Interaction between Catalyst and Support. 2. Low Coverage of Co and

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J. Phys. Chem. B 2001, 105, 2212-2221

Interaction between Catalyst and Support. 2. Low Coverage of Co and Ni at the Alumina Surface Qisheng Ma and Kamil Klier* Department of Chemistry and Zettlemoyer Center for Surface Studies, Lehigh UniVersity, Bethlehem, PennsylVania 18015

Hansong Cheng,* John W. Mitchell, and Kathryn S. Hayes Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, Allentown, PennsylVania 18195-1501 ReceiVed: October 6, 2000; In Final Form: January 9, 2001

The electronic and geometric structure of R-Al2O3 (0001) surface with and without adsorbed Co and Ni atoms has been investigated using the full-potential linearized augmented plane-wave density-functional theory method. It has been found that the truncated R-Al2O3 (0001) surface undergoes a large surface reconstruction from its bulk structure, which is further changed upon the metal atom adsorption. Geometries, energies, and electronic properties of the partially optimized and the truncated undistorted R-Al2O3 slabs are compared. Electronic “surface state levels” due to the unsatisfied bonding of the Al atoms at both ends of the slab are identified. Among several geometries, the 3-fold oxygen site has been found to be the only stable adsorption site for both Co and Ni atoms. Several factors determine the metal-support interaction between the Co (or Ni) atom and the R-Al2O3 substrate. Among these factors, the “screened ligand field” effects of partially occupied 3d electrons and the further relaxation of the R-Al2O3 substrate are shown to have the largest contributions to the adsorption energy.

1. Introduction Metal catalysts supported on alumina have found broad applications in many technological and industrial processes.1-3 Co/Al2O3 and Ni/Al2O3 have been widely used in the field of heterogeneous catalysis for hydrogenation, hydrotreating, combustion, and amination.4,5 The metal-support interactions, which might appreciably affect the catalytic reactivities, have been reported.6-8 However, the fundamental understanding of these metal-support interactions is still limited.9,10 The issues involved include understanding interactions of metallic or ionic metals with Al2O3; anchoring, clustering, or agglomerating of metals on the substrate; and the effects of the interactions on their catalytic properties. Some previous studies on the interaction of Co and Al2O3 found several “cobalt surface phases”, such as CoO, Co3O4, and CoAl2O4.11,12 However, cobalt exists in such phases in its ionic form, whereas it is in a metallic, nanoparticle form in the actual working catalysts. Therefore, the understanding of a direct interaction between zerovalent Co or other active metals such as Ni with Al2O3 will provide an insight into the effects of metal-support interactions on the catalytic properties. A theoretical study based on the density-functional theory (DFT) of interactions of metallic Co and Ni with the Al2O3 support is reported herein. The calculations were carried out in the full-potential linearized augmented plane-wave (FPLAPW)13 level with spin-polarization taken into account. The Al2O3 surface was taken from the truncated R-Al2O3 (0001) surface subject to geometry optimization of the top three layers. Large distortion of this surface has recently been reported.14-16 We have undertaken a study of the geometric, energetic, and * To whom correspondence should be addressed.

electronic properties before and after the metal adsorption. As a first attempt to quantify the interaction of Co and Ni with the Al2O3 support, we focus mainly on submonolayers at very low coverage. A single Co or Ni atom per unit cell is adsorbed on the optimized R-Al2O3 (0001) surface for the studies of interactions at the atomic level. The resulting adsorbed complex is further optimized, and its properties including magnetic state, total energy, and orbital energies are calculated. 2. Computational Method The DFT calculations were performed using the WIEN97.9 code with the FP-LAPW method.13 The Kohn-Sham equations17 are solved self-consistently in an iterative process under the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof functional for the exchange-correlation energy,18 subject to periodic lattice boundary condition. A dual basis set is used, consisting of a centrosymmetric set inside “muffin-tin” spheres surrounding individual atoms and a linear plane-wave set in the interstitial space between the “muffintin” spheres. Inside the “muffin-tin” radius, a linear combination of radial functions multiplied by spherical harmonics is used as the basis set:

φkn )

[Alm(kn)ul(r,El) + Blm(kn)u˘ l(r,El)]Ylm(rˆ ) ∑ lm

(1)

where ul(r,El) is the radial function for energy El and u˘ l is the energy derivative of ul taken at the same energy El. The coefficients Alm and Blm are functions of kn ) k + Kn, the sum of the wave vector k inside the first Brillouin zone and the reciprocal lattice vector Kn. The basis function φkn and its gradient are required to match the plane waves of the interstitial

10.1021/jp003673c CCC: $20.00 © 2001 American Chemical Society Published on Web 02/22/2001

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Figure 1. Geometry of the partially optimized R-Al2O3 surface. (a) Top view. The unit cell represented by the shadow area. (b) Side view. The distances of layers before and after partially optimized are listed on the right; the percentage changes are also given. (c) Local geometry indicating the 3-fold oxygen site. The bond lengths are in the unit of nm.

