J . Phys. Chem. 1989, 93,6145-6149 error sum
0.10
0.05
1 I
0'
P
d I
5
Figure 2. The error sum as a function of
T,, nr T ~ :
lo
(*) 1; (0)2; (0)3.
eter), 200 (Nicolet NT-200 spectrometer), and 300 MHz (Varian VXR-300 spectrometer). Typical parameters in the experiments were 10 pulse interval times from 0.005 to 15 s. The delay time between the pulse sequences was at least 5 times the longest T I . Good experimental fits were obtained. Using a series of different samples, the TI values were found to be accurate to within 4% at 200 and 300 MHz, and to within 6% at 90 MHz. All experiments were performed at 35 1 "C. The 2D NOE spectra
*
6145
were recorded on a Varian VXR-500 (500 MHz) instrument at 30 f 1 OC. A normal NOESY pulse sequence was employed. The mixing time was 0.1 s. Additional experimental details have been reported previously.' Computational Procedures. Equation 2 contains three unknown parameters (7s,rf, S),but T , is equal for all protons in the surfactant. When 7$ is chosen, the experimental T I values can be fitted to eq 2. Variation of 7Sproduces a shallow minimum in the error sum (Figure 2). The error sum is definedz0as the sum over all squares of the differences between the observed and calculated Tl-''s,divided by the square of the observed value. The 7, values obtained in this way are of moderate accuracy ( f l ns). The errors in 7f and S were estimated from a data set of 100 Ti's. These T,'s were calculated from the experimental Tl's and the experimental errors in TI using an inverse Gaussian function and a confidence interval of 80%.
Acknowledgment. The investigations were supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Foundation for Scientific Research (NWO). We thank Dr. R. M. Scheek and K. Dijkstra for expert assistance with the Varian VXR-500 spectrometer and with the data analysis of the NOESY experiments. Stimulating discussions with these scientists and with Professor H. J. C. Berendsen are also gratefully acknowledged.
Ethylene Hydrogenation Mechanism on the Pt( 111) Surface. Theoretical Determination Alfred B. Anderson* and S. J. Choet Chemistry Department, Case Western Reserve University, Cleveland, Ohio 441 06 (Received: December 12, 1988; In Final Form: March 28, 1989)
A (2 X 2) ethylidyne overlayer has been observed before and after ethylene hydrogenation to ethane over R(111) by Somorjai and co-workers. The role of ethylidyne in this catalytic process has been a topic of uncertainty. In this ASED-MO study it is shown that steric interactions prevent ethylene from approaching the ethylidynecovered surface but ethylidyne can readily shift laterally to new sites and open up space for ethylene to approach and be hydrogenated by the surface platinum hydride. Hydrogenation by the a-H of surface ethylidene, which has been postulated to be a hydrogenation intermediate, is found to proceed with a higher activation energy, forming nonadsorbed ethyl radicals. The higher activation energy and the formation of ethyl radicals are inconsistent with experimental results. Furthermore, ethylidene is predicted to have insufficient stability to be present in substantial concentrations on the surface. It is shown that ethylidene can readily exchange 8-H with the surface, but 8-H exchange is very slow in the experimental studies. From these results it is concluded that ethylidene plays no substantial role in ethylene hydrogenation.
Introduction Ethylene is readily hydrogenated to ethane over many transition-metal surfaces at near room temperature.' Vibrational analysis by high-resolution electron energy loss spectroscopy, HREELS, and structural analysis by low-energy electron diffraction, LEED, before and after the catalytic process indicate that during hydrogenation at 300 K the (1 11) Pt and Rh surfaces are probably covered by a partially ordered overlayer of ethylidyne, CCH3.2*3 When ethylene is adsorbed on these surfaces at temperatures of 200 K or below, it binds parallel to the surfaces with distortions indicating di-a-bonding to them, according to HREELS analysis$*s with carbon bond stretches from the gas-phase value (1.34 A) of 0.05 A for Rh( 1 11) and 0.17 A for Pt( 11 1) according to ref 5 . In the case of the Pt surface, a near-edge X-ray absorption fine structure (NEXAFS) study yielded a 0.16 0.03 A stretch.6 On heating to above 250 K the rearrangement to ethylidyne (CCH3) monolayer overtakes place, with the formation of (2 X 2)
*
* Address correspondence to this author. +Permanentaddress: Department of Chemistry, Inje College, Pusan, Korea 601.
