The photocatalytic autoxidation of sulfur oxoanions by water-soluble porphyrin complexes

Article abstract of DOI:10.1016/S1381-1169(98)00148-4

The photocatalytic autoxidation of S₄O₆^2- and S₂O₃^2- by water- soluble porphyrin complexes Fe(2-TMPyP), Mn(4-TMPyP), and Co(2-TMPyP) were compared under illumination with a 419 nm visible light in oxygen-saturated aqueous solutions at room temperature. The process involves S₂O₃^2- as an intermediate product and SO₄^2- is the final oxidation product. The trend in photocatalytic activity of the porphyrin complexes for S₄O₆^2- oxidation is Co(2-TMPyP) > Mn(4-TMPyP) > Fe(2-TMPyP). For S₂O₃^2- oxidation, the trend is Co(2-TMPyP) > Fe(2-TMPyP) > Mn(4-TMPyP).

Full text of DOI:10.1016/S1381-1169(98)00148-4

Ž
.
Journal of Molecular Catalysis A: Chemical 138 1999 1–13
The photocatalytic autoxidation of sulfur oxoanions by
water-soluble porphyrin complexes
)
Shen-Ming Chen
Department of Chemical Engineering, National Taipei UniÕersity of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 10643,
Taiwan
Received 3 January 1997; accepted 26 March 1998
Abstract
y
The photocatalytic autoxidation of S4O62 and S2O32y by water-soluble porphyrin complexes Fe 2-TMPyP , Mn 4-
Ž
.
Ž
.
Ž
.
TMPyP , and Co 2-TMPyP were compared under illumination with a 419 nm visible light in oxygen-saturated aqueous
y
y
solutions at room temperature. The process involves S2O32 as an intermediate product and SO42 is the final oxidation
product. The trend in photocatalytic activity of the porphyrin complexes for S4O62 oxidation is Co 2-TMPyP )Mn 4-
y
Ž
.
Ž
2
y
.
Ž
.
Ž
.
Ž
.
Ž
.
TMPyP )Fe 2-TMPyP . For S2O3 oxidation, the trend is Co 2-TMPyP )Fe 2-TMPyP )Mn 4-TMPyP . q 1999
Elsevier Science B.V. All rights reserved.
Keywords: Photocatalytic autoxidation; Sulfur oxoanions; Porphyrin complexes
w x
tochemical applications 3–5 . The photocat-
1. Introduction
w
alytic reaction by phthalocyanine complexes 6–
x
w
x
10 and porphyrin complexes 11,12 were pre-
S2O32y, S4O62y, and SO32 are the intermedi-
ates of sulfide oxidation by oxygen. These com-
pounds are found in partially oxidized sulfur in
the processing of sulfide ores, and in the treat-
y
viously reported.
The photooxidation of SO32 13,14 and the
y
w
x
y
photocatalytic autoxidation of SO32 15–17
w
x
y
and S2 2,9,18 have also been studied. The
photodecomposition or photocatalytic oxidation
of sulfite and sulfur oxoanions has potential
utility in solving the environmental pollution
w
x
w
x
ment of wastewater 1,2 .
Porphyrin complexes resemble phthalocya-
w x
nines 2 as active parts in visible light-driven
processes. These complexes exhibit many pho-
w
x
problem 19 .
This paper discuss the photocatalytic autoxi-
y
y
dation of S4O62 and S2O32 under illumination
with 419 nm light by water-soluble porphyrin
)
Ž
.
Ž
.
complexes Fe 2-TMPyP , Mn 4-TMPyP , and
Corresponding author.
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S1381-1169 98 00148-4

