Tungsten Carbides as Alternative Electrocatalysts: From Surface

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Ind. Eng. Chem. Res. 2011, 50, 16–22

Tungsten Carbides as Alternative Electrocatalysts: From Surface Science Studies to Fuel Cell Evaluation Alan L. Stottlemyer, Erich C. Weigert,† and Jingguang G. Chen* Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716

In the current paper we will provide a review of our recent efforts in experimental studies of tungsten carbides as alternative electrocatalysts for methanol electro-oxidation. We will first discuss ultrahigh vacuum (UHV) studies on single crystal surfaces to demonstrate the feasibility of using tungsten carbides for methanol decomposition. We then discuss UHV studies on polycrystalline thin films and foils to approximate commercially relevant catalysts, thus bridging the “materials gap” and demonstrating that the fundamental chemistry observed in UHV over single crystal surfaces is applicable to morphologically complex surfaces. Electrochemical studies of thin films will be discussed to bridge the “pressure gap” and to verify that tungsten carbides are both active and stable in an electrochemical environment. Finally, we will provide performance data from direct methanol fuel cell (DMFC) testing that incorporates tungsten carbides as the anode electrocatalysts. 1. Introduction The use of fuel cells for energy applications has seen many advances over the past few decades. For example, direct methanol fuel cells (DMFC) show promise for portable power applications because of the high gravimetric energy density of methanol, allowing for extended operating times compared to hydrogen fuel cells.1 However, technical hurdles remain for the large scale commercialization of the DMFC technology. Two of the main challenges associated with the anode electrocatalysts are (1) the high cost of Pt and Ru, which are the leading DMFC electrocatalysts, and (2) the strong binding of CO molecules on the Pt and Ru surfaces, which block active sites necessary for the electro-oxidation of CH3OH and contribute to the sluggish kinetics at the anode.2 Therefore, the commercialization of DMFC depends upon the development of new catalytic materials that should be more cost-effective and show lower binding energy for adsorbed CO. One class of potential alternative electrocatalyst is based on tungsten carbides, which have attracted attention from catalysis and electrocatalysis researchers since Levy and Boudart discovered that tungsten carbides behaved similarly to Pt for alkene isomerization reactions.3 For this reason, there has been considerable interest in developing tungsten carbides as alternative materials to replace Pt for catalytic and electrocatalytic applications.4-6 To determine the feasibility of using tungsten carbides for the decomposition of methanol, initial studies have focused on well-characterized single crystal surfaces in ultrahigh vacuum (UHV) experiments7-12 and density functional theory (DFT) calculations.13 While the studies on well-defined surfaces have provided fundamental insights into the Pt-like properties of tungsten carbides, in an industrially relevant catalyst the metal will be present in the form of nanoparticles. As shown in Figure 1, research efforts in our group involve three parallel approaches designed to bridge the “materials gap” and “pressure gap” between fundamental surface science studies and industrially relevant catalysts. In the current manuscript we will provide an * To whom correspondence should be addressed. E-mail: jgchen@ udel.edu. † Current address: Johnson Matthey Emission Control Technologies, Wayne, PA.

