Evidence for the distortion of C2H4 and C2H2 chemisorbed on W(100)

CHEMICAL PHYSICS LETTERS

Volume 46, number 1

EVIDENCE FOR THE DISTORTION OF C2H4 AND C,H, T.V. VORBURCER,

15 February 1977

CHEMISORBED ON W(IOD)

13.3.WACL.AWSKI

and

Received 5 November 1976

The adsorption of Cal14 on W(100) has been studied by uitravrolet photoelectron spectroscopy with hv = 21.22 ev. Thc spectrum measured sfter an mitial saturation esposure at 80 K exhíbits structure which correlates weIl with energy leveIs recently c&xrlated by Demuth and Eastman (DE) for sp3 rehybridized CzH4. Dehydrogenation of the adsorbate, eitber by subsequent heating to 295 K or direct adsorption at 295 K, yields a spectrum which correlates with DE’s caicutation for sp2 rehybridized CeHs. These resUbs suggest tbat CzH4 and CzHz may be distorted from their planar and Iincar structures respe:tiveIy and that the C-C bonds on these molecules are stretched by adsorption on ~V(lOO). Quahtative arguments suggest that the bonding site for both molecules is direc_tly over a W atom and that the DewarChatt model for x-d bonding IR organometailic compounds is appíicable.

In the search

to discover

the elementary

reaction

steps and the ï~te~ediates in catafytic reactions, it is important to know the geometries and energy leve& of chemisorbed mclecules. Ultraviolet photoelectron spectroscopy (UPS) has been increasingly used to characterize the electronic structure of chemisorbed species [ I-7J and may be used to identify bonding sites as wel1 [8,9J. In this letter, however, we report photoemission spectra for ethylene and acetylene on W( 100) which enable USto describe the distmted internalgeometry of these molecules. This is because the spectra show good agreement with energy leve1 splittings recently calculated by Demuth and Eastman (DE) for etbyfene and aeetylene mofecuies which are rehybridized from their planar and Iinear structures respectively f IOJ . Although, we have studied the chemisorption and decomposition of ethylene under a wíde range of adsorption conditions and subsequent heating temperatures by photoemission, work function measurements, and thermai desorption, we wil1 :on~entrate on tbree sets of resufts which bear directy on this rehybridization effect, namely, data for a sondensed muItiIayer (which has structure quite simi2

Iar to gas phase results), data for a monofayer of ethyfene chemisorbed at 80 K, and finalliy data for ethylene which has been deh~~drogenated to chemisorbed acetylene. Subsequently, we wil1 discuss some quaiitative arguments about the bonding mechanism of these hydrocarbons to the W(100) substrate. AI1 of tbe measurements were performed in an UItrahigh vacuum apparatus with a photoemission system operated in an angle ïntegrated mode using a hemispherical retarding analyzer which colIects electrons emitted at al1 angIes up to + 45” with respect to the sample normal 173. l3y way of introduction, fig. 1 shows the difference spectrum for ethyIene condensed on tungsten (100) at 80 K and a pressure of 1.33 X lom3 Pa ( lom5 torr). Above these data are plotted the peak positions from the gas phase data [ 1 I] which al1 Iine up with the peaks from the condensed Iayer shown here to within a tenth of an eV, provided a uniform refaxation shift of the gas phase Ievels by IS eV f2J is used to correct the ~neasnred binding energìes. The relaxation shift is determined by caIcuIating the binding energies of the adsorbate levels with respect to

CHEMICAL PHYSICS LETTERS

Volume 46, number 1

CLH4

CONDENSED

ON

WIIOO)

AT

hv=Zf

SOK 22eV

15 February

C2H4 CHEMISORBED hv = 21.22 eV

ON

1977

WttOO)

T’SOK GAS PEAK AER=

-15

-10 INITIAL

-5 ENERGY

PHASE

Cr a.

