Potential Application of Tungsten Carbides as Electrocatalysts. 2

The decomposition of water and CO over clean and carbide-modified W(111) is studied by using temperature-programmed desorption (TPD), high-resolution ...
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J. Phys. Chem. B 2001, 105, 10045-10053

10045

Potential Application of Tungsten Carbides as Electrocatalysts. 2. Coadsorption of CO and H2O on Carbide-Modified W(111) Henry H. Hwu, Brian D. Polizzotti, and Jingguang G. Chen* Center for Catalytic Science and Technology, Department of Materials Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: April 27, 2001; In Final Form: July 11, 2001

The decomposition of water and CO 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). On both clean and modified W(111) surfaces, the activity toward the decomposition of water is found to be significantly higher than Pt group metals. For the CO experiments, both molecular and dissociative adsorption are observed on W(111) and C/W(111). Approximately 52% and 10% of the adsorbed CO dissociates to produce atomic oxygen and carbon on W(111) and C/W(111), respectively. In contrast, CO molecules undergo reversible desorption on oxygen-modified C/W(111) at temperatures as low as 242 K. Finally, coadsorption experiments of water and CO on C/W(111) show that the presence of surface hydroxyls hinders the adsorption of CO, and that only trace amount of gas-phase CO2 is detected.

1. Introduction As interest in the development of the direct methanol fuel cell (DMFC) increases, one needs to assess the feasibility of its commercialization by accounting for the two disadvantages of Pt/Ru electrocatalysts: high costs and limited supplies.1 In the preceding paper, we examined the effectiveness of tungsten carbides toward the oxidation of methanol.2 This paper is a continuation of that investigation by studying the reactivity of tungsten carbides toward water and carbon monoxide. In general, the anode for the DMFC catalyzes three reactions:3,4

CH3OH f CO(ads) + 4H+ + 4e-

(1)

H2O f OH(ads) + H+ + e-

(2)

OH(ads) + CO(ads) f CO2v +H+ + e-

(3)

The effectiveness of tungsten carbide toward reaction 1 was studied in the preceeding paper,2 where we found the carbide surfaces to be highly active toward methanol oxidation. The goal of the current paper is to examine reactions 2 and 3, and to compare the results for tungsten carbide surfaces to that of Pt group metal surfaces. Additionally, experiments will also be carried out to determine whether adsorbed water can react with CO to form CO2 from the tungsten carbide surface. The chemistry of methanol and CO on the W(100)-(5 × 1)-C surface had been studied extensively by several authors.5-8 The interaction of methanol with the W(100) and the W(100)(5 × 1)-C surfaces was briefly summarized in part 1 of this investigation.2,5 TPD studies of CO on clean W(100) showed facile C-O bond scission at temperatures below 300 K.6 In contrast, no CO dissociation was observed on the W(100)-(5 × 1)-C surface; instead, only a single molecular desorption feature was present at ∼360 K.6 HREEL studies show two losses for a CO-saturated W(100)-(5 × 1)-C surface: (1) a ν(M-CO) * Corresponding author. E-mail: [email protected]. Fax: 302-831-4545.

mode at 383 cm-1 and (2) a ν(C-O) mode at 2100 cm-1.7 Additional results lead to the conclusion that CO is bonded normal to the surface in an atop site.7 The techniques used to conduct the current study involve temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). We will first report the TPD and HREELS results of H2O and CO on clean and various modified W(111) surfaces, followed by the findings from experiments involving coadsorbed CO and water. For discussion, we will provide quantitative analysis of the reaction products, and compare the activities of C/W(111) surfaces with C/W(100), Pt, and Ru toward CO and water. 2. Experimental Section 2.1. Techniques. The ultrahigh-vacuum (UHV) chamber used in the current study has been described in detail previously.9 Briefly, it is a three-level stainless steel chamber (base pressure of 4 × 10-10 Torr) which is equipped with Auger electron spectroscopy (AES), 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 × 105-2 × 106 cps, and the spectral resolution was between 35 and 50 cm-1 fwhm (fullwidth 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. Deionized water was purified by successive

10.1021/jp0116205 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/13/2001

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Figure 1. Temperature-programmed desorption spectra of water and hydrogen obtained following a 1 L exposure of water on clean W(111), C/W(111), and O/C/W(111).