region outside the “muffin-tin” radius

φkn )

1 exp(ik‚r) xΩ

(2)

where Ω is the volume of the unit cell. The solutions of the Kohn-Sham equations are expanded in this combined basis set (1) and (2), and the coefficients are determined by the Rayleigh-Ritz variational principle. A more detailed description is available elsewhere.19 For the calculations presented in this work, the plane-wave cutoff energy for the basis in the interstitial region was around 25 Ry. The core states were treated in a fully relativistic fashion, whereas the valence states were treated in a scalar relativistic fashion. Partial optimization20 was carried out by calculating the forces acting on the atoms after each convergence of SCF cycle. A modified Newton damping dynamic scheme21 was applied to move the specified atoms. Optimization convergence was assumed when the forces on specified atoms were less than 10 mRy/a.u. A modified tetrahedron integration scheme22 was used to generate the k-mesh in the irreducible wedge of the hexagonal Brillouin zone on a special point grid with a total of four k points of 2 × 2 × 1, (two k points along the in-plane x and y directions and one k point along the vertical direction). In total, 50 000 plane waves were generated to simulate the interstitial region. The computations were carried out on an SGI Origin 2000 machine and a cluster of SGI Octane workstations. 3. Results and Discussion 3.1. Surface Reconstruction of the r-Al2O3 (0001) Surface. The present model of the initial undistorted R-Al2O3 (0001)

surface is a six-layer slab obtained by truncating the bulk R-Al2O3 structure. It consists of a hexagonal unit cell of stoichiometry 2 × Al2O3, periodic and infinite in the two dimensions parallel to the (0001) plane. The unit cell has inplane lattice parameters a ) b ) 0.476 nm (as shown in Figure 1a), and the distance between the slabs along the vertical direction is c ) 2.000 nm, large enough to prevent layer-tolayer interactions. The coordinates of all four Al and six O atoms in this unit cell are given in Table 4 (see later). The Alterminated surface is chosen because it is electrically neutral.23,24 Whether the actual R-Al2O3 (0001) surface is terminated by the Al or the O atoms25 or a mixture of Al- and O-terminated26 is still being debated, but the present model includes both aluminum-exposed and oxygen-exposed sites after optimization, and therefore, the choice of initial surface termination may not be crucial for the description of the metal atom adsorption. As a matter of fact, the most energetic stable adsorption site for the metal atom is found to be above the 3-fold second-layer oxygen site, wherein the first layer Al has relatively small binding power for the transition metals. Previous theoretical and experimental studies indicated that the Al-terminated R-Al2O3 (0001) surface undergoes large relaxation from its bulk structure.14-16 The time-of-flight scattering and recoiling spectrometry (TOF-SARS) experiment performed by Ahn and Rabalais15 suggested a large relaxation of the first Al and the second O layer by 50-70% along the surface normal with respect to the ideal truncated bulk structure (decreasing from 0.08 to 0.03 nm). Similar results have also been found in a crystal truncation rod diffraction at a thirdgeneration synchrotron X-ray source by Eng et al.14 At the same

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Figure 2. Calculated band structure and several band structures with character plot of the perfect 3-D hexagonal R-Al2O3 crystal structure. (a) Regular band structure. (b) Contributions of O-p orbitals. (c) Contribution of Al-s orbital. (d) Contributions of Al-p orbitals.

time, the low-energy electron-diffraction (LEED) pattern remained hexagonal, indicating that lateral distortions are relatively small.15 To perform geometry optimization, we have fixed the bottom Al-O-Al layers (labeled as Al3, O2, and Al4 in Figure 1b) to maintain the R-Al2O3 bulk structure and allowed the top AlO-Al (labeled as Al1, O1, Al2 in Figure 1b) to be fully relaxed. Figure 1 shows the optimized geometry of the R-Al2O3 (0001) surface. As is apparent from the top view in Figure 1a, the hexagonal structure remains almost unchanged. This is consistent with the LEED measurement results. Yet the side-view in Figure 1b shows that the top three layers undergo very large relaxation along the 〈0001〉 direction normal to the surface plane. The Al atoms in the first and third layer relax inward, and the O atoms in the second layer relax outward. The relaxation of the first Al and the second O layer is about 90% with respect to the ideal bulk structure (decreasing from 0.084 to 0.008 nm). This relaxation places the first layer of Al atoms almost in the same plane as the O atoms, in a qualitative agreement with but quantitatively larger than the TOF-SARS measurement. Nevertheless, our calculations are consistent with several previous theoretical calculations23,27-28 in which larger contractions than the TOF-SARS result have also been found. A recent FPLAPW calculation by Wang et al.29 suggested that the presence of hydrogen could be the reason for these discrepancies on the magnitude and directions of surface relaxation between previous theoretical and experimental results. In fact, capping the Al2O3 slab structure by hydrogen did give rise to relaxation parameters that replicated the TOF-SARS experiment.29 In our present study, we first reproduced the results of Wang et al. for hydrogen-free Al2O3 slab. The relaxed geometry is identical between that in ref 29 and the current calculations. The local geometry illustrating the 3-fold oxygen site along with a list of geometry parameters is given in Figure 1c. Within the present optimization scheme, the reconstructed R-Al2O3 surface is more stable than the undistorted truncated R-Al2O3 surface by 1.94 eV per unit cell. The coordinates of the partially optimized geometry are listed in Table 4 as well as those of

