Solvent effects on Pt-Ru/C catalyst for methanol electro-oxidation

Available online at www.sciencedirect.comJOURNAL:OFScienceDirectNATURAL GASCHEMISTRYELSEVIERJournal of Natural Gas Chemistry 18(2009) 341- -345www.elsevier.com/locatejngcSolvent effects on Pt- Ru/C catalyst for methanol electro-oxidationJinwei Chen，Chunping Jiang，Hui Lu，Lan Feng，Xin Yang，Liangqiong Li，Ruilin Wang*College of Materials Science and Engineering, Sichuan University, Chengdu 610065, Sichuan, China[ Received February 27, 2009; Revised April 21, 2009; Available online July 28, 2009 ]AbstractAlloying degree, particle size and the level of dispersion are the key structural parameters of Pt-Ru/C catalyst in fuel cells. Solvent(s) usedin the preparation process can affect the particle size and alloying degree of the object substance, which lead to a great positive impact on itsproperties. In this work, three types of solvents and their mixtures were used in preparation of the Pt-Ru/C catalysts by chemical reductionof metal precursors with sodium borohydride at room temperature. The structure of the catalysts was characterized by X-ray diffraction(XRD) and Transmission electron microscopy (TEM). The catalytic activity and stability for methanol electro- oxidation were studied by CyclicVoltammetry (CV) and Chronoamperometry (CA). Pt-Ru/catalyst prepared H2O or binary solvents of H2O and isopropanol had largeparticle size and low alloying degree leading to low catalyability in. methanol electro-oxidation. When tetrahydrofuranwas added to the above solvent systems, Pt-Ru/C catalyst prA AliAand higher loying degee which rsuted in bteriditcatalytic activity, lower onset and peak potentials, comparM oreover, the catalyst prepared in ternary solven[of isopropanol, water and tetralhydrofuran had the smallestpying degree and the dispersion kept unchangeTherefore, this kind of catalyst showed the highest catalytichi y bruothanol electro oxidation.Key wordssolvent effect; fuel cell; methanol electro -oxidation; Pt-Ru/C catalyst; tetrahydrofuran1. IntroductionPt through the bifunctional mechanism or electronic effect[9,12-14]. In many possibilities, the binary Pt-Ru alloy isMethanol is a fundamental product of C1 chemistry in-generally regarded as the most promising candidate [15-17]. .dustry [1]. It can be economically produced from a varietyAnd numerous studies have been directed toward the synthe-of resources, such as natural gas, coal, heavy oil, and so forthsis and structure-activity relationship of this kind of catalyst[2]. Methanol has many applications in the chemical indus-[18- -21]. The dispersion, particle size and alloying degree atry. In recent years,fuel cell has been widely recognized as .important factors to be considered in order to obtain a goca very attractive device to obtain directly electric energy fromelectro-catalytic activity.the combustion of a chemical product, and has been exten-monly prepared starting from PuC followed by the depositiosively investigated [3- -8]. Among them, the direct methanol of the second metal (M) on PU/C and annealed at high tem-fuel cell has been developed for portable devices such as mo-peratures [22]. However, the heat treatment gives rise to un-bile telephones and laptops on account of its simple designdesired metal particles, which resulted in the decrease of Ptand operation. The direct oxidation of methanol in fuel cellsmass activity in methanol oxidation. An alternative way tohas been extensively studied [9-11]. Platinum is regardedprepare carbon supported Pt-based alloys with small particleas the most active metal for methanol oxidation. However, size is the simultaneous impregnation of Pt and M precursorsstrongly adsorbed CO-like species block the surface of Pt foron the carbon support, followed by reduction at low tempera-further methanol