J. Phys. Chem. C 2007, 111, 6781-6788
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First Row Transition Metal Atom Adsorption on Defect-Free MgO(100) Surface Se´ bastien Fernandez, Alexis Markovits,* Franck Fuster, and Christian Minot UniVersite´ Pierre et Marie Curie-Paris6 and CNRS, UMR 7616, LCT, Paris F-75005, France ReceiVed: December 8, 2006; In Final Form: February 9, 2007
Periodic density functional calculations have been used for a systematic study of the adsorption of metal atoms on the MgO(100) surface. The complete period of Mendeleev’s table, M ) K to Zn, has been considered. The evolution of the adsorption energies, along this series, is similar to that of the cohesive energies of the pure metals, showing two maxima for Ti and Ni. The origin of this behavior has been discussed, showing that, at high coverage, the M-M interaction contributes to this trend. However, we show that it is also found at low coverage when M-M interactions are negligible. The adsorption energy then results from two main interactions between M and the surface. That with Osurf is the dominant one, and the position on top of the Osurf position is always the most stable adsorption mode. Such interaction also leads to an energy curve presenting two maxima for Ti and Ni. A similar trend could be generalized to cases where M interacts with other bases than Osurf. The interaction of M with the Mgsurf adjacent to the adsorption site should also be taken into account when M is large (M ) K to V). The MO analysis of these interactions is supported by the presentation of the projected density of states.
1. Introduction The adsorption of metals on metal oxide surfaces has been the subject of many experimental1-5 and theoretical studies.6,7 The interaction strength and the adsorption site of several metal atoms, such as nickel,7-10 copper,7,8,10-13 and iron7,14 on perfect or defective MgO(100) have been studied using cluster or periodic models, using DFT/GGA or DFT/hybrid methods. Gaining fundamental insights on the adsorption behavior of metal atoms on oxide surfaces is of great importance in many technologies, like sensors15,16 or information storage.17 In particular, one of the major applications of the metal/oxide interface is heterogeneous catalysis;18,19 the metal oxide may enhance the activity of the metal supported. This is known as the strong metal support interaction, SMSI.20-25 Then, catalysts are composed of metal particles which are dispersed over a metal oxide support. The spreading lowers the cost of the catalyst and gives rise to various small metal nanoparticles whose size and morphology are determinant for the quality of the catalyst.24,26-28 As a first step to understand and control the metal-support interaction, it is of prime interest to study the nature of the metal-surface interaction. To do so, low coverage situations29-31 (the adsorption of a single metal atom being the extreme case) allow the understanding of the main features of the metal/oxide interaction before considering the adsorption of large particles and metallic interfaces that are more complex. Moreover, low coverage corresponds to nucleation processes for metal growth on a metal oxides.32 Experimental measurements of the interaction strength at low metal coverage have been challenging for a long time. Temperature-programmed desorption, TPD, is usually unable to measure the metal/oxide interaction strength at very low metal coverage. However, these studies have been considerably developed during the last decades.1,3,5,33-35 Recently, the use of TPD and calorimetric measurements of the heats of adsorp* To whom correspondence should be addressed. E-mail: alexis.
[email protected]. Fax: +33 144 27 41 17.
