Platinum Group Metal Adsorption on Clean and Hydroxylated

J. Phys. Chem. C 2009, 113, 16747–16756

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Platinum Group Metal Adsorption on Clean and Hydroxylated Corundum Surfaces Ludovic G. V. Briquet,†,‡ C. Richard A. Catlow,*,‡ and Samuel A. French§ Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, RG4 9NH, U.K., Department of Chemistry, UniVersity College London, Gower Street, London, WC1E 6BT, U.K., and Johnson Matthey Catalysts, Belasis AVenue, Billingham, TS23 1LB, U.K. ReceiVed: May 6, 2009; ReVised Manuscript ReceiVed: July 16, 2009

The adsorption of Ni, Pd, and Pt on the clean and hydroxylated (0001) and (1-102) R-alumina has been investigated using first principles methods. On both clean surfaces, the metals have very similar adsorption mechanisms where the metal promotes a charge transfer from a surface oxygen to a surface aluminum. The adsorption mechanism on the hydroxylated surfaces is different due to the unavailability of surface aluminum. Strong stabilization of the metal by the hydroxyl groups is observed for Pt and Ni due to the rupture of a surface hydroxyl group and the formation of a hydride species on the metal. The stabilization of Pd is found to be much less marked than Pt and Ni due to the lower strength of the Pd-H bond. 1. Introduction Interest in supported metal catalysts has grown considerably over the past decades.1,2 It is now established that the interactions between a catalyst metal particle and its support are of great importance for many industrial chemical process.3 Indeed, the chemistry of these nanoclustered materials can be distinctly altered by their interaction with the support, which can also play a role in the catalytic cycle by, for example, providing secondary adsorption sites for key intermediates. Alumina is commonly used as a support for a wide variety of reactions in industry.4 Computational studies allow us to provide detailed insight into the nature of the metal/metal oxide support interactions. For example, we recently reported a study describing the adsorption of 1/3 of a monolayer (ML) of Ni, Pd, and Pt on the clean R-alumina (0001) surface (in a 1 ML coverage, one metal atom is adsorbed per surface oxygen leading to three metal atoms per unit cell).5 In this study it was shown that, although Pd and Pt have similar adsorption sites (i.e., on top of a surface oxygen), Ni adsorbs on a different site (i.e., on a hollow between three oxygen atoms). We also demonstrated, for all three metals, that the adsorbed metal atom promotes electron transfer from the surface oxygen to the nearby aluminum, implying a backbonding type of interaction. The next step to increase the complexity of our model, in order to be closer to a “real” surface and to observe the effect of the surface on the adsorption properties of the metal, is to consider other R-alumina clean and hydrated surfaces. Therefore, in this paper, we have investigated the interaction between the three metal atoms on the clean (1-102) surface and on hydrated (0001) and (1-102) surfaces. Although the interaction of transition metals with metal oxide surfaces has been extensively investigated, only a few papers deal with the interaction of transition metals with alumina surfaces.6–11 Using first principles methods, Lodziana et al. studied the adsorption of 1 ML of Pd and Cu on (0001) R-AL2O3 * To whom correspondence should be addressed. E-mail: c.r.a.catlow@ ucl.ac.uk. † Johnson Matthey Technology Centre. ‡ University College London. § Johnson Matthey Catalysts.

surfaces containing 10 or 15 hydroxyl groups per unit cell.6 They showed that the metal monolayer adsorbs on top of the hydroxyl layer and that the energy cost for a hydrogen atom to move on top of the metal monolayer is between 0.5 and 1.0 eV. Valero et al. later investigated the interaction of a Pd monoatom and Pd clusters with several γ-alumina surfaces.8–10 They showed that Pd is less stable and less mobile on the hydrated (100) than on the clean surface. Very recently, Xiao et al. used ab inito methods to investigate Pt, Pd, Au, and Ag on both clean and heavily hydrated (15 OH/nm2) (0001) surfaces.11 In agreement with Valero’s results, lower adsorption energies for all metals, i.e., weaker binding, have been calculated on the hydroxylated than on the clean surface. They also note that the migration of a hydrogen atom from a hydroxyl group to the metal is possible, but only for Pt. Hydrogen transfer from a surface hydroxyl group to a metal particle is known as reverse spillover. The experimental detection of atomic hydrogen on metal particles is difficult because of the rapid migration of hydrogen atoms.12 It has, however, been determined that hydrogen reverse-spillover is mainly related to the number of hydroxyl groups present on the support and to a lesser extent to the surface acidity.13 Experimentalists suspect spillover hydrogen to play an important role in some catalytic processes, such as isocyanic formation by reaction of NO, CO, and H2 over Pt catalysts.14 In the next section, we will present the computational details used in this study. We will then briefly describe how hydrated surfaces have been modeled. Next we will compare the adsorption of Ni, Pd, and Pt on the clean (0001) and (1-102) surfaces and we will present our results for metal adsorption on hydrated surfaces. Finally, we will discuss our results in the light of previous studies. 2. Computational Details All calculations have been performed with the plane-wavebased density functional theory (DFT) package CASTEP15 using the Perdew-Wang generalized gradient approximation (GGA/ PW91).16 The electron density is determined by iterative diagonalization of the Kohn-Sham Hamiltonian. The ionic cores are represented by Vanderbilt’s ultrasoft pseudo potentials;17 a nonlinear core correction is used for the Ni atom. The

