New aspects of isopoly- and heteropolyanions in electrochemistry: chemically modified electrodes, catalysis

C. R. Acad. Sci. Paris, t. 1, Serie II c, p. 333-342,1998 ilectrochimie et photochimielE/ectrochemistry and photochemistry

New aspects of isopoly- and heteropolyanions in electrochemistry: chemically modified electrodes, catalysis Bineta KEITA”, Abderrahman Lids NADJOa*

BELHOUARI”,

Roland CONTANTb,

a Laboratoire d’tlectrochimie et de pharotlectrachimie, universitk Paris-Xl, bhtimenc 420. 9 1405 Orsay cedex, France ” Laboratoirr de chimie des mttaus de transition, LmiversicC Paris-VI, 4. place Jussieu, 75252 I’aria cedex 05, France (Received 14 February

1997,

accepted

24 October

1997)

Abstract - Two different aspectsof the work carriedout in our group on the electrochemistryof oxometalates are briefly reviewed.They concern first the STM observation of H,WIIO& on graphite electrodes during cyclic voltammetry experiments. These layers are believed to constitute the precursors for the electrochemical activation ofelectrode surfaces by oxomctalaws. The second part ofthe paper is devoted to the rlectrocatalysis of the reduction of-nitrite and nitric oxide by pol~~oxometalate/polylner systems.0 Academic desSciences/Elsevier.

Paris oxometalates I heteropolyanion microscopy / catalysis

/ electrochemistry

/ chemically

modified

electrode

I scanning

tunneling

R&urn6 - Rksultats rCcents dans Nlectrochimie des isopoly- et hCt&opolyanions : blectrodes modifiees et catalyse. Deux aspects diffkrenrs de I’ttude des oxomktallates sont bri&ement exposes dans ce m&moire. I,e premier concerne les possibilitks d’observation de ces entit6 mokulaires minbrales par microscopie de proximitk in situ, lorsqu’elles se deposent sur les surfaces d’tlectrodes au tours mime de leur etude &lectrochimique. Ces dCp6ts constituent, par exemple, les prkurseurs des catalyseurs a base d’oxomkallates qui activent la surface de matCriaux d’Clectrode ies plusdivers. Le deuxiPmeaspectd6crit Its propriCt& tlectrocacalytiques des oxom&allates en solution ou, plus particulikrement, lorsqu’ils servent de dopants immobilis& dans des polymkes fix&s sur les Clectrodes. L’exemple trait6 est celui de la rkduction catalptique du nitrite et de I’oxyde nitrique. 0 Academic des ScienceslElsevier, Paris oxomktallates

/ htkkropolyanion

Version franqaise

/ Clectrochimie

/ Plectrode

modifike

/ microscopic

Q effet tunnel

I catalyse

abrkgke

Les oxom&allates, en solution ou dtposts sur support, ont des prop&t& caralytiques reconnues pour de nombreux processusd’oxydation. LesCtudesdespropri&& catalytiques de leurs formes rCduites, en particulier cellesobtenues par e’lectrochimie, sont plus rtcentes et ne cessentde s’amplifier. C;e memoire presente brievement deux aspectsdu dCveloppement de l’tlectrochimie de ces composts. Le premier concerne les possibilitts d’observation de cesentitts moltculaires mintrales par microscopie B champ proche in situ lorsqu’elles se dtposent sur les surfacesd’Clectrodes au tours m&me de leur Ctude 6lectrochimique. En effet, il a Ctt montrt pr&tdemment qu’il est possible d’electrodtposer les oxomCtallates sur des surfacesd’&lectrodes. et que les materiaux ainsi trait& devenaient d’excellentes Clectrodes,par exemple, pour la r&action de dCgagementde I’hydrogkne ou la rkduction de l’oxyg&ne. L’e’tude proposCeici constitue une premitre &ape vers la description d&ill&e des processusq;i conduisent finalement ;I I’obtention de surf&es Clectrocatalytiques B base d’h&ropolyanions. Pour cette Ctude, l’anion choisi est H,W,20& et 1’Clectrodede travail est une surface de graphite pyrelytique. Le domaine de potentiel est choisi de maniere ?Ine pasaboutir au d&p& dlectrocatalytique &rCommunicated * Correspondence

1251-8069/98/00010333

by Franpis and

MATHEY.

reprints.

