TiO2 Catalyst from Periodic DFT

Apr 12, 2007 - Effect of Alkali Doping on a V2O5/TiO2 Catalyst from Periodic DFT Calculations ... Neutral alkali interact with several oxygen sites in...
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J. Phys. Chem. C 2007, 111, 6411-6417

6411

Effect of Alkali Doping on a V2O5/TiO2 Catalyst from Periodic DFT Calculations M. Calatayud* and C. Minot UniVersite´ Pierre et Marie Curie-Paris6, Laboratoire de Chimie The´ orique, UMR 7616 CNRS, Paris F-75005, France ReceiVed: December 6, 2006; In Final Form: February 2, 2007

The role of alkali (Li, Na, K) and H in a vanadia/titania catalyst model is discussed in terms of geometry, electronic structure, and reactivity. Neutral alkali interact with several oxygen sites involving active phase and support. The vanadyl VdO groups are affected by the presence of such dopants and elongate with respect to the undoped systems. Neutral alkali adsorption takes place through an electron transfer from the adatom to the surface vanadium atoms, pushing the Fermi level to higher energies. Alkali-doped slabs show a state in the gap lying 0.25 eV below the conduction band. This state appears for H, Li, and K only after using the GGA+U method. Undoped slabs are more active (higher adsorption energies) than the doped ones for electrostatic (methanol adsorption) and redox (hydrogenation) processes. Methanol adsorbs dissociatively, and the adsorption energy becomes less exothermic in the series H-Li-Na-K which corresponds to an increase in the basicity of the slabs and to a decrease of the hardness of the alkali dopant. Hydrogenation takes place by electron transfer to the vanadium sites and becomes less exothermic in the series H-LiNa-K. Bridging V-O-Ti sites are preferred to vanadyl VdO sites in both methanol and hydrogen adsorption final structures.

Introduction Alkali-modified supported metal oxides are systems of increasing interest in the last years due to the applications in electronic devices and catalysis. The degree of reduction of a metal oxide surface can be tuned by controlling the amount of metal deposed with promising applications in many technological fields. Fundamental and applied knowledge is needed for a better understanding of the processes taking place at the atomic level. Probably the most studied system involving alkali interaction with a metal oxide is rutile (110) TiO2. Many surface science techniques such as LEED, EXAFS, STM, or photoelectron spectroscopy have described the adsorption process and the presence of Ti3+ states due to an electron transfer from the alkali to the five-fold titanium atom (see ref 1 and refs therein). Theoretical investigations have addressed important features of the systems such as the effect of coverage and the reduction of the substrate by means of Hartree-Fock, DFT, and molecular dynamics methods.2-4 Another probe model is MgO and alkaliearth oxides where the adsorption of neutral metals cannot lead to a charge transfer due to the irreducibility of the metal cation in the oxide.5 Very recently, the structure of alkali-doped V2O5 (010) single crystal has been studied from DFT on a cluster model.6 More complex systems involve supported oxides. Potassium and sodium are among the strongest poisons for vanadium-supported catalysts, blocking Brønsted sites and leading to complete deactivation.7,8 On the other hand, the presence of potassium oxide is the origin of an increase in selectivity of many oxidation reactions such as methanol oxidation,9 o-xylene oxidation,10 or alkane dehydrogenation.11-13 The content of potassium oxide and the preparation method can be modified to obtain catalysts with the desired properties of * Corresponding author. Telephone: +33 (0) 1-44-27-26-82. Fax: +33 (0) 1-44-27-41-17. E-mail: [email protected].

