The electrocatalytic transformation of HS−, S2O32−, S4O62− and SO32− to SO42− by water-soluble iron porphyrins

Electrochimica Acta 45 (2000) 4399 – 4408 www.elsevier.nl/locate/electacta

The electrocatalytic transformation of HS−, S2O23 − , S4O26 − and SO23 − to SO24 − by water-soluble iron porphyrins Shen-Ming Chen *, Shiu-Wen Chiu Department of Chemical Engineering, National Taipei Uni6ersity of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC Received 12 November 1999; received in revised form 28 April 2000

Abstract The electrocatalytic transformation of S4O26 − , S2O23 − , SO23 − and HS− to SO24 − by water-soluble iron porphyrins was investigated. The electrocatalytic oxidation of S2O23 − to SO24 − can be performed by Fe(n-TMPyP) (n= 2,3, and 4). S4O26 − is reduced to S2O23 − by iron(I) species at weak basic aqueous solution, then S2O23 − is oxidized to SO24 − through iron(IV) porphyrin species. S4O26 − can be transformed to S2O23 − by a chemical processes first and then the electrocatalytic oxidation of S2O23 − to SO24 − is performed through iron(IV) species in a strong basic aqueous solution. HS− can be oxidized to S2O23 − in a solution containing O2 by chemical process, and then S2O23 − is oxidized by iron porphyrin using electrocatalytic process. At a weak basic solution, S4O26 − as the minor product can be reduced back to S2O23 − by electrocatalytic method and the cyclic process of oxidation and reduction repeated. The electrocatalytic oxidation of S4O26 − to SO24 − can be performed directly by Fe(n-TMPyP) (n = 2, 3, 4) in a strong basic aqueous solution. The electrocatalytic oxidation of SO23 − by Fe(n-TMPyP) through Fe(IV) was investigated at room temperature in aqueous solution. FeIII(n-TMPyP) (n= 2,3,4) can be oxidized to iron(IV) species by electrochemical method in the range of basic aqueous solution, then Fe(IV)(n-TMPyP) oxidizes SO23 − to SO24 − . The electrocatalytic oxidation activity is pH dependent in a wide pH range from 2.0 to 13.0, (O)FeIVP has more catalytic activity than (OH)(O)FeIVP. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrocatalytic; Transformation; Oxidation

1. Introduction The transformation of sulfur oxoanions (S2O23 − , S4O26 − , SO23 − ) and sulfides (HS−, S2 − ) is of great interest to workers in chemistry and biology [1–6]. Thiobacilli oxidize inorganic sulfur compounds such as S2O23 − , and HS− to SO24 − and use the energy for growth [2]. The reactions involve the following processes [2].

* Corresponding author. Fax: +886-2270-25238.

The oxoiron(IV) porphyrins have been investigated by various methods [7 – 15]. The water-soluble iron porphyrin Fe(2-TMPyP) has also been characterized [7 – 10] and the iron(I) porphyrins Fe(2-TMPyP) have been investigated in aqueous solution [8 – 10]. The electrocatalytic reduction of S4O26 − [16] and sulfur-containing compounds like L-cystine and oxidized form of glutathione [17] can be performed through iron(I) porphyrin species. This paper first describes the electrocatalytic oxidation of S2O23 − to SO24 − by Fe(n-TMPyP) (n=2, 3, 4) (Fig. 1) through Fe(IV) species. The second part describes the electrocatalytic transformation of S4O26 − to

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SO24 − by Fe(2-TMPyP) and Fe(3-TMPyP) through a two-step process. The third part covers the transformation of HS− to SO24 − through a chemical process containing O2 and then an electrocatalytic oxidation. The fourth part deals with electrocatalytic oxidation of S4O26 − to SO24 − directly by Fe(n-TMPyP) (n=2,3,4) in a strong basic solution. This paper also describes the 2− electrocatalytic oxidation of HSO− by Fe(23 and SO3 TMPyP) through Fe(IV) species. The reaction processes were proposed. This research deals with the selective oxidation of sulfur oxoanions to sulfate by various iron porphyrin catalysts. The transformation reactions are effective and selective by the catalyst. The electrochemical conversion (anodic oxidation and cathodic reduction) of these sulfur oxoanions species are important for environmental protection. Many of the concepts presented are also useful in electrocatalytic processes, material science, and electrochemical engineering to offer new and more effective processes.

