J. Phys. Chem. C 2008, 112, 16491–16496
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First Row Transition Metal Atom Adsorption On-Top of F°s Defects of a MgO(100) Surface Se´bastien Fernandez, Alexis Markovits,* and Christian Minot UniVersite´ Pierre et Marie Curie-Paris6, CNRS, UMR 7616, LCT, Paris F-75005, France ReceiVed: March 12, 2008; ReVised Manuscript ReceiVed: July 28, 2008
We performed periodic density functional calculations to study the reactivity of F°s defects on MgO(100) toward the first row of transition metal atoms at low coverage. The F°s center interaction strength with the first row of transition metal atoms is explained by the filling of bonding and antibonding orbitals and by the respective positions of the metal and vacancy levels. The vacancy levels are those of a dummy atom with a diffuse s-type orbital and empty p-type orbitals. Under adsorption, most of the metal atoms maintain their atomic configuration. Exceptions concern Ti, Co, and Ni that have different spin states. Across the period, the adsorption energy exhibits two maxima. The largest metal adsorption energies are found for Ti and V on one hand and Co, Ni, and Cu on the other hand. For these atoms, the highest antibonding σ-orbital is empty and the occupancy of π-orbitals provides additional stability. Interactions of metal orbitals of appropriate symmetry with empty p-type vacancy orbitals and surface Mg2+ orbitals yield stabilization. 1. Introduction One of the main applications of metal-supported particles is heterogeneous catalysis. Indeed, the activity of the metal may be enhanced by the support. This effect is known as the strong metal support interaction (SMSI), discovered 30 years ago by Tauster.1 The interaction with the support could affect the catalysis in different ways. The support could facilitate the adsorption of molecules from the gas phase, allowing then the activation of the molecule; it could allow spillover or interact with the metallic particle via electronic or magnetic effects, thus modifying the electronic structure of the particle.2,3 In order to deeply understand the catalytic activity of supported metal particles, it is necessary to start by studying the interaction between the metal and the support at the atomic level. Thus, the interaction of metal particles with oxide surfaces has been the subject of many studies, both experimental4-12 and theoretical.13-17 Numerous applications of technological interest lie on the properties of these systems, among them sensors,18,19 information storage,20-23 or molecular devices.24 An efficient way to anchor the metal on the metal oxide surface is to adsorb on O defects.25 In this paper, we address the reactivity of a neutral anion vacancy (noted as F°s in the following) at the surface of MgO(100) toward the first row of transition metal atoms. This defect corresponds to the removal a neutral oxygen atom, two electrons remaining trapped in the vacancy. This defect is also known as “color center” because of the two trapped electrons which give rise to electronic transitions in the visible region of the spectrum, thus changing the color of samples at a sufficient defect concentration. There is experimental evidence about nucleation processes occurring on F°s centers.25 Understanding the interaction of a single atom with the surface is then the first step in order to comprehend the growth of metal particles on oxide surfaces26 and the enhanced reactivity of these particles toward chemical reactions. Several studies have already addressed the characteristics of such a defect27-42 as well as the interaction of some atoms with it.3,43-54 To our knowledge, this paper is the first one to address * E-mail:
[email protected], fax: +33 144 274 117.
