Theoretical Investigation of the Structural Stabilities of Ceria Surfaces

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A Theoretical Investigation of the Structural Stabilities of Ceria Surfaces and Supported Metal Nanocluster in Vapor and Aqueous Phases Zhibo Ren, Ning Liu, Biao-Hua Chen, Jianwei Li, and Donghai Mei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10208 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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A 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

ABSTRACT

In the present work, the stabilities of three low-index ceria (CeO2) surfaces, i.e., (111), (110) and (100) in vapor and aqueous phases were studied using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. On the basis of calculated Gibbs surface free energies, the morphology and exposed surface structures of the CeO 2 nanoparticle were predicted using Wulff construction principle. It is found that the partially hydroxylated (111) and (100) are two major surface structures of the CeO2 nanoparticle in vapor phase at ambient temperature. As the temperature increases, the fully dehydrated (111) surface becomes the most dominant structure. While in aqueous phase, the exposed surface of the CeO 2 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. Due to the strong metal-support interaction, AIMD simulations show that the supported Pt13 nanocluster has the tendency to wetting the CeO2 surface in gas phase. The calculated interaction energies suggest 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 aqueous phase. Compared to the gas phase, more electrons are transferred from the Pt 13 nanocluster to the CeO2 support, implying the supported Pt13 nanocluster is further oxidized in aqueous phase.

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1. INTRODUCTION 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, watergas shift, automobile exhaust emission, and thermal condensation of biomass-derived oxygenates for its unique oxygen storage capability.1 In these applications, CeO2 is exposed in wet reaction conditions where water molecules act as the reactant and product or is immersed in aqueous phase where catalytic biomass conversion processes preferably are performed. Since water molecules can either molecularly or dissociatively adsorb on the CeO2 surfaces, it is expected that the existence of vapor or aqueous phase will influence the surface structure and stability of CeO2 materials. One of consequences is the surface active sites or anchoring sites for metal catalysts could be modified due to the surface hydroxylation and hydration to some extent. As a result, the catalytic activity and selectivity of some structure-dependent reactions over CeO 2 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, the hydrogenation takes place preferentially on the CeO 2(111) facet while 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 CeO 2 surface structures and its effects on the relative stabilities of surface structures that determine the shape/morphology of the synthesized CeO2 nanoparticle in hydrothermal condition. First principles density functional theory (DFT) calculation is a very useful tool to gain the fundamental insight into the interactions between water molecules and CeO2 surfaces. Most of DFT studies have been focused on addressing the water-CeO2 interaction under low coverage.4-7 The adsorption and dissociation of water molecule on the stoichiometric low index CeO 2



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surfaces, in particular, the CeO2(111) surface, have been studied. Although there are only a few studies on the 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 increasing water coverage on the CeO2 surface, the so-called water monolayer, bilayer, as well the interfacial water/CeO2 structures, which plays 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 using 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 water molecules in the water bilayer, which directly 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 forming hydrogen bonded networks over a fully hydroxylated CeO 2(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 CeO 2(100) surface while an amorphous molecular water layer is formed on the CeO 2(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, e.g., TiO2,13 ZrO2,14 Al2O315 and ZnO.16 For aqueous phase/CeO2 systems, Fabris et al.17 have reported that the fast diffusion of protons and hydroxide species along the water/CeO 2(111) interface.


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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 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 CeO 2(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 that the importance of the dynamics behavior of supported metal cluster catalysts during 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 adsorbed CO.20 This also inspires us to explore the possibility of water adsorption on the supported metal nanocluster in aqueous phase. In the aqueous environment, the supported metal nanoparticle simultaneously interacts with the support and surrounding water molecules. In this regards, although previous studies have suggested that the dynamic water/solid oxide interface has a significant impact on the metal-support interaction of Pt-CeO217 and Au-TiO221 systems, little knowledge about how the liquid water affects the structural property and stability of the supported metal cluster in aqueous phase have been discussed. In the present work, the stability of three low-index CeO2 surfaces, i.e., (111), (110) and (100) in the presence of various water concentrations (from vapor to liquid water) were studied using DFT based atomistic thermodynamic approach. The coverage-dependent water adsorption, the most stable water coverages and 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 CeO 2 nanoparticle in



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different wet conditions using Wulff construction principle. For aqueous phase/CeO 2 systems, a series of AIMD simulations were performed to explore dynamic interfacial structures between the liquid water phase and three low-index CeO2 surfaces. Similarly, the shape and structure of the CeO2 nanoparticle in aqueous phase were also predicted. Finally, the structural and stabilities a Pt13 nanocluster supported on CeO2 surfaces in both gas and aqueous phases were analyzed using AIMD simulations and DFT calculations.