freeze-pump-thaw cycles prior to their use. The purity was verified in situ by mass spectrometry. Carbon monoxide and oxygen were obtained from Matheson (99.99% pure) and was used without further purification. Doses are reported in langmuirs (1 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). The preparations of the clean and various modified W(111) surfaces have been described in detail previously.2,9-11 Briefly, the carbide surfaces, C/W(111), were prepared by exposing W(111) to ethylene at 90 K and then flashed to 1200 K; generally these procedures were repeated for three cycles, and produced 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). 3. Results and Interpretation 3.1. Reactions of Water. 3.1.1. TPD Results. In Figure 1 we show the hydrogen and water TPD spectra obtained following a 1 L exposure on the clean W(111), C/W(111), and O/C/W(111) surfaces. The clean W(111) surface is very active toward the dissociation of water. From Figure 1a, we observe an asymmetric molecular desorption feature at 168 K, as well as two hydrogen desorption peaks at approximately 213 and 267 K. The initial sharp H2 feature is likely due to the removal of a hydrogen from the decomposition of H2O to produce adsorbed OH, while the broader peak at 267 K is attributed to the subsequent decomposition of OH to hydrogen and surface

TABLE 1: Observed Vibrational Frequencies (cm-1) for Adsorbed Water and Hydroxyl Species mode ν(M-O) δ(HOH) δ(OH) ν(HOH) ν(OH)

O/Pt(111) O/Pd(100) Al(111) C/W O/C/W [27] [20] [12, 13] W(111) (111) (111) 430

445

1015 3480

930 3250

765 1620

893 (?) 1644

1691

1678

3450 3745

3578 3687

3599 3748

3572 3693

oxygen. These observations are consistent with the HREELS measurements, which will be discussed later. Similar to that on clean W(111), the dissociation of water is observed on both the C/W(111) and the O/C/W(111). On the carbide surface, the molecular desorption feature is relatively sharp and its temperature is the lowest among the three surfaces at 163 K. There appears to be three distinct hydrogen peaks at approximately 178, 295, and 377 K. The molecular desorption feature from the O/C/W(111) surface resembles that of the clean surface in both peak shape and temperature (∼170 K). However, the hydrogen peaks at 179 and 318 K are more similar to C/W(111) than to W(111) in terms of relative intensity and general spectral shape. 3.1.2. HREELS Results. The on-specular HREEL spectra of water adsorbed on clean W(111), C/W(111), and O/C/W(111) surfaces are shown in this section. The exposures of water are made with the crystal temperature at 90 K; the adsorbed layer is then heated to the indicated temperatures, held briefly for 5 s, and allowed to cool before the HREEL spectra are recorded. Table 1 summarizes the relevant vibrational assignments for these adsorbed layers. Finally, the height of the elastic peaks in all spectra has been normalized to unity, and that the expansion factor for each individual spectrum represents the multiplication factor relative to the elastic peak. Figure 2 shows the on-specular spectra following the decomposition of 1 L of water on W(111). At 90 K, the following features are observed: a weak feature at 304 cm-1, overlapping

Tungsten Carbides as Electrocatalysts. 2

Figure 2. HREEL spectra which monitor the thermal decomposition of 1 L H2O on clean W(111) following adsorption at 90 K.

broad bands at 474, 663, and 893 cm-1, a peak at 1644 cm-1, and a broad feature shouldered on a sharper peak at ∼3578 and 3687 cm-1, respectively. The two features at 1644, δ(HOH), and 3578 cm-1, ν(OH‚‚‚O), are characteristic of molecular H2O.12,13 The 3687 cm-1 feature is related to the ν(OH) of the surface hydroxyl groups; the presence of this vibrational mode indicates that some of the adsorbed molecules begin to dissociate

