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Future efforts involving the technique presented here will involve improvement of resolution using a smaller slit, improvement in alignment between slit and electrode, and the use of diffracted light to examine events close to the electrode. In its present form, the technique allows direct observation of concentrations of electroactive materials as a function of both time and distance with unprecedented sensitivity and selectivity. The long pathlength, selectivity gained from wavelength selection, and spatial resolution will allow much greater definition of the solution adjacent to an electrode surface. In addition to observing fundamental aspects of electrochemical mass transport, the technique will allow improved characterization of homogeneous reactions accompanying charge transfer.
(1) Winograd, N.; Kuwana, T. "Spectroektrat optically Transparent Electrodes", in "Electroanalytical Chemistry", Vol. 7, Bard, A. J., Ed.: Marcel Dekker: New York, 1974. (2) Kuwana, T. Ber. Bunsenges. Phys. Chem. 1973, 77, 858. (3) Heineman, W. R. Anal. Chem. 1978, 50, 390A. (4) Winograd, N.; Kuwana, T. J . Nectroanal. Chem. 1979, 23,333. (5) McCreery, R. L.; Pruiksma, R.: Fagan, R. Anal. Chem. 1979, 57, 749. (6) Muller, R. H. Adv. Electrochem. Electrochem, Eng. 1973, 9, 281. (7) Srinivasan, V. S. Ref. 6, p 369. (8) Obrlen, R. N.: Dieken, P. J . Electroanal. Chem. 1973, 42,25. (9) Obrien, R. N., et al. Can. J . Chem. 1965, 43,3304. (10) Obrien, R. N.; Rosenfield, C. J. Phys. Chem. 1963, 67, 643. (11) Michaelis, L.; Hill, E. S. J. Am. Chem. SOC. 1933, 55, 1481. (12) Feldberg, S. W. In "Electroat?alytical Chemistry", Bard, A. J., Ed.: Marcel Dekker: New York. 1970: Vol. 3. (13) Adam, R. N. "Electrochemisw at Solid Electrodes": Marcel Dekker: New York, 1969; p 254.
ACKNOWLEDGMENT The authors thank Robert Fagan for helpful discussion.
RECEIVED for review June 26, 1979. Accepted September 4, 1979. This work was supported by grants from NIMH (28412) and NSF (CHE-7828068).
LITERATURE CITED
Rotating-Ring-Disc Analysis of Iron Tetra(N-methylpyridy1)Porphyrin in Electrocatalysis of Oxygen Armand Bettelheim and Theodore Kuwana' Department of Chemistry, The Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210
When cyclic voltammetry and the rotating ring-disc electrode (RRDE) were used, the water soluble iron tetra( N-methylpyridyi)porphyrin (FeTMPyP) was found to catalyze O2 eiectroreduction by ca. 400 mV and produce H202,at a yield of 95%, as the lnltlal product. Using the RRDE method, the rate of removal of Fe(I1)TMPyP by O2 was estimated to be in the 107-108 M-l*s-' region. A mechanism invoivlng an "iron(I1I)auperoxideion" intermediate seems to be consistent with the above experlmentai results and wlth other data reported in the literature.
In a previous publication ( I ) , it was proposed that the electrocatalytic reduction of 02 occurred through an ec catalytic regeneration mechanism: Fe(II1)TMPyP e- = Fe(I1)TMPyP (1)
+
Fe(I1)TMPyP
+ 1/202 + H+ = Fe(II1)TMPyP +
(2) where the water soluble Fe(II1) tetra(N-methylpyridy1)porphyrin cation (abbreviated as Fe(II1)TMPyP) was reduced a t the electrode to Fe(I1)TMPyP and reacted rapidly in a homogeneous reaction with dissolved oxygen. The stoichiometry of the reaction as deduced from the analysis of the cyclic voltammetric waves indicated hydrogen peroxide as the major product. In this paper, the rotating-ring-disc electrode was employed to assess the production of hydrogen peroxide as indicated by reactions 1 and 2, and to evaluate, if possible, the rate parameters for reaction 2. Also, additional cyclic voltammetric results on the O2 concentration dependence and the effect of added catalase are presented to further characterize the iron porphyrin catalytic reduction of 02. EXPERIMENTAL Iron I11 tetra(N-methylpyridy1)porphyrinas the sulfate salt was prepared in our (henceforth abbreviated as F~(III)TMPYP+~) '/2H202
0003-2700/79/0351-2257$01 .OO/O
