Water Adsorption and Dissociation at Metal-Supported Ceria Thin Films

Article pubs.acs.org/JPCC

Water Adsorption and Dissociation at Metal-Supported Ceria Thin Films: Thickness and Interface-Proximity Effects Studied with DFT+U Calculations Lucie Szabová,†,‡ Yoshitaka Tateyama,†,¶ Vladimír Matolín,‡ and Stefano Fabris*,§,∥ †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University in Prague, V Holešovičkách 747/2, 180 00 Praha 8, Czech Republic ¶ PRESTO and CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 333-0012, Japan § CNR-IOM DEMOCRITOS, Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche, via Bonomea 265, I-34136 Trieste, Italy ∥ SISSA, Scuola Internazionale Superiore di Studi Avanzati, via Bonomea 265, I-34136 Trieste, Italy S Supporting Information *

ABSTRACT: The chemistry of several catalytic processes can be controlled by tuning metal−oxide interfaces, as demonstrated by fundamental studies on inverse model catalysts. We investigate the effects of the metal−oxide interface on the surface reactivity of ceria (CeO2) thin films supported by a copper metal surface. Our density functional theory (DFT+U) calculations reveal that the interface has impact on the surface water adsorption and dissociation when the thickness of the ceria film is below ≈9 Å. On thinner films, the energetics of adsorption and dissociation display a significant variation, which arises from a combination of thickness and interface-proximity effects, and which we rationalize in terms of charge-density response at the adsorbate-oxide and oxide-metal interfaces. The adsorption energy is maximized for film thicknesses of 5.5 Å (corresponding to two O−Ce−O trilayers), while thinner films affect primarily the relative stability between molecular, semidissociated, and dissociated water adsorption. These results provide useful insights into the effect of low-dimensional ceria species in Cu/CeO2 catalysts.

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INTRODUCTION

vacancies play a prime role in ceria O-buffering (the oxygen storage capacity of ceria) and they typically occur with chargecompensating excess electrons, which lead to the formation of Ce3+ ions in the oxide. Till now, some studies have focused on the impact of O vacancies on ceria chemistry, while the intrinsic role of the one-electron defects (Ce3+ sites) has been impossible to establish because their presence is usually associated with O vacancies,21 some adsorbates (for example H or metal adatoms),22,23 and highly reconstructed surface structures.24,25 On top of providing a model system suitable for surface science analysis, ultrathin ceria films grown on Cu offer the opportunity to disentangle the effects of one-electron defects from those of other point defects or of surface reconstructions. Previous works have shown that one monolayer (ML) of CeO2 grown on Cu(111) exposes a CeO2(111) surface which is almost isostructural to the corresponding one of a CeO2 single crystal, with the important difference that all the Ce ions

Copper-ceria (Cu/CeO2) systems have been shown to catalyze several industrially relevant chemical reactions, including preferential CO oxidation,1,2 water-gas-shift reaction,3−6 methanol steam reforming,7 and other processes for fuel cell technologies.8−10 Progress in our fundamental understanding of the reactivity of these complex catalysts relies on the synthesis and characterization of suitable model systems, which can mimic, in a controlled environment, the relevant properties of real catalysts. In this context, inverse model catalysts consisting of epitaxial oxide films grown on the flat surfaces of metal or oxide single-crystals allowed for unprecedented catalyst characterization at the atomic level.11−13 In particular, the highly ordered ceria thin films that can be grown on Cu(111) have become one of the standard inverse model catalysts for studying ceria chemistry.14−19 Several studies showed that the reactivity of ceria surfaces grown on metal substrates depends, among other variables, on the exposure of metal−oxide interfaces (i.e., on the continuity of the film), on the presence of surface defects (O vacancies or substitutional/interstitial metal ions) and of metal clusters, and on the degree of reduction of the film.20 Among these, O © 2015 American Chemical Society

Received: October 31, 2014 Revised: December 23, 2014 Published: January 7, 2015 2537

