Studies of the Adsorption Geometry and Decomposition Mechanisms

Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 ... properties on the carbide-modified Mo(110) su...
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J. Phys. Chem. B 1997, 101, 4044-4054

Studies of the Adsorption Geometry and Decomposition Mechanisms of Benzene on Clean and Carbide-Modified Mo(110) Surfaces Using Vibrational Spectroscopy Joseph Eng, Jr., and Brian E. Bent† Department of Chemistry and the Columbia Radiation Laboratory, Columbia UniVersity, New York, New York 10027

Bernd Fru1 hberger and Jingguang G. Chen* Corporate Research Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 08801 ReceiVed: October 28, 1996; In Final Form: March 11, 1997X

The bonding and reactivity of benzene on carbide-modified Mo(110) surfaces have been studied using highresolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) to investigate the effects of carbide formation on the chemistry of this early transition metal surface. For comparison, the reactions of benzene with both clean Mo(110) and oxygen-modified Mo(110) surfaces have been studied as well. We find that benzene adsorbs on all three types of surfaces at 80 K with the molecular plane parallel to the surface. For the case of benzene adsorption on the oxygen-modified surface, the similarity between the vibrational frequencies observed in the HREELS spectrum and infrared spectrum of liquid benzene indicates that benzene interacts only weakly with this surface. On the clean Mo(110) surface, the HREELS data show that adsorption occurs on only one type of surface site, and the benzene layer is stable to at least 325 K. Between 325 and 350 K, benzene decomposes to form benzyne, as proposed earlier by Liu et al. Finally, for the carbide-modified surface, it is observed that the benzene decomposes above ∼350 K to produce CxHy fragments. The latter reactivity is similar to what is observed for benzene on the platinum group metals, especially on Rh(111).

I. Introduction It has been well-documented in the catalysis literature that the chemical properties of early transition metals can be significantly modified upon the formation of the corresponding early transition metal carbides. For example, the catalytic activities of Mo in hydrogenation and dehydrogenation reactions can be significantly enhanced by the formation of carbide, Mo2C. Since these carbides, such as Mo2C, often demonstrate catalytic properties similar to those of the Pt-group metals,2,3 numerous surface science studies have been performed in order to understand the structural, electronic, and catalytic properties of carbide-modified early transition metal surfaces.4-7 Recent studies5-7 have shown that atomic carbon on a Mo(110) surface can be driven into the bulk by heating to temperatures above 1250 K to form an interstitial carbide overlayer which has a surface reactivity more similar to that of the Pt-group metals than that of clean Mo(110). The observation of Pt-like properties on the carbide-modified Mo(110) surface indicates that this surface can be used as a reliable model system to study the chemical properties of Mo2C. The present study of benzene adsorption on these surfaces represents a part of an ongoing effort to characterize the different reactivities of the clean and carbide-modified Mo(110) surfaces. As discussed below, this characterization can be better understood through comparisons to the large number of prior studies of benzene adsorption on various single-crystal transition metal surfaces. The presence of benzene in many industrially important heterogeneous catalytic reactions has led to numerous studies1,8-32 of benzene adsorption onto single-crystal transition metal surfaces over the past two decades. While these studies have been performed with a wide variety of surface analytical † Deceased. * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, May 1, 1997.

S1089-5647(96)03335-4 CCC: $14.00

techniques, one of the most informative probes of surface bonding and chemistry has been high-resolution electron energy loss spectroscopy (HREELS), which is the primary analysis technique in the present work. A survey of the previous HREELS studies of benzene adsorption on Ni,10,23-26 Pd,11-13, Pt,10,27,28 Ag,29 Rh,16,30,31 Os,19 Ru,20 and Re33 single-crystal surfaces reveals two interesting observations. The first observation is that benzene is always found to adsorb with the molecular plane nearly parallel to the metal surface at low coverages, regardless of the particular transition metal or crystallographic plane.10-13,16,19-25,27-30,33,34 This observation is based primarily upon on-specular HREEL spectra which show that this adsorption geometry is the only one that is consistent with the surface dipole selection rule. The second observation is that the frequency of the ν4 (Herzberg notation)35 out-of-plane C-H bending mode of an adsorbed benzene molecule appears to correlate with the strength of the benzene-metal interaction. As noted previously by Grassian et al.11 and Koel et al.,16 the frequency of the ν4 mode for benzene adsorbed on the (111) surfaces of the late transition metals correlates approximately linearly with not only the cohesive energy (heat of atomization) of the metal11 but also the heat of benzene adsorption and the surface work function change16 upon benzene adsorption. The primary focus of this work is to characterize the difference in reactivity between the clean and carbide-modified Mo(110) surfaces. Toward this goal, we have investigated whether or not the ν4 frequency of adsorbed benzene also correlates approximately linearly with the respective cohesive energy of these two surfaces. Importantly, we do not find a linear correlation between the absolute frequency of the ν4 mode and the respective cohesive energies of these surfaces, unlike what was observed for the late transition metals. We find, as discussed below, that the frequency of the ν4 mode increases from 704 to 731 cm-1 on going from the clean to the carbidemodified Mo(110) surface. This increase in the ν4 frequency © 1997 American Chemical Society

