J. Phys. Chem. B 2001, 105, 10037-10044
10037
Potential Application of Tungsten Carbides as Electrocatalysts. 1. Decomposition of Methanol over Carbide-Modified W(111) Henry H. Hwu and Jingguang G. Chen* Center for Catalytic Science and Technology, Department of Materials Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716
Kostantinos Kourtakis and J. Gerry Lavin DuPont Central Research & DeVelopment, Wilmington, Delaware 19880 ReceiVed: April 27, 2001; In Final Form: July 11, 2001
The decomposition of methanol over clean and carbide-modified W(111) is studied by using temperatureprogrammed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy (AES). The chemistry of methanol on unmodified W(111) is predominantly complete decomposition to produce atomic carbon and H2, with slightly less than 15% of the adsorbed methanol dissociating to form CO, CH4, and H2. Once the W(111) surface is carbide-modified, however, the most dominant reaction pathway is still the complete decomposition of CH3OH at ∼55%, but with significantly more CO and CH4 desorbing as gas-phase products. If the carbide surface is further modified with oxygen, the activity toward the production of CO is further enhanced and becomes the dominant pathway, while the yield of gas-phase CH4 is slighted reduced compared to the unmodified C/W(111) surface. These results will be compared to the activity of Pt group metal surfaces to explore the potential application of using tungsten carbides as an alternative to Pt group metal electrodes in fuel cells.
1. Introduction As the global supply of fossil fuels predictably diminishes, there has been overwhelming interest in developing the fuel cell technology. While there exist many types of fuel cells each requiring different electrocatalysts and fuels, systems involving the oxidation of methanol to obtain hydrogen fuel or electrons appear to exhibit potential advantages over others.1,2 In the case of the direct methanol fuel cell, the ideal chemistry at the anode involves the simultaneous oxidation of water and methanol to leave only CO2, electrons, and protons (eq 1):1,2
overall reaction: CH3OH + H2O f CO2v + 6H+ + 6e- (1) CH3OH f CO(ads) + 4H+ + 4e-
(2)
H2O f O (ads) + H+ + e-
(3)
OH(ads) + CO(ads) f CO2v + H+ + e(4) Under most conditions, the overall anode reaction is actually comprised of three separate steps. The first step is shown in eq 2, which involves the partial oxidation of methanol to produce protons, electrons, and chemisorbed CO; the subsequent two steps then entail the formation of surface OH groups followed by the combination with surface CO to yield gas-phase CO2 (eqs 3 and 4). Contrary to the behavior of CO2, CO often remains on the surface of the electrocatalysts and deactivates the catalytic activity at the anode.1,2 * Corresponding author. E-mail:
[email protected]. Fax: 302-831-4545.
Briefly, the oxidation of methanol can be described by the following steps. First, methanol adsorbs onto the electrocatalyst and is successively deprotonated. At this point the catalyst surface contains adsorbed CO species. On other sites, water is adsorbed and dehydrogenated to leave surface hydroxyl species. The two sites, Pt-CO and Pt-OH, can then combine to form Pt-COOH species, which subsequently decomposes to produce gas-phase CO2.3 Alternatively, researchers have also suggested that water directly reacts with Pt-CO without first adsorbing onto a neighboring site.3 Currently, the electrocatalyst of choice for methanol oxidation is the bimetallic system of Pt/Ru; this material is favored because it demonstrates significant activity for methanol oxidation as well as the dehydrogenation of water, which is critical for the removal of adsorbed CO species.2,4-6 Aside from its activity, however, the Pt/Ru system is disadvantageous in terms of its prohibitively high costs and limited supplies.7 As a result, the discovery of less expensive alternatives to the Pt/Ru catalysts would greatly facilitate the commercialization of the methanol-based fuel cell systems. This manuscript is the first of a series of papers aimed at examining the effectiveness of tungsten carbides as a methanol fuel cell catalyst. In this and the following paper, we will report our results on the interaction of CH3OH, H2O, and CO on tungsten carbide surfaces under ultrahigh vacuum (UHV) conditions. Results on the electrocatalytic properties of chemically modified tungsten carbides, both under UHV and in situ liquid conditions, will be described in subsequent papers. It is now well-known that the chemical properties of early transition metals often resemble that of Pt group metals when alloyed with carbon and nitrogen.8-13 The main reason for choosing tungsten carbides is that our earlier studies revealed that C/W(111) showed the characteristic Pt-like properties in the dehydroge-
10.1021/jp0116196 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001
