Insights into the Effects of Functional Groups on Carbon Nanotubes for

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Insights into the Effects of Functional Groups on Carbon Nanotubes for the Electrooxidation of Methanol Patricia Hern
andez-Fern
andez,† Steve Baranton,‡ Sergio Rojas,*,§ Pilar Oc
on,† Jean-Michel L
eger,‡ and Jose Luis G. Fierro§ †

Dpto. Química-Física Aplicada, Facultad de Ciencias, Universidad Aut
onoma de Madrid (UAM), C/Francisco Tom
as y Valiente 7, 28049 Madrid, Spain ‡ Laboratoire de Catalyse en Chimie Organique (LACCO), UMR-CNRS 6503, Universit
e de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France § Grupo de Energía y Química Sostenibles, Instituto de Cat
alisis y Petroleoquímica (CSIC), C/Marie Curie 2, 28049 Madrid, Spain ABSTRACT: Functionalized carbon nanotubes were used as a support for PtCo nanoparticles. Their performance as electrocatalysts for the electrooxidation of methanol was evaluated by cyclic voltammetry and in situ FTIR reflectance spectroscopy. The onset potentials for both the electrooxidation of methanol and the production of CO2 shifted to less positive values for catalysts prepared with more oxygen groups on the support. Furthermore, the production of CO2 was higher on catalysts prepared with functionalized carbon nanotubes. The functional groups play two different but complementary roles. On the one hand, they help to stabilize smaller PtCo particles of ca. 3 nm. On the other hand, they provide the OH groups necessary for the total oxidation of methanol to CO2 at potentials less positive than on nonfunctionalized supports. Remarkably, the consumption of carboxylic acid groups along with the production of water is observed in the infrared spectra of the functionalized supports recorded during the electrooxidation of methanol. This observation suggests that the OH groups of the support can also react with methanol, forming water and an ester.

1. INTRODUCTION The electrooxidation of methanol has attracted a great deal of attention because of its relevance for direct methanol fuel cells (DMFCs). The archetypal electrocatalysts for the methanol electrooxidation reaction (MOR) consist of nanosized Pt particles dispersed onto graphite-type carriers;1,2 however, the kinetics of the MOR reaction on Pt are rather slow. Several approaches to improving the MOR have been reported. On one hand, the activity can be improved by alloying Pt with metals that promote the formation of OHad species at potentials less positive than at Pt alone,3 5 which according to the so-called “bi-functional mechanism” will shift the onset of the MOR to less positive potentials.1,6 8 On the other hand, the mass activity of the electrocatalyst (or any catalyst for that matter) can be improved by increasing its dispersion, i.e., the fraction of sites exposed to the reactants.9 For the latter approach, particularly in the field of electrocatalysis, the choice of an adequate support is a delicate matter. The support ought to combine a number of properties, such as high specific surface area to accommodate high metal loadings, an electronic conductivity sufficient to support electrochemical reactions, and the proper hydrophilic/ hydrophobic balance. In addition, it should be stable under the harsh reaction conditions that occur within a fuel cell, particularly at the cathode. Those requirements make materials based on carbon black the most widely used supports,10,11 although other r 2011 American Chemical Society

materials, such as conducting polymers,12,13 TiO2,14,15 WO3,16 18 and carbon aerogels,19 are currently under study. Multiwalled carbon nanotubes (MWCNTs) are another family of materials that fulfill all the above requirements to be used as support for electrocatalysts. However, functionalization of the MWCNTs is necessary to incorporate a sufficient loading of metallic nanoparticles for fuel cell applications.20 The role of oxygen-containing groups on the surface of carbon supports has been studied by many researchers. Most studies only report the functional groups’ interactions with metal nanoparticles. Some authors affirm that oxygen-containing functional groups are the sites for nanoparticle anchoring, and increasing their surface abundance makes it possible to deposit large amounts of metallic nanoparticles on the carbon surface.20 22 Other groups report that the main role of the functional groups is to increase the metal dispersion.23 27 It is well-accepted that functional groups on the surface of carbon increase its hydrophilicity, favoring the diffusion of the reactants toward the active sites.24,28 Few studies address the roles of oxygen-containing groups in electrochemical reactions.29,30 Most studies report a general Received: March 28, 2011 Revised: June 1, 2011 Published: July 08, 2011 9621

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Table 1. Details of the Functionalization Treatments and Characterization of MWCNTs support

[H2SO4:HNO3] (M)