Benzene on Mo(110) Surfaces clearly shows a difference in the benzene-surface interaction for the two different surfaces and suggests that the strength of the benzene interaction with the carbide-modified Mo(110) surface may be more similar to that for the Pt-group metals (for which the ν4 frequency varies between 755 and 930 cm-1) than for the clean Mo(110) surface. As a calibration for these studies, the adsorption of benzene on an oxygen-modified Mo(110) surface has also been examined, since this surface has been shown to be relatively inert so that the vibrational frequencies for benzene adsorbed on this surface are expected to be close to those of benzene in the liquid phase. The rest of the paper is organized as follows. After briefly describing the methods used to prepare the clean, carbidemodified, and oxygen-modified Mo(110) surfaces, the results of selected temperature-programmed desorption studies are presented. Next follow the HREEL spectra for benzene on the three types of surfaces. Particular attention is given to the adsorption geometry and thermal decomposition mechanisms on these different surfaces. Finally, in the last section, the results are discussed in the context of previously reported results for benzene adsorption on single-crystal surfaces of the late transition metals. II. Experimental Methods 2.1. Techniques. The ultrahigh-vacuum (UHV) chamber used in these studies has been described previously in detail.5 It is a three-stage stainless steel chamber which has a base pressure of 2 × 10-10 Torr. The top two stages of the chamber are equipped with a Mg/Al anode for XPS studies, a doublepass cylindrical mirror analyzer for Auger and XPS studies, lowenergy electron diffraction (LEED) optics, and a shielded quadrupole mass spectrometer for performing thermal desorption studies. The bottom stage features an LK-3000 high-resolution electron energy loss spectrometer (HREELS). The electrons used in the HREEL studies had an energy of 6.0 eV and were incident on the surface at 60° from the surface normal. HREEL spectra were taken in both the specular and the +20° offspecular directions, where +20° refers to the angle where θ(detector) - θ(monochromator) ) 20°. For the clean, wellordered Mo(110) surface, the elastic peak typically has a width (fwhm) of 24-40 cm-1 and an intensity of 1 × 106 counts/s. The Mo(110) crystal (99.999% pure, 1.5 mm thick and 12 mm in diameter, from Metal Crystals and Oxides, Ltd., Cambridge, England) was mounted to the sample manipulator by spot-welding the crystal directly to two Ta posts which serve as electrical connections for resistive heating, as well as thermal contacts for cooling with liquid nitrogen. Using this mounting scheme, the temperature of the crystal could be varied between 80 and 1400 K. The temperature of the Mo(110) crystal was 80 K during the acquistion of all HREEL spectra. For the TPD experiments, a linear heating ramp of 3 K/s was used. The benzene-d0 (Aldrich, 99.99% pure) and the benzene-d6 (MSD Isotopes, 99.6% at. pure) were purified by successive freeze-pump-thaw cycles prior to their use. Their purity was verified in-situ by mass spectrometry. Oxygen was obtained from Matheson (99.99% pure) and used without further purification. Exposures are reported in langmuirs (1 langmuir ) 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 80 K with the crystal located in the top stage of the chamber. The gas exposures were made by backfilling the vacuum chamber. 2.2. Preparation of Clean, Carbide-Modified, and OxygenModified Surfaces. A clean Mo(110) crystal surface could be prepared by Ne+ bombardment at 1000 K (ion energy 5 kV,

J. Phys. Chem. B, Vol. 101, No. 20, 1997 4045 emission current 20 mA), followed by annealing at 1250 K for approximately 5 min. However, since it is somewhat difficult to remove carbon contamination by sputtering alone, thick carbide layers (which were present after some experiments) were removed by titrating with excess O2 at 1000 K. Oxygen reacts with the carbon-covered Mo(110) surface at this temperature to form CO, which then desorbs. Subsequent heating to 1250 K drives more carbon from the bulk to the near surface region. By repeating this process, the subsurface carbon is gradually removed and a molybdenum oxide layer is formed which is easily removed by the sputtering procedure mentioned above. Auger analysis shows that the C and O impurities are less than 2.0 and 1.0% of a monolayer, respectively, after this cleaning procedure. The preparation of carbide-modified Mo(110) surfaces using ethylene as a carbon source has been described previously.5 In the present work, however, the carbide-modified Mo(110) surfaces were prepared using benzene. Dosing/flashing cycles, in which the crystal was first exposed to 3 langmuirs of benzene at 600 K and then flashed to 1250 K, were repeated until a p(4×4) LEED pattern was obtained. Previous studies have shown that heating to 1250 K drives the surface carbon (which results from benzene decomposition at 600 K) into interstitial (subsurface) sites.6 Two to three cycles of dosing and annealing produces an Auger C(KLL 272 eV)/Mo(MNN 186 eV) peak ratio of ∼0.19, which corresponds to an atomic C/Mo ratio of 0.38 for the surface region probed by the Auger electrons in the energy range between 186 and 272 eV. Note that in this process the final step is always flashing to 1250 K, as opposed to dosing benzene at 600 K, so that the carbon is primarily subsurface and the outermost surface is active. As described previously,36,37 the oxygen-modified Mo(110) surface used in these studies was prepared by repeating cycles in which the crystal is first exposed to 10 langmuirs of O2 at 1000 K and then flashed to 1150 K. The dosing/flashing cycles were repeated until a p(2×2)-O LEED pattern was observed, which corresponded to an oxygen coverage of 0.25 ML,38 where 1 ML ) 1 O atom/per surface Mo atom on the (110) surface. III. Results and Interpretation 3.1. Temperature-Programmed Desorption (TPD). Figure 1 shows benzene and hydrogen TPD spectra obtained as a function of benzene exposure on clean Mo(110). The spectra, which are in good qualitative agreement with previous results,1 show that at low exposures of benzene (i.e., e 1 L) hydrogen is the only desorption product. At a benzene exposure of 3 L, weak benzene desorption peaks are observed at 199 (a1) and 365 K (a2) (see inset to Figure 1a), while the corresponding hydrogen TPD spectrum shows two distinct features, centered at 394 K and approximately 502 K. At an exposure of 10 L, the a1 peak shifts down to 181 K, likely due to repulsive interactions between the adsorbed benzene molecules, and its integrated area increases by a factor of 13 as compared to the 3 L exposure. (Note also the shoulder on the low-temperature side of this feature.) The a2 peak, on the other hand, remains at 365 K for the 10 L exposure, and its integrated area only increases by a factor of 2 relative to the peak observed in the 3 L exposure. Finally, for the 10 L benzene exposure, the broad benzene desorption feature at ∼800 K (Figure 1a) is attributed to desorption from parts of the manipulator, such as the Ta supports which hold the crystal. The sharp peak at 184 K in the H2 TPD spectrum is an experimental artifact arising from “crosstalk” between different channels of the multiplexing data acquisition system. A comparison with previous TPD results for benzene adsorption on Mo(110) by Liu et al.1 can be used to calibrate the

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Eng et al.

Figure 1. Temperature-programmed desorption of benzene adsorbed on clean Mo(110) at 80 K. The left panel shows molecular desorption (m/e ) 78) while the right panel shows hydrogen evolution (m/e ) 2).

