Periodic Density Functional Theory Study of VOn ... - ACS Publications

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Periodic Density Functional Theory Study of VOn Species Supported on the CeO2(111) Surface Cristina Popa, María Veronica Ganduglia-Pirovano,†,* and Joachim Sauer Humboldt University, Chemistry Department, Unter den Linden 6, 10099 Berlin, Germany

bS Supporting Information ABSTRACT: We model monomeric vanadia adspecies on the CeO2(111) surface of composition VOn 3 Ce12O24 (n = -1, 0, ..., 4) using the DFTþU approach and statistical thermodynamics. At low oxygen pressure (10-9 atm), VO4 is the most stable species below 400 K; in the 400-900 K range, VO2 is stable; and above 900 K, VO becomes stable. In all of these systems, vanadium is stabilized in the þ5 oxidation state. Using the energies of hydrogenation and oxygen vacancy formation as reactivity descriptors, we predict an enhanced reactivity of the vanadia/ceria system in Mars-van Krevelen-type oxidation reactions. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f states. We also calculate the frequencies of the normal vibrational modes of the supported VOn species and their infrared intensity.

1. INTRODUCTION Oxide-supported vanadium oxide catalysts have been studied quite extensively because of their high activity for selective oxidation reactions.1-4 The activity of supported vanadia is noticeably affected by the specific oxide used as support.2,4-6 In oxidative dehydrogenation (ODH) reactions occurring according to the Mars-van Krevelen mechanism, vanadia supported on ceria shows a remarkably high activity as compared to silica- and alumina-supported catalysts.4,6,7 Notwithstanding the well-known promoting effect of ceria in oxidation catalysis, which is typically attributed to its ability to store, release, and transport oxygen ions,8 the origin of the support effect on the activity of vanadia/ceria catalysts has remained a topic of continuous investigations. The complexity of the surface structure of supported powder catalysts has been partly responsible for the shallow understanding of the support effect. The study of welldefined model systems of increasing complexity, both experimentally and theoretically, is of importance for analyzing the support effect on the structure and properties of vanadia/ceria systems. Previously, the structure and reactivity of vanadia supported on CeO2(111) have been studied using photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD) of methanol.9-11 In particular, the results indicate that vanadium deposition is accompanied by the reduction of the CeO2(111) surface and the oxidation of vanadium, even when oxygen is not present in the gas phase during deposition. A similar effect was reported for ceria-supported vanadia powder catalysts.12 Hence, to gain insight into the specific interaction between vanadia and ceria from theoretical investigations, it is important that the computational method is able to provide an accurate description of the electronic structure of partially reduced ceria. Density functional theory augmented with a Hubbard-like U term describing the on-site Coulomb interactions (DFTþU) has proven to be adequate (see refs 13 and 14, and references therein). r 2011 American Chemical Society

CeO2(111)-supported isolated VOn (n = 1-4) vanadia species have been previously investigated15 using the Perdew-Wang 91 functional (PW91) which makes use of the generalized gradient approximation (GGA). But both LDA (local density approximation) and GGA functionals incorrectly predict reduced ceria to be metallic.14 The reason is well understood, namely, the incomplete cancellation of the Coulomb self-interaction stabilizes delocalized solutions. In this paper we describe and analyze in detail the structure and electronic properties of monomeric vanadia adspecies on the CeO2(111) surface of composition VOn 3 Ce12O24 (n = -1, 0, ...,4) using the DFTþU approach. Special attention is given to changes in the oxidation state of vanadium and cerium ions upon VOn adsorption. Statistical thermodynamics is applied to account for the effect of oxygen partial pressure at a given temperature (or temperature at a given pressure) on the stability of the supported vanadia species. Because little is known about the surface structure of supported metal oxides and vibrational spectroscopy is one of the major tools for structural characterization, we also present and analyze the results of the calculated vibrational frequencies. To discuss the catalytic activity of the supported VOn species for ODH reactions, we use two recently proposed reactivity descriptors, namely the energy of hydrogenation,16 which relates to the energy barrier of the rate-determining step,17,18 i.e., the hydrogen abstraction from the surface-bound reactant, and the energy of oxygen defect formation,19,20 which relates to the reaction energy in accord to the Mars-van Krevelen mechanism. Thus, the more exoenergetic the hydrogenation energy is (and the lower the defect

Received: August 28, 2010 Revised: January 25, 2011 Published: March 25, 2011 7399

dx.doi.org/10.1021/jp108185y | J. Phys. Chem. C 2011, 115, 7399–7410

The Journal of Physical Chemistry C formation energy), the higher the activity of the catalyst is (and the lower the barrier). Some of the results have recently been used in combination with experiments (high-resolution scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), PES with synchrotron radiation, and temperature-programmed desorption (TPD) of methanol) on model catalysts, where vanadia is deposited onto CeO2(111) thin films grown on Ru(0001) forming small V = 0 terminated species such as monomers, dimers, and trimers at low loadings.21,22 As a result of these efforts, the atomic structure of low coverage VOx species supported on CeO2(111) has been resolved to a far greater extent than in the past.21 Moreover, the calculated reactivity parameters have been shown to be consistent with the experimentally observed higher reactivity of vanadia/ceria systems toward methanol compared with that of vanadia and ceria surfaces.22 However, a comprehensive discussion of the structure, electronic, vibrational, and chemical properties of the stable CeO2(111)-supported monomeric species in thermodynamic equilibrium with an O2 environment at finite temperature, as obtain with the DFTþU approach, is still missing in the published literature.

