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Reply to the Comment on “Zeolite-Modified Electrodes: Intra- versus Extrazeolite Electron Transfer” Debra R. Rolison* Surface Chemistry Branch, Code 6170, NaVal Research Laboratory, Washington, DC 20375
Carol A. Bessel Department of Chemistry, VillanoVa UniVersity, VillanoVa, PennsylVania 19085
Mark D. Baker,* Chandana Senaratne,* and J. Zhang Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, UniVersity of Guelph, Guelph, Ontario N1G 2W1, Canada ReceiVed: February 1, 1996 Can supracommunal, electrode-addressable intracrystalline electron transfer occur in zeolites modified with internal redox solutes?1 Intrazeolite electron transfer can be observed by an external electrode as Mallouk and co-authors have previously demonstrated in a series of papers describing vectorial electron transport in zeolites.2-5 But in these studies a careful design of the electron transfer relays was necessary in order to establish (or sever) electron flow between the zeolite interior, the zeolite boundary, and an external electrode.2-5 In the absence of mobile electron transfer mediators which can access both the interior of the microporous zeolite and the boundary of the zeolite so that electrons may flow to an external electron transfer relay station (such as the electrode), the answersonce the experimental evidence is considered in totalsis no.1 Any claim of direct, nonmediated electron transfer to intrazeolite redox solutes, in the absence of mobile electron shuttles or molecular wires, must be tested with care and scepticism as it is a critical mechanism required for advanced applications of zeolite-modified electrodes and electrode-modified zeolites.6 In a series of papers describing the voltammetric behavior of transition-metal complexes synthetically entrained within zeolite crystals, Bedioui, Devynck, Balkus, and co-workers have claimed electron transfer to zeolite-encapsulated redox-active transition-metal complexes.7-15 It is unfortunate that much of the electrochemistry reported by Bedioui et al. does not follow experimental protocol that has been standard for the past 20 years in the study of chemically modified electrodes (CMEs). Such protocol includes (1) reporting the specific sources of all components in the modified electrode, (2) reporting the specifics of the modification procedure, which is necessary if other laboratories are to replicate the procedure, (3) reporting the appropriate voltammetric controls in the absence of the modifying redox species as the various components necessary to create the total modified electrode are added in turn into the modified interphase, and (4) integrating the Coulombic charge under the voltammetric waves obtained for a modified electrode so that the number of redox-active molecules can be calculated using Faraday’s law (q ) nFm). Bedioui et al. have been sparing in reporting the specifics of their procedure to prepare their zeolite-modified electrodes (ZMEs). Their first papers described their ZME formulation as a “graphite paste electrode”,7-10,14 but as none of the usually understood cohering components of a graphite paste electrode (i.e., an organic oil, grease, or polymer) are listed in their papers, replication of their ZME formulation is difficult. The paste S0022-3654(96)00722-8 CCC: $12.00
component was evidently never present as the authors finally dropped the misleading designation of their formulation as a “graphite paste electrode” to describe their ZMEs as “pressed powder composites”.11-13,15 In only one paper12 of their nine on the voltammetry of zeolite-encapsulated transition-metal complexes have Bedioui et al. provided a standard piece of informationsthe type of carbon used to make their pressed composite ZMEs; nor is it clear whether their prior studies7-11,13-15 were with this type of ultrapure graphite (Koch Light, 99.999%+). The variable surface area of and the oxygenated surface moieties on graphitic carbons are understood by electrochemists to influence the amperometric response and character of electrodes incorporating graphite.16,17 It is unfortunate Bedioui et al. only report an incomplete background voltammogram, Figure 1a,18 for their graphite in nonaqueous electrolyte in response to our discussion of the interplay of different graphites with the same redox modifier ({Co(salen)}NaY).1 Further, the data Bedioui et al. now report for a pressed powder composite containing their (again unspecified) graphite plus NaY, Figure 1b,18 should give them cause for concern as there appears to be no additional charging current over that at graphite alone; aluminosilicate zeolite NaY, with its high ionic strength surface diffuse layer, must contribute to the double-layer capacitance of the composite. Bedioui et al. have, to date, omitted the pressure and the time under pressure required to make their pressed powder composite ZMEssnor do they tell the interested reader whether their equal masses of carbon and redox-modified zeolite are ground together prior to pressing. In our hands, pressing equal weights of carbon and redox-modified