The adsorption of cyclic hydrocarbons on Ru(001)

Surface Science 130 (1983) 173-190 North-Holland Publishing Company

THE ADSORPTION II. Cyclohexane *

F.M. HOFFMANN W.H. WEINBERG

173

OF CYCLIC HYDROCARBONS

ON Ru(OO1)

**, T.E. FQLTER ***, P.A. THIEL *** and

Division of Chemistry and Chemical Engineering, Caiifonia Institute of ~echn~~~~, Pas&m, Carifornia Received

91 I25, USA 31 January

1983; accepted

for publication

6 April

1983

The adsorption of cyclohexane on Ru(001) at 90 K has been investigated by thermal desorption mass spectrometry, EELS, UV photoemission and LEED. Thermal desorption indicates the adsorption of the undissociated molecule first in a chemisorbed monolayer (Td = 200 K) with subsequent formation of multilayers (Td = 165 K) at higher exposures. The vibrational spectrum obtained by EELS is characterized by a frequency shift of the C-H stretching mode from 2920 cm-’ (multilayer) to 2560 em-’ for the chemisorbed monolayer. Off-specular EELS data indicate two different electron scattering mechanisms for the C-H stretching mode. Whereas for the C-H stretching mode of the multilayer, large angle electron impact scattering is observed, the C-H soft-mode of the monolayer is largely due to small angle dipolar scattering. The He I photoelectron spectra of cyclohexane multilayers are characteristic of the undissociated molecule. A new assignment of C(2s) and the lowest C(2p) level, based on a comparison with benzene, shows that the chemisorbed monolayer is characterized by the absence of emission or broadening of the 2a,, level. This is attributed to C,, symmetry of the chernisorbed layer and to a possible interaction of the 2a,, orbital with the metal surface.

1. Introduction Cyclohexane has been found to adsorb associatively at low temperatures ( < 200 K) on Pt(l11) [l], Nif 111) [2,3], Pd(l11) [4] and Ru(OO1) [S]. At higher temperatures (above room temperature), dehydrogenation to benzene was observed for Pt( 111) [If and Pd( Ill) [4]. High-resolution electron energy loss spectroscopy (EELS) has revealed an interaction of some of the C-H bonds with the metal surface through a “softening” of the C-H stretching frequency for Ni(ll1) and Pt( 111) [ 11. Preliminary results for adsorption on Ru(OO1) [6] * Supported by the National Science Foundation under Grant No. CHESZ-06387. ** Present address: Exxon Corporate Research Laboratory, P.O. Box 45. *** Present address: Physical Research Division, Sandia National Laboratory, Livermore, fornia 94550, USA.

0039-6028/83/~-~~/$03,00

0 I983 North Holland

Cali-

174

F.M. Hoffmann et

al. / Cyclic hydrocarbons on

Ru(OO1). II

have shown for submonolayer coverages a similar frequency shift to 2580 cm- ’ compared to a multilayer value of 2920 cm-‘. Geometrical information concerning the adsorption of C-C,H,, on the Ru(OO1) surface has been provided by Madey and Yates [5] through a study using electron stimulated desorption ion angular distributions (ESDIAD), low-energy electron diffraction (LEED) and thermal desorption mass spectrometry (TDMS) measurements. It was concluded from ESDIAD that the chemisorbed monolayer possesses short-range order with the molecule oriented essentially parallel to the surface with three axial hydrogen atoms directed into threefold hollow sites of the surface. Based on these findings, we have investigated the adsorption of C-C,H,, on Ru(OO1) in the temperature range betwen 90 and 300 K with EELS, UV photoelectron spectroscopy, LEED and TDMS. The primary focus of this work is directed toward relating information from vibrational spectroscopy to the geometrical information obtained by ESDIAD and their mutual relevance to possible bonding mechanisms.

