Effect of Reduction Temperature on the Preparation and

Feb 13, 2009 - Raghuram Chetty*, Wei Xia, Shankhamala Kundu, Michael Bron, Thomas Reinecke, Wolfgang Schuhmann and Martin Muhler*. Lehrstuhl für ...
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Langmuir 2009, 25, 3853-3860

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Effect of Reduction Temperature on the Preparation and Characterization of Pt-Ru Nanoparticles on Multiwalled Carbon Nanotubes Raghuram Chetty,*,†,| Wei Xia,† Shankhamala Kundu,† Michael Bron,†,‡ Thomas Reinecke,§ Wolfgang Schuhmann,‡ and Martin Muhler*,† Lehrstuhl fu¨r Technische Chemie, Analytische Chemie-Elektroanalytik and Sensorik, and Institut fu¨r Geologie, Mineralogie and Geophysik, Ruhr-UniVersita¨t Bochum, UniVersita¨tsstrasse 150, 44780 Bochum, Germany ReceiVed December 8, 2008. ReVised Manuscript ReceiVed January 16, 2009 Carbon nanotubes (CNT) supported platinum-ruthenium (Pt-Ru) catalysts were prepared by impregnation-reduction using an ethanolic solution of H2PtCl6 and RuCl3. The effect of reduction temperatures on particle size, surface area and their relationship to the electrocatalytic activity for methanol oxidation were investigated. Thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopy (XPS) were used for the catalyst characterization. XRD analysis showed that the PtRu/ CNT catalysts possibly consist of separate Pt and Ru phases. XPS analysis showed that the catalysts contain hydrous ruthenium oxide in addition to Pt and Ru metal and oxide species. The electrocatalytic activities of the catalysts were investigated in half-cell experiments using cyclic voltammetry, CO stripping voltammetry, chronoamperometry, and impedance spectroscopy. The results showed that the catalyst reduced at a temperature of 350 °C had the largest electrochemical surface area, lowest charge transfer resistance and the highest electrocatalytic activity for methanol oxidation. The superior catalytic activity is discussed based on the presence of appropriate amount of hydrated Ru oxide.

1. Introduction Electrooxidation of methanol has been receiving great attention over the last two decades due to the possible application in direct methanol fuel cells (DMFC), which show great potential as highefficiency, low-emission future power source.1,2 Electrochemical experiments have shown that CO, formic acid, formaldehyde, etc. are intermediates in the oxidation of methanol on a Pt electrode. Since CO is strongly adsorbed on the Pt surface reducing considerably its electroactivity, methanol oxidation at reasonable rates is convincible either by the addition of a second or third metal to the Pt catalyst3,4 or by increasing the operating temperature.5 Up to date, bimetallic Pt-Ru has been considered to be the most active catalyst for the oxidation of methanol, and is indeed the state of the art anode catalyst for DMFC.1-4,6 The oxidation of adsorbed CO is postulated to be the rate-determining step and Ru is widely accepted as a promoter for the CO oxidation, commonly explained on the basis of the bifunctional mechanism or the ligand effect. The bifunctional mechanism assumes that Ru promotes the oxidation of the strongly bound CO on Pt by * Corresponding authors. (R.C.) E-mail: [email protected]. (M.M.) E-mail: [email protected]: +91 44 22574178. Fax: +91 44 22574152. † Lehrstuhl fu¨r Technische Chemie, Ruhr-Universita¨t Bochum. ‡ Analytische Chemie-Elektroanalytik and Sensorik, Ruhr-Universita¨t Bochum. § Institut fu¨r Geologie, Mineralogie and Geophysik, Ruhr-Universita¨t Bochum. | Present address: Chemical Engineering Department, Indian Institute of Technology, Madras, India. (1) Scott, K.; Shukla, A. K. In Modern Aspects of Electrochemistry; White, R. E., Ed.; Springer: Berlin, 2007; Vol. 40, p 127. (2) Arico`, A. S.; Srinivasan, S.; Antonucci, V. Fuel Cells 2001, 1, 133. (3) Antolini, E. Mat. Chem. Phys. 2003, 78, 563. (4) Antolini, E. Appl. Catal., B 2007, 74, 338. (5) Lobato, J.; Canizares, P.; Rodrigo, M. A.; Linares, J. J.; Vizcaino, R. L. Energy Fuels 2008, 22, 3335. (6) Petrii, O. A. J. Solid State Electrochem. 2008, 12, 609.

supplying an oxygen source (Ru-OHad).7 According to the ligand effect, the electronic structure of the surface atoms is changed so that the binding strength of adsorbed CO is weakened, thereby reducing the oxidation overpotential.8 However, improvement of catalytic activity for the oxidation of methanol is an essential goal in the development of a practical DMFC. Recently, carbon nanotubes (CNT) as catalyst support have generated intense interest in fuel cell applications due to their unique properties, such as high external surfaces, good electronic conductivity, large surface to volume ratio and high stability, which make CNT an ideal supporting material.9,10 Additionally, the metal particles supported on the CNT seem to be less susceptible to CO poisoning than those deposited onto traditional carbon supports.11 Preparation of highly active Pt-Ru catalysts is an area of active research, and several methods are being adapted for the development of the bimetallic catalyst. It is well-known that the synthesis method has a strong influence on the composition, structure, dispersity, morphology and the performance of the catalyst.3,12,13 Hence, various synthetic techniques have been used for the preparation of Pt-Ru catalysts, such as impregnationreduction,13-19 colloidal chemistry,13,14,20-22 polyol pro(7) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (8) Ishikawa, Y.; Liao, M. S.; Cabrera, C. R. Surf. Sci. 2000, 463, 66. (9) Lee, K.; Zhang, J.; Wang, H.; Wilkinson, D. P. J. Appl. Electrochem. 2006, 36, 507. (10) Serp, P.; Corrias, M.; Kalck, P. Appl. Catal., A 2003, 253, 337. (11) Huang, J. E.; Guo, D. J.; Yao, Y. G.; Li, H. L. J. Electroanal. Chem. 2005, 577, 93. (12) Chan, K. Y.; Ding, J.; Ren, J.; Cheng, S.; Tsang, K. Y. J. Mater. Chem. 2004, 14, 505. (13) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D. P. J. Power Sources 2006, 155, 95. (14) Coutanceau, C.; Brimaud, S.; Lamy, C.; Le´ger, J. M.; Dubau, L.; Rousseau, S.; Vigier, F. Electrochim. Acta 2008, 53, 6865. (15) Yang, B.; Lu, Q.; Wang, Y.; Zhuang, L.; Lu, J.; Liu, P.; Wang, J.; Wang, R. Chem. Mater. 2003, 15, 3552.

