The Enhancement of Surface Reactivity on CeO2 (111) Mediated by

Nov 18, 2016 - The fixed effects for the slab were also taken into account. We calculated the formation energies of oxygen vacancies for the slab with...
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The Enhancement of Surface Reactivity on CeO2 (111) Mediated by Subsurface Oxygen Vacancies Jing Fan, Chengyang Li, Jinzhu Zhao, Yueyue Shan, and Hu Xu* Department of Physics, South University of Science and Technology of China, Shenzhen, China, 518055 S Supporting Information *

ABSTRACT: Surface reactivity on metal oxide surfaces and its enhancement play important roles in heterogeneous catalytic reactions. In this work, the interactions of O2 and H2O with reduced CeO2 (111) surface are studied by densityfunctional theory calculations. The corresponding adsorption geometries, adsorption energies, and reaction barriers are reported. It is found that the diffusion of subsurface oxygen vacancies toward surface can be promoted by the adsorption of O2 on the CeO2 (111) surface. Then those oxygen vacancies diffused onto surface sites will be healed by the adsorbed O2, leaving behind an O adatom on the surface. Interestingly, at moderate temperatures, the surface O adatom will swap positions with surface lattice O dynamically. The adsorption of H2O may also induce the diffusion of oxygen vacancies from subsurface to surface, leading to the formation of two hydroxyls on the CeO2 (111) surface. In addition, the interaction between the paired hydroxyl groups and O2 will result in the formation of water and oxygen adatom on the surface. Our results have revealed important roles played by the subsurface oxygen vacancies in the enhancement of surface reactivity, especially when involving the adsorption of water and oxygen.

1. INTRODUCTION Metal oxides have promising applications in a wide variety of fields.1−5 It is well-known that oxygen vacancies can strongly modify surface properties, and play key roles in surface reactions.1,6−8 For most metal oxides, oxygen vacancies usually prefer to be present at the surface region. For example, surface oxygen vacancies are energetically more favorable than the subsurface ones on rutile TiO2 (110) surface.9 By contrast, oxygen vacancies tend to locate at subsurface sites for anatase TiO2 (101) and CeO2 (111) surfaces,9−15 which are attributed to their larger structural relaxations induced by subsurface oxygen vacancies in the creation of oxygen vacancies. Recently, the interactions of adsorbates with subsurface oxygen vacancies have attracted intensive studies. For instance, Setviń et al.16 observed the induced migration of oxygen vacancies from subsurface site to surface site by the electronic field on anatase TiO2 (101) surface, and subsequently the surface oxygen vacancies are filled by the adsorbed O2. In 2014, Li et al.17 suggested that the relative stability of the subsurface and surface oxygen vacancies reverses in the presence of water. However, this reversal is in fact an artifact caused by an improper model using an insufficient number of layers.18 CeO2 has attracted considerable interest for decades due to its diverse applications, such as automotive exhaust catalysis, water−gas shift reactions, fuel cells, and hydrogen production.19,20 All these applications usually involve the interaction of oxygen vacancies with the adsorbed molecules. As water and molecular oxygen are always present in surface reactions, the interactions of H2O or O2 with oxygen vacancies play essential © XXXX American Chemical Society

roles in surface reactivity. However, similar to anatase TiO2 (101) surface, the existence of subsurface oxygen vacancies13−15 makes it complex to study the surface reaction of H2O or/and O2 on a CeO2 (111) surface. The adsorption of O2 on CeO2 (111) surface has been widely studied for decades due to its strong oxidizability and the important roles in surface catalytic reactions of O2 species. It is well-known that O2 interacts weakly with a stoichiometric CeO2 (111) surface,21 while O2 will adsorb on a Ce3+ ion to form superoxide O−2 on a reduced CeO2 (111) surface.21,22 As O−2 is a metastable state, it prefers to react with the surface oxygen 22 vacancy to form an O2− In addition, Li et al. also proposed 2 . 2− that O2 is much more stable than O−2 adsorbed on the CeO2 (111) surface with subsurface oxygen vacancy.23 However, the interactions of the adsorbed O2 with subsurface oxygen vacancies are still lacking. The interaction of water with surface oxygen vacancy on a CeO2 (111) surface has also been intensively studied.24−26 The results clearly showed that H2O prefers to heal the surface oxygen vacancy and leave two hydrogen atoms binding to surface O.24−28 Recently, Hansen and Wolverton proposed that the desorption of H2O and the diffusion of subsurface oxygen vacancy are likely to compete, which will prevent the dissociation of H2O and the formation of hydroxyls.27 However, the influence of H2O adsorption on the subsurface-to-surface diffusion is neglected.27 Therefore, it is Received: July 29, 2016 Revised: November 17, 2016 Published: November 18, 2016 A

