The Electronic Structure and Orientation of Styrene Adsorbed on FeO

The Electronic Structure and Orientation of Styrene Adsorbed on FeO(111) and. Fe3O4(111)sA Spectroscopic Investigation. M. Wu1hn,† Y. Joseph,‡ P. ...
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7694

J. Phys. Chem. B 2000, 104, 7694-7701

The Electronic Structure and Orientation of Styrene Adsorbed on FeO(111) and Fe3O4(111)sA Spectroscopic Investigation M. Wu1 hn,† Y. Joseph,‡ P. S. Bagus,†,§ A. Niklewski,† R. Pu1 ttner,| S. Reiss,† W. Weiss,‡ M. Martins,| G. Kaindl,| and Ch. Wo1 ll*,† Lehrstuhl fu¨ r Physikalische Chemie I, Ruhr-UniVersita¨ t Bochum, 44780 Bochum, Germany, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany, Fachbereich Physik, Freie UniVersita¨ t Berlin, Arnimallee 14, 14195 Berlin, Germany, and Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77842-3012 ReceiVed: February 18, 2000; In Final Form: May 5, 2000

The adsorption of styrene on (111)-oriented thin layers of FeO and Fe3O4 epitaxially grown on Pt(111) singlecrystal surfaces has been investigated using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). High-resolution gas-phase measurements were recorded, and precise ab initio electronic structure calculations were performed in order to aid the assignment of the NEXAFS resonances. Whereas on Fe3O4 (magnetite), styrene forms a chemisorbed monolayer that is stable up to room temperature, on FeO (wustite), a significantly more weakly bound (physisorbed) species is observed that is stable only for temperatures below 200 K. A planar adsorption geometry is observed in the case of styrene on Fe3O4 in the low-coverage regime (Θ < 0.5 ML); for higher coverages on the same surface and on FeO, the average tilt angle between the phenyl plane and the surface plane amounts to about 45°.

1. Introduction The dehydrogenation of ethylbenzene (EB) to styrene (see Figure 1) is an important industrial reaction and is carried out over iron oxide based catalysts in a mixture of EB and steam at temperatures of around 900 K.1-8 Despite the commercial importance of the reaction, the mechanism is not well understood. In the present study, we have used single-crystalline iron oxide films epitaxially grown on Pt(111) with FeO and Fe3O4 stoichiometry under UHV conditions. It has been demonstrated previously by scanning tunneling microscopy (STM)9-12 and low-energy electron diffraction (LEED)13,14 that these films exhibit a high structural quality. The FeO(111) (wustite) film consists of a single hexagonal close-packed iron-oxygen bilayer (1.24 Å) with a lateraly expanded lattice constant (3.11 Å vs 3.04 Å in the bulk) and is oxygen-terminated.15,16 In contrast, the thicker Fe3O4(111) (magnetite) film (100-200 Å) crystallizes in the cubic inverse-spinel structure in which the oxygen anions form a close-packed fcc sublattice with tetrahedrally and octahedrally coordinated Fe2+ and Fe3+ cations located in the interstitial sites. The surface consist of a hexagonal oxygen layer that is terminated by 1/4 ML of iron atoms. The electronic structure of the iron oxide films was previously investigated with photoelectron spectroscopy (UPS/XPS) 17 and X-ray absorption spectroscopy (NEXAFS)18 and is comparable to that of the corresponding bulk oxides.19 The adsorption of styrene on FeO and on Fe3O4 has previously been investigated by Kuhrs et al. using TDS.11,20 Their data show significant differences for the two iron oxide surfaces. On FeO, two different adsorbate species were found, whereas on Fe3O4, three different types could be identified. In * Author to whom correspondence should be addressed. † Ruhr-Universita ¨ t Bochum. ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft. § Texas A&M University. | Freie Universita ¨ t Berlin.

Figure 1. Principal catalytic reaction and required conditions for the conversion of ethylbenzene to styrene.

both cases, the desorption temperatures of the first and the second species amount to 150 K (condensed γ species) and 215 K (physisorbed β species). On Fe3O4, an additional species with a desorption temperature in the range between 220 and 420 K (chemisorbed R species) was reported. In recent years, it has been demonstrated that, for the identification and characterization of molecular adsorbates on metal-oxide surfaces, the application of NEXAFS (near-edge X-ray absorption fine structure) spectroscopy is particularly powerful, as the element-specific detection scheme allows for the separate determination of the contributions of the adsorbed molecule to the total electonic density of states. For example, see the case of CO and pyridine on different ZnO surfaces.21-23 In the present study, we have investigated the adsorption geometry of styrene on thin oxide layers of FeO (∼3 Å) and Fe3O4 (∼50-200 Å) by means of NEXAFS spectroscopy. The assignment of the different NEXAFS resonances is achieved through comparisons to high-resolution gas-phase data and to the results of ab initio Hartree-Fock calculations. 2. Experimental Section (a) Instrumentation. All experiments were carried out at the Berlin synchrotron radiation facility BESSY I. The NEXAFSs

