The adsorption of C2H4 on the Mo(110) surface and the evolution of the surface with temperature

Surface

56

The adsorption of C,H, on the Mo( 110) surface of the surface with temperature

Science 245 (1991) 56-64 North-Holland

and the evolution

M.B. Young and A.J. Slavin Departmenr of Physics, Trenr University, Peterborough, Ontario, Canada K9J 7BB and Department of Physics, Queens University, Kingston, Ontario, Canada K7L 3N6 Received

3 August

1990; accepted

for publication

2 November

1990

The adsorption of C,H, on the Mo(ll0) surface near room temperature and at 1100 K was studied with Auger electron spectroscopy, low energy electron diffraction, thermal desorption spectroscopy and electron energy loss spectroscopy. At both temperatures, adsorption saturated after exposures of about 6 langmuirs, (1 L = 10m6 Torr s), indicating the formation of a single layer which passivates the surface to further C2H, adsorption. During thermal desorption, following adsorption near room temperature, C,H, dissociates completely by 750 K to surface carbon and desorbed H,; no hydrocarbon desorption was observed. For adsorption above 750 K, the hydrogen desorbs at once leaving only carbon on the surface. No ordered overlayer was found at 240 K or during subsequent heating; however, following a vacuum anneal after saturation at either 800 or 1100 K a Mo(llO)-(4 X 4)-C structure was formed. The ratio of the number of carbon to molybdenum atoms on this surface was C : MO = 0.37 + 0.06, assuming unity sticking probability for C,H, on the clean surface. Thermal indiffusion of carbon from the (4 X 4) surface begins at about 1400 K and becomes very rapid by 1600 K. When the C : MO ratio fell to about 0.2, a second previously unreported structure, M~llO)-( -2)-C formed. These overlayers probably result from che~sorbed carbon rather than from the formation of a true surface carbide.

1. Introduction

This work is part of an on-going study of the adsorption of hydrocarbon gases of low molecular weight on Group VB and VIB metals, and of the carbon-bearing surfaces which result on heating; it follows work on Ta(ll0) [l] and W(110) [2,3]. The adsorption of ethylene (C,H,) [4] and acetylene (C,H,) I.51 on Malt) has been studied by other researchers, while the structures of the “carbided” Mo(100) and (111) surfaces have also been investigated [4,6,7]. To our knowledge the present study is the first published on C,H, adsorption on the Mo(l10) surface and the evolution of the surface on heating. MO and W have a number of very similar characteristics: both are group VIB transition elements with bee structure and lattice constants of 0.315 and 0.316 nm, respectively; both have simiIar band structures [8] and surface density-of-states 19,101. This suggests that a stable surface carbide 0039-6028/91/$03.50

8 1991 - Elsevier Science

Publishers

might exist on Mo(lf0) as has been studied on W(110) (see ref. [3], and references therein), While tungsten carbides have greater technological importance than molybdenum carbides, the catalytic properties of molybdenum and its compounds are also of interest [11,12].

The two molybdenum samples used were spark-cut from a single crystal ingot (5N pure, Metals Research, Cambridge, UK) to produce disks about 10 mm in diameter and 1.5 mm thick with faces parallel within 0.5 o to the (110) planes. The disks were ground and polished to mirror finish with 1 pm diamond paste, then electropolished using a solution of hydrochloric and sulphuric acids in methanol [13]. The samples were spot-welded to 0.5 mm diameter tantalum support wires, connected via copper braids to

B.V. (North-Holland)

