Interaction between Catalyst and Support. 1. Low Coverage of Co and

Bethlehem, PennsylVania 18015. Hansong Cheng,*,† John W. Mitchell, and Kathryn S. Hayes. Air Products and Chemicals, Inc., 7201 Hamilton BouleVard, ...
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10618

J. Phys. Chem. B 2000, 104, 10618-10626

Interaction between Catalyst and Support. 1. Low Coverage of Co and Ni at the Silica 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: July 5, 2000; In Final Form: September 12, 2000

A theoretical investigation of transition-metal interaction with a silica surface is reported herein. The study employs periodic density functional theory at the full-potential linearized augmented plane wave (FP-LAPW) level with spin polarization taken into account. Initial low coverages of Co and Ni metal are examined on a siloxane surface of a 2-dimensional periodic slab model with hexagonal unit cell of composition O3(top)Si2(OH)2. The geometry of the top oxygen layer is optimized before and after the metal adsorption along with the position of the metal atom. The preferred adsorption site is found to be a 3-fold hollow relaxed structural feature formed by the top layer O atoms without Si along inverted surface normal in the second layer. The calculated adsorption energies for Co are slightly larger than for Ni on all sites, while the differences among sorption sites are quite marked for both metals. The patterns of total energy are replicated by the stabilization of the occupied metal 4s orbital in forming a surface bond with the primary participation of the Si3s (and to smaller extent surface O2sp) empty antibonding orbital of the silica. The third layer of O atoms remains unaffected. The calculated energy band structure and densities of states yield useful insight into the detailed bonding, show a significant dispersion of the stabilized metal 4s orbital with average energy below the Fermi level, and symmetry splitting of the flat 3d band in the trigonal site. Partial occupancy of the 3d levels provides contributions to the adsorption energy which are much smaller than those due to the metal 4s-Si 3s/O 2sp interaction.

1. Introduction Heterogeneous catalysis by well-dispersed transition metal catalysts, such as Co and Ni on various supports, has widespread industrial applications in hydrogenation, amination, hydrotreating, and Fischer-Tropsch processes.1-6 The support effects have been reported as ranging from negligible to strong, and metalsupport interaction or its absence is considered of prime importance in catalyst function, patterns of activity and selectivity, and ensuing design.7-13 While attempts have been made to advance our understanding of such interactions, whether electronic, structural or textural,14-16 the detailed underlying mechanisms and even physical principles remain largely unknown. Among the many phenomena involved, agglomeration of catalyst particles is a severe problem encountered in many industrial processes, which usually results in surface shrinkage and catalyst deactivation. The problem is thought to be closely associated with the adhesive force of the catalyst to the supporting substrate. In addition, in semiconductor technology, the integration of cobalt silicate to ultra-large-scale integrated circuits (ULSI) has proven to be a great advantage for reducing the device size.17 Here again, a better understanding of interactions between the metal atoms and the silica substrate is critical. In this paper, we present a theoretical study based on a density functional theory (DFT) of the interactions of Co and Ni atoms with the SiO2 support at the full-potential linearized augmented †

E-mail: [email protected].

plane wave (FP-LAPW) level with spin polarization taken into account. The adhesive force of these metals on silica has not been addressed in the literature at the atomic level. As the first attempt to quantify the interaction, we focus mainly on the metal submonolayers at very low coverage at which theory is expected to predict the relative energies governing monatomic dispersion and onset of clustering. The SiO2 surface geometry was optimized to serve as a model substrate. Relative strengths of various adhesion sites for the metal atoms were then examined in terms of their adsorption energy and the calculated energy band structure. 2. Computational Details The DFT calculations were performed using the WIEN97.9 code with the full-potential linearized augmented plane-wave (FP-LAPW) method.18 Periodic boundary conditions were imposed in conjunction with a slab surface model of SiO2. In this way the Kohn-Sham orbitals19 are symmetrized to form the Bloch crystal orbitals (BCO) which are bases for irreducible representation of the crystal translation group.20 The computation was done under the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof functional.21 Full relativistic calculations of the core electrons and the scaled relativistic treatment of the valence electrons were included. A spinpolarized computational scheme was utilized to deal with the open-shell systems. A modified tetrahedron integration scheme22 was used to generate the k-mesh in the irreducible wedge of