2. Experimental procedures Details of the experimental system have been described previously [7]. Reagent grade cyclohexane (Merck), which was cleaned by repeated freeze-thaw cycles, was dosed directly on the surface through a directional beam doser consisting of a multichannel capillary array. As in the previous experiments [7], exposures are given in arbitrary units, and no absolute exposure calibration was made. The crystal was cleaned following standard procedures [7]. Particular care was taken to avoid contamination frcm carbon since cyclohexane dissociates partially at higher temperatures (on the order of 300 K) on Ru(OOl), leaving hydrogen and carbon on the surface. Hydrogen could be removed thermally at 400-500 K and carbon through treatment with oxygen (1 x 10Ms Torr) at 800- 1100 K. As a final step, the surface was heated to 1570 K to remove adsorbed oxygen. Occasionally, a tenacious surface oxide was observed, easily detectable by EELS, and removable by Ar+ sputtering. The thermal desorption experiments were performed by resistive heating of the crystal support (Ta wires, 0.010 inch dia). Typical heating rates of lo-30 K/s were linear within 10% over the temperature range investigated. The temperature was monitored by a W/S%Re versus W/26%Re thermocouple. The thermal desorption mass spectra were recorded with a quadrupole mass spectrometer as a function of the thermocouple voltage. For convenience, the mass spectrum was recorded at mass 56, the largest peak in the spectrum but occasionally a control spectrum was taken at mass 84. For correlation of thermal desorption and EELS spectra, the crystal was exposed to the gas, an EELS spectrum was recorded, and the crystal was heated for the TDM

F.M. Hoffmann et al. / Cyclic hydrocarbons on Ru(OO1). II

spectrum. residues.

Finally,

the crystal

was cleaned

with

oxygen

to remove

175

carbon

3. Results and discussion 3.1. TDMS and LEED In order to distinguish the chemisorbed monolayer from condensed multilayers, thermal desorption spectra of C-C,H,, adsorbed at 90 K have been measured. The results are shown in fig. 1, where desorption spectra were recorded for increasing exposures of the crystal to C-C,H,,. At low coverages,

1

I

I

C,H,,/Ru(OOl),

T = 90 K

2OOK FM

I

150

I

Layer

I

200 250 Temperature, K

J

I

300K

Fig. 1. Thermal desorption spectra of C-CsH,, adsorbed on Ru(OO1) at 90 K. The heating rate is 10 K/S.

176

F.M. Hoffmann et al. / Cyclic hydrocarbons on RufOOI). I1

the spectra show a desorption peak at 200 K which saturates with exposure, as well as the development of a feature between 160 and 180 K that does not saturate with exposure. In agreement with Madey and Yates [4], we assign these features to desorption from a chemisorbed monolayer (200 K) and physically adsorbed multilayers (160- 180 K). The spectra in fig. 1 also show a shoulder at 180 K on the low-temperature peak, which we ascribe to molecular c-C,H,, adsorbed in a second layer on the surface. Assuming a preexponential factor of lOi s- ‘, Madey and Yates [S] obtained an activation energy of desorption of 14.2 kcal/mol for the chemisorbed monolayer and 9.2 kcal/mol for the condensed multilayer. From the thermal desorption spectra of fig. 1, it is apparent that the multilayer forms in large amounts only after saturation of the monolayer. At submonolayer coverages, however, there is some desorption from the low-temperature state, indicating that a small fraction of the cyclohexane ( < 10%) is adsorbed on top of admolecules in the first layer prior to saturation of the latter. A simultaneous monito~ng of mass 2 in these measurements also showed desorption of hydrogen at 440-470 K, indicating that a small fraction of the cyclohexane dissociates upon heating in vacuum. A comparison of the mass spectrometric intensities of mass 2 and mass 56 indicates that the fraction of cyclohexane which dissociates into hydrogen and carbon is less than 5: As well be shown explicitly below when discussing adsorption at higher temperatures, this dissociation occurs as a result of heating the surface during the desorption reaction. Qualitative measurements of the change in the work function for increasing exposures of c-GH,, [8] show a continuous decrease of the work function of approximately 600 meV for submonolayer coverages with only a small further decrease during multilayer formation. LEED measurements were performed at submonolayer coverages of c-C,H,, in the electron energy range between 30 and 150 eV. No additional LEED beams due to an ordered overlayer were found. This is in agreement with earlier observations by Madey and Yates [5] and is indicative of the absence of long-range order at monolayer and submonolayer coverages. 3.2. EELS of cyclohexane adsorbed at 90 K Vibrational spectra were obtained with EELS at an incident energy of approximately 4 eV and an energy resolution (full-width at half-maximum of the elastically scattered beam) of lo-15 meV. Fig. 2 shows spectra of multilayer C-C,H,, (bottom) and C-C+D,, (top) adsorbed on Ru(~l) at 90 I(. For comparison, the gas phase spectra [9, lo] are indicated by vertical bars between the two spectra. The most notable feature in the spectra of the admolecule is a vibrational mode at 2560 cm-‘, which, due to its shift upon deuteration, is assigned to a C-H stretching mode. Otherwise, all modes of the adsorbed