J. Phys. Chem. B, Vol. 105, No. 41, 2001 10047 even at 90 K. The remaining features below 1000 cm-1 are related to hindered rotation (304 cm-1), hindered translations (474 and 893 cm-1), and ν(W-OH2) (609 cm-1) of molecular H2O.13 There are essentially no spectroscopic changes when the surface is heated to 140 K, which is consistent with the TPD results. After heating to 230 K, however, the following changes are observed: (1) the vibrational modes that are related to molecular H2O begin to diminish, (2) the intensity of the surface ν(OH) feature increases and shifts to 3619 cm-1, and (3) welldefined features at 446, 609, and 764 cm-1 start to appear. Upon further heating of this surface, one can identify the same three features between 400 and 800 cm-1 as well as the disappearance of the ν(OH) mode. At 450 K, the spectrum is characteristic of an oxygen-modified W(111) surface.14 The HREELS results indicate that water decomposes on clean W(111), producing surface oxygen and gas-phase hydrogen; these observations agree well with the TPD results. Parts a and b of Figure 3 show the on-specular spectra following the decomposition of 1 L of water on the C/W(111) and O/C/W(111) surfaces, respectively. At 90 K, water decomposition also begins on the carbide surface, as evidenced by the presence of the 3748 cm-1 ν(OH) feature. The other vibrational modes include two peaks at 1691 and 3599 cm-1 (molecular H2O), and the less resolved bands at energies below 1000 cm-1 (hindered rotation and translations). The peak positions remain the same after heating to 140 K, although the intensities associated with molecular H2O at below 1691 cm-1 all increase, most likely due to the ordering of adsorbed H2O upon heating to 140 K. By 230 K, the molecular H2O modes disappear, and the only well-resolved peak remaining is the ν(OH) feature at 3639 cm-1. The two broad features at 521 and 656 cm-1 are most likely related to ν(W-O) modes. After heating to 330 K, the ν(OH) diminishes, indicating the dissociation of the surface OH groups. Finally, at 450 K, we only observe significant modes at 656 and 967 cm-1, which are indicative of carbide surface with residual oxides on the

Figure 3. HREEL spectra which monitor the thermal decomposition of 1 L H2O on (a) C/W(111) and (b) O/C/W(111) following adsorption at 90 K.

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Figure 4. Temperature-programmed desorption spectra of CO and CO2 obtained following a 1 L exposure of CO on clean W(111), C/W(111), and O/C/W(111).

surface.14 The small peaks at ∼1624 and ∼2976 cm-1, observed at 330 and 450 K, are related to the adsorption of trace amounts of water and hydrocarbons, respectively, from the background during data acquisition. The HREEL spectra for water on the O/C/W(111) surface are very similar to that on C/W(111). As with the case of clean W(111) and C/W(111), we also observed some decomposition of water at 90 K. By 230 K, most of the H2O molecules have either molecularly desorbed or dissociated into surface hydroxyl groups and hydrogen. When heated to 330 K, the ν(OH) feature nearly disappears. Similar to the C/W(111), there is also a broad feature centered at 656 cm-1 for the O/C/W(111) surface with a shoulder at ∼950 cm-1 at all temperatures. Combining results from HREELS and TPD studies, we conclude that the decomposition of water occurs on the clean W(111), C/W(111), and O/C/W(111) surfaces, and that the decomposition occurs via identical pathways. 3.2. Adsorption and Reactions of CO. 3.2.1. TPD Results. Figure 4 shows TPD spectra following 1 L adsorptions of CO on the W(111), C/W(111), and O/C/W(111) surfaces. Both CO and CO2 are monitored in the TPD experiments, but only CO is observed as the desorption product. On the clean W(111), most of the monolayer CO desorbs at 292 K; in addition, a fraction of CO decomposes on W(111), as evidenced by the recombinanting desorption at approximately 874 K. The molecular desorption feature of CO on the C/W(111) surface appears to be an overlapping of two peaks centered at 330 and 355 K. Unlike the clean W(111) surface, there seems to be very little CO dissociation on C/W(111), as indicated by the relatively weak high-temperature recombination peaks. When the C/W(111) surface is modified by oxygen, the CO desorption temperature is shifted to 284 K, which is lower than the three C/W(111) surface. Additionally, the desorption feature is a relatively sharp peak at 284 K with a shouldered peak on each side (242 and 355 K). When heated to temperatures above 1000 K, one can observe another relatively intense CO peak, which

TABLE 2: Observed Vibrational Frequencies (cm-1) for Adsorbed CO mode ν(M-C) ν(M-O) ν(CO)

O/Pt(111) O/Pd(100) Mo(110) C/W O/C/W [40] [33] [15] W(111) (111) (111)

