Electrochemical Oxidation of Mn - unist

Electrochemical Oxidation of Mn 2¿ on Boron-Doped
Diamond Electrodes with Bi 3¿ Used as an Electron
Transfer Mediator
Joowook Lee, a,b Yasuaki Einaga, c Akira Fujishima, d, * and Su-Moon Park a, * ,z
a Department of Chemistry and Center for Integrated Molecular Systems and b School of Environmental
Science and Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea
c Department of Chemistry, Keio University, Yokohama 223-8522, Japan
d Department of Applied Chemistry, School of Engineering, University of Tokyo, Tokyo 113-8656, Japan
Electrochemical oxidation of Mn 2� with and without the presence of Bi 3� was studied using voltammetric and in situ spectroelectrochemical
techniques at boron-doped diamond �BDD� electrodes in 1.0 M HClO 4 . Electrochemical oxidation of only Mn 2�
resulted in the formation of mostly MnO 2 with MnO 4 � produced as a minor product. The MnO2 film formed on the electrode
surface, which is an inevitable part of Mn 2� oxidation, shows a blocking effect on the formation of MnO 4 � , and reduces the
overall current efficiency of MnO 4 � production. Higher Mn 2� concentrations result in less MnO4 � production due to the formation
of more MnO 2 . The addition of Bi 3� increased the current efficiency of MnO 4 � production. The Bi 3� is oxidized to Bi�V�, which
acts as an electrocatalyst in MnO 4 � production. The Bi�V� oxidizes MnO2 , formed on the electrode surface, to MnO 4 � . This
increases the production of MnO 4 � by removing the blocking film to provide an active electrode �bare BDD� surface, which is
available for further Mn 2� oxidation.
© 2004 The Electrochemical Society. �DOI: 10.1149/1.1765679� All rights reserved.
Manuscript submitted August 25, 2003; revised manuscript received January 27, 2004. Available electronically June 25, 2004.
Boron-doped diamond �BDD� electrodes have gained interest
from electrochemists in the past decade due to their unique electrochemical
properties. BDD electrodes display a very low capacitive
background current, 1 wide potential window in aqueous solution, 2
high corrosion resistance, 3 and high mechanical strength. 4 These
properties have allowed many investigators to use the BDD electrodes
in a wide variety of electrochemical applications, such as
electrochemical analysis, 5 electrosynthesis, 6 metal recovery, 7 and
oxidation of organic compounds. 8 Its wide potential window and
chemical inertness offer especially appealing characteristics for
studying electrochemically generated oxidants and/or reductants.
The wide potential window of BDD electrodes allows an oxidant to
be studied in aqueous solution without interference from oxygen
evolution. Electrochemical generation of strong oxidants such as
Ce�III�, 9 2� 10 11 12
peroxodisulfate (S2O8 ), Ag�II�, and ferrate�VI� has
already been studied at BDD electrodes.
The electrochemical oxidation of manganese�II� is an important
subject from practical and fundamental viewpoints. Its oxidation in
acidic solutions can produce various high valence products such as
Mn�III�, Mn�IV�, and Mn�VII�. 13 Mn�III� and Mn�VII� are strong
oxidants and have been used as titrants in analytical chemistry, 14
oxidants in indirect synthesis of organic compounds, 15 and also as
an oxidant for the destruction of organic compounds. 16 The Mn�IV�
species is available mainly as electrolytic manganese dioxide
�EMD� and has been studied extensively due to its application as
cathode material for high performance primary batteries. 17
Our current study focuses on the oxidation of Mn2� to perman-
�
ganate (MnO4 ) for use as a strong oxidant in mediated electrochemical
oxidation. We have been studying mediated electrochemical
oxidation of carbonaceous compounds in efforts to destroy
organic pollutants in water. 12,18 The electrochemical generation of
� o
MnO4 is unfavorable due to its high redox potential (EMn�VII�/Mn�II�
� 1.51 vs. NHE� and have only been studied using PbO 2 and
bismuth-doped PbO 2 electrodes, 19,20 where the oxygen overpotential
is very high, or in the presence of a catalyst such as Ag�II�. 21,22 In
our present study, voltammetric and in situ spectroelectrochemical
methods have been used to study the oxidation of Mn 2� on BDD
electrodes in 1 M HClO 4 with the presence of Bi 3� catalysts.
