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C catalyst for methanol electrooxidation in acidic medium
Journal of Alloys and Compounds 479 (2009) 395–400
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Effect of W on activity of Pt–Ru/C catalyst for methanol electrooxidation in acidic medium Zhen-Bo Wang ∗ , Peng-jian Zuo, Ge-Ping Yin School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 4 November 2008 Received in revised form 15 December 2008 Accepted 16 December 2008 Available online 25 December 2008 Keywords: Direct methanol fuel cell Pt–Ru–W/C catalyst Pt–Ru/C catalyst Methanol electrooxidation
a b s t r a c t The effect of W on the activity of Pt–Ru/C catalyst was investigated. The Pt–Ru–W/C and Pt–Ru/C-TR catalysts were prepared by thermal reduction method. Comparison was made to a homemade Pt–Ru/CCR catalyst prepared by chemical reduction. Their performances were tested by using a glassy carbon thin film electrode through cyclic voltammetric and chronoamperometric curves. The particle size, structure, composition, and surface state of homemade catalyst were determined by means of X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), transmission electron microscopy (TEM), and X-ray photoelectron spectrometry (XPS). The result of XRD analysis shows that the homemade ternary catalyst exhibits face-centered cubic structure and has smaller lattice parameter than Pt-alone and homemade Pt–Ru/C catalysts. The particle size of Pt–Ru–W/C catalyst is relatively large of 6.5 nm. Its electrochemically active specific area is 20 m2 g−1 less than that of Pt–Ru/C-CR, and much twice as big as that of Pt–Ru/C-TR. But, XPS analysis shows that the addition of W changes the surface state of Pt components in the alloy and can clean Pt surface active sites which are adsorbed by hydrogen. The electrocatalytic activity and tolerance performance to COads of Pt–Ru–W/C catalyst for methanol electrooxidation is the best due to the promoting function of W in comparison with homemade Pt–Ru/C ones. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Methanol is an interesting attractive fuel for electric vehicles, mobile telephone, and laptops. It is inexpensive and easily transported. So, proton exchange membrane fuel cells (PEMFCs) used methanol as fuel has much been studied in the past decades [1–4]. The main drawbacks limiting the broad commercialization of direct methanol fuel cell (DMFC) are the poisoning of the catalyst by COads species produced during methanol electrooxidation and the methanol crossover through the polymer electrolyte membrane from anode to cathode [5–7]. There is a need to improve the activity of anodic catalysts for methanol electrooxidation [8,9]. The only possibility to overcome this effect is to modify the catalysts surface in such a way to increase its coverage in oxygenated species coming from the dissociation of water at low potentials. As we know that the addition of metal (denoted as Me) to Pt-based catalysts significantly lowers the overpotential for methanol electrooxidation through a so-called bifunctional mechanism [10,11] that can
∗ Corresponding author at: Department of Applied Chemistry, School of Chemical Engineering and Technology, Harbin Institute of Technology, PO Box 411#, Harbin 150001, PR China. Tel.: +86 451 86417853; fax: +86 451 86413707. E-mail address: [email protected] (Z.-B. Wang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.061
be summarized as follows: Pt + CH3 OH → Pt–(CH3 OH)ads
(1) +
Pt–(CH3 OH)ads → Pt–COads + 4H + 4e
−
(2)
Me + H2 O → Me–(H2 O)ads
(3)
Me–(H2 O)ads → Me–(OH)ads + H+ + e−
(4) +
Pt–COads + Me–(OH)ads → CO2 + Pt + Me + H + e
−
(5)
At present, there are only a limited number of possible metals which are able to activate water at low potentials with a sufficient stability in acid medium. Pt–Ru alloy is still considered to be the best catalyst due to its tolerance to CO species, and is widely used in DMFC. But the performance of the Pt–Ru alloy catalyst cannot meet the demand of DMFC at low temperatures [9]. So, the addition of a third metal (Mo, Sn, Os or Ni,) was added to Pt–Ru alloy to enhance its activity for methanol electrooxidation [12–15], but the durability with time of such electrode needs further improvement. It has been claimed that tungsten oxide was a suitable promoter of Pt catalyst towards methanol oxidation showing a significant decrease in the amount of poisoning species compared with Pt-alone [16]. The presence of a ‘spillover effect’, caused by the formation of hydrogen tungsten bronze Hx WO3 during the dehydrogenation process of methanol, was postulated [17]. In situ observation method was used to prove that the transfer of hydrogen ions, produced on the
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platinum during methanol electrooxidation, to the tungsten oxide ensures that the active reaction sites on the platinum remain clean, thus enhancing the current density of methanol electrooxidation on Pt–WOx and Pt–Ru–WOx catalysts [17,18]. Gotz and Wendt [19] prepared carbon supported Pt–Ru–Mo, Pt–Ru–Sn, and Pt–Ru–W by impregnation and the colloid methods, and tested towards their activity for anodic oxidation of H2 containing CO of 150 ppm. The oxidation of H2 /CO on the Pt–Ru–W/C catalyst was superior to the other catalysts tested. Lima et al. [20] prepared Pt–Ru–X ternary metallic catalysts (with X = Au, Co, Cu, Fe, Mo, Ni, Sn, and W) by electrochemical deposition and dispersed in a conductive three-dimensional matrix, an electronic conducting polymer, polyaniline (PAni). Methanol electrooxidation was investigated on Pt–Ru–X/PAni in a solution of 0.5 mol L−1 HClO4 . Lima et al. [20] thought that Pt–Ru–Mo/PAni catalyst was the most efficient anode at potentials up to 500 mV (vs. reversible hydrogen electrode (RHE)) for methanol electrooxidation. Based on these literature findings we deem it meaningful to explore the use of thermal reduction to prepare carbon supported Pt–Ru–W catalyst for DMFC application. In the process we had investigated methanol electrooxidation on Pt–Ru–W/C catalyst. The performance of Pt–Ru–W/C catalyst was in comparison with Pt–Ru/C catalyst obtained by chemical reduction of H2 PtCl6 and RuCl3 as precursors with sodium borohydride as a reducing agent [14]. 2. Experimental 2.1. Preparation of catalysts Tungsten compounds cannot be reduced by chemical reduction with NaBH4 . A homemade Pt–Ru–W/C catalyst was prepared by thermal reduction. A described briefly, carbon black powder (Vulcan XC-72, Cabot, 250 m2 g−1 ) was used as the support for the catalyst. The catalyst contained 20 wt.