Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

J. Phys. Chem. C 2009, 113, 18191–18203

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Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces Alexander Hofmann,* M. Verońica Ganduglia-Pirovano, and Joachim Sauer Institut fu¨r Chemie, Humboldt-UniVersita¨t zu Berlin, Unter den Linden 6, D-10099 Berlin, Germany ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: July 28, 2009

The surface structure of vanadia supported on zirconia is examined by density functional theory with periodic boundary conditions. Adsorption of V2O5 and H2O on the (101) and (001) surfaces of tetragonal zirconia is studied for different loadings up to (V2O5)4 or V2O5 · 6H2O per eight Zr surface sites (monolayer coverage). The considered surface species include vanadia monomers, dimers, and polymers on water-free surfaces as well as different combinations of coadsorbed vanadia, hydroxyls, and water. Calculated vibrational spectra do not show unique features that could be used for identifying certain surface species. Statistical thermodynamics is used to evaluate the relative stability of different surface structures as function of vanadia loading, water partial pressure, and temperature (surface phase diagrams). The presence of water has a strong influence on the surface species. Under water free conditions and for Θ(V2O5) ) 0.25 vanadia is present as a monomeric species on the (001) surface and as dimeric species on the (101) surface. For water partial pressures typical of oxidative dehydrogenation of propane (10-2 bar, 775 K), hydrolysis of the dimeric species on the (101) surface is observed. The presence of water also affects the reactivity in partial oxidations as characterized by oxygen defect formation or hydrogen attachment energies. At Θ(V2O5) ) 0.25 the presence of water stabilizes less reactive species, whereas at high vanadia coverage more reactive species are present. 1. Introduction Vanadia catalysts supported on metal oxides such as SiO2, Al2O3, ZrO2, TiO2, and CeO2 are active catalysts for a variety of selective oxidation reactions, for example, the oxidative dehydrogenation (ODH) of light alkenes and the oxidation of methanol to formaldehyde.1-6 The support has a large effect on the catalytic activity,3,4,6 which is not well understood. The reason is the complexity of the surface structure of the supported catalysts in an environment that usually contains water. Few studies of catalytic oxidation reactions have focused on VOx species supported on ZrO2,7-15 which is a classical support for vanadia catalysts. It gives high vanadia dispersion, is thermally stable, is chemically inert in the ODH of hydrocarbons,2,4,16 and is very hard to reduce.17,18 The activity of zirconiasupported vanadia in the ODH exceeds those of many supports.10,19 The structure of the VOx species on ZrO2 has also been studied (see e.g., refs 13, 16, and 20-23) but not as thoroughly as it has for vanadia supported on SiO2, Al2O3, and TiO2. Under water-free conditions, isolated and polymeric surface species as well as V2O5 crystallites can occur. Vanadates, for example, ZrV2O7, form at high vanadia loading and elevated temperatures. Under ambient conditions the surface of the supported catalyst is partially hydrated and the specific species that can exist depend on the vanadium concentration and degree of hydration.22,23 Pure ZrO2 can exist in various polymorphic forms such as monoclinic,24 tetragonal,25 and cubic,26 depending on the temperature and this adds to the complexity of VOx/ ZrO2 catalysts. The phase composition of real catalysts is also influenced by the temperature and, in addition, by the presence of additives and the preparation conditions.16,27,28 * To whom correspondence should be addressed: E-mail: [email protected]. Present address: Umicore AG & Co. KG, AC-RT-R, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang, Germany.

An experimental strategy to reduce the complexity of supported catalysts is growing VOx species on well-defined oxide substrates, that is, single crystal surfaces or thin films. Whereas substrates like titania,29,30 alumina,31,32 silica,32,33 and ceria34 have been extensively studied by experimental and theoretical means, the difficulty to create crystalline high-quality ZrO2 films35-37 have prevented such experimental studies for ZrO2 so far. Theoretical studies are also scarce.15,38 The objective of this study is to investigate the nature of VOx species on the (101) and (001) oriented t-ZrO2 surfaces as function of the surface type and the degree of hydration. The tetragonal phase is chosen because it has a simpler structure than the monoclinic phase. Density functional theory39,40 (DFT) is used to determine the stable structures for a given composition. Statistical thermodynamics is applied to account for the effect of water partial pressure and vanadia loading at different temperatures. Because vibrational spectroscopy is a major tool for structural characterization, we determine the characteristic vibrational modes of vanadia species and examine how they change for different species on the support. Mechanistic studies for vanadia species on silica have shown that hydrogen transfer to vanadyl oxygen species is rate-limiting in the methanol oxidation41 and in the ODH of propane.42 This is consistent with a kinetic model for the ODH of propane on VOx/ZrO2.9 We therefore consider the energy of hydrogen chemisorption as a reactivity descriptor.43 The oxidation reaction leaves an oxygen defect on the vanadia species, see for example, refs 9, 41, and 42 Therefore, in accord with the Mars-van Krevelen mechanism, the energy of oxygen removal from the active phase (oxygen defect formation) has been suggested as a reactivity descriptor44 and is also used here. 2. Computational Details DFT calculations with periodic boundary conditions are carried out with the program package VASP 4.6.45-47 We use

10.1021/jp902755e CCC: $40.75  2009 American Chemical Society Published on Web 09/28/2009

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Hofmann et al. TABLE 2: Cumulative Adsorption Energies of nV2O5 and mH2O on the (1 × 4) t-ZrO2(101) and the (4 × 2) t-ZrO2(101) Surfaces (kJ/mol) t-ZrO2(101) · nV2O5 · mH2O t-ZrO2(101) · nV2O5 · mH2O

Figure 1. t-ZrO2(101) (left) and (001) (right) surface models. Zr atoms are blue, and O atoms are red.

sizes of unit cell (Å2) super cell size (Å2) area (nm2) composition

(101)

(001)

6.425 × 3.642 1×4 6.425 × 14.568 0.936 Zr40O80

3.642 × 3.642 4×2 14.568 × 7.284 1.061 Zr40O80

8 Zr(7) 16 O(3)

8 Zr(6) 16 O(2)

coordinatively unsaturated sitesa

a

0.25 1 -664 -837 -917 -971

0.5 2 -1075 -1140 -1289

1.0 4 -1466

0.25 1 -640 -765 -846 -896 -988 -1031

0.5 2 -1068 -1222 -1280 -1358 -1339

1.0 4 -1756 -1943 -1864

vanadia loading

TABLE 1: Surface Models surface

Θ (V2O5) ) n) m)0 +1 +2 +3 +4 +5

Coordination numbers are given as superscripts in parentheses.

