Modifying Surface Reactivity by Carbide Formation: Reaction

3894

J. Phys. Chem. B 2001, 105, 3894-3902

Modifying Surface Reactivity by Carbide Formation: Reaction Pathways of Cyclohexene over Clean and Carbide-Modified W(111)† N. Liu, S. A. Rykov, H. H. Hwu, M. T. Buelow, and J. G. Chen* Center for Catalytic Science and Technology (CCST), Department of Materials Science and Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: September 19, 2000; In Final Form: January 20, 2001

The decomposition and dehydrogenation of cyclohexene (c-C6H10) are used as probe reactions to compare the surface reactivities of clean and carbide-modified W(111). The reaction mechanisms have been studied using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and near-edge X-ray absorption fine structure (NEXAFS). On the clean W(111) surface, cyclohexene molecules decompose to produce hydrogen, atomic carbon and cyclohexane. In contrast, on the carbide-modified W(111) surface, cyclohexene undergoes primarily dehydrogenation to form benzene and hydrogen. The selectivity to the production of gas-phase benzene on C/W(111) is similar to that observed on the Pt(111) surface.

1. Introduction The carbides of groups IV-VI early transition metals often show intriguing catalytic properties.1-3 For example, transition metal carbides often demonstrate catalytic activities that are similar to those of the more expensive Pt-Group (Pt, Pd, Ir, Rh, and Ru) metals, especially in reactions involving C-H bond transformation, such as dehydrogenation and hydrogenation reactions.1,2 In the past few years, our research group has performed surface science investigations to determine how the reactivities of early transition metal surfaces are modified by the formation of carbides.3-13 For example, we have utilized several probe reactions to differentiate the reactivities of clean and carbide-modified Mo(110) surfaces,7-13 and to directly compare them to those of Pt-group metal surfaces. The probe reactions included the formation of ethylidyne from the decomposition of ethylene,7 the selective activation of the R-CH bonds of cis- and trans-2-butenes to produce 2-butyne,10 and the selective dehydrogenation of cyclohexene12,13 and cyclohexadiene13 to form gas-phase benzene. These results provided clear evidence that the surface reactivities of Mo(110) can be significantly modified by the formation of carbide, and that the reactivities of carbide-modified Mo(110) are very similar to those of Pt-group metals, especially to those of the Pt(111) surface. In the current paper, we report our comparative investigation of the reactivities of clean and carbide-modified W(111) surfaces. Our initial interest in studying carbide-modified W surfaces was to compare the similarities and differences between C/W(110) and C/Mo(110). As discussed in our earlier study on carbide-modified Mo(110),3,7 C/Mo(110) surfaces that are terminated by carbon atoms are inert toward the decomposition of unsaturated hydrocarbons; the C/Mo(110) surfaces become chemically active only after the incorporation of excess surface carbon atoms into the subsurface region (i.e., below the topmost metal layer) via a thermally induced diffusion at temperatures between 900 and 1200 K.7 In our preliminary study of carbide†

Part of the special issue “John T. Yates, Jr. Festschrift”. * Corresponding author. E-mail: [email protected]. Fax: 302-831-4545.

modified W(110)14 we found that the C/W(110) surface was essentially inert toward the decomposition of cyclohexene. Our speculation is that the C/W(110) surface is terminated by carbon atoms instead of by W atoms,14 and that annealing to 1200 K is not sufficient to induce the incorporation of carbon into the subsurface region. In the current investigation we utilized the more open-structured W(111) surface to help the incorporation of carbon atoms into the W lattice. As will be demonstrated in the current paper, after annealing to 1200 K the C/W(111) surfaces are chemically active toward the decomposition of cyclohexene. The selective dehydrogenation of cyclohexene to benzene is a characteristic reaction on Pt-group metal surfaces. For example, the reaction of cyclohexene on Pt(111) has been studied by several research groups using a variety of surface analytical techniques.15-19 Results from these studies indicated that cyclohexene underwent dehydrogenation to produce benzene via a c-C6H9 allyl intermediate, and that the selectivity to convert cyclohexene to gas-phase benzene is approximately 75% on Pt(111).12,17 In the current study, the decomposition and dehydrogenation of cyclohexene were used as probe reactions to directly compare the surface reactivities of W(111) and C/W(111) with Pt(111). We found that the reaction products were hydrogen, atomic carbon and cyclohexane on W(111). In contrast, the primary reaction pathway of cyclohexene on the carbide-modified W(111) surface was the production of gasphase benzene. 2. Experimental Section 2.1. Sample Preparation. The W(111) single crystal, 1.5 mm thick and 10 mm in diameter, was purchased from Metal Crystal & Oxide Ltd. The W(111) surface was cleaned by repeating cycles of Ne+ ion sputtering at 1000 K and annealing at 1200 K for 5 min. Titration with O2 at 1000 K and flashing to 1200 K were often employed to help remove carbon impurities that were not removed by sputtering alone.13 The cleanliness and the orderliness of W(111) were verified by AES and LEED, respectively, before the experiments. The W(111) crystal was mounted to the manipulator by spot-welding the crystal directly

10.1021/jp003390k CCC: $20.00 © 2001 American Chemical Society Published on Web 04/04/2001