zeolite around a noble-metal mesh electrode at 14 000 psig leads to a ZME that falls apart when placed in nonaqueous electrolyte. This friability makes a consistent and serious voltammetric characterization of zeolite-encapsulated transition-metal complexes, {M(L)}Z, impossible. As we discuss elsewhere,19 the voltammetry of zeolite-encapsulated Co(salen), in the absence of carbon and polymer binder, is affected deleteriously after the redox-modified zeolite is pressed at 14 000 psig. A greater lapse of standard CME procedure by Bedioui et al. was to present in multiple papers the voltammetry for their {M(L)}Z-modified electrodes without integrating the Coulombic charge under any of the voltammetric waves.7-10,14 Without this information they could not (and did not) report the number of M(L) molecules undergoing electron transfer. By neglecting this customary and elementary piece of electroanalysis, these authors could not (and did not) estimate the fraction of zeoliteencapsulated species communicating to the carbon electrodic interphase. This led them to claim that transition-metal complexes entrapped within the zeolite “can be readily examined by cyclic voltammetry”7-9 when, in reality, the Coulombic charge obtained for an oxidation or reduction of M(L) represents at best only a few percent of the complexes residing in the bulk of the zeolite crystal. When Bedioui et al. began to assess the Coulombic fraction represented in their voltammetric characterizations,11-13,15 they could only attribute the electroactive fraction of zeolite-encapsulated complexes to those complexes occluded in cages at the boundary of the zeolite crystal. It is not semantics to point out that this thin boundary layer reponse is not an intrazeolitic phenomenon, nor does the authors’ original claim that transition-metal complexes entrapped within the zeolite can be “readily examined by cyclic voltammetry”7-9 hold up. Additional analytical methods to probe redox-active species within the bulk of a zeolite crystal are greatly desired by the zeolite communityswhether these species are isomorphically substituted for Si or Al or are present as extraframework © 1996 American Chemical Society
Comments localized or cage-entrapped complexes. Cyclic voltammetry has not yet demonstrated its usefulness for such bulk intracrystalline electroanalyses of redox-modified zeolites in the absence of electron transfer mediators. We are in agreement with Bedioui’s and his co-authors’ revised assessment11-13,15 that those cage-entrapped redox species which are redox-active are present as boundary species. The issue now becomes one of whether such a limited sheath of electroactivity can be useful for electrocatalysis or selectivity in advanced sensors. We feel the former is indeed feasible, but only if the voltammetric response endures and does not readily or markedly decay with potential cycling or under potentiostatic conditions. The evidence presented by Bedioui et al.sin which the voltammetric signals obtained via their pressed-powder composite ZMEs are not temporally persistent,7-15 even on the voltammetric time scalesdoes not bode well for the undeniable advantages of site-isolated zeolite-supported electrocatalysis. We have found, however, in conditions atypical of ZMEs (i.e., those that do not involve mechanical working of the zeolites) that hours-long voltammetric durability can be obtained for {Co(salen)}NaY,19 with which effective electrocatalyses, and not just voltammetric scans in the presence of substrate, can be performed.20 With respect to several remaining issues raised in the comment by Bedioui et al., we would point out that we do state in our original paper that standard bulk analytical methods (such as UV-vis and infrared spectroscopies) were performed on our {Co(salen)}NaY and {[Fe(bpy)3]2+}NaY samples to establish that synthesis of the requisite transition-metal complex had occurred.1 But as the electrochemical response is surfaceoriented, and not bulk intrazeolitic, these traditional bulk analytical methods, also used by Bedioui et al.,7,9,11,12,14 are essentially useless other than to reassure oneself that most of the spectroscopically visible species are the expected ones. Those scientists who wish to study the electrochemistry of zeolite-encapsulated transition metal complexes need to employ and devise analytical methodologies that are more surface and near-surface specific if they wish to obtain information on the relevant electroreactants. This is why our study focused on the near-surface information provided by X-ray photoelectron spectroscopy (XPS) as correlated to the voltammetric response of {Co(salen)}NaY.1 As we show, once the XPS-derived nearsurface concentration of Co in {Co(salen)}NaY drops to that characteristic of the bulk concentration, the {Co(salen)}NaYmodified electrode is no longer electroactive.1 The assertion by Bedioui et al. that cation size-exclusion experiments (specifically those using the TBA+ cation) can be used to determine the electron transfer mechanism is fundamentally flawed. Variations in faradaic current with