tion, of Cu, Ag, and Pb on MgO(100) by Campbell et al. (see ref 36 and refs 5 and 16-22 therein), has quantified the interaction strength for highly dispersed metal particles. A significant conclusion which emerges from these studies is a correlation of the adsorption energies of the metals on MgO(100) with the enthalpies of formation of the corresponding metal bulk. Campbell and Starr conclude that as an immediate consequence the metal/oxide adhesion energy is ruled by the M-Mg bond rather than by the M-O bond. The situation is different at high coverage when the metal forms thick films covering the support. Then, both metal-Mg and metal-O interactions contribute to the interfacial bonding, since adhesion energies correlate with the sum of the metal’s sublimation energy and of the oxide’s enthalpy of formation. Such interpretations are based on the interactions at the atomic level. This is precisely one of the main goals of first principle calculations. A large number of theoretical studies have dealt with this subject. It is worth noticing that there remains a controversy in the evaluation of the adhesion energies within the framework of DFT because of the arbitrary choice of the functional.37,38 It has been shown that there is a quite significant dependence of the interaction energies and the electronic structure with respect to the functional.8,9,39 Even though hybrid density functionals in solid-state chemistry40 and surface chemistry8,13 are now improving, giving more accurate quantitative results, in this work we have used the generalized gradient approximation (GGA-PW91) as implemented in the VASP code. Moreover, the GGA method has already been successfully employed to study the adsorption of metal atoms of the first transition row on perfect MgO(100) such as nickel,9,10 copper10-12 and iron.14 MgO(100) can be considered as a prototype for metal oxides. It has been studied in a very large number of experiments and theoretical studies. MgO(100) has often been chosen as a model surface due to its simple geometry. The bulk has a simple rocksalt structure1 with an alternation of ions of opposite charges. The metal oxide corresponds to an ionic structure; the metal
10.1021/jp068451e CCC: $37.00 © 2007 American Chemical Society Published on Web 04/18/2007
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TABLE 1: Geometrical Parametersa geometrical parameter (Å) frozen surface atom K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn
atomic electronic configurationb 6 1
ps s2 p6d1s2 p6d2s2 p6d4s1 p6d5s1 d5s2 d6s2 d7s2 d9s1 d10s1 d10s2
optimized surface
d(M-O)
d(M-Mg)
d(M-O)
d(M-Mg)
3.29 2.59 2.22 2.05 2.15 2.42 2.15 1.97 1.90 1.82 2.03 3.09
3.91 3.34 3.06 2.94 3.01 3.21 3.01 2.88 2.83 2.79 2.93 3.74
3.10 2.21 1.99 1.85 2.08 2.29 2.06 1.93 1.87 1.80 1.99 2.99
3.89 (3.86) 3.36 (3.56) 3.15 (3.23) 3.09 (3.05) 3.12 (2.91) 3.25 (2.85) 3.13 (2.72) 3.01 (2.84) 2.93 (2.85) 2.85 (2.85) 2.99 (2.88) 3.72 (2.93)
a Adsorption at low coverage, θ ) 1/8. Geometrical parameters in Å. The metal is always on top of O, and dM-O is always optimized. When indicated “optimized surface”, the geometry of the surface atomic top layer is also optimized. The atomic configuration is the atomic ground state calculated with the GGA approximation and used as reference for the calculation of the adsorption energies. In parentheses, the standard Mg-M distances are defined as the average of Mg-Mg and M-M distances in Mg and M bulks.71 b As calculated by VASP (GGA).
cation is very electropositive and is not easily reduced. The (100) surface is stable, easy to prepare,41 and nonpolar. It is classified as type I according to Tasker,42 and no large relaxation takes place.43-48 Because MgO is a wide gap insulator, the perfect surface is rather inert, and the mechanism involved in the adsorption may hopefully be easily understood. In a forthcoming study, other oxides whose metal cations are reducible (e.g., TiO2) will be considered. The present work will serve as reference to emphasize the differences due to this property. On MgO(100), some systematic studies of several transition metal atoms are available. Yudanov et al.49 have studied the interaction of three triads of atoms using the cluster approach. They concluded that two mechanisms are involved in the adsorption. For Cr, Mo, Cu, Ag, and Au, the interaction is weak (less than 0.4 eV) and arises mainly from polarization and dispersion effects. On the contrary, for Ni, Pd, Pt, and W the interaction strength is large (g1 eV) and involves a mixing of one s-d hybrid orbital of the metal with a 2pz orbital of the surface oxygen atom. Several years later, the same authors developed their study adding eight other metals and using two functionals and a sophisticated embedding to model the surface.7,50 Goniakowski and Noguera51 have found the same feature for the adhesion energy of a metal (from Sc to Zn) monolayer deposited on MgO(100) and on MgO(111) from periodic calculations, namely a parabolic curve. The goal of this work is to present a systematic study of the adsorption of the third-row atoms from K to Zn on a perfect MgO(100) surface. A forthcoming paper will consider O vacancies, which are known to play an important role to anchor the metal on the surface. Considering Ni adsorption,52 we have previously addressed the role of the functional, the correspondence between the interaction strength, and the energy cost to change the atomic electronic state from high to low spin on the surface in comparison with the free gas phase atom. In this study, we focus on the explanation of the trend of the adsorption energy. Choosing several coverages, from one monolayer (θ ) 1) to the isolated adsorbed metal atom (θ ) 1/ ), we try to figure out what main parameter governs the 8 adsorbate/substrate interaction. In previous studies, we already showed the energetics of the interaction at low coverage.52,53 We now present the projected density of states (PDOS) and provide an explanation in order to rationalize the strength of the interaction. The PDOS shows that the metal orbitals split upon adsorption. Then, the interaction strength depends on whether a bonding or antibonding orbital is filled.