2. COMPUTATIONAL DETAILS 2.1. Methods and Models. Density functional theory (DFT) with the PBE (Perdew-Burke-Ernzerhof) functional23 as implemented in the Vienna Ab Initio Simulation Package (VASP, version 5.1.49)24-26 was used. A Hubbard U term was added to the PBE functional (DFTþU) employing the rotationally invariant formalism by Dudarev et al.,27 in which only the difference (Ueff = U - J) between the Coulomb U and exchange J parameters enters. The calculations were spin-polarized. The projector augmented wave method (PAW)28-30 was used to describe the interaction between the ions and the electrons with the frozen-core approximation.29 The Ce (4f, 5s, 5p, 5d, 6s), O (2s, 2p), and V (3p, 3d, 4s) electrons were treated as valence states using a plane-wave basis set with a kinetic energy cutoff of 400 eV. For Ce atoms, a value of Ueff = 4.5 eV was used, which was calculated self-consistently by Fabris et al.31 using the linear response approach of Cococcioni and de Gironcoli.32 This value is within the range 3.0-5.5 eV reported to provide localization of the electrons left upon oxygen removal from CeO2.33 Tests calculations for Ueff = 5.3 eV, as well as with Ueff = 4.5 eV and an additional Hubbard Ueff parameter of 2.0 or 4.5 eV for the V d-states were performed. In addition, tests were also performed with the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional34 (see Section 6 and Table 6 in Supporting Information). Ceria crystallizes in a cubic fluorite structure (face-centered cubic, Fm3m) with one formula unit per primitive unit cell. The thermodynamically most stable ceria surface is the oxygenterminated (111) face.35,36 For all surface calculations, the model adopted was that of a periodic slab with a p(2 2) surface unit cell, and for the Brillouin zone integration, a Monckhorst Pack 3 3 1 mesh was used. The CeO2(111) model includes three cerium layers and six oxygen layers, i.e., three stoichiometric OCe-O trilayers with a cell composition (CeO2)12, and a vacuum space of ∼13 Å. The bulk equilibrium lattice constant (5.49 Å) previously calculated by PBEþU (Ueff = 4.5 eV) was used.37 We also considered oxygen defective ceria surfaces with one surface or subsurface oxygen defect per p(2 2) surface unit cell, i.e., a

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surface defect concentration of 1/4. These calculations employed the models published by Ganduglia-Pirovano et al.,13 but in the present study the vacuum space is larger to avoid spurious interactions between facing surfaces of successive slabs upon the adsorption of VOn species. To find the most stable adsorbate structure for a given composition, {VOn 3 Ce12O24}CeO2(111), VOn clusters were added on one side of the clean and defective ceria slabs in many possible positions (above a surface or subsurface O atom, above Ce4þ and Ce3þ ions, etc.) and orientations. Many possible spin states were considered for each structure. The conjugate-gradient algorithm was used to optimize the ion positions. The VOn clusters and the six top atomic layers of the ceria support were allowed to fully relax, while the three bottom layers were kept fixed to their bulk positions. Atoms were moved until forces were smaller than 0.05 eV 3 Å-1. Vibrational frequencies and the normal modes are obtained by diagonalization of the (full) mass-weighted force constant matrix in Cartesian coordinates (Hessian). The force constants were obtained from finite differences of the forces with atomic displacements of (0.02 Å. Test calculations showed that the vibrations of the adsorbed VOn are mostly coupled to those of the first O-Ce-O trilayer. Thus, the two bottom O-Ce-O trilayers were kept fixed in the calculation of the vibrational frequencies. No imaginary frequencies were found. A scale factor was not applied to the calculated frequencies. The corresponding infrared intensities in the calculated spectra are proportional to the squared gradient of the dipole moment component perpendicular to the surface, each line of which is subject to Gaussian broadening. The description of the vibrations of all systems is based on visual inspection of the calculated normal modes. 2.2. Surface Free Energy. We are interested in the relative stability of different isomers with VOn 3 (CeO2)12 composition in thermodynamic equilibrium with an O2 environment at finite temperatures. The isomers can differ by the presence or not of an oxygen defect in the ceria layers, VOnþ1/Ce12O23 or VOn/ Ce12O24, by the position (surface or deeper oxygen layers) of the oxygen defect, if present, and by the position of the VOn adspecies on the surface. They can also differ by the number and localization of reduced Ce and V ions complying with total composition, and by the orientation of spins. The thermodynamic formalism used is described in detail and applied in several studies.38,39 The (formal) formation of a vanadium oxide species on the ceria surface from metallic vanadium and oxygen, n ð1Þ ðCeO2 Þ12 þ V þ O2 h VOn 3 ðCeO2 Þ12 2 is accompanied by the following surface free energy change Δγ: 1 ΔγðT, pÞ ¼ G½VOn 3 ðCeO2 Þ12 - G½ðCeO2 Þ12 - μ½V A n ð2Þ - μ½O2 ðT, pÞ 2 Here, G[VOn 3 (CeO2)12] and G[(CeO2)12] are the Gibbs free energies of the supercells representing the {VOn 3 Ce12O24}CeO2(111) system and the clean CeO2(111) surface respectively, μ[V] and μ[O2] are the (solid) vanadium and oxygen chemical potentials, respectively, and A is the area of the surface unit cell. Inserting the following: Δμ½OðT, pÞ ¼ 7400

1 ½μ½O2 ðT, pÞ - E½O2 2

ð3Þ

dx.doi.org/10.1021/jp108185y |J. Phys. Chem. C 2011, 115, 7399–7410

The Journal of Physical Chemistry C

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into eq 2, we obtain, 1 G½VOn 3 ðCeO2 Þ12 - G½ðCeO2 Þ12 - μ½V A n - E½O2 - nΔμ½OðT, pÞ ð4Þ 2 The oxygen chemical potential is related to the temperature and pressure conditions by assuming that the surface is in thermodynamic equilibrium with the gas phase. As the surrounding O2 atmosphere forms an ideal gas reservoir, the pressure dependence of Δμ[O](T, p) at a given temperature is given by the following: ΔγðT, pÞ ¼

Δμ½OðT, pÞ ¼

1 ½HðT, p0 Þ - Hð0K, p0 Þ - TSðT, p0 Þ 2 ð5Þ þ RTlnðp=p0 Þ

using μ[O2](0 K) = E[O2]. p0 is the pressure of a reference state (p0 = 1 atm). Tabulated values for the enthalpy, H, and entropy, S, at the temperature, T, were used.40 μ[V] is equated to the DFT total energy of the metallic bcc bulk vanadium (Ebulk[V]). The Gibbs free energies of the solid components in eq 4 are equated to the DFT total energies that are calculated for a fixed volume of the unit cell, V, and T = 0 K. Δγ ¼

1 E½VOn 3 ðCeO2 Þ12 - E½ðCeO2 Þ12 - Ebulk ½V A n - E½O2 - nΔμ½OðT, pÞ ð6Þ 2

This means that vibrational contributions and pV terms are neglected. It was shown previously that the vibrational contributions

to the entropy tend to cancel to a large extent, and that the influence of the pV term is even smaller.38,41 For a given chemical potential, we predict which {VOn 3 Ce12O24}CeO2(111) surface structure is the most stable by searching for the surface model with the lowest surface free energy.