10.1021/jp904217b CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009

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TABLE 1: Comparison of Bond Lengths (Å) and Adsorption Energies (Eads) (eV) for the Adsorption of Ni, Pd, and Pt on the Clean (0001) Surface Computed with a 700 eV and a 400 eV Cut-off Energy Ni M–O M–Al Eads

Pd

Pt

700 eV

400 eV

700 eV

400 eV

700 eV

400 eV

1.95 2.45 –1.76

1.95 2.45 –1.69

2.13 2.45 –1.47

2.13 2.46 –1.41

2.05 2.43 –1.99

2.05 2.42 –2.01

k-point mesh is 4 × 4 × 1. In our previous paper,5 we used a plane-wave cutoff of 700 eV. However, in order to speed up the calculations (hydrated surface unit cells can in some case be significantly larger than the clean (0001) surface unit cell), a lower plane-wave cutoff energy of 400 eV has been used for this study. Table 1 shows a comparison of adsorption energies and bond distances for Ni, Pd, and Pt on the clean (0001); no major difference can be seen. It should, however, be noted that there is a small energy difference of 0.07 eV observed for the adsorption energy of Ni; but as they are similar to the errors associated with the calculation of adsorption energies with DFT, we do not consider this to be significant. The self-consistent calculations have been performed with an energy threshold of 10-6eV/atom. Geometry optimization was performed using the BFGS algorithm,18 with an energy convergence criterion of 2 × 10-5eV/atom, a force convergence criterion of 0.05 eV/Å, and a displacement convergence criterion of 0.002 Å. The surfaces are built as a slab model, with a 10 Å wide vacuum gap between the two slabs. Both (0001) and (1-102) clean surface models are 1 × 1 unit cells. The (0001) surface model, containing 10 OH/nm2, is also built using a 1 × 1 unit cell (1 H2O/unit cell), while the (0001) surface model containing 5 OH/nm2 is built as a 2 × 1 supercell (0.5 H2O/unit cell). Both (1-102) surface models with 16 OH/nm2 (2 H2O/unit cell) and 8 OH/nm2 (1 H2O/unit cell) coverages are built using a 1 × 1 unit cell. Depending on the aim of the calculation, two kinds of slab model have been built. When computing a precise surface energy, the slab is symmetric with the central layer kept frozen. Using this methodology, the (0001) surfaces are composed of five Al-OOO-Al layers, while the (1-102) surfaces are composed of five OO-AlAl-OO-AlAl-OO layers. In order to investigate the metal adsorption properties, the size of the slab can be reduced and only the bottom layer is kept frozen to mimic the bulk. No dipole correction was used to correct the possible dipole induced by the asymmetry of the slab. This factor might influence the charge transfer analysis; however, tests on the clean and hydrated surfaces showed no major difference in their electronic properties when using a symmetric or an asymmetric slab. The (0001) surfaces are composed of four Al-OOO-Al layers while the (1-102) surfaces are composed of three OO-AlAl-OO-AlAl-OO layers. For each type of surface, all the possible metal adsorption sites have been screened. For the hydrated surfaces, both configurations with and without metal-hydrogen bonds in the system have been tested. All geometry optimizations have been initially performed without any spin polarization. Then, for the most stable configuration, a spin-polarized geometry optimization has been calculated, so we are sure we have the lowest energy for the system. Significant changes were only found in the adsorption energies of the Ni systems. To calculate binding energies of the metal on the surface, we need the energy of a single metal atom, which is calculated by placing an isolated atom at the center of a 10 Å wide cubic box. As the energy thus calculated is very important for the

subsequent calculation of the binding energy, the electronic configuration has been checked so that it corresponds to the experimental gas-phase electronic configuration for the metal atoms, i.e., 3d8 4s2 for Ni, 4d10 for Pd, and 5d9 6s1 for Pt. The binding energy is then computed as the difference between the energy of the single atom plus that of the surface and the energy of the total adsorbed configuration. It is worth noting that, in the CASTEP software package,15 partial densities of states (DOS), which allow us to represent the contribution of each band to a given atomic orbital, are based on the Mulliken population which involves a projection of the plane-wave basis set. The thermodynamic treatment developed by Scheffler and co-workers19,20 has been applied to compute the surface energy of hydrated surfaces at a finite temperature and water pressure. The general idea is to use the first-principles potential energy surface to calculate thermodynamic functions such as the Gibbs free energy. We consider the surface in contact with an atmosphere described by its temperature and the partial pressure of its components. The environment acts as a reservoir because it can give or take any amount of any component from or to the sample, without changing either the temperature or the pressure. The Gibbs energy can then be separated into the three contributions:

G(T, p, {Ni}) ) Gsolid(T, p, {Ni}) + Ggas(T, p, {Ni}) + ∆Gsurf(T, p, {Ni}) (1) where Gsolid is the contribution from the bulk of the solid phase, Ggas is the contribution from the gas phase, and ∆Gsurf is the contribution from the surface of the solid state. ∆Gsurf can be scaled linearly with the surface area, A, by considering an ideal single crystal in contact with a given environment. The surface free energy, γ, can then be introduced. For an alumina surface in contact with a humid environment, it takes the following form:

γ(T, p) )

1 (G(T, p, NAl, NO, NH) - NAl µAl(T, p) 2A NO µO(T, p) - NH µH(T, p))

(2)

As the chemical potential of these elements is related to the Gibbs free energy per formula unit, gi, of the three major species in the system and assuming a chemical equilibrium between the surface, the atmosphere, and the bulk oxide and the independence of the different gas phase reservoirs, eq 2 can be rewritten as

1 1 Gslab(T, p, NAl, NO, NH) - NAlgAl2O3(T, p) 2A 2 1 3 1 N µ (T, p) - NO - NAl - NH µO(T, p) (3) 2 H H2O 2 2

γ(T, p) )

[

(

]

)

Following Reuter et al.19 Gslab and gAl2O3 can safely be approximated by the DFT energy of the slab and bulk Al2O3, respectively. Because of the stoichiometry of all our models, the term containing the oxygen chemical potential is zero. The water chemical potential can be determined by

( )

µH2O(T, p) ) EHTotal + EHZPE + ∆µH2O(T, p0) + kBT ln 2O 2O

pH2O

p0 (4)

Platinum Group Metal Adsorption

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TABLE 2: Hydroxyl Bond Lengths (Å) for the Different Water Coverages Considered on the (0001) and (1-102) r-Alumina Surfaces 2

5 OH/nm (0001) 10 OH/nm2 (0001) 8 OH/nm2 (1-102) 16 OH/nm2 (1-102)

OHads

OHsurf

0.97 0.97 0.97 0.99

0.98 0.98 1.03 1.03

where EHTotal is the DFT energy of one water molecule in a 10 2O is the water zero-point energy Å wide cubic box, EHZPE 2O determined by DFT and ∆µH2O(T,p0) is the temperature dependence of the water chemical potential, available in thermodynamic tabulation.21 For a complete description of this approach we refer to refs 19, 20, 22, and 23. 3. Hydroxylation of Alumina Surfaces Many experiments have reported the presence of dissociated water at the alumina surfaces, suggesting strong interactions between water and aluminum oxide.24–26 These observations have been largely supported by extensive modeling studies.23,27–30 The purpose of this paper is therefore not to reinvestigate comprehensively the mechanisms of hydroxylation at the alumina surfaces; we instead decided to conduct an investigation of two moderate water coverages for both (0001) and (1-102) surfaces in order to determine the water coverage that should be taken into account in modeling metal adsorption. For the (0001) R-alumina surface, a 10 OH/nm2 (1 H2O/unit cell) and a 5 OH/nm2 (0.5 H2O/unit cell) were investigated while for the (1-102) surface we used an 8 OH/nm2 (1 H2O/unit cell) and a 16 OH/nm2 (2 H2O/unit cell) coverage. We observed the dissociation of water molecules on both surfaces. One OH group adsorbs on an available surface Al cation (Lewis acid) while the remaining proton forms another hydroxyl group by binding to a surface oxygen. Throughout this paper, we will denote the hydroxyl group adsorbed on the surface Al as OHads and the hydroxyl group formed by the remaining proton and a surface oxygen will be called OHsurf. On the (0001) surface, only one surface Al and three chemically equivalent surface oxygen ions per unit cell are available for water adsorption. Therefore, at 10 OH/nm2 coverage, the surface is saturated with hydroxyl groups and to increase further the hydroxyl coverage, we must remove Al3+ from the surface (possibly forming a Al(OH)3 molecule) and bind protons to all three surface oxygen ions, leading to a 15 OH/nm2 coverage. Metal adsorption on this heavily hydroxyated (0001) surface has been investigated by Xiao et al.11 The (1-102) surface possesses only two available pentacoordinated surface Al cations and two available tricoordinated surface oxygen anions per unit cell. With a 16 OH/nm2 coverage, the surface is completely saturated by hydroxyl groups. However, as for the (0001) surface, a higher hydroxyl coverage on a reconstructed (1-102) surface has been reported.23,25 This latter surface is also not considered further in this paper. As can be seen in Table 2, OHads and OHsurf are not equivalent. In all cases the hydroxyl bond of the OHads group is stronger than for the OHsurf. For the (0001) surface only a small difference of ∼0.01 Å in the bond length is observed; however, the difference is more noticeable for the (1-102) surface where the OHads bond length for the surface with 16 OH/nm2 is 0.99 Å and the OHsurf bond length is 1.03 Å. It is also worth noting that all hydroxyl bond lengths are longer on the heavily hydroxylated (1-102) surface than on the (0001) surfaces. This effect can be attributed to the formation of an

Figure 1. Representation of the H-bond network on the (1-102) R-alumina surface with a 16 OH/nm2 water coverage.

Figure 2. Evolution of the (0001) surface free energy with temperature for the clean (solid), 5 OH/nm2 (dotted), and 10 OH/nm2 (dashed) water coverage surfaces.

