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J. Phys. Chem. C 2008, 112, 14456–14463
Structure of Isolated Molybdenum(VI) Oxide Species on γ-Alumina: A Periodic Density Functional Theory Study Jarosław Handzlik† and Philippe Sautet*,‡ Institute of Organic Chemistry and Technology, Cracow UniVersity of Technology, ul. Warszawska 24, PL 31-155 Krako´w, Poland, and UniVersite´ de Lyon, Institut de Chimie de Lyon, Laboratoire de Chimie, E´cole Normale Supe´rieure de Lyon and CNRS, 46 Alle´e d’Italie, 69364 Lyon Cedex 07, France ReceiVed: March 18, 2008; ReVised Manuscript ReceiVed: July 9, 2008
A periodic density functional theory approach is used to investigate isolated monomeric Mo oxide species on γ-alumina. Eleven potential dioxo and monooxo Mo centers variously located on the (100) and (110) surfaces of γ-alumina are modeled. In these structures, the molybdenum is 2-, 3-, or 4-fold bonded to the surface. Thermodynamic stabilities of the Mo oxide species are compared for a wide range of temperatures, taking into account the hydration/dehydration state of the catalyst. It is predicted that in strict dehydrated conditions, square pyramidal monooxo species are dominant on the most exposed (110) surface of γ-alumina, while tetrahedral dioxo species and five-coordinate dioxo species are most probable on the minority (100) surface. The latter is the potential precursor of the most active sites for alkene metathesis. The presence of 4-fold coordinated monooxo Mo species, especially on the (100) facet, is also possible. At low water exposure, tetrahedral dioxo Mo species are present on both γ-alumina surfaces. It is also predicted that the Mo sites on the (110) γ-alumina are more stable than their analogues located on the (100) facet. A significant increase of the ModO stretching frequency is observed when going from the dioxo species to the monooxo structures on the majority (110) surface of γ-alumina. This agrees with the evolution of the Raman spectra upon calcinations. Such a frequency shift between the dioxo and the monooxo species does not happen on the minority (100) surface. 1. Introduction Supported molybdenum oxide systems have been extensively investigated because of their applications in a variety of catalytic processes, among others, selective oxidation reactions,1-3 dehydrogenation4 and oxidative dehydrogenation5 reactions, hydrodesulphurization,6,7 and olefin metathesis.8,9 The active surface Mo forms are highly dispersed and are often proposed to be isolated species, formed from a single MoOx unit.3,8,10 The knowledge of the catalyst structure at the atomic level is decisive for a good understanding of the mechanisms of the catalytic reactions, as well as extremely helpful in designing of new catalytic systems. In the case of molybdena-alumina systems, there is still a controversy concerning the structure of isolated surface molybdenum(VI) oxide species under dehydrated conditions. Many researchers, applying Raman, EXAFS, and NEXAFS techniques, concluded that the monomeric molybdenum oxide centers on alumina are tetrahedrally coordinated.11-15 Therefore, the obvious assumption is a dioxo structure of the sites (Figure 1a).11,12,14 This proposition is also supported by theoretical study of heterogeneous Mo systems,14 although very small cluster models were applied in that case. Other results obtained using Raman and IR spectroscopy indicate the presence of monooxo Mo centers on γ-alumina.16-21 The coincidence of the Raman and the IR fundamental ModO vibrations for the molybdena-alumina system and the presence of only one band in the overtone region is consistent with monooxo ModO functionality.21 This is also confirmed by the observation of only one shifted band after isotopic * To whom correspondence should be addressed. † Cracow University of Technology. ‡ Universite ´ de Lyon.