DOI: 10.1021/acs.jpcc.6b07650 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Formation Energy Per Oxygen Vacancy (unit in eV)a reference Ganduglia-Pirovano et al.14 Jerratsch et al.38 Li et al.23 this work this work this work this work this work

method

unit cell

U U U U U U U U

p(2 p(3 p(3 p(3 p(3 p(3 p(3 p(4

= 4.5 = 4.5 = 5.0 = 0.0 = 0.0 = 5.0 = 5.0 = 5.0

× 2) × 3) × 4) × 3) × 3) × 3) × 3) × 4)

fix or not

a

b

c

d

Yes No Yes No Yes No

2.50 2.22 2.31 2.76 2.77 2.32 2.32 -

2.40 2.06 2.30 2.52 2.53 2.13 2.13 -

2.30 2.13 2.15 2.15 2.12

1.89 1.95 1.87 1.87 1.90

a

The a, b, c, d in the columns below refer to structural configurations shown in Figure 1a−d, respectively. The prior results are also listed for comparison.

To study the dynamical behavior of O2− 2 on the reduced CeO2 (111) surface, ab initio molecular dynamics (MD) simulations39,40 have been performed. The initial states with an O adatom bridged on two surface cerium atoms at 0 K were heated to 200 and 400 K using the microcanonical ensemble, and the heating speed was 0.1 K/fs. In the following, canonical ensemble and Nosé−Hoover thermostat41−43 were used during a 30 ps of MD simulations. In MD simulations, O1 and O2 represent the topmost lattice O and the O adatom from the adsorbed O2 in the initial configuration, respectively. To trace the adsorbed O2, we have defined a 2D Gaussian-type probability density function for O1 and O2 as

interesting to investigate the interactions of subsurface oxygen vacancies with H2O on the reduced CeO2 (111) surface. Although many efforts have been made, it is still insufficient for us to know the interplay between the adsorbates and the reduced CeO2 (111) surface. Due to its diverse applications, it is of significant importance to study the interactions of O2 or/and H2O with subsurface oxygen vacancies on a CeO2 (111) surface.

2. COMPUTATIONAL METHODS All the calculations were carried out using Vienna Ab initio Simulation Package (VASP)28,29 within the framework of pseudopotential plane wave method. The diffusion pathways and barriers were calculated using climbing image nudged elastic band (CI-NEB) method.30 The Perdew−Burke−Ernzerhof (PBE) functional31 in the framework of General Gradient Approximation (GGA) was used, and the projector augmented wave (PAW)32,33 was used to describe the interaction between ions and electrons. The Ce 4f15s25p65d16s2 and O 2s22p4 electrons were treated as valence electrons. The cutoff energy of 400 eV, the vacuum region of 12 Å, and the Monkhorst−Pack sampling34 with grid spacing less than 0.04 × 2π Å−1 were used. The Hellman-Feynman forces acting on each atom were less than 0.02 eV/ Å. All the calculations are spin polarized. Due to the underestimation of electron localization effects for f electrons of Ce in standard PBE, the PBE+U method with U = 5 eV35 on f orbitals was used. The calculated PBE lattice constant for bulk CeO2 is 5.42 Å, which is in excellent agreement with the experimental value of 5.41 Å at room temperature.36 It should be noted that PBE+U calculations overestimate the lattice constants (5.46 Å).35 Because PBE+U calculations still cannot match all the practices,35 we also carried out the standard PBE calculations in this work for comparison. We employed the DFT-D337 method of Grimme to evaluate the van der Waals (vdW) effect on the adsorption of O2 on CeO2 (111) surface. The p(3 × 3) slab with five trilayers was cleaved to mimic CeO2 (111) surface. In all calculations, we relaxed all the atoms in the slab. The fixed effects for the slab were also taken into account. We calculated the formation energies of oxygen vacancies for the slab with a bottom trilayer fixed, which gave almost exactly the same result compared with those without a fixed layer (see Table 1). Therefore, we only focused on the results without a fixed layer in this work. To study the relative stability of different configurations, the adsorption energy (Eads) has been calculated as Eads = Etot − Esub − Emol

fO (x , y) =

1 N

N

∑ i=1

1 −((x − xi)2 + (y − yi )2 )/2σ 2 e 2πσ 2

(2)

where xi and yi are the projected coordinates of oxygen atom in O2 in the xy-plane for the ith MD step, N is the total number of MD steps, and σ is broadening width, which has been set to 0.1 Å.