10038 J. Phys. Chem. B, Vol. 105, No. 41, 2001 nation of cyclohexene9 and the decomposition of NO.13 Another reason is that tungsten carbide has been reported to show promising electrochemical properties for methanol oxidation,14-18 although little is known at present about the reaction mechanisms at the electrocatalyst surfaces. By using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS), we seek to obtain a fundamental understanding of chemical reactions of methanol, water, and carbon monoxide on W(111) and carbide-modified W(111) surfaces. In addition, oxygen-modified C/W(111) surfaces is also investigated since the presence of oxygen represents a more realistic fuel cell environment. In the current paper we will report the decomposition of methanol on various W(111) surfaces; the results of the reactions of water and carbon monoxide will be described in the following paper.48 As a comparison, we will provide a brief review of what is known about the reaction of methanol on other single crystal surfaces. Under UHV conditions, the observed chemistry of methanol on single crystals generally begins with either the scission of the O-H or the C-H bond. On late transition metals, methanol often adsorbs molecularly and eventually dissociates to form chemisorbed CO and H between 200 and 300 K.19 Additionally, the stable methoxy intermediate forms on many surfaces, for example, Ni(111), Pd(100), Ru(001), and Rh(111) and Rh(110).20-25 In the case of Pt(111) and Fe(100), preadsorbed surface oxygen can induce the formation of methoxy, which is not observed on unmodified surfaces.26,27 Furthermore, the presence of oxygen on Fe(100) and Rh(111) leads to the formation of a formaldehyde intermediate.23,27 For group IB metals such as Cu and Ag, methanol partially oxidizes to form formaldehyde as well;28-30 on Au, however, the methyl formate intermediate is formed.31 Studies have also been conducted on other tungsten surfaces,32 which will be compared to the current results in more detail in the Discussion. 2. Experimental Section 2.1. Techniques. The UHV chamber used in the current study has been described in detail previously.8,9 Briefly, it is a threelevel stainless steel chamber (base pressure of 4 × 10-10 Torr) which is equipped with Auger electron spectroscopy (AES), X-ray photoelectron spsectroscopy (XPS), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD) in the top two levels and high-resolution electron energy loss spectroscopy (HREELS) in the bottom level. The HREELS spectra reported here were acquired with a primary beam energy of 6 eV. Angles of incidence and reflection were 60° with respect to the surface normal in the specular direction. Count rates in the elastic peak were typically in the range of 5 × 105 to 2 × 106 cps, and the spectral resolution was between 35 and 50 cm-1 fwhm (full-width at half-maximum). For TPD experiments the W(111) sample was heated with a linear heating rate of 3 K/s. The single crystal sample was a [111] oriented, 1.5 mm thick tungsten disk (99.999%), 10 mm in diameter, and was purchased from Metal Crystals and Oxides, Ltd., Cambridge, England. The crystal was spot-welded directly to two tantalum posts that serve as electrical connections for resistive heating, as well as thermal contacts for cooling with liquid nitrogen. With this mounting scheme, the temperature of the crystal could be varied between 90 and 1300 K. Methanol (Aldrich, 99+% purity) was purified by successive freeze-pump-thaw cycles prior to their use. The purity was verified in situ by mass spectrometry. Oxygen was obtained from Matheson (99.99% pure) and was used without further purification. Doses are reported in Langmuirs (1 Lang-
Hwu et al. muir (L) ) 1 × 10-6 Torr s) and are uncorrected for ion gauge sensitivity. In all experiments, the gas exposures were made at a crystal temperature of 90 K with the crystal located in front of the leak valve. The gas exposures were made by backfilling the vacuum chamber. 2.2. Preparation of Clean and Modified W(111). A clean W(111) crystal surface is prepared by cycles of Ne+ bombardment at 500 K (sample current ∼6 µA) and flashing to 1200 K. These 5-min cycles are generally repeated three times before annealing at 1200 K for ∼5 min. To remove carbon contamination, excess O2 is used to titrate carbide layers at 1000 K. Oxygen reacts with carbon-covered W(111), which then desorbs as CO at temperatures above 1150 K. This oxygen treatment process is repeated several times to remove both surface and bulk carbon. Auger analysis shows that the C and O impurities are both less than 1% of a monolayer after the above cleaning procedure. The preparation of carbide-modified W(111) surfaces using ethylene or other unsaturated hydrocarbon molecules as a carbon source has been described previously;8,9,12 the carbide surface will be referred to as C/W(111). In this work, the carbide surfaces were prepared by exposing W(111) to ethylene at 90 K and then flashed to 1200 K; generally these procedures were repeated for 3 cycles. Three cycles of dosing and annealing produces a (x3 × x3)R30°-C/W(111) LEED pattern with an Auger C(KLL 272 eV)/W(MNN 182 eV) peak ratio between 0.55 and 0.7, corresponding to an atomic C/W ratio between 0.58 and 0.74. The preparation of oxygen-modified C/W(111) involves first making the carbide layer and then dosing 1 L of oxygen at 900 K; typically, the atomic ratios C/W and O/W are ∼0.45 and ∼0.25, respectively: this surface will be referred to as O/C/W(111). Finally, the oxygen-modified W(111) surface, or O/W(111), is made by exposing a clean W(111) surface to 30 L of oxygen at 600 K; the atomic O/W ratio for this surface is approximately 0.75. 