Supported Isolated Oxomolybdenum Species - American Chemical

Feb 9, 2007 - 3 × 1 × 1 Monkhorst-Pack special k-points for (001) and (101) anatase surfaces, respectively, which lead to the surface energies conve...
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J. Phys. Chem. C 2007, 111, 3963-3972

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Computational Investigation of TiO2-Supported Isolated Oxomolybdenum Species Karim Hamraoui, Sylvain Cristol, Edmond Payen, and Jean-Franc¸ ois Paul* Unite´ de Catalyse et Chimie du Solide, UMR CNRS 8181, Equipe Catalyse He´ te´ roge` ne, Baˆ timent C3 UniVersite´ des Sciences et Technologies de Lille, 59650 VilleneuVe d’Ascq, France ReceiVed: October 2, 2006

We present periodic density functional theory (DFT) calculations combined with thermodynamic analysis to study the structure of isolated molybdenum oxide entities supported on titania (anatase) under ambient and dehydrated conditions. The TiO2 support is represented by the perfect and hydrated (101) and (001) surfaces. The calculation of the vibrational wavenumbers of the stable structures under various conditions allows us to access to structural information by comparison with the experimental data obtained in in-situ conditions. The calculation of the most stable model on the (101) surface indicates that in dry conditions molybdenum is in a distorted tetrahedral environment with a single molybdenyl bond whereas dioxo entities are more stable on the (001) surface. Furthermore, it appears that the ModO stretching wavenumber is strongly influenced by the hydration state of the surface through formation of hydrogen bonds with the surface OH or H2O groups that induce a shift to lower wavenumber (more than 60 cm-1) in agreement with the Raman shift observed during the hydration-dehydration process.

1. Introduction Molybdenum oxide based catalysts are widely used in selective partial oxidation reactions of alkanes and alcohols. Both bulk and supported oxides are used for partial oxidation reactions, and it has been proposed that the same outermost overlayer is present in bulk catalysts and in supported ones.1 Supported molybdenum oxides are also very important as precursor of hydrodesulfurization catalysts and as catalysts for metathesis reactions. All these reasons make the knowledge of the molecular structure of supported molybdenum oxides a very important scientific task both on the fundamental and industrial point of view. Indeed, the knowledge of the local structural environment of the active-supported molybdenum sites is important for understanding the chemical properties that are relevant in heterogeneous catalysis. Numerous studies have concluded that the structures of the supported molybdenum oxide species are a function of the specific support, the extent of surface hydration, and the molybdenum oxide coverage but do not depend on the preparation method.2,3,4 Support effects are important as it has been proposed that the structure of the supported overlayer is dependent on the pH at zero point charge (ZPC) of the support in agreement with the phase diagram of molybdate in solution.3,5 This explains why monomeric (MoO42-like) entities are primarily formed on MgO6 (pH at ZPC ) 11) whereas polymolybdate (Mo7O246--like) entities appear on silica (pH at ZPC ) 4). The ZPCs of titania and γ-alumina are between these two extreme cases and the situation is then more complex. To complicate further the situation, support dissolution and heteroplyanion formation has been evidenced on alumina7,8 and silica.9 No such phenomena are observed on titania, where the structure of the supported molybdenum oxide depends only on the loading and on the environment, specifically on the hydration. It is now well recognized that the surface structures of the molybdenum oxide overlayers on oxide supports have to * To whom correspondence should be addressed. Tel: +33-3-20-3377-34; fax: +33-3-20-43-65-61; e-mail: [email protected].

be evaluated under two distinctly different environments: ambient and dehydrated conditions.2,3,10-13 Under ambient conditions, the moisture present in the atmosphere is sufficient to induce the hydration of the surface molybdenum oxides. At elevated temperatures, the catalyst surfaces are dehydrated, and the surface molybdenum oxide species undergo significant changes that are evidenced by a significant shift of the molybdenyl (ModO) Raman line located between 900 and 1000 cm-1. As far as titania-supported molybdenum oxides are concerned, mainly two techniques have been used to characterize the molybdenum overlayer: laser Raman spectroscopy (LRS) and X-ray absorption spectroscopy (XAS). Both Kim et al.,14 LRS studies, and Shimada et al.,15 based extended X-ray absorption fine structure (EXAFS), proposed that molybdenum is in a distorted octahedral environment under ambient conditions whatever the molybdenum loading. These conclusions are supported by subsequent XAS studies16 although the latter authors identify two structurally different molybdenum atoms by the mean of a detailed principal component analysis. These two components could well correspond to low and high coverage entities. On the other hand, Hu et al.5 proposed that both MoO42and Mo7O246- are present under ambient conditions at low loading whereas Mo7O246- and Mo8O264- are present at high loading under the same conditions. This conclusion is made mainly because of the similarity of the Raman lines of the supported species and the ions in solution. In the same article, convincing evidence is given for an octahedral coordination of the molybdenum atom in the high loading sample through LIIIedge X-ray absorption near-edge structure (XANES). Unfortunately, the spectrum of the low-loading sample under ambient atmosphere is not presented. Concerning the dry conditions, LRS suggests the presence of distorted octahedra at high loading2,5 and a mix of tetrahedra and octahedral at low loading5 whereas LIII-edge XANES supports the presence of octahedra at high loading and of tetrahedra at low loading. A mixed structure containing octahedra and tetrahedra on SiO2 is proposed through