Figure 2. Temperature-programmed desorption data obtained after exposing the clean, carbide-modified and oxygen-modified Mo(110) surfaces to 3 L of benzene. The exposure temperature was 80 K. The left panel shows molecular benzene desorption while the right panel shows hydrogen evolution.

surface coverage of benzene in the present studies. Liu et al. report that benzene desorption from the first monolayer occurs at 170 K, which is probably the same as the 181 K peak observed in these studies, after accounting for differences in sample heating methods. In addition, Liu et al. observe benzene desorption features at 140 and 155 K at even higher benzene exposures, which were attributed to desorption from the second layer and from multilayers, respectively. Based on these results, the absence of any desorption features below 180 K for the 3 L exposure suggests that the surface coverage at this exposure must be less than saturation coverage. Further quantification of benzene coverages can be achieved by measuring the surface carbon concentrations after the TPD experiments. For example, for the 3 L benzene exposure that is used in many of the experiments reported here, an Auger C(KLL 272 eV)/Mo(MNN 186 eV) peak ratio of 0.14 ( 0.01 is obtained after the TPD experiments. By using the standard Auger sensitivity factors,39

it can be determined that this peak ratio corresponds to a C/Mo atomic ratio of 0.28 ( 0.02, within the surface region probed by the Auger electrons in the energy range between 186 and 272 eV. Since the amount of molecularly desorbing benzene is small at this exposure (cf. Figure 1), and each decomposing benzene molecule produces six carbon atoms, the C/Mo atomic ratio of 0.28 suggests that the initial benzene coverage following a 3 L benzene exposure at 80 K is approximately 0.05 molecule per surface Mo atom. Figure 2 compares the benzene and hydrogen TPD spectra taken after a 3 L exposure of benzene onto clean, carbidemodified, and oxygen-modified Mo(110). The spectra clearly show that benzene interacts differently with each type of surface. For the oxygen-modified surface, a single intense benzene desorption peak is observed at 214 K, and the corresponding hydrogen TPD spectrum, although noisy, shows that there are no significant hydrogen desorption peaks. The spectra thus

Benzene on Mo(110) Surfaces

J. Phys. Chem. B, Vol. 101, No. 20, 1997 4047

Figure 3. (a) HREEL spectra (both on-specular and +20° off-specular) of C6H6, adsorbed on the oxygen-modified Mo(110) surface. (b) HREEL spectra (both on-specular and +20° off-specular) of C6D6 adsorbed on the oxygen-modified Mo(110) surface.

TABLE 1: Vibrational Assignments (in cm-1) for C6H6 and C6D6 on Clean, Carbide-Modified, and Oxygen-Modified Mo(110) mode description

free molecule17

symmetry D6h

ν(M-C) ν(M-O) γ(C-H)

ν4

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)

ν11 ν2 ν14 ν10 ν9 ν13 ν16 ν1

C2V

Cs

A2u

A1 A1 A1

A′

E1g A1g E1u B2u B2u E1u E2g A1g

B1 +B2 A1 B1 +B2 B2 B2 B1 +B2 A1 +B1 A1

A′ + A′′ A′ A′ + A′′ A′′ A′′ A′ + A′′ A′ + A′′ A′

Pt(111)5

O/Mo(110)

Mo(110)

C/Mo(110)

C6H6 (C6D6) ωH/ωD C6H6 (C6D6) C6H6 (C6D6) C6H6 (C6D6) ωH/ωD C6H6 (C6D6) ωH/ωD

673

(496)

1.37

845 993 1038 1150 1309 1486 1596 3074

(659) (946) (816) (827) (1286) (1335) (1552) (2303)

1.28 1.05 1.27 1.39 1.02 1.11 1.03 1.33

indicate that benzene desorbs without decomposing from the oxygen-modified Mo(110) surface. This behavior is analogous to that of ethylene adsorption onto oxygen-modified Mo(110) surfaces and demonstrates that oxygen poisons the Mo surface, making it relatively inert toward olefinic and aromatic compounds.5 On the other hand, the clean Mo(110) surface is quite active in decomposing benzene, as seen by the fact that the integrated area of the benzene desorption peaks for the clean surface is only one-fifth of that from the oxygen-modified surface. The presence of significant H2 desorption features at 394 and 502 K, resulting from benzene decomposition, is further evidence that benzene interacts strongly with the clean Mo(110) surface. Finally, the TPD spectra show that the reactivity of the carbide-modified Mo(110) surface is greater than that of the oxygen-modified surface, but less than that of the clean surface. The integrated peak intensity for benzene desorption from the carbide-modified surface is three-fifths that of the oxygen-modified surface and 3 times greater than that from the clean Mo(110) surface. The relatively weaker interaction between benzene and the carbide-modified surface, as compared to the clean Mo(110) surface, is also indicated by the smaller integrated area of the H2 peak, which is approximately twothirds of that for the clean Mo(110) surface. 3.2. High-Resolution Electron Energy Loss Spectroscopy (HREELS). 3.2.1. Vibrational Assignments and Molecular Orientation. In this section, HREEL spectra are presented for benzene adsorbed on the oxygen-modified, carbide-modified,

360

(350)

830 920

(610) (700)

379 589 690

846 967 1014 1130 (850) 1157 1420 (1350) 1366 1461 1569 3000 (2240) 3031

(385) (595) (507)

534

(589)

412

(392)

704

(487)

1.44

731

(528)

1.38

(656) 839 (589) (927) 988 (913) (798) 988 (724) (839) 1076 (805) (1312) 1340 (1326) (1319) (1529) (2259) 3003 (2246)

1.42 1.08 1.36 1.34 1.01

846 (643) 988 (913) 988 (731) 1089 (818) 1366 (1326)

1.31 1.08 1.35 1.33 1.03

1.34

3017 (2239)

1.35

and clean Mo(110) surfaces. Angle-dependent HREEL studies of both C6H6 and C6D6 adsorption on the three different types of surfaces have been used to determine the vibrational assignments and the bonding geometries. These spectra are shown in Figures 3-6, and the peak frequencies are assigned in Table 1. Although the exposures of C6H6 and C6D6 in these studies were made with the crystal temperature at 80 K, for the spectra shown in Figures 3, 4, and 6, the crystal is first heated to 150 K for 30 s prior to acquiring the HREEL spectra in order to remove any multilayers which may have formed as a result of slight variations in dosing. Finally, for each spectrum, the height of the on-specular elastic peaks has been normalized to unity, so that the expansion factor shown with each spectrum represents the expansion of the spectrum relative to the onspecular elastic peak. Oxygen-Modified Mo(110). Parts a and b of Figure 3 show the on-specular and +20° off-specular HREEL spectra obtained by dosing 3 L of C6H6 and C6D6, respectively, onto an oxygenmodified Mo(110) surface. As can be seen from Table 1, the observed vibrational frequencies in the +20° off-specular spectra of Figure 3a,b, are similar to those of liquid-phase C6H6 and C6D6,35,40 respectively, indicating that the benzene adsorbs intact and interacts only weakly with the oxygen-modified surface. This observation is consistent with the TPD data (Figure 2), which show no hydrogen desorption resulting from the decomposition of benzene.

