Published: December 21, 2011

r 2011 American Chemical Society 4097 dx.doi.org/10.1021/jp209549g | J. Phys. Chem. C 2012, 116, 4097–4104

ARTICLE

pubs.acs.org/JPCC

Electrochemical Characterization of PtRu Nanoparticles Supportedon Mesoporous Carbon for Methanol ElectrooxidationFederico A. Viva,† Mariano M. Bruno,*,†,‡ Matías Jobb�agy,§ and Horacio R. Corti†,§

†Grupo Celdas de Combustible, Departamento de Física de la Materia Condensada, Centro At�omico Constituyentes,Comisi�on Nacional de Energía At�omica (CNEA), Av General Paz 1499 (1650), San Martín, Buenos Aires, Argentina‡Escuela de Ciencia y Tecnología, Universidad de Gral. San Martín, Martin de Irigoyen 3100 (1650), San Martín,Buenos Aires, Argentina§Instituto de Química Física de los Materiales, Medio Ambiente y Energía (INQUIMAE), Universidad de Buenos Aires� CONICET,Ciudad Universitaria, Pabell�on II, 1428 Buenos Aires, Argentina

1. INTRODUCTION

The improvement of the efficiency of methanol electrooxida-tion continues to be a key research interest due to the role playedby this reaction in direct methanol fuel cell (DMFC), which isenvisioned as a convenient energy source for portable applica-tions. Pt�Ru alloy has known to be the best electrocatalyst forthe methanol electro-oxidation reaction for more than 30 years1�4

and has been used in fuel cells fed directly with methanol5 for∼25 years. However, DMFC still strives for a major breakout dueto the slow kinetics of methanol oxidation and the high catalystloadings needed to sustain reasonable power densities.6 Com-monly, the catalyst is supported over carbon particles to have agood dispersion, increase the metal particles surface area, andreduce the metal content.7�10 The carbon support also has toprovide an electrical connectivity between themetal particles andthe gas diffusion layer or current collector. Evidence demon-strates that the nature of the support and the interaction betweensupport and metal particles influence the morphology, disper-sion, and stability of the final catalyst,11�13 thus affecting thecatalytic activity of the metal particles.8,10,14�18

Recently, mesoporous carbon with controlled structure wasused as support for fuel cell electrocatalyst exhibiting promisingactivities in both half cell and single cell configuration.10,19�26

The control of structural parameters, such as surface area, poresize, and particle morphology, provides extra fine-tuning ofthe final catalyst electroactivity.7,8,10 In contrast, carbon powderslike Vulcan XC-72, the most common electrocatalyst carbon

support used in fuel cells have a much simple particle structure andlower surface area, which might not provide the best substrate forthe metal particles. Structured carbons allow the preparation ofhighly dispersed catalytic nanoparticles, whereas the mesoporosityguarantees facile diffusion path for reactants and byproducts.10,27

Moreover, the inherent surface microporosity provides goodphysical anchoring sites for the deposited catalyst layer.28

There are several methods to obtain structured carbon.29�31

One of the most used is by replication of inorganic template,which produces carbon particles with small size and narrowmesopore distribution (<15 nm). However, many steps are neces-sary, and consequently the cost of production increases.26,32 Struc-tured carbon fabrication using an organic template is a more robustand direct route to obtain a carbon with tunable distribution of largemesopore size (>15 nm).33,34

In the present work, we describe the preparation and character-ization of PtRu catalyst, prepared by the impregnation�reductionmethod, supported on a carbon with high surface area. The sub-strate, prepared by carbonization of a resorcinol-formaldehyde poly-mer with a soft template, has micropores (<2 nm) and largemesopores (15 to 30 nm). The supported catalyst was characterizedby powder X-ray diffraction (PXRD), transmission electron micro-scopy (TEM), and energy-dispersive X-ray spectroscopy (EDS).

Received: October 3, 2011Revised: November 25, 2011

ABSTRACT:Nanoparticles of PtRu supported on mesoporous carbon wereobtained by the impregnation and reduction method with NaBH4. The high-surface-area mesoporous carbon was obtained by carbonization of a resorcinol-formaldehyde polymer with a cationic polyelectrolyte as a soft template.Surface characterization performed by transmission electron microscopy andpowder X-ray diffraction showed a homogeneous distribution and highdispersion of metal particles. The PtRu catalyst shows an electrochemicalactive surface area, determined by CO stripping, 45% higher than PtRucatalyst synthesized by the same method on Vulcan. This translated in a 25%increase in the methanol oxidation current as well as a lower poisoning rateand higher turnover frequency, as was assessed by cyclic voltammetry and chronoamperometry. Differential electrochemical massspectroscopy indicated an 8% higher conversion efficiency of methanol to CO2, demonstrating the benefits of using a mesoporouscarbon as catalyst support.

