Transition State Modeling for Catalysis - American Chemical Society

Chapter 18

Modeling Transition States for Selective Catalytic Hydrogenation Paths on Transition Metal Surfaces Matthew Neurock and Venkataraman Pallassana

Downloaded by PENNSYLVANIA STATE UNIV on August 7, 2012 | http://pubs.acs.org Publication Date: April 8, 1999 | doi: 10.1021/bk-1999-0721.ch018

Department of Chemical Engineering, University of Virginia, Charlottesville, VA 22901

Density functional theory (DFT) quantum chemical calculations have been used to analyze the reaction coordinate forβ-hydrideelimination of ethyl on Pd(111), as a model for general C-H bond activation and C=C bond hydrogenation. The DFT computed activation barrier of +69 kJ/mol is comparable to the activation energy of 40-57 kJ/mol measured experimentally by Kovacs and Solymosi [1]. The role of electron -withdrawing substituents, such as -OH and - F , on the structure and energetics of adsorption and selective hydrogenation for a series of different substituted ethylene intermediates were examined in an effort to construct structure-reactivity relationships. Strong electron-withdrawing substituents were found to reduce the adsorption energy of the di-σ binding mode. These substituents were also found to raise the activation barrier forβ-hydrideelimination of the corresponding β-substituted-ethyl intermediates. The reaction mechanism and transition state structures for various other C-H bond activation reactions are compared. The results indicate that there is a noticeable similarity between the transition state structures for various C-H bond activation reactions. This suggests that there may be a universal mechanism that governs a series of relevant selective hydrogenation reactions.

The selective hydrogenation of multifunctional molecules is important in the synthesis of fine chemicals and pharmaceutical intermediates. The ability to selectively hydrogenate specific olefin, aldehyde or ketone moieties within a given structure could have tremendous commercial relevance. In the area of fine chemical synthesis, many

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© 1999 American Chemical Society

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


227 oxygenated intermediates are formed via the oxidation of hydrocarbons. Catalytic oxidation reactions are not easy to control and the natural tendency is to over-oxidize. Selective hydrogenation of specific moieties is often required to produce the desired product. The production of tetrahydrofuran, for example, first involves the oxidation of butane to maleic anhydride. Maleic anhydride is subsequently hydrogenated to THF by selectively saturating individual C=C and C=0 moieties. In addition to site specificity, enantiomeric selectivity is also important and often critical in the design and synthesis of new drugs. For many pharmaceutical applications, the desired compound is optically active. One enantiomer may possess the requisite properties while the other may have detrimental side-effects in clinical use. The ability to selectively synthesize the appropriate enantiomeric isomer would be of great importance. A fundamental understanding of the mechanisms for selective hydrogenation would, therefore, be invaluable in developing new strategies for the targeted synthesis of chemical intermediates. Model studies with a,P-unsaturated aldehydes on supported metal particles have been carried out to establish the mechanism for the selective hydrogenation of the C=C and C=0 bonds, and to design new heterogeneous catalysts that can selectively saturate one specific moiety [2-11], The ability to elucidate this chemistry under ultrahigh vacuum (UHV) conditions is difficult, because hydrogen will typically desorb before it will hydrogenate. To elucidate hydrogen addition to an adsorbed olefin, most of the surface science efforts have focused on analyzing the reverse reaction of the C-H bond activation of an adsorbed surface alkyl group. Surface alkyl groups are readily generated by decomposing the alkyl-halide reactant at low temperatures on the surface. By ramping the temperature, the products can be monitored to follow the C-H bond activation path. In this work we usefirst-principlequantum chemical Density Functional Theory (DFT) to probe the hydrogenation of a series of single functional moieties in an effort to understand the effect of substituents on the chemisorption and surface reactivity of these intermediates. We examine the hydrogenation of a series of substituted-ethylene molecules as well as the hydrogenation of CO. A close examination of the reaction coordinates, indicate that there is an elementary C-H bond formation step that is relevant in a number of other chemical processes. Modeling Transition States Up until the last decade, the application of theory to modeling surface chemistry has primarily been used in a qualitative way. Pioneering efforts by Hoffmann [72,75], van Santen [14,15], Newns [16], Anderson, Norskov [77,75], and numerous others, have shown how theory can provide tremendous insight into understanding the general concepts of bonding and how they relate to the governing catalytic mechanisms. The technological breakthroughs in quantum chemical methods and algorithms, that have occurred over the past decade, coupled with tremendous advances in CPU hardware, are now beginning to make it possible to also extract reliable energetic information for the chemisorption of small molecules on ideal systems, such as well-defined


