6822
J. Phys. Chem. C 2008, 112, 6822-6831
Intrinsic Metal Size Effect on Adsorption of Organic Molecules on Platinum V. Nieminen,*,†,§ K. Honkala,‡ A. Taskinen,† and D. Yu. Murzin† Laboratory of Industrial Chemistry, Process Chemistry Centre, Åbo Akademi UniVersity, Turku/Åbo, FI-20500, Finland, and Department of Physics/Department of Chemistry, Nanoscience Center, P.O. Box 35, UniVersity of JyVa¨skyla¨, FI-40014, Finland ReceiVed: September 17, 2007; In Final Form: February 6, 2008
Di-σ adsorbed ethene, bridge(30) adsorbed benzene (with four di-σ-type and two π-type interactions), and η1 and η2 adsorbed acetone on nanosized platinum clusters consisting of 19 to 38 Pt atoms were studied theoretically by density functional theory (DFT) calculations with general gradient approximation (GGA) utilizing plane wave and local basis sets. The cluster results were compared to plane wave calculations employed with periodic boundary conditions. It was found that the geometries obtained with different methods are very similar but the adsorption energy depends prominently on the cluster size. Adsorption was strongest on the 22- and 26-atom clusters and weakest on the 35- and 38-atom clusters. No correlation between the d-band center of the clusters and the adsorption energy was observed. The relative amount of adsorbed active and spectator species of an organic molecule may depend on the cluster size as exemplified by the η1 and η2 adsorption modes of acetone. It is also feasible that the reaction energies are size-dependent adhering to the Brønsted-Evans-Polanyi relationship. This indicates that the catalytic activity of Pt nanoparticles can be increased by carefully controlling their size.
Introduction Catalysis by dispersed metal particles on support materials is a very important method to produce a vast variety of organic chemicals. To enhance the catalytic activity, it is necessary to increase the accessibility of metal atoms, which could be achieved through the formation of very small particles with the diameter ranging 1-10 nm. This would lead to higher surface area of the metal particles on a support, i.e., increased dispersion (exposed surface area) resulting in higher reactivity and reduced catalyst preparation costs. However, certain reaction classes may be classified as structure-sensitive, as their rates depend on the size and shape of the catalyst particles. Reactions independent of the particle size are classified as structure-insensitive. Particles with nanodimensions exhibit properties that are between the properties of atoms and the bulk material; thus, nanoparticles should have an intrinsic effect on the reactivity. The nanosized particles have a large surface-to-volume ratio and an increased number of edges, corners, and faces leading to changes in the electronic structure and altered catalytic activity and selectivity (see, e.g., refs 1 and 2). A well-known example of the importance of size effect is gold, which was thought to be catalytically inert until the 1980s, when Haruta and co-workers discovered that nanosized gold particles are active in CO oxidation even at low temperatures.3,4 It was found that very small gold particles are active, probably due to quantum effects, but the activity is lost with increasing particle size. These remarkable results have led to an intensive research on gold catalysis. The effect of particle size on catalytic properties has been reported also for other metals.5 Several * Corresponding author. E-mail
[email protected] and ville.
[email protected]. † Åbo Akademi University. ‡ University of Jyva ¨ skyla¨. § Current address: Outotec Research Oy, Kuparitie 10, P.O. Box 69, FI-28101, Pori, Finland.
studies consider adsorption and/or reaction of atoms or rather small molecules, such as CO. For instance, it has been observed that at very low coverage the binding energy of CO increases with decreasing Pd cluster size below 5 nm.6 The size effects of small Pt clusters on oxygen adsorption have been studied theoretically, e.g., by Lin et al., who concluded that large differences in reactivity are seen between different cluster sizes.7 A variation in binding energy of oxygen as a function of Pt cluster size was also observed by Jacob et al.8 Masson et al.9 reported that the ethene hydrogenation rate has a maximum on Pt particles with a diameter ca. 0.6 nm. They suggested that the rise in activity with decreasing particle size is due to increased atom accessibility, while the decrease in activity on particles smaller than 0.6 nm is related to an insufficient number of Pt atoms. Dorling and Moss reported that benzene hydrogenation activity is higher on a Pt catalyst smaller than 5 nm.10 There are some reports on change in hydrogenolysis rates as a function of metal particle size. For example, a clear maximum on the rate-size curve in benzene and cyclopropane hydrogenation is observed on 1.2 nm nickel particles, but the reactions are structure-insensitive down to 1.5 nm.11 MgO film supported tiny Pd clusters consisting of only 1-30 atoms exhibit a pronounced size effect on the cyclotrimerization of acetylene to benzene.12 Furthermore, selectivity in asymmetric hydrogenation has been observed to depend on the catalyst particle size.13 In the literature, different explanations are proposed to account for the effect of the metal cluster size on reactivity. For instance, Zhdanov and Kasemo attributed the altered reaction rate for nanometer-size particles to changes in the ratio between different surface faces exhibiting different intrinsic kinetics.14 For reactions involving complex organic molecules and thus requiring multicentered adsorption, not only this ratio but also the size of a particle face per se could be a parameter influencing rates. Concerning the origin of size and structure effects in heterogeneous catalysis, the present paper focuses on the theoretical
10.1021/jp077486r CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008
Adsorption of Organic Molecules on Pt
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6823
Figure 1. Clusters employed to model the Pt(111) surface.
