Density functional theory study of the interaction of H 2 O, CO 2 and CO with the ZrO 2 (111), Ni/ZrO 2 (111), YSZ (111) and Ni/YSZ (111) surfaces

Thetriplephaseboundary(TPB),wherethegasphase,Niparticlesandtheyttria-stabilisedzirconia(YSZ)surface meet, plays a signi ﬁ cant role in the performance of solid oxide fuel cells (SOFC). Indeed, the key reactions take place at the TPB, where molecules such as H 2 O, CO 2 and CO interact and react. We have systematically studied the interaction of H 2 O, CO 2 and CO with the dominant surfaces of four materials that are relevant to SOFC, i.e. ZrO 2 (111), Ni/ZrO 2 (111), YSZ(111) and Ni/YSZ(111) of cubic ZrO 2 stabilized with 9% of yttria (Y 2 O 3 ). The study employed spin polarized density functional theory (DFT), taking into account the long-range dispersion was observed, Supported nanoparticles con ﬁ rming the in ﬂ uence of the surface on the molecules' intra-molecular bonds. These results are in line with Surface science the important role of Ni in this system, in particular for the CO adsorption and activation. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).


Introduction
The solid oxide fuel cell (SOFC) is an electrochemical device that converts electricity by oxidizing fuel, e.g. hydrogen, through an environmentally sustainable process [1]. A common anode used for this device is yttria-stabilized zirconia (YSZ) with supported Ni particles. The main advantages of the Ni/YSZ cermet (ceramic matrix (YSZ) bonded to metal particles (Ni)) [2] over other materials are, for instance, its good thermal expansion, ionic conductivity, high catalytic activity, mechanical/chemical stability and compatibility with the electrolyte [3,4]. Several experimental and theoretical studies of the Ni/YSZ cermet have endeavoured to gain understanding of the microstructure of this material, since the material's atomic level structure and processes lie at the core of the performance of the SOFC anode [5].
One of the species used as fuel in the SOFC are H 2 and CO which are oxidized at the anode of the SOFC [6]. Electrochemical oxidation of H 2 is a multistep process where adsorption, surface reactions and charge transfer are observed during the reaction [7]. The reaction at the anode is described by reaction (R.1) [8]: In the CO oxidation on Ni/YSZ, CO adsorbs associatively on Ni and then diffuses towards the triple phase boundary (TPB), where the gas phase, Ni particles and YSZ surface meet. Here, it is oxidized by O 2coming from the electrolyte i.e. the solid oxide (reaction (R.2)) [9].
Instead of pure H 2 or CO, H 2 /CO mixtures can be used leading to more complicated mechanisms since other reactions than direct oxidations are involved such as water-gas shifting [6] (reaction (R.3)) [10][11][12].
CO þ H 2 O ⇌ CO 2 þ H 2 ðR:3Þ All these molecules interact with the supported Ni and the surface, thus necessitating the study of these molecules in contact with the Surface Science 653 (2016) 153-162 TPB, which plays a key role in the performance of the SOFC [13]. In the past few years many researchers have investigated the microstructure and the chemistry of the TPB. For example, Liu et al. [14] have shown experimentally that impurities such as SiO 2 can degrade the Ni/YSZ grain boundaries, whereas Jensen et al. [15] have shown that impurities segregate from the bulk of the YSZ material and accumulate along the TPB.
To obtain YSZ, zirconia (ZrO 2 ) is doped with yttria (Y 2 O 3 ), thereby creating oxygen vacancies [16], which play a role in the ionic conductivity, where it is well known that the conductivity is maximised at a Y 2 O 3 concentration of around 8% [17]. In addition, experimental [18][19][20] and theoretical [21,22] studies have shown that the dopant (Y 3+ ) is located preferentially at the next nearest neighbour (NNN) site to the oxygen vacancy. We will therefore concentrate on the configuration with the dopant in the NNN site in our model of the YSZ cermet.
