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Platinized mesoporous tungsten carbide for electrochemical methanol oxidation
Electrochemistry Communications 9 (2007) 2576–2579 www.elsevier.com/locate/elecom
Platinized mesoporous tungsten carbide for electrochemical methanol oxidation Raman Ganesan, Dong Jin Ham, Jae Sung Lee
*
Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang University of Science and Technology, San 31 Hyoja-dong, Pohang 790-784, Republic of Korea Received 1 May 2007; received in revised form 1 August 2007; accepted 3 August 2007 Available online 10 August 2007
Abstract Mesoporous WC with hexagonal crystal structure was synthesized by a surfactant-assisted polymer method. A new electrocatalyst composed of a small amount of Pt supported on the mesoporous WC exhibited higher activity for electrooxidation of methanol than microporous Pt/WC or Pt/W2C as well as commercial Pt–Ru(1:1)/C catalysts. The mesoporosity and the phase of WC appear important for the high activity. Compared to the commercial Pt–Ru/C catalyst, the Pt/WC (mesoporous) showed the higher activity per mass of Pt by a factor of six even without Ru. Since the catalyst is also stable in electrochemical environment, it could become an alternative electrocatalyst for direct methanol fuel cells. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pt/WC; Electrocatalysts; Mesoporous; Methanol; Fuel cells
1. Introduction As electrocatalysts for low temperature fuel cells such as direct methanol (DMFC) and proton exchange membrane (PEM) fuel cells, Pt–Ru alloys show high and stable activity for electrooxidation of hydrogen and, in particular, methanol [1,2]. But these catalysts are expensive due to excessive use of noble metals, are susceptible to CO poisoning, and lose their catalytic activity with respect to time [3]. There is an urgent need to replace these noble metals [4,5]. Tungsten carbide-based materials have received considerable attention in recent years because of their resemblance to platinum in various catalytic reactions [6–8]. As electrocatalysts, they are known to be highly resistant to CO poisoning and stable in acidic and basic solutions, yet their electrocatalytic activity for methanol oxidation is very low [9,10]. However, this low activity of tungsten carbide could be improved dramatically by adding a *
Corresponding author. Tel.: +82 54 2792266; fax: +82 54 2795528. E-mail address: [email protected] (J.S. Lee).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.08.002
small amount of platinum to tungsten carbide [11,12]. We recently prepared tungsten carbide (W2C phase) microsphere by a polymer method with a high surface area (176 m2/g) and a high CO chemisorption capacity (18.7% of the total surface W atoms) [12]. The W2C alone did not show any activity of methanol electrooxidation. Loading platinum onto this W2C microsphere, however, produced an electrocatalyst that showed higher activity for methanol oxidation than a commercial Pt– Ru/C catalyst. As a step forward in the present work, we synthesized a mesoporous tungsten carbide by using ammonium metatungstate as a tungsten precursor and resorcinol–formaldehyde polymer as a carbon source in the presence of a surfactant. Further, new synthesis method yielded WC phase, which was reported to be most stable and most active for electrooxidation of methanol among the three major phases of tungsten carbide, W2C, WC and WC1x [9]. Indeed, slightly platinized mesoporous WC showed much higher activity for electrooxidation of methanol than the previous platinized W2C microsphere with microporosity and a commercial 20%Pt–Ru/C catalyst.
