Langmuir 1987, 3, 189-197
189
Properties of Chemisorbed Ethylene and Butadiene R. C. Baetzold Corporate Research Laboratories-Eastman
Kodak Company, Rochester, New York 14650
Received July 20, 1986. In Final Form: November 4, 1986 Tight-binding calculations employing two-three transition-metal atom layers of fcc(111)structure were used to consider the trends in properties of chemisorbed ethylene and butadiene. Ethylene undergoes a rehybridization upon bonding to the surface to approximately sp3 carbon. An increase in adsorption energy and metal-carbon overlap population is associated with this change. Several sites of chemisorption for s-cis or s-trans planar butadiene bound flat to the surface consistently give results that the molecule is an electron acceptor with a propensity to rehybridize at terminal carbon toward sp3. Upon chemisorption, the terminal C-C bond loses double-bond character, while the middle C-C bond gains double-bond character. A mechanism of C-H scission leading to vertical five-member metallacycles involving C4H4as the first dehydrogenation product was considered. Analysis of the ligand a and a* character in the adsorbate-metal Bloch functions shows extensive spreading and mixing of the a* ligand molecular orbital with the metal wave functions.
Introduction The chemistry of simple olefins chemisorbed to metal surfaces has been extensively studied, while the diolefins remain unstudied. Extensive spectroscopic studies of the simplest olefin, C2H4, have been carried out in ultrahigh vacuum (UHV). A low-temperature, intact molecule species is observed on transition-metal surfaces by electron energy loss (EELS) and ultraviolet photoemission spectroscopic (UPS) studies. For example, on P t ( l l l ) , evid e n ~ e l has - ~ been given to support a di-a-bound species with sp3 hybridization of carbon atoms. The evidence is consistent with the C-C bond parallel to the surface plane. This situation contrasts somewhat with Pd surfaces where the ethylene is relatively undistorted from gas-phase geometry on Pd(111)'j and Pd(l10)7but more distorted toward sp3 for terminal carbons on Pd(lOO).s Experimental studies have been supplemented by a variety of theoretical a n a l y s e ~ ~supporting *~,~ the rehybridized molecule. This rehybridization contrasts to chemisorption on noble metal surfaces such as Ag(ll0) or Cu(lOO), where the CzH4molecule retains carbon sp2 hybridization, probably indicative of a weaker interaction.lOJ1 In addition to the di-a-bound species on transition metals, the presence of purely a-bound (minor carbon rehybridization) species has also been reported. When the metal surface is varied, a change in the C hybridization has been noted through the vibration frequencies as a basis of the a l a classification developed by Stuve and Madix.8 In addition to the C2H4work, a particularly interesting lowtemperature study12of C5H8has appeared, which indicates considerable rehybridization in the low-temperature chemisorbed form. (1)Albert, M. A.; Sneddon, L. G.; Eberhardt, W.; Greuter, F.; Gustafsson, T.; Plummer, E. W. Surf. Sci. 1982,120, 19. (2)Baro, W.M.; Ibach, H. J. Chem. Phys. 1981,74,4194. (3)Demuth, J. E.Surf. Sci. 1979,84,315. (4)Kesmodel, L. L.;Dubois, L. H.; Somorjai, G. A. J. Chem. Phys. 1979,70,2180. (5)Felter, T. E.; Weinberg, W. H. Surf. Sci. 1981,103, 265. (6)Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1982,120, L461. (7)Chesters, M.A.; McDougall, G. S.; Pemble, M. E.; Sheppard, N. Appl. Surf. Sci. 1985,22, 369. (8) Stuve, E. M.; Madix, R. J. J. Phys. Chem. 1985,89,105. (9)Hiett, P. J.; Flores, F.; Grout, P. J.; March, N. H.; Martin-Rodero, A.; Senatore, G. Surf. Sci. 1984,140, 400. (10)Backx, C.; deGroot, C. P. M.; Biloen, P. Appl. Surf. Sci. 1980,6, 2.56.
(11)Nyberg, C.;Tengstal, C. G.; Andersson, S.; Holmes, M. W. Chem. Phys. Lett. 1982,87,87. (12) Avery, N. R. Surf. Sci. 1984,146,363.
It is ~ e l l - k n o w n that ' ~ as the surface temperature of ethylene chemisorbed to transition-metal surfaces is raised above about 250 K, C-H bond cleavage occurs and the -CCH3 ethylidyne species is formed. Several papers have summarizedsJ3the reaction sequences which take place on various surfaces. In this paper, we are interested in theoretical characterization of the low-temperature forms of chemisorbed conjugated molecules. Then, a possible mode of C-H cleavage with increasing temperature will be suggested. To my knowledge, theoretical studies of butadiene chemisorbed to extended metal surfaces have not appeared before. This is contrary to olefins such as C2H4, where a great number of cluster calculation^^^ have studied the carbon rehybridization and sites of preferred adsorption. Extended metal surface calculations are very germane to this work and have appeared for ethylene rehybridization on d-transition metal^.^ A particularly important analysis of the nodal properties of the metal/molecule wave functions involved in a* and a bonding to metals has recently appeared providing much insight into the adsorption sites.15 We use similar concepts, particularly emphasizing the bond-overlap population, to understand bonding in the butadiene system. The present work utilized band-structure calculations to obtain some preliminary information on properties of chemisorbed ethylene and butadiene. A variety of transition-metal surfaces and clusters have been examined. The latter have been included because of the number of butadiene complexes reported,"j in contrast to transition-metal surfaces, where butadiene has been scarcely studied experimentally. The interaction of ethylene with transition-metal surfaces is considered because of the wealth of experimental information available in this area. The ultimate goal of the present studies is to make some predictions of the general features of butadiene chemisorption under various situations. (13)Koestner, R. J.; Van Hove, M. A.; Somorjai, G. A. J. Phys. Chem. 1983,87,203. (14)(a) Itoh, H.; Kunz, A. B. Chem. Phys. Lett. 1979,66, 531. (b) Basch, H.; Newton, M. D.; Moskowitz, J. W. J.Chem. Phys. 1978,69,584. (c) Swope, W.C.; Schaefer, H. F., I11 Mol. Phys. 1977,34, 1037. (d) Gavezzotti, A.; Simonetta, M. Chem. Phys. Lett. 1980,99,453; 1979,61, 435. (e) Anderson, A. B.; Hubbard, A. T. Surf. Sci. 1980,99, 384. (0 Kobayahi, H.; Teramae, H.; Yamabe, T.; Yamaguchi, M. Surf. Sci. 1984, 141,580. (9) Kang, D. B.; Anderson, A. B. Surf. Sci. 1985,155, 639. (15) Silvestre, J.; Hoffmann, R. Langmuir 1985,I, 621. (16)Yasuda, H.; Tatsumi, K.; Nakamura, A. Acc.Chem. Res. 1985,18, 120 and references therein.
