Adsorption of Propane, Isopropyl, and Hydrogen on Cluster Models of

Feb 24, 2010 - The Mo−V−Te−Nb−O mixed metal oxide catalyst possessing the M1 phase structure is uniquely capable of directly converting propan...
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J. Phys. Chem. C 2010, 114, 4544–4549

Adsorption of Propane, Isopropyl, and Hydrogen on Cluster Models of the M1 Phase of Mo-V-Te-Nb-O Mixed Metal Oxide Catalyst Agalya Govindasamy,† Kaliappan Muthukumar,† Junjun Yu,† Ye Xu,*,‡ and Vadim V. Guliants*,† Department of Chemical and Materials Engineering, UniVersity of Cincinnati, Cincinnati, Ohio 45221, and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: October 30, 2009; ReVised Manuscript ReceiVed: January 26, 2010

The Mo-V-Te-Nb-O mixed metal oxide catalyst possessing the M1 phase structure is uniquely capable of directly converting propane into acrylonitrile. However, the mechanism of this complex eight-electron transformation, which includes a series of oxidative H-abstraction and N-insertion steps, remains poorly understood. We have conducted a density functional theory study of cluster models of the proposed active and selective site for propane ammoxidation, including the adsorption of propane, isopropyl (CH3CHCH3), and H which are involved in the first step of this transformation, that is, the methylene CsH bond scission in propane, on these active site models. Among the surface oxygen species, the telluryl oxo (TedO) is found to be the most nucleophilic. Whereas the adsorption of propane is weak regardless of the MOx species involved, isopropyl and H adsorption exhibits strong preference in the order of TedO > VdO > bridging oxygens > empty Mo apical site, suggesting the importance of TeOx species for H abstraction. The adsorption energies of isopropyl and H and consequently the reaction energy of the initial dehydrogenation of propane are strongly dependent on the number of ab planes included in the cluster, which points to the need to employ multilayer cluster models to correctly capture the energetics of surface chemistry on this mixed metal oxide catalyst. Introduction Heterogeneously catalyzed selective oxidation processes are responsible for a quarter of all industrial organic chemicals and intermediates and are of vital importance to modern economy.1-4 The selective oxidation and ammoxidation of alkanes catalyzed by mixed metal oxide catalysts5 represent new environmentally friendly green chemical processes that received considerable attention in recent years in the hope of replacing the current processes based on the more expensive and less abundant alkenes as feedstock. The Mo-V-Te-Nb-O mixed metal oxide discovered in 1995 by Mitsubishi Chemical Company possesses the unique ability to convert propane directly into partial (amm)oxidation products, acrylonitrile and acrylic acid. However, its activity and selectivity to acrylonitrile need to be further improved for a commercial application. Therefore, the optimization of the Mo-V-Te-Nb-O catalyst has been an important and active area of catalysis research.6-10 The bulk Mo-V-Te-Nb-O mixed metal oxide consists predominantly of two phases. The dominant M1 phase required for the activation of propane is responsible for its catalytic activity and selectivity to acrylonitrile.11-13 The crystal structure of the M1 phase has been determined by using a combination of high-resolution TEM, synchrotron X-ray, and powder neutron diffraction techniques.14,15 It has an orthorhombic structure with lattice constants of a ) 21.1717, b ) 26.6737, and c ) 4.0168 Å.11,16 The unit cell of the ab plane of the M1 phase has a bulk composition of (Mo7.8V1.2Nb)O28{TeO}0.94 and displays 13 crystallographically distinct metal lattice positions (M1-M13) * Corresponding authors. E-mail: [email protected], Phone: +1-865-5749761, Fax: +1-865-574-1753 (Y.X.); E-mail: [email protected], Phone: +1-513-556-0203, Fax: +1-513-556-3473 (V.V.G.). † University of Cincinnati. ‡ Oak Ridge National Laboratory.

