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Electrocatalytic oxidation of ethylene glycol on Pt and Pt–Ru nanoparticles modified multi-walled carbon nanotubes
Journal of Colloid and Interface Science 322 (2008) 537–544 www.elsevier.com/locate/jcis
Electrocatalytic oxidation of ethylene glycol on Pt and Pt–Ru nanoparticles modified multi-walled carbon nanotubes Vaithilingam Selvaraj, Mari Vinoba, Muthukaruppan Alagar ∗ Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai 600025, India Received 22 October 2007; accepted 25 February 2008 Available online 13 March 2008
Abstract The synthesis and characterization of catalysts based on nanomaterials, supported on multi-walled carbon nanotubes (CNT) for ethylene glycol (EG) oxidation is investigated. Platinum (Pt) and platinum–ruthenium (Pt–Ru) nanoparticles are deposited on surface-oxidized multi-walled carbon nanotubes [Pt/CNT; Pt–Ru/CNT] by the aqueous solution reduction of the corresponding metal salts with glycerol. The electrocatalytic properties of the modified electrodes for oxidation of ethylene glycol in acidic solution have been studied by cyclic voltammetry (CV), and excellent activity is observed. This may be attributed to the small particle size of the metal nanoparticles, the efficacy of carbon nanotubes acting as good catalyst support and uniform dispersion of nanoparticles on CNT surfaces. The nature of the resulting nanoparticles decorated multiwalled carbon nanotubes are characterized by scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis. The cyclic voltammetry response indicates that Pt–Ru/CNT catalyst displays a higher performance than Pt/CNT, which may be due to the efficiency of the nature of Ru species in Pt–Ru systems. The fabricated Pt and Pt–Ru nanoparticles decorated CNT electrodes shows better catalytic performance towards ethylene glycol oxidation than the corresponding nanoparticles decorated carbon electrodes, demonstrating that it is more promising for use in fuel cells. © 2008 Published by Elsevier Inc. Keywords: Multi-walled carbon nanotubes (CNT); Ethylene glycol (EG) oxidation, Pt nanoparticles; Pt–Ru nanoparticles; Fuel cells
1. Introduction Fuel cells employing alcohols (Direct Alcohol Fuel Cell— DAFC) are extremely attractive as power sources for mobile, stationary, portable applications such as cellular phones and laptop computers, etc. DAFCs can be made quite compact without a bulky external reformer, which is essential for polymer electrolyte membrane fuel cells (PEMFCs). Liquid fuels, such as small molecular weight alcohols, which have higher volumetric energy density and better energy efficiency, are easier to store and transport than gaseous fuels. The electrooxidation of organic compounds at noble metal electrodes using metal catalysts has been studied extensively for possible applications in electrochemical power sources [1] and electrochemical wastewater treatment [2]. These catalysts include * Corresponding author. Fax: +91 44 22352870.
E-mail addresses: [email protected] (V. Selvaraj), [email protected] (M. Alagar). 0021-9797/$ – see front matter © 2008 Published by Elsevier Inc. doi:10.1016/j.jcis.2008.02.069
platinum and platinum-based alloy catalysts [3,4]. In fact, the choice of a suitable supporting material is an important factor that may affect the performance of supported electrocatalysts owing to interactions and surface reactivity [5,6]. Carbon nanotubes (CNTs) play an important role as supporting materials in electrocatalysis [6,7] and nanoelectronic circuitry [8] due to their excellent mechanical, electronic and surface properties [9]. Several papers have been reported on the application of carbon nanotubes in fuel cells as catalysts supported and electrode materials [10–14]. The carbon nanotube material is considered to have several advantages over conventional support materials, which includes (i) having more defined crystalline structure with higher conductivity, (ii) containing little impurities, such as metals and sulfides, and thus eliminating potential poisoning effects to electrocatalysts, and (iii) possessing threedimensional structure thus favoring the flow of reactant and providing a large reaction zone when fabricated into electrodes. The carbon nanotubes are also chemically stable and resistant to thermal decomposition. Due to these distinctive characteris-
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tics, the carbon nanotube material is very suitable for use as a novel electrocatalyst support. There is a plethora of investigations on the potential use of nanotube/nanoparticles [15–17]. CNTs have been coated with various metal nanoparticles such as Au, Ti, Ni, Pd, Al, Fe, Pb and Pt by electron-beam evaporation or electroless metal deposition [18–23]. Highly dispersed nanoscale Pt particles have been an intensive research subject as the electrocatalyst for methanol oxidation [24,25]. The electrocatalytic activity of Pt nanoparticles for this reaction is dependent on various factors, which involves the size and dispersion of the particles, preparation methods, supporting materials and their surface conditions [26–35]. CNTs are good supporting materials, which can support a high dispersion of Pt nanoparticles due to their large surface area and particular morphology. In addition, the metal particles supported on the CNTs seemed to be less susceptible to CO poisoning than the traditional catalyst systems [36]. As a result, Pt nanoparticles decorated carbon nanotubes is of significant interest among the researches in the area of electrocatalysis. Among the different substances studied as a model fuel such as formic acid and methanol, ethylene glycol (simplest aliphatic diol) is also fascinating to investigate due to its high solubility in aqueous solutions and relatively high reactivity [37]. The electrochemical oxidation of ethylene glycol (EG) on platinum metal electrodes and catalysts has received significant attention during the last decades, both as a model compound for studies of the adsorption and electrooxidation behavior of CO-containing organic species and due to its potential application as attractive anodic fuel in direct oxidation fuel cells (acidic solution [38–45], alkaline solution [40,42]). Recently, Peled et al. [46,47] reported that methanol/oxygen and ethylene glycol/oxygen fuel cells equipped with a new nanoporous proton-conducting membrane and using PtRu/C (atomic ratio of 1:1) as anode catalyst provided a maximum power density of 400 and 300 mW cm−2 , respectively, which puts ethylene glycol in direct competition with methanol as a promising candidate for practical electric vehicles and stationary applications [48]. Kelaidopoulou et al. [49] observed that the addition of ruthenium and tin onto platinum dispersed in polyaniline increases the electro-oxidation of ethylene glycol in acid medium.