10.1021/jp002409g CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

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Figure 1. Silica surface structures: (a) structure I; (b) structure II. Shaded area represents the unit cell. The different adsorption sites are labeled as T (top), B (bridge), H (3-fold hollow O site) ,and H1 (3-fold O above Si).

the hexagonal Brillouin zone (BZ) on a special point grid with a total of four k-points of 2 × 2 × 1. The energy cutoff was about 20 Ry. 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. The present silica model is a slab with a hexagonal unit cell of stoichiometry O3(top)Si2(OH)2 (Figure 1) that contains three layers of SiO2 with the silicon layer sandwiched by two layers of oxygen. An additional fourth layer formed by hydrogen atoms was also included to terminate the dangling oxygen atoms in order to make the slab charge neutral. The introduction of hydrogen in the model is not expected to give rise to significant artifacts in the final results because the H atoms are considerably removed from the adsorption sites and the nature of the chemical bonding in the slab is highly covalent. The H atoms were not allowed to move in the geometry optimization schemes. The unit cell has in-plane lattice parameters a ) b ) 0.53 nm and the distance between the slabs c ) 1.5 nm, large enough to prevent appreciable interactions between them. The internal atomic coordinates were then optimized under the criterion that requires that all the forces acting on the atoms be less than 10 mRy/au.

3. Results and Discussion 3.1. Surface Structure and Adsorption Geometry. The geometry optimization on the SiO2 surface yields two minimum energy structures as illustrated in Figure 1. The less stable structure I contains a six-ring window of Si, with one O atom residing on top of each Si-Si bridge to form a Si-O-Si bond as shown in the top view of the structure in Figure 1a. This structure resembles the (111) surface of β-cristobalite. Although the Si-O-Si bond angle in an idealized β-cristobalite is 180°, it is in fact reported to be about 145° in the actual β-cristobalite silica structure.23 Our optimized SiO2 structure I gives an SiO-Si bond angle of approximately 140°, in reasonable agreement with the experimental value.23 In the optimized surface structure, each Si atom adopts a tetrahedral geometry with four covalent bonds to the nearest O atoms. One of the striking features of structure I is a large empty space in the hollow site at the center, despite being a potential minimum. By twisting the structure about the Si atoms 30° clockwise and anticlockwise alternatively, we obtained the second minimum energy structure (structure II) with a lower electronic energy by about 1.36 eV than that of structure I. This structure exhibits considerable lattice relaxation compared with

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TABLE 1: Adsorption of One Co or Ni Atom on the Different Sites of the Siloxane SiO2 Surfacea properties atom

structure

Co

I

II Ni

I

II

adsorption site

geometry (Å) M-Ob M-O⊥b

adsorption energy (eV)

T B H1 H

2.15 2.42 2.02 2.65

2.15 2.03 0.75 0.00

-0.14 0.06 -0.19 -0.45

T B H1 T B H1 H

1.89 2.19 2.05 2.16 2.45 2.03 2.65

1.89 1.56 0.76 2.16 2.06 0.77 0.00

-1.28 -1.31 -1.45 -0.08 0.05 -0.17 -0.44

T B H1

1.88 2.12 2.03

1.88 1.59 0.74

-1.19 -1.24 -1.40

a

T: on the top site; B: on the bridge site; H: on the 3(6)-fold hollow site without Si atom underneath; H1: on the 3-fold hollow site with one Si atom underneath. b M-O: metal (Co/Ni)-O bond length. M-O⊥: metal (Co/Ni) to O-plane distance.