228


organometallic clusters, zeolites and metal surfaces. The prediction of activation barriers on heterogeneous surfaces, however, is just beginning to emerge. Leading efforts by Ziegler [79], Siegbahn [20,21], Morokuma [22,23] and Goddard [24] for well-defined organometallic systems, van Santen [25] and Sauer [26] for zeolites, and Whitten [27], Ngfrskov [28,29], and van Santen [75] for metals, demonstrate the applicability of first-principle calculations toward computing activation barriers. Our focus here is on the application of first-principles methods to reactions on metal surfaces. Preliminary efforts in the theoretical study of metals demonstrated its application to the dissociation of dimer molecules such as H , O2, N2, CO, and NO. The application of theory to elucidate the chemistry of more complex surface intermediates has only just begun. In a previous work, we described the application of DFT methods to the prediction of chemisorption properties as well as the reactivity of maleic anhydride, vinyl acetate as well as a host of other commercially relevant intermediates [30,31]. A critical analysis of these intermediates, as well as a series of other model species, will enable us to establish a set of substituent effects and to develop structure-reactivity relationships. Herein, we map out the reaction coordinate for the hydrogenation of the C=C bond of ethylene, isolate the transition state for this reaction, and examine a series of substituted ethylene species to establish the effect of electron-withdrawing groups. 2

Computational Details Gradient corrected density functional theory (DFT) was used to compute all the structural and energetic results reported in this paper. The Vosko, Wilk and Nusair exchange-correlation functional was used within the local density approximation (LDA) [32]. Non-local gradient corrections of Becke (for exchange) and Perdew (for correlation) were explicitly incorporated in the exchange-correlation energy within each cycle of the self-consistent-field calculations [33-35]. The single-particle wavefunctions for the many-electron system are formed by a linear combination of an atomic orbital basis set. The basis sets used in our calculations are contracted Gaussian type functions of double zeta quality. For palladium, the core electrons are described by scalar-relativistically-corrected, frozen core pseudopotentials. The frozen core pseudopotential rmnimizes the CPU requirements for the self-consistent-field (SCF) calculations, as well as allows for an adequate inclusion of relativistic effects for the inner srjpll electrons. For all calculations, the SCF energy was converged to within 5.0x10" a.uând the geometry was considered optimized when the gradient was less than 5.0x10" a.u./A. Additional details on the implementation of DFT in the DGauss algorithm, which was used herein, can be obtained elsewhere [36,37]. In previous work, we have shown that constrained Pd(12,7) or Pd(12,6) clusters can be used to compute adsorption energies representative of that of the Pd(l 11) surface [30]. We allow for the complete relaxation of the adsorbate, while constraining the metal cluster to the bulk structure. Complete relaxation of the cluster can artef the energetics of adsorption by about 20-30 kJ/mol [30]. We have also performed periodic DFT-slab


229 calculations on some of adsorption systems and have confirmed that cluster edge effects are negligible for our Pdi8 and Pdi 9 cluster models. [30]. The binding and adsorption energies are computed as: E


AÂds = Pd(12,7)-adsorbate ~ Epd(12,7) ' Âdsorbate where, Epd(12,7)-adsorbate is the total energy at the optimized ground state of the adsorbate bound to the cluster, Epd(12,7) is the total energy of the bare Pd(12,7) cluster and E dsorbate is the total energy of the optimized adsorbate alone in the gas phase. Activation barriers for chemical reactions are computed by taking the differences in total energy for the structure at the transition state and the optimized reactant state. a

