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Feb 9, 2010 - The interaction of water molecules with the reduced and fully oxidized surface sites of the supported vanadium oxide catalyst VOx/TiO2 h...
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J. Phys. Chem. C 2010, 114, 3609–3613

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Water Effect on the Electronic Structure of Active Sites of Supported Vanadium Oxide Catalyst VOx/TiO2(001) Vasilii I. Avdeev and Vladimir M. Tapilin* BoreskoV Institute of Catalysis, Russian Academy of Science, NoVosibirsk 630090, Russian Federation ReceiVed: NoVember 23, 2009; ReVised Manuscript ReceiVed: January 24, 2010

The interaction of water molecules with the reduced and fully oxidized surface sites of the supported vanadium oxide catalyst VOx/TiO2 has been investigated by the Periodic DFT method. It has been found that the molecular structures of the surface VOx species are radically altered when adsorbed water is involved in the redox cycle. Water dissociates spontaneously on the reduced vanadium sites forming the surface hydroxyl groups OH. The following reoxidation by gas-phase oxygen results in the formation of active sites OdVO2(OH) including both the Bro¨nsted acid sites OH and the vanadyl oxygen VdO more reactive than on the dehydrated surface. Gas-phase oxygen, embedded on the surface under oxidation, does not take part in the formation of surface hydroxyl groups. The hydroxylation-hydration reaction path on the fully oxidized VOx/TiO2 surface has been calculated. It has been found that the recombination reaction of the two surface hydroxyl groups VsOH to form water with the following water desorption prevails over the reverse reaction of hydroxylation. In agreement with the experimental data we conclude that lattice oxygen of surface vanadia species VOx, rather than gas-phase oxygen, undergoes isotope exchange with that of the adsorbed water. 1. Introduction The great interest in the supported vanadium oxide catalysts is specified by their excellent catalytic properties for many important oxidation reactions of organic substances.1–3 Typical oxidation catalysts are known to be finely dispersed transition metal oxides as the active component supported on such oxides as SiO2, TiO2, ZrO2, CeO2, MgO, Al2O3, Nb2O5, etc. The surface structures of such catalysts depend on the active component concentration and the nature of the interface bonds between the active component and the support. For most supported transition metal oxides, interface bonds are strong, and oxygen atoms of the support appear to be incorporated into the active component, creating new surface molecular structures which are distinguished from those of the bulk samples. This determines a wide range of functional properties of the supported oxide active sites, which makes it possible to control the activity and selectivity of the whole oxidation process. The electronic nature of the active sites is of great interest in oxidation catalysis. The V2O5 supported on TiO2 seems to be the most interesting among the series of vanadia-based systems in heterogeneous catalysis.4 A number of studies of the supported vanadia systems were aimed at elucidating the structure of surface vanadium species and the nature of their catalytic action. Although the detailed molecular structure of active sites is still a matter of discussion, there is no doubt that functional groups VdO and V-O-V and their reduced forms V-OH and V-OH-V are the main components of the active sites in VOx/TiO2 catalysts.5–8 The above reviews show that the surface VOx species appear to be four coordinated and their structures can undergo strong modifications during oxidation reactions. It is generally accepted that the active sites contain vanadium in the highest oxidation states 5+, and in oxidation reactions surface V sites undergo a redox cycle, V5+-O-V5+ / V4+-O-V4+, in which the * To whom correspondence should be addressed. E-mail: tapilin@ catalysis.nsk.su. Fax: +7 3832-343056.

formation of surface oxygen vacancy is an important process.9,10 The recent theoretical works strongly suggest that the dynamic character of the VOx species during the redox cycle (creation and following annihilation of oxygen vacancy) can radically influence the stability of the surface sites VdO and V-O-V.11–16 The results of the analysis of some important oxidation reactions show that the molecular structures of the surface VOx species are generally altered in the presence of water molecules which inevitably exit in any real catalytic system as environment or reaction product.17,18 The drastic consequences of water effect on the formation of active sites and their catalytic behavior were reported in the literature on selective oxidation of hydrocarbons on the supported vanadium oxide.19–26 It is commonly accepted that the water effect is connected with the formation of the surface hydroxyl groups V-OH and generation of labile oxygen VOx species. Islam et al.27 have recently shown that in hydrated conditions the hydroxylated species V-(OH)2 and V-OH are predominant on the surface of silica-supported vanadium oxide catalysts. The isotopic methods seem to be the most appropriate for elucidating the role of V-OH in the mechanism of partial oxidation of hydrocarbons.28,29 The isotope exchange processes of the supported vanadium oxide catalysts have been investigated with in situ Raman spectroscopy by Wachs et al.30–33 It was found that the presence of water plays a determinative role in controlling the molecular structures of surface VOx species on TiO2, Al2O3, and CeO2 supports. In particular, it was shown that the vanadyl oxygen VdO readily undergoes oxygen exchange with water vapor. Sadovskaya et al.34 came to similar conclusions when studying the 18O exchange between H2O, O2, and V2O5/TiO2. We have recently reported that on the reconstructed anataseTiO2(001) surface dimeric and polymeric vanadium VOx species are formed which are exposed to strong structure relaxation in the redox cycle: V5+-O-V5+ / V4+-O-V4+.35 In the present paper we extend these investigations of the relaxation processes of the surface VOx species to the more realistic reaction