Modifying Surface Reactivity

J. Phys. Chem. B, Vol. 105, No. 18, 2001 3895

Figure 1. (a) C(KLL)/W(MNN) AES ratios obtained after TPD measurements of cyclohexene on W(111) as a function of exposure at 90 K (b) C(KLL)/W(MNN) AES ratios after TPD measurements of repeating cycles of exposing 2.2 L cyclohexene to W(111) at 90 K.

to two Ta posts, which served as electrical connections for resistive heating, as well as thermal contacts for cooling with liquid nitrogen. The carbide-modified W(111) surface was prepared by repeating cycles of exposing W(111) to 2.2 L cyclohexene at 90 K followed by flashing to 1200 K. After 3-4 cycles, a (x3×x3)R30° LEED pattern was obtained. The AES peakto-peak ratio of C(KLL)/W(MNN) of the 3-cycle surface was approximately 0.55, which corresponded to a C/W atomic ratio of 0.58, based on the standard sensitivity factors for the C(KLL) peak at 275 eV and W(MNN) feature at 182 eV.20 The oxygenmodified W(111) surface was prepared by exposing clean W(111) to 30 L O2 at 600 K. The single-crystal Pt (99.999%), oriented along the (111) direction, was 1.5 mm thick and 12 mm in diameter. As with the W(111) crystal, the Pt(111) sample could be heated resistively and cooled with liquid N2. The cleaning procedures were also similar to that of W(111): (1) repeated cycles of Ne+ ion sputtering at 1000 K, (2) annealing at 1100 K, and (3) removal of residual carbon by exposure to oxygen at 1000 K and subsequent heating to 1100 K. The resulting cleanliness and orderliness of the Pt(111) surface were verified by AES and LEED, respectively. Cyclohexene (99%), benzene (99.5%) and cyclohexane (99%) were purchased from Aldrich chemical company. They were purified by successive freeze-pump-thaw cycles. Oxygen (99.997%) and hydrogen (99.9995%) were obtained from Matheson and were used without further purification. The purity of the chemicals was verified in-situ by mass spectrometry. All the chemicals were introduced into the vacuum chamber via leak valves and the gas pressures were measured by uncorrected ion gauges. 2.2. Measurements. Most of the experiments were carried out in a multilevel UHV chamber, which was equipped with Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), and high-resolution electron energy loss spectroscopy (HREELS). The TPD data were collected using a Teknivent data acquisition system, which enabled us to collect up to 12 masses simultaneously. A linear heating rate of 3 K s-1 was used for all experiments. The beam energy in the HREELS measurements was approximately 4 eV with a typical resolution between 35 and 42 cm-1. The NEXAFS measurements reported here were performed in a UHV chamber at the U1A Beamline of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. The details of the apparatus have been described previously.5,6 In NEXAFS experiments, intense, tunable, and highly polarized synchrotron radiation is focused onto the

Figure 2. TPD spectra of H2 from the reaction of 2.2 L cyclohexene on clean and carbide-modified W(111) surfaces.

sample surface. The incident photon beam has an energy resolution of about 0.35 eV near the C K-edge region. The absorption of the X-ray photon is measured by following the annihilation of the core holes that are resulting from the X-ray absorption process, via either the Auger or the fluorescence decay channels. Electron yield spectra were obtained in the current study by using a channeltron multiplier located near the sample surface. A retarding potential of -100 eV at the entrance of the detector is used to repel bulk (low energy) electrons. 3. Results and Interpretation 3.1. TPD and AES Results. The atomic carbon is the only residual surface species on W(111) after TPD measurements, Figure 1a shows the C(KLL)/W(MNN) Auger ratio, recorded after each TPD measurement, as a function of cyclohexene exposure on clean W(111). The C(KLL)/W(MNN) AES ratio increases with the exposure of cyclohexene until it reaches to a value of 0.48 at the exposure of approximately 2.2 L of cyclohexene, which corresponds to approximately a C/W atomic ratio of 0.51 by using the standard Auger sensitivity factors.20 In the current study the cyclohexene exposure of 2.2 L was used for all surfaces. Figure 1b shows the C/W(KLL)/W(MNN) ratio as a function of the number of dosing/flashing cycles, which increases slightly as the number of reaction-cycle increases. Figure 2 shows the TPD spectra of H2 following the decomposition of 2.2 L cyclohexene on clean W(111) and on two carbide-modified W(111) surfaces. The two carbide-

3896 J. Phys. Chem. B, Vol. 105, No. 18, 2001

Liu et al.

Figure 3. TPD spectra of cyclohexene, benzene, and cyclohexane after exposing W(111) and C/W(111) surfaces to 2.2 L cyclohexene at 90 K.

