Theoretical Investigation of the Structural Stabilities of Ceria Surfaces

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

pubs.acs.org/JPCC

Theoretical Investigation of the Structural Stabilities of Ceria Surfaces and Supported Metal Nanocluster in Vapor and Aqueous Phases Zhibo Ren,†,‡ Ning Liu,†,‡ Biaohua Chen,† Jianwei Li,†,* and Donghai Mei‡,* †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

‡

S Supporting Information *

ABSTRACT: In the present work, the stabilities of three low-index ceria (CeO2) surfaces, that is, (111), (110), and (100) in vapor and aqueous phases were studied using ab initio molecular dynamics (AIMD) simulations and density functional theory calculations. On the basis of the calculated Gibbs surface free energies, the morphology and exposed surface structures of the CeO2 nanoparticle were predicted using the Wulff construction principle. It is found that the partially hydroxylated (111) and (100) are two major surface structures of the CeO2 nanoparticle in the vapor phase at ambient temperature. As the temperature increases, the fully dehydrated (111) surface becomes the most dominant structure. However, in the aqueous phase, the exposed surface of the CeO2 nanoparticle is dominated by the hydroxylated (110) structure. The morphology and stability of a cuboctahedron Pt13 nanocluster supported on CeO2 surfaces in both gas and aqueous phases were further investigated. Because of the strong metal−support interaction, AIMD simulations show that the supported Pt13 nanocluster has the tendency to wet the CeO2 surface in the gas phase. The calculated interaction energies suggest that the CeO2(110) surface provides the best stability for the Pt13 nanocluster. The CeO2-supported Pt13 nanoclusters are oxidized. The morphology of the CeO2-supported Pt13 nanocluster is less distorted because of the solvation effect in the aqueous phase. Compared with the gas phase, more electrons are transferred from the Pt13 nanocluster to the CeO2 support, implying the supported Pt13 nanocluster is further oxidized in the aqueous phase.

1. INTRODUCTION

for metal catalysts could be modified because of the surface hydroxylation and hydration to some extent. As a result, the catalytic activity and selectivity of some structure-dependent reactions over CeO2-based catalysts will be changed.2 The catalytic performance is also influenced by the nature of the exposed facets and their surface reconstruction under reaction conditions with different water concentrations. For example, hydrogenation takes place preferentially on the CeO2(111) facet, whereas oxidation reactions are favored on the more open CeO2(110) and CeO2(100) facets.3 Therefore, it is critical to understand the interaction of water with CeO2 surface

Ceria (CeO2) has been widely used as the catalysts or support materials in many heterogeneous catalysis applications such as steam reforming of hydrocarbons for hydrogen production, water−gas shift, automobile exhaust emission, and thermal condensation of biomass-derived oxygenates because of its unique oxygen storage capability.1 In these applications, CeO2 is exposed to wet reaction conditions where water molecules act as the reactant and product or is immersed in the aqueous phase where catalytic biomass conversion processes preferably are performed. Because water molecules can either molecularly or dissociatively adsorb on the CeO2 surfaces, it is expected that the existence of the vapor or aqueous phase will influence the surface structure and stability of CeO2 materials. One of the consequences is that the surface active sites or anchoring sites © XXXX American Chemical Society

Received: October 14, 2017 Revised: January 8, 2018 Published: February 13, 2018 A

DOI: 10.1021/acs.jpcc.7b10208 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

the supported metal nanoparticle simultaneously interacts with the support and surrounding water molecules. In this regard, although previous studies have suggested that the dynamic water/solid oxide interface has a significant impact on the metal−support interaction of the Pt−CeO217 and Au−TiO221 systems, little knowledge about how the liquid water affects the structural properties and stability of the supported metal cluster in the aqueous phase have been discussed. In the present work, the stability of three low-index CeO2 surfaces, that is, (111), (110), and (100) in the presence of various water concentrations (from vapor to liquid water) was studied using the DFT-based atomistic thermodynamic approach. The coverage-dependent water adsorption, the most stable water coverages, and the corresponding Gibbs surface free energies as a function of temperature and water partial pressure were calculated using DFT. These DFT data then have been used to predict the shape and exposed surface structures of the CeO2 nanoparticle under different wet conditions using the Wulff construction principle. For the aqueous phase/CeO2 systems, a series of AIMD simulations were performed to explore the dynamic interfacial structures between the liquid water phases and three low-index CeO2 surfaces. Similarly, the shape and structure of the CeO2 nanoparticle in the aqueous phase were also predicted. Finally, the structural properties and stabilities of a Pt13 nanocluster supported on the CeO2 surfaces in both gas and aqueous phases were analyzed using AIMD simulations and DFT calculations.

structures and its effects on the relative stabilities of surface structures that determine the shape/morphology of the synthesized CeO2 nanoparticle under hydrothermal conditions. The first-principles density functional theory (DFT) calculation is a very useful tool to gain the fundamental insights into the interactions between water molecules and CeO2 surfaces. Most of the DFT studies have been focused on addressing the water/CeO2 interaction under low coverage.4−7 The adsorption and dissociation of water molecules on the stoichiometric low-index CeO2 surfaces, in particular, the CeO2(111) surface, have been studied. Although there are only a few studies on water adsorption over CeO2(110) and (100) surfaces,8 it is suggested that the water/CeO2 interaction, as well as the stabilization, is strongly related to the surface structure and reduction.5 With the increasing water coverage on the CeO2 surface, the so-called water monolayer and bilayer, as well as the interfacial water/CeO2 structures, which play an important role in the surface stability and the catalytic reactivity, will be formed. Currently, an accurate and realistic description of the complex water/solid interface, however, remains a formidable challenge when using the DFT-based method.9 The CeO2(111) surface with ice water bilayers has been taken as a representative model to simulate the water/ CeO2 interface.10 Half of the water molecules in the water bilayer, which are directly in contact with the CeO2(111) surface, dissociates. The resulting surface hydroxyls not only enhance the water/CeO2 interaction but also further modify the interfacial structure. This scenario is even more complicated where the formed surface hydroxyls prefer to reorient themselves to form hydrogen-bonded networks over a fully hydroxylated CeO2(100) surface in wet environment.11 Very recently, Kropp et al. have studied water adsorption on the CeO2(111) and CeO2(100) surfaces at high coverages using DFT calculations.12 Their results clearly indicate that the surface structure affects the water dissociation behavior and the interfacial water structures. A square ice-like water layer is formed on the CeO2(100) surface, whereas an amorphous molecular water layer is formed on the CeO2(111) surface. Meanwhile, ab initio molecular dynamics (AIMD) simulations can provide the dynamic interfacial structure information and possible proton shuffling mechanisms in various water/metal oxide systems, for example, TiO2,13 ZrO2,14 Al2O3,15 and ZnO.16 For the aqueous phase/CeO2 systems, Fabris et al.17 have reported the fast diffusion of protons and hydroxide species along the water/CeO2(111) interface. The metal oxides have been commonly used as the support for metal catalysts. The shape/structure of the supported metal nanoparticle will undergo dynamic evolutions in the gas phase. It has been observed that the increase of hydrogen coverage induces a reconstruction of the γ-Al2O3 support Pt cluster from a biplanar to a cuboctahedral morphology.18 Ghosh et al. found that the small Au clusters supported on stoichiometric and defective CeO2(111) surfaces were strongly affected by reactant adsorption during CO oxidation.19 Upon CO adsorption, the supported Au clusters would break into smaller clusters, suggesting the importance of the dynamics behavior of supported metal cluster catalysts during the reaction. A recent DFT study further indicated that the gold cation can break away from the CeO2-supported gold nanoparticle, forming a single-atom catalytic active site on the CeO2(111) support with the assistance of the adsorbed CO.20 This also inspires us to explore the effect of water adsorption on the supported metal nanocluster in the aqueous phase. In the aqueous environment,

