Materials Focus Vol. 2, pp. 1–5, 2013 (www.aspbs.com/mat)
Copyright © 2013 by American Scientific Publishers All rights reserved. Printed in the United States of America
Poly(aniline)/MnO2 Supported Palladium–A Facile Nanocatalyst for the Electrooxidation of Methanol Ramanujam Kannan1, ∗ , Kulandaivelu Karunakaran1 , and Samuel Vasanthkumar2 ∗ 1
Department of Chemistry, Sona College of Technology, Salem 636005, India Department of Chemistry and Nanosciences and Technology, School of Science and Humanities, Karunya University, Coimbatore 641114, India
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ABSTRACT
1. INTRODUCTION Considerable attention has been paid to alternative energy sources in an attempt to relieve pollution and energy crises.1–7 One such promising source of clean and sustainable energy is Fuel cell technology. Recently, methanol, which is a high energy and low emission fuel, has drawn increased attention due to its possible application in the direct alcohol fuel cell (DAFC).2–6 Much attention has been focused on the preparation of metal catalysts at nanoscale levels for applications in fuel cell. Up until now, majority of research on DAFC has been done using Pt and Pt-based alloys as anode catalysts due to their high catalytic ability. However, its use has been restricted due to limited resources, higher adsorption of CO as poisoning agent etc.,3–6 Hence, non-Pt based catalysts such as palladium (Pd) and Pd based catalysts are being widely studied for methanol oxidation reaction (MOR).3–5 Pd is a promising substitute due to its higher abundance, lower cost, and excellent electrocatalytic activity towards alcohols.5–10 However, the poisoning of the active site of catalyst by the intermediate products such as CO is the major drawback in DAFC. In order to reduce this poisonous effect various methods were adopted.5 11–14 Recently, various materials including carbon nanotubes,12 carbon ∗
Authors to whom correspondence should be addressed. Emails:
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[email protected] Received: xx xxxx xxxx Accepted: xx xxxx xxxx
Mater. Focus 2013, Vol. 2, No. 4
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nanofibers,13 metal oxides14–16 etc., have been used as support for nobel metal catalyst for various DAFCs. Among these, metal oxides exhibit improved catalytic activity via the intermediate supply of oxygen to the catalyst.14 This work presents the electrocatalytic oxidation of methanol on a Pd dispersed Pani/MnO2 nanocomposite modified GC electrode.
2. EXPERIMENTAL DETAILS Palladium chloride, potassium permanganate, hydrogen peroxide (30% w/w), potassium hydroxide, formaldehyde, methanol and ethanol were purchased from Merck, India. All were of analytical grade and used without further purification. Manganese oxide (MnO2 nanotube was prepared by hydrothermal method.17 Pani/MnO2 nanocomposite was prepared as follows: about 1 ml of freshly distilled aniline was dissolved in 10 ml water and a few drops of dil.HCl. About 50 mg of prepared MnO2 was dispersed in the aniline solution and subjected to ultrasonication. After one hour, 2 ml of hydrogen peroxide was added drop wise into the aniline–MnO2 mixture and sonication was continued for a further three hours. The product was then separated by centrifugation, washed with double distilled water and dried in an oven at 60 C for 12 ‘hrs. Pani/MnO2 /Pd was prepared by in situ reduction method. In brief, suitable quantity of 0.5 mM PdCl2 solution was taken so as to achieve 5% of Pd in isopropanol. About 500 mg of prepared Pani/MnO2 was added to it doi:10.1166/mat.2013.1086
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Polyaniline/manganese oxide (Pani/MnO2 nanocomposite as a supportive material for Palladium (Pd) towards the electrooxidation of methanol was explored. Pd was dispersed on the Pani/MnO2 composite by in-situ reduction method. The structural details of the prepared nanocatalyst were charaterized by powder X-ray diffractogram (XRD), scanning slectron microscope equipped with energy dispersive X-ray analyzer (SEM– EDX) and transmission electron microscopy. The observed results indicate that the Pd nanoparticles were uniformly dispersed on the Pani/MnO2 to form Pani/MnO2 /Pd ternary nanocomposite. The catalytic activtiy of the Pani/MnO2 /Pd nanocomposite towards methanol electrooxidation exhibited superior catalytic activity compared to C/Pd, Pani/Pd, MnO2 /Pd. KEYWORDS: Manganese Oxide, Polyaniline, Electrooxidation, Methanol, Palladium.