3. Results and Interpretations 3.1. TPD Results. The TPD spectra from 1 L exposure of CH3OH on W(111), O/W(111), C/W(111), and O/C/W(111) are shown in Figure 1. The only detectable gas-phase products in the current study are H2, CH4, and CO. For the O/W(111) surface, methanol desorbs molecularly without producing any hydrogen or methane. The only other product from this layer is a small amount of CO at 207 K, which is likely due to the adsorption of CO impurities from the UHV background. The overall TPD results indicate that the O/W(111) surface is inert toward the decomposition of CH3OH. On clean W(111) we detected a relatively intense H2 desorption state at ∼270 K, followed by a second, more intense H2 peak at ∼360 K. In addition, we also observed the evolution of two more species on clean W(111): CO at ∼230 and ∼415 K, and CH4 at 222 and 414 K. By 861 K, an intense CO peak is observed, most likely resulting from the recombination of atomic C and O. From comparisons between the relative intensities of this recombinant CO desorption with the lowtemperature CO peaks, we conclude that the dominant pathway of methanol on W(111) is the complete decomposition to atomic C and O. After heating to 1200 K, AES measurements indicate that only a small amount of atomic oxygen (∼0.12 O/W atomic ratio) remains on the W(111) surface; this observation, combined with AES data at 600 K, is important in allowing us to estimate the product selectivity, which will be addressed in the Discussion. The thermal behavior of CH3OH on C/W(111) is somewhat similar to that of the clean W(111) surface. The H2 desorption
Tungsten Carbides as Electrocatalysts. 1
J. Phys. Chem. B, Vol. 105, No. 41, 2001 10039
Figure 1. Temperature-programmed desorption spectra of hydrogen, methane, and carbon monoxide obtained following a 1 L exposure of methanol on clean, carbide-modified, oxycarbide-modified, and oxygen-modified W(111).
features consists of an initial smaller peak at 300 K followed by a very broad peak centered at ∼460 K; the existence of a broad hydrogen TPD peak typically suggests that CHx fragments remain on the carbide surface even at relatively high temperatures.33 From the mass 16 and mass 28 spectra, we can identify a relatively intense, asymmetric CH4 peak (496 K) as well as some CO peaks (311 and 408 K). Further heating to 853 K produces a broad, recombinatory CO desorption feature, indicating that a significant fraction of the adsorbed CH3OH decomposes to produce atomic C and O. In the case of methanol on O/C/W(111) layer, the H2 peak shape is much sharper than that of C/W(111). Similar to the clean and carbide surface, methanol on O/C/W(111) also produces CO and CH4. However, on this surface, the CO peak is much more prominent while the intensity of the CH4 peak is less than that of C/W(111). After heating beyond 800 K, we observed an additional CO peak at 825 K, which is from the recombination of atomic C and O. Overall, the TPD results indicate that the decomposition of methanol occurs on all surfaces except the inert O/W(111) surface. The C/W(111) surface appears to produce more CH4 and less CO than the O/C/W(111) surface. The clean W(111) surface also produces both methane and carbon monoxide. Lastly, no detectable amounts of gas-phase formaldehyde, CO2, or H2O were detected on any of the four surfaces (spectra not shown). 3.2. HREELS Results. HREEL spectra for CH3OH adsorbed on the four surfaces are described in this section. Both the onspecular and off-specular HREEL spectra were measured on all surfaces. The exposures of methanol are made with the crystal temperature at 90 K; the adsorbed layer is then heated to the indicated temperatures, held for 5 s, and allowed to cool before the HREEL spectra are recorded. Finally, the height of the elastic peaks in all spectra has been normalized to unity, and the expansion factor for each individual spectrum represents the multiplication factor relative to the elastic peak. HREEL spectra of multilayer CH3OH on oxygen-modified W(111) at 90 K are shown in Figure 2. The relevant peak frequencies are assigned in Table 1. At this exposure the CH3OH/O/W(111) layer exhibited the following vibrational features: 778 cm-1, δ(OH); 1062 cm-1, ν(CO); 1143 cm-1, γ(CH3); 1488 cm-1, δ(CH3); 2070 cm-1, CO contaminants; 2990 cm-1, νas(CH3); and 3288 cm-1, ν(OH). The relatively weak feature at ∼2591 cm-1 is tentatively assigned to the
Figure 2. HREEL spectra recorded after exposing oxygen-modified W(111) to 10 L of CH3OH and CH3OD at 90 K.