MB. Young, A.J. Slavin / Adsorptionof C2H,, on the Mo(ilO] surface

liquid nitrogen cold fingers. A W-5%Re/W26%Re the~~ouple was spot-welded to the back of each sample, with a small (2 mm x 2 mm) piece of platinum foil between the disk and thermocouple to ensure good bonding. The samples could be heated resistively using AC current to about 1450 K; higher temperatures were obtained by electron bombardment from a thoriated tungsten filament. No conta~nation from this filament was ever detected with Auger electron spectroscopy (AES). The thermocouple was calibrated against an optical pyrometer for incandescent temperatures, and against a Chromel/Alumel thermocouple for temperatures from 90 to 500 K. These results gave excellent agreement with the calibration data supplied by the manufacturer, Hoskins Alloys. The temperatures quoted are believed to be accurate to +5 K. The ultrahigh vacuum system has been described elsewhere [1,2]; the base pressure with both the electron gun and mass spectrometer running was 5 X lo- lo Torr. A 4-grid retarding potential analyzer was used for AES, low energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS), using the coaxial electron gun and normal beam incidence in each case. LEED was performed with the sample at room temperature. The AES data were collected using a beam energy of 1500 eV, beam current of 15 PA, 6.0 V peak-to-peak modulation voltage and sweep rates of 2 eV/s. The EELS data were obtained with a beam energy of 70 eV, a beam current of 0.5 PA, 1.0 V peak-to-peak modulation voltage and sweep rates of 0.35 eV/s. These sweep rates are slow enough to avoid distortion of the spectroscopic peaks [14]. AES and EELS signals were detected using a lock-in amplifier tuned to the second and first harmonics, respectively, of the modulation signal. Gas pressures were monitored by an ion gauge which was calibrated using a capacitance manometer and are accurate to 10%. A Dycor quadrupole mass spectrometer was used for thermal desorption spectroscopy (TDS) and to monitor the residual gases. The sample was heated resistively during the TDS runs, using a constant applied voltage. This gave a heating rate which was acceptably linear with a value of 15 K-s-’ over the temperature range of interest. During the

57

desorption runs the sample was rotated to face the ionization chamber of the mass spectrometer, which was capable of sampling up to 8 mass/ charge channels as a function of time. An IBM-PC computer equipped with a Tecmar Labmaster interface board was used to control the AES, EELS and TDS runs and for data collection and analysis. The initial cleaning of the sample required several exposures of 1 to 2 h at 1200 K and lo-“ Torr of 0, to remove carbon, followed by 30 s flashes to 2000 K in vacuum to remove oxygen. The samples were judged to be clean when the carbon and oxygen Auger signals were below the noise level of the system, typically 1% of the 221 eV MO Auger peak. The clean samples were found to be highly reactive to the residual gases at room temperature, adsorbing both H, and CO. These background adsorbates could be desorbed by flashing the sample to 1200 K. Holding the sample above 1200 K for extended periods, to prevent H, and CO adsorption while allowing impurities to diffuse to the surface [15] showed no build-up of carbon, which suggests that there was no significant quantity of carbon in the bulk. C,H, was adsorbed onto the clean samples, held either between 240 and 300, or at 1100 K, with one exposure at 800 K. The C,H, pressure during the adsorption runs was typically 2 X 10B8 Torr. Following the low temperature runs, TDS was employed to observe the desorbing gases. The thermal indiffusion of the surface carbon above 1400 K was followed by AES and LEED taken near room temperature. During all the adsorption runs the carbon C(272 eV) Auger signal had the same three-lobed “carbidic” lineshape 116-181. Since no changes in the carbon lineshape were observed, the carbon coverages were taken to be directly proportional to the observed Auger peakto-peak heights. To account for system variations between runs, the carbon signal was normalized by dividing by the clean Mo(221 eV) Auger peakto-peak height. This normalized peak-height ratio will be denoted C/MO; the ratio of carbon-tomolybdenum atoms on the surface will be denoted C : MO. At saturation by C,H, at 1100 K, the Mo(221 eV) signal typically was attenuated by about 8% as compared to the clean sample,

M.B. Young, A.J. Slavin / A&orption of C, H4 on the Mo(Il0) surface

58

It was attempted to fit the adsorption runs to the equation for first order kinetics [1,2], h(t)

= h,(l

- eeKP’),

(I)

where h(t) is the C/MO peak-height ratio after an exposure of f s, h, is the saturation C/MO peakheight ratio, P is the partial pressure of C,H,, and K=

&h

200

where m is the mass of a C,H, molecule, k is Boltzmann’s constant, T is the adsorbing gas temperature, s is the sticking probability and N is the surface density of carbon atom sites. The factor of 2 in the numerator accounts for the number of carbon atoms per C,H, molecule.