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Poly(aniline)/MnO2 Supported Palladium–A Facile Nanocatalyst for the Electrooxidation of Methanol
vacuum. Electrochemical measurements were performed with a CHI 660 C electrochemical workstation (CH Instruments, USA) equipped with an undivided threeelectrode cell. The Pani/MnO2 /Pd nanocomposite modified GC electrode served as the working electrode, a Pt wire served as the counter electrode and a standard calomel electrode was used as the reference electrode. The electrocatalytic activity of Pani/MnO2 /Pd modified GC electrode towards methanol oxidation was compared with that of graphite/Pd modified GC electrode under similar method.
3. RESULTS AND DISCUSSION 3.1. Physical Characterizations of Prepared Pani/MnO2 /Pd Nanocomposite
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Fig. 1. Powder XRD patterns of (a) MnO2 , (b) Pani/MnO2 and (c) Pani/MnO2 /Pd.
and stirred well. Formaldehyde was added to the mixture drop wise and the reductive reaction was performed at room temperature. After 12 hours, the resulting product was filtered, washed with distilled water, and dried at 60 C for 6 h. The glassy carbon electrode (GC, 3 mm diameter) was initially polished with Al2 O3 slurry. Pani/MnO2 /Pd nanocomposite was ultrasonically dispersed in 0.5% Nafion in ethanol solution. An aliquot of Pani/MnO2 /Pd nanocomposite/Nafion suspension was dropped on the surface of the GC electrode and allowed to dry under
Powder XRD patterns of the synthesized MnO2 materials revealed a crytomelane type MnO2 material (Fig. 1(a)).18 Figure 1(b) shows Pani/MnO2 nanocomposite, which exhibits two sharp peaks at 2 of 25–30 indicating uniform distribution of MnO2 on Pani. Figure 1(c) shows the XRD patterns of the Pani/MnO2 /Pd compared with standard Pd (JCPDS: 88-2335), showing two characteristic diffraction peaks at ca. 40 (111), and 43 (200) belonging to the face-centered cubic phase of Pd deposited on the Pani/MnO2 nanocomposite and some of the parrticles were also present in the Pani matrix. The surface morphology of Pani/MnO2 /Pd nanocomposite was tested by SEM (Figs. 2(a and b)). Scanning electron microscopy revealed
Fig. 2. Scanning electron microscopic images of (A) MnO2 , (B) Pani/MnO2 , (C) transmission electron microscopic image of Pani/MnO2 /Pd; and (D) energy dispersive X-ray spectroscopy of Pani/MnO2 /Pd.
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Scheme 1. Schematic representation for the preparation of Pani/MnO2 /Pd nanocomposite material.
well–defined nanotubes with smooth faces. After composite formation with poly aniline, the structure remained unaltered (Figs. 2(a and b)). TEM image for Pani/MnO2 /Pd nanocomposite shows the presence of Pd and MnO2 nanotubes in the Pani matrix (Fig. 2(c)) and was represented by the scheme 1. In addition, the presence of Pd on the nanocomposite was confirmed by EDX analysis (Fig. 2(d)). 3.2. Electrooxidation of Methanol on Pani/MnO2 /Pd Nanocatalyst Electrode
Fig. 3. (A) CVs of different electrodes towards the electrooxidation of methanol (1 M CH3 OH/1 M KOH solution) at 50 mV/s; (a) Nafion, (b) Pani, (c) C/Pd and (d) Pani/MnO2 /Pd electrode. (B) CVs of different electrodes towards the electrooxdiation of methanol (1 M CH3 OH/1 M KOH solution) at 50 mV/s; (a) Pd/Pani, (b) Pd/MnO2 , and (c) Pani/MnO2 /Pd electrode. Mater. Focus, 2, 1–5, 2013
Fig. 4. CVs of electrooxidation of (a) methanol [5 M CH3 OH/1 M KOH], (b) ethanol [1 M C2 H5 OH/1 M KOH] at Pani/MN/Pd electrode at scanning potentials of (a) −1.1 to +0.2 V and (b) −1.1 to +0.4 V [Scan rate 50 mV/s].
