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Electrocatalytic properties of conducting polymer based composite film containing dispersed platinum microparticles towards oxidation of methanol
PERGAMON
Electrochimica Acta 44 (1999) 2131±2137
Electrocatalytic properties of conducting polymer based composite ®lm containing dispersed platinum microparticles towards oxidation of methanol Pawel J. Kulesza a, *, Monika Matczak a, Anna Wolkiewicz a, Bozena Grzybowska a, Mariusz Galkowski b, Marcin A. Malik b, Andrzej Wieckowski c a Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland Division of Chemistry, Department of Metallurgy and Materials Engineering, Technical University of Czestochowa, Armii Krajowej 19, PL-42-200 Czestochowa, Poland c Department of Chemistry, University of Illinois, 600 South Mathews, Urbana, Illinois 61801, U.S.A.
b
Received 27 July 1998
Abstract We demonstrate the usefulness of poly-N-methylpyrrole as a matrix for the fabrication of a composite ®lm containing spatially dispersed platinum and highly reactive Ru oxo centers in a form of polynuclear ruthenium(III, IV) oxide/cyanoruthenates. It comes from scanning electron micrography that spherical Pt microparticles are randomly dispersed in the ®lm. Cyclic voltammetry has been employed to characterize electrocatalytic oxidation of methanol at Pt-free and platinized composite ®lms at ambient (208C) and increased (608C) temperatures. The presence of ruthenium(III, IV) oxide/cyanoruthenate species in the composite ®lm increases the catalytic activity of Pt towards methanol oxidation. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Dispersed noble metal microparticles supported on high surface-area materials are of considerable interest to catalysis [1±3]. The primary function of the support is to separate the particles physically in order to diminish the rate of their degradation by agglomeration. In the case of electrocatalysis at modi®ed electrodes, good electronic/ionic conductivity of the ®lm is also of importance. Fabrication of rigid conducting matrices from polymeric materials has been proposed [4±6]. Ecient electrocatalytic systems would also involve mutual metal support interactions leading to activation of both dispersed metal and a matrix toward electrode processes [7]. Some inorganic oxides are known to in¯uence supported metal centers by aecting their che-
* Author to whom correspondence should be addressed.
misorptive and catalytic properties [3, 8]. Consequently, our strategy involves immobilization of Pt microparticles into the composite ®lm consisting of a conducting polymer matrix doped with reactive metal oxide species. Platinum has been recognized as a powerful, though expensive, electrocatalyst for the oxidation of methanol [9±18]. The fact, that the overall process is catalyzed on platinum, and it may involve six electrons per methanol molecule to produce carbon dioxide, makes methanol a promising fuel for the direct oxidation fuel cell. Among the pure noble metals, Pt has the highest catalytic activity for the oxidation of methanol in acidic media. The major limiting factor in the catalytic reaction is formation of the passivating methanolic intermediate on platinum. Although there is some controversy about the nature of the catalytic poison, most widely it is believed that the methanolic CO develops and impedes the oxidation. A great deal
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of research aims at providing the right catalytic surface morphology in order to reduce the CO poisoning. A number of studies indicate that, among such bimetallic combinations as Pt±Ru, Pt±Sn or Pt±Re, the mixture of Pt and Ru increases, in particular, the system's overall catalytic reactivity [9, 19±21]. The optimum content (in atomic percent) of ruthenium on the surface relative to platinum has been found to be from close to 10 atomic percent Ru at 258C to a value in the vicinity of 30 at 608C [15, 20]. Several mechanisms explaining the role of ruthenium in the activation of platinum have been suggested. They include modi®cation of the electronic nature of the surface, blocking of the poison formation reaction and participation of oxygen containing Ru species in the oxidation of methanol. It has been postulated that, under conditions of methanol oxidation, the oxidized ruthenium species, e.g. Ru(IV), are responsible for the enhancement of platinum activity [22, 23]. Modi®cation of electrode surfaces provides an attractive way of con®ning catalytic species to the eective spatial region and combines the experimental advantages of heterogeneous catalysis with the bene®ts of a three-dimensional distribution of active centers typically characteristic of homogeneous catalysis. In practice, a good electrocatalyst should both exhibit high reactivity towards the reagent molecules in solution and have the ability to distribute electrons rapidly in the microstructure in order to avoid chargetransport limitations. The aim of the present study is to identify conditions (choice of matrix, reactive components, and rigid anchoring groups) for preparation of thin, but three-dimensional, polymeric ®lms which are highly reactive towards electrooxidation of methanol. The attractiveness of mixed-valence inorganic materials [24, 25] for the fabrication of thin ®lms on electrode surfaces has been recognized in recent years. Among them, ruthenium(III, IV) oxide/cyanoruthenate, a mixed-valence polynuclear ruthenium oxide microstructure crosslinked with cyanide, has been shown to form robust thin ®lms on carbon substrates and to act as a more powerful electrocatalyst for the oxidation of methanol than a simple ruthenium oxide coating [26, 27]. In the present study, we explore the fact that ruthenium(III,V) oxide/cyanoruthenate, RuO/ CNRu, is negatively charged, and we demonstrate that this system can be incorporated into the matrix of such a conducting polymer as poly-N-methylpyrrole, PMPy [28]. It is noteworthy that this polymer is stable at positive potentials and, while oxidized, is in a conducting state. Following introduction of polynuclear RuO/CNRu, the overall stability of the composite ®lm is increased. These properties have allowed preparation of fairly thick ®lms in which active ruthenium oxo centers can be fully utilized during catalytic oxidations.
