Surfaces of Ceria - ACS Publications - American Chemical Society

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Interactions of Water with the (111) and (100) Surfaces of Ceria Thomas Kropp,† Joachim Paier, and Joachim Sauer* Institut für Chemie, Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany S Supporting Information *

ABSTRACT: This density functional theory study compares water adsorption on the ceria (111) and (100) surfaces. In the absence of water, the (111) surface is more stable than the polar (100) surface, but at higher water coverages the (100) surface is favored due to the formation of a highly stable “square ice” layer on top of the fully hydroxylated surface. At ceria (111), an amorphous ice layer is favored that consists predominantly of molecularly adsorbed water. This change in the water−surface interactions stems from the different surface terminations: the lower coordination numbers of ions at the (100) surface lead to higher water adsorption energies and a preference toward dissociative adsorption. Furthermore, the distance between neighboring surface oxygen ions (274 pm) is very close to the O−O distance in bulk ice, which facilitates the growth of a well-ordered ice layer. At the (111) surface, the distance between neighboring surface oxygen ions is significantly larger (388 pm), which hampers water−water interactions in the surface layer.

1. INTRODUCTION Because of its high oxygen storage capacity and low oxygen defect formation energy, ceria (CeO2) is an important oxide in heterogeneous catalysis with several applications such as the water−gas shift reaction, oxidative dehydrogenation of alcohols, and exhaust catalysts.1 These reactions involve water as a product, as a reagent, or as a solvent. Both theory2,3 and experiment4−6 relate different catalytic properties to different surface terminations.7,8 Hence, it is critical to understand the interactions of water with different ceria surfaces. The relative stability of the different surface terminations in the presence of water will also determine the shape of ceria nanoparticles obtained via hydrothermal synthesis.9,10 Here, we use density functional theory (DFT) and focus on a comparison of the most stable (111) with the least stable (100) surface. The latter is polar, and replacing the surface O2− ions by OH− ions is a common stabilization mechanism,11 which will significantly affect the water−surface interaction. Surface energies in the presence of water have not yet been reported. Experimentally, water adsorption at ceria surfaces has been studied using X-ray photoelectron spectroscopy (XPS),12−15 temperature-programmed desorption (TPD),12−16 scanning force microscopy (SFM),17 and nuclear magnetic resonance (NMR) spectroscopy.18 While these experiments refer to water loadings of about one monolayer (ML) and below, so far most computational studies19−24 considered monomeric water adsorption species. Only recently, monolayer structures were suggested for the (100)18 and (111)25 surfaces. Previous DFT studies employed PW91,20 PW91+U,19 PBE,21 PBE+U,22−24 and dispersion-corrected functionals,23 but doubts have been raised23 whether all reported structures are local minima on the potential energy surface (PES). Parker and co-workers24 simulated high water coverage by adding © XXXX American Chemical Society

monomers to small unit cells, which puts artificial strain on the structures; oligomeric adsorption species were not considered. They concluded that energies for molecular and dissociative adsorption on the (100) surface are similar at high coverage, but on our larger surface models dissociate adsorption is strongly favored at monolayer coverage. The predicted water desorption temperatures for their adsorption structures24 do not explain the two water desorption peaks observed for the ceria (111) surface (e.g., ref 13) which has been ascribed7 to an incomplete exploration of the PES of water on ceria surfaces under the constraints of small unit cells. As previously reported adsorption structures for low19−24 and high18,25 water coverage were obtained using different density functionals which should not be compared directly, we performed a comparative study of water adsorption on the (111) and (100) surfaces of ceria using a consistent methodology. Our p(4 × 4) and c(2 × 2) cells are large enough to consistently describe low- and high-coverage cases without geometric constraints for the high-coverage case and without artificial interactions for the low-coverage case. In addition to the monomeric water adsorption structures, this work provides new oligomeric and monolayer adsorption structures. We conclude that an amorphous water monolayer is formed on the (111) surface, which mostly consists of nondissociated water molecules, whereas on the fully hydroxylated (100) surface, water forms a well-ordered ice monolayer. Calculated surface energies indicate that (100) facets become more stable than (111) facets for high chemical Received: August 15, 2017 Revised: September 10, 2017