the bulk and can serve as a stable surface model for silica. Both the top view and the side view of structure II are displayed in Figure 1b. Structure II is similar to structure I from the side view with four atomic layers. However, as evidenced from the top view, three of the six O atoms now move closer to the center of the Si six-member ring to form a perfect 3-fold triangle, while the other three are pushed away from the center. The six O atoms are located essentially in the same plane. It is worth noting that both optimized SiO2 structures contain six-ring windows commonly seen in many natural silicates, including clays and micas. The optimized fractional coordinates of the atoms in the unit cells for I and II are listed in the Appendix. For the purpose of comparison, we investigated the metal adsorption on both surface structures while giving detailed analysis only on the energetically more favorable structure II. For a low coverage of the metal, each unit cell contained only one metal atom. We considered four possible adsorption sites as labeled in Figure 1: i.e., T: for the top site; B: for the bridge site; H1: for the 3-fold hollow site with a Si atom underneath; and H: for the 6(3)-fold hollow site without a Si atom below. To simplify the calculation, only the top layer O atoms as well as the metal atoms were relaxed in the geometry optimization. Significant structural relaxation of the second layer and below is not expected due to the fact that the interaction between the metal and the first silica layer yields the dominant contribution to the adsorption energy. This is confirmed by examining the residual forces, a sum of the Hellmann-Feynman and the Pulay forces,24 on both the “frozen” atoms (Si of the second layer and OH of the third and fourth layers) and the “moved” atoms (Co, Ni, and O of the first layer) after optimization has been completed. These forces were generally in the range 1-2 mRy/ au for Co, Ni, and the first layer O, 7-8 mRy/au for the third layer Si, and 2-3 mRy/au for the bottom OH groups. A few selected bond parameters of the optimized structures are shown in Table 1. The main features of the adsorption structures are summarized as follows: The large empty space at the 6-fold hollow site in structure I results in unusually large metal-oxygen bond lengths, i.e., 0.265 nm for both Co-O and Ni-O. The metal atom in structure I is located at the center of the O-plane. For adsorption on the 3-fold hollow H-site in structure II, both Co and Ni metal

Figure 2. Adsorption energy of a single Co and Ni atom adsorbed on the SiO2 surface structures at different adsorption sites specified in the legend to Figure 1.

atoms are located about 0.07 nm above the 3-fold O-plane with the metal-oxygen distances of about 0.190 nm. The three neighboring O atoms are slightly lifted by 0.003 nm, and about 0.002 nm closer toward the 3-fold center upon metal adsorption, and the plane formed by six O atoms becomes uneven as expected. 3.2. Adsorption Energy. The adsorption energy ∆E1 of a single metal atom is evaluated as

∆E1 ) Etot(adsorbed atom) - Etot(separated)

(1)

where Etot(adsorbed) is the total energy of the unit cell containing the adsorbed metal atom in its equilibrium position on the silica substrate and Etot(separated) is that of the system at a large separation of the metal atom from the surface. The energy of the single atom is computed by locating it in an empty cubic box whose dimensions are gradually increased until a stable Etot(atom) is achieved. Typically, the size of the simple cubic box is 1 × 1 × 1 nm3. Etot(separated) is the sum of Etot(atom) and the total energy Etot(SiO2) of the slab without the adsorbate. Figure 2 displays the calculated adsorption energies of Co and Ni on various sites of the support defined in Figure 1 using the two structural models. In general, structure I yields smaller adsorption energies for both metals. Of the hollow sites, H site yields higher adsorption energy than H1. Both the top and the bridge sites give even weaker bonding of the metal atoms. Surface structure II, which is much more stable than structure I, yields significantly larger adsorption energies for the metal atoms. In particular, the hollow site H gives the highest adsorption energy. Table 1 summarizes the adsorption energies of the two metals on the various sites examined. The metal adsorption at the atop and the bridge sites is considerably weaker as the number of coordinated O atoms decreases. We further note that the adsorption energies of Co and Ni on the same sites are comparable, with some 0.1 eV preference of the siloxane surface for cobalt over that for nickel. Analysis of changes of orbital energies upon adsorption reveals that, although many types of interactions are involved, there is one that dominates the patterns of adsorption energies and site preferences, namely the metal 4s electron interaction with the empty antibonding orbitals of the surface. This can be gleaned from the partial orbital energy correlation diagrams at the Γ-point of the Brillouin zone presented in Figure 3: when either metal atom is adsorbed, the largest stabilization occurs for its 4s orbitals. Furthermore, the lowering of the 4s energy, ∆E4s, is largest for Co on the H sites and follows the order ∆E4s-

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Figure 3. Energy diagrams (a) spin up, and (b) spin down of Co, (c) spin up, and (d) spin down of Ni, at 3-fold hollow H-site of SiO2. The numbers are energies (in eV) of energy levels with top of valance band (TVB) as reference. Within each diagram, the Si 3s level in the SiO2 surface is given in the right column, and 4s and 3d levels of single metal atom calibrated by their core levels are given in the left column. The center column displays the splitting of these orbitals when metals adsorbed.