Figure 1. (a) Unit cell of the ab plane of the M1 phase of the Mo-V-Te-Nb-O mixed metal oxide.15 The metal sites are labeled. The portion of the ab plane corresponding to the proposed active surface site modeled in this study is outlined by dashed lines. (b) Side and (c) top views of a multilayer cluster. In panel c, the experimentally determined15 distances (in Å) between the telluryl oxo, the vanadyl oxo, and the V-O-Mo bridging O species are indicated.

with several hexagonal and heptagonal channels formed by transition metal octahedra (Figure 1). Buttrey et al.14,15 and Grasselli et al.17 proposed that the octahedral lattice sites M1, 2, 3, and 7 were occupied by either V or Mo, that the M4, 5, 6, 8, 10, and 11 were occupied by Mo only, that the pentagonal bipyramidal lattice site M9 was occupied by Nb only, that the hexagonal and possibly heptagonal channel sites were occupied by Te, and that V, Mo, Nb, and Te were present in the 4+/5+, 5+/6+, 5+, and 4+ oxidation states, respectively. Recent highresolution STEM studies provided support for this structural model and further suggested that the M4 lattice site also contains some V and that the hexagonal channel (M12 site) is always

10.1021/jp910382x  2010 American Chemical Society Published on Web 02/24/2010

Cluster Models of Mixed Metal Oxide Catalyst occupied by Te, whereas the heptagonal channel (M13 site) is only partially occupied by Te.18 Grasselli et al. have proposed a hypothetical mechanism for propane ammoxidation on the surface ab planes of the Mo-V-Te-Nb-O M1 phase.11,19 The oxo group attached to V5+, through its resonance structure (V5+dO T V4+•sO•), is thought to be the propane-activating site capable of methylene-H abstraction. The adjacent Te4+dO performs the methyl-H and R-H abstraction to form propylene and then a π-allylic intermediate, whereas the adjacent Mo6+ catalyzes the insertion of NH into the π-allylic intermediate, followed by further H abstraction by adjacent Te4+. Nb, proposed to be in the pentagonal bipyramidal coordination, is thought to stabilize the M1 structure and provide site-isolating function without directly participating in any reaction step. Therefore, all required catalytic functions can be found in a small active site present in the surface ab planes (see Figure 1), and the close proximity of these different surface MOx species is thought to be a major reason for the efficient conversion of propane into acrylonitrile on the Mo-V-Te-Nb-O oxide.11 Extensive experimental work has been performed to probe the reaction mechanism and the roles of various surface metal oxide species. It has been established that the first step of the reaction is the oxidative dehydrogenation (ODH) of propane to propylene,20 and no dimerization or skeletal rearrangement occurs during subsequent transformation of the propylene intermediate.21 The rate of propane consumption was found to correlate with the surface concentration of V in the M1 phase.22 Te and Nb were shown to stabilize the Mo-V-Te-Nb-O M1 phase under reaction conditions,11,12 and Te in particular was shown to also promote the oxidation of propylene,23 consistent with the hypothetical mechanism proposed by Grasselli et al.17,24 Grasselli et al. have suggested that the improvement of catalytic efficiency may be realized by altering the elemental distribution in the active site.11 However, molecular-level information that links the structure and composition of the surface to the elementary steps in propane (amm)oxidation remains unavailable, in part because of the large number of metal lattice positions in the M1 ab plane and their variable occupation by different metal cations. Therefore, there is no fundamental basis for seeking further improvement of the reactivity of the active center in propane ammoxidation without detailed knowledge of surface sites responsible for propane activation, formation of the π-allylic intermediate, and NH insertion. Quantum chemical calculations have been well established as a powerful tool capable of generating mechanistic insights that are otherwise unavailable. So far, there has been no systematic quantum chemical investigation of the catalytic properties of the Mo-V-Te-Nb-O mixed metal oxide reported in the literature. We performed a series of density functional theory (DFT) calculations to explore the reactivity of different metal oxide moieties and the detailed mechanism of propane ammoxidation on the M1 ab plane, beginning with the initial step of propane activation, that is, the dissociative adsorption of propane into isopropyl and H. Because modeling the full unit cell of the M1 ab plane is computationally expensive and likely unnecessary for capturing the reactivity of the proposed isolated catalytic site, we chose to first explore finite cluster models of the proposed active site of the M1 ab plane (Figure 1), which contains all the key catalytic metal oxide species (Mo, V, and Te), and to investigate the adsorption sites and energies of propane and its dissociated products, isopropyl (CH3CHCH3) and atomic H, in order to lay the groundwork