10.1021/la804039w CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

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cesses,23,24 microemulsion,13,14,25 alcohol-reduction,26 sol-gel methods,27 etc. However, most of these preparation methods require repeated filtering and washing, and these procedures are time-consuming and easily cause loss of noble metals, some methods require the use of surfactants or polymers as stabilizers and the removal of these becomes a major problem.28-30 The most popular and simple among these preparation methods is impregnation, which is a straightforward chemical preparation technique involving an impregnation step followed by a reduction step. This method eliminates the need for tedious filtering and washing procedures and can also be used for large-scale production.15 Catalyst-support interactions play a fundamental role in catalysis, and the synthesis temperature may influence the surface functional groups responsible for anchoring the metal nanoparticles to the carbon nanotubes and in turn to their catalytic activity. In this work, emphasis was put on the optimization of the reduction temperature for the PtRu/CNT catalysts prepared by impregnation. The catalysts were characterized using various physical characterization methods such as thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy to determine the chemical, morphological, and crystallographic properties. Electrochemical methods such as cyclic voltammetry, linear sweep voltammetry, CO stripping voltammetry, chronoamperometry, and impedance spectroscopy were employed for the activity tests. It was found that the catalytic activity of the PtRu/ CNT toward the oxidation of methanol can be promoted by an appropriate reduction temperature.

2. Experimental Section Multiwalled carbon nanotubes having an inner diameter of 20-50 nm and an outer diameter of 50-200 nm were obtained from Applied Science Inc. (Ohio). As received CNT were first calcinated under high purity He (99.9999%) atmosphere for 60 min at 800 °C, and the CNT were functionalized by refluxing in 4 mol dm-3 H2SO4 + 4 mol dm-3 HNO3 at 90 °C for 5 h.31 The acid-treated CNT were diluted with water, filtered, washed with excess deionized water, and dried at 90 °C overnight. 2.1. Catalyst Synthesis. Pt-Ru catalysts were prepared by impregnation-reduction method. The required amount of H2PtCl6.6H2O (Alfa aesar) and RuCl3 · xH2O (Aldrich) were dissolved in ethanol, followed by the addition of functionalized CNT to the precursors solution to attain a nominal metal loading of 20 wt % and (16) Carmo, M.; Paganin, V. A.; Rosolen, J. M.; Gonzalez, E. R. J. Power Sources 2005, 142, 169. (17) Kawaguchi, T.; Sugimoto, W.; Murakami, Y.; Takasu, Y. J. Catal. 2005, 229, 176. (18) Wang, D.; Zhuang, L.; Lu, J. J. Phys. Chem. C 2007, 111, 16416. (19) Cui, Z.; Liu, C.; Liao, J.; Xing, W. Electrochim. Acta 2008, 53, 7807. (20) Vidakoviæ, T.; Christov, M.; Sundmacher, K.; Nagabhushana, K. S.; Fei, W.; Kinge, S.; Bo¨nnemann, H. Electrochim. Acta 2007, 52, 2277. (21) Han, K.; Lee, J.; Kim, H. Electrochim. Acta 2006, 52, 1697. (22) Bock, C.; Paquet, C.; Couillard, M.; Botton, G. A.; MacDougall, B. R. J. Am. Chem. Soc. 2004, 126, 8028. (23) Lee, D.; Hwang, S.; Lee, I. J. Power Sources 2006, 160, 155. (24) Tsuji, M.; Kubokawa, M.; Yano, R.; Miyamae, N.; Tsuji, T.; Jun, M. S.; Hong, S.; Lim, S.; Yoon, S. H.; Mochida, I. Langmuir 2007, 23, 387. (25) Rojas, S.; Garcı´a, F. J. G.; Ja¨ras, S.; Huerta, M. V. M.; Fierro, J. L. G.; Boutonnet, M. Appl. Catal., A 2005, 285, 24. (26) Sarma, L. S.; Lin, T. D.; Tsai, Y. W.; Chen, J. M.; Hwang, B. J. J. Power Sources 2005, 139, 44. (27) Suffredini, H. B.; Tricoli, V.; Avaca, L. A.; Vatistas, N. Electrochem. Commun. 2004, 6, 1025. (28) Pozio, A.; Silva, R.; De Francesco, M.; Cardellini, F.; Giorgi, L. Electrochim. Acta 2002, 48, 255. (29) Antolini, E.; Giorgi, L.; Cardellini, F.; Passalacqua, E. J. Solid State Electrochem. 2001, 5, 131. (30) Luna, A.; Camara, G.; Paganin, V.; Ticianelli, E.; Gonzalez, E. Electrochem. Commun. 2000, 2, 222. (31) Prabhuram, J.; Zhao, T. S.; Liang, Z. X.; Chen, R. Electrochim. Acta 2007, 52, 2649.