% metal. The 0.25 g Pt–Ru–W (with an atomic ratio of 6:3:1)/C catalyst, was obtained by thermal reduction of H2 PtCl6 , RuCl3 , and (NH4 )6 H2 W12 O40 as precursors. The carbon black was ultrasonically dispersed in a mixture of ultrapure water and isopropyl alcohol for 20 min. Then the precursors were added to the ink, and then mixed thoroughly for 20 min. The ink was dried with a magnetic stirrer at 60 ◦ C. The dried H2 PtCl6 , RuCl3 , and (NH4 )6 H2 W12 O40 compounds with carbon were put into a tube furnace and reduced at 400 ◦ C with a gaseous mixture of H2 and Ar with an atomic ratio of 1:9. The catalyst powder was stored in a vacuum vessel. Pt–Ru/C catalyst (denoted as Pt–Ru/C-CR) was prepared by chemical reduction of H2 PtCl6 and RuCl3 as precursors with NaBH4 according to the method mentioned in the literature [14]. The other Pt–Ru/C catalyst (denoted as Pt–Ru/C-TR) was prepared by thermal reduction of H2 PtCl6 and RuCl3 as precursors with H2 and Ar according to the method mentioned in the literature [21]. 2.2. Electrode preparation and electrochemical measurement 2.2.1. Preparation of working electrode Three millimeter diameter glassy carbon working electrodes (electrode area 0.0706 cm2 ) polished with 0.05 m alumina to a mirror finish before each experiment were used as substrates for the carbon supported catalysts. For the electrode preparation, 5 L of an ultrasonically redispersed catalyst suspension was pipetted onto the glassy carbon substrate. After the solvent evaporation, the deposited catalyst (28 gmetal cm−2 ) was covered with 5 L of a dilute aqueous Nafion® solution (5 wt.%). The resulting Nafion® film with a thickness of ≤0.2 m had a sufficient strength to attach the carbon particles permanently to the glassy carbon electrode without producing significant film diffusion resistances [22,23]. 2.2.2. Electrochemical measurements Electrochemical measurements were carried out with a conventional sealed three-electrode electrochemical cell at 25 ◦ C. The glassy carbon electrode as a working electrode was covered with the catalyst powder. A piece of Pt foil of 1 cm2 area was used as the counter one. The reversible hydrogen electrode was used as the reference one, with its solution connected to the working electrode compartment by a Luggin capillary whose tip was placed appropriately close to the working electrode. All chemicals used were of analytical grade. All the solutions were prepared with ultrapure water (MilliQ, Millipore, 18.2 M cm). Before measurements, a solution of 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 was stirred constantly and purged with ultrapure argon gas to expel oxygen, and then protected with Ar during measurements. Electrochemical experiments were performed by using a CHI630A electrochemical analysis instrument. Cyclic voltammograms (CV) were plotted within the potential range from 0.05 to 1.2 V with a scanning rate of 0.01 V s−1 . The chronoamperometric experiments were carried out by using CHI630A electrochemical analysis instrument controlled by an IBM PC. The potential jumped
from 0.1 to 0.7 V. Due to a slight contamination from the Nafion® film, the working electrodes were electrochemically cleaned by continuous cycling at 0.05 V s−1 until stable responses were obtained before the measurement curves were recorded. 2.2.3. CO stripping voltammetry The electrode was electrochemically cleaned in an Ar-degassed solution of 0.5 mol L−1 H2 SO4 at 25 ◦ C. The amount of the Pt–Ru–W/C catalyst as the working electrode with a geometric surface area of 0.0706 cm2 was 10 g (28 gmetal cm−2 ). CO was adsorbed on the surface of the catalyst at 0.08 V by bubbling CO gas through the 0.5 mol L−1 H2 SO4 solution for 25 min. CO dissolved in the solution was subsequently removed by bubbling argon gas of high purity for 35 min, keeping the potential also at 0.08 V. The potential was then cycled at a scanning rate of 0.02 V s−1 from 0.05 to 1.2 V for two oxidation and reduction cycles. 2.3. Characterization of physical properties 2.3.1. X-ray diffraction (XRD) measurements XRD analysis was carried out for the catalysts from different precursors with a D/max-rB (Japan) diffractometer using a Cu K␣ X-ray source operating at 45 kV and 100 mA. The XRD patterns were obtained at a scanning rate of 4◦ min−1 with an angular resolution of 0.05◦ of the 2 scan. 2.3.2. Energy dispersive analysis of X-ray (EDAX) Chemical composition analysis by EDAX was performed with an EDAX HitachiS-4700 analyser associated to a scanning electron microscope (SEM, Hitachi Ltd. S-4700). Incident electron beam energies from 3 to 30 keV had been used. In all cases, the beam was at normal incidence to the sample surface and the measurement time was 100 s. All the EDAX spectra were corrected by using the ZAF correction, which took into account the influence of the matrix material on the obtained spectra. 2.3.3. Transmission electron microscopy (TEM) After the catalyst sample was finely ground and ultrasonically dispersed in isopropanol, a drop of the resulting dispersion was deposited and then dried on the standard copper grid coated with polymer film. Transmission electron micrograph was taken with a JEOLJEM-1200EX Microscope (made in Japan), at the applied voltage of 100 kV, with a magnification of 200,000 and the spatial resolution of 1 nm. 2.3.4. X-ray photoelectron spectrometry (XPS) The surface composition XPS analysis was performed with the VG ESCALAB MKII X-ray photoelectron spectrometer, with the Al K␣ X-ray source (1486.6 eV). The XPS spectra were recorded at a 45◦ takeoff angle, while the chamber pressure was held below 5 × 10−9 Pa. The C 1s electron binding energy was referenced at 284.6 eV, and a nonlinear least-squares curve-fitting program was employed with a Gaussian–Lorentzian production function [24,25]. The deconvolutions of the XPS spectra were carried out according to the reported methods [16,26].
3. Results and discussion 3.1. Characterization of catalysts – X-ray diffraction XRD patterns reveal the bulk structure of the catalysts and their support. Fig. 1 presents the XRD patterns of Pt–Ru/C-TR (A) [21],
Fig. 1. XRD patterns of Pt–Ru/C-TR (A) [21], Pt–Ru/C-CR (B)[14], and Pt–Ru–W/C (C) [27] catalysts.
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397
Table 1 The particle sizes, specific areas, and lattice parameters of Pt–Ru–W/C and Pt–Ru/C catalysts. Catalysts a
Pt–Ru/C-CR Pt–Ru/C-TR Pt–Ru–W/Cc a b c
2 (◦ )
d-Value (nm)
Lattice parameter (nm)
Particle size (nm)
Specific area (m2 g−1 )
67.9 67.8 68.2
0.1379 0.1387 0.1375
0.3901 0.3923 0.3888
4.5 19.5b 6.5
74.1 17.1b 47.3
Data were taken from Ref. [14]. Data were taken from Ref. [21]. Data were taken from Ref. [27].
