Promoted dissociative adsorption of hydrogen peroxide and persulfate ions and electrochemical oscillations. Caused by a catalytic effect of adsorbed bromine

Article abstract of DOI:10.1149/1.1526112

Absorbed bromine on Pt and Au electrodes acts as a catalyst for dissociative adsorption of H₂O₂ and S₂O₈^2-, similar to adsorbed OH (and adsorbed iodine). This fact is revealed by analyses of newly found negative differential resistances and electrochemical oscillations, appearing in a potential region where the surface coverage of adsorbed bromine (θ_Br) decreases steeply. The present result further supports the generality of a catalytic effect of adsorbed electronegative species such as OH, Br, and I on the dissociative adsorption of peroxides such as H₂O₂ and S₂O₈^2-.

Full text of DOI:10.1149/1.1526112

Journal of The Electrochemical Society, 150 ͑1͒ E47-E51 ͑2003͒
E47
0013-4651/2003/150͑1͒/E47/5/$7.00 © The Electrochemical Society, Inc.
Promoted Dissociative Adsorption of Hydrogen Peroxide
and Persulfate Ions and Electrochemical Oscillations
Caused by a Catalytic Effect of Adsorbed Bromine
Shuji Nakanishi, Sho-ichiro Sakai, Michiru Hatou, Kazuhiro Fukami,
,z
*
and Yoshihiro Nakato
Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka,
Osaka 560-8531, Japan
Adsorbed bromine on Pt and Au electrodes acts as a catalyst for dissociative adsorption of H2O2 and S2O28Ϫ , similar to adsorbed
OH ͑and adsorbed iodine͒. This fact is revealed by analyses of newly found negative differential resistances and electrochemical
oscillations, appearing in a potential region where the surface coverage of adsorbed bromine (␪Br) decreases steeply. The present
result further supports the generality of a catalytic effect of adsorbed electronegative species such as OH, Br, and I on the
dissociative adsorption of peroxides such as H2O2 and S2O82Ϫ
.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1526112͔ All rights reserved.
Manuscript submitted March 14, 2002; revised manuscript received July 11, 2002. Available electronically December 3, 2002.
nism could be extended to the dissociative adsorption of S2O28Ϫ . In
the present work, we have studied the effect of adsorbed bromine to
obtain further confirmation and generalization of this mechanism.
Electrochemical oscillations in general appear in potential re-
gions of negative differential resistances ͑NDRs͒ of current density
͑j͒ vs. potential ͑U͒ curves,1,2 though the NDRs are sometimes hid-
den in the j-U curves by other electrochemical processes.2,3 The
oscillations that appear in the regions of nonhidden NDRs are called
NDR oscillators, whereas those appearing in the regions of hidden
NDRs ͑i.e., in the regions of apparently positive differential resis-
tances͒ are called HNDR oscillators.2,3 The important difference is
that the NDR oscillator appears only as a current oscillation,
whereas the HNDR oscillator appears as both current and potential
oscillations. There is another type of oscillation that appears in the
regions of normal positive differential resistances, called CNDR os-
cillators, which appears by coupling with NDRs in other potential
regions.4
The NDRs thus play a key role in the appearance of electro-
chemical oscillations. They arise from various factors2 such as the
formation of inactive layers at the electrode surface, the disappear-
ance of surface catalysts, and the electrostatic repulsion between
ionic electroactive species and polarized electrodes. Of these, the
NDRs arising from surface catalysts are especially interesting be-
cause their study can serve for exploration of new electrocatalytic
processes.
Experimental
Single-crystal Pt͑111͒, ͑100͒, and ͑110͒ electrodes with atomi-
cally flat surfaces were prepared by the method of Clavilier et al.9
The details of the preparation method were described elsewhere.5
Polycrystalline Au ͑99.99% in purity͒ disks of about 6.0 mm diam
were also used as the working electrode. The poly-Au disks were
polished with 0.06 ␮m alumina slurry and immersed in hot
HNO3 ϩ H2O2 for 10 min to remove surface contamination.
