Ag(100): Support Induced Stabilization or

Mar 30, 2010 - Water Dissociation on MgO/Ag(100): Support Induced Stabilization or Electron Pairing? ... The metal support enhances the adsorption ene...
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Water Dissociation on MgO/Ag(100): Support Induced Stabilization or Electron Pairing? Karoliina Honkala,† Anders Hellman,‡ and Henrik Gro¨nbeck*,‡ Department of Chemistry, Nanoscience Center, P.O. Box 35, UniVersity of JyVa¨skyla¨, FIN-40014 JyVa¨skyla¨, Finland, and Competence Centre for Catalysis and Department of Applied Physics, Chalmers UniVersity of Technology, SE-41296 Go¨teborg, Sweden ReceiVed: December 7, 2009; ReVised Manuscript ReceiVed: March 16, 2010

Density functional theory is used to compare water splitting on MgO(100) and MgO/Ag(100). Adsorption is considered on terraces and step edges. On the terrace, the limits of low and monolayer coverage are explored. The metal support enhances the adsorption energies of isolated OH and H through electron exchange with the oxide/metal interface, forming either OH- or H+. As a consequence, the nature of the bonding of dissociated H2O on MgO(100) is different as compared to that on MgO/Ag(100). The bonding on MgO(100) is governed by electron pairing in the oxide whereas bonding to MgO/Ag(100) is determined by the interaction with the oxide/metal interface. The difference in bond character is manifested in a red shift (∼200 cm-1) of hydrogenbonded O-H stretching vibrations when MgO is supported. Introduction Because most oxide surfaces are hydroxylated in air, an understanding of water adsorption and dissociation on oxide surfaces is important. Thanks to theoretical and experimental advantages, MgO(100) has become the prototypical model system for detailed investigations of hydrolysis. Ab initio molecular dynamics has been applied to compare low-coverage H2O adsorption on flat and stepped MgO(100) surfaces.1,2 Water was found to physisorb on MgO(100) whereas fast dissociation was predicted on stepped surfaces.1,2 Subsequent studies at higher H2O coverage on MgO(100) have discovered a collective dissociation process3-5 where one-third of a water monolyer is dissociated. Experimentally, LEED (low-energy electron diffraction)6 and HAS (helium atom scattering) measurements6 have revealed a (3 × 2) superstructure, and a combined HREELS (high-resolution electron energy loss spectroscopy) and UPS (ultraviolet photoelectron spectroscopy) study has indicated a partially dissociated water layer.7 In many applications, where heterogeneous catalysts is but one example, the oxide is in contact with a metal phase. Recently, studies of inverse catalysts have shown that ultrathin metal-supported MgO films have unique chemical properties. On the basis of density functional theory (DFT) calculations, it was predicted that gold atoms on Mo-supported MgO(100) should be charged and adsorbed with a higher adsorption energy than on bulklike (unsupported) MgO(100).8 The prediction of charging has been confirmed experimentally by STM (scanning tunneling microscopy) measurements,9 and the effect has been theoretically shown to apply to Au clusters of various sizes10-13 on MgO/Mo(100) as well as on MgO/Ag(100).14 The general nature of the phenomenon has been demonstrated by its presence for molecules such as NO2 adsorbed on different supported oxides (MgO,15 BaO,16 and Al2O317). The stabilization of NO2on MgO/Ag(100) was recently confirmed experimentally18 via core-level spectroscopy. The charge transfer between the oxide/ metal interface and the adsorbate was first shown for adsorbates with high electronic affinities (EA). The EAs for Au and NO2 * To whom correspondence should be addressed. E-mail: [email protected]. † University of Jyva¨skyla¨. ‡ Chalmers University of Technology.