0743-7463/87/2403-0189$01.50/0 0 1987 American Chemical Society
190 Langmuir, Vol. 3, No. 2, 1987
Baetzold b
a
I
I
0
c"'...
The geometry and chemisorption energy of butadiene are important factors in considering its reactivity on metal surfaces. Consider the well-known hydroformylation of simple olefins by CO/H2 leading to the aldehyde. A CO molecule is thought to insert into a C-metal bond formed by the olefin" and a theoretical analysis of this mechanism has been performed.ls In the case of butadiene, the geometry of the chemisorbed molecule should play a key role in single, or possibly even a double, hydroformylation reaction.19 If both C-end atoms are bound to the surface, the double hydroformylation is possible (other factors permitting); however, if one end above the surface is unattached, a second stage involving olefin adsorption and insertion would be required. In addition to the geometric considerations, there are always energetic factors to be evaluated. The simplest is the heat of chemisorption. If the heat of chemisorption is too large, the potential reactant will lie in a deep potential well making subsequent reaction slow. This well-known principle in catalysis makes knowledge of the heat of adsorption quite important. Method of Calculation. The method of calculation is tight-binding to represent the interactions of transitionmetal d orbitals with chemisorbed molecules. We and others have used this method to treat adsorption of saturated hydrocarbons,20,21C0,22and a variety of other molecules, such as NH3,23to extended metal surfaces. We employ two or three metal layers in a supercell approach. This is particularly necessary for a large molecule such as cis- or trans-butadiene, which we show in Figure 1 adsorbed to the six-atom unit mesh containing two metal layers used to represent the fcc(ll1) close-packed surface. Calculations for ethylene employ a four-atom mesh containing three metal layers. Details of the calculations, such as energy calculations and reciprocal-space averaging, may be found in ref 20. Parameter choices are discussed in Appendix 1, where we also show their influence on computed properties. In our calculation, the transition-metal surface is characterized by Nd, the average number of metal d electrons. We place particular emphasis on the d electrons, due to their central role in transition-metal binding. We attach some special significance to the bond-overlap population, following the emphasis placed on this quantity by Hoffmann and co-workers.15~21 We compute occ
PI] = 2CCdC*,ISI, 1
(1)
(17) Mills, G. A.; Stefgen, F. W. Catal. Reu. 1973, 8, 159. (18) Berke, H.; Hoffmann, R. J . Am. Chem. SOC.1978, 200, 7224. (19) Pelt, H.L.; DeMunch, N. A.; Verburg, R. P. J.; Brockhus, J. J. J. J.; Scholten, J. J. F. J. Mol. Catal. 1985, 31, 371. (20) Baetzold, R. C. J . Am. Chem. SOC.1983, 105, 4271. (21) Saillard, J. Y.; Hoffmann, R. J . Am. Chem. SOC.1984, 106, 2006. (22) Baetzold, R. C. Phys. Reu. B 1984, 30, 6870. ( 2 3 ) Baetzold, R. C. Phys. Reu. B 1984, 29, 4211.
i:/
1.51
1.7
y,
'...._......... .......
0
Figure 1. Six-atom metal planar unit mesh is shown with (a) s-trans-butadieneand (b) s-cis-butadienechemisorbed above the unit mesh.
M-C OVERLAP PWULAT,Y,,...,
x
I
I
2. I
2.5
c*c
x
M-C Distance (A) Figure 2. Heat released upon ethylene chemisorption on the fcc(ll1) transition-metalsurface with Nd = 7.9 is shown vs. M-C distance for approach with the C-C axis parallel to the surface for four sites: B, bridge; H, hollow; C, crossed; T, on-top. Also shown in the inset is the total M-C overlap population as it varies
with M-C distance.