There are numbers of methods reported in the literature on methanol oxidation using carbon nanotubes [50–53]. To the best of our knowledge, this is the first reports on the use of carbon nanotubes as supporting material towards ethylene glycol oxidation and nanoparticles were decorated on CNTs using glycerol as reducing agent (2–3 nm). This is the simplest method used to prepare the nanoparticles in the range of 2– 3 nm. In the present work, a new route has been adopted for the preparation of Pt and Pt–Ru nanoparticles deposited on the surface-oxidized MWCNTs (Pt/CNT & Pt–Ru/CNT) using glycerol as reducing agent by conventional heating method. The prepared Pt/CNT and Pt–Ru/CNT electrodes were used as a protocol towards the electrooxidation of ethylene glycol. This is the continuation of our previous work on the role of nanoparticles in fuel cell applications [54,55]. 2. Materials and methods 2.1. Materials Reagent grade glycerol and ethylene glycol (99.8%) are obtained from SRL and electrocatalyst precursor salts, i.e., H2 PtCl6 ·6H2 O (99.9%) and RuCl3 ·6H2 O (99%) are purchased from Alfa-Aesar. Multiwalled carbon nanotubes (99%) obtained from Sigma-Aldrich were rinsed with double distilled water and dried. They are used as received without any further purification. All the chemicals are reagent grade and doubledistilled water are used throughout the experiment. All experiments are carried out at 25 ◦ C. 2.2. Pretreatment of MWCNTs The carbon nanotubes were first oxidized in a hot conc. HNO3 solution, for 48 h under refluxing conditions to remove impurities and generate surface functional groups. Purification of CNT surfaces prevents self-poisoning by foreign impurities while functional group generation enhances electrocatalyst formation (Scheme 1). Examination on surfaces of acid oxidized carbon nanotubes was carried out using a Fourier transform infrared (FT-IR) spectrometer to ensure formation of desired surface functional groups. After the oxidation treatment, a surfaceoxidized MWCNTs sample was obtained.
Scheme 1. Schematic presentation of functional group formation and metal decoration on multi walled carbon nanotubes.
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2.3. Synthesis of Pt–Ru/CNT and Pt/CNT nanocomposites The modified procedure employed for the reduction deposition of Pt and Pt–Ru electrocatalysts on pretreated CNT is as follows. The CNT decorated Pt–Ru was obtained as follows, 140 mg of acid-oxidized CNT was added to 90 ml glycerol aqueous solution. Then, 54 mg of H2 PtCl6 ·6H2 O and 21 mg of RuCl3 as Pt–Ru electrocatalyst precursors were dissolved in 10 ml of aqueous glycerol (3:1 ratio of glycerol and H2 O). After that, the catalyst precursor solution was slowly added to the prepared CNT/glycerol and sonicated for 20 min in order to get uniform dispersed solution. The solution was then heated to reflux for 12 h at 120–130 ◦ C. After reduction reaction, the reacted mixture was filtered and washed with sufficient amounts of Millipore water and dried. This was used as the modified electrode for electrocatalysis reactions. Similarly, CNT decorated with Pt was obtained by refluxing acid treated nanotubes with H2 PtCl6 ·6H2 O in aqueous glycerol (Scheme 1).
Fig. 1. Scanning electron microscopic images of the synthesized catalyst material (a) Pt/CNT and (b) Pt–Ru/CNT.
2.4. Preparation of Pt/CNT and Pt–Ru/CNT catalyst electrode 5 mg of Pt/CNT catalyst, 50 µL of Nafion solution (5 wt%, Aldrich) and 1.0 ml of solvent were mixed using an ultrasonic bath. A measured volume of this mixture was transferred via a syringe onto a graphite electrode. The solvents (acetone or water or alcohol) were evaporated at room temperature for overnight. Similar experimental conditions were adopted for the preparation of Pt–Ru/CNT nanocatalysts supported on graphite electrodes. 2.5. Equipment and methods A CHI 660B instrument with a three-compartment cell is employed for the electrochemical measurement. The working electrode is a thin layer of Nafion impregnated nanoparticles modified CNT catalysts cast on a graphite electrode. A Pt foil and a saturated calomel electrode (SCE) are used as the counter and reference electrode, respectively. All potentials in this study are reported with respect to SCE under nitrogen atmosphere. For measurement of ethylene glycol oxidation reaction activities, cyclic voltammetry is performed in a solution containing 0.5 M ethylene glycol and 0.5 M H2 SO4 at room temperature. The surface analysis (SEM) is carried out using an LEO-stereoscan 440 microscope. The size of the particles is confirmed using HR-TEM (JEOL) with an accelerating voltage of 120 kV. 3. Results and discussion
Fig. 2. EDAX images of Pt/CNT and Pt–Ru/CNT.
has been well functionalized with appropriate functional groups (–COOH), which will favor the adherence of metal nanoparticles on its surface. It was reported [57] that the dominant functional group is carboxylic group generated by strong acid treatment. 3.2. Surface morphology of nanoparticles modified electrodes The micrographs of the Pt/CNT and Pt–Ru/CNT electrodes have been investigated by SEM (Fig. 1) and their corresponding EDAX images were shown in Fig. 2. The presence of Pt and Ru metal nanoparticle in the modified electrodes were confirmed by EDX Analysis (Fig. 2). From Fig. 1, it was observed that nanoparticles with less than 2 nm are deposited on the surface of carbon nanotubes. Pt/CNT and Pt–Ru/CNT composites have porous and three-dimensional structure. The three-dimensional structure, smaller particle size and high dispersion of nanoparticles may result in large valuable Pt surface area and good electrocatalytic properties towards ethylene glycol oxidation. Fig. 3 shows the TEM image of the purified MWCNT treated with Pt and Pt–Ru nanoparticles. It can be seen that well-dispersed, spherical particles were anchored onto the external walls of MWCNT with size ranging from 2–5 nm.
3.1. Spectral evaluation of CNTs 3.3. Electrooxidation of ethylene glycol A recent report by Han et al. [56] indicated that a stronger nitric acid solution generates more functional groups on CNT surfaces than a weaker one and results in better electrocatalyst formation. Therefore, concentrated nitric acid was employed in the present study for the purification of carbon nanotubes. The acid treated multi-walled carbon nanotubes (Scheme 1)
Fig. 4 shows the cyclic voltammograms of Pt/C, Pt/CNT and Pt–Ru/CNT in 0.5 M H2 SO4 solution in the absence of ethylene glycol at 50 mV/s in a potential window of −0.2 to 1.0 V vs SCE. In view of examining the role of carbon nanotubes towards EG oxidation, cyclic voltammograms were carried out
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Fig. 5. Electrooxidation of ethylene glycol on Pt nanoparticles modified carbon (Pt/C) in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
Fig. 3. HR-TEM images of catalyst materials (a) Pt/CNT, (b) Pt–Ru/CNT, (c) lattice structure of Pt/CNT, (d) lattice structure of Pt–Ru/CNT.