BET (m2g 1)

pore volume (cm3g 1)

O/Ca

Fe (wt%)b

Ni (wt%)b

CNT-MT

5.5:3.0

60

179

0.85

0.024

0.94

0.10

CNT-ST

18.3:15.5

110

93

0.25

0.167

0.50

0.04

MWCNT

-

-

145

0.78

0.013

218

-

0.003

Vu a

T (°C)

Surface atomic ratio by XPS. b Metallic content determined by TXRF.

increase in activity for CO and methanol oxidation reactions on functionalized carbon-based electrocatalysts.3,31 35 Such improvement has been attributed to either a better dispersion of the nanoparticles on the functionalized supports36 38 or improved stability of the metallic nanoparticles on the oxygencontaining groups of the carbon.39,40 Li et al.40 and Stevanovic et al.41 went further and suggested the direct participation of the oxygen-containing groups of the support in the MOR based upon cyclic voltammetry studies. However, conventional electrochemical methods alone are not accurate enough to establish whether the oxygen-containing groups actually participate in this reaction. To this end, a deeper inspection of the methanol oxidation reaction with in situ techniques is required. Herein, we report a thorough study of the role of the functional groups generated on the surface of carbon nanotubes by chemical methods in both the synthesis of PtCo/CNTs and performance for the MOR by combining information from cyclic voltammetry and in situ FTIR.

2. EXPERIMENTAL SECTION 2.1. Synthesis. Prior to the synthesis of the electrocatalysts, the multi wall carbon nanotubes (MWCNTs) (Sunnano, purity >90%, L = 10 30 nm) were functionalized by two different methodologies. Both methods involved refluxing the CNTs in a sulfonitric mixture at different temperatures. In the first method, the sulfonitric mixture contained 200 mL of each acid (H2SO4 and HNO3), and the temperature was set at 110 °C for 6 h. In the second approach, the sulfonitric mixture contained 84 mL of H2SO4 and 54 mL of HNO3 in 280 mL of H2O, and the temperature was set at 60 °C for 2 h. After each treatment, 1 L of distilled water was added to the suspension containing each set of nanotubes, being stirred for 12 h. Then, the nanotubes were recovered by filtration and treated with distilled water until the pH of the recovered water was 6. The nanotubes obtained were dried at 100 °C for 12 h. The MWCNTs obtained were designated as CNT-ST and CNT-MT in accordance with the severity of the treatment (ST is Severe Treatment, MT is Mild Treatment). Table 1 reports the conditions of the oxidation procedures. The preparation of the PtCo nanoparticles and their anchoring either on multiwalled carbon nanotubes or Vulcan XC-72R (Vu) was performed using the polyol methodology.42,43 Briefly, an ethylene glycol (EG) solution of each metal precursor (H2PtCl6 and CoCl2, Johnson Matthey) was added dropwise to a suspension of the carbonaceous support in EG with stirring. The mixture was stirred for 3 h. The pH of the mixture was adjusted to 13 with NaOH (2.5 M in EG). The total amount of water in the mixture was 5 vol %. The dispersion was refluxed for 3 h to ensure the total reduction of the metallic precursors. The process was conducted under an atmosphere of N2. The sample obtained was thoroughly rinsed with water and dried at 70 °C for 8 h; three electrocatalysts designated as PtCoCNT-MT, PtCoCNT-ST, and PtCo-Vu were obtained. Further details of the functionalization treatments of the support as well as the synthetic procedure for the electrocatalysts were reported previously.23,44 The most relevant physicochemical characteristics of the catalysts are shown in Table 2.

Table 2. Structural Parameters of the Electrocatalysts Pt catalysts

Co

(wt%)a (wt%)a

particle size (nm)b

Cπ Pt/Coc Pt/Cod (%)e

PtCoCNT-MT

26

10

3.0 ( 0.4

0.5

0.9

PtCoCNT-ST

12

7

2.8 ( 0.2

0.8

1.1

6

PtCo-Vu

30

10

5.1 ( 0.1

0.9

1.3

14

16

a

Metallic content determined by TXRF. b Mean particle size from TEM. Atomic ratio from TXRF. d Atomic ratio from XPS. e Relative amount of Cπ (C 1s species at 290.3 eV normalized to total C 1s). c