are 2.30 19 and 2.27 eV,20 respectively. However, recently a similar mechanism has been demonstrated to apply for O2 on MgO/Ag(100),21,22 and it has been proposed that CO oxidation at low temperatures should be possible directly over the supported oxide film.21 The low EA of O2 (0.45 eV23) stresses the consorted mechanism of stabilization24 where the polaronic distortion of the oxide surface is one crucial component. The stabilization of adsorbates on supported thin oxide films has also been predicted to occur via reverse charge transfer from the adsorbate to the oxide/metal interface. On the basis of first principles calculations, Giordano and Pacchioni demonstrated that a K cation is formed when K is adsorbed on MgO/ Ag(100).14 More recently, enhanced magnetization has been predicted for small Fe clusters supported on MgO/Fe(001) owing to a charge transfer from the cluster to the oxide/metal interface.25 Given the unique chemical properties of reversed catalysts,24 it is of interest to study water dissociation over metal-supported oxide films. For MgO/Ag(100), the submonolayer case has been investigated experimentally26-28 as well as theoretically.29 HREELS and XPS (X-ray photoelectron spectroscopy) measurements showed evidence of facile hydrolysis in this regime. The active sites for the reaction was proposed to be at the metal/ oxide interface,26,27 which was supported by the theoretical study of this system.29 Herein we present a density functional theory study of H2O adsorption and dissociation over thin MgO films supported on Ag(100). Adsorption is considered on both terraces and step edges. On the terrace, the limits of low and monolayer coverage are explored. The bonding of dissociated H2O on MgO/Ag(100) is found to be different in nature as compared to that on MgO(100). Computational Method and Systems DFT is used in a real-space grid implementation30,31 of the projector augmented wave (PAW) method.33 The exchange and correlation functional is approximated with the spin-polarized Perdew-Burke-Ernzerhof (PBE) formula.33 The frozen core and projectors are generated with scalar relativistic corrections for Ag. The grid spacing is set to 0.20 Å in all calculations.

10.1021/jp9116062  2010 American Chemical Society Published on Web 03/30/2010

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The lattice constants of Ag and MgO in the bulk are calculated to be 4.17 and 4.27 Å, respectively. A two-layer MgO(100) film is supported on a three-layer Ag(100) slab with the oxygen anions aligned with the metal atoms, which is known to be the preferred structure.34 The bare MgO(100) film is modeled with five atomic layers. Molecular adsorption is in most cases investigated in a (3 × 3) surface cell, and reciprocal space integration over the Brillouin zone is approximated with finite sampling of 4 × 4 × 1 special k points using the MonkhorstPack scheme.35,36 A set of comparative calculations are made for a (3 × 3) cell of MgO supported on Mo(100). The theoretical lattice constant of Mo is calculated to be 3.19 Å. The k-point sampling is in this case the same as for the corresponding MgO/ Ag(100) system. Activation barriers are evaluated within the nudge elastic-band method,37 and structures are regarded as structurally optimized when the largest element of the gradient is smaller than 0.05 eV/Å. The monolayer of water is studied using a range of different unit cells, namely, (1 × 1), (2 × 1), (3 × 1), (4 × 1), and (3 × 2). The reciprocal space integration over the Brillouin zone of the (1 × 1) unit cell is approximated with the finite sampling of 8 × 8 × 1 special k points. The sampling is scaled appropriately for the other unit cells. Stoichiometric monatomic steps edges in the [010] direction are modeled using a (2 × 5) unit cell. The adsorption energy of an adsorbate X is calculated according to

Eads ) EX + EMgO/Ag - EX/MgO/Ag

(1)

where X stands for H, OH, and H2O. The systems are treated in the lowest spin states: singlets or doublets. Charge transfer is analyzed according to the Bader method,38,39 which separates the total electron density onto the atoms in the system. Charge density differences are obtained using the frozen positions of the combined system also in the calculation of the separated systems. Low-Coverage Regime Adsorption of OH and H. As a first step, the adsorption of the H2O dissociation products (OH and H) is investigated in separate computational cells. The relaxed geometries are shown in Figure 1, and energy and structural data are collected in Table 1. Defect-free bulk MgO surfaces are not experimentally available, and it has been shown that cations with reduced coordination may act as electron trapping sites upon atomic adsorption.40 In this sense, the defect-free MgO(100) surface could be regarded as hypothetical. However, it is included in the study as a reference to the results for the supported ultrathin film. OH is adsorbed in a bridging configuration (between two Mg2+ cations) on the bare as well as the supported oxide. On MgO(100) [MgO/Ag(100)], the configuration with OH adsorbed atop Mg2+ is 0.18 (0.21) eV higher in energy. The adsorption energy of OH on MgO(100) differs considerably from that on MgO/Ag(100); the presence of the silver support stabilizes the bond by 1.8 eV. The enhanced bonding is connected to the increased charge transfer to the adsorbate. A Bader analysis reveals that OH is negatively charged by 0.6 electron on MgO(100) whereas it is charged by 0.9 electron on the supported oxide. In the latter case, the oxide and the metal are positively charged by 0.8 and 0.1 electron, respectively. (The supported oxide is positively charged by 0.5 electron in the absence of

Figure 1. Structural models and charge density difference maps for H and OH adsorbed on MgO(100) and MgO/Ag(100). The charge is projected onto a plane perpendicular to the surface plane. Red corresponds to charge gain, and blue corresponds to depletion. Isosurfaces are shown in the range of [-0.4, 0.4] (e/Å 2). Atomic color codes: blue (Ag), green (Mg), red (O), and white (H).