R. Ganesan et al. / Electrochemistry Communications 9 (2007) 2576–2579
2. Experimental 2.1. Electrocatalyst preparation Mesoporous WC (meso-WC) was prepared by polycondensation of resorcinol and formaldehyde in the presence of CTABr surfactant. In a typical synthesis, 6.5 g of cetyltrimethylammonium bromide (CTABr) was dissolved in 20 ml of water and stirred for 30 min. The solution containing 5 g of ammonium metatungstate salt (AMT) and 1.2 g of resorcinol and 1.8 ml of formaldehyde was added to the CTABr solution and heated at 358 K for 1 h. The resulting gel was autoclaved at 423 K for 48 h. The red coloured solid was dried at 383 K for 12 h and calcined at 1173 K for 1 h in Ar flow and 2 h in H2 flow (44.6 lmol/s). A reference sample, part-WC was prepared by the same procedure, but in the absense of CTABr. 2.2. Physicochemical characterization Before temperature-programmed desorption (TPD) experiments, the catalysts were activated at 473 K in hydrogen for 1 h. After cooling to room temperature CO chemisorption was done at room temperature for 1 h. Finally CO was desorbed by heating the catalysts from room temperature to 573 K in He flow. The amount of CO was measured by a mass spectrometer. The powder X-Ray diffraction (XRD) measurements were conducted using a Mac Science M18XHF diffractometer with Cu Ka radiation. The BET surface area and pore size distribution were calculated from nitrogen adsorption/desorption at 77 K in a constant volume adsorption apparatus (Micrometrics ASAP2012). The morphology of the sample was studied by high resolution transmission electron microscope (HRTEM, JEOL 2001) operating at 100 kV. The specimens for TEM analysis were prepared by ultrasonically suspending the electrocatalyst powder in ethanol. 2.3. Electrochemical characterization The working electrodes for the electrochemical measurements were fabricated by dispersing the catalyst in 1 ml of distilled water and 10 lL of 5 wt% Nafion. The dispersion was ultrasonicated for 15 min. A known amount of suspension was added on to the glassy carbon and solvent was slowly evaporated. 10 lL of 5 wt% Nafion was again added on the coatings and solvent was slowly evaporated. Pt foil and Ag/AgCl/3M NaCl were used as counter and reference electrodes, respectively. A solution of 1 M H2SO4-1 M CH3OH was used for all electrochemical experiments, which were performed on a Princeton Applied Research (PAR) voltammetry. 3. Results and discussion Two-types of WC powders were synthesized by controlling the amount of mesopores. The synthesis of both WC
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samples involved polycondensation of resorcinol–formaldehyde in the presence of AMT, followed by heat treatment under Ar flow at 1173 K. A difference in the preparation methods for the two WC samples was the presence or absence of a surfactant (CTABr) during the polycondensation step. As discussed later, the preparation with CTABr gave WC with ample mesoporosity (denoted as meso-WC) whereas that without CTABr yielded WC nanoparticles with a small pore volume (denoted as partWC). The XRD pattern of meso-WC sample in Fig. 1a corresponded to WC of a simple hexagonal phase with lattice ˚ and c = 2.83 A ˚ (JCPDS card parameters of a = 2.906 A no. 12070-12-1). There were no XRD peaks corresponding to carbon, tungsten trioxide or metallic tungsten. But mesoWC contained minor amounts of W2 C (hcp phase) and WC1x (fcc phase). The part-WC showed the same XRD pattern of hexagonal WC without the impurity phases (not shown). The nitrogen adsorption/desorption isotherms in Fig. 1b are identified as Type IV characteristic of mesoporous materials. The pore size distribution was calculated from the desorption isotherm as shown in the insets of Fig. 1b. Average pore sizes were similar, 4.6 nm for partWC and 4.3 nm for meso-WC. The BET surface areas were also similar, 95 m2/g for part-WC and 76 m2/g for mesoWC. The most significant difference between two samples was the pore volume. The part-WC showed a very small volume of 0.086 cm3/g, while that of meso-WC was ca. 3-fold higher at 0.24 cm3/g. Thus the surfactant CTABr introduced during the polycondensation reaction has increased the pore volume of the sample. The CO uptake was calculated from TPD by referring the area under the CO mass signal (28) to the known quantity of CO. The CO uptake value (106 lmol/g) of meso-WC corresponded to 10% of total tungsten atoms on the surface of the sample. The HRTEM images of meso-WC sample are shown in Fig. 2. The WC samples are collection of WC nanoparticles of ca. 20 nm, which appears highly crystalline as evidenced by clean lattice fringes. The spacing of the lattice fringes was about 0.25 nm (Fig. 2a) corresponding to the interplanar spacing of (100) planes of simple hexagonal WC. Selected area electron diffraction (SAED) pattern also shows that WC is well crystallized. The elemental analysis of meso-WC particle showed that the material had the composition of WC1.756 . Thus, there is an excess carbon on the surface that partly blocks the tungsten carbide surface. Now Pt particles were deposited on these WC materials by the conventional borohydride reduction method in alkaline media [13]. The HRTEM image (Fig. 2b) shows that platinum particles are finely dispersed on WC materials. The average Pt particle size is around 2 nm on WC, which is in good agreement with the value calculated from XRD using Debye–Scherrer equation (ca. 2 nm for 3.5 wt% Pt and ca. 6 nm for 7.5 wt% Pt both on part-WC and mesoWC). Finally, the platinized WC samples were tested for their activity in electrochemical oxidation of methanol in acidic
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R. Ganesan et al. / Electrochemistry Communications 9 (2007) 2576–2579
a
b
WC WC
160
0.006
Volume adsorbed /cm3/g
Intensity
120
WC
W
WC
C
W2C
100
Pore Volume /cm3/g
0.005
140
0.004 0.003 0.002 0.001
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Fig. 1. (a)XRD and structural model, and (b) N2 adsorption/desorption isotherms of meso-WC (inset: the BJH pore size distribution).