F.M. Ho@zann

Cyclohexane

et al. / Cyclic hydrocarbons

on Ru(OO1). II

177

/

Ru (00I) Tc90K Eo=4eV

Bi~60" A8, = 0" (specular

gas-phase

_:

120 0-f

t

I

I

(250 J

1 800

I 3000

I 2500

I

2000

Energy

I

1500

I

1000

I 500

X

I

0

Loss (cm-‘)

Fig. 2. Vibrational spectra of multilayers of cyclohexane c-C,H,, (lower) and C-C,D,, (upper) adsorbed on Ru(OO1) at T = 90 K. The spectra were obtained in the specular direction. The inset between the two spectra shows gas phase values after refs. [9] and [IO] (not all modes are shown). Dashed lines indicate modes of Es, E, symmetry (in D,,).

species in fig. 2 compare well with the gas phase spectrum and are assigned in table 1. In this table, the gas phase modes [9,10] and their respective symme-

Table 1 Vibrational spectrum of cyclohexane in the gas phase (refs. [9] and [lo]) and adsorbed on Ru(OO1) at 90 K (this work); for the gas phase spectrum not all Es and E, modes are shown; for the adsorbed molecule, the corresponding symmetry is given for the C,, point group C-C,H,,

Gas phase Mode number

D,, symmetry

Mode (cm-l)

1 12 2 14 3 20 28 21 29 4 15 22 30 31 5 23 16 24 6 32

Ais A zu Ais A Z” A& Es E” % E.,

2941 2932 2852 1456 1445 1348 1342 1267 1262 1158 1030 1029 905 862 802 785 521 427 384 248

AYg A Es? Eu J% A,, % A2u

% Al, Eu

on Ru(001)

symmetry

Mode (cm-‘)

Al

2920

C3”

A, f Al A, Al > E E E E

Al Al E 1 E E

C-H

stretch

2560

C-H

“soft”

1460

Scissor

1370

CH z wag

1210

1010

A, E

820

Al E

510

A, E 1

Description

CH,

of mode

mode

twist + C-C

stretch

CH 2 rock CH, rock C-C stretch + CH, wag CH 1 rock C-C stretch+CH, wag C-C stretch CH, rock + C-C stretch CH, rock + C-C-C deform. C-C-C

deformation

tries for the D,, point group {for the chair form of c-C,H,,) are listed. For adsorbed c-C,H 12, symmetries are given .for a tentative C,, point group (to be discussed below). The mode assignments of table 1 are largely in agreement with that obtained for c-C,H,, on Pt( 111) ]l] and Ni(li1) 131.The vibrational spectra indicate nondissociative adsorption of C-C,H,, on Ru(001) at 90 K for both monolayer and multilayer coverages. At submonolayer coverages, the vibrational spectrum is dominated by an intense C-H stretching mode at 2520-2600 cm-’ as may be seen in fig. 3. The thermal desorption spectrum corresponding to the low coverage EEL spectrum of fig. 3 shows a negligible contribution from multilayer clusters. Consequently, all cyclohexane molecules are chemisorbed in the first layer in direct contact with the metal surface. The vibrational spectrum obtained in the specular direction shows almost no intensity in the unshifted C-H stretching mode (near 2830 cm-‘). Unfortunately, no quantitative evaluation of the dynamic effective charge [20] associated with these two C-H stretching modes

F.M. Hoffmann et al. / Cyclic hydrocarbons on Ru(OO1). II

1

I

I

I

I

179

C,H,,/Ru(OOl), T = 90 K ei = es = 60°, E, = 4eV

x 100

600 s 3

.2

K f 400 s

Ji 1

3500

I

3000

-210

I

2500

cm.’