the gradient corrected Perdew-Wang functional PW91.48-50 Core electrons are described within the projector augmented wave scheme,51 improved and adapted into VASP by Kresse et al.52 We use an [Ar]3d10 core for Zr, whereas the 4s, 4p, 5s, and 4d electrons are treated explicitly. V is described by a [Ne]3s2 core, whereas 3p, 4s, and 3d are valence electrons, and a [He] core is adopted for O. The valence electrons are described by a plane wave basis set with an energy cutoff of 400 eV. Structure optimizations are carried out with a conjugate-gradient algorithm until convergence of 1 meV of the total energy is reached. The tetragonal zirconia slab structures are built from optimized bulk structures. Previous studies of the t-ZrO2(101) surface have shown that a five layer slab (Zr2O4 per layer) is sufficient to get converged surface energies.53 In the present models of both the (101) and (001) surfaces, we fix the positions of the atoms in the three bottom layers to their (optimized) bulk positions54 and study adsorption on the topmost layer of the slab only. The lattice parameter c was set to 30 Å. This results into a vacuum layer of approximately 15 Å, which is large enough to avoid surface-surface interactions. The cell parameters of our slab models are therefore a ) 6.425 Å, b ) 3.642 Å, and c ) 30.000 Å for the (101) surface and a ) b ) 3.642 Å and c ) 30.000 Å for the (001) surface (Figure 1). A Monkhorst-Pack mesh with 2 × 4 × 1 k-points, (101) surface, or with 4 × 4 × 1 k-points, (001) surface, is used.54 For low vanadia coverage, simulated annealing is used as a tool to find minimum energy structures. Harmonic force constants are obtained by numerical differentiation of gradients. The resulting frequencies are presented without any scaling. The intensities are scaled according to the cell size and therefore show the intensities of a (1 × 4) cell for the (101) surface, but of a (4 × 2) cell for the (001) surface (Table 1). 3. Surface Models Two different types of Zr-O bonds are found in bulk t-ZrO2. Each Zr atom forms four strong Zr-O bonds with a bond length

V atoms/nm2

2.1

4.2

8.4

1.9

3.8

7.6

of 2.10 Å and four weak bonds with bond lengths of 2.43 Å (calculated values).53,54 This results in two different types of Zr surface sites on the t-ZrO2(101) surface.53 One misses a short, strong Zr-O bond and is denoted Zr(s), the other a long, weak Zr-O bond and is denoted Zr(w). Adsorption is expected to be stronger at the (s)-site with respect to the missing short bond and weaker at the (w)-site with the missing long bond. The t-ZrO2 (101) (1 × 1) slab contains one of each of these zirconium sites. In contrast, on the t-ZrO2(101) surface each of the Zr surface atoms misses two weak Zr-O-bonds. So the t-ZrO2(101) surface features 7-fold coordinated Zr atoms whereas Zr atoms at the t-ZrO2(101) surface are 6-fold coordinated (Figure 1, Table 1). The reference cell for the stability analysis is a (1 × 4) cell in case of the (101) termination and the (4 × 2) cell for the (001) termination. Both cells have eight coordinatively unsaturated zirconium sites (Table 1, Figure 1). The cell composition in both cases is Zr40O80. The labeling of the adsorbate structures also refers to Zr40O80. For example, two V2O5 molecules adsorbed on the t-ZrO2(101) surface are labeled [101,V4O10]. The coverage is given with adsorbed vanadium atoms on (eight) zirconium atoms, here Θ ) 0.5, regardless of the cell size used for the specific calculation. 4. Results and Discussion The adsorption energy ∆Ea is defined as

∆Ea ) Eadsorbate - Epure - nEV2O5,gas - mEH2O,gas

(1) according to the following reaction for the reference cell

t-ZrO2(h01) + nV2O5,gas + mH2O h [t-ZrO2(h01),nV2O5,mH2O] (2) with h ) 1,0. The most stable V2O5 gas phase isomer is cyclic55 and is used as reference structure.56 The calculated energy of condensation of gaseous, cyclic V2O5 into R-V2O5,

V2O5,gas f R-V2O5,solid

(3)

is -397.7 kJ/mol. Another stable bulk phase is zirconium vanadate ZrV2O7. The computed energy for

t-ZrO2,bulk + V2O5,gas f ZrV2O7,solid is -497.5 kJ/mol.

(4)

Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

Figure 2. t-ZrO2(101) with one V2O5 unit per (1 × 4) cell (Θ ) 1/4), [101,V2O5].

Figure 3. t-ZrO2(101) with two V2O5 units per (1 × 4) cell (Θ ) 1/2), [101,V4O10]. Only half of the cell is shown.

Table 2 shows the adsorption energies and Figures 2-13 show vanadia and vanadia/water adsorption structures obtained (for additional structures see the electronic Supporting Information, Figures S1-S3). Zr atoms are blue, V are atoms green, O atoms originating from zirconia are red, O atoms originating from adsorbed V2O5 or water are black. This color code is used throughout the paper. For selected bonds of most important structures, distances are given in picometers. 4.1. Vanadia on Tetragonal Zirconia. 4.1.1. (101) Surface. The lowest vanadia coverage investigated is Θ ) 1/4. For the (1 × 4) supercell of t-ZrO2(101), this corresponds to adsorption of a V2O5 unit. The resulting most stable [101,V2O5] structure is a dimer. It has Pm symmetry and connects with five Zr-O-V bonds to the surface (Figure 2), occupying 3 Zr(7) surface sites. Four of these bridging oxygen atoms are 2-fold, O(2), and one is 3-fold coordinated, O(3). Two of the bridging O(2) atoms originate from O(3) surface atoms that have given up two of their three bonds to create a new one to vanadium. The other two bridging O(2) atoms are former vanadyl O atoms that coordinate to Zr surface sites. The two vanadium atoms are bridged by an O(3) atom that is also coordinated to a Zr surface atom. Each vanadium retains a terminal oxygen atom, O(1), which forms so-called vanadyl group. However, water-free monomers like on the (001) surface (see below) are less stable than dimers. For Θ ) 1/2 two V2O5 units are adsorbed per (1 × 4) ZrO2 surface cell. A symmetric adsorption structure is no longer possible. The resulting adsorption structure ([101,V4O10], Figure 3) resembles the less stable singly bridged (Od)2V-O-V(dO)2 gas phase cluster55 that attaches with three vanadyl oxygen atoms to Zr surface atoms. One of the vanadyls is retained and the V-O(2)-V bridge becomes nonsymmetric. One additional coordination is extended by the V-atoms to a O(3) surface atom.