Several ab initio investigations have been carried out on the interaction of molecules with the Ni/YSZ cermet [23][24][25][26]. Shishkin et al. [23] showed the oxidation of hydrogen at the TPB by the oxygen atoms bound to Zr (or Y) and Ni. Cucinotta et al. [24] showed that the hydrogen oxidation can be active away from the TPB, when water is adsorbed on the oxide surface. Ammal et al. [25] combined DFT and microkinetic studies to show that the O migration pathway is faster than the H spillover and OH migration pathway. Shishkin and Ziegler [26] have studied the oxidation of H 2 , CH 4 and CO at the Ni/YSZ interface and showed that the YSZ cermet is inert to fuel oxidation, while Ni is active towards fuel adsorption and oxidation. However, a systematic characterization of H 2 O, CO 2 and CO molecules at the TPB is still missing. We have therefore investigated the interaction of these three molecules on four relevant surfaces, i.e. ZrO 2 (111), Ni/ZrO 2 (111), YSZ(111) and Ni/YSZ(111), and compared our results with experimental data.
We have investigated up to five initial adsorption sites for each molecule on the pristine surfaces (ZrO 2 (111) and YSZ(111)) and two more on the surfaces with supported Ni (Ni/ZrO 2 (111) and Ni/YSZ(111)). We have elucidated the geometries and electronic structures of the most stable adsorption sites and compared these results with other reported computational models, where available, and supported our findings by experimental and computational infrared spectra (IR).

Models and computational methods
All the structural relaxations and frequency calculations have been performed using the Vienna Ab-initio Simulation Package (VASP) [27][28][29][30], solving the Kohn-Sham equations of the density functional theory (DFT) [31] in a periodic approximation. The calculations have been carried out adopting the generalized gradient approximation (GGA) [32] with the Perdew-Burke-Ernzerhof (PBE) density functional. The semi-empirical method of Grimme [33] was employed to describe and improve the long-range dispersion interactions. The H (1 s), C (2 s, 2 p), O (2 s, 2 p), Ni (3 d, 4 s), Zr (4 d, 5 s) and Y (4 s 4p 4 d 5 s) atomic orbitals have been treated as valence electrons, while the remaining electrons were considered as core electrons and kept frozen. To describe the interaction between the valence and the core electrons, we adopted the projected augmented wave method (PAW) [34]. The plane-waves kinetic energy cutoff was fixed at 500 eV, and all the calculations were spin-polarized. The conjugate gradient technique has been used for the geometry optimizations with an interatomic force threshold of 0.01 eV/Å. Monkhorst-Pack grids with a 7 × 7 × 1 k-point mesh were used to sample the reciprocal space for the 1 × 1 slab calculations. For the molecules in the gas phase a 1 × 1 × 1 mesh of k-points was used and each molecule was positioned in a large box (14 × 15 × 16 Å 3 ) in order to avoid lateral interactions. The rest of the settings were similar to those used for the slab calculations.
The procedure to compute the vibrational frequencies is implemented in the VASP software. The vibrational frequencies were evaluated by finite displacements of every coordinate and the vibrational frequencies thus correspond to the eigenvalues of the diagonal Hessian matrix (second derivative of the energy with respect to the atomic positions), where the eigenvectors are the vibrational normal modes of the system. Frequency changes reflect a structural modification of the molecules upon adsorption on the surfaces.
In Fig. 1 we show the fluorite crystal structure of cubic zirconia (c-ZrO 2 ), the unit cell of which is face cantered cubic (fcc) (space group Fm-3m). The fcc cube is formed by the Zr atoms which are coordinated to eight oxygens. The oxygen atoms are positioned on the diagonals of the cube.
To describe the (111) surfaces we have used a slab model with a perpendicular vacuum size of 15 Å, i.e. large enough to avoid interactions with its periodic images. Following previous work [22,35], the surfaces were O-terminated and contained 9 atomic layers (three O-Zr-O trilayers); the 5 top atomic layers were allowed to relax fully during geometry optimization, while the 4 layers at the bottom were kept fixed, thereby simulating the bulk material. The surface area is 44.96 Å 2 .
