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

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

Source and publish data:

Journal of Molecular Catalysis A: Chemical p. 1 - 13 (1999)

Update date:2022-08-11

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Chen, Shen-Ming

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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

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

The photocatalytic autoxidation of S₄O₆²and S₂O₃^2yby 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 S₂O₃²as an intermediate product and SO₄²is the final oxidation

product. The trend in photocatalytic activity of the porphyrin complexes for S₄O₆²oxidation is Co 2-TMPyP )Mn 4-

TMPyP )Fe 2-TMPyP . For S₂O₃oxidation, the trend is Co 2-TMPyP )Fe 2-TMPyP )Mn 4-TMPyP . q 1999

Keywords: Photocatalytic autoxidation; Sulfur oxoanions; Porphyrin complexes

w x

tochemical applications 3–5 . The photocat-

1. Introduction

alytic reaction by phthalocyanine complexes 6–

10 and porphyrin complexes 11,12 were pre-

S₂O₃²^y, S₄O₆^2y, and SO₃²are 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-

viously reported.

The photooxidation of SO₃²13,14 and the

photocatalytic autoxidation of SO₃²15–17

and S²2,9,18 have also been studied. The

photodecomposition or photocatalytic oxidation

of sulfite and sulfur oxoanions has potential

utility in solving the environmental pollution

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-

problem 19 .

This paper discuss the photocatalytic autoxi-

dation of S₄O₆²and S₂O₃²under illumination

with 419 nm light by water-soluble porphyrin

)

complexes Fe 2-TMPyP , Mn 4-TMPyP , and

Corresponding author.

PII: S1381-1169 98 00148-4

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

IR, and NMR spectra. Mn 4-TMPyP was pur-

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

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

sample tube containing S₄O₆²or S₂O₃^2y, and

catalyst in buffer solution. The irradiation light

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.

Fig. 1. Structure of Fe 2-TMPyP , Co 2-TMPyP , Mn 4-TMPyP .

Fig. 1 in oxygen-saturated

Co 2-TMPyP

aqueous solution at room temperature.

Electrochemistry was performed with a Bio-

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 Fe^III2-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

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 H₂2-TPyP , C₄₀N₈H₂₆.

Methylation was achieved by reacting H₂2-

TPyP with neat dimethyl sulphate to form

meso-tetrakis N-methyl-2-pyridyl porphyin

H₂2-TMPyP SO₄CH₃. Metallation was

achieved by refluxing H₂2-TMPyP with FeCl₂

PH₂O in distilled water for 10 h. Fe 2-

was precipitated by drops of satu-

rated NaClO₄solution and recrystalized with

with the use of a AgrAgClrKCl saturated KCl

.^y.

solution reference electrode. UV–visible spec-

tra were measured with a Hitachi Model U-3200

Spectrophotometer.

^IIIŽ

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

water. Co 2-TMPyP was prepared through

metallation by refluxing H₂2-TMPyP with

CoCl₂P6H₂O by a literature method 22 . The

products were identified by their UV–visible,

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

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 Na₂CO₃and

NaHCO₃buffer 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

3.1. The photocatalytic autoxidation of S₄O₆²

(

)

by Co 2-TMPyP

The decomposition of tetrathionate was car-

ried out under oxygen at pH 9.0 in the presence

fate. Thiosulfate and S₂O₆²were analyzed by

and absence of Co 2-TMPyP under illumina-

Fig. 2. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution using 10^y5M of Co 2-TMPyP with

^2yx

. w

^2yx Ž . w

^2yx Ž . w ^2yx Ž . w ^2yx

SO₄S₄O₆without

initial S₄O₆s2.5=10

M under illumination with 419 nm light.

S₄O₆

S₂O₃

(

Co 2-TMPyP .

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

M is present, the initial rate of S₄O₆

tion with a 419 nm monochromatic light. Fig. 2

shows the disappearance of S₄O₆²h , the rise

decomposition is 2=10^yMrmin and the pro-

of SO₄²B , and the presence of intermediate

duction rate of SO₄²is 8=10^yMrmin.

