J. Phys. Chem. B 2002, 106, 7489-7498
7489
Density Functional Study of Benzene Adsorption on Pt(111) Mark Saeys, Marie-Franc¸ oise Reyniers, and Guy B. Marin* Laboratorium Voor Petrochemische Techniek, UniVersiteit Gent, Krijgslaan 281 S5, B-9000 Gent, Belgium
Matthew Neurock Department of Chemical Engineering, School of Engineering and Applied Science, UniVersity of Virginia, Thornton Hall, CharlottesVille, Virginia 22904-4741 ReceiVed: January 16, 2002; In Final Form: May 14, 2002
The adsorption of benzene on Pt(111) was analyzed using first-principles density functional theoretical cluster and periodic slab calculations. The preferred adsorption site at low coverage is the bridge(30) site with an adsorption energy of 117 kJ/mol. At the bridge(30) site, two of the C pz orbitals are well aligned for overlap with the metal dz2 and dyz orbitals, leading to a strong C-Pt bond and a strong adsorption energy. The molecule’s second important site is the hollow(0) site with an adsorption energy of 75 kJ/mol. Comparing calculated and experimental vibrational frequencies confirms the preference for the bridge site at low coverage and also indicates that adsorption at the hollow(0) site becomes preferred at higher coverage. Adsorption at the hollow(30), the bridge(0) and at the atop sites was found to be unfavorable.
I. Introduction Platinum catalysts are widely used in the conversion of aromatic molecules.1,2 Hydrocracking of hydrocarbon feedstock is perhaps one of the most important processes in petroleum refining for the production of high-quality fuels. Hydrocracking is typically carried out over bifunctional Pt/zeolite catalysts.3 Environmental concerns as well as legislation continue to impose stricter limits on the aromatic content of fuels.4,5 Several possibilities currently exist for reducing the aromatic content to meet these legal restrictions. The first involves hydrotreating the fuel products. The second involves optimizing hydrocracking process conditions to reduce the aromatic fraction. Both will likely require a detailed understanding of the electronic interactions, chemisorption properties, and reaction mechanisms for aromatics over metal surfaces. The adsorption of benzene over different transition metal surfaces has been the subject of ongoing research for quite some time, for benzene is a prototypical aromatic compound. Ab initio studies of the adsorption of aromatic molecules on transition metals have been limited however, because of the large size of the aromatic molecules and consequently the large number of transition metal atoms that would be required to create an adequate model of the metal surface. Larger molecules also increase the number of possible adsorption orientations. Benzene is an ideal probe molecule for experimental and theoretical studies of aromatics because of its minimum number of atoms and its high symmetry. The fcc(111) surface is an ideal substrate in that it too is highly symmetric. In addition, it is also the closest-packed facet and the thermodynamically most stable. Of particular interest in these studies are questions of surface bonding and geometry and electronic and structural changes upon chemisorption. In this paper, we will explain the experimental observations and provide an electronic elucidation of the benzene-platinum system. * To whom correspondence should be addressed. E-mail: Guy.Marin@ rug.ac.be. Fax: ++32 9 2644999. Phone: ++32 9 2644516.
The adsorption of benzene on Ni(111) and Pt(111) has been examined experimentally as well as theoretically. Several experimental studies provide an invaluable background for the present theoretical work. The benzene/Pt(111) system has been studied in great detail using a wealth of analytical characterization techniques, including reflection absorption IR spectroscopy6 (RAIRS), scanning tunneling microscopy7,8,9 (STM), temperature programmed desorption10,11 (TPD), electron energy loss spectroscopy12 (EELS) and high-resolution EELS,13 near edge X-ray absorption fine structure14 (NEXAFS), diffuse low energy electron diffraction15 (LEED), and angle resolved ultraviolet photoelectron spectroscopy16 (ARUPS). Recently, Sheppard and de la Cruz17 reviewed and discussed the results of these spectroscopic studies for benzene adsorption on different transition metals. Theoretical calculations have also been performed on the benzene/Pt(111) system. Anderson et al.18 used semiempirical atom superposition and electron delocalization molecular orbital (ASED-MO) theory. Roszak and Balasubramanian19 performed ab initio calculations for the interaction of benzene with a single Pt atom. Recently, Kryachko et al.20 used hybrid density functional theory (DFT) for the same system, and Majumdar et al.21 performed a variety of ab initio calculations to study the interaction of 1-3 benzene molecules with Pt and Pt2 clusters. Benzene adsorption on Ni(111) was studied with embedded cluster calculations22 and recently with accurate periodic DFT23,24 calculations. Despite the wealth of experimental studies for the adsorption of benzene on platinum, there is still an open discussion concerning the preferred adsorption site. In this paper, we present results from DFT cluster and periodic slab calculations for the adsorption of benzene on the (111) surface of platinum. We calculate the energies of adsorption as well as the vibrational frequencies for benzene at various different sites on Pt(111) in order to compare with experimental TPD and spectroscopic studies. We analyze in detail the change in the electronic structure that occurs upon adsorption at different adsorption sites.