Pt–Ru/C-CR (B) [14], and Pt–Ru–W/C (C) [27] catalysts. It can be seen that the first peak in the XRD patterns is associated with the Vulcan XC-72 carbon support. The other four peaks are characteristic of face-centered cubic (fcc) crystalline Pt, corresponding to the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively, indicating that the catalyst is mainly single-phase disordered structure (i.e., solid solution). No diffraction peaks, which indicate the presence of either pure Ru and W metals or Ru-rich hexagonal close packed (hcp) phase, appear in the XRD pattern of Pt–Ru–W/C catalyst, suggesting that Ru and W atoms either form an alloy with Pt or exits as oxides in amorphous phase. Relative to the same reflections in Pt-alone (cf. the vertical lines) and Pt–Ru/C-CR [14], the diffraction peaks for Pt–Ru–W catalyst are shifted slightly to higher 2 values. The higher angle shifts of the Pt diffraction peaks reveal the formation of an alloy involving the incorporation of Ru and W atom into the fcc structure of Pt. The lattice parameter of Pt–Ru–W/C catalyst calculated using the Pt (2 2 0) crystal face is given in Table 1. The lattice parameter obtained for the P–Ru–W/C catalyst is smaller than those of Pt–Ru/C ones. In fact, the decrease in the lattice parameter of the alloy catalyst reflects the increase in the incorporation of Ru and/or W into the alloyed state. The average particle size d may be estimated according to Debye–Scherrer formula [14,28]: d=
k ˇ1/2 cos
(6)
S=
6000 d
(7)
Pt−Ru−W = XPt Pt + XRu Ru + XW W
26 m2 g−1 , bigger than that of the Pt–Ru–W/C catalyst, and four times bigger than that of the Pt–Ru/C-TR one. 3.2. EDAX measurement Chemical composition of the catalyst was determined by EDAX analysis. Typical values of the composition analysis of Pt–Ru–W/C, Pt–Ru/C-CR [14], and Pt–Ru/C-TR [21] catalysts are shown in Table 2. The EDAX analysis shows that the determined composition is quite similar to the nominal value, confirming the Pt, Ru or W is completely reduced in the as-prepared Pt–Ru/C-CR and Pt–Ru-W/C catalysts. But the practical composition is close to Pt2 Ru1 , confirming that Ru is not completely reduced in the Pt–Ru/C-TR one. 3.3. TEM manifestation TEM image was obtained on Pt–Ru–W/C catalyst, to examine the effect of mixing W into Pt–Ru catalyst on the particle morphology and the particle size, which are believed to strongly affect the property of catalyst. Fig. 2 shows the typical bright field TEM micrograph of Pt–Ru–W/C catalyst, with metal grains in black and carbon support in grey. The spherical metal particles in catalyst exist homogeneously on carbon support grains. It is easy to pinpoint the average particle size at about 7 nm. Its result is in agreement with that of XRD measurement.
(8)
where d is the average particle size (nm), the wavelength of Xray (1.5406 Å), the angle, at which the peak maximum occurs, ˇ1/2 the width (in radians) of the diffraction peak at half height, k a coefficient of 0.89–1.39 (0.9 here), the density of Pt–Ru or Pt–Ru–W alloy, Pt the density of Pt metal (21.4 g cm−3 ), Ru the density of Ru metal (12.3 g cm−3 ), W the density of W metal (19.3 g cm−3 ) and XPt , XRu , and XW are the weight percent of Pt, Ru, and W, respectively, in the catalysts. Table 1 gives the calculated average particle size and specific surface area of the catalyst according to the diffraction peak of Pt (2 2 0). For comparison, the data of Pt–Ru/C-CR [14] and Pt–Ru/C-TR [21] catalysts were also presented. The particle size of the Pt–Ru/CCR catalyst is the smallest. The particle size of the Pt–Ru/C-TR one is the biggest. The specific surface area of the Pt–Ru/C-CR is about
Table 2 The atomic composition of Pt–Ru–W/C and Pt–Ru/C catalysts (at.%). Catalysts
Pt–Ru/C-CRa Pt–Ru/C-TR Pt–Ru–W/C a
Nominal content
Determined by EDAX
Pt
Ru
W
Pt
Ru
W
50 50 60
50 50 30
– – 10
56.3 67.3 58.6
43.7 32.7 29.1
– – 12.3
Data were taken from Ref. [14].
Fig. 2. TEM micrograph of the Pt–Ru–W/C catalyst.
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Fig. 3. Cyclic voltammograms for CO electrooxidation on the Pt–Ru–W/C catalyst in 0.5 mol L−1 H2 SO4 at 25 ◦ C. Scan rate: 0.02 V s−1 [27].
3.4. The electrochemically active specific area of the catalyst Fig. 3 shows the cyclic voltammogram on the Pt–Ru–W/C catalyst for CO oxidation in a solution of 0.5 mol L−1 H2 SO4 at 25 ◦ C [27]. It can be seen that the onset potential and the peak potential for CO electrooxidation on the Pt–Ru–W/C catalyst is more negative than on the Pt–Ru/C-CR catalyst [14]. The electrochemically active surface area (SEAS ) of the catalyst is calculated by using Eq. (9) [29,30] and the cyclic voltammetry curve of CO adsorption and desorption electrooxidation: SEAS =
QCO G × 420
(9)
where QCO is the charge for CO desorption electrooxidation in microcoulomb (C), G represents the summation of Pt–Ru or Pt–Ru–W metal loading (g) in the electrode, and 420 is the charge required to oxidize a monolayer of CO on catalyst in C cm−2 . The SEAS of Pt–Ru–W/C catalyst is 44.7 m2 g−1 [27], and of the Pt–Ru/C-CR catalyst is 64.5 m2 g−1 [14], of Pt–Ru/C-TR catalyst is 15.5 m2 g−1 . The SEAS of Pt–Ru/C-CR catalyst is about 20 m2 g−1 bigger than that of Pt–Ru–W/C catalyst, and is four times as big as Pt–Ru/C-TR catalyst. The results are similar to the specific surface area values calculated from X-ray diffraction peaks and TEM image.
Fig. 4. Cyclic voltammetry curves of methanol electrooxidation in an Ar-saturated 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C on Pt–Ru–W/C (A), Pt–Ru/C-CR (B), and Pt–Ru/C-TR (C) catalysts. Scan rate: 0.01 V s−1 .
tial for methanol electrooxidation and the peak current density on the Pt–Ru/C-TR catalyst are about 0.874 V (vs. RHE) and only 3.56 mA mg−1 , respectively, during positive potential scanning as shown in curve C. The peak potential and the peak current density on the Pt–Ru/C-TR catalyst are about 0.724 V (vs. RHE) and 1.66 mA mg−1 , respectively, during its reverse scanning. The peak potential on the Pt–Ru–W/C catalyst during potential scanning is 10 mV lower than that on the Pt–Ru/C-CR one. But the peak current density on the Pt–Ru–W/C catalyst is about 19.80 mA mg−1 higher than that on the Pt–Ru/C-CR one. So, the performance of the Pt–Ru–W/C catalyst for methanol electrooxidation is much higher than that of the latter. The peak current density on the Pt–Ru–W/C catalyst is 12 times higher than that on the Pt–Ru/C-TR catalyst. It can be seen from the above results that the performance of the Pt–Ru–W/C catalyst is the best mainly due to the improving effect of W in the Pt–Ru/C catalyst for methanol electrooxidation. Fig. 5 shows the current densities measured at a constant potential jumping from 0.1 to 0.7 V in an Ar-saturated solution of 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C. Initial high current is ascribed mainly to double-layer charging. The currents decay with time in a parabolic style, and reach an apparent steady state
3.5. The electrochemical activity of the catalysts Fig. 4 shows the cyclic voltammograms on the Pt–Ru–W/C (A), Pt–Ru/C-CR (B), and Pt–Ru/C-TR (C) catalysts in a solution of 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C. It can be seen from Fig. 4 that the onset potential of a current rise for methanol electrooxidation on the Pt–Ru–W/C catalyst corresponds to that on the Pt–Ru/C catalysts, i.e., about 0.55 V. The peak potential for methanol electrooxidation, at which the peak current occurs is 0.868 V (vs. RHE), and the peak current density is 46.33 mA mg−1 during positive potential scanning on the Pt–Ru–W/C catalyst as shown in curve A. The peak potential and the peak current density on the Pt–Ru–W/C catalyst are about 0.749 V (vs. RHE) and 41.95 mA mg−1 , respectively, during its reverse scanning. The peak potential for methanol electrooxidation and the peak current density on the Pt–Ru/C-CR catalyst are about 0.878 V (vs. RHE) and 25.18 mA mg−1 , respectively, during positive potential scanning as shown in curve B. The peak potential and the peak current density on the Pt–Ru/C–CR catalyst are about 0.753 V (vs. RHE) and 26.53 mA mg−1 during its negative scanning. The peak poten-
Fig. 5. Chronoamperometric curves of methanol electrooxidation in an Ar-saturated 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C on Pt–Ru–W/C (A), Pt–Ru/C-CR (B), and Pt–Ru/C-TR (C) catalysts. Potential jumps from 0.1 to 0.7 V.