DOI: 10.1021/jp5109152 J. Phys. Chem. C 2015, 119, 2537−2544

Article

The Journal of Physical Chemistry C

Figure 1. Slab models used to simulate water adsorption on the unsupported CeO2 (111) surface (a) and 3 ML (b), 2 ML (c), and 1 ML (d) ceria films supported on Cu(111).

in the film are reduced to Ce3+ due to the charge transfer at the metal−oxide interface.26,27 A thicker 2 ML film instead exposes a stoichiometric O−Ce4+−O trilayer on top of a fully reduced O−Ce3+−O trilayer at the interface. In this paper we take advantage of this property of the CeO2/ Cu inverse model catalyst to investigate by means of density functional theory (DFT) calculations the intrinsic role of oneelectron defects and of charge transfer at the metal/oxide interface on the adsorption and dissociation of water on ceria surfaces. The study is motivated by XPS and TPD experiments on CeO2(111)/Cu(111)28 showing that molecular water adsorption is favored at low temperature while dissociation increases at higher temperatures or higher values of the Ce3+/ Ce4+ ratio (i.e., degree of ceria reduction). Water is reported to adsorb both as a molecule29 and in dissociated state30−32 on stoichiometric and reduced ceria surfaces. Several DFT studies addressed the interaction between water and ceria surfaces.33−40 A recent review that discusses these results also in the context of experimental data can be found in ref 41. For the case of stoichiometric CeO2 surfaces, several calculations report the stability of both molecular and dissociative water adsorption.33,34,36−38 In the molecular adsorption geometries, the O of the water is located on top of a surface Ce4+ site while the molecule can be oriented so as to form one, two, or no H-bonding with the surface O atoms. The partially dissociated water molecule with H atom neighboring the OH group was reported to be almost isoenergetic with the one H-bond molecular configuration, and lower in energy than the dissociated configuration.33,38 In this work we extend the study of water adsorption and dissociation to metal-supported ceria thin films and investigate the effects of thickness, of the proximity of the metal−oxide interface, and of one-electron defects.

To describe the insulating nature of reduced ceria and to limit the self-interaction effects in the PBE functional, a Hubbard U term was added to the DFT(PBE) functional as implemented by Cococcioni and de Gironcoli.45 The occupations of the Ce-f states were defined in terms of the atomic-like projector functions. The value of the Hubbard U parameter was set to 4.5 eV, in line with our previous works22,46−48 and with the values used by the current literature for ceria-based systems (4−5 eV).41,49−53 Water adsorption on the CeO2(111) and CeO2(111)/ Cu(111) systems was simulated with a periodic p(2 × 2) hexagonal slab supercell consisting of 3 CeO2 trilayers exposing the (111) surface (denoted as 3 ML CeO2, Figure 1a), and with orthorhombic supercells consisting of 3, 2, and 1 CeO2 trilayers supported by a Cu substrate (denoted as 3 ML CeO2/Cu, 2 ML CeO2/Cu, and 1 ML CeO2/Cu, respectively, Figure 1b− d), respectively. The Cu(111) substrate was simulated with a 4layer slab of Cu(111). Additional details on geometries, supercells, and convergency of the results with respect to the computational parameters can be found in ref 26. Integrals in the Brillouin zone were calculated numerically by using a finite Monkhorst−Pack54 (4 × 4 × 1) k-point mesh together with a Methfessel-Paxton55 smearing of 0.02 eV in the case of supercells containing Cu substrate and Gaussian energy broadening of 0.005 eV in the case of unsupported ceria slab. The atomic geometries were relaxed according to the Hellman-Feynman forces while constraining the position of the atoms in the lowermost O−Ce−O trilayer (for the 3 ML CeO2 system only, see below) or of the two lowermost Cu layers (for all other cases) to their corresponding bulk-like coordinates in order to simulate CeO2 and Cu crystal substrates, respectively. Searches for the minimum energy path and transition state for the water dissociation were performed with the nudged elastic band (NEB) algorithm using the Broyden scheme.56 The adsorption energy of water to the surface (Eads) was calculated as the following total-energy difference:

