Electrocatalytic effects of thionyl chloride reduction by polymeric Schiff base transition metal(II) complexes

Applied Catalysis A: General 252 (2003) 163–172

Electrocatalytic effects of thionyl chloride reduction by polymeric Schiff base transition metal(II) complexes Woo-Seong Kim a , Yong-Kook Choi b,∗ a

b

Department of Materials Science & Engineering, K-JIST, Kwangju 500-712, South Korea Department of Chemistry & RRC/HECS, Chonnam National University, KwangJu 500-757, South Korea Received 9 October 2002; received in revised form 28 March 2003; accepted 19 May 2003

Abstract Electrocatalytic effects for the reduction of thionyl chloride in LiAlCl4 /SOCl2 electrolyte solution containing polymeric Schiff base M(II) (M: Ni and Cu) complexes were evaluated by determining kinetic parameters with cyclic voltammetry at a glassy carbon electrode. The charge transfer process during the reduction of thionyl chloride was affected by the concentration of the catalyst. The catalytic effects were demonstrated from both a shift of the reduction potential for the thionyl chloride in a more positive direction and an increase in peak currents. The reduction of thionyl chloride was found to be diffusion-controlled. Catalytic effects are larger in thionyl chloride solutions containing (PVPS)M(II)(SALPR) rather than in those containing (PVPS)M(II)(SALPE). Such results are opposite to those for monomeric Schiff base complexes. Significant improvements in the cell performance were found in terms of both thermodynamics and kinetic parameters for the thionyl chloride reduction. An exchange rate constant, ko , of 1.62 × 10−8 cm/s was found at a bare electrode, while larger values of (4.69–8.18) × 10−8 cm/s were observed at the catalyst-supported glassy carbon electrode. © 2003 Elsevier B.V. All rights reserved. Keywords: Thionyl chloride; Electrocatalytic effect; Polymeric Schiff base; Transition metal

1. Introduction The system of lithium oxyhalide is one of the best for primary batteries. It combines the characteristics of high rate capability, high energy density, long shelf-life and low-temperature operation. Electrochemical studies on the lithium/thionyl chloride primary batteries have been of significant practical interest over the past several years and the mechanism of the reduction of thionyl chloride has been studied [1–14]. The system consists of a Li anode, a porous carbon cathode, and a LiAlCl4 /SOCl2 electrolyte ∗ Corresponding author. Tel.: +82-62-530-3375; fax: +82-62-530-3375. E-mail address: [email protected] (Y.-K. Choi).

solution, where SOCl2 acts as both a solvent and a cathode-active material. The Li anode is prevented from reacting with SOCl2 by virtue of the formation of a LiCl passivation film [1] on the Li as soon as it contacts the LiAlCl4 /SOCl2 electrolyte, according to reaction (1): 4Li + 2SOCl2 → S + SO2 + 4LiCl

(1)

The electrode kinetics of the cathode discharge reaction is rather poor due to the formation of passive LiCl films at the cathode as a result of reaction (1). The film is also the source of a voltage delay owing to its overly passive nature. Many investigators have tried to solve this high passivity problem. One approach to the enhancement of cell performance can be

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00414-9

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the addition of catalyst molecules, which accelerate the rate of electron transfer. Doddapaneni [13,14], after comparing several metal compounds as possible catalysts, found cobalt and iron phthalocyanines to be the most effective. Adding a small amount of metal phthalocyanines improves cell performance by changing both thermodynamic and kinetic parameters for the thionyl chloride reduction. Choi and coworkers [11,12] have observed the passivation and catalytic effect of the thionyl chloride solution containing cobalt phenylporphyrins. This paper reports catalytic effects for the thionyl chloride reduction obtained by evaluating electrokinetic parameters in LiAlCl4 /SOCl2 solution containing quadridentate polymeric Schiff base M(II) compounds as catalysts.