2.2. Physicochemical Characterization. X-ray diffractograms were collected on a Seifert 3000 powder diffractometer operating with Cu KR radiation (λ = 0.154 18 nm) generated at 40 kV and 40 mA. Scans were recorded at 0.02°/s for 2θ values between 10° and 90°. X-ray photoelectron spectra (XPS) of the samples were acquired with a VG Escalab 200R spectrometer fitted with an Mg KR (hυ = 1253.6 eV) 120 W X-ray source. The energy regions of the photoelectrons of interest were scanned until an acceptable signal-to-noise ratio was achieved. Intensities were estimated by calculating the integral of each peak, determined by subtraction of the Shirley-type background and fitting of the experimental curve to a combination of Lorentzian and Gaussian lines of variable proportions. Accurate binding energies ((0.2 eV) were determined by referencing to the C 1s peak at 284.6 eV. X-ray fluorescence analysis (TXRF) was performed on a Seifert EXTRA-II spectrometer equipped with two X-ray fine focus lines, Mo and W anodes, and a Si(Li) detector with an active area of 80 mm2 and a resolution of 157 eV at 5.9 keV (Mn KR). Direct solid analysis was employed to quantify the weight percentage of metal.45 Specimens for transmission electron microscopy (TEM) analysis were prepared by dispersing the powder samples in isobutanol. One drop of the resulting suspension was placed on a perforated carbon film supported by a copper grid. Samples were studied on a JEM 2100F device. Textural properties were evaluated by N2 adsorption desorption isotherms of the samples recorded at liquid N2 temperature with a Micromeritics ASAP 2000 apparatus. Samples were degassed at 150 °C under vacuum for 24 h. Specific areas were calculated by applying the BET method within the relative pressure range P/P0 = 0.05 0.30. 2.3. Electrochemical Experiments. The working electrode was prepared according to the thin-film method.46 The proper amounts of Nafion (Aldrich, 5 wt % in aliphatic alcohols), water, and electrocatalyst were mixed in an ultrasonic bath for 30 min. A homogeneous ink was obtained and deposited on a gold plate (0.38 cm2), resulting in a homogeneous coating. The amount of metal on all electrodes was 18 μg. The electrochemical measurements for the MOR were recorded in 0.1 M HClO4 (Merck, purity 70%) + 0.1 M CH3OH (Acros, HPLC grade) at room temperature using a normal hydrogen electrode (NHE) as a reference. Cyclic voltammetry (CV) experiments were performed with a μAutolab type III potentiostat/galvanostat controlled by a computer. The electrodes were cycled from 0.05 to 0.95 V at a scan rate of 1 mV 3 s 1. 9622

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2.4. Fourier Transform Infrared Reflectance Spectroscopy. In situ infrared reflectance spectroscopic measurements were acquired with a Fourier Transform Bruker IFS 66v spectrometer equipped with a specific reflection setup HgCdTe detector cooled with liquid nitrogen. Ar (Air Liquid) was bubbled through the solution for 30 min to deaerate the system. Then, the electrode surface was placed against the CaF2 optical window of the spectro-electrochemical cell. Single potential alteration infrared reflectance spectroscopy (SPAIRS)47 49 was used as the acquisition mode to characterize the catalysts. The SPAIRS technique consists of the acquisition of the reflectivities every 50 mV during the forward sweep at 1 mV 3 s 1. Each spectrum results from the coaddition of 256 interferograms. Spectra were then calculated as the REref)/REref, where the relative reflectivity change ΔR/R = (RE reference reflectivity REref is taken generally at the lowest potential limit of the forward scan (50 mV). This technique allows the detection of adsorbed species and reaction products near the electrode surface. For each catalyst, the experiments were repeated at least three times to ensure the reproducibility of the measurement.

3. RESULTS 3.1. Structure and Composition of CNTs and PtCo/CNTs Catalysts. After chemical treatment, the surface of the CNTs

