Mo(100) Surfaces - The

J. Phys. Chem. B 2000, 104, 3035-3043

3035

Surface Chemistry of Methyl Radicals on O/Mo(100) Surfaces† Seong Han Kim‡ and Peter C. Stair* Department of Chemistry, Northwestern UniVersity, EVanston, Illinois 60208 ReceiVed: August 2, 1999; In Final Form: NoVember 5, 1999

The chemistry of CH3 radicals on oxygen-modified Mo(100) surfaces (O/Mo(100)) has been studied using temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Gas-phase CH3 radicals were produced by pyrolysis of azomethane and dosed on O/Mo(100) at a surface temperature of 320 K. In TPD, O/Mo(100) with θO ) 1.4 monolayer (ML) produces exclusively CH4 and CO, but O/Mo(100) with θO ) 0.9 and 0.4 ML produce significant amounts of C2+ alkenes in addition to CH4 and CO. HREELS shows that the CH3 groups are bound to surface Mo atoms, not to surface oxygen. On 1.4 ML-O, the CH3 groups are stable at 320 K and have a symmetry lower than C3V. On 0.9 ML-O and 0.4 ML-O, some CH3 groups decompose to methylene groups, which react with intact CH3 groups to form surface alkyl groups. The surface species at 320 K appear to be controlled by the preadsorbed oxygen coverage, depending on whether θO < 1 ML or θO > 1 ML. CH4 is formed via hydrogenation of CH3 groups by surface hydrogen that is a product of CH3 decomposition. C2+ alkene products are formed by β-hydrogen elimination of surface alkyl groups. When atomic iodine is coadsorbed on O/Mo(100), the alkene yield in TPD is significantly reduced.

I. Introduction C1 fragments, CHx (x e 3), bound to metal surfaces are believed to be important intermediates in many heterogeneous catalytic processes such as oxidative coupling of methane to produce higher hydrocarbons, partial oxidation of methane to produce methanol or formaldehyde, and Fischer-Tropsch synthesis from CO and H2.1-4 However, a knowledge of their surface reactions at the molecular level is limited. To understand the elementary reactions of C1 intermediates on metal surfaces, the surface chemistry of methyl fragments has been studied under controlled ultrahigh vacuum (UHV) conditions.5 In many of the UHV studies, adsorbed methyl groups were produced by the dissociation of methyl halides on transition metal surfaces.6 Methyl halides can be adsorbed molecularly on metal surfaces at liquid nitrogen temperature and then activated, either thermally or photochemically, to produce methyl groups on the surface. With this procedure the retention of halogen atoms on the surface is inevitable. The coadsorbed halogen atom may limit the surface coverage of CH3 to a value significantly lower than that achieved under high-pressure catalytic reaction conditions.5 Moreover, coadsorbed halogen atoms may alter the surface chemistry of interest via siteblocking or electronic effects. In an attempt to avoid the potential problems of coadsorbed halogen, several methods have been developed to produce adsorbed methyl fragments in the absence of adsorbed halogen atoms: adsorption of gas-phase CH3 radicals produced by pyrolysis of azomethane,7 collision-induced dissociation of CH4,8 soft landing of CH3+ ion,9 and low-energy electron bombardments of CH4 adsorbed on metal surfaces.10 In this paper, the surface chemistry of CH3 on an oxygenmodified Mo(100) surface [hereafter referred to as O/Mo(100)] †

Part of the special issue “Gabor Somorjai Festschrift”. Present address: Department of Chemistry, University of California, Berkeley, CA 94720. * Corresponding author. Fax (847) 467-1018. E-mail [email protected]. ‡

has been studied using a methyl radical source derived from the pyrolysis of azomethane as the means to produce adsorbed methyl groups.7 A preliminary report of the temperature programmed desorption (TPD) data has been presented elsewhere.11,12 O/Mo(100) was chosen for study because molybdenum oxides are active catalysts for the partial oxidation of CH4 and the oxidative coupling of CH4.3,13-16 Molybdenum metal itself has also shown good catalytic activity for hydrogenation of CO under high-pressure condition.17 The geometric and electronic structure of O/Mo(100) surfaces have been characterized extensively by UHV surface science.18-22 CH3 radicals can be produced in the gas phase and allowed to impinge on O/Mo(100) under conditions where none of the side products of azomethane pyrolysis adsorb on the surface.11 Using TPD and high-resolution electron energy loss spectroscopy (HREELS), it was found that the surface reactions of CH3 on O/Mo(100) at 320 K are sensitive to the oxygen coverage (θO), particularly whether θO < 1 monolayer (ML) or θO > 1 ML. The effect of coadsorbed iodine was also investigated by dosing methyl radicals and atomic iodine from the gas phase. TPD data indicated that coadsorbed iodine hinders C-C bond formation on the O/Mo(100) surface. II. Experimental Section The UHV chamber used in this study has been described in detail elsewhere.23 The upper level of the chamber has instrumentation for surface cleaning with a cold-cathode argon ion gun, high temperature heating by electron bombardment, Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature-programmed desorption (TPD), and gas adsorption by directed dosing or back-filling. The lower level contains a high-resolution electron energy loss spectrometer (HREELS) housed in a µ-metal shield. The chamber is pumped with a 500 L/sec turbomolecular pump backed with an oil diffusion pump and a mechanical pump to a base pressure of ∼2 × 10-10 Torr.

