具有高活性和突出的抗中毒性能的Pt修飾Ru/C催化劑的制備和表征

高海麗 廖世軍 曾建皇 梁振興 謝義淳

(華南理工大學化學與化工學院,廣州 510641)

具有高活性和突出的抗中毒性能的Pt修飾Ru/C催化劑的制備和表征

高海麗 廖世軍*曾建皇 梁振興 謝義淳

(華南理工大學化學與化工學院,廣州 510641)

采用兩步浸漬-還原法制備了一種具有高Pt利用效率,高性能的Pt修飾的Ru/C催化劑(Ru@Pt/C).對于甲醇的陽極氧化反應,該催化劑的單位質量鉑的催化活性分別為Pt/C、自制PtRu/C和商業JM PtRu/C催化劑的1.9、1.5和1.4倍;其電化學活性比表面積分別為Pt/C和自制PtRu/C的1.6和1.3倍.尤為重要的是該催化劑對甲醇氧化中間體具有很好的去除能力,其正向掃描的氧化峰的峰電流密度(If)與反向掃描氧化峰的峰電流密度(Ib)之比可高達2.4,為Pt/C催化劑的If/Ib的2.7倍,表明催化劑具有很好的抗甲醇氧化中間體毒化的能力.另外,Ru@Pt/C催化劑的穩定性也高于Pt/C、自制PtRu/C和商業JM PtRu/C催化劑的穩定性.采用X射線衍射(XRD)、透射電鏡(TEM)和X射線光電子能譜(XPS)對催化劑進行了表征,Pt在Ru表面的包覆結構得到了印證.Ru@Pt/C的高鉑利用效率、高性能和高抗毒能力使其有望成為一種理想的直接甲醇燃料電池電催化劑.

直接甲醇燃料電池;Pt修飾Ru/C; 電催化劑; 甲醇氧化; 抗中毒能力

Abstract：A platinum-decorated Ru/C catalyst with high platinum utilization efficiency,high performance,and high poisoning tolerance was prepared using a two-stage impregnation reduction method.We found that for anodic methanol oxidation the catalyst activity in terms of the Pt load was 1.9 and 1.5 times as that of Pt/C and PtRu/C alloy catalysts,respectively.These values are also higher than that of the commercial JM PtRu/C catalyst.The electrochemically active surface area of Ru@Pt/C was found to be 1.6 and 1.3 times as those of Pt/C and PtRu/C alloy catalysts,respectively.Furthermore,we found that the ratio of the forward peak current density(If)to the backward peak current density(Ib)reached 2.4,which was 2.7 times as that of the Pt/C catalyst.This implies that the Pt-decorated Ru/C catalyst possesses a high tolerance for the intermediate poisoning species.In addition,the stability of Ru@Pt/C was higher than that of Pt/C,PtRu/C alloy and JM PtRu/C.The catalyst was characterized by X-ray diffraction(XRD),transmission electron microscopy(TEM),and X-ray photoelectron spectroscopy(XPS).The core-shell structure of the catalyst was determined by XRD and TEM.The high performance and high poisoning tolerance of Ru@Pt/C during the anodic oxidation of methanol make it a promising electrocatalyst for direct methanol fuel cells.

Key Words：Direct methanol fuel cell;Platinum-decorated Ru/C;Electrocatalyst;Methanol oxidation;Poison tolerance

Low-temperature fuel cells,including the H2/O2proton exchange membrane fuel cell,direct alcohol fuel cell,and direct formic acid fuel cell,have attracted considerable attention in recent years for their potential use in transportation and other mobile applications[1-4].

To this day,platinum has been the most common electrocatalyst for low-temperature fuel cells.However,it is readily poisoned by carbon monoxide in the reformate gas or by the intermediates generated during the oxidation of methanol or formic acid[5-8].In addition,the scarcity and high cost of Pt have preempted large-scale fuel cell development.Therefore,enhanced poison tolerance and increased Pt utilization efficiency are critical for the development and commercialization of low-temperature fuel cells[9].

Enormous effort has been put into increasing Pt utilization efficiency in fuel cells[10-12].The formation of Pt alloys is certainly one way to enhance Pt utilization and numerous reports have focused on this topic[13-15].In addition to Pt-based alloy catalysts,Pt decorated M(M@Pt catalysts,M denotes inexpensive metals)or Pt-skin Pt-M catalysts have proven effective in achieving high Pt utilization efficiency,since the electrocatalytic reaction occurs only on the electrocatalyst surface[16-18].For example,Ando et al.[19]prepared an electrocatalyst with ultralow Pt content via galvanic displacement of Pb by Pt and Ru:in this case the mass activity of Pt for the methanol oxidation reaction(MOR)was about 10 times higher than that of commercial PtRu/C.

