Theoretical Studies of Ethyl to Ethylene Conversion on Nickel and

Chapter 21

Theoretical Studies of Ethyl to Ethylene Conversion on Nickel and Platinum J. L. Whitten and H . Yang

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Department of Chemistry, North Carolina State University, Box 8201, Raleigh, NC 27695-8201

Ethylene productionfromethyl adsorbed on nickel and platinum surfaces is investigated usingfirst-principlestheory. Calculations are based on an embedded cluster formalism that permits an accurate determination of energies and adsorbate structure. The key issue is the nature of the activation barrier associated with the transfer of a beta hydrogen to the metal surface. The minimum energy pathway is found to require an initial repulsive interaction with the surface in which the adsorbed ethylfragmentis tilted until there is a slight penetration of the electron density of the metal surface by the ethyl beta hydrogen. The H atom can men be transferredfromthis more energetic configuration with an overall activation energy of 17 kcal/mol for Ni(100) and 14 kcal/mol for Pt(100). Differences between nickel and platinum are discussed.

1. Introduction The goal of this research is the development and application of theoretical techniques that will provide a molecular level understanding of surface processes, especially energetics, adsorbate structure and reaction mechanisms. The work relates to two broad subject areas: catalytic processes on metals and properties of electronic materials. First-principles theoretical methods are employed to obtain an accurate description of these systems (7,2). The difficulty in treating surface reactions usingfirst-principlestheory is that there are conflicting demands on the theory. At the surface, the treatment must be accurate enough to describe surface-adsorbate bonds and energy changes accompanying chemical reactions (generally, this is most readily achieved if systems are small); while, for a metal, a large number of atoms is required to describe conduction and charge transfer processes (in this case methods for treating large symmetric systems are most appropriate). The objective of the embedding theory employed in this work is to balance the accuracy and size aspects of the problem (2).

274

© 1999 American Chemical Society

In Transition State Modeling for Catalysis; Truhlar, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.


275 The present paper deals with catalytic reactions leading to ethylene formation on nickel and platinum. The ethylene study which involves consideration of a and P elimination pathways in the reaction of ethane with nickel is motivated by the short contact time catalytic studies of Schmidt and coworkers who observed a high degree of selectivity in several transition metal systems (3,4). In the formation of ethylene, the key issue is the nature of the activation barrier associated with the transfer of a beta hydrogen to the metal surface. In the present work, the minimum energy pathway for beta hydrogen transfer is shown to involve an initial repulsive interaction with the surface in which the adsorbed ethyl fragment is tilted until there is a slight penetration of the electron density of the metal surface by the ethyl beta hydrogen. Dissociation of CH then becomes more facile compared to H transfer from the equilibrium configuration of adsorbed ethyl. 2. Theory In order to describe reactions at a metal surface the theoretical treatment must be adequate to deal with the complexities of chemical bond formation or dissociation and accompanying effects of charge transfer, polarization and electronic correlation. Ab initio configuration interaction theory provides an attractive way to proceed only if systems are of manageable size. The purpose of the embedding approach adopted in the present work is to organize the theoretical treatment in such a manner that an accurate many-electron treatment of the adsorbate/surface portion of a system can be carried out while coupling this region to the bulk lattice (2). Localized orbitals extractedfroma treatment of the clean surface are used to define an electronic subspace encompassing the adsorbate and neighboring surface atoms. Calculations are carried out for the full electrostatic Hamiltonian of the system (except for core electron potentials), and wavefunctions are constructed by self-consistent-field (SCF) and multi-reference configuration interaction (CI) expansions,

!(f=E A det( k

k

k Xl

k

k

x ...X„ ). 2

In work to date, surfaces have been modeled by a large cluster of atoms and the system subsequently reduced to a smaller effective size by the embedding procedure. In the treatment of transition metals, the first stage deals only with the most delocalized part of the electronic system, the s,p band; d functions on surface atoms are added after a surface s,p electronic subspace is defined. SCF calculations are performed on the s band of the initial cluster and the resulting occupied orbitals are localized by a unitary transformation based on the maximization of exchange interactions with bulk atomic orbitals (2). Final electronic wavefunctions, including the adsorbate, are constructed by configuration interaction, and the coupling of the local electronic subspace and adsorbate to the bulk lattice electrons, j = 1, m}, is represented by the modified Hamiltonian, H = S ( -1/2V; + S N