2. COMPUTATIONAL METHODS 2.1. Model and Calculation Details. All periodic calculations were carried out using the spin-polarized, gradient-corrected functional of Perdew, Burke, and Ernzerhof (PBE)22 as implemented in the CP2K package.23 The core electrons were modeled by the Goedecker−Teter−Hutter pseudopotentials24 with 12, 6, 1, and 18 valence electrons for Ce, O, H, and Pt, respectively. The valence electrons were described by the double-ζ Gaussian basis sets,25 with an auxiliary plane wave basis set with a cutoff of 500 Ry and using the γ-point for Brillouin-zone integration. The DFT + U method on the basis of the Mulliken 4f state population analysis was used to describe the Ce 4f electrons. The U value was set to 7.0 eV.20 The transition states of the surface reactions were searched using the climbing-image nudged elastic band method.26 The maximum force was converged to less than 0.05 eV/Å. The semiempirical van der Waals correction proposed by Grimme has been included in all calculations.27 Three low-index CeO2 surfaces were taken into account in this work. The (111) and (100) surfaces were modeled as p(3 × 3) slabs consisting of 12 and 11 atomic layers, respectively. The (110) slab was modeled as a p(2 × 3) slab with six atomic layers. To maintain the stoichiometry of the CeO2(100) surface and avoid the dipole moment normal to the surface, half of the surface oxygen atoms were moved from the top surface layer to the bottom surface layer of the slab. A vacuum gap of 20 Å in the z direction was used to eliminate the unphysical interactions between the periodic slabs. After the optimization of each clean surface slab, the outermost six atomic layers in CeO2(111) and CeO2(100) and the outermost three atomic layers in CeO2(110) calculations were allowed to relax, whereas the rest of the atoms were kept fixed to their bulk positions in the calculations of water adsorption with different coverages. In the DFT calculations of the aqueous phase/CeO2 systems, the B


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between the clean oxide surface and water molecules in the vapor phase

same surface slab models were used. The liquid water phase between periodic CeO2(111), CeO2(110), and CeO2(100) surface slabs was represented by filling with 77, 84, and 89 H2O molecules, respectively, in the space of 20 Å in the z direction. All atoms in the aqueous phase/CeO2 systems were free to move during the AIMD simulations and further static DFT calculations. To understand the effects of the aqueous phase on the stability of metal nanoclusters on the different CeO2 surface structures, a cuboctahedron Pt13 cluster was used as the model metal nanocluster.18 The size of the Pt13 cluster requires larger surface supercell slabs. Therefore, the p(4 × 4) slab with nine atomic layers, the p(3 × 4) slab with four atomic layers, and the p(4 × 4) slab with seven atomic layers were used as CeO2(111), CeO2(110), and CeO2(100) model surfaces, respectively. Similarly, a vacuum of 15 Å in the z direction was inserted between the periodic Pt13/CeO2 surface slabs. For the aqueous phase/Pt13/CeO2 systems, this vacuum space was filled with a certain number of explicit water molecules (83 for Pt13/CeO2(111); 101 for Pt13/CeO2(110); and 96 for Pt13/ CeO2(100)) to mimic the liquid water with a density of ∼1.0 g/cm3 (Figure S1). In the AIMD simulations and further static DFT calculations, all atoms in the bottom six atomic layers of the CeO2(111) slab, the bottom two atomic layers of CeO2(110), and the bottom four atomic layers of CeO2(100) were kept fixed to their optimized bulk positions, whereas the rest of atoms in the systems were free to move. All periodic simulation box sizes, the sizes and atomic layers of surface slabs, as well as the numbers of explicit water molecules for each aqueous phase systems are summarized in Table S1. All AIMD simulations were performed at 393 K in a canonical ensemble constant volume constant temperature (NVT) employing Nose−Hoover thermostats28,29 with a time step of 0.5 fs. The optimized geometries obtained from static DFT calculations were used as the initial configurations for the AIMD simulations. The AIMD simulations were carried for at least 10 ps to ensure that the simulated liquid water phase reaches the equilibrium state with little effect derived from the initial water structure (Figure S6). The detailed information about AIMD simulations and liquid water phase equilibration is given in the Supporting Information. For the further energy analysis, 10 configurations with the local minima in the final 0.5 ps of AIMD simulations were selected as representative sample configurations.18 These “final” configurations were quenched to 0 K with a SCF convergence criteria of 1.0 × 10−6 eV to obtain the averaged total energies. The equilibrated AIMD trajectories of the last 2.5 ps were used for analyzing the structural and electronic properties. 2.2. Surface Free Energy Calculation. The stabilities of three low-index CeO2 surfaces in the vapor phase were investigated using the ab initio thermodynamics approach.30 The surface Gibbs free energy in the vapor phase (γvapor) is defined as γvapor = γ 0 +

ΔGads A

surface + nads H 2O(gas) ↔ [surface, nads H 2O]

(2)

The Gibbs free energy variation associated with water adsorption is then defined as ΔGads = G(surface + nads H 2O, solid) − G(surface, solid) − nadsμH O

(3)