Chetty et al. 1:1 Pt:Ru ratio. The mixture was sonicated for 3 h, ethanol was evaporated at 80 °C and the impregnated and dried CNT were ground in an agate mortar. The impregnated CNT were divided into several portions, placed in a quartz boat and submitted to an additional drying step under helium atmosphere at 100 °C for 1 h. Then the decomposition/ reduction of the metal precursors was carried out at various temperatures in a mixture of 1:1 He:H2 for 2 h at a flow rate of 100 cm3 min-1, and the catalysts were stored in a vacuum desiccator. 2.2. Characterization. Thermogravimetry (TG)/differential thermogravimetry (DTG) was performed with a Cahn TG-2131 thermobalance for the impregnated and dried CNT in a mixture of H2:He (1:1) at a flow rate of 100 cm3 min-1 and a heating rate of 5 K min-1 using a 20 mg sample. A Hitachi H-8100 microscope operated at 200 kV was used for transmission electron microscopy (TEM) measurement, and scanning electron microscopy (SEM) images were obtained with LEO Gemini 1530 microscope. The composition of the prepared catalysts was determined by energy-dispersive X-ray analysis (EDX) interfaced with the TEM, and was further confirmed by atomic absorption spectrophotometry. X-ray diffraction patterns were measured using a PANalytical theta-theta powder diffractometer equipped with a Cu KR radiation source, a secondary graphite monochromator and 0.04 rad incident/secondary beam soller slits. Measurement conditions were 45 kV, 40 mA, 0.5° divergence slit and 0.2 mm receiving slit width. To obtain a good signal-to-noise ratio which allows detection of minor phase fractions in nanocrystalline materials, scans were run from 10 to 60° 2θ with 0.04° step width and 90-130 s collection time/step and from 60 to 140° 2θ with 0.06° step width and 180-210 s/step. A pseudo-Voigt function was used to fit the experimental XRD data. X-ray photoelectron spectroscopy measurements were carried out in an ultrahigh vacuum setup equipped with a Gammadata-Scienta SES 2002 analyzer. A monochromatic Al KR (1486.6 eV; anode operating at 14 kV and 55 mA) X-ray source was used as incident radiation. The binding energies were calibrated based on the C1s peak of graphite at 284.5 eV ((0.5 eV energy resolution of the spectrometer at the setting employed) as a reference. The CASA XPS program with a Gaussian-Lorentzian mix function and Shirely back ground subtraction was employed to deconvolute the XP spectra. 2.3. Electrocatalytic Study. Electrochemical measurements were carried at room temperature (21 ( 1 °C) in a three-electrode cell connected to an Autolab PGSTAT12 potentiostat (Eco Chemie). The working electrode was a thin layer of Nafion-impregnated catalyst cast on a 3 mm glassy carbon rod (Sigradur G) held in a Teflon holder. The electrode was polished to a mirror finish with 0.1 and 0.03 µm alumina suspensions prior to catalyst coating. A catalyst ink was obtained by sonicating for 1 h a mixture of 2.5 mg of PtRu/C catalyst in 1 mL of deionized water and 0.5 mL of Nafion solution (Aldrich: 5 wt % Nafion). Then, 7 µL of the ink was pipetted and spread on the glassy carbon surface, and the electrode was dried at 90 °C for 1 h. Pt gauze and a commercial saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. For methanol oxidation, the electrolyte solution was 1 mol dm-3 CH3OH in 0.5 mol dm-3 H2SO4 and the solutions were purged with argon for 20 min prior to each measurement. Several activation scans were performed until reproducible voltammograms were obtained, and only the last cycles were used for comparison of the catalytic activity. The chronoamperometry tests were conducted between 0.3 to 0.5 V for a period of 30 min. The CO stripping voltammograms were conducted using the three-electrode cell and 0.5 mol dm-3 H2SO4 solution. Initially, Ar was purged to the electrolyte solution for 20 min. Afterward, CO gas was purged for 15 min by holding the electrode at a potential at -0.16 V. Then, the dissolved CO in the solution was removed by bubbling Ar into the solution for 20 min, and the stripping voltammograms were collected at a scan rate of 50 mV s-1. Electrochemical impedance spectroscopy (EIS) experiments were carried with a Solartron 1250 frequency response analyzer in 0.5 mol dm-3 H2SO4 with 1 mol dm-3 methanol at room temperature. The impedance spectra were recorded at frequencies from 103 to 0.01 Hz with a logarithmic data

Pt-Ru Nanoparticles

Figure 1. Thermoanalytical (TG and DTG) curves of Pt-Ru impregnated CNT in a 1:1 H2:He atmosphere at a heating rate of 5 K min-1.

collection at 10 steps per decade with an amplitude of 10 mV at the desired potential.