acid/base/redox.9 Note that the experimental studies of catalysts involve alkali oxides and not neutral alkali. To the best of our knowledge, no previous work has dealt with neutral alkali interaction with supported oxides. A deep understanding of the alkali interaction with metal oxide surfaces would be of great help to control the degree of reduction in metal oxides and to design systems with selected properties. The aim of the present paper is the investigation of alkali metal (Li, Na, K) interaction with a vanadia/titania system by means of periodic DFT calculations. H interaction is also included for comparison. We focus on the geometrical and electronic structure of the modified surfaces and discuss the implications in reactivity. Computational Details The Perdew-Burke-Ernzerhof functional14,15 has been used to compute total energy calculations as implemented in the VASP code.16,17 The core electrons are kept frozen and replaced by PAW-generated pseudopotentials,18,19 while the valence electrons are described with a plane-wave basis set (cutoff ) 400 eV). The valence electrons explicitly treated are the following: O, s2p4; Ti, s2d2; V, s2d3; M, s1 (M ) H, Li, Na); K, s1p6. The inclusion of p electrons in the valence for both transition and alkali metals does not involve significant changes in the results. A 3 × 3 × 1 Monkhorst-Pack sampling in the Brillouin zone is used. Geometry optimizations are carried out with the conjugate gradient algorithm, and the structures are checked to be minima in the potential energy hypersurface. All the atoms of the slab are free to relax. Spin-polarized calculations are carried out for the open-shell systems. Analysis of the atomic charges and spin is done within the Bader scheme20 as implemented by Henkelman et al.21 In the Bader approach the space is divided into regions separated by zero-flux surfaces where the charge density is a minimum. This partitioning of

10.1021/jp068373v CCC: $37.00 © 2007 American Chemical Society Published on Web 04/12/2007

6412 J. Phys. Chem. C, Vol. 111, No. 17, 2007

Calatayud and Minot TABLE 1: Selected Distances in Å, and in Parentheses the Corresponding Harmonic Frequencies for Vanadyl VdO Groups in cm-1 for the Most Stable M-V2O5/TiO2 Models (M ) H, Li, Na, K)a M)

H

Li

Na

K

bare

dM-O

0.977

1.859 1.906 2.033

2.199 2.262 2.341

dV1dO (υ) dV2dO (υ)

1.627 (1041) 1.609 (1081)

1.658b (978) 1.610 (1060)

1.652b (987) 1.610 (1072)

2.744 2.769 3.120 3.129 3.134 3.372 1.652b (976) 1.619 (1040)

1.615 (1068) 1.603 (1082)

a As a consequence of the interaction of M with the VdO bond, the latter elongates, and the corresponding frequency shifts to lower wavenumbers. b VdO in direct interaction with M.

interact with two neighboring vanadia units and the support. A detailed description of the geometry for the doped systems is done in the following section. Results and Discussion

Figure 1. Optimized structures for the M-doped vanadia/titania models (M ) H, Li, Na, K). Distances in Å. The neutral alkali atoms coordinate to several oxygen sites between vanadia units and support. The vanadia unit is represented in blue (large circles for O, small for V atoms), surface in gray (Ti) and red (O), alkali in yellow.

the charge density is more appropriated than the arbitrary Mulliken approach. The dispersed V2O5-TiO2-anatase catalyst has been previously modeled.22 The support is represented by the anatase (001) plane which is present in the commercial powders. A three TiO2layers-thick slab is used; the unit cell has dimensions 7.57 × 7.57 × 20 Å3 and contains four TiO2 units per layer. A V2O5 unit has been deposed on top of the surface with a favorable exothermic interaction. The coverage involves less than one monolayer which is found to report the best activity in catalytic reactions. The uppermost layer of TiO2, in contact with the active phase, rearranges substantially, the second presents smaller distortions, and the third one is only weakly affected by V2O5.22 Deeper layers would not have implication in the physics of the surface; we estimate that this model is thus a good compromise between accuracy and computational effort. Different sites are exposed, providing basic (oxygen) and acidic (metal) properties. Among the oxygen sites we find terminal vanadyl VdO, bridging V-O-V, interphase V-O-Ti, and support Ti-O-Ti sites. Metallic sites are Ti4+ and V5+. Neutral atoms M (M ) H, Li, Na, and K) are added to this surface with a surface ratio M/V/Ti of 1:2:4, very close to experimental preparations.9 Six different geometries have been considered for neutral M adsorption. After optimization, the systems where M interacts with both the vanadia unit and the support surface are favored. Figure 1 shows the most favorable geometry found for the doped systems. H adsorption takes place on the interphase bridging V-O-Ti site,23 while the alkali atoms