2. Experimental 1,3-Diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane, salicylaldehyde, nickel(II) acetate tetrahydrate, copper(II) acetate monohydrate, styrene, 4-vinyl-pyridine, 2,2 -azoisobutyronitrile, sodium hydroxide, toluene and ethanol were used as received from Aldrich. The elemental analysis (carbon, hydrogen, and nitrogen) was performed on a Foss Heraeus CHN Rapid (Analysentechnik GmbH), and the metal content was determined with a Perkin-Elmer model 603 atomic absorption spectrometer. Infrared and UV-Vis spectra were recorded on Shimadzu IR-430 infrared and Hitachi-557 UV-Vis spectrophotometers. Thermogravimetric analysis (TGA) was carried out using a Perkin-Elmer model 2 Thermogravimetric Analyzer. The molar conductance was measured in DMF at 25 ◦ C with a DKK model AO-6 Digital conductometer. 2.1. Preparation of poly(4-vinylpyridine-co-styrene) (PVPS) In toluene (50 ml) were dissolved 0.03 mol of styrene, 0.01 mol of 4-vinyl-pyridine and 2,2 -azoisobutyronitrile. This solution reacted for 10 h at 60 ◦ C under nitrogen atmosphere. A purple solid (PVPS) was precipitated. The precipitate was dried under reduced pressure at room temperature: 87.2% yield; anal. calcd. for C310 H320 N10 : C, 88.95; H, 7.71; N, 3.35; found: C, 89.11; H, 7.42; N, 3.39; UV-Vis/(DMF,

λmax , ε × 10−4 cm−1 M−1 ): 274/(2.44); IR/(KBr pellet, cm−1 ): 1602/(pyridine). 2.2. Preparation of polymer complexes The (PVPS)M(II)(SALPR), (PVPS)M(II)(SALBU) and (PVPS)M(II)(SALPE) complexes (M: Ni, Cu) were prepared by the addition of 1.25 mol of M(II)[SALPR], M(II)(1,3-bis(salicylidene imino) propane); M(II)[SALBU], M(II)(1,4-bis(salicylidene imino) butane); M(II)[SALPE], M(II)(1,5-bis(salicylidene imino) pentane) in PVPS solutions which were synthesized by the previous method [15,16] for 6 h under nitrogen atmosphere while stirring. The polymer complexes were precipitated. These complexes were dried under reduced pressure at 25 ◦ C. [(PVPS)Ni(II)(SALPR)(H2 O)]: 82% yield; anal. calcd. for C327 H337 N12 O3 Ni: C, 86.47; H, 7.48; N, 3.70; Ni, 1.29; found: C, 85.64; H, 7.32; N, 3.79; Ni, 1.30; UV-Vis/(DMF, λmax , ε × 10−4 cm−1 M−1 ): 272/(3.11); 350/(1.35); IR/(KBr pellet, cm−1 ): 1582/(C=N); 1599 (pyridine); 759/(Ni–N); 543/ (Ni–O); TGA/(weight loss, %): 0.40 at ∼150 ◦ C; 15.44 at 150–415 ◦ C; 98.18 at ∼538 ◦ C. [(PVPS)Ni(II)(SALBU)(H2 O)]: 81% yield; anal. calcd. for C328 H339 N12 O3 Ni: C, 86.50; H, 7.48; N, 3.69; Ni, 1.28; found: C, 86.55; H, 7.39; N, 3.72; Ni, 1.29; UV-Vis/(DMF, λmax , ε × 10−4 cm−1 M−1 ): 272/(2.77); 370/(1.11); IR/(KBr pellet, cm−1 ): 1584/(C=N); 1598 (pyridine); 762/(Ni–N); 541/ (Ni–O); TGA/(weight loss, %): 0.40 at ∼150 ◦ C; 15.79 at 150–446 ◦ C; 98.12 at ∼615 ◦ C. [(PVPS)Ni(II)(SALPE)(H2 O)]: 81% yield; anal. calcd. for C329 H341 N12 O3 Ni: C, 86.47; H, 7.50; N, 3.68; Ni, 1.28; found: C, 85.60; H, 7.48; N, 3.77; Ni, 1.30; UV-Vis/(DMF, λmax , ε × 10−4 cm−1 M−1 ): 272/(2.45); 378/(0.77); IR/(KBr pellet, cm−1 ): 1582/(C=N); 1599 (pyridine); 759/(Ni–N); 543/ (Ni–O); TGA/(weight loss, %): 0.40 at ∼150 ◦ C; 15.63 at 150–448 ◦ C; 98.24 at ∼551 ◦ C. [(PVPS)Cu(II)(SALPR)]: 88% yield; anal. calcd. for C327 H335 N12 O2 Cu: C, 86.72; H, 7.46; N, 3.71; Cu, 1.40; found: C, 86.94; H, 7.50; N, 3.81; Cu, 1.40; UV-Vis/(DMF, λmax , ε × 10−4 cm−1 M−1 ): 274/(1.74); 290/(0.35); 366/(0.21); IR/(KBr pellet, cm−1 ): 1584/(C=N); 1598 (pyridine); 752/(Cu–N); 543/(Cu–O); TGA/(weight loss, %): 0.04 at ∼150 ◦ C; 15.47 at 150–466 ◦ C; 98.28 at ∼642 ◦ C.

