First-Principles Periodic Density Functional Study of the

J. Phys. Chem. B 2000, 104, 9449-9459

9449

First-Principles Periodic Density Functional Study of the Hydrogenation of Maleic Anhydride to Succinic Anhydride over Palladium(111) Venkataraman Pallassana and Matthew Neurock* Department of Chemical Engineering, UniVersity of Virginia, CharlottesVille, Virginia 22903 ReceiVed: March 6, 2000; In Final Form: June 27, 2000

Gradient-corrected periodic density functional (DFT) calculations were carried out in order to examine the hydrogenation of maleic anhydride to succinic anhydride via a Horiuti-Polanyi like mechanism on the welldefined Pd(111) surface. Reaction path calculations were performed at a surface coverage of maleic anhydride corresponding to 0.11 monolayer. The hydrogenation of maleic anhydride to the maleic anhydryl surface intermediate was found to have the highest intrinsic barrier (+95 kJ/mol) on Pd(111), of all the elementary steps involved in the hydrogenation of the olefinic group of maleic anhydride. This step is endothermic by 17 kJ/mol. Calculations indicate that the second step in the mechanism, the hydrogenation of maleic anhydryl to succinic anhydride, has a slightly lower barrier (+89 kJ/mol) and is exothermic by 37 kJ/mol. In an effort to understand the effect of change in the electronic properties of the metal on the intrinsic barrier for maleic anhydride hydrogenation, the C-H bond formation reaction was reexamined on Re(0001), PdML/Re(0001), and Pt(111). Among the surfaces studied here, the hydrogenation reaction has the highest barrier on Re(0001) and the lowest barrier on Pt(111). The calculations suggest that the PdML/Re(0001) pseudomorphic overlayer has a lower intrinsic barrier than Pd(111) and Re(0001) for C-H bond formation during maleic anhydride hydrogenation. A model based on frontier orbital theory was developed in order to explain the trends in surface reactivity. As the average position of the d-band of the surface metal layer shifts away from the Fermi-energy, the intrinsic activation barrier for C-H bond formation is lowered. The converse is true for the microscopic reverse reaction of maleic anhydryl β C-H bond activation. Based on the model predictions, the intrinsic barrier for C-H bond formation is expected to be small on the noble metals. However, the group IB metals do not make effective hydrogenation catalysts, because the overall reaction rate is more likely limited by the dissociative chemisorption of dihydrogen, rather than by C-H bond formation.

1. Introduction The selective hydrogenation of olefinic substrates is of fundamental importance in a number of catalytic chemistries.1,2 A large number of experimental studies on ethylene adsorption3 and hydrogenation, on both well-defined single-crystal surfaces4-11 and supported metal particles,12-17 have vastly improved our understanding of olefin chemisorption and the mechanisms for hydrogenation. In recent years, theoretical quantum chemical methods have complemented these efforts by providing new and valuable information on the fundamental chemisorption and reaction of CdC unsaturated intermediates.18-26 It is generally established that the hydrogenation of the ethylenic fragment proceeds through the mechanism proposed by Horiuti and Polanyi.27,28 In this scheme, the olefin and dihydrogen chemisorb on the surface. The CdC bond is then hydrogenated through two successive, metal-mediated, CsH bond formation steps. The final paraffinic product is formed by hydrogenating the surface-bound alkyl intermediate. In the Horiuti-Polanyi reaction mechanism, the C-H bond formation steps are generally believed to be rate-determining. Our primary interest in examining CdC bond hydrogenation is to elucidate the surface reaction pathways that lead to the hydrogenation of maleic anhydride to tetrahydrofuran (THF). Commercially, in the DuPont process, it has been demonstrated * To whom correspondence should be addressed. E-mail: mn4n@ virginia.edu. Fax: (804) 982 2658.

that THF may be synthesized from maleic anhydride by hydrogenation over supported Pd-Re or Ru-Re catalysts.29-31 The hydrogenation reaction can either be carried out in the liquid or vapor phase. To simplify matters, we only consider the vaporphase chemistry. The overall reaction involves the saturation of the ring CdC double bond of maleic anhydride and the hydrogenolysis of the carbonyl (CdO) groups. Understanding the detailed mechanism for this reaction chemistry and the role of the bimetallic surface in catalyzing key steps in the reaction is expected to provide valuable insights into developing improved catalyst formulations. Toward this goal, we have been using nonlocal density functional theory (DFT-GGA) calculations to map out plausible elementary steps for the chemisorption and surface reaction of maleic anhydride on well-defined metal surfaces.32-34 Based on experiment, it is generally established that the hydrogenation of maleic anhydride starts with the saturation of the ring olefinic group. This results in the formation of succinic anhydride, which is an intermediate in the overall hydrogenation chemistry.35-37 In this paper, we examine a detailed pathway for hydrogenation of maleic anhydride to succinic anhydride on the well-defined Pd(111) surface. Detailed reaction coordinate search calculations were used to isolate the transition states for individual elementary surface reaction steps in the mechanism. Finally, we examine the effect of changing the metal on the intrinsic activation barrier for C-H bond formation, to probe the electronic effects of the metal on maleic anhydride initial