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CALCULATION DETAILS Our plane-wave spin-polarized calculations were based on DFT using the gradient corrected generalized approximation of Perdew−Burke−Ernzerhof (PBE)42 for the exchange-correlation functional and were performed with the PWscf code of the Quantum ESPRESSO distribution.43 The interaction between valence electrons and ionic cores was described by ultrasoft pseudopotentials.44 The energy cutoffs for plane-wave basis set and electron density representation were set to 30 and 300 Ry.

tot tot tot Eads = Ewat/slab − (Eslab + Ewat )

Etot wat/slab,

Etot wat,

(1)

Etot slab

where and are the total energies of the relaxed supercell containing the water molecule adsorbed on the substrate, the isolated water molecule in vacuum, and the substrate, respectively. 2538


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The Journal of Physical Chemistry C Table 1. H2O Adsorption Energies Eads [eV] on Clean Stoichiometric Ceria Surfacea configuration this work Férnandez-Torre et al.33 Kumar and Shelling34 Chen et al.35 Watkins et al.37 Fronzi et al.36 Yang et al.38 expt: Prin et al.58

MS1 −0.52 −0.54 −0.58 −0.49

MS2

MS3

HP1

HP2

−0.51 −0.51

−0.32 −0.32

−0.50 −0.52

−0.24 −0.24

−0.36 −0.33

−0.66 −0.36 −0.55

−0.16

−0.52 −0.49

−0.57 −0.56 to −0.61

a

The adsorption energies of the water molecule on the CeO2(111) surface in different configurations described in ref 33 as calculated in this work. For comparison we show values calculated in different theoretical publications as well as the experimental value of adsorption energy.

Figure 2. Top and side view of water adsorption configurations on 3 ML unsupported CeO2 slab simulating adsorption on bulk ceria. MS1 (a) represents the molecular adsorption, HP1 (b) the predissociated state, and HP2 (c) the dissociated water molecule. The nomenclature was adopted from ref 33. The green small circles correspond to Ce atoms, yellow are H atoms, and dark red, red, and orange circles represent surface and bulk O atoms and O atom in water molecule, respectively. Roman numbers on the elements denote the atomic layer as counted from the surface, while letters A−C refer to the atoms of the water molecule.

adsorption geometries reported in the extensive study of Férnandez-Torre et al.33 We adopt here the same notation of ref 33 for the adsorption configurations. MS1, MS2, and MS3 refer to molecular adsorption geometries in which the water molecule forms one, two, or no hydrogen bonding with the surface O atoms, respectively. HP1 and HP2 refer to adsorption geometries in which the water molecule is dissociated and differ in the distance between the OH and H fragments. The calculated binding energies to the stoichiometric CeO2(111) surface are reported in Table 1 together with the available experimental and theoretical values. Our calculations are in very good agreement with the most recent results, showing that the energies of the molecular (MS1 and MS2) and dissociated (HP1) adsorption modes are virtually degenerate (Eads between −0.50 and −0.55 eV), their difference being within the error bar of our DFT+U calculations. The MS3 and HP2 configurations are less bound, Eads = −0.32 and −0.24 eV, respectively. In the following analysis we will mostly focus on the MS1 molecular adsorption structure and on the dissociated structures HP1 and HP2 (Figure 2). We note that on stoichiometric ceria surfaces water always bind to a Ce site, while in the presence of O vacancies water binds more strongly

We note that, in some of the cases studied here, the adsorption of a water molecule on the thin metal-supported ceria films resulted in spontaneous structural rearrangements of the metal−oxide interface. This is in line with the reported small corrugation of the CeO2/Cu interface energy and with the ability of ceria to glide on the Cu substrate.57 The results described in the following always refer to the lowest-energy interface structures we could identify, also considering the interface-induced lateral translations eventually induced by the adsorbates (see the Supporting Information).