3. STRUCTURES AND RELATIVE ENERGIES For each composition the relative energies to the most stable {VOn 3 (CeO2)12}CeO2(111) (n = -1, 0, ..., 4) isomers are presented in Table 1. Figure 1 summarizes the most stable VOn (n = 0, ..., 4) adspecies. The most stable isomer of a given composition is labeled a, followed by b, c, ... for isomers of increasing energy. Note that there may be oxygen defects in the ceria slab. The defect can be at the surface or in deeper oxygen layers, i.e., subsurface. Hence, n = -1 means that a vanadium atom is adsorbed on a defective ceria surface. For example, for n = 2, the most stable isomer has a VO2 adspecies on the nondefective surface (VO2/Ce12O24), whereas VO3 adspecies on a defective surface, i.e., VO3 3 Ce12O23 structures, are less stable. Below selected {VOn 3 (CeO2)12}CeO2(111) structures are presented, see the Supporting Information for additional material. 3.1. Clean and Oxygen Defective CeO2(111) Surfaces. Figure 2 shows the low energy structures of the clean CeO2(111) surface and of surface and subsurface oxygen defects with (2 2) periodicity. The nondefective surface is nonmagnetic and insulating, with unoccupied Ce4f states in the O2p-Ce5d band gap. Upon oxygen defect formation, the electrons left in the system localize on cerium ions which are next nearest neighbor to the defect, reducing Ce4þ to Ce3þ.13,42 The subsurface oxygen vacancy is 0.45 eV (PBEþU) more stable than a surface vacancy.

Table 1. Energies (eV) of Different {VOn 3 (CeO2)12}CeO_2(111) Structures Relative to the Most Stable Isomer of a Given Composition.a adspecies/surface

n = -1

VOn/Ce12O24 (no defect)

a

n=0

n=1

n=2

n=3

n=4

1.43 (e) 1.48 (f) 1.58 (g) 1.65 (h) 2.35 (i)

0 (a) 0.25 (b) 0.52 (c) 0.70 (d) 0.78 (e) 0.79 (f) 1.17 (g)

0 (a) 0.07 (b) 0.24 (d) 1.36 (f)

0 (a) 0.13 (b) 0.17 (c) 1.56 (h)

0 (a) 0.36 (b)

1.39 (h) 1.40 (i) 1.55 (j) 1.90 (k)

0.14 (c) 0.66 (e)

0.64 (e) 0.67 (f) 0.98 (g)

VOnþ1/Ce12O23 (subsurface O defect)

0 (a)b 0.16 (b)b 0.79 (d) 1.14 (e) 1.30 (f)

0 (a)b 0.04 (b)b 0.49 (d)

VOnþ1/Ce12O23 (surface O defect)

0.32 (c)

0.24 (c)

0.55 (d)

The structures are alphabetically labeled according to increasing energy. b The O defect is located in the third O layer.

Figure 1. Schematic representation of the most stable {VOn 3 Ce12O24}CeO2(111) (n = 0-4) systems. Os denotes an O atom in a position that belongs to the oxide surface. 7401



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Figure 2. The first six atomic layers of the O-terminated CeO2(111) surface for a (2 2) surface unit cell. Large light and dark blue spheres represent Ce4þ and Ce3þ ions, respectively. This color code applies to all of the following figures.

Figure 5. {VO 3 (CeO2)12}CeO2(111) structures (perspective and top views). V is in the oxidation state þ5.

Figure 3. V adsorbed on oxygen defective CeO2(111) surfaces, V/ Ce12O23, (perspective and top views). Structures a and b have a subsurface oxygen defect, and structure c has a surface defect (see Table 1). V atoms are green.

Figure 4. {V 3 (CeO2)12}CeO2(111) structures (perspective and top views).

Lattice relaxation makes a large contribution to this preference. For both defective surfaces, the energy difference between the ferromagnetic and antiferromagnetic solutions is very small, of the order of 1 meV. 3.2. V Adsorbed on O Defective Ceria Surfaces. Figure 3 shows the three most stable structures of a vanadium atom on the defective ceria surfaces (n= -1 in Table 1). With the exception of structure c which has a surface oxygen vacancy, all others in Table 1 have a subsurface oxygen vacancy. The most stable structures a and b have the oxygen defect located in the third oxygen layer. V is located above a subsurface O and it coordinates to three surface O atoms in all structures. Its oxidation state is þ4 (a, c, d, e, f) or 5þ (b). In structure a five

of the six Ce3þ ions have R-spins. The other β-spin is on V (see Table 3 in the Supporting Information for the net magnetization of structures b-f). 3.3. {V 3 Ce12O24}CeO2(111). Most stable are structures a, b, and c which correspond to a O = V5þ(O-)3 surface species on a defective CeO2(111) surface, see Figures 4 and 1 above. Since V has given away its 5 valence electrons and V is in the oxidation state þ5, there are 5 Ce3þ ions in the cell. In structure a three of the five Ce3þ ions have R-spins. Structure b is very similar to a, with the defect in the third oxygen layer, but just differs in the position of one of the Ce3þ ions. Structure b has a Ce3þ below V5þ (cf. Figures 4a above and 2b in the Supporting Information). In structure c, the oxygen defect is in the outermost oxygen layer, whereas in structure d is in the second oxygen layer (cf. Figures 4a above and 2d in the Supporting Information). Structure c is high-spin and the net spin in structure d is 1, as in structures a and b (Table 3 in the Supporting Information). Structure e, which corresponds to a V4þ(O-)3 species on the nondefective CeO2(111) surface (Figure 4 e), is significantly less stable (1.43 eV) than structure a. The relative energies of the other V4þ(O-)3 species (f-i) to structure a lie within approximately a 1.5-2.4 eV range (Table 1). 3.4. {VO 3 Ce12O24}CeO2(111). The most stable structure a is high-spin and corresponds to a OdV5þ(O-)3 species on a nondefective CeO2(111) surface, with the VO group positioned in a 3-fold hollow site above the surface oxygen atoms and atop a subsurface oxygen atom of the ceria substrate (see Figures 5 a and 1 above). The three electrons removed from V2þ in VO when (formally) attaching it to the CeO2(111) surface create three Ce3þ ions in the first cerium layer and V is in the oxidation state þ5. The latter is true for all {VO 3 Ce12O24}CeO2(111) structures in Table 1 (n = 1). Structures b-f resemble structure a in the VO group position with somewhat different V-O bond distances in the OdV(O-)3 species (see Figure 5 above and Table 3 and Figure 3 in the Supporting Information). Additional differences among these structures (a-f) result from either the localization of the three electrons or their spin orientation, or from both. Structure g, which is 1.17 eV higher in energy than a, (also) has the VO group positioned in a 3-fold hollow site above the surface oxygen atoms forming a OdV(O-)3 species, but atop of a Ce4þ in the first cerium layer. Structure h with a subsurface O defect and VO2 adspecies, VO2/Ce12O23, is 1.39 eV higher in energy than structure a. The vanadium is in OdV(O-)3 7402