overview of recent results from our research group to demonstrate how these three parallel approaches can be used to rationally design novel anode electrocatalysts that are both less expensive and more CO tolerant than Pt. We will first investigate the reaction pathways of methanol on single crystal surfaces of tungsten carbide and Pt-modified tungsten carbide. The single crystal results will then be extended to the synthesis and electrochemical evaluation of polycrystalline tungsten carbide films.14-18 Finally, supported particles of tungsten carbide electrocatalysts will be tested in full cell measurements19 to demonstrate the feasibility to bridge the “materials gap” and “pressure gap” between fundamental UHV studies on model surfaces and electrochemical evaluation of supported electrocatalysts from both our group19 and others.20-27 2. Experimental and Theoretical Studies on Single Crystal Surfaces UHV studies of single crystal surfaces have provided fundamental understanding of the reaction pathways of methanol decomposition on tungsten carbides. For the UHV studies, the preparation and characterization of carburized W(111) [henceforth referred to as C/W(111)] has been described in detail previously.7 Briefly, the C/W(111) surface was generated by exposing clean W(111) to ethylene at 120 K and then flashed to 1200 K. Platinum was deposited to the surface of C/W(111) [referred to as Pt/C/W(111)] by resistively heating a W wire which was tightly wrapped with Pt wire.10 Previous Auger electron spectroscopy studies have determined that Pt grows on WC in a layer-by-layer fashion.28 Thus, at the submonolayer coverage Pt likely forms regions of monolayer on the WC surface and not nanoparticles or clusters. In this manner, Pt could be evaporated onto the carbide surface with precise control over the surface coverage. 2.1. TPD Studies on Single Crystal Surfaces. Temperature programmed desorption (TPD), Auger electron spectroscopy (AES), and high-resolution electron energy loss spectroscopy (HREELS) were used to determine the decomposition pathways of methanol on C/W(111) and Pt/C/W(111).7,10 Methanol is found to decompose to produce gas-phase CO, H2, and CH4,

10.1021/ie100441p  2011 American Chemical Society Published on Web 06/17/2010

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Figure 1. Parallel research approaches to bridge the “materials gap” and “pressure gap”.

and atomic C and O on the surface via the following net reaction pathways under UHV conditions: xCH3OH f xCO + 2xH2

(1)

yCH3OH f yCad + yOad + 2yH2

(2)

zCH3OH f zCH4 + zOad

(3)

where x, y, and z denote the amount of methanol molecules undergoing each pathway. TPD results following the reaction of 1.0 L of CH3OH on C/W(111) and 0.6 ML Pt/C/W(111) are compared in Figure 2.10 The C/W atomic ratio is approximately 0.55, based on standard AES sensitivity factors.29 In Figure 2a, there are two desorption peaks of CO (m/z ) 28), one centered at 402 K and the other at 896 K. The higher temperature peak is associated with the recombination and subsequent desorption of atomic C and O. Figure 2b illustrates the desorption of the CH4 product from the C/W(111) surface, which shows the characteristic cracking pattern of m/z ) 15. In comparison, the production of CH4 is nearly absent from the 0.6 ML Pt/C/W(111) surface. The quantification of TPD results from several single crystal surfaces is summarized in Table 1. These results indicate a synergistic effect by supporting submonolayer coverage Pt on WC; the Pt/WC surfaces retain the high activity for CH3OH decomposition, but without the production of the undesireable CH4. Control over methane production is critical for the application of WC as the DMFC electrocatalyst, as the production of each methane molecule consumes four of the six electrons available from the CH3OH electro-oxidation. 2.2. DFT Studies on Single Crystal Surfaces. Further evidence that Pt is modifying the electronic properties of the WC surface is from DFT calculations to monitor changes in the d-band density of states of the surface atoms before and after the addition of Pt to WC(0001).30 Adding Pt to WC(0001) promotes electron transfer of 0.22 e/W from W to Pt, causing a shift in the d-band center away from the Fermi level and weaker binding of adsorbates. This is manifested as changes in the reaction mechanism where WC(0001) breaks the C-O bond of CH3O but Pt/WC(0001) breaks the C-H bond. 2.3. Vibrational Studies of Single Crystal Surfaces. To further characterize the surface intermediates present during thermal decomposition, HREELS results following exposure to 1.0 L of CH3OH on C/W(111) and 0.6 ML Pt/C/W(111) were performed and are shown in Figure 3.9 At 90 K, the spectra on C/W(111) (Figure 3a) is indicative of adsorbed methoxy (CH3O), suggesting that methanol adsorbs dissociatively on the surface via O-H bond scission. On the other hand, the presence of the ν(OH) vibration at 3470 cm-1 on 0.6 ML Pt/C/W(111) indicates that at least some CH3OH adsorb molecularly at 90 K. After heating to 230 K and 330 K, spectroscopic features associated with methoxy, namely, ν(CO) at 1150 cm-1, γ(CH3) at 1028 cm-1, and δ(CH3) at 1454 cm-1, decrease in intensity on 0.6 ML Pt/C/W(111), while there is little change in the corresponding modes on the C/W(111) surface. The increase in intensity of the methoxy features from 230 K to 330 K on C/W(111) has

