Vanadium Oxides Supported on a Thin Silica Film Grown on Mo(112

8336

J. Phys. Chem. C 2009, 113, 8336–8342

Vanadium Oxides Supported on a Thin Silica Film Grown on Mo(112): Insights from Density Functional Theory Tanya K. Todorova, Jens Do¨bler, Marek Sierka, and Joachim Sauer* Humboldt-UniVersita¨t zu Berlin, Institut fu¨r Chemie, Unter den Linden 6, 10099 Berlin, Germany ReceiVed: December 24, 2008; ReVised Manuscript ReceiVed: March 11, 2009

Low-coverage vanadium oxide species (monomers and dimers) as well as larger VmOn clusters (m ) 4 and 6) supported on an ultrathin SiO2/Mo(112) film are investigated by density functional theory in combination with statistical thermodynamics. At low vanadium chemical potentials, monomeric species are stable, and with increasing vanadium or oxygen chemical potentials, dimeric VOx species become stable. These monomeric and dimeric species are created by replacing Si atoms in the two-dimensional SiO2 film by vanadyl (VdO) groups. At high vanadium chemical potentials (high vanadia loading), the largest vanadia clusters considered (V6O15) are stable. The frequencies calculated for the V4O10 cluster anchored to the silica surface by two V-O(2)-Si interface bonds account for all vibrational frequencies observed for the vanadia/SiO2/Mo(112) model system. Its calculated infrared spectrum shows an intense band at 1048 cm-1 corresponding to a stretching of the VdO groups and a broad band in the 630-730 cm-1 range attributed to V-O-V vibrations, wellknown for the V2O3 bulk. The characteristic vibrations of the ultrathin SiO2/Mo(112) support are infraredinactive, because of the metal surface selection rule, which is in full agreement with the experimental observations. I. Introduction Vanadium oxides supported on other oxides play a major role as catalysts for selective oxidation reactions, e.g., the oxidation of methanol to formaldehyde and the oxidative dehydrogenation of light alkanes.1-7 The activity and selectivity in oxidation reactions strongly depend on the chosen support, such as SiO2, Al2O3, TiO2, ZrO2, Nb2O5, and CeO2.7 Crucial for understanding the nature of the active species and the origin of the support effect is a profound knowledge of the atomic structure of the vanadium oxide under various conditions and the way it is anchored to the surface of the support material. The structure of the supported catalyst is, however, usually disordered and very complex. Therefore, experimental and theoretical studies have been made on well-defined model catalyst systems, which allow investigations at the atomic level while grasping essential aspects of the complexity of real systems.8,9 Experimentally, silica-supported vanadium oxide particles are prepared by physical vapor deposition of vanadium in an oxygen ambient onto a well-ordered thin silica film under ultrahigh vacuum conditions and are characterized as vanadyl-covered vanadium sesquioxide nanoparticles.10,11 Imaged on the atomically resolved silica substrate, the size of the particles was estimated rather precisely as 10-12 Å, suggesting vanadia aggregates such as V2O5 or V4O10.12 At increasing V coverages, formation of particles with apparent height and lateral size distribution in the range of 4-15 and 15-40 Å, respectively, was observed.11 Only if the supporting silica films were thick enough would they mimic the differently structured amorphous bulk materials that are widely used as supports. The well-ordered silica films we address are ultrathin13 and this may affect the size and distribution of active oxide particles as well as their properties and reactivity. Hence, they are supported systems in their own * To whom correspondence should be addressed. E-mail: js@ chemie.hu-berlin.de.

right.13 Understanding such “model” systems, which is the aim of the present study, may also help the understanding of the more complex (and disordered) “real” systems. Excellent agreement between the results of density functional theory (DFT) calculations and surface science experiments has shown that the ultrathin silica film grown on the Mo(112) substrate consists of a 2D monolayer network of corner-sharing [SiO4] tetrahedra, with one oxygen atom of each tetrahedron bonded to the protruding Mo atoms. The precise knowledge of the atomic structure of the SiO2 support allows detailed investigations of the VOx/SiO2/Mo(112) model catalysts. In this study, different models of silica-supported vanadium oxides are constructed and their structures, stabilities, and vibrational spectra are investigated using DFT. Statistical thermodynamics is applied to account for the effect of oxygen partial pressure and vanadium activity (loading) at a given temperature on the stability of the supported vanadia aggregates. We aim at identifying trends in the properties of systems as complex as supported vanadium oxides and at providing assignments of characteristic vibrational features of the experimental model systems.10,14,15 II. Methods Spin-polarized calculations based on density functional theory are performed using the Vienna Ab-Initio Simulation Package (VASP).16,17 A plane-wave basis set with a kinetic energy cutoff of 400 eV and the Perdew-Wang (PW91)18 exchange-correlation functional are employed. The electron-ion interactions are described by the projector augmented wave (PAW) method19 as implemented by Kresse and Joubert.20 Surface slabs using a (4 × 2) unit cell with respect to (1 × 1) Mo(112) and a corresponding composition of the silica support Si8O20/Mo(112) are separated by a vacuum region of 10 Å. The integrations in the Brillouin zone employ a (4 × 4 × 1) Monkhorst-Pack grid.21