as the overlap population between orbitals i and j , where C,l is the complex eigenvector and SLlis the overlap population. The atom-reduced overlap population A B OAB
=
CCP,,
(2)
1 1
is summed over all orbitals i on atom A and orbitals j on atom B. It serves as a measure of the A-B strength. Thus, double bonds will have a larger O A B value than the corresponding single-bond value. This quantity is particularly useful for comparison to trends in vibration frequencies, such as those measured in energy loss spectroscopy. We report the total carbon-metal overlap population in tables where the quantity O A B is summed over all metal atoms in the top surface layer. The bond-overlap population is compared to the heat of adsorption Q, which is the energy released on adsorbing a particular species. The metal parameters are chosen in these calculations to represent the final state that might result from some self-consistent computation. Thus, they should not be thought of relative to a vacuum level and are better thought of relative to the molecular orbitals of butadiene or ethylene. Estimates of the x* level in ethylene place it a few tenths of an electronvolt above or below the transition-metal Fermi level, depending upon the metal type.5 For all of the metals, we simulated this effect by keeping the Fermi energy EF = -9 eV constant, while varying the center of the d band to adjust band filling continuously, as in the past.20 We emphasize that this is a technique for exploring trends of diamagnetic metal surfaces, which is the virtue of this computational method, rather than looking at specific metals.24 This particular method of simulating extended metal surfaces of differing d occupation is in line with the analysis of Varma and Wilson.25 In that analysis of transition metals, the Fermi energy does not vary significantly across a given series while the positon of the mean band energy decreases rapidly to the right. We have explored this method earlier and also examine some other models in Appendix 1. Chemisorbed Ethylene. We begin by considering chemisorbed ethylene as a test of our method on systems for which experimental data are available. A t low tem(24) Baetzold, R. C. Solid State Commun. 1982, 44, 781. ( 2 5 ) Varma, C. M.; Wilson, A. J. Phys. Reu. B 1980, 22, 3795
Langmuir, Vol. 3, No. 2, 1987 191
Properties of Chemisorbed Ethylene and Butadiene perature, the ethylene molecule adsorbs intact and this species will be the focus of our attention to help us understand how the unsaturated a , ~system * interacts with the surface. We consider the potential energy curves for bringing the planar ethylene molecule toward the fcc(ll1) metal surface with Nd = 7.9. This provides a test of the computational method to distinguish different high-symmetry adsorption sites and to provide realistic potential curves. Figure 2 shows these curves for four different adsorption sites. The crossed site is unfavorable, while the bridge and hollow sites appear to give potential minima at realistic distances. The potential curve for the on-top site is unphysical, since a minimum is not found for realistic bond distances. The computed metal-carbon (M-C) overlap populations follow the heats of adsorption in trend, as shown at the inset. Both the criterion of maximum overlap population and heat of adsorption give realistic potential curves for the ethylene adsorption sites involving multiple M-C bonds. Nevertheless, the results of Figure 2 show the inability of this band-structure calculation to predict adsorption geometries reliably. The on-top adsorption site does not give a realistic potential curve in this computation. The bonding of ethylene to principally one metal center becomes dominated by orbital interactions involving a* molecular orbitals of ethylene with a single metal atom d,, orbital as sketched, 1. Reducing the M-C distance promotes this interaction,
1
t 0.5
$E) 0.25 0.5
0.25
-4
-2
-0.03 I
causing a greater flow of electrons to the a* molecular orbital with no repulsive force able to balance the attraction. The multiple metal-bonding sites do not possess this freedom, since orbitals on several metal atoms are involved, and their nodal properties are governed by the Bloch metal functions. We consider the interaction of a and a* molecular orbitals of ethylene with the metal surface. The molecule is first placed on the fcc(ll1) metal surface with its C-C axis centered and parallel to a metal-metal bond in the bridge site. The ethylene molecular orbitals mix with components of metal orbitals as they begin to interact and are shifted in energy. The degree of this interaction can be measured by projecting the molecular orbitals from the mixed molecular-metal Bloch wave functions according to procedures discussed earlier,23which are reviewed in Appendix 2. Figure 3 shows this result for a and a* molecular orbitals as the molecule is lowered closer to the surface. At infinity separation, the a,a*levels are unperturbed and consist of 6 functions of area corresponding to two electrons. Bringing the molecule closer to the surfaces gives splitting such that, at bonding distances, a single peak is no longer discernible. We note that the spreading of the a* molecular orbital is somewhat greater than the spreading of the a molecular orbital, a point that may be related to the magnitude of the free molecule normalization constant.20,26In fact, a large part of the a* level drops below the Fermi level of the metal, leading to a significant electron population in this molecular orbital. ( 2 6 ) Shustorovich, E.M.
J. Phys. Chem. 1983, 87,14.
E,mO
2
Energy (eV) Figure 3. Projection of the ethylene T and P* molecular orbital from the metal-adsorbate Bloch function is shown for ethylene adsorbed a t the bridge position for various distances above the plane of the fcc(ll1) Nd = 7.9 surface. Gas-phase ethylene levels are a t -8.51 and -13.12 eV.
-8
I
-6
-4
I
-2
,
EF.O
I
2
I
4
Energy (eV)
Figure 4. Components of the total M-C overlap population are shown vs. energy for ethylene adsorbed to the bridge position on the fcc(111) surface with Nd = 7.9.
Thus, a strong net acceptor role for ethylene results. The dependence of the strength of M-C interaction as a function of the degree of band filling is shown in Figure 4. The overlap population increases with metal electrons, up to a point, and then decreases as the band is filled further. This effect is corroborated in the computed heat of adsorption for an ethylene molecule chemisorbed to a metal surface shown in Table I. A perturbation on the ethylene molecule is caused by the chemisorption to the metal surface. This perturbation is measured in the extent of rehybridization of the molecule from sp2 toward sp3 at the C atoms. Four sites of chemisorption are considered on the (111) surface (as sketched before in Figure 2), where the C atoms and closest metal atoms are shown. The C atoms were considered in a plane parallel to the metal surface plane at a height of 1.9 A in all calculations. The H atoms are bent away from the surface to achieve hybridization changes from sp2 to sp3. We consider four equal increments for this change as shown in Table I. This metal surface with Nd = 7.9 promotes considerable rehybridization toward sp3 on the C atom attached to each site. The computed heats of adsorption and C-M overlap population track one another well and suggest that, in the hollow site, sp3 hybridization is preferred. The origin of this effect lies i n the population of a* molecular orbitals of ethylene; an effect that has been
192 Langmuir, Vol. 3, No. 2, 1987
Baetzold
BUTADIENE trans
1
%
CIS
--.-d*HI
- q H I
0,
101
Energy I ~
-5
t
m
0.5
- qd" 0
-lot 772 I
-6
1
U
Figure 5. Sketch of the energy levels of s-cis and s-transgas-phase butadiene is shown, along with the a and a* nodal wave functions.