analysis of the intrinsic cluster size effect on a (111) facet of Pt. Notably, in many cases it may not be possible in practice to assign a kinetic phenomenon to a single geometric, electronic, or confinement effect.15 Adsorption of a molecule is an initial step in any catalytic process. Therefore, understanding the adsorption of organic molecules can give valuable information on their possible reaction mechanisms and rates.16 In the late 1990s, the adsorption of small organic molecules on a variety of catalyst surfaces was studied theoretically, but only recently has it become possible to investigate large molecules (larger than benzene) on metal surfaces at the DFT level.17-21 Typically, two different approaches have been applied to describe the catalyst surface: a cluster and a slab model.22 The former uses a limited cluster of the surface atoms, while in the latter, the catalyst surface is described as a slab with a periodic structure along the surface. Localized functions are the usual choice for the basis set in cluster models, whereas both localized and plane waves are used in slab model calculations.22 Both approaches have benefits and drawbacks not discussed here, but when considering adsorption of molecules on metal surfaces, the predicted structures of the adsorbates and relative energies of various adsorption modes do not usually depend on the approach (see, e.g., refs 23-25). Employing the computationally rather cheap cluster approach is definitely tempting. However, the cluster size effect on adsorption has been studied only to a limited extent at clusters with more than 30 atoms and the diameter of ca. 1.5 nm. This is a typical diameter of catalyst particles in many chemical reactions. To our knowledge, no computational study exists where the effect of cluster size on chemisorption of various important organic molecules has been investigated and compared to the slab results. In this work, adsorption of ethene, acetone, and benzene on a close-packed platinum (Pt) with (111) surface is studied using density functional theory (DFT). The three main objectives are (i) to determine the effect of cluster size on
adsorption energy, (ii) to look for a general trend for adsorption energy as a function of cluster size, (iii) and, finally, to compare the obtained results to energetics calculated utilizing slab models to represent a Pt(111) surface. The number of atoms in the cluster is varied from 19 to 38. For comparison, the ethene adsorption energy is calculated on different clusters using both plane waves and the localized basis set. The adsorption energy is proven to depend on the cluster size in a specific way. Computational Details Calculations with the Localized Basis Set. The adsorption geometries and energies of ethene, acetone, and benzene were studied computationally using a cluster model approach. The calculations were performed with the TURBOMOLE program package26-28 and density functional theory together with the BP86 gradient-corrected exchange-correlation functional29-31 in combination with the RI technique for Coulomb integrals (resolution of the identity, i.e., the total density is approximated, which allows a very efficient treatment of Coulomb interactions).32,33 It is worth noting that the adsorption energies of benzene calculated previously using the BP86 functional were in good agreement with the experimental adsorption enthalpies on Pt(111).25 Relativistic effects were taken into account implicitly by using the relativistic effective core potential (ECP) from the TURBOMOLE library (“def-ecp”) to represent the 60 core electrons of Pt.34 The 18 valence electrons of platinum and all electrons of the other elements were treated explicitly using the SV(P) basis set (“def-SV(P)”).35 Eight different clusters consisting of 19, 22, 26, 30, 31, 35, and 38 Pt atoms and two or three layers were employed to model the Pt(111) surface (see Figure 1). The clusters with two layers (denoted formally as Pt12.7, Pt14.8, Pt16.10, Pt18.12, Pt19.12, Pt21.14, and Pt23.15 where X(Y) is the number of Pt atoms on the top (bottom) layer of the PtX.Y cluster) are hereafter labeled in the text as PtN, where N indicates the total number of Pt atoms in
