Theoretical Study of Trimethylacetic Acid Adsorption on CeO2 (111

Article pubs.acs.org/JPCC

Theoretical Study of Trimethylacetic Acid Adsorption on CeO2(111) Surface Weina Wang,†,‡ S. Thevuthasan,‡ Wenliang Wang,*,† and Ping Yang*,‡,§ †

Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710062, China ‡ Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *

ABSTRACT: Trimethylacetic acid (TMAA) adsorption on stoichiometric and oxygen-deficient CeO2(111) surfaces was investigated using density functional theory that accounts for the on-site Coulomb interaction via a Hubbard term (DFT +U) and long-range dispersion correction. Both the molecular state and dissociative state (TMAA → TMA− + H+) were identified on stoichiometric and oxygen-deficient CeO2(111) surfaces. For the stoichiometric surface, two thermodynamically favorable configurations with adsorption energies of the order of −30 kcal/mol are identified; one is a molecule adsorption state, and the other one is a dissociative state. For the oxygen-deficient surface, dissociative states are more favorable than molecular states. The most favorable configuration is the dissociative adsorption of TMAA with the adsorption energy of the order of −77 kcal/mol. The dissociated TMA moiety takes the position of oxygen vacancy, forming three Ce−O bonds. The signature vibrational frequencies for these thermodynamically stable structures are reported as well as their electronic structures. The effects of long-range dispersion interactions are found to be negligible for geometries but important for adsorption energies. been determined both theoretically8 and experimentally9 to be the most stable low-index surface. The three general adsorption modes being studied are monodentate, bridging bidentate, and chelating bonding, shown in Scheme 1.10,11

1. INTRODUCTION CeO2 is an important material with wide applications, including use as a catalyst and electrolyte and in solar cells because of its ability to accommodate varying charge states and facilitate relatively facile surface oxidation−reduction that allows it to mediate oxygen concentration. It is well-known that the concentration of Ce3+ ions increases with the reduction of the size of CeO2 nanoparticles, and consequently oxygen exchange and redox reactions occur easily.1 Because of this unique property, CeO2 nanoparticles have been recently identified for a wide range of biomedical applications in which the ligand−nanoparticle interactions are of paramount importance in controlling the required properties.2 For instance, CeO2 nanoparticles coated with appropriate ligands can work as effective drug carrier and delivery agents.3,4 Because their efficiency depends on the binding stability of the nanoparticles with the ligands, it is vital to understand the chemistry between the anchoring groups on ligands and the surfaces of CeO2 nanoparticles. One of the most commonly used anchoring groups is carboxylate; thus, the chemical adsorption of carboxylic acids on the nanoparticle surface is of great interest. Because of the complexity of nanoparticle surfaces, researchers have been using solid surfaces to study various facets of CeO2 nanoparticle surfaces. Carefully designed experimental studies have been undertaken on the adsorption of carboxylic acids and carboxylates, such as formic acid, acetic acid, and formate, on the CeO2(111) surface,5−7 which has © XXXX American Chemical Society

Scheme 1. Adsorption Mode of Carboxylate on Ceria Surface

However, concerning the most stable configuration of carboxylic acid on the CeO2(111) surface, varying results were reported. Stubenrauch et al.5 studied the reactions of formic and acetic acids on CeO2(111) and CeO2(100) surfaces using temperature-programmed desorption (TPD) and highresolution electron energy loss spectroscopy (HREELS). They found that carboxylic acids were dissociated on both surfaces to form surface carboxylates and hydroxyl groups and identified Received: October 7, 2015 Revised: December 13, 2015

A

DOI: 10.1021/acs.jpcc.5b09790 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C the formation of monodentate carboxylates using HREELS. These results are in contrast with other studies suggesting that the bidentate configuration is favored over the monodentate configuration.6,7 Recently, when Senanayake et al.6 studied the decomposition of HCOOH over stoichiometric and oxygendeficient CeO2 surfaces using TPD and soft X-ray photoelectron spectroscopy (XPS), they concluded that all formates on the ceria films should be bridging bidentate based on near edge X-ray absorption fine structure data and the monodentate configuration would be more likely only on the fully oxidized CeO2(111). Gordon et al.7 studied the adsorption of formic acid and formate on CeO2(111) surface using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculation. In this study, bridging bidentate formate was reported to be the most stable configuration on a stoichiometric CeO2 surface, in which the two oxygen atoms in formate are bound to two adjacent Ce atoms on the surface. The monodentate formate state that bonds to a single Ce atom via only one formate oxygen atom was found to be a saddle point on the local potential energy surface. Similar phenomena were observed for adsorption of carboxylic acids over TiO2 surfaces based on both experimental and theoretical studies.11−22 Intensive theoretical studies have investigated the adsorption and reaction of carboxylic acid on the TiO2 surface and provided great molecular-level insights.11,12,17−20,22 In contrast, computational studies of carboxylic acid adsorption on ceria surface are rather limited.6,10,23 The present study seeks to provide a molecular picture of the interaction between carboxylic acid and the CeO2(111) surface using density functional theory. Trimethylacetic acid (TMAA) is chosen as a model compound to investigate chemical adsorption of carboxylic acid on the stoichiometric and oxygendeficient CeO2 surfaces. It is well-known that surface reactions on CeO2 are significantly impacted by the surface oxygen vacancies,24,25 because these defects are often taken as preferential adsorption sites. Therefore, we considered both stoichiometric and oxygen-deficient CeO2(111) surfaces in this study. This work is organized as follows. The computational details are given in section 2. In section 3, we present the results and discussion, including the geometrical structures, adsorption energies, electronic structures and vibrational properties. Finally, the conclusions are summarized in section 4.

Figure 1. Models of stoichiometric and oxygen-deficient CeO2(111) surface.

studied in the current work. The oxygen-deficient CeO2(111) surface was generated by removing one surface oxygen atom, illustrated in Figure 1b,c. TMAA adsorption on a CeO2(111) surface has been investigated by means of spin-polarized density functional theory using the Vienna ab initio simulation program (VASP)32−35 with the projector-augmented wave (PAW) potential.36,37 Plane wave basis sets with a cutoff energy of 400 eV are used to expand the valence electron wave functions. The cerium 5s, 5p, 5d, 4f, and 6s electrons; the oxygen and carbon 2s and 2p electrons; and the hydrogen 1s electron were treated as valence electrons. The electron exchange and correlation were treated within the generalized gradient approximation using the optimized Perdew−Burke−Ernzerhor functional (optPBE-vdW)38,39 that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U) and longrange dispersion correction. We used a 3 × 3 × 1 Monkhorst− Pack grid for the surface Brillouin zone sampling. The geometries of surfaces and TMAA adsorption over surfaces were obtained until the forces on all relaxed atoms was less than 0.05 eV/Å. Vibrational frequencies were calculated within the harmonic approximation by numerical evaluation of the dynamical Hessian matrix elements using the first derivative of the atomic energy gradients, displacing each atom along the Cartesian coordinates by 0.02 Å. Gas-phase molecules of TMAA and O2 are calculated in a unit cell with the same size as in the surface-adsorbent calculations. Conventional DFT method with local-density approximation (LDA) or generalized gradient approximation (GGA) functional usually fails to describe the strong correlation interaction, which occurs during the reduction of Ce4+ to Ce3+ on the oxygen-deficient surface.8,40,41 In this study, we used the DFT +U method, in which a Hubbard U parameter was introduced for the Ce3+ to describe the on-site coulomb interaction. Therefore, it eliminates the self-interaction error and improves the description of correlation effects. The value of U is set to be 5.0 eV according to earlier theoretical works;40,42 hence, the localization of the Ce 4f electrons in the partially reduced ceria can be properly described.40 The calculated bulk lattice constant of CeO2, 5.437 Å, is in excellent agreement with the experimental value (5.411 Å)43 as well as the previously reported first-principles value of 5.436 Å.44 The energy gap of the bulk between O 2p and Ce 4f, 2.38 eV, also satisfactorily agrees with earlier theoretical (2.3 eV)44 and experimental values (3.0 eV).45 The chemical adsorption starts with plausible physical adsorption; therefore, various initial adsorption models were sampled based on electrostatic matching: negatively charged atoms interact with positively charged atoms, and vice versa. The classification of carboxylic acids adsorption, as suggested for the formic acid adsorption by