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RESULTS AND DISCUSSION Thin-Film Effects on Adsorption Structure and Energetics. We start our analysis by studying the adsorption of a water molecule on the stoichiometric CeO2(111) surface (3 ML CeO2, Figure 1a) and on a 8.5 Å thick ceria film supported by Cu(111) (3 ML CeO2/Cu, Figure 1b). The unsupported stoichiometric CeO2(111) surface is used as a baseline to establish interface and finite-size effects in the supported thinner films as well as to relate our results to the available theoretical and experimental studies. To this end, we consider as initial configurations the stable and metastable 2539


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The Journal of Physical Chemistry C at the point defect, with adsorption energies from −2.1138 to 2.12.39 The main bond lengths of the MS1, HP1, and HP2 relaxed configurations on the unsupported CeO2(111) surface (3 ML CeO2) are reported in Table 2 and are in very good agreement

are included in Table 2 and are very similar to the corresponding ones of the unsupported CeO2 surface. The bond lengths differ by less than 0.1 Å with an average difference within 0.02 Å for the 2−3 ML CeO2/Cu and 0.08 Å in the case of 1 ML CeO2/Cu. The main conclusion of this comparison is that the adsorption geometries of water at the surface of CeO2supported film are weakly dependent on the film thickness and are mostly equivalent to those at a single-crystal CeO2 surface. Despite this invariance in the adsorption geometry, adsorption energies of water on the supported ceria films, both in the molecular and dissociated states, display a considerable variation depending on the film thickness (Table 3). The adsorption energy of MS1 on the 3 ML CeO2/Cu

Table 2. Bond Lengths of H2O Adsorption Structures of the Water Molecule Adsorbed on Ceria Surfaces in the Molecular or Dissociative Statea configuration MS1

HP1

HP2

substrate 3 3 2 1 3 3 2 1 3 3 2 1

ML ML ML ML ML ML ML ML ML ML ML ML

CeO2 CeO2/Cu CeO2/Cu CeO2/Cu CeO2 CeO2/Cu CeO2/Cu CeO2/Cu CeO2 CeO2/Cu CeO2/Cu CeO2/Cu

HA−OI

OB−CeII

HC−OB

HA−OB

1.77 1.71 1.71 1.66 1.01 1.01 1.01

2.63 2.64 2.61 2.69 2.24 2.25 2.24

0.98 0.98 0.98 0.97 0.97 0.97 0.97

1.01 1.01 1.01 1.02 1.64 1.64 1.64

0.98 0.98 0.97 0.98

2.16 2.16 2.16 2.17

0.98 0.98 0.98 0.97

Table 3. H2O Adsorption Energies Eads [eV] on Ceria Surfacesa 3 3 2 1

4.36 4.40 4.39 4.67

configuration

MS1

MS2

MS3

HP1

HP2

ML ML ML ML

−0.52 −0.49 −0.99 −0.56

−0.51 −0.55 −0.95 −0.48

−0.32 −0.30 −0.77 −0.32

−0.50 −0.48 −0.99 unstable

−0.24 −0.29 −0.50 −0.39

CeO2 CeO2/Cu CeO2/Cu CeO2/Cu

a

Overview of the adsorption energies of the water molecule on CeO2(111) surfaces in different configurations described in ref 33 calculated within this study on unsupported ceria slab and thin ceria films on Cu(111). The dissociated HP1 configuration on the CeO2/ Cu system is unstable.

a

The lengths of bonds between the water molecule and the ceria surface (H−O; O−Ce) and OH bonds in the water molecule for each system in angstroms [Å].

