Zeolite-porphyrin modified electrodes

J. Electroanal Chem.. 187 (1985) 197- 202 Eisevier Seqnola S.A., Lausanne - Printed in The Netherlands

Preliminary

note

ZEOLITE-PORPHYRIN

BERTRAND

197

MODIFIED

ELECTRODES

DE VISMES, FETHI BEDIOUI and JACQUES

DEVYNCP

Laboratowe d%lectrochrmze Analytique et Applrquge, E.-V S C.P , 11, rue Pierre et Mane Cune, 75231 Pans Cedex 05 (France) CLAUDE

BLED-CHARRETON

Laboratoire de Chlmle de Coordrnatzon Bloorganique, Park-Sud, Centre d ‘Orsay, 91405 Orsay (France)

iJA No. 225 du CNRS-UniversiG

(Received 6th February 1985; in revised form 26th February 1985)

INTRODUCTION

Many kinds of electrodes, modified by linkage of coordination compounds, have been described during the last years. Recent reviews have given the “state of the art” and the potential possibilities of such materials [I_--21. One of the most promising domains of application is the electroassisted or electroinitiated catalysis of organic or inorganic reactions by these coordination compounds. If some examples have been proposed, there are few available applications. This lack of apphcabrlity observed with the early modified surfaces is lmked to their poor chemical stabrlity. Film deposits on electrodes give rather stable systems. Thus, coordination compounds linked on polystyrene, polyvinylpyridine or polymer conductors have been proposed. In some of these films (such as polystyrenesulfonate) the coordination compound can be flved by an ion-exchange process [3-53. Such a fixation by ion exchzillgc is also possible on inorganic polymers: Gosh and Bard [G] have shown thar; the electrochemical properties of Ru and Fe complexes fixed on natural clays such as montmorrllonite can be recovered when a layer of clay is deposited on the electrode surface. Zeolites are inorganic supports with well-known ion-exchange and catalytic properties [7]. The possibilities for direct electrochemical investigation of electroactive species on zeohte have been demonstrated in the case of metallic ions [8]. Furthermore, it was shown that a suspension of zeolite on which an electroactive compound is fixed can be oxidized and reduced at a platinum electrode [9] and that coordination compounds of the phthalocyanine-type can be fixed on zeolite [lo] _ These observations suggest that zeolite would be a good intermediate in modified electrodes. In this preliminary note. we describe such electrodes in which the electroactive compound is a multicharged metallic tetramethylpyridinium porphyrin, M(TMPyP) where M = Co, Mn or Fe. These M(TMPyP) are water-soluble *To whom correspondence should be addressed. 0022-0728/85/$03.30

0 1985 Ekevier Sequoia S.A

198

porphyrins [11-131.

whose

electrochemical

properties

have bee;i

investigated

recently

RESULTS

Polymer zeollte electrode meso-Tetrakis(4-N-methylpyrldyl)porphine (TMPyP) and the metal derivatives of Mn(III), CO(II1) and Fe(II1) were prepared according to the known method [14]_ The zeohte is a faulasite (Nay, IIZYS Union Carbide type). Polymer film electrodes m which zeoLte is coated with polystyrene at a platinum-surface electrode have been prepared in the followmg way: The electrode (total area = 2 cm* ) is dipped in a solution of 0.01 g polystyrene + 0.1 g zeolite in 1 ml of THF. The polymer zeolite electrode is then dipped m a 1W3 M TMPyP (with or without metal) aqueous solution_ The porphyrin is fixed onto the fauJaslte by ion exchange. The polymer zeolite electrode is examined by cyclic voltammetry m a 0.1 M EtqNC104 solution m acetonitrile and in an aqueous basic solution (0.1 M KOH). The best results are obtained in a non-aqueous solution with manganese porphyrin_ Typical voltammetnc curves are represented in Fig. 1. In both electrolytes, we obtain an electrochemical system characteristic of the Mn(III)/

2(

( Q >

//

-21

e---e A

E/V

I

-0.4

0

04

E/V F:g 1. Cyclic voltammetry of MnTMPyP fixed film electrode at various potential sweep rates; 0.1 M Et,NClO,; potential sweep rate (V s-’ ) same electrode in water + KOH (pH = 13). u =

on zeolite (faujasite) in a zeolite + polynxr electrode area* 2 cm’. (A) in acetonitrile + (1) 0.02; (2) 0.05, (3) 0.10, (4) 0.20. (B) the 0.050 V 8-l .

199

Mn(II) couple. The voltammetric peaks shift with potential sweep rate as can be seen in Fig. 1. The “equilibrium” potential, taken as half of the sum of the cathodic and anodic potential peaks is Ees = 0.0 + 5 mV/SCE in CH3CN + 0.1 M Et,NClO, , and Ees = 70 mV + 5 mV/SCE m Hz0 + 0.1 M KOH. The

number of manganese sites involved in the redox process is rather low (Q = 15 PC for 2 cm2 of electrode)_ It must be noted that the results were not reproducible in the case of iron or cobalt porphyrins with electrodes prepared accordmg to this procedure. Graphite-zeolzte

electrode

The best results were obtamed with electrodes prepared in the following way: po,-phyrin (with or without metal) was fured by ion exchange on the zeolite by contact with 400 mg of the Na’ form of faujasite mth 5 ml of a 10m3 M aqueous porphyrin solution. The zeohte was separated, washed and dried. 30 mg of zeolite were added to 30 mg of powder graphite, carefully mixed and pressed on a nickel or platinum grid (total area = 2 cm’ ). The electrochemical behavlour of such an electrode WE analyzed by cyclic voltammetry in the same electrolytes as above. Figure 2 shows the voltammograms of a zeolite graphite electrode: (1) without fixed TMPyP in CH3CN (A) and in water (B). (2) with TMPyP fixed on zeohte but without metal ion in Ci&CN and m water; these voltammograms show that no electrochemical signal appears in either electrolyte in the potential range charactenstic of M(iI)/III) redox

Fig. 2. Cyclic voltammograms of zeolite-modihed electrodes Et,NCIO,; (B) m water + 0.02 M KClO,. (1) Zeolite (wlthout 50%. (2) Zeolite (wAh metal-free porphyrin) 50% + graphite 10 mV s-’ .