0022-3654/89/2093-6145$01.50/0
layers. LEED analysis showed a C-C distance of 1.50 A on Pt(l1 1),v' 8 which is 0.16 8,greater than the distance in gas-phase ethylene and 0.04 8, less than the C-C single-bond distance in ethane. The same rearrangement occurs on Rh( 111) and the C 4 distance in the (2 X 2) ethylidyne overlayers was determined to be 1.45 k 8 s 9 When the surfaces are at 300 K and exposed to ethylene, hydrogenation immediately commences and after re(1) Horiuti, J.; Miyahara, K. "Hydrogenation of Ethylene on Metallic Catalysts"; Natl. Stand. Ref. Data Ser. 1969, NSROS-NBS-13. (2) Somorjai, G. A,; Van Hove, M. A.; Bent, B. E. J . Phys. Chem. 1988,
92, 973. (3) Zaera, F.; Somorjai, G. A. J . Am. Chem. SOC.1984, 106, 2288. (4) (a) Ibach, H.; Lehwald, S. J. Vacuum Sci. Technol. 1978,15,407. (b) Ibach, H.; Hopster, H.; Sexton, B. Appl. Surf.Sci. 1977, 1, 1 . Steininger, H.; Ibach, H.; Lehwald, S. Sur$ Sci. 1982,117,685. Felter, T. E.; Weinberg, W. H. Surf. Sci. 1981, 103, 265. ( 5 ) Bent, B. E. Ph.D. Thesis, University of California, Berkeley, 1986. (6) Stohr, J.; Sette, F.; Johnson, A. L. Phys. Reu. Lett. 1984, 53, 1684. (7) Kesmcdel, L. L.; Dubois, L. H.; Somorjai, G. A. Chem. Phys. Lett. 1987, 56, 261; J . Chem. Phys. 1979, 70, 2180. (8) Koestner, R. J.; Van Hove, M. A,; Somorjai, G. A. J . Phys. Chem. 1983, 87, 203. (9) Koestner, R. J.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1982, 121, 321.
0 1989 American Chemical Society
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action LEED shows the (2 X 2) ethylidyne pattern, though it is poorly ~ r d e r e d . ~ . ~ While the structures of adsorbed ethylene and ethylidyne are well characterized, the mechanism of ethylene hydrogenation is not certain. Ethylidyne at (2 X 2) quarter monolayer coverage is believed to block ethylene from reaching the surface and accepting hydrogen atoms from the metal, which is known to adsorb hydrogen under this condition.f3 There is a pressure dependence and an isotopic effect when D2 is used, and these imply hydrogen transfer is the rate-limiting step. The measured activation energy was 10.8 kcal/mol on Pt( 11 l)., The ethylidyne residence time, based on 14C labeling, was found to be lo6 longer than the rate of ethylene hydrogenation and the rate of H, D exchange in the methyl group was found to be s l o ~ e r . ~These . ~ findings exclude the possibility that ethylidyne hydrogenation to ethane is a major pathway or that ethylene gains its hydrogen atoms from the methyl groups of ethylidyne during hydrogenation. Therefore it was proposed that H could jump from the metal to the a-C position of CCH3, which is bonded to the surface, to form an ethylidene intermediate which would hydrogenate ethylene by transferring this H to it.3 More recently the possibility that ethylidyne species might move aside so that there would be room for ethylene to approach the metal surface for hydrogenation has been suggested.2 Available experimental data are consistent with both proposed mechanisms. The purpose of this paper is to present the results of a theoretical investigation into the question of the mechanism of ethylene hydrogenation over Pt(ll1). The atom superposition and electron delocalization molecular orbital (ASED-MO) theory and surface cluster models are used. The findings of this study should apply to Rh( 11 l), which shows similar behavior, and, if the suggestion of Zaern and Somorjai is correct, to other surfaces of these metals and to ethylene hydrogenation over other metal^.^
