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Langmuir 1990,6, 1338-1342
In Situ Mossbauer Spectroscopy of a Species Irreversibly Adsorbed on an Electrode Surface Cristian A. Fierro, Madan Mohan, and Daniel A. Scherson* Case Center for Electrochemical Sciences and the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 Received November 9, 1989. In Final Form: March 5, 1990 The redox properties of p-oxobis[iron meso-tetrakis(methoxyphenyl)porphyrin] (FeTMPP)20, adsorbed on high-area carbon in alkaline solutionshave been examined in situ by using MWbauer spectroecopy at cryogenic temperatures. The spectrum obtained for the electrode polarized at 0.0 V vs Hg/HgO,OHyielded at 100 K a doublet with parameters resembling those of bulk crystalline (FeTMPP)20. Similar measurements with the electrode polarized at -0.9 V at the same temperature displayed in addition to the original feature two new doublets with MBssbauer parameters consistent with the presence of a highspin ferrous (S= 2) and either a high-spin ferric ( S = 5/2) or an intermediate-spinferrous porphyrin speciea (S = 1). As the temperature was increased, these new features collapsed into a single doublet with an isomer shift intermediate between those observed at 100 K. Two possible models have been suggested to account for these results: (i) the formation of a mixed-valence p-oxo-type porphyrin, which below the spin trapping temperature would give rise to ferrous and ferric sites, and (ii) the full reduction and dissociation of the dimer following the first electron transfer, to yield an intermediate-spin and a high-spin ferrous porphyrin. The possibility of impurities being responsible for some of the spectroscopic features has also been considered.
Introduction A thorough in situ spectroscopic characterization of species adsorbed on electrode surfaces is of crucial importance to the further understanding of electrocatalytic phenomena at the metal-electrolyte interface. Indeed, much effort has been devoted during the past two decades toward the development and implementation of a host of techniques capable of providing information regarding electronic and vibrational properties of atomic and molecular species a t such interfaces.' This paper will present in situ Mossbauer spectra for a model transition-metal macrocycle irreversibly adsorbed on high-area carbon. The p-oxo-bridged form of bis[iron meso-tetrakis(methoxyphenyl)porphyrin],(FeTMPP)20, was selected for these studies on the basis of a number of considerations: (i) it is a fairly good electrocatalyst for t h e reduction of dioxygen in aqueous electrolytes, (ii) it displays a well-defined redox peak in a potential region in which the substrate exhibits purely capacitive behavior, and (iii) it shows good stability when placed in contact with alkaline solutions. The results obtained have indicated that electrodes polarized a t potentials positive to the voltammetric peak display at 100 K a single doublet attributed to adsorbed (FeTMPP)20, whereas two different doublets are found for the same specimens a t a potential negative enough for the complete reduction of the macrocycle to ensue. Two models have been suggested to explain the origin of these features: (i) the formation of a mixed-valence p-oxoporphyrin and (ii) the stepwise two-electron reduction of the dimer to yield two ferrous species with different spin states. Although some issues still remain to be resolved regarding these assignments, the results obtained in these experiments clearly indicate that the freezing of polarized (1) For a collection of monographs in the area of in situ application of spectroscopicmethods to the study of electrode4ectrolyt.einterfaces, see: Spectroelectrochemistry Theory andPmctice; Gale, R., Ed;Plenum Press: New York,1988.
high-area carbon electrodes does not appear to influence the state of charge of the interface.
Experimental Section Highly enriched (67FeTMPP)zOwas synthesized by the method of Torrens et al.2 using 2 mg of metallic 67Fe (ca. 99% isotopic purity, Du Pont, MA) to prepare anhydrous iron(I1) acetate. The macrocycle (14.4 mg) was adsorbed from a dichloromethane solution (1.25 X 1V M) onto Black Pearl (36 mg), BP, a high-area carbon with about 1500 m2.g-1. This corresponds to a 20% w/w loading and may be shown to be just sufficient to form a monolayer of macrocycle on the carbon assuming the rings lie flat on the support. Low-2, high-area, electronically conducting materials are essential in this kind of application to enable a sufficiently large number of adsorbed molecules to be probed without introducing excessive attenuation of the y-ray beam.3 The (FeTMPP)20/BP/CHaC12 slurry was dried at room temperature. The resulting powder was suspended in a dilute Teflon emulsion under ultrasonic agitation, which was then filtered, dried, and subsequently used to fabricate a thin, disk-shaped (17-mm diameter) Teflon-bonded electrode.4 The in situ MBssbauer measurements were conducted with a vacuum-tight electrochemical cell designed so as to fit the end section of the velocity transducer and to minimize attenuation of the y-ray beam. A flat, ring-shaped, macrocyclic-free,Teflonbonded BP carbon electrode prepared in the same fashion as the working electrode was employed as the counter electrode, whereas a Hg/HgO,OH--impregnated Teflon-bonded electrode with a diameter slightly smaller than the orifice of the counter electrode was utilized as the reference electrode. All electrodes were stacked in the cell normal to the y-ray beam in a sandwich-typearrangement, separated by cellulose(2) Torrens, M.A.; Straub, D. K.; E ~ s b i nL. , M.J. Am. Chem. SOC. 1972,94,4160. (3) &hereon, D. In Spectroelectrochemistry: Theory and Practice; Gale, R., Ed.; Plenum Press: New York, 1988. (4) Tanaka,A. A.; Fierro, C.; %hereon, D.; Yeager, E.B.J. Phys. Chem. 1987,91,3799.