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399
Fig. 6. di/dt (normalized scale) against t plots from t = 0 to t = 1 min for Pt–Ru-based/C catalysts with or without addition of tungsten. Slopes: 0.14 for Pt–Ru–W/C (A), 0.82 for Pt–Ru/C-CR (B), and 0.53 for Pt–Ru/C-TR (C) catalysts.
within 500 s. The current density of methanol electrooxidation on the Pt–Ru–W/C catalyst, which is a 11.1 mA mg−1 at 1000 s, is about 2.2 times higher than that on the Pt–Ru/C-CR catalyst at the same potential and the same time, is about 142 times higher than that on the Pt–Ru/C-TR catalyst, i.e., the activity of the Pt–Ru–W/C catalyst is the best. The results are similar to those of cyclic voltammetry measurement. Fig. 6, representing a (di/dt) = f(t) plot (normalized scale) at small time values allows evaluation of the initial poisoning rate for the catalysts during the methanol electrooxidation by comparing the three slopes: the greater the slope is, the easier the initial poisoning of the electrode surface is [31]. It appears that added tungsten leads to less poisoning since the experimental slope with added tungsten is lower by a factor of 6 and 4, compared to that the obtained Pt–Ru–W/C, Pt–Ru/C-CR, and Pt–Ru/C-TR catalysts, that is, 0.14, 0.82, and 0.53, respectively. 3.6. Characterization of X-ray photoelectron spectrometry The core level spectra of Pt 4f and W 4f for the Pt–Ru–W/C catalyst are depicted in Fig. 7. The two most intense peaks in Fig. 7A, located at the Pt 4f7/2 and Pt 4f5/2 , maintain an area ratio near 4:3 as expected theoretically for pure Pt [26,32,33]. The result of deconvolution indicates a phase composition which contains 61.32% of Pt in metallic Pt0 , 36.82% in Pt2+ (as PtO or Pt (OH)2 ), and 1.86% in Pt4+ as PtO2 . The content of Pt4+ decreases markedly due to W additive into Pt–Ru/C in comparison with that Pt–Ru/C catalyst [34]. It is interesting to observe that the W 4f signal is composed of two most intense peaks at binding energies of 34.3 and 35.1 eV (W 4f7/2 ) and at 36.5 and 37.4 eV (W 4f5/2 ). These peaks are ascribed to the oxidation states (W(IV) and W(VI)), respectively [16]. Yet, the W 4f7/2 signal at 34.3 eV may also be derived from the presence of W2 O5 or substoichiometric WO3−x (2 < x < 3) species. These tungsten oxides have comparable surface concentrations, i.e., WO3 (74.3%) and WO2 (23.6%). It is expected that the WO3 /WO2 system can act as a redox surface mediator for the oxidation of adsorbed CO-like from methanol electrooxidation. In fact, the redox potentials of these systems, calculated from the free energies, fall in the potential region where tungsten oxides redox couples may be active for methanol electrooxidation [16,28]. This means that proton conducting oxides, such as WOx , may play an important role in the enhanced methanol electrooxidation. In addition, the W 4f signal still consists of W metal peak at binding energy of 31.88 eV (W 4f7/2 line) with a small content of 2.1%.
Fig. 7. Typical XPS spectra of the Pt 4f and W 4f regions for the homemade Pt–Ru–W/C catalyst.
From the results above, it can be considered that the Pt–Ru–W electrode facilitates the generation of Me–OHads from water more than Pt–Ru due to the addition of W [28]. The methanol electrooxidation on the Pt–Ru–W/C catalyst adsorbs and reacts rapidly according to the so-called bifunctional mechanism. So, the peak potential of methanol electrooxidation on it is relatively negative, and its peak current is higher than that on Pt–Ru/C catalyst as shown in CV curves. The different tungsten oxides in Pt–Ru–W/C catalyst surface prepared by thermal reduction were formed. In addition, these materials (WOx ) were the good proton conducting oxides and could reduce the electrode potential of methanol electrooxidation and improve the reaction of methanol dehydrogenation. These WOx were electrochemically oxidized and reduced according to Eq. (10) [17,18,35]: WOx + yH+ + ye− ↔ Hy WOx
(10)
Hydrogen on the Pt-based catalyst would be transferred to the WOx in the Pt–Ru–W nanophase at a positive potential as shown in Eq. (11): Pt–H + WOx → Pt + H–WOx
(11)
This showed the occurrence of hydrogen spillover from Pt to WOx . As a result, the transfer of hydrogen ions, produced on the
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platinum during methanol electrooxidation, to the tungsten oxide ensured clean active reaction sites on the platinum thus enhancing the electrooxidation current density [17,18]. The tungsten oxides result in enhancing markedly the performance of the Pt–Ru–W/C catalyst for methanol electrooxidation, i.e., its activity and COtolerance are better than those of Pt–Ru/C catalyst due to the promoting function of W. The addition of W forms the alloyed state with Pt and Ru metals as shown in XRD pattern. In addition, the W cannot react with acid, i.e., the stability of W in acid medium is very good. The metal particles of catalyst during the duration of usage are not easily desquamated from carbon surface. So, the Pt–Ru–W/C catalyst shows the relatively good stability. The above preliminary results indicate that the addition of W into Pt–Ru catalyst can significantly improve the electrode performance for methanol dehydrogenation, and Ru metal improves COads oxidation from methanol dehydrogenation. The addition of W changes the surface state of Pt components in the alloy and cleans Pt surface active sites which are adsorbed by hydrogen. The improvement of electrode performance is more likely due to synergistic effects of Ru element and spillover effect of WOx oxides. The intrinsic mechanism study is in progress by in situ method. Experiments using the Pt–Ru–W/C catalyst in gas diffusion electrode for tests in single DMFC, as well as long-term investigations are currently in progress. 4. Conclusions The activity of the Pt–Ru–W/C catalyst, formed by thermal reduction with H2 of the inorganic salt precursors, was investigated with respect to methanol electrooxidation in acidic solution. The additions of W into Pt–Ru catalyst can remarkably improve the electrode performance for methanol dehydrogenation due to the spillover effect of WOx oxides. The performance of the Pt–Ru–W/C catalyst for methanol electrooxidation in acidic medium is much higher than that of the Pt–Ru/C-TR one. Compared to the Pt–Ru/CCR, the higher peak current density and the more negative peak potential for methanol electrooxidation were observed in CV curve on the Pt–Ru–W/C catalyst. The tolerance performance to CO from intermediates of methanol electrooxidation on the Pt–Ru–W/C catalyst is better than those of the Pt–Ru/C-CR and Pt–Ru/C-TR ones. Acknowledgments This research is financially supported by the National Natural Science Foundation of China (Grant No. 20606007), Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province of China (LBH-Q07044) and Harbin Innovation Science Foundation for Youths (2007RFQXG042).