Alloyed PtRu/C catalysts have been extensively investigated and it is generally accepted that Ru effectively improves the poisoning tolerance of catalysts for direct methanol fuel cells(DMFC)and reformate fuel cells.Compared with Pt,Ru is four times as abundant;In addition,Ru is just one-seventh the cost of platinum and possesses good tolerance to chemical and electrochemical corrosion.Based on this knowledge,we specially designed a catalyst with Ru as the core and Pt as the shell,to create a low-cost catalyst with high platinum utilization and high poison tolerance.

To date,only a few reports on platinum decorated Ru/C catalysts have been published.Alayoglu et al.[20]prepared Ru@Pt/C electrocatalyst and their experimental results indicated it had high performance for oxidizing CO in H2.Chen et al.[21]prepared Pt-decorated Ru nanoparticles with low Pt content using a redox-transmetalation process,and these catalysts had higher activity toward methanol oxidation than commercial Pt-Ru black catalyst.

In the present work,a platinum-decorated Ru/C with high performance and high poison tolerance was successfully prepared using a two-stage impregnation-reduction method.The activity of anodic methanol oxidation on the Ru@Pt/C catalyst was studied.

1 Experimental

All the reagents were of analytical purity and used as received without further purification.Chloroplatinic acid and ruthenium chloride were purchased from Shenyang Jinke Reagent Factory.Citric acid(≥99.5%)was purchased from Sinopharm Chemical Reagent Limited Corporation.Sulfuric acid(95%-98%)was provided by Guangdong Guanghua Chemical Factory Co.Ltd.and methanol(≥99.5%)was purchased from Guangzhou Donghong Chemical Factory.

1.1 Catalyst preparation

Vulcan XC72 carbon black was pretreated using a previously described procedure[22].Briefly,the carbon black was first pretreated at room temperature in acetone.Next,the carbon powder was added to a mixture solution of nitric acid and hydrogen peroxide and then refluxed at 80℃for 5 h to induce hydrophilization and remove metal residues.

Pt decorated Ru/C catalyst was prepared using a two-stage impregnation-reduction method.In the first stage,Ru/C was prepared by impregnating the carbon black with ruthenium chloride solution,followed by hydrogen reduction at 200℃for 2 h.In the second stage,platinum decoration was achieved by impregnating Ru/C with a solution of chloroplatinic acid and citric acid(the latter being used as a complexing and stabilizing agent),then reducing it in a hydrogen flow at 90℃for 5 h.The nominal Pt and Ru percentages in the resulting catalyst were 5%and 20%(w,mass fraction),respectively.For comparison,alloyed PtRu/C(atomic ratio of Pt:Ru is 1:1,metal loading 20%(w))and Pt/C(20%(w))were prepared via the same method.The prepared catalyst is denoted as Ru@Pt/C,although this does not necessarily mean that the active particles supported on the carbon possess a perfect core-shell structure.

1.2 Characterization

The morphology of the catalyst was observed using a transmission electron microscope(TEM;JEOL JEM-2010 HR)operating at 200 kV.Composition analysis of the catalyst was carried out using the energy-dispersive X-ray microanalysis(EDX)attached to the TEM.The preparation of samples for TEM and EDX observation was as follows:a small amount of electrocatalyst sample was added to a weighing beaker containing ethanol and placed in an ultrasonic bath for approximately 20 min.Carbon-coated copper grid(3 mm,200 mesh)was then immersed in the above solution and the solvent was allowed to evaporate from the grid.

X-ray powder diffraction(XRD)was carried out on a Shimadzu XD-3A,using filtered Cu Kαradiation and operating at 35 kV and 30 mA.The 2θ region between 20°and 80°was explored at a scan rate of 4(°)·min-1.

X-ray photoelectron spectroscopy(XPS)measurement was carried out in an ultrahigh vacuum on a Perkin Elmer PHI1600 system(PerkinElmer,USA)using a single Mg KαX-ray source operating at 300 W and 15 kV.The binding energies were calibrated based using the C 1s peak of graphite at 284.5 eV as a reference.

1.3 Electrochemical measurements

Electrochemical measurements were carried out at roomtemperature using an electrochemical work station(Ivium,Netherlands).A common three-electrode electrochemical cell was used for the measurements.The counter and reference electrodes were platinum wire and Ag/AgCl(in saturated KCl),respectively.The working electrode was a thin layer of Nafionimpregnated catalyst cast on a glassy carbon electrode with diameter of 5.0 mm.A thin-film electrode was prepared using standard thin-film method.Briefly,5.0 mg of catalyst was dispersed ultrasonically in 1.0 mL Nafion/ethanol(0.25%(w)Nafion)for 30 min.Then,6 μL ink solution was pipetted and spread on the glassy carbon surface and the electrode was air dried.Cyclic voltammetry(CV)experiments for the anodic oxidation of methanol were performed in a solution of 0.50 mol·L-1H2SO4+0.50mol·L-1CH3OHatascanrateof30mV·s-1.