J. Phys. Chem. C, Vol. 114, No. 10, 2010 4545 for further investigation of the mechanism of propane ammoxidation on the M1 ab plane of the mixed metal oxide by using DFT approaches. Methods The periodic DFT calculations were performed by using the Vienna ab initio Simulation Package (VASP).25-27 The exchange-correlation interaction was described by the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGAPBE).28 The core electrons were described by the projectoraugmented-wave (PAW) method,29 and the Kohn-Sham valence states (including Mo(4p4d5s), Te(5s5p), V(3s3p3d4s), O(2s2p), and H(1s)) were expanded in a plane-wave basis set up to 400 eV. The Brillouin zone was sampled at the Γ point only. Spin polarization was considered for all calculations. Different spin states for each cluster/adsorbate system were checked, and only the results of the ground state are reported below. The clusters employed in this study consisted of 1-5 identical truncated M1 ab planes (abbreviated henceforth as 1-5L), each containing the proposed active site present in the ab plane.11,15 The truncated M1 ab plane contained V in the M2 lattice site, Mo in the M4 and M7 lattice sites, and Te in the M12 channel sites (Figure 1). In the M1 crystal structure, the V and Mo cations are terminated by oxo groups on opposite sides of the bulk ab plane,11,16 but the nature of the oxo termination of the active ab surface remains a subject of ongoing research.30 To investigate propane activation, which was proposed to be successively catalyzed by the V and Te oxo groups, the V and Te were capped by O forming vanadyl (VdO) and telluryl (TedO) moieties, whereas the Mo had oxo groups on the opposite side of the ab plane. The eight terminating equatorial O atoms in each truncated ab plane were capped with H atoms. Therefore, each truncated ab plane contained a total of 36 atoms. The terminating O and H atoms were fixed at bond lengths of 0.96 Å during optimization, whereas the rest of the atoms in the plane were allowed to relax until the force in each of the x, y, and z directions on each relaxed atom fell below 0.05 eV/Å. The interplanar distance was fixed at the experimentally measured distance of 4.016 Å for adjacent ab planes. Each cluster was separated from its neighboring images by ∼11 Å in the x, y, and z directions. The adsorption energy was defined as ∆E ) Etotal - Ecluster - Eadsorbate, where Etotal, Ecluster, and Eadsorbate were the energies of the combined system (adsorbate and cluster), of the cluster, and of the adsorbate molecule in the gas phase in a neutral state (closed-shell or radical), respectively. Bader charge partition analysis31 was performed by using the code of Henkelman et al.32 to quantify the charge on individual atoms in the clusters. Selected clusters below were recalculated and reoptimized by using the HSE06 hybrid functional as implemented in VASP,33-35 which replaces 25% of the PBE exchange energy with exact Hartree-Fock (HF) exchange energy, with the Mo(4d5s), Te(5s5p), V(3d4s), O(2s2p), and H(1s) valence states expanded up to 283 eV. This was done in order to assess the self-interaction error introduced into adsorption energies calculated by the pure DFT methodology described above. Bulk VO2 and V2O5 were modeled in the rutile P42/mnm and orthorhombic Pmmn structures, respectively. The lattice constants for VO2 were calculated to be a ) b ) 4.63 Å and c ) 2.78 Å, in good agreement with the experimentally determined values of a ) b ) 4.5561 Å and c ) 2.8598 Å.36 The lattice constants for V2O5 were calculated to be a ) 11.66 Å, b ) 4.69 Å, and c ) 3.58 Å, in good agreement with the ex-

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Figure 2. Energy of adding an additional truncated ab plane (∆ExL, set to 0 for the 1L cluster), adsorption energies of propane, isopropyl, and H, and reaction energy of propane dehydrogenation into isopropyl and H, plotted against the number of truncated ab planes in the cluster.