Figure 4. TPD spectra of cyclohexane (a) and H2 (b) after exposing clean W(111) to 2.2 L cyclohexene and 2.2 L cyclohexane at 90 K.

modified W(111) surfaces corresponded to respectively the surfaces after one and three dosing/flashing cycles in Figure 1b. The desorption of the hydrogen product is observed from all three surfaces, indicating that all three surfaces are active toward the decomposition of cyclohexene. The H2 peak areas from C/W(111) are smaller than that from clean W(111). This is due to the different reaction pathways on clean and carbidemodified W(111), as will be quantified later. Figure 3 shows the hydrocarbon desorption products, cyclohexene (82 amu), benzene (78 amu), cyclohexane (84 amu), from the reaction of 2.2 L cyclohexene on clean and carbidemodified W(111). The molecular desorption of cyclohexene is shown in Figure 3a. For comparison, Figure 3a also shows a TPD spectrum obtained after exposing 10 L cyclohexene, which corresponds to the onset of multilayer desorption. The peak areas of cyclohexene, after 2.2 L exposures of cyclohexene to W(111) and C/W(111), are approximately less than 3% of the peak area of the 10 L TPD spectrum. This observation suggests that the amount of molecularly desorbed cyclohexene is nearly negligible after the exposure of 2.2 L cyclohexene, and that almost all chemisorbed cyclohexene undergoes dissociation. Figure 3b compares the production of benzene from the reaction of 2.2 L cyclohexene on W(111) and C/W(111). Benzene is not detected as the gas-phase product from the clean W(111) surface. In contrast, relatively intense benzene peaks are detected on carbide-modified W(111) surfaces. Finally, Figure 3c compares

the desorption of the cyclohexane product from the three surfaces. A relatively sharp cyclohexane peak is detected from clean W(111) at 190 K. In addition, weak peaks of mass 84 amu are also observed from the two C/W(111) surfaces, which appear at the similar temperature range as that observed for the desorption of cyclohexene (82 amu) on the corresponding surfaces. These relatively weak 84 amu peaks on C/W(111) are most likely due to the interference between masses of 82 and 84 amu in the multiplexing TPD measurements. To quantify the TPD peak area of cyclohexane we have performed TPD studies of the adsorption of cyclohexane on W(111). A relatively intense c-C6H12 desorption peak is observed at 190 K after exposing 2.2 L of cyclohexane on clean W(111), as shown in Figure 4a. Also compared in Figure 4a is the cyclohexane peak produced from the reaction of 2.2 L of c-C6H10 on clean W(111) surface. In addition, Figure 4b compares the H2 product from the reaction of cyclohexane and cyclohexene on clean W(111). The very weak H2 peak from cyclohexane suggests that the extent of cyclohexane decomposition on W(111) is rather small. We will use the peak areas of c-C6H12 and H2 to quantify the product yield in the Discussion. We also performed TPD measurements following the adsorption of benzene on carbide-modified W(111) at 90 K. As shown in Figure 5, the desorption temperature of chemisorbed benzene varies only slightly as a function of benzene exposure, which changes from 269 K at 1 L to approximately 240 K at 10 L.



Figure 5. TPD spectra obtained after exposing C/W(111) to benzene at 90 K.

An intense peak corresponding to the desorption of multiplayer benzene is also detected at 144 K. Furthermore, the desorption of benzene is not detected at a benzene exposure of 0.5 L, indicating that benzene undergoes decomposition on C/W(111). The most important observation in Figure 5 is that the desorption of chemisorbed benzene occurs at temperatures that are lower than the desorption temperature (300K) of the benzene product from the dehydrogenation of cyclohexene on carbide-modified W(111) in Figure 3. Such comparison suggests that the desorption of benzene, from the reaction of cyclohexene on C/W(111), is most likely a reaction-limited desorption process, as will be confirmed by the HREELS results below. 3.2. HREELS Results. We have performed HREELS measurements to characterize the surface intermediates of cyclohexene on clean and carbide-modified W(111). Figure 6 shows the HREEL spectra of molecularly adsorbed cyclohexene, benzene and cyclohexane. Figures 7 and 8 show the thermal behavior of cyclohexene on clean and carbide-modified W(111), respectively. To monitor thermal properties of cyclohexene on these surfaces, each of the surfaces is flashed to the indicated temperatures and cooled back to 90 K for HREELS measurements. The vibrational assignments are summarized in Table 1 and Table 2 for chemisorbed cyclohexene and benzene, respectively. The different temperatures, 90, 135, 240, 360, and 500 K, were chosen primarily based on the TPD results. These temperatures corresponded to the dosing, the beginning of cyclohexane desorption, the beginning of benzene desorption, and the end of benzene desorption temperatures, respectively. a. Molecularly Adsorbed Cyclohexene, Benzene, and Cyclohexane. The bottom HREELS spectrum in Figure 6 was obtained after exposing 2.2 L cyclohexene on oxygen-modified W(111) surface at 90 K, which was prepared by exposing W(111) to 30 L of O2 at 600 K. TPD and AES measurements suggested that cyclohexene underwent reversible molecular desorption on

Figure 6. HREEL spectra of 2.2 L cyclohexene adsorbed on the oxygen-modified W(111) surface. 90 K, and 2.2 L benzene and 3.0L cyclohexane adsorbed on the carbide-modified W(111) surface at 90 K.