Geometry. The interaction of neutral alkali with the vanadia/ titania model involves several oxygen sites, depending on the atom size. Figure 1 displays the most stable geometries obtained after optimization for the doped systems, and selected distances to the different sites are reported in Table 1. Thus, H interacts only with one oxygen atom, a bridging V2-O-Ti atom, forming an OH group. The interaction with a vanadyl VdO site is less favorable by 0.5 eV. Lithium interacts with three oxygen sites: it is located between two vanadia units through interaction with a vanadyl VdO and a V-O-Ti sites and forms a bond with a surface Ti-O-Ti site. The same arrangement is observed for Na, with larger M-O distances. Potassium interacts strongly with five oxygen sites, and three vanadia units are involved through VdO and V-O-Ti, as well as two support Ti-O-Ti sites. On a mixed vanadia/titania model the alkali atoms show then an affinity for both vanadia and support. The M-O distances increase from H to K due to a size effect, the crystal atomic radii of the cations being Li+: 0.68 Å, Na+: 0.97 Å, K+: 1.33 Å.24 In order to gain some insight in the nature of the alkali affinity for the vanadia unit, a computational experiment with lower coverage in vanadia has been carried out. A quadruple unit cell containing only one vanadia unit has been calculated (dimensions 15.4 × 15.4 × 20 Å3). Four different structures including those where K is close and where it is far from the vanadia unit are considered. Due to the size of the system only potassium, the biggest atom and most widely used promoter, has been calculated, but the conclusion should be extensive to lithium and sodium. Figure 2 displays the unit cell and the positions where the K atom has been placed. The best structures involve K is close to the vanadia unit, in interaction with terminal, bridging, and support oxygen sites. The energy difference between the positions of K close to and far from V2O5 varies from 0.3 to 0.5 eV. This confirms the affinity of alkali for the vanadia unit, and also the interaction with the support phase. A physical implication of this fact is the improvement in the dispersion of the vanadia units, or as it has been proposed, the prevention of polymerization. Another important feature of the alkali-doped systems is the perturbation of the vanadyl VdO bond. The VdO distances calculated in our models are shown in Table 1. They elongate with respect to the bare model; the closer to the alkali atom the

Alkali Doping on a V2O5/TiO2 Catalyst

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6413 TABLE 2: Absolute Adsorption Energy, in eV, and Relative Value with Respect to the Number of M-O Bondsa Formed H Li Na K

Eads

# M-O bonds

Eads/bond

-2.98 -4.33 -3.54 -3.71

1 3 3 4

-2.98 -1.54 -1.18 -0.93

a We consider a bond when the M-O distance is within 10% of the ionic crystal radii of M+ and O).

Figure 2. Model for the low-coverage vanadia-supported catalyst. Circles indicate positions for the K atom. The vanadia unit is represented in blue (large circles for O, small for V atoms), surface in gray (Ti) and red (O), potassium in yellow.

distance, and the influence is similar. Our calculations predict, then, a higher dispersion in the VdO distances for small atoms, although diffusion processes would equilibrate the situation. The elongation of the vanadyl bond is easily followed in Raman spectroscopy by a shift of the band at 1030 cm-1 toward lower energies9,25 in agreement with our calculated values shown in Table 1. The role of the vanadyl bond as active site for catalytic reactions is still a matter of debate. If this bond were the active site, it would disappear during the catalytic reaction, and the activity of the catalyst would increase with the content of alkali. This is not observed in methanol oxidation reactions on K-doped samples where the VdO bond is present even at high temperatures in the Raman spectrum; moreover, the activity of the catalyst decreases with the potassium content, and the samples become less reducible (and less reactive). These experiments would be in favor of interphase V-O-Ti sites as active sites in agreement with previous calculations.23 The adsorption energies Eads have been calculated as the difference between the doped system and the sum of the bare plus isolated alkali atom; negative values are exothermic. Although they do not show a trend in their absolute value, they decrease with the size of M when divided by the number of M-O bonds formed (see Table 2). With H interacting only with one site and with Li, Na, and K interacting with several, a correction must be included. The M-O bond strength decreases along the alkali column, the H-O bonds being the strongest and the K-O the weakest. This follows also the ionization potential trend for alkali atoms and H: H (13.598 eV), Li (5.392 eV), Na (5.139 eV), K (4.341 eV). Indeed, the bonding of an alkali atom to a reducible metal oxide involves an electron transfer from the alkali to the surface. This will be discussed in the next section. Regarding the M adsorption mechanism, it goes through the reduction of the V5+ to V4+ as confirmed by a charge analysis. This involves a decrease in the acidity of the vanadium site.