W.-S. Kim, Y.-K. Choi / Applied Catalysis A: General 252 (2003) 163–172

[(PVPS)Cu(II)(SALBU)]: 87% yield; anal. calcd. for C328 H337 N12 O2 Cu: C, 86.72; H, 7.48; N, 3.70; Cu, 1.40; found: C, 86.84; H, 7.49; N, 3.71; Cu, 1.41; UV-Vis/(DMF, λmax , ε × 10−4 cm−1 M−1 ): 274/(2.48); 292/(1.02); 368/(0.92); IR/(KBr pellet, cm−1 ): 1584/(C=N); 1600 (pyridine); 760/(Cu–N); 548/(Cu–O); TGA/(weight loss, %): 0.04 at ∼150 ◦ C; 16.02 at 150–432 ◦ C; 98.29 at ∼567 ◦ C. [(PVPS)Cu(II)(SALPE)]: 85% yield; anal. calcd. for C329 H339 N12 O2 Cu: C, 86.72; H, 7.48; N, 3.69; Cu, 1.39; found: C, 86.65; H, 7.54; N, 3.75; Cu, 1.41; UV-Vis/(DMF, λmax , ε × 10−4 cm−1 M−1 ): 274/(2.72); 292/(1.47); 372/(0.97); IR/(KBr pellet, cm−1 ): 1584/(C=N); 1600 (pyridine); 760/(Cu–N); 545/(Cu–O); TGA/(weight loss, %): 0.04 at ∼150 ◦ C; 16.42 at 150–462 ◦ C; 98.23 at ∼646 ◦ C.

research (PAR) 273 potentiostat/galvanostat interfaced with a microcomputer through an IEEE-488 bus was used for electrochemical measurements.

3. Results and discussion Quadridentate polymeric Schiff base metal(II) complexes were prepared and characterized by UV-Vis, IR, TGA, and elemental analysis. The results of elemental analysis of PVPS and polymeric complexes are in good agreement with the expected composition of the proposed complexes (Fig. 1). All complexes are insoluble in water but are soluble in aprotic solvents and SOCl2 . The positive charges of M(II) are perhaps neutralized by two phenoxy groups. The IR spectra of Ni(II) complexes show broad ν(OH) bands of the free ligands at 3400 cm−1 , but such bands were not found in spectra of Cu(II) complexes. The ν(OH) bands are from H2 O. All of the IR spectra of M(II) complexes show typical bands of the Schiff base with strong peaks assigned to ν(C=N) in the 1608–1628 cm−1 region. We can see that ν(C=N) bands in the complex are shifted to the lower energy regions of 24–46 cm−1 in the corresponding free ligands. According to Ueno and Martell [17], characteristic absorption bands for M(II)–N and M(II)–O bonds in the complexes appear, respectively, in the spectral regions of 650–850 cm−1 and 400–600 cm−1 . Here, two absorption bands at 752–762 cm−1 and 541–548 cm−1 are assigned to M(II)–N and M(II)–O bonds. The UV-Vis spectra of the M(II) complexes obtained in DMSO show a ␲–␲∗ ligand field

2.3. Electrochemistry Electrochemical reduction of thionyl chloride has been carried out by cyclic voltammetry. A jacketed single compartment cell housing a glassy carbon working (geometric area, 0.071 cm2 ), lithium foil counter, and lithium wire reference electrodes were used for electrochemical measurements. The glassy carbon electrode was polished to a mirror finish with 0.5 ␮m alumina powder, subsequently cleaned in an ultrasonic cleaning bath for removal of solid particles, and finally rinsed several times with doubly distilled deionized water before use. The 1.53 M LiAlCl4 /SOCl2 electrolyte solution (LITHCO) was used. All experiments were conducted in a glove box under the argon-gas atmosphere. A princeton applied

(C H

CH =N

O

2 )n

M( II)

OH2 N=

CH

N

N

O CH2

CH

30

CH2

CH

165

9

CH2

CH

1

Fig. 1. Structures for polymer complexes.