Finally, we show that codeposition of platinum centers, following inclusion of anionic chloroplatinate into the conducting polymer matrix, is also feasible. 2. Experimental Electrochemical measurements were performed with a Bioanalytical Systems BAS 100 B/W potentiostat. All experiments were carried out in three electrode con®guration. In all cases, potentials are reported versus the SCE. Unless otherwise stated, during measurements the temperature was maintained constant in the range 21±238C. All chemicals were commercial materials of the highest purity available: ACS reagent grade or Puratronic. The solutions were prepared from triply distilled or distilled/deionized (Millipore) water. Fabrication of PMPy ®lms was accomplished by performing 50 potential cycles within the limits from ÿ0.3 to 0.8 V in the freshly prepared solution obtained by adding 20 ml of methylpyrrole to 10 ml of 2.0 mol dm ÿ 3 H2SO4. As the electropolymerization reaction proceeds, anions of the supporting electrolyte are incorporated simultaneously into PMPy ®lm on the electrode. The positive charge on the chain of PMPy can be neutralized by the doping anionic species such as RuO/CNRu. A general method for the electrochemical preparation of electrodes covered with RuO/CNRu was described earlier [26, 27]. In the present report, the modi®cation of clean glassy carbon substrate, which had already been coated with PMPy was accomplished by cycling the potential at 50 mV s ÿ 1 between 0.35 and 1.05 V in a freshly prepared deaerated solution of 1.0 10 ÿ 3 mol dm ÿ 3 RuCl3 and 1.0 10 ÿ 3 mol dm ÿ 3 K4[Ru(CN)6] in 0.5 mol dm ÿ 3 KCl at pH 2. When PMPy is reduced, smaller doping anions such as Cl ÿ and Ru(CN)4ÿ 6 will be ejected from the PMPy phase. On the other hand, RuO/CNRu, which is generated during the oxidative potential cycles and has larger volume, cannot leave the reduced PMPy. Consequently, RuO/CNRu remains permanently in the ®lm. The cycling times were varied so that dierent loadings of RuO/CNRu could be obtained. Unless otherwise stated, eight full potential cycles were applied. Platinum microcenters in the ®lms were generated by electroreduction of chloroplatinic acid (0.012 mol dm ÿ 3) in 0.5 mol dm ÿ 3 sulfuric acid. Typically, each electroreduction step involved ten potential cycles in the range from 0.8 to ÿ0.3 V. Electrode surfaces were also examined using JEOL Model JSM-5400 scanning electron microscope of 3 nm resolution. The accelerating voltage of 25 kV was used. Application of microscopy to examine the deposition of platinum particles was established earlier [29].
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For scanning electron microscopy (SEM) measurements, pyrolytic graphite disks (from Union Carbide, Chicago) were used as electrode substrates.
3. Results and discussion Fig. 1 shows a typical cyclic voltammetric response of PMPy ®lm which was electrodeposited on glassy carbon in a manner described in the Experimental section. In 0.50 mol dm ÿ 3 H2SO4, the polymer ®lm becomes oxidized at potentials more positive than 0.5 V. Following incorporation of anionic RuO/CNRu dopant, the voltammetric characteristics of PMPy ®lm changes (Fig. 2), and it becomes dominated by peaks originating from RuO/CNRu. In view of the previous report [26, 27], the voltammetric peaks at about 0.80 and 1.05 V shall be interpreted in terms of the oxidative formation of Ru(III,IV) and Ru(IV) oxo species in the ®lm. The stability of RuO/CNRu in the ®lm was tested by measuring the decrease of voltammetric peak current at 1.060 V during potential cycling experiments in 0.50 mol dm ÿ 3 H2SO4. When the modi®ed electrode was subjected to potential cycling from 0.0 to 1.150 V at 50 mV s ÿ 1 for 1 h, virtually no peak current decrease was observed. Such observation shall be explained in terms of strong interaction between anionic RuO/CNRu sites and positively charged chains in the rigid polymer (PMPy) matrix. For comparison, it is well known that dissolution of Ru from Pt±Ru alloys occurs upon sweeping to oxidizing potentials, resulting in Pt-enriched alloy surfaces [33, 34]. Extension of the upper potential limit to 1.30 V during potential cycling of the composite ®lm led to 10±15% decrease of the peak current. It is unclear whether or not this behavior was due to the loss of entrapped
Fig. 1. Cyclic voltammogram for PMPy ®lm on glassy carbon. Scan rate, 50 mV s ÿ 1; electrolyte: 0.5 mol dm ÿ 3 H2SO4; electrode substrate area: 0.071 cm2.
Fig. 2. Cyclic voltammetry of a composite ®lm consisting of PMPy and RuO/CNRu. Other parameters as for Fig. 1.
