Electrocatalytic reduction of sulfur dioxide by iron phthalocyanine monolayer in acidic conditions

Article abstract of DOI:10.1246/cl.2002.648

The iron phthalocyanine (FePc) monolayer adsorbed on a graphite surface showed a remarkable activity for the SO₂ reduction in aqueous conditions. The highest activity was observed at strongly acidic solutions and decreased with a pH increase due to the H⁺ limitation of reduction. Only SO₂ and HSO^-₃ are reducible forms. The reduction mechanisms at strongly and moderately acidic conditions were suggested. The SO₂ reduction current was independent of the surface coverage and the ppb level detection was possible with amperometry.

Full text of DOI:10.1246/cl.2002.648

6
48
Chemistry Letters 2002
Electrocatalytic Reduction of Sulfur Dioxide
by Iron Phthalocyanine Monolayer in Acidic Conditions
ꢀ
Cheol Su Yu, Hyukjae Choi, and Sunghyun Kim
Department of Chemistry and Bio/Molecular Informatics Center, Konkuk University, Seoul 143-701, Korea
(Received February 25, 2002; CL-020179)
The iron phthalocyanine (FePc) monolayer adsorbed on a
by dipping the electrode for a given period of time in a DMSO
solution of FePc, followed by immediate washing in FePc-free
DMSO and then in the electrolyte. The surface coverage was
controlled by changing dipping time. In each experiment, the
electrode was ground against 1200 grit SiC paper and subject to
sonication to ensure the fresh surface. For an amperometric
detection of SO2, potential was applied where the reduction takes
place and current was measured while rotating the electrode. The
SO2 concentration was varied by the stepwise addition of the
concentrated solution into the SO2-free solution. All the
experiments were carried out in buffer solutions at room
temperature.
graphite surface showed a remarkable activity for the SO2
reduction in aqueous conditions. The highest activity was
observed at strongly acidic solutions and decreased with a pH
þ
increase due to the H limitation of reduction. Only SO2 and
À
HSO are reducible forms. The reduction mechanisms at strongly
3
and moderately acidic conditions were suggested. The SO2
reduction current was independent of the surface coverage and the
ppb level detection was possible with amperometry.
Electrochemical reactions of sulfur dioxide have attracted
much attention for their wide range of applications1 such as
energy generation, dithionite production, and electrowinning of
sulfide ores. While most studies have been directed toward the
{3
Figure 1 shows a series of cyclic voltammograms of SO2
(2 mM) reduction by the FePc-adsorbed electrode at different
À
pHs. It is evident that [Fe(II)Pc(-3)] , a reduced form of
4
{6
oxidation
nonaqueous solvents, the research on reduction is relatively rare.
at various metal electrodes both in aqueous and
Fe(II)Pc(-2), exhibits electrocatalytic activities whose peak
potential moves by 60 mV per pH. The reduction is energetically
favorable by ca. 200 mV compared to the bare electrode. Contrast
to the case in a nonaqueous solution, the reduction is highly
irreversible. The reduction current remains constant roughly up to
pH 3 and then decreases as the solution pH increases, indicating
In aprotic solvents, SO2 undergoes a reversible one-electron
7
À
reduction. Sawyer et al. showed that the formed SO2 binds to
SO2 to make a blue-colored species or dimerizes to dithionite ion
8
in a DMF solution. In aqueous media, however, SO2 undergoes
À
two-electron reduction, exhibiting a well-defined, diffusion-
controlled voltammetric response under highly acidic conditions.
The reduction product, H2SO2, is unstable and disproportionates
to SO2 and elemental sulfur whose presence was confirmed by the
both SO2 and HSO3 are irreversibly reduced in strongly acidic
conditions. No reduction wave was observed above pH 7. The
reduction is diffusionally controlled with elemental S as a final
product. The following reduction scheme could be written with
the formation of sulfoxylic acid, which disproportionates to SO2
and S.
II
voltammetric peak of Hg S on a mercury electrode. In early
9
polarographic studies, Kolthoff and Miller suggested the change
in the reduction mechanism as pH increased. Most recent
1
0
researches have been done using a Pt electrode. Quijada et al.
showed that SO2 was irreversibly adsorbed on both single- and
poly-crystalline Pt surfaces in a sulfuric acid medium. The
reduction proceeds in two steps at a polycrystalline Pt surface
where the adsorbed SO2 is reduced before the reduction of
solution phase SO2 takes place, yielding elemental sulfur and
polysulfide as ultimate products.
For a rapid decrease in current in moderately acidic
conditions, Kolthoff and Miller argued that it was due to the pH
effect on the position of equilibrium between two tautomeric
forms of H2SO3. This explanation, however, may not be plausible
as the equilibrium constant for the reaction, SO2 þ H2O ¼
While most researches to date have been carried out using
pure metallic electrodes in which a rather large overpotential is
