and Ordered PtâSn Surface Alloys - American Chemical Society

J. Phys. Chem. C 2008, 112, 555-566

555

New Insights in Adsorption and Dehydrogenation of Cyclohexene on Pt(111) and Ordered Pt-Sn Surface Alloys: Experiment and Theory Franc¸ oise Delbecq,*,† Fabienne Vigne´ -Maeder,† Conrad Becker,‡ Ju1 rgen Breitbach,‡ and Klaus Wandelt‡ Institut fu¨r Physikalische und Theoretische Chemie, UniVersita¨t Bonn, Wegelerstr. 12, 53115 Bonn, Germany, and UniVersite´ de Lyon, Institut de Chimie, Laboratoire de Chimie, Ecole Normale Supe´ rieure de Lyon/CNRS, 46 Alle´ e d’Italie, 69364 Lyon Cedex 07, France ReceiVed: July 23, 2007; In Final Form: October 17, 2007

The adsorption of cyclohexene (C6H10) on Pt(111) and two ordered PtnSn/Pt(111) surface alloys has been investigated experimentally using high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and temperature programmed desorption (TPD) as well as theoretically by ab initio density functional theory (DFT) calculations. On Pt(111) and Pt3Sn/Pt(111) a di-σ-bonding of cyclohexene has been found both experimentally and in the DFT calculations. In contrast, on the Pt2Sn/Pt(111) a mixture of weakly bound species (di-σ, π, and physisorbed) has been found. Whereas on Pt(111) a considerable fraction of the cyclohexene dehydrogenates during thermal treatment, the adsorption is completely reversible on the two surface alloys. A detailed theoretical study of the structure and vibrational spectra of the dehydrogenated species has been performed on Pt(111). This allowed the interpretation of the variations in the HREELS spectra when the flash temperature increases by the formation of some dehydrogenated products.

1. Introduction The adsorption of cyclohexene on Pt(111) was first studied by Gland et al., who identified the molecule as intermediate species during the dehydrogenation process of cyclohexane to benzene.1 An ordered phase with the following superstructure was found along with a work function decrease of -1.7 eV during adsorption.

| | 2 2 4 −2

Later the bonding was identified as being of di-σ type, and a coadsorbed agostic species was proposed.2 The latter assumption was corrected by Henn et al., who attributed their HREELS spectra to the presence of two different di-σ species depending on the sample temperature.3 This perception is supported by the data on the cyclohexene adsorption on the hexagonally reconstructed Pt(100)(5 × 20) surface, which showed also the presence of two conformers depending on the temperature.4 The adsorption of cyclohexene on the ordered Pt3Sn and Pt2Sn surface alloys has also been previously investigated and a di-σ bonding was proposed on Pt3Sn, whereas the bonding mode on Pt2Sn was identified as hydrogen platinum bond.5 Consequently, dehydrogenation of cyclohexene on the Pt3Sn surface alloy was found, but Pt2Sn turned out to be inactive for dehydrogenation. These findings were based on AES, TPD, and LEED results; no vibrational data of cyclohexene on the PtSn surface alloys is available so far. Cyclohexene in the gas phase has been studied both experimentally6-9 and theoretically.10-12 These studies have shown that the lowest energy conformations are the two-halfchair (or twisted) forms and that these two forms interconvert

via the boat conformation that corresponds to a saddle point at 21-25 kJ mol-1 above. When adsorbed on a surface, either on two metal atoms (di-σ geometry) or on one metal atom (π geometry), the sp2 carbons become pyramidal and the molecule tends to look like cyclohexane. In the gas phase, the chair conformation of cyclohexane is the most stable structure. Two other conformations exist, the twist and the boat ones at energies of 23 and 27 kJ mol-1 above the chair conformation, respectively, the latter being the transition state for the interconversion of the two twist forms. Between the chair and the twist forms, the transition state is a half-twist form at 46-50 kJ mol-1 above the chair conformation.13-17 The adsorbed forms of cyclohexene have also been studied by means of DFT calculations on Pt clusters18 but not on the alloys. To our knowledge, no theoretical investigations of the vibrational properties of these systems have been published so far. We present here the first complete study using periodic calculations of the adsorption of cyclohexene on Pt(111), Pt3Sn/Pt(111), and Pt2Sn/Pt(111), which relies on a joined experimental and theoretical approach. We compare the theoretical adsorption geometries of cyclohexene on these surfaces and attempt an assignment of the species actually present on the surface by comparing the calculated vibrational spectra of all of the stable adsorption forms with the experimental HREELS data. In order to understand the spectra obtained after flashes at various temperatures on Pt(111), we consider some dehydrogenated products resulting from cyclohexene by abstracting one or two hydrogen atoms, and we calculate their vibrational spectra. Based on the calculated spectra, an attempt to identify the intermediates and the pathways of dehydrogenation is presented. 2. Experimental and Computational Details

* Corresponding author. E-mail: [email protected]. † Universite ´ de Lyon. ‡ Universita ¨ t Bonn.

Experimental Details. The experiments were performed in an ultrahigh vacuum (UHV) system operating at base pressures

10.1021/jp075760b CCC: $40.75 © 2008 American Chemical Society Published on Web 12/22/2007