Figure 6. {VO2 3 (CeO2)12}CeO2(111) structures (perspective and top views). V is in the oxidation state þ5.

coordination, but with 2-fold local symmetry instead of 3-fold symmetry as in structure a. The VdO bond distance is 162 pm in both structures. 3.5. {VO2 3 Ce12O24}CeO2(111). For all of the structures in Table 1 (n = 2), one Ce4þ ion is reduced to Ce3þ and V is in the oxidation state þ5. Most stable is the VO2 adspecies coordinated to two O atoms of the nondefective ceria surface resulting in a OdV(O-)3 unit with local 2-fold symmetry (Figure 6 a, see also Figure 1 above). Structures b and d are similar to a with somewhat different V—O bond distances in the OdV(O-)3 species (Table 3 and Figure 4 in the Supporting Information). In structure d, the Ce3þ ion is in the same position as in a, whereas in structure b, it is in a different position. Structures c and e involve a subsurface oxygen defect in the ceria support and VO3 adspecies, VO3/Ce12O23. In structure c, the VO3 species is almost parallel to the ceria surface bridging with its three oxygen atoms to two Ce4þ and one Ce3þ surface ions, whereas in structure e, it has a tilted terminal VdO group and with its two other oxygen atoms bridges to Ce4þ surface ions. Structure f with VO2 adspecies lying almost parallel on the nondefective ceria support lies significantly higher in energy (1.36 eV) as compared to a. 3.6. {VO3 3 Ce12O24}CeO2(111). The more stable structures of this composition are presented in Figure 7. Most stable is a VO3 adspecies on the nondefective ceria surface with 2-fold symmetry which coordinates to one O2- and two Ce4þ surface ions resulting in a OdV(-O)3 surface species with an inclined VdO group (Figure 7 a, see also Figure 1 above). Since there are only 5 valence electrons on vanadium, one of the three oxygen atoms

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cannot be fully reduced in VO3, formally V5þ(O2-)2(O•-). The unpaired electron is not localized on one oxygen site, but equally distributed over the three O atoms of the VO3 unit. Structure b is similar to a with somewhat different V—O bond distances in the OdV(-O)3 surface species (Table 3 and Figure 5 in Supporting Information). In structure c, VO3 lies parallel to the ceria surface. Less stable (approximately 0.6-1.0 eV) are VO4/Ce12O23 structures (d-f) with VO4 adspecies on the defective surface. They feature a peroxo group on vanadium (O—O bond distance 142146 pm) and a surface (d) or subsurface (e, f, g) defect. The unpaired electron has been transferred into a cerium f-state, forming one Ce3þ ion according to the following: Ce4þ ðO2- ÞV 5þ ðO2 Þ2- ðO•- Þ f Ceðf 1 Þ3þ ðO2- ÞV 5þ ðO2 Þ2- ðO2- Þ Structure h which also possesses a peroxo group on vanadium (O-O bond distance 146 pm) but does not involve an oxygen defect (VO3/Ce12O24) has a significantly higher energy (∼1.6 eV). 3.7. {VO4 3 Ce12O24}CeO2(111). In isolated VO4 a superoxo group is present as shown by matrix isolation spectroscopy,43 confirmed by DFT calculations44 and suggested by the maximum oxidation state þ5 of vanadium, V5þ(O2-)2(O2-). Figure 8 shows two structures with a VO4 unit on the nondefective ceria surface which contain the superoxo group (O-O distance 135 pm) and are nonmagnetic. 7403



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4. THERMODYNAMICS Figure 9 (top panel) shows the surface free energy change for the formation of different {VOn 3 (Ce12O24)}CeO2(111) systems, Δγ (eq 6), as a function of the oxygen chemical potential, Δμ[O]. The latter is translated into an oxygen pressure scale for T = 700 K, using eq 5 (see the upper axis in top panel of Figure 9). For pressures below 10-12 atm the VO 3 (CeO2)12 structure (Figure 5 a) is the more stable. The VO2 3 (CeO2)12 structure (Figure 6 a) is the most stable over a wide pressure range, from 10-12 to 10-5 atm, whereas for pressures above 10-4 atm, the VO4 3 (CeO2)12 structure (Figure 8 a) is the most stable. Surface structures with the VO3 3 (CeO2)12 or V 3 (CeO2)12 composition are thermodynamically less stable. Only at temperatures around 1000 K and extremely low pressures (below 10-17 atm) is V 3 (CeO2)12 (Figure 4 a) present. Figure 9 also shows Δγ as function of the temperature for low oxygen pressure (10-9 atm). Below 400 K, the dominant structure is VO4 3 (CeO2)12. In the range 400-900 K, the VO2 3 (CeO2)12 structure is the most stable and above 900 K VO2 3 (CeO2)12 is reduced to VO 3 (CeO2)12. 5. VIBRATIONAL FREQUENCIES Table 2 compares the vibrational frequencies of the characteristic IR active modes of the most stable {VOn 3 (CeO2)12}CeO2(111) surface structures (n = 0-4), see Figure 1. Figure 10 presents the calculated IR spectra for those VOn 3 (CeO2)12 structures that appear as most stable in the stability plots of Figure 9 (n = 1, 2, 4). Although, in principle, normal modes of an oscillating system are delocalized over the whole system, some may be dominated by nuclear motions of a subset of atoms which allows an assignment to local internal coordinates. For example, the vibrations at 1055 and 1046 cm-1 of VO 3 (CeO2)12 and VO2 3 (CeO2)12, respectively, can be assigned to stretching motions of the surface vanadyl (VdO) groups, and the vibration at 1134 cm-1 of VO4 3 (CeO2)12 is localized in its superoxo (O—O) group. These high frequency vibrations are several orders of magnitude more intense than all the other vibrations in the respective spectra and therefore shown as lines in Figure 10, whereas all the lines corresponding to other normal modes of vibration are subject to Gaussian broadening. Intensities are normalized to the second most intense peak in each spectrum. For comparison, Figure 10 also shows the IR spectrum for the non-

Figure 9. Surface free energy change for the formation of the most stable structures of the {VOn 3 (CeO2)12}CeO2(111) systems, Δγ. Top: Δγ as a function of the oxygen chemical potential ΔμO(T, p). In the top x-axis, the chemical potential has been translated into a pressure scale (in atm) at T = 700 K (p0 = 1 atm). Bottom: Δγ as a function of the temperature for p = 10-9 atm. Δγ for the VOn 3 (CeO2)12 structures (n = 0-4) are shown as 0-red, 1-green, 2-blue, 3-magenta, and 4-cyan lines.