3. Results 3.1. Low temperature adsorption and thermal evolution of the surface The 240 K adsorption runs showed poor reproducibility during the initial uptake (fig. la), although saturation always occurred for C/MO ratios of h, = 0.11 + 0.02 after dosages of about 6 L. Eq. (1) did not give a reasonable fit to these runs. During the adsorption the initial sharp Mo( 1 lo)-(1 x 1) LEED pattern exhibited a marked

0.24

I4

I

300

400

500

600 T

700

800

900

1000

(K)

Fig. 2. (a) H, desorption following a 1 L dose of C,H, at 240 K (b) H, desorption following a 10 L dose of C,H, at 240 K.

increase in the background intensity, but no new spots were observed. This indicates a disordered overlayer. Subsequent TDS showed that for C,H, exposures of 1 L (fig. 2a) a single broad H, desorption peak occurred at 410 K. With increasing dosages this peak broadened and developed a long tail to higher temperatures (fig. 2b). No C,H, or hydrocarbon fragment desorption was observed. After desorption, annealing at temperatures up to 1600 K did not produce ordered overlayers. Indiffusion of the carbon started at 1400 K and was complete after 60 s at 1600 K. EELS for the clean surface (fig. 3a) gave essentially the same spectrum as measured by others [19,20]. There was little change in EELS for the saturated sample

I

-40 Exposure

CL)

Fig. 1. (a) C,H, uptake at 240 K, showing the ratio C/MO of the Auger peak-to-peak heights, normalized to the clean Mo(221 eV) peak, as a function of C,H, exposure. (b) C,H, uptake at 1100 K. The solid curve shows the fit to eq. (1). Different symbols represent separate runs.

-35

-30

-25

-20

loss

anwgy

-15

-10

-5

u

0

(OV)

Fig. 3. (a) EELS of clean Mo(ll0) and following a saturation dose (10 L) of C,H, at 240 K. The sample temperature was i 300 K for both spectra. (b) EELS of clean Mo(ll0) and following the formation of the (4x4) carbon overlayer. The sample temperature was 800 K for both spectra.

with only a slight attenuation energy of I1 eV.

of the peak at a loss

3.2. High temperature adsorption and thermal evolution of the surface

The adsorption runs at 1100 K (fig. lb) showed that saturation also occurred by about 6 L with

h, = 0.20 zt 0.02, a~~ro~mately twice the low temperature value. A sharp M~~l~~-~l X 1) LEED pattern (see fig, 4a for notation) with faint streaks aligned in the (OJ) direction was found at saturation. Subsequent vacuum annealing at 1100 K for 5 to 10 min produced a (4 x 4) pattern (fig. 4b) indicating an ordered overlayer. The C/MO ratio remained at 0.20 rt 0.02 with the C(272 eV) signal

M.B. Young, A.J. Slavin / Adsorption of C, H4 on theMo(lIO)surface

60

still showing a carbidic lineshape. When the sample was given a saturation dose (10 L) of C,H, at 800 K the C/MO ratio was identical to the 1100 K value but no additional LEED spots were observed. After a 10 min vacuum anneal at 1100 K the (4 X 4) pattern appeared. After saturation of the surface with carbon above 800 K, EELS again showed no significant changes beyond the slight attenuation of the 11 eV loss peak (fig. 3b). The (4 x 4) overlayer was not re-exposed to C,H, at lower temperatures, so it is not known if the surface is passivated to C,H, under such conditions. Thermal indiffusion of carbon from the (4 X 4) structure began to occur slowly at 1400 K, while at 1600 K indiffusion was complete after 60 s (fig. 5). A second LEED pattern (fig. 4c), resulting from a (_i i) ordered overlayer (see Ohtani et al. [21], for notation) formed during indiffusion at 1400 and 1450 K after the C/MO ratio had fallen to about 60% of the saturation value. This pattern persisted until the C/MO ratio reached about 35% of the initial value, after which only the Mo(llO)(1 x 1) pattern was observed. Above 1450 K, indiffusion was so rapid that the (_i i) pattern was not observed. The (4 x 4) surface was not totally passivated to residual gas adsorption at room temperature. Flashing the sample after it had been sitting for more than 15 min produced pressure bursts in the ion gauge readings with peaks at 480 and about 980 K. The first is due to H,; the second is 1. 2