2. Experimental All of the chemicals were of analytical grade. Aqueous solutions were prepared with doubly distilled deionized water. Solutions were deoxygenated by purging with pre-purified nitrogen gas. Buffer solutions were prepared from H2SO4, KHP, acetate, phosphate, borate, carbonate, and KOH for the pH range 0–14. The pH values were measured with a HANNA Model 8418 pH meter.

Fig. 1. Structure of Fe(2-TMPyP), (3-TMPyP), and Fe(4TMPyP).

A sample of FeIII(2-TMPyP) was prepared according to a literature method [18,19]. Pyrrole and 2-pyridine carboxaldehyde were refluxed in propionic acid to obtain the meso-tetrakis (2-pyridyl) porphyin (H2(2TMPyP), C40N8H26). Methylation was achieved by reacting H2(2-TMPyP) 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 FeC12 · xH2O in distilled water for 10 h. [FeIII(2-TMPyP)]5 + was precipitated by drops of saturated NaClO4 solution and recrystalized with water. The products were identified by their UV-visible, IR, and NMR spectra. Fe(3TMPyP) and Fe(4-TMPyP) were also synthesized by a literature method [18,19]. Electrochemistry was performed with a Bioanalytical system (West Lafayette, IN) Model CV-50W potentiostat and a BAS X-Y recorder. Cyclic voltammetry was conducted with the use of a three-electrode cell in which a BAS glassy carbon electrode (area 0.07 cm2) was used as the working electrode. The glassy carbon electrode was polished with 0.05 mm alumina on Buehler felt pads and ultrasonicated for 1 min. The auxiliary compartment contained a platinum wire, which was separated by a medium-sized glass frit. All cell potentials were taken with the use of a Ag/AgCl/KCl (saturated KCl solution) reference electrode. The spectroelectrochemical cell consisted of a 1 mm cuvette, a 100-mesh platinum gauze used as a working electrode, a platinum wire used as an auxiliary electrode, and an Ag/AgCl reference electrode. Bulk electrolysis was performed in a bulk electrolysis cell that consisted of a reticulated vitreous carbon (RVC) working electrode, a platinum wire auxiliary electrode in an isolation chamber and a reference electrode. The spectroelectrochemical cell used for the EMIRS (electrochemically modulated infrared spectroscopy) and SNIFTIRS (surface normalized interfacial Fourier transform infrared spectroscopy) consisted of a glassy carbon disc electrode or a tin dioxide electrode used as a working electrode, a platinum wire used as an auxiliary electrode, and a Ag/AgCl reference electrode. The IR spectrometer employed was a Jasco FTIR-300E with a horizontal specular reflectance accessory. UVvisible spectra were measured with a HITACHI Model U-3300 spectrophotometer. The ion chromatograph used in the experiments was a Dionex Instruments Ion chromatography DX-100 consisting of a pump, conductivity detector, an electrochemical detector, and a syringe loading system with 25-m one sample loop. The IC chromatograms were recorded using a Spectra-Physics Datajet computing intergrator. The columns used throughout were an IonPac AG4A guard column, an lonPac AS4A analytical column, and a self-regenerating suppressor column.

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temperature was at room temperature. These parameters were used for analyzing sulfate, sulfite and thiosulfate. Thiosulfate and S4O26 − were analyzed by an MPIC-NG1 guard column, an MPIC-NS1 analytical column, and a suppressor column. S2 − were analyzed by a PA1 guard column, a PA1 analytical column, and an electrochemical detector. An ion selective electrode was also used in the experiments for measuring the concentration of S2 − in the solutions.

3. Results and discussion

3.1. The electrocatalytic oxidation of S2O 23 − to SO 24 − by Fe(2 -TMPyP)

Fig. 2. Cyclic voltammograms of 3.0 ×10 − 4 M Fe(2-TMPyP) in various pH buffer solution. pH= (A) 2.0, (B) 7.0, (C) 9.0. Scan rate =0.1 V/s.

