A Density Functional Theory Analysis of the Reaction Pathways and

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A Density Functional Theory Analysis of the Reaction Pathways and Intermediates for Ethylene Dehydrogenation over Pd(111) Venkataraman Pallassana,† Matthew Neurock,*,† Victor S. Lusvardi,‡ Jan J. Lerou,§ David D. Kragten,| and Rutger A. van Santen| Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22903, DuPont Central Research and DeVelopment, Wilmington, Delaware 19808, NoVoDynamics, Inc., Ann Arbor, Michigan 48104, and Schuit Institute of Catalysis, EindhoVen UniVersity, The Netherlands ReceiVed: June 5, 2001; In Final Form: NoVember 2, 2001

DFT-GGA periodic slab calculations are used to examine ethylene dehydrogenation paths over Pd(111). The most favorable adsorption modes along with their corresponding binding energies for all C2Hx intermediates (acetylene, acetylidene, ethylene, ethyl, ethylidene, ethylidyne, vinyl, and vinylidene) are analyzed for 0.25 monolayer coverage on Pd(111). The binding energies are used to calculate the overall reaction energies for a number of elementary C-H bond activation and isomerization pathways that are likely involved in the decomposition of ethylene to ethylidyne over the well-defined Pd(111) surface. The intrinsic activation barrier for the dehydrogenation of ethylene to vinyl is determined using transition state search calculations. The stability of the surface vinyl species relative to ethylidyne is assessed by computing the activation barriers for the two-step conversion of vinyl to ethylidyne, via an ethylidene surface intermediate. Calculations indicate that the barrier for the conversion of vinyl to ethylidyne over Pd(111) is 84 kJ/mol, which is 67 kJ/mol lower than the computed barrier for vinyl formation from ethylene (151 kJ/mol). This is in agreement with UHV experimental literature that have consistently identified ethylidyne, but have not detected the vinyl species, during the thermal reactions of ethylene on the Pd(111) surface.

1. Introduction Ethylene dehydrogenation is important in both the selective and unselective paths for the synthesis of functionalized olefins such as vinyl chloride, vinyl alcohol, and vinyl acetate over supported metal catalysts. The selective paths for the catalytic production of vinyl acetate, vinyl alcohol, and vinyl chloride have all been speculated to involve the formation of a surface vinyl intermediate that subsequently reacts with a corresponding nucleophilic reagent such as acetic acid, water, or chlorine to form the functionalized olefin. Ethylene dehydrogenation, however, is also responsible for the production of the ethylidyne, which is believed to be a precursor to C1Hx decomposition products. These C1 fragments subsequently lead to catalyst deactivation. There have been numerous experimental studies aimed at elucidating the elementary paths for ethylene dehydrogenation over different single-crystal metal surfaces, including Pt,1-9 Pd,10-21 Ag,22 Rh,23 Ru,24 Ni,25,26 Mo,27 Fe,28 Cu,29,30 and Ir.31 Despite the large number of model studies, there are still several conflicting views on the actual mechanism for dehydrogenation. It is generally accepted, however, that ethylene decomposition over closed packed fcc (111) and hcp (0001) surfaces leads to the formation of the stable surface intermediate ethylidyne. Ethylidyne has been identified spectroscopically on Pt(111), Pd(111), Ir(111), Rh(111), and Ru(001) surfaces, all of which have hexagonal symmetry.32,33 Ethylidyne prefers to sit at the 3-fold adsorption sites, where it can satisfy its valence * To whom correspondence should be addressed. † University of Virginia. ‡ DuPont Central Research and Development. § NovoDynamics. | Eindhoven University.

by forming three bonds with the metal surface, in lieu of the three missing hydrogen atoms. On the more open surfaces such as Pt(100) (1 × 1), Ni(100), and Pd(100), however, vinylic (vinyl and vinylidene) species appear to be the more favored surface intermediates.13,32,33 Ethylidyne may also form on the more open surfaces such as Rh(100), but it is usually accompanied by metal surface reconstruction back to a more closed-packed structure. Mechanistically, there is evidence that the thermodynamically favored ethylidyne species may be formed through a sequence of steps that pass through a vinyl intermediate. Borg et al., for example, used temperatureprogrammed static secondary ion mass spectrometry and temperature-programmed desorption to show that ethylene reacts to form a vinylic intermediate which subsequently goes on to form ethylidyne over Rh(111).34 Similar paths have also been suggested over various other metals.8 Despite these general ideas, the intrinsic mechanism for ethylene dehydrogenation still remains to be fully resolved. In this paper, we are primarily interested in ethylene dehydrogenation over Pd(111) due to its importance in the synthesis of different functionalized olefins such as vinyl acetate. A common mechanistic feature for all of these reactions involves the formation of a surface vinyl group. We specifically probe how it may form and plausible decomposition routes. Ultrahigh vacuum (UHV) surface science studies of ethylene chemisorbed on close-packed surfaces such as Pd(111) and Pt(111) indicate that ethylene readily decomposes to form a surface-bound ethylidyne species at moderate temperatures (ca. 300 K).5,15-17 LEED and EELS analyses show that ethylidyne forms ordered (x3 × x3) overlayers on Pd(111).21 Ethylidyne (CH3-C) has one less hydrogen than ethylene (H2CdCH2). It is therefore

10.1021/jp012143t CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002

Ethylene Dehydrogenation over Pd(111)

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Figure 1. Elementary paths for the decomposition of ethylene to ethylidyne on Pd(111). Elementary reactions include C-H bond-formation, C-H bond-breaking, and H-1,2-shift reactions.