cation size merely point to the nature of the rate-determining step in the overall electrochemical process. It is impossible to determine whether charge compensation occurs prior to (or following) electron transfer using cation sieving arguments. It is vital to understand that it is not whether the cation can enter the zeolite pore system that is important here: it is the rate of sorption that matters. Bedioui et al. have consistently disregarded this distinction. Note also that the cation effects we have observed in the presence of a polystyrene binder21-23 conform to the known ion-sieving trends for both alpha (supercage) and beta (sodalite) cage species.24 This inability of a polystyrene binder to affect the cation series, especially for beta cages, is evidence that the polymers are not interfering in the fundamental character of the zeolite, as does our observation of enhanced charging current when NaY is held by a polyacrylic acid overlayer at a glassy carbon surface (see Figure 1e of ref 1). Visual observation of the color of a redox-modified zeolite offers near-surface rather than bulk information and does not
J. Phys. Chem., Vol. 100, No. 20, 1996 8611 provide the definitive evidence of uniform bulk modification of the zeolite crystal with ferricenium (Fc+) that Bedioui et al. think it does. We are not surprised that these authors find that Fc+-modified NaY is electroinactive when aqueous KCl was added to the electrolyte, as water (or polar aprotic solvents) will preferentially sorb into the zeolite and displace ferrocene. We report a similar effect for uncomplexed salen when {Co(salen)}NaY is stirred in water or DMF.1 The authors’ own data refute their claim that the electrochemistry they observed for Fc+modified NaY pressed powder electrodes is derived from ferricenium.11 The voltammetry they published (see Figure 1 of ref 11) is only consistent with the initial electrode process being the oxidation of ferrocene, not the reduction of intrazeolitic ferricenium.11 So we return to our initial question: Can supracommunal, electrode-addressable intracrystalline electron transfer occur in zeolites modified with internal redox solutes?1 In the absence of mobile electron transfer mediatorssand once the experimental evidence has been considered in totalselectrode-addressable intrazeolite electron transfer remains to be demonstrated. Acknowledgment. We gratefully acknowledge the Office of Naval Research, USA, the Natural Sciences and Engineering Research Council of Canada, and the Environmental Science and Technology Alliance Canada for financial support of our research programs. C.A.B. acknowledges a United States National Research Council/Naval Research Laboratory Postdoctoral Fellowship. References and Notes (1) Senaratne, C.; Zhang, J.; Baker, M. D.; Bessel, C. A.; Rolison, D. R. J. Phys. Chem. 1996, 100, 5849. (2) Li, Z.; Mallouk, T. E. J. Phys. Chem. 1987, 91, 643-648. (3) Li, Z.; Wang, C. M.; Persaud, L.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2592-2597. (4) Li, Z.; Lai, C.; Mallouk, T. E. Inorg. Chem. 1989, 28, 178-182. (5) Krueger, J. S.; Lai, C.; Li, Z.; Mayer, J. E.; Mallouk, T. E. In Inclusion Phenomena and Molecular Recognition; Atwood, J. E., Ed.; Plenum Press: New York, 1990; pp 365-378. (6) Rolison, D. R. Stud. Surf. Sci. Catal. 1994, 85, 543-586. (7) Bedioui, F.; De Boysson, E.; Devynck, J.; Balkus, K. J., Jr. J. Electroanal. Chem. 1991, 315 (1-2), 313-318. (8) Bedioui, F.; De Boysson, E.; Devynck, J.; Balkus, K. J., Jr. J. Chem. Soc., Faraday Trans. 1991, 87 (24), 3831-3834. (9) Gaillon, L.; Sajot, N.; Bedioui, F.; Devynck, J.; Balkus, K. J., Jr. J. Electroanal. Chem. 1993, 345, 157-167. (10) Mesfar, K.; Carre, B.; Bedioui, F.; Devynck, J. J. Mater. Chem. 1993, 3 (8), 873-876. (11) Bedioui, F.; Roue´, L.; Briot, E.; Devynck, J.; Bell, S. L.; Balkus, K. J., Jr. J. Electroanal. Chem. 1994, 373, 19-29. (12) Balkus, K. J., Jr.; Gabrielov, A. G.; Bell, S. L.; Bedioui, F.; Roue´, L.; Devynck, J. Inorg. Chem. 1994, 33, 67-72. (13) Bedioui, F.; Roue´, L.; Devynck, J.; Balkus, K. J., Jr. Stud. Surf. Sci. Catal. 1994, 84, 917-924. (14) Gabrielov, A. G.; Balkus, K. J., Jr.; Bell, S. L.; Bedioui, F.; Devynck, J. Microporous Mater. 1994, 2, 119-126. (15) Bedioui, F.; Roue´, L.; Devynck, J.; Balkus, K. J., Jr. J. Electrochem. Soc. 1994, 141, 3049-3052. (16) Kinoshita, K. Carbon: Electrochemical and Physicochemical Properties; J. Wiley: New York, 1988. (17) McBreen, J.; Olender, H.; Srinivasin, S.; Kordesch, K. V. J. Appl. Electrochem. 1981, 11, 787-796. (18) Bedioui, F.; Devynck, J.; Balkus, Jr., K. J. J. Phys. Chem. 1996, 100, 8607. (19) Bessel, C. A.; Rolison, D. R. Manuscript in preparation. (20) Bessel, C. A.; Rolison, D. R. Manuscript in preparation. (21) Baker, M. D.; Senaratne, C. Anal. Chem. 1992, 64, 697-700. (22) Senaratne, C.; Baker, M. D. J. Electroanal. Chem. 1992, 332, 357364. (23) Baker, M. D.; Senaratne, C.; Zhang, J. J. Chem. Soc., Faraday Trans. 1992, 88, 3187-3192. (24) Breck, D. W. Zeolite Molecular SieVes; R. E. Krieger: Malabar, FL, 1984.
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