6824 J. Phys. Chem. C, Vol. 112, No. 17, 2008 the cluster. The cluster consisting of three layers and 35 Pt atoms is labeled Pt14.13.8. Jacob and Goddard III have studied a similar type of cluster.36 The distance between the Pt atoms was fixed to the bulk value of 277.5 pm, and the central atoms (i.e., atoms not on edges) on the top layer were fully relaxed. This also clarifies the choice of the number of Pt atoms in the cluster; Pt19 has 3, Pt22 has 4, and finally Pt38 has 9 central atoms on the top layer. To study the effect of surface relaxation on adsorption, ethene adsorption was also studied on rigid clusters of all sizes keeping the distance between Pt atoms in the bulk value (277.5 pm). It should be noted that metal clusters are commonly applied on high-area porous metal oxide support. In highly dispersed catalysts, the number of atoms in the active catalyst particle can be as low as 10-30. Supported metal particles are nonuniform in size and shape, and even if the most stable structure of a cluster in the gas phase was known (e.g., from theoretical calculations), the detailed structure of the cluster on a catalyst support would be unknown since the interaction between the metal and the support influences the structural and electronic properties of the cluster. Therefore, some approximations regarding the structure of the particles has to be made in these kinds of theoretical studies. It is well-known that dispersion forces, which may be dominant in weakly bound cases (such as acetone), are not properly described in DFT. However, in this study this is not of major concern because the interest is not so much on absolute adsorption energies but rather on relative ones; the possible error in adsorption energy caused by the lack of modeling dispersion is expected to be of similar magnitude for every cluster size. All the calculations were performed spin-unrestricted with the spin state S ) 3 for the cluster and for the cluster with the adsorbate. The minimum energy spin states for Pt31 and Pt19 with all atoms fixed and for Pt31 with the 7 central top layer atoms relaxed were determined between 0 and 10. For both clusters, the spin state S ) 3 was found to be the minimum energy state, and therefore, it has also been used for other clusters. This choice is respected even if in some cases the minimum energy electronic state of the bare cluster and/or cluster plus adsorbate differs from S ) 3. Note that the effect of the spin state on the adsorption energy and the energy difference between the states close to spin state 3 is negligible as described below. Furthermore, a low-spin state should be chosen to represent the electronic structure of a nonmagnetic metal surface (as Pt), and it is customary to use the lowest energy low-spin cluster electronic state for the naked clusters as well as for the cluster plus its corresponding adsorbate.37 Calculations with the Plane Wave Basis Set. The DFT calculations with the periodic boundary conditions were performed using the DACAPO code,38,39 where Kohn-Sham equations are solved in plane wave basis restricted by a kinetic cutoff of 25 Ry. The PW91 generalized gradient approximation40 was employed self-consistently, and in some cases, the nonself-consistent energies for the PBE and RPBE functionals41,42 are also reported. The core electrons of all the atoms were treated with Vanderbilt ultrasoft pseudopotentials,43 and all the atoms except the bottom Pt layer were relaxed according to the Quasi Newton algorithm.44 The sampling of 16 Monkhorst-Pack k-points together with a Fermi smearing of 0.1 eV was applied. The vacuum region was at least 1.4 nm between adjacent slabs, and the adsorbate structures were created only on one side of the slab since the dipole correction was applied. The Pt(111) surface was modeled with two and three atomic layers thick
Nieminen et al.
Figure 2. RI-BP86/SV(P) optimized geometries of ethene, benzene, and acetone adsorbed on the Pt31 cluster.
