The Effects of Conducting Polymers on Formic Acid Oxidation at Pt Nanoparticles

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The Effects of Conducting Polymers on Formic Acid Oxidation at Pt Nanoparticles Reza B. Moghaddam, Osama Y. Ali, Mohammad Javashi, Peter L. Warburton, Peter G. Pickup ∗ Department of Chemistry, Memorial University, St. John’s, Newfoundland and Labrador, Canada A1B 3X7

a r t i c l e

i n f o

Article history: Received 11 July 2014 Received in revised form 14 August 2014 Accepted 16 August 2014 Available online xxx Keywords: electrocatalysis support effect density functional theory formic acid oxidation platinum nanoparticles

a b s t r a c t The effects of polyaniline, polypyrrole, polyindole and polycarbazole on formic acid oxidation at Pt nanoparticles are compared. The observed trend in activity (polypyrrole < polyaniline ∼ polyindole < polycarbazole) correlates with the decreasing LUMO energies of the monomers (pyrrole > aniline > indole > carbazole), supporting previous evidence of electron donation from Pt nanoparticles into the ␲-system of a polycarbazole support layer. Density functional theory calculations on CO and carbazole binding to Pt4 clusters show that the electronic effect of carbazole in a carbazolePt4 -CO ensemble considerably weakens the binding of CO. The magnitude of this effect is comparable to the effects of graphite and graphene supports reported by other researchers, and stronger than the effect calculated here for indole. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction Although it is well known that the incorporation of conducting polymers into fuel cell catalysts can enhance their activities [1], our understanding of the origins of these effects is very rudimentary. In part, this is due to the very diverse range of materials and synthesis methods that have been employed and difficulties in fully characterizing these systems. We have therefore adopted simple methodology for systematic investigation of these effects in which a thin layer of the conducting polymer, on a glassy carbon substrate, is coated with approximately a monolayer of preformed and well characterized Pt nanoparticles [2]. As illustrated in Fig. 1A, such a configuration maximizes the contact of the nanoparticles with the conducting polymer, while the use of thin films of polymer and nanoparticles minimizes transport (substrate, electrons, and protons) effects. In addition, the use of preformed metal nanoparticles removes any influence of the polymer on their size, composition, and morphology. The effects of polyaniline, polypyrrole, polyindole and polycarbazole on formic acid oxidation are compared here, and their mechanistic implications are discussed. It is curious that polymers with different backbone structures, chemical functionalities and electronic properties can exert

∗ Corresponding author. Tel.: +1 709 864 8657; fax: +1 709 864 3702. E-mail address: [email protected] (P.G. Pickup).

similar effects on the electrocatalytic properties of Pt nanoparticles. For example, polyaniline (Fig. 2) consists of phenyl rings linked by basic nitrogen atoms, while polycarbazole is more rigid, with fused phenyl-pyrrolyl rings containing nitrogen atoms that are not significantly acidic or basic. However, both of these materials enhance the rate of formic acid oxidation at Pt [3]. Understanding how this can be the case should help us develop better tools for the design of improved catalysts. Ultimately, it should be possible to tailor catalyst supports with different functionalities to optimize performances. The following examples illustrate the diverse roles that polymer supports can play. Polyaniline has been shown to control the dispersion of the catalyst [4], and influence electron transport [4] and methanol diffusion [5]. It also appears to inhibit the formation of strongly chemisorbed species at Pt during formic acid oxidation [6]. Polyparadimethoxybenzene can modify the approach and orientation of the formic acid and thereby decreases the concentration of adsorbed CO measured by in situ infrared reflectance spectroscopy [7]. Poly-(3,4-ethylenedioxythiophene) promotes the oxidation of CO during methanol oxidation, which could be due to electronic effects, increasing local lipophilicity, or activating water [8]. In general the support can influence:

(a) the nature of the catalyst particles (size, composition, morphology),

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A

B

Fig. 1. Schematic diagrams of Pt nanoparticles on glassy carbon coated with a thin conducting polymer film. A. Ideal structure. B: Structure revealed by AFM and SEM.

(b) access of reactants (including electrons and protons) to the catalytic sites, (c) the mechanism: by direct involvement (e.g. a bifunctional mechanism [9]) or indirectly via electronic (ligand) and/or geometric (e.g. third body) effects [10], etc.