0 A ca d Cmie

des ScienccslElsevier,

Paris

333

B. Keita

et al.

sistant. L’ttude simultanee des courbes intensitbpotentiel et des images par microscopic B effet tunnel (STM) de la surface de l’tlectrode montre que l’isopolyanion commence B se deposer sur la surface du graphite bien avant le passage du courant faradique. Le depot devient persistant et tres rtgulier dts que l’on atteint le domaine de potentiel de la troisitme vague de l’oxometallate. Les mesures faites sur les images donnent parfaitement le diametre cristallographique connu (lo,5 + 0,5 A) de l’anion Ctudit. Ces depots constituent ainsi les prtcurseurs des catalyseurs a base d’oxometallates qui activent la surface des materiaux d’tlectrodes les plus divers. Le deuxieme aspect de ce memoire se rapporte aux proprittts catalytiques des oxomttallates immobilists dans des polymtres fixes sur les electrodes. Le but est de reproduire, voire d’amplifier sur tlectrodes modifites, les proprietes catalytiques observees lorsque l’oxometallate est en solution. L’exemple trait6 est celui de la reduction catalytique du nitrite et de l’oxyde nitrique. Pour cela, avec des hettropolyanions varies, deux types de composites differents ont ttt prepares, l’un ayant un polymere conducteur, la polyaniline (PANI), comme matrice, et l’autre utilisant la poly-(4-vinyl yridine), f (QPVP), avec des heteropolyanions varies. Les differents composites sont : PANI/SiMo,,O&, PAN11 P,Mo,,O&, PANI/P,W,,O&, QPVP/SiMo,,O,&, QPVP/SiW,,O&, QPVP/P,Mo,,O& et QPVPIa,P,W,,VO&. L’ttude dttaillte d&rite Porte sur la catalyse de la reduction de l’anion nitrite en presence du composite PANI/SiMo,,O,&. L’Ctude voltametrique est completee par une tlectrolyse preparative. Le seul produit obtenu est N,O. Les autres composites conduisent B des rtsultats similaires.

1.

Introduction

The aim of this paper is to present briefly two new aspects of the work which is being carried out in our group on the electrochemistry of heteropolyanions (HPA). These chemicals show a series of physicochemical and chemical behaviours, which make this class of compounds unique in several domains. As a matter of fact, the ability of oxometalates, and particularly, heteropolyanions of the Keggin and Dawson types to serve as versatile oxidants and their reduced forms as reductants has been known for some time [l]. Hence, the impetus for the study exists in several fields. For instance, their redox chemistry and their catalytic and electrocatalytic behaviours towards substrates like the nitrite anion [2-81 or their catalytic atom transfer chemistry [9, lo] arouse much interest. By a smooth change of their composition, their redox potentials can be chosen to span a wide range and several electrons can be added to their oxidized forms or wichdrawn from suitable reduced species without a change in structure. Obviously, heteropolyanions (HPAs) offer numerous possibilities of composition variation. Their reduced forms are usually deep blue and are known as heteropolyblues (HPBs). Polyoxometalates show unusual combinations of properties, in catalysis and medicine [ 11, 121. Of particular interest is the redox chemistry of the first several electronation steps of a large number of HPAs, which makes them show behaviours parallel to those

334

of quinones: thus, depending on their composition, the electrolyte pH, these chemicals may act formally as one-electron, two-electron and even several-electron oxidants, or their reduced species as reductants [ 1-8, 13-151. In the present paper, two selected different aspects will be shown, ranging from the STM observation of the deposition of H,WrzO,bon graphite electrodes during cyclic voltammetry experiments to the electrocatalysis of the reduction of nitrite and nitric oxide by polyoxometslate/polymer systems. These examples illustrate briefly the versatility of oxometalates in two fields of electrochemistry.

2. STM evidence for the deDosition of H,W,,O& on graphite’ electrodes during cyclic voltammetry experiments [ 161 Electrochemical characterization of oxometalates on carbon electrodes has revealed the existence of two potential domains; starting from a 10m3 M solution of a selected oxometalate in a pH medium in which it is stable, the cyclic voltammograms (CVs) show that the first couple of voltammetric waves are apparently adsorption-free, while adsorption and/or deposition frequently appears at very negative potentials [13, 141. Al so, we have shown that adsorptive electrodeposition of a large variety of oxometalates on several electrode materials comprising semiconductors, modify these surfaces persistently and even make those usually

New aspects of isopoly- and heteropolyanions

not suitable for hydrogen evolution very good hydrogen cathodes [13, 141. However, little is known about the geometric factors within the molecules,or their spatial distribution or chemical transformation, which, altogether, could induce the observed catalytic properties. Then, near-field microscopies, many of which can be operated in situ during electrochemical experiments, appear asvery appropriate tools to try and monitor the stepwise activation of electrode surfaces at a molecular or even at an atomic level. As a first and necessarystep in this direction, we have proved that heteropoly- and isopolyanions can be imaged with a high spatial resolution [17-201. PW,202t;, which is also one of the precursorsused for electrode modifications, has been chosen as a representative example. Like most transition metal polyoxoanions, it is constituted by discrete molecules, with their structures basedon close-packed oxygen arrays containing interstitial metal centers. This very symmetrical structure is easily accommodated in a sphere of 1 I A diameter. A drop of methanolic solution of H iIIW,,O,+o(typically 1Ow4M) was deposited on‘ the freshly cleaved surface of a piece of highly oriented pyrolytic graphite (HOPG) and the solvent was allowed to evaporate. Figure 1 shows current-mode STM images of this surf&e. The observed structure is stable, well-ordered in severalareas, with parameters clearly different from those of the underlying HOPG. In rhe constant-current mode STM, the tunnel tip traces out contours of constant density of statesand thus probes the topology of the electron distribution above the surface of the sample.Therefore, the structures in $gure 1, which closely resemble what could be expected from the crystallographic shapeof PW120&‘, suggestthat individual oxometalate species are imaged. The mean diameter, deduced from numerous measurements[.I71 is 9.6 t 0.4 if, w h’ICh ISvery close to the crystallographic diameter. These results complemented by several others, including also AFM studies on a single crystal of Na6H2CeW, ,0,,.30H,O [ 191, establish the ability of STM and AFM techniques to show the shape of oxometalates and to allow for determinations of molecular dimensions. They also establish other conclusions: (i) fairly adherent deposits of the oxometalates on HOPG can be obtained and imaged by STM and AFM; (ii) in the case of oxometalates. a substrate like HOPG has no