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Figure 4. t-ZrO2(101) with four V2O5 units per (1 × 4) cell (Θ ) 1), [101,V8O20]. All VdO distances are about 160 pm. Side (left) and top view (right).

Figure 5. t-ZrO2(101) with one, fully dissociated V2O5 unit per (4 × 2) cell (Θ ) 1/4), [001,V2O5].

Fully V2O5 covered t-ZrO2(101) [101,V8O20] (Figure 4), has a wavelike, double-row shape and again mirror symmetry. Every vanadium atom carries a vanadyl oxygen atom and is connected to four additional O atoms as for Θ ) 1/4. In the right vanadia row in Figure 4 each V atom binds with one O(2) to a surface Zr atom, and its vanadyl group points nearly perpendicularly away from the surface. Neighboring V atoms are bridged by O(4) atoms that also connect with a V atom of the “left” row in Figure 4 and anchor to the surface with a rather long bond (252 pm). The fifth coordination of the V atoms is to O(3) atoms in the other (left) row, which bridge neighboring V atoms. The V atoms of the left rows are also 5-fold coordinated, one vanadyl group, two V-O(3)-V bridges, one V-O(4) bond to the right row and one bond to an O(3) surface atom that becomes O(4). The V2O5 adsorption energy for Θ ) 1/4 is -664 kJ/mol, that for Θ ) 1/2 is -537 kJ/mol and that for Θ ) 1 is -367 kJ/mol per V2O5 unit. Since the formation energy of bulk R-V2O5,solid from gas phase V2O5 is -398 kJ/mol, formation of V2O5 crystallites is energetically more favorable than formation of the Θ ) 1 structure. 4.1.2. (001) Surface. For the lowest coverage on the (4 × 2) supercell of t-ZrO2(101) (Θ ) 1/4), a similar adsorption energy is obtained, but the V2O5 adsorption mode is different from that on the (101) surface. In the [001,V2O5] structure (Figure 5) V2O5 dissociates, formally according to

V2O5 f VO3- + VO2+

(5)

VO3- attaches with two O atoms to Zr(6) sites and with V to an O(2) surface site, creating three O(3) atoms and exposing one vanadyl oxygen atom. VO2+ instead attaches with V on two surface O(2) atoms (which become 3-fold coordinated O(3)) and makes an V-O(2)-Zr(7) bond. The resulting most stable structure has P1 symmetry. A larger spatial separation of the two units leads to less stable isomers.

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V2O5 + H2O f 2VO2OH

Figure 6. t-ZrO2(101) with two V2O5 units per (4 × 2) cell (Θ ) 1/2), [001,V4O10]. Only half of the cell is shown.

Figure 7. t-ZrO2(101) fully covered with V2O5 (Θ ) 1, four V2O5 per (4 × 2) cell), [001,V8O20]. All VdO distances are about 159 pm. Two different side views.

With two V2O5 units adsorbed (Θ ) 1/2) a structure with just one vanadyl group forms ([001,V4O10], Figure 6) in a cell with Pm symmetry like on the (101) surface (Figure 3). The adsorption energies are also similar for both surfaces -1068 kJ/mol for the (001) versus -1075 kJ/mol for the (101) surface, (Table 2). In contrast to the fully covered t-ZrO2(101) surface, the fully V2O5 covered t-ZrO2(101) surface (Θ ) 1) does not show symmetry ([001,V8O20], Figure 7). The adsorption energy per V2O5 molecule, -439 kJ/mol, indicates high stability in comparison to bulk V2O5 (-398 kJ/mol for formation from gaseous V2O5) and to adsorption on the (101) surface (-367 kJ/mol). But as on the (101) surface dispersed vanadia (-640 kJ/mol for Θ ) 1/4) is favored over agglomerated vanadia. 4.1.3. Other Supported Vanadia Models. Structures in which VV replaces ZrIV atoms in surface positions have also been investigated but are found less stable than the presented isomers. Su and Bell proposed structures that show either hexagonal patterns or just rows of vanadia on the (001) surface of t-ZrO2.20 We considered also such models for both zirconia surfaces and examined different isomers with vanadyl oxygen atoms up or down in different combinations at different adsorption sites and for different loadings. They were all less stable than the structures discussed above (see Table 2). For example, hexagonal patterns have been found for Θ ) 2/3 on the (101) surface (Supporting Information, Figure S4, left) and the (001) surface. The cumulative adsorption energies corresponding to the surface areas discussed (surfaces with 8 Zr atoms) are -919 and -1251 kJ/mol, respectively. None of the structures have vanadyl oxygen atoms. The adsorption energies of the proposed structure with vanadia rows are -1045 and -1487 kJ/mol for the (101) and (001) surface, respectively (full loading). 4.2. Coadsorption of Water and Vanadia. Water has two possibilities to react with partially V2O5 covered t-ZrO2 surfaces. It may simply adsorb on a free Zr site, either molecularly or dissociatively, or it may hydrolyze V2O5 according to

(6)