The four surfaces (ZrO 2 (111), Ni/ZrO 2 (111), YSZ(111) and Ni/ YSZ(111)) were taken from our previous work [22]. There, we had substituted two Zr (from the topmost and the sub-surface O-Zr-O trilayers) by Y with the removal of one oxygen from the third atomic layer (the next nearest neighbour of Y atoms) to obtain the YSZ(111) surface (Zr 10 Y 2 O 23 (111) slab) ( yttria = 9.1 wt%). Then, Ni was deposited on the most favourable adsorption site on both the ZrO 2 (111) and YSZ(111) surfaces. On the ZrO 2 (111) surface, Ni sits preferentially on the O d ′ atom, i.e. slightly off from the perpendicular, Fig. 2, while on the YSZ(111), it adsorbs preferentially on top of the oxygen vacancy and away from the Y atoms (Fig. 2).
The magnetization of the Ni atom before adsorption was 0.57 μ B (Ni atom from bulk calculation). Upon adsorption on both ZrO 2 (111) and YSZ(111) surfaces, the calculated magnetisation for the adsorbed Ni atom was 0.0 μ B (singlet configuration). For comparison, we have also forced the Ni atom to be in a triplet configuration (2.0 μ B ) and the calculated total energies were 1.16 and 1.44 eV higher for Ni/ZrO 2 (111) and Ni/YSZ(111), respectively, compared with the values found for the singlet configuration. Moreover, a study made by Hahn et al. [36] took into account different spin states of Ni 10 cluster adsorbed on CeO 2 (111) surface and they showed that the singlet configuration of the cluster is stable, as it is only 0.01 eV higher in energy than the configuration where the multiplicity is 11.
The three molecules (H 2 O, CO and CO 2 ) were initially positioned at 2.0 Å above the surface and different orientations have been compared for each initial adsorption site. On the ZrO 2 (111) surfaces, four initial adsorption sites were tested for each molecule: on top of the Zr atom, above the outermost oxygen O u (up) or O d (down), and finally at the centre of the O u -O d -Zr "triangle", see Fig. 2. On the YSZ(111) surface, For each initial adsorption site, we have relaxed the molecules, as well as the five top layers of the slab (as described in the previous paragraph). On the Ni/ ZrO 2 (111) and Ni/YSZ(111) surfaces, we identified two initial adsorption sites: near the Ni atom and approximately over the most stable adsorption site found for the naked surface. The non-equivalent initial adsorption sites leading to unstable configurations are presented in the supporting information. We have calculated the adsorption energy, taking into account the zero point energy (ZPE), as the difference between the energy of the slab with the molecule adsorbed (E molecule/surface ) and the sum of the energies of the clean oxide surface (E surface ) and the isolated molecule (E molecule ), Eq. (1).
We have also calculated the Van der Waals adsorption energies as the difference between the estimated Van der Waals energy of the slab with the molecule adsorbed (E molecule/surface vdW ) and the sum of the estimated Van der Waals energies of the clean oxide surface (E surface vdW ) and We have analysed the charge transfer between the surface and the molecules using Bader analysis as implemented in the Henkelman algorithm [37]. To evaluate the interaction between the molecules and the surface atoms we have also calculated the electron localization function (ELF) which measures the probability of finding an electron in the neighbourhood of another electron with the same spin [38].