Nanoscopc U Parameters: Bias. Setpoint: XY:

Samples:

Figure 1. Top: The Keggin structure of H,PW120,,, shown in the coordination polyhedral model and the space-filling model respectively. rhe spheres representing close-packed oxygen atoms. Bottom: Fast-Fourier transform filtered STM image in air, of‘H,PW,2C140 deposited on HOPC;. Figure 1. En haut : structure de Keggin de H,P%‘iLC~4,~ sous forme d’assemblage de polyedres de coordination et sous forme de modele compact respectivement ; les sph& res rrpr&entcnt les empilements compacts d’atomes d’oxyg&ne. En has : image STM ?I I’air, de H,I’W,,O,o, d6posC sur HOI’G. L’image a 6t6 filtlde par transform& de Fourier rapide.

detectable influence on the STM image; (iii) the STM image corresponds to a purely topographic image of the surface; (iv) for oxometalates, an excellent correlation exists between the images obtained by the two techniques. To probe the dynamics of electrode/electrolyte interface in the presenceof an oxometalate, a cyclic voltammetry study with simultaneous STM imaging hasbeen undertaken with a solution containing 1O-’ M (NH,),H,W,z040 in a pH = 2 medium. The working electrode is the basal plane of a freshly etched HOPG of 0.4 1 cm2 surface area. Figure 2 gives a reference cyclic voltammogram of HZW,,Oi< in the pH = 2 medium, run in a convcnt;onal electrochemical cell and restricted to the first three waves. The first two waves are bielectronic and the third one features approximately ten electrons [2 1, 221. All the other cyclic voltammograms have been obtained in the ECSTM cell received with the Nanoscope. Under open-circuit conditions, the current-mode STM image

335

B. Keita

et al.

I1,

/ i

L

E/VVS -I.

1 -A-L

-09

SCE --I

-0.6

- 0.3

0.0

Figure 2. Cyclic voltammogram for 5 X IO-’ M (NH,),HzW,z04,, in HCI + 0.1 NaCl solution (pH = 2). Scan rate: 100 mV& ’ . F&we 2. Voltamogramme cyclique de 5 X 10 ” M (NH4)6HLW,204,, dam une solution de HCI + 0,l M NaCl (pH = 2). Vitesse de halayage du potentiel : 100 rnV.s- ’

of the atomic structure of the HOPG surface in the presence ofthe lo-” M HLW,?O&.I pH = 2 solution appears in -figure 3. No filtering has been applied. This image is identical with that obtained in air in the absence of liquid. Worth noting is the very good resolution of this image which underscores the excellent quality of the coated tips used in the present work. Then the potential control was set in. Four cathodic potential limits were fixed in order to

Figure 3. Current mode STM image of the atomic structure ofthe surface of HOPC in the presence ofan aqueous pH = 2 solution containing 10 .’ M (NH,,),H,W,,04,,. No filtering has been performed. Figure 3. Image STM en mode courant, de la structure atomique de la surface de HOPG en presence d’une solution de pH = 2 contenant lo-$ M (NH4)6HLW170i0. LJimage n’a pas et6 filtree.

