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shell nanocomposite as a Pd electro-catalyst support for ethanol oxidation
Materials Chemistry and Physics 128 (2011) 341–347
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Synthesis of a carbon-coated NiO/MgO core/shell nanocomposite as a Pd electro-catalyst support for ethanol oxidation C. Mahendiran a , T. Maiyalagan b , K. Scott b , A. Gedanken a,∗ a Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel b School of Chemical Engineering & Advanced Materials, University of Newcastle Upon Tyne, Newcastle Upon Tyne NE1 7RU, United Kingdom
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Article history: Received 2 June 2010 Received in revised form 13 February 2011 Accepted 16 February 2011 Keywords: Core–shell structure Nanoparticles Ethanol oxidation
a b s t r a c t Carbon coated on NiO/MgO in a core/shell nanostructure was synthesized by the single-step RAPET (reaction under autogenic pressure at elevated temperatures) technique, and the obtained formation mechanism of the core/shell nanocomposite was presented. The carbon-coated NiO/MgO and its supported Pd catalyst, Pd/(NiO/MgO@C), were characterized by SEM, HR-TEM, XRD and cyclic voltammetry. The X-ray diffraction patterns confirmed the face-centered cubic crystal structure of NiO/MgO. Raman spectroscopy measurements provided structural evidence for the formation of a NiO/MgO composite and the nature of the coated carbon shell. The high-resolution transmission electron microscopy images showed the core and shell morphologies individually. The electrocatalytic properties of the Pd/(NiO/MgO@C) catalyst for ethanol oxidation were investigated in an alkaline solution. The results indicated that the prepared Pd–NiO/MgO@C catalyst has excellent electrocatalytic activity and stability. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, the focus on the chemical synthesis of nanoparticles has shifted from single component nanoparticles to hybrid nanoparticles. Hybrid nanoparticles usually contain two or more different nanoscale domains, which lead to synergistic properties due to their interfacial interactions. Therefore, hybrid nanoparticles (non-symmetrical dimmers, symmetric core–shell and any other hetero-structures) are promising due to their multi-function and designable properties [1–3]. The design of core/shell structured composites has received much attention as a means to improve the stability and surface chemistry of the core materials, and as a way of obtaining unique structures, properties, and applications via a combination of the different characteristics of the components that are not available with their single-component counterparts [4–7]. The synthesis and characterization of hybrid nanostructure material is aimed at designing and arranging nanomaterials in complex functional structures, resulting in unique physical properties [8–11]. Carbon-coated nanomaterials are of great interest due to their stability towards oxidation and degradation [12–15]. The creation of core/shell nanostructures containing bi-metal oxides has greatly enhanced the efficiency of these structures over pure single metal oxide particles as destructive adsorbents for environmental pol-
∗ Corresponding author. E-mail address: [email protected] (A. Gedanken). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.02.067
lutants, such as SO2 and H2 S [16,17]. Magnesium oxide (MgO) is a typical wideband-gap insulator. It has found many important applications for use in catalysis, refractory materials, toxic waste remediation, paint, superconductors, and substrates for thin film growth [18–20]. NiO is a very successful material used in various fields, such as catalysis [21], battery cathodes [22] and fuel cell electrodes [23], and it is also widely studied as an ant ferromagnetic as it is a chemically-stable ionic insulator with high thermal stability and good resistance to corrosion. Many researchers have prepared NiO and MgO nanomaterials by various methods, e.g., the solvo-thermal technique [24], the laser technique [25], the sol–gel method [26] and thermal decomposition [27]. The preparation of core–shell nano-wire structures of ZnO/MgO [28] obtained by hydrothermal, atomic layer deposition [29] and chemical vapor deposition (CVD) [30] methods has already been reported. NiO/MgO thin films were prepared by an ion beam-assisted deposition technique and were used for chemical transformations and catalysis applications [31]. Direct ethanol fuel cells (DEFs) have attracted much attention because ethanol has a low toxicity, as compared to the methanol used in direct methanol fuel cells (DMFs). Ethanol can be easily produced in great quantities by the fermentation of sugar-containing raw materials [32]. Pt-based catalysts are extensively studied for electrocatalytic fuel cell applications. Indeed, platinum is very efficient for the different reactions involved in fuel cells, such as methanol oxidation and oxygen reduction [33]. However, it is well known that the anode activity of Pt is adversely affected by poisoning from CO species. Several studies have investigated the addition
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of Ru or other metals to Pt, in order to improve the activity of the catalysts [34–36]. Nevertheless, it has been shown recently that palladium is more active than platinum for ethanol oxidation in basic media [37–39]. As Pd is 50 times more abundant in the earth and cheaper than Pt, Pd-based nanomaterials are potential electrocatalysts for replacing Pt in DAFCs. Palladium also plays a vital role in catalysis and is involved in various reactions, such as Heck, Suzuki, and Stille coupling [40–43]. In order to reduce the amount of noble-metal loading on the electrodes, improvements to the dispersion of the noble metal on the support have been investigated. Metal nanoparticles have been dispersed on a wide variety of substrates, such as carbon nanomaterials [44,45], polymer matrices [46], and oxide matrices [47,48]. Recently, metal oxides such as CeO2 [49], NiO [50], MgO [51], TiO2 [52] and In2 O3 [53] have been used as electrocatalysts for the direct oxidation of alcohol, and which resulted in a significantly improved electrode performance in terms of the enhanced reaction activity. The noble-metal shell in the core/shell nanoparticles has two roles. First, it protects the non-noble core from contact with the alkaline electrolyte. Second, the coated Pd on the shell should improve the catalytic properties of the substrate. We therefore report herein on a one-stage, reproducible, solvent free, competent, and straightforward approach for the synthesis of NiO/MgO@C core shell nanostructures, followed by its decoration by Pd using simple impregnation. The prepared material was evaluated for the electrochemical ethanol oxidation reaction for fuel cell applications in alkaline media.
2. Experimental 2.1. Synthesis of a NiO/MgO@C core/shell nanocomposite The synthesis of a NiO–MgO@C core/shell nanocomposite was carried out by the thermal dissociation of a mixture of nickel acetate (Ni(C4 H6 O2 )2 ) and magnesium acetylacetonate (Mg(C5 H7 O2 )2 ), purchased from Sigma Aldrich and used as received. The 3 mL closed vessel cell was assembled from stainless steel Swagelok parts. A 1/2 union part was plugged from both sides by standard caps. In a typical synthesis of NiO/MgO@C core/shell material, 0.248 g of nickel acetate (0.072 mols) and 0.516 g of magnesium acetylacetonate (0.072 mol) were introduced into the cell at room temperature. The filled cell was closed tightly by the other plug and then placed inside an iron pipe at the center of the tubular furnace. The temperature was raised at a rate of 10 ◦ C min−1 . The closed vessel cell was heated at 600 ◦ C for 3 h. The reaction took place at the autogenic pressure of the precursor. The closed cell (Swagelok) was gradually cooled (∼5 h) to room temperature and opened. In fact, we expected to obtain 0.764 g of NiO/MgO@C for a 100% product yield, but during the chemical reaction inside the Swagelok some of the acetate molecules might have evaporated at the higher temperature. Thus, we achieved only 0.519 g (∼68%) of black-colored NiO/MgO@C. The synthesis was repeated five times in order to confirm the reproducibility of the process, and identical products were obtained.