RuO/CNRu or the dissolution of the polymer matrix itself. Methanol cannot be oxidized at bare carbon electrodes at least before the onset of electrolyte decomposition. The catalytic electrooxidation of methanol at glassy carbon electrode modi®ed with PMPy containing RuO/CNRu is demonstrated in Fig. 3 for a solution (0.5 mol dm ÿ 3 H2SO4) containing 0.5 mol dm ÿ 3 CH3OH. The dotted line (Curve B) shows the background voltammetric behavior of the derivatized electrode. Oxidation of methanol occurs at about 1.090 V (Curve A). The potential for the methanol oxidation peak, relative to the redox processes in ®lm (Fig. 2),
Fig. 3. Cyclic voltammogram for the oxidation of methanol at PMPy±RuO/CNRu composite ®lm on glassy carbon (Curve A). Methanol concentration; 0.5 mol dm-3; electrolyte; 0.5 mol dm ÿ 3 H2SO4. Dotted line (Curve B) represents the background response of the ®lm in the absence of methanol. Scan rate, 50 mV s ÿ 1. Electrode substrate area, 0.071 cm2.
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Fig. 4. (A) Cyclic voltammetric behavior of the composite PMPy-based ®lm into which RuO/CNRu and dispersed platinum particles (from chloroplatinate) were incorporated. (B) Voltammetric oxidation of methanol. Scan rate, 30 mV s ÿ 1. Other parameters as for Fig. 3.
suggests that electrochemically generated Ru(IV) oxo species [26, 27] participate in the overall mechanism. It shall be noted that the electrochemical characteristics of RuO/CNRu (Fig. 2) resemble the behavior of ruthenium oxide [30], except that, in the case of RuO/ CNRu, Ru(III, IV) redox reactions are better de®ned and they appear at approximately 100 mV less positive potentials. The latter property makes RuO/CNRu potentially more attractive, in comparison to Ru oxide, for electrocatalytic oxidations including methanol. Participation of the oxygen-containing ruthenium species in the activation of Pt towards oxidation of methanol has been postulated for bimetallic Pt±Ru catalysts (alloys) [15, 19±21]. It is likely that the ruthenium oxo species, though ``crosslinked'' by cyanoruthenates within RuO/CNRu, would also be able to induce activity of coexisting, dispersed, platinum for methanol oxidation. In this study, PMPy conducting polymer [31, 32] serves as anion exchange matrix for the incorporation of negatively charged RuO/CNRu and chloroplatinate. Mechanistically, these species are attracted into PMPy by cycling the potential of the respective, polymercoated, electrode, ®rst in the ruthenium chloride + hex-
acyanoruthenate mixture for modi®cation (from 0.350 to 1.050 V), and second in the chloroplatinic acid solution (from 0.400 down to ÿ0.400 V) to generate metallic Pt. Consequently, we can produce thin, but threedimensional, polymeric ®lms of dispersed platinum metal particles codeposited with ruthenium centers. Such systems are of considerable interest to the electrocatalysis of methanol oxidation. We have performed voltammetric measurements to provide a qualitative assessment of electroactivity and composition of the conductive polymer (PMPy) based, composite Pt±RuO/CNRu ®lm. Fig. 4A shows the cyclic voltammetric response of the ®lm of Fig. 3 except that it was subjected to a platinization step as described in the Experimental section. Consequently, the ®lm contained both platinum and RuO/CNRu centers within the PMPy matrix. Two sets of peaks appearing in the potential range from 1.150 to 0.700 V shall be attributed to the electroactivity of RuO/ CNRu [26, 27]. The peak at ca. 0.4 V re¯ects primarily reduction of PtO to metallic Pt. The reverse reaction of oxidation of Pt to PtO is not well de®ned: it presumably overlaps PMPy oxidation and contributes to the increased baseline at potentials more positive than 0.6 V. The presence of platinum is responsible for the system's ability to catalyze proton discharge dramati-
Fig. 5. Voltammetric oxidation of methanol (Curve B) and cyclic voltammetric behavior of PMPy±RuO/CNRu±Pt composite ®lm (Curve A) at 608C. Film and experimental parameters are as for Fig. 4.
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Fig. 6. Oxidation of methanol at the composite ®lm (as for Fig. 4) but subjected to additional platinization. Other parameters as for Fig. 4.