10.1021/jp0006734 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/25/2000

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Figure 2. XP spectra recorded for the two different substrate surfaces used in the present study, Fe3O4(111)/Pt(111) and FeO(111)/Pt(111). The element-specific lines for Pt, Fe, and O can be clearly identified. The inset in the upper right corner shows an enlarged view of the Fe2p lines, including their satellites.

and XPSsdata were measured under ultrahigh vacuum (UHV) conditions (base pressure better than 5 × 10 -10 mbar) at the beamline HE-TGM-2 with a resolution E/∆E ) 580.24 The main chamber was equipped with a sputter gun for ion bombardment (Ar+); an iron evaporator; a gas dosing line for argon, oxygen and hydrogen; a LEED unit, a twin anode X-ray source (VG); and a CLAM2 energy analyzer (VG). All XP spectra were recorded at hν ) 1253.6 eV (Mg KR anode). The organic molecules were adsorbed by backfilling the chamber through a leak valve. The binding energy and intensity of the C1s peaks were determined by fits to Gaussian peaks after subtraction of a Shirley background.25 The purity of the dosed gases was verified by a quadrupole mass spectrometer QMA (Pfeiffer). The NEXAFS spectra were recorded by a homemade detector based on a double-channel plate (Galileo). To increase the surface sensitivity, the contributions from inelastically scattered electrons was reduced by recording the spectra in the partialelectron-yield (PEY) mode with a retarding voltage of -150 eV. The normalization procedure for the raw data consists of the following steps: First, the signal of the clean substrate was subtracted. Then, the data were normalized by dividing through a spectrum recorded from a freshly sputtered Au wafer. Finally, a constant background was subtracted. In all NEXAFS spectra shown here, intensities are provided in units of the edge jump height (difference in intensity between 280 and 325 eV). A gold grid was used for simultaneous recording of the flux of the incident radiation (I0 signal), which serves as a flux monitor and is used for calibrating the exact energy position of the monochromator. The sample was heated to 1300 K by electron bombardment and could be cooled to 100 K using liquid

nitrogen, while the temperature was controlled via a Ni-NiCr thermocouple. The high-resolution gas-phase measurements of styrene were performed at the monochromator SX700/II, which is operated by the Freie Universita¨t Berlin.26 A gas ionization cell was employed and separated from the ultrahigh vacuum of the monochromator by an aluminum window with a thickness of 1000 Å. The gas ionization cell consisted of two stainless steel plates with an active length of 10 cm that functioned as a collector for the charged (photoionized) particles and was filled with styrene at a pressure of approximately 30 µbar. During the measurements, a constant gas flux through the gas ionization cell was ensured in order to avoid contributions of possible cracking products. The spectra were calibrated using the N 1s f π* excitation in N2 at a photon energy of 400.88 eV.26 From fits of the latter resonance with a constant natural line width of Γ ) 115 meV 27 and application of the formula ∆E ∝ E3/2 a resolution of 110 meV at the C1s ionization threshold was derived. (b) Preparation. Prior to growth of the iron oxide layers, the Pt(111) single crystal was cleaned by standard procedures involving sputtering with Ar+ ions and annealing at 1300 K. The iron films were grown epitaxially and subsequently oxidized, as described in detail in refs 9-14. The structural quality of the surfaces was monitored by LEED, and the cleanliness of the surface (in particular with regard to residual carbon contaminations) was checked with XPS and NEXAFS at the C K-edge. The adsorbed layers were prepared by exposing the surface to different amounts of styrene. The dosages required to form

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Figure 3. XP spectra recorded for different styrene coverages on FeO. Exposures and deposition temperatures are also provided. With increasing coverage, a shift toward higher energies is observed. The inset gives a schematic model of the different coverage regimes at different surface temperatures.