TABLE 1: Adsorption Properties of OH and H on MgO(100), MgO/Ag(100), and Ag(100)a OH/MgO OH/Ag OH/MgO/Ag H/MgO H/Ag H/MgO/Ag

Eads

dH-O

dHO-M

dAg-O

1.12 2.73 2.89 0.44 2.19 2.10

0.99 0.98 0.97 1.00

2.20 2.48 2.10

2.54

0.98

2.63

a

Eads is the adsorption energy (eV) and dH-O, dHO-M, and dAg-O are the distances between the adsorbed H and O in the OH group, OH and Mg (or Ag), and the Ag-MgO interlayer distance. The distances are in angstroms.

adsorbates.) Thus, the fairly high EA of OH (the experimental value is 1.83 eV41) renders a situation similar to the case of Au and NO2 adsorption on MgO/Ag(100). Also, the polaronic distortion is similar to that previously reported for Au or NO2 adsorption on MgO/Ag(100).14,15 Upon OH adsorption, the average O-Ag distance is reduced by 0.1 Å. However, the distortion has local character; the O-Ag bonds directly below the adsorbate are reduced to 2.37 Å whereas for the bare MgO/ Ag(100) system the O-Ag distance is 2.65 Å. The adsorption of H leads to the formation of an OsH- (where Os is a surface anion) group on both surfaces; however, the adsorption energy is 1.7 eV higher on the supported oxide. In this case, charge is transferred from the adsorption site to the oxide/metal interface. This is clearly demonstrated in the charge density difference analysis; charge is depleted from the O2- site and transferred to the interface. That the charge transfer from Os is more facile if the oxide is supported is demonstrated by the calculated O-H bond distances (Table 1), which are stretched on the bare oxide. The distances can be compared to the bond length of OH- in the gas phase, which is 0.98 Å. The structural distortion of the oxide film is also pronounced for H adsorption. However, the nature is qualitatively different as

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Honkala et al. TABLE 2: Adsorption Energy and Charging of Molecular and Dissociated H2O on MgO(100), MgO/Ag(100), and Ag(100)a Eads H2O/MgO (H + OH)/MgO H2O/MgO/Ag (H + OH)/MgO/Ag H2O/Ag (H + OH)/Ag

0.49 -0.64 0.46 -0.29 0.20 -0.52

q(OH)

q(Ag)

-0.88 -0.88 -0.59

-0.50 -0.55 0.00 0.77

q(MgO) 0.07 0.35 0.52 0.90

a

Eads is the adsorption energy (eV). q(OH), q(Ag), and q(MgO) are the charge on OH, on the silver slab, and on the MgO slab or film. The charge on the proton in OsH is included in the charge of the oxide.

Figure 2. Structural models of molecular and dissociated H2O on MgO(100) and MgO/Ag(100). Selected distances are reported in angstroms. Atomic color codes are the same as in Figure 1.

compared to that of OH adsorption. The Mg2+ cation below the OsH site is attracted to the surface, and the Mg-Ag distance is reduced from 3.34 to 3.11 Å upon H adsorption. The difference in the direction of charge transfer for OH and H adsorption can be monitored in the shift of the work function of MgO/Ag(100) upon adsorption. The calculated work function of MgO/Ag(100) is 3.1 eV, which agrees well with previous calculations.14,24 The work function is increased upon OH adsorption but reduced upon H adsorption. The calculated values are 3.6 and 2.6 eV, respectively. For comparison, OH and H adsorption directly onto Ag(100) is considered. The stable configuration for both adsorbates is the 4-fold hollow position, and the distances between the H and OH species and the surface plane are 0.38 and 1.81 Å, respectively. The calculated bond strengths are similar to those obtained for the supported oxide; the bonding of OH to the metal surface is 0.16 eV weaker than for the supported oxide whereas the H bond is 0.09 eV stronger. However, the character of the bonding is distinctly different. This is signaled by Bader analysis; both OH and H are negatively charged on the metal surface by 0.6 and 0.2 electron, respectively. The enhanced adsorption energy of H on MgO/Ag(100) is consistent with the previous prediction that the K adsorption energy on MgO/Ag(100) is higher than that on MgO(100).14 The enhancement was in that case reported to be 0.27 eV.14 Thus, the effect is considerably stronger for H. Adsorption of Water. The relaxed adsorption geometries of H2O on MgO(100) and MgO/Ag(100) are shown in Figure 2.