* Electrochemical Society Active Member.
z E-mail: smpark@postech.edu
Journal of The Electrochemical Society, 151 �8� E265-E270 �2004�
0013-4651/2004/151�8�/E265/6/$7.00 © The Electrochemical Society, Inc.
E265
Experimental
BDD electrodes were prepared by a chemical vapor deposition
�CVD� method 23 in the Fujishima Laboratory at the University of
Tokyo. The BDD electrodes used to obtain Fig. 2 and 6 were prepared
in the Einaga Laboratory of Keio University by etching the
BDD surfaces by an Ar ion glow discharge technique. 24 Etching of
BDD results in a very flat and smooth surface, ideal for use in
reflectance spectroscopy experiments. Shimizu et al. 24 reported that
Ar etching reduced the maximum peak-to-valley height of the electrode
surface from 1.49 �m to 613 nm, and the average surface
roughness, Ra , was reduced from 238 to 75 nm without affecting its
electrochemical performance. Etched BDD was used only in Fig. 2
and 6; all other spectra were obtained using nonetched BDD.
Janssen Chimica’s Manganese sulfate (MnSO4•H2O, 99%�, Aldrich’s
bismuth nitrate (Bi�NO3) 3•5H2O, 99.99%� and potassium
permanganate (KMnO4 ,99�%�were used as received. Perchloric
acid �special grade, 70%� was purchased from Samchun Pure
Chemicals Ltd. �Korea�. Doubly distilled, deionized water was used
for the preparation of all solutions.
The electrochemical cell was a single-compartment cell made of
Teflon, with the surface of the BDD working electrode exposed at
the bottom of the cell through an O-ring supported opening. The
counter electrode was a platinum foil, and the reference electrode
was a homemade Ag/AgCl �in saturated KCl� electrode. All potentials
mentioned in this paper are in reference to this electrode unless
otherwise stated.
Electrochemical measurements were made using an EG&G PAR
model 263 potentiostat/galvanostat. All measurements were taken at
room temperature without further temperature control. In situ absorption
spectra were taken with an Oriel InstaSpec® IV spectrometer
with a charge-coupled device �CCD� array detector, which was
configured in a near-normal incidence reflectance mode using a bifurcated
quartz optical fiber. The light from a 60 W xenon lamp was
brought onto the reflective electrode surface through a branch of the
bifurcated optical fiber probe and the reflected light off the electrode
surface was detected using the CDD detector at its other branch.
Thus, this spectrometer measures the absorbance of a species undergoing
a change between the probe and the electrode surface during
the electrolysis. The solution is not stirred during the spectroelectrochemical
measurements. The details of the setup have been
described elsewhere. 25,26 Measurements of permanganate concentra-

E266
Figure 1. Cyclic voltammograms at BDD electrodes in 1MHClO 4 containing
�a� 10, �b� 25, �c� 50, and �d� 100 mM MnSO 4 . The scan rate was 10
mV/s.
tions were made for current efficiency calculations using an S-2100
UV-vis Spectrophotometer from Scinco �Seoul, Korea�.
Results and Discussion
Figures 1a-d show the cyclic voltammograms �CVs� recorded for
oxidation of 10, 25, 50, and 100 mM MnSO4 , respectively, in 1.0 M
HClO4 on BDD electrodes. The CVs are characterized as having
one or two anodic peaks in the potential range of 1.4-1.8 V, followed
by a main peak at around 2.2-2.3 V, although none of the peaks are
observed in the 1.4-1.8 V range in the 10 mM Mn2� solution �Fig.
1a�. Note also that the main peak observed at about 2.2 V is hardly
proportional to the concentration of Mn2� . In fact, the appearance of
the peaks at less positive potentials is shown to suppress the main
anodic peak at 2.2 V. The peaks appearing at about 1.4-1.8 V had
been assigned to the oxidation of Mn2� to MnO2 �EMD� on the
electrode surface; 27,28 a few papers have dealt with the oxidation
mechanism of EMD formation in acidic media. 27,28 EMD formation
mechanism is not discussed in this paper, as our focus is on the
�
formation of MnO4 . On the reverse scan, a nucleation loop is observed
in all the CVs, indicating that the MnO2 film is stable and
continues to grow during the reverse scan until it gets reduced at ca.