the truncated bulk structure. The comparison shown here establishes the framework for an analysis of the band structure and surface states in the relaxed, hydrogen-free Al2O3 surface, as well as of the energetic, structural, and electronic changes upon adsorption of the transition metals Co and Ni. 3.2. Surface States in the Truncated and Optimized r-Al2O3 Slabs. Both the truncated and the relaxed Al2O3 slab structures possess electronic states in the band gap of threedimensional (3-D) R- Al2O3, which are identified as surface states associated with the broken bonds at the surface aluminum and oxygen atoms. To identify the surface states in the slab structures, electronic properties of the perfect 3-D R-Al2O3 structure were calculated first at the same level as that of the slabs. The crystal structure of bulk R-Al2O3 has rhombohedral symmetry (space group R3hc), with three Al2O3 formula units per primitive cell. Like all rhombohedral systems, this structure can be treated as hexagonally symmetric with atomic positions given in terms of the 3-D hexagonal unit cell. This choice of geometry facilitates the comparison with the 2-D hexagonal slab models created by truncation and relaxation of the 3-D structure. Figure 2 displays the calculated band structure of the 3-D R-Al2O3 in the hexagonal reciprocal lattice space. The regular band structure along the 〈001〉 direction, from the Γ point (000) to the Κ point (001), and along the 〈011〉 direction, from the Γ point (000) to the Μ point (011), is shown in Figure 2a. Contributions of the atomic orbitals O-2p, Al-3s, and Al-3p to the energy levels are then displayed using the “band structures with character plot”13 in Figure 2b-d. The O-2p orbitals are clearly the major contributors to the energy levels at the top of the valence band (TVB) and the bottom of the conduction band (BCB). The Al-3s orbital has also some contributions to the BCB. The calculated band structures of the truncated undistorted R-Al2O3 (0001) slab are shown in Figure 3a. One of the striking features is an appearance of additional energy levels within the band gap, which are identified as empty “surface levels”. In a truncated slab structure, the Al atoms have different surroundings. For example, in our slab, the top and bottom Al atoms

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Figure 3. Calculated band structure and some band structures with character plot of the truncated undistorted R-Al2O3 (0001) surface. (a) Regular band structure with several surface levels indicated. (b) Contribution of the top-layer Al atom. (c) Contribution of the middle-layer Al atom. (d) Contribution of the bottom-layer Al atom. EF is the Fermi level.

Figure 4. Calculated band structure and several band structures with character plot of the partially optimized R-Al2O3 (0001) surface. (a) Regular band structure with several surface levels indicated. (b) Contribution of the top-layer Al atom. (c) Contribution of the middle-layer Al atom. (d) Contribution of the bottom-layer Al atom. EF is the Fermi level.

are different from the middle-layer Al atoms. The band structures with character plot break down the contributions of different atomic orbitals to the calculated energy levels. Figure 3b-d displays these band structures with character plot of the top-layer Al (Figure 3b), the medium-layer Al (Figure 3c), and the bottom-layer Al (Figure 3d), showing that the surface states in the energy range 0∼1.2 eV above TVB are associated with the surface aluminum atoms. Figure 4a shows the calculated band structure of the partially optimized R-Al2O3 slab. Here the participation in the surface states of the orbitals of the Al atoms of the top-layer, which have relaxed along inward normal into the second-layer of

oxygen atoms, has significantly decreased compared with those of the bottom-layer Al atoms which have remained fixed in our optimization scheme. These surface levels are extraneous to the O-2p valence band, which contains 18 doubly occupied bands per unit cell by six 2p electrons per each of the six O atoms in their closed-shell anionic state O2-. There are two empty surface state levels within 0∼1.2 eV above the TVB in the truncated unrelaxed structure and one empty surface state in the structure relaxed on one but not the other Al surface side. Each of these surface states has an Al-3s character. In the truncated slab, one of the Al-3s surface state collapses into the O-2p TVB. Therefore, the truncated Al2O3 is a metal. On the contrary, the

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Ma et al. TABLE 1: Adsorption Energies of Single Co and Ni Atom on the 3-Fold Oxygen Site adsorption energy (eV) case

without further relaxation

with further relaxation

Co on Al2O3 Ni on Al2O3

0.80 0.95

1.10 1.09

with the transition metal atoms is independent of the pathway for reaching the stable adsorbate-substrate geometry. The adsorption energy ∆E is evaluated as

∆E ) Etot(adsorbed) - Etot(separated)