I

2000

Energy Loss (cm-‘) Fig. 3. C-H stretching modes of C-C,H,, adsorbed on Ru(001) at 90 K for increasing exposures (given in arbitrary units of the beam doser). The EELS spectra were taken in the specular direction. Intensities are normalized with respect to the elastic peak of the corresponding spectrum.

is possible. Apart from an uncertainty in determining the absolute coverage due to the lack of a LEED superstructure and .the uncertainty as to how many of the C-H bonds of the molecule contribute to either C-H stretching mode, these two loss features have different angular distributions. This can be seen clearly in fig. 4 where we present spectra taken 5’, 10” and 20” from the specular direction. The spectra show that whereas the C-H stretching mode at

180

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Hofmann

et ai. / Cyclic hydrocarbons on Ru@WI). If

CgH12/R~(00i)T=90K Bi=60",Eo=4eV

k! I

I

3500 3000

I

I

2500 2000 Energy

I

I

1500 iooo

I

I

500

0

Loss (cm-‘)

Fig. 4. EELS spectra of the C-H stretching modes taken at various collection angles Aff, = O’, 5O, 10” and ZOO from the specular direction toward the surface normal (see inset). The incident angle 8,‘was 60° in all cases. All spectra are normalized with respect to the elastic peak iii the specular direction.

F.M. Hoffmann et al. / Cyclic hydrocarbons on Ru(OO1). II

2920 cm-l

181

does not decrease in intensity appreciably in off-specular directions, the C-H soft-mode at 2560 cm-’ has virtually disappeared at Ad, = 20”. A comparison with the elastic peak is provided in fig. 5, where the absolute intensities of the elastic and various inelastic peaks are plotted as a function of the angular deviation from the specular direction. It is evident that almost all modes exhibit different angular profiles. This is probably a consequence of additive contributions from different electron scattering mechanisms. The theory of low-energy (l- 100 eV) inelastic electron scattering distinguishes between two scattering mechanisms according to the interaction of the electron with either the long-range or the short-range part of the potential of the admolecule. A long-range interaction or dipolar scattering [ 1 l] results in a scattering lobe peaked close to the specular direction with an angular half-width of A@,,, = t2w/2E0 [ll], where Aw is the energy loss due to the vibrational excitation, and E, is the impact energy. A further consequence of dipolar scattering theory is the “surface dipole selection rule” [ 121, which allows only excitation of those vibrational modes with a component of the dipole derivative perpendicular to the surface. This “surface dipole selection rule” is not valid for an interaction of the electron with the short-range part of the potential of the admolecule, i.e. impact scattering. Impact scattering occurs for all modes with a broad angular profile [ 13,141. In the absence of negative ion resonances [15], the cross section for this scattering mechanism is usually considerably lower (one or two orders of magnitude) than dipolar scattering. It can be competitive, however, if dipolar scattering of a particular mode is either forbidden or of low intensity [17]. In the latter case, the intensity from off-specular impact scattering causes an apparent broadening of weak dipolar modes. This might be the case in fig. 5 for the C-H stretching mode at 2920 cm-’ and for the modes at 1460 cm-’ (scissor) and 1030 cm-’ (ring mode + wag). Similar behavior has been observed for the C-H stretching mode as well as other modes of adsorbed cyclopropane [ 181 and acetylene [ 191. From fig. 5, it is evident that the mode at 510 cm-’ and the C-H stretching mode at 2560 cm-’ are dominated by dipolar scattering. There are two important consequences of a C-H stretching mode with a strong dynamic dipole moment. First, the molecule must be located close to the surface [21], with the C-H bond possibly vibrating into the surface. This will be discussed below in connection with a structure model supporting that notion. Second, it will affect the C-H stretching frequency which is important in understanding the mechanism of the interaction between the C-H bond and the surface. Two mechanisms can be attributed most reasonably to the large frequency shift of the C-H soft mode: a vibrational interaction with the image of the dipole in the metal surface, and/or a chemical interaction between the C-H bond and the metal surface. The chemical interaction will be discussed later. Dipole-image coupling shifts have been predicted theoretically [22,23] and can be observed (indirectly) experimentally for CO adsorbed on metals [24]

EM.

