Correlating Electronic Properties of Bimetallic Surfaces with Reaction

J. Phys. Chem. B 2006, 110, 11823-11831

11823

Correlating Electronic Properties of Bimetallic Surfaces with Reaction Pathways of C2 Hydrocarbons A. M. Goda, M. A. Barteau, and J. G. Chen* Center for Catalytic Science and Technology, Department of Chemical Engineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: September 26, 2005; In Final Form: March 31, 2006

The rate and selectivity of chemical reactions on transition-metal surfaces can be controlled by using different bimetallic combinations. The interaction of bimetallic components leads to a change in the electronic properties of the surface, which in turn produces a change in chemical reactivity. In the current paper, we illustrate the correlation of the electronic properties of bimetallic surfaces with the reaction pathways of C2 hydrocarbons. Density functional theory (DFT) was used to study the binding of hydrogen, ethylene, acetylene, ethyl, and vinyl on monometallic and bimetallic transition-metal surfaces. The binding energies of these species were found to correlate with the d-band centers of these surfaces. The binding energies for hydrogen atoms on bimetallic surfaces were lower than for those on the corresponding parent metal surfaces. This trend was consistent for ethylene and acetylene binding. Comparative studies between acetylene and ethylene revealed that acetylene was more strongly bonded to the monometallic and the bimetallic surfaces than was ethylene. Bond order conservation (BOC) theory was used to calculate the activation barriers for ethyl dehydrogenation to ethylene and vinyl dehydrogenation to acetylene. The activation barriers for these reactions were correlated with the surface d-band center of the substrates.

1. Introduction Transition metals have been used as catalysts for many important industrial applications. It has been observed that the catalytic properties of transition-metal surfaces can be altered to markedly different degrees by alloying them with a second metal, that is, by forming a bimetallic catalyst.1 This difference in reactivity occurs because bimetallic surfaces can exhibit quite different catalytic and electronic properties from the parent metal surfaces. A significant amount of work has investigated the catalytic, chemical, and electronic properties of bimetallic surfaces.2-4 Commercial applications include their use as reforming catalysts.1,5,6 Bimetallic combinations of Pt with Re, Sn, and Ir have been successfully used for naphtha re-forming. Bimetallic catalysts have also been investigated for important industrial chemistries such as hydrogenation, isomerization, hydrogenolysis, and oxidation.1 Roberti et al. have examined the use of Ni alloy catalysts for alkane isomerization.7 Their studies indicate that alloying Ni with Cu improves the selectivity for hexane isomerization. Linic et al. have investigated the use of bimetallic catalysts for ethylene epoxidation.8 They have shown by using theoretical calculations and experiments that Cu/Ag bimetallics are more selective for ethylene epoxidation than silver alone. Miyake and Asakawa have studied the use of Pd/Pb and Pd/Bi bimetallics as oxidation catalysts.9 Their studies indicated that adding Pb to Pd drastically increases the selectivity for the production of methacrylate from methacrolein, while adding Bi to Pd increases the activity for production of benzyl acetate from toluene. Conversely, in the case of hydrogenolysis of hydrocarbons, it has been seen that the Cu/Ni and Cu/Ru catalysts exhibit lower activity than either monometallic Ni or Ru.10,11 * To whom correspondence should be addressed. E-mail: [email protected].

There has been substantial research on alloy catalysts for many years, beginning in the late 1940s, with the goal of correlating the electronic and catalytic properties of these systems.3 With the advent of bimetallic catalysts for hydrocarbon re-forming in the petrochemical industry in the 1970s, further interest regarding catalytic properties of the bimetallics was generated.1,3,5,6 In recent years, a significant amount of work has been done using advanced experimental and theoretical methods to correlate the electronic properties of model bimetallic surfaces with their chemical reactivity. Sautet et al. have studied theoretically the electronic structure and local adsorption properties of the Pt80Fe20(111) alloy surface and compared these to the properties of a Pt(111) surface using DFT.12 They observed that iron atoms shift the d-band center away from the Fermi level and reduce the chemical reactivity of surface Pt atoms. Research done by Goodman et al. has shown that the CO desorption temperature in temperature programmed desorption experiments is strongly dependent on the core electron shift of bimetallic surfaces.13 Hammer and Nørskov have used DFT to develop correlations between the electronic and catalytic properties of transition-metal surfaces.14-16 They have convincingly shown that the valance-core level shift of monometallic and bimetallic transition-metal surfaces is related to the d-band center shift. Neurock et al. have shown correlations between the binding energies of ethylene and hydrogen and the d-band centers of bimetallic systems.17,18 Mavrikakis et al. have developed similar correlations for CO, oxygen, and hydrogen.19,20 Kitchin et al. have shown that the surface d-band is broadened and lowered in energy by subsurface 3d metals incorporated in the Pt(111) surface, resulting in weaker binding of hydrogen and oxygen on the bimetallic surfaces.21-24 Previous work has provided insights into the reaction pathways of C2 hydrocarbons on transition-metal surfaces. Neurock and coworkers have studied the reaction pathways and intermediates