10.1021/jp0664622 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/09/2007

3964 J. Phys. Chem. C, Vol. 111, No. 10, 2007 XANES, showing that such mixed structure can be evidenced by this technique, and is not proposed for TiO2 supported molybdenum at low loading. Finally, there have been different studies to elucidate the “mono-oxo” and “dioxo” nature of supported molybdenum species.10,11,17 They all agree that supported molybdenum oxides always show a single molybdenyl double bond. In this context, the investigation of the structural and vibrational properties with theoretical methods can be very useful to obtain a detailed picture of the structure of the catalyst on a molecular scale. Ab initio calculations are playing an increasingly important role in heterogeneous catalysis in the evaluation of reaction mechanisms and activation energies and in the identification of adsorbed reaction intermediates on metals,18 sulfides,19 and oxides.20 Despite the use of numerous experimental studies for the understanding of the structure of these supported oxide catalysts, the number of studies in modeling is relatively small and, to the best of our knowledge, no studies dealing with titania-supported molybdenum oxides have been published. Therefore, a systematic investigation of the surface molybdenum oxide on oxide support is needed to clarify the surface structure of supported molybdenum oxide catalyst and to reveal its structural dependence on the hydration/ dehydration conditions. This paper is devoted to the study of the structure of monomeric molybdenum oxide supported on titania under both ambient and dehydrated conditions. The starting point of such studies is the determination of the faces exposed by the support particle. Previously, different groups have studied anatase surface stability. It is generally found that only three types of face are stable: the (101), (001), and (100) surfaces. However, their surface energies are different and they do not contribute to the TiO2 particles exposed surface equally. Using periodic density functional theory (DFT) and plane-wave basis set, Arrouvel et al.21 and Lazzeri et al.22 reported that the most stable surface is the (101), followed by the (100), the (001) being the least stable. Both groups find, however, that the resulting crystal morphology is almost octahedral and exposes mostly the (101) surface. Depending on the calculation and on the amount of hydration taken into account, the octahedron can be truncated and can expose a minority (less than 10% of the total crystallite surface) of the (001) surface. Both surfaces will thus be considered in the present study to elucidate the structure of monomeric-supported molybdenum oxides that are expected to be present at low metal loading. 2. Computational Details and Models Total energy calculations are performed within the density functional theory (DFT) framework using the generalized gradient approximation (GGA) PW91.23 To solve the KohnSham equations, we used the Vienna Ab initio Simulation Package (VASP).24,25,26 The electron wave functions are expanded on a plane-waves basis set with an energy cutoff of 400 eV. Pseudopotentials are used to describe the electron-ion interactions within the projector augmented waves (PAW) approach.27 The convergence criterion for the electronic selfconsistent cycle is fixed at 0.1 meV per cell. The integrations in the Brillouin zone are preformed on a grid of 3 × 3 × 1 and 3 × 1 × 1 Monkhorst-Pack special k-points for (001) and (101) anatase surfaces, respectively, which lead to the surface energies convergences with respect to the k-point number. Geometry optimizations are carried out by means of the conjugate gradient technique using the exact Hellman-Feyman forces acting on the ions. The structure is considered as optimized when the