2063

355 390 1995

400 565 1970

406 676 2063

379 2070

365 643 2097

is not from dissociated CO, but from the recombination of O and C from the O/C/W(111) surface. 3.2.2. HREELS Results. The on-specular HREEL spectra collected following the adsorption of 1 L CO on the clean W(111), C/W(111), and O/C/W(111) surfaces are shown in this section. The exposures of CO are made with the crystal temperature at 90 K; the adsorbed layers are then heated to the indicated temperatures, held briefly for 5 s, and allowed to cool before the HREEL spectra are recorded. Table 2 summarizes the relevant vibrational assignments for these overlayers. The HREELS results of the thermal behavior of a CO/W(111) overlayer are shown in Figure 5. At 90 K, two dominant peaks are observed at ∼400 and ∼2060 cm-1. In addition, weaker features are also present at 1084, 1407, and 1603 cm-1, as well as some weak peaks at 2970, 3335, and 3585 cm-1, from the adsorption of hydrocarbon and water from the chamber background. The HREELS results correspond well with previous studies of CO on other early transition metals, most notably on Mo(110).15 The 2063 cm-1 feature is assigned to the ν(CO) mode of terminally bonded CO on the W(111) surface.16 The 400 cm-1 feature corresponds to the ν(W-CO) mode while the modes between 1000 and 1600 cm-1 are attributed to ν(CO) vibrations from inclined CO species.17-19 When heated to 230 K, no significant spectral changes are observed aside from a slight increase in the 1603 cm-1 peak. By 330 K, most of the 2063 cm-1 feature disappears, which corresponds well with the desorption of molecular CO at 292 K in the TPD measurement. Additionally, most of the 1084, 1407, and 1603 cm-1 features disappear with the appearance of two intense low-energy broad

Tungsten Carbides as Electrocatalysts. 2

Figure 5. HREEL spectra which monitor the thermal decomposition of 1 L CO on clean W(111) following adsorption at 90 K.

bands at 453 (ν(W-O)) and 676 (ν(W-C)) cm-1.11 These spectroscopic changes indicate that most of the remaining CO dissociate into surface C and O after heating to 330 K. Last, further heating to 450 and 600 K produced no significant changes. Parts a and b of Figure 6 show HREEL spectra following the adsorption of CO on C/W(111) and O/C/W(111) surfaces.

J. Phys. Chem. B, Vol. 105, No. 41, 2001 10049 At 90 K, the ν(CO) mode is observed at 2070 cm-1 for C/W(111) and 2097 cm-1 for O/C/W(111), while the ν(WCO) vibration is at 379 and 365 cm-1 for the C/W(111) and O/C/W(111) surfaces, respectively. In addition, these modified W(111) surfaces are different from that of the clean W(111) in that there are no significant features between 1000 and 1600 cm-1. When the CO/C/W(111) overlayer is heated to 230 K, no major changes are observed except for a small decrease in the 2070 cm-1 peak intensity. For the O/C/W(111) surface, however, heating to 230 K produces a much more noticeable decrease in the ν(CO) mode at 2097 cm-1; this difference between the C/W(111) and the O/C/W(111) surfaces is consistent with TPD results described earlier, where molecular CO desorbs as early as ∼240 K for O/C/W(111), but at a much higher temperature (∼330 K) for C/W(111). Upon heating the CO/C/W(111) layer to 330 K, a significant decrease of the 2070 cm-1 peak is observed without any shift in frequency. On the other hand, the CO/O/C/W(111) overlayer shows a shift of the ν(CO) mode from 2097 to 2036 cm-1 in addition to the decrease in intensity. Furthermore, three low-energy features are observed on both surfaces after heating to 330 K, at ∼400, ∼630, and ∼760 cm-1. For the O/C/W(111) surface, further heating to 450 and 600 K produced no dramatic spectral changes. The same can be stated for the CO/C/W(111) overlayer, with the exception of the appearance of a W-O mode (940 cm-1) at 450 K. The observation of this W-O feature suggests that a fraction of the adsorbed CO undergoes decomposition on the C/W(111) surface; the absence of a prominent recombinative CO peak in the TPD measurement (Figure 4) suggests that the fraction of CO undergoing dissociation is very small. 3.3. Coadsorption of H2O and CO. 3.3.1. TPD Results. The next series of data pertain to the coadsorption experiments of CO and H2O on C/W(111). The preparation of these overlayers are as follows: (1) introduce either low coverage (0.3 L) or near-saturation coverage (1 L) of water on C/W(111) at 90 K, (2) flash the H2O/C/W(111) overlayer to 230 K, the temperature

Figure 6. HREEL spectra which monitor the thermal decomposition of 1 L CO on (a) C/W(111) and (b) O/C/W(111) following adsorption at 90 K.