10.1021/jp0008703 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000

9450 J. Phys. Chem. B, Vol. 104, No. 40, 2000 hydrogenation activity. Using principles derived from frontierorbital theory38-40 along with a generalized chemisorptionreactivity model developed by Hammer-Nørskov,41,42 we begin to correlate the intrinsic surface reactivity for C-H bond formation with the local electronic properties of the metal surface, viz., the metal d-band position. While the ensuing analysis is primarily carried out for maleic anhydride hydrogenation, we demonstrate the remarkable resemblance in the reaction pathway for maleic hydrogenation with that for ethylene hydrogenation. The discussions are, therefore, likely to have more generalized implications for the hydrogenation of other unsaturated intermediates. 2. Computational Details Nonlocal gradient corrected, periodic density functional (DFTGGA) calculations were used to determine all the structural, electronic, and energetic results discussed in this paper. The Kohn-Sham equations43,44 were solved by using a plane-wave basis set having a maximum kinetic energy of 40 Rydberg. For a few cases it was verified that the total energy of the system was convergent at the chosen cutoff energy of the plane-wave basis set.45,46 The description of atom-centered, core electronic states by plane waves alone would require a huge cutoff energy. Since the electronic states actively involved in interaction with the adsorbate orbitals are in the valence shell, the nuclei and core orbitals may be described by an effective norm-conserving pseudopotential, without inducing significant error in the calculated adsorption and activation energies.45,46 The TroullierMartins norm-conserving pseudopotentials used here include scalar relativistic corrections for the core electrons of heavy metal atoms and were built by performing rigorous all-electron calculations on an isolated atom.46,47 The Vosko-Wilk-Nusair (VWN)48 exchange correlation potential in the local density approximation are augmented by Perdew-Wang (PW91)49 nonlocal gradient corrections in a self-consistent manner. For a (1 × 1) unit cell of Pd(111) and Re(0001), the total energy was found to be convergent by using 54 Chadi-Cohen k-points for sampling the first Brillouin zone.50 Maleic anhydride adsorption on the metal surfaces at low coverage was modeled by using (3 × 3) super-cells, containing 18 metal atoms per unit cell for a 2-layer slab model. For the (3 × 3) unit cells discussed in this paper, the reciprocal unit cell is nine times smaller than the corresponding reciprocal unit cell for a (1 × 1) real-space cell. Six Chadi-Cohen k-points are therefore estimated to be adequate for sampling the Brillouin zone of (3 × 3) super-cells of the Pd and Re surfaces studied in this paper and are used in all our calculations. Electronic occupations were Fermidistributed with an electronic kBT of 0.1 eV to stabilize the convergence scheme. All final energies, however, were extrapolated back to 0 K.45,46 For more details on the calculation scheme, see refs 45, 46, and 51. The most accurate calculations for small adsorbates, at high coverage, generally examine up to five metal layers. To obtain a reasonable description of metal-adsorbate binding at lowto-moderate coverage for large adsorbates such as maleic anhydride, we require unit-cells that contain 3-5 times the number of metal atoms typically used in periodic slab calculations for small molecules. For systems containing such a large number of heavy transition metal atoms, the problem can quickly become computationally intractable. To capture the essential surface chemistry for maleic anhydride, and yet keep the CPU expenditures manageable, we use a fixed two-layer slab model to describe the surface. All structural parameters related to the adsorbate were optimized during the calculations, while the slab itself was constrained to the bulk interatomic distance.

Pallassana and Neurock To estimate the error induced by using a two layered, constrained slab model of the surface, we examined maleic anhydride adsorption on the Pd(111) surface, using a three-layer surface model with the explicit inclusion of surface relaxation effects.34 It was found that the change in the adsorption energy due to allowing surface relaxation is less than 3 kJ/mol.34 Hydrogen chemisorption on Pd(111) was also previously examined, for thickness of the Pd(111) slab varying between two and four metal layers.52 The change in the binding energy of hydrogen by increasing the number of metal layers beyond two was found to be less than 5 kJ/mol.52 These differences in the adsorption energies are within the present-day limits of accuracy of the DFT methodology.53 In a number of cases, we have found that reasonable adsorption energies can be predicted by using the two-layer fixed surface model of a close-packed surface, because the metal atoms, to which the adsorbate is directly bound, have their complete set of nearest neighbors.25,34,54 The two-layer slab models used in this study are therefore expected to provide reasonable energetics for the closepacked surfaces examined in this paper. In periodic slab calculations, 3-dimensional periodicity is maintained by repeating the slabs at regular intervals in the direction normal to the surface. In our calculations, the vacuum region between adjacent slabs is 11 Å thick. We have verified that there are practically no interactions between adjacent slabs for this thickness of the vacuum region. Adsorption energies reported in this paper are calculated by using the equation

∆Eads ) ∆Eslab+adsorbate - ∆Eadsorbate - ∆Eslab where ∆Eads is the adsorption energy, ∆Eslab+adsorbate is the total energy of the optimized adsorbate on the slab, ∆Eadsorbate is the total energy of the geometry optimized adsorbate in the gasphase, and ∆Eslab is the total energy of the bare slab. By the above definition, a negative value for ∆Eads indicates an energetically favorable interaction between the surface and the adsorbate. Activation barriers reported in this paper are calculated as the difference between the total energy of the system at the transition state and the total energy at the reactant state. Transition state geometries were initially isolated by performing detailed reaction coordinate calculations with a Pd(12,7) cluster model. The principle component in the reaction coordinate for C-H bond activation in maleic anhydride hydrogenation involves the stretch of the C-H bond. The C-H bond length was therefore chosen as a trial reaction coordinate to determine the approximate structure of the transition state. The C-H bond distance was constrained to different values between the reactant and product structures, while the rest of the adsorbate structure was allowed to completely relax. The point of maximum energy along this trial reaction coordinate was chosen as an initial guess for detailed transition state search calculations to locate the saddle point. At the optimized transition state geometry, the energy gradients with respect to all nuclear displacements were determined, to verify that the structure is a stationary point on the potential energy surface. Vibrational frequencies were then computed for the transition state geometry to confirm the existence of a negative eigenmode. The presence of an imaginary vibrational frequency helps to confirm that the stationary point is in fact a saddle point on the potential energy surface. The transition states were reoptimized for the periodic slab geometry to determine the reaction energies for a fully periodic surface in the absence of cluster edge-effects.

Hydrogenation of Maleic Anhydride

J. Phys. Chem. B, Vol. 104, No. 40, 2000 9451

CHART 1

TABLE 1: DFT-GGA Computed Adsorption Energies for Maleic Anhydride (Di-σ Mode) and Atomic Hydrogen (3-fold fcc) on Re(0001), Pd(111), and PdML/Re(0001) surface

maleic anhydride chemisorption energy,a kJ/mol

atomic hydrogen binding energy,b kJ/mol

hydrogen dissociative adsorption energy, kJ/mol

Re(0001) Pd(111) PdML/Re(0001)

-200 -84 -72

-288 -267 -227

-139 -97 -18

a Coverage ) 0.11 monolayer. From: Pallassana, V.; et al. J. Phys. Chem. B 1999, 103, 8973. b Coverage ) 0.33 monolayer. From: Pallassana, V.; et al. J. Phys. Chem. 2000, 112, 12.