such an oxygen-modified W(111) surface, which will be discussed in more detail in a separate paper.21 In the current paper, we will use the weakly interacting cyclohexene/O/W(111) system to facilitate the assignment of vibrational features of molecularly adsorbed cyclohexene. The vibrational frequencies of 2.2 L c-C6H10 on O/W(111), listed in Table 1, are very similar to those reported for the liquid phase cyclohexene. This is consistent with the AES and TPD observation that cyclohexene interacts weakly with the oxygen-modified tungsten surface.21 Particularly, the presence of δ(CdC) at 737 cm-1, ν(CdC) at 1617 cm-1, and ν(dC-H) at 3024 cm-1 suggests that the Cd C double bond of cyclohexene is intact, and that cyclohexene is weakly π-bonded on the O/W(111) surface. The middle HREEL spectrum in Figure 6 was recorded after exposing 2.2 L benzene on a three-cycle carbide-modified W(111) surface. As summarized in Table 2, the vibrational frequencies of benzene on carbide-modified W(111) are very similar to those reported for the liquid phase benzene, which suggests that benzene is molecularly intact on the C/W(111) surface at 90 K. We will use this HREEL spectrum, especially the intense γ(C-H) mode at 710 cm-1 (out-of-plane C-H deformation), to determine the presence/absence of benzene surface intermediates during the reactions of cyclohexene on the clean and carbide-modified W(111) surfaces. Finally, the top HREEL spectrum was recorded after exposing 3 L cyclohexane on a 3-cycle carbide-modified W(111) surface at 90 K. TPD measurements indicate that the adsorption of c-C6H12 is reversible. More details about the vibrational assignment will be given in the Discussion. Similar to that of


Liu et al.

Figure 7. HREEL spectra monitoring thermal behavior of 2.2 L cyclohexane adsorbed on the clean W(111) surface.

Figure 8. HREEL spectra monitoring thermal behavior of 2.2 L cyclohexene adsorbed on carbide-modified W(111) surface at 90 K.

benzene, the HREEL spectrum of molecular cyclohexane will be used to verify the presence/absence of the c-C6H12 surface intermediate during the reaction. b. Cyclohexene on Clean W(111). Figure 7 shows the HREEL spectra following the thermal behavior of 2.2 L cyclohexene on clean W(111). At 90 K, there are no vibrational frequencies related to the CdC double bond, such as δ(CdC) at 737 cm-1, ν(CdC) at 1617 cm-1, and ν(dC-H) at 3024 cm-1. This observation suggests that cyclohexene is strongly bonded to W(111), most likely in a di-σ configuration via the CdC bond of cyclohexene. We will discuss more about the possible bonding configurations of cyclohexene at 90 K in the Discussion section. After heating to 240 K, a new vibrational feature appears at 3058 cm-1, which can be attributed to the C-H stretching mode of unsaturated hydrocarbon. The observation of this feature suggests the formation of dehydrogenated intermediates from the reaction of cyclohexene. The five relative sharp features of the C6 ring, in the frequency range of 1000-1500 cm-1, disappeared after heating to 360 K. This suggests that most of the C6 rings have been decomposed by this temperature. However, the presence of ν(C-H) modes at 2916 and 3058 cm-1 indicates that hydrocarbon fragments remain on the surface after the decomposition of the C6 ring. The HREEL spectra at 360 and 500 K are characterized by relatively intense vibrational features at 446 cm-1, 636, 812, and 920 cm-1, which are due to atomic carbon on W(111). Overall, the HREELS data in Figure 7 indicate that cyclohexene reacts strongly with the clean W(111) surface, leading to the production of dehydrogenated intermediates and to atomic carbon at higher temperature. It is

TABLE 1: Vibrational Assignments for Cyclohexene on Oxygen-Modified, Clean, and Carbide-Modified W(111) at 90 K mode description

liquid22

ring deformation 175 ring deformation 280 ring deformation 393 ring deformation 452 ν(metal-C) ν(metal-O) skeletal distortion 640, 670 δ(CdC) (out-of-plane) 720 ν(C-C) 810 ν(C-C) 905, 917 ν(C-C) + F(CH2) 1038 ωCH2(rock) 1138 ωCH2(twist) 1241, 1264 ωCH2(wag) 1321-1350 δ(CH2) (scissors) 1438-1456 ν(CdC) 1653 ν(-C-H) 2840-2993 ν(dC-H) 3026, 3065

c-C6H10 c-C6H10 c-C6H10 on O/W(111) on W(111) on C/W(111)

663 737 906 1001 1136 1251 1339 1441 1617 2922 3024

413

460

649 812 893 1042 1123 1251 1326 1434

656 737 819 913 1062 1143 1265 1333 1448

2916

2922

also important to point out that the intense γ(C-H) mode of benzene (at ∼710 cm-1) is not detected at any temperature in Figure 7. c. Cyclohexene on Carbide-Modified W(111). Figure 8 shows the HREEL spectra following the thermal behavior of 2.2 L cyclohexene on the carbide-modified W(111) surface. Besides a weak δ(CdC) mode at 737 cm-1, there are no other vibrational modes related to the CdC double bond at 90 K. This observation suggests that, similar to on clean W(111), cyclohexene interacts strongly with C/W(111) to produce most likely the di-σ bonded



TABLE 2: Vibrational Assignments for Benzene on Carbide-Modified W(111) mode description

free molecule23

ν(M-C) γ(C-H) γ (C-H) ν(C-C) δ(C-H) δ(C-H) δ(C-C) ν(C-C) + δ (C + C) ν(C-C) ν(C-H)