1.2 V. The MnO2 film undergoes electrochemical reduction during
the reverse scan with peak currents observed at around 1.0 V in all
the CVs. The second cathodic peak, immediately following the first
reduction peak, is also associated with the reduction of MnO2 film. 27
Figure 2 shows the in situ spectra obtained during oxidation of
10 mM MnSO4 on the BDD electrode by scanning the potential
from 1.0 to 2.5 V at a scan rate of 10 mV/s. The spectrum does not
change until the potential reaches ca. 1.7 V, when the region of
negative absorbance is observed between 300 and 450 nm, forming
a pit in the spectra �a dark spot of an oval shape�. The pit immediately
follows the first oxidation peak, which starts at ca. 1.6 V,
indicating that the negative absorbance is caused by the formation of
the MnO2 film. This negative absorbance is caused by the higher
reflectivity of the MnO2 film compared to that of the bare electrode
itself in this wavelength region. When the MnO2 film was physically
removed from the electrode, it exhibited a shiny dark red color. Due
to the nature of the spectroelectrochemical setup, the UV-vis light is
reflected off the electrode surface. 25,26 The reference spectrum was
obtained from the bare BBD electrode, which has a dark, matte
surface. The formation of a more reflective MnO2 film on the electrode
surface may cause the absorbance to drop to a negative value.
Previous studies 27,28 have noted that the overall oxidation of Mn�II�
Journal of The Electrochemical Society, 151 �8� E265-E270 �2004�
Figure 2. In situ UV-vis absorption spectra of 10 mM MnSO 4 on BDD
electrode in 1MHClO 4 at a scan rate of 10 mV/s. The spectra were recorded
during the oxidation scan between 1.0 and 2.5 V.
to MnO 2 occurs in multiple steps, with intermediates such as
MnOOH. Our spectroelectrochemical experiments did not detect
any intermediates, but that does not necessarily mean the oxidation
occurs without intermediates. From the spectra, the second oxidation
peak/shoulder of the CV following the formation of MnO 2 is assigned
to the formation of permanganate by comparing the spectrum
with that obtained from authentic MnO 4 � �not shown�. The permanganate
ion has a very distinct UV-vis spectrum with six wellresolved
vibrational energy levels between 500 and 600 nm. These
peaks start to arise at around 1.9 V, which coincides with the beginning
of the second oxidation peak in Fig. 1a.
Figure 3 shows a series of spectra singled out at labeled potentials
from �a� 25, �b� 50, and �c� 100 mM MnSO 4 solutions. The
negative pit appears between 300 and 450 nm for all concentrations
at 1.4-1.6 V depending on the concentration of Mn 2� , and the starting
potentials for the negative absorbance all correspond to the potentials
of the initial anodic current increase in the CVs. This indicates
that the product of the first oxidation peak is MnO 2 at all four
MnSO 4 concentrations. As the potential scan proceeds, the product
obtained at the main oxidation peak observed for the three higher
concentrations �25-100 mM MnSO 4) differs from that for the lowest
concentration �10 mM MnSO 4). The spectra shown for 2.2, 1.9, and
1.7 V in Fig. 3a, b, and c, respectively, indicate that the products of
the main oxidation wave is Mn�III�, not MnO 4 � , at higher Mn�II�
concentrations �25, 50, and 100 mM MnSO 4), whereas it is MnO 4 �
in the 10 mM MnSO 4 solution �Fig. 2�. The peaks arising at 475 nm
in all three figures indicate the formation of Mn�III� under these
conditions. 29 The difference in final products depending on the concentration
of Mn 2� can be explained by two reasons. The first is due
to the relative stabilities of Mn�III� and MnO 4 � . At high Mn 2� concentrations,
Mn�III� is stable, as electrogenerated MnO 4 � oxidizes
Mn 2� to Mn�III� according to the reaction
4Mn 2� � MnO 4 � � 8H � → 5Mn�III) � 4H2O �1�
The MnO 4 � is produced but immediately reacts with excess Mn 2� in
solution to form Mn�III�. As a result, mainly Mn�III� is produced at
higher Mn 2� concentrations. However, MnO 4 � is produced as a major
product at lower Mn 2� concentrations as there is not enough
Mn 2� for Reaction 1 to proceed. The second reason is the passivating
effect of the MnO 2 film. The MnO 2 film is always formed during
the oxidation process, which shows a blocking effect on the
production of MnO 4 � , as can be seen from the spectra.