Figure 3. (a) Side (slightly rotated) and (b) top views of the charge density difference in a 2L cluster (see Figure 1b) that illustrate the interaction between the two truncated ab planes (∆F ) Ftotal - Ftop Fbottom). The contours depicted are +0.008 (density increase, red) and -0.008 (density depletion, blue) e/Å3, respectively.

perimentally determined values of a ) 11.510 Å, b ) 4.369 Å, and c ) 3.563 Å.37 Results and Discussion Single-Layer versus Multilayer Cluster Models. We begin by exploring the energetic and electronic properties of the 1-5L clusters as a function of the cluster thickness, i.e., the number of truncated ab planes in the cluster. The energy of successively increasing the number of ab planes (∆ExL ) ExL - E(x-1)L E1L, with E0L set to 0) is shown in Figure 2 (solid diamonds). Adding one ab plane to the 1L cluster was calculated to be exothermic by -0.96 eV, and the energy of adding each subsequent ab plane quickly converged to -1.00 eV, which amounts to a surface energy of 5.6 meV/Å2 given the projected surface area of the cluster of ca. 90 Å2, excluding van der Waals (vdW) interactions which pure DFT methodology cannot capture. This interaction, evidenced by the slight rearrangement and accumulation of charge on the interplanar metal oxo groups, is relatively weak (Figure 3). The Bader charges of the various surface metal and O species are examined because surface reactivity may depend on their oxidation states (Table 1). As can be seen, the Bader charges of the surface species are also well converged with three ab planes, similar to the energy of adding ab planes. The oxidation state of the V atom in the M2 lattice site in the top surface plane is estimated by comparing its Bader charge to that of V in bulk VO2 and V2O5. The ratio of the average charges of V and O is -2.00 for bulk VO2 and -2.50 for bulk V2O5, consistent with the formal oxidation states of +4 and +5 for V and -2 for O in bulk phase. The average charge of V is calculated to be +2.09 and +2.22 in bulk VO2 and V2O5, respectively. The converged Bader charge of the surface V atom is +2.09, tentatively placing it in an oxidation state of +4, which is consistent with the previously proposed occupation of the M2 site by V4+ cation in the bulk M1 phase.11

Govindasamy et al. Previously, DeSanto et al. calculated the Madelung site potentials for the atomic species in the M1 phase and concluded that bridging O species were more nucleophilic than apical metal oxo groups; therefore, propane activation can occur via H abstraction by bridging O atoms.15 Our results indicate that although the vanadyl oxo is less electron-rich than the bridging O atoms, the telluryl oxo is by far the most electron-rich, and, therefore, the most nucleophilic O species, which has consequences for the adsorption of H, as discussed below. Incidentally, this order of nucleophilicity holds not only for the surface ab planes but for the interior planes as well (results not shown). Dependence of Adsorption Energies on the Number of ab Planes. The effect of cluster thickness on the adsorption energies of propane, isopropyl, and H is examined next. The adsorption energies of propane adsorbed on VdO, isopropyl adsorbed on VdO and TedO, and H adsorbed on TedO are plotted in Figure 2 as a function of the cluster thickness. The adsorption energy of propane is nearly zero, which agrees with the zero adsorption energy of propane on neutral bulk and molecular vanadia species reported by previous DFT calculations,38,39 and was insensitive to the number of ab planes (Figure 2, triangles). A vdW contribution of ca. -0.3 ∼ -0.4 eV is expected for the adsorption of propane (and likely also for isopropyl because of similar structures) based on experimental and calculated adsorption energies of alkenes on H-ZSM-5 zeolite40 and propane on V2O5(001).41 The vdW contributions for propane and isopropyl will not be further discussed below because they are not expected to vary significantly with the cluster thickness or the adsorption site and should largely cancel each other in the calculation of the reaction energies for H abstraction from propane. The adsorption energy of isopropyl on VdO converges to -2.0 eV and is within 0.12 eV of this value for the 3L cluster (Figure 2, solid squares). The adsorption energies of isopropyl and H on TedO (Figure 2, hollow squares and circles, respectively), on the other hand, depend strongly on the cluster thickness and converge slowly. It takes five ab planes in the cluster to reduce the change in adsorption energy with an additional ab plane to less than 0.15 eV for both adsorbates. Figure 2 suggests that three ab planes are needed to capture the majority of the change in the adsorption energies of isopropyl and H on TedO, which decrease (stronger adsorption) by 0.70 and 0.82 eV, respectively, from the 1L cluster to the 3L cluster. Pure DFT is known to favor the delocalization of metal d or f electrons in early transition metal oxides (e.g., VOx,42 CeOx,43 and other rare-earth oxides44), which destabilizes reduced states of the oxides. To assess the effect of this self-interaction error on the adsorption energies, we recalculated the adsorption energy of H on TedO by using the HSE06 hybrid functional. Because HF calculations using plane-wave basis sets are highly timeconsuming, this is done only for the 1-3L clusters. The results display the same trend of substantially decreasing adsorption energy with an increasing number of ab planes (Figure 2, plus signs), suggesting that this dependence is not of a theoretical origin. The HSE06 adsorption energy of H is considerably more exothermic than the corresponding PBE values (e.g., by 0.72 eV for the 3L cluster) because the HF exchange removes some of the self-interaction error and stabilizes reduced metal centers. The use of the hybrid functional is expected to systematically lower the adsorption energies of H and the reaction energies for H abstraction from propane (see below) on all cluster models compared to the PBE values. However, such systematic lowering of adsorption energies is not expected to affect the relative