slabs and a (3 × 3) unit cell. All energies are well-converged with respect to the plane wave cutoff energy and the number of k-points. In addition to slab calculations, some cluster calculations were also performed with DACAPO. Due to the plane wave basis and the periodic boundary conditions, clusters have been surrounded with enough vacuum space in all three dimensions to avoid interactions between clusters in different supercells. The minimum distance between clusters was at least 0.85 nm, and initial cluster geometries were taken from the structure optimization calculations performed by TURBOMOLE. The coordinates of the central Pt atoms on the top layer and the atoms of the adsorbates were fully relaxed in all calculations. For the k-point sampling of the Brillouin zone, Γ-point was used. Results and Discussion Ethene Adsorption. Ethene adsorption on Pt has been widely studied experimentally (see, e.g., reviews in refs 45 and 46) and theoretically (see, e.g., refs 47-51). Studies on ethene adsorption on Pt(111) have revealed that π-bonded ethene species are predominant at temperatures below 90 K46,49 and di-σ-bonded species are observed at the temperature range from 100 to 250 K and ethylidyne species from 280 to 450 K (see ref 50 and references therein). Since the main interest is the reactivity of organic molecules on Pt, di-σ-adsorbed ethene is studied as it is a preliminary reaction intermediate in the hydrogenation model by Horiuti and Polanyi.52 First, the TURBOMOLE calculations are considered. Figure 2 illustrates the optimized geometry of ethene on a Pt31 cluster. Adsorption geometries on different Pt clusters are almost identical with only negligible differences (Table 1). The C-Pt distances are 211-212 pm and the distance between the two Pt atoms that are bonded to the ethene carbon atoms varies from 269 to 276 pm. Upon adsorption, the carbon-carbon bond elongates from the gas-phase value of 134 pm to 150 pm. The bond length agrees very well with the experimental value 149 ( 4 pm53-55 and theoretical values 150 pm and 148-149 pm for cluster and slab models, respectively.47,48,51 Both carbons are rehybridized to nearly sp3, as expected, and seen from the C2-C1-H2-H1 torsional angle (Table 1) which deviates 4748° from that for the planar system (180°). Adsorption energy
Adsorption of Organic Molecules on Pt
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6825
TABLE 1: Adsorption Energy of Di-σ Adsorbed Ethene on Pt Clusters and Its Decomposition into Distortion Energy of the Molecule, Distortion Energy of the Cluster, and Interaction Energy between the Distorted Molecule and the Distorted Cluster (in kJ mol-1, See Also the Text) Calculated at the RI-BP86/SV(P) Levela ∆Eads ∆Eadsc Edist(molecule) Edist(cluster) Einteraction C1-C2 C1-Pt1 C2-Pt2 Pt1-Pt2 C2-C1-H2-H1
Pt19
Pt22
Pt26
Pt30
Pt31
Pt35
Pt14.13.8
Pt38
-153 -111 135 19 -307 150 212 212 274 132.9
-160 -122 134 9 -306 150 212 212 270 133.2
-162 -131 134 8 -304 150 212 212 269 133.4
-144 -120 137 7 -289 150 211 211 271 132.8
-139 -117 138 8 -284 150 212 211 273 133.0
-141 -113 142 12 -295 150 211 211 276 131.8
-130 n.c.b n.c. n.c. n.c. 150 212 212 268 133.5
-131 -104 140 11 -282 150 211 212 272 132.2
a Important geometric details are also given (distances A-B in pm and the torsional angle A-B-C-D in deg). For atom numbering, see Figure 3. b n.c. ) not calculated. c For a cluster with all atom positions fixed.
depends strongly on the cluster size and varies from -130 kJ mol-1 for Pt14.13.8 to -163 kJ mol-1 for Pt22. The relaxation of the central Pt atoms does not explain the size dependence, since ethene adsorption on frozen clusters gives a qualitatively similar trend but with weaker adsorption energies, which range from -104 kJ mol-1 for Pt38 to -131 kJ mol-1 for Pt26. A detailed energy decomposition analysis (see Table 1) shows that the variation of adsorption energy with cluster size does not depend on the number of free Pt atoms. Adsorption energy consists of three terms:53 (1) the distortion energy of the molecule that is the energy difference between the optimized gas-phase structure and the structure that the molecule has on the cluster, (2) the surface distortion energy that is the energy difference between the optimized clean surface and the optimized surface structure upon molecule adsorption, and (3) the interaction energy that is the difference between the adsorption energy and the distortion energies. As Table 1 shows, the distortion energies vary only between 134 and 142 kJ mol-1 for ethene and between 9 and 12 kJ mol-1 for the Pt22 to Pt38 clusters. For the Pt19 cluster, the distortion energy is slightly higher, 19 kJ mol-1, than that for Pt22 and Pt38, but it is compensated by the largest interaction energy of -307 kJ mol-1. In general, adsorption energy is a compromise between the destabilizing deformations and the stabilizing interactions.56 The latter term seems to increase with decreasing cluster size, with Pt35 being an exception (Table 1). The weaker ethene adsorption on Pt19 than on Pt22 and Pt26 could be related to the smaller number of relaxed central atoms. The distortion energy of the Pt19 cluster is 10 kJ mol-1 larger than that of Pt22, but the interaction energies are almost equal, explaining the observed difference in adsorption energies. One might assume that, with an increasing number of Pt atoms free to relax, the surface and the adsorbing molecule have more freedom to adjust their geometries, which then leads to a stronger adsorption. However, the adsorption energy is independent of the number of relaxed cluster atoms: the adsorption is strongest on Pt26 and weakest on Pt38, which has the largest number of relaxed atoms. The adsorption energy on frozen clusters follows the same trend: the strongest adsorption is found for Pt22 and Pt26. On average, ethene adsorption is 22-44 kJ mol-1 more exothermic on clusters with some free Pt atoms than on the frozen clusters. Interestingly, the adsorption energy of ethene on the Pt14.13.8 cluster (-130 kJ mol-1) with three atomic layers and four relaxed Pt atoms is closer to the adsorption energy on Pt38 (-131 kJ mol-1) with nine relaxed Pt atoms than that on Pt22 (-160 kJ mol-1) with four relaxed atoms. Obviously, the observed variation in adsorption energy depends on the cluster size (and shape) rather than the number of relaxed cluster atoms.