been attributed to a reorientation of the methoxy intermediate.7 By comparing Figure 3a with Figure 3b, it is clear that the thermal decomposition of CH3OH is significantly altered by supporting submonolayer Pt on C/W(111). These changes can be summarized by two important conclusions: (1) C/W(111) is more active to scission of the O-H bond to produce the methoxy intermediate, and (2) the presence of Pt on C/W(111) increases the subsequent decomposition of methoxy at temperatures greater than 230 K. In this manner, while Pt serves to reduce the activity of the C/W(111) surface toward O-H bond scission, it also promotes the dissociation of the C-H bond and the subsequent decomposition of methoxy, therefore preventing the formation of the gas-phase CH4 as shown earlier in the TPD results in Figure 2. 3. Surface Science and Half-Cell Measurements on Polycrystalline Surfaces Studies of carburized W single crystals, both with and without submonolayer coverages of Pt, provide a fundamental understanding of methanol decomposition under UHV conditions and suggest that tungsten carbide based electrocatalysts are promising materials to replace Pt in DMFC. However, to bridge the “materials” and “pressure” gaps, it is necessary to perform UHV and electrochemical evaluation of polycrystalline films of tungsten carbides to mimic the morphologically complex surfaces of supported cataysts. Chronoamperometry (CA), cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), and HREELS have been utilized to determine the surface and electrochemical properties of the carbide films toward the electro-oxidation of methanol. Phase-pure tungsten monocarbide (WC) films were synthesized by either depositing WC on a carbon substrate or by carburizing a W foil. The WC/C thin films were made on a glassy carbon substrate by magnetron sputtering of a WC target at 1000 K, followed by titration in O2 to remove excess surface carbon.14,17 The carburization of W was performed by first cleaning the W foil with cycles of Ne+ bombardment at 300 K, followed by annealing in vacuum to 1200 K. The clean W foil was carburized by cracking ethylene using a hot-filament sputter gun at 0.5 KV bias potential. The surface was then heated to 1200 K to form a carbide film with a C/W ratio of approximately unity, as confirmed by XPS.15 Pt was deposited to the WC surface by evaporation of Pt onto the substrate in the same manner as described previously for single crystal surfaces. The electrochemical studies were performed in a threeelectrode electrochemical cell composed of a saturated calomel electrode (SCE, -0.241 V wrt the normal hydrogen electrode) as the reference electrode, Pt gauze as the counter electrode, and either WC or Pt foil as the working electrode. The geometric surface area of the working electrode was ∼1 cm2. The electrolyte used to approximate DMFC operating conditions was 0.05 M H2SO4 and was continuously pumped at 10 mL/min by a peristaltic pump. 0.2 M CH3OH was added to the electrolyte solution for studies of methanol electro-oxidation. The electrochemical cell was attached to a UHV chamber via a gate valve and a load-lock transfer system between the cell and the UHV system to prevent exposure to air during or

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Figure 2. TPD spectra of characteristic cracking pattern of (a) CO and (b) CH4 on C/W(111) and 0.6 ML Pt/C/W(111) surfaces after exposure to CH3OH at 100 K. Table 1. Activity of Methanol Decomposition Products on Different Surfaces Summary of Methanol Decomposition [molecules per M atom] surface Pt(111)32 C/W(111)7 0.6 ML Pt/C/W(111)10 C/W(110)11 0.5 ML Pt/C/W(110)12

CO complete CH4 total activity (x) decomposition (y) activity (z) (x + y + z) ∼0 0.087 0.091