Figure 5. Local geometries of the Co and Ni atoms adsorbed on the 3-fold oxygen site of the partially optimized R-Al2O3 surface. (a) Al2O3 without Co/Ni adsorption. (b) Co on Al2O3. (c) Ni on Al2O3. The bond lengths and layer distances are in units of nm.

partially relaxed slab leaves a small band gap between the TVB and the single surface state that resides on the unrelaxed Al surface. Therefore the partially relaxed Al2O3 slab is a semiconductor with a narrow gap between O-2p and Al-3s orbitals on the unrelaxed surface. Relaxation of both Al surfaces removes the surface states and restores the insulating character, albeit in an entirely new 2-D structure. This inward relaxation of Al atoms has consequences for the reactivity toward the metal atoms, as described in the next section. 3.3. Adsorption of Co and Ni Atoms on the r-Al2O3 (0001) Surface. To calculate the adsorption energies and other properties of Co (or Ni) on the alumina substrate, a single Co (or Ni) atom was added to the partially optimized R-Al2O3 (0001) slab from the relaxed side. Afterward, the metal atoms, as well as the top Al1-O1-Al2 layers, were fully relaxed. Only one stable adsorption site was found, i.e., the 3-fold oxygen site as shown in Figure 1c. In this stable configuration, both the Co and Ni atoms are located above the oxygen plane, with the Co-O bond length of 0.222 nm and the Ni-O bond length of 0.227 nm. The first layer Al atoms, which are at a distance of 0.312 nm from the adsorption site, remain almost unaffected. The O atoms in the second, i.e., top, oxygen O1 layer also undergo relatively small relaxation with some movement toward the center of the 3-fold site. The most significant relaxation occurs with the thirdlayer Al2 atom. Upon both Co and Ni adsorption, this Al atom is pushed farther down by about 0.020 nm upon Co adsorption and 0.012 upon Ni adsorption. Figure 5 displays the local geometry of the optimized Al2O3 surface alone (Figure 5a), Co adsorbed on the Al2O3 surface (Figure 5b), and Ni adsorbed on the Al2O3 surface (Figure 5c). Adsorption of a Co or Ni atom on the originally truncated undistorted R-Al2O3 (0001) slab has also been calculated. When the top three Al1-O1-Al2 layers and the metal atom are fully relaxed, the stable configurations are the same as those obtained by the Co or Ni atom adsorption on the originally partially optimized R-Al2O3 (0001) slab. This result shows quite convincingly that the final optimized structure

(3)

where Etot(adsorbed) is the total calculated energy of the unit cell containing the adsorbed metal atom in its equilibrium position and Etot(separated) is that of the system at a large separation of the metal atom from the surface. Etot(separated) is equal to the sum of the calculated total energies of the partially optimized R-(Al2O3) slab and a single metal alone Etot(atom), which is computed by locating a single metal atom in an 1 × 1 × 1 nm3 empty cubic box. One of the main geometric features of the adsorbed metalalumina interaction is the nearest neighbor distance between the metal atom and a surface oxygen atom. In the present case, the Co-O (0.222 nm) and the Ni-O (0.227 nm) distances are larger than those on silica found previously,30 Co-O (0.195 nm) and Ni-O (0.190 nm), evidently because of the weaker bonding of the metals on alumina found here (vide infra) than on silica.30 A further comparison can be made with the Ni-O distance for nickel on Mg9O9 clusters calculated by Pacchioni and Ro¨sch,31 0.185 nm. Although experimental structural data regarding neutral atom adsorbates studied here are not available, a distance Co-Si in a 0.4 monolayer of cobalt on Si(100) has been reported as 0.23 nm.32 Therefore, the presently optimized surface structures yield distances within the range of experimental values for low coverage atomic adsorbates. Moreover, our further calculations in the entire [Ar]3dn4s1,2 series (n ) 0-10) show that the metal-oxygen distances are inversely related to the adsorption strength, consistent with the above findings. Table 1 lists the adsorption energies of the Co and Ni atoms on the 3-fold oxygen site. Initially atoms of the partially optimized R-Al2O3 slab were fixed with only Co and Ni atoms relaxed. The calculated adsorption energies are listed in the first column of Table 1 labeled as “without further relaxation”. To investigate the effect of the relaxation of the substrate, the top three Al1-O1-Al2 layers were then allowed to relax with the adsorbed atoms, and the adsorption energies are given in the second column of Table 1 labeled as “with further relaxation”. The adsorption of the Co atom causes larger distortions of the substrate and gives rise to about 0.30 eV stabilization energy and adsorption of the Ni atom to 0.14 eV stabilization compared to adsorption on a rigid “optimized” Al2O3 surface. It must be noted that the alumina slab has three surface oxygen atoms per unit cell, and therefore, the surface coverage is one transitionmetal atom per three oxygens, i.e., 33% surface coverage if one defines 100% coverage as that of the surface oxygen sites. At this surface coverage, lateral interactions among the metal atoms become a part of adsorption energy, but because of the large metal-metal distance (0.476 nm), these interactions constitute a small fraction of the adsorption energy. A detailed study of the dependence of the adsorption energy on metal coverage will be presented elsewhere. Upon adsorption of the Co or Ni atoms, the band structure of the alumina changes accordingly. One of the significant

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Figure 6. Several band structures with character plot of Co adsorbed on the 3-fold oxygen site of the partially optimized R-Al2O3 surface. (a) Contributions of Co-3d spin-up orbitals. (b) Contributions of Co-3d spin-down orbitals. (c) Contribution of Co-4s spin-up orbital. (d) Contribution of Co-3s spin-down orbital. EF is the Fermi level.