2. MODELS AND METHODS A surface is represented by a slab model with three-dimensional (3-D) periodic boundary conditions. The CeO2(111) surface, which is the most stable one among the low index surfaces of ceria, was modeled by a p(3 × 3) unit cell with three O−Ce−O trilayers, shown in Figure 1a. The periodic slabs were separated from its images in the direction perpendicular to the surface by a vacuum gap of 15 Å, resulting in the dimensions of a simulation cell of 11.48 × 11.48 × 22.81 Å3. The atoms in the upper two trilayers and the TMAA molecule were allowed to relax fully, whereas the bottom trilayer was fixed at the bulk positions during geometry optimization to mimic the bulk structure. The choice of this three-trilayer slab model is a compromise of the accuracy of the model and the computing cost. We examined the geometries and adsorption energies as a function of slab thickness using two-,26 three-,27−29 and fourtrilayer30,31 slab models. The detailed results are shown in Figure S1 and Table S1 in the Supporting Information. Both stoichiometric and oxygen-deficient CeO2(111) surface were B


Article

The Journal of Physical Chemistry C

Figure 2. Atom displacements of the oxygen-deficient surface: (a) top view of optimized structure (for clarity, only the top trilayer is shown) and (b) atom displacements of the top trilayer in the direction perpendicular to the surface.

Vittatini et al.,11 shown in Scheme 1, was adopted in the present work. The strength of adsorption can be quantified by adsorption energy, Eads, which was calculated according to

3. RESULTS AND DISCUSSION 3.1. Stoichiometric and Oxygen-Deficient CeO2(111) Surfaces. 3.1.1. Geometric Structure. The geometric structure of the stoichiometric CeO2(111) surface we obtained is in excellent agreement with published theoretical work.29,52 The relaxed stoichiometric CeO 2(111) surface possess little perturbation compared to the bulk structure. The surface atoms in the top trilayer were slightly contracted with a distance of 0.008 Å compared to the underlying atoms. The Ce−O bond length in the surface is 2.35 Å, which is nearly identical to 2.36 Å in the deeper layers. Previous theoretical works have discussed the geometry and electronic structure of the stoichiometric surface in detail; thus, our data are not repeated here given the good consistency with the reported results. The oxygen-deficient surface is generated by removing one oxygen atom (VO) from the optimized stoichiometric CeO2(111) surface, illustrated in Figure 1. Because each surface oxygen atom is bound to three Ce atoms, when one vacancy is formed, three Ce−O bonds are cleaved and the symmetry of these three Ce atoms is broken. The relaxed oxygen-deficient surface shows significant structural change induced by oxygen vacancy, shown in Figure 2. The top view of the relaxed structure, Figure 2a, shows that the three nearest neighboring Ce cations of the vacancy on the surface move away from the vacancy by 0.17−0.23 Å. The spin density analysis, shown in Figure 3, clearly illustrates that two of these

Eads = Eadsorbed slab − E bare surface − ETMAA

in which Eadsorbed slab, Ebare surface, and ETMAA are the electronic energies of adsorbed TMAA on a CeO2(111) surface, a bare CeO2(111) surface, and a TMAA molecule in vacuum, respectively. A more negative Eads value thus indicates stronger (exothermic) adsorption compared to isolated systems. The van der Waals (vdW) interactions resulting from dynamical correlations between fluctuating charge distributions likely affect adsorption energies.46−48 Given that the vdW interaction is poorly described in the standard DFT calculation, we are interested in understanding the effects of vdW interactions on geometries and energies of absorbed species. The adsorption energies were corrected using a nonlocal van der Waals density functional recently developed by Langreth and Lundqvist and co-workers (vdW-DF)49 and implemented in the VASP by J. Klimeš.50 We optimized stable structures using various functional including PBE, revPBE, and vdW corrected funcationals (optPBE-vdW and revPBE-vdW). The geometries are not sensitive to the choice of functional. In contrast, the choice of functional is important for absorption energies, as listed in Table S2 in the Supporting Information. We chose optPBEvdW exchange−correlation functional because the exchange functional was optimized for the correlation counterpart and vdW-DF.50,51 The nonlocal correction part of the energy is included by calculating the total energy in the following equation: Exc = ExGGA + EcLDA + Ecnl

where EGGA is the GGA exchange energy, ELDA the local x c correlation energy within the local density approximation (LDA), and Enlc the nonlocal correlation energy. Parallel to the pure GGA functional, the degree of long-range dispersion corrections on the adsorbed geometries and absorption energies will be quantified. Additionally, we assessed the effects of spin−orbit coupling (SOC) interactions on adsorption energies. Our results show that the SOC can be neglected in the CeO2 systems because the energies change less than 0.3 kcal/mol with inclusion of SOC. The detailed results are listed in Table S3 in Supporting Information. Therefore, the SOC interactions are not included in the following discussion.

Figure 3. Isosurface of spin densities for oxygen-deficient CeO2(111) surface, shown in cyan. The contour value is 0.002 e/Å3. For clarity, only the top trilayer is shown. The legend is the same as that applied in Figure 1.

neighboring cerium cations are reduced (denoted as Ce3+). In contrast, the next three neighboring ions (subsurface oxygen anions) move closer to the vacancy by 0.11−0.24 Å, illustrated in Figure 2a. These displacements are compensated by further smaller displacements of their neighboring atoms. These data are in excellent agreement with earlier theoretical calculations C


Article


Figure 4. Projected density of states (PDOS) for stoichiometric and oxygen-deficient CeO2(111) surfaces. Spin up/down densities of states are arbitrarily assigned positive/negative values along the ordinate axis. Panels a and b are for the stoichiometric surface; panels c and d are for the oxygen-deficient surface.