10.1021/jp811387z CCC: $40.75  2009 American Chemical Society Published on Web 04/16/2009

VmOn Supported on an Ultrathin SiO2/Mo(112) Film

Figure 1. Top and side views of the most stable monomeric (A) and dimeric (B) vanadia species on the SiO2/Mo(112) surface. V atoms are depicted in green, Si in orange, and O in red. The former Si-O-Mo interface oxygen atoms left on the Mo(112) surface are shown in dark red.

Vibrational spectra are calculated within the harmonic approximation using a central finite difference method with 0.02 Å displacements of the atoms in each Cartesian direction. Intensities are obtained from the derivatives of the dipole moment component perpendicular to the surface. To compensate for systematic errors of DFT, the calculated frequencies are scaled by an empirical factor derived from calculated and observed frequencies of known compounds.22 A scaling factor of 1.0312 is used to scale the modes of the SiO2 film (see refs 23, 24), whereas the vanadyl stretching vibrations are scaled by a factor of 0.985 derived from BP86 calculations on OdVF3, OdVCl3, and V4O10.25 III. Vanadium Oxide Surface Species Vanadium oxides supported on the ultrathin SiO2/Mo(112) film can be created either by replacement of silicon atoms of the 2D network by VdO groups or by “landing” gas-phase VmOn clusters on the clean silica surface. Monomeric and dimeric VOx species as well as larger clusters are thus created. A. Vanadium Oxides Modeled by Replacement. Figure 1 shows the optimized structures of low-coverage VOx species, such as monomers and dimers, constructed by replacement of Si atoms by VdO groups. The monomeric species (Figure 1A) are separated by 8.92 and 10.93 Å in the crystallographic [1j10] and [1j1j1] direction, respectively. As a result of the relaxation after the substitution, the bond to the interface oxygen is broken. It is retained in a short-bridge site on the Mo(112) surface (dark red atom in Figure 1A) with a distance of 3.05 Å from the vanadium atom. The V-O-Si angle is 152.2° and the three V-O bonds have the same length (1.78 Å). Thus, the VO4 unit maintains the tetrahedral coordination known from vanadia species on bulk silica.10 The calculated magnetic moment is 0 and the projected density of states (pDOS) shows no occupied vanadium d-states close to the Fermi level. This indicates that the vanadium is in the oxidation state VV, in agreement with the tetrahedral coordination by three 2-fold coordinated oxygen atoms and a vanadyl oxygen atom. The oxygen defect energy for removal of the vanadyl oxygen and formation of 1/2O2 is

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8337 2.54 eV. The lowest value found so far for (supported) vanadium oxides in tetrahedral coordination is 3.4 eV.26 The low defect energy can be explained with the structure of the supported site: Upon removal of the vanadyl oxygen, the vanadium atom relaxes inward and rebinds to the additional oxygen atom at the molybdenum surface. The vanadium thus retains a tetrahedral coordination even after reduction. Furthermore, we find a magnetic moment of 1 after reduction, indicating one unpaired electron and thus a VIV oxidation state. The second electron is not taken up by the vanadium, instead it is incorporated into the metallic state of the molybdenum support. The reduced structure is analogous to the original silica film with a VIV instead of silicon, which formally also has the oxidation state IV. Replacement of one additional Si atom in a nearest-neighbor position results in formation of two different dimeric VOx species with adjacent VdO groups along the [1j1j1] and [1j10] crystallographic direction, respectively. The dimer along the [1j10] direction (Figure 1B) is about 0.2 eV more stable. The separations between two dimeric sites are 5.56 in the [1j10] and 10.93 Å in the [1j1j1] direction. Again, upon replacement of Si atoms by VdO groups, the bonds to the interface are broken and the O atoms are left on the Mo(112) surface at a distance of 3.20 Å from the corresponding vanadium atoms. The substitution results in a relaxation of the vanadia unit away from the surface, creating a much more open structure as seen from the side view along the [1j10] direction. The V-O-V angle is 141.0°, but the two vanadyl groups are almost parallel. The structure resembles that of dimeric vanadium oxides in sesquioxide cluster models of silica; see ref 10. In this case, the oxygen defect formation energy is 2.38 eV for the oxidation of a single vanadium atom. Again, a relaxation leading to a rebinding of the vanadium atom to the oxygen atom on the Mo surface is found, and the defect energy is only slightly lower compared to the isolated vanadium site. The magnetic moment of the reduced structure is 1 and the pDOS indicates that similar to the isolated site one electron is located at the reduced vanadium site. For a consecutive reduction of the second V site, we find an even lower defect formation energy of 2.31 eV (with the single defect as a reference) with a magnetic moment of 2 and unpaired electron density at both vanadium atoms. In this state, the original morphology of the film is restored and both vanadium atoms are bonded to the interface oxygen atoms at the Mo surface. The result indicates that in the doubly reduced state two VIV are at SiIV positions and the lowered defect energy of the second reduction step can be explained by the relaxation that recovers the original structure of the film. B. Vanadium Oxides Modeled by Adsorption. A second approach to model silica-supported vanadia species is the adsorption of different VmOn gas-phase clustersswith vanadium in a VIV or VV oxidation stateson the SiO2/Mo(112) film surface. The adsorption energy, Ead, is defined as