3. RESULTS AND DISCUSSION 3.1. Stability of Oxygen Vacancy and Its Diffusion on Surface. It remains a very controversial topics concerning the relative stability and the localization of f electrons of oxygen vacancies at surface and subsurface sites on the reduced CeO2 (111) face. In 2005, Esch et al.13 pointed out that both surface and subsurface oxygen vacancies were reported by STM measurements, and the linear and trimeric surface defects were also observed. Subsequently, density functional theory (DFT) studies predicted that oxygen vacancies prefer to stay at the subsurface site rather than the surface site.14,15 Recently, the observed trimeric and linear surface defects by STM measurements are proposed to be fluorine impurities44 or hydroxylvacancy combined species.45 In addition, the exact degree of the localization of f electrons cannot be determined in experiments,38 thus it is necessary to study the distribution of excess f electrons. Prior PBE+U calculations suggested that the localized f electrons located at the next nearest neighbor (NNN) Ce near oxygen vacancy.14,23 We have also performed calculations to study the location of excess f electrons, and the corresponding atomic configurations are shown in Figure 1. Our calculated results are list in Table 1. Our PBE+U results showed that the formation energies of surface and subsurface oxygen vacancies are respective 2.32 and 2.13 eV if there exist excess f electrons located at the nearest neighbor (NN) Ce. By contrast, if the excess f electrons are located at the NNN Ce, these formation energies decrease to 2.15 and 1.87 eV, respectively. The calculated results clearly indicated that the excess f electrons

(1)

In eq 1, Etot is the total energy of the adsorption system, Esub refers the total energy of the substrate, and Emol is the total energy of the molecule in the gas phase. B

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The subsurface-to-surface diffusion pathway of oxygen vacancy on a p(3 × 3) surface is shown in Figure 2, which

Figure 2. Potential energy profile for the subsurface-to-surface migration of oxygen vacancy. Four selected images marked by a, b, c, and d are shown in the bottom panel. Each dot represents an image, which is the same in the following figures.

corresponds to the vacancy concentration (Θ) of 1/9. According to our calculations, the hopping of one localized f electron from NNN Ce (see Figure 2a) to NN Ce (see Figure 2b) will increase the total energy by 0.06 eV, and the energy barrier in this process is 0.33 eV. Another transition state appears in the process of the migration of oxygen vacancy toward surface to form surface oxygen vacancy. The corresponding reaction barrier is 0.30 eV, and there is no hopping of f electron in this stage. Finally, the third transition state corresponds to the hopping of another f electron from NN Ce to NNN Ce with an energy decrement of 0.04 eV, and the energy barrier is 0.26 eV. Therefore, the overall reaction barrier for the migration of oxygen vacancy from subsurface site to surface site is 0.60 eV, which is in excellent agreement with the experimental value of 0.52 eV.1 For comparison, the reaction pathway and energy barriers using a p(4 × 4) surface (Θ = 1/16) is presented in Figure S1. We find that the corresponding barriers are respectively 0.32, 0.24, and 0.25 eV, and the overall barrier is 0.53 eV. Our results clearly point out that the vacancy concentration has almost no influence on the migration of oxygen vacancy. For PBE calculations, the small polaron hopping disappears as the excess f electrons are fully delocalized. In this case, there only exists one energy barrier of 0.34 eV as shown in Figure S2. Based on the above results, our calculations show that oxygen vacancies tend to locate at the subsurface sites, and the low diffusion barrier indicates the subsurface oxygen vacancies may be healed by the adsorbates on the surface. 3.2. Interplay between O2 with Subsurface Oxygen Vacancies. Similar to a-TiO2 (101) surface, the subsurface site is also the most energetically favorable position for oxygen vacancy on the CeO2 (111) surface.13−15,51 Prior experimental measurements had detected the signals of O−2 and O2− 2 on nanocrystalline CeO2 by Raman spectroscopy; however, the surface index of CeO2 adsorbed by these O2 species cannot be determined in situ.52 In the aspect of calculations, prior studies suggested that O2 can stick to the NNN Ce of CeO2 (111) surface with the subsurface oxygen vacancy to form O−2 ,23 indicating that it seems to support the experiment finding of O−2 .