10.1021/jp9927193 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/28/1999

3036 J. Phys. Chem. B, Vol. 104, No. 14, 2000

Kim and Stair

Figure 1. TPD of CH4 from (a) 1.4 ML-O and (b) 0.9 ML-O as a function of CH3 exposure at 320 K. The insets show the exposure dependence of the area of mass 16 in TPD.

Figure 2. TPD of (a) H2 and (b) H2O after dosing CH3 on 0.9 MLO/Mo(100) at 320 K. The inset shows the exposure dependence of the area of mass 2 in TPD.

The Mo(100) crystal was press fit into a 0.5 mm Mo wire holder attached to a manipulator which provided translation and rotation of the sample, liquid nitrogen cooling to 125 K, and resistive heating to 1000 K for TPD experiments. A W-5%Re/ W-26%Re thermocouple was attached to the backside of the crystal. The (100) surface was cleaned with Ar+ sputtering to remove sulfur and calcium and flashed to 1100 K after exposure to O2 to remove carbon. The clean surface showed no detectable impurities, except oxygen, in AES. Oxygen overlayers were then formed on the surface by exposing the surface to O2 (∼5 × 10-9 Torr) at a crystal temperature of 320 K followed by annealing at high temperatures to desorb excess oxygen (1000 ∼1450 K depending on the desired oxygen coverage) and to sharpen the LEED patterns of the ordered overlayers. The LEED patterns of c(4 × 4)+Maltese Cross, (2 × 2)+weak(4 × 2), and diffuse(1 × 1) were determined to be 0.4, 0.9, and 1.4 ML of oxygen on the Mo(100) surface, respectively.18-20 A collimated effusive beam of methyl radicals was produced by low-pressure thermal decomposition of azomethane (CH3N2CH3) using a heated quartz tube at ∼1200 K, as described in detail elsewhere.7 The side products of azomethane pyrolysis (N2, CH4, and traces of C2H6 and H2) do not adsorb on the O/Mo(100) surface at 320 K.11 (The residual temperature of the crystal was ∼320 K due to the hot methyl radical source in the UHV chamber.) To produce methyl radicals and iodine atoms on the surface, methyl iodide was pyrolyzed using the same methyl radical source used for azomethane. Line-of-sight quadrupole mass spectrometry (QMS) analysis indicated that ∼10% of the CH3I was decomposed at a source temperature of

∼1300 K. The exposures reported in this paper [1 Langmuir (L) ) 1 × 10-6 Torr‚sec] were calculated from the background pressure rise and were not corrected for the yield of methyl radicals produced in the pyrolysis (typically ∼20% of the total product in the case of azomethane) or the pressure enhancement at the crystal surface caused by directed dosing. The TPD experiments were typically performed at 2 K/s after rotating the CH3-dosed surface to face the QMS. The desorption flux was spatially filtered by a 1 cm aperture ∼1 cm from the sample to minimize signals from the side and back of the sample. The coverage of desorbing CO and hydrocarbon molecules was quantified from TPD peak areas, corrected for QMS fragmentation and relative ionization sensitivity,24 using as a reference the CO TPD peak area measured for the saturated β-CO state on 0.3 ML-O/Mo(100), which corresponds to 0.35 ML of surface carbon.25 The HREELS experiments were performed at the specular direction with an incidence angle of 60°. The incident electron energy was 3.8 eV and the typical resolution was 7∼9 meV (57∼73 cm-1). All HREELS measurements were carried out at 320 K. For the annealing measurements, the crystal was briefly annealed to the desired temperature by resistive heating and allowed to cool to 320 K before the HREEL spectrum was recorded. III. Results 1. TPD. Figure 1 shows the TPD of CH4 (16 amu), which is the main hydrocarbon product, after dosing to various coverages

Methyl Radicals on O/Mo(100) Surfaces

J. Phys. Chem. B, Vol. 104, No. 14, 2000 3037

Figure 3. TPD from (a) 1.4 ML-O, (b) 0.9 ML-O, and (c) 0.4 ML-O after dosing CH3 to the saturation yield of CH4 formation. Note that only representative masses are shown.

of CH3 on O/Mo(100) with θO ) 1.4 ML (Figure 1a) and θO ) 0.9 ML (Figure 1b) at 320 K.11 Because H2 does not adsorb on these surfaces at 320 K, desorption of CH4 is an indication that adsorbed methyl, CH3(ads), dehydrogenates to produce hydrogen on the surface, which reacts with intact CH3(ads) to form CH4.11 Note that the amount of CH4 produced in TPD is much smaller from the surface with θO ) 1.4 ML than with θO ) 0.9 ML. As shown in the insets to Figure 1, the CH4 yield from 1.4 ML-O reaches saturation quickly at a CH3 exposure of 0.1 L while that from 0.9 ML-O levels off slowly over 1 L of CH3. Figure 2 shows H2 and H2O TPD after dosing CH3 on 0.9 ML-O/Mo(100). H2 desorbs over a very wide temperature range, fwhm ∼200 K, suggesting that there is more than one site for hydrogen formation. The dependence of H2 yield on CH3 exposure, shown in the inset to Figure 2a, is almost identical to that of the CH4 yield. Figure 2b compares the H2O (18 amu) TPD profiles after dosing 0.02 and 1.0 L of CH3 on the 0.9 ML-O surface. The desorption peak at ∼390 K originates from background water adsorption. However, the broad shoulder at higher temperature in the H2O TPD (T > 500 K) can be seen only after dosing CH3 on the surface. The difference profile after 1.0 and 0.02 L of CH3 exposure clearly shows that the low-temperature peak (background water) decreases and the high-temperature peak (T > 500 K) increases at higher CH3 exposure. For the 1.4 ML-O surface, the H2 and H2O(>500 K) yields are significantly reduced, but the general features of the TPD peaks are similar to those for 0.9 ML-O. Desorption of both H2 and H2O(>500 K) may be associated with dehydrogenation of CH3(ads) on O/Mo(100) to produce H-Mo and HO-Mo surface species, respectively. Figure 3 compares the TPD profiles of various masses after dosing CH3 to produce a saturation yield of CH4 on the 1.4 ML-O, 0.9 ML-O, and 0.4 ML-O surfaces. The 1.4 ML-O