Figure 7. Several band structures with character plot of Ni adsorbed on the 3-fold oxygen site of the partially optimized R-Al2O3 surface. (a) Contributions of Ni-3d spin-up orbitals. (b) Contributions of Ni-3d spin-down orbitals. (c) Contribution of Ni-4s spin-up orbital. (d) Contribution of Ni-3s spin-down orbital. EF is the Fermi level.

changes is the appearance of the additional energy levels in the band gap. These are mainly the Co (or Ni) 4s and 3d orbitals. Figure 6 displays the calculated band structure with orbital character plots of the spin-up (R) and spin-down (β) Co-4s and -3d levels, and Figure 7 shows analogous band structures for the adsorbed Ni atom. The 3d bands are flat throughout the Brillouin zone, and their energies are very close to those at the Γ point (k ) 0). For both Co and Ni, the R portions of the 3d band, which are fully occupied by five electrons, have lower energies than the β portions, which are partially occupied by two electrons for Co and three electrons for Ni. This separation of R- and β-spin manifolds originates from the strong correlation and exchange interactions of the 3d electrons. The separation of 3d R and β energies is greater in the Co case than in the Ni

case, because of one more spin-unbalanced 3d electron on Co than on Ni. On the other hand, the 4s bands show a strong dispersion along different k directions, whereas differences between the 4s R and β orbitals are relatively small because they are both occupied. A quantitative energy diagram illustrating the relative positions of the Co (or Ni) 4s and 3d levels at the Γ point (k ) 0) relative to the TVB is shown in Figure 8. Band structures with character plot of the second-layer O atoms, which form the 3-fold site, and the top-layer Al atom, middle-layer Al atom, and the bottom-layer Al atom to the band structure with adsorbed Co are presented in Figure 9 (spin-up portion) and Figure 10 (spin-down portion). The bonding relations are now quite complex, far more so than the simple Co-4s-Si-3s interaction found in our earlier work on silica-

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Figure 8. Energy diagram of Co (or Ni) 3d and 4s levels with respect to the TVB. Both spin-up (R) and spin-down (β) portions are shown. Occupancies of these energy levels are also indicated. The specific labels are approximate, for example, Co-4s (spin-down) is a mixture of dominant Co-4s plus nonnegligible Co-3dz2 and O-2p orbitals with their relative proportions listed in Table 2 column 7.

Figure 9. Spin-up portion of several band structures with character plot of Co adsorbed on the 3-fold oxygen site of the partially optimized R-Al2O3 surface. (a) Contributions of the second-layer O atoms. (b) Contribution of the top-layer Al atom. (c) Contribution of the middle-layer Al atom. (d) Contribution of the bottom-layer Al atom. EF is the Fermi level.

supported Co and Ni.30 The character plots of the “surface states” show that these surface states are associated primarily with the bottom-layer Al4 atoms (Figures 9d and 10d). The Al4 atom position, however, is not characteristic of a regular R-Al2O3 structure because its oxygen neighbors further down in the structure are missing. To remove this “slab termination problem” which is in some ways similar to cluster termination problems, we have calculated

two additional cases: one with all atoms including the Al4 atom fully relaxed and another with Co atoms attached symmetrically on both surfaces. In both cases, the surface states associated with Al4 disappeared. In the first case, the adsorption energy of the Co atom was 1.30 eV. In the second case of Co atoms adsorbed on both sides of the slab in a fully relaxed geometry, the total adsorption energy was 1.05 eV per Co atom. These energies are comparable to the 1.1 eV adsorption energy for

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Figure 10. The spin-down portion of several band structures with character plot of Co adsorbed on the 3-fold oxygen site of the partially optimized R-Al2O3 surface. (a) Contributions of the second-layer O atoms. (b) Contribution of the top-layer Al atom. (c) Contribution of the middle-layer Al atom. (d) Contribution of the bottom-layer Al atom. EF is the Fermi level.