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Figure 4. (a) HREEL spectra (both on-specular and +20° off-specular) of C6H6 adsorbed on the clean Mo(110) surface. (b) HREEL spectra (both on-specular and +20° off-specular) of C6D6 adsorbed on the clean Mo(110) surface.

The shift of the spectral features at 690 and 846 cm-1 in the +20° off-specular C6H6 spectrum to 507 and 656 cm-1 in the +20° off-specular C6D6 spectrum indicates that these modes are C-H (D) vibrations. In the following, the values for C6D6 are given in parentheses. As summarized in Table 1, a comparison with the vibrational assignments for liquid C6H6 and C6D635,40 indicates that the 690 (507) and 846 (656) cm-1 modes correspond to the ν4 and ν11 out-of-plane C-H (D) bending modes of C6H6 (C6D6). Other modes observed for C6H6 (C6D6) include a ν(C-C) mode (ν2) at 967 (927) cm-1, a δ(C-H (D)) mode (ν14) at 967 (798) cm-1, a δ(C-H (D)) mode (ν10) at 1163(850) cm-1, a ν(C-C) mode (ν9) at 1366 (1319) cm-1, a ν(C-C) + δ(C-C) mode (ν13) at 1461 (1319) cm-1, a ν(C-C) (ν16) mode at 1569 (1529) cm-1, and a ν(C-H (D)) mode (ν1) at 3030 (2259) cm-1. In addition, there are some features in Figure 3 that are not related to the vibrational modes of molecular benzene, namely, the mode near 590 cm-1, which is assigned to the Mo-O vibration and the mode at 380 cm-1 which is assigned to the Mo-C stretch of the adsorbed benzene. The sharp decrease in the relative intensity of the ν4 modes in the C6H6 and C6D6 spectra on going from on-specular to +20° off-specular detection indicates that the ν4 mode has a strong dipole scattering component, which suggests (on the basis of the dipole selection rule) that benzene adsorbs with its molecular plane parallel to the plane of the surface. This adsorption geometry is analogous to that found in previous studies of benzene on other single-crystal metal surfaces.10-13,16,19-25,27-30,33,34 Clean Mo(110). Parts a and b of Figure 4 show the HREELS spectra following the adsorption of C6H6 and C6D6, respectively, onto a clean Mo(110) surface. Though early transition metal surfaces generally react strongly with olefinic and aromatic compounds,1,5-7,41-44 the spectra show that neither isotope has decomposed significantly on the clean Mo(110) surface at 80 K. The presence of molecularly adsorbed benzene is indicated by the intense features at 704 (589) and 839 (656) cm-1, which are assigned to the ν4 and ν11 out-of-plane C-H (D) deformations of molecular benzene. Other vibrational features in Figure 4 are assigned as follows: 540 (480) cm-1 [Mo-C stretch]; 1076 (819) cm-1 [δ(C-H (D)) deformation]; 1340 (1326) cm-1 [δ(C-C) deformation], and 3003 (2246) cm-1 [C-H (D) stretch]. These

assignments are summarized in Table 1. In addition, the weak shoulders on the low-energy side of the C-H (D) stretching features at 2730 (2050) cm-1 in the off-specular spectra of Figure 4, might be the result of softened ν(C-H (D)) modes. As discussed previously, the observation of softened ν(C-H) modes is related to the strong interaction between C-H moieties and metal surfaces.45,46 As on the oxygen-modified surface, benzene bonds to the clean Mo(110) surface with its molecular plane essentially parallel to the surface. This bonding geometry is deduced from the strong angular dependence of the intensity of the ν4 mode in the on-specular versus the +20° off-specular spectra of Figure 4. Figure 4 also shows that the ν11 mode at 839 cm-1 is a very intense feature in the on-specular measurements. By comparing the symmetry representations in Table 1, it can be seen that the ν11 mode would become dipole-allowed only if the benzenesurface complex has a symmetry of Cs or lower. Furthermore, the relative intensities of the ν4 (704 cm-1)/ν11 (839 cm-1) peaks remain nearly constant as a function of exposure (Figure 5). This behavior is unlike that observed for benzene on Ni(111),10 Pt(111),10 and Rh(111),16 where the two strongest features in the on-specular spectra change in relative peak intensity ratio on going from low to high benzene coverages, indicating that the benzene molecules adsorb on two different types of surface sites.10 Since the relative peak intensity of the 704/839 cm-1 features shown in Figure 5 stays essentially constant even as the surface benzene coverage increases by a factor of 3 (cf. parts a and c of Figure 5), we conclude that these two features are related to the ν4 and ν11 modes of only one type of adsorbed benzene species, instead of being the ν4 modes of benzene species adsorbed on two types of surface sites. Carbide-Modified Mo(110). Figure 6a,b shows the onspecular and +20° off-specular spectra taken after the adsorption of 3 L of benzene onto a carbide-modified Mo(110) surface. As in the on-specular spectra of the oxygen-modified and clean Mo(110) surfaces, the intense ν4 features (observed at 731 cm-1 for C6H6 and at 528 cm-1 for C6D6 ) in the on-specular spectra indicate that the molecular plane of the adsorbed benzene molecules is essentially parallel to the plane of the surface.

Benzene on Mo(110) Surfaces

Figure 5. On specular HREEL spectra of C6H6/Mo(110) taken as a function of dose. Note that the relative intensity of the ν4 and ν11 modes (marked by dashed lines) do not change as a function of exposure, indicating that there is only one type of adsorption site on the clean Mo(110) surface.