Figure 1. The proposed structures for the monomeric Mo(VI) oxide species on the alumina surface.12,14,16,20,21 18O
2-
16O
2 exchange, whereas dioxo species are expected to show two additional Raman-active vibrational modes (16OdModO18 and 18OdModO18).16,21,22 On this basis, the species containing five-coordinated molybdenum was proposed (Figure 1b).16,20 However, Bell at al. recently showed from density functional theory (DFT) calculations for the molybdenasilica system that it is hard to distinguish between the monooxo and dioxo Mo species based purely on Raman spectroscopy.23 According to the theoretical predictions, there is only a small frequency difference between νs(16OdModO16) and νs(16OdModO18) modes, while a large shift is expected from νs(16OdModO18) to νs(18OdModO18). The DFT results were very recently confirmed by experimental isotopic 18O2-16O2 exchange study for molybdena-silica system,24 clearly indicating that detection of the intermediate 16OdModO18 species is indeed difficult. On the other hand, the high frequency of the ModO stretching vibration (over 1000 cm-1) reported for molybdena-alumina systems under dehydrated conditions supports the proposal of the monooxo Mo species.16 Finally, an isolated monooxo MoO4 site on alumina, having tetrahedral coordination and three bridging Mo-O-support bonds, was suggested as well (Figure 1c).21 This species seems to be in the best accordance with the mentioned experimental
10.1021/jp802372e CCC: $40.75 2008 American Chemical Society Published on Web 08/26/2008
Isolated Molybdenum(VI) Oxide Species on γ-Alumina results, because it has both tetrahedral geometry and only one oxo ligand. However, one should keep in mind that Mo(VI), not Mo(V), sites are considered here. Therefore, the Mo-Osupport linkages would have to be nonequivalent in the case of the center c. As the experimental results are somewhat ambiguous, theoretical calculations based on a reliable catalyst model seem to be required for better understanding the structure of the isolated Mo(VI) oxide species on the support. Therefore, the main aim of this work is to determine the geometry and location of a large family of potential Mo(VI) oxide forms on alumina, mostly under dehydrated conditions. We use a periodic slab model with a DFT approach to study the properties of both monooxo and dioxo isolated Mo(VI) surface species. The stability of the various proposed structures for the Mo sites on both (100) and (110) faces of γ-alumina are calculated, taking into account the hydration/dehydration state of the catalyst. Additionally, the theoretical ModO stretching frequencies are compared with reported experimental data to discuss possible monooxo or dioxo functionalities in the real catalyst. The recently reported model of γ-alumina25-29 is employed to enable realistic modeling of the molybdena-alumina system. Previously, such an approach was successfully applied to study the structure, thermodynamic stability and olefin metathesis activity of Mo-methylidene centers on γ-alumina.10 In many cases, the presently investigated Mo oxide species can be considered as the precursors of the previously studied surface Mo-methylidene forms. 2. Computational Methods and Models The periodic calculations have been performed in the framework of density functional theory, employing the Vienna Ab Initio Simulation Package (VASP).30-32 The Perdew and Wang (PW91) generalized gradient-corrected exchange-correlation functional is used.33 The one electron wave functions are developed on a basis set of plane waves. Atomic cores are described with the projector-augmented wave method (PAW).34 In most calculations, standard PAW atomic parameters were used, requiring a cutoff energy of 400 eV (fixed by the O atom) for a converged total energy. For Mo, the PAW is built with 12 e in the valence. Tests on molecular models were performed also with harder PAW parameters (using a core radius of 1.10 Å for O and 1.5 Å for Cl) and requiring a cutoff energy of 700 eV. A 331 Monkhorst-Pack mesh has been applied for Brillouinzone sampling. The models of the γ-Al2O3 surface, based on the developed nonspinel bulk structure,35 were previously validated.25,26 The shape of the γ-alumina particles is inherited from that of the boehmite precursor. Hence the (110) plane is the most exposed surface on the particles (70-83%) and the (100) surface is in minority (17%), while the (111) plane can be neglected.25,36,37 On the other hand, the calculated surface energy for dehydrated (100) γ-Al2O3 is lower than for dehydrated (110) γ-Al2O3.25,26 Both (100) and (110) facets are taken into account in this study. Surfaces have been modeled by a four layer slab. The two bottom layers have been frozen in the geometry of the bulk, while the two upper layers are relaxed. The surface unit cell dimensions (Å) are a ) 8.414, b ) 11.180 and a ) 8.097, b ) 8.414, for the former and the latter, respectively. Frequency calculations on the stable geometry have been carried out by numerical differentiation of the force matrix. To reduce the computational effort, only 27 to 42 coordinates of freedom were used for the frequency calculations (depending on the selected MoOx surface structure), i.e., the positions of
