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peaks in this low-field region could further result from the interaction of the spin with the nuclear moment of 0'7. The 0'' species has a nuclear spin of 5 / 2 which 1) states. could further split the S states into 2S(S 017-018, and 018-016 absorptions are These 016-017, not seen at the higher fields, corresponding to transitions within higher rotational states, because they are masked by the stronger 0'6-016 absorptions. These new peaks help explain the absorption at 4100 gauss, which did not fit the theoretical treatment for oxygen.6 Further work is going on to resolve the contributions of the various proposed species to the spectra.
+
35
30
1
0
t
Acknowledgment. The author wishes to thank Dr.
L. N. Mulay for many helpful discussions, the Paint Research Institute for a predoctoral fellowship, and the USAEC [Contract AT(30-1)-28511 for partial support of this research. MATERIALS RESEARCHLABORATORY PENXSYLVANIA STATEUNIVERSITY UNIVERSITY P.ARK,
PENNSYLV.4NI.A
L. K. KEYS*
16801
RECEIVED SEPTEMBER 9, 1966 ( 8 ) Nuclear Materials and Propulsion Operation, General Electric Co., Cincinnati, Ohio 45215. 0
0.08
0.04 w-
The Role of Hydrogen Peroxide i n the Reduction of Oxygen at Platinum Electrodes
Sir:
I n electrochemical reduction of oxygen from aqueous solutions, hydrogen peroxide has been reported to be either an intermediate or a product of the reHowever, other workers find that no hydrogen peroxide is produced in oxygen reduction and hence conclude that it is not an intermediate.4 I n this laboratory, a criterion has been devised which enables us to determine whether H20z is intermediate in a reaction path or a product in a reaction proceeding along a path parallel to that in which H2O is produced. This criterion utilizes the method of the rotating-disk electrode with a concentric ring. If oxygen is, at least partially, reduced a t the disk electrode to HzO?, a part of the produced H202 will diffuse to the ring electrode. When the potential of the ring electrode is kept at 1.4 v (us. h.e.), all of the hydrogen peroxide which diffuses to the electrode will be oxidized, and hence analyzed. If I d and I, are the measured currents a t the disk and the ring electrode, respectively, and if 5 is the ratio of the current at the disk for the reaction path which does not involve the formation of hydrogen peroxide to the current at the
0.12
0.16
1/2[*ecl/2].
Figure 1. The plot of the ratio of the current a t the disk to the current a t the ring electrode vs. the rate of disk rotation. Potentials of the disk electrode (21s.h.e.) are given in the figure. Solution is 11%'H2SOa.
disk due only to the reduction of O2 to hydrogen peroxide, then, using the equation
+
_ - _Z_ 1 Ir - N Id
+
K(X w'/2
2)
(1)
from I d / I r us. w-"' plots, 2 can be determined. I n this equation, w is the rate of disk rotation, N is a geometric constant (-0.35), and K is proportional to the rate of further reduction of H202 at the disk electrode. For oxygen reduction a t oxide-free platinum electrodes in sulfuric acid solution, the I d / I r plot consists of a series of lines parallel to the w-'" axis (Figure 1). Each line corresponds to a given potential of the disk electrode and has an intercept with the I d / I , axis greater than 1/N. Hence, two parallel paths for oxygen reduction are present. Hydrogen peroxide is produced in a path parallel to the main path of oxygen reduction (1) L. Muller and L. N. Nekrasov, Electrochim. Acta, 9, 1015 (1964). (2) A. Koaawa, J . EZectroanaZ. Chem., 8 , 20 (1964). (3) G. Bianchi and T. Xlussini, Electrochim. Acta. 10, 445 (1965). (4) J. J. Lingane, J. Electroanal. Chem., 2, 296 (1961).
Volume 70, NUmbeT 11 A-ooember 1966
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which does not involve HzOz as an intermediate. Since the slope of the lines is zero, hydrogen peroxide is not further reduced to water with any rate comparable to that by which it is produced ( K = 0). A complete analysis reveals, moreover, that the reduction of O2 to HzOz is due to a side reaction arising from the adsorption at the platinum surface of residual impurities of the solution. Thus, hydrogen peroxide i s not observed when the residual impurities of the solution are removed. I n alkaline solution, the Id/Ir plot reveals that two paths of oxygen reduction occur simultaneously at comparable rates (Figure 2). One of these paths has hydrogen peroxide (HOz-) formed as an intermediate, which partially reduces further at the disk electrode. A detailed account of this work and of the work on other noble metal electrodes will be given later.
60
60
40
r 9 30
20
io
I
I
C 0
0.04
0.08 w
1 0.12
I 0.16
- 1/2[sec1/z].
Figure 2. The plot of the ratio of the current a t the disk to the current at the ring electrode us. the rate of disk rotation. Potentials of the disk electrode (us. h.e.) are given in the figure. Solution is 1 N KOH.
The Journal of Physical Chemistry
Acknowledgments. We thank our sponsor, the U. S. Army Electronics Material Laboratory, Fort Monmouth, K. J. (Contract No. DA-36-039-Sc88921), for the support of this work. Thanks are also due to Dr. H. F. Hunger and Mr. J. Wynne for their advice and discussion. THEELECTROCHEMISTRY LABORATORY A. DAMJANOVIC THEUNIVERSITY OF PENNSYLVANIA M. A. GENSHAW J. O'iM.BOCKRIS PHILADELPHIA, PENNSYLVANIA 19104 RECEIVEDAUGUST22, 1966