(A) in CH,CN + 0 1 111 porphyrin) 50% + graphite 50%. Potential sweep rate-

20c

Fig 3 Cychc voltammetry of a MnTMPyP-eolite electrode + graphite flxed on platinum (electrode area. 1 cm’). (A) CH,CN + 0 1 M Et,NCIO, (B) H,O + 0 02 M KClO,, potential sweep rate/V 6-l 0 005.

systems [ll] .The fixation of TMPyP is characterized by a peak in water at -3.55 V vs. SCE. Examples of characteristic curves with Mn-porphyrin electrodes in CI-&CN and in water are reported in Fig. 3. The influence of potential sweep rate on the peak potential and on the quantity Q of charge involved in the electrochemical process is indicated in Tables 1 and 2. The mfluence of the nature of metal ion M on the equilibrium potential values was examined in the case of Mn(III), Co(III) and Fe(II1) in aqueous solution, where their electrochemical properties are well defined. The results are reported 111Table 2. Typical voltammetric curves are reported in Figs. 3 to 5. TABLE 1 Characterlstlcs of redox processes at a MnTMPyP-zeolite-graphlte CH,CN + 0 1 M Et,NClO, v/mV s-l

EplmV

E,@V

Q.&PC

QCAJC

2: 50 100 200

60 120 150 160 260

20 30 55 70 60

3400 2250 590 325 200

1545 980 270 135 85

modlfled electrode XI

201 TABLE

2

Charactenstics of redox processes at MTMPyP-zeolite-graphite Mn, Co, Fe) in water + 0.02 M KCIO, Denvative

vfmV s-’

A%/mV

MnTMPyP

2 5 10 20 50 100

20 32 44 80 168 236

98

CoTMPyP

2 10 20 50 100

30 45 77 100 135

295 290 297 297

FeTMPyP

2 10 20 50 109

30 90 170 380 470

-&ImV

96 98 96 96 94

295 -175 -175 -75 -150 -150

modtiled electrodes (M =

Q,hC

QcathC

1930 2180 1760 1910 2000 1410

1690 1930 1720 1845 1930 1365

980 760 800 740 560

710 745 800 565 400

1440 1510 1530 1660 920

2270 2090 1840 1905 830

The yreld of the electrochemical process at the momed electrode -7a.s calculated for ths case of the Mr.TMPyP-zeollte-graphite electrode. The capacity of the zeolite was obtained as follows: a known quantity of faujasite treated mth MnTMPyP was &pped in a 0.1 M KOH water :olution.

20-

$_ . Y

o-

-20 -

Fig 4 Cyclic voltammetry of CoTMPyP in a zeohte-graphite modrfled electrode m water + 0.02 M KClO, (electrode area. 2 cm-‘) at various potentul sweep rates/mV s-’ (1) 10; (2)20.(3)30,(4)50 Fig. 5. Cychc voltammetry of FeTMPyP in a zeohte-graph~te modified electrode (electrode area’ 2 cm-*) m water + 0.02 M KCIO, Potential sweep rate: 0 002 V s-l

202

MnTMPyP was quantitatively extracted by ion exchange. The total amount MnTMPyP was obtained by UV-visible absorptrometry (at 457 nm). The amount of electroactive complex was determined by chronoamperometry. The following results were obtained: 58% in HZ0 + KClO, (EfZed = -9.1 V) and 36% m CH3CN + Et4NC104 (E&d = -0.2 V).

of

CONCLUSION

These prehmmary results show that modified electrodes can be prepared m a simple way with multicharged metal-porphyrins fixed on a zeohte of the fauIa.slte type by ion-exchange. The ionic charge of the porphyrin is high enough to avold exchange unth the iomc species of the solution even in reduced forms of M. Modified electrodes were obtamed, either as polymer-coated electrodes, with polystyrene as coating agent, or by deposition of a mixture of zeolite and graphite: the electrochemical charactenstics are very well defined in the last case with Mn, Co and Fe porphyrms TMPyP. The position of the oxidation and reduction peaks is close to that expected [15] for the Mn(III)/Mn(II), Co(III)/Co(II) and Fe(III)/Fe(II) systems, and the values of the redox potentials are not far from those observed in homogeneous solutions. Very good yields of the electrochemical process were obtamed in the case of MnTMPyP, allowing us to investrgate some electroassisted reactions in which such compounds are involved. ACKNOWLEDGEMENTS

We thank Dr. D. Delafosse and Dr. R. Messina for fruitful discussion. This work was supported by the “Centre National de la Recherche Scientrfrque” and Agence Francaise pour la Maitrise de 1’Energie” (No. 980025). REFERENCES 1 2 3 4 5 6 7

8 9

10 11 12 13 14 15

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