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14457 all atoms, excluding the Mo site and its close neighbors, were frozen. For one selected structure, however, all the optimized degrees of freedom were also used to validate other results. The calculated ModO stretching frequencies differ by less than 1 cm-1. The surface models describe different dehydrated/hydrated states of the system and they are related by a dehydration/ hydration reaction. To calculate the Gibbs free energy for these reactions, the differences between the Gibbs free energies for condensed phases have been approximated by the differences between their calculated electronic energies. Only the rotational and translational contributions to enthalpy and entropy of water have been taken into account, that is, we assume that the vibrational contribution of a water molecule is not significantly influenced by its adsorption. Our efforts are mainly focused on the description of the molybdena-alumina system under dehydrated conditions. This usually means that the sample is calcinated at approximately 773 K before its in situ spectroscopic characterization.11,12,14,16-18,38,39 Thermal pretreatment of the catalyst prior to reaction can be carried out at even higher temperatures (820-1090 K).9,40,41 For the (100) surface of γ-alumina, a fully dehydrated surface is thermodynamically favored at 773 K, whereas a coverage of 3.0-5.9 OH per nm2 is present on the (110) surface, according to the periodic DFT and thermodynamical calculations if a water pressure of 1 atm is assumed.25,26 This is in good agreement with the experimental determination of ∼4 OH per nm2 after such a pretreatment. This coverage corresponds to 1-2 water molecules adsorbed per surface unit cell. Since the presence of the isolated Mo center on the surface is approximately equivalent to adsorption of one or two water molecules10 and the dehydrated conditions correspond to very low vapor pressure, in most cases no water molecules have been added to the alumina surface containing the models of the Mo species. The dehydrated (100) surface presents five-coordinated Al Lewis centers (AlV), arising from octahedral Al in the bulk, while tetrahedral Al are located below the surface plane. The dehydrated (110) surface shows a stronger Lewis acidity and presents AlIV (from octahedral Al in the bulk) and AlIII centers (from tetrahedral Al in the bulk). These various unsaturated sites will be used to attach the isolated monooxo and dioxo Mo species. All models will consider a Mo atom in the +VI oxidation state. 3. Results and Discussion 3.1. Calibration of the Theoretical Method. To validate the theoretical approach used here, calculations have been performed for the gas phase Mo compounds MoO2Cl2, MoOCl4, MoO2(OH)2, and H2MoO2 to compare the calculated geometrical parameters and/or theoretical vibrational frequencies with the corresponding experimental data. MoOCl4 has a square pyramidal monooxo structure, while the others are tetrahedral dioxo compounds. In Table 1, calculated and reported experimental42,43 bond lengths and angles are compared for MoO2Cl2 and MoOCl4. The agreement between theory and experiment is quite good. One can notice that the calculated metal-ligand bond lengths are slightly overestimated, which is expected for the GGA DFT methods.44 The theoretically predicted ModO distances are approximately 0.02-0.03 Å longer than the corresponding experimental values. Using the hard PAW parameters does not affect significantly the ModO bond lengths. Experimentally determined ModO stretching frequencies are available for all the considered gas phase Mo compounds.45-47
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TABLE 1: Calculated and Experimental Bond Lengths (Å) and Angles (°) for MoO2Cl2 and MoOCl4a MoO2Cl2
MoOCl4
ModO Mo-Cl OdModO Cl-Mo-Cl
ModO Mo-Cl OdMo-Cl Cl-Mo-Cl
calc PAW1
calc PAW2
exp42
1.707 2.262 106.6 112.4
1.705 2.263 106.6 112.4
1.686 2.258 106.3 113.9
calc PAW1
calc PAW2
exp43
1.685 2.306 104.2 86.6
1.683 2.306 104.2 86.6
1.658 2.279 102.8 87.2
a Normal (PAW1, 400eV cut-off) and hard (PAW2, 700 eV cut-off) PAW parameters are used.
TABLE 2: Calculated and Experimental ModO Stretching Frequencies (cm-1) for the Reference Compoundsa MoO2Cl2 MoOCl4 MoO2(OH)2 H2MoO2
calc. PAW1
calc. PAW2
exp.
1022 998 1046 1004 993 1015 1003
1010 982 1026 993 980 1001 986
997b 971b 1015c 979.3d 985.0d
a Normal (PAW1, 400eV cut-off) and hard (PAW2, 700 eV cut-off) PAW parameters are used. b Reference 45. c Reference 46. d Reference 47.