(
)
2
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
Ž
.
IR, and NMR spectra. Mn 4-TMPyP was pur-
Ž
.
chased from Porphyrin Products Logan, UT .
All chemicals were of analytical grade.
Aqueous solutions were prepared with doubly
distilled deionized water. Solutions were deoxy-
genated by purging with pre-purified nitrogen
gas. Buffer solutions were prepared from
Ž
.
H2SO4, KHP potassium hydrogen phthalate ,
acetate, phosphate, borate, carbonate, and KOH
for the pH range 0–14. The pH values were
measured with a HANNA Model 8418 pH me-
ter.
Irradiation was carried out on 15 ml quartz
y
sample tube containing S4O62 or S2O32y, and
catalyst in buffer solution. The irradiation light
Ž
.
was 419 nm 8 lamps RPR-4190 A, 112 W
equipped with a Rayonet photochemical cham-
ber reactor model RPR-100.
The thiosulfate and tetrathionate content were
quantified by measuring the absorbance at ls
215 nm through the use of a flow system with a
micro cell and a UV–visible detector. The sul-
fur oxoanions were also quantified by ion chro-
matography.
Ž
.
Ž
.
Ž
.
Fig. 1. Structure of Fe 2-TMPyP , Co 2-TMPyP , Mn 4-TMPyP .
Ž
.
Ž
.
Fig. 1 in oxygen-saturated
Co 2-TMPyP
aqueous solution at room temperature.
Electrochemistry was performed with a Bio-
Ž
.
analytical system West Lafayette, IN Model
CV-27 potentiostat and a BAS X-Y recorder.
Cyclic voltammetry was conducted with the use
2. Experimental
of a three-electrode cell in which a BAS glassy
A sample of FeIII 2-TMPyP was prepared
carbon electrode area 0.07 cm was used as
the working electrode. The glassy carbon elec-
trode was polished with 0.05 mm alumina on
Buehler felt pads and ultrasonicated for 1 min.
The auxiliary compartment contained a plat-
inum wire which was separated by a medium-
sized glass frit. All cell potentials were taken
2
Ž
.
Ž
.
w
x
according to a literature method 20,21 . Pyrrole
and 2-pyridine carboxaldehyde were refluxed in
propionic acid to obtain the meso-tetrakis 2-
pyridyl porphyin H2 2-TPyP , C40 N8H26 .
Methylation was achieved by reacting H2 2-
TPyP with neat dimethyl sulphate to form
meso-tetrakis N-methyl-2-pyridyl porphyin
H2 2-TMPyP SO4CH3 . Metallation was
achieved by refluxing H2 2-TMPyP with FeCl2
PH2O in distilled water for 10 h. Fe 2-
was precipitated by drops of satu-
rated NaClO4 solution and recrystalized with
Ž
.
Ž
Ž
.
.
Ž
.
Ž
.
.
Ž
with the use of a AgrAgClrKCl saturated KCl
Ž
w
Ž
x
Ž
.y.
Ž
.
solution reference electrode. UV–visible spec-
.
tra were measured with a Hitachi Model U-3200
Spectrophotometer.
w
IIIŽ
5
q
.
x
TMPyP
The ion chromatograph used in the experi-
ments was a Dionex Instruments ion chromatog-
raphy DX-100 consisting of a pump, conductiv-
ity detector, an electrochemical detector, and a
syringe loading system with 25-ml sample loop.
The IC chromatograms were recorded using a
Ž
.
water. Co 2-TMPyP was prepared through
Ž
.
metallation by refluxing H2 2-TMPyP with
CoCl2 P6H2O by a literature method 22 . The
products were identified by their UV–visible,
w
x