Fig. 1. XRD of NiO/MgO@C core/shell structure (a) and Pd–NiO/MgO@C (b).
2.4. Preparation of the working electrode Glassy carbon (GC) (Bas Electrode, 0.07 cm2 ) was polished to a mirror finish with 0.05 m alumina suspensions before each experiment and served as an underlying substrate of the working electrode. The working electrodes were fabricated by coating Nafion-impregnated catalyst ink on a glassy carbon electrode. Thin-film electrodes were prepared by dispersing 35 mg of the catalyst in 800 L of deionized water. 200 L of a 5 wt% Nafion solution was then added and ultrasonicated for 20 min. A known amount of the suspension was added to the glassy carbon (GC) electrode and slowly dried in air. The loading of the noble metal catalyst was 0.3 mg cm−2 . A solution of 1 M of KOH containing 1 M of ethanol was used to study the activity of ethanol oxidation. 2.5. Electrochemical measurements Electrochemical measurements were collected using a computer-controlled Gill AC potentiostat (ACM Instruments Ltd., Cumbria, UK). All experiments were carried out in a glass cell using a traditional three-electrode assembly incorporating a glassy carbon working electrode (0.07 cm2 ), a mercury–mercury oxide reference electrode, and a platinum mesh (25 mm × 25 mm) counter electrode. All electrochemical experiments were carried out at room temperature in 1 M of aqueous KOH. The electrolyte solution was purged with high pure nitrogen for 30 min prior to a series of voltammetric experiments. The current densities reported are based on the geometric area. Triply-distilled water was used throughout for the preparation of solutions.
3. Results and discussion 3.1. XRD
2.2. Preparation of Pd/(NiO/MgO@C) catalysts 3.13 mL of 0.045 mol L−1 PdCl2 were dissolved in 10 mL of ultrapure water and 60 mg of NiO/MgO@C was added. The solution was mixed by magnetic stirring for 3 h and ultrasonicated to achieve 20% Pd loading. A freshly prepared aqueous solution of NaBH4 was added drop wise under N2 atmosphere to the solution to reduce the Pd species to form the electrocatalyst, which was then repeatedly washed with distilled water to remove residual salts, centrifuged, and dried at 70 ◦ C.
2.3. Structural characterizations XRD patterns were collected using a Bruker AXS D* Advance Powder X-ray ˚ High-resolution scanning diffractometer (Cu KR radiation, wavelength 1.5406 A). electron microscopy (HRSEM) measurements of the products were carried out on a JEOL-JSM 840 scanning electron microscope operating at 20 and 30 kV. The morphologies and nanostructure were further characterized with a JEM-1200EX TEM model and a JEOL-2010 HR-TEM model using an accelerating voltage of 80 and 200 kV, respectively. SAEDS (selected area energy dispersive X-ray analysis) of one individual particle was conducted using a JEOL-2010 HR-TEM model. The Olympus BX41 (Jobin Yvon Horiba) Raman spectrometer was employed, using the 514.5 nm line of an Ar laser as the excitation source to analyze the nature of the carbon present around the core particle.
The X-ray diffraction (XRD) pattern of the as-prepared NiO/MgO@C core/shell nanocomposites is shown in Fig. 1(a). Sharp and well-defined peaks are observed at 2 values of 36.95, 42.91, 62.64, 75.00, and 79.00◦ corresponding to planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2), respectively, according to JCPDS No. 024-0712 of NiO/MgO. All these peaks could be indexed to a facecentered cubic structure with a lattice constant of a = 4.192A◦ of NiO/MgO. The obtained lattice constants matched well with the JCPDS values of the bulk materials of the NiO/MgO. Due to their very similar structure, not much difference is observed in the XRD patterns of NiO and NiO/MgO. Therefore, the formation of NiO/MgO can be identified using the above diffraction peaks. Fig. 1(b) shows the XRD patterns of the Pd doped on the NiO/MgO@C core/shell structure. The strong diffraction peaks observed at 2 values of 40.10◦ , 46.49◦ and 68.08◦ , correspond to the (1 1 1), (2 0 0) and (2 2 0) facets of Pd nanocrystals. The application of the Debye–Scherrer formula to the strongest diffraction peaks for Pd at 39.9◦ gives a crystallite size of 12.0 nm.
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Fig. 2. SEM images of NiO/MgO@C core/shell structure (a–c), EDAX spectrum of NiO/MgO@C core/shell structure (d).
3.2. SEM and TEM SEM and TEM images for the prepared materials are shown in Figs. 2 and 3. Fig. 2(a)–(c) shows the SEM images of NiO/MgO at different magnifications. The core–shell morphology is clearly distinct in the product. The obtained EDAX (Fig. 2(d)) analysis confirmed the presence of C, Ni, Mg and O elements in the synthesized composites. The calculated atomic ratio of MgO (0.071):NiO (0.069) from EDAX is more or less 1:1, which is comparable to the precursors ratio. The SEM and TEM images of the Pd/(NiO/MgO@C) catalysts are shown in Fig. 3(a)–(c). The SEM image (Fig. 3(a)) shows that the Pd nanoparticles are dispersed above the NiO/MgO@C core/shell structures. Because of the lower magnification, the shape and size of the Pd particles cannot be not clearly seen. The TEM images (Fig. 3(b) and (c)) of Pd/(NiO/MgO@C) catalysts show that the materials contain rod- and film-shaped Pd particles deposited on the core–shell structure of NiO/MgO@C. The measurements of the energy dispersive spectra of Pd (NiO/MgO@C) catalysts indicated that both the
Pd/C catalysts contain about 20.0 wt% of Pd, illustrating that almost all the PdCl2 has been reduced. The HRTEM images of NiO/MgO@C are shown in Fig. 4 and corroborate the morphologies observed in the SEM/HRSEM images in Fig. 2. The core/shell structure is clearly visible in the TEM pictures (Fig. 4(a)–(c)). The nearly spherical nanocrystals are agglomerated within the carbon shell (Fig. 4(a)). The dimensions of the nanocrystal core are estimated to be 20 ± 5 nm. The size of the C shell observed in Fig. 4(c) is 5.0 nm. Fig. 4(a)–(c) presents the HRTEM images of a nanocrystalline NiO/MgO@C composite. It illustrates the well-defined lattice fringes, indicating the high crystalline nature of the materials. The measured inter-planar spacing “d” ˚ which corresponds to the lattice plane of (2 0 0) value is 2.09 A, with the face-cantered cubic phase of the NiO–MgO material. These values are in agreement with the reported JCPDS value of d200 = 2.12 A˚ (JCPDS No. 024-0712 of NiO/MgO). The selected area electron diffraction (SAED) pattern (Fig. 4(d)) shows the formation of ring patterns, which infer the polycrystalline characteristic of the materials.
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Fig. 3. SEM images of Pd/(NiO/MgO@C) core/shell structure (a), TEM images of Pd/(NiO/MgO@C) (b and c) and EDAX spectrum of Pd/(NiO/MgO@C) core/shell structure (d).