Energetics and charging are reported in Table 2. On MgO(100) as well as on MgO/Ag(100), H2O is adsorbed atop an Mg cation. For MgO(100), the H atoms coordinate in the direction of surface anions whereas for MgO/Ag(100) the structure is asymmetric with one shorter HOH-Os distance. However, the potential energy surface is shallow, and the configuration of H2O on MgO/Ag(100) similar to that on MgO(100) is only 0.04 eV higher in energy. The molecule adsorbs weakly with similar strength on both surfaces. The adsorption proceeds without any marked charge transfer between the adsorbate and the surface and the water molecule has an excess charge of less than 0.1 electron in both cases. Note that the oxide is charged by 0.5 electron in the absence of adsorbates. The calculations predict that the dissociation of H2O is endothermic on MgO(100) as well as on MgO/Ag(100). The result for MgO(100) is in agreement with previous reports.1,42 The favorable structure for the dissociated molecule on MgO(100) is H adsorbed on a surface anion, forming OsH, and the (OH)ads group is adsorbed atop a surface cation. The situation with the OH group in a bridge configuration is calculated to be 0.06 eV higher in energy. On MgO/Ag(100), the OH group occupies a bridge configuration and the Mg atop site is 0.21 eV higher in energy. For the bare MgO(100) surface, the charge separation in the dissociated molecule is clearer than when H and OH are treated in separate surface cells. When H and OH are adsorbed in the same surface cell, the total charge on the OsH unit is 9.9 electrons. Thus, one electron has been transferred from the O2surface site to the adsorbed (OH)ads radical. The clearer charge separation is also observed in the O-H distances. The (O-H)ads and Os-H distances are calculated to be 0.97 and 0.98 Å, respectively. These should be compared with the slightly elongated distances calculated in the case of separated adsorbates. In contrast with the bare oxide surface, the structure of the dissociated molecule on MgO/Ag(100) is similar to that of the separated (OH)ads and OsH systems. The O-H distances and the polaronic distortions in the oxide film are similar. The O-Ag distance for anions below the (OH)ads group is calculated to be 2.44 Å, and the Mg-Ag distances below OsH are ∼3.11 Å. This indicates that the bond mechanisms for the dissociated H2O molecule are different on the bare and supported MgO. On MgO(100), the dissociated molecule is bonded to the surface via electron pairing in the oxide whereas in MgO/Ag(100) the oxide/metal interface takes an active part in the bond mechanism. Water interacts weakly with the Ag(100) surface (Table 2). The preferred adsorption site is atop Ag with an O-Ag distance of 2.61 Å, and the excess charge is calculated to be only 0.01 electron. The reaction energy for dissociation is endothermic and the corresponding activation barrier is 1.27 eV, thus water

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Figure 4. Structural model of H2O adsorbed at a monatomic step: MgO (left) and MgO/Ag(100) (right). Atomic color codes are the same as in Figure 1.

Figure 3. Enthalpy cycle for H2O dissociation on MgO(100) and MgO/ Ag(100). Gas-phase and adsorbed species are denoted g and a, respectively.