2. Surface Models and Computational Details WeperformedperiodiccalculationsusingtheVASPprogram54-56 together with a plane wave basis set with a kinetic energy cutoff of 396 eV to describe the valence electrons: two and six electrons for the surface atoms, Mg and O, respectively, and the 4s and 3d electrons for the metal atoms. For K, Sc, Ti, and V, the 3p electrons were also treated in the valence shell. The core electrons were replaced by ultrasoft pseudopotentials.57,58 The calculations were carried out within the generalized gradient approximation (GGA) using the Perdew-Wang exchange-correlation functional (PW91).59,60 Spin-polarization is crucial to get correct adsorption energies.9 It is important both on the supersystem and on the atomic value for metal atoms. For atomic reference, we have calculated the atomic spinpolarized energies in a cubic box. We refer to the GGA ground state even though it is not always possible to predict the correct atomic ground state. This issue has already been pointed out in previous studies.7,50,52 In Table 1, we display the nature of these ground states. All the adsorption energies were calculated as
Eads ) E(TM) + E(MgO) - E(TM/MgO) where E(TM) is the spin-polarized energy of the 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 supersystem. When studying the interaction of a full atomic metal layer, we also defined Eads relative to the metal atom; E(TM) includes the metal-metal interaction in an epitaxial layer, and Eads accounts for both the adsorption energy and the metal-metal interaction in the metal adlayer. On the contrary, we defined Eadh relative to the metal monolayer as the interaction of a metal monolayer with the metal oxide slab; this term only expresses the metal-surface interaction since the reference already contains the M-M interaction. Positive adsorption energies correspond to exothermic processes. We modeled the MgO(100) surface using a three-layer slab. Because the calculations are periodic in three dimensions, we imposed between two successive slabs a distance of more than 7.5 Å, in order to prevent any interaction with the periodic image in the z-direction. A test was performed to validate the model. In the case of the epitaxial metal monolayer (M ) K to Zn, coverage θ ) 1), a six-layer thick slab and a vacuum width of 19.5 Å between two MgO slabs was taken, and we could conclude that our small model allows for the desired accuracy
Adsorption on Defect-Free MgO(100) Surface
Figure 1. Top view of MgO(100). The (1 × 1) unit cell and the R45° 2x2 × 2x2 are shown at the right top side and the left bottom side, respectively.
of interaction energies. Our results show that the adsorption energies obtained with the surface model used in this work are then different by less than 1.4% with respect to the 6L-slab and the largest vacuum width. The largest energy difference is 25 meV, which does not change our conclusion. In all cases, the metal atoms are adsorbed vertically above the surface oxygen atoms. It is now well-established that the surface anions of MgO(100) are the most stable adsorption site for metals. We have checked that there was no tilting of the O-M bond. During the geometry optimization, only the adsorbate height was changed, and the positions of the substrate atoms were those of the bulk. This approximation seems reasonable since relaxation is known to be quite small.43-48 Hence, the cell parameters of the adsorbate layer are imposed by the metal oxide (epitaxial growth of the metal layer on the oxide). In the case of metals of large size (K, Ca) at θ ) 1, a double unit cell, R45° x2 × x2, containing two metal atoms was investigated in order to have two nonequivalent adsorbed metal atoms in the same unit cell with different heights. This allows a rumpling of the metal adlayer and an increase of the M-M distance relative to that imposed by epitaxy (a ) 2.98 Å). On the other hand, we did not allow dimerization or clustering for the metals of small size by studying supercells. At θ ) 1/8, we use an R45° 2x2 × 2x2 supercell (see Figure 1). The distance between two adsorbed metal atoms is 8.42 Å which is large enough to prevent any lateral interactions. In this case, the geometry optimization of the top layer was also performed, and we have checked whether the trend changes in comparison with the frozen substrate. The relaxation of the atomic positions in the supercell takes place until the subsequent step is smaller than 0.001 eV. The calculations were performed sampling the Brillouin zone in a 5 × 5 × 1 Monkhorst-Pack set. The densities of states were computed with a 8 × 8 × 1 grid to sample the Brillouin zone. We calculated the total and projected density of states. 3. Results and Discussion 3.1. Adsorption of an Epitaxial Metal Monolayer. First of all, we present the results for the adsorption of an epitaxial metal monolayer on the MgO(100) surface. There is one metal atom adsorbed on top of each surface oxygen atom; the coverage is