2

where G(surface + nadsH2O, solid) is the Gibbs free energy of the solid surface slab with a number of nads adsorbed water molecules, G(surface, solid) is the Gibbs free energy of the clean surface slab, and μH2O is the chemical potential of water in the vapor phase. For the solid phases, the entropic contribution, the pV term, and the thermal variations of internal energies are generally small32 and are being neglected in this work. Therefore, the Gibbs free energy for the solid phase can be approximated as the electronic energy from the DFT calculations, that is, G(solid) ≈ E(solid). The vapor phase under studied conditions is treated as an ideal gas. μH2O can be calculated using the following equation μH O(T , P) = E H2O + ΔμH O(T , P) 2

(4)

2

Then, eq 3 can be rewritten as ΔGads = ΔEads − nadsΔμH O

(5)

2

where the water adsorption energy (ΔEads) is defined as ΔEads = E H2O/slab − Eslab − E H 2O

(6)

Ewater/slab is the total electronic energy of the surface slab with H2O adsorption and Eslab and EH2O correspond to the energy of the surface slab and a single H2O molecule in vacuum. ΔμH2O is calculated as follows ΔμH O(T , P) = ΔμH0 O(T ) + RT ln 2

2

PH2O PH0 2O

(7)

where ΔμH2O(T) is the chemical potential of the vapor phase at PH0 2O = 1 bar, which is directly taken from NIST-JANAF thermochemical tables33 in this work. The surface Gibbs free energy in aqueous phase (γaqueous) is calculated as34 γaqueous = γ 0 + = γ 0 + γadh

ΔGadh ΔEadh − T ΔS = γ0 + 2A 2A T ΔS − 2A

(8)

where the entropic contribution (ΔS), which resulted from constrained water molecules in the interfacial layer, cannot be neglected. Instead, the melting entropy of ice (−22 J·mol−1 K−1) at 273.15 K, which is the energy required to convert the free water molecules in the liquid phase into “ice-like” constrained interfacial water molecules, is used to estimate this entropic contribution.35 A is the surface area of the oxide slab exposed to the aqueous phase (2A because of both the top and bottom surfaces of the slab being in contact with water molecules). γadh is the liquid water adhesion energy with the solid surface substrate, which can be estimated as follows

(1)

where A is the area of the exposed surface and γ0 is the surface free energy for the formation of a specific surface structure from the CeO2 bulk. The calculated γ0 of CeO2(111), CeO2(110), and CeO2(100) are 0.81, 1.26, and 1.51 J·m−2, respectively, which are consistent with the values from previous study.31 The water adsorption is described as the chemical equilibrium C


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Figure 1. Interaction energy analysis scheme for the aqueous phase/CeO2 system. Ce (light yellow), O (red), and H (white).

Figure 2. Interaction energy analysis scheme for the CeO2-supported Pt13 cluster in the aqueous phase. Ces (light yellow), Os (red), Pt (cyan), and H (white).

γadh = =

ΔEadh 2A Eaqueous phase/slab − Eoptimized slab − Eoptimized aqueous phase

aqueous phase γdef =

(9)

As shown in Figure 1, γadh can be decomposed into the deformation and interaction energy terms.

slab γdef =

(10)

Efixed slab − Eoptimized slab 2A

2A

(12)

aqueous phase/slab γinter Eaqueous phase/slab − Efixed slab − Efixed aqueous phase = (13) 2A The geometries extracted from the equilibrated AIMD trajectories are quenched to 0 K to obtain the total electronic energy of the aqueous phase/CeO 2 interface system (Eaqueous phase/slab). Efixed slab and Efixed aqueous phase are the singlepoint energy of the bare CeO2 slab and aqueous water with the same geometry as the above aqueous phase/CeO2 interface

2A

slab aqueous phase aqueous phase/slab γadh = γdef + γdef + γinter

Efixed aqueous phase − Eoptimized aqueous phase

(11) D


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The Journal of Physical Chemistry C system. Eoptimized slab and Eoptimized aqueous phase are the total electronic energy of the slab and free aqueous water obtained from the geometry optimization before AIMD simulations, respectively. 2.3. Stability of the Pt13 Cluster on the CeO2 Support. The stability of the CeO2-supported Pt13 cluster in the gas phase can be determined by the interaction energy (ΔEinter,gas) between the Pt13 cluster and the CeO2 surface support. At a specific temperature (e.g., 393 K), ΔEinter,gascan be calculated by ΔE inter,gas = E Pt13/slab − E Pt13 − Eslab

(14)

The Pt13 cluster and the CeO2 surface reconstruction occurs at 393 K. The deformation energy is defined as slab Edef = Eslab − Eoptimized slab

(15)

Pt13 Edef = E Pt13 − Eoptimized Pt13

(16)

Figure 3. Top view of the molecularly and dissociatively adsorbed single water molecule on three stoichiometric CeO2 surfaces. Ces (light yellow), Cesub (gray), Os (red), Osub (purple), Ow (blue), and H (white).

Finally, we examined the stability of the CeO2-supported Pt13 cluster in the aqueous phase. In this case, as shown in Figure 2, the interaction energy of the supported Pt13 cluster in aqueous phase at 393 K can be estimated as

barrier indicate that there is no strong preference for molecular and dissociative water adsorption on the CeO2(111) surface. On the CeO2(110) surface, a water molecule preferentially adsorbs at the Ces site in a titled configuration. Both hydrogen atoms of the adsorbed water molecule are in alignment with the Os shown in Figure 3. The calculated molecular and dissociative adsorption energies of a water molecule on the CeO2(110) surface are −0.79 and −1.03 eV, respectively. This is also consistent with the previously reported values of −0.85 and −1.12 eV.8 Clearly, the dissociative water adsorption is thermodynamically more favorable on the CeO2(110) surface. On the polar CeO2(100) surface, the water molecule adsorbs on the bridge site between two Ces atoms with an adsorption energy of −1.28 eV. Because of the removal of half Os atoms, there are intrinsic oxygen vacancy sites on the CeO2(100) surface. Water dissociatively adsorbs on the oxygen vacancy site forming an OwH group, filling the vacancy site and an H atom bonding to an Os with a strong adsorption energy of −1.86 eV. We note that the calculated adsorption energy on the CeO2(100) surface is slightly higher than the previously reported values of −1.00 and −1.57 eV from Molinari et al.8 using a (2 × 2) unit cell surface and PBE + U functional. The calculated activation barriers for water dissociation on the CeO2(110) and CeO2(100) surfaces are 0.15 and 0.05 eV, respectively. This suggests, unlike the CeO2(111) surface, that the dissociative state of water adsorption is both thermodynamically and kinetically more favorable than the molecular state on the CeO2(110) and CeO2(100) surfaces. 3.1.2. Water Adsorption at High Coverages. Considering the number of Ce atoms on the surface, the highest water coverages for CeO2(111), CeO2(110), and CeO2(100) surfaces are 7.76, 4.75, and 6.72 H2O nm−2, respectively, which are determined using the matching principle (Figure 4). On the CeO2(111) surface, the variation in the averaged water adsorption energy is less than 0.26 eV when the water coverage increases from 0.86 to 7.76 H2O nm−2 (Table S2). Although the hydrogen bonding interaction does not dramatically affect the water interaction with the CeO2(111) surface, it is noted that the water adsorption mode changes with the coverage (Figure S2). At the water coverage lower than 4.31 H2O nm−2, there is little hydrogen bonding interaction between the adsorbed H2O molecules. As the coverage is greater than 5.17 H2O nm−2, both dissociative and molecular water adsorption modes exist. The dissociative water adsorption

