Journal of Nanoparticle Research

, Volume 11, Issue 3, pp 707–712

Preparation of Pt–CeO2/MWNT nano-composites by reverse micellar method for methanol oxidation

Authors

    • College of Chemistry ScienceQufu Normal University
  • Shu-Kun Cui
    • Qufu Normal School
  • Hui Sun
    • Qufu Normal School
Research Paper

DOI: 10.1007/s11051-008-9430-z

Cite this article as:
Guo, D., Cui, S. & Sun, H. J Nanopart Res (2009) 11: 707. doi:10.1007/s11051-008-9430-z

Abstract

We report the preparation of Pt–CeO2 nanoparticles on the multi-walled carbon nanotubes (MWNTs) by a reverse micellar method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small Pt–CeO2 nanoparticles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the Pt–CeO2 nanoparticles on the MWNTs. Cyclic voltammetry (CV) results demonstrated that the Pt–CeO2/MWNT exhibited a higher methanol oxidation than did the Pt/MWNT catalyst. The CO stripping test showed that CeO2 can make CO stripped at a lower potential, which is helpful for CO and methanol electro-oxidation.

Keywords

Pt–CeO2/MWNTsMethanol electro-oxidationDirect methanol fuel cellsSynthesisNanomaterials

Introduction

Direct methanol fuel cells (DMFCs) are being widely studied for compact, high-power-density energy conversion (Scott et al. 1999; Baxter et al. 1999; Kelley et al. 2000; Lamy et al. 2002; Li et al. 2002; McNicol et al. 1999). Methanol has a low theoretical oxidation potential comparable to that of hydrogen, and in principle it can be used as an efficient fuel in low-temperature polymer electrolyte membrane fuel cells. Methanol possesses practical advantages over hydrogen as a fuel since it is a liquid at room temperature and pressure and requires no preprocessing modules such as reformers. Despite these advantages, high overpotentials for the methanol oxidation reaction prevent the use of DMFCs in practical applications. This is due to the presence of partial oxidation intermediates such as -CH3O, -CH2O, -CHO, and -CO, which adsorb strongly on the surface of the best-known cost-effective catalysts resulting in deactivation. Several studies have been performed on Pt-based multicomponent catalysts to improve the poison tolerance with the ultimate goal of creating cost-effective catalysts in practical fuel cells. Metal–metal oxide catalysts such as ZrO2, WO3, TiO2, etc. are being investigated for electro-oxidation reactions. (Park et al. 2006; Bai et al. 2005; Xiong and Manthiram 2004; Rajesh et al. 2002; Shim et al. 2001).

In recent years, much attention has been focused on CeO2-supported noble metal catalysts due to their applications in automotive exhausts. CeO2 is added as a promoter because of its unique redox properties, high oxygen storage capacity (OSC), and stabilization of the metal dispersion (Oh 1990). CeO2 is a promising catalyst with sites that promote CO adsorption for CO oxidation to take place and can provide oxygen from the support. Luengnaruemitchai et al. (2004) reported on the preparation of CeO2 using co-precipitation method which is an active support for this reaction. Summers and Ausen (1979) claimed that ceria donated oxygen to Pt for the oxidation of CO on Pt/CeO2 catalyst. This type of catalysts, if properly developed, would of course be much more economical. Ceria could be prepared by homogeneous precipitation (Chen and Chen 1996, 1997), hydrothermal synthesis (Wang and Feng 2003; Hirano and Kato 1996), gas condensation of Ce metal followed by oxidation using O2 gas (Guillou et al. 1997), homogeneous precipitation using hexamethylenetetramine (Zhang et al. 2004), sol–gel processing (Czerwinski and Szpunar 1997), and electrochemical synthesis (Zhitomirsky and Petric 2001). Recently, Liao et al. used microwave-aided hydrothermal method to rapidly prepare ceria (Liao et al. 2001). It appears that the activity and stability of ceria-based nanoparticles depend markedly on the preparation methods (Bunluesin et al. 1998; Bunluesin et al. 1997; Cordatos et al. 1996). However, the preparation of nanocrystalline CeO2 is very difficult using conventional conditions, and it is thus a challenge to find a novel approach to prepare the nanocrystalline oxide. Therefore, there is a need to better understand the way ceria-based materials operate to allow further improvements in the formulation and preparation of catalysts. Here we describe Pt–CeO2/MWNT catalysts for methanol oxidation. The materials were synthesized using a microemulsion technique, resulting in a nanostructured material with high surface area. The catalytic activity of these materials has been tested in comparison to a Pt–CeO2/MWCNT catalyst for methanol oxidation; the results indicate that methanol and CO studied are more active on Pt–CeO2/MWNT than on Pt/MWCNT. The smaller CeO2 nanoparticles may supply more active sites and contribute to the strong interaction between platinum and nanocrystalline CeO2, so the catalytic activity of as-prepared Pt–CeO2/MWNT composites with larger surface is much higher.

