3204
J. Phys. Chem. B 2001, 105, 3204-3210
Ion Exchange and Electron Transport at Methyl Viologen Y Modified Electrodes Tit-Wah Hui and Mark D. Baker* Department of Chemistry and Biochemistry, Guelph-Waterloo Centre for Graduate Work in Chemistry, Electrochemical Technology Centre, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1 ReceiVed: August 16, 2000; In Final Form: December 12, 2000
Cyclic voltammetry and chronocoulometry were used to study reduction of the methyl viologen dication to the cation radical at zeolite-Y-modified electrodes. Shifts in half wave potential as a function of both scan rate and MV2+ loading were shown to be due to a concentration overpotential produced by the ion exchange equilibrium at the zeolite-solution interface. Incorporation of the ion exchange-equilibrium constant for ion exchange between sodium cations and intrazeolite methyl viologen into the Nernst equation shows that equilibrium at the zeolite-solution interface is set up rapidly on the electrochemical time scale. Electron transport to the methyl viologen dication (MV2+) is also shown to occur via an extrazeolite mechanism. The electrochemical reaction rate is controlled by diffusion of methyl viologen dications from zeolite Y rather than by incorporation of charge balancing cations from the electrolyte solution. Chronoamperometric data are discussed in terms of several plausible electron transport mechanisms including percolation, coupled electron hopping, and ion exchange. The latter agrees well with the experimental data and is in keeping with rate control for an ion-exchange step from a spherical zeolite particle.
Introduction Several recent articles have discussed the locus of electron transfer to electroactive moieties originally placed within molecular sieve zeolites at electrode surfaces1-3 showing the dominance of interfacial electron transport. Interesting electrochemistry has surfaced showing significant shifts in redox potentials from the solution phase,4 nondiffusion controlled cation dynamics,5 and multiple redox waves.1,6 Complex electrochemistry associated with “well-behaved” redox couples introduced into zeolite molecular sieves is surprising in view of the interfacial nature of the redox requiring further thought and interpretation. In this paper and its companion,7 we re-visit the electrochemistry associated with the methyl viologen dication in the presence of zeolite Y.8-10 The first such study8 showed a continuous loss of methyl viologen dications (MV2+) from the zeolite into the acetonitrile solution phase but giving electrochemistry distinctly different to that observed in solution. Walcarius and co-workers9 however viewed solution-like cyclic voltammetry. Moreover, the solution-phase equilibrium concentration of MV2+ followed the voltammetric currents indicating that the ion exchange reaction preceded the electrochemical reduction (i.e., an extrazeolite electron-transfer mechanism). Recently, Calzaferri and co-workers studied MV2+ exchanged zeolite Y modified glassy carbon “monograin” electrodes in aqueous solution.10 Their results were similar to those obtained in acetonitrile. The voltammetric peak current for second reduction of MV2+ was significantly higher than that of the first reduction. Conproportionation of methyl viologen resulting in intrazeolite electron transfer was proposed. In this paper, we use chronocoulometry to determine the variation of the apparent diffusion coefficient of MV2+ with loading. This is then discussed in terms of electron transfer via either percolation, hopping coupled to physical diffusion, or ion * To whom correspondence should be addressed.