Hamraoui et al. forces acting on the atoms are smaller than 0.03 eV.Å-1 and the energies variation between successive geometries is below 1 meV. To mimic the bulk constraints, structural relaxation is performed only on the position of the adsorbed species and the two outermost TiO2 layers. Frequency calculations on the stable geometry have been carried out by numerical differentiation of the force matrix. All the optimized degrees of freedom are used in the frequency calculation. The unit cell dimension perpendicular to the surface plane has been chosen large enough to avoid interaction between slabs (i.e., a distance of at least 10 Å is always kept between successive slabs). A 2 × 2 and 1 × 2 cell are used for the (001) and (101) anatase surfaces, respectively (see Figure 1). The slabs are constituted of five stoichiometric layers for the (001) surface and six stoichiometric layers for the (101). The calculated relaxed surface energies were 1.14 J/m2 for (001) and 0.54 J/m2 for (101) surfaces (see Table 1). Thus, the stability of the surfaces increases in the order (001) < (101) in agreement with Arrouvel et al.21 and Lazzeri et al.22 The outermost atoms in the anatase surfaces are 2-fold O2c and 3-fold O3c coordinated oxygen and 5-fold Ti5c and 6-fold Ti6c coordinated titanium as shown in Figure 1. To take into account temperature and pressure effect for water adsorption, we define the Gibbs free energy according to equilibrium 1 and 2. The various surfaces will be referred hereafter according to the number of adsorbed water molecules and MoO3 species. We have chosen the dehydrated surfaces as a reference to analyze the influence of water molecules.

MoO3 - surface + nH2O ) nH2O - MoO3 - surface

(1)

∆rG ) µ(nH2O - MoO3 - surface) µ(MoO3 - surface) - nµ(H2O) surface + nH2O ) nH2O - surface

(2)

∆rG ) µ(nH2O - surface) - µ(surface) - nµ(H2O) Throughout this work, we will assume that for condensed phases, the differences between the chemical potentials can be approximated by the differences between their computed electronic energies. This approximation has been widely used21,28-30 and gives good results as the temperature- and pressure-dependent terms of each condensed phase are close and tend to cancel in the free-energy difference calculations. It is thus possible to simplify the expressions and to write

∆rG ) ∆E0 - nµ(H2O)

(3)

where ∆E° is the difference between the electronic energy of the considered surfaces and µ(H2O) is the water chemical potential. The temperature- and pressure-dependent terms in µ(H2O) can be obtained by the following equation:

µ(H2O) ) µ0(T) + RT ln

P(H2O) P0

µ° contains the temperature-dependent terms that can be computed with standard formulas of statistical thermodynamics31 using the partition functions of the water molecule in the gas phase.

µ0(T) ) E° + EZPE + Hvib + Hrot + Htr - T(Svib + Srot + Str)

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Figure 1. Schematic view of the (001) and (101) anatase surfaces. Light green circles: Ti; red circles: O.

TABLE 1: Geometric and Energetic Parameters of the Dehydrated Surfaces (hkl) before (Eunrl) and after (Erel) Relaxation hkl Nat Nlay K points mesh surface periodicity burface s areas (Å) Eunrl (J/m2) Erel (J/m2)

001

101

60 5 331 2a b × 2b B 57.03 1.39 1.14

72 6 311 b u × 2b B 77.49 1.15 0.54

In this equation, we decided not to take into account the vibrational contributions EZPE, ∆Hvib, and ∆Svib. In other words, we assume that the vibrational contribution of a water molecule will not be greatly affected by its adsorption. Hence, taking the vibrational degrees of freedom in the gas-phase partition functions and neglecting them for adsorbed molecules would result in an overestimation of the gas-phase term in eq 3. It is now possible to calculate ∆rG for various temperatures and various water pressures and to determine the stoichiometry of the stable surface for each experimental condition. The energy of interaction between the MoO3 units and the titania surfaces is evaluated taking bulk MoO3 as a reference. The adsorption energy is defined by the following equation:

Eads ) E(surface) + E(MoO3) - E(MoO3 - surface) A positive value corresponds to an exothermic adsorption. The energy (per MoO3) units computed with VASP with the same settings (except that 5 × 1 × 5 k-points grid is used) is -33.04 eV. This value is used throughout this study. 3. Results 3.1. Adsorption on the (101) Surface. 3.1.1. Adsorption of MoO3 on the Perfect Surface. Two different stable structures

Figure 2. Different structures of the monomeric MoO3 species on the (101) perfect surface. Light green circles: Ti; red circles: O (from TiO2); blue circles: Mo; violet circles : O (from adsorbate).