The other modes for benzene adsorbed on the carbidemodified surface are assigned as follows: 846 (643) cm-1 [δ(C-H (D) deformation]; 988 (994) cm-1 [ν(C-C) stretch]; 980 (730) cm-1 and 1109 (812) cm-1 [δC-H (D) deformation]; 1366 (1326) cm-1 [δ(C-C) deformation] and 3030 (2260) cm-1 [ν(C-H (D) stretch]. These assignments are summarized in Table 1. It is important to point out that, for strongly chemisorbed hydrocarbon molecules, both C-C stretching and CH (CD) bending internal coordinates often contribute strongly to the normal modes. Thus, the relative contributions are often different for perhydrogenated and perdeuterated hydrocarbons. As a result, the relative intensities of spectral features in the frequency range between 500 and 1450 cm-1 often differ substantially for perhydrogenated and perdeuterated hydrocarbons,40 as can be seen by comparing the spectra for C6H6 and C6D6 on Mo(110) (Figure 4) and on C/Mo(110) (Figure 6). 3.2.2. Thermal Decomposition of Benzene. Clean Mo(110). The detection of hydrogen desorption in the TPD experiments of Figure 1 indicates that benzene decomposes on the clean Mo(110) surface at or below 350 K. In order to investigate the thermal decomposition mechanism using HREELS, 3 L of C6H6 (or C6D6) were dosed onto a clean Mo(110) surface at 80 K. After heating the Mo(110) crystal to each specified temperature, the crystal was cooled to 80 K again before taking the corresponding HREEL spectrum. Figure 7 shows the HREEL spectra following the decomposition of C6H6. As seen by the similarity among parts a-c of Figure 7, the benzene molecules which do not desorb upon heating are stable and do not react or reorient up to at least 300 K. Upon heating to 350 K, the benzene layer undergoes a marked transformation which can be seen by the appearance of the intense C-H stretching modes in the on-specular spectrum shown in Figure 7d. Since the outof-plane C-H bending modes of the aromatic ring are still present, as seen by the peaks at 744 and 866 cm-1, the carbon ring must be still intact. The appearance of the C-H stretching

J. Phys. Chem. B, Vol. 101, No. 20, 1997 4049 features in the on-specular spectrum would therefore suggest that the entire aromatic ring tilts away from the surface. As explained more fully in the Discussion section, we believe that this species is an adsorbed C6H4 (benzyne) species, as originally proposed by Liu et al.,1 who used a combination of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS) to study the decomposition of benzene on Mo(110). The assignment of the vibrational features of the benzyne intermediate (Figures 7d,e) is facilitated by a comparison against the HREEL spectra of the deuterated benzyne species. The thermal decomposition of 3 L of C6D6 (initially adsorbed at 80 K) to form the corresponding C6D4 species has been monitored by HREELS, as shown in Figure 8. Since the modes related to C-H vibrations should shift upon deuteration, we conclude upon comparing Figure 7d with Figure 8d that the new modes which appear in Figure 7d at 1123, 1157, and 1211 cm-1 after heating to 350 K are C-H deformation modes. Table 2 summarizes the assignment of the vibrational modes for benzyne on Mo(110) as well as on other surfaces. Further studies to determine the temperature corresponding to the onset of benzene conversion to benzyne are shown in Figure 9. In these experiments, a clean Mo(110) surface was first exposed to a 6 L of C6H6 at 80 K and then heated in 25 K increments, starting at 300 K. Assuming that the surface is generally well-ordered, the larger exposure of C6H6 helps to ensure that the features in the subsequent HREEL spectra do not just reflect chemistry that is occurring at minority defect sites. As seen in Figure 9a,b, the C6H6 layer is stable to 325 K. Upon heating the C6H6 layer to 350 K, the HREEL spectrum (Figure 9c) is almost identical to those seen in Figure 7d,e. The similar results obtained for the 3 and 6 L exposures indicate that the underlying chemistry which causes the formation of the benzyne species does not just occur at the defect sites. Furthermore, the fact that the transition to the benzyne intermediate occurs over such a narrow temperature range (between 325 and 350 K) suggests that only one type of decomposition intermediate exists on the surface. This conclusion is based upon the assumption that the formation of more than one decomposition species would probably occur over a broader temperature range, reflecting the likely difference in the activation barriers (i.e., the kinetics) for the formation of the different species. Finally, note that further heating of the C6H6 layer to 375 K (Figure 9d) does not cause new spectral features to appear (cf. Figure 9c), again suggesting that only one type of decomposition intermediate exists on the surface. Additional studies rule out the possibility that upon decomposition a fraction of the benzene disproportionates in such a manner that neighboring benzene molecules become partially hydrogenated. This conclusion is reached by comparing the on-specular spectrum of benzyne (Figure 7d) with the spectra of more hydrogenated cyclic C6 molecules, such as 1,3cyclohexadiene, cyclohexene, and cyclohexane, as shown in Figure 10. A careful inspection reveals that the benzyne spectrum (Figure 7d) is not a superposition of any one of the spectra shown in Figure 10 with that of some other intermediates. Carbide-Modified Mo(110) Surface. Figure 11 shows that an adsorbed C6H6 layer, formed by a 3 L exposure on a carbidemodified Mo(110) surface at 80 K, is stable up to ∼300 K, which is similar to the clean Mo(110) surface. However, the subsequent thermal decomposition of C6H6 occurs by a different reaction pathway. As shown in Figure 11a-c, which are taken at 80, 150, and 300 K, respectively, the benzene layer does not react or reorient in this temperature range. Yet, upon

4050 J. Phys. Chem. B, Vol. 101, No. 20, 1997

Eng et al.

Figure 6. (a) HREEL spectra (both on-specular and +20° off-specular) of 3 L of C6H6 adsorbed on the carbide-modified Mo(110) surface. (b) HREEL spectra (both on-specular and +20° off-specular) of C6D6 adsorbed on the carbide-modified surface.

Figure 7. HREEL spectra monitoring the thermal decomposition of 3 L of C6H6 on the clean Mo(110) surface. The spectra taken at (a) 80, (b) 150, and (c) 300 K are similar, while decomposition to benzyne occurs at 350 K, as seen by the on- and off-specular spectra shown in (d) and (e), respectively.