appears enriched in O-bearing groups. The amount increases with the severity of the chemical treatment, as deduced by the surface atomic O/C ratio obtained from the XPS analysis: 0.167, 0.024, 0.013, and 0.003 for CNT-ST, CNT-MT, MWCNTs, and Vu, respectively (see Table 1). The BET specific surface area of the CNTs varies with the severity of the treatment reaching values of 179 and 93 m2 3 g 1 for CNT-MT and CNT-ST, respectively. The value of the nontreated nanotubes is 145 m2 3 g 1. Those values are both smaller than the 218 m2 3 g 1 recorded for Vu. Either treated or not, CNTs display a broad pore size distribution with the maximum value centered at 35 nm for CNT-MT and 25 nm for CTN-ST. However, as reported in Table 1, the pore volume of the severely treated sample (CNTST) is smaller than that of CNT-MT, 0.25 cm3 g 1 vs 0.85 cm3 g 1, respectively. The increase of the BET area of CNT-MT coincides with previous reports for similar treatments and is ascribed to the removal of impurities.23,43,50 CNT-ST display the lowest specific surface are value and pore volume in the series (Table 1). This feature probably accounts for the higher amount of functional groups (ca. 1 order of magnitude higher than for the CNT-MT and MWCNT as deduced from the O/C ratio reported in Table 1) which may block access to the pores or even fill most of the mesopores. Other authors have also observed a decrease of the BET area for HNO3-treated carbons and ascribed to the destruction of the mesopore network due to the thinning of the pore walls after the oxidation treatment.51,52 The actual metal loading and the Pt/Co atomic ratio derived content of the PtCo-CNTs are much lower than the nominal value of 40 wt % and the expected Pt/Co ratio. Remarkably, the metal loading decreases with increasing amounts of functional groups on the support. The metal loading on Vu, however, reaches the expected value of 40 wt %, and the Pt/Co wt % ratio is also in good agreement with the nominal value. Figure 1 shows representative TEM images of PtCo/CNTs displaying the multiwalled structure of the support. The images show that the dispersion of the PtCo nanoparticles is higher in PtCoCNTST than in PtCoCNT-MT, which presents some agglomerates of small nanoparticles. The mean sizes of the PtCo particles determined by counting more than 250 PtCo particles are

Figure 1. TEM micrographs of (a) PtCoCNT-MT and (b) PtCoCNT-ST.

2.8 ( 0.2 and 3.0 ( 0.4 nm for PtCoCNT-ST and PtCoCNTMT, respectively (Table 2), in good agreement with the sizes calculated from X-ray diffraction, and 3.2 and 3.6 nm for PtCoCNT-ST and PtCoCNT-MT, respectively.44 The mean particle size for PtCo-Vu is 5.1 nm. XPS analysis reveals that the surface of the catalysts is enriched in Pt, with the value of the surface atomic Pt/Co ratio higher than that of the bulk (Table 2). The Pt/Co ratio of both bulk and surface increases following the trend: PtCoCNT-MT < PtCoCNT-ST < PtCo-Vu. The electrode active surface area (EASA) of the PtCo catalysts has been determined by the evaluation of the Hupd charge.53 The values obtained in 0.5 M H2SO4 are 49, 45, and 30 m2 3 g 1Pt for PtCoCNT-ST, PtCoCNT-MT, and PtCo-Vu, respectively. 3.2. Methanol Electrooxidation: in Situ FTIR Studies. Figure 2a shows the voltammograms recorded during the methanol electrooxidation in 0.1 M HClO4. Clearly, the performance of the PtCo particles, in terms of both the onset potential and total current density (normalized to the total amount of Pt deposited onto each electrode), is better when supported on the functionalized CNTs. The inset of Figure 2a is a magnification of the potential window between 0.0 and 0.6 V, showing the onset potential for the electrooxidation of methanol. The onset potential for the methanol oxidation reaction follows the order PtCoCNT-ST < PtCoCNT-MT < PtCo-Vu. Selected mass current density values recorded at 500 and 650 mV are listed in Table 3. The better performance of the PtCo/CNTs samples could be related to either a higher amount of active sites or a higher intrinsic activity of those sites as compared to those on PtCo-Vu. Figure 2b shows the j E response of the catalysts normalized to the EASA, i.e., to the amount of active sites. Clearly, the trend for methanol electrooxidation activity remains unchanged, suggesting that the superior ability of the CNT-based catalysts should be ascribed to the higher intrinsic activity of active sites on the PtCo/CNTs catalysts. The methanol oxidation reaction was further studied by in situ infrared reflectance spectroscopy. Table 4 is a compilation of the positions of the more relevant bands detected in the FTIR spectra recorded during the MOR on the catalysts. Figure 3 shows the SPAIR spectra of PtCo-Vu recorded at potential intervals of 50 mV. The negative band at ca. 1100 cm 1 is assigned to perchlorate anions from the electrolyte.48 The positive band at 1640 cm 1 is ascribed to the consumption of interfacial water. The unipolar absorption band at 2035 cm 1 corresponds to COL. The bands at 1170 and 1280 cm 1 are assigned to formic acid and formaldehyde, respectively. The band ascribed to the formation of CO2 is centered at 2345 cm 1, being observed only at potentials higher than 500 mV. Figure 4 shows the SPAIR spectra for methanol oxidation on PtCoCNT-MT and PtCoCNT-ST. The spectra recorded for 9623

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Figure 2. Cyclic voltammetry recorded at 1 mV 3 s 1 in 0.1 M HClO4 + 0.1 M CH3OH. (red line) PtCo-Vu, (blue line) PtCoCNT-MT, and (green line) PtCoCNT-ST. Inset: Magnification of the onset potential window of the MOR area (forward cycle).

