稀有金属(英文版) 2015,34(01),12-16

收稿日期：12 June 2014

基金：financially supported by the National Natural Science Foundation of China (Nos. 51164017, 51374117, and 21363012);

Enhanced methanol oxidation activity of Au@Pd nanoparticles supported on MWCNTs functionalized by MB under ultraviolet irradiation

Ming-Li Xu Xi-Kun Yang Ying-Jie Zhang Shu-Biao Xia Peng Dong Guo-Tao Yang

Faculty of Science, Kunming University of Science and Technology

Research Center for Analysis and Measurement, Kunming University of Science and Technology

Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology

Abstract：

A successful approach to assemble Au core Pd shell(Au@Pd) nanoparticles on the surface of multi-walled carbon nanotubes functionalized by methylene blue(MB)(Au@Pd/f uv-MWCNTs) was reported. In this method,MWCNTs were functionalized under ultraviolet irradiation.UV–Vis analysis and high-angle annular dark-field transmission electron microscope(HAADF-TEM) image prove that core–shell structure of Au@Pd nanoparticles forms.TEM results indicate that Au@Pd nanoparticles(~5.2 nm)are well-dispersed on the surface of f uv-MWCNTs. X-ray photoelectron spectroscopy(XPS) reveals that ultraviolet irradiation can promote the interaction between Au@Pd nanoparticles and the functional groups on the surface of MWCNTs. Cyclic voltammograms(CV), chronoamperograms(CA), and electrochemical impedance spectroscopy(EIS) results demonstrate that the Au@Pd/f uv-MWCNTs catalysts show excellent electrocatalytic performance for methanol oxidation in alkaline media. The catalytic activity of the Au@Pd/f uv-MWCNTs is ~2 times higher than that of the commercial Pd/C catalysts. This is mostly attributed to that ultraviolet irradiation can make the moieties of MB provide a uniform surface with active and anchoring sites,and improves the functional effect of MB on the surface ofMWCNTs. Especially, ultraviolet irradiation modifies electronic structure of Pd and is beneficial for the enhancement of catalytic activity.

Keyword：

Au@Pd core–shell nanoparticle; Multi-walled carbon nanotube; Methylene blue; Electrocatalyst; Methanol oxidation; Ultraviolet irradiation;

Author： Ying-Jie Zhang,e-mail: zhangyingjie09@126.com;

Received： 12 June 2014

1 Introduction

Because direct methanol fuel cells (DMFCs) are prospective power sources for portable electronic devices, theelectro-oxidation of methanol attracts much attention [1].However, the commercialization of DMFC is facing serious difficulties due to poor methanol electro-oxidationkinetics in acid solution and the high cost of Pt-basedelectrocatalysts [2, 3]. In recent years, with the applicationof alkaline anion membrane in fuel cells [4, 5], a lot ofinterests are focused on the choice of Pd-based catalystsbecause it has a good electro-activity for methanol oxidation in alkaline media [6, 7]. Additionally, Masel et al.disclosed that Pd and Pd/C catalysts can overcome the COpoisoning effect and thereby yield high performance inDMFC [8]. So, Pd-based catalyst will be a promising anodecatalyst in an alkaline DMFC.

However, the activity and stability of the Pd-basedcatalyst are still in need of improvements. An effectivemethod of enhancing catalytic performance is the fabrication of Au@Pd core–shell structural catalyst because of theenhanced electronic coupling between the Au core and Pdshell [9].

In order to obtain the best catalytic performance whenkeeping a minimal mass of noble metals catalysts, nanoparticles are usually well dispersed on conductive carbonsupports. Multi-walled carbon nanotubes (MWCNTs), due tounique structure, electrical and mechanical properties, arefound to be usable as supports of electrocatalysts in DMFC[10, 11]. The functionalization of MWCNTs is required as aprimary step for enhancing the dispersibility of nanoparticles.Usually, it may be functionalized by harsh oxidative treatmentin acid solutions or by the surfactant [12–15]. If the surfactantcontains the donation/withdrawal electronic functionalgroups, the electronic structure of nanoparticles will be furthermodified. Wang et al. [15] proved the charge transferringbetween Pt nanoparticles and donation/withdrawal electronicfunctional groups on the surface of the functional MWCNTs.