(
)
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
3
Spectra-Physics DataJet computing integrator.
The columns used throughout were an IonPac
AG4A guard column, an IonPac AS4A analyti-
cal column, and a self-regenerating suppressor
column. Typical LC operational parameters were
as follows: mobile phase was a Na2CO3 and
NaHCO3 buffer solution, mobile phase flow
rate was 2 mlrmin; the column temperature was
at room temperature. These parameters were
used for analyzing sulfate, sulfite and thiosul-
an MPIC-NG1 guard column, an MPIC-NS1
analytical column, and a suppressor column.
3. Results and discussion
y
3.1. The photocatalytic autoxidation of S4 O62
(
)
by Co 2-TMPyP
The decomposition of tetrathionate was car-
ried out under oxygen at pH 9.0 in the presence
y
fate. Thiosulfate and S2O62 were analyzed by
and absence of Co 2-TMPyP under illumina-
Ž
.
Fig. 2. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution using 10y5 M of Co 2-TMPyP with
Ž
.
y4
w
2yx
Ž
h
. w
2yx Ž . w
2yx Ž . w 2yx Ž . w 2yx
SO4 S4O6 without
initial S4O6 s2.5=10
M under illumination with 419 nm light.
S4O6
,
v
S2O3
,
B
,
(
Ž
.
Co 2-TMPyP .

(
)
4
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
y
5
2y
Ž
.
M is present, the initial rate of S4O6
tion with a 419 nm monochromatic light. Fig. 2
10
y
6
shows the disappearance of S4O62 h , the rise
decomposition is 2=10y Mrmin and the pro-
Ž
.
y
y
6
of SO42 B , and the presence of intermediate
duction rate of SO42 is 8=10y Mrmin.
Ž
.
S2O32
v during the catalytic process. The
The photocatalytic oxidation of S4O62 by
y
y
Ž
.
half-life of the decomposition of S4O62 is about
Co 2-TMPyP can be formulated as follows:
y
Ž
.
90 min. All S4O62 is transferred to SO42 with
y
y
2S4O62y q7O2 q6H2O™8SO42y q12Hq
y
y5
a low concentration of S2O32 about 2=10
Ž
1
Ž .
.
M remaining.
When S4O62 is illuminated in the absence of
Co 2-TMPyP ( or if S4O6 is placed in the
y
3.2. The photocatalytic autoxidation of S4 O62
by Mn 4-TMPyP and Fe 2-TMPyP
y
2
y
Ž
. Ž .
(
)
(
)
Ž
.
dark but in the presence of Co 2-TMPyP with
saturated oxygen for 10 h, decomposition of
Fig. 3 shows the decomposition of tetrathio-
nate at pH 9.0 in the presence of oxygen using
y
S4O62 is slow. If the catalyst Co 2-TMPyP
Ž
.
Fig. 3. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution using 10y5 M of Fe 2-TMPyP and
Ž
.
y4
Ž
.
w
2yx
Ž
. w
. Ž . w
2yx
Ž .
B
Mn 4-TMPyP , with initial S4O6 s2.5=10
M under illumination with 419 nm light.
A
(
S4O6
without catalyst.
Ž
.
Ž
. w
2yx Ž . w
2yx Ž . w 2yx Ž .
Ž
.
Ž
2yx Ž . w
Fe 2-TMPyP present, e S4O6 , q S2O3 , l SO4 . C Mn 4-TMPyP present, ^ S4O6 , w S2O3 , ' SO4
2yx Ž . w 2yx
.