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Figure 7. Temperature-programmed desorption spectra of CO and CO2 obtained following a 1 L exposure of CO on C/W(111), 0.3 L H2O/C/ W(111), and 1 L H2O/O/C/W(111).

at which we observed the formation of surface OH species (see Figure 3a), and (3) allow the surface to cool to 90 K before exposing the OH/C/W(111) layer to 1 L of CO. Figure 7 shows the TPD results following the adsorption of 1 L CO onto the 0.3 and 1 L H2O modified C/W(111) surfaces. As discussed in section 3.2.1, the CO desorption peak from unmodified C/W(111) surface is two overlapping peaks centered at ∼330 and 355 K. In addition, very little dissociation of CO is observed, as indicated by the lack of an intense recombinative hightemperature peak, and by that no significant desorption of CO2 is detected. On the C/W(111) surface modified by low coverage of water (0.3 L), the CO desorption temperature is reduced to ∼300 K with a shoulder at about 355 K. There is also a very broad CO feature spanning over 300 K, starting at about 700 K, which can be attributed to the recombination of C from the surface and O from the decomposed hydroxyl groups. There is a broad CO2 desorption feature centered at ∼300 K, but more quantitative analysis concludes that the TPD peak area ratio of CO2/ CO is only ∼0.02. Finally, on the C/W(111) surface modified by 1 L of water, the CO desorption temperatures remain at 303 and 355 K (shoulder), but with significantly reduced intensity. Similar to that of the 0.3 L H2O/C/W(111) surface, one can also observe a broad high-temperature CO peak starting at ∼700 K on this surface. As for CO2 desorption, it appears that by increasing the coverage of water on C/W(111), the small amount of CO2 desorption observed on the 0.3 L H2O/C/W(111) surface now disappears. 3.3.2. HREELS Results. Parts a and b of Figure 8 show the HREEL spectra of the thermal behavior of coadsorbed water and CO on C/W(111). Similar to that observed previously (Figure 6a), at 90 K, the 1 L CO on 0.3 L H2O modified overlayer show the ν(W-O) modes at ∼440, 636, and ∼764 cm-1, the ν(CO) mode at 2036 cm-1, and the ν(OH) mode at 3599 cm-1. Heating to 230 K produced almost no spectrocopic change aside from a slight reduction in the intensities of the 2036 and 3599 cm-1 peaks. After heating to 330 K, most of

the CO desorbs and most of the OH dissociates. The diminishing of the 3599 cm-1 peak also corresponds with the appearance of the 927 cm-1 peak, which is a ν(W-O) mode. Subsequent heating to temperatures above 450 K resulted in no major changes. By increasing the water modification from 0.3 to 1 L, the main change is that the intensity of the ν(CO) mode at 2030 cm-1 decreases, suggesting the adsorption capacity of CO on this surface is less than the 0.3 L H2O/C/W(111) surface. The subsequent heating produces similar spectroscopic development as that observed on the 0.3 L H2O/C/W(111) surface. 4. Discussion 4.1. Reaction Pathways of H2O and CO. Aside from molecular desorption, the only products detected after exposing water to clean W(111), C/W(111), and O/C/W(111) are gasphase hydrogen and surface oxygen. After TPD measurements of H2O on W(111), we obtained an atomic O/W ratio of ∼0.32 from AES. The activity of water decomposition on clean W(111) is therefore ∼0.32 H2O molecules per W atom. Next, by comparing the H2 peak areas of C/W(111) and O/C/W(111) to that of clean W(111), we obtain the activity of water dissociation on C/W(111) and O/C/W(111) to be 0.18 and 0.095 molecules per W atom, respectively. On the basis of these results, we can conclude that while all three surfaces remain active, modifying the W(111) surface with carbon and oxygen significantly reduces its activity toward the dissociation of water. The reaction pathways of CO on the three surfaces are very straightforward: (1) molecular adsorption and desorption and (2) dissociation into surface C and O, which recombines to desorb as CO at high temperatures. ∆

CO(ads) 98 CO(g) ∆

(4) ∆

CO(ads) 98 C(ads) + O(ads) 98 CO(g)

(5)

To determine the selectivity toward each pathway, we relied

Tungsten Carbides as Electrocatalysts. 2

J. Phys. Chem. B, Vol. 105, No. 41, 2001 10051

Figure 8. HREEL spectra which monitor the thermal decomposition of 1 L CO on (a) 0.3 L H2O/C/W(111) and (b) 1 L H2O/C/W(111) following adsorption at 90 K.