Fig. 3. Cyclic voltammograms of 3.0 × 10 − 4 M FeIII(2TMPyP) at pH 9.0 borate buffer in the presence of different S2O23 − concentration. (a) 0.0 M; (b) 4 × 10 − 4 M; (c) 8 × 10 − 4 M; (d) 1.2 ×10 − 3 M; (e) 1.6 ×10 − 3 M; (f) 2.0 × 10 − 3 M. (a%) Only 2.0 × 10 − 3 M S2O23 − . Scan rate = 0.1 V/s.

Typical LC operational parameters were as follows: mobile phase was a Na2CO3 and NaHCO3 buffer solution, mobile phase flow rate was 2 ml/min; the column

The cyclic voltammetry of 3 ×10 − 4 M Fe(2-TMPyP) in various pH buffered solutions was studied. The pH range studied was between 2 and 13. The electrochemical response depends upon the acidity of the supporting electrolyte. Fig. 2(A) shows that the FeIII/II(2-TMPyP) redox couple [10] has a forma1 potentia1 E o% = + 0.11 V (vs. Ag/AgCl) in a pH 2.0 buffer solution. Fig. 2(B) shows that the FeIII/II(2-TMPyP) redox couple has a formal potential E o% = + 0.02 V(vs. Ag/AgCl) and the redox couple of FeII/I(2-TMPyP) has a formal potential E o% = − 0.76 V(vs. Ag/AgCl) in a pH 7.0 buffer solution. The irreversible reduction wave at about − 0.2 V in the CV indicates the existence of the m-oxo dimer [8]. The irreversible reduction wave at about − 0.2 V in the CV indicated the existence of the m-oxo dimer [8]. Fig. 2C shows two redox couples of FeIII/II(2-TMPyP) (E o% = − 0.22 V) and FeIII/I(2-TMPyP) (E o% = − 0.76 V) and an irreversible oxidation wave (Epa = + 0.63 V) of FeIII/IV(2-TMPyP) [7,8]. The electrocatalytic oxidation of S2O23 − by Fe(2TMPyP) at pH 9.0 buffer solution was studied by cyclic voltammetry using samples of S2O23 − between 0 and 2.0× 10 − 3 M. The electrochemical responses are shown in Fig. 3. The redox couples of FeIII/II(2-TMPyP) with formal potential E o% = − 0.14 V (vs. Ag/AgCl) and FeII/I(2TMPyP) (E o% = − 0.76 V) and an irreversible oxidation wave (Epa = + 0.75 V) of FeIII/IV(2-TMPyP) were observed [7,8] in a pH 9.0 buffer solution. The catalytic current of the FeIV/III(2-TMPyP) redox couple increases noticeably. This arised from the electrocatalytic oxidation of S2O23 − to SO24 − and S4O26 − through Fe(IV) species. The reversible electrocatalytic reduction of S4O26 − through FeII/I redox couple is not obvious. Bulk electrolysis was performed in a bulk electrolysis cell, using a reticular vitreous carbon electrode. Bulk electrolysis of S2O23 − oxidation catalyzed by Fe(2TMPyP) was performed at Eappl = + 1.10 and + 0.85

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respectively. Experiments performed using Fe(4TMPyP) and Fe(3-TMPyP), also show similar results to the influence of pH. The results are consistent with the following equation for S2O23 − oxidation.

V (pH 9.0), respectively. The products were analyzed using ion chromatography and UV-visible absorption spectroscopy. The only products obtained are SO24 − and S4O26 − . Quantitative analysis of SO24 − and S4O26 − are given in Table 1. After electrolysis at +1.10 V (vs. Ag/AgCl) for more than 40 mins (after S2O23 − is exhausted), 72% of the substrate was converted to SO24 − and 28% was converted to S4O26 − . If the electrolysis was performed at potential +0.85 V (vs. Ag/AgCl), S2O23 − was transformed to SO24 − and S4O26 − at the ratio of 44 and 56%, respectively. After the bulk electrolysis of S2O23 − , product S4O26 − can be returned back to S2O23 − by electrocatalytic reduction through the Fe(II/I) (2-TMPyP) redox couple. For example, after electrolysis at +1.10 V, S2O23 − is 28% converted to S4O26 − . Bulk electrolysis performed at −0.95 V, reduced all 28% S4O26 − to S2O23 − (Table 1). After the reduction process, S2O23 − can be oxidized to SO24 − and S4O26 − by Fe(2-TMPyP) at +1.10 V again. Total 92% S2O23 − can be transformed to SO24 − and 8% transformed to S4O26 − in three sucessive steps (oxidation “reduction “oxidation).