POSITIONS 1.5 eV

0 feVt

Fig. 1. UPS differente curve for Ca M4 condensed ou ~~(100) at = 1.33 x 10a3 Pa compared with the gas phase data measured by Turner et al. [ 111. A@ is the work function change induced by the adsorbate and AER ís the assumed relaxatíon shift. The initial energies are measured with respect to the Fermi cutoff of the photoelectron energy spectrum. The difference curve is taken by subtracting the spectrum measured for clean WflOO) from that measured during exposure to CaHa. Although physisorption takes piaee on top of the chemisorbed layer and, in al1 Irkeiihood, does not greatly perturb the ~~~emïsorbed tayer, the chemisorpt~on features of Gg. 2 are web masked by the streng features of the physisorption spectrum shown here.

the vacuum leve1 of the exposed surface 121. For CzH4 condensed on W( 100) the four peaks measured wîth respect to tbe Fermi energy are at -4.93, -7.08, -9.04, end -10.14 eV_ The work fu~ctioR for the exposed surface is measured to be 4.15 eV (whieh is srrder by 0.45 eV tban the measured work function for the clean surface). Therefore, the peaks are located at -9.08, -11.23, -13.19, and -14.29 eV with respect to the vacuum level. Since the measured peaks for gas phase CzH4 are at -10.51, -12.74, -14.76, and -15.82 eV, we conclude tbat tbere is an upward uniform relaxation sbift of the adsorbate Ievels with respect to the gas phase levels of = 1.5 eV _ Sïnce t_hiscondensed layer is physisorbed rather than chemisorbed, there is no evidente for any B bonding shìfts previously observed for chemisorbed layers on nickel(111) [2], iron, and copper [SI. The spectrum for the initial chemisorbed layer is entirely different_ Fig. 2 shows the differente curve

c3

r GAS

I~f~~~lll~~J -15

-10 tNfTfAL

-5

’

PHASE

rirf 0

ENERGY

Frg. 2. UPS differente curve for the imtial chemísorbed Yayet of CaH4 on W(100) eompared with the gas phase Levels and with the levels (arrows) caleuIated by Dcmuth and Eastman for an sp3 rehybridized ethylene molecule. The data points have been fitted to a theoretinl spectrum (solid line) with 4 gaussian peaks having fivhm = 1.5 ev. A sloping background (dashed Ene) is assumed. AE, is interpreted as the bonding shit in the rr orbital of the molecule_

tith respect to clean tungsten (100) for a saturat~o~ exposure (* 6.6 X 1W4 Fa s) to ethylene at a temperature of 80 K. This spectrum is quite different from any previously measured for chemisorbed ethylene. In particuiar, the data of Demuth and Eastman for Ni (111) [2] and of Yu et al. for iron and copper [5] aU show three web resolved cr tevels with spacirtgs wbieh are very similar to these for the gas phase levels shown again at the top. In the present experiment, instead of well resoived o structure, there is a main peak at -8.0 eV and two shoulders on each side wbich are depleted by subsequent heating to 295 K (see fìg. 3). There is an additional peak at -5.0 ev. Positive identi~catio~ of the four leveIs is made by resolving the spektrum into its komponent gaussian peaks as shown in fig. 2. Sï~~i~cantly better fits to tbe data are obtaíned with a theoretical curve having four rather than three peaks. The average positions of the leveIs are determined from several curve frts using slightly different values for the full widtb at half maximum of the peaks. This procedure yields values for the energy levels of -5 -04, -655, -7.98

43

CHEMICAL PHYSICS LETTERS

Voiume 46, number 1 C, H,

DEHYDROGENATED

ON GAS

-

7

$+;

.ADSORB -15

295

-JO INJTIAL

C2HE

A+=-0.37eV .-_-:. .. . 1

1 T

PHASE

_

-----

ADSORB 80K HEAT 295K I I

W(IOC 1

-

x -_f-

-

::

K -5 ENERGY

0 (eV1

Fig. 3. UPS differente curves for CzH4 on W(100) dehydrogenatcd to CzH2 under two sets of conditions. The spectra are compared with the levels cakulated by Demuth and Eastm,m for an _sp* rehybridized C2 Hz molecule.

