微波輔助加熱乙二醇法制備PtSn/CNT催化劑：pH值對其結構和電氧化甲醇性能的影響

黎海超,陳水挾,2,李啟漢,劉風雷

(1.中山大學 化學與化學工程學院，聚合物復合材料與功能材料教育部重點實驗室，廣東 廣州510275；2.中山大學 材料科學研究所，廣東 廣州510275)

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微波輔助加熱乙二醇法制備PtSn/CNT催化劑：pH值對其結構和電氧化甲醇性能的影響

黎海超1,陳水挾1,2,李啟漢1,劉風雷1

(1.中山大學 化學與化學工程學院，聚合物復合材料與功能材料教育部重點實驗室，廣東 廣州510275；2.中山大學 材料科學研究所，廣東 廣州510275)

采用微波輔助加熱乙二醇法制備了碳納米管(CNTs)負載的PtSn 雙組份催化劑。采用原子吸收光譜，X射線衍射儀和電子透射顯微鏡對產物進行了表征。結果表明，含金屬離子前驅體的乙二醇溶液的pH值對產物的金屬催化劑負載量、合金化程度和PtSn 粒子的形態有顯著的影響。在pH值為5時能得到組分配比為原始設計值的PtSn/CNT催化劑。在pH值2～7的范圍內納米粒子的尺寸較小，隨著pH值的進一步提高，納米粒子直徑變大且發生團聚。電化學測試表明在pH值為5時得到的PtSn/CNT催化劑對甲醇電化學氧化具有最佳的催化作用。合適的金屬負載比例和良好的納米顆粒形狀和尺寸分布控制是得到優異的催化性能的主要原因。

微波輻照；碳納米管；PtSn催化劑；甲醇電化學氧化

1　Introduction

As the promising power sources for portable electronics,direct alcohol fuel cells (DAFCs) using methanol,ethanol,ethylene glycol (EG) and glycerol as fuels have drawn a great deal of attention owing to their high power density,low operation temperature,no corrosion problem and so on[1,2].As the anode catalysts for DAFCs,bimetallic Pt-based alloys,such as PtSn,PtRu,PtCo with modified Pt electronic properties and surface chemistry,have been of continuing interest owing to their higher activity as compared with Pt catalyst[3-12].Formation of electrocatalysts on carbon materials for DAFC applications is commonly realized by reductive deposition method.But this method based on wet impregnation and chemical reduction is usually time-consuming,while do not provide adequate control of particle shape,size and size distribution.Researchers have been devoted to find a simple,fast and efficient way to control the size of Pt catalysts.A colloid formation method based on microwave-assisted reduction of metal salts in polyol solution is mainly used to prepare metal particles with narrow size distribution and specific shape owing to its speediness and energy efficiency[13,14].

PtSn catalyst for alcohol electrocatalytic oxidation has been extensively studied but few examinations investigated the pH influence on the PtSn catalyst.In this work,CNTs supported PtSn catalyst with a Pt/Sn atomic ratio of 3∶1 was prepared by intermittent microwave-assisted EG reduction method.PtSn/CNTs were synthesized at pH 2 to 12 in order to examine the influence of pH value.X-ray diffraction (XRD),transmission electron microscopy (TEM) and atomic absorption spectroscopy (AAS) were employed to characterize the structure and composition.The catalytic oxidation performance of this catalyst towards methanol was preliminary evaluated.

2　Experimental

2.1Materials

All the chemical reagents employed in this study were of analytical grade.Chloroplatinic acid was purchased from ShenYang Jin Ke Chemical Factory,China.Stannous chloride dihydrate was supplied by Guanghua Chemical Factory,China.Mutiwalled carbon nanotubes (MWCNTs) with tube diameters of 40-60 nm were purchased from Shenzhen Nanotech Port Co.,Ltd.,in China.20 wt% Pt supported on Vulcan carbon black (Pt/C) catalyst was from Johnson Matthey Company and Nafion 5 wt% solution from Dupont.