Figure 1. Top views of (a) surface oxygen vacancy with the NN Ce3+, (b) subsurface oxygen vacancy with the NN Ce3+, (c) surface oxygen vacancy with the NNN Ce3+ and (d) subsurface oxygen vacancy with the NNN Ce3+. The green, red and black spheres represent surface oxygen, subsurface oxygen, and cerium, respectively. The violet spheres represent the Ce3+ cations. The surface and subsurface oxygen vacancies are marked by green and red dashed circles, respectively. The following figures also use these representations.

prefer to locate at NNN Ce, which are in line with prior studies.14,23 It is worth mentioning that PBE calculations also give the correct trends in the relative stability of surface and subsurface oxygen vacancies, although PBE calculations cannot provide the correct description of the localization of excess f electrons. Now we turn to the migration of an oxygen vacancy from subsurface site to surface site. Prior theoretical studies suggested that the oxygen diffusion barriers are in the range between 0.46 and 1.08 eV,46−50 and the estimated value in the experimental measurement is 0.52 eV.1 However, the entire diffusion process and the related hopping of the excess f electrons (so-called “small polaron” hopping) have not been depicted. C

DOI: 10.1021/acs.jpcc.6b07650 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Indeed, we can reproduce O−2 by using the same slab model23 with three trilayers. However, O−2 disappears if we use the converged slab model with five trilayers. To identify the possible adsorption structures and the corresponding properties of the adsorbed O2, we listed the adsorption energies, bond lengths, Bader charges, and vibrational frequencies in Table 2.

that O−2 will form on the NN Ce according to our PBE calculations, as the interaction between the adsorbed O2 and the NN Ce seems stronger (see Table S1). To check the possibility of formation of O−2 , we artificially built a slab (see Figure 3c) by pushing the adsorbed O2 toward the subsurface oxygen vacancy slightly. In this situation, O2 will gain more electrons than that on the NN Ce, resulting in the formation of O−2 on CeO2 (111) surface due to the slightly stronger bonding between the adsorbed O2 and the NNN Ce. If O2 fills in the surface oxygen vacancy, there exist three adsorption configurations as shown in Figure 3d−f. PBE+U calculations show that the reaction barrier is 0.10 eV. If the vdW effect was taken into account, the corresponding barrier is 0.07 eV shown in Figure 4a, indicting a negligible effect on the

Table 2. PBE+U Results of Adsorption Energy (Eads), Bond Length (r(O−O) in Å), Charge Transfer and Vibrational Frequency of the Adsorbed O2 in Figure 3a−fa structure

Eads

r(O− O)

charge (e)

frequency (cm−1)

assignment

a b c d e f

−0.04 (−0.14) −0.10 (−0.23) −0.00 −1.48 −1.45 −1.36

1.24 1.26 1.33 1.44 1.45 1.44

0.02 0.17 0.63 1.24 1.20 1.20

1525 1457 1219 975 938 959

Oδ− 2 Oδ− 2 O−2 O2− 2 O2− 2 O2− 2

a

To calculate Eads, the total energy of clean surface with subsurface oxygen vacancy is used as Esub, respectively.

For PBE+U calculations, the adsorption energies for O2 adsorption on the NN Ce (see Figure 3a) and NNN Ce (see Figure 3b) are −0.04 and −0.10 eV, respectively. If the vdW effect is introduced, these values become −0.14 and −0.23 eV, respectively. In addition, bond lengths, Bader charges, and vibrational frequencies in Table 2 indicate that O−2 may not form for O2 adsorption on the NN Ce and NNN Ce ions. Instead, we propose another species of Oδ− 2 , in which 0 < δ < 1. It is noted

Figure 4. (a) Potential energy profile for the diffusion of subsurface oxygen vacancy toward surface induced by the adsorption of O2. PBE +U and PBE+U+vdW results are represented by blue dots and pink triangles, respectively. (b) The corresponding vibrational frequency variation along the pathway. (c) The initial and final states are labeled by a and b, respectively.