4098 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

Stripping of CO was used for the determination of the catalystelectrochemical surface area (ECSA), whereas the electrocatalyticactivity was determined by cyclic voltammetry (CV), chronoam-perometry, and potentiodynamic differential electrochemicalmass spectrometry (DEMS) measurements. For comparison, allsurface and electrochemical characterizations were also per-formed on a catalyst of PtRu of the same composition, preparedby the same reduction method and supported on commercialVulcan carbon.

2. EXPERIMENTAL SECTION

2.1. Mesoporous Carbon Preparation.Mesoporous carbon(MC) support was obtained using the method described else-where.34,35 In brief, a precursor was prepared by polymerizationof resorcinol (Fluka) and formaldehyde (Cicarelli, 37 wt %).Sodium acetate (Cicarelli) was used as catalyst, and a cationicpolyelectrolyte (polydilayl-dimethylammonium chloride, PDAD-MAC, Sigma-Aldrich) was used as a structuring agent. Thereactive mixture of resorcinol (R), formaldehyde (F), and sodiumacetate (C) was stirred at 40 �C for 10 min before the addition ofPDADMAC (P). The molar ratios of the components R/F/C/Pwere: 1:3:0.04:0.03, respectively. Once the mixture becamehomogeneous, the solution was heated to 70 �C for 48 h atatmospheric pressure. The resulting brown R-F piece was driedin air for 3 days. The polymer was then carbonized under nitro-gen stream in a tubular furnace from ambient temperature to1000 �C at a heating rate of 40 �C/h. An ASAP 2020 (Micro-metrics) instrument was used to measure the nitrogen adsorptionisotherms at �196 �C for determining the total volume ofmicropores (pores size <2 nm) applying theDubinin�Radushkevich(DR) equation and for determining the specific surface area bythe Brunauer�Emmett�Teller (BET) equation.36 Finally, thematerial was grinded and passed through a 40 μm pore sieve.2.2. Catalyst Preparation.A slurry of the mesoporous carbon

was prepared by adding the predetermined amount of the MC inmilli-Q water (Millipore) under vigorous stirring, followed by anultrasonic bath. Solutions of Pt (H2PtCl6 3 6H2O 5 g, Tetra-hedron) and Ru (RuCl3 3XH2O 5 g, Aldrich) metal precursorswere added in calculated amount to the slurry while stirring toachieve a final catalyst composition of Pt:Ru 1:1 atomic ratio anda metal loading of 60%. Then, the pH was adjusted to 8 with1 M NaOH (Pro Analysis, Merck) aqueous solution and heated.Once the temperature reached 80 �C, NaBH4 (granular 98%,Sigma-Aldrich) was added in a molar ratio of 3:1 (NaBH4 tometal salt), and the temperature was maintained for an additional2 h, followed by stirring for 12 h. The liquid was centrifuged, andthe solid was successively washed and centrifuged until thesupernatant showed a neutral pH and absence of Cl� by reactionwith AgNO3 (saturated solution). The solid was then separatedand dried in a vacuum oven at 60 �C overnight. PtRu supportedon Vulcan carbon (Vulcan XC-72, Cabot) was prepared follow-ing the same procedure described above. The Vulcan waspreviously washed by boiling in 30 wt % HCl aqueous solution.2.3. Catalyst Characterizations.PXRD pattern of the catalyst

was obtained using a Siemens D5000 diffractometer with a CuKα source operating at 40 kV and 30 mA. The angle extendedfrom 20 to 100� with a step size of 0.02� and a counting time of 2 s.TEM images were acquired with a Philips CM200, whereas EDSwas performed using an EDAX DC X4 to quantify the atomicratio of Pt and Ru in the catalysts.

Thermogravimetric analyses (TGA) were performed with aShimadzu TGA-51 instrument. Supported catalyst samples(5 to 10 mg) were heated up to 1000 �C at 5 �C per minuteon a titanium crucible in air atmosphere (flow: 100 mL 3min

�1).The metal content on the supported catalysts were calculatedfrom the difference between the initial and final weights.2.4. Electrochemical Measurements. A suspension of the

supported catalyst in milli-Q water and Nafion ionomer (5 wt %of Nafion dispersion, Aldrich) was prepared and spread over theworking electrode (WE) and a glass carbon (SPI) disk 5 mm indiameter mounted on a Teflon rod and dried in a vacuum oven at140�160 �C for 5 min. Two catalysts were tested: PtRusupported on the MC (PtRu/MC) and supported on Vulcancarbon (PtRu/C). The exposed face of the WE was sanded with400 grit paper to get a rough surface before covering withthe suspension. CO stripping, CV, and chronoamperometrywere performed in a jacked electrochemical cell, and the tem-perature was maintained at 25.0( 0.1 �C. The counter electrode(CE) consisted of a coiled Pt wire 0.5 mm in diameter and 30 cmlength, whereas a Ag/AgCl (sat. KCl) electrode was used as areference electrode (RE). All potentials were converted againstthe normal hydrogen electrode (NHE).The electrochemical active area wasmeasured byCO stripping

voltammetry. The cell was filled with 0.5 M H2SO4 (95�97%,Merck) solution and saturated with CO (RG, Indura) for 60 minwhile the WE potential was maintained at 0.2 V vs NHE. Afterthe time elapsed, and while maintaining the potential, thesolution was purged with N2 (RG, Indura) for 15 min, and twoscans between 0.05 and 0.8 V vs NHE were completed at a scanrate of 1mV 3 s