Finite-sized metal clusters, such as that of palladium, can have spin multiplicities different from that of the lowest energy state of the bulk system. To properly account for such unpaired electrons in finite sized metal clusters, all our calculations are spin unrestricted. The most favorable spin multiplicity for the bare Pd(12,7) cluster was determined to be the triplet. The spin multiplicities for all of the adsorbate/Pdl9 systems described herein remained the same as that reported for the Pdi9 cluster. The closed shell molecular adsorbates did not change the spin multiplicity of the system upon adsorption. Chemisorption of atomic hydrogen, however, lowered the ground state spin multiplicity of the Pd(12,7) cluster to a doublet. By carefully analyzing the resulting orbital spectrum we chose a series of other possible symmetry occupations to help isolate the lowest energy state. A complete analysis of all possible states on a Pdi9 cluster would be computationally infeasible. At 19 metal atoms, however, the energy differences between different states is very small. Results and Discussion A. C=C bond hydrogenation on Pd(lll) 1) di-a Adsorption on Pd(lll): Effect of Primary Substituents Theoretical density functional calculations have shown that the energetically most favorable adsorption mode for ethylene on Pd(lll) is di-a [30,38,39]. UHV experimental studies, however, have identified both n and di-a bound ethylene on single crystal Pd surfaces [40-42]. The optimized adsorption geometry along with its corresponding binding energy for ethylene on Pd(lll), in both the n and di-a chemisorption modes are depicted in Fig 1. Both cluster and periodic slab calculations were used to model this system. The adsorption energy computed from our Pd(12,6) cluster closely matches that of the periodic slab calculations, providing additional evidence that cluster edge effects for the Pdi8 cluster model are very minor. The predicted adsorption energy of the di-a



230

AE

arie

= -60 kJ/mol

C)

AE. . = -61 kJ/mol H

D)

AE „ = -27 kJ/mol ar)

AE_ „ = -30 kJ/mol H

Figure 1. The di-a and n adsorption modes of ethylene on Pd(l 11): Comparison of cluster and slab models for DFT. A) Di-a bound ethylene on Pd(l 11) 3 layer slab (V3 x V3 structure). B) Ethylene adsorption on Pd(12,6) cluster in the di-a mode. C) 7i bound ethylene on Pd(l 11) 3 layer slab (V3 x V3 structure). D) Ethylene adsorption on Pd(12,6) cluster in the n mode.


231


mode (-60 kJ/mol) is about 30 kJ/mol stronger than the 7t-adsorption mode in both the cluster and slab results. The DFT predicted di-a adsorption energy for ethylene agrees well with reported experimental values [40-43]. Our results are also consistent with the theoretical values reported by Sautet [38]. The frontier orbitals for the di-a adsorption mode show that there is electron-donation from the n orbitals of the adsorbate to the d orbitals of the metal and electron backdonationfromthe metal d orbitals to the n* (LUMO) orbital of the adsorbate. This is illustrated in Fig 2. The synergy of these bonding interactions result in a strong dative bond between the adsorbate and the metal. While these interactions are also present for the 7i-adsorption complex of ethylene, they are weaker than that for di-a adsorption. To understand the correlation between adsorbate structure and chemisorption energy, we examined the effect of electron withdrawing substituents (X) on the CH2=CHX adsorption of ethylene in the di-a mode (where X = -H, -OH and -F). The optimized geometry for each of these adsorbates on a Pd(12,7) cluster are shown in Fig 3. The vapor phase structures of the adsorbates were also optimized in order to compute the adsorption energies. The C=C bond distance for all of the vapor phase structures were between 1.34 and 1.35 A. There is considerable elongation of the C=C bond distance upon adsorption in the di-a mode, on account of rehybridization of the carbons from sp to sp , as is seen in the adsorbate/cluster complexes depicted in Fig 3. Interestingly, the C=C bond distance for each of these chemisorbed intermediates is almost identical (1.45 A) and appears to be independent of the nature of the substituent. Since the substituents examined here do not directly interact with the surface, the changes in the chemisorption energy of the adsorbates can be attributed to the electronic interactions between the substituent and the C=C moiety. To help quantify the electron-withdrawing capability of the various substituents, charges were assigned to the substituent groups based on a simple Mulliken population analysis of the adsorbates in the vapor phase. A more rigorous accounting of charge would have likely demonstrated similar trends. The analysis of the charge on each substituent indicates that the fluorine is the most and hydrogen is the least electron withdrawing substituent. This is consistent with expectations based on the electron affinity of these species. The adsorption energies of substituted ethylene on a Pd(12,6) cluster and the Mulliken charges on the substituent are summarized in Table 1. From Table 1, it is clear that an increase in the electron-withdrawing capability of the substituent (X) decreases the binding energy of the corresponding CH2=CHX species on Pd(lll). We have been unable to find experimental adsorption energies for vinyl alcohol and fluoro-ethylene bound to Pd(l 11) through the C=C moiety. The reported adsorption energies for allyl alcohol (E ds -50 kJ/mol) and acrolein (Eads -50 kJ/mol) on Pd(lll) are lower than that for ethylene, in spite of additional oxygensurface interactions for these molecules [44]. The lowered binding strength through the C=C moiety in these unsaturated oxygenates is consistent with our predicted trends for electron-withdrawing groups. The observed trends in the chemisorption energy can =