ΔE inter,aqueous = Eaqueous phase/Pt13/slab − Eaqueous phase/Pt13 − Eaqueous phase/slab

(17)

The adhesion energies of the Pt13 cluster and CeO2 surfaces in aqueous phase can be estimated as slab slab Eadh = γadh ·2A

= Eaqueous phase/slab − Eoptimized slab − Eoptimized aqueous phase (18) Pt13 Eadh = Eaqueous phase/Pt13 − Eoptimized Pt13

− Eoptimized aqueous phase

(19)

The geometries extracted from the equilibrated AIMD trajectories are quenched to 0 K to obtain the total electronic energies of the CeO2-supported Pt13, isolated Pt13, and bare CeO2 slabs in the gas or aqueous phase. Eoptimized slab, Eoptimized Pt13, and Eoptimized aqueous phase are obtained from the geometry optimization of the corresponding system before AIMD simulations.

3. RESULTS AND DISCUSSION 3.1. Water Interaction with CeO2 Surfaces in the Vapor Phase. 3.1.1. Single Water Adsorption. Both molecular and dissociative water adsorption on stoichiometric CeO2 surfaces can be found, as shown in Figure 3. On the CeO2(111) surface, a single water molecule molecularly adsorbs on the surface Ce site (Ces) via the oxygen atom of water (Ow) and one hydrogen atom (H) pointing to a surface oxygen atom (Os). For dissociative water adsorption, water dissociates into an hydroxyl (Ces−OH) at the Ces and an H atom bonding to an Os, forming the second hydroxyl (OsH). The calculated adsorption energies for molecular and dissociative water are −0.63 and −0.66 eV, which are in good agreement with the previously reported results of −0.67 and −0.61 eV.6 The water dissociation is found to be highly active on the CeO2(111) surface with an activation barrier of 0.11 eV. The very close adsorption energies and the low dissociation E


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Figure 4. Top view of the water adsorption at the highest coverage on three stoichiometric CeO2 surfaces.

configuration will convert into the molecular configuration because of an increase of the intermolecular hydrogen bonding. All adsorbed water molecules and formed hydroxyl groups rearrange themselves to form several H2O clusters. The dissociative water adsorption generating two hydroxyls is preferred on the CeO2(110) surface. Unlike on the CeO2(111) surface where the hydrogen bonding network forms at high water coverage, there is no hydrogen bonding interaction observed at all coverages. This is largely due to the openness of the surface Ce atoms on the CeO2(110) surface. All hydroxyl groups at high coverages are in the nearly same position with a slight orientation adjustment as the single water dissociation adsorption (Figure S3). The absence of hydrogen bonding interactions and pronounced surface reconstruction result in the coverage independent of water adsorption on the CeO2(110) surface. The coverage effect of the water adsorption on the CeO2(100) surface is pronounced with an adsorption energy difference of 0.44 eV. As the coverage increases to 2.99 H2O nm−2, the hydrogen bond interactions between neighboring hydroxyls are formed. The surface construction is also observed (Figure S4). At the highest coverage, all intrinsic oxygen vacancy sites on the polar CeO2(100) surface are filled by hydroxyl groups from water dissociation. As shown in Figure 4, the highly reconstructed CeO2(100) surface transforms into the bulk-like one because of the symmetric nature of the fully hydroxylated CeO2(100) surface. All hydroxyls on the CeO2(100) surface can be regarded as nine repeated hydroxyl pairs with the similar Os−H orientation. 3.1.3. Stability of Adsorbed Water on the CeO2 Surfaces in Vapor Phase. The relative stability of the specific surface structure is determined by the Gibbs surface free energy. The lower the surface free energy is, the more stable the surface structure is. Figure 5 displays the calculated Gibbs surface free energies as a function of temperature at constant water partial pressure (PH2O = 1 bar) and specific water coverages for three low-index CeO2 surfaces. In general, the water adsorption lowers the surface free energy for each surface structure and thus enhances the stability of the surface structure. As the temperature increases, the stable water coverage will change with the calculated surface energies under constant vapor pressure conditions. For the CeO2(111) surface, the most stable water coverage is 7.76 H2O nm−2 up to 400 K, where a fully hydroxylated surface is formed with six hydroxyl groups and three molecularly adsorbed water molecules. When the temperature reaches 500 K, the stable water coverage decreases to 5.17 H2O nm−2 due to the desorption of the molecularly adsorbed water molecules. Above 500 K, all hydroxyl groups are removed from the surface, leading to a fully dehydrated CeO2(111) surface.

Figure 5. Surface free energies as a function of temperature and water coverages at PH2O = 1 bar on three stoichiometric CeO2 surfaces.

