On the vibrational electron energy loss spectra of benzene chemisorbed on the (111) and (100) nickel faces

Surface Science 89 (1979) 4 6 7 - 4 7 6 © North-Holland Publishing Company

ON THE VIBRATIONAL ELECTRON ENERGY LOSS SPECTRA OF BENZENE CHEMISORBED ON THE (111) AND (100) NICKEL FACES J.C. BERTOLINI and J. ROUSSEAU * Institut de Recherches sur la Catalyse, 2, avenue Einstein, F-69626 Villeurbanne Cedex, France Received 26 February 1979; manuscript received in final form 12 April 1979

The vibrational electron energy loss spectra of benzene chemisorbed, at room temperature (3 L exposure) on the N i ( l l l ) and (100) faces, show the same loss peaks at 750, 845, 1115, 1325, 1425, 3025 cm -l (540, 820, 1225, 1365 and 2255 cm-~ with C6D6). The only one significant difference lies on the positions of the carbon-metal vibrations respectively at 290 and 360 cm-l on the N i ( l l l ) and (100) faces. These results provide evidence that the configuration of the adsorbed species are the same on the two surfaces. An associative adsorption, with chemisorbed benzene molecules rr bonded to the metallic substrate and with their rings parallel to the surface, is proposed. Alternative interpretations of the vibrational spectra are given according to the so called "surface selection rule", which implies that only totally symmetric modes are ELS active, or in terms of the most infrared active modes of the gaseous benzene.

1. |ntroductien

On nickel, the kinetic order of the catalytic hydrogenation of benzene is found near zero [ 1] and it is generally admitted that this reaction proceeds via chemisorbed benzene. This has been directly evidenced by hydrogenation of preadsorbed labelled benzene [2]. So, the knowledge of the adsorbed phase is of great interest for a better understanding of the mechanism of this reaction. In small nickel particles, benzene has been found to chemisorb associativeiy in presence of hydrogen [3], but some dehydrogenation occurs on bare surfaces [3,4], even at room temperature. Nevertheless, the assignment of the adsorbed complexes .!~ lit.Jr WtKJII UK;IUlK;Id

.

II lit;: U~t:; U I 311I~:rlt~

t;iy~tal~

[ O i lives

a

te H" i e c n e n n s o r p t ~ o n seems

3 be of interest since several techniques may be used to recognize the configuration and the bonding state of the stable adsorbed species. Benzene chemisorption has been previously studied on Pt(l 11) and Ni(111) by using LEED, Auger, UPS and work function changes measurements [5-7]. More

* Also in Laboratoire de Recherches Physicochimiques, UER de Physique, Universit6 Claude Bernard Lyon 1, 43, boulevard du 11 novembre 1918, I;'-69621 Villeurbanne, France. 467

468

J.C. Bertolini, J. Rousseau / Vibrational EEL spectra o f benzene on Ni

recent works have been done on these ordered surfaces by adding the vibrational electron energy loss spectroscopy [8,9], in order to try to determine: - t h e chemical nature of the adsorbed species (associative or dissociative chemisorption, ...); -the nlodifications of the intramolecular bonds (C-C, C-H)consecutive to the chem/sorption: -the geometry of the adsorbed species: "bent" configuration, orientation with respect to the surface ...; - the adsorption site. First results on Pt(111), Ni(111) and Ni(100) [8,9 ], agree together with benzf..ne molecu,.'s chemisorbed with their rings parallel to the surface. On the (111) faces, Lehwald et at. [8] suggest the existence of two zr bonded species, in differently rotated positions with respect to the ca axis of the substrate normal to the surface. In a firs', study devoted to the benzene chemisorption on Ni(111) and (100) faces, we mentioned the concordance of the LEED and of the thermal desorption data with benzene rings chemisorbed parallel to the surface [9]. However, the information deduced from the vibrational ELS results was limited by too poor resolution of the spectrometer. So the benzene-nickel systems have been reconsidered by working at higher resolution. In the present paper we discuss the configuration of the adsorbed benzene as a function of the more precise data supplied by vibrational electron energy loss spectroscopy.