Fig. 6. Electrooxidation of ethylene glycol on Pt nanoparticles modified multi walled carbon nanotubes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
Fig. 4. Cyclic voltammograms of Pt/C, Pt/CNT and Pt–Ru/CNT electrodes in 0.5 M H2 SO4 (scan rate = 50 mV s−1 ).
for Pt nanoparticles decorated on carbon and MWCNTs (Figs. 5 and 6). It can be seen from the figure that the electrocatalytic activity of Pt supported on CNTs was significantly improved and higher than that of carbon supported Pt (Fig. 6), thereby proving that CNTs acted as an efficient supporting material for effective dispersion of catalyst, which appears a promising field of research. The enhancement in catalytic activity is due to the following reasons. From the Fig. 4, it can be observed that the CV’s pertaining to Pt and Pt–Ru nanoparticles modified CNT electrodes overlap in the Hupd region (−0.2 < E < 0.2 V), which clearly indicates that the active surface area for both the electrodes are nearly the same. The charges and currents associated with the hydrogen adsorption/desorption regions of Pt–Ru
nanoparticles modified CNT composite electrodes were almost the same as for Pt nanoparticles modified CNT composites electrodes. Hence it could be understood that the addition of Ru to Pt did not change the active surface area. It is well documented that two or more electrodes with almost comparable currents for the formation and oxidation of adsorbed hydrogen atoms will have exactly same active surface area of metals [58,59]. However, in the case of Pt/C and Pt/CNT does not overlap in the Hupd region. Hence, the active surface area for Pt/C and Pt/CNT are different. Fig. 4 shows the increase in surface area for Pt/CNT compared to Pt/C. Hence, the difference in the active surface area is also responsible for enhanced catalytic activity. Another reason might be that the metal support interactions can significantly alter the activity of supported catalysts [60]. Since, CNTs have more organized graphitic structures than carbon black. They therefore would result in different interactions with the supported metal, which happen to be positive to the catalyst activity in the electrooxidation of EG. Hills et al. [61] reported a study on the carbon support effects on bimetallic Pt–Ru nanoparticles and found that the carbon black resulted in structural disorders in the alloy nanoparticles in which the
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nearest coordination numbers were smaller. In contrast, the nanoparticles deposited on fullerene soot have enhanced catalytic activity due to ordered crystalline structure. Similarly, the CNTs employed in the present study exhibited an enhanced catalytic activity towards electrooxidation of EG. The cyclic voltammogram of the graphite electrode modified by Pt/CNT in 0.5 M EG and 0.5 M H2 SO4 at 50 mV s−1 is shown in Fig. 6. As viewed from the figure, an oxidation peak around 0.6 V is observed in the anodic scan region, which corresponds to the oxidation of EG. Reductive peak in the reverse scan is obtained due to the reduction of Pt-oxide. The shape of the CV and the oxidation potentials are comparable with the previous reports. At lower potentials, EG adsorbs on the electrode surface and when the potential reaches 0.5 V, the current started increasing which is attributed to the electrooxidation of ethylene glycol, with a maximum current at 7.45 mA. As per the earlier reports, oxidation of ethylene glycol gives formic acid as one of the major product [62,63]. A probable mechanism as suggested by Bagotsky and Vijh [64], are given as follows (Steps 1–6). (CH2 OH)2 → (CH2 OH)2 → 1, (CH2 OH)2 → (:COH)2 → 2, (:COH)2 + 2H2 O → (HCOOH)ads + 2H+ + 2e− → 3, Pt(HCOOH)ads → Pt(CO)ads + H2 O → 4, Pt + H2 O → Pt(OH)ads → 5, Pt(OH)ads + Pt(CO)ads → Pt + CO2 + H+ → 6. Thus ethylene glycol oxidation takes place on Pt surfaces giving formic acid which in turn results in the formation of poisonous intermediate (CO)ads (Step 4) on the electrode which gets deposited on the active sites of Pt [42]. The only possibility to remove (CO)ads is to oxidize it to CO2 by reacting with OHads formed by the dissociation of water molecules on Pt surfaces (Steps 5 and 6). But the water activation process on Pt is very difficult and takes place at a higher potential and hence CO would be oxidized only at higher potentials. Due to the higher potential of CO oxidation on Pt, the electrode surfaces will be blocked by more amounts of CO species and thus the adsorption of other EG molecules on the electrode surface is hindered. This would facilitate the oxidation of only lesser number of EG moieties and the retardant CO remains on the electrode surface for a longer time occupying active catalyst sites and reducing the overall activity towards ethylene glycol oxidation. Hence, the current at 0.5 V due to EG oxidation depends mainly on the amount of CO oxidized from the active sites of Pt. In view of this poisoning effect, the present work is devoted to study the electrocatalytic activity of Pt–Ru nanoparticles modified carbon nanotubes for a comparative factor (Fig. 7). Before probing the catalytic activity of the bimetallic system in CNT, we have tested the atomic ratio of the metal precursors for the effective ratio required for the present investigation. Three catalysts of different Pt to Ru atomic ratios, namely, Pt50 Ru50 , Pt70 Ru30 , and Pt80 Ru20 , were prepared for investigation of the compositional effects. It was shown that highly dispersed bimetallic Pt–Ru alloy nanoparticles with no agglomeration can be synthesized on the carbon nanotubes. From the present investigation, we concluded that Pt50 Ru50 was found
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Fig. 7. Electrooxidation of ethylene glycol on Pt–Ru nanoparticles modified multi walled carbon nanotubes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
to be more stable, good long term stability and catalytic activity compare to other composition. However, the Pt70 Ru30 and Pt80 Ru20, catalysts showed poorer stability that can be justified by the bifunctional mechanism of bimetallic alloys. This was probably due to the composition effects of unpaired Pt and Ru atoms. Hence, we have chosen the Pt50 Ru50 composition for the following systems. Fig. 7 shows the electrocatalytic activity of Pt50 Ru50 nanoparticles modified carbon nanotubes towards ethylene glycol oxidation. It was found that the presence of Ru has a greater impact on EG oxidation. The rate of oxidation of EG was found enhanced by a large factor. In the case of Pt–Ru nanoparticle systems, the electrocatalytic activity towards EG oxidation is increased to 12.62 mA, which was 7.45 mA in the case of Pt modified electrodes. Thus when Ru added to Pt forming Pt–Ru systems, Ru helps in enhancing the oxidation rate to a considerable extent. To account for this effect, the mechanism of ruthenium enhancement on EG oxidation is explained as