10.1021/jp0554689 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/26/2006

11824 J. Phys. Chem. B, Vol. 110, No. 24, 2006 for ethylene dehydrogenation on the Pd(111) surface.18 Their results indicate that the formation of acetylidene (CCH) is unlikely to be directly involved in the formation of ethylidyne (CCH3) from ethylene on Pd (111). Other studies have investigated the stability and reactivity of C2 hydrocarbons on Pt clusters.25 With the use of DFT calculations, it has been shown that the primary reaction pathways for ethane hydrogenolysis on Pt involve highly hydrogenated species, such as C2H5(ads).25 Studies of acetylene adsorption on monometallic Pd, Pt, Ni, and Rh surfaces have indicated that acetylene binds very strongly on the Ni(111) surface compared to the other metal surfaces examined.26 Overall, these studies have demonstrated that to understand the reaction pathways of C2 hydrocarbons on transition-metal surfaces, it is essential to understand the binding of the corresponding surface intermediates. The current work is on investigation of the reaction pathways of the C2 hydrocarbons on several monometallic and bimetallic transition-metal surfaces, with the goal of correlating reactivity with the electronic properties of these surfaces. For this purpose, DFT was employed, beginning with the study of binding of probe molecules and intermediates such as hydrogen, ethylene, acetylene, ethyl, and vinyl on the monometallic and bimetallic systems. The monometallic substrates include closed-packed surfaces of Ni(111), Pt(111), Pd(111), Mo(110), W(110), and Fe(110). The bimetallic surfaces include Pt-Ni-Pt (subsurface Ni in Pt), Ni-Pt-Pt (monolayer Ni on Pt), Pd-Ni-Pd (subsurface Ni in Pd), Ni-Pd-Pd (monolayer Ni on Pd), Pd/ Mo (monolayer Pd on Mo), Pt/W (monolayer Pt on W), and Pt-Fe-Pt (subsurface Fe in Pt). In the cases of Pt-Ni and PdNi bimetallics, our previous studies have shown that Ni tends to migrate beneath the surface.27 After the study of the binding of the various intermediates on these substrates, activation barriers were calculated for selected C2 reactions. Finally, these barriers were correlated with the electronic properties of the substrates, represented by the d-band center. 2. Computational Methods 2.1. Density Functional Theory. DFT was employed to study the binding of atomic hydrogen and C2 intermediates on transition-metal surfaces. Self-consistent periodic slab calculations were carried out on the basis of gradient-corrected DFT to obtain all the theoretical results in this paper. The DACAPO code, developed at the Technical University of Denmark, was employed for all calculations.28 DACAPO is a total energy program that uses planewave basis sets to expand the valence electronic orbitals and describes the core electrons using Vanderbilt ultrasoft pseudopotentials. Calculations in DACAPO are performed using periodic boundary conditions. The adsorption of atomic hydrogen and the C2 intermediates was studied using 2 × 2 supercells that contained three atomic layers. The top layer was allowed to relax in each case. Previous work has shown that three metal layers are adequate to describe all the structural effects including surface segregation and surface relaxation.17,18,21-23,29 These studies showed that the binding energy difference was less than 10 kJ/mol when the number of layers was increased beyond three. Hence, the use of a three-layered surface was found to be adequate and efficient for the current work, which focuses on the general trends between different surfaces. A vacuum region of width equivalent to that of five metal layers was used to separate the slabs in order to avoid any electronic interactions between them. Because of the significantly greater computation time that would be required by the use of larger unit cells, we have utilized the 2 × 2 unit cell (a coverage of 1/4 ML) in our current

Goda et al.

Figure 1. DFT-calculated d-band centers using short cutoff radius vs infinite cutoff radius.

study. One of the justifications for use of the 2 × 2 unit cell (a coverage of 1/4 ML) was based on the coverage variation studies previously performed by Neurock and co-workers. For example, Neurock and van Santen carried out calculations using a (x3 × x3) R30° unit cell (a coverage of 1/3 ML), where repulsive interactions between neighboring ethylenes were observed on Pd(111).29 These authors then used the 2 × 2 unit cell to eliminate the repulsive interactions. In addition, Pallassana et al. have carried out calculations for all the C2Hx (x ) 1-5) intermediates on a 2 × 2 unit cell.17 Code input parameters were set according to the previous results.21-23 A planewave cutoff of 340 eV was found to be suitable for hydrogen and C2-species binding calculations. In accordance with our previous work, 18 Chadi-Cohen special k-points were found to be adequate, and maximum symmetry was applied to reduce them.21-23 An electronic temperature (kbT) of 0.2 eV was used with 15 additional bands to ensure sufficient vibrational freedom. The final total energy was extrapolated back to absolute zero. The PW91 generalized gradient approximation functional was used as the self-consistent exchange correlation functional. Calculations for gas-phase species were carried out implementing spin polarization, while the adsorbate-metal system calculations were carried out spin unpolarized. The neglect of spin effects will usually introduce only small errors into the calculated binding energies, but the deviations could be larger for metal surfaces containing significant quantities of Fe or Ni. More computationally intensive calculations are needed to determine the effect of spin polarization on the Fe or Ni surfaces. The electronic property used to correlate with the surface reactivity was the d-band center. The d-band center is defined as the average energy of the d-band and is generally calculated as the first moment of the projected density of states (DOS) on the surface atoms with reference to the Fermi level. In the case of DACAPO, one electron wave function can be projected onto spherical harmonic orbitals. However, the radial component of the atomic orbitals can be infinite, which might cause problems of overlap between the orbitals. To avoid this, a cutoff radius is defined. No contributions beyond this cutoff radius are included. We have compared calculations for both short and infinite cutoff radii. Figure 1 compares the d-band centers calculated for infinite cutoff radius versus those for short cutoff radius (1 Å) for a series of transition-metal and bimetallic surfaces. The approximately linear correlation between the results of the two calculation methods suggests that for purposes of predicting trends in surface reactivity, it should not matter