Ead ) EVmOn⁄Si8O20⁄Mo(112) - ESi8O20⁄Mo(112) - EVmOn(gas) (1) where EVmOn/Si8O20/Mo(112), ESi8O20/Mo(112), and EVmOn(gas) are the energies of the silica-supported vanadia slab model, the clean SiO2/Mo(112) surface, and the most stable VmOn cluster in the gas phase, respectively. The different VmOn gas-phase structures are calculated in a cubic box with a ) 15 Å using only the Γ-point. The obtained geometries are in a very good agreement with previously reported BP86 (TZVP basis set) results.27 Table 1 compiles the adsorption energies of the different VmOn clusters on the pristine silica-film surface given with respect to the corresponding most favorable VmOn in the gas phase. The total

8338

J. Phys. Chem. C, Vol. 113, No. 19, 2009

Todorova et al.

TABLE 1: Adsorption Energy (Ead, in eV) for the Different VmOn Clusters on the Pristine SiO2/Mo(112) Film Surface system

Ead

VO2/Si8O20/Mo(112) V2O4/Si8O20/Mo(112) V2O5/Si8O20/Mo(112) V4O8/Si8O20/Mo(112) (Figure 3A) V4O8/Si8O20/Mo(112) (Figure 4A) V4O8/Si8O20/Mo(112) (Figure 4B) V4O10/Si8O20/Mo(112) V6O12/Si8O20/Mo(112) V6O15/Si8O20/Mo(112)

-0.94 -1.08 -1.94 +1.78 -0.68 -0.89 -0.90 -0.08 -0.65

energies of the individual structures used in eq 1 are given in the Supporting Information. The most stable VO2/Si8O20/Mo(112) species, regarded as a monomeric vanadia site, is shown in Figure 2A. The VO2 adspecies retains one vanadyl (VdO) group and attaches with its second oxygen atom to a Si atom of the film, forming a V-O(2)-Si interface bond. The coordination of Si increases to 5. Although 5-fold coordinated silicon is not a common feature in crystalline phases, its existence has been reported for triclinic CaSi2O5,28,29 K-silicate glass quenched from high pressure,30 as well as for organosilane and organosilicate structures.31,32 The length of the new Si-O bond is 1.84 Å, which is about 0.2 Å elongated with respect to its typical value. The V atom initially in a VIV oxidation state forms an additional bond to an O atom of the silica network at a distance of 2 Å. The adsorption energy of this cluster given with respect to gas-phase VO2 is -0.94 eV. The magnetic moment of this structure is 1, indicating that vanadium is still in the VIV oxidation state. The vanadyl oxygen defect formation energy of this structure is 4.21 eV, considerably higher than that of gas phase VO2 (3.52 eV). This can be explained with the additional bond of one oxygen atom of VO2

to a surface silicon atom, which lessens the stabilization of the reduced vanadium. Two different dimeric clusters, V2O4 and V2O5, with vanadium in VIV and VV oxidation states, respectively, are adsorbed on the SiO2/Mo(112) surface. The most stable V2O4 cluster (Figures 2B) has a cyclic structure and two VdO bonds, one V-O(2)-V bond, and one V-O(3)-Si interface bond to a silicon atom, which becomes 5-fold coordinated (Si-O distance 1.81 Å). The two V atoms are bound to O atoms of the silica network. This cluster is slightly more strongly bound to the surface (Ead ) -1.08 eV) than VO2. The adsorption energy is calculated with respect to the most stable gas-phase V2O4 cyclic-trans isomer,33,34 in its open-shell singlet ground state. For the adsorbed structure we find a magnetic moment of 0, but the pDOS clearly shows occupied vanadium d-bands close to the Fermi level. This indicates that the adsorbed molecule is still in an open-shell singlet state with two VIV. The vanadyl oxygen defect formation energy is 4.49 eV, almost identical to that of the gas phase V2O4 (4.48 eV). The most stable V2O5 cluster is much more strongly bound to the silica surface (Ead ) -1.94 eV) than V2O4. It also has a cyclic structure (Figure 2C) and it is 1.43 eV more stable than the adsorbed linear V2O5 (not shown). It features two VdO bonds and two V-O(2)-V bonds. One of the V atoms is bound to an oxygen atom of the silica surface. The other V atom uses its second vanadyl oxygen to attach to a surface Si atom. In contrast to the previous two models (Figure 2A,B), the Si atom remains tetrahedrally coordinated, although a new V-O(2)-Si interface bond is formed. This is the result of a relaxation in which the Si atom moves up and the Si-O(-Mo) bond dissociates. The distance between silicon and the oxygen retained at the Mo surface (shown in dark red in Figure 2C) increases to 3.08 Å. Since a vanadyl is transformed into a V-O-Si linkage, vanadium has been reduced, and conse-