3.3. Raman spectra Fig. 5(a) and (b) shows the room-temperature Raman spectra of the products, where the bands in 100–1300 cm−1 and 1200–1700 cm−1 corresponding to the spectral regions of the NiO/MgO core and carbon shell, respectively. The Raman spectra of the synthesized NiO/MgO@C (Fig. 5(a)) show peaks at 530, 720, 880 and 1080 cm−1 , while the literature values of the Raman bands of NiO/MgO are 490, 720, 860, 980 and 1050 cm−1 [54,55]. The observed peaks of the NiO–MgO@C core/shell are shifted to the red, or unchanged, when compared to commercial NiO/MgO (Fig. 5(a)). This may be due the interaction between NiO and MgO when they are mixed together. The Raman spectrum in the region 1100–1700 cm−1 is characteristic of graphitic and disordered carbon [56]. The Raman peaks at 1330 and 1600 cm−1 are the D- and G-bands (Fig. 5(b)). The D-band is attributed to the Raman-inactive A1g vibration mode assigned to the vibrations of carbon atoms with dangling bonds in planar terminations of disordered graphite [57]. The G-band is the Raman active optical mode, E2g , of twodimensional graphite, and which is closely related to the vibrations in sp2 -bonded carbon atoms [58]. 3.4. Electrochemical ethanol oxidation The Pd/(NiO/MgO@C) catalyst performance for the electrocatalytic oxidation of 1 M of ethanol in 1 M of KOH was evaluated by cyclic voltammetry (Fig. 6) at a scan rate of 10 mV s−1 . The following parameters, including the onset potential (Eonset ), the
forward peak potential (Ef ), the backward peak potential (Eb ), and the forward peak current intensity (If ) expressed in mA cm−2 , are shown in Table 1. The Pd/(NiO–MgO@C) catalyst showed a better activity for ethanol oxidation than Pd/C. The onset potential for the ethanol oxidation reaction on a Pd/(NiO/MgO@C) electrocatalyst was 200 mV more negative in comparison to that of a Pd/C electro-catalyst. Values for Pd–NiO/C, similar to those of Pd–NiO/MgO@C, were previously reported [50]. It is clear that the involvement of metal oxides significantly increased the catalytic activity at the same Pd loadings. The peak current density for ethanol oxidation, 69.3 mA cm−2 for the Pd/(NiO–MgO@C) catalyst, was higher than on the Pd/C electrode (47.8 mA cm−2 ). The forward peak potential for the electro-oxidation of ethanol on the Pd/(NiO/MgO@C) electrode was −0.17 V (vs. Hg/HgO), lower than −0.03 V (vs. Hg/HgO) for the Pd/C catalyst. The high peak oxidation current and low anodic peak potential show that Pd/(NiO/MgO@C) electrocatalysts were more active than the Pd/C electrode. Moreover, the higher catalytic currents at more negative potentials on Pd/(NiO/MgO@C) electrocatalysts could potentially improve DEFC efficiency. Fig. 7 shows the current density–time curves measured at a constant potential of −0.2 V. The current density of ethanol oxidation for the Pd–NiO/MgO@C composite catalyst was higher than that for the Pd/C catalyst, as found above in the cyclic voltammetry tests. After polarization for 60 min, Pd/(NiO/MgO@C) and Pd/C catalysts reached their steady-state current densities of 33 and 25 mA cm−2 at a potential of −0.2 V, respectively. The data indicate that the Pd/(NiO/MgO@C) composite catalyst has a higher catalytic activity
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Fig. 4. HRTEM images of NiO/MgO@C core/shell structure (a–c), SAED patterns of NiO/MgO@C core/shell structure (d).
for ethanol oxidation than the Pd/C catalyst, owing to the addition of a core shell structure and oxide. 3.5. Formation mechanism of NiO/MgO@C core–shell nanocomposites The decomposition of metal acetates and metal acetylacetonates under their autogenic pressure at elevated temperatures yielded two different products, depending on the nature of the metal. The decomposition of the acetates of noble metals, or metals having a positive reduction potential, yields the metal as a core and carbon as the shell. These results were obtained for silver acetate, copper
acetate, and even for nickel and cobalt acetates. On the other hand, when the decomposition of the acetates of strong metals (standard reduction potential <−0.3 V) such as zinc and iron is conducted under the autogenic pressure, the core that is obtained is composed of their oxides. To obtain the metallic core for these strong metals, the decomposition of the alkyl-metal compound is conducted under their autogenic pressure, and metallic cores (zinc, cadmium, and aluminum) surrounded by a carbon shell are obtained. In the current case, magnesium as a strong metal would yield MgO as the core, but it is surprising to find nickel oxide as its partner, and not metallic nickel. It is suggested that the MgO aids (catalyzes) the oxidation of the nickel. It is also possible that since the reaction is
Table 1 Comparison of activity of ethanol oxidation between Pd/C and Pd/(NiO/MgO@C) electrodes. S. no.
Electrode
Onset potential (V)
Activitya Forward sweep If (mA cm−2 )
1 2 3 4 a b c
Pd/C Pd/(NiO–MgO@C) Pd–NiO(6:1)/Cb Pt–MgO(4:1)/Cc
−0.5 −0.7 −0.62 −0.58
Activity evaluated from cyclic voltammogram in 1 M KOH/1 M ETOH. From Ref. [50]. From Ref. [51].
47.8 69.3 95 51
Reverse sweep Ef (V) −0.03 −0.13 −0.08 –
Ib (mA cm−2 ) 72.5 52.9 – –
Eb (V) −0.14 −0.2 – –
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carried out at a lower temperature (600 ◦ C instead of 700 ◦ C), the carbon cannot reduce the nickel oxide to metallic nickel. 4. Conclusion Carbon-coated NiO/MgO core/shell nanostructures were synthesized by the single-step RAPET technique. For electrochemical ethanol oxidation, a Pd electrocatalyst was doped on NiO/MgO@C by a simple reduction method. XRD and Raman spectral studies of the as-prepared samples confirmed the formation of NiO/MgO@C, whereas TEM studies revealed a spherical shape NiO/MgO@C core/shell structure. The high order crystallinity of these materials has been confirmed by HRTEM images. The electrocatalytic properties of Pd/(NiO/MgO@C) composites for ethanol oxidation were investigated in an alkaline solution. Electrocatalytic studies show that the prepared Pd/(NiO/MgO@C) composites have an excellent electrocatalytic activity and stability in alkaline media. Fig. 5. Raman spectra of NiO/MgO@C core (a) and C-shell (b).
References 80
Current density (mA/cm 2)
(a) Pd/NiO-MgO@C (b) Pd/C
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(b) 40
20
0
-0.8
-0.6
-0.4
-0.2
0.0
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Potential (V) Vs Hg/HgO Fig. 6. Cyclic voltammograms for ethanol electro-oxidation on (a) Pd/(NiO–MgO@C) and (b) Pd/C in 1 M KOH/1 M ETOH solution, at a scan rate of 10 mV s−1 (recorded after 20 scans), 25 ◦ C.
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2
Current density (mA/cm )
60
50
40
30
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20
10 0
500
1000
1500
2000
2500
3000
3500
Time (seconds) Fig. 7. Current density–time curves at −0.2 V for 3600 s (a) Pd/(NiO–MgO@C) and (b) Pd/C in 1 M KOH/1 M ETOH solutions, 25 ◦ C.