2. Experimental The ultrahigh vacuum chamber combining LEED, work function changes, mass spectroscopy and vibrationaP ELS measurements has been largely described elsewhere [ 10]. The vibrational electron spectrometer uses concentric hemispheres and works typically under the following conditions: - incident current, 10 -~° A; - resolution power (FWHM)" 80 cm-1; - uncorrected incident energy: 2 eV; - half angles of aperture of the incident and analysed beams estimated nearby 5 6 °" The electron monochromator and analyser are fixed. Generally the measurements are done by analysing the reflected beam near the specular direction (0 = 0' = ~,,-' 9. ~,wever, measurements have been made out of the specular direction by rotating the sample (up to 0 = 54 °, 0 ' = 66°); the analysis axis remaining always in thc~ in~ ide~t plane. Tte samples are nickel single crystal discs, of 99.95% purity, issued from "Cristal I"ec". After X-ray orientation they are spark-cut, and mechanically and chemically polished. The orientation so obtained is better than _+0.5° . After baking at 750°C under vacuum, the contaminants (mainly sulfur) are LF.,~'X

~, ,,

__

J.C. Bertolini, J. Rousseau / Vibrational EEL spectra of benzene on Ni

469

eliminated by successive argon ion bombardments, oxygen and hydrogen treatments, and heatings. Then one obtains sharp LEED patterns and AES spectra showing no peaks other than those associated with nickel.

3. Results The following results are related to the irreversibly adsorbed benzene, at room temperature, remaining on the surface after pumping off the gas phase: - The oriented LEED patterns observed after 3 L exposure are interpreted as c(4 X a.

J

Ni ( 1 1 1 ) - C e l l s m

i

in

e,

It) ¢0

,,,g

0 O~ ¢q

o

,J (~-

--

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.1000 -

70

w

e

b. Ni (111)- c,6D6 0

,i

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.o

o ~

--

fO

"0

~

~,

i

1600

O~ ~

~, ~

~i

T O c m "1

H,

0 E0 Fig. 1. Vibrational electron energy loss spectra of benzene chemisorbed, at room temper;tture, on the Ni(111) face (3 L exposure): (a) C6H6; (b) C6D 6.

470

J.C. Bertolini, J. Rousseau / Vibrational EEL spectra of benzene on Ni

4)on Ni(100) and (2x/'3 × 2x/3)R30 ° on Ni(111) [9]. - O n the two low index faces, thermal desorption experiments show an unique mass 2 peak. ~.: 18G°C on Ni(l 11) and 210°C on Ni(100). A carbon residue is left on the surface having the features of the so-called "surface carbide", - At sat oration, the work function change reaches the values: -1.1 V on the (111) face and - 1 . 3 V on the (100) face. Typical vibrational loss spectra of benzene adsorbed on Ni(111) and Ni(100), recordtd around the specular direction (0 ''~ 0 "" 60 °) are shown in figs. 1 and 2. The spe,~.tra are very similar on the two surfaces and exhibit loss peaks at 750 +-5, 845 -+ 2f. ~ 115 _+20, 1325 +- 30, 1425 -+ 15 and 3025 -+ 10 cm -t (540 + 5 , 8 2 0 - +

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o

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~250

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O

Fig. 2. Vibrational electron energy loss spectra o f benzene chemisorbed at room temperature,

on the Nffl00) face (3 L exposure)- (a) C6H6; (b) C6D 6.

J.C. Bertolini, ]. Rousseau / Vibrational EEL spectra of benzene on Ni

471

15, 1225 -+ 30, 1365 + 30 and 2255 -+ 10 with the deuterated compound). The only one significant difference between the two spectra is found for the non shifted (H ~ D) loss peaks at 290 + 25 cm -1 on Ni(111) and at 360 -+40 cm -~ on Ni(100). By rotating the sample, the analysis out of the specular direction (0 " 54 °, 0' " 66 °) shows a large decrease of the specular beam intensity (nearby a factor 20), whereas the intensities of the losses decreas,; only slightly. The relative intensities of the loss peaks are not deeply modified; however, one can observe an increasing factor (1.5 to 2) for the peaks of low intensity (845, 1115, 1325 and 1425 cm -1) with respect to the main loss peak at 750 cm -1.