and -9.19 with an average 2 standard deviation statisticat uncertainty of 0.14 ev. The gas phase (r levels, shown at the top of fig. 2 cannot be shifted uniformly to line up with the expenrnental structure. In particular, the splitting of 2.0 eV between the gas phase levels (ra and u3 is much larger than the splitting of = 1.4 eV between the main peak and the upper shoulder in fhe experimental spectrum. Below the gas phase levels, however, the solid Iines show the spectrum calculated by DE for an ethylene molecule which has been rehybridized from the planar sp2 geometry to the sp3 geometry [ 101. In detail, the H-C-H angle is 109.5” and the carbon-carbon bond length has been stretched from the carbon-carbon double-bond Iength of 1.34 a to the carbon-carbon single-bond Iength of 1.54 w. In their calculation DE [10] used an ab initio self-consistent field LCAO scheme of Ditchfield et al. [ 12]_ The absolute valEes of their calculated ionization potentials are not expected to be accurate because of fìnal state effects. Rather, the positions of the states have been deterrnined by using the shifts which are calculated as the molecule3 geometry is changed from sp”. In particu-

lar, the energy of the “1 orbital is calculated to shift upward by 0.79 eV, and the splitting between the o1 44

15 February 1977

and 02 orbitals decreases by 0.94 ev. These calculated u orbitals line up with our data to within = 0.2 eV, provided a relaxation shift of 1.7 eV is assumed. The agreement is better for the sp3 geometry than for any of the other geometries caIculated by Demuth and Eastman. T%is result implies that the C-C bond length of ethylene on W( 100) is approximateIy fS A and that the CH bond is rotated by =S28” from the original plane of the molecule as shown in fig. 4a. The peak at -5.1 eV is ascribed to the n oibital of ethylene. If this assignment is correct, the n orbital is shifted downward in energy by about 1.3 eV from the calculated position as shown by the dotted Iine. This a shift is consistent with those observed previously for nickel(111) [Z] , iron, and copper [S] and is ïnterpreted as being indicative of 7r-d chemiso~tjve bonding [2] . There is one final point to make concerning fig. 2. The structure in the differente curve between -5 eV and the Fermi energy is ascribed to changes in the tungsten substrate d orbitals caused by adsorption rather than to the presence of ethylene energy levefs in this energy range. In particular, the dip at -0.5 eV and the increase between -1 and -3 eV are houdt to represent a depletion of the surface states of the clean metal near the Fermi leve1 and a redistribution

Fig. 4. (a) Proposed model for the structure of chemisorbed C2H4 on W(100). In the gas phase the mo!ecuIe is planar and the C-C bond ìength 1.34 A. (b) Proposed model for tbe structure of chemisorbed C2Hz on W(lO0). Xn the gas phase Q Hz is Iincdr and the C-C bond Iength is 1.2 1 A.

Volume 46, number 1

CHEMICALPHYSICSLETTERS

]13,14] to d states which are characteristic of the surface moIecuIar complex. We wil1 touch on this last point later. The evidente for rehybridization shown by these data is repeated for ethylene which is thermally dehydrogenated to chemisorbed acetylene. We show in fig. 3 photoemission data for ethylene dehydrogenated in two ways. The upper curve shcws the differente spectrum with respect to clean tungsten measured after a saturation exposure (z 6.6 X lom4 Pa s) to ethylene at 80 K and subsequent heating to 295 K. Barford and Rye have observed a hydrogen thermal desorption peak at 260 K from chemisorbed CzH4 on W( 100) [IS], and our own thermal desorption measurements agree with their observations. The lower curve shows the differente spectrum measured after adsorption at 295 K. As expected, these two curves are quite similar, insofar as both curves show three peaks due to the chemisorbed species at approximately -10.5, -7.8, and -5.7 ev. There are, however, smal1 differences in the work function change and in the positions of the peaks between the two sets of conditions. The differences are consistent with the interpretation that there is more chemisorbed atomic hydrogen on the surface after initial adsorption of C2H4 at 295 K than after adsorption at 80 K followed by heating. This is to be expected if dehydrogenation takes place at partial hydrocarbon coverages for initial adsorption at 295 K. Under these conditions, vacant surface sites should be available to chemisorb the hydrogen products of dehydrogenation. On the other hand, C2H4 adsorption at 80 K should form a saturated monolayer before dehydrogenation leaving few vacant surface sites for chemisorption of the free hydrogen. In either case the rehybridization of the product C,H2 is similar to that for C,H,. The lower two peaks are assigned to o orbitals. The peak at -5 eV is assigned to the carbon-carbon bonding n orbital which is doubly degenerate in the gas phase. The splitting between the two u peaks for the gas phase spectrum [l l] shown at the top is 2.1 eV, smaller than what we observe in either of the two spectra. However, if we use the shifts calculated by DE [ 101 for an acetylene molecule which is bent from an sp linear structure into an sp* structure we get the lower set of lines anä, in particular, a splitting between u1 and 02 of = 2.9 eV which compares quite well with the