250
(c)
200
Current /mAcm-2
150
(b) 100
(a) 50
Fig. 2. (a) Lattice TEM image of meso-WC (inset: SAED pattern of WC). (b) HRTEM images of Pt particles in Pt (3.5 wt%)/meso-WC. Scale bars represent 2 nm (a) and 20 nm (b).
0
-50
condition. This represents the half-cell anode reaction of DMFC. The activity was presented by the cyclic voltammogram (CV) in Fig. 3 and quantitatively in Table 1. The main feature in the CV between 0.4 V and 0.9 V represents oxidation/reduction of methanol (CH3OH + H2O ! CO2 + 6H+ + 6e). Table 1 also lists the electrochemical surface areas (ESA) extracted from the integration of the Pt–H adsorption/desorption peak in 0.2 to +0.2 V. The WC alone without Pt showed no activity. In contrast, both 3.5 wt% and 7.5 wt% Pt supported on these WC materials showed higher current density (taken at 0.75 V after cycle was stabilized) even in the absence of Ru than commercial 20%Pt–Ru E-Teck catalyst. Among tungsten carbide-based catalysts Pt/meso-WC performed better than Pt/part-WC and Pt/W2C microsphere [12]. In particular, the mass activity (mA/mg of Pt taken at 0.75 V) of 3.5% Pt/meso-WC (1851 mA/mg) was higher by a factor of six than that of the commercial 20%Pt–Ru/C E-Teck catalyst (307 mA/ mg). The activity data clearly show that Pt dispersed on tungsten carbides provides much better utilization of Pt than on conventional carbon support and that Ru could
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential /V vs Ag/AgCl
Fig. 3. Cyclic voltammogram of (a) 3.5 wt% Pt/part -WC, and (b) 20%Pt– Ru (1:1) /Vulcan XC-72 (commercial E-Teck catalyst) (c) 3.5 wt% Pt/ meso-WC (repeated scan) in a 1 M H2SO4 - 1 M CH3OH solution at a scan rate of 50 mV/sec at 298 K.
Table 1 Electrocatalytic activity of various catalysts for methanol oxidation Catalyst
ESA (m2/g–Pt)
Specific activitya (mA/cm2)
Mass activitya (mA/mg-Pt)
3.5%Pt/meso-WC 7.5%Pt/meso-WC 3.5%Pt/part-WC 7.5%Pt/part-WC 7.5% Pt/W2Cb 15% Pt/W2Cb 20%Pt–Ru/C(E-Teck)
336 316 289 337 327 189 71
185.1 294 61.4 261 156 224 114
1851 1373 614 1123 728 560 307
a b
At 0.75 V. From Ref. [12].
R. Ganesan et al. / Electrochemistry Communications 9 (2007) 2576–2579 2000
1600
(a) 11%Pt-9%Ru/Carbon Microsphere (b)13%Pt-7%Ru/Carbon commercial E-Teck (c) 15% Pt/W2C
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(d)7.5%Pt/W2C
Mass Activity /mA/mg
1800
1200 1000
(g)
(e)3.5%Pt/Part-WC (f) 7.5%Pt/Part-(WC (g)7.5%Pt/Meso-WC (h)3.5%Pt/Meso-WC
(f)
800
(e) (c)
600
(d)
400 200
(h)
(b) (a) 50
100
150
200
250
300
350
Electrochemical Surface area /m2/g
Fig. 4. Comparison of mass activity with electrochemical surface area of the catalysts.