673 845 993 1038 1150 1309 1486 1596 3074

C6H6 on C/Mo(110)10 412 731 846 988 988 1089 1366 3017

C6H6 on C/W at 90 K 710 859 1001 1001 1184 1353 1488 1590 3085

intermediate. In fact, there is a strong general similarity between Figures 7 and 8. The main difference is that the dehydrogenated intermediate is not observed at 240 K on C/W(111) (i.e. the absence of the ν(dC-H) mode at 3058 cm-1). The absence of the intense γ(C-H) modes of benzene in the 240 and 360 K spectra suggests that the benzene intermediate is not present on the surface. This is in agreement with the TPD results that the desorption of the benzene product is a reaction-limited process. On both clean and carbide-modified W(111) a trace amount of H2O is accumulated on the surface during the HREELS measurements, as indicated by the presence of the ν(OH) mode at 3578-3592 cm-1.24 The amount of H2O on the surface is estimated to be less than 3% based on AES measurements. d. Cyclohexene on Pt(111). For completeness, we have performed HREELS measurements of cyclohexene on the Pt(111) surface, using the same HREELS instrument resolution as that used in Figures 6-8. Our HREELS and TPD results following the reaction of cyclohexene on Pt(111) are similar to those reported earlier15-19 and will not be discussed here. The bottom HREELS spectrum in Figure 9 represents a previously assigned c-C6H9 intermediate, which is produced by heating cyclohexene/Pt(111) to 300 K.15-17 The HREEL spectra in Figure 9 also reveal the formation of the surface benzene intermediates at 350 and 400 K, as indicated by the sharp and intense γ(C-H) mode of benzene at 825 cm-1. In fact, the HREEL spectrum at 400 K is very similar to that reported for chemisorbed benzene on Pt(111),17 indicating that the dominant surface intermediate at 400 K is chemisorbed benzene. As shown in the inset of Figure 9, the detection of benzene at 350 K is before the onset of benzene desorption in the TPD measurements, confirming that the desorption of the benzene product from Pt(111) is a desorption-limited process.16,17 It should be pointed out that the frequency of the out-ofplane deformation mode of benzene appears at different frequencies on Pt(111) (at 825 cm-1) and on C/W(111) (at 710 cm-1). The possible origin of the different γ(C-H) frequency on different surfaces have been discussed in several previous papers10,25,26 and will not be discussed further in the current paper. 3.3. NEXAFS Results. We have performed polarization dependence NEXAFS measurements to further characterize the chemical identity and adsorption orientation of the reaction intermediates. Figure 10a shows the NEXAFS spectra of benzene on Pt(111), which is produced by heating a multilayer cyclohexene to 400 K. The assignment of NEXAFS features

Figure 9. HREEL spectra monitoring thermal behavior of 3 L cyclohexene adsorbed on Pt(111) surface at 90 K. The inset shows the benzene TPD spectrum following the reaction of 3 L cyclohexene on Pt(111).

of hydrocarbon molecules has be discussed in detail by Stohr.27 Briefly, the NEXAFS features of the multilayer cyclohexene, at 285.6, 288.4, and 292.4 eV, are due to the dipole transitions of C 1s electrons to the π*CdC, σ*C-H, and σ*C-C resonances, respectively. Similar to that reported by Friend and co-workers for the NEXAFS studies of benzene on Mo(110),28 the π* feature (contribution from both the π1* and π2* resonances) of benzene on Pt(111) is characterized by a broad peak centered at 286.0 eV. The features at 293 and 301 eV are related to the σ1* and σ2* of benzene, respectively.27,28 The π* feature of adsorbed benzene is much more intense in the glancing incidence spectrum, which is consistent with the adsorption geometry that the benzene ring is oriented nearly parallel to the Pt(111) surface. Figure 10(b) compares the thermal behavior of cyclohexene on the carbide-modified W(111) surface. The NEXAFS spectra were divided by the C K-edge spectra of C/W(111) to remove spectral contributions from the carbide substrate. The most important observation in Figure 10b is that the broad 286 eV π* feature of benzene is absent in either the 240 K or the 360 K spectra, which is consistent with the HREELS results in Figure 8 regarding the absence of surface benzene intermediates. Another important observation is that the π* feature at 285.4 eV is more intense at the glancing incidence at both 240 and 360 K. The intensity ratio of I(30°)/I(90°) is approximately 2.0 at 240 K and 3.5 at 360 K. By utilizing the relationship between the I(30°)/I(90°) ratio and adsorption geometry by Stohr,24 we estimate that the angle of the π* orbital with respect to the C/W(111) surface is approximately 40° at 240 K and 30° at 360 K.


Liu et al.

Figure 10. (a) NEXAFS spectra following the reaction of multilayer on Pt(111) (b) NEXAFS monitoring the thermal behavior of cyclohexene on the carbide-modified W(111) surface. The glancing and normal spectra refer to measurements with the crystal surface at a glancing angle (30°) and normal (90°) with respect to the incident photon beam.

4. Discussion 4.1. Product Yields on Clean and Carbide-Modified W(111) Surfaces. On the clean W(111) surface, cyclohexane and hydrogen are the only gas-phase products (Figures 2 and 3) and atomic carbon is the only surface species after the TPD measurements (Figure 1). Therefore, there are two reaction pathways for cyclohexene on clean W(111), as shown in eq 1 and 2. The variables a and b represent the amounts of c-C6H10 involved in each of the decomposition pathway:

c-C6H10 f 6C(a) + 5H2(g) 6a a 5a

(1)

c-C6H10 + H2 f C6H12 b b b

(2)

In addition, as described in the TPD results in Figure 4, C6H12 and relatively weak H2 peaks are detected after the adsorption of cyclohexane on W(111). The two reaction pathways, the reversible desorption of C6H12 and the decomposition of cyclohexane to produce H2, are expressed in eqs 3 and 4. The variables c and d represent the amounts of c-C6H12 involved in each of the reaction pathway.