15 February 1977

splitting of = 2.8 eV observed in both spectra here. If we now add a large relaxation shift of 3.3 eV and use the different work functions for the two conditions, the calculated levels line up with the data to approximately a tenth of an electron volt. The excellent agreement between the calculated and measured splitting of the o levels implies that the C-C bond length has been stretched to = 1.3 A and that the C-H bond has been rotated away from the surface by about 60” as shown in fig. 4b. The so-called bonding shift of the rt orbital shown by the dotted lines is approximately 2.7 eV which is much larger than the 1.3 eV shift measured for ethylene. Similar differences in the relaxation and bonding shifts between C2H4 and C,H, have been observed on Ni( 1 f 1) [2]. It should be noted that the degeneracy of the a orbital is removed by bending the molecule_ Accordingly two levels are predicted by the calculation of the rr orbital level. This n leve1 splitting is not expected to be resolvable, however, since the fwhm of the peak is expected to be at least 1.3 ev. The increase in the u leve1 splitting upon chemisorption bas also been observed for C2H2 chemisorbed on iridium (100) by Broden and Rhodin 1161. They interpreted their result as simply being due to a stretching of the carbon-carbon bond. As the carbon-carbon distance increases, the carbon-carbon bonding orbitals should undergo an increase in energy because of the smaller electronic overlap whereas the antibonding orbitals should undergo a decrease in energy. For both ethylene and acetylene this intuitive model predicts the same trends that we observe experimentally; however, their model is only qualitative. It is felt that quantitative agreement between the data and the rehybridization calculation substantiates the model that both carbon-carbon bond stretching and carbon-hydrogen bond bending away from the surface plane are taking place as shown in fig. 4. Questions now arise as to whether these observations of internal molecular geometry lead to any conclusions regarding the bonding site itself and whether the molecule is rr bonded or di-0 bonded to the substrate. The Dewar-Chatt model [17,18] for n-bonding of olefins or alkynes singly coordinated to transition metal ions is relevant to the rr-bonding sittiation. The rr orbital of the hydrocarbon farms u bond by donating electrons to an untïlled dZz orbital of the 45

Yd

__.- --_--_-

atom and would be centered directly above it. In the case of di-u absorption, however, the n-bond would be broken and each carbon atom would be coordinated to a different surface atom. From the observation of rehybridization one might be tempted to conciude that the di-0 mechanism is operative [ 101. However, we fee1 for two reasons that the more likely model is the n-d-n” bond to a single W atom as just described. First, there are a number of examples in organometallic chemistry of olefins and alkynes being distorted in the ways we have described, when these molecules are coordinated to a single transition metal ion. In particular, there are short compilations of Xray crystallographic data by Stahck and Ibers [ 191 and Manojlovic-Muir et al. [20]. There are also theoretical arguments by B!izzard and Santry [21] relating the degree of C-H bond bending to the amount of n* back bonding in coordinated acetylene. Second, th& bonding model is consistent with the dangling bond picture of the surface. The d orbitals for the bcc (100) surface, include a lobe of the d,z orbital which projects normal to the surface and four lobes of the dxz and dYz orbitals which project at an angle of 45” with respect to the surface normal. The hypothesis of donation from the 7rorbital of the molecule to unfuled dZ2 orbitals of the metal and Lack donation from d,,z or dxz orbitals to the z* orbital is consistent with the calculations of Desjonqusres and Cyrot-Lackmann [22] for the orbital symmetries of the surface states on W( 100) located just below the Fermi energy. Their calculations show that the surface state peak is composed mainly of dx,, , dxz, dyz, ad dx2_,, 2 orbitals and that the dZ2 states lie almost completely above the Fermi energy and hence are unfiiled. Therefore, we can speculate that the shift in electron density near the Fermi ievel for chemisorbed ethylene represents the stablization of the dxz or dYz orbitals of the W surface states by back donation to the vacant ethylene n* orbital and the shift of the II leve1 indicates its stabilization by donation to the vacant d,z surface state orbitals. It has been our interpretation that the observed e