10.1021/jp0201231 CCC: $22.00 © 2002 American Chemical Society Published on Web 07/09/2002
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Figure 1. Pt22 cluster model used in calculations.
All of this information is used to help explain the site preference of benzene on Pt(111). II. Computational Methods Accurate theoretical calculations for transition metal systems are still rather difficult. The near-degeneracy effects arising from the spacing of the d states, strong dynamical correlation effects, and relativistic effects (especially for heavy transition metals, such as platinum) are a challenge for computational studies.25 Recently, two papers investigated the quality of Hartree-Fock (HF) and DFT with different functionals for diatomic 3d transition metal molecules.26,27 Unrestricted HF fails to characterize transition metal molecules correctly, because it does not account for electron correlation. It was found that pure density functionals (e.g., Becke28 Perdew8629 (BP86) and Becke28LYP30 (BLYP)) performed better than the hybrid functionals for most transition metals. This is due to the fact that the hybrid functionals were parametrized for a test set of molecules which contains no transition metals. Moreover, they are biased by the poor description of transition metals by HF methods.26 In our study, the BP86 functional, which was found to yield the best results for 3d transition metals,26,27 was used. Philipsen et al.31 and Pacchioni et al.32 have shown the importance of relativistic effects for calculations on platinum. The scalar relativistic correction for the CO adsorption energy on platinum is as large as 70% of the total adsorption energy.31 Moreover, the inclusion of relativity in the calculation changes the preferred CO adsorption site.31 For all cluster calculations in this study, the scalar zero-order regular approximation33 (ZORA) Hamiltonian was applied. It was found that the Pauli formalism34 suffered from variational collapse35 when a large basis set was used for Pt. Two sizes of platinum clusters were considered. Small clusters of three and four Pt atoms, representing the adsorption site only, were used for preliminary calculations, for the computation of the vibrational frequencies, and for the study of orbital energies and orbital shape. Accurate adsorption energies were calculated with a large two-layered cluster of 22 Pt atoms (14 in the top layer and 8 in the second layer; Figure 1). This cluster was constructed in such a way that the four Pt atoms constituting the adsorption site have the maximal surface atom coordination. Two types of calculation were performed: one in which the Pt-Pt distance was kept fixed at the bulk Pt-Pt distance of 277 pm36 and the other in which the central part of the cluster was allowed to relax. To fully avoid cluster size effects, periodic DFT calculations were done for the dominant adsorption sites, using a four-layered slab of Pt.
Saeys et al. All cluster calculations were done with the Amsterdam density functional package35 (ADF2000.01). In this program, slater type orbitals (STO) are used as basis functions. Small double-ζ (DZ) and large triple-ζ augmented with two polarization functions (TZ+2P) basis sets were used. The innermost atomic shells were kept frozen and replaced by an effective core potential. The extent of these frozen cores was up to and including the Pt 4f and the C 1s shell. Decreasing the Pt frozen core to 4d, and thus allowing more electrons to participate in bond formation, increased the calculated adsorption energy by less than 5 kJ/mol. All calculations were performed unrestricted whereby the spin multiplicity was optimized to establish the lowest energy spin states. ADF standard SCF-convergence and geometry-convergence criteria were applied. Tightening these criteria changed the calculated energy by less than 0.1 kJ/mol. Symmetry was exploited whenever possible, which increased the computational efficiency significantly. Plane-wave periodic DFT calculations were performed using the Vienna ab initio simulation package (VASP).37,38 These calculations were performed using the Perdew Zunger local exchange-correlation functional.39 Nonlocal gradient corrections of the form of the generalized gradient approximation were calculated using the Perdew-Wang-91 potential.40 A 2 × 2 × 1 Monkhorst-Pack grid was used for Brillouin-zone integration. Platinum, carbon, and hydrogen atoms were described with ultrasoft pseudopotentials with a cutoff energy of 286.7 eV. A 3 × 3 unit cell size was chosen to try to minimize some of the lateral interactions between neighboring benzene molecules. The metal slab was described by 4 atomic layers, which required over 36 metal atoms (9 atoms per layer). The top two layers were allowed to fully relax in all calculations, whereas the bottom two layers were fixed to the bulk distance for Pt. A vacuum layer of 200 pm was chosen to insulate successive