A

DOI: 10.1021/acs.jpcc.7b08150 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

correlation functional by Perdew, Burke, and Ernzerhof (PBE)26 and an effective Hubbard-type U parameter of 4.5 eV for the Ce 4f electrons, i.e., PBE+U.41 The specific implementation of DFT+U used in this work follows Dudarev et al.42,43 A plane wave kinetic energy cutoff of 600 eV is used, and structure optimizations are performed until forces acting on the relaxed atoms are below 0.02 eV Å−1. Calculations with the hybrid functional by Heyd, Scuseria, and Ernzerhof (HSE)27 use identical settings except for structure optimizations, which employ the PRECFOCK = fast Fourier grid for Fock-exchange-related routines. HSE energies reported in this work are single-point energies using the dense PRECFOCK = normal Fourier grid. Dipole moments perpendicular to the surface are corrected with the approach presented in ref 44. For the (2 × 2) surface unit cells, integrals over k space were computed at 2 × 2 × 1 k points. The Brillouin zone sampling at the larger p(4 × 4) surface cell is restricted to the Γ point. Increasing the number of k points alters total energies by less than 0.01 eV. The semiempirical C6/R6 term parametrized by Grimme (DFT+D2) is added to total energies and gradients to correct for missing long-range dispersion-type interactions.45,46 The global scaling parameter s6 used within the Grimme approach is set to 0.75 for PBE+U and 0.6 for HSE; HSE uses the value originally obtained for PBE0.45 van der Waals parameters for Ce were derived and systematically tested in ref 47 (cf. SI of that reference).

potentials of water. Water desorption peaks observed in TPD experiments are assigned to water dimers and monolayers. We employ the widely used Perdew−Burke−Ernzerhof (PBE)26 functional augmented with an effective on-site Coulomb correlation term (PBE+U) under periodic boundary conditions and compare it with the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE).27,28 The surface models adopted are converged regarding the number of atomic layers and have been thoroughly tested; see refs 3 and 29.

2. SURFACE MODELS In the absence of water, density functional theory (DFT) predicts an increase in surface energies following (111) < (110) < (100).30−32 The ceria (111) surface consists of O−Ce−O trilayers and is oxygen terminated, whereas ceria (100) consists of alternating Ce and O layers, making it a polar surface.33 For the (100) surface, DFT predicts that an oxygen termination is favored, where only 50% of the surface lattice positions are occupied (red in Figure 1).30 Ions in the topmost layers of the (100) surface have lower coordination numbers, i.e., 6 (Ce) and 2 (O), than the ions in the (111) surface, i.e., 7 (Ce) and 3 (O).

4. RESULTS AND DISCUSSION 4.1. (111) Surface. Low water coverage of the ceria (111) surface (θ = 1/16 or 0.5 nm−2) is studied for the p(4 × 4) slab model, referred to as P1110; the letter P refers to a pristine (nondefective) surface. For n adsorbed water molecules we use the nomenclature P111n. A postpositioned letter m (or d) indicates molecular (or dissociative) adsorption; lower case acronyms such as p111n refer to a smaller p(2 × 2) model of the ceria surface with n adsorbed water molecules. Adsorption energies per molecule are defined as

Figure 1. Top views on the (a) (111) and (b) (100) surfaces of ceria using the following color code: Ce, blue; O, red. Subsurface ions are shown in grayscale. VESTA was used for visualizing structure models.34

The surface unit cells were generated by cutting bulk CeO2 with a lattice constant of a0 = 549 pm in (111) and (100) orientation, respectively. Structural parameters for the p(2 × 2) CeO2(111), the p(4 × 4) CeO2(111), and the c(2 × 2) CeO2(100) slab model are compiled in Table 1. These models are extensively discussed in refs 3 and 29.