In the following, we discuss water adsorption for the two surfaces with different vanadia loadings. On the (001) surface, it is possible to adsorb up to five water molecules in addition to the VO3-/VO2+ units on the (4 × 2) surface cell ([001,2O2VOH,4H2O], Figure 11); O2VOH occupies two Zr(6) sites and one O(2) site, and water occupies one Zr(6) site, where oxygen restores the bulk structure while making two bonds to two different Zr(6) sites. On the (101) surface, the maximum amount of coadsorbing water molecules is three, because one Zr(7) site per adsorbed O2VOH is blocked by a surface oxygen atom, which moved toward the vanadium atom. 4.2.1. Low Vanadia Loading (Θ ) 1/4). Figure 8 shows one of the two adsorbed O2VOH units formed according to eq 6 on the (101) surface resulting in the [101,2O2VOH] structure. Vanadium has a tetrahedral oxygen coordination, and the fourth ligand is the OH group. Alternative proton positions are less favorable. Moving the proton on an O(2) atom would require about 35 kJ/mol, and on an O(3) atom on the zirconia surface about 50 kJ/mol. The O2VOH unit attaches with two oxygen atoms to strong Zr sites, Zr(s), forming two V-O(2)-Zr(s) bonds. Coordination of V to an O atom creats a third V-O(2)-Zr(w) bond. Such bonds are also observed for dimeric vanadia species without water; see Figure 2 and for adsorbed SO3.54,57 Two Zr(w) sites remain unoccupied and unblocked (see above) in the (1 × 4) (101) cell, which provide adsorption sites for a second or third water molecule in the [101,2O2VOH,H2O] (Figure 9) and [101,2O2VOH,2H2O] (Supporting Information, Figure S1) structures. The adsorption energies are -173 kJ/ mol for the first “chemisorbed” water in [101,2O2VOH] (similar to 160 kJ/mol for pure zirconia (101)54), -80 kJ/mol for the second water in [101,2O2VOH,H2O] and -54 kJ/mol for the third water molecule in [101,2O2VOH,2H2O] (see Table 2). This means that vanadia wins the competition with water for strong Zr adsorption sites. On the (001) surface vanadia is already present as monomers, namely VO3- and VO2+. As for the (101) surface, dissociative water adsorption (H+ and OH-) yields two identically adsorbed O2 VOH species in the [001,2O2VOH] structure (Figure 10). Addition of two water molecules leads to the [001,2O2VOH,H2O] structure, which can be described as adsorbed VO3- plus water and adsorbed VO2+ plus water (Supporting Information, Figure S2). However, below we will show that only the [001,2O2VOH,4H2O] structure (Figure 11) is important, which consists of two identical O2 VOH units with four additional water molecules. The adsorption energies (Table 2) are -125 kJ/mol for the first “chemisorbed” water in [001,2O2VOH], -81 kJ/mol for the second water, -50 kJ/mol for the third water, -92 kJ/mol for the fourth water, and -42 kJ/mol for the fifth and last water in the [001,2O2VOH,4H2O] structure (Figure 11). The exceptional gain in adsorption energy for the fourth water molecule is due to formation of a network of hydrogen bonds in the [001,2O2VOH,3H2O] structure. Adsorption of the second water molecule reveals the different nature of both surfaces. On the (101) surface, the strong sites are already occupied by VO3H and only weak adsorption sites are available for additional water. In contrast, the (001) surface has only weak sites, on which both the O2VOH and the H2O molecule have to adsorb. As a result the adsorption structures are not longer well ordered; see Supporting Information, Figure S2 with the [001,2O2VOH,H2O] structure.


Figure 8. The [101,2O2VOH] structure. Only half of the cell is shown. The figure shows two strong Zr sites (left), but three weak Zr sites (right). This is because one Zr(w) is exactly on the boundary of the unit cell.

4.2.2. Higher Vanadia Loading (Θ ) 1/2). Higher vanadia loading prevents hydrolysis of V2O5 due to space restrictions. Water molecules attach to vanadium centers and create hydrogen bonded networks. On the (101) surface, the only free adsorption site for water is the vanadium atom without a vanadyl oxygen atom, [101,V4O10] (Figure 3), all the Zr sites are already occupied. The energy gain due to adsorption is small for the first water molecule (-65 kJ/mol, [101,V2O6H2,V2O5], Figure 12, left), but larger for the second one (-149 kJ/mol, [101,2V2O6H2], Figure 12, right). This indicates that hydrogen bonding plays a large role in the stabilization of the latter structure. The energy gained by adsorption of two water molecules on the (101) surface is similar to one water molecule on the Θ (V2O5) ) 0.5 (001) surface, namely -154 kJ/mol ([001,V2O6H2,V2O5], Figure 13, left), and on the pure ZrO2 (101) surface.54 Higher water loading on the [101,V4O10] system is also observed, namely three water molecules ([001,V2O6H2,V2O7H3-,H+], Figure 13, right). The average adsorption energy per water molecule is nearly 100 kJ/mol. 4.2.3. Maximum Vanadia Loading (Θ ) 1). In the [001,V8O21H2] structure (Supporting Information, Figure S3) water binds more strongly (-187 kJ/mol) on the fully vanadia covered (001) surface than on the Θ ) 0.5 surface (-154 kJ/mol, Figure 13, left)). Water adsorption on the fully vanadia covered (101) surface ([101,V8O20], Figure 4) was not investigated, because there is no coordinatively unsaturated site remaining on the zirconia surface, which could act as adsorption site. 4.3. Stability Analysis. We discuss the stability of different surface structures in terms of the Gibbs free energies of adsorption of V2O5 and H2O

∆Ga ) ∆Ea - n∆µ(V2O5) - m∆µ(H2O)

(7)

with ∆Ea given by eq 1. The chemical potential ∆µ is defined with respect to its value at 0 K, the sum of total and zero-point vibrational energy ∆µ(p,T) ) µ(p,T) - (Etotal + EZPV). Vibrational contributions of the solid are neglected. For justification and further details see ref 54. The stability diagrams in Figure 14 show regions of the chemical potential of water, ∆µ(H2O), and V2O5, ∆µ(V2O5), in the gas phase in which a given structure with a given composition has the lowest ∆Ga of all systems investigated. For comparison, the chemical potentials of V2O5 in R-V2O5 and zirconium vanadate ZrV2O7 are also shown on the left-most axis, because adsorption of V2O5 on the zirconia surface may compete with formation of these two bulk compounds. However, the vanadate formation would require major rearrangement of the structure and may therefore be kinetically hindered.


Figure 9. The [101,2O2VOH,H2O] structure. In case of the [101,2O2VOH,2H2O] structure the remaining empty space between the vanadia monomers would be filled (cf. Supporting Information, Figure S1).