be entirely replaced. Mesoporous WC was particularly effective for the purpose. The stability of the Pt/WC catalyst was tested by repeating electrochemical reaction cycles in a 1 M H2SO4-1 M CH3OH solution. As shown in Fig. 3c, the specific activity represented by the CV area increased initially, but was stabilized after ca. 30 cycles. We believe that surface oxygen species initially present on the catalyst is removed during this transient period [12]. Once a steady state was established, there was no sign of deactivation during 100 consecutive reaction cycles. Thus, Pt/WC catalyst performs better than the commercial catalyst even with the smaller Pt loadings and in the absence of ruthenium. According to Chen and coworkers [9,11], WC can activate methanol to form a methoxy intermediate, but the reaction stops there. Further decomposition of the methoxy species is promoted by Pt. Thus, initially inactive WC now actively participates in the reaction in the presence of Pt. In the present case, an indication of the active role of WC could be found in the ESA values of Pt/WC listed in Table 1. A spherical Pt particle with a diameter of 2 nm would have a physical surface area of 69.9 m2/g. All Pt/WC samples show higher ESA values even though observed Pt particle sizes are ca. 2 nm. In contrast, the commercial 10%Pt–Ru/C catalyst showed an ESA value (71 m2/g) expected from the Pt particle size. The higher ESA values for Pt/WC catalysts could be explained by active participation of WC in adsorption of hydrogen, probably through spillover from Pt sites. Along the same line, WC is expected to participate in methanol oxidation as well. Indeed there is a fine correlation between the activity of methanol oxidation and ESA values of the catalysts as summarized in Fig. 4. Thus, unlike carbon in Pt–Ru/C, the role of WC is not a support for Pt, but is an active phase working together with Pt for electrooxidation of methanol.
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The greatly improved activity of Pt/WC catalysts compared with Pt/W2C microspheres that we reported previously [12] appears to be due to the WC phase, which was reported to be most active for electrooxidation of methanol among the three major phases of tungsten carbide, W2C, WC and WC1x [9]. Finally, the present study has demonstrated that mesoporosity of the Pt/meso-WC catalyst is also important. Thus, Pt/meso-WC is more active than Pt/part-WC that has only a small amount of mesoporous. It is interesting to note that the two less active tungsten carbide catalysts possess higher surface areas than that of Pt/ meso-WC, but they are mostly due to micropores. Thus for high electrocatalytic activity, the facile mass transport of hydrated methanol to the reaction sites through mesoporous appear more important than the inaccessible high surface area. 4. Conclusions We have synthesized and characterized mesoporous WC phase by a surfactant-assisted polymer method. A new electrocatalyst composed of Pt supported on these materials shows higher activity for electrochemical oxidation of methanol than a commercial Pt–Ru/C catalyst by a factor of six (per mass of Pt) even without Ru. The mesoporosity and phase of WC appear important as Pt/meso-WC performs better than Pt/W2C-microsphere and Pt/part-WC. Since the catalyst is also stable in electrochemical environment, it could become an alternative electrocatalyst for DMFC with potential to replace current Pt–Ru catalysts. Acknowledgement This work has been supported by Hydrogen R&D Center funded by KOSEF and BK-21 program. References [1] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) 4245. [2] H. Zhang, Y. Wang, E.R. Fachini, C.R. Cabrera, Electrochem. Solid State Lett. 2 (1999) 437. [3] H.C. Yu, K.Z. Fung, Tz.C. Guo, W.L. Chang, Electrochim. Acta 50 (2004) 811. [4] V. Raghuveer, B. Viswanathan, Fuel 81 (2002) 2191. [5] V. Raghuveer, K.R. Thampi, J.M. Bonard, N. Xanthopoulos, H.J. Mathieu, B. Viswanathan, Solid State Ionics 140 (2001) 263. [6] J.S. Lee, S. Locatelli, S.T. Oyama, M. Boudart, J. Catal. 125 (1990) 157. [7] J.S. Lee, S.T. Oyama, M. Boudart, J. Catal. 106 (1987) 125. [8] R.B. Levy, M. Boudart, Science 181 (1973) 547. [9] M.B. Zellner, J.G. Chen, Catal. Today 99 (2005) 299. [10] D.R. McIntyre, G.T. Burstein, A. Vossen, J. Power Sources 107 (2002) 67. [11] H. Wu, J.G. Chen, J. Vac. Sci. Technol. A 21 (2003) 148. [12] R. Ganesan, J.S. Lee, Angew. Chem. Int. Ed. 44 (2005) 6557. [13] P.R. Van Rheenen, M.J. McKelvy, W.S. Glaunsinger, J. Solid State Chem. 67 (1987) 151.