Ea = 1/n(E(CeO2 ·nH 2O) − E(CeO2 ) − nE(H 2O))

and incremental adsorption energies are defined as ΔEa(n − 1) = E(CeO2 ·nH 2O) − E(CeO2 ·(n − 1)H 2O)

Table 1. Structure Parameters of the Three Slab Models Studied in This Work CeO2(111) atomic layers m (CeO2)a n (Ce4+)b surface area [nm2] vacuum layer [nm]

− E(H 2O)

CeO2(100)

p(2 × 2)

p(4 × 4)

c(2 × 2)

9 12 4 0.52 1.1

9 48 16 2.09 1.1

11 40 8 1.21 1.3

(1)

(2)

At the pristine (111) surface, P1110, water may adsorb molecularly (P1111m) atop a Ce ion with a Ce−O distance of 259 pm (Figure 2a). The molecule is tilted toward a surface O ion forming a H bond; structural parameters are given in the Supporting Information (SI). Molecular adsorption is moderately exothermic (−51 kJ/mol), and dissociative adsorption (P1111d, Figure 2b) is slightly favored by 6 kJ/mol. Upon protonation, the surface oxygen ion is lifted by 30 pm. For the water monomers, PBE+U water adsorption energies agree within 5 kJ/mol with the HSE values (Table 2). The water adsorption structures in refs 19 and 24 were obtained using PW91+U. The corresponding adsorption energies are within 13 kJ/mol of the values obtained using PBE+U (Table 2). PW91 and PBE are expected to yield very similar results, but other technical parameters of the calculations, e.g., the choice of the U value, may also contribute to the difference. Dispersion corrections increase the adsorption energies by 11− 14 kJ/mol but do not change adsorption structures

a

m is the number of CeO2 units per unit cell. bn is the number of cations in the surface layer.

3. COMPUTATIONAL DETAILS Calculations are performed using the projector augmented wave method (PAW)35,36 as implemented in the Vienna ab initio simulation package (VASP).37,38 The onsite Coulomb correlation of occupied f orbitals is corrected via the DFT+U approach39,40 employing the generalized gradient exchangeB


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PBE+U and PBE+U+D2, while HSE (+D2) predicts dissociative adsorption to be 9 (11) kJ/mol favored. Two water molecules at the (111) surface (coverage θ = 1.0 nm−2) form a dimer P1112, with one dissociatively and one molecularly adsorbed molecule as shown in Figure 2c. The adsorption energy is −66 kJ/mol per water molecule or −74 kJ/mol relative to P1111d. Highly stable dimer-type structures have also been reported at the Fe 3 O 4 (111) 48,49 and ZnO(101̅0)50 surfaces. An alternate adsorption structure P1112d with two dissociatively adsorbed water molecules is 17 kJ/mol less stable than the dimer P1112. While P1112 is always preferred over P1111d, it is less stable than P1111m at θ > 1/2, as dissociative adsorption is destabilized at high coverage (Figure 3). The adsorption energy for P1111m remains constant for θ ∈ [0, 1] due to the lack of lateral interactions between water molecules (Figure S1).

Figure 2. Top views on the water monomers (a) P1111d and (b) P1111m and the water oligomers (c) P1112 and (d) p1114 at the pristine ceria (111) surface using the following color code: H, white; O, red. Ce ions are represented by blue polygons; each vertex represents a surface O ion (surface ions participating in H bonds are shown explicitly). Length of H bonds (black line) is given in pm. Complete surface models of P1112 and p1114 are shown in the SI. Figure 3. Adsorption energies for molecular (black) and dissociative (red) water adsorption as well as water dimers (blue) on CeO2(111) at different water coverage. Linear fits are shown as dashed lines. Corresponding adsorption energies are given in Table 2.

significantly. This is shown for P1111d in Table S1 of the SI. Thus, calculating dispersion corrections using the PBE+U structure without further optimization is justified. Because dispersion corrections do not change the results qualitatively, we will use PBE+U without dispersion corrections in the following sections. For the (1 × 1) cell, molecular adsorption is 30 kJ/mol more stable than dissociative adsorption (PBE+U), which is in good agreement with the results of Parker and co-workers.24 For (2 × 2) and (2 × 3) cells, energies for molecular and dissociative adsorption are within 2 kJ/mol,23,24 whereas dissociative adsorption is favored by 5 kJ/mol for a (3 × 3) cell.23 For a (4 × 4) cell, we get a very similar difference (6 kJ/mol) with

Typically, water monolayers are assumed to contain one water molecule per surface cation. However, adding four water molecules to the p(2 × 2) slab model (coverage θ = 7.7 nm−2) yields the water tetramer p1114 (lower case letters indicate smaller surface cell), which does not form H bonds with its periodically repeated image (Figure S2). This indicates that water−water interactions are stronger than water−surface interactions. Only one water molecule of p1114 dissociates upon adsorption, and only two OHX moieties bind on top of

