had stabilized it could be used for a few days without a significant loss in current being observed. The reduction profile clearly shows the presence of two electrochemically distinct silver species, which we will denote as A and B as shown in Figure 2 . In the remainder of this paper for clarity in the discussion, the sites responsible for these waves will also be termed as A and B. In silver zeolite Y it is well established in the hydrated form and at less than 4 silver ions/unit cell that silver ions locate solely at site I, in the hexagonal prisms of the structure)) (see Figure 3) provided that the cocation is sodium.34 Beyond a limit of approximately 4 Ag' ions/unit cell, additional introduction of silver into the zeolite results in a further distribution over sites I' and 11, (see also Figure 3) with no further occupancy of site I . Larger cocations such as ammonium or cesium reduce the specificity of silver for site and this is discussed in the light of the electrochemical response of the electrodes later in the paper. Unless otherwise specified in this study the cocations were sodium (from the parent zeolite; see the Experimental Section) and potassium from the electrolyte solution. Even though the cocations in most of the experiments decribed in this study are a mixture of sodium and potassium, the overall affinity of site I for silver and the saturation of this site a t low exchanges of silver is still a n t i ~ i p a t e d . ) ~The . ~ ~effect of cocation on the appearance of the voltammograms is described later in this paper. The behavior of peaks A and B as a function of scan speed at 0 and 40 O C is shown in Figures 4 and 5 , respectively. At very slow scan speeds (e.g., 2 mV/s), and at 40 "C (and at room temperature'*) only peak A was observed. As the scan rate was increased, as well as the general predicted increase in peak currents,'-' peak B appeared and eventually dominated the voltammogram. A similar trend is also observed as a function of temperature as seen by a comparison of Figures 4 and 5: that is, peak B grows at the expense of peak A as the temperature decreases for the same scan rate. This behavior is reminiscent of electrochemical studies of conformational isomer^^^.^^ and is that quite generally expected for two coexisting, slowly interchanging species (35) Evans, D. H.:O'Connell. K. M. Electroanal. Chem. 1986. 14, 113. ( 3 6 ) Nelsen, S. F.; Echegoyan, E. L.; Clennen, E. L.; Evans, D. H.; Corrigan, D. A . J . Ani. Chmi. Sac. 1977, 99, 1130.
Baker and Zhang
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990
8706
CURRENT
CURRENT
T
1
POTENTIAL/ m V CURRENT
-300 POTENTIAL/ m V
-100
3
t
Ah
Figure 4. Behavior of the cathodic waves obacrvcd for AY5 as a function of scan rate at 0 O C . The scan rate is (a) 2, (b) 5 , (c) IO, and (d) 15
The cathodic waves and the sites responsible for them are labeled
mV/s.
A and B (see text). I
/-
CURRENT
i’o1”m.a i ,,,v Figure 6. Behavior of the cathodic waves as a function of hold time at 0 and 20 O C (see text). Note that the more cathodic peak (8) grows at longer hold times, concomitant with a relative decrease in the other peak: ( i ) hold time (a) 0, (b) 30, (c) 60, (d) 120, and (e) 300 s; (ii) (a) 0, (b) 30, (c) 60, (d) 120, and (e) 300 s. The scan speed in all cases was 10 mV/b.
POTENTlALI mV
Figure 5.
Behavior of the cathodic peaks observed for AY5 as a function
of scan rate at 40 O C . Note that the more cathodic peak (B) increases in relative intensity with decreasing temperature for any given scan speed by comparing with Figure 4 (and see text): (a) 2, (b) 5, (c) IO. and (d) 15 mV/s.
with different reduction potcntials. For example, consider the following cquilibrium of two electroactive silver ions (A and B) that havc distinct rcduction potcntials, i.e. Ag+(A)
Ag+( B)
Lct us supposc that Agf(A) is easier to reduce than type B. As thc cathodic scan proceeds. type A silver ions will be reduced first. If thc r:itc of interconversion is low enough so that not all the silver ions are rcduccd via thc first pcak (A), then some silver is left in site B, giving rise to ;I second, more cathodic peak. I f the scan ratc is slow enough relative to the time scale of the interconversion, thcn all of thc silvcr ions will be reduced at site A and peak B will not bc observed. A similar trend would be predicted for this simple cquilibriuni as a function of temperature: as the temperaturc dccrcascs, pcak B will increase in relative size to peak A sincc thc interconversion rate is lowered. The behavior observed in Figurcs 4 and 5 is therefore indicative of a slow exchange (on thc tinic sciilc of the voltammetry experiments, which take from 30 5 to 5 n i i n ) bctwccn silvcr ions in electrochemically distinct env i r o n m c n t s. At this point it is important to consider the anodic profiles obscrvcd in these experiments. In all cases only one anodic wave was obscrved with a markedly nondiffusive tail as described earlier. The obscrvation of one peak in the reverse scan has indeed been obscrvcd previously in the case of conformational isomers.35 when t b o pcaks wcrc observed in the forward scan. The conclusion was drawn that both conformations of this molecule were oxidized to thc same radical cation. In the case of the zeolite electrodes the conclusion is that both types of silver ions are reduced to the same
species. That is, they produce neutral silver in the same site or environment. The apparent migration of a t least one of the reduced silver atoms from its site following reduction, which must occur for this to be true, is addressed again later in this paper. Of additional interest is the observation that peak A shifts markedly with scan rate, whereas peak B is unaffected. This could have at least two sources as noted by Daum and Murray,27either being caused by film resistance effects or slow charge-transport kinetics, and additionally could be due to Donnan potential eff e c t ~ Type . ~ ~ A silver ions may be at a high-resistance surface site such as ridges in the film structure, whereas type B silver ions are situated in the “valleys” of the film. Alternatively, the charge-transport kinetics of site A may be considerably slower than for site B. The interpretation of the data therefore can be discussed in terms of macroscopic differences in the film (inhomogeneities in the surface morphology) or that the inhomogeneities are at the microscopic level and reflect distinct sites on the zeolite particles. As we now discuss, the second interpretation is apparently operative in this study. Further verification that the silver ions observed in these experiments are indeed located in two majority sites is reflected in Figure 6. These results were collected in the following manner. Subsequent to a cathodic and anodic scan, the potential was held at the anodic limit for different time periods. Following this delay, the cathodic scan was initiated. Note that at the end of the anodic scan the current had fallen to virtually zero (i.e., the background noise level). The data show that the relative heights of peaks A and B vary depending on the delay time in between the conclusion of an anodic scan and the next cathodic scan. (Note that all the voltammograms were recorded a t the same scan rate.) At zero delay time, peak B was not evident whatsoever, but as the delay time was increased, this peak grew at the expense of peak A. Apparently, site A converts to site B. The observation of an isopotential point in this data flags the presence of a mixture of two specie^.^^-^^ The interpretation of isopotential points in linear (37) Untereker, D. F.; Brukenstein, S. Anal. Chem. 1972, 44, 1009. (38) Abruna, H . D.; Walsh. J. L.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1981. 20. 1481.
Thr? Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8707
Zeolite Modified Electrodes CURRENT
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T
T
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b ,-d
300
200
0
100
-100
POTENTIAL/ mV
Figure 7. Relative intensities of the two waves as a function of temperature: (a) 40, (b) 30, (c) 20. and (d) 0 “C. Scan speed 5 mV/s, hold
POI‘I