Figure 3. Series of in situ UV-vis absorption spectra of �a� 25, �b� 50, and
�c� 100mMMn�II� in1MHClO 4 at noted potentials. The working electrode
was a BDD electrode.
To demonstrate the effect of the MnO 2 film, the film was deposited
on the BDD electrode by holding the potential at 1.7 V for 30 s
ina25mMMnSO 4 solution. The MnO 2-coated electrode was then
rinsed and placed in an electrochemical cell containing 10 mM
MnSO 4 and 1 M HClO 4 , and the potential was scanned from 1.0 to
2.5 V at 10 mV/s. The spectra obtained from this experiment are
shown in Fig. 4, which do not show any signs of MnO 4 � production,
although the experimental conditions were identical to those used
for Fig. 1. This illustrates the blocking effect of the oxide film. At
low Mn 2� concentrations, some of the active electrode �i.e., bare
BDD� surface may still be available for direct oxidation of Mn 2� to
MnO 4 � , because of an incomplete coverage of the electrode surface
Journal of The Electrochemical Society, 151 �8� E265-E270 �2004� E267
Figure 4. In situ UV-vis absorption spectra recorded at the MnO 2 film
coated BDD electrode, in solution containing 10 mM MnSO 4 and 1 M
HClO 4 . The spectra were recorded during the oxidation scan between 1.1
and 2.5 V at 0.5 s intervals.
by the MnO 2 film. At high Mn 2� concentrations, the film forms at
less positive potentials covering the entire surface with MnO 2 ,
which prevents the oxidation of Mn 2� to MnO 4 � . The MnO4 � produced
reacts with excess Mn 2� according to Eq. 1, producing
Mn�III�.
Figure 5 shows the cyclic voltammograms for oxidation of 5 mM
Bi 3� �a� and Mn 2� of various concentrations in the presence of 5.0
mM Bi 3� �b-e�, all recorded at 10 mV/s. The most distinct difference
between Fig. 1 and 5 is the absence of a nucleation loop when
Bi 3� is present, and the decrease in the size of the reduction peak
during the reverse �negative� scan. Also noted is that the peak responsible
for oxidation of Bi 3� is quenched as soon as Mn 2� is
added, indicating that the product of Bi 3� oxidation is immediately
consumed. Figure 6 shows the in situ spectra recorded during oxidation
of 10 mM MnSO 4 in the presence of 5 mM Bi�NO 3) 3 .Itis
almost identical to Fig. 2, except for the absence of the negative pit
corresponding to the formation of MnO 2 . Also, the absorbance for
the MnO 4 � peak is larger in Fig. 6 than in Fig. 2. Since the distance
between the optical fiber and the electrode surface was kept con-
Figure 5. Cyclic voltammograms of �a� 5mMBi�III� with �b� 10, �c� 25, �d�
50, and �e� 100 mM Mn�II� on BDD electrode in 1MHClO 4 . The scan rate
was 10 mV/s for all scans.

E268
Figure 6. In situ UV-vis absorption spectra of 10 mM MnSO 4 with5mM
Bi�III� on BDD electrode in 1MHClO 4 at a scan rate of 10 mV/s. The
spectra were recorded during the oxidation scan between 1.0 and 2.5 V at 0.5
s intervals.
stant, the higher intensity indicates that the amount of MnO 4 � produced
is greater when Bi 3� is present in the solution.