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Effect of W on activity of Pt–Ru/C catalyst for methanol electrooxidation in acidic medium Zhen-Bo Wang ∗ , Peng-jian Zuo, Ge-Ping Yin School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 4 November 2008 Received in revised form 15 December 2008 Accepted 16 December 2008 Available online 25 December 2008 Keywords: Direct methanol fuel cell Pt–Ru–W/C catalyst Pt–Ru/C catalyst Methanol electrooxidation
a b s t r a c t The effect of W on the activity of Pt–Ru/C catalyst was investigated. The Pt–Ru–W/C and Pt–Ru/C-TR catalysts were prepared by thermal reduction method. Comparison was made to a homemade Pt–Ru/CCR catalyst prepared by chemical reduction. Their performances were tested by using a glassy carbon thin film electrode through cyclic voltammetric and chronoamperometric curves. The particle size, structure, composition, and surface state of homemade catalyst were determined by means of X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX), transmission electron microscopy (TEM), and X-ray photoelectron spectrometry (XPS). The result of XRD analysis shows that the homemade ternary catalyst exhibits face-centered cubic structure and has smaller lattice parameter than Pt-alone and homemade Pt–Ru/C catalysts. The particle size of Pt–Ru–W/C catalyst is relatively large of 6.5 nm. Its electrochemically active specific area is 20 m2 g−1 less than that of Pt–Ru/C-CR, and much twice as big as that of Pt–Ru/C-TR. But, XPS analysis shows that the addition of W changes the surface state of Pt components in the alloy and can clean Pt surface active sites which are adsorbed by hydrogen. The electrocatalytic activity and tolerance performance to COads of Pt–Ru–W/C catalyst for methanol electrooxidation is the best due to the promoting function of W in comparison with homemade Pt–Ru/C ones. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Methanol is an interesting attractive fuel for electric vehicles, mobile telephone, and laptops. It is inexpensive and easily transported. So, proton exchange membrane fuel cells (PEMFCs) used methanol as fuel has much been studied in the past decades [1–4]. The main drawbacks limiting the broad commercialization of direct methanol fuel cell (DMFC) are the poisoning of the catalyst by COads species produced during methanol electrooxidation and the methanol crossover through the polymer electrolyte membrane from anode to cathode [5–7]. There is a need to improve the activity of anodic catalysts for methanol electrooxidation [8,9]. The only possibility to overcome this effect is to modify the catalysts surface in such a way to increase its coverage in oxygenated species coming from the dissociation of water at low potentials. As we know that the addition of metal (denoted as Me) to Pt-based catalysts significantly lowers the overpotential for methanol electrooxidation through a so-called bifunctional mechanism [10,11] that can
∗ Corresponding author at: Department of Applied Chemistry, School of Chemical Engineering and Technology, Harbin Institute of Technology, PO Box 411#, Harbin 150001, PR China. Tel.: +86 451 86417853; fax: +86 451 86413707. E-mail address: [email protected] (Z.-B. Wang). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.12.061
be summarized as follows: Pt + CH3 OH → Pt–(CH3 OH)ads
(1) +
Pt–(CH3 OH)ads → Pt–COads + 4H + 4e
−
(2)
Me + H2 O → Me–(H2 O)ads
(3)
Me–(H2 O)ads → Me–(OH)ads + H+ + e−
(4) +
Pt–COads + Me–(OH)ads → CO2 + Pt + Me + H + e
−
(5)
At present, there are only a limited number of possible metals which are able to activate water at low potentials with a sufficient stability in acid medium. Pt–Ru alloy is still considered to be the best catalyst due to its tolerance to CO species, and is widely used in DMFC. But the performance of the Pt–Ru alloy catalyst cannot meet the demand of DMFC at low temperatures [9]. So, the addition of a third metal (Mo, Sn, Os or Ni,) was added to Pt–Ru alloy to enhance its activity for methanol electrooxidation [12–15], but the durability with time of such electrode needs further improvement. It has been claimed that tungsten oxide was a suitable promoter of Pt catalyst towards methanol oxidation showing a significant decrease in the amount of poisoning species compared with Pt-alone [16]. The presence of a ‘spillover effect’, caused by the formation of hydrogen tungsten bronze Hx WO3 during the dehydrogenation process of methanol, was postulated [17]. In situ observation method was used to prove that the transfer of hydrogen ions, produced on the
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platinum during methanol electrooxidation, to the tungsten oxide ensures that the active reaction sites on the platinum remain clean, thus enhancing the current density of methanol electrooxidation on Pt–WOx and Pt–Ru–WOx catalysts [17,18]. Gotz and Wendt [19] prepared carbon supported Pt–Ru–Mo, Pt–Ru–Sn, and Pt–Ru–W by impregnation and the colloid methods, and tested towards their activity for anodic oxidation of H2 containing CO of 150 ppm. The oxidation of H2 /CO on the Pt–Ru–W/C catalyst was superior to the other catalysts tested. Lima et al. [20] prepared Pt–Ru–X ternary metallic catalysts (with X = Au, Co, Cu, Fe, Mo, Ni, Sn, and W) by electrochemical deposition and dispersed in a conductive three-dimensional matrix, an electronic conducting polymer, polyaniline (PAni). Methanol electrooxidation was investigated on Pt–Ru–X/PAni in a solution of 0.5 mol L−1 HClO4 . Lima et al. [20] thought that Pt–Ru–Mo/PAni catalyst was the most efficient anode at potentials up to 500 mV (vs. reversible hydrogen electrode (RHE)) for methanol electrooxidation. Based on these literature findings we deem it meaningful to explore the use of thermal reduction to prepare carbon supported Pt–Ru–W catalyst for DMFC application. In the process we had investigated methanol electrooxidation on Pt–Ru–W/C catalyst. The performance of Pt–Ru–W/C catalyst was in comparison with Pt–Ru/C catalyst obtained by chemical reduction of H2 PtCl6 and RuCl3 as precursors with sodium borohydride as a reducing agent [14]. 2. Experimental 2.1. Preparation of catalysts Tungsten compounds cannot be reduced by chemical reduction with NaBH4 . A homemade Pt–Ru–W/C catalyst was prepared by thermal reduction. A described briefly, carbon black powder (Vulcan XC-72, Cabot, 250 m2 g−1 ) was used as the support for the catalyst. The catalyst contained 20 wt.% metal. The 0.25 g Pt–Ru–W (with an atomic ratio of 6:3:1)/C catalyst, was obtained by thermal reduction of H2 PtCl6 , RuCl3 , and (NH4 )6 H2 W12 O40 as precursors. The carbon black was ultrasonically dispersed in a mixture of ultrapure water and isopropyl alcohol for 20 min. Then the precursors were added to the ink, and then mixed thoroughly for 20 min. The ink was dried with a magnetic stirrer at 60 ◦ C. The dried H2 PtCl6 , RuCl3 , and (NH4 )6 H2 W12 O40 compounds with carbon were put into a tube furnace and reduced at 400 ◦ C with a gaseous mixture of H2 and Ar with an atomic ratio of 1:9. The catalyst powder was stored in a vacuum vessel. Pt–Ru/C catalyst (denoted as Pt–Ru/C-CR) was prepared by chemical reduction of H2 PtCl6 and RuCl3 as precursors with NaBH4 according to the method mentioned in the literature [14]. The other Pt–Ru/C catalyst (denoted as Pt–Ru/C-TR) was prepared by thermal reduction of H2 PtCl6 and RuCl3 as precursors with H2 and Ar according to the method mentioned in the literature [21]. 2.2. Electrode preparation and electrochemical measurement 2.2.1. Preparation of working electrode Three millimeter diameter glassy carbon working electrodes (electrode area 0.0706 cm2 ) polished with 0.05 m alumina to a mirror finish before each experiment were used as substrates for the carbon supported catalysts. For the electrode preparation, 5 L of an ultrasonically redispersed catalyst suspension was pipetted onto the glassy carbon substrate. After the solvent evaporation, the deposited catalyst (28 gmetal cm−2 ) was covered with 5 L of a dilute aqueous Nafion® solution (5 wt.