I

0.0’ 0

J

200

400

Fig. 5. Thermal indiffusion ratio C/MO is normalized Sample at 1365 K ( * ). (b) at 1450 K (0, 0, A).

600 (-3) t,me

BOO

1000

1200

from the MO-(4 x 4)-C surface. The to the initial saturation values. (a) Sample at 1400 K ( x ). (c) Sample (d) Sample at 1500 K (-, 0).

attributed to desorbing CO [22,23]. When the (4 x 4) surface was exposed to the residual gases at room temperature, AES showed a gradual increase in the C signal and the corresponding appearance and growth of an 0 peak. Unfortunately, a failure of the mass spectrometer prevented any TDS studies of these adsorbates.

4. Discussion For the low temperature adsorption runs the carbon Auger strengths at saturation were quite reproducible, with a C/MO ratio of 0.11 + 0.02, representing a single layer of adsorbate that passivated the surface to further C,H, adsorption. This C/MO ratio gives an atomic ratio of C : MO = 0.20 k 0.07, obtained by scaling to the value obtained from fitting eq. (1) to the 1100 K runs, as discussed below. However, the room temperature adsorption runs could not be reasonably modelled by eq. (l), as was also found for both C,H, and C,H, adsorption at room temperature on W(110)

PI. The inconsistencies between adsorption runs near room temperature could be due to two causes. The first possibility is competition for adsorption sites by residual H, and CO during the cooling of the sample, which took about 10 min after being flashed to 1200 K, of which about 8 min was required for the temperature to fall from 600 to less than 300 K. Traces of C and 0 were commonly observed on the sample by AES by the time the C,H, was admitted to the chamber. The second possibility is the formation of different C,H, dissociation products due to variations of the sample temperature between runs. Studies of the Mo(ll0) surface [24,25], the Mo(100) surface [5,6,26] and the Mo(ll1) surface [4], indicate dissociative adsorption of hydrocarbons, thiophene, methanol, formaldehyde and carbon monoxide at low exposures for temperatures of 180 to 300 K. The absence of C,H, or any hydrocarbon fragments in the TDS data shows that C,H, completely dissociates before or during the TDS measurements. It has been shown [25,27,28] that saturation dosages of hydrocarbons adsorbed on transition metal surfaces generally undergo a frag-

M. B. Young, A.J. Slavin / Adsorption of C, H4 on the Mo(I JO)surface

mentation cascade to less hydrogenated species as the surface temperature is increased. This typically results in an H, TDS spectrum with a broad peak in the 300 to 400 K range and a long tail stretching to 700 or 800 K before desorption is complete, as also seen in this work. The similarity of fig. 2a with the H, desorption following H, adsorption on clean Mo(ll0) [24,29] suggests that for a 1 L C,H, exposure all the hydrogen desorbs directly from the MO surface with little interaction with the coadsorbed carbon. It is not possible to decide from these measurements whether dissociation occurs on adsorption or during heating. A reasonable explanation for the long tail to about 750 K for the 10 L C,H, exposure (fig. 2b) is that during higher exposures active sites on the MO surface become saturated with hydrogen and hydrocarbon fragments, preventing further dissociation. As H, desorbs during TDS, MO sites become available to assist in dehydrogenation. The concurrent build-up of carbon on the surface could both block MO sites and increase the time required for hydrogen atoms to migrate together on the surface to form H, and desorb. This hypothesis could be tested by an isothermal desorption study at 500 K to see if H, desorption goes to completion at this temperature. It is important to note that the final C: MO ratio after a 10 L C,H, exposure is only about 0.2, so the tail does not merely reflect H, desorption from a carbon-saturated surface rather than from a MO surface. It is not possible to provide a detailed discussion of the dehydrogenation process without the use of a vibrational spectroscopy, such as high resolution EELS, to identify the hydrocarbon fragments. For the adsorption runs at 1100 K, the formation of a single layer of carbon on the Mo(l10) surface is indicated by the good fit of eq. (1) (fig. lb), the decrease in the MO Auger signal of only 8% at saturation and the absence of evidence for indiffusion of carbon at this temperature. The first-order adsorption model of eq. (1) gives fitted parameters h, = 0.196 + 0.003 and KP = 0.011 -& 0.001 s-l. Assuming unity sticking probability as found on W(110) and Ta(ll0) [1,2], the carbon adsorption-site density N may be found from eq. (2). Using the known atomic density of the