556 J. Phys. Chem. C, Vol. 112, No. 2, 2008 of ≈1 × 10-8 Pa. Facilities in this chamber included a highresolution electron energy loss spectrometer (HREELS, VSW IB2000), an Auger electron spectrometer (AES), a back-view LEED optics, and a quadrupole mass spectrometer for temperature programmed desorption experiments (TPD). The experiments were performed using a disc-shaped Pt(111) sample of 7 mm diameter (MaTeck, Ju¨lich), which was mounted between tantalum wires wedged into grooves in the sides of the crystal. The mounting wires were also used for heating the sample by direct current through these wires. Cooling was provided by a LN2 reservoir, which was directly connected to the sample holder. The clean Pt(111) surface was prepared by cycles of Ar+ ion sputtering (10 min, 1 keV, 2.5 µA) at 900 K and subsequent annealing at 1100 K for 2 min. The cleanliness of the Pt(111) surface was monitored by AES and HREELS. The TPD experiments were performed using a constant heating rate of 2 K s-1, which was controlled by a Eurotherm 2408 temperature controller. Up to four different masses were detected concurrently. The HREELS spectrometer was operated at a resolution of 3-4 meV and a primary energy of 5 eV. Spectra were recorded in specular geometry at a scattering angle of 60° relative to the surface normal. An Al2O3 crucible was used for the evaporation of Sn, which was wrapped by a tungsten filament and temperature controlled by a thermocouple of type K. In order to ensure a constant Sn flux, the evaporator was operated at a temperature of 1300 K, which yielded a deposition rate of ∼0.001 ML s-1. The Pt-Sn surface alloys were prepared from thin Sn deposits on Pt(111), which were subsequently annealed in order to yield the desired surface structure and composition. Depending on the annealing temperature, ordered surface alloys of either Pt3Sn (Tann. ≈ 1000 K) or Pt2Sn (Tann. ≈ 800 K) stoichiometry are formed. This process was monitored by AES and LEED and has been described in detail in ref 19. Theoretical Details. The calculations were performed with the Vienna ab initio Simulation Package (VASP).20-22 This program performs periodic calculations based on the density functional theory (DFT) with a plane-wave basis set. The projector-augmented wave method (PAW)23 and the generalized gradient approximation (GGA) level with the functional of Perdew and Wang 9124 were used. A cutoff of 400 eV has been applied to the plane wave basis set. The surfaces were modeled by periodic four-layer slabs, with adsorption on one side of the slab. Since the prepared alloys are monolayer alloys deposited on platinum,19 only the first layer contains Sn atoms in a stoichiometry Pt3Sn for the (2 × 2) and Pt2Sn for the (x3×x3)R30° structures. Each slab is separated from its periodic image in the z direction by a vacuum space corresponding to five layers (11.5 Å). For the frozen part of the slab, the same metal interatomic distance (2.82 Å) was used in the case of pure platinum and in the case of the alloys (optimized from Pt bulk calculations, 2% larger than the experimental value 2.77 Å). This is justified by the fact that experimentally the lattice parameter is imposed by the underlying Pt bulk and the Sn atoms in the uppermost layer accommodate this constraint by an outward displacement, inducing a Pt-Sn distance of 2.86 Å. The electronic structures (DOS and electron transfer) of the two surface alloys have been published previously.25,26 One molecule is adsorbed per unit cell. For all structures, the geometry optimization included all degrees of freedom of the adsorbed molecule and of the two uppermost metal layers. The adsorption energy is defined as the difference between the

Delbecq et al. energy of the whole system and that of the bare slab and the isolated adsorbate. A negative value signifies stabilization. Since the molecules are adsorbed on one side of the slab only, the unit cell has a net dipole and a spurious electrostatic interaction between the slab and its periodic images can modify the total energy. A correction has been applied both on the energy and on the potential, in order to remove this effect. This correction does not exceed 35 meV for Pt and 70 meV for the alloys. The HREEL spectra have been simulated by calculation of the vibration frequencies and intensities.27 The vibrational analysis is based on the numerical calculation of the force constants around the minimum of the potential energy surface. The coupling with the surface phonons is neglected, and we have verified in two cases that this does not modify the spectrum, for frequencies higher than 30 meV. The force constant matrix is built with finite differences of the first derivatives of the total energy by geometrical perturbations in the two directions of the optimized Cartesian coordinates of the system ((0.02 Å at the harmonic level). The diagonalization of this matrix provides the harmonic molecular frequencies and the associated harmonic normal vibration modes. The intensities of the HEELS spectra are estimated by calculating the dynamic dipole moments of the vibrational modes. In the specular mode, only the modes giving rise to an oscillating dipole perpendicular to the surface are active. The calculation of the derivatives of the z component of the dipole moments is performed by finite differences in the Cartesian coordinate system. Then the absolute intensities Iloss of the energy losses normalized to the elastic peak intensity Ielastic are evaluated following the formula given in ref 28, a simplified version of which is given below

Ikloss Ielastic

)

(

3N

∑ i)1

Pki dµz

)

xmi d∆ri

2

F(νk) νk

where µz is the z component of the dipole moment, νk is the frequency associated to a given normal mode Qk, Pki/xmi is the mass weighted coordinate matrix of the normal mode and F(νk) is a function of the frequency and some fixed experimental parameters. The numerical accuracy of the vibration calculations is limited by two factors. The first one is due to the finite numerical accuracy of the calculations and depends for instance on the functional used. It can be quantified by comparison with experimental spectra. This comparison done for gas-phase cyclohexene showed that the frequency difference between experiment and simulation depends on the vibration modes and can be positive or negative. Hence, it is difficult to define a scaling factor, and we chose not to use any. The second one comes from the lack of anharmonicity in the calculations, which could result in a shift especially for the C-H stretching frequencies. 3. Experimental Results Cyclohexene on Pt(111). In order to study the thermal evolution of adsorbed cyclohexene on Pt(111), TPD spectra of different exposures were collected. A corresponding series is shown in Figure 1 for cyclohexene (m/e ) 82) and for hydrogen (m/e ) 2), which is a dehydrogenation product during heating. One can notice that at low coverage (2.0 L) desorption of cyclohexene is suppressed in favor of a complete dehydrogenation. Only at higher exposures a first desorption peak develops at 266 K. This temperature can be translated into a desorption

Cyclohexene on Pt(111) and Pt-Sn Surface Alloys

J. Phys. Chem. C, Vol. 112, No. 2, 2008 557

Figure 1. Temperature programmed desorption spectra of cyclohexene on Pt(111).

energy Edes ) 69 kJ mol-1 using the Redhead approximation29 under the assumption of first-order kinetics and a pre-exponential factor of 1013 s-1. At even larger exposure, a second desorption state is found at 224 K (Edes ) 58 kJ mol-1). These two peaks saturate at an exposure of about 5 L. At even higher exposure, two additional peaks are found, which correspond to desorption of the condensed (multilayer) phase (135 K) and desorption from edges of the sample (160 K). The intensity of the latter peak shows a strong dependence on the sample position, which suggests this assignment. A similar assignment has been proposed in ref 2. Our results on the cylcohexene desorption are in good agreement with those of Xu and Koel.5 The desorption trace of hydrogen is rather complex, which points to a stepwise dehydrogenation of the adsorbed cyclohexene during heating. The first peak at 284 K is clearly desorption limited since it occurs at a temperature, which is comparable to the desorption temperature of pure hydrogen from Pt(111).30 The subsequent peaks correspond to further dehydrogenation steps. Taking these results into consideration, HREELS experiments have been performed in order to associate the different desorption peaks to adsorbed species. These results are displayed in the temperature series in Figure 2. The spectra were acquired for a monolayer coverage of cyclohexene, which has been prepared by dosing 6 L at 100 K followed by a flash to 180 K in order to desorb the condensed phase. For the subsequent temperature steps, the sample was flashed to the corresponding temperatures and rapidly quenched to 100 K. The monolayer spectrum shows a multitude of well-resolved peaks, of which only the CH2 stretching vibrations at 355 meV (νsCH2), and 363 meV (νasCH2) can be easily assigned. A more complete assignment will be given in the discussion section by comparing the experimental results with our simulated spectra. Annealing to 260 K, that is above the first desorption state, results only in subtle changes of the vibration spectrum. The peak positions hardly change, but the relative intensities do vary. This is particularly pronounced for the low-lying vibration states around 60 and 69 meV. However, from the HREELS spectra alone, the two desorption states cannot be assigned to differently adsorbed species, as it was proposed by Henn et al.3 Further annealing to 330 K leads to changes in the relative intensities of the peaks. The most apparent difference seems to be the