(
)
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
5
Table 1
After 12 h of illumination, the percentage of
The initial rate of substrate decomposition and product formation
y
S4O62 transferred to S2O32y by Co 2-TMPyP ,
Ž
.
y4
2y
2y Ž
.
for photocatalytic oxidation of S4O6 2.5=10
M and S2O3
y4
y5
Ž
.
Ž
.
Mn 4-TMPyP , Fe 2-TMPyP are 0%, 44%, and
Ž
.
Ž
5=10
M by various catalysts of porphyrin complexes 10
.
M
7%, respectively.
Catalyst
Substrate Products
The photocatalytic oxidation can be formu-
lated as follows:
Ž
.
.
Ž
.
initial rate, Mrmin
initial rate, Mrmin
Co 2-TMPyP
S4O62y 2=10
SO4 8=10
SO4 2.6=10
SO4 2=10
y6
y6
Ž
Ž
Ž
.
2y Ž
.
Mn 4-TMPyP
Ž
S4O62y 1.3=10
Mn 4-TMPyP
y6
y6
Ž
2y Ž
.
Ž
.
y
S4O62
™
SO42y qS2O32y
.
2
Ž .
y8
y7
S4O62y 8=10
Ž
.
2y Ž
.
Fe 2-TMPyP
Ž
or Fe 2-TMPyP
y6
y8
y6
y6
y8
y7
Co 2-TMPyP
S2O32y 3.3=10
SO4 6.0=10
Ž
Ž
Ž
.
.
.
2y Ž
.
.
.
Mn 4-TMPyP
Ž
S2O32y 5.0=10
Ž
2y Ž
SO4 8.5=10
2y Ž
SO4 6.3=10
Fe 2-TMPyP
S2O32y 1.3=10
Ž
y
3.3. The photocatalytic autoxidation of S2 O32
Ž
.
Ž
.
Mn 4-TMPyP and Fe 2-TMPyP as catalysts.
Fig. 4 shows the decomposition of thiosulfate
y
The S4O62 decomposition initial rates are 1.3
at pH 9.0 using Co 2-TMPyP , Fe 2-TMPyP ,
Ž
.
Ž
.
=10y Mrmin, and 8=10y Mrmin, respec-
and Mn 4-TMPyP as catalysts under illumina-
6
8
Ž
.
2
y
Ž
.
tively Table 1 . S2O3 is an intermediate in
the photocatalytic reaction. The final concentra-
tion with a 419 nm light. The graph shows the
y
decrease of S2O32 and the rise of SO42y. Some
tions of SO42y, S2O32y, and S4O62 after 12 h of
intermediates are possibly present in the reac-
tion process.
y
illumination are shown in Table 2.
y
The data indicate that the phenomena of pho-
When S2O32 is placed in the dark in the
tocatalytic oxidation of S4O62 to SO42 by
presence of Co 2-TMPyP or when S2O3 is
y
y
2y
Ž .
Ž
.
Ž .
Co 2-TMPyP under illumination with 419 nm
light in a solution of pH 9.0 could be performed
illuminated in the absence of catalyst ( in
saturated oxygen for 10 h, only a small amount
2
y
y
efficiently the S4O6
decomposition initial
of S2O32 is decomposed Fig. 4 . If the catalyst
Ž
Ž
.
6
y
5
2y
rates are 2.0=10y Mrmin . Mn 4-TMPyP
Co 2-TMPyP 10
M is present, the S2O3
.
Ž
.
Ž
.Ž
.
y
and Fe 2-TMPyP also have photocatalytic ac-
tranferred to SO42 ` is fast with an initial
Ž .
y y6
Ž
.
tivity for S4O62 oxidation in pH 9.0 borate
rate of S2O32 decomposition v of 3.3=10
y
Ž
.
aqueous solution, and SO42 is the final oxi-
Mrmin, and SO42 production rate of 6.0=
y
y
6
dation product. The photocatalytic oxidation
activity of porphyrin complexes follows the
trend Co 2-TMPyP ) Mn 4-TMPyP ) Fe 2-
TMPyP .
10y Mrmin.
Fig. 4 also shows the decomposition of thio-
sulfate at pH 9.0 in the presence of oxygen with
Ž
.
Ž
.
Ž
.
Ž
. Ž
.
Ž
. Ž
.
Fe 2-TMPyP ' and Mn 4-TMPyP B as
Table 2
y4
y4
2yŽ
.
M
2yŽ
.
M by various catalysts of porphyrin complexes
The photocatalytic oxidation products of S4O6 2.5=10
and S2O3 5=10
after 12 h of illumination under 419 nm light
a
Ž
.
Ž
.
Catalyst survived
Substrate
Products, intermediate, and substrate final concentration
y3
S4O62y
S4O62y
S4O62y
SO4 1=10 M , S2O3 0.0 M , S4yO64 0.0 M
Ž
.
2yŽ
.
2yŽ
.
2yŽ
.
Co 2-TMPyP 60%
y4
Ž
.
2yŽ
.
2yŽ
.
2yŽ
.
Mn 4-TMPyP 100%
SO4 5.6=10 M , S2O3 2.2=10 M , S4O6 0.0 M
y4
y5
y4
Ž
.
2yŽ
.
2yŽ
.
2yŽ
.
M
Fe 2-TMPyP 100%
SO4 1.3=10 M , S2O3 3.5=10 M , S4O6 2.0=10
y3
S2O32y
S2O32y
S2O32y
SO4 1=10 M , S2O3 0.0 M
Ž
.
2yŽ
.
2yŽ
.
Co 2-TMPyP 65%
y4
y4
y4
Ž
.
2yŽ
.
2yŽ
.
M
.
M
Fe 2-TMPyP 100%
SO4 3.5=10 M , S2O3 2.5=10
y4
Ž
.
2yŽ 2yŽ
.
SO4 1.4=10 M , S2O3 4.0=10
Mn 4-TMPyP 100%
a Percentage of catalyst survived after 12 h illumination in the presence of substrate.