evident that the decomposition of ethylene to ethylidyne must, at some stage, involve C-H bond scission. Recent experimental work by Zaera and co-workers of ethylene on Pt(111) suggest that ethylene likely isomerizes to ethylidene, which then dehydrogenates to ethylidyne.8 This same direct path of ethylene to ethylidene to ethylidyne may also be important over Pd. For example, UHV studies indicate that the direct product of ethylene C-H bond activation i.e., vinyl (HCdCH2) is not detected on the (111) facets of Pd15,17,16 and Pt.8 In the absence of hydrogen, acetylene (HCtCH) reacts to form vinylidene.35-37 In the presence of surface hydrogen, however, acetylene reacts to form ethylidyne at 300 K.20,17 This requires C-H bond formation. Vinyl, which is the direct product of C-H bond formation of acetylene, is not measured in appreciable quantity on the surface. The above experimental observations suggest that even if vinyl is formed through reactions of ethylene or acetylene on Pd(111), it rapidly decomposes to a more stable surface intermediate such as ethylidyne. This was confirmed in a recent study by Tysoe et al., involving the thermal evolution of vinyl fragments, formed by the decomposition of vinyl iodide on Pd(111).20 The vinyl fragments were observed to rapidly convert to ethylidyne at temperatures as low as 160 K.20 The experiments of Tysoe and co-workers also suggest that acetylene decomposition to ethylidyne proceeds through a vinyl intermediate. Ethylidyne on Pd(111) decomposes at higher temperatures (400-500 K) by undergoing C-C bond scission to release CHx fragments on the metal surface.17 While UHV experimental evidence suggests that ethylene reacts directly to ethylidene and then on to ethylidyne, it does not rule out the possibility that vinyl may also be formed but quickly converts to ethylidene. In the presence of higher coverages of other nucleophilic intermediates, such as acetate, OH, or chlorine, the vinyl route could become an important route. This explanation might help to bridge the surface science results that indicate that ethylene reacts to ethylidyne and catalytic studies performed in the presence of a nucleophile that lead to the formation of functionalized olefins. Our specific interest in this paper is the decomposition of ethylene over Pd due to its relevance in the synthesis of vinyl acetate for both the selective paths as well as the unselective routes. Surface vinyl groups are considered to be crucial

intermediates necessary for the formation of vinyl acetate. These paths control not only the activity but the selectivity of this system as well. In an effort to better understand the mechanisms for ethylene dehydrogenation and to establish the factors that control the stability of the vinyl intermediate on Pd, we have performed first-principle density functional theory quantum chemical calculations. Density functional theory (DFT) methods are used herein to probe a model system, involving the surface chemistry of vinyl on Pd(111). We begin by examining the adsorption of the various different C2 fragments on Pd(111). The most favorable adsorption geometry and the binding energies of all the C2Hx (x ) 1-5) intermediates are calculated using density functional theory. DFT results are also used to help confirm the likelihood of the proposed mechanism which involves the formation of vinyl from ethylene and vinyl decomposition to ethylidyne on the well-defined Pd(111) surface. 2. Computational Methods Nonlocal, gradient corrected, density functional theory (DFT) calculations were used to determine all the structural and energetic results discussed in this paper. Fully periodic slab calculations along with cluster calculations were performed in this work. The cluster calculations were only used to help us isolate transition state intermediates. These transition state structures were subsequently optimized using periodic DFT calculations. In the cluster calculations, the molecular orbitals are generally localized to a finite region in three-dimensional space. They are therefore approximated by a linear combination of atom-centered Gaussian or Slater type functions, which constitute the atomic orbital basis set. In the periodic slab calculations, the metal eigenstates close to the Fermi level are best described by Bloch functions.38,39 The valence eigenstates are therefore approximated by a linear combination of a plane wave basis set with a maximum kinetic energy, typically on the order of 40 Rydberg.40 For our calculations, it was verified in a few cases that the total energy of the system did not change by increasing the cutoff energy beyond 40 Rydberg. The core orbitals are described by frozen-core, scalar-relativistic, normconserving pseudopotentials.41 The exchange-correlation po-

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TABLE 1: Periodic DFT Calculated Total Energies (in eV) for C2Hx (x ) 1-5) and Atomic Hydrogen in the Vapor Phasea species

spin multiplicity

total energy (eV)

ethylene ethyl ethylidene ethylidene ethylidyne ethylidyne vinyl vinyl vinylidene vinylidene vinylidene acetylene acetylidene acetylidene acetylidene atomic hydrogen

singlet doublet singlet triplet doublet quartet doublet quartet singlet triplet quintet singlet doublet quartet sextet doublet

-32.061 -34.902 -28.670 -28.989 -23.801 -22.537 -25.997 -22.379 -21.088 -19.194 -16.002 -23.066 -15.931 -11.425 -8.908 -1.1074

a The total energies are taken with reference to the zero-point state within the DACAPO. Taken alone, they have little significance. They can, however, be used to calculate reaction energy differences. They correspond to a species separation of 15 Å. Italicized values indicate the energetically most favorable spin state.

tential used in the local-density approximation is of the functional form proposed by Vosko, Wilk, and Nusair.42 Nonlocal contributions to the exchange-correlation energy were incorporated self-consistently while solving the Kohn-Sham equations,43,44 using the Perdew-Wang (PW91) functional.45 For ethylene on Pd(111), a (x3 × x3) unit cell was found to provide an optimal balance to minimize the size of the unitcell without encountering strong adsorbate-adsorbate repulsive interactions.46 Eighteen Chadi-Cohen special k-points were found to be adequate for sampling the first Brillouin zone corresponding to this (x3 × x3) supercell in real-space.47 The (1 × 1) adsorption of hydrogen on Pd(111) required 54 Chadiohen special k-points in order to adequately sample the first Brillouin zone.48 The adsorption of all C2Hx (x ) 1-5) intermediates were examined using (2 × 2) unit cells. For the (2 × 2) unit cell calculations, 16 k-points were used to sample the Brillouin zone and are expected to be adequate for predicting reliable energetics. All calculations discussed in this paper were performed on slabs that contained three atomic layers to represent the metal