dissociation on Ag(100) is not likely to occur. One important difference between the dissociation of H2O on bare and supported MgO(100) and on Ag(100) is the character of the barrier. In fact, on bare and supported MgO(100), there is no clear evidence of a barrier that separates the reactant from the products, and the dissociation proceeds by the transfer of H+ to an O2- surface anion whereby OH- is formed.43 On Ag(100), the calculations show that the H product is close to neutral during bond cleavage. The H2O dissociation on MgO(100) at low coverage was recently investigated theoretically within DFT.44 It was claimed that the barrier was 1.17 eV with an (HO)ads-H distance of 3.129 Å in the transition state. Because of the very long (HO)ads-H distance, we do not believe that this maximum on the potential energy surface should be related to the breaking of an HO-H bond that has a gas-phase bond length of 0.97 Å. The energetics of H2O adsorption and dissociation in the lowcoverage limit are summarized in Figure 3 with H2O in the gas phase as the zero reference. The gas-phase reaction energy for water dissociation is calculated to be 5.35 eV. Adsorption of H and OH in separate computational cells yields a total binding energy of 1.76 eV on MgO(100) whereas the combined binding energy of the dissociated molecule in one computational cell is 4.71 eV. Thus, the effect of electron pairing in MgO amounts to 2.95 eV. The dissociated molecule is endothermic with respect to H2O in the gas phase, and the stable configuration is represented by the adsorbed water molecule (with Eads ) 0.49 eV). The potential energy diagram is distinctly different for MgO/Ag(100). In this case, the combined adsorption energy of H and OH is calculated to differ by only 0.07 eV from the case when the dissociation products are treated in the same computational cell. The absence of an electron-pairing effect for MgO/ Ag(100) is rationalized by the fact that the closed-shell character of the electronic structure in the oxide is maintained by charge exchange with the oxide/metal interface. Although the Ag(100) support modifies the bond character of dissociated H2O on MgO, it does not change the thermodynamics. For NO2 and Au adsorption, it has been demonstrated that the bond strength could be tuned by the choice of supporting metal.24 However, because of the consorted action of the metal when stabilizing OH and H, it is not obvious how this should be done in the case of H2O. For OH, charge is transferred from the oxide/metal interface and is promoted by the low work function of the metal. However, for H adsorption, charge is transferred from the adsorbate to the metal. Thus, the binding

energy is reduced if the metal is replaced by a metal with a lower work function. To show this, the adsorption of (OH + H), OH, and H was calculated for MgO/Mo(100). The binding energies were calculated to be -0.06, 3.84, and 1.27 eV for (OH + H), OH, and H, respectively. Thus, even if the adsorption energy is enhanced for OH, it is reduced for H so that the binding energy of (OH + H) is still endothermic with respect to H2O in the gas phase. Photoelectron spectroscopy and Auger spectroscopy have been used to study hydrolysis on an 0.8 ML MgO film grown on Ag(100).28 The film was found to be strongly hydrated with 0.8 H2O per MgO unit. Because the 0.8 ML film is close to covering the Ag(100) surface, facile water splitting was attributed to H2O dissociation on MgO(100) terrace sites.28 We have investigated the case with 1 ML on Ag(100), and the results are close to those reported for 2 ML; adsorption as H2O is preferred over (OH + H) by 0.40 eV. This result, together with the high reactivity of MgO steps (see, for example, refs 26, 27, and 29), suggests that the high rate of dissociation on 0.8 ML MgO/Ag(100) could be ascribed to the edges of the MgO islands. Dissociation at Step Edges Water dissociates spontaneously on the considered step edges both in the bare and supported cases (Figure 4). The reaction is exothermic by 1.73 and 2.22 eV for MgO(100) and MgO/ Ag(100), respectively. On both bare and supported oxide, (OH)ads preferably occupies the side of the step edge in a way that it is coordinated toward one step Mg cation and one terrace Mg cation. The bond distance between O and the Mg step edge is 1.97 (1.95) Å on MgO (MgO/Ag), whereas the bond distance between O and the terrace Mg2+ is 2.15 (2.13) Å. The proton prefers to occupy a step-edge O anion, forming an OsH group. If (OH)ads and OsH are close, then they align to form a hydrogenbonded structure. The total charges on (OH)ads and OsH are 9.85 and 9.91 electrons on bare MgO and 9.86 and 10.12 electrons on Ag-supported MgO. Monolayer Adsorption Although H2O is molecularly adsorbed at low coverage, ab initio molecular dynamics simulations have predicted a mixed

Figure 5. Structural model of a (3 × 2) H2O monolayer on MgO/ Ag(100). The surface cell is indicated. Atomic color codes are the same as in Figure 1.

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TABLE 3: Adsorption Energy (eV) per H2O Molecule for a Monolayer Calculated with Different Surface Cellsa 1×1 MgO MgO/Ag(100) a

1×2

1×3

1×4

3×2

3×3

Ea

R

Ea

R

Ea

R

Ea

R

Ea

R

Ea

R

0.44

0/1

0.59

1/2

0.70 0.78

1/3 1/3

0.63

1/4

0.73 0.81

1/3 1/3

0.74 0.71

1/3 1/3

The fraction of dissociated molecules (R) is also indicated.

Figure 6. Vibrational wavenumbers and corresponding intensities (Debye2 angstrom-2 amu-1) for a monolayer of H2O on MgO(100) and MgO/Ag(100).