5

Q

2

k

- Z /r ) + E i N

k

ik

C H (ads) + OH (ads) 2

6

2

5

or possibly with adsorbed hydroxyl C H + OH (ads) -»C H (ads) + H 0 (ads) Downloaded by STANFORD UNIV GREEN LIBR on September 30, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch021

2

6

2

5

2

Following C H5 adsorption, gaseous ethylene can be formed by elimination of a Phydrogen: 2

C H (ads) + OH (ads) ->C H (gas) + H 0 (ads) 2

5

2

4

2

or C H (ads) + O (ads) ->C H (gas) + OH(ads) 2

5

2

4

or possibly by direct interaction of H with the surface. Elimination of an a-hydrogen leads to adsorbed ethylidene (-CHCH ) and eventually to undesired products. How these various pathways compete, through their different activation energies and probabilities of reaction, is the subject of the present theoretical study. The interaction of ethyl groups with transition metals has been examined experimentally under ultrahigh vacuum conditions at low temperature (100- 400 K) by temperature-programmed desorption (TPD), x-ray photoelectron spectroscopy, highresolution electron energy loss spectroscopy, and secondary ion mass spectroscopy (69). Thefirsthydrogen is apparently abstractedfromthe ethyl by p-elimination to form C H . The C H may desorb or react on the surface to form ethylidyne (-CCH ). In TPD studies, C H was shown to desorb at temperatures of 170 K; thus, at high temperature, one would expect C H to desorb immediately after being formed on the metal surface, thereby averting ethylidyne formation. These and other experimental studies suggest that the more noble metals tend to favor P-elimination rather than aelimination and cracking to carbon, but these metals are catalytically less active than nickel. In order to investigate the hierarchy in selectivity, high quality first-principles calculations on the adsorption of ethyl (-CH CH ) on nickel and platinum surfaces are being carried out and the energetics of P-hydrogen elimination by direct interaction with the surface 3

2

4

2

4

2

3

4

2

4

2

3

C H C H (ads) ->C H (ads) + H (ads) 2

3

2

4

calculated. In this paper, we report the results of calculations on Ni(100) and Pt(100). The goal is to understand the a and P-hydrogen elimination mechanisms by calculation of the reaction energetics, activation barriers and geometric features of the pathways. Insight into the reaction mechanism can be obtained by concentrating on two key questions: a) the energetics of adsorbed ethyl as it tilts toward the surface and b) the energetics and geometry of ethylene coadsorbed with hydrogen. We begin with a


279 consideration of ethylene on the surface. When ethylene adsorbs on Ni(100), several adsorption sites are plausible. Figure 1 gives the energies for planar and nonplanar ethylene at four-fold, bridge and other sites on Ni(100). The classical di-o configuration (C-C axis above a Ni-Ni bond) with the HCH plane of nuclei tilted away from the surface by 20 is found to be strongly bound, E = 8.9 kcal/mol. However, the di-o configuration at the four-fold site is slightly more stable, E = 9.8 kcal/mol. While the latter result is perhaps surprising, the opportunity to form one bond with a pair of surface Ni atoms (a three-center bond) can have advantages over forming a bond with a single Ni atom since there is a different pattern of rupture of surface Ni-Ni bonds. At all sites studied, going from a planar to nonplanar ethylene geometry lowers the energy by ~ 3 kcal/mol. The ethylene C-C bond lengthensfromthe gas phase value of 1.35 A to 1.43 A . Ethylene on Pt(100) is a d s


a d s

— o-c

8.9 kcal (20°)

8.0 kcal (10°)

6.1 k c a l ( 0 ° )

5.8 k c a l ( 0 ° )

9.8 kcal (20°)

7.9 kcal (20°)

6.5 kcal ( 0 ° )

5.6 kcal ( 0 ° )

Figure 1 Ethylene adsorption at several sites on Ni(100). Surface nickel atoms are located at the vertices of the boxes shown in the figure. Energies from embedded cluster calculations are reported for planar and nonplanar geometries. For the nonplanar geometry, the HCH tilt angle is 20 from the horizontal. Unless otherwise indicated, the C-surface distance is 2.35 A and the C-C distance is 1.38 A for the planar cases, and 2.28 A and 1.43 A , respectively, for the nonplanar cases. These values correspond to the equilibrium distances for the most favorable adsorption site.