the adsorption of the whole first transition metal atoms series. It is also the direct continuation of our previous study about the adsorption of the same atom series on the perfect surface.55 Previous studies concentrated on late transition atoms, starting with chromium48 and mainly on Ni and Cu groups.3,43-47,49-54 The recent discovery of the unexpected reactivity of gold also gives rise to abundant publications on this atom interacting with the F°s center.27,56-64 The first part is dedicated to computational details and the description of the models used. We then present results concerning the naked defective surface. We continue with the energetic and geometrical parameters of the adsorption of the metal atom series on the vacancy and finally we discuss the electronic analysis of theses systems at the light of the Frontier Orbital Theory.65,66 2. Models and Computational Details We carried out density functional theory (DFT) calculations using the VASP code.67-70 We used a plane-waves basis set with a kinetic energy cutoff of 396 eV. The core electrons were described by ultrasoft pseudopotentials.71 For the surface atoms, Mg and O, two and six electrons were explicitly treated in the valence shell, while the 3d and 4s electrons constituted the valence shell for the metal atoms. The 3p electrons were added to the valence shell for K, Sc, Ti, and V. Although there is still a controversy for the best choice of the exchange correlation functional72 in the framework of DFT adapted to the evaluation of adsorption energies, we performed spin-polarized calculations in the framework of generalized gradient approximation (GGA-PW91) which has already been successfully employed for similar systems.49,55,73,74 Our main purpose being the determination of a trend across the periodic table, we want to make possible the comparison with previous work using this method. We computed spin polarized isolated atom in a large parallelepiped-like box to get atomic references. We refer to GGA ground states (Table 1) although GGA may fail to predict some ground states accurately.55,75 The adsorption energies were evaluated according to the following formula: Eads ) E(TM) + E(MgO) - E(TM/MgO) where E(TM) is the spin polarized energy of the isolated atom
10.1021/jp802166y CCC: $40.75 2008 American Chemical Society Published on Web 10/01/2008
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TABLE 1: Adsorption Energies and Geometrical Parametersa atomic properties
geometrical parameters (Å)
Magnetic adsorption magnetic energy moment electronic Moment (eV) (µB) atom configuration (µB) K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
p6s1 s2 p6d1s2 p6d2s2 p6d4s1 p6d5s1 d5s2 d6s2 d7s2 d9s1 d10s1 d10s2
1 0 1 2 5 6 5 4 3 2 1 0
0.29 0.33 0.39 1.60 1.63 0.96 0.58 0.88 2.12 3.12 1.95 0.07
1 0 1/3 4 5 6 5 2/4 1 0 1 0
z
d(M-Mg)
3.69 3.50 2.58/2.53 2.27 2.04 2.31 2.60 1.97/2.09 1.51 1.29 1.75 3.27
4.29 4.13 3.40 /3.37 3.16 3.01 3.19 3.40 2.96/3.04 2.68 2.57 2.81 3.93
a The metal is always on top of the F°s center, and the distances are optimized. z is the height of the atom over the average plan of the surface atoms while d(M-Mg) is the distance between the metal atom and the vicinal Mg atom. For Sc and Fe, two distances are indicated, corresponding to the two degenerate spin states (respectively 1µB/3µB and 2µB/4µB). The electronic configuration is the atomic ground state calculated in the GGA approximation and used as reference for the calculation of the adsorption energies. The adsorption energies are calculated with respect to the bare slab and to the spin polarized isolated atom.
in the electronic configuration given in Table 1, E(MgO) is the energy of the bare MgO slab, and E(TM/MgO) is the spin polarized energy of the whole system. Positive adsorption energies account for exothermic processes. Total energies were evaluated within the tetrahedron method with Blo¨chl correction, Vosko-Wilk-Nusair interpolation was applied as well as monopole, dipole, and quadrupole corrections along the direction perpendicular to the slab. We modeled the MgO (100) surface using a three-layer slab as in our previous study dealing with the perfect surface.55 This is an accurate enough and widely used approach. Justifications originate both from the simple ionic rock-salt structure of MgO and from the slight relaxation of the surface76 even under adsorption or around a defect. We ran some tests using a sixlayer slab; the energy difference was less than 4 meV compared with the three-layer slab. The calculations being periodic in three dimensions, we imposed a vacuum of at least 7 Å between two consecutive slabs in order to eliminate any noticeable interaction with the periodic image along the z direction. The vacancy is modeled by removing a neutral oxygen atom. Considering adsorptions of isolated atoms (θ ) 1/8), we used a R45° 22 × 22 supercell, depicted in Figure 1. Hence, the distance between two adsorbed metal atoms is 8.42 Å, which is large enough to prevent any noticeable lateral interaction. We performed the full relaxation of the surface top layer; the second layer was only relaxed along the direction perpendicular to the slab. The relaxation of the atomic positions occurs until the subsequent energy steps are smaller than 0.001 eV. For geometry optimization, the Brillouin zone was sampled in a 5 × 5 × 1 Monkhorst-Pack set while the densities of states were evaluated with a 8 × 8 × 1 grid. We calculated the total, atom projected, and lm-decomposed densities of states. We also computed banddecomposed charge and spin densities. 3. Results and Discussion Description of the F°s Center. The density of states of the naked defective surface is displayed in Figure 2. The defect is
Figure 1. Sketch of the cell used to model the adsorption of isolated atoms (θ ) 1/8). We used an R45° 22 × 22 supercell. Oxygen atoms are in red, magnesium atoms are in green, and a metal atom (yellow) is on top of the F°s center. Color on line.