2. COMPUTATIONAL METHODS 2.1 Model and Calculation Details. All periodic calculations were carried out using the spinpolarized, gradient corrected functional of Perdew, Burke, and Ernzerhof (PBE)22 as implemented in the CP2K package.23 The core electrons were modelled by Goedecker-TeterHutter pseudopotentials24 with 12, 6, 1 and 18 valence electrons for Ce, O, H and Pt. 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 Gamma point for Brilluoin-zone integration. The DFT+U method on the basis of Mullikan 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 (TS) of surface reactions were searched using the climbing-image nudged elastic band method (CI-NEB). 26 The maximum force were converged to less than 0.05 eV/Å. The semi-empirical 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 twelve and eleven atomic layers, respectively. The (110) slab was modeled as a p(2×3) slab with 6 atomic layers. To maintain the stoichiometry of the CeO2(100) surface and avoid the dipole moment normal to the surface, half


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of 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 periodic slabs. After the optimization of each clean surface slab, the outermost six atomic layers in CeO2(111) and CeO2(100), the outermost three atomic layers in CeO2(110) calculations were allowed to relax, whereas the rest of atoms were kept fixed to their bulk positions in the calculations of water adsorption with different coverages. In the DFT calculations of aqueous phase/CeO2 systems, the 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 in the space of 20 Å in the z direction. All the atoms in aqueous phase/CeO2 systems were free to move during AIMD simulations and further static DFT calculations. To understand the effects of aqueous phase on the stability of metal nanocluster 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/Pt 13/CeO2 systems, this vacuum space was filled with a certain numbers of explicit water molecules (83 for Pt 13/CeO2(111); 101 for Pt13/CeO2(110) and 96 for Pt13/CeO2(100), respectively) to mimic the liquid water with density of ~1.0 g/cm3 (Figure S1). In the AIMD simulations and further static DFT calculations, all the atoms in the bottom six atomic layers of 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



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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 (NVT) employing Nose-Hoover thermostats28-29 with a time step of 0.5 fs. The optimized geometries obtained from static DFT calculations was used as initial configurations for AIMD simulations. The AIMD simulations were carried for at least 10 ps to ensure 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 was given in the supporting information. For the further energy analysis, ten 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 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 structural and electronic properties. 2.2 Surface Free Energy Calculation The stabilities of three low-index CeO2 surfaces in vapor phase were investigated using ab initio thermodynamics approach.30 The surface Gibbs free energy in vapor phase (𝛾

) is

defined as 𝛾

=𝛾 +

∆

(1)

where A is the area of the exposed surface, 𝛾 is the surface free energy for the formation of specific surface structure from the CeO2 bulk. The calculated 𝛾 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


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previous study.31 The water adsorption is described as the chemical equilibrium between the clean oxide surface and water molecules in vapor phase: 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑛

𝐻 𝑂(𝑔𝑎𝑠) ↔ [𝑠𝑢𝑟𝑓𝑎𝑐𝑒, 𝑛

(2)

𝐻 𝑂]

The Gibbs free energy variation associated with water adsorption is then defined as ∆𝐺

= 𝐺(𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑛

where 𝐺(𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝑛

𝐻 𝑂, 𝑠𝑜𝑙𝑖𝑑) − 𝐺(𝑠𝑢𝑟𝑓𝑎𝑐𝑒, 𝑠𝑜𝑙𝑖𝑑) − 𝑛

𝜇

(3)

𝐻 𝑂, 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 𝜇

is the chemical potential of water in vapor phase. For the solid

phases, the entropic contribution, the pV term, and the thermal variations of internal energies are generally small32 and being neglected in this work. Therefore, the Gibbs free energy for the solid phase can be approximated as the electronic energy from DFT calculations, i.e., 𝐺(𝑠𝑜𝑙𝑖𝑑) ≈ 𝐸(𝑠𝑜𝑙𝑖𝑑). The vapor phase under studied conditions is treated as an ideal gas. The 𝜇

can be

calculated using the following equation (𝑇, 𝑃) = 𝐸

𝜇

+ ∆𝜇

(4)