Results and discussion
3.1. Structural analysis and electronic structure The evaluation of the adsorption energies for the three molecules shows that the strength of their adsorption on the ZrO 2 (111) surface follows the trend: H 2 O N CO N CO 2 (see Table 1). H 2 O is likely to be dissociated, where one H is transferred to a surface oxygen with the OH remaining on top of the Zr atom (Fig. 3). The dissociative water adsorption on ZrO 2 was also observed in a previous investigation by Korhonen et al. [39], where they showed experimentally and theoretically that water, at low coverage, dissociates on the monoclinic zirconia surface. The most stable adsorption site found for CO 2 and CO molecules is on top of the zirconium atom (Fig. 3). The adsorption energy for CO 2 on ZrO 2 (111) (− 0.24 eV) is close to a previous study (− 0.26 eV) [ [40] showed that CO 2 is physisorbed at 2.70 Å away from the cluster and adopting a parallel position, comparable to our geometry (Fig. 3 [40] found a stronger CO 2 adsorption energy (− 0.34 and − 0.35 eV, respectively), but CO 2 is still physisorbed since for both clusters the CO 2 -cluster distance remains larger than 2.10 Å, indicating that the OH groups play a minor role in the CO 2 adsorption. On top of Zr 5 O 24 H 28 the CO 2 adsorption was much stronger (−0.67 eV) but the authors showed, using the multi-Langmuir model, that the Zr 5 O 24 H 28 site is less abundant than the three other sites. Thus, at high CO 2 loading, the authors [40] concluded that CO 2 adsorbs onto the Zr 2 O 14 H 20 ,    [41] (−0.14 eV), which in our model was evaluated to be −0.10 eV. Among the three molecules, dissociated H 2 O has a stronger affinity for the ZrO 2 (111) surface than CO 2 and CO. The variation of the interatomic distances is in good agreement with the adsorption energies and the transferred charges (see Table 1). For instance, [H + OH] is the closest species to the ZrO 2 (111) surface. Fig. 3 shows that O (of OH) is at 2.070 Å over the top zirconium atomic layer, while the closest atom, O for CO 2 and C for CO, are positioned at 3.009 Å and 2.500 Å, respectively, from the surface which, added to a lack of charge transfer, suggests that both molecules are physisorbed.

Ni/ZrO2(111)
On the Ni/ZrO 2 (111) surface, the strength of adsorption of the molecules near Ni is as follow: CO N H 2 O N CO 2 . Table 1 shows that, depending on the activation energies, H 2 O dissociation is practically in equilibrium with molecular adsorption since E ads (H 2 O)-Ni/ ZrO 2 (111)~E ads [H + OH]-Ni/ZrO 2 (111) (−1.39 and −1.33 eV respectively). The adsorption energy of CO 2 (−1.35 eV) is similar to H 2 O but considerably weaker than the one found for CO (− 2.70 eV), which will surround the single atom, replacing both CO 2 and H 2 O.
There is no obvious change in the H 2 O intra-molecular bond distances. However, the intra-molecular CO 2 bond length increases from 1.2 Å (gas phase) to 1.3 Å (CO 2 -Ni/ZrO 2 (111)) and the CO bond length increases from 1.1 Å to 1.2 Å (CO-Ni/ZrO 2 (111)). A previous study also reported these elongations of the intra-molecular distances upon interaction of the molecules with supported metal cluster on oxide surfaces [42]. Meanwhile, H 2 O remains practically unaltered, whereas CO 2 and CO indicate an electronic rearrangement which is not seen on the bare ZrO 2 (111). Charge analysis shows a gain of charge for CO 2 (− 0.5 e − ) and CO (−0.2 e − ) upon adsorption on the Ni/ZrO 2 (111). The outermost Zr atomic layer and the Ni atom lose +0.3 e − and +0.2 e − , respectively, while for CO-Ni/ZrO 2 (111) only the outermost Zr atomic layer transfers +0.2 e − , through the Ni adatom, to the CO molecule.