M f M + + eV5+ + e- f V4+

Figure 3. Site-projected DOS for the bare and the K-doped models. Only the uppermost layers (vanadia and one TiO2 layer) are shown.

longer the VdO distance. Thus, the smallest atoms H and Li show the highest influence in the closest V1dO bond. Note, however, that in our model there are two VdO groups; for small alkali one of them will be more perturbed than the other, for larger atoms such as K both VdO groups are at the same

The unpaired electron is located on the vanadium site V1 as shown in Table 3. Electronic Structure. In this section we analyze the electronic structure of the models in terms of Bader charges20 and density of states (DOS). The former are an indication of the acid/base character, whereas the latter are related to redox properties. Although they do not constitute quantitative measures for these properties, they are in, general, good indicators of the reactivity trends. Atomic charge and spin density calculated within the Bader approach are shown in Table 3. As mentioned in the Computational Methods section, the atoms are unambiguously defined in the Bader approach at variance with the Mulliken population analysis. The most negatively charged atoms are the oxygen sites of the support. The oxygen sites exposed on the surface

6414 J. Phys. Chem. C, Vol. 111, No. 17, 2007

Calatayud and Minot

TABLE 3: Bader Atomic Charges and Spin Density in |e| for the Bare and Doped Models (in Parentheses the Spin Density in |e|) bare OdV1 OdV2 V-O-V V1-O-Ti V2-O-Ti V1 V2 M Osupport Tisupport

-0.94 -0.87 -1.24 -1.16 -1.12 +2.58 +2.61

H -0.99 -0.91 -1.26 -1.63 -1.17 +2.54(0.78) +2.40 (0.18) +1.00

Li

Na

-1.17 -0.92 -1.26 -1.16 -1.33 +2.10(0.65) +2.12 (0.24) +1.00

K

-1.14 -0.95 -1.26 -1.29 -1.18 +2.09(0.67) +2.10 (0.21) +0.99

-1.07 -0.97 -1.14 -1.19 -1.23 +2.11(0.69) +2.14 (0.18) +0.92

-1.27/-1.34 +2.61/+2.66

are less coordinated and less charged than those embedded in the lattice; the negative charge in the bulk is indeed stabilized by the Madelung field. As a general rule in these systems the lower the coordination the lower the charge, it is not surprising to find single-coordinated vanadyl bonds less charged than bridging ones at ∼0.2 |e|. The V-O-V site presents the highest negative charge; indeed this atom binds to a surface titanium site and is not two-fold but three-fold coordinated. Vanadium and titanium sites present similar charges around 2.10-2.27 |e|. This trend is followed for all the doped models. However, note that the charge is not always correlated to reactivity and the most charged sites do not necessarily correspond to the most reactive ones.26,27 Thus, the V-O-V sites, despite being the most charged, do not report the highest adsorption energies in hydrogenation. We will discuss the hydrogenation energies in the next section. The charge on the alkali sites decreases in the series H-Li-Na-K, the smallest atoms being more ionized than the largest. It has been mentioned that the neutral alkali interaction with reducible metal oxides takes place through an electron transfer from the alkali to the surface. As a consequence, the final system is reduced and presents an unpaired electron. The spin density has been calculated for the doped models, and it is found that the unpaired electron is localized mainly in one vanadium site, V1 (see Figure 1, Table 3). The projected DOS is a powerful tool to analyze the energetic levels of our slabs. The region of the valence band VB (the HOMO in molecules) and the conduction band CB (LUMO in molecules) is usually related to reactivity: systems with a small band gap (HOMO - LUMO) are more reactive than those with a high value. Regarding redox processes those systems with a low CB are expected to be more reducible since this level is lower in energy and thus more accessible to electrons. Note that DFT underestimates the conduction band levels and gives lower band gaps; this is an inherent feature of the method. Calculations have been performed with finer k-meshes (5 × 5 × 1). The DOS projected on the outermost layers (vanadia and first TiO2 layer) is shown in Figure 3 for the bare and the K-doped models. The former is closed-shell and the latter contains an unpaired electron; only the alpha levels are displayed for simplicity. The valence band is composed of oxygen 2p levels and the conduction band of metallic V and Ti states. The Fermi energy is shifted to 0 in the figures and clearly moves from the top of the VB to the bottom of the CB when doping with alkali. For the K-doped system a state in the gap is observed, 0.25 eV below the CB, formed mainly of vanadium states and filled with the electron coming from potassium. This state is not observed in the H-, Li-, and Na-doped systems at the present approximation (pure DFT). However, upon introduction of the approximation of Dudarev et al.,28 with a (U - J) value of 1 eV for the vanadium d orbitals, the state in the gap appears also for H, Li,