required, we have tried to find efficient electrocatalysts made of
metallomacrocycles. Among many candidates, FePc, which has
been best known as an oxygen reduction catalyst and also found in
12
H2SO3, is known to be infinitesimally small. We attribute this to
1
1
our group to be an excellent electrocatalyst for NO reduction,
þ
the limitation to H , which is the requisite for the SO2 reduction.
was applied to the SO2 reduction in this work. We pursued the
characteristic behavior of SO2 reduction as a function of pH as
well as the possibility of this system functioning as a direct
electrochemical sensor in a ppb range.
To confirm this reasoning, we tried with more concentrated SO2
solution (20 mM) and obtained the same current-pH pattern but
þ
with a current drop at an early stage. That is because more H is
required to reduce higher concentration of SO2.
SO2 solutions were prepared either from SO2 gas or from
Na2SO3 just before use. Since the voltammetric features were
exactly the same, we used Na2SO3 to precisely adjust sulfite (or
equivalently SO2) concentration. Adsorption of FePc was
effected onto the edge plane of the ordinary pyrolytic graphite
In weakly acidic conditions beyond pH ca. 5, the oxidation
current was observed. This oxidation peak increases with a pH
increase although the reduction current rapidly drops down. We
attribute this to the formation and oxidation of dithionite ion.
Dithionite is very unstable in strongly acidic media but could be
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
649
to that of SO2 in solution to the electrode. In our case, it is not easy
to exactly calculate the distance between FePc molecules at a
submonolayer coverage due to the very rough nature of the
surface, but at least up to a tenth coverage of a full monolayer, the
surface diffusion is a determining factor. A rough calculation
shows that only an order of millisecond is needed for a SO2
molecule to reach the FePc site even at the lowest coverage
2
calculated from the equation, t ¼ Á =2Ds, where Á is the mean
square displacement and Ds the surface diffusion coefficient with
À11
2
À1
a typical value of 10 cm s . This observation has an
important implication for a sensor application in that the amount
of the catalyst can be reduced to achieve a maximum activity.
Figure 1. Cyclic voltammograms of FePc (dotted) and SO2
reduction (solid) at pH 1 (a), 3 (b), 5 (c), and 7 (d).
½
SO2ꢁ ¼ 2:0 mM, scan rate ¼ 0:1 V sÀ1, Electrode area ¼
2
0
:091 cm .
À
formed in weakly acidic media from HSO2 , which is produced
À
by the reduction of HSO3 . This explanation can be justified from
the fact that the same voltammetric feature was obtained by
dissolving Na2S2O4 in the electrolyte. The following reduction
scheme is written for pH > 5.
Figure 3. Plot of reduction current vs SO2 concentration
from amperometric experiments in 0.5 M H2SO4. The
electrode was rotated at 1000 rpm. R-factor of fit ¼ 0:999.
The possibility of a FePc-modified electrode as a SO2 sensor
has been demonstrated by the amperometric detection of SO2. A
concentrated solution of SO2 was added stepwise to the SO2-free
electrolyte and current was measured where the potential was
poised where the SO2 reduction took place. A linear response was
obtained down to ca. 30 ppb (Figure 3), demonstrating that this
type of sensor has enough sensitivity for the practical use.
Figure 2 shows a peak current dependency on the surface
coverage. Contrary to intuition, the peak current is virtually
independent of the surface coverage within experimental error. A
higher coverage does not necessarily mean higher catalytic
activity. Although some SO2 molecules reaching a bare graphite
site do not undergo immediate electrochemical reduction at a low
coverage, the same reduction current will be resulted if the rate of
surface diffusion of SO2 to the FePc site is fast enough compared
This work was supported by Korea Research Foundation
Grant. (KRF-2000-041-D00220.)
References and Notes
1
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2
3
C. Oloman, J. Electrochem. Soc., 117, 1604 (1970).
J. Gonzalez, C. Vapel-Boute, and C. Devroly, Electrochim. Acta,
1
0, 513 (1965).
R. M. Spontnitz, J. A. Volucci, and S. H. Langer, Electrochim. Acta,
8, 1053 (1983).
C. Korzeniewski, W. McKenna, and S. Pons, J. Electroanal. Chem.,
35, 361 (1987).
4
5
2
2
6
7
8
E. T. Seo and D. T. Sawyer, Electrochim. Acta, 10, 239 (1965).
R. P. Martin and D. T. Sawyer, Inorg. Chem., 11, 2644 (1972).
I. Jacobsen and D. T. Sawyer, J. Electroanal. Chem., 15, 181
(
1967).
9
1
I. M. Kolthoff and C. S. Miller, J. Am. Chem. Soc., 63, 2818 (1941).
0 C. Quijada, J. L. Vazquez, J. M. Perez, and A. Aldaz, J. Electroanal.
Chem., 372, 243 (1994).
Figure 2. Plot of peak current vs surface coverge of FePc
for the SO2 reduction in 0.5 M H2SO4. ½SO2ꢁ ¼ 2:0 mM,
1
1
1 S. Y. Ha and S. Kim, J. Electroanal. Chem., 468, 131 (1999).
2 F. A. Cotton and G. Wilkinson, ‘‘Advanced Inorganic Chemistry,’’
4th ed., Wiley and Sons, New York (1980).
À1
2
scan rate ¼ 0:1 V s , Electrode area ¼ 0:091 cm .