The electron localization function (ELF) plots in Fig. 4 show the effect of the Ni on the molecules' adsorption onto the surface. In the H 2 O-Ni/ ZrO 2 (111) case, the electron localization is negligible between H and the oxygen belonging to the surface, whereas the electrons localized on the oxygen atom of the water molecule interact favourably with the empty orbitals of the Ni atom, showing that Ni facilitates the adsorption of H 2 O on the Ni/ZrO 2 (111) surface. In Fig. 4 we note a bonding region between CO 2 and Ni, which is not observed in the CO 2 -ZrO 2 ( 111) system. This interaction explains the strengthening of the CO 2 adsorption over Ni/ZrO 2 (111) compared to the bare ZrO 2 (111) surface. The shape of the ELF around the carbon atom from CO changes in the presence of Ni: it points towards the Ni atom, showing the important effect of the metal atom.
In Fig. 5 we show the total density of states (DOS) of the three molecules (H 2 O, CO 2 and CO) adsorbed on ZrO 2 (111) and Ni/ZrO 2 (111) and the projected DOS on the three relevant atoms (C, O and H) and the Ni atom. A sharp peak (specific orbital) means that a specific orbital is located at this specific energy. Thus, when many orbitals of different atoms have the same energy (well localized) we consider that the interaction between those orbital atoms is favourable. On the other hand, the interaction is unfavourable if the peaks of the different orbital atoms are spread over a range of energies (delocalized).
We have shown, in the [H + OH]/ZrO 2 (111) density of states (DOS) (Fig. 5), that the peaks located below the Fermi energy correspond to the s and p orbitals of OH oxygen and are delocalized. However, in the H 2 O-Ni/ZrO 2 (111) system, they are localized and positioned at the same energy as the final two Ni s p d orbital peaks (− 0.26, − 0.20 and −0.01 eV). This favours the interaction between the Ni atom and the H 2 O oxygen orbitals which is in good agreement with the ELF analysis.
Similar situations occur with the CO 2 and CO molecules, i.e. their orbitals below the Fermi energy are dispersed over a range of energies when interacting with a naked surface, but are localized peaks in the presence of Ni atoms. The peaks at these energies correspond to Ni, O and C orbitals, indicating that an interaction exists between the molecule and Ni/ZrO 2 (111) and that this interaction is facilitated by the Ni atom, which confirms the ELF analysis discussed above.
We have shown that the adsorption of the molecules on ZrO 2 (111) and Ni/ZrO 2 (111) can lead to the dissociation of H 2 O, and causes the elongation of the intramolecular bonds of CO and CO 2 . In addition, the ELF plot indicates the significant effect of Ni on the electronic structure of the molecules, thereby enhancing their chemisorption. These results are in agreement with previous investigations, which have shown that metal particles can stabilize the adsorption of the molecule on the surface [43][44][45][46]. For example, Carrasco et al. [47] showed that CO prefers to bind to the Ni 1 /CeO 2 (111) surface, rather than on the pristine CeO 2 (111) surface, in a similar configuration as the one presented in Fig. 3.

YSZ(111)
We have followed the same procedure on the~9% yttria-doped ZrO 2 (111). The adsorption energies of these molecules on the YSZ(111) shows the following preference for binding: H 2 O N CO N CO 2 , see   Table 1. The CO 2 adsorption becomes~0.1 eV stronger compared with the pristine ZrO 2 (111). Both H 2 O and CO 2 adsorb near the yttrium atom in a slightly tilted position, whereas CO binds perpendicularly to the surface (Fig. 3). The CO adsorption energy is − 0.37 eV, i.e. very close to the one on ZrO 2 (111) and in good agreement with the DFT study by Yurkiv et al. (− 0.34 eV) [48].
The analysis of the interatomic distances from the structures in Fig. 3 agrees with the trend in adsorption energies, since H 2 O is stabilized by H bonds with the surface (H-O surf = 1.744 Å), and CO 2 and CO are not strongly bound. There is negligible variation in the intra-molecular distances or charge transfer for any of the three molecules. The electronic structure and the charge transfer compared with the pristine ZrO 2 (111) indicate that the interaction of CO 2 and CO molecules is barely modified by the presence of Ydopants. However, H 2 O prefers to adsorb molecularly on the YSZ(111) surface, whereas it dissociates upon adsorption on the ZrO 2 (111) surface.