have been found for the MoO3 adsorption on the anatase (101) (Figure 2). In both cases, the molybdenum is in a distorted tetrahedral environment. In the most stable geometry (structure 2a), the molybdenum center is bound to the surface through three Mo-O-Ti bonds, and there is only one molybdenyl Mod O double bond. The adsorption energy relative to bulk MoO3 is 0.71 eV reflecting the fact that one MoO3 unit is more stable on the surface than in bulk molybdenum oxide. This is in agreement with experimental studies that showed that mechanical mixtures of MoO3 and TiO2 heated at moderate temperature end up as supported molybdenum oxide.2,4,9 The ModO1 bond length in this structure is 1.71 Å. The computed wavenumber in the molydenyl region is 1004 cm-1. In structure 2b, the molybdenum is bound to the surface through two Mo-O-Ti bonds and presents two molybdenyl ModO double bonds. The optimized lengths of these two bonds are 1.72 Å. This results in much less stable structure with an adsorption energy of -0.35 eV. Symmetric and antisymmetric stretching vibrations are computed at 986 and 974 cm-1, respectively, indicating a coupling between the two molybdenyl bonds. The optimized lengths of all molybdenum-oxygen bonds in the adsorbed structure are given in Table 2. 3.1.2. Hydration of the (101) Surface. The results of the hydration of the (101) surface are represented in Figure 3, which shows the average adsorption energies for various coverages

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TABLE 2: Calculation Wavenumbers (cm-1), Adsorption Energies Eads/MoO3 (eV), and Molybdenum-Oxygen Bond Lengths (Å) of Different Adsorbed MoO3 Structures on the Anatase (101) Surface bond length surface

structure

Eads/MoO3

Mo-O1

Mo-O2

Mo-O3

Mo-O4

νModO1

(101)

Figure 2a Figure 2b

0.71 -0.35

1.71 1.72

1.82 1.72

1.82 1.87

1.91 1.93

1004 986/974

of water on the surface. The calculations show that water molecules are chemisorbed without dissociation. All the starting geometries tested with one water molecule adsorbed on the (101) surface lead to the same molecular adsorbed geometry. In this state, both hydrogens of the adsorbed molecule form hydrogen bonds with the bridging surface oxygens, and the Ti adsorption site becomes 6-fold coordinated (Figure 3a). The water adsorption geometry is similar for coverage up to four molecules (Figure 3a-d). For four H2O molecules (Figure 3d), all the Lewis Ti5c sites are saturated by one chemisorbed water molecule. For higher coverages, corresponding to 5H2O, 6H2O, 7H2O, and 8H2O molecules, new water molecules can only be adsorbed through hydrogen bonds with the basic Lewis sites of 2-fold coordinated oxygen atoms of the (101) surface. The saturation is thus reached for eight H2O molecules (Figure 3h). The results obtained for this surface are in good agreement with those of Vittadini et al.32 and Arrouvel et al.21 Figure 4 shows the ∆rG evolution as a function of the temperature for PH2O ) 0.01 bar, as expressed by eq 3, and each line corresponds to one of the water coverages detailed previously and for which the adsorption energy is given in Figure 3. The horizontal thick line represents the fully dehydrated surface, as described in the previous section. On such a diagram, the most stable state is given by the lowest Gibbs freeenergy value at a given temperature. At ambient temperature, the stable coverage is with eight H2O molecules, and there is no available Lewis site. For the same water partial pressure, when the temperature is between 345 and 405 K, only one H2O molecule remains adsorbed and the surface is completely dehydrated above 405 K. Of course, the complete dehydration temperature increases when the water partial pressure increases.

3.1.3. Hydration of the (101) Surface Containing MoO3. Because of the interaction of the water molecules with the Lewis center of the surface, it is important to determine the MoO3 adsorption geometries on the hydrated (101) anatase surface. The results are represented in Figure 5. All tested configurations lead to the formation of only one ModO double bond. At low water coverage, corresponding to one water molecule, the molybdenum is bound to the surface through three MoO-Ti bonds. The most stable geometry is the configuration without dissociation (Figure 5a). The water adsorption energy of this structure is 0.94 eV. This value is similar to the adsorption energy of the first molecule on the bare surface indicating that the electronic effect of the MoO3 adsorption is well localized. The bond length of the molybdenum bond Mod O1 in this structure is 1.71 Å (see Table 3), and the wavenumber of the ModO bond stretching is computed at 1007 cm-1. Both the structure and the vibration are very close to the value obtained on the dehydrated surface. Three geometries involving the water dissociation have been tested. The water adsorption energy is 0.04 eV if the proton is bonded to the oxygen atom in bridging position between the titanium and the molybdenum (Figure 5b), while it is only -0.29 eV, which corresponds to an endothermic adsorption, when the proton is localized on the ModO oxygen (Figure 5c). The calculations indicate clearly that the bridging oxygen atoms are more basic than the terminal ones. In any case, oxygen bound to the molybdenum atom is less basic than the TiO2 ones (Eads ) 0.49 for structure 5d). With two water molecules (Figure 5e), all 5-fold titanium sites are now saturated. The adsorption energy of the second water molecule in this structure is smaller than the first one (Eads/H2O) 0.80 eV) and the ModO stretching wavenumber is