∼0 0.155 0.086

∼0 0.038 0.000

∼0 0.280 0.177

0.060 0.054

0.176 0.075

0.068 0.000

0.304 0.129

after electrochemical measurements.15,18 After electrochemical testing, the chamber was evacuated, and the working electrode was moved directly into the UHV chamber via a long manipulator arm for surface characterization. In this way, the sample could be synthesized in the UHV chamber, moved to the electrochemical chamber for measurements at atmospheric pressure, and then returned to the UHV chamber for characterization without exposing the sample to air.15 3.1. Vibrational Studies of Polycrystalline Surfaces. After synthesis of the WC film, HREELS measurements were performed to compare the reaction pathways of methanol on single crystal and polycrystalline WC surfaces, as compared in Figure 4.16 Similar to single crystal C/W(111) and C/W(110) surfaces, the origin of the observed synergistic effect of supporting Pt on polycrystalline WC can be attributed to changes in the chemical properties of the surface. For example, HREELS results reveal the presence of the CH3O intermediate on Pt/WC/C, WC/C, and C/W(110) surfaces, indicating that these surfaces are active to the dissociation of the O-H bond at 100 K. On the other hand, the presence of molecular CH3OH on Pt(111) is evidenced by the detection of the ν (OH) vibrational mode at ∼3300 cm-1. The rather low activity of Pt(111) toward CH3OH decomposition is supported by previous UHV studies of CH3OH on Pt(111), which found that a very small fraction

(∼2%) of adsorbed CH3OH underwent decomposition to CO and H2 on the defect sites and step edges on the predominately planar Pt(111) surface.31,32 3.2. Electrochemical Evaluation of Polycrystalline WC. Surface science studies of both single crystal and polycrystalline surfaces suggested that WC and Pt/WC are promising DMFC electrocatalysts. However, the challenge is to bridge the “pressure gap” and to correlate the UHV surface science results to electrochemical activity. Previous CV studies of the electro-oxidation of methanol on Pt, WC, and Pt/WC (not shown) showed a common maxima for methanol oxidation occurred at about 0.65 V (NHE).15 Therefore, CA was used to evaluate the steady-state electrochemical activity of these surfaces at this potential and at room temperature, as compared in Figure 5.15 The current has been normalized to the geometric surface area of the working electrode, or ∼1 cm2. These results show that the steady-state methanol oxidation current on the Pt foil is significantly lower than that on the WC and 0.8 ML Pt/WC surfaces. The low electrochemical activity for methanol electro-oxidation on Pt foil is at least partially attributed to surface poisoning by strongly bonded CO reaction intermediates. Surface science studies have found that the binding energy of CO is much stronger on Pt compared to WC or Pt/WC.33 The weaker binding energy of CO on WC and Pt/ WC likely facilitates the oxidation of CO by surface OHspecies to produce CO2, thus increasing the availability of active sites as compared to Pt. The improved CO tolerance of WC and Pt/WC,33 combined with a lower onset potential for CH3OH electro-oxidation,16 probably accounts for the higher steadystate activity observed on these surfaces. One of the critical questions in utilizing carbide electrocatalysts is their stability under electrochemical environment. XPS measurements were performed to directly probe the oxidation state of the surface metal atoms before and after the CV measurements, without exposing the surfaces to air using the

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Figure 3. HREEL spectra following the thermal decomposition of CH3OH on C/W(111) and 0.6 ML Pt/C/W(111).

Figure 5. CA measurements of Pt, WC, and 0.8 ML Pt/WC surfaces exposed to 0.05 M H2SO4 with 0.2 M CH3OH at 0.65 V.

Figure 4. HREEL spectra after the adsorption of 1 L of CH3OH on Pt(111), C/W(110), WC, and 0.8 ML Pt/WC at 100 K.