(𝑇, 𝑃)

Then eq 3 can be rewritten as ∆𝐺

= ∆𝐸

−𝑛

(5)

∆𝜇

where the water adsorption energy (∆𝐸 ∆𝐸

=𝐸

The 𝐸 𝐸

(6)

−𝐸

/

is the total electronic energy of the surface slab with H2O adsorption and the

and 𝐸

correspond to the energy of the surface slab and a single H2O molecule in

vacuum. The ∆𝜇 ∆𝜇

−𝐸

/

) is defined as

is calculated as follows

(𝑇, 𝑃) = ∆𝜇

(𝑇) + 𝑅𝑇𝑙𝑛

(7)



where the ∆𝜇

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(𝑇) is the chemical potential of the vapor phase at 𝑃

= 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 𝛾

=𝛾 +

∆

=𝛾 +

∆

∆

=𝛾 +𝛾

−

∆

(8)

where the entropic contribution (∆𝑆), which is arisen from constrained water molecules in the interfacial layer, can not 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 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 aqueous phase (2A because of both top and bottom surfaces of the slab contacting with water molecules). 𝛾

is the liquid water adhesion

energy with the solid surface substrate, which can be estimated as follows 𝛾

=

∆

/

=

(9)

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

=𝛾

𝛾

=

𝛾 𝛾

+𝛾

+𝛾

/

(10) (11) (12)

= /

=

/

(13)


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

The geometries extracted from the equilibrated AIMD trajectories are quenched to 0 K to obtain

the

(𝐸

total /

electronic ). 𝐸

energy and 𝐸

of

the

aqueous

phase/CeO2

interface

system

are the single point energy of bare

CeO2 slab and aqueous water with the same geometry as the above aqueous phase/CeO 2 interface system. 𝐸

and 𝐸

are the total electronic energy of the slab and

free aqueous water obtained from the geometry optimization before AIMD simulation. 2.3 The Stability of the Pt13 Cluster on the CeO2 Support



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The stability of the CeO2 supported Pt13 cluster in gas phase can be determined by the interaction energy ( ∆𝐸

) between the Pt13 cluster and the CeO2 surface support. At

,

specific temperature (e.g., 393 K), ∆𝐸 ∆𝐸

,

=𝐸

−𝐸

/

,

can be calculated by (12)

−𝐸

The Pt13 cluster and the CeO2 surface reconstruction occurs at 393 K. The deformation energy is defined as: 𝐸

=𝐸

−𝐸

(13)

𝐸

=𝐸

−𝐸

(14)

Finally, we examined the stability of the CeO2 supported Pt13 cluster in aqueous phase. In this case, as shown in Figure 2, the interaction energy of the supported Pt 13 cluster in aqueous phase at 393 K can be estimated as ∆𝐸

,

=𝐸

/

/

−𝐸

/

−𝐸

/

(15)

The adhesion energies for the Pt13 cluster and CeO2 surfaces in aqueous phase can be estimated as: 𝐸

=𝛾

∙ 2𝐴 = 𝐸

𝐸

=𝐸

/

/

−𝐸

−𝐸

−𝐸

(16) (17)

−𝐸

The geometries extracted from the equilibrated AIMD trajectories are quenched to 0 K to obtain the total electronic energies of CeO2 supported Pt13, isolated Pt13 and bare CeO2 slab in gas or aqueous phase. 𝐸

,𝐸

and 𝐸

are obtained

from the geometry optimization of corresponding system before AIMD simulation.


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

3. RESULTS AND DISSCUSSION 3.1 Water Interaction with CeO2 Surfaces in 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, single water molecule molecularly adsorbs on the surface Ce site (Ces) via the oxygen atom of water (Ow) and one hydrogen atom (H) pointing bonding to a surface oxygen atom (O s). For dissociative water adsorption, water dissociates into an hydroxyl (Ce s-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 previously reported results of −0.67 and −0.61 eV.6 The water dissociation is found to be highly



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active on the CeO2(111) surface with an activation barrier of 0.11 eV. The very close adsorption energies and low dissociation barrier indicate there is no strong preference for molecular and dissociative water adsorption on the CeO2(111) surface. On the CeO2(110) surface, water molecule preferentially adsorbs at the Ce s site in a titled configuration. Both hydrogen atoms of the adsorbed water molecule are in align with the O s 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, water molecule adsorbs on the bridge site between two Ce s atoms with an adsorption energy of −1.28 eV. Due to the removal of half O s 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 an strong adsorption energy of −1.86 eV. We note that the calculated adsorption energy on the CeO2(100) surface are 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 CeO 2(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.