-
Anderson and Choe
-
Theoretical Method The ASED-MO t h e ~ r y ' ~was , ' ~ used in previous studies of acetylenelo and ethyleneI0J1adsorption on Pt( 11 1) and various rearrangements to ethynyl (CCH), vinylidene (CCH2), vinyl (CHCH2), ethylidyne, and ethylidene involving H transfer to and from the surface. The ASED-MO method is based in partitioning molecular electronic density functions into atomic and delocalization components and uses electrostatics to calculate the chemical binding energy by integrating the force on the nucleus of the less electronegative atom in each atom pair. The atom superposition two-body repulsion energy components are calculated directly from atomic charge density functions. The electron delocalization attraction energy component is approximated by a molecular orbital binding energy which is calculated by using a Hamiltonian dependent on valence orbital ionization potentials and Slater orbital exponents and similar in form to extended Huckel. The parameters used here were used in the previous studies of C2H, and in analysis of propylene rearrangement^'^ and benzene adsorptionls on Pt( 111) and of acetylene adsorption on other Pt surfaces.I6 Structure predictions were in close agreement with experiment. Acetylene was predicted to absorb in the triangular site with a C C bond stretch of 0.22 A from the gas-phase value, CCH angles of 125', and a tilt of the molecular plane of 30° from normal," all essentially in agreement with HREELS4*and NEXAFS6 analyses. For ethylidyne, the C-C stretch 0.30 A from the gas-phase acetylene value and the predicted %fold site adsorption with Pt-C distances of 2.0 A also agreed with e~periment.'.~ (10) Anderson, A. B.; Hubbard, A. J. Surf. Sci. 1980, 99, 384. ( 1 1 ) Kang, D. B.; Anderson, A. B. Surf. Sci. 1985, 155, 639. ( 1 2) Anderson, A. B. J . Chem. Phys. 1975, 62, 1187. ( 1 3 ) Anderson, A. B.; Grimes, R. W.; Hong, S. Y . J . Phys. Chem. 1987, 91. 4245. (14) Anderson, A. B.; Kang, D. B.; Kim, Y . J . Am. Chem. SOC.1984,106, 6597. (15) Anderson, A. B.; McDevitt, M. R.; Urbach, F. L. Surf.Sci. 1984, 146, 80. (16) Mehandru, S. P.; Anderson, A. B. Appl. Surf. Sci. 1984, 19, 116. Mehandru, S . P.; Anderson, A. B. J . Am. Chem. SOC.1985, 107, 844.
YY
C Figure 1. (a) Ethylidyne positions in (2 X 2) overlayer on Pt( 11 1). (b) Ethylidyne shift to open up an ethylene adsorption site. (c) Each shifted
ethylidyne moves closer to two neighbors as shown. Despite the excellent structure predictions of the past, the present study must be considered as qualitative. The modeling of several species on the surface and the need for a very large number of energy calculations to determine reaction pathways puts a constraint on the size of the surface cluster models, though past work usually shows comparable results with small and large clusters. Transition-state determinations are subject to uncertainty in principle because of the large number of coordinate variables treated, though a strong effort was made to find the lowest energy transition states. Generally in this work angles are optimized to the nearest 1O and distances to the nearest 0.01 A. Only De bond energies, uncorrected for zero-point vibrational energies, are reported. Theory parameters and structure details are given in the Appendix.