surface. In previous calculations involving hydrogen on Pd(111), we found that there was less than 10 kJ/mol change in the binding energy by increasing the number of metal layers from 3 to 5. The slab periodicity perpendicular to the surface was chosen appropriately to eliminate interactions between adjacent metal slabs. A vacuum region of thickness corresponding to 5 metal layers was found to be adequate for this purpose. Electronic occupations were Fermi-distributed with an electronic kBT ) 0.1 eV to stabilize the electronic convergence scheme, but all final total energies were extrapolated back to 0 K.48 All calculations on the gas-phase species were performed spin-polarized. Calculations on bulk Pd and the Pd(111) surface indicate that Pd is nonmagnetic. A comparison between spin polarized and unpolarized calculations for hydrogen on Pd(111) indicate that the relative change in the adsorption energy is less than 5 kJ/mol. In addition, the periodic slab calculations reported here were all within 15 kJ/mol of the spin-polarized cluster calculations. We therefore approximate the adsorbate-metal system here by spin unpolarized calculations. All calculations were carried out using the DACAPO program developed at the Technical University of Denmark.49,50 Transition states for C-H bond activation of ethylene, vinyl, and ethylidene were initially determined using reaction coordinate search calculations on a 19-metal atom Pd(12,7) cluster model of the Pd(111) surface.51 The cluster calculations were performed using the DGauss program.52,53 For the C-H bondbreaking reactions, the principal component to the reaction coordinate is along the C-H bond stretch. The C-H bond stretch was, therefore, chosen as a starting reaction coordinate to locate the transition state. The first step in our transition state search procedure was to map out a series of structures, having C-H bond distances intermediate to those of the reactant and product. For each of these structures, the geometry was optimized by minimizing the energy of the system along all internal modes except the C-H bond distance, which was kept constrained during the optimization procedure. This constrained geometry optimization scheme provided a series of structures and energies along the chosen trial reaction coordinate. The point of maximum energy along the trial coordinate provided a better

TABLE 2: Periodic DFT Calculated Binding Energies for C2Hx (x ) 1-5) Species on Pd(111) at 0.25 Monolayer Coverage

adsorbate

structural formula

mode

structure (Figure)

ethylene ethylene acetylene acetylene ethyl ethylidene ethylidene ethylidyne ethylidyne ethylidyne vinyl vinyl vinylidene vinylidene vinylidene vinylidene acetylidene acetylidene acetylidene hydrogen

H2CdCH2 H2CdCH2 HCtCH HCtCH H3C-CH2 H3C-CH H3C-CH H3C-C H3C-C H3C-C H2CdCH H2CdCH H2CdC H2CdC H2CdC H2CdC HCtC HCtC HCtC H

π di-σ di-σ η2η2(C,C) η1(C) η1(C) η2(C) η1(C) η2(C) η3(C) η1(C) η1η2(C,C) η1(C) η2(C) η1η3(C,C) η3(C) η1η3(C,C) η2η3(C,C) η3(C) η3

2a 2b 3b 3c 4 5a 5b 6a 6b 6c 7a 7b 8a 8b 8c 8d 9a 9b 9c -

adsorption site atop bridge bridge hollow atop atop bridge atop bridge hollow atop hollow atop bridge hollow hollow hollow hollow hollow hollow

cluster binding energies (kJ/mol, ref 63)

binding energy (kJ/mol)

-30 -60 -130 -237 -276

-27 -62 -136 -168 -140 -247 -334 -292 -461 -511 -207 -254 -188 -329 -363 -344 -411 -415 -399 -267



Figure 2. Adsorption modes for ethylene (CH2dCH2) on Pd(111) for a (2 × 2) coverage: (a) π-bound ethylene and (b) di-σ ethylene. Note that all adsorption geometries and energies were determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

guess for the transition state geometry. Detailed transition state search algorithms were then used to refine the transition state geometry, such that the energy gradients along all internal modes were close to zero. Vibrational frequencies were computed at the resulting transition state structure, to verify the existence of the negative eigenmode that corresponds to the reaction coordinate. All the transition states for C-H bond activation of ethylene, vinyl, and ethylidene were reoptimized for the fully periodic slab geometry, using the cluster optimized transition state as an initial guess structure. The calculation of forces in the periodic slab geometry, using the cluster-optimized transition state as a trial structure, indicated that the forces on the atoms were very small in all directions, except in the direction perpendicular to the slab. The transition state geometry was therefore reoptimized to minimize the forces along this direction. At the final slab-optimized transition state structure, the forces in all directions were within the convergence threshold of 0.2 eV/Å. Comparison of the cluster optimized geometries with the periodic slab structures showed very little difference for the adsorbate C-C and C-H bond distances. The metal-C and metal-H bond distances in the slab calculations, however, were observed to be consistently shorter (∼0.1 Å) than the corresponding distances in the cluster optimized geometry. This is

most likely because of the different pseudopotentials and basis functions used in the cluster and slab calculations. 3. Results and Discussion Ultrahigh vacuum (UHV) surface science studies of ethylene chemisorbed on Pd(111) suggest that ethylene will readily react to form ethylidyne at temperatures above 300 K.15-17 EELS and LEED analyses indicate that ethylidyne forms well-ordered (x3 × x3) overlayers following exposure of Pd(111) to ethylene.16,21 At much higher temperatures (ca. 400-500 K), ethylidyne decomposes completely to form CH fragments on the Pd(111) surface.17 Although the ethylidyne and other C2Hx species have been well-characterized, using spectroscopic methods, on the (111) surfaces of many metals, there is no definitive agreement on the detailed pathway for the transformation of ethylene to ethylidyne over metal surfaces.5,11,51-53 Figure 1 illustrates the network of steps by which ethylene may be converted to the ethylidyne on Pd(111). Each of the elementary steps shown in Figure 1 deals with either C-H bond formation (hydrogenation), C-H bond-breaking (dehydrogenation), or H-1,2-shift (isomerization) reactions. We have deliberately excluded C-C bondbreaking and C-C bond-formation steps in the processes shown in Figure 1. This is a reasonable assumption, since it is known


