Study of a 1 : 12 phosphomolybdic anion doped polypyrrole film electrode and its catalysis

87

J. Electroarud. Chem., 354 (1993) 87-97

Elsevier Sequoia S.A., Lausanne

JEC 02685

Study of a 1: 12 phosphomolybdic film electrode and its catalysis Shaojun Dong

l

anion doped polypyrrole

and Wen Jin

Laboratory of Electroanalytical Chemistry, Changchun Institute of Appiled Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin (People’s Republic of China)

(Received 11 August 1992; in revised form 4 January 1993)

Abstract

A phosphomolybdic anion doped polypyrrole (PMo,,O& + PPyJ film electrode has been prepared by electrochemical polymerization of pyrrole in an aqueous solution of 0.5 mol I-’ HaSO, or 0.5 mol anions, and characterized by scanning electron microscopy and 1-t KNO, containing PMo,,O& in-situ UV-visible spectroelectrochemical methods. The film electrode obtained is very stable upon potential cycling in acidic solution, but not in neutral solution. The catalytic effect of the film electrode on the reduction of ClO; and BrO; was studied.

INTRODUCTION

Recently heteropoly-acid-modified electrodes have been attracting much interest because of their good electrocatalytic properties [l-3]. There are three main strategies for modifying this type of species on electrode surfaces: electrochemical deposition [4,5], anion exchange [6,7] and immobilization of heteropoly anions as a dopant in a conducting polymer matrix [8-121. The third of these strategies seems most promising because the modified electrodes thus prepared have the characteristics of both the conducting polymer and the heteropoly acid. Most of the heteropoly anion doped conducting polymer film electrodes have been prepared in acetonitrile and a few in aqueous solution [10,13,14]. Lapkowski et al. [13] observed that the addition of 1: 12 phosphomolybdic anion to an aqueous solution of pyrrole caused immediate polymerization of pyrrole because of

l

To whom correspondence

0022-0728/93/$06.00

should be addressed.

0 1993 - Elsevier Sequoia S.A. All rights reserved

88

the oxidation capability of the 1: 12 phosphomolybdic anion, and black polypyrrole powder was obtained. In this paper we report the preparation of 1: 12 phosphomolybdic anion doped polypyrrole (PMo,,O$,- + PPyj film electrode by electropolymerization in an aqueous solution of KNO, or H,SO,. In order to avoid the direct chemical polymerization of pyrrole by 1: 12 phosphomolybdlc acid, a dilute solution of 1: 12 phosphomolybdic acid was used. Also, a freshly prepared solution was used to prevent the hydrolysis of 1: 12 phosphomolybdic anion. The electropolymerization of the film electrode and its catalysis have been studied. EXPERIMENTAL

Reagent grade 1: 12 phosphomolybdic acid (H,[PMo,,O,l . XH,O), KNO,, H,SO,, KClO, and KBrO, were used as received. Pyrrole was distilled and stored under a nitrogen atmosphere. Double-distilled water was used to prepare the solutions. The films were usually grown by potential scanning between 0.75 V and -0.10 V in H,SO, aqueous solution or between 0.75 V and -0.30 V in aqueous KNO, solution on a platinum plate or glassy carbon disc working electrode in a threeelectrode one-compartment cell with a platinum plate counter-electrode and an Ag/AgCl sat. KC1 reference electrode. After about 20 scanning cycles the film electrode was removed, thoroughly rinsed with water and transferred to an aqueous solution comaining 0.5 mol 1-l H,SO, or 0.5 mol I-’ KNO, which was used as the electrolyte for cyclic voltammetric measurements. The solution used for film preparation typically contained 5.78 X lop2 mol 1-l pyrrole, 5 x 10d3 mol 1-l phosphomolybdic acid and 0.5 mol 1-l KNO, or 0.5 mol 1-i H,SO,. The solution was degassed with argon before electrolysis and kept continuously under an argon flow at a positive pressure during the polymerization process. Cyclic voltammograms were recorded using a CV-47 corrosion voltammograph (BAS Co., USA) with a 60000 x-y recorder (Gould Electronics, China). A model ATA-1A rotating-disc electrode was used to study the catalytic kinetics. In-situ UV-visible spectroelectrochemical experiments were performed in a matched cell of path length 1.0 cm on a DMS-90 spectrometer (Varian, Australia). The working electrode was a piece of indium tin oxide glass coated with a thin layer of polypyrrole. RESULTS AND DISCUSSION