exchange (extrazeolite) mechanisms. In addition, we carefully consider the source of potential shifts for MV2+/•+ redox waves in terms of rapid interfacial ion exchange. Experimental Section Materials. NaY was from Union Carbide (LZY-52). Methyl viologen (N,N′-dimethyl-4,4′-bipyridyl dichloride) was purchased from Aldrich and used without further purification. All electrolytes (Aldrich) were reagent grade and were used as received. Water used in all experiments was from a Barnstead water purification system. The final resistivity was 18 MΩcm. Polystyrene (MW ) 32 600, Aldrich) was used as received. Prior to ion-exchange, zeolites were stirred in 1 M NaCl for 24 h to remove impurity cations and/or protons and were then filtered and washed. Different loadings of methyl viologen exchanged zeolite Y samples were prepared by ion exchanging a stoichiometric amount of methyl viologen (MV2+) into NaY. A total of 1 g of NaY was stirred in 250 mL of water containing methyl viologen dichloride for 24 h. The zeolite was then airdried and stored for future use. Loadings of methyl viologen were determined by dissolution of the zeolite in 2 M HCl followed by UV spectrophotometry. A value of 2 × 104 M-1 cm-1 at λmax 258 nm was used10 for the absorption coefficient. Loadings of MV2+ are reported as the percentage of Na+ ions exchanged. Crystallinity of the zeolites was assessed with XRD and suffered no measurable loss in lattice integrity. Monograin layer zeolite-modified electrodes were prepared using a modification of literature methods.10 Telfon sheathed glassy carbon rods (3 mm diameter) were polished with 0.05 mm alumina, ultrasonically washed, and then air-dried. A total of 5 mg of the zeolite was then dispersed in 3 mL of water and stirred vigorously for 30 min. A total of 3 µL of the dispersion was then applied onto the polished glassy carbon surface and air-dried. Onto the dried zeolite layer, 1 µL of 0.75 mg of polystyrene in 20 mL of tetrahydrofuran solution was added
10.1021/jp002973r CCC: $20.00 © 2001 American Chemical Society Published on Web 03/31/2001
Ion Exchange at Methyl ViologenY Modified Electrodes
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3205 longer times. This is likely due to depletion of viologen from the zeolite meaning that the concentration across the zeolite particles is not constant. These plots can nonetheless be used at short times to determine the variation of D with C. Since the effective electrode surface area and the concentration of electroactive species at the zeolite-modified electrodes are unknown, or difficult to determine, the absolute value of D cannot be calculated. However, at short times, the apparent diffusion coefficient variation with loading can be determined (see, for example, Figure 1). The Cottrell slope (CS) from eq 1 (i.e., for the Q vs t1/2) is
CS ) 2nFAC
Figure 1. Chronocoulometry in 0.05 M LiCl. Legend shows initial loading of MV2+. Potential step from -0.4 to -0.9 V for 10 s.
and allowed to evaporate. The final coating contained 5 µg of zeolite and 0.04 µg of polystyrene. Electrochemical data were recorded using an EG&G 273A potentiostat. A two-compartment, three electrode cell was used in all experiments. The counter electrode was a 1 cm2 platinum flag, and the reference electrode was a saturated calomel electrode (SCE). The working and counter electrode were placed in the working compartment with the SCE in a separate compartment. The electrolyte solution was purged with oxygen-free nitrogen for 20 min. All experiments were run without IR compensation. Cyclic voltammograms were recorded immediately following immersion of the electrodes. Chronocoulometry was recorded in a similar fashion. Step potentials are given in the body of the paper. All potentials are with respect to SCE. Results and Discussion Possible charge-transfer routes for the MV2+/MV•+ redox couple in zeolite Y entail either intrazeolitic or extrazeolitic mechanisms. In discussing these,1 related processes such as percolation, coupled electron hopping and the physical diffusion, and ion-exchange rate of MV2+ and MV•+ should be considered. Each will lead to distinct electrochemical behavior. In this paper, we distinguish between these general mechanisms (described in detail below) via the manner in which the electrochemically determined diffusion coefficient varies with the loading of methyl viologen in the zeolite. The variation of the apparent diffusion coefficient with loading is therefore diagnostic of the electron transfer mechanism, as discussed later, and can be conveniently determined chronocoulometrically via the Cottrell equation
Q ) 2nFAC
x
Dt π
(1)