%). The resulting Nafion® film with a thickness of ≤0.2 m had a sufficient strength to attach the carbon particles permanently to the glassy carbon electrode without producing significant film diffusion resistances [22,23]. 2.2.2. Electrochemical measurements Electrochemical measurements were carried out with a conventional sealed three-electrode electrochemical cell at 25 ◦ C. The glassy carbon electrode as a working electrode was covered with the catalyst powder. A piece of Pt foil of 1 cm2 area was used as the counter one. The reversible hydrogen electrode was used as the reference one, with its solution connected to the working electrode compartment by a Luggin capillary whose tip was placed appropriately close to the working electrode. All chemicals used were of analytical grade. All the solutions were prepared with ultrapure water (MilliQ, Millipore, 18.2 M cm). Before measurements, a solution of 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 was stirred constantly and purged with ultrapure argon gas to expel oxygen, and then protected with Ar during measurements. Electrochemical experiments were performed by using a CHI630A electrochemical analysis instrument. Cyclic voltammograms (CV) were plotted within the potential range from 0.05 to 1.2 V with a scanning rate of 0.01 V s−1 . The chronoamperometric experiments were carried out by using CHI630A electrochemical analysis instrument controlled by an IBM PC. The potential jumped
from 0.1 to 0.7 V. Due to a slight contamination from the Nafion® film, the working electrodes were electrochemically cleaned by continuous cycling at 0.05 V s−1 until stable responses were obtained before the measurement curves were recorded. 2.2.3. CO stripping voltammetry The electrode was electrochemically cleaned in an Ar-degassed solution of 0.5 mol L−1 H2 SO4 at 25 ◦ C. The amount of the Pt–Ru–W/C catalyst as the working electrode with a geometric surface area of 0.0706 cm2 was 10 g (28 gmetal cm−2 ). CO was adsorbed on the surface of the catalyst at 0.08 V by bubbling CO gas through the 0.5 mol L−1 H2 SO4 solution for 25 min. CO dissolved in the solution was subsequently removed by bubbling argon gas of high purity for 35 min, keeping the potential also at 0.08 V. The potential was then cycled at a scanning rate of 0.02 V s−1 from 0.05 to 1.2 V for two oxidation and reduction cycles. 2.3. Characterization of physical properties 2.3.1. X-ray diffraction (XRD) measurements XRD analysis was carried out for the catalysts from different precursors with a D/max-rB (Japan) diffractometer using a Cu K␣ X-ray source operating at 45 kV and 100 mA. The XRD patterns were obtained at a scanning rate of 4◦ min−1 with an angular resolution of 0.05◦ of the 2 scan. 2.3.2. Energy dispersive analysis of X-ray (EDAX) Chemical composition analysis by EDAX was performed with an EDAX HitachiS-4700 analyser associated to a scanning electron microscope (SEM, Hitachi Ltd. S-4700). Incident electron beam energies from 3 to 30 keV had been used. In all cases, the beam was at normal incidence to the sample surface and the measurement time was 100 s. All the EDAX spectra were corrected by using the ZAF correction, which took into account the influence of the matrix material on the obtained spectra. 2.3.3. Transmission electron microscopy (TEM) After the catalyst sample was finely ground and ultrasonically dispersed in isopropanol, a drop of the resulting dispersion was deposited and then dried on the standard copper grid coated with polymer film. Transmission electron micrograph was taken with a JEOLJEM-1200EX Microscope (made in Japan), at the applied voltage of 100 kV, with a magnification of 200,000 and the spatial resolution of 1 nm. 2.3.4. X-ray photoelectron spectrometry (XPS) The surface composition XPS analysis was performed with the VG ESCALAB MKII X-ray photoelectron spectrometer, with the Al K␣ X-ray source (1486.6 eV). The XPS spectra were recorded at a 45◦ takeoff angle, while the chamber pressure was held below 5 × 10−9 Pa. The C 1s electron binding energy was referenced at 284.6 eV, and a nonlinear least-squares curve-fitting program was employed with a Gaussian–Lorentzian production function [24,25]. The deconvolutions of the XPS spectra were carried out according to the reported methods [16,26].
3. Results and discussion 3.1. Characterization of catalysts – X-ray diffraction XRD patterns reveal the bulk structure of the catalysts and their support. Fig. 1 presents the XRD patterns of Pt–Ru/C-TR (A) [21],
Fig. 1. XRD patterns of Pt–Ru/C-TR (A) [21], Pt–Ru/C-CR (B)[14], and Pt–Ru–W/C (C) [27] catalysts.
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397
Table 1 The particle sizes, specific areas, and lattice parameters of Pt–Ru–W/C and Pt–Ru/C catalysts. Catalysts a
Pt–Ru/C-CR Pt–Ru/C-TR Pt–Ru–W/Cc a b c
2 (◦ )
d-Value (nm)
Lattice parameter (nm)
Particle size (nm)
Specific area (m2 g−1 )
67.9 67.8 68.2
0.1379 0.1387 0.1375
0.3901 0.3923 0.3888
4.5 19.5b 6.5
74.1 17.1b 47.3
Data were taken from Ref. [14]. Data were taken from Ref. [21]. Data were taken from Ref. [27].
Pt–Ru/C-CR (B) [14], and Pt–Ru–W/C (C) [27] catalysts. It can be seen that the first peak in the XRD patterns is associated with the Vulcan XC-72 carbon support. The other four peaks are characteristic of face-centered cubic (fcc) crystalline Pt, corresponding to the planes (1 1 1), (2 0 0), (2 2 0) and (3 1 1), respectively, indicating that the catalyst is mainly single-phase disordered structure (i.e., solid solution). No diffraction peaks, which indicate the presence of either pure Ru and W metals or Ru-rich hexagonal close packed (hcp) phase, appear in the XRD pattern of Pt–Ru–W/C catalyst, suggesting that Ru and W atoms either form an alloy with Pt or exits as oxides in amorphous phase. Relative to the same reflections in Pt-alone (cf. the vertical lines) and Pt–Ru/C-CR [14], the diffraction peaks for Pt–Ru–W catalyst are shifted slightly to higher 2 values. The higher angle shifts of the Pt diffraction peaks reveal the formation of an alloy involving the incorporation of Ru and W atom into the fcc structure of Pt. The lattice parameter of Pt–Ru–W/C catalyst calculated using the Pt (2 2 0) crystal face is given in Table 1. The lattice parameter obtained for the P–Ru–W/C catalyst is smaller than those of Pt–Ru/C ones. In fact, the decrease in the lattice parameter of the alloy catalyst reflects the increase in the incorporation of Ru and/or W into the alloyed state. The average particle size d may be estimated according to Debye–Scherrer formula [14,28]: d=
k ˇ1/2 cos
(6)
S=
6000 d
(7)
Pt−Ru−W = XPt Pt + XRu Ru + XW W
26 m2 g−1 , bigger than that of the Pt–Ru–W/C catalyst, and four times bigger than that of the Pt–Ru/C-TR one. 3.2. EDAX measurement Chemical composition of the catalyst was determined by EDAX analysis. Typical values of the composition analysis of Pt–Ru–W/C, Pt–Ru/C-CR [14], and Pt–Ru/C-TR [21] catalysts are shown in Table 2. The EDAX analysis shows that the determined composition is quite similar to the nominal value, confirming the Pt, Ru or W is completely reduced in the as-prepared Pt–Ru/C-CR and Pt–Ru-W/C catalysts. But the practical composition is close to Pt2 Ru1 , confirming that Ru is not completely reduced in the Pt–Ru/C-TR one. 3.3. TEM manifestation TEM image was obtained on Pt–Ru–W/C catalyst, to examine the effect of mixing W into Pt–Ru catalyst on the particle morphology and the particle size, which are believed to strongly affect the property of catalyst. Fig. 2 shows the typical bright field TEM micrograph of Pt–Ru–W/C catalyst, with metal grains in black and carbon support in grey. The spherical metal particles in catalyst exist homogeneously on carbon support grains. It is easy to pinpoint the average particle size at about 7 nm. Its result is in agreement with that of XRD measurement.