Modifications of the (2 X 2) Ethylidyne Overlayer Structure As was pointed out in ref 11, lateral interactions of adsorbed species must be considered in studying ethylene hydrogenation on the ethylidyne covered surface. A one-layer-thick Pt15cluster was used to examine ethylidyne interactions. A single CCH3 was calculated to bind to the center of this cluster with an energy of 5.87 eV (0, values are reported throughout this study) and at sites shown in Figure 1, a, b, and c, the energies were 5.71, 5.65, and 5.84 eV, respectively, illustrating the small magnitude of edge effects. Three CCH, were found to bind in the c(2 X 2) arrangement as shown in Figure 1 with an average energy of 5.43 eV. When these CCH, were moved apart as in Figure l b the average binding energy per CCH, decreased by 0.06 eV. This suggests the possibility of opening up a site on the (2 X 2) ethylidyne-covered surface for ethylene adsorption provided interactions with other neighboring CCH, allow it. These interactions were modeled as in Figure IC. The binding energy per CCH, increased 0.07 eV compared to the (2 X 2) model of Figure la. The calculated activation energy for a CCH3 to pass over the 2-fold bridging site to reach this structure was 0.43 eV, indicating the rearrangement should be possible since this is the same order of magnitude as the experimental activation energy for ethylene hydrogenation, 0.47 eV.3 Because of the limitations of the cluster model and the approximations of the theory, the slight stability gain calculated for the CCH3 rearrangement does not imply the extended surface overlayer should reorder. The fact the energy did not rise substantially is what is significant, for it suggests the possibility of local fluctuations in the (2 X 2) CCH, overlayer. Ethylene Hydrogenation by the Surface Metal Hydride Ethylene was found to be repelled by the (2 X 2) CCH3-covered surface. When brought down over the central triangular site of the cluster in Figure la, closed-shell interactions with the methyl groups prevented it from chemisorbing. However, on the open site of Figure 1b ethylene chemisorbed weakly, 0.93 eY, distorting
Ethylene Hydrogenation Mechanism on Pt( 11 1)
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6147
-81
Q--,
w
bW cFoq3Q&$q2 a
C
789 TET d
Figure 2. (a) Ethylene di-a-bonded on the open ethylidyne-covered surface. (b) Adsorbed ethyl radical. (c) Side view of adsorbed ethyl radical. (d) Ethyl radical dehydrogenation/ethylene hydrogenation transition state.
to the structure shown in Figure 2a, which is the same as predicted ear1ier.I' For this CCH3 arrangement, ethylene can approach the surface to pick up H from Pt. In fact, only two ethylidynes need to shift to open up the di-u adsorption site shown in Figure 2a. Some simplifications were introduced in modeling the first ethylene hydrogenation. Ethyl, CH2CH3, was placed in 1-fold coordination (Figure 2, b and c) and a hydrogen atom abstracted by a surface Pt atom as shown in Figure 2d. This structure choice has support from earlier methane CH activation studies on Pt( 11 1) and other metal surfaces where surface metal atom insertion into the C H bond was a low-energy pathway." In that case the Pt-C distance in the transition state was 2.3 8, and here it is 3.2 A, in part a result of the constraint of the Pt-a-C single bond. The calculated activation energy with respect to dehydrogenation is 0.91 eV, about twice the methane dehydrogenation barrier. The C H bond stretch of 0.68 8, compared to 0.36 8, for methane is symptomatic of the higher barrier for abstraction of the @ hydrogen from the adsorbed ethyl group. The electronic aspects of the activation may be understood by using the energy level correlation diagram of Figure 3. Energy levels for the adsorbed ethyl radical are in the first column and those for the transition state are in the third column. The second column of levels was calculated for a free ethyl radical in the transition-state structure. Compared with the equilibrium structure ethyl radical, the energy in the free transition-state structure molecule increases by 2.25 eV, largely the result of the destabilization of the C H orbital that is caused by increasing the C H distance by 0.68 8,. On the surface this orbital is stabilized (third column) by mixing with the Pt s-d band orbitals. The antibonding counterpart is doubly occupied, giving the overall interaction the form of a closed-shell repulsion. However, the C H u* orbital is greatly stabilized by the stretch (second column) and is therefore able to mix with the antibonding interaction and stabilize the antibonding counterpart. The net result is a strong C H u donation activation. Unlike the case of methane activation studied earlierI8 there is no Pt-C bonding in this transition state. Instead, binding through the other C to the surface restricts the C--H-Pt transition-state structure to more nearly linear and prevents C from bonding to the surface, which is the reason for the higher calculated activation energy. Also shown in Figure 3 is the ethyl radical u bonding to the surface in the transition state and in the equilibrium structure. The same transition-state structure applies to the ethylene hydrogenation reaction, and with respect to adsorbed ethylene and H adsorbed in a 3-fold site the activation energy is 0.94 eV. Experimental studies indicate H adsorbs on 3-fold sites of Pt( 111). (17) Christmann, K.; Ertl, G.; Pignet, T. Surf. Sci. 1976, 54, 365. (18) Anderson, A. B.; Maloney, J . J . J . Phys. Chem. 1988, 92, 809. (19) CRC Handbook of Chemistry and Physics; Weast, R. C., Ed.; Chemcial Rubber Co.: Boca Raton, FL, 1986.