on the results obtained from part 1 of this work.2 In that paper we were able to determine the amount of CO desorption from the decomposition of methanol on a per W atom basis. By comparing the CO desorption peak in Figure 4 to that from the decomposition of methanol, we estimate that the TPD areas of the 292 and 874 K peaks correspond to 0.068 and 0.072 CO molecules per W atom, respectively. These values indicate that ∼52% of the CO on W(111) decompose into adsorbed carbon and oxygen. We then compared the peak areas of CO desorption from the C/W(111) surface to those from the clean W(111) surface. Such comparison revealed that only 0.012 CO molecules per W atom (10%) dissociate into atomic O and C, and 0.103 CO molecules per W atom (90%) desorb molecularly. For the O/C/W(111) surface, we are unable to accurately assess the selectivity toward CO dissociation because of the complications in differentiating between the recombinant peaks of the CO adsorbates and the O/C/W(111) surface; the amount of lowtemperature CO desorption is estimated, based on the peak area of CO, to be ∼0.13 molecules per W atom. In the third part of this study we examined the effects of preadsorbing water before introducing CO onto the C/W(111) surface. By comparing the TPD peak areas of the coadsorbed overlayers with that of CO on unmodified C/W(111), we estimate that the amount of low-temperature CO desorption decreases with increasing H2O exposure. For the 0.3 L H2O/1 L CO on C/W(111), CO desorption near 300 K is only ∼55% of that on the surface without water (0.056 CO molecules per W atom). When H2O exposure is increased to 1 L, the CO desorption in that temperature regime drops even further to ∼18% (0.019 CO molecules per W atom). Once again, it was difficult to distinguish whether the high-temperature recombinant CO feature is from dissociated adsorbates or from decomposed water reacting with the carbide surface. The results presented in this section are summarized in Table 3a,b. Last, although a weak and broad CO2 peak is observed from the reaction of CO and OH groups on the 0.3 L H2O/C/W(111) surface, the amount of CO2 is negligible; the ratio of TPD peak areas of CO2/CO on this surface is ∼0.02.

TABLE 3: Activities of Clean W(111), C/W(111), and O/C/W(111) toward Decomposition of Water and Those of CO on W(111), C/W(111), O/C/W(111), and H2O/C/W(111) Surfaces (a) Activity of Clean W(111), C/W(111), and O/C/W(111 toward Decomposition of Water

surface

activity (molecules per W atom)

% decomposition relative to clean W(111)

W(111) C/W(111) O/C/W(111)

0.32 0.18 0.095

100 55 30

(b) Activity of CO on W(111), C/W(111), O/C/W(111), and H2O/C/W(111) surfaces

surface

molecular desorption: CO per W atom (%)

recombinant desorption: CO per W atom (%)

W(111) C/W(111) O/C/W(111) 0.3 L H2O on C/W(111) 1.0 L H2O on C/W(111)

0.068 (48.5) 0.103 (90) 0.133 0.056 0.019

0.072 (51.5) 0.012 (10) N/Aa N/A N/A

a

N/A ) not available.

4.2. Comparing C/W(111) to C/W(100). As mentioned previously, the chemistry of CO on the W(100) surfaces had been studied by Benziger et al.,6 Friend et al.,7 and Stevens et al.8 TPD studies of CO on clean W(100) showed that, at low coverages, all of the adsorbed CO dissociates to surface carbon and oxygen, which subsequently recombines when heated above 1000 K.6 At saturation exposure, an additional CO desorption peak is observed at 377 K.6 Benziger and co-workers then studied the interaction between CO and various C/W(100) surfaces; these authors reported that the amount of CO dissociation decreases with increasing extent of carburization.6 Benziger et al. concluded that ∼36% of the adsorbed CO dissociated on clean W(100); in contrast, the W(100)-(5 × 1)-C surface did not show any activity toward dissociation.6 For