cally at potentials more negative than ÿ0.15 V. Judging from the charge under the voltammetric peak at 0.4 V, the amount of platinum in the ®lm has been found to be on the level of 1.5 10 ÿ 8 mol cm ÿ 2. Upon consideration of the atomic weight of Pt, the latter value is equivalent to 3 mg cm ÿ 2. It comes from SEM examination that Pt forms spherical microparticles having diameters 20±30 nm. Determination of Ru loading is more dicult but we estimate the surface concentration of Ru oxo species to be at least 3± 4 10 ÿ 8 mol cm ÿ 2. It comes from Fig. 4B that methanol undergoes catalytic electrooxidation at approximately 0.7 and 1.1 V, i.e. the process proceeds at both Pt and Ru oxo centers. Due to very high reactivity of Ru(IV) oxo centers [26, 27] and because of their higher surface concentration relative to Pt, the methanol oxidation peak at 1.1 V is much higher than that at 0.7 V. The fact that PMPy matrix becomes more conductive at more positive potentials enhances the system's mediation capabilities and supports the overall catalytic process at 1.1 V. Fig. 5 summarizes data obtained upon increase of temperature to 608C. While the ®lm's cyclic voltammetric response recorded in supporting electrolyte (Fig. 5A) has not changed signi®cantly (compare to Fig. 4A), the methanol oxidation currents (Fig. 5B) have increased ca. three times (compare to Fig. 4B) at 608C. The results are consistent with (1) physicochemical stability of PMPy-based composite catalytic ®lm at 608C, and (2) decreased inhibiting eect of methanolic residues such as (COads) [10, 13] during oxidation of methanol at a higher temperature. Fig. 6 illustrates the voltammetric oxidation of methanol at the composite ®lm (as for Figs. 4 and 5) except that it was further platinized as described in the Experimental section. The micrograph of the resulting ®lm is shown in Fig. 7. Based on the previous
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report [29], the white spots in micrographs should be attributed to platinum microcenters. An eort was made to estimate the platinum loading in the composite ®lm matrix from SEM, assuming spherical particles and using micrographs of 10 K magni®cation. Most particles were approximately spherical, having diameters 30±40 nm. The amount of Pt was on the level of 0.015±0.020 mg cm ÿ 2. This loading of Pt centers is in agreement with the determination from integrating the charges under the respective voltammetric peak as described earlier. The peak potential of ca. 0.680 V for methanol oxidation (Fig. 6) is consistent with the reported electrochemical behavior of methanol on Pt electrodes [9±18]. We presume that Pt, rather than Ru oxo, centers in our composite ®lm are primarily responsible for the observed catalytic process. Indeed, it is apparent from Fig. 6 that a fairly large oxidation peak is obtained at ca. 0.420 V on the negative (reverse) potential scan. This peak immediately follows the rereduction of inhibiting oxides on Pt (Fig. 4A), and it appears in the potential region where active oxo-radicals are expected to exist [10, 13]. Methanol is believed to be electrooxidized over Ptbased catalysts in sulfuric acid via an initial, activating, chemisorption step, followed by interaction with the surface generated radicals, OHads or Oads, or with precursor adsorbed water molecules. The vital role of these chemisorbed oxo radicals is usually attributed to their ability to interact with the inhibiting methanolic residue, COads [10, 13]. By analogy to the activity of Ru, which was postulated for bimetallic Pt±Ru catalysts, we postulate that, due to electrochemical discharge of water by Ru(III) from RuO/CNRu in the ®lm [18, 19], the adsorbed radicals OHads or Oads are produced at less positive potentials than on Pt, and they reduce the COads poisoning. The latter ability to lower inhibition by methanolic residue is eective particularly at higher temperatures. It comes from the
Fig. 7. Scanning electron micrograph of Pt microparticles in the composite ®lm used for oxidation of methanol in Fig. 6.
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4. Conclusions
Fig. 8. Comparison of voltammetric behaviors of methanol at (A) 208C and (B) 608C. The PMPy±RuO/CNRu±Pt composite ®lm and experimental parameters were the same as for Fig. 6.
Such a conducting polymer as PMPy seems to be a promising material for a matrix into which reactive centers, dispersed Pt spheres and Ru oxo (RuO/ CNRu) species, can be introduced. RuO/CNRu is negatively charged and can be incorporated, together with chloroplatinate, into a thin ®lm of PMPy. From the viewpoint of an eective electrocatalytic oxidation, it is important to appreciate the fact that PMPy is, while oxidized, in a conducting state. The overall stability of PMPy ®lm is improved upon incorporation of polynuclear RuO/CNRu. This system shows redox reactions comparable to those of ruthenium oxide except that they are better de®ned, and they appear at less positive potential. By analogy to the activity of ruthenium in bimetallic Pt±Ru catalysts, RuO/CNRu enhances activity of Pt towards oxidation of methanol.
Acknowledgements comparison of curves A and B in Fig. 8 that the voltammetric currents for methanol oxidation have drastically increased upon increase of temperature from (A) 208C to (B) 608C. For well-characterized Pt±Ru alloys, the rate-determining step was believed to shift from methanol adsorption/dehydrogenation at lower temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen sites at higher temperature. It is likely that, in addition to blocking the formation of the poisoning intermediate, the oxygen-containing Ru species may take part in the main oxidation reaction. We postulate the ``synergistic'' eect of RuO/CNRu on Pt, i.e. that, in the presence of RuO/CNRu in the composite ®lm, the catalytic activity of Pt towards methanol oxidation increases. When the voltammetric experiment was performed in the analogous manner as for Fig. 6, i.e. using the same PMPy matrix containing approximately the same loading of Pt, except that no RuO/CNRu was introduced into the ®lm, we observed an almost twofold decrease of the voltammetric peak current at about 0.6±0.7 V. It is reasonable to expect that RuO/CNRu centers and Pt microparticles are close neighbors. They contact and interact with each other in a manner analogous to Ru oxo species and Pt on the surfaces of the alloys [15, 20]. Although we do not have any evidence for the formation of composite RuO/CNRu±Pt particles within the conducting polymer matrix, such a possibility cannot be excluded. Further research is in progress aiming at elucidation of the utility of the PMPy-based composite ®lm for electrocatalytic oxidation of methanol under steady state conditions at less positive potentials.
This work was supported by State Committee for Scienti®c Research (KBN), Poland under Grant 3 T09A 120 12 and by the NATO Collaborative Research Grant (HTECH.CRG 940808).