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I CSH,*,’

et al. /

Ru (001)

Cyclic

xix

I

IO’

I

-5

0

I

-10

on Ru(001).

II

T= 90K

0

-=I

h_ydrocarbons

I

-15

C-H stretch (2920 cm-‘)

C-H stretcfi (2560 cm-‘) -20

A@, (DEGREES)

102

.

I~

0

x_

-5

-10 AtI,

Fig. 5. Angular

dependence

cm-’

1030

cm“

510 cm-l

‘lx-

0

1460

-I5

-20

(DEGREES)

of the absolute

intensities

as a function

of the collection

angle A@,.

where they contribute to the overall downshift of the C-O stretching frequency compared to the gas phase value. Typical theoretically predicted downshifts are on the order of l-3% of the gas phase frequency, and they increase

F.M. Hoffmann

et al. / Cyclic hydrocarbons

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183

dramatically in magnitude for smaller dipole-image distances [22,23]. For the case of an adsorbed C-C,H,, molecule in contact with the Ru(001) surface, no more than IOO- 150 cm-’ of the observed 360 cm-’ downshift in frequency can be attributed to a dipolar-image coupling. Consequently, it is unreasonable to associate all of the downshift experienced by the C-H stretching mode in C-C,H,, adsorbed on Ru(OO1) to a dipole-image interaction. Finally, it should be noted that there is no shift of the C-H soft-mode to higher frequency with increasing surface coverage as is commonly observed for adsorbed CO [24,25]. This is not surprising since this shift (whether chemically or dipolar induced) is dependent on the distance between the vibrating dipoles. For cyclohexane, there exists within the molecule at least three contributing the effect of (axial) C-H dipoles at close distance (- 2.6 A). Consequently, increasing the coverage (and decreasing the intermolecular distance) is negligible by comparison. 3.3. EELS

of cyclohexane

adsorbed at temperatures

above 90 K

Slow annealing (heating rate < 1 K/s) of a multilayer of cyclohexane to 230 K resulted in a vibrational spectrum characteristic of adsorbed benzene. The C-H soft-mode near 2600 cm-’ disappeared and was replaced by a weak C-H stretching mode at approximately 3000 cm-‘. In addition, intense lowfrequency modes at 700 and 780 cm-’ appeared. This dehydrogenation of cyclohexane to benzene has been observed also with EELS for Pt( 111) and Ni[5(lll)x(liO)] [l], and for Pd(ll1) with UPS [4]. Since this dehydrogenation appears to depend on low heating rates, we have not pursued this subject. It is sufficient to note that our observation of dehydrogenation of cyclohexane can be related to the observed C-H soft-mode for the chemisorbed monolayer. 3.4. UV photoelectron

spectroscopy

The He I UV photoelectron spectrum of condensed cyclohexane [6], shown in fig. 6d, corresponds well to published gaseous spectra [26-291 where, however, all the levels have not been identified positively [30-321. An approximate identification of the gas phase levels, shown below fig. 6d, is due to Puttemans [33] who employed a simple, free electron model and assumed cylindrical symmetry. More advanced treatments of cyclohexane have been limited to the four high binding energy orbitals which are built up from C(2s) atomic orbitals [28,29]. As in the case of cyclohexane [28], the benzene photoelectron spectrum was initially interpreted [29] in terms of the four lowest lying levels being composed of C(2s) orbitals. In later work, however, Heilbronner and Maier [35] argued that a fifth level, the 3a,, composed of C(2p) orbitals should be inserted between the benzene 2b,, and 2e,, levels. This C(2p) derived molecular orbital

184

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et al. / Cyclic hydrocarbons on Ru(OO1). II

Ru(OOWC,H,, Normal

a=80” Emission

1:;,, /

200s

2a,,

P I

I

16

I

I

14

I

I 12

I

I IO

BINDING

30,” ’ I&

I

I,,

8

’