Fig. 3 Illustrates these roles for a Pt nanoparticle in a threedimensional catalyst layer. A previous study of the effects of polycarbazole on formic acid oxidation at Pt nanoparticles indicated that electron donation into the conjugated ␲-system of the polymer may be responsible for the increased activity [11]. This has been explored further here, both experimentally and computationally, by comparing polymers with different LUMO (lowest unoccupied molecular orbital) energies.

2. Experimental 2.1. Chemicals

2.2. Preparation of Pt nanoparticles NaBH4 (aq) (ca. 1 mL; 120 mM) was added dropwise to a stirred solution of 10 mL of 3 mM H2 PtCl6 (aq) mixed with 0.6 mL of 50 mM aqueous sodium citrate [12]. Following stirring for a further 2 h, the resulting grey colloidal Pt nanoparticle solution was stored in a fridge. X-ray diffraction (XRD) measurements indicated that the average particle diameter was 5.0 nm. 2.3. Working electrode preparation Glassy carbon electrodes (GC; CH Instruments; 0.071 cm2 ) were polished with 0.05 ␮m alumina and rinsed well with water before use. Polymer films were galvanostatically deposited as follows: (a) Polycarbazole was deposited at 0.28 mA cm−2 for 35 s from dichloromethane containing 0.01 M carbazole and 0.1 M Bu4 NPF6 . (b) Polyaniline was deposited at 0.17 mA cm−2 for 150 s from 0.1 M H2 SO4 containing 0.1 M aniline.

Sulfuric acid (Fisher Scientific), formic acid (Sigma Aldrich; 98100%), dichloromethane (Sigma Aldrich; ACS reagent, 99.9%), tetrabutylammonium hexafluorophosphate (Bu4 NPF6 ; Fluka; electrochemical grade, 99.0%), carbazole (Alfa Aesar; 95%), H2 PtCl6 ·6H2 O (Alfa Aesar), sodium borohydride (Sigma Aldrich), sodium citrate (Anachemia), and indole (Sigma Aldrich; > 99%) were used as received. Pyrrole (Sigma Aldrich) and aniline (Alfa Aesar) were purified by passing through alumina prior to use. All measurements were recorded at ambient temperature under a nitrogen atmosphere following purging for 15 min. Deionized water was used throughout the experiments.

Fig. 2. Structures of the polymers employed in this work.

Fig. 3. Schematic illustration of the roles of the support material in a threedimensional catalyst layer.

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Table 1 Approximate active areas and corresponding utilization values estimated from the H desorption charges in Fig. 5. Electrode

Active area/cm2

GC/Pt GC/polyindole/Pt GC/polycarbazole/Pt GC/polyaniline/Pt GC/polypyrrole/Pt

0.26 0.27 0.25 0.14 0.10

Utilization 76% 79% 75% 41% 29%

(c) Polyindole was deposited at 0.56 mA cm−2 for 60 s from dichloromethane containing 0.02 M indole and 0.1 M Bu4 NPF6 . (d) Polypyrrole was deposited at 0.070 mA cm−2 for 140 s from dichloromethane containing 0.02 M pyrrole and 0.01 M Bu4 NPF6 . These conditions were selected to give the thinnest stable film of each polymer (to minimize transport effects [13]), except for polyaniline where there was not a significant film thickness dependence, and the stated conditions gave better reproducibility than the thinnest films. The higher charge required for polyindole was due to a low polymerization yield due to the formation of soluble products. Each electrode, including a bare GC electrode, was drop coated with 0.60 ␮g of Pt nanoparticles. They are designated as GC/Pt, GC/polycarbazole/Pt, GC/polyaniline/Pt, GC/polyindole/Pt, and GC/polypyrrole/Pt, respectively. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) has shown that this procedure does not give the ideal structure shown in Fig. 1A, but produces clumps of particles as illustrated in Fig. 1B [14]. However, a large fraction of the particles are electrochemically active (Table 1) and this type of electrode shows strong support effects (see Fig. 6).

