Friendly Conditions Synthesis of Platinum Nanoparticles Supported on

Aug 1, 2007 - Friendly Conditions Synthesis of Platinum Nanoparticles Supported on a Conducting Polymer: .... Rajendar Bandari , Michael R. Buchmeiser...
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12454

J. Phys. Chem. C 2007, 111, 12454-12460

Friendly Conditions Synthesis of Platinum Nanoparticles Supported on a Conducting Polymer: Methanol Electrooxidation Horacio J. Salavagione,* Carlos Sanchı´s, and Emilia Morallo´ n Departamento de Quı´mica Fı´sica e Instituto UniVersitario de Materiales, UniVersidad de Alicante, Apartado 99, 03690 Alicante, Spain ReceiVed: February 6, 2007; In Final Form: May 17, 2007

An easy method to synthesize Pt nanoparticles supported on conductive polymers has been explored. The synthesis was carried out under “soft” conditions, i.e., at low temperatures and without using strong reducing agent. The optimal synthesis conditions have been established. Thus, the particles had almost monodisperse size distribution, around 2.4 nm, when the synthesis was carried out at 100 °C, and around 1.4 nm when it was done at 75 °C. Also, it has been demonstrated that the platinum loading and the pretreatment of the catalyst determine the catalyst activity. Cyclic voltammetry in sulfuric acid displayed the processes of adsorption-desorption of hydrogen. The PANI-Pt catalysts have been checked in the oxidation of methanol, and a low degree of poisoning due to carbon monoxide adsorption has been detected in comparison with bulk platinum.

Introduction Metals nanoparticles play an important role due to their potential applications in electronic and optical devices,1 biosensors,2,3 catalyst,4-7 etc. Because of environmental concerns and depletion in natural resources, the development of electrocatalysts for fuel cells has been the subject of most of the studies regarding metal nanoparticles. Considering the expensive price of precious metals used as catalysts and the need of catalysts with a high activity, most applications require the use of finely divided metal particles. In this way there are many synthetic procedures for obtaining metal nanoparticles:4,8 in aqueous solutions9,10 and organic solutions;11 by phase transfer of aqueous nanoparticles to organic media.12,13 However, these synthesis protocols require protecting agents to avoid the aggregation of the nanoparticles, which reduces the active surface of the metal and its catalytic activity. The immobilization of growing particles on solid supports emerges as an alternative procedure to overcome the loss of catalytic activity. Among the solid supports, carbon black has been the most utilized,14-17 although other carbon forms, such as carbon nanotubes18-21 (CNT) and fibers,22 have also been used. It has been reported the employment of viscous solvents (such as polyols) to minimize the particle diffusion and to inhibit the particle growth. Miyake et al. reported that Pt nanoparticles can be produced using short-chain alcohols as reducing agents and poly(N-vinyl-2-pyrrolidone) (PVP) to prevent nanoparticles aggregation.23 They demonstrated that the particle size could be controlled by changing both the kind of alcohol and the concentration of PVP. Chen and Xing reported a polymermediated synthesis to produce highly dispersed Pt nanoparticles.24 They used poly(ethylene glycol) as both solvent and reducing agent and PVP to control the distribution and the Pt nanoparticle size. This method is advantageous because PVP can be easily removed from the Pt surface by washing with * To whom correspondence should be addressed. Phone: +34 965903536. Fax: +34 965-903537. E-mail: [email protected].