TABLE 2: Adsorption Energies (in kJ mol-1) and Important Geometric Details (distances A-B in pm and torsional angle A-B-C-D in deg) for Di-σ Adsorbed Ethene on Two and Three Layer Thick Slabs of Pt Calculated with the PW91 Functional and the Plane Wave Basisa ∆Eads C1-C2 C1-Pt1 C2-Pt2 Pt1-Pt2 C2-C1-H2-H1
2-layer thick slab
3-layer thick slab
-148/-142b/-96c 148 212 212 277 133.8
-167/-157b/-93c 148 212 213 277 133.8
a For atom numbering, see Figure 3. b Non-self-consistent with the PBE functional. c Non-self-consistent with the RPBE functional.
TABLE 3: Adsorption Energies (in kJ mol-1) of Ethene Adsorption on Clusters Optimized with the PW91 Functional and the Plane Wave Basisa Pt19 Pt22 Pt26 Pt30 Pt35 Pt38
PW91
PBEb
BP86c
PBEd
BSSE/BP86c
BSSE/PBEd
-133 -130 -131 -123 -115 -113
-129 -125 -127 -119 -110 -108
-152 -160 -164 -141 -149 -130
-165 -173 -178 -154 -157 -144
30 20 29 29 29 26
33 22 32 31 31 28
a Obtained geometries were used to calculate non-self-consistent adsorption energies with the PBE functional and the plane wave basis as well as single-point adsorption energies with the PBE and BP86 functionals and the SV(P) basis set including basis set superposition error (BSSE, in kJ mol-1). b Non-self-consistent with the PBE functional and the plane wave basis. c Single-point with the BP86 functional and the SV(P) basis set. d Single-point with the PBE functional and the SV(P) basis set.
Next, we consider the DACAPO slab calculations. The main results are given in Tables 2 and 3. The geometries obtained are almost identical to those found with TURBOMOLE except that the C-C distance is slightly shorter, 148 pm. The slab calculations give adsorption energies -148 and -167 kJ mol-1 for di-σ-adsorbed ethylene on two and three layer thick slabs, respectively. Both values are close to those calculated with cluster models, although in the slab calculations, the Pt atoms of the bottom layer were frozen to the ideal bulk positions. The non-self-consistent adsorption energies calculated with the PBE functional are slightly smaller, being -142 and -157 kJ mol-1 for two and three layer thick slabs, respectively, but corresponding RPBE values are only -96 and -93 kJ mol-1. The PW91 optimized geometries from the cluster calculations are practically identical to those obtained with TURBOMOLE. However, the adsorption energy decreases with increasing cluster size; the adsorption energy varies from -133 kJ mol-1
6826 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Nieminen et al.
TABLE 4: Adsorption Energy of η1 Adsorbed Acetone on Pt Clusters and Its Decomposition into Distortion Energy of the Molecule, Distortion Energy of the Cluster, and Interaction Energy between the Distorted Molecule and the Distorted Cluster (in kJ mol-1, See Also the Text) Calculated at the RI-BP86/SV(P) Levela ∆Eads Edist(molecule) Edist(cluster) Einteraction C2-O O-Pt (C1)H-Ptc (C1)H-Ptc O-C2-C1-C3
Pt19
Pt22
Pt26
Pt30
Pt31
Pt35
Pt14.13.8
Pt38
-57 8 3 -68 125 224 251 258 179.1
-60 9 2 -70 125 223 251 256 179.9
-56 9 2 -67 125 226 253 239 179.0
-58 9 3 -70 125 223 260 252 179.8
-50 9 0 -58 125 223 259 266 179.3
-45 8 11 -64 125 227 261 265 179.2
-51 n.c.b n.c. n.c. 124 230 268 263 179.9
-39 8 2 -49 125 228 272 260 179.4
a Important geometric details are also given (distances A-B in pm and the torsional angle A-B-C-D in deg). For atom numbering, see Figure 3. b n.c. ) not calculated. c The shortest distance between hydrogen and Pt; see Figure 2.