H-bond network at the surface between all the hydroxyl groups (Figure 1). On the low coverage (1-102) surface, H-bonds are only observed between OHsurf and OHads, breaking the H-bond network. The computed surface free energies of the clean (0001) and (1-102) surfaces are 1.67 and 1.71 J/m2, respectively. We have calculated surface energies for the hydrated (0001) and (1-102) surfaces over a temperature range from 298 to 1500 K and under a water partial pressure of 3.2 kPa, which corresponds to an almost saturated atmosphere at ambient temperature (90.4% of relative humidity at 300 K). Figures 2 and 3 show the temperature dependence of the surface free energy of (0001) and (1-102) surfaces, respectively. At low temperatures, both surfaces are fully hydroxylated. Increasing the temperature results in dehydroxylation of the surface, and in a small window from 947 to 990 K, the hydroxyl coverage of the (0001) surface is reduced to 5 OH/nm2. Over 990 K, the surface is completely dehydrated. For the (1-102) surface, there is no transition to the medium hydroxyl coverage: at 969 K the surface dehydrates fully, the 8 OH/nm2 hydroxyl coverage not being the most stable surface configuration for the set partial pressure. The absence of the low hydroxyl coverage (1-102) surface is due to the H-bond network stabilizing the surface at high hydroxyl

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Figure 3. Evolution of the (1-102) surface free energy with temperature for the clean (solid), 8 OH/nm2 (dotted), and 16 OH/nm2 (dashed) water coverage surfaces.

TABLE 3: Bond Lengths (Å) and Adsorption Energies (eV) for Ni, Pd, and Pt Adsorption on (0001) and (1-102) r-Alumina Surfaces (0001)

M–O

(1–102)

M–Al Eads M–O M–Al Eads

Ni

Pd

Pt

1.95 2.11 2.51 2.45 –1.69 1.85 2.67 –1.70

2.13

2.05

2.46 –1.41 2.14 2.60 –1.58

2.42 –2.01 2.03 2.57 –2.26

coverage. Once the temperature is high enough to break the H-bond network, all the water molecules desorb from the surface. The vapor partial pressure has limited effect on the hydrated surface free energy. If we lower the pressure to 0.001 kPa, which

Figure 4. Top and side view of the electron density difference for Pt adsorbed on the clean (1-102) R-alumina surface. Blue and yellow represent electron density corresponding to 0.05 and -0.05 eV/Å3, respectively.

corresponds to a relative humidity of 0.03%, the transition temperature between the different water coverages reduces by only 12%. The 10 OH/nm2 (0001) to 5 OH/nm2 (0001) surface transition appears at 833 K, the 5 OH/nm2 (0001) to clean (0001) surface transition, at 990 K, and the 16 OH/nm2 (1-102) to the clean (1-102) surface, at 969 K. While the thermodynamic model presented certainly includes a number of significant approximations, including the limitations to the number of surface configurations that can be investigated, it shows the potential of these types of calculations to further our understanding of the surface relevant to the working conditions of materials used for catalysis. The temperature ranges presented in this section should not be considered as definite values, and further, more accurate calculations would be required to understand the associated errors. Nevertheless, this analysis leads to some very interesting conclusions. In particular, as it is unlikely that they will be observed experimentally, we are now able to omit the case of low hydroxyl coverage from our investigation of metal adsorption on hydrated surfaces. 4. Metal Adsorption on Alumina Surfaces 4.1. Clean Surfaces. As reported in our previous paper,5 the adsorbed metal promotes charge transfer between the surface oxygen and aluminum on the clean (0001) R-alumina surface. The strongest interaction for both Pt and Pd was found for adsorption on a surface oxygen, in close interaction with both oxygen and aluminum neighbors. The site-projected local density of states (LDOS) analysis showed clear bond formation with these two ions. Ni behaves slightly differently as, because of its smaller atomic radius, it sits in a hollow site between three oxygen ions and close to the surface aluminum. LDOS analysis shows bond formation between Ni and all three oxygen ions and between Ni and the surface aluminum. For all metals, the charge density difference plots reveal a depletion of electron density from the surface oxygen and from the metal d orbital and an increase in the electron density in the space between the metal and the aluminum, clearly supporting the charge transfer hypothesis. The adsorption of Ni, Pd, and Pt on the clean (1-102) surface follows a similar pattern. All three metal atoms are more strongly bound to a site between a surface aluminum and surface oxygen, with very similar bond lengths to those predicted on the (0001) surface (Table 3). Unlike the (0001) surface, no difference in the adsorption site is observed between Ni and the other metals. As the results in Table 3 show, the adsorption energies of Pd

Figure 5. Site-projected LDOS (arbitrary units) versus energy (eV) for the clean (1-102) R-alumina surface.