3. Results and Discussion Detailed theoretical and experimental results over the past two decades have verified that the hydrogenation of ethylene to ethane proceeds via the Horiuti-Polanyi mechanism.6,13,18,23,25,55 The CdC bond hydrogenation of other unsaturated olefins such as maleic acid is also speculated to proceed via this mechanism.56,57 In this reaction path, following the adsorption of maleic anhydride and the dissociative adsorption of hydrogen, succinic anhydride is formed through two sequential C-H bond formation steps. The elementary steps in the reaction mechanism are postulated to be:

MA(g) + * a MA* H2 + 2* a 2H*

maleic anhydride adsorption dihydrogen chemisorption

MA* + H* a MAH* + *

C-H bond formation

MAH* + H* a SA(g) + 2*

C-H bond formation

where MA ) maleic anhydride, MAH ) maleic anhydryl intermediate, and SA ) succinic anhydride. In a recent paper, we used DFT calculations to show that at low coverage the more favorable pathway for ethylene hydrogenation is through the more stable di-σ bound intermediate.26 At higher coverage, the more weakly bound intermediate appeared to be the reactive species for hydrogenation.6,26,58 In this paper, we examine the reaction pathway for maleic anhydride hydrogenation at a relative low coverage of 0.11 monolayer (ML). Since the most favorable chemisorption mode for maleic anhydride on Pd(111) at low coverage is through the ring olefinic group in a di-σ configuration,34 we analyze the hydrogenation pathway starting from the di-σ mode. In sections 3.1-3.3, we will analyze each of these steps on a well-defined Pd(111) surface. Finally, in section 3.4, we will examine the possibility of correlating the intrinsic activation

barrier for maleic anhydride hydrogenation to the bare metal electronic properties. 3.1. Chemisorption of Maleic Anhydride and Hydrogen. DFT-GGA periodic slab calculations were used to examine the chemisorption of maleic anhydride and atomic hydrogen on Pd(111), PdML/Re(0001), and Re(0001) surfaces. We present a detailed discussion of the adsorption modes, energetics, and geometric parameters elsewhere.34,52,59 Since the adsorption of maleic anhydride and hydrogen are relevant elementary steps in the proposed mechanism, we summarize the principal results from the adsorption study in this section. Calculations indicate that there are three primary modes for the adsorption of maleic anhydride on a metal surface.32-34 These are schematically illustrated below in Chart 1. In the η1 (or atop) adsorption mode, the interaction of maleic anhydride with the metal surface is through the ring oxygen atom. In the di-σ and π chemisorption modes, maleic anhydride binds through the olefinic group on the ring. Our calculations suggest that the most favorable adsorption mode for maleic anhydride on Pd(111), at low coverage, is the di-σ mode, with an adsorption energy of -84 kJ/mol.34 This is in good agreement with ultrahigh vacuum (UHV) temperature-programmed-desorption (TPD) estimates of the adsorption energy (-90 kJ/ mol).60,61 The calculated vibrational frequencies for di-σ bound maleic anhydride34 also agree well with the high-resolution electron energy loss spectroscopy (HREELS) measurements of Xu and Goodman.61 DFT calculations suggest that the η1 and π modes have significantly weaker binding energies of -28 and -34 kJ/mol, at low coverage. Since we are primarily interested in examining the hydrogenation mechanism at low coverage in this paper, we investigate the pathway starting from di-σ bound maleic anhydride. Table 1 summarizes the DFT-GGA computed di-σ adsorption energies for maleic anhydride on Pd(111), PdML/Re(0001), and Re(0001).34 Calculations indicate that the chemisorption is strongest on Re(0001) (-200 kJ/mol) and weakest on the PdML/

9452 J. Phys. Chem. B, Vol. 104, No. 40, 2000 Re(0001) (-72 kJ/mol) surface. In a recent paper, we carried out a detailed electronic analysis of maleic anhydride adsorption and demonstrated that the di-σ adsorption is principally controlled by the back-donation of electrons from the metal into the antibonding π* orbital of maleic anhydride.34 Since energetically the antibonding π* orbital of maleic anhydride is located slightly higher than the Fermi energy, the interaction with the filled portion of the metal d-band is stronger when the average position of the metal d-states is closer to the Fermi energy. In reference 52, we demonstrated that the d-band is further away from the Fermi energy for PdML/Re(0001) as compared to Pd(111) and Re(0001), because of strong Pd-Re interactions in the pseudomorphic PdML/Re(0001) surface. This is responsible for the weaker adsorption of maleic anhydride on the PdML/Re(0001) surface.34 In section 3.5, we will carry out a similar analysis to examine the effect of the surface electronic properties on the intrinsic activation barrier for C-H bond formation. The binding of atomic hydrogen on Pd(111), PdML/Re(0001), and Re(0001) exhibits very similar trends to that of maleic anhydride on the various surfaces. The DFT-computed adsorption energies for hydrogen on these surfaces at 0.33 ML coverage are tabulated in Table 1. The chemisorption of hydrogen is strongest on the Re(0001) surface with a binding energy of -288 kJ/mol.59 The adsorption is weakest on the PdML/Re(0001) surface with an adsorption energy of -227 kJ/ mol. The binding of hydrogen on Pd(111) (-267 kJ/mol) is of intermediate strength. The trends in the chemisorption energy correlate with the interaction of the metal d-band with the H 1s orbital state and are discussed in detail in the references.52,59 Analogous to trends in maleic anhydride chemisorption, the farther the d-band center is away from the Fermi energy, the weaker is the adsorption of hydrogen on the surface.59 The coadsorption of atomic hydrogen and maleic anhydride at neighboring sites is required before surface bound hydrogen can insert into the metal-C bond. When molecular and atomic adsorbates are bound to adsorption sites that share metal surface atoms, lateral interactions that occur either through-space or through the metal surface become important. We examined the chemisorption of atomic hydrogen (3-fold fcc site) and maleic anhydride (di-σ bound) in adjacent sites, where the two adsorbates share a single surface metal atom (see topmost panel in Figure 1). For this adsorption geometry, there are weak repulsive interactions between the two adsorbates. This is seen in the molecular geometry as a slight elongation of the Pd-C (∼0.03 Å) and Pd-H (∼0.05 Å) bonds corresponding to the shared Pd atom. The bond elongations reported are with reference to the individual adsorbates bound to the Pd(111) surface in the absence of metal atom sharing. Our calculations indicate that the pairwise adsorption energy is weakened by 21 kJ/mol due to these lateral repulsive interactions. 3.2. Hydrogenation of Maleic Anhydride to Maleic Anhydryl on PD(111). Following the coadsorption of maleic anhydride and atomic hydrogen, maleic anhydride can react with a neighboring atomic hydrogen to form the maleic anhydryl surface intermediate. This involves the insertion of hydrogen into the metal-C bond in order to form a new C-H bond. Periodic DFT calculations were used to isolate the transition state and compute the reaction path for C-H bond formation. The results are presented in Figure 1. The transition state (TS) (Figure 1b) is relatively early along the reaction pathway for C-H bond formation. The C-H bond distance (1.73 Å) is still quite long at the TS in comparison to the product maleic anhydryl state, where the C-H bond distance is about 1.1 Å.