combination mode from the 1062 and 1143 cm-1 features. To facilitate the assignment of vibration features, a comparison of the HREEL spectrum of multilayer CH3OD on O/W(111) recorded at 90 K is also shown in Figure 2. As expected, frequency shifts are observed for the two vibrational modes involving O-H and O-D bonds: (1) the δ(OH) mode shifts from 778 to 568 cm-1 and the ν(OH) mode shifts from 3288 to 2449 cm-1. The on-specular spectra recorded after heating the CH3OH/ W(111) layer to various temperatures are shown in Figure 3. At 90 K, the vibrational modes of this layer are observed at 494, 1028, 1157, 1461, 2983 cm-1, and a weak CO peak at
10040 J. Phys. Chem. B, Vol. 105, No. 41, 2001
Hwu et al.
TABLE 1: Vibrational Frequencies (cm-1) of Solid Phases CH3OH and CH3OD, and 10 langmuir CH3OH and CH3OD on Oxygen-Modified W(111) mode ν(W-O) δ(OH) ν(CO) γ(CH3) δ(CH3) νs(CH3) νas(CH3) ν(OH) a
CH3OH/ CH3OD/ CH3OH CH3OD (s)a (s)a νH/νD O/W(111)b O/W(111)b νH/νD 730 1032 1124 1452 2828 2951 3225
535 1032 1158 1470 2835 2957 2400
1.36 1.00 0.97 0.99 1.00 1.00 1.34
778 1062 1143 1488
568 1049 1143 1488
1.37 1.01 1.00 1.00
2990 3288
2990 2449
1.00 1.34
Reference 34. b This work.
Figure 3. On-specular HREEL spectra which monitor the thermal decomposition of 1 L CH3OH on clean W(111) following adsorption at 90 K.
2023 cm-1. The relatively weak feature at ∼2604 cm-1 is assigned to the combination mode from the 1157 and 1461 cm-1 features. It is important to note the lack of a ν(OH) feature around 3288 cm-1, which clearly indicates the scission of the OH bond of CH3OH on clean W(111) even at 90 K. The vibrational assignments of the resulting methoxy (CH3O) species are summarized in Table 2. Upon heating to 230 K, one cannot discern any dramatic spectroscopic changes with the exception of some small energy shifts. Further heating to 330 K produces
a new peak at 670 cm-1, while all other features remain nearly the same. On the basis of the observations from spectra a-c, we can conclude that most of the surface methoxy species is formed at 90 K and remains intact until 330 K. When the adsorbed layer is heated to 450 K, the following changes are observed: (1) a decrease in the intensities related to the CH3 modes and (2) an increase in the intensities of the modes at 453 and 676 cm-1, which are related to the metal-O and metal-C motions, respectively. By 600 K, all features associated with the methoxy species disappear, and only the features corresponding to the W-O and W-C vibrations remain. Figure 4 shows the on- and +20° off-specular spectra of the CH3OH/C/W(111) layer after heating to higher temperatures. The HREEL spectrum at 90 K shows features at 392, 521, 1035, 1157, 1454, 2002, and 2956 cm-1. Similar to that observed on clean W(111), the scission of the O-H bond also occurs on C/W(111) at 90 K, as suggested by the absence of the ν(OH) mode at ∼3288 cm-1. When heated to 230 K, the frequencies of methoxy features remain the same. Further heating to 330 K results in the following spectroscopic changes: (1) the intensities of the ν(CO), γ(CH3), δ(CH3), and ν(CH3) features increase, with the ν(CO) peak shifting from 1035 to 1082 cm-1, and (2) the ν(W-OCH3) peak at 521 cm-1 becomes well-resolved and its intensity increases. By 450 K, one can observe the significant overlap of the ν(CO) and γ(CH3) peaks, while the frequencies of all other modes remain nearly the same. Upon heating to 600 K, the HREEL spectrum is characteristic of oxygen modified C/W(111), with ν(W-C) at 501 cm-1, and ν(W-O) at 643 and 954 cm-1.33 The comparison in Figure 4 also reveals a strong similarity between the on- and off-specular spectra. This can be explained either by the lack of ordered orientation of the methoxy species or by the highly corrugated nature of the C/W(111) substrate. To facilitate the assignment of the methoxy vibrational features, Figure 5 shows HREEL spectra of 1 L exposure of deuterated methanols on C/W(111) at temperatures of 90 and 