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Electrochimica Acta 44 (1999) 2131±2137
Electrocatalytic properties of conducting polymer based composite ®lm containing dispersed platinum microparticles towards oxidation of methanol Pawel J. Kulesza a, *, Monika Matczak a, Anna Wolkiewicz a, Bozena Grzybowska a, Mariusz Galkowski b, Marcin A. Malik b, Andrzej Wieckowski c a Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland Division of Chemistry, Department of Metallurgy and Materials Engineering, Technical University of Czestochowa, Armii Krajowej 19, PL-42-200 Czestochowa, Poland c Department of Chemistry, University of Illinois, 600 South Mathews, Urbana, Illinois 61801, U.S.A.
b
Received 27 July 1998
Abstract We demonstrate the usefulness of poly-N-methylpyrrole as a matrix for the fabrication of a composite ®lm containing spatially dispersed platinum and highly reactive Ru oxo centers in a form of polynuclear ruthenium(III, IV) oxide/cyanoruthenates. It comes from scanning electron micrography that spherical Pt microparticles are randomly dispersed in the ®lm. Cyclic voltammetry has been employed to characterize electrocatalytic oxidation of methanol at Pt-free and platinized composite ®lms at ambient (208C) and increased (608C) temperatures. The presence of ruthenium(III, IV) oxide/cyanoruthenate species in the composite ®lm increases the catalytic activity of Pt towards methanol oxidation. # 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction Dispersed noble metal microparticles supported on high surface-area materials are of considerable interest to catalysis [1±3]. The primary function of the support is to separate the particles physically in order to diminish the rate of their degradation by agglomeration. In the case of electrocatalysis at modi®ed electrodes, good electronic/ionic conductivity of the ®lm is also of importance. Fabrication of rigid conducting matrices from polymeric materials has been proposed [4±6]. Ecient electrocatalytic systems would also involve mutual metal support interactions leading to activation of both dispersed metal and a matrix toward electrode processes [7]. Some inorganic oxides are known to in¯uence supported metal centers by aecting their che-
* Author to whom correspondence should be addressed.
misorptive and catalytic properties [3, 8]. Consequently, our strategy involves immobilization of Pt microparticles into the composite ®lm consisting of a conducting polymer matrix doped with reactive metal oxide species. Platinum has been recognized as a powerful, though expensive, electrocatalyst for the oxidation of methanol [9±18]. The fact, that the overall process is catalyzed on platinum, and it may involve six electrons per methanol molecule to produce carbon dioxide, makes methanol a promising fuel for the direct oxidation fuel cell. Among the pure noble metals, Pt has the highest catalytic activity for the oxidation of methanol in acidic media. The major limiting factor in the catalytic reaction is formation of the passivating methanolic intermediate on platinum. Although there is some controversy about the nature of the catalytic poison, most widely it is believed that the methanolic CO develops and impedes the oxidation. A great deal
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of research aims at providing the right catalytic surface morphology in order to reduce the CO poisoning. A number of studies indicate that, among such bimetallic combinations as Pt±Ru, Pt±Sn or Pt±Re, the mixture of Pt and Ru increases, in particular, the system's overall catalytic reactivity [9, 19±21]. The optimum content (in atomic percent) of ruthenium on the surface relative to platinum has been found to be from close to 10 atomic percent Ru at 258C to a value in the vicinity of 30 at 608C [15, 20]. Several mechanisms explaining the role of ruthenium in the activation of platinum have been suggested. They include modi®cation of the electronic nature of the surface, blocking of the poison formation reaction and participation of oxygen containing Ru species in the oxidation of methanol. It has been postulated that, under conditions of methanol oxidation, the oxidized ruthenium species, e.g. Ru(IV), are responsible for the enhancement of platinum activity [22, 23]. Modi®cation of electrode surfaces provides an attractive way of con®ning catalytic species to the eective spatial region and combines the experimental advantages of heterogeneous catalysis with the bene®ts of a three-dimensional distribution of active centers typically characteristic of homogeneous catalysis. In practice, a good electrocatalyst should both exhibit high reactivity towards the reagent molecules in solution and have the ability to distribute electrons rapidly in the microstructure in order to avoid chargetransport limitations. The aim of the present study is to identify conditions (choice of matrix, reactive components, and rigid anchoring groups) for preparation of thin, but three-dimensional, polymeric ®lms which are highly reactive towards electrooxidation of methanol. The attractiveness of mixed-valence inorganic materials [24, 25] for the fabrication of thin ®lms on electrode surfaces has been recognized in recent years. Among them, ruthenium(III, IV) oxide/cyanoruthenate, a mixed-valence polynuclear ruthenium oxide microstructure crosslinked with cyanide, has been shown to form robust thin ®lms on carbon substrates and to act as a more powerful electrocatalyst for the oxidation of methanol than a simple ruthenium oxide coating [26, 27]. In the present study, we explore the fact that ruthenium(III,V) oxide/cyanoruthenate, RuO/ CNRu, is negatively charged, and we demonstrate that this system can be incorporated into the matrix of such a conducting polymer as poly-N-methylpyrrole, PMPy [28]. It is noteworthy that this polymer is stable at positive potentials and, while oxidized, is in a conducting state. Following introduction of polynuclear RuO/CNRu, the overall stability of the composite ®lm is increased. These properties have allowed preparation of fairly thick ®lms in which active ruthenium oxo centers can be fully utilized during catalytic oxidations.