by Fabris et al.53 In their paper, the first cerium neighbors to the vacancy relax away from the oxygen vacancy O atoms by 0.11−0.16 Å, and the second cerium neighbors to the vacancy contact inward in the vacancy region by 0.06 and 0.18 Å. The ionic displacements of the oxygen-deficient surface along the direction perpendicular to the surface are shown in Figure 2b. Positive values indicate ions moving upward to the surface. The largest displacements come from the subsurface oxygen anions that are closest to the oxygen vacancy, which moves upward to the surface by 0.11−0.17 Å. For the surface Ce cations, except for the one cation that moves downward to the bulk with a displacement of 0.09 Å, others largely stay in the same surface as the stoichiometric case. The associated Ce−O bond lengths change accordingly to accommodate the changes of atomic positions; for example, the Ce−O bonds involving the Ce4+ cation are shortened by 0.04 Å and the Ce−O bonds involving the Ce3+ cation are elongated by 0.09 Å. Similar observations of bond length changes are also consistent with previous PW91+U calculations.40 3.1.2. Electronic Structure. Density of electronic states (DOS) for stoichiometric and oxygen-deficient CeO2(111) surface have been reported.40,53−55 Our results are consistent with the reported data, shown in Figure 4. For the stoichiometric CeO2(111) surface, the valence band is mainly derived from O 2p states, with small contributions from Ce 4f and 5d, and the characteristic narrow unoccupied Ce 4f band dominates the conduction band. When panels a and c of Figure 4 are compared, a new peak appears near the Fermi level upon introduction of an oxygen deficiency, which is the signature of the presence of reduced Ce3+ ions. These localized Ce 4f states, completely unfilled in the stoichiometric CeO2(111) surface, begin to be occupied in the oxygen-deficient surface. This phenomenon has been observed by X-ray photoelectron spectroscopy (XPS).56,57 Two cerium atoms should be reduced to Ce3+ upon removal of one oxygen atom. Spin density analysis for the oxygen-deficient CeO2(111) surface, shown in Figure 3, clearly illustrates that two of three cerium ions originally coordinated to the oxygen vacancy are reduced to Ce3+, leaving one as Ce4+. Furthermore, we investigated the magnetic coupling between the two reduced Ce3+ cations. The antiferromagnetic state

(AFM) is marginally higher in energy than the ferromagnetic state (FM) with less than 1.0 kcal/mol. This small energy difference is attributed to the weak coupling of the two unpaired electrons. This result can be further verified by the magnetization of the Ce3+ cations. In the FM state, each of the two reduced Ce3+ cations has a magnetic moment of 0.96 μB. In the AFM state, one Ce3+ possesses a magnetic moment of 0.96 μB and the other one has −0.96 μB. The adsorption of TMAA does not affect the magnetic states of the surface. The geometric parameters and adsorption energies are nearly identical for the AFM and FM states, shown in Tables S4 and S5 in the Supporting Information. Therefore, we focus on the FM states in the following discussion. 3.1.3. Formation Energy of Oxygen Vacancy. In the present study, the vacancy formation energy is defined by EV = E(VO) + 1/2E(O2) − E(clean slab), in which E(VO), E(O2), and E(clean slab) are the energies of the oxygen-deficient surface, gas-phase O2 molecule, and stoichiometric surface, respectively. For the gas-phase O2, the calculated bond length is 1.23 Å, which agrees with the corresponding experimental value, 1.21 Å. The calculated formation energy in this work is 59.39 kcal/mol (i.e., 2.58 eV), which is in excellent agreement with the previously reported result, 2.60 eV.40 Overall, the optPBE-vdW functional reproduces well the geometrical and electronic structures of stoichiometric and oxygen-deficient surfaces. 3.2. TMAA Adsorption on Stoichiometric CeO2(111) Surface. 3.2.1. Adsorption Configurations. The adsorption of TMAA can be categorized into two main types, molecular state and dissociative state, based on whether the hydroxyl group of carboxylic acid is dissociated. In the dissociative state (TMAA → TMA− + H+), according to the literature, the H+ can be bound to either nearby bridging surface oxygen or diffuse further from the TMA− moiety.10,11 We studied all possible adsorption modes, including monodentate, bridging bidentate, and chelating bidentate, as illustrated in Scheme 1. We sampled three main types of adsorption modes in both molecular state and dissociative state and on both stoichiometric and oxygendeficient surfaces. Five stable configurations for molecular state and eight stable configurations for dissociative state on stoichiometric surface D


Article


and the COT double bond length is 1.22 Å, respectively. The O−H bond length in the gas-phase TMAA is 0.98 Å but elongates to 1.05 in the molecular state mA. In dissociative states dA, dB, dC, dD, and dE, the two C−O bond lengths become nearly identical with a difference less than 0.02 Å, which is consistent with the dissociation of the proton forming a conjugated carboxylate group. The most stable configuration for molecular state is shown in Figure 5, mA, with adsorption energy of −26.23 kcal/mol. In mA, carboxylic acid group forms a coordination bond to a surface Ce atom (OT−Ce) and a hydrogen bond to its neighboring surface O atoms (OT−H···OS). The OT−Ce bond length is 2.47 Å, and the distances of OT−H and H−OS in the OT−H···OS hydrogen bond are 1.05 and 1.60 Å, respectively. TMAA is further stabilized by this strong hydrogen bond OT− H···OS, leaving the surface oxygen atom being pulled out of the surface plane by 0.22 Å. As a result, the distance between the bonding OS atom and bonding Ce atom is elongated by 0.12 Å after the adsorption of TMAA. The second stable configuration, mB, also forms a hydrogen bond, in which TMAA bonds on noncoordinated Ce and OS atoms therefore differ from the configuration mA, resulting in a less stable configuration with adsorption energy of −20.35 kcal/mol. Other molecular adsorptions without hydrogen bond formation are monodentate mC and mD and bidentate mE, which are all significantly less stable than mA and mB. As shown in configurations dA−dG in Figure 5, TMAA can also be adsorbed on the CeO2(111) surface in dissociative states. The most stable configuration is dA, with the adsorption energy of −31.59 kcal/mol. The TMA− moiety is bridged to the surface cerium atoms via two OT−Ce bonds with lengths of 2.49 and 2.46 Å. The dissociated proton stays close to the TMA− moiety to form a hydrogen bond to provide additional stability. The next stable configurations are dB and dC with nearly identical adsorption energies, −28.65 and −28.49 kcal/ mol, respectively. The geometry of configuration dB has a high similarity to that of dA, except with a longer hydrogen−oxygen length of 2.82 Å and therefore a weaker hydrogen bond. In configuration dC, TMAA bridges to the Ce and OS, with the OT−Ce bond length 2.36 Å. This geometry is very similar to the stable molecular state configuration mA, except that the OT−H bond was elongated to 1.42 Å and H atom dissociates from TMAA to the surface oxygen. The further the dissociated

were found, shown in Figure 5. The corresponding adsorption energies with inclusion of vdW correction are also given. It is

Figure 5. Adsorption configurations of TMAA on stoichiometric surface and their corresponding energies. The distances are in angstroms.

notable that no stable chelating mode was found even using a starting geometry that featured a chelating mode on both surfaces. The geometries became bridging bidentate or monodentate after optimization. The selected geometric parameters of adsorbed TMAA are list in Table 1. In all molecular-state cases, the TMAA forms a coordination bond, OT−Ce, to the surface with or without a hydrogen bond between a hydrogen atom in TMAA and a surface oxygen atom, H−OS. In this study, notation OT stands for an oxygen atom in TMAA and OS is an oxygen atom on CeO2(111) surface. The geometry of TMAA is evidently deformed upon adsorption. The largest change takes place in the carboxylate group, i.e., the bond lengths of C−OT and OT−H. For instance, in gas-phase TMAA, the C−OT single bond length is 1.37 Å