long known for cluster complexes.27 This also occurs on surfaces through the population of the K* molecular orbital as shown in Table I. We have also done a more limited set of calculations for the effect of metal d electron population on rehybridization. Considering only the bridging site, we found a tendency to rehybridize which does not seem to depend strongly upon the number of metal d electrons in the range Nd = 5.0-8.9. Trends in the properties of chemisorbed ethylene with Nd are quite consistent with one another. The planar molecule has an increasing P* population as Nd decreases. This tracks well with a reduction in the C-C overlap population and an increase in the heat of adsorption and the M-C overlap population. Clearly, the early-transition-metal surfaces destabilize the C-C bond the most, and they provide the greatest adsorption energy. We note that the rehybridization of ethylene from sp2 toward sp3 is accompanied by a weakening of the C-C bond (reduced overlap population) and a strengthening of the M-C bond (increased overlap population). This effect is not minor. Chemisorbed Butadiene. Let us consider the electronic structure of free butadiene before treating its interaction with metal surfaces and atoms. Figure 5 shows the frontier orbitals of the s-cis and s-trans isomers. The two K and two P* orbitals, composed of P, atomic orbitals perpendicular to the planar ground state, are sketched with regard to their nodal character. The nodal character is important when considering the symmetry relations involved in interactions with metal surfaces. For example, the symmetrical in-phase combinations of metal dZzorbitals, located at the bottom of the d band, will interact strongly with the lowest x1 state of either isomer. The other interactions are less clear, as they depend upon the atomic spacings and position of the adsorbed molecule, but the out-of-phase combinations in r3*and r4*will generally interact most strongly with the out-of-phase dZzorbitals a t the top of the d band. Several uCH orbitals are shown in the diagram to lie close in energy to the P orbitals and provide a complicated set of closely spaced levels. The uCH* levels may be worthy of attention in C-H bond-breaking processes. They lie just
n
-4
-2
0
,
2
4
Energy, eV Figure 6. Projections of the butadiene aI,a2,a3*,and y4*molecular orbitals from the metal-adsorbate Bloch functions are shown for butadiene adsorbed at the bridge site with a s-trans geometry. The metal surface has Nd = 7.9. Positions of energy levels for the free molecule are denoted by 7 , while projections are shown for the molecule adsorbed at a large distance above the surface plane Z = 2.9 A and at a chemisorption distance Z =
1.9 A.
I 0
p (E)
4
Energy (eV) Figure 7. Same as Figure 6 but for metal surface with Nd = 5.0 at Z = 1.9 A. above the vacuum level and, at this position, are more favorably positioned to accept electrons than the u C - ~ * levels in saturated hydrocarbons (CH4),20*21 which lie at +5 eV. The acceptor role of u ~ - in ~ *the C-H cleavage in CH, was discussed earlier. We consider mixing butadiene molecular orbitals with the metal surface by projecting these molecular orbitals from the total density of state. Figures 6 and 7 show these projections for the free ligand, the ligand at a large distance above the surface, 2.9 A, and the ligand strongly interacting with the surface a t a distance of 1.9 A. A t the large distance from the surface (2.9 A), the ligand molecular orbitals begin to spread in energy although their center of gravity remains at the position of the free ligand. When the interaction is strong, the ligand molecular orbitals are spread considerably in energy. The rl and r4*centers of gravity are shifted deeper and shallower in energy, respectively, while r2and P ~ *have completely lost their original shape. The stronger interaction with x2 and 7r3* compared to R~
Langmuir, Vol. 3, No. 2, 1987 193
Properties of Chemisorbed Ethylene and Butadiene
Table I. Properties of Chemisorbed Ethylene on fcc(ll1) Surface heat of adsorption
carbon
Nd 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 8.9 8.9 8.9 8.9 5.0
5.0 5.0 5.0
site bridge
hollow
on-top
crossed
bridge
bridge
H 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
hybridization SP2 SP3 SP2 SP3 SP2 SP3 SP2 SP3 SP2
SP3 SP2 SP3
charge
overlap population
ligand population
Q,eV
4
c-c
M-C
a
a*
0.78 1.18 1.33 0.33 0.43 0.62 0.62 1.12 1.08 1.14 0.96 0.44 -0.14 -0.04 -0.18 -0.61 0.39 0.80 0.91 0.63 1.37 1.68 1.82 1.65
-1.15 -1.17 -1.24 -1.37 -0.98 -1.08 -1.18 -1.36 -0.84 -0.90 -1.05 -1.24 -0.77 -0.84 -1.04 -1.25 -1.18 -1.19 -1.26 -1.31 -1.11 -1.14 -1.23 -1.34
1.00 0.98 0.95 0.92 1.10 1.02 1.01 0.91 1.07 1.05 1.01 0.94 1.11 1.12 1.02 0.97 1.09 1.07 1.03 0.98 0.93 0.92 0.89 0.86
0.15 0.21 0.26 0.21 0.07 0.14 0.16 0.29 0.11 0.13 0.15 0.15 0.03 0.06 0.08 0.09 0.09 0.15 0.21 0.24 0.15 0.21 0.26 0.28
2.00
1.28
2.00
1.02
2.00
0.92
2.00
0.76
2.00
1.15
2.00
1.49
Table 11. Properties of Chemisorbed Butadiene on fcc(ll1) Surface, Nd= 7.9 heat of overlap population geometry trans trans cis cis cis trans cis rehybridized trans rehybridized cis free molecule trans free molecule
ligand populationsb orienta- adsorption charge middle" terminaln middle terminal site tion Q, eV q M-C M-C C-C C-C a1 a2 a3* ad* uC-HI* bridge flat 0.50 -1.22 +0.03 +0.05 0.98 1.22 2.00 1.98 0.99 0.36 0.03 hollow flat 0.61 -1.35 +0.02 +0.07 1.00 1.20 2.00 1.98 1.07 0.38 0.04 bridge flat 0.34 -1.30 +0.03 +0.04 0.98 1.22 2.00 2.00 1.39 0.35 0.03 see Figure 1 flat 0.66 -1.46 +0.02 +0.11 1.04 1.16 2.00 1.97 1.39 0.35 0.03 hollow vertical 0.21 -1.20 -0.003 -0.01 1.01 1.25 2.00 2.00 0.89 0.06 0.21 hollow vertical 0.24 -0.97 0.00 +0.02 1.01 1.24 2.00 2.00 0.61 0.03 0.26 bridge vertical 0.30 -1.53 -0.02 +0.34 1.26 0.92 0.68
-1.26
+0.17
+0.17
1.03
1.16
0.92
1.34
0.93
1.33
uC-HZ*
0.03 0.02 0.01 0.01 0.04 0.07
"The total top metal surface atom-carbon overlap population. bThe free butadiene a orbitals in order of decreasing energy are al,a2,r3*, and a4*and the antibonding C-H orbitals in order of decreasing energy are UC-HI and U C - ~ Z(see Figure 5).