below. As well known, steady state EG oxidation involves generation of chemisorbed carbon monoxide on the Pt surface of the catalyst in the reaction. To sustain the steady-state current, or the continuous catalytic oxidation process, CO needs to be oxidatively removed from the surface in order to renew the surface sites needed to sustain the reaction. The addition of ruthenium to the Pt catalyst enhances CO removal from the surface since ruthenium promotes CO oxidation to CO2 via the mechanism (Steps 7 and 8), where OH symbolizes the oxygen-containing species produced by water activation process. The main reason for improved activity is due to the affinity of Ru towards the water molecules (Steps 7–8). Ru + H2 O → Ru(OH)ads + H+ + e− → 7, Ru(OH)ads + Pt(CO)ads → Pt + Ru + CO2 + H+ + e− → 8. On Pt–Ru, the main difference is the ability of Ru to activate water molecules at a lower potential than that on Pt (typically 0.35 V compared to 0.6 V with Pt) and hence CO was oxidized at lower potential. Thus the displacement of the onset and peak potentials in a negative direction in the case of the catalysts containing Ru compared to Pt/CNT is mainly related to
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Fig. 8. Effect of scan rate on ethylene glycol oxidation using (a) Pt/CNT and (b) Pt–Ru/CNT electrodes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
seen that the oxidation current increases with increasing the scan rate. This shows that EG oxidation at nanoparticles modified CNT electrodes is mainly controlled by diffusion. Further, Fig. 9 shows the typical chronoamperometric response curves at 0.5 M EG and 0.5 M H2 SO4 with Pt/CNT and Pt–Ru/CNT as the working electrodes (versus SCE). The current observed from chronoamperometric was found to be in good agreement with the current observed from cyclic voltammetry. The effects of different Pt/Ru ratio were also carried out (Fig. 10a), which gives information about the effective atomic ratio towards EG electrooxidation. 3.4. Long-term stability of the Pt/CNT and Pt–Ru/CNT electrodes Fig. 9. Chronoamperometric curves of ethylene glycol in a solution of 0.5 M EG and 0.5 M H2 SO4 at 25 ◦ C on the Pt/CNT (a) and Pt–Ru/CNT (b) catalysts.
the bi-functional mechanism as shown below, where the beneficial effect is related to the formation of Ru–OH species at low potentials, which aids in the complete oxidation of adsorbed CO as reported by others [65,66]. Hence a significant improvement is obtained when ruthenium sites are adjacent to Pt, which will initiate dissociative adsorption of H2 O (mainly on Ru sites and to some extent on Pt sites) and of ethylene glycol (mainly on Pt sites and to a some extent on Ru sites). Thus it seems that the presence of Ru limits the potential range of formation of poisoning species (adsorbed CO). At lower potentials itself, the surface becomes free of adsorbed CO and then is able to readsorb ethylene glycol molecules which can be further oxidized. However, Ru is not only used to oxidize CO to CO2 but also used as catalyst to oxidize the glycolic and oxalic acids, which is obtained as a byproduct from EG oxidation. Since such products like glycolic and oxalic acids also require oxygen donor species for its further oxidation, Ru helps in it. However, the exact reaction taking place between these poison intermediates such as glycolic acid and oxalic acids is still unclear [49]. Hence, the activity of Pt–Ru catalysts modified on carbon nanotubes was more efficient than Pt/CNT catalyst. This type of interaction becomes more significant as the monometallic was poisoned before reaching complete conversion. Fig. 8 shows the effect of scan rate for Pt/CNT and Pt–Ru/CNT electrodes towards ethylene glycol oxidation. From the figure, it can be
For the practical applications, long-term stability of the electrode is an important feature. The long-term stability of a Pt/CNT electrode was investigated in 0.5 M EG + 0.5 M H2 SO4 solutions (Fig. 10b). In the case of EG oxidation using Pt/CNT, the anodic current starts decreasing after 25th numbers of scans (Fig. 10b). After 25th numbers of scans, the peak current starts decreasing slightly. The loss of the catalytic activity may result from the consumption of EG during the CV scan. It also may be due to poisoning and the structural change of the platinum nanoparticles as a result of the perturbation of the potentials during the scanning in aqueous solutions, especially in the presence of an organic compound [1]. Another factor might be due to the diffusion process occurring between the surface of the electrode and the bulk solution. With the increase of scan number, ethylene glycol diffuses gradually from the bulk solution to the surface of the electrode. The peak current of the 500th scan is about 89.2% of that of the first scan for Pt nanoparticles modified CNT electrodes. In Pt–Ru/CNT electrodes, a sufficient improvement was noted, with the anodic current decreasing only after 50 numbers of scans. The peak current of the 500th scan is about 92.8% for Pt–Ru modified CNT electrodes than that of the first scans, which ensures a maximum stability. After the long-term CV experiments, the Pt/CNT and Pt– Ru/CNT electrodes were stored in double distilled water for a week; then ethylene glycol oxidation was carried out again by CV, and the catalytic activity was found to be around 95%, which was in good agreement with stable current achieved by chronoamperometric studies. This indicates that all NPs/CNT
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References
Fig. 10. (a) The effect Pt/Ru ratio towards EG oxidation. (b) Long term stability of Pt/CNT and Pt–Ru/CNT electrodes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
electrodes prepared in our experiment have good long-term stability and storage properties. 4. Summary The dispersion and electrocatalytic properties of Pt and Pt– Ru nanoparticles tailored multi walled carbon nanotubes towards ethylene glycol oxidation have been investigated. Results show that Pt–Ru acted as a more potent catalyst than the corresponding monometallic catalyst. This can be attributed to the nature of Ru species towards the surface poison COads intermediate oxidizing it at a lower potential itself. Such methods of fabricating metal nanoparticle catalysts using carbon nanotubes as a support produced a higher oxidation current. This may be ascribed to the unique structural and good electrical properties of MWCNT, and a high density of functional groups such as carboxyl, hydroxyl, and carbonyl groups on MWCNT. Further the long-term stability of Pt/CNT and Pt–Ru/CNT electrodes were studied and from the observation, it was found that Pt– Ru/CNT showed a maximum stability towards ethylene glycol oxidation. Acknowledgments The authors thank the Council of Scientific and Industrial Research (CSIR) Government of India, New Delhi-110 016, for the award of Senior Research Fellowship (SRF).