10.1021/jp911145c  2010 American Chemical Society Published on Web 02/09/2010

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conditions where water is involved in the redox cycle. This paper had two goals: to investigate the influence of the hydroxylation/ hydration processes on the electronic structure of the active sites of the supported vanadium catalyst VOx/TiO2 and to respond to the fundamental question of the oxygen isotope exchange experiments: why the lattice oxygen of surface vanadia species VOx, rather than gas-phase oxygen, undergoes rapid isotope exchange with adsorbed water. We first consider the adsorption of water on the reduced V4+-O-V4+ sites and the following modification of active sites at reoxidation by gas-phase oxygen. We show that adsorbed water dissociates spontaneously on the reduced V4+-O-V4+ sites and the following reoxidation of these sites results in the formation of the Bro¨nsted acid sites V-OH and the more reactive vanadyl oxygen VdO. The results show that during reoxidation the gas-phase oxygen connects dimeric species in the united polymeric chain and does not take part in the formation of the surface V-OH sites. In this paper, we present a detailed calculation of the reaction pathway of hydroxylation-hydration showing the interconversions of the surface hydroxyl groups OH into the adsorbed water. The dehydroxylation reaction pathway leads to recombination of the two surface hydroxyl groups V-OH to form water with the following water desorption. The results of this study give insight into the question concerning the nature of oxygen isotope exchange between H2O, gas-phase oxygen, and the lattice oxygen of an oxide catalyst. 2. Methods and Computational Details The calculations were made with the PWSCF program package36 based on the Density Functional Theory (DFT). Vanadium, titanium, and oxygen electron-core interactions were described by Vanderbilt Ultrasoft pseudopotentials37 with exchange-correlation Perdew-Burke-Ernzerhof (PBE) functionals.38 The wave functions of valence electrons were expanded in plane waves with a kinetic energy cutoff of 30 Ry, while the charge density cutoff was 120 Ry. The supported vanadium oxide catalyst VOx/TiO2 surface was simulated as a periodic slab with two Ti-O layers and VOx layer by using the (001)-(2×2) reconstructed anatase ADM model containing two V ions per unit cell. The model is built through isomorphic replacement of the surface top Ti atoms in a reconstructed three Ti-O layer slab by V atoms.39–44 Consequently, the rows with tetrahedrally coordinated V atoms in oxidation state V4+ are formed while the titanium atoms do not alter the valence states and remain in the highest oxidation states Ti4+. The present model is based on the AMD model proposed by Selloni et al.45,46 and its details and validity were described in ref 35. The atomic positions of the upper layers were obtained by the structure optimization. For the frozen bottom Ti-O layer the experimental anathase lattice parameters were used, a ) b ) 3.78 Å, c ) 9.51 Å. The resulting models are used in this paper as a starting surface for water adsorption. The energy reaction path of the hydroxylation-hydration processes are studied by the Climbing Image Nudged Elastic Band method (CI-NEB).47–51 The 13 images (11 movable ones) were specified to locate saddle points along the minimum energy path (MEP). The final saddle point images have a maximal force less than 0.04 eV/Å. The energies of the H2O molecule have been calculated at the Γ point of the Brillouin zone for the tetragonal unit cell with the parameters a ) 10.6 Å, c/a ) 1.25. In this approach the distances O-H are 0.98 Å and the angle H-O-H is 104.3°. The figures with the calculated structures presented below were obtained with the XCrySDen52 program.

Figure 1. Geometry (A) and electronic (B) structures of the surface V-OH species on the reduced V4+-O-V4+ sites. The curves above and below DOS ) 0.0 respectively show the spin-up and spin-down states. The Fermi energy EF was taken as the reference one. The coordinates of all atoms on this and the following figures in XYZformate are given in the Supporting Information.