(Co-H site) > ∆E4s(Ni-H site) > ∆E4s(Co-H1 site) > ∆E4s(Ni-H1 site), the same order as the total energy. Furthermore, both the spin-up and spin-down 4s orbitals exhibit the same effect, conserving the spin multiplicity of the metal 4s2 shell on the bond with the surface. The stabilization of the metal 4s levels involves admixture of the orbitals of the silica, the proportions of which are specified as contributions of the atomic populations in the stabilized metal “4s” orbital. Table 2 lists the contributions of the metal 4s, Si 3s, O 2s, and O 2p atomic orbitals to this BCO. Larger Si 3s populations occur in most stabilized bonds, paralleled by smaller O 2sp populations. The origin of the Si 3s and O 2sp orbitals is traced to the empty antibonding orbitals that are the counterpart of the occupied Si 3s-O 2sp bonding orbitals near the bottom of the valence band. More detailed analysis has also been performed by extracting the complex coefficients Alm, Blm, the radial functions u(r, El) and their derivatives with respect to the orbital energy El, u˘ l(r,El) ) ∂u(l,El)/∂El, at the surface of the muffin-tin spheres, and their phases are listed in Table 3. The overall linearized augmented BCO, φkn, inside each of the atomic spheres is defined by the expansion (2) for each kn ) k + Kn, where k is the wave vector inside the first BZ and Kn are the reciprocal lattice vectors.

φkn )

∑ lm

TABLE 2: Contributions of Metal 4s, Si 3s, O 2s, and O 2p Atomic Orbitals to the Bonding Metal 4s BCO at the Γ Pointa cases

adsorption sites

Co + SiO2

H

Ni + SiO2

H1 H H1

(2)

The phases of the coefficients Alm and Blm are listed in Table 3 for the Γ-point, kn ) 0. The bonding character is apparent from the presence or absence of nodes of the BCO φkn between neighboring atoms: if two atomic orbitals have the same sign of AlmulYlm + Blmu˘ lYlm at the surfaces of the muffin-tin spheres, the interaction is bonding unless additional even number of nodes is present in the interstitial spacesbut at the Γ-point no such nodes occur in the first BZ because kn ) 0. In the case of opposite signs of AlmulYlm + Blmu˘ lYlm of two atomic orbitals, the interaction is

Si 3s

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

up down up up down up

7.38 7.71 2.43 7.08 7.30 2.04

6.11 5.26 5.97 6.36 6.01 7.38

0.66 0.01 0.61 0.02 0.01 0.67

2.65 2.67 0.97 2.60 2.70 0.83

1.12 1.34 2.04 1.06 1.23 2.24

a

These are given as a sum of the complex squares of the coefficients Alm and Blm of eq 2.

TABLE 3: Phases of the Si 3s, Metal 4s, Metal 3dz2, O 2s, and O 2pz Atomic Orbital Contributions in the Co “4s” BCO at the Γ Point When Co Is Adsorbed on the H and the H1 Sites of SiO2a atomic sites orbitals Arelm Aimlm ure(R) uim(R) Brelm Bimlm u˘ re(R) u˘ im(R) φ4s H

H1

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

spin

Si-3s Co-4s Co-3dz2 O-2s O-2pz Si-3s Co-4s Co-3dz2 O-2s O-2pz

+ + + +

+ + + + + +

+ + + + + +

+ + + + + + -

+ + + + + -

+ + + + + -

+ + + + -

+ + + + -

+ + + + + + + + -

a The superscripts re and im denote the real and imaginary components of the coefficients Alm and Blm, and the radial functions u(R) and their energy derivatives u˘ (R) are at the periphery R of the muffin-tin spheres. In the last column is listed the overall phase of the contribution of a given atomic orbital to the BCO “Co 4s”

antibonding. The spherical harmonics Ylm are positive constants for the s-orbitals (l ) 0), affording a straightforward assessment of the bonding character involving the Co 4s, Si 3s, and O 2s orbitals. For the pz (l ) 1, m ) 0) and the dz2 orbitals (l ) 2, m

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Figure 4. Total charge density and the net spin charge density of valence electrons on the (001) O-plane: (a) and (b) on structure II alone; (c) and (d) with one Co atom adsorbed on the 3-fold hollow site of structure II; (e) and (f) with one Ni atom adsorbed on the 3-fold hollow H-site of SiO2.