and x4* is a consequence of their difference in energy with the metal d orbitals (Ed) and follows directly from second-order perturbation theory.28 In this particular calculation with Ed = -11 eV, there is roughly equal interaction of x 2 and a3* with the metal d orbitals. This type of interaction leads to pushing x 2 orbitals above the Fermi level and a donor function for the ligand, while the x3* orbitals are pushed below the Fermi level and they provide an acceptor function for the ligand. The latter function predominates for transition-metal elements. The interaction of x and x* ligand molecular orbitals with the surface is strongly dependent upon the position of the center of the metal d band (Ed). On middle-transition-metal elements, the Ed value is closer to the Fermi level than on late-transition-metal elements. This effect is apparent by comparing Figures 6 and 7. In Figure 7 where the middle transition-metal surface is considered, the x l and x2 orbitals are perturbed little by the metal surface, but the r3*and x4* levels show major perturbation. The electron-acceptor properties of butadiene will (28) Hoffmann, R. Acc. Chem. Res. 1971, 4, 1.
be greatly enhanced by this situation. The predominant acceptor function for chemisorbed butadiene on transition-metal elements is not parameter dependent for reasonable ranges of parameters. In Appendix 1, we show computational results derived from various metal parameter sets, including consideration of an averaging off-diagonal formula of Hoffmann,21J5which are completely consistent with these predictions. There is a preference for horizontal bonding of s-cis or s-trans isomers of butadiene to the fcc(ll1) surfaces, which is considered in Tables I1 and 111. We do not care to specify a preferred site for adsorption because of the molecular complexity of butadiene. The acceptor properties seem to be consistent for the various sites considered. We have tested the preference for horizontal bonding further by tipping the s-trans isomer away from the surface in small angular increments, while the closest carbon atom remains in place. A uniform decrease in the heat of adsorption results, supporting the view of preferred planar bonding. We have also considered possible rehybridization on terminal C atoms of butadiene to sp3. The appropriate cis isomer was bound to the surface in the bridge site,
194 Langmuir, Vol. 3, No. 2, 1987
Baetzold
Table 111. Properties of Chemisorbed Butadiene on fcc(ll1) Surface
Nd 9.0 9.0 9.0 9.0 9.0 9.0 5.0 5.0 5.0 5.0 5.0 5.0
geometry trans trans cis cis trans cis rehybridized trans trans cis cis trans cis rehybridized
heat of overlap population adsorption charge middle" terminal" middle terminal ligand populations site orientation Q,eV 4 M-C M-C C-C C-C TI TZ "3 ~4 uC-Hl* flat -1.14 -1.25 0.00 0.00 0.98 1.27 2.00 2.00 0.76 0.36 0.04 bridge -1.16 -1.49 f0.02 1.00 1.24 2.00 1.97 1.20 0.33 0.03 -0.02 hollow flat -1.02 -1.34 -0.01 +0.03 0.99 1.25 2.00 1.99 0.83 0.36 0.05 see Figure 1 flat -1.22 -1.44 -0.02 vertical hollow 1.00 1.30 2.00 2.00 0.63 0.06 0.23 -0.02 -0.03 0.98 1.28 2.00 2.00 0.36 0.03 0.07 -0.66 -0.90 -0.02 hollow vertical -2.29 -1.64 -0.02 +0.23 1.23 0.98 vertical bridge bridge hollow see Figure 1 hollow hollow bridge
flat flat flat vertical vertical vertical
1.73
1.80 1.54 1.10 0.64 0.03
-1.02 -1.03 -1.38 -1.39 -0.49 -1.56
f0.03 f0.02 f0.03 -0.02 -0.02 -0.02
0.99 1.01 1.06 1.06 0.98 1.22
+0.08 +0.09 f0.13 f0.02 0.00 f0.28
1.16 1.14 1.09 1.12 1.23 0.95
2.00 2.00 2.00 2.00 2.00
2.00 1.16 2.00 1.19 2.00 1.63 2.00 1.38 2.00 0.49
0.29 0.24 0.26 0.05 0.02
uC-HZ*
0.03 0.02 0.03 0.04 0.09
0.02 0.02 0.02 0.18 0.04
0.02 0.01 0.01 0.03 0.06
The total top metal surface atom-carbon overlap population.