As in the present work we focus just on the ModO stretching vibrations, the theoretical and the experimental ModO frequencies are compared in Table 2. It is seen that the calculated values are slightly overestimated (by ∼10-30 cm-1), compared to the experimental fundamentals; however, all trends are well reproduced. The largest overestimation is obtained for the chlorinecontaining compounds, while the other molecules give a better agreement (overestimation by 10-20 cm-1). Using harder PAW parameters (associated to a cutoff of 700 eV) reduces the Mo-O frequencies by 10-20 cm-1 and gives very good consistency for the latter case. The harder PAW reduces the overlap between the Mo and O atomic spheres in the short double ModO bond, yielding a somewhat more accurate description. However the high energy cutoff in this case is not practical for the extended periodic systems and hence the normal PAW set was kept. 3.2. Structures of Mo Oxide Species. Optimized structures for isolated Mo(VI) oxide species on the (100) surface of γ-alumina are shown in Figure 2. The molybdenum is pseudotetrahedrally coordinated in 100_1 and possesses two oxo ligands. However, one of them is involved in hydrogen bonding, which explains the significant elongation of the ModO bond (1.77 Å). The molybdenum atom is connected via two oxygen linkages with octahedrally coordinated Al atoms (previously AlV on the bare surface). The different lengths of the Mo-O single bonds can be mainly explained by the formation of a second hydrogen bond between one of the oxygen link atoms and the surface hydrogen (Figure 2). The formation of the center 100_1 can be formally described as the dissociative adsorption of a molybdic acid molecule MoO2(OH)2 on the bare surface, therefore two hydrogens per Mo site are present. The sites 100_2 to 100_5 can be formally regarded as the products of dehydratation of the 100_1 structure. These structures can be also viewed as a MoO3 unit distorted by interaction with the surface. 100_2 and 100_3 are fourcoordinate dioxo Mo species (compare with Figure 1a). In both
sites, the pseudotetrahedrally coordinated molybdenum is connected via only one oxygen bridge to an aluminum atom with a distorted octahedral structure. Additionally, a weak dative bond from an oxygen lone pair on the surface toward the Mo Lewis acid center is formed. 100_4 is an example of five-coordinate dioxo Mo species (Figure 2). In this structure the molybdenum atom shows an oxygen linkage with the octahedral Al atom, as before, with two additional dative O-Mo bonds from two surface oxygen atoms (2.18 and 2.26 Å). The Mo Lewis center is hence bridging two surface oxygen sites. Similar weak Mo-O bonds are present in bulk MoO3.48 Finally, 100_5 is a four-coordinate site, and this is the only example of a monooxo Mo species that we have obtained on the (100) surface of γ-alumina. Such a kind of isolated surface Mo form is presented in Figure 1c. The molybdenum atom is connected via two oxygen linkages with octahedrally coordinated Al atoms and forms additionally a dative bond with a surface oxygen. The Mo-O distance in the latter case (1.95 Å) is shorter than the corresponding Mo-O bond lengths for the other species. The other two Mo-O bonds exhibit a partial double bond character (1.78 and 1.79 Å), in accordance with the formal +VI oxidation state of the molybdenum atom. The predicted ModO distances do not differ significantly among the species 100_1-100_5, and they are in the range of 1.72-1.73 Å, excluding the oxo ligand involved in hydrogen bonding (100_1). These values are longer by 0.01-0.02 Å than the ModO length calculated for tetrahedral MoO2Cl2 (Table 1). Experimental ModO distances of 1.7049 or 1.74 Å,14 determined by EXAFS, were reported for calcinated molybdenaalumina systems. In the latter case, four equivalent ModO bonds were predicted for the surface species and a tetrahedral species, interacting by two oxygens with only one surface aluminum atom was proposed.14 However, our results confirm rather the former experimental value of the ModO bond length (1.70 Å), because one should take into account the correction of the calculated distance (0.02-0.03 Å overestimation from DFTGGA), already mentioned in Section 3.1. The optimized structures for isolated Mo(VI) oxide centers on the (110) surface of γ-alumina are presented in Figure 3. 110_1 to 110_3 are 4-fold coordinated dioxo Mo species (Figure 1a). Both 110_1 and 110_2 models are analogous to the structure 100_1 with two aluminoxi linkages and two oxo ligands. However, the molybdenum is connected with one tetrahedrally coordinated and one octahedrally coordinated aluminum atoms, instead of two octahedral Al for the (100) surface. 110_1 and 110_2 are the models of the same Mo site, but in the first case one water molecule per Mo center is additionally adsorbed on the surface. This results in a hydrogen bond formation between one oxo ligand and the surface hydroxyl group. Consequently, the ModO distance is elongated (1.74 Å), but to a less extent than in the case of the 100_1 species (Figure 2), where the corresponding hydrogen bond is stronger. Additionally, two other hydrogen bonds between the linkage oxygen atoms and surface hydrogens are observed (Figure 3). In the case of the species 110_3, the molybdenum has a distorted tetrahedral coordination and is connected via two oxygen atoms with one tetrahedrally coordinated aluminum atom (the same as in the case of 110_1 and 110_2) and two octahedrally coordinated aluminiums. The O atom is bridging two surface Al atoms and such a structure was also found for OH groups. In the latter case, the single Mo-O bond is elongated (2.03 Å). Considering the other Mo oxide species on the (110) surface (Figure 3),110_4 and 110_5 are four-coordinate monooxo Mo
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Figure 2. Optimized structures for the Mo oxide species on (100) γ-alumina. Bond lengths are given in Å.