S₂O₃²

v during the catalytic process. The

The photocatalytic oxidation of S₄O₆²by

half-life of the decomposition of S₄O₆²is about

Co 2-TMPyP can be formulated as follows:

90 min. All S₄O₆²is transferred to SO₄²with

2S₄O₆²^yq7O₂q6H₂O™8SO₄^2yq12H^q

a low concentration of S₂O₃²about 2=10

Ž .

M remaining.

When S₄O₆²is illuminated in the absence of

Co 2-TMPyP ( or if S₄O₆is placed in the

3.2. The photocatalytic autoxidation of S₄O₆²

by Mn 4-TMPyP and Fe 2-TMPyP

. 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

S₄O₆²is slow. If the catalyst Co 2-TMPyP

Fig. 3. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution using 10^y5M of Fe 2-TMPyP and

^2yx

. w

. Ž . w

^2yx

Ž .

Mn 4-TMPyP , with initial S₄O₆s2.5=10

M under illumination with 419 nm light.

(

S₄O₆

without catalyst.

. w

^2yx Ž . w

^2yx Ž . w ^2yx 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

Table 1

After 12 h of illumination, the percentage of

The initial rate of substrate decomposition and product formation

S₄O₆²transferred to S₂O₃^2yby Co 2-TMPyP ,

^2yŽ

for photocatalytic oxidation of S₄O₆2.5=10

M and S₂O₃

Mn 4-TMPyP , Fe 2-TMPyP are 0%, 44%, and

5=10

M by various catalysts of porphyrin complexes 10

7%, respectively.

Catalyst

Substrate Products

The photocatalytic oxidation can be formu-

lated as follows:

initial rate, Mrmin

Co 2-TMPyP

S₄O₆^2y2=10

SO₄8=10

SO₄2.6=10

SO₄2=10

^2yŽ

Mn 4-TMPyP

S₄O₆^2y1.3=10

Mn 4-TMPyP

^2yŽ

S₄O₆²

™

SO₄^2yqS₂O₃^2y

Ž .

S₄O₆^2y8=10

^2yŽ

Fe 2-TMPyP

or Fe 2-TMPyP

Co 2-TMPyP

S₂O₃^2y3.3=10

SO₄6.0=10

^2yŽ

Mn 4-TMPyP

S₂O₃^2y5.0=10

^2yŽ

SO₄8.5=10

^2yŽ

SO₄6.3=10

Fe 2-TMPyP

S₂O₃^2y1.3=10

3.3. The photocatalytic autoxidation of S₂O₃²

Mn 4-TMPyP and Fe 2-TMPyP as catalysts.

Fig. 4 shows the decomposition of thiosulfate

The S₄O₆²decomposition initial rates are 1.3

at pH 9.0 using Co 2-TMPyP , Fe 2-TMPyP ,

=10^yMrmin, and 8=10^yMrmin, respec-

and Mn 4-TMPyP as catalysts under illumina-

tively Table 1 . S₂O₃is an intermediate in

the photocatalytic reaction. The final concentra-

tion with a 419 nm light. The graph shows the

decrease of S₂O₃²and the rise of SO₄^2y. Some

tions of SO₄²^y, S₂O₃^2y, and S₄O₆²after 12 h of

intermediates are possibly present in the reac-

tion process.

illumination are shown in Table 2.

The data indicate that the phenomena of pho-

When S₂O₃²is placed in the dark in the

tocatalytic oxidation of S₄O₆²to SO₄²by

presence of Co 2-TMPyP or when S₂O₃is

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

efficiently the S₄O₆

decomposition initial

of S₂O₃²is decomposed Fig. 4 . If the catalyst

rates are 2.0=10^yMrmin . Mn 4-TMPyP

Co 2-TMPyP 10

M is present, the S₂O₃

.Ž

and Fe 2-TMPyP also have photocatalytic ac-

tranferred to SO₄²` is fast with an initial

Ž .

y y6

tivity for S₄O₆²oxidation in pH 9.0 borate

rate of S₂O₃²decomposition v of 3.3=10

aqueous solution, and SO₄²is the final oxi-

Mrmin, and SO₄²production rate of 6.0=

dation product. The photocatalytic oxidation

activity of porphyrin complexes follows the

trend Co 2-TMPyP ) Mn 4-TMPyP ) Fe 2-

TMPyP .