300 K. On the basis of the characteristic frequency shifts upon partial deuteration, and by comparing the values on Al(111),34 the vibrational features associated with methoxy are summarized in Table 2. In a previous vibrational study of methoxy on Al(111),34 the νH/νD ratios of the M-O and C-O modes were approximately 1, because these modes were not affected by the deuteration. For the methyl vibrational features, however, the νH/νD ratios were ∼1.35. The results obtained from this work are very similar to previously published data, as we also observed ratios ∼1.0 for the M-O and C-O modes, and ∼1.35 for the CH3 modes.20,35,36 Figure 6 shows HREEL spectra of 1 L CH3OH on O/C/ W(111) after heating to higher temperatures. The low-temperature spectrum at 90 K is very similar to that of the C/W(111) surface. Similar to the decomposition of methanol on the C/W(111) surface, the vibrational spectrum on O/C/W(111) is assigned to the formation of methoxy species on the surface at 90 K. When heated to 230 K there are no dramatic spectral
TABLE 2: Vibrational Frequencies (cm-1) of Methoxy (CH3O and CD3O) on Al(111), W(111), and C/W(111) mode
CH3O/Al(111)a
CD3O/Al(111)a
νH/νD
CH3O/W(111)b
CH3O/C/W(111)b
CD3O/C/W(111)b
νH/νD
ν(M-O) ν(CO) γ(CH3) δ(CH3) νs(CH3) νas(CH3)
655 1025 1170 1475
645 985
1.02 1.04
1080 2065 2235
1.37
467 1028 1157 1461
534 1021 1157 1448
1.10 1.05 1.28 1.35
1.33
2983
2956
487 974 906 1069 2063 2226
a
2970
Reference 34. b This work.
1.33
Tungsten Carbides as Electrocatalysts. 1
J. Phys. Chem. B, Vol. 105, No. 41, 2001 10041
Figure 4. On-specular and +20° off-specular HREEL spectra which monitor the thermal decomposition of 1 L CH3OH on carbide-modified W(111) following adsorption at 90 K.
Figure 5. HREEL spectra which compare 1 L exposures of CH3OH, CH3OD and CD3OH on carbide-modified W(111) at 90 and 300 K.
changes with the exception of diminished intensities for all peaks. At 330 K, however, two distinct changes were observed: (1) the appearance of a 636 cm-1 peak and (2) the appearance of the 1082 cm-1 mode at the expense of the 1035 cm-1 feature. At the same time, features at 1164, 1461, and 2963 cm-1 remain unchanged. After heating to 450 K, the intensities of the modes related to the surface methoxy begin to decrease while three other features at 446, 643, and 758 cm-1 become more prominent. By 600 K, there are no remaining traces of methoxy species. The features at 446, 643, and 947 cm-1 are related to the W-C and W-O features from the complete decomposition of methanol as those observed on the C/W(111) surface in Figure 4.
4. Discussions 4.1. Product Yields. On the clean W(111) surface, hydrogen, CH4, and CO are the gas-phase products, and atomic carbon and oxygen are the remaining surface species after TPD measurements to 600 K. The reaction pathways on W(111) are as follows: ∆
xCH3OH 98 xC(a) + xO(a) + 2xH2(g) ∆
yCH3OH 98 yCO(g) + 2yH2(g) ∆
zCH3OH 98 zCH4(g) + zO(a)
(5) (6) (7)
10042 J. Phys. Chem. B, Vol. 105, No. 41, 2001
Hwu et al. TABLE 3: Product Yield of Methanol on W(111), C/W(111), and O/C/W(111) Surfaces clean W(111)
reaction pathways complete decomposition activity per W(111) atom (%) CO activity per W(111) atom (%) CH4 activity per W(111) atom (%) total # of CH3OH reacting per W(111) atom
C/W(111) O/C/W(111)
0.350 (85) 0.155 (55)
0.050 (21)
0.048 (12) 0.087 (31)
0.16 (68)
0.013 (3)
0.038 (14)
0.026 (11)
0.411
0.280
0.236
To determine the product yields on the C/W(111) surface, one can simply determine the hydrogen and CO TPD peak areas relative to that from W(111). As before, the identical reaction pathways can be written as follows: ∆
aCH3OH 98 aC(a) + aO(a) + 2aH2(g) ∆
bCH3OH 98 bCO(g) + 2bH2(g) ∆
cCH3OH 98 cCH4(g) + cO(a)
(8) (9) (10)
On the bases of TPD data from Figure 1, the following relationships can be used to find “a”, “b”, and “c”: C/W(111)
Figure 6. HREEL spectra which monitor the thermal decomposition of 1 L CH3OH on oxycarbide-modified W(111) following adsorption at 90 K.