Pallassana and Neurock

Figure 1. DFT computed reaction pathway for the hydrogenation of maleic anhydride to maleic anhydryl on a Pd(111) surface. (a) The reactant state: di-σ bound maleic anhydride and atomic hydrogen (3fold fcc site) coadsorbed on Pd(111). (b) The transition state for C-H bond formation. (c) The product state: η1 bound maleic anhydryl. All distances are reported in angstroms.

The reaction proceeds via the weakening of the Pd-C with the concerted insertion of hydrogen into this bond. The result is a three-center (M-C-H) transition state. The M-C bond length at the transition state is considerably longer (by 0.23 Å) than that of the reactant geometry. The Pd-H bond distance at the TS (1.58 Å) is characteristic of hydrogen bound to a lowcoordination atop site on Pd.62 The CdC bond distance is elongated along the hydrogenation pathway as the predominantly double bonded CdC is transformed to a CsC single bond by hydrogenation. In Figure 2, we have compared the DFT-computed transition state geometry for hydrogenation of maleic anhydride to that determined earlier for ethylene hydrogenation.25,63 The figure shows remarkable resemblance between the transition state geometry for the two reactions. In both cases, the C-H bond distance at the TS is about 1.7 Å. The arrows shown in the figure indicate the normal mode eigenvectors corresponding to



Figure 2. Comparison of DFT-computed transition state structures for (a) ethylene hydrogenation to ethyl and (b) maleic anhydride hydrogenation to maleic anhydryl. Arrows indicate the displacement vectors corresponding to the negative frequency at the transition state geometry. All distances are reported in angstroms.

the imaginary frequency at the transition state geometry. The vectors show a simultaneous movement of the CH2 group away from the surface as the new C-H bond is formed. The movement of the CH2 group is commensurate with the change in hybridization of the carbon center from sp2-like to sp3. The hydrogenation of maleic anhydride to maleic anhydryl on Pd(111) at 0.11 monolayer (ML) coverage is endothermic by 17 kJ/mol. The DFT-computed activation barrier for hydrogenation is +95 kJ/mol. If the hydrogen atom is bound to the 2-fold bridge site, instead of the 3-fold fcc site, at the initial reactant state, the activation barrier for C-H bond formation is lowered from +95 to +57 kJ/mol. The bridgebound hydrogen may therefore be viewed as a precursor state for hydrogenation. The bridge-bound hydrogen species is likely to play an important role for hydrogenation at higher coverage, where lateral repulsive interactions significantly weaken the binding of adsorbates in multifold sites. Since the most stable adsorption site for hydrogen at low coverage is the 3-fold fcc site, in the following discussion we report the activation barrier from this state. In a recent paper, we analyzed the hydrogenation of ethylene to ethyl at low and high surface coverage.25,26 Our calculations showed that at low coverage the activation barrier for the hydrogenation of ethylene to an ethyl intermediate is +88 kJ/ mol.25,26 The reaction was endothermic by +25 kJ/mol.25,26 The intrinsic activation barrier and energy of reaction reported here for maleic anhydride hydrogenation to maleic anhydryl appear to be comparable to that for ethylene hydrogenation to ethyl. The activation barrier for the microscopic reverse C-H bond activation of maleic anhydryl is +78 kJ/mol, which is slightly higher than that for ethyl C-H bond activation (+63 kJ/mol). Kovacs and Solymosi have studied the C-H bond activation of ethyl fragments on a Pd(100) surface using temperature programmed reaction (TPR) spectroscopy.64 Based on their TPR studies, the activation barrier for C-H bond activation of ethyl is estimated to be 40-57 kJ/mol on Pd(100).64 On a more closepacked surface such as Pd(111), the C-H bond activation barrier is expected to be slightly higher and in better agreement with our DFT calculated result for ethyl C-H bond breaking.25,26 The higher barrier for C-H bond activation of maleic anhydryl can be rationalized by observing the structure of surface-bound maleic anhydryl on Pd(111) (Figure 1c). One of the carbonyl groups of the maleic anhydryl intermediate interacts with the surface as can be seen in Figure 1c. This interaction is absent at the reactant and TS geometry. The carbonyl oxygen-

metal distance in maleic anhydryl (2.37 Å) is significantly shorter than in maleic anhydride (2.71 Å). In comparison to ethyl, the elementary C-H bond breaking step in maleic anhydryl involves the additional breaking of this carbonyl group-metal interaction and is most likely the cause of the higher C-H bond activation barrier. The small differences in overall reaction energetics for maleic anhydride and ethylene hydrogenation could also be on account of substituent effects.54 The transition state for C-H bond formation occurs relatively early along this reaction pathway. The electronic effects of the substituent are therefore expected to be very similar at the reactant and transition states, thus having a nominal influence on the forward C-H bond formation barrier. Substituent effects on energetics are expected to be more conspicuous for the microscopic reverse C-H bond breaking step, where the transition state is relatively late along the reaction coordinate. In ref 54 we showed that an electron-withdrawing β-substituent on ethyl raises the activation barrier for β C-H bond activation. This has also been observed experimentally by Gellman and co-workers for the C-H bond activation of substituted propyl species on Cu surfaces.65 The slightly greater barrier for maleic anhydryl C-H bond activation (+78 kJ/mol) as compared to ethyl C-H bond activation (+63 kJ/mol) may also be partly due to the differences in the electronic effects of the substituent. At low surface coverage, the apparent actiVation barrier for hydrogenation would be lowered from the intrinsic actiVation barrier by the adsorption energy for the reactant species. The apparent activation energy for maleic anhydride hydrogenation to maleic anhydryl on a H-covered Pd(111) surface is therefore estimated to be about 32 kJ/mol. At higher surface coverage of ethylene, we find a slight lowering of the intrinsic activation barrier for hydrogenation of di-σ bound ethylene. This is primarily because the metal-H and metal-C bonds are weakened at higher coverage, due to adsorbate-adsorbate repulsive interactions, making the C-H bond formation step slightly more favorable. At higher coverage, however, the more dominant hydrogenation pathway for ethylene is through the π-bound surface intermediate.6,26 We have not explicitly analyzed the hydrogenation of maleic anhydride to succinic anhydride at higher surface coverage in this paper. 3.3. Hydrogenation of Maleic Anhydryl to Succinic Anhydride on PD(111). Maleic anhydryl can subsequently react with a second surface hydrogen atom to form succinic anhydride. This likely requires a similar pathway in which maleic