Figure 8. HREEL spectra monitoring the thermal decomposition of 3 L of C6D6 on clean Mo(110). The spectra taken at (a) 80, (b) 150, and (c) 300 K are similar, while decomposition to benzyne is observed at 375 K, as seen by the on- and off-specular spectra shown in (d) an d (e), respectively.

heating to 350 K, the intensities of the ν4 and ν11 modes decrease (Figure 11d) due to competing desorption and decomposition mechanisms, and by 425 K (Figure 11f) these modes have completely disappeared. Since the decrease in the intensity of these modes, as seen in the on-specular spectra, is not accompanied by a significant increase in the intensity of features in the C-H stretching region (∼3000 cm-1), it is clear that molecular reorientation is not a factor in decreasing the intensities of the ν4 and ν11 modes. The significant differences between Figure 11 and Figure 7 also rule out the possibility that benzene forms benzyne on the carbide-modified Mo(110) surface. We conclude that, on carbide-modified Mo(110),

benzene molecules decompose to produce CxHy species. These species subsequently dehydrogenate further to produce carbon and hydrogen, as seen in the corresponding H2 TPD spectrum shown in Figure 2. The absence of benzyne as a decomposition intermediate on carbide-modified Mo(110) is also supported by HREEL spectra which follow the thermal decomposition of C6D6, as shown in Figure 12. Similar to the case for C6H6, the C6D6 layer is stable in the temperature range from 80 to 300 K (Figure 12a-c) but the intensity of the ν4 mode at ∼528 cm-1 decreases by approximately a factor of 2 upon heating to 375 K. The +20° off-specular spectrum, shown in Figure 12e, only shows broad,

Benzene on Mo(110) Surfaces

J. Phys. Chem. B, Vol. 101, No. 20, 1997 4051

TABLE 2: Vibrational Assignments (in cm-1) of Benzyne on Mo(110) symmetry mode description ν(C-H) ν(C-H) ν(C-H) ν(C-C) ν(C-C) ν(C-C) ν(C-C) ν(C-C) δ(C-H) δ(C-H) ν(C-C) δ(C-H) γ(C-H) γ(C-H) ν(M-C)

Cs

C2V A1 A1

A′ A′

C6H4/Mo(110) C2

1,2-diiodobenzene53

C6H4 theory52

this work

previous work49

C6H4/Os(0001)19

A A

3065 3052

3262

3030

3105

3020

2922 2565 A 1 + B1 A 1 + B1 A1

A′ A′ A′

A B B

1556 1439

A1 B2 A1 A1 A2 B1

A′ A′ A′ + Α′′ A′ Α′′ Α′′

B A B B B A

1160 1100 1086

1415 1379 1113 1065

1576 1468 1387

1510

1157 1123

1130

1395 1290 1155 980

855

Figure 9. HREEL spectra monitoring the thermal decomposition of 6 L of C6H6 on clean Mo(110). The similarity between the spectra shown in this Figure with those taken after a 3 langmuir exposure (Figure 7d,e) indicate that the formation of benzyne does not occur at defect sites.

relatively weak loss features below 1500 cm-1, indicating that the carbon rings of a significant fraction of the benzene have been broken. Finally, it is interesting to note that at 375 K the on-specular spectrum shows only an extremely weak spectral feature in the C-D stretching region (∼2240 cm-1), while an intense feature is observed in the same frequency range in the +20° off-specular spectrum. Based on the surface dipole selection rule, the angular dependence of this spectral feature suggests that the C-D bonds of the CxDy thermal decomposition intermediate at 375 K are nearly parallel to the surface. IV. Discussion 4.1. Adsorption and Decomposition of Benzene on Clean Mo(110). The TPD data in Figure 1 show that benzene adsorbs irreversibly at low exposures (i.e., e1 L) on the clean Mo(110) surface and decomposes upon heating to form atomic carbon and evolve hydrogen. However, even though the adsorption is irreversible at such low exposures, the HREEL spectra in Figures

868 751 581

866 744 568

705

780 580

Figure 10. HREEL spectra of 3 L of 1,3-cyclohexadiene, cyclohexene, and cyclohexane on clean Mo(110). A comparison of the spectra shown in this figure with those shown in Figure 7d,e indicates that no benzene hydrogenation occurs concurrently with dehydrogenation.

4 and 5 suggest that benzene adsorbs molecularly at 80 K. In particular, even for a surface with a relatively low benzene coverage (0.5 L exposure), the HREEL spectra (Figure 5a) show the presence of the characteristic ν4 and ν11 modes of benzene, indicating that the benzene rings are intact on the surface. Furthermore, by comparing against HREEL spectra obtained in previous studies of H2 on Mo(110),47 we conclude that there are no Mo-H (D) vibrational modes in the spectra shown in either Figure 4 or 5, which indicates that decomposition to carbon and surface-bound hydrogen does not occur at 80 K. More specifically, previous HREEL studies showed that the Mo-H stretching vibrations occur at 800 cm-1 for H adsorbed on the pseudo-3-fold sites, at 1215 cm-1 for H adsorbed on the 2-fold “bridge” sites and at 1540 cm-1 for H adsorbed on terminal “atop” sites, with H preferentially residing in the pseudo-3-fold sites.47 Additionally, other HREEL studies have shown that surface-bound hydrogen atoms, derived from the decomposition of cyclohexene on Mo(110), also preferentially reside in the pseudo-3-fold sites of this surface, giving rise to an intense, broad spectral feature centered at approximately 810

4052 J. Phys. Chem. B, Vol. 101, No. 20, 1997

Figure 11. Thermal decomposition of 3 L of C6H6 on the carbidemodified surface. Notice that, unlike the case for the clean Mo(110) surface, a benzyne intermediate is not formed upon heating to 350 K.

Figure 12. Thermal decomposition of 3 L of C6D6 on the carbidemodified surface.

cm-1.48 The absence of such features between 683 and 839 cm-1 in the +20° off-specular spectrum of Figure 4a is a strong indication that the complete dissociation of benzene to form carbon and surface-bound hydrogen does not occur upon benzene adsorption at 80 K. Upon heating to 350 K, the adsorbed benzene forms a tilted ring intermediate which is assigned, as mentioned above, as belonging to a surface-bound benzyne species. This interpretation is based in part on a comparison with the HREEL spectrum of Roberts et al.,49 who studied the formation of surface-bound benzyne groups on Mo(110) from the decomposition of ben-