4. Discussion The similarity of the vibrational spectra of benzene adsorbed on Ni(111) and Ni(100) strongly implies the same stable chemisorbed species on these two faces. The explanation of this deep similarity laises the problem of the symmetry we have to consider for the two systems, since the geometrical arrangements of the metallic atoms are different on the two low index faces. 4.1. General features

Looking at the isotopic ( H - D ) shifts of the most intense losses, a rough examination of the vibrational spectra reveals the presence of stretching VcH(R ~ = 3025/ 2255 = 1.34) and Vcc(R2 = 845•820 = 1.03) modes and of an hydrogen bending mode (R3 = 750/540 = 1.38), by reference to the gaseous hydrocarbon molecules

[ll]. The existence of C-H and C - C bonds and the inobservance of hydrogen directly bonded to the metallic substrate - suggested by the lack of thermodesorption mass 2 peak near 100°C [12] - s h o w that benzene is associatively chemisorbed on these two surfaces. The thermal dehydrogenation curves of the adsorbed benzene exhibiting only one mass 2 peak (the carbon being left on the surface), one may suppose that the hydrogen-carbon bonds are energetically equivalent in the hydrocarbon surface complex. Then, the more probable chemisorption state is a benzene adsorbed with its ring parallel to the surface. Moreover, we have to notice that the unit meshes deduced from the observed LEED patterns [9] are compatible with the Van der Waals overspacing of the benzene lying flat, with one adsorbed molecule per unit mesh. 4.2. Tentative assignment o f the vibrational losses

Let us first consider a tentative interpretation of the loss spectra according to the so called "surface selection rule", often invoked, which implies that only totally

J.C. Bertolini, J. Rousseau / Vibrational EEL spectra of benzene on Ni

472

symmetric modes are ELS active. Effectively such modes have a normal component of their dynamic dipole moment, since the symmetry elements are perpendicflar to the surface. On the (111) face, one must consider the C av group if the benzene cycle is centered on on,• or three nickel surface atoms and the Cs group if the molecule is centered on ~wo nickel atoms. On the (100) face the lowest symmetry is C2v for the benzene cycle centered on one, two or four surface atoms. Whatever the symmetry subgrovp of D6h one has to consider, the correlation table (table 1) shows that one may attribute the three main loss peaks at 3025, 845 and 750 cm -~ to the excitation of the totally symmetric At (or A') modes: vt (symmetric Yen ), v2 (symmetric Vcc ) and v4 (symmetric bending 7cn) respectively at 3062, 993 and 675 cm -t in the gasec =s benzene. In fac~' the similarity of the vibrational spectra on Ni(111) and (100) surfaces, indicates r.uat in the case of benzene chemisorption the vibrational spectroscopy is either little sensible to the adsorption site, or that the adsorption site is the same on the two surfaces and is largely decoupled from the remaining of the substrate. So, it

Table 1 Correlation table between the D6h group and some o f its subgroups [13,14]

D6h Mode number

v1 *'2 ~'3 ~'4 v5 v6 v7 v8 v9 riO Vll v12 v13 v14 v 15 . . . v17 vI8 vl9 v20

Type o f symmetry in the subgroups

C6H6(C6D6) v (cm -t )

.