Cluster Models of Mixed Metal Oxide Catalyst

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TABLE 1: Average Bader Charges (in Number of Electrons) of the Various Moieties in the Surface Plane of the 1-5L Clustersa #L

V@M2

Te@M12

Mo@M4,7

O (VdO)

O (TedO)

O (V-O-Mo)

O (Mo-O-Mo)

1 2 3 3(HSE)b 4 5

+2.00 +2.07 +2.08 +2.54 +2.09 +2.09

+3.69 +3.71 +3.72 +3.82 +3.73 +3.73

+2.61 +2.57 +2.59 +3.61 +2.59 +2.58

-0.75 -0.68 -0.67 -0.98 -0.68 -0.67

-1.77 -1.78 -1.78 -1.84 -1.79 -1.79

-1.24 -1.50 -1.49 -1.66 -1.49 -1.49

-0.98 -0.97 -0.98 -1.35 -0.98 -0.98

a

O in terminal OH groups not included. b Results obtained using the HSE06 hybrid functional.

Figure 4. Top and side views of (a) propane (on TedO), (b) isopropyl (on VdO), and (c) H (on TedO) adsorbed on a multilayer cluster; (d) comparison of the 4L cluster without and with a H atom bonded to a TedO (marked ‘X’). The interplanar Te-Te distances (in Å) are labeled in panel d. Color code of the spheres: blue, Mo; gray, V; orange, Te; red, O; white, H. For clarity, propane and isopropyl are shown in ball-and-stick models.

ranking of various elementary transformation steps considered in this study or change its overall conclusions. Closer inspection of the clusters reveals that the positions of the four-fold-coordinated Te atoms in the hexagonal channel sites were relaxed to a much greater extent than those of the six-fold-coordinated V and Mo in the lattice sites. The TedO moieties in the top and bottom surface ab planes of the clusters without adsorbates are found to relax outward so that the Te-Te distance between the outermost and first subsurface ab planes increases by 0.2-0.3 Å and the distance between the first and second subsurface ab planes increases by ca. 0.1 Å, compared to the bulk interplanar distance of 4.016 Å. The adsorption of isopropyl and H on TedO causes the Te-Te distance between the outermost and first subsurface ab planes to contract by 0.2-0.3 Å and the Te-Te distance between the first and second subsurface ab planes to contract by ca. 0.1 Å (see Figure 4d), effectively restoring them to the bulk Te-Te distance. The adsorption-induced Te-O-Te interaction extends as far as two ab planes below the surface, which accounts for the slow convergence with cluster thickness for the adsorption energies of TedO-bound species. Site Preference of Adsorbed Propane, Isopropyl, And H. To understand how the M1 ab plane activates propane, it is necessary to explore the adsorption sites of propane and its dehydrogenation products, isopropyl and H, in other sites in the active center as well. The adsorption energies of propane, isopropyl, and H on different surface metal oxide moieties of the 3L cluster are listed in Table 2, and several optimized adsorption geometries are shown in Figure 4. Not surprisingly, propane, being a closed-shell saturated hydrocarbon, shows very weak chemical interaction with the cluster, and its adsorption is insensitive to both the nature of the adsorption site and the thickness of the cluster. On the other hand, the adsorption energies of isopropyl and H are strongly site-dependent. The overall site preference is the same for both species in the following order: TedO > VdO > bridging