(
)
6
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
Fig. 4. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution using 10y5 M of Fe 2-TMPyP ,
Ž
.
Ž .
.
y4
Ž
Ž
.
.
Ž
.
w
2yx
Ž
. Ž . w
2yx
Co 2-TMPyP , and Mn 4-TMPyP , S2O3 s5.0=10
M under illumination with 419 nm.
A
(
S2O3
.
without catalyst. B
Ž
. w
2yx Ž . w 2yx Ž .
Ž
.
Ž
. w
2yx Ž . w 2yx Ž
Ž
.
Ž
Fe 2-TMPyP present, ' S2O3 , ^ SO4 . C Mn 4-TMPyP present, B S2O3 , I SO4 . D Co 2-TMPyP present, v
w
2yx Ž . w 2yx
S2O3 , ` SO4
.
y
catalysts. S2O32 decomposition initial rates are
tion calculation, but are completely transferred
6
8
y
1.3=10y Mrmin and 5.0=10y Mrmin,
respectively. Some intermediates may exist in
the reaction process from the species concentra-
to SO42 after a long time of illumination.
The data indicate that the phenomena of pho-
y
y
tocatalytic oxidation of S2O32 to SO42 by

(
)
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
7
Ž
.
Co 2-TMPyP under illumination with 419 nm
in a solution at pH 9.0 could be performed
three porphyrin complexes have high absorption
coefficients at 419 nm. The Soret band of
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
effect. Fe 2-TMPyP and Mn 4-TMPyP also
Co 2-TMPyP , Fe 2-TMPyP , Mn 4-TMPyP
are at 423 nm, 409 nm, and 463 nm respec-
y
have photocatalytic activity for S2O32 oxida-
tion in pH 9.0 borate aqueous solution. SO42 is
the final product. The trend in photocatalytic
oxidation activity of the porphyrin complexes is
Co 2-TMPyP ) Fe 2-TMPyP ) Mn 4-
tively. The molar absorptivity of Co 2-TMPyP ,
y
Ž
.
Ž
.
Ž
.
Fe 2-TMPyP , Mn 4-TMPyP in pH 9.0 buffer
solution at 419 nm are 2.2=105, 9.1=104,
2.8=104, respectively.
Ž
.
Ž
.
Ž
.
TMPyP . After 12 h illumination, the undecom-
Fig. 6 shows the cyclic voltammograms of
posed percentage of S2O32 by Co 2-TMPyP ,
Fe 2-TMPyP , Mn 4-TMPyP , Co 2-TMPyP at
pH 9.0 buffer solution. The redox couples of
y
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Ž
.
Fe 2-TMPyP , Mn 4-TMPyP are 0%, 50%, and
80%, respectively.
Ž
.
.Ž
Ž
.
x
w
x
Fe IIIr II
2-T M PyP
23,24
and
Ž
.
w
Mn IIIrII 4-TMPyP 25,26 have close values
3.4. The properties of catalysts
of formal potential, and the redox couple of
Ž
.Ž
.
Ž
.Ž
Mn IIIrIV 4-TMPyP and Co IIrIII 2-
Ž
.
. w x
TMPyP 27,28 have similar values of formal
The absorption spectrum of Fe 2-TMPyP
Ž
.
Ž
.Ž
.
Ž
Ž .Ž
potential. The redox couple of Co IIrI 2-
, Mn 4-TMPyP - - - , and Co 2-
TMPyP . . . at pH 9.0 buffer aqueous solution
are shown in Fig. 5. The spectra show that the
. Ž
.
. Ž .Ž
TMPyP is more positive than Fe IIrI 2-
.
Ž
.Ž
.
TMPyP , and Fe IIIrIV 2-TMPyP is more
y6
y6
y5
M FeIII 2-TMPyP
. Ž
M CoII 2-TMPyP . . . , c 1.1=10
Ž .
Ž
. Ž .
, b 3.5=10
Ž
. Ž . Ž .
Fig. 5. Absorption spectrum of a 9.0=10
M MnIII 4-TMPyP - - - , at pH 9.0 buffer solution.
. Ž
Ž
.