9454 J. Phys. Chem. B, Vol. 104, No. 40, 2000

Figure 3. DFT computed reaction pathway for the hydrogenation of maleic anhydryl to succinic anhydride on a Pd(111) surface. (a) The reactant state: η1 bound maleic anhydryl and atomic hydrogen (3-fold fcc site) coadsorbed on Pd(111). (b) The transition state for C-H bond formation. (c) The product: succinic anhydride. All distances are reported in angstroms.

anhydryl and atomic hydrogen sit adjacent to one another on the surface, sharing a single metal atom site. Our calculations indicate that there are repulsive interactions for the coadsorption of maleic anhydryl and hydrogen at adjacent sites. The pairwise lateral repulsive interaction energy is estimated to be +30 kJ/ mol, which is slightly (ca. 9 kJ/mol) higher than the repulsive interaction between maleic anhydride and atomic hydrogen (3fold fcc site) adsorbed in neighboring sites. Figure 3 shows the DFT-computed reaction pathway for the hydrogenation of maleic anhydryl to succinic anhydride on Pd(111). This is the second C-H bond formation step in the hydrogenation of maleic anhydride to succinic anhydride via the Horiuti-Polanyi like mechanism. DFT calculations suggest that the reaction again proceeds through a three-centered transition state (TS), with hydrogen insertion into the Pd-C bond. The C-H bond distance at the TS is about 0.2 Å shorter as compared to the TS for maleic anhydride hydrogenation to

Pallassana and Neurock maleic anhydryl. In the path that we examined, the final succinic anhydride product is not directly bound to the surface. We have shown previously that the most favorable chemisorption mode for succinic anhydride is likely through the ring oxygen atom in an atop mode (η1), similar to the η1 adsorption of maleic anhydride (as illustrated in Chart 1).33 The binding energy for succinic anhydride in the atop mode was calculated to be -28 kJ/mol.33 In Figure 4, we compare the DFT-optimized TS for the hydrogenation of maleic anhydryl to succinic anhydride with that for the hydrogenation of ethyl to ethane on Pd(111).25 From Figure 4, it is evident that the TS for the two reactions look very similar. It thus appears that although the substituents may affect the energies for C-H bond formation, the reaction pathway and transition state geometry for hydrogenation of different CdC unsaturated intermediates are likely to be similar. Our calculations indicate that the intrinsic activation barrier for the hydrogenation of maleic anhydryl to succinic anhydride is +89 kJ/mol on Pd(111). The overall reaction energy for the hydrogenation of maleic anhydryl to succinic anhydride is 37 kJ/mol exothermic. The activation barrier is about 18 kJ/mol higher than that computed earlier for ethyl hydrogenation (+71 kJ/mol) on Pd(111).25 This is again most likely because of the interaction of the carbonyl group with the surface for the maleic anhydryl surface intermediate (as seen in Figure 3a). This interaction is absent at the transition state and for the optimized succinic anhydride product. The extra stabilization of the reactant state in maleic anhydryl hydrogenation as compared to ethyl hydrogenation is likely the cause for the increased activation barrier. The activation barrier for the microscopic reverse reaction of succinic anhydride C-H bond activation to maleic anhydryl on Pd(111) is +126 kJ/mol. This barrier is comparable to previously reported barriers for similar C-H bond breaking reactions of close-shelled molecules such as ethane on Pd(111) (+106 kJ/mol)25 and methane on Ni(111) (+121 kJ/mol).66-68 Figure 5 summarizes the reaction energetics for each of the elementary steps in the hydrogenation of maleic anhydride to succinic anhydride over a Pd(111) surface. The elementary step with the highest activation barrier is the hydrogenation of maleic anhydride to maleic anhydryl (+95 kJ/mol). The second C-H bond formation step has a slightly lower intrinsic barrier (+89 kJ/mol). The overall energy liberated in the catalytic cycle for maleic anhydride hydrogenation to succinic anhydride from DFT calculations is 150 kJ/mol (exothermic). This compares favorably with the net heat liberated in the hydrogenation of maleic anhydride to succinic anhydride, based on the standard heats of formation of the reactants and products in the vapor-phase, which is reported as 130 kJ/mol (exothermic).33,69 3.4. Electronic Factors Governing the Intrinsic Reactivity for C-H Bond Formation during Maleic Anhydride Hydrogenation to Maleic Anhydryl. In the preceding sections we examined the hydrogenation of maleic anhydride to succinic anhydride on Pd(111) and postulated a reaction pathway and determined the energetics of the elementary steps involved. Our calculations suggest that the step with the highest barrier for maleic anhydride hydrogenation to succinic anhydride is that which involves the first addition of hydrogen to form the maleic anhydryl intermediate. In an effort to understand how the intrinsic properties of the metal surface might affect catalytic selectivity, we examined the hydrogenation of maleic anhydride to maleic anhydryl on various different surfaces such as Re(0001), Pt(111), and a bimetallic pseudomorphic monolayer of Pd(111) on Re(0001) [PdML/Re(0001)].



Figure 4. Comparison of DFT-computed transition state structures for (a) ethyl hydrogenation to ethane and (b) maleic anhydryl hydrogenation to succinic anhydride. Arrows indicate the displacement vectors corresponding to the negative frequency at the transition state geometry. All distances are reported in angstroms.