336

restrict the potential cycling to the double layer region up to the very beginning of the first wave, then to the first wave itself, to the second one and finally up to the third wave. All the images shown throughout this paper are stable, reproducible and are, then, characteristic steady state images of the potential domain under investigation. Fipre 4 illustrates the main phenomena observed in the first potential regime. On cycling the electrode potential in this domain, a reduction in image quality is obtained in the long run. This unfiltered image shows valleys and ridges oriented from the rop left corner to the bottom right corner on the display, with a corrugation of about O.l0.2 nm between them. The image is stable as long as the potential cycling is maintained in the domain shown infigzlre 4. A very few randomly distributed spherical entities with a diameter which would fit that of H2Wr20& are detected on the surface. However, distance measurementson this image, after filtering by fast-Fourier transform, indicate that the presence of perturbed HOPG surface largely dominates. Scanning the potential to include the whole first wave does not fundamentally modify the phenomena observed. The alteration of the image continues steadily but remains dominated by the HOPG structure. The observed phenomena feature a smooth transition from the pattern obtained in jggre 4 and that obtained in the potential domain of the second redox couple. Therefore, this Iast situation will be described in more detail. The remarkable observation is that the cyclic voltammogram in jgure ELI shows no adsorption phenomena, while the STM image reveals a strong perturbation as appears in j&re 56. The pattern is very stable and doespersist upon prolonged cycling. Figure 5ib, which is fast-Fourier transform filtered, doesnot show any trend of domination by the structure of the underfying HOPG. By contrast with $gtire 4, a large number of spherical entities with a diameter in the range of 10.5 + 0.5 A can be seennow on the surface. Figures4 and 56 constitute a direct illustration of the potential dependence of the adsorptive deposition of H2W,20& and other oxometalates [ 13, 14, 23-251 on electrode surfaces.They also suggesta primary formation of islandson the bare surface for the oxometalate deposition process aslong as the layers remain thin enough (17, 261. The simultaneous recording of the cyclic voltammogram and of the STM image reveals a remarkable result:

New aspects of isopoly- and heteropolyanions

Figure 4. Combined display of the cyclic voltammogram of (NH,),H,W,z040 on HOPG and the current mode age captured simultaneously. The image selected has been obtained after numerous cycles of the electrode potential very beginning of the first reduction wave of the oxometalate. No filtering has been applied. Figure 4. Voltamogramme cyclique de (NHJ(,H,W,, I’tlectrode rtalisee simultanement. Kimage stlectionnee uric valeur correspondant au tout debut de I’observation

STM imup to the

et image STM en mode courant, de la surface de OdO sur HOPG a et6 prise aprts plusieurs cycles du potentiel de I’ttectrode jusqu’a du premier couple redox de I’oxometallate. I:image n’a pas Ccc filtree.

while adsorption is clearly shown by the STM image, the phenomenon is still not strong enough to distort the cyclic voltammogram which remains dominated by the solution phase process. Thus, it would appear that the sensitivity of the STM technique to track adsorption phenomena is largely better than that of cyclic voltammetry. This may help in clarifLing moreprecisely the various stepswhich end up in electrontransfer at the electrodesurface. Figure 6 shows the particularly well-ordered two-dimensional array which is observed when the potential limit is extended to comprise the third wave of H,W,,O&. The current output range of the potentiostat is exceeded and the current is saturated. This unfiltered image is displayed alone on figure 66 to give better evidence of the main features observed: the remarkably well-ordered domain is not entirely spreadout all over the surface and is adjacent to thicker deposits and/or disordered areas.A uniform diameter of 10.5 * 0.5 A is determined for the roughly spherical motives measuredfrom a zoomed and filtered, well-ordered area as appearsln$gure 6c. This value correspondsper-

fectly to the diameter of this Keggin anion. Even though the surface structure qualitatively resembles that of HOPG, the size of the motives and the smooth evolution of H,W,,O& deposition processstrongly suggest that indeed this anion is actually imaged. Among the main conclusions of this study, it is worth emphasizing the following points. I ) The adsorption of H,W 120,& on the electrode surface clearly appearsto depend on the applied potential. In the present work, the potential domain explored has been chosen so that completely ‘irreversible’ deposition of the molecules on the electrode surface does not occur. As a matter of fact, previous work from our laboratory [I 3, 14, 23-251 has demonstrated the irreversible electrochemical deposition of heteropoly- and isopolyanion-based catalysts on electrode surfaces at potentials more negative than used here. Such materials have been shown to be very efficient, for instance in the hydrogen evolution reaction 113, 14,23-251. Then, the present experiments suggestthat the catalysts attached to electrode surfacesbegin to build up even before their full

337

B. Keita

et al.

(b)

Figure

5. Combined display of the cyclic voltammogram on HOPG (a) and the current of (NHJ&~W,JI~,, mode STM image acquired simultaneously (b). The image selected has been obtained after several cycles of the electrode potential to include the first two redox systems of rhe oxometalate. Fast-Fourier transform filtering has heen applied to the image. Figure 5. Voltamogramme cyclique de (NH,J6H,W,, (I,,,, sur HOI’G (a) et image STM en mode courant, de la surface de I’electrode realisee simultanement (b). ITimage selectionn& L et6 prise apres plusieurs cycles du potentiel de l’electrode jusqu’a we valeur permetrant d’observer les deux premiers systemes redox de I’oxometallatc. L’image a ett: filtree par transformce de Fourier rapide.

becomes detectable at fairly negative potentials. 2) Undoubtedly, the ability of STM to detect very low level coverages on the electrode surface will be helpful to monitor surface morphology changes associated, eventually, with electron transfer processes; also, the study of electrode electrocatalytic surfaces at a molecular or even atomic resolution could be envisioned. Caution must be taken, however, that the area observed in STM be representative of the whole electrode surface. Work is in progress with STM and AFM techniques dynamically coupled in situ with electrochemistry to obtain qualitative and informations on oxometalatequantitative based materials. activity

3. Electrocatalysis by polyoxometalate/polymer reduction of nitrite and nitric oxide [4]

systems:

Another type of modified electrode is studied in the following which also exemplifies the versatile

338

use of oxometnlates.