combined UHV-half cell equipment described above,15,18 to study the susceptibility of WC and Pt/WC surfaces to surface oxidation by the electrochemical environment. Figure 6 shows the W 4f region of the XPS spectra of the WC and Pt/WC surfaces after CV measurements in 0.05 M H2SO4 to 1.241 V (NHE). While XPS confirmed phase pure WC before CV (figure not shown), after 30 cycles strong tungsten oxide features appear

at 36 and 38 eV, indicating that some of the surface WC is oxidized during CV measurements. On the other hand, after 50 CV cycles to 1.241 V (NHE) the Pt/WC surfaces show mostly carbidic tungsten 4f7/2 and 4f5/2 features at 32 and 34 eV, respectively. Thus, Figure 6 shows that the addition of Pt stabilizes the electrochemical stability of WC and that even submonolayer Pt coverage is sufficient to prevent substantial surface oxidation. 4. Full-Cell Measurements of Supported Catalysts In fuel cell devices catalysts are often supported nanoparticles which can vary in shape and size. While UHV and electrochemical studies of single crystal and polycrystalline surfaces demonstrated that single crystal studies could be used to predict

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Figure 6. XPS spectra of the W 4f region of WC, 0.8 ML Pt/WC, and 1.2 ML Pt/WC after CV measurements in 0.05 M H2SO4 to 1.241 V.

the catalytic properties of morphologically complex surfaces, questions still remained unanswered with respect to the feasibility of incorporating tungsten carbide based materials into realistic fuel cell devices. Attempts were made to examine the catalytic properties of WC and Pt/WC nanoparticles under fuel cell operating conditions.19 WC catalysts were obtained from Japan New Metals Company, with a surface area of 7.32 m2/g as measured by the Brunauer-Emmett-Teller (BET) approach, an average diameter of 50 nm, and a density of 15.6 g/cm3. The 10 wt % Pt-modified WC nanoparticles were prepared using the incipient wetness impregnation method, which was chosen both for the simplicity of the technique and because of the known stability of WC in aqueous solutions.34 The Pt impregnated WC nanoparticles were reduced in a H2/He mixture at 623 K for 6 h. Details about the preparation of the membrane electrode assembly (MEA) are described in detail elsewhere.19 Briefly, the WC and Pt/WC nanoparticles were coated directly onto Nafion 117 membrane, and the resulting catalyst coated membrane (CCM) and the gas diffusion layer (GDL) were hot pressed into the membrane electrode assembly (MEA). The MEA was tested using an Arbin Instruments fuel cell test stand, as described previously.35 4.1. Characterization of WC Nanoparticles. The WC nanoparticles were characterized using XPS, X-ray diffraction (XRD), and scanning electron microscopy (SEM). XRD and SEM (not shown) measurements confirmed the phase purity and particle size of the WC particles, respectively.19 XPS results of the W 4f and the C 1s region of the WC and Pt/ WC nanoparticles are shown in Figure 7.19 In agreement with the XRD data which suggested that the as-received catalyst consisted of single-phase tungsten monocarbide, XPS confirmed the carbidic C/W atomic ratio to be about unity. XPS also revealed that tungsten oxides contribute to about 25% of the total signal for the pure WC catalyst, as evidenced by oxide features at 36 and 38 eV in Figure 7a. The existence of some surface carbonaceous carbon is also revealed by the C 1s feature at 284.4 eV in Figure 7b. After the addition of Pt to the pure WC catalyst, the contribution from the W oxide features increases to almost 50% of the total XPS area. XPS of the corresponding Pt 4f region (not shown) show a feature at 71.2 eV, which is indicative of Pt in the metallic state. 4.2. Fuel Cell Studies of WC Anode Electrocatalysts. By withdrawing specific current density values for 5 min intervals, polarization and power density curves were generated. To test the capabilities of the MEA, the output was monitored at 323 K, 333 K, and 343 K, as shown in Figure

Figure 7. XPS spectra of the (a) W 4f and (b) C 1s region of WC nanoparticles.