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which cutoff radius is used as long as it is used consistently. Henceforth all results are reported for infinite cutoff radius calculations. 2.2. Bond Order Conservation (BOC) Theory. In our previous studies, we observed that the hydrogenation activity of several linear and cyclic alkenes is related to the strength of hydrogen-binding energies on various monometallic and bimetallic surfaces.30,31 The correlation between hydrogen-binding energy and hydrogenation activity is clearly an oversimplification. We use BOC analysis in the current study to understand the correlation between activation barriers and the hydrogenbinding energies on different surfaces. Bond order conservation Morse potential (BOC-MP), also known as the unity bond index-quadratic exponential potential (UBI-QEP) theory32-34 was used for the calculation of activation barriers. This theory is based on the bond order conservation postulate. It states that as a bond is stretched, its bond order decreases and finally becomes zero when the bond is broken. Also, simultaneously a new bond starts forming whose bond order increases from zero to a final value. According to bond order conservation, during this bond breaking and forming process; that is, as the molecule moves along the reaction coordinate, the total bond order is conserved.34 According to BOC, the single minimum pairwise interaction potential is expressed as a polynomial function of the bond order. In fact, the multibody potential energy can be expressed as the sum of pairwise interactions, according to the bond order conservation rule. BOC theory can be used to calculate many important physical quantities including estimation of the activation barriers for surface reactions. Bond order conservation was used to calculate activation energies for dehydrogenation of adsorbed ethyl groups to ethylene and of vinyl groups to acetylene. These steps can be written as

EAB )

[

EAB )

]

QAQB - QA - QB QA + QB

Ef ) EAB - (-DAB) + QAB ) DAB +

QAQB + QA + QB QAB - QA - QB (6)

The heat of chemisorption is defined as

∆Hrxn ) DAB + QAB - QA - QB

AB* + * T A* + B* For this type of reaction, the activation barrier is calculated using a variational procedure and interpolation.34 According to the UBI-QEP model, the expression for the interaction energy between a metal surface and a species AB is given as

EAB ) QA(2xA - xA2) + QB(2xB - xB2) + DAB(2xAB - xAB2) (1) subject to the constraint that

(2)

Also, the sum of the atom-surface energies of noninteracting adatoms, A and B, is expressed according to BOC as

(3)

This is not subject to any conservation constraints since there are no bonds. Minimizing EAB with respect to xA and xB gives

(7)

Hence,

Ef )

QAQB + ∆Hrxn QA + QB

(8)

To avoid overestimation of the actual activation barrier, the midpoint of the energy is chosen.34 Thus,

QAQB + 0.5∆Hrxn Ef ) 0.5 QA + QB

Both reactions are of the form

(5)

According to BOC, the activation barrier, Ef, for the forward reaction can be expressed as the difference between EAB and the gas-phase energy of the AB species. Hence,

C2H3* + * T C2H2* + H*

EA+B ) EA + EB ) QA(2xA - xA2) + QB(2xB - xB2)

[

This expression corresponds to the Leonard-Jones maximum, that is, the intersection of the dissociation energy curve and the A + B chemisorption energy curve. Since xAB is zero along the A + B chemisorption curve, at this intersection point it should be zero, too. Hence, EAB can be expressed as

C2H5* + * T C2H4* + H*

xA + xB + xAB ) 1

]

QAQB 2QAQB + DAB xAB2 + - DAB xAB + QA + QB QA + QB QAQB - QA - QB (4) QA + QB

(9)

The most critical inputs to the BOC theory are therefore the heats of chemisorption. The accuracy of the final activation barrier is directly dependent on the accuracy of the heats of chemisorption. We have used the heat of chemisorption values obtained by DFT as an input. This reduces many approximations encountered in the BOC theory when calculating heats of chemisorption. Also, it is more efficient to use DFT to calculate heats of chemisorption values as inputs for the BOC model rather than using DFT to compute the actual activation barrier values because the latter is highly computationally expensive. It is important to point out that the BOC formalism assumes a linear or nonlinear relationship between kinetics and thermodynamics of elementary reaction steps. As shown in eq 9, in the current study we adopted a linear relationship in the BOC analysis to qualitatively demonstrate the correlation between the activation barriers and binding energies. 3. Results 3.1. DFT Results for Hydrogen. Atomic hydrogen was chosen as the first probe adsorbate for studies correlating electronic and catalytic properties. The gas-phase energy for a hydrogen atom calculated using DFT (spin polarized) was found to be -13.68 eV. As far as the hydrogen-metal system is concerned, hydrogen was adsorbed at the 3-fold binding site of the fcc(111) surfaces and the quasi 3-fold site of the bcc(110) surfaces. (Figure 2a). This has been found to be a stable

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Figure 2. DFT calculations for hydrogen: (a) hydrogen-binding configuration on Pd(111); (b) hydrogen-binding energy vs surface d-band center for monometallic and bimetallic systems.