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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), H (white). 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 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 adsorbed H 2O molecules. As the coverage is greater than 5.17 H2O·nm-2, both dissociative and molecular water adsorption modes exist. The dissociative water adsorption configuration will convert into the molecular configuration due to an increase of intermolecular hydrogen bonding. All adsorbed water molecules and formed hydroxyl groups rearrange themselves to form several H 2O clusters. The dissociative water adsorption generating two hydroxyls is preferred on the CeO 2(110) surface. Unlike on the CeO2(111) surface where the hydrogen bonding network forms at high



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water coverage, there is no hydrogen bonding interaction is observed at all coverages. This is largely due to the openness nature of 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 for water adsorption on the CeO2(110) surface. The coverage effect on the water adsorption on the CeO2(100) surface is pronounced with the adsorption energy difference of 0.44 eV. As the coverage increases to 2.99 H 2O·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 due to the symmetric nature of fully hydroxylated CeO2(100) surface. All hydroxyls on the CeO2(100) surface can be regarded as nine repeated hydroxyl pairs with the similar O s-H orientation.

Figure 4. Top view of the water adsorption at the highest coverage on three stoichiometric CeO 2 surfaces. 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 surface structure is. Figure 5 displays the


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calculated Gibbs surface free energies as a function of temperature at constant water partial pressure (PH2O=1bar) and specific water coverages for three low-index CeO 2 surfaces. In general, the water adsorption lowers the surface free energy for each surface structure, thus enhances the stability of surface structure. As the temperature increases, the stable water coverage will change with the calculated surface energies at constant vapor pressure condition. 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 6 hydroxyl groups and 3 molecularly adsorbed water molecules. When the temperature reaches at 500 K, the stable water coverage decreases to 5.17 H2O·nm-2 due to the desorption of molecularly adsorbed water molecules. Above 500 K, all hydroxyl groups are removed from the surface, leading to a fully dehydrated CeO 2(111) surface. 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 CeO 2(110) surface becomes the dehydrated surface in vapor phase. While the CeO2(100) surface exhibits a contrasted behavior in comparison with the CeO 2(110) surface. Even at low temperature ranges, the fully hydroxylated CeO 2(100) state becomes slightly unstable due to the lack of hydrogen bonding interactions among surface hydroxyls. The hydroxyl pair is released one by one, resulting the 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 structures are 400, 550 and 900 K, respectively. This is in good agreement with the reported temperatures of 350, 600 and 850 K. 8

2.5

CeO2(111)

Surface energy (J•m-2)

2.0 1.5 1.0 0.5 0.00 3.45 6.9

0.0 -0.5

200

400

0.86 1.72 4.31 5.17 7.76 (H2O•nm-2)

600

800

2.59 6.03

1000

Temperature (K)

2.5

CeO2(110)

2.0 Surface energy (J•m-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 40

1.5 1.0 0.5 0.0 -0.5

0.00 3.17

200

400

0.79 3.96

600

1.58 2.38 4.75 (H2O•nm-2)

800

1000

Temperature (K)


Page 19 of 40

2.5

CeO2(100)

2.0 Surface energy (J•m-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5 0.00 2.99 5.97

0.0 -0.5

200

400

0.75 1.49 3.73 4.48 6.72 (H2O•nm-2)

600

800

2.24 5.23

1000

Temperature (K)

Figure 5. Surface free energies as a function of temperature and water coverages at P H2O = 1 bar on three stoichiometric CeO2 surfaces.