Figure 2. Experimental HREELS spectra of cyclohexene on Pt(111). The bottom spectrum represents the full monolayer coverage. The other spectra were taken after a flash to the indicated temperatures.

appearance of an additional CH2 stretching mode at 375 meV at temperatures of 290 K and above (seen as a shoulder after annealing to 310 K). After annealing above 360 K, the spectra change significantly showing a strong intensity increase for the peak at 103 meV and further decrease of the loss at 179 meV. After flashing to 450 K, a temperature that is above the next two dehydrogenation steps as judged from TPD, the HREELS spectrum is clearly totally different, and can easily been attributed to adsorbed benzene.31,32 Taking these results into account, the hydrogen desorption peaks at 284, 383, and 427 K can easily be assigned to the stepwise dehydrogenation of cyclohexene to benzene. The nature of the individual dehydrogenation steps is, however, not clear at this point. We will come back to this point in the discussion. Cyclohexene on Pt3Sn/Pt(111) and Pt2Sn/Pt(111). The adsorption behavior of cyclohexene on the Pt-Sn surface alloys deviates largely from the one encountered on Pt(111). First, already the desorption traces of cyclohexene from these surfaces show a markedly different behavior (Figure 3). Whereas on the

558 J. Phys. Chem. C, Vol. 112, No. 2, 2008

Delbecq et al.

Figure 3. Comparison of the thermal desorption spectra of cyclohexene on Pt(111) and the Pt-Sn surface alloys.

Pt3Sn/Pt(111) surface again two desorption states at 222 K (57 kJ mol-1) and 198 K (51 kJ mol-1) are found, only one state is present for Pt2Sn/Pt(111) at 197 K (51 kJ mol-1). Most importantly no hydrogen desorption is found on the surface alloys indicating that the cyclohexene adsorption is fully reversible. A small signal in the hydrogen desorption trace (≈380 K) from both surface alloys can be attributed to imperfections of the surface alloy, e.g., small Pt patches, which show some dehydrogenation activity. One can, therefore, exclude the dehydrogenation of cyclohexene on the Pt3Sn/Pt(111) surface, which has previously been found by Xu and Koel.5 The result presented here is in good agreement with the absence of dehydrogenation for other small alkenes, such as ethene33 and cyclopentene,26 on Pt3Sn/Pt(111). If one compares the desorption temperatures on the surface alloys with the one on Pt(111), two effects are clearly visible. First, the desorption temperature (adsorption energy) is lowered as the Sn content of the surface rises. Second, the hightemperature peak on Pt3Sn/Pt(111) is at the same temperature as the low-temperature peak on Pt(111) and the low-temperature peak on Pt3Sn/Pt(111) is at the same position as the desorption peak on Pt2Sn/Pt(111). Whether this observation is linked to a general lowering of the adsorption energy by alloying as it has been observed for cyclopentene26 or rather to a changed abundance of particularly adsorbed species on the different surfaces will be discussed below. A comparison of the HREELS spectra of cyclohexene on the three surfaces in question helps to clarify the issue (Figure 4). Immediately two important features are clearly observable. First, the spectrum on Pt3Sn/Pt(111) resembles the monolayer spectrum on Pt(111) except for the peak at 69 meV. Second, the spectrum of Pt2Sn/Pt(111) is very similar to the spectrum of the condensed (multilayer) phase, which is indicated by the strong peaks at 80 meV and the presence of the loss at 374 meV. From these observations, one is tempted to describe the adsorption state of cyclohexene on Pt3Sn/Pt(111) as being similar to the one on Pt(111) (di-σ) and the adsorption state on Pt2Sn/Pt(111) as being physisorbed. However, from these experimental findings alone, an unambiguous assignment of

Figure 4. Experimental HREELS spectra of a full monolayer of cyclohexene on Pt(111) and the Pt-Sn surface alloys. The topmost spectrum represents a multilayer on Pt(111).

adsorption modes and geometries is not possible. Only the comparison of the experimental HREELS spectra with calculated vibration spectra will allow us to assign the observed spectra to specific adsorption geometries. 4. Theoretical Results In order to provide a reference for the calculation of the adsorbed cyclohexene species, we have calculated with VASP the two known gas phase conformations: the boat and the halfchair. The boat conformation is indeed a transition state, 22 kJ mol-1 higher than the half-chair one, in agreement with previous experimental and theoretical results.11 Cyclohexene on Pt(111). By adsorption of cyclohexene through the double bond, two Pt-C bonds are created leading to a molecule with a structure comparable to cyclohexane. Hence, five conformations have been considered: two boats (up and down), two chairs (cis and trans), and one twist, all in the di-σ and π geometries. In the di-σ geometry, each ethylenic carbon atom is bound to a platinum atom. For the π geometry, where the two carbon atoms are bonded to the same Pt atom, two orientations have been considered: one in which the molecule lies on top of a hollow site and one where it lies above a Pt-Pt bond (see Scheme 1). Examples of the optimized structures that were obtained are given in Figure 5. The chair cis and trans differ by the position of the hydrogen atoms of the double bond. The chair trans form is likely difficult to obtain by adsorption but it has been envisaged in the past.5 Most calculations have been done for two different coverages, 1/9 ML corresponding to a 3 × 3 unit cell and 1/7 ML corresponding to a (x7 × x7)R19° unit-cell. The latter corresponds to the experimental saturation coverage of 0.14 ML.5 A schematic representation of these surfaces is given in Figure 6a,b. The Brillouin zone integration has been done on a 3 × 3 × 1 grid for coverage 1/9 ML and on a 4 × 4 × 1 Γ centered grid for coverage 1/7 ML. In the di-σ geometries, the C1-C2 double bond is more elongated than in the π geometries (1.50 vs 1.42 Å, Tables 1


Figure 5. Optimized geometries for different cyclohexene species adsorbed on Pt(111).


Figure 6. Representation of the surface coverage for adsorption of cyclohexene di-σ boat up: (x7 × x7)R19° (a) and (3 × 3) (b) on Pt(111); (4 × 2) (c) and (2x3 × 2x3) (d) on Pt3Sn/Pt(111); (3 × 3) (e) on Pt2Sn/Pt(111).