adsorption site for hydrogen.20,22,35 The coverage of atomic hydrogen was 0.25 ML on all surfaces. The adsorbate-slab system was then allowed to relax to its minimum energy configuration. The binding energy was calculated as the difference between the adsorbate-slab total energy and the sum of the total energy of the gas-phase adsorbate and the bare slab. Figure 2b shows the plot of hydrogen-binding energy (HBE) for various metal substrates as a function of their surface d-band centers. It can be seen that the HBE varies approximately linearly with the d-band center for the substrates studied. This is consistent with previous studies of HBE on various surfaces.20,22,35 It is clear from Figure 2b that, as the d-band center moves closer to the Fermi level, the hydrogen-binding energy increases. In general, when hydrogen adsorbs on the metal substrate, there are two major contributions to its binding. One contribution is from the s and p states of the metal, which is approximately the same for each transition metal. The second and the most important contribution is due to the interaction of H with the metal d-bands, whose contribution is different for different transition metals. This sensitivity may arise because the d-bands are narrow and are affected by small perturbations in the environment, while the s and p orbitals are broad and structureless.19 Hence, trends in binding energies can be attributed mainly to the coupling of adsorbate orbitals with the d-electrons of the metal. It can be also observed from Figure 2b that the thermodynamically stable bimetallic surfaces (Pt/W, Pt-Fe-Pt, Pd/ Mo(110), Pt-Ni-Pt, Pd-Ni-Pd) exhibit lower hydrogenbinding energies than the corresponding monometallic surfaces. For example, calculations for the Pd/Mo(110) bimetallic system

Goda et al. predict that the hydrogen-binding energy on a Pd monolayer on the Mo(110) surface will be significantly lower than that on either Pd(111) or Mo(110). This is consistent with experimental observations.36 It was shown that H2 desorbs from the 1.0 ML Pd/Mo(110) surface at 238 K, while the desorption temperature from clean Mo(110) was 400 K and from clean Pd(111) was 350 K. One can also see from Figure 2b that the HBE on the bimetallic Pt/W(110) surface is significantly lower than that on W(110) or Pt(111). This trend has also been confirmed experimentally.36 It was shown that H2 desorbs from the 1.0 ML Pt/W(110) surface at 142 K, while the desorption temperatures were 407 K from a clean W(110) surface and 300 K from a clean Pt(111) surface. For the Ni/Pt system, it has been observed experimentally that hydrogen atoms bind more weakly on the 1.0 ML Pt-Ni-Pt surface than on either Pt or Ni.22 When Ni is adsorbed on a Pt(111) surface and annealed, a significant fraction of the Ni atoms reside in subsurface sites of the 1.0 ML Ni/Pt(111) surface.22 As previously demonstrated, the location of the Ni atoms has a strong effect on the HBE. As illustrated by the results of the DFT calculations in Figure 2b, the HBE on a Ni monolayer on Pt(111) (designated Ni-Pt-Pt) is higher than that on either Ni or Pt, whereas for subsurface Ni (designated Pt-Ni-Pt), it is lower than on either Ni or Pt. For monometallic surfaces with the fcc structure, the trend in hydrogen-binding energies calculated is Pt < Pd < Ni. This is consistent with the results reported by Greeley and Mavrikakis.20 As one moves down the columns in the periodic table, the hydrogen-binding energy decreases. That means that 5d metals are more noble in character than the 4d metals which are in turn more noble than 3d metals. It is also clear that hydrogen-binding energy follows the trend of Pt < W and Pd < Mo. This means that as one moves farther to the left in the periodic table, the bond strength increases as expected qualitatively and as demonstrated by the DFT calculations of Hammer and Norskov.15 3.2. DFT Results for Ethylene. The adsorption of ethylene on various metal substrates was studied using periodic DFT calculations. The spin polarized gas-phase energy for an ethylene molecule was calculated to be -374.54 eV, using the spin multiplicity of 2. Ethylene was di-σ bonded to the metal surfaces (Figure 3a). The di-σ configuration has been reported as the most favorable adsorption state for ethylene on most transitionmetal surfaces.17,18,29 In this configuration, there is an interaction between the d-bands of the metal surface and the antibonding orbitals of the ethylene molecule. Ethylene shares its π electrons with the surface, and there is a flow of electrons into the antibonding orbitals of ethylene because of back-donation by the metal surface.15,17 Thus the extent of interaction with the d-orbitals determines the binding energy.36 Furthermore, as pointed out by Norskov and co-workers, the extent of interaction varies for the 3d, 4d, and the 5d metal surfaces, resulting from the Pauli repulsion of the electronic states between the metal surface and ethylene.15 Eventually, the kinetic energy is increased by an amount proportional to the square of the adsorbate-metal d-coupling matrix element, Vad2.15,37 This coupling element will vary because the 5d orbitals are more extended than the 4d orbitals which are in turn more extended than the 3d orbitals. The Pauli repulsion between electronic states of the metal surface and ethylene increases with the overlap. This effect is not as pronounced in the case of simple atomic adsorbates such as hydrogen, but must be taken into account when dealing with molecules such as ethylene,

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Figure 3. DFT calculations for ethylene: (a) ethylene binding configuration on Pd(111); (b) ethylene binding energy vs surface d-band center for monometallic and bimetallic systems. Three lines are drawn to show general trends for 3d, 4d, and 5d systems.