3. Results and Discussion 3.1. Formation of the V-OH Species on the Reduced VOx/TiO2 Sites. Our calculations of water adsorption on the reduced V4+-O-V4+ sites show that the adsorbed water dissociates spontaneously forming a polymeric chain of the hydroxylated species V4+-OH as shown in Figure 1A. The surface hydroxyl groups OH are not equivalent. The O-H bond lengths are 0.97 and 1.01 Å, and the lengths of the corresponding V-O bonds are 1.85 and 1.74 Å. The hydroxyl groups of the neighboring V-O-V dimer units involved in the strong hydrogen bonds have r(O · · · H) ) 1.48 Å. As it can be seen in Figure 1B, the DOS consists of two narrow surface bands at -7.8 and -6.5 eV, a valence band at -5.9 eV < E < -1.7 eV, and a conduction band with the gap of 2.6 eV. The surface bands consist of OH orbitals with small contributions of Vd states and arise from 3σ and 1π orbitals of isolated OH groups. Electrons at these orbitals are not involved in the bond of the OH group with the surface that follows from a small width of the surface bands and small contributions of V states to the surface band DOS. The valence band is formed by Vd states, the OH group oxygen electrons which are not involved in O-H intrinsic binding, and the bridge oxygen electrons. The bridge O contributes mainly at the lower part of the valence band while the oxygen of the OH group contributes at the upper part. Comparing relative contributions of V and O states to DOS, one can conclude that the covalent component of the V-O bond grows from the lowest surface band to the valence band. DOS for spin-up and spin-down states are practically the same, which leads to nearly equal populations of spin-up and spin-down states and small polarization of Vd

Interaction of Water Molecules with VOx/TiO2 Catalyst

Figure 2. Geometry (A) and electronic (B) structures of the hydroxylated monolayer on the fully oxidized V5-O-V5+ sites. The rectangle frame selects the four-coordinated monomeric OdVO2(OH) unit with labile vanadyl oxygen VdO.

states: 0.07 and 0.02 µB of the same direction for both V ions. The reason for such low values of magnetic moments for V4+ ions can be qualitative understood with the Anderson model.53 According to the model the possibility of an ion preserving its magnetic moment in the crystal depends on the relative values of an on-site repulsion term between localized electrons and the hopping integrals between the ion’s and neighboring site’s electrons. If this integral is big enough, magnetic electrons are delocalized and the magnetic moment disappears. In our case it occurs due to strong covalent interaction through the bridge oxygen between V ions. The calculations give water desorption energy on the reduced VOx species, Ed ) 1.67 eV (38.4 kcal/mol). We believe that dehydroxylation processes due to recombination of surface hydroxyl groups OH on the reduced V4+-O-V4+ sites cannot be right since the activation energy of this process should be greater than the desorption energy of water. However, the situation changes at coadsorbtion of water and oxygen on the reduced V4+-O-V4+ sites. In this case, involvement of oxygen in the processes results in the cardinal decrease of the surface hydroxyl groups OH stability. 3.2. Formation of the V-OH Species on the Fully Oxidized Surface VOx/TiO2 Sites. We have found that reoxidation of the reduced V4+-OH sites by gas-phase oxygen forms the new bridge oxygen V-O-V bond between the dimers and the surface hydroxyl groups OH are linked directly into the united polymeric chain. Figure 2A demonstrates the resulting structure of the surface hydroxylated monolayer. It is important to note that the gas-phase oxygen is embedded between two dimers occupying the position of a bridge oxygen. In fact, as one can see in Figure 2A, this oxygen binds strongly to only one of the vanadium cations, the bond length with the second vanadium cation being 2.20 Å. In essence, the hydroxylated

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Figure 3. Geometry (A) and electronic (B) structures of the hydrated surface on the fully oxidated V5+-O-V5+ sites appropriate to 0.5 monolayer coverage.