Platinized mesoporous tungsten carbide for electrochemical methanol oxidation Raman Ganesan, Dong Jin Ham, Jae Sung Lee
*
Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang University of Science and Technology, San 31 Hyoja-dong, Pohang 790-784, Republic of Korea Received 1 May 2007; received in revised form 1 August 2007; accepted 3 August 2007 Available online 10 August 2007
Abstract Mesoporous WC with hexagonal crystal structure was synthesized by a surfactant-assisted polymer method. A new electrocatalyst composed of a small amount of Pt supported on the mesoporous WC exhibited higher activity for electrooxidation of methanol than microporous Pt/WC or Pt/W2C as well as commercial Pt–Ru(1:1)/C catalysts. The mesoporosity and the phase of WC appear important for the high activity. Compared to the commercial Pt–Ru/C catalyst, the Pt/WC (mesoporous) showed the higher activity per mass of Pt by a factor of six even without Ru. Since the catalyst is also stable in electrochemical environment, it could become an alternative electrocatalyst for direct methanol fuel cells. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pt/WC; Electrocatalysts; Mesoporous; Methanol; Fuel cells
1. Introduction As electrocatalysts for low temperature fuel cells such as direct methanol (DMFC) and proton exchange membrane (PEM) fuel cells, Pt–Ru alloys show high and stable activity for electrooxidation of hydrogen and, in particular, methanol [1,2]. But these catalysts are expensive due to excessive use of noble metals, are susceptible to CO poisoning, and lose their catalytic activity with respect to time [3]. There is an urgent need to replace these noble metals [4,5]. Tungsten carbide-based materials have received considerable attention in recent years because of their resemblance to platinum in various catalytic reactions [6–8]. As electrocatalysts, they are known to be highly resistant to CO poisoning and stable in acidic and basic solutions, yet their electrocatalytic activity for methanol oxidation is very low [9,10]. However, this low activity of tungsten carbide could be improved dramatically by adding a *
Corresponding author. Tel.: +82 54 2792266; fax: +82 54 2795528. E-mail address: [email protected] (J.S. Lee).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.08.002
small amount of platinum to tungsten carbide [11,12]. We recently prepared tungsten carbide (W2C phase) microsphere by a polymer method with a high surface area (176 m2/g) and a high CO chemisorption capacity (18.7% of the total surface W atoms) [12]. The W2C alone did not show any activity of methanol electrooxidation. Loading platinum onto this W2C microsphere, however, produced an electrocatalyst that showed higher activity for methanol oxidation than a commercial Pt– Ru/C catalyst. As a step forward in the present work, we synthesized a mesoporous tungsten carbide by using ammonium metatungstate as a tungsten precursor and resorcinol–formaldehyde polymer as a carbon source in the presence of a surfactant. Further, new synthesis method yielded WC phase, which was reported to be most stable and most active for electrooxidation of methanol among the three major phases of tungsten carbide, W2C, WC and WC1x [9]. Indeed, slightly platinized mesoporous WC showed much higher activity for electrooxidation of methanol than the previous platinized W2C microsphere with microporosity and a commercial 20%Pt–Ru/C catalyst.