where n is the number of electrons involved in the electrochemical reaction, F is Faraday’s constant, A is the electrode surface area, Q is the charge passed at constant potential, and C is concentration. In Figure 1 we show Cottrell plots for four loadings of methyl viologen. For loadings of 37 and 25%, the Cottrell equation holds for the entire experimental time window. For loadings of 58 and 42%, the plots deviate from linearity at
xDπ
(2)
It is reasonable to assume that the concentration C is proportional to the initial loading of species in the zeolite. So the variation of the apparent diffusion coefficient with the concentration (loading) of the redox species can be assessed. At this point it is useful to collect together the diagnostic criteria for the various electron-transfer mechanisms together with the pertinent equations relating concentration to the diffusion coefficient. Intrazeolite Charge Transport Mechanisms. Percolation. Charge transfer via electron hopping between immobile redox centers is equivalent to percolation and can be described as a diffusion process,11 and thus diffusion control models developed from electrochemical theory apply. The apparent diffusion coefficient (Dapp) is given by
Dapp ) (keδ2C)/6
(3)
where ke is the second-order electron exchange rate constant. C and δ are the concentration and the distance between the centers of the chemically equivalent species involved in the electron exchange. The apparent diffusion coefficient is linearly proportional to the concentration of redox centers. However, long range conduction of electrons can only occur if the concentration of the redox center is above the percolation threshold. Then a random distribution of redox centers forms a collection of clusters between which electrons can hop. At lower concentrations, a decrease in connectivity between centers occurs, and the diffusion coefficient decreases. Below the percolation threshold, the system becomes nonconducting and a zero diffusion coefficient results. The apparent diffusion coefficient for electron percolation is proportional to the concentration of the electroactive species. Substitution of eq 2 into eq 3 gives
x
CS ) 2nAFC3/2
(keδ2) 6
(4)
Although not explicit from eq 4, CS plotted against loading will generate an x-intercept corresponding to the percolation threshold. That is current will only flow above a critical concentration of redox centers (vide supra). Free Diffusion and Electron Hopping. When the rates of diffusion toward the electrode-solution interface and electron exchange are comparable, electron transport can occur by electron hopping coupled to isothermal diffusion as shown by Dahms12 and by Ruff and co-workers.13,14 Electron transport is diffusion controlled with an apparent diffusion coefficient, Dapp equal to the sum of the diffusion coefficient for physical
3206 J. Phys. Chem. B, Vol. 105, No. 16, 2001
Hui and Baker
movement, Dphys and that for electron hopping, De, i.e.,
Dapp ) Dphys + De
(5)
where De ) (keδ2C)/6 as in eq 3. Although in the solution phase, Dphys . De and electron hopping is negligible, this may not be the case within the zeolite bulk. A common feature of the two models described above is that the apparent diffusion coefficient is proportional to the concentration of the redox centers. However, in this case a percolation threshold does not exist. The apparent diffusion coefficient for freely diffusing redox centers with electron hopping occurring should follow eq 5. The Cottrell slope will be given by
CS ) 2nFACx((Dphys/π) + (keδ2C)/6π)
(6)
If physical diffusion dominates, the apparent diffusion coefficient is insensitive to the loading and a linear relationship between the slope of the Cottrell plots and the initial loading should result. However, if electron hopping is a dominant process the slope should be proportional to the loading3/2. In both cases, a zero intercept is predicted, unlike the percolation model. Extrazeolitic Mechanisms. Ion Exchange. Extrazeolitic charge transfer can involve ion-exchange between cations in the electrolyte solution and MV2+ which subsequently undergo redox. This corresponds to redox of outer topological isomers described by Bessel and Rolison.2 In this case, the electrochemical response can be limited by the ion-exchange rate of the intrazeolite cation as we have shown previously.15 The fractional attainment of equilibrium, U(t), for an ion-exchange reaction, at a spherical particle of radius rc at a particular time t, at short times is to a very good approximation given by16
U(t) )
mt q 0 - qt 6 ) ≈ q0 - q∞ m∞ xπ
x
Dt rc2
(7)
x
Dt rc2
(8)
where m∞ corresponds to the number of ions exchanged into solution at infinite time which is the difference between the initial and final loadings.