The thermal stability of the hydroxylated CeO2(110) surface as a function of temperature is different from the CeO2(111) surface. The dissociative water monolayer (ML), which corresponds to the coverage of 4.75 H2O nm−2 (6 hydroxyl groups), is stable over a wide range of temperature (100−670 K). As the temperature reaches above 670 K, the CeO2(110) surface becomes a dehydrated surface in vapor phase. However, the CeO2(100) surface exhibits a behavior in contrast to that of the CeO2(110) surface. Even at low temperature ranges, the fully hydroxylated CeO2(100) state becomes slightly unstable due to the lack of hydrogen bonding interactions among surface hydroxyls. The hydroxyl pair is released one by one and the resulting stable water coverages are 5.97, 5.23, and 4.48 H2O nm−2 in the temperature range 200− 450, 450−700, and 700−800 K, respectively. The next two hydroxyl groups leave the surface sequentially in the temperature range 800−950 K. Consequently, the four residual hydroxyl groups desorb together at 950 K. In summary, our calculated Gibbs surface free energies suggest that water desorption peaks on three stoichiometric CeO2 surface F


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Table 1. Water Coverages (H2O nm−2), Gibbs Surface Free Energies (J·m−2) at Different Temperatures with Constant Water Partial Pressure (PH2O = 1 bar) and the Exposed Surface Termination Ratios of the Predicted CeO2 Nanoparticles via the Wulff Construction Principle water coverages/surface free energies/exposed surface termination ratios termination

>1000 K

700 K

500 K

300 K

CeO2(111) CeO2(110) CeO2(100)

0.00/0.81/100.0% 0.00/1.26/0.0% 0.00/1.51/0.0%

0.00/0.81/95.1% 0.00/1.26/0.0% 5.23/1.17/4.9%

5.17/0.79/61.8% 4.75/0.98/0.0% 5.23/0.80/38.2%

5.17/0.45/49.6% 4.75/0.66/0.0% 5.97/0.42/50.4%

the temperature further decreases to 500 K, both (111) and (100) terminations are hydroxylated with the stable coverages of 5.17 and 4.75 H2O nm−2. The increasing water coverages reduce the surface free energies of both terminations. Thus, the surface area of the (100) termination over the CeO 2 nanoparticle grows from 4.9 to 38.2%. At the room temperature (300 K), the water coverage on the CeO2(100) surface reaches as high as 5.97 H2O nm−2 and the exposed area of the (100) termination over the CeO2 nanoparticle increases to 50.4%. The variation in the proportion of the (111) and (100) surface terminations are attributed to the different stabilizing effects of water adsorption on these two surfaces. Our calculation results indicate that the CeO2 nanoparticle exhibits only the (111) and (100) surface terminations but not the (110) surface termination in the whole studied temperature range. This is because the CeO2(110) surface reaches its highest water coverage of 4.75 H2O nm−2, which is still too low to dramatically lower the surface free energy via hydroxylation like the other two surface terminations. At the coverage of 4.75 H2O nm−2, instead of being fully coordinated, each Ce on the CeO2(110) surface is heptacoordinated. When the CeO2(110) surface is exposed to the aqueous phase, the Ce atoms tend to achieve a fully restored octafold coordination with the formation of a high water coverage, and the CeO2(110) surface will be further stabilized. However, the structural properties and stability of the CeO2(110) and (100) surfaces in the aqueous phase have been rarely known. 3.2. CeO2 Surfaces in the Aqueous Phase. 3.2.1. Interfacial Structural Properties. A detailed atomic level understanding of both structural properties and dynamic behaviors of the aqueous phase/CeO2 interface is of significance to the stability of the CeO2 nanoparticle grown in the aqueous phase, as well as its catalytic properties. The AIMD simulations were carried for at least 10 ps to ensure that the simulated liquid water phase reaches the equilibrium state with little effect derived from the initial water structure (Figure S6). Generally

structures are 400, 550, and 900 K, respectively. This is in good agreement with the reported temperatures of 350, 600, and 850 K.8 3.1.4. Equilibrium Morphology of CeO2 Nanocrystals in the Presence of Water. The water adsorption on each CeO2 surface structure gives rise to the anisotropic changes in the surface energies of the exposed surfaces of the CeO 2 nanoparticle. The calculated surface Gibbs free energies of the three CeO2 surface structures at different temperatures in the vapor phase (PH2O = 1 bar) are listed in Table 1. With these surface Gibbs free energies, the equilibrium morphologies of the CeO2 nanocrystals at given temperatures can be predicted using the Wulff construction principle.36,37 As shown in Figure 6, the CeO2(111) surface is the dominant surface termination

Figure 6. Predicted shapes and surface terminations of the CeO2 nanoparticles as a function of temperature at PH2O = 1 bar.

over the CeO2 nanoparticle in the wet environment at high temperatures (>1000 K). This is due to the fact that all exposed surfaces over the CeO2 nanoparticle is fully dehydrated, which is the similar to the situation of the clean CeO2 surfaces at 0 K. With the decreasing temperature, the exposed CeO2 surfaces will be partially covered with either molecularly or dissociatively adsorbed water molecules. At 700 K, the CeO2(100) surface is partially hydroxylated with a coverage of 5.23 H2O nm−2. The adsorbed water molecules on the CeO2(100) surface reduce its surface energy. As such, the (100) surface termination appears with a small fraction of 4.9% over the CeO2 nanoparticle. As

Figure 7. Snapshots and the density profiles of the aqueous phase on the (a) CeO2(111) surface, (b) CeO2(110) surface, and (c) CeO2(100) surface. G


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Figure 8. Representative AIMD configurations showing the H2O dissociation at the aqueous phase/CeO2(110) interface and evolution of the ⟨H− O⟩, ⟨O−O⟩ distances during H2O dissociation.

Figure 9. (a) Snapshots of the atomic configurations for the aqueous phase/CeO2 interfaces. (b) Atomic density profiles of Ow (red) and H (blue) of the interface near the CeO2 surface as a function of the distance from the interface.

molecule (O2). At the same time, another proton (H2) from this adjacent H2O molecule (O2) transfers to the Ce−OwH (O3). The consequence of the fast proton shuffling between Ce−OwH2 and Ce−OwH via the water molecules in the vicinity dramatically promotes the proton-assisted catalytic reactions at different surface sites, which was also observed on the ZrO2 surface in the aqueous phase.38 The constrained interfacial H2O molecules lead to a significant density increase close to the CeO2 surfaces and exhibit different structural properties from the aqueous phase. The water density profiles along the z direction perpendicular to the CeO2 surfaces are shown in Figure 7. The pronounced peaks at ∼2 Å from the CeO2(111) and (110) surfaces correspond to the interface layer consisting of water molecules directly bonding to the surface Ce. The water density profiles are integrated from the CeO2 surface to the end of these peaks,

speaking, the interactions between liquid water molecules and the oxide surface induce the barrier-less water dissociation, leading to partial surface hydroxylation.17 Figure 7 shows the representative snapshots of the equilibrated aqueous phase/ CeO2 interfacial structures taken from AIMD simulations at 393 K. A general picture of the aqueous phase/CeO2 interface consists of the mixed OH and H2O layer that directly connects the oxide surface and liquid water molecules through a hydrogen bonding network. The H atoms bonding to the Os together with the adsorbed OwH at the Ce site are observed as a result of water dissociation. Besides water dissociation, the proton transfer between adsorbed water (Ce−OwH2) and hydroxyl (Ce−OwH) with the assistance of adjacent H-bonded H2O molecules is observed at the interface, which is shown in Figure 8. The O−H bond evolution suggests that the proton (H1) of the Ce−OwH2 (O1) transfers to the adjacent H2O H