Figure 4. DFT calculations for acetylene: (a) acetylene binding configuration on Pd(111); (b) acetylene binding energy vs surface d-band center for monometallic and bimetallic systems.

since molecular adsorption is much more complicated than atomic adsorption.36,38 Figure 3b shows the binding energy of ethylene on various metal substrates versus the surface d-band center. If all the results are considered together, there is at best a qualitative correlation of ethylene binding energy with d-band centers. However, within each row of surface metals (3d, 4d, or 5d) there is a fairly linear relationship between ethylene binding energy and the position of the d-band center. Overall the trend in Figure 3b shows that the binding energies of ethylene on the bimetallic surfaces (Pt-Ni-Pt, Pt/W, Pt-Fe-Pt, Pd-Ni-Pd, Pd/Mo) are less than those on the corresponding parent metals, consistent with the trend previously noted for hydrogen binding on these surfaces. Ethylene binding on bimetallic Pd/Mo was found to be much weaker than on either pure Pd(111) or pure Mo(110). This is consistent with experimental results.36 The results obtained for ethylene on the Pt/W(110) surfaces36 were also consistent with the DFT calculations; that is, the binding of ethylene on the bimetallic Pt/W(110) surface was much weaker than on either of the parent metals. The same is true for the Pt-Ni-Pt bimetallic surface. Experimental results clearly show that the ethylene desorption temperature is much less for the monometallic Pt(111) and Ni(111) surfaces than for the Pt-Ni-Pt bimetallic surface.39 It was also concluded from the experimental studies that activity toward ethylene decomposition on Ni/Pt bimetallics is much lower than that on either pure Ni or Pt. 3.3. DFT Results for Acetylene. The adsorption of acetylene on transition-metal surfaces was also studied using DFT. In the

case of ethylene, it was apparent that molecular adsorption is more complex than atomic adsorption, requiring the establishment of separate trends for 3d, 4d, and 5d surfaces. After completing these ethylene studies, we selected a few representative surfaces from each group (3d, 4d, and 5d) and used them as model surfaces for our studies on acetylene. One might expect that the binding of acetylene would be similar to that of ethylene, necessitating separate correlations for surfaces representing different rows of the periodic table because of different extents of Pauli repulsion. The substrates considered were Pt(111), Ni(111), Pd(111) and their bimetallic combinations. The gas-phase energy for acetylene was calculated to be -340.46 eV. This gas-phase calculation was carried out spin polarized with a spin multiplicity of 1. The coverage of acetylene was taken to be 0.25 ML. Acetylene was adsorbed in a 3-fold hollow adsorption site on the metal surface (Figure 4a). In this configuration, each carbon atom in acetylene is bonded to two adjacent surface metal atoms. When the molecule is oriented in this fashion over the metal surface, it can act as a four-electron donor because of the extra π-bond. Acetylene binding energies were calculated for monometallic and bimetallic surfaces and are plotted against the corresponding d-band centers in Figure 4b. As with the other adsorbates, the acetylene binding energy increases as the d-band center moves closer to the Fermi level. The number of surfaces considered was insufficient to establish separate trends for 3d, 4d, and 5d surfaces. However, among the limited number of substrates studied, the 5d (Pt) surfaces appear to be distinguishable from those with 3d or 4d metals (Pd or Ni) in the topmost layer.

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Figure 5. DFT calculations for ethyl: (a) ethyl binding on configuration Pd(111); (b) ethyl binding energy vs surface d-band center for monometallic and bimetallic systems. Three lines are drawn to show general trends for 3d, 4d, and 5d systems.

It can be seen from the figure that acetylene binds very strongly to Ni(111) as compared to both Pd(111) and Pt(111) surfaces. This is consistent with experimental data reported in the literature.26 Allendorf and Medlin have studied acetylene adsorption on these surfaces. Their studies indicate that acetylene binds in the µ-bridge configuration on Ni(111), and the high degree of overlap of Ni 3s orbitals with acetylene antibonding orbitals in this configuration is responsible for the high acetylene adsorption energy.26 Experiments done by Vattuone et al.40 have shown the same trend for (100) surfaces. They have shown that acetylene binds much strongly on Ni(100) than on Pd(100). It was observed that acetylene molecules do not decompose on the Pd(100) surface as they do on Ni(100) but are adsorbed in a strongly hybridized state.40 It is also apparent from Figure 4b that the binding of acetylene on the thermodynamically stable bimetallic surfaces is weaker compared to that on corresponding monometallic surfaces. This is consistent with the trends observed for hydrogen and ethylene. 3.4. DFT Results for Ethyl and Vinyl Species. DFT calculations were also performed for ethyl and vinyl species on metal surfaces. The ethyl group forms a single σ bond with a single surface metal. It prefers to bind atop the metal adsorption site (Figure 5a). As pointed out by Pallasana et al.,18 it does so to form an ethane-like intermediate to preserve its sp3 configuration (tetrahedral geometry). Thus, the surface metal site serves as the missing hydrogen atom in ethane. In this configuration, the ethyl group binds to the metal surface through electron donation.

Goda et al.