-18-
-ISl
- 20
u
t
t
Figure 3. Orbital correlation diagram for ethyl radical dehydrogenation/ethylene hydrogenation transition state. The C2H5* column of levels has the surface removed.
As this H moves into position to hydrogenate adsorbed ethylene the energy should increase as the ethylene binding to the surface is perturbed, until the transition state is reached. A hydrogen atom forms a bond to Pt( 111) with a De value of -2.6 eV,17 and the calculations overestimate this, yielding a value of 4.48 eV to the central 3-fold site of PtlSand 3.91 eV to the 3-fold site with an ethylene and three ethylidyne species present as shhoown in Figure 2a. This overestimate of the H binding energy to the surface suggests the possibility that the activation energy might be lower, perhaps approaching the experimental value of 0.47 eV, but there are enough uncertainties in the model that the proper conclusion is simply that the calculations support this mechanism as a possibility for ethylene hydrogenation. The second hydrogenation of adsorbed ethyl to yield desorbed ethane has not been examined but the previous study17of methane hydrogenation is relevant. There the activation energy for dehydrogenation was small, 0.45 eV, and the activation energy for ethane dehydrogenation will be close to this value, which means the first H-transfer step will be rate-limiting and the second H transfer will proceed rapidly. The ease by which ethylene hydrogenation is predicted to occur over bare regions of the Pt surface should not lead one to conclude that other mechanisms are unimportant. In the next section hydrogenation via a - H transfer from ethylidene intermediates is examined.
Ethylene Hydrogenation by Surface Ethylidene It is first necessary to explore the stability of adsorbed ethylidene compared to adsorbed ethylidyne and H. In the earlier study," employing a two-layer-thick Pt,8model and no coadsorbed species, the hydrogenation of ethylidyne by adsorbed H was uphill by 1.2 eV. It was suggested in ref 11 that high coverage might weaken the Hsurface bonds to aid ethylidene formation. With the present one-layer models we find that at the low-coverage limit the hydrogenation of ethylidyne by H adsorbed on 3-fold sites is uphill by 1.50 eV. The H adsorption energy on the (2 X 2) ethylidyne-covered surface, at a central 3-fold site, is calculated to be only 0.18 eV less than on the clean surface. Ethylidyne hydrogenation under this condition is uphill by 1.32 eV. Based on these approximate numbers it can be concluded that at ~ ( 2 x 2 ethy) lidyne coverage an equilibrium of the form H(a)
+ CCH,(a)
CHCH3(a)
(1)
6148
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989
Figure 4. (a) Ethylidene on a 2-fold site. (b) Transition state for a-H transfer from ethylidene to ethylene. (c) Side view of transition state for a-H transfer to ethylene. The methylene plane makes an angle of 39" to the surface plane.