Table 1. Selected Geometric Parameters of TMAA for Adsorption Configurations on the Stoichiometric Surfacea molecular state

dissociative state

PBE

TMAA

mA

mB

mC

mD

mE

dA

dB

dC

dD

dE

dF

dG

dH

CO (Å) C−OH (Å) O−H (Å) H−Os ∠OCO (deg)

1.22 1.37 0.98

1.25 1.32 1.07 1.52 121.8

1.24 1.33 1.04 1.65 124.0

1.23 1.35 0.98

1.22 1.38 0.99

1.22 1.37 0.98

1.27 1.29 1.80 0.99 124.6

1.27 1.28

1.28 1.27 1.41 1.08 123.0

1.27 1.28

1.28 1.28

1.23 1.34

1.23 1.32

1.23 1.32

1.00 122.9

0.98 123.5

0.98 123.3

optPBE-vdW

TMAA

mA

mB

mC

mD

mE

dA

dB

dC

dD

dE

dF

dG

dH

CO (Å) C−OH (Å) O−H (Å) H−Os ∠OCO (deg)

1.22 1.37 0.98

1.25 1.33 1.05 1.60 121.8

1.24 1.34 1.03 1.68 123.7

1.25 1.32 1.05

1.22 1.39 1.00

1.22 1.38 0.98

1.28 1.28

1.28 1.28

1.23 1.34

1.23 1.32

1.23 1.33

120.5

119.8

1.29 1.27 1.42 1.07 123.1

1.28 1.28

121.8

1.27 1.29 1.83 0.98 124.7

0.98 124.9

0.98 125.0

0.99 122.9

0.98 123.4

0.98 123.2

121.6

121.4

121.3 120.9 molecular state

119.8

0.98 125.2

0.98 124.9

0.97 0.97 125.0 125.0 dissociative state

a Os is the surface oxygen atom that H is bound to. The exchange functional of PBE is used for the geometry optimization without vdW corrections, and optPBE-vdW is used to include vdW corrections.

E


Article

The Journal of Physical Chemistry C proton diffuses away from the TMA−, the lower the adsorption energy. The same effects can also be observed in bidentate dissociative configurations, such as dD and dE. This result is not surprising because the positively charged proton can stabilize the negatively charged TMA− by staying close. Overall, the monodentate modes in dissociative bonding category, dF, dG, and dH, are less stable with lower adsorption energies. To further understand the energetics of the hydrogen bond in mA and dC, we scanned the energy profile by moving the hydrogen atom in mA and dC from OS to OT along the hydrogen bond, shown in Figure 6. During the energy profile

configurations using standard DFT (PBE) and then refined all the stable configurations using vdW corrected funcational (optPBE-vdW). The selected geometric parameters of adsorbate TMAA or TMA¯ are listed in Table 1. When the geometries of these two methods, PBE and optPBE-vdW, are compared, it is clear that the structures of adsorption configurations are not sensitive to inclusion of van der Waals interaction. The ∠OCO bond angle difference between the two methods is less than 0.5°, and the largest bond length difference is 0.02 Å. However, the effects of vdW interaction on the adsorption energies are important, because stronger binding between TMAA and the surface is suggested. Particularly, the order of the bonding configurations with small energy difference is altered when the vdW correction is included. For example, dC is more stable than dB using PBE+U, but dC is slightly less stable than dB with vdW correction. Based on PBE+U calculations, it can be deduced that the two configurations dA and dC can coexist because of the very small energy difference (−19.16 and −19.64 kcal/mol). In comparison, with the vdW correction, configuration dA is more favorable than configuration dC by 3.10 kcal/mol. The latter result is more consistent with earlier findings on formic and acetic acid which are adsorped on CeO2(111) surface in a bidentate dissociative manner.38 Thus, long-range dispersion interaction cannot be ignored in estimation of adsorption energies. For instance, the nonbonding configuration mE with a positive adsorption energy of 0.12 kcal/mol becomes a bonding configuration with binding energy of −12.34 kcal/mol when including vdW interaction. Compared to standard PBE+U calculation, the vdW-DF calculation shows a systematic increase of the order of 11 kcal/mol in the adsorption energies. As a result, all configurations have an attractive nature compared with isolated TMAA and CeO2(111) surface. 3.2.3. Vibrational Frequencies. Vibrational frequency analysis on the thermodynamically stable configurations can help to identify signature peaks for each binding mode and therefore will assist the assignment of the binding modes observed in experiments. We performed vibrational frequency calculations for representative stable states of TMAA on CeO2(111) surface, and the results are listed in Table 3. For the most stable configuration, dA, both the asymmetric and symmetric O−C−O stretching modes are found to be redshifted with respect to that of gas-phase TMA−. For instance, the asymmetric O−C−O stretching mode νas(O−C−O) is 1507 cm−1, whch is red-shifted by 134 cm−1 compared to TMA−. The symmetric stretching mode νs(O−C−O) (1329 cm−1) is red-shifted of 28 cm−1. Similar trends were found for other dissociative configurations, such as dB. The ν(OS−H) for dA is 3450 cm−1 and is red-shifted by 180 cm−1 compared to TMAA. This can be explained by the formation of a hydrogen bond between TMA− and H+ in dA, which weakens the OS−H bond and therefore makes the corresponding ν(OS−H) frequency red-shifted. For dC, the CO bond stretching mode, ν(CO), is 1403 cm−1. It is red-shifted by 340 cm−1

Figure 6. Energy profile of moving the hydrogen atom between OS and OT along the hydrogen bond. The black line is for the energy scan starting from the mA in Figure 5 and moving the hydrogen atom closer to the surface oxygen atom with a step size of 0.05 Å while keeping all other atoms frozen. The red line is the parallel scan starting from dC configuration in Figure 5 and moving the hydrogen atom away from OS to OT.

scan, all the atoms are frozen except the hydrogen atom between OS and OT atoms. It is worth noting that this is a double-well hydrogen bond because there is a low barrier on the order of 0.8 kcal/mol when moving hydrogen atom from one oxygen to another. Obviously, the energy is 2.32 kcal/mol lower with the hydrogen atom closer to the surface oxygen, suggesting that the H atom tends to dissociate from TMAA. In summary, on the CeO2(111) stoichiometric surface, the dissociative state is slightly more favorable compared to molecular state by 5.36 kcal/mol. The dissociated proton tends to stay close to the TMA− moiety to form a hydrogen bond to further stabilize the structure. Meanwhile, the TMA− moiety prefers bridging two surface Ce on CeO2(111) surface over chelating and monodentate modes. These conclusions are consistent with an experimental study of formate adsorption on CeO2(111) surface.7 Formate adopts a bridging bidentate configuration on CeO2(111) surface, and the deprotonated hydrogen of formic acid remains close to formate. 3.2.2. Importance of van der Waals Correction. To evaluate the long-range dispersion interactions, we quantify the impact of van der Waals interactions on the geometries and energies of TMAA adsorption. We optimized all possible