Figure 8. Power density curves (solid symbols) and polarization curves (open symbols) produced by Pt/WC anode MEA with fuel concentration of 2 M and flow rate of 4 mL/min at several operating temperatures. Table 2. Maximum Power Density and Corresponding Cell Voltage Values for Pt/WC and Pt/Ru Anode MEA Pt/WC and Pt/Ru Anode MEA [2 M CH3OH, 4 mL/min] at 333 K19 surface

power density [mW/cm2]

power per mg metal at anode [mW/mg M]

Pt/WC Pt/Ru

9.1 98.41

22.8 24.6

8 and summarized in Table 2 for Pt/WC.19 These temperatures were chosen because they are within the temperature range often used for DMFC evaluation.1,36-38 As the electrooxidation of methanol is often kinetically limited,2 it was

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not surprising that performance increases with increasing temperature. By increasing the temperature from 323 K to 343 K the power density of the MEA is increased by about a factor of 2, from 6.8 mW/cm2 to 14.5 mW/cm2. Figure 8 also shows that the open circuit voltage (OCV) increases with increasing temperature, from 460 mV at 323 K to 500 mV at 343 K. The position of the maximum power density falls clearly within the linear region of the corresponding polarization curve, which suggests that methanol electro-oxidation is reaction limited on Pt/WC. Furthermore, the lack of a masstransfer limited regime on the polarization curves suggests that inadequate surface area of the anode electrocatalyst likely limits the cell performance. The Pt/WC electrocatalyst was also benchmarked against an MEA prepared in a similar manner, but with a Pt/Ru bimetallic anode electrocatalyst instead of Pt/WC. After normalization of the maximum power density by metal loading, it was found that the Pt/WC electrocatalyst delivered approximately the same power as the Pt/Ru system on the basis of power per mg Pt/Ru, as shown in Table 2.19 As the MEA tested in this study was not optimized for performance, it is anticipated that with further refinement, especially in the synthesis of high surface area WC and Pt/WC, these catalysts could be promising substitutes to replace Pt/Ru in DMFC. 5. Conclusions In the current review we used methanol decomposition reactions to demonstrate the importance of a combined approach of UHV surface science studies and electrochemical evaluation to identify catalysts with desirable properties. The examples illustrate the possibility of bridging the “materials gap” and “pressure gap” by applying fundamental understanding from surface science studies to industrially relevant supported catalysts. For the case of DMFC, this approach demonstrates that it may be feasible to substitute the Pt/Ru electrocatalysts with WC-based materials for methanol electro-oxidation. Furthermore, the demonstration of WC and Pt/ WC as active and stable electrocatalysts should provide opportunities to explore their utilization in other electrochemical applications, including as the cathode materials in photoelectrochemical cells.39 Acknowledgment We acknowledge financial support from the Department of Energy (Grant DE-AC05-76RL01830). We also acknowledge partial support from the National Science Foundation (Grant CTS-0518900). A.L.S. also acknowledges support from the NASA Delaware Space Grant College Fellowship (NASA Grant NNG 05GO92H). Literature Cited (1) Carrette, L.; Friedrich, K. A.; Stimming, U. Fuel Cells- Fundamentals and Applications. Fuel Cells 2001, 1, 5. (2) Liu, H. A review of anode catalysis in the direct methanol fuel cell. J. Power Sources 2006, 155, 95–110. (3) Levy, R. B.; Boudart, M. Platinum-Like Behavior of Tungsten Carbide in Surface Catalysis. Science 1973, 181, 547–549. (4) Oyama, S. T. The Chemistry of Transitional Metal Carbides and Nitrides; Blackie Academic and Professional: Glasgow, 1996. (5) Chen, J. G. Carbide and Nitride Overlayers on Early Transition Metal Surfaces: Preparation, Characterization, and Reactivities. Chem. ReV. 1996, 96, 1477–1498. (6) Hwu, H. H.; Chen, J. G. Surface Chemistry of Transition Metal Carbides. Chem. ReV. 2005, 105, 185–212.