280 qualitatively similar. Figure 2 shows the energy variation with HCH angle for ethylene adsorbed at a four-fold site on Pt(100) with H coadsorbed at the center of an adjacent four-fold site. The angle variation is for an asymmetric location of ethylene in which one C is above the center of the four-fold site, corresponding to the product formed immediately after the transfer of Hfromethyl adsorbed at a four-fold site to the surface. Also shown in thefigureis the energy lowering that occurs when ethylene is moved so that the midpoint of the C-C bond is directly above the four-fold site. The C-C bond length for ethylene at the asymmetric location is shorter than that obtained for ethylene at the center of the four-fold site without H coadsorbed. The value is much shorter than the C-C bond length in adsorbed ethyl which is typical of that for a C-C single bond, 1.53 A. This observation is an important factor in explaining what geometry changes must accompany H transfer to the surface to produce ethylenefromadsorbed ethyl.

0

10 20 30 H tilt (degrees out of plane)

40

Figure 2. Ethylene coadsorbed with H on Pt (100). The energy variation is for an outof-plane tilting of ethylene hydrogens for ethylene adsorbed on Pt(100) with one C atom above the four-fold site and H coadsorbed above the center of an adjacent four fold site. The point labeled (• ) shows the energy when ethylene in its optimum geometry is shifted so that the center of the C-C bond is directly above the center of the four-fold site. The energy is in a.u. (1 .a.u. = 27.21 eV). Next we consider adsorbed ethyl. The calculations show that the C-C bond length remains close to that of a typical single bond. Figure 3 shows that tilting ethyl toward the surface gives an equilibrium geometry corresponding to a 50 tilt from perpendicular for ethyl/Ni(100). This tilt angle is primarily a consequence of the nearly tetrahedral orientation of bonds around the a carbon. On tilting further, the graph shows a sharp increase in energy as a hydrogen on the P carbon comes too close to the surface. For a tilt angle of 90 , the P hydrogen would be within 0.8 A of the surface which is


281


much too close for a hydrogen that remains bonded to another atom. For tilt angles of 60 and 90 , other symbols in thefiguredenote the change in energy when the distance from the a carbon to the surface is increased. At 60 , the energy increases slightly as the C-surface distance is increased from its equilibrium value. At 90 , the energy decreases as the C-surface distance is increased until the stabilization gained by moving the P hydrogen away from the surface is matched by the destabilization from the increased C-surface distance.

Figure 3. Energy change on C H tilt on Ni(100). Ethyl is adsorbed on Ni(100) at the center of a four-fold site. The equilibrium geometry corresponds to a 50 tiltfromthe surface normal. The sharp rise in energy at larger tilt angles is due to repulsion 2

5

o

o

between a p hydrogen and the surface. Symbols at 60 and 90 indicate changes in energy when the distancefromthe a carbon to the surface is increasedfrom3.8 a.u. (2.01 A ). The energy is in a.u. (l.a.u. = 27.21 eV). Figure 4 reports the variation in energy for ethyl tilt on Pt(100) in which the ethyl C is bonded at the center of the four-fold site. Since the Pt-Pt nearest neighbor distance is much longer than the Ni-Ni distance, both ethyl and adsorbed H are closer to the surface in their equilibrium geometries. However, the energy variation on tilting ethyl is very similar to that for Ni(100). Once H begins to penetrate the electron density of the surface, for example for a tilt greater than 70 , the energy increases sharply. The optimum tilt angle is 60 sightly larger than the value for Ni( 100). For tilt angles of 60 and 80 , other symbols in the figure denote the change in energy when the distance from the a carbon to the surface is increased. At 60 , the energy increases slightly as the Csurface distance is increasedfromits equilibrium value. At 80 , the energy decreases