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The Journal of Physical Chemistry C giving the water coverages of 7.33 H2O nm−2 and 7.52 H2O nm−2 on the CeO2(111) and CeO2(110) surfaces, respectively. In the case of the polar CeO2(100) surface, H2O dissociation occurs at the oxygen vacancy site, leading to the formation of the OsH and OwH pair. The OwH group that bridges the two exposed Ce atoms fills the vacancy site (Figure S8). Therefore, two peaks near the CeO2(100) surface are observed, which correspond to the dissociative H2O filling the oxygen vacancy site and the hydroxylated layer where H2O molecules interact with the CeO2(100) surface via strong hydrogen bonding. The integration results show that ∼70% of the oxygen vacancy sites are occupied by H2O molecules with the formation of a partially hydroxylated CeO2(100) surface. As shown in Figure 9a, three types of species are found on the CeO2 surfaces, that is, adsorbed molecular water (Ce− OwH2) and two types of surface hydroxyls (Ce−OwH and OsH). To gain further insights into the interfacial structures of the aqueous phase/CeO2 systems, the atomic density profiles of the interfacial regions are illustrated in Figure 9b. Similar to the density profiles shown in Figure 7, the reference for the atomic density profile was chosen at the center position of the topmost atomic layer on each CeO2 surface structure. For the CeO2(111) and CeO2(110) surfaces, a sharp Ow peak in the atomic density profile appears at ∼2.5 Å above the surface Ce atom, which is attributed to the adsorbed water (Ce−OwH2) and hydroxyl (Ce−OwH) on the surface Ce site. In addition, two H peaks are observed in the interface region. The first peak at ∼1.2 Å corresponds to the H directly bonding to the Os with OsH pointing out of the surface plane, whereas the second peak at ∼3.0 Å corresponds to the H atom belonging to the adsorbed species in the interfacial layer. In agreement with the previous study,17 the Ce atoms on the CeO2(111) surface impose their symmetric atomic arrangement to the adsorbed water/hydroxyl species, leading to a highly structured interface layer. However, the adsorbed species are relatively mobile on the CeO2 (110) surface because of the low coordination number of Ce. The species may move close to the surface and share the H-bonding to the neighboring OsH (Figure 7b). The small Ow peak at 1.8 Å is the consequence of such mobility. On the polar CeO2(100) surfaces, the water dissociation occurs at the vacancy site with the formation of surface hydroxyls (OsH and OwH), which has a characteristic Ow peak at the surface. The following peak at 2.5 Å corresponds to the Ow from water bound at the surface in the hydration layer. It is worth noting that four peaks are observed in the atomic hydrogen density profile. The first two peaks at the surface and 1.0 Å correspond to the OH group pointing to the adjacent Os and the OH group pointing out of the plane of the surface, as a kind of intrasurface hydrogen bonding network. The next two H peaks at 1.5 and 3.0 Å are observed on both sides of Ow peak at 2.5 Å, which suggest that the water molecule favors to stay in perpendicular to the surface plane with the H pointing toward the surface (Figure 7c). The orientations of the species in the interface region strongly depend on the surface structure. 3.2.2. Surface Species Evolution. The facial dissociation and recombination of water molecules result in the dynamic and complicated evolution patterns of surface species in the interfacial region. The CeO2 surfaces are terminated with Ce−OwH2, Ce−OwH, and Os−H (Figure 9a) where the partition of these surface species is governed by the dynamic equilibrium between the CeO2 surface and the liquid water phase. The time-dependent evolutions of surface species on three low-index CeO2 surfaces are calculated using the AIMD

simulation trajectories of the aqueous phase/CeO2 interfacial structures. The evolution of the surface species corresponds to the bond breaking and bond formation of Ce−Ow, Ow−H, and Os−H bonds. Herein, the upper limit of the first peak for the radial distribution functions (RDF) between corresponding atoms (Figure S9) is set as the cutoff distance for the bond formation. The potential Ce−Ow and Os−H bond lengths are defined as 3.0 and 1.2 Å, respectively. As shown in Figure 10,

Figure 10. Coverage evolutions of the surface species on three stoichiometric CeO2 surfaces in the aqueous phase.

the Ce−OwH and Os−H pairs appear at the beginning of the AIMD simulations, suggesting that some of the water dissociation has already occured when the water molecules first contacted the CeO2 surfaces. The partition of the surface species reaches at equilibrium after ∼5 ps simulation. It is also observed that the water assisted proton transfer between Ce− OwH2 and Ce−OwH. This is reflected by the coverage fluctuations in the surface species evolution. Statistically, the CeO2(111) surface slab is equilibrated with 11 Ce−OwH2 and 6 pairs of Ce−OwH and Os−H by the end of 10 ps simulation (Figure 10), which is considered to be the equilibrium state of the surface. The water coverage at the CeO2(111) surface reaches 7.33 H2O nm−2. Similarly, we note that the aqueous I


