高　麗，呂逍雨, 楊海堂, 楊敬賀,毛立群

(河南大學 化學化工學院,化工與清潔技術工程中心，河南 開封 475004)

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Au-Pd/石墨烯和Au-Pd/碳納米管催化電化學氧化甲酸

高麗，呂逍雨, 楊海堂, 楊敬賀*,毛立群

(河南大學 化學化工學院,化工與清潔技術工程中心，河南 開封 475004)

摘要：利用化學還原法合成了石墨烯和碳納米管負載的Au-Pd納米粒子. 石墨烯負載的Au-Pd納米粒子(Au-Pd/G)的粒徑遠小于碳納米管負載的Au-Pd納米粒子(Au-Pd/CNTs)的粒徑, 且Au-Pd納米粒子在復合材料上分布均勻. 與碳納米管負載的Au-Pd納米粒子催化劑相比, 石墨烯負載的Au-Pd催化劑對甲酸的催化顯示出更好的電催化活性, 結果表明作為Au-Pd納米粒子的基底,石墨烯可以明顯提高Au-Pd納米粒子的電催化活性. 在0.1 mol/L H2SO4中, 該納米修飾電極對甲酸有良好的電催化作用, 甲酸在電極上的氧化動力學過程為擴散控制過程.

關鍵詞：金-鈀納米粒子;碳納米管;石墨烯;甲酸氧化

Received date：2015-08-11.

Foundation item：Supported by the National Natural Science Foundation of China (21403053) and the Joint Funds of the National Natural Science Foundation of China (U1404503).

Biography：GAO Li(1982-), female, majoring in industrial crystallization and electrochemical research.*Corresponding author, E-mail: jhyang@henu.edu.cn.

As a kind of promising power source, direct formic acid fuel cell (DFAFC) offers a wide potential application. In recent years, Pd has been shown to be a good catalyst for the low temperature electro-oxidation of formic acid[1]. However, Pd catalyst loses easily a large amount of its activity owing to the oxidation of Pd surfaces and the poisoning adsorption of COadsspecies[2]. In order to improve the activity and stability of Pd catalyst, various Pd-based catalysts have been developed successfully[3-5]. Among various doping elements, Au is the very attractive element for Pd-based binary catalysts probably because Au can promotes electron transfer for Pd and accordingly enhances the catalytic activity[6]. Pd-Au bimetallic catalysts instead of monometallic Pd can enhance the electrochemical activity for fuel molecules oxidation[7]. As a new branch of carbon materials, carbon nanotubes (CNTs) and graphene (G) have been found numerous applications in catalytic field[8-9].

In this work, graphene supported Au-Pd nanoparticles (Au-Pd/G) and carbon nanotubes supported Au-Pd nanoparticles (Au-Pd/CNTs) were synthesized by a chemical reduction method. The Au-Pd/G catalyst shows a higher electrocatalytic activity for the formic acid electrooxidation compared to the Au-Pd/CNTs catalyst indicating the substrate graphene can obviously enhance the catalytic activity of Au-Pd nanoparticles.

1Experimental

1.1　Reagents

Nafion (5% ethanol solution, mass fraction) was purchased from Alfa Aesar, and diluted to 0.1% with doubly distilled water in use. H2SO4(98%) was purchased from Zhongping Engergy and Chemicals Group Kaifeng Dongda Chemical Co.,Ltd (Henan, China). (NH4)2PdCl4and (NH4)AuCl4·H2O(54.98%) were purchased from Alfa Aesar.

1.2　Synthesis of Materials

The Au-Pd/G catalyst with 20% Pd and 5% Au were prepared as follows. Firstly, 100 mg graphene, 2.9 mL NH4AuCl4·H2O (4.18 g·L-1), 12.5 mL (NH4)2PdCl4(5.709 g·L-1) and 8 mL THF were thoroughly mixed together. Then the ultrasonic dispersion suspension (1 h) was stirred for another 12 h in a flask. After that, 35 mL mixture solution of 0.1 mol/L NaBH4and 0.1 mol/L Na2CO3(pH=10) were added into the system which was kept stirring for one more hour at room temperature. Finally, the above mixture was filtered and sequentially washed with triply distilled water and ethanol. The precipitate was collected and dried in a vacuum oven at 333 K. For comparison, the Au-Pd/CNTs catalyst was prepared by the same method. And the control Pd/G and Pd/CNTs catalyst with 20% Pd or the Au/G and Au/CNTs catalyst with 5% Au was fabricated similarly except that only corresponding Pd or Au salt precursor was involved in the solution.

1.3　Apparatus and measurements

The surface morphology of the prepared nanocomposites was identified by using the scanning electron microscopy (SEM, Nova Nano SEM 450, FEI, USA). X-ray diffraction (XRD) patterns were obtained on an X-ray D8 Advance instrument (Bruker, Germany) . Electrochemical experiments were tested on a CHI660D electrochemical workstation (Shanghai, China) using three electrode system. The working electrode was glassy carbon electrode (GCE) (3 mm in diameter). A Pt wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively.

2Results and discussion

It can be observed from Fig.1A, 1B, 1C that the fabricated spherical nanoparticles aggregation on the carbon nanotubes, and that the fabricated uniform spherical nanoparticles on the graphene were well-distributed (Fig.1D, 1E, 1F). Moreover, the crystal structures of these synthesized nanocomposites were investigated by powder XRD method (Fig.2A). There was nearly no peak assignable to single Pd species in the Au-Pd/G catalyst. These results indicate that Pd has entered into the Au crystal lattice, forming the Au-Pd alloy. In addition, there appeared four obvious diffraction peaks of Pd in the Au-Pd/CNTs catalyst originated from the Pd (111), (200), (220) and (311) diffractions respectively, which belong to the fcc-type crystalline Pd as observed in the Pd/CNTs catalyst. This illustrate that only a thimbleful of palladium is involved in alloying and hence considerable extent of segregation of Au and Pd can be expected in the Au-Pd/CNTs catalyst, in which both individual Au and Pd nanoparticles retained its original fcc crystalline structure as demonstrated by XRD pattern.