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Figure 4. XP spectra recorded for different styrene coverages on Fe3O4, together with exposures and deposition temperatures. With increasing coverage, a shift toward higher energies is observed. The inset gives a schematic model of the different coverage regimes at different surface temperatures. For a direct comparison, see Figure 3.

multi-, mono-, and submonolayers were taken from previous investigations based on TDS.11,20 XPS was used to monitor the adlayer thickness. 3. Results (a) XPS Data. The photoemission spectra of the clean Fe3O4 and FeO surfaces, shown in Figure 2, reveal significant differences. In the case of the FeO film (lower trace B), the Pt lines 4f7/2 and 4f5/2, 4d5/2 and 4d3/2, and 4p3/2 are easily identified, whereas the 4p1/2 and the 4s lines are less intense. In addition, the spectrum exhibits iron and oxygen lines, namely, the O 1s peak (530.1 eV), the Fe 2p peaks (724.3 and 710.7 eV), and the Fe LMM Auger lines (655 eV, 606 eV, and 551 eV). Their energetic positions are in good agreement with literature data.18,28 In contrast the Fe 2s and the O KLL Auger lines can hardly be seen. In the spectrum recorded for the Fe3O4 film (upper trace A), the absence of the Pt peaks indicates a significantly greater thickness of the oxide film (>50 Å). The inset in the upper right corner of Figure 2 provides an enlarged view of the Fe 2p region around 724 and 710 eV. In addition to the broad, dominant Fe lines, additional shake-up satellites can be recognized. In the case of FeO, only contributions of Fe2+ ions are visible. In contrast, Fe2+ and Fe3+ satellites are observed for the Fe3O4 surface. These findings are in good agreement with previous results that were described in detail in refs 18 and 19. After characterization of the two clean iron oxide surfaces by LEED and XPS, styrene films of different coverages were prepared as described above. Figures 3 and 4 show the corresponding C1s XPS data recorded for different coverages. A schematic sketch of the different phases formed on the surface for different substrate temperatures is displayed as an inset. The binding energy scale was referenced to the substrate O1s lines (Ebind ) 530.1 eV).18,29 The resulting energetic positions of the

Figure 5. Multilayer NEXAFS spectra of styrene (above) and benzene (below). There are five main resonances labeled A-E. Note the weak shoulder at lower photon energies in the spectra of styrene.

C1s lines are also provided in the diagram. The thickness of the styrene adlayer was calculated from the attenuation of the Pt 4f and the O1s substrate peaks using the mean free paths reported in refs 30 and 31. Here, a styrene monolayer is defined as the saturation coverage on the FeO substrate at a substrate temperature of 200 K. (b) NEXAFS Data. In the upper part of Figure 5, a C1s NEXAFS spectrum recorded for a thick multilayer of styrene is presented, together with corresponding data for a benzene multilayer recorded at the same beamline (HE-TGM-2).24 In the spectra, five main features labeled A-E can be distinguished. The dominant resonance (A) is located at 285.0 eV for styrene and at 285.2 eV for benzene. This resonance is assigned to a C1s f π* transition for both molecules and corresponds to excitations into the lowest unoccupied molecular orbital (LUMO) of the phenyl ring and of benzene. In the case of styrene, an additional shoulder is present at 284.1 eV with an intensity ratio relative to resonance A of 0.12. In this work we also present ab

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TABLE 1: Energetic Positions of the Observed Resonancesa and Their Assignments32 resonance

EBenzene (eV)

EStyrene (eV)

∆E (eV)

assignment

A

285.2

B C D E

287.6 288.9 293.3 300.2

285.0 284.1 287.7 289.1 293.6 303.7

-0.2 +0.1 +0.2 +0.3 +2.5

π* (C1s f e2u) π* (C1s f e2u) Rydberg π* (C1s f b2g) σ* (C1s f e1u) σ* (C1s f e2g + a2g)

a For a detailed comparison of the corresponding NEXAFS spectra, see Figure 5.

Figure 7. NEXAFS data for the styrene/Fe3O4 system recorded for normal (90°) and grazing (30°) incidence. The data for the multilayer on the top can be compared to those for lower coverages (bottom). The film thickness was calculated from the XP spectra for which one monolayer (1.0 ML) is defined as the saturated coverage of styrene of FeO. Note the strong dichroism for the spectra at low coverage at T ) 240 K, which reveals a flat adsorption geometry.

Figure 6. NEXAFS data for the styrene/FeO system recorded for normal (90°) and grazing (30°) incidence. The data for the multilayer on the top can be compared to those for lower coverages (bottom). The film thickness was calculated from the XP spectra for which one monolayer (1.0 ML) is defined as the saturation coverage of physisorbed styrene layer of FeO at 200 K. The tilt angles between the molecular and surface planes, as obtained from an analysis of the dichroism (see text), are also provided.

initio Hartree-Fock calculations that allow for the identification of this feature as transitions from C1s core electrons localized at the carbon atoms C7 and C8 of the vinyl group (see Figure 1) into the LUMO (1s f π*). An assignment of all resonances is provided in Table 1. For benzene, the assignments are taken from ref 32. The assignments for styrene are based on a comparison to the results for benzene and on our ab initio calculations, which are presented below. Figures 6 and 7 shows the NEXAFS spectra for styrene adlayers of different thickness adsorbed on FeO and Fe3O4. The multilayer data displayed at the top of the two figures were recorded at an incident angle of 55°, whereas the monolayer data were recorded at normal (90°, straight line) and grazing (30°, dotted line) incidence. The film thickness decreases from the top of the figure to the bottom. In these films, the dominant first π* resonance exhibits a strong dichroism with a maximum in intensity at grazing incidence (30°). From the intensity ratios, the direction of the transition dipole moment (TDM), and thus of the tilt angle between the molecular plane and the surface normal, can be calculated.33 Figures 6 and 7 provide a list of tilt angles obtained from a corresponding analysis of the data. The accuracy is (2°.