Figure 3. Optimized structures for the Mo oxide species on (110) γ-alumina. Bond lengths are given in Å.
centers, while 110_6 is the only example in this work of fivecoordinate monooxo Mo species. They all are formally the products of the dehydratation of 110_2 or 110_3. Again, these sites can be regarded as a MoO3 molecule distorted by interaction with the bare surface, similarly to 100_2-100_5 structures. The species 110_4 and 110_5 are analogous to the 100_5 center (see also Figure 1c). The molybdenum possesses one oxo ligand and is 3-fold bonded to the surface. In 110_4, the Mo atom is connected via two oxygen linkages with tetrahedrally and octahedrally coordinated aluminum atoms. The former is the same atom as in the 110_1-110_3 models, while the latter is one of the octahedral Al atoms in the pair relevant for the structure 110_3. Additionally, the molybdenum in 110_4 interacts with a two-coordinated oxygen atom from the support. This oxygen was initially three-coordinated on the bare alumina surface; therefore, a reorganization of the surface takes place, resulting in strengthening of the dative Mo-O bond. The model 110_5 is quite similar to the 110_4 one, but with a different choice of octahedral Al atom on the surface for one aluminoxi linkage (now both considered tetrahedral and octa-
hedral aluminiums are the same like in the 110_1 and 110_2 models) and with a different surface oxygen involved in the dative bonding. This oxygen atom remains 3-fold coordinated, hence the Mo-O dative bond is weaker than in the case of 110_4 (Figure 3). Finally, the monooxo Mo species 110_6 is the only one where the molybdenum is 4-fold bonded to the alumina surface. This is an example of the structure proposed in Figure 1b. This molybdenum complex shows a distorted square pyramidal geometry with two single (1.81, 1.86 Å) and two dative (2.03, 2.16 Å) Mo-O bonds, as well as with the terminal oxo ligand on the top. The tetrahedral and octahedral Al atoms for the two aluminoxi linkages are still the same as in the models 110_1, 110_2, and 110_5. The calculated ModO distances for the dioxo species on the (110) surface of γ-alumina are in the range of 1.72-1.73 Å, excluding the oxo ligand involved in hydrogen bonding (110_1). Thus, they are very close to the corresponding bond lengths for both dioxo and monooxo Mo species on the (100) surface (Figure 2). On the other hand, the predicted ModO bond distances for the monoxo species on the (110) γ-alumina (1.71
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Figure 4. Relative stabilities of the Mo oxide species as a function of temperature for different water vapor pressures. The Gibbs free energy refers to the dehydration reaction indicated below.
Å, Figure 3) are approximately 0.01-0.02 Å shorter than the corresponding values for both the dioxo species on the same surface and for all the Mo species on the (100) surface. The bond lengths predicted for the molybdenyl ModO double bonds on the (110) face are well consistent with the reported value of 1.70 Å, determined by EXAFS.49 The obtained values are also close to DFT calculated ModO distances for isolated molybdenum oxide entities supported on titania.50 The most stable surface molybdenum oxide structure under dehydrated conditions, predicted in that work,50 is a distorted tetrahedral monooxo species, i.e., similar to the 100_5, 110_4 and 110_5 models from this present study. As we focus on the dehydrated and partially hydrated surface, we have not considered here potential Mo species having OH ligand. Such structures were proposed for molybdena-titania system, but only under ambient conditions.50 3.3. Relative Stabilities of the Mo Species. The energies of the surface Mo complexes of identical stoichiometry can be directly compared to each other. On the (100) surface of γ-alumina, the dioxo Mo center 100_3 and 100_4 are the most stable ones among the structures 100_2-100_5 (100_4 is only 2 kJ · mol-1 less stable than 100_3). The energies of the structures 100_2 and 100_5 are higher by 47 and 13 kJ · mol-1, respectively, compared to 100_3. The energies of 110_2 and 110_3 are also directly comparable and the former is predicted to be less stable by 57 kJ · mol-1. Among the species 110_4-110_6, the latter is clearly the most stable one: 110_4 and 110_5 are higher in energy by 99 and 74 kJ · mol-1, respectively. To study the relative thermodynamic stability of the Mo species of different stoichiometry, we have analyzed the energetics of adequate dehydration reactions. The left-side reaction presented in Figure 4 enables the determination of the relative stabilities of the sites 100_1 and 100_3. It is predicted that at 0 K, the former is more stable than the latter. An analogous situation takes place in the case of the Mo complexes on the (110) face, where the center 110_3 is thermodynamically preferred over the 110_6 one (Figure 4, the right-side reaction).