Table 2. Adsorption Energies of TMAA on Stoichiometric Surface (in kcal/mol) molecular state

dissociative state

Eads

mA

mB

mC

mD

mE

dA

dB

dC

dD

dE

dF

dG

dH

PBE optPBE-vdW

−15.16 −26.23

−9.47 −20.35

−5.76 −16.15

−5.31 −18.03

0.12 −12.34

−19.16 −31.59

−15.59 −28.65

−19.64 −28.49

−14.94 −27.56

−13.44 −26.26

−7.94 −19.13

−2.54 −14.00

−1.71 −12.99

F


Article


Table 3. Calculated Vibrational Frequencies of Isolated TMAA Molecule and Representative Surface Adsorption Configurations (in cm−1)a stoichiometric surface dissociative state vibrational mode ν(OS−H) ν(OT−H) νs(C−H) ν(OT···H−OS) ν(CO) νas(O−C−O) νs(O−C−O) a

TMAA 3631 2982

TMA−

dA

dB

3450

3722

2981

2984

2991

1641 1357

1507 1329

1498 1333

1743

dC

2974 1946 1403 1358

oxygen-deficient surface dissociative state r-dA

r-dB

3743

3688

2985

2993

1530 1387

1523 1348

All vibrational modes were stretching modes. νas is asymmetric mode, and νs is symmetric mode.

Figure 7. Projected density of states (PDOS) for representative configurations of TMAA adsorbed on stoichiometric CeO2(111) surface: (a) Total DOS and PDOS of Ce, oxygen, and sum of TMAA moiety of dA configuration; (b) PDOS for adsorbed TMAA moiety of dA configuration; (c) Total DOS and PDOS of Ce, oxygen, and sum of TMAA moiety of dC configuration; (d) PDOS for adsorbed TMAA moiety of dC configuration; (e) PDOS of gas-phase TMAA.

compared to the ν(CO) mode of gas-phase TMAA, 1743 cm−1, indicative of strong interactions with the surface. The ν(OT−H) mode of TMAA, 3631 cm−1, disappears in dC; meanwhile, a new mode, νas(OT···H−OS), with value 1946 cm−1 appears, suggesting formation of a hydrogen bond. 3.2.4. Electronic Structure. Figure 7 shows the projected density of state analysis of energetically favorable configurations dA and dC as well as gas-phase TMAA for comparison. For the gas-phase TMAA (Figure 7e), the peaks are very sharp, typical for an isolated molecule. For adsorbed TMAA, shown in either panel b or d of Figure 7, TMAA O p bands are broadened, and

two sharp peaks are merged into one broadening peak. The alternation of the O p bands indicates that strong interactions take place between the oxygen atoms in TMAA and surface cerium ions. These strong interactions can also be reflected by the charges on the ligand atoms. We studied the charges of adsorbed species (TMAA and TMA−) and the coordinated atoms on the surface using Bader’s atoms-in-molecules method,58,59 shown in Table 4. The charges on the TMA− moiety drop by 0.32−0.25 electrons after TMAA is adsorbed on the CeO2(111) surface, suggesting charge transfer from TMA− to the surface. Therefore, TMAA/TMA− binds to the G


Article

The Journal of Physical Chemistry C Table 4. Bader’s Charge Analysis of Representative Thermodynamically Favorable Configurations (in e)a configuration

TMA

C(OO)

O(−)

O()

Os

bound Ce

bound Ce*

TMAA TMA−

−0.81 −1

1.51 1.53

−1.37 −1.16

dA dB dC

−0.75 −0.70 −0.68

1.53 1.48 1.44

−1.16 −1.15 −1.07

−1.12 0.81 −1.19 Stoichiometric Surface −1.19 0.71 −1.15 0.68 −1.17 0.68

diss H

−1.38 −1.38 −1.34

2.41 2.39 2.39

2.41 2.40

r-dA r-dB

−0.78 −0.78

1.48 1.40

−1.17 −1.14

Oxygen-Deficient Surface −1.25 0.71 −1.23 0.78

−1.43 −1.45

2.38 2.37

2.11 2.12

bound Ce*

2.10 2.10

a TMA− is the dissociated TMA moiety, and C(OO), O(−), and O() is for the carbon atom, oxygen atom, and carbonyl oxygen atom in the carboxylate group, respectively. “Diss H” is for the dissociated proton and Os is for the surface oxygen atom. “Bound Ce” is for the surface Ce4+ atoms that are bound to the adsorbate. “Bound Ce*” is for the surface Ce3+ atoms that are bound to the adsorbate.

Figure 8. Optimized adsorption configurations of TMAA on oxygen-deficient surface. Molecular states are depicted from r-mA to r-mK, and dissociative states are depicted from r-dA to r-dD. Distances are in angstroms.

surfaces, where kinks and low coordinated Ce atoms are available upon further oxidization.10 Interactions of TMAA with the oxygen-deficient surface are much stronger than those with the stoichiometric surface. Among molecular adsorption states, the most stable configuration, r-mA, the carbonyl oxygen atom of TMAA replaces the vacancy oxygen forming three Ce−OT bonds with adsorption energy of −38.33 kcal/mol. The competition among the three Ce atoms results in slightly longer Ce−OT bond lengths compared to other configurations. The second stable configuration, r-mB, is formed via two OT-Ce bonds between the oxygen atoms of TMAA and the two reduced Ce cations resulting in an energy gain of −37.42 kcal/mol. This configuration resembles the similarity to the molecular adsorption mE on the stoichiometric surface but with shorter bond lengths. Overall, the molecular states on the oxygendeficient surface possess higher binding energies on average

surface via chemical interactions and the adsorption is chemical in nature. 3.3. TMAA Adsorption on Oxygen-Deficient CeO2(111) Surface. 3.3.1. Geometric Configurations. For the oxygen-deficient CeO2(111) surface, four categories of adsorption were examined: (i) O atom of TMAA filling in the O vacancy; (ii) TMAA bound via one OT−Ce bond in monodentate manner; (iii) forming a bridging bidentate binding mode via two OT−Ce bonds; and (iv) via one OT− Ce and one H−OS bond. Various initial structures are constructed and optimized to consider the difference between reduced and nonreduced Ce atoms on the surface. All stable configurations along with their binding energies are shown in Figure 8, and selected geometric parameters are listed in Table 5. No stable chelating bidentate mode is found on the partially reduced surface, while chelating mode is found on nanoparticle H


Article


Table 5. Selected Geometry Parameters for Different Adsorption Configurations of TMAA on Oxygen-Deficient Surfacea molecular states PBE

TMAA

r-mA

r-mB

r-mC

r-mD

r-mE

r-mF

r-mG

r-mH

r-mI

r-mJ

r-mK

CO (Å) C−OH (Å) O−H (Å) H−Os ∠O−C−O (deg)

1.22 1.37 0.98

1.24 1.34 0.98

1.23 1.35 0.98

1.22 1.38 0.99

1.23 1.35 0.98

1.23 1.35 0.98

1.22 1.37 1.00

1.21 1.39 0.98

1.22 1.36 0.98

116.4

115.6

116.3

1.26 1.29 1.72 1.00 125.0

1.22 1.38 1.02

121.6

1.26 1.29 1.53 1.05 123.1

121.6

121.5

119.2

120.2

r-dA 1.26 1.30

r-dB 1.26 1.30 2.09 0.98 120.4

r-dC 1.26 1.31

r-dD 1.26 1.31

0.97 123.9

0.97 124.0

PBE CO (Å) C−OH (Å) O−H (Å) H− Os ∠O−C−O (deg)