For a more detailed analysis of the electronic structure of adsorbed styrene, we have investigated the first π* resonance at 285 eV in more detail. In Figure 8, NEXAFS spectra of the saturated monolayers are shown for two different angles of incidence in a smaller energy range around the first π* resonance, together with the corresponding data for the multilayer. The peak at about 285 eV was fit by two Gaussians located at 284.1 and 285.0 eV ((0.1 eV) with a full width at half-maximum (fwhm) of 0.8 eV ((0.1 eV). The π* resonance at 284.1 eV has an intensity of about 0.1-0.2 when normalized to the main π* resonance at 285.0 eV and reveals the same dichroism. (c) Gas-Phase Spectra. Figure 9c displays the photoabsorption spectrum of gas-phase styrene in the region of the first C 1s f π* excitation. The photon energy was corrected with an accuracy of better than 20 meV. The spectrum of gas-phase styrene is very similar to that of condensed-phase (multilayer) styrene shown in Figure 9d, but reveals a considerably richer fine structure. This richer fine structure is attributed to the improved experimental resolution of the gas-phase spectrum (110 meV) as compared to that of the condensed-phase spectra with a resolution of 500 meV, because high-resolution condensedphase34,35 and gas-phase36 spectra of ethylene and benzene show the same fine structure in the C 1s f π* excitations. (d) Ab Initio HF Calculations. To investigate the origin of the dominant first π* resonance at about 285 eV, ab initio Hartree-Fock calculations were performed. Our theoretical study of the NEXAFS spectra of an isolated styrene molecule is based on the use of ab initio wave functions that describe excitations from a localized C1s level to an unoccupied valence level. Calculations were carried out for styrene in a planar geometry.37 In particular, the torsional angle of the vinyl group out of the ring plane is taken to be zero. Note that there is experimental and theoretical evidence 38 that this angle is ∼27°

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Figure 8. Simulation of the experimental NEXAFS data for the styrene/ FeO and styrene/Fe3O4 systems performed by fitting two Gaussians. The monolayer data recorded at two different angles of X-ray photon incidence (30° versus 90°) are compared to the corresponding multilayer data taken at 55°. See also Figures 6 and 7.

for gas-phase styrene. We have used the planar geometry in order to simplify the calculations. Ab initio Hartree-Fock, self-consistent-field (SCF) wave functions were determined in order to obtain the NEXAFS energies and intensities, as presented in Table 2. See, for example, ref 39 for general information about the calculation, interpretation, and accuracy of SCF wave functions. Because the excitations of C1s to π* place an additional electron in the valence space, a flexible basis set of better than triple-ζ quality in the valence region was used for the C atoms; the C atom basis set contained 5s and 4p contracted Gaussian-type orbitals, CGTOs. A smaller basis set of 2s and 1p CGTOs was used for the H atoms. In our previous theoretical work on NEXAFS excitations in alkanes,40,41 we used approximations that simplified the calculations of the excited-state wave functions and energies. First, we took the excited orbitals for the C1s f nλ NEXAFS wave functions to be the low-lying virtual, unoccupied orbitals in the SCF calculation for a C1s ion. Furthermore, we took the energies of the excited states as the SCF orbital energies, , of these virtual orbitals; of course,  represents term energies with respect to the ionization limit. These approximations have advantages when there is interest in several excited states that arise from a given core-level excitation because the orbitals and energies for all of the excited states are available from a single calculation for the ionic limit. However, when they are used, two potentially important aspects of the excitation are neglected. First, the splitting of the different, singlet and triplet, spin couplings of the singly occupied C1s level with the singly occupied excited level is neglected. The energy given by  is a weighted average,

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Figure 9. C 1s f π* excitation in the molecule styrene in detail: (a) Results of the Hartree-Fock calculations convoluted with a Lorentzian with a fwhm of 200 meV. (b) Theoretical results modified by inclusion of vibrational excitations (see text). (c) High-resolution gas-phase spectrum of styrene. The vertical-bar diagram indicates the presence of the C-H vibrational stretching mode. d) NEXAFS spectrum of a condensed multilayer of styrene. The theoretical spectra in panels a and b were shifted by 1.3 eV toward lower photon energy.