Experimental

Preparation of Pt–CeO2/MWNT composites

Multi-walled carbon nanotubes (MWNTs) were obtained from Chemical Engineering Department, Tsinghua University. The MWNTs were treated by boiling the as-received MWNTs in HNO3 for 3 h, rinsed with copious water, dried and ground. In the first step, MWNTs coated with CeO2, Cerium (III) nitrate · 6H2O was used as the starting material for the ceria nanoparticles, and sodium hydroxide was used as the reaction reagent. The microemulsion system used in the present study consisted of n-octane as the continuous oil phase, cetyl trimethyl ammonium bromide (CTAB) (Alfa Aesar 99%) as the surfactant, 1-butanol as the co-surfactant, and an aqueous solution as the dispersed phase. 0.6132 g of Ce(III) nitrate · 6H2O and suitable MWNTs in 10 ml of de-ionized water by sonication for 5 min and this solution was dispersed in a mixture of 14 g of n-octane, 3 g of 1-butanol, and 4 g of CTAB to form a microemulsion. Another microemulsion of similar composition was made using 10 ml of 0.69 M NaOH solution, and then the two stable microemulsions were mixed together using a magnetic stirrer for 1 h to obtain a CeO2/MWNT nanoparticles. which were extracted by centrifuging, then washed with methanol. The Pt–CeO2/MWNT catalysts were prepared by reducing of H2PtCl6 with excess hydrizine hydrate on CeO2/MWNT powders. The nominal loading of Pt in the catalyst was 20 wt%. The prepared catalysts were dried overnight at 70 °C in a vacuum oven. For comparison, Pt/MWNT nanoparticle catalysts (Saha et al. 2008; Chen and Lu 2008; Chen et al. 2005; Shao et al. 2006) were also obtained directly by reducing the Pt precursors in a carbon nanotube suspension using the dropwise addition of NaBH4 with stirring at room temperature.

Measurement

Electrochemical reactivity of the catalysts was measured by cyclic voltammetry (CV) using a three-electrode cell at the Solartron electrochemical workstation (Solartron 1287BZ). The working electrode was a gold plate covered with a thin layer of Nafion-impregnated catalyst. As a typical process, about 1 mg catalyst sample was ultrasonically mixed with Nafion EG solution to form homogeneous ink which was cast on the gold plate. Pt gauze and a saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. All potentials in this report are quoted versus SCE. CV test was conducted at 50 mV s−1 in a solution of 1 M HClO4 and 1 M CH3OH, potential ranging from −0.2 to 1.0 V. CO stripping experiments were performed as follows: after purging the solution with N2 gas for 20 min, gaseous CO was bubbled for 20 min to allow adsorption of CO onto the electro-catalysts while maintaining a constant voltage of 0.1 V versus SCE. Excess CO dissolved in solution was purged out with N2 for 20 min and CO stripping voltammetry was recorded in 1 M HClO4 solution at a scan rate of 20 mV s−1. The electrochemical measurements were conducted under 25 °C.

The morphology of Pt–CeO2/MWNT composites was investigated using transmission electron microscopy (TEM, JEOL model JEM-1200EX). The X-ray diffraction (XRD) analysis was performed using the Rigaku X-ray diffractometer with Cu Kα radiation source. The 2θ angular regions between 10° and 65° were explored at a scan rate of 6° min−1 with step of 0.02°.

Results and discussion

TEM analysis of Pt–CeO2/MWNT composites

The transmission electron microscopy (TEM) photographs of 20 wt.% Pt-loaded CeO2/MWNT are shown in Fig. 1, exhibiting the morphology and dispersion of Pt or CeO2–Pt on MWNT. The particle size distribution is uniform, and no agglomerates of particles are observed in the cases. The presence of CeO2 in the catalyst appears to have relatively little effect on the platinum particle size, with the average particle size being 2.0–6.0 nm in the samples.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9430-z/MediaObjects/11051_2008_9430_Fig1_HTML.jpg
Fig. 1