[1] P.D. Cozzoli, T. Pellegrino, L. Manna, Chem. Soc. Rev. 35 (2006) 119. [2] Y.W. Jun, J.S. Choi, J.W. Cheon, Y.W. Jun, J.S. Choi, J. Cheon, Chem. Commun. 121 (2007) 203. [3] H.W. Gu, M. Yang, J.H. Gao, C.K. Chang, B. Xu, J. Am. Chem. Soc. 127 (2005) 34. [4] F. Caruso, Adv. Mater. 13 (2001) 11. [5] A.H. Lu, E.L. Salabas, F. Schuth, Angew. Chem. Int. Ed. 46 (2007) 1222. [6] L.M. Liz-Marzaˇı n, P. Mulvaney, J. Phys. Chem. B 107 (2003) 7312. [7] Y.D. Yin, Y. Lu, Y.G. Sun, Y.N. Xia, Nano Lett. 2 (2002) 427. [8] F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111. [9] S.R. Hall, S.A. Davis, S. Mann, Langmuir 16 (2000) 1454. [10] O. Palchik, G. Kataby, Y. Mastai, A. Gedanken, Adv. Mater. 11 (1999) 1289. [11] J. Jang, B. Lim, Angew. Chem. Int. Ed. Engl. 42 (2003) 5600. [12] Z.H. Wang, C.J. Choi, B.K. Kim, J.C. Kim, Z.D. Zhang, Carbon 41 (9) (2003) 1751. [13] R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (5093) (1993) 346. [14] E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis, H. Mo¨hwald, Angew. Chem. Int. Ed. 37 (16) (1998) 2201. [15] I. Gill, A. Ballesteros, J. Am. Chem. Soc. 120 (34) (1998) 8587. [16] S. Decker, K.J. Klabunde, J. Am. Chem. Soc. 118 (1996) 12465. [17] C.L. Carnes, K.J. Klabunde, Chem. Mater. 14 (2002) 1806. [18] Y. Kawaguchi, Solid State Commun. 117 (2000) 17. [19] S.H.C. Liang, I.D. Gay, J. Catal. 101 (1986) 293. [20] A.N. Copp, Am. Ceram. Soc. Bull. 74 (1995) 135. [21] B. Sheela, H. Gomathi, G.P. Rao, J. Electroanal. Chem. 394 (1995) 267. [22] M. Yoshio, Y. Todorov, K. Yamato, H. Noguchi, J. Itoh, M. Okada, T. Mouri, J. Power Sources 74 (1998) 46. [23] R.C. Makkus, K. Hemmes, J.H.W.D. Wir, J. Electrochem. Soc. 141 (1994) 3429. [24] E.R. Beach, S. Krenar, S.E. Brown, S.J. Rozeveld, P.A. Morris, Mater. Chem. Phys. 115 (2009) 371. [25] K. Oka, T. Yanagida, K. Nagashima, H. Tanaka, T. Kawai, J. Am. Chem. Soc. 131 (2009) 3434. [26] E. Liu, W. Li, J. Li, X. Meng, R. Ding, S. Tan, Mater. Res. Bull. 44 (2009) 1122. [27] M. Salavati-Niasari, N. Mir, F. Davar, Polyhedron 28 (2009) 1111. [28] V. Plank, H.J. Snaith, C. Ducati, J.S. Bendal, L. Schmidt-Mende, M.E. Welland, Nanotechnology 19 (2008) 465603. [29] K.H. Woo, W.L. Jong, H.S. Seung, J. Phy, Chem. Solids 69 (2008) 1491. [30] Y.J. Zeng, Z.Z. Ye, F. Liu, D.Y. Li, Y.F. Lu, W. Jaeger, H.P. He, L.P. Zhu, J.Y. Huang, B.H. Zhao, Cryst. Growth Des. 9 (2009) 263. [31] A. Nakao, Y. Tanaka, A. Ide-Ektessabi, Surf. Coatings Technol. 203 (2009) 2731. [32] S.Q. Song, P. Tsiakaras, Appl. Catal. B 63 (2006) 187. [33] A.S. Arico, S. Srinavasan, V. Antonucci, Fuel Cells 1 (2001) 133. [34] B. Grgur, N. Markovic, P.N. Ross, Electrochim. Acta 43 (1998) 3631. [35] M. Gotz, H. Wendt, Electrochim. Acta 43 (1998) 3637. [36] J. Bett, H. Kunz, A. Aldykiewicz, J. Fenton, W. Bailey, D. McGrath, Electrochim. Acta 43 (1998) 3645. [37] S.S. Gupta, J. Datta, J. Power Sources 145 (2005) 124. [38] J. Liu, J. Ye, C. Xu, S.P. Jiang, Y. Tong, Electrochem. Commun. 9 (2007) 2334. [39] C. Xu, H. Wang, P.K. Shen, S.P. Jiang, Adv. Mater. 19 (2007) 4256. [40] M.T. Reetz, E. Westermann, Angew. Chem., Int. Ed. 39 (2000) 165. [41] D. Astruc, Inorg. Chem. 46 (2007) 1884. [42] R. Franzen, Can. J. Chem. 78 (2000) 957. [43] Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2 (2000) 2385. [44] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, Electrochem. Commun. 7 (2005) 905. [45] T. Maiyalagan, Appl. Catal. B: Environ. 89 (2008) 286. [46] T. Maiyalagan, J. Power Sources 179 (2008) 443. [47] T. Maiyalagan, B. Viswanathan, J. Power Sources 175 (2008) 789. [48] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, J. Nanosci. Nanotechnol. 6 (2006) 2067.
C. Mahendiran et al. / Materials Chemistry and Physics 128 (2011) 341–347 [49] F.P. Hu, C.L. Chen, Z.Y. Wang, G.Y. Wei, P.K. Shen, Electrochim. Acta 52 (2006) 1087. [50] C. Xu, P.K. Shen, Y.L. Liu, J. Power Sources 164 (2007) 527. [51] C. Xu, P.K. Shen, X.H. Ji, R. Zeng, Y.L. Liu, Electrochem. Commun. 7 (2005) 1305. [52] F. Hu, F. Ding, S. Song, P.K. Shen, J. Power Sources 163 (2006) 415. [53] D. Chu, J. Wang, S. Wang, L. Zha, J. He, Y. Hou, Y. Yan, H. Lin, Z. Tian, Catal. Commun. 10 (2008) 955.
347
[54] E. Cazzanelli1, A. Kuzmin, G. Mariotto, N. Mironova-Ulmane, J. Phys. Condens. Matter 15 (2003) 2045. [55] N.B. Manson, W. Von der ohe, S.L. Chodos, Phys. Rev. B 3 (1971) 1698. [56] V.G. Pol, S.V. Pol, N. Perkas, A. Gedanken, J. Phys. Chem. C 111 (2007) 134. [57] N. Shimodaira, A. Masui, J. Appl. Phys. 92 (2002) 902. [58] V. Barbarossa, F. Galluzzi, R. Tomaciello, A. Zanobi, Chem. Phys. Lett. 185 (1991) 53.