Figure 6. DFT calculations for vinyl: (a) vinyl binding on configuration Pd(111); (b) vinyl binding energy vs surface d-band center for monometallic and bimetallic systems.

Experimental studies have shown that it is not very easy to form an alkyl-metal bond by alkane dissociation. This reaction is activated on transition-metal surfaces, and it is therefore difficult to study the binding of alkyl ligands experimentally. The formation of alkyl groups on metal surfaces has been accomplished using oxidative addition of alkyl halides to metal centers, but the presence of residual halogen atoms may alter the chemistry of the surface.41 Hence, theoretical calculations may be the best tools to study the binding of these ligands on metal surfaces. The spin polarized gas-phase energy for a free ethyl radical was calculated to be -389.53 eV. The spin multiplicity was doublet for this case. The binding energy of this ligand was calculated on several transition-metal and bimetallic surfaces and is plotted against the d-band center of the substrates (Figure 5b). Within each row (3d, 4d, or 5d) there is a fairly linear relationship between ethyl binding energy and the position of the d-band center. This is consistent with the results obtained for ethylene. The ethyl binding energy increases as the d-band center moves closer to the Fermi level within each row. This is also consistent with previous calculations for ethyl binding on pseudomorphic Pd monolayers.17 Within a limited set of calculations, a similar trend appears to hold for the binding of vinyl ligands. Figure 6a shows the vinyl structure on the fcc(111) surfaces. Neurock et al. have referred to this structure as η1η2 binding and have shown it to be the most stable configuration for vinyl on the Pd(111) surface.17 Calculations were done for the binding energies of vinyl radicals on transition-metal surfaces using DFT. The gas-phase

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Figure 7. DFT-calculated binding energies for ethylene and acetylene vs surface d-band center for monometallic and bimetallic systems.

TABLE 1: Carbon-Carbon (C-C) and Carbon-Metal (C-M) Bond Lengths (Å) for Ethylene and Acetylene Binding on Transition-Metal Surfaces vs Binding Energy (eV) ethylene substrates Ni(111) Pd(111) Pt(111) Pt-Ni-Pt Pd-Ni-Pd

acetylene

C-C ∆ (C-C) C-M BE C-C ∆ (C-C) C-M BE 1.45 1.43 1.48 1.46 1.415

0.124 0.102 0.149 0.13 0.085

2.19 2.18 2.15 2.20 2.27

0.62 0.85 1.03 0.57 0.52

1.40 1.35 1.38 1.37 1.34

0.19 0.14 0.17 0.16 0.13

1.38 1.60 1.50 1.63 1.64

2.64 1.88 2.11 1.54 1.44

energy of the vinyl group was calculated to be -355.54 eV. This calculation was carried out spin polarized with doublet spin multiplicity. Figure 6b shows a plot of the binding energy of vinyl species against the d-band center of the substrates. It can be seen that the binding energy decreases as the d-band center moves away from the Fermi level. This is consistent with the results obtained for all the other species studied here. Pallasana and Neurock have established a similar correlation for vinyl species on pseudomorphic monolayers on Pd(111).17 4. Discussion 4.1. Ethylene versus Acetylene. Figures 3b and 4b illustrate the similar trends between the binding energies and surface d-band centers for ethylene and acetylene. However, it is important to recognize the significant difference in the binding energy scales of these two figures. Figure 7 shows the relative binding energies of ethylene and acetylene on the (111) surfaces of Pt, Ni, and Pd and their bimetallic surfaces on a common scale. It is immediately apparent that the overall variation in ethylene binding energies is much less than that of acetylene binding energies over a common set of metal surfaces. As noted earlier, whenever an unsaturated species such as ethylene or acetylene is bonded to a transition-metal surface, there is an interaction between the metal d-orbitals and the antibonding orbitals of the adsorbing species. This leads to donation and back-donation of electrons between the d-orbitals and the antibonding orbitals of adsorbate. Because of this process, there is a bond formation between the carbon atoms and the metal, which in turn weakens the C-C bond in adsorbed ethylene or acetylene species. The degree of distortion in the C-C bond is directly related to the strength of the metal-carbon bond. Table 1 shows the C-C and the C-M bond lengths calculated for acetylene and ethylene on the transition-metal surfaces in this work. When acetylene is adsorbed on the Ni(111) surface, the C-C bond length is calculated to be 1.40 Å and the C-Ni bond length