It will be hopeful that the catalytic performance ofAu@Pd core–shell nanoparticles may be further improvedvia tailoring catalyst-support interaction. Most of azo dyescontain donation/withdrawal electronic groups [16].Among them, methylene blue has small toxicity and can beused to effectively modify the surface of MWCNTs underultraviolet irradiation [17]. Herein, a successful methodwas described to prepare Au@Pd/f_uv-MWCNTs as electrocatalyst for DMFC. On the base of our previous work[17], Au@Pd core–shell nanoparticles synthesized byphotochemical method were uniformly assembled on thesurface of MWCNTs functionalized by MB under ultraviolet irradiation. Compared with commercial Pd/C (JM)catalysts, Au@Pd/f_uv-MWCNTs exhibit distinctly higheractivity and better stability toward methanol oxidation inalkaline media.

2 Experimental

2.1 Preparation of Au@Pd core–shell nanoparticles

Au@Pd core–shell nanoparticles were synthesized by a twostep ultraviolet (UV) irradiation method. First, Au seeds wereobtained in a quartz cuvette containing a mixture solution ofpolyethylene glycol (PEG 400, 0.05 moláL^-1), acetone(0.5 moláL^-1), and HAu Cl₄(4.88 9 10^-5moláL^-1) with UVirradiation of 300 nm after 24 min [18]. After that, the aboveAu hydrosols and H₂Pd Cl₄(1.952 9 10^-4moláL^-1)-PEGacetone aqueous solution were mixed in a quartz cuvette.After irradiation of 300 nm for 18 min, Au@Pd core–shellhydrosols (Au/Pd atomic ratio: 1:4) were obtained. Wine redsolution turned ash black.

2.2 Preparation of Au@Pd/fuv-MWCNTs catalysts

Au@Pd/f_uv-MWCNTs catalysts (20 wt% Pd) were prepared as follows: according to the same procedure as ourprevious work [17], the pristine MWCNTs were functionalized by MB under ultraviolet light, and the correspondingproduct was labeled as f_uv-MWCNTs. 12.2 mg of f_uvMWCNTs were added to 100 ml of the previously prepared Au@Pd hydrosols in a flask. The mixture was sonicated for 5 min and then magnetically stirred for 5 h atroom temperature. The catalysts were separated from thesolution in a centrifuge and thoroughly washed with doubledistilled water. The obtained Au@Pd/f_uv-MWCNTs catalysts (20 wt% metal) were dried in a vacuum oven at 60 °Cfor 12 h. As a comparison, pristine MWCNTs were alsofunctionalized by MB without UV irradiation using similarprocedure as described above (denoted as f-MWCNTs).Au@Pd/f-MWCNTs catalysts were prepared using similarprocedures.

2.3 Characterization

UV–Vis spectra were recorded on a lambda 900 UV/Vis/NIR spectrophotometer. Transmission electron microscopy(TEM) images were obtained on a Tecnai G²F20 S-Twin(200 k V). High-angle annular dark-field (HAADF) imageswere recorded with the Tecnai G²30 ST transmissionelectron microscope (300 k V). XRD data were obtained bya Bruker D8 Advance X-ray diffractometer with Cu Ka(0.15406 nm) radiation. XPS experiments were performedwith a PHI5500 spectrometer using Mg Ka (1,253.6 e V)radiation. Spectral correction was based on adventitiouscarbon, using the C1s binding energy of 284.8 e V.