Table 3. Kinetic Parameters for the MOR at Selected Potentials and Electrode Active Surface Area (EASA) of the PtCo Electrocatalysts j/A 3 gPt

1

j/A 3 gPt

Table 4. Assignments of the Most Relevant Species Detected in the in Situ FTIR Spectra of the Catalysts; (+) and (-) Are for Positive and Negative Bands, Respectively

1

wavenumbers/cm EASA/m2 3 gPt

1

1

catalyst

at 0.5 V

at 0.65 V

PtCoCNT-MT

1.4

10.7

45

PtCoCNT-ST

2.9

30.3

49

HClO4

1100 (-)

1100 (-)

30

H2O

1640 (+)

1642 (-)

COL

2035 (-)

2035 (-)

HCOOH

1170 (-)

1135 (-)

1180 (-)

HCHO

1280 (-)

1236 (-)

1300 (-)

Ads CO2 Free CO2

2345 (-)

2343 (-) 2360 (-)

2343 (-) 2360 (-)

1710 (+)

1710 (+)

PtCo-Vu

0.4

2.4

PtCoCNT-MT show an absorption band at 1100 cm 1, which is ascribed to perchlorate anions from the electrolyte. This band is not observed in the spectra recorded with PtCoCNT-ST. Negative bands of low intensity corresponding to the formation of formic acid and formaldehyde appear at 1135 and 1236 cm 1 in the spectra of PtCoCNT-MT, and at 1180 and 1300 cm 1 for PtCoCNT-ST. The negative band at 1642 cm 1 suggests the formation of interfacial water with both catalysts. Remarkably, a positive band at 1710 cm 1 appears in the spectra of the CNTsupported PtCo samples, ascribed to the consumption of carboxylic species. This band is associated with CdO stretching in COO-type structures.36,54 Figure 5 shows the variation of the intensities of the bands at 1642 and 1710 cm 1 with the applied potential. The consumption of the carboxylic groups and the production of water occur simultaneously and follow an identical trend with the potential. Furthermore, the intensity of the bands is higher in PtCoCNT-ST than in PtCoCNT-MT suggesting that the reaction is favored on the catalyst with the highest density of functional groups. This observation is further supported by the appearance of such bands at less positive potentials on PtCoCNT-ST. The band at 2035 cm 1 ascribed to the COad species is not observed in the spectra of the CNT-ST catalysts, appearing in the

species

CdO in

COO

PtCo-Vu

PtCoCNT-MT

PtCoCNT-ST

1642 (-)

spectra recorded at potentials higher than 350 mV with PtCoCNT-MT. The formation of CO2, indicated by the negative band at 2343 cm 1, begins at 500 mV and 450 mV on PtCoCNTMT and PtCoCNT-ST, respectively. This shift confirms that the electrooxidation of methanol is promoted on PtCoCNT-ST, which is in good agreement with the voltammetry results (Figure 2). Noticeably, the profile of the band associated with CO2 exhibits a second band at 2360 cm 1. This band, not observed in the spectra recorded with PtCo-Vu, is assigned to free CO2, and its appearance starts at potentials higher than 500 and 600 mV for PtCoCNT-MT and PtCoCNT-ST, respectively. The fabrication of multiwalled carbon nanotubes involves Fe, Co, Ni, or Al based catalysts;55,56 therefore, it is likely that trace amounts of these metals might be occluded in the tubes. The actual amount of Fe and Ni found varies with the severity of the treatment, and it decreases in the order CNT-ST < CNT-MT as shown in Table 1. The electrochemical performance of these 9624

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Figure 3. SPAIR spectra of PtCo-Vu recorded in 0.1 M HClO4 + 0.1 M CH3OH solution. Reference spectrum taken at 50 mV, scan rate 1 mV 3 s 1. Electrode potential (mV): (1) 100; (2) 200; (3) 300; (4) 400; (5) 500; (6) 600; (7) 700; (8) 800; (9) 900.