III patents issued from our laboratory [2, 31, it has been demonstrated that a large variety of oxometalates are suitable electron relays for the electrocatalytic reduction of nitrite and nitric oxide. In particular, Keggin- as well as Dawsontype heteropolyanions and their transitionmetal substituted derivatives show high activity in this issue. The conclusions in the patents were based mainly on unambiguous cyclic voltammetry experiments. Then, it was desirable to find out and design suitable experimental conditions in which the aforementioned catalytic procedures can be used to obtain sizeable amounts of chemicals. Our strategy was to entrap the polyoxometalates in polymeric matrices. The preparation of modified electrodes using polymers as supports for active catalysts is an area of continuing interest. The distribution of the oxometalates in these permeable matrices offers and enhances several advantages: the amount of catalyst can be diminished, while increasing substantially its concentration near the electron source. In some cases, a stabilization of the catalyst itself could result from the entrapment. Finally, the separation of the catalyst from reaction products is straightforward. The present paper illustrates these ideas and shows that redox as well as conducting polymers could be used successfully. Emphasis will be put only on some very efficient oxometalates and their main electrochemical behaviours will be described briefly, when necessary. Two kinds of polymeric matrices were used: polyaniline (PANI) was chosen as an example of conducting polymer and poly(4vinylpyridine) (PVP) slightly quaternized (QPVP) served to prepare redox assemblies. Several procedures to prepare electrodes can be found in our previous papers [27, 281.

PAN1 films are peculiar in being conductive only within a narrow potential domain ranging roughly from -0.2 V to +0.7 V vs. SCE in which rwo main redox systems appear [29-323. Then, the choice of appropriate oxometalates electroactive in the conduction and/or stability domain of the polymer is limited to those showing well-defined cyclic voltammetric wave(s) in the aforementioned potential range. Examples are SiMo ,?O& (abbreviated as SiMo, ? in the following), P,Mo,,O& (PZMo,,) and P,W,,O& (P,W,,). Beyond the negativepotential limit, PANI is an insulator, and can act, at best, as a polyelectrolyte with several drawbacks absent in usual polyectrolytes 127, 281. In the present experiments, PANI films on

New aspects of isopoly- and heteropolyanions

cv Pirrama E Ebb.5

I $0

2

5 : ‘lm

0 0I

9 -64x3.2 mv 50.0 mV

5 10

0I I

nm 0

5

10

Figure 6. (a) Combined

H~-n@l Bsmce (nm) vclticrd dislmx (ml) Angle (deg)

1 11 0:OO 0.06

1. I1 0.01 0.71

0.56 0.23 22.54

Spectral period (nm) 1.13

display of the cyclic wlrammogram of (NH,,),H,‘&‘,tOGo on HOPG and the current mode STM image acquired simultaneously. The potential domain has been programmed to include the third redox system of the oxometalate. No j&&zg hnc been wed, (b) l)isplay of the STM image alone. showing clearly thick and relatively disordered deposition domains adjacent to a remarkably well-ordered area. il%, fiiltrrir~~ hi beets itied, (c) Section through a wellordered area, to measure the molecular dimension. This image has been filtered by two-dimensional Fast-Fourier transforma[ion.

Figure 6. (a) Voltamogramme sur HOP<; I’@lectrode

cycliquc de j N H,),H, W,zC),,, et image STM en mode iourant de la surface de r6alisii-c aimultan6mcnc. Ix domainc de potentiel

explorC a 6t6 programme de mani& h inclure le troisitme sysc6me redox de I’oxom~tallarc. L’image n’a pas et6 filtr&. (b) L’image STM pr&dente est prCsentPe scuic, pour montrer clairemcnt les domaines dc dCp6rs i:pais ct rolarivernent d&ordonn&, adjacents g un domaine remarquabiement ordonnC. L’itnage n’a pas 6t6 filtrCc. (c) hilesure des dimensions molicuiaires, g&e 11une coupe Q travers Ir domainc ordonnc. (:ctte image a GtP filtr& par transformCc de Fourier rapide h dew dimensions.

glassycarbon electrodes were obtained by electrochemical oxidation of 0.1 M aniline solution in 1 M H,SO,, at 0.8 V vs. SCE. Typically a total charge Q = 3 mC was passedfor this synthesis. The films were characterized in 0.5 M H,SO, to ascertain that they exactly replicate literature results [28]. The doping was yerformed in 0.5 M H&1, containing 5 x 1 Om.M SiMo,2 by cycling at a scan rate of 100 mV/s between +0.5 V and -0.2 V. The doped film was stabilized and characterized in 0.5 M Hz SO,. It is worth noting that the cyclic voltammetry of the PANI/SiMo,? assembly is largely dominated by the redox chemistry of the oxometalate. The long-term stability of this assembly (up to 100 h under cycling in 0.5 M H2S0,, after which the test was arbitrarily stopped) has been demonstrated. Shorter, but also conclusive tests (10 h) have been performed on PANI/P,Mo,, and J’ANIIP,W,, assemblies.Further experimental details can be found in the original papers [4, 27, 281. To obtain QPVP/polyoxometalate systems equal volumes of PVP and 1 ,I Zdibro-