surface shows only a very small amount of C2H4 (26 amu) and no higher hydrocarbons. Most of the 28 amu peak at 380∼650 K is R-CO. However, the 0.9 ML-O and 0.4 ML-O surfaces show significant yields of higher hydrocarbons up to C5 over a wide temperature range from 380 to 620 K. The desorption peaks were identified as alkenes [C2H4 (26 amu), C3H6 (41 and 42 amu), C4H8 (56 amu), and C5H10 (70 amu)] from detailed analysis of the mass fragmentation patterns. It is also interesting to note that, regardless of the preadsorbed oxygen coverage, there is no desorption of C2+ alkanes (for example, 30 amu from C2H6, 43 and 44 amu from C3H8, 58 amu from C4H10). The small 44 amu peak at ∼600 K corresponds to a trace of CO2. The absence of C2H6 is strong evidence that direct coupling of methyl groups does not occur on O/Mo(100). Note also that desorption peaks for CH3O+(31 amu) and CHO+(29 amu) are not detected at any oxygen coverage, indicating the absence of alcohol or aldehyde formation via partial oxidation of CH3(ads) on O/Mo(100). The yields of the C1∼C4 hydrocarbon products detected by TPD, after dosing CH3 to the saturation yield of CH4, are shown in Figure 4. The yields were calculated by fitting the TPD peak areas taking into account the mass spectrometer sensitivity and fragmentation of methane and the linear alkenes. There is about 1 order of magnitude difference in the saturation yield of CH4 between 1.4 ML-O and 0.9 ML-O but no significant difference between 0.9 ML-O and 0.4 ML-O. C-C bond formation is much more probable on 0.9 ML-O and 0.4 ML-O, compared to 1.4 ML-O. Higher hydrocarbon yields for 1.4 ML-O are significantly smaller compared to 0.9 ML-O and 0.4 ML-O. The C2H4/CH4 ratio is ∼0.01 for 1.4 ML-O, while it is ∼0.08 for 0.9 ML-O and 0.4 ML-O. The yield for subsequent alkenes decreases by ∼70% for 0.9 ML-O and ∼60% for 0.4 ML-O,


Figure 4. Hydrocarbon product distributions in TPD measured after dosing CH3 on 1.4 ML-O, 0.9 ML-O, and 0.4 ML-O surfaces to the saturation of CH4 formation.

Kim and Stair

Figure 6. Carbon uptake of O/Mo(100) at 320 K from gas-phase CH3. Each data point is obtained by adding all the hydrocarbon and CO yields in TPD.

Figure 5. Total yield of CO and relative amount of R-CO and β-CO in TPD after dosing CH3 to the saturation yield of CH4 formation on O/Mo(100).

indicating the surface alkyl chain growth probability is ∼0.3 and ∼0.4, respectively. The desorption of CO (28 amu) in Figure 3 is proof that some of the CH3 decomposes completely to atomic carbon and hydrogen.11 Figure 5 shows the total CO yield and the relative fraction of R-CO (T < 750 K) and β-CO (T > 750 K) for various preadsorbed oxygen coverages. The total amount of CO desorption increases with decreasing oxygen coverage. This can be attributed to the dissociative adsorption of CH3 on bare metal patches.11,29 This interpretation is in line with the decreasing yield of β-CO with increasing θO. β-CO results from the reaction of atomic carbon with atomic oxygen on metallic molybdenum.25 When the oxygen coverage is 1.4 ML there is no desorption of β-CO; all of the atomic carbon desorbs as R-CO because at this high oxygen coverage the surface is covered by an oxide film.21,22,30 Figure 6 shows the carbon uptake curves on O/Mo(100) obtained by adding the hydrocarbon yields (Figure 4) and CO

Figure 7. TPD of 16 and 26 amu after dosing (a) 0.1 L of CH3 on 1.4 ML-O and (b) 0.04 L of CH3 on 0.9 ML-O.

yields (Figure 5) obtained from TPD. The carbon uptake on O/Mo(100) appears to be very sensitive to whether the preadsorbed oxygen coverage is higher or lower than 1 ML. From 1.4 ML-O to 0.9 ML-O, the saturation carbon coverage increases by 1 order of magnitude. The carbon uptake increases by only 30% between oxygen coverages of 0.9 and 0.4 ML. At similar hydrocarbon coverages, the formation of C2H4 from CH3 on O/Mo(100) is specifically sensitive to whether the oxygen coverage is above or below 1 ML. Figure 7 compares the C2H4 yields on 1.4 ML-O and 0.9 ML-O for CH3 coverages that produce nearly the same yield of CH4. Since CH4 is the major carbon containing product desorbed from the surface after CH3 exposure, the similar yield of CH4 implies a comparable coverage of hydrocarbon species on both surfaces. However, the yield of C2H4 is ∼8 times larger on the 0.9 ML-O surface than the 1.4 ML-O surface. These results suggest that the change in C2H4/CH4 ratio between 1.4 ML-O and 0.9 ML-O or 0.4



Figure 9. HREEL spectra of (a) CH3O and (b) C2H5O species produced by dosing CH3OH and C2H5OH on 0.9 ML-O/Mo(100) at 320 K, respectively.