Current density ͑j͒ vs. potential ͑U͒ curves were measured with a
potentiogalvanostat ͑Nikko-Keisoku NPGS-301͒ and a potential
programmer ͑Nikko-Keisoku NPS-2͒, using a Pt plate ͑10 ϫ 10
mm͒ as the counter electrode and a saturated calomel electrode
͑SCE͒ as the reference electrode. The data were recorded with a
data-storing system ͑instruNET, GW Instruments͒ with a sampling
frequency of 1 kHz. Electrolyte solutions were prepared using spe-
cial grade chemicals and pure water, the latter of which was ob-
tained by purification of deionized water with a Milli-Q water puri-
fication system. The electrolyte solutions were kept stagnant during
measurements. The ohmic drops in the solution between the work-
ing and the reference electrodes were not corrected in the present
work.
We reported previously5 that adsorbed OH on Pt electrodes pro-
moted the dissociative adsorption of H2O2 , causing the appearance
of an NDR and a current oscillation in the H2O2-reduction system.
The catalytic effect of adsorbed OH was explained5 in terms of
electrical positive polarization of surface Pt atoms lying near the
adsorbed OH, caused by a difference in the electronegativity be-
tween metal atoms and OH, the polarization accelerating the adsorp-
tion of negatively polarized O atoms of H2O2 . It was also shown5
that the extent of the catalytic effect strongly depended on the dif-
ference in the crystal-face ͑or the atomic-level͒ structure of the Pt
surface.
This mechanism for the catalytic effect of adsorbed OH is of
much interest, indicating a new type of surface process. It is also
interesting in that it leads to a unique feature of the appearance of
two stationary states ͑i.e., a high-current state with ␪OH Х 1 and a
low-current state with small ␪OH) at the same potential.6 These
points imply that the mechanism deserves further confirmation. If
the mechanism is true, a similar effect to adsorbed OH should be
observed for other adsorbed electronegative species such as ad-
sorbed I, Br, and Cl. Actually, we observed7 that adsorbed iodine
showed a similar effect. Moreover, it was shown8 that the mecha-
Results
Figures 1a and b show, for reference, reported j vs. U curves for
the H2O2 reduction on an atomically flat single-crystal Pt͑111͒ elec-
trode in relatively low and high H2O2 concentrations ͑0.2 M H2O2
ϩ 0.3 M H2SO4 and 1.0 M H2O2 ϩ 0.3 M H2SO4), respectively,
under controlled-potential conditions. The cathodic current in the
potential region from about ϩ0.55 to Ϫ0.08 V is due to the H2O2
reduction, which starts at about ϩ0.80 V. Hydrogen evolution starts
at about Ϫ0.1 V. The H2O2 reduction current in 0.2 M H2O2
ϩ 0.3 M H2SO4 shows two NDRs in regions from ϩ0.02 to Ϫ0.08
V and from ϩ0.25 to ϩ0.15 V, designated as NDR-H and NDR-OH
in Fig. 1a. The origins of the NDRs can be explained as follows. The
H2O2 reduction is initiated by dissociative adsorption of H2O2
2Pt ϩ H2O2 → 2Pt-OH
͓1͔
followed by electrochemical reduction of the resultant Pt-OH
* Electrochemical Society Active Member.
z E-mail: nakato@chem.es.osaka-u.ac.jp
Pt-OH ϩ Hϩ ϩ eϪ → Pt ϩ H2O
͓2͔

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Journal of The Electrochemical Society, 150 ͑1͒ E47-E51 ͑2003͒
Figure 1. Effect of BrϪ addition on j-U curves for H2O2 reduction on
Pt͑111͒ under controlled-potential conditions. Electrolyte: ͑a͒ 0.2 M H2O2
ϩ 0.3 M H2SO4 ; ͑b͒ 1.0 M H2O2 ϩ 0.3 M H2SO4 ; ͑c͒ 0.2 M H2O2
ϩ 0.3 M H2SO4 ϩ 2 ␮M KBr; and ͑d͒ 0.7 M H2O2 ϩ 0.3 M H2SO4
ϩ 2 ␮M KBr. Scan rate: ͑a, c͒ 100 and ͑b, d͒ 10 mV/s.
Figure 2. The same as Fig. 1, except that Pt͑100͒ is used and the H2O2
concentration in ͑b͒ is 0.7 M.
accordance with the appearance of the NDR, a new oscillation,
named oscillation F, appears in a solution of high H2O2 concentra-
tion ͑Fig. 1d͒.