layers. III. Results and Discussion This section consists of three parts. First, the computed adsorption energies and geometries at the different highsymmetry sites are presented. Second, the results from frequency calculations are compared with data from vibrational spectroscopy. In the final section, we analyze the change of the orbital energies upon adsorption in order to gain insight in the site preference of benzene. A. Adsorption Energy and Geometry. Background. It is fairly conclusively established that benzene adsorption on transition metals occurs mostly through the π electrons where the ring sits parallel to the surface. Recent NEXAFS studies by Lee et al.41 suggest a tilted adsorption mode for benzene at very high coverage on Pd(111). Johnson et al.42 mention tilted benzene adsorption on Pd(110), Cu(110), and Pt(110). Four different high-symmetry sites along with two different orientations are possible for benzene adsorbed on Pt(111) (Figure 2). The orientations are indicated by 0°, i.e., with C-C bonds parallel to the [1h10] direction, and 30°, i.e., with C-C bonds parallel to the [2h11] direction. Despite the number of studies, it is still not clear whether benzene has a preferred adsorption site or whether it occupies multiple adsorption sites with slightly different adsorption enthalpies. In addition, it is not known how site preference changes with changing coverage and temperature. TPD data10,11 indicate benzene is adsorbed at two different sites. The first mode occurs at high coverage, leading to adsorption enthalpies which range from 82 to 86 kJ/mol. The second mode occurs at lower coverage with adsorption energies that range from 117 to 129 kJ/mol. In vibrational spectroscopy
Study of Benzene Adsorption on Pt(111)
Figure 2. Schematic of high-symmetry benzene adsorption sites on a Pt(111) surface.
studies of adsorbed benzene, RAIRS,6 HREELS13 and EELS,12 two peaks related to the out-of-plane γ-CH vibrational mode (Herzberg mode43 number 4) are found. The first occurs at 900920 cm-1, and the second is at 820-840 cm-1. The relative intensity of these two modes is coverage dependent.17 Several explanations have been provided, the most cited being that the two peaks originate from benzene adsorbed at two different sites. Because both peaks are observed even at the lowest coverage, adsorption at both sites must then have comparable adsorption strengths. In addition, the high resolution of RAIRS allows the observation of the growth of a third peak at 820 cm-1 and the disappearance of the 840 and 920 cm-1 peaks at high coverage. In section B, it will be shown that the latter two peaks originate from the same species and that a second adsorbed state has a peak at the third position. Using STM at 4 K and very low coverage, Weiss and Eigler7 found that benzene adsorbs dominantly at the bridge site. Dosing at room temperature on the other hand and cooling to 4 K leads to a ratio of 2/3 for hollow/bridge adsorption. Atop adsorption was only found near defect sites or near other adsorbates. A diffuse LEED study15 suggests benzene adsorption at the bridge(0) site with large distortions of the C-C bonds, where with NEXAFS14 a distortion of less than 2 pm of the C-C bonds is found. ARUPS data at saturation coverage lead to the conclusion that the hollow-hcp site is the preferred adsorption site.16 Two semiempirical theoretical studies8,18 indicate that hollow and bridge site adsorption have similar adsorption energies. Two recent periodic DFT calculations for benzene adsorption on Ni(111) indicate the preferred adsorption site is the bridge(30) site.23,24 This is different from the qualitative results suggested from earlier embedded cluster and extended Hu¨ckel calculations, where the hollow(0) site was thought to be preferred. Adsorption Energies at Different Sites. To clarify which modes of adsorption are possible for benzene on Pt(111), we calculated the adsorption energies for benzene at the 8 highsymmetry adsorption sites. The calculations were performed on the large Pt22 cluster using the small double-ζ basis set to keep the computations tractable. All calculations were performed with Cs symmetry, with adsorption occurring at the central atoms of the cluster. To consider tilted adsorption, two benzene molecules were put on an Pt8 cluster with one benzene molecule at the hollow(0) site and the second tilted with one C-C bond over the Pt bridge (Figure 3). Table 1 lists the adsorption energies. The bridge(30) appears to be the preferred adsorption site. The hollow-hcp(0) and hollow-fcc(0) are also important adsorption sites. The hollow-hcp site is preferred over the hollow-fcc, although the difference in energy is small (6 kJ/mol). Because of the small difference in energy, both the hollow-hcp and the
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Figure 3. Tilted di-σ adsorption of benzene together with hollow site adsorption on Pt8.