0.97 121.5

121.2 118.9 dissociative state

molecular states optPBE-vdW

TMAA

r-mA

r-mB

r-mC

r-mD

r-mE

r-mF

r-mG

r-mH

r-mI

r-mJ

r-mK

CO (Å) C−OH (Å) O−H (Å) H−Os ∠O−C−O (deg)

1.22 1.37 0.98

1.25 1.34 0.98

1.24 1.36 0.98

1.22 1.39 0.99

1.23 1.36 0.98

1.23 1.36 0.99

1.22 1.38 1.00

1.21 1.41 0.98

1.22 1.37 0.98

115.7

115.1

115.8

1.26 1.30 1.72 1.00 125.0

1.22 1.39 1.02

121.4

1.27 1.29 1.56 1.04 123.0

121.6

121.3

118.6

119.6

r-dA 1.26 1.31

r-dB 1.26 1.31 2.11 0.98 120.1

r-dC 1.26 1.32

r-dD 1.26 1.32

0.98 123.7

0.98 123.9

optPBE-vdW CO (Å) C−OH (Å) O−H (Å) H−Os ∠O−C−O (deg)

0.98 121.2

120.8 118.5 dissociative state

a

The exchange functional of PBE is used for the geometry optimization without vdW corrections, and optPBE-vdW is used to include vdW corrections.

Table 6. Adsorption Energies on Oxygen-Deficient Surface (in kcal/mol) molecular states Eads

r-mA

r-mB

r-mC

r-mD

r-mE

PBE optPBE-vdW

−19.62 −38.33

−20.79 −37.42

−18.50 −34.67

−18.34 −30.76

−16.40 −28.19

Eads PBE optPBE-vdW

r-dA −59.23 −76.99

r-dB −55.48 −75.66

r-dC −36.09 −53.56

r-dD −34.66 −51.82

r-mF

r-mG

−11.56 −7.51 −27.01 −21.58 dissociative state

r-mH

r-mI

r-mJ

r-mK

−6.40 −18.06

−5.36 −18.99

−1.34 −17.72

2.03 −10.15

replace the oxygen vacancy by forming multibonds with the surface Ce atoms. In addition, the adsorption energies for the oxygen-deficient surface being higher than that of the stoichiometric surface suggests that the surface defects enhance adsorption. 3.3.2. Vibrational Frequencies. Vibrational frequency calculations for the most stable configurations on the oxygendeficient surface were analyzed and are listed in Table 3. Not surprisingly, the O−C−O asymmetric and symmetric stretching modes are red-shifted with respect to the gas-phase TMA− moiety, similar to the stoichiometric surface. More specifically, the frequency of O−C−O asymmetric stretching mode, νas(OCO), of the most stable configuration, r-dA, is 1530 cm−1, which is red-shifted by 111 cm−1 compared to that of TMA− and blue-shifted by ∼20 cm−1 compared to dA configuration for the stoichiometric surface. Therefore, by oxidizing stoichiometric ceria surface, one could observe a blueshift in the asymmetric O−C−O stretching mode region.

compared to the stoichiometric surface. It is also worth noting that the binding energies to the reduced Ce atoms are usually higher than to the nonreduced form. In the most stable dissociative configuration, r-dA, the two carboxylate oxygen atoms of TMA− moiety bind to the three Ce cations which are originally coordinated to the vacancy oxygen. The next stable configuration, r-dB, differs from r-dA mainly in the location of the dissociated proton. The closeness of the proton to the TMA− moiety, r-dB, causes slightly longer Ce−OT distances and a slightly lower energy of 1.33 kcal/mol. Hence, the dissociated proton is likely to diffuse away from TMA on the reduced surface, in marked contrast with the stoichiometric surface. Switching to binding to a Ce4+ atom causes a high energy penalty, as evidenced in configurations rdC and r-dD, which possess adsorption energies ∼25 kcal/mol higher. Clearly, the energy difference between molecular states and dissociative states is larger on reduced surface; therefore, the dissociative states are more preferable. The adsorbent will I


Article


Figure 9. Projected density of states of dissociative adsorption of TMAA on oxygen-deficient CeO2(111) surface: (a) Total DOS and PDOS of Ce, oxygen, and sum of TMAA moiety of r-dA configuration; (b) PDOS for adsorbed TMAA moiety of r-dA configuration; (c) Total DOS and PDOS of Ce, oxygen, and sum of TMAA moiety of r-dB configuration; and (d) PDOS for adsorbed TMAA moiety of r-dB configuration.

3.3.4. Electronic Structure. The projected density of states of configurations r-dA and r-dB are shown in Figure 9. The PDOS plots present high similarity to the bare oxygen-deficient surface by having the partial occupied Ce 4f states close to the Fermi level. Therefore, the reduced Ce3+ ions on the oxygendeficient surface are not being oxidized by the ligand upon adsorption of TMAA. The partial DOS of the TMAA moiety on the surface, shown in Figure 9b,d, suggest that strong interactions between the oxygen atoms of TMAA moiety and surface Ce atoms take place. The Bader charge analysis shows a small amount of charge transfer from ligand to the surface (∼0.22 electrons). Spin densities for the two favorable configurations, r-dA and r-dB, were also calculated. The reduced Ce atoms are illustrated by the isosurface of spin charge densities, shown in Figure 10

This is consistent with the fact that the 4f electron is mostly localized at the Ce atom.

4. CONCLUSIONS Trimethylacetic acid adsorption on the stoichiometric and oxygen-deficient CeO2(111) surfaces was investigated using density functional theory that accounts for the on-site Coulomb interaction via a Hubbard term (DFT+U). The vdW correction has little effect on the geometries, whereas it is important for the adsorption energy calculation, especially to get correct thermodynamically stable configurations. Hence, it is recommended that the long-range interaction be included for surface adsorption studies. Both molecular state and dissociative state (TMAA →TMA− + H+) were identified on the CeO2(111) surfaces. For the stoichiometric surface, two thermodynamically favorable configurations with adsorption energies of the order of −30 kcal/mol were found; one is molecule adsorption state and the other one is dissociative state. For the oxygen-deficient surface, dissociative states are more favorable than molecular states. The most favorable configuration is the dissociative adsorption of TMAA with adsorption energy of the order of −77 kcal/mol. Dissociated TMA− takes the position of oxygen vacancy, forming three Ce−O bonds. Therefore, surface oxygen defects significantly increase the adsorption energies. The signature vibrational frequencies for the thermodynamically stable structures show large red-shifts of the asymmetric stretching mode of (O−C−O) of the adsorbate upon adsorption. The charge analysis suggests the adsorption to the CeO2 surface is chemical adsorption because charge transfer is observed.

Figure 10. Isosurface of spin density for dissociative adsorption configurations of TMAA on oxygen-deficient CeO2(111) surface: for (a) r-dA and (b) r-dB in Figure 8. For clarity, only the top trilayer is shown. The legend is the same as that applied in Figure 1.