J. Phys. Chem. C, Vol. 111, No. 18, 2007 6783 θ ) 1. In two cases, namely K and Ca, the larger atoms, keeping the same coverage, we also considered a double unit cell containing two metal atoms to allow a rumpling; this leads to two optimized metal atom heights above the surface as discussed at the end of this section. The adsorption energies calculated with respect to the bare metal oxide slab and the isolated metal are shown in Figure 2, curve a. The shape of the curve of the cohesive energy presents two maxima, one around Ti and the other one around Ni. The trend along the row follows very closely that of the cohesive energies of the pure metal bulk (not shown) calculated within the local density theory61 or experimentally measured.62-69 We represent on curve 2b the cohesive energy of a single metal layer which is naturally also similar; the lattice has square symmetry, with a ) 2.98 Å, the MgO(100) cell parameter. Considering that the adsorption energies are due to two contributions (one arising from the metal-metal interaction and the other from the metal-surface interaction), the similarity of curves a and b suggests that the dominating term is the metal-metal interaction. This is also confirmed by considering the magnitude of the values: from Sc to Zn, the metal layer cohesive energy represents from 60% to 80% of the monolayer adsorption energy. It should be noted that if curves a and b have similar shape, this is also true for the third curve, c. Curve c shows the adhesion energies, Eadh, defined as the interaction of a metal monolayer with respect to the metal oxide slab and the metal monolayer. Therefore, Eadh values could not reflect any metal-metal interaction since this term is already enclosed in the reference. Consequently, even if at high coverage, the M-M interaction seems dominant, it is not exclusively responsible for the shape of the Eads curve. The adsorption energy of the metal monolayer, curve 2a, is endothermic for K and weakly exothermic for Ca. At the left side of the row of the periodic table, the size of the atoms is incompatible with the coverage imposed by the cell parameter of the oxide surface, 2.98 Å. As mentioned in the previous section, to decrease this mismatch maintaining epitaxy, we duplicated the unit cell and allowed the two metal atoms to have different optimized heights. Stabilization energies are 1.31 and 0.26 eV per metal atom for K and Ca, using a R45° x2 × x2 supercell (curve 2a with dotted line). The corresponding metal-metal atom distances are then 4.72 Å for K and 3.36 Å for Ca. For the atoms at the right side of Ca, we checked that no rumpling occurred. When the M atoms are smaller, a mismatch between the cell parameters of the adsorbate would be more realistic. It would require the use of supercells which could vary from one M to another. For simplicity, we have restricted our study to epitaxy. 3.2. Adsorption of Metal Atoms at Low Coverage: Energetics. To focus on the metal-oxygen interactions, the metal coverage was decreased. In this section, we considered a R45° 2x2 × 2x2 supercell with eight MgO units per layer. Adsorbing a single metal atom (θ ) 1/8) leads to a large adsorbate-adsorbate distance, 8.42 Å. This model represents the adsorption of isolated metal atoms. Curve d in Figure 2 shows results for the series of metals (K-Zn). The shape of curve d is again similar to others and cannot be attributed to adsorbate-adsorbate interactions. From a quantitative point of view, it is clear that the adsorption energies at θ ) 1 (curve a) are larger than at θ ) 1/8 (curve d), for instance by a factor of 3.3 for Ti. It shows, as already concluded in the previous section, that metal-metal interaction strength, which exists at high coverage but not at low coverage such as θ ) 1/8, dominates over the metal-surface interaction. It is worth mentioning that this domination is even underestimated by our model for which
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Figure 2. Metal adsorption at high coverage (θ ) 1) and low coverage (θ ) 1/8) on MgO(100). Eads (a) is the adsorption energy with respect to the bare slab and to the isolated metal atom. Curve b shows Elayer, the energy of the metal monolayer with respect to metal atom in absence of MgO; this expresses the M-M interactions which are dominant at θ ) 1. Curve c is the adhesion energy, Eadh, defined with respect to the bare slab and the metal monolayer. Positive values stand for exothermic processes. In all the cases, the cell parameters of the metal monolayer are imposed by the metal oxide surface. For K and Ca, we also used a double cell unit cell R45° x2 × x2 allowing a rumpling (see dotted line for curve a). Eads (d) is the adsorption energy of the isolated atom (the unit cell contains eight surface oxygen atoms per adsorbed transition metal atom θ ) 1/8), with respect to the bare slab and to the metal atom. Relaxation of the top MgO layer is not included. The total number of unpaired electrons, NR - Nβ, is indicated in the case of the isolated metal atom; see numbers in parentheses. Curve e shows the interaction energies of a metal atom with NH3 using VASP: the trend of the curve is similar.