laboratory (2). All other chemicals were analytical grade. Solutions were prepared with doubly distilled water. The concentration of the iron-porphyrin complexes in the solutions was determined by measuring the optical adsorption at the Soret band (at pH 1:, A = 398 nm, cm = 1 X lo5 M-' cm-') (3)using a Cary 15 spectrophotometer Nitrogen 99.9% pure was used for deaeration of all solutions. Solutions were saturated with oxygen and with mixtures of oxygen and nitrogen using Matheson's analyzed gases. The stock solutions were stored refrigerated (5 "C) and in the dark. Glassy carbon electrodes were obtained from Tokai Ltd. (Japan), Sigma (Germany), and Atomergic Chemetals (France). The electrodes were polished to a bright surface using alumina powder (final polish using 0.05-wm particle size, Buehler). The electrodes were then washed with 0.1 N H#04 and distilled water to remove any alumina. Elemental aluminum was found to be absent from the surface within limits of Auger and ESCA analysis. The rotating-ring-discelectrode (RRDE, Pine Instrument Co.) consisted of a glassy carbon disc surrounded by a platinum ring. The area of the disc was 0.48 cm2. The collection coefficient was determined to be 0.131 using the ferrocyanideferricyanide redox couple. Potentiostatic experiments were performed with a double potentiostat (RDE-3, Pine Instrument Co.). The RRDE was pretreated mechanically and electrochemically. The electrode was polished with alumina powder by rotating it on fine polishing cloth which was mounted on a glass disc. It was then washed with 0.1 N H2S04and doubly distilled water and introduced in a 0.1 N KC1 nitrogen saturated solution. The potentials of the disc and the ring electrodes were then cycled in the range 4 . 6 to +0.2 V, and 0 to +LO V, respectively, until a minimum residual current 5 density was obtained ( ~ 0 . bA.cm-2). Cyclic voltammetry was performed using a conventional three-electrode potentiostat. All reported potentials are with respect to a AgjAgC1 (sat'r.) reference electrode. Experiments were conducted at room temperature (20 i 1 "C).
RESULTS AND DISCUSSION Cyclic Voltammetry. As previously reported ( I ) , the presence of the water soluble porphyrin, Fe(III)TMPyP, greatly enhanced the rate of O2 reduction at a highly polished 0 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
,-I
+0.6
, +0.4
M.2
0
-0.2
E, volt.
, -44
,
~, -Q6
, -0.0
,
Figure 1. Cyclic voltammograms at a scan rate of 50 mV s-’ using glassy carbon as the working electrode ( A = 0.66 cm2)for: (a)a N2 saturated 0.1 N N2S04solution; (b) an air saturated 0.1 N H2S04solution; (c)a nitrogen saturated 0.1 N H2S0, solution containing 2.7 X M FeTMPyP; (d) same solution as for curve c but air saturated
% 0,
60 I
Figure 2. The cathodic peak current ( A = 0.13 cm2)as function of M FeTMPyP in phosphate buffer oxygen concentration for: (a) at pH 6.8, (b) the same solution (volume: 20 mL) in the presence of 2 mg of catalase glassy carbon electrode. (Similar results can be obtained at Pt, Au, or doped tin oxide electrodes.) Figure 1 shows typical current-potential (i-E) curves as obtained by cyclic voltammetry (CV) at glassy carbon: (1)Trace a is an electrode in 0.1 N H2S04aqueous solution after thoroughly purging and saturating with N,;(2) Trace b is for air-saturated solution where 0, concentration is ca. 2 x M; (3) Trace c is for 2.7 X 10“ M Fe(II1)TMPyP in absence of 0,; and (4) Trace d is the same as conditions for trace c except after air-saturation (20 “C). As may be seen from comparison of curves b, c, and d, the current is markedly enhanced and the overpotential is reduced by ca. 400 mV when 0, is reduced in the presence of the iron porphyrin. Consistent with the ec mechanism, reactions 1and 2, the potential of the catalytic 0, reduction is determined by the redox potential of the Fe(III/II)TMPyP couple (see traces c and d of Figure 1). (Analysis of the potential vs. pH of the reaction will be discussed in a separate publication.) The peak current, i,, was linearly proportional to the square root of the scan-rate, V I z ,for both FeTMPyP and 0, in the presence of FeTMPyP. The analysis of the i, vs. VI2was in agreement with previous results which were consistent with 0, being reduced by a two-electron overall process to hydrogen peroxide. The i, was found to be linearly dependent on the oxygen concentration as shown in Figure 2, trace a, for a M FeTMPyP solution in phosphate buffer a t pH 6.8. The choice of pH was dictated by our desires to test the effect of the catalase, a peroxide decomposing catalyst, on the height of the i,. The i, does increase slightly when catalase is present (trace b, Figure 2) consistent with a mechanism whereby more 0, becomes available due to the increased rate of HzOz dismutation:
(3) Rotating-Ring-Disc Electrode. T o further assess the proposed reactions 1 and 2 , rotating-ring-disc electrode experiments were conducted. In Figure 3, curves a, b, and c, are the disc currents, iD, a t different rotation speeds for air-saturated solutions of 10” M FeTMPyP at pH 9.0 (0.1 M KC1).
Figure 3. Polarization c w e s for the disc ( a x )and ri (a‘