10052 J. Phys. Chem. B, Vol. 105, No. 41, 2001 comparison, in the current study, approximately 10% of the adsorbed CO dissociates on the C/W(111) surface. This observed structure sensitivity is expected because C/W(111) has a more open-structure surface than C/W(100). The open-structured nature of the C/W(111) surface should facilitate more direct interaction between CO and W atoms, which in turns lead to a higher degree of CO dissociation than on the more closely packed C/W(100) surface. 4.3. Comparing C/W(111) to Pt and Ru Surfaces. As stated in the Introduction, the motivation of the current study is to explore the possible application of tungsten carbides in fuel cells. In this section a comparison of the reactivity of C/W(111) will be made with that of Pt and Ru surfaces, which are currently the most promising electrocatalysts used in direct methanol fuel cells. 4.3.1. Reaction of H2O. Adsorption studies of water on Ru(0001) were reported by Kretzschmar et al.,20 Thiel et al.,21 Doering and Madey,22 and Madey and Yates.23 The thermal desorption results of this system showed that at very low coverage of water, two molecular desorption states were observed at 155 and 200 K; at higher coverages, however, the 155 K feature remains unchanged while the 200 K feature splits into two distinct peaks at 180 and 215 K.22 Once the Ru(0001) surface has been preadsorbed with submonolayer amounts of O2 prior to exposure to water, only the low (155 K) and the high (215 K) temperature desorption states were observed. As the precoverage of O2 increased to beyond saturation level, the high-temperature desorption peak disappeared and was replaced by the intermediate peak at ∼180 K.22 HREEL studies further revealed that the adsorption of oxygen prior to water exposures causes the water to form adsorbed structures different from H2O on clean Ru(0001).21 In either case, the interactions of water with either clean or oxygen-modified Ru(0001) did not result in any significant dissociation products. The interaction of water on platinum surfaces has been studied in detail both in UHV24-28 and in electrochemical29-31 experiments. At low coverages, water desorbs from Pt surfaces at two temperatures, ∼180 and ∼195 K; at higher coverages, three physisorbed states can be observed which are ascribed to multilayer ice, a bilayer region, and a nonbilayer region.24 Furthermore, another higher temperature state of water desorption, at 185-196 K on Pt(111), is assigned to coadsorbed hydroxyls at defects.24 From HREEL studies conducted on the H2O/Pt(111) overlayer, dominating features near 3440 and 690 cm-1 were observed. While no significant decomposition was detected for water on unmodified Pt(111), one can induce the dissociation of water to adsorbed OH and H can be promoted by preadsorbing oxygen onto Pt(111).26 While the dissociated species were confirmed by HREELS results, the intermediates eventually recombine to desorb as water and leave behind adsorbed oxygen at elevated temperatures.26 H2O adsorption on Pt(100) has also been studied by Ibach and Lehwald.25 While the authors reported hydrogen bonding interactions between the water molecules and the Pt(100) surface atoms, no gas-phase products were observed aside from molecular H2O.25 Compared to the Pt and Ru surfaces, the results reported here indicate that the C/W(111) and O/C/W(111) surfaces have significantly higher activity toward the dissociation of water. While hydroxyl intermediates were detected on the surfaces of oxygen-modified Pt and Ru substrates, these adsorbed species eventually react to re-form water and surface oxygen. For the C/W(111) and O/C/W(111) surfaces, however, the decomposition of water and surface O-H groups is clearly indicated by the production of gas-phase hydrogen.

Hwu et al. 4.3.2. Adsorption and Desorption of CO. Numerous studies have also been conducted for CO on Ru and Pt surfaces.32-41 In UHV experiments with low to saturation coverage of CO on Ru(0001), CO desorption temperature is between 450 and 500 K; at elevated coverages, an additional desorption peak appears at about 400 K.33,37 For adsorbed CO on unmodified Ru(0001), no other desorption products were detected. On the Pt(001) surface, major CO desorption peaks at ∼475 and ∼550 K, and a small 400 K peak were observed.38 Crossley and King also reported a unique behavior for the Pt(001) surface in that the 550 K peak shifts to even higher temperatures with increasing coverage; this unusual shift suggests that the attractive interaction between adsorbed species increases with coverage,38 which is also consistent with vibrational studies.38,39 On the other hand, the desorption of CO on Pt(111) show only a single desorption peak between 420 and 505 K for all coverages, and that the peak shifts to lower temperatures with increasing coverage.39,41 As an alloy, the Pt/Ru bimetallic system has attracted special interest as a CO-tolerant anode catalyst in low-temperature fuel cell applications.42,43 Consequently, various UHV experiments have also been carried out to investigate the interaction of CO on Pt/Ru alloy surfaces. These investigations showed that the onset of CO desorption on Pt monolayer island covered Ru(0001) is shifted significantly lower than that of either pure surface.33 The reduction in CO desorption temperature from ∼400 to ∼325 K indicates that the adsorption energy on the bimetallic surface is reduced; similar observations were also reported for other Pt/metal bimetallic systems, where the energetic differences were attributed to electronic modifications resulting from two chemically different substrates.44 In general, CO begins to desorb from unmodified Pt group metals at ∼400 K. Because of the relatively high energy required to desorb CO from Pt group metals, fuel cells utilizing noble metal electrodes typically operate at temperatures above 360 K to avoid CO poisoning.1 On the other hand, our results on the C/W(111) surface show that the desorption of CO starts at ∼330 K. More importantly, the presence of oxygen on the O/C/W(111) surface further reduces the CO adsorption energy, leading to the onset of CO desorption at 242 K. The observation of the significant decrease in desorption temperature suggests the potential application of tungsten carbides at room temperature, without significant CO poisoning of the catalyst surfaces. 4.3.3. Oxidation of CO. As supported catalysts under atmospheric conditions, both Pt and Ru metals are known to be active toward the oxidation of CO.32 In fact, researchers found that supported and unsupported Ru catalysts are much more active toward CO oxidation than Pt, Pd, Ir, or Rh at high pressures.33,34,45 Under UHV conditions, however, Ru(0001) was found to be the least reactive.35,36 Studies have shown that oxygen precovered Ru(0001) exhibits dramatically increased oxidation activity that is not observed on unmodified Ru(0001). The reaction rate of CO oxidation exhibited a strong dependence on the coverage of oxygen.34 In addition, Thomas and Weinberg46 have shown that the presence of oxygen shifts the ν(CO) mode to higher frequency while the M-CO stretch moves lower; their observations indicate that the metal-CO bond is weakening, thus leading to lower desorption temperatures of CO.34,46 Similar to the Ru surfaces, the Pt(100) and (111) surfaces can become active toward CO oxidation when adsorbed or gasphase oxygen is present.47 In particular, Barteau and co-workers were able to detect significant CO2 desorption from either (1) adsorbing oxygen prior to CO exposures or (2) exposing the CO/Pt(001) overlayer to O2. In either case, the desorption