Figure 6. Site-projected LDOS (arbitrary units) versus energy (eV) for the Pt adsorbed on the clean (1-102) R-alumina surface. Al1 and Al2 are labeled following their distances from Pt.

and Pt are slightly lower on (1-102) compared with the (0001) surface, while for Ni, the energy difference is insignificant. All three metals have similar patterns in the charge density difference plots for the two surfaces, which are shown in Figure 4. There is a decrease in the electron density on both the metal d orbital and the surface oxygen, while the density increases in the space between the metal and the aluminum ion. We note that, for the (1-102) surface, the increase of the electron density in the space between the metal and the aluminum is observed in two directions, toward the two under-coordinated surface aluminum atoms in the unit cell. These charge density difference plots indicate a charge transfer mechanism promoted by the metal, which is very similar to that described for the clean (0001) surface. Figure 5 presents the LDOS of the clean (1-102) surface and Figure 6, the LDOS when Pt is adsorbed on the surface. For both surface aluminums, there is a shift of the sp band downward and the creation of two extra sp bands (one bonding and one antibonding orbital) overlapping with the metal d orbital. Unsurprisingly, the intensity of these two extra bands for the aluminum the furthest from the metal is much lower, showing a much weaker interaction of the metal with this ion. There is also a shift of the main p band of the oxygen toward lower energies and the creation of two new bands, overlapping with the metal d orbitals, revealing bond formation between Pt and O. The LDOS for the Pd and the Ni systems are very similar to the Pt system and are therefore not presented in this paper.

Figure 7. Adsorption sites and electron density difference for Pt adsorbed on the hydroxylated R-alumina surfaces. Blue and yellow represent electron density corresponding to 0.05 and -0.05 eV/Å3, respectively.

4.2. Hydroxylated Surfaces. 4.2.1. Pt. When the surface is hydroxylated, Lewis acid sites are no longer available to accept more electron density and to participate in the charge transfer mechanism, which occurs on the clean surfaces. As shown in Figure 7, Pt adsorbs at the same site on both

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TABLE 4: Bond Lengths (Å) and Adsorption Energies (eV) for Pt Adsorption on Hydroxylated r-Alumina Surfacesa

Pt–O1 Pt–O2 Pt–O3 Pt–H Eads a

5 OH/nm2 (0001)

10 OH/nm2 (0001)

16 OH/nm2 (1-102)

2.13 2.16 2.15 1.59 –3.07

2.13 2.19 2.13 1.58 –3.03

2.12 2.21 2.09 1.57 –2.93

Oxygen subscripts refer to configurations in Figure 7.

hydroxylated (0001) surfaces: in close contact with two surface oxygen ions and with the OHads group. On the hydroxylated (1-102) surface Pt is located between two OHads and in close contact with one surface oxygen ion. For all three surfaces, one OHsurf bond is broken and the proton migrates to the top of the metal atom. This migration of a hydrogen atom from a surface hydroxyl group to the metal has also been proposed by Xiao et al.,11 who calculated that proton transfer is favored by 0.30 eV. At the hydroxyl coverage investigated in this paper, the migration is also exothermic and barrierless: all configurations relaxed to one where the proton is bonded to Pt. Table 4 lists the bond lengths of the metal to the surface atoms and the related adsorption energies. The adsorption energies for Pt on hydroxylated surfaces are higher than those on clean surfaces: while for the clean (0001) and (1-102) surfaces, Pt adsorption energies are -1.99 and -2.26 eV, respectively, when the surface is wet, the adsorption

energies increase to -3.07 eV for the low-hydroxyl-coverage (0001) surface. The exothermicity of the proton migration suggests that the stabilization of Pt on the wet surface is mainly due to the formation of the bond with the hydrogen atom. Charge density difference plots for Pt adsorption on hydroxylated surfaces are presented in Figure 7. As for the clean surfaces, we observe a depletion of the electron density on the nearby oxygen atoms and the Pt d orbital. However, instead of transferring the density toward an aluminum ion, Pt transfers the density to the hydrogen atom, creating a hydride species. The driving force for the proton transfer is to coordinate Pt fully. The Mulliken population analysis available within the CASTEP code unfortunately gives little useful information on the charge distribution within the Pt-H bond given the difference in atomic radius of the two atoms. However, the electron density plots provide clear evidence for metal to hydrogen charge transfer. LDOS for the Pt/hydroxylated surfaces have been computed. Figure 8 presents the site-projected LDOS of both types of hydroxyl groups on the 10 OH/nm2 coverage (0001) surface. (The LDOS of the two other hydroxylated surfaces are very similar and are, therefore, not presented). Figure 9 shows the site-projected LDOS of the Pt/high-water-coverage (0001) system. On all surfaces, the changes to the LDOS following the adsorption of Pt are similar, suggesting similar interactions. The Pt main band is much broader than that observed for the clean surface: for the 10 OH/nm2 hydroxyl coverage (0001) surface, the d band stretches from about -8 to 1 eV, showing

Figure 8. Site-projected LDOS (arbitrary units) versus energy (eV) for the 10 OH/nm2 coverage (0001) R-alumina surface.