Table 2. Water Adsorption Energies Ea [kJ/mol] for the Monomeric Structures Shown in Figure 2a cell P1111m

P1111d

P1112

P1112d a

(1 (2 (2 (3 (4 (1 (2 (2 (3 (4 (2 (3 (4 (4

× × × × × × × × × × × × × ×

PW91+U 1) 2) 3) 3) 4) 1) 2) 3) 3) 4) 2) 3) 4) 4)

−55b −58b −56b

−14b −63d −57b

PBE+U (+D2) −52 −52c −53c −51 (−64) −22 −50c −58c −57 (−70) −53 −62 −66 −49

HSE (+D2) P1001m −47c

cell

PW91+U

PBE+U

(1 × 1) (2 × 2)

−86b −96b

−91

(1 × 1) (2 × 2)

−86b −151b

−159

−52 (−64) P1001d −45c

−61 (−75)

Values including dispersion corrections (single-point calculations) are given in parentheses. bReference 24. cReference 23. dReference 19. C


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Figure 4. Side and top views on water monolayers at the pristine ceria (111) and (100) surfaces, i.e., p1117 and P10024. For the (111) surface (a), O atoms of water molecules (on top of Ce ions) are (dark) red. For the (100) surface (b), surface OH groups are dark red and water O atoms are red. For the top views, Ce ions are represented by blue polygons; each vertex represents a surface O ion.

Ce ions (blue polygons in Figure 2d). The tetramer contains five H bonds leading to an adsorption energy of −53 kJ/mol per water molecule. An alternative adsorption structure p1114d with four dissociatively adsorbed water moieties was found to be significantly less stable with an adsorption energy of only −22 kJ/mol per water molecule. This adsorption structure corresponds to the dissociatively adsorbed water molecule at the p(1 × 1) surface cell (−14 kJ/mol in ref 24). Adding seven water molecules to the p(2 × 2) surface cell, which contains four Ce4+ ions, yields the water monolayer p1117 (Figure 4) with an adsorption energy of −55 kJ/mol per water molecule. The energy required to remove a single water molecule from p1117 amounts to 50 kJ/mol. Additional water molecules bind atop this layer. Thus, monolayer coverage corresponds to θ = 13.4 nm−2 or 1.75 water molecules per surface Ce ion. Each surface Ce ion (blue polygons in Figure 4a) is coordinated to exactly one OHn moiety (dark red oxygen ions in Figure 4a). These water molecules bind ca. 200 pm above the ceria surface; only 25% of them dissociate. The other water molecules (red in Figure 4a) connect these surface-bonded OHn moieties with H bonds forming multiple five-membered rings. Upon removing these connecting water molecules the remaining water molecules relax and form a tetramer similar to p1114 but 4 kJ/mol per water molecule less stable. The water monolayer at the ceria (111) surface (p1117) consists of 6 molecularly and 1 dissociatively adsorbed water molecule (∼14% dissociation). This is in agreement with Parker and co-workers,24 who predicted that molecular adsorption is favored at high water coverage based on their results for the p(1 × 1) surface cell. However, monolayer coverage significantly exceeds the one water per surface cation ratio that is commonly assumed. This is caused by the large cation−cation distance (388 pm) in this surface; the average O−O distance in the water monolayer is only 267 pm. In ice Ih, the average O−O distance amounts to 288 pm (PBE).51 4.2. (100) Surface. Water may adsorb molecularly into one of the empty lattice positions in the surface layer with an adsorption energy of −91 kJ/mol. This leads to the adsorption structure P1001m, which contains a H bond to a neighboring surface oxygen ion. Dissociative adsorption is highly favored with an adsorption energy of −159 kJ/mol; the resulting P1001d is depicted in Figure 5. P1001d does not contain H bonds, and the hydroxyl groups are tilted toward the surface (Figure 5b). Our PBE+U adsorption energies (Table 2) are in good agreement with the PW91+U values reported for a p(2 × 2) surface cell in ref 24 (−96 and −151 kJ/mol, respectively).