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Electrocatalytic oxidation of ethylene glycol on Pt and Pt–Ru nanoparticles modified multi-walled carbon nanotubes Vaithilingam Selvaraj, Mari Vinoba, Muthukaruppan Alagar ∗ Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai 600025, India Received 22 October 2007; accepted 25 February 2008 Available online 13 March 2008
Abstract The synthesis and characterization of catalysts based on nanomaterials, supported on multi-walled carbon nanotubes (CNT) for ethylene glycol (EG) oxidation is investigated. Platinum (Pt) and platinum–ruthenium (Pt–Ru) nanoparticles are deposited on surface-oxidized multi-walled carbon nanotubes [Pt/CNT; Pt–Ru/CNT] by the aqueous solution reduction of the corresponding metal salts with glycerol. The electrocatalytic properties of the modified electrodes for oxidation of ethylene glycol in acidic solution have been studied by cyclic voltammetry (CV), and excellent activity is observed. This may be attributed to the small particle size of the metal nanoparticles, the efficacy of carbon nanotubes acting as good catalyst support and uniform dispersion of nanoparticles on CNT surfaces. The nature of the resulting nanoparticles decorated multiwalled carbon nanotubes are characterized by scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis. The cyclic voltammetry response indicates that Pt–Ru/CNT catalyst displays a higher performance than Pt/CNT, which may be due to the efficiency of the nature of Ru species in Pt–Ru systems. The fabricated Pt and Pt–Ru nanoparticles decorated CNT electrodes shows better catalytic performance towards ethylene glycol oxidation than the corresponding nanoparticles decorated carbon electrodes, demonstrating that it is more promising for use in fuel cells. © 2008 Published by Elsevier Inc. Keywords: Multi-walled carbon nanotubes (CNT); Ethylene glycol (EG) oxidation, Pt nanoparticles; Pt–Ru nanoparticles; Fuel cells
1. Introduction Fuel cells employing alcohols (Direct Alcohol Fuel Cell— DAFC) are extremely attractive as power sources for mobile, stationary, portable applications such as cellular phones and laptop computers, etc. DAFCs can be made quite compact without a bulky external reformer, which is essential for polymer electrolyte membrane fuel cells (PEMFCs). Liquid fuels, such as small molecular weight alcohols, which have higher volumetric energy density and better energy efficiency, are easier to store and transport than gaseous fuels. The electrooxidation of organic compounds at noble metal electrodes using metal catalysts has been studied extensively for possible applications in electrochemical power sources [1] and electrochemical wastewater treatment [2]. These catalysts include * Corresponding author. Fax: +91 44 22352870.
E-mail addresses: [email protected] (V. Selvaraj), [email protected] (M. Alagar). 0021-9797/$ – see front matter © 2008 Published by Elsevier Inc. doi:10.1016/j.jcis.2008.02.069
platinum and platinum-based alloy catalysts [3,4]. In fact, the choice of a suitable supporting material is an important factor that may affect the performance of supported electrocatalysts owing to interactions and surface reactivity [5,6]. Carbon nanotubes (CNTs) play an important role as supporting materials in electrocatalysis [6,7] and nanoelectronic circuitry [8] due to their excellent mechanical, electronic and surface properties [9]. Several papers have been reported on the application of carbon nanotubes in fuel cells as catalysts supported and electrode materials [10–14]. The carbon nanotube material is considered to have several advantages over conventional support materials, which includes (i) having more defined crystalline structure with higher conductivity, (ii) containing little impurities, such as metals and sulfides, and thus eliminating potential poisoning effects to electrocatalysts, and (iii) possessing threedimensional structure thus favoring the flow of reactant and providing a large reaction zone when fabricated into electrodes. The carbon nanotubes are also chemically stable and resistant to thermal decomposition. Due to these distinctive characteris-
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tics, the carbon nanotube material is very suitable for use as a novel electrocatalyst support. There is a plethora of investigations on the potential use of nanotube/nanoparticles [15–17]. CNTs have been coated with various metal nanoparticles such as Au, Ti, Ni, Pd, Al, Fe, Pb and Pt by electron-beam evaporation or electroless metal deposition [18–23]. Highly dispersed nanoscale Pt particles have been an intensive research subject as the electrocatalyst for methanol oxidation [24,25]. The electrocatalytic activity of Pt nanoparticles for this reaction is dependent on various factors, which involves the size and dispersion of the particles, preparation methods, supporting materials and their surface conditions [26–35]. CNTs are good supporting materials, which can support a high dispersion of Pt nanoparticles due to their large surface area and particular morphology. In addition, the metal particles supported on the CNTs seemed to be less susceptible to CO poisoning than the traditional catalyst systems [36]. As a result, Pt nanoparticles decorated carbon nanotubes is of significant interest among the researches in the area of electrocatalysis. Among the different substances studied as a model fuel such as formic acid and methanol, ethylene glycol (simplest aliphatic diol) is also fascinating to investigate due to its high solubility in aqueous solutions and relatively high reactivity [37]. The electrochemical oxidation of ethylene glycol (EG) on platinum metal electrodes and catalysts has received significant attention during the last decades, both as a model compound for studies of the adsorption and electrooxidation behavior of CO-containing organic species and due to its potential application as attractive anodic fuel in direct oxidation fuel cells (acidic solution [38–45], alkaline solution [40,42]). Recently, Peled et al. [46,47] reported that methanol/oxygen and ethylene glycol/oxygen fuel cells equipped with a new nanoporous proton-conducting membrane and using PtRu/C (atomic ratio of 1:1) as anode catalyst provided a maximum power density of 400 and 300 mW cm−2 , respectively, which puts ethylene glycol in direct competition with methanol as a promising candidate for practical electric vehicles and stationary applications [48]. Kelaidopoulou et al. [49] observed that the addition of ruthenium and tin onto platinum dispersed in polyaniline increases the electro-oxidation of ethylene glycol in acid medium.