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 exposed surfaces of the CeO2 nanoparticle. The calculated surface Gibbs free energies of three CeO2 surface structures at different temperatures in vapor phase (P H2O =1bar) are listed in Table 1. With these surface Gibbs free energies, the equilibrium morphologies of CeO2 nanocrystals at given temperatures can be predicted using Wulff construction principle. 36-37 As shown in Figure 6, the CeO2(111) surface is the dominant surface termination over the CeO2 nanoparticle in the wet environment at high temperature (>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 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



Page 20 of 40

H2O·nm-2. The adsorbed water molecules on the CeO2(100) surface lower its surface energy. As such, the (100) surface termination appears with a small fraction of 4.9% over the CeO 2 nanoparticle. As 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 CeO2 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 (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 H 2O·nm-2, which is still too low to dramatically lower the surface free energy via hydroxylation like 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 hepta-coordinated. When the CeO2(110) surface is exposed in the aqueous phase, the Ce atoms tend to achieve a fully restored octa-fold coordination with the formation of high water coverage and the CeO2(110) surface will be further stabilized. However, the structural properties and stability of the CeO2(110) and (100) surface in the aqueous phase have been rarely known.


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Figure 6. The predicted shapes and surface terminations of the CeO2 nanoparticles as a function of temperature at PH2O =1 bar. Table 1. Water Coverages (H2O·nm-2), Gibbs Surface Free Energies (J·m-2) at Different Temperatures with Constant Water Partial Pressure (P H2O=1 bar) and the Exposed Surface Termination Ratios of the Predicted CeO2 Nanoparticles via Wulff Construction Pricnciple. water coverages / surface free energies/ exposed surface termination ratios termination

>1000 K

700 K

500 K

300 K

CeO2(111)

0.00 / 0.81 / 100.0%

0.00 / 0.81 / 95.1%

5.17 / 0.79 / 61.8%

5.17 / 0.45 / 49.6%

CeO2(110)

0.00 / 1.26 / 0.0%

0.00 / 1.26 / 0.0%

4.75 / 0.98 / 0.0%

4.75 / 0.66 / 0.0%

CeO2(100)

0.00 / 1.51 / 0.0%

5.23 / 1.17 / 4.9%

5.23 / 0.80 / 38.2%

5.97 / 0.42 / 50.4%

3.2 CeO2 Surfaces in 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 significant to the stability of CeO2 nanoparticle grown in aqueous phase, as well as its catalytic properties. The AIMD simulations were carried for at least 10 ps to ensure the simulated liquid water phase reaches the equilibrium state with little effect derived from the initial water structure (Figure S6). Generally speaking, the interactions between liquid water molecules and the oxide surface induces the barrierless water dissociation leading to partial surface hydroxylation. 17 Figure 7 shows the representative snapshots of the equilibrated aqueous phase/CeO 2 interfacial structures taken from AIMD simulations at 393 K. A general picture of the aqueous phase/CeO 2 interface consists of the mixed OH and H2O layer that directly connects the oxide surface and liquid water molecules through hydrogen bonding network. The H atoms bonding to the O s together with the adsorbed OwH at the Ce site are observed as a result of water dissociation. Besides water



Page 22 of 40

dissociation, proton transfer between adsorbed water (Ce-O wH2) 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 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 promote the proton assisted catalytic reactions at different surface sites, which was also observed on ZrO2 surface in aqueous phase.38

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


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

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 CeO2 surface to the end of these peaks, 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 OsH and OwH pair. The OwH group that bridges two exposed Ce atoms fills the vacancy site (Figure S8). Therefore, two peaks near the CeO 2(100)



Page 24 of 40

surface are observed, which correspond to the dissociative H 2O 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, i.e., adsorbed molecular water (Ce-OwH2), two types of surface hydroxyls (Ce-OwH and OsH). To gain further insight into the interfacial structures of the aqueous phase/CeO 2 systems, the atomic density profiles of 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-O wH2) 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, while 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 due to the low coordination number of Ce. The species may move close to the surface and share the H-bonding to the neighboring O sH (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


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at 2.5 Å corresponds to the Ow from water bonded 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 O s and the OH group pointing out of the plane of the surface, as kind of intra-surface hydrogen bonding network. The next two H peaks at 1.5 Å and 3.0 Å are observed on the both sides of O w peak at 2.5 Å, which suggest that 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.

Figure 9. (a) Snapshots of the atomic configurations for aqueous phase/CeO 2 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.