m ∞ ) L0 - L∞
(9)
where L0 is the initial MV2+ loading in the zeolite and L∞ is the loading at equilibrium. Combining eqs 8 and 9 gives
6 Q ) nF xπ
x
Dct rc
2
(L0 - L∞)
TABLE 1: Electrochemical Data Obtained by Exhaustive Electrolysis of Methyl Viologen Zeolite Y-Modified Electrodes in 0.05 M LiCl at -0.9 V for 10 min initial loading (%)
initial mol/g of zeolite (× 10-4)
charge consumed (mC)
mol/g of zeolite consumed (× 10-4)
equilibrium loading (%)
25 37 42 58
4.06 6.01 6.83 9.43
0.07883 0.2023 0.2116 0.3500
1.61 4.12 4.29 7.03
15.1 11.6 15.6 14.8
root of time and the slope of the Q versus t1/2 plot is given by
where mt and m∞ are the quantities of ions exchanged at time t and infinity, respectively, and q0, qt, and q∞ are the concentrations at these times. The apparent diffusion coefficient, D, is independent of the initial concentration of the electroactive species, in contrast to the electron hopping cases described above. Combining the Faraday law (Q ) nFmt) with eq 7, gives:
Q 6 ) nFm∞ xπ
Figure 2. Cottrell slope (CS, see text) as a function of initial loading of methyl viologen. Potential step from -0.4 to -0.9 V for 5 s. Solid points are experimental data. Fits to models for electron transport are indicated on the figure.
(10)
Equation 10 shows that the charge is proportional to the square
6 CS ) nF xπ
x
Dc
6 L0 - nF rc xπ 2
x
Dc
rc2
L∞
(11)
A plot of CS vs initial loading (L0) will be linear with a negative y-intercept indicative of L∞ and as with the percolation model a positive x-intercept. Figure 2 shows short-time slopes of the Cottrell plots against the initial loadings of methyl viologen, together with fits to the three cases above. The dotted line is calculated for electron percolation by fitting the experimental data with eq 4 and incorporating a fitting parameter equivalent to the percolation threshold. As can be seen from the plot, it was impossible to match both the x-intercept and the slope of the experimental plot with this model. Coupled physical diffusion and electron hopping did not fit the experimental data well. In particular, this model generated curvature at low loadings and does not predict an x-intercept. The linear fit assuming that the diffusion coefficient does not vary with loading (vide infra) is in line with eq 11 for ion exchange which holds when physical diffusion dominates. This fit is possibly the best, in view of the curvature of the other two functions. Also the x-intercept is close to that observed with the experimental data. In addition, the x-intercept from the percolation model does not fit other experimental data as we now show. The loading at equilibrium (L∞) can be obtained from the slope and the intercept of the
Ion Exchange at Methyl ViologenY Modified Electrodes
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3207
Figure 3. Cyclic voltammograms for 5 µg of MV2+ zeolite Y-modified glassy carbon electrodes with fractional loadings of (a) 58%, (b) 42%, (c) 37%, and (d) 25% in 0.05 M LiCl. Scan rates from bottom to top of each graph are 100, 50, 20, and 5 mV/s, respectively. Only the first scan is shown.
plot in Figure 2 and corresponds to about 15% of the cation exchange capacity. This value is consistent with observation of a negligible response in chronocoulometry at loadings of 12 and 8.3%. This gives further credence to ion exchange over percolation. To further reinforce the applicability of eq 11, the equilibrium loading from Figure 2 is now compared to that obtained by exhaustive electrolysis performed at -0.9 V until the current dropped to the background level (obtained from biasing a NaY electrode at the same potential). The equilibrium loading was then measured coulometrically by determining the quantity of viologen released into solution. The results are collated in Table 1 and agree well with the value obtained from Figure 2. Thus the value of L∞ predicted from the ion-exchange model applied to chronocoulometry agree extremely well with the experimental value determined by measuring the actual amount viologen released into solution. The chrononcoulometric response thus follows the ion-exchange kinetics. That the electrochemistry is controlled by ion exchange and that the electron transport is extrazeolitic imply that the redox occurs in the solution phase. We now show that potential shifts with loading are explicable in terms of rapid ion exchange. Figure 3 shows the cyclic voltammetry of zeolite Y-modified electrodes at four MV2+ loadings in an 0.05 M LiCl electrolyte.