water, ethanol, or acetone and particles do not aggregate since they are fixed to the carbon support. Conducting polymers are interesting supports for metal catalyst because they can be covalently modified by adding functional groups.25-27 Conducting polymers are usually used as matrix to noble metal catalysts for the oxidation of small organic molecules. Electrochemical techniquessusing both pulsed and cyclic methodsswere used to incorporate metal particlesontheelectrochemicallypreparedconductingpolymers.28-32 However, it has been demonstrated that the polymeric films do not cover the whole electrode surface, which makes the measured catalytic activity have some contribution from the bare electrode.33-35 Alternatively, PANI has been used to reduce platinum salts to bare metal in which the reaction of PANI implies the oxidation from the semiquinone state (emeraldine) to the fully oxidized state, pernigraniline.36 Moreover, a new method that describes the synthesis of metal nanoparticles (especially Ag) using polymer colloids composed of PANIs which acts as reducing agentsand poly(acrylic acid) has been reported.37 Kulesza et al.38 have studied the PMo12-protected Pt nanoparticles linked to PANI layers, and they control the layer-by-layer growth of the polymer/nanoparticle hybrid films in the electroreduction of oxygen. In this work, we study the chemical synthesis of Pt nanoparticles supported on polyaniline using PVP to control the size and distribution of Pt nanoparticles. The effects of the temperature of synthesis, the platinum loading, and the reducing agent on the properties of the catalyst are analyzed. Experimental Section Chemicals. Hexachloroplatinic acid (H2PtCl6‚6H2O), ethylene glycol, and poly(vinylpyrrolidone) (MW ) 40 000 Da) were purchased from Aldrich and used without further treatments. Platinum salt solutions were stored at low temperature in the dark. Aniline (from Aldrich) was distilled at lower pressure twice before use, and the water employed for the preparation

10.1021/jp071037+ CCC: $37.00 © 2007 American Chemical Society Published on Web 08/01/2007

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TABLE 1: Synthesis Parameters Employed for PANI-Pt Catalyst, Pt Loading, and Particle Size Obtained by TEM temp (°C)

Pt load in reactor (wt %)

media

Pt load in catal (wt %)

particle size (nm)

100 75 100 100 100

20 20 20 38 50

3/1 ethylene glycol/water 3/1 ethylene glycol/water water 3/1 ethylene glycol/water 3/1 ethylene glycol/water

16.8 22.8 17.2 37.4 46.4

2.7 1.4

of the solutions was obtained from an Elga Labwater Purelab Ultra system. Sulfuric acid was Merck suprapur, and methanol, Merck pa. Catalyst Synthesis. Polyaniline employed for catalysts preparation was synthesized using the standard procedure reported in the literature.39 The catalyst preparation was carried out adapting the procedure described in ref 24 for carbon blackPt catalysts. Briefly, polyaniline was dispersed in 50 mL of ethylene glycol/water solutions (volume ratio 3/1) containing the platinum precursor. The final concentration of Pt was always 0.002 M, and the ratio Pt/PANI was varied from 20 to 50 wt %. The mixture was sonicated for 1 min to pulverize the polymer grains. Then, the PVP was added to obtain a final PVP/Pt ratio equal to 0.2. The time of reaction was varied depending on the refluxing temperature. When the temperature was 100 °C, the time was 2 h, while when the reaction was performed at 75 °C, the time was 3 h. When the reaction was completed, the mixture was left to cool at room temperature, diluted by water, and then centrifuged. The precipitate was washed with water and centrifuged once again. The solid was washed with acetone to remove the rest of the PVP and the solvent. The catalysts were dried in dynamic vacuum at 30 °C during 12 h. The final products were stored as powder or suspended in water. To study the effect of PVP as reducing agent, the synthesis was carried out in 50 mL of water without ethylene glycol. The ratio Pt/PANI was 50 wt %, and the amount of PVP added was 8 mg. The rest of the experimental conditions was maintained as described above. Catalyst Characterization and Properties. The transmission electron microscopy (TEM) images were collected using a JEOL (JEM-2010) microscope, working at an operation voltage of 200 kV. The chemical analyses were made with an Oxford (INCA Energy TEM100) X-ray microanalizer (EDS) coupled to the microscope. The X-ray diffractograms (XRD) of the powder catalysts were obtained using a Bruker CCD-Apex with a X-ray generator (Cu KR) operated at 40 kV and 40 mA. The thermogravimmetric analysis (TGA) was done in a Mettler Toledo 851E/1600/LG equipment. Samples were dried under dynamic vacuum before the experiments and then placed in a crucible of alumina of 70 µL. The loss of weight was monitored from room temperature to 1000 °C using a heating rate of 10 °C/min. The experiments were done in a He/O2 (4/1) mixture. The flow rate employed was 100 mL/min. Fourier transform infrared (FTIR) spectra of the catalysts in the transmission mode were obtained in KBr pellets using a Nicolet Magna equipment with a 4 cm-1 resolution. For the in situ FTIR measurements the same equipment using a liquidnitrogen-cooled MCT detector was employed. The sample compartment was purged throughout the experiment using a 7550 Balston clean air package. The working electrode was a gold disc of 8 mm in diameter. The disc was mounted on a glass tube, and its surface was polished using alumina powder of decreasing particle size (1, 0.3, and 0.05 µm) before use. A platinum electrode was used as the counter electrode. The thinlayer spectroelectrochemical cell was made of glass and was