S2O23 − + 5H2O“ 2SO24 − + 10H+ + 8e −

(1)

The variation of conversion with the nature of catalysts at various potential and pH are summarized in Table 1.

3.3. Two-step electrocatalytic transformation of S4O 26 − to SO 24 − by Fe(2 -TMPyP) Fig. 4 shows the cyclic voltammograms of 3.0 × 10 − 4 Fe(2-TMPyP) in a pH 9.0 borate solution using S4O26 − between 0 to 2.2 × 10 − 3 M. In the presence of Fe(2TMPyP), the cathodic peak current of the FeII/I(2TMPyP) redox couple increases noticeably. This is caused by the reduction of S4O26 − to S2O23 − through iron(I) species. The reversible electrocatalytic oxidation through FeIII/IV redox couple corresponds to the oxidation of S2O23 − to SO24 − and S4O26 − . Comparing Fig. 4 and Fig. 3, the results show that the reversible electrocatalytic oxidation after S4O26 − reduction is obvious (Fig. 4), but the reversible electrocatalytic reduction after S2O23 − oxidation is not obvious (Fig. 3). The reason is that for the electrocatalytic oxidation of S2O23 − , a higher percentage was transferred to SO4− 2 and S4O26 − is the minor product. The reversible electrocatalytic reduction is not obvious because of the lower concentration of S4O26 − produced. Otherwise, the electrocatalytic reduction of S4O26 − is transferred to S2O23 − , and the reversible electrocatalytic oxidation current is obvious.

3.2. The electrocatalytic oxidation of S2O 23 − by 6arious catalysts at different pHs Bulk electrolysis of S2O23 − oxidation was conducted at different pH using various catalysts. Table 1 shows the electrocatalytic oxidation of S2O23 − by various catalysts at different conditions. Electrocatalytic oxidation of S2O23 − by Fe(2-TMPyP) was performed at pH 7, 9, and 13, and the percentage of substrate converted to SO24 − were 52, 72, and 100%

Table 1 The electrocatalytic oxidation of S2O2− by various catalysts at different conditionsa 3 Catalyst

V

Fe(2-TMPyP) Fe(2-TMPyP) Fe(2-TMPyP) Fe(2-TMPyP) Fe(2-TMPyP) Fe(3-TMPyP) Fe(3-TMPyP) Fe(4-TMPyP) Fe(4-TMPyP) Fe(4-TMPyP) Fe(II) (5-Clphen)3

+0.85 +1.10 +1.10c +1.00 +1.10 +1.10 +1.10 +1.10 +1.10 +1.10 +1.10

a

SO2− 4 (%) 44 72 92 100 52 70 90 46 67 100 5

S4O2− 6 (%) 56 28 8 0 48 30 10 54 33 0 95

S2O2− (reduction of S4O2− 3 6 ) (%)

28b

48b 30 54b 33

pH 9 9 9 13 7 9 9 7 9 13 1.5

The percentage shows the initial concentration S2O2− transformation. 5-Clphen: 5-chloro-1, 10-phenanthroline. Initial concen3 tration of S2O2− is 5×10−4 M. Concentration of catalysts: [Fe(2-TMPyP)] =10−6 M. After one cycle of electrocatalytic oxidation 3 of S2O2− was performed, 95% of Fe(2-TMPyP) catalyst remained. 3 b After the electrocatalytic oxidation of S2O2− was performed, the electrocatalytic reduction of S2O2− was performed, the 3 3 electrocatalytic reduction of S4O2− (the minor product) to S2O2− was performed. 6 3 c The final product of b is reoxidised.