However, the situation changes when the temperature increases, because of the entropic effect of the released water molecule. For both reactions, the change of the Gibbs free energy at various temperatures and under various water vapor pressures has been calculated (Figure 4). It is seen that under a vapor pressure of 0.01 atm (the moisture present in the atmosphere), the site 100_3 becomes more stable than 100_1 above a temperature of approximately 900 K. This threshold temperature decreases if the vapor pressure is reduced and in low pressure conditions the “dehydrated” site can be more stable than the “hydrated” one at temperatures above approximately 700 K. For the (110) surface, under a water vapor pressure of 0.01 atm the thermodynamic preference for the “hydrated” complex 110_3 is observed until approximately 850 K, while under strict dehydrated conditions the “dehydrated” site 110_6 can be more stable at temperature of about 650 K or higher (Figure 4). It can be noticed that these threshold temperatures for dehydrated conditions are lower than the temperatures usually applied for the in situ catalyst thermal pretreatment prior to spectroscopic measurements or before a catalytic reaction.9,11,12,14,16-18,38-41 Therefore, on the basis of the present calculations, it is predicted that in strict dehydrated conditions, the square pyramidal monooxo species 110_6 is dominant on the most exposed (110) surface of γ-alumina, while the tetrahedral dioxo site 100_3 and five-coordinate dioxo species 100_4 are mainly present on the minor (100) surface. The presence of small amounts of water (it does not mean yet the typical ambient conditions) leads to formation of the tetrahedral dioxo Mo species, presumably 100_1 and 110_3. The relative stabilities of the corresponding Mo forms on the (100) and (110) surfaces can be determined on the basis of the calculated energy for grafting the MoO2(OH)2 molecule to the alumina (Scheme 1). The obtained reaction energy is -224 and -441 kJ · mol-1 for the centers 100_1 and 110_3, respectively. Therefore, the latter is predicted to be more stable than the former. On the basis of the obtained energies for the reactions depicted in Scheme 2, for the dehydrated structures,
Isolated Molybdenum(VI) Oxide Species on γ-Alumina
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14461
SCHEME 1
SCHEME 2
it is also seen that the center 100_3 has a less stabilizing formation energy than the 110_6 one. Therefore, the present calculations indicate that the isolated molybdenum oxide species on the (110) γ-alumina are more stable than the corresponding ones on the (100) surface. The important difference between the two terminations of the alumina support is explained by the more unsaturated nature of the (110) surface, with lower coordination of the Al atoms (III and IV, compared to V on the (100) surface). As has been already mentioned, experimental researches lead to somewhat contradictory conclusions about the structure of isolated molybdenum oxide species on alumina under dehydrated conditions. It is usually concluded that molybdenum is in a distorted tetrahedral environment,11,12,14 although a distorted octahedral geometry was also suggested.16,17,20 On the basis of spectroscopic measurements, both dioxo (Figure 1a)11,12,14 and monooxo16-21 isolated Mo oxide species are proposed. The monooxo species can be either tetrahedral, 3-fold coordinated to the alumina surface (Figure 1c),21 or square pyramidol, 4-fold coordinated to the surface (Figure 1b).16,20 Because pyramidal geometry is rare among oxymolybdenum compounds, while tetrahedral or octahedral coordination is very common, the geometry of the latter Mo species is explained as a distorted octahedral with an additional weak axial Mo...O-Al bond existing opposite to the molybdenyl ModO bond.16 However, according to our calculations, the most stable Mo species on the dominant (110) face (110_6) has square pyramidal geometry (Figure 3), like MoOCl4, with no additional axial interaction with a surface oxygen. Nevertheless, our results are in excellent agreement with the proposition from ref 16 (Figure 1b), as the 110_6 structure has only one oxo ligand and is 4-fold bonded to the surface. Moreover, it was also reported16,20 that at low water exposure, two of the Mo-O-Al links hydrolyze and a tetrahedral dioxo species is formed. Our findings, presented in Figure 4, are in full accordance with those conclusions. On the basis of the present calculations, four coordinate monooxo Mo species (3-fold coordinated to the surface, Figure 1c), cannot be entirely excluded either. The structure 100_5 on the (100) γ-alumina (Figure 2) is only 13 kJ · mol-1 higher in energy than the species 100_3, which is the most stable under dehydrated conditions. On the other hand, similar structures on the (110) face, 110_4 and 110_5 (Figure 3), are much less probable, as they are clearly unstable compared to 110_6, by 99 and 74 kJ · mol-1, respectively.