There are numbers of methods reported in the literature on methanol oxidation using carbon nanotubes [50–53]. To the best of our knowledge, this is the first reports on the use of carbon nanotubes as supporting material towards ethylene glycol oxidation and nanoparticles were decorated on CNTs using glycerol as reducing agent (2–3 nm). This is the simplest method used to prepare the nanoparticles in the range of 2– 3 nm. In the present work, a new route has been adopted for the preparation of Pt and Pt–Ru nanoparticles deposited on the surface-oxidized MWCNTs (Pt/CNT & Pt–Ru/CNT) using glycerol as reducing agent by conventional heating method. The prepared Pt/CNT and Pt–Ru/CNT electrodes were used as a protocol towards the electrooxidation of ethylene glycol. This is the continuation of our previous work on the role of nanoparticles in fuel cell applications [54,55]. 2. Materials and methods 2.1. Materials Reagent grade glycerol and ethylene glycol (99.8%) are obtained from SRL and electrocatalyst precursor salts, i.e., H2 PtCl6 ·6H2 O (99.9%) and RuCl3 ·6H2 O (99%) are purchased from Alfa-Aesar. Multiwalled carbon nanotubes (99%) obtained from Sigma-Aldrich were rinsed with double distilled water and dried. They are used as received without any further purification. All the chemicals are reagent grade and doubledistilled water are used throughout the experiment. All experiments are carried out at 25 ◦ C. 2.2. Pretreatment of MWCNTs The carbon nanotubes were first oxidized in a hot conc. HNO3 solution, for 48 h under refluxing conditions to remove impurities and generate surface functional groups. Purification of CNT surfaces prevents self-poisoning by foreign impurities while functional group generation enhances electrocatalyst formation (Scheme 1). Examination on surfaces of acid oxidized carbon nanotubes was carried out using a Fourier transform infrared (FT-IR) spectrometer to ensure formation of desired surface functional groups. After the oxidation treatment, a surfaceoxidized MWCNTs sample was obtained.
Scheme 1. Schematic presentation of functional group formation and metal decoration on multi walled carbon nanotubes.
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2.3. Synthesis of Pt–Ru/CNT and Pt/CNT nanocomposites The modified procedure employed for the reduction deposition of Pt and Pt–Ru electrocatalysts on pretreated CNT is as follows. The CNT decorated Pt–Ru was obtained as follows, 140 mg of acid-oxidized CNT was added to 90 ml glycerol aqueous solution. Then, 54 mg of H2 PtCl6 ·6H2 O and 21 mg of RuCl3 as Pt–Ru electrocatalyst precursors were dissolved in 10 ml of aqueous glycerol (3:1 ratio of glycerol and H2 O). After that, the catalyst precursor solution was slowly added to the prepared CNT/glycerol and sonicated for 20 min in order to get uniform dispersed solution. The solution was then heated to reflux for 12 h at 120–130 ◦ C. After reduction reaction, the reacted mixture was filtered and washed with sufficient amounts of Millipore water and dried. This was used as the modified electrode for electrocatalysis reactions. Similarly, CNT decorated with Pt was obtained by refluxing acid treated nanotubes with H2 PtCl6 ·6H2 O in aqueous glycerol (Scheme 1).
Fig. 1. Scanning electron microscopic images of the synthesized catalyst material (a) Pt/CNT and (b) Pt–Ru/CNT.
2.4. Preparation of Pt/CNT and Pt–Ru/CNT catalyst electrode 5 mg of Pt/CNT catalyst, 50 µL of Nafion solution (5 wt%, Aldrich) and 1.0 ml of solvent were mixed using an ultrasonic bath. A measured volume of this mixture was transferred via a syringe onto a graphite electrode. The solvents (acetone or water or alcohol) were evaporated at room temperature for overnight. Similar experimental conditions were adopted for the preparation of Pt–Ru/CNT nanocatalysts supported on graphite electrodes. 2.5. Equipment and methods A CHI 660B instrument with a three-compartment cell is employed for the electrochemical measurement. The working electrode is a thin layer of Nafion impregnated nanoparticles modified CNT catalysts cast on a graphite electrode. A Pt foil and a saturated calomel electrode (SCE) are used as the counter and reference electrode, respectively. All potentials in this study are reported with respect to SCE under nitrogen atmosphere. For measurement of ethylene glycol oxidation reaction activities, cyclic voltammetry is performed in a solution containing 0.5 M ethylene glycol and 0.5 M H2 SO4 at room temperature. The surface analysis (SEM) is carried out using an LEO-stereoscan 440 microscope. The size of the particles is confirmed using HR-TEM (JEOL) with an accelerating voltage of 120 kV. 3. Results and discussion
Fig. 2. EDAX images of Pt/CNT and Pt–Ru/CNT.
has been well functionalized with appropriate functional groups (–COOH), which will favor the adherence of metal nanoparticles on its surface. It was reported [57] that the dominant functional group is carboxylic group generated by strong acid treatment. 3.2. Surface morphology of nanoparticles modified electrodes The micrographs of the Pt/CNT and Pt–Ru/CNT electrodes have been investigated by SEM (Fig. 1) and their corresponding EDAX images were shown in Fig. 2. The presence of Pt and Ru metal nanoparticle in the modified electrodes were confirmed by EDX Analysis (Fig. 2). From Fig. 1, it was observed that nanoparticles with less than 2 nm are deposited on the surface of carbon nanotubes. Pt/CNT and Pt–Ru/CNT composites have porous and three-dimensional structure. The three-dimensional structure, smaller particle size and high dispersion of nanoparticles may result in large valuable Pt surface area and good electrocatalytic properties towards ethylene glycol oxidation. Fig. 3 shows the TEM image of the purified MWCNT treated with Pt and Pt–Ru nanoparticles. It can be seen that well-dispersed, spherical particles were anchored onto the external walls of MWCNT with size ranging from 2–5 nm.
3.1. Spectral evaluation of CNTs 3.3. Electrooxidation of ethylene glycol A recent report by Han et al. [56] indicated that a stronger nitric acid solution generates more functional groups on CNT surfaces than a weaker one and results in better electrocatalyst formation. Therefore, concentrated nitric acid was employed in the present study for the purification of carbon nanotubes. The acid treated multi-walled carbon nanotubes (Scheme 1)
Fig. 4 shows the cyclic voltammograms of Pt/C, Pt/CNT and Pt–Ru/CNT in 0.5 M H2 SO4 solution in the absence of ethylene glycol at 50 mV/s in a potential window of −0.2 to 1.0 V vs SCE. In view of examining the role of carbon nanotubes towards EG oxidation, cyclic voltammograms were carried out
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Fig. 5. Electrooxidation of ethylene glycol on Pt nanoparticles modified carbon (Pt/C) in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
Fig. 3. HR-TEM images of catalyst materials (a) Pt/CNT, (b) Pt–Ru/CNT, (c) lattice structure of Pt/CNT, (d) lattice structure of Pt–Ru/CNT.