TABLE 4: Eigenvalues for the Top Valence Band and the Bottom of the Conduction Band for the Bare and Doped Systems, at the Γ-point, as Well as the Calculated Band Gap (in eV) bare H Li Na K

top VB

bottom CB

gap

-8.13 -8.02 -8.05 -7.93 -7.88

-6.00 -6.21 -5.95 -5.77 -5.80

2.13 1.81 2.10 2.16 2.08

and Na. The reason for this behavior is the small band gap given by the pure DFT methods: they describe the defective states in the conduction band as partly delocalized when they should be mostly localized below it. This problem has recently been discussed in a paper on the electronic structure of reduced rutile TiO2 by Di Valentin et al.29 In this paper it was shown that the use of a hybrid functional (B3LYP) gives the correct answer. The LDA+U and GGA+U methods are an alternative to the use of hybrid functionals. The presence of states in the gap region has been detected experimentally for alkali-doped TiO2.1 Table 4 shows the eigenvalues for the top VB and bottom CB calculated at the Γ point, as well as the band gap, for all the structures. Both the CB and the VB are progressively shifted toward higher energies for alkali-doped systems, although the band gap value does not change significantly. The hydrogenated slab makes an exception, showing a small band gap of 1.81 eV. Note that these values are absolute eigenvalues for the whole slab, including deeper layers that contribute to the top of the VB (not represented in the DOS, Figure 3), although the same trend should be expected. Reactivity. An important catalytic reaction is the oxidation of methanol to formaldehyde. This reaction often serves as a probe to test two types of reactivity on a catalytic surface: acid/ base and redox. The first step in the reaction is the adsorption of methanol that is governed by electrostatic interaction. The reduction of methanol to formaldehyde involves hydrogen and electron transfers to the surface:

CH3OH f H2CO + 2H+ + 2eV5+ + 2e- f V3+ or 2V5+ + 2e- f 2V4+ 2H+ + O)surf f H2Osurf CH3OH + V5+ + O)surf f H2CO + V3+ + H2Osurf The aim of the present work is not to provide an exhaustive study for the methanol reaction but to characterize first-stage intermediates and get some insights into the main properties controlling the reactivity, with special attention to the influence of the dopant. The study of the initial step, methanol adsorption on the surface, allows exploring the acid/base sites of the surface. The redox mechanism is involved when the surface oxygens

Alkali Doping on a V2O5/TiO2 Catalyst

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6415

Figure 4. Methanol adsorption on a Li-doped model. Molecular and dissociative models are considered.