�1. CV and choronoamperometry were performedon a 1 M methanol (Merck, HPLC grade) solution in 0.5 MH2SO4. CV was performed by sweeping the potential between0.05 and 0.8 V vs NHE at 1 mV 3 s

�1. Reproducible voltam-mograms were obtained after 3 to 5 scans. The chrono-amperometry was also performed at 0.5 V vs NHE for 1 h in a1 M methanol solution in 0.5 M H2SO4. The catalyst electro-chemical area was used to calculate the current density (j). Allelectrochemical measurements were performed with an AutolabPGSTAT302N potentiostat.2.5. DEMS Setup, Cell, and Calibration. The DEMS setup

consisted of two differentially pumped chambers and a quadru-pole mass spectrometer (100 amu, Pfeiffer). The primary vacuumchamber was pumped with a rotary vane pump (DUO 5,Pfeiffer), and a liquid nitrogen cold trap was included in thissection to avoid the oil vapors to enter the chamber. Thesecondary chamber was pumped by a 60 L 3 s

�1 turbomolecularpump backed by a dry diaphragm pump (turbo drag pumpingstation TSH/U 071 E, Pfeiffer). A gas-dosing valve (evn 116,Pfeiffer) connected the two chambers. The quadrupole massspectrometer, equipped with a continuous dynode secondaryelectron multiplier/Faraday cup detector having a sensitivity of200 A 3mbar�1 (QMS 200 M1, Prisma), was connected to theanalysis chamber (secondary chamber).The DEMS electrochemical cell was a flow-type cell of similar

design to the one used by Planes et al.37 sitting on top of theDEMS intake of the primary vacuum chamber. The same WEused for the electrochemical measurements was used for theDEMS analysis. Between the DEMS intake and the cell was theporous Teflon membrane that allowed the volatile products toenter the chamber. The WE and the cell were designed in such away that the electrode was separated 150 μm from the porousmembrane while a continuous stream of solution flowed between

4099 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

the electrode and the membrane. All measurements were carriedout with a flow solution of 2 μL 3 s

�1. CV were performed withan Autolab PGSTAT302N potentiostat analogous to Section 2.4,whereas the different products obtained were monitored by themass spectrometer.For quantitative detection of the CO2 formed during methanol

oxidation, a strip of CO with each catalyst was carried out beforeevery DEMS experiment to obtain the calibration constantK(44), as described by Wang et al.38

Kð44Þ ¼ 2QMSð44ÞQF

CO ð1Þ

whereQFCO is the Faradaic charge corresponding to the oxidation

of CO to CO2, QMS(44) is the integrated mass spectrometriccurrent of CO2, and 2 is the number of electrons for COoxidation.The average current efficiency of the complete scan (ηQ) for

the methanol oxidation to CO2 is given by

ηQ ¼ QF�QF

MeOH ð2Þ

where QFMeOH is the total Faradaic charge (i.e., forward and

reverse scan charge) and Q*F is the Faradaic charge correspond-ing to the formation of CO2 given by

QF� ¼ 6QMSð44ÞKð44Þ ð3Þ

where 6 is the number of electrons involved in the methanoloxidation to CO2.

3. RESULTS AND DISCUSSION

3.1. Mesoporous Carbon Characterization. The nitrogenadsorption�desorption isotherms of carbonized material at1000 �C is shown in Figure 1. An increase in the nitrogenadsorbed volume at low relative pressure and a hysteresis loop athigh relative pressure can be seen in the isotherm. According tothe IUPAC classification,39 the isotherm is of type-IV, and it has ahysteresis loop of type-H140 associated with mesoporous mate-rials. The analysis by the BET equation showed that a materialwith a high specific surface area (580m2

3 g�1) was obtained. The

total pore volume obtained (P 3 P0�1 = 0.986) was 0.99 cm3

3 g�1,

and the micropore volume (DR) was 0.23 cm33 g

�1. The poresize and cumulative volume distribution (inset Figure 1) werecalculated using original density functional theory (DFT)model assuming slit pore geometry. The Figure inset showsa maximum of the pore distribution around 20 nm. In the caseof porous carbon, the micro- (<2 nm) and meso-porosity(between 2 and 50 nm) are attributed to a structure consistingof clusters of porous uniform spheres in a fairly regular array.34