10^yMrmin.

Fig. 4 also shows the decomposition of thio-

sulfate at pH 9.0 in the presence of oxygen with

. Ž

Fe 2-TMPyP ' and Mn 4-TMPyP B as

Table 2

^2yŽ

M by various catalysts of porphyrin complexes

The photocatalytic oxidation products of S₄O₆2.5=10

and S₂O₃5=10

after 12 h of illumination under 419 nm light

Catalyst survived

Substrate

Products, intermediate, and substrate final concentration

S₄O₆^2y

SO₄1=10 M , S₂O₃0.0 M , S₄_yO₆₄0.0 M

^2yŽ

Co 2-TMPyP 60%

^2yŽ

Mn 4-TMPyP 100%

SO₄5.6=10 M , S₂O₃2.2=10 M , S₄O₆0.0 M

^2yŽ

Fe 2-TMPyP 100%

SO₄1.3=10 M , S₂O₃3.5=10 M , S₄O₆2.0=10

S₂O₃^2y

SO₄1=10 M , S₂O₃0.0 M

^2yŽ

Co 2-TMPyP 65%

^2yŽ

Fe 2-TMPyP 100%

SO₄3.5=10 M , S₂O₃2.5=10

^2yŽ ^2yŽ

SO₄1.4=10 M , S₂O₃4.0=10

Mn 4-TMPyP 100%

^aPercentage of catalyst survived after 12 h illumination in the presence of substrate.

(

)

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 10^y5M of Fe 2-TMPyP ,

Ž .

^2yx

. Ž . w

^2yx

Co 2-TMPyP , and Mn 4-TMPyP , S₂O₃s5.0=10

M under illumination with 419 nm.

(

S₂O₃

without catalyst. B

. w

^2yx Ž . w ^2yx Ž .

. w

^2yx Ž . w ^2yx Ž

Fe 2-TMPyP present, ' S₂O₃, ^ SO₄. C Mn 4-TMPyP present, B S₂O₃, I SO₄. D Co 2-TMPyP present, v

^2yx Ž . w ^2yx

S₂O₃, ` SO₄

catalysts. S₂O₃²decomposition initial rates are

tion calculation, but are completely transferred

1.3=10^yMrmin and 5.0=10^yMrmin,

respectively. Some intermediates may exist in

the reaction process from the species concentra-

to SO₄²after a long time of illumination.

The data indicate that the phenomena of pho-

tocatalytic oxidation of S₂O₃²to SO₄²by

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

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

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-

have photocatalytic activity for S₂O₃²oxida-

tion in pH 9.0 borate aqueous solution. SO₄²is

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 ,

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.

TMPyP . After 12 h illumination, the undecom-

Fig. 6 shows the cyclic voltammograms of

posed percentage of S₂O₃²by Co 2-TMPyP ,

Fe 2-TMPyP , Mn 4-TMPyP , Co 2-TMPyP at

pH 9.0 buffer solution. The redox couples of

Fe 2-TMPyP , Mn 4-TMPyP are 0%, 50%, and

80%, respectively.

.Ž

Fe IIIr II

2-T M PyP

23,24

and

Mn IIIrII 4-TMPyP 25,26 have close values

3.4. The properties of catalysts

of formal potential, and the redox couple of

.Ž

Mn IIIrIV 4-TMPyP and Co IIrIII 2-

. w x

TMPyP 27,28 have similar values of formal

The absorption spectrum of Fe 2-TMPyP

.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

TMPyP , and Fe IIIrIV 2-TMPyP is more

M Fe^III2-TMPyP

. Ž

M Co^II2-TMPyP . . . , c 1.1=10

Ž .

. Ž .

, b 3.5=10

. Ž . Ž .

Fig. 5. Absorption spectrum of a 9.0=10

M Mn^III4-TMPyP - - - , at pH 9.0 buffer solution.

. Ž

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

Fig. 7. Absorption spectra change of 3.8=10^y6M Co^II2-TMPyP

at pH 9.0 borate buffer in the presence of different S₄O₆^2y

Ž .

Ž . Ž .