Figure 2. Density of states of the bare slab. The energy zero is set at the vacuum level. The vacancy level appears in the gap of MgO, just before the Fermi level.
lying about 2.5 eV above the valence band. A Bader analysis of the charge density of the band in the gap gives around 1.4 electrons in the vacancy, the missing 0.6 electrons being shared between the surrounding atoms. Looking at the charge density of the highest occupied crystalline orbital, presented in Figure 3, we can observe that the density is not completely in the surface plane at the vacancy site, but protrudes notably outward, even covering the surrounding magnesium surface atoms. The orbital describing the F°s center is then very diffuse, and the center may be assimilated to a virtual atom of large size, with two electrons in an s-type orbital of high energy level. Such a comparison is used in studies dealing with spectroscopy of the F°s center.77 Adsorption Energies and Geometrical Parameters of Metal Atoms from K to Zn on top of the F°s Center. The adsorption energies and geometrical parameters of the adsorbed metal atom are reported in Table 1. First of all, half of the atoms of the considered series have adsorption energy smaller than 1 eV, and the height above the surface is quite large except for the triad Co, Ni, and Cu. This justifies considering that the F°s
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Figure 3. Charge density (top) and projected charge density (bottom) of the HOMO of the F°s center. We show charge densities for the HOMO of the F°s center. On top, from left to right, we present on top and side views of the isosurface plotted for a value of the charge density of 3.5 au. The oxygen atoms are in green, the magnesium atoms are in blue, and the vacancy is represented by a red atom. The associated projected charge densities are shown on the bottom, top view at left and side view at right. A Bader analysis shows that the vacancy contains about 1.4 electrons. Color on line.
center and the metal atom do not merge in a single moiety (a metal anion), and we may therefore analyze them as two interacting ones. Indeed the idea of a vacancy may suggest that an adsorbate could “fall” into the hole trapping the electrons from the F°s center. Distances on the order of interatomic distances show that this is not the case. The reason for this is the low electron affinity for the metals. A high affinity should be required to form a negatively charged moiety resulting from the fusion of M and F°s. In terms of MO analysis, the metal adsorption should then originate from the interaction of the atomic orbitals of the virtual atom (the highest occupied crystalline orbital of the naked surface) and those from the metal. The evolution of the adsorption energies on top the F°s center is plotted in Figure 4. It is worth noticing that this shape is quite similar to the curve obtained on the perfect surface:55 it exhibits two maxima, the second being larger than the first one. As already pointed out by several studies, Co, Ni, and Cu have large adsorption energies. In a first approach, the F°s center behaves as a basic center as the O2- anion does. We considered also the adsorption of the metal atom above an O2- anion and a Mg2+ cation. As expected, in agreement with our previous study, the F°s center is not systematically the adsorption site of lowest energy. Indeed, some atoms such as Ca or Sc more strongly adsorb above that of the oxygen anion of a clean terrace while some others (Ti, Mn, Fe, and Zn) are blind to the adsorption site (O2- or F°s). We have verified that the adsorption on-top Mg2+ was not preferred. The metal atoms which are the
Figure 4. Adsorption energies of the metals on the defective MgO(100) surface. We present the adsorption energy, with respect to the bare slab and to the isolated atom, of the first transition metal atoms series, including K and Ca. The total number of unpaired electrons, NR - Nβ, is reported in parentheses; for Sc and Fe, we found two degenerate spin states.