The electrochemical active surface area(SEA(m2·g-1))of Ptbasedcatalystscanbeestimatedfromtheintegratedchargeofthe hydrogen absorption region of the CV according to Gasteiger et al.[23]:

SEA=QH/0.21WPt

where QHis the charge exchanged during hydrogen desorption on the Pt surface(mC·cm-2),WPtis the Pt loading on the electrode(mg·cm-2),and 0.21 is the charge required to oxidize a monolayer of hydrogen on the Pt surface.

2 Results and discussion

Fig.1 shows the XRD patterns of Ru/C,Ru@Pt/C,alloyed PtRu/C,and Pt/C.The reflection peaks for Ru/C at 38.4°,44.0°,58.3°,69.3°,and 78.3°can be ascribed to Ru(100),Ru(101),Ru(102),Ru(110),and Ru(103),respectively,indicative of well reduced ruthenium at 200℃.

The XRD pattern of Ru@Pt/C is quite different from that of the alloyed PtRu/C.The main characteristic of the XRD pattern for alloyed PtRu/C is the disappearance of the ruthenium peak at 44.0°,in agreement with what has previously been reported[24-25].This is explicable by the incorporation of smaller ruthenium into the platinum lattice.In sharp contrast,from the XRD pattern of Ru@Pt/C we can clearly observe the independent peaks that belong to platinum and ruthenium,respectively.There is no notable shift in 2θ angle,implying that Pt did not alloy with Ru.

With the aid of a complexing agent and an appropriate solvent during preparation,the active component was highly dispersed on the support surface.The crystal sizes,read from the Jada software supplied by the vendor of the XRD instrument,were 2.8 and 3.0 nm for Ru/C and Ru@Pt/C,respectively.The larger particle size of Ru@Pt/C might have resulted from the decoration of Pt on Ru particles,and thus may serve as an indirect indication of Pt decoration on Ru.

Fig.2 shows the TEM/HRTEM images together with the particle size distribution of Ru@Pt/C catalyst.TEM images of Ru@Pt/C in Fig.2(A,B)show that the active components are well dispersed on the carbon support surface and the particle size distribution was quite uniform.EDX measurements reveal that the particles are composed of elemental Ru and Pt,and the mass ratio of Ru to Pt was 3.96,quite consistent with the preparation composition,indicative of complete reduction of Ru and Pt at a mild heating temperature under hydrogen.As is generally acknowledged,EDX analysis can only give information on the bulk composition of the nanoparticles.Fig.2C shows quite a narrow size distribution for the metal particles,with the average diameter ca 2.9 nm.

The insert in Fig.2B shows a magnified image of a particle,in which the crossover of the crystal lattice may result from the decoration of Pt on Ru.

Fig.3A shows the XPS results of Ru@Pt/C catalyst;the detected atomic ratio of Pt to Ru is ca 0.14,which is again consistent with the precursor composition.Since the detectable depth of XPS ranges from 1 to 10 nm,the measured composition can hardly reflect the surface composition of small particles(for example,3 nm for Ru@Pt/C in our work).

For alloyed PtRu/C catalysts it is widely accepted that there are changes in binding energy occur only for Ru,not for platinum[26-27].In sharp contrast,the lab-made Ru@Pt/C catalyst displayed quite different results(Fig.3B and Fig.3C):The binding energies of Pt 4f are 71.6 and 74.9 eV in Ru@Pt/C catalyst,obviously higher than those of Pt 4f in Pt/C catalyst(71.3 and 74.5 eV)and the shifts in binding energy are 0.3 and 0.4 eV,respectively.The binding energies of Ru 3p are 461.7 and 483.8 eV in Ru@Pt/C catalyst,lower than those of Ru 3p in Ru/C catalyst(462.0 and 484.0 eV)and the shifts in binding energy are 0.3 and 0.2 eV,respectively.The same phenomena were observed and reported by Alayoglu et al.for Ru@Pt/C catalyst[20].The higher shifts in the binding energy of Pt 4f may result from the high dispersion of Pt on Ru.Since the calculated thickness of a Pt shell on Ru particles is just 0.10-0.15 nm,these results imply a high dispersion of Pt as a monolayer of Pt atoms.