a


=


232

Figure 2. Frontier orbital interactions for the di-a adsorption mode of ethylene on Pd. a) Optimized geometry for ethylene-palladium complex. b) Molecular orbital corresponding to electron donationfromethylene 7i to metal d orbital. c) Molecular orbital corresponding to electron back-donation from metal d orbital to ethylene n* orbital.

Figure 3. Non-local DFT optimized structures for different substituted ethylene molecules (CH =CHX) on Pd(l 11). A) vinyl fluoride; B) vinyl alcohol and C) ethylene. Reproduced with permissionfromreference 30. 2



233

Table 1.

Effect of Direct Vinyl Substituents on the Adsorption Energy of Substituted Ethylene on Pd(l 11).

Adsorbate

Substituent

Charge on Substituent

Charge on the P-carbon

vinyl fluoride vinyl alcohol ethylene

-F

-0.210

+0.135

DFT computed Adsorption Energy kJ/mol -44

-OH

-0.029

-0.165

-54

-H

0.098

-0.196

-61


234 be explained by the fact that the presence of strong electron withdrawing substituents on the carbon atom reduce the electron donation capability of the adsorbate and consequently decreases the chemisorption energy.


2) C-H Bond Activation of Substituted Ethyl on Pd(lll) The selective hydrogenation of C=C unsaturated compounds on group VIII metals, encompasses several important reactions in the chemical industry. Although each of these processes may have characteristics that are specific to the chemistry, the fundamental moiety that undergoes hydrogenation still remains the same. We, therefore, decided to examine the mechanism for hydrogenation of the C=C moiety in ethylene. However, the only fundamental data by which we can gauge our results are present for the reverse reaction, P-hydride elimination. This is due to the fact that probing hydrogenation reactions under UHV conditions is rather difficult. Instead, microscopic reversibility allows us to examine the back reaction of P-hydride elimination in an effort to understand hydrogenation. In this section, we explore the reaction coordinate for C-H bond formation in C=C bond hydrogenation through a detailed analysis of the reverse reaction of P-hydride elimination. Changes in the activation barrier and transition state geometry, due to electron-withdrawing substituents on the P-carbon atom, are also investigated. The P-hydride elimination reaction proceeds through an agostic stretch of the C-H bond as the ethyl approaches the surface. Stretching the C-H bond lowers the energy of the crcH* bital of the ethyl group, allowing electron back-donation from the metal into this anti-bonding state. In addition to the C-H stretch, the C-C-Pd surface angle decreases which brings the "activated" CH group closer to the surface. The combination of both of these processes act to further weaken the C-H a bond, ultimately breaking it. The transition state for the reaction (refer figure 4) shows a long C-H bond (1.7 A). The carbon and hydrogen of the activated C-H bond are coordinated to the central metal atom via a 3-center transition state complex. This is a metal atom insertion process. The hydrogen atom is stabilized by the neighboring metal atom, forming the 2-fold bridge site. The hydrogen atom subsequently migrates to the 3-fold hollow site, where it is most energetically stable (figure 4). or

3

The reactant and product structures for p-hydride elimination of the different substituted ethyl (-CH2-CH2X) species (X = -F, -H and -CHO) were completely optimized on the fixed Pd(12,7) cluster. The transition state for the reaction was resolved by following the energy along a trial reaction coordinate. Since, for P-hydride elimination, the largest component of the reaction coordinate is the P C-H bond stretch, this internal mode was chosen as an approximate reaction coordinate in our transition state search procedure. The geometry of the adsorbate was optimized at a fixed C-H bond distance, to determine the lowest energy structure at that particular C-H bond stretch. This was repeated for selected points along the chosen reaction coordinate. The point of maximum energy along this trial reaction coordinate provides an approximate structure of the transition state and activation barrier for the reaction. To verify that our approximate transition state is close to the true transition state, we



Figure 4. Reaction path for C-H bond activation of ethyl on Pd(l 1 a) Optimized surface reactant (ethyl). b) Transition state for P-hydride elimination of ethyl. c) Optimized surface product (ethylene + atomic H). Reproduced with permission from reference 30.