TABLE 2: Electrochemical Data from Figure 3 loading (%)
scan rate (mV/s)
E1/2a (V)
∆E1/2b (V)
ipa/ipc
25
100 50 20 5 100 50 20 5 100 50 20 5 100 50 20 5
-0.752 -0.751 -0.757 -0.766 -0.722 -0.724 -0.728 -0.749 -0.717 -0.718 -0.725 -0.732 -0.714 -0.704 -0.711 -0.732
-64 -63 -69 -78 -34 -36 -40 -61 -29 -30 -37 -44 -26 -16 -23 -44
0.63 0.50 0.59 0.58 0.51 0.46 0.46 0.45 0.54 0.41 0.45 0.42 0.7 0.48 0.47 0.38
37
42
58
aE b 1/2 are calculated by (Epa + Epc)/2. ∆E1/2 are calculated by subtracting the E1/2(MV2+/MV•+) value (i.e., -0.688 V) in solution.
Data are collated in Table 2. The E1/2 of the of MV2+/•+ redox couple shifts negatively from 16 to 78 mV depending on the scan rate and loading. Similar data were also observed in NaCl
3208 J. Phys. Chem. B, Vol. 105, No. 16, 2001
Hui and Baker
Figure 4. Half wave potential, E1/2 as a function of log(L/(1 - L)2; see eq 20. L represents the initial MV2+ loading. Several concentrations of NaCl supporting electrolyte are shown. Scan rate: 200 mV/s.
supporting electrolytes (vide infra). The negative shift in halfwave potential (i.e., E1/2) is not uncommon for zeolite-modified electrodes. A 120 mV cathodic shift in reduction peak potential of Ru(NH3)63+ in zeolite Y17 was interpreted in terms of the negatively charged zeolite framework causing a reduction overpotential. Potential shifts with loading are also explicable in terms of ion exchange equilibria providing that equilibrium is rapidly achieved on the electrochemical time scale and can be predicted by combining the Nernst Equation with the ion-exchange equilibrium constant.18 The establishment of the ion exchange equilibrium associated with methyl viologen in zeolite Y is indeed rapidly attained.8,19 Following immersion of methyl viologen-containing zeolite particles in aqueous 0.05 M NaCl, a steady state of MV2+ in solution was obtained in less than one second.9 Since the zeolite contains sodium ions, for simplicity the supporting electrolyte considered in this section of the paper is NaCl. For the ion-exchange reaction:
2Na+(s) + MV2+(z) h 2Na+(z) + MV2+(s)
(12)
with concentration in the zeolite. However this variation makes no impact on the discussion (vide infra). So
Ke )
[MV2+]s[Na+]z2 Γ [MV2+]z[Na+]s2
(14)
For the electrochemical reaction:
MV2+(s) + e- T MV•+(s)
(15)
The apparent electrode potential Eapp is
Eapp ) E0 +
2+ RT [MV ]s ln nF [MV•+]
(16)
s
substituting eq 14 into eq 16 gives:
The equilibrium constant, Ke, is 2+
Ke )
+ 2
[MV ]s[Na ] [MV
2+
z ]z[Na+]2s
(
2
)
γNa+ fMV2+ γMV2+fNa+2
2+ 2+ 2 RT Ke RT [MV ]z RT [Na ]s + ln (17) Eapp ) E + ln + ln nF Γ nF [Na+] 2 nF [MV •+] 0
(13)
where γNa+ and γMV2+ are the activity coefficients in aqueous solution and fNa+ and fMV2+ are those in the solid state. The activity coefficients in aqueous solution of both ions are fairly constant regardless of the loading of MV2+ in the zeolite.20 However, in the solid state these are dependent on the selectivity of the ion exchanger and are not easily determined.21 These values have been assembled in the parameter Γ below. We note that Walcarius et al.22 have recently determined their variation
z
s
The loading of MV2+ in the zeolite can be defined as:
L)
2[MV2+]z S
(18)