yielding the data in Tables I1 and 111. We observe formation of a strong M-C bond through the values of the overlap population, while the middle C-C bond becomes more double bond in character and the terminal C-C bonds become more single bond in character. The computed heat of adsorption is roughly comparable to values for other geometries even though a destabilization in the gas-phase molecule of 4.06 eV is computed for going from sp2 to sp3. The M-C bond formation has offset most of this energy. A rehybridized structure of s-cis butadiene chemisorbed in a di-cr fashion to the fcc(ll1) surface was considered as an analogue to the ethylene (B) adsorption site in Figure 2. This type of 1-2-carbon bonding to the surface permits rehybridization of the 1-and 2-carbon atoms by bending 3-hydrogen atoms and the rigid CH-CH2 group away from the surface in equal increments. An energy minimum occurs a t one-third of the least-motion rehybridization from sp2 toward sp3 of the C atoms bound to the metal. The computed binding energy of this rehybridized species on the N d = 7.9 fcc(ll1) surface is 0.68 eV, which indicates greater stability than the other species. The entry for this species in Table I1 (rehybridized trans) indicates strong M-C bonding and considerable strengthening of the middle C-C bond. The C-C overlap population in butadiene is considerably altered upon chemisorption. Tables I1 and I11 show that the overlap population, relative to the gas-phase molecule, increases for the interior C-C bond and decreases for the exterior C-C bond upon chemisorption. The extent of this effect becomes particularly significant as the terminal C hybridization changes toward sp3, but it is also apparent even in the absence of rehybridization. This effect is caused by chemisorption leading to greater double-bond character in the interior C-C bond and the reverse for the outer C-C bonds, consistent with the population of r3*. The trends in properties of chemisorbed butadiene are shown vs. Nd for some select geometries in Figures 8 and 9. We observe stronger bonding on the middle- than on the late-transition-metal surfaces. This is apparent in the heat of adsorption and total M-C atom overlap population. The two trends with Nd are fairly consistent even though the slopes are quite different. The C-C overlap populations show a consistent change from gas-phase values in Figure 9. We observe that the middle C-C bond of chemisorbed butadiene is strengthened vs. the gas-phase value, fairly independently of N d . The terminal C-C bond is weakened more on the middle-transition-metal surfaces than on the late-transitionmetal surfaces.
I
I
I
I.\ 1 ,I g
,
,
-0.
-2.0 5
7
9
7
5
9
Nd
Nd
Figure 8. Plot of heat released on chemisorption (a) and total M-C overlap population (b) vs. Nd for s-cis (C), s-trans (T),and rehybridized (R) butadiene on fcc(ll1) surface. Middle C - C Bond
Terminal C-C Bond
-
Overlap populationI. 3
~~
t
0 5
-F
7
9
Nd
t 5
7
9
Nd
Figure 9. Plot of overlap population for middle C-C (a) and terminal C-C bond (b) of chemisorbed butadiene vs. Nd for s-cis (C), s-trans (T),and rehybridized (R)butadiene. Values of t h e computed gas-phase C-C overlap population are denoted as F.
We should consider the consequences of C-H bond scission in butadiene adsorbed to metal surfaces. This reaction is certainly anticipated due to the well-known C-H cleavage in ethylene, which occurs on various surfaces a t low-to-moderate temperatures. The reaction occurs below room temperature on Ir and Pt surfaces.29 Thermal desorption experiments have also provided evidence for this reaction.30 The mode of this reaction can certainly be thought of in terms of population of the crCH* anti(29) Klarup, D.; Gentle, T.; Muetterties, E. L., unpublished work. (30) Monnier, J. R., private communications.
Properties of Chemisorbed Ethylene and Butadiene Table IV. Properties of Chemisorbed C4H4 bridge site
heat of
orientation horizontal vertical ring vertical broken ring
adsorption Q,eV 4.85 6.65 5.43
overlap population terminal
middle
outer
M-C
C-C 0.99
C-C
0.14 0.37 0.39
0.99 0.98
1.32 1.22 1.22
bonding molecular orbitals, as has been shown in computations of saturated hydrocarbons to a metal surface.20i21 The details of this mechanism have been discussed quite fully and will not be repeated here. Rather, let us consider that the partial electron transferred from the surface could be most easily accommodated in terminal, rather than interior, uCH*antibonding levels as would be expected on general mechanistic organic chemistry grounds. This would first result in the formation of a C4H4species which should strongly interact with the surface. Let us consider various modes of interaction next. We have considered bonding of the C4H4fragment to the bridging site of the fcc(ll1) surface. Table IV shows properties of the fragment for various orientations, where we consider the ring structure 2 and the broken ring 3. H
7
/
\c -H
\ C-C.
H--6
Ring
2
rH
H
c
'c-
H--6
/ \
'
H
M
Broken Ring
3
There is a definite energetic preference for the ring metallacycle structure. This upright structure could well be an intermediate, prior to further dehydrogenation. It is strongly adsorbed to the surface, as deduced by comparing its heat of adsorption to the values for the intact butadiene. The strong adsorption energy provides a driving force for C-H cleavage on this clean metal surface. We note that a ring metallacycle PtC4H4 has been postulated from NEXAFS studies of thiophene desulfurization on Pt(lll).31 The five-member ring has also been postulated in thermal desorption studies.32
Discussion The computed potential energy curves for ethylene approaching a metal surface with a C-C axis perpendicular to the direction of approach are realistic for the multiple-atom binding sites. In this case, the nodal properties of the Bloch functions and a,r* molecular orbitals are (31) Stohr, J.; Gland, J. L.; Kollin, E. B.; Koestner, R. J.; Johnson, A. L.; Muetterties, E. L.; Sette, F. Phys. Reu. Lett. 1984,53,2161. (32)Wexler, R. M. Ph.D. Thesis, University of California, Berkley, 1983.