is 1.38 Å. In the case of ethylene, the C-C bond length is 1.45 Å while the C-Ni bond length is 2.05 Å. Since the C-C double bond length for the free ethylene molecule is 1.33 Å while the C-C triple bond length of acetylene is 1.21 Å, it is apparent that the lengthening in C-C bond for acetylene on Ni (0.19 Å) is larger than that of ethylene (0.12 Å). Thus, relative to the respective free molecules, the C-C bond is more elongated and weakened in adsorbed acetylene than in adsorbed ethylene. The C-Ni bond length for acetylene is less than that for ethylene. This confirms the weaker interaction of ethylene with Ni(111) compared to that of acetylene. This result is consistent with the calculations performed by Fahmi and van Santen.42 They have attributed this behavior to the presence of two active orbitals on acetylene that allow extra donation and back-donation. The result is that acetylene binding is more strongly dependent on the electronic properties of the surface than ethylene binding. This difference is also illustrated by the results for the Pd(111) surface. When acetylene is adsorbed on Pd(111), the change in the C-C bond length is 0.14 Å and that of the ethylene C-C bond is 0.10 Å. The carbon-metal bond distance is 1.60 Å in the case of acetylene while it is 2.18 Å for ethylene, again indicating that the binding of acetylene to the surface is stronger than that of ethylene. This is consistent with the experimental results obtained by Mittendorfer et el.43 They have found that the work function is decreased by a value of ∆Φ ) -1.16 eV after ethylene adsorption on Pd(111), whereas it is decreased by -1.31 eV after the adsorption of acetylene. On the basis of these values, they have concluded that the charge transfer from the molecule is more pronounced for the adsorption of acetylene than for that of ethylene on Pd(111). A similar observation was made for Pt(111). The work function decreased by -1.51 eV for ethylene on Pt(111), whereas it decreased by -1.77 eV for acetylene indicating that acetylene exhibits stronger binding.43 This is consistent with the results in Figure 7 and explains the lower carbon-metal bond distance and higher C-C distortion for acetylene as compared to that for ethylene on Pt(111) in Table 1. As shown in Figure 7, acetylene also binds more strongly on bimetallic surfaces with subsurface monolayers than does ethylene. In the case of Pt-Ni-Pt, the change in the C-C bond length is larger for acetylene (0.16 Å) than for ethylene (0.13 Å). The C-M bond length is likewise smaller for acetylene (1.63 Å) than for ethylene (2.2 Å). This explains the higher binding energy value for acetylene than for ethylene on PtNi-Pt. This trend remains the same for the subsurface PdNi-Pd surface. There the C-M bond distance for ethylene is 2.27 Å while that for acetylene is 1.64 Å. Also, the elongation is greater for acetylene on Pd-Ni-Pd (0.13 Å) than for ethylene (0.09 Å). The results in Table 1 indicate that one can correlate the binding energy for a particular species on a metal surface to the carbon-metal bond distance. For both acetylene and ethylene, the binding energy decreases as the carbon-metal bond distance increases. One can also see that acetylene is bonded most strongly on Ni(111), which has the shortest metalcarbon bond distance (1.38 Å), while it is bonded weakly on Pt-Ni-Pt and Pd-Ni-Pd surfaces where the bond distances exceed 1.6 Å. Similarly, ethylene is bonded most strongly on the Pt(111) surface where the carbon-metal bond distance is 2.15 Å, while it is bonded most weakly on Pd-Ni-Pd where the metal bond distance is 2.3 Å. It is also apparent from Table 1 that the variation in the ethylene-metal bond distances is much less than that in acetylene-metal bond distances, just as

11830 J. Phys. Chem. B, Vol. 110, No. 24, 2006

Goda et al.

Figure 8. BOC-calculated activation barriers for dehydrogenation of ethyl to ethylene as a function of (a) surface d-band center, (b) heat of surface reaction, and (c) hydrogen-binding energy.

Figure 9. BOC-calculated activation barriers for dehydrogenation of vinyl to acetylene as a function of (a) surface d-band center, (b) heat of surface reaction, and (c) hydrogen-binding energy.

the binding energy variations are much smaller for ethylene than for acetylene in Figure 7. 4.2. Ethyl Dehydrogenation to Ethylene. Activation energies for the dehydrogenation of ethyl to ethylene on monometallic and bimetallic surfaces were calculated using BOC. The inputs to the BOC theory were the heats of adsorption calculated using DFT, and the output was the activation barrier. Dehydrogenation of ethyl can take place via the elimination of a β-hydrogen atom to form ethylene or via elimination of an R-hydrogen atom to form ethylidene. These two reaction sequences can be represented as

As demonstrated by Figures 3b and 5b, the binding energies for ethylene and the ethyl group follow similar trends with changes in the surface d-band center. As a result, in the determination of the heat of reaction or the activation barrier for dehydrogenation, the binding energies for the adsorbed organic reactant and product will tend to cancel each other, yielding enthalpy changes that are dominated by the strength of the hydrogen atom binding to the surface. Figure 8c shows a plot of the activation barrier for dehydrogenation versus the HBE for different surfaces. There is a fairly linear correlation between the two quantities. The activation barrier decreases as the hydrogen-binding energy increases, that is, as dehydrogenation becomes more thermodynamically favorable. Therefore, the activation barrier on Pt(111) is higher than that on Pd(111) which in turn is higher than that on Ni(111). In addition, the subsurface bimetallic surfaces give rise to higher activation barriers compared to the monometallic surfaces, consistent with the lower hydrogen-binding energies on these surfaces. 4.3. Vinyl Dehydrogenation to Acetylene. The BOC model was also used to calculate the activation barriers for vinyl dehydrogenation to acetylene. Like ethyl dehydrogenation to ethylene, this reaction also occurs via the elimination of a β-hydrogen, in this case forming acetylene on the surface. As in case of ethyl dehydrogenation, there is also a possibility of R-hydrogen elimination to form vinylidene (CH2dC), which is not always desirable. The inputs to the BOC model were the heats of adsorption values calculated using DFT. Figure 9a shows a plot of activation barrier for vinyl dehydrogenation to acetylene as a function of the d-band center. The activation barrier varies fairly linearly with the d-band center of the substrate. As the d-band center moves closer to the Fermi level, the ease of dehydrogenation increases. This is consistent with the results obtained for ethyl dehydrogenation to ethylene. Activation barriers for vinyl dehydrogenation are also shown as a function of heats of the surface reaction in Figure 9b, which illustrates that the trend is the same as the one observed for ethyl dehydrogenation reaction. There is a linear relationship between the activation barrier and the heat of the surface reaction for vinyl dehydrogenation to acetylene. As the surface reaction becomes more endothermic, the activation barrier increases, consistent with the Polanyi relationship. As for the case of ethylene and ethyl species, the acetylene and the vinyl binding energies showed similar trends with surface d-band center. Hence, the intrinsic kinetics should vary with the HBE. As shown by Figure 9c, there is a fairly linear variation of the activation barrier with the HBE. The activation barrier decreases as the HBE increases, that is, as the product species is bonded more strongly to the surface. Thus the activation barrier for vinyl dehydrogenation to acetylene