systems without PtCo particles in methanol-containing and methanol-free electrolytes was also studied by cyclic voltammetry. The left side of Figure 6 shows the cyclic voltammograms of CNT-MT and CNT-ST recorded at 1 mV 3 s 1 in perchloric acid with and without methanol (0.1 M). The voltammograms of the supports recorded in 0.1 M HClO4 display anodic peaks at 0.64 and 0.69 V for CNT-ST and CNT-MT, respectively, and a cathodic peak at 0.6 V. The intensity of those peaks is higher on CNT-ST, in line with the higher amount of functional groups on this support. When methanol is added to the electrolyte, an oxidation wave at E g 0.5 V is observed. This oxidation wave reaches a higher current density in CNT-MT as compared to CNT-ST (34 vs 23 μA 3 cm 2). Although the support itself displays some activity for the MOR, it is ca. 2 orders of magnitude lower than that of PtCo catalysts (see inset in Figure 6 left for comparison). The poor electrooxidation of methanol by the CNTs was further confirmed by in situ FTIR studies. The right side of Figure 6 depicts the SPAIR spectra collected for CNT-MT in 0.1 M HClO4 + 0.1 M CH3OH at potential intervals of 50 mV highlighting the lack of bands ascribed to CO2.

4. DISCUSSION 4.1. Effects of the Functional Groups on the Nature of the Catalysts. The amount of oxygen-bearing surface groups on the

surface of the CNTs increases with the severity of the treatment at the expense of unsaturated CdC bonds, as reflected by the increasing O/C surface atomic ratio (Table 1) and the decrease in the intensity of the band at binding energy of ca. 290.3 eV (Table 2), ascribed to π electrons of the graphene sheets

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(Cπ sites).23,57 A severe decrease in the specific surface area of the CNTs is also observed after chemical treatment (Table 1). Contrary to the common perception that after functionalization more sites for metal anchoring are available, the observed amount of PtCo particles on the functionalized supports decreases. This finding is because the surface treatment decreases both the surface area of the supports and the abundance of the main (preferred) sites for metal anchoring, which are the Cπ sites.58 As studied by our group,23 the sites responsible for the metal C interaction are both the oxygen-bearing functional groups and the Cπ sites. The amount of Cπ sites, related to the amount of species located at 290.3 eV, decreases with the severity of the oxidation treatment, following the trend: PtCoCNT-MT ≈ PtCo-Vu > PtCoCNT-ST (Table 2). This trend matches the one obtained for the metal loading value obtained by TXRF: PtCo-Vu ≈ PtCoCNT-MT > PtCoCNT-ST. The XPS analysis yields similar values for the Pt/Co surface atomic ratio for all catalysts, displaying a higher value than that of the bulk analysis. The tendency of Pt to segregate when alloyed with Co has been attributed to its higher Wigner-Seitz radius.59 The higher Pt/C ratio found for PtCo-Vu implies that this catalyst has the highest amount of surface Pt atoms; however, the MOR is better on PtCoCNTs catalysts. The MOR activity is in agreement with the EASA values that show an increase of the exposed Pt sites following the order PtCoCNT-ST ≈ PtCoCNT-MT > PtCo-Vu (Table 3). Because the Hupd occurs exclusively on the Pt atoms,53 the EASA value accounts for the abundance of Pt atoms on the surface of the electrodes, which is higher on CNT-based catalysts. 4.2. Effects of the Functional Groups on the Methanol Oxidation Reaction. The performance of PtCo particles for the MOR is better when supported on CNTs than on Vulcan XC 72R, in terms of both the onset potential and total current density (Figure 2). Furthermore, the catalyst prepared with the support containing more functional groups, PtCoCNT-ST, exhibits the best performance for the MOR. According to the most accepted mechanism for the MOR, three neighboring Pt sites are necessary for the methanol adsorption dehydrogenation reaction.24,60 Also, it has been reported that the relationship between the electrocatalytic activity in the MOR and the particle size may be fitted to a volcano-type curve, whose maximum in activity is centered at ca. 3 nm.1,61 This feature explains the higher activity of the PtCo/ CNTs catalysts for the MOR. It fails to explain, however, the better performance of PtCoCNT-ST as compared to PtCoCNTMT. The increase could be ascribed to the increasing hydrophilicity of the surface-modified CNTs supports,62 which could enhance the diffusion of polar reactants such as methanol to the catalytic active sites. This finding could explain the higher current density recorded by PtCoCNT-ST, but it does not justify the shift to less positive values of the onset potential for the MOR observed in the voltammetry. Moreover, the in situ FTIR results show that the onset for the CO2 production is shifted by 50 mV to less positive potentials on PtCoCNT-ST. It is difficult to justify this shift by taking into account only the nature of the bimetallic PtCo particles. In fact, as discussed above, the performance of the PtCo particles for the Hupd is very similar for PtCoCNT-ST and PtCoCNT-MT; both catalysts have the same values for the EASA, i.e., they have the same amount of exposed Pt sites. This latter feature implies that the early stages of the methanol electrooxidation reaction, i.e., methanol adsorption and subsequent dehydrogenation steps, which are proposed to occur if three neighboring Pt sites are available, will proceed 9625