Tungsten Carbides as Electrocatalysts. 2 temperatures for CO2 was ∼300 K while CO still desorbs between 500 and 550 K.47 TPD measurements probing the interactions of hydroxyl radicals and CO on the Pt(111) surface have also been reported by Weibel et al.48 Using an intense molecular beam, OD radicals were dosed onto Pt(111) followed by exposures to CO.48 When the radicals were dosed with the surface held at 275 K, three CO2 desorption features were observed at 340, 440, and 650 K, with the 340 and the 440 K peaks being the dominating features.48 If OD was dosed with the Pt(111) crystal at 150 K, two minor CO2 peaks were observed at 185 and 230 K in addition to the previous three desorption peaks.48 These results clearly demonstrated that the oxidation of CO to CO2 could be facilitated by the coadsorption of surface hydroxyl groups. In general, Pt group metals exhibited oxidative activity toward CO, which can be further enhanced by covering the surface with submonolayer quantities of oxygen or surface hydroxyl groups. In contrast, the C/W(111) and the O/C/W(111) surfaces produced no detectable amounts of CO2. Even after coadsorbing water and CO on the C/W(111) surface, only trace amounts of CO2 was observed. One possible explanation is that CO is relatively weakly bonded on C/W(111) (desorbs at ∼330 K) and on O/C/W(111) (desorbs at 242 K). As a result, the desorption of CO occurs before the reactions between CO + O or CO + OH can take place. 5. Conclusions The reaction pathways observed for CO and water on clean W(111), C/W(111), and O/C/W(111) are very straightforward. On all three surfaces, water dissociation is evident by the production of gas-phase H2. On the C/W(111) and O/C/W(111) surfaces, CO desorbs at 330 and 242 K, which is significantly lower than that observed from Pt group metalsurfaces. Coadsorption experiments of CO and water on C/W(111) show no enhancement toward CO oxidation, which might be explained by the low desorption temperature of CO. Since current fuel cells operate at a relatively high temperature to avoid CO poisoning, the electrocatalytic application of carbide-based systems offers potential advantages in their ability to desorb CO at significantly lower temperatures than Pt-based electrodes. Acknowledgment. We acknowledge financial support from Department of Energy (DOE/BES Grant No. DE-FG0200ER15014). Two of us (H.H.H. and B.D.P.) also acknowledge financial support from the Dean’s Fellowship at the University of Delaware. H.H.H. also acknowledges financial support from the Presidential Fellowship from the University of Delaware. References and Notes (1) Stonehart, P. In Electrochemistry and Clean Energy; Drake, J., Ed.; Royal Society of Chemistry: Cambridge, 1994. (2) Hwu, H. H.; Chen, J. G.; Kourtakis, K.; Lavin, J. G. J. Phys. Chem. B 2001, 105, 10037.

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