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Figure 3. Adsorption modes for acetylene (CHtCH) on Pd(111) for a (2 × 2) coverage: (a) atop, (b) bridge, and (c) 3-fold (fcc) adsorption modes. Note that all adsorption geometries and energies were determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

that C-C bond breaking is relatively more difficult than C-H bond activation over the Pd group metals. C-C bond activation is therefore unlikely to be directly involved in the conversion of ethylene to ethylidyne on Pd(111), particularly so at lower temperatures. The mechanism for ethylene decomposition to ethylidyne could however involve the formation of various different C2Hx (x ) 1-5) intermediates, including vinyl (HCd CH2), ethyl (H2C-CH3), ethylidene (HC-CH3), acetylene (HCtCH), vinylidene (CdCH2), and acetylidene (CtCH). First-principle calculations are used in this paper to determine the binding energies for all relevant surface reactants, intermediates and products. The resulting values are then combined with the gas-phase reaction energies to calculate overall surface reaction energies. Detailed transition state searches were subsequently performed in order to establish the activation

barriers for the primary steps involved in the proposed mechanism for vinyl formation and decomposition to ethylidyne. 3.1. Adsorption of C2Hx (x ) 1-5) Species on Pd(111). Periodic DFT calculations were used to determine the adsorption modes and the corresponding energies of adsorption for all C2Hx (x ) 1-5) species on Pd(111). We examine a surface coverage of 0.25 monolayer (ML), corresponding to a (2 × 2) unit cell. For a number of individual species such as ethylene, acetylene and ethylidyne, the adsorption energies were also calculated for a larger (2 × 3) unit cell (i.e., lower surface coverage of 0.17 ML). The adsorption energies of these species were observed to change by less than 10 kJ/mol by increasing the unit cell size, which suggests that there are negligible lateral interactions between the adsorbates for a (2 × 2) unit cell. Figures 2 (ethylene), 3 (acetylene), 4 (ethyl), 5 (ethylidene), 6 (ethylidyne),



Figure 4. Atop (η1) adsorption of ethyl (CH3-CH2) on Pd(111) for a (2 × 2) coverage. Note that the adsorption geometry and energy was determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

7 (vinyl), 8 (vinylidene), and 9 (acetylidene) show the DFToptimized structural parameters for ethylene, ethyl, ethylidene, ethylidyne, vinyl, acetylene, vinylidene, and acetylidene for different possible adsorption modes on Pd(111) along with the changes in the surface Pd-Pd bonds. All of the structures and binding energies were determined using full periodic slab calculations. Herein we show only the top metal layer. We have removed the bottom metal layers for sake of visual clarity. The binding energy for each of the species on Pd(111) is listed in Table 2. It is important to note that the computed value of the binding energy is dependent on the reference state of the adsorbate.57 Table 1 shows the DFT calculated total energies for each of the adsorbates in the gas phase for various different spin states. The binding energies tabulated in Table 2 are with reference to the lowest-energy state of the adsorbate in the vapor phase, where the bond structure of the species is still retained. For example, the lowest-energy conformation for ethylidene in the gas phase would be to isomerize to ethylene. However, for the purposes of calculating the binding energy of ethylidene, we have chosen to maintain the structural identity of ethylidene in the vapor phase. We have only identified the spin multiplicity of ethylidene in the vapor-phase that would lead to its lowest energy. The most favorable spin-multiplicity for ethylene, ethyl, ethylidene, ethylidyne, vinyl, vinylidene, acetylene, acetylidene, and atomic hydrogen in the vapor phase, determined using periodic DFT calculations are singlet, doublet, triplet, doublet, doublet, singlet, singlet, doublet, and doublet, respectively (see Table 1). The total energy corresponding to these states are chosen as the adsorbate reference in the vapor state for the purposes of calculating surface binding energies. The binding energies for the C2Hx species in various different adsorption modes are tabulated in Table 2. The optimized structural features computed are illustrated in Figures 2-9. The most favorable adsorption state for ethylene on Pd(111) is the di-σ mode (Figure 2) with a binding energy of -62 kJ/mol. The calculated adsorption energy is in good agreement with UHV TPD experiments that have estimated a binding energy of -59 kJ/mol for ethylene on Pd(111).15,16 The adsorption in the π-bound mode is also relatively close in energy at -30 kJ/ mol. Acetylene is most favorably bound to the 3-fold fcc hollow site, where each of the carbon atoms forms bonds with two adjacent surface metal atoms (Figure 3C). The binding energy of acetylene in this η2η2(C,C) adsorption mode (Figure 3C) on