Eng et al. zenethiol. Since benzenethiol degrades the performance of HREEL spectrometers, it was difficult in those studies to obtain a well-resolved spectrum. Nevertheless, the general shape and position of the HREEL spectral features in those studies agree quite well with the spectra shown in Figure 7d,e. In previous studies of the thermal decomposition of benzene on Mo(110),1 it was shown using NEXAFS that the benzyne species that is formed is oriented such that the molecular plane is perpendicular to the surface. On the other hand, the remarkable similarity between the on-specular and +20° offspecular HREEL spectra of benzyne (Figure 7d,e, respectively) indicates that all of the spectral features exhibit an isotropic angular dependence. Since all of the benzyne vibrational modes are either parallel or perpendicular to the plane of the molecule, in order for all of the modes, both perpendicular and parallel, to exhibit the same angular dependence, the molecule would have to be oriented with the molecular plane tilted nearly 45° away from the surface and not 90° as was shown previously.1 However, the HREEL results presented here do not necessarily contradict the previous NEXAFS results. If the local symmetry (i.e., the symmetry of the adsorbate plus the surface) has been sufficiently reduced due to the presence of disordered coadsorbed species such as hydrogen, both perpendicular and parallel modes may become dipole-allowed, which would then also give rise to benzyne spectral features that have angle-independent intensities, even if the benzyne ring were tilted 90° relative to the surface plane. Analogous behavior has been observed previously for ethylidyne formation on the carbide-modified Mo(110) surface.5 Lastly, we note that studies of benzene decomposition on Os(0001) indicate that the benzyne intermediate that is formed during decomposition adopts an orientation in which the molecular plane is tilted 45° relative to the surface plane.19 Finally, we note that there are no spectral features in Figure 7d,e which correspond to CtC vibrations. Typically, vibrational frequencies for sp-bonded carbon atoms with carbon substituents fall in the 2100-2140 cm-1 range.50 However, gas phase photodetachment studies of the benzyne anion (C6H4-),51 along with recent high-level ab-initio normal mode calculations of benzyne,52 have indicated that the CtC stretching frequency of benzyne is unusually low and occurs at approximately 1850 cm-1. As shown in Figure 7d,e, we do not observe any vibrational features between 1600 and 2600 cm-1, which suggests that the benzyne is not oriented with the carbon-carbon triple bond away from the surface. Instead, the CtC bond probably interacts with the surface, causing the sp-hybridized carbon atoms to rehybridize to sp2. This rehybridization may be the reason that the vibrational frequencies of benzyne, as summarized in Table 2, are rather similar to the vibrational spectrum observed for 1,2-diiodobenzene.53 Also, note that the spectra shown in Figure 7d,e are qualitatively similar to the spectrum reported for benzyne on Os(0001)19 (see Table 2), the main difference being that the frequencies of the out-of-plane C-H deformations for benzyne on Os(0001) are higher than those observed for benzyne on Mo(110), as shown in Figure 7. 4.2. Adsorption and Decomposition on Carbide-Modified Mo(110). As seen in the TPD data (Figure 2) and HREEL spectra (Figure 11), the surface chemistry changes upon going from the clean Mo(110) surface to the carbide-modified Mo(110) surface. In particular, the hydrogen TPD data show that the peak area of H2 from the carbide-modified Mo(110) surface is about two-thirds of that from the clean surface, indicating that the amount of benzene decomposition is reduced by the formation of the carbide.

Benzene on Mo(110) Surfaces The HREEL spectra also reflect the altered surface reactivity that occurs on going from the clean to the carbide-modified Mo(110) surface. Most notably, unlike the case for benzene decomposition on the clean Mo(110) surface, we do not observe the formation of a benzyne intermediate on the carbide-modified surface upon heating to 350 K. In fact, the on-specular HREEL spectrum taken at 350 K looks remarkably similar to those taken in the temperature range between 80 and 300 K (Figure 11ac), aside from the changes in the relative intensity of the 744 and 846 cm-1 modes relative to the rest of the spectral features in Figure 11d. However, a closer inspection reveals that there are some important, though subtle, differences in the HREEL spectra taken between 80 and 350 K. In the spectra taken at 80 and 150 K (Figure 11, a and b), there are features at ∼400 and ∼530 cm-1 which are attributable to benzene-metal (carbide) modes. Upon heating to 300 K, a moderately intense broad feature appears at 453 cm-1 and obscures the modes at 400 and 530 cm-1. At 350 K, a new feature appears at 386 cm-1 and then becomes the dominant feature below 500 cm-1 upon heating to 425 K. These subtle changes in the HREEL spectra shown in Figure 11 are similar to those observed for benzene decomposition on Rh(111) by Koel et al.31 In their extensive studies of benzene decomposition on this surface, a variety of surface analytical techniques were used to show that a mixture of CH and C2H groups are generated at 470 K as the benzene decomposes. Importantly, they showed that the corresponding on-specular HREEL spectrum of this CxHy mixture is deceptively similar to the on-specular spectrum of undecomposed benzene taken at 300 K, except that the benzene-metal features at 345 and 550 cm-1 disappear, and a new mode appears at 475 cm-1. While we have not performed a similar detailed investigation to determine the surface intermediates that are formed during the decomposition of benzene on the carbidemodified surface, we believe that the decomposition mechanism for benzene on the carbide-modified Mo(110) surface is likely to be more similar to that for benzene decomposition on Rh(111) than to that for benzene decomposition on clean Mo(110). More specifically, since we did not detect the presence of any dehydrogenated benzene derivatives (e.g., phenyl or benzyne), the initial step of decomposition on the carbide-modified Mo(110) surface probably involves the breaking of the aromatic six-carbon ring, instead of breaking the C-H bonds. 4.3. Benzene-Metal Interaction Strength and Correlations to the Frequency of the ν4 Mode. As mentioned in the Introduction, the absolute frequency of the ν4 mode for adsorbed benzene has been shown to correlate with certain physical parameters of the late transition metals, such as the cohesive energy (heat of atomization) of the metal,11 the heat of benzene desorption,16 and the change in surface work function upon benzene adsorption.16 Despite their empirical nature, such trends suggest that these physical parameters indirectly reflect the strength of the benzene-metal interaction through the degree of perturbation of the ν4 mode. In the discussion below, we will examine whether or not the approximately linear correlation between the frequency of the ν4 mode and the cohesive energy of the substrate can be extended to describe benzene adsorbed on clean and carbide-modified Mo(110). To our best knowledge, this is the first time that such a correlation has been extended to early transition metal surfaces. As pointed out earlier by Grassian et al.,11 for benzene adsorption onto the (111) faces of certain late transition metals (specifically, Ag, Pd, Ni, Rh, and Pt), the frequency of the ν4 mode increases with the heat of atomization of the respective metal. Figure 13 summarizes this trend and also shows that

J. Phys. Chem. B, Vol. 101, No. 20, 1997 4053

Figure 13. Frequency of the ν4 (out-of-plane C-H deformation) plotted against the cohesive energy (∆Hatomization) of the metal substrate.