TABLE 2: Adsorption Energies of Isopropyl and H on Five Different Surface Moieties of the 3L Clustera O (dTe) O (dV) O (V-O-Mo) O (Mo-O-Mo) Mo propane isopropyl H

+0.04 -2.06 -3.63

-0.01 -1.88 b

-0.11 -0.89 -2.01

-0.14 -0.61 -2.29

+0.05 -0.22 -0.75

a Adsorption energies are in eV and are relative to gas-phase species. b Spontaneously moved to TedO.

oxygens > empty Mo apical site. Isopropyl prefers to adsorb on TedO with an adsorption energy of -2.06 eV, followed by VdO, where the adsorption energy is -1.88 eV. The bridging O atoms, being coordinately saturated, bind isopropyl less strongly at -0.61 eV (Mo-O-Mo) and -0.89 eV (V-O-Mo). The empty Mo apical site is by far the least stable for isopropyl adsorption (-0.22 eV) because of a lack of available electrons for back-donation to form metal-carbon bonds. The site preference of atomic H is similar to that of isopropyl, with one key exception. Because of the close proximity of the vanadyl and telluryl oxo groups and the strong nucleophilicity of the telluryl oxo, atomic H, which loses its electron upon bonding to metal oxo groups and becomes a proton, is spontaneously transferred from the VdO to the TedO. The adsorption energy of H is -3.63 eV on TedO, substantially lower than that on the bridging O atoms (ca. -2 eV), which in turn bind H much more strongly than does the apical Mo site (-0.75 eV). H adsorbed on Mo becomes a hydride species (Hδ-). The strong preference for H adsorption on the telluryl oxo suggests that TedO plays an important role in H abstraction not only from propyl and propylene as previously proposed11,19 but also from propane. As reported previously, the M2 and M7 lattice sites in the M1 crystal structure can be occupied by either V or Mo.19 Because the clusters of this study consisted of stacked identical ab planes, the influence of elemental composition of the active site was probed by adsorbing isopropyl on two modified 3L clusters, one with V and Mo exchanged in the second ab plane

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Govindasamy et al.

Figure 5. Illustrations of different product states of the initial methylene C-H bond scission in propane ODH. Numbering corresponds to Table 3. In panel 4, the cluster is rotated around the z axis by 90° to better show the C-O bond. Color code of the spheres: blue, Mo; gray, V; orange, Te; red, O; white, H. For clarity, propane and isopropyl are shown in ball-and-stick models. Panels 4, 5, and 8 are rotated 90° clockwise about the surface normal compared to the rest of the panels.

TABLE 3: Adsorption Energies for Different Co-adsorption States of Isopropyl and H on the 3L Clustera 1 2 3 4 5 6 7 8

isopropyl location

H location

energy (eV)

gas-phase radical TedO VdO V-O-Mo Mo-O-Mo VdO TedO V

TedO TedO VdO TedO TedO TedO VdO VdO

+0.68 -0.33 -0.20 +1.12 -0.14 -1.16 -0.74 +2.13

a Adsorption energies are in eV and are relative to gas-phase propane. Numbering corresponds to Figure 5.