[27],

mododecane in 2-propanol solutions were deposited on the glassy carbon surface and left to come to dryness in air. Then, the electrodes were maintained at 80 “C for 18 h to promote the desired ‘solid phase’ cross-linking between the alkylhalide and pyridine residues.The polymer, thus slightly cross-linked with I, I2-dibromododecane (I 4 % cross-linking) is designated asQPVJ? Previous protonation of QPVP favours its fast doping and equilibration. Typically, doping is accomplished in 1 M HCI + 5 x IOY3M SiW,,O& (SiW, ?), ‘l-he stabilization and characterization of the electrode were performed in pure 0.2 M CF,COONn + CF,COOH (pH = 2). The same technique was useful for all the oxometalates of this work

14, 271. 3.1. ElectrocataZytic and nitric

reduction of nitrite oxide with polyoxometuhtes

‘-L’ypically, &ure i’ shows a representative example of the main solution electrochemical behaviours of SiMo, 2 in the absenceand presence of the nitrite ion. The observed cyclic

339

B. Keita

et al.

were the same as in their solution electrochemistry, thus suggesting that the bonding of the polyoxometalates to the polymer introduces no appreciable difference in the electronic structures of the oxidized or reduced forms of the redox couples.

3.2. P!I assembly

E/V YS SCE I

I -0.7

I

I -0.3

I

I 10.1

I

I +0.5

7. Cyclic voltammograms showing the electrocatalytic reduction of nitrite by SiMo,,. Electrolyte: 0.2 M Na,SO, + H,SO, (pH = 2). Working electrode: glassy carbon disk (A = 0.07 cm’). Scan rate: v = 5 mV s-‘. Curve (a): background current in the supporting electrolyte; curve (b): reduction wave of 5 x 10e3 M NaNO, in the same electrolyte; curve (c): first three redox systems for 10e3 M SiMo,,; curve (d): effect of the addition of 5 x 10e3 M NaNO, in the solution (c); curve (e): effect of the addition of lo-* M NaNO, in the solution (c). Figure

Figure 7. Voltamogrammes cycliques montrant la reduction Clectrocatalytique du nitrite par SiMo,,. glectrolyte : 0.2 M Na2S04 + H,SOd (pH = 2). Ijlectrode de travail : disque de carbone vitreux (A = O,O7 cm*). Vitesse de balayage du potentiel : 5 mV s-‘. Courbe (a) : courant de fond dans Ptlectrolyte-support ; courbe (b) : vague de reduction de 5 x 10m3 M NaNO, dans le m&me electrolyte ; courbe (c) : trois premiers systtmes redox de SiMo,, dans le meme electrolyte ; courbe (d) : effet de l’addition de 5 x 10e3 M NaNO* dans la solution (c) ; courbe (e): effet de l’addition de lo-’ M NaNO, dans la solution (c).

voltammograms clearly feature the electrocatalytic process involving the nitrite ion. Double potential step chronocoulommetry was used to determine the rate constant of the catalytic process, under pseudo-first-order conditions. The second-order rate constant was then derived to be k = (1.6 f 0.1) x lo3 M-’ s-l. A previous observation indicates that this catalytic process could eventually be transferred to electrode surfaces modified with polymer/polyoxometalate systems: the standard potentials of the various redox systems in this polymer films

340

electrode;

I??4NI/SiMoo,

The PAN1 film studied in Jigure 8 has been obtained with Q = 3mC. The potential domain has been chosen to ascertain the stability of the film. The first,cyclic voltammetric run in the presence of the nitrite ion would indicate that PAN1 alone strongly catalyzes the reduction of nitrite (curve b). However, after a few runs the activity of the electrode towards nitrite is no longer observed. Even, the cyclic voltammetry pattern of the PAN1 film itself vanishes as appears from the seventh and tenth runs in $gure 8 (curve c and d). However, curve (a) of jgure 8 could be restored by cycling the electrode in plain supporting electrolyte. By contrast, the PANUSiMo,, assembly appears to catalyze also the electroreduction of nitrite and to stand repetitive cycling in the presence of NaNOz without any deactivation of the electrode. As appearsinjgure 9, a potential gain in the range of 600 mV is obtained in comparison with the direct reduction of nitrite on the glassy carbon. The electrode stability was easily confirmed as the solid line curve of figure 9 is readily observed again when the electrode was transferred to the plain supporting electrolyte. The catalytic efficiency in this system can be defined as: CAT

= 1oo

x ~,W-‘A,

NaNW

- Z,(HPA)