with those of Fér nandez-Torre et al. 33 In the MS1 configuration, the OB atom in the water molecule is located on top of a CeII atom, with a CeII−OB distance of 2.63 Å, which is 10% longer than the Ce−O bond in ceria bulk. (Roman numbers on the elements denote the atomic layer as counted from the surface, while letters A−C refer to the atoms of the water molecule, see Figure 2.) The orientation of the water molecule is such that one of its H atoms points toward a surface O atom. The resulting HA−OI distance is 1.77 Å, which is well in the range of the low-temperature H-bonding distances (e.g., 1.75 Å in ice59). In the HP1 configuration, the OH molecular fragment (OB− HC) binds on top of the CeII ion with a OB−CeII bond length of 2.24 Å. As expected, this Ce−O bond is significantly shorter than the bond in the case of molecularly adsorbed H2O and about 10% shorter than the Ce−O bond in the bulk ceria. The HA atom is adsorbed at the surface oxygen directly neighboring the adsorbed OH group forming a HA−OI bond 1.01 Å long, comparable to the length of the OH group. The distance between the oxygen OB in the OH group and the dissociated hydrogen atom HA is 1.64 Å. These results are in line with those reported in the literature for molecular and dissociated water adsorption on unsupported ceria surfaces. They are therefore a suitable reference to evaluate the effects of the metal−oxide interface and of the ceria-film thickness on water adsorption. We note here that the extensive test with different approximations and approaches performed in ref 33 demonstrated that the inclusion of van der Waals dispersion terms into the exchange and correlation functional does not affect the relative binding energy of the molecular and the dissociated configurations. According to ref 33 the van der Waals contribution to the adsorption energies is 180 meV per water molecule. The same molecular and dissociated adsorption configurations were also found on the metal-supported ceria films. The bond lengths for the MS1, HP1, and HP2 water configurations

system (thickness of ceria film ≈8.5 Å) is virtually the same as on the reference unsupported single-crystal CeO2 surface. The 2 ML CeO2/Cu system (thickness of ceria film ≈5.5 Å) displays the largest variation in the adsorption energy of water: Eads for all configurations (MS1, HP1, and HP2) increases by more than 90%, up to 0.99 eV in the case of MS1 configuration. Upon further reduction of the film thickness (1 ML CeO2/Cu system ≈2.3 Å thick), the strength of the interaction is reduced to 0.56 eV, almost to the reference value for the unsupported ceria surface. The enhanced interaction displayed by the supported 2 ML CeO2 film is not a structural effect (the surfaces of all supported films are substantially isostructural) but is necessarily due to finite-size and interface-proximity effects. As explained in the following, we show that these changes have electrostatic origins due to the different response of the charge density at the surface and at the metal−oxide interface in the supported films. Charge Density and Electrostatics Analysis. To gain a deeper insight into these effects we have performed a chargedensity analysis. The bonding between water and ceria surfaces was characterized by atom-resolved projected density of states (pDOS) together with the Löwdin charges (for details see the Supporting Information). The calculated pDOS shows that the stoichiometry and degree of reduction is preserved upon water adsorption on the surface of all systems considered here (see Figures S2−S4 in the Supporting Information). The pDOS for these systems are however very similar to each other and therefore not useful to identify the origins of the thicknessdependent enhanced binding. Instead the charge-population analysis reveals distinct differences. We plot in Figure 3 the charge reorganization due to water adsorption in the MS1 and HP1 configurations analyzed in terms of Löwdin-charges differences: red and blue colors represent charge accumulation and depletion, respectively. 2540