282 considerably as the C-surface distance is increased until the stabilization gained by moving the P hydrogen awayfromthe surface is matched by the destabilization from the increased distance of the a carbonfromthe surface. We now explore possible transition states for beta H elimination. If we consider ethyl on Ni(100), tilted 50 , the distancefromthe P H to the surface is 2.44 A which is much larger than the equilibrium distance for a completely dissociated H of 1.0 A . If the C-H bond is simply stretched in order to move H toward the surface, the energy increases sharply. The same is found for ethyl on Pt( 100): transferring Hfromethyl adsorbed at its equilibrium geometry is a very energetic process. Although the delectrons assist in preferentially stabilizing the transition state, their bonding is insufficient to allow H to be transferred through spacefromthe beta C.

-0.08-1

Figure 4. Energy change on C H tilt on Pt(100). Ethyl is adsorbed on Pt(100) at the center of a four-fold site. The equilibrium geometry corresponds to a 60 tiltfromthe surface normal. The sharp rise in energy at larger tilt angles is due to repulsion between a p hydrogen and the surface. Symbols at 60 and 80 indicate changes in energy when the distancefromthe a carbon to the surface is increasedfrom3.3 a.u. (1.75 A ) to the values shown in the inset. The total energy of H and ethylene coadsorbed at the centers of adjacent four-fold sites is indicated. The energy is in a.u. (l.a.u. = 27.21 eV). 2

5

Of course, it is an oversimplification to consider only the CH stretch reaction coordinate. In order to calculate the energy barrier accurately, it is necessary to allow simultaneously a relaxation of the resulting - C H - C H geometry as H is transferred to the surface. As noted earlier, one of the principal coordinates affecting the energy is the C-C distance where a large decrease in energy is expected in goingfroma single to double bond distance; likewise, angle variations accompanying the transition from 2

2



283 tetrahedrally bonded C to that corresponding to ethylene with tilted hydrogens are expected to be important. The present work explores only a few dissociation pathways, but one feature of the (100) surface seems clear: the dissociation is facilitated if the H and ethylene products are in different four-fold cells on the surface. In all of the pathways investigated, CH bond stretch occurs across a metal-metal bridge. Equally promising are pathways in which CH bond stretch and dissociation take place across a single surface atom site, and such possibilities will be considered in future studies. After carrying out rather extensive geometry optimizations, it turns out that there is a simple way to describe the motion leading to the transition state for H transfer. The key point is not to focus on H transferfromthe equilibrium geometry of adsorbed ethyl but rather to allow ethyl to vibrate into or otherwise penetrate the electron density of the metal before transfer of hydrogen. Specifically, by tilting ethyl beyond its equilibrium geometry to 80 , a more energetic configuration is created that lies ~ 5 kcal/mol above that of the equilibrium geometry. Hydrogen is now closer to the surface (at 1.32 A on Ni and 1.06 A on Pt) and CH bond stretch is found to occur with a much smaller increase in energy. This is true for ethyl onbothNi(lOO) and Pt(100). The key steps in such a reaction pathway are summarized in Tables 1 and 2 for nickel and

Table 1. Ethylene formationfromethyl adsorbed on Ni( 100). The energy increase for key steps in the lowest energy reaction pathway for beta hydrogen transferfromethyl to the surface are shown. The perpendicular distances of the alpha carbon and beta hydrogenfromthe surface plane of nuclei are denoted by and R , respectively. The AE value is the increase in energy compared to the previous geometry; the first entry, 5.5 kcal/mol, is relative to ethyl in its equilibrium geometry on the surface. H

AE (kcal/mol) Ethyl bends toward surface (a = 80 ) slightly penetrating repulsive region R =1.53A, R = 2.0lA R^ =1.09A, R =1.32A CC

5.5

c

H

H

CH stretch 0.26 A (24%) R = 1.36 A, R = 1.09 A, ACC = -0.05 A

5.8

CH stretch 0.43 A (40%) R =1.52A, R =1.09A HCH angles optimized

4.7

CH stretch 0.64 A (58%) R =1.73 A, R = 0.95A

0.8

C H

CH

CH

H

H

H

Overall activation barrier (sum of steps)