Page 26 of 40

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 CeO 2 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 aqueous phase/CeO2 interfacial structures. The evolution of surface species is corresponding to the bondbreaking 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-O w and Os-H bond lengths are defined as 3.0 Å and 1.2 Å, respectively. As shown in Figure 10, the Ce-O wH and Os-H pairs appear at the beginning of AIMD simulations, suggesting that some of water dissociation already occurs when the water molecules firstly contact with the CeO2 surfaces. The partition of surface species reaches at equilibrium after ~5 ps simulation. It is also observed that the water assisted proton transfer between Ce-O wH2 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 CeOwH2 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 CeO 2(111) surface reaches7.33 H2O·nm-2. Similarly, we note that the aqueous phase CeO2(110) surface is terminated by 10 Ce-OwH2 and 9 pairs of Ce-OwH and Os-H, resulting 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


Page 27 of 40

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 coverage on three CeO2 surfaces are all in good agreement with the result integrated from the atomic density profile.

6

CeO2(111)

Coverage (species•nm-2)

4

0.5ML

H Os

2 6

H

4 2 9

6 4 2

O Ce

0.5ML

0

2

4 6 Time (ps)

8

CeO2(110) H Os

6

1ML

O Ce

2 9

H

H O Ce

6 3 0

10

1ML

H

4

0

H

H

6 0

0.5ML

O Ce

3

Coverage (species•nm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1ML

2

4 6 Time (ps)

8

10



9

H

CeO2(100)

6 Coverage (species•nm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Os Ce

3 9

1 ML Ce

H Ow Ce Ce

6 3

Page 28 of 40

1 ML

6 Ce

2 0

H

H

4

0

2

O

0.5 ML

Ce

4 6 Time (ps)

8

10

Figure 10. The coverage evolutions of surface species on three stoichiometric CeO 2 surfaces in aqueous phase.

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/CeO 2 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 H 2O/TiO2 system.39 The Hartree potential differences show different extent increase at the H 2O/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 Aqueous Phase. The structural property and dynamics of the liquid/solid interface strongly affect surface Gibbs free energy, which is required to determinate the equilibrium shape of a nanocrystal in aqueous phase.34 As shown in Figure 1, the


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deformation of the liquid water, which is in contact with the CeO 2(111) surface, results in a large energy cost of 2.53 J·m-2. While 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 of 3.27 J·m-2 for the aqueous phase and 0.47 J·m-2 for the CeO2(110) surface slab. Due to 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 due to the formation of aqueous phase/CeO 2 interface. The statistically averaged water molecule numbers of 17, 19 and 13, which directly 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) system. 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. This suggest that the CeO2(110) surface is the most stable surface structure among three low-index surfaces in aqueous phase at 393 K.



Page 30 of 40

It is noted that the surface free energies of three low-index CeO 2 surfaces in aqueous phase are lower than that those in vacuum. On the basis of calculated surface free energies, the equilibrium morphology of CeO2 nanocrystals in aqueous phase at 393 K is predicted using Wulff construction principle. As shown in Figure S11, the exposed surface area of the CeO 2 nanoparticle is dominated by the (110) structure (94.0%) with a small area of the (100) plane (6.0%).

Table 2. Calculated deformation energies, interaction energies, adhesion energies, entropic contributions and surface free energies for the aqueous phase/CeO 2 systems. All energies are in J·m-2. (111)

(110)

(100)

𝛾

0.27

0.47

1.25

𝛾

2.53

3.27

3.82

−3.41

−4.89

−6.50

𝛾

−0.61

−1.15

−1.43

𝑇∆𝑆/2𝐴

−0.07

−0.08

−0.15

𝛾

0.81

1.26

1.51

𝛾

0.27

0.19

0.23

CeO2 surface

𝛾

/

3.3 CeO2 Supported Pt13 Cluster in 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 liquid water environment not only modify the electronic structure of Pt catalyst, but also affect the stability of the supported metal catalyst on the oxide supported. In the present work, the CeO 2