M; c 1.6=10 M; d

concentration. a 0.0 M; b 4.0=10

3.3=10^y3M.

of S₄O₆²increases gradually from 0 M to

Fig. 6. Cyclic voltammograms of A 2.5=10 M Fe 2-TMPyP ,

Ž . Ž .

3.3=10^yM, the new spectrum appear with

5.0=10^y4M Mn 4-TMPyP ,

2.0=10y4 M Co 2-

TMPyP , in pH 9.0 buffer solution. Scan rate 0.1 Vrs.

isosbestic points at 356, 418, 503, 532, 553, 570

nm. There is coordination or interaction be-

tween Co 2-TMPyP and S₄O₆^y, but no obvi-

.Ž

positive than Mn IIIrIV 4-TMPyP and

ous interaction between Co 2-TMPyP and

.Ž

Co IIrIII 2-TMPyP . The formal potentials of

the different redox couples of porphyrin com-

plexes are summarized in Table 3.

S₂O₃²under the same conditions.

The result can be expressed as follows:

Co^II2yTMPyP qS O²

Fig. 7 shows the absorption spectra of Co 2-

™Co^II2yTMPyP S O²

Ž .

TMPyP at pH 9.0 buffer solution with different

concentrations of S₄O₆²^y. 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

The absorption spectra show that Co 2-

. x

M IVrIII P

M IIIrII P

M IIrI P

TMPyP has higher molar absorption at 419 nm.

Fe 2-TMPyP

q0.57^a

q0.47

y0.14

y0.17

q0.48

y0.76^b

When the photocatalytic autoxidation of S₄O₆²

Mn 4-TMPyP

No^c

Co 2-TMPyP

y0.61^d

was performed with the catalysts at the same

absorbance at 419 nm As1.0 , the trend in

Refs. 23,29 , data taken from spectroelectrochemical method.

^bEp_catfor the electrocatalytic reduction of S₄O₆^2yis y0.80 V

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

vs. AgrAgCl .

^cEp_catfor the electrocatalytic reduction of S₄O₆^2yis y0.72 V

vs. AgrAgCl .

^dEp_catfor the electrocatalytic reduction of S₄O₆^2yis y0.60 V

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

SO₄²

production are 9.4 = 10^yMrmin,

Co 2-TMPyP )Fe 2-TMPyP )Mn 4-TMPyP

3.2=10^yMrmin and 2.6=10^yMrmin for

the catalysts Mn 4-TMPyP , Co 2-TMPyP , and

Fig. 8 .

For the photocatalytic reaction:

. Ž

Fe 2-TMPyP A₄₁₉s1.0 , respectively.

S₄O₆²^y™SO₄²

In the case of the photocatalytic autoxidation

Ž .

of S₂O₃²with the catalysts at the same ab-

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

Fig. 8. Concentration of sulfur oxoanions vs. time of oxygen present at pH 9.0 borate buffer solution with A₄₁₉s1.0 of Fe 2-TMPyP ,

^2yx

. Ž . w

^2yx

Co 2-TMPyP , and Mn 4-TMPyP , with initial S₂O₃s5.0=10

M under illumination with 419 nm light. A

(

S₂O₃without

Ž .

. w

^2yx Ž . w ^2yx Ž .

. w

^2yx Ž . w ^2yx Ž

catalyst. B Fe 2-TMPyP present, ' S₂O₃, ^ SO₄. C Mn 4-TMPyP present, B S₂O₃, I SO₄. D Co 2-TMPyP

. w

^2yx Ž . w ^2yx

present, v S₂O₃, ` SO₄

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

For the photocatalytic reaction:

The production rate of SO₄²is directly pro-

portional to the initial concentration of sub-

strate.

S₂O₃²^y™SO₄²

Ž .

the trend in catalytic activity is Co 2-TMPyP

Fig. 11 shows the photocatalytic autoxidation

of S₄O₆²by varying the concentration of

)Fe 2-TMPyP )Mn 4-TMPyP .

Mn 4-TMPyP . In all the different concentra-

3.6. Effect of substrate concentration on the

photocatalytic reaction

tions of Mn 4-TMPyP performed, SO₄pro-

duction rate is directly proportional to the con-

centration of Mn 4-TMPyP .

