Titania Catalysts at High Loading: A

Article pubs.acs.org/JPCC

Hydration Dynamics for Vanadia/Titania Catalysts at High Loading: A Combined Theoretical and Experimental Study Anna E. Lewandowska,*,† Mònica Calatayud,*,‡,§,∥ Frederik Tielens,⊥,# and Miguel A. Bañares*,† †

Catalytic Spectroscopy Laboratory, Institute of Catalysis and Petroleum Chemistry, CSIC, E-28049 Madrid, Spain Laboratoire de Chimie Théorique, UPMC Univ Paris 06, UMR 7616, F-75005 Paris, France § Laboratoire de Chimie Théorique, CNRS, UMR 7616, F-75005 Paris, France ∥ Institut Universitaire de France, F-75005 Paris, France ⊥ Laboratoire de Réactivité de Surface, UPMC Univ Paris 6, 3 rue Galilée, 94200 Ivry-Sur-Seine, France # Laboratoire de Réactivité de Surface, CNRS, UMR 7197, 3 rue Galilée, 94200 Ivry-Sur-Seine, France ‡

S Supporting Information *

ABSTRACT: Periodic DFT calculations are used to simulate the early stages of hydration of dehydrated high-loading vanadia/titania catalysts. The hydration of molecularly dispersed vanadia is studied by successive additions of water molecules to initially dehydrated models. Special attention is paid to the formation and transformation between different surface species, monovanadates and polyvanadates, and the role of V−OH in the hydration process. It is found that two physical surface processes occur at high vanadia coverage and that their importance depends on the surface water content. First, under mild hydration conditions, a polymerization process increases the number of polyvanadates chains. Polyvanadates formation is preceded by an initial generation of monomers with OV(OH)O2 and OVO3 pyramidal structures. Second, with higher number of water molecules, a solvation process increases the coordination number of vanadium. The interconversion between different surface vanadium oxide species occurs through a fast hydrogen transfer mechanism and depends on water content. Theoretical results are combined with in situ Raman spectra acquired at several temperatures in dry and humid environment during stepwise dehydration. The experimental data indicate that the redshift of the vanadyl band upon exposure to increasing humidity and decreasing temperature is associated to such progressive interaction with water molecules, which weakens the vanadyl VO bond. maleic anhydride.23 The presence of such polymeric species is directly related to the surface loading of vanadia on the oxide support. There is a lack of information concerning both the molecular mechanism of polymerization of vanadium oxide moieties, and the role of water in the formation and interconversion of monoand polymeric vanadia surface species. In that context, the present work aims at gaining a better knowledge about the impact of water on the surface species on titania-supported catalyst at high vanadia loading by means of periodic calculations combined with in situ Raman experiments. Theoretical ab initio calculations have proven to be a valuable tool for describing structural and mechanistic aspects of supported catalysts at the molecular level.24−27 A successful approach consists of mimicking the grafting preparation process.28,29 Thus, isolated VOx units are obtained via the condensation reaction of a model VO(OH)3 precursor molecule with adjacent OH groups of the hydroxylated (001)

1. INTRODUCTION The attention focused on vanadium oxide containing materials in the last years results from their wide used in different catalytic technologies including chemical, petrochemical, and environmental processes.1 Especially, titania-supported vanadium oxide catalysts find application in selective oxidation reactions, such as o-xylene oxidation to phthalic anhydride,2−4 methanol oxidation,5−7 toluene oxidation to benzoic acid,8,9 and the selective catalytic reduction of NO (DeNOx process).10−12 The efficiency of such catalysts is related to many factors, among them the hydration conditions and the polymerization degree of the active phase; both are related to the presence of surface (poly)vanadates. Indeed, water vapor addition to reactant stream enhances the catalytic properties of vanadia/ titania system in the selective oxidation of functional hydrocarbons such as 3-picoline oxidation13 and toluene oxidation14 reactions. The role of polyvanadates vs monovanadates depends on specific reactions, some are not affected by the polymerization degree, like alkane oxidative dehydrogenation (ODH) reactions,15−22 and others depend strongly on the polymerization degree, such as the oxidation of n-butane into © 2013 American Chemical Society