adsorption and result in low DMFC powerture with reducing agents. In the impregnation method at lowdensity. Therefore, Pt-based bimetal, ternary, even quater-temperature, solvent is one of the important factors in obtain-nary catalysts have been used to mitigate CO poisoning ofing catalysts with appropriate structural parameters. The" Corresponding author. Tel&Fax: +86-028-8541801 8; E-mail: rl.wang @ scu.edu.cnThis work was supported by 863 Project (No. 2006AA05Z102), and the Cultivation Fund of the KeY Seintific and Terhnical Inovation Project, Ministryof Education of China (No. 707050), Specialized Research Fund for the Doctoral Program of Higher Educa中国煤化工du Natural ScienceFoundaion (Nos.06GGYB449GX 030, and 07GGZD139GX).fYHCNM HGopyright@2009, Dalian Institute of Chemical Physics, ChineseAcademy of Sciences. All ighs rserve.doi: 10.1016/S 10039953(08)601 14-0342Jinwei Chen et al./ Journal of Natural Gas Chemistry Vol. 18 No.3 2009mixture solvents of H20 and isopropanol have been com-monly employed as the solvent of the precursors [23], andpatterns were obtained at a scanning rate of 4/min with ancatalysts with small particle size are obtained, however, theangular resolution of 0.06° of the scan in the range of 10°alloying degree is low. Recently, Chen et al. [24] reported aand 90°. Transmission electron microscopy (TEM) experi-significant increase in alloying degree when the mixture solu-ments were performed with a JEOL, JEM-100CX II micro-tion of H2O and tetrahydrofuran (THF) was used as the sol-scope. Before taking the electron micrographs, the catalystvent. However, the pH levels and different solvents in prepa-samples were finely ground and ultrasonically dispersed inration process can result in low activity. A modified routeethanol. Then a drop of the resultant dispersion was depositedwas proposed in this paper. Pt-Ru/C catalysts with high ac-on a standard copper grid and dried in air.tivity were obtained using the mixture solvents of H2O, iso-propanol and THP with appropriate pH. The structural param-2.3. Electrode preparation and electrochemical measurementeters of Pt-Ru/C catalyst relied on the solvents used in the pre-pared process, and the high catalytic activity was confirmed inGlassy carbon working electrode (φ3， surface areamethanol eletr-oxidation.0.070 cm2) was used as the support for the electro-oxidationstudies of PtRu/C catalyst. PtRu/C catalyst with 5.0 mg2. Experimentalwas dispersed in a solution containing 1.2 ml DI water and0.80 ml isopropanol and 60 pμl Nafion solution (5wt%) with2.1. Preparation of catalysts .30 min of ultrasonication to make a uniform catalyst suspen-sion. Then 6 μl of dispersed catalyst suspension was pipettedCarbon supported Pt- Ru (atomic ratio of 1 : 1) catalyston the glassy carbon electrode. And the metal loading waswas prepared by chemical reduction of H2PtCl6 and RuCl364 μg/cm2. Then the prepared electrodes were dried in a vac-(AR, Shenyang Research Institute of Nonferrous Metals,China) precursors with sodium borohydride at room tem-All measurements were carried out in the conventionalperature. All the prepared samples consisted of 30% metalthree-electrode electrochemical cell at 25 °C. The electrodes(Pt + Ru) in weight, and 70% carbon black powder (Vulcanprepared with the above procedure served as the workingXC-72, Cabot) which were heat- treated at 600°C for 4h inelectrode. The counter electrode was a graphite electrode,ultra-pure argon gas before use. Isopropanol, tetrahydrofu-and the reference electrode was a saturated calomel electroderan, sodium borohydride, sulfuric acid, methanol (AR, Kelong(SCE), and all the potentials reported in this paper were re-Ltd, China), sodium hydroxide, and sodium carbonate (AR,ferred to SCE. The cyclic voltammograms were carried outChanglian Ltd, China) were