Figure 8. HREEL spectra of (a) clean 0.9 ML-O/Mo(100) and after dosing CH3 at 320 K of (b) 0.06 L, (c) 0.21 L, and (d) 0.53 L. Also shown are HREEL spectra measured after dosing 0.53 L of CH3 at 320 K followed by annealing to (e) 430 K and (f) 650 K.

ML-O is not just due to differences in CHx coverage, i.e., the increase in C2H4 yield is not due simply to the second-order kinetics of C-C bond formation. 2. HREELS. Figure 8 shows HREEL spectra measured after dosing CH3 on 0.9 ML-O/Mo(100) at 320 K, as a function of CH3 exposure and annealing temperature. On the 0.9 ML-O surface before CH3 dosing (Figure 8a), the strong peak at 545 cm-1 and the weak peak at 735 cm-1 are assigned to the vibrations of oxygen atoms located in 3-coordinate sites on the Mo(100) surface which exhibits a (2 × 2)+weak (4 × 2) LEED pattern.31,32 After a very small dose of CH3 (Figure 8b), this peak shifts to 580 cm-1 and peaks at 450 and 980 cm-1 grow significantly. The low loss energy peaks are assigned to 3-coordinate oxygen. The peak at 980 cm-1 is assigned to oxygen terminally bonded to a single molybdenum atom, ν(ModO).31,32 These changes in the Mo-O vibrational modes suggest that adsorption of CH3 induces a restructuring of the O/Mo(100) surface at 300 K. With increasing CH3 exposure (Figures 9b-d), the Mo-O vibrational modes (450, 575, and 980 cm-1) become weaker and peaks at 940, 1160, 1445, 2950, and 3610 cm-1 become stronger. The peak at 3610 cm-1 can be assigned to the O-H stretching mode of surface hydroxyl groups. The peaks at 940, 1160, 1445, and 2950 cm-1 are C-H vibrations attributed to hydrocarbon species which will be described later in this section. If the CH3-dosed surface is annealed to 440 K, corresponding to the peak desorption temperature of CH4 (Figure 8e), the hydrocarbon vibrational modes become weaker, without any shift in frequency or growth of any new peaks, and the Mo-O vibrational modes become stronger. Finally, after annealing at 650 K (Figure 8f), all of the hydrocarbon vibrational modes disappear, and the 3-coordinate Mo-O vibrational modes are recovered.

Figure 10. HREEL spectra for (a) 1.4 ML-O, (b) 0.9 ML-O, and (c) 0.4 ML-O after dosing CH3 to the saturation yield of CH4 formation.

It is interesting to note that, for all CH3 exposures, there is no strong vibrational loss peak at ∼1050 cm-1 which corresponds to the C-O stretching mode of alkoxy species adsorbed on the O/Mo(100) surface as shown in Figure 9.33-35 Also, the TPD of methoxy and ethoxy groups, produced by dosing methyl


Kim and Stair

TABLE 1: Comparison of HREELS Data from CH3 + 1.4 ML-O/Mo(100) with Adsorbed CH3 Groups with a Symmetry Lower than C3W Mo(100)-O 1.4 ML Cu(111)b 3585 vwa 3290 s, sh 2975 s, br 1430 s 1140 s 985 w

2945 m, 2790 w 1380 s 1190 s

Cu(100)c

Ru(001)d

2915, 2910 m 2760 s, br 1430s 1340 m, sh 1150s 1180 s

890 w

mode Pt(111)e assignments

2950 s, 2790 w 1410 s 1180 s 820 m

760 m 490-590

ν(OH) ν(OH) ν(CH3) δa(CH3) δs(CH3) ν(ModO)f Fs(CH3) ν(Mo-O)g ν(Mo-O)g

a s ) strong; m ) medium; w ) weak; vw ) very weak; sh ) shoulder; br ) broad. b CH3 from pyrolysis of azomethane.38 c Thermal decomposition of CH3I/Cu(100).28 d Thermal decomposition of CH3I/ Ru(001).39 e Thermal decomposition of CH3I/Pt(111).40 f Oxygen at the terminal site.31 g Oxygen at multiple sites.31