Figure 2. Top and side views of the most stable monomeric (VO2) and dimeric (V2O4 and V2O5) clusters adsorbed on the SiO2/Mo(112) surface. See Figure 1 for color coding.


Figure 3. Top and side views of the V4O8 (A) and V6O12 (B) clusters adsorbed on the SiO2/Mo(112) surface. See Figure 1 for color coding.

quently, we find occupied vanadium d-states close to the Fermi level. The magnetic moment is 0.8 and the pDOS shows that the spin density is delocalized over both vanadium atoms, with a slightly lower contribution of the V atom that originally had two vanadyl bonds. The vanadyl oxygen defect formation energy is 4.09 eV and the reduction leads to a different structure than adsorption of V2O4 (shown in Figure 2B) due to the breaking of the Si-O bond upon adsorption. If we take the more stable structure of adsorbed V2O4 as the final state, the defect formation energy is reduced to 3.64 eV. However, both values are considerably larger than for gas phase V2O5 (2.78 eV). This is probably a result of the lower oxidation state of vanadium upon adsorption on the silica film surface. Larger VmOn clusters, with m ) 4 and 6, are also investigated. Attempts to find structures with strong interaction to the silica surface resulted in models shown in Figure 3. Adsorbed V4O8 is a flat cluster located on top of the sixmembered silica rings of the [SiO4] network. It has four VdO bonds, two V-O(2)-V bonds, and two V-O(3)-V bonds. The O(3) atoms in the latter form interface bonds by extending a third coordination to surface Si atoms, which increases their coordination to 5-fold (cf. Figure 3A). The structure, however, is quite unstable with respect to the cagelike gasphase V4O8 structure, with an adsorption energy of +1.78

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8339 eV. Increasing the dispersion of V4O8 clusters along the [1j 10] direction by one-half slightly decreases the endothermicity of the adsorption to Ead ) +1.56 eV. The V6O12 cluster is constructed by placing a V2O4 cyclic unit on top of the V4O8/Si8O20/Mo(112) structure; see Figure 3B. This creates a 3D particle, which is about 6 Å in height. However, similar to the aforementioned V4O8 structure, the cluster is weakly bound to the silica surface; the adsorption energy is only -0.08 eV. The latter is given with respect to the V6O12 gas-phase cluster obtained by removal of three vanadyl oxygen atoms from the most stable V6O15 cagelike structure.27 Our studies indicate that VmOn clusters do not bind strongly to the coordinatively saturated and oxygen-terminated SiO2/ Mo(112) film. Obviously, the interaction with the surface does not outweigh the energy increase due to distortion from the most favorable structure in the gas phase. Therefore, we use yet another approach to anchor vanadia particles more strongly to the silica support. One Si atom from the 2D network is pulled out of the plane, and its bond with the interface oxygen atom is broken. The dangling bond is then saturated by forming a bond with vanadyl oxygen from the vanadia cluster. Note that species anchored in such a way have been obtained by “landing” the V2O5 cluster (cf. Figure 2C). The most stable V4O8 cluster with a single link to the silica film has a cagelike structure (Figure 4A) and one VdO group, six V-O(2)-V bonds, and one V-O(2)-Si interface bond. This model is 2.45 eV more stable than the V4O8 cluster shown in Figure 3A. In contrast to the latter, its adsorption on the silica surface is exothermic (Ead ) -0.68 eV). Another possibility is to anchor the V4O8 cluster on the SiO2/ Mo(112) surface by its two vanadyl oxygen atoms. Depending on the orientation of the vanadia cluster, four different structures are thus obtained, as shown in Figure 5. However, their energies differ by less than 0.1 eV, because of the similar distances between any two Si atoms involved in interface bond formation. As a result of the strong binding to the surface, the adsorption energy of such a cluster (Figure 4B) increases to Ead ) -0.89 eV. Thus, the energy gain due to formation of the second V-O-Si interface bond is about 0.2 eV. Moreover, V4O10 clusters are considered, and the most favorable V4O10/Si8O20/Mo(112) structure is shown in Figure 4C. It has two surface VdO groups and is anchored to the silica surface by two V-O(2)-Si interface bonds. The bonding geometry is virtually identical with the V4O8 structure (Figure 4B), and its adsorption energy is Ead ) -0.90 eV. The magnetic moment is 2, and for the two vanadium atoms anchored to the film, occupied d-states close to the Fermi level are found. This indicates that these two atoms are reduced to VIV upon adsorption on the silica support. The

Figure 4. V4O8 cluster anchored to the SiO2/Mo(112) surface by one (model A) and two (model B) V-O(2)-Si interface bonds and V4O10 (model C) and V6O15 (model D) clusters.