Preparation of a PMo,,O,3mol I-’ H2S0,

+ PPy film electrode in an aqueous solution of 0.5

The PMo,,O$- anion is unstable in aqueous solution and undergoes a series of hydrolysis processes [151, but it is fairly stable in acidic solution. When pyrrole was

89

,

I

0.6

0.6

0.4

0.2

E/V

vs.Ag/AgCL

I

I

0.0

-0.2

Fig. 1. Cyclic voltammograms recorded during the electrochemical polymerization of pyrrole in 0.5 mol I-’ HZSO, aqueous solution containing 5~10~~ mol I-’ PMo,,O&. Scan rate, 100 mV SC’; [pyrrole] = 5.78 x 10W2 mol 1-l.

added to an aqueous solution of 1: 12 phosphomolybdic acid, the colour of the solution changed from yellow to dark green. Bidan et al. [8] attributed the colour change observed on addition of pyrrole or N-methyl pyrrole to acetonitrile solution to the formation of pyrrole and heteropoly anion adducts, e.g. PW,,Oi;--’ (H,O), _,(Py)L-. Because PMo,,O$ is a strong oxidation agent, we consider the colour change of the solution in this case to be due to the strong oxidation capability of PMo120&. When pyrrole is oxidized, phosphomolybdic anion itself is reduced to “heteropoly blue”, causing the colour change of the solution from yellow to green. Figure 1 shows the cyclic voltammograms recorded during the electrochemical polymerization of pyrrole in aqueous solution containing phosphomolybdic acid. Three redox couples appear in the potential range 0.75 V to - 0.10 V and the peak currents increase with the number of scans. After the electrochemical polymerization has proceeded for a certain time, a bright green polymer film is formed on the platinum electrode surface and the solution becomes very dark.

90

lSOmV/s 90 mV/s 50 mV/s 30mV/s I OmV/s

v, ,

m

II

I

0.7

.

8

0.5

0.3 E/V

0.1

-0.1

vs.Ag/AgCl

Fig. 2. Cyclic voltammograms of PMo,,O& scan rates.

+PPy film electrode in 0.5 mol I-’ H,SO,

at different

Figure 2 shows the cyclic voltammograms of the film electrode in an aqueous solution of 0.5 mol I-’ H,SO, at different scan rates. The peak currents are proportional to scan rates, and the film electrode is very stable upon cycling in this solution. Four reversible redox peaks appear in the potential range of 0.65 V to -0.10 V. The mean peak potentials E1,2 = (E,, + E&/2 are 0.0 V (I), 0.25 V (II), 0.38 V (III) and 0.50 V (IV) respectively. The shape of the cyclic voltammograms is very similar to that of phosphomolybdic anion doped polyWmethylthiophene) film electrode in a solution of 0.5 mol 1-l in LiClO, [9]. Redox peaks III-III’, II-II’ and I-I’ correspond to reduction and oxidation through two-, fourand six-electron processes respectively [16,17]. The peak potentials of these three peaks are very similar to those observed in homogeneous phosphomolybdic aqueous solution and at a PMo,,O$modified glassy carbon electrode [4]. The cause

91

J

0.8

’

’

I

0.6

t

0.4 E/V

I

I

0.2

I

t

0.0

I

I

I

-0.2

w.Ag/AgCL

Fig. 3. Cyclic voltammograms during the electrochemical polymerization of pyrrole in 0.5 mol I-’ MO, aqueous solution containing SX 1O-3 mol I-’ PMo120&. Scan rate 100 mV s-‘. [Pyrrole]= 5.78x lo-* mol I-‘.

of peak IV-IV’ PMo i20&-.