(
)
8
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
Fig. 7. Absorption spectra change of 3.8=10y6 M CoII 2-TMPyP
at pH 9.0 borate buffer in the presence of different S4O62y
Ž
.
y4
y3
Ž .
Ž .
Ž . Ž .
M; c 1.6=10 M; d
concentration. a 0.0 M; b 4.0=10
3.3=10y3 M.
y
of S4O62 increases gradually from 0 M to
y4
Ž
.
.
Ž
.
Fig. 6. Cyclic voltammograms of A 2.5=10 M Fe 2-TMPyP ,
Ž . Ž .
3
3.3=10y M, the new spectrum appear with
5.0=10y4 M Mn 4-TMPyP ,
C
2.0=10y4 M Co 2-
TMPyP , in pH 9.0 buffer solution. Scan rate 0.1 Vrs.
Ž
Ž
B
.
isosbestic points at 356, 418, 503, 532, 553, 570
nm. There is coordination or interaction be-
2
tween Co 2-TMPyP and S4O6y, but no obvi-
Ž
.
Ž
.Ž
.
positive than Mn IIIrIV 4-TMPyP and
Ž
.
ous interaction between Co 2-TMPyP and
Ž
.Ž
.
Co IIrIII 2-TMPyP . The formal potentials of
the different redox couples of porphyrin com-
plexes are summarized in Table 3.
y
S2O32 under the same conditions.
The result can be expressed as follows:
y
CoII 2yTMPyP qS O2
Ž
.
Ž
Fig. 7 shows the absorption spectra of Co 2-
4
6
.
™CoII 2yTMPyP S O2
3
Ž .
y
TMPyP at pH 9.0 buffer solution with different
Ž
.
Ž
.
4
6
concentrations of S4O62y. As the concentration
3.5. Catalysts concentration and the photocat-
alytic autoxidation
Table 3
The formal potentials of various porphyrin complexes at pH 9.0
borate buffer solution
Ž
The absorption spectra show that Co 2-
w
Ž
. x
w
Ž
. x
w
Ž
. x
M IVrIII P
M IIIrII P
M IIrI P
.
TMPyP has higher molar absorption at 419 nm.
Fe 2-TMPyP
q0.57a
q0.47
No
y0.14
y0.17
q0.48
y0.76b
Ž
.
When the photocatalytic autoxidation of S4O62
y
Mn 4-TMPyP
Noc
Ž
.
Co 2-TMPyP
y0.61d
Ž
.
was performed with the catalysts at the same
Ž
.
absorbance at 419 nm As1.0 , the trend in
a
w
x
Refs. 23,29 , data taken from spectroelectrochemical method.
b Epcat for the electrocatalytic reduction of S4O62y is y0.80 V
Ž
.
photocatalytic oxidation is Mn 4-TMPyP )
Co 2-TMPyP )Fe 2-TMPyP , because the
concentration of Mn 4-TMPyP is 3.3 times that
of Fig. 3 and the concentration Co 2-TMPyP
Ž
.
Ž
.
Ž
.
vs. AgrAgCl .
c Epcat for the electrocatalytic reduction of S4O62y is y0.72 V
Ž
.
Ž
.
vs. AgrAgCl .
Ž
.
d Epcat for the electrocatalytic reduction of S4O62y is y0.60 V
Ž
.
is 0.4 times that of Fig. 2. The initial rates of
vs. AgrAgCl .