(OH)ads (∼3800 cm-1), H2O not coordinating to (OH)ads (∼3700 cm-1), and Os-H (∼3550 cm-1). The wavenumbers for these modes are similar for MgO(100) and MgO/Ag(100). The set of modes with wavenumbers between 2000 and 3000 cm-1 are due to O-H stretching vibrations on H2O that are coordinated to (OH)ads. The scissors modes of the H2O molecules are located at ∼1650 cm-1, and the wavenumbers around ∼1000 cm-1 are due to different types of wagging modes. Despite the large difference in intensity, the wavenumbers of the vibrations are similar for MgO(100) and MgO/Ag(100) with the exception of the modes corresponding to water vibrating toward (OH)ads. In this case, the modes on MgO/Ag(100) are red shifted by ∼200 cm-1. The red shift is an indication of a stabilization of the (OH)ads groups on the supported oxide. Because the softened vibrations are important for proton diffusion in the partially dissociated water overlayer, we may speculate that proton transfer is faster on the metal-supported oxide surface. Conclusions

dissociated/molecular phase to be preferred at monolayer coverage.3,4 One monolayer is defined here as one water molecule per MgO unit on the surface. The reason for the preference of a mixed dissociated/molecular layer is the strong O-H bonds formed between molecular H2O and (OH)ads.4 The prediction of a partially dissociated monolyer is supported by HREELS experiments.7 In the present study, the monolayer regime has been studied with different surface cells. For MgO(100), the (1 × 1), (1 × 3), (1 × 4), (3 × 2), and (3 × 3) cells are investigated, whereas (1 × 3), (3 × 2), and (3 × 3) are considered for MgO/Ag(100). The structure for the (3 × 2) case on MgO/Ag(100) is shown in Figure 5. The structural motifs observed in this case are representative of what is obtained for the other systems; a part of the monolayer is dissociated, and molecular H2O is adsorbed over Mg2+ with one H coordinating toward the (OH)ads. The adsorption energies per H2O molecule are reported in Table 3. Except for the smallest cell, the binding energy per water molecule is calculated to be higher than in the low-coverage regime. This can be attributed to the hydrogen bonds formed in the monolayer network. The fraction of dissociated molecules (R) is shown in Table 3. In agreement with previous calculations,3,4 one-third of the molecules in the monolayer are preferably dissociated. This is related to the two strong HOH-(OH)ads bonds formed for each dissociated water molecule. The number of such bonds is highest for a dissociation ratio of one-third. In agreement with the low-coverage regime, there is a slight stabilization of the dissociated water layer when the Ag(100) support is present. Again, this could be attributed to the different bond mechanism for MgO(100) and MgO/Ag(100). To investigate if the difference in bond character could be observed in a property that is experimentally accessible, the vibrational spectrum has been calculated for the (3 × 2) monolayer on MgO(100) and MgO/Ag(100) (Figure 6). The modes at the highest wavenumbers are due to O-H stretching vibrations for

During the past few years, it has become clear that thin metalsupported oxide films exhibit properties in variance with bulk oxide surfaces. The phenomenon is electrostatic in nature and originates from charge transfer from the metal/oxide interface to the adsorbate. It was first observed to yield charged Au atoms on MgO/Mo(100)8 and has later been shown to influence molecular reactions on thin oxide surfaces such as NO2 disproportionation15,18 and CO oxidation.21 In this work, we have investigated whether the stabilization mechanism influences water splitting on MgO/Ag(100) by comparisons with the well-known system of MgO(100). We find that the metal support enhances the adsorption energies of isolated OH and H. In the case of OH, charge transfer is from the oxide/metal interface to the adsorbates, yielding OH-. For H, however, the charge transfer is reversed, leading to adsorbed H+ forming an OH- group on the surface. However, despite the enhanced bonding for OH- and H+ on supported MgO, an intact H2O molecule is the preferred species at low coverage. In agreement with previous predictions, a partially dissociated water overlayer is preferred at monolayer coverage. The different bond character of the dissociated (OH + H) pairs on MgO(100) and MgO/Ag(100) is predicted to be manifested by a red shift of O-H stretching vibrations corresponding to hydrogen-bonded water molecules. Acknowledgment. Support from the Academy of Finland, the Swedish Research Council, and the COST D41 network is gratefully acknowledged. CPU time has been provided by CSC (Espoo), NSC (Jyva¨skyla¨), C3SE (Go¨teborg), and NSC (Linko¨ping). The Competence Centre for Catalysis is hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and member companies AB Volvo, Volvo Car Corporation, Scania CV AB, GM Powertrain Sweden AB, Haldor Topsoe A/S, and the Swedish Space Corporation.

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