61

Mo(ll0) surface 1301, gives C : MO = 0.37 I 0.06. A similar study [2] of C,H, adsorption at 1100 K on the W(110) surface found a concentration of C: W = 0.64 rfr 0.05 for the saturated surface and complete passivation against the uptake of H, and CO. The higher reactivity of the saturated Mo( 110) surface to residual H, and CO, compared to the saturated W(110) surface, may be partially due to the lower C concentration on MO. These results are similar to those for carbon saturation of the Mo(l11) surface [4] which resulted in a reduction in oxygen uptake, but not complete passivation. The EELS results following both low and high temperature saturation exposures show a decrease in the loss peak at 11 eV but no evidence of either peak shifts or new features. The 11 eV peak has been attributed to the excitation of surface plasmons [19,20,31,32], even though its energy does not agree well with the theoretical value Aw,/fi= 16.3 eV, where Aw, = 23.1 eV is the bulk plasmon energy. The discrepancy is due to the influence of inter-band transitions at nearby energies 131,331, which can cause significant shifts in the energies of both the surface plasmon and the inter-band transitions. That the 11 eV peak is slightly attenuated is reasonable in the low temperature case where the atomic ratio C : MO = 0.20 is low. Essentially identical results are obtained for the high temperature case, where the C: MO ratio is approximately double the room temperature value and a (4 x 4) overlayer has formed. The observed relative insensitivity of EELS to carbon adsorption is similar to results reported for chemisorbed carbon on Fe(lOO) [18]. In that study the EELS spectra of clean iron, a chemisorbed carbon c(2 x 2) overlayer and a bulk iron carbide (Fe, Cr),C were compared. The clean iron and chemisorbed carbon spectra were similar, the presence of carbon producing a slight attenuation of features below about 25 eV, whereas the 15 eV peak, attributed to a combination of a bulk transition to a band above the Fermi level and a surface plasmon, is almost completely absent in the bulk carbide spectra. This suggests that the (4 x 4) overlayer found in this work is due to chemisorbed carbon and not to a surface carbide layer. Unfortunately we are not aware of any reported EELS studies of the bulk molybdenum carbides.

62

M.B. Young, A.J. SIavin / Adwrption of C, Ha on the Mo(ll0)

The shoulder at about 8 eV (fig. 3a) is probably due to the adsorption of residual H,, since no other contaminants were observed with AES. A peak from hydrogen on Mo(ll0) has been reported near this position [29]. The curves of fig. 3b were both obtained with the sample held at 800 K, above the H, desorption temperature, which would explain the absence of this peak in these curves. Proposed structural models for the surfaces detected by LEED are shown in fig. 6. The adsorbed carbon atoms are assumed to lie in hollows at the centers of the primitive unit cells of the Mo(ll0) substrate lattice. The (4 x 4) pattern is generated by a coincidence surface lattice with a 5-carbon atom basis as indicated by the diamonds. The primitive unit cell of the surface lattice contains 16 molybdenum atoms, giving C : MO = 0.31 which is just within the uncertainty of the experimental value of C : MO = 0.37 + 0.06 calculated above. A 6-carbon atom basis, formed by adding one more carbon atom to the proposed 5-carbon atom basis, (as indicated by the X’s in fig. 6) gives a ratio of C : MO = 0.38 which is in better agreement with