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Synthesis of a carbon-coated NiO/MgO core/shell nanocomposite as a Pd electro-catalyst support for ethanol oxidation C. Mahendiran a , T. Maiyalagan b , K. Scott b , A. Gedanken a,∗ a Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan University Center for Advanced Materials and Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel b School of Chemical Engineering & Advanced Materials, University of Newcastle Upon Tyne, Newcastle Upon Tyne NE1 7RU, United Kingdom
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Article history: Received 2 June 2010 Received in revised form 13 February 2011 Accepted 16 February 2011 Keywords: Core–shell structure Nanoparticles Ethanol oxidation
a b s t r a c t Carbon coated on NiO/MgO in a core/shell nanostructure was synthesized by the single-step RAPET (reaction under autogenic pressure at elevated temperatures) technique, and the obtained formation mechanism of the core/shell nanocomposite was presented. The carbon-coated NiO/MgO and its supported Pd catalyst, Pd/(NiO/MgO@C), were characterized by SEM, HR-TEM, XRD and cyclic voltammetry. The X-ray diffraction patterns confirmed the face-centered cubic crystal structure of NiO/MgO. Raman spectroscopy measurements provided structural evidence for the formation of a NiO/MgO composite and the nature of the coated carbon shell. The high-resolution transmission electron microscopy images showed the core and shell morphologies individually. The electrocatalytic properties of the Pd/(NiO/MgO@C) catalyst for ethanol oxidation were investigated in an alkaline solution. The results indicated that the prepared Pd–NiO/MgO@C catalyst has excellent electrocatalytic activity and stability. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, the focus on the chemical synthesis of nanoparticles has shifted from single component nanoparticles to hybrid nanoparticles. Hybrid nanoparticles usually contain two or more different nanoscale domains, which lead to synergistic properties due to their interfacial interactions. Therefore, hybrid nanoparticles (non-symmetrical dimmers, symmetric core–shell and any other hetero-structures) are promising due to their multi-function and designable properties [1–3]. The design of core/shell structured composites has received much attention as a means to improve the stability and surface chemistry of the core materials, and as a way of obtaining unique structures, properties, and applications via a combination of the different characteristics of the components that are not available with their single-component counterparts [4–7]. The synthesis and characterization of hybrid nanostructure material is aimed at designing and arranging nanomaterials in complex functional structures, resulting in unique physical properties [8–11]. Carbon-coated nanomaterials are of great interest due to their stability towards oxidation and degradation [12–15]. The creation of core/shell nanostructures containing bi-metal oxides has greatly enhanced the efficiency of these structures over pure single metal oxide particles as destructive adsorbents for environmental pol-
∗ Corresponding author. E-mail address: [email protected] (A. Gedanken). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.02.067
lutants, such as SO2 and H2 S [16,17]. Magnesium oxide (MgO) is a typical wideband-gap insulator. It has found many important applications for use in catalysis, refractory materials, toxic waste remediation, paint, superconductors, and substrates for thin film growth [18–20]. NiO is a very successful material used in various fields, such as catalysis [21], battery cathodes [22] and fuel cell electrodes [23], and it is also widely studied as an ant ferromagnetic as it is a chemically-stable ionic insulator with high thermal stability and good resistance to corrosion. Many researchers have prepared NiO and MgO nanomaterials by various methods, e.g., the solvo-thermal technique [24], the laser technique [25], the sol–gel method [26] and thermal decomposition [27]. The preparation of core–shell nano-wire structures of ZnO/MgO [28] obtained by hydrothermal, atomic layer deposition [29] and chemical vapor deposition (CVD) [30] methods has already been reported. NiO/MgO thin films were prepared by an ion beam-assisted deposition technique and were used for chemical transformations and catalysis applications [31]. Direct ethanol fuel cells (DEFs) have attracted much attention because ethanol has a low toxicity, as compared to the methanol used in direct methanol fuel cells (DMFs). Ethanol can be easily produced in great quantities by the fermentation of sugar-containing raw materials [32]. Pt-based catalysts are extensively studied for electrocatalytic fuel cell applications. Indeed, platinum is very efficient for the different reactions involved in fuel cells, such as methanol oxidation and oxygen reduction [33]. However, it is well known that the anode activity of Pt is adversely affected by poisoning from CO species. Several studies have investigated the addition
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of Ru or other metals to Pt, in order to improve the activity of the catalysts [34–36]. Nevertheless, it has been shown recently that palladium is more active than platinum for ethanol oxidation in basic media [37–39]. As Pd is 50 times more abundant in the earth and cheaper than Pt, Pd-based nanomaterials are potential electrocatalysts for replacing Pt in DAFCs. Palladium also plays a vital role in catalysis and is involved in various reactions, such as Heck, Suzuki, and Stille coupling [40–43]. In order to reduce the amount of noble-metal loading on the electrodes, improvements to the dispersion of the noble metal on the support have been investigated. Metal nanoparticles have been dispersed on a wide variety of substrates, such as carbon nanomaterials [44,45], polymer matrices [46], and oxide matrices [47,48]. Recently, metal oxides such as CeO2 [49], NiO [50], MgO [51], TiO2 [52] and In2 O3 [53] have been used as electrocatalysts for the direct oxidation of alcohol, and which resulted in a significantly improved electrode performance in terms of the enhanced reaction activity. The noble-metal shell in the core/shell nanoparticles has two roles. First, it protects the non-noble core from contact with the alkaline electrolyte. Second, the coated Pd on the shell should improve the catalytic properties of the substrate. We therefore report herein on a one-stage, reproducible, solvent free, competent, and straightforward approach for the synthesis of NiO/MgO@C core shell nanostructures, followed by its decoration by Pd using simple impregnation. The prepared material was evaluated for the electrochemical ethanol oxidation reaction for fuel cell applications in alkaline media.
2. Experimental 2.1. Synthesis of a NiO/MgO@C core/shell nanocomposite The synthesis of a NiO–MgO@C core/shell nanocomposite was carried out by the thermal dissociation of a mixture of nickel acetate (Ni(C4 H6 O2 )2 ) and magnesium acetylacetonate (Mg(C5 H7 O2 )2 ), purchased from Sigma Aldrich and used as received. The 3 mL closed vessel cell was assembled from stainless steel Swagelok parts. A 1/2 union part was plugged from both sides by standard caps. In a typical synthesis of NiO/MgO@C core/shell material, 0.248 g of nickel acetate (0.072 mols) and 0.516 g of magnesium acetylacetonate (0.072 mol) were introduced into the cell at room temperature. The filled cell was closed tightly by the other plug and then placed inside an iron pipe at the center of the tubular furnace. The temperature was raised at a rate of 10 ◦ C min−1 . The closed vessel cell was heated at 600 ◦ C for 3 h. The reaction took place at the autogenic pressure of the precursor. The closed cell (Swagelok) was gradually cooled (∼5 h) to room temperature and opened. In fact, we expected to obtain 0.764 g of NiO/MgO@C for a 100% product yield, but during the chemical reaction inside the Swagelok some of the acetate molecules might have evaporated at the higher temperature. Thus, we achieved only 0.519 g (∼68%) of black-colored NiO/MgO@C. The synthesis was repeated five times in order to confirm the reproducibility of the process, and identical products were obtained.
Fig. 1. XRD of NiO/MgO@C core/shell structure (a) and Pd–NiO/MgO@C (b).