Figs. 9 and 10 show the photocatalytic autox-

Fig. 12 shows the photocatalytic autoxidation

idation of S₄O₆²and S₂O₃²with constant

of 5.0=10^yM, 2.5=10^yM, 1.2=10^y

amount of Co 2-TMPyP and varying the initial

concentration of substrates.

M, and 0.6=10^yM S₄O₆²by constant

amount of Mn 4-TMPyP . For initial concentra-

vs. time of oxygen present at pH 9.0 borate buffer solution using 8=10^y6M of Co 2-TMPyP and

^2yx

Fig. 9. Concentration of SO₄

^2yx

illumination with 419 nm light. S₄O₆s B 2.5=10

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

I have reported in previous papers that the

electrocatalytic reduction of S₄O₆²to S₂O₃²

by porphyrin complexes 24,25,27 is through

Ž .Ž

the Fe I 2-TMPyP and Co I 2-TMPyP

species. For Mn 4-TMPyP , catalytic wave po-

.Ž

tential is after the Mn IIIrII 4-TMPyP redox

couple with a Ep_catsy0.72 V vs. AgrAgCl .

Electrocatalytic peak potential for transferring

S₄O₆²to S₂O₃²at pH 9.0 is y0.60 V, y0.72

V, and y0.80 V by Co 2-TMPyP , Mn 4-

TMPyP , Fe 2-TMPyP as catalyst, respectively

Table 3 . The electrocatalytic reaction peak

potentials are consistent with the photocatalytic

decomposition rate of S₄O₆²^y. The coordination

^IIŽ

or interaction between Co 2-TMPyP and

S₄O₆²may enhance the photocatalytic autoxi-

dation of S₄O₆²

I have recently shown that the electrocat-

alytic oxidation of S₂O₃²and S₄O₆^2yby Co 2-

^2yx

vs. time of oxygen present at

Fig. 10. Concentration of SO₄

pH 9.0 borate buffer solution using 8=10^y6M of Co 2-TMPyP

^2yx

Ž .

under illumination with 419 nm light. S₂O₃s B 5.0=10

Ž .

M; v 2.5=10

M; ' 1.2=10

M; l 0.6=10

tions of 1.2=10^yM, 0.6=10^yM, SO₄²

and S₂O₃²production concentration is increas-

ingly dependent to the concentration of sub-

strate. For initial concentrations of 5.0=10^y

M, 2.5=10^yM, SO₄^2y, production concentra-

tion is increasingly independent to the concen-

tration of substrate.

3.7. The photocatalytic autoxidation mecha-

nisms

Products analysis by ion chromatography

identified S₂O₃²as the intermediate of photo-

catalytic reaction of S₄O₆²by Co 2-TMPyP ,

Mn 4-TMPyP and Fe 2-TMPyP . Photodecom-

position of S₄O₆²follows a stepwise decompo-

Fig. 11. Concentration of sulfur oxoanions vs. time of oxygen

present at pH 9.0 borate buffer solution with 2.5=10^y4

sition and oxidation to SO₄²by oxygen as

S₄O₆^2yunder illumination with 419 nm light. Mn 4-TMPyP s

Ž .

shown in Eq. 6 .

. w

^2yx Ž . w ^2yx Ž .

A 2.5=10 M, ` S₂O₃, v SO₄, B 1.2=10 M,

S₂O₃

0.6=10^y5M,

. w

^2yx

. w ^2yx Ž .

SO₄

. w

^2yx

S₂O₃,

S₄O₆²^y™S₂O₃^2y™SO₃^2y™SO₄²

Ž .

. w ^2yx

SO₄

(

)

S.-M. ChenrJournal of Molecular Catalysis A: Chemical 138 1999 1–13

tions of photocatalysis and autoxidation, S₄O₆²

decomposes to thiosulfate and sulfate. Co 2-

TMPyP completely converts S₄O₆to SO₄^2y

while Fe 2-TMPyP and Mn 4-TMPyP are only

partially effective, thus producing S₂O₃²

Acknowledgements

This work was supported by the National

Science Council of the Republic of China.

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Article Doi

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

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

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