Fig. 4. Cyclic voltammograms (1st scan at 100 mV s−1 ) of GC/polycarbazole, GC/polyaniline, GC/polyindole, and GC/polypyrrole electrodes in 0.1 M H2 SO4 (aq).

where E(Pt4 -CO) is the total energy of the Pt4 -CO adduct, E(Pt4 ) is the energy of the Pt4 cluster, and E(CO) is the energy of molecular CO. For comparison with literature results, the adsorption energy of CO on Pt4 was also calculated by utilizing a DFT framework based on plane-wave pseudopotentials with plane basis sets as implemented in the Vienna ab initio simulation package (VASP) program [18–20]. Therefore, the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional were used to describe the interaction between core and valance electrons [21,22]. A plane-wave of 400 eV cutoff energy was used. A 15 A˚ vacuum slab was employed to separate the repeated slabs.

2.4. Instrumentation

3. Results and discussion

An EG&G Model 273A Potentiostat/Galvanostat run by a PC through M270 commercial software was used for voltammetry and chronoamperometry. A saturated calomel electrode (SCE) and a platinum wire formed the reference and counter electrode, respectively. Each freshly prepared electrode was cycled in 0.1 M H2 SO4 to establish a stable voltammetric pattern (typically 5 cycles), following which formic acid was injected to give a 0.5 M concentration and voltammograms were recorded over a number of cycles between -0.25 and +0.8 V. Currents were not normalized for either the geometric or active area.

3.1. Polyaniline, polycarbazole, polyindole and polypyrrole supported Pt nanoparticles

2.5. Theoretical methods and computational details Calculations for all proposed structures were carried out using the Gaussian 09 suite of programs [15]. The final structures were optimized and the harmonic vibrational frequencies of the structures were calculated using Becke’s three-parameter exchange functional with the correlation functional of the Lee, Yang, and Parr method (B3LYP). The LANL2DZ effective core potential basis set [16] was used for platinum, the 6-31G(d,p) basis set of double zeta quality with p polarization functions was used in hydrogen atoms, and d polarization functions in carbon, nitrogen and oxygen atoms. The LANL2DZ basis set was used to reduce the computational demands without having much effect on the binding energy of small molecules [17]. The lowest energy structures found were used in calculating of the adsorption energy Eads which is defined for CO adsorption as: Eads (CO) = E(Pt4 − CO) − E(Pt4 ) − E(CO)

(1)

Fig. 4 shows cyclic voltammograms of GC/polycarbazole, GC/polyaniline, GC/polyindole, and GC/polypyrrole electrodes in 0.1 M H2 SO4 . Although each polymer has distinctive electrochemical behavior, with peaks at different potentials, the peak currents are similar. This indicates that similar amounts of polymer were deposited in each case. Polypyrrole and polyaniline are the most easily oxidized, while the onset potential for oxidation is highest for polycarbazole. Fig. 5 compares cyclic voltammograms of GC/Pt, GC/polycarbazole/Pt, GC/polyaniline/Pt, GC/polyindole/Pt, and GC/polypyrrole/Pt electrodes in 0.1 M H2 SO4 . Less positive upper limits were used for the polymer coated electrodes to avoid oxidative degradation of the polymer. At potentials below 0 V, these voltammograms show characteristic peaks for H adsorption (cathodic scan) and H desorption (anodic scan) that can be used to assess how much of the Pt applied to the electrode was electrochemically active. At potentials above 0 V, the GC/Pt electrode (dotted line) shows a small charging current followed by oxide formation (0.5-1 V) and stripping (cathodic peak at 0.45 V) at higher potentials. The GC/polycarbazole/Pt electrode (dashed line) gave a similar response. Notably, the charge in the H adsorption/desorption region was not changed significantly by the presence of the polycarbazole film, indicating that a high fraction of the Pt nanoparticles were electrochemically active [13]. The other electrodes all showed enhanced currents at potentials above ca. 0 V due to the electrochemistry of the polymer film.

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Fig. 5. Cyclic voltammograms (approximately steady-state at 100 mV s−1 ) of GC/Pt, GC/polycarbazole/Pt, GC/polyaniline/Pt, GC/polyindole/Pt, and GC/polypyrrole/Pt electrodes in 0.1 M H2 SO4 (aq).