Electrochemical experiments were carried out with aCHI 760C electrochemical workstation at room temperature. A conventional three-electrode cell consisting ofglassy carbon (GC, with a diameter of 3 mm) as theworking electrode, Pt foil as the counter electrode andsaturated calomel electrode (SCE) as the reference electrode, was used. The preparation of the working electrodeis the same as our previous work [17]. All the potentials inthis paper were quoted against SCE. The elelctrocatalyticperformance for the methanol oxidation was measured in0.5 moláL^-1KOH ? 2 moláL^-1CH₃OH solution. Thetesting solution was purged by nitrogen for 20 min beforethe electrochemical measurement.

3 Results and discussion

3.1 UV–Vis analysis

Figure 1 describes UV–Vis absorption spectra of Au, Pd,and Au@Pd hydrosols after the photochemical reactionconducts completely. It can be observed that the absorptionpeak at 515 nm in Curve 1 is the characteristic peak of Auhydrosols. The characteristic spectrum of Pd hydrosols is adeclining curve (Curve 2). It is noted that the characteristicspectrum of Au@Pd hydrosols is also a declining curve andthe characteristic peak of Au disappears. In addition, Curve3 is extremely similar to Curve 2, which suggests thatPd Cl₄²-is reduced on the surface of Au seeds. Au coresare covered completely by Pd shell, and Au@Pd (Au/Pd1:4 atomic ratio) nanoparticles form.

3.2 TEM analysis

Figure 2 shows TEM images of Au@Pd/f-MWCNTs,Au@Pd/f_uv-MWCNTs, particle size distribution of Au@Pdnanoparticles (inset in Fig. 2a), and HAADF-TEM imageof the corresponding Au@Pd hydrosols (inset in Fig. 2b). Itcan be observed that Au@Pd nanoparticles with aggregations are loaded on the surface of the f-MWCNTs and arewell dispersed on the surface of the f_uv-MWCNTs. Theaverage particle size measured from counting 300 particlesis about 5.2 nm. In addition, in HAADF-TEM image, core–shell structure can be clearly observed. This can beattributed to the different atomic numbers and imagingcontrast between Au and Pd. It is in good agreement withthe conclusion from UV–Vis analysis. The higher dispersion of Au@Pd/f_uv-MWCNTs catalysts compared with thatof Au@Pd/f-MWCNTs catalysts results from the bettermodified effect of MB with ultraviolet irradiation. Themoieties of MB maybe provide a uniform surface withactive and anchoring sites which can effectively adsorbAu@Pd nanoparticles and prevent adjacent nanoparticlesfrom aggregating.

Fig. 1 UV–Vis absorption spectra of Au, Pd, and Au@Pd colloidalsolution after ultraviolet irradiation

3.3 XRD analysis

Figure 3 shows XRD patterns of the Au, Pd, and Au@Pd/f_uv-MWCNTs catalysts. In the XRD pattern of Au@Pd/f_uvMWCNTs catalyst, the peak locating at 2h of about 26.3°originates from the (002) plane of graphite [17]. The otherfour peaks are the characteristic peaks of fcc crystalline Au(JCPDS 04–0784) corresponding to the (111), (200), (220),and (311) planes at 2h of about 38.7°, 44.5°, 65.1°, and78.1°, respectively. However, we can not find the characteristic peaks of fcc crystalline Pd (JCPDS-ICDD, cardNo.46-1043). It shows that Pd shell is very thin so that itcannot be observed evidently in XRD patterns.

3.4 XPS analysis

In our previous work, Fourier transform infrared spectroscopy (FT-IR) analysis indicates that anchoring sitesform on the surface of f_uv-MWCNTs under ultravioletirradiation [17]. To further confirm the impact of UVirradiation on the interaction between nanoparticles andfunctional groups, XPS experiments were performed. Figure 4 shows XPS spectra of Pd3d in the Au@Pd/fMWCNTs and Pd3d in the Au@Pd/f_uv-MWCNTs. FromFig. 4a and b, it can be observed that the binding energy ofPd (0) increases from 335.35 to 335.6 e V, and that of Pd(2) apparently increases from 336.97 to 337.3 e V. It furtherindicates that UV irradiation can promote the interactionbetween Au@Pd nanoparticles and functional groups onthe surface of MWCNTs. In addition, the electronicstructure of Pd is modified by the interaction.