used as received without furtherby CHI 760B (CHI Instruments) electrochemical analysis in-treatment.strument. Before the CV measurement, the ultra- pure argonPretreated carbon blacks were dispersed in deionized (DI)gas was used to eliminate oxygen. Methanol electro-ox idationwater with 20 min of ultrasonication to make an uniform car-on Pt-Ru/C catalyst was carried out in a solution containedbon ink, and then the appropriate amount of H2PtCl6 and1.0 mol/L CH3OH and 0.5 mol/L H2SO4 by a CV techniqueRuCl3 precursors were added into the ink with further 20 minbetween -0.24 to 1.0 V with a scan rate of 50 mV/s, and theof ultrasonication. The mixture was mechanically stired forstable CV curves after 10 cycles were obtained and used for5 h, and then was added NaOH to adjust the pH to 8. The :explaining the catalytic activity. And the chronoamperomet-above complex solution was then reduced by slowly adding aric curves were obtained at a potential of 0.5 V with a polar-mixture solution of NaBH4 and Na2CO3. After string forization time of 1000s. All the electrodes and solutions for3h at room temperature, an aqueous solution of HCI waselectro-oxidation measurements were fresh-made.added into the reaction solution to neutralise the excess of0H-. Finally, the suspension was filtered and washed with3. Results and discussionDI water thoroughly, and then dried in vacuum oven at 80。Covernight. This catalyst was denoted as Pt-Ru/C-1. The other3.1. Physicochemical characterization of catalyststhree catalysts were also prepared by the same process as de-scribed above, but the solvents of precursors were different.Figure 1 shows the XRD patterns of Pt-Ru/C-1 (H2O),They were marked as Pt-Ru/C-2 prepared in DI water and iso-Pt-Ru/C-2 (H2O + isopropanol), Pt-Ru/C-3 (H2O + THF) andpropanol solution(1 : 1), Pt-Ru/C-3 prepared in DI water andPt-Ru/C-4 (H2O + isopropanol + THF) catalysts. For pure PtTHF solution (1 : 1), and Pt-Ru/C-4 prepared in the mixturecrystal, the peaks at 39.76°, 46.240, 67.450 and 81.280 are as-of DI water, THF and isopropanol with equal volume ratiosigned to the (111), (200), (220) and (31 1) planes of the face-(1:1: 1), respectively.centered cubic (fcc) structure [25]. And these four character-istic pealks are also observed in Figure 1. In addition, there2.2. Physicochemical characterization of the catalystsare no distinct pe中国煤化工2 or hexagonalclose packed (hcpYHCN MH Ges the absenceThe synthesized Pt-Ru/C catalyst was characterized byof metallic Ru andUi uiaiivyuu Ru most prob-X-ray diffraction (XRD) (DX- 2000, Dandong Ltd, China)ably in amorphous oxides states [26,27]. The peaks of theJournal of Natural Gas Chemistry Vol. 18 No.3 2009343XRD pattern of the as-prepared catalysts are slightly shifted3.2. Effect of solvents on physicochemical characterization ofto higher angles from Pt, which can be interpreted as evidencecatalystsof alloying.Figure 2 shows the linear sweeping voltammograms ofH2PtCl6 both in H2O and in mixture solvent of H2O andTHF with the volume ratio of 1 : 1 on the glassy carbon elec-(11)trode. The reduction reaction of Pt4+ in H2O may proceed ataround 400 mV vs SCE, whereas the onset reduction poten-(200)(311)tial of H2PtCl6 in mixture solvent of H2O and THF is around200 mV, negatively shifted 200 mV. Figure 3 shows voltam-富mograms of RuCl3 in different solvents on the glassy carbonelectrode. RuCl3 has almost the same reduction peak poten-tial at around 200 mV both in H2O and in mixture solvent ofH2O and THF. Therefore, Pt4+ and Ru'+ can be reduced atalmost the same potential in mixture solvent of H2O and THF,suggesting the highest alloying degree of Pt-Ru/C-3 preparedin mixture solution of H2O and THF.20) 50)70.20.