Received: September 3, 2013 Revised: November 3, 2013 Published: November 5, 2013 25535

dx.doi.org/10.1021/jp408836d | J. Phys. Chem. C 2013, 117, 25535−25544

The Journal of Physical Chemistry C

Article

type of oxygen was higher on the partially dehydrated catalyst surface, especially at high vanadia coverage. In this work, a microsolvation model is employed to mimic the hydration process of vanadia/titania system at high vanadia loading, in a similar manner as we did for low vanadia content.31 We focus on the V−OH bond formation and the transformation between polymeric and monomeric species induced by successive water addition to the systems, modeling the early stages of the hydration process. The transformations between different molecularly dispersed vanadium oxide species are investigated by ab initio molecular dynamics at different temperatures. Standard ab initio optimizations are carried out to study in detail the geometry and energetics of the solvated structures. An atomistic thermodynamic approach is used to build a temperature-dependent diagram of stability. In addition, in situ Raman spectra are acquired at controlled temperatures and humidity levels to obtain experimental insight of the processes taking place during progressive hydration.

TiO2-anatase surface. The formation of polyvanadates species with increasing vanadia loading is then modeled. The simplest polyvanadate, divanadate, is achieved via dimerization of two OV(OH)(O−Ti)2 units, which evolve into V(O)−O−V(O) moieties. Trimers of different composition are also obtained following a similar procedure. Tetrahedrally coordinated dimers show favorable formation energy for divanadate, V(O)−O− V(O), hydroxo-divanadate, VO(OH)−O−V(O), and pyrovanadate, VO(OH)−O−VO(OH), structures. The energetically favorable trimers contained tetrahedral coordinated VOx species, V(O)−O−VO(OH)−O−V(O), while models with 5fold coordinated central V atoms, V(O)−O−V(OH)−O− V(O), were less stable. The stability of polyvanadate trimers was found to be lower with respect to isolated monomers and dimers. To study the stability of surface species, a thermodynamic approach considers the external conditions, i.e., temperature and partial pressure (related to the chemical potential). With this technique, vanadium oxide monomers have been found stable on silica30 and titania31 surfaces. Gryboś and Witko32 investigated the vanadia surface species supported on (001) TiO2 system at high vanadia coverage. They found three polyvanadates with vanadium ions bonded together by oxygen bridges and connected to the support by one or two V−O−Ti bonds as stable structures. Such structures differed mostly on the number and position of hydrogen atoms. The surface hydroxyl groups (V−OH) were easily removed during alkane oxidative dehydrogenation (ODH) reaction and under ultrahigh vacuum (UHV) conditions. The bridging V−O−Ti oxygen atom was labile and active, allowing easy interconversion between surface species. A microsolvation model has also been used to explicitly consider the presence of water molecules in the structures. For the simplest case, a V2O5 cluster is progressively exposed to an increasing number of water molecules resulting in the change of the vanadium oxide local coordination and the appearance of hydrogen-bonded molecular water. 33,34 The dissociative adsorption of water was more favorable than its molecular adsorption at the early hydration stages, i.e., the formation of V−OH groups is preferred. The addition of a third water molecule breaks V−O−V bonds forming two monomeric OV(OH)3 units. The fourth water molecule adsorbs molecularly, stabilizing two monomeric units by hydrogen bonds. Recently, we reported the stability of monomeric species on a titania-supported model with low vanadia content (0.84 V· nm−2).31 Experimentally, isotopic methods studied the promoting role of water and the formation of V−OH hydroxyl group. The conditions of 18O exchange between H2O or O2 and V2O5/ TiO2 were close to those for selective oxidation of hydrocarbons.35 The exchange involved two types of catalyst oxygen sites, differing on their substitution rate, V−OH and VO. First, a fast isotope exchange would occur between water and V−OH group. The redshift of the vanadyl bond in Raman spectra reflects the interaction of water molecules with surface vanadium oxide species, which remains even after a long-term heating.36 Second, a slow substitution of the catalyst oxygen involved both terminal (VO) and bridging (V−O−V) bonds.35 Surface vanadium oxide species coordinated to OH groups showed the highest activity in the oxygen exchange with water. It was also assumed that isotope exchange of water with terminal and bridging oxygen occurred during formation and decomposition of the V−OH groups. The concentration of this