Figure 5. DFT-computed reaction energetics for the hydrogenation of maleic anhydride to succinic anhydride on a Pd(111) surface. MA ) maleic anhydride; MAH ) maleic anhydryl; SA ) succinic anhydride. MA*H refers to maleic anhydride and atomic hydrogen coadsorbed on neighboring sites, sharing a surface metal atom. MAH*H refers to maleic anhydryl and atomic hydrogen coadsorbed on neighboring sites, sharing a surface metal atom. Reaction energetics correspond to 0.11 monolayer coverage of maleic anhydride.

The maleic anhydride and hydrogen adsorption energies are representative of M-C and M-H bond strengths for the adsorbates, respectively. Earlier we stated that the chemisorption energy for maleic anhydride and hydrogen on Pd(111), Re(0001), and PdML/Re(0001) (Table 1) correlated quite well with the surface metal d-band center.34,52 We showed that the adsorbatemetal bond strength of both maleic anhydride34 and hydrogen52,59 weaken as the center of the d-band of the metal surface layer shifts away from the Fermi energy. A detailed discussion of metal electronic effects on adsorption is presented in ref 34 and 52. Since the hydrogenation reaction involves the breaking of the metal-C bond in maleic anhydride and the metal-H bond for atomic hydrogen, one would expect an analogous relationship between the C-H bond formation barrier and the surface metal d-band center. In this section, we examine the possibility of establishing such a correlation using frontier orbital theory. Table 2 summarizes the DFT-computed activation barriers and reaction energies for maleic anhydride hydrogenation to maleic anhydryl on Pd(111), Re(0001), PdML/Re(0001), and Pt(111). The table shows that the hydrogenation of maleic anhydride to maleic anhydryl is strongly endothermic on Re(0001) (+68 kJ/mol), while it is moderately exothermic on

Pt(111) (-11 kJ/mol). The reaction energies on Pd(111) and PdML/Re(0001) are +17 kJ/mol and -7 kJ/mol, respectively. The structural parameters related to the reactant, TS, and product on the different metal surfaces were found to be very similar. Structurally similar reactions are often observed to conform to the Evans-Polanyi linear free energy relationship. From Table 2, we see that the calculated activation energies for hydrogenation of maleic anhydride scale with the overall reaction energies. The C-H bond formation reaction has a lower intrinsic barrier on surfaces where the reaction is also more exothermic. The activation barrier for hydrogenation is therefore lowest on the Pt(111) surface (+82 kJ/mol) and highest on the Re(0001) surface (+118 kJ/mol). There is also a significant variation in the activation barriers for the microscopic reaction of β C-H bond activation on the various surfaces. C-H bond breaking appears to be more facile on surfaces such as Re(0001), where the bond-breaking reaction is more exothermic. This is again in agreement with the Evans-Polanyi relationship. By comparing Tables 1 and 2, we see that the hydrogenation of maleic anhydride to maleic anhydryl has a lower activation barrier on surfaces where both maleic anhydride and hydrogen are weakly adsorbed. This is actually quite intuitive. The weaker Pd-C and Pd-H bonds allow for a more facile insertion of H into the Pd-C bond, lowering the activation barrier for hydrogenation. In a previous paper, we examined the hydrogenation and dehydrogenation of ethylene on pseudomorphic overlayers of Pd(111) on different metal substrates.70 We showed that the hydrogenation of ethylene was more favorable on surfaces where both ethylene and hydrogen are relatively weakly bound.70 Our results for maleic anhydride hydrogenation exhibit almost identical behavior. To develop a formal theoretical basis to explain the trends in C-H bond activation energy, we examine the electronic factors that control the C-H bond activation of maleic anhydryl on metal surfaces. Figure 6 shows the electronic density-ofstates (DOS) projected to the maleic anhydryl β-C 2p and H 1s states. The two molecular orbitals of fundamental interest are the σCH and σCH* states. These are the bonding and antibonding orbitals that correspond to the β C-H σ bond. The C-H bond activation reaction proceeds via an agostic stretch of the C-H bond, with little initial participation by the metal surface states. Stretching the C-H bond weakens the overlap of the C sp3

9456 J. Phys. Chem. B, Vol. 104, No. 40, 2000

Pallassana and Neurock

TABLE 2: DFT-GGA Computed Activation Barriers and Overall Energies of Reaction for Maleic Anhydride Hydrogenation to Maleic Anhydryl on Re(0001), Pd(111), and PdML/Re(0001) surface

∆Erxn (hydrogention), kJ/mol

Eact (maleic anhydride hydrogenation), kJ/mol

Eact (maleic anhydryl dehydrogenation), kJ/mol

fa,b

V2 b,c

d-band center,d eV

Re(0001) Pd(111) Pt(111) PdML/Re(0001)

+68 +17 -11 -7

+118 +95 +82 +89

+50 +78 +93 +96

0.6 0.9 0.9 0.9

6.04 2.78 3.90 2.78

-1.16 -1.98 -2.25 -2.70

a d-band filling. b Values are from Ruban et al. (ref 77). c d-band coupling matrix element. d d-band center is for the bare metal surface and is reported relative to the Fermi energy.

[

δEts ) -2 f

V2σCH*

σCH* - d

[

]

+ fSσCH*VσCH* -

]

V2σCH

2 (1 - f ) + (1 + f )SσCHVσCH (1) d - σCH

Figure 6. Frontier orbital interactions in the C-H bond activation of maleic anhydryl on Pd(111). The dark shaded area corresponds to the density-of-states (DOS) projected onto the H 1s state. The lightly shaded region corresponds to the DOS projected to the C 2p orbital. The DOS projected to the d-band of the surface metal atom at the transition state on Pd(111) (solid line) is depicted in the middle and lower panels. The bare surface d-band (dotted line) is shown for comparison.