Ni/YSZ(111)
Once Ni is deposited on YSZ(111), the trend in the adsorption of the molecules changes from the naked surface and the preference becomes: CO N H 2 O N CO 2 . As on YSZ(111), H 2 O adsorbs molecularly on Ni/ YSZ(111), although the difference in adsorption energy compared to the dissociated configuration is only 0.15 eV, indicating a close thermodynamic equilibrium (depending on the energy barrier to dissociation). H 2 O adsorbs near the Y and away from Ni, while CO 2 and CO prefer to adsorb near the Ni atom with adsorption energies of −0.31 and −2.32 eV respectively. Only CO has more affinity for the Ni/YSZ(111) surface than for YSZ(111), releasing almost 2 eV more than on the bare YSZ(111) surface. Thus, as a general trend, H 2 O and CO 2 adsorb on the bare surface, while CO is strongly attached to Ni/YSZ(111), see Table 1.
The distance analysis of the structures in Fig. 3 shows that H 2 O and CO 2 are at approximately 2.0 Å from the surface, while the C atom (of CO) is 1.696 Å from the Ni atom. There is no obvious increase in either H 2 O or CO 2 intra-molecular bond lengths. However, the CO bond increases by 0.1 Å upon adsorption, in agreement with the strong adsorption and the charge transfer of − 0.3 e − from the outermost zirconium atomic layer. As such, Ni acts as an electrode, which is its main function in SOFC: electrons are transferred from To investigate further the role of Ni, we have compared the ELF for the YSZ(111) and Ni/YSZ(111) systems. Fig. 4 shows that the perturbation of the ELF upon adsorption of H 2 O and CO 2 is negligible, which agrees with physisorption and the insignificant effect of Ni. However, the perturbation of the electrons surrounding the CO molecule is considerable in the presence of Ni.
In the H 2 O-Ni/YSZ(111) DOS (Fig. 6) we observe five localized peaks (− 0.89, − 0.85, − 0.67, − 0.12 and − 0.04 eV) below the Fermi level, corresponding to the Ni s p d orbitals. The contribution of the H 2 O orbitals to these peaks is negligible, indicating that H 2 O interacts weakly with both YSZ(111) and Ni/YSZ(111) surfaces, in good agreement with the low adsorption energies and the ELF analysis. The conclusion is similar for the CO 2 /YSZ(111) and CO 2 -Ni/YSZ(111) systems, where the DOS plots show that the CO 2 orbitals do not contribute to the valence band, in agreement with the weak adsorption and the ELF plots. However, the presence of Ni causes the CO orbitals to be localized at the same energy as the Ni orbitals below the Fermi level (− 0.72, − 0.62, and − 0.48 eV), leading to a stronger interaction. Thus, Ni stabilizes CO adsorption on the Ni/YSZ(111) surface, as was also observed from the ELF analysis.

Infrared spectra
We have calculated and analysed the IR spectra of the three molecules in the gas phase and compared them with the molecules adsorbed on the four surfaces (Fig. 7), which provides data that are directly comparable with simple experimental characterization, thus providing circumstantial evidence of the geometries and electronic structures described above.  Table 2 Theoretical vibrational frequencies (ω) in cm −1 for H 2 O, CO 2 and CO in the gas phase and adsorbed on the surfaces (ZrO 2 (111), Ni/ZrO 2 (111), YSZ(111) and Ni/YSZ(111). ν, ν s , ν as , and δ denote stretching, symmetrical stretching, asymmetrical stretching and bending modes, respectively. We show in parenthesis the infra-red experimental values from the literature. For example, the molecularly adsorbed water is confirmed by the presence of a bending mode around 1600 cm −1 , since there is no OH-related vibrational mode at 1600 cm −1 [49]. Similarly, analysing the IR spectra of adsorbed CO 2 and CO indicates a potential dissociative chemisorption of CO 2 , as was shown by previous studies where CO 2 is adsorbed on top of silica-supported iron [50]. In Table 2 (111), there are two different stretching modes, one each corresponding to ν(H\ \O surf ) and ν(H\ \O water ). In addition, Table 2 shows, that from these stretching modes ν(H\ \O surf ) has the lowest wavenumber, which is predictable since the H\ \O surf bond is weaker than H\ \O water . The wavenumber of the two stretching modes of H 2 O over Ni/ZrO 2 (111) are even lower than the ones found for ZrO 2 (111). This is in good agreement with the larger adsorption energy of H 2 O-Ni/ ZrO 2 (111): H 2 O binds more strongly to Ni/ZrO 2 (111) than ZrO 2 (111), which weakens its internal bonding by electronic rearrangement, explaining the decrease of the stretching modes of H 2 O-Ni/ZrO 2 (111).