236


compute the vibrationalfrequenciesfor our predicted transition state geometry. Vibrationalfrequencieswere computed on a Pd(7,0)fragmentof the Pd(12,7) cluster so as to minimize the computational resource requirements. The frequency calculations led to the prediction of the imaginary modes that correspond to translation along the reaction coordinate. This confirmed that our optimization process brought us to within the vicinity of the true transition state for the reaction. Detailed transition state search calculations on the smaller Pd(7,0) cluster resulted in a geometry that was very similar to our "hand-optimized" transition state. The transition state structures for the various substituted ethyl species, depicted in Fig 5, show remarkable similarity. The Pd-H bond distance at the transition state was found to be between 1.58-1.62 A while the C-H bond distance was between 1.681.70 A. This outstanding similarity between transition state structures for similar chemical reactions, if generic, presents an interesting opportunity in reaction coordinate analysis. The transition state for the reaction of one member of a homologous series can be used to estimate the structure and predict the properties of additional members of the series, without having to employ the detailed reaction coordinate analysis procedure. As mentioned earlier, vibrationalfrequencycalculations enabled us to confirm that we indeed isolated a saddle point on the potential energy surface. The normal mode eigenvectors corresponding to the reaction coordinate for ethyl p C-H bond breaking are shown in figure 6. Thefrequencycorresponding to the reaction coordinate for P-hydride elimination of ethyl is -228 cm" . It is clear that the largest component to the reaction coordinate is the p C-H bond stretch. There is also a slight CH2 bending element to the reaction coordinate, which corresponds to the change in hybridization of the P-carbon atomfromsp to sp . The activation barriers for P-hydride elimination of the substituted-ethyl (-CH2-CH2X) species on a Pd(12,7) cluster are summarized in Table 2. The charges on the substituents (X) and the P-carbon atom are, again, based on Mulliken population analysis of the adsorbates in the vapor phase. The activation barrier for P-hydride elimination of ethyl is computed to be +69 kJ/mol. This is slightly higher than the experimental activation barrier of +40-57 kJ/mol measured by Kovacs and Solymosi for ethyl on the Pd(100) surface [1]. Since the surface metal atoms have a higher coordination number on the [111] surface, one would expect a slightly higher activation barrier for the P-hydride elimination process on this surface, which would agree more closely with our computed barrier. Temrjerature-programmed-desorption (TPD) studies of acroleinfromthe Pd(l 11) surface showed reaction-limited desorption of propanal at 340 K [44]. Based on this observation, the overall activation barrier for the hydrogenation reaction is estimated to be +85 kJ/mol. The DFT computed heat of reaction for C-H bond formation, in the hydrogenation of acrolein, is -7 kJ/mol [30]. If we assume that the C-H bond formation step is rate limiting, which is typically true for such reactions, then the DFT predicted activation barrier of +82 kJ/mol, for hydrogenation, is in good agreement with experiment [30,44].



237

Figure 5. Non-local DFT optimized transition state structures for p-hydride elimination of substituted ethyl (-CH -CH X) on Pd(l 11). A) vinyl fluoride; B) acrolein and C) ethylene. 2

2

Figure 6. Normal mode eigenvectors corresponding to the reaction coordinate for P-hydride elimination, a) acetate and b) ethylene. Reproduced with permissionfromreference 30.



238

Table 2.