Pd(111) is -168 kJ/mol. The calculated binding energy (B.E.) as well as the structure is consistent with that determined theoretically by Pacchioni and Lambert (-184 kJ/mol).58 The adsorbed structure is also consistent with known experimental results which indicates that the C-C bond lengthens upon adsorption from 1.20 to 1.42 Å. The preferred adsorption site is the 3-fold fcc site. Acetylene is tilted by 17° from the surface normal. It is difficult to compare the calculated adsorption energy with the experimental value from TPD since acetylene decomposition occurs before thermal desorption can take place. TPD spectroscopy indicates that acetylene converts to an adsorbed intermediate which is speculated to be vinylidene at 480 K. This translates into an activation barrier of 115 kJ/mol. The energy of desorption should therefore be somewhat greater than 115 kJ/mol.59 Table 2 also summarized the binding energy of the other unsaturated C2Hx species on Pd(111) for different adsorption geometries. Ethyl (CH3-CH2) (Figure 4) prefers the atop adsorption site where a surface metal atom essentially replaces the missing hydrogen atom in order to form an ethane-like surface intermediate which preserves its sp3 symmetry. The binding energy for ethyl was calculated to be -140 kJ/mol. Ethylidene (CH3-CH) (Figure 5) is missing two hydrogens from the R carbon atom and therefore prefers to bind at a bridge site on the surface. The two metal atoms act to replace the two missing hydrogen atoms and preserve its sp3 structure. Its binding energy is -334 kJ/mol, which is over 2 times that of ethyl, which formed only a single M-C bond. Ethylidyne (CH3-C) (Figure 6) is missing three hydrogen atoms and therefore appears to favor the 3-fold fcc surface sites whereby it can form three new sigma bonds. The binding energy for ethylidyne at the 3-fold fcc site is calculated to be -511 kJ/ mol. The vinyl intermediate (CH2dCH) (Figure 7) is missing one hydrogen at the R carbon and two hydrogens from the β carbon. It therefore prefers to sit in a η1η2 fashion in order to complete its valence configuration at each carbon atom, with a binding energy of -254 kJ/mol. Vinylidene (CH2dC) (Figure 8) is missing three hydrogens from the R carbon and one hydrogen from the β carbon. It therefore, prefers to sit η1η3 with an adsorption energy -363 kJ/mol. Finally, acetylidene (CHdC) (Figure 9) which is missing all three hydrogens from the R carbon and two hydrogens from the β carbon, prefers the η2η3 adsorption mode. It binds at -415 kJ/mol.


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Figure 5. Adsorption of ethylidene (CH3-CH) at the (a) atop and (b) η2 bridge site on Pd(111) for a (2 × 2) coverage. Note that the adsorption geometries and energies were determined using periodic slab calculations using a 3 layer metal slab. Only the top metal surface layer is shown here for sake of visual clarity.

The general trends show that as we continue to remove hydrogen atoms from the R carbon atom, the binding energies of the resulting adsorbate scale approximately as N × metalcarbon bond strength. N here refers to the number of M-C (metal-carbon) sigma bonds that are formed. The resulting adsorbate attempts to satisfy its valence by creating N sigma bonds with the surface to replace its missing hydrogens. The binding energies therefore scale as

CH3-CH3 (MC ) 0) < CH3-CH2 (MC ) 1) < CH3-CH (MC ) 2) < CH3-C (MC ) 3) MC here refers to the number of metal-carbon (sigma) bonds the intermediate forms with the metal. While these simple ideas seem to hold for CH3CHx species over Pd(111), this concept may not hold when applied to other adsorbates, or other metals. As we begin to remove the β hydrogen atoms, the adsorbatesurface interaction is actually weakened since the carboncarbon bond order is increased. This decreases the number of sigma bonds that the R carbon can form with the surface. All these ideas agree with the principles of bond-order conservation. The final optimized structures for all of the C2Hx species at different adsorption sites are shown in Figures 2-9.

3.2. Ethylene Decomposition Paths on Pd(111): Surface Reaction Energy Analysis. On the basis of the binding energies of the various C2Hx (x ) 1-5) intermediates calculated in section 3.1, we have determined the overall surface reaction energies for the different elementary steps shown in Figure 1. It is assumed that all reactant and product species are bound to their most favorable adsorption site in a (2 × 2) coverage. We further assume that there are no lateral interactions between the species at both reactant and product geometry. The surface reaction energies are calculated using the expression vapor-phase ∆Esurface + reaction ) ∆Ereaction

∑

products

(B.E.products) -

∑

(B.E.reactants)

reactants

where, ∆Ereaction is the energy of reaction. The superscript “vapor-phase” indicates the reaction energy in the absence of the metal surface, and the superscript “surface” indicates the surface reaction energy. B.E.products and B.E.reactants are the surface binding energies of the product and reactant species, respectively, for a (2 × 2) surface coverage.



Figure 6. Adsorption modes for ethylidyne (CH3-C) on Pd(111) for a (2 × 2) coverage: (a) atop (η1), (b) bridge (η2), and (c) 3-fold fcc (η3). Note that all adsorption geometries and energies were determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

Table 3 summarizes the DFT-computed reaction energies for the various elementary steps depicted in Figure 1. The surface reaction energetics suggest that the formation of acetylidene on Pd(111) is unlikely at low surface coverage. The formation of acetylidene (CtCH) by dehydrogenation of acetylene (HCt CH) or vinylidene (CdCH2) is energetically uphill by +68 or +72 kJ/mol, respectively. We therefore suspect that acetylidene is unlikely to be involved in the direct pathway for the decomposition of ethylene to ethylidyne to Pd(111). Most of the other steps listed in Table 3 are either slightly endothermic or exothermic and would therefore still be possible steps in the mechanism of ethylene decomposition to ethylidyne. Recent experimental results by Tysoe and co-workers suggest that vinyl species adsorbed onto the Pd(111) surface are readily

transformed to ethylidyne at temperatures as low as 160 K.20 On the basis of their UHV experiments, they also suggest that the rate-limiting step in acetylene hydrogenation to ethylene, is the hydrogenation of acetylene to vinyl.20 This implies that surface vinyl species, in the presence of hydrogen, will easily convert to ethylidyne or ethylene. 3.3. Dehydrogenation of Ethylene to Vinyl on Pd(111). The reaction path calculated using periodic DFT calculations for ethylene dehydrogenation to vinyl over Pd(111) is shown in Figure 10. The reaction path was computed for a (x3 × x3) unit cell. The smaller size unit cell was chosen to minimize the computational effort involved in carrying out transition state search calculations The (x3 × x3) unit cell corresponds to 0.33 monolayer (ML) coverage of ethylene on the Pd(111)