Figure 3. Adsorption energy of water on the (101) surface as a function of coverage. Same color code as in Figure 2, plus white circles: H.

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Figure 4. Variation of the Gibbs free energy of the (101) surface of anatase-TiO2 as a function of temperature for different coverages of H2O and the stability domain of the various surface stoichiometry, (PH2O ) 0.01 bar).

Figure 5. Different structures of the monomeric MoO3 species on the hydrated (101) anatase surface. Same color code as in Figure 3.

lowered by more than 20 cm-1 (ν(ModO) ) 981 cm-1) compared to the structure obtained on the dehydrated surface. With three water molecules, a mixed structure combining water molecules adsorbed directly on the Lewis center and one water molecule forming hydrogen bond with the surface oxygen atoms is found (see Figure 5f). The average adsorption energy of the water molecules for this structure is 0.83 eV and the wavenumber of the ModO1 is almost unchanged (980 cm-1). The fourth water molecule is adsorbed on the molybdenum center (Figure 5g). This adsorption mode is due to the almost complete saturation of the surface. All the Ti atoms are now 6-fold coordinated and only one surface oxygen atom is accessible to form a hydrogen bond. The adsorption energy of the fourth molecule is 0.51 eV (leading to an average adsorption energy of the H2O molecules of 0.75 eV) indicating that the Mo center is less acidic (in Lewis’

TABLE 3: Calculated Vibrational Wavenumbers (cm-1), Adsorption Energies Eads/H2O (eV), and Molybdenum-Oxygen Bond Lengths (Å) of Different Adsorbed MoO3 Structures on the Hydrated (101) Anatase Surface bond length nads H2O

structure

Eads/H2O

Mo-O1

Mo-O2

νModO1

1H2O

Figure 5a Figure 5b Figure 5c Figure 5d Figure 5e Figure 5f Figure 5g Figure 5h Figure 5i

0.94 0.04 -0.29 0.49 0.80 0.83 0.75 0.70 0.71

1.71 1.71 1.88 1.71 1.72 1.72 1.74 1.74 1.74

1.82 1.96 1.76 1.82 1.82 1.82 1.84 1.80 1.80

1007

2H2O 3H2O 4H2O 5H2O 6H2O

981 980 923 943 934

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Figure 6. Variation of the Gibbs free energy of the adsorbed MoO3 species on the anatase (101) surface as a function of temperature for different coverages of H2O and the stability domain of the various surface stoichiometry (PH2O ) 0.01 bar).

Figure 7. Different structures of the monomeric MoO3 species on the (001) perfect surface. Same color code as in Figure 3.

definition) than the Ti ones. Water adsorption on the Mo atom induces a small increase of the ModO1 and Mo-O2 bond lengths (+0.02 Å) and lowers the ModO1 stretching wavenumber by 80 cm-1 (ν ) 923 cm-1) compared to the dehydrated surface (1004 cm-1). With five water molecules (Figure 5h), the environment around the Mo atoms is changed. The Mo geometries are still tetrahedral but only two Mo-O-Ti bonds remain. The MoO3 adsorption on a surface on which all the Lewis centers are already saturated leads to the displacement of two water molecules from the Ti atoms to the oxygen atoms and the dissociation of one of the molecule previously in hydrogen interaction with the surface. The interaction of the water molecule with the 3-fold coordinated Mo atom is strong enough to induce this dissociation. The proton is then bounded to the surface 2-fold-coordinated oxygen atom. The Mo-O2 distance decreased from 1.84 to 1.80 Å. The average water adsorption energy is 0.70 eV, and the ModO1 stretching wavenumbers is calculated at 943 cm-1. With six water molecules (Figure 5i), we have found the same environment around the Mo atoms, and the ModO1 stretching wavenumber is computed at 934 cm-1. Focusing on the stable states as a function of temperature (Figure 6) at partial pressure of water equal to 0.01 bar when