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supported Pt13 cluster in aqueous phase was used as the example for understanding the aqueous phase effects on the morphology and stability of the supported Pt 13 cluster. 3.3.1 Structural Properties of the Supported Pt13 Cluster. A series of AIMD simulations of the supported 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, an isolated free Pt 13 cluster in vacuum and liquid water were also simulated at 393 K. Firstly, we note that the initial cuboctahedron Pt 13 cluster evolves into a pyramidal structure, with the coordination number of all surface Pt atoms on the cluster decrease from penta- to the tetra-coordinated. This is consistent with the previous study.41 While in aqueous phase, no pronounced structure deformation of the free cuboctahedron Pt13 cluster is found after 10 ps AIMD simulation. The presence of 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 Pt 13 cluster flattens and expands its contact area “wet” on the CeO2 surfaces in gas phase (Figure 11). However, in the presence of 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 AIMD simulation trajectories of Pt13/CeO2(111) and Pt13/CeO2(100) systems, showing only slight distortions of the supported Pt 13 cluster structure, where most of Pt atoms are still penta-coordinated. For the supported Pt 13 cluster on the CeO2(110) surface, although a large structural distortion is observed, the Pt 13 cluster does not show the tendency of flattening and become more two-dimensional structure as it in gas phase. The structural difference of the supported Pt13 cluster in gas and aqueous phase are further reflected the calculated structural



Page 32 of 40

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 gas phase because of the strong Pt-O interactions. While the Pt13 cluster would maintain its original three dimensional morphology, although some structure distortion occur due to the interactions between surface Pt atoms with the surrounding liquid water molecules.

Figure 11. Optimized geometries of isolated and CeO2 supported Pt13 cluster in vacuum or aqueous phase after 10 ps AIMD simulations. Ce (light yellow), O (red), Pt (cyan), H (white). 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 gas phase, the total of fractional electrons of −0.24, −0.53 and −0.18 |e| are found to be transferred from the supported Pt 13 cluster to the CeO2(111), CeO2(110) and CeO2(100) surfaces, respectively. This indicates that the supported Pt 13 cluster on the CeO2 support is partially oxidized in gas phase, independent of exposed surface structures. As shown in Figure 12, the electron transfer mainly occurs at the interface between the contacting Pt atoms


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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/Pt 13/CeO2 systems, the supported Pt13 clusters also exhibit being oxidized by losing electrons. More electron transfer occurs between the CeO2 support and the Pt13 cluster in aqueous phase. The total of fractional electrons of −0.49, −0.76 and −0.59 |e| are found to be transferred from the Pt 13 cluster to the CeO2(111), CeO2(110) and CeO2(100) surfaces, respectively, indicating that the supported Pt 13 cluster in aqueous phase is more oxidized than it in gas phase. Furthermore, we note that the existence of 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.

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



Page 34 of 40

3.3.3 Stability of the Supported Pt13 Cluster. The stability of the supported Pt13 cluster on the CeO2 support in both gas and aqueous phases is investigated on the basis of the interaction energy shown in Table 3. In gas phase, the isolated free Pt 13 cluster reconstruction occurs at 393 K with a deformation energy of −2.89 eV, while three CeO 2 surface slabs remain 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) support 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 aqueous phase, both the CeO2 surface and the Pt13 cluster are inevitably solvated. The estimated adhesion energies for 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 Pt 13 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 Pt 13 cluster on the CeO2(111), CeO2(110) and CeO2(100) support in aqueous phase are −11.95, −14.03 and −11.65 eV, respectively. Compared to the gas phase, the existence of aqueous phase clearly enhances the stability of the Pt 13 cluster on the CeO2 support. Again, the Pt13 cluster deposited on the CeO2(110) surface is the most stable in aqueous phase.

Table 3. Estimated energies (eV) for the Pt13 cluster supported on three low-index CeO2 surfaces in gas and aqueous phases.