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The electrocatalytic activity of the Pani/MnO2 /Pd electrode towards methanol oxidation reaction (MOR) was tested by
cyclic voltammetry (CV). The voltammetric behavior of C/Pd, Pani/MnO2 /Pd, Pani and Nafion modified GC electrodes were tested in 1 M CH3 OH/1 M KOH. Pani and Nafion modified GC electrode did not show any significant peak. C/Pd and Pani/MnO2 /Pd modified electrodes exhibited well defined oxidation peaks at −0.175 V and −0.245 V respectively, indicating that the catalytic activity was only due to the presence of Pd metal. The potential shift of 70 mV for Pani/MnO2 /Pd electrode compared to Pd/C electrode indicates that the Pani/MnO2 influences methanol oxidation. For systematic evaluation of catalyst activity, the CVs of Pani/Pd, MnO2 /Pd and Pani/MnO2 /Pd were studied under identical conditions (1 M CH3 OH/1 M KOH). Figure 3(B) a–c shows the electrooxidation of methanol. In the forward
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Poly(aniline)/MnO2 Supported Palladium–A Facile Nanocatalyst for the Electrooxidation of Methanol
scan, MOR produces a significant anodic oxidation peak; a peak potential of −0.274 V for MnO2 /Pd, −0.245 V for Pani/MnO2 /Pd and −0.170 V for Pani/Pd electrode was observed. In the reverse scan, a prominent oxidation peak potential of −0.38 V for C/Pd and −0.47 V for Pani/MnO2 /Pd electrode was observed. The oxidation peak for Pd/MnO2 electrode was insignificant. The oxidation peaks observed during the forward scan can be attributed to the oxidation of freshly adsorbed methanol on the surface of the catalytic electrode, while the oxidation peaks observed during the reverse scan can be attributed to the oxidative removal of the incomplete or partially oxidized carbonaceous species formed during the forward scan.11 It is possible that the carbonaceous species that accumulate during the forward scan occupy the active sites of the catalyst, suppressing the catalytic activity. The ratio of forward scan peak curve density (If to the backward scan current density (Ib , can be used to find out the tolerance limit of the catalyst on the carbonaceous species absorbed. A high If /Ib ratio indicates that the oxidation of methanol to carbon dioxide is favorable.15 Based on the CV results, three important points were observed during the MOR. First, from the qualitative analysis of these voltammograms, the If /Ib ratio for C/Pd, MnO2 /Pd, Pani/MnO2 /Pd was calculated to be 1.66, 6.15 and 7 respectively. This indicates that the carbonaceous species formed during the forward scan was effectively oxidized on the surface of the Pani/Pd electrode. Pani/Pd exhibits a lower If /Ib ratio compared to Pani/MnO2 /Pd and MnO2 /Pd, indicating that the presence of MnO2 in the composite helps reduce the poisonous intermediates effectively. Secondly, the current density of
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the Pani/MnO2 /Pd is higher than the C/Pd and MnO2 /Pd. Third, the Pani/MnO2 /Pd electrode exhibits lower peak potential of 75 mV (−0.245 V) compared to Pani/Pd (−0.170 V). From the above results, we conclude that Pani/MnO2 acts as a excellent support for Pd for methanol elecctro oxidation. Figure 4(A)(a, b) shows the CVs of Pani/MnO2 /Pd nanocomposite electrode in 5 M CH3 OH/1 M KOH in the scanning potential reanges of (a) −1.1 to +0.2 V, and (b) −1.1 to +0.4 V. In both the CVs, an anodic oxidation peak appears in the −0.32 to −0.17 V region with vertex potential at −0.247 V. In the −1.1 to +0.2 V range a prominent backward oxidation peak appears in the −0.35 V region. However, the −1.1 to +0.4 V range shows comparatively inferior current response