c-C6H12 f 6C(a) + 6H2(g) 6c c 6c

(3)

c-C6H12 f c-C6H12 d d

(4)

The values of a and b can be estimated by using the AES results in Figure 1b and the TPD data in Figure 4 as follows. After the TPD measurement of 2.2 L c-C6H10 on clean W(111) surface, AES results reveal a C(KLL)/W(MNN) ratio of 0.41 ( 0.08, which corresponds approximately to a C/W atomic ratio of 0.43 ( 0.08 by using the standard Auger sensitivity factors. The error bar in the AES ratio was obtained by averaging six sets of experiments. Because each cyclohexene molecule produces six atomic carbon atoms, the C/W atomic ratio of 0.43 ( 0.08

corresponds to the complete decomposition of 0.072 ( 0.013 cyclohexene molecule per W atom. In other words, the value of a in eq 1 equals to 0.072 ( 0.013 cyclohexene molecule per W atom.

a ) 0.072 ( 0.013

(5)

We needed several equations to quantify the amount of cyclohexane product in eq 2. We performed AES measurements after exposing 2.2 L cyclohexane on clean W(111) at 90 K. The AES measurements show a C(KLL)/W(MNN) AES ratio of ∼0.50, which corresponds to a C/W atomic ratio of ∼0.53 and a surface coverage of 0.088 cyclohexane molecule per W atom on the clean W(111) surface. As described earlier, cyclohexane undergoes both reversible molecular desorption and decomposition on the clean W(111) surface. Therefore, the eq 6 is obtained:

c + d ) 0.088

(6)

By comparing the TPD area of the c-C6H12 product from 2.2 L cyclohexene to that from the molecular desorption of 2.2 L cyclohexane on clean W(111) in Figure 4a, a ratio of 0.19 ( 0.01 is obtained, which leads to the following relationship:

b/d ) 0.19 ( 0.01

(7)

Furthermore, by comparing the TPD area of the H2 product from 2.2 L cyclohexane to that from 2.2 L cyclohexene on clean W(111) in Figure 4b, the ratio of 0.092 is obtained. It leads to the relationship:

6c/(5a - b) ) 0.092

(8)

By solving the eqs 5-8, we determine a ) 0.072 ( 0.013, b ) 0.0157 ( 0.001, c ) 0.0053 ( 0.001, and d ) 0.0827 ( 0.001. We therefore obtain that the dominant reaction pathway (∼82% selectivity) of cyclohexene on clean W(111) is the complete dissociation to form surface carbon and hydrogen. The



TABLE 3: Vibrational Modes between 1000 cm-1 and 1500 cm-1 mode description

π-bonded c-C6H10 on O/W(111)

σ-bonded c-C6H9 on Pt(111)

weakly bonded c-C6H12 on C/W(111)

σ-bonded c-C6H9/c-C6H10 onW(111)

σ-bonded c-C6H9/c-C6H10 on C/W(111)

ν(C-C) + F(CH2) ωCH2(rock) ωCH2(twist) ωCH2(wag) δ(CH2) (scissors)

1001 1136 1251 1339 1441

1062 1150 1238 1319 1448

1035 1103 1265 1346 1448

1042 1123 1251 1326 1434

1062 1143 1265 1333 1448

other ∼18% of cyclohexene reacts with hydrogen to produce gas-phase cyclohexane. On the carbide-modified W(111) surface, the reaction is qualitatively different from the clean W(111) surface. Benzene and hydrogen are the only gas-phase products in the TPD measurements (Figure 2 and Figure 3), and atomic carbon is the only surface species on the C/W(111) surface after TPD measurements (Figure 1b). This is due to the onset of a new dehydrogenation pathway to produce benzene and hydrogen. The decomposition and dehydrogenation pathways are shown in eq 9 and 10. The variables e and f represent the amount of c-C6H10 involved in each reaction.

c-C6H10 f 6C(a) + 5H2(g) 6e e 5e

(9)

c-C6H10 f C6H6 (g) + 2H2 (g) f f 2f

(10)

As estimated from eqs 1-8, the total amount of cyclohexene on clean W(111), after exposing to 2.2 L at 90 K, is a + b ) 0.0877 ( 0.014 cyclohexene per W atom. If we assume that the sticking coefficient of cyclohexene on clean and carbonmodified W(111) surfaces is similar at 90 K, we obtain

a + b ) e + f ) 0.0877 ( 0.014 cyclohexene molecule per W atom (11) By comparing the H2 TPD area from 2.2 L c-C6H10 on carbidemodified W(111) to that from 2.2L c-C6H10 on clean W(111) (Figure 2), we estimate that the ratio of H2 peak areas on the two surfaces is 0.76, which leads to the following relationship (see eqs 1-10):

(5e + 2f)/(5a - b) ) 0.76

(12)