defective ceria surface. The vibration at 522 cm-1 corresponds to the surface localized antisymmetric O—Ce—O stretching and its IR intensity is 107 times higher than that of the vibration at 438 cm-1 which corresponds to the rocking of the surface and subsurface O atoms. The vibrations at 809 and 891 cm-1 of the VO2 3 (CeO2)12 and VO4 3 (CeO2)12 species, respectively, are assigned to V—O stretching in V—O(—Ce) bonds. The frequency increase from VO2 to VO4 is explained by the decrease of V—O the bond distance from 173 to 167 pm (Figure 1). For VO3 the vibration at 852 cm-1 belongs to the stretching of the terminal V-O bond, which is with 166 pm, longer than a VdO bond, but shorter than a typical V—O single bond. All species in Table 2 and Figure 1 show vibrations in the 631-711 cm-1 range, which are assigned to V—Os(—Ce) bonds (bond distance between 180 and 184 pm). For VO3 the vibration at 620 cm-1 corresponds to the 7404



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Table 2. Calculated Harmonic Frequencies (cm-1) of IR Active Modes for the Most Stable Structures a of the {VOn 3 (CeO2)12}CeO_2(111) Systemsa stretch mode/adspecies

Vb

VO

VO2

VO3

1046

c

O—O superoxo VdO

VO4

Table 3. Oxygen Defect Formation Energy (Edef(1/2O2), in eV per 1/2O2 Molecule, eq 10), and Number of Ce3þ Ions for the Most Stable {VOn 3 (CeO2)12}CeO_2(111) (n > 0) Structures (a in Table 1)a

1134 1054

1055

V—O(—Ce) in-phase V—Os(—Ce) in-phase

703

711

703

V—Os(—Ce) out-of-phase V—O(—Ce) out-of-phase

668

676

631

Edef(1/2O2)

852

809

No. Ce3þb

891 708d

701d

620

a

Os (O) denotes an atom that does (not) belong to the oxide surface. b The surface species is VO with a subsurface defect, see Figure 1. c The terminal V—O bond is longer than a VdO bond (see text). d There is only one V—Os(—Ce).

CeO2(111)

n=1

n=2

n=3

n=4

1.88 (3.93)

2.19

0.79 (3.99)

0.25

0.40

3

1

0

0

a

In parentheses, Edef(1/2O2), for unrelaxed geometries. CeO2(111) corresponds to the bare support. b V oxidation state is always þ5.

hydrogen abstraction can happen, the reactant molecule needs to bind to the surface. This initial binding is of van der Waals type in the case of propane,18 but it is an acid-base type chemisorption in the case of methanol, leading to a hydroxy and a methoxy group on the catalyst surface.17 Thus, in the case of methanol, the hydrogen atom is abstracted from a surface bound methoxy group. In both cases, the reaction energy can be decomposed into the C—H bond dissociation, which remains constant for different catalysts, and the hydrogenation energy of the surface oxygen species. Hence, according to the BEP principle, a linear relation between the energy barrier and the hydrogenation energy is expected. In the present study we consider the hydrogenation reaction: 1 VOn =ðCeO2 Þ12 þ H2 f H 3 VOn =ðCeO2 Þ12 2

ð7Þ

The corresponding reaction energy is as follows: Eads ð1=2H2 Þ ¼ E½H 3 VOn =ðCeO2 Þ12 - E½VOn =ðCeO2 Þ12 1 - E½H2 ð8Þ 2 Figure 10. Calculated IR spectra of the nondefective CeO2(111) surface and the VO a, VO2 a, and VO4 a structures on CeO2(111). The peaks at 522, 1055, 1046, and 1134 cm-1 have intensities of a few orders of magnitude higher than the other peaks. Intensities are normalized to the second most intense peak in each spectrum.

out-of-phase stretching of V—O(—Ce) bonds. The structures and vibrational frequencies of the VO, VO2 and VO3 surface species agree very well with the values calculated by Shapovalov and Metiu,15 who report for the VdO stretching mode 1051, 1040, and 876 (V-O) cm-1, respectively. For comparison, we performed the vibrational analysis for the ceria bulk as well. As already mentioned, bulk CeO2 has fluoritetype face-centered cubic Fm3m structure, with one formula unit per primitive unit cell (point group Oh). There is an infrared active T2g band at 455 cm-1 and a Raman active T1u band at 290 cm-1. Our prediction is in excellent agreement with the experimental Raman spectrum of Pushkarev et al.,45 who reported a strong band at 465 cm-1 assigned to a T2g mode.

6. REACTIVITY Hydrogen abstraction by a surface oxygen species has been identified as the rate determining step in the oxidative dehydrogenation reaction of propane and methanol,17,18 and thus, it has been proposed16 to use the energy of hydrogenation as a reactivity descriptor. The underlying rationale is the linear relation between energy barriers and reaction energies (Brønsted-Evans-Polyani (BEP) Principle). Before the

where E[H 3 VOn/(CeO2)12], E[VOn/(CeO2)12], and E[H2] are total energies of the H 3 VOn/(CeO2)12 and VOn/(CeO2)12 structures, and the free H2 molecule, respectively. After the oxidation half-cycle has been completed, a water molecule is formed and an oxygen defect is created on the catalyst (Mars-van Krevelen). Therefore, the oxygen defect formation energy, 1 VOn =ðCeO2 Þ12 f VOn - 1 =ðCeO2 Þ12 þ O2 2

ð9Þ

Edef ð1=2O2 Þ ¼ E½VOn =ðCeO2 Þ12 - E½VOn - 1 =ðCeO2 Þ12 1 ð10Þ - E½O2 2 is used as an additional descriptor for the activity of the oxidation catalyst.19,20 E[O2] is the total energy of the free O2 molecule. The more exoenergetic the hydrogenation energy is (and the lower the defect formation energy), the higher the activity of the catalyst is (and the lower the barrier). Hydrogen transfer leaves one electron on the catalyst, whereas oxygen defect formation (completing the oxidative dehydrogenation) leaves two electrons, which can occupy either V 3d or Ce 4f states. 6.1. Defect Formation Energy. On the pristine CeO2(111) surface, the lowest oxygen defect formation energy (1.88 eV) is found for a subsurface oxygen site (Table 3 and Figure 11, top right),13 which is consistent with recent experimental findings.46 Creation of an oxygen defect in the {VO2 3 Ce12O24}CeO2(111) (a) structure (Figure 6), which is the most stable structure over a 7405