The high BET surface area of the material can only beaccounted for by the mentioned structure.41 Therefore, theobtained material has a hierarchical pore structure (micro- andmeso-porosity).3.2. Catalyst Surface Characterization. Dispersion and size

of metal nanoparticles on the two carbon support was assessed byTEM images, as shown in Figure 2. Low-magnification TEMimages show a better dispersion of catalyst particles across thesupport forMC (Figure 2A) than for carbon Vulcan (Figure 2D).Figure 2B,E shows the higher magnification images used forthe size diameter determination of PtRu/MC and PtRu/C,respectively. The catalyst particle size diameter distribution wasobtained by measuring the diameter of 100 randomly selectedparticles. Figure 2C,F presents the bar plots of PtRu/MC andPtRu/C, respectively. Both catalyst exhibits for the particle sizea Gaussian Distribution; however, PtRu/MC shows a narrowdispersion with a 4.0 nm mean particle diameter and a medianof 3.7 nm with few particles presenting sizes over 5 nm. PtRu/Chas a 5.0 nm mean particle diameter, and the median isalso 5.0 nm, meaning that 50% of the measured particles arelarger than 5.0 nm and with particles up to 8.0 nm in size.Moreover, Figure 2E shows some PtRu clusters 10 to 15 nm indiameter.The EDS results indicated that the ratio of Pt/Ru present on

the catalysts was 60:40 atomic for PtRu/MC and 55:45 for PtRu/C.For both catalysts, the values were close to the nominal precursoramount of 1:1 atomic ratio. The thermograms indicated metalloadings of 62 wt % for PtRu/MC and 58 wt % for PtRu/C in thefinal catalysts, also close to the intended load.The PXRD patterns of the prepared catalysts are shown in

Figure 3. The diffractograms show the characteristic diffractionpeaks of face centered cubic (fcc) crystalline Pt42 correspondingto the planes (111), (200), (220), and (311) shifted to a highervalues of 2θ as expected due to the presence of Ru.43�47 Thepeaks corresponding to the diffractogram of PtRu/MC are widercompared with PtRu/C, indicating smaller crystallites. Thelattice parameters were calculated by indexing the first threepeaks, that is, (111)�(200)�(220), yielding a value of 0.3900.001 nm for PtRu/MC and 0.392( 0.001 nm for PtRu/C, whilethe crystal sizes were calculated using Scherrer’s formula48 givinga value of 4.8( 0.5 nm for PtRu/MC and 5.8( 0.5 nm for PtRu/C.The lattice parameter value was used to estimate the Ru atomicfraction alloyed with Pt according to the formula proposed byAntolini and coworkers.49,50 Using the reference value of 0.3923 nmfor Pt/C, the fraction of alloyed Ru was 17% for PtRu/MC and5% for PtRu/Cwith an error of 2%, indicating that most of the Rupresent in both catalysts was found in an amorphous phasealthough in a major proportion for PtRu/C.3.3. Electrochemical Active Surface Area Determination

by CO Stripping Voltammetry. Previously to the catalyst acti-vity studies, the ECSA was determined by CO stripping voltam-metry. Figure 4 presents theCO stripping voltammograms in 0.5MH2SO4 for the PtRu/MC and PtRu/C. For all electrochemical

Figure 1. N2 adsorption isotherm of the carbon. Inset: pore size andpore volume distribution (DFT) of a typical carbon sample.

4100 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

measurements in this work the upper potential limit was setto 0.8 V vs NHE to avoid the formation of irreversible rutheniumoxides or dissolution.1,51 The low current at the beginning of thefirst scan for both catalysts indicated that the surface was coveredwith adsorbed CO. It has been found that the shape and position

of the CO oxidation peak on PtRu surfaces depend on the metalcontent and composition.51�53 The voltammograms in Figure 4present a peak potential of 0.42 V for PtRu/MC and 0.44 V forPtRu/C, whereas the onset potential (the potential at which10% of the maximum current was reached) was ca. 0.32 V for

Figure 2. TEM images at different magnifications and histogram of particle size distribution of PtRu/MC (a�c) and PtRu/C (d�f).

4101 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

PtRu/MC and 0.36 V for PtRu/C. The results obtained for thecatalysts prepared in this work fall within the values expected for aPtRu catalyst of the same composition and metal content,51,52

with themesoporous catalyst exhibiting a slightly lower potential,thus indicating a more facile CO oxidation. ECSA was calculatedbased on the catalyst amount deposited onto the WE and thepeak integration using the reference charge value of 420 μC 3 cm