alcohol and ethyl alcohol on O/Mo(100) at 320 K (data not shown), reveals product distributions and desorption profiles that are totally different from those observed after adsorption of CH3 on O/Mo(100). The absence of the alkoxy C-O vibrational mode is consistent with previous X-ray photoelectron spectroscopy (XPS) data that showed no C(1s) peak corresponding to CH3O(ads) after dosing CH3 on 0.9 ML-O/Mo(100).11 Both the changes in Mo-O vibrational modes and the absence of alkoxy species indicate that CH3 is bound to surface metal atoms rather than to surface oxygen atoms. Figure 10 compares the HREEL spectra measured after dosing 1 L of CH3 on the three different O/Mo(100) surfaces. First, note that the HREEL spectra of CH3-dosed O/Mo(100) surfaces, at 320 K, do not change for up to 5 h in UHV after dosing. This suggests that chemical equilibrium is reached during or immediately after dosing CH3. Second, the C-H deformation region (1100 ∼ 1460 cm-1) after dosing CH3 on the 1.4 ML-O surface differs from those on the 0.9 ML-O and 0.4 ML-O surfaces, indicating that different hydrocarbon species are present on the surface depending on whether the oxygen coverage is above or below 1 ML. The vibrational mode assignments have been carried out by comparison with the published HREEL spectra for hydrocarbon species on metal surfaces.36-48 The C-H vibrational modes on 1.4 ML-O are given in Table 1. The relative intensity of δs(CH3) and δa(CH3) indicates that the symmetry of the CH3

adsorbed on 1.4ML-O is not C3V. If the CH3(ads) has C3V symmetry with the figure axis perpendicular to the surface, the δa(CH3) mode will produce a vibrational dipole moment parallel to the surface. Since the vibrational dipole moments from such “parallel modes” are screened by image dipoles in the metal surface (dipole selection rule), their intensity in HREELS will be very weak along the specular direction. This has been observed for HREEL spectra from CH3 on Ni(111), Cu(111), and Rh(111) where C3V symmetry for CH3(ads) is compatible with the C3V structural symmetry of the fcc(111) surfaces.27,36,37 For CH3 on 1.4 ML-O/Mo(100), however, the δs(CH3) and δa(CH3) modes are both intense, suggesting that CH3(ads) has a symmetry lower than C3V. HREELS data for CH3(ads) with a symmetry lower than C3V are shown in Table 1 for comparison. The relative intensities in the C-H deformation region measured by HREELS after dosing CH3 on 0.9 ML-O/Mo(100) and 0.4 ML-O/Mo(100) surfaces, are not compatible with the vibrational modes of C1 species (CH3, CH2, and CH) adsorbed on metal surfaces.41-44 Instead, the general features in this fingerprint region appear to be a combination of vibrational spectra for CH3 and C2+ alkyl species on metal surfaces.45-48 Though the relative coverage of alkyl groups on the surface is expected to be small compared to adsorbed methyl groups (Figure 4), the presence of these alkyl groups is revealed by differences in the C-H deformation vibration region evident from Figure 10 spectrum a compared to spectra b and c. The vibrational band assignments from the 0.9 and 0.4 ML O/Mo(100) surfaces are summarized in Table 2 and compared to the band assignments reported for ethyl and propyl species on various metal surfaces. 3. Coadsorbed Iodine. Iodine was coadsorbed on the 0.9 ML-O/Mo(100) surface by pyrolyzing ∼10% of the CH3I flux impinging on the surface. The formation of adsorbed methyl groups by C-I bond dissociation in molecular CH3I on the surface occurs only to a very small extent as indicated by a negligible amount of CH4 observed in TPD (dotted line in Figure 11). Atomic iodine adsorbed on metallic surface sites desorbs at temperatures higher than 670 K with a peak temperature of 750 K. The small 127 amu peaks at 430 and 600 K correspond to desorption of molecular CH3I and iodine adsorbed on small islands of high oxygen coverage, respectively.49 The iodine adsorbed on metallic sites significantly reduces the yield of C2H4. For two surfaces giving nearly the same yield of CH4,

TABLE 2: Comparison of HREELS Data of CH3 + 0.9 ML-O/Mo(100) and 0.4 ML-O/Mo(100) with Alkyl Groups Adsorbed on Metal Surfaces Mo(100)-O 0.9ML

C2H5 on Cu(111)b

C 2H 5 on Pt(111)c

3270 vwa 2950 s, br

3290 w 2955 s, br

2935 s

2918 s, br

2780 w, sh 1430 s

2780 w, sh 1440 s

2745 w 1430

1450 s, br

1120 w, br

1355 w 1140 vw, br

1140 m

930 m

920 w

475-595

475-555

C3H7 on Cu(111)d

C3H7 on Al(100)e

2950 s

2945 s

2740 w

2885 br 1450 s

1376 s, br 1173 m

1385 s 1165 vw

1385 m, sh 1160 w

950 s, br

1022 s

880 s

806 s, br

710 w

941 s

720 w

mode assignments ν(OH) ν(CH3) ν(CH2) soft ν(C-H) δa(CH3) δ(CH2) δs(CH3) ω(CH2), τ(CH2) ν(C-C), F(CH3) F(CH3), F(CH2) ν(Mo-O)

s ) strong; m ) medium; w ) weak; vw ) very weak; sh ) shoulder; br ) broad. b Thermal decomposition of C2H5I/Cu(111).45 Photodissociation of C2H5Cl/Pt(111).46 d Thermal decomposition of 1-C3H7Br/Cu(111).47 e Thermal decomposition of 1-C3H7I/Al(100).48 a

c

Mo(100)-O 0.4ML



CH3(g) f CH3(ads) CH3(g) f CH2(ads) + (1-x)H(ads) + xOH(ads)

(S1) (S2)