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absorption band at 270–292 nm and a d–␲∗ charge transfer band at 350–378 nm. Ni(II) complexes show the TGA curve decreasing in weight at ∼150 ◦ C, with subsequent decomposition. Thermal gravimetric analysis data support the conclusion that the Ni(II) complexes contain water molecules. Catalytic activity for electrochemical reduction of thionyl chloride by metal phthalocyanines has been demonstrated by Doddapaneni [14]. Doddapaneni reported two voltammetric peaks for the reduction of thionyl chloride in the presence of these catalysts and ascribed the catalytic activity to the formation of an adduct of thionyl chloride with a phthalocyanine molecule, followed by a two step fast electron transfer to the adduct. The two consecutive electron transfer reactions were described as: M-Pc · SOCl2 + e− → M-Pc · SOCl• + Cl−

(2)

M-Pc · SOCl• + e− → M-Pc + 21 S + 21 SO2 + Cl− (3) These explain the two voltammetric peaks observed. Bernstein and Lever [18] described the reaction as a typical catalytic EC reaction after running an extensive number of experiments in 1,2-dichlorobenzene. Thus, two cyclic voltammetric peaks are observed during the catalytic reduction of Co(III)-Pc first to Co(II)-Pc and then to Co(I)-Pc. The overall catalytic cycle is Co(III)-Pc

+

2e-

Co(I)-Pc + SOCl2

Co(I)-Pc

Co(III)-Pc + 1/2S + 1/2SO2 + 2Cl-

(4) These investigators have shown convincingly that SOCl2 oxidizes Co(I)-Pc through a two-electron transfer reaction. Bernstein and Lever’s conclusions of the thermodynamic and kinetic requirements for electrocatalysts show that the thermodynamic reduction potential of the SOCl2 must be more positive than the oxidation potentials of metal phthalocyanines and also that the electron transfer kinetics must be more favorable at a given electrode for these compounds than for thionyl chloride. While the thermodynamic requirement depends on the electron affinity of the

central metal ion of a catalyst molecule, the latter would be met easily, since most metal phthalocyanines are known to undergo reversible reactions due to their favorable molecular sizes as well as their molecular geometry. Catalytic effects of (PVPS)M(II)(SALPR), (PVPS)M(II)(SALBU) and (PVPS)M(II)(SALPE) complexes on the reduction of thionyl chloride at a glassy carbon electrode were evaluated by determining the kinetic parameters using cyclic voltammetry. Peak currents and peak potentials obtained from the cyclic voltammograms are plotted as a function of the catalyst concentration for (PVPS)Ni(II)(SALPR) complexes. These results are shown in Fig. 2. The magnitude of the reduction current appears to be dependent on the catalyst concentration in the thionyl chloride solution. These phenomena are observed for all complexes, although the extent of the effects is different. There is an optimum concentration for each catalyst at around 0.8 mM because the rate-limiting step for reduction of the thionyl chloride depends on the electron transfer at the electrode surface [19]. Fig. 3 shows cyclic voltamograms of thionyl chloride reduction in the presence of the optimum concentration of polymeric metal(II) complexes at a glassy carbon electrode. As shown in Fig. 3, a sharp current drop at the less positive side seems to be due to passivation of the electrode by lithium chloride [7]. The catalytic effects are clearly seen in the shift of the reduction potential for thionyl chloride towards a positive direction, resulting in the decrease of the overpotential. This indicates that Schiff base complexes used in this study behave as catalysts for the reduction of thionyl chloride. Especially, the peak potential was shifted about 156 mV toward the positive direction and the peak current was increased about 179% in (PVPS)Ni(II)(SALPR). Therefore, we conclude that these compounds show good catalytic activities at a glassy carbon electrode. These results are in accordance with previous results reported by Doddapaneni [13] on the reduction of the SOCl2 solution containing metal phthalocyanine. An extensive number of cyclic voltammetric experiments were conducted to evaluate kinetic parameters only at the “optimum” catalyst concentration. Based on these results, the peak currents and potentials observed for the reduction of thionyl chloride are summarized in Table 1. Fig. 4 shows a series of