) 0), the phase is positive in the z-direction, which is chosen to be the outward normal to the silica slab surface that adsorbs the metal atom. Here also the assessment of bonding character is straightforward from the values of Alm, ul, Blm, and u˘ l only. A more complicated is the analysis of the px and py contributions because Y11 and Y1-1 have opposite signs. However, the contributions of these in-plane orbitals only occur with oxygen and are small compared to those of the metal 4s, Si 3s, O 2s O 2pz, and metal 3dz2 orbitals. The net result of this orbital analysis for the stabilized “metal 4s” orbital is apparent from the magnitude of the contributing orbitals listed in Table 2, and the signs of the coefficients Alm, Blm and the radial functions ul and u˘ l listed in Table 3. For both the H and the H1 sites, the Si3s, metal 4s and to a lesser extent O 2s are contributing in a bonding fashion. Further, the contributions of Co 3dz2 or Ni 3dz2 are very small at the H-site but become large at the H1site. In the latter geometry, the metal 3dz2 orbitals hybridize with metal 4s to form an extended bonding orbital along the direction perpendicular to the SiO2 surface that further binds albeit weakly, with the underlying Si 3s. The O 2p orbitals contribute less in the more strongly bonding H-site and their phase indicates antibonding contribution. The partially filled metal 3d orbitals also undergo changes, the most notable of which is the splitting into a nondegenerate dz2 and two doubly degenerate sets {dx2-y2, dxy} and {dzx, dyz}. This is the characteristic “ligand-field” d-orbital splitting in the trigonal symmetry of the H or the H1 site. The spin-up d-orbitals are 5-fold occupied for each metal, and their center-of-weight is slightly higher than in free metal atom with ensuing slight destabilization. The spin-down d-orbitals also have higher center of weight than in free atom, but because of their fractional occupancy (2 electrons in Co and 3 electrons in Ni), they provide a small contribution to the stabilization of the adsorbed state. The analysis based on the correlation diagrams at the Γ-point

TABLE 4: Calculated Net Spins within Spheres of Co/Ni Atom, One Nearest Neighbor O Atom, One Next Nearest Si Neighbor and the Interstitial Spacea net spin cases

interstitital space

Co/Ni

SiO2 single Co atom single Ni atom Co + SiO2

0.00 0.31 0.36 0.32

1.19 0.64 0.82

each O

each Si

0.00

0.00

0.05

0.03

a

There are 3 first O neighbors, and 2 next Si neighbors in the unit cell when Co/Ni atom adsorbed on the 3-fold hollow site.

provides the essential picture of the orbital nature of the adsorption process and is refined further by a full analysis of the densities of states and band structure calculated here for 100 k-points of the Brillouin zone (Figures 6-9). 3.3. Electron Density Analysis. The adsorption behavior of the transition metal atoms on silica can be understood based on the electron density distribution and, in particular, the net spin density since both Co and Ni in their free atom ground electronic states maintain unpaired electrons with the net spin of 3/2 and 1, respectively. Figure 4a shows the calculated total charge density of the valence electrons (the spin-up charge density R plus the spindown charge density β) of the SiO2 structure II. The three O atoms that form a 3-fold hollow site are indicated by arrows. Three adsorption sites are also highlighted. Large electron density is concentrated around the O atoms as expected. Not surprisingly, the net spin density, shown in Figure 4b for the structure II, all vanishes. Upon the metal adsorption, the electron density and the net spin change accordingly. Figure 4c-f displays the valence electron density in the presence of Co and Ni. Note that the

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Figure 5. “Band character” plots (a) SiO2 structure II; contributions of (b) the Si s orbitals; (c) Si p orbitals; and (d) O p orbitals, and (e) the total density of states (DOS). The special k-directions represented by labels are: Γ ) (0,0,0); Σ ) (1/4,0,0); Μ ) (1/2,0,0); Κ ) (1/3,1/3,0), and Λ ) (1/6,1/6,0). The top of valence band (TVB) is chosen to be zero for reference.