2. METHODOLOGY 2.1. Computational Details. The Perdew−Becke−Erzernhof functional PBE37 has been used as implemented in the VASP code.38−41 The core electrons are represented by pseudopotentials generated by the PAW (projector augmented wave) method. The valence electrons (V, 3p64s23d3; Ti, 4s2 3d2; O, 2s2 2p4; H, 1s1) are described by plane-wave basis sets. Only the gamma-point is considered in the Brillouin zone (dimensions of the cell: 15.4 × 15.4 × 25 Å3, including a vacuum of ∼10 Å). All atoms were allowed to relax until the total energy difference was below 1 meV. For geometry optimization, the conjugate gradient method was employed. The model is built on a 4 × 4 anatase (001) five TiO2-units thick slab, on top of which four V2O5 dimeric units (the dehydrated system) and water (hydrated systems) are deposited. The bare vanadia/titania slab contains 268 atoms and represents a vanadium coverage of 3.37 atoms·nm−2 of titania support. The effect of water is calculated adding consecutively 1 to 4 H2O molecules to the bare system; for comparison purposes, higher water content is modeled by including six and eight H2O molecules. Two sets of calculations have been carried out: (i) ab initio molecular dynamics at different temperatures to follow the dynamic behavior of the systems upon hydration and (ii) standard ab initio geometry optimizations, to analyze the structure and energetics of the most representative systems. Molecular dynamics (MD) calculations were performed using the microcanonical ensemble (constant energy molecular dynamics). The PAW pseudopotentials O_s, V, Ti, and H are used together with a plane wave basis set of 283 eV. The calculated Hellmann−Feynman forces serve as acceleration acting onto the ions. The total free energy (i.e., free electronic energy + Madelung energy of ions + kinetic energy of ions) is conserved. The following methodology was used: a first run (1 fs steps, during 1 ps simulation) was done starting at 800 K. The final structure and velocities were used as starting points at 500 K (1 fs step, during 1 ps simulation); the same procedure was repeated at 300 K. The runs were analyzed with regard to the surface species present (monomers, dimers, trimers, etc., with different V coordination) and their dynamics (surface mobility, hydrogen transfer, and desorption). More than 20 MD runs were analyzed considering several starting geometries. Then, the most representative structures were fully optimized at 0 K in a standard DFT energy calculation in order to 25536



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Scheme 1. Structure of 4-Coordinated Monomeric (mTx) Surface Vanadium Oxide Species, That Corresponds to T1, T3, and T4 Structures at Low Vanadia Coverage System19 (Top); Structure of 4-, 5-, and 6-Coordinated Dimeric (dTx; dPx; dHx) Surface Vanadium Oxide Species (Middle); Structure of 4-, 5-, and 6-Coordinated Trimeric (tTx; tPx; tHx) Surface Vanadium Oxide Species (Bottom)a

a

mPHx indicate combination of both P and H types in the same vanadia unit. White small balls are Ti atoms belonging to the (001) TiO2 anatase surface. Green balls are V atoms, red balls are O atoms, and yellow balls are H atoms.

surface; these are denoted as m, d, and t, respectively. Scheme 1 illustrates selected vanadium conformations obtained during MD calculations. Monomers evolve into three different tetracoordinated structures upon the addition of water molecules, (tetrahedral, denoted as mTx). They differ by the number of bridging V−O−Ti bonds (from 1 to 3) formed with the support, and the number of V−OH bonds (0 to 2) pointing out of the surface. All structures possess a vanadyl VO group. Tetrahedral monomers form upon hydration of titaniasupported vanadium oxide species at high coverage; these are also typical during hydration at low vanadia coverages.31 The OVO3 pyramid and the hydroxo-monovanadate OV(OH)O2 (Scheme 1, mT1 and mT3, respectively) are found to be the most stable conformations, while the OV(OH)2O umbrella-like structure (Scheme 1, mT4) forms mainly at higher water content. Dimeric and trimeric surface vanadium oxide species are tetra-, penta-, and hexa-coordinated, being denoted as dTx, dPx, dHx (dimers) and tTx, tPx, tHx (trimers), respectively (Scheme 1). These are the simplest models for polyvanadates. Vanadium atoms exhibit tetrahedral coordination with different numbers of V−OH bonds and one vanadyl VO group per vanadium unit in tetracoordinated polyvanadates (Scheme 1; dT1, dT2, and tT1). The structures corresponding to

characterize their structure and stability. The PAW pseudopotentials O, V, Ti, and H are used together with a plane wave basis set of 400 eV. The most stable systems were used to construct the stability diagram following the atomistic thermodynamics principles described elsewhere.31 2.2. Experimental Details. A titania-supported vanadium oxide catalyst was synthesized as described elsewhere.42 Its surface density was 3.5 V atoms·nm−2 of TiO2 support (named 3.5VTi) and was chosen to be close to the coverage modeled in the calculations, 3.37 V atoms per nm2 of TiO2. Raman spectra were acquired with a single monochromator Renishaw System-1000 microscope Raman equipped with a cooled CCD detector (200 K) and an Edge filter. The powder samples were excited with a 514 nm Ar+ line and 1 mW power on the sample; spectral resolution was near 3 cm−1 and spectrum acquisition consisted of 20 accumulations of 10 s. The sample was exposed to synthetic dry and humid air (0.03 at H2O partial pressure) in a Linkam TS-1500 in situ hot stage. The spectra were recorded during stepwise heating from 298 K up to 673 K every 50 K.