Figure 11. Top: The fully oxidized CeO2(111) surface (middle) as well as the unrelaxed (left) and the relaxed (right) reduced CeO2(111) surfaces with a subsurface oxygen vacancy. Bottom: The most stable VO2 adspecies on the CeO2(111) surface (middle) as well as the unrelaxed (left) and the relaxed (right) VO adspecies. The reaction energies are given in eV.

wide oxygen pressure range (from 10-12 to 10-5 atm) at 700 K and over a wide temperature range (400-900 K) under 10-9 atm (Figure 9), requires 0.79 eV (Table 3 and Figure 11, bottom right) and yields the supported VO/(CeO2)12 (a) species (Figure 5). Hence, formation of an oxygen defect in the VO2/ (CeO2)12 system requires ∼1.1 eV less than for the uncovered CeO2 support. The two electrons left in the system upon oxygen removal localize in 4f states on two Ce sites (namely, B- and C-sites, Figure 11, bottom right), reducing them to Ce3þ. The important point here is that ceria is stabilizing the highest oxidation state of vanadium, V(d0)5þ; i.e., vanadia species on the ceria support are not reduced, but the support is. The oxygen defect formation energy reactivity parameter indicates an enhanced activity of VO2/(CeO2)12 monomeric species as compared to the bare support (cf. 0.79 (VO2/CeO2) and 1.88 eV (CeO2)). The relaxation effects upon defect formation are crucial to the localization phenomenon and lead to a more facile reduction of the supported VO2/(CeO2)12 species, which has a terminal VdO group and with its second O atom it bridges to a Ce4þ ion (namely, A-site, Figure 11, bottom middle). To illustrate this, we remove the bridging O atom and performed a DFTþU calculation for the unrelaxed structure. For this unrelaxed structure, the two excess electrons are localized on the Ce and V atoms which belonged to the 5þ bridge, thus creating one additional original Ce4þ A —O—V 1 3þ Ce(f ) and one V(d1)4þ ions (Figure 11, bottom left). The defect formation energy is 3.99 eV. Note that upon lattice relaxation the two electrons localize on two surface Ce sites (i.e., B and C), 5þ bridge. which do not belong to the original Ce4þ A —O—V For the bare CeO2 support, if lattice relaxations are not considered, the creation of a subsurface defect requires 3.93 eV;13 the two electrons left in the system are equally shared by the four nearest-neighbor Ce atoms to the defect, but localize on cerium ions which are next nearest neighbor to the defect upon lattice relaxation (Figure 11, top left). Thus, with 3.2 eV, the VO2/CeO2 structure has the largest energy gain due to relaxation as compared to ∼2.1 eV for the bare support. Hence, our results predict that the two electrons left in both sytems, VO2/CeO2 and CeO2 support, localize in f-states on two Ce sites, reducing them to Ce3þ, and that the relaxation effects are essential to the stabilization of the highest oxidation state of vanadium and the determining factor in the lower oxygen defect formation energy for VO2/CeO2 as compared to the CeO2 support.

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Figure 12. H adsorbed on the nondefective CeO2(111) surface (perspective and top views). Small white spheres represent the H atoms. Structure labeling as in Table 4.

The reduction of the structure with VO 3 (CeO2)12 composition (Figure 5 a) requires 2.19 eV (Table 3) producing the supported V/(CeO2)12 (a) species with vanadium in the oxidation state þ5 (Figure 4), and two additional Ce3þ ions. The {VO 3 Ce12O24}CeO2(111) structure is stable under low oxygen pressures (900 K) under 10-9 atm (Figure 9). Oxygen removal from the supported VO4/(CeO2)12 (a) species with a superoxo group (Figure 8), is significant less costly than from the VO/(CeO2)12 and VO2/(CeO2)12 species (cf. Table 3). The defect formation energy in VO4/(CeO2)12 is only 0.25 eV. The resulting {VO3 3 Ce12O24}CeO2(111) structure is not thermodynamically stable (Figure 9) and its reduction would require 0.40 eV. 6.2. Hydrogenation Energy. For the CeO2(111) support, a hydrogenation energy of -1.21 eV is obtained (1 in Figure 12). The surface hydrogenation is accompanied by the creation of a Ce(f1)3þ ion, which is a next-nearest neighbor to the surface oxygen forming a OH group. Structure 2 (Figure 12), which corresponds to a nearest-neighbor position of the Ce(f1)3þ ion to the OH group, is only 0.04 eV less stable (Table 4). The hydrogenation of the defective CeO2(111) support is by 0.30.4 eV less exoenergetic than that of the nondefective support (Table 4); it is also accompanied by the creation of a Ce(f1)3þ ion (Figure 13). The hydrogenation energy of the supported {VO n 3 (CeO2)12}CeO2(111) species was calculated for the thermodynamically most stable structures (i.e., n = 1, 2, and 4, Figure 9). For the VO/(CeO2)12 structure (Figure 5 a), which corresponds to a OdV5þ(O-)3 species on the nondefective CeO2(111) surface, a hydrogenation energy of -1.09 eV is obtained (Table 4). The surface oxygen to which VO is not bonded, is the most stable site for H adsorption (Figure 14 1), whereas the remaining three surface sites, forming the OdV5þ(O-)3 species, are not stable (structure 5, see the Supporting Information). The second-most stable adsorption site is the vanadyl O atom (-0.62 eV, Figure 14 2). Upon hydrogenation, V remains in the þ5 oxidation state, whereas the ceria support is reduced. Structures 3 and 4 resemble structures 1 and 2, respectively, however, with somewhat different V-O bond distances in the OdV(O-)3 species (Supporting Information). Additional differences among these structures result from the spin orientation of the 4f electrons at the surface Ce sites. The hydrogenation of the VO 2 /(CeO 2 )12 structure (Figure 6a) yields a VO2H/(CeO2)12 structure (Figure 15 1) 7406



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Table 4. Hydrogenation Energies (Eads(1/2H2), in eV per 1/2H2 Molecule, eq 8) for the Most Stable {VOn 3 (CeO2)12}CeO_2(111) (n = 1,2, and 4) Structures (a in Table 1)a

Eads(1/2H2)

surf.

sub.