CH3(ads) h CH2(ads) + (1-y)H(ads) + yOH(ads) (S3) CH2(ads) h C(ads) + (2-z)H(ads) + zOH(ads) (S4) CH2(ads) + CH3(ads) h CH3CH2(ads)

(S5)

CH2(ads) + CH3CH2(ads) h CH3CH2CH2(ads) (S5′) During TPD at T > 350 K:

Figure 11. TPD of 16, 26, and 127 amu from 0.9 ML-O/Mo(100) (a) without iodine (pyrolysis of azomethane) and (b) with surface iodine (pyrolysis of CH3I). Also shown is the TPD of 16 amu from 0.9 ML-O after dosing CH3I without pyrolysis (dotted line).

the yield of C2H4 (26 amu in Figure 11) is ∼15 times lower with atomic iodine on the surface than in the absence of atomic iodine. IV. Discussion 1. Reaction Mechanisms for CH3 on O/Mo(100). The evolution of C2+ alkenes from 0.9 ML-O and 0.4 ML-O surfaces at temperatures higher than 350 K reveals many aspects of the CH3 surface chemistry on O/Mo(100). First, the presence of C3+ alkenes indicates that C-C bond formation is not the ratedetermining step. Otherwise, C-C bond formation would lead to immediate desorption of C2H4 and no C3+ species would be produced. Second, the desorption of alkenes in this temperature range is a reaction-limited process. The desorption of molecularly adsorbed C2H4 and C3H6 is complete below 280 and 330 K, respectively (data not shown). If these alkenes were produced during CH3 dosing at 320 K, they would immediately desorb into the gas phase before the TPD experiments. Instead, the desorption kinetics of C2+ alkenes after dosing CH3 are in good agreement with C2H4 desorption from β-hydrogen elimination of C2H5 directly adsorbed on O/Mo(100).50 Combined with the HREELS results, it can be concluded that C2+ alkyl groups are formed on the 0.9 ML-O and 0.4 ML-O surfaces during CH3 dosing at a surface temperature of 320 K. The mechanism of alkyl growth on the surface is most likely methylene insertion reactions, as observed on Cu surfaces by Bent and coworkers.26-28 Methylene groups, CH2(ads), produced by dehydrogenation of CH3 on O/Mo(100), insert into the carbonsurface bond of adsorbed alkyl groups. A sequence of reactions consistent with the surface species observed in HREELS and products monitored by TPD follows. During dosing at 320 K:

CH3(ads) + H(ads) f CH4(g)

(S6)

CH3CH2(ads) f C2H4(g) + H(ads)

(S7)

CH3CH2CH2(ads) f C3H6(g) + H(ads)

(S7′)

H(ads) + OH(ads) f H2O(g)

(S8)

2H(ads) f H2(g)

(S9)

C(ads, oxide) + O(ads) f R-CO(g)

(S10)

C(ads, metallic) + O(ads) f β-CO(g)

(S11)

At 320 K, CH3(g) reacts with Mo metal atom(s) to form CH3-Mo, but not with oxygen to form CH3O-Mo (reaction S1). In reactions S2 and S3, CH3 dissociates to produce methylene groups, CH2(ads), at 320 K. Reaction S2 is a direct dissociation channel (Eley-Rideal reaction) and reaction S3 is an adsorption-mediated dissociation channel (Langmuir-Hinshelwood reaction). Reaction S2 may be important because the gas-phase CH3 radicals impinging on the surface are at the elevated temperature of the pyrolysis doser (∼1000 K).51 Further dehydrogenation of CH2(ads) can occur to produce atomic carbon and either surface hydrogen or hydroxyl groups (reaction S4). At the same time, CH2(ads) can insert into carbon-metal bonds, forming adsorbed C2+ alkyl groups (reaction S5). Reaction S5 must be competitive with reaction S4.12 Otherwise, the adsorbed CH3 would all dehydrogenate to atomic carbon. There is currently no direct evidence for the reversibility of reactions S3-S5′; however, we include this possibility for completeness. Hydrogenation of CH3(ads) to form CH4 occurs as the surface temperature increases in the TPD process (reaction S6). Reaction S7 is β-hydrogen elimination of the alkyl groups to release the corresponding alkenes.26-28 Reaction S8 accounts for the formation of H2O. Reaction S9 is the association of surface hydrogen to form H2. Reactions S6-S9 are kinetically favored due to irreversible removal of products from the surface and drive the equilibrium of reactions S3 and S4 to the right. Since the amount of H2O and H2 detected in TPD is very small for all oxygen coverages used in this study, most of surface hydrogen appears to be consumed by reaction S6. In fact, reactions S8 and S9 are largely delayed until temperatures are reached where all of the CH3 is consumed (see Figure 3). Reactions S10(R-CO) and S11(β-CO) represent the formation of CO from oxygen and atomic carbon located on surface oxide sites and metallic sites, respectively. The formation of C2+ alkyl groups is remarkably sensitive to whether the preadsorbed oxygen coverage is above or below 1 ML. To understand this it is important to know what step or steps in the surface reaction mechanism are responsible for the difference in products formed. A priori, formation of C2+ alkyl groups on 1.4 ML-O could be suppressed, compared to 0.9