TABLE 5: Adsorption Energy in eV Calculated as Eads ) (Esubstrate+n·adsorbate - Esubstrate - nEadsorbate)/n a bare

H

Li

Na

K

CH3OH molecular on V1 -1.04 -0.99 -0.86 -0.39 -0.47 d(V1-OCH3OH) 2.107 2.080 2.117 2.066 2.070 CH3O-/V H+/OV1Ti -1.46 -1.50 -1.23 -1.00 -0.72 d(V1-OCH3) 1.768 1.782 1.867 1.855 1.846 d(C-O) 1.416 1.421 1.436 1.426 1.418 d(M-OCH3) 2.589 2.098 2.546 2.724 OH-/V1 CH3+/OV1Ti -1.09 -1.23 -1.22 -0.69 -0.43 d(C-O) 1.434 1.432 1.429 1.428 1.426 d(V1-OH) 1.789 1.791 1.887 1.875 1.867 d(M-OH) 2.434 1.910 2.436 2.634 H on V2-O-Ti H on V1dO

-2.95 -2.37

1H -2.73 -2.04

-2.70 -2.34

-2.63 -2.31

-2.54 -2.34

OV2Ti OV1Ti OV2Ti OdV2

-2.74 -2.53

2H -2.50b -2.55c

-2.48 -2.28

-2.43 -2.23

-2.41 -2.28

a Negative values are exothermic; isolated H has been taken as reference for hydrogenation (EH ) -1.115 eV, EH2 ) -6.71 eV). Selected distances in Å are also presented for the methanol adsorption. b OV2Ti OV1Ti OdV1. c OV2Ti OV1Ti OdV2.

are protonated; redox sites will be then studied through the analysis of the hydrogenated (reduced) surface. The adsorption energy of the adsorbates is evaluated as:

Eads ) E[substrate-adsorbate] - E[Substrate] - E[adsorbate] by calculation of the total energy of selected structures. Negative values indicate exothermic processes (at 0 K). Note that several interactions contribute to the stability of the systems: the methanol bond breaking, the M-O, C-O, O-H bond formations, and in the case of hydrogenation, the electron transfer. Methanol Adsorption. We have considered the interaction of one CH3OH molecule per unit cell (per V2O5 unit). Several systems [including molecular, formally dissociated CH3O- + H+, CH3+ + OH- fragments in interaction with different surface sites (two vanadium, terminal, and bridging V-O-Ti oxygen atoms)] have been scanned. Cationic fragments interact with oxygen sites, and anionic fragments, with vanadium sites. Note that for dissociative adsorption the same products (methoxy and hydroxyl groups) are obtained irrespective of the initial methanol bond break; the difference in the adsorption sites, however, leads to different adsorption modes (see Figure 4). Table 5 presents heats of adsorption and selected distances for the most favorable

dissociative and molecular modes, and Figure 4 displays the most favorable geometries obtained for the Li-doped system; analogous geometries are also obtained for Na and K-doped systems. Dissociative modes are preferred to the molecular one. Molecular adsorption takes place by electrostatic interaction of the methanol oxygen with a surface vanadium atom, without direct interaction between the methanol molecule and the metal atoms. The V-O distances are, in consequence, rather insensitive to the nature of the alkali atom. Adsorption energy values are exothermic by ∼1 eV and become less exothermic when the alkali size increases (bare: -1.04 eV, H: -0.99 eV, Li: -0.86 eV, Na: -0.39 eV, K: -0.47 eV). According to these results, the bare slab leads to more negative adsorption energies (more exothermic processes) than the alkali-doped ones. Since exothermic heats of adsorption involve higher residence time of such species on the surface, the possibility of parallel reactions increases for the bare slab, and the doped models are more selective. The trend within the alkali-doped slabs can be explained in terms of hard and soft concepts: the smaller alkali atoms are harder, thus favoring electrostatic interactions, whereas larger systems are softer, decreasing the electrostatic interactions and exothermicity. These trends are also observed for the dissociative modes and are discussed below. The most favorable adsorption mode is found upon formal breaking of the OH bond and adsorption as CH3O-/V1, H+/ OV1Ti thus forming a V1-C bond and a OV1Ti-H bond. The interaction of the H+ fragment with terminal VdO sites is less favorable by 0.6-0.8 eV (not shown). As can be seen from Figure 3, the methoxy group interacts with the alkali metal; this is a stabilizing electrostatic interaction. However, the bond of the alkali atom to a second vanadia unit is weakened. The OH group forms hydrogen bonds with a neighboring V2dO site. Thus, new stabilizing interactions appear, but others disappear, and the final stability will be a balance of all of them. The methoxy C-O bond elongates for the doped systems with respect to the bare model, being longer for the Li-doped slab. The adsorption energy is exothermic and becomes less exothermic with the alkali size (bare: -1.46 eV, H: -1.50 eV, Li: -1.23 eV, Na: -1.00 eV, K: -0.72 eV). The alkali-doped slabs present in all cases lower exothermic heats of adsorption than the bare one. The higher hardness of the small alkali makes the electrostatic interactions with the methoxy group favorable, and in the case of Li the longest C-O distance is obtained (see Table 5). The interaction of the methoxy group directly with the alkali atom, thus forming M-OCH3, has also been explored, but the systems were by far less stable than the V-OCH3 ones.