CeO2(111)

n=1

n=2

n=4

-0.91 (1)

-0.80 (1)

-1.21 (1)

-1.09 (1)

-1.45 (1)

-2.06 (1)

-0.90 (2)

-0.77 (2) -0.27 (3)

-1.17 (2)

-0.62 (2) -0.20 (3)

-1.33 (2) -1.33 (3)

-1.74 (2) -1.45 (3)

þ0.07 (4)

-1.29 (4)

-1.30 (4)

þ0.12 (5)

-1.26 (5) -0.98 (6) -0.91 (7) -0.38 (8) -0.33 (9)

No. Ceini3þ No. CeþH3þb

2 3

2 3

0 1

3 4

1 2

0 0

a

Structure (1) is the most stable isomer of a given composition, followed by (2), (3), etc. CeO2(111) corresponds to the non-defective support. The number of Ce3þ ions before (Ceini3þ) and after hydrogenation (CeþH3þ) is the same for all structures of a given composition. The labeling for the defective support corresponds to the (surface/subsurface) position of the oxygen vacancy. b V oxidation state is always þ5.

Figure 14. {VOH 3 (CeO2)12}CeO2(111) structures (perspective and top views). Structure labeling as in Table 4.

Figure 13. H adsorbed on defective CeO2(111) surfaces (perspective and top views). Structure labeling as in Table 4.

with vanadium in the oxidation state þ5, one additional Ce3þ ion, and a reaction energy of -1.45 eV (eq 8). The relative energies of other VO2H/(CeO2)12 structures (2-9) to structure 1 lie within approximately a 0.1-1.1 eV range. For these less stable structures, V also remains in the þ5 oxidation state, whereas ceria is reduced. The structures differ by the H adsorption site or the position of the two Ce3þ ions, or both. See the Supporting Information for additional material. The hydrogenation of supported VO4/(CeO2)12 species with a superoxo group (Figure 8 a), is significantly more exoenergetic than that of the VO/(CeO2)12 and VO2/(CeO2)12 species (cf. Table 4).

The surface oxygen sites that are not covered by the VO4 species, are the most stable sites for H adsorption (Figure 16). Comparison of the hydrogenation for the VO2/(CeO2)12 species with that for the CeO2 support yields the same picture as discussed in the previous section for the oxygen defect formation. The hydrogenation of VO2/(CeO2)12 is 0.24 eV more favorable than that of the uncovered CeO2 support, which also indicates an enhanced reactivity of the VO2/(CeO2)12 species as compared to the bare support. V remains in the þ5 oxidation state, whereas the ceria support is reduced. Hence, the effect of the support is, in fact, a cooperative effect between vanadia and ceria which can be considered as a promoting effect of the vanadia species on the activity of the uncovered ceria support. To test the dependency of the results on the actual value of the Ueff parameter for the Ce f-states, in particular the predicted enhanced activity of VO2/(CeO2)12 monomeric species as compared to the bare support, selected calculations were repeated with Ueff(Ce) = 5.3 eV, as well as with Ueff(Ce) = 4.5 eV and an additional Hubbard Ueff(V) parameter of 2.0 or 4.5 eV for the V d-states, and compared to the results of the present choice, i.e., Ueff(Ce) = 4.5 and Ueff(V) = 0 eV. Similar to cerium, 7407



Figure 15. {VO2H 3 (CeO2)12}CeO2(111) structures (perspective and top views). Structure labeling as in Table 4.

vanadium also exhibits multiple valences. Thus, the idea behind adding a Ueff parameter for the V d-states is to test how robust is the finding that upon VO2/(CeO2)12 reduction V remains in the þ5 oxidation state, whereas the ceria support is reduced. For Ueff(Ce) = 5.3 eV (Ueff(V) = 0 eV), the formation of an oxygen defect in the VO2/(CeO2)12 system requires ∼1.0 eV less than for the bare CeO2 support (cf. 0.57(VO2/CeO2) and 1.53 eV (CeO2)). This is consistent with the above presented results obtained for Ueff(Ce) = 4.5 (cf. 0.79 (VO2/CeO2) and 1.88 eV (CeO2)). Moreoever, the values of Edef(1/2O2) for the VO2/(CeO2)12 species with Ueff(V) = 2.0 and 4.5 eV (Ueff(Ce) = 4.5 eV) are 1.60 and 1.58 eV, respectively, i.e., the formation of an oxygen defect requires ∼0.3 eV less than for the bare CeO2 support (1.88 eV). On the CeO2(111) surface, the (subsurface) oxygen defect formation energy with the HSE06 hybrid functional is 2.69 eV, in agreement with the previously published result.13 The HSE value of Edef(1/2O2) for the VO2/(CeO2)12 species is 1.61 eV. Thus, the formation of an oxygen defect in the VO2/(CeO2)12 system is energetically more favorable by 1.1 eV with HSE, which supports the results obtained PBEþU for our original choice, Ueff(Ce) = 4.5 and Ueff(V) = 0 eV. Hence, the conclusions made in this work are robust with respect to the computational method. All of these tests led to the same localization in Ce f states of the electrons resulting from reduction as the calculation with Ueff = 4.5 eV and Ueff(V) = 0 eV.

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Figure 16. {VO4H 3 (CeO2)12}CeO2(111) structures (perspective and top views). Structure labeling as in Table 4.

7. COMPARISON WITH EXPERIMENT The results of this study of theoretical models of ceria supported monomeric vanadia adspecies are consistent to those recently obtained for experimental model systems where vanadia is deposited onto CeO2(111) thin films grown on Ru(0001),21,22,47 as well as those for powder VOx/ceria catalysts.48,49 As for the experimental model systems, for low vanadia coverage, the formation of highly dispersed and randomly distributed vanadyl-terminated monomeric species have been demonstrated using a combination of high-resolution scanning tunneling microscopy, infrared reflection absorption spectroscopy, and PES with synchrotron radiation.21,47 The absence of preferential nucleation sites is indicative of a strong interaction between the vanadia species and the underlying ceria support. Photoelectron spectroscopy of the VOx/ceria samples at low coverages has revealed vanadium only in a fully oxidized state, þ5 state, as shown by a single peak at approximately 517 eV for the V 2p3/2 core level. Concomitantly, it has been shown that Ce is reduced, as reflected by the an additional peak above the top of the O 2p band in the bare ceria band gap. Similar results were previously reported by Vohs et al.10 With increasing vanadia coverage, the density of monomeric species increases, which agglomerate to form dimers and trimers which are anchored flat on the surface. According to the VOn/CeO2(111) system phase diagram (Figure 9), the VO2 3 (CeO2)12 structure (Figure 6a) is the most stable over a wide pressure range (10-12 to 10-5 atm). The VO2 7408