where S is the total number of occupied ion exchange sites by MV2+ and Na+. Assuming that the change of loading is
Ion Exchange at Methyl ViologenY Modified Electrodes
J. Phys. Chem. B, Vol. 105, No. 16, 2001 3209
TABLE 3: Slopes Obtained from Figure 4 conc of NaCl (M)
slope of E1/2 versus ln(L(1 - L)2 (mV)
0.01 0.025 0.05 0.075 0.1
66.6 60.1 62.5 55.7 58.0
insignificant on the experimental time scale, and combining eqs 16-18 gives:
Eapp ) E0 +
RT L RT Ke RT 1 ln + ln + ln + nF Γ nF 2S nF (1 - L)2 2+ 2 RT [Na ]s (19) ln nF [MV•+] s
Since E1/2 and Eapp are related via the Nernst equation Eapp ) E1/2 + RT/nFln C, where C is a concentration dependent term, we can equate C with [Na+]2/[MV•+], since L is unchanged on the time scale of the experiment. This leads to
Ke 0.0592 1 0.0592 log + log + E1/2 ) E0 + n Γ n 2S 0.0592 L (20) log n (1 - L)2 So a plot of E1/2 versus log(L/(1 - L)2) will be linear with a slope of 59.2 mV (see Figure 4, Table 3 and ref 18). Note that, E1/2 should not change significantly with the concentration of Na+ ions in solution, and indeed shifts of less than 2 mV occurred for all NaCl concentrations as shown in the figure and in Table 3. Furthermore, at low loadings, log(L/(1 - L)2) is negative and a cathodic shift in E1/2 will result. When L is large, log(L/(1 - L)2) is positive, and E1/2 will shift anodically. Shifts in E1/2 follow eq 20 for negative values of log(L/(1 - L)2) but deviate for positive values. This may be due to rapid ionexchange at high Na+ concentrations causing a significant loss of methyl viologen from the zeolite, negating the assumptions used in generating eq 19. These results suggest that redox potential shifts are the result of ion-exchange concentration overpotentials. The last term on the right-hand side of eq 20 shows that the applied potential causes a change in the concentration of Na+ at the electrode surface by removing MV2+ from the zeolite. The other terms on the right-hand side of the equation containing the ionexchange equilibrium constant, the initial loading of the redox species, and the total number of ion-exchange sites constitute the concentration overpotential. This can be viewed as an activation energy required to drive mass transfer at the rate needed to support the current. The lower the loading of MV2+, the slower the mass transfer rate and the higher overpotential that is required. These experimental results are consistent with an extrazeolite electron-transfer processes. Counter diffusion coefficients can, however, be a function of the concentration and self-diffusivity of the ions involved in the ion-exchange reaction. Ion-exchange kinetics are affected by several factors such as the requirement to preserve charge balance, cation hydration and ion sieving effects, and presence of different cation sites. Systems conforming to a simple ionexchange model however are not uncommon. Ion exchange pairs Ca2+-Sr2+, Ca2+-Ba2+ in chabazite and Na+-Sr2+, Na+Ca2+, and Na+-Mg2+ in zeolite Y conform reasonably closely to the simple ion-exchange model, but for other systems such
Figure 5. Cottrell slope as a function of initial loading of methyl viologen. Data are shown for different concentrations of LiCl electrolyte as indicated in the figure. Potential step from -0.4 to -0.9 V for 5 s.