2.4. Preparation of the working electrode Glassy carbon (GC) (Bas Electrode, 0.07 cm2 ) was polished to a mirror finish with 0.05 m alumina suspensions before each experiment and served as an underlying substrate of the working electrode. The working electrodes were fabricated by coating Nafion-impregnated catalyst ink on a glassy carbon electrode. Thin-film electrodes were prepared by dispersing 35 mg of the catalyst in 800 L of deionized water. 200 L of a 5 wt% Nafion solution was then added and ultrasonicated for 20 min. A known amount of the suspension was added to the glassy carbon (GC) electrode and slowly dried in air. The loading of the noble metal catalyst was 0.3 mg cm−2 . A solution of 1 M of KOH containing 1 M of ethanol was used to study the activity of ethanol oxidation. 2.5. Electrochemical measurements Electrochemical measurements were collected using a computer-controlled Gill AC potentiostat (ACM Instruments Ltd., Cumbria, UK). All experiments were carried out in a glass cell using a traditional three-electrode assembly incorporating a glassy carbon working electrode (0.07 cm2 ), a mercury–mercury oxide reference electrode, and a platinum mesh (25 mm × 25 mm) counter electrode. All electrochemical experiments were carried out at room temperature in 1 M of aqueous KOH. The electrolyte solution was purged with high pure nitrogen for 30 min prior to a series of voltammetric experiments. The current densities reported are based on the geometric area. Triply-distilled water was used throughout for the preparation of solutions.
3. Results and discussion 3.1. XRD
2.2. Preparation of Pd/(NiO/MgO@C) catalysts 3.13 mL of 0.045 mol L−1 PdCl2 were dissolved in 10 mL of ultrapure water and 60 mg of NiO/MgO@C was added. The solution was mixed by magnetic stirring for 3 h and ultrasonicated to achieve 20% Pd loading. A freshly prepared aqueous solution of NaBH4 was added drop wise under N2 atmosphere to the solution to reduce the Pd species to form the electrocatalyst, which was then repeatedly washed with distilled water to remove residual salts, centrifuged, and dried at 70 ◦ C.
2.3. Structural characterizations XRD patterns were collected using a Bruker AXS D* Advance Powder X-ray ˚ High-resolution scanning diffractometer (Cu KR radiation, wavelength 1.5406 A). electron microscopy (HRSEM) measurements of the products were carried out on a JEOL-JSM 840 scanning electron microscope operating at 20 and 30 kV. The morphologies and nanostructure were further characterized with a JEM-1200EX TEM model and a JEOL-2010 HR-TEM model using an accelerating voltage of 80 and 200 kV, respectively. SAEDS (selected area energy dispersive X-ray analysis) of one individual particle was conducted using a JEOL-2010 HR-TEM model. The Olympus BX41 (Jobin Yvon Horiba) Raman spectrometer was employed, using the 514.5 nm line of an Ar laser as the excitation source to analyze the nature of the carbon present around the core particle.
The X-ray diffraction (XRD) pattern of the as-prepared NiO/MgO@C core/shell nanocomposites is shown in Fig. 1(a). Sharp and well-defined peaks are observed at 2 values of 36.95, 42.91, 62.64, 75.00, and 79.00◦ corresponding to planes of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2), respectively, according to JCPDS No. 024-0712 of NiO/MgO. All these peaks could be indexed to a facecentered cubic structure with a lattice constant of a = 4.192A◦ of NiO/MgO. The obtained lattice constants matched well with the JCPDS values of the bulk materials of the NiO/MgO. Due to their very similar structure, not much difference is observed in the XRD patterns of NiO and NiO/MgO. Therefore, the formation of NiO/MgO can be identified using the above diffraction peaks. Fig. 1(b) shows the XRD patterns of the Pd doped on the NiO/MgO@C core/shell structure. The strong diffraction peaks observed at 2 values of 40.10◦ , 46.49◦ and 68.08◦ , correspond to the (1 1 1), (2 0 0) and (2 2 0) facets of Pd nanocrystals. The application of the Debye–Scherrer formula to the strongest diffraction peaks for Pd at 39.9◦ gives a crystallite size of 12.0 nm.
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Fig. 2. SEM images of NiO/MgO@C core/shell structure (a–c), EDAX spectrum of NiO/MgO@C core/shell structure (d).
3.2. SEM and TEM SEM and TEM images for the prepared materials are shown in Figs. 2 and 3. Fig. 2(a)–(c) shows the SEM images of NiO/MgO at different magnifications. The core–shell morphology is clearly distinct in the product. The obtained EDAX (Fig. 2(d)) analysis confirmed the presence of C, Ni, Mg and O elements in the synthesized composites. The calculated atomic ratio of MgO (0.071):NiO (0.069) from EDAX is more or less 1:1, which is comparable to the precursors ratio. The SEM and TEM images of the Pd/(NiO/MgO@C) catalysts are shown in Fig. 3(a)–(c). The SEM image (Fig. 3(a)) shows that the Pd nanoparticles are dispersed above the NiO/MgO@C core/shell structures. Because of the lower magnification, the shape and size of the Pd particles cannot be not clearly seen. The TEM images (Fig. 3(b) and (c)) of Pd/(NiO/MgO@C) catalysts show that the materials contain rod- and film-shaped Pd particles deposited on the core–shell structure of NiO/MgO@C. The measurements of the energy dispersive spectra of Pd (NiO/MgO@C) catalysts indicated that both the
Pd/C catalysts contain about 20.0 wt% of Pd, illustrating that almost all the PdCl2 has been reduced. The HRTEM images of NiO/MgO@C are shown in Fig. 4 and corroborate the morphologies observed in the SEM/HRSEM images in Fig. 2. The core/shell structure is clearly visible in the TEM pictures (Fig. 4(a)–(c)). The nearly spherical nanocrystals are agglomerated within the carbon shell (Fig. 4(a)). The dimensions of the nanocrystal core are estimated to be 20 ± 5 nm. The size of the C shell observed in Fig. 4(c) is 5.0 nm. Fig. 4(a)–(c) presents the HRTEM images of a nanocrystalline NiO/MgO@C composite. It illustrates the well-defined lattice fringes, indicating the high crystalline nature of the materials. The measured inter-planar spacing “d” ˚ which corresponds to the lattice plane of (2 0 0) value is 2.09 A, with the face-cantered cubic phase of the NiO–MgO material. These values are in agreement with the reported JCPDS value of d200 = 2.12 A˚ (JCPDS No. 024-0712 of NiO/MgO). The selected area electron diffraction (SAED) pattern (Fig. 4(d)) shows the formation of ring patterns, which infer the polycrystalline characteristic of the materials.
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Fig. 3. SEM images of Pd/(NiO/MgO@C) core/shell structure (a), TEM images of Pd/(NiO/MgO@C) (b and c) and EDAX spectrum of Pd/(NiO/MgO@C) core/shell structure (d).