Langmuir, Vol. 3, No. 2, 1987 195 responsible for the repulsive component of the potential, unlike the on-top site. Extensive rehybridization of ethylene carbon atoms toward sp3takes place on the bridge and hollow sites at all Ndvalues between 5.0 and 8.9. This is in accord with experimental data, as discussed by Stuve and Madix? for the low-temperature form of chemisorbed ethylene. Their U ~ Tparameter varied between 0.43 and 0.92 for several metals, where 0.0 corresponds approximately to the sp2 carbon and 1.0 corresponds approximately to the sp3 carbon. It is difficult to compare experimental low-temperature heats of adsorption to our calculations. Some clean surfaces have two forms of C2H4adsorbed simultaneously, such as Pd(100): reported to have a T and di-a form. The adsorption energy should also depend upon coverage with decreasing values as the coverage increases. In fact, the orientation of the C-C bond has been shown to change with c0verage.I Some reported values for ethylene heats of adsorption are P t ( l l l ) - Q = 12.0 f 3.2 kcal/mol,13Ni(111)-Q = 12.0 kcal/m01,~~ and Pt(ll1)-Q = 13.0 kcal/ mol.34 The Ni data were concluded to result from a physisorbed layer, while the Pd data refer to a chemisorbed layer. Despite the uncertainties in how to compare the experimental data to our calculated values of the heat of adsorption for unrehybridized ethylene, they do agree well, while the rehybridized ethylene values which we calculated are larger. This observation, together with the experimental fact that u and T forms of chemisorbed ethylene coexist (at least on Pd(100)), suggests that there may be an activation barrier for going between the two forms if they occupy different sites. Our calculation procedure was not able to detect such an activation barrier. The calculations for planar butadiene give strong evidence for its low-temperature adsorption to be flat on the (111) surface plane although the 1,2-diadsorbed species may be favored on some surfaces. We are not able to make a conclusive statement regarding preferred adsorption sites at this time, because they all lie close in energy within the error of the computational method. It is clear, however, that butadiene accepts electrons from the metal surface. The M-C overlap populations with terminal carbon atoms increase as the number of metal d electrons decreases. At the same time, the C-C overlap populations are strongly affected. The overlap population increases for the middle C-C bond and decreases for the terminal C-C bond for chemisorption. These effects are strongest on the Nd = 5.0 surface. These trends should be experimentally measurable in the EELS vibrational spectrum. Our calculations of a rehybridized sp3 form of cis-butadiene bonding vertically to the surface give a heat of adsorption of 0.30 eV on the Nd = 7.9 surface. This value is comparable to heats of adsorption computed for unrehybridized butadiene on these surfaces. The fact that our calculations indicate that the undistorted planar geometry is 4.06 eV more stable in the gas phase suggests that considerable rehybridization toward sp3 is possible for the terminal carbons of adsorbed butadiene. A small population of the u C - ~ * molecular orbitals of butadiene is computed for various geometries and sites in Tables I1 and 111. Certainly, this could lead to weakening of the C-H bond and eventual scission by the same mechanism we have discussed for adsorbed saturated hydrocarbons. The metallacycles hypothesized involving C4H4are a likely first stage, but further C-H bond scission is certainly possible. Our computed heats of adsorption (33) Zuhr, R.A.;Hudson, J. B. Surf. Sci. 1977,66,405. Nyberg, G. L.; Lambert, R. M. J. Phys. Chem. 1984, (34) Tysoe, W.T.; 88,1960.
196 Langmuir, Vol. 3, No. 2, 1987
Baetzold
have been considered. The s-cis metallacyclopentene structure (4) is consistent with our computed results for chemisorbed butadiene. The overlap populations reported16were 0.338 for terminal M-C and 0.060 for middle M-C in the ZrCpzCIHscomplex. The Zr complex features more C-C double-bond character in the middle C-C bond than in the outer C-C bond. This effect is found in our chemisorption calculations only for the sp3 rehybridized geometry and is not found for the flat sp2hybridized butadiene. It is interesting to note that the complexes undergo a fluxional behavior in which the butadiene passes through a planar metallocyclopentene transition state as shown in 5. The experimental activation barrier varies
Table V. Parameters of the Calculation Energies param set -EF,eV -ee, eV -eD, eV -ed, eV Nd I 9.00 3.00 2.00 12.00 8.9 9.00 3.00 2.00 11.00 7.9 9.00 3.00 2.00 9.00 5.0 I1
8.53 7.80 8.08 7.60 8.00 3.00 9.00 3.00 9.50 3.00 orbital exponents
I11
I, 111 11, Ni 11, Fe
2.25 2.1 1.9
ap
adi
2.25 2.1 1.9
5.98 5.75 5.35
3.70 3.80
9.90 9.20
8.00 5.1 11.00 7.9 12.50 8.8 coefficients
2.00 2.00 2.00
2.61 2.00 1.80
9.5 7.5
cdi
cdz
0.5296 0.5683 0.5366
0.6372 0.6292 0.6678
for these species are so large that, even with the inaccuracies of this method, it is not likely that they could be very reactive. Thus, for purposes of high reactivity, the intact butadiene (or possibly physically adsorbed layers on top of the metallacycle layer) would be a better candidate. Molecular complexes of transition-metal atoms with butadiene ligands offer some means of comparison with the chemisorbed molecule. A review of several of these complexes, including computations of bonding properties, has recently appeared.16 An interesting geometry (s-cis) reported for s-cis ligands involves T bonds from inner C atoms and u bonds from outer C atoms to the Zr, Hf, or Ta metal center as shown in 4. Other bonding schemes I
I
M
4
5