SCHEME 1: Schematic Representation of the Various Adsorption Positions of Cyclohexene on Pt(111) and the Pt-Sn Surface Alloys

Figure 7. Adsorption energy per surface unit area as a function of the coverage for the di-σ boat up geometry.

and 2) and the two carbons C1 and C2 are more pyramidal corresponding to an sp3 hybridization. For example, the bond angles around C1 in the di-σ boat up geometry are 111.8°, 111.3°, and 113.8° for HC1C2, HC1C6, and C6C1C2, respectively, close to the values for a tetrahedral carbon. In the case of the π chair cis geometry, the same angles are 116.9°, 115.9°, and 121.4°, respectively. Moreover the Pt-C bonds are longer for the π geometries (2.23-2.25 vs 2.11-2.15 Å) indicating a weaker bonding. Hence in the di-σ geometries, the molecule looks rather like gas-phase cyclohexane, and in the π geometries, it partially keeps the structure of gas-phase cyclohexene. Nevertheless, due to the bonding to the surface, the molecule is less flexible than cyclohexane in the gas phase and not all conformations are found. The boat up and down forms have a real boat conformation both in the di-σ and in the π geometry,

as evidenced by the dihedral angles (Table 1) that are 0° and (52° in the gas phase. The boat down form is a little deformed to avoid interactions with the surface. The di-σ twist and chair cis structures evolve during the optimization to the same intermediate form with dihedral angles differing from the real chair form of cyclohexane where all angles are alternatively (54° (this work) and from the twist form (angles given in Table 1). On the contrary, the di-σ chair trans form looks like a real chair (angles close to (54°). The π chair cis form looks rather like a half-chair cyclohexene (see the dihedral angles in Table 2). The π chair trans form is not stable contrary to its di-σ counterpart. This can be explained by geometrical considerations: to obtain two hydrogen atoms trans to each other when the molecule is adsorbed on the surface, the two Pt-C bonds must be equatorial (Figure 5). In the di-σ geometry, such an orientation can adapt to the orbitals of the two interacting Pt atoms, whereas in the π geometry, where only one metal atom is involved, the orbital overlap is difficult to obtain. Both orientations A and B for the π geometry are equivalent (Table 2).


Delbecq et al.

TABLE 1: Theoretical Key Data of Di-σ-Bonded Cyclohexene on Pt(111) Compared to Gas-Phase Cyclohexanea gas-phase cyclohexaneb

adsorbed

EA (1/7 ML) EA (1/9 ML) C1-C2 Pt-C C1C2C3C4 C2C3C4C5 C3C4C5C6 C4C5C6C1 C5C6C1C2 C6C1C2C3 a

boat up

boat down

chair cis

chair trans

TS boat-chair

-75 -73 1.50 2.13 54 -53 0 53 -54 0

-62 -60 1.50 2.15 -44 45 0 -44 44 0

-68 -65 1.50 2.15 -27 48 -63 60 -37 22

-63 -61 1.50 2.11 -62 53 -52 53 -62 70

1.50 2.14 13 11 -52 69 -45 5

twist

1.55 -26 62 -33 -28 64 -35

half-twist

1.53, 1.56 14 7 -48 69 -48 7

Adsorption energies EA in kJ mol-1, bond lengths in Å, and angles in degrees. b Calculated with Gaussian 98.36

TABLE 2: Theoretical Key Data for π-Bonded Cyclohexene on Pt(111) Compared to Gas-Phase Valuesa adsorbed boat up

boat down

chair cis(A)

-42 -54 EA (1/7 ML) -40 EA (1/9 ML) -38 -40 -52 C1-C2 1.42 1.41 1.41 Pt-C 2.23 2.24 2.23 2.24 C1C2C3C4 52 -37 -17 C2C3C4C5 -51 36 46 C3C4C5C6 0 0 -61 C4C5C6C1 51 -36 45 C5C6C1C2 -52 37 -16 C6C1C2C3 0 0 2

gas phase chair cis(B) half-chair -54 -52 1.41 2.25 2.27 -17 46 -60 44 -15 1

1.34 -14 44 -61 44 -14 2

boat

1.34 45 -43 0 43 -45 0

a Adsorption energies E in kJ mol-1, bond lengths in Å, and angles A in degrees.

Concerning the adsorption energies, the most stable adsorption geometry is the di-σ boat up one followed by the di-σ chair cis, which is less stable by 7-8 kJ mol-1 (compare Tables 1 and 2). The order is inverted compared to the gas-phase cyclohexane for which the chair is the most stable conformation (by 20 kJ mol-1) and the boat is a transition state. With the help of the nudged elastic band method (NEB),34 we have investigated the reaction path leading from the di-σ boat to the di-σ chair conformation. The transition state has been located 15 kJ mol-1 above the boat form. The dihedral angles show that it has the half-twist conformation (see Table 2). The two other di-σ conformations (boat down and chair trans) are close in energy (12 kJ mol-1 less stable than di-σ boat up). The highest adsorption energy of -75 kJ mol-1 is in good agreement with the value obtained from the TPD experiment for the peak at 266 K (69 kJ mol-1), which could mean that it corresponds to di-σ boat up cyclohexene. From an energetic point of view, the second TPD peak at 224 K could be attributed to di-σ chair cis (58 kJ mol-1). All the π structures are less stable by, at least, 20 kJ mol-1. The stability order is the same as for cyclohexene in gas phase, with the chair cis conformation (half-chair) more stable than the boat one by 14 kJ mol-1. The relative energies are in agreement with those found previously,18 although the absolute values are slightly different, owing to the use of periodic calculations instead of clusters. To explain the second peak observed in TPD, some authors suggest the existence of a species weakly adsorbed through the aliphatic H atoms, as alkanes do.2 Such a species has been calculated for cyclohexane with an adsorption energy of -36.5 kJ mol-1.18 For the half-chair form of cyclohexene, we have found a similar structure but with a smaller adsorption energy

of -11 kJ mol-1. Hence for energetic reasons, this form bound by H atoms must be ruled out. The influence of the coverage has been studied for the most stable structure, the di-σ boat up one. The following unit cells have been considered: x3 × x7 (1/5 ML), 3 × 2 (1/6 ML), 2x3 × x7 (1/8 ML), 3 × 4 (1/12 ML), 3 × 3 and x7 × 3 (1/9 ML). For each unit cell, the geometry has been optimized and the adsorption energy per surface unit has been evaluated. These energies are plotted in Figure 7 as a function of the coverage. The minimum of the curve corresponds to a coverage of 0.14 ML, which is in agreement with the experimental saturation coverage.5 Moreover a physisorbed stable structure has been found with a binding energy of -17 kJ mol-1. In this geometry, the ethylenic hydrogen atoms (H1 and H2) point toward the surface with a Pt-H distance of 2.9 and 3.1 Å, respectively (Figure 5). Cyclohexene on Pt3Sn/Pt(111) and Pt2Sn/Pt(111). The same adsorption structures as on Pt(111) have been calculated on the surface alloys: the (2 × 2)-Pt3Sn/Pt(111) and the (x3 × x3)R30°-Pt2Sn/Pt(111). In the case of the Pt3Sn alloy, two coverages have been considered: 1/8 ML corresponding to a (4 × 2) unit cell and 1/12 ML corresponding to a (2x3 × 2x3) unit cell. In the case of the Pt2Sn alloy, only a coverage of 1/9 ML (3 × 3 unit cell) has been studied. The geometries of these systems are illustrated in Figure 6c-e. The coverages of 1/8 ML for Pt3Sn and 1/9 ML for Pt2Sn are close to the experimental saturation coverages of 0.13 and 0.12 ML, respectively. The Brillouin zone integration has been done on a 3 × 3×1 grid for coverage for 1/9 and 1/12 ML and on a 2 × 4×1 Γ-point centered grid for 1/8 ML coverage. The adsorbed geometries (boat or chair) are the same as on Pt(111) and are not described in detail. However, because of the presence of tin, the sites are not equivalent and a larger variety of structures exists, which are depicted in Scheme 1. The results of the structure optimizations are collected in Table 3. The geometries where a carbon interacts with a tin atom (e.g., di-σ(B) on Pt2Sn) are not stable and are not considered further. The optimized geometries on the alloys are similar to those found on Pt(111) in terms of bond lengths and dihedral angles. However, the adsorption energies are smaller on the alloys as it was expected from the TPD results and from our previous results for other unsaturated molecules.25,26 For example, the adsorption energy of the di-σ boat up geometry is only -27 and -13 kJ mol-1 at the saturation coverage on Pt3Sn and Pt2Sn, respectively, compared to -75 kJ mol-1 on Pt(111). Among the π chair cis structures, only one is stable on Pt3Sn at coverage 1/12 ML, π(C) with an adsorption energy of -11 kJ mol-1 compared to -54 kJ mol-1 on Pt(111). For these π chair cis structures, an important influence of the coverage