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Figure 7. Adsorption modes for vinyl (CH2dCH) on Pd(111) for a (2 × 2) coverage: (a) η1-atop and (b) η1η2 (C,C). Note that all adsorption geometries and energies were determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

surface. DFT calculations suggest that the dehydrogenation of ethylene may occur as a single elementary step on Pd(111). Ethylene is bound to Pd(111) in the di-σ mode in the initial reactant state. The C-H bond is activated by insertion of a single surface metal atom into the C-H bond and proceeds via a threecentered transition state (Figure 10b). Along with the C-H bond activation, there is concurrent migration of the carbon toward the Pd-Pd bridge site, which is seen as an increase in the C-C-Pd-Pd dihedral angle from 0° to 30° along the reaction coordinate. This torsion is necessary because the product vinyl species is most favorably bound in the η1η2(C,C) mode (see Figure 10c). The C-H bond distance at the transition state is 1.76 Å. This is very similar to other C-H bond activation reactions such as methane activation and ethyl C-H bond activation, where the C-H bond distance at the transition state is between 1.7 and 1.8 Å.46,51,60 The transition state also shows the initial formation of C-Pd and H-Pd bonds, with a slightly shorter H-Pd bond length (1.74 Å), which is characteristic of H bound to a lower coordination site. The H-Pd bond distance in the product state is 1.8 Å. On the basis of our DFT calculations, for a (x3 × x3) geometry, the activation barrier for C-H bond breaking of ethylene is +151 kJ/mol. The elementary reaction is endothermic by +73 kJ/mol.61 In the previous section, we estimated the reaction energy for ethylene dehydrogenation to vinyl to be +20 kJ/mol (see Table 3). The reaction energy in that instance excluded the effect of lateral interactions and was based solely

on the vapor-phase reaction energies and low coverage binding energies of the reactants and products. For the (x3 × x3) geometry, the lateral interactions between vinyl species and hydrogen are quite strong in the product state. This would raise the total energy of the product (vinyl + H) state relative to that of the reactant (ethylene) state, resulting in an increase in the overall endothermicity of the reaction. On the basis of some of our previous work, it appears that bond-breaking reactions, such as vinyl formation, where there are more adsorbate-metal bonds at the product state, become less favored as the surface coverage is increased because of lateral repulsive interactions.46,61 The activation barrier and reaction energy for vinyl formation from ethylene are expected to be lower than +151 kJ/mol and +73 kJ/mol, respectively, at lower surface coverage. Low surface coverage, however, may not be very representative of what one would see in most reactions involving the functionalization of hydrocarbons. On the basis of the surface-science work of Stuve and Madix13 and assuming a first-order bond dissociation process, the activation barrier for the formation of vinyl on Pd(100) is estimated to be about 65-75 kJ/mol. Our calculated activation barrier is significantly higher than this value. This apparent discrepancy may be partly reconciled by considering the fact that the (100) surface is a more open surface than the (111) surface. Since each surface metal atom in the (100) surface is only surrounded by eight metal atoms, as opposed to nine metal



Figure 8. Adsorption modes for vinylidene (CH2dC) on Pd(111) for a (2 × 2) coverage: (a) η1-bound vinylidene, (b) η2-bound vinylidene, (c) η1η3(C,C)-bound vinylidene, and (d) η3(C)-bound vinylidene. Note that all adsorption geometries and energies were determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

atoms in the (111) surface, bond dissociation reactions such as vinyl formation are expected to be more facile on the Pd(100) surface. 3.4. Vinyl Decomposition to Ethylidyne on Pd(111). On the basis of UHV surface science studies of ethylene on Pd(111), there is no direct evidence for the formation of vinyl on Pd(111).15,16 This suggests that vinyl, if formed on Pd(111), rapidly decomposes to some other surface bound species. Since ethylidyne is thought to dominate the surface on Pd(111), it is likely that vinyl may be rapidly transformed to ethylidyne on Pd(111). This speculation is ratified by recent UHV experimental studies by Tysoe and co-workers which show that vinyl species, generated by grafting vinyl iodide on Pd(111), are readily transformed to ethylidyne at temperatures as low as 160 K.20 It is important to clarify at this point that we do not seek to prove that the decomposition of ethylene to ethylidyne necessarily proceeds through the formation of the vinyl species. We simply wish to explore the fate of the vinyl entity if and when it is formed on Pd(111). It is proposed as an important intermediate in a number of metal catalyzed chemistries but has not been directly isolated in UHV studies over Pd(111). On the basis of Figure 1, it appears that surface vinyl species may be transformed to ethylidyne by one of following three routes. They are: (i) Hydrogenation of vinyl to ethylidene, followed by dehydrogenation of ethylidene to ethylidyne. (ii) Isomerization (1,2 H-shift) reaction of vinyl directly to ethylidyne. (iii) Dehydrogenation of vinyl to acetylene (or vinylidene), followed by hydrogenation of vinylidene to ethylidyne. We can eliminate the third possibility on the basis of experimental evidence which suggests that acetylene hydrogenates to the vinyl species, and ethylene, before the appearance