allows the possibility of only a very small concentration of CHCH3 on the Pt( 11 1) surface. The first ethylene hydrogenation by transfer of an a - H from cluster with a single adsorbed HCCH3 was modeled by using a PtI3 ethylidene molecule bound to it in a 2-fold bridging site. Ethylidene binds most stably here and it should have no steric difficulty occupying this site rather than the 3-fold one favored by ethylidyne because of the small distance it has shifted from the 3-fold site. This structure is shown in Figure 4a. If neighboring ethylidene shifts in the same way, ethylene can approach the surface in Figure 4a. Initially it was thought that the ethylene molecule might bind to the surface through one C while the other C receives the a - H from ethylidene, so a pathway with the ethylene C2 and the ethylidene Cz in the same plane was tried. It became evident that this structure was too unstable to pursue because of ethylene H interactions with the surface. It was then decided to try an ethylene orientation with ethylene in the gas phase rather than chemisorbed through a methylene end. A transition state was found with the structure shown in Figure 4b,c. In this transition state the H-a-C bond is stretched 0.41 A. The a-C-ethylene C distance is 2.95 A and the ethylene C to H distance is 0.20 8, longer than the calculated methyl group C H distance. The bond angles around the ethylene C accepting the H are within a few degrees of tetrahedral. The ethylene C-C distance is increased 0.12 A, which is over halfway to the single bond value. The CCH, framework structure is unchanged but the C-C axis is tilted 10' toward the surface normal from the ethylidene value. The CCH, fragment height is decreased from 1.6 to 1.4 8, and has shifted laterally 0.07 A toward the 3-fold site favored by the ethylidyne product. Using this transition-state structure without the Pt surface present, the energy increase is 1.73 eV but the activation energy on the surface is 1.49 eV. The surface itself does not provide much activation for H transfer but it may allow ethylidene to form. The calculations indicate that in the gas phase the products, ethylidyne and the ethyl radical, are 2.62 eV less stable than the reactants in the gas phase, which means the greater adsorption energy of ethylidyne over ethylidene, 5.48 vs 3.01 eV on this cluster, is the driving force for this process on the surface. The product of this reaction is a gas-phase ethyl radical and the overall reaction is somewhat destabilizing, uphill by 0.22 eV. The ethyl adsorption energy to a I-fold site neighboring the atoms making up the 3-fold site where the ethylidyne is adsorbed is 2.21 eV, so the overall reaction energy to yield this product is -1.99 eV. As discussed in the previous section, the hydrogenation of the adsorbed ethyl radical by adsorbed H will proceed easily with an activation energy of about 0.5 eV. However, for the ethyl radical to adsorb, two neighboring ethylidyne species will either have to convert to ethylidene and shift to 2-fold sites or they have to move in order to open up sufficient space on the surface. As discussed above, this seems to be feasible and will be aided by the stability of the ethyl radical chemisorption bond. Another pathway to consider for the hydrogenation of the ethyl radical is H transfer from a second adsorbed ethylidene. This would not require the more massive reordering of the surface ethylidene overlayer that was proposed in the previous section.
Anderson and Choe
Figure 5. Effect of a second ethylidene on the transition-state structure for a-H transfer shown in Figure 4,a and b. The CH, group on ethylene has been allowed to rotate to an equilibrium position.
The calculations indicate that in the transition state for the first hydrogenation the structure is favorable for the second hydrogenation to commence. The methylene end of the forming ethyl radical is forced by the second ethylidene to rotate as shown in Figure 5. This costs 0.34 eV in the absence of the ethylidene but when it is present a weak bond with a Mulliken bond order of 0.02 forms between the a - H and the ethylene C. As a result, the activation energy for the first H transfer increases by a small amount, 0.03 eV, to 1.5 1 eV even though the alignment is not in the nearly linear form as for the first H transfer. If the energy released following the transition state for the first hydrogenation, 1.29 eV, could be coupled to activate the second hydrogenation, then the second step might be activated to proceed more easily than the first. The chances for this seem small because a substantial fraction of this energy will be carried away by the first ethylidyne as it forms and slips into the 3-fold site. Consequently, hydrogenation of the ethyl radical by a-H transfer from ethylidene is expected to have an activation energy comparable to the first a-H transfer.