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trilayer, which is fully reduced due to its contact with the metal support. Although this plane of Ce3+ ions lies 4 Å below the surface and 5.5 Å from the water molecule, it still plays an important effect on the surface properties of the film. The charge-difference population analysis for the MS1 water adsorption on the 2 ML CeO2/Cu system is displayed in Figure 3c,g and clearly identifies two regions, the surface O−Ce4+−O trilayer and the interface O−Ce3+−O one. The charge rearrangement due to adsorption in the surface trilayer follows the same pattern described for the surface of thicker ceria films, i.e. an increase in the dipole moment of the adsorbed water molecule and the polarization response of the surface atoms. In addition, due to the presence of the metal−oxide interface, this system can further respond to water adsorption by polarizing the metal−oxide interface (Figure 3g) to a higher degree than the 3 ML/Cu system. The actual charge-difference pattern is complex, with characteristic polarization including reoxidation of the CeV layer and charge accumulation at the interfacial OVI layer. The interfacial CuVII layer exhibits on average a charge depletion. This polarization of the metal−oxide interface additionally screens the surface dipoles and thus reduces the electrostatic interactions in the oxide film. We ascribe the higher adsorption energy of water on the 2 ML CeO2/Cu system to this charge redistribution at the interface, that is active only for the ultrathin supported ceria films and that rapidly decreases for thicker films. At variance with all the thicker films, the surface of the 1 ML CeO2/Cu system is fully reduced. The charge-difference population analysis due to water adsorption in the MS1 configuration is displayed in Figure 3d,h. In this case, likely due to the proximity of the layer of Ce3+ ions, the charge reorganization is reduced with respect to the thicker films. The same effects govern the thickness dependency of the water binding energy in the dissociated states. The bottom panels of Figure 3 display the charge-difference population analysis for the HP1 configuration (a metastable configuration is displayed for the 1 ML CeO2/Cu system, see below). Also these cases respond to the presence of the adsorbate with charge rearrangements at the adsorbate−surface layer and at the metal−oxide interface, following the pattern and relative magnitude described above. The orientation of the water molecule affects only the in-plane polarization at the surface layer, while the polarization at the interface remains unchanged in 2 ML CeO2/Cu substrate for all studied water molecule orientations. Thickness and Interface Effects on Water Dissociation. We now turn to the effects of the film thickness and of the metal−oxide interface on the activation energy for water dissociation and on the relative energetics between the molecular and dissociated states. The relevant potential energy diagram for the dissociation of water from the MS1 initial state (IS) to the HP1 final state (FS) is displayed in Figure 4. The energy of the MS1 state is taken as a reference for all systems. First we focus on the relative energy difference between the initial and final states. In the limits of the DFT+U approach, these are essentially isoenergetics for film thicknesses larger than 2.3 Å, corresponding to 1 CeO2 ML. Instead, the fully reduced Ce3+ layer exposed by the 1 ML CeO2/Cu system disfavor the dissociated HP1 state, which is unstable and spontaneously converts into the molecular MS1 one. In addition, for a ceria/copper interface with higher symmetry (see the Supporting Information), we could also identify a metastable HP1 state (see Figure 4), which can however

Figure 3. Difference in the Löwdin charges induced by water adsorption in MS1 (top view − top panels, a−d; side view − middle panels, e−h) and HP1 (bottom panels, i−l) configurations. Red/blue color denotes charge accumulation/depletion.

The MS1 water adsorption on the stoichiometric 3 ML CeO2 and 3 ML CeO2/Cu systems leads to similar charge reorganization (Figure 3a,b,e,f). It is mostly local since it involves primarily the water O atom (which becomes more negative), the surface OI atom engaged in the H bonding (which becomes less negative), and an additional neighboring surface O atom (which becomes more negative). According to this analysis, surface adsorption induces an increase in the intrinsic dipole of water and an electrostatic response of the surface, which optimally couples with the water dipole via an inplane polarization. Quite importantly, there is no significant charge reorganization below the outermost CeO2 ML, i.e. from 1.6 Å below the surface in the case of unsupported 3 ML CeO2 except from a minor reoxidation of the atomic CeVIII layer in the 3 ML CeO2/Cu system. The surface exposed by the 2 ML CeO2/Cu system has the same structure and degree of reduction of the one exposed by thicker films. Important differences compared to the 3 ML and 3 ML/Cu systems arise only in the subsurface O−Ce−O 2541


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unstable and recombines into an adsorbed water molecule. In parallel, the 1 ML-thin ceria film displays a high structural flexibility that is evident in the fully dissociated water configuration, where the lattice undergoes sizable distortions so as to optimize the Ce−OH bonding. With respect to stoichiometric ceria surfaces, ultrathin ceria films supported by copper are shown to selectively promote water interaction, even in the absence of surface O vacancies. More reactive than thicker films, in particular the 2 ML film is likely to preferentially attract water, which then can be easily dissociated on the thinner 1 ML film. In the context of the water-gas-shift reaction, the presence of low-dimensional ceria species on the surface of the copper metal has been shown to be an important factor controlling the reactivity of ceria− copper catalysts.62 The interface effects, which we have revealed here for the case of water adsorbates, may therefore be significant also in the context of other reactions where water molecules are present as a reactant or as a product and where metal/ceria interfaces form on the catalyst surface.