the MoO3 species is present on the (101) surface, three stability domains are found on the Gibbs free energy diagram, as expressed by eq 3. For T smaller than 375 K, the surface exhibits three water molecules in the presence of MoO3 species. For T between 375 and 440 K, only one water molecule remains on the surface and above 440 K, the surface is completely dehydrated. 3.2. Adsorption on the (001) Surface. 3.2.1. Adsorption of MoO3 on the (001) Perfect Surface. Three different structures, shown in Figure 7 of the monomeric MoO3 species on the anatase (001) surface, were obtained after geometry optimization. In the structure in Figure 7a, one oxygen atom of MoO3 is bound to one neighboring 5-fold coordinated surface titanium atom. The adsorption energy of this endothermic process is -2.38 eV, and the ModO stretching wavenumbers are calculated as 989 and 958 cm-1 (see Table 4). In the structure shown in Figure 7b, two oxygen atoms of MoO3 are bound to two neighboring 5-fold coordinated surface titanium atoms, and a bond between the molybdenum atom and the surface oxygen O4 is formed. This structure is found to be 2.35 eV lower in energy (Eads ) -0.03 eV) than the structure in Figure 7a, and the ModO stretching wavenumber is calculated as 984 cm-1. The system is further stabilized by 0.97 eV if the surface oxygen

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TABLE 4: Calculated Wavenumbers (cm-1), Adsorption Energies Eads/MoO3(eV), and Molybdenum-Oxygen Bond Lengths (Å) of Different Adsorbed MoO3 Structures on the Anatase (001) Surface bond length surface

structure

Eads/MoO3

Mo-O1

Mo-O2

Mo-O3

Mo-O4

νModO1

(001)

Figure 7a Figure 7b Figure 7c

-2.38 -0.03 +0.94

1.72 1.72 1.72

1.72 1.85 1.72

1.77 1.83 1.89

1.85 1.89

989/958 984 1002/980

atom O4 bound to molybdenum is displaced from its lattice position with the formation of an oxygen vacancy on the anatase surface (Figure 7c). This structure is the most stable one. Molybdenum is 4-fold coordinated, and the MoO4 tetrahedron is distorted in this structure. The two ModO double bonds have the same length (1.72 Å) resulting in a dioxo structure with two stretching vibrations in the region of 1002 (s) and 980 (as) cm-1. 3.2.2. Effect of the Hydration on the (001) Surface. The most stable hydrated (001) surfaces are represented in Figure 8, which shows the water adsorption energy as a function of water surface coverage. The successive average water adsorption energies are 1.35, 1.22, 1.00, and 0.90 eV. These values are significantly higher than for the adsorption on the (101) surface. The calculations indicate that the two first water molecules are dissociated. At a low coverage, corresponding to one water molecule, no molecular adsorption is found whatever the starting geometry. The structure of the dissociated state (Figure 8a) is characterized by the formation of two hydroxyl groups. For two H2O molecules (Figure 8b), all 5-fold titanium sites are saturated. For coverage greater than two H2O molecules per unit cell, mixed structures combining dissociated and nondissociated adsorbed H2O are found (see Figure 8c and 8d). These calculations are consistent with the results published by Arrouvel et al.21 and Vittadini et al.32 who obtain the same stable states. Figure 9 shows the ∆rG evolution as a function of temperature for a PH2O ) 0.01 bar. We observe three stability domains. For PH2O ) 0.01 bar and temperatures between 300 and 500 K, the stable surface is covered with two adsorbed H2O molecules. When the temperature rises to 500 K, the coverage decrease to one H2O molecule per unit cell, and above 600 K, all water molecules are removed from the surface. For a water partial pressure of 0.1 bar, the configuration with six water molecules would be stable at low temperature. 3.2.3. Hydration of the (001) Surface Containing MoO3. Several structures were used in the modeling of the MoO3 adsorbed on the hydrated (001) surface. Figure 10 shows the model structures after geometry optimization. Bond lengths, calculated wavenumbers, and adsorption energies are summarized in Table 5. Different types of configurations have been considered for the adsorption of the first water molecules. The geometries 10a and 10b correspond to a nondissociative and disssociative adsorption, respectively. In each case, the vacancy formed by the MoO3 adsorption on the dehydrated (001) surface remains below the molybdenum atom. The adsorption energies are relatively small (0.62 and 1.00 eV). The molybdenum atom is in a tetrahedral environment, with two double bonds and two single Mo-O bonds. The dissociative adsorption is favored in the same way as on the bare surface. The three other adsorption geometries (Figure 10c-e), for which the vacancy on the surface has been filled, differ only by the position of the proton on the surface. The most stable structure is obtained when one proton is bonded to an oxygen of the surface while the second one is placed on the oxygen in bridging position between the Mo and the Ti atom (Figure 10c). The water adsorption

