J. Phys. Chem. 1995,99, 12368-12369
12368
Reply to “Comment on Intrazeolite Electron Transport Mechanism”. The Importance of the Manner To Prepare Zeolite-Modified Electrodes
a)
Jian-wei Li, Klaus Pfanner, and Gion Calzaferri* Institute for Inorganic and Physical Chemistry, University of Berne, Freiestrasse 3, CH-3000 Beme 9, Switzerland Received: April 13, I995
We observed that the voltammetric comportment of Ag+-A zeolite monograin modified electrodes depends considerably on the Ag+-exchange degree, the scan rate, and the electrolyte cations.’ Eight distinct waves were found in the first cathodic scans. An intrazeolite electron transport mechanism was proposed as the dominant process and discussed by assuming site-specific reduction potentials, site-to-site interconversion of Ag+ ions, and formation of silver clusters. When writing our paper, we were fully aware of the work published by Baker. This is reflected in the citation (refs 3, 4, and 6 in our paper). In their Comment, Baker et al. state2 that we have not provided evidence that the zeolite monograin layers prepared by us’ behave differently from the zeolite-modified electrodes used for example in his laboratory. In fact, it is easy to see that the electrochemical behavior of the two types of electrodes is completely different by just comparing the cyclic voltammograms (CVs) recorded by us (see Figures 3 and 4 in ref 1) and the ones recorded by them (see refs 3, 4, and 6 in our article). The most striking features of our CVs are the multiple cathodic waves in the potential range from 200 to -800 mV vs SCE, whereas in the CVs reported by Baker et al. only one or in some cases two cathodic waves are present. This different behavior originates from the different morphology of the zeolite layers prepared by Baker et al. and the ones prepared by us. The main differences of the resulting electrodes are illustrated in Figure 1. We show an “ideal” zeolite layer (a), the compact zeolite monograin layer (b) as prepared by us, and the porous polystyrene/graphite/zeolite layer (c) which corresponds to the electrodes used by Baker et al. Electrodes with an “ideal” zeolite layer (a) would facilitate mechanistic studies; they have, however, not been prepared so far. Electrodes of type (b) are obtained by very slow evaporation of a zeolite/water suspension on a substrate. A dense zeolite monograin layer results and is stabilized with a little amount of a polymer. With this procedure a maximum coverage and direct contact between almost each individual zeolite particle and the substrate electrode surface were reached; see SEM micrographs in refs 1 and 3. The zeolite grain layer structures of type (c) are obtained from a suspension of zeolite microcrystals and graphite powder in a solution of polystyrene in THF. SEM micrographs of such electrodes have been reported by Shaw et a1.: and Baker et al. have used this type of electrodes; see refs 3,4, and 6 in our article. The same procedure was tried by us several years ago. We were not able to prepare a monograin layer in this way, and we always observed a bad contact between the zeolite particles and the substrate electrode surface. The reason is that the polymer shows a stronger adhesion to the electrode surface than to the zeolite microcrystals: and thus they aggregate and are embedded in a polymer matrix. Good direct contact between the zeolite microcrystals and the substrate electrode is, however, crucial in our experiments. Bad contact results in the extra zeolite mechanism (c) in Figure 7 of ref 1 as dominant process, and only few of the Ag+ ions initially present in the zeolite reach
* To whom correspondence should be addressed. 0022-3654/95/2099- 12368$09.00/0
0 zeolite
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polymer
Figure 1. Different zeolite layers on substrate electrodes: (a) “ideal” zeolite layer, (b) dense zeolite monograin layer, and (c) porous zeolite/ polystyrene layer.