monolayer corresponds to the weakly interacting monomeric OdVO2(OH) units. The monomeric unit is stabilized by two interface bonds V-O-Ti. Calculations give the vacancy formation energy of the bridge vanadyl oxygen E(VdO) ) 4.21 eV. This value is significantly less than the dissociative energy of vanadyl bond VdO of the clean (dehydrated) VOx/TiO2 surface where E(VdO) is 5.74 eV.35 Thus, the vanadyl bond VdO becomes more labile. The DOS shown in Figure 2B demonstrates the features of structural relaxation. In comparison with Figure 1B, the surface OH bands have shifted toward the valence band bottom and now were located at -6.8 and -6.2 eV. The upper surface band overlaps with the valence band and the splitting between the bands is significantly reduced. The attention was drawn to the decrease of the bridge oxygen resonance peak (see Figure 1B) and behavior of the bridge oxygen DOS becomes similar to that of the terminal oxygen DOS. It reflects the monomeric unit formation mentioned above in which the bridge O significantly shifted toward one of the V ions. Thus, inclusion of water in the redox cycle V4+-O-V4+ T V5+-O-V5+ results in cardinal transformation of the surface VOx species. Water promotes the formation of the Bro¨nsted acid sites V-OH and the vanadyl oxygen VdO is more reactive in comparison with the dehydrated surface. Note that gas-phase oxygen, embedded into the bridge position, does not take part in the formation of surface OH sites. Gas-phase oxygen carries out another important role, namely, it links the monomeric OdVO2(OH) units in polymeric VOx species on the fully oxidized V5+-O-V5+ sites. 3.3. Molecular Adsorption of Water on the Fully Oxidized VOx/TiO2 Sites. In addition, we considered water adsorption on the fully oxidized reconstructed VOx/TiO2 surface. As is seen

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Figure 5. Structure of metastable intermediate (INT) forming at the first step of the H-transfer reaction. The trajectory of the proton movement of the second step is traced with an arc line. Figure 4. Minimum energy path (MEP) of the H-transfer reaction between surface hydroxyl groups to form the adsorbed water on fully oxidized VOx/TiO2 sites calculated by the CI-NEB method.

in Figure 3A, the resulting structure of the adsorbed water is stabilized by the formation of an H-bond between the water proton and vanadyl oxygen of the adjacent dimer. It is seen that the water binds to one of the vanadium dimer sites, forming 0.5 monolayer coverage. The appropriated vanadyl oxygen VdO is shifted from the plane V-O-V. Such a configuration initiates breaking of the water O-H bond and hydrogen transfer to form the hydroxylated VOx/TiO2 surface. These process will be analyzed below in detail. Calculations give the water desorption energy, Ed ) 0.44 eV (10.2 kcal/mol). Figure 3B demonstrates that water creates a surface band at -8.6 eV, resonance states -5.9 eV at the bottom of the valence band, and a wide structure above the resonance states. Hydrogen states contribute only to the surface state band. Only water oxygen states of the valence band contribute to the covalent bond of water with the V cation. Resonance states at -5.5 eV are formed by the bridge O states. Equal contributions to DOS of these states, as well as the compared contribution of the bridge O states in the remaining part of the valence band with Vd states, point to a significant covalent component to bind the bridge oxygen with V ions. 3.4. The Reaction Path of Hydroxylation-Hydration Transition on the Fully Oxidized VOx/TiO2 Surface. The resulting reaction pathway of the hydroxylation-hydration processes on the fully oxidized VOx/TiO2 surface is demonstrated in Figure 4. The starting state of this pathway is the stable configuration shown in Figure 2 and the final state corresponds to the molecular adsorbed water shown in Figure 3. Hydroxylation-hydration is a two-step process. The reaction path of the first step begins with displacement of the bridging oxygen between dimers from the V-O-V plane (see Figure 2) and through the saddle point TS1 ends with the formation of a metastable intermediate INT with terminal vanadyl oxygen shown in Figure 5. Now, the dimers are bound by the H-bond. The second step initiates H-transfer to oxygen of the hydroxyl group V-OH of the right dimer through the second saddle point TS2 and ends with the formation of adsorbed water. Our calculations give the activation energy E* ) 0.33 eV (7.6 kcal/ mol) for the direct hydroxylation-hydration reaction and for the reverse one, E* ) 0.54 eV (12.1 kcal/mol). The reaction barrier for the second step is less than the barrier of the first one. Thus, the INT intermediate formation processes seem to be a rate-determining step of the hydration reaction on the fully oxidized VOx/TiO2 surface.