best candidates for interacting with such an adsorption site are the most electropositive ones. Indeed, only electropositive metals can lose electrons for Mg2+. For K and Ca, we have found that
16494 J. Phys. Chem. C, Vol. 112, No. 42, 2008 the adsorption strength on a Mg2+ adjacent to an oxygen vacancy is larger by 0.07 and 0.05 eV for K and Ca, respectively. However, for Sc and Ti, M is not electropositive enough and moves from Mg2+ to F°s during the geometry optimization. As reported in Table 1, most of the atoms maintain their atomic spin. The case of Ni has already been treated elsewhere.75 Electronic Analysis. In the following, we consider the vacancy as a virtual atom, possessing an s-type orbital, noted as “vacancy orbital” hereafter. The virtual atom also possesses p-type orbitals that are empty and higher in energy. These levels have been shown to play a role in electronic transitions.77 The vacancy orbital is quite high in energy while the energy levels of the metal orbitals vary. Compared with a 2p(O) orbital, the vacancy orbital is more diffuse and does not point in the z direction. The F°s center appears more basic than O2- of the perfect surface by characterization with an electron pair of very high energy level. The metal then appears as an electron acceptor which necessitates that its electronic affinity is reasonably large.27,48 The metal-F°s center interaction can be decomposed into two terms: The first one is the interaction with the two trapped electrons. Dominantly, the nearly pure 4s(M) orbital interacts with the vacancy orbital. An attraction or a repulsion follows according to the number of electrons; a repulsion arises as soon as the antibonding vacancy-4s(M) orbital is filled by two electrons. The large distance z with respect to the surface results from the diffuseness of the vacancy orbital, maximizing the overlap at a large distance; we have indeed recognized the F°s center as a virtual atom of large size. The second term is the interaction with the four surrounding Mg2+ ions, stabilizing when the dxz and the dyz orbitals (π orbitals) are filled. Such an interaction was only seen for the clean surface55 in the case of δ orbitals. In the case of π orbitals, it was counterbalanced by the repulsion of M with the 2px and 2py orbitals of the oxygen site of adsorption. For the defective surface, the p-type orbitals of the virtual atom of the F°s center are empty and thus cooperate with the stabilization by the surrounding magnesium atoms. We provide a diagram (Figure 5) showing the different molecular orbitals. Basically, the interaction of a metal atom with the vacancy is due to the σ interaction of the 4s(M) orbital with the vacancy orbital. The dz2 orbital is also σ; however, in a first approach, it can be considered as poorly interacting due to a weak overlap with the vacancy orbital. The vacancy orbital and 4s(M) interaction gives rise to two σ orbitals, σ1 and σ2, on the left-hand side of the periodic table, σ1 and σ3, on the righthand side, the label changing due to the dz2 orbital. The two π are stabilized by the F°s center and by the surrounding Mg2+ ions. One delta orbital, δ1, that has the correct geometry, is stabilized by the surrounding Mg2+ions. The other one, δ2, that does not have the symmetry to interact with the Mg2+ions remains nonbonding. From K to Zn, the electropositivity of the metal atoms decreases and there is a lowering of the metal levels that pass below the vacancy orbital level for Ti. For K, the σ1 orbital corresponds essentially to the virtual atom (vacancy) while the σ2 orbital is localized on the metal. Considering Cu for instance, it is the opposite; the main amplitude of σ3 orbital is localized on the vacancy. When the σ orbital with large antibonding character is empty, the interaction is stabilizing. It is particularly
Fernandez et al.
Figure 5. Molecular orbitals diagram. This diagram reports a simplified picture of the interaction of a metal with the F°s center. The F°s center (right) has s-type orbital called l. The metal (left) has an s orbital (σ symmetry) and five d orbitals (σ, 2π, 2δ). The orbitals interact and give rise to a strongly bonding, σ1, a weakly bonding, σ2, and a strongly antibonding, σ3, orbital. In this figure, the two s orbitals are represented with the same energy. The dz2 orbital of the metal (σ symmetry) is stabilized by the pz-type orbital of the F°s center. The dxz, dyz (π) orbitals of the metal are stabilized by interacting with the surrounding Mg2+ and by the px and py orbitals of the F°s center. The dx2-y2 orbital leads to the δ1 orbital stabilized by the surrounding Mg2+. The last metal orbital dxy remains almost nonbonding and is labeled δ2.