3062 993 1346 675 3048 1010 991 707 1309 1146 849 3057 1479 1035 3047 . 1~77 607 969 404

(2294) (945) (1055) (497) (2275) (970) (830) (599) (1282) (823) (663) (2276) (1330) (812) (2267) ~*.~-~-,; (868) (579) (789) (351)

Type of symmetry

Alg A~g A2g A2u Blu Blu B2g B2g B2u B2u Elg Elu Elu Elu E2g E2g E2g E2g E2u E2u

C6v

Al A~ A2 A~ B1 BI B1 B1 B2 B2 El El E1 E1 E2 rJ 2 E2 E2 E2 E2

C3v

C2v

Ov

od

Al Al A2 A1 A1 A1 A1 A1 A2 A2 E E E E E E E E E E

A1 Al A2 A~ A2

Cs od

Ov

A1 A1 A2 A1 B2

A' A~

Ar At

A" A' A"

A" Ar Ar

A2

B2

A2 A2 Al At E E E E E

B2 B2 Bl Bt Bt Bt Bt Bt At

A" A" A"

A~ Ar Ar

['i E E E E

At

AI At At

A) A'

A"

A" Ar

+B2 +B2 +B 2 +B 2

A'+A" A'+A" A'+A" A'+A"

A'+A" A'+A" A'+A" A'+A"

+A 2 +A 2 +A 2 +'%2 +A,2 +A~

A'+A" A' +A" A'+A" A'+A" A'+A"

A'+A" A' + A" A'+A" A' + A" A'+A"

A'+A"

A'+A"

d.C. Bertotini, J. Rousseau / Vibrational EEL spectra o f benzene on Ni

473

seems reasonable to search a same symmetry group by confining the vibrating system to the molecule plus its bonding site. To conciliate the geometry on the two surfaces, one may consider a preferential bonding of the benzene molecule to a single nickel atom (symmetry C6v) or to two adjacent surface metal atoms (symmetry C2v). These symmetry considerations remain valuable if the H atoms of the benzene molecule are "in" or somewhat "out of plane" of the ring. Only three A~ modes belong to the C6v symmetry (vt, v2 and v4). Starting with the C2v symmetry, the Vts --} v20 vibrations change to At modes in addition to vt, v2 and v4 (table 1) and one can explain the observed spectra in terms of only At modes (table 2). However, in the light of recent vibrational ELS observations [ 15,16], it has been shown that non-totaUy-symmetric vibrational modes can interact with the incident electrons. Such effects are well evidenced mainly by detection of the scattered electrons out of the specular direction. It is suggested that they are issued from "short range scattering processes". Due to the large angular width of the incident and analysed beams used in our experiments, the observation of various types of vibration cannot be ruled out. For example, taking into account the electronvibration interaction picture pointed out by Sokcevic al. [17], we may consider "a priori" the infrared active modes enhanced, or only partially screened, according to their orientation perpendicular, or parallel, to the surface. So, do we see normal modes with low dynamic dipole moment, enhanced by the surface selection rule, or parallel modes with large dynamic dipole moment, partially screened? This can explain the detection of small peaks associated with the excitation of non-totallysymmetric modes besides the At modes, in the different symmetry subgroups of D6h previously considered. Moreover, this point of view induces uncertainties even on the attribution of the two intense loss peaks at 3025 and 845 cm -~ , if one takes into account the magnitude of the dynamic dipole moment associated to the symmetric or antisymmetric stretching vcH and Vcc modes. The experimental vibraTable 2 Tentative interpretation of the vibrational specta according to a C2v symmetry. (only the A I modes are supposed ELS active) Experimental loss peak

Tentative attribution

Values in gaseous or condensed

frequencies (cm-t)