Substituent Effects on p-hydride Elimination of P-substituted Ethyl on Pd(lll). Experimental Activation Energy kJ/mol

Adsorbate

Substituent

Charge on Substituent

Charge on P-carbon

DFT computed Activation Energy kJ/mol

vinyl fluoride acrolein ethylene

-F

-0.210

+0.135

+90

-

-CHO -H

-0.108 0.098

-0.165 -0.196

+75

+77*

+69

+40to+57§

5

from [1] •estimated from TPD data of Davis and Barteau [44]



239 It is evident,fromTable 2, that the presence of electron-withdrawing substituents raise the activation barrier for the P-hydride elimination reaction. Strong electron withdrawing groups decrease the electron density on the P-carbon atom, making the process of p-hydride elimination (i.e. H transfer to metal) more difficult. This is in consonance with the conclusions of Gellman and co-workers, who observed that substitution of fluorine at the y-carbon of the propyl group increases the barrier for P-hydride elimination over copper, and associated it with the decreased electron density on the p-carbon atom [45]. They found that the barrier increased by 20-30 kJ/mol per fluorine substituent. If we neglect the differences between Cu and Pd, this is in general agreement with our results where there is a 20 kJ/mol increase in the addition of a single fluorine substituent. B. Comparison of Transition States for Different C-H Bond Activation Reactions In the previous section, we demonstrated the noticeable similarities between the transition state structures for P-hydride elimination of ethyl species, with different P-carbon substituents. The trends in the activation barriers were also rationalized based on the electron-withdrawing nature of the substituent groups. Comparison of the p-hydride elimination reaction with other C-H bond activation reactions reported in the literature, such as methane activation on Ni, or acetate C-H bond activation, show outstanding congruence in the reaction pathway and structure of the transition state. To test this assumption and illustrate the resemblance, the transition state structures and activation barriers for different C-H bond activation reactions are compared in this section. For multifunctional adsorbates with C=C and C=0 moieties, analyzing the differences between C=C and C=0 hydrogenation mechanisms is crucial in elucidating the key factors that control hydrogenation selectivity. In an initial effort to understand the differences in hydrogenating these moieties, we examine the mechanism for CO hydrogenation and compare it with that for C=C hydrogenation. DFT optimized transition state structures for the C-H bond activation processes of additional hydrocarbon molecules are depicted in Fig 7. C-H bond activation of acetate is believed to be an important precursor in its decomposition to C02 on metal surfaces such as Pd(lll). By performing detailed reaction coordinate calculations, we have isolated the reaction pathway for this C-H bond activation process [30,31]. The activation barrier for the C-H bond breaking reaction is computed to be about +115kJ/mol, which is considerably higher than that of ethyl on Pd(lll) at +69 kJ/mol. The acetate group is initially bound perpendicular to the Pd(l 11) surface in a di-a conformation. These results are consistent with experimental evidence from HREELS measurements [46]. The reaction pathway first involves the tilting of the acetate group away from the surface normal and in a direction perpendicular to the initial plane of the COO group. When the terminal CH3 group is close enough to the surface, the mechanism is quite similar to ethyl C-H bond activation. The reaction coordinate again involves a C-H bond stretch coupled with the bending mode to bring the CH3 group toward the surface [30,31]. The transition state is late along the C-H bond stretch coordinate. The C-H bond distance at the transition state (1.76 A) is slightly longer than that found for ethyl (1.7 A) on Pd(l 11). The dissociating hydrogen



240

Figure 7. Comparison of transition states for different C-H bond activation reactions. A) ethyl; B) acetate; C) formyl and D) methane.