8340


Todorova et al.

Figure 5. Top view of the differently oriented V4O8 clusters anchored to the SiO2/Mo(112) surface by two interface bonds.

energy for removal of a single vanadyl oxygen is 3.98 eV. The reduced state has a magnetic moment of 4, which is consistent with VV being reduced to VIII. The defect energy is larger than for V4O10 in the gas phase (3.5 eV), but is identical to V4O8(OH)2 (4.0 eV), which resembles the oxidation states of the vanadium atoms in the supported cluster more closely. The second defect formation energy is considerably lower (3.30 eV), and the reduced state has a magnetic moment of 0. The pDOS shows that all four vanadium atoms are in a reduced state, but the electrons are delocalized over all vanadium atoms and coupled into a lowspin state. The summed defect energy for both defects (7.3 eV) is identical to the reduction of V4O10 to V4O8 in the gas phase (7.3 eV), while the value for V4O8(OH)2 is higher (8.1 eV). This might indicate that for the doubly reduced cluster an electronic interaction with the metallic support exists, so that the support takes up two electrons. Finally, we consider the even larger V6O15 species, and the most stable structure is illustrated in Figure 4D. It is terminated by four VdO groups and is anchored to the SiO2/Mo(112) surface by two V-O(2)-Si interface bonds. The adsorption energy is -0.65 eV and is given with respect to the most stable V6O15 trigonal prism in the gas phase. IV. Thermodynamic Stability As with our previous studies on SiO2/Mo(112) films23,24 and alumina-supported vanadium oxides,34 statistical thermodynamics is applied to account for the effect of oxygen partial pressure and vanadium concentration at a given temperature on the stability of the vanadia aggregates supported on the thin crystalline SiO2 film grown on Mo(112). The following equilibrium reaction is considered

1 mV + n O2 + Si8O20 ⁄ Mo(112) / VmOn ⁄ Si8O20 ⁄ Mo(112) 2 (2) The corresponding reaction energy is

1 ∆E ) EVmOn⁄Si8O20⁄Mo(112) - ESi8O20⁄Mo(112) - mEVbulk - n EO2 2 (3) where EVmOn/Si8O20/Mo(112) and ESi8O20/Mo(112) are the total energies of the system with a given vanadia/Si8O20/Mo(112) composition and the clean silica slab, respectively. EVbulk and EO2 are the total energies of the metallic body-centered cubic (bcc) bulk vanadium and the oxygen molecule, and m and n are the number of V and O atoms in the supported vanadium oxides, respectively. The total energies are given in the Supporting Information. This equation, however, holds only when the number of Si atoms is constant. If one or two Si atoms are replaced by

Figure 6. Phase diagram as a function of the ∆µO and ∆µV chemical potentials for vanadia aggregates supported on the SiO2/Mo(112) surface. ∆µO is translated into a pressure scale at T ) 800 K.

VdO groups forming monomeric and dimeric VOx species, eq 2 is reformulated as

mV +

2m + n O2 + Si8O20 ⁄ Mo(112) / 2 VmOn ⁄ Si8-mO20 ⁄ Mo(112) + m(SiO2)R-quartz

(4)

where (SiO2)R-quartz is the thermodynamically most stable SiO2 bulk phase. The accompanying change in the surface-related free energy ∆γ is given by

1 ∆γ(T, p) ) [∆E - m∆µV - n∆µO] A

(5)

where A is the area of the surface unit cell. Here, the chemical potential differences, ∆µi, are defined as

∆µV(T, aV) ) µV(T, aV) - EVbulk

(6)

1 ∆µO(T, p) ) [µO2(T, p) - EO2] 2

(7)

The reaction energy ∆E is the energy required to form a silica-supported vanadium oxide from a silica thin film, metallic vanadium, and oxygen. The most favorable structure is the one that minimizes ∆γ for given values of vanadium and oxygen chemical potentials. The resulting phase diagram is shown in Figure 6 with values of the oxygen potential related to an oxygen pressure at T ) 800 K. At low vanadium chemical potentials, monomeric VOx species obtained by replacement of Si atoms of the SiO2/ Mo(112) film by VdO groups (see Figure 1A) become energetically stable. Upon increase of the ∆µV values, dimers