Figure 9. Site-projected LDOS (arbitrary units) versus energy (eV) for Pt adsorbed on the 10 OH/nm2 coverage (0001) R-alumina surface. Oxygen subscripts refer to configurations in Figure 7.

a strong interaction of the metal with another species and has a sharp end at higher energies, just under the Fermi level. This main band consists of 5d orbitals with a little participation of the 6s orbital. No interaction of the metal with surface aluminum ions can be seen. A splitting of the neighboring oxygen sp band in both bonding and antibonding orbitals is observed, while the main sp band is shifted toward lower energies. The bonding orbital is located just below the Fermi level, overlapping the sharp d Pt band. The hydride hydrogen presents a very broad s band, completely overlapping the Pt d orbital. Overlapping between unoccupied bands above the Fermi level is also

observed. This large overlap of the hydrogen and platinum bands confirms the strong interaction between the two atoms and the resulting strong stabilization of the adsorbed metal. 4.2.2. Ni. As observed for Pt, Ni only has interactions with oxygen and hydrogen atoms. The adsorption sites on the different hydroxylated surfaces are very similar to Pt. Due to its smaller radius, on the (0001) surface, Ni is in close contact with three surface oxygens instead of two for Pt and is also in close contact with one OHads group. As for Pt, one OHsurf bond is broken and the resulting proton migrates on top of Ni. On the hydroxylated (1-102) surface, Ni is between two OHads

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Figure 10. Adsorption sites and electron density difference for Ni adsorbed on the hydroxylated R-alumina surfaces. Blue and yellow represent electron density corresponding to 0.05 and -0.05 eV/Å3, respectively.

Figure 11. Adsorption sites and electron density difference for Pd adsorbed on the hydroxylated R-alumina surfaces. Blue and yellow represent electron density corresponding to 0.05 and -0.05 eV/Å3, respectively.

TABLE 5: Bond Lengths (Å) and Adsorption Energies (eV) for Ni Adsorption Hydroxylated r-Alumina Surfacesa

TABLE 6: Bond Lengths (Å) and Adsorption Energies (eV) for Pd Adsorption Hydroxylated r-Alumina Surfacesa

Ni–O1 Ni–O2 Ni–O3 Ni–O4 Ni–H Eads a

5 OH/nm2 (0001)

10 OH/nm2 (0001)

16 OH/nm2 (1-102)

1.92 2.09 2.11 1.99 1.45 –3.34

2.00 2.39 1.96 1.95 1.49 –3.13

1.94 2.00 1.90 – 1.45 –2.79

Pd–O1 Pd–O2 Pd–O3 Pd–H Eads a

5 OH/nm2 (0001)

10 OH/nm2 (0001)

16 OH/nm2 (1-102)

2.36 2.28 2.45 – –1.76

2.21 2.13 2.12 1.57 –1.83

2.11 2.16 2.09 1.56 –1.59



groups and in close contact with a surface oxygen. Again, one proton migrates from a OHsurf group to bind to Ni. Figure 10 shows the stable Ni adsorption sites on all hydroxylated surfaces and Table 5 presents bond lengths and adsorption energies for Ni on the hydroxylated surfaces. Ni adsorption energies on hydroxylated surface are also much higher than those calculated for clean surfaces: on the clean (0001) and (1-102) surface, Ni adsorption energies are -1.96 and -1.70 eV, respectively, while on the hydroxylated surfaces, the adsorption strengthens with binding energies of -3.34 eV for the 5OH/nm2 hydroxyl coverage on the (0001) surface. As for Pt, all starting configurations relaxed to a configuration having one metal-hydrogen bond, indicating a barrierless transfer of the hydrogen from the surface to the metal atom. The charge density difference plots presented in Figure 10 show the same kind of interactions as Pt: electron depletion is observed on the nearby oxygen and on the nickel d orbital, and hydrogen plays the role of a Lewis acid by accepting extra electron density from Ni. The LDOS plots for the Ni/OH-(0001) surface systems are very similar to those observed for Pt and are therefore only briefly described. The Ni d band is much broader than on clean surfaces and overlaps with the s band of the hydride hydrogen, showing very strong interactions. The oxygen p band is shifted toward lower energies, and two extra bands have been created below and above the Fermi level overlapping with the nickel

3d orbital; they correspond to bonding and antibonding orbitals. No interaction can be observed with aluminum. 4.2.3. Pd. In the case of Pd adsorption on hydroxylated surfaces, the adsorption mechanism depends on the surface and on the hydroxyl coverage. For the hydroxylated (1-102) surface and the 10 OH/nm2 (0001) surface, the adsorption mechanism is very similar to that described for Ni and Pt: Pd is in close contact with a surface oxygen and with OHads groups, and one proton migrates from a OHsurf group to the metal. On the lowwater-coverage (0001) surface, Pd is located between two adjacent OHads groups and in close interaction with one surface oxygen. Unlike the other systems, the OHsurf bond is not broken and Pd does not create bonds with hydrogen. Figure 11 presents the adsorption sites and Table 6 gives both bond lengths and adsorption energies. It is worth noting that for the low-watercoverage (0001) surface, Pd-O bond lengths are noticeably longer than on other systems. The stabilization of Pd on the hydroxylated surfaces over clean surfaces is much less marked than for Ni or Pt. On clean (0001) and (1-102) surfaces, the adsorption energies are -1.41 and -1.58 eV, while hydroxylation increases the adsorption energies to only -1.83 eV. This weaker stabilization together with the absence of proton migration for the lowest hydroxyl coverage system underlines the small affinity of Pd for hydrogen compared to the other two metals. Unlike Ni and Pt, not all starting configurations relaxed to systems where a hydrogen atom has been transferred to the metal. The energy difference