The adsorption of CO 2 is weak on ZrO 2 (111), with the wavenumbers of the three different vibrational modes remaining similar to the gas phase (Table 2). However, it adsorbs strongly on Ni/ZrO 2 (111), lowering the wavenumbers of the asymmetrical and symmetrical stretching modes by 571 and 148 cm −1 , respectively. Indeed, the stronger adsorption of CO 2 on top of Ni/ZrO 2 (111) affects the strength of the CO 2 bonds, which explains the drop of the wavenumbers of the stretching modes observed in CO 2 -Ni/ZrO 2 (111). The bending mode of CO 2 -Ni/ZrO 2 (111) increases by 103 cm −1 compared to CO 2 -ZrO 2 (111). The reason for this increase is explained by the rearrangement of the electronic structure of CO 2 upon adsorption on Ni/ZrO 2 (111), where CO 2 gains a charge of 0.5 e − . As to CO, the difference in stretching mode upon adsorption on ZrO 2 (111) is negligible compared with the gas phase: Δν CO− ZrO 2 (111) = −3 cm −1 , confirming the physisorbed state, which is in good agreement with the experimental study made by Dulaurent et al. [53] (ν(CO) = 2185 cm −1 ). On Ni/ZrO 2 (111), the wavenumber of the stretching mode is even lower (Δν CO−Ni/ZrO 2 (111) = 131 cm −1 ), in good agreement with the strong adsorption energy and increase of the C\ \O distance.

YSZ(111) and Ni/YSZ(111)
Vibrational modes of H 2 O on YSZ(111) and Ni/YSZ(111) show a decrease in wavenumbers compared to the gas phase, although they remain higher than on ZrO 2 (111). The asymmetrical and symmetrical stretching and bending modes of H 2 O are located at lower wavenumber, when it is adsorbed on YSZ(111) and on Ni/YSZ(111) ( Table 2). This analysis agrees with the weakness of the intramolecular bonds on both doped surfaces, as described before. CO 2 adsorbs on both YSZ(111) and Ni/YSZ(111) surfaces, leading to a decrease in the wavenumber of each vibrational mode (Table 2). These shifts of the vibrational modes upon adsorption are related to the decreased strength of CO 2 bonds due to the interaction with the surfaces and the occupation of the molecular antibonding orbitals leading to a bent structure. Our calculated value for the symmetrical stretching mode of CO 2 on YSZ(111) (1317 cm −1 ) is in the same range as the experimental study made by Köck et al. [54], who found a peak of a bridged carbonate at 1305 cm −1 . The adsorption energies of CO-YSZ(111) and CO-Ni/YSZ(111) agree with the CO affinity for these two surfaces. For example, on Ni/YSZ(111) the stretching mode of the CO molecule is observed at significant lower wavenumber than in the gas phase (Table 2), having decreased by 135 cm −1 . The increase in the length of the CO bond upon adsorption and its weakening further shows the strong interaction between CO and the bare and Ni-decorated YSZ surfaces.