TABLE 3: Theoretical Key Data for Adsorbed Cyclohexene on the Pt-Sn Surface Alloysa

a

di-σ (A)

di-σ (B)

di-σ (A)

π(A)

π(B)

π(C)

boat up

boat up

chair cis

chair cis

chair cis

chair cis

EA (1/8 ML) EA (1/12 ML) C1-C2 Pt-C

-27 -25 1.51 2.14

-24

Pt3Sn/Pt(111) -9 -14 1.50 2.15, 2.16

not stable -3 1.41 2.27

not stable not stable

not stable -11

EA (1/9 ML) C1-C2 Pt-C

-13 1.51 2.14

Pt2Sn/Pt(111) -8 1.50 2.15

-15 1.41 2.23, 2.24

-14 1.42 2.23

-11 1.42 2.23

1.50 2.14

Adsorption energies EA in kJ mol-1 and bond lengths in Å.

can be noticed since, in the case of 1/8 ML, none is stable. This is due to the fact that the chair cis adsorbed structure requires more space on the surface (see Figure 5). Such an influence of the coverage is not observed for the di-σ forms. On Pt3Sn the di-σ boat up geometry remains the most stable but the energetic difference to the π cis geometry (for 1/12 ML) is reduced compared to Pt(111). In the case of Pt2Sn, the di-σ and π geometries have roughly the same adsorption energy, π(A) and π(B) forms being even more stable than the di-σ boat up one. These results are in agreement with the TPD experiments (this work and ref 5) although the energy decrease is not as large experimentally observed, as it has been noticed previously.25,26 5. Discussion Cyclohexene on Pt(111). The vibrational frequencies of all of the theoretically investigated structures have been calculated in order to simulate the HREELS spectra and thereby identify the species that are actually present on the surface. In the following, the atom numbering used to describe the normal modes is the one given in Figure 5. In the case of Pt(111), we have chosen a coverage of 1/7 ML corresponding to experimental findings of Xu and Koel.5 The corresponding spectra, drawn with identical scales on the y axis, are shown in Figures 8 and 9 for the di-σ and the π geometries, respectively. The spectra can be divided into several characteristic energy ranges. The first one, between 340 and 375 meV, covers the νCH stretching vibrations. For the boat up geometry, either di-σ or π, there is only one intense νCH peak at 367 meV that corresponds to the in-phase stretching of the C4-H4 and C5H5 bonds. For the other geometries, the νCH stretching vibrations give several peaks. For the chair cis form, either di-σ or π, some low energies (350 and 342-347 meV, respectively) are found, which indicates the interaction of H atoms with the surface as it had been already observed for cyclopentene.30 Indeed for the di-σ chair cis geometry, one Pt-H distance is only 2.48 Å and the corresponding C-H bond is 1.11 Å, which corresponds to a slight elongation compared to the other Csp3-H bond lengths (1.10 Å). Other peaks present in all spectra are located near 180 meV. They correspond to combinations of CH2 scissoring located on C3, C4, C5, and C6, in some cases mixed with the νC1C2 stretching mode. For the π adsorption (boat up and chair cis) these peaks are particularly intense because a molecule that is adsorbed on a single Pt atom is less rigid. Between 80 and 170 meV a large number of intense peaks is found for the di-σ geometries whereas fewer and less intense peaks are present for the π geometries. For the boat conformation, these peaks correspond mostly to in-phase and out-of-phase combinations of the νCC stretching modes of the cycle. In the

case of the chair cis form, either di-σ or π, the small peaks in this energy range are mainly in-phase and out-of-phase combinations of the CH2 rocking modes, which is a consequence of the greater flexibility of the chair compared to the boat conformation. In addition, the spectrum of the di-σ chair cis geometry is dominated by an intense peak at 95 meV, a mixture of νC1C2, νC1C6, and νC2C3, rocking of C4H44′ and C5H55′ and of the out-of-plane movement of H1 and H2. The di-σ chair trans structure shows a simpler spectrum with a limited number of intense peaks corresponding to combinations of the νCC modes. This is indeed a real symmetrical chair oriented along the vertical. Below 80 meV, the last interesting part of the spectra concerns the νPtC stretching modes. Their positions and intensities depend much on the conformation and on the adsorption geometry. In the case of the boat up species, there is only one intense peak at 66 meV (di-σ) and 62 meV (π) corresponding to the symmetric νPtC1 + νPtC2 mode. For the chair cis geometry, several peaks correspond to the νPtC stretching modes because they mix with some CH2 rocking modes (41 meV for π and 46, 53, 57, and 77 meV for di-σ). By comparison of all the presented spectra with the experimental ones, one can conclude that the spectrum of the di-σ boat up form matches well the experimental monolayer spectrum, in terms of the positions of the peaks and of their respective intensities. This concerns in particular the loss peaks at 69 and 180 meV. Other small and not well-defined peaks at 45 and 59 meV and a shoulder at 80 meV match quite well the peaks of the di-σ chair cis form in the theoretical spectra (46, 57, and 77 meV, respectively). Therefore, the comparison of the experimental and theoretical spectra leads to the conclusion that the monolayer is formed by a majority species (di-σ boat up) and a minority species (di-σ chair cis). This assignment is corroborated by the fact that these two species are the most stable ones with theoretical adsorption energies of -73 and -65 kJ mol-1, respectively. We can exclude the presence of the chair trans form, which had been postulated in an earlier study.5 The π species can also be excluded because of their low calculated adsorption energies (see Table 2) compared to the adsorption energies of the di-σ species (Table 1). The spectrum for the physisorbed species is given in Figure 9. It is characterized by two intense peaks at 78 and 200 meV, corresponding to the symmetric out-of-plane movement of H1 and H2 and to the νC1C2 stretching mode, respectively. These two peaks are visible in the experimental spectrum of the multilayer at 80 and 205 meV (see Figure 4). The spectrum of the species adsorbed by an aliphatic H atom (not shown here) presents an intense softened νCH peak at 325 meV, as it is usual for adsorbed alkanes (321 meV has been found experimentally for cyclohexane).35 The absence of such a peak in the


Delbecq et al.