I

6

ENERGY

I 4

I

I

2

,I

E,

(eV)

Fig. 6. Normal emission He I photoemission spectra of: (a) clean Ru(OO1); (b) 200 s exposure to C,H,, (monolayer); (c) 400 s exposure; (d) 800 s exposure (multilayer). Exposures arc given in arbitrary units of a directional beam doser.

in benzene (3a,,) appears very much like the corresponding one in cyclohexane (also denoted 3a,,). In both cases, the molecular orbital can be described as a small torus located within and surrounded by the carbon ring, which in turn is surrounded by six lobes. The six lobes are of opposite phase from the small internal torus. The 3a,, orbital may be pictured as the in-phase combination of 2p orbitals directed radially from the center of the molecule. The gas phase spectra of benzene and cyclohexane are very similar (cf. fig. 4 of ref. [29]). For both benzene and cyclohexane the ordering in decreasing binding energy and increasing intensity is the 2a,, (25 eV), 2e, (23 eV) and 2es (19.5 eV). Approximately 2 eV above the latter is a more narrow, less intense peak, with an even less intense shoulder at 1.7 eV higher energy. In benzene, these features are assigned by Heilbronner and Maier [35] to be the 3a,, and

F.M. Hoffmann

et al. / Cyclic hydrocarbons

on Ru(001). II

185

2b,, levels, respectively. The similarity in intensity and position leads to the assignment of these features in the cyclohexane spectrum as the 3a,, and 2a,, molecular orbitals, respectively. Thus, for cyclohexane, we propose the following new assignment in decreasing binding energy: 2a, g, 2e,, 2es, 3a1 g and 2a I “. To summarize, based on the shape of the molecular orbitals and the similarity in the photoelectron spectra, we agree with Potts’ claim that each of the four C(2s) derived molecular orbitals in benzene correspond well with their counterparts in cyclohexane [29]. Furthermore, we believe the deepest C(2p) molecular orbital also falls within this correspondence and, in fact, lies between the highest and next highest lying C(2s) derived molecular orbitals. Thus, we believe that the assignment by Heilbronner and Maier [35] for benzene applies equally well to cyclohexane. This assignment is shown below spectrum b of fig. 6. Inserting the 3a,, level of cyclohexane between the 2a ,” and 2es levels amounts to reversing the order of filling of the 2a,, and 3a,, levels. Our assignment differs from the order given by Jorgensen and Salem [34] in that the 2a,, and 3a,, levels are reversed. However, our assignment agrees with the symmetries of Puttemans [33]. Of the five deeply bound levels discussed above, only three are observed in the multilayer He I photoelectron spectrum of fig. 6d. They are located at 14.3, 13.1 and 11.1 eV and correspond to the es, as, and a, levels, in Puttemans’ notation [33]. The other carbon 2s derived levels are too deeply bound to be photoionized by He I radiation. In going from the multilayer spectrum of fig. 6d to the submonolayer exposure of fig. 6b, three important changes can be seen. First, the shoulder on the high binding energy side of the peak at 9.5 eV disappears. Second, the peak at 9.5 eV narrows by approximately 0.3 eV. Finally, the peak at approximately 6.7 eV disappears. All of these changes can be explained by appropriate levels being symmetry forbidden for the chemisorbed monolayer, but allowed for the multilayer. This would occur if the molecules in the first layer are oriented with respect to the surface, whereas subsequent layers involve randomly oriented molecules. For cyclohexane in the chair form of Dsd symmetry, adsorption parallel to the surface reduces the symmetry to C,,. For our geometry (grazing incidence photons and normal emission), levels of the A, irreducible representation in C 3v symmetry will be forbidden. In fig. 6, the gas phase spectrum, represented by verticle bars, is aligned under curve b. The bars are labeled with the convention that if the label is placed above the bar, then upon symmetry reduction from D,, to C3”, the level is not symmetry forbidden. The three levels labeled below the bar fall in the A, irreducible representation when the symmetry is lowered from D,, to C,,. It is readily seen that it is precisely the disappearance of these levels which accounts for the changes in going from the 800 s to the 200 s exposure. Finally, in order to clarify further the role of the 2a,, orbital in the bonding of cyclohexane to Ru(OOl), the He I ultraviolet photoelectron spectrum is