R. Ganesan et al. / Electrochemistry Communications 9 (2007) 2576–2579
2. Experimental 2.1. Electrocatalyst preparation Mesoporous WC (meso-WC) was prepared by polycondensation of resorcinol and formaldehyde in the presence of CTABr surfactant. In a typical synthesis, 6.5 g of cetyltrimethylammonium bromide (CTABr) was dissolved in 20 ml of water and stirred for 30 min. The solution containing 5 g of ammonium metatungstate salt (AMT) and 1.2 g of resorcinol and 1.8 ml of formaldehyde was added to the CTABr solution and heated at 358 K for 1 h. The resulting gel was autoclaved at 423 K for 48 h. The red coloured solid was dried at 383 K for 12 h and calcined at 1173 K for 1 h in Ar flow and 2 h in H2 flow (44.6 lmol/s). A reference sample, part-WC was prepared by the same procedure, but in the absense of CTABr. 2.2. Physicochemical characterization Before temperature-programmed desorption (TPD) experiments, the catalysts were activated at 473 K in hydrogen for 1 h. After cooling to room temperature CO chemisorption was done at room temperature for 1 h. Finally CO was desorbed by heating the catalysts from room temperature to 573 K in He flow. The amount of CO was measured by a mass spectrometer. The powder X-Ray diffraction (XRD) measurements were conducted using a Mac Science M18XHF diffractometer with Cu Ka radiation. The BET surface area and pore size distribution were calculated from nitrogen adsorption/desorption at 77 K in a constant volume adsorption apparatus (Micrometrics ASAP2012). The morphology of the sample was studied by high resolution transmission electron microscope (HRTEM, JEOL 2001) operating at 100 kV. The specimens for TEM analysis were prepared by ultrasonically suspending the electrocatalyst powder in ethanol. 2.3. Electrochemical characterization The working electrodes for the electrochemical measurements were fabricated by dispersing the catalyst in 1 ml of distilled water and 10 lL of 5 wt% Nafion. The dispersion was ultrasonicated for 15 min. A known amount of suspension was added on to the glassy carbon and solvent was slowly evaporated. 10 lL of 5 wt% Nafion was again added on the coatings and solvent was slowly evaporated. Pt foil and Ag/AgCl/3M NaCl were used as counter and reference electrodes, respectively. A solution of 1 M H2SO4-1 M CH3OH was used for all electrochemical experiments, which were performed on a Princeton Applied Research (PAR) voltammetry. 3. Results and discussion Two-types of WC powders were synthesized by controlling the amount of mesopores. The synthesis of both WC
2577
samples involved polycondensation of resorcinol–formaldehyde in the presence of AMT, followed by heat treatment under Ar flow at 1173 K. A difference in the preparation methods for the two WC samples was the presence or absence of a surfactant (CTABr) during the polycondensation step. As discussed later, the preparation with CTABr gave WC with ample mesoporosity (denoted as meso-WC) whereas that without CTABr yielded WC nanoparticles with a small pore volume (denoted as partWC). The XRD pattern of meso-WC sample in Fig. 1a corresponded to WC of a simple hexagonal phase with lattice ˚ and c = 2.83 A ˚ (JCPDS card parameters of a = 2.906 A no. 12070-12-1). There were no XRD peaks corresponding to carbon, tungsten trioxide or metallic tungsten. But mesoWC contained minor amounts of W2 C (hcp phase) and WC1x (fcc phase). The part-WC showed the same XRD pattern of hexagonal WC without the impurity phases (not shown). The nitrogen adsorption/desorption isotherms in Fig. 1b are identified as Type IV characteristic of mesoporous materials. The pore size distribution was calculated from the desorption isotherm as shown in the insets of Fig. 1b. Average pore sizes were similar, 4.6 nm for partWC and 4.3 nm for meso-WC. The BET surface areas were also similar, 95 m2/g for part-WC and 76 m2/g for mesoWC. The most significant difference between two samples was the pore volume. The part-WC showed a very small volume of 0.086 cm3/g, while that of meso-WC was ca. 3-fold higher at 0.24 cm3/g. Thus the surfactant CTABr introduced during the polycondensation reaction has increased the pore volume of the sample. The CO uptake was calculated from TPD by referring the area under the CO mass signal (28) to the known quantity of CO. The CO uptake value (106 lmol/g) of meso-WC corresponded to 10% of total tungsten atoms on the surface of the sample. The HRTEM images of meso-WC sample are shown in Fig. 2. The WC samples are collection of WC nanoparticles of ca. 20 nm, which appears highly crystalline as evidenced by clean lattice fringes. The spacing of the lattice fringes was about 0.25 nm (Fig. 2a) corresponding to the interplanar spacing of (100) planes of simple hexagonal WC. Selected area electron diffraction (SAED) pattern also shows that WC is well crystallized. The elemental analysis of meso-WC particle showed that the material had the composition of WC1.756 . Thus, there is an excess carbon on the surface that partly blocks the tungsten carbide surface. Now Pt particles were deposited on these WC materials by the conventional borohydride reduction method in alkaline media [13]. The HRTEM image (Fig. 2b) shows that platinum particles are finely dispersed on WC materials. The average Pt particle size is around 2 nm on WC, which is in good agreement with the value calculated from XRD using Debye–Scherrer equation (ca. 2 nm for 3.5 wt% Pt and ca. 6 nm for 7.5 wt% Pt both on part-WC and mesoWC). Finally, the platinized WC samples were tested for their activity in electrochemical oxidation of methanol in acidic
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R. Ganesan et al. / Electrochemistry Communications 9 (2007) 2576–2579
a
b
WC WC
160
0.006
Volume adsorbed /cm3/g
Intensity
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WC
W
WC
C
W2C
100
Pore Volume /cm3/g
0.005
140
0.004 0.003 0.002 0.001
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0.000 20
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Fig. 1. (a)XRD and structural model, and (b) N2 adsorption/desorption isotherms of meso-WC (inset: the BJH pore size distribution).