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3.4. The electrocatalytic transformation of HS− to SO 24 − by iron porphyrins The transformation of HS− to SO24 − can be performed by a two step process. The first step is HS− oxidation in a solution containing O2. HS− is oxidized to S2O23 − by a chemical process. 2HS − + 2O2 “ S2O23 − + H2O

(3)

The second step is the electrocatalytic oxidation of S2O23 − by iron porphyrins through Fe(IV) species. The transformation of S2O23 − to SO24 − is pH dependent and catalyst dependent. After HS− was added to the solution containing O2 for 30 min, the bulk electrolysis was performed. The percentage of S2O23 − converted to SO24 − by Fe(2-TMPyP) is about 50, 70, and 100% at pH 7.0, 9.0, 13.0, respectively. S4O26 − is proFig. 4. Cyclic voltammograms of 3.0 ×10 − 4 M FeIII(2TMPyP) at pH 9.0 borate buffer in the presence of different S4O26 − concentration. (a) 0.0 M; (b) 4.5 × 10 − 4 M; (c) 9 × 10 − 4 M; (d) 1.4× 10 − 3 M; (e) 1.8×10 − 3 M; (f) 2.2× 10 − 3 M. (a%) Only 2.2 ×10 − 3 M S4O26 − . Scan rate = 0.1 V/s.

From the results, the electrocatalytic transformation of S4O26 − to SO24 − occurs in two steps. The first step is the electrocatalytic reduction of S4O26 − to S2O23 − , and the second step is the electrocatalytic oxidation of S2O23 − to SO24 − . The minor product is S4O26 − and the two step reactions can be repeated.

(2) The successive steps of transformation of (1) to (4) are as follows: 1. reduction through Fe(I) species; 2. oxidation through Fe(IV) species; 3. reduction through Fe(I) species; 4. oxidation through Fe(IV) species. Bulk electrolysis was performed and chemical analysis of the reduction and oxidation products indicated the presence of S2O23 − , SO24 − , and S4O26 − (Fig. 5). Fig. 6 shows IR reflection spectra of sulfur oxoanions in aqueous solution with Fe(2-TMPyP) adsorbed on SnO2 electrode and different potentials applied on the working electrode. Fig. 6(A) show IR peaks at 1197 and 1032 cm − 1, indicating S4O26 − present. When Eappl = −0.9 V was applied at working electrode, S4O26 − is transformed to S2O23 − with peaks at 987 and 1127 cm − 1. Fig. 6 (B) spectrum shows S2O23 − present. When Eappl = +0.8 V was applied at working electrode S2O23 − is transformed to SO24 − with peaks at 1138 cm − 1 and minor product S4O26 − with peaks at about 1030 and 1240 cm − 1.

Fig. 5. The electrocatalytic transformation of 2.5 ×10 − 4 M S4O26 − by FeIII(2-TMPyP) at pH 9.0 buffer solution over time. Eappl. (reduction) = − 0.95 V (vs. Ag/AgCl). Eappl. (oxidation) = + 1.10 V (vs. Ag/AgCl).

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Fig. 6. IR reflection spectra of sulfur oxoanions adsorbed on SnO2 electrode in 0.1 M KCl at pH 9.0 buffer solution and Fe(2-TMPyP) present at different potentials. (A) 0.01 M S4O26 − present. (B) Eappl = −0.9 V, S4O26 − transformed to S2O23 − . (C) Following (B) Eappl = + 0.8 V, S2O23 − transformed to SO24 − .

duced as the minor product at a weak basic aqueous solution. The electrocatalytic reduction of S4O26 − to S2O23 − through Fe(II/I) redox couple can be performed, and the cycle of oxidation through Fe(IV) and reduction through Fe(I) repeated. The percentage of S4O26 − , transferred to SO24 − resemble the electrocatalytic oxidation S4O26 − to SO24 − in Section 3.3. Table 2 shows that the concentration of S2O23 − and SO24 − versus time, for HS− oxidized to S2O23 − followed by the electrocatalytic oxidation of S2O23 − to SO24 − at pH 13.0.

(4) The successive processes of (1)–(4) are as follows; 1. oxidation of HS− in a solution containing O2;

2. oxidation through Fe(IV) species; 3. reduction through Fe(I) species; 4. oxidation through Fe(IV) species. Table 2 The transformation of HS− to SO2− by a two-step process at 4 pH 13.0 Time (min)

S2O2− 3

(a) (Non-catalytic oxidation) 0 30

0 3.0×10−4

(b) (Electrocatalytic oxidation) 0 15 20 30 50

3.0×10−4 1.8×10−4 7×10−5 3×10−5 0

SO2− (M) 4

0 0 0 1.4×10−4 4.0×10−4 5.2×10−4 6.0×10−4

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Thirty minutes after S4O26 − is added, about 70% of S4O26 − is converted to S2O23 − and 10% of S4O26 − is converted to SO24 − . The results show all species of sulfur oxoanions were converted to SO24 − . S4O26 − can be converted to S2O23 − and S3O26 − , S3O26 − can transfer to S2O23 − then S2O23 − can be converted to SO24 − by the electrocatalytic oxidation through Fe(IV) species.