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by mixing of these states with the electronic states of the metal. Such u leve1 shifts were not observed, however, for CzH4 and C,H, adsorbed on Ni(ll1) [2] , Fe, and Cu [5 ] although the n bonding shifts accompanying chemisorption were observed. It is felt, therefore, that the same mechanism of rr-d-n* bonding takes place for CZHZ and C2H4 adsorbed on W( IOO), Ni(ll1) [2] , Fe, and Cu [5] . However, in the case of W(lOO)(and probably for CzH2 on Ir( 100) [16] as well) the extent of n-d or n*-d bonding is different than in the case of Ni( 11 l), Cu, and Fe. This differente is manifested in the geometrical distortion of CzH4 and CzHz on W( 100). Indeed, the data suggest some correlation between the heat of chemisorption and the amouni of distortion or rehybridization. The heat of chemisorption of ethylene on tungsten has been measured to be 4.4 eV [233 whereas on Ni, Fe, and Cu, which do not show any rehybridization effects in UPS [2,5], the heats of chemisorption are only 2.5,3.0, and 0.8 eV respectively [23] _However, it has been argued that for the singly-coordinated olefins and alkynes in transition metal complexes the degree of bending is determined primarily by the population of the ?T~orbital, i.e., only by the degree of back bonding [2 l] _ Before one can establish any trends, experimental results for heats of chemisorption are needed for these hydrocarbons on the single crystal faces of tungsten, iridium, and nickel which have been studied by UPS. In summary, then, the UPS data for C,H, and CzHL on W( 100) show agreement with the calculations of Demuth and Eastman [ 101 and enable USto describe the internal geometry of the chemisorbed molecules. Qualitative arguments suggest that the bonding site is directly over a W atom and that the Dewar-Chatt model [17,18] of x bonding and n* back bonding is applicable.

The authors are grateful to J.W. Gadzuk for a fruitful discussion and to C.E. Kuyatt, C.J. Powell, TE. Madey, and W. Goodman for their careful reading of the manuscript.

1CJ

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(1974) 1123. [31 E. Umbach, J.C. Fuggle and D. hlenzel, J. Electron Spectry., to be published. 141 E_W. Plummer, B.J. Waclawskl and T.V. Vorburger, Chem. Phys. Letters 28 (1974) 5 10. 151 K.Y. Yu, W.E. Spicer, 1. Lindau, P. Pianetta and SF. Lin, 3. Vacuum Sci. Technol. 13 (1976) 277. [61 T.V. Vorburger, D.R. Sacdstrom and B.J. Waclawski, J. Vacuum Sci. Technol. 13 (1976) 287. 171 E.W. Plummer, B.J. Waclawski, T.V. Vorburger and CE. Kuyatt, Progr. Surface Sci. 7 (1976) 149. 181 J.W. Gadzuk, Solid State Commun. 15 (1974) 1011; Phys. Rev. Bl0 (1974) 5030; Surfacc Sci. 53 (1975) 132. PI A. Liebsch, Phys. Rev. Letters 32 (1974) 1203. Wl J.E. Demuch and D.E. Eastman, Phys. Rev. Bl3 (1976) 1523. 1111 D.W. Turner, C. Baker, A.D. Baker and CR. Brundie,

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urn Sci. Technol. 13 (1976) 349. B.D. Barford and R.R. Rye, J. Chem. Phys. 60 (19741 1046. G. Broden and ‘l- Rhodii, Chem. Phys. Letters 40 (1976) 247. M.J.S. Dewar, BUIL Sec. Chim. France LS (19.51) C71. J. Chatt and L.A. Duncanson, J. Chem. Sec. (1953) 2939. J.K. Stalick and J.A. Ibers, J. Am. Chem. Sec. 92 (1970) 5333. L. Manojlovic-hluir, K.W. hluir and J.A. Ibers, Discussions Faraday Sec. 47 (1969) 84. A.C. Blizzard and D.P. Santry, J. Am.Chem. Sec. 90 (1968) 5749. M.C. Desjonquères and F. Cyrot-Lackmann, 5. Phys. F 6 (1976) 567. D.O. Hayward and B.bf.W_ Trapneil. Chemisorption (Butterworths, London, 1964).

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