2.4 2.1

provided with a prismatic CaF2 window beveled at 60°. Spectra were collected at 8 cm-1 resolution and are presented as ∆R/R. The electrochemical measurements were carried out using a conventional cell of three electrodes. The counter and reference electrodes were a platinum foil and a hydrogen reversible electrode immersed in the same working solution, respectively. The working electrode was a glassy carbon disk polished successively with diamond paste of 3 and 0.5 µm particles. Catalyst-modified electrodes were prepared by casting 10 µL of a suspension containing 0.7 mg of catalyst dispersed in 1 mL of water. The catalysts were tested in 0.5 M H2SO4 and 0.5 M H2SO4 + 0.1 M methanol solutions at 50 mV s-1. Results and Discussion To determine the adequate conditions to obtain good catalytic activity, the effects of some synthesis conditions were analyzed, that is, the temperature of synthesis, the platinum loading, and the presence of ethylene glycol as reducing agent. Catalysts Characterization. The catalysts were synthesized in 3/1 ethylene glycol/water at two different temperatures, 75 °C (PANI-Pt75) and 100 °C (PANI-Pt100). Table 1 shows the synthesis parameters employed for the different catalysts prepared in this work and the amount of platinum in weight percentage. Figure 1 shows the TEM images of two catalysts with 20 and 38 wt % of Pt. For PANI-Pt100 (38 wt %) catalyst (Figure 1a and Figure 1b (higher magnification)) the particles are well distributed, covering uniformly the entire polymer surface; however, in the PANI-Pt75 catalyst (Figure 1c) a worse particle distribution is observed. EDS analysis were performed to ensure that the dark points correspond to Pt. The EDS results confirm that the dark points are Pt nanoparticles (data not shown). The platinum loading in the catalyst, obtained by TGA measurements, is in both cases close to the desired during the synthesis (Table 1). Figure 2 shows the particle size distribution obtained analyzing 300 nanoparticles for the two catalysts in Figure 1. Table 1 shows the average particle size determined analyzing the TEM images for different catalysts synthesized in this work. The average particle size is 2.4 nm for PANI-Pt100 (38 wt %) and 1.4 nm for PANI-Pt75 (20 wt %). For PANI-Pt100 (Figure 2a) a particle size distribution from 1.8 to 3 nm is observed, which is narrower than that for carbon-black-supported platinum nanoparticles produced at similar conditions.24 However, the average particle size obtained from the TEM analysis (2.4 nm) resembles that reported for carbon-black-supported platinum nanoparticles. Therefore, the nature of the solid support does not affect the average particle size, which is only controlled by the PVP/solid support ratio. For PANI-Pt75 the particle size distribution is quite different suggesting a random distribution of sizes. The temperature is a parameter that affects the particle size; however, the kinetics of the process is complex because not only nucleation and growth occur but also the presence of the protecting polymer can have an influence in the platinum particle size. Then, the ratio protecting-polymer to platinum precursor to obtain similar particles size could be different when the temperature changes as it is known for other systems.40,41

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Figure 1. TEM images of PANI-Pt100 (38 wt %) (a), PANI-Pt100 (38 wt %) higher magnification (b), and PANI-Pt75 (20 wt %) (c) synthesized in 3/1 ethylene glycol/water.

Figure 3. XRD pattern of PANI base powder and PANI-Pt100 catalyst. Figure 2. Histogram of Pt nanoparticle size for PANI-Pt100 (38 wt %) (a) and PANI-Pt75 (20 wt %) (b) catalysts measured from TEM images.