benzene (cm-1) [ 14,81

C6||6

C6D6

C6tt6

C6D6

3025 1425 1325 1115 845 750

2250 1365 1225 820 820 540

3062, 3047 1595

2294, 2267 1553

v!, v! s vt6 (vl 6) v17, v19 v2 v4

1177, 993 675

969

868, 945 497

769

474

J.C Bertolini, J. Rousseau / Vibrational EEL spectra o f benzene on Ni

tional loss spectra may be then tentatively compared to the infrared spectrum of gaseous benzene, merely if the benzene molecule is only little distort with respect to its planar geometry, and the detected loss peaks may be connected to the most active infrared medes of the gaseous benzene [ 11,13,14] (table 3), inas.much as the observed speci.ra have similarities with the vibrational ELS spectra of multilayer condensed benzene [8]. In summary, the largest peak (750 ~ 540 cm -~) can be surely connected to the u4 mode (hydrogen bending vibration perpendicular to the benzene ring), since this totally sy.vJnetric mode is polarized normally to the surface and has the strongest infrared activity. An alternative remains for the assignment of the two other intense loss peaks ',c~ween symmetric (with a low dynamic dipole moment normal to the surface) or antisymmetric (with a large dynamic dipole moment parallel to the surface) s~retching modes vCH at 3025 cm -t (vl or vt2) and Vcc at 845 cm -t (v2 or v6). The suggested attributions of the other losses must be regarded cautiously. The energetic position of the 845 cm -~ loss peak and of its deuterated homolog at 820 cm -t, and the same order of magnitude of their intensities has allowed to assign these peaks to a Vcc stretching vibration. However, an additional contribution, at the 820 cm -t loss peak, of the isotopicaUy shifted low intensity peak at 1115 cm -t cannot be neglected since the isotopic ratio (R4 = 1115/820 = 1.35) is typically the one observed for "H type" vibrations.

4.3. Bonding and configuration of the adsorbed benzene molecule An adsorbed benzene species lying parallel to the surface agrees well with the thermodesorption, LEED and vibrational ELS data. Work function changes indicate l'able 3 Comparison of the observed losses to tile infrared active modes of the gaseous benzene Gaseou,~ ,~r cc,ad 'n~cd benzene [ 14,8] Mode number

Type of vibration

Frequencies (cm -1) C6H6

v4 v6 v14 vl0 v~4 ~'1 "; v9 ~'13 vl2

3,CH vCC 8CH 5CH ~'CH 7Ch 6CC '~c'~' vCH

675 1010 1035 1146 1035 969 1309 1479 3057

Experimental data Infrared activity

Frequencies (cm -1 )

Relative intens' ies

C6D6

497 970 812 823 812 789 t282 1330 2276

Very strong Weak Medium Weak Medium Weak Weak Medium Strong

C6H6

C6D6

750 845 845 1115 1115 111,5 1325 1425 3025

540 820 645 820 820 820 1225 1365 2250

Very strong Medium Very weak Weak Very weak Weak Strong

The stretching modes, the "in-plane" and "out-~_ff-plane" bending modes are designated by v, and y respectively.

J.C. Bertolini, J. Rousseau / Vibrational EEL spectra o f benzene on Ni

475

an electronic transfer from the adsorbate to the metal, and, consequently, a partial unfflling of the rr ring orbitals may be supposed. This supports a predominent n bonding with the nickel substrate. So, a weakening of the internal molecular bond is expected, as a consquence of the bond competition accompanying the chemisorption. Thus, an internal bond weakening must induce a frequency shift downwards for the correspon~ding Vcc and vCH stretching modes. Whatever their symmetric or antisymmetric character, the different stretching Vcc modes (v2 or v6) have quite comparable frequencies, just as the various stretching Vcn modes (vl, us, v~2 or vls), in the gaseous benzene. Consequently, although these data introduce uncertainties upon the attribution of the loss peaks, the following conclusions, deduced from the frequency shifts consecutive to the chemisorption, remain valid. (i) The shifts of the stretching modes are less important for the VcH (from 3 0 4 7 3062 to 3025 cm -1) than for the Vcc (from 993-1010 to 845 cm-~). It is a consequence of the change of the electron density on the rr ring orbital, consecutive to the carbon-metal chemisorption bond formation. The lowering of the Vcc frequency allows to estimate to nearby 40%, the decrease of the C - C force constant and consequently of the C - C bond. A complete calculation of the whole vibrational system should be done to precise this rough information. (ii) The v4 C - H perpendicular bending vibration increase (from 675 to 750 cm -l) may be also correlated to the occurred rehybridization, as generally observed in the frce molecules [ 11 ]. However, one may also invoke some hateraction between the H atoms and the metallic substrate to explain such an increase, as previously suggested [91. Finally, the only one difference between the vibration~d spectra registered on the two faces, lies on the frequency position of the usually called carbon-metal vibration. It probably characterizes the oscillating displacement of the whole benzene residue normal to the surface, but the observed frequency value also surely depends on the coupling of this mode with the substrate phonons. So, it is perhaps more convenient to consider this mode as a surface phonon of the benzene-nickel systems. According to this last point of view the higher frequency value observed on the (100) face could be related to the lesser surface density of the Ni atoms on the (100) face than on the (111). It is then interesting to note that the frequency of this mode is found experimentally (by neutron diffusion measurements) to be higher [18] (445 cm -1) on samples such as Raney nict el having a priori a lower density of atoms than the ordered faces.