000*0

0

X/B

0 0 ,0 0 c O~oooepoooqoosooo dooh 0 0 0 x ,0’-0 -0 0 0 0 p \8 osdoo~~~eoopsoshoo 0 k ,0’ 0 0 x;+ 0,o 0 l 0 0 O ,O 0 Z-Q. C 0 s&e 0 ‘0. ,0’ 0 0 X‘@,O0 0 0 : 0 Q% O 6-@ooo‘@h~OOAY’~OOOOO 0 \O 0 0 oxaeo 000.0 0 0 0 0 ‘r 0~000~0~000~0~000 0 0 0 0 OXOOO 0

*

0

\8,

-0

0

0

0

x;!Q,o

0

0 Q

O

0

‘3, O oO,@I 0 ‘0 0 0 +3’0 p.0 O

\O r\D 0 0’ ep’o C 0 d 0 0 0 0

0’

0

0 0

0

0

1( 0 0 C

,0

0’

0

0 0

0

0

0

0 0

0

0 I 0

0 0

0

C 0 0

0 OXOOO

0

0 0

Fig. 6. Proposed structures for the (4X4) and (_: :) surfaces. Primitive unit meshes for the coincidence lattices are indicated by dashed and by dotted lines respectively. Substrate molybdenum atoms are indicated by o; 0 indicates carbon atoms belonging to the (4x4) surface; and 0 indicates carbon atoms belonging to the (-ii) surface. The arrows indicate carbon atoms which must relocate to form the second surface. The positions denoted by x are discussed in the text.

surface

the experimental value. However, a simple structure-factor calculation based on this 6-atom basis predicts the extinction of some fractional order spots which was not observed. It is interesting to note that a very similar LEED patern has been reported [34] for carbon segregated to a Mo(ll0) surface. The (_: :) pattern is generated by a coincidence lattice with a 5-carbon atom basis as shown in fig. 6. The primitive unit cell of the coincidence surface lattice contains 24 molybdenum atoms, giving a ratio of C : MO = 0.21 which agrees well with the value of C : MO = 0.22 f 0.06 at which the second LEED pattern was first observed. The transformation from the (4 x 4) to the (_: :) pattern requires the indiffusion of about 35% of the carbon atoms and a rearrangment of about 6% of the remaining ones, as indicated by the arrows in fig. 6. C,H, exposures at 1100 K which resulted in a C/MO ratio of only 60% of the saturation value did not produce either the (4 X 4) or (_: :) pattern. A saturation dose of C,H, near room temperature, followed by an 1100 K anneal also failed to produce either pattern, even though the amount of carbon present is greater than that at which the (_: :) pattern disappeared during the indiffusion runs. Unfortunately, no adsorption experiments were carried out near 1400 K to test whether the (4 x 4) surface was a necessary precursor to the second pattern. The thermal indiffusion of carbon from the (4 x 4) surface does not follow the theory for either classical diffusion from a plane source [35] or the model of indiffusion from carbide edges proposed for the W(110) surface [3]. The (4 x 4) overlayer is also less stable than the surface carbide on W(llO), for which the (15 X 3)R14” LEED pattern persists until the C/W ratio falls to about 10% of the saturation value, and thermal indiffusion remains slow below about 1600 K [3]. In comparison, carbon deposited on the less densely packed Mo(ll1) surface [4] by the thermal decomposition of C,H, was observed to begin rapid indiffusion at about 1300 K. Based on our results we therefore suggest that the two ordered overlayers for carbon on the Mo(ll0) surface result from chemisorbed carbon and not from a surface carbide as is found on

MB.