Potentiodynamic polarization curves for HCOOH oxidation recorded on these electrodes were presented in Fig.2B. On Pd/CNTs and Pd/G, a plateau followed by high peak was obtained. However, with the addition of Au, the reaction started earlier, the plateau transforms to the peak at 0.22 V on Au-Pd/CNTs. The potentiodynamic curves in Fig.2B show that the current on Au-Pd/G was higher and the reaction started at 0.28 V which meant more HCOOH molecules were oxidized, which demonstrated that of the substrate material has an effect on HCOOH and the supported noble metals. These results confirm that the graphene supported Au-Pd bimetallic catalyst was more active toward HCOOH oxidation.

Fig.1　SEM images of the Au/CNTs (A), Pd/CNTs (B), Au-Pd/CNTs (C), Au/G (D), Pd/G (E) and Au-Pd/G (F)

Fig.2　XRD patterns of the materials (A). Polarization curves for the oxidation of HCOOH in 0.1 mol/L H2SO4 + 0.5 mol/L HCOOH electrolyte recorded on different catalyst electrodes. Positive-going potential sweeps recorded at 50 mV·s-1 (B). Electrochemical impedance spectra of GCE, Au/G, Pd/G and Au-Pd/G (C) Au/CNTs, Pd/CNTs and Au-Pd/CNTs (D) in 0.5 mol/L HCOOH + 0.5 mol/L H2SO4 at -0.2 V

Fig.2C, 2D show the Nyquist plots of the modified electrodes measured at -0.2 V (vs. Ag/AgCl) in the frequency ranging from 100 kHz to 1 Hz in 0.5 mol/L HCOOH + 0.5 mol/L H2SO4. The impedance of Au-Pd/G is lower than that of the Pd/G and Au/G catalysts, indicating a lower electrochemical polarization impedance (Fig.2C). The very small arc of in Au-Pd/G indicates a very small polarization impedance in the case of HCOOH oxidation, indicating the layer of Au-Pd/G could form on the electrode surface and exhibited a lower electrochemical polarization impedance. As seen in Fig. 2D, in the low-frequency range, a peculiar diffusion control impedance appears for Pd-Au/CNTs, which suggests the electrochemical reaction is very fast. Consequently, the Pd-Au/CNTs catalyst presents a lower charge-transfer resistance compared to others during formic acid oxidation.

Fig.3A, 3B show the CVs measured on modified electrodes. No anodic peak is observed at the Au/G and Au/CNTs catalysts in the forward direction scan, indicating that the Au/G and Au/CNTs catalysts have no electrocatalytic activity for the formic acid electrooxidation. During the forward direction scan, the peaks of the formic acid oxidation at the Pd/G and Pd/CNTs catalysts are detected at about 0.32 and 0.32 V and the apparent peak currents are 1.7 and 0.82 mA, respectively. While at the Au-Pd/G and Au-Pd/CNTs catalysts electrodes, the anodic peaks were located at about 0.28 and 0.22 V. Meanwhile, compared with graphene and carbon nanotubes supported monometallic Au or Pd nanoparticles, the graphene supported Au-Pd bimetallic catalyst was active for the oxidation of formic acid, the anodic peak current of Au-Pd/G modified electrode was much higher which gives us a hint that Au can promote the electrocatalytic activity of Pd catalysts for the oxidation of formic acid. Hence, compared to carbon nanotubes, graphene is more suitable for Au-Pd nanoparticles as substrate.

Fig.3　Cyclic voltammograms of 0.5 mol/L HCOOH in 0.1 mol/L H2SO4solutoin at Pd/G, Au-Pd/G and Au/G catalyst electrodes (A) and at Pd/CNTs, Au-Pd/CNTs and Au/CNTs catalyst electrodes (B). Cyclic voltammograms at Au-Pd/G (C) and Au-Pd/CNTs (D) catalyst electrode in the presence of 0.5 mol/L HCOOH in 0.1 mol/L H2SO4 solution at different scan rates (mV·s-1). The inset is the relation of square root of scan rate and peak current

As shown in Fig.3C, the electro-catalytic properties of Au-Pd/G and Au-Pd/CNTs were evaluated by cyclic voltammetry (CV). The CV behaviors of the Au-Pd/G and Au-Pd/CNTs at different scan rates were displayed. It can be seen that oxidation peak current (Ip) for formic acid oxidation become larger with the increase of the scan rate. The line relation between peak current (Ip) and square root of the scan rate (v1/2) were shown in the inset of Fig.3C and 3D. The anodic peak currents increase linearly with the square root of scan rate indicating the electrochemical reaction controlled by the semi-infinite linear diffusion from the electrolyte to the electrode.

3Conclusions

In this study, Au-Pd/CNTs and Au-Pd/G catalysts were synthesized via conventional NaBH4reduction method. Au-Pd/CNTs and Au-Pd/G catalysts were promising catalysts with excellent catalytic and stability for formic acid oxidation. Moreover, as substrate for Au-Pd nanoparticles, graphene has better performance than carbon nanotubes. The HCOOH oxidation of Au-Pd/CNTs and Au-Pd/G was a diffusion controlled behavior in the range of scan rate from 80 mV·s-1to 300 mV·s-1and 40 mV·s-1to 240 mV·s-1, respectively. All this proves that graphene supported Au-Pd nanoparticles is a promising candidate as an anode catalyst of direct formic acid fuel cell.

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