and H 1s orbitals and the energy difference between the σCH and σCH* states is lowered. When the states are relatively close to the Fermi energy, they begin to interact strongly with the s-, p-, and d-states of the metal, resulting in electron donation and back-donation interactions. The coupling with the sp-band is approximately the same for all the transition metal surfaces. The more dominant differences between transition metals are in the valence d-band. Using frontier orbital theory, the fundamental interactions at the transition state (see Figure 6 mid-panel) can be described by the donation of electrons from the σCH-state to the empty metal states and the back-donation of electrons from the filled metal d-state to the antibonding σCH*-state.70 The surfaces examined here have more filled d-states than empty states. In addition, the average location of the d-band for the metals is closer to the antibonding σCH*-state. Therefore, the primary interaction stabilizing the TS is the back-donation of electrons from the metal to the σCH* state.70 This is very similar to the analysis originally proposed by Hoffmann,71 Baetzold, Shustorovich, and Muetterties72-74 for the activation for methane. Based on frontier orbital theory38-40,75 and the NewnsAnderson chemisorption model,76 Hammer and Nørskov have developed a simplified reactivity model to describe the reactivity of CO, NO, and N2 on metal surfaces.41,42 With this model as a basis, the interaction energy at the transition state for C-H bond activation of maleic anhydryl can be described by eq 142,41

where, f is the d-band filling; V2σCH and V2σCH* are the d-band coupling matrix elements for interaction with the renormalized σCH- and σCH*-states, respectively; SσCH and SσCH* are overlap matrix elements, related to the coupling elements V, by the expression S ) -RV (R is a constant for the metal-adsorbate system); d is the location of the metal d-band center relative to the Fermi-energy; σCH and σCH* are the renormalized energies of the σCH and σCH* orbitals of maleic anhydryl, with respect to the Fermi energy, after interaction with the sp-band of the metal. The values of the f and V2 terms for the different surfaces are tabulated in Table 2.77 The term enclosed by the first set of square brackets is a balance between the rehybridization gain and orthogonalization cost (Pauli repulsion) associated with electron back-donation.42 The term enclosed in the second set of square brackets is the balance of the rehybridization gain and orthogonalization cost associated with electron donation.42 For C-H bond activation of maleic anhydryl on metals having almost completely filled d-states, the dominant contribution is from the electron back-donation term. Since the product fV2 is approximately constant for all the surfaces examined here,42 the Pauli repulsion term is approximately the same. Given these approximations, in the perturbation limit, eq 1 can be simplified to the following expression:

[

∆δEts ) -2 f

V2σCH*

]

(σCH* - d)2

∆d

(2)

If the C-H bond activation barrier is assumed to be proportional to the interaction energy at the transition state (δEts), as defined in eq 1, the activation energies for C-H bond activation on different metal surfaces can be correlated to the surface metal d-band center (d) through eq 2. In Figure 7, we have plotted the activation barrier for C-H bond breaking of maleic anhydryl as a function of the bare surface d-band center for the various metal surface examined in this paper. The near linear correlation between the two parameters verifies the effectiveness of the Hammer-Nørskov analysis41,42 for this reaction system. The small difference between the DFT-computed barrier for Pt(111) and the simplified model predictions is because of the differences in the V2 values of Pt(111) and Pd(111) (see Table 2).41 Earlier we stated that the C-H bond activation of maleic anhydryl on metal surfaces also satisfies the Evans-Polanyi relationship. In Figure 7, the activation barrier for the microscopic reverse reaction of maleic anhydride hydrogenation is also plotted as a function of the surface d-band center. Figure 7 suggests that C-H bond


Figure 7. DFT-GGA computed activation barriers for maleic anhydride hydrogenation (2) and maleic anhydryl C-H bond activation (b) as a function of the surface metal d-band center.

Figure 8. DFT-GGA computed activation barriers for C-H bond formation in maleic anhydride hydrogenation (2) and hydrogen dissociative adsorption (9) as a function of the surface metal d-band center. The activation barriers for hydrogen dissociative adsorption are from Hammer and Nørskov (Surf. Sci. 1995, 343, 211).

formation reactions are favored on metal surfaces where the surface metal d-band center is further away from the Fermi energy. Conversely, C-H bond breaking reactions are more favorable when the metal d-band center is closer to the Fermi energy. The bond-breaking reaction is more facile because the back-donation of electrons to the antibonding σCH* orbital is favored when the d-band is in resonance with this antibonding state. The average position of the d-band (i.e., the d-band center), for transition elements having more than five d-electrons, shifts away from the Fermi energy to lower binding energies as one moves from left to right across the transition metal series.77 The d-band center is thus farthest away from the Fermi energy for group IB metals such as Au, Ag, and Cu.77 On the basis of the Hammer-Nørskov analysis, it appears that the noble metals should have relatively low intrinsic barriers for surface catalyzed C-H bond formation. But it is well-known that metals such as Au are not as effective as group VIII metals for hydrogenation. The primary reason for this is shown in Figure 8. As we move from group VIII to group IB in the periodic table, the valence d-band for the metals becomes completely occupied. Hammer and Nørskov have shown that for noble metals such as Cu and Au, the dissociative adsorption of dihydrogen is a highly activated process and is also endothermic.42,78 The binding of molecular adsorbates is also relatively weak on noble metal surfaces. As a result, even though coupling reactions such as C-H bond formation are likely to have lower intrinsic barriers on Au, Ag, and Cu, they are not effective catalysts for hydrogenation. The reaction rate on these metals is more likely to be limited by the dissociative adsorption of dihydrogen rather than by C-H bond formation. From Figure 8, it is observed