16.8


284


platinum surfaces, respectively, and motions leading to the transition state for beta hydrogen transfer for Pt(100) are depicted in Figure 5. On both surfaces, the HCH angles remain nearly tetrahedral in the early stages of CH stretch, e.g., up to an increase in CH bond length of 24% from the equilibrium value of 1.09 A . For the reaction on Pt(100), as shown in Table 2, most of the barrier is found to occur during the early stages of CH stretch. Between CH bond length increases of 24% and 40%, HCH angle variations begin to be important. The calculations reveal a significant difference between Ni and Pt, however. For ethyl on Pt(100) it is found that as the CH bond is stretched the emerging ethylene and H separate by ethylene sliding toward a more favorable site on the surface whereas for Ni(100) the H transfer occurs with less motion of the ethylene fragment. Table 2 shows a decrease in the intermediate and final contributions to the barrier height for Pt due to this effect. More work is currently underway in an effort to understand such subtle distinctions of the dissociation process on these two metals.

Table 2. Ethylene formationfromethyl adsorbed on Pt( 100). The energy increase for key steps in the lowest energy reaction pathway for beta hydrogen transferfromethyl to the surface are shown. The AE value is the increase in energy compared to the previous geometry; the first entry, 5.0 kcal/mol, is relative to ethyl in its equilibrium geometry on the surface. The perpendicular distances of the alpha carbon and beta hydrogenfromthe surface plane of nuclei are denoted by and R , respectively. The lateral shift moves ethylene toward the center of the four-fold site increasing its distance from the H being transferred to the surface. H

AE (kcal/mol) Ethyl bends toward surface (a = 80 ) slightly penetrating repulsive region R = 1.53 A,R =1.75 A R =1.09A, R = 1.06 A c c

CH

5.0

C

H

CH stretch 0.26 A (24%) R =1.36A, R =0.82A ACC = -0.05 A

8.1

CH stretch 0.55 A (40%) R =1.53 A, R = 0.67A 0.21 A lateral shift of C H,

1.0

CH stretch 0.66 A (53%) R =1.68A, R = 0.48A 0.21A lateral shift of C H,

-0.1

Overall activation barrier (sum of AE > 0 steps)

14.1

CH

CH

H

H

2

CH

H

2


285

CH stretch


penetration of density lateral motion

Figure 5. Depiction of the pathway leading to the transition state for the transfer of a beta hydrogen from adsorbed ethyl to the surface of Pt(100). Ethyl is tilted beyond its equilibrium geometry until one of the beta hydrogens penetrates the repulsive region of the metal electron density corresponding to a 5 kcal/mol energy increase for a tilt angle a = 80°. CH stretch is accompanied by a lateral separation of the emerging ethylene and adsorbed H. Contributions to the overall activation barrier of 14 kcal/mol are indicated at points along the reaction pathway in Table 2.

Acknowledgment Support of this research by the U.S. Dept. of Energy is gratefully acknowledged. Literature Cited (1) Whitten, J. L.; Yang, H. Surf. Sci. Reports 1996, 24, 55.. (2) Whitten, J. L.; Yang, H. Int. J. Quantum Chem. 1995, 29, 41. (3) Huff, M.; Schmidt, L. D. J. Phys. Chem. 1993, 97, 11815; Goetsch, D. A.; Schmidt, L. D. Science 1996, 271, 1561. (4) Huff, M.; Torniainen, P. M.; Schmidt, L. D. Catal. Today 1994, 21, 113. (5) Okada, S.; Matsuoka, O. J. Chem. Phys. 1989, 91, 4183. (6) Zaera, F. J. Am. Chem. Soc. 1989, 111, 8744; J. Phys. Chem. 1990, 94, 8350. (7) Jenk, C. J.; Chiang, C. M.; Bent, B. E. J. Am. Chem. Soc. 1991, 113, 6308. (8) Liu, Z. M.; Zhou, X. L.; Buchanan, D. A.; White, J. M. Chem. Ind. 1994, 53, 521. (9) Zhou, X. L.; Solymosi, F.; Blass, P. M.; Cannon, K. C.; White, J. M. Surf. Sci., 1989, 219, 294.


Theoretical Studies of Ethyl to Ethylene Conversion on Nickel and

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