$W’A)

where I,(HPA) is the peak current for the reduction of the heteropolyanion (HPA) in the absenceof NaNO,; Ip (HPA, NaNO,) is the value of the peak current observed in the presence of NaNO,. Also, mainly due to the stabilization of the HPA by the polymer matrix, the influence of pH on the catalytic process could be studied up to pH = 5. At this pH, the experiment was short enough to ensure the stability of the system. The catalytic efficiency was found to decreasewhen the pH increases.The CAT values were I30 %, 50 % and 30 % respectively at pH = 2, 3 and 5. The catalytic current increased when the concentration of nitrite was increased. All these observations would fit in a mechanism in which the active

New

aspects

of isopoly-

and

heteropolyanions

PAN1

I

3.2pA

IC

1

I

E/VvsSCE

E/V I -0.5

I

I 0.0

vs 53ZE

I

-

I +0.5

Figure 8. Cyclic voltammograms showing the electrocatalytic reduction of nitrite by polyaniline film (PANI). Electrolyte: 0.2 M NaZSO, + H$O, (pH = 2). Working electrode: glassy carbon electrode (A = 0.07 cm’) covered with PANI. Scan rate: u = 2 mV s-‘. Curve (a): cyclic volcammogram of PAN1 in the supporting electrolyte; curve (b): first cyclic voltammogram after the addition of 5 x IO-” M NaNO, in the preceding solution; curve (c) and (d): seventh and tenth cyclic voleammograms in the solution containing 5 x 10-j M NaNO>. Figure 8. Voltamogrammes cycliques montrant la reduction electrocatalytique du nitrite par un film de polyaniline (PANI). 6lectrolyte : 0,2 M Na$O, + H$O, (pH = 2). Electrode de travail : carbone vitreux (A = 0.07 cm”), recouvert de PANI. Vitesse de balayage du porentiel : 2 mV SC’. Courbe (a) : voltamogramme cyclique de PAN1 darts I’electrolyte-support ; courbe (b) : premier voltamogramme cyclique apt& I’addition de 5 x 1 Om’ M NaNO, dam la solution precedente : courbcs (c) et (d) : septieme et dixieme volramogrammes cycliques dam la solution contenant 5 x IO-’ M NaNC$.

species should be HNO, and/or NO, at least up to pH I 3 and NO, at pH = 5. As a matter of fact, by replacing nitrite by NO gas in several experiments corresponding to pH 5 3, exactly the same catalytic waves with the same potential locations were obtained as with NaN02.

. .~.._LJ -013

--? +O.l

-

I________ +0.5

Figure 9. Cyclic voltammograms showing the electrocatalytic reduction of nitrite at PANUpolyoxometalatecovered electrodes. Electrolyte: 0.2 M Na,S04 + H,SO,% (pH = 2). Scan rate: v = 5 mV s-‘. Cyclic voltammetry of the PANUSiMo,, assembly in the supporting electrolyte, restricted to the stability domain of PAN1 (-); effect of the addition of lo-’ M NaNO? in the electrolyte (- - -). Figure 9. Voltamogrammes cycliques montrant la reduction electrocatalytique du nitrite sur des electrodes recouvertes du composite I-‘ANIlpolyoxomCtallate. l?lectrolyre: 0,2 M Na2S04 + HaSO, (pH = 2). Vitesse de balayage du potentiel : D = 5 mV 5 I. Voltamogramme cyclique du composite PANI/SiMo,L dans I’electrolpte support, restreint au domaine de stabilitt de PAN1 (-) ; effet de l’addition de lo-’ M NaNO, dans l’electrolyte

c---j. Other possible mechanistic been checked in this work.

pathways

have not

Preliminary large-scale electrolyses were performed with a 2.4 cm2 surface area glassy carbon electrode covered with the PANI/SiMol, system, at pH = 2, with NaN02 or NO bubbled for 3 min. The working electrode was poised 100 mV negative of the peak potential of the first redox couple of SiMo12. Whatever the starting chemical, N,O gas was the main product detected, with a fairly high current effi-

341

B. Keita

et al.

ciency. Work is in progress to detect other possible products, but this remains a formidable task 151. Completely similar observations are made with PANI/P2M~,8, PANI/P,W,,, QPVPt QPVPISiy,,, QPVP/P,Mo,s and SiMo,,, QPVPIa,I’zW,7VO&. Further details can be found in the original papers [2-4, 271. To conclude this part, it has been shown that unsubstituted as well as substituted Keggin- and Dawson-type oxometalates can be entrapped in polymeric matrices on electrodes and keep their activity in the electrocatalytic reduction of nitric and nitric oxide. Both conducting and redox polymers were found to be suitable. The stability of polymer/oxometalate assemblies and their efficiency were also assessed as a prerequisite for the viability of the electrocatalytic systems in large scale electrolysis.

have been studied. The first part pertains to the early stages of the electrochemical deposition of oxometalates, which, in acid media and at suitably negative potentials, ends up in the very efficient hydrogen cathodes patented by our group. The second part explores another kind of immobilization of oxometalates on the electrode surface and demonstrates that the catalytic properties which could be found in solution electrochemistry are maintained with these chemicals entrapped in polymeric matrices. It is also worth noting that this second kind of modified electrodes can be driven as well to become very efficient hydrogen cathodes. The present examples partly illustrate the versatility of oxometalate use in electrochemistry. Acknowledgement

4. Conclusion In the present paper, two aspects of the modification of electrode surfaces with oxometalates

This work was supported by the CNRS the University Paris XI, Orsay.