Figure 6. Spin-up portion of “band character” plots with contributions of (a) Co 3d; (b) Co 4s; (c) Si 3s; and (d) O 2p with Co atom adsorbed on 3-fold hollow H-site of structure II defined in Figure 1. The total density of state (DOS) is given in (e) with Fermi energy Ef labeled. The k-directions are the same as in the legend to Figure 5.

contour plot is taken for the (001) plane formed by the three O atoms and that the metal atom is located above this plane. The electron density around the Co atom shown in Figure 4c and around Ni in Figure 4e thus appears much smaller than what it really is. It is seen that the electron density around the O atoms is enhanced, due to the charge transfer from the electron-rich transition metal atoms to the electron-deficient O atoms. This is more evident from Figure 4d for Co and Figure 4f for Ni, where the net-spin density of the atoms is displayed. The net spin density of the metal atom is apparently spilled over to the adjacent O atoms as they all gain an appreciable amount of net spin density upon the metal adsorption. Table 4 summarizes the calculated values of net electron spin for both metals. The proximal O atoms are clearly seen to be net spin acceptors, as are to a smaller extent the next-nearest Si atoms. Thus spin delocalization is part and parcel of the transition-metal sorption process, which adds to the interaction

energy the metal 3d-O2p component, while the main source of the adsorbate bonding is the “spinless” metal 4s-Si 3s/O 2sp interaction. 3.4. Band Structure Analysis. The adsorption mechanism can be readily understood from the electronic structure analysis using the calculated density of states (DOS) and the band structure. In particular, detailed analysis can be carried out by breaking down the contributions to the band structure from specific orbitals. To facilitate the presentation, we use circles with their radii proportional to the contribution from a particular atomic orbital component at each k-point to describe the band structure. The resultant “character” band structure plots thus yield information on both the orbital energy dispersion in the k-space and the partial densities of state in the energy space. Figure 5a shows the calculated band structure of the SiO2 structure II. The contributions of the atomic orbitals of O and Si to the valence band are displayed in Figure 5b-d. The total

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Figure 7. Spin-down portion of “band character” plots with contributions of (a) Co 3d; (b) Co 4s; (c) Si 3s; and (d) O-2p with Co atom adsorbed on 3-fold hollow H-site of structure II defined in Figure 1. The total density of state (DOS) is given in (e) with Fermi energy Ef labeled. The k-directions are the same as in the legend to Figure 5.

Figure 8. Spin-up portion of “band character” plots with contributions of (a) Ni 3d; (b) Ni 4s; (c) Si 3s; and (d) O 2p with Ni atom adsorbed on 3-fold hollow H-site of structure II defined in Figure 1. The total density of state (DOS) is given in (e) with Fermi energy Ef labeled. The k-directions are the same as in the legend to Figure 5.

density of states (DOS) is also displayed in Figure 5e. The top of the valence band (TVB) contains mainly the O 2p orbitals. The lower portion of the valence band is composed of the Si 3sp and O 2sp bands. On the other hand, the bottom of the conduction band (BCB), about 3 eV above the TVB, is primarily composed of the antibonding Si 3s and to a lesser extent O 2sp orbitals. These features are in good agreement with the known band structure of bulk SiO2.25 The band structure changes substantially upon the Co and Ni adsorption. Figures 6 and 7 display the calculated band structure with orbital character plots of the spin-up and spindown BCOs for Co at the hollow site H of structure II together with the DOS plots and the position of the calculated Fermi level marked as EF. Figures 8 and 9 show those for the Ni atom. Several features of the band structure and DOS plots reinforce and refine the physical picture that has emerged already from the analysis near the Γ-point. New occupied levels originating

from the metal 4s and 3d orbitals appear near the bottom of the formerly empty conduction band of the silica. The 3d bands are flat throughout the Brillouin zone and their energies are very close to those at the Γ-point. On the other hand, the stabilized “4s” orbital shows a strong dispersion, consistent with its significant participation in surface bonding. As one proceeds from the Γ-point in the M (100) or the K (110) direction, the energy of the “4s” orbital increases till it reaches that of the metal d-levels. The “4s” aVerage energy is below that of the 3d levels in contrast with the free atom in which the spin-up 4s level is slightly higher than 3d and spin-down 4s level is nearly equal to spin-down 3d level (Figure 3). The band structure and DOS plots also confirm why Ni is bonded more weakly than Co: The position of the “4s” level in the adsorbate is less separated from the 3d flat band. The main reason for this difference lies in the fact that the atomic Ni 4s level does not match the empty orbitals at the BCB of