3.3. Raman spectra Fig. 5(a) and (b) shows the room-temperature Raman spectra of the products, where the bands in 100–1300 cm−1 and 1200–1700 cm−1 corresponding to the spectral regions of the NiO/MgO core and carbon shell, respectively. The Raman spectra of the synthesized NiO/MgO@C (Fig. 5(a)) show peaks at 530, 720, 880 and 1080 cm−1 , while the literature values of the Raman bands of NiO/MgO are 490, 720, 860, 980 and 1050 cm−1 [54,55]. The observed peaks of the NiO–MgO@C core/shell are shifted to the red, or unchanged, when compared to commercial NiO/MgO (Fig. 5(a)). This may be due the interaction between NiO and MgO when they are mixed together. The Raman spectrum in the region 1100–1700 cm−1 is characteristic of graphitic and disordered carbon [56]. The Raman peaks at 1330 and 1600 cm−1 are the D- and G-bands (Fig. 5(b)). The D-band is attributed to the Raman-inactive A1g vibration mode assigned to the vibrations of carbon atoms with dangling bonds in planar terminations of disordered graphite [57]. The G-band is the Raman active optical mode, E2g , of twodimensional graphite, and which is closely related to the vibrations in sp2 -bonded carbon atoms [58]. 3.4. Electrochemical ethanol oxidation The Pd/(NiO/MgO@C) catalyst performance for the electrocatalytic oxidation of 1 M of ethanol in 1 M of KOH was evaluated by cyclic voltammetry (Fig. 6) at a scan rate of 10 mV s−1 . The following parameters, including the onset potential (Eonset ), the
forward peak potential (Ef ), the backward peak potential (Eb ), and the forward peak current intensity (If ) expressed in mA cm−2 , are shown in Table 1. The Pd/(NiO–MgO@C) catalyst showed a better activity for ethanol oxidation than Pd/C. The onset potential for the ethanol oxidation reaction on a Pd/(NiO/MgO@C) electrocatalyst was 200 mV more negative in comparison to that of a Pd/C electro-catalyst. Values for Pd–NiO/C, similar to those of Pd–NiO/MgO@C, were previously reported [50]. It is clear that the involvement of metal oxides significantly increased the catalytic activity at the same Pd loadings. The peak current density for ethanol oxidation, 69.3 mA cm−2 for the Pd/(NiO–MgO@C) catalyst, was higher than on the Pd/C electrode (47.8 mA cm−2 ). The forward peak potential for the electro-oxidation of ethanol on the Pd/(NiO/MgO@C) electrode was −0.17 V (vs. Hg/HgO), lower than −0.03 V (vs. Hg/HgO) for the Pd/C catalyst. The high peak oxidation current and low anodic peak potential show that Pd/(NiO/MgO@C) electrocatalysts were more active than the Pd/C electrode. Moreover, the higher catalytic currents at more negative potentials on Pd/(NiO/MgO@C) electrocatalysts could potentially improve DEFC efficiency. Fig. 7 shows the current density–time curves measured at a constant potential of −0.2 V. The current density of ethanol oxidation for the Pd–NiO/MgO@C composite catalyst was higher than that for the Pd/C catalyst, as found above in the cyclic voltammetry tests. After polarization for 60 min, Pd/(NiO/MgO@C) and Pd/C catalysts reached their steady-state current densities of 33 and 25 mA cm−2 at a potential of −0.2 V, respectively. The data indicate that the Pd/(NiO/MgO@C) composite catalyst has a higher catalytic activity
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Fig. 4. HRTEM images of NiO/MgO@C core/shell structure (a–c), SAED patterns of NiO/MgO@C core/shell structure (d).
for ethanol oxidation than the Pd/C catalyst, owing to the addition of a core shell structure and oxide. 3.5. Formation mechanism of NiO/MgO@C core–shell nanocomposites The decomposition of metal acetates and metal acetylacetonates under their autogenic pressure at elevated temperatures yielded two different products, depending on the nature of the metal. The decomposition of the acetates of noble metals, or metals having a positive reduction potential, yields the metal as a core and carbon as the shell. These results were obtained for silver acetate, copper
acetate, and even for nickel and cobalt acetates. On the other hand, when the decomposition of the acetates of strong metals (standard reduction potential <−0.3 V) such as zinc and iron is conducted under the autogenic pressure, the core that is obtained is composed of their oxides. To obtain the metallic core for these strong metals, the decomposition of the alkyl-metal compound is conducted under their autogenic pressure, and metallic cores (zinc, cadmium, and aluminum) surrounded by a carbon shell are obtained. In the current case, magnesium as a strong metal would yield MgO as the core, but it is surprising to find nickel oxide as its partner, and not metallic nickel. It is suggested that the MgO aids (catalyzes) the oxidation of the nickel. It is also possible that since the reaction is
Table 1 Comparison of activity of ethanol oxidation between Pd/C and Pd/(NiO/MgO@C) electrodes. S. no.
Electrode
Onset potential (V)
Activitya Forward sweep If (mA cm−2 )
1 2 3 4 a b c
Pd/C Pd/(NiO–MgO@C) Pd–NiO(6:1)/Cb Pt–MgO(4:1)/Cc
−0.5 −0.7 −0.62 −0.58
Activity evaluated from cyclic voltammogram in 1 M KOH/1 M ETOH. From Ref. [50]. From Ref. [51].
47.8 69.3 95 51
Reverse sweep Ef (V) −0.03 −0.13 −0.08 –
Ib (mA cm−2 ) 72.5 52.9 – –
Eb (V) −0.14 −0.2 – –
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carried out at a lower temperature (600 ◦ C instead of 700 ◦ C), the carbon cannot reduce the nickel oxide to metallic nickel. 4. Conclusion Carbon-coated NiO/MgO core/shell nanostructures were synthesized by the single-step RAPET technique. For electrochemical ethanol oxidation, a Pd electrocatalyst was doped on NiO/MgO@C by a simple reduction method. XRD and Raman spectral studies of the as-prepared samples confirmed the formation of NiO/MgO@C, whereas TEM studies revealed a spherical shape NiO/MgO@C core/shell structure. The high order crystallinity of these materials has been confirmed by HRTEM images. The electrocatalytic properties of Pd/(NiO/MgO@C) composites for ethanol oxidation were investigated in an alkaline solution. Electrocatalytic studies show that the prepared Pd/(NiO/MgO@C) composites have an excellent electrocatalytic activity and stability in alkaline media. Fig. 5. Raman spectra of NiO/MgO@C core (a) and C-shell (b).
References 80
Current density (mA/cm 2)
(a) Pd/NiO-MgO@C (b) Pd/C
(a)
60
(b) 40
20
0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Potential (V) Vs Hg/HgO Fig. 6. Cyclic voltammograms for ethanol electro-oxidation on (a) Pd/(NiO–MgO@C) and (b) Pd/C in 1 M KOH/1 M ETOH solution, at a scan rate of 10 mV s−1 (recorded after 20 scans), 25 ◦ C.
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(a) Pd/NiO-MgO@C (b) Pd/C
2
Current density (mA/cm )
60
50
40
30
(a) (b)
20
10 0
500
1000
1500
2000
2500
3000
3500
Time (seconds) Fig. 7. Current density–time curves at −0.2 V for 3600 s (a) Pd/(NiO–MgO@C) and (b) Pd/C in 1 M KOH/1 M ETOH solutions, 25 ◦ C.