To determine the crystalline structure of the catalysts, X-ray diffraction was used. The XRD pattern of the polymer (PANI) and a catalyst (PANI-Pt100) are shown in Figure 3. Peaks consistent with the face-centered cubic (fcc) expected for Pt are clearly observed for the PANI-Pt100 catalyst.23,24 XRD

patterns were used to estimate the average particle size using the Scherrer equation:

D)

Kλ β2θ cos θ

(1)

where λ is the wavelength of radiation (0.154 nm), K is the Scherrer constant (whose accepted value for spherical particles

Pt Nanoparticles Supported on a Conducting Polymer

Figure 4. TGA curves for PANI-Pt100 catalysts with different platinum loadings.

Figure 5. FTIR transmission spectra of PANI base powder and PANIPt100 and PANI-Pt75 catalysts.

is 0.9), β2θ is the half-peak width for Pt(111), and θ is the Bragg angle. An average particle size of 2.5 nm has been obtained for PANI-Pt100 (20 wt %), in agreement with the values observed in TEM images (Table 1). Platinum loading and the stability of the catalysts were also analyzed by TG experiments in an oxygen-containing atmosphere. The TG curves for the PANI-Pt100 catalyst with different Pt/PANI ratios are shown in Figure 4. At low Pt/PANI ratio (20 wt %), the weight loss is small until the temperature reaches 400 °C. Then a fast weight loss occurs due to the polymer burning. However, at higher Pt/PANI ratio, the curves show an additional weight loss step at around 170-180 °C which can be related to the removal of small molecules coming from the platinum salt precursor, such as hydrogen chloride. Moreover, the main weight loss related to PANI occurs at lower temperatures, indicating a decrease in the stability of the catalyst which can be due to a catalytic effect of the platinum in the combustion of the support.42 Using the residual weight values from the TG experiments, the Pt loading in the catalysts was calculated. The obtained values are very close to the Pt/PANI ratio in the reactor (Table 1). Taking into account that the polymer could suffer some degradation during the synthesis, the polymer structure was studied by FTIR spectroscopy. The vibration spectra for PANI, PANI-Pt75, and PANI-Pt100 catalysts are shown in Figure 5. The spectra for the catalysts resemble those obtained for PANI, displaying one additional band at 1660 cm-1 in the case of PANI-Pt100, which can be attributed to CdO stretching of carbonyl groups coming from either the remaining PVP or

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12457 quinones produced by some degradation of the support.43 However, the FTIR spectrum of PANI-Pt75 is similar to that for PANI powder without the presence of the band at 1660 cm-1, suggesting that this carbonyl band in PANI-Pt100 corresponds to some degradation of the polymer. The band at 1504 cm-1 corresponds to the stretching of Cd C in bencenoid (B) units (related to the reduced state of the polymer), while the band at 1595 cm-1 corresponds to CdN stretching of quinonimine (Q) units produced when PANI is oxidized.44 Thus, the ratio between the areas of these bands (B/ Q) gives information about changes in the oxidation state of the polymer. For PANI-Pt100 the B/Q ratio is 10% higher than the B/Q ratio in the starting PANI suggesting that the polymer is reduced during the Pt nanoparticles synthesis. Recently it has been reported that PVP can act not only as stabilizer but also as a mild reducing agent in the aqueous synthesis of metal nanoparticles.45 Therefore, the synthesis of Pt nanoparticles supported on PANI using water as solvent, without ethylene glycol, has been performed at 100 °C. We used a Pt/PANI ratio of 20 wt % and a large excess of PVP to ensure the total reduction of the platinum salt precursor. The FTIR spectrum of PANI-Pt100 obtained in absence of ethylene glycol shows the same bands that the catalysts in Figure 5 with a B/Q ratio similar to that of the starting polymer, proving that the ethylene glycol can also reduce the solid support. The TEM image of the as-obtained catalyst (figure not shown) shows that the particles are not well dispersed and clusters of around 50 nm can be observed. The clusters are formed by densely packed particles like those in Figure 1a. The packing in clusters could be due to the high amount of PVP that would produce a strong interaction between the polymeric chains capping the nanoparticles. Using a Pt/PANI ratio of 20 wt % in the reactor, the platinum loading in the catalyst obtained by TG measurements was 17.2 wt % for the synthesis carried out in water and 16.8 wt % for that in ethylene glycol/water (Table 1). A more detailed study is necessary to obtain the optimal PVP/Pt ratio to produce well-dispersed nanoparticles using only water as solvent. Methanol Electrooxidation. In this section, the electrochemical characterization of the catalysts obtained in 3/1 ethylene glycol/water at 100 °C with a platinum loading of 20, 38, and 50 wt % is presented. Cyclic voltammetry is a common technique to study the electrocatalytic properties of Pt nanoparticles. Differently from the carbon-supported metal nanoparticles, the PANI support displays faradic processes because of polymer electroactivity in acidic media. Therefore, the electrochemical response of the catalytic reaction test could be overlapped with the polymer process. The catalyst electrodes were cycled in sulfuric acid between 0.05 and 1.0 V at 50 mV s-1 for different times to clean the platinum particles surface, although the PANI polymer can be partially degraded. Figure 6 shows the cyclic voltammograms of PANI-Pt100 (50 wt %) electrodes treated during 15 min (a) and 120 min (b) in 0.5 M sulfuric acid (black line) and 0.5 M sulfuric acid + 0.1 M methanol (gray line). In sulfuric acid the PANI-Pt100 catalyst displays the characteristic peaks between 0.06 and 0.4 VRHE corresponding to the characteristic adsorption-desorption processes on platinum (Figure 6a). Also, the response in presence of methanol is similar to that observed on bare platinum electrodes in the same conditions. After 15 min of cycling the polymer remains electroactive due to the low degree of degradation. Further cycling of the catalyst up to 120 min increases the response toward methanol oxidation and diminishes the PANI signal until it disappears (Figure 6b).