(
)
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
9
y
6
SO42
production are 9.4 = 10y Mrmin,
Co 2-TMPyP )Fe 2-TMPyP )Mn 4-TMPyP
Ž
.
Ž
.
Ž
.
3.2=10y Mrmin and 2.6=10y Mrmin for
the catalysts Mn 4-TMPyP , Co 2-TMPyP , and
Fig. 8 .
6
7
Ž
.
Ž
.
.
Ž
.
For the photocatalytic reaction:
Ž
. Ž
Fe 2-TMPyP A419 s1.0 , respectively.
y
S4O62y ™SO42
4
In the case of the photocatalytic autoxidation
Ž .
of S2O32 with the catalysts at the same ab-
y
Ž
.
Ž .
the catalytic activity of Co 2-TMPyP and
Ž . Ž .
Mn 4-TMPyP 4Fe 2-TMPyP .
sorbance at 419 nm As1.0 , the results show
that the trend in photocatalytic oxidation is
Ž
.
Fig. 8. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution with A419 s1.0 of Fe 2-TMPyP ,
y4
Ž
.
Ž
.
w
2yx
Ž
. Ž . w
2yx
Co 2-TMPyP , and Mn 4-TMPyP , with initial S2O3 s5.0=10
M under illumination with 419 nm light. A
(
S2O3 without
Ž .
Ž
.
Ž
. w
2yx Ž . w 2yx Ž .
Ž
.
Ž
. w
2yx Ž . w 2yx Ž
.
Ž
.
catalyst. B Fe 2-TMPyP present, ' S2O3 , ^ SO4 . C Mn 4-TMPyP present, B S2O3 , I SO4 . D Co 2-TMPyP
Ž
. w
2yx Ž . w 2yx
present, v S2O3 , ` SO4
.

(
)
10
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
y
For the photocatalytic reaction:
The production rate of SO42 is directly pro-
portional to the initial concentration of sub-
strate.
y
S2O32y ™SO42
5
Ž .
.
Ž
the trend in catalytic activity is Co 2-TMPyP
Fig. 11 shows the photocatalytic autoxidation
of S4O62 by varying the concentration of
y
Ž
.
Ž
.
)Fe 2-TMPyP )Mn 4-TMPyP .
Ž
.
Mn 4-TMPyP . In all the different concentra-
2
y
Ž
.
3.6. Effect of substrate concentration on the
photocatalytic reaction
tions of Mn 4-TMPyP performed, SO4 pro-
duction rate is directly proportional to the con-
Ž
.
centration of Mn 4-TMPyP .
Figs. 9 and 10 show the photocatalytic autox-
Fig. 12 shows the photocatalytic autoxidation
y
y
4
4
4
idation of S4O62 and S2O32 with constant
of 5.0=10y M, 2.5=10y M, 1.2=10y
4
y
Ž
.
amount of Co 2-TMPyP and varying the initial
concentration of substrates.
M, and 0.6=10y M S4O62 by constant
Ž
.
amount of Mn 4-TMPyP . For initial concentra-
vs. time of oxygen present at pH 9.0 borate buffer solution using 8=10y6 M of Co 2-TMPyP and
w
2yx
Fig. 9. Concentration of SO4
Ž
M.
.
y4
y4
y4
y4
w
2yx
illumination with 419 nm light. S4O6 s B 2.5=10
Ž
.
Ž
.
Ž
.
Ž .
M; v 1.2=10
M; ' 0.6=10
M; l 0.3=10