The value of “x” in eq 5 can be determined by using AES ratios obtained after TPD experiments to 600 K. At this temperature, AES results reveal a C/W atomic ratio ∼0.35, or “x”. Since AES results after heating the CH3OH/W(111) layer to 1200 K show that no carbon remains on the surface, the C/W ratio of ∼0.35 corresponds to the CO desorption peak at 861 K in Figure 1i. Next, by comparing the smaller CO peaks at 229 and 417 K to the 861 K peak, we determined that the value of “y” in eq 6 is ∼0.048. Last, the value of “z” from eq 7 can be estimated based on the TPD area ratio of CH4/CO. By considering different ion gauge sensitivity factors for CO (1.05) and CH4 (1.6),37 we obtain the mass spectrometer intensity ratio of CH4/CO to be ∼1.26 at equal pressures of CO (1.05 × 10-8) and CH4 (1.6 × 10-8). From this ratio we determine the value of “z” to be 0.013, leading to the following selectivity values:
selectivityCH4 ≈ 0.013/(0.013 + 0.35 + 0.048)100% ) 3.2% selectivityCO ≈ (0.048/0.411)100% ) 11.7% selectivitycomp.decomp. ≈ (0.35/0.411)100% ) 85.1% The dominant reaction pathway (∼85% selectivity) of methanol on clean W(111) is the complete dissociation to form surface carbon, oxygen and gas-phase hydrogen. The remaining methanol decomposes to CO and hydrogen (∼12%), and to methane and atomic oxygen (∼3%). Furthermore, the overall activity on the W(111) surface, defined as the total number of CH3OH undergoing decomposition, is determined to be 0.411 (x + y + z) CH3OH per W atom. The activity and selectivity values are summarized in Table 3.
a + b areaH2 ≈ 0.608 f a + b ) 0.242 (11) ) x+y areaW(111) H2
C/W(111)
b areaCO ) [temp < 600 K] ≈ 1.8478 f b ) 0.087 y areaW(111) CO
(12) C/W(111)
c areaCH4 ) ≈ 2.9325 f c ) 0.038 z areaW(111)
(13)
CH4
From eqs 11-13, we then determine that a + b + c equals 0.280, and that ∼55.4% of the methanol completely decomposes while ∼31% and ∼13.6% react to CO and CH4, respectively. Similar analysis can then be applied to the O/C/W(111) surface; on this surface, we find that a + b + c equals 0.236, and that only ∼21% of the methanol completely dissociated while ∼68% and 11% react to CO and CH4, respectively. A summary of the above analysis is also included in Table 3. 4.2. Reaction Pathways of Methanol on Clean and Modified W(111) Surfaces. On the clean W(111), C/W(111), and O/C/W(111) surfaces, the O-H bond cleaves to form methoxy at temperatures as low as 90 K. On the basis of the TPD results (Figure 1 and Table 3), there appear to be three main reaction pathways leading to different groups of surface species and gasphase products:
C (ads) + O (ads) + H2 (g)
complete decomposition (1)
CH4 (g) + O (ads)
decomposition of C-O bonds (2)
CO (g) + H2 (g)
decomposition of C-H bonds (3)
The respective percentage yields for the three reaction channels for each surface is summarized in Table 3. Although the
Tungsten Carbides as Electrocatalysts. 1 HREELS results confirm the identical methoxy species on all three surfaces at 90 K, there are noticeable differences in the spectral changes between 230 and 330 K among the three surfaces. For example, the ν(C-O) vibrational mode shifts by approximately 50 cm-1 on the C/W(111) surface between 230 and 330 K. On the O/C/W(111) surface, there appears to be an overlap of two ν(C-O) features at 1035 (original) and 1082 (shifted) cm-1. In contrast, no frequency shift is observed on clean W(111) at the same temperature range. Hrbek et al. observed a shift of the ν(C-O) mode to lower energy when methanol is adsorbed onto Ru(001); these authors attributed this frequency shift to either a weakening of the C-O bond in the methoxy species or to the reduced intermolecular vibrational coupling at lower methoxy coverages.22 In the current study the shift in the ν(CO) mode is to higher energy on both C/W(111) and O/C/W(111). We tentatively attribute the frequency shift of the ν(C-O) mode to a change of the CO bond in methoxy from an inclined (1035 cm-1) to a more perpendicular (1082 cm-1) position. The strength of the C-O bond should be enhanced since a perpendicular C-O bond would exhibit less interaction with the surface, which is consistent with the frequency increase of the ν(C-O) mode. Furthermore, a perpendicular methoxy would favor the production of CH3 and subsequently gas-phase CH4. This is consistent with the TPD results shown