3. Results and Discussion The nature and concentration of the surface functional groups responsible for anchoring the metal precursors during catalyst preparation can be modified during thermal treatment/reduction conditions, which in turn may vary the electrocatalytic activity. In the following section, physical characterization and electrochemical activity of PtRu/CNT catalysts subjected to various reduction temperatures are examined and compared. 3.1. Thermogravimetry. TG analysis was carried out to investigate the reduction steps at a heating rate of 5 K min-1 from room temperature to 800 °C. Figure 1 shows the TG-DTG profile of the CNT impregnated with H2PtCl6 and RuCl3. As can be seen from the curves, the decomposition of metal precursors occurs at two or more steps and the reduction was complete at about 300 °C. The first minor weight loss in the curve can be mainly associated with the loss of physisorbed water,32 and the second major weight loss is due to the reduction of both the ruthenium and platinum chloride precursors. With further increase in the temperature a mass gain is observed in the range of 300-500 °C, which could be due to oxidation of metallic Ru to RuO2 by oxygen traces in the carrier gas.33 The final mass loss observed above 500 °C is attributed to the decomposition of the functional groups such as phenol, carbonyl, quinone, lactone, ether, etc. in the acid treated CNT.32,34 From the TG profile, four different temperatures, i.e., 150, 250, 350, and 450 °C, were chosen for the reduction in order to compare their influence on the catalytic activity toward CO and methanol oxidation. 3.2. Transmission and Scanning Electron Microscopy. TEM micrographs taken after the deposition of Pt-Ru on the carbon nanotubes and at reduction temperatures of 150, 250, 350, and 450 °C are shown in Figure 2. It can be seen that aggregation of the metallic nanoparticles is minimal at lower temperature (150 °C) and highly dispersed Pt-Ru nanoparticles are obtained over the CNT. The nanoparticle sizes seem to increase with the increase in reduction temperature, and the increase of particle size can be attributed to the fact that agglomeration of metallic particles can occur at higher temperatures. The particle size range was found to be 3-5 nm for the catalyst reduced at (32) Toebes, M. L.; van Heeswijk, J. M. P.; Bitter, J. H.; van Dillen, A. J.; de Jong, K. P. Carbon 2004, 42, 307. (33) Long, J. W.; Stroud, R. M.; Lyons, K. E. S.; Rolison, D. R. J. Phys. Chem. B 2000, 104, 9772. (34) Kundu, S.; Wang, Y.; Xia, W.; Muhler, M. J. Phys. Chem. C 2008, 112, 16869.

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150 °C, whereas it increased to 10-12 nm when the reduction temperature was 450 °C. A histogram of the metal particle size distribution for catalyst reduced at 350 °C is shown in the Supporting Information. The TEM particle size ranges determined for all the reduction temperatures are given in Table 1. SEM images of the PtRu/CNT catalysts prepared at various reduction temperatures are given in the Supporting Information, where well dispersed metallic nanoparticles on the surface of the CNT can be clearly seen, but no noticeable difference in the particle size can be observed from these images. 3.3. X-ray Diffraction. The XRD patterns of the Pt-Ru catalysts prepared at various reduction temperatures are shown in Figure 3. In the XRD patterns, the diffraction peaks of the face-centered cubic (fcc) Pt lattice are clearly recognizable, as indicated by the characteristic peaks around 39.7°, 46.3°, 67.5° 81.3° and 85.7° due to Pt (111), (200), (220), (311), and (222), respectively. The peak at ca. 2θ ) 26° is attributed to the (002) of the graphitic structure of CNT. Diffraction peaks due to Ru were not observed in the XRD spectra of the catalyst reduced at 150 and 250 °C. The absence of Ru peaks could indicate the presence of Ru in an amorphous form.22 By increasing the reduction temperature to 350 °C, a new peak appears at 2θ ) 44.0° which is identified as the Ru (101) plane (which represents the most intense reflection of the hexagonal Ru pattern), and the peak increases with the increase in reduction temperature. For the catalyst reduced at 450 °C, additional Ru diffraction lines at 38.4, 69.4, 84.7° 2θ for Ru (100), (110), and (112), respectively, occur as shoulders on the Pt (111), (220), and (311) diffraction lines, suggesting the presence of a separate Ru phase. As reported by Roth et al.,35 separation of Ru with the increase in temperature evidence the presence of unalloyed Pt and Ru phase. This is further confirmed by the percentage of Ru in the Pt-Ru solid solution calculated from the lattice parameter-composition relationship,36 which was in the range 2-5% for the PtRu/CNT samples reduced at 150 to 450 °C. In order to obtain the lattice parameter (a) of a particular sample, the apparent lattice parameter (ao) was calculated from the KR1 positions (λ ) 0.15406 nm) of 5-6 Pt reflections in the 60-140° 2θ range. A linear least-squares fit to the Nelson-Riley function37 and extrapolation to 2θ ) 180° gave the true (a). The crystallite size was determined using the integral breadth method of the Pt (220) reflection which was corrected for instrumental broadening. The crystallite size and the calculated lattice parameters obtained for all the catalysts are given in Table 1. The crystallite sizes were 7-8 nm for the PtRu/CNT catalysts reduced between 150 to 350 °C, and increased to 11 nm when the reduction temperature was 450 °C. There is a slight discrepancy between the particle size obtained from XRD and TEM measurements for the catalyst reduced at 150 °C, but as reported by Ozkaya et al.,38 a more precise estimate of the smaller particles can be expected from the TEM measurements. 3.4. X-ray Photoelectron Spectroscopy. The nature of surface species on the PtRu/CNT catalysts was investigated by XPS. The elemental surface compositions of all the components derived from XPS for the PtRu/CNT catalysts subjected to various reduction temperatures are given in Table 2. XPS spectra for the catalyst reduced at 150 °C showed the presence of Cl-, with a peak at 198 eV typically observed for the Cl 2p core level, as it is possible that complete removal in such low temperature heat treatment could be difficult. However, no Cl- was observed for the catalysts reduced at higher reduction temperatures. Figure 4 (35) Roth, C.; Martz, N.; Fuess, H. Phys. Chem. Chem. Phys. 2001, 3, 315. (36) Gasteiger, H. A.; Ross, P. N., Jr.; Cairns, E. J. Surf. Sci. 1993, 293, 67. (37) Nelson, J. B.; Riley. D. P. Proc. Phys. Soc., London. 1945, 57, 160. (38) Ozkaya, D. Platinum Met. ReV. 2008, 52, 61.