250
(c)
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Current /mAcm-2
150
(b) 100
(a) 50
Fig. 2. (a) Lattice TEM image of meso-WC (inset: SAED pattern of WC). (b) HRTEM images of Pt particles in Pt (3.5 wt%)/meso-WC. Scale bars represent 2 nm (a) and 20 nm (b).
0
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condition. This represents the half-cell anode reaction of DMFC. The activity was presented by the cyclic voltammogram (CV) in Fig. 3 and quantitatively in Table 1. The main feature in the CV between 0.4 V and 0.9 V represents oxidation/reduction of methanol (CH3OH + H2O ! CO2 + 6H+ + 6e). Table 1 also lists the electrochemical surface areas (ESA) extracted from the integration of the Pt–H adsorption/desorption peak in 0.2 to +0.2 V. The WC alone without Pt showed no activity. In contrast, both 3.5 wt% and 7.5 wt% Pt supported on these WC materials showed higher current density (taken at 0.75 V after cycle was stabilized) even in the absence of Ru than commercial 20%Pt–Ru E-Teck catalyst. Among tungsten carbide-based catalysts Pt/meso-WC performed better than Pt/part-WC and Pt/W2C microsphere [12]. In particular, the mass activity (mA/mg of Pt taken at 0.75 V) of 3.5% Pt/meso-WC (1851 mA/mg) was higher by a factor of six than that of the commercial 20%Pt–Ru/C E-Teck catalyst (307 mA/ mg). The activity data clearly show that Pt dispersed on tungsten carbides provides much better utilization of Pt than on conventional carbon support and that Ru could
-0.2
0.0
0.2
0.4
0.6
0.8
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Potential /V vs Ag/AgCl
Fig. 3. Cyclic voltammogram of (a) 3.5 wt% Pt/part -WC, and (b) 20%Pt– Ru (1:1) /Vulcan XC-72 (commercial E-Teck catalyst) (c) 3.5 wt% Pt/ meso-WC (repeated scan) in a 1 M H2SO4 - 1 M CH3OH solution at a scan rate of 50 mV/sec at 298 K.
Table 1 Electrocatalytic activity of various catalysts for methanol oxidation Catalyst
ESA (m2/g–Pt)
Specific activitya (mA/cm2)
Mass activitya (mA/mg-Pt)
3.5%Pt/meso-WC 7.5%Pt/meso-WC 3.5%Pt/part-WC 7.5%Pt/part-WC 7.5% Pt/W2Cb 15% Pt/W2Cb 20%Pt–Ru/C(E-Teck)
336 316 289 337 327 189 71
185.1 294 61.4 261 156 224 114
1851 1373 614 1123 728 560 307
a b
At 0.75 V. From Ref. [12].
R. Ganesan et al. / Electrochemistry Communications 9 (2007) 2576–2579 2000
1600
(a) 11%Pt-9%Ru/Carbon Microsphere (b)13%Pt-7%Ru/Carbon commercial E-Teck (c) 15% Pt/W2C
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(d)7.5%Pt/W2C
Mass Activity /mA/mg
1800
1200 1000
(g)
(e)3.5%Pt/Part-WC (f) 7.5%Pt/Part-(WC (g)7.5%Pt/Meso-WC (h)3.5%Pt/Meso-WC
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(d)
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Electrochemical Surface area /m2/g
Fig. 4. Comparison of mass activity with electrochemical surface area of the catalysts.