Figure 8. Calculated HREEL spectra for the various conformations of cyclohexene adsorbed on Pt(111) in the di-σ geometry at coverage 1/7 ML.

experimental spectrum excludes definitively this structure, that had also been postulated previously.2,5 The experimental spectrum after a flash to 260 K (Figure 2) is characterized by very intense νCH stretching modes around 363 meV. The peak at 180 meV is relatively more intense than in the monolayer spectrum and the peak at 69 meV almost totally disappears, while a small one at 60 meV becomes more apparent. The change in the other peaks is not characteristic. This indicates that, by desorbing part of the cyclohexene, the di-σ boat up form disappears and a reordering of the layer takes place, which leads to the variation of the loss intensities. This is evidently not reflected by the DFT calculations since mixed layers have not been calculated. However, the comparison of the HREELS spectra clearly shows that the first desorption peak cannot be related to the desorption of a specific cyclohexene species as it was suggested before.2,5 One can notice that the spectra taken after a flash to higher temperatures are similar in the temperature range from 260 to 330 K with only a small changes in the relative intensity of the peaks, particularly that at 180 meV, and with an additional shoulder at 373 meV. Above 360 K, important changes occur in the spectra: between 164 and 180 meV, a new peak appears at 172 meV, and the peaks between 130 and 150 meV disappear progressively. New peaks are also present between 40 and 80 meV whose intensity increases with the temperature. However the most important change is the appearance of a high peak at 103 meV accompanied by a smaller one at 112 meV. These two peaks are the main ones visible in the spectrum taken after a flash to 450 K that corresponds indeed to the spectrum of benzene.32

Figure 9. Calculated HREEL spectra at coverage 1/7 ML for the various conformations of cyclohexene adsorbed on Pt(111) in the π geometry and for the physisorbed structure.

Since it has been observed that cyclohexene dehydrogenates on the platinum surface, these spectra could correspond to dehydrogenated species finally leading to benzene at 450 K. Hence we have optimized the structure of several mono- and didehydrogenated compounds (C6H9 and C6H8, respectively). These compounds have been considered in previous works as intermediates in the hydrogenation of benzene18,36-38 or dehydrogenation of cyclohexane,38 but their HREELS vibrational spectra have not been calculated so far. The optimized structures that we found are given in Figures 10 and 11 for different C6H9 and C6H8 species, respectively, along with their relative energies. These results are very similar to those published previously.18,36-38 The 1,2,3-σ chair mono-dehydrogenated compound and the 1,2π,4,6-σ di-dehydrogenated compound, which both possess an allylic arrangement, are the most stable of the C6H9 and C6H8 species, respectively. The calculated HREELS spectra are given in Figure 12 for the C6H9 compounds. They are characterized by relatively small peaks around 180 meV, corresponding to combinations of CH2 scissoring, as in the case of cyclohexene, and, on the contrary, more intense peaks near 47 or 60-69 meV depending on the species, corresponding to combinations of the stretching νPtC modes. The intense peak at 81 meV in the spectrum of 1,2,3-σ chair cyclohexenyl is assigned to the rocking mode of C5H55′. The νCH stretching modes give peaks with weak intensity and some are at low energy (341 and 332 meV for 1,2,3-σ chair and 1,2-π-3-σ, respectively) because a hydro-



Figure 10. Adsorbed structures of the C6H9 species on Pt(111). Relative energies in kJ mol-1.

Figure 12. Calculated HREEL spectra for the C6H9 species adsorbed on Pt(111) at coverage 1/9 ML.

Figure 11. Adsorbed structures of the C6H8 species on Pt(111). Relative energies in kJ mol-1.

gen atom bound to C5 is close to the surface (see Figure 10). The dehydrogenated species lie flatter on the surface and have more bonds with it, which induces a weak intensity for the CH2 scissoring modes (around 180 meV) and for the νCH stretching modes and a high intensity for the PtC stretching modes. The disappearance of the CH2 scissoring modes is still more visible on the spectra calculated for the C6H8 species, given in Figure 13. The 1,2-σ,3,4-π geometry (1-3 bridge cyclohexadiene18) has only very small peaks. On the contrary, the spectrum of the 1,4-σ,2,3-π species (1-3 hollow cyclohexadiene18) is characterized by two peaks of high intensity, one at 102 meV, corresponding to the symmetric out-of-plane movement of H1 and H2 mixed with the stretching mode νC4C5, and one at 124 meV, corresponding to the symmetric out-of-plane movement of H3 and H6 mixed with the stretching mode νC4C5. An intense peak at 839 cm-1 (104 meV) is indeed present in the experimental spectrum of 1-3 cyclohexadiene.40 In the case of 1,2,4,5-σ geometry (bridge quadra-σ 1-4 cyclohexadiene18), the spectrum is dominated by a peak at 116 meV corresponding to the in-phase out-of-plane movements of H2 and H5, mixed with the stretching modes νPtC2 and νPtC5 and the rocking modes of C3H2 and C6H2. Hence, the spectra of molecules having two double bonds, conjugated or not, are characterized

by intense peaks corresponding to the in-phase out-of-plane movements of the ethylenic hydrogen atoms (102 meV for the 1,4-σ,2,3-π species and 116 meV for the 1,2,4,5-σ geometry). In contrast, the spectrum of the most stable C6H8 isomer, 1,2π,4,6-σ geometry, characterized by a double bond and two isolated C radicals, presents a smaller peak at 101 meV (symmetric out-of-plane movement of H1 and H2 mixed with the rocking mode of C3H2) and is characterized by two peaks of medium intensity at 122 and 129 meV (symmetric rocking modes of C3H2 and C5H2 and out-of-plane movement of H4 and H6, respectively). If we consider the appearance of the loss at 373 meV in the experimental HREELS spectra (see Figure 2) at temperatures above 290 K, the most probable candidates for intermediate species are the 1,2,3-σ chair and the 1,2-π-3-σ forms. These two forms are the only ones, which show a peak at such high energy in the theoretical spectra. Furthermore, they are also the energetically most favorable species. However, the latter shows a low νCH peak at 332 meV that does not exist experimentally, which rules it out. The spectrum of the 1,2,3-σ chair form, shown in Figure 12, matches the experimental one particularly in the region 40-80 meV with three intense peaks at 47, 69, and 81 meV. In the νCH region also, the three experimental losses are well reproduced. Moreover a peak at 176 meV is also present. Hence the 1,2,3-σ chair form is certainly present at these temperatures. The only calculated spectrum that is dominated by a peak at 102 meV is the one of the 1,4-σ-2,3-π form that is the second most stable among the di-dehydrogenated species (Figure 13). In this spectrum, one observes peaks at 145, 165, and 177 meV that are also found in the experimental


Delbecq et al.