Finally, we show that codeposition of platinum centers, following inclusion of anionic chloroplatinate into the conducting polymer matrix, is also feasible. 2. Experimental Electrochemical measurements were performed with a Bioanalytical Systems BAS 100 B/W potentiostat. All experiments were carried out in three electrode con®guration. In all cases, potentials are reported versus the SCE. Unless otherwise stated, during measurements the temperature was maintained constant in the range 21±238C. All chemicals were commercial materials of the highest purity available: ACS reagent grade or Puratronic. The solutions were prepared from triply distilled or distilled/deionized (Millipore) water. Fabrication of PMPy ®lms was accomplished by performing 50 potential cycles within the limits from ÿ0.3 to 0.8 V in the freshly prepared solution obtained by adding 20 ml of methylpyrrole to 10 ml of 2.0 mol dm ÿ 3 H2SO4. As the electropolymerization reaction proceeds, anions of the supporting electrolyte are incorporated simultaneously into PMPy ®lm on the electrode. The positive charge on the chain of PMPy can be neutralized by the doping anionic species such as RuO/CNRu. A general method for the electrochemical preparation of electrodes covered with RuO/CNRu was described earlier [26, 27]. In the present report, the modi®cation of clean glassy carbon substrate, which had already been coated with PMPy was accomplished by cycling the potential at 50 mV s ÿ 1 between 0.35 and 1.05 V in a freshly prepared deaerated solution of 1.0 10 ÿ 3 mol dm ÿ 3 RuCl3 and 1.0 10 ÿ 3 mol dm ÿ 3 K4[Ru(CN)6] in 0.5 mol dm ÿ 3 KCl at pH 2. When PMPy is reduced, smaller doping anions such as Cl ÿ and Ru(CN)4ÿ 6 will be ejected from the PMPy phase. On the other hand, RuO/CNRu, which is generated during the oxidative potential cycles and has larger volume, cannot leave the reduced PMPy. Consequently, RuO/CNRu remains permanently in the ®lm. The cycling times were varied so that dierent loadings of RuO/CNRu could be obtained. Unless otherwise stated, eight full potential cycles were applied. Platinum microcenters in the ®lms were generated by electroreduction of chloroplatinic acid (0.012 mol dm ÿ 3) in 0.5 mol dm ÿ 3 sulfuric acid. Typically, each electroreduction step involved ten potential cycles in the range from 0.8 to ÿ0.3 V. Electrode surfaces were also examined using JEOL Model JSM-5400 scanning electron microscope of 3 nm resolution. The accelerating voltage of 25 kV was used. Application of microscopy to examine the deposition of platinum particles was established earlier [29].
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For scanning electron microscopy (SEM) measurements, pyrolytic graphite disks (from Union Carbide, Chicago) were used as electrode substrates.
3. Results and discussion Fig. 1 shows a typical cyclic voltammetric response of PMPy ®lm which was electrodeposited on glassy carbon in a manner described in the Experimental section. In 0.50 mol dm ÿ 3 H2SO4, the polymer ®lm becomes oxidized at potentials more positive than 0.5 V. Following incorporation of anionic RuO/CNRu dopant, the voltammetric characteristics of PMPy ®lm changes (Fig. 2), and it becomes dominated by peaks originating from RuO/CNRu. In view of the previous report [26, 27], the voltammetric peaks at about 0.80 and 1.05 V shall be interpreted in terms of the oxidative formation of Ru(III,IV) and Ru(IV) oxo species in the ®lm. The stability of RuO/CNRu in the ®lm was tested by measuring the decrease of voltammetric peak current at 1.060 V during potential cycling experiments in 0.50 mol dm ÿ 3 H2SO4. When the modi®ed electrode was subjected to potential cycling from 0.0 to 1.150 V at 50 mV s ÿ 1 for 1 h, virtually no peak current decrease was observed. Such observation shall be explained in terms of strong interaction between anionic RuO/CNRu sites and positively charged chains in the rigid polymer (PMPy) matrix. For comparison, it is well known that dissolution of Ru from Pt±Ru alloys occurs upon sweeping to oxidizing potentials, resulting in Pt-enriched alloy surfaces [33, 34]. Extension of the upper potential limit to 1.30 V during potential cycling of the composite ®lm led to 10±15% decrease of the peak current. It is unclear whether or not this behavior was due to the loss of entrapped
Fig. 1. Cyclic voltammogram for PMPy ®lm on glassy carbon. Scan rate, 50 mV s ÿ 1; electrolyte: 0.5 mol dm ÿ 3 H2SO4; electrode substrate area: 0.071 cm2.
Fig. 2. Cyclic voltammetry of a composite ®lm consisting of PMPy and RuO/CNRu. Other parameters as for Fig. 1.