References

[ 161 Keita B., Nadjo 1171 Keita B., Nadjo

[I] Pope M.T. Hereropoly Springer-, Berlin, 1983.

and

Isopoly

Oxometalates,

R.. Fournier M., HcrvC G., ET’ 382, 644. Chem. Ahsr.

Chem

(1993)

I... Surf. Sci. 254 (1391)

1181 Kcita B.. Kjoller L613.

[ZJ Keita B., Nadjo L., Contanr R., Fournicr M., Hewe G., French Parent (CNRS) 8911. 728; 10 February 1989. [3] Keita B., Nadjo L.., Contant Eur. Patent (CNRS), Appl. 114: 191882 L,.

I., J Electroanal K., Nadjo

I.., Surl:

354,295.

L443.

Sci. 256 (1991)

1171 Keira B., Chauveau F.. Theobald F., Belanger Nadjo L.., Surf. Sci. 264 (1992) 271. [ZO] Keita B., Kjoller I;., Chimiquc, mars-avril

Paris t.., Nadjo 1902.

L., I.‘Acrualit&

K.. J, Elec-

121 ] Keira 787/

[5] lbth J.L., Anson F.C., J. Am. (:hcm. 2444.

Sot.

11 1 (1989)

[22] Keita B., Nadjo L., Krier G., M&r (:hcm. 223 (1987) 287.

J.F., J, Electroanal.

Chcm.

756 (1989)

IL.31 Keita B., h’adjo 441.

I... J, Electroanal.

Chcm.

191 (1985)

[24] licit.1 B., Nadjo 87.

L., J. Electroanal.

Chem.

243 (1988)

[25] Keita B., Nadjo 157.

L., J. Electroanal.

Chem.

247 (1988)

Anson l:.C..J.

[7] Xlrh J.E., Melron Anron t,.C.. Inorg.

Electrtranal.

J.D., (:abclli I).. Biclski Chem. 29 (1990) 1952.

B.H.J..

[8] Chng S., Liu M., J. Electroanal. Chcm. ,372 (1994) 95. [9] I..xm D.K., Warren W.K.. No\w ‘I:, Domaillc l?l.% Fvitr E., Johnson D.C., E‘inkc I~.(;.. ,l. Am. Chem. Sot. 113 (13‘)l~ 7209. lO]Mansuy L)., Bartoli J.E. Finkc R..G., J, Am. Chcm.

Bartioni I!, I.yon I).K., SOL. I 1.3 (1991 ) 7222.

111 Pope M.T.. Muller A., Angw. Chcm. Int. Ed. Engl. 30 (1991) 34. 121 Pope M.T., Muller A. (Eds.), I’olyoxomctalatrs. From platonic solids to antire~roviral activity. Kluwcr Acddemic. Dordrecht, Netherlands. 1994. 1.31 Keita 13.. Nadjo 141 Keita B., Nadjc) ( 199.3) 77.

L., Mater.

Chcm.

L.. Tbpics

Phys. 22 (1989)

(:urrent

151 Keita B., Essaadi K., Nadjo l.., Conrant J, Elcctroanal. Chem. 404 (19%) 271,

342

I%xrrochcm. R.. Justum

77, 2 Y.,

I,.. J, Elecrnranal.

I>..

[4] Keita B., Belhouari A., Nadjo L., Conrant troanal. Chem. 18 (19951 243.

[S] TorhJ.II.. 361.

B., Nadjo

and

Chcm.

227 (1 9S7i

3261 Warson B.A., Barteau M.A.. Hdggerty L., 1.enhoffA.M.. W&r R.S., Langmuir. 8 (1992) 145. 1271 Keita B.. Essaadi K., Nadjo L.. J. hlectroanal. Chrm 259 (1989) 127. 128) Keita B.. Bouaziz I)., Nadjo l.., J. Electroanal. Chem 255 (1988) 303. I.?91 Paul E.W. Kicco A.J.. Wrighton M.S.. J. Phys. Chem. 89 (1985) 1441. [30] Focke W.W., Wuck (1987) 5813.

G.E..

Wci

Y,, J. Phys. Chem.

91

I31 I Kobayashi ‘T., Yoneyama H., ‘[unum <:hrm. 161 (1984) 419.

H., J. Eleccroanal.

I.121 Kobayashi *I’., Yoneyama H., ‘[‘dmura <:hcm. 177 (1984) 293.

H.. J. Elcctroanal.