of ethylidyne on the Pd(111) surface. Surface science experiments also suggest that vinylidene does not readily hydrogenate under UHV conditions.20,62 Our efforts to isolate a pathway for the isomerization of vinyl to ethylidyne suggest that the reaction has a very high barrier on Pd(111). In our calculations, the hydrogen atom that shifts from the β to the R carbon atom during the reaction did not want to interact with the metal surface. The reaction proceeded more like a vapor phase reaction path. This is likely the reason for the high calculated barrier. We were unable to isolate a transition state geometry where the H atom migration could be stabilized by the metal surface. ASED-MO calculations by Anderson and co-workers also suggest that the isomerization path may have a high activation barrier.56 We believe that we do not have sufficient basis to completely discount the elementary isomerization path as a possibility. We simply suggest that our preliminary calculations indicate that it is likely to have a high barrier. In the following sections, we examine the conversion of vinyl to ethylidyne via the third remaining possibility. This is through the two-step pathway involving an ethylidene intermediate. 3.4.1. Vinyl Hydrogenation to Ethylidene on Pd(111). In the first step of the proposed two-step path for the decomposition of vinyl to ethylidyne, vinyl is first hydrogenated to an ethylidene intermediate. The DFT computed reaction pathway is shown in Figure 11. The calculations were again performed for a (x3 × x3) unit cell. At the reactant state, the vinyl species is bound in its most favorable η1η2(C,C) mode at the 3-fold fcc site. Atomic hydrogen is bound to a neighboring 3-fold site (see Figure 11a). The reaction proceeds by a concurrent breaking of the metal-H and metal-C bonds, together with the formation of the C-H bond. This again involves a 3-centered transition state (TS), where the metal-H bond distance (1.56 Å) is


Pallassana et al. TABLE 3: Periodic DFT Calculated Surface Reaction Energies for the Dehydrogenation of Ethylene on Pd(111)a reaction number (refer to Figure 1) 1 2 3 4 5 6 7 8 9 10 11 12 13

surface reaction

reaction

∆Erxn(DFT) (kJ/mol)

ethylene(a) + H(a) f ethyl(a) ethylene(a) f ethylidene(a) ethylene(a) f vinyl(a) + H(a) ethyl(a) f ethylidene(a) + H(a) vinyl(a) + H(a) f ethylidene(a) vinyl(a) f ethylidyne(a) vinyl(a) f vinylidene(a) + H(a) vinyl(a) f acetylene(a) + H(a) acetylene(a) f acetylidene(a) + H(a) acetylene(a) f vinylidene(a) acetylidene(a) + H(a) f vinylidene(a) vinylidene(a) + H(a) f ethylidyne(a) ethylidene(a) f ethylidyne(a) + H(a)

hydrogenation isomerization dehydrogenation dehydrogenation hydrogenation isomerization dehydrogenation dehydrogenation dehydrogenation

+21 +25 +20 +4 +5 -46 -9 -5 +68

isomerization hydrogenation

-4 -72

hydrogenation

-37

dehydrogenation

-51

a Calculated surface reaction energies are in the absence of lateral surface interaction, and assuming all reactants and products are in their energetically most favorable conformation in a (2 × 2) surface arrangement.

Figure 9. Adsorption modes for acetylidene (CHtC) on Pd(111) for (2 × 2) surface coverage: (a) η1η3(C,C)-bound acetylidene, (b) η2η3(C,C)-bound acetylidene, and (c) η3-bound acetylidene. Note that all adsorption geometries and energies were determined using periodic slab calculations. Only the top metal surface layer is shown here for sake of visual clarity.

characteristic of hydrogen bound to the atop site. The metal-C bond length (2.26 Å) at the transition state is about 0.13 Å longer than the metal-C bond distance in the reactant vinyl species. The transition state occurs fairly early along the reaction coordinate. This is evident from the C-H bond distance (1.7 Å) at the TS, which is still significantly longer than the C-H bond distance in the final product ethylidene (1.1 Å). The product of hydrogenation, ethylidene, is favorably bound to the bridge site (see Figure 7c). The C-C bond length is also elongated along the reaction coordinate from 1.44 Å for surfacebound vinyl to 1.51 Å for ethylidene. In an earlier paper, we discussed the hydrogenation of ethylene to an ethyl species on Pd(111).51,61 Our DFT calcula-

tions suggest that there is a remarkable congruence between the transition states for vinyl hydrogenation to ethylidene, and ethylene hydrogenation to ethyl. In both cases, the C-H bond distance at the transition state is 1.7 Å. The changes in the metal-C and metal-H bond distances along the reaction coordinate are also very similar. The DFT-computed intrinsic activation barrier for vinyl hydrogenation to ethylidene is +84 kJ/mol, and the energy of reaction is -17 kJ/mol. In Table 3, we reported the surface reaction energy for vinyl hydrogenation to ethylidene to be +5 kJ/mol. This difference in the overall reaction energies is due to lateral repulsive interactions between vinyl and hydrogen in the reactant state for a (x3 × x3) geometry. The lateral repulsive interactions are weaker at the product state as compared to the reactant state, thus making the reaction more exothermic, for the (x3 × x3) coverage. The activation barrier for the β-C-H bond activation of ethylidene to form surface vinyl is +101 kJ/mol. This is about 20 kJ/mol higher than the activation barrier for the β-hydride elimination of ethyl on Pd(111) (+82 kJ/mol), reported by us earlier.46,61 3.4.2. Ethylidene Dehydrogenation to Ethylidyne on Pd(111). The second step in the proposed mechanism for the decomposition of vinyl to ethylidyne on Pd(111), involves the metal catalyzed dehydrogenation of the ethylidene species. The reaction pathway calculated using DFT for this elementary step is shown in Figure 12. At the initial reactant state, ethylidene is bound to the metal surface at the bridge site. This is the most favorable adsorption geometry for ethylidene on Pd(111). The reaction proceeds via a tilting of the C-C axis of ethylidene toward the surface normal. The R carbon atom simultaneously starts migrating toward the 3-fold hollow site. The tilting of the ethylidene species away from the plane of the surface, lowers the R H atom sufficiently for it to begin to interact with the metal surface. A 4-center like transition state forms whereby the C-H bond is activated over the Pd-Pd bridge site (see Figure 12b). Rudiments of metal-H bond formation are evident at the transition state structure. The metal-H bond length (1.94 Å) is about 0.1 Å longer than that at the final product state, where the metal-H bond distance is closer to 1.8 Å. The ethylidyne product is most favorably bound to the 3-fold hollow site, with