be entirely replaced. Mesoporous WC was particularly effective for the purpose. The stability of the Pt/WC catalyst was tested by repeating electrochemical reaction cycles in a 1 M H2SO4-1 M CH3OH solution. As shown in Fig. 3c, the specific activity represented by the CV area increased initially, but was stabilized after ca. 30 cycles. We believe that surface oxygen species initially present on the catalyst is removed during this transient period [12]. Once a steady state was established, there was no sign of deactivation during 100 consecutive reaction cycles. Thus, Pt/WC catalyst performs better than the commercial catalyst even with the smaller Pt loadings and in the absence of ruthenium. According to Chen and coworkers [9,11], WC can activate methanol to form a methoxy intermediate, but the reaction stops there. Further decomposition of the methoxy species is promoted by Pt. Thus, initially inactive WC now actively participates in the reaction in the presence of Pt. In the present case, an indication of the active role of WC could be found in the ESA values of Pt/WC listed in Table 1. A spherical Pt particle with a diameter of 2 nm would have a physical surface area of 69.9 m2/g. All Pt/WC samples show higher ESA values even though observed Pt particle sizes are ca. 2 nm. In contrast, the commercial 10%Pt–Ru/C catalyst showed an ESA value (71 m2/g) expected from the Pt particle size. The higher ESA values for Pt/WC catalysts could be explained by active participation of WC in adsorption of hydrogen, probably through spillover from Pt sites. Along the same line, WC is expected to participate in methanol oxidation as well. Indeed there is a fine correlation between the activity of methanol oxidation and ESA values of the catalysts as summarized in Fig. 4. Thus, unlike carbon in Pt–Ru/C, the role of WC is not a support for Pt, but is an active phase working together with Pt for electrooxidation of methanol.
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The greatly improved activity of Pt/WC catalysts compared with Pt/W2C microspheres that we reported previously [12] appears to be due to the WC phase, which was reported to be most active for electrooxidation of methanol among the three major phases of tungsten carbide, W2C, WC and WC1x [9]. Finally, the present study has demonstrated that mesoporosity of the Pt/meso-WC catalyst is also important. Thus, Pt/meso-WC is more active than Pt/part-WC that has only a small amount of mesoporous. It is interesting to note that the two less active tungsten carbide catalysts possess higher surface areas than that of Pt/ meso-WC, but they are mostly due to micropores. Thus for high electrocatalytic activity, the facile mass transport of hydrated methanol to the reaction sites through mesoporous appear more important than the inaccessible high surface area. 4. Conclusions We have synthesized and characterized mesoporous WC phase by a surfactant-assisted polymer method. A new electrocatalyst composed of Pt supported on these materials shows higher activity for electrochemical oxidation of methanol than a commercial Pt–Ru/C catalyst by a factor of six (per mass of Pt) even without Ru. The mesoporosity and phase of WC appear important as Pt/meso-WC performs better than Pt/W2C-microsphere and Pt/part-WC. Since the catalyst is also stable in electrochemical environment, it could become an alternative electrocatalyst for DMFC with potential to replace current Pt–Ru catalysts. Acknowledgement This work has been supported by Hydrogen R&D Center funded by KOSEF and BK-21 program. References [1] M. Winter, R.J. Brodd, Chem. Rev. 104 (2004) 4245. [2] H. Zhang, Y. Wang, E.R. Fachini, C.R. Cabrera, Electrochem. Solid State Lett. 2 (1999) 437. [3] H.C. Yu, K.Z. Fung, Tz.C. Guo, W.L. Chang, Electrochim. Acta 50 (2004) 811. [4] V. Raghuveer, B. Viswanathan, Fuel 81 (2002) 2191. [5] V. Raghuveer, K.R. Thampi, J.M. Bonard, N. Xanthopoulos, H.J. Mathieu, B. Viswanathan, Solid State Ionics 140 (2001) 263. [6] J.S. Lee, S. Locatelli, S.T. Oyama, M. Boudart, J. Catal. 125 (1990) 157. [7] J.S. Lee, S.T. Oyama, M. Boudart, J. Catal. 106 (1987) 125. [8] R.B. Levy, M. Boudart, Science 181 (1973) 547. [9] M.B. Zellner, J.G. Chen, Catal. Today 99 (2005) 299. [10] D.R. McIntyre, G.T. Burstein, A. Vossen, J. Power Sources 107 (2002) 67. [11] H. Wu, J.G. Chen, J. Vac. Sci. Technol. A 21 (2003) 148. [12] R. Ganesan, J.S. Lee, Angew. Chem. Int. Ed. 44 (2005) 6557. [13] P.R. Van Rheenen, M.J. McKelvy, W.S. Glaunsinger, J. Solid State Chem. 67 (1987) 151.