is still not clear; it is probably due to some hydrolysis products of

Preparation of a PMo,,O& mol I-’ KN03

+ PPy film electrode in an aqueous solution of 0.5

In contrast with the H,SO, solution system, when pyrrole is added to 0.5 mol 1-i KNO, solution containing PMo,,O&, the colour of the solution is almost unchanged, showing that the oxidation capability of the 1: 12 phosphomolybdic anion in neutral solution is much less than that in acidic solution. Figure 3 shows the cyclic voltammograms recorded during the electrochemical polymerization of pyrrole in an aqueous solution of 0.5 mol 1-l MO, containing 5 x lop3 mol 1-i 1: 12 phosphomolybdic acid. In contrast with the case of 0.5 mol

92

I

L

0.6

0.2

0.4 E/V

I

0.0

I

I

I

-0.2

vs.Ag/AgCl

Fig. 4. Cyclic voltammograms of a PMo,,O& mol I-’ H,SO,. Scan rate, 100 mV s-l.

+ PPy film electrode in (a) 0.5 mol

1-l KNO, and (b) 0.5

1-l H,SO, solution, three distinct peaks are not observed during the positive scans. After the electrochemical polymerization process, the film is rinsed with water and then transferred to an aqueous solution of 0.5 mol I-’ KNO,. Figure 4(a) shows the cyclic voltammogram of the film which has an ill-defined form in this solution. The film electrode is not stable upon potential cycling in an aqueous solution of 0.5 mol 1-i KNO,. If the film electrode is transferred into an aqueous solution of 0.5 mol 1-l H,SO,, the cyclic voltammogram changes dramatically to give four sharp redox peaks as shown in Fig. 4(b). The film electrode is fairly stable and can withstand potential cycling in H,SO, solution. Characterization of films prepared in 0.5 mol I- I H,SO,

and 0.5 mol l- I KNO,

Films prepared in 0.5 mol 1-l H,SO, and 0.5 mol 1-l KNO, were characterized by scanning electron microscopy (SEM) and in-situ spectroelectrochemical methods. Figure 5 shows scanning electron micrographs of the two films; it can be seen that their morphology is very similar. Figure 6 shows in-situ UV-visible spectra of films prepared in 0.5 mol 1-l H,SO,. Absorptions at 380 nm and 460 nm are due to polypyrrole [18]. As the

(B)

(Al

Fig. 5. Scanning electron micrograph of PMo,,O$~ and (B) 0.5 mol I-’ KNO,.

+PPy films prepared in (A) 0.5 mol I-’ H,SO,

potential decreases, a new absorption due to the reduction of phosphomolybdic anions appears at ‘about 530 nm [19]. Similar behaviour is observed in the spectra of films prepared in 0.5 mol I-’ KNO,. There seem to be no differences between films prepared in these two solutions.

450550

350

X/nm

Fig. 6. W-visible spectra of a PMo,,O$- +PPy film prepared in 0.5 mol I-’ H,SO,: 0.00 V, c, 0.10 V; d, 0.20 V; e, 0.30 V; f, 0.60 V.

a, -0.02 V, b,

94

Electrocatalytic effect of a PMo,,O&and ClO; anions

+ PPy film electrode on the reduction of BrO;

Unoura et al. [20] have studied the catalytic effect of PMo,,O&- on the reduction of ClO; in homogeneous aqueous solution. They found that the cathodic wave corresponding to the reduction of the phosphomolybdic anion from four-electron to six-electron reduction products (peak I) has catalytic properties in the presence of ClO;. Wang and Dong [21] have studied the catalytic reduction of ClO; and BrO; on PMo,,O$--modified carbon fibre electrodes and have found that the catalytic effect on the reduction of ClO; is more obvious than that

C

(B)

I b

-0.1 0.1 0.3 E/V Fig. 7. (A) Cyclic voltammograms of a PMoIzO.$ + PPy film electrode in 0.5 mol I-’ H,SO, aqueous solutions containing (a) 0.0 mol I-’ BrO; and (b) 0.03 mol I-’ BrO; (scan rate, 10 mV s- ‘1. (B) Cyclic voltammograms of a PMo,,O.& +PPy film electrode in 0.5 mol 1-l H,SO, aqueous solutions containing (a) 0.0 mol I-’ ClO,; (b) 0.02 mol I-’ CIO; and (cl 0.04 mol 1-l ClO; (scan rate, 10 mV s-1). 0.7