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(7) Hwu, H. H.; Chen, J. G.; Kourtakis, K.; Lavin, J. G. Potential Application of Tungsten Carbides as Electrocatalysts. 1. Decomposition of Methanol over Carbide-Modified W(111). J. Phys. Chem. B 2001, 105, 10037–10044. (8) Hwu, H. H.; Polizzotti, B. D.; Chen, J. G. Potential Application of Tungsten Carbides as Electrocatalysts. 2. Coadsorption of CO and H2O on Carbide-Modified W(111). J. Phys. Chem. B 2001, 105, 10045– 10053. (9) Hwu, H. H.; Chen, J. G. Potential application of tungsten carbides as electrocatalysts. J. Vac. Sci. Technol. A 2003, 21, 1488–1493. (10) Liu, N.; Kourtakis, K.; Figueroa, J. C.; Chen, J. G. Potential application of tungsten carbides as electrocatalysts: III. Reactions of methanol, water, and hydrogen on Pt-modified C/W(111) surfaces. J. Catal. 2003, 215, 254–263. (11) Hwu, H. H.; Chen, J. G. Potential application of tungsten carbides as electrocatalysts: 4. Reactions of methanol, water, and carbon monoxide over carbide-modified W(110). J. Phys. Chem. B 2003, 107, 2029–2039. (12) Zellner, M. B.; Chen, J. G. Potential Application of Tungsten Carbides as Electrocatalysts: Synergistic Effect by Supporting Pt on C/W(110) for the Reactions of Methanol, Water, and CO. J. Electrochem. Soc. 2005, 152, A1483–A1494. (13) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Trends in the chemical properties of early transition metal carbide surfaces: A density functional study. Catal. Today 2005, 105, 66–73. (14) Zellner, M. B.; Chen, J. G. Surface science and electrochemical studies of WC and W2C PVD films as potential electrocatalysts. Catal. Today 2005, 99, 299–307. (15) Weigert, E. C.; Stottlemyer, A. L.; Zellner, M. B.; Chen, J. G. Tungsten Monocarbide as Potential Replacement of Platinum for Methanol Electrooxidation. J. Phys. Chem. C 2007, 111, 14617–14620. (16) Weigert, E. C.; Zellner, M. B.; Stottlemyer, A. L.; Chen, J. G. A Combined Surface Science and Electrochemical Study of Tungsten Carbides as Anode Electrocatalysts. Top. Catal. 2007, 46, 349–357. (17) Weigert, E. C.; Humbert, M. P.; Mellinger, Z. J.; Ren, Q.; Beebe, T. P., Jr.; Bao, L.; Chen, J. G. Physical vapor deposition synthesis of tungsten monocarbide (WC) thin films on different carbon substrates. J. Vac. Sci. Technol. A 2008, 26, 23–28. (18) Weigert, E. C.; Esposito, D. V.; Chen, J. G. Cyclic voltammetry and X-ray photoelectron spectroscopy studies of electrochemical stability of clean and Pt-modified tungsten and molybdenum carbide (WC and Mo2C) electrocatalysts. J. Power Sources 2009, 193, 501–506. (19) Weigert, E. C.; Arisetty, S.; Advani, S. G.; Prasad, A. K.; Chen, J. G. Electrochemical Evaluation of Tungsten Monocarbide (WC) and Platinum-modified WC as Alternative DMFC Electrocatalysts. J. New Mater. Electrochem. Syst. 2008, 11, 243–251. (20) Angelucci, C.; Deiner, L.; Nart, F. On-line mass spectrometry of the electro-oxidation of methanol in acidic media on tungsten carbide. J. Solid State Electrochem. 2008, 12, 1599–1603. (21) Ganesan, R.; Lee, J. S. Tungsten Carbide Microspheres as a NobleMetal-Economic Electrocatalyst for Methanol Oxidation. Angew. Chem., Int. Ed. 2005, 44, 6557–6560. (22) Izhar, S.; Yoshida, M.; Nagai, M. Characterization and performances of cobalt-tungsten and molybdenum-tungsten carbides as anode catalyst for PEFC. Electrochim. Acta 2009, 54, 1255–1262. (23) Joo, J. B.; Kim, J. S.; Kim, P.; Yi, J. Simple preparation of tungsten carbide supported on carbon for use as a catalyst support in a methanol electro-oxidation. Mater. Lett. 2008, 62, 3497–3499. (24) Meng, H.; Shen, P. K.; Wei, Z.; Jiang, S. P. Improved Performance of Direct Methanol Fuel Cells with Tungsten Carbide Promoted Pt/C Composite Cathode Electrocatalyst. Electrochem. Solid-State Lett. 2006, 9, A368–A372. (25) Nagai, M.; Yoshida, M.; Tominaga, H. Tungsten and nickel tungsten carbides as anode electrocatalysts. Electrochim. Acta 2007, 52, 5430–5436. (26) Wang, Z. B.; Zuo, P. J.; Liu, B. S.; Yin, G. P.; Shi, P. F. Stable PtNiPb/WC Catalyst for Direct Methanol Fuel Cells. Electrochem. SolidState Lett. 2009, 12, A13–A15. (27) Zhao, Z.; Fang, X.; Li, Y.; Wang, Y.; Shen, P. K.; Xie, F.; Zhang, X. The origin of the high performance of tungsten carbides/carbon nanotubes supported Pt catalysts for methanol electrooxidation. Electrochem. Commun. 2009, 11, 290–293. (28) Humbert, M. P.; Menning, C. A.; Chen, J. G. Replacing bulk Pt in Pt - Ni - Pt bimetallic structures with tungsten monocarbide (WC): Hydrogen adsorption and cyclohexene hydrogenation on Pt-Ni-WC. J. Catal. 2010, 271, 132–139. (29) Childs, K.; Carlson, B.; LaVanier, L.; Moulder, J.; Paul, D.; Stickle, W.; Watson, D. Handbook of Auger Electron Spectroscopy; Physical Electronics Industries: Eden Prairie, MN, 1995.