for Pt19 to -113 kJ mol-1 for Pt38, with the non-self-consistent values with the PBE functional being 4-5 kJ mol-1 smaller. The adsorption energies calculated in this work are in good agreement with the values given in the literature. Experimental determination of adsorption energy is difficult, as several adsorbed species may be present, decomposition can occur, and surface structure may vary and differ from (111). The following initial heats of adsorption at 300 K have been reported: -148 kJ mol-1 on Pt film, -160 kJ mol-1 on Pt black (-120 kJ mol-1 at 173 K), as well as -157 kJ mol-1 on Pt/SiO2.45 Unfortunately, the adsorption geometries corresponding to the aforementioned energies were not identified, and thus, it is not known if these values correspond to only di-σ-bonded ethene. Definitely, substantial decomposition must have occurred at higher temperatures and the presence of π-bonded ethene is probable at lower temperatures. Heats of molecular adsorption of the diσ-bonded ethene of -120 kJ mol-1 for Pt powders and -136 kJ mol-1 for Pt(110) have been reported.57,58 However, several DFT results for di-σ-bonded ethene on Pt(111) have been reported. Miura et al.59 calculated adsorption energies of -95 and -56 kJ mol-1 for frozen Pt7 and Pt7.3, respectively. Dumesic et al.50 obtained -116 kJ mol-1 for di-σ-bonded ethene on three layer thick, frozen Pt6.6.7 and -149 kJ mol-1 for totally relaxed Pt6.3.1 (a value of -171 kJ mol-1 for the heat of adsorption was given in ref 51). Goddard III and co-workers found an adsorption energy of -111 kJ mol-1 on the Pt14.13.8 cluster and -151 kJ mol-1 on a single-layer Pt8 cluster.36,60 With the slab model, the following energies have been obtained for the frozen surface: -101, -106, -107, and -109 kJ mol-1.23,47,48,58 Upon the relaxation of the surface, the adsorption becomes stronger, -122 and -127 kJ mol-1, when the top layer and all of the atoms in the Pt(111) slab were relaxed, respectively.48,61 These slab values differ somewhat from those obtained in the present study, but this is mainly due to the different unit cell and the full relaxation of the surface. Acetone Adsorption. The thermal desorption and HREELS experiments of acetone adsorption on single crystals suggested that the dominant species on the Pt(111) surface has an η1 (endon) adsorption geometry, where the molecule is attached to the surface via the oxygen atom with the CdO bond nearly perpendicular to the surface, while the other species, assigned to an η2 (also referred as a side-on mode and di-σ-bonded acetone), has been identified to be adsorbed parallel to the surface.62,63 Although the surface concentration of the latter is very low, it has been suggested that η2 modes are relevant for the hydrogenation.64,65 Therefore, we study here both η1 and η2 adsorption modes. The results obtained with TURBOMOLE are given in Tables 4 and 6, and those with DACAPO in Tables 5 and 7.
TABLE 5: Adsorption Energies (in kJ mol-1) and Important Geometric Details (distances A-B in pm and torsional angle A-B-C-D in deg) for η1 Adsorbed Acetone on Two and Three Layer Thick Slabs of Pt Calculated with the PW91 Functional and the Plane Wave Basisa ∆Eads C2-O O-Pt (C1)H-Ptd (C1)H-Ptd O-C2-C1-C3
2-layer thick slab
3-layer thick slab
-65/-58b/-20c
-43/-32b/+19c 125 226 259 284 182.3
125 225 276 268 180.6
a
For atom numbering, see Figure 3. bNon-self-consistent with the PBE functional. c Non-self-consistent with the RPBE functional. d Shortest distance between hydrogen and Pt; see Figure 2.
Figure 2 illustrates the optimized η1 adsorption mode of acetone on Pt31. The optimized η1 adsorption structures are similar to those reported in the literature.64,66 One methyl group is closer to the surface than the other one, and the geometry is only slightly perturbed from the gas-phase structure, which is seen from a small (