Energy 𝐸

(111) 0.00

Gas phase (110) (100) 0.01 0.14

Energy 𝐸

(111) −16.18

Aqueous phase (110) (100) −35.84 −41.91


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𝐸 ∆𝐸

,

−2.89 −9.83

−12.55

−10.45

𝐸 ∆𝐸

,

−8.88 −11.95

−14.03

−11.65

4. CONCLUSIONS In the present work, the effects of aqueous phase on the morphology and stability of the CeO 2 nanoparticle, as well as the stability of metal nanocluster catalyst on the ceria support were studied using ab initio based thermodynamic approach. Gibbs surface free energies of 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 surface free energies of CeO 2 surfaces. With calculated Gibbs surface free energies at different conditions, the shape and the ratio of exposed surface structures of the CeO2 nanoparticle were predicted using Wulff construction principle. At low temperature, the partially hydroxylated (111) and (100) are two major surface structures of the CeO2 nanoparticle. While the fully dehydrated (111) surface becomes the most dominant surface as the temperature increases. In aqueous phase, the dynamical interfacial structures, proton transfer, and electronic properties of aqueous phase/CeO 2 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 vapor phase, the hydroxylated (110) structure is the dominant surface of the CeO2 nanoparticle in aqueous phase. A cuboctahedron Pt 13 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 gas phase. While in aqueous phase, although the supported Pt 13 nanocluster is further oxidized, the three-dimensional Pt13 structure is less distorted, suggesting the solvation effect induced by



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surrounding water molecules play a role in maintaining the supported Pt 13 nanocluster structure and stability.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications. Supplementary Figures, Figure S1−S14; Supplementary Note 1, Discussion of the slab model used to simulated CeO2(100) surface, Figure S15−S16; Supplementary Note 2, H2O dissociation at O vacancy on Pt13/CeO2 surface, Figure S15−S16. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (D. Mei). *E-mail: [email protected] (J. Li). 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. ACKNOWLEDGMENT


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The work was financially supported by the National Natural Science Foundation of China (No. 21476012, 21571012, and 91534201). Z. Ren 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. Mei is supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. REFERENCES (1) Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P. Fundamentals and Catalytic Applications of CeO2-Based Materials. Chem. Rev. 2016, 116, 5987-6041. (2) Wu, K.; Sun, L. D.; Yan, C. H. Recent Progress in Well-Controlled Synthesis of Ceria-Based Nanocatalysts towards Enhanced Catalytic Performance. Adv. Energy. Mater. 2016, 6, 1-46. (3) Trovarelli, A.; Llorca, J. Ceria Catalysts at Nanoscale: How Do Crystal Shapes Shape Catalysis? ACS Catal. 2017, 7, 4716-4735. (4) Fronzi, M.; Piccinin, S.; Delley, B.; Traversa, E.; Stampfl, C. Water Adsorption on the Stoichiometric and Reduced CeO2(111) Surface: A First-Principles Investigation. Phys. Chem. Chem. Phys. 2009, 11, 9188-9199. (5) Yang, Z. X.; Wang, Q. G.; Wei, S. Y.; Ma, D. W.; Sun, Q. A. The Effect of Environment on the Reaction of Water on the Ceria(111) Surface: A DFT plus U Study. J. Phys. Chem. C 2010, 114, 14891-14899. (6) Fernández-Torre, D.; Kośmider, K.; Carrasco, J.; Ganduglia-Pirovano, M. V. n.; Pérez, R. n. Insight into the Adsorption of Water on the Clean CeO2(111) Surface with van der Waals and Hybrid Density Functionals. J. Phys. Chem. C 2012, 116, 13584-13593. (7) Hansen, H. A.; Wolverton, C. Kinetics and Thermodynamics of H 2O Dissociation on Reduced CeO2(111). J. Phys. Chem. C 2014, 118, 27402-27414. (8) Molinari, M.; Parker, S. C.; Sayle, D. C.; Islam, M. S. Water Adsorption and Its Effect on the Stability of Low Index Stoichiometric and Reduced Surfaces of Ceria. J. Phys. Chem. C 2012, 116, 7073-7082. (9) Björneholm, O.; Hansen, M. H.; Hodgson, A.; Liu, L.-M.; Limmer, D. T.; Michaelides, A.; Pedevilla, P.; Rossmeisl, J.; Shen, H.; Tocci, G. Water at Interfaces. Chem. Rev. 2016, 116, 7698-7726. (10) Carchini, G.; García-Melchor, M.; Łodziana, Z.; López, N. r. Understanding and Tuning the Intrinsic Hydrophobicity of Rare-Earth Oxides: A DFT+ U Study. ACS Appl. Mater. Inter. 2015, 8, 152-160. (11) Gill, L.; Beste, A.; Chen, B.; Li, M.; Mann, A. K.; Overbury, S. H.; Hagaman, E. W. Fast MAS 1H NMR Study of Water Adsorption and Dissociation on the (100) Surface of Ceria



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TOC Graphic


Theoretical Investigation of the Structural Stabilities of Ceria Surfaces

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