Figure 5. (a and b) Top and (c and d) side views on dissociative water adsorption structures at the ceria (100) surface. O ions originating from water molecules are red, surface O ions are dark red, and protons are black. Top views show surface ions only.

In contrast to the (111) surface, two water molecules do not form a dimer at the (100) surface. Instead, both adsorb dissociatively (P1002d) with an adsorption energy of −148 kJ/ mol per water molecule or −137 kJ/mol relative to P1001d. The dimer similar to P1112 (one dissociatively and one molecularly adsorbed molecule) was found to be 132 kJ/mol less stable. Upon depositing 8 water molecules at the c(2 × 2) surface model, all previously empty lattice positions are occupied with hydroxyl groups, and all surface oxygen ions are protonated forming a (1 × 1) overlayer (P1008d in Figure 5). The OH groups are tilted into the surface and form a network of H bonds (227 pm). The adsorption energy is −130 kJ/mol per water molecule, which is significantly lower than the adsorption energy for the first water molecule (P1001d, −159 kJ/mol). The desorption energy for a single water molecule of P1008 amounts to 110 kJ/mol. For a p(1 × 1) surface cell, Parker and coworkers24 report an adsorption energy of −86 kJ/mol for both adsorption modes. This small cell has the same water coverage as P1008d, but the adsorption energy of P1008d is significantly more exothermic (−130 kJ/mol). The p(1 × 1) surface cell is too small to reproduce the H-bond network of P1008d (Figure 5) that forms a c(1 × 1) overlayer. Further water molecules adsorb molecularly atop this fully hydroxylated surface. One particularly stable structure is obtained at θ = 13.3 nm−2, i.e., P10016. The additional 8 water molecules form a stripe atop the hydroxylated surface that consists of interconnected four-membered rings. The adsorption energy is −97 kJ/mol per water molecule or −64 D


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where R is the gas constant and qrot and qtrans are the partition functions for rotations and translations, respectively. Figure 6 shows the surface energy as a function of the chemical potential of water. For small values of Δμwater, the

kJ/mol per water molecule relative to P1008, which is comparable to the sublimation energy of ice Ih, i.e., −65 kJ/ mol (PBE).51 The energy required to remove a single water molecule of P10016 amounts to 80 kJ/mol. 1 H NMR spectra of CeO2(100) exposed to water indicate that molecular water does not fully desorb below 298 K under mild vacuum.18 The presence of molecular water greatly increases the number of crystallographically nonequivalent H atoms (Table 3), which could explain the large number of peaks in the 1H NMR spectra of ref 18. Table 3. Water Adsorption Energies [kJ/mol] and Number of Crystallographically Nonequivalent H Atoms nH for Water Adsorbed on CeO2(100) P1001d P1002d P1008 P10016 P10024 a

nH

−ΔEa(n − 1)

−ΔEa

2 2 1 8 6

159 137 110 80 53

159 148 130 97 (64a) 82 (58b)

Figure 6. Surface energies (PBE+U) of different water adsorption structures as a function of the chemical potential of water. Dashed lines refer to the (100) surface, whereas solid lines refer to the (111) surface. Using eq 4, the chemical potential has been translated into the temperature at 10−5 bar.

Difference P10016 − P1008. bDifference P10024 − P10016.

By adding another 8 water molecules to P10016, a second stripe may form, completing a water monolayer on top of the fully hydroxylated surface (P10024; cf. Figure 4b). The adsorption energy is −82 kJ/mol per water molecule, and the desorption energy for a single water molecule of P10024 amounts to 53 kJ/mol, which is comparable to the desorption energy at p1117. This two-dimensional “square ice” monolayer represents a (1 × 1) overlayer and translates into a water coverage of θ = 19.9 nm−2 (θ = 13.3 nm−2 for the second layer). Every other surface OH group rotates toward this monolayer to form H bonds (169 pm), and all oxygen ions of P10024 are coordinated by 4 Ce cations or protons forming highly distorted tetrahedra. Such “square ice” monolayers have been recently observed in graphene nanocapillaries,52 and the authors predict that they can be formed in any hydrophobic nanochannel. In contrast, the ceria (100) surface enables this type of ice through a template effect: surface OH groups form H bonds to the water molecules, and the O−O distance in the (100) surface (274 pm) is very close to the lattice constant of the square ice reported in ref 52 (283 pm). The importance of the lattice match between ice and the surface has been reviewed in refs 53 and 54. For example, a good lattice match may lead to higher water adsorption energies53 and to the formation of well-ordered nanostructures, which have been observed for α-Al2O3(0001),55 CaO(100),56 Cu(110),57 MgO(100),58 and TiO2(110).59 4.3. Surface Energies in the Presence of Water. For the structures considered in this work, the surface energies γ = A−1[Eslab − mE bulk − nEwater − nΔμwater (T , p)]