energy is 1.29 eV. This most stable structure leads to stretching wavenumbers of 988 and 908 cm-1 for the symmetric and antisymetric stretching. The water adsorption energy is reduced by 0.14 eV when the proton is displaced to a terminal oxygen atom (Figure 10d). The ModO1 bond length is slightly enlarged by the formation of a hydrogen bond with the proton bonded to the surface, which explains the decrease of the ModO stretching wavenumber (934 cm-1). The last structure (Figure 10e) is obtained by the displacement of the surface hydroxyl proton on the surface. The adsorption energy is 0.84 eV. This low value is due to the weakness of the bond between the hydrogen atom and already 3-fold coordinated oxygen atom. Adsorption structures were tested with two (Figure 10f) and three (Figure 10g) water molecules, respectively. The adsorption of the second molecule is again dissociative and induces the cleavage of one bond between one titanium atom and one oxygen atom located in the surface as for the surface without MoO3 species. The adsorption energy of this second water molecule is 0.65 eV whereas the adsorption of the third one is only 0.46 eV. This last molecule is adsorbed nondissociatively on the last Lewis site in the unit cell. In these structures, the molybdenum is 5-fold coordinated but the MoO3 geometries are close to a tetrahedral one. Figure 11 shows the ∆rG evolution as a function of temperature for a water partial pressure equal to 0.01 bar. Within the range of temperatures we are studying, there are only three stable geometries. At low temperatures (T < 370 K), the stable phase exhibits two water molecules in the presence of the MoO3

Figure 8. Adsorption energy of water on the (001) surface as a function of coverage. Same color code as in Figure 4.

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Figure 9. Variation of the Gibbs free energy of the (001) surface of anatase-TiO2 as a function of temperature for different coverages of H2O and the stability domain of the various surface stoichiometry (PH2O ) 0.01 bar).

Figure 10. Different structures of the monomeric MoO3 species on the hydrated (001) anatase surface. Same color code as in Figure 3.

species. In the intermediate temperature range (370 < T < 580 K), the stable phase is the surface with only one water molecule. At higher temperature (T > 580), all water molecules are removed from the surface. The presence of the MoO3 species on the surface decreases slightly (20 K) the dehydration temperature.

4. Discussion According to our calculation, the (101) surface is dehydrated above 405 K at P(H2O) ) 0.01 bar. By coupling ab initio calculations and a thermodynamic model in a way similar to this work, Arrouvel et al.21 found a dehydration temperature of

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TABLE 5: Calculated Wavenumbers (cm-1), Adsorption Energies Eads/H2O(eV), and Molybdenum-Oxygen Bond Lengths (Å) of Different Adsorbed MoO3 Structures on the Hydrated Anatase (001) Surface bond length Eads/H2O

structure

Mo-O1

Mo-O2

Mo-O3

Mo-O4

νModO1

1H2O

0.62 1.00 1.29 1.15 0.84

Figure 10a Figure 10b Figure 10c Figure 10d Figure 10e

1.75 1.72 1.75 1.73 1.71

1.75 1.76 1.72 1.88 1.80

1.85 1.86 2.09 1.91 1.86

1.85 1.85 1.91 1.89 1.84

943/916 997/950 988/908 934 1001

2H2O

0.97

Figure 10f

1.71

1.89

1.83

2.10

1000

3H2O

0.80

Figure 10g

1.71

1.90

2.07

1.84

943

nads H2O

375 K at 0.01 bar. The slight discrepancy between the two studies might come from the inclusion or not of the vibrational degrees of freedom in the gas-phase partition functions (see section 2). The molybdenum supported on the (101) surface is completely dehydrated above 440 K with the same water partial pressure. This result shows that, as far as low loading is considered, addition of molybdenum on the surface does not significantly modify water adsorption. Our calculations also indicate that the H2O groups are completely removed from the molybdenum supported on the minority (001) surface above 580 K at PH2O ) 0.01 bar. This temperature is higher than observed in the (101) surface. On the basis of LIII-edge XAS spectroscopies studies, Hu et al.5 reported that, at low molybdenum loading and under dehydrated conditions, the surface molybdenum oxide species are primarily isolated tetrahedrally coordinated ones. These conclusions are supported by our results. On the predominant (101) surface, the molybdenum atom is 4-fold coordinated, bound to the surface through three Mo-O-Ti bonds, and there is only one molybdenyl ModO double bond (Figure 2a). The computed ModO vibrational wavenumber of 1004 cm-1 is also in good agreement with the experimentally observed Raman spectra. The most stable structure obtained on the marginal (001) surface contains two bridging Mo-O-Ti bonds and two Mod O double bonds, to which correspond two stretching modes