reaction barrier. In addition, there is no reaction barrier in this process in PBE calculations, which corresponds to a spontaneous process as shown in Figure S3. During this process, the vibrational frequencies of the adsorbed O2 vary from 1428 to 974 cm−1 shown in Figure 4b, indicating that O2 − 2− changes from Oδ− 2 to O2 and then to O2 . Our PBE and PBE+U − calculations indicate that O2 is unstable, and it will transfer to O2− 2 by filling the surface oxygen vacancy. In addition, the adsorption form of O 22− is further confirmed by the corresponding bond lengths, Bader charges, and vibrational frequencies in Table 2. We can find that bond lengths and charge gains are around 1.40 Å and 1.20 e, respectively. The vibrational frequencies are in the range of 930−980 cm−1. All

Figure 3. Adsorption configurations of O2 on the reduced CeO2 (111) surface. O2 adsorption on (a) the NN and (b) NNN Ce in the presence of subsurface oxygen vacancy. The direction of the spin state of NNN Ce3+ will be changed by the adsorption of O2, and this Ce3+ is marked by dark blue sphere. (c) O2 adsorbs on the NNN Ce3+ as O−2 . (d−f) O2 fills the surface oxygen vacancy as O2− 2 . D

DOI: 10.1021/acs.jpcc.6b07650 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C these parameters are in accord with the typical characteristics of 53 O2− 2 . Generally speaking, the dynamical behaviors of adsorbates on surface will affect the catalytic reactions. The adsorption energies per O2 for structures shown in Figure 3d−f are −1.48, −1.45, and −1.36 eV, respectively. These small differences in adsorption energy indicate that they may change from one to another at moderate conditions. DFT calculations show that it is very easy for oxygen adatom to rotate.21 The diffusion of this oxygen adatom at room temperature is probably hindered due to the large energy barrier of 1.39 eV, which is in line with prior value of 1.42 eV.21 In addition to the rotation of the oxygen adatom, we have found that the adsorbed O2 would swap between O1 and O2, and the barrier for the site exchange is only 0.13 eV (see Figure 5). Moreover, MD simulations were

Figure 6. Position probability densities of two oxygen atoms (the topmost lattice O (O1) and adatom O (O2) represented by orange and blue, respectively) of O2 on the CeO2 (111) surface at 200 K (a and b) and 400 K (c and d).

Figure 7. (a) Projective distance to center site of oxygen vacancy of O1 (R1) and O2 (R2) as a function of time at 400 K. The sites of O1 and O2 and definitions of R1 and R2 are shown in panel b.

has received considerable attention due to the important catalytic properties of ceria.24−27 We have also performed calculations to study the adsorption behaviors of water on a reduced CeO2 (111). Eight different adsorption sites are compared to obtain the possible stable adsorption structure of H2O, and the corresponding structures are shown in Figure S4. For both PBE and PBE+U calculations, the corresponding adsorption energies per H2O are presented in Table S2. The results clearly show that the relative stability is not changed. It can be concluded that water adsorption in molecular form tends to bind to the NNN Ce shown in Figure S4g, and the corresponding adsorption energy per H2O is −0.59 eV, which is in good agreement with the prior study.27 Recent studies have suggested two different reaction pathways for H2O to interact with subsurface oxygen vacancy on anatase TiO2 (101) surface.17,18 For a reduced CeO2 (111) surface with subsurface oxygen vacancy, it seems that there exists only one possible pathway. We find that the dissociation of water on surface can facilitate the diffusion of subsurface oxygen vacancy, and the corresponding energy barriers for water dissociation and subsurface oxygen vacancy diffusion are respectively 0.09 and

Figure 5. Potential energy profile for the site exchange of O1 (the orange sphere) and O2 (the blue sphere).

carried out at 200 and 400 K. As there is no excess f electrons on the surface when O2− 2 forms on surface, only PBE calculations are presented here. As shown in Figure 6a,b, from the plotted position probability densities of O1 and O2 we can clearly see that O2 (blue region) rolls around O1 (orange region) at 200 K, which gives the triangular trajectory for O2. When the temperature is increased to 400 K, we have found the site exchange between O1 and O2 shown in Figure 6c,d, which can be confirmed by the variation of projective distances from O1 (R1) and O2 (R2) to the center site of oxygen vacancy in the time range of 30 ps (see Figure 7). From Figure 7, we find that the site exchange between O1 and O2 occurs many times in the time range of 30 ps, e.g., at around 17 ps. 3.3. Reactions of H2O with Subsurface Oxygen Vacancy. The interaction of water with CeO2 (111) surface E

DOI: 10.1021/acs.jpcc.6b07650 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C 0.07 eV, as shown in Figure 8. Alternatively, it seems less possible for the subsurface oxygen vacancy to diffuse to the

Figure 9. PBE results of H2O adsorption on the CeO2 (111) surface with the preadsorbed surface O adatom. The corresponding adsorption energies are labeled. Here, Emol is total energy of both H2O and O2, and Esub is the total energy of CeO2 (111) surface with one subsurface oxygen vacancy. The adsorption energies in the following discussions also use this definition.