(therefore, having V at M7 and Mo at M2) and another one with V and Mo exchanged in the third ab plane. The V-Mo exchange in the second plane was found to weaken the adsorption energy of isopropyl from -1.88 to -1.77 eV, whereas the exchange in the third plane had a smaller effect (-1.86 eV). The Bader charge of V in the surface M2 site is +2.05 and +2.09 for metal atoms exchanged in the second and third plane, respectively. Thus, the metal species in the first subsurface ab plane slightly affects the adsorption on the surface plane, whereas the effect of the metal species in still lower ab planes is almost completely screened. Product States of Initial C-H Bond Scission in Propane. The knowledge of the adsorption energies of isopropyl and H in different adsorption sites allows one to determine where the products of the initial dehydrogenation of propane are likely adsorbed in the active center. This provides useful information for determining the reaction pathways as well as for exploring the subsequent steps in propane ammoxidation, which is the subject of our ongoing research. The initial methylene C-H bond scission in propane ODH (C3H8(g) f C3H7+H) on this mixed metal oxide was previously proposed to proceed via H abstraction by VdO.19 The analogous process of propane ODH on bulk and single-site vanadia catalysts has been proposed to proceed via H abstraction by VdO or bridging O forming isopropyl38,39,45-47 or via the addition of the methylene C-H bond across VdO or bridging O forming propanol adduct,38,47 in previous DFT studies. The addition of the methylene C-H bond across a VdO bond has been suggested for alkenes.46 On the basis of these different reaction mechanisms, the energies of several different isopropyl and H coadsorption states (illustrated in Figure 5) are calculated (Table 3). States 1 (H on TedO, isopropyl as a free radical), 4 (H on TedO, isopropyl on V-O-Mo bridging O), and 8 (H on VdO,

isopropyl on V) are endothermic. Among these, states 4 and 8 are energetically more costly. State 4 is unfavorable because the O bridging V and Mo must be significantly displaced from its equilibrium position in which it is already coordinated to three cations, in order to form the O-C bond with isopropyl. In state 8, the V cation at the M2 site, which is fully coordinated to six O in a octahedral configuration, forms an extra bond with isopropyl in a highly strained manner. Given that the activation barrier for propane ODH has been determined to be ca. 1 eV on VOx and V2O5(001) in previous DFT studies, we tentatively rule out these two states as final states for propane dehydrogenation on the mixed metal cluster because the thermodynamic barriers that they impose alone are greater than 1 eV, any additional kinetic barrier notwithstanding. States 2 and 3 are essentially propanol adsorbed directly on Te and V cations. States 6 and 7 are by far more stable than the rest, suggesting that adjacent VdO and TedO groups in the active site may be an important propane-activating motif. Similar to the adsorption energies of isopropyl and H, the reaction energy for the initial H abstraction step also exhibits a strong dependence on the thickness of the cluster. On the basis of the double-oxo state 6 (C3H8(g) f C3H7*OdV+H*OdTe), the reaction energy is calculated to be endothermic by +0.14 eV on the 1L cluster but becomes significantly exothermic on the multilayer clusters: it decreases to -1.14 eV on the 3L cluster and further to -1.20 eV on the 5L cluster (Figure 2, star symbols). Similar to the case of the adsorption energies, at least three ab planes are needed to capture the substantial exothermicity of this step on the Mo-V-Te-Nb mixed metal oxide. Conclusions Cluster models of the M1 phase of the Mo-V-Te-Nb-O mixed metal oxide, which is active and selective for direct propane ammoxidation to acrylonitrile, have been used to investigate the surface moieties involved in the initial activation of propane based on DFT calculations. These clusters consist of 1-5 truncated M1 ab planes containing V in the M2 lattice site, Mo in the M4 and M7 lattice sites, and Te in the M12 channel sites. The adsorption of propane is found to be siteinsensitive, but both isopropyl and H display a clear adsorption site preference in the order of TedO > VdO > bridging oxygens > empty Mo apical site. H is unstable on the vanadyl oxo and transfers spontaneously to the telluryl oxo because the latter is by far the most nucleophilic O species in the truncated ab plane. On the basis of the calculated adsorption sites and corresponding adsorption energies, several different final states for the initial H abstraction from propane are considered. The double-oxo states in which the products, isopropyl and H, are bound to adjacent VdO and TedO groups are by far the more favorable energetically. The results suggest that TeOx located in hexagonal channels may play an important role in H abstraction not only from propyl and propene as previously suggested but from propane as well. Although the interaction between ab planes is found to be relatively weak, in the clusters without adsorbates, the adsorption energies of isopropyl and H on TedO decrease significantly with increasing number of ab planes (indicating stronger adsorption). This dependence is not due to the electron selfinteraction error of DFT but rather to the enhanced Te-O-Te bonding induced by the adsorbates, which extends across several ab planes. Consequently, the calculated reaction energy for initial propane dehydrogenation into a double-oxo state changes from slightly endothermic on the single-layer cluster to strongly exothermic on the thicker clusters. At least three ab planes are

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