Finally, according to our calculations, the most stable isolated Mo form on the minor (100) γ-alumina face is the tetrahedral dioxo species 100_3 (Figure 2), a representative of the often proposed structure depicted in Figure 1a.11,12,14 On the other hand, the five-coordinate dioxo form 100_4 is predicted to be only 2 kJ · mol-1 less stable than 100_3; hence, its existence is highly probable. Such a structure for the isolated surface Mo oxide species has not been proposed, so far. Thus, the picture of the isolated molybdenum oxide structures on γ-alumina seems to be rather complex and the reported contradictory conclusions concerning their geometries are not surprising in the light of our results. Recently, we investigated the structure and metathesis activity of Mo-methylidene species on both the (100) and (110) surfaces of γ-alumina.10 Such centers are usually formed from their Mo oxide precursors, often reduced, after the contact with alkene.40,41 Among many considered structures only one, located on the (100) surface, was found to be active in alkene metathesis at room temperature. The dioxo species 100_4 studied in the present work can be the initial Mo(VI) precursor of that active site, because both are identically located on the γ-alumina. Therefore, the existence of the metathesis active sites on the (100) face is quite probable, as the molybdenum oxide form 100_4 is one of the two most stable Mo species on this surface under dehydrated conditions. On the other hand, the most stable Mo species on the (110) surface (110_6) is the potential precursor of a Mo-methylidene center that was predicted to be inactive in alkene metathesis at low or moderate temperatures.10 3.4. ModO Vibrational Frequencies. In situ Raman spectroscopy allows the observation of changes in the structure of molybdenum oxide species on alumina that occur when ambient conditions are replaced by dehydrated conditions. Oyama et al. proposed that ModO stretching frequencies in the range 940-970 cm-1 correspond to dioxo species surrounded by varying amounts of waters coordination.16 The molybdenum is supposed to be in a distorted octahedral environment. After partial calcination, at low water exposure the Raman band frequency increases to 970-990 cm-1.16 It was proposed that at this stage water molecules are removed from the surface and tetrahedral dioxo Mo species (Figure 1a) are surrounded by hydroxyl groups. Finally, after full calcination monooxo Mo species 4-fold coordinated to the surface (Figure 1b) were postulated, which are characterized by a ModO vibrational frequency in the range 1006-1012 cm-1.16 Other reported
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TABLE 3: Calculated ModO Stretching Frequencies (cm-1) for the Surface Mo Oxide Forms on γ-Alumina label
type
conditions
νcalc
100_1 100_2 100_3 100_4 100_5 110_1 110_2 110_3 110_4 110_5 110_6
dioxo dioxo dioxo dioxo monooxo dioxo dioxo dioxo monooxo monooxo monooxo
hydrated dehydrated dehydrated dehydrated dehydrated hydrated hydrated hydrated dehydrated dehydrated dehydrated
996/897 981/960 980/958 987/961 995 993/953 990/944 998/968 1010 1016 1018
ModO vibrational frequencies for low molybdenum loading under dehydrated conditions are in the broader range 985-1012 cm-1.11,12,14,17-20,38,39 The calculated ModO vibrational frequencies for all the Mo oxide species studied in this work are listed in Table 3. If we first concentrate on the dioxo species, only a small difference (10 cm-1) is observed between the frequencies of the “hydrated” and “dehydrated” dioxo species, unless the oxo ligand is strongly hydrogen bonded (the lower frequency of 100_1). However, the frequency for the oxo ligand, which is involved in weaker hydrogen bond, is close to the lower frequency of the free ModO bonds (compare 110_1 and 100_3). In the case of both 100_1 and 110_1 species, the stretching frequency corresponding to the oxo ligand that is not hydrogen bonded is among the highest frequencies calculated for the dioxo species. A more significant increase of the ModO stretching frequency (about 20 cm-1) is observed when going from the dioxo species to the monooxo structures on the (110) surface. On the other hand, this effect is not present on the (100) surface. The ModO frequency for the monooxo model 100_5 is quite similar to the values calculated for the dioxo species. This can be explained by the different strength of the MoOx/Al2O3 interface, associated with a bond order conservation effect. On the (100) surface, the interaction is rather weak and the Al-O bonds of the aluminoxi linkages are long (1.93 and 2.06 Å for 100_5). As a result the associated O-Mo distances for these Al-O-Mo linkages are short (1.78-1.79 Å for 100_5) and hence, by a bond order conservation effect around Mo, the oxo bond is elongated (1.72 Å for 100_5). In