3. RESULTS 3.1. Structures of Vanadium Oxide Species. Mono-, di-, and trimeric surface vanadium oxide species form on titania 25537



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Figure 1. Global view on polymerization process of surface VOx species after water addition to high vanadia/titania model: monomers of VOx species (light blue); dimers of VOx species (brown); trimers of VOx species (orange). Molecular dynamic calculated at 300, 500, and 800 K. The % represents the number of times a given species appears in the MD run with respect to the total number of structures counted.

abundance decreases with the water content for all temperatures ranging from 25% to 40% in the 300 to 800 K interval. The general trend for penta- and hexa- coordinated dimers is an increase with water content, except at 300 K, which exhibits a minimum for 2 water molecules. The abundance of highly coordinated dimeric species reaches 45% (300 K), 35% (500 K), and 25% (800 K) at high water content (4 water molecules), showing a clear dependence on temperature. 3.3. Evolution of Dispersed Vanadium Oxide Species during Hydration. The analysis of the MD runs allows investigating the evolution of different dispersed vanadium oxide species during hydration early stages. Two phenomena are apparent: (i) the formation of monomers and (ii) the polymerization of the surface vanadia units. Figure 3 depicts the process upon the addition of two water molecules at 800 and 500 K at a femtosecond (fs) time scale. Before water addition, the initial structure at 800 K is composed of four V2O5 dimers. These initial structures break their polymeric V−O−V bonds into V−(OH)−V and V−(OH)−Ti hydroxyl groups in 200 fs (Figure 3, 800 K). Such a process delivers monomeric species (black circles): dehydrated OVO3 pyramid and the OV(OH)O2 hydroxo-monovanadate. Later, at 400 fs, monomeric OVO3 and OV(OH)O2 species predominate. These monomers exhibit distorted tetracoordinated structures, and they change their locations through mutual interaction. At 500 K, dispersed monomers condense and lead to the formation of new dimeric species (Figure 3, 500 K, 1 fs). Then, at 100 fs, the remaining monomers bind to dimers forming larger oligomeric structures, marked by blue circles in Figure 3. This process clearly indicates a tendency to polymerization characterized by V−O− V−O−V skeleton bonded to VO and V(OH) groups. A significant distortion in the first atomic layer of the support is observed as a consequence of the shifts and the mutual interaction of VOx units. Two opposite processes seem to occur upon hydration: (i) the break of oligomeric V−O−V bonds into monomeric moieties forming hydroxyl groups (V−OH) and (ii) the condensation of dispersed vanadia groups into oligomeric species.

pentacoordinated dimers (Scheme 1, dP3) and trimers (Scheme 1, tP2) show conformations consisting of vanadium units in square pyramidal and trigonal bipyramidal geometries. The coexistence of these pentahedral geometries in the same dimeric or trimeric structures results from the geometric match of VOx unit to the (001) anatase surface. All mixed dimeric and trimeric tetra-, penta-, and hexa-coordinated structures contain exclusively one vanadyl VO group per vanadium atom. 3.2. Abundance of Dispersed Vanadium Oxide upon Progressive Hydration. Molecular dynamics (MD) simulated the molecular structures of dispersed vanadium oxide species: these are classified as monomers, dimers, or trimers, and the number of times they appear during MD runs gives their statistical presence. Figure 1 illustrates the polymerization degree of dispersed vanadium oxide species upon hydration at different temperatures. Temperature has a strong effect on the polymerization degree. Trimeric moieties formation exhibits a volcanic dependence with water presence, reaching a maximum at 2 water molecules per unit cell (Figure 1, all temperatures). Their maximum predominance, for 2 water molecules per unit cell, ranges from 50% at 300 K to ∼40% at 500 K and drop to ∼6% at 800 K. Concomitantly to this trend for trimers, the population of dimeric structures exhibits an inverse volcanic dependence with the water content. Dimeric moieties population sharply drops to ∼30% at 300 K, this minimum value rises to 40% at 500 K and becomes more abundant (∼60%) at 800 K. Dimers predominate with low water content at the calculated temperatures. It should be noted that dimers are the predominant structure at 800 K, regardless of the hydration levels imposed. Finally, the population of monomeric vanadia species grows continuously with water loading up to ∼35% at 300 and 500 K, leveling off at ca. 35%. Figure 2 illustrates the structural distribution of surface vanadium oxide species determined by the MD runs. Among the monomers, dehydrated OVO3 pyramid (mT1) and the OV(OH)O2 hydroxo-monovanadate (mT3) moieties predominate in the presence of water at the temperatures considered (Figure 2, top). The mT4 hydroxylated species appears upon a third or fourth water molecule addition at 500 and 800 K, with an abundance below ∼5%. Tetrahedral dimeric polyvanadates 25538