TABLE 2: Calculated Energy Positions and Relative Intensities of the Contributing C Atoms

a

carbon atom

energy position (eV)a

intensity (arb units)

C5 (phenyl) C1 (phenyl) C4 (phenyl) C3 (phenyl) C2 (phenyl) C6 (phenyl) C7 (vinyl) C8 (vinyl)

(0 (286.49 eV) -0.01 -0.12 -0.19 -0.30 -0.38 -1.06 -1.46

1.00 1.10 1.07 0.99 0.94 1.04 1.16 1.16

Relative to C1.

∆E, of the singlet-coupled, ∆ES, and triplet-coupled, ∆ET, excitation energies such that  ) 1/4(∆ES) + 3/4(∆ET) (see refs 40 and 41 and references therein. However, only the excitation to the singlet-coupled excited state is dipole allowed in the NEXAFS spectra, and the measured energies are closer to ∆ES rather than to the weighted average. For alkanes, which are saturated hydrocarbons, the only NEXAFS excitations bound with respect to the C1s ionization limit are to Rydberg excited levels, and the singlet-triplet splitting is negligibly small. Thus, the use of ∆E (or ) rather than the excitation energy to the singlet-coupled excited state does not introduce any significant uncertainty. For the C1s to π* excitations in styrene, the excited π* orbitals are much more compact than Rydberg levels and the singlet-triplet splittings are ∼0.3-0.4 eV; thus, ∆E differs from ∆ES by ∼0.25 eV. The second approximation introduced by the use of the virtual orbitals of the ionic wave function is that the relaxation of the orbitals due to the occupation of the excited level is neglected. For the diffuse

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excited Rydberg orbitals found for the bound NEXAFS excited states of alkane molecules, this additional relaxation is small and leads to a lowering of the excited-state energies, denoted ER, which is ER e 0.1 eV and can be neglected. For the π* excited levels in styrene, this additional relaxation energy is much larger, ER ∼ 1.0 eV. Furthermore, ER is not constant but varies by ∼0.8 eV from the smallest to the largest values for the various C1s excitations; in particular, the ER values for the excitations of the two vinyl C1s to π* are larger than those for the six excitations of phenyl C1s to π*. Because the approximations related to the neglect of the singlet-triplet splitting and of the additional ER are large for the NEXAFS excitation energies in styrene, it is not appropriate to use the same approach as for the Rydberg excited states of the alkanes. Instead, we have directly computed the SCF wave functions and energies for the singlet-coupled C1s to π* excited states; thus, we take explicit account of the singlet-triplet splitting and the additional ER contribution. Furthermore, we computed many-electron transition matrix elements in order to obtain the relative intensities of the NEXAFS excitations. In particular, the relative intensity, Irel, is taken to be proportional to the square of the matrix element of z

Irel ∝ |〈Ψ(GS)|z|Ψ(1s f π*)〉|2

(1)

where Ψ(GS) and Ψ(1s f π*) are the many-electron SCF wave functions for the ground and excited states, respectively, and z ) Σzi with i summed over all electrons. Because the styrene molecule is assumed to be in the x,y plane, the only nonzero component of the dipole transition matrix element is for z. The expression of eq 1 does not include the contribution of the excitation energy, E(1s f π*) - E(GS), to the intensity. However, because the Irel values of eq 1 are for NEXAFS states whose energy changes by ∼1 eV out of a total excitation energy of ∼285 eV, this neglect is entirely acceptable. Figure 9a shows a theoretical absorption spectrum that was obtained using Lorenzians with a fwhm of 0.2 eV as a simulation for both the experimental resolution of 110 meV for the gasphase spectrum and the natural line width. The lines are centered at the positions computed for the different C atoms with the intensities obtained from the calculations. 4. Discussion The present results reveal significant differences concerning the adsorption of styrene on the two investigated iron oxide surfaces, in agreement with the TDS measurements reported previously by Kuhrs et al.11,20 A monolayer is stable for temperatures up to 220 K in the case of Fe3O4(111), whereas on FeO(111), desorption takes place already at 200 K. For both surfaces, the formation of a multilayer is observed for temperatures below 160 K. On the Fe3O4 substrate an intermediate species was detected at 200 K, the same temperature as that at which the styrene monolayer desorbs from the FeO surface. Noting that the FeO surface is terminated by oxygen atoms only whereas the Fe3O4 top layer consists of iron (1/4) and oxygen (3/4) atoms, the following assignment is proposed: At a temperature around 240 K, a chemisorbed species is identified on Fe3O4 but not on FeO (γ species) by an enlarged fwhm of the first π* resonance (see below). At lower temperatures (200 K), a physisorbed species (β species) is detected on both surfaces, and for temperatures below 160 K, the onset of condensation (R species, formation of multilayers) is observed. The absence of the γ species on FeO indicates a significantly