Figure 12. Site-projected LDOS (arbitrary units) versus energy (eV) for Pd adsorbed on the 5 OH/nm2 coverage (0001) R-alumina surface. Oxygen subscripts refer to configurations in Figure 11.

between the hydrogen-transferred and the non-hydrogentransferred systems is in the range of a few tenths of an eV for all three hydrated systems. Charge density differences for the three Pd systems are presented in Figure 11. For the two systems where proton migration is observed, the same kind of electronic rearrangement is observed. Pd promotes charge transfer from the nearby oxygen to a hydride species, which plays the role of an electron acceptor. For the low-water-coverage (0001) system, there is a complex reorganization of the electron density around the metal. There is little depletion of the electron density on the nearby oxygen due to the greater separation between the species. For the hydroxylated (1-102) surface and the high-watercoverage (0001) surface, we observe similar behavior of the LDOS to Pt and Ni: Pd has a broad d band overlapping with the s band of the hydride. The sharp edge of the d band, just below the Fermi level, overlaps with the new p band on the neighboring oxygen ions; antibonding orbitals are created above the Fermi level. The Pd s band is not occupied and the d band is almost full. For the low-water-coverage (0001) surface (Figure 12), the Pd d band is sharp and located just below the Fermi level, as observed on the clean surfaces. Pd only interacts with oxygen. Both the surface oxygen and the oxygen involved in the OHads bond show a shift of their main p band toward lower energies and small extra bands above and below the Fermi level. These bands, overlapping with the Pd d orbitals, are the bonding and antibonding orbitals describing the Pd-O bonds, respectively.

5. Discussion Platinum group metals, when adsorbed as monoatoms on an alumina surface, need to bond with their environment in order to complete their coordination shell and to increase their stability. On the clean surfaces, there are only two types of atoms available at the surface: electron-rich oxygen, playing the role of Lewis basic site, and electron-poor aluminum, playing the role of a Lewis acid site. As described in our earlier paper, the metal, when adsorbed on such a surface, promotes charge transfer between the two basic and acid sites.5 When the surface is hydroxylated, acidic aluminum sites are no longer available, which can lead to either a destabilization of the metal as observed by Xiao et al. for Pt and Pd on heavily hydroxylated R-alumina (0001) surfaces11 or to the rupture of a hydroxyl group and the transfer of the remaining proton to the metal in order to complete its coordination. Depending on the strength of the hydroxyl group and the affinity between the metal and the hydrogen atom, one scenario or the other will occur. We have shown that the OHsurf bond is slightly weaker than OHads (due to the higher coordination number of the oxygen in the OHsurf bond), which is the bond that breaks to release the proton. It is also evident from our calculations and from earlier results11 that Pd and Pt do not have the same affinity for hydrogen. Xiao predicted the migration of the proton from a surface hydroxyl group to Pd on the heavily hydroxylated (0001) surface to be endothermic.11 Similar conclusions can be drawn

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from the results presented by Valero et al., concerning the adsorption of Pd on hydrated γ-alumina (100) surfaces, as this study did not find evidence for the formation of a hydride species on the Pd atom.8–10 Vayssilov et al., investigating reverse spillover for Group 8-11 metals adsorbed on a zeolite support, concluded that there was an energetically favorable reverse spillover for all the metals investigated except for silver and gold. Although still energetically favorable, hydrogen reverse spillover on Pd was predicted to be significantly less stable than other metals such as Pt or Ni.31 We showed that the migration of the proton to the Pd atom is only possible for selected surfaces and selected hydroxylation coverages. The formation of a Pd-H bond is much less favorable than the formation of a Pt-H or a Ni-H bond and does not always bring enough stabilization to the system to make the hydroxyl bond breaking energetically favorable. It is worth noting that for the low hydroxyl coverage (0001) surface, the system with the proton on the Pd atom is only a few tenths of an eV higher in energy, showing a close balance between the bond strengths at the surface. In addition, our calculations show that, when bonded to the metal, the electron density of the hydrogen increases, suggesting the creation of an hydride specie on the metal. To the best of our knowledge no current experimental data can support this observation. The main reason is the difficulty of experimentally detecting and characterizing atomic hydrogen adsorbed on finely dispersed metal particles. 6. Summary and Conclusion The adsorption mechanisms of Ni, Pd, and Pt on the clean and hydroxylated (0001) and (1-102) R-alumina surfaces have been investigated and compared. Three types of hydroxylated surfaces have been considered: low-water-coverage (0001), highwater-coverage (0001), and high-water-coverage (1-102), as according to our thermodynamic model, the low-water-coverage (1-102) is not stable. On both clean surfaces, the metals have the same adsorption mechanisms as previously described.5 The metal promotes charge transfer from the electron-rich oxygen to the electronpoor aluminum through its d orbitals. Due to the lack of available aluminum Lewis acid sites on the hydroxylated surfaces, the adsorption mechanism is different from that on the clean surfaces. For both Pt and Ni, we observe strong stabilization of the metal by surface hydroxylation. This stabilization has mainly been attributed to the migration of a proton from a surface hydroxyl group to the metal, with the proton accepting electron density. Large overlap of the LDOS bands of the metal and the hydride is observed. For Pd, the adsorption mechanism depends on the surface and the water coverage. Proton migration and hydride formation have been observed for the hydroxylated (1-102) surface and for the high water coverage (0001) surface. On the low-water-coverage (0001) surface, no proton transfer has been observed and Pd interacts only with the surface hydroxyl groups. The stabilization of Pd by surface hydroxylation is much lower than for Pt and Ni, suggesting a weaker affinity for Pd to form metal-H bonds. The changes in the nature and strength of metal adsorption on wet surfaces have clear implications for the catalytic application of these systems.