241 atom is observed to be close to the 2-fold bridge site, similar to that for ethyl, at the transition state. If the CO2" group is viewed as an electron withdrawing substituent, it is easily envisioned why the C-H bond activation process has higher activation energy for acetate, as compared to ethyl. There is also an additional energy expense associated with the significant tilting of the acetate species away from the surface normal. A notable distortion about the O atoms occurs to tilt the acetate species towards the surface. In addition, there is a repulsive interaction associated with bringing a methyl group towards the surface. This repulsive interaction remains high until there is enough energy for electrons from the surface to be donated to the C T C H * orbital. The predicted activation barrier of+115 kJ/mol for C-H bond activation of acetate is a little higher than the overall barrier of +85 kJ/mol measured by Davis and Barteau for the decomposition of acetate [46\. C-H bond activation of methane is another example where the structure of the transition state is likely to be similar to that of ethyl and acetate. Methane activation over supported Ni particles has received considerable attention, due to its relevance in steam reforming. The reaction has been studied extensively on single metal atoms [47-49], well-defined clusters [50-53] and extended slabs [54]. The transition state for the reaction is shown in figure 7. The reaction pathway for C-H bond activation of methane on Ni is very similar to that of ethyl on Pd(l 11) and involves metal insertion into the C-H bond. The transition state is late along the C-H stretch coordinate and occurs at a C-H bond distance of 1.4-1.8 A. The metal-H bonding is also evident at the transition state with metal-H bond distances of 1.7-1.8 A . The C-H bond activation barrier for ethane or methane (+121 kJ/mol) [54] is considerably higher than that of ethyl on Pd(l 11). This is possibly due to the additional stability provided by the CH2 moiety, that anchors the ethyl species to the surface [30]. The hydrogenation of CO to form oxygenate intermediates occurs readily over supported Pd clusters. The initial step of CO hydorgenation to form the surface formyl intermediate has been speculated to be the rate determining in this chemistry. The reaction path for CO hydrogenation involves a CO and hydride migration. Surface CO and atomic hydrogen react either over one or two metal atom centers. The preferred path is shown in Fig 7C. A closer analysis of this reaction indicates that the reverse reaction involves metal insertion into the CH bond. The mobility of the CO intermediate enables the HCO complex to easily shift to a di-a like intermediate bridge complex where the CO group is stabilized by interacting with one metal atom center while the atomic hydrogen can stabilize by interacting with an adjacent metal atom. This helps to lower the barrier. The structure itself is quite similar, but now involves a four-center transition state where both metal atoms contribute to lowering the barrier. Despite these minor changes the mechanism still follows the basic pattern of an agostic C-H bond stretch and the rotation of the hydrocarbon intermediate species toward the surface to help stabilize the C-H bond activation.


242


Conclusions The selective hydrogenation of C=C and C=0 unsaturated adsorbates finds numerous applications in the fine-chemicals synthesis industry. Understanding the mechanistic pathways for selective hydrogenation is crucial in the design of effective catalysts for specific applications. In this paper, we have detailed the reaction pathway for the Phydride elimination of ethyl, which is the microscopic reverse reaction of ethylene hydrogenation. The mechanism involves a coupled C-H stretch and a bending of the C-C-Pd angle. The computed activation barrier of +69 kJ/mol for the P-hydride elimination reaction on Pd(lll) is comparable to the experimentally measured reaction barrier of 40-57 kJ/mol [7]. By studying the effect of substituents on the C=C bond adsorption and ethyl P-hydride elimination, we have speculated a generalized fragment based approach in analyzing activation barriers for geometrically similar reactions. In general, strongly electron-withdrawing substituents tend to raise the activation barrier for p-hydride elimination of P-substituted ethyl species. For instance, the activation barrier for p-hydride elimination of -CH2-CH2F species on Pd(lll) is about 20 kJ/mol higher than that for ethyl. Electron-withdrawing groups also lower the energy of adsorption through the C=C bond for substituted-ethylene. For example, the adsorption energy of vinyl fluoride is about 15 kJ/mol lower than that of ethylene. Finally, we have compared the reaction mechanism and transition state structure for ethyl P-hydride elimination with other C-H bond activation reactions, reported previously in the literature. Comparison of transition states for different kinds of C-H bond formation reactions, brings out the inherent similarities in the reaction mechanism and the structure of the transition state. The mechanism for all the C-H bond activation reactions show metal insertion into the C-H bond and a late transition state along the C-H bond stretch, similar to ethyl P-hydride elimination. These observations seem to suggest an intercomparable mechanism governing a large number of C-H bond activation processes relevant to the chemical process industry. Acknowledgments The authors would like to thank Prof. Jens Norskov, Bjork Hammer and Lars Hansen from the Center for Atomic Scale Materials Physics, Technical University of Denmark, for help with the use of their plane wave pseudopotential program and Dr.George Coulston (DuPont Chemical Company) for helpful discussions. The DuPont Chemical Company and NSF (Career Award CTS-9702762) are also acknowledged forfinancialsupport. References 1. 2. 3. 4. 5. 6.

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243 7. 8. 9. 10. 11. 12. 13.


14.

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

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