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Figure 6. CV of PANI-Pt100-modified glassy carbon electrodes in 0.5 M H2SO4 (black) and 0.5 M H2SO4 + 0.1 M CH3OH (gray) after cycling in 0.5 M H2SO4 during 15 (a) and 120 min (b). Scan rate ) 50 mV s-1.

Figure 7. TEM image of PANI-Pt100 catalyst after cycling 120 min in 0.5 M H2SO4 solutions at 50 mV s-1 between 0.05 and 1 V.

However, the electrochemical treatment could lead to some aggregation of Pt nanoparticles diminishing the active surface. The TEM image shows nanoparticles aggregation due to part of the polymer being decomposed (Figure 7). However, the clusters or aggregates retained nanoscale dimension suggesting that the remaining polymer plays a stabilizing role although its structure has changed. Here it is important to remark that the electrochemical behavior was not reproduced when lower platinum loading amounts (20 and 38 wt %) were employed, suggesting that the content of platinum on the catalysts represents a critical point. The broad oxidation peaks at 0.75 V in Figure 6a have been deconvoluted using the Origin 7.0 program to subtract the PANI contribution. The results show that the curves are almost identical for both cycling times, and thus, the charge involved in the methanol oxidation is almost the same. The oxidation current for methanol oxidation is 16.1 A g-1 for PANI-Pt100 treated for 15 min and 16.9 A g-1 for the same electrode treated for 120 min. Therefore, we suggest that 15 min of pretreatment of the catalyst is enough to attain good catalytic activity without affecting the chemical structure and properties of the support. Figure 8 shows the chronoamperometric curves obtained at different potentials for PANI-Pt100 50 wt % catalysts in the oxidation of methanol. All the curves show a typical chronoamperogram with a sudden increase in the current corresponding

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Figure 8. Chronoamperometric experiments obtained with PANIPt100 50 wt % at different potentials in 0.5 M H2SO4 + 0.1 M CH3OH solution.