TABLE 2: Contributions of Metal-4s, Metal-3dz2, and O-2p Atomic Orbitals to the Metal-4s and TVB Energy Levelsa case

Al2O3

energy level

TVB

spin O-2p metal-4s metal-3dz2

Co on Al2O3

Ni on Al2O3

TVB

Co-“4s”

TVB

Ni-“4s”

up

down

up

down

up

down

up

down

up

down

48.33

47.31

17.21 0.49 4.16

18.01 0.36 0.76

6.80 14.11 0.83

5.98 11.88 5.96

18.54 0.51 2.00

19.05 0.45 1.00

6.80 16.43 1.25

6.77 15.57 2.90

a The contributions from O-2p orbitals are the sum of three O atoms, which form the 3-fold site. The contributions from other atoms in the unit cell, such as Al atoms, to these energy levels are relatively smaller.

Co on the slab with unrelaxed bottom Al4 (Table 1). This analysis shows that the artificially introduced surface states associated with Al4 play a minor role on the adsorption energy of the transition metal. 3.4. Orbital Interactions. Band structure analysis indicates that the Co (or Ni) 4s and 3d orbitals participate the most in the interaction with the Al2O3 substrate. The Wien program used in this study provides a detailed analysis tool to access the contributions from individual atomic orbitals within the “muffintin” spheres to the energy levels at each k point. Because of the presence of the interstitial charge, the absolute values of these weight contributions do not have a clear meaning. However, the relative weight values provide the information about the atomic orbital interactions and hybridization. A detailed breakdown orbital contributions of the Co (or Ni) 4s and 3d and O-2p atomic orbitals to the metal-4s level and the TVB level at Γ point (k ) 0) is presented in Table 2. For both spin-up and spin-down portions, the metal-4s and -3dz2 orbitals form hybridized orbitals which interact with the Al2O3 substrate to the largest extent. The O-2p atomic orbitals are major contributors to these orbital interactions with metals, whereas the contributions from other atoms, such as Al atoms, are much smaller and have not been listed. We notice that both the metal4s and O-2p TVB levels are occupied before and after adsorption, and there is no net charge transfer among these atoms. In our earlier study of adsorption of Co and Ni atoms on the SiO2 surface, similar 3-fold oxygen adsorption sites have been

found30 and the metal 4s orbitals were dominant in the metalsupport interaction. There is a substantial difference, however, in the nature of orbitals of the silica or alumina support that participate in the bonding of the metal adatoms: on silica, the empty Si-3s orbitals contributed significantly, whereas on alumina, the Al-3s contribution is negligible. A simple explanation of this difference is based on the difference of the orbitals energies of the Si-3s and Al-3s levels in the oxide material with respect to the metal 4s energies, e.g., at the Γ point, (Si-3s) (Co-4s) ) -1.24 eV, (Si-3s) - (Ni-4s) ) -0.71 eV, (Al3s) - (Co-4s) ) +3.50 eV, and (Al-3s) - (Ni-4s)) +4.00 eV. The magnitude of the metal-4s-Al-3s gap is 2∼3 eV larger than that with Si-3s, which results in a smaller interaction even from the standpoint of perturbation theory. We now address the specific way in which the 3d orbitals of the metal atom participate in the interaction with the oxide support. The following features assist an interpretation of such participation. (1) The 3d bands of the adsorbed complex are flat (Figures 6 and 7), indicating little if any dispersion, (2) the d levels are split into the nondegenerate dz2 and two doubly degenerate sets {dxy, dx2-y2} and {dzx, dxy} typical of screened crystal-field splitting in a trigonal geometry,33 (3) the 3d spinup and spin-down manifolds are separated by a larger energy difference than the amount of crystal-field splitting (Figure 8), and (4) the {3dxy, 3dx2-y2} orbitals are highest in the one-electron energy scale in each of the separate spin manifolds. Furthermore, the energy splitting of the 3d orbitals is small compared to that between the spin-up and spin-down d manifolds, indicating that

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TABLE 3: Net Spin Charge Densities within Certain Spheres of Atoms and an Interstitial Regiona case

interstitial

Al2O3 Co+Al2O3 Ni+Al2O3

0.00 0.83 0.78

Co/Ni

O

Al

2.09 1.03

0.00 0.01 0.01

0.00 0.00 0.00

The radii of the atomic spheres are chosen as follow: Co/Ni ) 2.0 a.u.; O ) 1.4 a.u.; and Al ) 1.6 a.u. a