(2)_Figure 1. X-ray diffraction patterns of Pt Ru/C catalysts prepared in differentsolvents. (1) Pt-Ru/C-1 (H2O), (2) Pl-Ru/C-2 (H2O + isopropanol), (3) PI--0.Ru/C-3 (H2O + THF), (4) Pt-Ru/C-4 (H2O + isopropanol + THF)目-0.41)/In the XRD pattern of Pt-Ru/C catalyst, the Pt (220)peak is far from the background signal of the carbon support.三-0.6Therefore, the average particle size may be roughly calculated-0.8from Pt (220) FWHM using Debye Scherrer Equation [28].The results are shown in Table 1. The average sizes of the par-ticles are 4.5, 4.2, 3.6 and 3.1 nm for PtRu/C-1, Pt-Ru/C-2,.1.2 EPt-Ru/C-3 and Pt-Ru/C-4, respectively. The average particle-20020040000size of Pt- Ru/C-4 catalyst from TEM is 3.3 nm (2.4- 4.3 nm),E1(mV vs. SCE)in good agreement with the result calculated from XRD.Figure 2. Linear sweeping voltammograms of 2.0 mmol/L H2PtCl6 in (1)A convenient approach to obtain the alloying degree ofH2O and (2) binary solvents of H2O and THF with the volume ratioof 1 : 1Pt-Ru/C catalyst is to measure the lattice constant changeon the glassy carbon electrodes. Test conditions: 0.05 mol/L KCl as elec-caused by alloying [29]. The Pt fcc lattice parameter can betrolyte solution, scan rate: 50 mV/s, 25 °Croughly calculated from the diffraction peak position. And thePt (220) peak was chosen for the calculation of the lttice pa-0os -rameter (a) [28]. Then, the alloying degree of Pt-Ru/C catalystis defined as the Ru atomic fraction (XRu) in the Pt-Ru alloy,which is related to the lattice parameter through the following0.00 fequation proposed by Antolini and co-workers [30,31].a= ao- 0.124xRuξ -0.05 FWhere, ao = 0.39155 nm, the lattice constant of pure Pt.Table 1 summarized the structural data of the differentPt-Ru/C catalysts. It can be clearly seen that, the alloying de--0.10-(2gree of Pt-Ru/C-1 and Pt-Ru/C-2 catalysts are low whereas(1the particle size are large. Of them, the alloying degree ofPt-Ru/C-3 catalyst is the highest. Pt-Ru/C-4 catalyst has a-0.15similar alloying degree as the Pt-Ru/C-3, but the average par-80ticle size is the smallest.E/ (mV vs. SCE)Figure 3. Linear sweping voltammograms of 2.0 mmo/L RuCl3 in (1) H2OTable 1. Structural data of the different Pt-Ru/C catalystsand (2) binary solvents of H2O and THF with the volume ratioof 1:1 onCatalysdxRn/nm20max/(*)a/nmthe glassy carbon elec中国煤化工KCl as electrolyte .t-Ru/C-0.3894solution, scan rate: 50Pt-Ru/C-2.268.040.17THCNMHG68 300.28When the isopauucy出uu mixture solvent20.43.1 306827480.265 0 H2O and THI66Qre were sale7pQmicle size and i&t@2秒/(o)344Jinwei Chen et al./ Journal of Natural Gas Chemistry Vol. 18 No.3 20093 5orsion of Pr-RwC4 ealst obainede. even if the lyward anodic peak (I) is more than 1.0, indicating the CO re-degree was decreased slightly. Xie et al. [32] confirmedsistance to Pt-Ru/C catalyst. The stability of Pt-Ru/C catalyststhat the catalyst dispersion was inversely proportional to vis-in methanol oxidation can be characterized by chronoamper-cosity of the solvent. The dielectric constant and viscosity ofometry at a potential of 500 mV. As shown in Figure 5, for theisopropanol were 20.1 and 2430 Pals, respectively. When iso-four catalysts, the onset oxidation current densities and thepropanol was added into the system, the high viscosity of thedecay current densities are different. The onset oxidation cur-solvent blocked the aggregation of the catalyst particle to arent density on Pt-Ru/C-4 is the highest (310 mA/mg) whilecertain extent. Therefore, the Pt-Ru/C prepared in a mixturethese on both Pt-Ru/C-1 and Pt-Ru/C-3 catalysts are similar,3 OOnt of H2O, THF and isopropanol had the smallest particlebut are lower than that on Pt-Ru/C-4. Pt-Ru/C-2 has the lowestsize, high alloying degree and good dispersion.on the onset oxidation current density. After decay of 1000 s,the decay current densities of the four catalysts have the order3.3. Methanol electro-oxidationof Pt-Ru/C-4> Pt-Ru/C-3> Pt-Ru/C-2>Pt-Ru/C-1, which