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Figure 2. TEM micrographs (scale bar ) 100 nm) of Pt-Ru nanoparticles deposited on carbon nanotubes at varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C. Table 1. Crystallite Size Obtained from Pt Reflection and Lattice Parameters for the PtRu/CNT Catalysts Prepared at Varying Reduction Temperaturesb catalyst

(220) peak position (2θ)

PtRu/CNT-150 PtRu/CNT-250 PtRu/CNT-350 PtRu/CNT-450

67.59 67.58 67.56 67.60

phase detected C, C, C, C,

Ptss Ptss Ptss, Ruhex Ptss, Ruhex

lattice parameter (nm)

crystallite sizea (nm)

TEM particle size range (nm)

0.3923 0.3922 0.3922 0.3920

7 7 8 11

3-5 5-7 8-10 10-12

a Calculated from reciprocal integral breadth (β*f)-1 of the Pt (220) reflection, β*f ) (βf π cos θ)/(180 λCuKR1), assuming spherical crystallites. Ptss: Pt solid solution. Ruhex: hexagonal Ru. b Diffraction data were compared to the TEM particle size.

Table 2. XPS Surface Atomic Concentration of PtRu/CNT Subjected to Various Reduction Temperatures

Figure 3. X-ray diffraction patterns of Pt-Ru deposited on carbon nanotubes at varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C. Diffractions due to separate Ru phase marked as *.

shows the Pt 4f region of the catalysts. The spectra of the Pt 4f region were fitted with Doniach-Sunjic profiles39 convoluted with Gaussian distribution. The Pt 4f spectrum showed a doublet containing a lower energy (Pt 4f7/2) band and a higher energy (Pt 4f5/2) band at around 71.5 and 74.9 eV, respectively. To identify the different oxidation states of Pt, the spectrum was deconvoluted into two pairs of peaks at around 71.5 and 74.8 eV and at 72.8 (39) Doniach, S.; Sunjic, M. J. Phys. C 1970, 3, 285.

reduction temperature/°C

C/%

O/%

Pt/%

Ru/%

Cl/%

150 250 350 450

84.5 87.2 87.3 90.4

8.1 8.8 8.5 5.9

1.5 1.6 1.5 1.6

2.6 2.4 2.6 2.1

3.2

and 75.9 eV. These two pairs of peaks indicate that Pt is present in two different oxidation states, Pt0 and Pt2+.40 Binding energies (BE) and relative percentages of Pt0 and Pt2 species obtained from the respective intensities are listed in Table 3. Pt0 is found to be the predominant species in all the catalysts. The Pt0 percentage for the catalyst reduced at 150 °C was 63.6% while it increased to 83.8% when the reduction temperature was 450 °C. Figure 5 shows the Ru 3p region of the PtRu/CNT catalysts. Since the C 1s peak covers the Ru 3d3/2 signal and partially overlaps with the Ru 3d5/2 peak, a quantitative estimation of the oxidation states is not possible from those spectra. Hence the Ru 3p3/2 region was chosen for the analysis. The Ru 3p3/2 signal is deconvoluted into three peaks of different intensities located around 461.5, 463.7 and 467.1 eV. The lower binding energy components were assigned to Ru0 and anhydrous RuO2 species, whereas the component with BE 467.1 eV matches the character (40) Raman, R. K.; Shukla, A. K.; Gayen, A.; Hegde, M. S.; Priolkar, K. R.; Sarode, P. R.; Emura, S. J. Power Sources 2006, 157, 45.

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Figure 4. X-ray photoelectron spectra of PtRu/CNT subjected to varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C showing the Pt 4f region. The spectra were deconvoluted into two pairs of peaks which are assigned to Pt0 (71.5, 74.8 eV) and Pt2+ (72.8, 75.9 eV) species. Key: points, experimental data; lines, fitting curves.

Figure 5. X-ray photoelectron spectra of PtRu/CNT subjected to varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C showing the Ru 3p region. The spectra were deconvoluted into three peaks which are attributed to Ru0 (461.5 eV), RuO2 (463.7 eV), and RuOxHy (467.1 eV) species. Key: points, experimental data; lines, fitting curves.

Table 3. Binding Energy and Relative Intensity Values for Various Pt Species As Observed from the Pt 4f Spectra of the PtRu/CNT Catalyst Reduced at Different Temperatures

Table 4. Relative Intensity Values for Various Ru Species As Observed from the Ru 3p Spectra of the PtRu/CNT Catalysts Reduced at Different Temperatures

reduction temperature/°C 150 250 350 450

Pt species

peak position/eV

relative concentration/%

reduction temperature/°C

Pt0 Pt2+ Pt0 Pt2+ Pt0 Pt2+ Pt0 Pt2+

71.5 72.8 71.4 72.8 71.4 72.9 71.5 72.8

63.6 36.4 79.7 20.3 80.5 19.5 83.6 16.4

150

of hydrous RuO2 (also written as RuOxHy), which is reported to have a higher BE than its anhydrous form.41 The presence of RuO3 was excluded in favor of the hydrous amorphous RuO2 · xH2O, since the former is thermodynamically unstable under the reduction temperatures employed here.41 The relative percentages of all the three Ru components are given in Table 4. The table reveals that Ru0 was enriched on the surface of PtRu/CNT from 32% to 55% by increasing the reduction temperature from 150 to 450 °C. Another predominant form of surface ruthenium was RuO2, which decreased in favor of metallic Ru with the increase in reduction temperature. The surface concentration of RuOxHy remained constant at 15% for 150 °C250 °C, and increased to 20% when increasing the reduction temperature to 350 °C before decreasing to 16% at 450 °C. The decrease in RuOxHy with increasing reduction temperature could be due to dehydration of the hydrous ruthenium oxide. 3.5. CO Stripping Study. CO stripping voltammetry was performed in order to determine the electrochemical surface area (ESA) of the PtRu/CNT catalysts prepared at various reduction temperatures. Figure 6 compares the CO stripping voltammograms and the subsequent cyclic voltammograms (CV) in 0.5 (41) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W. Langmuir 1999, 15, 774.