�2

for the oxidation of a COmonolayer.54,55 The calculated ECSAs

were 58 m23 g

�1 for PtRu/MC and 40 m23 g

�1 for PtRu/C.These values are expected because a high metal loading (60% inthis work) tends to generate large metal nanoparticle agglomer-ates decreasing the ECSA, as reported, for Pt56 and PtRu53

supported catalysts. Our ECSA results indicate that the MCsupport allows a better dispersion of the PtRu nanoparticles onthe surface than the Vulcan carbon, as shown in the TEM images(Figure 2).3.4. Electrochemical Catalyst Characterization. The cata-

lytic activity of the prepared catalyst was assessed in a 1 Mmethanol + 0.5 MH2SO4 as supporting electrolyte. As in the COstripping determination, the upper potential limit was 0.8 V vsNHE to avoid catalyst degradation. Figure 5 shows the voltam-mograms for the PtRu/MC catalyst at different sweeping rates.All voltammograms show the same shape and reach the samevalue for the oxidation current at the anodic limit, indicating thatthe entire catalyst surface is accessible to methanol oxidation.Figure 6 shows the voltammograms of PtRu/MC and PtRu/C at 1 mV 3 s

�1. The onset potential of methanol oxidation for

Figure 3. PXRD patterns of PtRu/MC and PtRu/C.

Figure 4. CO stripping voltammograms in 0.5 H2SO4 at 1 mV s�1.(A) PtRu/MC. (B) PtRu/C.

Figure 5. CV of PtRu/MC at different sweeping rates in 1 M methanol +0.5 M H2SO4.

Figure 6. PtRu/MC and PtRu/C CVs at 1 mV 3 s�1 in 1 M methanol +

0.5 M H2SO4.

4102 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

PtRu/MC was 0.41 V, whereas that for PtRu/C was 0.46 V,indicating a slightly lower overpotential on PtRu/MC.Moreover,the maximum current attained by the PtRu/MC represents 25%with respect PtRu/C, suggesting a better catalytic performancefor PtRu/MC.Figure 7 depicts the chonoamperometric measurements of

the catalysts at 0.5 V vs NHE for a 1 h period. The plot shows ahigher oxidation current for the PtRu/MC during the entiremeasurement. The current decay observed between 100 and 500 sis steeper for PtRu/C than for PtRu/MC. Because the methanoloxidation is controlled by diffusion, the rapid current decay forPtRu/C indicates a more restricted diffusion of methanol in thiscatalyst as compared with PtRu/MC. The slope of the currenttransient for periods between 500 and 1800 s has been attributed

to catalyst poisoning by CO, whereas at longer periods the decayis related to anion adsorption according to Jiang and Kucernak.57

The poisoning rate (δ) of the catalysts was calculated using58

δ ¼ 100Io

dIdt

� �t > 500

ð4Þ

where (dI/dt)t>500 is the slope of the linear portion of the currentdecay obtained from fitting a linear regression between 500 and1800 s, and Io is the current at the start of the polarization obtainedfrom the linear fit.The turnover frequency (TOF),57,59 that is, the number of

methanol molecules that reacts per catalyst surface site per second,was calculated60 using

TOF ¼ I � NA

n� F � mð5Þ

where I is the steady-state current density measured at 1800 s andconsidering a complete oxidation of methanol to CO2, n is thenumber of electrons produced by the oxidation of one methanolmolecule (6), NA is the Avogadro constant, F is the Faradayconstant, and m is the mean surface atomic density of PtRu(1.23 � 1015 site 3 cm

�2).61

The values obtained for the poisoning rate and TOF, respec-tively, were 0.0011% s�1 and 0.0355 molecules 3 site

�13 s�1 for

the PtRu/MC and 0.0037% s�1 and 0.0186 molecules 3 site�1

3 s�1

for PtRu/C. The results indicate that on PtRu/MC the number ofmolecules per second reacting is almost twice that on PtRu/C,giving rise to the higher oxidation current on the former one. Thatresult is related to the lower poisoning rate due to the higheroxidation current obtained. Overall, the results from the CVand chronopotentiometry indicate that the PtRu/MC presents abetter accessibility of methanol toward the catalyst while showinglower poisoning due to formed CO and a higher catalytic activity.The results could be explained due to the high surface area andmesoporosity of the carbon support, which in turn allows a better

Figure 7. PtRu/MCandPtRu/C chronoamperometry in 1Mmethanol +0.5 M H2SO4.

Figure 8. Potentiodynamic DEMSmeasurement of PtRu/MC and PtRu/C in 1Mmethanol + 0.5MH2SO4 with the signal corresponding tom/z = 44(CO2) and m/z = 60 (methyl formate).