large when the main amplitude of σ3 orbital is on the vacancy; then, the vacancy is depopulated. This is the case for Co, Ni, and Cu. We can follow the relative position of the energy levels of the metal with respect to the one of the vacancies by looking at the charge density of the σ orbitals. We display the projected density of states on the orbitals and the associated charge densities in the case of Ca, Sc, and Cu as an illustration (Figure 6). The 4s(Ca) energy level lies less than 1 eV below the energy level of the vacancy. The consequence is that we found density in the vacancy for the two first σ orbitals. Starting with Sc, the metal levels begin to get closer in energy to the vacancy orbital level. The σ1 orbital is still more localized on the vacancy while σ2 is more on the scandium atom. At the right-hand side of the periodic table, the metal levels become significantly lower than the vacancy level; the vacancy appears associated with σ3 orbital and a strong antibonding character. This is shown on the example of Cu. For K (σ12σ21), the interaction is weak. The vacancy orbital level is below that of the metal ones; thus, the magnitude of the interaction is controlled by the filling of the σ metal level which is antibonding. This is a 3e- interaction that is weakly favorable. For Ca (σ12σ22), the σ metal level is completely filled, resulting in a very weak interaction. Hybridization is not efficient to stabilize the σ2 level. This 4e- occupation forbids any electron redistribution and this atom, weakly bonded, remains neutral. We found two degenerate states for Sc, (σ12σ22π1) and (σ12σ21π1δ11), with 1 µB and 3 µB, respectively. The σ22 occupancy explains a week interaction for Sc (doublet), as for Ca. The transfer of one electron to the metal (from σ2 to δ1) does not improve the energy since the scandium levels are still a little too high in energy relative to the vacancy orbital level to bring energy. This transfer becomes favorable for Ti and beyond. For Ti (σ12σ21π2δ11) and V (σ12σ21π2δ11δ21), we find the situation (σ12σ21), a 3e- interaction that was favorable for K. The π and the δ orbitals are filled. Since these orbitals are stabilized by their interaction with the surrounding Mg2+ ions
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Figure 6. Decomposed density of states and band decomposed charge densities. We present decomposed density of states for Ca, Sc, and Cu. The vacuum level is set to zero, and the Fermi level is represented by a vertical line. Band-decomposed charge densities are plotted for a few σ orbitals. These sketches follow the relative energy position of the metal levels and the vacancy level. From left to right in the periodic table, the metal levels decrease in energy. Thus, the vacancy level appears in σ orbitals of increasingly higher energy. Color on line.
and by the p orbitals of the F°s center, the filling of these orbitals leads to a substantial gain in adsorption energy. The δ2 orbital is mainly nonbonding, explaining the small difference between the adsorption energy of Ti and V. Next, the filling of the antibonding σ3 orbital for Cr (σ12σ21π2δ11δ21σ31), Mn (σ12σ22π2δ11δ21σ31), and the high spin state (4 µB) of Fe (σ12σ21π4δ11δ21σ31) implies a reduction of the adsorption energies that become smaller than 1 eV. We have found two degenerate states for Fe, the high spin state (4 µB) mentioned above and a 2 µB state, Fe (σ12σ22π4δ11δ21). The σ3 orbital and the δ2 orbital (non bonding) are then close in energy, meaning that the vacancy orbital level is close in energy to the d levels of the iron atomic orbitals. Beyond the metal transition series, the σ3 orbital will be above the δ2 orbital. Considering Co (σ12σ22π4δ12δ21) and Ni (σ12σ22π4δ12δ22), the metal levels are far below the vacancy orbital level associated to the vacancy. For these atoms the highest antibonding σ-orbital is empty, resulting in high adsorption energy. The occupancy of π-orbitals provides additional stability. The σ3 orbital is filled with one electron for Cu (σ12σ22π4δ12δ22σ31), reducing dramatically the adsorption energy. Finally, the interaction vanishes for Zn (σ12σ22π4δ12δ22σ32) where it is fully occupied. The Co and Ni atoms exhibiting the largest adsorption energies are also among those exhibiting high electronic affinity. However, one can notice that the affinity of Cu (1.226 eV) is larger than those of Ni and Co (1.15 and 0.7 eV, respectively). This criteria would also lead to the sequence Cr > V > Ti whereas the calculated adsorption energies are Ti ∼ V > Cr. 4. Conclusion We have studied the adsorption of the metal atoms from K to Zn on top of a F°s defect on the MgO(100) surface, at low coverage. The vacancy may be assimilated to a virtual atom of large size, possessing an s-type orbital and empty p-type orbitals. The large height of adsorption is a direct consequence of the diffuseness of the s-type orbital, maximizing the overlap at a large distance. The s-type orbital of the vacancy interacts with the σ orbitals of the metal. The adsorption is large when the antibonding
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