250 350 450

Ru species

peak position/eV

relative concentration/%

Ru0 RuO2 RuO2 · xH2O Ru0 RuO2 RuO2 · xH2O Ru0 RuO2 RuO2 · xH2O Ru0 RuO2 RuO2.xH2O

461.8 463.6 466.8 461.5 463.7 466.7 461.5 463.7 467.1 461.7 463.9 466.7

32.3 52.7 15.0 47.3 37.2 15.5 46.3 33.7 20.0 55.3 28.6 16.1

mol dm-3 H2SO4 at a scan rate of 50 mV s-1. It can be observed from the voltammograms that for all the catalysts, in the first scan the hydrogen desorption peaks are completely suppressed in the lower potential region, due to the saturated coverage of COads species on the Pt sites. All CVs show a single oxidation peak, whereas no CO oxidation was observed in the second scan confirming the complete removal of the adsorbed CO species. The ESA of all the catalysts were measured using the CO oxidation charge after subtracting the background current of the subsequent CV with the assumption of 420 µC cm-2 as the oxidation charge for one monolayer of CO on a smooth Pt surface.31 The measured ESA value for all the catalysts along with the onset and peak potentials of CO oxidation are given in Table 5. As can be seen, the electrochemical surface area increased with the increase in reduction temperature and the catalyst prepared at 350 °C showed the highest ESA. The decrease in surface area for the catalyst reduced at 450 °C indicates the existence of optimum condition for the enhanced CO oxidation. In other words, according to the reduction temperature, the Pt and Ru nanoparticles contain favorably or unfavorably modified properties for CO oxidation,

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which also depend on the interaction with the functional groups of the CNT support, as higher reduction temperatures can cause loss of oxygen functional groups.32 For the catalyst prepared at 250 °C a positive shift of 20 mV is observed in the onset potential compared to the catalyst reduced at 350 °C. This is probably due to the lower particle size of the former catalyst affecting the CO oxidation onset potential as reported in the literature.42 The low surface area obtained for catalyst reduced at 150 °C is most likely due to the incomplete reduction of the Pt and Ru precursors. As evident by the XPS suggesting the presence of Cl- ions, could have resulted in low surface area owing to residual species covering the electrocatalyst. The PtRu/CNT catalyst reduced at 350 °C showed the highest surface area, probably as a result of good metal-support interaction and dispersion of the Pt and Ru nanoparticles with the CNT surface. 3.6. Methanol Oxidation. The electrocatalytic activities of the PtRu/CNT catalysts for methanol oxidation reaction were studied by cyclic voltammetry in 1 mol dm-3 methanol/0.5 mol dm-3 H2SO4 at a scan rate of 50 mV s-1. The resulting voltammograms are shown in Figure 7, which from the overall shape are in good agreement with those reported in the literature.31 The peak in the forward scan can be attributed to the oxidation of methanol and in the reverse scan to the removal of the incompletely oxidized carbonaceous species formed during the forward scan. The catalyst reduced at a temperature of 150 °C, showed very low current density for methanol electrooxidation. This is in agreement with the presence of impurities such as Clas discussed in the CO oxidation study, which might have suppressed the catalytic activity. The onset potentials for all the other catalysts were between 200 to 250 mV. The methanol oxidation current increased with the increase in reduction temperature, with the catalyst prepared at 350 °C showed the maximum current density. Furthermore, linear sweep voltammograms were also performed at a pseudosteady state scan rate of 1 mV s-1 in the potential range of -0.24 to 0.7 V as shown

in Figure 7 inset. The same trend was observed also for the lower scan rate, with the catalyst reduced at 350 °C showing the highest methanol oxidation current. Table 5 also gives a comparison of the catalytic activity for all the PtRu/CNT catalysts in terms of forward anodic peak potential for methanol oxidation, and the ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) to indicate the catalytic performance. The ratio of If to Ib can be used to describe the tolerance of catalysts to the accumulation of carbonaceous species.43 A high If/Ib ratio implies an effectively complete oxidation of methanol to carbon dioxide during the anodic scan and the removal of poisoning species from the surface of the catalyst. In contrast, a low If/Ib ratio implies the reverse. The forward anodic peak current density of the methanol oxidation for PtRu/CNT catalysts increased in the order PtRu/CNT-150 °C < 450 °C < 250 °C < 350 °C. As can be seen from Table 5, the catalyst reduced at 350 °C showed the highest If/Ib ratio of 2.01, suggesting an improved tolerance to carbonaceous species poisoning. In addition to the large ESA as discussed in the CO stripping study, the accumulation of intermediate carbonaceous species on the Pt surface leading to catalytic poison can be partly overcome by the presence of appropriate amount of oxygen source (RuOxHy) present at the vicinity for the catalyst reduced at 350 °C could be a reason for the observed high activity. 3.7. Chronoamperometry. In order to compare the longterm performance of the PtRu/CNT catalysts prepared at various reduction temperatures toward methanol oxidation reaction, the catalysts were biased at potentials between 0.3 and 0.5 V, and changes in the oxidation current with time were recorded. Figure 8 shows the chronoamperometric response recorded for 1 mol dm-3 methanol in 0.5 mol dm-3 H2SO4 at room temperature. Prior to the electrochemical measurements, the electrolyte was deaerated by Ar and the working electrodes were subjected to the following pretreatment.43 The potential was stepped up from the open circuit voltage to 0.8 V. Two seconds later, the potential was instantaneously lowered to 0.2 V for 2 s to remove any adsorbed oxides or hydroxides44 formed on the electrode at 0.8 V. Recording of the current transient was then performed at 0.3, 0.4, or 0.5 V, which was maintained for 1800 s. As can be seen from Figure 8, the oxidation current decreased continuously for all the catalysts, supposedly because of catalyst poisoning by chemisorbed carbonaceous species formed during the oxidation of methanol.31 The PtRu/CNT catalyst reduced at 350 °C was able to maintain the highest current density among all the catalysts at all potentials confirming the best electrocatalytic performance. The metal-support interaction between the nanoparticles and the functional group of carbon nanotubes for the catalyst reduced at 350 °C could have been stronger owing to the higher stability of the deposited nanoparticles in comparison to the other reduction temperatures. As discussed in the thermogravimetry analysis, high reduction temperature results in loss of functional groups from the CNT surface, which is further confirmed by the XPS analysis (Table 2) where lower surface oxygen was observed for the catalyst reduced at 450 °C. 3.8. Electrochemical Impedance Spectroscopy. EIS was used to evaluate the activity for methanol oxidation of the Pt-Ru/ CNT catalysts prepared at various reduction temperatures. Figure 9 shows the Nyquist plots obtained in a solution of 0.5 mol dm-3 H2SO4 and 1 mol dm-3 CH3OH at various bias potentials for the PtRu/CNT catalysts reduced at 150 °C, 250 °C, 350 and 450 °C. At potentials lower than 0.3 V, big open arcs are observed, in which double layer charging followed by adsorption of methanol