TABLE 1: Adsorption Energy (kJ/mol) at Different High Symmetry Sites
a
adsorption mode
adsorption energy
bridge (30) bridge (0) hollow-hcp (0) hollow-hcp (30) hollow-fcc (0) hollow-fcc (30) atop (0) atop (30) tilted di-σ atop-defect
101.8 65.7 70.6 51.1 64.6 54.4 not stable not stable 113.2a 97.6b
For two benzene molecules. b Overbinding due to small cluster.
hollow-fcc sites will often be treated together. Adsorption at the atop site does not occur on the perfect Pt(111) surface. Because adsorption at the atop site was observed in STM at defect sites, we calculated the adsorption energy at the corner of a Pt4-tetraeder, using C3V symmetry. The adsorption energy of 97.6 kJ/mol is considerably higher than that for benzene atop of Pt without a neighboring defect. This value is lower than the 123 kJ/mol obtained by Kryachko et al.20 with the B3LYP method for the adsorption on a single Pt atom, as expected. Tilted di-σ adsorption might be possible, but with a small adsorption energy of 42.6 kJ/mol for the tilted molecule. This value is most likely an overestimation, because the calculations were done on a small cluster, which is known to cause overbinding. Adsorption at the bridge(0) and hollow-hcp(30) site yielded rather low adsorption energies. However, after a small distortion away from the optimized geometry, the bridge(0) adsorbed benzene relaxed to the hollow-hcp(0) geometry, and the hollow-hcp(30) relaxed to the bridge(30) mode. This indicates that the only stable adsorption sites are the bridge(30) and the hollow(0) sites and that the bridge(0) and the hollow(30) modes are actually transition states during surface diffusion. From the difference in adsorption energy between the hollow(0) and the bridge(0) site (Table 1), it can be estimated that diffusion from the hollow(0) site has a small activation barrier. Diffusion from the bridge(30) site, via the hollow(30), has an important barrier. At this point, we can conclude that there are two important adsorption sites, bridge(30) and hollow(0) with adsorption energies of 101.8 and 70.6 kJ/mol respectively. The agreement with the experimental adsorption enthalpies of 117-126 kJ/ mol and 82-86 kJ/mol is quite satisfactory. Influence of Computational Procedure. Benzene adsorption at the hollow-hcp(0) and the bridge(30) site was studied in more detail using different computational approaches. An overview of the results is given in Table 2. It was found that the double-ζ basis set was rather incomplete, giving rise to a basis set
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Saeys et al.
TABLE 2: Influence of Computational Approach on Obtained Adsorption Energy (kJ/mol) type of calculation
bridge (30)
hollowhcp(0)
diff
Pt4-Pt3 cluster/DZ basis Pt22 cluster/DZ basis Pt22 cluster, relaxed site/DZ basis Pt22 cluster/TZ+2P basis Pt22 cluster, relaxed site/TZ+2P basis Periodic calculation experimenta
182.0 101.8 131.4 49.1 85.1 116.7 117-129
124.4 70.6 99.2 16.0 55.3 74.6 82-86
57.6 31.2 32.2 33.1 29.8 42.1 32-43
a
References 10 and 11.