in Figure 1, which shows that more CH4 is produced on C/W(111) than clean W(111). Applying the above correlation to the O/C/W(111) surface, one can reason that since the gas-phase CH4 yield is on the order of C/W(111) > O/C/W(111) > W(111), methoxy is likely be in both the inclined and perpendicular orientations; this is consistent with the observation in Figure 6, which shows ν(C-O) modes at both 1035 and 1082 cm-1. Finally, the observation of an increase in the intensity of the ν(W-OCH3) mode at 330 K (at 521 cm-1 on C/W(111) and near 535 cm-1 on O/C/W(111)) is also consistent with the orientation change of the CO bond in methoxy. As demonstrated in an earlier study of CO on Mo(110), the ν(Mo-CO) stretching mode is much more intense for perpendicularly bonded CO than that in the inclined configuration.38 The comparison of HREELS and TPD results suggests a possible correlation between the orientation of the methoxy species and the amount of gasphase the methane product. One possible explanation is that, in the inclined orientation, at least one of the C-H bonds in the CH3 group will be effectively interacting with the surface, leading to the dehydrogenation to CHx (x < 3) species. On the other hand, the relatively weaker interaction between the perpendicular CH3 group and the surface should favor the production of the CH3 group upon the C-O bond cleavage. The CH3 group should be more effective in the production of gas-phase methane than the CHx (x < 3), which might explain the observation of higher methane yield from the more perpendicular methoxy species on C/W(111). However, a more definitive correlation would require a detailed comparative investigation of the dehydrogenation and hydrogenation activities of the methoxy species on W(111) and C/W(111). 4.3. Comparison with Methanol Decomposition on W(100) and C/W(100) Surfaces. In a detailed investigation,32 Ko and co-workers have shown that low coverages of CH3OD on clean W(100) yielded hydrogen, carbon, and oxygen. At higher exposures of methanol, additional products such as methane, formaldehyde, and methanol (OD and OH) were observed. Ko et al. observed the desorption of CH3OD and CH3OH at 370 and 490 K, suggesting the presence of a stable methoxy intermediate.32 If the numerical analysis by these authors is
J. Phys. Chem. B, Vol. 105, No. 41, 2001 10043 converted to identical units as in the current work, the product selectivity is as follows: 79% toward complete decomposition, 5% toward CO, 7% toward CH4, 7% toward methanol-OH, and 2% toward formaldehyde. Combining the above results to our derivations in section 4.1, the product distributions of methanol on clean W(100) and W(111) appear to be very similar. Ko et al. then prepared a carbide surface on W(100) by carburization; this surface subsequently showed greater yields of hydrocarbon products even though the overall methanol decomposition decreased from ∼97% on the clean surface to ∼44% on the C/W(100) surface.32 Additionally, the C/W(100) surface no longer exhibits the complete decomposition pathway,32 which is distinctly different from the W(100) surface (79% selectivity), and more importantly, the C/W(111) surface. Again, if converting Ko et al.’s analysis to the same units as presented in this work, the product distribution of CH3OD on C/W(100) is as follows: 30% toward CO, 28% toward CH4, 33% toward CH3OH, 6% toward formaldehyde, and 3% toward CO2, H2O, and methyl formate. In contrast, the product selectivity in the current work on C/W(111) is 55% toward complete decomposition, 31% toward CO, and 14% toward methane. The above comparison demonstrates the structure sensitivity of C/W(100) and C/W(111) surfaces. 4.4. Comparison with Methanol Decomposition on Pt and Ru Surfaces. In this section, we will briefly compare our results with those on the two commonly used Pt group metal surfaces in fuel cells: Pt and Ru. Comparing against previous UHV studies of methanol on Pt and Ru surfaces, the results presented in this work show that C/W(111) and O/C/W(111) surfaces are very active toward the decomposition of methanol. On Pt surfaces, methanol adsorbs without dissociation at temperatures ∼100 K. Slight heating to 140 K desorbs the multilayer, followed by the molecular desorption of the monolayer at 180 K.19,39-43 Prior vibrational studies also show that the spectrum