By solving eqs 11 and 12, we determine that the value of e is 0.0287 ( 0.0087 and that of f is 0.0590 ( 0.064. These values correspond to a selectivity of ∼33% cyclohexene undergoing complete decomposition, with the other ∼67% cyclohexene dehydrogenating to form benzene. The selectivity of ∼67% is very similar to that observed on Pt(111), where the selectivity to gas-phase benzene from cyclohexene is about 75%.12,16 Furthermore, the selectivity to benzene on C/W(111) is nearly identical to that observed in the dehydrogenation of cyclohexene on carbide-modified Mo(110) (∼70% selectivity12). 4.2. Reaction Mechanisms. Despite the systematic HREELS and NEXAFS measurements, at present we cannot conclusively identify all the surface intermediates. For example, in Table 1 we assigned the vibrational spectra, obtained after exposing 2.2 L cyclohexene to clean and carbide-modified W(111), to those of di-σ bonded cyclohexene. However, the vibrational frequencies of these two spectra are also very similar to those in the HREEL spectrum of the allyl c-C6H9 species, as shown in Figure 9 after heating the cyclohexene/Pt(111) to 300 K. In fact, except for minor changes in the relatively intensities, the frequencies of the vibrational modes, especially those of C6 ring

modes in the frequency range between 1000 and 1500 cm-1, are very similar for the π-bonded c-C6H10 on O/W(111) (bottom spectrum in Figure 6), σ-bonded c-C6H9 on Pt(111), and the σ-bonded c-C6H10 or c-C6H9 on clean and carbide-modified W(111) (Figures 7 and 8). Furthermore, the vibrational modes between 1000 and 1500 cm-1 are also similar to those of cyclohexane on carbide-modified W(111) (top sepectrum in Figure 6). The most obvious difference in c-C6H12 is the onset of a softened ν(C-H) mode at 2672 cm-1. The origin of the softened ν(C-H) mode in c-C6H12 has been the subject of several papers.15,29-32 In an earlier paper we have provided examples on several surfaces to demonstrate that the presence of softened ν(C-H) does not always lead to C-H bond dissociation.29,32 Table 3 compares the five C6 ring (between 1000 and 1500 cm-1) vibrational modes in Figures 6-9. The frequencies of these modes are within 30 cm-1 in most cases. Therefore, a conclusive assignment of whether the low-temperature intermediates are c-C6H9 or c-C6H10 in Figures 7 and 8 can only be achieved with the aid of theory, such as density functional theory (DFT) calculations. Comparing to HREELS, the NEXAFS measurements are even less sensitive in differentiating different intermediates in the reaction of cyclohexene. For example, as shown in Figure 10, the peak positions of the NEXAFS features are essentially indistinguishable between multilayer cyclohexene and the intermediates obtained after heating the surface to 240 and 360 K. However, the clear advantage of NEXAFS is its capability to determine the adsorption orientation of the surface intermediates. With these precautions, we will attempt to proposed the most plausible surface reaction mechanisms of cyclohexene on clean and carbide-modified W(111). On the clean W(111) surface, assuming that the 90 K adsorbates is σ-bonded c-C6H9, the production of cyclohexane should occur via the reaction between c-C6H9 with surface hydrogen (from the dissociation of c-C6H10 to c-C6H9 at 90 K). On the other hand, assuming that σ-bonded c-C6H10 is the adsorbate, cyclohexane can be produced via the disproportionation mechanism, leading to the production of cyclohexane and dehydrogenated surface intermediates. The latter mechanism seems to be more plausible based on the HREELS observation of dehydrogenated intermediates, i.e., the onset of the relatively intense ν(dC-H) mode at 3058 cm-1 at 240 K (Figure 7). The observation that the cyclohexane product desorbs at the same temperature as adsorbed cyclohexane (Figure 5a) suggests that the desorption of the c-C6H12 product is a desorption-limited process. However, this cannot be conclusively confirmed by the HREELS measurements because of the similar vibrational frequencies between c-C6H12 and σ-bonded c-C6H10 (or c-C6H9). Furthermore, the HREELS and TPD results indicate that most of the C6 rings are decomposed between 240 and 360 K on clean W(111), as indicated by the disappearance of the C6 ring modes in the HREEL spectra and by the onset of an intense H2 desorption peak at 300 K (Figure 3d). The resulting intermedi-

3902 J. Phys. Chem. B, Vol. 105, No. 18, 2001 ates at 360 K are likely small CxHy fragments. The presence of the 3058 cm-1 mode indicates that they are dehydrogenated, and the polarization dependence of the NEXAFS features suggests that the orientation of the π* orbitals are approximately 30° away from the surface. Finally, the broad tail of the H2 TPD peak indicates that these intermediates gradually decompose at temperatures up to 600 K. The reaction mechanism on carbide-modified W(111) is more straightforward. The absence of benzene features in either the HREELS or NEXAFS measurements indicates that benzene desorbs from C/W(111) as soon as it is produced. More importantly, the combined HREELS and TPD measurements also reveal the differences in the production of gas-phase benzene on C/W(111) and Pt(111). For example, the desorption temperature of benzene is significantly different on the two surfaces. As shown in Figure 5, the desorption temperature for chemisorbed benzene from C/W(111) is in the range of ∼240269 K, which is lower than the benzene feature at 300 K in the reaction of 2.2 L c-C6H10 on C/W(111). Therefore, the desorption of benzene product is a reaction-limited process on this surface. On the other hand, the desorption peak of benzene from Pt(111) appears at ∼404 K (Figure 9), which is higher than the onset temperature for the conversion of cyclohexene to benzene in the HREELS measurements (∼350 K). As a result, the desorption of the benzene product is a desorption-limited process on Pt(111). 5. Conclusions By using the decomposition and dehydrogenation of cyclohexene as probe reactions, different reaction pathways are revealed on W(111) and C/W(111) surfaces. On the clean W(111) surface, ∼82% cyclohexene undergoes completely decomposition to form atomic carbon and hydrogen, and ∼18% cyclohexene reacts with hydrogen to produce cyclohexane. The formation of carbide significantly changes the surface reactivity of W(111). On the carbide-modified W(111) surface, ∼33% cyclohexene undergoes decomposition to produce atomic carbon and hydrogen, while the other ∼67% cyclohexene dehydrogenates to form benzene and hydrogen. The ∼67% gas-phase benzene yield on carbide-modified W(111) is very similar to that observed on Pt(111). The results reported here provide a clear example that the formation of carbide converts the surface reactivities of early transition metals to those of Pt-group metals. On the other hand, our results also demonstrate that the reaction mechanisms of cyclohexene are not identical on C/W(111) and Pt(111). For