6416 J. Phys. Chem. C, Vol. 111, No. 17, 2007 The formal dissociation of the methanol C-O bond leads to the formation of OV1Ti-CH3 and V1-OH bonds. Alkali form a chemical bond with the OH group, and the hydroxyl fragment is also stabilized by a hydrogen bond. The alkali bond to a second vanadia unit is lost. The adsorption energy for this mode is intermediate between the O-H and the molecular modes (bare: -1.09 eV, H: -1.23, Li: -1.22 eV, Na: -0.69 eV, K: -0.43 eV). In this case the hardest dopants H and Li present values higher than the bare slab, the trend within the alkali being again the increased (less exothermic) adsorption energy. According to these results, the formal C-O bond breaking is less favored than the O-H bond breaking in methanol. However, the C-O cleavage is easier in gas phase, and on other oxides such as TiO2 or SnO2 (see refs 26 and refs 7, 10, 84, 85 therein). The surface acidic-basic properties play a key role in stabilizing the fragments: the bonds formed in V2O5-TiO2 compensate for the more energetic O-H break, while in TiO2 or SnO2 those interactions are not so favorable. Hydrogenation. The interaction with hydrogen has been modeled in two steps: first, one hydrogen is added to the unit cell, and for the best situation, the second step involves the addition of a second hydrogen atom. We have explored the five oxygen sites in the vanadia unit. The bridging ones are found to be preferred to the terminal ones in agreement with previous results.23 The difference in energy between the two sites is 0.58 eV for the bare system and 0.20 eV for the K-doped. The adsorption energy for the first H atom on the V2-O-Ti site increases (is less exothermic) within the series bare (-2.95 eV) < H (-2.73) < Li (-2.70) < Na (-2.63) < K (-2.54). According to these results, the most reducible system is the undoped slab, followed by H- and Li-doped structures, and the least reducible would be the K-doped system. The doped systems are less reducible than the bare one since neutral alkali adsorption already involves a first reduction of the substrate. In order to check the effect of alkali without previous reduction we have performed calculations on the same systems doped with M+ species instead of M. The slabs doped with M+ are still less reducible than the bare one. This conclusion is also reached from models including alkali hydroxides MOH. The presence of alkali thus decreases reducibility of the system as observed in experiments with vanadia-silica and vanadia-alumina catalysts.9,11 Further reduction (adsorption of a second H atom in the same unit cell) takes place on the available V1-O-Ti bridging site and is less favorable as corresponds to a strongly reduced system. The doped systems show similar values around -2.4 eV, less exothermic than the bare slab (-2.74 eV). Full spin relaxation leads to magnetization values around 1, 2, and 3 unpaired electrons for the alkali-doped, 1H-alkalidoped, and 2H-alkali-doped slabs respectively. The unpaired electrons are located on the vanadium atoms, and for the most reduced systems (2H-alkali-doped slabs) also the surface titanium atoms possess spin density. Note that the degree of reduction for the calculated slabs is high, although experimentally the complete reduction of the samples is affordable.9,11 Conclusion The alkali metals bind to oxygen sites by electrostatic interaction. Both the vanadia unit and the support interact with the alkali; a physical effect of the addition of alkali could be the easier dispersion of the V2O5 units since alkali would be stabilized in the interphase between active phase and support. The M-O bond strength decreases in the series H to K. When alkali interact with vanadia, an elongation of the VdO bonds of ∼0.20-0.40 Å is found with respect to the bare