the electrode because of the stacked arrangement of the zeolite microcrystals. The charges in the cathodic and the anodic processes are expected to be about the same because in this case the Ag+ ions are reduced on the surface of the electrode where the silver remains until it is oxidized in the anodic scan. In electrodes of type (c), hence, the zeolite layer serves only as a reservoir for the Ag+ ions. This is what Baker et al. have found and how they correctly interpret their data. Our electrodes and our observations are, however, completely different. We measured large currents in the first cathodic scans corresponding to 50-100% of the Ag+ initially present in the zeolite. This supports an intrazeolite mechanism. Recent measurements made in our laboratory at methylviologen zeolite Y monograin electrodes fully confirm our previously reported results, and they further underline the importance to prepare electrodes with a large direct contact area between the zeolite microcrystals and the substrate ele~trode.~ In their Comment, Baker et al. state that our data do not survive a detailed analysis. They argue that (1) the reduction potential for each peak is not constant and that (2) the influence of the concentration and of the scan rate on the peak potential has not been considered. They state that our peak assignment is arbitrary and just at our convenience and that the data in Figures 3 and 4 of ref 1 only show the presence of three or four distinct reduction peaks. It is well-known that the effect of the solution resistance R, between the electrodes is to flatten the wave and to shift the reduction peak toward a more negative potential. The larger the scan rate and the larger the concentration, the more the peak potential is shifted; see e.g. ref 6. The influence of the resistance R, of the zeolite Y monograin layers on the peak potential shift in an intrazeolite process was recently discussed by US.^ We found that R, depends on the mobility of cations in the zeolite and that the peak potential shift is mainly caused by the uncompensated iR,. The cages and the channels of zeolite A are much smaller than those of zeolite Y, and hence, a significantly larger resistance R, is expected for it. This is why the reduction potential of each peak in Figures 3 and 4 of ref 1 is not constant but shifts in the negative direction with increasing scan rate and increasing concentration because no iR compensation was applied, as stated in the experimental part. H is the first reduction peak. It matches that of Ag+ at the blank glassy carbon electrode (see Figure 5 in ref l), and its peak potential shifts only very slightly. We have assigned H to an extrazeolite process which should not be influenced by R, of the zeolite. The other peaks were assigned one by one 0 1995 American Chemical Society
Comments
J. Phys. Chem., Vol. 99,No. 32, 1995 12369 I
0.6
0.2
G
-0.2
-0.6
E / V vs. (SCE)
Figure 2 3xpanded cathodic waves of two voltamogramms of Figure 3 in ref 1. Dashed lines I and I1 correspond to = 12 and6.5, respectively, at the scan rate of 5 mVls. Solid line 11' is a current axis
x
expansion of curve 11.
from G to A according to the following criterion: If, for two adjacent peaks, the current of the peak at a more negative potential was equal to or smaller than the other one, it was interpreted as a new peak. This reasonable criterion leads very clearly to eight electrochemically distinct silver species. In reply to the doubts expressed by Baker et al. that no silver ions are leaching in the slow scan regimes for the Ag6,sA electrodes, we show an expanded cyclic voltammogram in Figure 2. The waves I and I1 correspond to the CVs at x = 12
and 6.5, respectively, at the scan rate of 5 mV/s in Figure 3 of ref 1. Curve II' is the expansion of J.I. It shows that the current increases sharply in the potential region of the peak H where the leached Ag+ ions start to be reduced. At the slow scan rates this behavior was also observed for smaller exchange degrees than 6.5. It shows that ion exchange always happens during the measurement, as discussed in our article. The corresponding currents are, however, too small to be visible in the Figures 3 and 4 of ref 1, and they are too small to be considered in the discussion of the dominant electrochemical processes. Baker et al. also state that for redox couples with one insoluble species the reduction potentials shift to more negative potentials with decreasing concentrations. This aspect is discussed in detail in our article, and we see no need to repeat all the arguments.
References and Notes (1) Li, J.;Pfanner, K.; Calzafem, G. J. Phys. Chem. 1995, 99, 2119. (2) Baker, M. D.; Senaratne, C.; McBrien, M. J. Phys. Chem. 1995, 99, xxxx. (3) Li, J.; Calzafem, G. J. Electroanal. Chem. 1994, 377, 163. (4) Shaw, B. R.; Creasy, K. E.; Lanczycki, C. J.; Sargeant, J. A,; Tirhado, M. J. Electrochem. SOC. 1988, 135, 869. (5) Calzafem, G.; Lanz, M.; Li, J. J . Chem. SOC., Chem. Commun. 1995, 1313. ( 6 ) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications; Wiley: New York, 1980.
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