J. Phys. Chem. B, Vol. 104, No. 40, 2000 9457 that the Pd group metals provide relatively low barriers for hydrogenation, while still being able to dissociatively chemisorb hydrogen. The PdML/Re(0001) surface appears to provide a lower intrinsic barrier for C-H bond formation as compared to Pd(111) and Re(0001). 3.5. Effect of Subsurface Hydrogen on Maleic Anhydride Hydrogenation. Experimental evidence presented by Ceyer and co-workers indicates the participation of subsurface hydrogen in the hydrogenation of surface bound methyl and ethylene species over Ni.79,80 Theoretical work by Sautet and co-workers81 and Hu et al.82 have attempted to resolve the role of subsurface hydrogen in the hydrogenation mechanism. The theoretical work suggests that the direct attack of subsurface hydrogen on surfacebound species during hydrogenation is less likely. They suggest that the more favorable mechanism for hydrogenation probably involves the transport of subsurface hydrogen to the surface, followed by surface-catalyzed hydrogenation.81,82 There may be yet another possible situation, with the indirect influence of subsurface hydrogen on hydrogenation kinetics. As compared to the adsorption energy on the pristine metal surface, the binding energy of adsorbates on a Pd or Ni surface is likely to be different, when hydrogen is bound to a nearby subsurface site. Since surface-catalyzed hydrogenation entails breaking of metal-adsorbate and metal-hydrogen bonds, the changes in metal-adsorbate bond strengths, due to the presence of subsurface hydrogen, will most likely affect the thermodynamics and kinetics of the surface hydrogenation steps. In other words, the kinetics of surface hydrogenation steps may be modified, even though there may be no direct participation of subsurface hydrogen in the hydrogenation event. The effect may occur through changes in the metal-adsorbate bond energies. To explore this possibility, we examined the hydrogenation of maleic anhydride to maleic anhydryl over Pd(111) in the presence of latent subsurface hydrogen. The subsurface hydrogen was placed in all of the octahedral subsurface sites. Our DFT calculations suggest that the presence of subsurface hydrogen, at 100% subsurface coverage, significantly weakens the adsorption energy of maleic anhydride on Pd(111) from -84 kJ/mol [for the clean Pd(111) surface] to -48 kJ/mol. The activation energy for hydrogenation of maleic anhydride to maleic anhydryl on Pd(111) is lowered from +95 kJ/mol [over clean Pd(111)] to +56 kJ mol [in the presence of subsurface hydrogen]. This is consistent with our earlier observation that coupling reactions such as hydrogenation are favored when the adsorbates are more weakly bound. Recent preliminary results suggest that the changes in adsorption and activation energies are a more complex function of the location and coverage of subsurface bound species. This will be presented in a forthcoming publication. 4. Conclusions Gradient-corrected density functional theory (DFT-GGA) slab calculations were performed to analyze the detailed mechanism for the hydrogenation of maleic anhydride to succinic anhydride over Pd(111). The reaction was studied for a surface coverage of 0.11 monolayer. The postulated reaction path is analogous to the Horiuti-Polanyi mechanism for ethylene hydrogenation. Calculations indicate that the elementary step with the highest activation barrier on Pd(111) is the initial addition of hydrogen to maleic anhydride to form a maleic anhydryl surface intermediate. This reaction step has a computed activation barrier of +95 kJ/mol and is endothermic by +17 kJ/mol. The hydrogenation of maleic anhydryl to succinic anhydride over Pd(111) has a slightly lower activation barrier of +89 kJ/mol and is exothermic by 37 kJ/mol.

9458 J. Phys. Chem. B, Vol. 104, No. 40, 2000 Toward developing structure-property relationships, the hydrogenation of maleic anhydride to a maleic anhydryl intermediate was reexamined on the well-defined Re(0001), Pt(111), and PdML/Re(0001) surfaces. DFT calculations indicate that the hydrogenation of maleic anhydride has a much lower activation barrier (80-90 kJ/mol) on Pt(111) and PdML/Re(0001). The same step has a very high intrinsic barrier (+118 kJ/mol) on Re(0001). The microscopic reverse reaction of maleic anhydryl C-H bond activation exhibits a completely opposite trend. C-H bond breaking of maleic anhydryl is relatively facile on Re(0001) (∆Eact ) 50 kJ/mol) but has higher activation barriers on Pt(111) (+93 kJ/mol) and PdML/Re(0001) (+96 kJ/ mol). Frontier orbital theory was used to explain the trends in C-H bond activation energies over the different metal surfaces. A detailed electronic analysis was performed to demonstrate that C-H bond activation is primarily controlled by the backdonation of electrons from the metal d-band to the antibonding σCH* orbital of maleic anhydryl. This implies that the closer energetically the average position of the d-band is to the σCH* orbital of maleic anhydryl, the stronger is the back-donation interaction stabilizing the transition state. As a consequence the activation energy for C-H bond breaking is also lowered. The exact opposite appears to be true for the microscopic reverse C-H bond formation reactions. Pt(111) and PdML/Re(0001) surfaces have d-band centers that are further away from the σCH* orbital and therefore exhibit higher activation barriers for C-H bond breaking and lower barriers for hydrogenation. Based on our analysis it appears that group IB metals such as Au, Ag, and Cu are likely to provide lower intrinsic barriers for C-H bond formation. However, these metals are not as good as group VIII metal catalysts for hydrogenation, because the dissociative adsorption of dihydrogen on these group IB metals has a high activation energy. Acknowledgment. We thank Professor Jens K. Nørskov, Professor Bjørk Hammer, Dr. Lars B. Hansen, and the Center for Atomic Scale Materials Physics (CAMP) for use of their plane-wave pseudopotential program DACAPO as well as for helpful technical discussions. Dr. George W. Coulston, Dr. Victor S. Lusvardi, Professor Robert J. Davis, Dr. Jan J. Lerou, Dr. Kathy Saturday, and Dr. Bruce Smart are also thanked for their insightful comments and discussions. The National Science Foundation (NSF-CTS 9702762), the ACS-Petroleum Research Fund (Grant No. 31342G5), and the DuPont Chemical Co. are gratefully acknowledged for financial support. We also kindly acknowledge the NCSA Supercomputing Center at the University of Illinois for computational time. V.P. acknowledges support from the IBM-Minnesotta Shared Research Project, ACS, and NSF (CDA-9502979), through the 1998 IBM Graduate Student Award in Computational Chemistry. References and Notes (1) Bond, G. C. Catalysis by Metals; Academic Press: London, 1962. (2) Ponec, V.; Bond, G. C. Catalysis by Metals and Alloys; Elsevier: New York, 1995. (3) Gates, J. A.; Kesmodel, L. L. Surf. Sci. 1982, 120, L461-L467. (4) Zaera, F. Langmuir 1996, 12, 88-94. (5) Yang, M. X.; Bent, B. E. J. Phys. Chem. 1996, 100, 822-832. (6) Cremer, P. S.; Su, X.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1996, 118, 2942-2949. (7) Cremer, P. S.; Somorjai, G. A. J. Chem. Soc., Faraday Trans. 1995, 91, 3671-3677. (8) Sekitani, T.; Takaoka, T.; Fujisawa, M.; Nishijima, M. J. Phys. Chem. 1992, 96, 8462-8468. (9) Windham, R. G.; Koel, B. E.; Paffett, M. T. Langmuir 1988, 4, 1113-1118.

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