Polyaniline gave a well-defined peak at +0.11 V while polypyrrole gave a broad region of enhanced current with an irreversible oxidation beginning at ca. 0.5 V. Polyindole produced a broad region of enhanced electroactivity with an anodic peak at +0.78 V and cathodic peak at +0.48 V. These potentials and the wave shapes are very similar to those for the GC/Pt electrode indicating that this region was dominated by the Pt/oxide electrochemistry. The H adsorption/desorption region was similar for all electrodes, suggesting that none of the polymers suppressed H adsorption greatly. However, the high “charging currents” (seen between 0 and +0.25 V) in some cases create significant uncertainly and add to the total charge in the H adsorption/desorption region. For the GC/polypyrrole/Pt electrode in particular, the H adsorption/desorption peaks are superimposed on the very broad redox waves of the polypyrrole that extend into the -0.25 to 0 V region. If the charge for this process is subtracted from the total charge in this region, the resulting charge for the H adsorption/desorption processes on Pt are much lower than for the other electrodes. Similarly for polyaniline, the H adsorption/desorption is significantly lower when the charge for the polymer redox is subtracted. Table 1 gives approximate active Pt areas and percent utilizations (active area/theoretical area for 5.0 nm spherical particles) for each electrode. Although the subtraction of the background current of the polymer is imprecise, these data provide important clues to the mechanistic effects of each polymer. It can be seen from the data that the polycarbazole and polyindole films did not significantly change the Pt active area, while the active area decreased in the order polyaniline > polypyrrole for the other two polymers. Fig. 6 compares linear sweep voltammograms for GC/Pt, GC/polycarbazole/Pt, GC/polyaniline/Pt, GC/polyindole/Pt, and GC/polypyrrole/Pt electrodes in 0.1 M H2 SO4 (aq) containing 0.5 M formic acid. First scans are shown in order to compare inherent activities, although decreases during subsequent scans were minor. The peak at ca. 0.2 V seen for all of the electrodes can be attributed to the direct oxidation of formic acid to carbon dioxide, while the peak at ca. 0.6 V (shoulder for GC/polycarbazole/Pt) is due to the indirect oxidation via adsorbed carbon monoxide (COads ) [23]. It can be seen that polyanaline, polycarbazole, and polyindole enhance both peaks relative to GC/Pt while polypyrrole decreases both peaks. In the case of polypyrrole, the decrease of about 60% is similar to the decrease in active area, and so can reasonably be attributed to simple blocking of Pt sites by adsorbed polypyrrole. The promoting effects of polyaniline and polyindole seen in Fig. 6 are similar at 0.2 V, but polyindole has a greater effect at 0.6 V. This difference

Fig. 6. Linear sweep voltammograms (1st scan at 10 mV s−1 ) of GC/Pt, GC/polycarbazole/Pt, GC/polyaniline/Pt, GC/polyindole/Pt, and GC/polypyrrole/Pt electrodes in 0.1 M H2 SO4 containing 0.5 M FA.

at 0.6 V can be adequately explained by the higher active area of Pt on the polyindole modified electrode. This suggests then, that the activating effects of polyaniline at 0.2 V is stronger than that of polyindole. However, polycarbazole has a much greater activating effect at 0.2 V than polyaniline. In the case of polycarbazole it was found in ref. 13 that the use of thicker films caused peak shifts that could be attributed to increasing film resistance. We therefore infer here that the absence of significant peak shifts indicates that the effects of film resistances were negligible at the peak potentials for formic acid oxidation. The observation of different degrees of activation of Pt for formic acid oxidation by the four polymers studied here provides important clues to the origins of the effects. Polyindole and polycarbazole have the same pyrrole ring functionality as polypyrrole, but with fused benzene rings that extend the conjugation (Fig. 2). The order of their activities suggests that this extended conjugation could be an important factor in the activation of the Pt surface. The structure of polyaniline also contains a benzene ring, but its nitrogen functionality is very different, being external to the ring system and quite basic. In contrast, the aromatic nitrogen atoms in polypyrrole, polyindole, and polycarbazole are not significantly basic or acidic. This suggests that acid/base properties are not important, unless activation of Pt by polyaniline occurs by a different mechanism. 3.2. Methanol oxidation at polycarbazole and polyaniline supported Pt nanoparticles Further insight into the nature of the effects of polycarbazole were sought by exploring its influence on methanol oxidation at Pt nanoparticles. Fig. 7 shows linear sweep voltammograms for methanol oxidation at GC/Pt, GC/polycarbazole/Pt and GC/polyaniline/Pt electrodes. Previously [2], we have found that polyaniline does not significantly improve the activity of Pt nanoparticles for methanol oxidation and data from that work is included in Fig. 7. The small enhancements in current observed with polyaniline over the 0.2 V to 0.35 V region can be attributed mainly to the electrochemistry of the polyaniline. Fig. 7 presents a stunning contrast between the effects of polycarbazole and polyaniline on methanol oxidation at Pt nanoparticles. While polycarbazole has a large activating effect, similar in magnitude to its effect on formic acid oxidation (Fig. 6), polyaniline inhibits methanol oxidation. The effect of polyaniline on methanol oxidation also contrasts sharply with its significant promoting effect on formic acid oxidation (Fig. 6). These