epitaxy is imposed. Indeed, aggregation of metal atoms should stabilize more the free metal monolayer than the adsorbed one. Curves c and d are remarkably close to each other; decreasing the coverage or referring to the M layer are two modes of eliminating the M-M interactions. Similar curves are also found when a metal atom is involved in other types of bonding as in the interaction of M with a base. We calculated the interaction strength of the series of metal atoms (K-Zn) with NH3 as representative of a Lewis base. The shape of curve e shown in Figure 2 is again that of the previous curves. We obtained similar results for other bases including anions such as O2- using VASP or Gaussian.70 It is not the mere consequence of metal-metal interactions. Obviously, the trend is due to the spin polarization (see Figure 3). In GGA calculations, the atomic spin is preserved under adsorption except for Ni.52 For Co the doublet and the quartet are quasidegenerate. The curve without spin polarization (curve f) does not present a depletion for Cr. The spin influence is larger for the isolated metal atom than for that involved in strong binding (adsorbed). Spin polarized calculations show a decrease of the adsorption energies that is especially large in the middle of the metal series (V, Cr, Mn, Fe) where the spin is particularly high. It is also the case for the adsorption of the nickel atom9 even though GGA calculations overestimate the stability of the low spin state for this atom. The curve corresponding to the non-spin-polarized calculations has a parabolic shape like that obtained by Goniakowski and Noguera.51 To estimate relaxation effects, we repeated these calculations allowing the relaxation of the top-layer of MgO(100). Curve g displayed in Figure 3 shows these results (to be compared with curve d for the case without relaxation). There is no effect on the shape of the curve confirming the conclusions drawn from
Figure 3. Importance of spin polarization and surface relaxation. Adsorption energy of the metal atoms on MgO(100) at θ ) 1/8. Calculation without spin polarization (f) and with spin polarization (d) already shown in Figure 2 (same labeling). Adsorption energies of the isolated metal atom (θ ) 1/8) with relaxation (g) of the MgO top layer. The relaxation effects are moderate. The increase of the adsorption energy indicates that relaxation is induced by adsorption. That for naked MgO is negligible.
the results with the frozen substrate. However, there is an increase of all adsorption energies up to 0.4 eV. Because the relaxation of the clean MgO slab in the reference is weaker than that upon adsorption, Eads increases. The metal-surface distances do not either vary by much (see Table 1). This is in agreement with previous results.7,50 3.3. Adsorption of Metal Atoms at Low Coverage: Projected Density of States (PDOS) and Molecular Orbitals. Let us now sketch an MO analysis. The metal-surface interaction may be decomposed into two interactions according to which
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Figure 4. Scheme of the two metal δ orbitals and of the four surrounding Mg2+ empty orbitals. Among the two δ orbitals, only the 3dx2-y2 (right-hand side) one has the right symmetry to interact with an empty orbital localized on the four adjacent Mg2+ ions.