A similar result was obtained for Pt͑100͒ electrodes, as shown in
Fig. 2. The NDR-OH is not so prominent for Pt͑100͒ ͑Fig. 2a͒,5 and
oscillation E appears only in some experiments ͑Fig. 2b͒. When BrϪ
was added to the solutions, the NDR-Br and oscillation F appear
͑Fig. 2c and d͒, similar to the case of Pt͑111͒. It may be noted that
another oscillation, named oscillation G in Fig. 2d, is observed for
Pt͑100͒ in a region of ϩ0.52 to ϩ0.46 V, though the origin is un-
known at present.
The potential-independent H2O2-reduction current in the ϩ0.55 to
Ϫ0.08 V region indicates that Reaction 1 is the rate-determining
step ͑rds͒.5,6,10 The NDR-H arises from the suppression of Reaction
1
by the formation of underpotential-deposited hydrogen
͑upd-H͒.10,11 The NDR-OH arises from a decrease in the coverage
(␪OH) of adsorbed OH ͑Pt-OH͒, which acts as an autocatalyst for
Reaction 1, with a negative potential shift.5-7 Namely, the rate con-
stant, k1 , for Reaction 1 is expressed as follows
Figure 3 shows results of similar experiments for Pt͑110͒ elec-
trodes. Neither NDR-OH nor oscillation E appears for Pt͑110͒, con-
trary to the cases of Pt͑111͒ and Pt͑100͒. Furthermore, neither
NDR-Br nor oscillation F appears when BrϪ was added.
Figure 4a shows a j-U curve for S2O28Ϫ reduction on a polycrys-
talline Au-disk electrode under a controlled-potential condition. Two
NDRs, NDR-OH and NDR-X ͑the origin of NDR-X is unknown͒,
are observed in 0.5 M Na2S2O8 ϩ 0.5 M HClO4 . The reduction of
S2O28Ϫ is reported12,13 to proceed via the following mechanism in
the potential region of NDR-OH
k1 ϭ k10 ϩ ␬␪
͓3͔
OH
where k10 is a normal rate constant, and ␬ is a proportional constant.
A negative shift in U leads to a decrease in ␪OH , which in turn leads
to a decrease in k1 ͑Eq. 3͒ and hence a decrease in j. Thus, an NDR
͑NDR-OH͒ appears. In a high H2O2 concentration ͑1.0 M H2O2
ϩ 0.3 M H2SO4), two oscillations, called oscillation A and E, ap-
pear in the regions of NDR-H and NDR-OH ͑Fig. 1b͒. Slight shifts
in potential between the NDRs and oscillations in Fig. 1a and b can
be attributed to ohmic drops in the solution.
S2O28Ϫ → 2SOϪ4
͓4͔
͓5͔
ad
Figure 1c and d shows j-U curves when 2 ␮M KBr was added to
0.2 M H2O2 ϩ 0.3 M H2SO4 and 0.7 M H2O2 ϩ 0.3 M H2SO4 ,
respectively. A new NDR, named NDR-Br in Fig. 1c, appears. In
2SOϪ4 ϩ 2eϪ → 2SO24Ϫ
ad
ad

Journal of The Electrochemical Society, 150 ͑1͒ E47-E51 ͑2003͒
E49
Figure 3. The same as Fig. 2, except that Pt͑110͒ is used.
Thus, the NDR-OH arises8 from a catalytic effect of adsorbed OH
on the dissociative adsorption of S2O28 ͑Reaction 4͒, similar to the
case of the H2O2-reduction system. Under controlled-current condi-
tions, a potential oscillation ͑named oscillation ␥͒ appears in the
potential region of NDR-OH8 ͑Fig. 4c͒.
When BrϪ ions were added to the solution, a new NDR, named
NDR-Br, appears between NDR-OH and NDR-X ͑Fig. 4b͒, with the
NDR-X shifted to more negative potential. In accordance with the
appearance of the NDR-Br, a new oscillation, named oscillation ␨, is
observed under controlled-current conditions ͑Fig. 4d and e͒. Note
that the appearances of NDR-Br and oscillation ␨ were well repro-
duced, but the shape of NDR-Br ͑or the shape of the j-U curve in a
region below Ϫ0.1 V͒ changed from experiment to experiment, sug-
gesting that the surface processes in this region are complicated. The
wave shape of oscillation ␨ was also very complex, strongly depend-
ing on the regulated current density. More experiments are necessary
to obtain a definite conclusion. In the present paper, we focus on the
fact that NDR-Br and oscillation ␨ appear upon addition of BrϪ.