J. Phys. Chem. C, Vol. 113, No. 19, 2009 8341

TABLE 2: Calculated Scaled Harmonic Vibrational Frequencies (cm-1) and IR Intensities (in parentheses, normalized to 1.00) for the Most Intense Vibrational Modes of the Pristine SiO2/Mo(112) Film, Monomeric and Dimeric VOx Species, Supported V4O10 and V6O15 Clusters mode

model support

V-O-Si stretch VdO stretch

monomer

dimer

1075 (0.01) 947 (0.008) 1056 (0.30)

1100 (0.21) 1064 (0.15)

Si-O-Mo stretch

1061 (0.79)

1032 (1.00)

1004 (0.83)

Si-O-Si stretch

779 (0.05)

801 (0.03) 757 (0.04)

812 (0.04)

V-O-V stretch Si-O-Si bending

672 (0.07)

659 (0.06)

that are also modeled by replacement are predicted to be stable (Figure 1B). At even higher vanadium chemical potentials, the largest vanadia clusters considered (V6O15) form. A characteristic feature of all stable structures is the presence of surface vanadyl groups. Furthermore, among the various structure models investigated here, the clusters “landed” on the silica surface do not appear in the stability plot. The reason is that the thin-film silica support is oxygen-terminated and coordinatively saturated; thus, the binding of the vanadia species is very weak. The results suggest that, initially, deposition of vanadium in an oxygen atmosphere would result in formation of lowcoverage VOx species, such as monomers and dimers created by replacement of Si atoms from the crystalline SiO2/Mo(112) film. Upon increase of the amount of deposited vanadium, large vanadia clusters that are anchored to the surface via interface V-O(2)-Si bonds would form. Note, however, that in this work clusters anchored to already existing VOx sites on the film surface have not been investigated. At low V exposures, the formation of vanadia particles consisting of one to two V atoms was reported, based on the amount of deposited vanadium and the determined particle density.10 In ref 11, high-resolution STM images showed atomic sized features, suggested to be monomeric VOx species. Registry analysis revealed that they are located above the outermost Si-O1-Si bonds of the [SiO4] network (see Figure 1 for atom labeling). Our calculation can rationalize these experimental observations in terms of formation of monomeric VOx species created by replacement (cf. Figure 1A). Thus, the protrusions imaged by STM in ref 11 might be attributed to the vanadyl oxygen atoms of the monomers, which are indeed located above the O1 atoms of the SiO2 network. V. Vibrational Analysis Table 2 shows the frequencies and intensities obtained for selected models, i.e., low-coverage (monomeric and dimeric) VOx species and large V4O10 and V6O15 clusters, which are differently supported on the silica surface. Statistical thermodynamics suggests that these structures are stable phases. Note, however, that at high ∆µV values, V4O10 is always less favorable than V6O15. For the clean SiO2/Mo(112) film, a very intense band at 1061 cm-1 and two weak signals at 779 and 672 cm-1 were obtained, in perfect agreement with the experimental IRAS results.23,24 They were assigned to Si-O-Mo asymmetric stretching, Si-O-Si symmetric stretching coupled with Si-O-Si bending, and to a coupling of Si-O-Si bending modes, respectively. Upon creation of monomeric VOx species (cf. Figure 1A), the main phonon of the silica film is strongly influenced by the

V4O10

V6O15

1048 (0.18) 1034 (0.05) 1015 (0.002) 933 (0.003)

1037 (0.05) 1006 (0.11)

726 (0.02) 637 (0.02)

858 (0.13) 805 (0.11) 733 (0.03)

655 (0.04)

replacement of silicon by vanadium and shifts ∼30 cm-1 toward lower wavenumbers (1032 cm-1). Concomitantly, a new vibration, corresponding to stretching of the VdO bond, emerges at 1056 cm-1, with three times lower intensity than the Si-O-Mo phonon. The coupled Si-O-Si symmetric stretching and bending mode is split into two bands at 801 and 757 cm-1, and the Si-O-Si bending mode appears at 659 cm-1. Table 2 shows that the intensities of these modes are almost unchanged compared to the corresponding modes in the silica support. In addition, two weak signals at 1075 and 947 cm-1 are attributed to the in-phase V-O-Si vibration coupled with VdO stretching and to out-of-phase V-O-Si stretching modes, respectively. Upon formation of dimeric VOx species, the main silica phonon shifts to even lower wavenumbers (1004 cm-1) and its intensity is slightly reduced. The vanadyl stretching mode appears at similar frequencies (1064 cm-1) as in the monomeric species, but its intensity is reduced by one-half (see Table 2). In fact, this vibration corresponds to an in-phase stretching of the two vanadyl groups. The corresponding out-of-phase mode at 1055 cm-1 is IRAS inactive, because no change of the dipole moment component perpendicular to the surface occurs. The coupled Si-O-Si symmetric stretching and bending vibration appears at 812 cm-1, and the coupling of bending modes is redshifted (655 cm-1), compared to the same mode in the silica film. The vibration at 1100 cm-1 in the dimeric structure is an in-phase V-O-Si stretching mode and has a significant intensity, whereas the V-O-V vibration at 795 cm-1 is parallel to the surface and is IRAS inactive. None of these models can explain the experimental vibrational spectra of the vanadia/silica systems,10-12 for which the single band at 1046 cm-1 is assigned to stretching vibration of VdO species, whereas the broad signal at 720 cm-1 that appears at higher vanadium coverages is attributed to the V-O-V stretching vibrations. However, it is important to realize that the IRAS spectra reported in the literature are always taken on vanadia particles with higher vanadia coverage than that of the monomeric and dimeric VOx species. Thus, as a next step, the vibrational frequencies of large vanadia clusters, such as V4O10 and V6O15, anchored to the SiO2/ Mo(112) surface by two V-O(2)-Si interface bonds are calculated. The spectrum of the former structure is characterized by a vibrational mode at 1048 cm-1 assigned to the in-phase stretching of the two vanadyl groups, and a mode at 1034 cm-1, which corresponds to the out-of-phase vibration. Interestingly, the asymmetric Si-O-Mo stretching modes at 1015 and 933 cm-1, which are strongly coupled with the V-O(2)-Si interface modes, have negligible intensities (see Table 2) because they are not connected to a change in the dipole moment component