weaker interaction of styrene with the substrate when compared to Fe3O4. Similar results are observed for the adsorption of ethylbenzene using UPS42 and TDS.43 Also, the coverage estimated for the chemisorbed monolayer on Fe3O4(111), namely, 70% of the physisorbed monolayer on FeO, is consistent with the result for ethylbenzene.42 For thin (monolayer and submonolayer) films on both iron oxide surfaces the NEXAFS spectra recorded under normal (90°) and grazing (30°) incidence of the exciting X-rays show a significant dichroism. The analysis of the spectra taken for films in the submonolayer regime,33 as shown in Figures 6 and 7 for the FeO(111) and Fe3O4(111) surfaces, respectively, reveals that the molecules adsorb with a distinctly different orientations. In the case of Fe3O4, for the unsaturated monolayer at the 240 K surface temperature, the average tilt angle of the molecular plane relative to the substrate surface is 28°. With increasing coverage the tilt angle increases and reaches a value of 42° for the saturated monolayer at T ) 225 K. Subsequent dosing of styrene onto the substrate at the same surface temperature changes neither the XP nor the NEXAFS spectra. Additional dosing at lower temperatures (T < 220 K) leads to the growth of bilayers with an average tilt angle close to the magic angle of 54.6°, indicating the presence of a random distribution of molecular orientations. At surface temperatures T < 160 K, the formation of multilayers is observed. In the case of FeO, for the unsaturated styrene monolayer at a surface temperature of T ) 200 K, the average tilt angle of the molecular plane relative to the substrate surface is 41°. With increasing coverage up to 1 ML, no significant change is observed. At saturation coverage, the average tilt angle is 43°. Exposing the surface to styrene vapor at temperatures below 160 K again results in the growth of multilayers, and the average tilt angle of the TDM approaches the magic angle of 54.6 °. The adsorption of styrene on the iron oxide surfaces can be compared to the results for benzene adsorbed on different metal surfaces in which a generally flat adsorption geometry is observed. In the case of a weak interaction with the substrate, e.g., on Au(111), almost no intensity is observed for the first π* resonance for spectra taken under normal incidence (90°), and the resonance width shows no broadening relative to that of the multilayer.44 For a strong adsorbate-substrate interaction, e.g., on the Rh(111) and Pt(111) surfaces, the electronic structure changes strongly as a result of the increased chemical interaction between the molecule and the substrate. Particularly obvious is the broadening of the π* resonance.44 Weiss et al. demonstrated that these observations are consistent with an out-of-plane molecular distortion, in which the C-H bonds are bent away from the surface plane.45 As a result, the symmetry is reduced, and the C2px and C2py orbitals can mix into the π* MO. In analogy to these previous observations, we relate the decreased dichroism (small intensity in the normal incidence spectra in Figure 7 at 285 eV) seen for the chemisorbed styrene species on Fe3O4 to the formation of a weak chemical bond. This conclusion is corroborated by a line-shape analysis, in which the first π* resonance at 285.0 eV was determined by fitting a Gaussian. The resulting value of 1.1 eV is slightly, but significantly, larger than those observed for the higher coverages of styrene on Fe3O4 and those seen for all adlayers on FeO (0.9 eV). Because in the case of benzene adsorbed on various transition metals the width of the π* resonance has been shown to correlate with the presence of a significant chemical bonding to the substrate,44 we propose that, for low coverages (Θ < 0.5 ML), styrene weakly chemisorbs on Fe3O4 with a significant interaction between the molecular π system and the substrate,

