Electrochemical Oxidation of Mn - unist

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