Their detailed behavior and the relation to NDR-X will be discussed
elsewhere.
Figure 4. Effect of BrϪ addition on j-U curves for S2O28Ϫ reduction on
poly-Au under ͑a, b͒ controlled-potential, and ͑c-e͒ controlled-current con-
ditions. Electrolyte: ͑a, c͒ 0.5 M Na2S2O8 ϩ 0.5 M HClO4 ; ͑b, e͒ 0.5 M
Na2S2O8 ϩ 0.5 M HClO4 ϩ 15 ␮M KBr; and ͑d͒ 0.5 M Na2S2O8 ϩ 0.5 M
HClO4 ϩ 12.5 ␮M KBr. Scan rate: ͑a, b͒ 100 mV/s and ͑c, d, e͒ 1 mA/s.
changes largely. For example, Gasteiger et al. reported14 that ad-
sorbed Br on Pt͑111͒ in 0.1 M HClO4 was desorbed in a potential
region from ϩ0.10 to Ϫ0.12 V. Mrozek and Weaver reported,15 by
surface-enhanced Raman spectroscopy ͑SERS͒, that adsorbed Br on
Pt was desorbed in a region more negative than Ϫ0.17 V. They also
reported16 that adsorbed Br on Au was desorbed in a region of
Ϫ0.17 to Ϫ0.57 V. We can thus conclude that NDR-Br and oscilla-
tions F and ␨ arise from adsorbed Br.
The appearance of NDR-Br can be explained quite in the same
way as that of NDR-OH.5,7,8 If adsorbed Br acts as a catalyst for the
dissociative adsorption of H2O2 ͑Reaction 1͒ and S2O28Ϫ ͑Reaction
4͒, the rate constants for these reactions can be expressed as
Discussion
The experimental results show that the addition of BrϪ ions to
the solution causes the appearances of NDR-Br ͑Fig. 1c, 2c, and 4b͒
and the corresponding oscillations ͑oscillations F and ␨, Fig. 1d, 2d,
4d, and 4e͒. The potential regions of NDR-Br for Pt and Au elec-
trodes ͑Fig. 1c, 2c, and 4b͒ are in good agreement with the reported
potential regions in which the surface coverage (␪Br) of adsorbed Br

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Journal of The Electrochemical Society, 150 ͑1͒ E47-E51 ͑2003͒
potentials. This means that a lot of empty sites (1 Ϫ ␪Br) are left,
which allows the appearance of NDR-OH and oscillations E or ␥,
even in the presence of BrϪ ͑Fig. 1, 2, and 4͒.
The catalytic effect of adsorbed Br ͑non-zero ␬ and ␬ in Eq. 6
Ј
Љ
and 7͒ can also be explained in the same way as that of NDR-OH,5
because Br is an electronegative element and adsorbed Br can in-
duce positive polarization of surface Pt or Au atoms in the neigh-
borhood, similar to adsorbed OH, as schematically shown in Fig.
5A. The NDR-Br and oscillation F appear most prominently for
Pt͑111͒ ͑Fig. 1-3͒, also similarly to NDR-OH and oscillation E.5
This crystal-face dependence can be attributed5 to the difference in
the number of positively polarized Pt ͑or Au͒ atoms near the ad-
sorbed Br among the ͑111͒, ͑100͒, and ͑110͒ faces ͑Fig. 5B͒.
Both oscillation F in the H2O2 system and oscillation ␨ in the
S2O28Ϫ system appear from a common NDR ͑NDR-Br͒, as men-
tioned earlier, but their appearance mechanisms are quite different.
The appearance mechanism for oscillation F can be estimated to be
the same as that for oscillation E,5 because both oscillations E and F
appear in the region of the j-U curve of the same characteristics
͑Fig. 1͒. The only difference is that adsorbed OH in oscillation E5 is
replaced by adsorbed Br in oscillation F. Thus, oscillation F can be
regarded as an NDR oscillator with adsorbed Br as the NDR-
inducing species and the H2O2 diffusion as a slow process.