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Figure 2. Statistical distribution of surface species after water addition to high coverage vanadia/titania model, calculated at 300, 500, and 800 K: 4coordinated monomeric surface VOx species (mTx, top); 4- and 5,6-coordinated polymeric surface VOx species (dTx, dPHx, tTx, and tPHx, bottom). The % represents the number of times a given structure appears in the MD run with respect to the total number of structures counted.

V−OH + V−OH ⇄ V−O−V + H 2O

coverage. The surface species stabilize the system by forming hydrogen bonds. Figure 5 depicts the adsorption energy for the most stable conformations per water molecule as a function of the number of water molecules added (Table 1). The adsorption energy per water molecule is exothermic, and it becomes less favorable as the number of H2O molecules per unit cell increases. The most important gain in energy, −2.22 eV, occurs after adsorption of the first water molecule. Beyond four H2O molecules, the gain in energy is found to reach the value of about −0.95 eV. The adsorption energy per water molecule essentially levels off above four water molecules per unit cell. This result corresponds to the lack of dissociative adsorption of water molecules; this could be indicative of a transition from dissociative adsorption to condensation of water on the surface. In order to estimate the relative stability between the different species we use an atomistic thermodynamic approximation, as described elsewhere.43 We consider the vanadia/titania system in contact with a gaseous H2O reservoir. From the electronic energy, the free energy of the water/

The abundance of the different species depends on the balance between competitive reactions and therefore depends upon the water content and temperature. 3.4. Energetics and Structures of Vanadia/Titania Models under Hydrated Conditions. In order to investigate the stability of the different surface moieties, the most representative structures obtained from the MD runs are optimized at 0 K. The most energetically favorable systems are displayed in Figure 4, and the energetic parameters obtained are shown in Table 1. The mT3 moiety is the main monomeric species. Dimeric (dTx, dTPx, and dPx) and trimeric (tTx, tTPx, and tPx) structures mainly with one V−OH hydroxyl group appear as the most favorable conformations. They appear to coexist with monomeric structures on the titania surface. While water is mostly found to dissociate and form hydroxyl groups, its molecular form (black circle in Figure 4) is stabilized at eight H2O molecules per unit cell. Interestingly, no surface Ti−OH groups are found to be stable in any model probably due to the low support area exposed in the presence of high vanadia 25539



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Using the above-mentioned formalism, the free energy of reaction for the formation of the hydrated structures at equilibrium conditions, can be then expressed as ΔGn = E(titania−V2O5 −( 8 − n)H 2O) − nΔG(H 2O) − E(titania−V2O5)