3042 J. Phys. Chem. B, Vol. 104, No. 14, 2000 ML-O and 0.4 ML-O, by one or a combination of factors: [A] The coverage of adsorbed CH2 on 1.4 ML-O is much smaller than on 0.9 ML-O and 0.4 ML-O because (1) reactions S2 and S3 are slow, (2) the S3 equilibrium favors CH3, or (3) the S4 equilibrium favors complete dehydrogenation of adsorbed CH2. [B] On 1.4 ML-O the rate of CH2 insertion into carbon-surface bonds (reactions S5 and S5′) is much slower than is CH2 dehydrogenation (reaction S4). [C] The rate of β-hydrogen elimination is much faster on 1.4 ML-O compared to the 0.9 ML-O and 0.4 ML-O surfaces so that when CH3CH2(ads) is formed during methyl radical dosing at 320 K it rapidly converts to ethylene which desorbs immediately. Among these possibilities the experimental evidence favors explanation B. Possibilities A1 and A2 would reduce the coverage of adsorbed hydrogen as well as the coverage of CH2(ads). Such a reduction in hydrogen coverage should be observable as a lower rate or higher threshold temperature for CH4 TPD on 1.4 ML-O compared to the 0.9 ML-O and 0.4 ML-O surfaces. This is not the case. The threshold temperature and rate of methane formation TPD are essentially the same on all three surfaces. If the S4 equilibrium favored complete dehydrogenation much more on 1.4 ML-O than on the submonolayer O/Mo(100) surfaces (possibility A3), then the yield of CO should be correspondingly higher, but exactly the opposite trend is seen in Figure 5. Finally, β-hydrogen elimination from CH3CH2(ads) on 1.4 ML-O actually has a higher threshold temperature than on 0.9 ML-O, ruling out possibility C.50 The production of alkenes, but no alkanes, from alkyl groups on O/Mo(100) is similar to the results from high-pressure reactions. Somorjai and co-workers reported that, for the catalytic reaction of CO and H2 (CO:H2 ) 1:2, 6 atm, 625 K), Mo(100) produces primarily CH4, C2H4, and C3H6, while other transition metals produce solely CH4 (Ni) or a distribution of alkanes (Fe, Ru, Rh, and Co).17 This result implies that Mo is not a good hydrogenation catalyst even in the presence of hydrogen. For the catalytic oxidative coupling of CH4,14,15 Kiwi and co-workers also found that molybdate catalysts have a relatively high selectivity to C2H4 over C2H6, compared to alkaline earth metal oxide and rare earth metal oxide catalysts. Further study is necessary to understand why alkyl groups on O/Mo(100) undergo the β-hydrogen elimination reaction in preference to hydrogenation by surface hydrogen. 2. Role of Preadsorbed Oxygen. From the arguments above it appears that the rate of methylene insertion, reaction S5, is Very sensitiVe to whether the preadsorbed oxygen coVerage is below or aboVe one monolayer. However, the present results do not allow us to separately assess the effects of surface structure and electronic properties on reactions S2-S5. Both factors change significantly as the oxygen coverage increases from below 1.0 ML to above 1.0 ML. For 0.3 ML e θO < 1 ML, the average oxidation state of surface molybdenum is about +1.3 and the electronic polarizability is very close to the value of the clean Mo(100) surface, whereas the Mo(100) surface with θO ) 1.4 ML exhibits a surface Mo oxidation state and polarizability of MoO2.21,22 In the same oxygen coverage region the surface undergoes significant restructuring.31 Comparison of the alkene yields for 0.4 ML-O and 0.9 ML-O in Figure 4 suggests that the presence of a certain coverage of preadsorbed oxygen is not critical for alkyl chain formation so long as the coverage is below 1.0 ML. The LEED pattern of 0.4 ML-O is c(4 × 4)+Maltese Cross and that of 0.9 ML-O is (2 × 2)+weak(4 × 2). This difference in surface structure results in changes in the TPD profiles as shown in Figure 3. On 0.4 ML-O, the hydrocarbon products desorb with much narrower