pristine ceria (111) surface (P1110) is the most stable surface termination as previously reported in refs 30−32 for water-free surfaces (black lines in Figure 6), but the hydroxylated (100) surface (P10016) becomes more stable at Δμ(H2O) = −63 kJ/ mol (ca. 240 K at 10−5 bar following eq 4). This is due to polarity compensation at the (100) surface upon hydroxylation (see, e.g., ref 11). For chemical potentials larger than Δμ(H2O) = −58 kJ/mol (ca. 220 K at 10−5 bar), P10024 is the most stable structure. On the (111) surface, with increasing chemical potentials of water, first the dimer structure P1112 appears as most stable one. Then the amorphous water monolayer p1117 follows, which is more stable than the well-ordered monolayer proposed in ref 25, p1118 following the naming convention of this paper (pink in Figure 6). However, p1117 is less stable than P10016 over a wide range of values for Δμ(H2O). Both p1117 and P10016 (full and broken red lines in Figure 6, respectively) have similar water coverage (θ ≈ 13.3 nm−2). Thus, hydroxylated ceria (100) facets are stable in the presence of water even though the water-free (100) surface is significantly less stable than the pristine (111) surface. Using force fields, Sayle and co-workers61 modeled ceria nanoparticles submersed in water. According to their models, water interacts more strongly with the (110) and (100) surfaces than with the (111) surface, thus stabilizing the corresponding facets. However, they do not report surface energies in the presence of water. 4.4. Comparison with TPD Experiments. Peden and coworkers13 report water desorption at 265 K for ceria (111) films at low water coverage. At higher coverage, water desorbs already at 195 K, and the TPD spectrum has a shoulder at 265 K. This shoulder is more pronounced for partially reduced ceria films. Mullins et al.14 confirmed a peak at 200 K, and based on XPS results obtained at different temperatures,14 this desorption peak is assigned to a molecular adsorption species. For the two TPD peaks observed for CeO2(111),13,14 Campbell and Sellers16 derived desorption enthalpies of 57 and 80 kJ/mol (Table 4).

(3)

depend on the chemical potential μ of water, which is referenced to the electronic energy Ewater. Here, A is the surface area, m is the number of CeO2 units in the slab model, and n is the number of adsorbed water molecules. Vibrational contributions as well as volume changes of the solid are neglected, whereas pressure and temperature effects are incorporated in Δμ (e.g., ref 60) via Δμwater (T , p) = RT − RT ln(qrot ·qtrans)

(4) E


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The Journal of Physical Chemistry C Table 4. Assignment of TPD Peaks at Tdes [K] Obtained with a Heating Rate of β [K/s]a surface

β

Tdes

Hd

(111)

1 2 1 2 2 2 2

195b 200c 265b 200c 300c 400c 460c

57d 58e 80d 58e 87e 117e 134e

(100)

assignment

−ΔEa(n − 1)

−Ea

H2O monolayer (p1117)

50

55

H2O dimer (P1112) H2O monolayer (P10024) H2O stripe (P10016) OH monolayer (P1008d) OH monomer (P1001d)

74 53 80 110 159

66 58f 64g 130 159

Ed 46 47 69 47 76 106 123

± ± ± ± ± ± ±

8 8 8 8 8 8 8

a Desorption enthalpies, Hd, estimated desorption energies, Ed, as well as calculated incremental, ΔEa(n − 1), and average adsorption energies per water molecule, Ea, all in kJ/mol. bReference 13. cReference 14. dEnthalpies calculated in ref 16 using the Redhead equation. eEnthalpies obtained using the Redhead equation with the pre-exponential factors obtained in ref 16. fDifference P10024 − P10016. gDifference P10016 − P1008.

the O−O distance in ice Ih (288 pm), the hydroxylated surface acts as a template facilitating the growth of a square ice monolayer of H2O molecules. The predicted binding of H2O molecules in this monolayer onto the fully hydroxylated (100) surface is about as strong (58 kJ/mol) as the binding predicted for the molecules in a H2O monolayer on the pristine (111) surface (55 kJ/mol), which explains that TPD peaks are observed at about the same temperature for the (100) and (111) surfaces, 20014 and 195− 200 K,13,14 respectively. The TPD peak at 265 K observed for water on the (111) surface is assigned to water dimers.