Figure 4. Barriers for partial dissociation of the water molecule on different substrates as calculated by NEB.

convert into the molecular MS1 state with a negiglible barrier that is smaller than 0.05 eV. Hence, the partially dissociated water configuration is destabilized by the one-electron defects so that the fully dissociated HP2 configuration is the only stable dissociated state on the supported 1 ML ceria film. Quite interesting, this ultrathin layer displays here its high structural flexibility: the molecular OH fragment pulls up the underlying Ce3+ ion by 1 Å through the surface O atoms. The calculated activation energy for the dissociation of a water molecule from the MS1 to the HP1 state is also reported in Figure 4. The barrier for the stoichiometric unsupported surface is 0.11 eV, and is consistent with the result reported in ref 33. The thermodynamics of the process for the supported thin films is thickness dependent and nicely follows the Brønsted−Evans−Polany relationship between the activation energy and the energy difference E(FS) − E(IS).60,61 There is a transition in the value of the barrier for the reverse process from 0.10 to 0 eV when the thickness decreases from 2 to 1 ML CeO2/Cu. As explained above, this is a clear effect of the proximity of the metal−oxide interface to the adsorption/ reaction sites. The very small values of these energy barriers, which may actually be dependent by the inclusion of van der Waals contributions and by the functional choice, show that they can be easily overcome at room temperature and below.

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ASSOCIATED CONTENT

S Supporting Information *

Details of bonding charge and projected density of states analysis of water adsorption on supported thin ceria films and detailed analysis of adsorbate-induced structure rearangement in thin ceria films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the FP7-NMP-2012 project chipCAT under Contract No. 310191, the EU FP7 COST action CM104, and the Czech Science Foundation grant No. 13-10396S for financial support. Part of the work has been performed under the Project HPC-EUROPA2 (Project number 228398), with the support of the European Community-under the FP7 Research Infrastructures Programme. This work was supported by the Strategic Programs for Innovative Research (SPIRE), MEXT, and the Computational Materials Science Initiative (CMSI), Japan. The calculations in this work were carried out on the supercomputer centers in NIMS, ISSP, and ITC in the University of Tokyo, Kyushu University as well as the K computer at the RIKEN AICS through the HPCI Systems Research Projects (Proposal Nos. hp140110 and hp140232).

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CONCLUSIONS In summary, we studied the water adsorption and dissociation on metal-supported thin films of ceria. Our study shows that the energetics of adsorption, the relative energetics between molecular/dissociated states, and the activation energy for surface dissociation depends on the film thickness. The effect becomes significant for film thicknesses below 8.5 Å. Thin films can increase the water−ceria interaction by a factor of 2. The binding energy displays a maximum for a thickness of 5.5 Å that corresponds to 2 CeO2 MLs. This preferential adsorption is not an effect of surface structure nor an effect of electron confinement in the ceria film. It arises from the proximity between the surface and the underlying metal− oxide interface that responds to water adsorption and dissociation with charge rearrangement, thus optimizing the electrostatic interactions. For film thicknesses down to 2.3 Å the effect equally alters the energetics of both the molecular and dissociated configurations. In addition to this charge transfer at the metal−oxide interface, the thinnest ceria films display an additional feature which modifies the relative stability of the adsorption configurations. In particular, the semidissociated state is

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Water Adsorption and Dissociation at Metal-Supported Ceria Thin Films

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