Discussion It seems most likely that ethylene hydrogenation generally occurs by the first mechanism studied here, surface metal hydride H transfer to adsorbed ethylene to form adsorbed ethyl radicals which are subsequently hydrogenated to ethane: C2H4(a) + 2H(a)
-
C2H5(a) + H(a)
-
C2H6(g)
(2)
The first step is rate limiting and, although an activation energy about double the experimental value of 0.47 eV is calculated, the process is clearly highly activated and the inaccuracy can be attributed to the approximations of the theory. In this mechanism the adsorbed ethylidynes, which are predicted to shift easily to allow space for the reaction, are merely spectators. Whether defects in the (2 X 2) ethylidyne overlayer appear spontaneously during ethylene approach to the surface or whether they are already present is not clear. The fact that the (2 X 2) ethylidyne-covered surface disorders partially during hydrogenation, as noted in ref 2 and 3, suggests there is some dynamic shifting during hydrogenation. It might be asked if this loss of order might be due to the formation of additional ethylidyne. The calculations indicate that ethylidyne absorbs fairly strongly, with a binding energy of 4.04 eV, to a 3-fold site centered between three ethylidynes of the (2 X 2) structure. This is 1.83 eV less than for a single ethylidyne at low coverage. For the gas-phase reaction CzH4(g)
-+
H(g) + CCHS(g)
(3)
the calculated reaction energy is 8.18 eV. This is close to the estimate of 8.3 eV (which includes zero-point vibrational energies) based on the sum of the H-vinyl bond strength of 4.7 eV and the difference in carbon double and single bond strengths, 3.6 eV.I8 At low coverage the binding energies of CCH, and H to 3-fold sites are calculated to be 5.87 and 4.48 eV, respectively. Therefore
The Journal of Physical Chemistry, Vol. 93, No. 16, 1989 6149
Ethylene Hydrogenation Mechanism on P t ( l l 1 )
TABLE I: Atomic Parameters Used in the Calculations: Principal Quantum Numbers, n , Ionization Potentials, IP (ev), Slater Orbital Exponents, (nu), and Linear Coefficients, c S P d
atom
n
IP
z
n
IP
z
Pt C
6 2 1
10.0 19.0 12.6
2.55 1.6583 1.2
6 2
5.96 10.26
2.25 1.6180
H
n
IP
CI
z1
c2
(2
5
10.6
0.6562
6.013
0.5711
2.396
the energy of adsorbed ethylidyne formation according to the reaction (4) is calculated to be -2.17 eV, meaning the reaction will proceed. Correcting for the theoretically overestimated H-surface bond strength would reduce this stability, but it is not known if the calculated ethylidyne adsorption energy is similarly overestimated. Calculated binding energies of CCH3 and H at 3-fold center sites on the (2 X 2) ethylidyne-covered surface are 4.04 and 3.05 eV, respectively. The energy of dissociative chemisorption (eq 4) in this case is 1.09 eV uphill, so the reaction will not go. If ethylidyne were to adsorb in the 3-fold site at the center of three ethylidynes shifted as in Figure lb, the reaction might go. In this case the CCH3adsorption energy is calculated to be 5.47 eV, so the reaction energy to form this and H adsorbed at a 3-fold site from ethylene is -0.34 eV. The long observed ethylidyne residence time suggests this reaction will be infrequent. High coverage of H will favor ethylene hydrogenation over adsorption and rearrangement at these sites. Some comments on the mechanisms of ethylidyne formation from adsorbed ethylene are in order. According to the earlier study in ref 11 the most likely mechanism for forming ethylidyne is first vinylidene formation with an activation energy of 0.9 eV:
-
C2H4(a)
CHCH2(a) + H(a)
(5)
The reaction is downhill. The activation energy for methyl group dehydrogenation for adsorbed ethylidene was 0.7 eV and at the high coverage modeled here this will also be approximately the activation energy for ethylidene formation according to CHCH2(a) + H(a)
-
CHCH,(a)