(42) Maillard, F.; Savinova, E. R.; Stimming, U. J. Electroanal. Chem. 2007, 599, 221.

(43) Gu, Y. J.; Wong, W. T. Langmuir 2006, 22, 11447. (44) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004, 108, 8234.

Figure 6. CO-stripping voltammograms of PtRu/CNT subjected to varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C in 0.5 mol dm-3 H2SO4 at room temperature and 50 mV s-1 scan rate. The dotted line voltammograms refer to the second cycle (after removal of the adsorbed CO).

Pt-Ru Nanoparticles

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Table 5. Electrochemical Surface Area, Onset/Peak Potential of CO Oxidation, Methanol Oxidation Peak Potential and If/Ib Ratio for the PtRu/CNT Catalysts catalyst reduction temperature/°C

CO oxidation onset potential/V

CO oxidation peak potential/V

electrochemical surface area/m2 g-1

MeOH oxidation peak potential

If/Ib

150 250 350 450

0.31 0.18 0.20 0.21

0.51 0.33 0.33 0.34

4.9 41.1 57.4 54.2

0.71 0.71 0.66 0.69

1.45 1.14 2.01 1.68

on the electrode surface influences the impedance.45 With increasing potential, the diameter of the arc decreased sharply, indicating the increasing driving force for the methanol oxidation. These Nyquist plots are indicative of the electrode/electrolyte interface in which the charge-transfer resistance predominates. A reason for the observed well defined semicircle with the increase in potential could be due to the formation of more OHads groups on the catalyst surface at higher potential. The as-formed OHads groups are used to oxidize the COads species to decrease the CO

Figure 7. Cyclic voltammograms for methanol oxidation (1 mol dm-3) in 0.5 mol dm-3 H2SO4 at room temperature and 50 mV s-1 scan rate for PtRu/CNT subjected to varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C. Inset showing the linear sweep voltammograms at 1 mV s-1 scan rate.

Figure 8. Chronoamperometric response recorded at 300, 400, and 500 mV (vs SCE) in 0.5 mol dm-3 H2SO4 and 1 mol dm-3 methanol at room temperature for PtRu/CNT subjected to varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C.