The Journal of Physical Chemistry C adspecies are coordinated to two O atoms of the nondefective ceria surface resulting in a OdV(O-)3 unit with local 2-fold symmetry (Figure 1). In agreement with experiment, stable monomeric species are vanadyl-terminated. Moreover, vanadia species are not reduced (þ5 oxidation state), whereas the ceria support is. The activity of the experimental model systems has been investigated for the oxidation of methanol to formaldehyde as a prototype ODH reaction.22,47 Temperature-programmed reaction experiments have revealed that at low vanadia coverages the reaction may occur at much lower temperature than for the pure ceria. Using the calculated energies of hydrogenation and oxygen defect formation as descriptors for reactivity, this finding can be explained.22 As discussed above, the more favorable hydrogenation of the VO2/CeO2(111) system and its less energy requirement for a defect creation as compared to the bare support, are indicative of a superior activity of the vanadia/ceria system. Upon hydrogenation and defect formation, V remains in the þ5 oxidation state. Hence, the reduction of the ceria support and a positive cooperative effect between vanadia and ceria, are key factors determining the observed higher reactivity of vanadia/ceria systems compared with that of ceria surfaces. As for the powder vanadia/ceria catalysts, the strong interaction between vanadia species and the ceria support, the presence of VdO groups, the stabilization of the þ5 oxidation state, and the effect of vanadia promoting ceria reduction, have also been demonstrated using a combination of in situ Raman, XANES, and EPR spectroscopies.48,49 Moreover, the activity of the powder catalysts has been investigated for the oxidative dehydrogenation of ethane and a key role of the ceria sites at the V—O—Ce interface toward defining the active site have been suggested.

8. DISCUSSION AND CONCLUSIONS Monomeric vanadia species (VOn) supported on ceria (CeO2(111)) have been investigated using the DFTþU approach and statistical thermodyanamics. Computational models have been established, the properties of which are consistent with experimental knowledge for model and powder vanadia/ceria catalysts. Specifically, the stability of vanadyl-terminated monomeric species is shown, and the structure is resolved. The most stable is a structure with VO2 3 (CeO2)12 composition with the VO2 adspecies coordinated to two O atoms of the nondefective ceria surface resulting in a OdV(O-)3 unit with local 2-fold symmetry. The ceria support has a strong influence on the reactivity of the VO2/(CeO2)12 species, as probed by the oxygen defect formation energy (Edef(1/2O2), which relates to the reaction energy) and the energy of hydrogenation (Eads(1/2H2), which relates to the energy barrier of the rate-determining step). The comparison of the Edef(1/2O2) and Eads(1/2H2) values for the VO2/(CeO2)12 species and the bare ceria support indicates that the former system is expected to be more reactive than the latter one. The oxidation state of vanadium in any of the ceria supported VOn and VOnH species is þ5, i.e., upon both the oxygen removal and the hydrogenation reactions, vanadia is not reduced, but ceria is. Hence, the reducibility (and the reactivity) of ceria increases in contact with vanadia. The higher reactivity of the VO2/(CeO2)12 species as compared to the bare ceria support and the stabilization of the þ5 oxidation state of vanadium are explained in terms of defect induced lattice relaxations. The VO2/(CeO2)12 species are stable over a wider oxygen pressure range (from 10-12 to 10-5 atm) at 700 K and over a wider temperature range (400-900 K) under 10-9 atm, as

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compared to the supported VO/(CeO2)12 and VO4/(CeO2)12 species (Figure 9), and thus we have recently considered the synergy between the ceria support and the VO2/(CeO2)12 species to explain an unusual low-temperature peak (370 K) in the TPD spectra of adsorbed methanol, which is observed at coverages for which primarily monomeric species have been identified.22 We believe that the cooperativity effect between vanadia and ceria is special compared to other supports, even among redox active supports such as titania. A similar conclusion has been reached in a study on mixed metal oxide gas phase clusters50 which produced spectroscopic evidence for the larger stability of Ce3þ/ V5þ compared to Ce4þ/V4þ in vanadia-ceria clusters, whereas calculations showed that in vanadia-titania clusters, reduction of V5þ to V4þ is competitive with that of V4þ to V3þ.

’ ASSOCIATED CONTENT

bS

Supporting Information. Tables of total energies, selected bond distances, and total magnetic moments for all structures studied. Structures of the VOn/(CeO2)12 and VOnH 3 (CeO2)12 systems not shown in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Addresses †

Institute of Catalysis and Petrochemistry-CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain

’ ACKNOWLEDGMENT This was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 546, Transition Metal Oxide Aggregates). The calculations were carried out at the Norddeutscher Verbund f€ur Hoch- und H€ochstleistungsrechnen (HLRN). C.P. thanks V. Brazdova and J. L. F. Da Silva for the help in analyzing the symmetry of the vibrations of bulk CeO2. ’ REFERENCES (1) Blasco, T.; Nieto, J. L. Appl. Catal., A 1997, 157, 117–142. (2) Ba~ nares, M. A. Catal. Today 1999, 51, 319–348. (3) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78, 25–46. (4) Wachs, I. E. Catal. Today 2005, 100, 79–94. (5) Khodakov, A.; Olthof, B.; Bell, A. T.; Iglesia, E. J. Catal. 1999, 181, 205–216. (6) Dinse, A.; Frank, B.; Hess, C.; Habel, D.; Schom€acker, R. J. Mol. Cat. A: Chem. 2008, 289, 28–37. (7) Daniell, W.; Ponchel, A.; Kuba, S.; Anderle, F.; Weingand, T.; Gregory, D.; Kn€ozinger, H. Top. Catal. 2002, 20, 65–74. (8) Catalysis by Ceria and Related Materials; Trovarelli, A., Ed.; Imperial College Press: London, 2002; Vol. 2. (9) Wong, G. S.; Concepcion, M. R.; Vohs, J. M. J. Phys. Chem. B 2002, 106, 6451–6455. (10) Vohs, J. M.; Feng, T.; Wong, G. S. Catal. Today 2003, 85 303–309. (11) Feng, T.; Vohs, J. J. Catal. 2004, 221, 619–629. (12) Martínez-Huerta, M. V.; Coronado, J. M.; Fernandez-García, M.; Iglesias-Juez, A.; Deo, G.; Fierro, J. L. G.; Ba~ nares, M. A. J. Catal. 2004, 240–248. (13) Ganduglia-Pirovano, M. V.; Da Silva, J. L. F.; Sauer, J. Phys. Rev. Lett. 2009, 102, 026101-1–026101-4. 7409


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(49) Martínez-Huerta, M. V.; Deo, G.; Fierro, J. L. G.; Ba~ nares, M. A. J. Phys. Chem. C 2008, 112, 11441–11447. (50) Jiang, L.; Wende, T.; Claes, P.; Bhattacharyy, S.; Sierka, M.; Meijer, G.; Lievens, P.; Sauer, J.; Asmis, K. submitted for publication.

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