Comment on "Intrazeolite electron transport mechanism"

Abstract

We report significant errors, resulting in the misinterpretation of the cyclic voltammetry of Ag+A monograin layers deposited on glassy carbon electrodes.' The electrochemical reduction of extraframework cations was proposed to occur while the silver cation was present within the pore and channel system of zeolite A on the basis of multiple cathodic waves. The interpretation, however, does not take into account the known ion exchange properties of zeolite A in aqueous solution and ignores previous compelling evidence that redox occurs outside of the zeolite Moreover, the data presented do not survive a detailed analysis based on site-specific redox potentials. In this brief comment we will indicate how the data of Li et al.' have been misinterpreted.The comerstone of Li et al.' is the proposition of site-specific reduction potentials. However, the data of Li et al. show quite a different behavior. Their data purport to point to the observation of four distinct extraframework cations and three types of clusters. However, the reduction potential for each is not constant. Referring to Figures 3 and 4 of the original paper, the following spans in peak potential are readily observed: peak A, 200 mV; peak B, 200 mV; peak C, 125 mV; peak D, 125 mV. The peak assignments are seemingly arbitrary. For example, Li et al. have recognized differences of 40 mV to assign site reductions to peaks A, B, and C in Figure 3 at a scan rate of 20 mV/s and x = 2.6,4.0, and 6.5, respectively. In the same figure they ignore differences of the order of 250 mV when assigning peaks to the same site reduction (peak A at 2 mV/s and x = 2.6 and 50 mV/s and x = 4.0). Clearly, the peak assignments are of an arbitrary nature dictated solely by the necessity of agreement with the hypothesis.Peaks C-E in this work are ascribed to redox processes of electrogenerated intrazeolite clusters. However, these peaks often arise before any prior reduction of silver is indicated. (See Figure 3, x = 6.5, v = 50 mV s-I, and Figure 4, x = 4.0, v = 20 mV SKI.)The authors, already claiming to demonstrate intrazeolite redox, which would open up a new vista on this field, now form clusters with no evidence of reduction. The absence of "a solution peak" (reduction of solution phase silver) is startling. In the slow scan regimes for the Ag6,sA we are asked to believe that following a 5 min immersion in 0.1 M NaC104 no silver ions are produced in solution.In all the voltammograms shown in Figures 3 and 4 of ref 1 the maximum number of peaks present is four. The authors have not considered the possibility of concentration or scan rate dependent shifts in peak potentials. It could well be that only three or four distinct reduction peaks are present. For redox couples with one insoluble species (e.g., metal deposition) the * To whom correspondence should be addressed. 0022-3654/95/2099-12367$09.00/0 reduction potentials shift to more negative potentials with decreasing concentration^.^ Although the authors leave the impression that monograin electrodes may behave differen...

Host–Guest Antenna Materials

“…Intrazeolite charge transport on Cu 2+ ‐Y zeolite, on Ag + ‐A zeolite, and on MV 2+ ‐Y zeolite electrodes, in which the zeolite microcrystals are deposited as monolayers on glassy carbon‐disc electrodes, was reported by us 73. Baker et al questioned the importance of preparing zeolite electrodes as monolayers,76 a question which we have countered 73c. He and others later focussed mainly on conditions in which in the intrazeolite charge transport by the guest does not play a significant role 77.…”

Zeolite-Modified Electrodes: Intra- versus Extrazeolite Electron Transfer

“…Indeed, we have exactly reproduced the data obtained for AgA by Calzaferri's group 25,35 using non-monograin electrodes prepared according to the formulation given in the Experimental Section. 65 Thus, the notion that monograin electrodes and those used here for transition metal studies behave differently is invalid.…”

Wirt‐Gast‐Antennenmaterialien

“…Wir haben über intrazeolithische Ladungstransportexperimente mit Cu 2+ ‐Y‐Zeolith‐, Ag + ‐A‐Zeolith‐ und MV 2+ ‐Y‐Zeolith‐Elektroden berichtet, bei denen die Zeolithkristalle als Monoschichten auf Scheibenelektroden aus glasartiger Kohle deponiert waren 73. Baker et al stuften die Herstellung von Zeolithmonoschicht‐Elektroden als unwichtig ein,76 eine Auffassung, der wir widersprochen haben 73c. Er und andere konzentrierten ihre Arbeiten später vor allem auf Bedingungen, bei denen der intrazeolithische Ladungstransport der Gäste eine untergeordnete Rolle spielt 77.…”