■

using cyan. The spin density remains localized on the Ce atoms originally coordinated to the oxygen vacancy, in the same positions as the bare oxygen-deficient surface. Hence, the spin density distribution is dominated by the oxygen vacancy and is not sensitive to the adsorption of TMAA. Our calculations show that the magnetic moments of the two Ce3+ are not affected by the TMAA adsorption because the magnetization of each Ce3+ atom is identical to the bare oxygen-deficient surface.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09790. Optimized geometries of slab models with various thicknesses (Figure S1); adsorption energies of dA on J


Article


■

(6) Senanayake, S. D.; Mullins, D. R. Redox Pathways for Hcooh Decomposition over CeO2 Surfaces. J. Phys. Chem. C 2008, 112, 9744−9752. (7) Gordon, W. O.; Xu, Y.; Mullins, D. R.; Overbury, S. H. Temperature Evolution of Structure and Bonding of Formic Acid and Formate on Fully Oxidized and Highly Reduced CeO2(111). Phys. Chem. Chem. Phys. 2009, 11, 11171−11183. (8) Nolan, M.; Grigoleit, S.; Sayle, D. C.; Parker, S. C.; Watson, G. W. Density Functional Theory Studies of the Structure and Electronic Structure of Pure and Defective Low Index Surfaces of Ceria. Surf. Sci. 2005, 576, 217−229. (9) Nörenberg, H.; Briggs, G. A. D. Defect Formation on CeO2(111) Surfaces after Annealing Studied by Stm. Surf. Sci. 1999, 424, L352− L355. (10) Lu, Z.; Karakoti, A.; Velarde, L.; Wang, W.; Yang, P.; Thevuthasan, S.; Wang, H.-f. Dissociative Binding of Carboxylic Acid Ligand on Nanoceria Surface in Aqueous Solution: A Joint in Situ Spectroscopic Characterization and First-Principles Study. J. Phys. Chem. C 2013, 117, 24329−24338. (11) Vittadini, A.; Selloni, A.; Rotzinger, F. P.; Grätzel, M. Formic Acid Adsorption on Dry and Hydrated TiO2 Anatase (101) Surfaces by Dft Calculations. J. Phys. Chem. B 2000, 104, 1300−1306. (12) Lyubinetsky, I.; Deskins, N. A.; Du, Y.; Vestergaard, E. K.; Kim, D. J.; Dupuis, M. Adsorption States and Mobility of Trimethylacetic Acid Molecules on Reduced TiO2(110) Surface. Phys. Chem. Chem. Phys. 2010, 12, 5986−5992. (13) Grinter, D. C.; Nicotra, M.; Thornton, G. Acetic Acid Adsorption on Anatase TiO2(101). J. Phys. Chem. C 2012, 116, 11643−11651. (14) Thomas, A. G.; Syres, K. L. Adsorption of Organic Molecules on Rutile TiO2 and Anatase TiO2 Single Crystal Surfaces. Chem. Soc. Rev. 2012, 41, 4207−4217. (15) Gutierrez-Sosa, A.; Martinez-Escolano, P.; Raza, H.; Lindsay, R.; Wincott, P. L.; Thornton, G. Orientation of Carboxylates on TiO2(110). Surf. Sci. 2001, 471, 163−169. (16) Tanner, R. E.; Liang, Y.; Altman, E. I. Structure and Chemical Reactivity of Adsorbed Carboxylic Acids on Anatase TiO2(001). Surf. Sci. 2002, 506, 251−271. (17) Nilsing, M.; Persson, P.; Ojamäe, L. Anchor Group Influence on Molecule-Metal Oxide Interfaces: Periodic Hybrid Dft Study of Pyridine Bound to TiO2 Via Carboxylic and Phosphonic Acid. Chem. Phys. Lett. 2005, 415, 375−380. (18) Gong, X.-Q.; Selloni, A.; Vittadini, A. Density Functional Theory Study of Formic Acid Adsorption on Anatase TiO2(001): Geometries, Energetics, and Effects of Coverage, Hydration, and Reconstruction. J. Phys. Chem. B 2006, 110, 2804−2811. (19) Ojamäe, L.; Aulin, C.; Pedersen, H.; Kall, P. O. Ir and QuantumChemical Studies of Carboxylic Acid and Glycine Adsorption on Rutile TiO2 Nanoparticles. J. Colloid Interface Sci. 2006, 296, 71−78. (20) McGill, P. R.; Idriss, H. Dft Study of Carboxylic Acids Modes of Adsorption on Rutile TiO2(011) Surfaces. Surf. Sci. 2008, 602, 3688− 3695. (21) Tao, J.; Luttrell, T.; Bylsma, J.; Batzill, M. Adsorption of Acetic Acid on Rutile TiO2(110) Vs (011)-2 × 1 Surfaces. J. Phys. Chem. C 2011, 115, 3434−3442. (22) Muir, J. M. R.; Idriss, H. Study of the Interaction of Formic Acid, Formaldehyde and Formamide with the Bulk Terminated (1 × 1) and Reconstructed (2 × 1) Surfaces of Rutile TiO2(011). Surf. Sci. 2013, 607, 187−196. (23) Neitzel, A.; Lykhach, Y.; Johánek, V.; Tsud, N.; Skála, T.; Prince, K. C.; Matolín, V.; Libuda, J. Decomposition of Acetic Acid on Model Pt/CeO2 Catalysts: The Effect of Surface Crowding. J. Phys. Chem. C 2015, 119, 13721−13734. (24) Trovarelli, A. Catalytic Properties of Ceria and CeO2Containing Materials. Catal. Rev.: Sci. Eng. 1996, 38, 439−520. (25) 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.

slab surfaces with various thicknesses (Table S1); adsorption energies of TMAA on surface as a function of density functionals (Table S2); spin−orbit coupling effects on the adsorption energies of r-dA and r-dB (Table S3); geometry parameters and magnetic moments for the ferromagnetic and antiferromagnetic states of rdA and r-dB configurations (Table S4); relative energies and adsorption energies for the ferromagnetic and antiferromagnetic states of r-dA and r-dB configurations (Table S5) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Ajay S. Karakoti for helpful discussions on the role of ligands on ceria surfaces. Portions of this research (Yang) were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Biosciences and Geosciences Division (CSGB), Heavy Element Chemistry Program and was performed at Los Alamos National Laboratory (LANL) under Contract DE-AC52-06NA25396. LANL is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy. Portions of this research were supported by the intramural program of William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. DOE BER and located at Pacific Northwest National Laboratory (PNNL). The calculations were performed using the Molecular Science Computing (MSC) Facilities in the EMSL. Weina Wang is grateful for the Alternate Sponsored Fellowship program at PNNL. Portions of this work were supported by the National Nature Science Foundation of China (21173139), the Fundamental Research Funds for the Central Universities (GK:201101004, 201303004) and Shaanxi Innovative Team of Key Science and Technology (2013KCT-17).