contrast, the Al atoms on the (110) surface are more unsaturated resulting in stronger Al-O bonds (1.83 and 1.94 Å for 110_4). In a cascade effect, the single O-Mo bonds are weakened (1.82 and 1.83 Å for 110_4) and the Mo-O distance is shortened (1.71 Å for 110_4). If we now come back to the experimental Raman data, they should be related to the majority sites and hence to the (110) surface. The upshift of 30 cm-1 observed between partial and full calcinations should then be associated with the transformation from the dioxo species (type 110_3) to the fully dehydrated monooxo form (type 110_6) in line with our thermodynamic analysis. However, within the dioxo species Raman should not be able to distinguish between the dehydrated and the hydrated forms unless strong hydrogen bonding effects are involved. 4. Conclusions A periodic slab model has been used for the first time with a DFT approach to investigate isolated Mo oxide species on γ-alumina. A large number of potential dioxo and monooxo Mo centers differently located on the (100) and (110) surfaces have been modeled. In these structures, the molybdenum is 2-, 3-, or 4-fold bonded to the surface. According to the DFT
calculations and thermodynamic analysis, the relative stabilities of different Mo oxide species depend on the temperature and water vapor pressure. It is also predicted that the Mo sites on the (110) surface are more stable than their analogues located on the (100) facet. On the basis of the present calculations, it is concluded that in strict dehydrated conditions, the square pyramidal monooxo species is dominant on the most exposed (110) surface of γ-alumina, while tetrahedral dioxo species and five-coordinate dioxo species are most probable on the minor (100) surface. However, the presence of monooxo Mo species 3-fold bonded to the surface, especially on the (100) facet, cannot be entirely excluded. At low water exposure, tetrahedral dioxo Mo species are present on both γ-alumina surfaces. A significant increase of the ModO stretching frequencies (about 20 cm-1) is observed when going from the dioxo species to the monooxo structures on the (110) surface of γ-alumina, in accordance to the reported experimental results. On the other hand, the ModO vibrational frequency for the monooxo species on the minority (100) surface is close to the frequencies calculated for the dioxo Mo species. These Mo-oxo species are the precursors for the oxo-carbene active sites in the olefin metathesis reaction. The most abundant surface structure predicted from our calculations in dehydrated conditions, the monooxo 110_6, is the precursor of a oxocarbene, which is only moderately active. A structure for a fivecoordinate dioxo Mo form on γ-alumina has not been proposed in the literature so far. As this species 100_4 can be in minority on the minor (100) face, its detection by spectroscopic techniques is very difficult or impossible. However, this species seems to be the best candidate for the precursor of the active site of alkene metathesis, which is known to involve only a small fraction of the deposited Mo atoms.51 Therefore, DFT calculations give detailed insights in the structure and properties of the surface sites and allow the investigation of surface sites that are in minority and not easily detectable by spectroscopic techniques. Such centers can be the active site precursors, hence being decisive for the catalytic activity. Therefore, quantum chemistry methods appear as a very useful tool for description of the supported catalytic systems. The theoretical approach is not only helpful in the interpretation of the experimental results, but it also brings complementary insights. Acknowledgment. The work has been performed under the Project HPC-EUROPA (RII3-CT-2003-506079), with the support of the European Community - Research Infrastructure Action under the FP6 “Structuring the European Research Area” Programme. Computing resources from Institut du De´veloppement et des Ressources en Informatique Scientifique (IDRIS) and Academic Computer Centre CYFRONET AGH (Grants MEiN/SGI3700/PK/021/2006 and MNiSW/SGI4700/PK/044/ 2007) are gratefully acknowledged. References and Notes (1) Zhang, W.; Desikan, A.; Oyama, S. T. J. Phys. Chem. 1995, 99, 14468. (2) Liu, H.; Iglesia, E. J. Catal. 2002, 208, 1. (3) Ohler, N.; Bell, A. T. J. Phys. Chem. B 2006, 110, 2700. (4) Harlin, M. E.; Backman, L. B.; Krause, A. O. I.; Jylha¨, O. J. T. J. Catal. 1999, 183, 300. (5) Chen, K.; Bell, A. T.; Iglesia, E. J. Catal. 2002, 209, 35. (6) Bergwerff, J. A.; Visser, T.; Leliveld, R. G.; Rossenaar, B. D.; de Jong, K. P.; Weckhuysen, B. M. J. Am. Chem. Soc. 2004, 126, 14548. (7) Bergwerff, J. A.; Jansen, M.; Leliveld, R. G.; Visser, T.; de Jong, K. P.; Weckhuysen, B. M. J. Catal. 2006, 243, 292.
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