Figure 5. Charge isodensity calculated in the energy range of the Sc 3dx2-y2 orbital. Side view and top view.
surface atoms are involved, O or Mg. It is obvious that the interaction with O is dominant since M is vertical upon an O surface atom; however, the four M-Mg distances to the surface Mg atoms adjacent to O may also be considered as short. Except for weak adsorptions (Cr, Mn, and Zn), the calculated M-Mg distances are close to the standard Mg-M distances (see Table 1) inferior or superior by less than 7%. The case of Mn represents an exceptional increase of 15%. In the following, we comment on the two interactions before presenting the PDOS. Interaction with O. In MgO, the valence band is mainly bonding and localized on the 2p orbitals of O2-. The energy gain due to the adsorption of a metal on top of a surface oxygen is due to a further stabilization of these orbitals relative to that already existing in the naked surface. This stabilization is larger when the metal orbitals are lower in energy. It therefore increases when the metal electropositivity decreases (from K to Zn). The atomic levels of the metal are then primarily shifted up by this interaction; this destabilization partially compensates the energy gain of the 2p(O2-) orbitals when the antibonding M levels are populated. To understand the global energy gain, we have thus to consider the population of the M levels. The interaction splits the metal levels which take a more or less pronounced antibonding character according to the nature σ, π, or δ of the overlap with the surface oxygen atoms. The σ interaction is the strongest; the π interaction comes next, and the δ interaction may be neglected in a first approach. According to the number of electrons and to the spin state, antibonding levels will be populated or not. The magnitude of the adsorption energy depends on the filling of the antibonding metal orbitals; in particular, it is very small when the highest σ hybrid is populated. The σ interaction indeed involves two transition metal hybrid orbitals, made of the mixing of 3dz2 and 4s atomic orbitals; one of them pointing at the surface oxygen strongly interacts and generates the highest σ antibonding orbital, that most significantly raised in energy. When this upper orbital is occupied, the global adsorption energy must drop. Interaction with Mg. Let us now consider the stabilizing metal interaction with the four Mg2+ vicinal surface cations. This interaction is weak and masked for the σ and π orbitals for two reasons: the overlap is weak, and the interaction with O prevails. On the contrary, it appears for the δ orbitals which lift their degeneracy. Indeed, only the dx2-y2 orbital matches the x and y directions where the Mg atoms are, as shown in Figure 4. A strong Mg-M interaction should correspond to a low-lying metal dx2-y2 orbital. This interaction is also evidenced by the density of charge calculated in the energy range of the dx2-y2 orbital. Figure 5 presents the result for the case of Sc. Then, there is a clear overlap between the adsorbed metal atom and the four nearest surface Mg atoms.
Projected Densities of States. To shed light on each electronic structure, we have calculated the projected densities of states (PDOS) (Figure 6). For the sake of clarity, only the majority spins projected densities of states are shown. For K and Ca, interactions are dominated by the interaction of the 4s orbital with the oxygen atom at the surface. The two σ orbitals of the metal (called σ and σ′), 4s and 3pz, hybridize and interact with the surface oxygen 2pz orbital. Only the lower one, σ, with predominant 4s character is populated by one electron or two. Figure 6 clearly shows a single peak (R spin for K, two electrons for Ca). The hybridization increases the amplitude in the plane parallel to the surface and decreases the extension in the direction perpendicular to the surface and the overlap with O. This decreases the antibonding character and increases the bonding character with the four neighboring Mg atoms. The global result is a weak interaction. From Sc to Zn, the electropositivity of M decreases, and there is a lowering of the 3d levels that participate in the interaction. This yields a stronger interaction leading to an increase of the adsorption energy as long as the antibonding levels are vacant. Simultaneously, the size of the metal atom decreases, reducing the stabilizing interaction with the Mg atoms. For Sc (σ2δ1) and Ti (σ2δ1π1), the increase of the interaction energy is due to the following: (1) an increase of the interaction with the O atom [The 3d levels are lower in energy and interact more, stabilizing the 2pz orbital. The antibonding levels remain empty (except the π level for Ti).] and (2) an Mg-M interaction which appears in the splitting of the δ orbitals and in the fact that the π level is lower than the dxy level [For the π orbital, the bonding interaction (Mg-M) dominates over the antibonding one (O-M), and the π level is below the nonbonding dxy one.]. It is worth noticing that the contribution of the Mg-M interaction is not negligible. It cannot occur in the case of the interaction between M and NH3 studied in paragraph 3.2 (curve e Figure 2). Curves d and e are similar. However, the strength of interaction is weaker with NH3 (curve d is above curve e) despite NH3 being a stronger base than the surface. The apparent higher value for the surface is not due to a larger basicity of the surface oxygens; the Madelung field stabilizes these anions that are not particularly reactive. The surface is more reactive because of additional contributions of the Mg-M interaction. From V, the Mg-M interaction becomes weak as seen in the PDOS by the small splitting of the δ levels. The high spin situation forces the antibonding levels to be filled, and the adsorption energy decreases. It reaches a minimum for Cr when the σ′ orbital is occupied by one electron. From Mn (σ2δ1π2δ′1σ′1), we now show in Figure 6 the PDOS for the β electrons since the R part remains completely filled. The adsorption energy increases again. The M-O interaction increases because of the lowering of the energy level of the
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Figure 6. Part 1 of 2.