Briquet et al. With the increase in computer power and the development of modeling methods, we are able to consider more realistic problems, such as metal adsorption on hydrated surface. Our calculations have shown new insights into how the wetting conditions at metal oxide surfaces may influence the metal atom adsorption properties. The present study underlines the importance of accurately modeling the support surface at the relevant environmental conditions, as this will have a large influence on processes such as metal adsorption or catalytic reactions occurring on supported metals. Acknowledgment. L.G.V.B. thanks Johnson Matthey and the Marie Curie Early Stage Training for financial support via the Framework 6 program. References and Notes (1) Chorkendorff, I.; Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics; Willey-VCH: Weinheim, 2003. (2) van Santen, R. A.; Neurock, M. Molecular Heterogeneous Catalysis; Wiley-VCH: Weinheim, 2006. (3) Henry, C. R. Surf. Sci. Rep. 1998, 31, 235. (4) Misra, C. Industrial Alumina Chemicals; American Chemical Society: Washington, DC, 1986. (5) Briquet, L. G. V.; Catlow, C. R. A.; French, S. A. J. Phys. Chem. C 2008, 112, 18948. (6) Lodziana, Z.; Norskov, J. K. J. Chem. Phys. 2001, 115, 11261. (7) Jennison, D. R.; Mattsson, T. R. Surf. Sci. 2003, 544, L689. (8) Valero, M. C.; Digne, M.; Sautet, P.; Raybaud, P. Oil Gas Sci. Technol. 2006, 61, 535. (9) Valero, M. C.; Raybaud, P.; Sautet, P. J. Phys. Chem. B 2006, 110, 1759. (10) Valero, M. C.; Raybaud, P.; Sautet, P. Phys. ReV. B 2007, 75, 045427. (11) Xiao, L.; Schneider, W. F. Surf. Sci. 2008, 602, 3445. (12) Martin, D.; Duprez, D. Stud. Surf. Sci. Catal. 1993, 77, 201. (13) Miller, J. T.; Meyers, B. L.; Modica, F. S.; Lane, G. S.; Vaarkamp, M.; Koningberger, D. C. J. Catal. 1993, 143, 395. (14) Dumpelmann, R.; Cant, N. W.; Trimm, D. L. J. Catal. 1996, 162, 96. (15) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.; Payne, M. C. Z. Kristallogr. 2005, 220, 567. (16) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (17) Vanderbilt, D. Phys. ReV. B 1990, 41, 7892. (18) Pfrommer, B. G.; Cote, M.; Louie, S. G.; Cohen, M. L. J. Comput. Phys. 1997, 131, 233. (19) Reuter, K.; Scheffler, M. Phys. ReV. B 2002, 65, 035406. (20) Reuter, K.; Scheffler, M. Phys. ReV. Lett. 2003, 90, 046103. (21) Stull, D. R.; Prophet, H. U.S. National Bureau of Standards, Washington, DC, 1971. (22) Wang, X. G.; Chaka, A.; Scheffler, M. Phys. ReV. Lett. 2000, 84, 3650. (23) Marmier, A.; Parker, S. C. Phys. ReV. B 2004, 69, 115409. (24) Schildbach, M. A.; Hamza, A. V. Surf. Sci. 1993, 282, 306. (25) Trainor, T. P.; Eng, P. J.; Brown, G. E.; Robinson, I. K.; De Santis, M. Surf. Sci. 2002, 496, 238. (26) Fu, Q.; Wagner, T.; Ruhle, M. Surf. Sci. 2006, 600, 4870. (27) Nygren, M. A.; Gay, D. H.; Catlow, C. R. A. Surf. Sci. 1997, 380, 113. (28) Shapovalov, V.; Truong, T. N. J. Phys. Chem. B 2000, 104, 9859. (29) Lodziana, Z.; Norskov, J. K.; Stoltze, P. J. Chem. Phys. 2003, 118, 11179. (30) Ranea, V. A.; Schneider, W. F.; Carmichael, I. Surf. Sci. 2008, 602, 268. (31) Vayssilov, G. N.; Rosch, N. Phys. Chem. Chem. Phys. 2005, 7, 4019.

JP904217B

Platinum Group Metal Adsorption on Clean and Hydroxylated

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