In this approximation, we consider that the energies of the hydroxylated species are independent of the degree of hydration of the titania surface. Figure 6 shows the free energy, ΔG, of the complexes on titania surface as a function of temperature (T) for a water partial pressure (p) equivalent to that of the ambient air water partial pressure (pw = 1500 Pa at 25 °C 50% humidity).44 From our set of structures the most stable isomeric forms are selected for the phase diagram. At the above-mentioned conditions, the structure with eight water molecules is the most stable up to 325 K; then, the structure containing two water molecules up to 640 K; then, it loses one more water molecule. The thermodynamic approach suggests that the surface is highly hydrated up to 325 K and that it is difficult to obtain a complete dehydrated sample even at 800 K. 3.5. In Situ Raman Evolution of Vanadyl Bands during Dehydration in Dry and Humid Air. The vanadyl, VO, bond gives rise to the principal Raman mode of supportedvanadia catalysts. Its Raman shift is sensitive to the local environment, temperature, and humidity; especially the presence of hydrogen bonds, and can thus monitor hydration/dehydration processes. The experimental in situ Raman spectra during dehydration of 3.5VTi were acquired at increasing temperatures in a stream of dry and humid air. Figure 7 (left) exhibits representative spectra of the effect of humid and dry synthetic air on the Raman spectra of 3.5VTi at 473 K; the Raman shift of the vanadyl group appears at 1029 and 1013 cm−1 in dry air; and at 1012, 1006, and 992 cm−1 in humid air. Figure 7 (right) illustrates the evolution of vanadyl Raman mode upon progressive heating in dry and in humid synthetic air. Apart from the VO mode, an additional Raman band appears at 920−915 cm−1, which is assigned to V−O−V mode due to polymerization.45 The vanadyl and the V−O−V modes exhibit higher wavenumber in dry air than in humid air at any temperature studied. The difference in the vanadyl mode frequencies decreases with increasing temperature. Literature reports vanadyl bond redshifts upon exposure to humidity.45 According to that, a Raman band at 1029−1021 cm−1 would correspond to the vibrational mode of VO bond of polyvanadates species, while a band at 1013−1006 cm−1 would be related to the VO mode of monovanadates species. Both vanadyl vibrational modes are red-shifted in the presence of water vapor.36 On the basis of our theoretical calculations, humidity is to be invoked as a key factor on the vanadyl bond vibration. The latter bands are probably related to the VO mode of polyvanadates and/or monovanadates species in closer interaction with hydroxyl groups or water molecules with the formation of hydrogen bonds.31 This is coherent with an elongation of the VO bond observed in the model systems that increases from average 1.62 Å (dehydrated) to average 1.64 Å (hydrated), mainly because of H-bonding with neighboring hydroxyl groups. The close relationship between the VO bond vibration mode and bond length was clearly demonstrated by Hardcastle and Wachs.46

Figure 3. Evolution of vanadia surface species after two H2O molecules are added to high coverage vanadia/titania model. Molecular dynamics runs calculated at 800 and 500 K in femtoseconds (fs) time scale. Monomers are marked by black circles and trimers are marked by blue circles.

Figure 4. Most stable models of the high loading vanadia/titania catalyst after water molecules addition; optimized at 0 K. Water molecules are marked by a black circle.

vanadia/titania interface may be estimated under known thermodynamic conditions. It consists in the neglect of the variation of the chemical potentials of the surfaces with the adsorption and the consideration of the gas phase as a perfect gas. In the proposed scheme, the free energy of water (including the zero potential energy, ZPE, correction) in the gas phase is ΔG(H 2O) = E(H 2O) − ((ΔHG − T ΔSG(T )) + RT ln(p /p°))

(2)

(1)

where E(H2O) is the electronic energy of water calculated at 0 K, ΔHG and ΔSG(T) are the enthalpy and entropy of gaseous species, calculated with the Gaussian03 code37 as a function of the temperature, p is the partial pressure of water vapor, and p° is the standard pressure (1 bar). The minor entropy contributions to the vibrations of the surface are neglected.39 25540



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Table 1. Total Energy Optimized at 0 K for the 4VOx/TiO2 Model with Different Number of Water Molecules; All Energies in eV number of H2O molecules 0a 1a 1 1 2a 2 2 3a 3 3 4a 4 6 6 8a 8 a

Etotal, (relative) −2329.99 −2346.43 −2346.15 −2345.76 −2362.09 −2361.86 −2359.82 −2376.94 −2376.74 −2376.58 −2391.68 −2391.64 −2421.41 −2421.21 −2451.35 −2449.47

Eads per H2O molecule

VOx structure

physical effect

0.00 −2.22 −1.94 −1.55 −1.83 −1.72 −0.70 −1.43 −1.36 −1.31 −1.20 −1.19 −1.02 −0.98 −0.95 −0.72

mT1; dTx mT2; dTx; tTx mT2; dTPx; tTPx mT1; mT2; dTx; dTPx mT2; tTPx; tPx mT2; dT1; dTPx; tTx dTx; dT1 mT2; dPx; tTPx mT1; mT2; dT1 mT1; mT2; dTPx; tTPx dTPx; dPx mT1; mT3; dTx; dTPx dTx; dTPx dTx; dPx dPx dTPx; dPx

polymerization polymerization solvation polymerization polymerization solvation polymerization solvation polymerization solvation solvation solvation solvation solvation solvation

(0.00) (0.00) (0.28) (0.67) (0.00) (0.23) (2.27) (0.00) (0.20) (0.36) (0.00) (0.04) (0.00) (0.20) (0.00) (1.88)

See this model in Figure 4.