(7)

Fig. 7. Cyclic voltammograms of 3.0 × 10 − 4 M FeIII(2TMPyP) at pH 13.0 buffer solution performed immediately after adding different S4O26 − concentration. (a) 0.0 M; (b) 5× 10 − 4 M; (c) 2.2×10 − 3 M. (a%) Only 2.2× 10 − 3 M S4O26 − . Scan rate =0.1 V/s.

3.5. The electrocatalytic transformation of S4O 26 − to SO 24 − by Fe(n-TMPyP) at pH 13 Thirty minutes after adding S4O26 − to a solution of FeIII(2-TMPyP), all of S4O26 − are transferred to other species. The electrocatalytic reduction through iron(I) species is followed by the electrocatalytic oxidation, but no electrocatalytic reduction signal was observed for S4O26 − transferred to S2O23 − . The magnitude of the electrocatalytic reduction current of S4O26 − through Fe(I) (2-TMPyP) is not significant at pH 13. It is known that S4O26 − disproportionates in alkaline to form S2O23 − and SO24 − by the following equations [20]. 4S4O26 − +6OH − “2S3O26 − +5S2O23 − +3H2O 2− 6

S3O

+2OH “S2O −

2− 3

+SO

2− 4

(5)

+H2O

(6)

Fig. 7 shows the electrocatalytic reduction after adding S4O26 − immediately, followed by the reversible electrooxidation of S2O23 − (the product of the electrocatalytic reduction). The electrocatalytic reduction current is obvious only after S4O26 − addition. The reason is that S4O26 − easily transfers to S2O23 − or S3O26 − (S3O26 − also can transfer to S2O23 − ) in a strong basic aqueous solution. Table 3 shows the direct electrocatalytic oxidation of S4O26 − by Fe(n-TMPyp) at pH 13.0 and pH 9.0. S4O26 − is easily transferred to S2O23 − and S3O26 − , and further transferred to SO24 − . The electrocatalytic oxidation of S4O26 − is very slow when the experiment was performed at pH 9.0.

3.6. Electrocatalytic oxidation of sulfite through Fe(IV) (2 -TMPyP) Fig. 8(A) shows the cyclic voltamograms of 3 × 10 − 4 M Fe(2-TMPyP) in a pH 9.0 borate solution using samples between 0 and 2.0 × 10 − 3 M SO23 − . The catalytic peak current of the FeIII/IV(2-TMPyP) redox couple increases noticeably with increasing sulfite concentration. The electrocatalytic reduction of SO23 − through FeII and FeI species is not obvious. Fig. 8(B) shows the cyclic voltammograms of Fe(2TMPyP) in a pH 13.0 aqueous solution using SO23 − concentrations corresponding to figure Fig. 8(A). The electrocatalytic oxidation current decreases as the pH

Table 3 The directly electrocatalytic oxidation of S4O2− by various catalysts at different conditionsa 6 Catalyst

V

Fe(2-TMPyP) Fe(2-TMPyP) Fe(3-TMPyP) Fe(4-TMPyP)

+1.00 +1.00c +1.00 +1.00

SO2− (%) 4 100 10 100 100

S4O2− (%) 6 0 90 0 0

S2O2− (%) 3 70b 0b 70b 70b

pH 13 9 13 13

a The initial concentration of S4O2− is 1.5×10−4 M. There are no significant amounts of S4O2− present after 30 min. Catalysts 6 6 concentration are 10−6 M. After one cycle of electrocatalytic oxidation of S2O2− was performed, about 95% of catalyst remained, 3 b The initial concentration of S2O2− 30 min after adding S4O2− 3 6 . c Bulk electrolysis was performed for 40 mins at pH 9.0.