Figure 9. CV of bare Pt electrode (a) and PANI-Pt100-modified glassy carbon electrode (b) in 0.5 M H2SO4 + 0.1 M CH3OH recorded after cycling in the same solution during 1 min (black) and 30 min (gray) between 0.05 and 1 V.

to the double layer formation and oxidation of methanol, and then a decay with time is observed that corresponds to a diffusion control of the reaction. The residual current at 20 s is higher at higher potentials and reaches a value near 2 mA cm-2 or 9 A g-1 at 0.8 V. The last point discussed in this study is the stability of the catalyst as electrode for methanol oxidation. It is know that the signal of methanol oxidation on polycrystalline platinum electrode decreases as the number of cycles increases because methanol is dissociatively and irreversibly adsorbed, forming linear and multibonded carbon monoxide.46 The dependence of the electrochemical response with the number of cycles for both PANI-Pt100-modified glassy carbon and bare Pt electrodes is shown in Figure 9. It can be seen that, under the same conditions, the PANI-Pt100 electrode retains almost completely the activity while the response of the bare platinum electrode drops dramatically. While the bare Pt electrode loses 83% of its activity after 30 min of cycling, PANI-Pt100 electrode only loses 17%. In situ FTIR measurements were performed to check the CO adsorption on the nanoparticles. A PANI-Pt100-modified gold electrode was immersed in a 0.1 M methanol solution for 5

Pt Nanoparticles Supported on a Conducting Polymer

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12459 produces minimal degradation in the polymer support and the efficiency to oxidize methanol is almost the same as that for other electrodes in which the solid support has been degraded. Furthermore, the loss of catalytic activity of PANI-Pt100 due to CO poisoning is 5-fold lower than those in bare Pt. Therefore, the new catalysts could be used as electrodes for methanol oxidation. To apply the catalyst to other reactions of interest, we are extending the work to the modification of the solid support to enhance its selectivity to other molecules. Acknowledgment. Financial support by the Ministerio de Educacio´n y Ciencia (Granrt MAT2004-1479) and the FEDER funds is acknowledged. H.J.S. thanks the Ministerio de Educacio´n y Ciencia of Spain for a Juan de la Cierva Contract. C.S thanks the MEC for a FPU fellowship.

Figure 10. (a) In situ FTIR spectra of PANI-Pt100-modified Au electrode after 5 min immersed in 0.1 M CH3OH in 0.5 M H2SO4 solution (100 interferograms) at different sample potentials. (See text in figure.) Reference potential: 0.9 V. (b) Dependence of the wavelength on the potential.

min. Then, the electrode was removed and introduced in the spectroelectrochemical cell in 0.5 M H2SO4 at controlled potential (0.2 V). Then the potential was increased until 0.9 V in steps of 0.1 V and the FTIR spectra were collected at each potential. The FTIR spectra of CO adsorbed on the PANIPt100/Au electrode in 0.5 M H2SO4 solution are shown in Figure 10a. The reference spectrum was the one obtained at 0.9 V, to ensure that all adsorbed CO has been removed as carbon dioxide. The spectra in Figure 10a contain an absorption band at around 2050 cm-1, attributed to linearly adsorbed CO, which is similar to the results previously reported for single-crystal and polycrystalline platinum electrodes.47,48 Moreover, the band shifts to higher frequencies when increasing the potential, as generally occurs for CO adsorbed in Pt. Figure 10b shows the dependence of the frequency of the linearly adsorbed CO band on the applied potential for methanol adsorption in 0.5 M sulfuric acid. The ν-E slope calculated was 59 cm-1 V-1, which is almost double those reported for the same CO-adsorbed band on both single crystals and polycrystalline Pt electrodes.47 This difference can be explained considering the effect of coverage since, it has been established that the ν-E slope increases as the coverage decreases.49 Conclusions Pt nanoparticles supported on polyaniline with narrow size distribution have been chemically synthesized using PVP as size mediator. The polymer does not suffer drastic structural changes during the synthesis due to the soft conditions employed. Several parameters have been analyzed to find the optimal conditions of synthesis. The temperature introduces a new parameter to control the particle size and the polymer stability which needs to be deeply studied. The catalysts were characterized by TEM, XRD, FTIR spectroscopy, and TG techniques. The TEM results suggested that the particles produced at 75 °C (PANI-Pt75) have smaller size than those obtained at 100 °C (PANI-Pt100). Catalyst obtained in 3/1 ethylene glycol/water at 100 °C with a platinum loading of 50 wt % exhibits the best results. Pt nanoparticles have a narrow particle size distribution and are homogeneously dispersed within the polymer, and the catalyst has a good electrochemical response in the presence of methanol after a pretreatment in sulfuric acid. Furthermore, this treatment

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