Figure 13. Calculated HREEL spectra for the C6H8 species adsorbed on Pt(111) at coverage 1/9 ML.

spectrum at 143, 168, and 180 meV. Hence the 1,4-σ-2,3-π form is likely present when the temperature increases. Therefore it seems that the spectra taken after a flash to temperature between 360 and 450 K correspond to the dehydrogenated 1,2,3-σ chair and 1,4-σ-2,3-π species. Moreover the presence of some benzene, already formed at these temperatures, cannot be excluded owing to the very high loss at 102 meV. These results suggest that, in the present experimental conditions, the dehydrogenation of cyclohexene involves these two compounds as intermediates. These findings disagree with results obtained previously for the dehydrogenation of cyclohexene, where the most probable intermediates were calculated to be the 1,2,3-σ cyclohexenyl and the 1,2-π,4,6-σ forms.39 In fact, in the latter study, the starting point was cyclohexane adsorbed in the chair conformation. Hence cyclohexene, resulting from the first dehydrogenation of cyclohexane, is also adsorbed in the same conformation (di-σ chair). Starting from this species, the comparison of the activation energies shows that the best dehydrogenation path goes via the 1,2,3-σ chair cyclohexenyl and the 1,2-π,4,6-σ chair forms that are both the most stable of the mono and didehydrogenated species, respectively. However, in the present study, the starting point is the di-σ boat up conformation of adsorbed cyclohexene. The mono-dehydrogenated product should therefore be the 1,2,3-σ boat cyclohexenyl, whose structure and spectrum are given in Figures 10 and 12, respectively. For this species, the hydrogens on C4 and C6 are the closest to the surface and the dehydrogenation leads to 1,4σ,2,3-π cyclohexadiene. Owing to the low stability of the 1,2,3-σ boat cyclohexenyl conformation, its dehydrogenation should be easy. This could explain why the fingerprint of this form is not seen in the spectra (no intense peak near 66 meV). The 1,2,3-σ chair cyclohexenyl form can be obtained by interconversion between the boat and the chair conformations, either from cyclohexene by dehydrogenation of the di-σ chair adsorbed form or directly from 1,2,3-σ boat cyclohexenyl. We have seen before

Figure 14. Calculated HREEL spectra for the boat up and the chair cis conformations of cyclohexene adsorbed on Pt3Sn/Pt(111) at coverage 1/8 ML and on Pt2Sn/Pt(111) at coverage 1/9 ML. At the top, calculated HREEL spectrum for physisorbed cyclohexene on Pt2Sn/Pt(111).

that this interconversion requires 15 kJ mol-1 in the case of adsorbed cyclohexene. From 1,2,3-σ chair cyclohexenyl only, the 1,2-π,4,6-σ form can be obtained, owing to the position of the hydrogen situated on C5 (Figure 10). The foregoing could explain why the di-dehydrogenated species in the spectra taken after flashes at temperature higher than 350 K is 1,4-σ,2,3-π cyclohexadiene. Cyclohexene on Pt3Sn/Pt(111) and Pt2Sn/Pt(111). For the Pt3Sn surface alloy, two spectra are shown in Figure 14, corresponding to the most stable di-σ(A) boat up and the less stable di-σ(A) chair cis structures. These spectra correspond to a coverage of 1/8 ML, close to the experimental one. The spectrum of the di-σ(B) boat up structure is similar to that of the di-σ(A) boat up one, with some small differences in the relative intensities, and it is therefore not reported in the figure. The spectra shown in Figure 14 look very similar to those obtained in the case of Pt(111) for the same structures (Figure 8): the peaks are observed at approximately the same energies, which reflects the similar adsorption geometries (compare Tables 1 and 3). Nevertheless there are some differences in the intensities. For example, the peak at 67 meV in the spectrum of the di-σ(A) boat up structure is less intense compared to the one at 180 meV and the νCH stretching modes while it is the

Cyclohexene on Pt(111) and Pt-Sn Surface Alloys most intense on Pt(111). In the case of the di-σ(A) chair cis structure, the spectrum on the alloy is dominated by the peak at 95 meV as it is on Pt(111). The only difference is the absence of the νCH mode below 350 meV. Indeed, to avoid interaction with the Sn atom, the molecule in its chair conformation is tilted away from the surface compared to the situation on Pt(111), which is reflected in the increase of the PtPtC1C6 dihedral angle from 114° to 125°. Hence no hydrogen atom is close enough to interact with the surface. The comparison of the simulated spectrum for the di-σ(A) boat up structure on Pt3Sn/Pt(111) with the experimental one (Figure 4) shows a good agreement. Indeed the experimental spectra on Pt(111) and on Pt3Sn/Pt(111) differ mainly by the intensity of the peak at 69 meV as do the simulated spectra, the other peaks being similar. Hence, as on Pt(111), the majority species on Pt3Sn/Pt(111) is the di-σ boat up form. In the case of the (x3 × x3)R30° Pt2Sn/Pt(111) alloy, four structures are competitive in terms of the calculated adsorption energies (Table 3). Because the calculated spectra of all three π structures are similar, only two spectra are given in Figure 14: that of the di-σ(A) boat up and that of the π(A) chair cis. Once again, the spectrum of the di-σ(A) boat up form is similar to those obtained for the same form on the two previous surfaces, with the characteristic peaks at 66 and 180 meV. These peaks are found in the experimental spectrum (69 and 180 meV), which is an indication of the presence of this structure. The spectra corresponding to the π chair species are similar to each other, as said before, with peaks of rather small intensity. Two peaks stand out at 95 and 180 meV, the former being formed by several peaks corresponding to CH2 rocking and ΣνCC. Small peaks characterizing the π chair forms are also present at 42 and 50-55 meV. All of these peaks are visible in the experimental spectrum at 43, 56, 99, and 180 meV, which means that some π chair forms are adsorbed on Pt2Sn/Pt(111) in the present experimental conditions. In fact, owing to the similar adsorption energies of the π chair and di-σ(A) boat up structures, all these species have the same probability to exist. The experimental spectrum is dominated by a high peak at 80 meV accompanied by a less intense one at 89 meV. Moreover a small peak is visible at 204 meV. The presence of these three peaks can be the signature of a physisorbed species similar to that found on Pt(111) and whose spectrum is given in Figure 14 (peaks at 78, 87, and 204 meV with relative intensities matching with the experimental spectrum). This physisorbed species has an adsorption energy of 7.3 kJ mol-1, not too far from the other forms, taking into account that physisorption is not correctly described by DFT. Hence on Pt2Sn/Pt(111) it seems that there is a mixture of species having similar small adsorption energies: a di-σ boat up, several π chair cis, and a physisorbed one. 6. Conclusion We have used a combined experimental and theoretical approach to elucidate the adsorption structures of cyclohexene on Pt(111), Pt3Sn/Pt(111), and Pt2Sn/Pt(111) as well as the first steps of the decomposition of the molecule on Pt(111). On Pt(111) two adsorption structures have been identified, which are present in the monolayer: the majority di-σ boat up species and a minority di-σ chair cis species. On Pt3Sn/Pt(111) the di-σ boat up species is by far the most stable one, and the presence of this species is confirmed by HREELS. The adsorption of cyclohexene on Pt2Sn/Pt(111) is quite different from the other surfaces. No unambiguous assignment of the bonding mode is possible, but both the relative energies and