poisoning, generating more refreshed Pt sites for methanol adsorption.46 The charge transfer resistance (Rct) values obtained, e.g., at 0.35 V with the equivalent circuits reported in the literature47 are 185, 105, 44, and 48 Ω cm-2 for PtRu/CNT reduced at 150, 250, 350, and 450 °C, respectively. A significant decrease of Rct values for PtRu/CNT reduced at 350 °C indicates a smaller reaction resistance and higher catalytic activity of the electrode, compared to the catalysts reduced at other temperatures. A reasonable explanation for this observation could be the presence of higher amount of RuOxHy in the catalyst reduced at 350 °C as evident by the XPS. RuOxHy is reported to be a mixed proton-electron conducting species and believed to have low ohmic resistance, as it contains liquid-like regions of water to facilitate proton conduction within an electron-conducting matrix.48 Several groups have investigated the origin of the Ru enhancement by using various types of bimetallic Pt-Ru catalyst, including bulk and surface alloys, plain and decorated single crystals, etc.49 Maillard et al.50 reported enhanced kinetics for methanol electrooxidation on the Ru-decorated Pt nanoparticles compared to the Pt-Ru alloy. Similarly, Dubau et al.51 observed that for the same Pt:Ru ratio, a mixture of Pt and Ru colloids supported on Vulcan displayed enhanced kinetics for both CO and methanol electrooxidation as compared to the Pt-Ru alloys. Thus, an alloy formation is not a prerequisite for obtaining effective CO tolerant electrocatalysts, but the Pt and Ru atoms have to be in close proximity to act catalytically for the CO removal from the surface.49-53 Although Pt-Ru is the preferred anode catalyst for methanol oxidation, the active state of ruthenium is still a subject of intense debate. While most authors believe that metallic Ru0 is the active component, some authors suggest oxides of Ru54-56 and others refer to hydrous ruthenium oxides.41,48,57,58 Long et al.33 have proposed that RuOxHy speciation of Ru in Pt-Ru nanoparticles affords a much more active catalyst for methanol oxidation than does Ru0 as part of a bimetallic alloy. It has also been reported that hydrous ruthenium oxides in the (45) Chen, L.; Guo, M.; Zhang, H. F.; Wang, X. D. Electrochim. Acta 2006, 52, 1191. (46) Wu, G.; Xu, B. Q. J. Power Sources 2007, 174, 148. (47) Ocampo, A. L.; Hernandez, M. M.; Morgado, J.; Montoya, J. A.; Sebastian, P. J. J. Power Sources 2006, 160, 915. (48) Dmowski, W.; Egami, T.; Lyons, K. E. S.; Love, C. T.; Rolison, D. R. J. Phys. Chem. B 2002, 106, 12677. (49) Maillard, F.; Lu, G. Q.; Wieckowski, A.; Stimming, U. J. Phys. Chem. B 2005, 109, 16230. (50) Maillard, F.; Gloaguen, F.; Leger, J. M. J. Appl. Electrochem. 2003, 33, 1. (51) Dubau, L.; Hahn, F.; Coutanceau, C.; Leger, J. M.; Lamy, C. J. Electroanal. Chem. 2003, 554, 407. (52) J. Kaiser, J.; Colmenares, L.; Jusys, Z.; Mo¨rtel, R.; Bo¨nnemann, H.; Ko¨hl, G.; Modrow, H.; Hormes, J.; Behm, R. J. Fuel Cells 2006, 6, 190. (53) Roth, C.; Papworth, A. J.; Hussain, I.; Nichols, R. J.; Schiffrin, D. J. J. Electroanal. Chem. 2005, 581, 79. (54) Hamnett, A.; Kennedy, B. J.; Wagner, F. E. J. Catal. 1990, 124, 30. (55) Lasch, K.; Jo¨rissen, L.; Friedrich, K. A.; Garche, J. J. Solid State Electrochem. 2003, 7, 619. (56) Gu, Y. J.; Wong, W. T. J. Electrochem. Soc. 2006, 153, A1714. (57) Chen, Z.; Qiu, X.; Lu, B.; Zhang, S.; Zhu, W.; Chen, L. Electrochem. Commun. 2005, 7, 593. (58) Cao, L.; Scheiba, F.; Roth, C.; Schweiger, F.; Cremers, C.; Stimming, U.; Fuess, H.; Chen, L.; Zhu, W.; Qiu, X. Angew. Chem., Int. Ed. 2006, 45, 5315.

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Figure 9. Nyquist plot of EIS as a function of bias voltage for PtRu/CNT subjected to varying reduction temperatures: (a) 150 °C, (b) 250 °C, (c) 350 °C, and (d) 450 °C in 1 mol dm-3 methanol and 0.5 mol dm-3 H2SO4.

Pt-Ru electrode play an important role in mediating proton transport during methanol electro-oxidation.30,41 High catalytic activity toward methanol oxidation can be achieved as these species can adsorb large quantities of OH species at low potentials.30,41 RuOxHy is a mixed proton and electron conducting species and it innately expresses the Ru-OH, which is of key importance in the bifunctional mechanism of methanol oxidation. Thus the higher activity of PtRu/CNT catalyst obtained at a reduction temperature of 350 °C can be ascribed to the presence of abundant RuOxHy (evident by XPS) or an appropriate amount of Pt to RuOxHy boundaries57 to be active for methanol oxidation. As can be seen from the result and discussions above, the reduction temperature has a great influence on the activity of the PtRu/CNT catalyst. The superior activity of the PtRu/CNT catalyst reduced at 350 °C can be rationalized by taking few factors into account, such as (i) the functional groups formed on the CNT surfaces creating a favorable electrochemical reaction environments on the electrode, (ii) the catalyst possessing the appropriate surface composition that enhances its electrocatalytic activity, and (iii) the presence of appropriate amount of hydrous ruthenium oxide in the catalyst increasing the electronic conductivity and lowering the resistance of the catalyst. These results prove that electrocatalytic activity can be tuned by the impregnationreduction temperature.

4. Conclusion Carbon nanotubes supported Pt-Ru catalysts were prepared by impregnation-reduction method from chloride precursors. XRD

analysis showed that the PtRu/CNT catalyst particles possibly consist of separate Pt and Ru phases. XPS analysis showed that the catalyst contains substantial amount of hydrous ruthenium oxide (RuOxHy) in addition to Pt, Ru metal and oxide species. From the electrochemical measurements, it can be concluded that the catalyst prepared at a reduction temperature of 350 °C had the highest electrochemical surface area showing the maximum catalytic activity for CO and methanol oxidation. Electrochemical impedance spectroscopy showed that the catalyst had the lowest charge transfer resistance compared to the catalysts prepared at other reduction temperatures. The low charge transfer resistance and the increase in conductivity of the catalyst due to the presence of hydrous ruthenium oxide could have helped for the removal of CO adsorbed fragments from the Pt surface, thereby increasing the catalytic activity. Acknowledgment. We are thankful for the financial support by the MIWFT-NRW within the “Nachwuchsgruppe: Mikroelektrochemie zur Optimierung von Heterogenkatalysatoren fu¨r PEM-Brennstoffzellen” and the project “Lokale mikroelektrochemische Visualisierung von Katalysatoraktivita¨ten zur Optimierung von Direkt-Methanol-Brennstoffzellen (DMFC)” in cooperation with the Forschungszentrum Ju¨lich. Supporting Information Available: Figures showing a TEM histogram and SEM images. This material is available free of charge via the Internet at http://pubs.acs.org. LA804039W