*C2H5 + * f *C2H4 + *H (β-scission)

(I)

*C2H5 + * f *CHCH3 + *H (R-scission)

(II)

where reaction I is the desired reaction, since ethylidene might lead to formation of many other undesirable products.44 The activation barriers calculated for dehydrogenation of ethyl to ethylene are plotted against the d-band center of the substrate in Figure 8a. There is a fairly linear relationship between the activation barriers and the d-band centers of the substrates. As the d-band center moves closer to the Fermi level, the activation barrier decreases. Neurock has developed similar correlations for the reverse reaction. They have examined ethylene hydrogenation to ethyl on pseudomorphic Pd/Re, Pd/Ru, Pd, and Pd/Au surfaces17 and have observed that the calculated activation barriers for hydrogenation reactions vary linearly with the d-band center. They have explained this correlation by the fact that the back-donation into the σCH* orbital controls the C-H bond activation process and hence is more favored when the d-band center is in resonance with the antibonding state. Activation barriers are also correlated to the heat of surface reaction, as given in eq 8. Activation barriers for dehydrogenation were plotted against the heats of reaction on metal surfaces (Figure 8b). As expected from the BOC formulation, there is a linear variation between the activation barriers and the heat of reactions. This is also consistent with the Polanyi relationship

E ) Eint + γp∆H

(10)

where Eint is the intrinsic activation barrier and γp is the transfer coefficient. The transfer coefficient varies between 0 and 1, and its value depends on whether the transition state is more reactant-like or product-like. It is also evident from Figure 8b that the activation barrier increases as the reaction becomes more endothermic, as expected.

Bimetallic Surfaces and C2 Hydrocarbons decreases in the order of Pt > Pd > Ni, since hydrogen is bonded more strongly to Ni as compared to Pd which in turn binds hydrogen more strongly than Pt. 5. Conclusions Periodic DFT theory calculations were used to correlate the chemical (e.g., binding energy) and electronic (e.g., d-band center) properties of several monometallic and bimetallic transition-metal surfaces. The combination of DFT and BOC was used to study the relationship of reaction kinetics (activation barriers) to surface d-band center. On the basis of the results and the analysis provided above, we can conclude the following: (1) Hydrogen-binding energies on the transition-metal surfaces vary linearly with the d-band center of the substrate. The binding strength increases as the d-band center moves closer to the Fermi level. The binding energy of hydrogen on thermodynamically stable bimetallic surfaces is less than that on the corresponding parent metals. As far as the monometallic systems are concerned, the binding strength increases as one moves upward in the periodic table. Within a particular row, the binding strength increases toward the left of the periodic table. (2) Trends in molecular adsorption are more complex than for those in atomic adsorption. Ethylene adsorption on transitionmetal surfaces gives rise to separate correlations for 3d, 4d, and 5d surfaces. Within a particular row, that is, 3d, 4d, or 5d, ethylene binding energies vary linearly with the d-band center of the substrate. A similar trend within each row was observed for binding of ethyl groups to the surface. Acetylene and vinyl binding energies can also be correlated with the d-band center of the substrate. (3) Activation barriers for ethyl dehydrogenation to ethylene and vinyl dehydrogenation to acetylene also exhibit linear correlations with the d-band centers of the substrates. The activation barriers were also found to vary linearly with the heats of the corresponding surface reactions. This is consistent with the Polanyi relationship. (4) Activation barriers for these dehydrogenation reactions scale nearly linearly with hydrogen-binding energies on the surfaces considered in this study. This trend arises because the variations in ethylene binding energies are very similar to those for ethyl, and likewise the trend for vinyl is very similar to that for acetylene. Hence, the barrier to dehydrogenation decreases when hydrogen is more strongly bonded to the surface. Acknowledgment. We would like to acknowledge the Department of Energy, Office of Basic Energy Sciences (DOE/ BES Catalysis Science Grant No. DE-FG02-03ER15468) for financial support of this research. We also thank Dr. John Kitchin for his initial help in DFT modeling. References and Notes (1) Sinfelt, J. H. Bimetallic Catalysts: DiscoVeries, Concepts, and Applications; John Wiley & Sons: New York, 1983. (2) Hwu, H. H.; Eng, J.; Chen, J. G. G. J. Am. Chem. Soc. 2002, 124, 702-709.

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