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Figure 9. Spin-down portion of “band character” plots with contributions of (a) Co 3d; (b) Co 4s; (c) Si 3s; and (d) O 2p with Ni atom adsorbed on 3-fold hollow H-site of structure II defined in Figure 1. The total density of state (DOS) is given in (e) with Fermi energy Ef labeled. The k-directions are the same as in the legend to Figure 5.

the silica as well as that of Co, giving rise to smaller separation of the bonding and antibonding Ni 4s-Si 3s than that in similar orbitals of adsorbed cobalt. 4. Summary The calculated adsorption energies of Co and Ni atoms on siloxane surface of silica have been found to be comparable for the two metals but to vary on different sites. The strongest adsorption site is a 3-fold hollow ensemble of oxygen atoms with no silicon present along inward surface normal underneath, but with 6 silicon neighbors in the “line-of-sight” at 0.33 nm. The adsorption process is dominated by the interaction of the occupied metal 4s electron levels with empty Si 3s orbitals of the silica, with a smaller contribution from O 2sp, at the bottom of its conduction band. The sorption energies of ∼-1.9 to -1.76 eV are sufficient to sustain atomically dispersed metal at low coverages. In all calculations the spin state of the metal is found to have a subtle role in the adsorption process. The net spin due to partial occupancy of the 3d orbitals of the metal is to a small extent delocalized to the proximal oxygens, but the concomitant energy effects are significantly smaller than those of the metal 4s-Si 3s/O 2sp interaction. The orbital density of states and energy band structure place the partially filled Co 3d and Ni 3d orbitals well above the O 2p valence band, making the present theoretical prediction amenable to valence band XPS and UPS detection, quantitative analysis, and validation of the theory. It remains to be noted that the single atom adsorption energies 1.90 eV for Co and 1.76 eV for Ni on the H sites correspond to fairly strong interactions, in chemical units to 183 and 170 kJ/mol, respectively. For larger metal loadings an intriguing question arises whether such an adsorption energy will sustain a high dispersion of the metal for catalytic applications. Calculations of cluster nucleation on top of the single-atom adsorption site are expected to resolve this issue. Referring to experiment, Riva et al.12 investigated the Co-silica interaction by using XPS, TPR, TPD, XRD, and TEM and found no conclusive proof of any force between them due to the fact that Co tends to sinter on the silica surface. In the ionic precursor state, Co(II) was found to be readily reducible by H2, giving

rise to agglomerated metal particles. In view of these experimental findings, clustering can occur either on other than the relatively strong 3-fold hollow sorption sites or the clustering energy will overcome the sorption energy of 1.7-1.9 eV on the trigonal sites reported and modeled here. Calculations of the adsorption of metal atoms of the entire first-row transition series [to be reported elsewhere] reveal a regular periodic pattern with several elements bonded more strongly than Co and Ni. This opens up an opportunity to study bimetallic systems, in which an anchoring metal and an active metal make a cluster alloy of superior properties than each of the separate components. 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. (APCI). The 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. We also value the discussion with Professor W. B. Fowler of Lehigh University’s Physics Department regarding solid-state physics of silica materials. APPENDIX: Optimized Fractional Coordinates of Atoms within the Unit Cell of the SiO2 Surface: (a) Structure I, (b) Structure IIa (a) structure I

(b) structure II

atoms

X

Y

Z

atoms

X

Y

Z

Si1 Si2 O1 O2 O3 O4 O5 H1 H2

0.0000 0.3333 0.1667 0.6667 1.1667 0.0000 0.3333 0.0000 0.3333

0.0000 0.6667 0.3333 0.8333 0.6667 0.0000 0.6667 0.0000 0.6667

0.1687 0.1687 0.2080 0.2080 0.2080 0.0620 0.0620 0.0000 0.0000

Si1 Si2 O1 O2 O3 O4 O5 H1 H2

0.0000 0.3333 0.0000 0.3333 0.6667 0.0000 0.3333 0.0000 0.3333

0.0000 0.6667 0.3333 0.0000 0.6667 0.0000 0.6667 0.0000 0.6667

0.1711 0.1711 0.2072 0.2072 0.2072 0.0627 0.0627 0.0000 0.0000

a X and Y are fractions of the in-plane lattice parameters a ) b ) 0.53 nm, and Z the fractions of c ) 1.5 nm. Note that the angles ∠(a, b) ) 120° and ∠(b, c) ) ∠(c, a) ) 90°.

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