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Figure 6. Part 2 of 2. Projected densities of states.
metal orbitals; the extra electron (β) occupies a nonbonding level. The increases continue up to Fe (σ2δ2π2δ′1σ′1) and Co (σ2δ2π3δ′1σ′1). We have already mentioned that, according to GGA, Ni (σ2δ2π4δ′2) is in a high spin state prior to adsorption and changes to a low spin state under adsorption.9 The σ′ antibonding orbital is then empty, and a significant energy gain takes place. One can expect that a high spin configuration for Ni would result in a much smaller adsorption and a longer O-Ni distance in agreement with the reference.9 The population of the σ′ antibonding orbital is unavoidable for Cu (σ2δ2π4δ′2σ′1) and Zn (σ2δ2π4δ′2σ′2), reducing dramatically the adsorption energy. For Zn, the interaction nearly vanishes. 4. Summary and Conclusion We have studied the adsorption of M ) K to Zn on MgO(100) surface using DFT-GGA. In all cases except Ni and possibly Co, the metal spin is unchanged under adsorption. The trend of the adsorption energy at high coverage (epitaxial growth of the metal monolayer with one metal atom on top of each oxygen surface atom, one monolayer, θ ) 1) resembles that of the cohesive energy of the metal bulk, suggesting that the adsorption is dominated by the metal-metal interaction. This is true for high coverage; however, the trend is invariant when there is no M-M interaction. Relaxation of the surface top layer is moderate. Taking into account relaxation effects slightly increases the adsorption energies. Spin polarization is crucial in order to give the evolution of the adsorption energies. The depletion for Cr and Mn is due to the spin property of the atoms.
The shape of the different curves giving adsorption energies for M ) K-Zn reveals an increase followed by a decrease with a depletion in between for M ) Cr-Mn. The increase is due to the lowering of the metal levels associated with a decrease in electropositivity. The decrease is due to the filling of antibonding orbitals by electrons; the adsorption energy drops for Zn, representing the saturation of these orbitals. MO analysis of the adsorption energies at low coverage allows us to go further and show finer interplay of two sets of interactions, O-M and M-Mg. The decrease of electropositivity implies an increase of the interaction which is counterbalanced by the filling of O-M antibonding levels. The splitting of the d orbitals primarily originates from the σ interaction with surface oxygen; the filling of the resulting antibonding σ′ orbital weakens the interaction. The Mg-M interactions, however, also contribute to the binding especially to the left-hand side of the period. Their contributions are necessary to understand some splitting of the metal orbitals, in particular that of dx2-y2 and dxy in the case of Ti. The role of the Mg-M interaction is a consequence of the small size of the Mg-M distances when M is atop of O; these distances are comparable to standard Mg-M bond lengths. Such an Mg-M contribution has been postulated by Campbell and Starr36 who found from calorimetric measurements a correlation between the low-coverage heats of adsorption and the bulk sublimation energy. However, these authors invoke bonding at defective sites. According to our results, the M-Mg interaction is not specific to oxygen vacancies even if it is certainly more important on FS centers. A forthcoming study will present the M adsorption of the same series in presence of defects.
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