0.5

95 8-

/a

6a

I

0.7

*

’

’

0.5

n

’

’

E

0.1

0.3

-0.1

t

E/V vs.Ag/AgCl

Fig. 8. Cyclic voltammograms of a PMo,rO& +PPy film electrode in 0.5 mol 1-r H,SO, containing BrO; concentrations of 0.0025, 0.005, 0.01, 0.02, 0.04 and 0.05 mol 1-l (reading from bottom to top). Scan rate, 10 mV s -l. The inset shows catalytic current Ip vs. BrO; concentration c.

observed in a homogeneous aqueous solution. The catalytic reduction of ClO; and BrO; on PMo,,O$ + PPy film electrodes can be seen clearly in Fig. 7: the cathodic currents rise while the anodic currents decrease and disappear. Unlike

I

0.5

I

*

I

0.3 E/V

0.1

,

-d.l

vs.Ag/AgCl

Fig. 9. Cyclic voltammograms of a PMo,,O& + PPy film electrode rotating at w = 122 rev min-’ in 0.5 mol I-’ H,SO, solutions containing (a) 0.0 mol 1-r BrO; and (b) 0.02 mol 1-l BrO;. Scan rate, 10 mV s-l.

96

.

0 Fig. 10. Koutecky-Levich BrO; concentrations.

0.01

0.03 ” -"2/(rev/min) plots of a PMo,,O&

moL/L

0.06

0.09

92

+ PPy film electrode in 0.5 mol I-’ H,SO,

at different

the catalytic reduction of ClO; and BrO; in homogeneous PMo,,Oj{ solution or on a PMo,,O&-modified carbon fibre electrode, the catalytic wave appears on the second reduction wave (peak II) of PMo,,O&-, corresponding to two-electron to four-electron reduction of PMo,,O&, and peak I is almost unaffected by the addition of BrO; or ClO;. Figure 8 shows cyclic voltammograms of a PMo,,Oz; + PPy film electrode in solutions of different BrO; concentrations. The catalytic current is proportional to BrO; concentration in the range 2.5 X 10e3-4.0 X lo-’ mol l-‘, as shown in the inset to Fig. 8. We used a rotating-disc electrode to investigate the kinetics of this electrocatalytic reaction. Films used in the kinetic study were grown on glassy carbon electrodes (A = 0.13 cm*) in an aqueous solution of 0.5 mol 1-l H,SO, + 5 X 10m3 mol l-‘, 1: 12 phosphomolybdic acid. Figure 9 shows cyclic voltammograms of the rotating film electrode obtained at a rotational speed w of 122 rev min-’ in 0.5 mol 1-l H,SO, and 0.03 mol 1-l BrO;. A plateau appears on peak II. We chose the current measured at a potential of 0.06 V as the limiting current I, and varied the rotating speed to examine the kinetic process. Figure 10 shows the KouteckyLevich plot at different BrO; concentrations. IL1 is proportional to w-‘I’, and the slope and intercept of the plot are proportional to the BrO; concentration, indicating that the reaction is the first order with respect to BrO;. This can be expressed by the equation [221 l/Z, = l/(nE4kr,,,cb)

+ l/Z,,

where ZLev = 0.62nFAD2/3v-1/6cb.w1/2

97

Here IL is the limiting current, k is the catalytic constant, I,,, is the surface concentration of the catalyst, cb is the bulk concentration of BrO;, ZhV is the Levich current, A is the area of the electrode, D is the diffusion coefficient and u is the kinematic viscosity of the solution. When w + ~0,i.e. ww112+ 0, k = Z,/nFAI’,,,cb In the case of the PMo,,O& + PPy film electrode, the charge corresponding to the second reduction peak is equal to 0.7 mC (Q = &“I,,,). Hence values of k = 3 x lo5 cm3 mol-’ s-l and I,,, = 3 X lo-* mol cme2 are obtained. ACKNOWLEDGEMENT

This work was supported China.

by the National

Natural

Sciences Foundation

of

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