of methanol on Pt is nearly identical to the gas-phase molecule.19,43 Aside from the Pt(110)-(1 × 2) surface, the only cases where methanol decomposition is observed by coadsorbing oxygen onto either Pt(111) or Pt(100).19,43-46 The reaction of methanol on Ru surfaces, particularly Ru(001), have been studied in detail. When clean, Ru(001) adsorbs methanol dissociatively at low coverages and molecularly at high coverages. The first channel for decomposition involves the cleavage of the OH bond to form methoxy (80% selectivity); additionally, the CO bond can also be broken to produce water, hydrogen and surface carbon (20% selectivity). When heated to above 220 K, the methoxy intermediate either recombines with hydrogen followed by desorption (33% selectivity), or decomposes to form adsorbed CO and hydrogen.22 When preadsorbing the Ru(001) surface with a low coverage of oxygen (atomic O/Ru ratio ∼0.25), methoxy formation is promoted with respect to the clean surface. Upon heating, the decomposition proceeds only through the dissociation of the CH and OH bonds, leading to the formation of CO (ads) and H2.47 By comparing the results of this work with that from Pt group metals, it appears that the C/W(111) and O/C/W(111) surfaces are at least as active as Pt group surfaces toward methanol decomposition. These results suggest that tungsten carbides are potential materials as anode catalysts for direct methanol fuel cells. More discussions will be included in the companion paper regarding the dissociation and reaction of water and CO on C/W(111) and O/C/W(111).48 5. Conclusions Three reaction pathways were identified following the decomposition of methanol on W(111), C/W(111), and O/C/
10044 J. Phys. Chem. B, Vol. 105, No. 41, 2001 W(111) surfaces. All three pathways were present on each surface, but with varying selectivities. The C/W(111) surface showed 14% selectivity toward methane, with 31% and 55% going to CO and complete decomposition, respectively. After modifying the C/W(111) surface with atomic oxygen, the selectivity values on O/C/W(111) change to 21% complete decomposition, 68% CO, and 11% methane. The results on C/W(111) and O/C/W(111) surfaces are very different from that on clean W(111), where the selectivity toward total decomposition is at 85%, with 12 and ∼3% selectivity toward CO and CH4, respectively. In terms of overall activity, the clean W(111) surface is the most active, with 0.411 CH3OH molecules reacting on a per W atom basis; after modifying the clean surface with carbon and oxygen, the overall activity drops to 0.280 and 0.236 CH3OH per W atom for C/W(111) and O/C/W(111), respectively. These results show that the carbide surfaces are highly active toward the decomposition of methanol. The behavior of CO and H2O over carbide surfaces will be the subject of a following paper to further illustrate that tungsten carbides possess the necessary catalytic activity as potential substitutes for Pt-based electrodes. Acknowledgment. We acknowledge financial support from the Department of Energy (DOE/BES Grant No. DE-FG0200ER15014). The authors acknowledge partial financial support from DuPont and Delaware Research Partnership (DRP). One of us (H.H.H.) also acknowledges financial support from the Dean’s Fellowship at the University of Delaware. References and Notes (1) Hamnett, A. Catal. Today 1997, 38, 445. (2) Parsons, R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (3) Acres, G. J. K.; et al. Catal. Today 1997, 38, 393. (4) Hamnett, A.; Kennedy, B. J. Electrochim. Acta 1988, 33, 1613. (5) Janssen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976, 21, 869. (6) Campbell, S. A.; Parsons, R. J. Chem. Soc., Faraday Trans. 1992, 88, 833. (7) Stonehart, P. In Electrochemistry and Clean Energy; Drake, J., Ed.; Royal Society of Chemistry: Cambridge, 1994. (8) Fruhberger, B.; Chen, J. G. J. Am. Chem. Soc. 1996, 118, 11599. (9) Liu, N.; Rykov, S. A.; Hwu, H. H.; Buelow, M. T.; Chen, J. G. J. Phys. Chem. B 2001. In press. (10) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides; Blackie Academic and Professional: Glasgow, 1996. (11) Chen, J. G. Chem. ReV. 1996, 96, 1477. (12) Fruhberger, B.; Chen, J. G. Surf. Sci. 1995, 38, 342. (13) Zhang, M.; Hwu, H. H.; Buelow, M. T.; Chen, J. G. Catal. Lett. 2001, submitted.
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