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The Journal of Physical Chemistry C phase CeO2(110) surface is terminated by 10 Ce−OwH2 and 9 pairs of Ce−OwH and Os−H, resulting in a water coverage of 7.52 H2O nm−2. The higher water coverage on the CeO2(110) surface is attributed to the low coordination of surface atomic Ce. On the polar CeO2(100) surface, water dissociation at the vacancy site leads to the formation of two surface hydroxyls at the expense of molecularly adsorbed water molecules. At equilibrium, there are 11 hydroxyl pairs (OsH and OwH) and 2 OwH2 species on the CeO2(100) surface. Consequently, a total of 13 vacancy sites are filled by water molecules. Again, the water coverages on the three CeO2 surfaces are all in good agreement with the result integrated from the atomic density profile. 3.2.3. Electron Transfer at the Aqueous Phase/CeO2 Interface. Except for the surface species evolution, the electronic properties at the interfacial areas of aqueous phase/CeO2 systems were explored using the total electron density difference. As shown in Figure S10, the calculated profiles of total electron density difference suggest that all three CeO2 surfaces can transfer electrons to the aqueous phase, which is also observed in the H2O/TiO2 system.39 The Hartree potential differences show different extent increase at the H2O/ CeO2 interface close to the surface, which indicates the electron loss from the CeO2 surface.40 The calculated Bader charges of the CeO2(111), CeO2(110), and CeO2(100) surfaces are +0.73, +2.20, and +1.52 |e|, respectively. 3.2.4. Surface Stability in the Aqueous Phase. The structural property and dynamics of the liquid/solid interface strongly affect surface Gibbs free energy, which is required to determine the equilibrium shape of a nanocrystal in the aqueous phase.34 As shown in Figure 1, the deformation of the liquid water, which is in contact with the CeO2(111) surface, results in a large energy cost of 2.53 J·m−2, whereas the deformation of the CeO2(111) surface only causes a relatively small energy change of 0.27 J·m−2. With an interaction energy of −3.41 J·m−2, the calculated adhesion energy γadh is −0.61 J· m−2 for the aqueous phase/CeO2(111) system. For the aqueous phase/CeO2(110) system, the calculated deformation energy changes to 3.27 J·m−2 for the aqueous phase and 0.47 J· m−2 for the CeO2(110) surface slab. Because of high water coverage and strong water adsorption energy on the CeO2(110) surface, the interaction energy, however, is significantly increased to −4.89 J·m −2 . On the polar CeO2(100) surface, the dissociative water molecules on the vacancy sites not only strongly interacts with the CeO2(100) surface but also leads to a significant surface reconstruction. As a result, the even higher interaction energy at the aqueous phase/CeO2(100) interface is calculated to be −6.50 J·m−2. Besides the energy change, the entropy change is introduced into the system because of the formation of the aqueous phase/ CeO2 interface. The statistically averaged water molecule numbers of 17, 19, and 13, which are directly in contact with the CeO2(111), CeO2(110), and CeO2(100) surfaces, are obtained, respectively. In addition, a H-bonding hydration layer with 25 H2O (corresponding to 6.14 H2O nm−2) is observed above the CeO2(100) surface, which makes a significant entropy contribution to the aqueous/CeO2(100) system. As listed in Table 2, the entropic contribution is estimated to be −0.07, −0.08, and −0.14 J·m−2 for the aqueous phase/ CeO2(111), aqueous phase/CeO2(110), and aqueous phase/ CeO2(100) systems, respectively. Using eq 8, the surface free energies γaqueous for CeO2(111), CeO2(110), and CeO2(100) surfaces in the aqueous phase are 0.27, 0.19, and 0.23 J·m−2,

Table 2. Calculated Deformation Energies, Interaction Energies, Adhesion Energies, Entropic Contributions, and Surface Free Energies for the Aqueous Phase/CeO2 Systemsa

a

CeO2 surface

(111)

(110)

(100)

γslab def phase γaqueous def aqueous phase/slab γinter γadh TΔS/2A γ0 γaqueous

0.27 2.53 −3.41 −0.61 −0.07 0.81 0.27

0.47 3.27 −4.89 −1.15 −0.08 1.26 0.19

1.25 3.82 −6.50 −1.43 −0.15 1.51 0.23

All energies are in J·m−2.

respectively. This suggests that the CeO2(110) surface is the most stable surface structure among the three low-index surfaces in the aqueous phase at 393 K. It is noted that the surface free energies of three low-index CeO2 surfaces in the aqueous phase are lower than those in vacuum. On the basis of the calculated surface free energies, the equilibrium morphology of CeO2 nanocrystals in the aqueous phase at 393 K is predicted using the Wulff construction principle. As shown in Figure S11, the exposed surface area of the CeO2 nanoparticle is dominated by the (110) structure (94.0%) with a small area of the (100) plane (6.0%). 3.3. CeO2-Supported Pt13 Cluster in the Aqueous Phase. As aforementioned, the wet environments either gas or liquid water could dramatically affect the shape and exposed surface structures of the CeO2 support. It is expected that the existence of the liquid water environment not only modifies the electronic structure of the Pt catalyst but also affects the stability of the supported metal catalyst on the oxide surface. In the present work, the CeO2-supported Pt13 cluster in the aqueous phase was used as the example for understanding the aqueous phase effects on the morphology and stability of the supported Pt13 cluster. 3.3.1. Structural Properties of the Supported Pt13 Cluster. A series of the AIMD simulations of the Pt13 cluster supported on three low-index CeO2 surface structures in both gas and aqueous phases were performed at 393 K. The snapshots from the 10 ps AIMD simulation trajectories are shown in Figure 11. For comparison, isolated free Pt13 clusters in vacuum and liquid water were also simulated at 393 K. First, we note that the initial cuboctahedron Pt13 cluster evolves into a pyramidal structure, with the coordination number of all surface Pt atoms on the cluster decreasing from the penta- to the tetracoordinated structures. This is consistent with the previous study.41 While in the aqueous phase, no pronounced structure deformation of the free cuboctahedron Pt13 cluster is found after 10 ps AIMD simulation. The presence of the aqueous phase maintains the symmetric morphology of the initial Pt13 cluster. Upon deposition of the Pt13 cluster on the CeO2 surface, the strong Pt−O bonds between the Pt atoms and Os atoms form at the interface. As a result, the supported cuboctahedron Pt13 cluster flattens and expands its contact area “wet” on the CeO2 surfaces in the gas phase (Figure 11). However, in the presence of the aqueous phase, the Pt atoms in the cluster not only interact with the support surface but also interact with the water molecules in the aqueous phase. This is clearly depicted by the AIMD simulation trajectories of the Pt13/CeO2(111) and Pt13/ CeO2(100) systems, showing only slight distortions of the J


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Figure 11. Optimized geometries of isolated and CeO2-supported Pt13 cluster in vacuum or the aqueous phase after 10 ps AIMD simulations. Ce (light yellow), O (red), Pt (cyan), and H (white).

Figure 12. Electron density difference plots for the optimized CeO2-supported Pt13 cluster in the gas and aqueous phases. The positive (negative) electronic density change difference is represented by yellow (blue).