exchange interactions among the d electrons localized on the metal atom are stronger than the crystal-field effects. These exchange interactions also contribute to the ordering of d levels because of electron occupancies in the following manner: the “natural order” for equally occupied d orbitals, such as in the spin-up manifolds in Figure 8, is (dzx,yz) > (dxy,x2-y2) > (dz2) because electrons in the dzx,yz orbitals that point toward the support oxygens are repelled most and those in the dz2 orbital which point away from the oxygen “ligands” are repelled least. The spin-down d orbitals are partially occupied by two electrons in Co and three electrons in Ni. Exchange interaction forces the two electrons in Co into the degenerate set {dxy, dx2-y2} to maintain spin-parallel configuration which stabilizes this subset and shifts its energy below that of the unoccupied dz2 orbital. In addition, the occupied Co-4s spin-down orbital has a considerable admixture of Co-3dz2 (Table 2, column 7), which reflects hybridization Co-4s (occupied) ) a × Co-4s + b × Co-3dz2, a > b, and Co-3dz2 (unoccupied) ) c × Co-4s + d × Co-3dz2, c < d. Such a hybridization is most pronounced in the Co spin-down case, because the 4s and the 3dz2 orbitals are closer in energy than in all other cases investigated. One additional d electron in Ni occupies the dz2 orbital and restores its lower energy while keeping maximum spin alignment because of exchange forces in this {dxy, dx2-y2}, dz2 spin-down configuration. The screened ligand field split of the spin-up portion of the 3d orbitals does not contribute to the adsorption of metal atoms because they are fully occupied. The partially occupied spindown portion, however, plays a key role in stabilizing the system. The stabilization energy can be estimated by finding the “center-of-weight” of the 3d orbitals indicated with dashed lines in Figure 8. The spin-down stabilization of the {dxy, dx2-y2} configuration in Co, and the {dxy, dx2-y2, dz2} configuration in Ni amounts to 0.46 eV (Co) and 0.48 eV (Ni). Both of these values account roughly for one-half of the adsorption energy. The remainder is attributed to a combination of geometric relaxation phenomena, exchange-correlation effects beyond those included in the one-electron Kohn-Sham orbital energies, and lateral interactions among the metal atoms. The spin localization on the transition metal is also apparent from the net spin-charge densities within the atomic spheres and interstitial space before and after the metal atom adsorption shown in Table 3. The Al2O3 surface has zero net spin density before the Co/Ni adsorption. Even after the metal adsorption, the net spin densities within the oxygen and aluminum atomic spheres remain almost unaffected, whereas the net spin charge of the system resides inside the Co/Ni atomic spheres and the interstitial region. 4. Conclusions Adsorption of atomic Co or Ni on a partially optimized R-Al2O3 surface takes place with nearly the same energy of ∼1.0 eV. The Al2O3 surface is strongly relaxed from the initial structure of R-Al2O3 truncated by the (0001) planes and undergoes a further smaller relaxation upon adsorption of the

TABLE 4: Fractional Coordinates of the Al2O3 Surface Structure Truncated from the Bulk Structure and Optimized Surface Structurea trancated structure atoms (layer) Al(1) O(2) O(2) O(2) Al(3) Al(4) O(5) O(5) O(5) Al(6)

relaxed surface structure

X

Y

Z

X

Y

Z

0.667 0.694 0.000 0.306 0.333 0.000 0.639 0.333 0.027 0.667

0.333 0.000 0.694 0.306 0.667 0.000 0.667 0.973 0.361 0.333

0.254 0.210 0.210 0.210 0.126 0.084 0.042 0.042 0.042 0.000

0.663 0.672 0.013 0.303 0.332 0.000 0.639 0.333 0.027 0.667

0.332 -0.016 0.691 0.214 0.666 0.000 0.667 0.973 0.361 0.333

0.152 0.149 0.149 0.149 0.092 0.084 0.042 0.042 0.042 0.000

transition metal. Simulating the surface using truncated slabs from the bulk structure has been widely used in many calculations. Our calculations clearly demonstrate that unsaturated surface bonds result in the surface levels which will affect the surface geometry and electronic properties. Although surface reconstruction removes the surface levels from the relaxed end of the slab, the terminations of slabs are similar to the terminations of clusters, valid only when the terminations are far enough from the main region of chemical actions. Both the adsorbed Co and Ni atoms are in their high-spin state, and the spin charge is strongly localized on the 3d orbitals of those atoms. The orbital nature of the metal-support interactions is complex. The “ligand field” splitting of partially occupied 3d orbitals screened by the bonding orbital involving the 4s electron in a 3-fold oxygen site has been shown to have a significant contribution to the metal-support interaction, accompanied by the stabilization because of reconstruction of the alumina substrate. Compared with the results of adsorption of Co and Ni atoms on silica presented in our previous study,30 the adsorption energies of Co and Ni on alumina are significantly smaller. This, perhaps, is due to the “shielding effect” of the top layer aluminum atoms adjacent to the adsorption sites, which repel the incoming transition metal atoms. As a consequence, the metal atoms are less tightly bound by the surface oxygen atoms. The cause of the aluminum-metal repulsion is traced to the high energy of the Al-3s orbital, well above the Si-3s energy which gives rise to an attraction interaction between the metal4s and the metal-4s and the Si-3s levels.30 Acknowledgment. This research was carried out under the Grant PPDO-001 of the Pennsylvania Infrastructure Technology Alliance (PITA) program with financial support from PITA and Air Products and Chemicals, Inc. Support of scientists Drs. K. Anselmo, J. Armor, J. Tao, R. Pierrantozzi, and C. Valenzuela and their commitment to computational approaches to practical aspects of material science of catalysis is highly appreciated. References and Notes (1) Imelik, B., et al., Eds., Metal-Support and Metal-AdditiVe Effects in Catalysts; Elsevier Scientific Publishing Co., Amsterdam, The Netherlands, 1982. (2) Gunter, P. L. J.; Niemantsverdriet, J. W.; Ribeiro, F. H.; Somorjai, G. A. Catal. ReV. 1997, 39, 77. (3) Gates, B. C. Chem. ReV. 1995, 95, 511. (4) Sewell, G. S.; O’Connor, C. T; van Stern, E. Appl. Catal. A 1995, 125, 99. (5) Gardner, D. A.; Clark, R. T. Catalytic Process for Preparing Ethylamines. U.S. Patent 4,255,357, March 10, 1981. (6) Reuel, R. C.;Bartholomew, C. H. J. Catal. 1984, 85, 63, 78. (7) Iglesia, E.; Soled, S. L.; Fiato, R. A. J. Catal. 1992, 137, 212.