7700 J. Phys. Chem. B, Vol. 104, No. 32, 2000 leading to a mainly planar adsorption geometry. When the coverage is increased further, the interaction between the molecule and the surface is reduced, leading to a less specific adsorption, which is consistent with the large tilt angle of 42° and the reduced line width. On the FeO surface, the more strongly bound, flat-lying species is not observed. We propose that the stronger binding energy of the chemisorbed species is due to an interaction between the styrene π system and the Fe 3+ ions present at the Fe3O4 surface. These ions are absent on the FeO surface, which is terminated by O2ions and for which the spectroscopic data point toward the absence of a pronounced adsorbate/substrate interaction. The gas-phase spectrum (Figure 9c) shows a main peak at around 285 eV and a shoulder located approximately 0.8 eV below. The presence of two lines with different intensities can be readily understood by considering that styrene consists of a phenyl ring and a vinyl group. The shoulder at 284.1 eV can be assigned to C1s excitations localized at the C atoms in the vinyl group (C7 and C8), and the main peak at 285 eV to excitations at the C atoms of the phenyl ring (C1-C6). This assignment is supported by the fact that the splitting between the π* resonance for the core holes localized at the vinyl C atoms and the phenyl C atoms is similar to that between ethylene and benzene (∼1 eV).34,35 On the basis of this simple model, the intensity ratio can be explained by the ratio of the numbers of carbon atoms in the vinyl group and the phenyl ring. Of course, the actual electronic structure of styrene is more complicated than just the simple addition of the electronic features of ethylene and benzene because (i) the molecule possesses eight nonequivalent carbon atoms and (ii) there is an interaction between the π system of the vinyl group and the phenyl group. Precise Hartree-Fock calculations explicitly considering the presence of the core hole are, therefore, required for a reliable assignment. The high-resolution NEXAFS data shown in Figure 9c also reveal a series of structures with a spacing of around 320 meV and with decreasing intensity for increasing photon energy, as indicated by the vertical bar diagram. To understand these structures, we note that the C 1s f π* excitations in the photoabsorption spectra of benzene and ethylene reveal a strong vibrational fine structure.34-36 Both molecules exhibit a number of vibrational modes with vibrational energies in the ground state of around 375 meV for the C-H stretching modes, around 120-200 meV for the C-C stretching modes, and around 100 meV for the bending modes.46 For a number of small molecules (e.g., N2, CO, and NO), it has been demonstrated previously that a core excitation into a π* level leads to a weakening of the bond and a corresponding decrease in the vibrational energies as compared to the ground state.47-49 As a consequence of the arguments listed above, we also expect vibrational fine structures for the C 1s f π* excitation in styrene with vibrational energies below the values given above and assign the structures observed in the spectrum of styrene with a splitting of around 320 meV to an excitation of the C-H stretching mode. The presence of the vibrational structure can also explain the differences between the experimental gas-phase spectrum (Figure 9, top two spectra) and the calculated spectrum convoluted with a Lorenzian of 200 meV to simulate the natural line width and the experimental resolution (Figure 9, bottom). Here, the most striking difference is that, in the calculated spectrum, the C7,8 1s f π* excitation can be observed as a separate peak, whereas in the gas-phase spectrum, these excitations are only visible as a shoulder. To underline that these differences can be explained with vibrational bands, a simple

Wu¨hn et al. simulation was performed. In this model, we only used the HF calculations displayed in Figure 9 and one vibrational progression with a splitting of 320 meV to simulate the C-H stretching modes. From spectra available in the literature,34-36 we estimated the intensity ratio of the vibrational bands to be 1:0.6: 0.4:0.2:0.1 for the v′′ ) 0 f v′ ) 0, 1, 2, 3, 4 transitions, respectively, where v′′ and v′ represent the vibrational states of styrene in the electronic ground state and the C 1s f π* coreexcited state, respectively. The described vibrational progression is used for all eight Cn 1s f π* excitations. The vibrational energy of the symmetric C-C stretching modes in the phenyl ring amounts to 120 meV. These modes are neglected in the simulation as the splitting is smaller than the convoluted line width of 200 meV. The C-C stretching mode in the vinyl group with a vibrational splitting of 200 meV is taken into account by using a larger line width of 300 meV for all lines related to the C7,8 1s f π* excitations. The result of this simulation is shown in Figure 9b and reveals a significantly better agreement with experiment. The vibrational progression in the high-energy regime of the C7,8 1s f π* excitations of the phenyl ring are described correctly, and the presence of the C7,8 1s f π* excitations as shoulders instead of separate peaks is also reproduced. Although the agreement between this simple simulation and the experimental gas-phase spectrum is certainly not perfect, a more detailed analysis of the vibrational fine structure is rather difficult and beyond the scope of this work, as the vibrational bands for all eight Cn 1s f π* (n ) 1-8) excitations are all expected to be different. For example, atom C5 has no neighboring H atom and, as a consequence, only weak C-H stretching modes are expected for the C5 1s f π* excitation. Contrary to this, for all other Cn 1s f π* excitations, the C-H stretching mode is expected to be significantly stronger. This is particularly important for the C8 1s f π* excitation, as there are two H atoms bonded to this C atom. 5. Summary The present detailed spectroscopic study of the adsorption of styrene on iron oxide surfaces reveals significant differences between the different oxides in their chemical behavior in agreement with previous TDS measurements by Kuhrs et al.11,20 A chemisorbed species on Fe3O4, but not on FeO, could be observed, whereas physisorbed and condensed species were detected on both surfaces. For Fe3O4 (magnetite), a mostly planar adsorption geometry (28° between molecular and surface plane) is observed for the low-coverage regime. For FeO (wustite), on the other hand, a significantly larger tilt angle of about 41° is seen. When the coverage approaches 1 ML on both surfaces, similar tilt angles (42° for Fe3O4 and 43° for FeO) are observed. Together with a small but distinct broadening of the π* resonance of styrene in the low-coverage regime on Fe3O4, this observation strongly indicates that a weak bond is formed between the Fe3+ ions present on this surface and the phenyl π systems, which is found to be absent on the FeO surface. These observations lead to the conclusion that the iron cations on the oxide surfaces are necessary for chemisorption and must play an important role in the catalytic dehydrogenation of ethylbenzene to styrene. The assignments of the NEXAFS resonances are supported by high-resolution gas-phase spectra, in which vibrational fine structure could be observed, and ab initio Hartree-Fock calculations. A shoulder at 284.1 eV of the dominant π* resonance at 285.0 eV could be assigned to (1s f π*) excitations located at the vinyl carbon atoms. This resonance can thus be used to discriminate between ethylbenzene and