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Figure 4. SPAIR spectra recorded in 0.1 M HClO4 + 0.1 M CH3OH solution. Reference spectrum taken at 50 mV, scan rate 1 mV 3 s 1. Electrode potential (mV): (1) 100; (2) 150; (3) 200; (4) 250; (5) 300; (6) 350; (7) 400; (8) 450; (9) 500; (10) 550; (11) 600; (12) 650; (13) 700; (14) 750; (15) 800; (16) 850; (17) 900; (18) 950. PtCoCNT-MT (left) and PtCoCNT-ST (right).

Figure 5. (b) Carboxylic group consumption (positive band at 1710 cm 1) and (red b) water production (negative band at 1640 cm 1) vs potential for (a) PtCoCNT-MT and (b) PtCoCNT-ST.

similarly on the PtCo-CNTs catalysts. On the other hand, it is known that OHad species are necessary for the complete electrooxidation of methanol to CO2. Given that the PtCo particles on PtCoCNT-ST and PtCoCNT-MT are essentially the same, the more facile CO2 production may be attributed to the support itself, by providing OH species before they are

actually nucleated on the metallic sites. At potentials more positive than 500 mV, the OH species can be nucleated on the bimetallic particles, and the participation of the support is not predominant. This idea has been suggested by other groups. Hsieh et al.63 studied the electrochemical activity of oxidized CNTs supported 9626

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Figure 6. Left: Cyclic voltammetry recorded at 1 mV 3 s 1 in 0.1 M HClO4 [(blue line) CNT-MT, (green line) CNT-ST] and in 0.1 M HClO4 + 0.1 M CH3OH [(blue circle) CNT-MT, (green circle) CNT-ST]. Inset: Cyclic voltammetry recorded at 1 mV 3 s 1 in 0.1 M HClO4 + 0.1 M CH3OH of (red line) PtCo-Vu, (blue line) PtCoCNT-MT, and (green line) PtCoCNT-ST. Right: SPAIR spectra of CNT-MT recorded in 0.1 M HClO4 + 0.1 M CH3OH solution. Reference spectrum taken at 50 mV, scan rate 1 mV s 1. Electrode potential (mV): (1) 100; (2) 150; (3) 200; (4) 250; (5) 300; (6) 350; (7) 400; (8) 450; (9) 500; (10) 550; (11) 600; (12) 650; (13) 700; (14) 750; (15) 800; (16) 850; (17) 900; (18) 950.

PtFe, PtCo, and PtNi in the MOR, finding better activity for PtCo and PtNi catalysts. They proposed that oxidized groups on the carbon surface could act as OH donors, helping in the CO oxidation reaction. G
omez de la Fuente et al.64 also observed an increase in the activity of the MOR on PtRu-Vu when the support had been subjected to an oxidation treatment. This finding was attributed to participation of the functional groups in the oxidation of the adsorbed intermediate species. The onset potential for the production of COL species is 350 mV for PtCoCNT-MT and 150 mV for PtCo-Vu. However, CO species are not observed on PtCoCNT-ST (Figures 3 and 4). The absence of absorption bands corresponding to methoxide species65,66 suggests that the mechanism for the MOR at the Vu and CNTs supported catalysts occurs via dissociation of C H bonds.67 Therefore, it is reasonable to assume that the lack of CO species on PtCoCNT-ST should be explained by a more facile oxidation to CO2. This idea is in good agreement with the shift to less positive potentials observed for the production of CO2 on PtCoCNT-ST. Another interesting feature of the in situ FTIR spectra recorded during the MOR on the CNTs catalysts is the splitting of the CO2 band, showing two peaks at 2345 and 2360 cm 1. This finding is due to the presence of CO2 species trapped on the carbon pores and is related to the pore size distribution of the support. This free CO2 may be formed and diffuse into the inner pores of the carbon, or may be formed directly in the pores. Colmenares et al.68 suggested that carbonaceous materials with both high BET specific area and porosity can trap CO2 on their structure. 4.3. Formation of H2O and Consumption of OxygenContaining Groups in CNT Supported PtCo Catalysts. As discussed in the section above, the O-bearing groups of the