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Table 2 Calculated (Gaussian) adsorption energies for CO, carbazole and indole on Pt4 , and for CO on carbazole-Pt4 and indole-Pt4 adducts. Substrate

adsorbate

adsorption energy/eV

Pt4 Pt4 Pt4 carbazole-Pt4 indole-Pt4

carbazole indole CO CO CO

-2.07 -1.79 -2.77 -1.84 -2.48

The adsorption energy of CO onto the carbazole-Pt4 adduct was calculated as Eads (CO) = E(CZ − Pt4 − CO) − E(CZ − Pt4 ) − E(CO)

Fig. 7. Linear sweep voltammograms (1st scan at 10 mV s−1 ) for methanol oxidation (0.1 M H2 SO4 +0.2 M MeOH) at GC/Pt, GC/polycarbazole/Pt and GC/polyaniline/Pt electrodes.

observations suggest that the mechanisms of action of the two polymers on formic acid oxidation may be different. For example, if both polymers increased the rate of formic acid oxidation via an electronic effect, they would both (or neither) be expected to increase the rate of methanol oxidation. 3.3. Density functional theory (DFT) calculations Previously [11], we have suggested that the high activity of polycarbazole supported Pt may be due to electron donation from Pt nanoparticles into the conjugated ␲-system of the polycarbazole. The new results presented here are consistent with this type of electronic (ligand) effect and also suggest that polyindole has a weaker electronic effect, polypyrrole has an insignificant electronic effect, and that polyaniline influences formic acid oxidation by a different mechanism. In order to explore the possibility of this type of electronic effect for polycarbazole, and possibly explain the weaker effects of the other polymers, we have initiated a DFT study of the interactions of Pt clusters with carbazole and indole, and how these influence CO binding. Preliminary results are reported here to support a mechanistic discussion of the electrochemical results. Fig. 8 shows the optimized structures of the systems that have been investigated. These consist of a Pt4 cluster bound to either a carbazole (A) or indole (B) molecule, and a terminal CO molecule, and individual components of these systems. The use of Pt4 clusters for this type of study has been shown to provide useful insights into support effects [24,25], and allows us to make valuable comparisons with other systems that have been studied in this way. The strength of binding of CO to metal clusters provides a useful probe of electronic effects that are relevant to the oxidation of small organic molecules such as formic acid and methanol. Clearly this approach will overestimate the effect of the modifier, but the trends should be correct, and the literature work with graphene and graphite provided excellent benchmarks. The adsorption energies of CO (Eads (CO)), carbazole (Eads (CZ)), and indole (Eads (IN)) on Pt4 were calculated from the energies of optimized structures C (E(Pt4 -CO)), D (E(CZ-Pt4 )), and E (E(IN-Pt4 )), in Fig. 8, by subtraction of the energies of optimized structures of the individual components (E(Pt4 ), E(CO), E(CZ), E(IN)) as in equations 1-3. Eads (CZ) = E(CZ − Pt4 ) − E(Pt4 ) − E(CZ)

(2)

Eads (IN) = E(IN − Pt4 ) − E(Pt4 ) − E(IN)

(3)