is inagreement with their behavior in the cyclic voltammograms.The catalytic activities of the Pt-_Ru/C catalysts preparedSo the Pt-Ru/C-4 catalyst prepared in the mixture of DI water,in different solvents for methanol electro-oxidation were an-THF and isopropanol with equal volume ratio (1: 1 : 1) has2 5@d by cyclic voltammograms in an aqueous solution con-the good catalytic activity and longer stability for methanoltaining 0.5 mol/L H2SO4 and 1.0 mol/L CH;OH. From Fig-oxidation.ure 4, it can be seen that the current of the methanol oxidationbecomes apparent as the potential rises above 200 mV. In the350forward scan, methanol oxidation produces an anodic peak at---- Pr-Ru/C-1300Pl-Ru/C-2around 600 mV. In the backward scan, an anodic peak appears---- Pr-Ru/C-3at around 400 mV, which can be atributed to the removal ofPr-Ru/C-42 tgcompletely oxidied carbonaceous species formed in theard scan. For comparison, the onset potential of methanoloxidation on Pt-Ru/C-3 catalyst is approximately similar to香150that on Pt-Ru/C-4, that is, 200 mV, which is negatively shifted100 mV from 300 mV on Pt-Ru/C-1 and Pt-Ru/C-2 catalysts100due to the higher alloying degree. Moreover, Pt-Ru/C-3 cat-alyst has the lowest oxidation peak potential because of itshighest alloying degree. Of them, Pt-Ru/C-4 catalyst has thebgbest current density atributed to the smallest particle size00800and good dispersion. It also can be seen that there are lower/scurrent densities on Pt-Ru/C-1 and Pt-Ru/C-2 catalysts thanFigure 5. Chronoamperometry curves of methanol electro-oxidation on Pt-on Pt-Ru/C-3 and Pt-Ru/C-4 catalysts. Therefore, the catalyticRu/C catalyst. Test conditions: 1.0 mol/L CH3OH + 0.5 mol/L H2SO4 solu-tion, potenial held at 500 mV vs. SCE, at 25。Cactivities for methanol electro-oxidation are in the order of Pt-Ru/C-4>Pt-Ru/C-3> Pt-Ru/C-2> Pt-Ru/C-1.4. Conclusions .1C40Pt-Ru/C-1p-RuC2Solvent has played an important role on the Pt- Ru/C cat-Pt-Ru/C-3alysts prepared by chemical reduction with sodium borohy-P-RuwC-4 .dride for fuel cells' application. Catalysts prepared in H2O orH2O + isopropanol had large particle size and low alloying200degree which resulted in low catalytic activity in methanolelectro-oxidation. When THF was added to the above sol-5vent system, the catalysts displayed higher alloying degree,and showed the lowest onset and peak potential for methanolelectro-oxidation. Moreover, the smallest particle size andhigh level of dispersion of the catalysts were obtained when600800 1000the ternary solvents of isopropanol, H2O and THF were usedE/(mV vs. SCE)in the preparation process. This result can be explained in away that the solvents in the mixture can cooperate one anotherFigyre 4. Cyclic voltammograms of methanol eletro-oxidation on Pt-Ru/Cto block the aggtalyst Darticles. In summary, thecatlyt. Test conditions: 1.0 mol/L CH3OH + 0.5 mo/L H2SO4 solution,scan rate: 50 mV/s, 10h scan, at 25°Ccatalytic activities中国煤化工ts for methanolelectro- oxidation IYHCN MH Gr-Ru/C-4 (H2OAGording to Figure 4, it 260 be observed that 400+ isopropanol 62σ0)>Pt-Ru/C-2 1 00(tio of the forward anodic peak Current density (If) to the back-(H2O + isopropanol)>Pt Ru/C-1 (H2O).t |SJournal of Natural Gas Chemistry Vol. 18 No.32009345References[17] Spendelow J s, Babu P K, Wieckowski A. Current Opinion inSolid State & Materials Science, 2005, 9(1-2): 37[1] Ye F, Chen sZ, Dong X F, Lin W M. J Natur Gas Chem, 2007,[18] Wang K, Yin H, Sha W, Huang J, Fu H. J Phys Chem B, 2007,111(45): 1299716(2): 1622] TjmP J A, Waller F J, Brown D M. Appl Catal A: General,[19] Rolison D R, White H S. Langmuir, 1999, 15(3): 64920] Wang D, Zhuang L, LuJ T. J Phys Chem C, 2007, 1144):2001, 221(1-2): 27516416[3] Wilson M S, Valerio J A, Gottesfeld S. 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