RuO/CNRu or the dissolution of the polymer matrix itself. Methanol cannot be oxidized at bare carbon electrodes at least before the onset of electrolyte decomposition. The catalytic electrooxidation of methanol at glassy carbon electrode modi®ed with PMPy containing RuO/CNRu is demonstrated in Fig. 3 for a solution (0.5 mol dm ÿ 3 H2SO4) containing 0.5 mol dm ÿ 3 CH3OH. The dotted line (Curve B) shows the background voltammetric behavior of the derivatized electrode. Oxidation of methanol occurs at about 1.090 V (Curve A). The potential for the methanol oxidation peak, relative to the redox processes in ®lm (Fig. 2),
Fig. 3. Cyclic voltammogram for the oxidation of methanol at PMPy±RuO/CNRu composite ®lm on glassy carbon (Curve A). Methanol concentration; 0.5 mol dm-3; electrolyte; 0.5 mol dm ÿ 3 H2SO4. Dotted line (Curve B) represents the background response of the ®lm in the absence of methanol. Scan rate, 50 mV s ÿ 1. Electrode substrate area, 0.071 cm2.
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Fig. 4. (A) Cyclic voltammetric behavior of the composite PMPy-based ®lm into which RuO/CNRu and dispersed platinum particles (from chloroplatinate) were incorporated. (B) Voltammetric oxidation of methanol. Scan rate, 30 mV s ÿ 1. Other parameters as for Fig. 3.
suggests that electrochemically generated Ru(IV) oxo species [26, 27] participate in the overall mechanism. It shall be noted that the electrochemical characteristics of RuO/CNRu (Fig. 2) resemble the behavior of ruthenium oxide [30], except that, in the case of RuO/ CNRu, Ru(III, IV) redox reactions are better de®ned and they appear at approximately 100 mV less positive potentials. The latter property makes RuO/CNRu potentially more attractive, in comparison to Ru oxide, for electrocatalytic oxidations including methanol. Participation of the oxygen-containing ruthenium species in the activation of Pt towards oxidation of methanol has been postulated for bimetallic Pt±Ru catalysts (alloys) [15, 19±21]. It is likely that the ruthenium oxo species, though ``crosslinked'' by cyanoruthenates within RuO/CNRu, would also be able to induce activity of coexisting, dispersed, platinum for methanol oxidation. In this study, PMPy conducting polymer [31, 32] serves as anion exchange matrix for the incorporation of negatively charged RuO/CNRu and chloroplatinate. Mechanistically, these species are attracted into PMPy by cycling the potential of the respective, polymercoated, electrode, ®rst in the ruthenium chloride + hex-
acyanoruthenate mixture for modi®cation (from 0.350 to 1.050 V), and second in the chloroplatinic acid solution (from 0.400 down to ÿ0.400 V) to generate metallic Pt. Consequently, we can produce thin, but threedimensional, polymeric ®lms of dispersed platinum metal particles codeposited with ruthenium centers. Such systems are of considerable interest to the electrocatalysis of methanol oxidation. We have performed voltammetric measurements to provide a qualitative assessment of electroactivity and composition of the conductive polymer (PMPy) based, composite Pt±RuO/CNRu ®lm. Fig. 4A shows the cyclic voltammetric response of the ®lm of Fig. 3 except that it was subjected to a platinization step as described in the Experimental section. Consequently, the ®lm contained both platinum and RuO/CNRu centers within the PMPy matrix. Two sets of peaks appearing in the potential range from 1.150 to 0.700 V shall be attributed to the electroactivity of RuO/ CNRu [26, 27]. The peak at ca. 0.4 V re¯ects primarily reduction of PtO to metallic Pt. The reverse reaction of oxidation of Pt to PtO is not well de®ned: it presumably overlaps PMPy oxidation and contributes to the increased baseline at potentials more positive than 0.6 V. The presence of platinum is responsible for the system's ability to catalyze proton discharge dramati-
Fig. 5. Voltammetric oxidation of methanol (Curve B) and cyclic voltammetric behavior of PMPy±RuO/CNRu±Pt composite ®lm (Curve A) at 608C. Film and experimental parameters are as for Fig. 4.
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Fig. 6. Oxidation of methanol at the composite ®lm (as for Fig. 4) but subjected to additional platinization. Other parameters as for Fig. 4.