(4)

where E(CZ-Pt4 -CO) is the energy of optimized structure A. The adsorption energy of CO onto the indole-Pt4 adduct was calculated similarly from the energy of optimized structure B. The results are summarized in Table 2. The calculated adsorption energy of CO on Pt4 was -2.77 eV using Gaussian and -2.63 eV using VASP. These values are close to previously reported values of -2.54 eV [24] and -2.92 eV [25] from VASP calculations. The calculated adsorption energy of carbazole on Pt4 was -2.07 eV using Gaussian, indicating a significant interaction of the Pt4 cluster with the carbazole ␲-system. This interaction weakens the binding of CO to the Pt4 cluster, resulting in a calculated Eads (CO) of only -1.84 eV (Gaussian) for binding of CO to the carbazole-Pt4 adduct. The decrease in Eads (CO) due to the carbazole was 0.93 eV, which is comparable to a calculated decrease in Eads (CO) of 1.01 eV reported for CO binding to Pt4 on graphene [24], and indicates a significant electronic (ligand) effect of carbazole on the Pt4 cluster. A similar effect has been calculated for Pt4 on graphite [25], with Eads (CO) decreasing by 0.73 eV. In the case of Pt4 on graphite, it has been shown that there is electron donation from the cluster to the graphite surface and that the atoms contacting the graphite become positively charged while the top Pt atom becomes negative [26]. These findings, together with analogy with the binding of CO to metals, imply that back donation of electron density into the LUMO is a key feature in the binding of Pt to carbazole. It is therefore pertinent that the calculated energy of the LUMO of carbazole (-0.66 eV) is very similar to that of CO (-0.58 eV). Since back donation of electron density to CO strengthens the Pt-CO bond, donation from the Pt4 cluster to carbazole appears to “compete” with this and thereby weakens the CO binding. This suggests that the strength of the electronic effect of different polymer supports on Pt nanoparticles would depend on the LUMO energy of the monomer, which is the main factor that determines the LUMO energy of the polymer [27]. It can be seen from the data in Table 2 that the interaction of indole with the Pt4 cluster is significantly weaker than for carbazole, and that indole has a much smaller effect on CO binding. This weaker electronic effect of indole is consistent with the lower effect of polyindole on formic acid oxidation, providing further evidence that the activation of Pt for formic acid oxidation by polycarbazole and polyindole is due to an electronic effect. The calculated LUMO energy of indole is 0.55 eV higher than that of carbazole (Table 3), which is consistent with its weaker electronic effect relative to carbazole. This trend is followed by pyrrole, which has a calculated Table 3 Calculated (Gaussian) HOMO and LUMO energies for carbazole, indole, and pyrrole. Compound

HOMO energy/eV

LUMO energy/eV

carbazole indole pyrrole aniline

-5.46 -5.42 -5.50 -5.39

-0.66 -0.09 +1.34 +0.25

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Fig. 8. Optimized structures used for calculation of the binding of CO, carbazole and indole to Pt4 clusters. C (Black), H (White), Pt (Silver), N (Dark-gray), O (Light-gray).

LUMO energy of +1.34 eV (2.00 eV higher than carbazole), with polypyrrole not promoting formic acid oxidation at all (Fig. 6). These three monomers have similar HOMO energies (Table 3), so it appears to be the LUMO energy that is most important. Although the LUMO of aniline is significantly higher than that of indole (Table 3), polyaniline has a similar activating effect on formic acid oxidation to polyindole (Fig. 6) (a greater effect on an active area basis). This may be due to differences in LUMO energies between the monomers and polymers, but may indicate that the mechanism of action of polyaniline is different (i.e. not an electronic effect). The computational results on CO binding presented above provide a useful proxy for understanding the effects of the polymers on formic acid oxidation. CO binding provides a sensitive, well studied, and relatively simple probe for electronic effects that can be explored both experimentally and computationally. In contrast, the more complex and less understood mechanism of formic acid oxidation [28–30] makes it very difficult to unambiguously identify electronic effects. We therefore make the assumption that if electronic effects are shown to influence CO binding, they will likely play a role in formic acid oxidation either by decreasing the CO coverage or via an influence on another adsorbed species [28]. Previous work has provided significant evidence that polycarbazole can exert a significant electronic effect on Pt nanoparticles