Figure 8. Potential energy profile for the diffusion of subsurface oxygen vacancy toward the surface induced by H2O.

rotation of an O adatom. In addition, we have also performed PBE+U calculations for all these structures shown in Figure S6, which give similar adsorption energies and relative stabilities. In another case, if H2O adsorbs on the surface first, the oxygen vacancy will be filled by H2O and leave two surface hydroxyls. The most stable adsorption structure is shown in Figure 10a, and the subsequent O2 tends to land on the nearest Ce ion to form one hydrogen bond. The corresponding adsorption energy is −2.05 eV. Now we have also considered the possibility of H2O2 formation by the interaction of O2 and H2O on the reduced CeO2 (111) surface. Three possible coadsorption configurations, i.e., the formation of two OHs, H2O2, and OOH species, are shown in Figure 10d−f, respectively. The calculations clearly indicate that the formation of H2O2 on the surface is not the most energetically favorable. Among all the possible coadsorption configurations shown in Figure 9 and Figure 10, the structure shown in Figure 9c has the lowest total energy. The results strongly indicate that the sequence of water and O2 arrival on the reduced CeO2 (111) is not important, and it is possible for surface hydroxyls to react with the adsorbed O2 to form water and surface oxygen adatom. The most possible reaction pathways are shown in Figure 11. In the beginning of the reaction, the adsorbed O2 abstracts a hydrogen atom forming the OOH group from the surface hydroxyl, and the reaction barrier is 0.08 eV for this process, as shown from IS to M1 in Figure 11. Afterward, by climbing over a very small barrier of 0.08 eV, structure M2 forms on the surface. Then, the OOH group is split into one OH group and one oxygen adatom by striding over an energy barrier of 0.76 eV. In the following, water forms on the CeO2 (111) surface when the OH receives another H from the surface hydroxyl. Finally, water and oxygen adatom can also form on the surface by the interaction of the surface hydroxyl groups and molecular oxygen. To evaluate the localization effect of excess f electrons, the potential energy profile calculated by using PBE+U is also shown in Figure 11. It is found that the localization effect of excess f electrons may increase the reaction barrier from 0.76 to 1.05 eV. Interestingly, the relative stabilities of different adsorption configurations do not depend on the functional used.

surface site first, and, in the following, water dissociates via the interaction of water with surface oxygen vacancy due to high barrier of 0.60 eV. For PBE results, it also indicates that the first pathway is more likely to happen than the second one. In the case of PBE calculations shown in Figure S5, if the H2O dissociate prior to the oxygen vacancy diffusion, the two barriers are 0.09 and 0.10 eV, respectively. For another pathway of PBE results, the energy barriers for the diffusion of the subsurface oxygen vacancy and water dissociation are 0.22 and 0.08 eV (see Figure S5), respectively. Based on the discussions above, water is the most likely to dissociate first, and then subsurface oxygen vacancies tend to diffuse toward the surface. When the oxygen vacancy reaches the surface, the dissociated H2O can heal the vacancy barrierlessly. Moreover, it seems less possible for H2O desorption from the surface due to the large adsorption energy of −0.59 eV. The results imply that the adsorbed H2O on CeO2 (111) surface strongly affect the distribution of oxygen vacancies. 3.4. Co-adsorptions and Reactions of H2O and O2. As it is analyzed above, the subsurface oxygen vacancy can be pulled out by the adsorption of O2, and then the oxygen vacancy will be filled leaving behind an oxygen adatom. Alternatively, if water lands on a reduced CeO2 (111) surface first, water may dissociate on the surface in the beginning, and then the subsurface oxygen vacancy diffuse to the surface. Finally, two surface hydroxyl groups form by the interaction of the dissociated water with the surface oxygen vacancy. As water and molecular oxygen play crucial roles in catalytic reactions, it is important to investigate the coadsorption of O2 and H2O on the reduced CeO2 (111) surface. If O2 preadsorbs on the reduced surface, the possible adsorption configurations of the subsequent arrival of water are shown in Figure 9a−d. The adsorption structure shown in Figure 9c has the lowest adsorption energy of −2.84 eV, and there forms a hydrogen bond between the adsorbed water and the oxygen adatom. It is worth noting that, in this configuration, the O adatom locates at the bridge site between the surface O and Ce in the second layer, which implies that the formation of one hydrogen bond can cover the increased energy of the F