as Zn2+ in zeolite A, well-defined deviations from the simple model have been observed. The concentration dependence of diffusion coefficients for a variety of different systems has been reported.23 A useful review including an extensive tabular summary of reported diffusivity data has been given by Barrer.24 The models used above require that an ingress for electrolyte ions occurs sufficiently rapidly to maintain charge balance. The diffusion of MV2+ is in fact rate determining as shown by Figure 5. Here, CS is plotted as a function of the supporting electrolyte concentration. The linear and parallel plots support the notion that the apparent diffusion coefficient is a constant. In addition, it is reasonable to expect that the diffusion of hydrated lithium ions within the zeolite pores would be much faster than the diffusion of methyl viologen dications based upon the sizes and charges of the two ions. Note that the electrolyte concentration does affect the rate of ion-exchange by changing the equilibrium loading of MV2+ in zeolite Y. For lower electrolyte concentrations, more MV2+ ions are retained in the zeolite at equilibrium causing a lowering of the ion-exchange rate. The changes in equilibrium loading for low concentrations are reflected by the more positive x-intercept in Figure 5. When the electrolyte concentration increases, the value of the equilibrium loading decreases and tends to a constant value. Thus, the plots for 0.05 and 0.1 M LiCl are closer to each other consistent with the results obtained by Walcarius et al.22 References and Notes (1) Senaratne, C.; Zhang, J.; Baker, M. D.; Bessel, C. A.; Rolison, D. J. Phys. Chem. 1996, 100, 49. (2) Bessel, C. A.; Rolison, D. J. Phys. Chem. B 1997, 101, 1148. (3) Brouwer, D. H; Baker, M. D. J. Phys. Chem. B. 1997, 101, 10390. (4) Li, J. W.; Calzaferri, G. J. Electroanal. Chem. 1994, 377, 163. (5) Baker, M. D.; McBrien, M.; Burgess, I. J. Phys. Chem B. 1998, 102, 2905. (6) Li, J. W.; Pfanner, K.; Calzaferri, G. J. Phys. Chem. 1995, 99, 2119. (7) Hui, T. W.; Baker, M. D., manuscript in preparation. (8) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. 1986, 208, 95. (9) Walcarius, A.; Lamberts, L.; Derouane, E. G. Electrochim. Acta 1993, 38, 2257. (10) Calzaferri, G.; Lanz, M.; Li, J. W. J. Chem. Soc. Chem. Comm. 1995, 1313,
3210 J. Phys. Chem. B, Vol. 105, No. 16, 2001 (11) Ratner, M. A.; Nitzan, A. Faraday Discuss. Chem. Soc. 1989, 88, 19. (12) Dahms, H. J. Phys. Chem. 1968, 72, 362. (13) Ruff, L. J. Phys. Chem. 1971, 75, 3297. (14) Ruff, L.; Friedrich, J.; Demeter, K.; Csailag, K. J. Phys. Chem. 197, 75, 3303. (15) Baker, M. D.; Senaratne C.; Zhang, J. J Phys. Chem. 1994, 98, 1668. (16) Barrer, R. M.; Hinds, L. J. Chem. Soc. 1953, 1879. (17) Walcarius, A.; Lamberts, L. J. Electroanal. Chem. 1997, 422, 77. (18) Naegeli, R.; Redepenning, J.; Anson, F. J. Phys. Chem. 1986, 90, 6227.
Hui and Baker (19) Li, C.; Wang, M.; Persaud, L.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2592. (20) Walcarius, A.; Lamberts, L.; Derouane, E. G. Electrochim. Acta 1993, 38, 2267. (21) Breck, D. W. Zeolite Molecular SieVes, Wiley: New York, 1984. (22) Walcarius, A.; Mariaulle, P.; Lamberts, L. J. Electroanal. Chem. 1999, 463, 100. (23) Rees, L. V. C. Proceedings of the 6th International Zeolite Conference, Olson, D., Bisio, A., Eds.; Reno. Butterworth Press: Guildford, U.K., 1984, p 626. (24) Barrer, R. M. Proceedings of the 5th International Zeolite Conference, Rees, L. V. C., Ed., Heyden, London, 1980, p 213.