■

REFERENCES

(1) Paun, C.; Safonova, O. V.; Szlachetko, J.; Abdala, P. M.; Nachtegaal, M.; Sa, J.; Kleymenov, E.; Cervellino, A.; Krumeich, F.; van Bokhoven, J. A. Polyhedral CeO2 Nanoparticles: Size-Dependent Geometrical and Electronic Structure. J. Phys. Chem. C 2012, 116, 7312−7317. (2) Vincent, A.; Inerbaev, T. M.; Babu, S.; Karakoti, A. S.; Self, W. T.; Masunov, A. m. E.; Seal, S. Tuning Hydrated Nanoceria Surfaces: Experimental/Theoretical Investigations of Ion Exchange and Implications in Organic and Inorganic Interactions. Langmuir 2010, 26, 7188−7198. (3) Chen, J.; Patil, S.; Seal, S.; McGinnis, J. F. Rare Earth Nanoparticles Prevent Retinal Degeneration Induced by Intracellular Peroxides. Nat. Nanotechnol. 2006, 1, 142−150. (4) Karakoti, A.; Singh, S.; Dowding, J. M.; Seal, S.; Self, W. T. Redox-Active Radical Scavenging Nanomaterials. Chem. Soc. Rev. 2010, 39, 4422−4432. (5) Stubenrauch, J.; Brosha, E.; Vohs, J. M. Reaction of Carboxylic Acids on CeO2(111) and CeO2(100). Catal. Today 1996, 28, 431− 441. K


Article

The Journal of Physical Chemistry C (26) Bruix, A.; Neyman, K. M.; Illas, F. Adsorption, Oxidation State, and Diffusion of Pt Atoms on the CeO2(111) Surface. J. Phys. Chem. C 2010, 114, 14202−14207. (27) Li, H. Y.; Wang, H. F.; Gong, X. Q.; Guo, Y. L.; Guo, Y.; Lu, G. Z.; Hu, P. Multiple Configurations of the Two Excess 4f Electrons on Defective CeO2(111): Origin and Implications. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 193401. (28) Liu, B.; Liu, J.; Li, T.; Zhao, Z.; Gong, X.-Q.; Chen, Y.; Duan, A.; Jiang, G.; Wei, Y. Interfacial Effects of CeO2-Supported Pd Nanorod in Catalytic Co Oxidation: A Theoretical Study. J. Phys. Chem. C 2015, 119, 12923−12934. (29) Mei, D.; Deskins, N. A.; Dupuis, M.; Ge, Q. Methanol Adsorption on the Clean CeO2(111) Surface: A Density Functional Theory Study. J. Phys. Chem. C 2007, 111, 10514−10522. (30) Capdevila-Cortada, M.; García-Melchor, M.; López, N. Unraveling the Structure Sensitivity in Methanol Conversion on CeO2: A DFT + U Study. J. Catal. 2015, 327, 58−64. (31) Lu, Y.-H.; Chen, H.-T. Computational Investigation of NO2 Adsorption and Reduction on Ceria and M-Doped CeO2 (111) (M = Mn, Fe) Surfaces. J. Phys. Chem. C 2014, 118, 10043−10052. (32) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (33) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (34) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (35) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 14251− 14269. (36) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (37) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (38) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (39) Perdew, J. P.; Burke, K.; Ernzerhof, M. Errata: Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1997, 78, 1396. (40) Nolan, M.; Parker, S. C.; Watson, G. W. The Electronic Structure of Oxygen Vacancy Defects at the Low Index Surfaces of Ceria. Surf. Sci. 2005, 595, 223−232. (41) Fabris, S.; de Gironcoli, S.; Baroni, S.; Vicario, G.; Balducci, G. Taming Multiple Valency with Density Functionals:A Case Study of Defective Ceria. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 71, 041102. (42) Li, H. Y.; Wang, H. F.; Gong, X. Q.; Guo, Y. L.; Guo, Y.; Lu, G. Z.; Hu, P. Multiple Configurations of the Two Excess 4f Electrons on Defective CeO2(111): Origin and Implications. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 193401. (43) Gerward, L.; Olsen, J. S. Powder Diffraction Analysis of Cerium Dioxide at High Pressure. Powder Diffr. 1993, 8, 127−129. (44) Chen, H. T.; Choi, Y. M.; Liu, M. L.; Lin, M. C. A Theoretical Study of Surface Reduction Mechanisms of CeO2 (111) and (110) by H-2. ChemPhysChem 2007, 8, 849−855. (45) Wuilloud, E.; Delley, B.; Schneider, W. D.; Baer, Y. Spectroscopic Evidence for Localized and Extended F-Symmetry Sates in CeO2. Phys. Rev. Lett. 1984, 53, 202−205. (46) Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (47) Wesolowski, T. A.; Parisel, O.; Ellinger, Y.; Weber, J. Comparative Study of Benzene···X (X = O2, N2, CO) Complexes Using Density Functional Theory: The Importance of an Accurate Exchange-Orrelation Energy Density at High Reduced Density Gradients. J. Phys. Chem. A 1997, 101, 7818−7825.

(48) Zhang, Y.; Pan, W.; Yang, W. Describing van der Waals Interaction in Diatomic Molecules with Generalized Gradient Approximations: The Role of the Exchange Functional. J. Chem. Phys. 1997, 107, 7921−7925. (49) Lee, K.; Murray, É. D.; Kong, L.; Lundqvist, B. I.; Langreth, D. C. Higher-Accuracy van der Waals Density Functional. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 081101. (50) Klimeš, J.; Bowler, D. R.; Michaelides, A. Van der Waals Density Functionals Applied to Solids. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 195131. (51) Klimeš, J.; David, R. B.; Angelos, M. Chemical Accuracy for the van der Waals Density Functional. J. Phys.: Condens. Matter 2010, 22, 022201. (52) Chen, H.-L.; Chen, H.-T. Role of Hydroxyl Groups for the O2 Adsorption on CeO2 Surface: A Dft+U Study. Chem. Phys. Lett. 2010, 493, 269−272. (53) Fabris, S.; Vicario, G.; Balducci, G.; de Gironcoli, S.; Baroni, S. Electronic and Atomistic Structures of Clean and Reduced Ceria Surfaces. J. Phys. Chem. B 2005, 109, 22860−22867. (54) Shi, S. Q.; Tang, Y. H.; Ouyang, C. Y.; Cui, L. X.; Xin, X. G.; Li, P. J.; Zhou, W. W.; Zhang, H.; Lei, M. S.; Chen, L. Q. O-Vacancy and Surface on CeO2: A First-Principles Study. J. Phys. Chem. Solids 2010, 71, 788−796. (55) Beste, A.; Mullins, D. R.; Overbury, S. H.; Harrison, R. J. Adsorption and Dissociation of Methanol on the Fully Oxidized and Partially Reduced (1 1 1) Cerium Oxide Surface: Dependence on the Configuration of the Cerium 4f Electrons. Surf. Sci. 2008, 602, 162− 175. (56) Henderson, M. A.; Perkins, C. L.; Engelhard, M. H.; Thevuthasan, S.; Peden, C. H. F. Redox Properties of Water on the Oxidized and Reduced Surfaces of CeO2(111). Surf. Sci. 2003, 526, 1− 18. (57) Mullins, D. R.; Radulovic, P. V.; Overbury, S. H. Ordered Cerium Oxide Thin Films Grown on Ru(0001) and Ni(111). Surf. Sci. 1999, 429, 186−198. (58) Bader, R. F. W. Atoms in Molecules. Acc. Chem. Res. 1985, 18, 9−15. (59) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360.

L


Theoretical Study of Trimethylacetic Acid Adsorption on CeO2 (111

Recommend Documents