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of the aqueous solution increases from 9 to 13. Table 4 shows the catalytic current (Ipcat) of the Fe(2-TMPyP) redox couple at different pH with 2.0× 10 − 3 M of SO23 − . The pH has important effects on the efficiency of electrocatalytic oxidation. The electrochemical oxidation of SO23 − to SO24 − can be written as follows: SO23 − + H2O“ SO24 − + 2e − + 2H+

(8)

From this equation, as the pH value of the solution increases, we predict the magnitude of electrocatalytic current to increase, but the experimental results showed otherwise. The electrocatalytic oxidation of SO23 − proceeds through the iron(IV) species, where FeIV(2TMPyP) exhibits pKa of 10.0 in aqueous solution [8], the oxidized forms are FeIV(O)(2-TMPyP) and (OH)FeIV(O)(2-TMPyP). The possibility is that the electrocatalytic efficiency SO23 − with FeIV(O)(2-TMPyP) is greater than (OH)FeIV(O)(2-TMPyP). The formal potential of FeIII/II(2-TMPyP) and FII/I(2TMPyP) was also observed to be dependent on the sulfite concentration in the 0 and 2 ×10 − 3 M range at pH 9.0. The formal potential of FeIII/II(2-TMPyP) shifts to a more positive value with increasing concentration of sulfite. The formal potential of FeII/I(2-TMPyP) is more negative when sulfite concentration increases. FeII(2-TMPyP) forms stronger complexes with SO23 − than do Fe(III) and Fe(I). FeIIIP+ e − + SO23 − = FeIIP(SO23 − ) Fig. 8. Cyclic voltammograms of 3.0 × 10 − 4 M FeIII(2TMPyP) at (A) pH 9.0 borate buffer (B) pH 13.0, in the presence of different SO23 − concentration. (a) 0.0 M; (b) 4×10 − 4 M; (c) 8× 10 − 4 M; (d) 1.2×10 − 3 M; (e) 1.6× 10 − 3 M; (f) 2.0×10 − 3 M. (a%) Only 2.0× 10 − 3 M SO23 − . Scan rate = 0.1 V/s.

Table 4 −3 The electrocatalytic oxidation Ipcata (m A) of [SO2− 3 ] = 2×10 M in various buffer solution with [Fe(2-TMPyP)] = 3×10−4 M pH 2.0 3.0 4.0 5.0 9.0 11.5 13.0 a

Ipcat (m A) 0.5 3 12 22 34 18 4

Ipcat is the anodic peak current increasing in the presence of catalyst.

FeIIP(SO23 − ) +e − = FeIP+ SO23 −

(9) (10)

3.7. Electrocatalytic oxidation of sulfite in acidic aqueous solution Fig. 9 show that the electrocatalytic oxidation of sulfite in acidic aqueous solution. Though the iron(IV) species of Fe(2-TMPyP) was stable in basic aqueous solution [8], the oxidation wave of FeIII(2-TMPyP) to FIV(2-TMPyP) is not as obvious in acidic media, nevertheless, the electrocatalytic oxidation of sulfite by Fe(2TMPyP) is also active. Fig. 9(A) – (D) show that the electrocatalytic activity of sulfite oxidation at pH 2.0, 3.0, 4.0, 5.0, respectively, with the same concentration of SO23 − . The electrocatalytic activity is increasing with increasing pH of the buffer soltuion. The electrocatalytic activity in acidic aqueous solution is also through a iron(IV) species after the iron(III) species is oxidized to iron(IV) species with a slower reaction rate. The processes are as follows: 1. electrochemical step Fe(III)(2-TMPyP)“ Fe(IV)(2-TMPyP)+ e − (11)

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Fig. 10. Cyclic voltammograms of 3.0 ×10 − 4 M FeIII(3TMPyP) at pH =(A) 9.0 (B) 5.0 buffer solution in the presence of SO23 − . (a) 0.0 M; (b) 4×10 − 4 M; (c) 8×10 − 4 M; (d) 1.2 × 10 − 3 M; (e) 1.6 ×10 − 3 M; (f) 2.0 × 10 − 3 M. (a%) Only 2.0 × 10 − 3 M SO23 − . Scan rate =0.1 V/s.

Fig. 9. Cyclic voltammograms of 3.0 ×10 − 4 M FeIII(2TMPyP) at various pH buffer solution in the presence of SO23 − concentration between 0.0 M and 2.0 × 10 − 3 M. pH= (A) 2.0, (B) 3.0, (C) 4.0, (D) 5.0. Blank only 2.0× 10 − 3 M SO23 − scanned to + 1.05 V. Scan rate= 0.1 V/s.