To summarize the general trend of the vibrational modes of the molecules upon adsorption, we have plotted in Fig. 8 the variation of the stretching modes as a function of the adsorption energies. Note that the H 2 O dissociation is not represented here. We have observed that, Fig. 8. Plot of the variation of the symmetric (ν s ) and asymmetric (ν as ) stretching modes upon adsorption. •, ■, ▲ and ♦ represent ZrO 2 (111), Ni/ZrO 2 (111), YSZ(111) and Ni/YSZ(111) surfaces, respectively. E ads corresponds to the total adsorption energy. Since H 2 O adsorbs dissociatively on ZrO 2 (111) the variation of H 2 O stretching mode on ZrO 2 (111) is not represented here. ν as is not represented for CO as in the gas phase CO has one stretching mode. when the molecules adsorb weakly onto the surface, the shift in the wavenumber of the stretching mode is negligible and the moleculesurface interaction is mainly due to van der Waals interactions. In Table 3 we compare E ads with (E ads − E ads vdW ), where E ads and E ads vdW correspond to the total and Van der Waals adsorption energies, respectively. We observe from Table 3 that for the weak adsorptions, such as CO 2 -ZrO 2 (111) and CO 2 -Ni/YSZ(111), (E ads − E ads vdW ) is approximately equal to zero, indicating that the molecule-surface interaction arises mainly from the Van der Waals interactions. For other systems, such as H 2 O-Ni/YSZ(111), CO 2 -YSZ(111) and CO-ZrO 2 (111), the contribution of the Van der Waals interactions is still non-negligible since (E ads − E ads vdW ) is approximately twice as high as E ads . For the strong interactions (H 2 O-Ni/ZrO 2 (111), CO 2 -Ni/ZrO 2 (111), CO-Ni/ZrO 2 (111) and CO-Ni/YSZ(111)), the shift of the stretching mode is considerable (Fig. 8) and the contribution of the Van der Waals interactions is negligible, since the negative value of (E ads − E ads vdW ) is lower than −1 eV (Table 3).

Conclusion
We have studied systematically the interaction of H 2 O, CO 2 and CO on the naked and Ni-decorated ZrO 2 (111) and YSZ(111) surfaces, where we have employed DFT with long-range interaction corrections to characterize the bonding character of these molecule-metal-support interfaces. We found that thermodynamically H 2 O adsorbs dissociatively on ZrO 2 (111), but in the presence of Ni both dissociated and molecular H 2 O can coexist. On the other hand, molecular water is likely to be present on YSZ(111) and Ni/YSZ(111), depending on the experimental conditions. Regardless of which surface is considered, the charge transfer is negligible upon H 2 O adsorption, except for the dissociated state. CO 2 shows very weak adsorption on the surfaces studied with the exception of Ni/ZrO 2 (111), where the molecule is activated by the transfer of 0.5 e − . Here, the metal is slightly more oxidized leading to a decrease in the stretching modes of CO 2 . The CO adsorption depends strongly on the presence of Ni at the surface; On the Ni-decorated surfaces, CO adsorbs more strongly than H 2 O and CO 2 , while on the naked surfaces CO binds mainly by long-distance interactions. The interatomic distance analysis agrees with the trend in adsorption energies, since on Ni-decorated surfaces the CO-surface distance is smaller than both H 2 O-surface and CO 2 -surface distances. Thus, it appears that the role of Ni at the TPB is to provide electrons to CO with a subsequent shift in the IR spectrum. We have also explored the relationship between the binding energy and the variation in the stretching modes where we noted a negligible shift in the wavenumber of the stretching mode for the molecules interacting weakly with the surface.
Clearly, the incorporation of a single nickel atom is an approximate model for the triple-phase boundary, and our results may change if the metal-oxide systems were to include finite nickel clusters or even nanoparticles. In future work, we will therefore investigate how Ni clusters of increasing sizes affect the structure and properties of the interface with the zirconia materials, and if the size of the cluster will affect the interaction of the metal clusters with the gaseous molecules.