Styrene Adsorbed on FeO(111) and Fe3O4(111) styrene and will be used to directly follow the conversion reaction in future work. Acknowledgment. This project was funded by the German BMBF (05625VHA3 and 05-SR8KE1-1) and supported by the Deutsche Forschungsgemeinschaft (Contract No. WE 1372/5-1 and Do-561/1-3). The authors thank M. Mast and W. Braun at the Berlin synchrotron facility BESSY I and W. Ranke, M. Swoboda, and R. Schlo¨gl from the Fritz-Haber Institut, Berlin, for their excellent technical and scientific support. We also acknowledge partial computer support provided by the National Center for Supercomputing Applications, Urbana-Champaign, Illinois. References and Notes (1) Addiego, W. P.; Estrada, C. A.; Goodman, D. W.; Rosynek, M. P.; Windham, R. G. J. Catal. 1994, 146, 407. (2) Coulter, K.; Goodman, D. W.; More, R. G. Catal. Lett. 1995, 31, 1. (3) Hirano, T. Appl. Catal. 1986, 26, 65. (4) Hirano, T. Appl. Catal. 1986, 29, 119. (5) Lee, E. H. Catal. ReV. 1973, 8, 285. (6) Muhler, M.; Schu¨tze, J.; Wesemann, M.; Rayment, T.; Dent, A.; Schlo¨gl, R.; Ertl, G. J. Catal. 1990, 126, 339. (7) Muhler, M.; Schlo¨gl, R.; Ertl, G. J. Catal. 1992, 138, 413. (8) Zscherpel, D.; Weiss, W.; Schlo¨gl, R. Surf. Sci. 1997, 382, 326. (9) Ritter, M.; Ranke, W.; Weiss, W. Phys. ReV. B 1998, 57, 7240. (10) Weiss, W.; Ritter, M. Phys. ReV. B 1999, 59, 5201. (11) Shaikhutdinov, S. K.; Joseph, Y.; Kuhrs, C.; Ranke, W.; Weiss, W. Faraday Discuss. 1999, 144, 363. (12) Shaikhutdinov, S. K.; Weiss, W. Surf. Sci. Lett. 1999, 432, L627. (13) Ritter, M.; Weiss, W. Surf. Sci. 1999, 432, 81. (14) Weiss, W.; Barbieri, A.; VanHove, M. A.; Somorjai, G. A. Phys. ReV. Lett. 1993, 71, 1848. (15) Galloway, H. C.; Sautet, P.; Salmeron, M. Phys. ReV. B 1996, 54, 145. (16) Fadley, C. S.; VanHove, M. A.; Hussain, Z.; Kaduwela, A. P. J. Electron Spectrosc. Relat. Phenom. 1995, 75, 273. (17) Cai, Y. Q.; Ritter, M.; Weiss, W.; Bradshaw, A. M. Phys. ReV. B 1998, 58, 5043. (18) Schedel-Niedrig, T.; Weiss, W.; Schlo¨gl, R. Phys. ReV. B 1995, 52, 17449. (19) Wandelt, K. Surf. Sci. Rep. 1982, 2, 1. (20) Kuhrs, C.; Weiss, W. Stud. Surf. Sci. Catal., in press. (21) Ho¨vel, S.; Kolczewski, C.; Wu¨hn, M.; Albers, J.; Weiss, K.; Staemmler, V.; Wo¨ll, C. J. Chem. Phys. 2000, 112 (8), 3909. (22) Lindsay, R.; Gutierrez-Sosa, A.; Thornton, G.; Ludviksson, A.; Parker, S.; Campbell, C. T. Surf. Sci. 1999, 439, 131. (23) Walsh, J. F.; Davis, R.; Muryn, C. A.; Thornton, G.; Dhanak, V. R.; Prince, K. C. Phys. ReV. B 1993, 48, 14749.

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