The CV shown in Fig. 5a indicates that Bi 3� oxidation produces
its higher oxidation state on the BDD electrode with a distinct oxi-
dation peak current at ca. 2.2 V. This value corresponds well to the
o
thermodynamic value, EBi�III�/Bi�V) � ca. 2.0 � 0.2 vs. NHE. 30
Note also that no reversal peak is observed for Bi�V� produced,
indicating that it is unstable because it is a strong enough oxidant to
oxidize water. Due to this high oxidation power, oxidation of Mn 2�
to MnO 4 � by Bi�V� in the form of sodium bismuthate (NaBiO3) in
sulfuric or nitric acid solution was a standard procedure for the
qualitative detection 31a and quantitative spectrophotometric analysis
of Mn�II�. 31b Zhang and Park 29 reported that Bi�V�, generated by
oxidizing Bi 3� doped into PbO 2 electrodes, facilitates the oxidation
of Mn 2� to MnO 4 � . In our experiment, electrochemically produced
Bi�V� in solution facilitates the oxidation of Mn 2� to MnO 4 � by two
pathways. The first is the aforementioned oxidation of Mn 2� via
Bi�V� according to Reaction 2
2Mn 2� � 5HBiO 3 � 9H � → 2MnO 4 � � 5Bi 3� � 7H2O �2�
The second is via oxidation of MnO 2 formed on the electrode. At
higher potentials, the MnO 2 can be electrochemically oxidized to
MnO 4 � . This oxidation reaction not only oxidizes the MnO2 film to
MnO 4 � , but simultaneously removes the blocking film, allowing
oxidation of Mn 2� at the bare BDD electrode surface. There should
also be some contribution from the mediated oxidation of MnO 2 by
electrogenerated Bi�V�
2MnO2 � 3HBiO3 � 3H� � 3� → 2MnO4 � 3Bi � 3H2O
�3�
albeit small, as the reaction would be occurring in the solid state.
This reaction is also thermodynamically favorable because E 0 for
oxidation of MnO 2 to MnO 4 � is 1.70 V. 32
The effect of Bi�V� on the MnO 2 film is shown in Fig. 7. The
MnO 2 film was first deposited onto the electrode as for the experiment
in Fig. 4, which was then rinsed thoroughly with deionized
water and immersed in a solution containing only 5.0 mM Bi 3�
without Mn 2� . The CV and the spectra obtained at this
MnO 2-covered electrode in this solution by scanning the potential
from the open circuit potential �OCP� to 2.5 V are shown in Fig. 7.
The spectra recorded concurrently with the CV show the appearance
of MnO 4 � past 2.1 V. The oxidation of MnO2 to MnO 4 � must have
Journal of The Electrochemical Society, 151 �8� E265-E270 �2004�
Figure 7. Cyclic voltammograms and in situ UV-vis absorption spectra of
MnO 2 film, deposited on BDD electrode, in solution containing 5 mM Bi�III�
and1MHClO 4 . The spectra were recorded during the oxidation scan between
1.2 and 2.5 V at 0.5 s intervals. The scan rate was 10 mV/s for the CV.
been caused by direct oxidation of MnO 2 and/or indirect oxidation
via Bi�V�. Figure 8 shows the direct oxidation of the MnO 2 film at
the electrode without Bi 3� . The MnO 2 film was formed according to
the procedure mentioned above, and the potential was scanned from
the OCP to 2.5 V in a solution containing only 1 M HClO 4 ; the
spectra were recorded between 1.2 and 2.5 V. The CVs and in situ
spectra shown in Fig. 7 (Bi 3� added� and Fig. 8 �no Bi 3� added� are
identical up to 2.3 V, indicating that Bi�III� is not oxidized prior to
2.3 V. The production of MnO 4 � between 2.1 and 2.3 V is only due
to the direct oxidation of MnO 2 . The large amount of MnO 4 � produced
past 2.3 V in Fig. 7 is attributed to indirect oxidation via
Bi�V�. It appears Bi 3� cannot be oxidized on MnO 2 , so the CV and
spectra are identical up until 2.3 V, where MnO 2 is oxidized directly.
The electrochemical oxidation of MnO 2 to MnO 4 � provides active
�bare BDD� sites for Bi 3� oxidation, and Bi�V� oxidizes and removes
the MnO 2 film according to Eq. 3. The appearance of a
reduction peak during the negative scan in Fig. 8a indicates that
direct electrochemical oxidation does not completely remove the
MnO 2 from the electrode surface. However, indirect oxidation via
Bi�V� completely removes the MnO 2 film, as seen from the absence
of the reduction peak in Fig. 7a. The Bi�V� is reduced back to Bi 3�
and available for repeated oxidation at the electrode surface. The
large increase in current between 2.3 and 2.5 V is attributed to the

Figure 8. Cyclic voltammograms and in situ UV-vis absorption spectra of
the MnO 2 film, deposited on BDD electrode, in solution containing 1 M
HClO 4 without Bi 3� . The spectra were recorded during the oxidation scan
between 1.2 and 2.5 V at 10 s intervals. The scan rate was 10 mV/s for the
CV.