3.5. Why Oxygen from Water H2O, Rather than GasPhase Oxygen, Undergoes Exchange with Lattice Oxygen of the Catalyst VOx/TiO2 in the Isotopic Transient Experiment. In Figure 5 one can see that the INT configuration is stabilized due to the strong hydrogen bond between the hydroxyl group V-OH of the left dimer and the vanadyl oxygen VdO of the right dimer. The H-bond length r(H · · · O) equals 1.35 Å, which allows the OH bond to begin the dissociation process. It would be expected that the proton transfer will be to the vanadyl oxygen of the right dimer, which originates from the gas-phase oxygen. In this case, each adsorbed water molecule would generate two surface hydroxyl groups OH. The oxygen of the first hydroxyl group comes from the water, while the oxygen of the second one arises from the gas-phase oxygen. Both hydroxyl groups would be connected by the same vanadium cation. The following recombination of two surface hydroxyl groups V-OH would lead to gas-phase oxygen exchange with the water oxygen. However, our calculations show that the proton movement along MEP through the second saddle point TS2 corresponds to H transfer to oxygen of the hydroxyl group V-OH of the right dimer, but not the vanadyl oxygen. The trajectory of the proton movement of the second step is denoted with an arc line in Figure 5. In this case, the hydration reaction generates the adsorbed water including the oxygen originating from the water only. The gas-phase oxygen occupied the position of the vanadyl oxygen and does not take part in the formation of the surface-hydrated monolayer. As far as the water desorption energy (0.44 eV) being less than the activation energy of the reverse reaction (0.55 eV), the recombination reaction of two surface hydroxyl groups V-OH with water desorption prevails over the reverse reaction of hydroxylation. Therefore, the hydroxylation-hydration processes lead to water desorption. The present results demonstrate that lattice oxygen of the surface vanadia species can be exposed by oxygen exchange with the oxygen of water, but not with the gas-phase oxygen. Thus, exchange of the lattice oxygen with adsorbed water H218O involved in the redox cycle might be represented as in Scheme 1. In Scheme 1, the oxygen vacancy is marked as “0”. In the scheme only the H-shift from the marked OH group to the unmarked OH group is presented. The opposite transfer can also take place; however, it does not lead to the hydrogen exchange process. The V4+-O-V4+ sites are oxidized by dioxygen, which does not take part in the formation of surface hydroxyl groups. This scheme conforms to the kinetic study of the H218O/H216O isotope exchange over vanadia-titania catalyst performed by Sadovskaya et al34 at conditions close to those of selective oxidation of hydrocarbons. The authors34 have found that the

Interaction of Water Molecules with VOx/TiO2 Catalyst

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interaction of adsorbed water with VOx species results in their hydrolysis to form V-OH groups, which recombination underlies the isotope exchange with the activation energy of ca. 16.7 kcal/mol. Our calculations give the value of the activation energy as 12.1 kcal/mol. 4. Conclusions The influence of water on the electronic structure of VOx species supported on the reconstructed anatase-TiO2(001) surface was investigated by a periodic DFT method within the ADM model. The involvement of water in the redox cycle leads to a pronounced effect on the structures of the surface vanadium oxide VOx/TiO2 species. The water dissociates spontaneously on the reduced vanadium sites forming the surface hydroxyl groups HO-V4+-O-V4+-OH which are too stable to desorb in the gas phase as H2O. The following reoxidation by gasphase oxygen forms the hydroxylated monolayer on the fully oxidized VOx/TiO2. In this case, desorption energy of water significantly decreases from 38.4 to 10.1 kcal/mol. These results demonstrate that water forms with Lewis acids sites V5+ the weak coordination bonds. The vanadyl bond dissociation energy E(VdO) is 4.21 eV, unlike dehydrated surfaces for which the energy is E ) 5.74 eV. The detailed calculation of the reaction pathway of hydroxylation-hydration shows that the dehydroxylation reaction pathway leads to recombination of the two surface hydroxyl groups V-OH to form water with the following water desorption in the gas phase. As a result, the lattice oxygen of the surface vanadia species is exchanged with water oxygen rather than with the gas-phase oxygen. Acknowledgment. The work is partially supported by the Russian Federal Innovation Agency via the program “Scientific and Educational cadres” and by Integration Projects of SB RAS Nos. 26 and 40. Supporting Information Available: Listing of atomic coordinates for the models discussed. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: New York,1994. (2) Kung, H. H. AdV. Catal. 1994, 40, 1. (3) Chambers, S. A. Surf. Sci. Rep. 2000, 39, 105. (4) Wainwright, M. S.; Foster, N. R. Catal. ReV. 1979, 19, 211. (5) Deo, G.; Wachs, I. E.; Haber, J. Crit. ReV. Surf. Chem. 1994, 4, 141. (6) Coudurier, G. J. C.; Vedrine, J. C. Catal. Today 2000, 56, 415. (7) Briand, L. E.; Faneth, W. E.; Wachs, I. E. Catal. Today 2000, 62. (8) Vedrine, J. C. Top. Catal. 2002, 21, 97. (9) Mars, P.; van Krevelen, D. W. Suppl. Chem. Eng. Sci. 1954, 3, 41. (10) Vannice, M. A. Catal. Today 2007, 123, 18.

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