4.3.1. (101) Surface. The left part of Figure 14 shows the phase diagram for the (101) surface. We start the discussion at very low water chemical potential corresponding to low water pressure and high temperature at the top left of the diagram. The pure t-ZrO2(101) surface is followed by structures with increased vanadia loading as ∆µ(V2O5) increases. The value of ∆µ(V2O5) for which R-V2O5 might compete with the VOx/ZrO2 systems (-400 kJ/mol) is close to the phase boundary between Θ ) 1/4 ([101,V2O5]) and Θ ) 1/2 ([101,V4O10]). Moving along the ∆µ(V2O5) ) -400 kJ/mol line from ∆µ(H2O) ) -400 kJ/mol to µ(H2O) ) 0 kJ/mol, we observe first the appearance of [101,2O2VOH] (Figure 8), then [101,2O2VOH,H2O] (Figure 9), and finally [101, 2O2VOH,2H2O] (Supporting Information, Figure S1). All these structures are stable at reasonable and experimentally accessible chemical potentials of water in the gas phase. At the bottom of the figure, temperature scales for isobaric conditions are plotted, which allow for assessing water adsorption/desorption isobars. For example, the ambient pressure of water at 298 K is roughly 10-2 bar. Keeping this ambient pressure constant (and ∆µ(V2O5) at the above value of -400 kJ/mol), we find that the following structures are stable in the following temperature ranges: [101,2O2VOH,2H2O] until about room temperature, [101,2O2 VOH,H2O] until about 400 K, [101,2O2VOH] until about 775 K, and [101,V2O5] above 775 K. This dehydration temperature is far higher than, for example, the system silica/vanadia/water.58 Moving further along the ∆µ(V2O5) ) -400 kJ/mol line to very high water pressures vanadia disappears and leaves only water on the zirconia surface, 2[101,2OH-,2H+,2H2O]. For 398 K, this would happen at a water partial pressure of about 103 bar, which is not realistic. A typical water partial pressure during a selective oxidation of propane at about 10% conversion at 100% selectivity to propene for a feed mixture of 29.1/14.5/ 56.4 of C3H8/O2/N2 as applied, for example, in ref 19, would be 10-2 bar. At V2O5 chemical potentials above -400 kJ/mol and assuming that neither ZrV2O7 nor R-V2O5 form, the [101,V4O10] structure is stable at low water chemical potentials. At higher water chemical potentials, one of the hydrated structures already discussed above with quarter vanadia loading ([101,2O2VOH], [101,2O2VOH,H2O], [101,2O2VOH,2H2O]) or the [101,2 V2O6H2] structure with half vanadia loading (Figure 12) become stable. 4.3.2. (001) Surface. The right part of Figure 14 shows the stability plot of the (001) surface. The pure surface is stable in the high temperature and low pressure region. Increasing just

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Figure 10. The [001,2O2VOH] structure. Only half of the cell is shown.

Figure 11. The [001,2O2VOH,4H2O] structure. Only half of the cell is shown.

Figure 12. The [101,V2O6H2,V2O5] structure (left) and the [101,2V2O6H2] structure (right).

the chemical potential of V2O5 yields, as in the case of the (101) surface, surfaces loaded with increasing amounts of vanadia. At the ∆µ(V2O5) value at which R-V2O5 would be stable, a half monolayer vanadia coverage is stable. The stability region of this structure is much smaller than in the case of the (101) surface, the transition to the fully vanadia covered surface occurs at a 200 kJ/mol lower ∆µ (V2O5) value than on the (101) surface. Zirconium vanadate formation could compete with Θ ) 0.25 structure, but it requires major structural changes and is therefore probably connected with a high activation barrier. The other striking difference compared to the (101) surface is that vanadia-free hydroxylated surfaces occur at much lower chemical potentials and that one and the same structure, [101/ 001,2OH-,2H+], is stable over a broader range.

In a small region around ambient water pressure (about 10-2 bar at 298 K) and around ∆µ(V2O5) ) -400 kJ/mol, structures with the same vanadia loading and different levels of hydration compete in small stability areas with each other. 4.3.3. Comparison with Experiment. Our computational study provides information about possible structures of nV2O5 · mH2O species on the (101) and (001) surfaces of tetragonal-ZrO2 up to monolayer coverage. Direct comparison with experiments is difficult, because (i) they are performed on powder samples that expose a mixture of different crystal surfaces and (ii) the distribution of species may not be thermodynamically controlled, but at least partially kinetically controlled. This means that on the powder surface we may find a mixture of species that our calculations



+ Figure 13. The [001,V2O6H2,V2O5] structure (left) and the [001,V2O6H2,V2O7H3 ,H ] structure (right).

predict to be thermodynamically most stable under certain conditions (Figure 14). In Table 3, every row shows all V-O bond lengths of one vanadium center. VdO(-H) indicates vanadyl group bond lengths (with or without hydrogen atom(s) attached), whereas V-OM is the bond length with O attached to a metal center (M ) V, Zr). Av(V-OM) shows the average over V-OM bond lengths of one vanadia center. Medium long VdO(-H) bonds (about 180 pm) belong to a hydroxy group, long VdO(-H) bonds (about 200 pm) belong to water adsorption on a vanadium site. In agreement with experimental studies, we find V mostly in tetrahedral oxygen coordination, typically with one vanadyl bond (161 pm) and three longer bonds (average about 175 pm, see Table 3). In case of higher loading, e.g., Θ ) 1/2, we also find a trigonal bipyramidal coordination. However, the latter appears only at high vanadia and high water loadings (Figure 14). The calculated bond distances are systematically longer than observed by, for example, EXAFS,66 which is typical of DFT with GGA functionals.

Moreover, the level of hydration considered in the calculations is very different from the multilayer situations encountered when hydrated species in solution interact with the fully hydroxylated surface.22,23 We rather refer to the conditions during the catalytic reaction (H2O partial pressure 10-2 bar, T ) 598-798 K) under which only one H2O molecule per surface cell (8 Zr sites) will be present. The predicted [101,2O2VOH] surface species does not exhibit a free vanadyl group, but this does not mean that the vanadyl band would not be observed for a powder sample. Under the same conditions, species with vanadyl groups may be present on other surfaces, for example, monomeric species with vanadyl groups (Figure 5) would be present under similar conditions on the (001) surface. 4.4. Vibrational Spectroscopy. Figure 15 shows infrared (IR) spectra for the structures presented above. Tables 4-6 list the wavenumbers and assign them to vibrational modes. For the water-free surface modes, the highest wavenumbers (1052-1100 cm-1) are the vanadyl modes, clearly separated from V-O(2)-Zr (763-957 cm-1) and V-O(3)-Zr interphase

Figure 14. Stability plot of the t-ZrO2(101)/V2O5/H2O(left) and t-ZrO2(101)/V2O5/H2O systems (right). p0 is 1 bar. The chemical potentials refer to V2O5 and H2O in the gas phase. For comparison, the formation energies of R-V2O5 and zirconium vanadate from V2O5,gas are shown (left side). Bold numbers are oxygen defect formation energies with respect to 1/2 O2, italic numbers are H attachment energies with respect to 1/2 H2 in kJ/mol. Θ is given with respect to a monolayer (ML) V2O5.