(8)
where d is the average particle size (nm), the wavelength of Xray (1.5406 Å), the angle, at which the peak maximum occurs, ˇ1/2 the width (in radians) of the diffraction peak at half height, k a coefficient of 0.89–1.39 (0.9 here), the density of Pt–Ru or Pt–Ru–W alloy, Pt the density of Pt metal (21.4 g cm−3 ), Ru the density of Ru metal (12.3 g cm−3 ), W the density of W metal (19.3 g cm−3 ) and XPt , XRu , and XW are the weight percent of Pt, Ru, and W, respectively, in the catalysts. Table 1 gives the calculated average particle size and specific surface area of the catalyst according to the diffraction peak of Pt (2 2 0). For comparison, the data of Pt–Ru/C-CR [14] and Pt–Ru/C-TR [21] catalysts were also presented. The particle size of the Pt–Ru/CCR catalyst is the smallest. The particle size of the Pt–Ru/C-TR one is the biggest. The specific surface area of the Pt–Ru/C-CR is about
Table 2 The atomic composition of Pt–Ru–W/C and Pt–Ru/C catalysts (at.%). Catalysts
Pt–Ru/C-CRa Pt–Ru/C-TR Pt–Ru–W/C a
Nominal content
Determined by EDAX
Pt
Ru
W
Pt
Ru
W
50 50 60
50 50 30
– – 10
56.3 67.3 58.6
43.7 32.7 29.1
– – 12.3
Data were taken from Ref. [14].
Fig. 2. TEM micrograph of the Pt–Ru–W/C catalyst.
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Fig. 3. Cyclic voltammograms for CO electrooxidation on the Pt–Ru–W/C catalyst in 0.5 mol L−1 H2 SO4 at 25 ◦ C. Scan rate: 0.02 V s−1 [27].
3.4. The electrochemically active specific area of the catalyst Fig. 3 shows the cyclic voltammogram on the Pt–Ru–W/C catalyst for CO oxidation in a solution of 0.5 mol L−1 H2 SO4 at 25 ◦ C [27]. It can be seen that the onset potential and the peak potential for CO electrooxidation on the Pt–Ru–W/C catalyst is more negative than on the Pt–Ru/C-CR catalyst [14]. The electrochemically active surface area (SEAS ) of the catalyst is calculated by using Eq. (9) [29,30] and the cyclic voltammetry curve of CO adsorption and desorption electrooxidation: SEAS =
QCO G × 420
(9)
where QCO is the charge for CO desorption electrooxidation in microcoulomb (C), G represents the summation of Pt–Ru or Pt–Ru–W metal loading (g) in the electrode, and 420 is the charge required to oxidize a monolayer of CO on catalyst in C cm−2 . The SEAS of Pt–Ru–W/C catalyst is 44.7 m2 g−1 [27], and of the Pt–Ru/C-CR catalyst is 64.5 m2 g−1 [14], of Pt–Ru/C-TR catalyst is 15.5 m2 g−1 . The SEAS of Pt–Ru/C-CR catalyst is about 20 m2 g−1 bigger than that of Pt–Ru–W/C catalyst, and is four times as big as Pt–Ru/C-TR catalyst. The results are similar to the specific surface area values calculated from X-ray diffraction peaks and TEM image.
Fig. 4. Cyclic voltammetry curves of methanol electrooxidation in an Ar-saturated 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C on Pt–Ru–W/C (A), Pt–Ru/C-CR (B), and Pt–Ru/C-TR (C) catalysts. Scan rate: 0.01 V s−1 .
tial for methanol electrooxidation and the peak current density on the Pt–Ru/C-TR catalyst are about 0.874 V (vs. RHE) and only 3.56 mA mg−1 , respectively, during positive potential scanning as shown in curve C. The peak potential and the peak current density on the Pt–Ru/C-TR catalyst are about 0.724 V (vs. RHE) and 1.66 mA mg−1 , respectively, during its reverse scanning. The peak potential on the Pt–Ru–W/C catalyst during potential scanning is 10 mV lower than that on the Pt–Ru/C-CR one. But the peak current density on the Pt–Ru–W/C catalyst is about 19.80 mA mg−1 higher than that on the Pt–Ru/C-CR one. So, the performance of the Pt–Ru–W/C catalyst for methanol electrooxidation is much higher than that of the latter. The peak current density on the Pt–Ru–W/C catalyst is 12 times higher than that on the Pt–Ru/C-TR catalyst. It can be seen from the above results that the performance of the Pt–Ru–W/C catalyst is the best mainly due to the improving effect of W in the Pt–Ru/C catalyst for methanol electrooxidation. Fig. 5 shows the current densities measured at a constant potential jumping from 0.1 to 0.7 V in an Ar-saturated solution of 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C. Initial high current is ascribed mainly to double-layer charging. The currents decay with time in a parabolic style, and reach an apparent steady state
3.5. The electrochemical activity of the catalysts Fig. 4 shows the cyclic voltammograms on the Pt–Ru–W/C (A), Pt–Ru/C-CR (B), and Pt–Ru/C-TR (C) catalysts in a solution of 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C. It can be seen from Fig. 4 that the onset potential of a current rise for methanol electrooxidation on the Pt–Ru–W/C catalyst corresponds to that on the Pt–Ru/C catalysts, i.e., about 0.55 V. The peak potential for methanol electrooxidation, at which the peak current occurs is 0.868 V (vs. RHE), and the peak current density is 46.33 mA mg−1 during positive potential scanning on the Pt–Ru–W/C catalyst as shown in curve A. The peak potential and the peak current density on the Pt–Ru–W/C catalyst are about 0.749 V (vs. RHE) and 41.95 mA mg−1 , respectively, during its reverse scanning. The peak potential for methanol electrooxidation and the peak current density on the Pt–Ru/C-CR catalyst are about 0.878 V (vs. RHE) and 25.18 mA mg−1 , respectively, during positive potential scanning as shown in curve B. The peak potential and the peak current density on the Pt–Ru/C–CR catalyst are about 0.753 V (vs. RHE) and 26.53 mA mg−1 during its negative scanning. The peak poten-
Fig. 5. Chronoamperometric curves of methanol electrooxidation in an Ar-saturated 0.5 mol L−1 CH3 OH and 0.5 mol L−1 H2 SO4 at 25 ◦ C on Pt–Ru–W/C (A), Pt–Ru/C-CR (B), and Pt–Ru/C-TR (C) catalysts. Potential jumps from 0.1 to 0.7 V.