2.2Synthesis and characterization of the catalysts

Oxidation treatment with concentrated HNO3and H2SO4was employed to purify the MWCNTs and introduce some oxygen-containing groups on the carbon surface.

The 20 wt% PtSn/CNT with a Pt/Sn atomic ratio of 3∶1 was prepared by intermittent microwave-assisted EG reduction method.This catalyst was named as M-PtSn/CNT.The typical preparation procedure is as follows:1.12 mL of chloroplatinic acid in EG solution (3.7 mg Pt/mL EG) and 1.6 mg of stannous chloride dihydrate (SnCl2·2H2O) were quantitatively added into 40 mL of EG in a flask.20 mg of acid-treated MWCNTs were mixed with the solution of metallic precursors under ultrasonic treatment for 3 h.The synthesis solution pH was adjusted to 5 by adding 1.0 M NaOH EG solution.The microwave treatment was accomplished in a household microwave oven (Midea,PJ17C-M,2.45 GHz,700 W) for 3 times with 30 s irradiation on and 60 s irradiation off.The resulting suspension was filtered and the residue was washed thoroughly with deionized water.The solid product recovered as such was dried at 60 ℃ over night in a vacuum oven.As-prepared catalyst was denoted as PtSn/CNT.Four other such catalysts were prepared at the pH values of 2,7,9 and 12 to study the effect of pH value on the structure and electrocatalytic activity of PtSn/CNT catalysts.

X-ray diffraction (XRD) patterns were obtained on a D8 ADVANCE (BRüCKNER Textile Technologies GmbH & Co.,KG) X-ray diffractometer using Cu Kαradiation (λ= 0.154 056 nm).The tube voltage was maintained at 40 kV and tube current at 40 mA.The 2θangles ranging from 20° to 70° were covered at a scan rate of 10(°)/min.Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010HR operating at 200 kV.For the atomic absorption spectroscopy (AAS) analysis,PtSn/CNTs samples were immersed in aqua regia for 24 h to dissolve the PtSn particles.The undissolved CNTs were filtered by using a millipore membrane filter.The clear solution was then diluted to an appropriate concentration before the measurement.Zeta potential measurement was performed on a Zetaplus,Brookhaven Instruments Corp.Holtsville,NY.

2.3Measurement of the electrochemical properties of the catalysts

All electrochemical measurements were performed in a three-electrode electrochemical cell on an IM6ex electrochemical workstation (Zahner-Electrik,Germany) at room temperature.For the preparation of working electrodes,1 mg of catalyst and 0.5 mL of isopropyl aqueous solution (Visopropanol∶Vwater= 2∶1) were mixed ultrasonically.The well-mixed electrocatalyst ink (10 μL) was deposited onto the surface of a freshly polished glassy carbon disk (GC,3 mm in diameter and 0.070 65 cm2) and dried at 60 ℃ for 30 min.3 μL of Nafion solution was then sprayed on the PtSn/MWCNT catalyst surface to form a protective layer to avoid loss of catalyst during the test.A Pt foil and a saturated calomel electrode (SCE) were used as the counter and the reference electrodes,respectively.N2gas was purged for 30 min before the experiment.

3　Results and discussion

3.1Effect of pH value on metal loading of PtSn

Metal catalyst loading is defined as the weight fraction of PtSn over the weights of the catalyst.The metal loading and compositions were analyzed by AAS (Table 1).It is found that the deposition efficiency and Pt/Sn weight ratio of the particles were sensitive to the pH values of EG solution.The initial composition based on precursors are 16.6 wt% and 3.4 wt% for Pt and Sn,respectively.Metal deposition efficiency could be over 95% for catalyst prepared at pH 5 and weight ratio of Pt/Sn of as-prepared catalyst was very close to the intended one.But catalysts prepared at pH 2,7 and 9 show deposition efficiencies of 60% to 90%,indicating that there were metals remained in the solution.And we found that metal loading on CNTs prepared at pH 12 is extremely low,only 1.6 wt% Pt and 0.15 wt% Sn.