superposition error (BSSE) of 54.0 kJ/mol for the benzene basis and 11.9 kJ/mol for the Pt basis, for adsorption at the hollow site of a small Pt3 cluster. Increasing the basis set to triple-ζ augmented with two polarization functions decreased the BSSE to 2.9 kJ/mol for the benzene basis and to 7.4 kJ/mol for the Pt3 basis. As can be seen from Table 2, increasing the basis set lowers the benzene adsorption energy significantly, from 70.6 to 16.0 kJ/mol for the hollow site on the large cluster, and from 101.8 to 49.1 kJ/mol for the bridge site. These lowerings are nearly equal to the difference in BSSE for the large and the small basis, namely, 55.6 kJ/mol. Notice that the relatiVe adsorption energies are affected much less by changing the computational procedure than the absolute values. However, the absolute values obtained with the more complete basis set disagree more with the experimental data. A factor that was neglected in these calculations was relaxation of the metal cluster upon adsorption. Allowing for the full relaxation of the metal cluster in combination with a large basis set makes such a calculation prohibitively expensive. Moreover, this causes a very strong distortion of the metal cluster, resulting in adsorption energies and geometries that should not be compared to single crystal experimental data. A way to avoid the unrealistic distortions but still have surface relaxation in a cluster calculation is to allow relaxation only for the four central atoms of the top layer, the ones that form the adsorption site, and freezing the 18 surrounding Pt atoms at their bulk positions. This was done in two steps. First a calculation was carried out with the small basis, and starting from these results, a calculation with the large basis was done. Results from both computations are reported in Table 2. As can be noticed, allowing for surface relaxation increases the adsorption energies by about 30-40 kJ/mol, bringing the results from the larger cluster and larger basis set again in better agreement with the experiment, although they are still too low. The changes of the adsorption energy on the inclusion of surface relaxation are due to two competing effects. The reference state for the adsorption becomes the relaxed surface, in which the surface atoms are at their minimum energy configuration, in contrast to the bulk terminated (not relaxed) case, resulting in their reactivity being reduced. This is offset by the extra interaction of the surface atoms with the benzene that can be gained by the relaxation of the surface atoms because of adsorption, leading to an increase in adsorption energy. The inclusion of surface relaxation again does not strongly influence the relatiVe strength of the hollow and the bridge site adsorption. It does increase the computational effort significantly, however. Finally, periodic calculations were performed in order to eliminate any cluster size effects.44,45 To illustrate these effects, calculations on small clusters are included in Table 2. They gave overbinding, and also the difference in adsorption strength is overestimated. The periodic calculations also include surface relaxation. They are the most accurate, but also the most
TABLE 3: Geometric Properties (Figure 4) for Bridge and Hollow Adsorption Site, Obtained with Pt22-Cluster, Relaxed Site/TZ+2P Basis Calculations and with Periodic Calculations bridge (0)
hollow-hcp (30)
variable
cluster
periodic
r1 (pm) r2 (pm) R1 (pm) R2 (pm) R (°) β (°) Pt1-Pt2 (pm) Pt2-Pt3 (pm) benzene height (pm) buckling (pm)
148 143 216 221 36.2 15.6 282 286 208 and 197 26 and 19
145 143 220 227 35.5 16.1 280 291 209
cluster 147 143 221 18.5
periodic 146 144 222 18.1
285
283
205 25
211
demanding computations reported. An adsorption energy of 116.7 kJ/mol was obtained for the bridge site and 74.6 kJ/mol for the hollow-hcp site. These results are in very good agreement with the experimental values. Again, the relative values are similar to the results from the large cluster calculation. For simplicity, we decompose the interactions that govern the adsorption into orbital overlap and Pauli repulsion contributions that arise from interactions with the metal d states and with metal sp band.46 The major fraction of the bonding strength is derived by the interaction of the adsorbing molecule with the transition metal sp band. For a good description of the interaction with the sp band, periodic calculations are required. The second contribution comes from the d band. We believe that an accurate description of this interaction can be obtained from cluster calculations, because of the localized nature of the d orbitals. Although the majority of the bonding is determined by the sp band interaction, this interaction is rather insensitive to the type of transition metal and to the specific adsorption site. The difference in bonding energy in going from one transition metal to the next can be predicted by analyzing changes in the d band.47,48 Our Pt22 cluster, with a maximal surface coordination of the adsorption site atoms, seems to be sufficient for the description of the d band interaction, even with a fixed cluster and a small basis set. Adsorption Geometry. The distortion of benzene and the Pt surface upon adsorption at the hollow-hcp(0) and at the bridge(30) site are summarized in Table 3 and Figure 4. Gas phase benzene has a calculated C-C distance of 139.7 pm and a C-H distance of 109.0 pm. These values compare well with recent recommendations of 139.14 pm and 108.02 pm.49 Upon adsorption, the C-C distances increase significantly, indicating a substantial filling of the antibonding π orbitals. The most substantial increase is found for the C-C-bonds bridging 2 platinum atoms (r1 in Table 3 and Figure 4), as well at the hollow as at the bridge site. These bond lengths attain a value closer to the ethane single C-C bond distance of 153 pm. This interaction can be compared to the di-σ adsorption mode of ethene on Pt(111)50 and Pd(111).45,48 The increase of the C-C bond length over one Pt atom, r2 in Table 3, is not so substantial, indicating less back-donation into the antibonding orbitals. This C-C bond can be compared to the weaker π-adsorption mode of ethene on Pt(111). Experimental results indicate lower C-C bond lengths (