Interaction between Catalyst and Support (8) Riva, R.; Miessner, H.; Vitali, R.; Del Piero, G. Appl. Catal. A 2000, 196, 111. (9) Stakheev, A. Yu.; Kustov, L. M. Appl. Catal. A 1999, 188, 3. (10) Blyholder, G. J. Mol. Catal. 1997, 119, 11. (11) Ji, L.; Lin, J.; Zeng, H. C. J. Phys. Chem. B 2000, 104, 1783. (12) Chokkaram, S.; Srinivasan, R.; Milburn, D. R.; Davis, G. H. J. Mol. Catal. A 1997, 121, 157. (13) Blaha, P.; Schwarz, K.; Luitz, J. WIEN97, A Full Potential Linearized Augmented Plane Wave Package for Calculating Crystal Properties Karlheinz Schwarz, Techn., Universita¨t Wien, Austria, 1999; ISBN 3-9501031-0-4. (14) Eng, P. J.; Trainor, T. P.; Brown, G. E., Jr.; Waychunas, G. A.; Vewville, M.; Sutton, S. R.; Rivers, M. L. Science 2000, 288, 1029. (15) Ahn, J.; Rabalais, J. W. Surf. Sci. 1997, 388, 121. (16) Suzuki, T.; Hishita, S.; Oyoshi, K.; Souda, R. Surf. Sci. 1999, 437, 289 (17) Hohenberg, P.; Kohn, W. Phys. ReV. B 1964, 136, 864. Kohn, W.; Sham, L. J. Phys. ReV. A 1965, 140, 1133. (18) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (19) Singh, D. Plane WaVes, Pseudopotentials and the LAPW Method; Kluwer Academic: Boston, 1994. (20) Yu, R.; Singh, D.; Krakauer, H. Phys. ReV. B. 1991, 43, 6411. (21) Stumpf, R.; Scheffler, M. Comput. Phys. Commun. 1994, 79, 447. (22) Blo¨chl, P. E.; Jepsen, P.; Anderson, O. K. Phys. ReV. B 1994, 49, 16223.

J. Phys. Chem. B, Vol. 105, No. 11, 2001 2221 (23) Gordin, T. J.; LaFemina, J. P. Phys. ReV. B 1994, 49, 7691. (24) Guo; Ellis, D. E.; Lam, D. J. Phys. ReV. B 1983, 45, 175. (25) Goodman, D. W. J. Vac. Sci. Technol. A 1996, 14, 1526. (26) Wander, A.; Searle, B.; Harrison, N. M. Surf. Sci. 2000, 458, 25. (27) Manassidis, I.; et al. Surf. Sci. Lett. 1993, 285, L517. (28) Verdozzi, C.; Jennison, D. R.; Schultz, P. A.; Sears, M. P. J. Phys. ReV. Lett. 1997, 5, 321. (29) Wang, X. G.; Chaka, A.; Scheffler, M. Phys. ReV. Lett. 2000, 84, 799. (30) Ma, Q.; Klier, K.; Cheng, H.; Mitchell, J. W.; Hayes, K. S. J. Phys. Chem. B 2000, 104, 10618. (31) Pacchioni, G.; Ro¨sch, N. J. Chem. Phys. 104, 7329. (32) Watson, P. R.; Van Hove, M. A.; Hermann, K. Atlas of Surface Structures. J. Phys. Chem. Ref. Data 1, 769. (33) The separation of the highest from the lowest 3d levels is 0.48 eV for Co and 0.51 eV for Ni. These values are smaller than expected for the Co2+ and Ni2+ ions in trigonal oxygen sites, cf., R. Kellerman, R.; Klier, K. Surface and Defect Properties of Solids. J. Chem. Soc. (London) 1975, 4, 1-33. Taking octahedral splitting of the 3d levels in trigonal complexes is estimated to be even larger, by a factor up to 5. Therefore, in the present case of neutral Co and Ni atoms, the 4s electrons appear to screen the action of the crystal field on their 3d electrons compared to the ions which are free of such screening.