Fig. 6. Electrooxidation of ethylene glycol on Pt nanoparticles modified multi walled carbon nanotubes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
Fig. 4. Cyclic voltammograms of Pt/C, Pt/CNT and Pt–Ru/CNT electrodes in 0.5 M H2 SO4 (scan rate = 50 mV s−1 ).
for Pt nanoparticles decorated on carbon and MWCNTs (Figs. 5 and 6). It can be seen from the figure that the electrocatalytic activity of Pt supported on CNTs was significantly improved and higher than that of carbon supported Pt (Fig. 6), thereby proving that CNTs acted as an efficient supporting material for effective dispersion of catalyst, which appears a promising field of research. The enhancement in catalytic activity is due to the following reasons. From the Fig. 4, it can be observed that the CV’s pertaining to Pt and Pt–Ru nanoparticles modified CNT electrodes overlap in the Hupd region (−0.2 < E < 0.2 V), which clearly indicates that the active surface area for both the electrodes are nearly the same. The charges and currents associated with the hydrogen adsorption/desorption regions of Pt–Ru
nanoparticles modified CNT composite electrodes were almost the same as for Pt nanoparticles modified CNT composites electrodes. Hence it could be understood that the addition of Ru to Pt did not change the active surface area. It is well documented that two or more electrodes with almost comparable currents for the formation and oxidation of adsorbed hydrogen atoms will have exactly same active surface area of metals [58,59]. However, in the case of Pt/C and Pt/CNT does not overlap in the Hupd region. Hence, the active surface area for Pt/C and Pt/CNT are different. Fig. 4 shows the increase in surface area for Pt/CNT compared to Pt/C. Hence, the difference in the active surface area is also responsible for enhanced catalytic activity. Another reason might be that the metal support interactions can significantly alter the activity of supported catalysts [60]. Since, CNTs have more organized graphitic structures than carbon black. They therefore would result in different interactions with the supported metal, which happen to be positive to the catalyst activity in the electrooxidation of EG. Hills et al. [61] reported a study on the carbon support effects on bimetallic Pt–Ru nanoparticles and found that the carbon black resulted in structural disorders in the alloy nanoparticles in which the
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nearest coordination numbers were smaller. In contrast, the nanoparticles deposited on fullerene soot have enhanced catalytic activity due to ordered crystalline structure. Similarly, the CNTs employed in the present study exhibited an enhanced catalytic activity towards electrooxidation of EG. The cyclic voltammogram of the graphite electrode modified by Pt/CNT in 0.5 M EG and 0.5 M H2 SO4 at 50 mV s−1 is shown in Fig. 6. As viewed from the figure, an oxidation peak around 0.6 V is observed in the anodic scan region, which corresponds to the oxidation of EG. Reductive peak in the reverse scan is obtained due to the reduction of Pt-oxide. The shape of the CV and the oxidation potentials are comparable with the previous reports. At lower potentials, EG adsorbs on the electrode surface and when the potential reaches 0.5 V, the current started increasing which is attributed to the electrooxidation of ethylene glycol, with a maximum current at 7.45 mA. As per the earlier reports, oxidation of ethylene glycol gives formic acid as one of the major product [62,63]. A probable mechanism as suggested by Bagotsky and Vijh [64], are given as follows (Steps 1–6). (CH2 OH)2 → (CH2 OH)2 → 1, (CH2 OH)2 → (:COH)2 → 2, (:COH)2 + 2H2 O → (HCOOH)ads + 2H+ + 2e− → 3, Pt(HCOOH)ads → Pt(CO)ads + H2 O → 4, Pt + H2 O → Pt(OH)ads → 5, Pt(OH)ads + Pt(CO)ads → Pt + CO2 + H+ → 6. Thus ethylene glycol oxidation takes place on Pt surfaces giving formic acid which in turn results in the formation of poisonous intermediate (CO)ads (Step 4) on the electrode which gets deposited on the active sites of Pt [42]. The only possibility to remove (CO)ads is to oxidize it to CO2 by reacting with OHads formed by the dissociation of water molecules on Pt surfaces (Steps 5 and 6). But the water activation process on Pt is very difficult and takes place at a higher potential and hence CO would be oxidized only at higher potentials. Due to the higher potential of CO oxidation on Pt, the electrode surfaces will be blocked by more amounts of CO species and thus the adsorption of other EG molecules on the electrode surface is hindered. This would facilitate the oxidation of only lesser number of EG moieties and the retardant CO remains on the electrode surface for a longer time occupying active catalyst sites and reducing the overall activity towards ethylene glycol oxidation. Hence, the current at 0.5 V due to EG oxidation depends mainly on the amount of CO oxidized from the active sites of Pt. In view of this poisoning effect, the present work is devoted to study the electrocatalytic activity of Pt–Ru nanoparticles modified carbon nanotubes for a comparative factor (Fig. 7). Before probing the catalytic activity of the bimetallic system in CNT, we have tested the atomic ratio of the metal precursors for the effective ratio required for the present investigation. Three catalysts of different Pt to Ru atomic ratios, namely, Pt50 Ru50 , Pt70 Ru30 , and Pt80 Ru20 , were prepared for investigation of the compositional effects. It was shown that highly dispersed bimetallic Pt–Ru alloy nanoparticles with no agglomeration can be synthesized on the carbon nanotubes. From the present investigation, we concluded that Pt50 Ru50 was found
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Fig. 7. Electrooxidation of ethylene glycol on Pt–Ru nanoparticles modified multi walled carbon nanotubes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
to be more stable, good long term stability and catalytic activity compare to other composition. However, the Pt70 Ru30 and Pt80 Ru20, catalysts showed poorer stability that can be justified by the bifunctional mechanism of bimetallic alloys. This was probably due to the composition effects of unpaired Pt and Ru atoms. Hence, we have chosen the Pt50 Ru50 composition for the following systems. Fig. 7 shows the electrocatalytic activity of Pt50 Ru50 nanoparticles modified carbon nanotubes towards ethylene glycol oxidation. It was found that the presence of Ru has a greater impact on EG oxidation. The rate of oxidation of EG was found enhanced by a large factor. In the case of Pt–Ru nanoparticle systems, the electrocatalytic activity towards EG oxidation is increased to 12.62 mA, which was 7.45 mA in the case of Pt modified electrodes. Thus when Ru added to Pt forming Pt–Ru systems, Ru helps in enhancing the oxidation rate to a considerable extent. To account for this effect, the mechanism of ruthenium enhancement on EG oxidation is explained as below. As well known, steady