Similarly, the appearance mechanism for oscillation ␨ can be
estimated to be the same as that for oscillation ␥8 because these
oscillations are located in the region of the j-U curve of the same
characteristics ͑Fig. 4͒. Previous work revealed8 that oscillation ␥
was classified as an HNDR oscillator, for which the NDR-OH is
hidden by an increase in j ͑or empty surface sites͒ by desorption of
adsorbed SO24Ϫ ͑as a product of the S2O28Ϫ reduction͒ with a nega-
tive shift in U. The appearance mechanism for oscillation ␨ can thus
be explained in the same way, by replacing adsorbed OH in oscilla-
tion ␥ by adsorbed Br in oscillation ␨.
Figure 5. ͑A͒ Schematic illustration of a catalytic effect of adsorbed Br on
the dissociative adsorption of H2O2 on Pt. ͑B͒ The difference in the number
of Pt pairs on which accelerated adsorption of H2O2 can occur among
Pt͑111͒, ͑100͒, and ͑110͒.
k1 ϭ k10 ϩ ␬ ␪
͓6͔
͓7͔
Ј
Br
Br
k4 ϭ k40 ϩ ␬ ␪
Љ
where k10 and k40 are normal rate constants for Reactions 1 and 4,
and ␬ and ␬ are proportional constants. Thus, a decrease in ␪Br by
Ј
Љ
Finally, we summarize a variety of electrochemical oscillations
observed for the H2O2 and S2O82Ϫ reduction systems in Table I. For
oscillations whose mechanisms have not yet been fully clarified, the
characteristics such as the NDR-inducing species, the slow pro-
cesses, and the mechanical classification are indicated in parenthe-
a negative shift in U leads to a decrease in k1 or k4 and hence a
decrease in j, resulting in an NDR ͑NDR-Br͒. The BrϪ concentration
is very low ͑2 or 15 ␮M͒ in the present work and thus ␪ is con-
Br
siderably smaller than unity, even for the saturated ␪ at positive
Br
Table I. A variety of oscillations appearing in the H2O2 reduction on Pt and the S2O28À reduction on Pt or Au, together with their characteristics
and classification.
Name of oscillationa
NDR-inducing
species
Slow process
S2O28Ϫ system
Classification
H2O2 system
Adsorbed-OH
Oscillation E5
Enhanced oscillation E7,b
Oscillation C10,17
¯
¯
NDR ͑class III͒
NDR ͑class III͒
HNDR ͑class IV.2͒
H2O2 diffusion
H2O2 diffusion
Desorption of adsorbed
Oscillation ␥8
ClϪ, BrϪ, or SO24Ϫ
Adsorbed-Br
upd-H
Oscillation Fc
¯
NDR ͑class III͒
HNDR ͑class IV.2͒
(H2O2 diffusion͒
c
͑Desorption of adsorbed
¯
Oscillation ␨
SO24Ϫ
)
Oscillation A18-20
Oscillation B4
Oscillation ␣8
NDR ͑class III͒
CNDR ͑class V͒
H2O2 diffusion
H2O2 diffusion
¯
¯
Adsorbed OH
and upd-H
Desorption of
adsorbed BrϪ
Oscillation D10,17
HNDR ͑class IV.4͒
͑Unknown͒
¯
¯
Oscillation ␤8
Oscillation ␦8
HNDRd ͑class IV.3͒
HNDRd ͑class IV.2͒
(S2O28 diffusion͒
͑Desorption of adsorbed
SO24Ϫ
)
a Numerals attached on the names of oscillations refer to the reference numbers.
b Enhanced by a catalytic effect of adsorbed iodine.
c The present work.
d Determined by impedance spectroscopy ͑an unpublished result͒.

Journal of The Electrochemical Society, 150 ͑1͒ E47-E51 ͑2003͒
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473, 156 ͑1999͒.
Osaka University assisted in meeting the publication costs of this article.
11. Y. Mukouyama, S. Nakanishi, and Y. Nakato, Bull. Chem. Soc. Jpn., 72, 2573
͑1999͒.
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