0 1990 American Chemical Society
In Situ Messbauer Study of Electrode-Adsorbed Species
1m
SWRCE
CWNTER ELECTRODE
REFERENCE ELECTROE
GAMMA RAY SCAM
~
ACRYLIC CELL WMlKlNG ELECTROE
ELECTROLYTE
0-
DETECTOR
Figure 1. Schematicdiagram of the electrochemical cell for in situ Mhbauer measurements.
I . I . I . I . ) . I . I I I I I -1.2 -1.0 -0.8 4 . 6 -0.4 -0.2 0.0
E(V) VS. SCE
Figure 2. Cyclic voltammetry of (FeTMPP)20adsorbed on a BP Teflon-bondedelectrode in 0.1 M NaOH (see text for details). based films,which served both as electrolyteretainers and electronic insulators. A schematic diagram of the cell is shown in Figure 1. After assembly, the in situ M6ssbauer electrochemicalcell was coupled to the end section of the transducer and placed in the bore of the Dewar. A Ranger Scientific MS-900 M6ssbauer spectrometermounG ed vertically on a Janis 12CNDT 'Supervaritemp" cryostat was used in the experiments. The electrochemical data were collected with a Pine RDE-3hipotentiostat at room temperature. All measurements were carried out in 0.1 M NaOH.
Results and Discussion 1. Cyclic Voltammetry. Prior to the in situ MBssbauer measurements, the electrochemicalproperties of the electrode were examined by cyclic voltammetry in 0.1 M NaOH in a cell open to the atmosphere. As shown in Figure 2, the curves yielded two well-defined redox peaks with peak potentials a t -0.75 and -0.60 V vs Hg/HgO,OHin the scans in the negative and positive directions, respectively. Differences in the values of the peak potentials have also been observed for coatings of closely related macrocycles in slightly less alkaline solution^.^ This effect may he particularly pronounced in the present study due to the highly convoluted structure of the Teflonbonded high-area carbon electrode, a factor that precludes the facile transport of ions in and out of the matrix. The average peak potential compares favorably with that reported by Shigehara and Anson for coatings of FeTPPC1 in the same media6 (ca. 0.72 V vs SSCE). (5) Shigeha, R.;h n , F. J. Phys. Chem. 1981,86,"76.
Langmuir, Vol. 6, No. 8, 1990 1339
The fraction of electrochemicallyactive (5"FeTMpP)~0 molecules, as determined by a simple integration of the voltammetric peak, was less than one-third (ca. 30%) of that expected on the basis of the total number of iron sites in the macrocycle adsorbed originallyon the carhon support (vide infra). Essentially identical voltammetric curves were observed for the electrode in the in situ MBssbauer cell mounted on the transducer immediately prior to spectral acquisition. In the case of closely related macrocycles, this redox process has heen attributed to the one-electron reduction of the monomeric form of the macrocycle, to yield the correspondingferrous species.5 This igsue will be discussed in more detail later. 2. In S i t u M6sshauer Spectroscopy. Freezing of Polarized Electrodes. The (FeTMPP)zO/BP electrode at open circuit under ambient conditions failed to yield any detectable Mdsshauer spectra after a collection time of about 24 h. The solubility of (FeTMPPhO and closely related porphyrins in aqueous electrolytes is negligibly small, as judged by the fact that no detectable losses of material have been observed upon continuously cycling coatings on the order of a monolayer adsorbed on smooth carbon electrode surfaces in electrolytes free of ~ x y g e n . ~ It may thus be concluded that although the effect may be accentuated in the presence of a liquid, the absence of a MBssbauer effect at room temperature appears to be a property intrinsic to the adsorbed species and not caused by the dissolution of the macrocycle in the electrolyte. Support for this view is provided by the fact that no spectra could be detected in earlier studies for the same macrocycle adsorbed on high-area carbon Vulcan XC-72 at room temperature in the form of a dry powder. In an effort to overcome these difficulties, without compromising the in situ character of the measurements, the cell was slowly cooled down to cryogenic temperatures under potential control before any further Mdssbauer measurements were attempted. As soon as the electrolyte resistance reached a high enough value, the potentiostat was turned off and the leads were disconnected from the cell. From a fundamental viewpoint, charged interfaces of the type examined in this work may be regarded as ideally polarizable, and therefore the applied potential in the absence of faradaic processes is expected to be preserved even if the external source is removed. In fact, experiments of this kind yielded for the assembled in situ cell drifts of less than 10 mV over periods of time exceediing 1 h at room temperature for electrodes polarized at potentials negative to the redox peak. Such low rates of discharge are not surprising since the electrode area is very large (ea. 50 m2) and the volume of electrolyte (and thus the total amount of oxygen and other impurities is the solution) is very small (