[1] P.D. Cozzoli, T. Pellegrino, L. Manna, Chem. Soc. Rev. 35 (2006) 119. [2] Y.W. Jun, J.S. Choi, J.W. Cheon, Y.W. Jun, J.S. Choi, J. Cheon, Chem. Commun. 121 (2007) 203. [3] H.W. Gu, M. Yang, J.H. Gao, C.K. Chang, B. Xu, J. Am. Chem. Soc. 127 (2005) 34. [4] F. Caruso, Adv. Mater. 13 (2001) 11. [5] A.H. Lu, E.L. Salabas, F. Schuth, Angew. Chem. Int. Ed. 46 (2007) 1222. [6] L.M. Liz-Marzaˇı n, P. Mulvaney, J. Phys. Chem. B 107 (2003) 7312. [7] Y.D. Yin, Y. Lu, Y.G. Sun, Y.N. Xia, Nano Lett. 2 (2002) 427. [8] F. Caruso, R.A. Caruso, H. Mohwald, Science 282 (1998) 1111. [9] S.R. Hall, S.A. Davis, S. Mann, Langmuir 16 (2000) 1454. [10] O. Palchik, G. Kataby, Y. Mastai, A. Gedanken, Adv. Mater. 11 (1999) 1289. [11] J. Jang, B. Lim, Angew. Chem. Int. Ed. Engl. 42 (2003) 5600. [12] Z.H. Wang, C.J. Choi, B.K. Kim, J.C. Kim, Z.D. Zhang, Carbon 41 (9) (2003) 1751. [13] R.S. Ruoff, D.C. Lorents, B. Chan, R. Malhotra, S. Subramoney, Science 259 (5093) (1993) 346. [14] E. Donath, G.B. Sukhorukov, F. Caruso, S.A. Davis, H. Mo¨hwald, Angew. Chem. Int. Ed. 37 (16) (1998) 2201. [15] I. Gill, A. Ballesteros, J. Am. Chem. Soc. 120 (34) (1998) 8587. [16] S. Decker, K.J. Klabunde, J. Am. Chem. Soc. 118 (1996) 12465. [17] C.L. Carnes, K.J. Klabunde, Chem. Mater. 14 (2002) 1806. [18] Y. Kawaguchi, Solid State Commun. 117 (2000) 17. [19] S.H.C. Liang, I.D. Gay, J. Catal. 101 (1986) 293. [20] A.N. Copp, Am. Ceram. Soc. Bull. 74 (1995) 135. [21] B. Sheela, H. Gomathi, G.P. Rao, J. Electroanal. Chem. 394 (1995) 267. [22] M. Yoshio, Y. Todorov, K. Yamato, H. Noguchi, J. Itoh, M. Okada, T. Mouri, J. Power Sources 74 (1998) 46. [23] R.C. Makkus, K. Hemmes, J.H.W.D. Wir, J. Electrochem. Soc. 141 (1994) 3429. [24] E.R. Beach, S. Krenar, S.E. Brown, S.J. Rozeveld, P.A. Morris, Mater. Chem. Phys. 115 (2009) 371. [25] K. Oka, T. Yanagida, K. Nagashima, H. Tanaka, T. Kawai, J. Am. Chem. Soc. 131 (2009) 3434. [26] E. Liu, W. Li, J. Li, X. Meng, R. Ding, S. Tan, Mater. Res. Bull. 44 (2009) 1122. [27] M. Salavati-Niasari, N. Mir, F. Davar, Polyhedron 28 (2009) 1111. [28] V. Plank, H.J. Snaith, C. Ducati, J.S. Bendal, L. Schmidt-Mende, M.E. Welland, Nanotechnology 19 (2008) 465603. [29] K.H. Woo, W.L. Jong, H.S. Seung, J. Phy, Chem. Solids 69 (2008) 1491. [30] Y.J. Zeng, Z.Z. Ye, F. Liu, D.Y. Li, Y.F. Lu, W. Jaeger, H.P. He, L.P. Zhu, J.Y. Huang, B.H. Zhao, Cryst. Growth Des. 9 (2009) 263. [31] A. Nakao, Y. Tanaka, A. Ide-Ektessabi, Surf. Coatings Technol. 203 (2009) 2731. [32] S.Q. Song, P. Tsiakaras, Appl. Catal. B 63 (2006) 187. [33] A.S. Arico, S. Srinavasan, V. Antonucci, Fuel Cells 1 (2001) 133. [34] B. Grgur, N. Markovic, P.N. Ross, Electrochim. Acta 43 (1998) 3631. [35] M. Gotz, H. Wendt, Electrochim. Acta 43 (1998) 3637. [36] J. Bett, H. Kunz, A. Aldykiewicz, J. Fenton, W. Bailey, D. McGrath, Electrochim. Acta 43 (1998) 3645. [37] S.S. Gupta, J. Datta, J. Power Sources 145 (2005) 124. [38] J. Liu, J. Ye, C. Xu, S.P. Jiang, Y. Tong, Electrochem. Commun. 9 (2007) 2334. [39] C. Xu, H. Wang, P.K. Shen, S.P. Jiang, Adv. Mater. 19 (2007) 4256. [40] M.T. Reetz, E. Westermann, Angew. Chem., Int. Ed. 39 (2000) 165. [41] D. Astruc, Inorg. Chem. 46 (2007) 1884. [42] R. Franzen, Can. J. Chem. 78 (2000) 957. [43] Y. Li, X.M. Hong, D.M. Collard, M.A. El-Sayed, Org. Lett. 2 (2000) 2385. [44] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, Electrochem. Commun. 7 (2005) 905. [45] T. Maiyalagan, Appl. Catal. B: Environ. 89 (2008) 286. [46] T. Maiyalagan, J. Power Sources 179 (2008) 443. [47] T. Maiyalagan, B. Viswanathan, J. Power Sources 175 (2008) 789. [48] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, J. Nanosci. Nanotechnol. 6 (2006) 2067.
C. Mahendiran et al. / Materials Chemistry and Physics 128 (2011) 341–347 [49] F.P. Hu, C.L. Chen, Z.Y. Wang, G.Y. Wei, P.K. Shen, Electrochim. Acta 52 (2006) 1087. [50] C. Xu, P.K. Shen, Y.L. Liu, J. Power Sources 164 (2007) 527. [51] C. Xu, P.K. Shen, X.H. Ji, R. Zeng, Y.L. Liu, Electrochem. Commun. 7 (2005) 1305. [52] F. Hu, F. Ding, S. Song, P.K. Shen, J. Power Sources 163 (2006) 415. [53] D. Chu, J. Wang, S. Wang, L. Zha, J. He, Y. Hou, Y. Yan, H. Lin, Z. Tian, Catal. Commun. 10 (2008) 955.
347
[54] E. Cazzanelli1, A. Kuzmin, G. Mariotto, N. Mironova-Ulmane, J. Phys. Condens. Matter 15 (2003) 2045. [55] N.B. Manson, W. Von der ohe, S.L. Chodos, Phys. Rev. B 3 (1971) 1698. [56] V.G. Pol, S.V. Pol, N. Perkas, A. Gedanken, J. Phys. Chem. C 111 (2007) 134. [57] N. Shimodaira, A. Masui, J. Appl. Phys. 92 (2002) 902. [58] V. Barbarossa, F. Galluzzi, R. Tomaciello, A. Zanobi, Chem. Phys. Lett. 185 (1991) 53.