W.-S. Kim, Y.-K. Choi / Applied Catalysis A: General 252 (2003) 163–172

167 0.076 0.074

2.666

0.072

Potential

2.664

0.070 0.068 0.066

2.660

0.064 2.658

0.062

2.656

0.060

Current, A/cm2

Potent ial, V vs.Li/ L i

+

Current

2.662

0.058 2.654

0.056 0.054

2.652

0.052 2.650 0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

Concentration, mM Fig. 2. Plots of peak currents and peak potentials vs. the concentration of the catalyst for the reduction of SOCl2 solution containing [(PVPS)Ni(II)(SALPR)(H2 O)]. Scan rate was 50 mV/s.

cyclic voltammograms recorded at various scan rates in thionyl chloride solution containing Ni(II) complexes. The current decays faster than the Cottrellian fashion beyond the cyclic voltammogram peak potential. As discussed above, these phenomena seem to be due to some passivation of the electrode surface during the reduction of thionyl chloride. To establish, whether the electron transfer is a diffusion-controlled or a surface process, we plotted cyclic voltammetric peak currents against scan rates. Fig. 5 shows a plot of peak current versus ν1/2 (ν: scan rate) from voltam-

metric results obtained at the glassy carbon electrode under “optimum” conditions. The result of the plots in Fig. 4 indicates that the thionyl chloride reduction at the glassy carbon electrode is controlled by diffusion of the electroactive compound. The peak current from cyclic voltammetry for an irreversible case is given as follows [20]: ip = (2.99 × 105 )n(αna )1/2 AC∗o Do ν1/2 1/2

(5)

where n is the number of electrons transferred, α the transfer coefficient, na is the apparent number of

Table 1 Peak potentials and peak currents on SOCl2 reduction at the glassy carbon electrode Catalysts

Peak potential, Ep (V)

Peak current, ip (␮A)

Bare

2.649

243

Ni(II)(SALPR)(H2 O)2 Ni(II)(SALBU)(H2 O)2 Ni(II)(SALPE)(H2 O)2

2.656 2.712 2.713

Cu(II)(SALPR) Cu(II)(SALBU) Cu(II)(SALPE)

2.709 2.715 2.716

Scan rate was 50 mV/s.

Catalysts

Peak potential, Ep (V)

Peak current, ip (␮A)

378 411 452

(PVPS)Ni(II)(SALPR)(H2 O) (PVPS)Ni(II)(SALBU)(H2 O) (PVPS)Ni(II)(SALPE)(H2 O)

2.805 2.801 2.790

434 367 365

358 378 449

(PVPS)Cu(II)(SALPR) (PVPS)Cu(II)(SALBU) (PVPS)Cu(II)(SALPE)

2.789 2.789 2.765

408 389 310

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0.006

(a)

Bare Ni(II)(SALPR) Ni(II)(SALBU) Ni(II)(SALPE) (PVPS)Ni(II)(SALPR) (PVPS)Ni(II)(SALBU) (PVPS)Ni(II)(SALPE)

Cathodic current, A

0.005

0.004

0.003

0.002

0.001

0.000 3.6

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

+

Potential, V vs. Li/Li

0.006

(b)

Bare Cu(II)(SALPR) Cu(II)(SALBU) Cu(II)(SALPE) (PVPS)Cu(II)(SALPR) (PVPS)Cu(II)(SALBU) (PVPS)Cu(II)(SALPE)

Cathodic current, A

0.005

0.004

0.003

0.002

0.001

0.000 3.6

3.4

3.2

3.0

2.8

2.6

2.4

2.2

2.0

1.8

+

Potential, V vs. Li/Li

Fig. 3. Cyclic voltammograms for the reduction of SOCl2 solution containing (a) Ni(II) and (b) Cu(II) complexes. Scan rate was 50 mV/s.