TEM image of Pt–CeO2/MWNT composites

XRD analysis of Pt–CeO2/MWNT composites

Figure 2 shows the XRD patterns of Pt–CeO2/MWNT. For the patterns of Pt–CeO2/MWNT, both of the diffraction peaks of Pt and CeO2 can be observed indicating their coexistence in the sample. The XRD diffraction patterns of the Pt–CeO2/MWNT catalysts revealed the presence of a crystalline phase of CeO2, only the extremely broad Pt(111) peak was obtained for the presence of Pt. The characteristic diffraction peaks were detected at 2θ = 28.55, 33.07, 47.48, and 56.34, corresponding to (111), (200), (220), and (311) planes of the cubic fluorite structure of CeO2. The particle sizes calculated from the XRD patterns by the Debye-Scherrer equation were 5.7 and 2.1 nm for the CeO2 and Pt, respectively. The first peak is merged with the MWNT(002) peak.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9430-z/MediaObjects/11051_2008_9430_Fig2_HTML.gif
Fig. 2

XRD patterns of Pt–CeO2/MWNT composites

Electrochemical properties of Pt–CeO2/MWNT composites

Figure 3 shows the cyclic voltammograms on the Pt–CeO2/MWNTs and Pt/MWNT catalysts with a solution of 1 M CH3OH and 1 M HClO4. The performance of the Pt–CeO2/MWNT catalysts is better than that of Pt/MWNT catalysts. Noble metal (NM)/ceria-based catalysts are among the systems long known to exhibit strong metal-support interaction (SMSI) effects (Peil et al. 1989). These catalysts have been shown to give high activity for hydrogenation reactions, water–gas shift reactions, CO and hydrocarbon oxidation, etc. (Nwalor et al. 1989; Stockwell et al. 1988; Efstathiou et al. 1994; Agnelli et al. 1998; Balakos et al. 1993; Schuurman and Mirodatos 1997). Generally accepted methanol oxidation behaviors on Pt–CeO2 are as follows. Reactants and reaction intermediates, i.e. formic acid, formaldehyde, and carbon monoxide, are adsorbed on the catalyst surface at lower potentials to form a Pt-bound carbonyl species, which then reacts with an oxygen atom coming from the ceria to form CO2. The reduced ceria is subsequently reoxidized by water, and hydrogen is produced as a result (Li et al. 2000). The electrochemical behaviors of Pt/MWNTs were similar to that of Pt–CeO2/MWNTs. However, the peak current density of Pt–CeO2/MWNTs is significantly higher than that of Pt/MWNTs. The peak current density for Pt–CeO2/MWNTs was about 575 mA mg−1 Pt, while the peak current density for Pt/MWNTs was only 325 mA mg−1 Pt. The forward scan peak potential of the methanol electro-oxidation for the two catalysts is almost the same, but the onset potential (in the positive going scan) of methanol electro-oxidation on Pt–CeO2/MWNTs was more negative than on Pt/MWNTs. The results not only show that the peak current density is much higher on Pt–CeO2/MWNT electrode than that on Pt/MWNT electrode, but also show that the starting potential for methanol oxidation shifts to more negative direction, indicating methanol electro-oxidation is more active on Pt–CeO2/MWNT electrode than that on Pt/MWNT electrode.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9430-z/MediaObjects/11051_2008_9430_Fig3_HTML.gif
Fig. 3

Voltammetry curves for Pt–CeO2/MWNTs and Pt/MWNTs in 1 M HClO4 + 1 M CH3OH solutions with a scan rate of 50 mV s−1

Since CO species are the main poisoning intermediate, a good catalyst for methanol electro-oxidation should possess excellent CO electro-oxidizing ability, which can be reflected from CO stripping test. The CO stripping voltammogram curves are shown in Fig. 4; significant differences in the onset potential and peak potential for CO oxidation between the catalysts containing CeO2 and those of pure platinum were observed. The peak potential of CO oxidation in the forward scan on Pt–CeO2/MWNTs was at 0.52 V, which was about 50 mV lower than that measured on Pt/MWNT electrode, thus illustrating the beneficial role of CeO2 for CO oxidation. Such a result indicates that the ceria-supported catalyst could participate in the CO oxidation reaction.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9430-z/MediaObjects/11051_2008_9430_Fig4_HTML.gif
Fig. 4

CO stripping curves for Pt–CeO2/MWNTs and Pt/C in 1 M HClO4 solutions with a scan rate of 20 mV s−1

Conclusion

In this study, Pt–CeO2/MWNT nanoparticles have been synthesized using a reverse micellar method. The XRD and TEM showed that the prepared catalysts had narrow particle size distribution. Compared with Pt/MWNT, Pt–CeO2/MWNT exhibited higher catalytic activity for methanol electro-oxidation, and the corresponding CO stripping potential shifted to a lower value, indicating that CeO2 can make CO electro-oxidation easier. The higher electrochemical activity can attribute to that the CO poisoning of the platinum was being decreased by the presence of CeO2.

Acknowledgment

This project was supported by the Scientific Research Foundation of Qufu Normal University.

Copyright information

© Springer Science+Business Media B.V. 2008