Fig.4 shows the cyclic voltammograms of Pt/C,alloyed PtRu/C,and Ru@Pt/C catalysts in 0.50 mol·L-1H2SO4in a N2-purged atmosphere.Evidently,the hydrogen desorption of Ru@Pt/C is similar to that of the alloyed PtRu/C catalyst but different from that of Pt/C,which may indicate that there are some interactions between Pt and Ru.

Generally,the electrochemical active surface area(SEA)of acatalyst is related to its catalytic activity.The values of SEAcalculated from the hydrogen desorption area in Fig.4 are 123.3 m2·g-1for Pt/C,150.9 m2·g-1for PtRu/C,and 190.3 m2·g-1for Ru@Pt/C,the last of these approaches the maximum section surface area of a gram of Pt,and the SEAof Ru@Pt/C is 1.6 and 1.3 times as those of Pt/C and PtRu/C alloy catalysts,respectively,implying that Pt was highly dispersed on the Ru surface,approaching a monolayer dispersion of Pt.

Fig.5 shows CV curves of Ru/C,Pt/C,alloyed PtRu/C,Ru@Pt/C,and JM PtRu/C in 0.50 mol·L-1CH3OH+0.50 mol·L-1H2SO4solution.Clearly,Ru/C displays almost no activity towards the anodic oxidation of methanol compared with the other three catalysts whilst Ru@Pt/C shows excellent activity.The forward-sweeping mass peak current of Ru@Pt/C is 0.49 A·mg-1,about 1.9,1.5,and 1.4 times as those of Pt/C,alloyedPtRu/C,and JM PtRu/C,respectively.Furthermore,the onset potential of the MOR with Ru@Pt/C is 0.18 V,negatively shifted 40 mV and 20 mV compared with Pt/C and alloyed PtRu/C.The superior catalytic activity of Ru@Pt/C catalyst,its lower onset potential and its higher oxidation current density,may result from the high dispersion of Pt on the Ru and the interaction between Pt and Ru at their interface.

It is perhaps worth noting that Ru@Pt/C can achieve a very high ratio of forward peak current to backward peak current(If/Ib)towards the anodic oxidation of methanol.This ratio is generally associated with tolerance for CO-like oxidative intermediates[28]:a high ratio is an indication of more effective removal of the poisoning species from the catalyst surface[29].As is shown in Fig.5,for the Ru@Pt/C catalyst,the Ifand Ibare 0.49 and 0.19 A·mg-1,respectively:the If/Ibvalue thus reaches 2.4,which is higher than that of Pt/C catalysts(0.90),alloyed PtRu/C(1.5),and JM PtRu/C(1.3).The If/Ibvalue is 2.7 times as that of the Pt/C catalyst.In other words,this Ru@Pt/C catalyst should exhibit excellent poison tolerance.

Above results show that the decoration of Pt on the Ru particles not only significantly boosted the utilization efficiency but also enhanced its poison tolerance.We speculate that this exceptionally high tolerance may result from the synergistic effect of Ru and Pt at their interface.

To investigate the stability of Ru@Pt/C catalyst,we compared several catalysts using chronoamperometry in 0.50 mol·L-1CH3OH+0.50 mol·L-1H2SO4solution for 1800 s at a constant potential of 0.6 V.Fig.6 shows the chronoamperometric spectra of the catalysts,and reveals that the Ru@Pt/C catalyst shows better stability than Pt/C and PtRu/C catalysts,further confirming the conclusion derived from If/Ibvalues.

3 Conclusions

A Ru@Pt/C electrocatalyst with low Pt content(5%(w).)was prepared using a two-stage impregnation-reduction method.XRD observations found that Ru@Pt/C displayed quite a different pattern from that of alloyed PtRu/C,suggesting a different structure.Decoration of Pt on the Ru particles enhanced not only the catalyst′s Pt utilization and catalytic activity towards the MOR,but also its poison tolerance.The experimental results suggest that the design and preparation of Ru@Pt/C is a promising approach for reducing the cost of high performance DMFCs.

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Preparation and Characterization of Platinum-Decorated Ru/C Catalyst with High Performance and Superior Poison Tolerance

GAO Hai-Li LIAO Shi-Jun*ZENG Jian-Huang LIANG Zhen-Xing XIE Yi-Chun
(School of Chemistry and Chemical Engineering,South China University of Technology,Guangzhou510641,P.R.China)

O643

Received:May 25,2010;Revised:September 29,2010;Published on Web:November 3,2010.

?Corresponding author.Email:chsjliao@scut.edu.cn;Tel/Fax:+86-20-87113586.

The project was supported by the National Natural Science Foundation of China(20673040,20876062)and National High-Tech Research and Development Program of China(863)(2009AA05Z119).

國家自然科學基金(20673040,20876062)和國家高技術研究發展計劃項目(863)(2009AA05Z119)資助