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TABLE 3: Calculated V-O Bond Distances for Different Models Compared to EXAFS Results (pm) Θ

structure

VdO(-H)

Av(V-OM)

V-OM

V-OM

V-OM

180.0 179.0 179.0 179.7 173.5 181.7 174.0 171.0 171.0 171.0 182.8

191 178 183 184 166 186 169 172 169 169 174

175 178 182 182 177 180 181 172 171 172 183

173 181 172 173 177 179 177 169 173 172 186

191.3 186.3 178.0 180.3 173.8 180.3 176.3 177.8 179.0 177.5 175

171 170 169 180 168 178 175 167 178 168

188 170 170 180 168 178 176 170 179 168

202 186 184 181 178 185 178 181 180 186

0.25

[101,V2O5] [001,V2O5]

0.5

[101,V4O10]

161 161 161 161

[001,V4O10]

161

[101,2O2VOH] [101,2O2VOH,H2O] [101,2O2VOH,2H2O] [001,2O2VOH 4OH-,4H+]

179 179 180 180

[101,2V2O6H2]

163 178 207 161

0.25

0.5

[001,V2O6H2,V2O5]

[001,V2O6H2,V2O7H3-,H+]

0.323a

EXAFS66

163 166 207 164 207 158

V-OM

174 169

188 204 219 189 181 193 188

a Θ ) 0.323 calculated with 0.23 VOx/nm2 for monomeric monolayer covergage.67 Our value would be about 1/8 VOx/nm2 (cf. Table 1) and hence Θ (EXAFS) ) 0.16.

Figure 15. Infrared spectra of different vanadia coverages on t-ZrO2(101). Intensities are given in arbitrary units (a.u.), but the plot range is the same for all spectra. In the lower part of the figure frequencies of all modes (including Raman active ones) are shown as lines. The top-down sequence is the same as in the legend shown in the respective top panels. Top spectra, water-free structures (cf. Table 4); bottom spectra, watercontaining structures. Left, (101) surface (cf. Table 5); right, (001) surface (cf. Table 6).

modes (706-834 cm-1). There is no monotonic trend of these modes with respect to loading, which is explained by the different structure types of supported vanadia at Θ ) 1/4, 1/2,

and 1. The vanadyl modes, ν(VdO(1)) for Θ ) 1/4, [V2O5], show up at 1074 and 1068 cm-1 for the (101) and (001) surfaces, respectively. Higher loading, [V4O10], leads to lower wavenum-

Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces TABLE 4: Frequencies (cm-1) and Assignment of Different Modes for Water Free Vanadia Covered Zirconiaa,b t-ZrO2(101)

[V2O5]

[V4O10]

VdO(1)

1074.1 ip 1058.2 oop

1051.6

V-O(2)-Zr

889.8 861.8 805.7 792.8

956.9 877.6 V-O(3)-Zr

V-O(2,3)-Zr

ip oop oop oop

V-O(3)-Zr t-ZrO2(101) VdO(1)

706.0 [V2O5] 1067.8 ip 1058.7 oop

V-O(2)-Zr

854.9 VO2+

V-O -V

834.8 V-O(2)-V 784.3 762.5 712.5 Zr-O [V4O10] 1053.1

Zr-O

806.7 795.0 757.1 746.9 717.3 724.2 706.0

VO3VO3VO2+ VO2+ VO3-

ip oop oop oop

729.3 [V8O20] 1100.3 ip 1085.5 oop 1070.3 oop 1065.0 oop

917.3 ip 908.0 oop

(2)

V-O(3)-Zr

[V8O20] 1097.3 1070.4 1065.9 1063.6 807.4 775.8

787.9 779.6 756.6 716.9

V-O(2)-V ip oop Zr-O

V2O(4)Zr2

756.9 ip 748.3 oop

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18199 Water coadsorption changes the picture significantly. The spectra for the (101) surface with low vanadia coverage, [101,2O2VOH,nH2O], n ) 0, 1, 2, do not show bands in the vanadyl region, because VdO(1) bonds are replaced by V-OH bonds. This change may be used to distinguish (hydrated) monomeric from (water-free) dimeric species on partially hydrated surfaces. In the [101,2V2O6H2] structure (Θ ) 0.5) a vanadyl bond is retained (Figure 12) and the IR spectrum shows a vanadyl vibration (1025 cm-1) at the expected position above 1000 cm-1. Vanadia and water on the (001) surface show rather complex spectra that may prevent the identification of specific structures. Low coverage vanadia plus water [001,2O2VOH,4OH-,4H+] shows, like in the case of the (101) surface, no vanadyl vibration. The peak at 958 cm-1 is due to the V-OH vibration. Three modes contribute to the peaks from 750 to 850 cm-1. Higher vanadia loading on the (001) surface leads to separation of these peaks in the region around 800 cm-1 ([001,V2O6H2,V2O5]), more water broadens them again ([001,V2O6H2,V2O7H3-,H+]). The [001,V8O21H2] structure leads in comparison to [001,V8O20] to two more features, namely in the interval 730 to 900 cm-1. 4.5. Reducibility of Vanadia on Zirconia. The reducibility of vanadia is taken as a descriptor for its activity in the oxidative dehydrogenation of propane.44,43 Two parameters are calculated, (i) energy of removing an oxygen atom as a “half” oxygen molecule

725.9

a

Coupling: ip, in phase coupling; oop, out-of-phase coupling; more complex coupling is described in detail. b See Figure 15 top.

bers, 1052 and 1053 cm-1, respectively, whereas maximum loading, [V8O20], leads to the highest wavenumbers, 1097 and 1100 cm-1, respectively. The full range of vanadyl wavenumbers for [V8O20], 1064-1100 cm-1, is similar to the range reported previously for crystalline R-V2O5 (1079-1095 cm-1) and vanadia thin films on R-Al2O3 (0001) (1058-1076 cm-1).59,61 The trend for vibrations of bridging oxygen atoms V-O(2)-Zr for different loadings is opposite to the one for vanadyl stretching vibrations; the structure with the lowest ν(VdO(1)) has the highest ν(V-O(2)-Zr) (bold wavenumbers in Table 4). This is also seen in Figure 15, top. The gap between vanadyl and V-O(2)-Zr interphase modes is smallest for [V4O10], next is [V2O5] and it is largest for [V8O20].

[ZrO2,nV2O5,mH2O] h 1 [ZrO2,(n - 1)V2O5,V2O4,mH2O] + O2,gas 2

and (ii) energy of hydrogen attachment with respect to “half” an hydrogen molecule

1 [ZrO2,nV2O5,mH2O] + H2,gas h 2 [ZrO2,(n - 1)V2O5,V2O5H, mH2O] (9) Structures of the products are discussed in only exceptional cases; see Figures 16-19, and for additional figures see Supporting Information, Figures S5-S16 (oxygen detachment) and Figures S17-S28 (hydrogen attachment).

TABLE 5: Frequencies (cm-1) and Assignment of Different Modes for Water and Vanadia Covered (101) Surfacesa,b t-ZrO2(101)

[2O2VOH]

VO -H

3755.8

(2)

V(O(3)-H)V O-HH2O δ(O-H) V-O(2)-Zr

934.7 ip 857.4 oop 834.7 oop

V-O(2)H

758.6

Zr-O a

See Table 4. b See Figure 15 bottom left.