4103 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

dispersion of the metal particles and also helps to the diffusion ofreacting species and products.3.5. DEMS Analysis. DEMS experiments were carried out to

monitor the reaction products for methanol oxidation on both,PtRu/MC and PtRu/C, catalysts. Methanol oxidation on Pt-basedcatalyst produces a series of products: CO, formaldehyde, formicacid, and CO2,

62 where CO is the main poisonous species.1

DEMS has been used extensively to follow and quantify thereactions products of methanol oxidation; however, because ofsoluble intermediates formation63 and signal overlapping offormaldehyde (m/z = 30), CO (m/z = 28), and formic acid(m/z = 46) with lines corresponding to the mass spectra ofmethanol (m/z=32) and CO2 (m/z=44),52,59 only the mainreaction product CO2 (m/z = 4 4) andmethyl formate (m/z = 60),a product of the reaction between the present methanoland the formic acid produced in the oxidation, can bedetected.52,59,62,64

Results obtained on the DEMS experiments are presented inFigures 8 for PtRu/MC and PtRu/C. The shape of the massspectrum, including the hysteresis loop, is similar to that pre-viously reported for supported Pt and PtRu catalysts.65,66 Theonset potentials of the voltammograms maintain the trendobtained in regular electrochemical cells where PtRu/MC showsa lower onset. The measured current densities are low comparedwith those of Figure 6 because a lower catalyst load was used inthe DEMS electrode. The mass spectrum shows that the onsetfor methyl formate starts at slightly higher potential comparedwith CO2 onset, as previously observed.

52 The mass signals ofCO2 and methyl formate for PtRu/MC do present a smallhysteresis as compared with PtRu/C, although such hysteresisis absent in the voltammograms for both catalysts. The hysteresisin the reverse scan originates from the formation and reductionof oxides on Pt.52,67 During the reverse scan, the higher rate of thePt oxide reduction at the higher overpotential causes a morerapid increase in the methanol oxidation resulting in a highercurrent than in the forward scan.52 The obtained average currentefficiency toward the CO2 formation was 71% for PtRu/MC and63% for PtRu/C. The results clearly indicate that the conversionof methanol to CO2 is higher on PtRu/MC. Once again, thebetter catalytic property of the MC catalyst can be explained bythe higher surface area and higher dispersion of metal particles onthe MC. Moreover, in the case of the higher oxidation efficiencyof methanol to CO2, the mesoporosity of the carbon supportextends the residence time of intermediates species close to thecatalyst allowing them to reabsorb and further oxidize to CO2.

62,68

4. CONCLUSIONS

Mesoporous carbon are demonstrated to be a suitable supportfor the in situ generation of PtRu nanoparticles. The supportedbimetallic nanoparticles showed an improved catalytic activity formethanol oxidation by CV, chronoamperometry, and DEMSstudies when compared against PtRu supported on Vulcancarbon. Both CV and DEMS studies showed a lower onsetpotential for CO2 formation for PtRu/MC, indicating a morefacile oxidation of methanol compared with the PtRu/C catalyst.Moreover, chronoamperometry showed a lower poisoning rateand a higher TOF of methanol oxidation toward CO2 at the sametime that maintained a higher current along the whole measure-ment. Quantification by DEMS of the current efficiency for thereaction of methanol to CO2 was found to be 8% higher onPtRu/MC than on PtRu/C

The structured carbon used in the present work as support forPtRu allowed us to prepare a high metal loading catalyst withhigher ECSA than that expected for Vulcan type of supports, aswas shown by Maillard et al.53 The better performance of thePtRu/MC compared with the PtRu supported on Vulcan mayoriginate on the structure of the carbon support, creating asynergetic effect with the catalytic properties of PtRu 1:1 formethanol oxidation.

’AUTHOR INFORMATION

Corresponding Author*Tel: +54-11-6772-7100. Fax: +54-11-6772-7121. E-mail: mbruno@tandar.cnea.gov.ar.

’ACKNOWLEDGMENT

We acknowledge financial support from Agencia Nacional dePromoci�on Científica y Tecnol�ogica (ANPCyT) (PICT 2097,PAE 36985). Surface characterization was performed at Labor-atorio deMicroscopía Electr�onica of Centro AtomicoConstituyentes(CAC) with the help of P. Bozzano and G. Zbihlei. FAV, MMB,MJ, and HRC are permanent research fellows of CONICET.

’REFERENCES

(1) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys.Chem. 1993, 97, 12020–12029.

(2) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 121–229.(3) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9–45.(4) Jansen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976,

21, 869–878.(5) Surampudi, S.; Narayanan, S. R.; Vamos, E.; Frank, H.; Halpert,

G.; Laconti, A.; Kosek, J.; Prakash, G. K. S.; Olah, G. A. J. Power Sources1994, 47, 377–385.

(6) Carrette, L.; Friedrich, K. A.; Stimming, U. Chem. Phys. Chem.2000, 1, 162–193.

(7) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Appl. Catal., A 1998,173, 259–271.

(8) Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Roy, C. Appl. Surf. Sci.2003, 271, 181–193.

(9) Lizcano-Valbuena, W. H.; de Azevedo, D. C.; Gonzalez, E. R.Electrochim. Acta 2004, 49, 1289–1295.

(10) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D.J. Power Sources 2006, 155, 95–110.