Figure 5. Adsorption energy per H2O molecule related to the most stable models of polymerized and hydrated structures of the high loading vanadia/titania catalyst; optimized at 0 K.

Figure 6. Free energy ΔG, of the complexes on titania surface as a function of temperature (T) for a water partial pressure (p) equivalent to the ambient air water partial pressure (pw = 1500 Pa).33 From our set of structures, the most stable isomeric forms are retained to draw the phase diagram. At the above-mentioned conditions, the structure with eight water molecules is the most stable until T = 325 K, followed by the structures containing two and one water molecules.

4. DISCUSSION 4.1. Nature of the Hydrated Titania Surface at High Vanadia Coverage. In situ spectroscopic studies of supported vanadia systems evidence differences between hydrated and dehydrated spectra related to different supported vanadium oxide species. These underline the complexity of the local structure of surface vanadium oxide species, as evidenced by the broadening of Raman and FT-IR bands. Such observations led Busca to propose a model of supported vanadia possessing a vanadyl and a V−OH group.47 Our results provides a rationale for this complex scenario in which the nature of the surface species at high coverage depends on their proximity and on the hydration degree of the surface. At low hydration extent, tetrahedral monomers and V−OH groups are easily formed and become predominant, while higher water content leads to their polymerization. This is mainly due to the high surface density of molecularly dispersed vanadium oxide species that facilitates their interaction, mainly driven by surface hydroxyl groups. These hydroxyl groups seem to play a key role both in the nature of the surface species and

on their dynamics. FT-IR spectroscopy detects the presence of V−OH species present under hydrated conditions in VOx/ TiO211,48 and VOx/SiO249−51 catalysts. The presence of a large number of hydroxyl groups in highly hydrated surfaces creates an amorphous vanadia surface stabilized by a H-bonding network. This structure resembles that of a gel. Indeed, amorphous V2O5·nH2O gels have been reported in the literature52 for the hydrolysis of vanadates. Structures similar to V2O5·nH2O gels were also suggested for hydrated surface vanadium oxide species.45,53 The experimental studies suggest a composition around V2O5·1.8H2O, which is close to that found in our calculations for high water content. Additionally, experiments52 suggest that vanadium displays a distorted square pyramidal VO5 environment with one short VO double bond as our model predicts. The presence of penta- and hexa-coordinated surface vanadium oxide species has been 25541



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Figure 7. (left) Representative Raman spectra of a catalyst containing 3.5 V atoms per nm2 of TiO2 (3.5VTi) in dry (green trace) and humid (blue trace) air flow at 473 K. (right) Raman vanadyl vibration mode vs temperature for 3.5VTi in dry (green trace) and humid (blue trace) air.

inate on the surface with stable V−OH group at mild hydration conditions, in which the hydrogen transfer is limited. 4.3. Comparison with Low Coverage Vanadia Catalysts. In a previous work, we studied the hydration of titaniasupported vanadium oxide at low coverage.31 The structural changes of high vanadia loading structures upon hydration present some similarities with the low loading ones. First, hydration is thermodynamically favorable although the adsorption energies are more exothermic for the low vanadia loading structures (−2.67 eV low loading, −2.22 eV high loading with one water molecule). Also, at hydration with low water concentration (up to 4 molecules per unit cell) water is found to adsorb dissociatively, forming V−OH and favoring the formation of monomers, whereas hydration at higher water concentration leads to the formation of a hydrogen-bond network. The main difference in the vanadia coverage is the role of the support, which is much more exposed at low vanadia content: in this case, TiO2 participates in the stabilization of dissociated water. The V−O−Ti to V−O−V population ratio is much higher at low vanadia loading; the V−O−Ti sites hydrolyze at higher water concentrations. In such a case, vanadium keeps a tetrahedral environment, and high water content solvates vanadium oxide moieties into leached OV(OH)3. Conversely, at high vanadia coverage, the vanadium sites are closer to each other, favoring the formation of polymeric vanadia units, which results in a much lower V−O− Ti to V−O−V population ratio. There are significantly less V− O−Ti sites available; thus, water adds to the vanadium centers increasing their coordination rather than hydrolyzing these bonds. For either system, interconversion between different surface species occurs rapidly through a rapid proton transfer between VO and V−OH sites, which depends on the temperature and on the water content.