support promote the formation of CO2 from COL at 450 mV on PtCoCNT-ST. A closer inspection of the SPAIR spectra reveals that the oxygen groups on the support could be participating in other processes. The SPAIR spectra of PtCoCNT-ST and PtCoCNT-MT show the appearance of positive bands at 1710 cm 1 at E g 500 and 650 mV, respectively. This band, assigned to carboxylic groups,36,54 is not observed in the spectra of PtCo-Vu. Because positive bands correspond to disappearing species, it appears that such carboxylic groups are being consumed during or in parallel to the MOR. In the spectra of the CNTs, the band at 1640 cm 1 is also positive, in contrast to the negative band observed for PtCo-Vu. This observation implies that water is produced on the CNTs catalysts in parallel to the MOR. The different profile of the carboxylic acid and water FTIR bands is shown more clearly in Figure 7, which compares the SPAIR spectra of the PtCo catalysts recorded in 0.1 M HClO4 + 0.1 CH3OH at 800 mV. Figure 5 shows the variation of the intensity of the bands at 1710 cm 1 (consumption of carboxylic groups) and 1640 cm 1 (water formation) with the applied potential for PtCoCNT-ST and PtCoCNT-MT. Clearly, both processes occur simultaneously and cannot be related to the methanol oxidation reaction alone, as the MOR implies a consumption of water. These observations lead to the conclusion that a reaction other than methanol electrooxidation reaction is occurring simultaneously. This process could be the reaction between the acid groups from the support and methanol (from the reaction medium) to produce water and esters. This observation is further supported by the SPAIR spectra recorded in methanolfree 0.1 M HClO4 solutions; in this case, the band at 1640 cm 1 is positive and the band at 1710 cm 1 (carboxylic acid) is not 9627

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In summary, the effect of the functional groups is twofold. On one hand, the presence of oxygen-bearing groups on the surface of the support results in the formation of smaller metal particles, increasing the number of active sites. On the other hand, they participate in the reaction promoting the formation of CO2 at less positive potentials.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], Telephone: +34 91 585 4632.

’ ACKNOWLEDGMENT This work is supported by the projects S2009/ENE-1743 from the Comunidad de Madrid and ENE2010-15381 from the Spanish Ministry of Science and Innovation. ’ REFERENCES

Figure 7. SPAIR spectra recorded at 800 mV in 0.1 M HClO4 + 0.1 M CH3OH solution. Reference spectrum taken at 50 mV, scan rate 1 mV s 1. (red line) PtCo-Vu, (blue line) PtCoCNT-MT, and (green line) PtCoCNT-ST. The top spectrum (green circle) corresponds to PtCoCNT-ST in 0.1 M HClO4 (without CH3OH).

observed. Admittedly, further experiments are needed to determine the precise reactions involved in the consumption of the carboxylic groups from the support. In summary, the production of water and consumption of carboxylic groups (observed by in situ FTIR) occurs only in the presence of methanol with catalysts based on the modified carbon nanotubes. Water production is higher on the severely treated nanotubes, i.e., those containing more functional groups as shown in Figure 5a and b.

5. CONCLUSIONS The electrooxidation of methanol is improved considerably in chemically treated CNTs supported PtCo compared to PtCoVu; the onset potential of the methanol oxidation reaction shifts toward less positive potentials and the current density is higher. The performance of the catalysts for the MOR improves with the amount of oxygen surface groups on the support. The role of the oxygen-containing groups on the carbon nanotube surfaces can be summarized as follows: (i) the number of active sites on PtCo/CNTs, especially on CNT-ST, is higher than on PtCo-Vu, because smaller PtCo particles are stabilized on the functionalized CNTs, (ii) the free fraction of functional groups could be assisting methanol oxidation by supplying OH groups to the intermediates species (Pt-(CHO)ads or Pt-(CO)ads), hence promoting the production of CO2 at less positive potentials, and (iii) in the presence of methanol, carboxylic groups react to form water and an ester.

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