(11) Cui, Z.; Liu, C.; Liao, J.; Xing, W. Electrochim. Acta 2008,53, 7807–7811.

(12) Figueiredo, J.; Pereira, M.; Serp, P.; Kalck, P.; Samant, P.;Fernandes, J. Carbon 2006, 44, 2516–2522.

(13) Zhou, W. J.; Li, W. Z.; Song, S. Q.; Zhou, Z. H.; Jiang, L. H.;Sun, G. Q.; Xin, Q.; Poulianitis, K.; Kontou, S.; Tsiakaras, P. J. PowerSources 2004, 131, 217–223.

(14) G�omez de la Fuente, J. L.; Martinez-Huerta, M. V.; Rojas, S.;Terreros, P.; Fierro, J. L. G.; Pena,M. A.Catal. Today 2006, 116, 422–432.

(15) Rao, V.; Simonov, P.; Savinova, E.; Plaksin, G.; Cherepanova,S.; Kryukova, G.; Stimming, U. J. Power Sources 2005, 145, 178–187.

(16) Arbizzani, C.; Beninati, S.; Soavi, F.; Varzi, A.; Mastragostino, M.J. Power Sources 2008, 185, 615–620.

(17) Kim, H.; You, D.; Yoon, H.; Joo, S.; Pak, C.; Chang, H.; Song, I.J. Power Sources 2008, 180, 724–732.

(18) Hepel, M.; Dela, I.; Hepel, T.; Luo, J.; Zhong, C. J. Electrochim.Acta 2007, 52, 5529–5547.

(19) Calvillo, L.; L�azaro, M. J.; García-Bordej�e, E.; Moliner, R.;Cabot, P. L.; Esparb�e, I.; Pastor, E.; Quintana, J. J. J. Power Sources 2007,169, 59–54.

4104 dx.doi.org/10.1021/jp209549g |J. Phys. Chem. C 2012, 116, 4097–4104

The Journal of Physical Chemistry C ARTICLE

(20) Joo, S. H.; Lee, H. I.; You, D. J.; Kwon, K.; Kim, J. H.; Choi,Y. S.; Kang, M.; Kim, J. M.; Pak, C.; Chang, H.; Seung, D. Carbon 2008,46, 2034–2045.(21) Joo, S. H.; Pak, C.; You, D. J.; Lee, S.-A.; Lee, H. I.; Kim, J. M.;

Chang, H.; Seung, D. Electrochim. Acta 2006, 52, 1618–1626.(22) Lei, Z.; Bai, S.; Xiao, Y.; Dang, L.; An, L.; Zhang, G.; Xu, Q.

J. Phys. Chem. C 2008, 112, 722–731.(23) Liu, S. H.; Yu, W. Y.; Chen, C. H.; Lo, A. Y.; Hwang, B. J.;

Chien, S. H.; Liu, S. B. Chem. Mater. 2008, 20, 1622–1628.(24) Nam, J.-H.; Jang, Y.-Y.; Kwon, Y.-U.; Nam, J.-D. Electrochem.

Commun. 2004, 6, 737–741.(25) Zhou, J. H.; He, J. P.; Ji, Y. J.; Dang, W. J.; Liu, X. L.;

Zhao, G. W.; Zhang, C. X.; Zhao, J. S.; Fu, Q. B.; Hu, H. P. Electrochem.Solid-State Lett. 2007, 10, B191–195.(26) Ding, J.; Chana, K. Y.; Rena, J.; Xiao, F. S. Electrochim. Acta

2005, 50, 3131–3141.(27) Sahu, A. K.; Sridhar, P.; Pitchumani, S. J. Indian Inst. Sci. 2009,

89, 437–445.(28) Bruno, M. M.; Franceschini, E. A.; Planes, G. A.; Corti, H. R.

J. Appl. Electrochem. 2010, 40, 257–263.(29) Gierszal, K. P.; Kim, T.W.; Ryoo, R.; Jaroniec, M. J. Phys. Chem.

B 2005, 109, 23263–23268.(30) Jin, J.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Microporous

Mesoporous Mater. 2009, 118, 218–223.(31) Wan, Y.; Shi, Y.; Zhao, D. Chem. Mater. 2008, 20, 932–945.(32) Lin, M. L.; Lo, M. Y.; Mou, C. Y. J. Phys. Chem. C 2009,

113, 16158–16168.(33) Bruno,M.M.; Cotella, N. G.;Miras, M. C.; Koch, T.; Seidler, S.;

Barbero, C. Colloids Surf., A 2010, 358, 13–20.(34) Bruno, M. M.; Cotella, N. G.; Miras, M. C.; Barbero, C. Colloids

Surf., A 2010, 362, 28–32.(35) Bruno, M. M.; Corti, H. R.; Balach, J.; Cotella, G. N.; Barbero,

C. A. Funct. Mater. Lett. 2009, 2, 135–138.(36) Hutson, N. D.; Yang, R. T. Adsorption 1997, 3, 189–195.(37) Planes, G.; Garcia, G.; Pastor, E. Electrochem. Commun. 2007,

9, 839–844.(38) Wang, H.; Alden, L.; DiSalvo, F. J.; Abru~na, H. D. Phys. Chem.