5. Conclusions Experimental data proviue convincing evidence in favour of a predominantly n bonded benzene molecule on the two faces, the ring lyiaag parallel to the surface. The deep similarity of the vibrational electron loss spectra strongly reveals an equal

476

J.C. Bertolini, J. Rousseau / Vibrational EEL spectra o f benzene on Ni

configuration for the adsorbed species, on the (111) and (100) low index faces, insensitive to the differences of geometry and symmetry of the surfaces. This implies: - Either that the molecule is not sufficiently coupled with the substrate in order for the vibrating system to be sensible to the surface geometry. - O r that the chemisorption bond "isolates" the adsorption site from its neighbours Ni atoms. Then, a surface complex, combining the benzene molecule plus the same adsorption site, would be formed on the two faces.

Reference,

111 R.C.Z. van Meerten, PhD Thesis, Nijmegen, Netherlands (1975). [21 J.P. Candy and P. FouiUoux, J. Catalysis 38 (1975) 110. J.P. Candy, P. Fouilloux and B. Imelik, Nouveau J. Chim. 2 (1978) 45.

[31 R.B. Moyes and P.B. Wells, Advan. Catalysis 23 (1973) 121. [41 G.A. Martin and B. Imelik, Surface Sci. 42 (1974) 157. [5] P.C. Stair and G.A. Somorjai, J. Chem. Phys. 67 (1977) 4361. J.L. Gland and G.A. Somorjai, Surface Sci. 38 (1973) 157;41 (1974) 387.

[61 D.E. Eastman and J.E. Demuth, Japan. J. Appl. Phys. Suppl. 2 part 2 (1974) 827. [71 G. Dalmai-lmelik and J.C. Bertolini, J. Vacuum Sci. Technol. 9 (1972) 677. [81 J. Lehwald, H. Ibach and J.E. Demuth, Surface Sci. 78 (1978) 577. [91 J.C. Bertolini, G. Dalmai-lmelik and J. Rousseau, Surface Sci. 67 (1977) 478. [10l J.C. Bertolini, G. Dalmai-lmelik and J. Rouscau, J. Microsc. Spectrosc. Electron. 2 (1977) 575. [111 H. Herzberg, Infrared and Raman Spectra (Van No.strand, New York, 1956). [121 K. Chri~tmann, O. Schober, G. Ertl and M. Neumann, J. Chem. Plays. 60 (1974) 4528; G, Dalmai-lmelik, ].C. Bertolini, J. Massardier, J. Rousseau and B. Imelik, in: Proc. 71h Intern. Vacuum Congr. and 3rd lnte:n. Conf. on S~lid Surfaces (Vienna, 19771 p. 1179. ~-:.B. Wilson, J.C. l)echls and P.C, Cross, Molecular Vibrations; I'tle Theory of Infrared and Raman Vibrational Spectra (McGraw-Hill, New York, 1955). [14] J. Favrot, P. Caillet and M.T. Forel, J. Chim. Phys. 71 (1974) 1337. [151 M.R. Barnes and R.F. Willis, Phys. Rev. Letters 41 (1978) 1729. [16l H. Ibach and S. Lehwald, Surface Sci. 89 (1979) 425. [17] D. Sokcevic, Z. Lenac, R. Brako and M. Sunjic, Z. Physik B28 (1977) 273. [181 H. 5obic, P. FouiUoux and A. Renouprez, private communication.