186

F.M. Hoffmann

et al. / Cyclic hydrocarbons

on Ru(001). II

shown in fig. 7a for the Ru(OO1) surface after saturation exposure to oxygen. By comparison to the clean surface, spectrum (a) shows a greatly attenuated peak near the Fermi level and a pronounced change in the shape of the peak at approximately 5 eV. Fig. 7b is the spectrum measured after multilayers of cyclohexane have been adsorbed at 80 K onto this oxygen treated surface. This spectrum exhibits a more pronounced shoulder at 9.4 eV and an upward shift (toward EF) by 1.0+0.2 eV of all the features with respect to multilayer adsorption on the clean surface (cf. fig. 6~). Heating to 180 K (fig. 7~). i.e. temperature where the multilayer is desorbed from the clean surface, produces a spectrum which shows a slight “rounding” of the features between 3 and 7 eV and a greater d-band emission than in (b). Comparing the features and in

Ru(OOl) He I CL= 80” Normal Emission 90K

I

I,,

16

I,

14

I

12

BINDING

j

IO

,

I,

,

8

6

ENERGY

,

,

4

(

/

2

,

/

E,

(eV)

Fig. 7. Normal emission He I photoemission spectrum of: (a) Ru(001) after standard oxygen treatment (see text); (b) exposure to C,H,, at 90 K; (c) annealing to 180 K; (d) annealing to 210 K.

F.M. Hoffmann

et al. / Cyclic hydrocarbons

on Ru(001). II

187

particular the intensity of the d-band emission of the spectrum in fig. 7c with those in fig. 6, we can determine the coverage in fig. 7c to correspond to that of a clean surface between 200 and 400 s. There is, however, an important difference between the spectra of the clean and the oxygen precovered surface: the latter (fig. 7c) shows emission from the 2a,, level at 9.4 eV, whereas in both the 200 s and the 400 s spectra of the clean surface (figs. 6b and 6c), this feature is not observable. Finally, with further heating to 210 K, the multilayer desorbs giving the spectrum of fig. 7d characteristic of the oxygen precovered surface (cf. fig. 7a). The work function, as determined from the energy cut-off of the secondary emission, has decreased, however, by approximately 0.7 eV. To summarize, preadsorbed oxygen suppresses the type of chemisorbed layer that is found on the clean surface and which is characterized by the absence or broadening of the 2a, u level. Rather, we observe after annealing to 180 K (where only a monolayer is left), a spectrum which is similar to that of the multilayer, but with an increased d-band emission which indicates monolayer coverage. This suggests site-blocking or a ligand effect of the adsorbed oxygen, a notion which is supported by recent EELS results for adsorbed cyclopentane [36] and cyclohexane [37] on Ru(OO1) where precoverage of oxygen resulted in the suppression of the CH soft-mode in the vibrational spectra. 3.5. Discussion A structural model, shown in fig. 8, has been proposed by Madey and Yates for cyclohexane chemisorbed on Ru(OO1) [5]. This model is based on ESDIAD results which show a H+ emission pattern consisting of a strong central lobe normal to the surface and a weaker pattern of sixfold symmetry rotated by 30” with respect to the ruthenium substrate unit cell. This pattern can be accounted for by a molecule adsorbed on top of a ruthenium atom with three equatorial C-H bonds pointing toward two-fold metal (bridge) sites and three axial C-H bonds pointing into threefold metal (bridge) sites (cf. fig. 8). Our vibrational data strongly support this model. The large downshift (over 300 cm- ‘) for the C-H stretching mode compared to the value for the gaseous molecule (cf. fig. 3), as well as the strong dipolar scattering (cf. figs. 4 and 5) are attributable to a C-H group vibrating into the surface. A further consequence of the structural model shown in fig. 8 is C 3v symmetry of the admolecule. Therefore, it should be possible to confirm this symmetry by observing dipolar active modes in EELS. For C,, symmetry, only the A, -modes in table 1 should be dipolar active. Our vibrational data at submonolayer coverages suggest C,, symmetry, although the features below 1500 cm-’ are neither sufficiently well resolved nor sufficiently intense for an unambiguous assignment. At higher coverages, where multilayer formation occurs, E modes are also observed (cf. table 1: modes 20, 28 and 21, 29), indicating a symmetry lower than C,, for the