Chem. Phys. 2008, 10, 3739–3751.(39) Sing, K. S.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti,

R. A.; Rouquerol, T.; Siemieniewska, T. Pure Appl. Chem. 1985,57, 603–619.(40) Rouquerol, F.; Rouquerol, J.; Sing, K.Adsorption by Powders and

Porous Solids; Academic Press: San Diego, 1999.(41) Gavalda, S.; Gubbins, K. E.; Hanzawa, Y.; Kaneko, K.; Thomson,

K. T. Langmuir 2002, 18, 2141–2151.(42) Chu, D.; Gilman, S. J. Electrochem. Soc. 1996, 143, 1685–1690.(43) Deivaraj, T.; Lee, J. J. Power Sources 2005, 142, 43–49.(44) Ma, L.; Liu, C.; Liao, J.; Lu, T.; Xing, W.; Zhang, J. Electrochim.

Acta 2009, 54, 7274–7279.(45) Neto, A. O.; Dias, R. R.; Tusi, M. M.; Linardi, M.; Spinace, E. V.

J. Power Sources 2007, 166, 87–91.(46) Nitani, H.; Nakagawa, T.; Daimon, H.; Kurobe, Y.; Ono, T.;

Honda, Y.; Koizumi, A.; Seino, S.; Yamamoto, T. A. Appl. Catal., A 2007,326, 194–201.(47) Wang; Liao, J.; Liu, C.; Xing, W.; Lu, T. Electrochem. Commun.

2009, 11, 198–201.(48) Liu, Z.; Ling, X. Y.; Su, X.; Lee, J. Y. J. Phys. Chem. B 2004,

108, 8234–8240.(49) Antolini, E.; Cardellini, F. J. Alloys Compd. 2001, 315, 118–122.(50) Antolini, E.; Giorgi, L.; Cardellini, F.; Passalacqua, E. J. Solid

State Electrochem. 2001, 5, 131–140.(51) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys.

Chem. 1994, 98, 617–625.(52) Jusys, Z.; Kaiser, J.; Behm, R. J. Electrochim. Acta 2002,

47, 3693–3706.(53) Maillard, F.; Bonnefont, A.; Chatenet, M.; Guetaz, L.;

Doisneaucottignies, B.; Roussel, H.; Stimming, U. Electrochim. Acta2007, 53, 811–822.

(54) Takasu, Y.; Fujiwara, T.; Murakami, Y.; Sasaki, K.; Oguri, M.;Asaki, T.; Sugimoto, W. J. Electrochem. Soc. 2000, 147, 4421–4427.

(55) Takasu, Y.; Kawaguchi, T.; Sugimoto, W.; Murakami, Y.Electrochim. Acta 2003, 48, 3861–3868.

(56) Nores-Pondal, F. J.; Vilella, I. M. J.; Troiani, H.; Granada, M.;de Miguel, S. R.; Scelza, O. A.; Corti, H. R. Int. J. Hydrogen Energy 2009,34, 8193–8203.

(57) Jiang, J.; Kucernak, A. J. Electroanal. Chem. 2003, 543, 187–199.(58) Jiang, J.; Kucernak, A. J. Electroanal. Chem. 2002, 520, 64–70.(59) Jusys, Z.; Kaiser, J.; Behm, R. J. Langmuir 2003, 19, 6759–6769.(60) Chrzanowaki, W.; Wieckowski, A. In Interfacial Electrochemistry:

Theory, Experiment, Aplications; Wieckowski, A., Ed.; Dekker: New York,1999.

(61) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J.J. Electrochem. Soc. 1998, 145, 925–931.

(62) Wang, H.; Wingender, C.; Baltruschat, H.; Lopez, M.; Reetz,M. T. J. Electroanal. Chem. 2001, 509, 163–169.

(63) Wang, H.; Baltruschat, H. J. Phys. Chem. C 2007, 111, 7038–7048.

(64) Iwasita, T.; Vielstich, W. J. Electroanal. Chem. 1986, 201, 403–408.

(65) Jusys, Z.; Behm, R. J. J. Phys. Chem. B 2001, 105, 10874–10883.(66) Jusys, Z.; Massong, H.; Baltruschat, H. J. Electrochem. Soc. 1999,

146, 1093–1098.(67) Angerstein-Kozlowska, H.; Conway, B. E.; Sharp, W. B. A.

J. Electroanal. Chem. 1973, 43, 9–36.(68) Cohen, J. L.; Volpe, D. J.; Abru~na, H. D. Phys. Chem. Chem.

Phys. 2007, 9, 49–77.