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Figure 10. Top views of (a−c) O2 adsorption on the CeO2 (111) surface with two preadsorbed hydroxyls, (d) two OHs, (e) H2O2, and (f) OOH adsorption on the stoichiometry CeO2 (111) surface. The corresponding adsorption energies are also labeled.

ORCID

Hu Xu: 0000-0002-2254-5840 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS In summary, the diffusion mechanism of subsurface oxygen vacancy and the surface reactivity enhancement induced by subsurface oxygen vacancy have been studied by DFT calculations. The adsorption of O2 or H2O will significantly reduce the energy barriers for the diffusion of subsurface oxygen vacancy toward CeO2 (111) surface. When the oxygen vacancy reaches the surface, O2 adsorbed on the surface will fill the oxygen vacancy leaving behind an O adatom, while water will also heal the oxygen vacancy by forming a pair of surface hydroxyls. MD simulations showed that surface O adatom prefers to roll around the surface lattice O at low temperatures. It is also possible for surface O adatom to swap positions with surface lattice O as temperatures increase. In addition, it is found that H2O may dissociate prior to the migration of subsurface oxygen vacancy. The formation of water and O adatom on the surface due to the interaction between the adsorbed O2 and surface hydroxyls is also revealed.

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REFERENCES

(1) Paier, J.; Penschke, C.; Sauer, J. Oxygen defects and surface chemistry of ceria: quantum chemical studies compared to experiment. Chem. Rev. 2013, 113, 3949−3985. (2) Dresselhaus, M. S.; Thomas, I. L. Alternative energy technologies. Nature 2001, 414, 332−337. (3) Ganduglia-Pirovano, M. V. The non-innocent role of cerium oxide in heterogeneous catalysis: A theoretical perspective. Catal. Today 2015, 253, 20−32. (4) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Active nonmetallic Au and Pt species on ceria-based water-gas shift catalysts. Science 2003, 301, 935−938. (5) Gorte, R. J. Ceria in catalysis: From automotive applications to the water-gas shift reaction. AIChE J. 2010, 56, 1126−1135. (6) Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J. Oxygen vacancies in transition metal and rare earth oxides: Current state of understanding and remaining challenges. Surf. Sci. Rep. 2007, 62, 219− 270. (7) Wu, X. P.; Gong, X. Q.; Lu, G. Z. Role of oxygen vacancies in the surface evolution of H at CeO2 (111): A charge modification effect. Phys. Chem. Chem. Phys. 2015, 17, 3544−3549. (8) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive devices for computing. Nat. Nanotechnol. 2013, 8, 13−24. (9) Cheng, H. Z.; Selloni, A. Surface and subsurface oxygen vacancies in anatase TiO2 and differences with rutile. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 092101. (10) He, Y.; Dulub, O.; Cheng, H.; Selloni, A.; Diebold, U. Evidence for the predominance of subsurface defects on reduced anatase TiO2 (101). Phys. Rev. Lett. 2009, 102, 106105. (11) Scheiber, P.; Fidler, M.; Dulub, O.; Schmid, M.; Diebold, U.; Hou, W.; Aschauer, U.; Selloni, A. (Sub)surface mobility of oxygen vacancies at the TiO2 anatase (101) surface. Phys. Rev. Lett. 2012, 109, 136103.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07650. More discussions on the PBE calculations of adsorption properties and potential energy profiles (PDF)

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ACKNOWLEDGMENTS

This work is supported by the National Natural Science Foundation of China (NSFC, Grant Nos. 11674148, 11204185, 11334003 and 11404159) and the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (Grant No. JCYJ20160531190054083).

Figure 11. Potential energy profile for the reactions of O2 with a pair of hydroxyl groups on the CeO2 (111) surface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-755-88018210. G

DOI: 10.1021/acs.jpcc.6b07650 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b07650 J. Phys. Chem. C XXXX, XXX, XXX−XXX