(
)
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
11
I have reported in previous papers that the
y
y
electrocatalytic reduction of S4O62 to S2O32
w
x
by porphyrin complexes 24,25,27 is through
Ž .Ž
.
Ž .Ž
.
the Fe I 2-TMPyP and Co I 2-TMPyP
Ž
.
species. For Mn 4-TMPyP , catalytic wave po-
Ž
.Ž
.
tential is after the Mn IIIrII 4-TMPyP redox
Ž
.
couple with a Epcat sy0.72 V vs. AgrAgCl .
Electrocatalytic peak potential for transferring
S4O62 to S2O32 at pH 9.0 is y0.60 V, y0.72
y
y
Ž
.
Ž
V, and y0.80 V by Co 2-TMPyP , Mn 4-
.
Ž
.
TMPyP , Fe 2-TMPyP as catalyst, respectively
Ž
.
Table 3 . The electrocatalytic reaction peak
potentials are consistent with the photocatalytic
decomposition rate of S4O62y. The coordination
IIŽ
.
or interaction between Co 2-TMPyP and
y
S4O62 may enhance the photocatalytic autoxi-
dation of S4O62
.
y
I have recently shown that the electrocat-
y
alytic oxidation of S2O32 and S4O62y by Co 2-
Ž
w
2y x
vs. time of oxygen present at
Fig. 10. Concentration of SO4
pH 9.0 borate buffer solution using 8=10y6 M of Co 2-TMPyP
Ž
.
y4
w
2y x
Ž .
under illumination with 419 nm light. S2O3 s B 5.0=10
y4
y4
y4
Ž
.
Ž
.
Ž .
M; v 2.5=10
M; ' 1.2=10
M; l 0.6=10
M.
4
4
y
tions of 1.2=10y M, 0.6=10y M, SO42
and S2O32 production concentration is increas-
y
ingly dependent to the concentration of sub-
4
strate. For initial concentrations of 5.0=10y
M, 2.5=10y M, SO42y, production concentra-
4
tion is increasingly independent to the concen-
tration of substrate.
3.7. The photocatalytic autoxidation mecha-
nisms
Products analysis by ion chromatography
y
identified S2O32 as the intermediate of photo-
y
catalytic reaction of S4O62 by Co 2-TMPyP ,
Ž
.
Ž
.
Ž
.
Mn 4-TMPyP and Fe 2-TMPyP . Photodecom-
position of S4O62 follows a stepwise decompo-
y
Fig. 11. Concentration of sulfur oxoanions vs. time of oxygen
present at pH 9.0 borate buffer solution with 2.5=10y4
M
sition and oxidation to SO42 by oxygen as
y
S4O62y under illumination with 419 nm light. Mn 4-TMPyP s
w
Ž
.x
Ž .
shown in Eq. 6 .
y5
y5
Ž
Ž
Ž
.
Ž
. w
2y x Ž . w 2y x Ž .
A 2.5=10 M, ` S2O3 , v SO4 , B 1.2=10 M,
S2O3
,
,
C
0.6=10y5 M,
. w
^
l
2y x
Ž
'
. w 2y x Ž .
SO4
Ž
e
. w
2y x
S2O3 ,
y
S4O62y ™S2O32y ™SO32y ™SO42
6
Ž .
. w 2y x
SO4
.

(
)
12
S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13
y
tions of photocatalysis and autoxidation, S4O62
Ž
decomposes to thiosulfate and sulfate. Co 2-
2
y
TMPyP completely converts S4O6 to SO42y
,
.
Ž
.
Ž
.
while Fe 2-TMPyP and Mn 4-TMPyP are only
partially effective, thus producing S2O32
.
y
Acknowledgements
This work was supported by the National
Science Council of the Republic of China.
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