with 10 mV negative shift in potential. The availability of active oxygen is comparatively less at +0.2 V. However, a slight increase in the potential i.e., +0.4 V leads to the availability of active oxygen through oxygen evolution reaction occurs and the poisonous intermediates are oxidized more effectively. Recent studies suggest that the metal oxides such as MnO2 , Co3 O4 , CeO2 etc., will supply oxygen from the material to the catalyst, thereby increasing the catalytic activity of the catalyst.13–15 MnO2 present in the nanocomposite provides the active oxygen to Pd catalyst and may extract oxygen atom from the electrolyte thereby the intermediates get oxidized. This kind of effect doesnot observed in the C/Pd electrode (supplement informations). Similarly, for the ethanol oxidation reaction almost fifty percentage of secondary oxidation of the poisonous intermediates was observed as shown in Figure 4(B) (a, b) and explained by the following section.
Fig. 5. CVs of electrooxidation of ((a–e) 1–5 M L−1 CH3 OH/1 M KOH at Pani/MnO2 /Pd electrode. [Inset: (A) Calibration plot peak current density versus concentration of methanol and (B) Chronoampherometric curve for the electrooxidation of methanol at Pani/MnO2 /Pd electrode].
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The beneficial oxidation of CO is driven by the supply of active oxygen atom by the MnO2 through the reversible redox transformation of MnOOH to MnO2 .19 The possible reaction mechanism is as follows: In aqueous medium MnO2 + H+ + e− −→ MnOOH+
(1)
Pd/Pani/MnO2 electrode in alkaline medium MnOOH/Pd + OH− ←→ Pd/MnO2 + H2 O + e−
(2)
MnO2 /Pd–COads + OH− −→ Pd/MnOOH + CO2 + e− (3)
for the electrooxidation of methanol and ethanol was explored. Pd metal provides good catalytic activity towards methanol and ethanol oxidation. In Pani/MnO2 , in addition to its function as a catalyst support, MnO2 helps enhance the MOR activity by extracting active oxygen atoms and supplying it to the Pd active sites, helping the complete oxidation of poisonous intermediates. Acknowledgment: The authors thank the Management and the Authorities of Sona College of Technology, Salem and Karunya University, Coimbatore for their kind support.
The overall reaction Pd–COads + 2OH− −→ Pd + CO2 + 2e−
References and Notes (4)
4. CONCLUSION Ability of palladium dispersed polyaniline/manganese oxide nanocomposite to function as good support material
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Based on the above mechanism, the removal of CO by facilitating the oxygen supply through the reversible redox system of MnO2 (Mn3+/4+ . The effects of methanol concentrations of 1 to 5 M L−1 on Pani/MnO2 /Pd electrode were also studied. Figure 5 shows a monotonic peak current density increase with an increase in methanol concentration. Chronoampherometric tests to study electrode stability show that the Pani/MnO2 /Pd electrode was highly stable (Fig. 5(B) inset). The stability of the Pani/MnO2 /Pd catalyst for electrooxidation of methanol was tested by varying the scan rate (supplement informations). The calibration plot between peak current density was proportional to the square root of scan rate ranging from 25 to 250 mV/s (r = 099), which is ascribed to diffusion controlled electrode process. The electrocatalytic activity towards MOR was nearly the same even after the electrode was stored in de-ionized water for a week. Thus, the modified electrode is highly stable and has potential applications as catalyst in direct methanol fuel cell applications. The mechanistic studies for the MOR at the Pani/MnO2 /Pd nanocomposite electrode are underprogress.
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