188

EM. Hoffmann el al. / Cyclic hydrocarbons on Ru(OO1). II

condensed multilayer. This suggests a loss of orientation in the multilayer, a conclusion which is in agreement with our photoemission data presented above as well as the previous ESDIAD results of Madey and Yates [5].

Fig. 8. Schematic

orientation

of cyclohexane

chemisorbed

on Ru(001)

[5].

The question of the type of chemical interaction between the admolecule and the metal surface atoms is most intriguing. Cyclohexane is a saturated molecule and has no ring strain in its chair form. Thus, in contrast to cyclopropane where ring strain induces some delocalized ucc charge which could participate in the bonding 17,181, for cyclohexane bonding to the metal might occur via a C-H-metal interaction. Arguments for this point of view include the proximity of the C-H group to the surface (strong dipolar character of the C-H stretching loss) and the large downshift in frequency of the C-H stretching mode. C-H-metal interactions are also known to occur in metal clusters. Early results have been reported for molybdenum clusters by Trofimenko [38], who observed C-H stretching modes at 2704 and 2664 cm-i. Cotton et al. 1391 investigated further the interaction of aliphatic C-H bonds with the metal atom in molybdenum clusters and found an H-MO distance of 2.27 to 2.15 A. Based on their findings, a three-center, two-electron bond encompassing the C-H-MO atoms was postulated, resulting in the activation of the C-H bond by the metal atom. Even stronger C-H-M interactions have been reported by Schultz et al. [40] who reported for tantalum-neopentylidene complexes activated C-H bonds with C-H stretches of 2400-2600 cm-’ and a H-Ta distance of 2.119 A. An example of a closed three-center C-II-M bond,

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et al. / Cyclic hydrocarbons

on Ru(001). II

189

i.e. where both the carbon and the hydrogen atoms are in bonding distance to the metal, has been reported recently by Muetterties et al. [41] for HFe,CH(CO),, clusters. There, the methylidyne group is tipped toward one of the iron atoms to give distances of 1.80 w (Fe-H) and 1.926 A (Fe-C), which led to the suggestion that this species is a model of a transition state for carbon-hydrogen bond breaking. Finally, Churchill et al. [42] have reported an extraordinarily low C-H stretching frequency of 2200 cm-’ for Ta(CHCMe,)(dmpe),Cl. There, the a-hydrogen in the Ta = C, = Cp ligand is in a “bridging” position between Ta and C, at distances of 1.850 P\ (Ta-C) and 1.796 A (Ta-H), but with an C-Ta-H angle of 100.4”, suggesting that this species is an intermediate in hydrogen abstraction. Two mechanisms are of interest in the bonding of cyclohexane to the Ru(OO1) surface and to the C-H bond weakening: charge transfer from a CH u-orbital to the metal and back-donation of electron charge into antibonding unoccupied CH orbitals. The former is suggested in analogy to metal clusters [42,43] and is supported by the observation of an overall decrease of the work function for the first chemisorbed layer of cyclohexane. This mechanism will lead to a C-H bond weakening as charge is removed from bonding C-H orbitals. In turn, this charge can be transferred to carbon-metal bonds to strengthen bonding to the surface. Evidence that the 2a,,(uc,J orbital plays an important role can be derived from the photoemission data, On the other hand, back-donation into unoccupied, antibonding C-H orbitals (e.g. 4a,,, ?~:u,) may also be important. This again, in analogy to CO adsorption [44]), will stabilize bonding to the surface and weaken the C-H bond.

Acknowledgments The authors are grateful for helpful discussions F. Robbins and T.H. Upton, and to E.L. Muetterties prior to publication.

with T.E. Madey, G. Rice, for supplying a manuscript

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hydrocarbons

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II

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