electrons transferred, A the electrode area in cm2 , Co∗ the bulk concentration of an electroactive compound in mol/cm3 , and Do the diffusion coefficient of the electroactive compound or other charge carrier in cm2 /s. Eq. (5) reveals that a plot of ip versus ν1/2 allows us to obtain the diffusion coefficient of thionyl chloride with known αna . From the relationship of ip

with Ep [20]:


   αna F o o ip = 0.227nFAC∗k exp − (E − E ) p o RT (6) ko

is the exchange rate constant in cm/s and where  Eo is the standard electrode potential in V. We can

W.-S. Kim, Y.-K. Choi / Applied Catalysis A: General 252 (2003) 163–172 0.012

0.009

20 mV/s 50 mV/s 100 mV/s 200 mV/s

0.008

20 mV/s 50 mV/s 100 mV/s 200 mV/s

0.010

0.006

0.008

Cathodic current, A

Cathodic current, A

0.007

0.005 0.004 0.003 0.002 0.001 0.000

0.006

0.004

0.002

0.000

-0.001 3.6

3.4

3.2

3.0

2.8

2.6

2.4

Potential, V vs. Li/Li

(a)

2.2

2.0

1.8

+

3.6

3.4

3.2

(c)

3.0

2.8

2.6

2.4

Potential, V vs. Li/Li

2.2

2.0

1.8

+

0.010

0.012

20 mV/s 50 mV/s 100 mV/s 200 mV/s

20 mV/s 50 mV/s 100 mV/s 200 mV/s

0.009 0.008

Cathodic current, A

0.010

Cathodic current, A

169

0.008

0.006

0.004

0.007 0.006 0.005 0.004 0.003 0.002

0.002 0.001 0.000

0.000 3.6

(b)

3.4

3.2

3.0

2.8

2.6

2.4

Potential, V vs. Li/Li

2.2

2.0

-0.001 3.6

1.8

3.4

3.2

(d)

+

3.0

2.8

2.6

2.4

Potential, V vs. Li/Li

2.2

2.0

1.8

+

Fig. 4. Scan rate dependency of the voltammograms for the reduction of the SOCl2 solution containing (a) no complexes; (b) (PVPS)Ni(II)(SALPR)(H2 O) complexes; (c) (PVPS)Ni(II)(SALBU)(H2 O) complexes and (d) (PVPS)Ni(II)(SALPE)(H2 O) complexes. Scan rates were (a) 20; (b) 50; (c) 100 and (d) 200 mV/s.



calculate ko . A plot of ln(ip ) versus Ep −Eo should yield a straight line with a slope, αna F/RT, and an intercept, ln(0.227nFACo∗ ko ); from these, αna and ko values can be calculated, respectively. In these calculations, the thermodynamic Eo , value of 3.734 V versus Li/Li+ is used as it is obtained from Eq. (1) using free energies of formation for reactants and products  [6]. The ln(ip ) versus Ep −Eo plots are shown in Fig. 6 for the reduction of SOCl2 at the glassy carbon electrode. The αna value obtained from this equation can 1/2 be used to determine Do in Eq. (5). Kinetic parameters calculated from these plots at “optimum” catalyst concentrations are listed in Table 2. As shown in Table 2, exchange rate con-

Table 2 Kinetic parameters on SOCl2 reduction at the glassy carbon electrode Catalysts

na

Do (cm2 /s)

ko (cm/s)

Bare

0.18

9.15 × 10−9

1.62 × 10−8

(PVPS)Ni(II)(SALPR)(H2 O) (PVPS)Ni(II)(SALBU)(H2 O) (PVPS)Ni(II)(SALPR)(H2 O)

0.16 0.17 0.17

2.07 × 10−8 1.81 × 10−8 1.76 × 10−8

6.32 × 10−8 5.13 × 10−8 4.69 × 10−8

(PVPS)Cu(II)(SALPR) (PVPS)Cu(II)(SALBU) (PVPS)Cu(II)(SALPE)

0.16 0.17 0.17

1.45 × 10−8 1.31 × 10−8 1.17 × 10−8

8.18 × 10−8 5.17 × 10−8 4.85 × 10−8

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W.-S. Kim, Y.-K. Choi / Applied Catalysis A: General 252 (2003) 163–172

0.012 0.011

peak curre nt , A/ cm

2

0.010

Bare (PVPS)Ni(II)(SALPR) (PVPS)Ni(II)(SALBU) (PVPS)Ni(II)(SALPE)