The electrocatalytic activity increasing in higher pH in acidic aqueous solution is consistent with the easier formation the iron(IV) species in basic aqueous solution. The Ipcat versus pH data are shown in Table 4.

2. chemical step

3.8. The electrocatalytic reaction of SO 23 − by Fe(3 -TMPyP) and Fe(4 -TMPyP)

Fe (IV)(2-TMPyP) + SO23 − “Fe(III) (2-TMPyP) +SO3−

(12)

Fe(IV)(2-TMPyP) +SO3− +H2O “Fe(III)(2-TMPyP) +SO24 − +2H+

(13)

Table 5 The formal potential of Fe(n-TMPyP) in pH 9.2 borate buffer solution Fe[I/II)] Fe(2-TMPyP) −0.76a Fe(3-TMPyP) −0.93c Fe(4-TMPyP) No a

See reference [8]. See reference [7]. c See reference [21]. b

[Fe(II/III)P]

[Fe(III/IV)P]

−0.15b −0.23c −0.24c

+0.57a +0.59c +0.60b

The electrocatalytic oxidation of sulfite by Fe(3TMPyP) and Fe(4-TMPyP) was also investigated. Table 5 shows the formal potential of Fe(n-TMPyP) (n= 2,3,4) at various oxidation states. Both Fe(2TMPyP) and Fe(3-TMPyP) have a Fe(I/II)P redox couple but Fe(4-TMPyP) has an irreversible reduction wave which corresponds to the ring decomposition. Fe(n-TMPyP) all have a Fe(III/IV) redox couple in a basic aqueous solution. Fig. 10(A) shows the cyclic voltammograms of Fe(3TMPyP) in a pH 9.0 borate solution using samples between 0 and 2.0×10 − 3 M SO23 − . The anodic peak current of the FeIII/IV(3-TMPyP) redox couple increases noticeably. The electrocatalytic reduction of SO23 − through FeII and FeI is no obviously. Fig. 10(B) show that the electrocatalytic reduction of sulfite in pH 5.0 acidic aqueous solution. The electrocatalytic reduction of sulfite is more active in acidic

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aqueous solution and almost no obvious activity by Fe(3-TMPyP) in basic aqueous solution. The electrocatalytic reduction of SO23 − by Fe(4-TMPyP) is not obvious because the redox couple of Fe(II/I) is not present and only an irreversible reduction wave of the ring decomposition.

Acknowledgements This work was supported by the National Science Council of the Republic of China.

References 4. Conclusion The results presented in this study have shown that S4O26 − , S2O23 − , SO23 − , and HS− can be transformed to SO24 − by the catalysts Fe(2-TMPyP) and Fe(3TMPyP). Fe(4-TMPyP) does not present a clear Fe(II/ I) redox couple but only performed the electrocatalytic oxidation of S2O23 − to SO24 − in a weak basic aqueous solution. The transformation of S2O23 − by Fe(n-TMPyP) (n= 2,3,4) through Fe(IV) species can be performed in a basic aqueous solution. The results show that S4O26 − is transformed to SO24 − following a two-step electrocatalytic processes. The first step involves the electrocatalytic reduction of S4O26 − to S2O23 − , and the second step involves the electrocatalytic oxidation of S2O23 − to SO24 − . The minor product produced from the second step can be converted to SO24 − by repeating the cycle of reduction and oxidation. HS− can be transformed to SO24 − by a two-step process including a chemical process containing O2 in the solution and followed by the electrocatalytic oxidation of S2O23 − to SO24 − . The percentage yield of SO24 − is dependent on S2O23 − converted to SO24 − . S4O26 − can also be converted to SO24 − by a chemical process and an electrocatalytic oxidation processes in a strong basic aqueous solution. S4O26 − disproportionates to form S2O23 − , S3O26 − and SO24 − . S3O26 − can convert to S2O23 − and SO24 − .On the second step S2O23 − can be converted to SO24 − by electrocatalytic oxidation with the highest percentage yield. SO23 − may be electrocatalytically oxidized by Fe(2TMPyP) in basic solutions. For pH values 8.0–14.0, FeIII (2-TMPyP) can form stable FeIV(2-TMPyP). This FeIV(2-TMPyP) complex can oxidize SO23 − to SO24 − according to the proposed mechanisms.

.

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