8342


perpendicular to the surface. The frequency analysis also reveals no IRAS active Si-O-Si symmetric stretching and bending modes, which is in agreement with the experimental spectra obtained upon deposition of vanadium onto the pristine silica surface. The lack of these two signals was interpreted as an indication of long-range order loss in the crystalline silica film.11 Therefore, very recently, a new ice-assisted preparation of silicasupported vanadia particles has been developed.11 Although we do not investigate the role of the H2O layer in protecting the silica film and the properties of vanadia grown on ice-assisted silica support in this work, the present calculations suggest that the loss of the three IRAS active bands characteristic for the silica support does not necessarily indicate a destruction of the film. Instead, the quenching of these modes upon formation of vanadia-deposited particles is governed by the metal surface selection rule. Finally, the new bands that emerge at 726 and 637 cm-1 are attributed to vibrations of the entire V-O-V framework and the outermost V-O-V bond, respectively. Thus, the V4O10 cluster anchored to the SiO2/Mo(112) surface by two V-O(2)-Si interface bonds, despite the fact that is not the most thermodynamically stable phase (see Figure 6), can account for all vibrational features in the spectra of the experimental vanadia/silica model systems. This contradiction might be explained by the fact that the phase diagram shows only thermodynamic stability under equilibrium conditions, while the exact mechanism of vanadia particle formation is unknown. It is plausible to assume that the nature of the surface species formed upon the deposition does not depend on stability alone but also on kinetic factors. Similar to V4O10, the vibrational spectrum of V6O15/Si8O20/ Mo(112) reveals neither IRAS active Si-O-Mo stretching modes nor vibrations associated with Si-O-Si symmetric stretching and bending modes of the thin silica film. The highest frequency vibrations involve stretching motions of different vanadyl groups present in the structure, whereas those at 858, 805, and 733 cm-1 are stretching modes of the V-O-V framework (see Table 2). The existence of two very intense bands in the region of 800-850 cm-1 is a main discrepancy with the measured IRAS spectra. Therefore, despite the thermodynamic stability of the V6O15 clusters at high ∆µV values, the frequency analysis indicates that these are not the experimentally formed vanadia species. VI. Summary and Conclusions Low-coverage vanadia species (monomers and dimers), as well as large vanadia clusters, differently anchored to the ultrathin silica layer of the SiO2/Mo(112) support have been investigated by DFT in combination with statistical thermodynamics. The vanadia/silica phase diagram as a function of vanadium activity and oxygen partial pressure shows that, at low ∆µV values, first monomeric and then, with increasing ∆µV values, dimeric VOx species become stable. Such structures could be obtained experimentally by, for example, codeposition of V and Si in oxygen atmosphere. They are modeled by replacement of Si atoms in the 2D network of the SiO2/Mo(112) film by VdO groups. At high values of the vanadium chemical potentials, the largest vanadia clusters considered (V6O15) form. A characteristic feature of all stable species is the presence of surface vanadyl groups. Vibrational frequency analysis of different models investigated in this work provides valuable insight into the structural features of the vanadia/silica model catalysts. A V4O10 cluster anchored to the SiO2/Mo(112) surface by two V-O(2)-Si interface bonds can account for all vibrational frequencies of the experimental model