the ν4 mode correlates approximately linearly with the heat of atomization. (The line shown in Figure 13 was generated by a linear least-squares fit to the data for the late transition metals.) Thus, for Ag, which has the lowest heat of atomization (68 kcal/ mol) of the five metals that were studied, the ν4 frequency is 675 cm-1 and is barely shifted relative to the gas phase value of 673 cm-1. This extremely small perturbation of the ν4 mode suggests that the adsorbed benzene interacts only weakly with the Ag(111) surface, a prediction which is confirmed by TPD studies of benzene on Ag(111) which show that molecular benzene desorbs from this surface without undergoing thermal decomposition.54 On the other hand, the benzene ν4 frequency is shifted to 830 cm-1 (157 cm-1 higher relative to ν4 in gas phase benzene) when benzene is adsorbed onto Pt, a result which is consistent with the fact that the heat of atomization of Pt is much larger (135 kcal/mol) than that of Ag. Also, it should be noted that this approximately linear relationship between the frequency of the ν4 mode and heat of atomization of the metal substrate appears to be still true upon going to different crystallographic faces, as seen by the similarity in the frequencies of the ν4 mode for benzene on Pd(110) versus Pd(111) and for benzene on Ni(110) versus Ni(111).13,25 We find, however, that Mo does not fit this trend. The heat of atomization of Mo is relatively high (167 kcal/mol),55 and on the basis of the foregoing arguments, one would expect that the ν4 frequency of benzene adsorbed on the Mo(110) surface would be even higher than that observed for Pt(111) (830 cm-1), since the heat of atomization of Mo is 32 kcal/mol higher than that for Pt. However, as seen in Figures 4 and 13, the frequency of the ν4 mode is 704 cm-1 for benzene adsorbed onto the Mo(110), which is close to the value observed for benzene adsorbed on Ag(111).11 Yet, it is clear by H2 TPD spectra shown in Figure 1 that the Mo(110) surface is not inert toward benzene decomposition like the Ag(111) surface, but instead actively decomposes benzene. Thus, we conclude that, while the empirical trend observed by Grassian et al.1 is certainly true for the late transition metals, it does not hold for the early transition metals. Finally, it is interesting to compare the ν4 frequency for benzene adsorbed on a carbide-modified Mo(110) surface with that for benzene adsorbed on a clean Mo(110) surface. In making this comparison, we are assuming that the catalytic activity of the interstitial carbide overlayer used in these studies is similar to that for bulk molybdenum carbide (Mo2C). Thus, we can plot the ν4 frequency for benzene on the carbidemodified surface against the heat of atomization for the bulk molybdenum carbide (∆Hatomization Mo2C ) 527 kcal/mol), as shown in Figure 13.56 Interestingly, although the ν4 frequency for benzene on the carbide-modified surface does not fall on the line shown in Figure 13, it does fall higher than that observed for benzene on the clean Mo(110) surface and is closer in frequency to what is observed for benzene on Rh or Pt surfaces.

4054 J. Phys. Chem. B, Vol. 101, No. 20, 1997 V. Conclusions Benzene adsorbs molecularly at 80 K on clean, carbidemodified, and oxygen-modified Mo(110) surfaces with the benzene molecular plane parallel to the surface. This behavior is similar to what has been observed previously in other HREEL studies of benzene adsorption on low-index transition metal surfaces. Furthermore, only one type of adsorption site is found for benzene adsorption on the clean Mo(110) surface. The interaction of benzene with the oxygen-modified surface is relatively weak, as seen by the reversible molecular benzene adsorption/desorption on this surface in TPD experiments, as well as by the fact that the vibrational frequencies for adsorbed benzene are barely perturbed relative to those for the free molecule. In contrast, the interaction between benzene and the clean Mo(110) surface is considerably stronger, as indicated by the detection of H2 in the TPD measurements and by the observation of benzyne as a stable intermediate around 350 K in the HREELS measurements. The HREEL spectra indicate that the molecular plane of the benzyne species is tilted away from the plane of the surface. The surface reactivity of the clean Mo(110) surface is changed upon the formation of an interstitial carbide layer. The TPD data show a decrease in the H2 peak area and a concomitant increase in the molecular benzene desorption peak area from the carbide-modified surface relative to that for the clean surface. Furthermore, the HREEL spectra show that, unlike on the clean Mo(110) surface, the decomposition mechanism of benzene on the carbide-modified surface does not involve the formation of a benzyne intermediate. Instead, the subtle changes in the spectra are reminiscent of benzene decomposition on Rh(111),31 in which C2H and CH fragments are formed upon heating to 470 K. Finally, the approximately linear correlation between the absolute frequency of the ν4 mode and the cohesive energy of the metal substrate, which is observed for benzene adsorbed on certain late transition metals,11 cannot be extended to include the cases in which benzene is adsorbed on either clean Mo(110) or carbide-modified Mo(110). In fact, the ν4 frequency for benzene adsorbed on the clean Mo(110) surface falls roughly 150 cm-1 lower than what would be expected based on the trend observed for the late transition metals. This behavior may reflect the influence of a variety of competing effects, and more experimental and theoretical work is needed in order to understand the origins of such trends. Acknowledgment. Financial support from the National Science Foundation (Grant CHE-93-18625) is gratefully acknowledged. One of us (J. Eng, Jr.) gratefully acknowledges the financial support given by the Exxon Summer Internship Program. References and Notes (1) Liu, A. C.; Friend, C. M. J. Chem. Phys. 1988, 89, 4396. (2) Levy, R. L.; Boudart, M. Science 1973, 181, 547. (3) The Chemistry of Transition Metal Carbides and Nitrides; Oyama, S. T., Ed.; Blackie Academic and Professional: Glasgow, 1996. (4) Chen, J. G. Chem. ReV. 1996, 96, 1477. (5) Fru¨hberger, B.; Chen, J. G. Surf. Sci. 1995, 342, 38. (6) Fru¨hberger, B.; Chen, J. G.; Eng Jr., J.; Bent, B. E. J. Vac. Sci. Technol. A 1996, 14, 1475. (7) Fru¨hberger, B.; Chen, J. G. J. Am. Chem. Soc. 1996, 118, 11599. (8) Huber, W.; Steinru¨ck, H. P.; Pache, T.; Menzel, D. Surf. Sci. 1989, 217, 103. (9) Ramsey, M. G.; Steinmu¨ller, D.; Netzer, F. P.; Schedel, T.; Santaniello, A.; Lloyd, D. R. Surf. Sci. 1991, 251/252, 979. (10) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577. (11) Grassian, V. H.; Muetterties, E. L. J. Phys. Chem. 1987, 91, 389. (12) Wadill, G. D.; Kesmodel, L. L. Phys. ReV. B 1985, 31, 4940.

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