[11]. This includes a negative shift in the peak potential for CO stripping, an increase in the amount of Cu underpotential deposition, and a small increase in the Pt 4f7/2 binding energy. These observations all suggest that polycarbazole acts as an acceptor of electron density from Pt. Similar binding energies for Pt nanoparticles supported on graphene have been taken as evidence of “interaction between Pt and graphene via ␲–d hybridization”, which was used to explain the higher activities for CO oxidation observed for smaller nanoparticles [31]. The computational results presented above show that the electronic effects for a carbazole molecule are very similar to those of graphene. It appears that the close match of the carbazole LUMO with that of CO could be a key factor here, and that donation of electrons from the Pt4 cluster into the carbazole LUMO may weaken the back-donation of electron density from Pt to CO. These hypotheses are supported by the computational results for indole and pyrrole, and the correlation of their increasing LUMO energies (carbazole < indole < pyrrole) with the decreasing activities for formic acid oxidation of Pt nanoparticle supported on their polymers (polycarbazole > polyindole > polypyrrole). Aniline/polyaniline does not fit the above trend, and the basic nature of the nitrogen functionality and the failure of polyaniline to promote methanol oxidation suggest that factors other than an electronic effect may be involved. These could include a third-body

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effect, or influences on the adsorption of water, formic acid, and formate via H-bonding or local lipophilicity [32]. 4. Conclusion The electrocatalytic activities for formic acid oxidation of Pt nanoparticles on polycarbazole, polyindole, and polypyrrole correlate with the LUMO energy of the monomer. This supports previous evidence that donation of electron density from the nanoparticle to the polymer may increase activity. Such an electronic effect is also indicated by the calculated effects of carbazole and indole on CO binding to a Pt4 cluster. Polyaniline appears to behave differently than the pyrrole based polymers, suggesting an alternate mechanism. Acknowledgments This work was supported by the Natural Sciences and Engineering Research Council of Canada and Memorial University. We also thank the Atlantic Computational Excellence Network (ACENet) for providing computational resources. References [1] E. Antolini, Composite materials. An emerging class of fuel cell catalyst supports, Applied Catalysis B-Environmental 100 (2010) 413–426. [2] R.B. Moghaddam, P.G. Pickup, Support effects on the oxidation of methanol at platinum nanoparticles, Electrochemistry Communications 13 (2011) 704–706. [3] R.B. Moghaddam, P.G. Pickup, Influences of aniline, carbazole, indole, and pyrrole monomers and polymers on formic acid oxidation at Pt electrodes, Electrochimica Acta 107 (2013) 225–230. [4] H.H. Zhou, S.Q. Jiao, J.H. Chen, W.Z. Wei, Y.F. Kuang, Effects of conductive polyaniline (PANI) preparation and platinum electrodeposition on electroactivity of methanol oxidation, Journal of Applied Electrochemistry 34 (2004) 455–459. [5] H. Gharibi, K. Kakaei, M. Zhiani, Platinum nanoparticles supported by a Vulcan XC-72 and PANI doped with trifluoromethane sulfonic acid substrate as a new electrocatalyst for direct methanol fuel cells, Journal of Physical Chemistry C 114 (2010) 5233–5240. [6] M. Gholamian, J. Sundaram, A.Q. Contractor, Oxidation of formic-acid at polyaniline-coated and modified-polyaniline-coated electrodes, Langmuir 3 (1987) 741. [7] M. Choy, F. Hahn, J.M. Leger, C. Lamy, J.M. Ortega, In situ Fourier transformed infrared reflectance spectroscopy study of the effect of poly-pDMB film modified platinum electrodes on the electrooxidation of formic acid, Thin Solid Films 515 (2007). [8] H.V. Patten, E. Ventosa, A. Colina, V. Ruiz, J. Lopez-Palacios, A.J. Wain, S.C.S. Lai, J.V. Macpherson, P.R. Unwin, Influence of ultrathin poly-(3,4ethylenedioxythiophene) (PEDOT) film supports on the electrodeposition and electrocatalytic activity of discrete platinum nanoparticles, Journal of Solid State Electrochemistry 15 (2011). [9] J.H. Ma, Y.Y. Feng, J. Yu, D. Zhao, A.J. Wang, B.Q. Xu, Promotion by hydrous ruthenium oxide of platinum for methanol electro-oxidation, Journal of Catalysis 275 (2010) 34–44.

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Please cite this article in press as: R.B. Moghaddam, et al., The Effects of Conducting Polymers on Formic Acid Oxidation at Pt Nanoparticles, Electrochim. Acta (2014), http://dx.doi.org/10.1016/j.electacta.2014.08.029