Todorova et al. systems. The spectrum reveals a single band at 1048 cm-1 corresponding to a stretching of the VdO groups and a broad band in the range of 630-730 cm-1 attributed to V-O-V vibrations, well-known for the V2O3 bulk. Moreover, the three characteristic bands of the ultrathin silica film of the SiO2/Mo(112) support are IRAS inactive when covered with vanadia particles because of the metal surface selection rule, which is in full agreement with the experimental observations. Acknowledgment. The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft (DFG) through the Sondersforschungsbereich SFB 546. T.K.T. thanks the International Max Planck Research School “Complex Surfaces in Materials Science” for a fellowship. The calculations were carried out at the IBM p690 system of the Norddeutscher Verbund fu¨r Hoch- and Ho¨chstleistungsrechnen (HLRN). Supporting Information Available: Table of total energies for all structures studied. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Mamedov, E. A.; Corberan, V. C. Appl. Catal., A 1995, 127, 1. (2) Blasco, T.; Nieto, J. M. L. Appl. Catal., A 1997, 157, 117. (3) Banãres, M. A. Catal. Today 1999, 51, 319. (4) Banãres, M. A.; Martıńez-Huerta, M. V.; Gao, X.; Fierro, J. L. G.; Wachs, I. E. Catal. Today 2000, 61, 295. (5) Khodakov, A.; Olthof, B.; Bell, A. T.; Iglesia, E. J. Catal. 1999, 181, 205. (6) Weckhuysen, B. M.; Keller, D. E. Catal. Today 2003, 78, 25. (7) Wachs, I. E. Catal. Today 2005, 100, 79. (8) Baümer, M.; Freund, H.-J. Prog. Surf. Sci. 1999, 61, 127. (9) Campbell, C. T. Surf. Sci. Rep. 1997, 27, 1. (10) Magg, N.; Immaraporn, B.; Giorgi, J. B.; Schroeder, T.; Baümer, M.; Do¨bler, J.; Wu, Z.; Kondratenko, E.; Cherian, M.; Baerns, M.; Stair, P. C.; Sauer, J.; Freund, H.-J. J. Catal. 2004, 226, 88. (11) Kaya, S.; Sun, Y.-N.; Weissenrieder, J.; Stacchiola, D.; Shaikhutdinov, S.; Freund, H.-J. J. Phys. Chem. C 2007, 111, 5337. (12) Guimond, S.; Abu Haija, M.; Kaya, S.; Lu, J.; Weissenrieder, J.; Shaikhutdinov, S.; Kuhlenbeck, H.; Freund, H.-J.; Do¨bler, J.; Sauer, J. Top. Catal. 2006, 38, 117. (13) Freund, H.-J. Surf. Sci. 2007, 601, 1438. (14) Bra´zdova´, V.; Ganduglia-Pirovano, M. V.; Sauer, J. Phys. ReV. B 2004, 69, 165420. (15) Bra´zdova´, V.; Ganduglia-Pirovano, M. V.; Sauer, J. J. Phys. Chem. B 2005, 109, 394. (16) Kresse, G.; Furthmu¨ller, J. Comput. Mater. Sci. 1996, 6, 15. (17) Kresse, G.; Furthmu¨ller, J. Phys. ReV. B 1996, 54, 11169. (18) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (19) Blo¨chl, P. E. Phys. ReV. B 1994, 50, 17953. (20) Kresse, G.; Joubert, D. Phys. ReV. B 1999, 59, 1758. (21) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (22) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (23) Weissenrieder, J.; Kaya, S.; Lu, J.-L.; Gao, H.-J.; Shaikhutdinov, S.; Freund, H.-J.; Sierka, M.; Todorova, T. K.; Sauer, J. Phys. ReV. Lett. 2005, 95, 076103. (24) Todorova, T. K.; Sierka, M.; Sauer, J.; Kaya, S.; Weissenrieder, J.; Lu, J.-L.; Gao, H.-J.; Shaikhutdinov, S.; Freund, H.-J. Phys. ReV. B 2006, 73, 165414. (25) Do¨bler, J.; Sauer, J. Unpublished. (26) Sauer, J.; Do¨bler, J. Dalton Trans. 2004, 3116. (27) Vyboishchikov, S. F.; Sauer, J. J. Phys. Chem. A 2001, 105, 8588. (28) Kanzaki, M.; Stebbins, J. F.; Xue, X. Geophys. Res. Lett. 1991, 18, 463. (29) Angel, R. J.; Ross, N. L.; Seifert, F.; Fliervoet, T. F. Nature (London) 1996, 384, 441. (30) Stebbins, J. F.; McMillan, P. Am. Mineral. 1989, 74, 965. (31) Turley, J. W.; Boer, F. P. J. Am. Chem. Soc. 1968, 90, 4026. (32) Tacke, R.; Burschka, C.; Richter, I.; Wagner, B.; Willeke, R. J. Am. Chem. Soc. 2000, 122, 8480. (33) Pykavy, M.; van Wu¨llen, C.; Sauer, J. J. Chem. Phys. 2004, 120, 4207. (34) Todorova, T. K.; Ganduglia-Pirovano, M. V.; Sauer, J. J. Phys. Chem. C 2007, 111, 5141.

JP811387Z

Vanadium Oxides Supported on a Thin Silica Film Grown on Mo(112

Recommend Documents