Kim and Stair fwhm than on 0.9 ML-O, consistent with a more homogeneous distribution of active sites. The complete dissociation channel (reaction S4) is favored on 0.4 ML-O and the desorption of β-CO is increased by a factor of 3.5. As a result, more surface hydrogen is produced resulting in recombinative desorption into H2,52,53 especially at temperatures higher than 500 K where no more CH3 is available to produce CH4. However, there is no significant change in the distribution of hydrocarbons as seen in the comparison of 0.9 ML-O and 0.4 ML-O in Figure 4. The relative alkene yields to methane are similar on both surfaces. The geometry of the basal surface, Mo(100) vs Mo(110), also appears to be critical for the interaction of gas-phase CH3 radicals and oxygen-modified Mo surfaces. Friend and coworkers have studied the reaction of gas-phase CH3 with O/Mo(110) where oxygen occupies the quasi three-fold site.54 When CH3 is dosed on this surface, CH3 reacts with oxygen to form CH3O-Mo. A significant fraction of the methoxy groups formed on Mo(110) undergoes dehydrogenation and hydrogenation, producing methane and atomic carbon, and the rest desorbs as CH3 radicals at high temperatures (T > 500 K). This behavior is in contrast to that of O/Mo(100) where CH3 reacts preferentially with Mo to form CH3-Mo. Further study is necessary to understand why the surface chemistry of CH3 on oxygenmodified Mo surfaces in UHV is so sensitive to the atomic structure of the molybdenum surface. 3. Effect of coadsorbed iodine. Coadsorption of iodine hinders the bimolecular C-C bond formation reaction on the O/Mo(100) surface. As shown in Figure 11, the yield of C2H4 is significantly reduced by the presence of atomic iodine on the 0.9 ML-O surface. Since the desorption temperature of CH4 is lower when surface iodine is present (Figure 11), the main effect of coadsorbed iodine may be to reduce the rate of methylene insertion. Therefore, coadsorbed iodine appears to exert an effect similar to preadsorbed oxygen (Figure 7). It should be noted that the effects of coadsorbed iodine on C-C bond formation between adsorbates on metal surfaces appear to be metal specific. On Cu(111), where methylene insertion occurs immediately following dehydrogenation of CH3(ads) above 400 K, the coadsorbed iodine has no significant effect on the surface chemistry, except to limit the surface coverage of CH3(ads).38 However, on Ag(111) where dehydrogenation of CH3(ads) does not occur, the presence of surface iodine lowers the barrier for the direct coupling reaction to form ethane.55-57 V. Conclusion On oxygen-modified Mo(100) surfaces, gas-phase CH3 radicals react with Mo atom(s) to form CH3-Mo, but not with O-Mo to form CH3O-Mo. For θO > 1 ML, CH3(ads) is stable at a surface temperature of 320 K and has a symmetry lower than C3V. Upon heating, it decomposes to atomic carbon and surface hydrogen or hydroxyl groups. The surface hydrogen reacts rapidly with intact CH3(ads) to form CH4(g). There are additional reaction channels for CH3(ads) on O/Mo(100) with θO < 1 ML. At 320 K, some CH3 groups dehydrogenate to methylene groups, CH2(ads), which react with intact CH3(ads) to form C2H5(ads) and subsequently C3+ alkyl groups. These surface alkyl groups on O/Mo(100) desorb as the corresponding alkenes via β-hydrogen elimination at higher temperature. Coadsorbed atomic iodine lowers the yield of C2H4 on O/Mo(100).

Methyl Radicals on O/Mo(100) Surfaces Acknowledgment. The authors gratefully acknowledge support for this work by the National Science Foundation under Grant No. CHE-9305937. References and Notes (1) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (2) Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. Chem. Soc. ReV. 1989, 18, 251. (3) Pitchai, R.; Klier, K. Catal. ReV.-Sci. Eng. 1986, 28, 13. (4) Rofer-DePoorter, C. K. Chem. ReV. 1981, 81, 447. (5) Bent, B. E. Chem. ReV. 1996, 96, 1361. (6) Zaera, F. Acc. Chem. Res. 1992, 25, 260. (7) Peng, X. D.; Viswanathan, R.; Smudde, G. H., Jr.; Stair, P. C. ReV. Sci. Instrum. 1992, 63, 3930. (8) Lee, M. B.; Yang, Q. Y.; Tang, S. L.; Ceyer, S. T. J. Chem. Phys. 1986, 85, 1693. (9) Strongin, D.; Mowlem, J. Chem. Phys. Lett. 1991, 63, 175. (10) Alberras-Sloan, D. J.; White, J. M. Surf. Sci. 1996, 365, 212. (11) Smudde, G. H., Jr.; Yu, M.; Stair, P. C. J. Am. Chem. Soc. 1993, 115, 1988. (12) Kim, S. H.; Stair, P. C. J. Am. Chem. Soc. 1998, 120, 8535. (13) Exdohelyi, A.; Fodor, K.; Solymosi, F. J. Catal. 1997, 166, 244. (14) Kiwi, J.; Thampi, K. R.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1990, 1690. (15) Kiwi, J.; Thampi, K. R.; Gratzel, M.; Albers, P.; Seibold, K. J. Phys. Chem. 1992, 96, 1344. (16) Driscoll, S. A.; Gardner, D. K.; Ozkan, U. S. J. Catal. 1994, 147, 379. (17) Logan, M.; Gellman, A.; Somorjai, G. A. J. Catal. 1985, 94, 60. (18) Bauer, E.; Poppa, H. Surf. Sci. 1979, 88, 31. (19) Ko, E. I.; Madix, R. J. Surf. Sci. 1981, 109, 221. (20) Zhang, C.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1985, 149, 326. (21) Fryberger, T. B.; Stair, P. C. Chem. Phys. Lett. 1982, 93, 151. (22) Grant, J. L.; Fryberger, T. B.; Stair, P. C. Surf. Sci. 1985, 159, 333. (23) Parker, B. R.; Jenkins, J. F.; Stair, P. C. Surf. Sci. 1997, 372, 185. (24) Beran, J. A.; Kevan, L. J. Phys. Chem. 1969, 73, 3866. (25) Felter, T. E.; Estrup, P. J. Surf. Sci. 1976, 76, 464. (26) Chiang, C.-M.; Wentzlaff, T. H.; Bent, B. E. J. Phys. Chem. 1992, 96, 1836. (27) Lin, J.-L.; Bent, B. E. J. Vac. Sci. Tech. 1992, A10, 2202. (28) Lin, J.-L.; Chian, C.-M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. Catal. 1994, 147, 250. (29) Deffeyes, J. E.; Smith, A. H.; Stair, P. C. Surf. Sci. 1985, 163, 79.

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Mo(100) Surfaces - The

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