0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.10

0.15

0.20

0.25

0.30

0.35

(scan rate, V/sec)

(a)

0.40

0.45

1/ 2

0.012 0.011

peak curr ent, A/ cm

2

0.010

Bare (PVPS)Cu(II)(SALPR) (PVPS)Cu(II)(SALBU) (PVPS)Cu(II)(SALPE)

0.009 0.008 0.007 0.006 0.005 0.004 0.003 0.002 0.001 0.10

(b)

0.15

0.20

0.25

0.30

(scan rate, V/ sec)

0.35

0.40

0.45

1/ 2

Fig. 5. Plots of peak current vs. ν1/2 for the reduction of SOCl2 at optimum concentrations containing (a) Ni(II) and (b) Cu(II) complexes.

stants, ko , are determined to be 1.62 × 10−8 cm/s at the bare glassy carbon electrode at room temperature. On the other hand, these values are determined to be (4.69–8.18) × 10−8 cm/s at the catalyst-supported

glassy carbon electrode. The increase in exchange rate constant indicates significant improvements in cell performance. Therefore, most of the enhancement is attributed to the catalytic effects of the polymeric

W.-S. Kim, Y.-K. Choi / Applied Catalysis A: General 252 (2003) 163–172

171

-4.2 -4.4

2

ln(peak current, A/cm )

-4.6 -4.8 -5.0 -5.2 -5.4 -5.6 -5.8 -6.0 Bare (PVPS) Ni( II)(SALPR) (PVPS)Ni(II)(SALBU) (PVPS)Ni(II)(SALPE

-6.2 -6.4 -6.6

-1.20

-1.15

-1.10

-1.05

-1.00

-0.95

o

(a)

E p-E ', V vs. Li/Li

-0.90

-0.85

+

-4.2 -4.4

2

ln(peak current, A/cm )

-4.6 -4.8 -5.0 -5.2 -5.4 -5.6 -5.8 -6.0 Bare (PVPS)Cu(II)(SALPR) (PVPS)Cu(II)(SALBU) (PVPS)Cu(II)(SALPE)

-6.2 -6.4 -6.6 -1.20

-1.15

-1.10

-1.05 o

(b)

-1.00

-0.95

-0.90

-0.85

+

E p-E ', V vs. Li/Li 

Fig. 6. Plots of 1n(ip ) vs. (Ep −Eo ) for the reduction of SOCl2 at optimum concentration containing (a) Ni(II) and (b) Cu(II) complexes.

Schiff base complexes. As shown in Tables 1 and 2, catalytic effects are slightly larger in thionyl chloride solution containing (PVPS)M(II)(SALPR) complexes than in those containing (PVPS)M(II)(SALBU) and

(PVPS)M(II)(SALPE) complexes, due probably to steric reasons. These results are opposite to our previous results [15]. The steric factors of the polymeric Schiff base metal(II) complexes are larger than those

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W.-S. Kim, Y.-K. Choi / Applied Catalysis A: General 252 (2003) 163–172

of the monomeric Schiff base metal(II) complexes in shorter chains.

4. Conclusions It is clear that some polymeric Schiff base transition metal compounds show sizable catalytic activity for the reduction of thionyl chloride. From our results, we conclude that: 1. The catalyst molecules are reduced on the electrode surface, which in turn reduces thionyl chloride, resulting in the generation of oxidized catalyst molecules; thus the catalytic cycle is completed. 2. There is an optimum concentration for each catalyst. 3. Relative catalytic effects are slightly larger in the thionyl chloride solution containing (PVPS)M(II)(SALPR) complexes compared with those in solutions containing (PVPS)M(II)(SALBU) and (PVPS)M(II)(SALPE). Acknowledgements This work was supported by the Korean Science and Engineering Foundation through the Region Research Center of HECS of Chonnam National University (2002). WSK acknowledge the financial support from the BK 21 project of the Ministry of Education. References [1] Y.-K. Choi, W.-S. Kim, K. Chung, M.-W. Chung, H.-P. Nam, Microchem. J. 65 (2000) 3. [2] W.-S. Kim, M.-S. Shin, Y.-K. Choi, K.-H. Chjo, Bull. Korean Chem. Soc. 14 (1993) 313.

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