catalytic �EC�� reaction between Bi�V� and MnO 2
The overall effect of Bi 3� on the oxidation of Mn 2� is shown in
Fig. 9 as the current efficiency of Mn 2� oxidation to MnO 4 � . Potentiostatic
electrolysis of 10 mM Mn 2� was carried out with and without
the addition of 2.0 mM Bi 3� for 10 min. The amount of MnO 4 �
generated was measured with a UV-vis spectrophotometer, from
which the current efficiency was calculated. In both circumstances,
with and without Bi 3� , the current efficiency increases as higher
potential is applied. The slight decrease in current efficiency at 2.4 V
may be due to the increase in oxygen evolution. At potentials below
2.0 V, only a slight advantage in current efficiency is observed when
Journal of The Electrochemical Society, 151 �8� E265-E270 �2004� E269
Figure 9. Current efficiencies for MnO 4 � generation at different potentials in
solutions containing 10 mM MnSO 4 �-�-� and 10 mM MnSO 4 � 2 mM
Bi�III� �-�-�.
Bi 3� is added. The difference in current efficiency increases drastically
when applied potential increases beyond 2.0 V. From the CV, it
can be seen that Bi 3� oxidation starts slightly past 1.9 V; this potential
corresponds to the potential, at which the current efficiency
of MnO 4 � production starts to increase. When the electrodes were
observed after electrolysis, the MnO 2 film formed on the electrode
surface can be seen in solutions containing only Mn 2� . The MnO 2
film was absent on electrodes where Bi 3� was added and potentials
higher than 2.0 V were applied. The removal of MnO 2 from the
electrode surface greatly increases the current efficiency of MnO 4 �
production.
It should be pointed out that the overpotentials for generation of
higher valence states of manganese, such as Mn�III�, MnO 2 , and
MnO 4 � , observed here at the BDD are significantly higher than
those at Bi-doped PbO 2 electrodes. 19,20,29 This is because oxidation
of metal ions to higher oxidized states are mostly described as the
oxygen transfer reactions as pointed out by Johnson et al. 19,20 and,
thus, their oxidation would be more efficient at the oxide electrodes.
At BDDs, oxidation of ions to higher states probably occurs first,
which is followed by an oxygenation reaction with water molecules.
The electron transfer from the metal ions to the electrode surface via
the electrical double layer of the BDD must be an inefficient process.
Nonetheless, the BDD appears to be the only nonoxide electrode
capable of oxidizing a number of compounds and ions to their
high oxidation states in its natural state.
Conclusion
The oxidation of Mn 2� to MnO 4 � was carried out on BDD electrodes.
Due to its high oxygen overpotential, current efficiencies as
high as 37% were obtained on the BDD without Bi 3� . The formation
of the MnO 2 film, which shows a blocking effect on the production
of MnO 4 � , reduced the current efficiency of MnO4 � production.
Higher Mn 2� concentrations led to thicker MnO 2 film
formation, which reduced MnO 4 � production. The addition of Bi 3�
increased the overall current efficiency of MnO 4 � production. When
2.0 mM Bi 3� was added to 10 mM Mn 2� , the current efficiency
increased by an average of 250% at potentials greater than 2.0 V.
The current efficiency was increased by the removal of the MnO 2
film, which displays a blocking effect on the generation of MnO 4 � ,
from the electrode surface. The MnO 2 film is removed by a chemical
reaction with Bi�V�, which is generated electrochemically at the
active �bare BDD� electrode surface.

E270
Finally, the direct oxidation of Bi3� to Bi�V�, which has not been
reported in the literature, was shown to be observed at the BDD
electrode. Thus far, its oxidation potential has been estimated from
the thermodynamic data. 30 Our present results demonstrate that the
BDD electrodes can be very useful for finding redox potentials that
are difficult or impossible to obtain with other types of electrodes.
Acknowledgments
A grateful acknowledgment is made to the KOSEF for supporting
this research through its grant to the Center for Integrated Molecular
Systems and to the KRF for supporting graduate students
through the BK-21 program.
Pohang University of Science and Technology assisted in meeting the
publication costs of this article.
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