This reaction will be approximately energy neutral. The activation energy for transferring a-H to the surface to yield ethylidyne according to eq 1 was 0.8 eV. As discussed above, this reaction is stabilizing so eq 5 , 6 , and 1 are the probable route to ethylidyne. The activation energy for ethylidene methyl group dehydrogenation to yield vinyl was calculated to be 0.7 eV in ref 11, which is relatively small. The activation energy for ethylidyne dehydrogenation to vinylidene was large at 1.3 eV. This large barrier is consistent with the very slow observed rate of H, D exchange in the ethylidyne methyl g r o ~ p . ~If. ethylidene ~ were to form in equilibrium with adsorbed ethylidyne and H according to eq 1, then methyl H, D exchange would be expected because of the low activation energy. Since this exchange was very slow, it is unlikely that much ethylidene forms on the surface. This is a strong argument against importance of the ethylidene a - H transfer mechanism for ethylene hydrogenation. There are additional arguments against the ethylidene a - H transfer hydrogenation mechanism. Not only is the calculated activation energy for ethyl radical formation by this mechanism relatively large, but since steric constraints require the ethylene not be bonded to the metal surface in the transition state, there is a likelihood that many of the ethyl radicals would desorb. Such radicals would be expected to dehydrogenate the ethylidyne methyl groups and enter into polymerization and other reactions. No such reactions were observed in ref 2 and 3. The structure insensitivity of the ethylene hydrogenation reaction over many metals2 also implies such a specific mechanism as the sequential
CH2CH(a)
CH2CH . C&i$
Figure 6. Structure details; distances in angstroms and angles in degrees.
transfer of a-H from the two adjacent ethylidene as in Figure 5 can be ruled out. Finally, the in situ infrared spectroscopic study of Beebe and Yates20 showed that on alumina-supported Pd catalysts whether or not ethylidyne forms depends on whether there is an excess of ethylene relative to H2 and that the initial rates of ethylene hydrogenation are the same in the presence and absence of preadsorbed ethylidyne. Taking into account the dependence of calculated adsorption energies on coverage, the observations in ref 19 are understandable. Conclusions
It is concluded that ethylene hydrogenation takes place over H-covered metal surface regions and that ethylidyne is an inactive spectator. It seems probable that these conclusions will apply generally to the catalytic hydrogenation of ethylene over other transition-metal surfaces.
Acknowledgment. S.J.C. is grateful for academic research support from the Educational Ministry of the Republic of Korea. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the America1 Chemical Society, for support which aided in the preparation of the manuscript. Appendix
Parameters used in the calculations are in Table I. As in past studies of Pt surfaces, the clusters were assigned high-spin orbital occupations so that each d-type cluster orbital contained at least one electron. For the PtI3 and PtI5 clusters 12 electrons were therefore unpaired and when adsorption resulted in an odd total number of electrons 11 electrons were unpaired. Structure details are given in Figure 6. The gas-phase ethylidyne CC bond is 0.07 A longer and the C C H angles are 1' larger. The gas-phase ethylidene CC bond is 0.03 A longer, the H-a-C-C and a-C-C-H angles are 1O smaller and the H-a-C bond is 0.02 A longer. The gas-phase ethyl radical structure is the same. ASED-MO calculations with this parameter set produce a molecular C2 bond length close to experiment but systematically overestimate hydrocarbon C=C, C=C, and C-H bond lengths by 0.1,0.15,0.2, and 0.1 A, respectively. Registry No. Pt, 7440-06-4; ethylene, 74-85-1. (20) Beebe, Jr., T. P.; Yates, J. T. J . Am. Chem. SOC.1986, 108, 663.