Table 1　Structure and compositions of PtSn/CNT and Pt/C catalysts.

3.2Effect of pH value on structure of PtSn/CNT

The X-ray diffraction patterns of PtSn/CNT electrocatalysts prepared in different pH values are shown in Fig.1.For the sake of comparison,the pattern of commercial Pt/C catalyst (Johnson Matthey,Pt:20 wt%) is also shown in the same figure.The peak at about 2θ= 25° was associated with C (200) plane.All the of the PtSn/CNTs catalysts,except the one prepared at the pH vuale of 12,showed peaks at approximate 2θ= 39°,45°,66° and 79°,which were the main characteristic peaks of crystalline Pt and Pt alloys.The absence of Pt diffraction peaks for the catalyst prepared at pH 12 (Fig.1f) may be attributed to a poor deposition efficiency.All these peaks shifted to lower 2θvalues for PtSn/CNTs electrocatalysts as compared with the commercial Pt/C catalyst,which is caused by the formation of an alloy due to incorporation of Sn atom into the Pt fcc structure,resulting in a lattice expansion[5].No distinct peaks of SnO2were detected possibly because the particles were amorphous or too small.It should be noted that as the pH value increased,the PtSn phase diffraction peaks shifted to high 2θangle,which revealed that the alloying degree of PtSn decreased.From literature data[21],a linear relationship of the lattice parameter and alloyed Sn atomic ratio xSnhas been proposed by the following equation.

aPtSn=kxSn+aPt

where aPt= 0.391 4 nm is the lattice parameter of Pt/C,aPtSnis the lattice parameter of PtSn,which can be evaluated according to the angular position of the Pt (220) peak,and k is a constant = 0.352.

Table 1 clearly shows that alloyed Sn atomic ratio xSndecreased with the pH value.The average size of the catalysts was calculated from XRD data based on the broadening of the Pt (220) peak from the Scherrer equation[22].It was found that the PtSn/CNT catalysts had a crystallite size of around 3.6 nm.We could not obtain the information for the sample prepared at pH of 12 due to the absence of Pt diffraction peaks as lattice parameter,alloyed Sn atomic ratio and XRD mean particle size were calculated based on the AAS and XRD data.

Fig.1　XRD patterns of (a) commercial Pt/C catalyst and PtSn/CNT prepared at different pH values: (b) 2,(c) 5,(d) 7,(e) 9 and (f) 12.

3.3Effect of pH on morphology of PtSn/CNT

Besides loading amount and composition,nanoparticle size,distribution and morphology are also vital to the electrochemical properties of the catalysts.Morphology of the CNT-supported PtSn catalysts observed by TEM was presented in Fig.2.The corresponding mean particle size of catalysts were also obtained by measuring over 100 particles from TEM and presented in Table 1.It can be seen that PtSn catalysts prepared at pH 5 and 7 showed the most satisfied distribution on CNTs,except for a slight particle agglomeration (Fig.2b and c).For the PtSn/CNT catalysts prepared at pH 2 and 9 (Fig.2a and d),nanoparticle agglomeration was easily observed.PtSn particles synthesized at pH 12 were rarely detected,and those located on the surface of the CNTs were large and agglomerated particles and as shown in the selected area (Fig.2e).A broader particle size distribution from 2.0 to 13.0 nm with a mean particle size of 7.6 nm was obtained.

Fig.2　TEM images and corresponding particle size distribution histograms of PtSn/CNT prepared at different pH values:(a) 2; (b) 5; (c) 7; (d) 9 and (e) 12.

3.4Insight into the reduction and deposition mechanism

Fig.3　Zeta potential as a function of pH for acid-treated CNTs in EG solution.

3.5Electrocatalytic properties

The effect of pH values on the electrocatalytic activity of PtSn/CNT for methanol oxidation was examined by cyclic voltammetry and the result is presented in Fig.4.