Calatayud and Minot substrate, displacing the corresponding vanadyl frequencies to lower energies. The VdO groups are more perturbed in the presence of polarizing atoms (H, Li). Neutral alkalis make the charges on the vanadia O atoms increase and those on vanadium atoms slightly decrease. This corresponds to an increase in basicity observed in experiments. Upon adsorption, the neutral metal reduces vanadium. H, Li, and Na are completely ionized, while K shows a charge of +0.92. The electron transferred from the alkali atom is localized on one of the vanadium atoms. This process also shifts the Fermi level in the DOS from the top of the valence band to the bottom of the conduction band. A state in the gap, below the conduction band, appears for the doped systems as observed in experiments for similar systems. This state is only observed for the K-doped in the pure DFT description, but clearly appears for all dopants when using GGA+U. The best adsorption mode for methanol adsorption corresponds to formal dissociation into methoxy CH3O-/V5+ and H+/OVTi. The bridging V-O-Ti sites would be the most reactive. The alkali-doped slabs present higher (less exothermic) heats of adsorption than the undoped one. Since low Eads values are associated with selective catalytic processes, the most selective catalyst would be obtained after K-doping. The adsorption energy values become less exothermic within the series Li-Na-K for the molecular and dissociative adsorption modes. This trend is explained in terms of hard and soft concepts: hard systems favor electrostatic interactions, thus the methanol adsorption on hard slabs (as Li-doped) is favored with respect to soft ones (K-doped). Regarding hydrogenation, the bare substrate is more reducible than the doped systems. The best adsorption energy is obtained for hydrogenation of the bridging V-O-Ti sites. This interaction is accompanied by an electron transfer from H to the vanadium atoms and the consequent reduction. A decrease in |Eads| is observed for the bare-H-Li-Na-K series, the K-doped system being the less reducible in agreement with experiments. Acknowledgment. We thank Dr. M. A. Ban˜ares and Dr. I. E. Wachs for stimulating discussions and Dr. B. Diawara for the ModelView visualization program. M.C. is grateful to Dr. V. Bra´zdova´ for help in the vibrational analysis. Computational facilities provided by CCRE and IDRIS are also acknowledged. References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Muscat, J.; Harrison, N. M.; Thornton, G. Phys. ReV. B 1999, 59, 15457. (3) San Miguel, M. A.; Calzado, C. J.; Sanz, J. F. J. Phys. Chem. B 2001, 105, 1794. (4) Bredow, T.; Apra`, E.; Catti, M.; Pacchioni, G. Surf. Sci. 1998, 418, 150. (5) Chiesa, M.; Giamello, E.; Di Valentin, C.; Pacchioni, G.; Sojka, Z.; Van Doorsaler, S. J. Am. Chem. Soc. 2005, 127, 16935. (6) Witko, M.; Grybos, R.; Tokarz-Sobieraj, R. Top. Catal. 2006, 38, 105. (7) Bartholomew, C. H. Appl. Catal., A 2001, 212, 17. (8) Kamata, H.; Takahashi, K.; Odenbrand, C. U. I. J. Mol. Catal. A: Chem. 1999, 139, 189. (9) Wang, X.; Wachs, I. E. Catal. Today 2004, 96, 211. (10) Jime´nez-Jime´nez, J.; Me´rida-Robles, J.; Rodrı´guez-Castello´n, E.; Jime´nez-Lo´pez, A.; Lo´pez Granados, M.; del Val, S.; Melia´n Cabrera, I.; Fierro, J. L. G. Catal. Today 2005, 99, 179. (11) Zhao, Z.; Liu, J.; Duan, A.; Xu, C.; Kobayashi, T.; Wachs, I. E. Top. Catal. 2006, 38, 309. (12) Garcia Cortez, G.; Fierro, J. L. G.; Ban˜ares, M. A. Catal. Today 2003, 78, 219. (13) Lemonidou, A. A.; Nalbandian, L.; Vasalos, I. A. Catal. Today 2000, 61, 333. (14) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865.

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