On the (100) surface, water desorption is observed at 460 K at low coverage.14 This peak shifts to 400 K as the coverage increases, and two additional desorption peaks appear at 200 and 300 K. Furthermore, XPS14 and 1H NMR18 experiments indicate a high percentage of dissociated species above 298 K. Using the Redhead equation with the pre-exponential factor obtained in ref 16 (υ ≈ 3 × 1014), from the desorption temperatures of 200, 300, 400, and 460 K we obtain desorption enthalpies of 58, 87, 117, and 134 kJ/mol, respectively. Since pre-exponential and desorption energies are difficult to determine independently, we assume an uncertainty of ±6 kJ/mol for these values which corresponds to a change of the pre-exponential of 1 order of magnitude. To assign the peaks, we first convert these desorption enthalpies into desorption energies by subtracting 11 ± 2 kJ/ mol to account for changes in zero-point vibrational energies and thermal enthalpy contributions (see, e.g., ref 60). Comparison is made with two types of calculated adsorption energies, the average energy per water molecule, Ea, and the incremental value for the adsorption of the last molecule in a given structure, ΔEa(n − 1). Since we cannot expect quantitative agreement between experimentally derived values and DFT results, our assignment is based on relative values. For the (111) surface, we assign the desorption peak at 265 K (estimated Ed = 69 ± 8 kJ/mol) to the P1112 dimer structure (74 kJ/mol) and the peak at 195 K (estimated Ed = 46 ± 8 kJ/ mol) to desorption from the p1117 monolayer (50 kJ/mol). This monolayer structure contains predominantly water molecules, which is in agreement with the XPS spectra at high coverage.14 On the hydroxylated (100) surface, desorption from the square ice water monolayer of P10024 occurs at about the same temperature (peak at 200 K, 47 ± 8 kJ/mol), whereas the peak at 300 K (76 ± 8 kJ/mol) is assigned to desorption from the interconnected four-membered rings of P10016. The peak at 400 K (106 ± 8 kJ/mol) is assigned to desorption of dissociated species (Table 4) from the fully hydroxylated surface P1008d. As the coverage decreases, the desorption energy increases to 123 ± 8 kJ/mol (P1001d).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08150. Structure parameters for monomeric water adsorption structures; top views on P1112, p1114, and p1111m; table of total energies; optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas Kropp: 0000-0002-9166-566X Joachim Sauer: 0000-0001-6798-6212 Present Address †

T.K.: Department of Chemical and Biological Engineering, University of WisconsinMadison, 1415 Engineering Drive, Madison, Wisconsin 53706-1607, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for computing time at the highperformance computer centers HLRN (North-German Supercomputing Alliance in Berlin and Hannover). We acknowledge support from the German Research Foundation (CRC 1109), COST action CM1104, and the “Fonds der Chemischen Industrie”. We thank Prof. Núria López for discussing her monolayer model p1118 with us. J.P. is grateful for financial support by the Humboldt-Innovation GmbH.

5. CONCLUSIONS While the pristine CeO2(111) surface has a lower surface energy than the polar (100) surface, the hydrated (100) surface is the most stable surface for higher chemical potentials of water. Due to the coordinative unsaturation of the (100) surface, dissociative adsorption is favored and a fully hydroxylated (100) surface is formed. This OH surface layer likely causes the observed water TPD peaks at 400−460 K.14 Since the O−O distance at the surface (274 pm) is very close to

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REFERENCES

(1) Trovarelli, A. Catalysis by ceria and related materials; Imperial College Press: London, 2002.

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Surfaces of Ceria - ACS Publications - American Chemical Society

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