J. Phys. Chem. C, Vol. 112, No. 2, 2008 565 the comparison between the calculated and the experimental HREELS spectra suggest that there is a mixture of several bonding modes including di-σ and π types and a physisorbed state. On Pt(111), the spectra obtained after a flash to temperatures between 260 and 330 K show the disappearance of the most stable di-σ boat up form. However they cannot unambiguously be attributed to any of the calculated species of either cyclohexene or a dehydrogenation product. In contrast, the spectra obtained after a flash to temperatures between 360 and 450 K correspond to dehydrogenated species. The simulated spectra allowed us to assign them to the stable 1,2,3-σ cyclohexenyl and 1,4-σ,2,3-π cyclohexadiene forms, without excluding the presence of some benzene. Contrary to Pt(111) the decomposition of cyclohexene on the Pt3Sn and Pt2Sn surface alloys does not occur and adsorption of cyclohexene on these surfaces is completely reversible. Acknowledgment. The authors gratefully acknowledge the financial support from the Deutsche Forschungsgemeinschaft (DFG) and the Centre National de la Recherche Scientifique (CNRS) through a binational grant. They also thank IDRIS at Orsay (Project 609) and CINES at Montpellier for CPU time. References and Notes (1) Gland, J. L.; Baron, K.; Somorjai, G. A. J. Catal. 1975, 36, 305. (2) Rodriguez, J. A.; Campbell, C. T. J. Catal. 1989, 115, 500. (3) Henn, F. C.; Diaz, A. L.; Bussell, M. E.; Hugenschmidt, M. B.; Domagala, M. E.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5965. (4) Lamont, C. L. A.; Borbach, M.; Martin, R.; Gardner, P.; Jones, T. S.; Conrad, H.; Bradshaw, A. M. Surf. Sci. 1997, 374, 218. (5) Xu, C.; Koel, B. E. Surf. Sci. 1994, 304, 249. (6) Sharpen, L. H.; Wollrab, J. E.; Ames, D. P. J. Chem. Phys. 1968, 49, 2368. (7) Anet, F. A. L.; Haq, M. Z. J. Am. Chem. Soc. 1965, 87, 3147. (8) Rivera-Gaines, V. E.; Leibowitz, S. J.; Laane, J. J. Am. Chem. Soc. 1991, 113, 9735. (9) Laane, J.; Choo, J. J. Am. Chem. Soc. 1994, 116, 3889. (10) Saebø, S.; Boggs, J. E. J. Mol. Struct. 1981, 73, 137. (11) Anet, F. A. L.; Freedberg, D. I.; Storer, J. W.; Houk, K. N. J. Am. Chem. Soc. 1992, 114, 10969. (12) Shishkina, S. V.; Shishkin, O. V.; Leszczynski, J. Chem. Phys. Lett. 2002, 354, 428. (13) Allinger, N. L. AdV. Phys. Org. Chem. 1976, 13, 1. (14) Anet, F. A. L.; Anet, R. Dyn. Nucl. Magn. Reson. Spectrosc. 1975, 543. (15) Geise, H. J.; Buys, H. R.; Mijlhoff, F. C. J. Mol. Struct. 1971, 9, 447. (16) Allinger, N. L.; Miller, M. A.; Van Catledge, F. A.; Hirsch, J. A. J. Am. Chem. Soc. 1967, 89, 4345. (17) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127. (18) Saeys, M.; Reyniers, M.-F.; Neurock, M.; Marin, G. B. Surf. Sci. 2006, 600, 3121. (19) Paffett, M. T.; Gebhard, S. C.; Windham, R. G.; Koel, B. E. Surf. Sci. 1989, 223, 449. (20) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (21) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 48, 13115. (22) Kresse, G.; Hafner, J. Phys. ReV. B 1994, 49, 14251. (23) Kresse, G.; Hafner, J. Phys. ReV. B 1998, 59, 1758. (24) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (25) Delbecq, F.; Sautet, P. J. Catal. 2003, 220, 115. (26) Becker, C.; Delbecq, F.; Breitbach, J.; Hamm, G.; Franke, D.; Ja¨ger, F.; Wandelt, K. J. Phys. Chem. B 2004, 108, 18960. (27) Loffreda, D.; Jugnet, Y.; Delbecq, F.; Bertolini, J.-C.; Sautet, P. J. Phys. Chem. B 2004, 108, 9085. (28) Morikawa, Y. Phys. ReV. B 2001, 63, 033405. (29) Redhead, P. A. Vacuum 1962, 12, 203. (30) Christmann, K.; Ertl, G.; Pignet, T. Surf. Sci. 1976, 54, 365. (31) Breitbach, J.; Franke, D.; Hamm, G.; Becker, C.; Wandelt, K. Surf. Sci. 2002, 507, 18. (32) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf. Sci. 1998, 78, 577. (33) Tsai, Y.-L.; Xu, C.; Koel, B. E. Surf. Sci. 1997, 385, 37. (34) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. J. J. Chem. Phys. 2000, 113, 9901.

566 J. Phys. Chem. C, Vol. 112, No. 2, 2008 (35) Bussell, M. E.; Hennard, F. C.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5978. (36) Morin, C.; Simon, D.; Sautet, P. Surf. Sci. 2006, 600, 1339. (37) Saeys, M.; Reyniers, M.-F.; Neurock, M.; Marin, G. B. J. Phys. Chem. B 2005, 109, 2064.

Delbecq et al. (38) Saeys, M.; Reyniers, M.-F.; Marin, G. B.; Neurock, M. Surf. Sci. 2002, 513, 315. (39) Saeys, M. Ph.D. Thesis, University of Gent, 2002. (40) Hugenschmidt, M. B.; Diaz, A. L.; Campbell, C. T. J. Phys. Chem. 1992, 96, 5974.

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