Figure 10. DFT-calculated reaction coordinate for the dehydrogenation of ethylene to vinyl on Pd(111), for a (x3 × x3) unit cell: (a) di-σ bound ethylene on Pd(111), (b) transition state for C-H bond activation, and (c) η1η2-bound vinyl and 3-fold hydrogen on Pd(111) [Pallassana, V.; Neurock, M. J. Catal. 2000, 191, 301-317].

the C-C bond parallel to the surface normal (see Figure 12c). The hydrogen atom is bound to a neighboring 3-fold site. The DFT computed activation barrier for ethylidene C-H bond activation to form ethylidyne is +75 kJ/mol, and the energy of reaction is -22 kJ/mol, for a (x3 × x3) unit cell. The reaction energy for the (x3 × x3) unit cell is 29 kJ/mol less exothermic than the reaction energy listed in Table 3 (-51 kJ/mol). The reaction energy listed in Table 3 is computed at low coverage where there are no lateral adsorbate-adsorbate interactions. For the ethylidene C-H bond-breaking reaction, the lateral repulsive interactions are greater at the product state, where there are more adsorbate-metal bonds, than at the reactant state. The reaction energy is therefore less exothermic for the (x3 × x3) unit cell as than reaction energies at the low coverage. This is again consistent with the postulate that bond-breaking reactions that result in more adsorbate-metal bonds in the product state are less favored at higher surface coverage. In Figure 13, we summarize the calculated reaction energy diagram for the formation of vinyl on Pd(111) and its decom-


Figure 11. DFT-calculated reaction coordinate for the hydrogenation of vinyl to ethylidene on Pd(111), for a (x3 × x3) unit cell: (a) η1η2bound vinyl and 3-fold hydrogen on Pd(111), (b) transition state for C-H bond formation, and (c) η2-bound ethylidene product on Pd(111).

position to ethylidyne via the proposed two-step pathway. It is evident that the overall activation barrier for the decomposition of vinyl to ethylidyne is significantly lower than the activation barrier for the dehydrogenation of ethylene to vinyl on Pd(111). We were not able to isolate a direct isomerization route for vinyl to ethylidyne, with a lower activation barrier than the two-step mechanism computed here. Such a path, if it exists and if favorable, could further lower the activation barrier for the transformation of vinyl to ethylidyne. The DFT calculations suggest that the vinyl species, if formed on the clean Pd(111) surface, would either hydrogenate back to ethylene, or rapidly decompose to ethylidyne. This is consistent with experimental observation, where the ethylidyne species and not the vinyl species, is detected on the Pd(111) surface following its exposure to ethylene.15-17,20 The challenge in functionalizing vinyl groups involves stabilizing the vinyl intermediate relative to ethylidyne. This


Pallassana et al. relative to ethylidyne. Au acts to shut down some of the 3-fold sites that would accommodate the ethylidyne product. 4. Conclusions

Figure 12. DFT-calculated reaction coordinate for the dehydrogenation of ethylidene to ethylidyne on Pd(111), for a (x3 × x3) unit cell: (a) η2-bound ethylidene reactant on Pd(111), (b) transition state for R C-H bond breaking of ethylidene, and (c) η3-bound ethylidyne and 3-fold hydrogen products on Pd(111).

would lead to a greater probability that vinyl would couple with the functional group, such as acetate, to form the vinylsubstituted product. In a forthcoming paper, we discuss how alloying of Pd and Au, increases the stability of the vinyl species

Gradient corrected periodic density functional theory calculations were used to examine the binding of all C2Hx (x ) 1-5) species on Pd(111). The adsorption modes and energies for ethylene, ethyl, ethylidene, ethylidyne, vinyl, acetylene, vinylidene, and acetylidene were all examined at a surface coverage of 0.25 monolayers (ML) corresponding to a (2 × 2) unit cell. Binding energies and the energetically most favorable adsorption sites were determined for all the above species. Using the most favorable binding energies, low-coverage surface reaction energies are reported for 13 distinct elementary reactions that may potentially be involved in the decomposition of ethylene to ethylidyne on Pd(111). DFT calculations suggest that the formation of acetylidene from acetylene or vinylidene is energetically unfavorable by ∼70 kJ/mol at low coverage. This indicates that the formation of acetylidene is unlikely to be directly involved in the formation of ethylidyne from ethylene on Pd(111). Although vinyl has not been observed on Pd(111), it is still strongly believed to be the precursor for selective as well as unselective surface reaction paths. Periodic DFT transition state search calculations were used to isolate a path for the formation of vinyl from ethylene on Pd(111). Calculations indicate that the formation of vinyl from ethylene has a barrier of +151 kJ/ mol and is endothermic by +73 kJ/mol at 0.33 ML coverage of ethylene on Pd(111). The two-step conversion of vinyl to ethylidyne, via an ethylidene intermediate, was also examined using DFT. Calculations indicate that the hydrogenation of vinyl to ethylidene on Pd(111) has an activation barrier of +84 kJ/ mol and is exothermic (-17 kJ/mol) for a (x3 × x3) unit cell. The activation barrier for the dehydrogenation of (x3 × x3) ethylidene to ethylidyne on Pd(111) was calculated to be +75 kJ/mol while the surface reaction energy was -22 kJ/mol. The activation barrier for the decomposition of vinyl to ethylidyne is therefore roughly half the barrier for its formation from ethylene on Pd(111). This helps to explain why the vinyl species is never detected spectroscopically, but ethylidyne is observed at moderate temperatures, following ethylene adsorption on Pd(111).

Figure 13. Reaction energy diagram for the formation of vinyl from ethylene on Pd(111), and its decomposition to ethylidyne via ethylidene. The activation barrier and reaction energies were computed for a 0.33 monolayer (ML) coverage of ethylene on the surface, corresponding to a (x3 × x3) unit cell.

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