from 6.5 to 12.4 kcal/mol.16 This transition-state structure bears some resemblance to sp3 rehybridization cis geometry, considered a chemisorption state. Despite the fact that in the gas phase the sp3rehybridized butadiene is 4.06 eV less stable than the planar ground state, we compute that upon complexation to a metal atom, the sp3 is only a few tenths of an electronvolt less stable than the planar geometry. Our calculations seem to agree qualitatively with experiment that the transition state 5 is close in energy to the s-cis ligand. The qualitative comparison of phenomena in cluster complexes and chemisorption to surfaces was probed further by calculations of binding energy and overlap populations vs. the number of metal d electrons. The s-cis geometry of the Zr complex and a hypothetical trans geometry of butadiene were considered for this complex. Figure 10 shows an increase in ligand binding energy as Nd decreases. The overlap populations show a crossover in double-bond character with N,+ There is more terminal double-bond character on the late transition metal compared to middle C-C double-bond character. This is a consequence of greater electron transfer to butadiene on the early-transition-metalatom. Of course, this conclusion
Table VI. Properties of Chemisorbed Butadiene Computed with Parameter Set I1 heat of overlap population adsorption Nd
9.5 9.5 9.5 9.5 7.5 7.5 7.5 7.5
geometry trans trans cis cis rehybridized trans trans cis cis rehybridized
site bridge hollow see Figure 1 bridge bridge hollow see Figure 1 bridge
orientation flat flat flat vertical flat flat flat vertical
Q,ev -1.21 -1.02 -1.08 -0.65 -1.13 -0.94 -1.00 -0.73
charge 4
-1.24 -1.06 -1.24 -1.37 -1.46 -1.54 -1.66 -2.09
middle M-C 0.08 0.06 0.11 -0.03 +0.04 +0.06 +0.07 -0.07
term M-C 0.10 0.23 0.09 0.38 +0.13 +0.14 +0.10 +0.36
middle
term
0.97 1.00 1.00 1.20 0.98 1.00 1.03
1.13 1.12 1.10 0.95 1.06 1.05 1.02 0.98
c-c
1.11
c-c
Table VII. Properties of Chemisorbed Butadiene Comuuted with Parameter Set 111
Nd
8.8 8.8 8.8 7.9 7.9 7.9 5.1 5.1 5.1
geometry trans cis cis rehybridized trans cis cis trans cis cis rehybridized
site bridge see Figure 1 bridge bridge see Figure 1 bridge bridge see Figure 1 bridge
orientation flat flat vertical flat flat vertical flat flat vertical
heat of adsorption Q,eV -0.82 -0.86 -2.45 0.50 0.66 0.30 2.87 3.22 1.98
charge 4
-1.24 -1.49 -1.62 -1.22 -1.46 -1.53 -1.74 -1.85 -1.87
middle M-C -0.10 -0.10 -0.03 -0.07 -0.06 -0.02 -0.07 -0.05 -0.03
overlap population term middle M-C c-c -0.07 0.99 -0.07 1.03 -0.15 1.23 -0.03 0.98 -0.05 1.04 0.21 1.26 0.02 1.05 0.00 1.09 0.19 1.15
term
c-c
1.31 1.28 0.97 1.22 1.16 0.92 1.08
1.04 0.97
Langmuir, Vol. 3, No. 2, 1987 197
Properties of Chemisorbed Ethylene and Butadiene
Terminal
c
3
5
7
9
Nd
1fl -
Terminal
6
8
D
1
-Hii
1
2
(eV)
m e oxidized M Figure 10. Properties of a butadiene ligand adsorbed to a metal Cp, complex in a s-cis and s-trans geometry as a function of the number of d electrons of metal Hi; value or oxidation state. Heat
of adsorption and C-C terminal and middle-bond overlap populations are considered.
assumes similar metal oxidation states, which may be difficult to achieve experimentally for the complexes. As the oxidation state of the metal increases, the terminal C-C bond strengthens, while the middle C-C bond weakens.
Acknowledgment. I am grateful to Ron Wexler for many interesting discussions concerning this work. Appendix 1 The metal-ionization parameters (parameter set I) used in this work are shown in Table V. As indicated earlier, we keep the Fermi level constant in these calculations. Other parameter sets have been explored as noted in Table V. The parameters of Saillard and Hoffmann21(parameter set 11) and an earlier set (111) that we employed20were examined. Tables VI and VI1 contain the results of calculations with these parameters. In the case of parameter set 11, the butadiene is not calculated to have a positive adsorption energy even though the M-C overlap populations indicate good bond formation. Despite this, bonding is'stronger to Fe vs. Ni, complying with the trend found in parameter set I. We also see that butadiene is a strong electron acceptor, and that chemisorption increases the center C-C overlap population, while decreasing the terminal C-C overlap population. We should point out that in this calculation with parameter set 11, the averaging
off-diagonal formula of Hoffmann used to suppress "counter-intuitive" orbital mixing effects was employed. Parameter set I11 is applied to test specifically the effect of Fermi-level changes across a transition series. The change in Fermi level with position of the center of the d band has been modeled similarly in the past.M The varying Fermi level leads to a larger increase in heat of adsorption as the d occupation decreases. This effect is due to electron transfer from the metal at the Fermi level to the butadiene molecule. The trend with d occupation is the same as the trend we calculate for constant Fermi level. Also the increase in C-C overlap population for the middle C-C bond during chemisorption occurs at the expense of the outer C-C bond overlap population. The chemisorbed molecule is a strong electron acceptor, which accounts for these effects.
Appendix I1 The weight of given ligand molecular orbital M in a particular metal-molecule Bloch function i is computed from Wi(W = (aMii,R)2 + ((YMiMi")'
+
metal ligand
i
s
bsMSsj(CjiRaMi'R + C"aMi") 11
In this equation, which results from the projection of the ligand molecular orbital from the normalized integral of the wave function, Ssj is an orbital overlap integral, C..R is the real part of the Cji eigenvector element, and Cj$s the corresponding imaginary part coefficient. The ligand molecular orbital $'M
= CblMXl 1
is expanded in atomic orbitals with blM as the known eigenvector elements. The real and imaginary part coefficients aM1are defined from ligand
aMi,I
=
C blMCCj/Slj 1
I
where (@M~#iBloch) = LyM4R
+ icyMiJ
Registry No. C2H4,74-85-1; butadiene, 106-99-0.