state EG oxidation involves generation of chemisorbed carbon monoxide on the Pt surface of the catalyst in the reaction. To sustain the steady-state current, or the continuous catalytic oxidation process, CO needs to be oxidatively removed from the surface in order to renew the surface sites needed to sustain the reaction. The addition of ruthenium to the Pt catalyst enhances CO removal from the surface since ruthenium promotes CO oxidation to CO2 via the mechanism (Steps 7 and 8), where OH symbolizes the oxygen-containing species produced by water activation process. The main reason for improved activity is due to the affinity of Ru towards the water molecules (Steps 7–8). Ru + H2 O → Ru(OH)ads + H+ + e− → 7, Ru(OH)ads + Pt(CO)ads → Pt + Ru + CO2 + H+ + e− → 8. On Pt–Ru, the main difference is the ability of Ru to activate water molecules at a lower potential than that on Pt (typically 0.35 V compared to 0.6 V with Pt) and hence CO was oxidized at lower potential. Thus the displacement of the onset and peak potentials in a negative direction in the case of the catalysts containing Ru compared to Pt/CNT is mainly related to
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Fig. 8. Effect of scan rate on ethylene glycol oxidation using (a) Pt/CNT and (b) Pt–Ru/CNT electrodes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
seen that the oxidation current increases with increasing the scan rate. This shows that EG oxidation at nanoparticles modified CNT electrodes is mainly controlled by diffusion. Further, Fig. 9 shows the typical chronoamperometric response curves at 0.5 M EG and 0.5 M H2 SO4 with Pt/CNT and Pt–Ru/CNT as the working electrodes (versus SCE). The current observed from chronoamperometric was found to be in good agreement with the current observed from cyclic voltammetry. The effects of different Pt/Ru ratio were also carried out (Fig. 10a), which gives information about the effective atomic ratio towards EG electrooxidation. 3.4. Long-term stability of the Pt/CNT and Pt–Ru/CNT electrodes Fig. 9. Chronoamperometric curves of ethylene glycol in a solution of 0.5 M EG and 0.5 M H2 SO4 at 25 ◦ C on the Pt/CNT (a) and Pt–Ru/CNT (b) catalysts.
the bi-functional mechanism as shown below, where the beneficial effect is related to the formation of Ru–OH species at low potentials, which aids in the complete oxidation of adsorbed CO as reported by others [65,66]. Hence a significant improvement is obtained when ruthenium sites are adjacent to Pt, which will initiate dissociative adsorption of H2 O (mainly on Ru sites and to some extent on Pt sites) and of ethylene glycol (mainly on Pt sites and to a some extent on Ru sites). Thus it seems that the presence of Ru limits the potential range of formation of poisoning species (adsorbed CO). At lower potentials itself, the surface becomes free of adsorbed CO and then is able to readsorb ethylene glycol molecules which can be further oxidized. However, Ru is not only used to oxidize CO to CO2 but also used as catalyst to oxidize the glycolic and oxalic acids, which is obtained as a byproduct from EG oxidation. Since such products like glycolic and oxalic acids also require oxygen donor species for its further oxidation, Ru helps in it. However, the exact reaction taking place between these poison intermediates such as glycolic acid and oxalic acids is still unclear [49]. Hence, the activity of Pt–Ru catalysts modified on carbon nanotubes was more efficient than Pt/CNT catalyst. This type of interaction becomes more significant as the monometallic was poisoned before reaching complete conversion. Fig. 8 shows the effect of scan rate for Pt/CNT and Pt–Ru/CNT electrodes towards ethylene glycol oxidation. From the figure, it can be
For the practical applications, long-term stability of the electrode is an important feature. The long-term stability of a Pt/CNT electrode was investigated in 0.5 M EG + 0.5 M H2 SO4 solutions (Fig. 10b). In the case of EG oxidation using Pt/CNT, the anodic current starts decreasing after 25th numbers of scans (Fig. 10b). After 25th numbers of scans, the peak current starts decreasing slightly. The loss of the catalytic activity may result from the consumption of EG during the CV scan. It also may be due to poisoning and the structural change of the platinum nanoparticles as a result of the perturbation of the potentials during the scanning in aqueous solutions, especially in the presence of an organic compound [1]. Another factor might be due to the diffusion process occurring between the surface of the electrode and the bulk solution. With the increase of scan number, ethylene glycol diffuses gradually from the bulk solution to the surface of the electrode. The peak current of the 500th scan is about 89.2% of that of the first scan for Pt nanoparticles modified CNT electrodes. In Pt–Ru/CNT electrodes, a sufficient improvement was noted, with the anodic current decreasing only after 50 numbers of scans. The peak current of the 500th scan is about 92.8% for Pt–Ru modified CNT electrodes than that of the first scans, which ensures a maximum stability. After the long-term CV experiments, the Pt/CNT and Pt– Ru/CNT electrodes were stored in double distilled water for a week; then ethylene glycol oxidation was carried out again by CV, and the catalytic activity was found to be around 95%, which was in good agreement with stable current achieved by chronoamperometric studies. This indicates that all NPs/CNT
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References
Fig. 10. (a) The effect Pt/Ru ratio towards EG oxidation. (b) Long term stability of Pt/CNT and Pt–Ru/CNT electrodes in 0.5 M H2 SO4 and 0.5 M EG (scan rate = 50 mV s−1 ).
electrodes prepared in our experiment have good long-term stability and storage properties. 4. Summary The dispersion and electrocatalytic properties of Pt and Pt– Ru nanoparticles tailored multi walled carbon nanotubes towards ethylene glycol oxidation have been investigated. Results show that Pt–Ru acted as a more potent catalyst than the corresponding monometallic catalyst. This can be attributed to the nature of Ru species towards the surface poison COads intermediate oxidizing it at a lower potential itself. Such methods of fabricating metal nanoparticle catalysts using carbon nanotubes as a support produced a higher oxidation current. This may be ascribed to the unique structural and good electrical properties of MWCNT, and a high density of functional groups such as carboxyl, hydroxyl, and carbonyl groups on MWCNT. Further the long-term stability of Pt/CNT and Pt–Ru/CNT electrodes were studied and from the observation, it was found that Pt– Ru/CNT showed a maximum stability towards ethylene glycol oxidation. Acknowledgments The authors thank the Council of Scientific and Industrial Research (CSIR) Government of India, New Delhi-110 016, for the award of Senior Research Fellowship (SRF).
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