l%ung, A.J. Siavin / Adrotption

W(llO), in spite of the presence of a “carbidic” Auger lineshape. A recent study 1181 of the Fe(lOO)-C system has shown that the presence of such a lineshape does not necessarily imply the formation of a surface carbide. Althou~ the Auger carbon lineshapes in this study are similar to reported lineshapes for a bulk MO& sample [16], they are not identical. One clear difference is the large variation in the asymmetry factors (the ratio of the heights of the positive and negative lobes of the derivative mode lineshapes (see ref. 1361, and references therein) of 1.7 for the bulk carbide and 0.8 for this study, which cannot be solely attributed to differences in instruments. Moreover no shift was seen in the MO peak on carbon adsorption, whereas a -1.5 eV shift has been observed between clean MO and bulk MqC [16]. The lack of persistence to low carbon coverage of either LEED pattern on Mo(llO), the non-passivation to adsorption of H, and CO and the slight effect on the MO surface plasmon peak suggest a relatively weak interaction between the C and MO. It would be very interesting to see if the W(lI0) surface plasmon peak is significantly perturbed by the formation of the (15 x 3)R14O carbon overlayer, which appears to be unambiguously that of a true surface carbide.

5. Conclusions Adsorption of C,H, on the Mo(ll0) surface near room temperature likely results in a complex series of hydrocarbon fragments, the exact nature of which depends critically on the the adsortion temperature and the presence of coadsorbed residual gases. The surface saturates by an exposure of 6 L. Heating this sample results in the emission of only H,, with desorption being complete by about 500 K for low (1 L) exposures and by about 7.50 K for saturation exposures. Saturation dosages at temperatures above 800 K, followed by annealing at 1100 K, results in a (4 X 4) overlayer which transforms to (_: :) as carbon in-diffuses. These overlayers are quite unlike the W(110) surface carbide, despite the close similarities between the geometries of the clean metal surfaces, the electronic configurations of the

ofC,H,

on the Mo(liO)

surface

63

atoms and the band structures of the metals. In particular, the indiffusion of the carbon from the carbide on W(llO> occurs at carbide edges, allowing the (15 X 3)R14’ carbide to persist as islands down to carbon coverages as low as 10% of the saturation value. In contrast, the presence of an order-order transition for the Mo(llO)-C system and the smaller carbon concentration at saturation indicate substantial differences in the carbonmetal interactions between the two metals. We suggest that the ordered carbon overlayers on M~llO) result from chemisorbed carbon and not from a true surface carbide as occurs on W(110).

The authors gratefully acknowledge F. Londry for the development of some of the data acquisition and analysis software used and K. Fowler, W. King and J. Tomlinson for their technical assistance. Financial assistance has been provided by NSERC and Trent University. One of us (M. B. Y.) thanks the Ontario Ministry of Colleges and Universities for scholarship support.

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M.B. Young, A.J. Slaoin / Adsorption

[15] Y. Viswanath and L.D. Schmidt, J. Chem. Phys. 59 (1973) 4184. [16] T.W. Haas, J.T. Grant and G.J. Dooley, J. Appl. Phys. 43 (1972) 1853. [17] M.A. Smith and L.L. Levenson, Phys. Rev. B16 (1977) 1365. [18] G. Panzer and W. Diekmann, Surf. Sci. 160 (1985) 253. [19] W.K. Schubert and E.L. Wolf, Phys. Rev. B 20 (1979) 1855. [20] M.J. Lynch and J.B. Swan, Aust. J. Phys. 21 (1968) 811. [21] H. Ohtani, C.-T. Kao, M.A. Van Hove and G.A. Somorjai, Prog. Surf. Sci. 23 (1986) 155. [22] T.E. Felter and P.J. Estrup, Surf. Sci. 76 (1978) 464; 54 (1976) 179. [23] E. Gillet, J.C. Chiarena and M. Gillet, Surf. Sci. 66 (1977) 596. [24] J.T. Roberts and CM. Friend, Surf. Sci. 186 (1987) 201; J. Am. Chem. Sot. 109 (1987) 4423. [25] J.W. Erickson and P.J. Estrup, Surf. Sci. 167 (1986) 519. [26] D.G. Kelly, M. Salmeron and G.A. Somorjai, Surf. Sci. 175 (1986) 465.

of C, H4 on the Mo(ll0)

surface

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