Fig.4　Catalytic activity of PtSn/CNT prepared at various pH values towards methanol electro-oxidation in 0.5 M H2SO4+ 1.0 M methanol with a sweep rate of 20 mV·s-1.

The current values were normalized by the loading amount of Pt metal,taking account of the alcohol adsorption and dehydrogenation occurring on the Pt sites[29].Distinct changes in the peak currents for the catalysts prepared at different pH values were observed.The catalyst prepared at pH 5 showed the highest peak current density of 223 mA·mg-1Pt at 0.61 V.The mass activity decreased as the pH value increased.The peak currents were 191 and 153 mA·mg-1Pt for the catalysts prepared at pH 7 and 9,respectively.The catalyst prepared at pH 12 had nearly no activity.This result indicated that pH 5 is the optimum value for preparing the PtSn/CNT with a high electrocatalytic activity.

4　Conclusions

A microwave irradiation assisted EG reduction method was employed to prepare CNT-supported PtSn binary catalyst with high electrocatalytic activities for glycerol oxidation.It was found that pH value of the EG solution influenced significantly on the loading efficiency,compositions and morphology of as-prepared PtSn nanaparticles via influencing the adsorption condition of metallic precursors and stabilizing effect of glycolate.Desired catalyst with a composition close to the intended weight ratio of Pt to Sn of 16.6∶3.4 (wt/wt) was obtained by adjusting the pH value to about 5,near the IEP of the acid-treated CNTs.The PtSn nanoparticles displayed the most satisfying size distribution at pH 5 and 7.Overall the PtSn/CNT catalyst prepared at pH 5 exhibited the best catalytic activity for methanol electro-oxidation at room temperature mainly due to a high loading efficiency and adequate control of particle shape and size distribution.

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Effect of the pH of the preparation medium on the microstructure and electrocatalytic activity of carbon nanotubes decorated with PtSn nanoparticles for use in methanol oxidation

LI Hai-chao1,CHEN Shui-xia1,2,LI Qi-han1,LIU Feng-lei1

(1.PCFM Lab,School of Chemistry and Chemical Engineering,Sun Yat-Sen University,Guangzhou510275,China;2.Materials Science Institute,Sun Yat-Sen University,Guangzhou510275,China)

Carbon nanotubes (CNTs) decorated with PtSn nanoparticles (PtSn/CNT) were prepared by the microwave-assisted ethylene glycol reduction method and characterized by atomic adsorption spectroscopy,X-ray diffraction and transmission electron microscopy.Results indicated that the loading efficiency of the metal catalyst,and the degree of alloying and morphology of the PtSn nanoparticles were significantly affected by the solution pH value of the metallic ions in the ethylene glycol.The required composition of the PtSn/CNT catalysts could be obtained by adjusting the pH value to about 5,which is almost the isoelectric point of the acid-treated CNTs.The size of the PtSn nanoparticles decreased with the pH value in the range 2 to 7,but they became large and agglomerated when the pH value was greater than 7.Electrocatalytic activity tests indicated that the PtSn-CNTs prepared at pH 5 had the best catalytic performance towards methanol oxidation.The improvement in catalytic activity was mainly attributed to a high loading efficiency and control of particle shape and size distribution.

Microwave irradiation; Carbon nanotubes; PtSn catalyst; Methanol electro-oxidation.

date:2016-05-07;Revised date:2016-06-05

National Natural Science Foundation of China (50373053); Science and Technology Project of Guangdong Province (2012B091000080).

CHEN Shui-xia.E-mail:cescsx@mail.sysu.edu.cn

1007-8827(2016)03-0293-08

TB333

A

國家自然科學基金(50373053)；廣東省科技計劃項目(2012B091000080).

陳水挾，教授.E-mail:cescsx@mail.sysu.edu.cn

English edition available online ScienceDirect (http:www.sciencedirect.comsciencejournal18725805 ).

10.1016/S1872-5805(16)60014-8