Liu et al. example, the desorption of the benzene product is reactionlimited on C/W(111) but desorption-limited on Pt(111). Acknowledgment. We acknowledge the financial support from DOE/BES (Grant DE-FG02-00ER15104). We also acknowledge Dr. Joseph Eng, Jr., of Lucent Technology for participating in the NEXAFS measurements on Pt(111). References and Notes (1) Oyama, S. T.; Haller, G. L. In Catalysis, Specialist Report; Bond, G. C., Webb, G., Eds.; The Chemical Society: London, 1981; Vol. 5, p 333. (2) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides; Blackie Academic and Professional: Glasgow, 1996. (3) Chen, J. G. Chem. ReV. 1996, 96, 1477 and references therein. (4) Chen, J. G.; Furhberger, B.; Eng, J., Jr.; Bent, B. E. J. Mol. Catal. A 1998, 131, 285. (5) Chen, J. G. Surf. Sci. Rep. 1997, 30, 1. (6) Chen, J. G.; Eng, J., Jr.; Kelty, S. P. Catal. Today 1998, 43, 147. (7) Fruhberger, B.; Chen, J. G.; Eng, J., Jr.; Bent, B. E. J. Vac. Sci. Technol. A 1996, 14, 1475. (8) Fruhberger, B.; Chen, J. G. J. Am. Chem. Soc. 1996, 118, 11599. (9) Fruhberger, B.; Chen, J. G. Surf. Sci. 1995, 342, 38. (10) Eng, J., Jr.; Bent, B. E.; Fruhberger, B.; Chen, J. G. Phys. Chem. B 1997, 101, 4404. (11) Eng, J., Jr.; Chen, J. G. Surf. Sci. 1998, 414, 374. (12) Chen, J. G.; Fruhberger, B. Surf. Sci. 1996, 367, L102. (13) Eng, J., Jr.; Bent, B. E.; Fruhberger, B.; Chen, J. G. Langmiur 1998, 14, 1301. (14) Zhang, M. H.; Hartmam, R.; Chen, J. G. Unpublished results. (15) Land, D. P.; Erley, W.; Ibach, H. Surf. Sci. 1993, 289, 237. (16) Rodriguez, J. A.; Campbell, C. T. J. Catal. 1989, 115, 500;. (17) Henn, F. C.; Diaz, A. L.; Bussell, M. E.; Hugenschmidt, M. B.; Domagala, M. E.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5965 and references therein. (18) Davis, S. M.; Somorjai, G. A. Surf. Sci. 1980, 91, 73. (19) Davis, S. M.; Somorjai, G. A. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1984; p 4. (20) Childs, K. D.; Carlson, B. A.; LaVanier, L. A.; Moulder, J. F.; Paul, D. F.; Stickle, W. F.; Watson, D. G. Handbook of Auger Electron Spectroscopy, Physical Electronics, 3rd ed.; 1995. (21) Liu, N.; Rykov, S. A.; Chen, J. G. Surf. Sci., submitted. (22) Neto, N.; Dilauro, C.; Castellucci, E.; Califano, S. Spectrochim. Acta 1967, 23A, 1763. (23) Nieuwenhuys, B. E.; Somorjai, G. A. Surf. Sci. 1978, 72, 8. (24) Hwu, H. H.; Chen, J. G. In preparation. (25) Grassian, V. H.; Muetterties, E. L. J. Phys. Chem 1987, 91, 389. (26) Koel, B. E.; Crowell, J. E.; Mate, C. M.; Somorjai, G. A. J. Phys. Chem. 1984, 88, 1988. (27) Stohr, J. NEXAFS Spectroscopy; Springer-Verlag: New York, 1992. (28) Liu, A. C.; Friend, C. M. J. Chem. Phys. 1998, 89, 4396. (29) Raval, R.; Chesters, M. A. Surf. Sci. 1989, 219, L505. (30) Weldon, M. K.; Uvdal, P.; Friend, C. M.; Wiegand, B. C. Surf. Sci. 1996, 355, 71. (31) Hoffmann, F. M.; Upton, T. H. J. Phys. Chem. 1984, 88, 6209. (32) Teplyakov, A. V.; Bent, B. E.; Eng, J., Jr.; Chen, J. G. Surf. Sci. 1998, 399, L342.

Modifying Surface Reactivity by Carbide Formation: Reaction

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