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Fig. 6. di/dt (normalized scale) against t plots from t = 0 to t = 1 min for Pt–Ru-based/C catalysts with or without addition of tungsten. Slopes: 0.14 for Pt–Ru–W/C (A), 0.82 for Pt–Ru/C-CR (B), and 0.53 for Pt–Ru/C-TR (C) catalysts.
within 500 s. The current density of methanol electrooxidation on the Pt–Ru–W/C catalyst, which is a 11.1 mA mg−1 at 1000 s, is about 2.2 times higher than that on the Pt–Ru/C-CR catalyst at the same potential and the same time, is about 142 times higher than that on the Pt–Ru/C-TR catalyst, i.e., the activity of the Pt–Ru–W/C catalyst is the best. The results are similar to those of cyclic voltammetry measurement. Fig. 6, representing a (di/dt) = f(t) plot (normalized scale) at small time values allows evaluation of the initial poisoning rate for the catalysts during the methanol electrooxidation by comparing the three slopes: the greater the slope is, the easier the initial poisoning of the electrode surface is [31]. It appears that added tungsten leads to less poisoning since the experimental slope with added tungsten is lower by a factor of 6 and 4, compared to that the obtained Pt–Ru–W/C, Pt–Ru/C-CR, and Pt–Ru/C-TR catalysts, that is, 0.14, 0.82, and 0.53, respectively. 3.6. Characterization of X-ray photoelectron spectrometry The core level spectra of Pt 4f and W 4f for the Pt–Ru–W/C catalyst are depicted in Fig. 7. The two most intense peaks in Fig. 7A, located at the Pt 4f7/2 and Pt 4f5/2 , maintain an area ratio near 4:3 as expected theoretically for pure Pt [26,32,33]. The result of deconvolution indicates a phase composition which contains 61.32% of Pt in metallic Pt0 , 36.82% in Pt2+ (as PtO or Pt (OH)2 ), and 1.86% in Pt4+ as PtO2 . The content of Pt4+ decreases markedly due to W additive into Pt–Ru/C in comparison with that Pt–Ru/C catalyst [34]. It is interesting to observe that the W 4f signal is composed of two most intense peaks at binding energies of 34.3 and 35.1 eV (W 4f7/2 ) and at 36.5 and 37.4 eV (W 4f5/2 ). These peaks are ascribed to the oxidation states (W(IV) and W(VI)), respectively [16]. Yet, the W 4f7/2 signal at 34.3 eV may also be derived from the presence of W2 O5 or substoichiometric WO3−x (2 < x < 3) species. These tungsten oxides have comparable surface concentrations, i.e., WO3 (74.3%) and WO2 (23.6%). It is expected that the WO3 /WO2 system can act as a redox surface mediator for the oxidation of adsorbed CO-like from methanol electrooxidation. In fact, the redox potentials of these systems, calculated from the free energies, fall in the potential region where tungsten oxides redox couples may be active for methanol electrooxidation [16,28]. This means that proton conducting oxides, such as WOx , may play an important role in the enhanced methanol electrooxidation. In addition, the W 4f signal still consists of W metal peak at binding energy of 31.88 eV (W 4f7/2 line) with a small content of 2.1%.
Fig. 7. Typical XPS spectra of the Pt 4f and W 4f regions for the homemade Pt–Ru–W/C catalyst.
From the results above, it can be considered that the Pt–Ru–W electrode facilitates the generation of Me–OHads from water more than Pt–Ru due to the addition of W [28]. The methanol electrooxidation on the Pt–Ru–W/C catalyst adsorbs and reacts rapidly according to the so-called bifunctional mechanism. So, the peak potential of methanol electrooxidation on it is relatively negative, and its peak current is higher than that on Pt–Ru/C catalyst as shown in CV curves. The different tungsten oxides in Pt–Ru–W/C catalyst surface prepared by thermal reduction were formed. In addition, these materials (WOx ) were the good proton conducting oxides and could reduce the electrode potential of methanol electrooxidation and improve the reaction of methanol dehydrogenation. These WOx were electrochemically oxidized and reduced according to Eq. (10) [17,18,35]: WOx + yH+ + ye− ↔ Hy WOx
(10)
Hydrogen on the Pt-based catalyst would be transferred to the WOx in the Pt–Ru–W nanophase at a positive potential as shown in Eq. (11): Pt–H + WOx → Pt + H–WOx
(11)
This showed the occurrence of hydrogen spillover from Pt to WOx . As a result, the transfer of hydrogen ions, produced on the
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platinum during methanol electrooxidation, to the tungsten oxide ensured clean active reaction sites on the platinum thus enhancing the electrooxidation current density [17,18]. The tungsten oxides result in enhancing markedly the performance of the Pt–Ru–W/C catalyst for methanol electrooxidation, i.e., its activity and COtolerance are better than those of Pt–Ru/C catalyst due to the promoting function of W. The addition of W forms the alloyed state with Pt and Ru metals as shown in XRD pattern. In addition, the W cannot react with acid, i.e., the stability of W in acid medium is very good. The metal particles of catalyst during the duration of usage are not easily desquamated from carbon surface. So, the Pt–Ru–W/C catalyst shows the relatively good stability. The above preliminary results indicate that the addition of W into Pt–Ru catalyst can significantly improve the electrode performance for methanol dehydrogenation, and Ru metal improves COads oxidation from methanol dehydrogenation. The addition of W changes the surface state of Pt components in the alloy and cleans Pt surface active sites which are adsorbed by hydrogen. The improvement of electrode performance is more likely due to synergistic effects of Ru element and spillover effect of WOx oxides. The intrinsic mechanism study is in progress by in situ method. Experiments using the Pt–Ru–W/C catalyst in gas diffusion electrode for tests in single DMFC, as well as long-term investigations are currently in progress. 4. Conclusions The activity of the Pt–Ru–W/C catalyst, formed by thermal reduction with H2 of the inorganic salt precursors, was investigated with respect to methanol electrooxidation in acidic solution. The additions of W into Pt–Ru catalyst can remarkably improve the electrode performance for methanol dehydrogenation due to the spillover effect of WOx oxides. The performance of the Pt–Ru–W/C catalyst for methanol electrooxidation in acidic medium is much higher than that of the Pt–Ru/C-TR one. Compared to the Pt–Ru/CCR, the higher peak current density and the more negative peak potential for methanol electrooxidation were observed in CV curve on the Pt–Ru–W/C catalyst. The tolerance performance to CO from intermediates of methanol electrooxidation on the Pt–Ru–W/C catalyst is better than those of the Pt–Ru/C-CR and Pt–Ru/C-TR ones. Acknowledgments This research is financially supported by the National Natural Science Foundation of China (Grant No. 20606007), Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province of China (LBH-Q07044) and Harbin Innovation Science Foundation for Youths (2007RFQXG042).
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