Fig. 2 TEM images of a Au@Pd/f-MWCNTs (inset: particle size distribution of Au@Pd nanoparticles) and b Au@Pd/fuv-MWCNT (inset:HAADF-TEM images of Au@Pd nanoparticles)

3.5 Electrochemical measurement

The electrochemical properties of Au@Pd/f_uv-MWCNTcompared with Au@Pd/f-MWCNT and the commercialPd/C (JM) were investigated. As shown in cyclic voltammograms (CV) curves in Fig. 5a, it can be observed thatthe mass-specific activity of Au@Pd/f_uv-MWCNTs catalysts (785.7 m Aámg^-1) is higher than that of Au@Pd/fMWCNTs (565.8 m Aámg^-1) and is twice as much as thatof the commercial Pd/C catalysts (397.6 m Aámg^-1). It maybe attributed to the well dispersion of Au@Pd nanoparticles and electronic structure modification by the support[15].

Fig. 3 XRD patterns of Au@Pd/fuv-MWCNTs

Figure 5b shows chronoamperometric (CA) curves of theAu@Pd/f_uv-MWCNTs, Au@Pd/f-MWCNTs, and the commercial Pd/C catalysts for methanol oxidation. The decreaseof current density indicates the poisoning of catalysts, whichresults from the adsorbed intermediates in methanol electrooxidation. Nevertheless, Au@Pd/f_uv-MWCNTs catalystsare able to maintain the higher current density in the wholeprocess. This indicates that Au@Pd/f_uv-MWCNTs catalystsare more stable and poison-tolerant compared with Au@Pd/f-MWCNTs and the commercial Pd/C catalysts.

Figure 5c presents the EIS results of the Au@Pd/f_uvMWCNTs, Au@Pd/f-MWCNTs, and the commercial Pd/Ccatalysts, respectively. The diameter of the semicircle isconsidered as the charge transfer resistance, representingthe rate of charge exchange between ions in aqueous andcatalysts at electrochemical interface, and indicating thecatalytic activity of methanol oxidation reaction [19]. It canbe observed that the semicircle diameter of the impedancespectrum of the Au@Pd/f_uv-MWCNTs is much smallerthan that of the commercial Pd/C catalysts, indicating asmaller reaction resistance in the Au@Pd/f_uv-MWCNTthan that in the commercial Pd/C catalysts. This fact maysuggest that the strong interaction between f_uv-MWCNTsand Au@Pd nanoparticles can facilitate the effectivedegree of electron delocalization and enhance the conductivity of catalysts [20].

Fig. 4 XPS spectra of a Pd3d in f-MWCNTs and b Pd3d in fuv-MWCNTs

Fig. 5 CV curves at 50 m V?s-1a, CA curves at -0.22 V b, and EIS at -0.23 V c of Au@Pd/fuv-MWCNTs, Au@Pd/f-MWCNTs, andcommercial Pd/C in 0.5 mol?L-1KOH ? 2.0 mol?L-1CH3OH solution

4 Conclusion

In this paper, Au@Pd/f_uv-MWCNTs catalysts were successfully prepared by assembling Au@Pd nanoparticles onthe surface of MWCNTs functionalized with MB under UVirradiation. UV–Vis analysis and HAADF-TEM imageprove that Au@Pd (1:4) nanoparticles (*5.2 nm) withcore–shell structure form. XPS analysis indicates that UVirradiation can facilitate the interaction between MB andMWCNTs and the electron structure of Pd is modified. UVirradiation is beneficial for the well-dispersion of Au@Pdnanoparticles on the surface of f_uv-MWCNTs. The electrocatalytic activity of Au@Pd/f_uv-MWCNTs is *1.4 timeshigher than that of Au@Pd/f-MWCNTs and *2 timeshigher than that of the commercial Pd/C catalysts in alkalinemedium. It is believed that the Au@Pd/f_uv-MWCNTs is avery promising catalyst in an alkaline DMFC.