cally at potentials more negative than ÿ0.15 V. Judging from the charge under the voltammetric peak at 0.4 V, the amount of platinum in the ®lm has been found to be on the level of 1.5 10 ÿ 8 mol cm ÿ 2. Upon consideration of the atomic weight of Pt, the latter value is equivalent to 3 mg cm ÿ 2. It comes from SEM examination that Pt forms spherical microparticles having diameters 20±30 nm. Determination of Ru loading is more dicult but we estimate the surface concentration of Ru oxo species to be at least 3± 4 10 ÿ 8 mol cm ÿ 2. It comes from Fig. 4B that methanol undergoes catalytic electrooxidation at approximately 0.7 and 1.1 V, i.e. the process proceeds at both Pt and Ru oxo centers. Due to very high reactivity of Ru(IV) oxo centers [26, 27] and because of their higher surface concentration relative to Pt, the methanol oxidation peak at 1.1 V is much higher than that at 0.7 V. The fact that PMPy matrix becomes more conductive at more positive potentials enhances the system's mediation capabilities and supports the overall catalytic process at 1.1 V. Fig. 5 summarizes data obtained upon increase of temperature to 608C. While the ®lm's cyclic voltammetric response recorded in supporting electrolyte (Fig. 5A) has not changed signi®cantly (compare to Fig. 4A), the methanol oxidation currents (Fig. 5B) have increased ca. three times (compare to Fig. 4B) at 608C. The results are consistent with (1) physicochemical stability of PMPy-based composite catalytic ®lm at 608C, and (2) decreased inhibiting eect of methanolic residues such as (COads) [10, 13] during oxidation of methanol at a higher temperature. Fig. 6 illustrates the voltammetric oxidation of methanol at the composite ®lm (as for Figs. 4 and 5) except that it was further platinized as described in the Experimental section. The micrograph of the resulting ®lm is shown in Fig. 7. Based on the previous
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report [29], the white spots in micrographs should be attributed to platinum microcenters. An eort was made to estimate the platinum loading in the composite ®lm matrix from SEM, assuming spherical particles and using micrographs of 10 K magni®cation. Most particles were approximately spherical, having diameters 30±40 nm. The amount of Pt was on the level of 0.015±0.020 mg cm ÿ 2. This loading of Pt centers is in agreement with the determination from integrating the charges under the respective voltammetric peak as described earlier. The peak potential of ca. 0.680 V for methanol oxidation (Fig. 6) is consistent with the reported electrochemical behavior of methanol on Pt electrodes [9±18]. We presume that Pt, rather than Ru oxo, centers in our composite ®lm are primarily responsible for the observed catalytic process. Indeed, it is apparent from Fig. 6 that a fairly large oxidation peak is obtained at ca. 0.420 V on the negative (reverse) potential scan. This peak immediately follows the rereduction of inhibiting oxides on Pt (Fig. 4A), and it appears in the potential region where active oxo-radicals are expected to exist [10, 13]. Methanol is believed to be electrooxidized over Ptbased catalysts in sulfuric acid via an initial, activating, chemisorption step, followed by interaction with the surface generated radicals, OHads or Oads, or with precursor adsorbed water molecules. The vital role of these chemisorbed oxo radicals is usually attributed to their ability to interact with the inhibiting methanolic residue, COads [10, 13]. By analogy to the activity of Ru, which was postulated for bimetallic Pt±Ru catalysts, we postulate that, due to electrochemical discharge of water by Ru(III) from RuO/CNRu in the ®lm [18, 19], the adsorbed radicals OHads or Oads are produced at less positive potentials than on Pt, and they reduce the COads poisoning. The latter ability to lower inhibition by methanolic residue is eective particularly at higher temperatures. It comes from the
Fig. 7. Scanning electron micrograph of Pt microparticles in the composite ®lm used for oxidation of methanol in Fig. 6.
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4. Conclusions
Fig. 8. Comparison of voltammetric behaviors of methanol at (A) 208C and (B) 608C. The PMPy±RuO/CNRu±Pt composite ®lm and experimental parameters were the same as for Fig. 6.
Such a conducting polymer as PMPy seems to be a promising material for a matrix into which reactive centers, dispersed Pt spheres and Ru oxo (RuO/ CNRu) species, can be introduced. RuO/CNRu is negatively charged and can be incorporated, together with chloroplatinate, into a thin ®lm of PMPy. From the viewpoint of an eective electrocatalytic oxidation, it is important to appreciate the fact that PMPy is, while oxidized, in a conducting state. The overall stability of PMPy ®lm is improved upon incorporation of polynuclear RuO/CNRu. This system shows redox reactions comparable to those of ruthenium oxide except that they are better de®ned, and they appear at less positive potential. By analogy to the activity of ruthenium in bimetallic Pt±Ru catalysts, RuO/CNRu enhances activity of Pt towards oxidation of methanol.
Acknowledgements comparison of curves A and B in Fig. 8 that the voltammetric currents for methanol oxidation have drastically increased upon increase of temperature from (A) 208C to (B) 608C. For well-characterized Pt±Ru alloys, the rate-determining step was believed to shift from methanol adsorption/dehydrogenation at lower temperature to the surface reaction between the dehydrogenated intermediate and surface oxygen sites at higher temperature. It is likely that, in addition to blocking the formation of the poisoning intermediate, the oxygen-containing Ru species may take part in the main oxidation reaction. We postulate the ``synergistic'' eect of RuO/CNRu on Pt, i.e. that, in the presence of RuO/CNRu in the composite ®lm, the catalytic activity of Pt towards methanol oxidation increases. When the voltammetric experiment was performed in the analogous manner as for Fig. 6, i.e. using the same PMPy matrix containing approximately the same loading of Pt, except that no RuO/CNRu was introduced into the ®lm, we observed an almost twofold decrease of the voltammetric peak current at about 0.6±0.7 V. It is reasonable to expect that RuO/CNRu centers and Pt microparticles are close neighbors. They contact and interact with each other in a manner analogous to Ru oxo species and Pt on the surfaces of the alloys [15, 20]. Although we do not have any evidence for the formation of composite RuO/CNRu±Pt particles within the conducting polymer matrix, such a possibility cannot be excluded. Further research is in progress aiming at elucidation of the utility of the PMPy-based composite ®lm for electrocatalytic oxidation of methanol under steady state conditions at less positive potentials.
This work was supported by State Committee for Scienti®c Research (KBN), Poland under Grant 3 T09A 120 12 and by the NATO Collaborative Research Grant (HTECH.CRG 940808).
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