TABLE 6: Experimental Vibrational Frequencies (cm-1) at Low Coverage of the Molybdenum-Titania (MoOx-TiO2) Catalyst under Dehydrated and Ambient Conditions conditions

MoO3 wt (%)

νModO (cm-1)

ref

dehydrated

1.25 1

996 993

4 5

ambient

1 2.4

934 948

5 14

calculated (Figure 7c) at 1002 cm-1 for symmetric stretch and at 980 cm-1 for an antisymmetric one. Although the symmetric stretch could be responsible for the observed Raman line, this structure is probably not relevant. Indeed, the antisymmetric stretch is not visible in infrared spectroscopy whereas it should be as pointed out by Busca.12 Furthermore, isotopic substitution experiments with 18O isotopic experiments would lead to three distinct Raman bands corresponding to the symmetric stretching. Such a situation is not observed.17 Hence, as far as spectroscopic data are concerned, this minority surface does not contribute to the observed signals. Under ambient conditions (PH2O ) 0.01 bar), the most stable structures on the (101) surface contains three H2O molecules and tetrahedral molybdate species with only one ModO bond, and the molybdenum center is bound to the surface through three Mo-O-Ti bonds. The ModO stretching wavenumber is

Figure 11. Variation of the Gibbs free energy of the adsorbed MoO3 species on the anatase (001) surface as a function of temperature for different coverages of H2O and the stability domain of the various surface stoichiometry, (PH2O ) 0.01 bar).

3972 J. Phys. Chem. C, Vol. 111, No. 10, 2007 calculated at 980 cm-1. This value is too far from those available in the experimental literature (see Table 6) to correspond to the structure present on the anatase surface. However, the energy differences between the structure with three and six adsorbed water molecules are small. A limited increase of the water partial pressure will stabilize the structure with six absorbed molecules. Since the conversion of the water chemical potential to the water partial pressure might not be very precise, we can propose two other models which are only slightly less stable, with five (Figure 5h) and six (Figure 5i) adsorbed water molecules per unit cell. In both structures, one proton is located on the MoO3 species which has thus one ModO bond, one Mo-OH bond, and two Mo-O-Ti bonds. The molybdenum is in a distorted tetrahedral environment. The vibrational wavenumbers of Mod O bond are calculated at 943 and 934 cm-1 for five and six water molecules, respectively. These results are in better agreement with the experimental data (Table 6) that correspond to solid exposed in the moistened atmosphere. It appears that the extent of hydration strongly influences the vibrational properties of the molybdenyl without significant changes in the coordination number of the molybdenum. Dissolution of the molybdenum entities is thus not required to explain the shift of the Raman line at ca. 930 cm-1, as previously suggested in the literature.3,5 Indeed, the symmetric stretching mode of these dissolved species would have been observed at lower wavenumber (typically 898 cm-1). Our results show that a simple hydration of the surface species can account for the 60-70 cm-1 downward shift of the molybdenyl Raman line as proposed by Busca.12 5. Conclusions The structure of isolated molybdenum oxide entities supported on titania (anatase) has been studied through DFT calculations. The results clearly show that in dry conditions molybdenum is in a distorted tetrahedral environment with one single molybdenyl bond. The study on the (001) surface shows that dioxo structures, with two ModO double bonds, would show an important coupling of the two molybdenyls groups. As such coupled stretchings are not observed, these structures can be ruled out confirming that this surface is very marginal. The study of the hydration of the structures evidences that there is no need to take dissolution into account to explain the shift of the main Raman line. As a consequence, we propose two tetrahedral structures that could be present on the low-loading titaniasupported molybdenum oxide in ambient conditions. This study demonstrates that an evolution of the ModO stretching wavenumber is not always correlated to an evolution of the geometry of the Mo coordination number. This lowering may also be due to the formation of weak interactions between the molybdenum and adsorbed water molecules.

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