[2O2VOH,H2O]

[2O2VOH,2H2O]

3753.6 3747.8

3751.7

3670.5 3571.3 1570.6

3673.2 oop 3581.5 ip 1579.6

923.0 ip 919.8 ip,oop 855.2 ip,oop 848.2 ip,oop 843.6 oop 823.8 oop 774.2 760.2 703.3 702.8

(8)

[2V2O6H2]

3725.0 3075.7 O(2)-H · · · O(1)

928.0 ip 852.9 oop 825.0 oop

1025.4 948.1 V-O(2)-Zr 717.0 δ(V(O(3)-H)V) 906.0 δ(O-H · · · O(1)) 843.7 836.0

751.7

762.0 737.2

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TABLE 6: Frequencies (cm-1) and Assignment of Different Modes of Water and Vanadia Covered (001) Surfacesa,b t-ZrO2(001) Zr2O-H

VO-H

δ(O-H)

[2O2VOH,4OH-,4H+] 3782.2 3749.6 3377.1 2858.1 Zr2O-H · · · O(2)Zr2 3753.9

1136.9 840.2 δ(O(2)-H) 814.7 δ(O(2)-H) 786.1 δ(Zr2O-H · · · O(2)Zr2)

V-O(1)

[V2O6H2,V2O7H3-,H+]

[V2O6H2,V2O5]

3558.7

3761.1 VO(3)-H 2874.2 VO(3)-H · · · O(1)

1506.1

3766.5 3753.5 3674.8 2692.7 2454.6 1538.9 1518.9

VO(3)-H VO(3)-H VO(2)-H VO(2)-H · · · O(1) VO(2)-H · · · O(1)

1035.0 V-O(1) 1008.4 V-O(1) · · · H

1013.8 · · · H and δ(H2O) 973.6 · · · H and δ(H2O) 956.7 · · · H and δ(H2O) 924.3 · · · H and δ(H2O)

925.0 886.1 880.5 δH2O 732.1 Zr-O

942.2 917.8 902.8 817.9 789.6 886.8 885.5 756.6

V-O(2)-V

V-O(2)-Zr

958.4 929.9 δ(O(2)-H) 925.0 δ(O(2)-H)

δ(VO-H) V-O(2,3)-Zr V-O(3)-Zr

Zr-O

a

745.6 726.2

[V8O21H2]

927.7 919.8 796.5 787.8 780.1 768.6 754.0 714.9 703.9 701.8

V-O(2)-V V-O(2)-V V-O(2)-V Zr-O Zr-O V-O(3)-Zr V-O(2)-V V-O(2)-V

V-O(2)-V VO(3)-H δ(H2O) V-O(2)-V V-O(2)-V V-O(2,3)-Zr,V V-O(2,3)-Zr,V V-O(2)H

781.0 V-O(2)-V 761.4

729.9 722.7 717.4 708.9 707.9

V-O(2,3)-Zr,V V-O(2,3)-Zr,V and δ(H2O) V-O(2,3)-Zr,V and δ(H2O) V-O(2,3)-Zr,V and δ(H2O)

3428.4 VO(2)-H 3022.9 VO(2)-H

1090.2 V-O(1) 1082.2 V-O(1) ip 1080.0 V-O(1) oop 1067.3 V-O(1) oop 1036.4 V-O(1) 998.0 V-O(1) · · · H 931.6 ip 880.7 δ(VO(2)-H) 862.9 δ(VO(2)-H) 819.0 805.3 V-O(2)H 781.2 765.1 V-O(2)H

808.4 V-O(2)H

741.8 Zr-O 735.6 Zr-O 727.6 720.7 701.5 V-O(3)-Zr Zr-O 759.6

See Table 4. b See Figure 15 bottom right.

Figure 16. Dimeric [101,V2O5] with oxygen defect. Black numbers are spin densities.

Figure 17. Monomeric [101,2O2VOH] with oxygen defect.

4.5.1. Oxygen Defect Formation. Energies of oxygen defect formation, eq 8, are shown in Table 7. For dry surfaces they decrease with higher vanadia loading. The drop is largest from Θ ) 1/2 to Θ ) 1. The defect formation energies are larger on

the (001) surface, except for full loading, for which it is 45 kJ/mol lower. The reducibility of [001,V8O20] (145 kJ/mol) is even higher than the one of pure R-V2O5 (about 186 kJ/mol).60 On the (101) surface, water increases the defect formation energy. The change from water-free V2O5 dimer to the


J. Phys. Chem. C, Vol. 113, No. 42, 2009 18201 TABLE 8: Hydrogen Attachment Energies of Structures Appearing in the Stability Plot of Figure 14 (kJ/mol)a t-ZrO2(101) · nV2O5 · mH2O t-ZrO2(001) · nV2O5 · mH2O Θ (V2O5) ) 0.25 n) 1 m)0 +1 +2 +3 +5

Figure 18. Dimeric [101,V2O5] with one attached hydrogen atom.

Figure 19. Monomeric [101,2O2VOH] with one attached hydrogen atom.

TABLE 7: Oxygen Defect Formation Energies (kJ/mol) of Structures Appearing in the Stability Plots of Figure 14 t-ZrO2(101) · nV2O5 · mH2O t-ZrO2(101) · nV2O5 · mH2O Θ (V2O5) ) 0.25 n) 1

0.5 2

1.0 4

0.25 1

0.5 2

1.0 4

m)0

333.3

192.5

406.1

358.0

145.0

354.3

83.0

+1 +2 +3 +5

389.0 (374)a 409.5s (405)a 412.1s 402.2s

346.9 306.2 443.6O(2)

a

PBE result (single point energy calculation on PW91 structure). closed shell singlet ground state is more stable than a triplet ground state. O(2) bridging oxygen atom defect is more stable than the default O(1) vanadyl oxygen atom removal. s

hydrated O2 VOH monomers increases the oxygen defect formation energy by 20 kJ/mol. The differences between the structures with different water loading (n ) 1-3) are (Figures 8, 9, Supporting Information, Figure S1) small and the defect formation energies fall into a narrow range of 402-412 kJ/ mol. Protons originally attached to the oxygen atom that is removed, move to the zirconia surface or to another oxygen atom of adsorbed vanadia. In the case of Θ ) 0.5 plus water, coadsorption increases the defect formation energy from 333 to 347 kJ/mol for the [101,2V2O6H2] structure, Figure 12. For the vanadia loaded (001) surface, water has a nonuniform effect on the defect formation energies. At Θ (V2O5) ) 1/4 the energy for oxygen defect formation increases in the [001,2O2VOH,4H2O] structure (Figure 11). At Θ (V2O5) ) 1/2, the defect formation energy drops from 358 kJ/mol (no water, Figure 6) to 354 kJ/mol (one water,

0.5 2

1.0 4

0.25 1

-0.4 -42.6 -146.5 -11.5 (-1.3)b 44.2c (43.5)b,c 57.0c -50.8c 21.8d 31.0c

0.5 2

1.0 4

-10.6

-150.8

-8.0b

-127.0

-71.0

a All systems are doublets. Hydrogen is attached to the vanadyl oxygen atom, O(1), except otherwise noted. b PBE result (single point energy calculation on PW91 structure). c Hydrogen is attached to O(2). d Hydrogen is attached to O(3).