Table 3. Estimated Energies (eV) for the Pt13 Cluster Supported on Three Low-Index CeO2 Surfaces in the Gas and Aqueous Phases gas phase energy

(111)

Eslab def Pt13 Edef

−2.89

ΔEint,gas

−9.83

0.00

(110) 0.01 −12.55

aqueous phase (100)

energy

0.14 −10.45

(111)

(110)

(100)

Eslab adh Pt13 Eadh

−16.18

−35.84

−41.91

ΔEint,aqueous

−11.95

−14.03

−11.65

−8.88

3.3.2. Electronic Properties of the Supported Pt13 Cluster. The possible electron transfer between the CeO2 support and the supported Pt13 cluster can be tracked on the basis of calculated Bader charge analysis.42 In the gas phase, the total of fractional electrons of −0.24, −0.53, and −0.18 |e| is found to be transferred from the supported Pt13 cluster to the CeO2(111), CeO2(110), and CeO2(100) surfaces, respectively. This indicates that the supported Pt13 cluster on the CeO2 support is partially oxidized in the gas phase, independent of the exposed surface structures. As shown in Figure 12, the electron transfer mainly occurs at the interface between the contacting Pt atoms in the Pt13 cluster and the CeO2 support, while the electronic densities associated with the Pt atoms away from the support is less affected. In aqueous phase/Pt13/CeO2 systems, the supported Pt13 clusters also become oxidized by losing electrons. In the aqueous phase/Pt13/CeO2 systems, more electron transfer occurs between the Pt13 cluster and

supported Pt13 cluster structure, where most of the Pt atoms are still penta-coordinated. For the supported Pt13 cluster on the CeO2(110) surface, although a large structural distortion is observed, the Pt13 cluster does not show the tendency of flattening and becomes more two-dimensional in structure as it is in the gas phase. The structural difference of the supported Pt13 cluster in the gas and aqueous phases are further reflected by the calculated structural parameters, such as averaged coordination numbers (Figure S12) and the distribution (Figure S13) of the Pt atoms, obtained from our AIMD simulations. In summary, our AIMD simulation results suggest that the Pt13 cluster would wet the CeO2 support surfaces in the gas phase because of the strong Pt−O interactions. The Pt13 cluster would maintain its original three-dimensional morphology, although some structural distortion occurs due to the interactions between the surface Pt atoms and the surrounding liquid water molecules. K


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vapor phase, the hydroxylated (110) structure is the dominant surface of the CeO2 nanoparticle in the aqueous phase. A cuboctahedron Pt13 nanocluster supported on three CeO2 surfaces in both gas and aqueous phases were studied using AIMD simulations. It is noted that the supported Pt13 nanocluster not only melts down on CeO2 surfaces but also is oxidized in the gas phase. While in the aqueous phase, although the supported Pt13 nanocluster is further oxidized, the three-dimensional Pt13 structure is less distorted, suggesting that the solvation effect induced by the surrounding water molecules plays a role in maintaining the supported Pt13 nanocluster structure and stability.

surrounding. The Pt13 cluster deposited on the CeO2(111), CeO2(110), and CeO2(100) surfaces lose fractional electrons of −0.49, −0.76, and −0.59 |e|, respectively, indicating that the supported Pt13 cluster in the aqueous phase is more oxidized than it is in the gas phase. Furthermore, we note that the existence of the surrounding aqueous phase at the Pt13/CeO2 interfaces induces a significant electronic redistribution. The electron depletion occurs on most of Pt atoms in the cluster, leading to slightly positively charged Ptδ+ atoms in the Pt13 cluster. 3.3.3. Stability of the Supported Pt13 Cluster. The stability of the supported Pt13 cluster on the CeO2 support in both the gas and aqueous phases is investigated on the basis of the interaction energy shown in Table 3. In the gas phase, the isolated free Pt13 cluster reconstruction occurs at 393 K with a deformation energy of −2.89 eV, whereas three CeO2 surface slabs retain their initial configuration with negligible deformation energy (Figure S14). The interaction energies of the equilibrated Pt13 cluster on the CeO2(111), CeO2(110), and CeO2(100) supports are −9.83, −12.55, and −10.45 eV, respectively. This indicates that the CeO2(110) surface provides the most stable support for the Pt13 cluster at 393 K in gas phase. When the CeO2 surface slabs are immersed in the aqueous phase, both the CeO2 surface and the Pt13 cluster are inevitably solvated. The estimated adhesion energies for the aqueous phase/CeO2 interfaces are −16.18, −35.84, and −41.91 eV, respectively. Similar to the previous study,43 the adhesion and reorganization of the liquid water environment was observed over the Pt13 cluster surface with the estimated adhesion energy of −8.88 eV. Therefore, according to the energy analysis scheme displayed in Figure 2, the calculated interaction energy of the Pt13 cluster on the CeO2(111), CeO2(110), and CeO2(100) supports in the aqueous phase are −11.95, −14.03, and −11.65 eV, respectively. Compared with the gas phase, the existence of the aqueous phase clearly enhances the stability of the Pt13 cluster on the CeO2 support. Again, the Pt13 cluster deposited on the CeO2(110) surface is the most stable in the aqueous phase.

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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.7b10208. Discussion of the slab model used to simulated CeO2(100) surface and H2O dissociation at O vacancy on Pt13/CeO2 surface (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.M.). *E-mail: [email protected] (J.L.). ORCID

Zhibo Ren: 0000-0001-8034-9752 Donghai Mei: 0000-0002-0286-4182 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Natural Science Foundation of China (no. 21476012, 21571012, and 91534201). Z.R. appreciates the joint Ph.D. scholarship support from the China Scholarship Council. The computing time was granted by a scientific theme user proposal in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), which is a U.S. Department of Energy national scientific user facility located at PNNL in Richland, Washington. D.M. is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences.

4. CONCLUSIONS In the present work, the effects of the aqueous phase on the morphology and stability of the CeO2 nanoparticle, as well as the stability of the metal nanocluster catalyst on the ceria support were studied using the ab initio based thermodynamic approach. Gibbs surface free energies of the CeO2(111), CeO2(110), and CeO2(100) surfaces as a function of temperature, water partial pressure, and surface coverage were calculated using DFT. We found that the adsorbed water molecules, whether molecular or dissociative, would lower the surface free energies of the CeO2 surfaces. With the calculated Gibbs surface free energies at different conditions, the shape and the ratio of the exposed surface structures of the CeO2 nanoparticle were predicted using the Wulff construction principle. At low temperature, the partially hydroxylated (111) and (100) are the two major surface structures of the CeO2 nanoparticle, whereas the fully dehydrated (111) surface becomes the most dominant surface as the temperature increases. In the aqueous phase, the dynamical interfacial structures, proton transfer, and electronic properties of the aqueous phase/CeO2 systems were investigated using AIMD simulations at 393 K. Different hydroxylation behaviors at the aqueous phase/CeO2 interface were statistically analyzed. Instead of the (111) surface as the dominant structure in the

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REFERENCES

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Theoretical Investigation of the Structural Stabilities of Ceria Surfaces

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