proposed in the literature based on IR and Raman measurements reported by Busca and workers, who indicated that molecularly dispersed vanadium oxide on titania is composed of penta-coordinated mono-oxo vanadyl species along with a V− OH group.54 4.2. Dynamics of the Surface Species. Interconversion between different surface species occurs easily and reversibly depending on temperature. An important feature of the dynamic changes of vanadium oxide species during hydration is the mobility of hydrogen atoms (protons). During the MD runs they move from V−OH to vanadyl (VO) group and Ti−OH surface hydroxyl groups. If the transfer occurs to the bridging V−O−V and V−O−Ti bonds, they break up forming monomers. Hydrogen migration decreases with temperature and occurs mainly between V−OH and VO groups. The hydrogen mobility is also responsible for the formation of water molecules. Water recombines without preference on Ti−OH or V−OH sites at high temperature, while mostly forming on Ti− OH sites at low temperature. The changes in the surface vanadium oxide species resulting from the hydrolysis effect were observed by different spectroscopic techniques.36,53,55−57 The evolution of vibrational mode of VO bond from dry to humid environment was studied by Raman spectroscopy.36,45 The redshift of Raman bands appears during the hydration process of high loading titania-supported vanadium oxide catalysts. The spectroscopic study under controlled environment shows a displacement of vibrational VO mode from 1032 cm−1 for dehydrated surface to 1018 cm−1 for partially hydrated until 1000 cm−1 for highly hydrated surface. The changes in Raman spectra are induced by formation of hydrogen bonds between vanadyl group (VO) and surface hydroxyl group (V−OH) or water molecule. The extensive hydration can also result in the formation of solvated vanadium oxide species, which are closer to hydrated vanadates. The changes induced by hydrogen mobility were also mentioned for FT-IR studies of V2O5/TiO2 samples during temperature treatment.48 The downward frequency shift of the VO−H feature in FT-IR band position was discussed as a result of new Ti−OH and VO group formation by hydrogen atom transfer independently on the surface hydration degree.48 The present molecular dynamics calculations confirm the hydrogen transfer between all surface species at higher hydration conditions. It appears that hydrogen bond networks predom-

5. CONCLUSIONS The molecular dynamics calculations of high loading vanadia/ titania catalyst reveal the important role of the hydration degree and the temperature on the global changes of surface vanadium oxide species. The addition of water to high-loading vanadia/titania catalyst model is thermodynamically favorable, resulting in dissociative water adsorption and subsequent formation of hydroxylcontaining species. High water content (2H2O/V2O5) stabilizes a hydrogen-bonded network structure where molecular water is 25542



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stabilized. The interaction of surface hydroxyl groups with a neighboring VO group results in its elongation. This is consistent with the redshift of the VO Raman band observed in humid conditions compared to dry conditions. Two competitive physical processes, polymerization (formation of V−O−V bonds) and solvation (formation of vanadium penta- and hexa-coordinated), occur on the surface of vanadia/titania system. The abundance of the surface species depends on the water content and on temperature. The monomers, mainly tetrahedrally coordinated vanadium oxide species, form rapidly upon splitting of dimeric units in the presence of low water content, especially at high temperature. Polymerization is favored in mild hydration conditions (0.5H2O/V2O5) and temperatures up to 500K. Vanadium coordination higher than four is stabilized only at high water content. High and low vanadia coverages on titania systems exhibit similar behavior regarding the formation of V−OH groups and their stabilization upon hydration. The most important difference stems from the role of the support, which is hardly exposed at high vanadia coverage; thus, it only plays a significative role at low loading vanadia coverage. This induces different hydration mechanisms depending on vanadia content; mainly V−O−Ti hydrolysis at low vanadia coverage on titana and polymerization/solvation at higher vanadia coverages on titania support.

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ASSOCIATED CONTENT

S Supporting Information *

Periodic model of the high loading vanadia/titania catalyst. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*(A.E.L.) E-mail: [email protected] or [email protected]. *(M.C.) E-mail: [email protected]. *(M.A.B.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Spanish Ministry project PRO-CTQ201125517 and ESF COST Action D36, COST CM1104. A.E.L. acknowledges the European Commission, Capacities Area, Research Infrastructures for HPC-EUROPA2 project (nr: 228398) and Spanish Ministry of Education and Science for ″Juan de la Cierva″ postdoctoral contract. M.C. and F.T. thank HPC resources from GENCI-CINES/IDRIS (Grants 2010x2010082131 and 2011-x2011082022), the CCRE-DSI of Université P. M. Curie, and Institut Universitaire de France for a junior position. Dr. B. Diawara from LCPS ENS Paris is kindly acknowledged for providing us with ModelView used in the visualization of the structures.

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Titania Catalysts at High Loading: A

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