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(30) Stottlemyer, A. L.; Liu, P.; Chen, J. G. Controlling Bond Scission Sequence of Methanol through Synergistic Effect of Supporting Platinum on Tungsten Monocarbide Surfaces. 2010, in preparation. (31) Gibson, K. D.; Dubois, L. H. Step effects in the thermal decomposition of methanol on Pt(111). Surf. Sci. 1990, 233, 59–64. (32) Sexton, B. A. Methanol decomposition on platinum (111). Surf. Sci. 1981, 102, 271–281. (33) Mellinger, Z. J.; Weigert, E. C.; Stottlemyer, A. L.; Chen, J. G. Enhancing CO Tolerance of Electrocatalysts: Electro-oxidation of CO on WC and Pt-Modified WC. Electrochem. Solid-State Lett. 2008, 11, B63. (34) Bozzini, B.; Pietro De Gaudenzi, G.; Fanigliulo, A.; Mele, C. Electrochemical oxidation of WC in acidic sulphate solution. Corros. Sci. 2004, 46, 453–469. (35) Arisetty, S.; Prasad, A. K.; Advani, S. G. Metal foams as flow field and gas diffusion layer in direct methanol fuel cells. J. Power Sources 2007, 165, 49–57. (36) Cheng, X.; Peng, C.; You, M. D.; Liu, L.; Zhang, Y.; Fan, Q. B. Characterization of catalysts and membrane in DMFC lifetime testing. Electrochim. Acta 2006, 51, 4620–4625.

(37) Reshetenko, T. V.; Kim, H. T.; Krewer, U.; Kweon, H. J. The effect of the anode loading and method of MEA fabrication on DMFC performance. Fuel Cells 2007, 7, 238–245. (38) Wee, J. H. Which type of fuel cell is more competitive for portable application: Direct methanol fuel cells or direct borohydride fuel cells. J. Power Sources 2006, 161, 1–10. (39) Esposito, D. V.; Dobson, K. D.; McCandless, B. E.; Birkmire, R. W.; Chen, J. G. Comparative Study of Tungsten Monocarbide and Platinum as Counter Electrodes in Polysulfide-Based Photoelectrochemical Solar Cells. J. Electrochem. Soc. 2009, 156, B962–B969.

ReceiVed for reView February 27, 2010 ReVised manuscript receiVed May 24, 2010 Accepted June 1, 2010 IE100441P