Figure 13, left) and to 306 kJ/mol (three water, Figure 13, right). For full V2O5 coverage the trend is the same from 145 (no water, Figure 7) to 83 kJ/mol (one water, Supporting Information, Figure S3). This might be interpreted as autocatalytic behavior because water is formed in the reaction of interest, the oxidative dehydrogenation of, for example, propane. Shah et al.62 studied redox isotherms of vanadia supported on zirconia for vanadia loadings of 2.5 and 5 wt %, correponding to 2.9, and 5.8 V/nm2, respectively. Since the fully vanadia loaded surfaces host about 8 V/nm2 (Table 1), these samples correspond to Θ ) 0.36 and 0.72, respectively, and fall in between our Θ ) 0.25, 0.5, and 1.0 models. The numbers given in ref 62 are free energies and refer to one O2 molecule. With additional data for the corresponding enthalpies from ref 63 and assuming that the entropy change is the same for the different materials (-212 J/molK), we arrive at the following enthalpies of reduction per 1/2 O2 molecule (eq 8) from refs 62 and 63 -225 kJ/mol (2.5%VZr), -129 kJ/mol (5%VZr), -113 kJ/mol (ZrV2O7), -143 kJ/mol (V2O5), which can be compared with the calculated DFT(PW91) data in Table 7, that is, about -390 to -410 kJ/mol for small (isolated and dimeric) species, about -150 to -190 kJ/mol for polymeric species, and -186 kJ/mol for V2O5 (ref 18). Although the PW91 results in Table 7 are overestimated as we already know from previous studies for V2O5,18,44 they are in reasonable agreement with the trend in the above experimental data. 4.5.2. Hydrogen Attachment. Table 8 shows hydrogen attachment energies for the vanadia/water loaded surfaces. For water-free vanadia/zirconia we compute small energy gains of up to 43 kJ/mol for Θ (V2O5) ) 1/4 and 1/2 and larger energy gains of about 150 kJ/mol for Θ ) 1. The hydrogenation of model systems for isolated vanadia sites, OdV(OCH3)3 (+44 kJ/mol, PBE) and OdVSi7O12H7 (+19 kJ/mol, PBE) is endoenergetic,43 whereas for the V2O5 (001) surface an energy gain of 75 kJ/mol (PW91) has been calculated.64 The hydrogenation energy of the water-free [101,V2O5] system (quarter loading) is -0.4 kJ/mol. The presence of one and two water molecules yields positive values for the hydrogenation reaction energy, +44 and +57 kJ/mol, respectively. A further increase in the water concentration to three molecules supports the reaction slightly (∼+22 kJ/mol). At half V2O5 loading and two water molecules on the (101) surface, [101,2 V2O6H2], the attachment energy (-51 kJ/mol) is similar to that for the water free system [101,V4O10] (-43 kJ/mol). At the (001) surface at half loading, like in the case of quarter loading on the (101) surface, increasing the water concentration

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(from one to three molecules), supports the attachment of an additional hydrogen atom (cf. -8 and -71 kJ/mol). 5. Summary and Conclusion The equilibrium structure of the t-ZrO2/V2O5/H2O system depends on the support structure, on the vanadia loading, as well as on the temperature and water pressure. For very low values of the water chemical potential and -600 j ∆µ(V2O5) j -400 kJ/mol, the formation of a quarter monolayer structure on both zirconia surfaces is energetically preferred. It corresponds to dimeric species on the (101) surface and to monomeric species on the (001) surface. Differences are also predicted at higher values of ∆µ(V2O5). On the (001) surface, formation of a monolayer structure appears possible for ∆µ(V2O5) J -400 kJ/mol, in competition with the formation of crystallites of bulk vanadia. On the (101) surface, the stability range of a monolayer structure lies between -200 j ∆µ(V2O5) < 0 kJ/mol. If water is present, new structures will form. For example, on the (101) surface for a water pressure which is typical for partial oxidation (10-2 bar), the dimeric species that are stable at high temperature (T > 775 K), hydrolyze to monomeric species at lower temperatures. The latter are stable between 400 < T < 775 K. The reducibility of the model systems of zirconia supported vanadia catalysts changes upon water coadsorption. This may be one of the reasons why the activity of a catalyst under operating conditions, where stochiometric amounts of water are produced, is different from that of a freshly activated catalyst. For instance, the difference in oxygen defect formation and H attachment energies between the [101,V2O5] dimeric species and the [101,2O2VOH] monomeric species obtained by hydration is 21 kJ/mol (oxygen detachment) and 45 kJ/mol (hydrogen attachment) with the monomers being more difficult to reduce. These differences are larger than those obtained for dimeric species on two different supports, namely ZrO2 and SiO2. For dimeric species, the O defect formation and H attachment energies change by -13 and -5 kJ/mol when passing from ZrO2 (PBE numbers in Tables 5 and 6) to SiO265 as support. We conclude that the change in the surface structures, (101) versus (001), and the change due to the presence of some water for a given support (ZrO2) can have a more significant effect in the reactivity of the system than the change in the support material (such as ZrO2 vs SiO2). However, the nature of the support controls the amount of water present on the surface. Acknowledgment. This work has been supported by Deutsche Forschungsgemeinschaft (SFB 546) and by a grant of computer time at the Norddeutscher Verbund fu¨r Hoch- und Ho¨chstleistungsrechnen (HLRN). Supporting Information Available: Optimized structures not shown in the main article and table of total energies of all investigated structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) 127, 1. (2) (3) Wachs,

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JP902755E

Vanadia and Water Coadsorption on Tetragonal Zirconia Surfaces

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